Comprehensive Organometallic Chemistry IV [Volume 8. Groups 8 to 10. Part 2] 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 8

GROUPS 8 TO 10 - PART 2 VOLUME EDITOR

TIMOTHY H. WARREN Michigan State University, East Lansing, MI, United States

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

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

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

CONTENTS OF VOLUME 8 Editor Biographies

vii

Contributors to Volume 8

xiii

Preface 8.01

xv

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

1

Jooyeon Lee, Changho Yoo, Jaesung Kwak, and Min Kim

8.02

Half-Sandwich Rhodium and Iridium Complexes

55

Wen-Xi Gao, Peng-Fei Cui, Zheng Cui, and Guo-Xin Jin

8.03

Group 9 Boryl Complexes

188

Makoto Yamashita

8.04

Group 9 and 10 Carbonyl Clusters

205

Cristina Femoni, Cristiana Cesari, Maria Carmela Iapalucci, Silvia Ruggieri, and Stefano Zacchini

8.05

Nickel-Carbon s-Bonded Complexes

271

Clifton L Wagner and Tianning Diao

8.06

Cyclopentadienyl Nickel Complexes

357

Buchowicz Włodzimierz

8.07

N-Heterocyclic Carbene Complexes of Nickel

427

Irene Ligielli, Andreas A Danopoulos, Pierre Braunstein, and Thomas Simler

8.08

Palladium and Platinum NHC Complexes

575

Fabian Mohr, Nicole S Gawlik, and Bernd Mell

8.09

Allyl-Palladium Complexes in Organic Synthesis

632

Rodney A Fernandes, Praveen Kumar, and Naveen Chandra

8.10

Zerovalent Nickel Organometallic Complexes

680

Jorge A Garduño and Juventino J Garcí a

8.11

Monovalent Group 10 Organometallic Complexes

733

K Matsubara

8.12

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

783

Andrei N Vedernikov

v

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

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Editor Biographies

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

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

Editor Biographies

ix

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

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Editor Biographies

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

Editor Biographies

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

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

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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 8 Pierre Braunstein Laboratoire de Chimie de Coordination, CNRS, Institut de Chimie UMR 7177, Université de Strasbourg, Strasbourg, France Cristiana Cesari Department of Industrial Chemistry “Toso Montanari”, University of Bologna (IT), Bologna, Italy Naveen Chandra Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, India Peng-Fei Cui Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai, PR China Zheng Cui Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai, PR China Andreas A Danopoulos Laboratory of Inorganic Chemistry, National and Kapodistrian University of Athens, Athens, Greece Tianning Diao Department of Chemistry, New York University, New York, NY, United States Cristina Femoni Department of Industrial Chemistry “Toso Montanari”, University of Bologna (IT), Bologna, Italy Rodney A Fernandes Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, India Wen-Xi Gao Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai, PR China

Juventino J García Facultad de Quí mica, Universidad Nacional Autónoma de México, Mexico City, Mexico Jorge A Garduño 6128 Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, NH, United States Nicole S Gawlik University of Wuppertal, Inorganic Chemistry, Wuppertal, Germany Maria Carmela Iapalucci Department of Industrial Chemistry “Toso Montanari”, University of Bologna (IT), Bologna, Italy Guo-Xin Jin Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai, PR China Min Kim Department of Chemistry, Chungbuk National University, Cheongju, Korea Praveen Kumar Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, India Jaesung Kwak Infectious Diseases Therapeutic Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon, Korea Jooyeon Lee Department of Chemistry, Chungbuk National University, Cheongju, Korea Irene Ligielli Laboratory of Inorganic Chemistry, National and Kapodistrian University of Athens, Athens, Greece K Matsubara Fukuoka University, Fukuoka, Japan

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Contributors to Volume 8

Bernd Mell University of Wuppertal, Inorganic Chemistry, Wuppertal, Germany

Clifton L Wagner Department of Chemistry, New York University, New York, NY, United States

Fabian Mohr University of Wuppertal, Inorganic Chemistry, Wuppertal, Germany

Buchowicz Włodzimierz Faculty of Chemistry, Warsaw University of Technology, Warsaw, Poland

Silvia Ruggieri Laboratory of Luminescent Materials, Department of Biotechnology, University of Verona, Verona, Italy

Makoto Yamashita Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya, Aichi, Japan

Thomas Simler Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Institut Polytechnique de Paris, Palaiseau, France Andrei N Vedernikov Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, United States

Changho Yoo Green Carbon Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon, Korea Stefano Zacchini Department of Industrial Chemistry “Toso Montanari”, University of Bologna (IT), Bologna, Italy

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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8.01

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Jooyeon Leea, Changho Yoob, Jaesung Kwakc, and Min Kima, aDepartment of Chemistry, Chungbuk National University, Cheongju, Korea; bGreen Carbon Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon, Korea; cInfectious Diseases Therapeutic Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon, Korea © 2022 Elsevier Ltd. All rights reserved.

8.01.1 Introduction 8.01.2 Rh and Ir complexes with monodentate NHCs 8.01.2.1 Syntheses of Rh and Ir complexes with monodentate NHCs 8.01.2.1.1 Syntheses of Rh and Ir complexes with monodentate NHCs from [M(COD)X]2 8.01.2.1.2 Syntheses of Rh and Ir complexes with monodentate NHCs from [M(alkene)2X]2 8.01.2.1.3 Syntheses of Rh complexes with monodentate NHCs from Rh2(O2CR)4 8.01.2.1.4 Syntheses of Rh and Ir complexes with monodentate NHCs from [Cp MCl2]2 8.01.2.1.5 Syntheses of chiral Rh and Ir complexes with monodentate NHCs 8.01.2.2 Reactivities of Rh and Ir complexes with monodentate NHCs 8.01.2.2.1 Syntheses of Rh(NHC)– and Ir(NHC)–carbonyl complexes 8.01.2.2.2 Rh(NHC)– and Ir(NHC)–N2 complexes 8.01.2.2.3 Reactions of Rh(NHC) and Ir(NHC) complexes with O2 8.01.2.2.4 Reactions of Rh(NHC) and Ir(NHC) complexes with H2 8.01.2.2.5 Reactions of Rh(NHC) and Ir(NHC) complexes with X–H bonds (X ¼ C, O, B) 8.01.2.2.6 Syntheses and reactivities of Rh(NHC) Fisher carbenes 8.01.2.2.7 Syntheses and reactivities of Rh(NHC)(COD)X and Ir(NHC)(COD)X (X ¼ OH, F) 8.01.2.2.8 Redox reactions of Rh(NHC) and Ir(NHC) complexes 8.01.3 Rh and Ir complexes with chelating bis-NHC ligands 8.01.3.1 Bis-NHC ligands with alkyl linkers 8.01.3.1.1 Bis(imidazolidene) ligands with alkyl linkers 8.01.3.1.2 Bis(abnormal carbene) ligands with alkyl linkers 8.01.3.1.3 Bis(triazolidene) ligands with alkyl linkers 8.01.3.1.4 Other bis-NHC ligands with alky linkers 8.01.3.2 Annulated bis-NHC ligands 8.01.3.3 Bis-NHC ligands with rigid linkers 8.01.3.4 Bis-NHC ligands with chiral linkers 8.01.3.5 Pincer bis-NHC ligands 8.01.3.6 Scorpionate-bis-NHC ligands 8.01.4 Rh(NHC) and Ir(NHC) complexes with connected external coordinating groups 8.01.4.1 Rh(NHC) and Ir(NHC) complexes with connected external phosphine groups 8.01.4.1.1 Rh(NHC) and Ir(NHC) complexes with connected external triarylphosphine groups 8.01.4.1.2 Rh(NHC) and Ir(NHC) complexes with connected external trialkylphosphine groups 8.01.4.1.3 Rh(NHC) and Ir(NHC) complexes with connected external mixed phosphine groups 8.01.4.2 Rh(NHC) and Ir(NHC) complexes with connected external Cp groups 8.01.4.3 Rh(NHC) and Ir(NHC) complexes with connected external heterocycles 8.01.4.3.1 Rh(NHC) and Ir(NHC) complexes with connected external pyridine groups 8.01.4.3.2 Rh(NHC) and Ir(NHC) complexes with connected external oxazoline groups 8.01.4.3.3 Rh(NHC) and Ir(NHC) complexes with connected external imidazole, pyrimidine, and pyrazole groups 8.01.4.3.4 Rh(NHC) and Ir(NHC) complexes with connected external triazole and quinoline groups 8.01.4.4 Amines, alcohols, thiols, and their derivatives 8.01.4.4.1 Rh(NHC) and Ir(NHC) complexes with connected external amine groups 8.01.4.4.2 Rh(NHC) and Ir(NHC) complexes with connected external alcohol, ether, and carboxylate groups 8.01.4.4.3 Rh(NHC) and Ir(NHC) complexes with connected external sulfur functional groups 8.01.4.5 Olefins and other teathers 8.01.5 Conclusion Acknowledgment References

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00085-8

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N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.1

Introduction

Phosphines are widely used as ligands to govern the stability and reactivity of metal species but are easily oxidized to phosphine oxides under ambient conditions, whereas electron-rich N-heterocyclic carbenes (NHCs) can strongly bind metals to afford extraordinary stable metal NHC complexes. Consequently, since Arduengo’s discovery of stable NHCs, numerous metal NHC complexes have been developed as alternatives to metal phosphine complexes. Given the importance of Rh and Ir complexes for both academia and industry, the NHC complexes of these metals have been extensively studied.1–4 Herein, we review the syntheses, structures, reactivities, and catalytic applications of Rh(NHC) and Ir(NHC) complexes reported since the 1990s. However, we do not cover the general information on NHC ligands themselves, as their electronic and steric properties have been comprehensively reviewed elsewhere. Specifically, this chapter describes the syntheses and reactivities of Rh and Ir complexes with (1) monodentate NHCs, (2) chelating bidentate NHCs, and (3) NHCs bearing external coordinating groups.

8.01.2

Rh and Ir complexes with monodentate NHCs

8.01.2.1

Syntheses of Rh and Ir complexes with monodentate NHCs

8.01.2.1.1

Syntheses of Rh and Ir complexes with monodentate NHCs from [M(COD)X]2

M(COD)(NHC)X (M ¼ Rh or Ir, COD ¼ cyclooctadiene, X ¼ halogen), which are the most versatile Rh/Ir NHC complexes used to benchmark the properties of newly synthesized NHC ligands and catalyze a variety of chemical transformations, are synthesized from [Rh(COD)Cl]2 (A1a) or [Ir(COD)Cl]2 (A1b) and appropriate NHC precursors (Scheme 1). The direct ligation of 1 equiv. of free carbene to A1a or A1b affords M(COD)(NHC)Cl (Scheme 1A). Given the low stability of free carbenes toward O2, their direct use requires air-free conditions. Consequently, free carbenes can be masked with other molecules to avoid such limitations (Scheme 1B). For example, free carbenes can bind CO2 to afford imidazolium carboxylates and be regenerated upon CO2 release by heating.5 In addition, the reaction of free carbenes with pentafluorobenzene gives C–H insertion products that can be used as efficient carbene transfer reagents.6 The alcohol adduct A2 prepared by reacting KOtBu with SIMes
HX can also be used as a carbene transfer reagent.7 Free carbenes are most frequently generated in situ by the deprotonation of their precursors with suitable bases such as alkoxides, sodium hydride, and carbonates8 (Scheme 1C). As a modification of this method, one can mention the reaction of [M(COD) OR]2 (R ¼ H or alkyl) produced via the reaction of A1a or A1b with metal hydroxides or metal alkoxides with the subsequently added NHC
HX (Scheme 1D).9 Alternatively, the desired species can be obtained by transmetalation reactions between A1a or A1b and suitable metal (e.g., Ag) NHC complexes (Scheme 1E). Among the metal NHC complexes, Ag(NHC) complexes are the most versatile carbene transfer reagents10,11 and can be prepared in situ by reacting Ag2O with NHC
HX. Finally, double C–H bond activation of cyclic H2C(NRCH2)2 by [M(COD)Cl]2 can generate metal carbene bonds (Scheme 1F). For this purpose, diolefin additives such as cyclooctene (COE) and norbodiene (NBD) are required as H2 acceptors.12 Due to the cis-coordination mode of the COD ligand, the synthesis of bis-NHC-ligated Rh or Ir COD complexes requires the presence of a small alkyl substituent at the nitrogen atom of NHC ligands to reduce steric conflict between NHC ligands. For example, the complexation of 2 equivalent of IMe with A1a afforded the bis-NHC complex [Rh(IMe)2(COD)]Cl (A3) (Scheme 2).13 The reactions of the Ag bis-NHC complex A4 with A1a and A1b afforded Rh and Ir bis-NHC complexes A5a and A5b, respectively (Scheme 3).14 Heteroleptic bis-NHC complexes were prepared by reacting Ag NHC complexes with Ir(NHC)(COD)Cl. For example, the reactions of [Ag(InBu)2]PF6 (A7) and [Ag(IMe)2]PF6 (A8) with Ir(COD)(NHC1)Cl (A6) afforded [Ir(COD)(NHC1)(NHC2)] PF6 complexes A9 and A10, respectively (Scheme 4).15 Cationic [Ir(SIMes)(COD)(py)]PF6 (A11) was prepared by treating the cationic Ir precursor, [Ir(COD)(py)2]PF6, with SIMes (Scheme 5),16 while cationic [Ir(COD)(NHC)L]X complexes were prepared by abstracting the halide ligand of Ir(COD)(NHC)X. The reaction of Ir(COD)(NHC)Cl with AgX in the presence of ligand L afforded cationic [Ir(COD)(NHC)L]X complexes A13, A16–A17 (Scheme 6).17,18 When the same reaction was performed in the absence of additional ligands, the N-aryl ring of the NHC ligand was coordinated to the vacant site of the metal center to afford a square planar geometry. Several cationic Rh and Ir COD complexes bearing no additional L-type ligands have been reported (A18–A21, Fig. 1).19–22 [Rh(COD)]+ or [Ir(COD)]+ fragments complexed with neutral NHCs had weakly coordinating counter-anions such as BF−4 and PF−6, whereas those complexed with zwitterionic NHCs did not. The zwitterionic complex A18 had a high solubility in nonpolar solvents because of its overall neutral charge, and the zwitterionic NHC ligand could stabilize electrophilic metal species. In view of these properties, complex A18b is a highly active olefin hydrogenation catalyst.19

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 1

3

4

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 2

Scheme 3

Scheme 4

Scheme 5

Scheme 6

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

5

Fig. 1 Cationic Rh and Ir COD complexes bearing no additional L-type ligands.

Halide ligands can be exchanged for pyridine ligands without using halide-abstracting reagents, e.g., the chloride ligand in Ir(IMes)(COD)Cl (A15) was replaced with pyridine in methanol to afford [Ir(IMes)(COD)(py)]Cl (A22) (Scheme 7).23

Scheme 7

8.01.2.1.2

Syntheses of Rh and Ir complexes with monodentate NHCs from [M(alkene)2X]2

Except for p-acidic CO and P(OPh)3,24 other ligands cannot easily substitute COD because of its bidentate coordination mode. For example, the reaction of Rh(IPr)(COD)Cl (A23) with 2,20 -bipyridine (bpy) or phenanthroline (phen) does not afford ligand-exchanged products, which can only be obtained from [M(alkene)2Cl]2 (M ¼ Rh or Ir, alkene ¼ ethylene or cyclooctene) as precursors. IPr (2 equiv.) was reacted with [Rh(ethylene)2Cl]2 to exchange one ethylene and thus afford [Rh(IPr)(ethylene) Cl]2 (A24).25,26 The treatment of A24 with bpy or phen afforded Rh(IPr)(bpy)Cl (A25a) or Rh(IPr)(phen)Cl (A25b), respectively (Scheme 8).27

Scheme 8

Numerous ligands have been incorporated into [M(NHC)(alkene)Cl]2 (Scheme 9). The reactions of [Rh(NHC)(ethylene) Cl]2 with PPh3 afforded tetra-heteroleptic complexes Rh(NHC)(ethylene)(PPh3)Cl (A26a–A26d).28 The analogous pyridine-coordinated complexes Rh(IPr)(ethylene)(py)Cl (A26e) were obtained by reacting [Rh(IPr)(ethylene)Cl]2 with pyridine.27 The treatment of [Rh(NHC)(COE)Cl]2 with bisphosphine ligands (P-P) generated Rh(NHC)(P-P)X (A27a–A27d).29 Finally, the exchange of the chloride ligand of [Rh(IPr)(alkene)Cl]2 for acetylacetonate (acac) afforded Rh(IPr)(acac)(alkene) (A28). The acac ligand was removed by TfOH in MeCN to obtain a solvated Rh(NHC) complex, [Rh(IPr)(MeCN)3]OTf (A29).30

6

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 9

Treatment of [M(alkene)2Cl]2 with 4 equiv. of NHC affords either bis-NHC complexes or intramolecular cyclometalated complexes, depending on the N-substituent of the NHC ligand (Scheme 10, Fig. 2). If the peripheral C–H bond of the NHC ligand is not in close proximity to the metal center, a bis-NHC complex is obtained. Ir(IiPrMe)2(COE)Cl (A30), Ir(ICy)2(COE)Cl (A31), and Ir(IDD)2(COE)Cl (A32) were obtained via free carbene transfer methods, whereas Ir(IiPr)2(COE)Cl (A33) and Ir(IsB)2(COE)Cl (A34) were obtained via transmetalation of Ag(NHC) complexes (Scheme 11).31

Scheme 10

Fig. 2 NHC-Cyclometalated Rh(NHC) and Ir(NHC) complexes.

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

7

Scheme 11

The reaction of [Rh(COE)2Cl]2 with 4 equiv. of IMes in hexane solution afforded Rh(IMes)2(COE)Cl (A35),25 whereas the same reaction in benzene or THF generated cyclometalated Rh(IMes’)(IMes)(H)Cl (A36a).32 Similar Ir(NHC) complexes, Ir(IMes’)(IMes) Cl (A36b)33 and Ir(SIMes’)(SIMes)(H)Cl (A37), 34 were obtained from Ir(IMes)2(COE)Cl (A38) and Ir(SIMes)2(COE)Cl (A39), respectively, in toluene. The Ir–C bond of A36b was further reacted with LiBH4 to borylate the methyl group of the mesityl ring. On the other hand, no solvent-dependent intramolecular cyclometalation was observed in the synthesis of Rh(IPr)2(COE)Cl (A40).25 Intramolecular cyclometalation of the diisopropylphenyl (DIPP) group (A41) with the formation of Rh black occurred in the toluene solution of [Rh(SIPr)(ethylene)Cl]2 (A42) at 80 C.26 Interestingly, the reaction of [Ir(COE)2Cl]2 and IPr in THF solution resulted in the dehydrogenation of the isopropyl group of DIPP to afford Ir(IPr00 )(IPr)Cl (A43).35 This complex was presumably obtained through the initial oxidative addition of the isopropyl C–H bond to Ir followed by b-hydride elimination. To avoid the above intramolecular cyclometalation, bis-NHC complexes can be synthesized under H2. In this case, H2 addition results in the hydrogenation of the cyclometalated M–C bond to afford M(NHC)2(H)2Cl.32,35,36 In special cases, doubly cyclometalated bis-NHC complexes have been reported. The reaction of six- or seven-membered saturated NHC ligand (6-Mes or 7-Mes; 6 equiv.) with [Ir(COE)2Cl]2 in THF solution gave doubly cyclometalated complexes Ir(6-Mes’)2(H) (A44) and Ir(7-Mes’)2(H) (A45), respectively, with elimination of HCl (Fig. 3).37 The difference in reactivity (compared to that observed in the case of A36b) was ascribed to the increased NCN angles of 6-Mes and 7-Mes and the thus decreased distance between the peripheral C–H bond and the metal center.34 The same reaction with DIPP wingtips resulted in backbone decomposition.38 The reaction of ItBu with [M(COE)2Cl]2 initially afforded cyclometalated M(ItBu’)(ItBu)(H)Cl (A46a for Rh and A46b for Ir), and the additional reaction gave doubly cyclometalated complexes M(ItBu’)2Cl (A47a for Rh and A47b for Ir) and H2.39–41 The free rotational motion of the tert-butyl group of ItBu mediated the intramolecular oxidative addition of the C–H bond to the metal center, in line with the resistance of the conformationally rigid ItBu analogue iBioxMe4 to intramolecular oxidative addition (Scheme 11).42,43 tris-NHC complexes were obtained using iBioxMe4 as a ligand. The reaction of [Rh(IBioxMe4)(COE)Cl]2 (A48) with 4.6 equiv. of IBioxMe4 afforded trans-[Rh(IBioxMe4)2(COE)Cl] (A49, 47%) and [Rh(IBioxMe4)3Cl] (A50, 24%) (Scheme 11). The abstraction of chloride from A50 by Na[BArF] (BArF ¼ B(3,5-C6H3(CF3)2)4) afforded cationic three-coordinated [Rh(IBioxMe4)3][BArF] (A51a) with a T-shaped geometry.42 The corresponding T-shaped Ir complex [Ir(IBioxMe4)3]BArF (A51b) was also reported.44 The Rh tris-ICy complex [Rh(ICy)3(CO)]PF6 (A52) and [Rh(IiPrMe2)3(CO)]PF6 (A53) were prepared by reacting Rh(H)(PPh3)3(CO) with 6 equiv. of ICy or IiPrMe2, respectively, followed by treatment with KPF6.45

Fig. 3 Doubly cyclometalated bis-NHC iridium complexes.

8

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.2.1.3

Syntheses of Rh complexes with monodentate NHCs from Rh2(O2CR)4

The coordination of NHC ligands at the axial position of paddlewheel dirhodium(II) carboxylates affords either [Rh2(O2CR)4(NHC)] or [Rh2(O2CR)4(NHC)2] (Fig. 4). The reaction of Rh2(O2CtBu)4(H2O)2 and 2 equiv. of IMeMe afforded monoligated Rh2(O2CtBu)4(IMeMe) (A54).46 The bis-NHC complexes Rh2(TFA)4(IPr)2 (A55), Rh2(TFA)4(SIPr)2 (A56), Rh2(OAc)4(IPrCl)2 (A57), and Rh2(OAc)4(SIPrCl)2 (A58) were prepared by the same method.47 Syntheses relying on the in situ generation of carbenes by treatment of carbene precursors with bases are also effective. Rh2(OAc)4(IPr)2 (A59) and Rh2(OAc)4(SIPr)2 (A60) were prepared by reacting Rh2(OAc)4 with 3 equiv. of IPr
HCl or SIPr
HCl, respectively, and KOtBu.48 These complexes catalyzed the arylation of aldehydes, with mono-NHC complexes envisioned as key catalytic intermediates.

8.01.2.1.4

Syntheses of Rh and Ir complexes with monodentate NHCs from [Cp MCl2]2

Pentamethylcyclopentadienyl (Cp ) metal halide dimers are frequently used as precursors for the syntheses of M(III)-NHC complexes. Cp M(NHC)Cl2 complexes are obtained from[Cp MCl2]2 by methods similar to those described in Section 8.01.2.1.1, e.g., via the use of free carbenes,49,50 in situ generated carbenes,51 or Ag(NHC) complexes,52 and catalyze a variety of reactions such as transfer hydrogenation,53 Oppenauer-type oxidation,49 water oxidation,54 CO2 reduction,55 and acceptorless dehydrogenation.56 As in the case of M(I)-NHC complexes, intramolecular C–H activation of NHCs is also possible in the Cp M(NHC)Cl2 platform. Given the high oxidation state of these complexes, the spontaneous cyclometalation of NHC ligands via oxidative addition is less likely. However, the exposure of Cp M(NHC)Cl2 to bases often leads to intramolecular cyclometalation.57,58

8.01.2.1.5

Syntheses of chiral Rh and Ir complexes with monodentate NHCs

Given the umbrella-type structure of monodentate NHC ligands, chiral alkyl substituents were introduced at the peripheral position of NHCs to form chiral ligands (Fig. 5). However, the resulting chiral NHC complexes generally exhibit low enantioselectivity in a variety of catalytic reactions. For example, the hydrosilylation of acetophenone with diphenylsilane catalyzed by A61a afforded 32% ee,59 and the transfer hydrogenation of phenones catalyzed by A61b gave 52.6% ee.60 The low enantio-induction observed for complexes of this type is partly due to the rotational flexibility of N-alkyl substituents around the metal center.61 When planar chiral NHC A62 was used as a ligand for the Rh-catalyzed 1,4-addition of arylboronic acids, high enantioselectivities of up to 95% ee were obtained.62 The backbone of saturated NHCs can be modified to maximize chiral induction around the metal center (Fig. 6), and the cis-substitution pattern at this backbone transfers the chiral information to N-subsitituents.61 This structural feature enables N-aryl substitution of NHC ligands (A64–A66). Hydrosilylation catalyzed by A64a proceeded with moderate (up to 58% ee) enantioselectivity.63 The cationic chiral Ir NHC complex A66 demonstrated excellent enantioselectivity in the intramolecular hydroamination of aminoolefins64 and also effectively promoted ring-opening aminations.65

Fig. 4 Coordination of NHC ligands at the axial position of paddlewheel dirhodium(II) carboxylates.

Fig. 5 Chiral alkyl substituents at the peripheral position of NHCs.

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

9

Fig. 6 Chiral introduction at the backbond of saturated NHCs.

Fig. 7 Rotatable N-substitution at NHCs.

The tethering of N-alkyl substituents to the NHC backbone allows the rotatable N-substituents to gain conformational rigidity and thus enhances the chiral environment around the metal center (Fig. 7). Transfer hydrogenation catalyzed by complex A67 proceeded with high enantioselectivity, which was ascribed to the fact that in this complex, a peripheral alkyl group was tethered to the NHC backbone to effectively promote chiral induction.66 As described in Section 8.01.2.1.2, the peripheral gem-dimethyl substituent of the iBioxMe4 ligand is conformationally rigid, which makes the iBiox ligand an effective platform for enantioselective reactions. The Rh(Ibiox[(-)-menthyl])(CO)2Cl complex (A68) effectively catalyzed the enantioselective hydroarylation of azabicycles.67

8.01.2.2 8.01.2.2.1

Reactivities of Rh and Ir complexes with monodentate NHCs Syntheses of Rh(NHC)– and Ir(NHC)–carbonyl complexes

The COD ligand of M(NHC)(COD)X is readily replaced by CO to afford M(CO)2(NHC)X complexes with a cis arrangement of the two carbonyl groups (Scheme 12). The carbonyl ligand trans to the NHC ligand is readily replaced by DMSO and PPh3.68 M(CO)2(NHC)X are benchmark compounds for gauging the electronic properties of the bound NHC ligand.69,70 The dicarbonyl complexes feature two carbonyl stretches with respect to the relative position of the NHC ligand, and the average v(CO) values are important parameters describing the electronic structure of NHC ligands. The average v(CO) values of Rh(CO)2(NHC)X and Ir(CO)2(NHC)X were found to be linearly correlated with each other and were converted to the Tolman electronic parameter (TEP), which is a universal parameter describing the electronic properties of ligands.10,71,72

Scheme 12

Bis-NHC carbonyl complexes M(NHC)2(CO)Cl were prepared by treating M(NHC)2(H)2Cl with CO, e.g., Rh(IPr)2(CO)Cl (A70) was obtained from Rh(IPr)2(H)2Cl (A69).36 The reaction of the cyclometalated bis-NHC complex Rh(IMes’)(IMes)(H)Cl (A36a) with CO afforded the carbonyl complex Rh(IMes)2(CO)Cl (A71) and H2.32

8.01.2.2.2

Rh(NHC)– and Ir(NHC)–N2 complexes

The treatment of [Rh(ethylene)2Cl]2 with 4 equiv. IPr in THF under N2 generated Rh(IPr)2(N2)Cl (A72a) (Scheme 13).73 Rh(SIPr)2(N2)Cl (A72b) was prepared in a similar way except for the use of benzene instead of THF.26 The N–N bond length of bound N2 in A72a (1.100(6) A˚ ) was slightly longer than that in free N2 (1.0975 A˚ ). The vibrational frequency of bound N2 equaled

10

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 13

2103 cm−1, where those of free N2 is 2330 cm-1. While A72a is stable as a solid under an inert atmosphere, the bound N2 of A72a is readily replaced by O2, H2, and CO in solution to afford Rh(IPr)2(O2)Cl (A73), Rh(IPr)2(H)2Cl (A74), and Rh(IPr)2(CO)Cl (A75), respectively. The reaction of Ir(IMes)2(H)2Cl (A76) with Na[BArF] gave [Ir(IMes)2(H)2Cl(Na)]BArF (A77), in which the sodium cation was intercalated between the mesityl rings of two IMes ligands (Scheme 14).74 When a toluene solution of A77 was heated at 80 C under N2, NaCl and H2 were eliminated to give [Ir(IMes)2(N2)2]BArF (A78a). [Ir(IMes)2(N2)(THF)]BArF (A78b) was obtained similarly to A78a and recrystallized from THF/pentane. The N–N bond length of A78a was not reported because of high disorder, whereas that of A78b was obtained as 1.109(8) A˚ . The v(N–N) of A78a and A78b equaled 1993 and 1995 cm−1, respectively.

Scheme 14

8.01.2.2.3

Reactions of Rh(NHC) and Ir(NHC) complexes with O2

The reactivity of Rh(NHC) and Ir(NHC) complexes (viewed as highly stable analogues of Rh and Ir phosphine complexes) toward O2 was explored (Fig. 8). Rh(NHC)-O2 adducts were synthesized by treatment of suitable Rh(NHC) precursors with air or O2. Rh(IMes)(PPh3)(O2)Cl (A80) was obtained by the oxygenation of Rh(IMes)(PPh3)2Cl with loss of OPPh3,75 whereas the analogous reaction of Rh(NHC)(P-P)Cl led to decomposition with the formation of diphosphine oxide.29 Other bidentate ligands were also employed as supporting ligands for O2 adduct formation. Rh(NHC)(P-N)(O2)Cl (NHC ¼ IMes, A81a; NHC ¼ IPr, A82b; P-N: chelated o-(diphenylphosphino)-N,N-dimethylaniline) was obtained by stirring a toluene solution of Rh(NHC)(P-N)Cl under 1 atm of O2. In addition, Rh(IPr)(bipy)(O2)Cl (A82a) and Rh(IPr)(phen)(O2)Cl (A82b) were prepared from Rh(IPr)(bipy)Cl (A25a) and Rh(IPr)(phen)Cl (A25b), respectively.27 Rh(NHC)2(O2)Cl (NHC ¼ IMes, A83a; NHC ¼ IPr, A83b; NHC ¼ SIPr, A83c) was prepared from the corresponding N2 complexes Rh(NHC)2(N2)Cl in either solution or solid states.26,76 Alternatively, A83a and A83b were also accessed by treatment of Rh(IMes)2(H)2Cl (A84) and Rh(IPr)2(H)2Cl (A74), respectively, with O2 (1 atm).73,76 Rh(IPr)(py)(O2)Cl (A86) was obtained by bubbling O2 into a toluene solution of Rh(IPr)(COE)(py)Cl (A85).77 The addition of pyridine to A86 afforded the bis-pyridine adduct Rh(IPr)(O2)(py)2Cl (A87). Cationic Rh bis-NHC complexes of O2 were synthesized by treatment of Ag salts with Rh(NHC)2(O2)Cl.78 The reaction of A83b and AgBF4 in acetonitrile gave [Rh(IPr)2(MeCN)2(O2)] BF4 (A88a), while the reaction of A83c and AgOTf in acetonitrile gave [Rh(SIMes)2(MeCN)2(O2)]OTf (A88b). While complexes with short O–O bond distances (1.24–1.39 A˚ ) and square planar geometry at Rh are classified as RhI{1O2} complexes, those with elongated O–O bond distances (1.43–1.5 A˚ ) and octahedral geometry at Rh are classified as RhIII peroxo complexes.75,76 IR and Raman spectroscopy were used to acquire additional information on the bound O–O bond, as exact O–O bond metrics were difficult to determine from crystal structures.79 The O–O bond of A83b was short (1.267(13) A˚ ) and exhibited a stretching frequency of 1010 cm−1. Although these features are generally observed in metal-superoxide complexes, Rh L-edge X-ray absorption spectroscopy clearly assigned A83b as square planar Rh(I) bound to singlet oxygen. A80,75 A83a,76 A83b,76 A83c,76 and A8677 were classified as RhI{1O2} complexes, whereas octahedral A81a,25 A81b,25 A82a,27 A82b,27 A87,77 A88a,78 and A88b78 were classified as RhIII peroxo complexes.

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

11

Fig. 8 O2 adducts of Rh(NHC) and Ir(NHC) complexes.

Ir(NHC)-O2 adducts have been less studied. Ir(IPr)2(O2)Cl (A89) was obtained by exposure of Ir(IPr)2(H)2Cl (A90) to air35 and featured an O–O bond length of 1.361(7) A˚ (determined from the crystal structure) and an O–O stretching frequency of 863 cm−1. These data were consistent with those of the IrIII peroxo complex. The exposure of [Cp Ir(IMe)(Me)(CD2Cl2)]BArF (A91) to air afforded [(Cp Ir(IMe))2(m-O)](BArF)2 (A92) (Scheme 15).80 The reaction proceeded via the formation of a transient m-peroxo complex followed by homolytic cleavage of the O–O bond. The resulting terminal Ir oxo species were trapped by A91 to give A92.

Scheme 15

8.01.2.2.4

Reactions of Rh(NHC) and Ir(NHC) complexes with H2

The reaction of Ir(IPr)(COD)Cl (A12) with H2 results in COD hydrogenation, and the resulting intermediates further react with H2 to afford a dihydride complex (Fig. 9).81 In a non-coordinating solvent, the dihydride complex aggregates to form a trinuclear

Fig. 9 Dihydride Ir(NHC) complexes.

12

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

[Ir(IPr)(H)2Cl]3 complex (A93). The reaction of Ir(IMes)(COD)Cl (A15) with H2 afforded a trinuclear complex, Ir3(IMes’) (IMes)2(H)5Cl3 (A94), in which one of the Ir(IMes) fragments was cyclometalated. The hydrogenation of [Rh(COE)(NHC) Cl]2 afforded dimeric [Rh(NHC)(H)2Cl]2 (NHC ¼ IMes, A95a; NHC ¼ IPr, A95b).36 When the chloride of Ir(IPr)(COD)Cl (A12) was removed by treatment with Na[BArF], DIPP ring dehydrogenation occurred to afford A95 (Scheme 16).81 The hydrogenation of this complex in fluorobenzene afforded [Ir(IPr)(H)2(6-C6H5F)]BArF (A96), and the subsequent thermolysis of A96 in dichloromethane gave binuclear [Ir(IPr)(H)2](BArF)2 (A97) featuring an 6-coordination of the metal center to the DIPP ring.

Scheme 16

The hydrogenation of zwitterionic Ir(WCA-NHC)(COD) (A18a) in benzene afforded [Ir(WCA-NHC)(H)2(6-C6H6)] (A98) and

Scheme 17

cyclohexane (Scheme 17).19 When the same reaction was performed in cyclohexane, binuclear [Ir(H)(WCA-NHC)]2 (A99) featuring an 6-coordination of Ir to the DIPP ring was generated. The reaction of cationic [Ir(IMes)(COD)(acetone)]PF6 (A100) with H2 in benzene gave [Ir(IMes)(6-C6H6)(H)2]PF6 (A101) (Scheme 18).82 The benzene ligand of A101 was readily replaced by acetone to give [Ir(IMes)(H)2(acetone)3]PF6 (A102a), in which the acetone ligand could be further exchanged upon exposure to acetonitrile to give [Ir(IMes)(H)2(MeCN)3]PF6 (A102b). When A101 was treated with propylene in either C6Me6 or MeCN, all peripheral mesityl C–H bonds were doubly cyclometalated to afford [Ir(IMes00 )(6-C6Me6)]PF6 (A103a) or [Ir(IMes00 )(MeCN)3]PF6 (A103b), respectively.83 On the other hand, the reaction of [Ir(IPr) (COD)(acetone)]BF4 (A13) with H2 in acetone gave the binuclear complex [Ir(IPr)(H)]2(BF4)2 (A104), in which the DIPP ring acted as an 6 arene ligand.18

8.01.2.2.5

Reactions of Rh(NHC) and Ir(NHC) complexes with X–H bonds (X ¼ C, O, B)

As evidenced by their facile intramolecular cyclometalations, Rh(NHC) and Ir(NHC) complexes are capable of activating C–H bonds through oxidative addition. The reaction of [Rh(IPr)(COE)Cl]2 (A105) with 2-phenylpyridine afforded a cyclometalated Rh(III)-hydride complex, A106, and its Rh–H bond underwent addition into C–C multiple bonds to form Rh–alkyl/alkenyl bonds (Fig. 10).84 The resulting species afforded hydroarylated products via reductive elimination. This reaction could also be performed catalytically. [Rh(IPr)(H)2Cl]2 (A95b) is also a suitable catalyst precursor used for the catalytic alkene/alkyne hydroarylation of 2-(2-thienyl)-pyridine derivatives.85 In this case, the reaction was initiated by the k1N coordination of 2-(2-thienyl)-pyridine, with the subsequent elimination of H2 affording the catalytically active species. The substrate scope of this catalytic hydroarylation can be extended to multidentate compounds such as 2,2’-bipyridine owing to the strong s-donating ability of the NHC ligand.86,87 The reaction of [Rh(IPr)(ethylene)Cl]2 (A24) with 2,2’-biquinoline afforded rollover cyclometalated compound A107.88

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

13

Scheme 18

Fig. 10 Cyclometalated Rh(III)-hydride complexes.

The oxidative addition of directing-group free aromatic compounds was also demonstrated, e.g., the reaction of [Ir(COE)2Cl]2 with IBioxMe4 in the presence of Na[BArF] in 1,2-difluorobenzene afforded trans-[Ir(IBioxMe4)3(H)(C6H3F2)]BArF (A108) as the oxidative addition product (Scheme 19).89 The oxidative addition of the aromatic C–H bond was probably triggered by chloride abstraction from the initially formed Ir(IBioxMe4)2(COE)Cl (A109). The oxidatively added aromatic compounds in A108 could be exchanged with external arene substrates, which highlighted the reversibility of C–H activation in the case of A108 (Scheme 20).

Scheme 19

14

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 20

Fig. 11 8-Quinolinol-bearing Rh(III)-hydride complex.

The reaction of [Rh(IPr)(COE)Cl]2 (A105) with 8-quinolinol afforded Rh(IPr)(k2O,N-C9H6NO)(H)Cl (A110) (Fig. 11),90 which promoted the selective H/D exchange at the b-position of styrene derivatives. The hydride ligand of A110 was readily deuterated in CD3OD solution, and the resulting Rh–D bond was added to styrene in both Markovnikov and anti-Markovnikov fashion to give branched (kinetic product) and linear (thermodynamic product) Rh alkyl species, with subsequent b-hydride elimination affording b-deuterated styrene derivatives. Treatment of M(NHC)2(H)2Cl with Na[BArF] afforded cationic [M(NHC)2(H)2]BArF complexes that are highly active for X-H activation. Although the reaction of Ir(IPr)2(H)2Cl (A90) with Na[BArF] afforded highly reactive cationic [Ir(IPr)2]BArF (A111), that of M(IMes)2(H)2Cl gave a NaBArF adduct, [M(IMes)2(H)2Cl(Na)]BArF (M ¼ Rh, A112; M ¼ Ir, A77).91 The prolonged reaction of A90 with Na[BArF] gave a product containing a dehydrogenated isopropyl group, Ir(IPr00 )(IPr)(H)2 (A113).74 The 6 h reaction of A112 with tBuNH2
BH3 afforded the corresponding 2-aminoborane adducts (A114) upon the loss of NaCl (Scheme 21). The 48 h

Scheme 21

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

15

reaction of 112 with tBuNH2
BH3 generated a dehydrogenated aminoborane adduct (A115). When 3,30 -dimethylbutene was treated with [Ir(IMes)2(H)2(m-H)2B(H)
NCy2]BArF (A116b), intramolecular cyclometalated complex A117 was obtained upon the loss of H2. Upon further heating at 65 C, the toluene solution of the resulting complex underwent additional H2 elimination to yield A118.

8.01.2.2.6

Syntheses and reactivities of Rh(NHC) Fisher carbenes

The reaction of Rh(IMes)(COD)Cl (A119) with Cr(CO)5(C(OMe)CH¼ CHPh) afforded Rh(IMes)(C(OMe)CH ¼ CHPh)(CO)Cl (A120a) (Fig. 12),92 while the same reaction with Cr(CO)5(C(NMe2)CH ¼ CHPh) generated Rh(IMes)(C(NMe2)CH ¼ CHPh)(CO) Cl (A120b). The heating of a toluene solution containing A120a afforded the dimerization product (A121) of bound carbene as well as [Rh(IMes)(CO)Cl]2.

Fig. 12 Rh(NHC) Fisher carbenes.

8.01.2.2.7

Syntheses and reactivities of Rh(NHC)(COD)X and Ir(NHC)(COD)X (X ¼ OH, F)

NHC ligands are well suited for the stabilization of Rh and Ir hydroxide species. Rh(NHC)(COD)OH complexes were prepared by treating THF/H2O solutions of Rh(NHC)(COD)Cl with CsOH.93 Rh(ICy)(COD)OH (A122) catalyzes the conjugation of arylboronic acids to a,b-unsaturated ketones. The reaction proceeds via the formation of an arylrhodium intermediate followed by its conjugate addition to a,b-unsaturated ketones. The stoichiometric reaction of A122 with phenylboronic acid afforded square planar Rh(ICy)(COD)(Ph) (A123), which slowly reacted with cyclohexanone to give an oxa-p-allyl Rh intermediate (Scheme 22).

Scheme 22

Ir(NHC)(COD)OH complexes were synthesized using a strategy similar to that employed for Rh analogs94 and exhibited similar reactivities toward arylboronic acids. The reaction of Ir(IiPr)(COD)OH (A124) with arylboronic acids afforded square planar Ir(IiPr)(COD)(Ar) (Ar ¼ Ph, A125a; Ar ¼ p-MeOC6H4, A125b). A124c was capable of activating silanes, e.g., the reaction of A124 with PhSi(OMe)3 afforded Ir(IiPr)(COD)(OSi(OMe)2Ph) (A126) with the concomitant formation of MeOH. The treatment of Ph3SiH with A124 gave Ir(IiPr)(COD)(OSiPh3) (A127) and H2. Finally, the reaction of A124 with TMSCN gave Ir(IiPr)(COD)(CN) (A128) with the elimination of TMSOH (Scheme 23). The Ir hydroxide complex was also subjected to C–H, N–H, and O–H bond activation. The reaction of A124 with MeOH, PhCO2H, or (p-BrC6H4)NH2 generated Ir(IiPr)(COD)(OMe) (A129a), Ir(IiPr)(COD) (O2CPh) (A129b), or Ir(IiPr)(COD)(NH(p-BrC6H4)) (A129c), respectively, and H2. A124 could also activate the C–H bonds of phenylacetylene, nitromethane, acetone, dimethylmalonate, and pentafluorobenzene (A130a–A130e, Scheme 24).

16

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 23

Scheme 24

Upon treatment with CO2, A124 gave a carbonate-bridged Ir(IiPr) dimer complex, [Ir(IiPr)(COD)]2(m-k1:k2-CO3) (A131) (Scheme 25).95 When 131-13C obtained by the reaction of A124 with 13CO2 was exposed to 12CO2, the content of 13C decreased, which indicated the reversibility of CO2 insertion/elimination. The reactions of A129a and A129c with CO2 afforded carbonate

Scheme 25

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

17

(A132a) and carbamate (A132b) complexes, respectively. However, the insertion of CO2 into Ir complexes containing carbanion ligands did not afford the desired carboxylate complexes. This result was ascribed to the high kinetic barrier of this reaction, as supported by the absence of 13CO2 scrambling in the case of A129b. The carboxylation of phenylacetylide complex A130a was analyzed by DFT calculations,96 and the insertion of CO2 into the Ir–C bond was computed to be endothermic by 18.7 kcal/mol and have an energy barrier of 37.5 kcal/mol. Rh(NHC) and Ir(NHC) fluoride complexes, namely Ir(IPr)(COD)F (A133a), Ir(IiPr)(COD)F (A133b), Rh(ICy)(COD)F (A133c), and Rh(IPr)(COD)F (A133d), were obtained by treating the corresponding hydroxide complexes with potassium difluoride.97 Ir(IPr)(COD)F (A133a), Ir(IDD)(COD)F (A133e), Ir(IiPr)(COD)F (A133b), and Ir(ICy)(COD)F (A133f) were also prepared by reacting Ir(NHC)(COD)Cl with 5 equiv. of AgF. The fluoride complexes could also be synthesized using Et3N
3HF. Treatment with 0.33 equiv. of Et3N
3HF afforded metal NHC fluoride complexes, whereas the use of 0.66 equiv. of Et3N
3HF give rise to bifluoride complexes M(NHC)(COD)(HF2). Complexes M(NHC)(COD)OH, M(NHC)(COD)F, and M(NHC)(COD)(HF2) could be interconverted using proper stoichiometric amounts of either Et3N
3HF or KOH. The reaction of a 1:2 mixture of cis-Rh(6-iPr)(PPh3)2H (A134a) and trans-Rh(6-iPr)(PPh3)2H (A134b) with C6F5CF3 afforded a 4:1 mixture of cis-Rh(6-iPr)(PPh3)2F (A135a) and trans-Rh(6- iPr)(PPh3)2F (A135b) with the selective formation of 2,3,5,6-C6F4HCF3.98,99 When a mixture of A135a and A135b was treated with Et3SiH upon heating, Rh(6-iPr)(PPh3)2H (a mixture of A134a and A134b) was regenerated (Scheme 26). Although A134 promoted the stepwise stoichiometric defluorohydrogenation of C6F5CF3, the same reaction with a catalytic amount of A134 was not effective. The low catalytic efficiency might be due to the deactivation of A134 via the C–H activation of 2,3,5,6-C6F4HCF3. The resulting arylrhodium species Rh(6-iPr)(PPh3)2(C6F4CF3) (A136) did not show reactivity toward Et3SiH, which suggested that the formation of A136 in the catalytic cycle was a dead end. Rh(NHC)- and Ir(NHC)-trifluoromethyl complexes were synthesized by treating M(NHC)(COD)Cl with AgCF3.97 Specifically, AgCF3 was produced by reacting AgF with TMSCF3 in MeCN at −40 C and further reacted with M(NHC)(COD)Cl to afford M(NHC)(COD)CF3 (Scheme 27). The synthesis of trifluoromethyl complexes from either Rh(IPr)(COD)Cl (A23) or Ir(IPr)(COD) Cl (A12) was unsuccessful. The crystal structure of Ir(IiPr)(COD)CF3 (A137c) indicated that two fluorine atoms and the methyl group of the NHC ligand were in close proximity. In addition, one methyl carbon peak in the 13C{1H} NMR spectrum was split into a quartet because of through-space coupling. These results implied that the transfer of the CF3 group to metal NHC complexes with sterically bulky NHCs would result in significant steric clashes.

Scheme 26

Scheme 27

18

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.2.2.8

Redox reactions of Rh(NHC) and Ir(NHC) complexes

Neutral Ir(NHC) complexes were oxidized with XeF2 at low temperatures to give Ir(III) NHC difluoride complexes Ir(NHC)(COD) ClF2 and Ir(NHC)(CO)2ClF2 (Scheme 28).100 Due to low stability of the difluoride complexes, only spectroscopic analysis was performed to obtain structural information. The COD-containing complexes Ir(IMes)(COD)ClF2 (A138a) and Ir(IPr)(COD) ClF2 (A138b) decomposed at room temperature, whereas the CO-containing complexes Ir(IMes)(CO)2ClF2 (A139a) and Ir(IPr) (CO)2ClF2 (A139b) were more stable. The 19F NMR spectra of oxidized products featured peaks with a lower chemical shift at d  −390 ppm and 2JFF interactions on Ir (III). These data revealed that the relative position of the fluoride ligand was trans to Cl, CO, or COD. The oxidation of cationic Ir(NHC) complexes [Ir(IMes)(CO)2(PPh2Et)]BF4 (A140a) and [Ir(IMes)(CO)2(PPh2CCPh)] BF4 (A140b) with XeF2 was also demonstrated. Both reaction afforded 1:1 mixtures of isomeric difluoride complexes that exhibited 2 JPF coupling constants diagnostic of F-trans-F and F-trans-CO relative geometries at Ir(III) (A141, A142).

Scheme 28

The chemical reduction of in situ generated Rh(IPr)(COD)Cl with KC8 in the presence of 18-crown-6 ether resulted in the dearylation of the IPr ligand to give [K(18-crown-6)(THF)][(2:2-COD)2Rh2(m-C3H2N2DIPP-kC2,kN3)2(m-DIPP)2K] (A143) (Scheme 29).101 This product was presumably produced via the intramolecular C–N oxidative addition of the highly reducing Rh(−1) intermediate [K(18-crown-6)(THF)][Rh(IPr)(COD)]. The isolation of Rh(0)/Rh(−1) intermediates was unsuccessful because of their low stability, whereas such intermediates could be isolated when COD was replaced with divinyltetramethyldisiloxane (dvtms) (Scheme 30). The reaction of [Rh(dvtms)Cl]2 with 2 equiv. of IPr and KC8 gave Rh(IPr)(dvtms) (A144), whereas the same reaction with 4 equiv. of KC8 and 2 equiv. of 18-crown-6 afforded [K(18-crown-6)][Rh(IPr)(dvtms)] (A145). Alternatively, A144 and non-encapsulated A145 (A146) were obtained via the stepwise reduction of [Rh(IPr)(dvtms)Cl]2 (A147).102 The EPR spectrum of A144 revealed that the unpaired electron is predominantly located at the metal dz2 orbital, and this electronic structure was further confirmed by DFT calculations. The reaction of A146 with AuCl(PPh3) afforded a Rh–Au bimetallic complex (A148) with Au coordinated to the axial position of Rh (Scheme 31).

Scheme 29

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

19

Scheme 30

Scheme 31

8.01.3

Rh and Ir complexes with chelating bis-NHC ligands

In this section, we discuss Rh and Ir complexes supported by chelatingligands containing two NHC units. Compared to monodentate ligands, bis-NHC ligands exhibit chelating effects that can improve catalyst stability and reactivity while also offering electronic, geometric, steric, and chiral parameter tuning.103,104 Biscarbene ligands comprise three components, namely the backbone, linker and backbone (Fig. 13). As in the case of monodentate carbenes, the backbone determines the electronic properties of biscarbene ligands. In general, biscarbene ligand structure is significantly affected by the topology of the linkers, which connect two backbones. Linker length, shape, and rigidity strongly influence the structure of metal complexes. The bite angle, chirality, and anisotropy of azole rings can be tuned using different linkers.103,104 The wingtip, i.e., the N-substituent on the NHC backbone, typically affects the steric and electronic properties of the ligand much less than the backbone and linker; however, bulky wingtips may change the geometry of the metal complex or affect substrate binding by interacting with other ligands and/or external substrates.103,104

Fig. 13 Structure of bis-NHC ligands.

Several structural parameters have been introduced to describe the structures of metal complexes supported by biscarbene ligands (Fig. 14), namely the bite angle (b), tilt angle (a), and yaw angle (y).105,106 The bite angle is defined as the Ccarbene–M–Ccarbene angle, while the tilt angle is defined as the average angle between two imidazolium ring planes and the xy plane of the metal complex (dihedral angle of Ccarbene–M–Ccarbene–N) and is strongly dependent on linker length. 105 The yaw angle describes the conformational strain imposed by the metalacycle due to the steric interaction between the bulky wingtip and the metal center. 106 Given that the linker is a unique component of biscarbenes largely determining the structure of their complexes, biscarbenes are classified according to their linkers in this section, with representative examples of each category shown in Scheme 32.

Fig. 14 Structural parameters of bis-NHCs, namely bite angle (b), tilt angle (a), and yaw angle (y).

20

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 32 Representative examples of bis-NHC ligands.

8.01.3.1

Bis-NHC ligands with alkyl linkers

Alkyl chain linkers are the ones most extensively studied. The first examples of Rh and Ir biscarbene complexes supported by methylene-bridged bis(imidazolidene) ligands were reported in 2002.107,108 Since then, over 70 articles have been published on the synthesis, reactivity, and catalytic performance of Rh and Ir complexes with biscarbene ligands containing C1–C4 alkyl linkers (–(CH2)n–, n ¼ 1–4). Most of these complexes feature imidazolidene backbones, although other (e.g., abnormal carbene, triazole, and bicyclic NHC) backbones have also been investigated (Scheme 33).

Scheme 33 Common backbones found in biscarbene ligands.

8.01.3.1.1

Bis(imidazolidene) ligands with alkyl linkers

Because the bis(imidazolidene) with alkyl linkers were introduced first, the synthesis of new bis(imidazolium) precursors, metalation strategies, binding modes, structure, and molecular behavior have been intensively investigated with various linkers and wingtips in the early stages.105–117 Tables 1 and 2 summarize Rh and Ir complexes supported by bis(imidazolidene) with alkyl linkers and their structural parameters. In general, the bite angle (b) is restricted by the length of the linker, that is, ligands short linkers only allow small bite angles, while long linkers offers more flexibility to wider bite angles. Furthermore, biscarbene ligands with short linkers prefer a conformation with the two azole rings in the xy plane (small a), while those with long linkers tend to align the azole rings along the z-axis (large a) (Table 1). Linker length and the steric effect of the wingtip also result in a distortion of the M–Ccarbene bond (yaw distortion, y).106 Yaw distortion decreases with increasing metalacycle ring size with long linkers but significantly increases in the presence of the bulky wingtips such as tBu group (Table 1 and 2). The effects of linker length, wingtip sterics, and anions on the formation of monodentate vs. chelate species have been systematically researched.105,106,115,117 The metalation of a biscarbene with nBu wingtips by [Rh(COD)Cl]2 successfully yielded a chelate compound [(biscarbene)Rh(COD)][PF6] with long linkers (Q ¼ –(CH2)3–, B10b or –(CH2)4–, B12).105 However, short linkers align azole rings close to the xy plane and therefore lead to large steric congestion in this plane. Consequently, dirhodium complexes bridged by biscarbene ligands were formed with short linkers (Q ¼ –CH2– or –(CH2)2–).105 This reactivity trend was reversed when the tBu wingtip was employed.106 For bulky wingtips, the steric interaction between them is significant when the azole rings are aligned along the z-axis in the case of long linkers. As a result, for the tBu wingtip, long linkers favored dirhodium complexation (Q ¼ –(CH2)2–, –(CH2)3– and –(CH2)4–), while short linkers allowed for chelate compound formation (Q ¼ –(CH2)–, B7f).106 The exclusion of halide anions with PF6 salts allowed the formation of C2-bridged complexes B9a and B9c-f. Therefore, the synthetic method is also an important factor determining the selectivity for chelate vs. dinuclear species.117 In the case of Ir metalation, linker length affects the oxidation state of the Ir product.115 That is, Ir(III) (biscarbene)Ir(H)(COD) (Cl) complexes were obtained with a short linker (Q ¼ –CH2–, B28), whereas Ir(I) (biscarbene)Ir(COD) complexes were obtained for long linkers (Q ¼ –(CH2)2–, B30a, –(CH2)3–, B32b, and –(CH2)4–, (B39a).115 Again, this finding was ascribed to the fact that for long linkers, large steric hindrance in the z-axis direction leads to the reductive elimination of axial ligands (H and Cl).

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Table 1 Structure

21

Rh complexes supported by bis(imidazolidene) ligands bridged by alkyl linkers. Comp. #

Linker (Q)

Wingtip (R)

X

L

A

b ( )

a ( )

y ( )

References

B1a B1b B1c B1d B1e B1f B2a B3 B4 B5 B6a B7 B6b B6c

–CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –(CH2)2– –(CH2)3– –(CH2)3– –(CH2)4– –CH2– –CH2– –CH2– –CH2–

Me n Bu i Pr Bn polybenzyl ether (G1) polybenzyl ether (G2) n Bu n Bu n Bu n Bu Me Me n Bu Mes

2I 2I 2I 2I 2I 2I 2I 2I 2 Br 2I 2I H, Cl 2I 2I

– – – – – – – – – – 2 MeCN COD 2 MeCN 2 MeCN

– – – – – – – – – – PF6 PF6 I I

86.6 87.3 – – – – – – – – 87.8 – – –

20.8 19.4 – – – – – – – – 26.3 – – –

0.3 0.8 – – – – – – – – 4.1 – – –

114,116 105,107,112 107 116 116 116 3 3 118 3 112 115 118,119 119

B7a B7a B8a B8a B7b B7c B7d B7e B7f B8b B9a B9b B9c B9d B9e B9f B9g B9g B10a B11a B10b B11b B10c B12 B13 B14a B14a B14b B15 B16

–CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)3– Me –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)4– –(CH2)4– –CH2– –CH2– –CH2– –CH2– –(CH2)2–

Me Me Me Me Et n Bu Mes Cy t Bu t Bu Me n Bu Ph Mes i Pr Cy t Bu t Bu Me –(CH2)3– n Bu n Bu i Pr n Bu n Bu Me Me Ph –(CH2)3SO3Na i Pr

– – – – – –

– – – I I I Cl Cl

COD COD 2 CO 2 CO COD COD COD COD COD 2 CO COD COD COD COD COD COD COD COD COD 2 CO COD 2 CO COD COD 2 CO – – – – –

PF6 BPh4 PF6 BPh4 I PF6 BF4 I PF6 PF6 PF6 PF6 PF6 PF6 PF6 PF6 PF6 Br PF6 PF6 PF6 PF6 PF6 PF6 PF6 PF6 I I – (Cp )RhCl3

– 83.2 83.5 – – – 85.3 – 81.4 81.5 84.0 84.0 88.0 85.2 – 81.6 – – 83.8 – 87.6 – – – 91.0 86.0 – – – 92.2

– 45.7 41.4 – – – 37.8 – 55.3 53.7 59.1 61.2 55.5 55.7 – 64.0 – – 78.1 – 78.0 – – – 86.3 28.8 – – – 45.0

– 8.3 6.8 – – – 7.8 – 13.8 12.4 7.4 6.1 6.9 25.8 – 4.9 – – 1.4 – 1.3 – – – 0.6 3.2 – – – 0.3

112,120 120 120 120 121 106 113 121 106,111 106 117,121 105 117 117,122 117 117 106 121 123 123 105,106 105 106 105,106 105 124 124 124 125 109

B17a B17b

–CH2– –CH2–

Me n Bu

2 Cl 2 Cl

– –

– –

– 83.7

– 37.0

– 5.3

114 114

– – – – – – – – – –

22

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Table 2 Structure

Ir complexes supported by bis(imidazolidene) ligands bridged by alkyl linkers. Comp. #

Linker (Q)

Wingtip (R)

X

L

A

b ( )

a ( )

y ( )

References

B26a B26b B26c B26d B26e B26f B26g B26h B27 B28 B29

–CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2CMe2CH2– –CH2– –CH2–

Me n Bu i Pr neopentyl Bn t Bu –CH2OEtOMe –(CH2)3SO3K neopentyl Me n Bu

2I 2I 2I 2I 2I 2I 2I 2I 2I H, Cl 2I

– – – –

– – – –

– – – – COD 2 MeCN

– – – – PF6 PF6

– – 86.4 – – – – – 87.2 84.6 87.4

– – 20.4 – – – – – 21.6 39.3 30.1

– – 0.3 – – – – – 1.1 7.2 4.6

108,126 108,127–129 108 108,130 108 108 126 127–129,131–134 130,135 115 136

B30a B30b B30d B30c B31 B32a B32b B32b B32c B33a B33a B33a B33a B33a B33a B34 B35 B33b B33c B36 B37 B38 B39a B39a B39b B40a B41a B41b B42 B41c B41c B41d B41e B41f B41g B41h B41i B43a B44 B43b B45

–CH2– –CH2– –CH2– –CH2– –CH2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –(CH2)3– –CH2CMe2CH2– –CH2CMe2CH2– –(CH2)4– –(CH2)4– –(CH2)4– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –CH2– –(CH2)2– –(CH2)2– –(CH2)2– –(CH2)3–

Et Cy DIPP t Bu t Bu Me n Bu n Bu t Bu Me Me Me Me Me Me Me Me Et n Bu n Bu neopentyl neopentyl Me Me n Bu Me Me Me Me Et n Bu n-octyl Ph Cy t Bu –CH2CH2OH –(CH2)3SO3Na Me Me i Pr Me

– –

COD COD COD COD 2 CO COD COD COD COD COD COD COD COD COD COD CO CO COD COD 2 CO COD 2 CO COD COD COD – – – H2 – –

I I PF6 Cl Cl PF6 PF6 Br Br PF6 BF4 CF3COO OTf I Br – – I PF6 PF6 PF6 PF6 PF6 CF3COO PF6 PF6 PF6 Cl 2B(C6F5)4 PF6 PF6 PF6 PF6 PF6 PF6 Cl PF6 PF6 OTf PF6

– – 87.4 81.1 81.6 – – – – – – – – – – – – – 84.6 – –

– – 31.2 55.2 51.7 – – – – – – – – – – – – – 79.8 – –

– – 7.5 13.0 11.5 – – – – – – – – – – – – – 1.6 – –

93.9 – – 86.4 85.3 – – – 84.5 – 86.8 84.1 84.9 – – 92.0 – 91.1 95.7

87.8 – – 27.8 31.1 – – – 32.8 – 27.4 35.8 44.6 – – 42.6 – 44.8 37.4

0.6 – – 3.4 3.5 – – – 3.7 – 4.9 4.4 11.6 – – 3.4 – 3.1 3.9

121,137 121 110 111 111 115,138 115 121 121 115,137,138 138 138 138 137,138 137 137 137 137 115 139 135 135 115 138 115 140–143 55,140,144,145 140 140 143 131,143 145 143 145 145 146 125,131,132 144,145,147 144 109 145

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

– – – – – MeCN – –

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

23

Both Rh and Ir complexes have been widely studied as (transfer) hydrogenation catalysts. For example, Rh complexes B1b and B1c were used for the transfer hydrogenation of ketones and imines using iPrOH as the H2 source (TON up to 19000).107 The water-soluble Rh complex B15 containing sulfonate groups is a highly efficient catalyst for the hydrogenation of CO2 (TOF ¼ 100 h–1, TON ¼ 3600) and H2 production from formate (TOF ¼ 39000 h–1, TON ¼ 449000) in aqueous solution.125 B8a was shown to promote the intermolecular hydroamination of 4-pentyn-1-amine to 2-methyl-1-pyrroline, although the rate of this transformation was not improved compared to that observed for the analogous bis(N-methyl-imidazol-2-yl)methane ligand.120 Ir complexes have been more intensively studied as transfer hydrogenation catalysts because of their high activity.55,108,125–135,137,142 In particular, transfer hydrogenation reactions with iPrOH were performed for a series of methylene-bridged biscarbene Ir(III) complexes B26a–f (Scheme 34),108 in which case steric modifications of the wingtip enabled activity and selectivity tuning. The catalyst containing neopentyl wingtips (B26d) was more efficient than those containing Me, nBu, i Pr, and benzyl wingtips for the transfer hydrogenation of benzophenone, achieving a TON of 5300 and a TOF of 50,000 h–1, while the catalyst with iPr wingtips (B26c) showed higher regioselectivity for a,b-unsaturated ketones.142 Methylene-bridged (B26d) and neopentanediyl-bridged (B27) complexes were also used for the transfer hydrogenation of various aldehydes, ketones, and imines with iPrOH (Scheme 34).108,130,135

Scheme 34 Transfer hydrogenation of ketones, imines, and aldehydes.

The hydrogenation of CO2 with H2 was studied using the half-sandwich Ir complex B40a.55 The activity of this complex exceeded that of the complex containing a monodentate NHC, and a maximum TON of 1600 was achieved in the former case. However, only one turnover was observed in the transfer hydrogenation of CO2 with iPrOH (Scheme 35). The hydrogenation of CO2 was also attempted using a catalyst that contained alcohol functionalities in its wingtips (B41h) to enhance solubility in water.146 In aqueous solution, B41h achieved a TON of 9500 with H2 and 203 with iPrOH, while the less stable mono-NHC complex decomposed and showed poor activity under the same conditions. A highly water-soluble catalyst (B26h) was prepared by introducing a sulfonate functionality,128 achieving TONs of 82300 and 1730 for the hydrogenation of CO2 with H2 and iPrOH, respectively. The solubility of B26h in water also enabled its use for the deuteration of pyridines in D2O.128 The high solubility of B26h in polar solvents also allowed the use of glycerol, which is an abundant byproduct of the biodiesel industry as an H2 source. B26h catalyzed the transfer hydrogenation of ketones, olefins, aldehydes, and imines performed using glycerol as a solvent and an H2 source.127,129,131,133,134 The simultaneous conversion of CO2 and glycerol promoted by B26h showed TONs of 226 and 885 for formate and lactate, respectively.132 Several C3-bridged biscarbene compounds, B33a, B33b, B34, and B35, were screened as catalysts for the CO2/glycerol reaction,137 showing higher activity and better recyclability than C1-bridged biscarbene (B30a) and monodentate NHC complexes. High turnover was obtained using the bromide-containing compound B33a (TON ¼ 32609 and TOF ¼ 1630 h−1 for lactate, TON ¼ 16856 and TOF ¼ 843 h−1 for formate). The role of the linker was evaluated using DFT calculations. As the C3 linker more strongly tilts the azole rings from the xy plane to the z-axis than the C1 linker, the vacant coordination site in the xy plane of the former is more widely open to the incoming bicarbonate (or CO2). In addition, the planarity of the methylene-bridged biscarbene causes p-conjugation and thus decreases the electron density on the Ir center.

Scheme 35 Transfer hydrogenation of CO2 using iPrOH and glycerol as H2 sources.

Other reactions include hydrosilylation,116–118,122 hydroborylation,121,138 hydroformylation,120 and water oxidation.141,144,145,147 Biscarbene Rh(III) complexes with dendritic wingtips (B1d–f) were used to promote the hydrosilylation of 2-cyclohexen-1-one116 and showed high activity, affording a mixture of cyclohexanone and 2-cyclohexen-1-ol in 91–99% yield with 79–81% selectivity for cyclohexanone. When a methyl wingtip was employed, the yield (62%) and selectivity (65%) significantly decreased. Several biscarbene Rh(I) compounds with different wingtips (B9a, B9c–f) were used for the hydrosilylation of 4-fluoroacetophenone with diphenylsilane to yield the corresponding silyl ether and silyl enol ether (Scheme 36).117 Catalyst activity and silyl ether selectivity increased with the increasing steric effect of the wingtip. Mechanistic study with B9d suggested the formation of a Rh silylene intermediate.122

24

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 36 Hydrosilylation of 4-fluoroacetophenone with diphenylsilane.

Rh(I) complexes B10a and B11a were also tested for the hydroformylation of 1-hexene, showing activities and selectivities similar to those of monoNHC analogs, although biscarbene ligands were not the main focus of this study.120 The ability of Ir to activate C–H bonds was utilized in C–H borylation. C1-bridged (B30a–b) and C2-bridged (B32b–c) Ir biscarbene complexes catalyzed the borylation of aromatic C–H and C–halogen bonds (Scheme 37).121 The C–H borylation of benzene was further studied with C2-(B32a), C3-(B33a), and C4-bridged (B39a) NHCs.138 The results obtained for various ligands and counter ions revealed that counter ion basicity was the most important feature of the complex design motif. The most active catalyst was the C3-bridged complex (B33a) with a trifluoroacetate anion.

Scheme 37 C–H borylation of benzene.

The half-sandwich Ir complexes have been studied as water oxidation catalysts. The C1-bridged Ir complex B40a was tested as a catalyst for water oxidation with NaIO4 as an oxidant, achieving a TOF of 0.13 s–1.141 Similar complexes with C1-bridged (B41a) and C2-bridged (B43a) biscarbenes were used to promote water oxidation using (NH4)2[Ce(NO3)6] or NaIO4 as the oxidant, achieving a TOF of 0.20 s–1.144 B43a also promoted the photooxidation of water in the presence of [Ru(bpy)3]+ as a sensitizer and persulfate as a sacrificial electron acceptor, achieving a quantum efficiency of 3.8%.144 EPR spectroscopy revealed the formation of an Ir(IV) intermediate via photoinduced electron transfer. Later, the activity of B43a was optimized to realize a quantum efficiency of 34%.147 Comparison with analogous carbene ligands demonstrated that O2 evolution activity increased with increasing electron-donating ability. The influence of the ligand was further studied for various biscarbene ligands including bis(imidazolidene) with different linker lengths and wingtips (B42, B41d, B41f, B41g, B43a, and B45).145 Notably, catalytic activity was not dependent on electronic properties but was rather influenced by the steric and hydrophobic properties of wingtips.

8.01.3.1.2

Bis(abnormal carbene) ligands with alkyl linkers

Abnormal carbenes (imidazolidene bound through C4 or C5) have received attention because of their strong s-donating ability and constitute a new family of NHCs.148–150 Although the use of normal imidazolidene can sometimes result in the formation of abnormal carbenes, their generation can be forced by substitution at C2.

The first biscarbene Ir complexes containing abnormal carbenes were prepared from C2-Me-substituted bisimidazolium salts.151 The mixed normal and abnormal NHC compound B46 was obtained by metalation of a mono-C2-Me-substituted salt with [Cp IrCl2]2. In the case of di-C2-Me-substituted bisimidazolium salts, the outcome of metalation depended on linker length. The methylene-bridged salt afforded a chelating ligand through an abnormal NHC and a methylene group resulting from the C–H activation of C2-Me (B47). A similar product was obtained for the ethylene linker (B48) together with the chelating bis(abnormalNHC) species (B49).

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

25

Several Rh bis(abnormal-NHC) complexes were prepared from bisimidazolium salts with different C2- and N-substituents (B50a-d, B51a-d, B52b).119 XRD structures revealed that abnormal carbenes have a higher trans-influence than analogous normal imidazolidenes. The strong donor ability of abnormal carbenes was also demonstrated by the halide substitution reaction of B53b–c with chloroalkanes to give B54b–c and the corresponding iodoalkanes.152 The halide exchange in chloroalkanes is not a metal-catalyzed reaction, however halide dissociation is promoted by the strong trans-influence. The strong donor ability of abnormal carbenes has been widely exploited in catalytic transfer hydrogenation. B50–B52 catalyzed the transfer hydrogenation of benzophenone with iPrOH, with a TON of 330 obtained for B52b.119 B50b–c were reacted with diphosphine ligands to give B55b–c, which displayed dynamic cis/trans isomerization in solution.153 The diphosphine compounds B55b-c showed higher activity for the transfer hydrogenation of benzophenone, achieving a TON of 4600. The pyridyl complexes B56c, B57c, and B58c were synthesized by treating B50c with pyridine, phenanthroline, and terpyridine, respectively, and also effectively promoted transfer hydrogenation.154 The catalyst state could be monitored based on a strong color change, which was therefore concluded to be a useful probe for catalyst recycling. The activity for benzophenone reduction was sustained over five consecutive substrate additions, and the maximum TON equaled 3100. The water-soluble bis(abnormal-NHC) complex B59 was prepared by introducing a sulfonate group128 and used for the hydrogenation of CO2 with H2 and iPrOH in aqueous solution,128 the transfer hydrogenation of ketones and aldehydes with glycerol and imines as the hydrogen source and solvent.128,129,134 In general, the activity of B59 was much higher than that of the analogous normal biscarbene compound B26h.

8.01.3.1.3

Bis(triazolidene) ligands with alkyl linkers

Triazolidenes are an attractive but underexplored class of mesoionic carbenes. The electron donor ability of 1,2,4-triazol-5-ylidene is slightly lower than that of imidazolidene. More recently, much attention has been drawn to 1,2,3-triazole-5-ylidene, as 1,2,3-triazolium precursors are easily accessible through click chemistry and exhibit strong s-donor properties. 155 The first bis(triazolidene) complex was reported in 2003. Metalation of bis(1,2,4-triazolium) salts with [Rh(norbornadiene) Cl]2 gave bis(1,2,4-triazol-5-ylidene)Rh(I)2(OAc) complexes (B60a–b) together with nortricyclyl species (B61a–b) derived from the rearrangement of the norbornadiene ligand.156 In the silylation of 4-fluoroacetophenone with diphenylsilane, Rh(I) bis(triazolylidene) complexes (B62, B63a–d, and B64a–b) showed lower catalytic activity than imidazolidene compounds (B8 and B9).117 The Ir (III) and Ir (I) complexes of methylene- and pentadienyl-bridged bis(triazolylidene) ligands were prepared (B65, B66, and B67) and compared with the corresponding bis(imidazolidene) compounds.135 The CO stretching frequency of the biscarbonyl compound B67 (av. 2051 cm–1) revealed that triazolidene is less electron donating than the imidazolidene in B38 (av. 2036 cm–1). Hence, B65 and B66 were less active in the transfer hydrogenation of aldehydes and imines with iPrOH than their imidazolidene analogs B26d and B27. The transfer hydrogenation of acetophene with iPrOH was tested with various mono- and biscarbene ligands, but both bis(triazolylidene) (B68) and bis(imidazolidene) (B40a) species showed poor activity. When the same compounds were tested as water oxidation catalysts, the activity of the Cp Ir triazole compound B68 (0.20 s–1) was higher than that of B40a (0.13 s–1).141

26

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

For 1,2,3-triazole-5-ylidene, mixed imidazolidene, and 1,2,3-triazole-5-ylidene ligand and its Rh and Ir complexes were synthesized (B69–B72), and the CO stretching frequencies of B69 (av. 2034 cm–1) and B70 (av. 2024 cm–1) revealed that the electron-donating ability of 1,2,3-triazole-5-ylidene exceeds that of imidazolidene but is weaker than that of abnormal carbenes.157 The catalytic activity of Ir and Rh complexes (B69–B72) for the transfer hydrogenation of ketones evaluated with different wingtips, but the activity was not significantly affected by wingtip structure.

Several bis(1,2,3-triazolium) precursors were synthesized in one step from commercially available diynes using the click reaction.158 The wingtips critically dictate the reactivity of Rh and Ir metalation, e.g., the phenyl wingtip underwent C–H activation at the ortho position to result in a tridentate coordination mode (B73 and B74), whereas the ethyl substituent afforded chelate complexes B75a–b via the transmetalation of the Ag complex with [Cp RhCl2]2. The synthetic method was further modified to selectively obtain chelating complexes (B76a–d), monodentate complexes (B77b–d), and bimetallic complexes (B78b-c).159 The transfer hydrogenation of benzophenone with iPrOH was conducted using B76a–d and B77b–d. Catalytic activity increased with increasing linker length but was not positively affected by the presence of an ether moiety in the linker. Compound B76a was also an active catalyst for the hydrogenation/dehydrogenation of quinoline/hydroquinoline.160

8.01.3.1.4

Other bis-NHC ligands with alky linkers

The electronics of NHCs can be modified by introducing the substituents at imidazolidene C4 and C5 positions. The electron donor abilities of Rh(I) and Ir(I) complexes containing bis(dichloroimidazolyidene) ligands (B79–B84) were evaluated based on their CO stretching frequencies.139 In the cases of B80 (av. 2059 cm–1) and B81 (av. 2044 cm–1), these frequencies were hypsochromically

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

27

shifted by 18 cm–1 relative to those of non-chlorinated bis(imidazolidene) complexes B11b and B36, respectively. The electronic modification of the ligand improved catalytic activity for the hydrosilylation of terminal acetylenes and the cyclization of acetylenic carboxylic acids, probably because of the weaker coordination capability of COD in the case of chlorinated biscarbene compounds. More recently, mixed NHC compounds with different wingtips were prepared by the alkylation of B82a–b with HBF4 or Et3OBF4 to give B83a–b and B84a–b, respectively. The bromo-substituted compound of Rh (B85) was prepared and used as a water oxidation catalyst.161 The weaker electron donating ability of B85 compared to that B43a was reflected by differences in the carbene 13C shift. Water oxidation activity could be enhanced by stabilizing the Ir(IV) intermediate with strongly electron-donating ligands, i.e., B85 was less active than B43a.

Protic NHCs are an emerging class of carbene ligands that can be involved in the reaction by activating or recognizing substrates via hydrogen bonding.162–165 A non-functionalized bis(imidazole) ligand precursor was directly metalated using Cp Ir(OAc)2 to give a mixture of bis(protic-NHC) complexes B86a and B87a.166 Ligand exchange to X-and L-type ligands gave a series of bis(proticNHC) complexes B86a–e and B87a–c. B86b showed promise as a bifunctional catalyst for the selective hydrogenation of unsaturated ketones, e.g., it catalyzed the selective hydrogenation of double bonds in a,b-unsaturated ketones, while carbonyl group hydrogenation was observed for non-conjugated ketones (Scheme 38).

Scheme 38 Chemoselective transfer hydrogenation of unsaturated ketones.

Other heterocycles such as benzimidazole, dihydropyrroloimidazole, dihydroimidazooxazole, theophylline, and adenine have been used to prepare biscarbene complexes (Scheme 33) with tunable electronic and chiral properties and develop a new class of carbenes. Benzimidazolylidenes are slightly less electron-donating than imidazolidenes. Bis(benzimidazolylidene) Rh complexes B88a–c and B89a–c were prepared and evaluated in terms of their ability to promote silylation reactions.117 The activities of B88a–c and B89a–c were much higher than those of imidazolidene (B9a, B9c–f) and triazolylidene compounds (B62, B63a–d, and B64a–b). The Ir complex B90 was used for the C–H borylation of benzene (Scheme 38), exhibiting an activity comparable to that of the bis(imidazolidene) complex.138 The half-sandwich Ir complexes B91 and B92 promoted water oxidation.145,147 In this case, activity seemed to be influenced not only by the electron donating nature of biscarbenes,147 but also by the hydrophobicity of the wingtip.145

28

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

The chiral bicyclic carbene complex B93 was prepared using pyrrolidinoimidazole backbone for asymmetric catalysis, although it did not show enantioselectivity in the transfer hydrogenation of acetophene.66 As a potential candidate for non-innocent carbenes,162–165 the generation of an azolato carbene featuring a basic ring-nitrogen atom was attempted with theophylline derivatives.167a The metalation of imidazolium-functionalized theophylline with [Cp RhCl2]2 or [Cp RhCl2]2 resulted in the formation of a C-metalated azolato complex (B94 or B95) via the direct C–H activation at the C8 position of theophylline. Other azolato carbene complexes (B96 and B97) based on adenine were also prepared.

8.01.3.2

Annulated bis-NHC ligands

In annulated biscarbene ligands, two azole rings are directly connected through N–N or C–C bonds. Table 3 summarizes the structural parameters of annulated biscarbene complexes. As the two azole rings are in the same plane because of p-system delocalization, the tilt angle a is very small (7.1 at most). On the other hand, the carbene direction is constrained by annulation; therefore, a large yaw angle was observed even for small wingtips. The first example of an annulated bis-NHC complex was reported by the Crabtree group in 2007.168 Methylation of 4,4’bi-1,2,4-triazole gave a bitriazolium precursor, which was metalated with [RhCl(COD)]2 to afford a Rh(III) complex (B98) supported by a 4,4’-bis(1,2,4-triazol-5-ylidene) ligand (called “bitz”). The reaction conditions and the metal precursors differ in the product. The reaction with [RhCl(COD)2] in the presence of NaOAc led to the formation of a complex with two bitz ligands (B99). Metalation with [Rh(CO)2(OAc)]2 and NaOAc afforded dirhodium(II) complex B100, while metalation with [Rh(OEt) (COD)2] gave dirhodium(I) complex B101.172 DFT calculations revealed that bitz has a weaker donor ability than diphosphines and bipyridines, because (a) the extra electronegative nitrogen atom in triazolidene leads to a significantly lower donor power compared to that of imidazolidene and (b) the two azole rings are delocalized, i.e., each azole ring acts as an electron-withdrawing substituent with respect to the other ring. B98 and B99 showed catalytic activity for the transfer hydrogenation of ketones and imines with iPrOH; however, the rates were lower than those observed for related biscarbene compounds.172 Table 3 linkers.

Rh and Ir complexes supported by bis-NHC ligands bridged by annulated

Comp. #

b ( )

a ( )

y ( )

References

B98 B99 B102b B103a B103b B104a B104b B105a

79.5 78.6 77.0 76.1 77.1 80.6 80.0 79.5

2.8 3.0 3.3 7.1 2.7 0.7 3.9 3.5

13.8 9.4 11.5 10.0 12.1 20.4 20.0 19.1

168 168 169 170 170 171 171 171

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

29

The Bertland group prepared similar bitriazole ligands (called “i-bitz”) and their Rh(I) biscarbonyl complexes (B102a and B102b) from 1,2,3-triazole.169 The CO stretching frequencies of 2047 cm–1 (B102a) and 2048 cm–1 (B102b) indicated electron donor abilities stronger than those of dcpe (2059 cm–1) and bpy (2069–2073 cm–1), which is in contrast to the results obtained for the abovementioned bitz ligand. Ir compounds (B103a–b) were used in the catalytic oxygenation of cyclooctane and transfer hydrogenation catalysis.170

The Kunz group reported pyridazine-annelated biscarbene ligands (called “vegi”) and their Rh (B104a–b) and Ir (B105a–b) complexes.171 The bite angle of 80.7 was slightly larger than those of bitz and i-bitz. The CO stretching frequencies indicated that vegi had a stronger electron donor effect than bipyirdine and a weaker donor effect than i-bitz.171 The above Rh(I) compounds (B104a and B104b) were used for the transfer hydrogenation of ketones with isopropanol,171 while the Ir complexes (B105a and B105b) were used to catalyze the isomerization of 1,5-cyclooctadiene.173

8.01.3.3

Bis-NHC ligands with rigid linkers

As alkyl linker flexibility impedes the fine-tuning of structural parameters, the introduction of a rigid linker may allow better structural control. Several aromatic rings were utilized as linkers, as exemplified by bis(imidazolidene) bridged by ortho-xylene and their Rh and Ir complexes (B106–B108).121,135,138,174,175 Triazole (B109)135 and other bicyclic NHC (B110)66 complexes with an ortho-xylene linker have also been reported. Other linkers include phenyl (B111–B114),107,176,177 diphenyl (B115),178 substituted ferrocene (B116, B117),179 and calix[4]arene (B118).180 The corresponding structural parameters are summarized in Table 4. In general, rigid linkers tend to have a larger bite angle (>90 ) than alkyl linkers. In the case of the phenyl linker, small tilt angles are observed (33 ), presumably because of the preference for planar geometry required for efficient delocalization in the p-system. Table 4 Rh and Ir complexes supported by bis-NHC ligands bridged by rigidified linkers. Comp. #

b ( )

a ( )

y ( )

References

B106a B106b B107b B111 B113 B115 B116 B119 B120 B121 B122 B123 B124 B125

90.5 89.9 92.0 92.1 82.4 96.3 92.7 78.1 83.1 81.1 83.7 83.7 81.2 83.8

86.6 84.6 86.6 33.3 62.2 43.6 88.6 84.1 87.0 86.0 86.8 86.3 86.0 86.7

1.3 2.1 0.4 2.5 2.6 4.4 0.7 0.0 0.8 0.9 1.7 0.7 0.2 1.5

174 175 121 107 176 178 179 181 182 182 182 182 182 182

30

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Similar to those with alkyl linkers, compounds with aryl linkers can be used for the transfer hydrogenation of ketones,135,174 the hydrosilylation of alkynes and olefins,174 and the hydroborylation of aromatic C–H bonds.121,138 In most cases, the reactivity is comparable to that of analogs with alkyl linkers, although the effect of aryl linkers seems to be underexplored.

Macrocyclic carbenes in which the macrocycle comprises two carbenes are another interesting structures. The first example was reported in 1980, i.e., before the emergence of NHC ligands.181 The reaction of octahydrotetraazapyrene with [Rh(COD) Cl]2 resulted in the cleavage of the C¼ C bond in the tetraazacycle to give the dicarbene species B119. More recently, imidazolium-linked cyclophane precursors and their Rh(I) and Ir(I) complexes (B120-B125) have been prepared, and their redox behavior was studied.182

8.01.3.4

Bis-NHC ligands with chiral linkers

Rh and Ir complexes are widely used in asymmetric catalysis, and their selectivity can be enhanced by the choice of suitable ligands.183–186 A common strategy involves the use of a chiral moiety in the heterocyclic backbone or the N-substituent. In the biscarbene system, an axially chiral moiety can be employed in the linker to form chiral catalysis. The first example was reported by the Shi group,187 who metalated a binaphtyl-based dibenzimidazolium salt with Rh(COD)Cl to form B126.187 The octahydro-binaphthyl (B127) and biphenyl (B128) variants were also prepared.188,189 Both B126 and B127 catalyzed the asymmetric hydrosilylation of methyl ketones with enantioselectivities of up to 98% ee.187,188 The hydrosilylation of b-ketoesters, i.e., 3-oxo-3-arylpropionic acid methyl/ethyl esters, in the presence of B126 and B128 afforded b-hydroxyesters with excellent chemo- and enantioselectivity (up to 98% ee).189

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

31

The Zi group reported the synthesis of a biscarbene ligand with a 2,2-dimethylbiphenyl linker and its metalation (B129)190 and also prepared ligands containing two extra pyridine donors in the wingtip.191,192 These ligands afforded tetradentate coordination modes for Rh complexes (B130–B132). Three compounds catalyzed the asymmetric hydrosilylation of acetophenone with a moderate enantioselectivity of up to 73%. The Elsevier group prepared a Rh complex supported by a bis(triazolylidene) with a binaphthyl linker (B133)193 and showed this complex exhibited a higher activity for the hydrogenation of acetophenone than B126, although enantioselectivity was much lower (up to 51%). trans-1,2-Diaminocyclohexane and L-valinol are easily accessible chiral moieties that can be installed as linkers in biscarbene ligands. Riederer et al. synthesized bis(triazolidene) ligands with a chiral diaminocyclohexane linker and the corresponding Rh complexes B134a and B134b,194 which promoted the hydrogenation of dimethylitaconate and methyl-2-acetamidoacrylate and achieved enantioselectivities of up to 61% ee. Notably, the bulky substituent in B134b improved the enantioselectivity. Gigler et al. prepared bis(imidazolidene) ligands with a diaminocyclohexane linker; however, B135a and B135b did not achieve significant asymmetric induction in the hydrosilylation of 4-F-acetophenone.194

A series of chiral biscarbene Ir complexes containing L-valinol linkers (B136a–b and B137a–b) were prepared and used for the asymmetric transfer hydrogenation of ketones.195 Chiral alcohols were obtained in high yield and moderate to good enantioselectivities of up to 68%. The enantioselectivities were strongly affected by the combination of the substrate and wingtips. The Veige group used trans-9,10-dihydro-11,12-ethanoanthracene (DEA) as a chiral linker. The metalation of a DEA-tethered chiral enetetramine with [Rh(norbornene)2]BF4 produced bis(benzimidazolidene) complexes B138a.196a Compounds with different wingtips are prepared and used for hydrogenation of olefin and ketones, albeit with very low enantioselectivity.196b Bis(imidazolidene) compounds B139 and similar compounds in which the carbenes are directly connected to DEA positions 11 and 12 were also synthesized (B139 and B140).197-199 B138-B140 were tested for the catalytic 1,4-conjugate addition of phenylboronic acid to cyclohex-2-enone, and good enantioselectivities of up to 82% ee.199

32

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.3.5

Pincer bis-NHC ligands

A pincer ligand is a tridentate ligand that forms meriodinal geometry. An additional donor can be employed in the linker between two NHCs to form a pincer-type ligand. Peris et al. first reported a pincer biscarbene ligand using a pyridine linker.174 A Rh(III) tribromide complex with a CNC pincer ligand (B141) was prepared. Rh(I) complexes with the same ligand B142a–c were prepared and their nucleophilicitywas evaluated based on the kinetics of their reaction with MeI.200 The rate constants of MeI oxidative addition significantly exceeded that of known nitrogen, phosphine, and sulfur-based Rh(I) carbonyl complexes. The nucleophilic Rh(I) complex B143 oxidatively added CD2Cl2 to form an octahedral Rh(III) complex. In addition, several Rh(III) Z2-fluorobenzene adducts (B144a–e) related to the intermediates formed during aromatic C–H activation were obtained.201 The Ir(I) complex with CNC ligand B145 activated the C–H bond of the ligand iPr group to form B146.202 B145 was also found to oxidatively add CH2Cl2.203 Cationic Ir(I) adducts with p-acidic ligands such as pyridine, MeCN, CO (B147a–c), and ethylene (B148) were also prepared,202,203

Chaplin et al. reported a Rh complex with a macrocyclic pincer ligand tethered with a C12 alkyl chain wingtip connecting two NHCs (B149). The alkyl chain of B149 showed dynamic atropoisomerism, as observed by variable-temperature NMR and IR spectroscopy,204,205 and was converted into a dearomatized complex (B150) upon the addition of a base.204 Rh(III) and Ir(III) complexes B151 and B152 are known for their agnostic interactions involving the coordination of the flexible alkyl chain.206 The Rh(I) ethylene adduct B153 was used to promote terminal alkyne coupling reactions, selectively affording E-enynes (Scheme 39a).207 In contrast, the acyclic congener (B154) selectively gave the gem-isomer (Scheme 39b). In a stoichiometric

Scheme 39 (a) Selective coupling of terminal alkynes for E-enyne and (b) gem-enyne synthesis. (c) Zipper annulation of gem-enynes.

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

33

study, the E-isomer adduct B155 was obtained, which revealed that the steric constraints imposed by the flexible alkyl chain increased the barrier for the elimination of the gem-enynes. The selective production of gem-enyne in the presence of B154 was used in the synthesis of tetraaryl-substituted bicyclo[4.2.0]octa-1,5,7-trienes via terminal alkyne coupling followed by zipper annulation (Scheme 39c).208 Another Rh(I) complex with methyl wingtips (B156) was used to promote the oxidative addition of biphenylene and chlorobenzene for selective C–C and C–Cl bond activation.209 Another type of biscarbene CNC ligands, namely the carbazole-based biscarbene ligand called “bimca,” has been studied by Kunz et al. The kinetics of oxidative MeI addition to the Rh(I) carbonyl compound B157 proceeded faster than with any other Rh(I) complex reported so far and thus indicated the strong electron donor ability of bimca.210 The reaction of B157 with allylic chloride gave allyl complex B158 via SN2-type oxidative addition.211 The synthesis of the Ir(I) complex B159 and its reaction with allylic chloride (B160) was also studied.212 The strong nucleophilicity of the bimca Rh(I) complex was used in the Meinwald rearrangement reaction.213a In the presence of Lewis acid cocatalysts such as lithium salts, B157 catalyzes the rearrangement of terminal epoxides into methylketones. A more nucleophilic CO-free catalyst (B161) was prepared using an N-homoallyl-substituted bimca ligand and used as a catalyst for the Meinwald rearrangement under mild conditions even without a Lewis acid cocatalyst. The mechanism and catalyst deactivation pathways were studied using a similar compound having only one homoallyl substituent (B162).213b The CO-free Ir(I) complex B163 was also prepared, although its reactivity was much lower than that of the Rh complex B161. The isomerization of terminal aziridine to enamide was also realized using B162 and B163 (Scheme 40).214 The carbonyl compound B157 catalyzed the deoxygenation of epoxide with CO.215

Scheme 40 (a) Meinwald rearrangement of terminal epoxides into methylketones, (b) isomerization of a terminal aziridine to an enamide, and (c) deoxygenation of an epoxide with CO.

Biscarbene ligands with a phenylene linker are known to form CCC pincer complexes via C–H activation. Hollis et al. first reported a CCC biscarbene pincer complex of Rh.216 As the preparation of pincer complexes from a phenylene-bridged bis(imidazolium) salt requires the formation of two NHCs and the activation of an aryl C–H bond, Rh complexes B164 and B165 were synthesized by the reaction of bis(imidazolium) with Zr(NMe2)4 followed by transmetalation with [Rh(COD) Cl]2.216,217 B165 was used as a catalyst for hydroamination,217 Michael addition,218 and b-borylation (Scheme 41).219 Interestingly, the analogous ligand with adamantyl wingtips produced a mixed normal/abnormal carbene complex (B166) because of the large steric bulk of the adamantyl group. Dihydroimidazolidene-based CCC ligands and their Ir complexes (B167a–b) were used for the transfer hydrogenation of cyclooctane with tert-butylethylene.220 Benzimidazole-based CCC pincer complexes were studied by the Chianese group. Octahedral Ir(III) compounds (B168a–d) and five-coordinate species (B169a–b) with bulky tBu and Ad wingtips were obtained. 221,222 B168a–d were used for the borylation of aromatic C–H bonds,221 transfer hydrogenation,221 alkene isomerization,222,223 and alkene dehydrogenation.222,224

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N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

Scheme 41 (a) Hydroamination/cyclization of alkenylamines, (b) 1,4-addition of aryl boronic acids, (c) b-borylation of a,b-unsaturated ketones.

As the metalation of phenylene-bridged bis(imidazolium) salts often gives bimetallic complexes,225 the Braunstein group introduced a 4,6-dimethyl-1,3-phenylene linker to prevent C–H activation at undesired positions to selectively prepare a mononuclear (CCC)Ir complex (B168).226 With the adamantyl wingtip, metalation afforded an adduct featuring hydrogen bonding between the imidazolium salt and the Ir halide, which was metalated upon the addition of an appropriate base at only one imidazolium to give a monocarbene species (B169) instead of a pincer complex.227,228 A pincer biscarbene complex with a 4,6-trifluoromethyl phenylene linker (B170) was also reported.229

8.01.3.6

Scorpionate-bis-NHC ligands

Scorpionates are ligands capable of tridentate fac coordination, which occurs when the linker has an additional donor that is not in the plane of the metal and the two carbenes. In this section, we discuss not only complexes with tridentate coordination, but also the biscarbene complexes with potential donors that can either bind to the metal center or are substrate-bound on the metal center under appropriate conditions. The additional donor accommodates the primary or secondary coordination sphere interactions. The first scorpionate biscarbene complex containing a phenol-incorporated linker (B173) was reported by the Peris group.230 The same group reported a pyridine-incorporated complex (B174), which forms a polymeric species (B175) by coordination of pyridine to another Ir center.231 Other pyridine-functionalized complexes (B176a–c and B177a–c) were prepared for the transfer hydrogenation of ketones.232 Hydroxy-functionalized complexes (B178a–e) and their use in transfer hydrogenation and silylation have also been reported by Herrman, Kuhn, and co-workers.233,234

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Albrecht et al. reported the formation of alkyl-bridged CCC scorpionate complexes during the synthesis of a bis(abnormal carbene) with a propylene linker. The C–H activation at the C2 carbon of the propylene linker gave a mixture of (CCC)Rh complexes B179a–d and B180a–d, which were in equilibrium with each other.235,236 Deuterium exchange on the carbene ligand was observed upon the addition of D3PO4.236 B179b catalyzed the reaction of alcohols with silanes to form silyl ethers as well as the hydrosilylation of olefins and ketones.118 The diphosphine-bound species (B181a–b) were also synthesized and used to promote the transfer hydrogenation of ketones.153

Pérez-Torrente et al. reported carboxylate-functionalized methylene-bridged biscarbene ligands, showing that the carboxylate group exhibited hemilabile behavior. In half-sandwich compounds B182–187 and 16-electron Rh(I) (B188) and Ir(I) (B189) compounds, the carboxylate anion remained unbound.237 When the electrophile reacted with B189 to produce Ir(III), the carboxylate anion was bound to the metal center to form 18-electron octahedral Ir(III) complexes (B191–B193). The zwitterionic nature makes these Ir complexes soluble in water. Compound B189 catalyzed the hydrogenation of CO2 to formate in water in the presence of NEt3 as a base, reaching a TON of 1500.238. DFT calculations suggest that the formation of dihydrido Ir(III) intermediates is favored by the coordination of the carboxylate anion. The water-soluble zwitterionic Rh and Ir compounds B183, B184, and B189 were used as water oxidation catalysts,239 achieving excellent yields and TOF50 values of up to 1000 h−1 in the presence of (NH4)2[Ce(NO3)6] or NaIO4 as sacrificial oxidants. Changes in the wingtips of the biscarbene ligand or the catalyst framework did not improve catalytic performance. The reaction mechanism was believed to involve IrIII/IrIV/ IrV intermediates, similar to those proposed for biscarbene Cp Ir(III) catalysts,144,147 therefore stabilized by the carboxylate-functionalized biscarbene ligand. B183 also efficiently catalyzed the selective hydrosilylation of CO2 to afford the corresponding silylformate240 and was more active than the related carboxylate-free Ir complex B41a. The carboxylate moiety in B183 was found to be involved in the precatalyst activation step, leading to the formation of an Ir(III) hydrido active species with a silyl-carboxylate moiety acting as a silyl carrier. Carboxylate-mediated silylation allows the selective hydrosilylation of alkynes.241 B183 catalyzed the conversion of terminal alkynes with excellent regio- and stereoselectivity for the less thermodynamically stable b-(Z)-vinylsilane, probably via the transfer of silylium from the carboxylate to the alkyne.

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N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.4

Rh(NHC) and Ir(NHC) complexes with connected external coordinating groups

This section is divided into five sections. The first part deals with complexes featuring external phosphine groups, while the second part deals with those where the Cp moiety is connected to the NHC, and the third part covers heterocyclic molecules with the NHC ligand. The fourth part deals with amine-, alcohol-, and sulfide-based coordinating groups, and the fifth part covers olefin coordination along with NHC and miscellaneous coordinating groups with NHC moieties. The C–H activation for Rh–C and Ir–C coordination is not covered, as it is a reaction-based process. Tethered external coordination groups are also handled.

8.01.4.1

Rh(NHC) and Ir(NHC) complexes with connected external phosphine groups

This section is further divided on the basis of phosphine substituents, with Section 8.01.4.1.1 covering triarylphosphine–type ligands, Section 8.01.4.1.2 covering trialkylphosphine–type ligands for Rh and Ir, and Section 8.01.4.1.3 covering aryl and alkyl mixed phosphines.

8.01.4.1.1

Rh(NHC) and Ir(NHC) complexes with connected external triarylphosphine groups

Before extensive work on Rh(NHC) and Ir(NHC) chemistry, Rh and Ir phosphine complexes had been widely developed for various organometallic systems. Therefore, combinatorial studies of NHCs and phosphines for Rh and Ir have been naturally initiated by multiple research teams. In 2004, the Bolm group reported the synthesis of planar chiral chelating phosphinyl-imidazolyidene ligands and their Ir complexes (C1a).242 A planar chiral [2.2] paracyclophane backbone was used for the phosphine moiety, and triarylphosphines and substituted aryl rings were installed onto imidazole to afford NHCs. C1a displayed good catalytic activity and stereoselectivity for hydrogenation. A similar unsymmetrical installation of a phosphine moiety on the NHC ligand was also attempted by the Poli group in 2007.243 In this study, ferrocene-functionalized diphenylphosphine was installed onto imidazole, and the remaining part was substituted by a mesitylene group (C2a).

Andersson et al. continuously designed more general triphenylphosphine–type NHC ligands for Rh and Ir complexes (C3a, C3b, C3c, and C3d, respectively). Pristine C3a was stabilized by a bidentate system between the NHC and the phosphine and successfully

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catalyzed the selective alkylation of aromatic anilines to afford secondary amines as well as the cyclization of amino alcohols at room temperature.244 Two years later, the NHC backbone was modified. Complex C3c was developed as a hydrogen transfer catalyst for the N-monoalkylation of amides.245 Finally, the chiral version of C3d was reported in 2020.246 This bidentate NHC-phosphine Ir system showed good catalytic activity for the enantioselective hydrogenation of ketones. The main reaction was performed at room temperature under a hydrogen pressure of 1 bar and a catalyst loading of 1 mol%. A slightly modified Ir-NHC-phosphine complex C3e was prepared by Kerr and used to promote hydrogen isotope exchange in aryl sulfones.247

8.01.4.1.2

Rh(NHC) and Ir(NHC) complexes with connected external trialkylphosphine groups

Dicyclohexylphosphine derivatives have been intensively studied by Hahn et al. In 2011, a dicyclohexylphosphine-connected benzimidazole-based NHC ligand was prepared and studied as a ligand for Rh complexes (C4a).248 In this study, the phosphine connected to the NHC ligand and the separated tricyclohexylphosphine and triphenylphosphine were independently controlled and investigated. In addition, the stereoconfiguration (cis-P,P and trans-P,P) depended on the steric demand of the NHC ligand and spacer alkyl groups. Three years later, the same group reported hydrogen transfer in NHC ligands using complex C4b.249 The protonation and deprotonation of the benzimidazole ring were conducted using Ir(NHC)-Cl and Ir(NHC)-H species.

The trialkylphosphine with NHC ligands was then expanded to a tert-butyl system. The Braunstein group reported Ir(NHC) complexes with phenolates, and the phenolate end was connected to di-tert-butylphosphine (C5a and C5b).250 In addition, Hofmann’s team reported the more general tert-butyl-functionalized NHC-phosphine complexes for Rh and Ir (C6a and C6b).111 The sterically bulky and electron-rich ligand featuring tert-butyl- and di-tert-butylphosphine groups as NHC side chains was successfully introduced into both Rh and Ir complexes. The comparison of Rh(NHCP) and Ir(NHCP) complexes with traditional Rh(NHC) and Ir(NHC) complexes furnished useful structural and spectroscopic information.

8.01.4.1.3

Rh(NHC) and Ir(NHC) complexes with connected external mixed phosphine groups

As the phosphine group generally has three substituents, one group is used for connection with the NHC part, which leaves two groups attached to the P atom. Therefore, the simplest synthetic approach is to use one alkyl substituent to connect with the NHC ring and leave two aryl substituents on the phosphine. In 2005, Danopoulos reported a Rh(acac) complex with an NHC ligand featuring diphenylphosphine appended through an ethylene bridge (C7a) and used this complex to promote transfer hydrogenation.251 In the same year, the Messerle group prepared Rh(COD) and Ir(COD) complexes with NHC ligands having an ethylene linker and diphenylphosphine groups (C8a and C8b).252 In 2019, the same group expanded this system to a perimidine-based NHC with two diphenylphosphine groups. Two phosphine groups were connected through a methylene bridge in this case (C9a).253 In addition, a dicyclohexylphosphine derivative was also prepared using a similar strategy. These Rh complexes were applied to the selective formylation of amines or methylation.

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N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.4.2

Rh(NHC) and Ir(NHC) complexes with connected external Cp groups

Cyclopentadienyl (Cp) and pentamethylcyclopentadienyl (Cp ) are the most common organic ligands for group 9 metals. Therefore, the external coordination of Cp series functionalities with the NHC ligands was also investigated. The Royo group significantly contributed to the development of Cp-functionalized NHC ligands and the related Rh and Ir chemistry. In one of the initially prepared complexes, the IrI2-based NHC-Cp complex C10a,254 the methyl group in the Cp ligand is connected to the imidazole-type NHC through the ethylbenzene moiety, and the other part of the NHC ligand is substituted by a methyl group. This complex was used to promote the transfer hydrogenation and b-alkylation of primary alcohols. Subsequently, the Royo group expanded this Cp -functionalized NHC system to both RhCl2 and IrCl2 complexes, and Cp and NHC moieties were connected with simple ethylene groups (C10b and C10c).255 These dichloro complexes also showed good catalytic activity for the b-alkylation of primary alcohols. Two years later, Royo and co-workers developed Rh and Ir complexes with enantiomerically pure Cp-functionalized NHC ligands (C10d and C10e).256

8.01.4.3

Rh(NHC) and Ir(NHC) complexes with connected external heterocycles

This section is further divided based on the heterocycle type. Section 8.01.4.3.1 covers pyridine-type ligands, Section 8.01.4.3.2 deals with oxazoline-type ligands for Rh and Ir, while Section 8.01.4.3.3 covers Rh(NHC) and Ir(NHC) complexes with external coordination through imidazole, pyrimidine, and pyrazole groups. Finally, triazole-and quinoline-type functionalities are covered in Section 8.01.4.3.4.

8.01.4.3.1

Rh(NHC) and Ir(NHC) complexes with connected external pyridine groups

Pyridine-containing NHC ligands for Rh and Ir complexes are divided into those with a pyridine connection to the NHC scaffold and those with a methylene (or alkylene) connection between the pyridine and NHC parts. The first significant contribution to this topic was reported by Crabtree et al. in 2001.257 In their Ir complex C11a, IrH was bonded to two triphenylphosphines and NHCpyridine. However, the metalated position of imidazole moved to the abnormal C5-position from the C2-position between two nitrogen atoms. In 2006 and 2007, Liu et al. reported Rh(COD) and Ir(COD) complexes with pyridine-containing NHC ligands (C12a and C12b).258,259 In these studies, an IMe-type NHC was used, and pyridine also had mesitylene substituents. In particular, the Ir complex C12b was used as an efficient catalyst for nitroarene reduction. In 2017, the Suárez group reported variations in the pyridine moiety.260 Simple pyridine, 2-phenylpyridine, and dimethoxyphenyl-substituted pyridine were investigated for Rh(COD) complexes with NHC ligands (C13a, C13b, and C13c). In addition, stereoselective (E-selective) alkyne hydrosilylation was studied using these catalyst series. Generally, the steric hindrance of the pyridine moiety is strongly related to the efficiency of catalytic hydrosilylation. In 2011, Pérez-Torrente et al. studied a simpler Ir(COD) system with an NHC-pyridine ligand. Methyl-and picoline-substituted NHC ligands were used for metalation (C14a), and the obtained Ir species was used as a transfer hydrogenation catalyst.261 In the same year, Cavell reported a unique asymmetric bicyclic NHC with two pyridyl groups for the Ir complex C15a.262 This pincer-type ligand features one NHC and two pyridyl groups, and the other two coordinations are occupied with additional COD ligands. The above complex catalyzed the transfer hydrogenation of ketones and the hydrogenation of alkenes; however, high stereoselectivity was not achieved under high-temperature conditions. Finally, the IrCp complex was investigated for the

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39

NHC-pyridine ligand in 2019. Ke et al. prepared 2-pyridone-containing NHC ligands and applied them to the IrCp system (C16a),263 revealing that this Ir species showed good catalytic activity for the N-alkylation of aniline derivatives.

At the same time, the pyridine moiety could be directly connected to NHC as an aryl substituent. In 2005, the Danopoulos group reported a Rh(COD) complex with picoline and a 2,6-diisopropylbenzene-substituted imidazole-type NHC ligand (C17a).251 In 2008, Jin and co-workers prepared pyridine-connected and methylene-bridged-type NHCs for IrCp complexes (C18a and C18b).264 Both of these Ir complexes were used to catalyze the polymerization of norbornene in the presence of a methylaluminoxane co-catalyst. Interestingly, methylene-bridged C18a showed better catalytic activity for polymerization than the complex with a direct connection (C18b). In 2014, Kühn et al. prepared an Ir(COD) complex with two NHC-bearing picoline moieties (C19a),265 revealing that only one pyridyl group was coordinated to the Ir center. This Ir(NHC) complex with hemilabile coordination was studied as a transfer hydrogenation catalyst. Lugan et al. reported a pincer-type NHC ligand with phosphine and pyridine266 and used it to prepare the RhCl complex C20a and the RhI3 complex C20b, which were subsequently subjected to structural analysis.

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N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.4.3.2

Rh(NHC) and Ir(NHC) complexes with connected external oxazoline groups

The oxazoline moiety is an important motif in organic synthesis in view of its interesting chiral environment and relatively easy preparation. Therefore, the external coordination of the NHC ligand with the oxazoline moiety was initiated in the early stages of Rh(NHC) and Ir(NHC) chemistry. There are two classes of oxazoline-bearing NHC ligands, namely alkyl and aryl ones. The first oxazoline-bearing NHC was reported by Herrmann in 1998. Benzyl- and isopropyl-substituted enantiomerically pure oxazoline-substituted imidazolium salts were prepared and used to synthesize the corresponding Rh(COD) complexes (C21a, C21b, C21c, and C21d).267 Interestingly, the coordination of Pd with the same ligand afforded dimeric species. After 2001, the Burgess group significantly contributed to the development of Ir(NHC-oxazoline) complexes and their catalytic applications.268–272 Ir(COD) complex C22a had an NHC ligand with an oxazoline moiety and an ethylene connection, displaying good catalytic activity and stereoselectivity for the hydrogenation of aryl alkenes.268 Interestingly, the sterically bulky adamantyl substituent in the oxazoline group generally afforded better stereoselectivity than other groups. Moreover, the Ir(COD) complex with NHC-bearing chiral oxazolines showed catalytic activity for the reduction of enol ethers.271 In 2006, Pfaltz et al. reported an Ir(COD) complex with an NHC-oxazoline ligand. In this system, the NHC ligand and the oxazoline moiety were connected by a methylene bridge (C23a), and the corresponding complex effectively catalyzed the hydrogenation of a,b-unsaturated esters, allylic alcohols, and imines.273 The Gade group prepared a Rh complex with directly connected NHC-oxazoline ligands (C24a),274 revealed that this complex showed good catalytic activity for hydrosilylation, and probed the correlation between the rigidity of the ligand backbone and catalytic efficiency. In general, the direct connection between oxazoline and NHC allowed better catalytic performance than the bridged system in this study.

The oxazoline moieties were also connected through the aromatic ring system. In 2003, the Bolm group used a planar chiral [2.2] paracyclophane system to connect chiral oxazoline and NHC molecules (C25a).275 This series of Ir(COD)(NHC-oxazoline) complexes was applied to the asymmetric hydrogenation of alkenes. In 2004, Crudden and co-workers successfully prepared an NHC-bearing 2-phenyl oxazoline moiety and metalated it with Pd and Rh (C26a).276 The Rh(NHC-oxazoline) complex was used to promote the hydrosilylation of acetophenone and the hydroboration of styrene. In 2019, Hou et al. appended a 2-phenyl benzoxazoline group to NHC and subsequently performed metalation with the IrCp complex (C27a).277 In addition to aryl substituents, fused arenes were also utilized in NHC scaffolds to establish a connection with oxazoline. In 2020, Ruiter and co-workers reported an Ir(NHC) complex bearing a chiral oxazoline group and a COD ligand (C28a)278 and used it to catalyze the direct asymmetric hydrogenation of aryl ketones and their dynamic kinetic resolution. Overall, high enantioselectivity and good diastereoselectivity were achieved for a broad range of substrates. Finally, a pincer-type NHC-phenyl-oxazoline ligand was developed for Rh by Nishiyama in 2016 (C29a).279 This CCN pincer-type Rh complex was prepared through C–H bond activation and applied to hydrogenation and reduction.

N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.4.3.3

41

Rh(NHC) and Ir(NHC) complexes with connected external imidazole, pyrimidine, and pyrazole groups

Relatively stable carbenes could be generated at the C2 position of imidazole through deprotonation, and the N atom on the imidazole ring could act as a donor for metal coordination. The NHC-connected imidazole moiety was independently reported by the Liu and Choudhury groups in 2018. The Liu group mainly focused on the antitumor effect of Ir(NHC-benzimidazole) complexes (C30a, C30b, C30c, and C30d) and showed that they exhibited good activity (three times higher than cisplatin) against the A549 cell line.280 The Choudhury group used the Ir(NHC-benzimidazole) complex to promote base-free hydrogenation under ambient pressure, revealing that various aldehydes, including aromatic, heteroaromatic, and aliphatic ones, were successfully reduced under mild reaction conditions.281 NHC-pyrimidine ligands were developed by Grotjahn in 2011, who prepared RhCp and IrCp complexes coordinated with NHC-pyrimidine (C31a and C31b) and analyzed them by X-ray crystallography and NMR spectroscopy. In addition, the Ir complex C31a was studied as a catalyst for intramolecular hydroamination.282 Pyrazole-bearing NHCs were mainly developed by the Messerle group from 2006.255,283,284 Interestingly, Rh(NHC) and Ir(NHC) showed quite different behaviors for coordination with NHC-pyrazole. Although Ir preferred monodentate coordination through the Ir-NHC bond, coordination in cases of Rh-NHC and Rh-pyrazole featured bridge controls (C32a and C32b). The above complexes were used as catalysts for the hydrogenation of styrene and employed in mechanistic studies of alkene hydrogenation.

8.01.4.3.4

Rh(NHC) and Ir(NHC) complexes with connected external triazole and quinoline groups

Triazoles are also important heterocycles for Rh and Ir complexes. Ir(NHC) complexes with connected triazoles were reported by Burley in 2012.285 Although N-triazole-substituted benzimidazole afforded only the undesired Ir-N-coordinated complex, N-methylated and N-triazole-substituted benzimidazole afforded an Ir complex upon reaction with [IrCl2Cp ]2 (C33a). In this complex, the Ir center was coordinated with Cp , iodide, NHC from benzimidazole, and the N atom of triazole. In 2019, Barnard prepared Ir(BPY) complexes with NHC-triazole ligands and studied them as luminescent materials.286 Quinoline-bearing NHC ligands were prepared by Li in 2008 and applied to the Rh(COD) complex.287 Interestingly, the molar ratio between the metal, ligand, and base controlled the formation of the C34a complex and monodentate Rh species. The same ligand was also used in the case of the IrCp complex, and the obtained Rh(NHC-quinoline) and Ir(NHC-quinoline) complexes were studied in terms of their electrochemical properties (C34b).

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N-Heterocyclic Carbene (NHC) Complexes of Rhodium and Iridium

8.01.4.4

Amines, alcohols, thiols, and their derivatives

This section is further divided based on the type of heteroatom. Section 8.01.4.4.1 covers amine-type external coordination, while Section 8.01.4.4.2 deals with alcohol- and ether-based functionalities for Rh(NHC) and Ir(NHC) and external carboxylate coordination groups. Finally, thiol, sulfide, and S-related functionalities are covered in Section 8.01.4.4.3.

8.01.4.4.1

Rh(NHC) and Ir(NHC) complexes with connected external amine groups

The strong metal-coordinating ability of amines is an important issue in Rh(NHC) and Ir(NHC) chemistry. In general, there are two types of external amine coordination groups in NHC molecules, namely aromatic (aniline-type) and aliphatic amines. The first efficient aromatic amine–bearing NHC for Rh chemistry was developed by the Fryzuk group in 2008.288 2,4,6-Trimethylaniline was connected with an NHC through an ethylene bridge, and Rh(COD) was coordinated with the amine-bearing NHC ligands (C35a). Although the desired structure was obtained and analyzed, these Rh(NHC-amine) species did not promote alkene hydrogenation, which was ascribed to the absence of the corresponding active sites due to the tight binding of all groups coordinated to Rh centers. In 2011, Cross and co-workers introduced RhCp and IrCp complexes for amine-bearing NHCs (C36a and C36b).289 Isopropyl and ortho-aminobenzene-substituted NHCs were prepared and applied to Rh, Ir, and Ru. A major difference was observed in deprotonation behavior. Although Rh bound to the NH2 group with a total positive charge, aniline deprotonation occurred in the case of Ir to afford neutral Ir complexes (C36b). These Rh(NHC-amine) and Ir(NHC-amine) complexes showed moderate reactivity for the transfer hydrogenation of acetophenone. In 2013 and 2014, the Elsevier research team investigated the structural features of Rh(NHC-amine) and Ir(NHC-amine) complexes.290,291 Both Cp -type complexes (C37a and C37b) and COD-based Rh and Ir complexes (C38a and C38b) were carefully studied and analyzed. Interestingly, while the Cp system allowed a positive complex without the deprotonation of aniline, the COD system favored deprotonated neutral metal complexes.

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Aliphatic amine–tethered NHC ligands were also studied for external coordination with Rh(NHC) and Ir(NHC) systems. Sánchez et al. developed an NHC-pyridine-proline-type ligand for Rh chemistry.292 The N-alkyl pyrrolidine moiety in proline bound to the Rh center as the third coordinating species to afford a CNN-type pincer ligand with Rh(COD) (C39a and C39b). Au, Pd, and Rh complexes were synthesized using this ligand and studied as enantioselective hydrogenation catalysts. In 2013, the Ru˚ ži
cka team introduced a N,N-dimethylbenzylamine moiety into a Rh(COD) complex (C40a).293 In general, the monodentate/bidentate coordination pattern of Rh was controlled by the coordinating partner and the environment, and unusual seven-membered rhodacycles were obtained through the external coordination of the N,N-dimethylbenzylamine moiety. Martín-Matute reported pendent amine group–containing NHC ligands and their sandwich-type Ir complexation.294,295 An unsymmetrically functionalized NHC (alkyl amine and n-butyl) Ir complex (C41a) was studied as a catalyst for water oxidation and alcohol dehydrogenation. These complexes were also expanded to pendent alcohol-type molecules.

Imine-bearing NHC ligands have also been intensively applied to Rh(NHC) and Ir(NHC) chemistry. Imine condensation from cyclohexyl diamine molecules is one of the most well-known chemistries for Schiff base synthesis. In 2008 and 2009, the Douthwaite team developed chiral cyclohexyl diamine–based NHC complexes with imine moieties and applied them to Rh and Ir complexes (C42a).255,296 In general, Ir(NHC-imine) complexes displayed better catalytic activity than Rh(NHC-imine) complexes for asymmetric transfer hydrogenation. In 2011, Tilset et al. developed chelating imine-functionalized NHC ligands and metalated them with Rh species.297 This approach was based on computational studies and explained the structural aspects of this unique Rh(NHC-imine) complex (C43a). In addition, the Danopoulos group synthesized similar chelating imine-functionalized NHC ligands and Ir complexes (C44a).298 The monomeric/dimeric complexes and their C–H/N–H activation abilities were compared and deeply studied.

8.01.4.4.2

Rh(NHC) and Ir(NHC) complexes with connected external alcohol, ether, and carboxylate groups

The tethered alcohol and ether moieties on NHC ligands can be divided into aromatic, aliphatic types, and ether groups. An aromatic alcohol-type tethered group was first reported by Crabtree in 2005,299 who prepared chiral BINAP-containing NHC ligands and applied them to Rh(NHC-alcohol) and Ir(NHC-alcohol) complexes (C45a, C45b, C45c, and C45d). These complexes were tested as catalysts for the asymmetric hydrosilylation of ketones, with Ir complexes generally displaying better efficiency than Rh ones. In 2010, Bercaw and co-workers reported diphenolate-functionalized NHC ligands and their Ir complex (C46a).300 Interestingly, this NHC system was efficient for both Ir(I) and Ir(III) systems and successfully catalyzed the hydrogenation of olefins. DFT calculations supported the experimental findings. In 2018, Liu et al. reported a monophenolate-functionalized NHC and its Ir complex (C47a).301 This half-sandwich-type Ir(III) complex was tested for anticancer activity. For all aromatic alcohol–tethered groups, deprotonated alkoxides were employed for Rh(NHC) and Ir(NHC) complexes.

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NHC ligands with aliphatic alcohol moieties were developed with aliphatic amine-bearing NHCs (Section 8.01.4.4.1). Martín-Matute et al. reported aliphatic alcohol-tethered NHC ligands for Ir complexes in 2012 (C48a).302 Although the original study used a mono-aliphatic alcohol group as the NHC substituent, Martín-Matute developed di-aliphatic alcohol-substituted NHCs for IrCp in 2018 (C48b).295 Generally, this alcohol-functionalized Ir(NHC) complex showed better catalytic efficiency than amine-functionalized Ir(NHC) complexes (e.g., C41a) for the dehydrogenation of alcohols. The aliphatic alcohol–functionalized NHC ligands were also applied to Rh(COD) by Denialti, and the obtained Rh(NHC-alcohol) complex showed good catalytic activity for the arylation of aldehydes (C49a).

Ether-tethered NHC ligands for Rh and Ir chemistry were also developed by the Oro group.303 In 2013, bis-NHC ligands with ether functionalities were developed and subjected to metalation with Rh and Ir (C50a and C50b). The resulting Rh(NHC-ether) and Ir(NHC-ether) complexes were studied as catalysts for the selective hydrosilylation of terminal alkynes.

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The ability of carboxylate groups to strongly coordinate Rh and Ir has inspired works on carboxylate-tethered NHC ligands. In 2011, the Peris team developed a simple acetate-bearing NHC ligand and used it to prepare a RhCp complex (C51a) that was tested as a catalyst for the intramolecular oxidation of alcohols.304 In addition, the Basl_e group contributed to the development of chiral carboxylate-bearing NHC ligands for Rh and Ir chemistry. In 2018, the RhCp Cl(NHC-carboxylate) complex was obtained, showing good catalytic activity for regio-selective ortho C–H borylation under mild conditions (C52a).305 In 2019, a Rh(COD) (NHC-carboxylate) complex was developed (C53a) and applied to regioselective C–H borylation along with a photocatalytic process including direct visible-light absorption.306 At the same time, an Ir(BPY)(NHC-carboxylate) complex was synthesized by the Basl_e team (C54a) and applied to photocatalytic intermolecular [2 +2] cycloaddition and enantioselective Friedel-Craft addition.307

8.01.4.4.3

Rh(NHC) and Ir(NHC) complexes with connected external sulfur functional groups

Sulfur-related functionalities have a unique ability to coordinate heavy metals and exhibit interesting behaviors. In Rh(NHC) and Ir(NHC) chemistry, the external coordination of sulfide, sulfonate, and sulfonamine groups was investigated. Rh chemistry has mainly been studied in the case of sulfide-bearing NHC ligands. In 2007, the Labande group reported Rh(COD)(NHC-sulfide) complexes (C55a and C55b) and studied their ability to promote the hydrosilylation of ketones.308 At the same time, the Fernández team prepared a more fused and complex Rh(COD)(NHC-sulfide) system (C56a). The structural aspects were analyzed, and the main catalytic activity was studied using the Pd(NHC) complex.309 In 2008 and 2010, Lassaletta et al. studied the stereoselective synthesis of Rh(COD)(NHC-sulfide) complexes. A triazole-based NHC was utilized in 2008 (C57a),310 and a simple imidazole-based NHC was used in 2010 (C57b) for metalation with Rh.311 Rh(NHC-sulfide) complexes were applied to the asymmetric hydrosilylation of acetophenone and dynamic kinetic resolution.

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In 2015, Achard’s group introduced a sulfonate functionality into an NHC ligand and applied it to the IrCp complex (C58a),312 revealing that this unique IrCp (NHC-sulfonate) complex promoted hydrogen transfer and the N-alkylation of amines. A sulfonamine-tethered NHC was prepared by Bera et al. and applied to a Rh(CO) complex (C59a).313 This triflimide-tethered Rh(NHC) complex was developed for hydroalkoxylation and showed good catalytic activity for both intermolecular and intramolecular hydroalkoxylations of terminal alkynes and alkynols.

8.01.4.5

Olefins and other teathers

C¼ C bonds are important p-coordinating groups. Therefore, NHCs with external olefins were also synthesized and investigated because of their unique properties. In 2005, Oro et al. prepared Ir(COD) complexes with allyl-tethered NHC ligands (C60a and C60b)314 and studied them as transfer hydrogenation catalysts, revealing that the fifth coordination plays a key role in determining the reaction mechanism. Five-coordinated complexes generally showed lower catalytic activities than four-coordinated Ir(NHC) complexes. In 2008, Mata et al. prepared diallyl-substituted NHC ligands and applied them to Ir(COD) complexes (C61a, C61b, and C61c).315 Fluxional processes for extra coordination and decoordination were intensively studied, and monocoordinated Ir(NHC) complexes generally showed good catalytic activity for the hydrosilylation of terminal alkynes. The Aldridge team investigated olefin-coordinated Ir(NHC) complexes (C62a).35 Although the original structure corresponded to Ir(IPr)2Cl, C–H activation produced a C¼ C bond that coordinated to the Ir center. In addition, this phenomenon was explained and supported by quantum chemical simulations.

The carborane cage is a molecular cluster consisting of 10 boron and two carbon atoms. In 2016, Willans et al. developed a carborane-tethered NHC ligand and applied it to a Rh(COD) complex (C63a).316 Although nido-carborane produced a bimetallic dimeric Rh complex, closo-carborane afforded an interesting seven-membered metalacycle (C63a).

8.01.5

Conclusion

The development of rhodium and iridium chemistry for organometallics and organic methodologies has been significantly accelerated by the evolution of NHC ligands. Various important aspect including syntheses and reactivities of Rh(NHC) and Ir(NHC) are summarized in this chapter with NHC ligand types. NHC ligands are commonly thought as analogues to phosphine ligands, therefore the classical rhodium and iridium chemistry such as hydrogenation/dehydrogenation, hydrosilylation, water oxidation, and C-H activations has been widely studied with a variety of NHC ligands. However, the versatility of NHC ligands additionally provides a high degree of ligand tunability including electronic and steric properties along with external donor groups. Further fascinating developments and researches for new methodology may be expected with Rh(NHC) and Ir(NHC).

Acknowledgment Jooyeon Lee were supported by the National Research Foundation of Korea (NRF) Global Ph.D. Fellowship program (2019H1A2A1076014) funded by the Ministry of Education. Our studies on Rh(NHC) chemistry was supported by the Basic Science Research Program (2019R1A2C4070584), the Science Research Center (2016R1A5A1009405) through the NRF (funded by the Ministry of Science and ICT), and the R&D Program of Institutional Research Program of the KRICT (SI2111-40).

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A Bifunctional Strategy for N-Heterocyclic Carbene-Stabilized Iridium Complex-Catalyzed: N -Alkylation of Amines with Alcohols in Aqueous Media. Green Chem. 2019, 21, 219–224. 264. Xiao, X. Q.; Jin, G. X. Functionalized N-Heterocyclic Carbene Iridium Complexes: Synthesis, Structure and Addition Polymerization of Norbornene. J. Organomet. Chem. 2008, 693, 3363–3368. 265. Riener, K.; Bitzer, M. J.; Pöthig, A.; Raba, A.; Cokoja, M.; Herrmann, W. A.; Kühn, F. E. On the Concept of Hemilability: Insights into a Donor-Functionalized Iridium(I) NHC Motif and Its Impact on Reactivity. Inorg. Chem. 2014, 53, 12767–12777. 266. Valyaev, D. A.; Willot, J.; Mangin, L. P.; Zargarian, D.; Lugan, N. Manganese-Mediated Synthesis of an NHC Core Non-Symmetric Pincer Ligand and Evaluation of Its Coordination Properties. Dalton Trans. 2017, 46, 10193–10196. 267. Herrmann, W. A.; Goossen, L. J.; Spiegler, M. Chiral Oxazoline/Imidazoline-2-Ylidene Complexes. Organometallics 1998, 17, 2162–2168. 268. 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Synthesis and Application of Chiral N-Heterocyclic Carbene-Oxazoline Ligands: Iridium-Catalyzed Enantioselective Hydrogenation. Chem. Eur. J. 2006, 12, 4550–4558. 274. César, V.; Beliemin-Laponnaz, S.; Wadepohl, H.; Gade, L. H. Designing the “Search Pathway” in the Development of a New Class of Highly Efficient Stereoselective Hydrosilylation Catalysts. Chem. Eur. J. 2005, 11, 2862–2873. 275. Bolm, C.; Focken, T.; Raabe, G. Synthesis of Iridium Complexes with Novel Planar Chiral Chelating Imidazolylidene Ligands. Tetrahedron Asymmetry 2003, 14, 1733–1746. 276. Ren, L.; Chen, A. C.; Decken, A.; Crudden, C. M. Chiral Bidentate N-Heterocyclic Carbene Complexes of Rh and Pd. Can. J. Chem. 2004, 82, 1781–1787. 277. Huang, S.; Hong, X.; Cui, H. Z.; Zhou, Q.; Lin, Y. J.; Hou, X. F. N -Methylation of Ortho -Substituted Aromatic Amines with Methanol Catalyzed by 2-Arylbenzo[d]Oxazole NHC-Ir(III) Complexes. Dalton Trans. 2019, 48, 5072–5082. 278. Ayya Swamy, P. C.; Varenikov, A.; de Ruiter, G. Direct Asymmetric Hydrogenation and Dynamic Kinetic Resolution of Aryl Ketones Catalyzed by an Iridium-NHC Exhibiting High Enantio- and Diastereoselectivity. Chem. Eur. J 2020, 26, 2333–2337. 279. Ito, J. I.; Ubukata, S.; Muraoka, S.; Nishiyama, H. Enantioselective Direct Alkynylation of Ketones Catalyzed by Chiral CCN Pincer RhIIIComplexes. Chem. Eur. J. 2016, 22, 16801–16804. 280. Han, Y.; Liu, X.; Tian, Z.; Ge, X.; Li, J.; Gao, M.; Li, Y.; Liu, Y.; Liu, Z. Half-Sandwich Iridium(III) Benzimidazole-Appended Imidazolium-Based N-Heterocyclic Carbene Complexes and Antitumor Application. Chem. Asian J. 2018, 13, 3697–3705. 281. Garhwal, S.; Maji, B.; Semwal, S.; Choudhury, J. Ambient-Pressure and Base-Free Aldehyde Hydrogenation Catalyst Supported by a Bifunctional Abnormal NHC Ligand. Organometallics 2018, 37, 4720–4725.

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282. Specht, Z. G.; Cortes-Llamas, S. A.; Tran, H. N.; Van Niekerk, C. J.; Rancudo, K. T.; Golen, J. A.; Moore, C. E.; Rheingold, A. L.; Dwyer, T. J.; Grotjahn, D. B. Enabling Bifunctionality and Hemilability of N-Heteroaryl NHC Complexes. Chem. Eur. J. 2011, 17, 6606–6609. 283. Messerle, B. A.; Page, M. J.; Turner, P. Rhodium(I) and Iridium(I) Complexes of Pyrazolyl-N-Heterocyclic Carbene Ligands. J. Chem. Soc. Dalton Trans. 2006, 2006, 3927–3933. 284. Mancano, G.; Page, M. J.; Bhadbhade, M.; Messerle, B. A. Hemilabile and Bimetallic Coordination in Rh and Ir Complexes of NCN Pincer Ligands. Inorg. Chem. 2014, 53, 10159–10170. 285. Burley, G. A.; Boutadla, Y.; Davies, D. L.; Singh, K. Triazoles from N-Alkynylheterocycles and Their Coordination to Iridium. Organometallics 2012, 31, 1112–1117. 286. Karmis, R. E.; Carrara, S.; Baxter, A. A.; Hogan, C. F.; Hulett, M. D.; Barnard, P. J. Luminescent Iridium(III) Complexes of N-Heterocyclic Carbene Ligands Prepared Using the “Click Reaction.”Dalton Trans. 2019, 48, 9998–10010. 287. Peng, H. M.; Webster, R. D.; Li, X. Quinoline-Tethered N-Heterocyclic Carbene Complexes of Rhodium and Iridium: Synthesis, Catalysis, and Electrochemical Properties. Organometallics 2008, 27, 4484–4493. 288. Jong, H.; Patrick, B. O.; Fryzuk, M. D. Amine-Tethered N-Heterocyclic Carbene Complexes of Rhodium(I). Can. J. Chem. 2008, 86, 803–810. 289. Cross, W. B.; Daly, C. G.; Boutadla, Y.; Singh, K. Variable Coordination of Amine Functionalised N-Heterocyclic Carbene Ligands to Ru, Rh and Ir: C-H and N-H Activation and Catalytic Transfer Hydrogenation. Dalton Trans. 2011, 40, 9722–9730. 290. Jansen, E.; Jongbloed, L. S.; Tromp, D. S.; Lutz, M.; De Bruin, B.; Elsevier, C. J. Ligand Effects on the Hydrogenation of Biomass-Inspired Substrates with Bifunctional Ru, Ir, and Rh Complexes. ChemSusChem 2013, 6, 1737–1744. 291. Jansen, E.; Lutz, M.; Bruin, B. D.; Elsevier, C. J. Charge-Delocalized k2 C, N-NHC-Amine Complexes of Rhodium, Iridium, and Ruthenium. Organometallics 2014, 33, 2853–2861. 292. Boronat, M.; Corma, A.; González-Arellano, C.; Iglesias, M.; Sánchez, F. Synthesis of Electron-Rich CNN-Pincer Complexes, with N-Heterocyclic Carbene and (S)-Proline Moieties and Application to Asymmetric Hydrogenation. Organometallics 2010, 29, 134–141.
293. Turek, J.; Panov, I.; Horá
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elková, Z.; Ruiszicka, A. Amino Group Functionalized N-Heterocyclic 1,2,4-Triazole-Derived Carbenes: Structural Diversity of Rhodium(I) Complexes. Organometallics 2013, 32, 7234–7240. 294. Mahanti, B.; González Miera, G.; Martínez-Castro, E.; Bedin, M.; Martín-Matute, B.; Ott, S.; Thapper, A. Homogeneous Water Oxidation by Half-Sandwich Iridium(III) N-Heterocyclic Carbene Complexes with Pendant Hydroxy and Amino Groups. ChemSusChem 2017, 10, 4616–4623. 295. González Miera, G.; Martínez-Castro, E.; Martín-Matute, B. Acceptorless Alcohol Dehydrogenation: OH vs NH Effect in Bifunctional NHC-Ir(III) Complexes. Organometallics 2018, 37, 636–644. 296. Dyson, G.; Frison, J. C.; Simonovic, S.; Whitwood, A. C.; Douthwaite, R. E. Synthesis and Structural Variation of Iron, Rhodium, Palladium, and Silver Complexes of a Chiral N-Heterocyclic Carbene - Phenoxyimine Hybrid Ligand. Organometallics 2008, 27, 281–288. 297. Rosenberg, M. L.; Krapp, A.; Tilset, M. On the Mechanism of Cyclopropanation Reactions Catalyzed by a Rhodium(I) Catalyst Bearing a Chelating Imine-Functionalized NHC Ligand: A Computational Study. Organometallics 2011, 30, 6562–6571. 298. He, F.; Braunstein, P.; Wesolek, M.; Danopoulos, A. A. Imine-Functionalised Protic NHC Complexes of Ir: Direct Formation by C-H Activation. Chem. Commun. 2015, 51, 2814–2817. 299. Chianese, A. R.; Crabtree, R. H. Axially Chiral Bidentate N-Heterocyclic Carbene Ligands Derived from BINAM: Rhodium and Iridium Complexes in Asymmetric Ketone Hydrosilylation. Organometallics 2005, 24, 4432–4436. 300. Weinberg, D. R.; Hazari, N.; Labinger, J. A.; Bercaw, J. E. Iridium(I) and Iridium (III) Complexes Supported by a Diphenolate Imidazolyl-Carbene Ligand. Organometallics 2010, 29, 89–100. 301. Zhang, Y.; Zhang, S.; Tian, Z.; Li, J.; Xu, Z.; Li, S.; Liu, Z. Phenoxide Chelated Ir(III) N-Heterocyclic Carbene Complexes: Synthesis, Characterization, and Evaluation of Their in Vitro Anticancer Activity. Dalton Trans. 2018, 47, 13781–13787. 302. Bartoszewicz, A.; Marcos, R.; Sahoo, S.; Inge, A. K.; Zou, X.; Martín-Matute, B. A Highly Active Bifunctional Iridium Complex with an Alcohol/Alkoxide- Tethered N-Heterocyclic Carbene for Alkylation of Amines with Alcohols. Chem. - A Eur. J. 2012, 18, 14510–14519. 303. Iglesias, M.; Sanz Miguel, P. J.; Polo, V.; Fernández-Alvarez, F. J.; Pérez-Torrente, J. J.; Oro, L. A. An Alternative Mechanistic Paradigm for the Β- Z Hydrosilylation of Terminal Alkynes: The Role of Acetone as a Silane Shuttle. Chem. Eur. J. 2013, 19, 17559–17566. 304. Benítez, M.; Mas-Marzá, E.; Mata, J. A.; Peris, E. Intramolecular Oxidation of the Alcohol Functionalities in Hydroxyalkyl-N-Heterocyclic Carbene Complexes of Iridium and Rhodium. Chem. Eur. J. 2011, 17, 10453–10461. 305. Thongpaen, J.; Schmid, T. E.; Toupet, L.; Dorcet, V.; Mauduit, M.; Baslé, O. Directed: Ortho C-H Borylation Catalyzed Using Cp∗ Rh(III)-NHC Complexes. Chem. Commun. 2018, 54, 8202–8205. 306. Thongpaen, J.; Manguin, R.; Dorcet, V.; Vives, T.; Duhayon, C.; Mauduit, M.; Baslé, O. Visible Light Induced Rhodium(I)-Catalyzed C− H Borylation. Angew. Chem. Int. Ed. 2019, 58, 15244–15248. 307. Manguin, R.; Pichon, D.; Tarrieu, R.; Vives, T.; Roisnel, T.; Dorcet, V.; Crévisy, C.; Miqueu, K.; Favereau, L.; Crassous, J.; et al. A Kinetic Resolution Strategy for the Synthesis of Chiral Octahedral NHC-Iridium(III) Catalysts. Chem. Commun. 2019, 55, 6058–6061. 308. Wolf, J.; Labande, A.; Daran, J. C.; Poli, R. Nickel(II), Palladium(II) and Rhodium(I) Complexes of New NHC-Thioether Ligands: Efficient Ketone Hydrosilylation Catalysis by a Cationic Rh Complex. Eur. J. Inorg. Chem. 2007, 5069–5079. 309. Roseblade, S. J.; Ros, A.; Monge, D.; Alcarazo, M.; Álvarez, E.; Lassaletta, J. M.; Fernández, R. Imidazo[1,5-a]Pyridin-3-Ylidene/Thioether Mixed C/S Ligands and Complexes Thereof. Organometallics 2007, 26, 2570–2578. 310. Ros, A.; Alcarazo, M.; Iglesias-Sigüenza, J.; Díez, E.; Álvarez, E.; Fernández, R.; Lassaletta, J. M. Stereoselective Synthesis of Rhodium(I) 4-(Dialkylamino)Triazol-5-Ylidene Complexes. Organometallics 2008, 27, 4555–4564. 311. Ros, A.; Alcarazo, M.; Monge, D.; Álvarez, E.; Fernández, R.; Lassaletta, J. M. Stereoselective Synthesis of Cationic Heterobidentate C(NHC)/SR Rhodium(I) Complexes Using Stereodirecting N, N-Dialkylamino Groups. Tetrahedron Asymmetry 2010, 21, 1557–1562. 312. Rajaraman, A.; Sahoo, A. R.; Hild, F.; Fischmeister, C.; Achard, M.; Bruneau, C. Ruthenium(II) and Iridium(III) Complexes Featuring NHC-Sulfonate Chelate. Dalton Trans. 2015, 44, 17467–17472. 313. Sarbajna, A.; Pandey, P.; Rahaman, S. M. W.; Singh, K.; Tyagi, A.; Dixneuf, P. H.; Bera, J. K. A Triflamide-Tethered N-Heterocyclic Carbene–Rhodium(I) Catalyst for Hydroalkoxylation Reactions: Ligand-Promoted Nucleophilic Activation of Alcohols. ChemCatChem 2017, 9, 1397–1401. 314. Hahn, F. E.; Holtgrewe, C.; Pape, T.; Martin, M.; Sola, E.; Oro, L. A. Iridium Complexes with N-Allyl-Substituted Benzimidazol-2-Ylidene Ligands and Their Application in Catalytic Transfer Hydrogenation. Organometallics 2005, 24, 2203–2209. 315. Zanardi, A.; Peris, E.; Mata, J. A. Alkenyl-Functionalized NHC Iridium-Based Catalysts for Hydrosilylation. New J. Chem. 2008, 32, 120–126. 316. Holmes, J.; Pask, C. M.; Fox, M. A.; Willans, C. E. Tethered N-Heterocyclic Carbene-Carboranes: Unique Ligands That Exhibit Unprecedented and Versatile Coordination Modes at Rhodium. Chem. Commun. 2016, 52, 6443–6446.

8.02

Half-Sandwich Rhodium and Iridium Complexes

Wen-Xi Gao, Peng-Fei Cui, Zheng Cui, and Guo-Xin Jin, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai, PR China © 2022 Elsevier Ltd. All rights reserved.

8.02.1 Introduction 8.02.2 Half-sandwich Rh/Ir complexes with bidentate ligand of N and/or O atoms 8.02.2.1 N^N0 -bidentate ligands 8.02.2.2 N^O-bidentate ligands 8.02.3 Half-sandwich Rh/Ir complexes with phosphorus ligands 8.02.3.1 Phosphite ligands 8.02.3.2 Ligands bearing P and P^P coordination sites 8.02.3.3 P^N bidentate ligands 8.02.3.4 Ligands bearing P and C coordination sites 8.02.3.5 Ligands bearing P, H/P, O and other coordination sites 8.02.4 Half-sandwich Rh/Ir complexes with sulfur ligands 8.02.4.1 S^S0 -bidentate ligands 8.02.4.2 S, N-coordination sites ligands 8.02.4.3 S,C/S,B-coordination sites ligands 8.02.4.4 S, P-coordinates sites ligands 8.02.5 Half-sandwich rhodium-carbon or iridium-carbon bonded complexes 8.02.5.1 Monodentate ligands with C atom as coordination site 8.02.5.2 Bidentate ligands 8.02.5.2.1 C^N five-membered metallacycles 8.02.5.2.2 C^N six-membered metallacycles 8.02.5.2.3 C^N seven-membered metallacycles 8.02.5.2.4 C^C five-membered metallacycles 8.02.5.2.5 C^C six- or seven- membered metallacycles 8.02.5.3 Multidentate ligands 8.02.6 Half-sandwich Rh/Ir complexes bearing hydrogen, borane and metal groups 8.02.6.1 Half-sandwich Rh/Ir hydride complexes 8.02.6.2 Rhodium-metal or iridium-metal bonded complexes 8.02.6.3 Metallaborane complexes based on half-sandwich rhodium and iridium 8.02.7 Half-sandwich Rh/Ir fragments in supramolecular chemistry 8.02.7.1 Supramolecular macrocycles 8.02.7.2 Supramolecular cages 8.02.7.3 Molecular knots 8.02.7.4 Molecular links 8.02.8 Conclusion Acknowledgment References

8.02.1

55 56 56 62 64 64 67 70 73 79 82 82 91 95 102 102 102 104 104 122 124 127 133 135 139 139 144 151 163 163 166 172 175 181 181 181

Introduction

Like the most different areas of organometallic chemistry, fundamental and applied studies on half-sandwich iridium and rhodium complexes have had a tremendous expansion in the decade that separated the preparation of this review from COMC (2007). Taking advantage of their accessibility, robustness, air-stability and water-solubility, half-sandwich iridium and rhodium fragments have been widely used in the construction of versatile organometallic compounds and applied in synthesis, catalysis, or as building blocks in supramolecular chemistry. Recently, they have also been explored as promising anticancer drugs due to their biological behavior and have shown structure-activity relationships, guiding new design concepts. Due to the huge amount of scientific literature dealing with half-sandwich iridium and rhodium complexes and their application in different disciplines, the authors target to highlight a few selected works reflecting general interests offered by half-sandwich iridium and rhodium complexes. The material in this chapter is organized in much the same way as in COMC (2007). It is composed of six sections organized by different ligand types, reflecting the chemical behaviors of half-sandwich iridium or rhodium complexes with different coordination atoms (B, C, N, O, P, S and/or H donor atoms). At the end of this chapter, a separate section contains the discussion on rational design and synthesis of various supramolecular architectures based on half-sandwich iridium and rhodium species.

Comprehensive Organometallic Chemistry IV

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8.02.2

Half-sandwich Rh/Ir complexes with bidentate ligand of N and/or O atoms

8.02.2.1

N^N0 -bidentate ligands

In half-sandwich Cp Rh(III)/Ir(III) complexes, the p-bound negatively charged pentamethylcyclopentadienyl (Cp ) ligand occupies one face of the octahedron (3 coordination sites). The chelating L^L0 ligands can be accommodated in the remainder of the coordination sphere to form a variety of complexes. The chelating L^L0 ligand provides additional stability for the complex and contributes to tuning the electronic properties of the rhodium and iridium center. The monodentate ligand X, such as chloride, can provide a labile site for substitution reactions with target sites. The general structure of these pseudo-octahedral complexes is shown in Fig. 1. A bipyridine-based functional ligand, such as 6-hydroxy-2,20 -bipyridine or 6,60 -dihydroxy-2,20 -bipyridine that should lead to stable catalysts ligated by N, N0 -chelation. Treatment of Cp Ir precursor, water-soluble aqua complex [Cp Ir(H2O)3](OTf )2 with 6-hydroxy-2,20 -bipyridine or 6,60 -dihydroxy-2,20 -bipyridine in water at room temperature gave dicationic Cp Ir complexes 1 in high yield.21–23 The analogue compounds 2–6 involving half-sandwich Ir(III) fragments have been demonstrated to be used in various catalytic systems as shown in Fig. 2.24–35 Through the screening of the substituents (-CO2, Me, OMe, or O−) on 2, 20 -bipyridine ligands at different positions,36–42 these ligands show promise for supporting catalysts and allowing acid/base-sensitive properties. The oxyanion (O−) which have the highest electron-donating substituents among them was found to be the most effective substituent in CO2 hydrogenation under basic conditions in water. The oxy-anion substituent was generated by the deprotonation of the OH group under basic conditions and highly electron push derived from the resonance through pyridine ring and O−. With these compounds used as catalysts, a variety of primary and secondary alcohols have been efficiently converted to aldehydes and ketones, respectively, in aqueous media without using any oxidant.43,44 In addition, dihydroxybipyridine complexes of iridium are highly active for homogeneous catalytic water oxidation,33 and these complexes function best in relatively mild conditions including near-neutral pH with less oxidizing reagents. It is believed that OH groups of dihydroxybipyridine are doubly deprotonated at near-neutral-pH values, causing increased electron-donating character and heightened catalytic activity.

Fig. 1 General structure of half-sandwich Cp Rh(III)/Ir(III) complexes.

Fig. 2 Complexes 1–6 based on functional bipyridine ligands.

Half-Sandwich Rhodium and Iridium Complexes

57

The half-sandwich iridium complex 3 was obtained by reacting the dimer [(Cp Ir(m-Cl)Cl]2 with 4-methyl-40 -carboxy-2,2-bipyridine, and subsequent anion exchange to a PF−6 salt. The carboxylic acid group in 3 was then reacted with the amine group of dibenzocyclooctyne-amine (DBCO-NH2) to afford the clickable analogue so that the strain-promoted copper free azide-alkyne cycloaddition strategy could been utilized to conjugate iridium complex to more functional groups.45 Using Cp Ir complex 6 with bipyridonate ligand as catalyst, an efficient catalytic system for the dehydrogenative oxidation of primary and secondary alcohols was developed. It could be carried out under mild conditions (reflux in pentane for secondary alcohols and reflux in tBuOH for primary alcohols). Furthermore, the reversible transformation between 2-propanol and acetone by catalytic dehydrogenation and hydrogenation as a prototype for hydrogen storage was also described.46 Triphenylamine groups were introduced to the bipyridine chelating ligand through a single bond and coordinated with Ir centers to obtain half-sandwich Ir(III) complexes 7–9 as shown in Fig. 3, which was revealed to effectively improve the antitumor activity of these complexes.47,48 The pyridyl pyrimidine derived heterocyclic molecules synthesized as neutral ligand to coordinate to Ir (III) and Rh (III) centers with structural features analogs to half sandwich compounds.49 To a solution of the binuclear dichloro complex [Cp M (m-Cl)]2Cl2 (M ¼ Rh/Ir) in CH2Cl2, an excess of the pyrimidine-2-amine ligands was added. The resulting mixture was stirred at room temperature and recrystallized by methanol and diethyl ether affording complex 10 (Fig. 4). The bidentate ligands coordinate to a metal ion with pyrimidine and pyridine nitrogen donors and occupy piano stool geometry.50,51 Complexes [Cp Ir(phen)(H2O)]2+ (phen ¼ 1,10-phenanthroline) was synthesized as PF−6 salts, which could be carried out in catalytic reductions of quinones without an enzyme through hydride transfer from nicotinamide adenine dinucleotide(NADH). On account of the easy modification of the phenanthroline chelating ligands, a variety of functional groups have been introduced into half-sandwich Ir(III) compounds (11) as shown in Fig. 5.52–56 The compounds of the type [Cp IrCl(pp)](CF3SO3) (12–14) (pp ¼ dpq, dppz, dppn) were prepared by refluxing the solvent complex [Cp IrCl(acetone)2](CF3SO3) with the appropriate polypyridyl ligand (pp) in CH3OH/CH2Cl2. [Cp IrCl(acetone)2]+ can be obtained in situ by addition of two equivalents of Ag(CF3SO3) to a solution of the dimeric starting compound [Cp Ir (m-Cl)]2Cl2 in acetone as shown in Fig. 6.57–60 A series of Cp Ir(III) complexes 15–18 coordinated by five-membered heterocycles such as imidazole or imidazoline was synthesized as shown in Fig. 7. It was presumed that the imidazole-type ligand had a high electron-donating effect and was more efficient than the pyridine ligands. 0

Fig. 3 Complexes 7–9 based on bipyridine ligands with triphenylamine groups.

Fig. 4 Complex 10 based on pyridyl pyrimidine ligand.

Fig. 5 Complexes 11 based on 1,10-phenanthroline derivatives.

Fig. 6 Complexes 12–14 based on polypyridyl ligands.

Fig. 7 Complexes 15–18 based on five-membered heterocycles ligands.

Half-Sandwich Rhodium and Iridium Complexes

59

As a result of CO2 hydrogenation, complex 15 showed higher catalytic activity than analogue bearing bipyridine.61 In addition, [Cp Ir(bimH2)Cl]Cl (bimH2 ¼ 2,20 -bisimidazole) (15) and [Cp IrCl(m2-k2-k1-bimH)IrCl2Cp ] (16) were found to be effective catalysts for water oxidation to molecular oxygen driven by cerium ammonium nitrate.62 The high catalytic activity was observed for complexes bearing the strong electron-donating bisimidazole ligands, which is likely due to an enhanced oxidation propensity of the catalyst induced by the ligand. The latter may be beneficial not only for the generation of species with metal in the highest oxidation states but also for the oxidative transformation of the catalyst leading to the real active species. The pyridyl-imidazoline ligand and pyrazole moiety of the imidazole isomer were also disclosed to be effective ligand bones (18).63–65 When the Cp Ir(III) precursor is complexed with an unsymmetrical bidentate ligands, a chiral iridium complex can also be constructed due to the difference in the coordinating groups. For example, the reaction of [Cp IrCl2]2 with the 2-(20 -pyridyl) imidazole and 2-(20 -pyridyl)benzimidazole ligands resulted in the formation of racemic, cationic complexes 19 and 20 in the presence of NH4PF6.66 Similar compound 21 with methyl 1-butyl-2-pyridyl-benzimidazole carboxylate ligand have also been reported.67 The iridium(III) complexes 22 of substituted 2-(4,5-dihydro-1H-imidazol-2-yl)pyridines could serve as highly efficient catalysts for dehydrogenation of formic acid, transfer hydrogenation of aldehydes, and aerobic oxidation of aldehydes.68–70 Treatment of [Cp IrCl2]2 with 2,3-di(2-pyridyl)pyrazine (dpp) or 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz) afforded the complexes 23 and 24 as shown in Fig. 8.71 Similarly, complex 25 was prepared in the presence of NaBF4 in refluxing methanol.72

Fig. 8 The structures of complexes 19–32.

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Half-Sandwich Rhodium and Iridium Complexes

The iridium catalyst, [Cp Ir(2,20 -bi-1,4,5,6-tetrahydropyrimidine)Cl]Cl (26), was used to catalyze the direct hydrogenation of CO2 to formic acid in water in the absence of a base.73 Reactions of the binuclear complexes [Cp Ir(m-Cl)]2Cl2 with 1-(pyridin-2-yl)-7-azaindole or 2-substituted-1,8-naphthyridine ligands, lead to the formation of the mononuclear cationic complexes 27–30.74,75 The complexes [Cp Ir(pyridyltriazol)Cl]PF6 (31) and [Cp Ir(bis-triazole)Cl]PF6 (32) were synthesized from the reactions of the [Cp Ir(m-Cl)]2Cl2 precursors with the corresponding nitrogen-donating ligands in methanol and subsequent salt metathesis with Bu4NPF6.76 The pyridyltriazol and bis-triazole ligands were chosen with the same substituent, 2,6-diisopropylphenyl on the triazole rings. Schiff bases (dRC]Nd), obtained by the condensation of amines (hydrazones, thiosemicarbazones, shrinkage amines, amino acids, and heterocyclic amines) and aldehydes/ketones, possess good coordination ability with Cp Ir(III) centers. In view of the promising applications of coumarin derivatives in the field of antitumor drugs, Schiff base chelating ligands were obtained by condensation with salicylaldehyde and further reacted with an iridium(III) dimer to construct half-sandwich iridium(III) Schiff base compounds 33–35.77–79 These compounds showed favorable stability in the biological environment and laid a foundation for the biological performance test.80 Using enantiopure imines, the reaction with [Cp Ir(m-Cl)]2Cl2 in the presence of NaSbF6 afforded the Cp Ir(III) complexes 36–45 as shown in Fig. 9.81,82

Fig. 9 Complexes 33–45 based on Schiff bases ligands.

Half-Sandwich Rhodium and Iridium Complexes

61

The Cp Ir(III) complexes 46 containing (N,N)-bound picolinamide ligands have been prepared for use as anticancer agents, which were prepared according to two different methods as shown in Scheme 1.83–86 Also they were demonstrated to be capable of mediating catalytic hydride transfer from either NADH or formate to aldehydes in phosphate-buffered saline (PBS) buffer and cell culture media. The iridium catalysts are tolerant of up to moderate concentrations of biological nucleophiles, including thiols such as glutathione and cysteine.87–90 Catalyst 47 with picolinamide derivative substituted of the OH group on 4-position of pyridine moiety exhibited high activity in CO2 hydrogenation under pressurized conditions.

Scheme 1 Synthesis of complexes 46 and the structure of complexes 47–49.

A series of efficient chiral aqua iridium(III) complexes 50 for asymmetric transfer hydrogenation were prepared by mixing [Cp Ir(H2O)3]SO4 with a collection of chiral diamines in water at room temperature (Scheme 2).91,92 This catalytic system has shown high reactivity, leading to satisfactory enantioselectivities (up to 99% ee) for various aromatic a-cyano and a-nitro ketones. Also they provide the ortho-substituted aromatic alcohols and the diamines can be used as the chiral ligands without conversion to the corresponding monosulfonylated diamines.

Scheme 2 Synthesis of chiral aqua iridium(III) complexes 50 and the structure of different ligands.

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Half-Sandwich Rhodium and Iridium Complexes

Half-sandwich iridium complexes containing monotosylated diamine ligands is another group of important iridium complex. A library of half-sandwich Cp Ir(III) chloride complexes (51) were obtained from pyridinesulfonamide ligands containing an ethylene linker with different functional groups. These Cp Ir(pyridinesulfonamide)Cl complexes could serve as precatalysts for the transfer hydrogenation of electron-rich and -poor acetophenone derivatives.93–95 The complexes 52–54 were generated by reacting the appropriate achiral tosyl diamines ligand with [Cp IrCl2]2 and base in an organic solvent, or in water without base.96–100 The tosyl diamines ligand is demonstrated to be crucial for efficient transfer hydrogenation. Using the Cp Ir(III) complexes 55 and 56 containing a N-triflyl-1,2-diphenylethylenediamine (TfDPEN) ligand shown in Fig. 10,101 catalytic hydrogen evolution was achieved.102 Moreover, they were explored in a mild catalytic asymmetric transfer hydrogenation of b, b-disubstituted nitroalkenes.103 Formic acid is used as a reductant in combination with an Ir catalyst. The reaction is conducted in water at low pH and open to air to give adducts in preparatively useful yield and selectivity. Such half-sandwich iridium complexes have also shown promise in metallo-enzymatic catalysis. Docking of the biotinylated, racemic Cp Ir-diamine complexes 57 into a variety of protein and DNA hosts afford artificial metalloenzymes, which catalyze enantioselective transfer hydrogenation of ketones.104–107

8.02.2.2

N^O-bidentate ligands

The reaction of [Cp Ir(m-Cl)]2Cl2 with sodium pyridine-2-carboxylate in methanol gives the orange crystalline compounds chloro Cp (Z2-pyridine-2-carboxylato)iridium(III) (58) in 89% yield as shown in Fig. 11, where the sodium salts of anionic bidentate ligands can act as both incoming ligand and metathesis reagent.108,109 Iridium complexes 59 containing hydroxy–pyridine– carboxylate ligands are active water oxidation catalysts with performances tunable by a rational selection of the nature and position of the pyridine-substituent and pH value.110–112 The structurally similar iridium carboxylato pyrazine complex 60 was prepared from [Cp Ir(m-Cl)]2Cl2 and pyrazine-2-carboxylic acid in the presence of sodium methoxide. The Cp Ir complex 61 bearing a 2-hydroxypyridine ligand has been designed, by which various secondary alcohols can be dehydrogenatively oxidized to ketones under neutral conditions with high turnover numbers.113 The complexes 64 were prepared from [Cp Ir(m-Cl)]2Cl2 by treatment with 2-(20 -pyridyl)-2-propanol and sodium bicarbonate in refluxing acetone, which are demonstrated to be active water oxidation catalysts.114,115

Fig. 10 The structures of complexes 51–57.

Half-Sandwich Rhodium and Iridium Complexes

63

Fig. 11 Complexes 58–64 based on N^O-bidentate ligands.

Fig. 12 Complexes 65 with aminoacidato ligands.

Multinuclear iridium complexes are generated when multi-dentate ligands are employed. The reaction of [Cp Ir(m-Cl)]2Cl2 with pyrazine-2,5-dicarboxylic acid or pyrazine-2,3-dicarboxylic acid afforded the binuclear complexes 62 and 63.116 Cp Ir(III) half-sandwich complexes 65 (Fig. 12) with aminoacidato ligands are demonstrated to be highly active and selective catalysts for the alkylation of amines with alcohols.117 Cp Ir(III) complexes 66–70 (Fig. 13) containing other N^O-, O^O-, or N^N-chelating ligands were reported to differ in their anticancer activity.118–122

64

Half-Sandwich Rhodium and Iridium Complexes

Fig. 13 Structures of complexes 66–70.

8.02.3

Half-sandwich Rh/Ir complexes with phosphorus ligands

8.02.3.1

Phosphite ligands

Half-sandwich phosphite complexes of iridium Cp IrCl2[P(OR)3] (71, 72) were prepared by reacting the dimeric species [Cp IrCl2]2 with phosphites in refluxing methanol or ethanol, as shown in Scheme 3.123 Treatment of chloro complex 71 and 72 with an excess of hydrazine R1NHNH2 in the presence of NaBPh4 yielded hydrazine complexes [Cp IrCl (R1NHNH2){P(OR)3}] BPh4 (73–75). When treatment of Cp IrCl2 [P(OR)3] with two equivalents of AgOTf, followed after filtration by the reaction with an excess of hydrazine (Scheme 4), substitution of the triflate ligand by hydrazine afforded bis(hydrazine) complexes [Cp Ir(R1NHNH2)2{P(OR)3}](BPh4)2 (76,77), which were isolated as BPh4 salts and characterized.

Scheme 3 Synthesis and reactivities of half-sandwich phosphite complexes of iridium Cp IrCl2[P(OR)3].

Scheme 4 Substitution reaction of half-sandwich phosphite complexes of iridium Cp IrCl2[P(OR)3].

Half-Sandwich Rhodium and Iridium Complexes

65

Fig. 14 Structures of the metalated azine complexes of half-sandwich iridium containing the phosphite ligand.

Albertin and coworkers developed the metalated azine complexes of half-sandwich iridium containing the phosphite ligand. Half-sandwich complexes Cp IrCl2[P(OR)3] react first with one equivalent of AgOTf and then with both aldazines (R’C6H4)(H)C] NdN]C(H)(R’C6H4) and the ketazine (CH3)2dC]NdN]C(CH3)2 to give k1-azine complexes (78–86; M ¼ Ir and Rh) which were isolated as BPh4 salts and characterized (Fig. 14).124,125 Carmona and coworkers also developed the chiral phosphramidites (Fig. 15) as coordination ligand to react with the half-sandwich metal (Ir and Rh) complexes (Scheme 5). They anticipated that these complexes and the analogs containing related phosphoramidites ligands are well-suited candidates to behave as LAB catalysts.124 As shown in Scheme 5, treatment of chiral phosphramidites complex 87 with the hydroxymethylpyridine (NOH) ligand in the presence of AgSbF6 yielded the complexes [Cp M(NOH){(R)-P1}][SbF6]2 (88).124,126

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Half-Sandwich Rhodium and Iridium Complexes

Fig. 15 Structures of chiral phosphramidites ligand.

Scheme 5 Reactivity of chiral phosphramidites ligand with half-sandwich metal (Ir and Rh) complexes.

Mezzetti and coworkers used the phosphoramidite ligand P5 to react with the [Cp MCl2]2 in CDCl3 at room temperature to give [Cp MCl2(P5)] (89 (M ¼ Rh), 90 (M ¼ Ir)) (Scheme 6). The neutral complex 89 reacts with trimethylsilyl triflate (CF3SO3SiMe3) in CH2Cl2 to give the cationic derivative (91) (Scheme 4). The complex 92 was freshly prepared in CD2Cl2 solution by treating complex 90 with AgSbF6 (Scheme 6). The complex 91, 92 are mononuclear, six-coordinate complex in which the coordination sphere of the 16-electron fragment is saturated by means of an interaction between the rhodium (III) or iridium (III) atom and the phenyl ring of one of the two CH(Me)Ph substituents at nitrogen.127

Half-Sandwich Rhodium and Iridium Complexes

67

Scheme 6 The reactivity of [Cp MCl2(P5)] (89 (M ¼ Rh), 90 (M ¼ Ir)) complexes.

An Arbuzov-type reaction with the bis(alkynyl) iridium complex [Cp Ir(L)(C^CAr)2] (L ¼ P(OMe3)) was reported by Leong group. They use the iridium complex [Cp Ir(L)(C^CAr)2] to react with the phosphites to give four-membered iridacyclic phosphonates [Cp Ir(L){C(Ar)]C[P(]O)(OR)2]C]CHAr}]. It can also isomerize to a five-membered iridafuran in the presence of a strong acid (Scheme 7).128

Scheme 7 An Arbuzov-type reaction with the bis(alkynyl) half-sandwich metal complexes.

8.02.3.2

Ligands bearing P and P^P coordination sites

The coordination capabilities of the P atom with half-sandwich iridium complex have also been studied by organophosphorous ligands. The complexes 96, 97 and 98 show that the organophosphorous ligand can act as the monodentate ligand to coordinate with iridium (Fig. 16).129,130

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Half-Sandwich Rhodium and Iridium Complexes

Fig. 16 Structures of half-sandwich iridium complexes containing organophosphorous ligands.

Helm and coworkers developed the unique P,P0 -diphenyl-1,4-diphospha-cyclohexane ligand to react with the [Cp MCl2]2 (M ¼ Ir, Rh) and form the dinuclear complex 99 (Scheme 8A). The synthesis of complex 99 result in formation of two distinct isomers arising from the configuration of the 6-membered ring of the phosphorus ligand, the major (cis) isomer and minor (trans) isomer (99a and 99b, respectively).131 The complex 99 can convert to cationic mono-metric diphosphine complex [Cp Rh(Cl) (dpdpc)][PF6] (100), as shown in Scheme 8B.131

(A)

(B)

Scheme 8 Structures and reactivity of half-sandwich metal complexes containing P,P0 -diphenyl-1,4-diphospha-cyclohexane ligands.

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69

Fig. 17 Structures of Iridium and rhodium “PNP” aminodiphosphine complexes.

Iridium and rhodium “PNP” aminodiphosphine complexes are also a classical model of double phosphorus coordination. And these complexes are used extensively in ethylene oligomerization studies. The general structures of these complexes are shown in Fig. 17.132,133 The reaction of the iridium complex 102 with AgBF4 in CH2Cl2 provided a mixture of complexes [Cp Ir(Maxphos)][BF4] (103a and 103b) in which the cyclometallation of the diphosphane ligand occurs through one of the methyl group of one of the tert-butyl substituents (Scheme 9). The irradiation of the pro-S proton of the metalated methylene of the major isomer enhances the resonance of tBu, but not that of the Me, bonded to the P atom. This suggest that the absolute configuration of the major (103a) and minor (103b) isomers is SIr,RP1,SP2 and RIr,RP1,RP2, respectively.134

Scheme 9 The reactivity of the iridium “PNP” aminodiphosphine complex 102 with AgBF4.

Liu and coworkers studied the half-sandwich iridium complexes containing P ^P ligands exhibit very promising anticancer properties.135,136 The half-sandwich iridium complexes 104a, 104b were synthesized by reacting the ligands 1,2-bis (diphenylphosphino)benzene (dppbz) and 1,8-bis(diphenylphosphino)naphthalene (dppn) and the dinuclear iridium precursors in methanol at ambient temperature, which was isolated as PF−6 salts, as shown in Scheme 10. The reaction of CabPP (Diphosphineo-carborane ligand) with the half-sandwich [Cp MCl2]2 (M ¼ Ir, Rh) in CH3OH solution led to the yellow and orange crystalline zwitterionic complex [Cp M(Cl)(7,8-(PPh2)2–7,8- C2B9H10)] 105a (M ¼ Ir, Rh). The metal center is coordinated by the two P atoms of the nido ligand and a folded five-membered IrP2C2 ring is formed. Refluxing the nido complex with Et3N in THF led to new products 105b. Exposuring solutions of 105b to a H2 atmosphere (1 atm) at room temperature afforded Cp M(H)(7,8(PPh2)2-7,8-C2B9H10) 105c (Scheme 10B).137,138

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Half-Sandwich Rhodium and Iridium Complexes

(A)

(B)

Scheme 10 (A) Structures of half-sandwich iridium complexes containing P ^P ligands. (B) The reactivity of CabPP (Diphosphine-o-carborane ligand) with the half-sandwich [Cp MCl2]2 (M ¼ Ir, Rh) in CH3OH solution.

8.02.3.3

P^N bidentate ligands

The half-sandwich iridium complexes can also adapt in the three-leg piano-stool featuring five-membered P∧ N chelates. Treatment of the phosphinepyridonate ligand139 with [Cp IrCl2]2 in methanol solution resulted in sensitive species 106 and 107 in dynamic equilibrium (Scheme 11). It is noteworthy that, in these cases, a molecule of methanol was observed in a 1:1 ratio of MeOH/ complex, demonstrating hydrogen-bonding interactions. However, the complex 108 was synthesized through treatment of the phosphinepyridonate ligand with potassium tert-butoxide and [Cp IrCl2]2 in methanol solution (Scheme 11). And the complex 108 adopted in neutral piano-stool conformation.139

Scheme 11 Reactivities of the phosphinepyridonate ligand with [Cp IrCl2]2 in methanol solution.

Half-Sandwich Rhodium and Iridium Complexes

71

Lysosome-targeted phosphine-imine half-sandwich iridium anticancer complexes were synthesized by Liu and coworkers. They use the P ^N ligands below shown in Scheme 12 to react with the [Cp IrCl2]2 in CH2Cl2 solution to form the complexes 109–114. These complexes were successfully prepared in moderate yields as their PF−6 salts.140

Scheme 12 Synthesis of Lysosome-targeted phosphine-imine half-sandwich iridium anticancer complexes.

The diphenylphosphine-containing complex [Cp Ir(P-NH2)Cl]PF6 (115; P-NH2 ¼ 2-(diphenylphosphino)-benzylamine) shown in Scheme 12 was also prepared by the reaction of the P-NH2 ligand and the [Cp IrCl2]2, followed by the by addition of AgPF6 to the reaction mixture (Fig. 18).141

Fig. 18 Structure of diphenylphosphine-containing complex [Cp Ir(PdNH2)Cl]PF6 (115).

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Half-Sandwich Rhodium and Iridium Complexes

Four-membered metallacycle 116 with P,N ligand can also achieved by deoxygenation of Ph2PyP ¼ O with emphasis on the reductive cleavage of P]O bonds (Scheme 13), while the relatively weaker P]S and P]Se bonds are stable under similar reaction environment.142

Scheme 13 Deoxygenation of Ph2PyP]O bond.

New heterodonor NPPN tetradentate ligands, 2-PyCH2(Ph)P-(CH2)n-P(Ph)CH2-2-Py (meso- and rac-Ln; n ¼ 2–4, Py ¼ pyridyl) were prepared and reacted with [Cp MCl2]2 (M ¼ Ir, Rh) in the presence of NH4BF4 to afford a series of dinuclear complexes 117–122 with different configurations (Fig. 19).143

Fig. 19 Structures of half-sandwich metal complexes containing heterodonor NPPN tetradentate ligands.

Half-Sandwich Rhodium and Iridium Complexes

73

The racemic Schiff base from 2-(diphenylphosphino) benzaldehyde and a-amino acid esters react as many other N and P donors with the [Cp MCl2]2 (M ¼ Ir or Rh) under the cleavage of the Cl bridges to give the complexes 122–131. And there are some differences between Iridium and rhodium center to coordinate with the negative N atom (Scheme 14). The neutral complexes 122–126, 130 and 131 are converted into the cationic complexes 132–138 by treatment with NH4PF6. Abstraction of all chloride ligands in 122, 124–127, 131 and 137 using AgBF4 affords the dicationic complexes 139–145, whereby in 139–143 coordination of the ester group takes place whereas in the C-allylglycinecontaining complexes 144 and 145 the C]C double bond is coordinated to the metal atoms (Scheme 14).144

Scheme 14 Reaction of 2-(diphenylphosphino) benzaldehyde and a-amino acid esters with the [Cp MCl2]2 (M ¼ Ir or Rh).

8.02.3.4

Ligands bearing P and C coordination sites

The CdH bond activation can also take place by half-sandwich iridium or rhodium complex chelating PPh2Me or PMe3 ligands. Treatment of [Cp IrCl(NCMe)L]PF6 (L ¼ PPh2Me, PMe3) with 2-methyl-3-butyn-2-ol in methanol at room temperature gives the (methoxy)- alkenylcarbeneiridium complexes [Cp IrCl{]C(OMe)CH]CMe2}L]PF6 (L ¼ PPh2Me, PMe3) (146) in high yields. The addition of AgPF6 to a solution of 146 in CH2Cl2 gives the cyclic carbine complexes 147, through an intramolecular CdH activation of one of the methy groups of alkenyl fragments (Scheme 15). When the iridacyclopenta-1,3-diene complexes 147 are treated with KOtBu, the methylene group is deprotonated, giving the new iridacyclopenta-2,4-dine complex 148 (Scheme 15). This type of metallacyclopentadiene complex is interesting, due to its implication in the cyclooligomerization of alkynes.145–147

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 15 Synthesis and reactivity of [Cp IrCl{]C(OMe)CH]CMe2}L]PF6 (L ¼ PPh2Me, PMe3) (146) complex.

Reaction of [Cp MCl2]2 (M ¼ Ir and Rh) with (1-naphthyl) diphenyl phosphine in the presence of sodium acetate at room temperature result in corresponding five-membered cyclometalated complexes 149a (M ¼ Ir and Rh) in good yields.148 Interestingly, the complex 149a go through an intramolecular C(sp2)dH bond activation. Additionally, the reaction also affords small amount of complex 149b when treatment with [Cp IrCl2]2, a normal phosphine-substituted iridium dichloride (Scheme 16A). Monophosphine-o-carborane CabPH (1-PPh2-1,2-C2B10H11) is of particular interest as a precursor because of its electronic and

(A)

(B)

Scheme 16 (A) Reaction of [Cp MCl2]2 (M ¼ Ir, Rh) with (1-naphthyl) diphenyl phosphine. (B) The reactions of CabPH with [Cp MCl2]2 (M ¼ Ir, Rh).

steric effects that stabilize the metal center.148 The reactions of CabPH with [Cp MCl2]2 (M ¼ Ir, Rh) in the presence of Et3N give [Cp M(Cl)2(1-PPh2-1,2-C2B10H11)] 150a in almost quantitative yields (Scheme 16B). The influence of the solvent is obvious because the metallacarboranes 150b 1-PPh2–3-Cp -3,1,2-MC2B9H10 were only obtained in CH3OH (Scheme 16B).149,150 Insertion of alkynes into M-C(sp2) bonds of cyclometalated complex 149a are also studied. The reaction of 149a with dimethyl acetylenedicarboxylate (DMAD) were tried. It underwent the normal 1,2-insertion and afforded corresponding seven-membered cyclometalated complexes 151 (Scheme 17).148

Half-Sandwich Rhodium and Iridium Complexes

75

Scheme 17 Insertion of alkynes into M-C(sp2) bonds of cyclometalated complex 149.

However, complex 149 with different metal centers showed different reactivity and insertion modes when reacted with diphenylacetylene and phenylacetylene. Reaction of 149 with diphenylacetylene and phenylacetylene in methanol at room temperature resulted in five- and six-membered doubly cyclometalated complexes 152 (Scheme 18). While reactions of 149 of rhodium metal center with diphenylacetylene and phenylacetylene in methanol produced corresponding seven-membered cyclometalated complexes by means of normal 1,2-insertion mode.148

Scheme 18 Reaction of 149 with diphenylacetylene and phenylacetylene in methanol solution.

In order to explore applicable scope of this type of cyclometalation, the ligand was extended to phosphinite. Therefore, reactions of [Cp MCl2]2 (M ¼ Ir and Rh) with (1-naphthyl)-diisopropylphosphinite were examined, which produced the corresponding five-membered cyclometalated complexes 153 (Scheme 19). Theoretically, another C(sp2)dH bond at an adjacent a position of naphthyl ring could also be activated; however, no corresponding six-membered cyclometalated complex was detected. It is interesting to note that the reactions also afforded methyl diisopropylphosphinite substituted iridium and rhodium dichloride 154 (Scheme 19).148

Scheme 19 Reaction of [Cp MCl2]2 (M ¼ Ir and Rh) with (1-naphthyl)-diisopropylphosphinite ligand.

A series of 2,4,6-triarylphosphinines were prepared and investigated in the base-assisted cyclometallation reaction using [Cp IrCl2]2 as the metal precursor (Fig. 20). The cyclometallation reaction turned out to be very sensitive to steric effects as even small substituents can have a large effect on the regioselectivity of the reaction.151

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Half-Sandwich Rhodium and Iridium Complexes

Fig. 20 Structures of half-sandwich iridium complexes containing 2,4,6-triarylphosphinines.

Inspired by the C(sp2)dH bond activation through phosphine ligand, Zhu’s group decided to test whether it could also promote activation of a C(sp3)dH bond. Reactions of [Cp MCl2]2 (M ¼ Ir and Rh) with diisopropyl (o-methylphenyl) phosphine in the presence of sodium acetate were carried out. The corresponding five-membered cyclometalated complex 160 (M ¼ Ir and Rh) were synthesized.148 It is note that complex 160 is not quite air stable, which undergoes further oxidation to give complex 161. (Scheme 20).

Scheme 20 Reactions of [Cp MCl2]2 (M ¼ Ir and Rh) with diisopropyl (o-methylphenyl) phosphine ligand.

The similar reaction also take place in the reaction of phosphine PMe(Xyl)2 (Xyl ¼ 2,6-C6H3Me2) ligand and half-sandwich complex [Cp MCl2]2 (M ¼ Ir and Rh). Reaction of [Cp MCl2]2 (M ¼ Ir and Rh) with one equivalent of PMe(Xyl)2 in the presence of the weakly coordinating base 2,2,6,6-tetramethylpiperidine (TMPP) yields the cyclometalated complex 162 (Scheme 21).152,153

Scheme 21 Reaction of [Cp MCl2]2 (M ¼ Ir and Rh) with PMe(Xyl)2 ligand.

Half-Sandwich Rhodium and Iridium Complexes

77

Metathesis reactions of complex 162 of iridium metal center with LiBr, MgI2 or NH4SCN afford corresponding complex 163, 164 and 165 (Scheme 22). The complex 162 is also a good starting material for the synthesis of complexes that contain IrdH and IrdC bonds. Iridium hydride complex 166 is prepared by reaction of complex 162 of iridium metal center and LiAlH4 in THF solution at 45  C for 2 h (Scheme 22). The methylderivative 167 is prepared by reaction of 162 with ZnMe2 or Mg(Me)Br (Scheme 22). Alkylation of 162 can be readily extended to other alkyl groups. This is shown in Scheme 22 for the synthesis of the IrdCH2SiMe3 complex 168.153,154

Scheme 22 The reactivities of cyclometalated complex 162.

It is worth mentioning that the stronger methylating reagent, LiMe, also yields 167 but accompanied by product with three IrdC bonds 169, as a consequence of metallation of the P-bound methyl group (Scheme 23).154

Scheme 23 The reaction of 167 with stronger methylating reagent.

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Half-Sandwich Rhodium and Iridium Complexes

Reaction of the neutral complexes 162 with 1 equiv. of NaBArF4 (BArF4 ¼ [B(3,5-C6H3(CF3)2)4]), in CH2Cl2 as solvent, led to complexes 170 isolated as the BArF4 salt (Scheme 24).155,156

Scheme 24 Reaction of the neutral complex 162 with 1 equiv. of NaBArF4.

Exposure of a CH2Cl2 solution of 170 to 1 bar of H2 at 20  C allowed the formation of the cationic bis(hydride) complex 171. Interestingly, formation of complex 171 was found to be reversible, and removal of H2 under vacuum regenerated cleanly the complex 170 (Scheme 25).155

Scheme 25 Reaction of the neutral complex 170 with H2.

When complex 170 was treated with a saturated aqueous solution of piperidinium/piperidine in catalytic amounts in CH2Cl2 solution, the complex 172 was synthesized and the complex 171 was observed as a side product of this rearrangement (Scheme 26).155

Scheme 26 Reaction of the neutral complex 170 with a saturated aqueous solution of piperidinium/piperidine in catalytic amounts in CH2Cl2 solution.

Rheingold and coworkers have developed a new family of metal perfluoroalkyl fluoro complexes by treatment of the iodo precursors Cp Ir(PMe3)(RF) with AgF in CH2Cl2 solution in the dark to give Cp Ir(PMe3)(RF)F(RF ¼ CF3, CH2CF3, CF2CF2CF3, CF(CF3)2, CF(CF3)(CF2CF3)) (Fig. 21).156,157 The transition-metal-catalyzed coupling reaction to synthesis of organometallic complexes has also been developed to the synthesis of the IrdC bond, complexes of which contains the PMe3 ligand.156

Half-Sandwich Rhodium and Iridium Complexes

79

Fig. 21 Structures of half-sandwich iridium complex containing perfluoroalkyl fluoro ligands.

The complexes [Cp M(Z2-C2H4)(PPh3)] react with iodoperfluoroalkanes to give [Cp M (CH2CH2RF)I(PPh3)] (M ¼ Ir, RF ¼ t-C4F9 (179), i-C3F7 (180); M ¼ Rh, RF ¼ t-C4F9 (181)) (Scheme 27A). The similar reaction can also take place in [Cp Rh (Z2-C2H4) (PMe3)] to form the complex [Cp Rh (n-C4F9)I(PMe3)] (182) (Scheme 27B). The reaction of these complexes with AgOTf in CH2Cl2 gave AgI and the complexes [Cp IrH(Z2-CH2CHRF)(PPh3)]OTf (RF ¼ c-C6F11, i-C3F7) (183) (Scheme 27C).158

Scheme 27 (A) Reaction of the complexes [Cp M(Z2-C2H4)(PPh3)] (M ¼ Ir, Rh) with iodoperfluoroalkanes. (B) Reaction of [Cp Rh (Z2-C2H4) (PMe3)] with (n-C4F9) (CH2)2I ligand. (C) The reaction of these complexes with AgOTf to give AgI and the [Cp IrH(Z2-CH2CHRF)(PPh3)]OTf complexes.

8.02.3.5

Ligands bearing P, H/P, O and other coordination sites

The Bolaño group synthesized and characterized the new pentamethylcyclopentadienyl iridium hydride complexes.159 Protonation of the dihydride [Cp Ir(H)2(PPh2Me)] 177 with HBF4 at low temperature gave the classical trihydride complex [Cp Ir(H)3 (PPh2Me)]BF4 178 that displays quantum mechanical exchange coupling (Scheme 28). And the reaction is revisable when treatment with NEt3.

(A)

(B)

(C)

Scheme 28 Acid-base response of pentamethylcyclopentadienyl iridium hydride complex.

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Half-Sandwich Rhodium and Iridium Complexes

P,O-chelating ligands with the half-sandwich iridium and rhodium complexes are also well known due to their unique characteristics. Liu and coworkers developed the fluorescent half-sandwich phosphine-sulfonate iridium complexes as potential lysosome-targeted anticancer agents.160 The complexes were prepared from the reaction of the appropriate [Cp IrCl2]2 dimer with the corresponding phosphine-sulfonate ligand and Na2CO3 in CH2Cl2 (Scheme 29).160

Scheme 29 Reaction of [Cp IrCl2]2 dimer with the phosphine-sulfonate ligand.

Sulfonamidophosphine ligands have been demonstrated to combine proton responsive character at nitrogen with hydrogen bonding properties upon coordination to transition metals.161 Mixing the sulfonamidophosphine ligand with [Cp IrCl2]2 in CH2Cl2 instantly led to complete consumption of starting materials and formation of a single species 185 (Scheme 30). Addition of 185 to a suspension of sodium acetate in CH2Cl2 led to a new species 186. Then the ligand assisted intermolecular activation of alkynes was studied and the complex 187 was formed. The Irdphenylacetylide species 188 was generated via proton transfer from the terminal alkyne to the proton responsive ligand (Scheme 30).162

Scheme 30 The synthesis and reactivities of single species 185.

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81

Mixing 1 equiv. of the ligand (Ph2PNHS(O)2NHPPh2) and 1 equiv. of iridium precursor [Cp IrCl2]2 instantly led to the formation of a single species 189 (Scheme 31). When complex 189 was stirred at room temperature in CH2Cl2 with excess NaOAc, both dNH groups were deprotonated and generated different coordination pockets. It cleanly converted to a new species 190. Reek and coworkers then studied the difference in reactivity in the two metal centers. Reaction of complex 190 with 1 equiv. of HCl led to reprotonation of the sulfimine nitrogen and formed the complex 191. This transformation is fully reversible when added the NaOAc as base source. When the complex 190 was treated with the 5 bar of H2 at room temperature. Then heating the reaction mixture to 50  C for a period of 20 h, resulting in the conversion to Ir-hydride complex 192 (Scheme 31).163

Scheme 31 The synthesis and reactivities of species 190.

The rhodium and iridium pentamethylcyclopentadienyl complexes (193 and 194) can also be prepared in high yield from the reaction of the appropriate dimer, [Cp MCl2]2, (M ¼ Rh or Ir) with the appropriate pro-ligand and KOH in methanol (Fig. 22).164 Other types of [(Cpx/arene)M(P^O)Cl]PF6 (M ¼ Ir, Cpx ¼ pentamethylcyclopentadienyl (Cp ) or its phenyl (Cpxph ¼ C5Me4C6H5) or biphenyl (Cpxbiph ¼ C5Me4C6H4C6H5) (P^O ¼ phosphine phosphonic amide ligand (PPOA)) derivatives are shown in Fig. 23. All of the complexes show remarkable anticancer activities toward HeLa and A549 cancer cells, activities which are higher than that of the clinical anticancer drug cisplatin. It indicates the diverse properties of the half-sandwich metal complexes with phosphorus ligands.165

Fig. 22 Structures of half-sandwich metal complexes containing P, O coordination sites.

82

Half-Sandwich Rhodium and Iridium Complexes

Fig. 23 Other types of [(Cpx/arene)Ir(P^O)Cl]PF6 complexes.

8.02.4

Half-sandwich Rh/Ir complexes with sulfur ligands

8.02.4.1

S^S0 -bidentate ligands

The unique and peculiar reactivity of the 16-electron half-sandwich metal (iridium and rhodium) complexes based on dithiolato ligands have been reported. Here are some examples of these complexes based on benzene, naphthalene, biphenyl, carborane and other ligands (Fig. 24).5,166–169 And the 16-electron half-sandwich metal (iridium and rhodium) complexes contains benzene- and derivatives of benzene- dithiolato ligands can also convert to dinuclear complexes bridging chlorine or S atom (Fig. 25).168

Fig. 24 Structures of half-sandwich metal complexes containing S^S0 -bidentate ligands.

Half-Sandwich Rhodium and Iridium Complexes

83

Fig. 25 Structures of half-sandwich metal complexes containing benzene- and derivatives of benzene- dithiolato ligands.

The complex 198 also has the potential to form the metal-metal bonds with another transition metal carbonyl compounds. Nishihara and coworkers reported the synthesis of the first series of triangular cluster complexes of [MCo2] (M ¼ Rh, Ir) 207 with a planar metalladithiolene ring coordinating in the Z3-bonding mode (Scheme 32).170 And the p-conjugated trinuclear iridium and cobalt dithiolenes undergo multiple metal-metal bond formation have also been developed through this type (Scheme 33).171 Complex 209 retains a planar framework and intense p conjugation across the three iridadithiolenes and the phenylene bridge, which results in intense electronic communication among the three Co2(CO)5 units in reduced mixed-valent states.

Scheme 32 Synthesis of triangular cluster [MCo2] (M ¼ Rh, Ir) 207.

84

Half-Sandwich Rhodium and Iridium Complexes

Scheme 33 Synthesis of the p-conjugated trinuclear iridium and cobalt dithiolenes undergo multiple metal-metal bond.

The reactivity of the complex 198 has also been studied by the Yan group.172 The Cp M-type half-sandwich dichalcogenolate complexes bearing either benzene moieties show diverse reactivity patterns toward two selected 2,6-disubstituted aryl azides under thermal or photolytic conditions. The interaction of 198 with 2-Me-6-NO2C6H3N3 led to the generation of an analogous product 210 and a new species 211. Interestingly, a rather different reactivity was observed in the reaction of complex 198 with excess 2,6-Me2C6H3N3, which resulted in the formation of new complex 212 in moderate yield (Scheme 34).

Scheme 34 The reactivity of complex 198.

The other types of 16 electron Cp M-type half-sandwich complexes bearing benzene moieties shows the reactivity with PEt3 to form the 18 electron complexes 213, 214 and 215 (Fig. 26).173 Other methods to form the 18 electron complexes have also been studied. 1,2-bis(phenylchalcogenomethyl) benzene can also act as the bidentate ligand to react with the [Cp MCl2]2 (M ¼ Ir and Rh) complexes. The formation of complexes 216 and 217 (Fig. 27) have been found efficient for catalytic transfer hydrogenation (TH) of aldehydes and ketones in glycerol, which acts as a solvent and hydrogen source.174 Sheldrick and coworkers also developed

Fig. 26 The other types of Cp M-type half-sandwich complexes bearing benzene moieties and PEt3.

Half-Sandwich Rhodium and Iridium Complexes

85

Fig. 27 Structures of half-sandwich metal complexes containing 1,2-bis(phenylchalcogenomethyl) benzene and diethyldithiocarbamate ligands.

tridentate diethyldithiocarbamate ligand to react with the [Cp IrCl2]2 and formed the complex 218, whose S atoms coordinate individual iridium atoms to afford four-membered chelate rings (Fig. 27).175 The superheated methanol solution of the dimeric complex [Cp Ir(m-(C2H5)2NCS2)]2(CF3SO3)2 218 resulted in the cleavage of the bridging IrdS bonds by CO and the complex 219 was obtained. (Fig. 27).175 The Jin group has developed the organochalcogen ligands derived from 3-methyl-imidazole-2-thione groups, Mbit and Ebit [Mbit ¼ 1,10 -methylenebis (3-methyl-imidazole-2-thione); Ebit ¼ 1,10 -(1,2-ethanediyl) bis(3-methyl-imidazole-2-thione).176 Reactions of [Cp IrCl2]2 and [Cp RhCl2]2 with Mbit, and Ebit result in the formation of the complexes [Cp M(Mbit)Cl]Cl (220
Cl) and [Cp M(Ebit)Cl]Cl (221
Cl), respectively (Fig. 28). Reactions of [Cp MCl2]2 (M ¼ Ir, Rh) with tri-dentate organochalcogen ligand results in the 18-electron half-sandwich complex (222
Cl).177 As for the 16-electron half-sandwich metal (iridium and rhodium) complexes containing carborane dithiolato ligands, the reactivity of these complexes is diverse. Jin and coworkers developed the metal-metal bond formation based this type and selected examples are shown in Fig. 29. A series of other homo- and hetero- bi-, tri- and tetra-nuclear complexes have been synthesized exhibiting diverse structures and bonding situations (Table 1).178–191

Fig. 28 Structures of half-sandwich metal complexes containing organochalcogen ligands derived from 3-methyl-imidazole-2-thione groups.

86

Half-Sandwich Rhodium and Iridium Complexes

Fig. 29 Structures of half-sandwich metal (complexes containing carborane dithiolato ligands.

Half-Sandwich Rhodium and Iridium Complexes

87

Table 1 Selected complexes with M-M bonds containing S,S; Se,Se and S,Se chelating modes based on carborane backbone (Cp ¼ 5-pentamethylcyclopentadienyl, Cptt ¼ 5-1,3-ditert-butylclopentadienyl, Cp ¼ 5-cyclopentadienyl, COD ¼ 1,5-cyclooctadiene). M-M’

Complexes

References

Ir-Ir

(Cp Ir)2[Se2C2(B10H10)] [(COD)Ir](Cp Ir)(-OCH3)[S2C2(B9H8)] [(COD)Ir](Cp Ir)[S2C2(B10H9)] cis-{Cp Ir[Se2C2(B10H10)]}-{Cp Ir(OCH3)[Se2C2(B10H8)]}Ir trans-{Cp Ir[Se2C2(B10H10)]}-{Cp Ir(OCH3)[Se2C2(B10H9)]}Ir Cp Ir[(COD)Rh][S2C2(B10H9)] Cp Ir[(COD)Rh][S2C2(B10H10)] Cp Ir[(COD)Rh](-OCH3)[S2C2(B9H9)] Cp Ir[CpRh][S2C2(B10H10)] Cp Ir[CpRh][Se2C2(B10H10)] {Cp Rh[Se2C2(B10H10)]}2Ir {Cp Ir[S2C2(B10H10)]}{Cp Ir(m-OCH3)-[S2C2(B10H8)]}Rh {Cp Ir[S2C2(B10H10)]}{Cp Ir(m-OCH3)-[S2C2(B10H8)]}Rh {(Cp Ir)2Rh[(COD)Rh][S2C2(B10H10)]-[S2C2(B10H9)] {(Cp Ir)2Rh[(COD)Rh][Se2C2(B10H10)]-[Se2C2(B10H9)] Cp Ir[S2C2(B10H10)][Co2(CO)5] Cp Ir[Se2C2(B10H10)][Co2(CO)5] {Cp Ir(S2C6H4)}Co2(CO) Cp Ir[(COD)Ru][S2C2(B9H8)(H)] Cp Ir[(COD)Ru](OCH3)[S2C2(B9H8)] (Cp Rh)2[S2C2(B10H10)] (Cp Rh)2[Se2C2(B10H10)] (CpRh)(Cp Rh)[S2C2(B10H10)] (CpRh)(Cp Rh)[Se2C2(B10H10)] cis-{Cp Rh[S2C2(B10H10)]}2Rh trans-{Cp Rh[S2C2(B10H10)]}2Rh (Cp Rh)2Rh2(CO)[Se2C2(B10H10]3 (Cp Rh)2(m-H)[7,8-(S)2-7,8-C2B9H10] Cp Rh[CpCo][S2C2(B10H10)] Cp Rh(CpCo)[Se2C2(B10H10)] Cp Rh[S2C2(B10H10)][Co2(CO)5] Cp Rh[Se2C2(B10H10)][Co2(CO)5] Cp Rh[(COD)Ru][S2C2(B10H9)(m-H)] Cp Rh[(COD)Ru][S2C2(B9H8)(m-H)] Cp Rh[(COD)Ru](-OCH3)[Se2C2(B9H8)] {Cp Ir2[S2C2(B10H10)]2}W(CO)2 {Cp Ir2[Se2C2(B10H10)]2}W(CO)2 {Cp Rh[S2C2(B10H10)]}{W(CO)2[S2C2(B10H10)]} {Cp Rh[S2C2(B10H10)]}{W(CO)2[S2C2(B10H10)]} {Cp Rh2[S2C2(B10H10)]2}W(CO)2 {Cp Rh2[Se2C2(B10H10)]2}W(CO)2 {Cp Ir2[S2C2(B10H10)]2}Mo(CO)2 {Cp Ir2[Se2C2(B10H10)]2}Mo(CO)2 {Cp Rh[S2C2(B10H10)]}{Mo(CO)2[S2C2(B10H10)]} {Cp Rh2[S2C2(B10H10)]2}Mo(CO)2 {Cp Rh2[Se2C2(B10H10)]2}Mo(CO)2 {Cp Ir[Se2C2(B10H10)]}Fe(CO)3 {Cp Rh[S2C2(B10H10)]}Fe(CO)3 {Cp Rh[Se2C2(B10H10)]}Fe(CO)3

178 179 179 180 180 179 179 179 181 181 182 182 182 183 183 184 184 185 186 186 187 187 181 181 188 188 188 189 181 181 187 187 186 186 186 188 188 190 190 187 187 182 182 191 191 191 184 190 190

Ir-Rh

Ir-Co

Ir-Ru Rh-Rh

Rh-Co

Ir-Ru

W-Ir W-Rh

Mo-Ir Mo-Rh

Fe-Ir Fe-Rh

Except for the metal-metal bond formation in 16-electron half-sandwich metal (iridium and rhodium) complexes, insertion reactions192 can also take place in these complexes. Selected examples reported by Yan group are shown in Scheme 35.168,193

88

Half-Sandwich Rhodium and Iridium Complexes

(A)

(B)

(C)

(D)

Scheme 35 Insertion reactions of 16-electron half-sandwich metal (iridium and rhodium) complexes containing carborane dithiolato ligands.

Half-Sandwich Rhodium and Iridium Complexes

89

Similar to 16-electron Cp M-type half-sandwich complexes bearing benzene moieties, the 16-electron half-sandwich metal (iridium and rhodium) complexes bearing carborane ligands can also form an 18-electron complex. The Jin group has synthesized many examples through this method and get the supermolecular complexes.13,194–196 Some examples are shown in the Fig. 30.

Fig. 30 Structures of pyridyl bridged 16-electron half-sandwich metal complexes bearing carborane ligands.

90

Half-Sandwich Rhodium and Iridium Complexes

Fig.30—Cont’d

The Jin group also synthesized a new type of alkene metal complexes using face-capping thione-alkene ligands. A series of chelating thione-alkene S(Z2dC]C)S tridentate ligands bound to half-sandwich metals are shown in Scheme 36. They illustrate that the counter-anions play an important role in the binding of the alkene moiety and different solvents are observed to affect the stability of the rhodium complexes.197

Scheme 36 Synthesis of chelating thione-alkene S(Z2dC]C)S tridentate ligands bound to half-sandwich metals.

Monosubstituted carborane-thiol ligands can also react efficiently with the half-sandwich metal (M ¼ iridium and rhodium) complexes (Scheme 37). The organophosphorous ligand can replace one of the monosubstituted carborane-thiol ligand to form the complex 256. The chlorine atom can then be replaced by silver salts to form a univalent cation 257 (Scheme 37).198

Half-Sandwich Rhodium and Iridium Complexes

91

Scheme 37 Substitution reaction of half-sandwich metal complexes bearing Monosubstituted carborane-thiol ligand.

8.02.4.2

S, N-coordination sites ligands

Except for the dithiolato ligands, the S, N chelating coordination mode can also serve as the important ligands for halfsandwich metal (M ¼ iridium and rhodium) complexes. Singh and coworkers developed the half-sandwich rhodium and iridium complexes of half-pincer chalcogenated pyridines (Fig. 31).199 And they also studied the catalysis of oxidation of secondary alcohols with N-methylmorpholine-N-oxide and transfer hydrogenation reaction of ketones with 2-propanol. The variable structural bonding modes of thiosemicarbazone and their thiosemicarbazone derivatives of rhodium and iridium metal complexes have been reported by the Kollipara group.200,201 The structures are shown in Fig. 32. They also studied the thiosemicarbazone ligand and (3-piolyl)-thiourea derivatives as S, N bidentate donor ligands (Fig. 33).202,203

Fig. 31 Structures of the half-sandwich rhodium and iridium complexes of half-pincer chalcogenated pyridines.

Fig. 32 Structures of thiosemicarbazone and their thiosemicarbazone derivatives of rhodium and iridium metal complexes.

92

Half-Sandwich Rhodium and Iridium Complexes

Fig. 33 Structures of half-sandwich metal complexes containing thiosemicarbazone and (3-piolyl)-thiourea derivatives as S, N bidentate donor ligands.

Besides that, Nakajima and coworkers developed the homo- and heterodinuclear rhodium and Iridium complexes supported by SNn mixed donor ligands (n ¼ 2–4) (Fig. 34).204 Compared these complexes they found that higher affinity of the Cp Ir fragment to both the NS and py sites relative to the rhodium analogue. The Lai group also reported a series of half-sandwich iridium complexes with thiosemicarbazone ligands in two types of coordination modes. The structures of them are characterized (Fig. 35).205 Tiherrien and coworkers synthesized the pyrenyl-derived thiosemicarbazone half-sandwich complexes and their biological activities were also studied (Fig. 36).206 Half sandwich complexes of iridium containing cysteine-derived ligands are developed by the Carmona group.207 Select examples are shown in Fig. 37.

Half-Sandwich Rhodium and Iridium Complexes

Fig. 34 Structures of half-sandwich metal complexes containing S, N bidentate donor ligands.

Fig. 35 Structures of half-sandwich iridium complexes with thiosemicarbazone ligands.

93

94

Half-Sandwich Rhodium and Iridium Complexes

Fig. 36 Structures of pyrenyl-derived thiosemicarbazone half-sandwich complexes.

Fig. 37 Structures of half sandwich complexes of iridium containing cysteine-derived ligands.

Half-Sandwich Rhodium and Iridium Complexes

95

The synthesis of half-sandwich transition-metal complexes containing the CabN,S chelate ligand (LiCabN,S ¼ LiSC2B10H10 CH2C5H4N) is described by the Jin group.208 The ligand was treated with [Cp RhCl2]2 complex to give the [Cp RhCl(CabN,S)] 286. Addition reaction of LiCabS (CabS ¼ SC2(H)B10H10) to the rhodium complex 286 yields [Cp Rh (CabS)(CabN,S)] (287). (Scheme 38).208

Scheme 38 The synthesis of half-sandwich rhodium complex containing the CabN,S chelate ligand.

The S, N chelating coordination mode with half-sandwich metal (M ¼ iridium and rhodium) complexes can form the organometallic rings. The Jin group has developed this method by pyridine-4-thiolato ligands. They have synthesized the neutral organometallic cyclic tri- and tetra-nuclear half sandwich iridium complexes. The structures of them are characterized by X-ray crystallography and are shown in Scheme 39. Through different solvent and different PH, they could adjust the formation of the organometallic cycles.209 Rhodium complexes could also be synthesized and were reported by the Hou group.210

Scheme 39 The synthesis of organometallic rings based on S, N chelating coordination mode with half-sandwich metal complexes.

8.02.4.3

S,C/S,B-coordination sites ligands

The S atom chelating ligand with half-sandwich metal (M ¼ iridium and rhodium) complexes are also helpful for inducing the CdH or BdH bond activation and form the M-C or M-B bonds. Tatsumi and coworkers synthesized the coordinatively unsaturated rhodium and iridium complex 291 bearing a bulky thiolate ligand (Scheme 40). This kind of complex weakly interact with the ipso-carbon of one of the mesityl groups in the SDmp ligand (Dmp ¼ 2,6-(mesityl)2C6H3) and form complex 292. While the

96

Half-Sandwich Rhodium and Iridium Complexes

Scheme 40 The reactivity of coordinatively unsaturated rhodium and iridium complex 291 bearing a bulky thiolate ligand.

ipso-carbon dissociates from the coordination sphere in the reactions with donor ligands to form complexes 293 and 294. (Scheme 40).211 Interestingly, the coupling of terminal alkynes and SDmp took place upon the addition of excess 1-pentyne or phenylacetylene to solutions of 292, affording cationic complexes with an S-arylated thiophene group (Scheme 41).211

Scheme 41 The reactivity of complex 292.

The Ison group reported the ortho-C-H bond activation of thiobenzoic acid and the reactivity of iridium thiobenzoate complexes. Based on the complex 297, the reaction with the stronger donor ligand, CO, was examined. The new complex 298 was formed in quantitative yields (Scheme 42). The reactions of 297 with electrophilic methyl substrates were examined. The reaction of excess methyl iodide with 297 in benzene resulted in the new iridium complex 299 (Scheme 42). The reaction of 297 with methyl triflate required milder conditions than with methyl iodide and resulted in the formation of the cationic dimer 300 (Scheme 42).212

Half-Sandwich Rhodium and Iridium Complexes

97

Scheme 42 Reactivity of iridium thiobenzoate complexes.

The Singh group reported the Schiff base [PhS(CH2)2C]N-9-C14H9] and [PhSe(CH2)2C]N-9-C14H9] to react with [Cp IrCl2]2 and CH3COONa at 50  C followed by treatment with NH4PF6, iridacycle 301 results. The similar reaction in the absence of CH3COONa gives complex in which the Schiff base ligand ligates in a bidentate mode 302 (Scheme 43).213 The same reaction can also take place in half-sandwich rhodium complex.214

Scheme 43 Synthesis of half-sandwich metal complexes based on Schiff base [PhS(CH2)2C]N-9-C14H9] and [PhSe(CH2)2C]N-9-C14H9] ligands.

The carboranylamidinate sulfide ligand can also react with [Cp MCl2]2 (M ¼ Ir and Rh) to induced to BdH bond activation (Scheme 44). And If NaH was changed to other base, such as CH3ONa and NEt3, the products could be produced with slightly higher yields.215

Scheme 44 Synthesis of half-sandwich metal complexes based on carboranylamidinate sulfide ligand.

98

Half-Sandwich Rhodium and Iridium Complexes

The Jin group developed the carboranylthioamide ligand with half-sandwich iridium complexes for selective C-H/B-H bond activation.216,217 The [Cp IrCl2]2 complex reacts with the carboranylthioamide ligand to form the 16-electron complex 304. Then the reactivity of this complex was studied in detail. The complex 304 can react with the donor ligands, such as CO, CN-tBu to afford the stable 18-electron configuration 305 and 306. It can also undergo an IrdS bond insertion reaction with DMAD to produce an acetylene insertion product 307. An additional equivalent of [Cp IrCl2] 2 reacts with complex 304 produced the unique binuclear species 308 with metal-metal bond (Scheme 45).

Scheme 45 Reactivity of 16-electron complex 304.

The m-carborane bearing the carboranylthioamide ligand can lead to the formation of metalloradicals upon oxidation.218 The reaction of m-carborane bearing the carboranylthioamide with [Cp IrCl2]2 (0.5 equiv.) in CH2Cl2 leads to the selective formation of the single B(2)dH bond-activated complex 309 (Scheme 46). The half-sandwich iridium(III) complex 309 bears an S^B^S pincer ligand.

Scheme 46 Synthesize of B(2)dH bond-activated complex 309.

The unsymmetrical structure of complex 309 suggests that the IrdS coordination bonds are of different strengths. Accordingly, carbon monoxide was passed though the solution of complex 309 in CH2Cl2 to form the complex 310 (Scheme 47). Subsequently, a drop of trifluoromethanesulfonic acid (HOTf ) was added to a solution of complex 309 in CH2Cl2 (Scheme 47), providing after workup pincer complex 311. The complex 309 is not stable, as it gradually converts to the radical 312• (Scheme 47) with concomitant loss of dihydrogen. The [Cp IrCl2]2 (0.5 equiv.) could also react with complex 309 to form the dinuclear complex 313 (Scheme 47).218 Then they sought to oxidize complex 313 with iodine in an attempt to synthesize a dinuclear radical complex 314, via one-electron transfer from complex 309 to the iodine (Scheme 48).

Half-Sandwich Rhodium and Iridium Complexes

99

Scheme 47 Reactivity of unsymmetrical structure 309.

Scheme 48 Synthesize of a dinuclear radical complex 314 via one-electron transfer.

They also used the carboranylthioamide ligand based on para-carborane to synthesize metallacycles with different cavity size (Fig. 38).219 They utilize the dihydrogen bond interactions of these metallacycles to achieve alkane recognition and separation of hexane isomers.

100 Half-Sandwich Rhodium and Iridium Complexes

Fig. 38 Structures of metallacycles bearing carboranylthioamide ligand.

Half-Sandwich Rhodium and Iridium Complexes

101

The Jin group also developed the monophosphine-o-carborane sulfide as a noninnocent ligand for C,S; S,S0 and B,S,S0 coordination modes of half-sandwich iridium and rhodium complexes. Treatment of the dimeric metal complexes [Cp M(m-Cl) Cl]2 (M ¼ Ir, Rh) with the lithium salt of monophosphine-o-carborane sulfide generate the C,S coordinated complexes [Cp IrCl(CabC,S)] (320) and [Cp RhCl(CabC,S)] (321) (CabC,S ¼ (1-SPPh2-1,2-closo- C2B10H10)), respectively (Scheme 49).220 The half-sandwich complexes 322 and 323 were prepared in a one-pot reaction by the in situ formation of the S-lithium salt of a monophosphine-o-carborane sulfide ligand (Scheme 50).220 The complex 323 can further react with the lithium salts of o-carborane and monosubstituted carborane-thiol ligand in the THF solution to form complex 324 and 325 (Scheme 50).221

Scheme 49 Reactivity of monophosphine-o-carborane sulfide ligand with half-sandwich metal complex.

Scheme 50 Synthesis and reactivity of complex 323.

The rhodium complex 326 of a redox-active, chelating N-heterocyclic carbine/thioether ligand was reported (Fig. 39). Electrochemical analysis reveals reversible redox behavior about the iron center in complex 326.222

Fig. 39 Structures of half-sandwich metal complexes chelating N-heterocyclic carbine/thioether ligand and P,S chelating modes.

102

8.02.4.4

Half-Sandwich Rhodium and Iridium Complexes

S, P-coordinates sites ligands

The P,S chelating mode with half-sandwich metal (metal ¼ iridium and rhodium) complexes attracts much attention due to its multiple coordination configurations. Werner and coworkers reported a series of half-sandwich type complexes with [Cp RhPR’3] and [Cp Ir(PiPr3)] as building blocks. Selected examples are shown in Fig. 39.223 The Z2-P,S ligand were also reported by the Schmidt group to react with the half-sandwich iridium complex to form the chloro Cp [Z2-2-(diphenylphosphanyl) thiophenolato]iridium(III) [Cp Ir (Z2-2-Ph2PC6H4S)Cl] complex 329. The iridium atoms in 329 are chiral and both enantiomers are present in the unit cell. The reaction of complex 329 with LiAlH4 or MeLi in tetrahydrofuran results in the iridium-hydrogen complex 330 and iridium-methyl complex 331, respectively (Scheme 51).108

Scheme 51 Reactivity of the chloro Cp [Z2–2-(diphenylphosphanyl) thiophenolato]iridium(III) complex 329.

8.02.5

Half-sandwich rhodium-carbon or iridium-carbon bonded complexes

8.02.5.1

Monodentate ligands with C atom as coordination site

The reaction of Cp Ir(CO)2 332 with pentafluorobenzonitrile in the presence of water produced the metallacarboxylic acid Cp Ir(CO)(p-C6F4CN)(COOH) 333, which can then be further reacted in the presence of a base to obtain the diaryl complex Cp Ir(CO)(p-C6F4CN)2 334, the hydride Cp Ir(CO)(p-C6F4CN)(H) 335 and the complex Cp Ir(CO)(p-C6F4CN)(C(OH)(NH2) C6F5) 336, respectively (Scheme 52).224

Scheme 52 Synthetic routes to the complexes 333–336.

A series of complexes in the form [Cp M(bpy)(CH3)]I 337 (M ¼ Rh, Ir, bpy ¼ 2,20 -bipyridyl) can be obtained by treating CpM(bpy) or Cp M(bpy) with methyl iodide (Fig. 40).225 After deprotonation, the 3-ethylthiazolium peptide salts prepared based on the unnatural amino acid thiazolylalanine can quickly react with (Cp RhCl2)2 through silver carbene transfer reaction to obtain the corresponding thiazole-based carbene complexes 338 and 339.226 A class of complexes Cp Ir(NHC)Cl2 340 (Fig. 41) synthesized using tunable chiral hydroxy-amide functionalized N-heterocyclic carbene ligands can be used as catalyst precursors for stereoselective transfer hydrogenation of acetophenone in the presence of KOH, and exhibits moderate enantioselectivity.227

Half-Sandwich Rhodium and Iridium Complexes

103

Fig. 40 Structure of the complexes 337–339.

R1,R1 H,H

R2 Bn

H,H

CH2(1-naphthalenyl) Me Me Me Me Me

R3 Bu

t

Bu

t

Me Et i Pr i Bu t Bu

R1,R1

R2 Me CH2(1-naphthalen Bn Bn Me Me

Fig. 41 Structure of a class of compounds 340.

The aldehyde-terminated self-assembled monolayers reacted with the complex [Cp Ir(PMe3)(CH3)](CF3SO3) to anchor the organometallic complex on the surface of the film 341 (Fig. 42).228

Fig. 42 The film 341 loaded with organometallic complexes.

The reaction of the half-sandwich lutetium dialkyl complexes (C5Me4R)Lu(CH2SiMe3)2(THF) with 1 equivalent of the iridium imido complex Cp Ir]NtBu produced the corresponding heterobimetallic LudIr complexes Cp Ir(m-NtBu)(m-CH2SiMe2CH2)Lu (C5Me4R) 343 bridged by imido group and silylmethyl group (Scheme 53). In this process, the CdH bond of a methyl group in one CH2SiMe3 group is activated by another CH2SiMe3 group to release SiMe4. In the reaction of the scandium dialkyl complex (C5Me4R)Sc(CH2SiMe3)2(THF) with Cp Ir]NtBu, the CdH bond of the a-CH2 unit in a CH2SiMe3 group is activated to generate

104

Half-Sandwich Rhodium and Iridium Complexes

Scheme 53 Syntheses of the complexes 343–349.

the imido and trimethylsilylmethylidene bridged ScdIr complex Cp Ir(m-NtBu)(m-CHSiMe3)Sc(C5Me4R) 344. Complex 344 was recrystallized from toluene in the presence of pyridine to obtain a pyridine coordinated complex Cp Ir(m-NtBu)(m-CHSiMe3)Sc (NC5H5)(C5Me4R) 345. The LudIr heterobimetallic complex 343 (R ¼ Me) reacted with two equivalents of PhCN to synthesize complex Cp Ir(m-NtBu)(m-CH2SiMe2)N(C(]CH2)Ph)Lu(PhCN)Cp 346 through inserting one molecule of PhCN into the terminal LudCH2 bond of 343 (R ¼ Me) and coordinating the other molecule of PhCN to the Lu atom. The intramolecular nucleophilic addition of the enamido unit to the coordinated PhCN in 346, and then after isomerization (1,3-hydrogen transfer), produced the diazalutenacyclohexadienyl complex Cp Ir(m-NtBu)(m-CH2SiMe2NC(Ph)CHC(Ph)NH)LuCp 347. Complex 343 (R ¼ Me) reacted with carbon monoxide (1 atm) at room temperature to obtain the enolate complex Cp Ir(m-NtBu)(m-CH2SiMe2C(]CH2)O)LuCp 348. After the latter reacted with PhCN, the adduct complex Cp Ir(m-NtBu)(m-CH2SiMe2C(]CH2)O)Lu(NCPh)Cp 349 was formed through the coordination of PhCN and Lu atom. In any case, no reaction at the bridged imido ligand NtBu was observed.229

8.02.5.2 8.02.5.2.1

Bidentate ligands C^N five-membered metallacycles

The chloro-bridged dimeric complexes (Cp MCl2)2 (M ¼ Rh, Ir) reacted with Schiff base ligands (p-RPhCH])N(p-PhtBu) in CH2Cl2/MeOH in the presence of NaOAc at room temperature to afford complexes 350 (Fig. 43). Efficient binding of complexes

Fig. 43 Structure of a series of complexes 350.

Half-Sandwich Rhodium and Iridium Complexes

105

350 with calf thymus DNA (CT DNA) have been established by UV–vis and emission spectroscopic studies. Protein binding (bovine serum albumin, BSA) has been investigated by UV–vis, fluorescence, synchronous, and 3D fluorescence spectroscopy. Furthermore, the complexes 350 exhibited significant cytotoxicity against the human lung cancer cell line (A549).230 Structures similar to complex 350 were also synthesized by Jones et al. using phenylimine ligands through CdH activation.231 The ligands (p-PhR)CH]NdN]CH(p-PhR) and one equivalent of half-sandwich metal complexes [Cp MCl2]2 (M ¼ Ir, Rh) were reacted in methanol to obtain mononuclear complexes 351 through mono CdH bond activation (Fig. 44). No biscyclometalated products were found in the reaction. Similarly, complex 352 was synthesized. The iridium complex 351 (M ¼ Ir, R ¼ Cl) exhibited good catalytic activity for the reduction of both electron-rich and electron-poor aryl imines with low catalyst loading in the presence of formic acid/triethylamine (F/T) azeotropic mixture.232

Fig. 44 Structure of the complexes 351 and 352.

R1

H

R2,R3 H,OMe

R4 CN

H,OMe

CF3

H,OMe H,OMe H,Me H,Me H,Me H,NO2

OMe H Cl H Me Me

R1

H

R2,R3 H,H

R4 Me

H,H

Cl

H,NO2 H,Cl H,F

H H H H

Cl

H,H

H

Fig. 45 Structure of a series of complexes 353.

Through the reaction of (Cp IrCl2)2 and ketimine ligands, a series of half-sandwich Ir(III) complexes 353 were synthesized (Fig. 45), and their in vitro activity on leukemia K562 cell line was investigated. These compounds demonstrated antiproliferative activities against K562 cells with IC50 values of 0.26–4.77 mM. In particular, compound 353 (R1, R2, R3, R4 ¼ H, H, OMe, OMe) showed cytotoxicity against five cancer cell lines/sublines and stronger activities than cisplatin in K562, K562/A02, MCF-7, MCF-7/ ADM, and A549 cells.233 Xiao and coworkers also demonstrated the use of (Cp IrCl2)2 for the cyclometallation of ketimine ligands.234–236 Half-sandwich cyclopentadienyl Ir(III) complexes 354 and 355 have been synthesized and characterized (Fig. 46). X-ray crystal structures of three complexes have been determined. All complexes showed potent cytotoxicity, with IC50 values ranging from 32.9 to 1.7 mmol
L−1 toward Hela human cervical cancer cells. Their potency is in the trend: Cpxbiph > Cpxph > Cpx > Cp . Complexes 354 (CpR ¼ Cpxbiph) and 355 (CpR ¼ Cpxbiph) displayed the highest potency, more than 4 times more active than the clinical platinum drug cisplatin.237 Through metal-mediated activation of the CdH bond, the dimeric metal complex (Cp IrCl2)2 is treated with Schiff base ligands to obtain the corresponding air-stable C,N-chelated mononuclear half-sandwich iridium complexes 356 (Fig. 47). These iridium complexes exhibit high catalytic activity for norbornene polymerization. Both the steric and electronic effects of the substituents affect the behaviors of the polymerization process.238 Cyclometalated iridium complexes 357–362 (Fig. 48) are highly efficient chemically selective catalysts that can be used in the transfer hydrogenation of a wide range of carbonyl groups with formic acid in water. Examples include a-substituted ketones (a-ether, a-halo, a-hydroxy, a-amino, a-nitrile, or a-ester), a-ketoester, b-ketoester, and a,b-unsaturated aldehydes. The reduction was carried out at substrate/catalyst ratios of up to 50,000 at pH 4.5, and no organic solvent is required. This provides a practical, simple and effective method for the synthesis of b-functionalized secondary alcohols (such as b-hydroxy ethers, b-hydroxyamines and b-hydroxy halogenated compounds), which are valuable intermediates in pharmaceutical, fine chemical, perfume and agrochemical synthesis.239

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Half-Sandwich Rhodium and Iridium Complexes

CpR

tetramethylcyclopentadienyl (Cpx)

pentamethylcyclopentadienyl (Cp*)

tetramethyl(phenyl)cyclopentadienyl (Cpxph)

tetramethyl(biphenyl)cyclopentadienyl (Cpxbiph)  Fig. 46 Structure of the complexes 354 and 355.

Fig. 47 Structure of a series of complexes 356.

Fig. 48 Structure of the complexes 357–362.

Sodium acetate promotes the interaction of pyrene-based imine ligands with (Cp IrCl2)2 to achieve regiospecific o-C2pyrene-H bond activation, forming half-sandwich complexes 363 and 364. Then it was found that the internal and terminal alkynes were inserted into the IrdC2pyrene bond of the cycloiridation complex 364, thereby inducing another regiospecific p-C8’naphthyl-H bond

Half-Sandwich Rhodium and Iridium Complexes

107

activation (365 and 366): the different coordination modes of the alkynes group were captured (Scheme 54). All the intermediate compounds (363–367) after CdH activation, alkyne insertion and reduction were fully characterized, including X-ray structure determination.240 Other structures similar to 363 were also reported later.241

Scheme 54 Syntheses of the complexes 365–367.

Ferrocenyl (Fc) and phenyl (Ph) containing imines FcCH]NCH(R)Ph and FcCH(R)N]CHPh (R ¼ H and Me; 368, 370, 372) were cycloiridated using (Cp IrCl2)2 with NaOAc in CH2Cl2. The complexes 369 and 371 derived from 368 and 370 were obtained as a mixture of E and Z imine isomers respectively (Scheme 55), and the product obtained from the S-370 was isolated by

Scheme 55 Synthetic routes to the complexes 369, 371, and 373.

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Half-Sandwich Rhodium and Iridium Complexes

recrystallisation as a single diastereoisomer. The configuration was determined by an X-ray crystal structure analysis as SC,RIr,E. In addition, the synthesis from 372 (R ¼ Me) to complex 373 (R ¼ Me) also showed diastereoselectivity.242 Reaction of (Cp IrCl2)2 with ferrocenylimines results in ferrocene CdH activation and the diastereoselective synthesis of half-sandwich Ir monocyclic complexes 374 of relative configuration Sp,RIr (Fig. 49). Extension to (S)-2-ferrocenyl-4-(1methylethyl)oxazoline gave highly diastereoselective control over the new elements of planar chirality and metal-based pseudo-tetrahedral chirality, to give both neutral and cationic half-sandwich Ir monocyclic complexes 375 and 376 of absolute configuration Sc,Sp,RIr. Substitution reactions proceed with retention of configuration, with the planar chirality controlling the metal-centered chirality through an iron–iridium interaction in the coordinatively unsaturated cationic intermediate. In addition, the related Sc,Rp,SIr configuration complexes 377 were synthesized by using methyl or TMS blocking group.243

Fig. 49 Structure of the compounds 374–377.

Cationic half-sandwich CdN chelating Ir and Rh complexes 378 (Fig. 50) are synthesized by AgSbF6 mediated halide abstraction from neutral azametallacycles derived from tritylamine or cumylamine and are fully characterized by NMR spectroscopy and X-ray crystallography. The treatment of the cationic complex 378 (M ¼ Ir, R ¼ C6H5) with H2 gas under ambient conditions in the presence of triethylamine in THF-d8 quantitatively yielded hydrido(amine) complex 379. The complexes 378 are effective hydrogenation catalysts. Actually, the Ir complexes serve as highly efficient catalysts for the imine hydrogenation instead of the complex bearing N-sulfonyldiamine ligands. Furthermore, the cationic Rh complex 378 (M ¼ Rh, R ¼ C6H5) was successfully used for the hydrogenation of nitriles.244 Previously, Ikariya et al. have conducted detailed studies on the structure and properties of complex 378 and its analogs.245–248

Fig. 50 Structure of the complexes 378 and 379.

Cationic half-sandwich iridacycle complexes 380–383 (Fig. 51) are highly active and efficient in the racemization of chiral alcohols and amines. Upon activation with base, these complexes are able to selectively racemize alcohols, whereas the

Fig. 51 Structure of the complexes 380–383.

Half-Sandwich Rhodium and Iridium Complexes

109

non-activated complexes are selective catalysts for the racemization of amines. The iridacycles have been used in the dynamic kinetic resolution of racemic b-chloroalcohols to produce chiral epoxides in a biphasic system in good yields and high enantiomeric excess (ee).249,250 Complexes with a structure similar to 380 have also been reported by Pfeffer and coworkers.251 Cyclometallation of (Cp IrCl2)2 with methyl (S)-2-phenyl-4,5-dihydrooxazole-4-carboxylate in the presence of NaOAc under anhydrous conditions selectively led to a N,C-chelated Cp Ir(III) complex 384 (Fig. 52).252 Through metal-mediated CdH bond activation based on benzothiazole ligands, several C,N-chelate cyclometalated half-sandwich iridium-based catalysts 385 have been prepared. These complexes exhibited good catalytic activity for hydrogenation of imines and quinoline derivatives by using H2 as the hydrogen source. A survey of various kinds of substrates suggested the good tolerance of the catalytic system for the hydrogenation process. In addition, complex 385 (R ¼ 4-OMe) could be reused for at least three cycles without any loss of activity in the hydrogenation of quinoline.253

Fig. 52 Structure of the complexes 384 and 385.

Cyclometalation of (2R,5R)-2,5-diphenylpyrrolidine with (Cp MCl2)2 using NaOAc in CH2Cl2 at room temperature, followed by cationization with KPF6 in CH3CN, cleanly yielded the cationic cyclometalated amines 386 (Fig. 53).254,255 Successful application of the NaOAc/CH2Cl2 methodology to the cycloiridation and cyclorhodation of the imidazolines led to the neutral half-sandwich chloro complexes 387, which were completely characterized, with yields ranging from 44% to 82%. In these complexes, the labile NH proton was localized in a g position with regard to the metal.254 CdH activation (388–395) by acetate-assisted cyclometallation of a phenyl group and half-sandwich complexes (Cp MCl2)2 (M ¼ Ir, Rh) can be directed by a variety of nitrogen donor ligands (Fig. 54). All the tested ligands can be cyclometalized on iridium,

Fig. 53 Structure of the complexes 386 and 387.

Fig. 54 The complexes 388–395.

110

Half-Sandwich Rhodium and Iridium Complexes

Fig. 55 Structure of a series of complexes 396.

but rhodium cannot cause cyclometalization in some cases.256 Fukuzumi and coworkers studied the catalytic performance of a water-soluble mononuclear iridium complex similar to complex 388.257 Cyclometallation of 2-phenylbenzimidazole ligands through sodium acetate-mediated CdH activation reaction generated corresponding cyclometalated complexes 396 (Fig. 55). The antiplasmodial activity of these complexes against the chloroquinesensitive (NF54) strain of the human malaria parasite Plasmodium falciparum was also studied.258 A series of pyrazole and pyrazolato complexes 397–400 with structural diversity have been synthesized (Scheme 56), and their bifunctional addition-elimination chemistry is essential to achieve effective catalysis. The intramolecular hydroamination of unactivated aminoalkenes catalyzed by 397 and 398 suggested the importance of the acidic b-NH functionality.259 Subsequent work also further clarified the mechanism and substrate scope of the catalytic hydroamination.260,261

Scheme 56 Syntheses of the complexes 398–400.

A series of cationic chlorido arene-iridium(III) complexes 401 and 402 with bidentate pyridyl functionalized mesoionic carbenes (MIC) of the 1,2,3-triazol-5-ylidene type have been prepared (Fig. 56). All complexes have been studied for their cytotoxic activity on a human cervical carcinoma HeLa cell line. Two of the compounds, 402 (R ¼ C6H5, 2,6-di(iPr)C6H3), were the most cytotoxic with IC50 values of 7.33 mM and 2.01 mM, respectively. Examination of their cytotoxic effect on different cell lines revealed that they preferentially kill cancer over normal cells.262

Half-Sandwich Rhodium and Iridium Complexes

111

Fig. 56 Structure of the complexes 401 and 402.

Treatment of (Cp IrCl2)2 with 2-phenylpyridine yields complex 403 which is readily converted to its analogs 404–406 (Scheme 57). Heating a solution of 405 and PhI(OAc)2 for an hour at 45  C gives a single new iridium species 407.263

Scheme 57 Synthetic routes to the complexes 404–407.

Using pyridine-N-oxide and 4-methoxy-pyridine-N-oxide to react with Cp Ir(phpy) (phpy ¼ cyclometalated 2-phenylpyridine) cations can form stable pyridine-N-oxide adducts 408 and 409, respectively (Fig. 57). In complexes 408 and 409, the oxygen atom transfer (OAT) reaction initiated by pyrolysis or photolysis has also been studied.264

Fig. 57 Structure of the complexes 408 and 409.

In the presence of ligands L, complexes 410 and 411 were synthesized by halide abstraction from 403 with Na[B(ArF)4] (B(ArF)4 ¼ tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) (Scheme 58). In subsequent experiments, a silver-bridged binuclear Ir species 412 was accidentally synthesized. The Ag+ source of complex 412 was identified as AgCl.265

112

Half-Sandwich Rhodium and Iridium Complexes

Scheme 58 Synthetic routes to the complexes 410–412.

When [Cp Rh(phpy)(NCArF)][BArF4] (413, NCArF ¼ 3,5-bis(trifluoromethyl)benzonitrile) was treated with the soluble oxygen atom transfer reagent 2-tert-butylsulfonyliodosylbenzene (sPhIO), oxygen atom insertion into the rhodium-carbon bond of coordinated phpy was observed (Scheme 59). This resulted in the formation of a k2 2-(2-pyridyl)phenoxide ligand. Following insertion to form a new bidentate ligand, a second equivalent of sPhIO, acting as a neutral, two-electron donor ligand, coordinated to the rhodium center through the iodosyl oxygen. Over time, the sPhIO ligand dissociates and dimerization occurs to generate a phenoxide-bridged dinuclear species 415.266

Scheme 59 Syntheses of the complexes 414 and 415.

The oxidation of [Cp Ir(phpy)(NCArF)][BArF4] (410) with the OAT reagent sPhIO yielded a single, molecular product at −40  C. Monitoring the [Cp Ir(phpy)(NCArF)]+ oxidation reaction by low-temperature NMR techniques revealed that the reaction involved two separate OAT events. An intermediate 416 was detected, synthesized independently with trapping ligands, and characterized (Scheme 60). The first oxidation step involves direct attack of the sPhIO oxidant on the carbon of the coordinated nitrile ligand. Oxygen atom transfer to carbon, followed by insertion into the iridium–carbon bond of phpy, formed a coordinated organic amide. A second oxygen atom transfer generated an unidentified iridium species (the “oxidized complex”). In an attempt to determine the structure of oxidized complex, HCl(g) was purged through a freshly prepared sample of the oxidized complex in dichloromethane-d2 at −40  C. This led to a new iridium product 417 with the Cp , nitrile, and phenylpyridine ligand components within the coordination sphere.267

Half-Sandwich Rhodium and Iridium Complexes

113

Scheme 60 Syntheses of the complexes 416 and 417.

According to the CdH activation mechanism of the inner sphere, the p-toluenesulfonamido (NTs) nitrene group was inserted into the bidentate phpy of the complex 406 to form an iridium complex 420 with a pyridyl-amide bidentate ligand (Scheme 61).268,269 Stirring 2.5 equiv. of the nitrene transfer reagent PhI]NTs (p-toluenesulfonyliminoiodobenzene) with the iridium complex 410 for 3 days at room temperature yielded a single iridium product, 419. Roughly 1 equiv. of the amine, H2NTs, was generated as a byproduct. Compound 418 was an undetected intermediate, because it formed cleanly in a 1:1 reaction of PhI] NTs and 410. The structure of the final iridium product 419 reveals that an outer-sphere CdH insertion reaction is accessible to form amidation products.268

Scheme 61 Synthetic routes to the complexes 418–420.

Two iridium-BODIPY dyads were synthesized by a procedure involving the replacement of the anionic chlorido ligand of Cp Ir(phpy)Cl (403, phpy ¼ cyclometalated 2-phenylpyridine) by neutral meso-pyridyl-BODIPY (BODIPY ¼ 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dye) ligands to afford the monocationic, highly lipophilic complexes 421 and 422 (Fig. 58). The molecular

Fig. 58 Structure of the complexes 421 and 422.

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Half-Sandwich Rhodium and Iridium Complexes

structure of both complexes was confirmed by X-ray crystallography. Judicious positioning of the BODIPY entity allowed the dyads to maintain a high quantum yield as well as potent antiproliferative activity on a range of cancer cell lines.270 The easily obtained complexes 403, 423, and 424 (Fig. 59) from the commercially available (Cp IrCl2)2 proved to be highly active catalyst precursors for water oxidation.271 Subsequently, Crabtree et al. reported complete details about the homogeneous water oxidation catalysis of 403 and 424 and compared their behavior with related similar complexes.23 In addition, complex 403 precatalyst can also be used for selective CdH oxidation with sodium periodate as the terminal oxidant.272 Freixa et al. investigated the hydrosilylation of enolizable imines catalyzed by iridacycle complex 403. For comparison, a series of similar half-sandwich iridium complexes have also been synthesized, such as complexes 425–428.273 The catalysis of Cp Ir complex 425 with functional C,N-chelate ligand for the dehydrogenative oxidation of alcohols has also been reported.274

Fig. 59 Structure of the complexes 423–428.

Four half-sandwich iridium triphenylamine or carbazole-modified 2-phenylpyridine (TPA/Cz-PhPy) complexes 429–431 were synthesized and characterized (Fig. 60). Thereinto, triphenylamine is attached with phenylpyridine ligand through trans-double bond, which effectively improves the electron-donating ability of C^N-chelating ligands, and eventually improve the anticancer activity of complexes. Target complexes 429–431 show potential activity against A549, HepG2 and HeLa cells, and effectively prevent the migration of cancer cells.275 Subsequently, Liu et al. prepared and characterized six N-phenylcarbazole/triphenylamineappended (PhCz/TPA) half-sandwich iridium(III) 2-phenylpyridine complexes 432 and 433 (Fig. 61). The introduction of PhCz/ TPA unit effectively improves the anti-tumor activity of these complexes on A549 and HeLa tumor cells, the best of which has an activity close to eight times that of clinical cisplatin, and effectively prevents the migration of cancer cells.276

Fig. 60 Structure of the complexes 429–431.

Half-Sandwich Rhodium and Iridium Complexes

115

Fig. 61 Structure of the complexes 432 and 433.

R1 H

R2 R3 F H

R4 H

R5 H

R1 CH2OH

R2 H

R3 H

R4 H

R5 H

F

H

H

H

H

H

H

H

CH2OH

H

H H CHO H NO2 H

H H H H H H

F H H H H H

OH H H H H

H H CH3 H H

H H H H H

H H H CH3 H

H OH H H CH3

H H H F H H H CHO H H NO2 H

Fig. 62 Structure of a series of complexes 434.

In order to study the effect of substituents on C,N-chelated 2-PhPy ligands and their complexes, Sadler et al. reported the synthesis, characterization, and antiproliferative activity of 15 iridium(III) half-sandwich complexes 434 (Fig. 62) of the type Cp Ir(2-(R-phenyl)-R-pyridine)Cl bearing either an electron-donating (dOH, dCH2OH, dCH3) or electron-withdrawing (dF, dCHO, dNO2) group at various positions on the 2-phenylpyridine (2-PhPy) chelating ligand giving rise to six sets of structural isomers.277 The reaction of complex 435 with phenylacetylene formed the double insertion product 436 (Scheme 62). The reaction of complex 437 with phenylacetylene produced a mixture containing 436 and 438. Complex 437 can readily promote the tandem transformation of terminal alkynes into N-phenylamines and catalyze the “one-pot” intermolecular hydroamination-hydrosilation/ protodesilation of terminal alkynes.278

Scheme 62 Synthetic routes to the complexes 436 and 438.

116

Half-Sandwich Rhodium and Iridium Complexes

In the presence of sodium acetate, planar-prochiral 2-phenylpyridine and 3-methyl-2-phenylpyridine complexes 439 undergo cyclometallation reaction with (Cp MCl2)2 (M ¼ Rh, Ir) to obtain complexes 440–443 in a yield of 60% to 92% (Scheme 63). The reactions exhibit stereoselectivity. The chlorine ligands that bind to Rh and Ir in the resulting diastereomers are trans relative to Cr(CO)3, that is, the Cp ligands are syn relative to the tricarbonylmetal moiety.279 To understand the factors that influence the stereochemical course of ligand exchange at a planar-prochiral iridacycle, a series of transformations of substrate 441 into methyl and phenyl iridium compounds 444 were achieved. Structural and analytical information indicated the exclusive formation of the anti stereoisomer in which the methyl and phenyl groups occupy the anti position with respect to the Cr(CO)3 moiety.280 Subsequently, Djukic et al. also used complex 435 and its analogs to react with the labile solvato complexes of “Cp Ru+” and “Cp Ir2+” to stereospecifically synthesize cationic and bis-cationic iridacycles derived from 2-anilinylpyridines.281,282

Scheme 63 Syntheses of the complexes 440–444.

Purine metallanucleosides 446 were obtained as 1:1 diastereomeric mixtures by reaction of 9-b-D-ribofuranosyl-6-phenylpurine 445 with (Cp MCl2)2 (M ¼ Ir, Rh) in the presence of sodium acetate, in dichloromethane, at room temperature (Scheme 64).283,284 The preparation of the analogous pyrimidine nucleosides 449 and 450 from b-D-ribofuranosyl-4-phenylpyrimidine 448 was carried out in a sealed tube, at 90  C for 16–48 h. The cyclometalations of pyrimidine nucleoside 448 were diastereoselective, leading to isomeric ratios of 70:30 for the Ir(III) derivative 449 and 90:10 for the Rh(III) compound 450. This trend is general for the structurally related pyrimidine nucleosides, and their cyclometallation also occurs with good diastereomeric ratios. Heterobimetallic complexes 447 were prepared by reaction of cyclometalated purines 446 (1:1 diastereomeric mixtures) with 1.0 equiv. of the

Scheme 64 Syntheses of the complexes 446, 447, and 449–451.

Half-Sandwich Rhodium and Iridium Complexes

117

complex OsH2Cl2(PiPr3)2, in the presence of 4.0 equiv. of Et3N, in dichloromethane, at room temperature. The dinuclear complexes were obtained as dark red (447, M ¼ Ir, 77%) and orange (447, M ¼ Rh, 42%) solids by precipitation with pentane as 7:3 and 8:2 diastereomeric mixtures, respectively. In a similar manner, reaction of Ir(III) pyrimidine nucleoside 449 (7:3 isomeric mixture) led to the formation of dinuclear derivative 451, which was obtained by precipitation in pentane in 86% yield, as a brown-reddish solid (8:2 diastereomeric mixture). This compound decomposed in solution.283 Eight half-sandwich cyclopentadienyl Ir(III) pyridine complexes of the type [CpxphIr(phpy)L]PF6 were synthesized and characterized (Scheme 65), in which Cpxph ¼ C5Me4(C6H5) (tetramethyl-(phenyl)cyclopentadienyl), phpy ¼ 2-phenylpyridine as C^N-chelating ligand, and L ¼ pyridine (py) or a pyridine derivative. The monodentate py ligands blocked hydrolysis; however, antiproliferative studies showed that all the Ir compounds are highly active toward A2780, A549, and MCF-7 human cancer cells. In general the introduction of an electron-donating group (e.g., Me, NMe2) at specific positions on the pyridine ring resulted in increased antiproliferative activity, whereas electron-withdrawing groups (e.g., COMe, COOMe, CONEt2) decreased anticancer activity. Complex 453 (L ¼ 4-NMe2-py) displayed the highest anticancer activity, exhibiting submicromolar potency toward a range of cancer cell lines in the National Cancer Institute NCI-60 screen, ca. 5 times more potent than the clinical platinum(II) drug cisplatin.285 An organometallic half-sandwich iridium Ir(III) complex 454 with a similar structure has also been synthesized (Fig. 63). It shows higher potency toward A2780 human ovarian cancer cells compared with cisplatin, and also shows higher potency in the National Cancer Institute NCI-60 cell-line screen.286,287 The complex 454 undergoes rapid hydrolysis of the chlorido ligand. In contrast, the pyridine complex 455 aquates slowly, and is more potent (in nanomolar amounts) than both 454 and cisplatin toward a wide range of cancer cells. The pyridine ligand protects 455 from rapid reaction with intracellular glutathione.288

Scheme 65 Syntheses of a series of complexes 453.

Fig. 63 Structure of the complexes 454 and 455.

Two rhodamine-modified half-sandwich Ir(III) complexes 456 and 457 were synthesized and characterized (Fig. 64). Both complexes showed potent anticancer activity against A549, HeLa, and HepG2 cancer cells and normal cells, and altered ligands had an effect on proliferation resistance. The complex enters cells through energy dependence, and because of the different ligands, not

Fig. 64 Structure of the complexes 456 and 457.

118

Half-Sandwich Rhodium and Iridium Complexes

Fig. 65 Structure of the compounds 458–463.

only could it affect the anticancer ability of the complex but also could affect the degree of complex lysosome targeting, lysosomal damage, and further prove the antiproliferative mechanism of the complex. Excitingly, antimetastatic experiments demonstrated that complex 456 has the ability to block the migration of cancer cells.289 The half-sandwich iridium complex 458 (Fig. 65) is highly cytotoxic: 15–250 more potent than clinically used cisplatin in several cancer cell lines.290 Subsequently, Pizarro et al. synthesized five other similar complexes 459–463. The cyclopentadienyl ligand bears a tethered pyridine that binds to the metal center, resulting in an Ir(Z5:k1-C5Me4CH2pyN) tether-ring structure, as confirmed by the X-ray crystal structures of 458, 459, 461, 462, and 463. Nontether versions of 458 and 459 were synthesized to aid unambiguous correlation between structure and activity. While nontether complexes are highly potent toward MCF7 cancer cells (similar to cisplatin), complexes bearing the tether-ring structure, 458–463, are exceptionally more potent (1–2 orders of magnitude).291 Corresponding neutral metallacyclic compounds 465 can be prepared from the stoichiometric cyclometallation reaction of metal precursors of Rh and Ir with benzo[h]quinoline 464 (Scheme 66). In the presence of Cu(OAc)2 (0.5 equivalent) and ammonium fluoride promoter (TBAF, 2.0 equiv.), (4-trifluoromethyl)phenyl neopentyl-glycol borate as a coupling partner can smoothly react with the metallacycles 465 to form the corresponding complexes 466. In addition, when 465 (M ¼ Rh) is treated with 2,5,5-trimethyl-1,3,2-dioxaborane in the presence of tBuOK at room temperature, a cyclometalated rhodium-methyl complex 467 is formed.292

Scheme 66 Syntheses of the complexes 465–467.

Novel cyclopentadienyl C^N chelated Rh(III) anticancer complexes 468 and 469 can be easily synthesized (Fig. 66). The reactivity and cytotoxicity of these complexes can be reasonably modulated by selection of the monodentate ligand as chloride

Half-Sandwich Rhodium and Iridium Complexes

119

Fig. 66 Structure of the complexes 468 and 469.

or pyridine. The hydrolysis rate of chlorido complexes increases in the order Cp Cpxph < Cpxbiph, showing that incorporation of the extended CpR ring confers more labile kinetics on the monodentate chloride ligand. On the contrary, when the chloride is replaced with pyridine, the rate of hydrolysis is slowed down by orders of magnitude and decreases in the order Cp > Cpxph > Cpxbiph. This difference in hydrolysis kinetics for the chlorido and pyridine complexes leads to the differences in reactivity, and subsequent differences in cytotoxicity. The pyridine complex 469c reacts more slowly with glutathione than the chlorido analogue, resulting in less deactivation and an order of magnitude greater potency toward lung cancer cells. Meanwhile, complex 469c accumulates to a much lesser extent than the chlorido analogue in cancer cells at equipotent IC50 concentrations, indicating that complex 469c requires a lower dose than 468c to achieve the same therapeutic potency.293 A new family of half-sandwich iridium(III) complexes 470 (Fig. 67) bearing versatile imine-N-heterocyclic carbene ligands has been synthesized. With the exception of 470a, 470b, and 470 k, each complex shows potent cytotoxicity, with IC50 (half-maximum inhibitory concentration) values ranging from 1.99 to 25.86 mM toward A549 human lung cancer cells. Complex 470 h (IC50 ¼ 1.99 mM) displays the highest anticancer activities, whose cytotoxicity is more than 10-fold higher than that of the clinical platinum drug cisplatin against A549 cancer cells.294 Among the subsequently synthesized complexes 471 (Fig. 68), the complex 471f displays the greatest cytotoxic activities (IC50 ¼ 0.85 mM), whose anticancer activity is more approximately 25-fold higher than that of cisplatin. The initial cell death mechanistic insight displays that this group of iridium(III) complexes exerts anticancer effects via cell cycle arrest, apoptosis induction and loss of the mitochondrial membrane potential. In addition, the confocal microscopy imaging shows that the complex 471f can damage lysosome.295

No. 470a

CpR R1 R2 Cp* 2,6-Me2C6H3 Me

R3 Me

No. 470g

CpR Cp*

R1 2,6- Pr2C6H3

R2 Me

470b Cp* 2,6-Me2C6H3 Me

Et

470h

Cp*

2,6-iPr2C6H3

Me

470c 470d 470e 470f

Pr n Bu Me Et

470i 470j 470k 470l

Cp* 2,6-Me2C6H3 Cp* 2,6-Me2C6H3 Cp* 2,6-iPr2C6H3 Cp* 2,6-iPr2C6H3

Fig. 67 Structure of a series of complexes 470.

Me Me Me Me

i

i

Cp* 2,6-Me2C6H3 Ph Cp* 2,6-Me2C6H3 Ph Cp* Ph Ph xbiph Cp 2,6-Me2C6H3 Me

R3 Pr

i

Bu

n

Pr Bu i Pr Me i

n

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Half-Sandwich Rhodium and Iridium Complexes

No. 471a

R1 2,6- Pr2C6H3

R2 R3 Me Me

471b

2,6-iPr2C6H3

Me

i

471c 2,6-iPr2C6H3 Me 471d 2,6-Me2C6H3 Me 471e 2,6-Me2C6H3 Me 471f 2,6-Me2C6H3 Ph

Et Pr Et i Pr i Pr i

Fig. 68 Structure of a series of complexes 471.

The unhindered N-pyrid-2-yl imidazolidene NHC ligand has been shown to chelate to a Cp Ir fragment (472) (Scheme 67).296 With the goal of weakening the coordination of the pyridyl substituent and enabling its role as pendant base or hemilabile ligand, a tert-butyl group at C-6 next to the pyridyl N was installed. Attempted coordination of the newly synthesized carbene ligand avoided N-coordination entirely, but led to unexpected C-metalation at C-3 by the Cp Ir center (473). Successful formation of a weakly N-coordinated analogue was achieved by synthesizing a ligand with a second tert-butyl group at C-4. The complexes were studied using X-ray crystallography and NMR spectroscopy. The X-ray crystal structure of di-tert-butyl analogue 474 showed that in the solid the complex existed as a chloride-bridged dimer 475, with the pyridyl nitrogen uncoordinated. In solution, 15N chemical shift information revealed that 474 existed with the pyridyl substituent N-coordinated, presumably as a monomer, but that addition of an amine ligand readily opened the chelate. Complex 474 can be used as a catalyst for intramolecular hydroamination of primary and secondary alkenylamines.297

Scheme 67 The complexes 472–475.

The NHC-pyOR complexes 476 of iridium(III) obtained by using ligands with an NHC ring bound to a pyridinol ring are moderately active precatalysts for the hydrogenation of CO2 (Scheme 68). Low activity is observed when these NHC-pyOR complexes are used for formic acid dehydrogenation. NHC complexes 476 all underwent transformations under basic CO2 hydrogenation conditions.298

Scheme 68 Synthetic routes to a series of complexes 476.

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The NHC pyrimidine iridium complexes 478 were prepared by in situ transmetalation from the silver carbene complexes of compounds 477 (Scheme 69). Compounds 477 were treated with Ag2O in CH2Cl2 at room temperature in the absence of light to form the presumed silver carbenes, which were then added to (Cp IrCl2)2 and KPF6 to react to obtain orange-yellow solids 478.299

Scheme 69 Synthetic routes to a series of complexes 478.

Fig. 69 Structure of the complexes 479, 472, and 480.

Picolyl, pyridine, and methyl functionalized N-heterocyclic carbene iridium complexes 479, 472 and 480 have been synthesized by transmetallation from Ag(I) carbene species (Fig. 69). The iridium carbene complexes 479 and 472 show moderate catalytic activities (3.03  105 g PNB (mol Ir)−1 h−1 and 1.70  106 g PNB (mol Ir)−1 h−1) for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as co-catalyst.296 The reaction of 2-acetylpyridine with (Cp RhCl2)2 in the presence of sodium acetate gave 481b in good yield (Scheme 70), while the similar reaction with iridium gave an inseparable mixture. The lithium enolate was prepared in situ by reacting 2-acetylpyridine with LiHMDS in THF at −80  C, and then adding (Cp MCl2)2 (M ¼ Ir, Rh) to produce N,C chelate complexes 481.300

Scheme 70 Syntheses of the complexes 481a and 481b.

Reactions of (Cp MCl2)2 (M ¼ Ir, Rh) with carboranylamidinate ligand 482 in the presence of the BuLi excess produced 16-electron half-sandwich iridium and rhodium complexes 483 (Scheme 71).301

Scheme 71 Synthetic routes to the complexes 482 and 483.

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Half-Sandwich Rhodium and Iridium Complexes

8.02.5.2.2

C^N six-membered metallacycles

The half-sandwich iridium(III) complexes 484 and 485 containing deprotonated 1-phenyl-7-azaindole and 1-(thiophen-2-yl)7-azaindole, respectively, were prepared (Fig. 70). For the electroneutral complexes 485, although no base was added to the reaction mixture in fact, they still contained 1-(thiophen-2-yl)-7-azaindole as a deprotonated ligand coordinated through the N7 atom of the 7-azaindole moiety and C12 atom of the pendant thiophenyl group.75

Fig. 70 Structure of the complexes 484 and 485.

Treatment of benzo[h]quinoline with (Cp RhCl2)2 afforded cyclometalated Rh(III) complex 468a, which reacts with diazomalonate in 1,2-dichloroethane to afford s-alkyl −Rh(III) complex 486 in 69% yield (Scheme 72). The structure of 486 was determined by X-ray crystallography.302

Scheme 72 Synthetic route to the complex 486.

S and R configurations of chiral-at-metal Ir(III) and Rh(III) complexes (487 and 488) were selectively obtained by using chelate-type NHC ligands with a- and b-glucopyranosyl units, respectively (Fig. 71). In the 1H NMR spectra of the complexes, only two sets of the signals for the Cp and sugar-coated NHC ligands with ratios of 95:5 for 487 (M ¼ Ir), 85:15 for 488 (M ¼ Ir), 90:10 for 487 (M ¼ Rh), and 85:15 for 488 (M ¼ Rh) were observed. Subsequent circular dichroism (CD) spectroscopy and X-ray single crystal diffraction analysis confirmed the diastereoselectivity of the synthesis.303

Fig. 71 Structure of the complexes 487 and 488.

Eight half-sandwich iridium(III) benzimidazole-appended imidazolium-based N-heterocyclic carbene (NHC) antitumor complexes 489 were synthetized and characterized (Fig. 72). The complexes showed potential antitumor activity toward A549 cells, and the activity of complex 489 (CpR ¼ Cpxbiph, R1 ¼ nBu) is even 3 times higher than that of the clinical antitumor drug cisplatin.304 Subsequent reports of similar anticancer complexes 490 and 491 also showed interesting antitumor activity against A549 cells.305

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Fig. 72 Structure of the compounds 489–491.

Complexes 492 were prepared from the isolated silver carbene precursor, followed by reaction with the (Cp MCl2)2 (M ¼ Rh, Ir) precursor in the presence of a large excess of KPF6 (Scheme 73). They can catalyze the hydrogenation of acetophenone and hydrogen.306 Six-membered cyclometalated complexes similar to 492 derived from primary amine-functionalized imidazolium salt have been previously reported by Cross and coworkers.307

Scheme 73 Syntheses of the complexes 492.

Complex 493 were prepared by treatment of (Cp RhCl2)2 with N-tosyl-1,2-diphenylethylenediamine and NEt3 in CH2Cl2 and were crystallized as red-orange needles. The chloride ligand in 493 has been shown to be very labile; reaction with CO afforded the cationic complex 494 and 495 as a mixture of diastereomers, S,S,RRh and S,S,SRh, with opposite chirality at Rh (Scheme 74).308 The insertion reaction of carbon monoxide and other unsaturated molecules to the M-C bond of five-membered iridium and rhodium cyclometalated complexes has also been studied.309

Scheme 74 Reaction of 493 with CO.

The C,N-chelated metal complex 497 was prepared by using the dimeric metal complex (Cp IrCl2)2 and ligand 496 (Scheme 75). In the presence of methylaluminoxane (MAO) as a co-catalyst, carborane complex 497 shows high catalytic activity for the polymerization of ethylene.208

Scheme 75

Synthetic route to the complex 497.

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Fig. 73 Structure of the complexes 498–502.

Fig. 74 The complexes 503 and 504.

A series of Cp Ir complexes with hydroxy- (498, 499) and amino- (500) functionalized NHC ligands as molecular water oxidation catalysts were reported (Fig. 73). Among these species, complex 498 is the most active catalyst. The importance of the pendant functionalities, in particular the hydroxy groups, for catalytic activity is shown in comparison with a reference Cp Ir catalyst (501) that lacks these groups. Compared to that of 501, the rate of oxygen evolution catalyzed by 498–500 is up to 15 times higher. The importance of chloride ligands coordinated to the Ir centers was established by comparison with complex 502, which contains the hydroxy-functionalized NHC but lacks chloride ligands.310 The study by Peris et al. showed that complex 501 can be used for the deuteration of a wide range of organic molecules, using CD3OD as a deuterium source, providing high activities under relative mild conditions.311 Twelve half-sandwich Ir(III)-NHC complexes 503 (CpR ¼ Cp ) and 504 were synthesized and characterized (Fig. 74). These complexes showed higher cytotoxic activity toward A549 cells and HeLa cells than cisplatin. An increase in the number of contained phenyl groups was related to better anticancer activity.312 Subsequently, Liu et al. studied the anticancer and antimetastasis properties of the complexes 503 (CpR ¼ Cpxbiph). Compared with cisplatin, these Ir(III) complexes have stronger anti-proliferative activity and can fight against 7 cancer cell lines. Complex 503 (CpR ¼ Cpxbiph, R1 ¼ Bn) is 8 times more potent than cisplatin, and it can significantly inhibit tumor growth in mouse xenograft models of colon cancer in vivo.313

8.02.5.2.3

C^N seven-membered metallacycles

The synthesis of iridium and rhodium complexes 506 and 508 containing pyrazolyl-N-heterocyclic carbene donor ligands was achieved via the transmetalation of the carbene from an in situ-generated silver complex (Scheme 76). These complexes yield active hydroamination catalysts upon abstraction of the chloride coligand with AgBF4. Among them, compared with Rh(III) complexes, the corresponding Ir(III) complexes are far superior catalysts for the hydroamination of both aliphatic amines and anilines. And the steric shielding of the metal center by the inclusion of large substituents on the ligand scaffold was shown to have an adverse effect on the catalyst activity, with the complex containing the sterically smaller ligand 507 affording a catalyst with higher activity.314

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Scheme 76 Syntheses of the complexes 506 and 508.

The air-stable iridium(III) complex 510 was prepared by a transmetalation reaction of 509 and (Cp IrCl2)2 in refluxing acetonitrile (Scheme 77). Similarly, the iridium(III) complex 512, which contains a cyclopentadienyl ligand (Cp), can be obtained by carrying out a transmetalation reaction from 509 to (CpIrCl2)2 in refluxing acetonitrile. The iridium(III) hydride-amine complex 511 was prepared from a warm 2-propanol solution of 510 containing 3 equiv. sodium 2-propoxide. The analogous iridium(III) hydride-amine complex 513 which contains a Cp ligand was prepared in a fashion similar to that of 511 starting from the Cp complex 512 and 2-propoxide in 2-propanol. Complex 514 can be synthesized from 1-(N,N-dimethylamino)propyl-3methylimidazolium chloride hydrochloride. The complex 510, when activated by an alkoxide base, catalyzed the H2 hydrogenation of acetophenone and benzophenone under 25 bar of H2 pressure at 50  C, achieving a maximum turnover frequency (TOF) of 416 h−1. In comparison, the complex 512 has higher activity in the catalytic H2 hydrogenation of acetophenone, with a TOF as high as 687 h−1.141

Scheme 77 Syntheses of the complexes 510–514.

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Half-Sandwich Rhodium and Iridium Complexes

The cyclometalated complexes 388 readily undergo insertion reactions with RC^CR (R ¼ CO2Me, Ph) to give mono insertion products, the rhodium complex also reacts with PhC^CH regiospecifically to give an analogous product (Scheme 78). The reaction of the cyclometalated imine complexes 517 with PhC^CPh produces monoinsertion products 518, whereas the reaction of complex 517 (M ¼ Ir) with PhC^CH produces a di-insertion product 519.315

Scheme 78 Syntheses of the complexes 515 and 517–519.

The ligand 4,4-dimethyl-2-oxazolinylbenzene is easily cyclometalated by (Cp IrCl2)2 in the presence of sodium acetate. The resultant complex 520 dissolves in acetonitrile in the presence of KPF6 to give an acetonitrile-coordinated cationic complex 521 (Fig. 75). This labile cationic complex 521 undergoes insertion reactions with internal and terminal alkynes. Internal alkynes give only monoinsertion products 522, whereas terminal alkynes give mono- (523) or di-insertion (524) products. The cations will also react with CO, but no insertion occurs in this case (525).316 The similar reactions of the unsaturated molecules inserting the M-C bonds (M ¼ Rh, Ir) of the cyclometalated complexes also resulted in the seven-membered metallacycles.317

Fig. 75 Structure of the complexes 520–525.

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127

A detailed investigation of the mechanism of the iridium and rhodium-catalyzed oxidative annulation of isoquinolones and alkynes was reported by Wang and coworkers. In the presence of NaOAc and Et3N at room temperature in CH2Cl2, the isoquinolones 526 were treated with (Cp MPyCl2) (Py ¼ pyridine, prepared in situ from (Cp MCl2)2 and pyridine) to obtain the metallacycles 527 with coordinated pyridine groups in moderate-to-excellent yields (Scheme 79). The alkyne-insertion complexes 528–530 were formed in good-to-excellent yields from the reactions between cyclometalated complexes 527 and diphenyl acetylene or 1-phenyl-1-propyne for both rhodium and iridium complexes. Two types of molecular structures were found for these compounds. The structures of complexes 528 and 530 contain a seven-membered metallocycle with the formation of M-C(alkyne), M-N, and M-O bonds. The structures of complexes 529 also contain a seven-membered metallocycle with the formation of M-C(alkyne) and M-N bonds, but there is no M-O bond and the phenyl group of the isoquinolone moiety has Z2-coordinated with the metal.318

Scheme 79 Syntheses of the complexes 527–530.

8.02.5.2.4

C^C five-membered metallacycles

Treatment of the imidazolium salt 531 with (Cp IrCl2)2 in the presence of potassium tert-butoxide generates 532 (Scheme 80), which is a precursor to a catalyst that can oxidize water to dioxygen.319

Scheme 80 Synthetic route to the complex 532.

The imidazolium pyridinium salt 533 reacted with (Cp IrCl2)2 in refluxing acetonitrile in the presence of Cs2CO3 to form the corresponding 535, wherein the pyridylidene coordinates to the metal through the para-carbon atom (Scheme 81). For the reaction with (Cp RhCl2)2, a mixture of complexes 536–538 was obtained. For the reactions carried out with the salt 534, in which the C2 position of the imidazolium is blocked by a methyl group, the reaction with (Cp IrCl2)2 led to the formation of complex 539, in which the chelate ligand is coordinated through the pyridylidene and a methylene group resulting from the CdH activation of the Me group at the C2 of the imidazolium ring. For the reaction carried out with (Cp RhCl2)2, a high-yield complex 540 was obtained, resulting from the reductive coupling between the Cp and the pyridinium rings, and the imidazolium ring is bound to the metal as

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 81 Syntheses of the complexes 535–540, 542, and 543.

a result of the activation of the Me group at C2. In the presence of Cs2CO3 and KI, 541 reacted with (Cp IrCl2)2 in refluxing acetonitrile to form complex 542 and a small amount of bimetallic species 543.320 Complex 545 was readily available from the corresponding 1,3-dimethyl-4-phenyl-1,2,3-triazolium salt 544 via Ag2O-mediated proton abstraction and in situ metallation with (Cp IrCl2)2 in a one-pot procedure (Scheme 82). Under basic conditions, the

Scheme 82 Syntheses of the complexes 545, 546, and 548.

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129

C-bound phenyl group is readily cyclometalated to form complex 546, while under acidic conditions, cyclometalation is reversed.321 Transmetalation of a 1,4-diphenyl-substituted 1,2,3-triazolylidene silver complex 547 with an electrophilic metal center Ir(III) or Rh(III) induces spontaneous and chemoselective cyclometallation (548) involving CdH bond activation of the N-bound phenyl group exclusively.322 Sulfur-based chiral triazolium salts are adequate chiral auxiliaries for the preparation of enantiopure chiral-at-metal Ir(III) and Rh(III) half-sandwich complexes. Enantiopure five-membered metallacycles (549–551) were obtained completely diastereoselectively through aromatic CdH activation (Fig. 76). The formation of six-membered metallacycles (552) was considerably less diastereoselective; however, the resultant diastereoisomers of the metal complexes were separable by column chromatography, which allowed access to both enantiomers at the metal center with reasonable yields of the isolated product. Both the formation of cationic Ir(III) complexes (553) and the insertion of alkynes into the Ir(III)dC bond (554) occurred with excellent yields and complete retention of configuration at the metal center.323

Fig. 76 Structure of the compounds 549–554.

Metalization of pyridinium-functionalized triazolium salt 555 with (Cp IrCl2)2 induced double CdH bond activation to give the C,C-bidentate complexes 556 and 557 (Scheme 83). Complex 556 comprises two different abnormally bound N-heterocyclic carbene ligands, that is, a triazolylidene and a 3-pyridylidene, while complex 557 features a rare ylide bonding mode of the pyridinium ligand precursor, along with the abnormal triazolylidene. Complexes 556 and 557 show excellent activity in catalytic water oxidation.324,325

Scheme 83 Synthetic route to the complexes 556 and 557.

The treatment of 2 equivalent of naphthalimide imidazolium salt with (Cp IrCl2)2 in the presence of Ag2O and NBu4I in CH3CN for overnight reflux afforded in a one-pot reaction the iridium carbene complexes (Cp Ir(NI-NHC)I) (558 and 559) in a 1:1 ratio (Fig. 77). The optically active compounds S-558, R-558, S-559, and R-559 were obtained by resolving the racemic samples using chiral column chromatography techniques.326

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Fig. 77 Structure of the complexes 558 and 559.

The reaction of 1-tert-butyl-3-methylimidazolium iodide 560 with Ag2O in CH2Cl2 and the further addition of (Cp IrCl2)2 provided complex 561, in which the cyclometallation of the ligand through one of the methyl groups of the tert-butyl group has been produced (Scheme 84). The reaction of 1-benzyl-3-tert-butylimidazolium chloride 562 with Ag2O and further addition of (Cp IrCl2)2 selectively proceeded to the cyclometalated species 563, without aromatic CdH activation.327

Scheme 84 Syntheses of the complexes 561 and 563.

Reaction of 564 with MeONa (1 equivalent) gave the cyclometalated carbene complex 565 in 99% yield by an intramolecular CdH bond activation reaction (Scheme 85).328 The reaction of 564 with 2 equivalents of AgOTf followed by addition of acetonitrile gave the non-cyclometalated complex 566.329 Unstable cyclometalated complex 567 was prepared by the reaction of the dicationic complex 566 with MeONa or NEt3. In the presence of acetonitrile, the reaction of 565 with AgOTf also gave 567.

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Scheme 85 Syntheses of the complexes 565–567.

For the reactions of (Cp IrCl2)2 and the hydroxyethyl-substituted azolium salts 568 (X ¼ CH), the NHC-acyl cyclometalated species 569 (X ¼ CH) were afforded in the presence of Cs2CO3 (Scheme 86). In the reaction with (Cp IrCl2)2, the use of a triazolium salt 568 (X ¼ N) with weaker electron donating properties can also isolate the similar cyclometalated complex 569 (X ¼ N). All these reactions illustrate that in the presence of a weak base, N-hydroxyethyl group has a high tendency to undergo oxidation and cyclometalation.330

Scheme 86 Syntheses of the complexes 569.

The binuclear species (Cp IrCl2)2 reacts with terminal alkynes in the presence of methanol to afford iridium methoxy-carbene complexes 570 (Scheme 87). However, the reaction of (Cp IrCl2)2 with terminal alkynes (R1C^CH) and anilines (R2C6H4NH2) resulted in a variety of orthometalated iridium amino-carbene complexes 571 of anilines and terminal alkynes, with yields ranging from 30% to 80%.331 In contrast, the reaction of (Cp RhCl2)2 with an aniline (R2NH2) and a terminal alkyne (R1C^CH) afforded the N-containing cyclometalated rhodium complexes Cp Rh(Cl)(N(R2)]C(CH2R1)CH]CR1) 572 via a hydroamination and a 1,2-insertion of an alkyne.332

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 87 Syntheses of the complexes 570–572.

The complex 574, with the chelating NHC ligand 573, was synthesized via the transmetalation route with Ag2O (Scheme 88). The silver complex was first generated by the reaction of 573 with Ag2O in the presence of a chloride source in the dark. The thus generated silver complex was reacted with the chloro-bridged dimeric metal precursors. Salt metathesis with KPF6 resulted in the generation of the metal complexes in good yields.76

Scheme 88 Synthetic route to the complex 574.

Enantiopure bimetallic systems (such as 576 and 578) containing three different elements of chirality, namely a main-groupbased chiral center (sulfur), a transition-metal chiral center (rhodium or iridium), and a planar chiral element (ferrocene or ruthenocene), have been prepared by a sequence of diastereoselective reactions (Scheme 89). The chirality of the chiral sulfur center attached to C-5 of a 1,2,3-triazolylidene mesoionic carbene ligand coordinated to a metal (Ir, Rh) was transferred through the formation of bimetallic complexes having a chiral-at-metal center and a planar chiral metallocene by CdH activation of the sandwich moiety (M ¼ Fe, Ru). The sense of the planar chirality formed in this sequence of reactions depended on the nature of the ligands at the metal center of the starting complex (such as 575 and 577).333

Half-Sandwich Rhodium and Iridium Complexes

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Scheme 89 Syntheses of the complexes 576 and 578.

Reactions of dilithium salt of 1,1-bis-o-carborane with half-sandwich transition metal complexes (Cp MCl2)2 (M ¼ Ir, Rh) in THF at room temperature afforded the C,C-chelated mononuclear metal complexes 579 in moderate yields (Fig. 78). They are 16-electron “pseudo-aromatic” bis-o-carborane iridium and rhodium complexes, and can be converted to the saturated 18-electron complex (580) in the presence of small two-electron donor.334

Fig. 78 Structure of the complexes 579 and 580.

8.02.5.2.5

C^C six- or seven- membered metallacycles

Half-sandwich pseudo-octahedral pentamethylcyclopentadienyl Ir(III) complexes 581 were synthesized and characterized (Fig. 79).335,336 Except for complex 581 (CpR ¼ Cp , R1 ¼ CH3), the other 11 complexes all showed potent cytotoxicity, with IC50 values ranging from 2.9 to 46.3 mM toward HeLa human cervical cancer cells. The potency toward HeLa cells increased with additional phenyl substitution on Cp : Cpxbiph > Cpxph > Cp , and increased with the size of chain substitution on the C^C-ligand in the order: ph > butyl > ethyl > methyl. Complex 581 (CpR ¼ Cpxbiph, R1 ¼ Ph) displayed the highest potency, and was about 3 times more active than the clinical platinum drug cisplatin. The subsequently reported complexes 582 and 583 also showed potent antitumor activity, with IC50 values ranged from 3.9 to 11.8 mM against A549 cells.337 Analogs of complex 582 have also been synthesized by Peris et al.327

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Fig. 79 Structure of the complexes 581–583.

The reaction of (S,S)-1,3-di(methylbenzyl)imidazolium chloride with (Cp IrCl2)2 in acetonitrile in the presence of NaOAc afforded compound 584 in high yield (63%, Scheme 90). The coordination of the NHC ligand, together with the orthometalation of the phenyl ring, provides the chelating coordination of the ligand. The crystal structure of 584 confirms that only one enantiomer (SIr,SC,SC)-584 is present in the crystal. The complex has been used in the catalytic diboration of olefins, providing high efficiencies and chemoselectivities on the organodiboronate products.338

Scheme 90 Synthetic route to the complex 584.

N-heterocyclic bis-carbene ligand (bis-NHC) which was derived from 1,10 -diisopropyl-3,30 -ethylenediimidazolium dibromide via silver carbene transfer method, reacted with (Cp MCl2)2 (M ¼ Rh, Ir) afforded complexes 585 (M ¼ Rh) and 586 (Fig. 80). When (Cp IrCl2)2 was treated with 2 equiv. AgOTf at first, and then reacted with bis-NHC ligand, 585 (M ¼ Ir) was obtained.339

Fig. 80 Structure of the complexes 585 and 586.

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135

The metalation of a series of C2-Me-substituted monoimidazolium and bisimidazolium salts to (Cp IrCl2)2 was studied (Scheme 91). The reaction of the monoimidazolium salt provides the species Cp Ir(NHC)Cl2 587, in which the NHC shows an abnormal coordination mode. The use of the bisimidazolium salt provides different reaction patterns depending on the linker length between the two azolium rings. For the methylene-linked bisimidazolium salt, the only compound 589 obtained shows an unusual type of coordination in which the chelating ligand is coordinated through an abnormal NHC and a methylene group resulting from the CdH activation of the C2dMe group. For the ethylene-linked bisimidazolium salt, a similar product 590 is obtained, together with the chelating bis-abnormal-NHC species 591 and a neutral species 592 with a 1,2-dimethylimidazol ligand. In addition, the complex 588 coordinated by both the abnormal and normal modes chelating dicarbene ligand was also synthesized by 1,10 -ethylene-2,3,30 -trimethylbis(1H-imidazolium) dibromide.340

Scheme 91 Synthetic routes to the complexes 587–592.

8.02.5.3

Multidentate ligands

The cyclometalated complex 594 was obtained as a dark brown solid in 64% yield from the reaction of the chloro complex 593 with sodium hydroxide in acetonitrile at room temperature (Scheme 92).341

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 92 Synthetic route to the complex 594.

The purple-colored 16-electron Ir amide complex 595 in CF3CH2OH as solvent transformed into a yellow-colored cyclometalated complex Cp Ir(k3(N,N0 ,C)-(S,S)-CH3C6H3SO2NCHPhCHPhNH2) 596 at 50  C, as a single diastereomer almost quantitatively (Scheme 93). The complex 596 can also be conveniently obtained by reaction of Cp IrCl(k2(N,N0 )-(S,S)-CH3C6H 3SO2NCHPhCHPhNH2) 597 with C6H5ONa in good yield. Similarly, the complex 598 can be obtained by the reaction of the complex 493 with C6H5ONa. The Ir alkoxide complex without NH protons in the diamine ligand, Cp Ir(OCH2CF3)(k2(N,N0 )CH3C6H3SO2NCH2CH2N(CH3)2) 599, was separated from the reaction of the Ir chloride complex, Cp IrCl(k2(N,N0 )CH3C6H3SO2NCH2CH2N(CH3)2), with CF3CH2OH containing the base KOtBu. And the separated Ir alkoxide complex 599 underwent intramolecular cyclometalation at 50  C, leading to the metallacycle product Cp Ir(k3(N,N0 ,C)-CH3C6H3SO2NCH2 CH2N(CH3)2) 600.342

Scheme 93 Synthetic routes to the complexes 596, 598, and 600.

The reaction of compound 601 with (Cp IrCl2)2 (1 equivalent) and AgOTf (4.0 equivalent) led to the formation of complex 602 in the THF solution (Scheme 94). (Cp MCl2)2 (M ¼ Ir, Rh), AgOTf and nBuLi were used to treat (2-pyridine)(o-carboranyl) methanol ligand 601 in THF, which resulted in the formation of complexes 603. Ligand 604 was treated with the full equivalent of (Cp IrCl2)2 dimer, which led to the generation of complex 605 via dual CdH and BdH activation.343

Half-Sandwich Rhodium and Iridium Complexes

137

Scheme 94 Syntheses of the complexes 602, 603, and 605.

In the presence of Ag2O in CH2Cl2, the reaction of compound 606 with (Cp IrCl2)2 yielded Ir(III)-NHC complex 608 (Scheme 95). In MeCN, 606 reacted with excess Ag2O to form carborane cyclometalated 607, and the closo-dicarbadodecaborane anion was coordinated with the metal through the boron atom. Ligand precursor 609 reacted with (Cp IrCl2)2 in CH2Cl2 to obtain

Scheme 95 Syntheses of the complexes 607, 608, and 610–612.

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Half-Sandwich Rhodium and Iridium Complexes

Cp Ir(NHC)Cl2 complex 610 with excellent yield. Similar to complex 608, the Ir complex 610 underwent cyclometallation upon reaction with Ag2O in MeCN to obtain a mixture of Ccarborane- and Bcarborane-cyclometalated complexes 611 and 612.344 Iridium(III)-induced selective B(2,3)dH and CdH bond activations at mono-p-methoxybenzeneazo-substituted m-carborane have been investigated (Scheme 96). Mononuclear, dinuclear and trinuclear complexes (614–616) featuring m-carboranyl ligands have been prepared in single- or multi-step procedures. The experimental results highlight that the base employed in the reaction plays a key role in the formation and the structures of the complexes.345 Iridium(III)-induced selective B-H/C-H bond activations on p-methoxyphenylazo substituted o- and p-carboranes were also studied by Jin and coworkers.346

Scheme 96 Syntheses of the complexes 614–616.

The reaction of the benzene derived trisimidazolium salt 617 (X− ¼ PF−6) with (Cp MCl2)2 (M ¼ Ir, Rh) resulted in the formation of dinuclear M(III) complexes 618 where each metal center is coordinated by an NHC donor and orthometallates the central phenyl ring (Scheme 97). The more reactive salt 617 (X− ¼ Br−) reacts with (Cp RhCl2)2 to give initially the dimetalated complex 618 (M ¼ Rh) which upon reaction with Ag2O followed by addition of 0.5 equivalent (Cp RhCl2)2 yields the trinuclear triply orthometalated neutral complex 619 in 53% yield.347 Hahn and coworkers subsequently used the unsymmetrical tris (imidazolium) salt 620 featuring a 1,2,4-substitution pattern of the central phenyl ring. It reacted with Pd(OAc)2 and (Cp MCl2)2 (M ¼ Ir, Rh) in a one-pot reaction to obtain heterobimetallic complexes 621, in which the Pd(II) ion is chelated by two ortho-position N-heterocyclic carbene (NHC) donors while the third NHC donor coordinates to the M(III) center, which orthometalates the central phenyl ring.348

Half-Sandwich Rhodium and Iridium Complexes

139

Scheme 97 Syntheses of the complexes 618, 619, and 621.

The N-alkylation of monometallic di-NHC complexes 622 (Fig. 81) at the unsubstituted imidazolato ring-nitrogen atom of one of the diazaheterocycles is an efficient and useful way to obtain polymetallic complexes in single- or multi-step procedures. This methodology has been used for the preparation of the dinuclear species (623 and 624), as well as for the trimetallic complex 625. Since 623 (M1/M2 ¼ Rh/Ir) cannot be accessed by direct site-selective metallation of a tetrakisazolium salt, the synthesis of the mixed Rh(III)/Ir(III) NHC complex 623 (M1/M2 ¼ Rh/Ir) is ingenious.349 The development of cyclometalated rhodium and iridium complexes from naphthaldimine-based poly(propyleneimine) dendrimer scaffolds of the type, DAB-(NH2)n (n ¼ 4, 8, DAB ¼ diaminobutane) has been accomplished. Four metallodendrimers 627 and 628 (Fig. 82) were synthesized by first reacting DAB-(NH2)n with napththaldehyde and subsequently metallating the Schiff-base dendrimers with the dimers (Cp MCl2)2 (M ¼ Rh, Ir). Related mononuclear complexes 626 were obtained in a similar manner. The in vitro anticancer activities of 626–628 were evaluated against the A2780 and A2780cisR human ovarian carcinoma cell lines.350

8.02.6

Half-sandwich Rh/Ir complexes bearing hydrogen, borane and metal groups

8.02.6.1

Half-sandwich Rh/Ir hydride complexes

In the process of iridium aqua complexes (Cp Ir(L)(OH2))2+ (L ¼ 2,20 -bipyridine, 4,40 -dimethoxy-2,20 -bipyridine) catalyzing the hydrogenation of CO2 into HCOOH at pH 3.0 in H2O, the active hydride catalysts 629 and 630 can be successfully isolated (Fig. 83).351 The hydrides similar to 629 and 630 have also been described and studied by many research groups.22,24,25,28,39,352–357 When studying the interaction between NADH and (CpRIr(L)(OH2))2+ (CpR ¼ Cp , Cpxph; L ¼ 1,10-phenanthroline) by 1H NMR spectroscopy, iridium hydride complexes 631 can be observed.52

Fig. 81 Structure of the complexes 622–625.

Fig. 82 Structure of the complexes 626–628.

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141

Fig. 83 Structure of the complexes 629–631.

The parent metal-chloride complexes are treated with sodium borohydride or sodium formate to obtain hydrides 632 or 633 (634), respectively, in good yields. In order to obtain the complex 635, the starting IrdCl compound was first reduced with sodium borohydride under basic conditions, and then protonated by reaction with hydrogen chloride (Scheme 98).358

Scheme 98 Syntheses of the complexes 632–635.

The formation of hydride 636 was observed by monitoring the reaction between the parent iridium chloride complex and NADH by 1H NMR (Fig. 84).359 Hydrides similar to 636 have also been described and studied.360

Fig. 84 Structure of the complex 636.

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Half-Sandwich Rhodium and Iridium Complexes

The corresponding hydrido(amine) complex 637 can be obtained quantitatively by treating the amido complex with formic acid in [D8]THF. This has been confirmed by NMR experiments, and a single crystal structure of 637 was also obtained.102 Treatment of hydroxido-amine complex in 2-propanol at ambient temperature resulted in the smooth formation of a hydrido complex 638 in an almost quantitative yield (Scheme 99).361

Scheme 99 Synthetic routes to the complexes 637 and 638.

Using ReddAl (sodium bis(2-methoxyethoxy)aluminum hydride) with good solubility in THF to treat the parent iridiumchloride complexes at 50  C for 4–6 h can cleanly obtain hydrides 639 (Scheme 100). In comparison, the parent rhodium-chloride complexes were completely converted into rhodium hydrides 640 in THF using NaBH4 at 65  C for 2 days.362

Scheme 100 Syntheses of the complexes 639 and 640.

The stoichiometric reaction of 387a with tetra-n-butylammonium formate in CH3CN at room temperature gave the corresponding solution of hydride complex 641 (Scheme 101). Alternatively, by reacting 387a with sodium formate in in a biphasic DCM-water mixture with tetra-n-butylammonium formate as a phase transfer catalyst, a pure solid form of 641 could be obtained. Similarly, the hydride 642 could also be isolated as a pure solid.363

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143

Scheme 101 Synthetic route to the complex 641.

The chloride-oxime complex 643 underwent facile dehydrochlorination with an equimolar amount of a base to afford the oximato-bridged dinuclear complexes 644 (Scheme 102). Complex 644a reacted with an amine hydrochloride to regenerate 643a as expected. In contrast, dehydrochlorination of 643 in a hydrogen donor solvent, 2-propanol, resulted in the formation of the hydrido-bridged oxime-oximato complexes 645. Complex 645a could also be obtained cleanly by transfer hydrogenation of the oximato-bridged dinuclear complex 644a with 2-propanol at 50  C.364

Scheme 102 Syntheses of the complexes 644 and 645.

The reaction of complex 646 with one equivalent of base (iPrONa) in isopropyl alcohol did not result in CdH activation, instead a hydride complex 647 was obtained (Scheme 103). However, by reacting 646 and two equivalents of iPrONa in isopropyl alcohol, the cyclometalated hydrido complex 648 could be obtained in high yield via intramolecular CdH activation of the ethyl group. The complex 648 was stable in isopropyl alcohol, but it was slowly converted into dihydrido complex 649 in methanol without treatment with any other reagents. The reaction of 650 with an excess amount of base (MeONa) also gave the similar cyclometalated complex 651 in high yield. Similar to complex 646, the reaction of complexes 652 and 501 with one equivalent of i PrONa also produced corresponding hydrides similar to 647. However, when complexes 652 and 501 reacted with two equivalents of iPrONa, in addition to a small amount of chloro hydrido complexes 654, dihydrido complexes 653 were the main products.365

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 103 Syntheses of the complexes 647–649, 651, 653, and 654.

The hydride 656 was obtained by reacting bis(N-heterocyclic carbene) complex 655 and NaBH4 in isopropanol at room temperature for 2 h (Scheme 104).22

Scheme 104 Synthetic route to the complex 656.

8.02.6.2

Rhodium-metal or iridium-metal bonded complexes

A half-sandwich acetatoiridium(III) complex can activate two geminal C(sp3)dH bonds of 2-methylaniline derivatives to produce new dinuclear azametallacycles 657 (Scheme 105). The CdN chelate iridium displaying an amido-carbene structure favors formation of the saturated 34-electron complex via p-coordination to the other Cp Ir fragment.366

Half-Sandwich Rhodium and Iridium Complexes

145

Scheme 105 Syntheses of the complexes 657.

Visible light irradiation of Cp Ir(CO)2 332 in pentafluorobenzontrile resulted in the formation of the two isomeric diiridium(II) complexes (Cp Ir(m-CO)(C6F4CN))2 658 and (Cp Ir(CO)(C6F4CN))2 659 (Scheme 106). And under visible light irradiation, the complex 659 is isomerized to 658. The complex 332 reacted with hexafluorobenzene under UV irradiation to obtain (Cp Ir(CO) (C6F5))2 660 and Cp Ir(CO)(Z2-C6F6) 661 in a ratio of 1/5.367

Scheme 106 Syntheses of the complexes 658–661.

The sulfonylimido-bridged diiridium complex (Cp Ir(m2-NTs)2IrCp ) (662; Ts ¼ p-CH3C6H4SO2), readily accessible from the reaction of (Cp IrCl2)2 with TsNH2, reacted with P(CH3)3 and HOTf (Tf ¼ CF3SO2) to afford the adduct 663 and cationic amidoimido complex 664 (Scheme 107), respectively. On the other hand, the reaction of 662 with benzoic acid resulted in intramolecular CdH bond activation, giving the cyclometalated complex 665.368

146

Half-Sandwich Rhodium and Iridium Complexes

Scheme 107 Syntheses of the complexes 662–665.

The synthesis and structural characterization of a photochromic dirhodium dithionite complex: (CpxphRh)2(m-CH2)2 (m-O2SSO2) (Cpxph ¼ C5Me4Ph) was reported by Isobe group. The reaction of trans- (CpxphRh)2(m-CH2)2Cl2 with Na2S2O4 in MeOH under N2 in the dark led to the formation of the dithionite complex 666. The crystal of complex 666 consists of two independent dithionite complexes, 666-cis and 666-trans (Fig. 85).369,370 The photoisomerization of 667 to 668 proceeds without degradation of the single-crystal form (Scheme 108), though complex 667 has a considerable bulky ligand of Cpxbn (C5Me4Bn) ring which can not rotate in the crystal.371 The rhodium dinuclear complex with two Cp and photoreactive dithionite (m-O2SSO2) ligands (Cp Rh)2(m-CH2)2(m-O2SSO2) (669), shows intriguing photochromic performance, such as reversible 100% photo- and thermo-chemical conversions in the crystalline-state.

Fig. 85 Structure of the complex 666.

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147

Scheme 108 Syntheses of the complexes 668 and 670.

The reaction of Cp Ir(CO)2 with Ru3(CO)12 under a hydrogen atmosphere afforded the heterometallic clusters Cp IrRu3 (m-H)2(CO)10 671 and a small amount of tetrahydrido cluster Cp IrRu3(m-H)4(CO)9 672, which can be obtained in high yields by the hydrogenation of 671. The reaction of Os3(m-H)2(CO)10 with Cp Ir(CO)2 afforded the osmium analogue Cp IrOs3 (m-H)2(CO)10 673 in 70% yield, along with a trace amount of the pentanuclear cluster Cp IrOs4(m-H)2(CO)13 674 (Scheme 109). Solution-state NMR studies show that the hydrides in the iridium-ruthenium clusters are highly fluxional even at low temperatures while those in the iridium-osmium clusters are less so.372

Scheme 109 Synthetic routes to the complexes 671–674.

m3-Silyl complexes (Cp Ru)2(Cp M)(m3-H2SiR)(m-H)3 (677, M ¼ Rh; 678, M ¼ Ir) were synthesized by the reaction of trinuclear heterometallic clusters of Ru and group 9 metals, (Cp Ru)2(Cp M)(m-H)3(m3-H) (675, M ¼ Rh; 676, M ¼ Ir), with primary silanes (Scheme 110). The m3-silyl complex 678b was transformed into the m-silyl complex (Cp Ru)2(Cp Ir)(m-H2SiPh)(tBuNC)(m-H)3 679 upon treatment with tBuNC. Complex 678b also reacted with CO to afford the m3-silylene complex (Cp Ru)2(Cp Ir)(m3-HSiPh) (m-CO)(m-H)2 680.373

148

Half-Sandwich Rhodium and Iridium Complexes

Scheme 110 Syntheses of the complexes 677–680.

The trigonal bipyramidal clusters M2Ir3(m-CO)3(CO)6Cp2CpR (681, CpR ¼ Cp , Cpx (Cpx ¼ C5Me4H); M ¼ Mo, W) reacted with isocyanides to give ligand substitution products 682 (Scheme 111), in which core geometry and metal atom locations are maintained, whereas reactions with PPh3 afforded 683, with retention of core geometry but with effective site-exchange of the precursors’ apical Mo/W with an equatorial Ir. Similar treatment of trigonal bipyramidal MIr4(m-CO)3(CO)7CpCp (686, M ¼ Mo, W) with PPh3 afforded the mono-substitution products 687, and further reaction of the molybdenum example 687a with excess PPh3 afforded the bis-substituted cluster 688. Reaction of 681a with diphenylacetylene proceeded with alkyne coordination and C^C cleavage, affording 684 together with an isomer 685. In addition, the reaction between clusters 686 and alkynes was also investigated.374 Humphrey et al. also used 681a to react with CpxIr(CO)2 to provide a four-valence-electron-deficient butterfly cluster 689 (Scheme 112). Reaction of 686a with [N(PPh3)2][Ir(CO)4] afforded the capped octahedral cluster 690, which possesses three semi-face-capping CO ligands.375

Half-Sandwich Rhodium and Iridium Complexes

Scheme 111 Syntheses of the complexes 682–685, 687, and 688.

149

150

Half-Sandwich Rhodium and Iridium Complexes

Scheme 112 Syntheses of the complexes 689 and 690.

The syntheses of rhodium(III) and iridium(III) half-sandwich complexes containing tertiary arsine and stibine ligands of the form Cp M(L)Cl2 (M ¼ Rh, Ir; L ¼ AsEt3, AsPh3, SbPh3) 691 are reported in Scheme 113.376 These compounds represent infrequent examples of rhodium and iridium metal complexes bearing arsenic or antimony ligands.

Scheme 113 Syntheses of a series of compounds 691.

Coordination of SnCl−3 ligands to (Cp MCl2)2 (M ¼ Rh, Ir) has been reported by the Abramov group. The structure of (Cp M(SnCl3)3)− (692) anion is shown in Fig. 86. The complexes (Cp MCl2)2 (M ¼ Rh and Ir) react with SnCl2 2H2O in

Fig. 86 Structure of the complexes 692–694.

Half-Sandwich Rhodium and Iridium Complexes

151

CH2Cl2/CH3OH with the formation of (Cp MCl3-n(SnCl3)n]2− species (n ¼ 1, 2, 3). Despite the presence of several species in the solution only a 1:3 complex [Cp 2Rh2(m-Cl)3][Cp Rh(SnCl3)3] was crystallized and characterized by x-ray diffraction. Analogous iridium complex was crystallized with iridicinium cation as [Cp 2Ir][Cp Ir(SnCl3)3].377 Earlier, Abramov et al. also reported two complexes 693 and 694. The crystal structure of 693 and 694, obtained in the reaction of [CpxetRh(C6H6)](PF6)2 (Cpxet ¼ C5Me4Et) with ZnSe in 4M HCl under hydrothermal conditions, was determined.378

8.02.6.3

Metallaborane complexes based on half-sandwich rhodium and iridium

Free borylenes are highly reactive hypovalent chemical species that can only be generated as an intermediate under drastic conditions. Stabilization of borylenes into the coordination sphere of transition metals is well developed and systematically studied by Braunschweig et al.,379–381 Aldridge et al.382,383 and Ghosh et al.384 Recently, a series of homometallic and heterometallic triply bridging borylene complexes that contain a “parent” borylene ligand (BH) have been reported by the Ghosh group.385 Thermolysis of nido-695 in the presence of [M’(CO)5
THF] (M’]Mo and W) at 60  C led to the formation of trimetallic triply bridging borylene complexes of [(Cp Rh)2(m3-BH)(m-CO)M’(CO)5}] (696: M’]Mo; 697: M’]W; Scheme 114). Alternatively, the geometry of 696 and 697 can be viewed as a tetrahedral structure with three metals and one BH ligand. Unlike [Mo(CO)5
THF] or [W(CO)5
THF], treatment of nido-695 with [Cr(CO)5
THF] gave 698 as a red solid (Scheme 114). The geometry of 698 can be viewed as a condensed polyhedron composed of a tetrahedron ([Rh2Cr(m3-BH)]) and a square-pyramidal core ([Rh2CrB2]).

Scheme 114 Syntheses of the complexes 696–698.

The room-temperature reaction of nido-695 with excess of [Fe2(CO)9] led to the formation of [(Cp Rh)2Fe(CO)3(m3-CO) B3H2Cl] (699) and metallaborane [(Cp Rh)2Fe(CO)3Fe(CO)2(m-CO)2B2H2] (700) (Scheme 115A).386 The equatorial plane of 700 consists of two iron and two boron atoms and the axial positions are occupied by two rhodium atoms. And the reaction of nido695 with [Fe2(CO)9] at moderate temperature led to the isolation of [(Cp Rh)3Fe(CO)2(m3-CO)2B2HX] 701 (X ¼ H) and 702 (X ¼ Cl), [(Cp Rh)3(RhCO)3Fe(CO)3(m-CO)3B3H2] (703) as well as the formation of 700 (Scheme 115A).386 The solid-state X-ray diffraction structures of 701 and 702 can be viewed as cubane clusters having 62 cluster valence electrons with five metal–metal bonds.387 Likewise, the reaction of nido-695 with [Mn2(CO)10] at 90  C led to the formation of 704 and 705 (Scheme 115B). Both complex 703 and [(Cp Rh)3Rh(CO)2-{Mn(CO)3}2B4H3] (704) can be seen as tricapped trigonal prisms. In 703, six metal atoms occupy the corners of the trigonal prism array and two borons and one metal atom cap the square faces of the trigonal prism geometry. In 704, all the six metal atoms occupy the corners of the trigonal prism with three boron atoms capping the three square faces of the trigonal prism geometry. The molecular structure of 705, can be viewed as having a trigonal planar geometry with 48 cluster valence electrons.

152

Half-Sandwich Rhodium and Iridium Complexes

(A)

(B)

Scheme 115 Syntheses of the complexes 699–705.

The reaction of nido-695 with [M(CO)5.THF] (M ¼ Mo or W) yielded the trimetallic metallaborane clusters [(Cp Rh)2 M(CO)3(m-CO)(m3-BH)(B2H4)] (706: M ¼ Mo; 707: M ¼ W) having a capped borylene fragment and trimetallic triply bridging borylene complexes [(Cp Rh)2(m3-BH)(m-CO)M(CO)5] (708: M ¼ Mo; 709: M ¼ W) (Scheme 116). The chemistry of trimetallic triply bridging borylene complexes (708 and 709) were explored with Lewis bases such as tert-butyl isocyanide ligands. Photolysis of 708 and 709 with tert-butyl isocyanide yielded [(Cp Rh)2(m3-BH)(m-CO)M(CO)4(CN-tBu)] (710: M ¼ Mo and W) (Scheme 116).388

Scheme 116 Syntheses of the complexes 706–710.

The nido-695 was also explored with the 2-mercaptobenzothiazole (2-mbtz) and 2-mercaptobenzoxazole (2-mboz) ligands. The reaction occurs in a more straightforward way to generate the intermediate [(Cp Rh)2B3H6L] (711; L]C7H4NS2) (Scheme 117). Compound 711, upon reaction with one molecule of 2-mbtz, yielded [Cp RhBH(L)2], 712 (L]C7H4NS2) (Scheme 117). The

Half-Sandwich Rhodium and Iridium Complexes

153

Scheme 117 Syntheses of the complexes 711–713.

molecular structure of 712 shows the presence of a direct metal-boron bond, which is further supported by two benzothiazolyl bridges.389 Interestingly, the reaction of 711 with excess 2-mbtz ligand, which could isolate a highly polar compound 713 in moderate yield (Scheme 117). The thermolysis of nido-695 with S or Se powder resulted in the formation of [(Cp Rh)2(m-E)2-(m3-E)4B2H2] (714a: E ¼ S; 714b: E ¼ Se) (Scheme 118). The molecular geometries of 714a and 714b show an open-core structure derived from a [Rh2E4B2] cubane with two elongated B–E edges bridged by one E atom, a geometry that is analogous to the classic organic cage hydrocarbon 1,3-bishomocubane.390

Scheme 118 Syntheses of the complexes 714.

The methodologies to the iridium system arachno-Cp IrB3H9 (715) was also studied by Ghosh group. The reaction of arachno715391 with [Fe2(CO)9] (Scheme 119A) yielded compounds 716 and 717 in moderate yields (Scheme 119A). Both the clusters [Cp Ir{Fe(CO)3}2Fe(CO)2(m-CO)2] (716) and [(Cp Ir)2-{Fe(CO)3}2(m3-CO)2] (717) can be viewed as tetrahedral clusters with 60 cluster valence electrons.386 The reaction of arachno-Cp IrB3H9 (715), with a ruthenaborane, nido-(Cp RuH)2B3H7 (718), was explored and the products were shown in Scheme 119B. Reflux of an equimolar mixture of 715 and 718 in hexane for 14 h gave modest yields of two related products, 719 and 720.392 Thermolysis of arachno-Cp IrB3H9 (715), with BH3
THF at 100  C for 18 h followed by chromatography allows the isolation of three new stable metallaboranes (Cp Ir)B5H9, 722, nido-(Cp Ir)B9H13, 723, and nido-(Cp Ir)2B8H12, 724 each in a yield of approximately 10% (Scheme 119C). The known sole product at lower temperature (Cp Ir)B4H10, 721, is also formed.391

154

Half-Sandwich Rhodium and Iridium Complexes

(A)

(B)

(C)

Scheme 119 Syntheses of the complexes 716, 717, and 719–724.

Fehlner et al. have reported the synthesis of complex 721 as starting materials.393 Scheme 120 can be used as a guide to the following sections. The reaction of [Cp IrB4H10] (721) with [{Z6-(C6H4)(CH3)2}Mo(CO)3] in tetrahydrofuran (THF) leads to [Cp Ir(CO)3(THF)MoB4H8] (725).387 However, the unstable tetrahydrofuran derivative 725 could be characterized only by NMR spectroscopy in solution. Cluster 721 also is a catalyst or catalyst precursor for the isomerization of olefins, namely, hex-1-ene to cis-hex-2-ene and tetramethyl allene to 2,4-dimethylpenta-1,3-diene. These novel results also establish that [1-Cp -2,2,2-(CO)32-(alkyne)-nido-1,2-IrMoB4H8], 726, forms from 725 and constitutes a logical precursor to 727. The stoichiometric mechanism for the generation of [1-Cp -5,6,7,8-(R)4-nido-1,5,6,7,8-IrC4B3H3], 730, from the reaction of 721 with RC^CR, R ¼ Me, Ph, has been identified. For R ¼ Me, the major product in solution is [1-Cp -5,6,7,8-(CH3)4-closo-1,5,6,7,8-IrC4B3H3Mo(CO)3], 729, which is in equilibrium with 730. And under alkyne deficient conditions, the product [1-Cp -2,2,2-(CO)3-(í -CO)-3,4-(CH3)2-closo-1,2,3,4IrMoC2B3H3], 731, can be isolated.394

Half-Sandwich Rhodium and Iridium Complexes

155

Scheme 120 Syntheses of the complexes 725–731.

As shown in Scheme 121A, reaction of [Cp RhCl2]2 with 6-fold excess of [LiBH4
THF] followed by thermolysis in presence of excess [BH3
THF] at 105  C yielded yellow 732, colorless 733 and 734, purple 735, and known [(Cp Rh)3B4H4], I.395,396 And a room temperature reaction of 732 with [Fe2(CO)9] yielded a condensed product [Fe2(CO)6(Cp Rh)2B6H10], 736 (Scheme 121B). Compound 736 displays a condensed structure of rhodaborane [(Cp Rh)2B6] core and an external {Fe2B2} unit through two common boron atoms.396 The sequential replacement of hydrogen atom(s) in polyboranes by isolobal LAu unit(s) is a typical reaction in borane chemistry. Thus, the reaction of 732 with [AuPPh3Cl] with an excess of sodium hydride was performed (Scheme 121B). After thin-layer chromatography, the reactions yielded isomer [(Cp Rh)2(AuPPh3)2B6H8] (737a, 737b).397 The room-temperature reaction of 732 with [Fe2(CO)9] in hexane yielded the 12-vertex cluster [(Cp Rh)2B6H6{Fe(CO)2}2{Fe(CO)3}2] (738) and the 7-vertex cluster [(Cp Rh)2B3H3{Fe(CO)3}2] (739). In parallel to the formation of 737 and 738, the 10-vertex cluster [(Cp Rh)2B6H10{Fe(CO)3}2] (736) has been isolated in good yield.396 Interestingly, a non-boron cluster, found to be a tetrahedral heterometallic carbonyl cluster, [(Cp Rh)2{Fe-(CO)3}2(m3-CO)2] (740), has been isolated by a cluster degradation reaction.397 A room temperature reaction with 732 and [Co2(CO)8] led to the isolation of the first M8-boride, [(Cp Rh)2{Co6(CO)12}(m-H)(BH) B], 741 (Scheme 121C).398 The reaction complex 742 with [AuPPh3Cl] under an excess of sodium hydride resulted in the [(Cp Rh)2(AuPPh3)2B8H10] (743) (Scheme 121D).397

156

Half-Sandwich Rhodium and Iridium Complexes

(A)

(B)

(C)

(D)

Scheme 121 Syntheses of the complexes 732–741 and 743.

The reaction of 732 and [Cp IrH4] (744) afforded an extremely polar yellow solid [(Cp Ir)2H3][B(OH)4], (745) (Scheme 122). The molecule in its asymmetric unit contains two units, [Cp Ir]2 and [B(OH)4].399 The reaction of 732 with Li[BH2Se3] afforded the yellow complex 746 (Scheme 123).400

Half-Sandwich Rhodium and Iridium Complexes

157

Scheme 122 Synthetic route to the complex 745.

Scheme 123 Synthetic route to the complex 746.

A proposed intermediate [(Cp Rh)(m6-B) {Ru(CO)3}4{RuH(CO)2}], 747 can obtained from the reaction of [Cp RhCl2]2 and [LiBH4.THF], with [Ru3(CO)12] (Scheme 124).401

Scheme 124 Synthetic route to the complex 747.

The development of isolobal analogy discloses the similarities between electronic structures of compounds of different atomic compositions.402 Applying the same strategies, we have isolated the fused metallaborane clusters [(Cp Rh)2B4H4Rh{Cp RhB3H7}], 748, [(Cp Rh)B3H7{Ru(CO)2}(Cp RhCO)2], 749 and [(Cp Rh)2B3H3{Ru(CO)3}2], 750 in moderate yields from the reaction of [(Cp Rh)2B2H6] and [Ru3(CO)12] (Scheme 125). As [Ru(CO)3] fragment is isolobal to [Os(CO)3], we put forward our aim toward [Os3(CO)12]. The reaction yielded octahedral boride cluster [(Cp Rh)2{Os4(CO)12}(B)H], 751 along with two tetrahedral heterometallic hydrido clusters, [(Cp Rh){Os(CO)3}3(m-H)4], 752 and [(Cp Rh)2{Os(CO)3}2(m-CO) (m-CO) (m-H)2], 753 (Scheme 125).403

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 125 Syntheses of the complexes 748–753.

As shown in Scheme 126, thermolysis of [Cp IrCl2]2 with excess of [BH3
THF] in toluene for 30 h yielded [nido-5-(Cp Ir)3B7H11], 754 in good yield404 along with known [nido-3,4-(Cp Ir)2B8H12],391 [nido-3-(Cp Ir)B9H13]391 and [nido-5,7-(Cp Ir)2B8H12].405

Scheme 126 Synthetic route to the complex 754.

The formation of novel rhodaborane compounds 755–759 (Scheme 127)406 proceeds via the reaction between [Cp RhCl2]2 and excess of [LiBH4
THF] followed by thermolysis with excess of [BH3
THF] at 105  C for 5 days. In parallel to the formation of 755–759, known rhodaboranes ([(Cp Rh)2B6H10], (732)395 and [(Cp Rh)3B4H4]) have also been isolated in moderate yields.

Half-Sandwich Rhodium and Iridium Complexes

159

Scheme 127 Synthetic route to the complexes 755–759.

The 11-vertex hypho family of tricarbaboranes [2,5,12-C3B8H15][X] (X][NMe4]+ or [PPh4]+) can be also used as ligand for use in metal complexation reactions. As a result of these initial experiments, the reaction between this anion with [Cp RhCl2]2 in the presence of 1,8-bis-(dimethylamino) naphthalene, ‘protonsponge,’ yielded several compounds, one of which has been isolated and characterized as 11-vertex rhodadicarbaborane [1-Cp -4-CH3-closo-1-2,3-RhC2B8H9] (760) (Scheme 128).407

Scheme 128 Synthetic route to the complex 760.

Treatment of [Cp IrCl2]2 with Li[BH3(SePh)] (3 equiv.) in toluene led to the formation of hexaborane derivative nido-[(Cp Ir) (m-SePh)2Ir{(Cp Ir)-SePh}B4H8] (761, Scheme 129). And the room-temperature reaction of [Cp RhCl2]2 with Li[BH3(SePh)] (3 equiv.) in toluene led to the formation of arachno-[{(Cp Rh)(m-SePh)3}Rh(m-SePh)-B3H6] (complex 762) (Scheme 129).408 Reaction of (Cp IrCl2)2 with Li[BH3TePh] yielded tellurolato-bridged diiridium half sandwich complex Cp IrH(m-TePh)2HIrCp 763 along with known arachno-(Cp IrH2)B3H7 764 and Cp IrH4 744 (Scheme 130).409 And when treatment of [Cp MCl2]2 (M ¼ Rh or Ir) and [LiBH4
THF], followed by thermolysis in the presence of Se powder, yields corresponding dimetallaheteroboranes nido-[(Cp M)2B2H2Se2] (765: M ¼ Rh, 766: M ¼ Ir) in moderate yields (Scheme 131).410 The room-temperature reactions of 765 with [Fe2(CO)9] resulted in the formation of nido-[(Cp Rh)Fe(CO)3B2H2Se2], 767.

160

Half-Sandwich Rhodium and Iridium Complexes

Scheme 129 Syntheses of the complexes 761 and 762.

Scheme 130 Synthetic route to the complexes 763, 764, and 744.

Scheme 131 Syntheses of the complexes 765–767.

Half-Sandwich Rhodium and Iridium Complexes

161

Ghosh group carried out the reactivity of [Cp IrCl2]2 with Li[BH2E3] (E ¼ S or Se) that indeed led to the formation of a series of bis- and tris-homocubane analogs, 768–770. To demonstrate the uniformity of this method, they further carried out the reaction with [Cp RhCl2]2 that resulted in the formation of bishomocubanes, 771, and a trishomocubane, 772 (Scheme 132).411

Scheme 132 Syntheses of the complexes 768–772.

The pyrolysis of in situ generated [Cp2Zr(BH4)2]412 and [Cp IrB3H9]413 in the presence of excess BH3
THF yielded yellow arachno-[(Cp2Zr)(Cp Ir)B4H10] (773).414,415 The yield of the minor product arachno-[(Cp2Zr) (Cp Ir)B3H9] (774), which has a higher Rf value than that of 773, was enhanced over time (Scheme 133). Similarly, the reaction of in situ generated [Cp2Zr(BH4)2] and [(Cp Rh)2B2H6] yielded arachno-[(Cp2Zr)-(Cp Rh)B4H10] (775) (Scheme 133).415

Scheme 133 Syntheses of the complexes 773–775.

The most widely employed synthetic routes to closo- and the related pseudocloso-rhodacarboranes with ŋ5-cyclopentadienyltype ligands at the metal vertex rely on the reaction between the dimeric rhodium complexes [(ŋ5-C5R5)2Rh2Cl4] (R ¼ H or Me) and the open-faced dianions [7; 8-R’2–7; 8-nido-C2B9H9]2− (where R’ is H or bulky substituent). Treatment of A with 2.2–2.5 M excess of the K+ salts of the nido-carborane monoanions [7,8-R1,R2-7,8-nido-C2B9H10]_ ligand (a, R1 ¼ R2 ¼ H; b, R1 ¼ R2 ¼ Me; c, R1 ¼ H, R2 ¼ PhCH2; d, R1 ¼ R2 ¼ PhCH2) in solution of C6H6-EtOH (4:1) mixture for c.a. 5 days at ambient temperature afforded the desired closo-rhodacarboranes [3-Cp -1,2-R2-3,1,2-closo-RhC2B9H9] (776a-d), respectively (Scheme 134).416

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 134 Syntheses of a series of complexes 776.

The similar reaction can also take place on half-sandwich Iridium complex. The complex 777 can be obtained by the reaction of [Cp IrCl2]2 with the sodium salt of carborane anion B (Scheme 135).301

Scheme 135 Synthetic route to the complex 777.

The Jin group also developed the metallacarboranes to induce the CdC bond cleavage. Their previous study showed that the carboranylamidinates could gradually lose the B3/B6 atom to generate the nido ligand in good yields upon heating in neat methanol. Treatment of [(Cp IrCl2)2] with AgOTf in CH3OH for 6 h, followed by the addition of DcabNH as a solid at room temperature, afforded mixed sandwich metallacarboranes (778a and 778b) in good yields, respectively (Scheme 136).417 When they treat the diazo-substituted o-carborane ligand418 with [Cp IrCl2]2 in CH3OH, the complex 779 could be formed as an air-stable yellow solid with CdC bond cleavage (Scheme 137).419

Scheme 136 Syntheses of the complexes 778.

Scheme 137 Synthetic route to the complex 779.

Half-Sandwich Rhodium and Iridium Complexes

8.02.7

Half-sandwich Rh/Ir fragments in supramolecular chemistry

8.02.7.1

Supramolecular macrocycles

163

An efficient route for synthesizing air- and moisture-stable 16-electron M2L2-type metallacycles under very mild conditions has been developed.420 The binuclear metallacycles favor the binding of small ligands such as MeCN, Cl−, CO, and pyridine. Furthermore, H2 were found to be able to initiate unusual transformations between organometallic assemblies. The reaction of a coordinatively unsaturated 16-electron M2L2-type macrocyclic complex 780 featuring thione ligands with 1 atm of H2 leads to the isolation of an 18-electron M2L3-type cylinder 781, along with hydride species 782 (Scheme 138). Remarkably, the obtained mixture underwent loss of H2 in a facile manner upon heating to reform the starting M2L2-type complex.

Scheme 138 (A) Synthesis of coordinatively unsaturated 16-electron organometallic metallacycles 780 and H2-mediated transformation reactions from 16-electron to 18-electron species 781; (B) Single crystal structures of complex 780, 781 and 782. Color code: Ir, dark blue; S, yellow; N, blue; C, gray; H, white.

The 60-membered macrocyclic structures were obtained from half-sandwich Cp Rh cores and 2-(pyridin-4-yl)-1H-imidazole4,5-dicarboxylate ligand. The octadecanuclear metallamacrocycle 784 was obtained through a two-step reaction. As shown in Scheme 139, the dilithium salt of 2-(pyridin-4-yl)-1H-imidazole-4,5-dicarboxylate was added to a solution of [Cp RhCl2]2 in MeOH. The reaction mixture was then stirred with silver salt AgOTf yield orange solid 784 (83%), which is soluble in water and most common polar organic solvents.421 Moreover, a series of Cp Rh complex with different topologies was obtained when similar imidazole-dicarboxylate ligands were choose as the organic linker.422–424

164

Half-Sandwich Rhodium and Iridium Complexes

Scheme 139 (A) Synthesis of trinuclear complex 783 and octadecanuclear metallamacrocycle 784; (B) Single crystal structures of complex 783 and 784. Color code: Rh, violet; O, red; N, blue; C, gray.

A series of different type of multinuclear organometallic assemblies were constructed through stepwise coordination-driven self-assembly.425,426 Based on two similar multifunctional hydroxamate ligands pyrazine-2-hydroxamic acid and 4,40 -bipyridine2-hydroxamic acid featuring one monodentate site and two pairs of chelating sites. The Rh(III)dPd(II) heterometallic macrocycles could be constructed by using the semi-open palladium(II) source [Pd(en)Cl2] (en ¼ ethylenediamine) with free acceptor sites. Meanwhile, only one kind of macrocycle 785 was found when the shorter pyrazine-2-hydroxamic acid ligand was used, while in for the larger ligand, various spectroscopic techniques demonstrated the coexistence of hexanuclear macrocycles 786 and octanuclear macrocycles 787 in solution and the proportions of both components depended on concentration and temperature (Scheme 140).427

Half-Sandwich Rhodium and Iridium Complexes

165

Scheme 140 (A) Synthesis of heterometallic macrocycle 785 and interconversion between macrocycles 786 and 787; (B) Single crystal structures of complex 785 and 787. Color code: Rh, violet; Pd, green; O, red; N, blue; C, gray.

By using the pyrazine-2,5-dihydroxamic acid ligand featuring two pair of chelating sites together with half-sandwich iridium and rhodium fragments, complexes 788–793 were constructed (Scheme 141). Remarkably, the tetranuclear half-sandwich iridiumbased building block can be structurally regulated through the selective introduction of chloride ions to the N, N0 -bonded iridium centers, resulting in the formation of “open” structures. Such an interesting selectivity is rare for most homometallic systems in which all metal ions have the same coordination environment. Furthermore, by making use of the different chemical hardness of the two types of metal-binding sites, heterometallic macrocycle 792 was constructed. This heterometallic macrocycles is large enough to be efficient hosts for the recognition of p-donor guests. It shows that the differences in the electron-donating ability of the two kinds of chelating sites played an important role in the selective formation of these heterometallic assemblies.428

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 141 (A) Synthesis of octanuclear macrocycle 789, dodecanuclear cage 790 and host-guest adduct 793; (B) Single crystal structures of complex 790, 792 and 793. Color code: Ir, dark blue; Pd, green; O, red; S, yellow; N, blue; C, gray.

8.02.7.2

Supramolecular cages

Driven by supramolecular coordination driven self-assembly, a series of organometallic cages was obtained from the reaction of [Cp IrCl2]2 and 2,4,6-tri(4-pyridyl)-1,3,5-triazine with various bidentate ligands in the presence of AgOTf (Fig. 87).429–431 These metallaprisms have shown a good ability to encapsulate planar electron-donor aromatic molecules such as pyrene, coronene, [Pt(acac)2] (acac ¼ acetylacetone), and hexamethoxytriphenylene as shown in Fig. 87. Their 1:1 adducts 794 and 795 were confirmed by 1H NMR, elemental analyses and single-crystal X-ray diffraction analyses and the most important driving force for the formation of guest–host complexes is regarded to be the donor–acceptor p ⋯p stacking interaction, including the charge-transfer interaction between the electron-donating and electron-accepting aromatic components.

Fig. 87 The structures of host-guest adducts 794 and 795.

Half-Sandwich Rhodium and Iridium Complexes

167

A series of M–Cu(II) (M ¼ Rh(III), Ir(III)) heterometallic cages 797 were obtained in stepwise approach, by utilization of the cooper-contained metalloligand as building block.432–436 Single-crystal X-ray diffraction indicated that all the cages have distorted cuboid-shaped structures. By increasing the volume of inner cavity of such cages using longer ligands, the organometallic boxes are made large enough to accommodate pyrene and [Pt(acac)2] as guests (Scheme 142).

Scheme 142 (A) Synthesis of heterometallic cage 797 and host-guest adduct 798; (B) Single crystal structures of complex 797a and 798a. Color code: Rh, violet; Pt, white; Cu, orange; Cl, green; N, blue; C, gray.

In light of the encouraging synthesis of host-guest adduct 798, two sets of heterometallic molecular capsules with analogous open-ended cavities were prepared based on the half-sandwich rhodium fragments.437,438 In the case of [Rh4Cu4] cages 799–802, up to six-fold-stacked host-guest structures were formed by varying the cavity’s dimensions. Moreover, the series of capsules were demonstrated to self-fine-tune to form multi-heteroguest arrays via favorable donor-acceptor p interactions. The longer bidentate ligand 4,40 -bis(dipyrromethan-5-yl)biphenyl was chosen to bridge two electron-deficient surfaces with a distance of 17.06 A˚ (Rh ⋯ Rh non-bonding distance). Gratifyingly, the results of single-crystal X-ray analysis revealed that a four-layer guest array was captured inside the open-ended cavity (Scheme 143). The distance between two opposite copper centers increased to 16.87 A˚ , resulting in an average interplanar distances of 3.4 A˚ .

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 143 (A) Synthesis of heterometallic cage 799 and host-guest adducts 800–802; (B) Single crystal structures of complex 800, 801 and 802. Color code: Rh, violet; Cu, orange; O, red; N, blue; C, gray.

Inspired by the encapsulation of multiple guests in the large cavity of heterometallic cage 800, another coinage metal, silver(I), was selected to serve as an anchor to bind aromatic guests inside similar molecular capsules 803 and 804, which would possess analogous nano-cavities to the copper(II)-containing cages described above (Scheme 144). Such an assembly would take advantage of metal −p interactions, a well-known class of bonding between metal centers and aromatic systems, and has been utilized to facilitate the formation of a wide range of metal-arene complexes.

Half-Sandwich Rhodium and Iridium Complexes

169

Scheme 144 (A) Synthesis of heterometallic host-guest adducts 803 and 804; (B) Single crystal structures of complex 803 and 804. Color code: Rh, violet; Ag, white; O, red; N, blue; C, gray.

The construction of a series of discrete Cp Rh-based metallosupramolecular architectures featuring stable heteroarylrhodium(III) units has been reported, ranging from metallarectangles to heterometallic macrocages and hexanuclear prisms. They were prepared from heterocyclic boronic acids and rhodium precursors under mild conditions (Scheme 145). The heterometallic capsule 805 was proved to be not only suitable as a host for the encapsulation of a series of aromatic compounds, the receptor also shows widely differing specificity for the various isomers. Size and shape matching, as well as the metal−p interactions, are proposed to be the main forces governing the extent of molecular recognition. These results showcase an attractive methodology to selectively extract one isomer from mixtures, providing a platform to design tuned capsules for selective recognition of specific target molecules. The selected guest can then be released by using a simple solid−liquid solvent washing strategy or an external light stimulus.439,440

Scheme 145 Synthesis of heterometallic capsule 805 and its host-guest chemistry.

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Half-Sandwich Rhodium and Iridium Complexes

Selected CdH activation was observed when an unsaturated dicarboxylic acid (i.e., fumaric acid) was used to build macrocycles in mild conditions (Scheme 146).441,442 Using trans-1,2-bis(4-pyridyl)ethylene as bridge ligand, complex 808 could be transformed into 809 through the [2 +2] photocycloaddition of the olefin induced by UV/visible-light irradiation.

Scheme 146 The [2+2] photocycloaddition of complex 808.

Double-site CdH activations of aromatic bis-imine substrates were involved in the construction of macrocycle 811 (Scheme 147).443 In addition, the CdH activation-directed self-assembly is also demonstrated to allow precise control over molecular cage formation.444–447 Treatment of [Cp IrCl2]2 with 2,4,6-tri(4-pyridyl)-1,3,5-triazine at room temperature resulted

Scheme 147 Synthesis of complex 811 and 812 by double-site CdH activations of aromatic bis-imine substrates.

Half-Sandwich Rhodium and Iridium Complexes

171

in the formation of trinuclear complex, which further react with AgOTf. The color of the suspension changed to red, accompanying by addition of sodium acetate and terephthal-bis-aromatic imine ligands, indicating the formation of CdH activation-directed complexes 812. Through alteration of planar polyaromatic ligands with gradually enlarged p systems, a series of discrete supramolecular architectures with different topologies has been rationally constructed due to the different strengths of stacking interactions (Scheme 148),448 including macrocycle 813 and duplex metallotweezer stacked structures 814. The construction of the duplex assemblies can be attributed to noncovalent inter-molecular interactions inducing self-complementarity. Moreover, selective CdH activation has been observed in these assemblies. By employing the larger conjugated proligands, exclusive formation of duplex metallotweezer structures through unilateral activation of each ligand was observed, thanks to noncovalent interactions favoring the discrete quadruple stack structures. This contribution has demonstrated a self-assembly approach to stacking interactions directed by selective CdH activation and thus provides a foundation for the design and construction of molecular tweezers and discrete aromatic stacks.

Scheme 148 (A) Synthesis of macrocycle 813 and duplex metallotweezer stacked structures 814; (B) Single crystal structures of complex 813 and 814. Color code: Rh, violet; N, blue; C, gray.

172

8.02.7.3

Half-Sandwich Rhodium and Iridium Complexes

Molecular knots

A few 31 knots (trefoil knots) have been prepared to date based on half-sandwich rhodium or iridium fragments. Utilization of a class of building blocks featuring a varying number of flexible methylene groups are demonstrated to be a practical approach to design and synthesis molecular knots (Table 2). The flexible ligands 1,4-phenylenebis(methylene) diisonicotinate were first chosen because of the high degrees of rotational freedom of its ester functional groups (Scheme 149), which can allow the ligand to present a variety of configurations and induce hydrogen-bond interactions. In addition, its p-conjugated phenyl and pyridyl moieties can engender favorable aromatic p-p stacking and CH-p interactions. In addition, an unusual trefoil knot induced by similar stacking I interactions were realized based on the heterobimetallic tetranuclear (IrIII 2 + Ag2) complex featuring argentophilic interaction and 0 0 the nonrigid dipyridyl ligand 4,4 -bis(pyridin-4-ylmethyl)-1,1 -biphenyl. Moreover, the reversible topological transformation between the Solomon link and an unusual trefoil knot can be achieved by utilizing the chemical reactivity of silver(I) ions under mild conditions.

Table 2

Synthesis of molecular 31 knots from different building block combinations.

Binuclear building blocks

Flexible ligands

References 449

450

451

452

453

454

455

Half-Sandwich Rhodium and Iridium Complexes

173

Scheme 149 (A) Synthesis of trifoil knot 816; (B) Single crystal structures of complex 816. Color code: Rh, violet; O, red; N, blue; C, gray.

The 41 knot, had always existed as a mathematical symbol due to its extremely complex entanglement characteristics, until its serendipitous synthesis by Jin group. This figure-eight knot was self-assembled from three main components: 2, 20 -bisbenzimidazole as chelating ligand, an amidopyridine ligand as molecular linker and a half-sandwich Cp Rh building block that joins them together (Scheme 150). The formation reason is mainly based on noncovalent interactions including stacking and hydrogen-bond interactions. In particular, these NdH⋯ O intramolecular hydrogen bonds induced by amide groups play a crucial role in the formation of molecular figure-eight knots. Moreover, ligands 6,11-dihydroxy-5,12-naphthacenedione and 1,5-dihydroxy-anthraquinone, due to their significant transverse width and longitudinal length, were subsequently used to build edge units. In addition, the rational design of the organic (pyridyl) linkers is also pivotal for the synthesis of a molecular figure-eight knot. The flexible naphthalene diimide (NDI)-based pyridyl ligands have significant advantages in constructing diverse complicated discrete p-stacked assemblies as their planar, electron-deficient aromatic surface can engender favorable aromatic p-p stacking interactions, while the sp3-hybridized carbon atoms of their methylene or ethylene spacers allow the two pyridyl arms to rotate freely (Table 3).

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Half-Sandwich Rhodium and Iridium Complexes

Scheme 150 Synthesis of the molecular figure-eight knot 821.

Table 3

Synthesis of molecular 41 knots from different building block combinations.

Binuclear building blocks

Flexible ligands

References 451

456

456

456

Half-Sandwich Rhodium and Iridium Complexes

8.02.7.4

175

Molecular links

The rigid conjugated pyridyl ligands, upon coordination with binuclear half-sandwich rhodium building blocks, are shown to provide a [2]catenane (Scheme 151), whereby the stacking interactions between p-electron-rich units act as driving forces, inducing the formation of an interlocked topology containing two identical interlocked tetranuclear rectangles. Furthermore, favorable Donor-Acceptor stacking interactions between dinuclear edge unit 823 and the flexible ligand also enable the self-assembly of a hopf link, wherein a pair of catenated binuclear trapezoids make up an inseparable ensemble. The length of the naphthalenediimide (NDI) edge unit is 11.9 A˚ (RhdRh nonbonding distance), which is large enough to allow the phenyl group of the flexible ligand to pass through (Table 4).

Scheme 151 (A) Synthesis of [2]catenane 822; (B) Single crystal structures of complex 822. Color code: Ir, dark blue; O, red; N, blue; C, gray; Cl, green.

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Table 4

Synthesis of [2]catenane from different building block combinations.

Binuclear building blocks

Flexible ligands

References 457

458

459

460

449

461

By introducing flexible ligands containing methylene units, the possibilities of the self-assembly can be substantially diversified due to the increased conformational freedom of the ligands (Scheme 152). Using flexible dipyridyl ligands and rigid binuclear organometallic Rh(III)/Ir(III) fragments, a family of template-free self-assembled molecular Solomon links are obtained in high yield (Table 5). The p-p stacking and edge-to-face CH-p interactions are supposed to drive the formation of the doubly-interlocked topology. The inherently chiral nature of such a Solomon link complex is illustrated by single-crystal X-ray diffraction. Moreover, alteration of solvent or concentration is shown to induce dynamic interconversion between Solomon link and metallacycle ensembles in solution accompanying conformational change of these flexible ligands.

Scheme 152 (A) Synthesis of Solomon link 825; (B) Single crystal structures of complex 825. Color code: Ir, dark blue; N, blue; C, gray.

Table 5

Synthesis of molecular Solomon links from different building block combinations.

Binuclear building blocks

Flexible ligands

References 453

462

463

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Half-Sandwich Rhodium and Iridium Complexes

Table 6

Synthesis of [3]catenane from different building block combinations.

Binuclear building blocks

Pyridine ligands

References 457

464

To promote the formation of a linear topology (Table 6), thiophene groups were employed in the form of dipyridyl donors (Scheme 153). The self-assembly process of equimolar amounts of binuclear half-sandwich Rh(III) fragment 796 and thiophene-contained bridge ligand affords the interesting trimeric structure 826 in highly concentrated methanol solution, where the interlocking of one ring with two identical rings is observed. The presence of multiple p-p interactions between the alkynyl groups and the conjugated planes formed by the dithienyl moieties are thought to drive the preorganization of the rings.465

Scheme 153 (A) Synthesis of [3]catenane 826; (B) Single crystal structures of complex 826. Color code: Rh, violet; O, red; S, yellow; N, blue; C, gray.

Half-Sandwich Rhodium and Iridium Complexes

179

Borromean rings, the simplest type of Brunnian link, evolve into a set of trivial unlinked circles upon breaking or removal of any one constituent ring. As a topological isomer of [3]catenane, Borromean rings are described as a 632 link in the Anderson-Briggs notation. The most common noncovalent interactions used to construct these elegant architectures is parallel-displaced aromatic stacking between two overlapped conjugate ligands in the resulting Borromean ring complex (Table 7). According to the conventional Table 7

Synthesis of molecular Borromean rings from different building block combinations.

Binuclear building blocks

Flexible ligands

References 466

467 468

469

457

460

460

470

471

(Continued )

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Half-Sandwich Rhodium and Iridium Complexes

Table 7

(Continued)

Binuclear building blocks

Flexible ligands

References 468

472

472

467

469

473

474

effective distance of p-p interactions (3.5 A˚ ), the difference between the length and width of a rectangle should be roughly equal to 7 A˚ (double the effective distance of stacking interactions) in order to favor a Borromean ring complex (Scheme 154). By using binuclear half-sandwich Cp Rh fragments, the first examples of molecular Borromean ring complexes prepared by template-free self-assembly. One of the three available coordination sites of each rhodium center is occupied by a bridging monodentate pyridine

Scheme 154 (A) Supramulecular transformation from Borromean ring 828 to monomer 829; (B) Single crystal structures of complex 828. Color code: Rh, violet; O, red; N, blue; C, gray.

Half-Sandwich Rhodium and Iridium Complexes

181

derivative, either 1,4-bis(4-pyridyl)benzene or N, N0 -4-dipyridyloxalamide, both being of similar length (ca. 15.5 A˚ ), while the other sites are chelated by the oxalate-like binding sites of the copper-containing metalloligand.

8.02.8

Conclusion

Research in the chemistry of organo- iridium and rhodium compounds has evolved dramatically, as half-sandwich fragments are becoming more common and important role in constructing these organometallic compounds. The half-sandwich group is conducive to the directional coordination of the metal center and to improving the solubility of the compounds. These unique advantages will facilitate the rational design and selective synthesis of the half-sandwich structure organometallic compounds, benefiting the development of their potential application value. Remarkably, the combination of half-sandwich metal units and supramolecular chemistry greatly promoted the construction of new topological architectures. In the future, we believe that half-sandwich iridium and rhodium complexes will be certainly sparkling and versatile in the construction of new topological structures, catalysis, medicine, and other fields.

Acknowledgment We appreciate the continuous funding from the National Science Foundation of China (21531002, 22031003, 21720102004).

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Half-Sandwich Rhodium and Iridium Complexes 402. 403. 404. 405. 406. 407. 408. 409. 410. 411. 412. 413. 414. 415. 416. 417. 418. 419. 420. 421. 422. 423. 424. 425. 426. 427. 428. 429. 430. 431. 432. 433. 434. 435. 436. 437. 438. 439. 440. 441. 442. 443. 444. 445. 446. 447. 448. 449. 450. 451. 452. 453. 454. 455. 456. 457. 458. 459. 460. 461. 462. 463. 464. 465. 466. 467. 468. 469. 470. 471. 472. 473. 474.

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8.03

Group 9 Boryl Complexes

Makoto Yamashita, Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya, Aichi, Japan © 2022 Elsevier Ltd. All rights reserved.

8.03.1 8.03.2 8.03.3 8.03.4 8.03.5 References

8.03.1

Introduction Ir-Boryl complexes Rh-Boryl complexes Co-Boryl complexes Summary

188 188 196 200 203 203

Introduction

Transition metal boryl complexes are defined as complexes having an anionic sp2-hybridized boron atom as an X-type ligand, that coordinates to transition metals through 2-center-2-electron bond (Fig. 1, left). Thus, these complexes formally contain a boryl anion. This chapter will describe chemistry of group 9 boryl complexes, which have been structurally characterized, including their spectroscopic properties and application to organic synthesis. It should be noted that caged borane and boride complexes are not included to this chapter because these compounds have an electron-deficient shared B-M bond rather than 2-center-2-electron B-M bond. Additionally, this chapter will not cover chemistry of neutral base- and anion-stabilized boryl complexes (Fig. 1, center and right) because they have an sp3-hybridized boron atom and without the characteristic Lewis acidity on the boron center. It should be noted that the anion-stabilized boryl complex can also be considered as a Z-borane complex1 that forms through a coordination of d-orbital on the transition metal atom to a vacant orbital of a neutral borane. The chemistry of transition-metal-borylene complexes, outside the scope of this chapter, is covered in a number of reviews.2

8.03.2

Ir-Boryl complexes

The first structurally characterized examples of the transition-metal-boryl complexes were reported for group 9 metals. Two boryliridium complexes 1 and 2 were synthesized by an oxidative addition of BdH bond to Ir(I) precursor (Scheme 1).3 Both of them were structurally characterized by a single-crystal X-ray diffraction analysis to show their six-coordinate Ir(III) structure. It should be noted that the strong trans-influencing boryl ligand avoids trans position to the hydride ligand in both cases. The former compound 1 was applied as a catalyst for hydroboration of alkynes as an insertion of alkynes to the IrdH bond and subsequent CdB bond-forming reductive elimination were directly observed to liberate alkenylborane product.4 Reaction of an electron-rich biphenyl-2,20 -diyl Ir complex with NOBF4 followed by ligand exchange reaction with NaBPh4 afforded cationic Ir(IV) complex 3 having an (aryl)(fluoro)boryl ligand (Scheme 2).5 The boryl ligand probably formed through an electrophilic reaction of BF3 derived from BF−4 anion with the IrdC bond in the starting complex, initiated by a single-electron transfer from Ir(III) to NO+ followed by a formal insertion of “BF” unit into the IrdC bond. Due to odd number of electrons, 3 exhibited an EPR signal, supporting an Ir oxidation state of four. Similar to the initial synthesis of 1, a series of (catecholato)boryl complexes were synthesized by oxidative addition of BdH bond to Ir(I) precursors (Scheme 3).6 These complexes 4–6 were also active for hydroboration of alkenes using catecholborane. Reaction of an indenyl-Ir(I) precursor with excess catecholborane furnished (arene)iridium tris(boryl) complexes 7a-7d (Scheme 4).7 Through the formation of 7a-7d, the indenyl ligand dissociated from the Ir atom and three catecholborane molecules should react with Ir(I) center. As a result, indane- and cyclooctadiene-derived products were observed in GC/MS analysis for this reaction. It should be noted that the supporting information of this report showed the existence of a small amount of (tolyl)boronic ester was observed by GC/MS analysis probably though CdH borylation reaction of toluene.

188

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00150-5

Group 9 Boryl Complexes

(A)

(B)

(C)

Fig. 1 Classification of boryl complexes having B-M single bond (R: covalently bonded substituent, LB: neutral Lewis base).

Scheme 1 The first reports for structurally characterized boryliridium complexes 1 and 2.

Scheme 2 Insertion of “BF” unit into IrdC bond to form boryl complex 3.

Scheme 3 Synthesis of (catecholato)boryl complexes 4–6 with various phosphine ligands.

Scheme 4 Synthesis of (arene)iridium tris(boryl) complexes 7a-7d.

189

190

Group 9 Boryl Complexes

Changing the boron reagent to diborane(4) having a BdB bond provides a synthetic route to bis(boryl) complexes (Scheme 5). Reaction of IrCl(PMe3)3(coe) with bis(catecholato)diborane(4) afforded a six-coordinate Ir(III) bis(boryl) complex 8.8 The molecular structure of 8 was confirmed by single-crystal X-ray diffraction analysis to show that three PMe3 ligands coordinate to Ir in mer-fashion. Reaction of Ir precursor having slightly bulkier PEt3 ligands with bis(catecholato)diborane(4) furnished a six-coordinate Ir(III) bis(boryl) complex 9 as characterized by NMR spectroscopy in solution.9 However, crystals obtained from the reaction mixture showed the existence of five-coordinate complex 10. This rapid dissociation of PEt3 ligand from 9 would reflect the strong trans-influence of boryl ligand.

Scheme 5 Synthesis of bis(boryl)iridium complexes 8, 9, and 10 via oxidative addition of BdB bond.

According to the established procedure for preparation of diborylplatinum complexes,10 one of the simplest diborane(4)s, B2F4, reacted with Vaska’s complex, trans-Ir(Cl)(CO)(PPh3)2, to give a six-coordinate tris(boryl)iridium complex 11 (Scheme 6).11 X-ray diffraction analysis of 11 revealed the existence of three BF2 ligands coordinated in a fac-fashion. Since introduction of three BF2 unit cannot be explained by a simple oxidative addition of BdB bond toward the iridium center, the authors proposed two possible mechanisms. One is s-bond metathesis between BdB bond of B2F4 and IrdCl bond of an intermediate Ir(Cl)(CO) (PPh3)2(BF2)2 12, which should form by the oxidative addition of B2F4 to trans-Ir(Cl)(CO)(PPh3)2. The other involves reductive elimination of CldBF2 from 12 to result in the formation of 13 followed by the second oxidative addition of BdB bond of B2F4 to 13.

Scheme 6 Synthesis of tris(boryl)iridium complex 11 via oxidative addition of BdB bond.

Reaction of an Ir hydride complex with hydroborane gave boryliridium complex (Scheme 7). Treatment of Cp IrH4 with 1 equiv. of HBpin resulted in the formation of monoboryl complex 14 through loss of H2.12 Subsequent reaction of 14 with the second equivalent of HBpin afforded bis(boryl) complex 15. Monoboryl complexes 16–18 were also synthesized by a treatment of Cp IrH4 with nBuLi followed by reaction with haloboranes. It should be noted that the mono- and bis-pinacolatoboryl complexes 14 and 15 reacted with hydrocarbons, such as benzene and n-alkanes, to provide the corresponding borylated hydrocarbons via CdH bond activation.

Scheme 7 Synthesis of mono- and bis-(boryl) and mono-(boryl)-Cp Ir(V) complexes 14–18, and their reaction with hydrocarbons.

Group 9 Boryl Complexes

191

Reaction of [Ir(coe)2Cl]2 with bis(pinacolato)diborane(4) (pinB–Bpin) and 4,4’-tBu2bipyridine afforded tris(boryl)iridium(III) complex 16 in 15%, which was characterized by a single-crystal X-ray diffraction analysis to show its six-coordinate structure (Scheme 8).13 This compound was assumed as an intermediate for the Ir-catalyzed CdH borylation of arenes. In fact, treatment of 19 with benzene at room temperature resulted in the formation of borylated benzene in 83% yield.

Scheme 8 Synthesis of tris(boryl)iridium(III) complex 19 and its reaction with benzene at room temperature.

Oxidative addition of B–halogen bond is also an effective method to prepare B–M bond (Scheme 9). Reaction of Ir(coe) (PMe3)3Cl with B-chlorocatecholborane afforded (boryl)(dichloro)iridium(III) complex 20-PMe3-Cl.14 Using an excess amount of B-bromocatecholborane for the reaction of Ir(coe)(PMe3)3Cl furnished (boryl)(dibromo)iridium(III) complex 20-PMe3-Br, where the excess amount of bromoborane induced halogen exchange on the iridium center. Similarly, (boryl)(dihalo)iridium(III) complexes 20-PEt3-Cl and 20-PEt3-Br were synthesized from Ir(PEt3)3Cl precursor and haloboranes.

Scheme 9 Synthesis of (dihalo)(boryl)iridium(III) complexes 20 via oxidative addition of B-X (X ¼ halogen) bond.

Ligand exchange of boryl complexes is another method to prepare new boryl complexes. Addition of 4,4’-tBu2bipyridine and cyclooctene to (p-xylene)iridium tris(boryl) complex 7e, prepared as described in Scheme 4, gave six-coordinate complex 190 (Scheme 10).15 Similarly, five-coordinate diphosphine-ligated iridium tris(boryl) complexes 21a and 21b could be synthesized. Since these complexes 190 , 21a, and 21b can be considered as an intermediate for CdH borylation of arenes (Scheme 8), reaction of 190 and 21b with benzene were examined. Interestingly, dtbpy-coordinated 190 reacted with benzene with faster rate than

Scheme 10 Ligand exchange reaction of preformed boryliridium complexes with retaining of BdIr bond.

192

Group 9 Boryl Complexes

diphosphine-coordinated 21b. To compare the electron-donating effect of Bpin and Bcat ligand toward the metal center, two (carbonyl)iridium tris(boryl) complexes 22 and 220 were generated by ligand exchange reaction with CO. The lower carbonyl stretching frequency nCO (1987 cm−1) for 22 than that of 220 (2017 cm−1) indicates that the Bpin ligand donates electrons much stronger than Bcat . Reflecting this stronger donor ability of Bpin ligand than that of Bcat ligand, diphosphine-ligated tris(pinacolatoboryl)iridium complexes 19 exhibited higher reactivity toward benzene than that of 190 .16 It should be noted that the bulkiness of alkyl group on the phosphorus atoms also affects the reaction rate (21c vs. 21d, Scheme 11). Similar dippe-ligated tris(catecholatoboryl) complexes 21e and 21f were also prepared by ligand exchange reaction.17

Scheme 11 Ligand exchange reaction of preformed boryliridium complexes that retain BdIr bonds.

Oxidative addition of the BdH bond is also effective method to prepare boron-containing pincer-Ir complexes (Scheme 12).18 Six different pincer ligand precursors 22a-f having a diazaborole ring were prepared. Treatment of tBu- and iPr-substituted precursors 22a and 22d with [Ir(cod)Cl]2 gave the corresponding oxidative addition products, PBP-pincer (hydrido)(chloro)iridium complexes 23a and 23d. Exposure of 23a to CO furnished six-coordinate carbonyl complex 24a, which was compared with a previously reported PCPdIr complex possessing a same set of ligands (H, Cl, CO) to show a longer IrdCl bond in 24a, reflecting the strong trans-influence of boryl ligand. In contrast, reaction of Ph-substituted PNP pincer ligand precursor 22b with chloroiridium alkene complexes afforded a complex mixture, probably due to a less-sterically hindered phosphorus environment causing a variety of oligomerization reaction. Instead, reaction of 22b with Ir[P(o-tolyl)3]2(CO)Cl gave PBPdIr(H)(Cl)(CO) complex 24b. The precursor 22c was also active for complexation with Ir to result in the formation of PBPdIr(H)Cl complex 23c. Two PBP-pincer ligand precursors 22e and 22f possessing longer phosphorus sidearms also reacted with [Ir(coe)2Cl]2 to furnish the corresponding PBPdIr(H)Cl complexes 23e and 23f. It was found that these complexes reacted with nBuLi to give PBPdIr(H)2 complexes 25e and 25f via substitution of chloride with nBu group followed by a b-hydride elimination and dissociation of the resulting 1-butene. Five-coordinate PBPdIr(H)Cl complexes 23a,c,d could be converted to the corresponding PBPdIr(I) ethylene complexes 26a,c,d by a treatment with LiTMP in the presence of ethylene. It should be noted that complexes 23d/LiTMP, 23e/KHMDS, 25e, and 26d showed a catalytic activity (TON ¼ 43, 113, 126, and 33) for transfer dehydrogenation of cyclooctane at 160–220  C in the presence of 3,3-dimethylbutene as a hydrogen acceptor. Also, combination of 23f (0.5 mol%) with KOtBu (1.5 equiv. vs. 23f) catalyzed dehydrogenating coupling of Me2NH-BH3 to afford a cyclic dimer (Me2N-BH2)2 quantitatively.

Group 9 Boryl Complexes

193

Scheme 12 Synthesis and reactivity of PBP-pincer iridium complexes.

Heating a cationic iridacycle complex possessing a m2-aminoborane ligand induced loss of dihydrogen molecule to give complex 27.19 Considering 1H and 11B NMR spectra along with structural parameters obtained by a single-crystal X-ray analysis, the authors proposed two limiting resonance forms: an a-agostic iridium (amino)(hydrido)boryl complex 27 or an (hydrido)iridium (amino) borylene complex 270 stabilized by intramolecular coordination of an iridium-bound hydride to the boron atom (Scheme 13).

Scheme 13 Spontaneous dehydrogenation of aminoborane-coordinated cationic iridacycle.

Treatment of a SiNN-pincer Ir(I) hydride complex having cod ligand with H–Bpin afforded the corresponding SiNN-pincer bis(boryl)iridium hydride 28 (Scheme 14).20 Considering the structural parameters obtained by a single-crystal X-ray diffraction analysis, NMR spectroscopic analysis, and DFT calculations, 28 can be described as SiNN-bis(boryl)iridum(V) silyl/hydride complex or bis(boryl)iridium(III)-silane complex. Interestingly, 28 and its precursor coe complex catalyze the dehydrogenative

194

Group 9 Boryl Complexes

Scheme 14 Synthesis of SiNN-pincer bis(boryl)iridium hydride.

coupling between terminal alkynes and pinacolborane to furnish the corresponding alkynylborane in the absence of hydrogen acceptor with liberation of dihydrogen as a sole byproduct. The substrate scope involves aryl-, alkyl-, silyl-, haloalkyl-, alkoxyalkylacetylenes in high yields. Formation of boryl complexes possessing BN bidentate ligand was achieved by using appropriate hydroborane, diborane(4), and silylborane precursors with preformed BdN bonds (Scheme 15). A phosphorus-tethered hydroborane precursor of a bidentate BN ligand reacted with Ir(I) source to give bis(phosphine-boryl)iridium chloride complex 29 through complexation of two BN ligands onto the iridium center.21 A pyridine-substituted diborane(4) underwent oxidative addition of BdB bond to [Ir(cod)Cl]2 to form a cationic bis(boryl)iridium complex 30.22 Silylboranes may also serve as a precursor to a boryl complexes. Pyridine-tethered silylboranes 31a and 31b having benzo- and naphtho-diazaborole backbones oxidatively added to Ir(I) source to form the corresponding pyridine-tethered (boryl)(silyl)iridium complexes 32a and 32b.23 A chiral variant of BN bidentate ligand 31c was also introduced to Ir to furnish a chiral (boryl)(silyl)iridium complex 32c.24 This study and additional subsequent reports showed that other chiral variants of the BN ligand were effective for asymmetric borylation of organic molecules (Fig. 2).24,25

Scheme 15 Synthesis of boryliridium complexes having BN-bidentate ligand(s).

Group 9 Boryl Complexes

195

Fig. 2 Chiral BN bidentate boryl ligand precursors.

A PNP-pincer (boryl)iridium complexes 33 and 34 were synthesized (Scheme 16) and structurally characterized as a counterpart in the study of PNP-cobalt complex-catalyzed CdH borylation chemistry (vide infra). It should be noted that these Ir complexes showed no turnover for CdH borylation in contrast to Schemes 10 and 11.

Scheme 16 Synthesis of boryliridium hydride chloride complex 33 having a neutral PNP-pincer ligand and its reduction to the dihydride complex 34.

Oxidative addition of the BdB bond in B2pin2 occurred at a PCP-pincer Ir complex (Scheme 17).26 Reaction of alkenecoordinated PCPdIr(I) complex with bis(pinacolato)diborane afforded the corresponding PCP-pincer bis(boryl)iridium complex 35. In the presence of H2, bis(boryl) complex 35 is in equilibrium with its H2 adduct. Similarly, treatment of Xantene-based diphosphine-iridium(III) trihydride or metallacyclic iridium(hydride)(chloride) with bis(catecholate)diborane resulted in the formation of tris(boryl)iridium complex 36 through an oxidative addition of BdB bond following the formation of an Ir(I) intermediate.27

Scheme 17 Synthesis of PCP-pincer bis(boryl)iridium complex 35 and tris(boryl)iridium complex 36.

Coordination-induced oxidative addition of the BdC bond in a phosphorus-tethered triarylborane led to a formation of PBP-pincer Ir(phenyl)(chloride) complex 37 (Scheme 18).28 Exposure of 37 to H2 at 100  C resulted in hydrogenolysis of the IrdC bond to afford PBP-iridium(hydride)(chloride) 38. Treatment of 38 with NCS (N-chlorosuccinimide) resulted in the formation of PBP-iridium dichloride complex 39. On the other hand, treatment of 36 with NaEt3BH led to a substitution of chloride with hydride, furnishing a hydride-bridged PBP-iridium(phenyl)(hydride) 40. Subsequent exposure of 40 to carbon

196

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Scheme 18 Synthesis and reactivity of aryl-substituted PBP-pincer iridium complexes.

monoxide induced a BdC bond forming reductive elimination to give Ir(I) dicarbonyl complex 41 possessing a Z-type triarylborane ligand. Further heating of 41 furnished PBPdIr(I) dicarbonyl complex 42 with an elimination of benzene.

8.03.3

Rh-Boryl complexes

Soon after the first structural characterization of a boryl(iridium) complex, the first structurally characterized (boryl)rhodium complex was reported (Scheme 19). Reaction of (iPr3P)2Rh(N2)Cl with catecholborane gave boryl(rhodium) complex 43a through an oxidative addition of BdH bond and loss of N2 ligand.29 Same type of reaction with pinacolborane was also reported to form 43b.30 Similarly, reaction of the previously synthesized five-coordinate borylrhodium complex31 with catecholborane furnished bis(boryl)rhodium complex 44a through a formation of the second RhdB bond.32 Employing H–BBzpin (tetraphenyl H–Bpin), Wilkinson’s complex (PPh3)3RhCl also underwent oxidative addition of BdH bond to form 44c as a phenyl-substituted derivative of 44a.33

Scheme 19 Synthesis of boryl(rhodium) complexes via oxidative addition of BdH bond.

The oxidative addition of a BdB bond is also an effective route to form boryl(rhodium) complexes (Scheme 20). Treatment of (Me3P)4RhMe with bis(catecholato)diborane(4) afforded the corresponding five-coordinate mono(boryl) complex 45. Subsequent

Scheme 20 Synthesis of boryl(rhodium) complexes via oxidative addition of a BdB bond.

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197

treatment of 45 with one more equivalent of this diborane(4) furnished mer-tris(boryl)rhodium complex 46. Similarly, Wilkinson’s complex (Ph3P)3RhCl reacted with bis(catecholato)diborane(4) gave bis(boryl)rhodium complex 47. Treatment of the resulting complex 47 with PEt3 induced ligand exchange to give 48. Similar to Ir complexes (Scheme 9), haloboranes are also active for oxidative addition to Rh(I) complex, providing the corresponding borylrhodium complexes 49dCl and 49dBr (Scheme 21).14 In each case, three phosphine ligands coordinate to the Rh center in a meridional fashion.

Scheme 21 Synthesis of Rh-boryl complexes via oxidative addition of a B–halogen bond.

Reaction of a four-legged piano-stool bis(silyl)rhodium(V) complex with pinacolborane led to a formation of (boryl)(silyl) rhodium complexes 50a and 50b via s-bond metathesis (Scheme 22).34 Interestingly, these compounds reacted with alkanes under heating to produce borylated alkanes through CdH bond activation. Similarly, bis(boryl)rhodium complex 51 can be prepared by reaction of Cp Rh(Z4-C6Me6) with pinacolborane. Further treatment of 51 with pinacolborane under neat conditions produced tris(boryl)rhodium complex 52. It should be noted that 51 and 52 underwent BdH bond forming reductive elimination upon treatment with phosphine ligand to furnish Rh(III) complexes 53 and 54, respectively. Reaction of 52 with hydrocarbons gave alkylborane product with higher yield than those with 50. A photochemically generated coordinatively unsaturated 16e Rh(I) species from (C5H4X)Rh(PR3)(C2H4) complex also underwent oxidative addition of BdH or BdB bonds to form mono(boryl)rhodium complexes 55a-c or bis(boryl)rhodium complexes 56a-c (Scheme 23).35

Scheme 22 Synthesis of Cp Rh-boryl complexes and their reaction with hydrocarbon.

Scheme 23 Synthesis of Rh-boryl complexes through photochemical dissociation of ethylene.

The reaction of dihydride (IMes)Rh(H)2Cl with dimethylamine-borane in the presence of NaBArF4 gave the cationic (IMes)2Rh(H)(boryl) complex 57 through an oxidative addition of a BdH bond and loss of H2 (Scheme 24).36 The positions of hydride ligand and BdH hydrogen in 57 were unambiguously determined by a neutron diffraction analysis. Similar reaction of cationic bis(phosphine)Rh(arene) complex with dimethylamine-phenylborane afforded cationic (iPr3P)2Rh(H)(boryl) complex 58.37 It should be noted that a significantly short interatomic distance [2.634(8) A˚ ] between the Rh atom and the ipso carbon atom of the phenyl group in the crystal structure of 58. Treatment of cationic bis(phosphine)Rh complex having a bidentate dcpm

198

Group 9 Boryl Complexes

IMes H Rh Cl H IMes F iPr

3P

iPr

3P

Rh BArF4

H3B NHMe2 NaBArF4

Ph H2B NHMe2

BArF4 H Rh B N IMes H 57

IMes =

IMes

N

N

ArF = 3,5-(CF3)2C6H3 F Cy2 F H B NHMe P 3 2 Rh P Cy2 [Al(OC(CF3)3)4]

PiPr3 NMe2 H Rh B BArF4 PiPr3 58

Cy2P PCy2 H H H Rh Rh B N Cy2P PCy2 [Al(OC(CF3)3)4] 59

PPh2 H2B NiPr2 H BH2NMe3 –H2 O Rh H H PPh2

PPh2 H BH2NMe3 NCMe O Rh BH H NiPr2 PPh2 60

PPh2 NCMe O Rh BH H NiPr2 PPh2 61

Scheme 24 Synthesis of Rh-boryl complexes through BdH oxidative addition of amine-borane.

(bis(dicyclohexyl)phosphinomethane) ligand with dimethylamine-borane induced double BdH oxidative addition to form a dinuclear complex 59 possessing a bridging borandiyl ligand through a change of the coordination mode of dcpm ligand from bidentate to bis(monodentate) fashion.38 It was also demonstrated that (Xantphos)Rh(H)2(H3B-NMe3) reacted with N, N-diisopropylaminoborane gave aminoboryl complex 60 through BdH oxidative addition.39 This complex 60 was characterized by NMR spectroscopy as an equilibrium mixture with its starting complex. Further ligand exchange reaction of 60 was observed to replace s-amine-borane ligand with acetonitrile to form 61. The hydroborane precursor 22a of the PBP-pincer ligand underwent an oxidative addition of BdH bond toward Rh(I) complex to furnish (PBP)Rh(H)Cl complex 62 having interaction between B and H atoms (Scheme 25).40 Ligand exchange of 62 by a treatment with AgOTf afforded the corresponding (PBP)Rh(H)OTf complex 63. Addition of sterically hindered base, LiTMP (TMP ¼ 2,2,6,6-tetramethylpiperidide), to 63 led to a formation of T-shaped 14e Rh(I) complex 64, structurally characterized

Scheme 25 Synthesis and reactivity of PBP-pincer Rh complexes.

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199

with an infinite chain structure via intermolecular CdH agostic interactions. This “naked” T-shaped complex 64 coordinated with p-acceptor ligands such as CO, ethylene, and N2 to form the corresponding adducts 65, 66, and 67. Interestingly, the T-shaped complex 64 reacted with phenol to give OdH oxidative addition product 68. In the case of reaction with aliphatic alcohol, carbonyl complex 65 was isolated probably via dehydrogenation of alcohol and carbonyl extrusion of aldehyde. The “naked” complex 64 was also cleaved the CdC bond of benzocyclobutenone to give a five-membered acylrhodium complex 69.41 All of these characteristic features of the present complexes would arise from the strong s-donating property of the ancillary boryl ligand. The SiNN pincer ligand (Scheme 14) also enabled the synthesis of a borylrhodium complex (Scheme 26). Treatment of the SiNN-ligated Rh(hydride) complex with pinacolborane gave (boryl)(silyl)Rh(H) complex 70 through a loss of H2.42 It should be noted that the complex 70 could catalyze borylation of deuterated benzene by using pinacolborane.

Scheme 26 Synthesis of SiNN-pincer bis(boryl)rhodium hydride.

An unsymmetrical diborane(4) reagent underwent s-bond metathesis with Rh complexes to form RhdB bond (Scheme 27).43 Treatment of pinacol- and benzenediamine-substituted unsymmetrical diborane(4) with (Me3P)4RhMe complex afforded a square planar (diaminoboryl)rhodium(I) complex 71, which is in equilibrium with five-coordinate complex 72 through a coordination of an additional equivalent of PMe3 ligand. In contrast, reaction of the same diborane(4) reagent with (Me3P)4RhOtBu complex furnished a square pyramidal (pinacolatoboryl)rhodium(I) complex 73. Photochemical metathesis reaction of RhdCH3 complex is also effective to construct RhdB bond (Scheme 28).44 Irradiation to Tp’Rh(H)(CH3)(PMe3) in the presence of pinacolborane led to a formation of borylrhodium complex 74.

Scheme 27 Metathesis reaction of unsymmetrical diborane(4) with RhdCH3 and Rh-OtBu complexes to form borylrhodium complexes.

Scheme 28 Metathesis reaction of catecholborane with RhdCH3 complex to form BdRh bond.

The reactivity of POP-pincer Rh complexes toward hydroborane and diborane(4) was studied in detail (Scheme 29).45 Reaction of (POP)RhCl complex with B2pin2 gave a six-coordinate bis(boryl)rhodium 75 through BdB oxidative addition, which was characterized by NMR analysis. This 75 was spontaneously converted to 76 by a reductive elimination of chloroborane. Interestingly, this 76 rapidly reacted with benzene to liberate a CdH borylated product, Ph–Bpin, and (POP)Rh hydride complex 77, which was independently synthesized and characterized. The hydride complex 77 further reacted with chloroborane to form (POP)Rh(H) (Bpin)Cl complex 78 via BdCl oxidative addition. Similarly, 77 reacted with pinacolborane or catecholborane to form (POP)Rh(H)2(boryl) complexes 78 and 79, which immediately release dihydrogen gas through reductive elimination to give (POP)Rh(boryl) complexes 76 and 80.

200

Group 9 Boryl Complexes

Scheme 29 Synthesis and reactivity of POP-pincer Rh complexes.

8.03.4

Co-Boryl complexes

The first example of a borylcobalt complex was synthesized by a ligand exchange of CpCo(CO)2 with an isolated aminoboraalkene to provide complex 81 with an Z2-interaction with the BdC bond (Scheme 30).46 This complex 81 has a single BdCo bond in a three-membered ring, where the CodB bond [2.000(8) A˚ ] was shorter than the CodC bond [2.074(6) A˚ ] reflecting the strong backdonation from the cobalt atom.

Scheme 30 Synthesis of the first Co-boryl complex.

Similar to chemistry observed for Ir and Rh, phosphine-cobalt complexes in low oxidation states (0 and 1) react with BdB bonds to form borylcobalt complexes (Scheme 31). Treatment of tetrakis(phosphine)cobalt complex with diborane(4) led to a formation of diborylcobalt complexes 82a and 82b through oxidative addition of the BdB bond.47 Methylcobalt(I)-phosphine complex reacted with catB–Bcat to furnish borylcobalt(I)-phosphine complex 83 through a metathesis reaction of CodMe and BdB bond.48 This complex is further treated with catB–Bcat to afford tris(boryl)cobalt complexes mer-84 and fac-84 as an isomeric mixture.

Scheme 31 Synthesis of phosphine-ligated Co-boryl complexes.

Borylcobalt complexes possessing an anionic or a neutral PNP-ligand have been extensively studied (Scheme 32). A cobalt alkoxide complex having an anionic PNP-pincer ligand reacted with B2pin2 to give a square planar PNP-pincer borylcobalt complex 85 via a s-bond metathesis reaction.49 Reaction of a PNPdCodCH2SiMe3 complex having a neutral pyridine-based PNP ligand

Group 9 Boryl Complexes

201

Scheme 32 Synthesis of Co-boryl complexes having an anionic or a neutral PNP-pincer ligand.

with pinacolborane also led to the formation of a PNP-pincer borylcobalt complex 86.50 Similar reaction of the PNPdCodCH2SiMe3 complex with B2pin2 under N2 atmosphere gave borylcobalt-N2 complex 87. The weakly bound N2 ligand in 87 was easily substituted by CO ligand to form carbonyl complex 88. Interestingly, the reaction of the PNPdCod CH2SiMe3 complex with an excess amount of B2pin2 afforded borylcobalt-N2 complex 89 with the additional introduction of a Bpin group at the 4-position of pyridine ring. The Bpin substituent at the pyridine backbone in 89 was introduced via CdH borylation catalyzed the PNP-cobalt complex itself. Reaction of 89 with excess pinacolborane resulted in the borylcobalt dihydride complex 90a. Similar borylcobalt dihydride complexes 90b-90d were synthesized by reaction of PNP-CoCl2 complex with pinacolborane and NaHBEt3. It should be noted that 90d is the most active catalyst for CdH borylation reaction among 90a90e due to increase of electron density at the Co center by pyrrolidine substituent on the pyridine ring. A similar pyrrolidine-substituted PNPdCo(N2) complex 92 was synthesized by s-metathesis reaction of methylcobalt precursor. Reaction of cationic PNPdCo(N2) complex with B2pin2 also afforded cationic borylcobalt complex 93.51 A related SiNSi-pincer borylcobalt dihydride complex 94 having bis(silylene) sidearms was accessible by a reaction of a SiNSi-pincer cobalt trihydride/NaHBEt3 complex with B2pin2 (Scheme 33).52

Scheme 33 Synthesis of Co-boryl complex having a neutral SiNSi-pincer ligand.

Hydroborane precursors to the PBP pincer ligand may also be used to form borylcobalt complexes (Scheme 34).53 Treatment of Bu-substituted precursor 22a with CoBr2 followed by a reduction with Na/Hg under N2 atmosphere afforded borylcobalt(I) dN2 complex 95. Dihydrogen reversibly added to 95 form an equilibrium with borate-coordinated cobalt dihydride complex 96.

t

202

Group 9 Boryl Complexes

Scheme 34 Synthesis and reactivity of PBP-pincer Co complexes.

In contrast, reaction of 95 with dimethylamine-borane gave PBP-hydroborane-cobalt complex 97 with the k2-coordinated BH4 anion. Both 95 and 97 serve as active catalysts for dehydrogenation of dimethylamine-borane to give cyclic dimethylaminoborane dimer. The CodB distances in 95, 96, and 97 were 1.9463(13) A˚ , 1.908 (av.) A˚ , and 1.9077(15) A˚ , although the structural drawing of 96 and 97 did not illustrate BdCo bonds in the original paper. Similarly, Cy-substituted precursor 22c reacted with CoI2 followed by a treatment with Na/Hg afforded a dinuclear complex 98. Two boron atoms had different BdCo bond lengths [2.049(2) and 2.006(2) A˚ ], indicating their behavior as a bridging boryl ligand and a Z2-hydroborane ligand in 98. Addition of ferrocenyl cation to 98 led to a formation of cationic complex 99. Complexes 95 and 98 were catalytically active for hydrogenation of alkenes. Nucleophilic borylation of cobalt complexes using boryllithium, which was developed as a boron-based nucleophile,54 afforded the corresponding borylcobalt complexes (Scheme 35).55 Treatment of PPh3-ligated cobalt-substituted ester with boryllithium similarly furnished PPh3-ligated borylcobalt tricarbonyl complex 100a through a loss of ethoxycarbonyl group. In contrast, reaction of (PhO)3P-ligated cobalt-substituted ester with boryllithium gave borylcobalt tetracarbonyl complex 100b via elimination of a P(OPh)3 ligand. The carbonyl ligand trans to boryl group in 100b could be substituted with an additional ligand such as PPh3, PMe3, and the N-heterocyclic carbene IPr to form tricarbonyl complexes 100a, 100c, and 100d. The PMe3-ligated complex 100c further reacted with a strongly acidic proton source, [H(OEt2)2]BArF4 [ArF ¼ 3,5-(CF3)2C6H3] to afford cationic complex 101 via protonation of the enamine backbone of boryl ligand.

Scheme 35 Synthesis and reactivity of borylcobalt complexes derived from boryllithium.

Group 9 Boryl Complexes

8.03.5

203

Summary

In this review, all structurally characterized examples of boryl-group 9 metal complexes were collected and described. Three different type of methods may be used to form the B-M bond: (a) oxidative addition of BdH, BdB, and B–X (X ¼ halogen) bond to low-valent metal species, (b) s-bond metathesis of precursor complexes with a BdB bond, and (c) nucleophilic borylation by using “boryl anion.” These complexes find extremely useful application in organic synthesis, especially by enabling the construction of new boryl-substituted bonds to sp, sp2, and sp3 carbon centers for further synthetic elaboration.

References 1. (a) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859–871; (b) Bouhadir, G.; Bourissou, D. Coordination of Lewis Acids to Transition Metals: Z-Type Ligands. In The Chemical Bond III: 100 Years Old and Getting Stronger; Mingos, D. M. P., Ed.; Springer International Publishing: Cham, 2017; pp 141–201. 2. (a) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1–51; (b) Braunschweig, H.; Colling, M. Eur. J. Inorg. Chem. 2003, 2003, 393–403; (c) Aldridge, S.; Coombs, D. L. Coord. Chem. Rev. 2004, 248, 535–559; (d) Braunschweig, H. Adv. Organomet. Chem. 2004, 51, 163–192; (e) Braunschweig, H.; Rais, D. Heteroat. Chem. 2005, 16, 566–571; (f ) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding 2008, 130, 1–27; (g) Braunschweig, H.; Dewhurst, R. D.; Gessner, V. H. Chem. Soc. Rev. 2013, 42, 3197–3208; (h) Braunschweig, H.; Shang, R. Inorg. Chem. 2015, 54, 3099–3106. 3. (a) Knorr, J. R.; Merola, J. S. Organometallics 1990, 9, 3008–3010; (b) Baker, R. T.; Ovenall, D. W.; Calabrese, J. C.; Westcott, S. A.; Taylor, N. J.; Williams, I. D.; Marder, T. B. J. Am. Chem. Soc. 1990, 112, 9399–9400. 4. Merola, J. S.; Knorr, J. R. J. Organomet. Chem. 2014, 750, 86–97. 5. (a) Lu, Z.; Jun, C.-H.; de Gala, S. R.; Sigalas, M.; Eisenstein, O.; Crabtree, R. H. J. Chem. Soc., Chem. Commun. 1993, 1877–1880; (b) Lu, Z.; Jun, C.-H.; de Gala, S. R.; Sigalas, M. P.; Eisenstein, O.; Crabtree, R. H. Organometallics 1995, 14, 1168–1175. 6. Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C. Can. J. Chem. 1993, 71, 930–936. 7. Nguyen, P.; Blom, H. P.; Westcott, S. A.; Taylor, N. J.; Marder, T. B. J. Am. Chem. Soc. 1993, 115, 9329–9330. 8. Dai, C.; Stringer, G.; Marder, T. B.; Baker, R. T.; Scott, A. J.; Clegg, W.; Norman, N. C. Can. J. Chem. 1996, 74, 2026–2031. 9. Lawlor, F.; Marder, T.; Norman, N.; Guy Orpen, A.; Quayle, M.; Rice, C.; Robins, E.; Scott, A.; Souza, F. S.; Whittell, G. J. Chem. Soc., Dalton Trans. 1998, 301–310. 10. Kerr, A.; Marder, T. B.; Norman, N. C.; Orpen, G. A.; Quayle, M. J.; Rice, C. R.; Timms, P. L.; Whittell, G. R. Chem. Commun. 1998, 319–320. 11. Lu, N.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Timms, P. L.; Whittell, G. R. J. Chem. Soc., Dalton Trans. 2000, 4032–4037. 12. Kawamura, K.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 8422–8423. 13. Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390–391. 14. Souza, F. E. S.; Nguyen, P.; Marder, T. B.; Scott, A. J.; Clegg, W. Inorg. Chim. Acta 2005, 358, 1501–1509. 15. Liskey, C. W.; Wei, C. S.; Pahls, D. R.; Hartwig, J. F. Chem. Commun. 2009, 5603–5605. 16. Chotana, G. A.; Vanchura, I. I. B. A.; Tse, M. K.; Staples, R. J.; Maleczka, J. R. E.; Smith, I. I. I. M. R. Chem. Commun. 2009, 5731–5733. 17. Vanchura, I. I. B. A.; Preshlock, S. M.; Roosen, P. C.; Kallepalli, V. A.; Staples, R. J.; Maleczka, J. R. E.; Singleton, D. A.; Smith, I. I. I. M. R. Chem. Commun. 2010, 46, 7724–7726. 18. (a) Segawa, Y.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 9201–9203; (b) Segawa, Y.; Yamashita, M.; Nozaki, K. Organometallics 2009, 28, 6234–6242; (c) Tanoue, K.; Yamashita, M. Organometallics 2015, 34, 4011–4017; (d) Kwan, E. H.; Kawai, Y. J.; Kamakura, S.; Yamashita, M. Dalton Trans. 2016, 45, 15931–15941; (e) Kwan, E. H.; Ogawa, H.; Yamashita, M. ChemCatChem 2017, 9, 2457–2462. 19. Tang, C. Y.; Thompson, A. L.; Aldridge, S. J. Am. Chem. Soc. 2010, 132, 10578–10591. 20. Lee, C.-I.; Zhou, J.; Ozerov, O. V. J. Am. Chem. Soc. 2013, 135, 3560–3566. 21. Schubert, H.; Leis, W.; Mayer, H. A.; Wesemann, L. Chem. Commun. 2014, 50, 2738–2740. 22. Wang, G.; Xu, L.; Li, P. J. Am. Chem. Soc. 2015, 137, 8058–8061. 23. Wang, G.; Liu, L.; Wang, H.; Ding, Y.-S.; Zhou, J.; Mao, S.; Li, P. J. Am. Chem. Soc. 2017, 139, 91–94. 24. Zou, X.; Zhao, H.; Li, Y.; Gao, Q.; Ke, Z.; Xu, S. J. Am. Chem. Soc. 2019, 141, 5334–5342. 25. (a) Shi, Y.; Gao, Q.; Xu, S. J. Am. Chem. Soc. 2019, 141, 10599–10604; (b) Song, P.; Hu, L.; Yu, T.; Jiao, J.; He, Y.; Xu, L.; Li, P. ACS Catalysis 2021, 11, 7339–7349; (c) Yang, Y.; Chen, L.; Xu, S. Angew. Chem. Int. Ed. 2021, 60, 3524–3528; (d) Du, R.; Liu, L.; Xu, S. Angew. Chem. Int. Ed. 2021, 60, 5843–5847; (e) Chen, X.; Chen, L.; Zhao, H.; Gao, Q.; Shen, Z.; Xu, S. Chin. J. Chem. 2020, 38, 1533–1537. 26. Press, L. P.; Kosanovich, A. J.; McCulloch, B. J.; Ozerov, O. V. J. Am. Chem. Soc. 2016, 138, 9487–9497. 27. Esteruelas, M. A.; Fernández, I.; Martínez, A.; Oliván, M.; Oñate, E.; Vélez, A. Inorg. Chem. 2019, 58, 4712–4717. 28. (a) Shih, W.-C.; Gu, W.; MacInnis, M. C.; Timpa, S. D.; Bhuvanesh, N.; Zhou, J.; Ozerov, O. V. J. Am. Chem. Soc. 2016, 138, 2086–2089; (b) Shih, W.-C.; Ozerov, O. V. Organometallics 2017, 36, 228–233. 29. Westcott, S. A.; Taylor, N. J.; Marder, T. B.; Baker, R. T.; Jones, N. J.; Calabrese, J. C. J. Chem. Soc., Chem. Commun. 1991, 304–305. 30. (a) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B. Angew. Chem. Int. Ed. 2001, 40, 2168–2171; (b) Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z.; Marder, T. B.; Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre, G. J. Organometallics 2003, 22, 4557–4568. 31. (a) Hiromichi, K.; Kazushi, I.; Yoichiro, N. Chem. Lett. 1975, 4, 1095–1096; (b) Männig, D.; Nöth, H. Angew. Chem. Int. Ed. Engl. 1985, 24, 878–879. 32. Baker, R. T.; Calabrese, J. C.; Westcott, S. A.; Nguyen, P.; Marder, T. B. J. Am. Chem. Soc. 1993, 115, 4367–4368. 33. Fritschi, C. B.; Wernitz, S. M.; Vogels, C. M.; Shaver, M. P.; Decken, A.; Bell, A.; Westcott, S. A. Eur. J. Inorg. Chem. 2008, 2008, 779–785. 34. Cook, K. S.; Incarvito, C. D.; Webster, C. E.; Fan, Y.; Hall, M. B.; Hartwig, J. F. Angew. Chem. Int. Ed. 2004, 43, 5474–5477. 35. Câmpian, M. V.; Harris, J. L.; Jasim, N.; Perutz, R. N.; Marder, T. B.; Whitwood, A. C. Organometallics 2006, 25, 5093–5104. 36. Tang, C. Y.; Phillips, N.; Bates, J. I.; Thompson, A. L.; Gutmann, M. J.; Aldridge, S. Chem. Commun. 2012, 48, 8096–8098. 37. Kumar, A.; Priest, I. K.; Hooper, T. N.; Weller, A. S. Dalton Trans. 2016, 45, 6183–6195. 38. Colebatch, A. L.; McKay, A. I.; Beattie, N. A.; Macgregor, S. A.; Weller, A. S. Eur. J. Inorg. Chem. 2017, 2017, 4533–4540. 39. Johnson, H. C.; Leitao, E. M.; Whittell, G. R.; Manners, I.; Lloyd-Jones, G. C.; Weller, A. S. J. Am. Chem. Soc. 2014, 136, 9078–9093. 40. Hasegawa, M.; Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem. Int. Ed. 2012, 51, 6956–6960. 41. Masuda, Y.; Hasegawa, M.; Yamashita, M.; Nozaki, K.; Ishida, N.; Murakami, M. J. Am. Chem. Soc. 2013, 135, 7142–7145. 42. Lee, C.-I.; Hirscher, N. A.; Zhou, J.; Bhuvanesh, N.; Ozerov, O. V. Organometallics 2015, 34, 3099–3102. 43. Borner, C.; Brandhorst, K.; Kleeberg, C. Dalton Trans. 2015, 44, 8600–8604. 44. Procacci, B.; Jiao, Y.; Evans, M. E.; Jones, W. D.; Perutz, R. N.; Whitwood, A. C. J. Am. Chem. Soc. 2015, 137, 1258–1272. 45. (a) Esteruelas, M. A.; Oliván, M.; Vélez, A. Organometallics 2015, 34, 1911–1924; (b) Esteruelas, M. A.; Oliván, M.; Vélez, A. J. Am. Chem. Soc. 2015, 137, 12321–12329. 46. Helm, S. W.; Linti, G.; Nöth, H.; Channareddy, S.; Hofmann, P. Chem. Ber. 1992, 125, 73–86.

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47. (a) Dai, C.; Stringer, G.; Corrigan, J. F.; Taylor, N. J.; Marder, T. B.; Norman, N. C. J. Organomet. Chem. 1996, 513, 273–275; (b) Adams, C. J.; Baber, R. A.; Batsanov, A. S.; Bramham, G.; Charmant, J. P. H.; Haddow, M. F.; Howard, J. A. K.; Lam, W. H.; Lin, Z.; Marder, T. B.; Norman, N. C.; Orpen, A. G. Dalton Trans. 2006, 1370–1373. 48. Drescher, W.; Schmitt-Monreal, D.; Jacob, C. R.; Kleeberg, C. Organometallics 2020, 39, 538–543. 49. Tran, B. L.; Adhikari, D.; Fan, H.; Pink, M.; Mindiola, D. J. Dalton Trans. 2010, 39, 358–360. 50. Obligacion, J. V.; Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 4133–4136. 51. Rummelt, S. M.; Zhong, H.; Léonard, N. G.; Semproni, S. P.; Chirik, P. J. Organometallics 2019, 38, 1081–1090. 52. Arevalo, R.; Pabst, T. P.; Chirik, P. J. Organometallics 2020, 39, 2763–2773. 53. (a) Lin, T.-P.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310–15313; (b) Lin, T.-P.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 13672–13683. 54. (a) Segawa, Y.; Yamashita, M.; Nozaki, K. Science 2006, 314, 113–115; (b) Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem. Int. Ed. 2007, 46, 6710–6713; (c) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 9570–9571; (d) Segawa, Y.; Suzuki, Y.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2008, 130, 16069–16079; (e) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K. Chem. Lett. 2008, 37, 802–803; (f ) Nozaki, K.; Aramaki, Y.; Yamashita, M.; Ueng, S.-H.; Malacria, M.; Lacôte, E.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 11449–11451; (g) Yamashita, M.; Nozaki, K. J. Synth. Org. Chem., Jpn. 2010, 68, 359–369; (h) Hayashi, Y.; Segawa, Y.; Yamashita, M.; Nozaki, K. Chem. Commun. 2011, 47, 5888–5890; (i) Yamashita, M. Bull. Chem. Soc. Jpn. 2011, 84, 983–999; (j) Dettenrieder, N.; Aramaki, Y.; Wolf, B. M.; Maichle-Mössmer, C.; Zhao, X.; Yamashita, M.; Nozaki, K.; Anwander, R. Angew. Chem. Int. Ed. 2014, 53, 6259–6262; (k) Yamashita, M.; Nozaki, K. Boryl Anions. In Synthesis and Application of Organoboron Compounds; Fernández, E., Whiting, A., Eds.; Springer International Publishing, 2015; pp 1–37; (l) Asami, S.-S.; Okamoto, M.; Suzuki, K.; Yamashita, M. Angew. Chem. Int. Ed. 2016, 55, 12827–12831; (m) Ohsato, T.; Okuno, Y.; Ishida, S.; Iwamoto, T.; Lee, K.-H.; Lin, Z.; Yamashita, M.; Nozaki, K. Angew. Chem. Int. Ed. 2016, 55, 11426–11430; (n) Asami, S.-S.; Ishida, S.; Iwamoto, T.; Suzuki, K.; Yamashita, M. Angew. Chem. Int. Ed. 2017, 56, 1658–1662; (o) Kisu, H.; Kosai, T.; Iwamoto, T.; Yamashita, M. Chem. Lett. 2021, 50, 293–296. 55. (a) Frank, R.; Howell, J.; Campos, J.; Tirfoin, R.; Phillips, N.; Zahn, S.; Mingos, D. M. P.; Aldridge, S. Angew. Chem. Int. Ed. 2015, 54, 9586–9590; (b) Frank, R.; Howell, J.; Tirfoin, R.; Dange, D.; Jones, C.; Mingos, D. M. P.; Aldridge, S. J. Am. Chem. Soc. 2014, 136, 15730–15741.

8.04

Group 9 and 10 Carbonyl Clusters

Cristina Femonia, Cristiana Cesaria, Maria Carmela Iapaluccia, Silvia Ruggierib, and Stefano Zacchinia, aDepartment of Industrial Chemistry “Toso Montanari”, University of Bologna (IT), Bologna, Italy; bLaboratory of Luminescent Materials, Department of Biotechnology, University of Verona, Verona, Italy © 2022 Elsevier Ltd. All rights reserved.

8.04.1 Introduction 8.04.2 Cobalt 8.04.2.1 Homometallic cobalt carbonyl clusters: DFT studies on unsaturated Co4(CO)n species 8.04.2.2 Homometallic cobalt clusters containing main-group elements 8.04.2.3 Heteroleptic cobalt carbonyl clusters 8.04.2.4 Heterometallic cobalt carbonyl clusters containing main-group elements 8.04.2.5 Cobalt carbonyl clusters as precursors in the synthesis of metal nanoparticles 8.04.2.6 Cobalt carbonyl clusters in catalysis 8.04.3 Rhodium 8.04.3.1 Homometallic rhodium carbonyl clusters 8.04.3.2 Heteroleptic rhodium carbonyl clusters 8.04.3.3 Heterometallic rhodium carbonyl clusters 8.04.3.4 Homometallic rhodium carbonyl clusters containing post-transition metals 8.04.3.5 Carbonyl fluxionality studies in rhodium clusters 8.04.3.6 Kinetic studies on Rh-nitride cluster formation 8.04.3.7 Rhodium carbonyl clusters in catalysis 8.04.4 Iridium 8.04.4.1 Homometallic iridium carbonyl clusters 8.04.4.2 Heteroleptic iridium carbonyl clusters 8.04.4.3 Heterometallic heteroleptic iridium carbonyl clusters 8.04.5 Nickel 8.04.5.1 Homometallic nickel carbonyl clusters containing main-group elements 8.04.5.2 Homometallic nickel carbonyl clusters containing post-transition metals 8.04.5.3 Heterometallic nickel carbonyl clusters 8.04.5.4 Heterometallic nickel carbonyl clusters containing main-group elements 8.04.6 Palladium 8.04.6.1 Homometallic heteroleptic palladium carbonyl clusters 8.04.6.2 Heteroleptic palladium carbonyl clusters containing post-transition metals 8.04.6.3 Heterometallic heteroleptic palladium carbonyl clusters 8.04.7 Platinum 8.04.7.1 Chini clusters 8.04.7.2 Other homometallic platinum carbonyl clusters 8.04.7.3 Homometallic platinum carbonyl clusters containing post-transition metals 8.04.7.4 Heteroleptic platinum carbonyl clusters 8.04.7.5 Heterometallic platinum carbonyl clusters 8.04.8 Conclusion Acknowledgment References

206 207 207 208 209 209 213 214 215 215 217 218 219 222 224 225 225 225 226 229 236 237 239 241 243 248 248 250 250 252 252 255 258 260 263 266 266 266

Nomenclature en dppm dppe dppa dppp dppmMe dpam dppmb dbu bpcd

Ethylenediamine Ph2PCH2PPh2 Ph2P(CH2)2PPh2 Ph2PNHPPh2 Ph2P-(CH2)3PPh2 Ph2PCH(CH3)PPh2 Ph2AsCH2AsPh2 1,4-Bis(diphenylphosphinomethyl)benzene 1,8-Diazabicyclo[5.4.0]undec-7-ene 4,5-Bis(diphenylphosphino)-4-cyclopenten-1,3-dione

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00151-7

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acac P^P dppf IPr IMes PTA Cp COD Bipy Bpy Pyz p-DCB tpy macro 2-PTZ EtV TBA THF DMSO MCM-41 CPO IR CV ESI-MS MALDI EPR SQUID DLS XPS OTTLE Cell TEM HRTEM EDS CVE EHMO

8.04.1

Acetylacetonate CH2]C(PPh2)2 Fe(C5H4PPh2)2 C3N2H2(C6H3iPr2)2 C3N2H2(C6H2Me3)2 1,3,5-Triaza-7-phosphaadamantane Z5-C5Me5 Cyclooctadiene 4,40 -Bipyridine 2,20 -Bipyridyl Pyrazine p-Dicyanobenzene 2,20 :60 ,200 -Terpyridine 5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraaza-4,11-cyclotetradecadiene 5-(2-Piridyl)-tetrazolate 1,1-Diethyl-4,4-bipyridilium cation Tetrabutylammonium cation Tetrahydrofuran Dimethyl sulfoxide Mobil Composition of Matter n. 41. It is a mesostructured silica Catalytic partial oxidation Infrared Cyclic voltammetry Electro spray ionization mass spectrometry Matrix-Assisted Laser Desorption/Ionization Electron paramagnetic resonance spectroscopy Superconducting QUantum Interference Device Dynamic Light Scattering X-ray photoemission spectroscopy Optically Transparent Thin Layer Electrochemical Cell Transmission Electron Microscopy High-resolution transmission electron microscopy Energy Dispersive X-ray Spectrometry Cluster Valence Electrons Extended Hückel Molecular Orbitals

Introduction

The chemistry of carbonyl clusters of group 9 and 10 metals presented in this chapter aims to cover the scientific literature within this field that has been published from 2006 onwards, as the preceding years are reported in COMC III. This chapter focuses on the latest findings regarding heteroleptic and homoleptic species with a nuclearity of four or more metal atoms. The chapter is divided in separated sections, one for each metal, and in sub-sections gathering the reported compounds in categories, hopefully making them easier to find for the reader. “Heterometallic species” refer to compounds of the main group 9 or 10 metals together with either other transition or with post-transition metals. Conversely, in the sub-sections stating “containing post-transition metals” we intend clusters of the specific element where post-transition metals are actually contained inside the metal cluster frame. Nonetheless, in some cases the two sub-categories are entangled. The same logic has been applied when considering clusters containing main-group elements, that is elements belonging to either the 2nd or 3rd row of the periodic table. As the structural aspects are crucial, we reported various crystal structures of selected compounds.1 The chemistry of metal carbonyl clusters started with the pioneering work of Walter Otto Hieber, and it highly developed in the 1970s and 1980s especially thanks to the systematic studies of the research groups of Brian F. G. Johnson and Lord Jack Lewis in Cambridge, Paolo Chini in Milan and, later, his pupil Giuliano Longoni, who after the premature demise of Chini continued his studies in Bologna. The concurrent improvement of crystallographic diffractometers was essential, as they allowed to determine the molecular structures of the isolated compounds that, owing to their dimensions, were very challenging for the existing techniques. Among the many crystallographers that worked in the field we cannot fail to mention Lawrence F. Dahl, who recently passed away. Together with the late Paolo Chini, the world of metal clusters, especially of high nuclearity, has lost another one of its more esteemed representatives.

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After a decrease of interest in the 1990s, cluster chemistry gained a renewed attention in the 2000s, also due to the outbreak of nanotechnology, and the realization that large molecular carbonyl clusters are, in the end, metal nanoparticles stabilized by CO ligands with the advantage of being atomically precise.

8.04.2

Cobalt

Over the last fifteen years, the chemistry of cobalt carbonyl clusters has been greatly developed mainly by the Italian research groups in Bologna and Milan, but also by eminent groups led by Braunstein and de Jong, among others, as well as by the work of Berben on catalysis. In this chapter a survey of the latest findings will be performed. In this time frame, no new purely tetra-nuclear (or greater) homometallic compounds have been reported, and the only related reported papers regard theoretical studies.

8.04.2.1

Homometallic cobalt carbonyl clusters: DFT studies on unsaturated Co4(CO)n species

The first homoleptic tetranuclear metal carbonyl to be prepared was the neutral cobalt Co4(CO)12 species, which was first reported in 1932.2 The subsequent crystallographic studies3,4 unraveled its full molecular structure, including the distribution of the carbonyl groups, nine of which are terminally coordinated, while the remaining three are edge-bridging ligands, paralleling those of Rh4(CO)12. A possible isomer with a full tetrahedral symmetry, therefore all terminal carbonyl groups like in the case of Ir4(CO)12, has not been isolated yet. Irrespective of the carbonyl coordination, all above isomers follow the 18-electron configuration. Removal of CO groups from Co4(CO)12 to give Co4(CO)n (n ¼ 11, 10, 9, 8, etc.) is expected to lead to unsaturated derivatives, possibly compensated by multiple metal bonding or structural rearrangement. A modern DFT study5 on Co4(CO)11 revealed, among other possibilities, a butterfly array of cobalt atoms with an unexpected new structural feature, namely a m4-carbonyl group bridging all four metal atoms (see Fig. 1). Such a CO group is of particular interest since its carbon–oxygen bond order is relatively low, as indicated by its predicted n(CO) IR stretching frequency of 1636 cm−1 and CdO distance of 1.18 A˚ , against the experimental values of 1866 cm−1 and 1.133 A˚ , respectively, for edgebridging COs. Related m4-carbonyl groups are found in the more complicated heterometallic derivatives (Z5-C5H5)4Mo2 Ni2S2(m4-CO)6 and Rh4(CO)4(m-CO)4(m4-CO)(PtBu3)2[Pt(PtBu3)].7 In the latter compound, for sake of comparison, the n(CO) IR stretching frequency of the m4-carbonyl group is 1704 cm−1, and its CdO distance 1.168 A˚ . Such configuration could be stabilized by replacing some of the terminal CO groups with small-bite strong p-acceptor ligands such as CH3N(PF2)2, which can stabilize metal–metal bonds by forming stable five-membered M2P2N ring systems.8 In a following paper, the further unsaturated Co4(CO)n clusters (n ¼ 10, 9) have been compared with the analogous species with the heavier Ir atoms.9 As mentioned earlier, the Ir4(CO)12 cluster firstly discovered by Hieber and Lagally in 1940,10 possesses a tetrahedral geometry like the cobalt congener, but shows no bridging CO groups as each Ir atom is coordinated with three terminal carbonyl ligands.11

Fig. 1 Calculated Co4(CO)11 structure with a m4-carbonyl group bridging the four metal atoms. Reproduced with permission from Zhang, X.; Li, Q.; Xie, Y.; King, R. B.; Schaefer III, H. F. Eur. J. Inorg. Chem. 2008, 2158–2164.

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From a DFT investigation all low-energy Co4(CO)n structures were calculated to have several bridging carbonyl groups including face-bridging m3-COs. However, most of the low-energy Ir4(CO)n structures have only one or two edge-bridging m-CO groups and no face-bridging carbonyls. Furthermore, many low energy Co4(CO)n structures have central Co4 butterflies, whereas almost all of the low energy Ir4(CO)n ones show central Ir4 tetrahedra.

8.04.2.2

Homometallic cobalt clusters containing main-group elements

Transition metal clusters are at the interface between molecular and solid-state chemistry. Such compounds are of great interest also because of their potential to exhibit several electrochemically reversible or quasi-reversible redox systems without structural rearrangement of the cluster skeleton, thus being multivalent.12 When metal carbonyl clusters contain main-group elements, giving rise to, for instance, carbide species, they strengthen the metal core thus such multivalence properties are enhanced, or even permitted.13 Over the years, some alternative routes have been uncovered to synthetize metal carbonyl clusters, in some cases by avoiding high temperature and/or pressure conditions. While in most cases the designed strategy succeeded, some unexpected results also were obtained. For instance, in the attempt of preparing cobalt compounds containing interstitial boron atoms, Co2(CO)8 was reacted with Cp NbBH4.14 However, the net result was the formation of the already known [Co6(CO)15C]2−15 and [Co13(CO)24C2]3−16 carbide clusters, while boride compounds were only presented in minor fractions. Several homometallic cobalt carbide carbonyl clusters are known, and most of them can be prepared starting from the trigonal prismatic [Co6C(CO)15]2− compound.17 As a matter of fact, in 2014 a thorough chemical investigation reported the synthesis and characterization of several high nuclearity cobalt carbide species, not only via thermal reactions but also by redox transformations (Fig. 2). Among these species, six were new ones, namely [Co11C2(CO)23]−, [Co11C2(CO)23]2−, [Co10(C2)(CO)21]2−, [Co8C(CO)17]4−, [Co6C(CO)12]3−, and [Co7C(CO)15]3−.18 All clusters were fully structurally characterized, and the paramagnetic [Co11C2(CO)23]2− and [Co7C(CO)15]3− species were analyzed via electron paramagnetic resonance (EPR) spectroscopy. Moreover, [Co11C2(CO)23]− was also studied via electrochemical experiments, which allowed to observe that the cluster can undergo one reversible oxidation and one reversible reduction step. Not only can a carbide C atom be found inside metal carbonyl clusters, but also other main-group elements belonging to the 2nd and 3rd row, and well as beyond. In 2010 the isolation of a cobalt carbonyl compound containing semi-interstitial phosphorous atoms19 was reported from the reaction between the [(CO)4W(PH3)2] complex with [Co2(CO)8]. Out of the new species, namely [Co10(m7-P)2(CO)24], [Co8(m6-P)2(CO)19], and [[Co3(CO)8{m4-PW(CO)5}][(m4-P)Co3(CO)9]], the former and higher nuclearity cluster is isostructural with the analogous [HCo10(m7-P)2(CO)23] hydride derivative, and possesses a metal framework made of a central Co4P2 octahedron to which two triangular Co3 fragments are joint, resulting in an epta-coordination of the P atoms.20 Its molecular structure is depicted in Fig. 3. The novelty of this work stands in the use of the [(CO)4W(PH3)2] as a source of phosphorous atoms in the synthesis of new cluster species. The possible multivalence properties of this new phosphide cluster have not been explored. The presence of main-group elements in carbonyl clusters may favor their multivalence by strengthening the metal core. Carbonyl-telluride cobalt clusters have also demonstrated multivalence features, such as in [Co11Te7(CO)10]n− (n ¼ 1, 2).21 In 2007, the same authors identified a novel species, namely [Co11Te5(CO)15]n− (n ¼ 1, 2),22 that could be obtained in very low yield in the same and previously studied reaction between the di-nuclear Co2(CO)8 and Cp 2Nb(Te2H). Similarly, this cluster possesses a Co-centered pentagonal prismatic geometry made by the cobalt atoms and stabilized by five tellurides capping as many prismatic faces, and fifteen terminal carbonyl ligands (see Fig. 4).

Fig. 2 Reaction scheme starting from the [Co6C(CO)15]2− cluster carbide precursor. Reproduced with permission from: Ciabatti, I.; Femoni, C.; Hayatifar, M.; Iapalucci, M. C.; Longoni, G.; Pinzino, C.; Solmi, M. V.; Zacchini, S. Inorg. Chem. 2014, 53, 3818–3831.

Group 9 and 10 Carbonyl Clusters

209

Fig. 3 Molecular structure and metal core of [Co10(m7-P)2(CO)24] (Co atoms in blue; P atoms in green; C in gray; O in red).

Fig. 4 Molecular structure of [Co11Te5(CO)15]n-: (left) top and (right) side view (Co atoms in blue; Te atoms in yellow; C in gray; O in red).

The importance of [Co11Te5(CO)15]n− lies in its remarkable multivalence behavior, as it can sustain electron addition, going from [Co11Te5(CO)15]− to [Co11Te5(CO)15]5−. The higher negative charge is accompanied by an increasing distortion of the metal architecture. This electron-sponge behavior is observed more and more in clusters with a nuclearity starting from around 10 metal atoms.13 Studies on predictions of the magnetic behavior for clusters with similar geometries have been carried out in the same reported research, and experimentally verified for previously known [Co11Te7(CO)10]n-.

8.04.2.3

Heteroleptic cobalt carbonyl clusters

In 2008, Braunstein et al.23 reported a work on the preparation of a beautiful, organized molecular structure composed of four Co4 cluster units around an organic core bearing four -N(PPh3)2 donors and sulfur spacers, forming a nano-object with possible novel properties and applications. This new architecture was synthesized by reacting carbonyl-substituted Co4(CO)10(dppx) (x ¼ m and a; dppm ¼ Ph2PCH2PPh2; dppa ¼ Ph2PNHPPh2) with the novel 1,2,4,5-{(Ph2P)2NCH2CH2SCH2}4C6H2 ligand.24 The crystal structure of the dppa-substituted oligomeric structure was determined by X-ray diffraction studies (Fig. 5). Notably, the ancillary dppa ligand prevented further substitution and played the role of a stopper.

8.04.2.4

Heterometallic cobalt carbonyl clusters containing main-group elements

The synthesis of heterometallic carbonyl clusters can be also achieved by redox-condensation methods that involve reaction of a carbonyl cluster precursor of one specific metal, where the metal possesses a negative oxidation state, with a different-metal complex where the oxidation state is, conversely, positive.25 This synthesis has proven to be very successful for many mixed-metal species of groups 9 and 10 elements. In the case of cobalt, for instance, by reacting the [Co6C(CO)15]2− carbide species with PtCl2(Et2S)2 it is possible to obtain the heterometallic even-electron [Co8Pt4C2(CO)24]2− cluster.26 Its metal cage consists of a tri-octahedron with shared faces, resulting in a hexagonal close-packed fragment containing an abab stacking of four triangles. The outer ones are exclusively made by Co atoms, whereas the six inner positions, corresponding to the central octahedron, are disordered Pt and Co sites, giving rise to three possible isomers (Fig. 6). Electrochemical studies via CV experiments on [Co8Pt4C2(CO)24]2− revealed that the cluster can reversibly undergo two reduction and one oxidation processes, therefore showing multivalence features. Notably, the paramagnetic monoanion could be also obtained by chemical reduction, and its isolation allowed its full structural characterization by X-ray single-crystal diffraction. EPR experiments carried out on [Co8Pt4C2(CO)24]− confirmed the presence of an unpaired electron. However, the most striking

210

Group 9 and 10 Carbonyl Clusters

Fig. 5 Molecular structure of {Co4(CO)10(dppa)}4{(Ph2P)2NCH2CH2SCH2}4C6H2. Phenyl rings and hydrogen atoms have been omitted for sake of clarity (Co atoms in blue; P atoms in green; S atoms in yellow; N in orange; C in gray; O in red).

Fig. 6 The possible isomers of [Co8Pt4C2(CO)24]2−. Only the metal skeleton is shown (Co atoms in blue; Pt atoms in magenta). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Wolowska, J.; Zacchini, S.; Zanello, P.; Fedi, S.; Riccò, M.; Pontiroli, D.; Mazzani, M. J. Am. Chem. Soc. 2010, 132, 2919–2927.

results of this study are that EPR analysis revealed the presence of unpaired electrons also in the even-electron [Co8Pt4C2(CO)24]2−, unraveling a S ¼ 1 triplet state, confirmed by SQUID measurements. The same triplet state was observed for the isostructural [Fe6Ni6N2(CO)24]2−27 and [Co10Rh2N2(CO)24]2−28 nitride compounds, suggesting that such magnetic feature is independent from the metal composition and the interstitial atoms. The latter new Co-Rh nitride cluster was obtained, together with its [Co10RhN2(CO)21]3− and [Co11RhN2(CO)24]2− congeners, by reacting the [Co10N2(CO)19]4−/[Co11N2(CO)21]3− nitride precursors29,30 with a Rh(I) complex, like [Rh(CO)2(MeCN)2]+ or [Rh(CO)2Cl2]−, with different stoichiometric ratios. The dodeca-heterometallic [Co10Rh2N2(CO)24]2− and [Co11RhN2(CO)24]2− clusters exhibit a virtual identical CV profile and undergo three reduction and one oxidation processes, all reversible (Fig. 7), while the redox profile of the [Co10RhN2(CO)21]3− species shows a more complicated pattern. Like their CV profiles, the molecular structures of the dodeca-nuclear [Co10Rh2N2(CO)24]2− and [Co11RhN2(CO)24]2− are also very similar and resemble that of the previously mentioned [Co8Pt4C2(CO)24]2−. Likewise, the occurrence of a positional disorder of the heavier Rh atoms over the inner octahedral positions, is in line with their higher metal-metal bond energies,31 as the central octahedron shows the largest metal connectivity. A comparative analysis of M12C2 metal structures, mostly Co-containing, was performed in 2012, when the novel [Co10Pt2C2(CO)22]4−, [Co8Pt4C2(CO)20]4− and [Co10–xPt2+ xC2(CO)24]2− (x ¼ 0–2) clusters were obtained by chemical reduction of [Co8Pt4C2(CO)24]2− with Na/naphthalene.32 Notably, the two [Co10Pt2C2(CO)22]4−, [Co8Pt4C2(CO)20]4− species show multivalence behavior, like their parent compound. In particular, the former cluster can reversibly release up to one electron and accept up

Group 9 and 10 Carbonyl Clusters

211

10 PA E (V, vs. SCE) –2.0

–1.5

–1.0

–0.5

0.0

+0.5 2−

Fig. 7 Cyclic voltammetric response recorded at a platinum electrode in a MeCN solution of the [Co10Rh2N2(CO)24] dianion (1.2  10−3 mol dm−3) with [NBu4] [PF6] (0.2 mol dm−3) supporting electrolyte and a scan rate of 0.2 V s−1. Reproduced with permission from: Costa, M.; Della Pergola, R.; Fumagalli, A.; Laschi, F.; Losi, S.; Macchi, P.; Sironi, A.; Zanello, P. Inorg. Chem. 2007, 46, 552−560.

to two more, therefore with a charge going from −3 to −6, without breaking of the metal structure; the latter, conversely, shows the same propensity in oxidation but it is able to exist also with a charge up to −7, at least within the time scale of the electrochemistry experiments. With respect to the structural comparative analysis of dodeca- di-carbide metal clusters, the experimental evidence reveals a wide series of possible arrangements. These are illustrated in Fig. 8. The [Co8Pt4C2(CO)24]2−, [Co10–xPt2+ xC2(CO)24]2−, [Co10Rh2N2(CO)24]2−, [Co11RhN2(CO)24]2− and [Fe6Ni6N2(CO)24]n– (n ¼ 2–4) clusters possess a number of CVE of 168–170, show multivalent behavior, are all isostructural and based on the same stacking of three face-sharing octahedra (Fig. 8A). The [Ru10Pt2C2(CO)28]2− species33 is not isoelectronic with those just mentioned, having 166 CVE, but again its structure is based on three octahedra, this time edge-sharing (Fig. 8B). According to the condensed polyhedral approach proposed by Mingos,34 a three-face-sharing-octahedra cluster should have 162 CVE, so all above-mentioned

Fig. 8 Possible structural combination for dodeca-nuclear dicarbide carbonyl clusters from experimental data (metal atoms in gray; C atoms in white). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S.; Fedi, S.; Fabrizi de Biani, F. Eur. J. Inorg. Chem. 2012, 2243–2250.

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clusters are more electron rich, and this may account for their redox properties. Another possible structural arrangement is shown by the isostructural and isoelectronic (166 CVE) [Co10Pt2C2(CO)22]4−, [Rh12C2(CO)24]2− and [Rh12C2(CO)23]4− clusters,35 with the carbide atoms inside two edge-sharing capped trigonal prisms (Fig. 8C). The condensed polyhedral approach would predict in this case four extra electrons (i.e., 170 CVE). This structural motif is also observed in [Rh12C2(CO)24(AuPPh3)]− (166 CVE)36 but the prisms are connected through one edge, thus generating a third (empty) one (Fig. 8D). The [Co8Pt4C2(CO)20]4− anion (164 CVE) shows a structure based on two capped trigonal prisms sharing a basal edge (Fig. 8E). Conversely, [HRh12N2(CO)23]3− (168 CVE)37 derives by the condensation of a capped trigonal prism with a capped distorted square antiprism (Fig. 8F). The mono-acetylide [Ni12(C2)(CO)16]4−38 displays a structure composed of two face-sharing capped trigonal prisms further capped by two additional Ni units on opposite triangular faces (Fig. 8G). Similarly, the mono-acetylide [Rh12(C2)(CO)25]39 (Fig. 8H) is structurally related to [HRh12N2(CO)23]3− (Fig. 8F) but with a strong distortion, allowing the formation of a bonded C2 unit. In conclusion, despite their same nuclearity, these M12E2 clusters display very different structures based on the fusion of E-centered edge- or face- sharing octahedra, trigonal prisms and square antiprisms, and with a different number of carbonyl ligands. Their structural diversity is most likely due to subtle balances among the M–M, M–E and M–CO interactions, coupled with metal size, and indicates the large adaptability of metal clusters cages. In 2013, by exploiting the redox condensation between the [Co6(CO)15]2− carbide cluster and a Pd(II) complex, the new high-nuclearity heterometallic [H6− nCo20Pd16C4(CO)48]n− (n ¼ 3–6) carbide species had been prepared.40 While the presence of the hydride has been established by indirect methods, such as electrochemical studies on the different hydride species and chemical deprotonation and protonation reactions, the crystal structures of the bi and mono-hydrides have been fully elucidated by X-ray diffraction analyzes. Both possess the same metal framework, shown in Fig. 9, which is based on a Pd16 core arranged in a cubic close packing fashion, decorated by four square-pyramidal {Co5C(CO)12} moieties. Notably, the chemical reduction of the [H2Co20Pd16C4(CO)48]4− dihydride and its reaction with PPh3 allowed to isolate several other both homoleptic and heteroleptic clusters, more specifically [H6−nCo16Pd2C3(CO)28]n− (n ¼ 5, 6), [Co4Pd2C(CO)11(PPh3)2], [Co2Pd5C(CO)8(PPh3)5], and [Co4Pd4C2(PPh3)4(CO)10Cl]−. A year later, another new cluster was prepared by using the same CoPd carbide tetra-anionic precursor, that is [HCo15Pd9C3(CO)38]2−.41 Despite its even-electron count, SQUID measurements proved that the cluster is paramagnetic with two unpaired electrons, in analogy with what found for the earlier-mentioned [Co8Pt4C2(CO)24]2−. The crystal structure determination of the four [H3−nCo15Pd9C3(CO)38]n− (n ¼ 0–3) species allowed to unravel a significant structural rearrangement of the Pd9 metal core, which goes from a tri-capped octahedron when n ¼ 0, 1 to a tri-capped trigonal prism when n ¼ 2, 3. This change suggests that interstitial hydrides may have relevant stereochemical effects even in large carbonyl clusters. However, packing effect due to the counterion and/or the solvent molecules within the unit cell may result in similar structural isomerism, as inferred by further studies on [H2Co15Pd9C3(CO)38]2− from the same group, reported in 2018.42 The molecular structure of [HCo15Pd9C3(CO)38]2− is presented in Fig. 10. Several new Co-Au carbide clusters have also been prepared in the last fifteen years, again exploiting the ease of redox condensation exhibited by [Co6C(CO)15]2−. Its reaction with 2 equiv. of [AuCl4]− gave rise to the new [{Co5C(CO)12}Au {Co(CO)4}]− bimetallic cluster. From this species, the dimeric [{Co5C(CO)12}2Au]− can be obtained by treatment with HBF4, while the new [Co5C(CO)12(AuPPh3)], and [Co5C(CO)11(AuPPh3)3] derivatives can be isolated upon addition of [Au(PPh3)Cl] to the dimeric cluster.43 Interestingly, both [{Co5C(CO)12}Au{Co(CO)4}]− and [{Co5C(CO)12}2Au]− display a very rich redox activity, as inferred by spectroelectrochemical studies coupled with CV analyzes. A very interesting structural analysis was performed successively on three different solvates of the Co6C(CO)12(AuPPh3)4 derivative.44 Notably, this cluster can be seen as the adduct product of the [Co6C(CO)12]4− unit (which is actually not know, as opposed to its parent tri-anion) and four [Au(PPh3)]+ fragments. The Co6C(CO)12(AuPPh3)4 cluster has been structurally characterized in three isomers, which present different distribution of the [Au(PPh3)]+ ligands (see Fig. 11).

Fig. 9 Metal skeleton of the [H6− nCo20Pd16C4(CO)48]n− (n ¼ 4, 5) (Co atoms in blue; Pd atoms in green; C in gray).

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213

Fig. 10 Molecular structure of [HCo15Pd9C3(CO)38]2− (Co atoms in blue; Pd atoms in green; C in gray; O in red).

Fig. 11 Co6CAu4 cores of the three isomers of Co6C(CO)12(AuPPh3)4. Reproduced with permission from:Ciabatti, I.; Femoni, C.; Hayatifar, M.; Iapalucci, M. C.; Ienco, A.; Longoni, G.; Manca, G.; Zacchini, S. Inorg. Chem. 2014, 53, 9761− 9770.

Those different structural arrangements appear to be a consequence of the number of co-crystallized THF molecules (0, 1, and 4). Theoretical investigations suggest that the formation in the solid state of the three isomers is governed by packing and van der Waals forces, as well as aurophilic and weak p–p and p–H interactions. It is, however, very important to underline that the CodCo bond distances in the three isomers are virtually the same, indicating the robustness of the cluster metal core, enhanced by the carbide presence. An expansion of the investigation of the reaction between [Co6C(CO)15]2− and [Au(PPh3)Cl] under different conditions has been carried out a year later, in 2015, and several new adducts have been isolated and characterized, namely [Co5C (CO)11(AuPPh3)3], [Co5C(CO)11(AuPPh3)2]−, [Co5C(CO)10(PPh3)2(AuPPh3)], [Co5C(CO)12(AuPPh3)], [Co5C(CO)11(PPh3) (AuPPh3)], and [Co4C(CO)10(AuPPh3)2].45

8.04.2.5

Cobalt carbonyl clusters as precursors in the synthesis of metal nanoparticles

Metal carbonyl compounds can generally be used as valid precursors in the preparation of metallic nanoparticles. Moreover, when they possess a mixed metal composition, a retention of their atomic ratio may be envisaged, as carbonyl clusters are molecular compounds of specific composition. For instance, in 2006, THF solutions of the already known homoleptic [Co3Ru(CO)12]4−46 have been deposited into mesoporous silica matrixes via impregnation techniques,47 and subsequently thermally decomposed to obtain bimetallic nanoparticles.48 The already low oxidation states of the metals in the carbonyl cluster precursor are essential to avoid further reducing atmosphere during the thermal treatment. Notably, while the pure unsupported [Co3Ru(CO)12]4− cluster

214

Group 9 and 10 Carbonyl Clusters

starts to degrade at 150  C and it is fully decomposed at 220  C, when it is incorporated inside mesoporous silica its stability is significantly enhanced, as the full degradation temperature raises by over 100  C. The obtaining of bimetallic nanoparticles was confirmed by magnetic measurements. In fact, the [Co3Ru(CO)12]4− species is diamagnetic, however a rapid increase of magnetization was observed after decomposition, consistent with the formation of the ferromagnetic Co3Ru nanoparticles. However, segregated Co and Ru nanoparticles were observed before formation of the final alloy with the same composition as the original cluster.

8.04.2.6

Cobalt carbonyl clusters in catalysis

One of the applications of transition-metal carbonyl clusters is their use as precursors for preparing supported metal catalysts, as pointed out in the previous paragraph. As a main advantage in exploiting such species there is the low oxidation state of the metal (often either zero or negative), ensured by the nature of the CO-metal bond, which means that there is no need for carbonyl clusters to undergo high-temperature reduction processes. Moreover, heterometallic carbonyl clusters have a potential more efficient metal synergy. Fischer-Tropsch (F-T) synthesis is one of the most promising alternative routes for converting natural gas and coal into liquid fuels. Cobalt-based catalysts are the preferred ones due to high activity and selectivity towards linear hydrocarbons, and low activity for the water-gas shift (WGS) reaction. Recent studies have been carried out in order to compare the catalytic performances of Co-based catalysts supported on g-Al2O3 prepared by using various cobalt carbonyl compounds as precursors, like Co2(CO)6HCCCOOH, Co3(CO)9CCH2COOH, and Co4(CO)10HCCCOOH,49 and their activity was also compared with those obtained by employing Co(NO3)2.50 The outcomes pointed out that the dispersion of the species derived from the cobalt carbonyl clusters was better than that observed with Co(NO3)2, which resulted in better catalytic behavior in the F-T reaction. As for the surface acidity, the Co3/g-Al2O3 catalyst with less acid sites gave higher CO conversion and C5+ selectivity than the Co2/g-Al2O3 and Co4/g-Al2O3 ones. Another very recent interesting work is by Berben et al., who reported the use of high-nuclearity cobalt carbide clusters, namely [Co13C2(CO)24]4−51 and [Co11C2(CO)23]3−,52 to study the kinetics of proton transfer (PT) occurring in Hydrogen Evolution Reactions (EHR). Metal carbonyl clusters (MCCs) can be exploited as molecular electrocatalysts promoting fast PT reactions, as they form hydride species.53 Also, as their nuclearity increases, they possess a lower energy associated with electron transfer (ET), with respect to single-site molecular electrocatalysts, as illustrated by the cobalt carbide [Co6C(CO)12]−/2−/3−, [Co11C2(CO)24], and [Co13C2(CO)24]3−/4−/5−. Their first very thorough work reports studies on the proton and electron transfer of [Co13C2(CO)24]4−, which was found to be able to catalyze hydrogen evolution at −1.06 V vs. SCE with observed PT that is diffusion-limited. The heterogeneous ET rate constant (ks) for the cluster reduction was obtained from variable scan rate CVs recorded under N2, and its determined value of 0.35 cm s−1 appeared to be quite fast compared with that of the CoIII/II (0.003–0.051 cm s−1). The cluster proton affinity was measured by reaction of [Co13C2(CO)24]4− with anilinium tetrafluoroborate, monitored via IR spectrometry, and the PT process was found to be limited by diffusion with a determined rate of 2.3  109 M−1 s−1. The two ET and PT analyzes allowed to formulate a possible mechanism for H2 evolution due to the multivalence and proton acceptor properties of the [Co13C2(CO)24]4− anion (Fig. 12). A further deepening of this studies was carried out in 2021 also on the [Co11C2(CO)23]3− cluster compound, with a separated analysis of the two-step proton transfer process that occurs in HER catalyzed reactions. The overall results pointed out that the first fast protonation reaction is supported by large carbonyl clusters because of the presence of many protonation sites at the cluster surface, while the second PT event, which proceeds through hydride intermediates and after which H2 evolves, is the rate-limiting step.

Fig. 12 Proposed mechanism for electrocatalytic HER by [Co13C2(CO)24]4−. Pink highlights the starting point in the cycle. Sequential ET-PT pathways shown at right side of figure may be concerted. Reproduced with permission from: Carr, C. R.; Taheri, A.; Berben, L. A. J. Am. Chem. Soc. 2020, 142, 12299–12305.

Group 9 and 10 Carbonyl Clusters

8.04.3

215

Rhodium

Over the last fifteen years many papers have been published on the chemistry of rhodium carbonyl clusters, also in consideration of its high catalytic activity in several homogeneous and heterogeneous reactions. As well as the Italian groups in Bologna and Milan, Gates and co-workers in the US largely contributed to enrich the scientific literature on this subject, along with Heaton (UK) and Gracheva (Russia), among others.

8.04.3.1

Homometallic rhodium carbonyl clusters 54

In 2019 and 2020,55 Gates et al. reported an alternative way to synthesize homometallic clusters exploiting the size-controlling role of zeolites. They observed the conversion of Rh(I)(CO)2 complexes into Rh4(CO)12 and Rh6(CO)16 (minor product) within the supercages of zeolite HY, by reproducing the conditions of the water gas shift reaction. After this conversion, when the zeolite incorporating Rh4(CO)12, Rh6(CO)16, and unconverted Rh(I)(CO)2 was exposed to flowing helium saturated with water vapor at 80  C, the IR bands representing Rh4(CO)12 were gradually replaced by those characterizing Rh(I)(CO)2 and Rh6(CO)16 (Fig. 13). The authors’ hypothesis that some tetranuclear species would break under into Rh(I) species which would in turn migrate and recombine to intact Rh4(CO)12 to give the hexanuclear cluster was confirmed by experimental IR studies. Over the last decade, the reinvestigation of the redox chemistry of [Rh7(CO)16]3−56 has resulted in finding new alternative syntheses for a series of both previously reported and new homometallic Rh-centered carbonyl clusters. In 2007 Longoni et al. carried out an electrochemical study on the long-term known [Rh7(CO)16]3− cluster, in order to explain the mechanism that allowed it to be used as a reactant for the synthesis of new carbonyl clusters.57 More specifically, during such reactions the hepta-nuclear species oxidized, originating higher nuclearity species like [H4−nRh14(CO)25]n− (n ¼ 3 and 4),58 [Rh15(CO)27]3−,59 [Rh17(CO)30]3−,60 and the new [Rh15(CO)25(MeCN)2]3− and the conjunto [Rh17(CO)37]3−, even when it was reacted with complexes containing other metals, like Ni2+ hydrated halides. Therefore, it was unclear whether the oxidation, for instance, of [Rh7(CO)16]3− to [H4−nRh14(CO)25]n− (n ¼ 3 and 4) was to ascribe to the Ni2+/Ni couple or the Brønsted acidity of the [Ni(H2O)6]2+ aquo complex. The first oxidation of [Rh7(CO)16]3− occurs at −0.23 V vs. SCE (formally, +0.04 V vs. the hydrogen electrode) and it is irreversible at any scan rate. Taking the standard redox potentials in water of the Ni2+/Ni (−0.23 V) and H+/H2 (0.0 V) couples as rough references (even if the pertinent potentials are not comparable and the redox couples are not in standard conditions), the H+/H2 redox couple should be more effective. In this case, the resulting products will more likely be homometallic. This study has been extended to other metal complexes and all experimental evidence sustained the above conclusion, which allows to predict whether the reaction of [Rh7(CO)16]3− with aqueous metal complexes leads to homo- or heterometallic clusters. Notably, the reaction between [Rh7(CO)16]3− and ZnCl2 resulted in the new conjunto [Rh17(CO)37]3−. Its molecular structure consists of a {Rh6(CO)15} unit connected to a {Rh11(CO)22} truncated n2 trigonal bipyramid via a RhdRh bond edged by two m-CO ligands (Fig. 14). A work of Dragonetti et al.61 showed a different approach from the previously described one to prepare [Rh14(CO)25]4−. More specifically, by bubbling carbon monoxide at atmospheric pressure into an ethylene glycol solution of RhCl3
nH2O (as opposed to

Fig. 13 Reaction network of the reversible cycle of formation and fragmentation of Rh4(CO)12 and Rh6(CO)16 facilitated by the half-reactions of the water gas shift and converting metastable Rh4(CO)12 to the mixture of Rh6(CO)16 and Rh(I)(CO)2. Reproduced with permission from: Fang, C.-Y.; Valecillos, J.; Conley, E. T.; Chen, C.-Y.; Castaño, P.; Gates, B. C. J. Phys. Chem. C 2020, 124, 2513–2520.

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Group 9 and 10 Carbonyl Clusters

Fig. 14 Molecular structure of [Rh17(CO)37]3− (Rh atoms in blue; C in gray; O in red).

Fig. 15 Scheme of the anionic rhodium carbonyl clusters syntheses in ethylene glycol solution. Reproduced with permission from: Dragonetti, C.; Garlaschelli, L.; Mussini, P.; Roberto, D. J. Organomet. Chem. 2009, 694, 3718-3724.

on silica surface, reported elsewhere by the same author62), at 150  C for about 8 h, and in presence of a base like CH3CO2Na, the reductive carbonylation gave rise to the formation of the [Rh14(CO)25]4− cluster. Notably, If the reaction is carried out at 50  C for a longer period of time and with a stronger base (as Na2CO3), the [Rh7(CO)16]3− species is formed. Further studies suggested that the latter species is actually an intermediate in the formation of [Rh14(CO)25]4− as illustrated in Fig. 15. This latter procedure reduces the passages required by the original preparation of [Rh7(CO)16]3−, that is rather resource and time consuming as it needs Rh4(CO)12 as main reactant which, in turn, is obtained by reducing hydrated RhCl3 with copper under CO atmosphere. An alternative starting material to prepare high-nuclearity carbonyl clusters is represented by the neutral [Rh4(CO)12] species, as reported in 2012.63 When its isopropanol solution is refluxed in presence of NaOH and under either nitrogen or hydrogen atmosphere, it is possible to isolate the new [Rh19(CO)31]5− and [Rh33(CO)47]5− carbonyl compounds, respectively. The metal structure of the former has not been properly determined due to the poor quality of its crystals. Conversely, a full X-ray structural determination was performed on the latter species, which is the largest homometallic rhodium carbonyl cluster to date. Its metal framework consists of a sequence of interpenetrated centered Rh13 icosahedra so to create a Rh19 core. Then, this grows along the C5 axis on both sides by interpenetration of two centered arachno Rh11 moieties, to give the Rh27 rod. The remaining six Rh atoms evenly decorate its sides. The breakout of the metal structure of [Rh33(CO)47]5− is depicted in Fig. 16.

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Fig. 16 Breakout of the metal structure of [Rh33(CO)47]5−.

8.04.3.2

Heteroleptic rhodium carbonyl clusters

Carbonyl groups are among the strongest ligands when coordinated to rich-electron transition metals, therefore their substitution may require high-energy methods. Conversely, more labile ligands like acetonitrile can be easily removed. In 2007 Gracheva (also reported as Grachova) et al. investigated substitution reaction by adding InCp to [Rh6(CO)15(CH3CN)] in mild conditions, which resulted in the formation of the new [Rh6(CO)15(InCp )] cluster (shown in Fig. 17).64 Like its congener GaCp , InCp is capable of occupying the m3-bridging positions of a transition metal core in carbonyl clusters. This results in the formation of cluster compounds with direct bonds between the transition metal and the In atom. This strategy can be used to create mixed-metal cluster compounds with direct bonds between transition and group 13 metals. Other substitution reactions exploiting the labile acetonitrile ligand in [Rh6(CO)15(CH3CN)] have been studied by the same research group. For instance, its reaction with the hetero-dentate Ph2PC6H4NMe2 ligand resulted in the obtaining of the monosubstituted Rh6(CO)15[(C6H5)2PC6H4N(CH3)2] cluster.65 Beside the [Rh6(CO)15(CH3CN)] carbonyl-substituted cluster, over the last fifteen years further species worth mentioning have been reported. One is the already mentioned [Rh15(CO)25(MeCN)2]3−, where the acetonitrile ligands occupy two terminal positions on the capping rhodium atoms, at 90 with respect to each other. A second species, namely [Rh26(CO)29(CH3CN)11], has been obtained by controlled addition of H2SO4 to CH3CN solutions of [H3Rh22(CO)35]5−. Its beautiful metal structure is highly symmetrical and consists of four interpenetrating centered Rh13 icosahedra that, as a result, create an inner Rh4 tetrahedron (Fig. 18).63 A ligand substitution may also be performed on homoleptic high-nuclearity carbonyl clusters such as in the formation of the [Rh10Sb(CO)21PPh3]3− species, which originated from adding roughly 1 equiv. of PPh3 to the icosahedral [Rh12Sb(CO)27]3− under N2 atmosphere.66 However, the ligand substitution caused a partial breaking of the original metal framework, probably due to the steric hindrance of the phosphine ligand. It is important to mention that by changing the atmosphere from N2 to CO, different and possibly unpredicted products were obtained. In fact, the addition of the same quantity of phosphine under CO resulted in the formation of the dimeric [{Sb@Rh12Sb(CO)25}2Rh(CO)2PPh3]7− homoleptic cluster, where one Rh(CO)2PPh3 unit connects two Sb-mono-capped Sb-centered icosahedral fragments.67

Fig. 17 Molecular structure of [Rh6(CO)15(InCp )].

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Fig. 18 Metal frame of [Rh26(CO)29(CH3CN)11] pointing out the interstitial Rh4 tetrahedron.

8.04.3.3

Heterometallic rhodium carbonyl clusters

A large number of articles have appeared within the last twenty years reporting the preparation and characterization of high-nuclearity mixed-metal clusters. The interest in these compounds is due to their fascinating metal core, potential application in catalysis, and ability to act as an electron reservoir, and more recently, as valuable precursors to support nanocatalysts and nanosized metal particles.68 We are now going to overview the latest findings within the heterometallic rhodium cluster chemistry, starting from the combination of Rh with other transition metals, and then moving to clusters containing post-transition elements. As for the first category, in the last fifteen years there is quite a limited number of papers describing the synthesis of such compounds, and those with a nuclearity equal or above four metal atoms will be reported in this book chapter. One of these papers regards the isolation of a novel dodeca-metal Rh-Os carbonyl cluster, namely [Os9Rh3(m-CO)2(CO)26]−,69 achieved by thermolysis of the neutral [Os3Rh(m-H)3(CO)12].70 This cluster presents a molecular structure that differs from those earlier described when discussing the dodeca-nuclear cobalt-based species, despite having the same metal nuclearity. The metal framework of [Os9Rh3(m-CO)2(CO)26]− can be described as two fused octahedra that share a common equatorial Rh–Rh edge, with two additional osmium atoms capping the opposite triangular faces, as presented in Fig. 19. This metal structure resembles those of the Os-, or the 8-group-element-based carbonyl clusters, such as [Os10(CO)24{Au(PPh2R)}4]71 and [Ru8Pt2(CO)23(m3-H)2].72 Notably, the [Os9Rh3(m-CO)2(CO)26]− cluster presents an exceptional low electron count, and it may be also for this reason that it acts as an “electron reservoir” by reversibly accepting up to four extra electrons, without breaking of the cluster. The multivalence behavior was ascertained by CV studies (Fig. 20) and controlled potential coulometry. Transition metal Au-containing carbonyl clusters have been reported in the past, however the larger ones are mainly based on Fe, Ni, and Pd.73 In 2018, the first high-nuclearity bimetallic Rh-Au cluster has been reported to our knowledge, resulting from the reaction between the [Rh7(CO)16]3− precursor and Au(III) halides: [Rh16Au6(CO)36]6−.74 Its fascinating structure is composed of an

Fig. 19 Molecular structure of [Os9Rh3(m-CO)2(CO)26]− (Os atoms in orange; Rh atoms in blue; C in gray; O in red).

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5 µA

20 mV s–1 50 mV s–1 100 mV s–1 200 mV s–1 –1.6

–1.2

–0.8

–0.4

0.0

E / V (Ag/AgNO3) Fig. 20 Cyclic voltammogram of [PPN][Os9Rh3(m-CO)2(CO)26] in CH2Cl2 containing 0.1 M tBu4NPF6 in CH2Cl2; glass carbon electrode, platinum auxiliary electrode and Ag/AgNO3 reference electrode; scan rate that varied from 20 to 200 mV s−1. Reproduced with permission from: Lau, J. P.-K.; Gu, Y.-J.; Wong, W.-T. Eur. J. Inorg. Chem. 2007, 19, 3011–3014.

Fig. 21 Structure of [Rh16Au6(CO)36]6−: metal skeleton with the highlighted Au6 octahedron (left); the whole molecular structure (right) (Rh atoms in blue, Au atoms in yellow, C in gray, O in red). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Acc. Chem. Res. 2018, 51, 2748–2755.

Au6 octahedron sharing four out of its 12 edges, all laying on the same plane, with as many Rh4Au6 octahedra, each stabilized by nine CO ligands equally divided into four terminal- and five edge-bridging groups. In analogy with [Ni32Au6(CO)44]6−, this cluster is an example of aurophilicity, as gold atoms are segregated and bonded with each other, and forming a core inside the rhodium cluster. The metal framework and molecular structure of [Rh16Au6(CO)36]6− are reported in Fig. 21.

8.04.3.4

Homometallic rhodium carbonyl clusters containing post-transition metals

In the case of rhodium-based carbonyl clusters containing post-transition elements, a comprehensive paper has been published in 2018.70 Over the past fifteen years various rhodium carbonyl compounds containing transition or post-transition metals have been characterized, and they all have been synthetized by reacting the versatile [Rh7(CO)16]3− cluster precursor with halides of various post-transition elements like Ge, Sn, Sb, Bi, as well as with Au(III) salts, by exploiting redox condensation reactions.75 As already mentioned in this paper, the advantage of using [Rh7(CO)16]3− as starting material lays on the fact that (i) being in a negative state, it is easily oxidized to higher-nuclearity species even at room temperature; (ii) it serves as source of CO, thus avoiding working under high pressures of carbon monoxide. In 1981 Vidal et al. had reported the synthesis and characterization of the Sb-centered icosahedral [Rh12Sb(CO)27]3− cluster, via reaction of Rh(CO)2(acac) with SbCl3 under 400 atm of CO/H2 (1:1) and at 140–160  C.76 But when the reaction is carried out by using SbCl3 with [Rh7(CO)16]3− instead, it occurs in very mild conditions

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Fig. 22 Molecular structure of the [Rh12E(CO)27]n− family of clusters (E atom in red). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Acc. Chem. Res. 2018, 51, 2748–2755.

of pressure (1 atm of CO) and temperature (25  C). Notably, if the same reaction is carried out under N2, the [Rh12Sb(CO)24]4− unsaturated species has been isolated.66 When SbCl3 is substituted with Sn(II), Bi(III) or Ge(II) halides, by keeping the same molar ratio and reaction conditions the same centered icosahedral species are isolated. As a matter of fact, it is possible to obtain the [Rh12E(CO)27]n− family of clusters, with n ¼ 4 when E ¼ Ge or Sn, and with n ¼ 3 when E ¼ Sb or Bi. These carbonyl compounds share both the molecular structure, which consists of a E-centered Rh12 icosahedral cage stabilized by twenty-seven CO ligands (Fig. 22), and the electron count, possessing all 170 CVE, in agreement with the Polyhedral Skeletal Electron Pair (PSEP) approach derived from the borane analogy.77 The electron count is obtained by considering the nine valence electrons of each Rh atom (9  12), the valence electrons of E (4 in the case of Ge and Sn, 5 in the case of Sb and Bi), the two electrons donated by each ligand (2  27), and the negative charge (4 in the case of Ge and Sn, 3 in the case of Sb and Bi). After the isolation of the E-centered icosahedral cluster, the four Rh-Ge, Rh-Sn, Rh-Sb and Rh-Bi chemical parallel systems start diverging, and further addition of E halides leads to very different species. In the case of Rh-Ge,78 beside [Rh12Ge(CO)27]4− which was isolated under CO atmosphere, the reaction between [Rh7(CO)16]3− and GeBr2 with a growing equivalent amount of the latter, under N2 atmosphere, results in the formation of the [Rh13Ge(CO)25]3− and [Rh14Ge2(CO)30]2− derivatives. In these clusters, the Ge atoms are still interstitial but occupy different, smaller cavities than the icosahedral one. More specifically, the metal structure of [Rh13Ge(CO)25]3− consists of a Ge-centered Rh8 cubic unit capped by five Rh atoms on as many square faces, in analogy with the [Rh14(CO)25]5− homometallic compounds. Notably, when [Rh13Ge(CO)25]3− is put under CO atmosphere in the presence of halide ions, it turns into the icosahedral [Rh12Ge(CO)27]4− congener. This rearrangement indicates, once more, the importance of the energetic balance between the metal-metal and metal-ligand bonds which, coupled with electronic and steric factors, drives the metal structure arrangement. As for the [Rh14Ge2(CO)30]2−, its metal skeleton consists of two mono-capped square antiprisms fused via one face, with one germanium atom incapsulated in each antiprismatic cavity. When the reaction that leads to the formation of [Rh13Ge(CO)25]3− is carried out under CO atmosphere, the larger [Rh23Ge3(CO)41]5− nanocluster is obtained, albeit in low yields. Its molecular structure, reported in Fig. 23, consists of three equivalent Rh10Ge polyhedra fused by one Rh2 edge and sharing in pairs another Rh atom. Each Rh10Ge unit is a convex solid possessing two square faces and seven triangular ones, with an overall C2v symmetry, and therefore can be identified as a sphenocorona centered by Ge. The cluster overall contains three fully interstitial Ge atoms and two semi-interstitial Rh ones, the latter capped by two hexagonal faces. This compound seems to represent the top of the nuclearity growing path curve of the reaction between the epta-nuclear precursor and the GeBr2 salt. When moving to the Rh-Sn system, a further equivalent addition of SnCl2 with respect to that needed to obtain the icosahedral [Rh12Sn(CO)27]4− species results in the RhCl-capped [Rh12Sn(RhCl)(CO)27]4− derivative if the reaction is carried out under CO atmosphere, and in the Sn-centered uncomplete and distorted icosahedral [Rh12SnCl2(CO)23]4− adduct when the reaction is carried out under N2 atmosphere.79 Subsequent research on the [Rh12Sn(CO)27]4− cluster revealed that it is possible, via thermolysis, to remove up to two carbonyl ligands to enable the isolation of the isostructural but electronic-deficient [Rh12Sn(CO)26]4− (168 CVE) and [Rh12Sn(CO)25]4− (166 CVE) parent anions.80 These species have been characterized by Band-target Entropy Minimization (BTEM) IR studies, 13C NMR spectroscopy and single crystal X-ray diffraction. The former analysis was performed on both the thermal decomposition of [Rh12Sn(CO)27]4− and the carbonylation of [Rh12Sn(CO)25]4−, in order to confirm the intermediate formation of the elusive [Rh12Sn(CO)26]4− compound. The deconvoluted IR spectra of the [Rh12Sn(CO)25+x]4− (x ¼ 0, 1, 2) species, illustrated in Fig. 24, are in good agreement with those recorded on crystalline samples.

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Fig. 23 Structure of [Rh23Ge3(CO)41]5−: one highlighted sphenocorona Rh10Ge unit out of the three within the metal framework (left); the molecular structure (right) (Rh atoms in blue, Ge atoms in green, C in gray, O in red). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Acc. Chem. Res. 2018, 51, 2748–2755.

Fig. 24 Pure component spectral estimates of the primary organometallic complexes as obtained from BTEM analysis. Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Tiozzo, C.; Zacchini, S.; Heaton, B. T.; Iggo, J. A.; Zanello, P.; Fedi, S.; Garland, M. V.; Li, C. Dalton Trans. 2009, 2217–2223.

In the case of the Rh-Sb system, very recent investigations of the reaction between the epta-nuclear [Rh7(CO)16]3− precursor and SbCl3 allowed to unravel the existence of other high-nuclearity nanocarbonyl species, namely [Rh20Sb3(CO)36]3−, [Rh21Sb2(CO)38]5− and [Rh28−xSbx(CO)44]6−.66 The molecular structures of the former two are very similar and consist of a Rh-centered icosahedron where two opposite vertexes are occupied by Sb atoms and these same vertexes are capped by two pentagonal Rh5 units. In the case of the Sb-richer [Rh20Sb3(CO)36]3− derivative, illustrated in Fig. 25, the third Sb atom occupies one of the vertexes of the pentagon forming the central icosahedron. The exact atomic ratio of [Rh28−xSbx(CO)44]6− could not be unambiguously determined by X-ray diffraction, due to the poor quality of the obtained crystals, even if all experimental data, including ESI-MS spectra and Energy Dispersive X-ray Spectrometry (EDS) analysis, pointed towards a [Rh25Sb3(CO)44]6− formula. Nonetheless, its metal structure is certain and can be described as a centered [RhSb]12 icosahedron where three vertexes are capped each by a pentagonal face (Fig. 26).

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Fig. 25 Metal skeleton of [Rh20Sb3(CO)36]3− (left), and its breakdown into a Rh-centered Rh9Sb3 icosahedron with the two opposite Sb vertexes capped by pentagonal Rh5 faces (right) (Rh atoms in blue, Sb atoms in yellow). Reproduced with permission from: Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Inorg. Chem. 2020, 59, 4300− 4310.

Fig. 26 Metal skeleton of [Rh28−xSbx(CO)44]6− (left), and its breakdown into a centered [RhSb]12 icosahedron capped by three pentagonal faces (right). Reproduced with permission from: Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Inorg. Chem. 2020, 59, 4300−4310.

The electronic properties of the high-yield [Rh21Sb2(CO)38]5− cluster were studied through cyclic voltammetry and in situ infrared spectroelectrochemistry. Despite the unconclusive CV profile, due to a very low current associated with the redox processes, the in situ infrared spectroelectrochemistry allowed to unravel the rich multivalent nature of the nanocluster, which can stably exist with several different negative charges without breaking of the metal structure. In Fig. 27 selected IR spectra as function of the different potentials E(V), registered during the spectroelectrochemistry experiments, are reported, and the progressive downshift of the n(CO) stretching frequencies is in agreement with the increasing negative cluster charge. In the case of the Rh−Bi system,81 the cluster nuclearity grows with the addition of further BiCl3 equivalents with respect to that needed to obtain [Rh12Bi(CO)27]3−. For instance, the first extra equivalent generates both the dimeric [(Rh12Bi(CO)26)2Bi]5− and the [Rh14Bi(CO)27Bi2]3− nanoclusters. The former consists of two icosahedral moieties joint by a Bi atom; the latter presents two additional Bi-Rh capping fragments on two different sides of the parent polyhedron. When a further half equivalent of BiCl3 is added, the [Rh17Bi(CO)33Bi2]4− nanocluster is obtained. This compound possesses a structure based on a Bi-centered icosahedron of Rh atoms decorated by additional Rh3Bi and Rh2Bi fragments. Fig. 28 illustrates all of the above-described systems by collecting the key species encountered in the path of the buildup of Rh carbonyl clusters containing post-transition metals. The latest findings regarding the highest-nuclearity Rh-Sb nanoclusters are not included in the diagram, as they were characterized afterwards. New investigations on heterometallic Rh-In clusters are in progress by the same authors; earlier results unraveled the existence of a rich binary system, including the presence of the icosahedral [Rh12In(CO)28]3− species, a new member of the E-centered family of dodeca-nuclear rhodium carbonyl clusters.

8.04.3.5

Carbonyl fluxionality studies in rhodium clusters

The Rh4(CO)12 and Rh6(CO)16 clusters were among the earliest transition metal carbonyls to be discovered.82 The X-ray structure of Rh4(CO)12, later ascertained83 and confirmed by spectroscopic studies,84 has shown a tetrahedral arrangement of the four Rh atoms

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Fig. 27 Selected IR spectra of [Rh21Sb2(CO)38]n− as a function of the potential E (vs. Ag pseudoreference electrode). The initial spectrum (n ¼ 5) is at −0.60 V. Reproduced with permission from: Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Inorg. Chem. 2020, 59, 4300−4310.

Fig. 28 Growing of heterometallic Rh carbonyl clusters vs. added equivalents of EXn with respect to the initial [Rh7(CO)16]3−. Rh–Sb clusters are indicated with a yellow mark, Rh–Ge clusters and Ge atoms in green, Rh–Sn clusters and Sn atoms in orange, and Rh–Bi clusters and Bi atoms in magenta. Reproduced with permission from:Femoni, C.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Acc. Chem. Res. 2018, 51, 2748–2755.

224

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stabilized by nine terminal- and three edge-bridging carbonyls, viz., Rh4(m-CO)3(CO)9, to give an overall C3v symmetry. However, from variable temperature 13C NMR measurements it is known that there is carbonyl scrambling in solution and that, at +60  C, there is a single 13C resonance, a quintet, as a result of coupling to four 103Rh nuclei that have become equivalent on the NMR time scale. Two mechanisms for this intramolecular exchange have been so far proposed. The first proposal, known as the merrygo-round process, was made by Cotton in 196685 and involves opening and closing of the m-CO units so as to move them around the cluster in concerted steps. The other proposed mechanism envisages concerted movements of the metal skeleton within carbonyl polyhedron.86 In 2006, Heaton et al. unambiguously determine the accordance with the merry-go-round mechanism via 2D NMR measurements.87 The same studies were performed on the hexanuclear Rh6(CO)16 species, and the same type of carbonyl dynamics was suggested. The mechanism likely occurs via a “local all-terminal” intermediate, followed by a rotation of the “Rh(CO)3” fragment to eventually restore the Rh3(t-CO)6(m3-CO) structure (TSR mechanism). The other pathway, FET, which involves the rupture of two Rh-(m3-CO) bonds to give the “local-all-terminal” intermediate, is less likely to occur. Fig. 29 illustrates the TSR and FET mechanisms of the face-bridging to terminal COs for the Rh6 octahedron. For a deepening on ligand fluxionality, Authors suggest enjoying the reading of “Comment on the ligand polyhedral model approach to the mechanism of complete carbonyl exchange in [Rh4(CO)12] and [Rh6(CO)16].”88

8.04.3.6

Kinetic studies on Rh-nitride cluster formation

As far as carbonyl clusters are concerned, much has been done in terms of synthesis of new species and their spectroscopic and electrochemical properties, as well as ligand fluxionality. However, less attention has been paid to the kinetic investigation of their reaction mechanisms and energetics. In 2008 Poe et al. performed a kinetic study89 on the reaction between the neutral [Rh6(CO)16] octahedral cluster90 and NO−2 to give the trigonal prismatic [Rh6N(CO)15]−.91 In this case, the reaction involved the reduction of the nitrogen atom from plus three to minus three, with its concomitant insertion into the center of the [Rh6(CO)16] cluster, and formation of CO2. Its kinetic study, performed via NMR, temperature-controlled IR and UV–visible spectroscopic analyzes, and stopped-flow measurements, showed that the reaction proceeds in six kinetically distinguishable steps, followed by other four steps that are kinetically unidentifiable but that can be related through analogies with other similar reactions. The first step involves the nucleophilic attack of NO−2 (via one of its oxygen atoms) on a carbon atom belonging to a CO ligand. Subsequently, after CO loss, the intermediate [Rh6(CO)14(OCONO)]− species is formed. This has found to react relatively slowly with CO, but once the equilibrium has been reached, other faster steps lead to the final [Rh6N(CO)15]− product. Fig. 30 reports a proposed reaction mechanism that takes into consideration two main elements. The first one, previously ascertained,92–94 is that nucleophilic attack on carbonyl clusters usually occurs concurrently with metal-metal bond breaking. The second one is that the conversion from the initial octahedral to the final trigonal prismatic geometry is accompanied by a decrease in the overall Rh-Rh connectivity, from 12 to 9. In the scheme, the candidate bonds for the breakage are indicated with dotted lines.

Fig. 29 TSR and FET mechanism of the face-bridge $ terminal CO exchange of a face of the Rh6 octahedron. For the sake of simplicity only one face of the octahedron is shown. Reproduced with permission from: Heaton, B. T.; Jacob, C.; Podkorytov, I. S.; Tunik, S. P. Inorg. Chim. Acta 2006, 359, 3557–3564.

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Fig. 30 Proposed reaction mechanism for the formation of [Rh6(CO)17(NO)]−. Reproduced with permission from: Babij, C.; Farrar, D. H.; Poe, A. J.; Tunik, S. P. Dalton Trans. 2008, 5922–5929.

In the proposed reaction mechanism, it is the trigonal prismatic [Rh6(CO)17(NO)]− species that undergoes reduction of the NO ligand into the nitride ion, which penetrates the cluster center, with concurrent loss of CO2.

8.04.3.7

Rhodium carbonyl clusters in catalysis

Rhodium carbonyl clusters may be used as precursors for preparing supported catalysts. The catalytic activity of rhodium, in fact, is well known. The Rh4(CO)12 cluster95 had been studied almost thirty years ago for catalyzing different reactions, such as the hydrosilylation of isoprene, and cyclohexanone,96 all in homogeneous catalysis. Rh4(CO)12-derived catalysts, supported on Al2O3, MgO, and CeO2, had also been tested in 2001 in the catalytic partial oxidation (CPO) process for syngas production.97 However, the CPO conditions and the related generated hot spot do not allow a simple comparison among catalysts. Furthermore, no tests on heterogeneous supported catalysts based on the Rh4(CO)12 cluster have been reported yet. In 2019, Basile et al.98 reported the synthesis, characterization, and catalytic behavior of Rh-based catalysts, obtained by using the Rh4(CO)12 neutral cluster as the active-phase precursor. In particular, the preparation method allowed the deposition of the cluster on the surface of Ce0.5Zr0.5O2 and ZrO2 supports, which were synthetized by w/o microemulsion technique. The catalysts were found to be active in the low-temperature steam reforming process for syngas production. Moreover, at concentrations of 0.05% and high temperature, the Ce0.5Zr0.5O2-supported cluster samples showed better results with respect to a classical Rh-impregnated CeZr catalyst.

8.04.4

Iridium

The iridium carbonyl cluster chemistry over the last fifteen years has been mainly dominated by the extensive work of R. D. Adams in the US and M. G. Humphrey in Australia, along with R. Ros and W. H. Watson. In this section the main iridium-based carbonyl clusters are reported, leaving out those where iridium represents a minority in terms of atomic presence.

8.04.4.1

Homometallic iridium carbonyl clusters

Despite the wide variety of new iridium compounds that have been discovered in the last fifteen years, to the best of our knowledge, there has been only a new one with a homometallic nature and a nuclearity equal or greater than four. In 2019, thus very recently,

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Fig. 31 Molecular structure of [Ir8(CO)18]2− (Ir atoms in blue; C in gray; O in red).

Konarev et al. reported the synthesis of new homometallic carbonyl cluster species with a very interesting approach, that is by using a known reductant, decamethylchromocene (Cp 2Cr, Eox ¼ −1.04 V), to reduce neutral clusters like Co4(CO)12, Ir4(CO)12 and Rh6(CO)16 and selectively produce higher nuclearity anionic compounds. Among these, and along with the already known [Co6(CO)15]2− and [Rh11(CO)23]3− clusters, they reported for the first time the isolation of the [Ir8(CO)18]2− homometallic species.99 Its metal structure, illustrated in Fig. 31, consists of a Ir6 octahedron bi-capped on two opposite triangular faces, laying on the same side of the polyhedron, by the two additional Ir atoms. In 2012 Dixon et al. published a paper on the stability of neutral homo-metallic Irn(CO)m carbonyl compounds (n ¼ 1, 2, 3, 4, and 6) ascertained by computational methods. The results pointed out that Ir4(CO)12 is predicted to be the most favored species for reactions of Irn(CO)m with CO at low temperature, while Ir6(CO)16 is formed above room temperature. Smaller Irn(CO)m clusters will nucleate to form Ir4(CO)12 spontaneously.100

8.04.4.2

Heteroleptic iridium carbonyl clusters

In 2006, Ros et al. extended their work on the di-substituted Ir4(CO)10(L-L) cluster (L-L ¼ bidentate phosphine ligand) and synthesized a new series of anionic decarbonylated [Ir4H(CO)9(m− L−L)]− hydride derivatives by reacting the former species with 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu) at room temperature.101 The different L-L ligands, namely Ph2PCH(CH3)PPh2 (dppmMe), Ph2P(CH2)2PPh2 (dppe), Ph2P-(CH2)3PPh2 (dppp), and Ph2AsCH2AsPh2 (dpam), gave rise to as many new compounds, which were characterized by IR, 1H, 31P and 13C NMR, and X-ray diffraction analyzes. The structural characterization allowed to confirm the preliminary hypothesis, formulated on the basis of spectroscopical evidence, that they all display a single conformation in solution, such as three edge-bridging COs around the triangular basal face and both the hydride and the bidentate ligands located in axial positions with respect to this face. Fig. 32 shows the molecular structure of [Ir4H(CO)9(dppmMe)]−. Notably, neutron diffraction studies performed in 2004102 allow the determination of the IrdH bond distance of 1.618(14) A˚ in the [Ir4H(CO)9(dppm)]− derivative, which had been previously reported by the same authors.103 Another interesting paper was reported in 2008 by Watson et al.,104 where the homoleptic Ir4(CO)12 was activated by (CH3)3NO or thermolysis and reacted with 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd), generating the Ir4(CO)10(bpcd) and Ir4(CO)8(bpcd)(m-bpcd) heteroleptic derivatives. Notably, the latter (cluster 4 in Fig. 33) is not stable at room temperature and, through CO loss, it affords the hydride-bridged HIr4(CO)7(bpcd)[m-PhP(C6H4)C]C(PPh2)C(O)CH2C(O)] species (cluster 5 in Fig. 33).

Fig. 32 Molecular structure of [Ir4H(CO)9(dppmMe)]− (Ir atoms in blue; P atoms in green; Hydride atom in white; C in gray; O in red). Hydrogen atoms on the ligands have been omitted for sake of clarity.

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Fig. 33 Formation of HIr4(CO)7(bpcd)[m-PhP(C6H4)C]C(PPh2)C(O)CH2C(O)]. Reproduced with permission from: Watson, W. H.; Wu, G.; Richmond, M. G. J. Organomet. Chem. 2008, 693, 1439–1448.

CV experiments were performed on all three clusters and the obtained results, coupled with DFT studies, revealed their reduction properties, which are localized on the bpcd, in accordance with previous findings on other metal clusters coordinated to the same chelating ligand.105 By exploiting substitution reaction with bidentate ligands, it is also possible to build finite and/or infinite networks of metal carbonyl clusters connected by bridging linkers. For instance, in case of mixed Fe-Cu compounds, the substitution of the labile acetonitrile ligands in Fe4Cu2C(CO)12(MeCN)2 with 4,40 -bipyridine (bipy), pyrazine (pyz) or p-dicyanobenzene (p-DCB) gave rise to complex polymeric and oligomeric structures as a function of the crystallization solvent, in which the bidentate ligands linking two cluster units.106 In the case of iridium clusters, the work of Sironi et al. reported the synthesis of a remarkable network made by linking four Ir4(CO)9 cluster units with six 1,4-bis(diphenylphosphinomethyl)benzene (dppmb) ligands.107 This new nanomaterial was initially obtained by thermal treatment of the homoleptic Ir4(CO)12 with the bidentate dppmb; however, a better yield was achieved by using the Br-substitute [Ir4Br(CO)11]− cluster as starting material. The crystal structure of this cyclic assembly is pretty peculiar and is reported in Fig. 34. The three dppmb ligands on each cluster unit are all coordinated to the basal plane of the bridging COs, however two of them connect two cluster moieties, and the resulting two pairs of adducts are in turn linked with the third dppmb ligand.

Fig. 34 Molecular structure of {Ir4(CO)9(dppmb)3/2}4. The four Ir4 units are highlighted in purple. The CO ligands are omitted for clarity. The diphosphino ligands are in blue (those forming a short circuit between two Ir4 units) or in gray (simple connection between two Ir4 units). Reproduced with permission from: Peli, G.; Daghetta, M.; Macchi, P.; Sironi, A.; Garlaschelli, L. Dalton Trans. 2010, 39, 1188–1190.

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Fig. 35 Molecular structure of tert-butyl-calix[4]arene-(OMe)2(OCH2PPh2)2(Ir4(CO)11)2 (Ir atoms in blue; P atoms in green; C in gray; O in red). Hydrogen atoms have been omitted for sake of clarity.

In 2010, a similar concept of connecting iridium cluster units via organic linkers was reported by Katz et al., who built a fascinating assembly of two Ir4(CO)11 cluster units joint together via a calixarene phosphine ligand, again from the [Ir4(CO)11Br]− cluster.108 In Fig. 35, the tert-butyl-calix[4]arene-(OMe)2(OCH2PPh2)2[Ir4(CO)11]2 is illustrated. In 2011, Adams reported new iridium-tin species obtained by reacting the same labile-substituted [Ir4(CO)11Br]− cluster with SnPh3OH and SnPh4.109 Notably, in both cases a transmetalation reaction did not occur, as the main product, [Ir4(CO)11Ph]−, contains a terminally coordinated phenyl in place of the bromide anion instead. However, this species turned out to be quite reactive: when put in contact with PPh3 it generated the [Ir4(CO)10(PPh2C6H4)]− derivative, where the edge-bridging PPh2C6H4 group derived from the PPh3 reagent that became ortho-metalated to one of the Ir atoms. Interestingly, when the anion was treated with HBF4 under CO atmosphere, it was neutralized by the addition of H+ to the carbon atom of the metalated phenyl ring, a CO ligand coordinated to the cluster and the known compound [Ir4(CO)11(PPh3)]− was obtained. Other species were isolated within the same work, namely the new penta-nuclear Ir5(CO)12Ph(PPh3) and Ir5(CO)11(PPh3)(PPh2C6H4) compounds, by reacting [Ir4(CO)11(PPh3)]− with the Ir(CO)(PPh3)2Cl complex in refluxing benzene. The overall reaction scheme is outlined in Fig. 36.

Fig. 36 Reaction scheme. Reproduced with permission from: Adams, R. D.; Chen, M. Organometallics 2011, 30, 5867–5872.

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Fig. 37 Molecular structure of Ir9(CO)15(Ph)(m3-C8H10)(COD) (Ir atoms in blue; C in gray, O in red).

The following year, Adams exploited the reactivity of [Ir4(CO)11Ph]− by treating it with an Ir(I) complex, [Ir(COD)Cl]2.110 The reaction resulted not only in the formation of the penta-nuclear Ir5(CO)11(Ph)(COD) and, Ir5(CO)9(Ph)(COD)2 species, but also in the isolation of the higher nuclearity Ir9(CO)15(Ph)(m3-C8H10)(COD), all retaining the s-phenyl ligand. Notably, it turned out that the latter cluster derived from a secondary condensation reaction between the former two penta-nuclear compounds. Fig. 37 shows the molecular structure of Ir9(CO)15(Ph)(m3-C8H10)(COD). Quite interestingly, the obtaining of the side products Ir4(CO)10(COD) and Ir4(CO)7(COD)(m4-C8H10), which were already known clusters,111 could be greatly enhanced by the addition of H+ to the main reaction, which is believed to facilitate the removal of the phenyl ligand, presumably as benzene. In 2015, Humphrey et al. obtained new carbonyl-substituted Ir clusters, namely Ir4(CO)6(Z5-C5Me4H)2 and Ir7(m3-CO)3(CO)12 5 (Z -C5Me5), but by using mixed-metal Ir-Mo precursors.112 While the authors recognized that this methodology lacked predictive merit or broad applicability, the clusters obtained showed notable features. Ir7(m3-CO)3(CO)12(Z5-C5Me5) possesses three semi-face capping CO ligands (see Fig. 38, dotted lines), whereas Ir4(CO)6(Z5-C5Me4H) is formally four-valence-electron deficient; theoretical studies suggest that it is presumably stabilized by significant intracluster multiple IrdIr bonding.

8.04.4.3

Heterometallic heteroleptic iridium carbonyl clusters

As already mentioned throughout this book chapter, mixed-metal clusters are of considerable interest for a variety of reasons, for instance, clusters coupling early to mid-transition metals with late transition metals may be effective precursors to catalysts for a range of heterogeneously catalyzed transformations. One efficient route into clusters of this type is by way of “metal exchange” reactions, where one or more metal-ligand groups of a cluster are replaced by a different metal-ligand group to afford a new cluster containing the same total number of metal atoms.113 However, most examples involved trinuclear clusters114 and metal exchange at higher nuclearity had yet to be exploited. In 2006, Humphrey et al. reported the syntheses of medium-nuclearity heterometallic metal clusters by a combination of core expansion and metal exchange, upon reacting MIr3(CO)11(Z-L) (M ¼ Mo, W; L ¼ C5H5, C5HMe4, C5Me5) with [M(CO)3(Z-L)]-(M ¼ Mo, W; L ¼ C5HMe4, C5Me5),115 in continuation with their work published in 2002 that reported the preparation of homologous heterometallic Ir-based carbonyl-substituted clusters (Fig. 39).116

Fig. 38 Molecular structure of Ir7(m3-CO)3(CO)12(Z5-C5Me5). Dotted lines indicate semi-face capping CO ligands (Ir atoms in blue; C in gray; O in red).

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Fig. 39 Syntheses of M2Ir3 (M ¼ Mo, W) clusters. Reproduced with permission from: Usher, A. J.; Lucas, N. T.; Dalton, G. T.; Randles, M. D.; Viau, L.; Humphrey, M. G.; Petrie, S.; Stranger, R.; Willis, A. C.; Rae, A. D. Inorg. Chem. 2006, 45, 10859–10872.

Interestingly, the in depth study on the formation of the homovertex clusters, which represent the highest-yielded products when Mo is involved, revealed that the core expansion process is not a simple addition of a molybdenum-containing fragment to the triangle of iridium atoms, but that a mechanism involving core expansion and vertex replacement must occur. A few years later, in 2013, Humphrey reported the synthesis of higher nuclearity Ir-M clusters (M ¼ Mo, W) by reacting the same MIr3(CO)11(Z5-C5Me4R) (R ¼ H, Me) clusters but, this time, with the Ir(CO)2(Z5-C5Me5) carbonyl complex.117 This investigation gave rise to an impressive series of compounds, which are summarized in the scheme reported in Fig. 40. The redox behavior of the new clusters was also examined through cyclic voltammetry. The results pointed out that they all exhibit oxidation and reduction processes, and that the latter are enhanced by decreasing the Ir content of the clusters, replacing W by Mo, and increasing alkylation of the cyclopentadienyl ligands, as alkyl substituents make them more effective electron donors.

Fig. 40 Syntheses of Ir-M clusters (M ¼ Mo, W). Reproduced with permission from:Randles, M. D.; Simpson, P. V.; Gupta, V.; Fu, J.; Moxey, G. J.; Schwich, T.; Criddle, A. L.; Petrie, S.; MacLellan, J. G.; Batten, S. R.; Stranger, R.; Cifuentes, M. P.; Humphrey, M. G. Inorg. Chem. 2013, 52, 11256−11268.

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Fig. 41 Syntheses of Ir-Mo and Ir-W clusters. Reproduced with permission from: Simpson, P. V.; Randles, M. D.; Gupta, V.; Fu, J.; Moxey, G. J.; Schwich, T.; Morshedi, M.; Cifuentes, M. P.; Humphrey, M. G. Dalton Trans. 2015, 44, 7292–7304.

The research of Humphrey and his co-workers on Ir-Mo and Ir-W carbonyl-substituted clusters continued to produce many new heterometallic species. In 2015 they reported the ligand substitution reactions on pentanuclear Mo-Ir and Mo-W clusters with phosphine, isocyanide, and arylalkynes ligands.118 While in the two former cases a single or multiple CO replacement took place, accompanied by cluster-core isomerization with isocyanides, in the case of arylalkynes the C^C bond cleavage occurred (Fig. 41). Electrochemical studies were performed on all the above-described compounds, and the overall results pointed out that the incorporation of isocyanides, phosphines or alkyne residues in those pentanuclear clusters increased the propensity to oxidation and decreased that of reduction, therefore they can tune the electron richness of the clusters. This is due to the electron-donor properties of the PPh3 ligands or the isocyanide substituents, which lower the oxidation potentials. In the same year, Humphrey and co-workers extended their investigation on the core expansion of Ir-Mo cluster by reacting Ir(CO)2(Z5-C5HMe4) with Mo2Ir2(CO)10(Z5-C5H5)2, affording three new clusters with different nuclearities which were isolated via thin-layer chromatography (Fig. 42). The first species, Mo3Ir3(m3-O)(CO)11(Z5-C5H5)3, possesses trigonal bipyramidal core with the face-capping oxo ligand located in a Mo2Ir2 butterfly cleft in the structure. The second one, Mo4Ir4(CO)13(Z5-C5H5)4 displays a capped pentagonal bipyramidal metal framework and is four-electron deficient; the third cluster, Mo3Ir5(CO)9(Cl)(Z5-C5H5)3(Z5-C5HMe4) possesses a tetra-capped tetrahedral core and has six electrons less than the PSEPT-predicted electron count.119 In 2017, Humphrey also reported the synthesis of new Ir-Mo and Ir-Mo-Rh heterometallic clusters, obtained by thermolysis of the Mo2Ir2(CO)10(Z5-C5H5)2 precursor and subsequent reaction with Rh(CO)2(Z5-C5Me5) (Fig. 43).120 The trimetallic cluster adopts a Mo2Ir3 trigonal bipyramidal geometry with the molybdenum atoms in apical and equatorial sites, a rhodium capping an IrMo2 face, and the third molybdenum capping the unique Ir3 one. However, such metal structure is unusual for hepta-metallic clusters, as a capped octahedral arrangement is more common. A DFT comparative analysis between Mo3RhIr3(CO)11(Z5-C5H5)3(Z5-C5Me5), which exhibits a precise 96 electron count, and the isostructural W3Ir4(m-H)(CO)12 (Z5-C5H5)3, which possesses 94 CVE, has been performed, and the results are consistent with the greater stability of these isolated species as opposed of congeners with one CO less or more, respectively. Along with the comprehensive work of Humphrey on Ir-Mo and Ir-W carbonyl clusters, a very extensive research on Ir-based heterometallic species has also been carried out by Adams. In 2011, Adams et al. reported the transmetalation reaction of Ir4(CO)12 with Ph3SnOH in the presence of [Bu4N]OH.121 The importance of Sn-doped transition metal clusters derives from the fact that the resulting heterometallic species can improve the selectivity of certain types of catalytic hydrogenation reactions, also in a form of supported metal nanoparticles.122 The above

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Fig. 42 Synthesis of Mo3Ir3(m3-O)(CO)11(Z5-C5H5)3 (1), Mo4Ir4(CO)13(Z5-C5H5)4 (2) and Mo3Ir5(CO)9(Cl)(Z5-C5H5)3(Z5-C5HMe4) (3). Reproduced with permission from: Fu, J.; Moxey, G. J.; Cifuentes, M. P.; Humphrey, M. G. J. Organomet. Chem. 2015, 792, 46–50.

Fig. 43 Mo3Ir3(m4-Z2-CO)(CO)11(m5-C5H5)3 (1) and Mo3RhIr3(CO)11(m5-C5H5)3(Z5-C5Me5) (2). Reproduced with permission from: Fu, J.; Randles, M. D.; Moxey, G. J.; Stranger, R.; Cifuentes, M. P.; Humphrey, M. G. J. Organomet. Chem. 2017, 829, 66–70.

Fig. 44 Molecular structure of [Ir4(CO)10(SnPh3)2(m-H)]− (Ir atoms in blue; Sn atoms in green; H atom in white; C in gray; O in red).

reaction resulted in the formation of the mono- and bi-substituted [Ir4(CO)11(SnPh3)]− and [Ir4(CO)10(SnPh3)2(m-H)]− products. Beside their structural characterization via X-ray diffraction, the hydride nature of the latter (Fig. 44) had been established by 1H NMR studies. For comparison, the reaction with Ph3SnOH was expanded to the Ir4(CO)11(PPh3) derivative,123 and the new tin complex Ir4(CO)10(SnPh3)(PPh3)(m-H) was isolated. On the basis of the obtained results and other findings reported in the previous literature by Chini et al.,124 the authors were able to formulate a hypothesis of the possible mechanism that could lead to the formation of the above-mentioned Sn-containing species (Fig. 45).

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Fig. 45 Proposed mechanism for the formation of Ir4(CO)10(SnPh3)(PPh3)(m-H). Reproduced with permission from: Adams, R. D.; Chen, M.; Trufan, E.; Zhang, Q. Organometallics 2011, 30, 661–664.

For sake of completeness, in the same year Adams also studied the reaction of the mixed-metal Ir-Ru carbonyl compounds with HGePh3, and this investigation gave rise to several tetranuclear clusters coordinated with germyl ligands. As they are mostly Ru-based species, we will not expand their description in this book chapter. In 2012, Adams et al. published their research on heterometallic Ir-based clusters exploiting the viability of the carbonyl substituted [Ir4(CO)11(Ph)]− and [Ir4(CO)11Br]− to react with gold complexes like Au(PPh3)+ and RAu(PPh3) (R ¼ C6H5; CH3; 2-C16H10), unraveling a wide series of compounds. Like Sn, in fact, iridium is an important element in catalysis and its use is growing more and more in several applications, mainly in homogeneous125 but also heterogeneous reactions, even in combination with other metals.126 The reaction of [Ir4(CO)11(Ph)]− with [Au(PPh3)][NO3] led to the [Ir4(CO)11(Ph)(m-AuPPh3)] mono-adduct, while the analogous reaction with the hydride derivative [HIr4(CO)11]− precursor resulted in the formation of the pentanuclear Ir4(CO)11(AuPPh3)2 and Ir4(CO)10(AuPPh3)2 compounds (clusters 2 and 3 in Fig. 46, respectively).127 The latter can be converted

Fig. 46 Reaction scheme in the synthesis of clusters 2–4. Reproduced with permission from: Adams, R. D.; Chen, M.; Yang, X. Organometallics 2012, 31, 3588−3598.

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into the former by a reversible CO addition involving an interesting metal framework transformation. Likewise, the addition of the 2-electron donor ligand PPh3 to the same Ir4(CO)10(AuPPh3)2 led to the analogous addition product (cluster 4 in Fig. 46). Finally, when Ir4(CO)10(PPh3)(AuPPh3)2 is heated, it eliminates CO in a process that results in a transformation and degradation of the PPh3 ligand into a triply bridging PPh(C6H4) ligand (cluster 5 in Fig. 46). A comprehensive scheme of the above reactions is illustrated in Fig. 46. Thorough DFT molecular orbital calculations on Ir4(CO)11(AuPPh3)2 and Ir4(CO)10(AuPPh3)2 have also been performed and reported, in order to elucidate the metal −metal bonding and the mechanism of their interconversion by CO addition and elimination. In the same year, Adams et al. reported the reaction between [Ir4(CO)11Br]− and (R)Au(PPh3) (R ¼ C6H5, CH3, 2-C16H10), further extending the chemistry of Ir-Au carbonyl clusters.128 This way, not only the easily replaceable bromide ligand is substituted with the R unit carried by the golden complex, but the [Au(PPh3)]+ moiety adds to the iridium cluster through a bridging coordination. Notably, when the reaction with (R)Au(PPh3) (R ¼ C6H5; CH3) was carried out by using [HIr4(CO)11]− as starting material, the higher nuclearity species Ir4(CO)9(CH3)2(AuPPh3)4, Ir4(CO)9(PPh3)(Ph)(AuPPh3)3 and Ir4(CO)9(Ph)2(AuPPh3)4 were also obtained. The former is illustrated in Fig. 47. Other interesting works again by Adams and his co-workers were published over the same period and reported the synthesis of heterometallic Ir carbonyl clusters combined with Ge-, Sn- and Bi-bearing ligands, as well as Cu and Ag. The reaction of the previously obtained Ir3(CO)9(m3-Bi) cluster129 with Ph3GeH and Ph3SnH was thoroughly investigated, and the new mixed-metal tri-iridium based Ir3(CO)6(GePh3)3(m3-Bi)(m-H)3, Ir3(CO)6(SnPh3)3(m3-Bi)(m-H)3 and Ir3(CO)6(m-SnPh2)3(m3-Bi) clusters were obtained.130 Subsequently, the same precursor was reacted with gold complexes to obtain the trinuclear Ir3(CO)8(Ph) (m3-Bi)[m-Au(NHC)] which, in turn, led to other tin derivatives upon reaction with HSnPh3.131 Conversely, a series of tetra nuclearity species (based on iridium atoms) was obtained by using Ir4(CO)11(PPh3) in combination with GePh3H and at different temperatures, namely Ir4(CO)10(PPh3)(GePh3)(m-H), Ir4(CO)10(PPh3)(m-GePh2), Ir4(CO)7(PPh3) (GePh2)(GePh3)(m3-Z2-GePhC6H4)(m-H)2 and Ir4(CO)7(PPh3)(GePh2)2(m3-Z2-GePhC6H4)(m-H). In addition, Ir4(CO)10(PPh3) (m-GePh2) was reacted with GePh2H2 to yield the new compound Ir4(CO)6(PPh3)(GePh2)3(GePh2H)(m-H)3. Some of them contain a rare m3-Z2-GePh(C6H4) ligand formed by orthometallation of a germyl phenyl ring.132 All these new compounds were characterized through IR and 1H NMR spectroscopy, single-crystal X-ray diffraction analyzes, and mass spectrometry. Fig. 48 illustrates the molecular structure of Ir4(CO)7(PPh3)(GePh2)(GePh3)(m3-Z2-GePhC6H4)(m-H)2. The importance of such clusters is reinforced by recent studies demonstrating the higher catalytic activity of tetra-iridium clusters for ethylene hydrogenation, which is typically several times greater than that of hexa-iridium and mononuclear iridium complexes.133 Several other tetra-iridium-based carbonyl clusters containing Cu and Ag were isolated by Adams in 2013, where the reaction of the versatile and already mentioned [Ir4(CO)11(Ph)]− (cluster 1 in Fig. 49) with [Cu(NCMe)4][BF4] and Ag[NO3], led to the formation of Ir4(CO)11(m-Z1-Ph)[m3-Cu(NCMe)] (cluster 2 in Fig. 49) and [{Ir4(CO)11Ph}2(m4-Ag)]− (cluster 3 in Fig. 49) species, respectively. Furthermore, by adding another equivalent of the silver compound and, subsequently, other 2-electron donor ligands, the new [Ir4(CO)11]2(m4-Ag)(m-Ag)(m3-Ph)(m-Ph) (cluster 4) Ir4(CO)11(Z1-Ph)[m3-Ag(NCMe)] (cluster 5) and Ir4(CO)11(m-Z1-Ph) [m3-Ag(PPh3)] (cluster 6) species were obtained. All compounds were structurally characterized by single-crystal X-ray diffraction analyzes, and DFT studies were carried out to analyze the bonding of the bridging phenyl ligands.134 Fig. 49 illustrates the complex system in a very clear scheme proposed by the Authors. Notably, in what it is indicated as compounds 4 there is a second and rare triply bridging phenyl ligand that, on the basis of the DFT calculation results, formally serves as a three-electron donor.

Fig. 47 Molecular structure of Ir4(CO)9(CH3)2(AuPPh3)4 (Ir atoms in blue; Au atoms in yellow; P atoms in green; C in gray; O in red; H in white).

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Fig. 48 Molecular structure of Ir4(CO)7(PPh3)(GePh2)(GePh3)(m3-Z2-GePhC6H4)(m-H)2 (Ir atoms in blue; Sn atoms in cyano; P atom in green; H atoms in white; C in gray; O in red). The H atoms of the phenyl ligands have been omitted for sake of clarity.

Fig. 49 Reaction scheme for the synthesis of clusters 2–6. Reproduced with permission from: Adams, R. D.; Chen, M.; Elpitiya, G.; Yang, X.; Zhang, Q. Organometallics 2013, 32, 2416−2426.

In 2016 Adams and his co-workers continued their extensive investigation on Ir-Bi compounds and reported that the thermal treatment of Ir3(CO)9(m3-Bi) led to the formation of Ir6(CO)13(m3-Bi)(m4-Bi) (see Fig. 50), through CO loss and subsequent condensation of the unsaturated fragments. Moreover, the reaction of the tri-nuclear and the new hexanuclear derivatives with PPh3 resulted in the ligand-substituted Ir3(CO)9−n(PPh3)n(m3-Bi) (n ¼ 1–3) and Ir6(CO)12(PPh3)(m3-Bi)(m4-Bi), respectively.135

Fig. 50 Molecular structure of Ir6(CO)13(m3-Bi)(m4-Bi) (Ir atoms in blue; Bi atoms in magenta; C in gray; O in red).

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The large Ir6(CO)13(m3-Bi)(m4-Bi) possesses a metal structure made of a Ir-capped Ir5 square pyramid with one quadruply bridging bismuth ligand capping its base and a triply bridging bismuth ligand across a triangular iridium face. The authors’ conclusion on this work is very interesting, as they attribute to the lone pair on the Bi ligand of Ir3(CO)9(m3-Bi) the propensity for its self-condensation to form the hexa-iridium complex species, in analogy with what had been found happening with sulfide ligands in osmium and ruthenium carbonyl clusters.136 The molecular structure of Ir6(CO)13(m3-Bi)(m4-Bi) resembles that of the Ir5(CO)10(m3-Bi)2(m4-Bi) congener, reported by Adams and Raja in the same period,137 as far as the metal framework is concerned. Notably, this compound was used as precursor to prepare bimetallic nano-catalysts for the oxidation of 3-picoline to niacin. When different reagents are used, the cluster formation may take another path, even if the involved metals and ligands are the same. For instance, in the same year Adams reported the synthesis of the new iridium–bismuth [Ir4(CO)10(m-BiPh2)(m-H)]2 heterocyclic neutral cluster, obtained by reacting [HIr4(CO)11]− with Ph2BiCl.138 Its formation was hypothesized to originate via a formation of an electron-rich Ir4(CO)11(m-BiPh2)(m-H) monomer (62 valence electrons), which retained the original tetrahedral structure undergoing CO elimination and then self-dimerization to produce the new compound. This species, in fact, is composed of two tetrahedral {[Ir4(CO)10(m-H)]}− moieties connected by two bridging [BiPh2]+ units (see Fig. 51). Unfortunately, due to its low yield, mechanistic studies could not be performed. Three years later, Xu et al. synthetized further heterometallic Ir-Bi carbonyl clusters via a different approach. More specifically, they used Ir(CO)2(acac) as starting material and made it react with K5Bi4 in ethylenediamine (en), forming the deltahedral [(Z3-Bi3)2(IrCO)6(m4-Bi)3]3− cluster.139 The importance of the ethylenediamine ligand derives from the fact that, in such medium, 140 the K5Bi4 is known to fragment in Bix− which are those effectively reacting with the Ir(I) complex. x anions (x ¼ 1 or 3), The crystal structure of [(Z3-Bi3)2(IrCO)6(m4-Bi)3]3−, shown in Fig. 52, consists of a central {Ir6(CO)6} trigonal prism whose three side faces are capped by three naked Bi atoms; two additional cycloBi3 anions on top and bottom of the prism complete the structure. The net tri-anionic charge can be formally derived by considering six [Ir(CO)]+ moieties and nine Bi− ions, and the 114 CVE are consistent with the Wade–Mingos’ rules141 and Polyhedral Skeletal Electron Pair Theory.

8.04.5

Nickel

Over the last fifteen years, the chemistry of nickel carbonyl clusters has been enriched by several new bimetallic or Ni-based compounds containing main group elements, mainly thanks to the research work carried out in Bologna by G. Longoni and his co-workers. Notably, there are no examples of new pure homometallic species reported in this time frame.

Fig. 51 Molecular structure of [Ir4(CO)10(m-BiPh2)(m-H)]2.

Fig. 52 Molecular structure of [(Z3-Bi3)2(IrCO)6(m4-Bi)3]3− (Ir atoms in blue; Bi atoms in magenta; C in gray; O in red).

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8.04.5.1

237

Homometallic nickel carbonyl clusters containing main-group elements

Several examples of nickel carbonyl clusters containing elements of the main groups, almost exclusively carbides, have been reported in the last fifteen years. For instance, in 2008 the new [HNi25(C2)4(CO)32]3− and [Ni22(C2)4(CO)28Cl]3− species have been unraveled thank to the reaction of the [Ni6(CO)12]2− cluster precursor with CCl4 (Fig. 53).142 Besides the high nuclearity displayed by these clusters, they are noteworthy because possessing interstitial Ni(Z2-C2)4 and Ni2(m-Z2-C2)4, respectively. However, it has been demonstrated that tightly bonded acetylide units are less effective than isolated carbide atoms in stabilizing cluster cages. By changing the stoichiometric ratio between the [Ni6(CO)12]2− cluster precursor with CCl4, more specifically by increasing the amount of the latter, and by using also C4Cl6, the new [Ni25(C2)4(CO)32]4− and [Ni17(C2)2(CO)24]4− poly-acetylide tetra-anions, respectively, were reported in 2012.143 A few years later the largest nickel carbide carbonyl cluster has been isolated.144 The reaction of [Ni10C2(CO)16]2− with CuCl afforded, in fact, the new [Ni45C10(CO)46]6− deca-carbide, along with the bimetallic [HNi42C8(CO)44(CuCl)]7− octa-carbide as a side product. The latter species is isostructural to the previously reported [Ni42C8(CO)44(CdCl)]7− and [HNi42C8(CO)44(CdBr)]6− compounds, which will be discussed later in this chapter book. Finally, two more carbide clusters were reported in the same paper, that is the homoleptic [Ni32+xC6(CO)36+x]6− (x ¼ 0–2) and the heteroleptic [Ni38C6(CO)36(MeCN)6(CuMeCN)2x]2− clusters. Considering the 10 interstitial carbides in [Ni45C10(CO)46]6−, five are enclosed within distorted Ni6C octahedral cavities, two within Ni6C trigonal prismatic cavities and three inside Ni7C mono-capped trigonal prismatic cavities. The molecular structure of [Ni45C10(CO)46]6− is a very complex assembly of various metal units, based on the same Ni32C8 framework composed of two differentially arranged Ni18C4. For a more detailed description of the crystal structure, reported in Fig. 54, authors refer to the comments illustrated in the main paper. Oxidation and reduction reactions of readily available low nuclearity metal carbonyl clusters may be exploited for the synthesis of new clusters, as recently demonstrated for already mentioned [Co6C(CO)15]2−. Analogously, the redox chemistry of [Ni9C(CO)17]2− and [Ni10(C2)(CO)16]2− has been explored and led to the new [Ni12C(CO)18]4− and [Ni22(C2)4(CO)28(Et2S)]2− species.145 The former cluster was obtained by reduction of [Ni9C(CO)17]2− with Na/naphthalene, whereas the latter, which recalls the above-mentioned [Ni22(C2)4(CO)28Cl]3−, originated by the oxidation of [Ni12C(CO)18]3− and [Ni12C(CO)18]5−, albeit in low yields, due to its limited stability. Conversely, the [Ni12C(CO)18]4− monocarbide can be obtained in satisfactory yields and displays an interesting redox chemistry. In fact, the related [Ni12C(CO)18]3− and [Ni12C(CO)18]5− anions have been obtained both by chemical and electrochemical oxidation/reduction of [Ni12C(CO)18]4−. In 2018, the reactions of [Ni16(C2)2(CO)23]4− and [Ni38C6(CO)42]6− with CuCl afforded mixtures bimetallic octa-carbide clusters, among which the new [HNi43C8(CO)45]7− and [HNi44C8(CO)46]7− species, which were also found in co-crystals with the CuCl-decorated congeners.146 In the last few years, a couple of papers reported the isolation of nickel carbonyl clusters containing fully interstitial phosphorous atoms. The first one describes the reaction between the versatile [Ni6(CO)12]2− cluster precursor with PCl3, which subsequently gave rise to two new carbonyl species, that is [Ni11P(CO)18]3− and the large [HNi31P4(CO)39]5−, whose metal structure is illustrated in Fig. 55.147 The Ni31P4 cage can be viewed as composed of two distorted and P-centered Ni9P mono-capped square antiprisms and

Fig. 53 (Left) The Ni(Z2-C2)4 interstitial moiety (A), the metal framework (B) and the complete structure (C) of [HNi25(C2)4(CO)32]3−. (Right) The Ni2(m-Z2-C2)4 interstitial moiety (A), the metal framework (B) and the complete structure (C) of [Ni22(C2)4(CO)28Cl]3− (Nickel atoms in green; Cl atom in yellow; C in gray; O in red). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Chem. Commun. 2008, 3157–3159.

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Fig. 54 Formal building up of the metal carbide cage of [Ni45C10(CO)46]6−. (A) The Ni32C8 fragment (Ni atoms in green, yellow, orange and magenta highlighting the polyhedral whose cavities host the carbides). (B) Ni36C8 framework by adding four Ni atoms (light blue). (C) Ni40C9 unit by adding a Ni4C fragment (Ni in red, C in white). (D) Ni44C10 framework obtained by adding a Ni4C fragment (Ni in blue, C in dark gray). (E) The final Ni45C10 cage resulting from the addition of a further Ni (brown). Reproduced with permission from: Bernardi, A.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. J. Organomet. Chem. 2016, 812, 229–239.

Fig. 55 Two different views of the Ni31P4 core of [H6−nNi31P4(CO)39]n− (n ¼ 4 and 5) (Ni atoms in green; Ni atoms fully interstitial in blue; Ni atoms not bonded with P in yellow; P atoms in magenta). Reproduced with permission from: Capacci, C.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Funaioli, T.; Zacchini, S.; Zanotti, V. Inorg. Chem. 2018, 57, 1136−1147.

two distorted and P-centered Ni10P bicapped square antiprisms fused together resulting in a Ni29P4 framework. This, in turn, is capped by two additional Ni-atoms not bonded to any phosphorus. The cluster also possesses two fully interstitial Ni atoms. The hydride nature of the cluster was inferred by chemical means via protonation and deprotonation reactions, which led to the [H2Ni31P4(CO)39]4− and [Ni31P4(CO)39]6− derivatives, respectively, and through electrochemical studies on the mono- and di-hydride species. In fact, their different redox behavior indirectly proved their hydride nature. The direct detection of the hydrogen atoms via 1H NMR spectroscopy was inconclusive, as already previously observed for such large compounds. Interestingly, even the lower nuclearity [Ni11P(CO)18]3− is multivalent, being able to exist as the tetra-anion congener. These findings, once again, sustain the evidence that interstitial elements (carbide, phosphides, nitrides, etc.) can strengthen the metal skeleton of a carbonyl cluster and prevent its breakage upon reduction or oxidation reactions. Two years later, a further investigation of the Ni-P system was carried out. First of all, it was repeated the same reaction described above but conducted in a different solvent, more specifically CH2Cl2 as opposed to THF. Second of all, a different P-bearing reactant, POCl3, was employed instead of PCl3.148 This study led to the isolation and full characterization of several high-nuclearity cluster phosphides, namely [Ni14P2(CO)22]2−, [Ni22−xP2(CO)29−x]4− (x ¼ 0.84), [Ni23−xP2(CO)30−x]4− (x ¼ 0.82), [Ni22P6(CO)30]2− and [Ni39P3(CO)44]6−.

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Fig. 56 Syntheses of Ni-P-CO clusters. Reproduced with permission from: Capacci, C.; Cesari, C.; Femoni, C.; Iapalucci, M. C.; Mancini, F.; Ruggieri, S.; Zacchini, S. Inorg. Chem. 2020, 59, 16016− 16026.

The scheme reported in Fig. 56 resumes the reaction conditions in which all above clusters were obtained. Such species have all been characterized by single-crystal X-ray diffraction, IR and 31P NMR spectroscopy. The metal frameworks of the new clusters are quite diverse. The smaller [Ni14P2(CO)22]2− one consists of two P-centered mono-capped square Ni9P antiprisms sharing one square face. The [Ni23−xP2(CO)30−x]4− cage is composed of one distorted P-centered Ni9P mono-capped square antiprism and one distorted P-centered Ni10P sphenocorona; the resulting Ni18P2 is completed by the addition of four further Ni atoms, obtaining the metal framework of Ni22P2(CO)29]4− (x ¼ 0). The further addition of another Ni atom completes the [Ni23P2(CO)30]4− metal cage (x ¼ 1). The [Ni22P6(CO)30]2− hexa-phosphide displays a metal Ni12 central polyhedron of pseudo D3d symmetry, with the six P atoms capping its pentagonal faces. The ten remaining Ni atoms complete the metal framework organized in two Ni3 and two Ni2 capping moieties. Finally, the [Ni39P3(CO)44]6− species, which so far is the largest nickel phosphide carbonyl cluster reported in the literature, shows a fascinating metal skeleton composed of three interpenetrating P-centered Ni12P icosahedra creating a Ni33P3 core with three fully interstitial nickel atoms. The remaining six cap in pairs each icosahedron. It is very interesting to notice that the P atoms in all those clusters are lodged in cavities with different geometries (see Fig. 57), and even more so it is to find that they are also able to occupy the large icosahedral cavities, which are usually the perfect hosts for bigger atoms like Ga, Ge, Sn and, to a lesser extent, Sb (see next section).

8.04.5.2

Homometallic nickel carbonyl clusters containing post-transition metals

Besides main group elements, also post-transition metals have been reported to insert into nickel carbonyl cluster cages, such as Ge and Sn, in the corresponding icosahedral [Ni12(m12-E)(CO)22]2− species.149 In 2007 new Ni-Ga carbonyl clusters were reported, all showing interstitial Ga atoms inside icosahedral-based nickel frameworks.150 More specifically, by reacting either [Ni6(CO)12]2− or [Ni5(CO)12]2− with GaCl3, under nitrogen atmosphere, a mixture of the [Ni12+x(m12-Ga)(CO)22+x]3− (x ¼ 0–3) tri-anions was obtained. By a short exposure under carbon monoxide atmosphere, exploiting the following equilibrium, the pure icosahedral [Ni12(m12-Ga)(CO)22]3− was isolated:
3
3 Ni12 +x ðm12 − GaÞðCOÞ22 + x + 3xCO⇆ Ni12 ðm12 − GaÞðCOÞ22 + xNiðCOÞ4 This cluster is isostructural to its Ge and Sn congeners, being composed of a Ge-centered icosahedron made by the nickel atoms, and it is also isoelectronic, as all possess 170 CVE, in compliance with the PSEP theory; it also resembles those metal structures related to the [Rh12E(CO)27]n– family of clusters. Notably, under protonation it is possible to obtain the [HNi12(m12-Ga)(CO)22]2− hydride congener, which was isolated and fully characterized not only via X-ray diffraction but also via 1H NMR spectroscopy, which gave one single signal at d ¼ − 13.56 ppm. In turn, by exposing the icosahedral cluster to Ni(CO)4, it is possible to push the above equilibrium backwards and maximize the formation of the [Ni12+x(m12-Ga)(CO)22+x]3− (x ¼ 2, 3) species, which co-crystallized as [Ni14(m12-Ga)(CO)24]3− and [Ni15(m12Ga)(CO)25]3− in a 70:30 ratio. The crystal structure of the former is illustrated in Fig. 58. It consists of a Ga-centered Ni icosahedron bi-capped by two Ni(CO) fragments on opposite triangular faces. Nickel carbonyl clusters containing Sb had been reported in the past.151 A reinvestigation of that system led to the isolation and characterization of the new [Ni19Sb4(CO)26]4−, along with the physical and spectroscopic characterization of the viologen salts of the [Ni10{SbNi(CO)3}2(m12-Ni)(CO)18]n– (n ¼ 2, 3) clusters.152 The new cluster was isolated by degradation of the previously known [Ni15Sb(CO)24]2− or [Ni13Sb2(CO)24]2− species with PPh3, via elimination of Ni(CO)4–n(PPh3)n (n ¼ 2, 3) and subsequent condensation of the unsaturated fragments. Its molecular structure, illustrated in Fig. 59, consists of a Ni-centered

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Fig. 57 Diverse environments of P atoms in Ni–P–CO clusters. Ni5P pentagonal pyramid in [Ni22P6(CO)30]2− (A); Ni7P, monocapped trigonal prism in [Ni22P6(CO)30]2− (B); Ni8P bicapped trigonal prism in [Ni22P6(CO)30]2− (C); Ni9P monocapped square antiprism in [Ni14P2(CO)22]2− (D), [Ni22−xP2(CO)29−x]4− (E), [Ni23−xP2(CO)30−x]4− (F) and [H6−nNi31P4(CO)39]n− (n ¼ 4, 5) (G); Ni10P, sphenocorona in [Ni11P(CO)18]3− (H) and [Ni23−xP2(CO)30− x]4− (I); Ni10P bicapped square antiprism in [Ni22−xP2(CO)29−x]4− (J) and [H6−nNi31P4(CO)39]n− (n ¼ 4, 5) (K); Ni12P icosahedron in [Ni39P3(CO)44]6− (L). Reproduced with permission from: Capacci, C.; Cesari, C.; Femoni, C.; Iapalucci, M. C.; Mancini, F.; Ruggieri, S.; Zacchini, S. Inorg. Chem. 2020, 59, 16016−16026.

Fig. 58 Molecular structure (left) and metal skeleton (right) of [Ni12+x(m12-Ga)(CO)22+x]3− (x ¼ 2 at 70%, 3 at 30%) (Ni atoms in green; Ga atom in turquoise; C in gray; O in red).

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Fig. 59 Molecular structure of [Ni19Sb4(CO)26]4− (Ni atoms in green and cyano; Sb atoms in yellow; C in gray; O in red).

Ni9Sb4 icosahedral inner core around which eight additional Ni(CO)n fragments, four on each side, are symmetrically coordinated. The metal framework is then completed by two m3-capping Ni atoms. The Sb atoms are semi-interstitially lodged in incomplete Ni9Sb(m10-Sb) and Ni8Sb(m9-Sb) icosahedral moieties, related to [B10H14]2− and [B9H13]2− arachno boranes.

8.04.5.3

Heterometallic nickel carbonyl clusters

Many Ni-Pt carbonyl clusters had been reported prior to 2006, among which the first large [HNi38Pt6(CO)48]5−,153 [Ni36Pt4(CO)45]6−,154 [Ni24Pt14(CO)44]4−,155 [Ni35Pt9(CO)48]6−156 and the highest nuclearity one to date, [Ni32Pt24(CO)56]6−.157 In the last 15 years there has been no report of new Ni-Pt carbonyl species. However, in 2006 Dahl and co-workers finally determined the crystal structure of the [Ni38Pt6(CO)48]6− hexaanionic species derived from deprotonation of the former above-mentioned [HNi38Pt6(CO)48]5− cluster. An alternative aprotic synthesis, therefore more straightforward, was also reported and it involves the reaction of Pt(COD)Cl2 with [Ni6(CO)12]2− in DMSO, with OH− added to eliminate the formation of protonated species.158 The metal structure of [Ni38Pt6(CO)48]6−, illustrated in Fig. 60, is composed of a Pt6 octahedron fully encapsulated in a n3 Ni38 octahedron of nickel atoms and stabilized by 38 carbonyl ligands. A remarkable result out of this work is that it was finally possible to compare the structure of the hexa-anion with the mono-hydride penta-anion, and their close geometrical agreement is consistent with the original suggestion by Longoni and co-workers, based upon their electrochemical-IR studies, that the hydrido-like proton interstitially occupies one of the octahedral metal cavities, as previously ascertained in the [HnNi12(CO)21]n−4 series (n ¼ 1, 2) from combined neutron-X-ray diffraction measurements.159 Among the best nickel carbonyl precursors to prepare heterometallic clusters there are the [Ni6(CO)12]2− and [Ni9(CO)18]2− dianions. In analogy with what happens with [Rh7(CO)16]3−, in fact, they are in an enough negative oxidation state to be easily oxidized by complexes of the metal of choice to couple with nickel, resulting in the formation of heterometallic cluster via redox condensation reactions. However, when those cluster precursors are combined with soft acids like Cd(II), then a Lewis acid-base reaction preferably occurs. Therefore, by reacting [Ni6(CO)12]2− with CdCl2 the adduct {Cd2Cl3[Ni6(CO)12]2}3− is formed,160 in analogy with what happened with InBr3.161

Fig. 60 Metal structure of [Ni38Pt6(CO)48]6− (Ni atoms in green; Pt atoms in magenta).

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Conversely, when [Ni6(CO)12]2− reacts with an Au(III) derivative, the inclusion of gold atoms within the nickel metal framework occurs, as a result of a redox condensation reaction. This is how the already known [Ni32Au6(CO)44]6− cluster was more recently prepared,162 whereas its original synthesis reported by Dahl and co-workers in 1999 was carried out by reacting an Au(I) complex, AuPPh3Cl, with the same cluster precursor but in presence of Ni(OAc)2.163 Notably, the other previously known Ni-Au cluster, [Au6Ni12(CO)24]2−, was again reported by Dahl and his co-worker in 1991.164 The reinvestigation of the synthesis of the larger cluster was driven by the desire to study its possible redox activity, given the presence of six interstitial gold atoms, which was then unprecedented in the transition metal carbonyl cluster chemistry (the [Rh16Au6(CO)36]6− species had not been reported yet). The crystal structure of [Ni32Au6(CO)44]6− is illustrated in Fig. 61. Another smaller cluster was isolated in the same reaction, that is the dimeric [Ni12Au(CO)24]3− bimetallic species, whose structure formally consists of two [Ni6(CO)12]2− cluster units joint by an Au(I) atom. Notably, this cluster is an adduct product between the homometallic precursor and Au(I), which originated from the reduction of the employed Au(III) that took place in the reaction environment. The electrochemical investigation of [Ni32Au6(CO)44]6− was carried out by CV experiments, and it turned out that the cluster can undergo five reversible reduction steps without any breakage of the molecular structure. However, the observed current was rather low that prevented form ascertain the number of electrons involved in each step. This is an occurrence that is not unprecedented in high-nuclearity transition-metal carbonyl clusters. An interesting combination of Ni and Pd was reported very recently, where the bimetallic [Ni22−xPd20+x(CO)48]6− (x ¼ 0.62), [Ni29−xPd6+x(CO)42]6− (x ¼ 0.09) and [Ni29+xPd6−x(CO)42]6− (x ¼ 0.27) were obtained by reacting [Ni6(CO)12]2− with Pd(Et2S)2Cl2 in either dichloromethane (in the case of the former) or acetonitrile (the two latter species).165 They were the first ones to be isolated since the report of [Ni36Pd8(CO)48]6−,156 [Ni13Pd13(CO)34]4−,166 [Ni16Pd16(CO)40]4− and [Ni26Pd20(CO)54]6−,167 that dates back nearly twenty years ago. Notably, while Pd does not form homoleptic carbonyl clusters due to the low PddCO bond energy, and Ni does not form high-nuclearity species, the synergic combination of the two gives rise to very large carbonyl clusters, often of nanometric dimensions. Notably, in every bimetallic Ni-Pd cluster the Pd atoms are never terminally coordinated to the carbonyl ligands, therefore generating a metal segregation within the clusters themselves (see introduction of the Pd section). The molecular structure of [Ni22−xPd20+x(CO)48]6− (x ¼ 0.62) consists of a five-layer close-packed stacking arrangement of 40 metal atoms capped by two additional ones on the outer and opposite triangular faces. Moreover, it presents both compositional and substitutional disorder. The molecular structures of [Ni29−xPd6+x(CO)42]6− and [Ni29+xPd6−x(CO)42]6− are basically identical, as they only slightly differ in the Ni/Pd composition. Their metal architecture, illustrated in Fig. 62, is a chunk of a hexagonal close packing lattice composed of three layers, thus forming an inner trigonal bipyramid made by five Pd atoms, three of which are fully interstitial. It also presents substitutional disorder. This metal framework closely resembles that of the bimetallic [CuxNi35−x(CO)40]5− (x ¼ 3 or 5) cluster.168 The redox properties of [Ni22−xPd20+x(CO)48]6− and [Ni29−xPd6+x(CO)42]6− were studied through CV experiments and in-situ spectroelectrochemistry. In the first case the very low current associated with the redox processes impaired a direct determination of the number of electrons involved. However, the spectroelectrochemistry proved to be a valid tool to correctly interpret the multivalence behavior of the cluster even with unconclusive CV experiments. Fig. 63 displays the four negative charges in which [Ni22−xPd20+x(CO)48]n− can stably exist without disruption of its molecular structure. The downshift of the n(CO) stretching frequencies has an almost steady value of 14 cm−1 for every additional negative charge, fully consistent with monoelectronic steps.

Fig. 61 Molecular structure (left) and metal core (right) of [Ni32Au6(CO)44]6− (Ni atoms in green; Au atoms in yellow).

Group 9 and 10 Carbonyl Clusters

243

Fig. 62 Metal core of [Ni29−xPd6+x(CO)42]6− and [Ni29+xPd6−x(CO)42]6− (Ni atoms in green; Pd atoms in magenta; Ni/Pd in yellow). Reproduced with permission from: Berti, B.; Cesari, C.; Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Zacchini, S. Dalton Trans. 2020, 49, 5513–5522.

Fig. 63 Selected infrared spectra of [Ni22−xPd20+x(CO)48]n− as a function of the cluster charge n and potential E (vs. the Ag pseudo-reference electrode), in CH3CN containing 0.1 M [NnBu4][PF6]. The absorptions of the solvent and the supporting electrolyte have been subtracted. Reproduced with permission from: Berti, B.; Cesari, C.; Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Zacchini, S. Dalton Trans. 2020, 49, 5513–5522.

The same number of redox couples, albeit at different potentials, and the same stable negative charges were found for [Ni29−xPd6+x(CO)42]n−. These findings corroborate the fact that multivalence features in metal carbonyl clusters are promoted not only by the presence of inner elements of the main groups, but also by interstitial metal atoms, and that high-nuclearity carbonyl clusters can behave as electron-sponge compounds.

8.04.5.4

Heterometallic nickel carbonyl clusters containing main-group elements

Various new Ni-Co heterometallic poly-carbide clusters have been prepared over the last decade, either by exploiting the redox-condensation method or by thermolysis. Notably, some of these new species possess a poly-hydride nature, like [H2Ni22Co6C6(CO)36]4−.169 This cluster can be obtained by reacting [Ni10C2(CO)16]2− with Co3(m3-CCl)(CO)9. The CCl ligand is the source of carbide atoms, while the presence of hydrides most likely derives by water used over the reaction workup. The hydride-free [Ni22Co6C6(CO)36]6− cluster can be obtained by deprotonating the parent tetra-anion. The detection of hydrogen atoms in high-nuclearity transition-metal clusters has been a matter of discussion. Eventually, it has been experimentally demonstrated that, starting from nuclearity of around 22 metal atoms, all 1H NMR resonances disappear since they become so broad as to get blurred in the baseline of the spectrum.170 Therefore, the hydride nature of large metal carbonyl clusters can be only indirectly inferred, for instance, from joined chemical and electrochemical experiments, and sometimes by the aid of mass spectrometry. In the case of [H2Ni22Co6C6(CO)36]4−, the hydride was confirmed both by ESI-MS analysis and via comparative electrochemistry studies on the di- and mono-hydride species. As a matter of fact, [H2Ni22Co6C6(CO)36]4− displays two reduction processes with features of chemical reversibility, whereas [HNi22Co6C6(CO)36]5− displays three reversible reduction steps. It is remarkable that multivalence behavior can also be exploited to differentiate hydride clusters and serve whereas other techniques fail.

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Fig. 64 Metal framework of [H6−nNi22Co6C6(CO)36]n− (n ¼ 4, 6) (Ni atoms in green; Co atoms in blue; C in gray).

The molecular structure of the [H6−nNi22Co6C6(CO)36]n− (n ¼ 4, 6) cluster is rather complex and formally results from the condensation of six Ni7CoC distorted square antiprismatic C-centered cages (Fig. 64). The controlled reaction of the [Ni22Co6C6(CO)36]6− with OH− led to the formation of the lower nuclearity [Ni9CoC2(CO)16−x]3− (x ¼ 0, 1). Conversely, the thermal treatment of [H2Ni22Co6C6(CO)36]4− resulted in the mono-hydride and octa-carbide [HNi36Co8C8(CO)48]5− cluster, from which the [H6−nNi36Co8C8(CO)48]n− (n ¼ 3–6) derivative can be prepared by protonationdeprotonation reactions.171 The structure of the Ni-Co hexacarbide species is quite complex, but its core resembles that of the [Ni32C6(CO)32]6−,172 as it is composed of a Ni8C6 unit consisting of a cube of nickel atoms whose six faces are capped by as many carbon atoms. These carbides are then encapsulated in six square antiprisms, although in [H6−nNi36Co8C8(CO)48]n− four of them appear elongated. The structure is completed by the Co atoms, capping four out eight hexagonal faces described by the central core, and by the remaining two Ni2Co2C units capping two opposite hexagonal faces. The chemistry of mixed rhodium-nickel carbonyl clusters has been extensively studied over the past years,173 but more recently new Ni-Rh carbide clusters have been synthetized, in analogy with the Ni-Co congeners.174,175 More specifically, from the reaction of the [Ni9C(CO)17]2− carbide precursor with a Rh(I) complex containing labile ligands, namely [Rh(COD)Cl]2, the heterometallic [Ni10Rh2C(CO)20]2− and [Ni9Rh3C(CO)20]3− clusters could be obtained.176 They are both isoelectronic and isostructural; in fact their metal geometries consist of a distorted square antiprism tetra-capped by Ni(CO)n units on two alternate pairs of adjacent triangular faces. The Rh atoms are, in both cases, disordered over the internal prismatic positions. Interestingly, the degradation of [Ni10Rh2C(CO)20]2− under carbon monoxide atmosphere resulted in the formation of the bis-acetylide [Ni6Rh8(C2)2(CO)24]4− species (Fig. 65). Its metal structure is composed of three metal layers, of which the external ones are made of Rh atoms, while the central layer contains the six Ni atoms describing an elongated hexagon. The two carbide pairs are encapsulated within the metal cage and their distance [C–C 1.382(19) A˚ ] suggests a bond order of ca. 2. The relative location of the Ni and Rh atoms within the metal cages seems to be the result of a subtle balance of the M–M, M–C and M–CO bonding energies. Over the last fifteen years the investigation of the reaction between nickel carbide cluster precursors and Cd(II) complexes gave rise to the isolation and characterization of a wide series of heterometallic compounds. For instance, in 2008170 it was reported that

Fig. 65 Degradation of [Ni10Rh2C(CO)20]2− (left) under CO atmosphere and formation of [Ni6Rh8(C2)2(CO)24]4− (right). Only metal frameworks are depicted (Ni atoms in white; Rh atoms in gray; C in black). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Eur. J. Inorg. Chem. 2009, 17, 2487–2495.

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Fig. 66 Molecular structure of [H2Ni30C4(CO)34(CdCl)2]4− (Ni atoms in green; Cd atoms in magenta; Cl in turquoise; C in gray; O in red).

the reaction of [Ni9C(CO)17]2− with CdCl2 in THF affords the novel heterometallic [H6−nNi30C4(CO)34(CdCl)2]n− (n ¼ 3–6) carbide clusters, related by protonation-deprotonation equilibria, and three of them (with n ¼ 4–6) were also fully structurally characterized. Once again, electrochemical studies not only unraveled their multivalence, but allowed an indirect, albeit unconfutable, proof of their hydride nature, as 1H NMR spectrometry fails when dealing with such large metal carbonyl clusters. The metal structure of [H6−nNi30C4(CO)34(CdCl)2]n−, illustrated in Fig. 66, is composed of a Ni20 cubic close-packed (CCP) core which presents 28 triangular and four square faces on its surface. The four carbide atoms are located over the latter faces, and a trigonal prismatic cage can be obtained around each by condensing two more Ni atoms. The resulting Ni28C4 fragment is then completed by two additional Ni atoms onto two opposite square faces, while the two [CdCl]+ moieties cap two opposite pentagonal faces. Two years later, the related [Ni30C4(CO)34(CdX)2]6− hexaanion (n ¼ Cl, Br, I) was converted in the mixture of the new [Ni31C4(CO)35(CdX)]7−, [Ni32C4(CO)36(CdX)]7− and [Ni33C4(CO)37(CdX)]7− (X ¼ Cl, Br, I) tetra-carbide clusters, which only differ for the presence/absence of Ni(CO) fragments, via nucleophilic attack with X− or OH−.177 Still in 2009, a quite relevant study on the new [HNi22(C2)4(CO)28(CdBr)2]3− tetra-acetylide species, obtained via reaction of [Ni10C2(CO)15]2− with a large excess of CdBr2, was carried out in order to elucidate once more the problem in detecting hydrides in metal carbonyl clusters by 1H NMR.178 In fact, as mentioned in several occasions in this chapter book, the large size of the metal core in the highest nuclearity metal carbonyl cluster anions and its incipient metallization affects their spectroscopic behavior. While 1H NMR spectra of lower-nuclearity cluster species, e.g., [H4−nNi12(CO)21]n−,179 [H4−nNi9Pt3(CO)21]n−180 [H4−nNi9Pt3(CO)21]n−181 are diagnostic of the presence of hydride atoms, beyond a nuclearity of ca. 20–22 several spin-active nuclei in metal carbonyl cluster anions become partially or completely silent in NMR experiments, with the most remarkable exception being represented by the neutral H12Pd28Pt13(PPh3)12(PMe3)(CO)27.182 A thorough 1H NMR study pointed out, first of all, a solvent- and temperature dependence of the hydride signal of [HNi22(C2)4 (CO)28(CdBr)2]3− (see Fig. 67), which suggested the occurrence of dynamic site-exchange processes in solution.

Fig. 67 Variable temperature 1H NMR spectra recorded at 600 MHz of [HNi22(C2)4(CO)28(CdBr)2]3− in d6-acetone. Reproduced with permission from: Bernardi, A.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Dalton Trans. 2009, 4245–4251.

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Second of all, the analyzes showed an anomalous broadness of signals (the half-height widths are in the 500–800 Hz range), whereas the resonances of lower nuclearity species, for instance [H4− nNi12(CO)21]n−, are relatively sharp (5–10 Hz). In order to explain this phenomenon, acetonitrile solutions of [HNi22(C2)4(CO)28(CdBr)2]3− and [Fe4(CO)13]2− (for comparison), have been investigated by Dynamic Light Scattering (DLS). The results have pointed out that in solution the hydrodynamic diameters are considerably greater than the free cluster ions, especially those of the larger cluster. It seems conceivable that these enormous particles, displaying a size ca. 30 and 7 times that of their respective free ion, respectively, might arise by assembly in solution of fractal clusters of clusters or extensive chunks of ionic lattice by electrostatic interactions. The slow tumbling of these huge aggregates, in combination or not with dynamic hydride site-exchange processes, might account for the observed 1H NMR behavior (broadening, as well as large chemical-shift drifts with solvent and temperature) of the [HNi22(C2)4(CO)28(CdBr)2]3− ion. In 2010, several other Cd-decorated Ni carbide clusters have been reported after further investigation of the reaction of [Ni10C2(CO)15]2− in THF with a large excess of CdCl2, and subsequent treatment of the obtained product with OH-.183 These new compounds, [Ni36C8(CO)36(Cd2Cl3)]5−, [Ni36–yC8(CO)34–y(MeCN)3(Cd2Cl3)]3− (y ¼ 0.61), [Ni42+yC8(CO)44+y(CdCl)]7−, and [HNi42+yC8(CO)44+y(CdBr)]6− have been structurally characterized by single-crystal X-ray diffraction. The investigation of the multivalence of [HNi42+yC8(CO)44+y(CdBr)]6− and [Ni36C8(CO)36(Cd2Cl3)]5- through spectroelectrochemistry and CV analyzes confirmed their rich redox activity, as expected from clusters of such nuclearity containing interstitial main-group elements. Fig. 68 displays the IR spectral sequences stepwise upon reduction (a) and reoxidation (b) of a DMF solution of [Ni36C8(CO)36(Cd2Cl3)]5− recorded in an Optically Transparent Thin Layer Electrochemical (OTTLE) cell in the region of the terminal n(CO) frequencies. Their values show a downshift of about 20 cm−1 when the cluster is reversibly reduced from the pentato the hexaanion, consistent with mono-electronic processes. The use of [Ni9C(CO)17]2− as cluster carbide precursor has proved to be very successful in synthetizing new heterometallic species. Its reaction with Au(PPh3)Cl allows, in fact, to obtain several bimetallic Ni-Au carbide compounds. In 2013 the investigation of this reaction led to the formation of the Ni6C(CO)9(AuPPh3)4 neutral species. This was structurally characterized in two isomers and their main difference laid in the Au ⋯ Au distances, which DFT calculations attributed solely to packing effects.184 Notably, Ni6C(CO)9(AuPPh3)4 represented the very first isolated octahedral mono-carbide carbonyl species, soon replicated by the obtaining of the [Ni6(C)(CO)8(AuPPh3)8]2+ cationic compound. To the best of our knowledge, the other example of a nickel octahedral monocarbide cluster, albeit non carbonylic, is the neutral Ni6C(Cp)6 species.185 The novel [Ni6(C)(CO)8(AuPPh3)8]2+ cluster was synthesized by adding HBF4
Et2O to Ni6C(CO)9(AuPPh3)4, during the investigation of its chemical reactivity.186 In the same paper, the preparation of the new [Ni12(C)(C2)(CO)17(AuPPh3)3]− anionic species was also reported, done by reacting [Ni9(C)(CO)17]2−, [Ni10(C2)(CO)16]2− and Au(PPh3)Cl in a 1:1:3 molar ratio. The obtained higher-nuclearity species is a unique example of a carbonyl cluster concurrently containing one carbide atom and one tightly bonded C2-unit; it is worth noting that sub-van der Waals contacts, evinced from DFT calculations, are present between the unique carbide and the C2 one, suggesting the incipient formation of more extended CdC bonding. The molecular structure of [Ni12(C)(C2)(CO)17(AuPPh3)3]− shows a metal cage composed of a Ni10(C2) monoacetylide fragment fused with a Ni5(C)Au octahedron through three shared Ni atoms, completed by two further Au-atoms capping two Ni4-butterfly surfaces. The metal framework of [Ni12(C)(C2)(CO)17(AuPPh3)3]− with the highlights of the above-mentioned fragments is illustrated in Fig. 69.

Fig. 68 IR spectroelectrochemical trends recorded in the region of the terminal n(CO) frequencies for [Me4N]5[Ni36C8(CO)36(Cd2Cl3)] in DMF solution by using an OTTLE cell. (A) Stepwise variation of the potential from Ew ¼ − 0.70 to −1.15 V vs. pseudo-reference Ag electrode. (B) Reoxidation from Ew ¼ −1.15 to −0.70 V. [Et4N][PF6] (0.1 mol dm−3) as the supporting electrolyte. Reproduced with permission from: Bernardi, A.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S.; Fedi, S.; Zanello, P. Eur. J. Inorg. Chem. 2010, 4831–4842.

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Fig. 69 Metal framework representations of [Ni12(C)(C2)(CO)17(AuPPh3)3]− showing (A) its Ni10(C2) fragment (Ni atoms in blue; C in back); (B) the Ni5(C)Au octahedron (Ni atoms in magenta; Au (1) in orange; C in black); (C) the weak C–(C2) interactions (dotted lines) (Ni atoms in green; Au atoms in yellow; C in gray). Reproduced with permission from: Bortoluzzi, M.; Ciabatti, I.; Femoni, C.; Hayatifar, M.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Dalton Trans. 2014, 43, 13471–13475.

In 2013 a rich series of new Cu(CH3CN)-decorated Ni carbide carbonyl clusters has been reported.187 They had all been synthesized by reacting the versatile [Ni9C(CO)17]2− with [Cu(CH3CN)]+ in different stoichiometric ratio, and in some cases with a subsequent addition of p-NCC6H4CN that replaced the acetonitrile ligand bore by Cu. The clusters obtained by dosing the reactant in an almost equal ratio are [H2Ni30C4(CO)34{Cu(CH3CN)}2]4− co-crystallized with [H2Ni29C4(CO)33{Cu(CH3CN)}2]4−, then by increasing the [Cu(CH3CN)4]+/[Ni9C(CO)17]2− ratio to ca. 1.7/1 the closely related [H2Ni30C4(CO)35{Cu(CH3CN)}2]2−, [H2Ni29C4(CO)34{Cu(CH3CN)}2]2− and [H2Ni29C4(CO)32(CH3CN)2{Cu(CH3CN)}2]2− dianions were isolated; finally, replacement of Cu-bonded CH3CN with p-NCC6H4CN afforded, after protonation of the former tetra-anion, mixtures of [H3Ni30C4 (CO)34{Cu(NCC6H4CN)}2]3− and [H3Ni29C4(CO)33{Cu(NCC6H4CN)}2]3−. For a more detailed structural description of the above clusters, authors suggest the reading of the full paper. The compounds that crystallized as pure species, namely [H6−nNi30C4(CO)34{Cu(CH3CN)}2]n− (n ¼ 4, 5, 6), [H2Ni29C4(CO)34 {Cu(CH3CN)}2]2− and [H2Ni29C4(CO)32(CH3CN)2{Cu(CH3CN)}2]2−, were also investigated through CV experiments, to evaluate their redox activity. Indeed, the species turned out to be multivalent as they did undergo different reversible redox processes. This property could be the result of incipient metallisation of the clusters’ metal cores, as revealed by the small values of DE between consecutive redox couples determined through EHMO calculations. Finally, once again electrochemical experiments were essential to discriminate the hydride nature of high-nuclearity species, as the cluster which only differ by their hydride numbers behaved in different ways (Fig. 70). As far as heterometallic nickel nitrides, in 2008, Della Pergola et al.188 reported the synthesis of two new Fe-Ni carbonyl clusters by reacting two cluster precursors of [Ni6(CO)12]2− with [Fe4N(CO)12]−. In the end, it was the iron that prevailed over nickel in the resulting species, most likely due to its stable nitride nature, as the obtained products are the [HFe5NiN(CO)14]2− and [HFe4Ni2N (CO)13]2− nitride hydride clusters. Notably, in the latter, thanks to the excellent quality of the crystallographic data, the hydrogen

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Fig. 70 Formal electrode potentials (in V, vs. SCE) for the redox changes exhibited by [H6−nNi30C4(CO)34{Cu(CH3CN)}2]n− (n ¼ 4, 5, 6) (1), [H2Ni29C4(CO)34{Cu(CH3CN)}2]2− (4) and [H2Ni29C4(CO)32(CH3CN)2{Cu(CH3CN)}2]2− (5). Reproduced with permission from: Bernardi, A.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Dalton Trans. 2013, 42, 407–421.

atom had been located in the Fourier map and properly refined. This is a quite remarkable occurrence that does not happen in high-nuclearity carbonyl clusters, irrespective of the quality of the X-ray diffraction collected data.

8.04.6

Palladium

In the last fifteen years there are a few papers that report works on Pd carbonyl clusters. It is important to underline that palladium is the only late-transition metal that does not form homoleptic homometallic carbonyl clusters of general type [Pdx(CO)y]n− (x, y > 1; n > 0),189,190 owing to rather weak Pd–Pd and Pd–CO (terminally coordinated) bonding. Indeed, Pd–Pd bonds are the weakest metal–metal bonds in the iron, cobalt and nickel triads.191 For these reasons, the work that will be discussed below only regards heteroleptic palladium carbonyl clusters, often of very high nuclearity. Those compounds where Pd is not the preponderant element but contain one of the subject metals of this book chapter will not be inserted here but in the respective section. The chemistry of palladium carbonyl clusters in the last fifteen years, and beforehand, has been nearly exclusively dominated by the work of the late Prof. L. F. Dahl (University of Wisconsin-Madison, United States), who only recently passed away. His outstanding career in the field of transition-metal carbonyl clusters dates back in the late 1950s, and the scientific community has been enlightened by his contribution since. One quote from an article appeared in the Journal of Chemical Education in 2009 wonderfully reveals Dahl’s (and his long-time co-worker E. G. Mednikov) passion for palladium cluster chemistry: “Is there a metal element that allows a chemist to create and enjoy an unexpected variety of extraordinary molecular architectures that are different from those obtained from all other metal elements? Our separate and currently combined research over three decades has revealed the existence of one such element: palladium”.190 For a further deepening of the Pd carbonyl chemistry up to 2010 we suggest enjoying the reading of a comprehensive review by G. Mednikov and L. F. Dahl, where species also prior to 2006 are reported.192

8.04.6.1

Homometallic heteroleptic palladium carbonyl clusters

To our knowledge, there have been only two new homometallic species reported within the time frame of this chapter book, the latest and largest one being Pd145(CO)x(PEt3)30 (x  60).193 In 2008, in fact, Dahl and co-workers published the synthesis and characterization of the new homometallic species, Pd37(CO)28{P(p-Tolyl)3}12, obtained by thermolysis of Pd10(CO)12{P(p-Tolyl)3}6 in THF solution.194 Notably, if the P(p-tolyl)3 is substituted by PPh3, the immediately formed product of the thermolysis is determined not to be the PPh3-ligated Pd37 analog but instead the known Pd12(CO)12(PPh3)6 species,195 owing to its precipitation in the reaction medium due to its insolubility in organic solvents. The molecular structure of Pd37(CO)28{P(p-tolyl)3}12 possesses a metal core resembling the one of [Rh26(CO)29(CH3CN)11], as it too is composed of three interpenetrating Pd-centered icosahedra forming a Pd23 kernel, in this case furtherly asymmetrically capped by the remaining fourteen Pd atoms (Fig. 71).

Fig. 71 The Pd23 metal core (left) and the full metal structure (right) of Pd37(CO)28{P(p-tolyl)3}12.

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249

Fig. 72 Metal structure of [Pd10(CO)12(PPh3)6] (Pd atoms in orange; P atoms in green; C in gray; O in red).

In 2011 Hermans et al. reported the synthesis in mild conditions of heteroleptic palladium clusters containing phosphine ligands, by bubbling CO into a solution of [Pd2(dba)3] (dba ¼ dibenzylideneacetone) with 3, 1 or 0.5 equiv. of PPh3, that is the already known [Pd4(CO)5(PPh3)4],196 and the new [Pd10(CO)12(PPh3)6] (albeit already characterized in 1983, but with PBu3 ligands instead197) and [Pdn(CO)x(PPh3)y] (n  24).198 The former two species were characterized also by X-ray diffraction, while the latter was identified by MALDI mass spectrometry and elemental analysis, as well as IR and NMR. Fig. 72 illustrates the crystal structure of [Pd10(CO)12(PPh3)6], whose metal frame consists of a Pd6 octahedron with the remaining Pd atoms capping two opposite triangular faces on the same octahedral side and two bridging two opposite Pd-Pd edges. All obtained clusters were tested by reduction with NaBH4, and only the deca-nuclear species gave identifiable reduction products, among which [Pd12Hx(CO)12(PBu3)6]−. In 2016 Dahl and co-workers published another beautiful work where they reported the synthesis and characterization of another homometallic Pd nanocluster, more specifically a Mackay 55-metal-atom two-shell icosahedron Pd55L12(CO)20 (L ¼ PiPr3) (L ¼ PiPr3) neutral compound.199 Mackay stated that close-packed assembly of equal-sized spheres can form concentric shells of either regular icosahedra or cuboctahedra.200 As a result, the second shell of spheres, packed over the first shell (i.e., the centered 13-atom icosahedron or cuboctahedron), would consist of 42 spheres. The 55-metal-atom two-shell cluster is a “magic number” in clusters, as it is presumed to be especially stable because of its complete outer geometry. This finding gave a significant contribution also on the fields of nanoparticles and intermetallic clusters, and quasicrystals.201 In the past, Schmid et al. reported the synthesis of powdered samples of several M55L12Clx clusters, where L was a bulky phosphine ligand and M ¼ Au, Rh, Ru, Pt. Despite the unsuccessful attempts to crystallize them, spectroscopic characterization coupled with molecular weight determinations, elemental analyzes, and high-resolution transmission electron microscopy (HRTEM) had suggested to the Authors that these clusters were “understandable” only if they had cuboctahedral metal architectures. Pd clusters have revealed to form an extraordinary diversity of nanosized CO/PR3-ligated icosahedral species. Among them, two contain a 55 inner core that conforms to the Mackay model, namely the above-mentioned Pd145(CO)x(PEt3)30 (x  60) cluster and the bimetallic (m12-Pt)Pd164−xPtx(CO)72(PPh3)20 (see later). Two recent independent investigations of Au133(SC6H4-p-But)52 unraveled an isostructural interior 55-metal-atom two-shell Mackay icosahedron with shell 3 in an anti-Mackay atom arrangement.202,203 The new Pd55(PiPr3)12(CO)20 cluster exhibited a rather high stability compared with the species isolated by Schmidt, most likely because the tri-isopropylphosphine ligands, provided by the Pd10(CO)12(PPri3)6 precursor, would sterically prevent further sequential conversions of the Pd55 into other Pdn clusters. The metal structure of Pd55(PPri3)12(CO)20 is reported in Fig. 73.

Fig. 73 Metal framework (A) and molecular structure (B) of Pd55(PPri3)12(m3-CO)20 (Pd atoms in red and blue; P atoms in pink; C in gray; O in pale blue) The organic groups bonded to the phosphorous atoms have been omitted for sake of clarity. Reproduced with permission from: Erickson, J. D.; Mednikov, E. G.; Ivanov, S. A.; Dahl, L. F. J. Am. Chem. Soc. 2016, 138, 1502−1505.

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For a thorough comparative analysis of the Pd55(PiPr3)12(CO)20 metal structure with those of Pd145, Pt-centered [Pd–Pt]165 and Au133 nanoclusters Authors refer to the published paper.

8.04.6.2

Heteroleptic palladium carbonyl clusters containing post-transition metals

The use of Pd10(CO)12(PPh3)6 as precursor to obtain new palladium cluster containing post-transition metals was exploited in 2012 by Dahl and co-workers to prepare Pd9[m3/3-Tl(acac)]6(CO)9(PPh3)6 through reaction with TlPF6, in presence of PPh3, acetylacetone (Hacac) and base (NEt3), or by its direct reaction with PPh3 and Tl(acac).204 A thorough structural analysis with the parent Pd9[m3-TlCo(CO)3L](CO)9(PEt3)6 and [Pd12Tl2(CO)9(PEt3)9]2+ is reported. In the case of the former, the comparison has been extended to solution dynamics. Notably, variable-temperature 31P{1H} NMR solution data of Pd9[m3/3-Tl(acac)] (CO)9(PPh3)6 indicate that no Pd–Tl detachment occurs. Conversely, corresponding temperature-dependent 31P and 13C NMR data of Pd9[m3-TlCo(CO)3L](CO)9(PEt3)6 are consistent with rapid, reversible dissociation/association of the entire [m3-TlCo(CO)3L] ligand. In continuation with the investigation of the Pd-Tl chemistry, 3 years later Dahl and his co-workers reported the synthesis of a sandwiched Pd9Tl compound were the Tl(I) acts as a linker between two a Pd9 and a Pd3 units, giving rise to the new {[Pd9(CO)9(PMe3)6]Tl[Pd3(CO)3(PMe3)3]}+ species. This time the used cluster precursor was Pd8(CO)8(PMe3)7, which reacted with TlPF6.205 DFT calculations suggested that high stability of this compound, as well as the stabilization of Tl(I), was achieved via donation of its 6s2 electron-pair to the Pd octahedron augmented with three wingtip Pd3 atoms and a significant back-donation from the Pd entities onto the empty Tl(I) 6p valence orbitals. The molecular structures of Pd9[m3/3-Tl(acac)](CO)9(PPh3)6 and {[Pd9(CO)9 (PMe3)6]Tl[Pd3(CO)3(PMe3)3]}+ are illustrated in Fig. 74.

8.04.6.3

Heterometallic heteroleptic palladium carbonyl clusters

In 2007 Dahl and co-workers reported the synthesis and characterization of the Pd-Au nanosized Au4Pd28(CO)22(PMe3)16 cluster,206 continuing his exploration of new synthetic pathways involving the use of new precursors to ensure the increase of cluster nuclearity. This similar strategy had previously produced the large Au4Pd32(CO)28(PMe3)14, which originated by the reaction between Pd10(CO)12(PMe3)6 and Au(SMe2)Cl (for the synthesis of the latest species, Au(PPh3)Cl can be also employed).207 Before that, the Au2Pd21(CO)20(PEt3)10208 and Au2Pd41(CO)27(PEt3)15209 compounds had been obtained by reduction of the monometallic Pd(PEt3)2Cl2/Au(PPh3)Cl precursors under CO in NaOH-containing DMF solutions. Notably, this same reaction had also produced for the first time the above-mentioned Pd145 cluster, albeit in very low yield. Prior to those works, the largest cluster was represented by [Au2Pd14(CO)9(PMe3)11]2+, prepared by reacting Pd8(CO)8(PMe3)7 with an Au(I) complex.210 The molecular structure of the Au4Pd28(CO)22(PMe3)16 neutral compound is depicted in Fig. 75, and shows a metal cage composed of an Au4 tetrahedron encapsulated in a complex Au4Pd24 architecture of interpenetrating, highly deformed Au-centered three-layer (Pd3)A(Au(n)Pd6)B(Au3)C cuboctahedra (n ¼ 1–4), capped by the remaining four Pd atoms. In the same year appeared the report by Dahl and co-workers of the largest heterometallic palladium carbonyl species ever isolated and characterized to date, as well as the largest crystallographic example of a discrete transition metal cluster with direct metal-metal bonding: (m12-Pt)Pd164−xPtx(CO)72(PPh3)20 (x  7).211 They had put many efforts in preparing other multi-shell homopalladium CO/PR3-ligated clusters after the isolation of Pd145(CO)x(PEt3)30. Because PR3 ligands with bulky R substituents were generally found to give rise to clusters with smaller Pdn core-geometries, their attempts had primarily involved reactions of palladium precursors with smaller PEt3, PMe3, PMe2Ph3 ligands. However, in the end the large cluster has been synthesized by reacting Pd10(CO)12(PPh3)6 with Pt(CO)2(PPh3)2, as it turned out that the large PPh3 protects the Pd-Pt core-geometry.

Fig. 74 Metal structure of Pd9[m3/3-Tl(acac)](CO)9(PPh3)6 (left) and {[Pd9(CO)9(PMe3)6]Tl[Pd3(CO)3(PMe3)3]}+ (right) (Pd atoms in orange; Tl atom in violet; P atoms in green; C in gray; O in red). The organic ligands bonded to the phosphorous atoms have been omitted for sake of clarity.

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251

Fig. 75 Metal structure of Au4Pd28(CO)22(PMe3)16 (Pd atoms in orange; Au atoms in yellow).

Fig. 76 Four-shell anatomy of 165-atom Pt-centered PtxPd165−x core (x  8) in (m12-Pt)Pd164−xPtx(CO)72(PPh3)20 (x  7). (A) Shell 1: the central Pt atom enclosed in a PdxPt12−x (x  1.2) icosahedron. (B) Shell 2: PtxPt42−x (x  3.5) n2 tetrahedral cage. (C) Shell 3: 60-atom semi-regular icosahedral PtxPd60−x cage (x  2.2) called rhombicosidodecahedron. (D) Shell 4: n2 pentagonal dodecahedral Pd50 cage. (E) Shell 3 inside Shell 4. Reproduced with permission from: Mednikov, E. G.; Jewell, M. C.; Dahl, L. F. J. Am. Chem. Soc. 2007, 129, 11619–11630.

The metal structure of (m12-Pt)Pd164−xPtx(CO)72(PPh3)20, reported in Fig. 76, is a beautiful example of a multi-shell architecture. Notably, the outer metal shell is all composed of Pd atoms, and the metal structure is stabilized by PPh3 and edge-bridging CO ligands. For a much more detailed analysis of the cluster structure the Authors refer to the literature article. The (m12-Pt) Pd164−xPtx(CO)72(PPh3)20 nanocluster was also characterized by both 1H and 31P NMR in CD2Cl2 and solid-state IR spectroscopy. In 2013, Huynh, Leong and co-workers reported a research work that resulted in the isolation of Os carbonyl clusters decorated by Pd-NHC (N-heterocyclic carbene) ligands (NHC ¼ SIPr [N,N0 -bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene] or IPr [N,N0 -bis(2,6-diisopropylphenyl)imidazolin-2-ylidene], namely [Os3(CO)12(PdNHC)n] (n ¼ 1–3).212 In Fig. 77 is illustrated the molecular structure of the Os3Pd3 congener, whose metal framework consists of a n2 triangle where the inner one is made by the three Os atoms and where the Pd ones bridge the Os-Os edge.

Fig. 77 Molecular structure of [Os3(CO)12(PdNHC)3] (Pd atoms in orange; Os atoms in turquoise; N in blue; C in gray). Hydrogen atoms have been omitted for sake of clarity.

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8.04.7

Group 9 and 10 Carbonyl Clusters

Platinum

This section gathers the findings on platinum carbonyl clusters over the last fifteen years, both homometallic and heterometallic, whenever Pt is the predominant species in terms of atomic count. Within this chemistry, the late P. Chini (University of Milan, Italy) has been an outstanding pioneer and the so-called Chini clusters, which have the general formula [Pt3n(CO)6n]2− (n ¼ 1–10), represent a milestone in inorganic and cluster chemistry. Later, his inheritance was taken by G. Longoni and, subsequently, by his co-workers both in Bologna and in Milan, as inferable from the literature references. Nevertheless, the Pt carbonyl clusters are also attracting several other research groups throughout the world, including L. F. Dahl, also due to the high catalytic activity of Pt.213

8.04.7.1

Chini clusters

The [Pt3n(CO)6n]2− oligomers now called Chini clusters, which replaced the previous “platinum carbonyls” denomination, had been reported many years ago.214,215 However, up until 2006, only those with n ¼ 1–5 had been properly characterized.216 The great interest they gathered over the last 15 years allowed to unravel much more of their chemistry and their chemical and physical properties. In 2006 a reinvestigation of the synthetic path allowed to isolate and characterize the new oligomer with n ¼ 8, more specifically [Pt24(CO)48]2− in its [NBu4]+ salt, which was selectively obtained by the oxidation of [Pt3n(CO)6n]2− precursor in THF by SbCl3.217 Its molecular structure does not represent an exception to the metal architecture of all Chini clusters, as it too consists of a pile of slightly tilted triangular Pt3(CO)6 units (Fig. 78). The clockwise/anticlockwise twist is probably necessary to release the steric hindrance of the carbonyl ligands between adjacent triangular layers, and accounts for the quite long distances among the {Pt3(CO)6} planes, especially between the top one from the {Pt21(CO)42} group. However, and this is the fascinating aspect of the molecular structure, the long distance is repeated both below and above the single triangular fragment, therefore the whole crystal packing is an infinite self-assembly of {Pt21(CO)42} moieties and {Pt3(CO)6} units. The counterion was, in fact, the only element that allowed to discriminate the molecular formula and define the value of n ¼ 8. Such an infinite stack had been predicted by Hoffmann over twenty years before.218 In continuing the investigation of Chini clusters oxidation to higher-nuclearity species, the following year the same group reported that [Pt9(CO)18]2− with tropylium tetrafluoroborate self-assembles into infinite semicontinuous or continuous {[Pt3n(CO)6n]2−}1 conductor wires upon crystallization.219 Basically, depending on the nuclearity of the cluster, the packing arrangement significantly changes. For instance, the [NMe4]2[Pt12(CO)24] salt exhibits discrete [Pt12(CO)24]2− cluster units separated by the counterions but aligned on top of each other. Notably, with respect to the previous structural determination,220 the Pt3(CO)6 units are perfectly ordered. The [NEt4]2[Pt15(CO)30] salt exhibits incipient columnar self-assembly of [Pt15(CO)30]2− units, separated from each other by a distance of ca. 3.45–3.54 A˚ (intramolecular Pt3(CO)6 distances are in the range of 3.06–3.15 A˚ ). When moving to [NMe4]2[Pt18(CO)36]
2Me2CO, this intermolecular distance disappears and infinite pillars made of [Pt18(CO)36]2− portions self assemble in the solid state in a one-dimensional fashion (see Fig. 79). Basically, the single di-anionic cluster is no longer distinguishable in the crystal packing.

Fig. 78 Ball-and-stick (A) and space-filling (B) models of the molecular structure of the [Pt24(CO)48]2− ion. The interplane distances between the {Pt3(CO)6} units are indicated in A˚ (Pt atoms in green, C in gray, O in red). Reproduced with permission from: Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Mehlstäubl, M.; Zacchini, S.; Ceriotti, A. Angew. Chem. Int. Ed. 2006, 45, 2060–2062.

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Fig. 79 The packing of the [NMe4]2[Pt18(CO)36]
2Me2CO salt: (left) one-dimensional packing of infinite columnar stacks of [Pt18(CO)36]2− dianions, (right) a portion of the stack that corresponds to [Pt18(CO)36]2−. Reproduced with permission from: Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Eur. J. Inorg. Chem. 2007, 1483–1486.

The same infinite self-assembly, this time in a bi-dimensional fashion, is also observed for [Pt24(CO)48]2− but in its [NEt4]+ salt (in the earlier-mentioned work the counterion was [NBu4]+). This denotes the high importance of the counterion in the solid state. Notably, resistivity measurements carried out under a nitrogen atmosphere and at room temperature with a four-point probe on pressed pellets (ca. 13  1 mm) of powdered samples pointed out the difference between the isolated [Pt3n(CO)6n]2− clusters (n ¼ 4, 5), which showed values greater than 108 O cm, and those creating infinite stacks (n ¼ 6, 8), which exhibited resistivity in the 105–102 O cm range. In the [Pt3n(CO)6n]2− Chini cluster series mentioned above, those with n values from 2 to 8 had been structurally characterized with the exception of n ¼ 7. Nonetheless, its existence, as well as that of larger oligomers with n values up to 10, have been detected during ESI-MS analysis in acetone solutions of the [Pt18(CO)36]2− species, as displayed in Fig. 80.221 Notably, oligomers with n ¼ 2–4 gave simpler spectra, pointing out their greater stability comparing to those with a greater value of n.

Fig. 80 ESI-MS spectrum of [NBu4]2[Pt18(CO)36] in acetone solution. Reproduced with permission from: Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. J. Clust. Sci. 2014, 25, 115–146.

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Reviews on Chini clusters have been published recently and can serve as a complementary source of information.221–223 In 2010, the use of [Ru(tpy)2]2+ (tpy ¼ 2,20 :60 ,200 -terpyridine) allowed to crystallize and therefore structurally characterize the [Pt27(CO)42]2− species.224 This was, actually, a fallout out of a comprehensive survey on the solid-state assembly of the [Pt3n(CO)6n]2− clusters (n ¼ 4–8) focused on its dependence on the nature of the counterions. The results pointed out that all species with n  4 do not form cluster chains, most likely due to electrostatic repulsions between these small and negatively charged anions. However, as the cluster nuclearity increases (n > 5), being the negative charge always the same but delocalized over a larger species, those repulsions decrease and the cluster anions can approach each other, giving rise to infinite discontinuous, semicontinuous or continuous chains. A summary for [Pt3n(CO)6n]2− with n ¼ 4–7 is reported in Fig. 81.

Fig. 81 Crystal packing of (A) [Ru(tpy)2][Pt12(CO)24] 4Me2CO (view along the a axis); (B) [Ni(macro)][Pt12(CO)24] 2 Me2CO (view along the a axis), (C) [Ru(tpy)2] [Pt15(CO)30] 3DMF (view along the a axis); (D) [Ru(bpy)3][Pt15(CO)30] 3 Me2CO (view along the c axis), (E) [Ru(bpy)3][Pt18(CO)36] (view along the a axis); (F) [Ru(bpy)2(2-PTZ)]2[Pt18(CO)36] (view along the b axis), and (G) [Ru(tpy)2][Pt21(CO)42] 2DMF (view along the main axis of the hexagonal channels). (tpy ¼ 2,20 :60 ,200 -terpyridine; bpy ¼ 2,20 -bipyridyl; macro ¼ 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraaza-4,11-cyclotetradecadiene; 2-PTZ ¼ 5-(2-piridyl)tetrazolate). Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Lovato, T.; Stagni, S.; Zacchini, S. Inorg. Chem. 2010, 49, 5992–6004.

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DLS measurements performed on the above-mentioned Chini clusters suggested that some aggregation already occurs in solution, as a function of the cluster nuclearity, concentration, solvent, and temperature. Quite interestingly, Remita et al. the following year reported photophysical studies of platinum carbonyl clusters, more specifically on Chini clusters with n ¼ 3 and 4, showing that when deposited on glass they self-assemble into long nanowires or spherical aggregates.225 Moreover, these aggregates exhibit fluorescence properties analogous to those of the individual clusters, that in turn possess double emitting properties in the visible and near-infrared spectral region for single excitation wavelength, behaving as quantum dots. TEM images of the self-assembly of [Pt12(CO)24]2− also showing its luminescence properties are reported in Fig. 82, while UV-visible absorption spectra for [Pt9(CO)18]2− and [Pt12(CO)24]2− are shown in Fig. 83. In 2007, Ugo and co-workers published a complete different approach to synthesize the Chini clusters. Through reductive carbonylation of silica-supported Na2[PtCl6], K2[PtCl4], [Pt(CH3CN)2Cl2] or [Pt(COD)Cl2] they were able to isolate the [Pt3n(CO)6n]2− (n ¼ 6, 5, 4, 3) clusters.226 Remarkably, the silica surface plays a key role in these reductive carbonylation reactions as no carbonyl cluster is obtained by reductive carbonylation of solid Na2[PtCl6] in the absence of silica. The selectivity can be easily tuned by controlling the surface metal loading, the basicity of the surface, and the nature of the platinum precursor. Like all transition metal carbonyl clusters, Chini clusters can be used as precursors to prepare catalysts for several catalytic applications. In 2011 Inoue et al. prepared various [Pt3n(CO)6n]2− compounds (n ¼ 3–8), by bubbling CO through [PtCl6]2− solutions with different solvents and applied them to the preparation of Pt-nanoparticle-loaded carbon black (Pt/CB). The mean size of the resultant Pt nanoparticles increased with n, whereas their size distribution remained narrow (0.3 nm), irrespective of n. The resulting nanoparticles were successfully used in oxygen reduction reaction.227

8.04.7.2

Other homometallic platinum carbonyl clusters

Beside the Chini clusters there are several other Pt carbonyl compounds which possess more compact structures, namely [Pt19(CO)22]4−,228 [Pt24(CO)30]2−,12,75 [Pt26(CO)32]2−,12,75 and [Pt38(CO)44]2−.229 Notably, they do possess multivalence behavior, established from previously reported analyzes.12 On this matter, a very interesting electrochemical and spectroelectrochemical study was reported by Ceriotti et al. in 2008, with a revisitation, and in some cases a completion, of the redox activity of [Pt19(CO)22]4−, [Pt24(CO)30]2−, and [Pt38(CO)44]2−. Moreover, the EPR investigation on the electro-generated monoanion [Pt24(CO)30]− represents the first successful study on the paramagnetism of homoleptic platinum carbonyl clusters (Fig. 84).230

Fig. 82 Images of the one-dimensional assembly of [Pt12(CO)24]2−: (A) transmission mode image; (B–C) false-color luminescence images: (B) excitation around 440 nm and emission around 480 nm, (C) excitation around 500 nm and emission above 530 nm; (D) TEM images of the linear aggregates in low magnification. Inset: high magnification of one such linear aggregate. Reproduced with permission from: Selvakannan, P. R.; Lampre, I.; Erard, M.; Remita, H. J. Phys. Chem. C 2008, 112, 18722–18726.

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1.60 1.35

Absorbance

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[Pt3(CO)6]42–

1.10 0.85 3

2

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Wavelength (nm) Fig. 83 UV-visible absorption spectra of the alkaline methanol solutions of platinum carbonyl clusters [Pt3n(CO)6n]2− (n ¼ 3 and 4) synthesized from 2.5  10−3 M H2PtCl6 (spectrum 1, red curve, path length 1 mm) and 10−3 M H2PtCl6 solutions (spectrum 2, green curve, path length 2 mm) and from 2.5  10−3 M Pt(acac)2 (spectrum 3, blue curve, path length 2 mm), dose rate ¼ 2.3 kGy/h, dose ¼ 4600 Gy. Inset: the structures of Chini clusters and photographs of vials containing the clusters [Pt3n(CO)6n]2− (n ¼ 3 and 4) in alkaline methanol. Reproduced with permission from: Selvakannan, P. R.; Lampre, I.; Erard, M.; Remita, H. J. Phys. Chem. C 2008, 112, 18722–18726.

Fig. 84 X-band EPR spectrum of the monoanion [Pt24(CO)30]− electrogenerated in CH2Cl2 solution. Experimental (A) and simulated (B) lineshapes. T ¼ 100 K; n ¼ 9.448 GHz. Reproduced with permission from: Fedi, S.; Zanello, P.; Laschi, F.; Ceriotti, A.; El Afefey, S. J. Solid State Electrochem. 2009, 13, 1497–1504.

A few more structurally compact species have been obtained over the last few years. The first was isolated and characterized in 2011, over the investigation of the reduction of the Chini clusters. It was known that the interconversion of the [Pt3n(CO)6n]2− (n ¼ 3–5) can be promoted by the presence/absence of molecular hydrogen according with the following equilibrium: h i2−
2− ðn − 1Þ Pt3n ðCOÞ6n + H2 ⇆n Pt3ðn − 1Þ ðCOÞ6ðn − 1Þ + 2H + However, it appears that this is no true for [Pt6(CO)12]2− (where n ¼ 2).215

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In 2011, Heaton, Garland, Longoni and co-workers published a study using in situ FTIR in combination with band-target entropy minimization (BTEM) to elucidate the mechanism of the [Pt3n(CO)6n]2− interconversion.231 The above equilibrium was confirmed, but under H2 pressure it was possible to observe the formation of [Pt3n(CO)6n]2− (n ¼ 2), which gets quickly oxidized in the absence of hydrogen, albeit only in acetonitrile solution and not in THF. This phenomenon can be explained by the following equilibrium involving the solvent, with the formation of protonated acetonitrile cluster cations, [(CH3CN)mH]+ (m ¼ 1–3): h i2−
+
2− + H2 + 2mCH3 CN ⇆ n Pt3ðn − 1Þ ðCOÞ6ðn − 1Þ + 2 ðCH3 CNÞm H ðn − 1Þ Pt3n ðCOÞ6n In addition, evolution of CO2 was estimated by BTEM even if the experiments had been conducted in absence of carbon monoxide atmosphere, with a concurrent formation of IR peaks of an unknown species. Thanks to these results, the reduction of [Pt3n(CO)6n]2− (n ¼ 3) with OH− in THF was reinvestigated, and that species was characterized as the new [Pt15(CO)19]4−. Its molecular structure, depicted in Fig. 85, can be derived from a centered bicapped pentagonal prismatic [Pt13(CO)17] unit, corresponding to the building block of the known interpenetrated double bi-capped pentagonal prismatic [Pt19(CO)22]4− cluster; the structure is then completed by the addition of two Pt(CO) moieties on opposite sides of the prism. Other new very large homometallic and homoleptic cluster species were reported in 2016, namely [Pt33(CO)38]2− and [Pt40(CO)40]6−. The former, in analogy with the above-mentioned [Pt15(CO)19]4− species, has been obtained from the [Pt15(CO)30] Chini cluster, this time by thermolysis. The latter, on the contrary, has been prepared by reaction of [Pt19(CO)22]4− with acids.232 Their molecular structures, characterized via single-crystal Xray diffraction, are illustrated in Fig. 86. The [Pt33(CO)38]2− cluster shows a metal skeleton that is a chunk of a ccp metal lattice, but that can also be seen a defective derivation of the [Pt38(CO)44]2− species, whose metal frame is a truncated octahedron. The [Pt40(CO)40]6− cluster, on the other hand, exhibits a rare bcc packing metal arrangement.

Fig. 85 Molecular structure of [Pt15(CO)19]4− (Pt atoms in magenta; C in gray; O in red).

Fig. 86 Molecular structure of [Pt33(CO)38]2− (left) and [Pt40(CO)40]6− (right). (Pt atoms in magenta; C in gray; O in red).

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Fig. 87 Selected infrared spectra of [Pt40(CO)40]n as a function of the cluster charge n, and of the potential E, in MeCN containing 0.1 M [NBu4][PF6]. The absorptions of the solvent and the supporting electrolyte were subtracted. Reproduced with permission from: Cattabriga, E.; Ciabatti, I.; Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Zacchini, S. Inorg. Chem. 2016, 55, 6068− 6079.

Their redox activity was tested via electrochemical and spectroelectrochemical experiments and for both it turned out to be very rich. In fact, it was possible to identify nine different IR spectra for [Pt33(CO)38]n−, with n going from 0 to 9, and eight IR spectra for [Pt40(CO)40]n− (n ¼ 4–11), shown in Fig. 87. Two years later, further investigation of thermal treatment of the [Pt3n(CO)6n]2− Chini clusters by the same group led to the isolation of the new [Pt14+x(CO)18+x]4− (x ¼ 0, 1), which is actually a mixture of the previously reported [Pt15(CO)19]4− (18%) and the new [Pt14(CO)18]4− (82%), and [Pt44(CO)45]n−, as well as the previously reported [Pt15(CO)19]4−, [Pt19(CO)22]4−, [Pt24(CO)30]2−, [Pt26(CO)32]2−, [Pt33(CO)38]2− and [Pt38(CO)44]2−. Owing to the poor quality of the obtained crystals, however, the full structure of [Pt44(CO)45]n− could not be elucidated.233 Chemical reactivity tests on the obtained species allowed to isolate the monoanionic [Pt26(CO)32]− and the new [Pt36(CO)44]2− and [Pt23(CO)27]2− dianion, which had only been partially structurally characterized. For a more detailed description of the metal arrangements in the above-mentioned clusters authors advise to refer to the main paper. In 2020 an interesting research has been published by Pontiroli et al., where [Pt19(CO)22]4− served as precursor for the decoration of defective graphene layers with single Pt atoms, with the aim of use it in catalytic reactions. The choice of this cluster was driven by its high solubility in CH3CN and thermal stability. The resulted materials were investigated by transmission electron microscopy and X-ray photoemission spectroscopy (XPS). Unexpectedly, the process of aggregation of Pt into larger clusters was not only inhibited, but fragmentation even into single metal atoms was observed onto the defective-graphene surface.234 Fig. 88 shows a detailed XPS of the thermally exfoliated graphite oxide (TEGO), with and without Pt.

8.04.7.3

Homometallic platinum carbonyl clusters containing post-transition metals

Over the last fifteen years the only high-nuclearity Pt clusters containing post-transition elements are those decorated by various Sn-bearing ligands. In 2011 it was reported that the reaction of [Pt15(CO)30]2− Chini cluster with SnCl2, in mixture of ROH/acetone (R ¼ Me, Et, iPr) and in presence of Na2CO3 caused a breakage of the precursor and led to the new trigonal bi-pyramidal [Pt5(CO)5{Cl2Sn(m-OR) SnCl2}3]3− species.235 Such geometry, rare for group 10 metals, is supported by an interpenetration with a non-bonded trigonal prism of tin atoms.

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Fig. 88 C1s (A), O1s (B), and Pt 4f (C) lineshape analysis of as-prepared TEGO, Pt-TEGO, Pt-TEGO after thermal treatment at 573 K, and [TBA]4[Pt19(CO)22] cluster. The background has been subtracted and the intensity normalized for each spectrum. Reproduced with permission from: Gaboardi, M.; Tatti, R.; Bertoni, G.; Magnani, G.; Della Pergola, R.; Aversa, L.; Verucchi, R.; Pontiroli, D.; Riccò, M. Surf. Sci. 2020, 691, 121499.

A few years later, a similar investigation with variable amounts of SnCl2 allowed to isolate a series of compounds variously decorated by miscellaneous Sn(II) ligands (SnCl2, [SnCl3]−, [Cl2Sn(OR)SnCl2]−, and [Cl2SnOCOSnCl2]2−), specifically the already reported [Pt5(CO)5{Cl2Sn(OR)SnCl2}3]3− (R ¼ H, Me, Et, and iPr) and [Pt8(CO)10(SnCl2)4]2−,236 and the new [Pt10(CO)14 {Cl2Sn(OH)SnCl2}2]2−, [Pt6(CO)6(SnCl2)2(SnCl3)4]4− and [Pt9(CO)8(SnCl2)3(SnCl3)2(Cl2SnOCOSnCl2)]4−.237 From a structural point of view, these species show a neat segregation of the two metals, as they are composed of a low-valent Pt core surfacedecorated by Sn(II) fragments behaving as two-electron donor ligands. Among them, [Cl2SnOCOSnCl2]2− is rather unique. All clusters have also been characterized by IR spectroscopy, whereas their bonding features have been studied by theoretical DFT calculations. The molecular structure of [Pt10(CO)14{Cl2Sn(OH)SnCl2}2]2− is reported in Fig. 89.

Fig. 89 Molecular structure of [Pt10(CO)14{Cl2Sn(OH)SnCl2}2]2− (Pt atoms in magenta; Sn atoms in turquoise; Cl atoms in blue-gray; C in gray; O in red).

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298 K

243 K

213 K

193 K 270

260

250

240

13

230

220

210

200

190

180

170

2− 13

Fig. 90 Variable-temperature C NMR spectra of [Pt6(CO)8(SnCl2)(SnCl3)2(PPh3)2] - CO in CD3COCD3. Reproduced with permission from: Bortoluzzi, M.; Ceriotti, A.; Cesari, C.; Ciabatti, I.; Della Pergola, R.; Femoni, C.; Iapalucci, M. C.; Storione, A.; Zacchini, S. Eur. J. Inorg. Chem. 2016, 3939–3949.

In the same year the reaction on [Pt6(CO)6(SnCl2)2(SnCl3)4]4− with nucleophiles such as CO and PPh3 resulted in the new [Pt6(CO)8(SnCl2)(SnCl3)4]4− and [Pt6(CO)8(SnCl2)(SnCl3)2(PPh3)2]2− species, which were characterized by means of spectroscopic methods (IR, variable-temperature 13C NMR, and 31P NMR), their structures by single-crystal X-ray diffraction, and their bonding was theoretically investigated by DFT calculations.238 Fig. 90 reports the variable-temperature 13C NMR spectra of [Pt6(CO)8(SnCl2)(SnCl3)2(PPh3)2]2−-13CO in CD3COCD3. This analysis was possible after stirring the cluster under an atmosphere of 13CO for a few days, obtaining a ca. 100% 13CO enrichment. The results pointed out that there is a coalescence of the m-CO resonance at 243 K (dC ¼ 227.5 ppm), while the t-CO one (dC ¼ 193.1 ppm) can still be observed. Interestingly, four resonances appear at dC ¼ 192.8 ppm at 193 K, in agreement with the solid-state structure of [Pt6(CO)8(SnCl2)(SnCl3)2(PPh3)2]2−, which contains four different types of CO ligands. The last reported paper on Sn-decorated Pt carbonyl clusters appeared in 2020, where the previously mentioned [Pt6(CO)6 (SnX2)2(SnX3)4]4− (X ¼ Cl, Br) species were used as precursors to prepare new clusters. Indeed, through their reaction with acids it was possible to isolate the new [Pt12(CO)10(SnCl)2(SnCl2)4{Cl2Sn(m-OH)SnCl2}2]2− and [Pt7(CO)6(SnBr2)4{Br2Sn(m-OH)SnBr2} {Br2Sn(m-Br)SnBr2}]2−.239 A full characterization of these clusters has been carried out by IR and variable-temperature 13C NMR spectroscopy, X-ray diffraction, while DFT calculations have been performed to theoretically describe their bonding. The solid state structures of these clusters revealed the presence of hydrogen bonds involving the OH-groups of the {X2Sn(m-OH) SnX2}– ligands. Fig. 91 depicts the solid-state architecture driven by intermolecular H-bonds in [Pt12(CO)10(SnCl)2(SnCl2)4 {Cl2Sn(m-OH)SnCl2}2]2−.

8.04.7.4

Heteroleptic platinum carbonyl clusters

There are many reported examples of heteroleptic platinum clusters, and in the last fifteen years the majority of the new compounds derive from CO substitution of Chini clusters. The exception is represented by a research work reported by Leoni and co-workers in 2008, which exploited the previously known [Pt6(PtBu2)4(CO)6]2+ cluster240 to prepare several differently functionalized hexanuclear platinum derivatives.241 All the following reported heteroleptic species have been synthetized, either directly or indirectly, from Chini clusters. A tetra-nuclear heteroleptic Pt cluster was obtained in 2013 by reacting [Pt12(CO)24]2− with CH2]C(PPh2)2(P∧ P), giving rise to the Pt4(CO)4(P∧P)2 species, from which the mono- and di-hydride derivatives, confirmed via 1H NMR analyzes, could be obtained upon protonation with HBF4.242 In the same year, the reaction of the [Pt3n(CO)6n]2− (n ¼ 2–6) Chini clusters with increasing amounts of PPh3 has been investigated by combining FT-IR, 31P{1H} NMR, and electrospray ionization-mass spectrometry (ESI-MS) studies.243 The results pointed out that up to three CO ligands were gradually substituted by PPh3, resulting in isonuclear phosphine-substituted anionic clusters of general formula [Pt3n(CO)6n−x(PPh3)x]2− (n ¼ 2–6; x ¼ 1–3). Notably, the addition of small amounts of PPh3 to an acetone solution of [Pt12(CO)24]2− showed the gradual and continuous lowering of both terminal and bridging n(CO) stretching frequencies, as displayed in Fig. 92. The CO-substituted [Pt12(CO)22(PPh3)2]2− and [Pt9(CO)16(PPh3)2]2− were structurally characterized, and they maintained the same structure as the parent homoleptic clusters, with the two PPh3 ligands in both cases coordinated to the two external Pt3-triangles. On the contrary, the crystal structure of [Pt6(CO)10(PPh3)2]2− transformed its arrangement from trigonal prismatic to trigonal antiprismatic after CO/PPh3 exchange.

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Fig. 91 H-bonds involving the [Pt12(CO)10(SnCl)2(SnCl2)4{Cl2Sn(m-OH)SnCl2}2]2− anion. Five molecules are represented (Pt atoms in magenta; Sn atoms in orange, Cl atoms in green; H atoms in white; C in gray; O in red. H-bonds are represented as dashed lines). Reproduced with permission from: Berti, B.; Bortoluzzi, M.; Cesari, C.; Femoni, C.; Iapalucci, M. C.; Zacchini, S. Inorg. Chim. Acta 2020, 503, 119432.

Fig. 92 IR spectra in the n(CO) region obtained by the stepwise addition of PPh3 to an acetone solution of [Pt12(CO)24]2−: (1) starting material; (2) +0.5 equiv.; (3) +1.0 equiv.; (4) +1.5 equiv.; (5) + 2.0 equiv.; (6) +3.0 equiv.; (7) +3.5 equiv.; (8) +4.5 equiv.; (9) +5.0 equiv.; (10) +6.5 equiv.; (11) +7.5 equiv. Reproduced with permission from: Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Lovato, T.; Zacchini, S. Inorg. Chem. 2013, 52, 4384−4395.

Analogous reactions between Chini clusters (n ¼ 2–4) with the bulkier Ph2PCH2CH2PPh2 (dppe) ligand resulted in the formation of the correspondent [Pt6(CO)10(dppe)]2−, [Pt9(CO)16(dppe)]2−, and [Pt12(CO)20(dppe)2]2− heteroleptic compounds.244 Notably, the more sterically hindered [Pt6(CO)10(dppe)]2−, due to the smaller cluster size, and [Pt12(CO)20(dppe)2]2−, due to the double substitution, possess a metal structure modified with respect to the original ones. In fact, the former went from a trigonal prism to an antiprism, whereas the latter is composed of two trigonal prismatic [Pt6(CO)10(dppe)] units rotated of 180 and joined by the two PtdPt bonds.

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Fig. 93 31P{1H} NMR spectrum of [Pt9(CO)16(dppe)]2− in CD3CN at 298 K. The main peak of the resonances of the two inequivalent P atoms is indicated with ▪, P (1), and ▲, P (2). Arrows are used for the coupling constants: 1JPtP (1), red; 2JPtP (1), yellow; 1JPtP (2), blue; 2JPtP (2), green. The intensities of some Pt satellites are lower than expected because of second-order effects. Reproduced with permission from: Cesari, C.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Mancini, F.; Zacchini, S. Inorg. Chem. 2017, 56, 1655− 1668.

Compounds were also characterized through NMR analyzes. The 31P{1H} NMR spectrum recorded in CD3CN at 298 K displays two complex resonances, and both show a large 1JPtP coupling (5060 and 4995 Hz for the two resonances, respectively) and two very different 2JPtP coupling constants. The larger 2JPtP values correspond to coupling to the two Pt atoms to which the P atom is directly bonded, whereas the smaller 2JPtP ones correspond to inter-triangle coupling, due to the bidentate nature of the dppe ligand. The 31P{1H} NMR spectrum of [Pt9(CO)16(dppe)]2− is reported in Fig. 93. The possibility to substitute some of the carbonyl ligands in Chini clusters is quite a powerful tool as it allows their functionalization and the modifications of some of their physical properties. For instance, by using the PTA ligand (PTA ¼ 1,3,5-triaza-7phosphaadamantane) it is possible to impart to [Pt3n(CO)5n(PTA)n]2− (n ¼ 2–5). solubility in water.245 All compounds have been characterized by spectroscopic techniques (IR, 31P{1H} NMR, ESI-MS), and in the case of Pt12(CO)20(PTA)4]2− and [Pt15(CO)25 (PTA)5]2− also by crystallographic analysis via X-ray diffraction. Thanks to their acquired solubility in water, their cytotoxicity towards human ovarian (A2780) cancer cells and their cisplatin-resistant strain (A2780cisR) was evaluated. The results allowed to conclude that, compared to cisplatin, both CO-substituted Chini clusters are considerably less cytotoxic to the A2780 cell line, whereas they both are slightly more cytotoxic to A2780cisR cells. The molecular structure of [Pt15(CO)25(PTA)5]2− is illustrated in Fig. 94.

Fig. 94 Molecular structure of [Pt15(CO)25(PTA)5]2− (Pt atoms in magenta; P atoms in green; N atoms in blue; C in gray; O in red; H in white).

Group 9 and 10 Carbonyl Clusters

263

Several other ligands have been tested in CO-substitution reactions with Chini clusters. The latest work in 2018 reports the reaction of [Pt6(CO)12]2− with CH(PPh2)2 (dppm), CH2]C(PPh2)2 (P^P), and Fe(C5H4PPh2)2 (dppf ), which gave rise to di-carbonyl-substituted [Pt6(CO)10(dppm)]2−, [Pt6(CO)10(P^P)]2−, and [Pt6(CO)10(dppf )]2− derivatives. Notably, when [Pt6(CO)12]2− was reacted with Ph2P(CH2)4PPh2 (dppb) and Ph2PC^CPPh2 (dppa), redox-fragmentation products originated instead, namely Pt(dppb)2, Pt2(CO)2(dppa)3, and Pt8(CO)6(PPh2)2(C^CPPh2)2(dppa)2, most likely because of the steric properties of dppb (very long backbone) and dppa (perfectly linear and rigid backbone). Finally, oxidative oligomerization of [Pt6(CO)10(dppm)]2− by short contact with air was observed, resulting in the larger heteroleptic Chini-type clusters [Pt12(CO)20(dppm)2]2−, [Pt18(CO)30(dppm)3]2−, and [Pt24(CO)40(dppm)4]2−.246

8.04.7.5

Heterometallic platinum carbonyl clusters

During the investigation of bimetallic Au-Pt clusters, whose interest lays not only within the base chemistry but also in the catalytic activity of both metals, in 2006 Dahl et al. published the obtainment of several new heteroleptic species, like the bimetallic [Pt3(AuPPh3)5(CO)4PPh3]+ and [(m6-Au){Pt3(CO)3(PMe3)4}2]+ compounds,247 and the trimetallic Pt3(Pt1−xNix)(AuPPh3)2(CO)5 (PPh3)3 and Pt2(Pt2−yNiy)(AuPPh3)2(CO)6(PPh3)2 species.248 They were obtained by reacting a mixture of solid Pt(COD)Cl2, Au(I) complex and PPh3 in DMSO with [Ni6(CO)12]2−, the same synthetic strategy that earlier on allowed to isolate the higher nuclearity Pt7(AuPPh3)2(CO)8(PPh3)4249 and Pt13[Au2(PPh3)2]2(CO)10(PPh3)4.250 Within the chemistry of bimetallic platinum carbonyl species, the physical properties imparted by the viologen cations EtV (EtV ¼ 1,10 -diethyl-4,40 -bipyridilium cation) prompted an investigation of already known Fe-Au and Fe-Pt clusters.251 Notably, [EtV]2[Fe4Pt(CO)16] and [EtV][Fe3Pt3(CO)15]
THF derivatives display resistivities three and four orders of magnitude less than those of their corresponding ammonium salts, respectively, putting them in the category of semiconductor materials. EPR analysis on solutions of [EtV][Fe3Pt3(CO)15]
THF displayed quite intense signals due to the [Fe3Pt3(CO)15]%− and [EtV]%− in a  1;1 ratio, in agreement with the occurrence of the following equilibrium:
2 −
− − Fe3 Pt3 ðCOÞ15 + ½EtVŠ2 + ⇆ Fe3 Pt3 ðCOÞ15 ˙ + ½EtVŠ˙ The analysis repeated in the solid-state gave a much broader signal, which is consisted with a retro-shift of the above equilibrium with respect to the solution. The EPR spectra of [EtV][Fe3Pt3(CO)15]
THF registered in solution and on crystalline sample are reported in Fig. 95. New Pt-Ag carbonyl clusters were reported in 2017, originated from the reaction of [Pt3n(CO)6n]2− Chini clusters (n ¼ 2–4) with Ag(IPr)Cl, namely [Pt6(CO)12(AgIPr)2] and [Pt9(CO)18(AgIPr)2], de facto representing Lewis acid-base adducts. The structure of the latter is illustrated in Fig. 96. By thermal treatment, some of these gave rise to homometallic species bearing carbene ligands, among which [Pt3(CO)4(IPr)2] and [Pt4(CO)4(IPr)3].252 Notably, the reaction of the same Chini clusters with Ag(IMes)Cl [IMes ¼ C3N2H2(C6H2Me3)2] took a different path, as the only isolated product was [Pt3(CO)3(IMes)3(AgCl)], therefore suggesting the absence or high instability of Lewis acid-base adducts with such ligand. Moreover, under CO atmosphere, [Pt3(CO)3 (IMes)3(AgCl)] slowly led to the pentanuclear [Pt5(CO)7(IMes)3]. Other Lewis acid-base adducts were prepared by reacting Chini clusters with Cd(II) halides. In 2006 the reaction of 253 While this product [NBu4]2[Pt9(CO)18] with the soft Lewis acid CdCl2 resulted in the formation of [Pt9(CO)18(m3-CdCl2)2]2− 2 . may have been predicted, due to the electron-donor ability of [Pt9(CO)18]2−, the unexpected and remarkable feature is that, via formation of chloride bridges involving the Cd atoms, in the solid state the adduct forms infinite {[Pt9(CO)18(m3-CdCl2)2]2−}1 chains (Fig. 97). Reaction of [Pt12(CO)24]2− with CdBr2 at high temperature under nitrogen, conversely, took a complete different route and resulted in the formation of the new [Pt13(CO)12{Cd5(m-Br)5Br2(DMF)3}2]2− and [Pt19(CO)17{Cd5(m-Br)5Br3(Me2CO)2} {Cd5(m-Br)5Br(Me2CO)4}]2− heterometallic species decorated by [Cd5(m-Br)5Br5− x(solvent)x]x+ fragments.254 The metal structure of [Pt13(CO)12{Cd5(m-Br)5Br2(DMF)3}2]2− consists of a Pt-centered Pt13(CO)12 icosahedron sandwiched between two {Cd5(m-Br)5 Br2(DMF)3} fragments. Notably, the cluster possesses 162 CVE, whereas established electron-counting rules would predict a value

Fig. 95 The EPR spectra of [EtV][Fe3Pt3(CO)15]
THF (A) in acetonitrile and (B) in a solid microcrystalline state. Reproduced with permission from: Femoni, C.; Iapalucci, M. C.; Longoni, G.; Tiozzo, C.; Wolowska, J.; Zacchini, S.; Zazzaroni, E. Chem. Eur. J. 2007, 13, 6544–6554.

264

Group 9 and 10 Carbonyl Clusters

Fig. 96 Molecular structure of [Pt9(CO)18(AgIPr)2] (Pt atoms in magenta; Ag atoms in gray-white; N atoms in blue; C in gray; O in red; H in white).

Fig. 97 Inset of the infinite {[Pt9(CO)18(m3-CdCl2)2]2−}1 chains within the crystal packing of [Pt9(CO)18(m3-CdCl2)2]2− 2 (Pt atoms in magenta; Cd atoms in yellow; Cl atoms in blue; C in gray; O in red).

of 170 CVE for icosahedral species (like those described earlier in this book chapter), behaving like most gold icosahedral metal compounds, e.g., [Au13Cl2(PMe2Ph)10]3+255 and [Au9M4Cl4(PMe2Ph)8]+ (M ¼ Cu, Ag, and Au).256,257 The low electron count of clusters with heavier elements has been explained by Mingos as arising from a predominance of radial over tangential bonding.258 The molecular structure of [Pt19(CO)17{Cd5(m-Br)5Br3(Me2CO)2}{Cd5(m-Br)5Br(Me2CO)4}]2−, reported in Fig. 98, consists of a {Pt19(CO)17} interpenetrated double-icosahedron sandwiched along the idealized C5 axis by the Cd5(m-Br)5Br3(Me2CO)2 and one

Group 9 and 10 Carbonyl Clusters

265

Fig. 98 Molecular structure of [Pt19(CO)17{Cd5(m-Br)5Br3(Me2CO)2}{Cd5(m-Br)5Br(Me2CO)4}]2−. The acetone molecules have been omitted for sake of clarity.

Cd5(m-Br)5Br(Me2CO)4 rings. Notably, this metal framework diverges from that of the already known [Pt19(CO)22]4− homoleptic species,228 which exhibits a pentagonal prismatic-based structure. It is worth mention that 1H and 13C NMR studies indicate that the terminal DMF and Me2CO ligands may be easily exchanged with N-donor molecules, such as pyridines and amines. By continuing the investigation of the reaction between [Pt3n(CO)6n]2− (n ¼ 4, 5) and hydrated CdBr2, further two Pt carbonyl clusters decorated by Cd-bearing fragments were reported a year later, namely [H2Pt26(CO)20(CdBr)12]8− and [H4Pt26(CO)20 (CdBr)12(PtBr)x]6− (x ¼ 0–2), which share the same Pt26Cd12 core but differ in the charge and the presence of further PtBr capping groups.259 The hydride nature of these clusters was evinced through IR-monitored deprotonation/protonation reactions. Even though the absolute number of hydride ligands could not be ascertained for sure, the proposed formulation is in agreement with all the information available, and in keeping with the best knowledge in the field. For those interested in a detailed analysis of their electron counts, author refer to the main paper. The molecular structure of [H2Pt26(CO)20(CdBr)12]8−, reported in Fig. 99, is reminiscent of that of [Pt38(CO)44]2−, as the [Pt26Cd12] skeleton is too a truncated n3-octahedron incapsulating a Pt6 octahedron. The molecular structure of [H4Pt26(CO)20 (CdBr)12(PtBr)x]6− (x ¼ 0–2) is basically the same but with additional PtBr fragment(s) capping opposite faces of the truncated octahedron.

Fig. 99 Molecular structure of [H2Pt26(CO)20(CdBr)12]8− (Pt atoms in magenta; Cd atoms in yellow; Br atoms in blue; C in gray; O in red).

266

Group 9 and 10 Carbonyl Clusters

8.04.8

Conclusion

The outbreak of nanotechnology gave a new life to the chemistry of metal carbonyl clusters, and significantly contributed to its expansion over the last fifteen years, as evidenced by the countless papers appeared in the scientific literature and reported herein. Authors wish that more and more research groups throughout the world will continue to keep this field active and prolific, by discovering new compounds and means to implement their syntheses and applications.

Acknowledgment The financial supports of MIUR (PRIN 2017 “Nemo” 20173L7W8K) and of the University of Bologna are gratefully acknowledged.

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Synth. 1989, 26, 316–319; (b) D’Aniello, M. J.; Carr, C. J.; Zammit, M. G. Inorg. Synth. 1989, 26, 319–323; (c) Calabrese, J. C.; Dahl, L. F.; Chini, P.; Longoni, G.; Martinengo, S. J. Am. Chem. Soc. 1974, 96, 2614–2616; (d) Bengtsson-Kloo, L.; Iapalucci, M. C.; Longoni, G.; Ulvenlund, S. Inorg. Chem. 1998, 37, 4335–4343; (e) Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Mehlstäubl, M.; Zacchini, S. Chem. Commun. 2005, 5769–5771. 217. Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Mehlstäubl, M.; Zacchini, S.; Ceriotti, A. Angew. Chem. Int. Ed. 2006, 45, 2060–2062. 218. Underwood, D. J.; Hoffmann, R.; Tatsumi, K.; Nakamura, A.; Yamamoto, Y. J. Am. Chem. Soc. 1985, 107, 5968–5980. 219. Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Eur. J. Inorg. Chem. 2007, 1483–1486. 220. Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Mehlstäubl, M.; Zacchini, S. Chem. Commun. 2005, 5769–5771. 221. Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. J. Clust. Sci. 2014, 25, 115–146. 222. Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Zacchini, S. La Chimica & L’Industria 2017, 3, 36–40. 223. Berti, B.; Femoni, C.; Iapalucci, M. C.; Ruggieri, S.; Zacchini, S. Eur. J. Inorg. Chem. 2018, 3285–3296. 224. Femoni, C.; Iapalucci, M. C.; Longoni, G.; Lovato, T.; Stagni, S.; Zacchini, S. Inorg. Chem. 2010, 49, 5992–6004. 225. Selvakannan, P. R.; Lampre, I.; Erard, M.; Remita, H. J. Phys. Chem. C 2008, 112, 18722–18726. 226. Dragonetti, C.; Ceriotti, A.; Roberto, D.; Ugo, R. Organometallics 2007, 26, 310–315. 227. Higuchi, E.; Taguchi, A.; Hayashi, K.; Inoue, H. J. Electroanal. Chem. 2011, 663, 84–89. 228. Washecheck, D. M.; Wucherer, E. J.; Dahl, L. F.; Ceriotti, A.; Longoni, G.; Manassero, M.; Sansoni, M.; Chini, P. J. Am. Chem. Soc. 1979, 101, 6110–6112. 229. (a) Lewis, G. J.; Roth, J. D.; Montag, R. A.; Safford, L. K.; Gao, X.; Chang, S.-C.; Dahl, L. F.; Weaver, M. J. J. Am. Chem. Soc. 1990, 112, 2831–2832; (b) Ceriotti, A.; Masciocchi, N.; Macchi, P.; Longoni, G. Angew. Chem. Int. Ed. 1999, 38, 3724–3727. 230. Fedi, S.; Zanello, P.; Laschi, F.; Ceriotti, A.; El Afefey, S. J. Solid State Electrochem. 2009, 13, 1497–1504. 231. Gao, F.; Li, C.; Heaton, B. T.; Zacchini, S.; Zarra, S.; Longoni, G.; Garland, M. Dalton Trans. 2011, 40, 5002–5008. 232. Cattabriga, E.; Ciabatti, I.; Femoni, C.; Funaioli, T.; Iapalucci, M. C.; Zacchini, S. Inorg. Chem. 2016, 55, 6068–6079. 233. Cattabriga, E.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Inorg. Chim. Acta 2018, 470, 238–249. 234. Gaboardi, M.; Tatti, R.; Bertoni, G.; Magnani, G.; Della Pergola, R.; Aversa, L.; Verucchi, R.; Pontiroli, D.; Riccò, M. Surf. Sci. 2020, 691, 121499. 235. Ceriotti, A.; Daghetta, M.; El Afefey, S.; Ienco, A.; Longoni, G.; Manca, G.; Mealli, C.; Zacchini, S.; Zarra, S. Inorg. Chem. 2011, 50, 12553–12561. 236. Brivio, E.; Ceriotti, A.; Garlaschelli, L.; Manassero, M.; Sansoni, M. J. Chem. Soc. Chem. Commun. 1995, 2055–2056. 237. Bortoluzzi, M.; Ceriotti, A.; Ciabatti, I.; Della Pergola, R.; Femoni, C.; Iapalucci, M. C.; Storione, A.; Zacchini, S. Dalton Trans. 2016, 45, 5001–5013. 238. Bortoluzzi, M.; Ceriotti, A.; Cesari, C.; Ciabatti, I.; Della Pergola, R.; Femoni, C.; Iapalucci, M. C.; Storione, A.; Zacchini, S. Eur. J. Inorg. Chem. 2016, 3939–3949. 239. Berti, B.; Bortoluzzi, M.; Cesari, C.; Femoni, C.; Iapalucci, M. C.; Zacchini, S. Inorg. Chim. Acta 2020, 503, 119432. 240. Leoni, P.; Marchetti, F.; Marchetti, L.; Pasquali, M.; Quaglierini, S. Angew. Chem. Int. Ed. 2001, 40, 3617–3618. 241. Bonaccorsi, C.; Fabrizi de Biani, F.; Leoni, P.; Marchetti, F.; Marchetti, L.; Zanello, P. Chem. A Eur. J. 2008, 14, 847–856. 242. Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Organometallics 2013, 32, 5180–5189. 243. Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Lovato, T.; Zacchini, S. Inorg. Chem. 2013, 52, 4384–4395. 244. Cesari, C.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Mancini, F.; Zacchini, S. Inorg. Chem. 2017, 56, 1655–1668. 245. Batchelor, L. K.; Berti, B.; Cesari, C.; Ciabatti, I.; Dyson, P. J.; Femoni, C.; Iapalucci, M. C.; Mor, M.; Ruggieri, S.; Zacchini, S. Dalton Trans. 2018, 47, 4467–4477. 246. Berti, B.; Cesari, C.; Conte, F.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Vacca, F.; Zacchini, S. Inorg. Chem. 2018, 57, 7578–7590. 247. de Silva, N.; Laufenberg, J. W.; Dahl, L. F. Chem. Commun. 2006, 4437–4439.

270 248. 249. 250. 251. 252. 253. 254. 255. 256. 257. 258. 259.

Group 9 and 10 Carbonyl Clusters de Silva, N.; Nichiporuk, R. V.; Dahl, L. F. Dalton Trans. 2006, 2291–2300. Ivanov, S. A.; de Silva, N.; Kozee, M. A.; Nichiporuk, R. V.; Dahl, L. F. J. Clust. Sci. 2004, 15, 233–261. de Silva, N.; Dahl, L. F. Inorg. Chem. 2005, 44, 9604–9606. Femoni, C.; Iapalucci, M. C.; Longoni, G.; Tiozzo, C.; Wolowska, J.; Zacchini, S.; Zazzaroni, E. Chem. A Eur. J. 2007, 13, 6544–6554. Bortoluzzi, M.; Cesari, C.; Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Zacchini, S. Inorg. Chem. 2017, 56, 6532–6544. Femoni, C.; Kaswalder, F.; Iapalucci, M. C.; Longoni, G.; Zacchini, S. Chem. Commun. 2006, 2135–2137. Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S.; Zarra, S. J. Am. Chem. Soc. 2011, 133, 2406–2409. Briant, C. E.; Theobald, B. R. C.; White, J. W.; Bell, L. K.; Mingos, D. M. P. J. Chem. Soc. Chem. Commun. 1981, 201–202. Copley, R. C. B.; Mingos, D. M. P. J. Chem. Soc. Dalton Trans. 1996, 491–500. Copley, R. C. B.; Mingos, D. M. P. J. Chem. Soc. Dalton Trans. 1992, 1755–1756. Mingos, D. M. P. J. Chem. Soc. Chem. Commun. 1985, 1352–1354. Ciabatti, I.; Femoni, C.; Iapalucci, M. C.; Longoni, G.; Zacchini, S.; Zarra, S. Nanoscale 2012, 4, 4166–4177.

8.05

Nickel-Carbon s-Bonded Complexes

Clifton L Wagner and Tianning Diao, Department of Chemistry, New York University, New York, NY, United States © 2022 Elsevier Ltd. All rights reserved.

8.05.1 8.05.2 8.05.2.1 8.05.2.2 8.05.2.3 8.05.2.4 8.05.2.4.1 8.05.2.4.2 8.05.2.4.3 8.05.2.5 8.05.2.6 8.05.2.7 8.05.3 8.05.3.1 8.05.3.1.1 8.05.3.1.2 8.05.3.2 8.05.3.3 8.05.3.4 8.05.3.5 8.05.3.6 8.05.3.6.1 8.05.3.6.2 8.05.3.6.3 8.05.3.6.4 8.05.3.6.5 8.05.3.7 8.05.3.8 8.05.3.8.1 8.05.3.8.2 8.05.3.8.3 8.05.3.9 8.05.4 8.05.4.1 8.05.4.2 8.05.4.3 8.05.4.3.1 8.05.4.3.2 8.05.4.4 8.05.5 8.05.5.1 8.05.5.2 8.05.5.2.1 8.05.5.2.2 8.05.5.2.3 8.05.6 8.05.6.1 8.05.6.2 8.05.7 8.05.7.1 8.05.7.2 8.05.7.3

Introduction Organonickel(II) complexes stabilized by tridentate ligands (PCP)Ni complexes (PNP)Ni complexes PCN, NCN, PPC, SCS, NNN, and GeCGe pincer nickel complexes Common reactivity Migratory insertion s-Bond metathesis Reductive elimination Nickel alkylidene complexes, ligand redox-activity, and metal-ligand cooperativity Biomimetic (pincer)Ni complexes Selected catalytic reactions Organonickel(II) complexes stabilized by bidentate ligands Schiff base complexes Monoanionic Schiff base complexes Neutral Schiff base complexes Bisphosphine nickel complexes Phosphine-oxo nickel complexes Bipyridyl nickel complexes Synthesis of bidentate nickel(II) complexes Common reactivity Alkyl abstraction and protonation Reductive elimination Insertion b-H elimination Photoexcitation Nickelacycles Catalytic reactivity Olefin polymerization Cross-coupling CO2 conversion Biomimetic bidentate nickel complexes Organonickel(II) complexes stabilized by monodentate ligands (NHC)nickel complexes Phosphine nickel complexes Common reactivity Cycloaddition Oxidative addition Selected catalytic reactions Low-valent nickel complexes Nickel(0) complexes Nickel(I) complexes and representative reactivity Two-electron oxidative addition and reductive elimination One-electron oxidative addition and reductive elimination CO2 insertion High-valent nickel complexes Nickel(III) complexes Nickel(IV) complexes Dinuclear and mixed-valent nickel complexes Cycloaddition catalysts Reductive elimination Olefin polymerization catalysts

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00141-4

272 273 274 274 282 287 287 288 289 290 292 293 295 306 307 307 307 308 308 309 311 311 312 313 314 315 315 317 317 318 320 320 321 324 327 327 327 328 328 328 328 330 330 331 335 335 336 337 338 341 341 341

271

272

Nickel-Carbon s-Bonded Complexes

8.05.7.4 8.05.7.5 8.05.8 References

8.05.1

Biomimetic nickel complexes Nickel alkylidenes Conclusions and outlook

342 344 346 346

Introduction

With a global production value of 2.1 Mt in 2017,1 nickel is amongst the most mined metals in the world primarily for its use in the production of high-grade steel. The majority of the nickel imported by the United States is in the form of alloys such as stainless steel. While the average cost of palladium in 2016 grew to approximately $20 million per metric ton, nickel prices have remained economical at $9600 per metric ton.2 As the demand for more earth abundant transition-metal catalysts increases, it has become increasingly pertinent to understand the chemistry behind organonickel catalysis. Organonickel complexes have garnered increasing attention since the turn of the century as technology has accelerated their unambiguous characterization. Contemporaneously, the implication of their intermediacy in the catalysis of cross-coupling and polymerization reactions as well as the identification of organonickel active sites in metalloenzymes has fueled investigations into the synthesis of a wide variety of organonickel species. As an electron-rich first-row transition metal, nickel has access to up to five different oxidation states and high-spin and low-spin electronic states. The precise electronic behavior of organonickel complexes has spurred research into the characterization and determinants of organonickel complexes. Nickel has been traditionally viewed as a potential surrogate to palladium in organometallic chemistry, but its unrivaled combination of chemical properties has long warranted its distinction as a unique transition metal.3 As in the case of palladium, nickel has access to several even-numbered oxidation states (e.g., Ni(0), Ni(II), Ni(IV)). The higher pairing energy of nickel compared to palladium enables access to stable odd-numbered electron complexes (e.g., Ni(I), Ni(III)). Open-shell nickel complexes help to diversify the electrophile activation pathways by including single-electron radical pathways in addition to the palladium-like two-electron oxidative addition pathway. This metalloradical chemistry is employed in the active sites of metalloenzymes including NiFe-hydrogenase and acetyl-CoA synthase.4 The atomic radius of nickel in comparison to palladium results in an inferior ability to promote b-H elimination. Finally, nickel has a low reduction potential, allowing for activation of traditionally inert substrates via oxidative addition. Accordingly, these properties enable complex mechanisms of Ni-catalyzed cross-coupling reactions through both one- and two-electron pathways mediated by organometallic nickel intermediates. Upon application, these methods augment the current arsenal for the design and synthesis of new products that have proven difficult for palladium catalysts. This chapter documents nickel alkyl and aryl complexes categorized by their oxidation states and the type of supporting ligands, including tetradentate macrocyclic ligands, tridentate ligands such as pincer and tripodal ligands, bidentate ligands and monodentate ligands. Pincer nickel alkyl and aryl complexes have limited applications toward catalysis but have provided fundamental insight into the activation of CO, CO2, H2, and ethylene. Rich catalytic reactivities are observed with bidentate nickel alkyl and aryl complexes in the arena of cross-coupling and olefin polymerization.5 Monodentate nickel alkyl and aryl complexes are dominated by bulky NHC ligands which allow for the isolation of low-coordinate nickel complexes that react with small molecules and serve as precursors for the synthesis of other organometallic nickel complexes.6 They also display important catalytic reactivity in cross-coupling of inert substrates. This chapter is limited to nickel alkyl and aryl complexes, which are bound through an Z1 coordination mode. Nickel-Z3-allyl and Z3-Cp complexes are not within the scope of this chapter. Catalytic reactions that involve isolated and characterized nickel alkyl and aryl complexes are also documented herein. This past decade has witnessed the emergence of prominent studies on radical organonickel complexes. These efforts have led to the isolation of tetravalent, trivalent, monovalent, and zerovalent organonickel complexes. The high-valent derivatives have proven competent models for understanding reductive elimination to form new bonds. Studies on the low-valent derivatives have elucidated the mechanisms of substrate activation. Synthetic routes toward organonickel complexes are demonstrative of their reactivity (Fig. 1). The most common pathway is through transmetalation with an organometallic reagent (M ¼ Li, Mg, Zn, Zr). Alternatively, low-valent nickel species can undergo oxidative addition with a variety of C–E bonds (E ¼ C, H, O, Cl, S, amide, ammonium, pyridinium) to form nickel carbyl complexes. Migratory addition of a Ni species to an unsaturated bond can afford Ni-alkyl molecules. Finally, cycloaddition of unsaturated molecules can afford nickelacycle complexes with nickel alkyl or aryl bonds. [Ni] X

M R M = Li, Mg, Zn, Zr X = F, Cl, Br transmetalation oxidative addition [Ni] R [Ni] R X migratory insertion X = Cl, Br, I, CN, SR', NHR' R” C E [Ni] R’ R” E = C, N, O

Fig. 1 Common synthetic strategies for nickel alkyl and aryl complexes.

X [Ni] cycloaddition

[Ni]

X

Nickel-Carbon s-Bonded Complexes

R

O R

273

[Ni] + R

[Ni] CO

radical ejection

oxidation

carbonylation [Ni] R R = alkyl or aryl reductive elimination

CO2 insertion CO2

O

[Ni]

olefin insertion

R R’ R

O

[Ni] R

Fig. 2 Commonly-observed organometallic reactions with nickel alkyl and aryl complexes.

Organonickel complexes can proceed to various organometallic transformations, based on the nucleophilicity of the carbyl group attached to nickel. Common reactions include the insertion of CO or CO2 to the NidC bond, migratory insertion of olefins or other unsaturated bonds, reductive elimination to form new bonds, and reduction and oxidation reactions. The stability of open-shell electronic states and the accessibility of high-valent oxidation states allows for radical ejection. These fundamental reactions empower nickel as a catalyst in cross-coupling reactions. The reactivity of olefin insertion contributes to its performance as both an olefin-polymerization catalyst and as an alkene functionalization catalyst (Fig. 2).

8.05.2

Organonickel(II) complexes stabilized by tridentate ligands

Pincer ligands have a stronger binding affinity compared to the mono- and bi-dentate ligand classes. Over the previous decade, a variety of methods emerged to diversify the available types of pincer ligands. A common approach to stabilize organometallic complexes is to incorporate aryl or alkyl groups into the backbone of pincer ligand scaffolds. The PCP and POCOP ligands are common for stabilizing nickel alkyl and aryl complexes. By varying the phosphine or phosphine chloride used in their synthesis, the steric bulk about the nickel center can be modified. The aryl derivatives, PC(sp2)P and POC(sp2)OP, can be substituted to moderate the ligand’s electronic properties. The trans influence of these pincer complexes are measured by the shortening of the NidC bond with more electronegative substituents on nickel. The incorporation of p-acceptor groups, such as pyridyl and imino, into these ligand scaffolds introduce redox-activity into their corresponding nickel complexes. The pincer complexes of the PCP, PCN, CCC, and SCS ligands are commonly synthesized through a CdH nickelation reaction with a nickel(II) precursor (Fig. 3). Pincer ligands with aromatic backbones are more efficient at the nickelation reaction than the aliphatic backboned pincer ligands. The identity of the aryl-phosphine linker also determines the effectiveness of the nickelation reaction, where methylene-linked PCP ligands nickelate faster than the POCOP ligands.7 The inability of NCN ligands to undergo the CdH nickelation reaction is attributed to weak NidN bonds, which prevents the consistent chelation required for the CdH activation process.8 Alternatively, the central aryl group can be metalated by lithium-halogen exchange or Grignard formation and followed by transmetalation to a nickel halide. Pincer ligands with a central amido group such as monoanionic PNP and NNN ligands can be deprotonated with a strong base such as nBuLi. The subsequent installation of alkyl groups to these complexes usually proceeds through a transmetalation reaction with an organolithium or Grignard reagent or an olefin insertion reaction into a nickel-hydride bond. Some alkyl substrates, such as acetonitrile, are sufficiently acidic to undergo deprotonation with a pincer nickel amide or alkoxide to form a pincer nickel alkyl complex. Olefin insertion is another method to prepare nickel-alkyl complexes. A monoanionic amido PNP pincer complex 1 of nickel can form nickel alkyls that are stable to b-H elimination (Fig. 4). The monoanionic amido PNP pincer ligand stabilizes a nickel(II) hydride species 1, which is use as a synthon in the hydronickelation of alkenes. The observed divergent regioselectivity depends on the olefin. An aryl-substituted terminal alkene undergoes 2,1 olefin insertion, whereas an alkyl-substituted terminal alkene undergoes 1,2 olefin insertion is used.9

E

NiX 2 base

E

H E Fig. 3 CdH nickelation reaction of ECE-type pincer ligands.

+

Ni E

X

baseH+X -

274

Nickel-Carbon s-Bonded Complexes

iPr P iPr

N C6H6 45 h N

iPr P iPr NiII

P iPr

NiII

P iPr

H

iPr

1 13 31 iPr XRD, H NMR, C NMR, P NMR, EA P iPr

N

1 red

C6H6 18 h

NiII

P iPr

2 red 86 % yield

iPr

iPr

3 red 90% yield XRD, 1H NMR, 13C NMR, 31P NMR, EA

Fig. 4 Regioselective olefin insertion by a pincer nickel alkyl complex.

8.05.2.1

(PCP)Ni complexes

The phosphine-carbyl-phosphine (PCP) ligand scaffold is commonplace in pincer complexes. The carbon binding position of the PCP ligand is typically either an aryl or cycloalkyl moiety. The aryl derivative is noted for the enhanced stability induced upon its metal complexes, while the electron-rich cycloalkyl derivatives have been very reactive, occasionally resulting in decomposition of the intended metal complex. The stability of the PCP ligand framework has allowed for the isolation of a variety of nickel alkyl complexes, a majority of which are methyl derivatives. Unfortunately, the stability induced by the PCP ligand scaffold precludes significant reactivity of these complexes. This is observed in the synthesis of many nickel methyl derivatives of PCP ligands, which are plagued by contamination with nickel halide precursors.10,11 Several of the crystal structures of these compounds contain the nickel halides as co-crystallites. The stability of the PCP ligand framework enables the isolation and characterization of a diverse set of functionalized nickel complexes. The binding of different nickel substituents impacts the 13C NMR signal of the ipso carbon of the pincer ligand as well as the meta and para carbons. While the ipso carbon’s chemical shift strongly indicates the s-basicity, the meta and para carbon chemical shifts are more influenced by the p-basicity of the substituent. These are the manifestation of two organometallic interactions (Fig. 5). The first is the push-pull effect in which a trans substituent adds electron density to the nickel d-orbital which is then pulled into the p orbital of the aryl group of the PCP ligand. The second effect is the contribution of a paramagnetic excited state; the ground state of many low-spin nickel(II) pincer complexes is diamagnetic, however, an MLCT excited triplet state can significantly contribute, resulting in paramagnetic shifting of the 13C NMR signal.12 A large variety of square planar nickel(II) complexes are supported by PC(sp2)P ligands featuring substituents including: halides, alkyls, hydrides, alkoxides, thiolates, pseudohalides, aryls, alkynyls, heteroaryls, alkenyls, and silyls (Table 1). Abstraction of the halide with silver or thallium salts gives cationic nickel complexes with common counterions including borate, triflate, antimonate, and tungstate salts. The vacant coordination sites are typically occupied by organonitriles but can be substituted by many Lewis bases. The PC(sp3)P ligand scaffold similarly forms square planar nickel complexes with halide, pseudohalide, alkyl, aryl, acyl, amido, alkoxy, alkynyl, phenoxy, hydride, and siloxy derivatives (Table 2). The halides derivatives can undergo halogen abstraction reactions with silver and thallium salts to form nickel cations coordinated with Lewis bases. The central carbon is commonly a secondary alkyl group, which upon reaction with a strong base can form the nickel alkylidene complex.98 This carbon has also demonstrated the ability to undergo insertion reactions with saturated substrates such as carbonyls, nitriles and olefins.101,107

8.05.2.2

(PNP)Ni complexes

The PNP pincer ligands contain a central nitrogen atom, which is sometimes incorporated within a pyridine ring, and flanking phosphine groups that also bind to the metal (Table 3).122 They typically behave as either neutral or monoanionic ligands

π*

dπ

pπ

π* σ

C

M

push-pull effect

L

C

M

L

contribution of the paramagnetic excited state

Fig. 5 Electronic effects of metal aryl interactions including the push-pull effect and the contribution of a paramagnetic excited state.

Nickel-Carbon s-Bonded Complexes

Table 1

275

PC(sp2)P pincer nickel complexes. ˚) Ni–C (A

Complex

R'

E P E R'

R'

Ni P

R

R' R F Me H Me Ph Cl OCHO OMe OCHO Br H Br OEt OnBu OiPr OCH2CH2OH (m-H2)BBN (m-H2)BH2 (m-H2)BH2 (m-H2)BH2 SH CH2CN OH S(p-Tol) S(p-CF3-Ph) (m-H2)Bcat S(p-OMe-Ph) SPh SBn SBn (m-H2)BH2 H OCHO I F Cl OCHO Cl I Me Et Me SH SH SH OH S(p-Tol) S(p-Cl-Ph) S(p-CF3-Ph) S(p-OMe-Ph) S(p-Tol) SPh S(p-Cl-Ph) S(p-CF3-Ph)

R0 iPr iPr iPr iPr iPr Ph tBu iPr c-C5H9 Ph iPr iPr iPr iPr iPr iPr c-C5H9 c-C5H9 iPr tBu tBu iPr Ph tBu tBu iPr tBu tBu tBu iPr iPr iPr iPr tBu tBu iPr Cy iPr iPr iPr iPr iPr iPr iPr tBu iPr Ph Ph Ph Ph iPr iPr iPr iPr

E CH2 CH2 CH2 CH2 CH2 CH2 O O O C]O O CH2 CH2 CH2 CH2 CH2 O O O O O CH2 O O O O O O O O NMe NMe NMe O O S CH2 O O O O O CH2 O O CH2 O O O O O O O O

1.894(4); 1.895(3); 1.908(5)12 1.991(2)12 1.907(1); 1.945(1); 1.890(1)12 1.991(3); 1.992(3)12 1.941(2), 1.952(2)12 1.992(3)13 1.886(3)14 1.883(2)15 1.872(3); 1.873(3)15 1.923(3)16 1.894(2); 1.892(2)17 1.91518 1.926(2); 1.927(2)19 1.919(5)19 1.915(6)19 1.918(3)19 1.901(3)20 1.892(2)20 1.900(5); 1.898(4)20 1.901(2)20 1.901(2)21 1.910(2)22 1.898(2)23 1.906(2)24 1.898(2)24 1.880(2)24 1.899(3); 1.899(4); 1.898(4)24 1.900(1)24 1.899(2)24 1.899(2); 1.895(2)24 1.906(2); 1.904(2)25 1.899(3)25 1.894(2); 1.897(2)25 1.892(2)26 1.874(1)26 1.919(1)27 1.933(3)28 1.879(2); 1.975(2)29 1.880(7)29 1.901(2); 1.903(2)29 1.911(2)29 1.994(2)29 1.937(6); 1.932(6)30 1.895(9); 1.898(9)30 1.942(2)30 1.918(2)31 1.907(2)32 1.9088(17)32 1.910(3)32 1.8988(19)32 1.907(2)32 1.8987(19)32 1.900(2)32 1.899(3)32 (Continued )

276

Table 1

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C (A

Complex OSiMe3 OMes OSiPh3 N3 Cl SPh S(p-OMe-Ph) P(O)Ph2 CN NCO Br NO3 OTf C(O)OMe CN C^CPh N3 SCN SCN N3 SCN NH2 OMe H H HCO3 F SCS SCS 3,6-tBu2-1,2-benzoquinone 3,6-tBu2-1,2-benzoquinone Me Z1-allyl HCO2 OC(O)CHCH]CH2 Me OPh OC(O)Ph 2-Me-N-Im OH (COOMe)2CH 2-ox C^CPh N3 OC(O)NH2 HCO3 Br Cl H CF3 F Ph SiHPh2 SiH2Ph Cl 1-Styrene 2-Styrene C^CPh C^CPh S(C2B10H11) Cl Cl Cl

iPr iPr iPr iPr Me Ph Ph Ph iPr iPr Ph Ph Ph Ph Ph Ph tBu tBu iPr Ph Ph iPr iPr tBu Cy iPr tBu tBu iPr Cy iPr tBu tBu tBu tBu tBu iPr iPr tBu tBu tBu tBu tBu tBu tBu tBu tBu tBu tBu iPr iPr tBu iPr iPr iPr c-C5H9 iPr iPr c-C5H9 iPr Ph Ph tBu

O O O O O O O O O O O O O O O O O O O O O CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 O O CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 O O O O O O O O NMe O O O O O O NH NH

1.8762(12)33 1.967(4)33 1.883(2)33 1.891(3)33 1.872(2)34 1.898(2)34 1.909(2)34 1.9193(19)34 1.893(2)35 1.885(2)35 1.874(3)36 1.875(2)36 1.870(2)36 1.866(3)36 1.891(3)36 1.893(2)36 1.884(3)37 1.8787(19)37 1.8854(11)37 1.880(3)37 1.889(2)37 1.931(2)38 1.922(3)38 1.920(2)39 1.9263(14)39 1.909(5)40 1.917(4)41 1.9051(19)42 1.8975(19)42 1.9156(14)43 1.9218(12)43 1.941(3)44 1.981(6)44 1.922(6)44 1.923(4)44 2.026(4)44 1.8818(15)45 1.881(2)45 1.918(11)46 1.939(10)46 1.929(2)46 1.953(2)46 1.942(5)46 1.935(3)46 1.930(5)46 1.916(3)46 1.887(2)47 1.887(2)48 1.892(3)48 1.907(2)49 1.875(2)49 1.971(3)10 1.9188(13)50 1.9124(12)50 1.915(2)51 1.909(4); 2.023(4)52 1.903(4); 1.924(4)52 1.895(4)52 1.889(3)52 1.896(3)53 1.879(2)54 1.894(3)55 1.912(2)55

Nickel-Carbon s-Bonded Complexes

Table 1

277

(Continued) ˚) Ni–C (A

Complex Ph I

iPr iPr

R2

R1

1.911(2)56 1.921(6); 1.923(6)57

R2 tBu H H tBu H H H H H Me Me H H H tBu tBu H H H H H H H H H tBu H H tBu COOMe COOMe Cl H H H H H tBu tBu H H

1.89258 1.875(1)59 1.877(1)59 1.893(2)59 1.875(4)60 1.866(5)60 1.874(5); 1.881(4)60 1.885(3)60 1.881(3)60 1.96061 1.945(2)61 1.888(3); 1.889(3)30 1.884(2)62 1.876(2)63 1.8982(19)63 1.899(2)63 1.8765(17)37 1.8760(18)37 1.884(6)37 1.8866(18)37 1.886(3)37 1.8775(14)37 1.875(3)37 1.885(3)47 1.872(2)47 1.892(4)47 1.877(2)47 1.877(2)47 1.909(2)64 1.895(3)64 1.8906(17)64 1.879(2)65 1.903(2)49 1.876(2)66 1.8772(16)66 1.8877(16)66 1.8876(14)66 1.8964(17)66 1.8898(19)67 1.869(9)68 1.877(2)69

R'

E P

R2

R'

Ni

E R'

O CH2

R

P R'

R I Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl SH Cl Cl Cl I SCN SCN N3 N3 SCN N3 Cl Br Br Br Br Br I I Cl Br CF3 Cl Cl H H H F Cl Br

R0 tBu iPr iPr iPr tBu tBu iPr tBu Ph tBu Ph tBu Ph c-C5H9 tBu Ph tBu Ph tBu Ph iPr iPr tBu iPr iPr iPr tBu iPr tBu tBu tBu iPr iPr tBu tBu tBu tBu tBu tBu iPr iPr

E O O O O O O O O O CH2 CH2 O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

R1 H COOMe OMe H OC(O)Ph OC(O)(p-OMe-Ph) OH OH OH H H COOMe OH COOEt H H COOMe COOMe COOMe COOMe COOMe COOMe COOMe Me COOMe H COOMe Me H H H H OMe OMe Me OMe Me H H OPiPr2 Br

R2

R1

R

E P

R2

A

Ni

E R

R

P

L

R

(Continued )

278

Table 1

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C (A

Complex L R E MeCN tBu CH2 MeCN iPr O (NH]C(Me)(c-NC4H8O)) iPr O MeCN tBu CH2 MeCN tBu O N^CCH]CHPh iPr O N^CCH]CHPh iPr O NH3 iPr O NH3 iPr O N^CCH]CH2 iPr O H2O iPr O N^CtBu iPr O H2O tBu CH2 H2 tBu CH2 N2 tBu CH2 MeCN tBu O MeCN iPr CH2 MeCN iPr O C^O iPr NMe MeCN iPr O MeCN iPr O MeCN iPr O MeCN iPr O MeCN iPr O MeCN tBu O MeCN tBu O MeCN Ph O NCCH2CH2N(H)Ph Ph O CF2 iPr O [k3-P,C,P0 -[1,3-(OP(iPr)2)2-naph(Z6-FeCp)]NiCl][PF6] [k3-P,C,P0 -[1,3-(OP(iPr)2)2-naph(Z6-RuCp)]NiCl][PF6] k3-P,C,P0 -[1,3-(OP(iPr)2)2-naph]NiCl k3-P,C,P0 -[1,3-(PPh2)2-2-naph]NiCl k3-[P,C,P0 -[2-OPPh2-6-(NC]CN(Me)C]PPh2)Ph]NiBr][OTf] k3-P,C,P0 -[2-OPPh2-6-(NC]CN]CPPh2)-Ph]NiBr k3-P,C,P0 -[2-CH2OPiPr2-6-OPiPr2-Ph]NiBr k3-P,C,P0 -[2-CH2OPiPr2-6-OPiPr2-Ph]NiO(p-Ts) k3-[P,C,P0 -[2-CH2OPiPr2-6-OPiPr2-Ph]NiNCMe][BPh4] k3-P,C,P0 -[2-CH2OPiPr2-6-OPiPr2-Ph]NiMe k3-P,C,P0 -[2-CH2OPiPr2-6-OPiPr2-Ph]NiCCMe k3-P,C,P0 -[2,6-(OP(N(iPr)C2H4N(iPr))2-3,5(tBu)2-C6H1]NiBr k3-[P,C,P0 -[Cp-Ru-Z6-(2,6-(OPiPr2)2-Ph)]NiCl][PF6] [k3-P,C,P0 -[Ph(CH2PiPr2)2]Ni]2(m-CO3) k3-P,C,P0 -[2-(SPiPr2)-6-(OPiPr2)-Ph]NiCl k3-P,C,P0 -[2,6-(CH2P(2,4-Me2-C4H6))2-Ph]NiBr k3-P,C,P0 -[Ph(CH2PtBu2)2]Ni(m-(C^O))W(Cp)(C^O)2 k3-P,C,P0 -[Ph(OPtBu2)2]Ni(m-C^O)WCp(C^O)2 k3-P,C,P0 -[1,3-(OP(iPr)2)2-naph(Z6-Cr(C^O)3)]NiCl k3-P,C,P0 -[R,R-Ph(CH(CH(COOMe)2)PPh2)2]NiCl k3-P,C,P0 -[Ph(OP((tBuN)2PNHtBu))2]NiI [k3-P,C,P0 -[2-(2-PiPr2-N-Im)-6-OPiPr2-Ph]Ni(MeCN)][OTf] k3-P,C,P0 -[2-(2-PiPr2-N-Im)-6-OPiPr2-Ph]NiBr k3-P,C,P0 -[2-(2-PiPr2-N-Im)-6-OPPh2-Ph]NiBr k3-P,C,P0 -[2,6-(CH2PMe(tBu))2-3,5-Me2-C6H1]NiCl k3-P,C,P0 -[2,6-(OP(tBu)(Ph))2-Ph]NiCl k3-P,C,P0 -[2,6-(o-(PiPr2)-Ph)2-4-(NMe2)-Ph]NiI k3-P,C,P0 -[2,8-(OPiPr2)2-naph]NiCl k3-P,C,P0 -[2,8-(OPCy2)2-naph]NiCl k3-P,C,P0 -[2-(OPtBu2)-6-(SPtBu2)-Ph]NiCl

R1 H H H H H COOMe OMe H COOMe COOMe H H H H H Me H H H OMe COOMe Br H Me H COOMe H H H

Naph, naphthylene; ox, oxazoline; Im, imdazole; cat, catechol; Bn, benzyl; Mes, 2,4,6 trimethylphenyl.

R2 H H H H H H H H H H H H H H H H H H H H H H tBu H H H H H H

A BF4 OTf OTf CpW(C^O)3 CpW(C^O)3 OTf OTf OTf OTf OTf OTf OTf BF4 B(C6F5)4 B(C6F5)4 BF4 BPh4 BPh4 SbF6 OTf OTf OTf OTf OTf OTf OTf OTf OTf BF(C6F5)3

1.914(6)70 1.883(2); 1.883(2); 1.881(2)29 1.888(3)71 1.925(3)72 1.925(3)73 1.878(2)74 1.881(2)74 1.894(2)74 1.890(2)74 1.877(2)74 1.882(3)74 1.891(1)74 1.905(4)41 1.905(5)75 1.9335(17)75 1.8861(14)66 1.916(3)65 1.8839(14)65 1.915(2)51 1.884(2)69 1.880(2)69 1.879(3)69 1.890(2)69 1.879(3)69 1.885(2)69 1.881(2)69 1.885(3)76 1.883(2)76 1.902(3)77 1.873(7)78 1.871(3); 1.883(3)78 1.876(2); 1.881(2)78 1.885(4)79 1.932(2)80 1.945(2)80 1.920(2)81 1.916(2)81 1.919(2)81 1.947(2)81 1.936(3); 1.936(3)81 1.882(5)82 1.866(6); 1.847(6)83 1.911(2)40 1.894(8); 1.894(2)27 1.928(3)84 1.924(13)72 1.922(12)73 1.872(3)78 1.924(3)85 1.868(4)86 1.944(2)87 1.939(3)87 1.943(2)87 1.926(3)61 1.895(2)88 1.9188(14)89 1.9232(2)90 1.9410(2)90 1.899(7)91

Nickel-Carbon s-Bonded Complexes

Table 2

PC(sp3)P pincer nickel complexes.

Nickel complex

Ni–C (A)

R'

E Ni E R' R Br Me Ph C^CPh OH I Cl O3SCF3 Br OSiPh3 N3 CF3 Cl OSiMe3 OMes NPh2 C^CH N(H)Dipp OP(O)iPr2 C(O)N(H)Dipp OC(O)N(H)Dipp OSiPh2OSiMe3

R

P R'

R0 iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr tBu iPr iPr iPr iPr iPr iPr iPr iPr iPr

Ni E R"

P R"

R00 iPr iPr iPr

E CH2 O O

2.039(3)92 2.012(2)93 2.011(5)93

A BF4 BF4 BF4 BF4 BPh4 BPh4

1.965(8)92 1.981(3)92 1.989(2)92 2.000(2)92 1.9751(10), 1.9787(10)92 1.978(3)65

R R

P

Ni E R

1.9820(30)18 2.0040(20)18 1.942(2)18 1.9917(17)18 1.9856(16)92 1.986(7)92 1.957(3)92 1.922(5), 1.949(4)92 1.964(3)93 1.9561(19), 2.010(2)33 1.988(3), 1.939(2)33 1.986(2), 2.009(2)33 1.974(4)94 1.967(4)95 1.9739(14)95 1.986(6), 1.982(8)95 1.969(2), 1.9734(19), 1.972(2), 1.9716(19)95 1.992(3), 1.995(3)95 2.009(7), 1.958(3)96 1.982(3)96 1.945(3)96 1.941(19), 1.986(16)50

R'

R0 Br Cl Br

E

E CH2 CH2 CH2 CH2 CH2 O O O O O O O NH O O O O O O O O O

R" P R" R

E

L H2O MeCN NH(iPr)2 C^O MeCN N(H)(Ph)EtCN

R'

P

H

R Br Cl Br

279

A

L

P R

R iPr iPr iPr iPr iPr iPr

E CH2 CH2 CH2 CH2 CH2 CH2

R'

E P Ni

E R'

R'

P

R

R' (Continued )

280

Table 2

Nickel-Carbon s-Bonded Complexes

(Continued)

Nickel complex

Ni–C (A)

R R0 Br tBu Cl tBu H tBu OH tBu NO3 tBu COOH tBu HCO3 tBu Cl tBu Ph tBu C^CPh tBu Br tBu O2CH tBu Me tBu OAc tBu k3-P,C,P0 -[C(SiPh3)(o-(PiPr2)-Ph)2]NiH k3-P,C,P0 -[C((p-OMe-Ph)3Si)(o-(PiPr2)-Ph)2]NiH k3-P,C,P0 -[C((p-MePh)3Si)(o-(PiPr2)-Ph)2]NiH

E CH2 CH2 O O O O O O O O O O O O

1.998(2)97 1.990(4)97 1.948(3)97 1.942(4)97 1.913(3)97 1.956(4)97 1.942(4)97 1.939(3)97 1.970(3)97 1.942(3)97 1.939(2)97 1.948(2)97 1.966(3)97 1.948(3)97 2.043(4)98 2.0483(18)98 2.055(9)98

R' P

R'

Ni P

R

R' R' R R0 OPh iPr SiPh3 iPr O3SCF3 iPr Br Cy Cl tBu N(H)C(O)Ph iPr OAc iPr CN iPr Br iPr H iPr OH iPr [k3-P,C,P0 -[C(H)(o-(PiPr2)-Ph)2]Ni(THF)][SbF6] k3-P,C,P0 -[C(H)(2-(PiPr2)-4-NMe2-Ph)2]NiBr k3-P,C,P0 -[C(H)(2-(PiPr2)-4-NMe2-Ph)2]Ni(O3SCF3) k3-P,C,P0 -[C(H)(2-(PiPr2)-c-C8H4S)2]NiBr [k3-P,C,P0 -[C(H)(2-(PiPr2)-C8H4S)2]Ni(THF)][SbF6] [k3-P,C,P0 -[C(H)(2-(PiPr2)-c-C8H4S)(2-(P(O)iPr2)-C8H4S)]Ni(ONMe3)][SbF6] k3-P,C,P0 -[C(H)(2-(PiPr2)-Im)2]NiBr [k3-P,C,P0 -[2,6-(OPtBu2)2-C6H9]Ni]2CO3 [k3-P,C,P0 -[C(H)(P(CH2PPh2)Ph2)2]NiCl][BF4] k3-P,C,P0 -[1,8-(PiPr2)2-C12H6-OCNiPr]NiNCtBu k3-P,C,P0 -[1,8-(PiPr2)2-9,9-Me2-C14H7]NiBr k3-P,C,P0 -[1,8-(PiPr2)2-9,9-Me2-C14H6CO2]NiPMe2Ph k3-P,C,P0 -[C(H)(CH2OPiPr2)2]Ni(O2CC(H)(CH2OPiPr2)2Ni(CO)2 k3-P,C,P0 -[Ph(OPiPr2)(CH2OPiPr2)]NiMe [k3-P,C,P0 -[C(PPh2CH2PPh2)2]NiMe][AlMe2Cl2] k3-P,C,P0 -[C(H)(CH2CH2PtBu2)2]NiBr k3-P,C,P0 -[C(H)(CH2CH2PtBu2)2]NiCl k3-P,C,P0 -[C(H)(CH2CH2PtBu2)2]NiI [k3-P,C,P0 -[C(H)(CH2CH2PtBu2)2]Ni(MeCN)][BPh4] [k3-P,C,P0 -[C(H)(CH2CH2PtBu2)2]Ni(NCC(H)]CH2)][BPh4] Dipp, 2,6-diisopropylphenyl

1.983(3)98 2.030(8)98 1.963(3)99 1.975(5)100 1.982(1)100 1.973(4)100 1.965(5)100 1.979(5)101 1.973(3)101 2.000(3)101 1.981(3)101 1.976(2)99 1.949(6)99 1.975(6)99 1.965(7)99 2.028(6)99 1.956(4)99 1.965(3)102 NA103 1.990(3)104 1.996(7)18 1.974(2)105 2.020(2)105 1.958(2)96 2.065(2)81 1.986(4)104 1.971(4)106 1.979(3)106 1.985(7)106 1.969(3)106 1.982(2)106

Nickel-Carbon s-Bonded Complexes

Table 3

281

PNP pincer nickel aryl complexes. ˚) Ni–C (A

Complex

R'

E N E R' R o-F-Ph Ph Ph Me

R'

P

Ni

R

P R'

R0 iPr iPr Ph iPr

E NH CH2 CH2 CH2

E H E R'

N

Ni

1.914(5)108 1.916(1)108 1.903(1)71 1.968(2)17

R' P

R'

A

R

P R'

R R0 E Ph iPr CH2 Ph iPr CH2 Ph iPr CH2 Ph iPr CH2 Me iPr CH2 Me iPr CH2 Me iPr CH2 k3-P,N,P0 -[N(CH2CH2PiPr2)(CHCHPiPr2)]NiPh [k3-P,N,P0 -[2-(N(H)PPh2)-6-(o-PPh2-Ph)-pyrimidine]Ni(2-NH2-6-(o-Ph)-pyrimidine)][ClO4] k3-P,N,P0 -[N(o-PhPCy2)2]Ni(C6F5) k3-P,N,P0 -[N(o-PhPiPr2)2]Ni(p-Tol) k3-P,N,P0 -[N(o-PPh2-Ph)(C(Ph)]C(H)PiPr2)]NiPh k3-P,N,P0 -[2,6-(OPtBu2)2-Py]NiPh k3-P,N,P0 -[2,6-(CH2PPh2)-pyrr]Ni(p-OMe-Ph) k3-P,N,P0 -(NiXantphos)NiMe k3-P,N,P0 -[N(2-PiPr2-4-Me-Ph)2]NiMe k3-P,N,P0 -[NC4H2(CH2PCy2)2]NiMe k3-P,N,P0 -[NC4H2(CH2PPh2)2]NiMe k3-P,N,P0 -[NC4H2(CH2PtBu2)2]NiMe k3-P,N,P0 -[2,2,4-Et3-1,5-(]NPtBu2)2NC5H]NiMe k3-P,N,P0 -[1-(]C(H)PiPr2)-5-(CH2PiPr2)-NC5H3]NiMe [Li(dme)3][k3-P,N,P0 -[1-(C(H)]PiPr2)-5-(CH2PiPr2)-c-NC5H3]NiMe] [Li(Et2O)2][k3-P,N,P0 -[1-(C(H)]PtBu2)-5-(]C(H)PtBu2)-c-NC5H3]NiMe] k3-P,N,P0 -[N(2-PPh2-Ph)2]NiMe k3-P,N,P0 -[N(2-PPh2-4-Me-Ph)2]NiMe k3-P,N,P0 -[N(2-PCy2-4-Me-Ph)2]NiMe k3-P,N,P0 -[N(2-PPh2-4-Me-Ph)2]NinBu k3-P,N,P0 -[N(2-PiPr2-4-Me-Ph)2]NinBu k3-P,N,P0 -[N(2-PPh2-4-Me-Ph)2]NiCH2SiMe3

A Br PF6 BPh4 NiCl4 Cl Br BPh4

1.909(3)109 1.899109 1.903(2)109 1.906(2)109 1.9488(17)17 1.9756(18)17 1.9608(15)17 1.907(2)109 1.903(3)110 1.934(7)111 1.909(8)111 1.903(2)112 1.959(4)113 1.9129(19)114 1.967(5)115 2.004(2)116 1.967(2)71 1.999(2)117 1.957(2)71 1.945(2)118 1.955(2)119 1.954(2)119 1.959(2)119 1.940(8)120 1.967(11)121 1.995(7)121 1.971(3)121 1.963(3)121 1.944(7)121

pyrr, pyrrole; dme, 1,2-dimethoxyethane.

dependent on the identity of the nitrogen binding fragments. While pyridyl groups are neutral, amido and pyrrole groups are typically monoanionic.123 It is the redox non-innocence involving these nitrogen groups that distinguishes this ligand class (Fig. 6).124 This is exemplified by the transformation of a monoanionic ligand to a neutral ligand through protonation as well as the dearomatization of neutral pyridyl based ligands to form a monoanionic amido based PNP ligand.125,126 Transition metal complexes of monoanionic PNP ligands frequently adopt an amido-centered HOMO, in contrast to the metal-centered HOMO of most PCP ligand complexes.127 This is particularly prominent during the formation of the open-shell metalloradical complexes in which the complex possesses significant aminyl radical character.11 These electron-rich nitrogen atoms can also be protonated to convert the monoanionic ligand into a neutral donor ligand.

282

Nickel-Carbon s-Bonded Complexes

R P R

N Nin

P R R

R P R

H

N Nin

R P R

+ H+

P R R

- H•

R P R

H N Nin

P R R

N (n+1) Ni P R R

Fig. 6 Ligand chemistry of PNP nickel complexes.

8.05.2.3

PCN, NCN, PPC, SCS, NNN, and GeCGe pincer nickel complexes

Whereas phosphines serve as strongly coordinating and largely inert donors, the N donors in NCN ligands are typically considered hemilabile and vulnerable to b deprotonation reactions. The hemilability of the NidN bond, especially in the presence of additional hard metals (Li, Zn, Mg), increases their functionality as catalysts.11 Meanwhile their susceptibility to b deprotonation results in metal-ligand cooperativity via proton transfer reactions.128 The asymmetric PCN class of ligands combines many of the characteristics of the PCP and the NCN ligand classes (Fig. 7). The stability and the opportunity for 31P NMR characterization is similar to the PCP ligand class while the hemilability of the amino group and the metal-ligand cooperativity is reminiscent of the NCN ligand class. The nickel chemistry of the PCN ligand class is significantly less developed with its representation limited to the alkyl, aryl, halide and pseudohalide complexes (Table 4). The nickel complexes of the NCN ligand class have been limited to halide and phenoxy derivatives (Table 5). Several nitrogen donor groups have been utilized for these ligands including oxazolines, imines, amines, phosphaimines, pyrazoles, and piperidines. While most of the ligands included in this chapter have aryl or alkyl groups incorporated into the backbone of the ligand, several do not and are able to stabilize alkyl or aryl derivatives including NNN, PNP, PSiP, NNO, PPP, PSbP, and PBP pincer ligands (Table 6). The limited number of (SCS)Ni complexes were synthesized for bioinorganic investigations of metalloenzymes. The (NNN)Ni complexes have been applied in organometallic catalysis, where their redox-activity and hemilability helps to facilitate organic transformations. The (PPC)Ni and (NNC)Ni complexes commonly serve as bidentate ligands to investigate the reductive elimination and b-hydride elimination of tethered organic substrates. The (CCC)Ni, (PSiP)Ni, (SiCSi)Ni, (GeCGe)Ni, and (PBP)Ni complexes are members of a growing class of new ligands that incorporate low-valent main group atoms as coordination sites including carbenes, silylenes, germylenes, and borylenes.209 Novel tetrylene based pincer ligands were developed with silicon 4 and germanium 5 derivatives (Fig. 8).82 The nickel complexes of these ligands were able to catalyze a Sonogashira cross coupling reaction. The silicon- and germanium-based pincer ligands allowed for the isolation of a nickel copper alkynyl intermediate in the cross coupling of the alkyne and the alkenyl halide. The stoichiometric addition of 1-iodo-1-octene to these intermediate complexes resulted in formation of the coupled product in yields of 80–95%, as well as the corresponding nickel iodide, demonstrating their relevance as catalytic intermediates. Nickel complexes featuring tripodal ligands occur as both tridentate and tetradentate complexes. Some tripodal ligands possess alkyl backbones that result in extremely long NidC bonds in their nickel complexes (Table 7). The most common class of tripodal ligands are the tris(pyrazolyl)borates, also known as scorpionates.225 These ligands have allowed for isolation of nickel complexes in a variety of oxidation states. Many tripodal ligands featuring sulfur chelators have been utilized in the synthesis of biomimetic nickel complexes to imitate the sulfur ligands frequently seen in enzymes such as sulfides, cysteine, and methionine.226

R N R P R

Ni

N R R

H R P R

Ni

N

Fig. 7 Hemilability and deprotonation of PCN nickel complexes.

R

R P R

Ni

R P R

Ni

- H+ N R

R

Nickel-Carbon s-Bonded Complexes

Table 4

283

PCN nickel complexes. ˚) Ni–C (A

Complex

N Ni

E R'

R"

R

P R'

R Br Me (m-H2-BH2) (m-F-BF3) Cl Cl Br Br2 Br Br Br Br Br2 SCN

0

R00 Dipp Dipp Dipp Dipp Dipp Dipp Bn Bn Ph tBu Cy Ph Ph Ph

R iPr iPr iPr iPr Ph iPr iPr iPr Ph Ph Ph iPr iPr iPr

E O O O O O O O O O O O O O O

1.856(3)129 1.869(5); 1.881(5)129 1.865(1)129 1.842(2)129 1.885(2)130 1.847(3)130 1.855(5); 1.856(5); 1.860(6)131 1.895(2)131 1.856(1)132 1.859(2)132 1.863(2)132 1.851(3)132 1.902(4)132 1.849(2)132

Nflank Ni

O R'

P

R

R' R R0 Nflank Br iPr c-NC4H8O Br iPr NEt2 Br2 iPr NMe2 Br2 iPr c-NC4H8O Br2 iPr NEt2 Br iPr NMe2 Br iPr N(15-crown-5) Br iPr N(15-crown-5)LiPF6 Br iPr N(18-crown-6) Br iPr N(15-crown-5)Li(OTf ) Br tBu NiPr2 Me tBu NiPr2 Br iPr NHPh k3-P,C,N-[2-(2-Ph)-6-(OPtBu2)-Py]NiBr k3-P,C,N-[2-N(H)P(Ph)2-6-C]NC(iPr)C(H2)C(p-Tol)-Ph]NiCl k3-P,C,N-[2-(OPPh2)-6-(C]NC(H)iPrCH2N(p-OMe-Ph))-Ph]NiCl k3-P,C,N-[2-(OP(NC4H7)(OCPh2)-6-(C]NC(H)iPrCH2N(p-OMe-Ph))-Ph]NiCl k3-P,C,N-[Ph(OPiPr2)(CH2N(H)Bn)]NiBr [k3-P,C,N-[Ph(OPiPr2)(CH2N(H)Bn)]Ni]2 k3-P,C,N-[Ph(OPiPr2)(CH2N(allyl)Bn)]NiBr k3-P,C,N-[Ph(OPiPr2)(CH2NBn2)]NiBr k3-P,C,N-[2-(CH2PtBu2)-6-pz-Ph]NiF k3-P,C,N-[2-(CH2PtBu2)-6-pz-Ph]NiCl k3-P,C,N-[2-(CH2PtBu2)-6-pz-Ph]NiBr k3-P,C,N-[2-(CH2PtBu2)-6-pz-Ph]NiI k3-P,C,N-[2-pz-6-(CH2PtBu2)-Ph]Ni(m-H2-BH2) k3-P,C,N-[2-(2-py)-6-(CH2PCy2)-Ph]NiBr k3-P,C,N-[2-(2-py)-6-(CH2PtBu2)-Ph]NiBr k3-P,C,N-[2-(2-py)-6-(CH2PtBu2)-Ph]NiEt k3-P,C,N-[2-PtBu2-6-OBzTz-Ph]NiBr k3-P,C,N-[2-PtBu2-6-OBzTz-Ph]NiCl [k3-P,C,N-[2-PtBu2-6-OBzTz-Ph]Ni(OH2)][BF4]

1.853(2)133 1.8557(19)133 1.897(2)133 1.883(4)133 1.8907(19)133 1.859(2)133 1.854(3)134 1.860(3)135 1.8593(18)135 1.858(3)135 1.881(2)136 1.905(2), 1.981(2) (CMe)136 1.858(3)137 1.922(4); 1.926(4)138 1.873(2)139 1.869(6)140 1.851(5)141 1.8540(50)142 1.8638(18)142 1.8492(17)142 1.8560(2)142 1.856(3)143 1.885(5)143 1.880(6)143 1.875(10)143 1.886(1)144 1.878(3)145 1.881(3)145 1.899(15), 2.064(12) (CEt)145 1.888(3)146 1.8850(15)146 1.866(13)146 (Continued )

284

Table 4

Nickel-Carbon s-Bonded Complexes

(Continued)

Complex

˚) Ni–C (A

k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiBr2 k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiPh k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiMe k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiBr k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiNO3 [k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]Ni]2(m-CO3) k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiC^C(o-Tol) k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiCl2 k3-P,C,N-[2-(CH2PtBu2)-6-(CH2NMe2)-Ph]NiCl [k4-P,C,N,O-[2-(OPiPr2)-6-N(15-crown-5)-Ph]Ni][PF6] [k3-P,C,N-[2-(OPiPr2)-6-N(15-crown-5)Na-Ph]Ni][BArF4]

1.928(5)147 1.950(5); 1.915(5)147 1.902(2)147 1.8922(18)147 1.8782(13)147 1.873(3)147 1.902(3)147 1.928(3)147 1.881(3)147 1.8479(18)135 1.863(3)135

BzTz, benzo[d]thiazole.

Table 5

NCN pincer nickel complexes. ˚) Ni–C (A

Complex

E

R''

N E

R'

Ni N

R

R' 0

R R I iPr Br Ph Br Ph Br iPr Br Ph k3-N,C,N0 -[1,6-(1-C5H11-benzoylbenzimidazolyl)2-4-tBu-Ph]NiBr k3-N,C,N0 -[2,6-(CH2-indazole)2-Ph]NiBr k3-N,C,N0 -[2,6-(3-(p-Tol)-5-(Bn)-2-Im)2-Ph]NiCl k3-N,C,N0 -[2,6-(3-(Cy)-5-(iPr)-2-Im)2-Ph]NiCl k3-N,C,N0 -[2,6-(3-(p-Tol)-5-(iPr)-2-Im)2-Ph]NiCl [k3-N,C,N0 -[2,6-(3-(p-Tol)-5-(Bn)-2-Im)2-Ph]Ni(MeCN)][BF4] k3-N,C,N0 -[2,6-(CH2N]PPh3)2-Ph]NiBr k3-N,C,N0 -[2,6-(CH2N]PPh2Cy)2-Ph]NiBr k3-N,C,N0 -[2,6-(CH2N]PPhCy2)2-Ph]NiBr k3-N,C,N0 -[2,6-(CH2N]PCy3)2-Ph]NiBr k3-N,C,N-[2,6-(CH2Pipe)2-Ph]NiClBr k3-N,C,N-[2,6-(N-pz)2-Ph]NiBr k3-N,C,N-[2,6-(N-pz)2-4-(OMe)-Ph]NiBr k3-N,C,N-[2,6-(N-pz)2-Ph]Ni(O(2,6-tBu2-Ph)) [k3-N,C,N0 -[2,6-(NOC3H3iPr)2-Ph]Ni(OH2)][ClO4] k3-N,C,N0 -[2,6-(C(Me)]N(2,6-Me2-Ph))2Ph]NiBr k3-N,C,N0 -[2,6-(CH2NMe2)2-Ph]NiBr2 k3-N,C,N0 -[2,6-(C(H)]N(2,6-Me2-Ph))-Ph]NiBr k3-N,C,N0 -[2,6-(C(H)]N(2,6-Et2-Ph))-Ph]NiBr k3-N,C,N0 -[2,6-(C(H)]N(2,6-iPr2-Ph))-Ph]NiBr k3-N,C,N0 -[2,6-(CH2NMe2)2-Ph]NiCl k3-N,C,N0 -[C(H)(P(]NSiMe3)Ph2)2]NiBr [k3-N,C,N0 -[C(H)(P(]NSiMe3)Ph2)2]NiCl]2 k3-N,C,N0 -[N,N0 -(MeC]NDipp)2-1,3-C3N2H3]NiCl k3-N,C,N0 -[2,6(40 ,40 -Me2-20 -ox)2Ph]NiI k3-N,C,N0 -[2,6-(2-py)-Ph]NiCl k3-N,C,N0 -[2,6-(2-py)-Ph]NiBr k3-N,C,N0 -[2,6-(2-py)-Ph]NiI k3-N,C,N0 -[2,6-(2-py)-Ph]Ni(OC(O)Me) [k3-N,C,N0 -[2,6-(CH2NMe2)2-Ph]Ni(MeCN)][OTf] [k3-N,C,N0 -[2,6-(CH2NMe2)2-Ph]Ni(MeCN)3][SbF6] [k3-N,C,N0 -[2,6-(CH2NMe2)2-Ph]NiBr(MeCN)][SbF6] Pipe, piperidine.

R00 tBu H tBu H H

E O O O O S

1.841(6)148 1.844(3)148 1.835(10)148 1.849(19)148 1.837(2)149 1.846(4)150 1.899(3)151 1.836(4)152 1.848(4)152 1.846(4)152 1.831(6)152 1.834(4)153 1.833(4)153 1.833(4)153 1.831(4)153 1.881(2)154 1.825(5)155 1.822(4)155 1.822(2)155 1.8333(4)148 1.819(2)156 1.894(3)156 1.827(7)157 1.840(6)157 1.823(4)157 1.830(1)158 2.205(5); 2.193(5)159 2.3099(19)159 1.828(3)107 1.859(3); 1.857(4)160 1.836(7)161 1.8298(16)161 1.829(4)161 1.8136(19)161 1.8245(14)162 1.907(5)162 1.873(4)162

Nickel-Carbon s-Bonded Complexes

Table 6

285

Nickel pincer aryl/alkyl complexes. ˚) Ni–C (A

Complex

R' S Ni

R'

R

S

R R0 I c-NC4H8 I c-NC5H10 I NHCy Br NHBn k3-S,C,S0 -[Ph(CH2SCH2CH2OCH2CH2)2O]NiBr k3-S,C,S0 -[Ph(CH2SCH2CH2OCH2CH2)2O]NiBr(LiN(SO2CF3)2) k3-S,C,S0 -[2,6(CH2SMe)2Ph]NiBr k3-S,C,S0 -[3,5-(C(S)NEt2)2-4-py]NiCl [k3-S,C,S0 -[3,5-(C(S)NEt2)2-4-py]NiCl][I] [PPh4][k3-S,C,S0 -[N-Me-3,5-(C(O)S)2-4-py]NiBr] [PPh4][k3-S,C,S0 -[N-Me-3,5-(C(O)S)2-4-py]NiCl] [k3-S,C,S0 -[N-Me-3,5-(C(O)S)2-4-py]Ni(Im) [k3-P,C,O-[C(H)(o-(PiPr2)-Ph)(o-(P(O)iPr2)-Ph)]Ni(ONMe3)][OTf] [k3-P,C,O-[C(H)(2-(PiPr2)-4-NMe2-Ph)(2-(P(O)iPr2)-4-NMe2-Ph)]Ni(ONMe3)] [OTf] k3-N,N0 ,N00 -[2,5-(]CC]NCH(Ph)CH2O)2-NC4H4]Ni(4-F-Ph) k3-N,N0 ,N00 -[2,5-(]CC]NCH(Ph)CH2O)2-NC4H4]NiPh k3-N,N0 ,N00 -[2,5-(]CC]NCH(Ph)CH2O)2-NC4H4]Ni(4-OMe-Ph) k3-N,N0 ,N00 -([2,5-(]CC]NCH(Ph)CH2O)2-NC4H4]Ni)4(1,3,8,10tetraazaperopyrene) k3-N,N0 ,N00 -[2-C(O)NnBu-6-C(Me)]N(2,6-Et2-Ph)-py]Ni(2-naph) k3-N,N0 ,N00 -[2-C(O)NnBu-6-C(Me)]N(Dipp)-Py]Ni(2-naph) k3-N,N0 ,N00 -[2-C(O)NBn-6-C(Me)]N(Dipp)-Py]Ni(2-naph) [(4-Cl-terpy)Ni(Mes)][Br] [(4-Cl-terpy)Ni(2,6-Me2-Ph)][Br] [(terpy)Ni(2,6-Me2-Ph)][PF6] [(4-Cl-terpy)Ni(2,6-Me2-Ph)][PF6] k3-N,N0 ,N00 -[2,5-(CMe2(5-iPr-ox))2-pyrr]Ni(p-F-Ph) k3-[N,Fe,N0 -[1,10 -(N]PPh3)2-Fc]NiPh][BPh4] k3-[N,N0 ,N00 -[NH((C2H4)N]PPh3)2]Ni(2-F-Ph)][PF6] k3-[N,N0 ,N00 -[NH((C2H4)N]PPh3)2]NiPh][Br] k3-[N,N0 ,N00 -[2,6-(C]NC(H)(iPr)CH2O)2-Py]NiPh][BArF] k3-N,N0 ,N00 -[2,6-(C]NC(H)(iPr)CH2O)2-Py]NiPh k3-N,N0 ,N00 -[N(o-Ph(NOC3H3iPr))2]NiMe k3-N,N0 ,N00 NC5H3(C(Me)]NDipp)2]NiMe [(terpy)Ni(Mes)][Br] k3-N,N0 ,N00 -[N(2-(NMe2)-Ph)2]NiPh k3-N,N0 ,N00 -[2,6-(4-tBu-2-Py)-4-tBu-Py]NiMe [k3-N,N0 ,N00 -[2,6-(4-tBu-2-Py)-4-tBu-Py]NiMe][I]

1.874(5)163 1.887(5)163 1.866(13)163 1.8651(17)164 1.895(3)165 1.901(3)165 1.900(2)166 1.859(3)167 1.842(3) 1.8462(17)168 1.857(4)168 1.860(3)168 1.948(2)99 1.945(5)99 1.907(3); 1.902(2)169 1.902(3); 1.906(3); 1.906(3); 1.911(3); 1.913(3); 1.896(3)169 1.898(2)169 1.941(8), 1.880(7), 1.904(6), 1.912(6); 1.892(4), 1.902(7), 1.903(4), 1.896(7)169 1.913(5)170 1.896(5)170 1.908(4); 1.898(6)170 1.916(6)171 1.864(14)171 1.898(3)171 1.890(3)171 1.902(3)172 1.891(3)173 1.900(2)174 1.886(2)174 1.881(3)175 1.896(3); 1.894(3)175 2.077(4)176 1.922(4)177 1.899(5)171 1.886(1)178 1.944(4)179 1.921(9)179

tBu P

Ni

P tBu

R

tBu

R CH2SiMe3 NHDipp Br Me SiPr N(Me)Ph k3-P,Si,P0 -[Si(o-PiPr2-Ph)2(NHTripp)]NiPh

1.9807(13)180 1.9692(7)180 1.964(3)181 1.975(2) 1.9480(11)181 1.951(3)181 1.974(4)182 (Continued )

286

Table 6

Nickel-Carbon s-Bonded Complexes

(Continued)

Complex

˚) Ni–C (A

k3-P,C,C0 -[2-(N-Me-N-Im)-6-OPiPr2-Ph]NiBr [k3-P,C,C0 -[2-(N-Me-N-Im)-6-OPiPr2-Ph]Ni(MeCN)][OTf] k3-C,O,C0 -[o-(CMe(O)(CH2C(O)(o-Ph)))]Ni2(PMe2Ph)2(m-OH) k3-N,N0 ,C-[Py2CFPh]Ni(CF3)2F k3-N,N0 ,C-[Py2CF(3-OMe-Ph)]Ni(CF3)2F k3-N,N0 ,C-[Py2CF(3-F-Ph)]Ni(CF3)2F k3-N,N0 ,C-[3-(N]P(Ph)(NHtBu)Ph)-quin]NiCl k3-N,N0 ,C-[2-o-py-3-CF3-6-o-Ph]NiBr k3-N,N0 ,C-[6-(p-OMe-o-Ph)-bpy]NiBr k3-N,N0 ,C-[3-(m-OMe-Ph)-6-(o-Ph)-bpy]NiBr k3-N,N0 ,C-[3-CF3-6-(o-Ph)-bpy]NiBr k3-N,N0 ,C-[4-(p-OMe-Ph)-6-(o-Ph)-bpy]NiBr k3-N,N0 ,C-[6-Ph-bpy]NiBr k3-N,N0 ,C-[3-NCO(Ph)-quin]Ni(3-NCOPh-quin) k3-N,N0 ,C-[1-(o-Ph)-3-(o-Tol)-bpy]NiBr k3-O,N,N0 -[2-(CH2C(Ph2)O)-6-(3,5-Me2-N-pz)-py]Ni(naph) k3-O,N,N0 -[2-(CH]C(Ph)O)-6-(3,5-Me2-N-pz)-py]Ni(o-Tol) [k3-O,C,O0 -[2,6-(C(H)]N(tBu)O)2-Ph]Ni(H2O)]Br k3-P,N,N0 -[N(o-(PiPr2)-4-Me-Ph)(o-(P(iPr2)]NH)-4-Me-Ph)]NiPh k3-N,N0 ,C-[8-(NC(O)(o-Ph))-quin]Ni(8-(NC(O)Ph)-quin) k3-P,C,C0 -[2-OPPh2-6-(NC]CN(Me)C)Ph]NiBr k3-C,C0 ,C00 -[2,6-(N-nBu-Im)2-Ph]2Ni2(NCMe)2 k3-[C,C0 ,C00 -[2,6-(N-nBu-Im)2-Ph]2Ni2(NCMe)2][BPh4] k3-[C,C0 ,C00 -[2,6-(N-nBu-Im)2-Ph]2Ni2(NCMe)2][PF6]2 k3-[C,C0 ,C00 -[2,6-(N-nBu-Im)2-Ph]Ni(NCMe)][PF6] k3-C,C0 ,C00 -[2,6-(N-nBu-Im)2-Ph]NiCl [k3-C,C0 ,C00 -[2,6-(N-nBu-Im)2-Ph]Ni(MeCN)][PF6] k3-C,C0 ,C00 -[Ph(N-Dipp-BzIm)2]NiCl3 k3-C,C0 ,C00 -[Ph(N-Dipp-BzIm)2]NiBr k3-C,C0 ,C00 -[Ph(N-Dipp-BzIm)2]NiBr3 k3-C,C0 ,C00 -[2,6-(N-Dipp-N-BzIm)2-Ph]NiCl k3-C,C0 ,C00 -[2,6-(N-Dipp-N-BzIm)2-Ph]NiH k3-C,C0 ,C00 -[2,6-(R-5-Ph-6,8-H2-5H-[1,4]oxazino-[1,2,4]triazol-2-yl)2-Ph]NiCl k3-C,C0 ,C00 -[Ph(N-Dipp-BzIm)2]NiMe k3-Ge,C,Ge0 -[2,6-(OGe(Z3-N(tBu)C(Ph)N(tBu)))2-3,5(tBu)2-Ph] Ni(CC(1,6-Ph2-Ph) k3-Ge,C,Ge0 -[2,6-(OGe(Z3-N(tBu)C(Ph)N(tBu)))2-3,5-(tBu)2-Ph]NiBr k3-Si,C,Si0 -[2,6-(OSi(N(tBu)C(Ph)N(tBu)))2-3,5(tBu)2-Ph]NiI k3-Si,C,Si0 -[2,6-(OSi(N(tBu)C(Ph)N(tBu)))2-3,5(tBu)2-Ph]NiBr k3-Si,C,Si0 -[2-(OSi(N(tBu)C(Ph)N(tBu)))2-6-(OSi(N(tBu)C(Ph)N(tBu)CuBr))-3,5 (tBu)2-Ph]NiCCPh k3-C,C0 ,N-[2-EtNHC-6-(C]NCH(tBu)CH2N(p-Tol))-Ph]NiCl k3-P,C,S-[o-(PPh2)(C(H)P(S)Ph2)-Ph]NiCl k3-P,C,S-[o-(PPh2)(C(H)P(S)Ph2)-Ph]NiOPh k3-P,C,S-[o-(PPh2)(C(H)P(S)Ph2)-Ph]NiN(H)(p-MePh) k3-P,C,S-[o-(PPh2)(C(H)P(S)Ph2)-Ph]NiNH2 k3-N,C,N0 -[N,N0 -(PhC]NDipp)2-1,3-C4N2H7]NiCl k3-P,Sb,P0 -[Sb(1,2-O2-Ph)(2-PPh2-Ph)2]Ni(2-PPh2-Ph) k3-P,N,N0 -[N(8-quin)(C(O)CN(H)PPh2)]NiPh k3-P,P0 ,P00 -[PPh(o-P(p-Tol)2-Ph)2]NiI(p-CF3-Ph) k3-C,C0 ,O-[2-N(BuNHC)-6-(BuNHC)2(2-O-3-BuNHC-Ph)-Ph]Ni(NCMe) k3-P,O,P0 -[O-(2-C(CH3)2-6-PPh2-Ph)2Ni(o-Tol)Cl k3-P,N,C-[2-OPtBu2-6-(o-Ph)-Py]NiCF3 k3-P,N,C-[2-OPtBu2-6-(o-Ph)-Py]NiMe k3-N,N0 ,N00 -[N(o-(5-iPr-ox)-Ph)2]NiPh [Li(O]PPh3)4][k3-N,N0 ,C-[1-(5-CH2iPr-ox)-2-(NCH(Me)(o-Ph))-Ph]NiCl] [k3-P,P0 ,P00 -[PC6H5(C2H4PPh2)2]NiMe][OTf] k3-P,C,N-[Ph(OPiPr2)(CH]NDipp)]NiMe k3-P,C,N-[Ph(CH2PtBu2)(CH2NMe2)]NiMe k3-P,N,C-[NC5H3(OPtBu2)(o-Ph)]NiMe k3-P,N,N0 -[N(C2H4NMe2)(o-Ph-PPh2)]NiMe k3-P,B,P0 -[N,N-c-BN2Ph(CH2PtBu2)2]NiMe

1.870(2)87 1.876(4)87 1.8790(14); 1.8839(14)183 1.963184 1.956(2)184 1.953184 1.886(4)185 1.900(5)186 1.882(7)186 1.900(11)186 1.901(5)186 1.921(10)186 1.947(5)187 1.941(5)188 1.905(8)186 1.900(3)189 1.886(6)189 1.834(4)32 1.893(2)190 1.9481(14)191 1.874(2)80 1.855(1), 1.855(1); 1.857(2), 1.857(2)192 1.855(1), 1.855(1)192 1.854(4), 1.860(4)192 1.859(4)192 1.854(3)193 1.854(2)193 1.8504(16)194 1.8528(18)194 1.8994(14)194 1.8504(16)195 1.873(4)195 1.825(11)196 1.98(3)195 1.985(3); 1.970(3)82 1.960(3)82 1.927(1)82 1.927(2)82 1.939(4)82 1.860(6)197 1.9735(2)198 1.9657(13)198 1.9797(15)198 1.995(4)198 1.848(5)199 1.966(3)200 1.895(2)201 1.944(9)202 1.919(2)192 1.887(2); 1.898(2)203 1.944(3)204 1.933(4), 1.943(4)204 1.910(2)176 1.887(5)205 1.963(9)206 1.959(4)129 1.983(2)147 1.933(4)204 1.979(5)207 2.059(2)208

BArF4, tetrakis(3,5-trifluoromethyl-phenyl)borate; quin, quinoline; terpy, terpyridine; tripp, 2,4,6-triisopropylphenyl; BzIm, benzimidazole; dme, 1,2-dimethoxyethane; OTf, trifluoromethane sulfonate.

Nickel-Carbon s-Bonded Complexes

Ph

tBu tBu

N O E N

tBu

tBu NiBr2(dme) or Ni(cod) 2

X tBu

tBu O E N N tBu Ph

THF, reflux 4 h

4 Si: yellow 5 Ge: red

N O E N

hexyl

tBu

53% yield

tBu

NiII Br

E = Si or Ge

Ph

Ph

tBu

287

tBu

O E N N tBu Ph 6 Si: 70% yield, yellow 7 Ge: 57% yield, red

7 (5-10 mol%) CuI (5 mol%) PhCCH + 5 equiv hexyl

I

XRD, 1H NMR, 13C NMR, 29Si NMR, APCI-MS, EA

Fig. 8 A pincer ligand scaffold featuring flanking tetrylene units forms a nickel bromide complex that catalyzes a Sonogashira coupling.

Table 7

Tripodal nickel aryl and alkyl complexes. N

E

N N

N

N N

Ni

R” R’

R

E R R0 Ph B CF3 B CF3 Ph B CF3 CF3 k3-N,N0 ,N00 -[Tpz]Ni(2,20 -biphenyl) k3-N,N0 ,N00 -[Tpz]Ni(biphenyl)(OC(O)CF3) k3-N,N0 ,N00 -[Tpz]Ni(biphenyl)(CF3) k3-N,N0 ,N00 -[Tpz]Ni(biphenyl)Cl [k3-N,N0 ,N00 -[C(H)Pz3]Ni(Py)(o-PhCMe2CH2)][BF4] [k3-N,N0 ,N00 -[Tpz]Ni(MeCN)(o-PhCMe2CH2)][BF4] [k3-N,N0 ,N00 -[C(H)Pz3]Ni(Py)(o-PhCMe2CH2)][BF4] [k3-N,N0 ,N00 -[(MeNCH2CH2)3]Ni(o-CMe2CH2-Ph)][PF6] [k3-N,N0 ,N00 -[(MeNCH2CH2)3]Ni(MeCN)(o-CMe2CH2-Ph)][BF4] k3-N,N0 ,N00 -[Tpz]Ni(o-CMe2CH2-Ph) k3-N,N0 ,N00 -[Tpz]Ni(CF3)(o-CMe2CH2-Ph) [k3-N,N0 ,N00 -[HC(2-Py)3]Ni(CF3) (o-CMe2CH2-Ph)][OTf] k3-[N,N0 ,N00 -[C(H)Pz3]Ni(2-(C(Me2)CH2)Py][BF4] k3-[N,N0 ,N00 -[Tpz]Ni(2-(C(Me2)CH2)(NCMe)][BF4] k3-P,P0 ,P00 -[B(Ph)(CH2PPh2)3]Ni(Z2-Bn) k3-N,C,N0 -[CH2CMe2CF(2-Py)2]NiCl(CF3)2 k3-N,C,N0 -[CH2CMe2CF(2-Py)2]NiOTf(CF3)2 k3-N,C,N0 -[CH2CMe2CF(2-Py)2]NiF(CF3)2 k3-N,N0 ,N00 -[HB(2-Me-4-Ph-Pz)3]NiCH2SiMe3 [k4-C,N,N0 ,N00 -[HC(S(2-Py))3]NiCl]2 [k4-C,N,N0 ,N00 -[HC(S(2-Py))3]NiBr]2 [k4-C,N,N0 ,N00 -[HC(S(2-Py))3]NiBr]2[PF6]2 [k4-C,N,N0 ,S-[HC(S(2-Py))2(2-S-N-Py)]NiBr][PF6] k4-C,N,N0 ,N00 -[HC(S(2-Py))3]NiCl(S(2-PyH)) [Ph3P]N]PPh3][k4-P,S,S0 ,S00 -[P(2-S-3-SiMe3-Ph)3]NiMe] [Ph3P]N]PPh3][k4-P,S,S0 ,S00 -[P(2-S-3-SiMe3-Ph)3]NiEt] k4-C,N,N0 ,N00 -[C(SiMe2(N-iPr-N-BzIm))3]NiBr [Ph3P]N]PPh3] [k4-P,S,S0 ,S00 -[P(2-S-3-SiMe3-Ph)3]NiCH2CN]

R00 PMe3 N/A Ph

1.950(3)210 1.917(5)210 2.0002(16)211 1.946(2), 1.941(2)210 1.970212 1.965212 1.952, 1.952213 1.985(3)214 2.031(5)214 1.985(3)214 1.9755(9)215 2.032(9)215 1.966(6), 1.963(4)210 2.006(2), CAr 1.952(2), CCF3 1.956(2)216 2.006(2)216 1.935(3)214 1.952(5)214 2.226(3)217 2.022(2)218 2.000(3)218 1.998(3)218 1.9995(18)219 2.070(2)220 2.078(4)220 2.023(8)220 1.950(4)220 2.081(3)221 1.994(3)222 2.015(3)222 2.2197(16)223 2.037(3)224

pz, pyrazole; Tpz, tris(pyrazolyl)borate; py, pyridine.

8.05.2.4 8.05.2.4.1

Common reactivity Migratory insertion

The total anthropogenic CO emissions in 2011 was 604.64 Tg, primarily from vehicular consumption and industrial processes, such as the production of lime, cement, petroleum cracking, iron, steel, and ammonia.227 Industrial synthesis incorporating CO is limited to the Fischer-Tropsch process for gas-to-liquid (GTL) fuel conversion and the aforementioned production of methanol, supplied as a reductant of metal catalysts of other industrial scale reactions.228

288

Nickel-Carbon s-Bonded Complexes

Ph P Ph N

NiII

Ph P Ph

CO

N

P Ph

NiII

Me Ph

8 red

Ph P Ph

Me N

O

P Ph

O CO

Ph

CO

CO

P

Me Ph

Ph 10 yellow 75% yield

9 yellow 92% yield

XRD, 1H NMR, 13C NMR, 31P NMR, EA

Ni0

Fig. 9 Carbonyl insertion into the nickel-methyl bond of a pincer nickel complex followed by reductive elimination to form a CdN bond.

A number of pincer nickel alkyl complexes can undergo CO insertion to generate nickel acyl complexes.71 A monoanionic diphenylamido-based PNP nickel methyl complex 8 undergoes CO insertion to form the nickel acyl complex 9 (Fig. 9).207 Furthermore, the complex proceeds to reductively eliminate the acyl-amido bond to form the amide product 10 despite the substituents being trans to each other in complex 9. Carbon dioxide is a greenhouse gas with annual emissions in the United States topping 5000 MT.229 It also serves as an abundant C1-building block that is used in industrial synthesis on the scale of 110 MT per year.230 The growing uses of carbon dioxide for the synthesis of organic chemicals has expanded to include the carboxylation of unsaturated species, organic carbonates, methylation reactions, and carbamates. In order to inform catalysis, nickel-mediated CO2 insertion has drawn considerable attention. In this regard, CO2 insertion has been studied with a variety of nickel complexes include hydrides, alkyls, amides, allyls, and hydroxyls, to generate their corresponding nickel carboxylates or carbonate in the case of the nickel hydroxyl.10,12,44,103 The stabilization of the PC(sp2)P ligand allows for the isolation of nickel-methyl, nickel-allyl, and nickel-hydride complexes and their reactivity with CO2 was investigated (Fig. 10).44 While many PC(sp2)P nickel alkyl complexes are stable in air, the tert-butyl derived nickel hydride 11, methyl 12, and Z1-allyl 13 readily react with CO2 to afford the corresponding carboxylates.12 DFT calculations reveal that the three nickel complexes react with CO2 via distinct mechanisms. Nickel-hydride 11 coordinates CO2 and conducts hydronickelation via a four-membered transition state to form the nickel formate; complex 12 activates the CO2 via nucleophilic attack of the methyl group to CO2 in an SN2 fashion; with respect to the nickel-allyl 13, it is C3 of the allyl group that attacks CO2. Thus far there has been no observation of CO2 insertion to Ni-aryl complexes.231

8.05.2.4.2

s-Bond metathesis

CdH activations can lead to sustainable functionalization of organic molecules and has been extensively studied with precious metal catalysts, such as palladium and iridium.232 First row transition metals have shown limited ability to activate any CdH bonds.233 In general, nickel is substantially less reactive with regards to CdH activation compared to palladium. Therefore, high temperature is usually required for such catalytic reactions.234 A few stoichiometric Ni-mediated CdH activation reactions have been reported. tBu P

P CO2

NiII

NiII

H P tBu

tBu

tBu tBu

tBu O

NiII

H

P O tBu tBu

tBu

P

tBu

H

O P

C

tBu

O

tBu 14

11 XRD, 1H NMR, 13C NMR, 31P NMR, EA, IR tBu

tBu P

P

tBu CO2

NiII Me P tBu

tBu

NiII P tBu tBu

12 XR D, 1H NMR, 13C NMR, 31P NMR, EA tBu P tBu CO2 II Ni P tBu

tBu

tBu tBu

H C H H

P

O

O P

O tBu

tBu

15

tBu

tBu P

P

tBu

O P

P tBu tBu O

Fig. 10 Calculated transition states of CO2 insertion by pincer nickel complexes.

tBu O

NiII

NiII

13 XR D, 1H NMR, 13C NMR, 31P NMR, EA

tBu O

NiII

tBu O

tBu 16

Me

Nickel-Carbon s-Bonded Complexes

iPr

iPr P N

iPr

P

AlMe3, C6H6 N

NiII

NiII

100 °C

iPr

Ph

Me P

P iPr

289

iPr

iPr 17 red

18 yellow quantitative

iPr

XR D, 1H NMR, 13C NMR, 31P NMR, EA

Fig. 11 Benzene activation by a pincer nickel alkyl complex.

A PNP nickel methyl complex 17 can activate benzene in the presence of Lewis acids such as AlMe3 and B(C6F5)3 (Fig. 11).111 When this reactivity was extended to toluene, only the m-tolyl and p-tolyl nickel complexes were observed, demonstrating a preference for C(sp2)dH over C(sp3)dH bonds. The byproducts of the reaction were methane and dimethylaluminium hydride; hydrogen gas was not detected. Metal thiolates are ubiquitous in biochemistry both for their electron transfer abilities and their involvement in alkylation reactions.235 PC(sp2)P nickel thiolate 19 undergoes s-bond metathesis with HdB(cat) (cat ¼ catechol) to afford (PCP)NidH 20 and (cat)BdSPh (Fig. 12).21

8.05.2.4.3

Reductive elimination

Directing groups have frequently been employed to enhance the selectivity of cross-coupling reactions, but they also inhibit the rotation of the metal-carbon bond to achieve their coordination geometry, from which CdC bond forming reductive elimination can occur via a concerted three-membered transition state.236 In order to investigate the reductive elimination process of nickel-catalyzed directed cross-coupling reactions, (PNC)Ni 21 was synthesized via cyclometallation when binding to nickel (Fig. 13).204 The coordinated aryl tethered to the pincer ligand can be reversibly protonated using HBF4. The oxidation of 21 by ferrocenium results in the reductive elimination to form the methylated aryl Ni(I) product 22. In addition to oxidation, addition of coordinating ligands and elevated temperature can promote reductive elimination as well (Fig. 14). (PPC)Ni 23 proceeds to reductively eliminate the methylated arene when promoted by heating and external triphenylphosphine.181 While alkyl derivatives undergo reductive elimination, amido and alkoxy derivatives instead undergo b-hydride elimination. O

P

iPr iPr

O O +

NiII O

benzene-d6

H B

SPh

O

NiII

RT

O

P

19 orange XRD, 1H NMR, 13C NMR, 31P NMR, EA, HRMS iPr

P

iPr

iPr

P

iPr iPr

O +

PhS B O

H

20 iPr orange-yellow

XRD, 1H NMR, 13C NMR, 31P NMR, EA, HRMS

Fig. 12 s-bond metathesis of nickel thiophenol with HBcat.

O PtButBu NiII Me

O PtButBu NiII Me Me

[Fe(η1-C5H4C(O)Me)(η5-C5H5)][BF4]

+ BF4

21 XRD,

1H

22

NMR, 13C NMR, UV-Vis

Fig. 13 Oxidation induced reductive elimination.

Me tBu

P P tBu

PPh 3

NiII

tBu

Me

50 °C, 48 h C6D6

23 yellow XR D, 1H NMR, 13C NMR, 31P NMR, EA Fig. 14 Base-induced reductive elimination.

tBu

P

Ni0

PPh 3

P tBu

tBu

24 red-brown 88% yield 1H NMR, 13C NMR, 31P NMR, HRMS

290

8.05.2.5

Nickel-Carbon s-Bonded Complexes

Nickel alkylidene complexes, ligand redox-activity, and metal-ligand cooperativity

Ligands were initially considered simple point charges with ionic bonding to an electropositive metal, but modern molecular orbital theory helped to define the covalent bonding between a metal and its ligands.237 The introduction of orbital symmetry aided in the explanation of p-accepting ligands. Recently, ligand design concepts such as metal-ligand cooperativity and redox-active ligands have come to the forefront. These principles rely on the localization of the frontier orbitals onto the ligands rather than the metal. Metal-ligand cooperativity describes the phenomenon in which both the metal and the ligand participate in either a bond formation or bond cleavage process.238,239 The common ligand motifs that allow for the cooperativity are metal ligand multiple bonds, ligands with remote lone pairs or vacant p-orbitals, and metal bound pyridyl systems vulnerable to dearomatization. Common substrates for bond activation are H2, CO2, alcohols, amines, boranes, and silanes.240,241 This strategy can introduce new steps into the catalytic cycle and may increase the stability of the catalyst when the metal-based intermediate is more susceptible to decomposition than the ligand-based intermediate. Among the recent advances in this area is the incorporation of low-valent main group elements into ligand scaffolds.209 A difluorocarbene nickel complex 26 can be synthesized via fluoride abstraction (Fig. 15).77 A trifluoromethyl group is initially introduced to a (POCsp2OP)NiCl complex via Me3SiCF3. While fluoride abstraction can be accomplished with reagents such as Me3SiOTf and BF3OEt2, the use of B(C6F5)3 allows for a more stable complex, presumably due to the steric bulk of the anion. This allows for exploration of the reactivity of the nickel difluorocarbene complex, wherein the LUMO of the complex is determined to be at the carbene resulting in the formation of the pyridine adduct 27. Metal-ligand cooperativity has been introduced through a variety of mechanisms, including proton and electron transfer.242 Another less common mechanism is the 1,2 addition across a metal ligand multiple bond (Fig. 16). Traditionally, these systems have been stabilized through a rigid ligand donor environment such as a pincer ligand with flanking phosphines and a central metal-element double bond or through charge delocalization by having the ligand scaffold assist in the polarization of the metal-element double bond.243 These strategies were both implemented in the isolation of a stable nickel alkylidene complex 29 (Fig. 17).198 Initially the secondary nickel alkyl complex 28 was formed, however upon deprotonation with NaOtBu, the nickel alkylidene was achieved. This complex undergoes the 1,2 addition of ammonia, phenol, and p-toluidine. The ligand cooperative reversible activation of ammonia is an important step for the utilization of ammonia in a ligand cooperative catalyst system. The PC(sp3)P nickel alkylidene species 32 reacts with a variety of small molecules across the Ni]C

O NiII O

iPr P iPr

iPr

P

B(C 6F 5) 3 CF 3

P

iPr

O NiII

CH2Cl2

O

25 yellow

iPr pyridine

O

FB(C 6F 5) 3

CF2

CH2Cl2

P

iPr

iPr

iPr F F N

FB(C 6F 5) 3

iPr

27 yellow-orange

26 orange 98% yield XRD, 19F NMR

XRD, 1H NMR, 19F NMR, 31P NMR

P

NiII

P

iPr

iPr

O

iPr

19F

NMR, 31P NMR

Fig. 15 Synthesis and reactivity of a difluorocarbene nickel complex.

A R P R

Ni

A B

R

P

R P R

R

Ni B

P

R R

Fig. 16 Ligand cooperative bond activation by nickel alkylidenes.

Ph Ph

H

P S

NiII Cl

Ph

Ph Ph P(C H3) 3, NaOtBu P

Ph Ph

28 XRD, 1H NMR, 13C NMR, 31P NMR, HRMS, EA

P S

THF

Ph NiII

NH3 P

Ph P(C H3) 3 Ph

THF, -78 °C

29 red 64% yield

XR D, 1H NMR, 13C NMR, 31P NMR, HRMS, EA

Fig. 17 Metal-ligand cooperativity in the addition of ammonia and hydrogen to nickel pincer complexes.

H

P S

NiII NH2

P

Ph Ph

30 brown 32% yield XR D, 1H NMR, 13C NMR, 31P NMR, HRMS, EA

Nickel-Carbon s-Bonded Complexes

H

H

iPr NiII

P iPr

P

iPr

H2 (1 atm)

iPr

C6D6

P

H

iPr

NiII

iPr

iPr

NH3 (1 atm)

iPr

NiII

P

C6D6 iPr

PPh 3

iPr

31 brown quant. yield

P

291

iPr

P

iPr

NH2

33 yellow quant. yield

32 brown

XR D, 1H NMR, 13C NMR, 31P NMR, HRMS, IR, EA

XR D, 1H NMR, 13C NMR, 31P NMR

Fig. 18 Hydrogen and ammonia insertion into Ni¼C bonds.]C

double bond, including hydrogen, alkynes, alcohols, and ammonia (Fig. 18).98 The ammonia complexation in complex 33 was reversible at 1 atm. This reactivity of 1,2-addition can be extended to silanes using a PCP-based alkylidene complex, although the polarity of the SidH bond results in the silyl group adding to carbon and the hydride to nickel. There are few metal complexes that can add to the E–H (E ¼ B, N, O, Si) bonds reversibly. The alkyl-backboned PC(sp3)P nickel amide complex undergoes a monomer-dimer equilibrium (Fig. 18).101 The amide complex 33 is synthesized by the addition of ammonia to a nickel diphenylcarbene complex 32 decorated with diisopropyl phosphine substituents, which coordinate to the nickel atom. The dimer is the result of the lability of the nickel-coordinated phosphine, which is replaced by a bridging interaction with a neighboring amido group. The NdH insertion into the nickel-carbon bond is determined to be reversible via the incorporation of deuterium upon reaction with ND3. The reactivity of CO, CO2, and their sterically hindered isocyanide analogs isocyanide and isocyanate, respectively, with PC(sp3)PNi were also investigated.101 CO2 and the tert-butyl isocyanate undergoes a 2 + 2 cycloaddition to the Ni]C double bond 35 (Fig. 19). However, both the CO and the tertbutyl isocyanide cleave the Ni]C double bond to form a ketene product 34. All the reactions perturb the square planar geometry of the nickel center. The PNP ligand of 37 can be deprotonated, resulting in anion 38 (Fig. 20). The backbone can nucleophilically attack CO2 to afford 39.119 In another example, a (phosphine-amido-phosphine)Ni complex demonstrates metal-ligand cooperativity through the protonation of the amide with Brønsted acids.109

O

O

O

C

C

P

Ni0 P P

O

iPr CO (1 atm)

iPr iPr

iPr

NiII CO2 (1 atm)

NiII N

THF

P

iPr

iPr

C

tBu

P

THF P

iPr

iPr

N C tBu iPr iPr

iPr iPr

35 orange

34 yellow-brown 57% yield XR D, 1H NMR, 13C NMR, 31P NMR, IR, EA

36 red

XRD, 1H NMR, 13C NMR, 31P NMR, EA

1H

NMR, 13C NMR, 31P NMR

Fig. 19 Metal-ligand cooperativity in the addition of CO and CO2 to a nickel pincer complex.

O

H

tBu P

N

NiII

MeLi

P tBu

tBu

37 orange XRD, 1H NMR, 13C NMR, 31P NMR, EA

tBu

tBu

tBu Me

O

P

N

Et2O

NiII

tBu

CO2

Me

tBu

38 red 60% yield XRD, 1H NMR, 13C NMR, 31P NMR, EA

Fig. 20 Ligand-centered CO2 activation in a pincer nickel methyl complex.

NiII

tBu Me

P

P tBu

P

N

tBu

tBu 39 yellow

XRD, 1H NMR, 13C NMR, 31P NMR, EA

292

Nickel-Carbon s-Bonded Complexes

tBu

N

tBu

P tBu

N B

NiII

N B

H2 (2 bar) N C6D6 70 ºC, 12h

Me

tBu P

P tBu

P tBu

N B

N

Ni0

tBu P tBu

CH4

tBu

H

tBu

NiII

H

tBu P tBu

40 yellow XRD, 1H NMR, 13C NMR, 31P NMR, 11B NMR, EA

41 yellow 90% yield XRD, 1H NMR, 13C NMR, 31P NMR, 11B NMR, IR, EA

Fig. 21 Boryl-assisted activation of dihydrogen by nickel.

The development of NHC ligands as strong s-donors and mild p-acceptors has been utilized throughout synthetic inorganic chemistry and organometallic catalysis.244,245 In 2006, a boryl anion was introduced by Yamashita and Nozaki,246 and its electronic configuration is isolobal to that of an NHC although it is an X-type ligand. A boryl anion-based pincer ligand was applied to nickel, where the methyl derivative 40208 reacts with hydrogen to produce methane and a nickel hydride complex 41 (Fig. 21). Upon addition of cyclooctadiene, the hydride transfers to the boron while the nickel is coordinatively saturated by the cyclooctadiene, suggesting the intermediacy of a BdH agostic interaction with nickel before the nickel hydride formation. The electronic effects of the boryl pincer ligand were particularly pronounced, resulting in a downfield 13C NMR signal of 8.94 ppm for the methyl carbon. Nickel(0) compounds can be added to imidazolium salts to generate carbene complexes through CdH oxidative addition.247 In the case of the bis(imino)-functionalized imidazolium ligand, the nickel(0) only oxidatively adds to the carbon and chloride anion to afford Ni-alkyl complex 43 (Fig. 22).107 The CdH bond of 43 is activated and can insert into ethylene to form complex 44. The (POCsp3OP)Ni siloxy complex 45 undergoes CO insertion to the backbone carbon to form a carboxylate bridged dimer 46 (Fig. 23).95 The ligand can decompose to a nickel p-allyl complex 47 via CdO bond cleavage. This decomposition pathway is more pronounced in the presence of O2, KOtBu, or KN(SiMe3)2.

8.05.2.6

Biomimetic (pincer)Ni complexes

Transition-metals play an important role in biological processes. The desire to identify and investigate the role of transition metals in biology has spurred the synthesis of inorganic analogs of metalloenzyme active sites, which perform with similar efficacy as the metalloenzymes themselves.248 Their synthesis has, in many cases, required the development of novel synthetic strategies and

N

N

Dipp

N Ni(cod) 2

H

N

Cl

N

N

H

N

NiII

N

42

H

Dipp

Cl

N

toluene

Dipp 43 orange-red 62% yield

Dipp

N NiII

Cl

N

pentane

N

Dipp

Dipp 44 red 80% yield

XR D, 1H NMR, 13C NMR, EA

XR D, 1H NMR, 13C NMR, EA

Fig. 22 Olefin insertion through the alkyl backbone of a pincer ligand.

O iPr

P iPr

O iPr P II iPr Ni OSiMe3

O CO

iPr

toluene RT, 10d

45 yellow-orange XRD, 1H NMR, 13C NMR, 31P NMR, EA

O iPr P iPr NiII O O

P iPr

O iPr O II P iPr P Ni iPr iPr OC CO

46 yellow 14% yield

Fig. 23 Reaction of CO with a PC(sp3)P nickel siloxy derivative.

iPr

1H

XR D, NMR, NMR, EA

31P

13C

+

NiII iPr

NMR,

iPr P O

O P iPr

iPr NiII

iPr P P

O O iPr

47 green

NiII iPr

Nickel-Carbon s-Bonded Complexes

293

ligand designs. Several important metalloenzymes possess active sites with nickel, including NiFe hydrogenase, acetyl-CoA synthase, methyl-coenzyme M reductase, and lactate racemase.4 It is paramount to identify and characterize the intermediate nickel states during enzymatic processes. However, the limited accessibility of nickel Mossbauer, the difficult discernment of oxidation states by XAS and EPR, and the existence of multiple commonly diamagnetic oxidation states make analytical characterization problematic.249 Carbon monoxide dehydrogenase is a metalloenzyme that governs the transformation of carbon monoxide to carbon dioxide and the reverse reaction.249,250 The anaerobic enzyme is nickel dependent and helps organisms when deciding whether to produce energy in the form of CO or utilize CO2 as a C1 source. A proposed active site of this enzyme contains a square planar nickel complex. Therefore, investigation into both the CO and CO2 insertion chemistry at square planar nickel is important to understanding the possible chemistry taking place in the enzyme. (PPP)Ni(0) complex 48 is a mimic of the nickel cofactor in carbon monoxide dehydrogenase.206 The phosphine ligands are representative of soft bridging thiolato ligands found in the carbon monoxide dehydrogenase active site for nickel. In the enzymatic process it is known that methyl cobalamin transfers a methyl group to the nickel site. This reaction is plausible with 49 to generate nickel(II) methyl 50 (Fig. 24). The nickel(0) complex 48 acts as the nucleophile in this SN2 reaction with methyl cobaloxime 49, and upon addition of carbon monoxide the methyl transfer is not observed. This contrasts with the analogous methyl cobaloxime reaction with nickel tetramethylcyclam, which was found to occur through an electron transfer mechanism.251 Lactate racemase is an enzyme that governs the transformation between benzyl alcohol and benzaldehyde.252 The enzyme active site 52 includes a nickel atom bound in a pincer conformation with two flanking sulfides and a central pyridyl group. The enzyme moves through a redox cycle including the dearomatization and rearomatization of the central pyridine ring as a means of hydrogen transfer. A biomimetic nickel pincer complex 53 exhibits similar dehydrogenation activity, oxidizing benzyl alcohol to benzaldehyde (Fig. 25).167,168

8.05.2.7

Selected catalytic reactions

The nitrile functional group is commonly incorporated into modern pharmaceuticals.253 An important goal is the installation of nitrile groups to molecules using less toxic reagents than the traditional cyanide salts. The air- and moisture-stable complex 55 can catalyze the cyanomethylation of aldehydes (Fig. 26).22 The nickel-alkyl bond is sufficiently polarized to allow for addition into benzaldehyde. Furthermore, the vacancies at the apical sites allows coordination of an acetonitrile, lowering its pKa to allow for deprotonation by a nickel-alkoxide. Derivatization of the PCP framework by introducing a different connecting group between the carbon and phosphorus binding positions was critical in the development of Guan’s cyanomethylation catalyst 55.22 The stability conferred by the PCP ligand framework prevents catalyst degradation, while the oxygen bridge between the carbon and phosphorus binding positions enhances the reactivity of the nickel complexes with regards to the cyanomethylation of aldehydes.95 Hydrogen is a commodity synthetic precursor to ammonia, hydrogen peroxide, methanol, and saturated oils.254 Efficient electrolysis of water to hydrogen and oxygen is important for sustainable energy conversion. However, its commercial application F2 B

Ph Ph

PH Ph

P

+

Ni0

PPh 3

P

N O

Ph

Ph

O N

48 orange

Ph

P

CD3CN/ C6D6

P NiII

P Ph

F2 B

+

Ph

O CH3 N Co N Py O B F2 49 yellow

O N

Ph +

Co N O

CH3

O N N O

B F2 51 blue quantitative

Ph

50 yellow quantitative XRD, 1H NMR, 31P NMR, ESI -MS

Fig. 24 A triphosphine pincer nickel complex undergoes a methyl transfer reaction with a methyl cobaloxime.

Arg75 O HO

P

I

N

O N Lys184

O HO

NiII NH N

N NiI O

+ Cl

S

SH

HO 52

S

S

O

S

53 black XRD, 1H NMR, 13C NMR, HRMS

OH

O DBU (1 equiv), CH3CN 100 °C, 10 h

+

H

S

N

N N

64% yield 21% yield 54

Fig. 25 The structure of the lactate racemase active site and the stoichiometric dehydrogenation of benzyl alcohol by a biomimetic pincer nickel complex.

294

Nickel-Carbon s-Bonded Complexes

55 (0.01 mol%) O

O

OH

+ MeCN

CN

P

iPr iPr

NiII

MeCN, 72h RT

O 83% yield

C

N

P

iPr

iPr

55

XRD, 1H NMR, 13C NMR, 31P NMR, IR, EA 2

Fig. 26 Catalytic cyanomethylation of aldehydes by a (PC(sp )P)Ni alkyl complex.

tBu P tBu

electroreduction H2O

56 (0.2 mM)

-0.6 V vs. NHE, TOF: 209 rate: 1.045 M s-1

PF 6-

NiII

H2

N Me

P tBu

s-1,

tBu

56

XR D, 1H NMR, 13C NMR, 31P NMR, FT-ICR, MS Fig. 27 Electrocatalytic hydrogen evolution by a pincer nickel complex.

is limited by the necessity of precious metal catalysts such as platinum and iridium. Pincer nickel complex 56 can serve as an electrocatalyst for hydrogen evolution (Fig. 27).70 Notably, the reduction was determined to be metal centered, in contrast to previous investigations into nitrogen based ligand systems, suggesting that ligand redox-activity is nonessential for hydrogen evolution. The overpotential of −0.345 V, however, suggests that improvement is needed for this nickel catalyst to be competitive with precious metal catalysts. Polysilanes are inorganic polymers containing silicon-based backbones, widely applied as precursors to silicon carbide.255 The difference between the aryl PC(sp2)P nickel siloxide 57 and the Csp3 PCP nickel siloxide results in the formation of different polymerization products with phenyl silane (Fig. 28).33 The aryl PCP complex 57 is limited to the production of dimers and redistribution products, while the Csp3 PCP nickel catalyst 59 forms polysilanes. Mechanistic investigation suggests that the Csp3 PCP nickel catalyst operates through a nickel(0) intermediate. Hydroboration and hydrosilylation are both useful methods for selectively reducing alkenes or carbonyl groups.256 Recently, first-row transition metals have been applied to hydroboration and hydrosilylation. A hybrid iminophosphonite pincer ligand that includes phosphorus, carbon, and nitrogen chelation sites supports complex 60, which serves as an efficient hydroboration catalyst

O

P

iPr iPr

O PhSiH3 (3 equiv)

NiII O iPr

OSiMe3

NiII O

toluene

P iPr

iPr

57 yellow

P

iPr iPr

O PhSiH3

H

P

8d

iPr

P

iPr iPr

NiII O iPr

SiPhH2 P iPr yellow 21% yield

58 orange-yellow

XR D, 1H NMR, 13C NMR, 31P NMR, EA XRD, 1H NMR, 31P NMR, EA

XRD, 1H NMR, 13C NMR, 31P NMR, EA O

P

iPr iPr PhSiH3 (excess)

NiII O iPr

59 (cat.)

OSiMe3

- H2, RT

P iPr 59 yellow

XRD, 1H NMR, 13C NMR, 31P NMR, EA

Fig. 28 PC(sp3)P nickel siloxides catalyzes the polymerization of organosilanes, whereas PC(sp2)P nickel siloxides do not.

Ph Si H

n

Nickel-Carbon s-Bonded Complexes

N

Dipp

NiII

O iPr

P

295

Me

O

60 (5 mol%) HBPin (4.2 equiv)

N

O

N

O

iPr

OBPin C6D6 RT 1h

60 dark red

> 99% yield

XRD, 1H NMR, 13C NMR, 31P NMR, EA Fig. 29 Hydrosilylation and hydroboration reactions catalyzed by PC(sp2)P nickel complexes.

of succinimide, although Ni(cod)2 is generally more effective than the nickel pincer complex (Fig. 29).129 Mechanistic investigations suggests that the imino group of the pincer ligand is hemilabile.

8.05.3

Organonickel(II) complexes stabilized by bidentate ligands

Nickel alkyl and aryl complexes coordinated by bidentate ligands constitute a plurality of the genre. They have found widespread utility amongst olefin polymerization and cross-coupling reactions. They also serve as platforms for catalysis mechanistic investigations and as synthetic precursors for other nickel complexes. The identity of the chelating atoms has a significant impact on the electronic properties of the nickel complexes as well as the general stability. For example, oxygen ligands tend to be more labile. Many classes of bidentate ligands have been used to stabilize nickel aryl and alkyl complexes (Tables 8–15). These ligand classes can be categorized into those with neutral ligands and with monoanionic ligands. The bis(phosphine) ligand scaffold has stabilized Table 8

Monoanionic Schiff base Nickel Aryl complexes. ˚) Ni–C (A

Complex

R'

R"

N

R2

O

R Ni

L

R1 R Ph Ph Ph Ph Ph Ph Ph Ph Ph 2-naph o-Tol o-Tol Ph Ph Ph 2-naph 2-naph Ph Ph Ph Ph Ph Ph Mes Ph naph naph

R0 Dipp Dipp Dipp 3,5-(CF3)2-Ph Dipp 2-naph 2-naph Ph 3,4,5-F3-Ph 2,6-Br2-4-Me-Ph 2-bpy(ZnCl2) 2-bpy 2,6-Me2-Ph Mes Mes 2,6-(C6F5)2-4-Me-Ph 2,6-(30 ,40 ,50 -F3-Ph)2-4-Me-Ph Dipp Dipp Dipp Dipp Dipp Dipp Dipp Dipp Mes 2,6-Et2-Ph

R00 Me Me H H H H H H H H H H H H H H H H H H H H H H H H H

R1 H 2-naph tBu I I H H H H Cl tBu anth tBu tBu tBu tBu tBu NO2 NO2 o-NO2-Ph C6F5 o-OH-Ph 2-OH-6-(CH]NDipp)-Ph (OCH2CH2)2OMe Cy CHO CHO

R2 H H H I I H OMe H H CHO NO2 H Me tBu tBu H H H NO2 H H H H H Me Cl Cl

L PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 Py Py PPh3 N-iPr-N-Bn-NHC iPr NHC PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3

1.896(2)324 1.905(2)324 1.916(4)325 1.908(3)325 1.905(2)325 1.892(3)326 1.887(4)326 1.887(2)327 1.888(2)327 2.011(8)328 1.899(2)329 1.911(3)329 1.887(3)330 1.891(3)331 1.888(3)331 1.902(2)332 1.902(4)332 1.896(4)333 1.893(4)333 1.881(4)334 1.901(11)334 1.894(3)335 1.891(7)335 1.912(1)336 1.889(4)337 1.922(3)338 1.903(9)338 (Continued )

296

Table 8

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C (A

Complex

R"

R"'

R' N

R Ni

O

L

R R0 R00 R000 Ph Dipp Me Me Ph Dipp H tBu Ph Dipp H naph Ph Dipp CF3 2,20 -biphenyl Ph Dipp H 2,20 -biphenyl Ph Dipp CF3 Ph Ph Dipp Me Ph Ph Dipp Me CF3 Ph 2,6-Me2-Ph Me CF3 o-Tol Dipp Me CH2 C]N(Dipp)(Me) p-CF3-Ph 8-(3,5-Cl2-Ph)-naph CF3 8-(3,5-Cl2-Ph)-naph k2-N,O-[1-O-2,6-(C]N(Dipp))2-3,5-O-4-C(H)NH(Dipp)-C6]NiPPh3(p-Tol) k2-N,O-[1-O-2-ox-Ph]NiPh(PPh3) k2-N,O-[2-N]C(Ph)O-5-NO2-py]NiPPh3(2-naph) k2-N,O-[2-N]C(Ph)O-py]NiPPh3(2-naph) k2-N,O-[2-N]C(Ph)O-5-NO2-py]NiPPh3Ph k2-N,O-[OC(C4H8)]CC(H)]N(Dipp)]NiPhPPh3 k2-N,O-[OC(C8H12)]CC(H)]N(Dipp)]NiPhPPh3 k2-N,O-[OC(C9H14)]CC(H)]N(Dipp)]NiPhPPh3 k2-N,O-[OC(C7H10)]CC(H)]N(Dipp)]NiPhPPh3 k2-N,O-[N(napthoquinone)(2,6-(CHPh2)2-4-Me-Ph]NiPh(PPh3) k2-N,O-[N(napthoquinone)(2,6-(OMe)2-Ph]NiPh(PPh3) k2-N,O-[N-Dipp-(8-O-[1,5-a]benzoquinone)-Im]NiPh(PPh3) [K2][k2-N,O-[N(Dipp)C(Me)C(H)]C(O)CH2C(Me)]N(Dipp)]Ni(o-Tol)(py)]2 [Li][k2-N,O-[N(Dipp)C(Me)C(H)]C(O)CH2C(Me)]N(Dipp)]Ni(o-Tol)(py)] [Na][k2-N,O-[N(Dipp)C(Me)C(H)]C(O)CH2C(Me)]N(Dipp)]Ni(o-Tol)(py)] k2-N,O-[N(napthoquinone)(2,6-Me2-Ph)]NiPh(PPh3) k2-N,O-[N(napthoquinone)(Mes)]NiPh(PPh3) k2-N,O-[N(napthoquinone)(2,6-Et2-Ph)]NiPh(PPh3) k2-N,O-[1-(N](MeNHC))-2-O-Ph]NiPh(PPh3) k2-N,O-[1-(N](MeNHC))-2-O-3,5-tBu2-Ph]NiPh(PPh3) k2-N,O-[1-(CH2N](MeNHC))-2-O-3,5-tBu2-Ph]NiPh(PPh3) [k2-N,O-[2-(C(H)]N(Dipp))-6-(OCH2CH2OCH2CH2OMe)-PhO]Ni(Ph)(PPh3)Na][BArF4] [k2-N,O-[2-(C(H)]N(Dipp))-6-(OCH2CH2OCH2CH2OMe)-PhO]Ni(Ph)(PPh3)K][BArF4] [k2-N,O-[2-(C(H)]N(Dipp))-6-(OCH2CH2OCH2CH2OMe)-PhO]2Ni2Ph2(PPh3)2Na][BArF4] k2-N,O-[2-(C(H)]N(Dipp))-6-(tBu)-PhO]NiMe(Py) k2-N,N0 -[MeC(]NDipp)C(O)NDipp]Ni(CH2Ph)(Py) k2-N,N0 -[Ph2P(NSiMe3)2]NiPh(PPh3) k2-N,N0 -[C(p-Tol)(NC6F5)(NDipp)]NiPh(PPh3) k2-N,O-[N(2,6-Me2-Ph)]CHC(COOEt)]C(O)Me]NiMes(PPh3) k2-N,O-[N(p-OMe-Ph)]CHC(CN)]C(O)OEt]NiMes(PPh3) k2-N,O-[N(2,6-Me2-Ph)]CHC(COOEt)]C(O)nPr]NiMes(PPh3) k2-N,O-[N(2,6-Et2-Ph)]CHC(CN)]C(O)OEt]NiMes(PPh3) k2-N,O-[N-Me-2-C(O)O-N-Im]Ni(o-Tol)(PPh3) k2-N,O-[N-Me-2-C(O)Ph2-N-Im]Ni(o-Tol)(PPh3) k2-N,N0 -[N-CH2Ph-5-C(O)NDipp-Trz]NiPh(PPh3)

R' N

L Ni

N R"

R

L PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 Py MeCN

1.881(4)330 1.884(3)339 1.886(17)339 1.908(3)340 1.905(7)340 1.910(3)341 1.906(3)341 1.913(6)341 1.889(3)341 1.9098(13)342 1.9249(15)343 1.899(6)344 1.886(3)345 1.951(1)346 1.939(7)346 1.887(3)346 1.893(4)347 1.897(4)347 1.887(3)347 1.893(4)347 1.871(5)348 1.877(2)348 1.905(2)349 1.914(3)342 1.935(3)342 1.9126(16)342 1.886(2)350 1.878(4)350 1.877(2)350 1.8865(16)351 1.897(2)351 1.882(6), 1.884(7)351 1.899(3)336 1.889(3)336 1.898(7), 1.890(6)336 1.926(5)352 1.966(2)352 1.8850(18)353 1.883(3)354 1.9029(17)355 1.892(4)355 1.882(7)355 1.904(4)355 1.870(11)356 1.896(2)356 1.8846(15)357

Nickel-Carbon s-Bonded Complexes

Table 8

(Continued) ˚) Ni–C (A

Complex R Ph Ph Ph Ph Ph Ph Ph o-Cl-Ph o-Cl-Ph

297

R0 Ph Ph Ph 3,5-Me2-Ph 3,5-Me2-Ph 3,5-(CF3)2-Ph Tripp p-OMe-Ph 9-anth

R00 2,6-Me2-Ph Dipp C6F5 2,6-Me2-Ph Dipp Dipp Dipp Dipp Dipp

L PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3

1.899(15)358 1.9029(17)358 1.888(3)358 1.8988(17)358 1.908(3)358 1.897(12)358 1.889(3)358 1.909(6)359 1.880(5)359

bpy, 2,20 -bipyidine; anth, anthracene; NHC, N-heterocyclic carbene imdazol-2-ylidene.

Table 9

Monoanionic Schiff base Nickel Alkyl complexes. ˚) Ni–C (A

Complex

R'

R"

N

R Ni

O

L

R"' R Me Me Me Me Me Me Me Et Me Me CH2Ph Me Me

0

R00 H H H H CF3 H H H H H H H H

R 2,6-(OPh)2-Ph 2,6-(O(3,5-Me2-Ph))2-Ph) 2,6-(CH2(3,5-(CF3)2-Ph))2-Ph) 2,6-(CH2(3,5-Me2-Ph))2-Ph) Dipp 2,6-(3,5-Me2-Ph)2-Ph 2,6-(3,5-(CF3)2-Ph)2-Ph 2,6-(3,5-(CF3)2-Ph)2-Ph Dipp 2,6-(3,5-(OMe2)-Ph)2-Ph Dipp o-SO2Me-Ph o-CH2Ph-Ph

R"

R' N

R000 naph anth naph naph tBu tBu tBu tBu tBu tBu C6F5 tBu tBu

L Py Py Py Py PMe3 PMe3 PMe3 2,4-Lutidine Py Py PMe3 PMe3 PMe3

1.932(5)360 1.935(2)360 1.932(3)360 1.922(2)360 1.931(2)361 1.937(3)361 1.953(2)361 1.926(5)361 1.936(2)362 1.955(3)362 1.945(3)363 1.949(2)363

R Ni

R"'

O

L

R R0 R00 R000 CH2Ph Dipp Ph ]NDipp CH2Ph Dipp iBu ]NDipp CH2Ph Dipp Et ]NDipp CH2Ph Dipp iPr ]NDipp CH2Ph Dipp Me ]CH2 CH2Ph Dipp Me ]NDipp CH2Ph Dipp Me ]NDipp k2-N,O-[(Dipp)N]CH(8-PhC10H5)O]NiMe(Py) k2-N,O-[1-(O)-2-(C(H)]N(Dipp))-8-(SiMe3)-9,9-Me2Xanthene]NiMe(Py) k2-N,O-[2-(C(H)]N(Dipp))-4-(tBu)-6-CPh2OH-PhO]NiMe(PMe3) k2-N,O-[2-(C(H)]N(2-(3,5-(CF3)2Ph)-8-(3,5-Me2C6H5)C10H5))-6-(C14H9)PhO]NiMe(Py) k2-N,O-[o-N(H)-PhC(O)Ph]NiMe(PMe3) k2-N,O-[1-(O)-2-(]N(Dipp)C7H5]NiMe(Py) k2-N,O-[6- tBu-2-(C(H)]N(o-SO2Ph))-PhO]NiMe(PMe3) k2-N,O-[2,4-tBu2-6-(C(H)]N(2,6-(CHPh2)2-4-OMe-Ph))-PhO]NiMe(PMe3) k2-N,O-[2,4- tBu2-6-(C(H)]N(2,6-(CHPh2)2-4-Me-Ph))-PhO]NiMe(PMe3)

L PMe3 PMe3 PMe3 PMe3 PMe3 Py 2,6-lutidine

1.945(4)364 1.946(4)364 1.958(3)364 1.951(3)364 1.965(3)365 1.943(3)366 1.9647(17)366 1.946(2)334 1.928(4)367 1.932(2)368 1.931(3)369 1.929(4)370 1.918(3)371 1.949(2)372 1.932(3)373 1.937(3)373 (Continued )

298

Nickel-Carbon s-Bonded Complexes

Table 9

(Continued) ˚) Ni–C (A

Complex [k2-N,O-[MeC(]NDipp)C(O)NDipp]Ni(CH2)(PMe3)]2(Ph-Ph) k2-N,O-[o-(N](N,N0 -(Dipp)2-Im))PhO]NiMe(PMe3) k2-N,O-[MeC(O)C(CN)C(]N(2,6-(3,5-(CF3)2Ph)2Ph))Me]Ni(Bn)(PMe3) k2-N,O-[2-(C(H)]N(2,6-(4-F-Ph)2Ph))-4,6-(I)2-PhO]NiMe(Py) k2-N,O-[2-(C(H)]N(2,6-(3,5-Me2-Ph)2Ph))-4,6-(I)2-PhO]NiMe(Py) k2-N,O-[2-(C(H)]N(2,6-(3,5-(NO2)2-Ph)2Ph))-4,6-(I)2-PhO]NiMe(PPh3) [k2-N,O-[2-C(H)]N(2,6-(3,5-(CF3)2Ph)2Ph)-6-NaO(C2H4O)4Me-PhO]NiMe(Py)] [BArF4] k2-N,O-[MeC(O)C(CN)C(]N(Dipp))Me]Ni(CH2Ph)(PMe3) k2-N,O-[2-(C(H)]N(2,6-(3,5-(But)2Ph)2Ph))-4,6-I2-PhO]NiMe(Py) k2-N,O-[N(H)C(Me)C(CN)C(CF2CF2CF3)O]Ni(PPh3)Mes k2-N,O-[N(C3H6)CC(CN)C(C6F11)O]Ni(PPh3)(o-Tol) k2-N,O-[N-Ph-2-(N]C(O)Ph)-Im]NiPh(PPh3) k2-N,O-[N-Ph-2-(N]C(O)Ad)-Im]NiPh(PPh3) k2-N,O-[N(Dipp)]CHC(C(O)CF3)]C(O)CF3]NiPh(PPh3) k2-N,N-[(Dipp)N]C(Me)C(O)NPh]NiBn(PMe3) k2-N,N-[(Ph)N]C(Me)C(O)NDipp]NiBn(PMe3) k2-N,O-[(2,6-Me2-Ph)N]C(Me)C(O)]N(2,6-Me2-Ph)]NiBn(PMe3) k2-N,O-[(2,6-Me2-Ph)N]C(Me)C(O)N(2,6-Me2-Ph)]NiBn(PMe3)

R"

R"' R Me Me Me Me Me Me

1.973(6), 1.973(6)374 1.921(2)375 1.9820(13)376 1.928(4)377 1.940(4)378 1.945(7)379 1.938(4)380 1.948(4)381 1.991(9)362 1.891(5)382 1.890(5)382 1.8766(17)383 1.883(4)383 1.900(3)384 1.9479(19)385 1.9533(17)385 1.960(3)385 1.9714(14)385

R' R

N

Ni

O

R0 Dipp Dipp Dipp Dipp Dipp Dipp

R00 H H H H H Me

L R000 Ph naph anth 4-OMe-Ph 4-CF3-Ph C(H)]C(NHDipp)Me

L Py Py Py Py Py Py

1.929(3)339 1.924(2)339 1.913(2)339 1.924(2)339 1.911(4)339 1.935(2)342

Ad, adamantyl.

Table 10

Neutral Schiff base organonickel complexes. ˚) Ni–C (A

Complex

N Ni N

R R'

R" R R0 Mes Cl Mes Br Mes Br Mes Br Mes Br Mes Br Mes Br Mes Br k2-N,N0 -[1,2-(]NDipp)-C2H2]Ni(CH2(N-Ts-4-(]C(p-CF3-Ph)))c-NC4H5)) [k2-N,N0 -[1,2-(]N(Dipp))2C6H8]NiEt][B(3,5-(CF3)2Ph)4] [k2-N,N0 -[1,2-(]N(Dipp))2C6H8]NiiPr][B(3,5-(CF3)2Ph)4] k2-N,N0 -[1,2-(]N(Dipp))2C6H8]NiEt2 k2-N,N0 -[2-(C(H)]NDipp)-Py]Ni(CH2CH]CHCH2NTs) k2-N,N0 -[2,3-(]NCy)2C4H6]Ni(CH2SiMe3)2 k2-N,N0 -[2-(C(H)]NDipp)-5-(p-OMe-Ph)-pyrr]Ni(o-Cl-Ph)(PPh3)

R00 Ph Ph 4-NO2-Ph 2-CF3-Ph 2-NO2-4-Me-Ph Mes Dipp 3,5-iPr2-Ph

1.899(2)390 1.887(3)390 1.885(6)390 1.907(8); 1.907(9)390 1.891(4)390 1.896(1)390 1.911(4)390 1.903(9)390 1.934(3), 1.943(3), 1.936(3)391 1.901(4)392 1.921(3)392 1.949(2), 1.933(2)392 1.937(3)393 1.943(1), 1.953(1)394 1.900(6)359

Nickel-Carbon s-Bonded Complexes

Table 10

299

(Continued)

Complex

˚) Ni–C (A

k2-N,N0 -[2-(C(H)]NDipp)-5-(9-anthracene)-pyrr]Ni(o-Cl-Ph)(PPh3) k2-N,N0 -[1,2-(]N(Mes))2-anaph]NiBr(o-(OMe)-Ph) k2-N,N0 -[Me2C(4,4,5-Ph2-ox)2]NiBr(Ph) k2-N,P-[1-NDipp-1-Ph-2-PPh2-Et]NiCH2SiMe3(Py) k2-N,P-[1-NDipp-1-Ph-2-PiPr2-Et]Ni(p-OAc-Ph)(m-OH)(Na(OEt2)2) [k2-N,P-[DippN]C(Ph)CH2PPh2]Ni(CH2SiMe3)][OTf] k2-N,P-[DippN]C(Ph)CH2PPh2]NiBr(p-OAc-Ph) k2-N,N0 -[(S)-tBupyrox]Ni(CH2SiMe3)2 [K(18-crown-6)][k2-N,N0 -[(S)-tBupyrox]Ni(CH2SiMe3)2] k2-N,N0 -[(S)-tBupyrox]Ni(Dipp)2 [K(18-crown-6)][k2-N,N0 -[(S)-tBupyrox]Ni(Dipp)2] k2-N,N0 -[(S)-tBupyrox]NiBr(Dipp) k2-N,N0 -[1,2-(]NDipp)-C2H2]Ni(CH2CH2C(O)O)

1.881(5)359 1.908(8)395 1.906(4)396 1.9593(13)397 1.896(3)397 1.9512(17)397 1.8855(19)397 1.925(7), 1.943(8)398 1.934(6), 1.952(6)398 1.909(5), 1.923(7)398 1.916(9), 1.940(9)398 1.871(13)398 1.913(2)399

Ts, p-toluene sulfonyl; anaph, acenanaphthylene.

Table 11

Bis(phosphine) organonickel complexes. ˚) Ni–C (A

Complex

R R R' P Ni R" P R R R iPr

R0 CF3

Cy CF3 Cy naph tBu naph tBu Ph Ph 2-PPh2-C6F4 Cy 2-naph iPr naph iPr 5-Me-naph iPr 5-CN-naph iPr 3-CN-Ph iPr 2-naph iPr 2-CN-Ph iPr Ph Cy o-Tol iPr Ph iPr p-OMe-Ph iPr p-Tol Cy Ph Cy F Cy C^C-Ph iPr F Ph Br Cy Ph Cy Ph Ph Br k2-P,P0 -[2,3-PAd2-Py]NiCl(o-Tol) k2-P,P0 -[2,3-PAd2-quinax]NiCl(o-Tol) k2-P,P0 -[1,2-PAd2-Ph]NiCl(o-Tol) k2-P,P0 -[1-P(o-Tol)2-2-PAd-Ph]Ni(o-Tol)Cl

R00 naph naph OPiv OPiv OPiv 2-PPh2-C6F4 OPiv CN CN CN CN CN CN Cl I (m-C^N)BPh3 (m-C^N)BPh3 (m-C^N)BPh3 CF(CF3)2 3,4,5,6-F4-2-Py 3,4,5,6-F4-2-Py 2,3,4,5-F4-Ph o-OMe-Ph OPh indole o-(CH(Me)OSiMe3)-Ph

1.931(8) & 1.923(7) 1.931(1) [CF3]403 1.937(5)404 1.933(4); 1.941(1)405 1.929(2)406 1.931(6); 1.928(5); 1.928(5); 1.935(5)407 1.950(2), 1.940(2)275 1.941(2)408 1.952(1), 1.923(6)409 1.935(4)409 1.933(3)409 1.927(2)409 1.953(4)409 1.910(4)409 1.933410 1.946(2)411 1.9302(17)412 1.9315(14)412 1.9263(15)412 2.00, 2.04413 1.917(2)414 1.919(10)414 1.957(8)415 1.929(2)416 1.9211(17) 1.926(5)417 1.924(4)418 1.948(6)419 1.947(4), 1.917(8)419 1.924(4)419 1.971(3), 1.826(2)406 (Continued )

300

Table 11

Nickel-Carbon s-Bonded Complexes

(Continued)

Complex

˚) Ni–C (A

k2-P,P0 -[3,30 -(PPh2)2-(2-naph2)]Ni(p-CF3-Ph)Cl k2-P,P0 -[1-P(o-Tol)2-2-PAd-Ph]NiPhCl k2-P,P0 -[1-PCy2-2-CH(Me)PCy2-Fc]Ni(p-Tol)Cl k2-P,P0 -[P(o-Tol)2C]CSC]C-PAd]Ni(o-Tol)Cl k2-P,P0 -[(P(o-Tol)2)2C]CSC]C]Ni(o-Tol)Cl k2-P,P0 -[1-P(o-Tol)2-2-PAd-Ph]Ni(o-Tol)(N(H)SO2(p-OMe-Ph))

1.936(3)420 1.954(3)421 1.917(5)422 1.905(2), 1.953(1)423 1.939(4)423 1.9450(19)424

R" R" R

P Ni

Fe P

R'

R"

R o-Tol o-Et-Ph o-Tol naph naph Tripp 2,6-Me2-Ph Mes Tripp CF3 naph 2,4-Me2-Ph 2,6-Me2-4-F-Ph 2-Me-6-CF3-Ph 2-CF3-Ph 2-Et-Ph 2-Me-4-CF3-Ph 2-Me-4-NO2-Ph 2-Me-5-CF3-Ph 2-OMe-Ph Mes

R"

0

R00 Ph Cy Cy Cy iPr Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph

R Cl Br Cl Cl Cl Br N/A N/A N/A Ph Br Br Br Br Br Br Br Br Br Br Br

1.919(6)425 1.892(7)426 1.896(1)203 1.884(8)427 1.897(3)427 1.956(6)428 1.954(7)428 1.948(2)428 1.9765(15)428 1.918(3)429 1.924(6)430 1.889(6)430 1.914(2)430 1.932(4)430 1.898(4)430 1.905(14)430 1.890(6)430 1.87(3)430 1.888(4)430 1.898(17)430 1.906(6)430

R" R" P Ni

O

R R'

P R" R"

R R0 naph CF3 naph N/A Ph N/A o-Tol N/A 2-Me-4-CF3-Ph N/A o-Tol I p-(pyrr)-Ph Br Ph Br k2-P,P0 -[1,4-(o-PiPr2-Ph)2-Ph]Ni(2,20 -biphenyl) k2-P,P0 -[3,4-(PCy2)2-SC4H2]Ni(naph)(OC(O)tBu) k2-P,P0 -[CpFe(1-PCy2-2-(CH(Me)PCy2)-Cp)]NiCl(o-Tol) k2-P,P0 -[1,2-(PPh2)2-Ph]Ni(o-(C(O)N(p-Tol))-Ph) k2-P,P0 -[O(o-PPh2-Ph)2]Ni(2,3,5,6-F4-C6H)(C^NBPh4) k2-P,P0 -[1-PPh2-2-(CH(Me)PtBu2)-Fc]NiCl(p-(C^N)-Ph) k2-P,P0 -[1-PPh2-2-(CH(Me)PCy2)-Fc]Ni(OPiv)(p-C^N-Ph) k2-P,P0 -[1-PPh2-2-(CH(Me)PCy2)-Fc]NiCl(Ph) k2-P,P0 -[1-PPh2-2-(CH(Me)PCy2)-Fc]NiCl(p-(C^N)-Ph) k2-P,P0 -[1,2-(PtBu2)2-C2H4]Ni(2-(]O)-6-(O)-C6H8)

R00 iPr iPr tBu tBu tBu tBu tBu tBu

1.958(3)427 1.896(3)427 1.9795(16)431 1.998(3)431 1.983(2)431 1.916(13)431 1.904(3)431 1.8970(17)431 1.9154(10), 1.9141(10)432 1.912(5)433 1.925(3)434 1.9585(15)435 1.925(6)436 1.921(4)437 1.929(3)437 1.900(11)438 1.911(5)438 2.014(1)439

Nickel-Carbon s-Bonded Complexes

Table 11

301

(Continued)

Complex

˚) Ni–C (A

k2-P,P0 -[1,2-(PtBu2)2-C2H4]Ni(2-(]O)-6-(O)-6-Me-C6H7) (Xantphos)NiCl(o-Tol) k2-P,P0 -[1,2-(PCH(Me)C2H4CH(Me))2-Ph]NiCl(Ph) k2-P,P0 -[(2-naph-3-PPh2)2]Ni(o-Tol)Cl k2-P,P0 -[N-PiPr2-2-PPh2-pyrr]NiCl(o-Tol) [(Dippe)Ni(o-(OAc)-Ph)][BPh4] (Dippe)Ni(o-(C(O)]CHMe)-Ph) k2-P,P0 -[O(o-PPh2-Ph)2]Ni(2,3,5,6-F4-Ph)2

2.039(2)439 1.867(6)440 1.939(3)441 1.906(2), 2.010(1)203 1.929(3)442 1.939(2)443 1.913(4)443 1.931(2), 1.9129(19)444

PAd, trioxaphosphaadamantane; quinax, quinoxalyl; NiXantphos, 4,6-Bis(diphenylphosphino)-10H-phenoxazine; dippe, 1,2 bis(diisopropylphosphino)ethane.

Table 12

Bis(phosphine) nickel alkyl complexes. ˚) Ni–C (A

Complex

R R R' P Ni R" P R R R iPr Ph Ph Ph Ph Et Et Et Et tBu tBu iPr Me iPr iPr iPr iPr iPr iPr Cy Cy Cy Ph Cy Cy Et iPr iPr

R0 Me Me Me Me Me Me Me Me Me Me Me Me Me Me Me Cl Me Me Me Br N(Me)Ph SMe CF3 SMe SAd CH2CMe2Ph Me Br

R00 Me Me SC6F5 SPh o-(N(H)C(O)tBu)-S-Ph SPh SC6F5 o-(N(H)C(O)tBu)-S-Ph p-(N(H)C(O)tBu)-S-Ph SC2H5 SPh CH2CH2Ph PPh2CH2CH2B(Cy2)Me CH2C(O)OMe O2CC(H)C(O)Ph CH2CN N(H)(CH2CH2Ph) OMe SC(]NPh)(pyrrolidine)) Ph Ph Ph Ph p-(SO2Me)-Ph p-(SO2Me)-Ph CH2CMe2Ph CN Bn

1.975(3)403 1.977(2), 1.986(2)445 1.959(3)445 1.983(2)445 2.092(4)445 1.965(2)445 1.9717(14)445 1.966(6)445 1.9755(19)445 1.979(3)407 1.980(5), 1.994(4)407 1.987(4), 2.039(3)446 1.970(4)447 1.9894(18)448 1.956(3), 1.956(3), 1.926(2), 1.926(2)448 2.0135(14)449 1.977(3)450 1.954(2)450 1.9998(11)450 1.9263(14)451 1.926(3)451 1.9153(12)451 1.918(2)451 1.8947(12)452 1.919(3)452 2.0079(19)260 2.122(13)453 1.99(1)454

R" R" R P Ni R' P R"' R"' R Me Me Me Me CH2Ph CH2Ph

R0 Me Me Me Me CH2Ph CH2Ph

R00 tBu tBu tBu Cy tBu tBu

R000 tBu iPr Ph Cy iPr Ph

1.957(3), 1.953(3)455 1.968(3), 1.960(3)455 1.958(2), 1.959(2)455 1.969(2), 1.956(2)455 1.996(3), 1.990(3)455 1.993(2)455 (Continued )

302

Table 12

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C (A

Complex Bn Br [k2-P,P0 -[1,2-(PtBu2)2-C2H4]Ni(dcp)][BF4] [k2-P,P0 -[1,2-(PtBu2)2-CH2]NiMe(THF)][B(3,5-(CF3)2Ph)4] [k2-P,P0 -[1,2-(PtBu2)2-CH2]NiMe(PPh3)][B(3,5-(CF3)2Ph)4] [k2-P,P0 -[1-(PtBu2)-2-(PPh2)-CH2]NiMe(THF)][B(3,5-(CF3)2Ph)4] k2-P,P0 -[MeN(CH2PiPr2)2]NiMe2 k2-P,P0 -[PhCH2N(CH2PiPr2)2]NiMe2 k2-P,P0 -[iPr2N(CH2PiPr2)2]NiCl(CH2SiMe3) k2-P,P0 -[iPrN(CH2PiPr2)2]Ni(CH2Ph)2 [k2-P,P0 -[MeN(CH2PiPr2)2]NiMe(THF)][B(3,5-(CF3)2Ph)4] k2-P,P-[P(o-PhOMe)2(2-(PiPr2)-4-(B(PhF2)3)-Ph)]NiMe(CO) k2-PP0 -[tBuXantphos]NiMe k2-P,P0 -[MeN(CH2PiPr2)2]NiMe2 k2-P,P0 -[PhCH2N(CH2PiPr2)2]NiMe2 k2-P,P0 -[PCy(CH2C(Me)]C(Me)CH2)2PCy]Ni(CH2CH2C(O)O) k2-P,P0 -[1,2-(PPh2)2-C2H4]Ni(1,2-CH2CHnBu-C2B10H10) k2-P,P0 -[1,2-(PCy2)2-C2H4]Ni(a-CH3CHC(O)O) k2-P,P0 -[1,2-(PCy2)2-C2H4]Ni(b-CH3CHC(O)O) k2-P,P0 -[1,2-(PMetBu)2-Ph]Ni(b-CH2CH2C(O)O) k2-P,P0 -[1,2-(PtBu2)2-C2H4]Ni(CH2C(Ph)C(Me)(Ph)O) k2-P,P0 -[1,2-(PtBu2)2-C2H4]Ni(b-CH2CH2C(O)O) [k2-P,P0 -[1,2-(PtBu2)2-C2H4]Ni(CH2CMe2Ph)][BArF4] k2-P,P0 -[1,2-(PiPr2)2-C2H4]NiMe(O2CC(H)C(O)Ph) k2-P,P0 -[1,2(PPh2)2-C2H4]Ni(CH2CH2C(O)O) k2-P,P0 -[1,2(PCy2)2-C2H4]Ni(CH2CH2C(O)O) k2-P,P0 -[1,2(P(3,4,5-F3-Ph)2)2-C2H4]Ni(CH2CH2C(O)O) k2-P,P0 -[(Ph2PCH2N(CH2PPh2)2]Ni(CH2CH2C(O)O) k2-P,P0 -[1,2(PPh2)2-C4H8]Ni(CH2CH2C(O)O) k2-P,P0 -[1,2(P(4-F-Ph)2)2-C2H4]Ni(CH2CH2C(O)O) [k2-P,P0 -[1,2-(PCy2)2-C2H4]Ni(CF2C(O)Ph)(CNtBu)][FB(C6F5)3]

iPr

2.005(7)454 1.9543(5)456 1.941(3)455 1.984(3)455 1.941(8)455 1.989(3)457 1.985(14), 1.973(15)457 1.976(1)457 2.020(2); 2.008(2)457 1.965(5)457 1.971(3)458 2.055(7)459 1.989(3)457 1.985(14), 1.973(15)457 1.983(7)460 1.953(2)461 1.980(5)462 1.930(7)462 1.955(4)463 1.963(2), 1.958(2)464 1.944(3)465 1.955(3)466 1.956(3), 1.956(3), 1.926(2), 1.926(2)448 1.965(3)399 1.950(3)399 1.957(5)399 1.955(4)399 1.949(4)399 1.955(5)399 1.966(7)467

iPr

R

Xantphos, bis(R2phosphino)-9,9-dimethylxanthene.

Table 13

Phosphino-oxo nickel aryl/alkyl complexes. ˚) Ni–C (A

Complex

R' R"

R Ph p-F-Ph p-F-Ph p-F-Ph Ph

R' P O

R

Ni

R0 tBu p-Menthane tBu tBu Cy

L R00 H H C6F5 tBu H

R"

L PEt3 PPh3 Py Py PMe3

1.901(4)473 1.911(3)473 1.891(2)473 1.900(5)473 1.893(1)474

L PPh3 PPh3 PPh3 PPh3

1.861(4)475 1.864(1)475 1.899(3)476 1.893(5)476

R'

P

R Ni

O

S

O

L

O R Ph Ph Ph Ph

0

R Ph Ph Ph Ph

R00 2-Benzofuran-Ph 2-Furan-Ph o-(2,6-F2-Ph)-Ph 2,20 -Biphenyl

Nickel-Carbon s-Bonded Complexes

Table 13

303

(Continued) ˚) Ni–C (A

Complex Ph Ph Ph p-OMe-Ph Ph p-NMe2-Ph Ph o-OMe-Ph Ph o-OCH2CH2OMe-Ph Ph Ph Ph Ph k2-O,P-[OS(O2)C5H3(FeCp)P(Ph)(2-(2,6-OMe2)-Ph)]Ni(PPh3)Ph k2-P,O-[PPh2C]C(2-NHPh-Ph)O]NiPh(P(4-F-Ph)3) k2-P,O-[PPh2(C(COOEt)]C(CF3)O)]NiPh(PPh3) k2-P,O-[PPh2(C(COOEt)]C(Ph)O)]NiPh(PPh3) k2-P,O-[PPh(o-(2,6-(OMe)2-Ph))(C(H)]C(Ph)O)]NiPh(PPh3) k2-P,O-[N-Ph-3-Me-4-PPh2-5-O-pz]NiPh(PPh3) k2-P,O-[1-PPh2-2-(PPh(O)(NiPr2))-Ph]NiCl(Ph) k2-P,O-[1-(PPh(o-(2,6-OMe2-Ph)-Ph))-2-(PPh(O)(NiPr2))-Ph]NiCl(Ph) k2-P,O-[PPh2(o-(C(O)N(H)iPr-Ph)]Ni(C6F5)2 k2-P,O-[PPh2(o-(C(O)N(H)Ph-Ph)]Ni(C6F5)2 k2-P,O-[P(o-OEt-Ph)2(C(H)]C(O)Ph)]Ni(o-OEt-Ph) k2-P,O-[P(o-OiPr-Ph)2(C(H)]C(O)Ph)]Ni(o-OiPr-Ph) k2-P,O-[P(o-O(n-Pentyl)-Ph)2(C(H)]C(O)Ph)]Ni(o-O(n-pentyl)-Ph) k2-P,O-[P(o-OMe-Ph)2(C(H)]C(O)Ph)]Ni(o-OMe-Ph) k2-P,O-[PCy(o-2,6-OMe2Ph-Ph)(o-PhSO3)]NiMe(2,6-lut) k2-P,O-[2-PPh2-6-tBu-PhO]NiMe(Py) k2-P,O-[2-PPh2-6-C6F5-1-O-Ph]NiMe(Py) k2-P,O-[P(o-OMe-Ph)2(o-SO3-Ph)]NiMe(Me2SO) k2-P,O-[2-tBu-6-(P(o-PhPh)2)-1-O-Ph]NiMe(Py) k2-P,O-[PCy2(o-PhSO3)]NiMe(2,6-lutidine) k2-P,O-[P(menthyl)(o-(2,6-F2-Ph)Ph)(o-SO3-Ph)]NiMe(Py) k2-P,O-[P(o-(OMe)-Ph)2(o-SO3-Ph)]NiMe(Py) k2-O,P-[1-SO3-2-SiMe3-6-(PPh(Ph-2-(2,6-OMe2-Ph))-Ph]NiPhPPh3 k2-P,O-[OC(2-OMe-Ph)]C(H)PPh2]NiPhPPh3 k2-P,O-[1-O-2-PtBu(2,4,6-(OPh)3-Ph)-6-tBu-Ph]NiPh(PPh3) k2-P,N-[PPh2(2-(C(O)N(H)Ph)-Ph)]NiBn(PMe3) k2-P,N-[PPh2(2-(C(O)N(H)tBu)-Ph)]NiBn(PMe3

o-(2,6-(OiPr)2-Ph)-Ph 2,6-(OMe)2-Ph 2,6-(OMe)2-Ph o-OMe-Ph o-OCH2CH2OMe-Ph p-Et-Ph o-(O(2,6-Me2-Ph))-Ph

1.903(7)476 1.895(3)477 1.901(3)477 1.939(5)478 1.899(4)479 1.890(5)480 1.888(6)481 1.867(1), 1.908(1)482 1.892(7)483 1.894(2)484 1.891(2)484 1.865(9)485 1.885(2)486 1.861(4)487 1.898(7)487 1.872(2), 1.931(2)488 1.880(2), 1.951(2)488 1.898(6)489 1.890(2)489 1.881(5)489 1.889(3)489 1.939(3)490 1.929(4), 1.926(4)352 1.934(4)352 1.915(5)491 1.927(3)492 1.944(5)493 1.925(5)494 1.930(2)495 1.894(4), 1.914(5)490 1.887(2)496 1.900(4)497 1.981(8)498 1.989(4)498

PPh3 PPh3 PPh3 PPh3 PPh3 PPh3 PPh3

Lut, lutidine.

Table 14

Polypyridyl nickel alkyl/aryl complexes. ˚) Ni–C (A

Complex

R" N N

R Ni

R'

R" R Cl o-PPh2-C6F4 Mes Tripp I I Cl Br I I

R0 o-Tol o-PPh2-C6F4 Mes Tripp o-Mes-Ph Mes Mes 2,4,6-Cy3-Ph o-iPr-Ph p-tBu-Ph

R00 tBu H tBu tBu Me H H H tBu tBu

1.872(6)502 1.883(2), 1.901(2)275 1.911(2), 1.916(2)503 1.932(2), 1.918(2)503 1.890(5)504 1.901(3)505 1.900(5)505 1.911(8)506 1.901(2), 1.897(2)507 1.901(2), 1.897(2)507 (Continued )

304

Table 14

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C (A

Complex Br p-CF3-Ph Br p-CF3-Ph Br 4-CF3-Ph CH2CF3 CH2CF3 CHF2 CHF2 CHF2 OC(O)CHF2 Ph I k2-N,N0 -[50 ,50 -Me2-20 ,20 -bpy]NiBr(Ph-p-CF3) k2-[N,N0 -[bpy]NiMes2][K(2,2,2-crypt)] k2-N,N0 -[bipym]NiMe2 k2-N,N0 -[bipym]NiMe2Yb(Me5Cp)2 k2-N,N0 -[2,20 -Me2-phen]Ni(2,4,6-iPr3-Ph) k2-N,N0 -[bpy]Ni(CH(Ph)CH2CF2CF2) k2-N,N0 -[1,2-phen]Ni(CH2CH(CH2Ph)NTs) [3,4,7,8-Me4-phen]Ni(Mes)2 k2-N,N0 -[bpy]Ni(biphenyl) 2-(p-tBu-o-Ph)-phen]Ni(OTf ) 2-(p-tBu-o-Ph)-phen]Ni(OPiv) 2-(p-tBu-o-Ph)-phen]NiN(SiMe3)2 [k2-N,N0 -[4,4-Me2-bpy]Ni(o-NH2-Ph)]2 k2-N,N0 -[5,50 -Me2-bpy]NiBr(p-CF3-Ph) k2-N,N0 -[2,9-Mes2-phen]NiCH2tBu k2-N,N0 -[phen]Ni(Mes)Br k2-N,N0 -[Me2Si(2-Py)2]Ni(CH2CH2C(O)O) k2-N,N0 -[2,9-Me2-phen]Ni(CH2CH2C(O)O)

tBu H OMe H tBu tBu H

1.879(5)508 1.896(4)509 1.881(1)510 1.947, 1.947511 1.8845(16), 1.8867(16)512 1.881(5)513 1.887(5)514 1.885(4)515 1.935516 1.931(3), 1.928(3); 1.927(3), 1.935(3)517 1.924(4), 1.926(3)517 1.959(6)503 1.978(6)310 1.912(2)518 1.914(4), 1.911(4)519 1.898(4), 1.902(3); 1.900(3), 1.898(3)213 1.901(2)520 1.8983(18)520 1.9038(14)520 1.913(7), 1.909(7)521 1.884(4)522 1.961(3)523 1.887(3)524 1.922(3)525 1.932(4)399

bipym, bipyrimidine; phen, 1,10-phenanthroline; crypt, cryptand.

Table 15

Bidentate nickel alkyl/aryl complexes. ˚) Ni–C bond distance (A

Complex

Me Me

N

R' Ni

Me

N

R

Me R o-Tol o-Tol Ph CH2SiMe3 CH2tBu CH2CMe2Ph Ph k2-C,N-[PhPy]-Ni(p-Me-py)Br k2-C,C-[biphenyl]Ni(Me2NHC)2 2,6-[k2-N,O-([O(2-Ph-6-C]N-Ph)]NiPhPPh3]-Mes k2-C,C0 -[1,10 -binaph ]Ni(Ni(Z4-binaph )Cp)(Cp) k2-[C,C0 -[1,10 -binaph ]Ni(Ni(1,2,20 -binaph )Cp)(Cp)][PF6] k2-P,N-[1-PPh2-2-N(Dipp)-Ph]NiPMe3Ph k2-C,C0 -[Z2-(2,20 ,3,30 -F4-biphenyl)]Ni2(PEt3)4 k2-P,P0 -[1,10 -(PiPr2)2-Fc]NiCl(o-Tol) k2-[C,N-[2-(p-tBu-N-Py)-Py]Ni(acac)][BPh4] k2-P,C-[2-OPiPr2-Ph]NiBr(NCMe) k2-P,C-[2-OPiPr2-4,6-(OMe)2-Ph]NiBr(NCMe) k2-N,N0 -[N(Dipp)C(O)C]CN(2-py)N]N]NiPh(PPh3) k2-P,N-[1-NH2-2-PPh2-Ph]NiPhPPh2

R0 OPh Cl Cl CH2SiMe3 CH2tBu CH2CMe2Ph I

1.893(2)257 1.9079(19)258 1.891(2)259 1.9486(13)260 1.9505(16), 1.9550(16)260 1.9543(12)260 1.894(2)109 1.894(1)261 1.934(4), 1.940(4)262 1.905(3), 1.905(3)263 1.893(7), 1.936(8); 1.905(8), 1.936(7)264 1.974(4), 1.895(4)264 1.923(7)265 1.952(3), 1.965(3)266 1.899(2)267 1.852(2), 1.852(2)268 1.916(2)269 1.932(2)269 1.885(2)270 1.909(2)271

Nickel-Carbon s-Bonded Complexes

Table 15

305

(Continued)

Complex

˚) Ni–C bond distance (A

k2-C,C0 -[N-(CH2(3,5-tBu2-Ph))-N0 -Dipp-Im]Ni(PMe3)Br k2-C,C0 -[2-(NC]C(Ph)N(Dipp)]C(Ph))-Ph)]Ni(Z3-C8H13) k2-C,C0 -[2-(C]CN(Dipp)C(Ph)]N(Dipp))-Ph)]Ni(Z3-C8H13) k2-C,N-[2-(C(H)]NN]C(H)(2,6-Cl2-Ph))-3-Cl-Ph]Ni(PMe3)2Cl k2-C,N-[2-(C(H)]NN]C(H)(2-Cl-Ph))-Ph]Ni(PMe3)2Cl k2-C,N-[2-(C(H)¼N-C2H4-N]C(H)(2,6-Cl2-Ph))-3-Cl-Ph]Ni(PMe3)2Cl k2-C,N-[2-(C(H)¼N-C2H4-N]C(H)(2-Cl-Ph))-Ph](Ni(PMe3)2Cl)2 k2-C,P-[2-PPh2-C6F4]2Ni k2-P,C-[2-OP([1,10 -Biphenyl]-2,20 -diol)-Ph]Ni(DippNHC) k2-S,S0 -[(N,N-diethyl-3,7-diazanonane-1,9-dithiolate)Ni]NiI(C6F5) k2-S,S0 -[(N,N-diethyl-3,7-diazanonane-1,9-dithiolate)Ni]Ni(S(2,6-Mes2-Ph))(C6F5) k2-N,P-[(4-PPh2)phenanthridine]Ni(2-naph)Cl k2-S,C-[2-(2-S-Ph)-Ph]Ni(iPrNHC)2 k2-N,N0 -[2-(2,4-Ph2-pyrr)-Py]NiPh(2,4-Me2-Py) k2-N,N0 -[2-(2,4-Ph2-pyrr)-Py]NiPh(phen) k2-N,N0 -[2-(2-NSO2(2-NO2-Ph)-Ph)-Py]Ni(3-indole(NCO2tBu))Py (1,2-(PPh2)2-CH]CH)Fe(C^O)2(m2-1,3-S2C3H6)Ni(C6F5)2 k2-S,S0 -[S](3-S-4-(SMes)-c-C3S2]NiMes(PPh3) k2-P,S-[1-(PPh(C(SiMe3)]CH2)-2-(S)-3-(SiMe3)-Ph]NiPh(PMe3) k2-P,S-[1-(PPh(C(SiMe3)]CH2)-2-(S)-4-Me-Ph]NiPh(PMe3) k2-C,C0 -[N,N0 -Mes2-H4Pyrimidine]NiF(C6F5)(PPh3) k2-C,O-[N-(Dipp)-N-(2-O-3-Ad-5-Me-Ph)-Im]Ni(Mes)(PPh3) k2-C,O-[N-Mes-N-(C(H)]C(O)Ph)-Im]NiPh(py) k2-P,N-[PPh2(C(H)]C(Me)NDipp)]NiPh(py) k2-P,N-[PPh2(C(H)]C(Me)N(2,6-Me2-Ph))]NiPh(py) k2-P,N-[PPh2(C(H)]C(Me)NPh)]NiPh(py) k2-C,C0 -[1-Ph-N,N0 -Dipp2-4-(4-OMe-o-Ph)-3-Im]Ni(c-C8H13) k2-P,C-[2-PPh2-binaph]NiCl(PCy3) k2-P,C-[o-(OPiPr2)-Ph]NiBr(PiPr2(OPh)) k2-P,N-[PiPr2(CH2CH2NMe2)]NiF(2,3,5,6-F4-4-py) k2-P,N-[PiPr2(CH2CH2NMe2)]NiF(3,5,6-F3-2-py) k2-N,N0 -[a:2,3-py2-phenazine]NiBr(Mes) k2-P,N-[1-PiPr2-2-(N(2,6-Me2-Ph))-Ph]NiPh(PMe3) [k2-N,N0 -[2-(o-(NSO2(o-NO2-Ph))-Ph)-py]Ni(o-(CH2O(2-py))-Ph)][BF4] k2-P,N-[2-(C(Ph)]PCH2SiMe3)-py]NiBr(2,4,6-tBu3-Ph) k2-C,C0 -[N-o-Ph-N-Me-4-Ph-1,2,3-triazole]NiCp k2-N,N0 -[2-(Ph-(o-NSO2-Ph-2-NO2))-Py]NiPy[CC(OtBu)NC(OtBu)NC(H)] k2-S,S-[S2(k4-Pt)2(PPh3)4]Ni(C6F5)2 k2-P,N-[2-CH2CH2PPh2-Py]Ni(o-Tol)Cl Ni(o-OPiPr2-Ph)2 k2-N,N0 -[3,5-Me2-2-(2-Py)Pyrr]NiMe(2,4-lut) k2-O,O0 -[MeC(O)C(H)C(O)N(H)(3,5-Me2Ph)]Ni(CH2Ph)(PMe3) k2-N,P-[o-PiPr2-N(2,6-Me2Ph)-Ph]NiMe(PMe3) k2-N,P-[o-PPh2-N(2,6-iPr2Ph)-Ph]Ni(CH2SiMe3)(PMe3) k2-N,P-[o-PiPr2-N(2,6-Me2Ph)-Ph]Ni(CH2SiMe3)(PMe3) k4-N,O,N0 ,O0 -[2,3,5,6-Me4-1,4-(2-(O)-3-(C(H)]N(Dipp))-5-(tBu)-Ph)2C6]Ni2Me2(NH2(1,1-Me2C3H5))2 k2-N,P-[2-(C(Ph)(]P(2,4,6-tBu3-Ph))Py]NiMe2 k2-N,P-[2-(C(Ph)(]P(2,4,6-tBu3-Ph))Py]Ni(CH2SiMe3)2 k2-N,N0 -[1,2-(NMe2)C2H2]Ni(CH2CH2C(O)O) (4-Me-Py)2Ni(CH2CH2C(O)O) (4-C(O)OMe-Py)2Ni(CH2CH2C(O)O) (4-NMe2-Py)2Ni(CH2CH2C(O)O)304 k2-N,N0 -[2-(CH2N(H)SiMe2tBu)-Py]Ni(CH2CH2C(O)O) k2-N,N0 -[2-(CH2NH2)-Py]Ni(CH2CH2C(O)O) (4-NH2-Py)2Ni(CH2CH2C(O)O) k2-N,P-[2-(CH2PtBu)-Py]Ni(CH2CH(Me)C(O)O) k2-N,P-[2-(C(H)]NCy)-PPh2Ph]Ni(CH(Ph)C(COOEt)]C(Me)O) k2-N,N0 -[(MeNCH2CH2)3]Ni(o-CMe2CH2-Ph) k2-P,P0 -[1,6-(PiPr2)2-Cp2Fe]Ni(b-CH2CH2C(O)O) (DippNHC)Ni(CH2CH2CH]C(Ph)O)

1.906(2)272 1.927(3)273 1.945(5)273 1.890(2)274 1.886(2)274 1.882(3)274 1.905(5), 1.905(5)274 1.941(4), 1.934(4)275 1.915(3)276 1.905(3)277 1.901(3)277 1.900(3)278 2.130(2)279 1.885(6)280 1.906(3)280 1.901(4); 1.893(4)281 1.917(3)282 1.934(3)283 1.9123(15)284 1.911(2)284 1.9008(17)285 1.905(2)286 1.891(2)287 1.910(4)288 1.906(5)288 1.928(5)288 1.954(2)289 1.8978(18)290 1.935(2)291 1.892(3)292 1.891(4)292 1.893(8)293 1.916(7)294 1.968(6)295 1.912(3)296 1.898(1)297 1.903(3)298 1.913(4), 1.914(4)299 1.903(2), 1.931(6)203 1.932(2), 1.930(2)300 1.950(2)280 1.957(2)301 1.980(4)294 1.979(4)294 1.978(4)294 1.941(3), 1.941(3)302 1.943(4), 1.931(4)296 1.958(2), 1.970(2)296 1.922(2), 1.925(3)303 1.915(3)304 1.926(6)304 1.920(3)304 1.918(5), 1.921(5)304 1.910(3)304 1.916(5)304 1.936(2)305 N/A306 1.929(4)215 1.926(7)307 1.888(2)308 (Continued )

306

Table 15

Nickel-Carbon s-Bonded Complexes

(Continued)

Complex

˚) Ni–C bond distance (A

k2-N,N0 -[1,2-(NMe2)C2H2]Ni(C(Ph)HC(C(O)OEt)]C(Me)O) k2-N,N0 -[1,2-(NMe2)C2H2]Ni(CH(Ph)CH2CF2CF2) (NH2CH2Ph)2Ni(CH(Ph)CH2CF2CF2) (Py)2Ni(N(SiMe3)(SiMe2CH2)) k2-N,N0 -[2-C2H4NH2-Py]NiMe2 k2-P,S-[1,2-PPh2-(S)-Ph]NiMe(PMe3)2 k2-P,S-[1,2-PPh2-(S)-C10H6]NiMe(PMe3)2 k2-P,S-[1,2-PPh2-(S)-C10H6]NiMe(PMe3) k2-S,P-[S(CH2CH2NHPiPr2)]Ni(CH2CH2NHPiPr2) k2-S,P-[S(CH2CH2NHPiPr2)]Ni(CH2CH2NHOPiPr2) [k2-C,N-[CH2-3,5-Me2-2-(2-NH2-3-Mes-Ph)-Ph]Ni(DippNHC)][BArF4] [k2-C,N-[CH2-3,5-Me2-2-(2-NH2-3-Mes-Ph)-Ph]Ni(DippNHC)(THF)][BArF4] k2-C,P-[N-Mes-N0 -(PtBu2)-Im]NiMe2 k2-C,C0 -[N-Mes-N0 -(CH2CH2CHC^N)-Im]Ni(acac) k2-C,C0 -[N-(4,6-Me2-2-(CH2)-Ph (C8H13))-H4Pyrimidine]Ni(Z3-C8H13) k2-C,C0 -[CH2(N-tBu-Im)2]NiPh(CN) [k2-N,C-[2-(o-Ph)-Py]Ni(2-Ph-Py)2][OTf] Ni(2-Py-Ph)2 (2-Ph-Py)NiBr(2-Py-Ph) Ni(2-(o-(C(Ph)]C(Ph))-Ph)-Py)(2-Py-Ph) (benzoquinone)NiBr(Py) (benzoquinone)2Ni (benzoquinone)Ni(OC(O)CF3)(Py) (benzoquinone)NiCl(Py)

1.961(3)309 1.986(2)310 1.973(5)310 1.971(2)311 1.921(9), 1.936(9)312 2.007(5)313 1.977(5)313 1.975(3)313 1.987(3), 1.993(3)108 1.987(1)108 1.866(6)314 1.931(3)314 1.920(6), 1.946(6)315 1.961(2)316 1.942(2)317 1.934(6)318 1.9093(18)319 1.892(2), 1.887(2)319 1.9044(18)319 1.8911(11)319 1.899(3)320 1.889(2), 1.895(2)320 1.891(3)320 1.9014(11)320

acac, acetylacetonate; Fc, ferrocene.

nickel aryl and alkyl complexes with the second substituent being aryls, alkyls, halides, pseudohalides, carboxylates, phenoxides, alkoxides, amides, or thiolates. The bis(phosphine) ligands are primarily ethylene-bridged derivatives, although ferrocene, xanthene, methylene, and propyl bridged derivatives are also common. Neutral dinitrogenous donor ligands include bipyridine, phenanthroline, pyridyl-imine, pyridyl-oxazoline, and a-diimine derivatives.321 The alkyl and aryl nickel complexes of these derivatives are predominantly diaryl, dialkyl, or organo-halide derivatives.322 Other neutral donor ligands used to stabilize nickel alkyl and aryl complexes include bis(carbenes). Monoanionic ligands include salicylaldiminate, b-ketiminate, phosphino-sulfonate, phosphino-phenolate, and phosphino-sulfide ligands. They generally form nickel aryl and alkyl complexes with square planar geometries with a Lewis base (PPh3 or Py) trans to the neutral donor atom and the alkyl or aryl substituent trans to the monoanionic donor atom.

8.05.3.1

Schiff base complexes

Ligands containing imino groups, commonly referred to as Schiff bases, are incorporated into a large amount of organonickel complexes due to their ease of synthesis (Fig. 30). They are typically synthesized via the condensation between a carbonyl and a Ar N

O Ni

Ni

N

N Ar

β-diketiminate

Ar N Ni N Ar

α-diimine Fig. 30 Common bidentate Schiff base ligand scaffolds.

R

salicylaldiminate

Ni O

N R

pyridyl-imine (pyridyl-oxazoline)

Nickel-Carbon s-Bonded Complexes

307

primary amine. The increased s-donicity of the imine group compared to carbonyl groups enables its utility as a chelating ligand with most transition and main-group metals. The identity of the primary amine can dictate the steric bulk of the ligand, and the organic groups at the a position of the imine group can determine the redox activity of the Schiff base ligand.

8.05.3.1.1

Monoanionic Schiff base complexes

Monoanionic Schiff base ligands are a robust platform.323 In the construction of bidentate Schiff base ligands it is common to use monoanionic chelation partners such as phenoxides, oxides, pyrrolides and b-diketiminates (Tables 9 and 10). Conjugation through these anionic partners can result in a delocalized anionic charge through the imine group, weakening the N]C bond but strengthening the M–N bond. Among the most implemented Schiff base ligands, salicylaldimines can be prepared by a condensation reaction between an aldehyde and a primary amine. Aniline derivatives, in particular, diversify the structures and have aided in the application in olefin polymerization.386 The catalyst structure typically comprises a salicylaldiminate ligand with an organometallic substituent trans to the oxygen donor and a Lewis base (PPh3, Py, etc.) trans to the imino group. The modular nature of their construction allows for significant customization of sterics and electronic to tailor their catalytic performance (Fig. 31).387 b-diketiminates have been used to stabilize unprecedented coordination environments and low-valent derivatives for metals throughout the periodic table.388 Recently, the application of such complexes to the synthesis of fundamental organometallic species has grown immensely. Their ease of synthesis, once again by the condensation of an aniline onto a carbonyl (in this case acetylacetonate), enables tunable electronic and steric properties.

8.05.3.1.2

Neutral Schiff base complexes

The incorporation of multiple imine groups into the ligand scaffold is represented by the a-diimine ligand class. Synthesized by the condensation of a primary amine or aniline onto 2,3-butadione, these ligands retain conjugation through their sp2 hybridized atoms, which results in ligand-redox activity.389 The imine lone pairs behave as good s-donors, while the conjugation in the backbone allows for their behavior as p-acceptors. These complexes have gained prominence due to their implementation in olefin polymerization (Table 11).400 Pyridyl-oxazoline ligands have recently found an abundance of applications in catalyzing cross-coupling reactions.401

8.05.3.2

Bisphosphine nickel complexes

Diphosphine ligands are strong s-donors and weak p-acceptors. Both the electronic and steric parameters of the ligand can be influenced by the P-M-P bite angle.402 Diphosphine ligands are tailored by the incorporation of sterically bulky groups to change the cone angle and affect the electronics. The increased PdNidP angle of the ethyl bridged diphosphine compared to the methyl-bridged diphosphine results in a shorter NidC bond length. Additionally, substituents on phosphine can affect the electronic properties of the ligand, ranging from electron-withdrawing to electron-donating groups (alkoxy > amino > aryl > alkyl). Bidentate phosphine ligands are well represented amongst stabilized nickel alkyl and aryl complexes (Tables 12 and 13). The customization of diphosphine ligands is important for the design of catalytic systems. For example, a wide bite angle mimics the migratory insertion transition state. Therefore, to prevent migratory insertion from being the rate limiting step a diphosphine ligand with a wide bite angle should be used.468 Phosphaalkene derivatives are underdeveloped ligands, which have additional p-backbonding character. The Ozawa group uses a neutral PN ligand featuring a phosphaalkene moiety.296 Interestingly, the combination of the phosphaalkene donor and the strong anionic s-donating methyl group causes a distortion of the geometry (Fig. 32). The p-backbonding of the phosphaalkene unit with nickel results in the orientation of the s symmetry to no longer lie in the molecular plane. Thus, the anionic s-donor alkyl ligand does not bind within the plane of the ligands.

Incorporation of donor atoms - Reduced chain transfer and molecular weight - hydrogen bonding increases thermal stability of catalyst Electronic tuning - Electron withdrawing groups increase activity Steric hindrance - Increases activity but reduces thermal stability

R Me

N O

Ni R

R'

Incorporation of donor atoms - Promotes hydrogen bonding, allows incorporation of polar monomers Fig. 31 General design features in salicylaldimino nickel complexes for olefin polymerization.

L

Steric hindrance of spectator ligand - Less hindered ligands increase catalyst lifetime

308

Nickel-Carbon s-Bonded Complexes

Me *Mes P Ph

Ni N

Me

Fig. 32 Electronic description of the nickel bonding in a phosphaalkene nickel complex.

8.05.3.3

Phosphine-oxo nickel complexes

Phosphino-sulfonates are widely applied to nickel-catalyzed olefin polymerization. However, their effectiveness is dwarfed by the resilient performance of palladium phosphino-sulfonate complexes in the presence of polar monomers.469 The incorporation of the phosphino group allows tracking of catalytic reactions using 31P NMR spectroscopy.470 PO based ligands are amongst those initially successful for the SHOP (Shell Higher Olefin Process) nickel catalysts (Fig. 33).471 Further development over the decades has resulted in the development of phosphine phenolate nickel complexes that serve as olefin polymerization catalysts in the SHOP reactions.472 These ligand scaffolds have allowed for the development of nickel olefin polymerization catalysts that have limited resilience in the presence of polar monomers (Fig. 34) (Table 14).

8.05.3.4

Bipyridyl nickel complexes

Bypyridine ligands, such as 2,20 -bipyridine and 1,10-phenanthroline, serve as neutral s-donors and p-acceptors (Fig. 35).389 Their extended aromaticity results in rigid planar coordination that is difficult to perturb under common reaction conditions.499 Additionally, increasing the steric bulk of these ligands has little effect on their bite angle. Many of these ligands are redox-active.500 The performance of bipyridines as L2 type ligands as well as their ability to perform MLCT explains their prominent role in homogeneous catalysis, electrochemical studies and luminescent properties.501 The difficulty in synthesizing sufficiently bulky R P

R R' P Ni O

R Ni

O

O

S O

phosphine phenolate

phosphino-sulfonate Fig. 33 Common bidentate phosphino-oxo ligand scaffolds.

MeO

Sterics at Ortho Position - Increases polymer molecular weight

MeO R' Me P Ni R S O O L O

Sterics at Axial Position - Increases rate of chain transfer. - Increases molecular weight of polymer. Electronic Tuning - Tunes oxophilicity of the nickel center. - Tunes molecular weight of polymer. - Tunes resilience to polar monomer polymerization

Labile Base - Ease of activation. - Allows polar monomer polymerization.

Fig. 34 General design features of phosphino-sulfonate based nickel complexes in olefin polymerization.

R

R N

N Ni

N R 2,2’-bipyridine Fig. 35 Common polypyridine ligand scaffolds.

Ni N R 1,10-phenanthroline

Nickel-Carbon s-Bonded Complexes

309

derivatives commonly seen with bis(phosphines) and a-diimines, as well as the vulnerability of the polypyridine ligands themselves to side reactions has left the organonickel chemistry of these ligand complexes underexplored compared to bis(phosphine) and salicylaldiminate ligands (Table 15).

8.05.3.5

Synthesis of bidentate nickel(II) complexes

Bidentate ligand platforms are generally less stabilizing than tridentate and macrocyclic ligand platforms, while generally enforcing a cis square planar geometry in contrast to many monodentate ligands.237 Two common synthetic pathways to form organonickel(II) complexes are oxidative addition of an alkyl- or aryl-E bond (E ¼ Cl, Br, I, S, O, or N) to a nickel(0) precursor and transmetalation of an organometallic reagent M-R (M ¼ Li, Mg, Zr, or Zn) with a nickel(II) salt with anions such as halides, acetylacetonates, and pseudohalides (Fig. 36). The aforementioned routes are general in scope and have little dependence on the bidentate ligand scaffold. Conversely, ligand exchange reactions between monodentate ligands such as PMe3, PPh3, and pyridine and bidentate ligands capitalize upon the chelation effect to out-compete monodentate ligands and install the desired bidentate ligand (Fig. 37). The trans PMe3 adducts of nickel alkyl halides have been commonly used to synthesize bidentate nickel alkyl halides, while the trans PPh3 nickel aryl halides are used to the same effect for bidentate nickel aryl halides. For monoanionic ligands, inclusion of a base such as NaH or LiN(SiMe3)2 during the reaction will result in the bidentate organonickel complex coordinated by the corresponding phosphine. The use of the dipyridine adduct of dimethylnickel(II) in the ligand exchange reaction will form the bidentate dimethylnickel(II) complex for neutral ligands, but the protonated monoanionic ligand is commonly used to form methane and give the bidentate nickel methyl pyridine adduct. Cycloaddition of the phosphorus ylide aldehyde complex is a common synthetic route for bidentate phosphino-oxo nickel aryl complexes. The reaction results in an aryl transfer from the phosphorus to the nickel while nickel is oxidized from the 0 to the +2 oxidation state. The NN class of ligands are varied as a result of the diversity of nitrogen binding groups where the nitrogen atoms can bind neutrally or as a monoanionic atom. Additionally, several nitrogen containing functional groups are commonly incorporated into ligands including pyridyl, amido, anilido, and amino groups. The employment of transition metal complexes as reagents and catalysts as well as the synthesis of transition metal complexes can rely on the homogeneous nature of the starting material. As such, the advent of metal bis(trimethylsilyl)amides as lipophilic sources of metal ions was welcomed. Unfortunately, for the 2nd row E Ni E E

X Ni

E

X

E

R–X R = aryl or alkyl

R Ni

E

X

E

M–R R = aryl or alkyl

R +

Ni E

M–X

X

Fig. 36 Oxidative addition and transmetalation are common synthetic routes for the synthesis of nickel aryl and alkyl complexes.

E E

E

(PMe 3) 2NiRX R = alkyl

(tmeda)NiMe2 or py2NiMe2

E

E

E

PR 3

E

R

E

X

E

Me

R = aryl

Ni0, L

+ tmeda or 2 py

Ni E

(PPh 3) 2NiRX

+ 2 PMe3

Ni

X

E

R + 2 PPh 3

Ni E

X

R2 P

R NiII

O

O

Fig. 37 Common synthetic pathways employed for bidentate nickel alkyl and aryl complexes.

L

310

Nickel-Carbon s-Bonded Complexes

N N NiII

HN

NiI Me

N

NH

N N

N

N 62 black 82% yield

63 91% yield

N

NH2 Me

Metallation

N

XR D, UV-Vis, EPR , EA

Me Me

N

Disproportionation

NiII Me

N

N

Me

H2N

Me

HN P

61

Ligand Substitution

H2 N N

Reductive Elimination

Me NiII

Bn Ph HN P Ph

Me

Ni0 4

64 red 70% yield

65 yellow 40% yield

XRD, 1H NMR, 13C NMR, MS, IR, EA Fig. 38 Performance of (tmeda)NiMe2 as an organometallic nickel starting material.

transition metals and nickel the metal bis(trimethylsilyl)amides are unstable.526 Therefore, alternative lipophilic sources of these metal ions may allow for the synthesis of novel transition metal complexes. (tmeda)NiMe2 61 has been demonstrated to be an organic soluble nickel starting material similar to the M{N(SiMe3)2} systems for other first-row transition metals (Fig. 38).312 Through the introduction of different ligands, they demonstrate varied reactivity: terpyridine results in the formation of the terpyridine nickel(I) derivative 63, 2-aminomethylpyridine results in the substitution of the tmeda group, N-diphenylphosphanyl-2-aminomethylpyridine results in the reductive elimination of the methyl groups and production of the tetrakis(N-diphenylphosphanyl-2-aminomethylpyridine)nickel(0) 65, and the addition of 8-aminoquinoline results in the metalation reaction to form the nickel(II) bis(8-amidoquinoline) 62. A (phosphine-thiolato)nickel complex mediates hydrophosphination of an alkyne resulting in a nickel aryl 66 via PdAr bond cleavage (Fig. 39). The proposed mechanism proceeds through an initial oxidative addition of the SdH bond by nickel(0) to generate the nickel hydride. Transmetalation and oxidative addition require the existence of stable nickel(II) and nickel(0) precursors, respectively. Several groups have circumvented these limitations through clever strategies in redox chemistry. The synthesis of (bpy)Ni(Mes)Br was implemented through the electrochemical reduction of a (bpy)NiBr2 to generate an in situ nickel(0) species, which was reacted with MesBr.524 Alkene p-complexation to nickel is commonly found amongst stable nickel(0) complexes. Stoichiometric protonation of a nickel alkene complex results in the formation of a cationic nickel alkyl complex 68 (Fig. 40).456 Isolation of such complexes has allowed for spectroscopic and structural analysis of agostic interactions at the nickel center. DFT calculations on the complex 68 differentiated between the agostic interactions of early transition metals with those of late transition metals which possess a s-interaction component to their agostic interactions.456

CH2

PPh 2 +

Ni(PMe 3) 4

Me3Si

H

SH

Me3Si

Ph NiII S PMe3 P

66 yellow 64% yield XRD, 1H NMR, 13C NMR, 31P NMR, MP, IR, EA

Fig. 39 A unique synthesis of a nickel aryl complex through P–C bond activation.

Nickel-Carbon s-Bonded Complexes

311

Fig. 40 Synthesis of a cationic nickel alkyl complex via protonation of a nickel(0) alkene complex.

8.05.3.6 8.05.3.6.1

Common reactivity Alkyl abstraction and protonation

The polarity of the nickel alkyl bond enables alkyl abstraction as a viable pathway for removing or exchanging a ligand from a nickel center. This process is important for accessing 14e− species as active polymerization catalysts. In many cases additives are included in olefin polymerization reactions to activate the catalyst through alkyl/aryl ligand abstraction. Cationic nickel methyl complexes can be stabilized with the support of bidentate PP ligands (Fig. 41).455 The bidentate ligand containing a methylene linker is amongst the diphosphine ligands with the smallest bite angles. A bidentate phosphine ligand enables the isolation of a series of nickel alkyl complexes, including the dimethyl 71 and dibenzyl 69 derivatives.457 Application of the non-coordinating Brønsted acid [H(Et2O)2][BArF] to the dimethyl and dibenzyl complexes results in the formation of cationic nickel complexes, and in the case of the benzyl derivative 69, the coordination mode changes from Z1 to Z3 binding upon the formation of toluene. The methyl derivative coordinates a THF molecule to maintain a four-coordinate nickel complex 72. The bite angle of the ligands in these complexes is significantly larger than those of the methylene bridged diphosphines, resulting in a lower rate of chain transfer, allowing for isolation of some of the intermediate products from reactions of the cationic nickel methyl species and unsaturated substrates, including the first example of allene insertion into the nickel-methyl bond.

tBu P NiII P tBu tBu

tBu

[H(Et 2O) 2][BArF4] -PhMe Et2O

tBu Me P NiII P Me tBu tBu

[H(Et 2O) 2][BArF4] - MeH

tBu Me P NiII P O tBu tBu

tBu

Et2O

71 yellow XR D, 1H NMR, 13C NMR, 31P NMR, IR, MP, EA

[BArF4]

70 orange 25% yield XRD, 1H NMR, 13C NMR, 31P NMR, IR, MP, EA

69 XRD, 1H NMR, 13C NMR, 31P NMR, IR, MP, EA

tBu

tBu P NiII P tBu tBu

tBu

[BArF4]

72 71% yield XRD, 1H NMR, 13C NMR, 31P NMR, IR, MP, EA

Fig. 41 Diphosphine nickel complexes undergo protonation reactions to make cationic nickel complexes.

312

Nickel-Carbon s-Bonded Complexes

Fig. 42 Alkyl protonation reactions on a nickel dialkyl complex. Bond distances shown in A˚ .

When catalyzed by late transition metal complexes, olefin polymerization proceeds through a series of b-H elimination and alkene insertion steps. The frequency of b-H elimination directly corresponds to the degree of branching in the polymer produced and inversely effects the molecular weight of the polymer.527 The strength of the metal b-hydrogen agostic interaction increases the rate of b-H elimination by weakening the CdH bond. In a study featuring an a-diimine nickel ethyl complex, Diao and coworkers evaluated the nature of nickel b-hydrogen agostic interactions (Fig. 42).392 As in the bidentate nickel benzyl complexes, reduction of the coordination number via the incorporation of a non-coordinating anion can change the binding mode of the substituent. While the benzyl group can change the binding from Z1 to Z3, the ethyl group 74 accommodates the low-coordinate nickel system by strengthening the b-hydrogen agostic interaction to such a degree that the interaction is observable in both the single crystal X-ray structure and the 1H NMR spectrum.

8.05.3.6.2

Reductive elimination

Reductive elimination is an elementary reaction, in which the metal center undergoes a 2e− reduction and forms a new s-bond between two ligands on the metal. In addition to conducting the formation of new bonds, another important consequence of this process is the reduction of organometallic complexes. This allows for the synthesis of low-valent intermediates which may be unstable under other reduction conditions (i.e., alkali metals, electrochemical reduction). Homogeneous nickel(0) complexes are reactive organometallic species implicated in the mechanisms of many nickel catalyzed organic transformations. Additionally, many nickel(0) complexes serve as catalysts and stoichiometric reactants for the activation of organic substrates.528 This is a result of their ability to perform oxidative addition on strong bonds, such as CdOMe, CdNMe+3 and CdCl. Unfortunately, the most practical nickel(0) precursor, Ni(cod)2, is air and water sensitive, and the dissociation of cod can be slow.529 Several groups have developed alternative nickel(0) precursors via reductive elimination of nickel(II) precursors. The groups of Cornella and Engle have used alkene containing ligands with unique electronic properties to stabilize nickel(0) complexes to the extent that they remain air stable.530,531 Another strategy has been the development of nickel(II) precursors that upon activation will form nickel(0) species in situ. Jamison and coworkers synthesized a bis(phosphino) nickel tolyl chloride complex 75 that is air stable (Fig. 43). When the complex is reacted with trimethylsilyl triflate the chloride is abstracted, and a disproportionation of the ligands occurs to form the bis(tolyl) nickel complex.276 This complex is prone to reductive elimination to form 2,20 -bitolyl and the in situ-generated nickel(0) species 76. They demonstrate that the catalyst system efficiently performs the selective benzylation of terminal alkenes. They further expand the scope of activating groups to Grignard reagents, organozinc reagents, organoboranes, organolithium reagents, and silanes. This effort was furthered by implementation of the dcpf diphosphine ligand to eliminate the dependence on trimethylsilyl triflate for activation. This catalyst was shown, under elevated temperatures, to catalyze the cross coupling of secondary amines with aryl chlorides. Doyle and Pfizer made a significant leap in this field through the introduction of TMEDA as a ligand for the nickel tolyl chloride 77 (Fig. 43).258,532 This complex is made by the methylation of Ni(acac)2 with Al(OEt)Me2 in the presence of TMEDA and o-tolyl chloride. The complex can self-activate with heat or can generate a diaryl species through transmetalation with a boronic acid to form the coupled aryl product and the active nickel(0) complex. CdN cross-coupling is a useful method for the construction of CdN bonds that is typically done through palladium catalysis.533 The earth abundance of nickel has prompted interest in nickel’s performance as a CdN cross-coupling catalyst. These nickel complexes commonly incorporate sterically hindered diphosphine ligands that operate in Ni(II)/Ni(0) catalytic cycles (Fig. 44).451 The stoichiometric reductive elimination of the nickel phenyl amide complex 78 was shown to generate a nickel(0) complex 79 in 95% yield and the reductive elimination of nickel phenyl amide complex 80 was shown to generate N-phenyl-indole in 65% yield to demonstrate the steps involved in the catalytic CdN bond formation process.534

Nickel-Carbon s-Bonded Complexes

313

Fig. 43 Nickel(II) complexes serving as nickel(0) precatalysts.

Fig. 44 Bidentate nickel aryl amides serving as C–N coupling intermediates.

8.05.3.6.3

Insertion

Fast and efficient isotope labeling of potential pharmaceuticals is an important process for the investigation of their metabolism and mechanism of action.535 b-amino acids are both important biomolecules and components of pharmaceutically active compounds. The Skyrdstrup group is able to synthesize an azanickelacycle 82 by reaction of a nickel(0) source with an aziridine (Fig. 45).518 The phenanthroline bound azanickelacycle undergoes CO insertion into the nickel-carbon bond and is functional for both 13CO and

314

Nickel-Carbon s-Bonded Complexes

Fig. 45 Nickel mediated carbonylation of a protected aziridine.

Cy Cy P NiII P Cy Cy 86 Cy Cy P NiII P Cy Cy

Cy Cy P I NiII P Cy Cy

MeI

I H C MeI

H Cy Cy H P NiII P Cy Cy

Cy

Cy P

I

NiII P Cy Cy 87

Cy

Cy P

I

NiII P Cy Cy

Fig. 46 Proposed mechanism of nickel mediated benzyne alkylation.

14

CO. This insertion product 83 can serve as a building block for a variety of products including amides, thioesters, esters, alcohols, b-lactams, and b-amino acids. Transition metal aryne complexes serve as potent intermediates in aryl transformation reactions. The stability conferred upon the benzyne by the coordination to a transition metal allows for the introduction of subsequent reactivity that may elude common synthetic pathways.536 As such, the investigation of a nickel benzyne complex 86 by Hatakeyama was a welcome demonstration of the nickel mediated alkylation of the benzyne (Fig. 46).411 DFT calculations suggest that the alkylation reaction moves through an SN2 mechanism wherein the HOMO of the benzyne complex is localized on one of the aryne carbons.

8.05.3.6.4

b-H elimination

The phosphino-amide ligand system was explored for the development of stable nickel alkyl complexes.294 Previous work with pincer ligands of the PNP and PNN variety demonstrated that occupation of the equatorial nickel sites shuts down the b-H elimination pathway.9,207 However, the question remained as to whether a coordinatively unsaturated system would be susceptible to a competing b-H elimination process (Fig. 47). The diarylamidophosphine ligands behave as anionic ligands through the nitrogen atom. The anionic alkyl group is installed trans to the anionic nitrogen of the ligand upon transmetalation. While the bidentate nature of the ligands makes the resulting nickel complexes more vulnerable to b-hydride elimination, it also results in them being more active Kumada coupling catalysts. As these conclusions seem at odds with each other, they attribute the stability of the catalyst to the formation of an anionic dialkylated product 89 that is no longer vulnerable to b-hydride elimination.

Nickel-Carbon s-Bonded Complexes

315

MgBr N P R2

NiII

PMe3 H

N P R2

88

NiII

89

β- Hydrogen elimination

N P R2

NiII

PMe3

+

H

90 Fig. 47 Proposed rationale on the stability of bidentate nickel alkyl complexes to b-hydride elimination as a result of the formation of a nickelate complex.

tBu

tBu

N N

NiII

Cl

o-Tol

N

hν

N

THF tBu

tBu 91 square planar

Cl

NiII o-Tol

92 tetrahedral

Fig. 48 Excited state behavior of (4,40 -tBu2-bpy)NiCl(o-Tol).

8.05.3.6.5

Photoexcitation

The modern incorporation of nickel complexes into photoredox catalysis reactions has necessitated investigations into the excited state properties of organonickel complexes. The study of the complex 91 indicates that the photoexcited MLCT state gives access to a tetrahedral 3d-d state (Fig. 48).537 Although the tetrahedral isomer was not isolated, analysis of excited state lifetimes as a function of solvent basicity and the observed cis, trans-isomerization of the EXSY spectrum suggest that the excited state is tetrahedral.538

8.05.3.7

Nickelacycles

Metallocycles have demonstrated an aptitude in the catalytic transformation of organic substrates. This proficiency is a result of the ability of metallocycles to stabilize reactive intermediates and form organometallic structures that kinetically favor selective difunctionalization of a substrate.539 Furthermore, they have found utility in bioorganometallic chemistry, as chemotherapeutics, and in materials chemistry as molecular switches and light harvesters.540,541 While commonly synthesized by the appropriately named cyclometallation reaction, many strategies have been developed for their synthesis including oxidative addition, cycloaddition, and hydrometalation reactions. A unique synthetic method entails the addition of Ni(cod)2 to a 1,2,3-benzotriazin-4 (3H)-one 93, which upon denitrogenation forms a nickelacycle 94 (Fig. 49).435

Fig. 49 A unique nickelacycle synthesis through dinitrogen elimination.

316

Nickel-Carbon s-Bonded Complexes

R E

L Ni

R

L

cyclization n

L

L

Ni

E

L X

n

R

R

L

E

Ni +

R directing group

L L

n

R X E Ni

n

R'

R' Fig. 50 Common reactions of nickelacycles.

Nickelacycles are generally defined as a nickel substituted carbo- or heterocyclic complex. For the purposes of this section, this definition will be further limited to monocyclic complexes containing covalent linkages to the nickel center to preclude many aryl and alkyl backboned ligand complexes mentioned elsewhere in this chapter. Nickelacycles commonly serve as intermediates in two types of reactions: the intramolecular bond activation and formation, as in cyclization reactions, and chelation-assisted bond activation, as in directed cross-coupling reactions (Fig. 50). Cyclometalation reactions have been explored as a mild method for the synthesis of metal alkyl bonds.542 In this context, the metallocycles produced can be used to stabilize reactive intermediates during metal-mediated organic transformations. Furthermore, through the incorporation of heteroatoms, these organic transformations can be made selective due to the strength of the metal-heteroatom bond.236 Multicomponent cross-coupling reactions are efficient and economical means of synthesizing complex molecules. Nickel’s access to multiple oxidation states and both one and two e− chemistry make it a strong candidate as a catalyst for multicomponent cross-coupling reactions. Xie’s group isolated a nickel intermediate in the [2 + 2 + 2] cycloaddition of carboryne, alkenes, and alkynes (Fig. 51).461 The nickel metallocycle 97 is synthesized via transmetallation of a zirconium carborane metallocycle 96 with (dppe)NiCl2. This carborane nickelacycle can further react with an alkyne to form a 7-member carborane nickelacycle which undergoes reductive elimination to form dihydrobenzocarborane 98. There has been an ongoing search for efficient catalysts for the synthesis of acrylic acid from carbon dioxide and ethylene. Acrylic acid is a commodity chemical that is commonly used as a polar monomer in the synthesis of a variety of polymers.543 Nickelalactones have been investigated as both intermediates and catalysts for the conversion of olefins and CO2 into commodity chemicals. A bidentate phosphinopyridine ligand forms a nickelalactone 101 from ethylene/propylene and carbon dioxide.305 Subsequent reaction with carbon monoxide provides a succinic anhydride or, when reacted with diethyl zinc, results in the synthesis of g-ketoacids. One of the limitations of nickel-catalyzed synthesis of acrylic acid from carbon dioxide and ethylene is the high energy barrier for b-H elimination of the intermediate nickelalactone.544 Several nickel complexes can react with ethylene and CO2 to generate ligated nickelalactone complexes (Fig. 52).303 The oxidative addition of methyl iodide serves to change the geometry about the metal center, lowering the energy barrier to b-H elimination. The rate of methylation of these complexes to produce methyl acrylate was influenced by the nature of the ligand on nickel. While large bite angle ligands appear entirely unreactive to the methylation reaction, bidentate amino-ligands like TMEDA are significantly more reactive than bidentate phosphine ligands such as dppe.

Cp2Zr

Cl

Li(OEt2) 2

Cp2Zr

nBu

toluene, Δ

96 light yellow

95 NiCl2(dppe) toluene, Δ 2h nBu nBu

nBu

nBu

nBu (dppe)Ni

nBu

THF, Δ

98 86% yield

97 light brown 48% yield XRD, 1H NMR, 13C NMR, 11B NMR, EA

Fig. 51 Stoichiometric reactions involved in the nickel catalyzed [2 + 2 + 2] cycloaddition of carboryne, alkenes, and alkynes.

Nickel-Carbon s-Bonded Complexes

tBu

tBu P

tBu P

Ni0

C6D6

N

tBu

tBu

tBu P

CO2

Ni0

O 101 yellow 76% yield

100 purple

99 blue

Me NiII

N

N

317

O

XRD, 1H NMR, 13C NMR, 31P NMR, EA CO

O Et

HO Me O

O

ZnEt2

tBu O

tBu

THF

O

P N

O

NiII O

Me O

102

Me Fig. 52 Nickel activation of an olefin followed by CO2 insertion to synthesize a nickelacycle.

8.05.3.8 8.05.3.8.1

Catalytic reactivity Olefin polymerization

The global production of plastics reached 359 Mt in 2018 and continues to grow despite deleterious effects on the environment.545 Polyolefins are the most common subcategory of plastics made by the polymerization of ethylene and propylene; monomers obtained from petroleum cracking. The most common method of polymerization of olefins is catalyzed by the Ziegler-Natta catalyst, which is usually a titanocene or zirconocene complex and methylaluminoxane (MAO). The desire for more control over the structure of the produced polymer has promoted intense research in both industry and academia.546 A significant advance was made by the discovery of an a-diimine nickel complex that could catalyze olefin polymerization through a different mechanism that utilized chain-walking and chain transfer discouraging termination.547 However, this type of catalyst gained commercial prominence with the application of the salicylaldimino548 ligands to nickel which increased the stability of the nickel alkyl intermediates, resulting in high molecular weight polyolefins.549 The rate of chain transfer is directly related to the bite angle of the ligand about the nickel center, as small bite angle nickel complexes have lower rates of chain transfer, leading to higher molecular weight and less branched polymers.550 A prominent challenge is the incorporation of polar monomers to diversify surface properties of polyolefins. Although late-stage functionalization of polymers is possible, the extreme conditions required make the process prohibitively expensive and limit the size of the polymers.551 Strategies to circumvent these constraints include the development of ring-opening metathesis polymerization and acyclic diene metathesis.527 The key to direct polymerization of polar monomers is to overcome the oxophilicity of the catalyst. The cationic palladium diimine catalyst by Brookhart can catalyze the copolymerization of ethylene and polar acrylates to form branched copolymers.552 Subsequently, the application of phosphine-sulfonate palladium catalysts has enabled the polymerization of ethylene and polar acrylates to give linear copolymers. The economical substitution of palladium by nickel toward the polymerization of polar monomers has garnered ample interest in the past two decades.547 The development of (phosphine sulfonate)Ni and Pd methyl complexes as efficient polymerization catalysts is a milestone achievement.491 While the palladium catalyst 104 can polymerize even polar monomers like acrylates, the nickel analog is limited to non-polar olefins. Structural analysis indicates that while the nickel catalyst 103 oxophilic nature causes it to bind DMSO through the oxygen atom, the soft nature of palladium allows it to bind the DMSO through the sulfur atom (Fig. 53). The lack of oxophilicity observed by the palladium catalyst explains its effectiveness at polymerizing polar monomers.

OMe MeO P O S

O

NiII

Me O

O Me

S

[P,S] M = Ni DMSO

Me

103

Me

Me N MII N

Me

Me

Me Me

OMe MeO P

[P,S] M = Pd O S

DMSO

O

Me

PdII

O

O S

Me

Me

104

XRD, 1H NMR, 31P NMR, EA

Fig. 53 The coordination of DMSO by analogous nickel and palladium complexes demonstrates the hard and soft acidity of the metal centers, respectively.

318

Nickel-Carbon s-Bonded Complexes

The salicylaldiminato ligand is commonly used in nickel polymerization catalysts.553 These complexes are known for their ease of synthesis and air and moisture stability. Tuning the electronics of the salicylaldiminate ligand affects the lability of the spectator ligand, which is critical to both the activity of the catalyst and the molecular weight of the polymers produced.492 Similarly, less sterically hindered spectator ligands result in longer lived catalysts. The incorporation of an aryl ether in the imino group of a salicylaldiminate results in suppressed b-hydrogen interactions and the chain transfer mechanism, due to a competition between the oxygen donation and the olefin-bound nickel complex.360 This causes chain termination to be more likely to occur. Therefore, the inclusion of these aryl ether functionalized salicylaldiminate ligands results in oligomerization and hyperbranching rather than linear polymerization of ethylene while maintaining similar TONs. An aryl sulfone functionalized salicylaldiminate ligand enhanced both the activity and thermal stability of the corresponding ethylene polymerization catalyst.363 At the extremes of steric bulk, the incorporation of m-terphenyl groups on the imino group of the salicylaldiminate ligand gives very active nickel methyl pyridine precatalysts for olefin polymerization, but only the 3,5-CF3 substituted m-terphenyl variant resulted in a stable catalyst under the polymerization reaction conditions.362 The influence of hydrogen bonding from CF3 groups on the flanking rings of the m-terphenyl groups restricts the geometry of the nickel olefin polymerization catalysts, thus mitigating the syn-periplanar conformation required for b-hydride elimination.554 This effect results in higher thermal stability and greater molecular weights, as the chain termination process involves b-hydride elimination. However, Marks and coworkers found a different result when the hydrogen bonding fragment was attached to the phenoxy group of the salicylaldiminate ligand.368 While hydrogen bonding increases the tolerance of the catalyst toward polar monomers, it results in faster b-hydride elimination and a less stable catalyst. This suggests that the directionality of the hydrogen bonding can determine whether the system increases or decreases the rate of b-hydride elimination. The Do group developed a salicylaldiminate ligand platform that includes a polyethylene glycol chain meant to coordinate alkali metals.380 The incorporation of this equatorially-coordinated sodium ion serving as a proximal Lewis acid site demonstrates significantly higher activity relative to when the sodium ion is not present. An a-iminocarboxamide nickel benzyl complex serves as a polymerization catalyst for norbornenyl acetate and ethylene.365 As bulky groups are added to the backbone of the ligand, the nickel complexes deviate from square planar geometry. An a-iminoenolato nickel benzyl complex 105 was also developed for the polymerization of ethylene (Fig. 54). Upon activation with B(C6F5)3, the oxygen atom switches coordination from the nickel to the borane and the alkene unit of the ligand coordinates to the nickel center. This activated zwitterionic nickel complex can generate very high molecular weight polyethylene without the associated steric hindrance typically required for nickel polymerization catalysts. When phosphine coordinating bases are incorporated into the nickel complexes, an activating Lewis acid is required. However, when pyridyl bases are used, the complexes can serve as a single-component olefin polymerization catalyst. Their effectiveness is dependent on the lability of the pyridyl base, with sterically hindered bases proving more labile.

8.05.3.8.2

Cross-coupling

Nickel-catalyzed cross-coupling reactions has been pioneered by Fu, Jamison, and others. Many early studies form the catalyst in situ by mixing a nickel precursor, such as NiCl2(DME) and Ni(cod)2, with ligands. Catalytic reactions with well-defined nickel complexes have emerged in the past two decades. The bis(phosphine) ligands JosiPhos and DalPhos-ligated nickel complexes such as 107 are compelling catalysts for Buchwald-Hartwig amination (Fig. 55).434 The scope of this reaction includes primary amines as well as ammonia. DalPhos is better suited for use with aryl bromide substrates than JosiPhos, but how the secondary

N O

NiII

N

2 B(C 6F 5) 3 PMe3

NiII

O

toluene 1.5 h

B(C 6F 5) 3

105 orange

106 purple 93% yield

XR D, 1H NMR, 13C NMR, 31P NMR, EA

XR D, 1H NMR, 13C NMR, 31P NMR, EA Fig. 54 The use of tris(pentafluorophenyl)borane to determine the coordination sphere of an olefin polymerization catalyst.

Cl

Me

Fe

Cy2 P P Ni Cy2 Cl Me

107 orange XR D, 1H NMR, 13C NMR, 31P NMR, EA

HN

O

107 (0.5 mol%) + H2N

O

NaOtBu, 16h, 25 ºC toluene

Fig. 55 JosiPhos nickel-catalyzed Buchwald-Hartwig coupling between aryl halides and primary amines.

90% yield

Nickel-Carbon s-Bonded Complexes

S

Cy Cy P NiII O P Cy Cy

319

O H

Ph O tBu

Ph

+ tBu

O

108 orange XRD, 1H NMR, 31P NMR, HRMS

Ph

108 (10 mol%)

Ph

O

K3PO 4, 150 ºC toluene

82% yield

O

Fig. 56 Diphosphine nickel catalyzed a-arylation of a ketone with an ester.

coordination sphere creates this distinction remains uncertain. The dppf ligand, on the other hand, has generally proven ineffective in the coupling of primary amines to aryl halides, although it shows some reactivity in the coupling of secondary amines to aryl halides. Cross-coupling reactions featuring phenols are useful because of their natural abundance in comparison to the unnatural aryl halides.555 However, activation of the CdO bond is a limiting factor for the cross-coupling of alcohols with other substrates. A common strategy to circumvent this limitation is the binding of an activating group to the alcohol to make the CdO bond cleavage more facile and selective (Fig. 56). The a-arylation of benzyl-phenyl ketone with naphthalene-2-yl pivalate demonstrates this strategy, in which nickel catalyst cleaves the CdO bond to generate a nickel aryl intermediate 108 that is then available to facilitate a cross-coupling reaction.433 This unusual oxidative addition of CdO bonds exhibits a strong ligand dependence. While PCy3 and dppe give no conversion, dcype enables complete conversion in 8 h. This reaction provides the basis for catalytic cross-couplings of phenol derivatives. The use of nickel in the place of palladium as a catalyst for Suzuki coupling has the advantage of employing a significantly cheaper transition metal and, for Csp2-Csp3 couplings, decreased proclivity for b-H elimination.556 However, nickel-catalyzed Suzuki couplings are still susceptible to significant drawbacks, including air sensitivity, the high commercial cost of nickel(0) catalysts, and the insolubility of nickel(II) precatalysts that can hinder the efficiency of catalytic reactions.557 During the investigation of bipyridine diethyl nickel(II) as a potential Suzuki coupling catalyst, Yang and coworkers determined that bipyridine bis(2,2,2-trifluoroethyl) nickel(II) serves as a suitable precatalyst for Suzuki coupling wherein a fluorine atom migrates to nickel to generate geminal difluoroethylene.511 This nickel fluoride species can then react with aryl boronic acids and organohalides to produce a bipyridine nickel(I) halide catalyst. This nickel(I) halide undergoes transmetallation with an aryl boronic acid to generate an organonickel(I) species, which then undergoes oxidative addition with an alkyl iodide through a radical mechanism. This species subsequently reductively eliminates the aryl alkyl product to regenerate the nickel(I) halide catalyst. Difluoromethylation and trifluoromethylation have drawn significant interest due to their pharmaceutical applications. Currently difluoromethylation reactions are limited by the harsh temperatures required for their installation of the difluoromethane group.558 The limited ability of nickel to catalyze Negishi couplings was probed using a difluoromethyl zinc reagent with aryl iodides.512 Judicious ligand choice is critical to the success of the reaction. The dppf ligand promotes the reaction even at room temperature, while 4,40 -ditertbutylbipyridine inhibits it and results in isolation of a nickel bis(difluoromethyl) species, which is not reactive with aryl iodides. This ligand also inhibits the cross-electrophile coupling of aryl chlorides with chlorodifluoromethane catalyzed by nickel using zinc as a reductant.513 While 4,40 -ditertbutylbipyridine does not enable catalysis, several other bipyridine ligands were effective albeit at 80  C. Surprisingly 4,40 -diaminobipyridine was the most effective even though amino groups traditionally poison nickel catalysts. The selective activation of CdH bonds serves as a potential method for the facile conversion of petroleum products into reagents toward the synthesis of fine chemicals. A few groups have demonstrated the activation of the relatively inert CdH bonds of methane via borylation.559,560 Chirik and coworkers synthesized an a-diimine nickel dineosilyl complex 109 that is a competent catalyst in the triborylation of toluene with bis(pinacolato)diborane (Fig. 57).394 The borylation is more effective with the bench-stable (a-diimine)nickel dipivalate catalysts.

BPin BPin 109 (25 mol%) excess B2Pin2 CPME, 80 ºC, 48 h

Fig. 57 Nickel alkyl catalyzed triborylation of toluene.

BPin

110 84% yield

Cy N N Cy

NiII

CH2TMS CH2TMS

109 dark blue XRD, 1H NMR, 13C NMR, EA

320

Nickel-Carbon s-Bonded Complexes

O O NiII P C2H4

P

+

10 bar

CO2 10 bar

111 (2 mol%) NaO-2-F-Ph, Zn THF 100 ºC, 24h

O ONa TON: 19

111 yellow XR D, 1H NMR, 31P NMR, IR

Fig. 58 Catalytic synthesis of sodium acrylate from ethylene and CO2.

8.05.3.8.3

CO2 conversion

Bis(phosphine) nickel complexes have been shown to catalyze the reaction between ethylene and CO2 to form sodium acrylate.463 A macrocyclic diphosphine ligand synthesized from white phosphorus enabled the isolation of a nickelalactone intermediate 111 from the reaction of ethylene and CO2 (Fig. 58).460 This species catalyzes the reaction of ethylene and CO2 to form sodium acrylate. As the product is an acrylate anion, a cation source is required. Exploration of different sodium phenoxides determined that sodium o-fluoro-phenoxide was an optimal cation source for this reaction.

8.05.3.9

Biomimetic bidentate nickel complexes

Acetyl CoA synthase is a prominent enzyme in methanogenic bacteria that allows for the reduction of carbon dioxide into an acetate, which the bacteria can metabolize to form methane. For the utilization of CO2 as a C1 building block as well as establishing methane (natural gas) as a renewable resource, understanding of the mechanism of acetyl CoA synthase is important. The enzyme active site is composed of an Fe4S4 cluster and two distinct nickel sites. It is believed that most of the chemistry occurs at the nickel centers while the iron cluster serves as a conduit for electron transfer. There are two competing theories on the role of nickel in the mechanism of acetyl CoA synthase.249 Some research suggests that a Ni(I) intermediate likely binds a methyl radical generated from the cobalamin unit of a corrinoid iron-sulfur protein in what is called the paramagnetic mechanism. Alternatively, in the diamagnetic mechanism, a Ni(0) intermediate, could undergo an oxidative addition mechanism with the cobalamin unit of the corrinoid iron-sulfur cluster to generate the nickel(II)-methyl species, although such a Ni(0) species has never been observed in a biological system.249 In an elegant study Riordan and coworkers synthesized a variety of PP nickel methyl species with varying thiolato groups to serve as biomimics of an acetyl CoA synthase intermediate (Fig. 59).445 As in acetyl CoA synthase, the nickel thiolato species 113 undergoes a reversible CO insertion. However, in the presence of excess CO the methyl thioester 116 is reductively eliminated and a Ni(0) species 115 is generated. While this step was not reversible, the converse reaction was observed when an electron-withdrawing thioester MeC(O)SC6F5 was decarbonylated with nickel(0) to generate a nickel alkyl thiolato species. This chemistry lends support to the diamagnetic mechanism by demonstrating the ability of a nickel(II) complex to generate a nickel(0) species under only mildly reductive conditions. Et

Et P

NiII

P Et

Me Me

Ar = C6H4-ο-NHC(O)tBu

Ar 2S2

Et

CH3SAr

113 yellow 88% yield XR D, 1H NMR, 13C NMR, 31P NMR, MP, IR, EA

112 yellow 1H

Et Et Me P NiII SAr P Et Et

NMR, 13C NMR, 31P NMR

CO

Et Et CO P Ni0 CO P Et Et 115

O ArS

CO (excess) Me

Et Et O Me P NiII SAr P Et Et

116 114

Fig. 59 A diphosphine nickel complex undergoing disulfide addition, carbonyl insertion, and reductive elimination as in acetyl CoA synthase.

Nickel-Carbon s-Bonded Complexes

8.05.4

321

Organonickel(II) complexes stabilized by monodentate ligands

Nickel complexes featuring monodentate ligands have varied chemistry although they are mostly limited to complexes featuring stabilizing phosphine or NHC (N-heterocyclic carbene) ligands. Few reactions beyond transmetalation with other classes of monodentate donor ligands were identified (Table 16). While olefin insertion is observed with both phosphine and NHC nickel complexes, they are poor olefin polymerization catalysts. The insertion of carbon monoxide between both nickel aryl and nickel alkyl complexes is facile and, in many cases, reversible under specific conditions. Phosphines and NHCs are the most common monodentate ligands used to stabilize nickel alkyl (Tables 17 and 19) and aryl (Tables 18 and 20) complexes. They tend to form square planar nickel alkyl and aryl complexes wherein the monodentate ligands typically bind trans to each other. This structure results in the monodentate ligands having little determination on the Ni–Calkyl/ Ni–Caryl bond length. High coordinate nickel alkyl and aryl complexes (CN  4) display longer Ni–C bond lengths than low coordinate nickel alkyl and aryl complexes (CN  3). Most complexes are of nickel dialkyls, diaryls, and organo-halides although some silyl, phosphide and thiolate derivatives also exist. Table 16

Monodentate and Z5 organonickel complexes. ˚) Ni–C bond distance (A

Complex

R N

Ni N

R'

R

0

R Dipp Dipp Mes Mes Mes Mes (Z5-CpCH2CH2CH2-Z1-Cp)Ni(MesNHC) 1,3-((MesNHC)NiCp)2-CH2C(O)CH2 (m3:Z2 1,10 biphenyl)(Ni(MeCp))3 (m3:Z2 2,3 biphenyl)(Ni(MeCp))3 (Cp Ni)2(o-(C(Et)]C(Ph))-Ph) [Li2(OEt2)4][(p-terphenyl)(NiCp)2] [Li(OEt2)2][NiCp(biphenyl)] [Li(DME)3][NiCp(biphenyl)] [Li(DME)][NiCp(biphenyl)] [MesNHC][(Cp)Ni(biphenyl)] [2,6-Et2-PhNHC][(Cp)Ni(biphenyl)] [MesH2NHC][(Cp)Ni(biphenyl)] (MesH2NHC)NiMe(Cp) [N-Mes-N-nBu-Im][(Cp)Ni(biphenyl)]

R Z1-Cp CH2C(Me)]CH2 CH2CN Ph o-Ph-Ph Me

R'

Ni

R

N N

n

R0 Me Mes Mes Mes

R CN CN CN CN

2.021(3)561 1.9714(19)561 1.961(2)562 1.908(2)563 1.9211(11)564 2.033(3)565 2.032(2)566 1.977(2)567 1.898(2), 1.871(2)568 1.862(5), 1.884(5)568 1.924(4)569 1.893(3), 1.895(3)570 1.894(3), 1.889(3)571 1.880(6), 1.885(6)571 1.892(2), 1.885(2)571 1.893(5)1.892(5)564 1.8806(12), 1.8853(13)564 1.882(6), 1.893(6)564 1.9667(18)565 1.8869(9), 1.8860(9)564

n 1 1 4 2

R'

1.9697(11)572 1.980(7)572 1.987(6)573 1.8560(19)562

R Ni

Me

L (Continued )

322

Table 16

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C bond distance (A

Complex R R0 SiMe3 SiMe3 SiMe3 H CH2CH2P(tBu)2 H [Li(12-crown-4)2][NiPh(Z3-(C3H4)CH2CH2C2H2CH2)] Ni((2,3,6,7-dibenzo-C8H4)2)2 (N(SiMe3)Dipp)2NiMe KNiMe(N(SiMe3)Dipp)2 (DippNHC)NiC(SiMe3)3 (iPr4Cp)NiMe(PMe3) [(iPr4Cp)Ni]2CH2 (2,6-(Dipp)2-PhN^C)2NiCl(CHCl2) [Li(12-crown-4)2][Ni(Z3-(C3H4)CH2CH2C2H2CH2)(2,6-Me2-Ph)] [k2-N,C]C0 -[1-(O)-1-(]CH2)-2,2-Me2-3-(]NDipp)-C3H]NiCH2Ph]2 [Li(THF)4][[Z5-1,2,3,4-dibenzo-Cp]Ni(Z1-2,3,4,5-dibenzo-Cp)2] (N-Me-4-iPr-py)2NiF(C6F5) (AsPh3)2Ni(2,4,6-F3-3,5-Cl2-C6)2 (SbPh3)2Ni(2,4,6-F3-3,5-Cl2-C6)2 (N-Me-4-iPr-py)2Ni(SnPh3)(C6F5) trans-(c-NC4H9)2NiBr(2,4-(CF3)2-Ph) trans-(NH2nPr)2NiBr(2,4-(CF3)2-Ph) [(NH2nPr)3Ni(2,4-(CF3)2-Ph)][Br] [N(nBu)4][Ni(C6F5)4] [Li2(THF)4][(cod)Ni(biphenyl)] (TMEDA)LiNiPh(CH2]CH2)2 (Py)2Ni(CH2CH2C(O)O) (Py)2Ni(CH(Et)CH2C(O)O) (C^N(2,6-Mes2-Ph))2NiCl(C2Cl5)

L PPh3 PMe3 N/A

1.9591574 1.9456(19)574 1.978(2)575 1.979(2), 1.928(2)576 2.005(2), 2.006(2), 2.008(2), 2.004(2)577 1.922(2)578 1.879(2)579 1.989(3)580 1.9936(19)581 1.886(14), 1.869(13)581 1.9160(19)582 1.917(5), 2.007(6)583 1.978(4)584 2.013(2), 1.996(2)585 1.903(3)586 1.933(3)587 1.926(5)587 1.931(9)588 1.880(3)589 1.884(3)589 1.902(4)589 1.919(5), 1.923(5), 1.931(5), 1.920(5)590 1.978(3), 1.967(4)591 1.9634(12)592 1.917(2)525 1.909(4)525 1.976(2)593

Cp, cyclopentadienyl; cod, 1,5-cyclooctadiene; Cp , pentamethylcyclopentadienyl; H2NHC, saturated N-heterocyclic carbene 4,5-dihydro-imidazol-2-ylidene.

Table 17

NHC nickel alkyl complexes. ˚) Ni–C bond distance (A

Complex

R' R'

R

N N R

R1

R2 Ni

R

R

N R'

N R'

R1 Me Et Et Me Me CN CH2Ph CH2Ph C6F5 C6F5 CH2SiMe3 CH2SiMe3 F F F Me nBu CH2SiMe3 CH2SiMe3 (DippNHC)Ni(CH(SiMe3)2)

R2 S(O)Me C6F5 4-C6F5-C6F4 4-CF3-C6F4 4-C6F5-C6F4 nBuCN Cl Br Me Z1-Cp CH2SiMe3 Br CF2Ph CF2(m-CF3-Ph) CF2(1-naph) Me nBu CH2SiMe3 CH2SiMe3

R iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr

R0 H H H H H H H H H H Me Me H H H Me Me H Me

2.000(3)640 1.986(3)660 1.991(2)660 1.975(8)660 1.967(3)660 1.982(2)646 1.979(3)661 2.007(7)661 2.053(3)649 2.079(5)649 2.0349(11), 2.0317(11)260 1.9768(13)260 1.910(2)662 1.900(4)662 1.905(3)662 1.966(2), 1.975(3)663 2.0060(15)663 1.9920(13)260 2.0027(15), 2.0046(15)260 1.968(3)664

Nickel-Carbon s-Bonded Complexes

Table 17

323

(Continued)

Complex

˚) Ni–C bond distance (A

(DippNHC)Ni(CH(SiMe3)2)(C^NtBu) (DippNHC)NiBr(CH(SiMe3)2) (DippNHC)Ni(Z1-Cp)(TEMPO) (DippNHC)Ni(CH2C(Me)]CH2)(Z3-2-Me-allyl) (DippNHC)NiCl(PMe3)(CH2Ph) [(DippNHC)Ni(CH2CH2C(H)]C(H)O)]2 (DippNHC)Ni(Z3-O]C(Me)]CH(CH(Me)CH(C(O)Ph)CH2CH2) (DippNHC)Ni(Z3-O]C(Me)]CH(CH(Ph)CH(C(O)Ph)CH2CH2) [(DippNHC)Ni(N(SiMe3)2)Me][BArF4] (DippNHC)Ni(2-(]O)-6-(C(Ph)]C(2-Me-allyl))-Cy) (DippNHC)NiCp(CH2C(Me)]CH2) k2-C,O-[1-O-2-CH2-indane]Ni(tBuNHC)

2.008(3)664 1.902(8)664 2.034(2)665 1.966(3)666 1.966(4)667 1.9144(18)308 1.923(3)308 1.932(4)308 1.907(7)668 2.047(4)669 1.971(2)670 1.918(6)671

Table 18

NHC nickel aryl complexes. ˚) Ni–C bond distance (A

Complex

R' N R

R' N R

Ni R1

R2 R N R'

R N R'

R iPr iPr iPr iPr iPr iPr iPr iPr iPr iPr Me Me iPr Mes Cy Cy Cy Cy Me iPr iPr iPr Cy iPr iPr iPr iPr iPr iPr iPr

R0 H H H H H H H H H H Me Me H H H H H H Me H H H H H H H H H H H

R1 p-CHO-Ph p-OMe-Ph p-NH2-Ph p-CF3-Ph p-Cl-Ph m-Cl-Ph 6-Cl-2-Py 4-NMe2-Ph Ph Ph p-CF3-Ph p-OMe-Ph Mes 3,5,6-F3-Ph 5-Me-2-naph 2-naph p-(p-Tol)-Ph p-(p-Tol)-Ph Ph Ph 2,4-(OMe2)-Ph 2-Py 3,5-F2-Ph C6F5 2,3,5,6-F4-4-SiMe3-Ph C6F5 C6F5 C6F5 C6F5 C6F5

R2 Cl Cl Cl Cl Cl Cl Cl Br S(O)Me OSMe Br Br SMe F Cl Cl F Cl PPh2 CN CN CN Cl F F OTf Z1-Cp S(nPr) Se(iPr) H

1.882(2)638 1.901638 1.937(3)638 1.890(2)638 1.891(2)638 1.890(5)638 1.907(2)638 1.910(3)639 1.930(3)640 1.898(5)640 1.891(3)641 1.904(3)641 1.935(5)279 1.897(1)642 1.889643 2.023(8), 2.031(4)644 1.901(3); 1.887(3)645 1.903(5)645 1.9371(15)285 1.928(2)646 1.937(4)646 1.920(8)646 1.888(2)647 1.907(2)648 1.911(6)648 1.883(8)649 1.946(5)649 1.950(2)649 1.941(7)649 1.968(3)649 (Continued )

324

Table 18

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C bond distance (A

Complex iPr H iPr H iPr H iPr H iPr H (iPrNHC)2NiH(C6F4-p-CF3) [DippNHC]Ni(CH2CH2CHCHNPh) [DippNHC]Ni(CH2CH2CHCHNCH2CH2NMe2) [DippNHC]Ni(b-(PhCH2CH2C(O)NPh) (MesNHC)Ni(C6F5)(C8H10) [(iPrNHC)2NiCl]2C6F4 [(DippNHC)Ni(o-(N(H)C(O)Me)-Ph)(C(O)Me2)][I] (DippNHC)Ni(CH2CH(n-hexyl)(o-(N(H)C(O)Me)-Ph) k2-C,O-[2-O-biphenyl]Ni(DippH2NHC) (iPrNHC)2NiF(2,3,5,6-F4-4-CF3-C6) (DippNHC)Ni(CH2CH]NC(CF3)2(Ph))(N(H)C(CF3)2(Ph)) (MesH2NHC)NiCl(P(OPh)3)(p-C^N-Ph) [(iPrNHC)2Ni(C6F5)(PPh3)][(C2F5)2PF3] cis-[(iPrNHC)2Ni(C6F5)(DippNHC)][(C2F5)2PF3] [(iPrNHC)2Ni(p-C6F5-C6F5)(OH2)][(C2F5)2PF3]

Table 19

2,20 -biphenyl C6F5 p-C6F5-C6F4 p-CF3-C6F4 p-C6F5-C6F4

Phosphine nickel alkyl complexes. ˚) Ni–C bond distance (A

Complex

R 3P R"

R' Ni PR 3

R R0 R00 Me Me Me Me Me 3,4-(CF3)2-pyrr Et Me o-F-Ph Me Me m-F-Ph Et Me p-F-Ph Me Me 2,3-F2-Ph Me Me 2,5-F2-Ph Me Me 3,5-F2-Ph Me Me N(H)C(O)(2,6-F2-Ph) Et Me Me Me Me Cl k2-C,O-[CH2(o-C(O)tBu-Ph)]Ni(OTf )(PPh2Cy) [Ni(PPh3)2(CH2PPh3)2(m-Cl)]2[Al(OC(CF3)3)4] (PCy3)NiPh(Z3-(C3H4)CH2CH2C2H2CH2) k2-P,Cp-[1-(CH2CH2PtBu2)-(Z5-Cp)]NiMe (PPh2(o-OMe-Ph))Ni(Py)(CH2CH2C(O)O) (PCy3)Ni(Py)(CH2CH2C(O)O)

8.05.4.1

1.942(11), 1.949(10)650 1.971(3), 1.986(3)650 1.9770(17), 1.9913(18)650 1.952(7), 1.971(8)650 1.954(4), 1.967(4)650 1.944651 1.908(2)652 1.957(5)652 1.942(4), 1.946(4)561 1.958(5)653 1.919(3), 1.919(3)654 1.8931(5)655 1.9229(10)655 1.849(2)656 1.870(8)656 1.928(2)657 1.898(3)658 1.942(4)659 1.9548(18)659 1.8851(18)659

N/A Et Et Me Me

1.987(15)612 1.943(3)612 1.932(3), 1.979(3)672 1.9369(13), 1.9796(15)672 1.9373(3), 1.9838(16)672 1.931(3), 1.984(3)672 1.931(4), 1.9851(16)672 1.9221(14), 1.9839(17)672 1.946(3)673 2.010(2)674 1.921(9)675 1.915(3)676 1.9560(16), 1.9579(16)677 1.996(5)678 1.972(11)679 1.939(5)680 1.927(3)680

(NHC)nickel complexes

N-heterocyclic carbenes are versatile ligands that can be easily synthesized from the corresponding aniline or primary amine to produce imidazolium salts.681 Deprotonation of the air stable imidazolium salt allows for isolation of the resultant carbene which can serve as an organometallic ligand or as a catalyst in its own right. Use of different anilines allow for facile tuning of the steric properties of N-heterocyclic carbenes. The electronic properties of these ligands stay largely similar, although the use of a saturated vs. unsaturated imidazole display marked differences in their corresponding metal complexes. Additionally, some groups have incorporated six and seven membered N,N-heterocycles to adjust the electronic properties of the carbene ligand.317

Nickel-Carbon s-Bonded Complexes

Table 20

Phosphine nickel aryl complexes. ˚) Ni–C (A

Complex

R 3P R" R iPr iPr iPr Ph Cy Bn Me c-C5H9 Cy Cy Et Et Et Et Et Et Et Cy Ph Ph Ph Ph Ph Me Me Me Me Me Et Et nBu iPr iPr iPr Et Ph Et Et Et Me Me Ph Ph Et Cy c-C5H9 Ph Ph Ph p-OMe-Ph Ph Ph Ph Ph Cy

325

R0 2,3,5,6-F4-Ph 2,3,4,6-F4-Ph 2,3,5,6-F4-Ph phenanthrene-9-yl p-CF3-Ph o-Tol o-N(H)C(O)Me-Ph 2,4-F2-3-CF3-Py 2,4-F2-3-CF3-Py 2,3,4,5-F4-Py p-I-C6F4 p-I-C6F4 p-I-C6F4 p-I-C6F4 o-I-C6F4 o-I-C6F4 o-I-C6F4 (k-N)C12N2F8 2-(OC(H)]C(H)Me)-naph 2-(O(CH2)3CH]CH2)-naph o-(CH2C(H)]C(H)Me)-Ph o-(CH2C(H)]CMe2)-Ph 2,6-Me2-4-OSiMe3-Ph o-(C(O)NHMe)-Ph o-(C(O)NH(nBu))-Ph o-(C(O)NH(p-Tol))-Ph p-OMe-Ph Ph 2-I-3-F-Ph p-(CH]CH(p-Py))-Ph o-(CH2CH2C(Me)]CH2)-Ph 2,3,6-F3-Ph C6F5 C6F5 2,3,5-F3-4-NH2-Ph naph 3,5,6-Py 3,4,5,6-Py 1-corannulene Ph Ph 2,4,6-F3-3,5-Cl2-Ph 2-(C^CsiMe3)-5-Me-Ph 2,20 -biphenyl naph 2,3,5,6-F4-Ph Ph 4-Ac-Naph o-OMe-Ph naph Mes Mes 2-F-5-Cl-6-Ph-4-Pyrimidine 2,6-F2-5-Cl-4-pyrimidine Ph

R' Ni PR 3 R00 H H Z3-C8H13 Cl Cl Cl Cl F F F F Cl Br I F Cl I F Cl Cl Cl Cl Br Cl Cl Cl C(CF3)]CF2 C(CF3)]CF2 I Br (m-(C^N)-AlMe2Cl F F N/A F Cl N3 OAc Br Ph 3,4-(CF3)2-pyrr 2,4,6-F3-3,5-Cl2-Ph Br N/A F (m-(C^N)-BPh3 Cl Cl Br Cl N^C-(1-OEt-1,3-(]NDipp)2-iPr) N^C-(1-OEt-1-(]NDipp)-3-(2,6-Me2-Ph)-iPr) Cl Cl F

1.874(2)594 1.934(3)594 1.937(3)594 1.902(2)595 1.884(1)596 1.886(8), 1.943(8), 1.855(1)203 1.892(2)597 1.861(2)598 1.873(2)598 1.865598 1.873599 1.881(2)599 1.881(2)599 1.877(6)599 1.891(7)599 1.899(7), 1.880(1)599 1.907599 1.875(3)600 1.890(4)601 1.895(2)601 1.898(3)601 1.899(3)601 1.907(3)602 1.877(5); 1.886(5); 1.884(6)603 1.885(4)603 1.895(2)603 1.920(4); 1.934(4)604 1.912(2)604 1.863(6)605 1.888(8)606 1.914(5)607 1.883(2)608 1.888(5)608 1.973(2)608 1.864(2)609 1.896(3)610 1.881(2)414 1.8753(14)414 1.880(4)611 1.9377(18), 1.9366(18)612 1.908(4)612 1.938(2), 1.951(2)587 1.875(7)613 1.971(5)605 1.8881(16)427 1.924(4)436 1.895(2)614 1.879(7)615 1.884(3)418 1.929(4)616 1.919(3)355 1.899(2)355 1.8640(16)617 1.8541(17), 1.879(14)617 1.8828(18)557 (Continued )

326

Table 20

Nickel-Carbon s-Bonded Complexes

(Continued) ˚) Ni–C (A

Complex Me 2,3,4,5,6-Cl5-Ph Et 2,3,4,5-F4-Ph [(N-Me-3-Py)Ni(PPh3)2Cl][OTf] (P(CH3)2Ph)2Ni(2,6-(OMe)2-Ph)Br (PiPr3)2Ni2(SiHPh2)Ph (3-naph)Ni(PCy3)2Cl (m-(OH)2)Ni2(3-naph)2(PCy3)2 (2-naph)Ni(PEt3)2CN (2,4-H2-C6F3)Ni(PEt2)2F (h2-(2,20 -F2-Ph))Ni2(PEt3)4 (2,6-Me-4-OH-Ph)2Ni2(m-(OH)2)PPh3 (2-SMe-Ph)Ni(PMe3)2I (C6F5)Ni(1,2,3,4-(nPr)4-Z3-c-C4H1)PCy3 [NiPEt3]2(Z4-(4,40 -ditriphenylene)) (m-OPiv)[Ni(PCy3)]2(Z2-1-naph) (2-(3-(OC(H)]C(H)Me)-naph))Ni(PPh2Me)2Cl (PCy3)Ni(N](2-Me-2-(CH2)-indane)(HOTf ) (PCy3)Ni(N](2-(CH2)-indane)(HOTf ) (PPh3)2Ni(CH2CH2CF2CF2) [(PCy3)Ni(3-Me-4-CH2-(a-naphthalenone))]2 [(PnBu3)Ni(3-Me-4-CH2-(a-naphthalenone))]2 (PPhEt2)2NiI(4-I-C6F4) 1,2-Ni(PEt3)2-C6F4 1,4-[(PEt3)2NiBr]2-C6F4 [(PEt3)2Ni]2[m-Z1:Z1-(Ph-3,4-F2)]2 [(PEt3)Ni]3(m3-4,5-F2Ph)(m3-4,5-F2Ph-40 ,50 -F2Ph) (2-(C(CN)]C(O)CF2CF2CF3)-NC4H6)Ni(PPh3)Mes (2-(C(CN)]C(O)CF2CF2CF3)-NC5H8)Ni(PPh3)Mes (2-(C(CN)]C(O)CF2CF2CF3)-NC6H10)Ni(PPh3)Mes (PCy2Ph)2NiCl(o-Tol) (1-PiPr2-2-BF3-Ph)NiPh(PPh3)(MeCN) (1-PiPr2-2-BF3-Ph)NiPh(2,6-lut) (1-PiPr2-2-BF3-Ph)NiPh(PPh3) (m-(o-Ph4))Ni2(PiPr3)2 (m-(2-20 -biphenyl))Ni2(PiPr3)2 (PCy3)Ni(2-Me-4-F-Ph)(Me2NSO3) (PiPr2(CH2CH2OMe))2NiF(3,5,6-F3-2-Py) (PMe3)2NiCl(2-(C(O)Et)-Ph) (PMe2Ph)2Ni(o-(C(O)]C(H)Me)-Ph) (dcypp)NiCl(p-CF3-Ph) (PMe3)2NiCl(2-(CH]N(o-Cl-Ph))3-Cl-Ph)

Cl F

1.883(3)618 1.858(4)619 1.874(3)620 1.897(1)203 1.892(5)621 1.927(1), 1.935(3)622 1.887(5), 1.887(5)622 1.974(6)623 1.878(3); 1.878(3)624 1.916(2), 1.962(2); 1.920(2), 1.963(2)266 1.900(2), 1.901(1)602 1.916(3)625 1.949(2)626 1.928(4), 1.928(4)627 1.857(6)408 1.892(1)601 1.918(9)628 1.938(5)628 1.995(3)629 1.945(3), 1.945(3)630 1.940(2), 1.940(2)630 1.902(2)599 1.870, 1.870599 1.898(2), 1.898(2); 1.899(2), 1.899(2)599 1.916(2), 1.920(2), 1.963(2), 1.962(2)605 1.974(3), 1.980(3)605 1.884(3)631 1.886(4)631 1.892(2)631 1.877(13)632 1.898(3)633 1.892(3)633 1.891(3)633 1.930(3), 1.919(3)634 1.993(3), 1.915(3)634 1.907(4)635 1.875(5)292 1.885(3)183 1.932(2)183 1.922(2)636 1.8930(19)637

dcypp, 1,2-bis(dicyclohexylphosphino)propane.

Metallopolymers are an important class of materials with unique applications due to their electronic and magnetic properties. NHC can stabilize the initial step of the ring-opening polymerization of nickelocene.566 Upon introduction of the NHC a ring slip occurs, in which one of the cyclopentadienyl groups undergo Z5 to Z1 shift. After heating, the Z1-cyclopentadienyl group undergoes ring opening to extend the length of the chain between the cyclopentadienyl groups. Magnetic data of the soluble polynickelocene indicates paramagnetic contribution corresponding to four unpaired electrons from the dinuclear complex. Chetcuti and coworkers investigated the poor performance of nickelocene based catalysts.572 By using an NHC functionalized with a tethered alkyl cyano group, the cyclopentadienyl ligand of 118 can be removed under acidic conditions without severing the nickel NHC bond to generate the acetonitrile adduct 119 (Fig. 60). The tethered alkyl cyano group of 117 can react with KOtBu to form nickel alkyl 118. This method works with a variety of alkyl cyanides including acetonitrile. While the isonitrile adducts are typically formed, further reaction with potassium acetylacetonate results in the square-planar NHC nickel alkyl acetylacetonate complex.316 This CdH activation reaction can also be extended to acetone to form a bridged oxaallyl nickel dimer.573 The cyclopentadienyl NHC cyano alkyl nickel complex can be further reacted with potassium acetylacetonate to generate the NHC bound nickel acetylacetonate complex 120. However, even after the removal of the coordinatively-saturated cyclopentadienyl ligand, these nickel complexes were not catalytically active toward cross-coupling reactions.

Mes

Nickel-Carbon s-Bonded Complexes

Me NiII NC Cl

KHMDS Mes N

N

HCl (1 equiv.) Mes KPF6 (1 equiv.) II Ni N CH3CN 20 min Me N 97% yield

NC

toluene 66% yield

117

118 dark green

Mes

Mes N II N Ni

327

N

PF6

HCl (1 equiv.) KPF6 (1 equiv.)

N

O

CH3CN

NC

N

O NiII

N

NC 120

119 dark yellow XRD, 1H NMR, 13C NMR, IR, HRMS

Fig. 60 Method of removal of the cyclopentadienyl ligand while maintaining nickel alkyl bond.

8.05.4.2

Phosphine nickel complexes

Organophosphines are traditional inorganic ligands that can be easily synthesized and have tunable electronic and steric characteristics.402 They behave as L-type ligands and have mild p-accepting character which makes them strong field ligands. For practical applications they have been largely usurped by diphosphine ligands due to their tendency to form multiple chelates in solution. This makes reactions of organophosphine nickel complexes difficult to monitor by 31P NMR and results in varied reactivity due to the occasionally vacant coordination site.

8.05.4.3 8.05.4.3.1

Common reactivity Cycloaddition

Transition-metal catalyzed hydroacylation reactions are plagued by decarbonylation side products which also serve to deactivate the catalyst through binding of carbon monoxide. Ogoshi and coworkers demonstrated the intramolecular alkene hydroacylation to synthesize benzocyclic ketones catalyzed by a nickel(0) NHC system.671 Isolation of the oxanickelacycle 122 intermediate resulted in a dimeric nickel complex that reductively eliminates at 130  C to give the desired benzocyclic ketone product 123 (Fig. 61). [2 +2] cycloadditions are thermally forbidden according to the Woodward-Hoffmann rules, but this can be circumvented through the use of a metalacyclic intermediate.682 The nickel(0) NHC system was also found to catalyze the [2 + 2] cycloaddition of conjugated enynes with a,-b-unsaturated ketones.669 Additionally, it allows for isolation of the nickelacycle intermediate 125 in which the nickel is bound to the a position of the cyclohexanone substrate, while with acyclic enones the oxygen-bound nickel intermediate is formed (Fig. 61). Upon heating to 60  C in the presence of ethyl acrylate the cyclobutene product 126 is formed.

O Ni0

22 ºC toluene 33d

O NiII

130 °C toluene 24h

tBuNHC

tBuNHC

122 yellow 84% yield XRD, 1H NMR, 13C NMR, EA

121 yellow

O 123 32% yield

O O

DippNHC

Ni(cod) 2 DippNHC +

Ph 124

toluene 30 min

NiII

O

Ph Ph 125 orange 45% yield XR D, 1H NMR, 13C NMR, EA

Fig. 61 NHC-stabilized cycloaddition intermediates.

CO2Et

(5 equiv.)

126

328

Nickel-Carbon s-Bonded Complexes

F F Ni(cod) 2 + 2 PPh 3

F

O

F F

F

Ph F F Ph3P

NiII

PPh 3 127 yellow quantitative yield

toluene 40 ºC 48h 58% yield

XR D, 1H NMR, 13C NMR, 31P NMR, 19F NMR, EA

F F F F Ph3P

NiII O PPh 3 128 yellow

Ph

XRD, 1H NMR, 13C NMR, 31P NMR, 19F NMR, EA

Fig. 62 Olefin oligomerization mediated by nickelacycles.

The metallocyclic intermediate 127 in the olefin trimerization of ethylene with tetrafluoroethylene was isolable (Fig. 62).629 Furthermore, polar monomers such as ethyl acrylate are incorporated when combined with the metallocyclic intermediate. When styrene and tetrafluoroethylene are oligomerized and Et2OBF3 is added, a-fluorine elimination occurs to produce 1,2-difluorocyclobutenes.310

8.05.4.3.2

Oxidative addition

Low-valent metal complexes have frequently been investigated for their ability to activate strong bonds. NHCs have facilitated these investigations through their ability to stabilize a variety of low-valent metal complexes.683 The Radius group uses a nickel(0) NHC dimer to demonstrate the oxidative addition of the sulfur-carbon bond of both alkyl and aryl sulfoxides to generate the nickel(II) alkyl 132 and aryl complexes respectively (Fig. 63).649 The sulfur atom of the sulfoxide remains bound in the case of DMSO and phenyl methyl sulfoxide, while in the case of diphenyl sulfoxide the sulfoxide group rearranges to bind to the nickel through the oxygen atom. Moreover, this nickel(0) NHC dimer serves as a competent catalyst for the Hiyama coupling of triethoxy or trimethoxy phenyl silane with hexafluorobenzene or perfluoro toluene.660 However, attempts to catalyze the Negishi coupling using dimethyl and diethyl zinc were unsuccessful due to the stability of the generated nickel alkyl complex 131.661 Bis(allyl) nickel complexes bind allyl groups in both the Z3 and Z1 fashion.684 The inclusion of a sterically hindered L-type ligand—either NHC’s or phosphines—allows for the formation of a 16e− bis(allyl) complex through Z1 allyl coordination.670 By incorporating the electron-rich methallyl substituent, the Z3/-Z1 nickel bis(methallyl) carbene complex is isolable, although low temperature 1H NMR data suggest that some of the Z3/-Z1 isomer exists for several ligated nickel bis(allyl) complexes. DFT calculations suggest that CO2 insertion occurs at the Z1 NidC bond. Similar conclusions were found with NHC nickel benzyl complexes, for which introduction of an addition PMe3 ligand allowed for the isolation of the Z1 coordinated benzyl complex. Palladium and nickel have distinct chemistry with regards to this Z3/-Z1 isomerization as well as cyclopentadienyl ring slippage. The synthesis of the NHC stabilized palladium cyclopentadienyl methallyl complex results in an Z3 methallyl substituent and an Z1 cyclopentadienyl substituent. Alternatively, the NHC stabilized nickel cyclopentadienyl methallyl complex gives an Z1 methallyl substituent and an Z5 cyclopentadienyl substituent. Regardless of which substituent is bound through an Z1 coordination mode, it remains the site of reactivity toward electrophiles as suggested by the aforementioned CO2 insertion reaction.

8.05.4.4

Selected catalytic reactions

5

Z -Indenyl nickel methyl complexes coordinated by phosphines, such as 136, serve as catalysts for the hydrosilylation of styrene (Fig. 64).574 The dissociation of the phosphine ligand is a critical step in the hydrosilylation reaction. Surprisingly, changing the methyl group with a chloride did not affect the catalytic reaction, even though the methyl derivative would be expected to have a less labile NidP bond. Nickel(II) complex 137 is an effective nickel(0) precatalysts, which activates through intramolecular alkene insertion, followed by b-H elimination (Fig. 65). The addition of Et3SiOTf or heat can cause the Heck reaction to proceed and produce an in situ nickel(0) complex. This nickel(0) complex is able to catalyze a carbonyl-ene cross-coupling reaction. The Et3SiOTf likely serves as a halogen-atom abstraction agent to provide a vacant coordination site on nickel, this allows the Heck reaction to proceed.

8.05.5

Low-valent nickel complexes

8.05.5.1

Nickel(0) complexes

Ni(0) complexes play a vital role in oxidative activation of electrophilic organic molecules. Its low redox potential enables Ni(0) to activate substrates that are conventionally inert to Pd(0), such as CdO and CdCN bonds. The Love group demonstrates that Ni(cod)2 can catalyze the coupling of aryl thioesters with aryl boronic acids.407,439 (dtbpe)Ni(0)(Z6-benzene) 138 can undergo oxidative addition, followed by decarbonylation, of thioesters to generate nickel alkyl thiolates 139 (Fig. 66). Phenyl thioether then forms upon transmetalation of phenyl boronic acid. (dtbpe)Ni(0) 138 can also react with esters to produce aryl nickel-acetate 140 without decarbonylation. Upon heating, complex 140 can cross-couple with boronic acids to form biphenyl, possibly via transmetalation.

iPr N

iPr N

F NiII

N iPr F

N iPr F

iPr N

ZnEt2 F

iPr

NiII

N

F

NC

N iPr

toluene

F F 131 yellow 44% yield

NMR, 13C NMR, 19F NMR, IR, MS, EA

THF -78 ºC iPr

iPr

CN

NiII N iPr

toluene

N

N

iPr N

1H

N iPr F

F

135 orange 66% yield XRD, 1H NMR, 13C NMR, IR, EA CN

N iPr F F

F

iPr

iPr N

NiII

THF 12h

F 130 yellow 56% yield XRD, 1H NMR, 13C NMR, 19F NMR, IR, MS, EA F F F

N N iPr iPr N

F

iPr N

Et

iPr N

N iPr

Me

N Ni0

Ni0 129

N

iPr

iPr

iPr iPr N

O S

iPr N

Me

iPr N

NiII N N S Me iPr iPr O

toluene

N 132 yellow 76% yield

CN Cl

134 colorless 80% yield XRD, 1H NMR, 13C NMR, IR, MS

Me

XRD, 1H NMR, 13C NMR, IR, EA Et2O iPr N

Cl

iPr N

NiII N iPr

N iPr

133 orange 92% yield XRD, 1H NMR, 13C NMR, IR, EA

Fig. 63 Bond cleavage reactions mediated by the nickel(0) dimer [(iPrNHC)4Ni2(cod)].

H2PhSi Me3Si

136 (1 mol%)

SiMe3

+

NiII Me

PhSiH3

PPh 3

C6H6 RT, 5h 79% yield

136 XRD, 1H NMR, 13C NMR, 31P NMR, EA

Fig. 64 Catalytic hydrosilylation of styrene by a nickel complex.

Ph3P

NiII

Cl PPh 3

Et3SiOTf or heat

(PPh 3) 2Ni0

137

137 (5 mol%) Et3N (6 equiv) Et3SiOTf (1.75 equiv)

O +

O

SiEt3 n-pent

n-hex

MeO

toluene, RT 18 h

MeO 98% yield

Fig. 65 A nickel(II) precatalyst that forms nickel(0) species.

330

Nickel-Carbon s-Bonded Complexes

Fig. 66 A diphosphine nickel complex demonstrating CdS bond formation and decarboxylative CdC bond formation.

Me

Me

Me

N Li + Ni0(CDT) 141

ethylene tmeda

Li Me

Ni0

N

N

Et2O -60 to 0 °C

Me

N

Me

-tmeda Me

Me

Me Me

Li N N

Ni0 CDT = 142 yellow 48% yield 1H

143 XRD

NMR, 13C NMR, 7Li NMR

solv

solv

M(solv)n

M

Ni0

Ar-Br Ni2-

solv

solv 144

-C2H2 -MBr

M

Me

Me

M = Li, MgX

M R

Ni0

R-M

Ni0

M

Ph-R

M

solv solv

146

145

solv

solv 147

Fig. 67 Isolation of a nickel(0) aryl complex and its postulated role in CdC bond formation.

The doubly-reduced Ni(cod)2 complexes 142 and 143 were reported in the 70s by Jonas and Porscheke.685 Cornella and coworkers discovered that 143 can catalyzed the Kumada coupling of alkenyl bromides and alkyl Grignards at −65  C,592 while traditional nickel(0) catalysts fail. In a stoichiometric study, Ni(2–) complex 144 undergoes oxidative addition to bromobenzene to afford Ni(0)-phenyl anion 146, which proceeds to transmetalate with Grignard reagents to deliver the cross-coupling product (Fig. 67).

8.05.5.2 8.05.5.2.1

Nickel(I) complexes and representative reactivity Two-electron oxidative addition and reductive elimination

The growing recognition of the involvement Ni(I) and Ni(III) intermediates in catalysis has sparked the investigation of Ni(I) complexes. The study of low-coordinate transition metal complexes can aid in understanding the intermediate complexes that exist in the solution state. Isolable low-coordinate complexes are typically stabilized through the use of sterically saturated, inert ligands.686 The Tilley group synthesized the Ni(I) complex 148 stabilized by bulky amido ligands (Fig. 68).578,579 Complex 148 can undergo oxidative addition with methyl iodide to afford nickel(III) alkyl complex 149. This nickel(III) complex adopts a low-spin configuration and can be reduced by KC8 to produce the anionic nickel(II) alkyl species 150, which serves as an isolable intermediate of the coupling of methylmagnesium halides with aryl halides.

Nickel-Carbon s-Bonded Complexes

iPr iPr K iPr

N

iPr MeI

NiI

iPr

N

Si

Me

THF -78 ºC

iPr

NiIII

N

Si

N

iPr

KC8

Si iPr

Si

Me

toluene 25 min

iPr

NiII

N Si

Si N

iPr

iPr 148 yellow

331

iPr K

150 blue 64% yield

149 green 80% yield XRD, 1H NMR, µeff , EA

XRD, 1H NMR, 13C NMR, UV-Vis, EA

Fig. 68 Oxidative addition to a two-coordinate nickel(I) complex to form a nickel(III) complex.

Br

Dipp N NiII N Dipp

SiMe3

Me

Ph

SiMe3

152 yellow XRD, 1H NMR, µeff, EA

ClMgCH(SiMe3)2 Et2O -35 ºC 82% yield Dipp Mes N NiI N Dipp Mes

Li(2,6-Mes2-Ph) Et2O 70% yield

Dipp N NiI N Dipp

Cl Cl

Dipp N NiI N Dipp

NaN(SiMe3)2 Et2O -35 Â C

151 pale yellow

155 yellow XRD, 1H NMR, µeff, EA

SiMe3 Dipp Ph N SiMe3 + 1/2 Me NiII Me N Ph Br Dipp 153 blue 70% yield

XRD, 1H NMR, 13C NMR, EA

Dipp N NiI N Dipp

SiMe3 N SiMe3

154 yellow 72% yield

Fig. 69 Transmetalation to form two-coordinate nickel alkyl and aryl complexes.

8.05.5.2.2

One-electron oxidative addition and reductive elimination

The majority of Ni(I)-mediated oxidative addition of alkyl halides proceed through single-electron pathways and form radical intermediates. Hillhouse pioneered the synthesis of Ni(I) complexes with NHC ligands (Fig. 69).314,664,668 Transmetalation to (DippNHC)Nickel(I) 151 dimer gave Ni(I)-amido 154, terphenyl complex 155, and alkyl 152. Complex 152 can readily activate sec-phenethyl bromide to afford Ni(II) 153 and the corresponding benzylic dimer, an indication of radical formation. Complex 63 is one of the landmark examples of organonickel(I) complexes. Complex 63 is prepared from ligand exchange of (tmeda)NiMe2 61 with terpyridine followed by methyl radical ejection.687 Characterization by EPR established that the terpyridine ligand is reduced and the nickel center remains in the +2 oxidation state.688 This complex 63 is a viable catalyst for Negishi coupling reactions (Fig. 70). The Lee group isolated a T-shaped nickel(I) pincer complex 156 featuring an amido PNP ligand that can undergo bimolecular oxidative addition with both MeI and MeCN to generate (PNP)nickel(II)-methyl and (PNP)Ni-iodide or cyanide. (Fig. 71).115 Its intriguing reactivity includes the activation of ethylene by two molecules of 156 cooperatively to afford a bridged ethyl nickel dimer 160. Complex 156 binds to p-acceptors, such as pyridine and CO. The EPR spectra show organic radicals, suggesting the delocalization of the radical to the ligands. In order to distinguish the reactivity of these nickel oxidation states, MeI was introduced

I +

ZnBr

N THF, 23h RT

Fig. 70 Ligand redox-activity in (terpy)NiMe.

N

63 (5 mol%)

64% yield

N NiII Me

63 dark violet XRD, UV-Vis, EA, EPR

332

Nickel-Carbon s-Bonded Complexes

iPr

iPr P

MeI or MeCN iPr P N

N

C6H6 10 min

NiII

P iPr

iPr

iPr

NiI

+ Me 157

iPr

iPr

P

156 red

N

C6D 6 CO

P iPr

N

NiII

iPr I/CN

P iPr

158/159 iPr

XR D, 1H NMR, 13C NMR, 31P NMR, EA, UV-Vis

P iPr

P

iPr

iPr iPr

NiII iPr iPr

iPr P

NiII N

160 brown

P iPr

XR D, 1H NMR, 13C NMR, 31P NMR

iPr

iPr P iPr N NiI CO

P

MeI

N

P N

Me

P

P iPr

NiII

iPr iPr

iPr

iPr 161 and 162

NiII

P iPr

163 red (13%)

iPr

iPr

iPr P iPr

iPr N

O iPr

164 yellow (37%)

NiII

P iPr

iPr 166 purple

iPr 165 green (50%)

-

iPr N

I

P

Me

+ P

NiII

iPr iPr

P

MeI No Reaction

CO

N

NiII

iPr

MeI 163

CO

P iPr iPr 167 orange

Fig. 71 A T-shaped nickel(I) metalloradical demonstrating CdCN bond cleavage and ethylene insertion to form nickel alkyl complexes.

to these complexes. Ni(II) 166 is inert to MeI, while Ni(I) 162 reacts with MeI to generate a mixture of (PNP) Ni(II)dMe 163, (PNP)Ni(II)–acyl 164, and (PNP)Ni(II)-iodide 165. (PNP)Ni(0) 167, on the other hand, only generated the nickel methyl complex through two-electron SN2 oxidative addition This divergent nickel(I) and nickel(0) reactivity did not extend to other alkyl iodides, as both oxidation states favor the CO insertion product.116 Fu and coworkers developed (pybox)Ni complexes for a variety of enantioselective cross-coupling reactions (Fig. 72).175 During the course of their mechanistic investigation, they isolated (iPrpybox)nickel(I)dBr 168 and (iPrpybox)nickel(II)Ph complex 172 and evaluated their reactivity in catalysis. Studies reveal that the activation of propargylic bromides by Ni(I)dBr 168 precedes transmetalation of Ph2Zn to (iPrpybox)nickel(II)Br2 169 to afford (iPrpybox)nickel(II)(Ph)Br 170 as an intermediate. In contrast, (iPrpybox)Ni(I)dPh 172 was not an intermediate, since its reaction with propargylic bromides forms the cross-coupling product in a lower yield and ee compared to the catalytic reaction. Four pathways are possible for Ni(I)-mediated activation of alkyl halides to form radicals: SN2 oxidative addition followed by radical ejection, outer-sphere electron transfer preceding homolytic cleavage of carbon-halogen bonds, the inner-sphere variant, and concerted halogen-atom abstraction.3 Diao and coworkers isolated a series of three-coordinate nickel(I) aryl complexes supported by tBuXantphos (Fig. 73). These metalloradicals are unreactive with aryl halides but can perform halogen-atom abstraction on alkyl bromides and iodides. Kinetic studies show that Ni(I)dPh 173 favors the activation of alkyl bromides that can form stable radicals, and the rate of bromide abstraction is sensitive to steric effects. These results rule out oxidative addition and electron transfer pathways, suggesting that concerted halogen-atom abstraction is the operational mechanism. DFT calculations validate this hypothesis and demonstrate that electrons are donated from Ni to the s of the CdBr bond to initiate homolytic cleavage.

Nickel-Carbon s-Bonded Complexes

333

O N O

nBu

NiI

nBu TMS

Br

iPr

N

Ph

TMS

Br

N

168 purple

iPr

O

O N

NiIII

171

iPr

N N

TMS Ph

Br O

TMS

N

O

nBu

iPr

N

nBu iPr

NiII

Br

Br

N

169

iPr O O

PhZnAr

N O

N

NiII

N iPr

BrZnAr

Ph

O

NiI

N

Br

iPr

170 red

XRD, 1H NMR, 13C NMR, EA

iPr

N

iPr

N

Ph 172

not an intermediate XR D, EPR

Fig. 72 Nickel-catalyzed Negishi coupling proceeding through a nickel(I) intermediate.

O

NiI

tBu P tBu

Br

Br

Ph

O

tBu

173 dark brown XRD, 1H NMR, EA, HRMS

Ni Ph

P tBu

tBu P tBu

P tBu

tBu

tBu P tBu O NiII Br + P Ph tBu tBu 174 85% yield XRD, 1H NMR, 31P NMR, EA, HRMS

Fig. 73 Halogen-atom abstraction by a nickel(I) aryl complex.

The Hazari group isolated several nickel(I) aryl complexes stabilized by the dppf ligand (Fig. 74).428 Ni(I)-Tripp 175 can disproportionate with Ni(I)dBr to afford Ni(II) 177 and Ni(0) 178. Over time, Ni(I)-Tripp 175 can decompose to generate biaryls, presumably via a bimolecular mechanism. Both TEMPOH and a pyridinium acid can generate TrippH by H-atom transfer and protonation, respectively. Ni(I)dCl exhibits catalytic reactivity toward Suzuki biaryl couplings and is detected as the active catalytic species in reactions with Ni(0) and Ni(II) as the pre-catalysts. The Klein group showed that (2-phosphino-thiophenolato)nickel(I)(bis-trimethylphosphine) 179 can be obtained from the comproportionation of Ni(II) 180 and Ni(PMe3)4, dehydrogenation of Ni(II)-hydride 181, or the UV photolysis of Ni(II)-methyl 183.313 Ni(II)-hydride can undergo insertion with trimethylsilyl acetylene to generate a nickel aryl complex 182 (Fig. 75). The catalytic relevance of the pyrox (pyrox ¼ pyridine-oxazoline) ligand has prompted questions about the redox-activity of this ligand scaffold.398 The reduction of both dialkyl 184 and diaryl derivatives of nickel pyrox gave radical complexes 185 and 186, both demonstrating ligand-centered reduction (Fig. 76).

334

Nickel-Carbon s-Bonded Complexes

PhPh

iPr

P

PhPh P

NiI

Fe

P Ph Ph orange

iPr

+

NiI

Fe

Ni0

+ Tripp

P

P

177 yellow

176 orange

175 XRD, 1H NMR, IR, EPR , μeff

P

NiII

C6D6 30 min

P Ph Ph

iPr

Br

P

cod

Br

178

XRD, 1H NMR, 13C NMR, 31P NMR, EA OMe

Cl

B(OH) 2 +

(dppf)NiICl (2.5 mol %) K3PO 4 (4.5 equiv) toluene, RT, 4 hr

MeO 2.5 equiv.

90% yield Fig. 74 Reactions of diphosphine nickel(I) aryl complexes.

Ph2 S P NiII P S Ph2 180 green SiMe3

Me3Si

Ph Ph

P

H

NiII PMe3 THF 0 ºC S 3d PMe3 182 yellow 64% yield

Ph2 H P - 10 ºC NiII PMe 3 S - H2 PMe3

Ni(PMe 3) 4

+

Ph2 P PMe3 NiI PMe3 S

Ph2 CH3 P NiII PMe3 S PMe3

hv - CH3

179 red

181 red

XRD, 1H NMR, 13C NMR, 31P NMR, MP, IR, EA

183 orange

XR D, 1H NMR, 13C NMR, 31P NMR, MP, IR, EA

Fig. 75 A phosphino sulfide ligand that allows nickel to eject hydrogen or methyl radicals to form a nickel(I) complex.

O

tBu O

N N

Ns NiII

Ns

KC8, 18-crown-6 (1 equiv) Et2O/THF

184 olive-green XRD, 1H NMR, 13C NMR, IR, UV-Vis

Fig. 76 Reduction of

tBu

O O O K

N

O O O

N

NiII

CH2TMS CH2TMS

185 chartreuse 43% yield XRD, EPR , UV-Vis

O O O O K O O O

tBu

-

N N

Dipp NiII

Dipp

186 olive-green 25% yield XR D, EPR , UV-Vis

tBu

pyrox coordinated nickel dialkyl and diaryl complexes to give ligand-centered radicals.

A novel strategy in the synthesis of biomimetic complexes has been the development of protein-based model complexes wherein proteins or large peptides act as ligands to a metal to form the biomimic.689 These systems are more comparable in both size and complexity to the naturally occurring metalloenzymes than traditional metal complexes. A protein-based model of acetyl coenzyme A synthase was developed by incorporating nickel into a mutant of Pseudomonas aeruginosa azurin (Fig. 77).690 Upon reduction, the model complex 187 can react with MeI to give a nickel methyl complex 189. EPR studies reveal the methyl carbon to have a trigonal planar rather than tetrahedral geometry, suggesting that the methyl group is experiencing an inverted ligand field similar to that seen in high-valent trifluoromethyl derivatives of nickel.

Nickel-Carbon s-Bonded Complexes

His117

His117

N

NiII S

HN

eCys112

N

-630 mV

HN

vs. NHE

N

HN

His117 NiI S Cys112

N

MeI

H N

HN HN

His46

His46

His46

188

187

H H NiI S Cys112

N

HN

335

I

189 EPR , UV-Vis, MCD, EXAF S

Fig. 77 Methylation of a protein model complex for the acetyl coenzyme A synthase active site demonstrates ligand field inversion.

8.05.5.2.3

CO2 insertion

A growing number of catalytic studies to incorporate CO2 as a building block to organic molecules has inspired the investigation of CO2 insertion to nickel complexes.691 While CO2 insertion to Ni(II)-methyl complexes require a high temperature,10 computational studies suggest that the more nucleophilic Ni(I)-alkyl complexes should enable a more facile CO2 insertion.692 The Diao group demonstrates that (Xantphos)Ni(I)-methyl 190 can mediate rapid CO2 insertion at room temperature to afford the corresponding Ni(I)-acetate 191 in a high yield (Fig. 78). In the (Xantphos)Ni(I)-methyl and ethyl complexes, the NidO distances of 2.6018(11) A˚ and 2.6737(16) A˚ suggest that the oxygen atom of xantphos is weakly coordinating to stabilize the molecule.459 In contrast to (Xantphos)Ni(I)-methyl, (Xantphos)Ni(I)-phenyl 192 shows no reactivity toward CO2, suggesting that the insertion of CO2 may proceed in an SN2 fashion rather than a concerted pathway. While diphosphine ligands are useful for isolating reactive organometallic intermediates, N,N bidentate ligands such as bipyridines and phenanthroline derivatives are more relevant to modern nickel cross-coupling catalysis.693 Martin, Hazari, and coworkers synthesized a bulky (phen)Ni(I)-neopentyl complex 193 via transmetalation with Grignard reagents (Fig. 79).523 This complex can undergo CO2 insertion to afford a Ni(I) species, assigned to (phen)Ni(I)-neopentyl carboxylate 194 based on EPR spectroscopy. Protonation of 194 afforded t-butyl acetic acid in 52% yield.

8.05.6

High-valent nickel complexes

High-valent nickel complexes are frequently proposed intermediates in nickel catalysis reactions prior to reductive elimination steps to form new bonds.694 The high oxidation potentials of these nickel species promote facile reductive eliminations that remain

Fig. 78 CO2 insertion into a XantPhos nickel(I) methyl complex.

Mes N N

NiI

Mes CO2 (1 bar) Et2O, -60 °C to RT

Mes 193 green XRD, 1H NMR, EPR , µeff Fig. 79 CO2 insertion into a bulky phenanthroline nickel(I) neopentyl complex.

N N

O

2M HCl tBuCH2CO2H

NiI O Mes 194 EPR

52% yield

336

Nickel-Carbon s-Bonded Complexes

elusive for nickel(II) complexes and other first row transition metal complexes. Since the publication of the previous edition of Comprehensive Organometallic Chemistry, significant advances have been made in the isolation and study of high-valent nickel complexes, which help to elucidate the role of high valent nickel complexes in the bond-forming steps in organonickel catalysis.

8.05.6.1

Nickel(III) complexes

Zargarian and coworkers synthesized a variety of derivatives featuring an aliphatic backboned PCP ligand.18 Transmetalation of (PCP)nickel(II)Br 195 with n-butyllithium gives (PCP)nickel(II)(n-Bu) 196, which is resistant to b-hydride elimination at room temperature. Complex 196 is active in catalyzing the Kumada coupling of methyl Grignards with chlorobenzene, with (PNP)Ni(II)-methyl found to be the catalyst resting-state using 31P{H} NMR spectroscopy. Iron(III) or copper(II) halides can oxidize (PCP)nickel(II)Br 195 to give (PCP)Ni(III)Br2 197 as a red compound (Fig. 80). The synthesis of organometallic Ni(III) complexes and the study of their reductive elimination were pioneered by Mirica, who developed an N4 macrocyclic ligand to stabilize a nickel(III) cationic complex with 4-fluoro-phenyl and bromide substituents.695 Transmetalation with methyl Grignard allowed for the isolation of the methyl derivative which is thermally unstable at room temperature and decomposes to form the reductively eliminated 4-fluoro-toluene (Fig. 81). The methylated complex can be synthesized by using the dibromide starting material. Investigation of the reductive elimination of this complex demonstrated that the nickel(III) derivative formed ethane in 54% yield and methane in 30% yield. Nickel(III) in the presence of the oxidant Ac FcPF6 formed an 84% yield of ethane. These observations indicate that 198 may undergo a methyl radical ejection while a nickel(IV) intermediate undergoes a traditional reductive elimination pathway. Many nickel(III) complexes are synthesized via chemical oxidation methods. In order to investigate the mechanism of nickel catalyzed fluorination reactions, the Ritter group synthesized a nickel(II) complex featuring a tethered aryl substituent (Fig. 82).281,295 This compound was subsequently oxidized using N-fluoropyridinium tetrafluoroborate 201 and BF3Et2O to give the nickel(III) tetrafluoroborate salt 202. While this nickel(III) complex is isolable when the aryl group is tethered to a bischelating substituent, the corresponding complex with a monochelating substituent is only observable by EPR and produces the aryl fluoride product, whereas complex 202 does not undergo reductive elimination. In situ oxidation of 203 forms a putative Ni(III) intermediate that undergoes facile CdF bond-forming reductive elimination.

P NiII nBu

P iPr

iPr iPr

nBuLi P

C6D6

iPr

iPr

196 1H

P NiII Br

iPr iPr

FeBr3 CH2Cl2

iPr

P iPr

NiIII

iPr P iPr Br

iPr Br

195 yellow

197 red

XRD, 1H NMR, 13C NMR, 31P NMR, EA

XRD, EA

NMR, 13C NMR, 31P NMR

Fig. 80 Oxidation of a nickel(II) complex to form a nickel(III) dibromide.

tBu

tBu

N

N N

MeMgI

Br

N

THF, -50 ºC N

F tBu

MeCN, RT

N

NiIII

198 yellow XRD, 1H NMR, 19F NMR, UV-Vis, HR-ESI-MS, μeff

Br

Me NiIII

N N

F tBu

199 red XR D, 1H NMR, 19F NMR, UV-Vis, HR-ESI-MS, μeff

THF, RT

Me F 54% yield

Fig. 81 Reductive elimination from nickel(III) complexes.

F 48% yield

Nickel-Carbon s-Bonded Complexes

337

BF 4 +

N

NO2 O N S O NCMe

N NO2 F 201 O N S O BF3•Et2O NiII N MeCN -40ºC O

N O

BF4

NiIII N 202

200

XR D, EPR O N S O NiII N

N

18F

PhI(4-OMe-pyridine) 2(OTf) 2

tBuO N

N

203 OtBu peach 46% yield

18F K3PO 4, 18-crown-6 MeCN (0.5% H2O) 23 °C (99%) and stereoselectivities (E/Z >99:1).

Scheme 45 Pd-catalyzed intermolecular allylic C–H amidation by Cook.

The first allylic C–H azidation of allylarenes 48 via Pd(II)-catalysis using NaN3 in the presence of O2 as terminal oxidant to produce the allyl azides 101 was described by Jiang and co-workers67 (Scheme 46). A wide range of allyl benzenes 48 with various substituents reacted with NaN3 for allylic C–H azidation to deliver a range of allyl azides 101 in good yields (52%–92%). This can be taken for the one-pot allylic C–H azidation and cycloaddition reaction under click-conditions with alkynes to furnish 102 in moderate to good yields (47%–90%).

Scheme 46 First allylic C–H azidation of allylarenes by Jiang.

Allyl-Palladium Complexes in Organic Synthesis

649

In 2019, Ohkuma and co-workers68 reported the palladium catalyzed allylic isocyanation. The allylic phosphonates 103 were treated with trimethylsilyl cyanide in presence of catalytic amount of Pd(OAc)2 to deliver exclusively allylic isonitriles 104 (no allylic nitriles were seen, Scheme 47). The regioselectivity of the cyanide as the N-terminus nucleophile was achieved by the use of phosphate as the leaving group. The reaction was exposed to a wide range of aromatic-, heteroaromatic-, vinylic-, and aliphaticsubstituted allylic phosphates to deliver the corresponding allyl isonitriles in good to excellent yields.

Scheme 47 Palladium catalyzed allylic isocyanation by Ohkuma.

8.09.3.2

Intramolecular allylic amination

In 2007 Fraunhoffer and White69 disclosed the oxidative Pd-catalyzed intramolecular direct allylation of N-sulfonylcarbamates 105 having terminal unsaturation to deliver oxazolidinones 106 (Scheme 48A). The optimized reaction condition involved the 1,2-bis(phenylsulfinyl)ethane as ligand and phenyl p-benzoquinone (PBQ) as the terminal oxidizing agent. The distereoselectivity of the reaction depends on the substituent at the homoallylic position in the substrate (Scheme 48A). As the bulkiness of the substituent increases the preference of anti-product over the syn-product occurs. Later in 2009, the same group modified the reaction to improve the reactivity by switching the N-protecting group from tosyl to the more electron-poor nosyl protecting group 107 (Scheme 48B).70 This imparts an increase in acidity of the N-attached hydrogen atom which results in better and easy nucleophile to deliver the 1,3-oxazinones 108 in good yields and high diastereoselectivities.

Scheme 48 Oxidative Pd-catalyzed intramolecular direct allylation of N-sulfonylcarbamates (PhBQ ¼ phenyl benzoquinone).

In 2009, Poli and co-workers71 documented the Pd-catalyzed intramolecular direct allylation of N-sulfonylcarbamates 109 (Scheme 49). Their studies showed that the presence of AcOH in the reaction medium would be expected to improve the reactivity for reasons, such as (a) the Pd(0) reoxidation depends on the pH of the reaction,72 (b) the acidic medium promotes the protonation

Scheme 49 Poli’s Pd-catalyzed intramolecular direct allylation of N-sulfonylcarbamates.

650

Allyl-Palladium Complexes in Organic Synthesis

of the acetate ligand bound to Pd, followed by formation of a reactive cationic complex,73 and (c) the acidic medium prevents the decomposition of HPdOAc and the following deterioration of the (
3-allyl)palladium complex intermediate.74 By both experiments and theory, they concluded that a change in solvent to AcOH brought about inimitable improvement in reaction yields and diastereoselectivities. Later in 2017, the Poli’s group75 described the palladium-catalyzed cyclization of unsaturated Nsulfonylamides elevated by hypervalent iodine reagent, (diacetoxyiodo)benzenes (Scheme 49). The reaction of N-tosylglycine-N0 crotyl-N0 -benzylamide 111 with Pd(OAc)2/PhI(O2CCH3)2 delivered vinylpiperazinone 112 in 50% yield instead of the expected corresponding aminoacetoxylated product. Whereas the reaction of N-tosylglycine-N0 -crotyl-N0 -benzylamide 111 with Pd(OAc)2/ PhI(O2CCF3)2 furnished 2-vinylimidazolidinone 113. Liu and co-wokers76 reported the aerobic Pd-catalyzed intramolecular direct allylation of d,e-unsaturated N-tosyl-carboxamide 114. Here the basic reaction conditions influenced the dihydroazepinone 116 formation via 7-endo cyclization, whereas the use of CrIII-(salen)Cl delivered a vinylpyrrolidone 115 as the major product by favoring 5-exo ring closure (Scheme 50). They concluded that the basic conditions might be responsible for the generation of an anti-configured
3-allylpalladium complex which promotes an intramolecular amide ligand, resulting in the observed dihydroazepinone moiety.

Scheme 50 Intramolecular direct allylation of d,e-unsaturated N-tosyl-carboxamides.

Recently, the Zhang’s group77 described the synthesis of indolines 118 by using Pd(II)-catalyzed aerobic intramolecular allylic C–H amination of alkenes 117 (Scheme 51). So far, the synthesis of indolines have been reported through aromatic C–H activation, Wacker-type reaction and allylic substitution reactions. Zhang work achieved the synthesis of indolines through Pd-catalyzed direct intramolecular allylic C–H amination. Various aniline tethered terminal olefins 117 were taken for the Pd(II)-catalyzed allylic C–H amination to deliver indoline derivatives 118 in good yields (33%–81% yield).

Scheme 51 Zhang’s synthesis of indolines via aerobic intramolecular allylic C–H amination.

Abrunhosa-Thomas and co-workers78 synthesized 2,6-disubstituted-1,2,3,6-tetrahydropyridines 120/121 diastereoselectively from non-activated alcohols 119 via Pd-catalyzed intramolecular allylic amination (Scheme 52). The N-protected b-amino-allylic alcohols 119 which can be prepared from the corresponding phosphonates were treated with Pd-catalyst under mild conditions to undergo intramolecular cyclization and produce tetrahydropyridines derivatives in good yields and high diastereoselectivities.

Scheme 52 Diastereoselective synthesis of 2,6-disubstituted-1,2,3,6-tetrahydropyridines (DCE ¼ 1,2-dichloroethane).

Allyl-Palladium Complexes in Organic Synthesis

8.09.3.3

651

Asymmetric allylic amination

In 2008, Shi and co-workers79 disclosed the asymmetric C–H diamination of terminal olefins catalyzed by the palladium complex in presence of H8-BINOL-based phosphorus amidite ligand L11 (Scheme 53). The di-tert-butyldiaziridinone 123 worked as the nitrogen source as well as oxidant and gave diaminated products 78 with terminal olefins in high yields and excellent regio-, diastereo- and enantioselectivities.

Scheme 53 Asymmetric C–H diamination of terminal olefins.

In 2017, Han and co-workers80 established the unique chiral palladium-catalyzed Csp2-H functionalization/intramolecular asymmetric allylation between aryl urea derivatives and diene ester (Scheme 54). After screening of several chiral ligands they concluded that chiral sulfoxide-oxazoline (Sox) L12 is the best. The sulfoxide ligand having single chiral center on sulfur is well capable of C–H bond cleavage and stereocontrol in the asymmetric allylation step. The substituted aryl urea 124 and 1,3-diene ester 125 delivered a wide range of indoline derivatives 126 in good yields and enantioselectivities.

Scheme 54 Asymmetric allylation between aryl urea derivatives and diene ester.

In 2019 Zhang and co-workers81 described the Pd-catalyzed asymmetric allylic cycloaddition for the enantioselctive synthesis of cyclic urea (Scheme 55). The nitrogen-containing allylic carbonates 127/130 were treated with the isocyanates 128 catalyzed by the Pd-complex generated in situ form Pd2(dba)3
CHCl3 and phosphoramidite ligand L13/L14 to deliver imidazolidinones 129 and tetrahydropyrimidinones 131 in good yields and excellent enantioselectivities. This mild reaction conditions have good tolerance toward various substitution on the isocyanates.

Scheme 55 Asymmetric allylic cycloaddition for the enantioselctive synthesis of cyclic urea.

The Guo’s group82 unfolded the remarkable synthesis of functionalized chiral hexahydropyrazolo[5,1-a]isoquinoline derivatives via Pd-catalyzed asymmetric tandem [3 + 2] cycloaddition/allylation of methylene-trimethylenemethane (Scheme 56). The methylene-TMM 133 on treatment with azomethine imine 132 underwent asymmetric tandem [3 + 2] cycloaddition/allylation catalyzed by Pd-complex with the chiral phosphoramidite ligand L15 to deliver the functionalized hexahydropyrazolo[5,1-a] isoquinoline derivatives 134 in excellent to good yields and markable enantioselectivity.

652

Allyl-Palladium Complexes in Organic Synthesis

Scheme 56 Guo’s synthesis of functionalized chiral hexahydropyrazolo[5,1-a]isoquinoline.

In 2020, Guo and co-workers83 established the Pd-catalyzed asymmetric allylic amination of alicyclic Morita–Baylis–Hillman (MBH) adducts providing the carbocyclic nucleosides with high enantioselectivities (Scheme 57). The use of Pd-catalyst in combination with (S,Sp)-phosferrox ligand L16 gave in situ Pd-complex which exhibited excellent N9/N7-selectivities, high enantioselectivities and good yields. This mild reaction conditions showed good tolerance to various functional groups.

Scheme 57 Synthesis of carbocyclic nucleosides.

Qian and Tang84 reported the unexcelled regio- and enantioselective synthesis of chiral vinyl-substituted heterocycles via Pd-catalyzed tandem allylic substitution reaction (Scheme 58). The Pd-catalyst in combination with achiral bisphosphorus ligand WingPhos L17 (only 1 mol%) catalyzed the reaction between N,N0 -(1,2-phenylene)bis(4-methylbenzenesulfonamide) 139 and 2-butenylene dicarbonate 140 to produce a series of chiral vinyl-substituted heterocycles 141 in good yields and notable enantioselectivities. The mild reaction conditions were implicated for the synthesis of various heterocycles such as tetrahydroquinoxalines, piperazines, dihydro-2H-benzo[b][1,4]-oxazines, and morpholines in good yields.

Scheme 58 Synthesis of chiral vinyl-substituted heterocycles.

Recently, Wolf and co-workers85 established the utilitarian methodology for the asymmetric allylic amination of isatins 142, sulfonamides, imides, carbamates, amines and N-heterocycles by using Pd-catalysis (Scheme 59). The Pd-catalyst along with the 4-tert-butyl-2-[2-(diphenylphosphino)phenyl]-2-oxazoline ligand L18 catalyzed the reaction at room temperature in presence of mild base to render asymmetric allylic C − N bond formation with remarkably high enantioselectivities and good yields.

Scheme 59 Asymmetric allylic amination of isatins, sulfonamides, imides, carbamates, amines and N-heterocycles.

Allyl-Palladium Complexes in Organic Synthesis

8.09.4

653

Allylic alkylation

The Tsuji-Trost reaction86 is quite applauded in construction of new C–C bonds. However, it requires prefuctionalized allylic substrates. Later in 1979, Trost and co-workers developed a direct allylic C(sp3)–H alkylation, without the use of prefunctionalized substrates.4a–c The transformation required stoichiometric amounts of palladium catalyst and hence a practical approach was still desired. In 2003, Franzén and Bӓckvall87 demonstrated for the first time the catalytic version of this reaction by forming
3-allylpalladium intermediate via Pd-catalyzed insertion of p-bond. Following this, many explored the Pd-catalyzed direct allylic C–H alkylation, arylation, carbonylation, etc., by trapping the allylic C–Pd intermediates with various reagents. Allylic alkylation section is further divided as intermolecular, intramolecular and asymmetric allylic alkylations.

8.09.4.1

Intermolecular allylic alkylation

The active methylene nucleophiles having ester, cyano, keto, aldehyde or amide groups have been widely used in C–C bond formation via p-allylpalladium intermediates. Shi and co-workers in 2008 reported the first example of intra/intermolecular direct allylic C–H alkylation of alkenes with 1,3-diketones (Scheme 60).88 Preliminary mechanistic studies indicated initial generation of p-allylpalladium species via allylic C–H activation and then subsequent nucleophilic attack delivered alkylated products 146 or 148 in good yields.

Scheme 60 Intra/intermolecular allylic alkylation.

Young and White89 in 2008 demonstrated an intermolecular allylic alkylation method by using a-nitrocarbonyl derivatives 149 as nucleophiles having two strong electron deficient groups (Scheme 61, condition A). Later in 2011 they further extended this protocol to unactivated a-olefins 4 (Scheme 61, condition B).90 Thus, the intermolecular allylic C–H alkylation reaction involving unactivated as well as activated olefin substrates was achieved.

Scheme 61 Pd(II)-catalyzed allylic C–H alkylation.

654

Allyl-Palladium Complexes in Organic Synthesis

Trost and co-workers91 in 2012 developed two independent tandem Pd(0)- and Pd(II)-catalyzed allylic alkylation protocols using Pd(PPh3)4 and Pd(OAc)2 catalysts, respectively with 1,4 dienes 152 and tertiary nucleophiles 151 (Scheme 62). Using different substrates it was easy to understand which catalyst could get engaged using their oxidation level. This protocol however had limited substrate and nucleophile scope.

Scheme 62 Tandem Pd(0) and Pd(II)-catalyzed allylic alkylation of 1,4-dienes.

Guo et al.92 successfully expanded Trost oxidative allylic alkylation method to prepare fluorine-containing aromatic compounds 159 having quaternary carbon center that were useful for drug discovery via allylarenes allylic C–H activation with CF3-containing nucleophiles (Scheme 63).

Scheme 63 Allylic C–H alkylation to fluorine-containing compounds (NMP ¼ N-methyl-2-pyrrolidone).

Trost and co-workers93 in 2015 disclosed a new mode of activation of an imine via a rare aza-substituted p-allyl complex. Pd-catalyzed allylic C–H activation of N-allyl imine 160 and then nucleophilic attack of a-alkyl cyanoester 161 furnished exclusively 2-aza-1,3-diene 162 in excellent yield, which further was utilized in an inverse-electron-demand Diels–Alder reaction for hetero-biaryl molecules synthesis (Scheme 64).

Scheme 64 Synthesis of a-cyano b-amino esters by allylic C–H activation.

Wang and co-workers94 in 2017 reported the deacylative allylic C −H alkylation by using decarboxylative nucleophiles (Scheme 65). In this method, the less stabilized tertiary nucleophile generated in situ from the deacylative retro-Claisen

Scheme 65 Allylic C–H alkylation using [Pd(
3-cin)(IPr)Cl] complex.

Allyl-Palladium Complexes in Organic Synthesis

655

condensation of functionalized ketones 164 participated in the subsequent functionalization of
3-allylpalladiun intermediate. Notably, the tertiary a-acetyl carbonyl compounds also generated alkylated products in good yields. A wide range of nucleophiles were tolerated and densely functionalized alkylated products 165 were obtained in moderate to good yields. Zhong and co-workers95 in 2019 described a Pd(PPh3)4/acid-catalyzed mono- or di-selective allylic substitution reaction of benzothiazolylacetate 166 and allylic alcohols 167 to deliver various mono- and diallylated products (168 or 169) in good to excellent yields and selectivities (Scheme 66). The mono- and di-selectivity was efficiently controlled by changing reaction conditions, i.e., solvent and temperature. Both linear as well as branched allyl alcohols were compatible in the reaction.

Scheme 66 Mono- or di-allylation of benzothiazolylacetate with allylic alcohols.

Zhou and co-workers96 in 2019 developed a Pd-catalyzed a-allylation of a-amino acids using vinylcyclopropanes 171 with aldimine esters 170 (Scheme 67). The transformation consisted of mild reaction conditions and had a broad substrate scope to afford a-allylated a-amino acids 172 in high yields and excellent stereoselectivities. The reaction could be scaled up to gram scale with good yield and stereoselectivity.

Scheme 67 Allylic alkylation of aldimine esters with vinyl-cyclopropanes.

Yang and co-workers97 in 2019 reported the Pd-catalyzed decarboxylative transformation of vinyl cyclic carbonates 173 into highly substituted allylic alcohols 175 having new Csp3–Csp3 bond with quaternary carbon center (Scheme 68). This method has good yields, stereoselectivities, mild reaction conditions, easy to handle procedure and a wide substrate scope. All these features afforded a practical and efficient way to generate multi-substituted olefins having a wide range of functionalities.

Scheme 68 Decarboxylative allylation of vinyl cyclic carbonates (dppp ¼ 1,3-bis-diphenylphosphino propane).

Gong and co-workers98 in 2019 revealed the Pd-catalyzed tandem decarboxylation and subsequent allylation of vinyloxazolidinones 176 to afford highly substituted allylic amines 177 bearing a tertiary or quaternary center (Scheme 69). This methodology offered mild conditions, a wide substrate scope and delivered good to excellent yields and selectivities.

656

Allyl-Palladium Complexes in Organic Synthesis

Scheme 69 Decarboxylative allylation of vinyloxazolidinones.

The Chen’s99 work reported in 2020 involves the Pd-catalyzed cross-coupling of vinylethylene carbonates 178 with ketimine esters 179 to deliver stereoselectively the (Z)-tri- or tetrasubstituted allylic amino acid derivatives 180 in good to excellent yields, regio- and stereoselectivities (Scheme 70). This methodology was further utilized for a variety of functional groups having mild reaction conditions and scalability.

Scheme 70 Pd-catalyzed cross coupling of vinylethylene carbonates with ketimine esters.

Wang et al. in 2019 established a highly efficient allylic C −H alkylation of terminal alkenes under mild conditions by palladium-monodentate phosphoramidite catalysis (Scheme 71).100 A wide range of alkenes 4 and carbon nucleophiles 181 were compatible for this transformation. Mechanistic studies revealed that Pd(0) complex coordinated with a monodentate phosphorus ligand, benzoquinone and alkene to generate the active site. The chiral phosphoramidite ligand L19b was able to trigger catalytic activity of Pd-catalyst and promote C–H allylation to deliver asymmetric allylic C–H functionalization of unactivated terminal alkenes with pyrazol-5-ones in good yield and high enantioselectivity.

Scheme 71 Ligand-enabled allylic C −H alkylation of terminal alkenes.

Lei et al. in 2014 reported the first oxidative allylic alkylation of unactivated ketones 184 via novel dual activation strategy for allylarenes 163 allylic C–H alkylation (Scheme 72).101 In this reaction, Pd catalyzed the allylic C −H activation and proline catalyzed

Scheme 72 Synergistic Pd/enamine catalysis for allylic C–H arylation.

Allyl-Palladium Complexes in Organic Synthesis

657

the ketone nucleophilic activation that were proposed to synergistically proceed leading to allylarene C–H alkylation. In this protocol, the Pd-catalyst, amine-catalyst, and p-benzoquinone were all crucial for the direct oxidative coupling. Lin and co-workers102 in 2016 developed the Pd/L-proline-catalyzed direct allylic alkylation of ketones 186 with alkynes 187 with a broad substrate scope and having wide functional group tolerance (Scheme 73). Since no leaving group was liberated in the reaction, this process exhibited high atom economy (with no extra amount of oxidant being required).

Scheme 73 Pd/L-proline-catalyzed direct allylic alkylation of ketones with alkynes.

You and co-workers103 in 2019 described the cascade Pd-catalyzed intermolecular allylic alkylation/dearomatization reaction of electron poor N-heteroarenes 189 including pyridines, pyrazines, and quinolines (Scheme 74). Interestingly, the reaction has mild conditions, easily available substrates and delivered structurally diverse heterocycles such as 2,3-dihydroindolizine, 6,7-dihydropyrrolo[1,2-a]pyrazine, and 1,2- dihydropyrrolo[1,2-a]quinolin derivatives 190. Trisubstituted alkene containing dicarbonate yielded products having quaternary stereocenter.

Scheme 74 Allylic alkylation/dearomatization reaction of N-containing heterocycles.

Yang and co-workers104 in 2019 demonstrated the palladium − NHC-catalyzed allylic alkylation of various pronucleophiles 191 with alkynes 187 (Scheme 75). This transformation exhibited good substrate scope, wide functional group compatibility and high atom economy. Interestingly, 2-butynoic acid derivatives and skipped enynes were also tolerated and the reaction of b-naphthol with alkyne afforded dearomatized products.

Scheme 75 Pd −NHC-catalyzed allylic alkylation of alkynes (IPr.HCl ¼ 1,3-bis-(2,6-diisopropylphenyl) imidazolium chloride).

Feng and co-workers105 in 2019 developed the novel Pd-catalyzed nucleophilic addition induced allylic alkylation reaction (NAAA) of allenoates 193 (Scheme 76). Bimetallic (Pd/Zn) system suppressed direct allylic sulfonylation. Allenoates 193 underwent allyl-sunfonylation at the internal double bond and delivered structurally diversified a-allyl-b-sufonylbut-3-enoate derivatives 195. ZnCl2 promoted Micheal addition with allylic alkylation.

Scheme 76 Nucleophilic addition induced allylic alkylation (NAAA) of allenoates.

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Allyl-Palladium Complexes in Organic Synthesis

Nakao and co-workers106 in 2019 achieved the carboallylation of electron-deficient alkenes 196 by cooperative Pd/Cu catalysis, which gave access to a variety of carbon skeletons 199 (Scheme 77). This method is efficient and convenient than previous methods based on organosilicon reagent since a broad range of organoborane compounds 197 are cost-effective and commercially available.

Scheme 77 Pd-catalyzed carboallylation of electron-deficient alkenes.

In 2017 Yang and co-workers107 revealed that catalytic [Pd(
3-cin)(IPr)Cl] complex was efficient catalyst for allylic C–H alkylation by using oxindoles 200 as carbon nucleophiles, affording a broad scope of alkylated products 202 with high regioand stereoselectivity (Scheme 78). Interestingly, unactivated alkenes including internal olefins were also suitable substrates. In addition, pyrazolones 201 as nucleophilic reagents were also tolerated very well under the reaction conditions.

Scheme 78 Allylic C–H alkylation using [Pd(
3-cin)(IPr)Cl] complex.

Lu and co-workers108 in 2020 demonstrated the Pd-catalyzed allylic alkylation of oxindoles 204 with cyclopropyl acetylenes 205 to afford 1,3-diene oxindole framework 206 bearing a quaternary stereocenter at the C-3 position with high regio- and stereoselectivities under mild conditions (Scheme 79). This protocol possessed wide substrate scope, functional group compatibility and high atom economy.

Scheme 79 Allylation of cyclopropyl acetylenes with oxindoles.

Chen and co-workers109 in 2019 reported an unprecedented Pd(PPh3)4-catalyzed auto-tandem cooperative catalysis (ATCC) for Morita–Baylis–Hillman carbonates from isatins 207 and allylic carbonates 208 (Scheme 80). Dissociated phosphine delivered

Scheme 80 Allylic alkylation via auto-tandem cooperative catalysis.

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phosphorus ylides and palladium formed the p-allylpalladium intermediate that underwent g-regioselective allylic–allylic alkylation reaction. Finally, a tandem intramolecular Heck-type coupling furnished spirooxindoles 209 incorporating a 4methylene-2-cyclopentene unit. Further investigation revealed that both Pd and phosphine play key role in the catalytic cycle. Zhong and co-workers110 in 2019 demonstrated Pd(PPh3)4 and Brønsted acid-catalyzed efficient allylic substitution of benzothiazolylacetamides 210 with allylic alcohols 211 in water to afford various allylated products 212 with good to excellent yields and high regioselectivities (Scheme 81). This transformation has mild reaction conditions, scalability and good functional group compatibility.

Scheme 81 Pd-catalyzed allylation of benzothiazolylacetamide with allylic alcohols.

Lin and co-workers111 in 2017 developed unique Pd-catalyzed allylic alkylation of pronucleophiles 191 with unactivated skipped enynes 213 (Scheme 82). This protocol gave access to wide range of 1,3 dienes 214. Without any leaving group requirement and extra amount of oxidants, this method displayed high atom economy, excellent functional group tolerance, regioselectivities and scalability.

Scheme 82 Allylic alkylation of pronucleophiles with skipped enynes.

In 2014 Gong and co-workers112 demonstrated that
3-allylpalladium intermediate undergoes a carbene insertion reaction with a-diazo esters 215 to furnish conjugated polyenes 216 in moderate to high yields and having excellent stereoselectivities (Scheme 83). Interestingly, the co-catalysts, (salen)CrCl, triggered the formation of p-allylic palladium carbenoid by coordinating to the nitrogen of the a-diazo ester to enhance the nucleophilicity rather than the activation of the p-allyl-Pd species by coordination to BQ. Disulfoxide ligand not only played key role in allylic C–H activation but also enhancement of stereoselectivity.

Scheme 83 Allylic C–H olefination using a-diazo esters.

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Allyl-Palladium Complexes in Organic Synthesis

A few ylides based on P and S have served as excellent nucleophiles in alkylation of p-allylpalladium intermediates. Tang and co-workers113 in 2019 reported the Pd-catalyzed dehydrogenative alkylation of stabilized phosphonium ylides 217 with allylic alcohols 167 in water (Scheme 84). A wide range of allyl alcohols reacted with ylides to deliver functionalized skipped dienes 218 in moderate to high yields having excellent regioselectivities. Further studies disclosed that water is necessary for the formation of p-allylpalladium complex via hydrogen bond. This method is limited to water-sensitive phosphonium ylides.

Scheme 84 Dehydrogenative alkylation of stabilized phosphonium ylides (dppf ¼ 1,10 -bis-diphenylphosphino ferrocene).

In 2019 Jiang and co-workers114 reported the Pd-catalyzed synthesis of conjugated dienones using sulfoxonium ylides 219 as the precursor of carbenes (Scheme 85). A wide range of functionalized conjugated dienones 220 were obtained in moderate to good yields and excellent regioselectivities. Mechanistic studies revealed that after allylic C–H activation, a palladium carbenoid species was formed in situ with the release of DMSO, and subsequent migratory insertion and b-hydride elimination afforded the products.

Scheme 85 Pd-catalyzed oxidative allylation of conjugated sulfoxonium ylides with allylarenes.

Dithianes, halogenated arynes and aryliodonium compounds have also served as nucleophiles. Walsh and co-workers115 in 2019 developed Pd-catalyzed allylic alkylation of 2-aryl-1,3-dithianes 221 at room temperature with a variety of cyclic and acyclic electrophiles to deliver allylated products 223 in excellent yields (Scheme 86). These products on deprotection generated b,g-unsaturated ketones. Further investigation disclosed that 2-sodio-1,3-dithiane nucleophile behaves as a “soft” nucleophile, which attack on p-allyl palladium complex with retention of stereochemistry via double inversion. This methodology was further applied to synthesize asterogynin derivatives, which are important bioactive compounds through a sequential one-pot allylation-Heck cyclization.

Scheme 86 Allylic alkylation of 2-aryl-1,3-dithianes.

In 2014 Jiang and co-workers116 revealed the allylic C–H arylation of alkenes 4 with polyfluorobenzenes 224 in good yields and high regioselectivities (Scheme 87A). Polyfluorobenzenes reacted with silver salt to produce aryl nucleophile via deprotonation. Here dioxygen was used as terminal oxidant, which resulted in an atom-economical approach for allylic C–H bonds arylation. Similarly, at the same time Yang and co-workers117 (2014) developed the Pd-catalyzed allylic C − H arylation reaction with a series of electron-deficient arenes (polyflurobenzenes) showing good to excellent regio- and stereoselectivity (Scheme 87B). The first successful use of 1,1-bi-2-naphthol as the ancillary ligand in allylic C − H activation with high selectivity was demonstrated. Allylic C −H acetoxylation and amination was also achieved under this transformation.

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Scheme 87 Aerobic allylic C–H arylation using polyfluorobenzenes.

Liu and co-workers118 in 2020 described an efficient Pd-catalyzed C-glycosylation of iodonium salts 228 with glycals 227 to deliver 2,3-dideoxy C-aryl glycosides 229 in high yields and excellent stereoselectivities (Scheme 88). A wide range of glycals, including D-glucal, D-galactal, D-allal, L-rhamnal, L-fucal, L-arabinal, D-maltal and D-lactal, were tolerated with broad substrate scope and exceptional a-stereoselectivity useful in carbohydrate chemistry.

Scheme 88 Pd(II)-catalyzed stereoselective synthesis of C-glycosides from glycals with diaryliodonium salts (TBAC ¼ tetrabutylammonium chloride).

An interesting example of aryl nucleophile through ortho-C–H activation is also reported. Wang and co-workers119 in 2020 developed Pd-catalyzed cascade dehydrogenative [4 + 2] annulation of terminal olefins 231 with N-sulfonyl amides 230 via C(sp2) −H activation, allylic C−H activation and homoallylic C(sp3)−H elimination (Scheme 89). A variety of various benzamides, heterocyclic arylamides, alkenyl carboxamides and commercial olefins were employed efficiently with high E-stereoselectivity. The reaction was accelerated by DMSO ligand and air served as terminal oxidant, which made this method environmentally friendly.

Scheme 89 Pd-catalyzed tandem dehydrogenative [4 + 2] annulation.

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Allyl-Palladium Complexes in Organic Synthesis

Boron-based nucleophiles are another source for C–C bond formation. In 2015 Uozumi and co-workers120 reported the allylic arylation of various allylic substrates 233 with sodium tetraarylborates 234 in the presence of palladium NNC-pincer complex A at loadings of the order of ppb to ppm (molar) to proceed smoothly under mild conditions and furnish the desired arylated products 235 in good-to-excellent yields (up to 99%, Scheme 90). Further studies by same group involved allylic arylation in water with vesicular self-assembled amphiphilic palladium NNC-pincer complex to deliver arylated products in high yields, whereas the same complex as an amorphous powder did not initiate the reaction.121

Scheme 90 Allylic arylation of various allylic compounds catalyzed by palladium NNC-pincer complex.

Yin and Hyland122 in 2015 disclosed a new protocol for ring opening of activated vinylcyclopropanes 236 with boronic acids 237 in water (Scheme 91). Regioselectivity of the reaction depends on the substitution on vinylcyclopropane dicarboxylates. Aryl substituents on alkene formed branched products 239 while unsubstituted vinylcyclopropanes delivered linear allylic alkylation products 238 in reductive elimination step. Palladium nanoparticles formed from Pd(OAc)2 act as a catalyst under ligand free condition, which made this process more practical and economical.

Scheme 91 Ring-opening of vinylcyclopropane-1,1-dicarboxylates by boronic acids.

Jiang and co-workers123 in 2018 demonstrated Pd-catalyzed oxidative a-boroalkylation reaction of simple olefins 48 with 1,1-bis[(pinacolato)boryl]methane 240 to generate functionalized homoallylic boronic esters 241 in good to excellent yields (Scheme 92). Interestingly, an efficient construction of the Csp3–Csp3 bond has been achieved via the Pd-catalyzed oxidative functionalization of the allylic C–H bond. The protocol afforded broad substrate scope and excellent functional group compatibility.

Scheme 92 Oxidative allylation of bis[(pinacolato)boryl]methane (NQ ¼ 1,4-naphthoquinone).

Gong and co-workers124 in 2019 demonstrated a novel Pd-catalyzed cross-coupling reaction of gem-difluorinated cyclopropanes 242 with boronic acid derivatives 237 (Scheme 93). A wide range of aryl- and heteroarylboronic acids were coupled with gem-difluorinated cyclopropanes with excellent functional group tolerance. This protocol gave access to a diverse number of monofluoroalkenes 243, especially conjugated and skipped fluorodienes having high Z-selectivity.

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Scheme 93 Defluorinative arylation/alkenylation/alkylation of gem-difluorinated cyclopropanes.

Yamaguchi and co-workers125 in 2020 described the dearomative allylation of aromatic cyanohydrins 244 by palladium catalyst with allyl borates 245 and allyl stannanes (Scheme 94). Initially the dearomative reaction (C-4 substitution of the aromatics) was competing with benzyl substitution. This was resolved by combination of palladium and m-disubstituted triarylphosphines L20 in 1:1 to deliver dearomatized products 246 with good selectivities. Further these products were derivatized to a variety of substituted alicyclic systems.

Scheme 94 Pd-catalyzed dearomative allylation of naphthyl cyanohydrins.

Heterocycles like indoles, azoles, furan and alkynes are also explored as nucleophiles. In 2016 Yang and co-workers126 disclosed a dearomatization strategy to form carbon nucleophiles via deprotonation with silver salt of indoles for allylic C–H alkylation of allylarenes 163 (Scheme 95A). Addition of 2,5-DMBQ raised the yields. The cascade allylic activation and subsequent dearomatization gave access to various 3,3-disubstituted indolines 249. Later in 2016, Lin and co-workers reported the Pd-catalyzed dearomative allylic alkylation of indoles 250 with alkynes 251 to construct indolenines 252 with C3-quarternary centers (Scheme 95B).127 The transformation resulted in good functional group tolerance and high atom economy.

Scheme 95 Dearomatization strategy for allylic C–H alkylation using indoles.

Breit and co-workers128 in 2019 established the Pd-catalyzed allylation, olefin isomerization/double C(sp3)–H allylation of azoles 253 and alkyne 251 to deliver corresponding C2-alkenylated azoles 254 in good yields (Scheme 96). A wide range of substituted trisallylated azoles bearing all quaternary carbon centers has been synthesized efficiently and with high atom economy.

Scheme 96 Pd-catalyzed trisallylation of azoles.

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Allyl-Palladium Complexes in Organic Synthesis

Wang and co-workers129 in 2019 developed the Pd-catalyzed oxidative cross-coupling of conjugated enynones 255 with allylarenes 163 to deliver furylsubstituted 1,3-dienes 256 (Scheme 97). The transformation exhibited wide substrate scope and good functional group compatibility. Further studies revealed palladium carbene migratory insertion as the key step with conjugated enynones serving as the carbene precursors.

Scheme 97 Pd-catalyzed oxidative allylation of conjugated enynones with allylarenes.

Gong and co-workers130 recently (2020) described the Pd-catalyzed alkynylation of gem-difluorinated cyclopropanes 242 via C −C bond activation/C − F bond cleavage, followed by C −C(sp) coupling (Scheme 98). This mild reaction conditions gave easy access to diversely fluorinated enynes 258 and arenes in good yields and high stereoselectivities.

Scheme 98 Pd-catalyzed ring-opening alkynylation of gem-difluorinated cyclopropanes.

An interesting example of umpolung addition of aldehyde via carbene intermediate is also reported. Ohmiya and co-workers131 disclosed synergistic palladium-bisphosphine-catalyzed allylation of allylic amines 260 and aldehydes 259 to deliver b,g-unsaturated ketones 261 using thiazolium N-heterocyclic carbene L21a or L21b (Scheme 99). b,g-Unsaturated ketones are valuable intermediates for various transformation in organic synthesis.

Scheme 99 Synergistic NHC/Pd-catalyzed allylic alkylation (dippf ¼ 1,10 -diisopropylphosphino ferrocene).

Jiang and co-workers132 in 2011 revealed the Pd-catalyzed direct oxidative carbonylation of allylic C–H bonds of 36 with carbon monoxide to result in b-enoic acid esters 262 with high regioselectivity and good yields (Scheme 100). Deuterium labelling experiments indicated that allylic C–H activation process is an irreversible as well as rate-determining step.

Scheme 100 Oxidative allylic C–H carbonylation of olefins.

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Scheme 101 Cascade allylic C–H activation and Diels–Alder reaction.

In 2011 Stang and White133 disclosed that
3-allylpalladium species undergo further b–H elimination to furnish 1,3-dienes, which were further used in Diels–Alder reaction with a series of maleimide dienophiles to construct more complex cyclohexenyl rings 264 (Scheme 101). Sterically bulky oxidant 2,6-dimethyl-1,4-benzoquinone (2,6-DMBQ) instead of BQ was used to decrease cycloaddition between BQ and diene, it also suppressed reductive elimination of acetate from p-allylpalladium species. p-Nitrobenzoic acid promoted the reoxidation of Pd(0) into Pd(II). Furthermore, tertiary carbon nucleophiles were also compatible with the oxidative system, constructing a challenging quaternary carbon center.134

8.09.4.2

Intramolecular allylic alkylation

Ramasastry and co-workers135 in 2019 presented an efficient palladium-catalyzed intramolecular Trost −Oppolzer-type Alder-ene approach for the synthesis of carbazoles, naphthalenes, dibenzofurans, and cyclopenta[b]indoles from easily accessible(3-allyl-1H-indol-2-yl)methyl acetates (Scheme 102). Notably, this divergent approach was substituent dependent with tolerance to a wide range of functional groups, scalability and gave easy access to many new heterocycles 266 or 267.

Scheme 102 Pd-catalyzed ene reaction for the synthesis of carbazoles and cyclopenta[b]indoles.

Dong and co-workers136 in 2019 developed the Pd-catalyzed intramolecular a-allylic alkylation of unactivated ketones with pendant alkynes 268 (Scheme 103). Both endo- and exo-bridged cyclohexanones 269 or 2690 were obtained in good diastereoselectivity, which was controlled by the ligands and acid additives. Monodentate DTBMPP ligand favored formation of endo isomer, while the bidentate DIOP ligand preferentially delivered the exo isomer. Further deuterium labeling studies revealed that the reaction proceeds through an alkyne/allene isomerization and Pd-p-allyl complex formation. A wide range of functional groups were compatible to afforded [3.2.1] bicyclic skeletons.

Scheme 103 Pd-catalyzed intramolecular a-allylic alkylation of unactivated ketones with alkynes (HFIP ¼ hexafluoro-2-propanol).

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Allyl-Palladium Complexes in Organic Synthesis

Arisawa and co-workers137 in 2019 reported the first Pd-catalyzed migratory cycloisomerization of N-allyl-o-allenyl aniline derivatives 270 to deliver 2-substituted indoles 271 (Scheme 104). Mechanistic studies revealed that the intramolecular cyclization required sufficiently small ligand and negative charge on the nitrogen of the p-allyl intermediate. Electron-withdrawing group was needed and if there was substituent at 3-position on aromatic ring the reaction didn’t occur due to steric hindrance.

Scheme 104 Pd-catalyzed migratory cycloisomerization of N-allyl-o-allenylaniline derivatives.

8.09.4.3

Asymmetric allylic alkylation

The asymmetric allylic alkylation is one of the most difficult task aspired in p-allylpalladium chemistry as the reaction involves C-nucleophiles. Various active methylene nucelophiles with aldehyde, keto, amide, nitro, ester groups have been explored. In 2007 Mukherjee and List138 disclosed the Pd-catalyzed enantioselective a-allylation of branched aldehydes 272 with allyl amines 273 (Scheme 105A). The phosphoric acid TRIP (as Brønsted acid) acts as proton donor as well as ligand for the cationic p-allyl-Pdintermediate. This marked the beginning of asymmetric induction achieved in palladium-catalyzed allylic alkylation using the chiral anionic ligand. Later in 2011,139 List reported the Pd-catalyzed enantioselective direct a-allylation of branched aldehydes 272 with the readily available allyl alcohol 3 (Scheme 105B). The reaction conditions were successfully applied to a wide range of substrates to deliver the desired products 274 in good yields and enantiomeric ratio.

Scheme 105 Asymmetric a-allylation of branched aldehydes.

Gong and co-workers140 employed terminal alkenes in the palladium and chiral phosphoric acid ligand-catalyzed asymmetric allylic alkylation of aldehydes in high enantioselectivities (Scheme 106). A wide spectrum of branched aromatic aldehydes 272 and terminal alkenes 275 delivered allylation products 276 in good yields and excellent enantioselectivities.

Scheme 106 Gong’s asymmetric allylic alkylation of branched aromatic aldehydes.

In 2013 Trost et al.141 established the Pd-catalyzed enantioselective allylic C–H alkylations using chiral phosphoramidite ligands (Scheme 107). The 2-acetyl-1-tetralone 277 was taken for alkylation with allylbenzenes 48 in presence of 2,6-dimethylbenzoquinone catalyzed by palladium acetate and chiral phosphoramidite ligand L22 to deliver the desired a-alkylated carbonyl products 278 in good yields and enantioselectivities.

Scheme 107 Trost’s allylic C–H alkylations using chiral phosphoramidite ligands.

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Gong and co-workers142 developed the Pd-catalyzed highly efficient asymmetric allylic C − H alkylation of allyl ethers 280 using chiral phosphoramidite ligand L23 (Scheme 108). This transformation was compatible with a wide range of substrates 279 and offered moderate to good yields of products 281. Further, a concise total synthesis of tachykinin receptor antagonist 283 was achieved by this method.

Scheme 108 Asymmetric allylic C −H alkylation of allyl ethers.

Tian and co-workers143 in 2019 described the Pd-catalyzed asymmetric allylation of a-branched b-ketoesters 285 in presence of chiral a-amino acid to deliver enantioselectively allylated a,a-disubstituted b-ketoesters 286 having quaternary stereocenters (Scheme 109). Formation of byproducts such as amine or ammonia has slight effect on enantioselectivity in this method.

Scheme 109 Pd-catalyzed allylic alkylation of a-branched b-ketoesters.

Gong and co-workers144 disclosed the synthesis of chiral a,a-disubstituted b-keto esters 289 by asymmetric allylic C−H alkylation of 1,4-dienes 288 with cyclic b-keto esters 287 catalyzed by the combination of palladium catalyst and chiral phosphoramidite ligand L24 (Scheme 110). This transformation afforded chiral a,a-disubstituted b-keto esters 289 in excellent yields, regioselectivities, E/Z selectivity, and enantioselectivities. The mild reaction conditions show high tolerance for 1,4-dienes with different functional groups such as as ketone, chloride, ester, amide, etc.

Scheme 110 Gong’s approach toward the synthesis of chiral a,a-disubstituted b-keto esters.

In 2013 Gong and co-workers145 revealed the asymmetric allylic alkylation of pyrazol-5-ones 290 with allylic alcohols 291 catalyzed by the Pd-complex with chiral phosphoramidite ligand L25 and chiral phosphoric acid CPA1 (Scheme 111). This

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Allyl-Palladium Complexes in Organic Synthesis

Scheme 111 Asymmetric allylic alkylation of pyrazol-5-ones with allylic alcohols and allylarenes.

delivered highly functionalized heterocyclic products 292 in good yields and high enantioselectivities. Many of the chiral heterocycles could be used for the synthesis of the drugs molecules. Later, in 2016 the same group expended the work by using similar reaction conditions for the enantioselective allylic C−H alkylation reaction of pyrazol-5-ones 290 with allylarenes 4 using L26 and CPA2.146 This afforded a wide range of highly functionalized chiral N-heterocycles 292 with excellent enantioselectivities (up to 96% ee). Chen and co-workers147 in 2019 reported the Pd-catalyzed ligand-free direct vinylogous umpolung protocol for deconjugated b,g-unsaturated carbonyl compounds under O2 atmosphere (Scheme 112). The chiral phosphonium phase-transfer catalyst incorporated the g-regioselectivity in the asymmetric allylic alkylations with 3-substituted oxindoles 293. This mild reaction conditions provided a wide spectrum of alkylated all-carbon-based quaternary center compounds 295.

Scheme 112 Chen’s approach toward alkylation of deconjugated b,g-unsaturated carbonyl compounds.

Trost and co-workers148 in 2019 demonstrated the Pd-catalyzed decarboxylative asymmetric allylic alkylation of dihydroquinolinones 296 to generate quaternary stereocenters 297 with high enantioselectivity in presence of chiral anthracenyl ligand L9 (Scheme 113). This methodology was applicable to the synthesis of norepinephrine reuptake inhibitor 299. Further studies disclosed that the role of chiral anthracenyl ligand is significant for desired reactivity and enantioselectivity.

Scheme 113 Pd-catalyzed decarboxylative asymmetric allylic alkylation of dihydroquinolinones.

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In 2020 Yang and co-workers149 reported the enantioselective allylic alkylation of isoquinolinediones 300 catalyzed by Pd in combination with the phosphinooxazoline based chiral ligand L27 (Scheme 114). A wide spectrum of iso-quinolinediones 300 having various functional groups along with allyl partner 5 gave rapid access to the iso-quinolinediones, dihydroisoquinolones and THIQ derivatives 301 in high yields and enantioselectivities.

Scheme 114 Enantioselective allylic alkylation of isoquinolinediones.

In 2019 Stoltz and co-workers150 described the asymmetric allylic alkylation of 1,4-diazepan-5-ones 302 by Pd-catalysis delivering the gem-disubstituted diazepanone heterocycles 303 (Scheme 115). The use of chiral phosphoric acid provided the gem-disubstituted diazepanone heterocycles with various functional groups in excellent yields and high enanatioselectivities. Notably, the p-anisoyl protection of lactam imparted good yields and enantioselectivities.

Scheme 115 Asymmetric allylic alkylation of 1,4-diazepan-5-ones.

Huang and Marek151 in 2020 disclosed the Pd-catalyzed allylation of stereodefined polysubstituted ketene aminals 304 to deliver the vicinal tertiary and quaternary carbon stereocenters in acyclic compounds 306 in good yields, excellent diastereo- and enantioselectivities (Scheme 116). The reaction was catalyzed by Pd in combination with Segphos chiral ligand.

Scheme 116 Allylation of stereodefined polysubstituted ketene aminals (TBAT ¼ tetrabutylammonium difluorotriphenyl silicate).

Cossy and co-workers152 in 2019 revealed the Pd-catalyzed highly regio- and enantioselective asymmetric allylic alkylation of a,g-disubstituted 2-silyloxypyrroles 307 to provide enantioenriched g-lactam derivatives 309 bearing a quaternary stereogenic center in presence of chiral Trost ligand L28 (Scheme 117). The method was easy to handle and scalable giving excess to various a-allylated b,g-unsaturated g-lactams 309 in high yields. These could be further functionalized to chiral pyrrolidinones and pyrrolidines.

Scheme 117 Pd-catalyzed allylic alkylation by Cossy.

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Allyl-Palladium Complexes in Organic Synthesis

In 2019 Colombo and co-workers153 developed the Pd-catalyzed enantioselective allylation of azlactone allyl enol carbonates 312 to afford chiral quaternary 4-allyl oxazol-5-ones 313 using (R,R)-DACH-phenyl Trost chiral ligand L28 (Scheme 118). Various allylated derivatives were tolerated in this transformation to give good yields up to 98% and enantioselectivities up to 85% ee.

Scheme 118 Pd-catalyzed asymmetric decarboxylative allylation of azlactone enol carbonates.

Trost and Li154 documented the palladium-catalyzed asymmetric allylic alkylation reaction involving 2-aza-p-allyl palladium intermediates (Scheme 119). This involved the C(Sp3)-H activation of N-allyl imines 314 by Pd catalyst in the presence of chiral bidentate (S,S)-Cy-DIOP ligand L29 followed by the nucleophilic attack of glycinates 315 to furnish vicinal diamino compounds 316 as single regioisomers with high diastereo- and enantioselectivities.

Scheme 119 Synthesis of vicinal diamino compounds via Pd-catalyzed asymmetric allylic alkylation.

Guo and co-workers155 in 2019 disclosed the first combination of triple catalytic system having a chiral aldehyde L30, a Lewis acid and a palladium metal to enable a-allylation reactions of N-unprotected amino esters 317 and allyl acetates 5 to deliver chiral a,a-disubstituted a-amino acids 318 in moderate to good yields (Scheme 120). Mechanistic studies indicated that the chiral aldehyde L30 was crucial for activating the amino acids through Schiff base formation and enhancing the rate of reaction via co-ordinating with the p-allyl Pd(II)-intermediates.

Scheme 120 Pd-catalyzed enantioselective a-allylic alkylation of amino acid asters.

In 2020 Yang and Xing156 documented the Pd-catalyzed allylic alkylation of oxazolones 319a with 1,3-dienes 319b in presence of DTBM-SEGPHOS chiral ligand to deliver the enantioenriched oxazolones 320 having tertiary carbon center in good diastereoand enantioselectivities (Scheme 121). Here the palladium-hydride formed by reaction of Pd(OAc)2 with CSA activates the 1,3-dienes in a base-free reaction.

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Scheme 121 Palladium catalyzed allylic alkylation of oxazolones with 1,3-dienes (CSA ¼ camphor sulfonic acid).

Later, Gong and co-workers157 (2019) demonstrated the Pd-catalyzed asymmetric allylic C − H alkylation of 1,4-dienes 288 with azlactones 321 using chiral phosphoramidite L31 (Scheme 122). This methodology having mild reaction conditions displayed wide substrate scope delivering structurally divergent a,a-disubstituted a-amino acids 322 in high yields, excellent diastereo-, Z/E-, regio-, and enantioselectivities. Experimental as well as computational studies revealed that the catalytic cycle consisted of a novel concerted proton and two-electron transfer process for the allylic C −H cleavage. This method was applied in the synthesis of key chiral intermediates for an efficient synthesis of lepadiformine A (323a) and B (323b) marine alkaloids.

Scheme 122 Pd-catalyzed asymmetric allylic C −H alkylation of 1,4-dienes.

In 2020 Stolz and co-workers158 described the Pd-catalyzed enantioselective synthesis of substituted a-N-pyrrolyl and indolyl ketones 325 (Scheme 123). The acyclic ketones underwent decarboxylative allylic alkylation in presence of an electron-deficient phosphinooxazoline ligand L32 to deliver the a-N-pyrrolyl and indolyl ketones 325 in good to excellent yields and high enantioselectivities.

Scheme 123 Synthesis of substituted a-N-pyrrolyl and indolyl ketones.

Ziegler and co-workers159 in 2019 revealed two D-fructose based spirofused PHOX ligands (example L33) having bulky benzyl groups at position 3 of the fructose moiety for asymmetric allylic alkylation with various nucleophiles to deliver diarylallylated products 327 having high enantiomeric ratios (up to 93:7 er, Scheme 124).

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Allyl-Palladium Complexes in Organic Synthesis

Scheme 124

D-Fructose

based spiro-fused PHOX Ligands enabled AAA.

Trost and co-workers160 (2019) demonstrated the Pd-catalyzed decarboxylative asymmetric allylic alkylation of a-nitroallyl esters 328 to furnish tetrasubstituted nitroalkanes 329 in good to excellent yields and high enantiomeric ratio using (R,R)DACH-phenyl Trost chiral ligand L28 (Scheme 125). Further studies revealed that the combination of ligand and solvent is crucial for achieving chemo- and enantioselective C-alkylation of electronically challenging benzylic nitronates and sterically encumbered 2-allyl esters.

Scheme 125 Pd-catalyzed decarboxylative asymmetric allylic alkylation of a-nitroesters.

In 2020 Zhang and co-workers161 documented the enantioselective asymmetric allylation of N-fluorenyl trifluoromethyl imine 330 with allylic acetates 331 catalyzed by the combination of Pd-catalyst with (R)- or (S)-t-Bu-PHOX chiral ligand to deliver the polysubstituted chiral a-trifluoromethyl amines 333 after hydrolysis (Scheme 126). The polysubstituted chiral a-trifluoromethyl amines bearing two adjacent stereocenters along with one allyl group were synthesized in excellent yields, good regio-, diastereo-, and enantioselectivities.

Scheme 126 Enantioselective asymmetric allylation of N-fluorenyl trifluoromethyl imine with allylic acetates.

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Trost and co-workers162 described the synthesis of chiral organophosphorus compounds via the Pd-catalyzed in situ-generation of phospha-TMM (trimethylenemethane) species (Scheme 127). The unique transformation delivered a wide spectrum of chiral organophosphorus containing carbo- and heterocyclic compounds 337 in good yields, high regio-, diastereo- and enantioselectivities (>99% ee).

Scheme 127 Trost’s synthesis of chiral organophosphorus compounds.

Zhang and co-workers163 evolved a new pathway for the vinylogous C-glycosylation of a,b-unsaturated lactones 338 under Pd-catalysis in presence of chiral xantphos ligand to deliver the alkylated products 340 in excellent regio- and stereoselectivities (Scheme 128). The mild reaction conditions rendered a wide spectrum of C-glycosylated a,b-unsaturated lactones in good to excellent yields.

Scheme 128 Vinylogous C-glycosylation of a,b-unsaturated lactones.

In 2019 Stoltz and co-workers164 unfolded a unique route for the synthesis of tetrasubstituted benzoin derivatives 342 via the Pd-catalyzed decarboxylative allylic alkylation of fully substituted a-hydroxy acyclic enol carbonates 341 (Scheme 129). The use of chiral bisphosphine ligand L9 in combination with Pd-catalyst delivered the products in high enantioselectivities.

Scheme 129 Pd-catalyzed decarboxylative allylic alkylation of a-hydroxy acyclic enol carbonates.

Trost and co-workers165 later developed the Pd-catalyzed asymmetric allylic alkylation (AAA) of acyclic a-hydroxy ketones 343 with boronic acids in presence of chiral Trost ligand L280 to furnish alkylated products 344 in good yields and high regio- and enantioselectivities (Scheme 130). Boronic acids reacted with hydroxyketones to deliver 1,3-dioxaboroles which were used as

Scheme 130 Pd-catalyzed allylic alkylation of preformed 1,3-dioxaboroles.

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pre-functionalized substrates for AAA. This protocol was first example in Pd-catalyzed AAA for inducing point chirality on a nucleophile simultaneous to stereoinduction on an axial chiral allene. Trost and co-workers166 in 2019 also disclosed the first Pd-catalyzed asymmetric allylic trifluoromethylation reaction with allyl fluorides 345 leading to formation of p-allyl intermediates by eliminating fluoride with TMSCF3 in ionization and nucleophilic activation (Scheme 131). This transformation gave access to various fluoroalkylated carbo and heterocycles 346 with high functional group compatibility and enantioselectivities. Mechanistic studies revealed that the overall stereochemistry in the process is retention of configuration.

Scheme 131 Pd-catalyzed allylic fluoroalkylation.

In 2017 Gong and co-workers167 established the Pd-catalyzed allylic C −H borylation in combination with bis(pinanediolato) diboron 347 and a chiral Brønsted acid CPA3 (Scheme 132). The aldehydes 259 reacted with simple alkenes 4 in an enantioselective carbonyl allylation to deliver the homoallylic alcohols 348 in good yields and excellent diastereo- and enantioselectivities.

Scheme 132 Gong’s Pd-catalyzed allylic C–H borylative carbonyl allylation (NFSI ¼ N-fluorobenzenesulfonimide).

8.09.5

Miscellaneous nucleophiles in allylic substitution

Other than oxygenation (C–O bond formation), amination (C–N bond formation) and alkylation/arylation (C–C bond formation) other carbon-heteroatom bonds (C–F, C–Si, C–S, etc.) formed via Pd-catalyzed allylation are categories in this section. In 2011 Szabó and co-workers168 developed an allylic C–H silylation with hexamethyldisilane as the silyl source to give rise to linear allylsilane products 349 (Scheme 133). Strong oxidants such as hypervalent iodine reagents and benzoyl peroxide along with BQ,

Scheme 133 Pd-catalyzed oxidative allylic C–H silylation.

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delivered allylsilanes in moderate to good yields. The reaction was efficient on substrates bearing allylic position activated with electron withdrawing functional groups. Mechanistic studies suggested that PdII/PdIV catalytic cycle to be involved to form
3-allylpalladium intermediates. Braun and Doyle169 in 2013 inspired by White’s work developed the allylic C–H fluorination by use of nucloephilic fluoride source (Et3N.HF) based on the Pd(II)/bis-sulfoxide/Lewis acid catalysis (Scheme 134). Further studies revealed that Pd(TFA)2 was more efficient than Pd(OAc)2 and the additive chromium salen was crucial for enhancing the electrophilicity of
3-allylpalladium complex for nucleophilic functionalization. A wide range of branched fluorinated products 350 were obtained in good yields and selectivities.

Scheme 134 Pd-catalyzed direct allylic C–H fluorination.

Huang and co-workers170 in 2019 demonstrated a cascade reaction for the synthesis of naphthyl sulfones 353 from obromobenzaldehydes 351 and N-sulfonylhydrazones 352 (Scheme 135). The reaction consisted of tandem palladium-carbene migratory insertion/allylic alkylation and base-promoted aldol condensation. Mechanistic studies showed that sulfonylation of the allylpalladium intermediate is reversible and a thermodynamically stable naphthalene ring 353 was formed regioselectively.

Scheme 135 Pd-catalyzed direct allylic sulphonylation of o-bromobenzaldehydes with N-sulfonylhydrazones.

Recently in 2020 Breinbauer and co-workers171 reported the Pd/BIPHEPHOS catalyst system to generate thioethers 356 from S-allylation of thiols 354 with allylcarbonates 355 and allylacetates having excellent n-regioselectivity (Scheme 136). A broad range of substrates were tolerated with respect to thiols as well as allyl substrates. This transformation was also used for late stage diversification of antibiotic cefalotin. Mechanistic studies revealed that the reaction is reversible under applied reaction conditions.

Scheme 136 Pd-catalyzed direct allylic S-allylation to form thioethers.

Gong and co-workers172 in 2015 described the allylic C–H borylation and subsequent carbonyl allylation with acyclic alkenes 36 in high yields and good diasterioselectivity (Scheme 137). The oxidant played an important role in allylic C–H activation step, while the Brønsted acid triggered the rate of reaction).

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Scheme 137 Allylic C–H borylation of acyclic alkenes and subsequent allylation.

8.09.6

Conclusions and outlook

The p-allylpalladium chemistry has evolved over a period into a major area of research in catalysis and organic synthesis. While it begin modestly in the 1950s as a stoichiometric variant with respect to palladium, the later catalytic version gave a major boost and hence several ways of achieving the transformation via the p-allylpalladium intermediate/complex became a reality. While variation in nucleophile from C-based to O and N saw majority of the research, it also allowed the employment of halides, Si and S as nucleophiles. The compatibility of various chiral catalysts and the co-operative mode of different catalyst allowed asymmetric synthesis of valuable scaffolds. The p-allylpalladium chemistry has been well utilized in the total synthesis of natural products giving a new dimension. The decarbonylative allylation have also been well explored. The present day allylic C–H activation with hydrogen as leaving group has several advantages over the traditional leaving group ionization and has been realized under catalytic conditions. The redox-neutral mode of generating p-allylpalladium from 1,3-dienes and 1,2-dienes (allenes) is another feat achieved in this chemistry that has the potential to omit the use of terminal oxidants. p-Allylpalladium chemistry though has several avenues for its generation has future potential toward the synthesis of valuable scaffolds, heterocycles, drug candidates and intriguing natural products.

Acknowledgments The Authors thank DST-SERB grant No. EMR/2017/000499 for financial support. P.K. and N.C. thank the University Grant Commission (UGC) India and the Council of Scientific and Industrial Research (CSIR) New Delhi, India, respectively, for research fellowships.

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8.10

Zerovalent Nickel Organometallic Complexes

Jorge A Garduñoa and Juventino J Garcíab, a6128 Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, NH, United States; bFacultad de Química, Universidad Nacional Autónoma de México, Mexico City, Mexico © 2022 Elsevier Ltd. All rights reserved.

8.10.1 Introduction 8.10.2 Nickel(0) complexes with s-carbon-bound carbonyl and isocyanide ligands 8.10.2.1 Carbonyl complexes 8.10.2.2 Isocyanide complexes 8.10.3 Nickel(0) s-adducts with E-H bonds (E ¼ B, Si, Mg) and hydride-bridging ligands 8.10.3.1 s-Adducts with E-H bonds (E ¼ B, Si, and Mg), and hydride-bridging ligands 8.10.4 Nickel(0) complexes with olefin ligands 8.10.4.1 Complexes with the COD (1,5-cyclooctadiene) ligand 8.10.4.2 Ethylene complexes 8.10.4.3 Styrene and stilbene complexes 8.10.4.4 Polyene and polyenyne complexes 8.10.4.5 Vinyl complexes 8.10.4.6 Alkene complexes with miscellaneous p-coordinating groups 8.10.5 Nickel(0) complexes with alkyne ligands 8.10.5.1 Complexes with alkyne ligands 8.10.6 Nickel(0) complexes with p-arene ligands 8.10.6.1 Arene complexes with benzene-derived ligands 8.10.6.2 Arene complexes with pyridine, thiophene and pyrrole ligands 8.10.6.3 p-Fullerene complexes 8.10.6.4 Aryne complexes 8.10.7 Side-on nickel(0) complexes with C ¼ E (E ¼ O, S, N) and C  N moieties 8.10.7.1 Side-on carbonyl and thiocarbonyl complexes 8.10.7.2 Side-on borane-containing complexes 8.10.7.3 Side-on imine complexes 8.10.7.4 Side-on nitrile complexes 8.10.7.5 CO2 and CS2 complexes Acknowledgment References

8.10.1

680 681 681 686 688 688 690 690 694 696 698 700 701 705 705 707 708 712 713 714 715 715 719 720 723 725 726 726

Introduction

This article covers most of the literature published between the years 2005 and 2020, on the synthesis and characterization of isolated zerovalent nickel organometallic complexes, that is, Ni(0)-compounds bearing at least one carbon-bound ligand, either through a s- or a p-bonding interaction. Also included are low-valent nickel s-adducts, not necessarily containing Ni(0)-carbon bonds, but three-center two-electron interactions with at least one hydride-like moiety. Examples of Ni(0)-organometallics studied by DFT, serving as intermediates in some reaction mechanisms, can as well be found throughout this article without details on computational methods. Information from the literature presented herein is categorized according to the main type of carbon-bound ligand; therefore, covered in the following subchapters: I. Nickel(0) complexes with s-carbon-bound carbonyl and isocyanide ligands; II. Nickel(0) s-adducts with H-E bonds (E ¼ B, Si, Mg) and hydride-bridging ligands; III. Nickel(0) complexes with olefin ligands; IV. Nickel(0) complexes with alkyne ligands; V. Nickel(0) complexes with p-arene ligands; and VI. Side-on nickel(0) complexes with C ¼ E (E ¼ O, S, N) and C N moieties. Each subchapter starts with general remarks and is divided into subsections according to specific types of organonickel bonding interactions (i.e., “III. Nickel(0) complexes with olefin ligands” includes subsections such as “Ethylene complexes” or “Vinyl complexes”). Subsections introduce descriptions of the corresponding zerovalent nickel complexes appearing in chronological order of publication. Figures accompanying each section cover specific groups of compounds classified according to common features (i.e., “Fig. 1. Nickel(0) carbonyls with monodentate phosphine ligands,” or “Fig. 50. Lewis acidN-adducts of side-on nickel(0) nitrile complexes”). In the cases where more than one kind of organometallic bond is present, compound is classified according to relevance of either functionality in terms of reactivity or the main topic of the source paper. Likewise, in the few cases when more than one kind of organometallics are closely related, they appear in the same subsection regardless the type of ligand they exhibit. Out of the scope of this article are zerovalent nickel compounds with N-heterocyclic carbene (NHC) ligands, although a list of references can be found at the end of this article;3–51 most of Ni(I) intermediates, even if stemming from Ni(0) precursors, when

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Fig. 1 Nickel(0) carbonyls with monodentate phosphine ligands.1,2

Ni(0) is not the theme of discussion; and reviews published during the period of time covered in this text,52–73 including topics such as bond and small-molecule activation,52,55,56,73 structural and electronic description of Ni(0)-organometallics,53,54,58 and several catalytic transformations relevant to organic synthesis.57,59–72

8.10.2

Nickel(0) complexes with s-carbon-bound carbonyl and isocyanide ligands

Zerovalent nickel carbonyl complexes comprise a diverse family of compounds having in common at least one s-carbon-bound carbon monoxide (C  O) ligand either as a terminal or bridging ligand. Likewise, zerovalent nickel isocyanide complexes also comprise a diverse family of compounds, which have in common at least one s-carbon-bound isocyanide (C  N-R) (R ¼ alkyl or aryl) ligand. Coordination of CO and (C  N-R) (R ¼ alkyl or aryl) to Ni(0) is usually accompanied by partial loss of bond-order of the C-E (E ¼ O, N) triple bond, as a consequence of p-back bonding into the p-antibonding orbitals of the CO and C  N-R ligands. Tetracoordinated Ni(0)-carbonyl and isocyanide complexes, bearing up to four terminal carbonyl or isocyanide ligands, are usually diamagnetic and exhibit tetrahedral coordination geometry. As detailed in this section, methods to synthesize nickel(0) carbonyls can include the use of an already formed Ni(0) source, namely but not restricted to [Ni(CO)4], [Ni(COD)2], [LnNi2(m-arene)], or [(LnNi2(m-N2)], which participated in ligand exchange upon exposure to CO. In some cases, deoxygenation of CO2 in the presence of Ni(0) and a sacrificial ligand, was the source of CO. Synthesis of Ni(0) carbonyls also included the use of Ni(II) or Ni(I) sources under several reducing conditions, followed by exposure to CO. Similarly, methods for preparing nickel(0) isocyanides included either ligand exchange with a Ni(0) source, or reduction of Ni(II) precursors and treatment with isocyanide ligands. Characterization of carbonyl and isocyanide complexes typically included determination of the C-E (E ¼ O, N) stretching frequencies by FTIR, molecular structure determination by X-ray crystal diffraction, and solution characterization by NMR spectroscopy.

8.10.2.1

Carbonyl complexes

Carbonyl complexes 1 and 2 (Fig. 1) were synthesized by treatment of [Ni(CO)4] with (Me2P-6-C6H5)(6-C6H6)M (M ¼ V or Cr) at room temperature.1 In the solid state, complex 2 showed pseudo tetrahedral geometry around the Ni(0)-center. The molecular structure for 2 featured an inter-chromium distance of 947 pm. EPR studies of heterometallic 1, bearing the ligand (Me2P-6-C6H5) (6-C6H6)V, supported conformational flexibility for such a vanadium(d5)-based oligoradical species. The complex [Ni(PhBP3)(CO)][Li(TMEDA)] (3) was obtained by reaction of [Li(TMEDA)][PhB(CH2PPh2)3] with Ni(COD)2 under 1 atm of CO at room temperature (Fig. 2).74 IR analysis in THF for complex 3 featured blueshift of the CO stretching band relative to its value in the solid state, consistent with the transformation of an alkali metal ion isocarbonyl to a solvent-separated ion pair in solution. In the solid state, the nickel center exhibited pseudo tetrahedral geometry with the three phosphorus atoms disposed in a facial arrangement, and a terminal CO completing the coordination sphere. The structure of the anion of 3, [Ni(PhBP3)(CO)]−, was also calculated by DFT, which showed that the HOMO orbital was mainly composed of the nickel dp orbitals featuring back bonding to the CO p orbital. A mixture of [Ni(COD)2] and [C5H4CH¼ N(C6F5)]Fe[-C5H4PPh2] reacted under atmospheric CO to give the Ni(0) dicarbonyl complexes, [C5H4CH¼ N(C6F5)]Fe[-C5H4PPh2]Ni(CO)2 (4a) (Fig. 3),77 and {[C5H4CH¼ N(C6F5)]Fe[-C5H4PPh2]}2Ni(CO)2 (4b) (Fig. 1).77 The IR and mass spectral data of 4a indicated it was a mononuclear compound with bidentate iminophosphine and

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Fig. 2 Nickel(0) ate carbonyl complexes.74–76

Fig. 3 Nickel(0) dicarbonyl compounds with chelating ligands.2,76–84

terminal carbonyls, with no conclusive evidence of p-imine complexation. For 4b, neither spectroscopic evidence supported imine coordination. In the solid state, 4b showed a tetrahedral Ni(0) with two extended side-arms represented by the imine-phosphine ferrocene ligands, which coordinated through their phosphorous atoms. The two Ni-C bonds were significantly shorter than those in isonitrile complex 41a (vide infra, Fig. 8), which was consistent with stronger p-acidity of CO, and hence stronger M-C, compared to RNC. In the preparation of 4b, compound 4a was the major product; even in excess of [C5H4CH¼ N(C6F5)]Fe[-C5H4PPh2], formation of chelated 4a was unavoidable. Replacement of the toluene ligand in a silylene-containing Ni(6-toluene)-complex by three CO ligands yielded complex 5 in high yield (Fig. 4).85 Silylene ligand in 5 reacted as a nucleophile toward Lewis acids, such as tris(pentafluorophenyl)borane and

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683

Fig. 4 Heterometallic nickel(0) carbonyls. 85–87

[H(OEt2)2][B(C6F5)4], yielding complexes 6 and 7 (Fig. 4),85 respectively. On the other hand, the electron withdrawing CO ligands in 5 ultimately caused an increased Lewis acidity of the silicon center due to decreased Ni-to-Si p-backbonding. Thus, the silicon (II) hydroxide complex 8a (Fig. 4),85 was prepared by slow diffusion of water into a hexane solution of complex 5. Complex 5 also reacted with trifluoromethanesulfonic acid to yield 8b (Fig. 4).85 The weakly coordinating triflate anion was bound to the highly electrophilic Si center in solution and in the solid state. A toluene solution of [(dippe)Ni(m-H)]2 reacted with excess CO2 at room temperature to yield dicarbonyl dinuclear complex, [{Ni(dippe)(CO)}2(m-dippe)] (9) (Fig. 5),88 and monocarbonyl complex [Ni(dippe)(CO){dippe(O)}] (10) (Fig. 5),88 in 26% and 50% yield, respectively, along with [Ni(dippe)2] and dippe(O)2 in additional 15% and 9% yield. In the solid state, complex 9 displayed two nickel centers in a tetrahedral geometry bridged through a diphosphine ligand. The CO stretching frequencies observed for complexes 9 and 10 suggested strong p Ni-CO backbonding for both species. Additionally, when complex 10 was treated with a stream of CO gas, ligand substitution occurred, since compounds [Ni(dippe)(CO)2] and dippe(O) were spectroscopically identified.

Fig. 5 Nickel(0) monocarbonyl complexes with chelating phosphines.82,88–90

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Zerovalent Nickel Organometallic Complexes

Carbonyl complex [(dippe)Ni(CO)2] (11) was obtained after treatment of a MeOH solution of [(dippe)Ni(2-C,C-styrene)] (12) with a CO2 stream at room temperature. Coproduction of styrene hydroesterification products was also observed, along with formation of the Ni(II) complex, [(dippe)Ni(CO3)], and dippe(O)2.91 Complex [(dippe)Ni(CO)2] (11) was also obtained from the reactivity of CO2 and Et3SiH in the presence of substoichiometric amounts of [(dippe)Ni(H)]2.92 The reaction between [(dtbpe)Ni(H)]2, styrene and CO2 in MeOH, at room temperature, generated [(dtbpe)Ni(2-C,C-styrene)] (13). Upon heating complex 13 to 120  C for 36 h, styrene converted to their corresponding hydroesterification products, and the Ni(0) complex, [(dtbpe)Ni(CO)2] (14), the Ni(II) carbonate, [(dtbpe)Ni(CO3)], as well as dtbpe(O) and dtbpe(O)2 were also produced. 91 In the solid state, complexes [(dippe)Ni(CO)2] (11) and [(dtbpe)Ni(CO)2] (13) showed nickel-centers in a tetrahedral geometry, coordinated both by two P atoms from dtbpe and two terminal CO ligands.91,92 Treatment of a bright green solution of Ni(II) complex, 14a, with carbon monoxide in a flow reactor produced the Ni(0) complex, 14b (Scheme 1),93 along with the formation of uncoordinated 2-(1-propenyl)-[1,10]phenantroline, acrylic aldehyde and Ni(CO)4. According to structural analysis, complex 14b showed distorted tetrahedral geometry at the Ni-center, and the Ni-N bond lengths in 14b were longer than those for 14a. The CO stretching vibrations for complex 14b appeared at lower wavenumber values compared to related complex [Ni(phen)(CO)2].

Scheme 1 Synthesis of dicarbonyl complex 14b.93

Suitable crystals for X-ray diffraction analysis of compound [Fe(dsdm)]2Ni(CO)2 (15) (Fig. 4),86 were obtained from the reaction mixture between [Ni(dsdm)] and [Et4N][Fe(CN2(CO)3I]. The trinuclear Fe-Fe-Ni complex was not symmetric. The Ni atom appeared four-coordinated in a distorted tetrahedral geometry with two sulfur atoms and two carbonyl ligands. The Ni(CO)2 unit was bridged by two sulfur atoms from one Fe(dsdm) unit. One such S-atom formed an additional bond to the adjacent iron, and all four sulfur atoms in this molecule were different in their metal-coordination nature. Treatment of benzene solutions of [({2-R2PC6H4}2N)Ni(H)] with an atmosphere of carbon monoxide at room temperature led to the generation of diamagnetic crystalline compounds 16a–d (Fig. 3).78 No carbonyl insertion products (i.e., formyl complexes) were detectable by NMR. Upon CO coordination, the hydride resonance of the starting Ni(II) compounds disappeared, indicating that the formation of zerovalent nickel dicarbonyl complexes, 16a–d, was a consequence of a reductive elimination process. Solid-state structures for 16b and 16c indicated the Ni center was surrounded by two phosphorus donors and two terminal carbonyl ligands in a distorted tetrahedral geometry. Carbonylation of [({2-Ph2PC6H4}2N)Ni(R)] (R ¼ Me, Et) proceeded through intermediacy of acyl species, [({2-Ph2PC6H4}2N)Ni{C(O)R}] (R ¼ Me, Et) to yield final products 17a and 17b (Fig. 3).78 Both 17a and 17b showed inequivalent phosphorus atoms, apparently as a consequence of larger N-substituents in the diphosphine ligands compared with compounds 16a–d. Additionally, 17a and 17b featured rapid inversion at the pyramidal diarylamino nitrogen atom. 17b displayed tetrahedral coordination geometry in the solid state. Compound [(PiPr3)2Ni(CO)2] (18) was synthesized from the reaction between [{(iPr3P)2Ni}2N2] and CO. [(PiPr3)2Ni(CO)2] (18) exhibited very distorted tetrahedral geometry in the solid state and featured strong bands of nearly equal intensity at 1987 and 1926 cm−1 for the symmetric and asymmetric CO stretches.94 Synthesis of [{iPr2PCH2)2NMe}Ni(CO)2] (19) (Fig. 3),79 was performed from ethylene complex, [{iPr2PCH2)2NMe}Ni(2-ethylene)], in the presence of CO gas at room temperature. FTIR spectrum for 19 featured two bands of strong intensity at 1983 and 1920 cm−1 consistent with stretching for two terminal CO ligands. Metalation of 1,4-(2-iPr2PC6H4)2C6H4 with 3 equiv. of [Ni(COD)2] in the presence of carbon monoxide provided access to the trinuclear nickel complex 20a (Fig. 6),95 in 74% yield. Consistent with its solid-state structure, the solution IR spectrum of 20a showed bands indicative of terminal and bridging carbonyl coordination. Terminal carbonyl ligand in complex 20a was readily substituted upon addition of PMe3 to produce complex 20b, featuring an intact triangular-trinuclear core with Cs symmetry and strong metal-arene interactions, according to spectroscopic characterization. Dinuclear Ni-complex 21 (Fig. 6),96 was isolated in 11% yield after the reduction of the NiI-NiI dichloride, [{1,4-(2-iPr2PC6H4)2C6H4}Ni2Cl2], with Na[Co(CO)4], which served as a source of both reducing equivalents and carbonyl ligands. Complex 21 could also be synthesized through the addition of 4 equiv. of CO to 2 equiv. of [Ni(COD)2] and 1,4-(2-iPr2PC6H4)2C6H4, albeit with a lower yield.96 Single crystal X-ray diffraction studies confirmed the stabilization of the dinuclear Ni-Ni fragment by metal-arene interactions. Additionally, VT NMR studies showed decoalescence of the central arene protons into a pair of doublets at −20  C. IR spectroscopy displayed bands for two terminal and one bridging CO ligands.

Zerovalent Nickel Organometallic Complexes

685

Fig. 6 Nickel(0) complexes with bridging carbonyl ligands.95,96

Dicarbonyl complex 22 (Fig. 3),80 was obtained by treating a THF solution of Ni(0) bischelate, [({2-Ph2PC6H4}2SiMe2)2Ni], with CO. Complex 22, isolated as pale yellow crystals from diethyl ether at −30  C, exhibited distorted tetrahedral geometry around the Ni center in the solid state. X-ray crystallography allowed to discard any Ni-Si bond interaction in complex 22. Diamagnetic nickel carbonyl species 23 (Fig. 2),75 was produced by reduction of paramagnetic Ni(I) complex, [{(2-iPr2P-4Me-C6H4)2N}Ni(CO)] in the presence of 1 equiv. of sodium naphthalide (NaC10H8), followed by treatment with 2 equiv. of 12-crown-4. In the solid state, ate complex [{(2-iPr2P-4-Me-C6H4)2N}Ni(CO)]− displayed pseudo tetrahedral coordination geometry. Spectroscopic data indicated that 23 possessed a very electron-rich zero-valent nickel anion. Regeneration of Ni(I) species, [{(2-iPr2P-4-Me-C6H4)2N}Ni(CO)], from 23 was accomplished in 74% yield by the oxidation with 1 equiv. of silver trifluoromethanesulfonate.75 Complex 24 (Fig. 7) was obtained as a yellow powder in high yield from the reaction between tripodal phosphine (Ph2PCH2)3 C(CH3) and [Ni(CO)2(PPh3)2] in toluene at 90  C.97 In the solid state, complex 24 featured a nickel center tetrahedrally coordinated by the three phosphines of the tripodal ligand and one CO group. Alternatively, melting complex 25 (Fig. 7) or heating it to 90  C for a few hours, led to quantitative production of 24. Complex 25 was obtained selectively in high yield from the reaction between (Ph2PCH2)3C(CH3) and [Ni(CO)2(PPh3)2] at room temperature in the absence of light.97 Compound 25 showed only two phosphine groups of the tripodal ligand coordinated to the nickel center. Uncoordinated phosphine in complex 25 reacted with [Au(THT)Cl] producing colorless heterobimetallic complex 26 (Fig. 7) in 58% yield.97 Compound 26 showed a shelf life of several months at ambient temperature under an inert atmosphere; decomposition only occurred by heating 26 up to its melting point. The nickel complex 27 (Fig. 7) was selectively synthesized in 25% yield through ligand exchange between tripodal phosphine (Ph2PCH2CH2)3Si(CH3) and [Ni(CO)(PPh3)3]. Such [Ni(CO)(PPh3)3] starting material was in turn synthesized in 62% yield from a molten mixture of [Ni(CO)2(PPh3)2] and PPh3 in the absence of solvents. Attempts to prepare complex 27 directly from [Ni(CO)2(PPh3)2] and (Ph2PCH2CH2)3Si(CH3) led to a mixture of oligomeric byproducts.97 Complexes 28a–d (Fig. 3),81 were used for probing the degree of ligand p-acceptor character of their parent ligands by measuring the corresponding carbonyl stretching frequencies by means of IR spectroscopy. These nickel-carbonyl complexes were prepared by reaction of the respective bidentate ligands with [Ni(CO)2(PPh3)2]. Molecular structure for complex 28a showed the Ni(0) center in a tetrahedral geometry. Generation of complex 29a (Fig. 5) was observed after reaction of the Ni(II) complex, [{(2-iPr2PC6H4)2P}Ni(OPh)], in the presence of carbon monoxide at room temperature in C6D6.89 In contrast, species 29b (Fig. 5) was produced by the addition of carbon monoxide to a C6D6 solution of the Ni(0) complex, [{(2-iPr2PC6H4)2P(OMe)}2Ni2(m-N2)].89 Both complexes were characterized spectroscopically, displaying a single carbonyl ligand at each nickel(0) center, with stretching frequencies indicating increased back bonding in 29b when compared with 29a. Both solid-state structures of 29a–b revealed a phosphinite coordination at the pseudo tetrahedral nickel(0) centers. DFT analysis on both species supported the zerovalent nature of the nickel atom.89

Fig. 7 Mono- and dicarbonyl nickel(0) complexes with a tripodal phosphine ligand.97

686

Zerovalent Nickel Organometallic Complexes

The structures for the Ni(0) anions [Ni(CN)(CO)3]− (30) and [Ni(CN)2(CO)2]2− (31) were determined crystallographically. [PPh4][Ni(CN)(CO)3] (30a) was prepared from reaction between NiCl2
6H2O and NaCN with excess NaOH in aqueous media under 1 atm of CO at 25  C, followed by treatment with Ba(OH)2 and PPh4Cl. [Ni(CN)(CO)3]− (30) further reacted with 1 equiv. of cyanide to form [Ni(CN)2(CO)2]2− (31) either in aqueous solution or dichloromethane solution. Both nickel-ate compounds had tetrahedral geometry, thus only one structural isomer was possible for either. [Ni(CN)(CO)3]− (30) showed C3v molecular symmetry and [Ni(CN)2(CO)2]2− (31) displayed C2v symmetry, which was also consistent with spectroscopic characterization. Ate complexes 30 and 31 showed air- and light-sensitivity.98 During the study of the ligand properties of the germylene phosphane adduct, (C2F5)2Ge
PMe3, the tricarbonyl nickel(0) complex 32 (Fig. 4) was synthesized by the reaction between (C2F5)2Ge ∙PMe3 and Ni(CO)4.87 The molecular structure of 32 featured a both germanium and nickel atoms each in the center of a distorted tetrahedron. A Tolman cone angle of 168 was estimated for the ligand (C2F5)2Ge
PMe3 in complex 32, and as for the p-acceptor character, such ligand was similar to PMe3, but better p-acceptor than PCy3 and an inferior p-acceptor than P(C2F5)3. Preparation of complexes 33a–d (Fig. 1) was performed by addition of [Ni(COD)2] to THF solutions of ligands (P{RNCH2CH2}3N) (R ¼ Me, iPr, iBu, and Bz), followed by treatment with 1 atm of CO, yielding the corresponding products in 84–93% yield.2 Solution IR spectra showed two CO vibrational stretches assigned to the A1 and E modes. A1 resonances appeared at 2059.1–2054.6 cm−1, whereas E resonances showed at 1981.1 to 1974.7 cm−1. Crystallographic data for 33b–d showed near ideal tetrahedral geometries. Complex [Ni(TPAP)(CO)2] (34) (Fig. 3) was synthesized in 77% yield from [Ni(COD)2] and the ligand TPAP, in THF, in the presence of 1 atm of CO, and displayed two vibrational stretches at 1981.7 and 1906.5 cm−1.2 Complex 35 (Fig. 1) was obtained in 62% yield from a 2:1 ratio of (P{MeNCH2CH2}3N) and [Ni(COD)2] in THF and 1 atm of CO.2 For compound 35, IR signals for two CO stretches were observed at 1976.8 and 1909.3 cm−1. Molecular structures of 34 and 35 displayed tetrahedral geometries. Exposure of a degassed THF solution of Ni(0) pincer ate complex 36 (Fig. 2),76 to CO2 at room temperature yielded several diamagnetic species. Major product was species 37 (Fig. 3), obtained in 35.5% yield,76 which in the solid state revealed a Td symmetric tetrameric nickel cluster, in which a CO2 molecule was incorporated into each nitrogen atom of a PNP-pincer ligand. Carbamate vibration appeared at 1626 cm−1. In the nickel cluster, each sodium ion bound to four oxygen atoms from carbamates and terminal CO groups to construct a tetrameric structure. NMR experiments suggested that the tetrameric structure of 37 was maintained in a benzene solution. CO ligands present in compound 37 originated from reductive disproportionation of CO2, yielding CO and carbonate, whose in situ generation was also confirmed. In addition, when compound 36 was treated with a 1:1 mixture of CO/CO2, compound 37 was obtained in higher yield (66.6%).76 Compounds 38a (Fig. 5),82 and 38b (Fig. 3),82 were synthesized from complex [(dppbz)Ni(PMe3)2] (dppbz ¼ 1,2-bis (diphenylphosphino)benzene) by sequential substitution of PMe3 ligands by CO. Complex 38b was also obtained by treatment of [Ni(PPh3)2(CO)2] with dppbz. Likewise, a more convenient approach to obtain 38a was the reaction of [(dppbz)Ni(CO)2] (38b) with PMe3. The molecular structures of 38a–b were determined by X-ray diffraction. The asymmetric unit of 38b contained two crystallographically independent molecules differing in the conformations of the phenyl substituents. The geometry of the metal center was pseudo tetrahedral in both cases. Formation of dimeric mixed-valence Ni(0)Ni(II) complex 39 (Fig. 3) was observed after exposure of a C6D6 solution containing Ni(COD)2 and (2-Ph2PC6H4)N ¼ CH(2-Ph2PC6H4), to 1 atm of CO.83 Complex 39 was a minor species present in the reaction mixture, but crystals were obtained and analyzed by X-ray diffraction showing a Ni(0) center bound to two terminal CO groups and a Ni(II) center surrounded by two neutral phosphine ligands and two anionic amido ligands. Geometry around the Ni(0) center was pseudo tetrahedral. IR analysis of 39 showed signals for the two terminal CO ligands. The formal oxidation state of such Ni(0) center remained unchanged, starting from the Ni(0) precursor [Ni(COD)2]. In contrast, the additional Ni(II) center was formed as the product of intramolecular oxidative coupling of two former imine-moieties, originally present in the ligand (2-Ph2PC6H4) N ¼ CH(2-Ph2PC6H4).83 Reaction of [Ni(CO)4] in the presence of the ligand 2, 20 -bis(diphenylphosphanyl)-3, 30 -bibenzo[b]thiophene in CDCl3 produced the tetrahedral dicarbonyl diphosphine complex 40 (Fig. 3).84 The IR spectrum for 40 showed two signals consistent with the symmetric and anti-symmetric stretching of the two terminal CO ligands. In the solid state, the asymmetric unit consisted of a dicarbonyl Ni(0) complex with the nickel center tetracoordinated by two carbonyls and the two phosphorus atoms of the chelate ligand.

8.10.2.2

Isocyanide complexes

Ligand replacement of [Ni(COD)2] with [{5-C5H4CH¼ N(C6H5)}Fe{5-C5H4P(tBu)2}] and [{5-C5H4CH ¼ N(C6F5)}Fe {5-C5H4PPh2}] in the presence of CNtBu in hexane, yielded dark-red solids [{5-C5H4CH ¼ N(C6H5)}Fe{5-C5H4P(tBu)2}Ni [CNtBu)3] (41a) (Fig. 8),99 and [{5-C5H4CH¼ N(C6F5)}Fe{5-C5H4PPh2}Ni(CNtBu)3] (41b) (Fig. 8),99 respectively. The solid state structure of 41b revealed a tetrahedral Ni(0) complex supported by three isocyanide ligands, with the iminophosphane in a monodentate coordination mode. Complex 41b exhibited catalytic activity toward ethylene oligomerization in the presence of EtAlCl2, with TOF values of up to 120,600 h−1 at 30  C. THF solutions of [(DME)NiBr2] were treated with KC8 in the presence of both a tripodal phosphine and the isonitriles, cHexNC and tBuNC, to yield compounds 41 and 42 (Fig. 8).100 The ethylene derivative 43 (Fig.16),100 was obtained by performing the same

Zerovalent Nickel Organometallic Complexes

687

Fig. 8 Nickel(0) isocyanides with phosphine ligands. 90,99–102

reduction with KC8 under an atmosphere of ethylene instead of argon. Compounds 41–43 were isolated as yellow to orange solids in fair yields and analytically pure crystalline form. In the solid state, they showed pseudo tetrahedral geometry of the coordination sphere. Treatment of p-methoxy isocyanobenzene with an ether suspension of [Ni(COD)2] at room temperature produced a yellow precipitate characterized as tetra(p-isocyanoanisole)nickel(0) (44) (Fig. 9) in 87% yield.103 Compound 44 crystallized in a chiral orthorhombic setting and showed an approximately tetrahedral geometry. Treatment of [Ni(COD)2] with 2 equiv. of CNArDipp2 in THF solution yielded the orange bis-isocyanide complex [Ni(COD)(CNArDipp2)] (45) (Fig. 9).104 IR analysis of complex 45 indicated p-back donation from the Ni center to the isocyanide ligands, and solid-state analysis revealed pseudo tetrahedral geometry around the nickel center. In species 45, steric properties of CNArDipp2 prevented formation of a tetrakis-coordinated Ni(0) complex. Compound 46 was obtained through intermediacy of the Ni(II) complex, [Ni(CNArDipp2)2(I)2], formed first by oxidation of 45 in the presence of I2. Subsequent addition of magnesium to [Ni(CNArDipp2)2(I)2] in THF solution selectively produced three-coordinate [Ni(CNArDipp2)3] (46) (Fig. 9) in 53% yield, or 83% yield when excess CNArDipp2 was present.104 Solid-state structure of 46 featured trigonal-planar geometry. Species [{(2-PiPr2-C6H4)2P(OiPr)}Ni(CO)] (47) (Fig. 5),90 was cleanly synthesized by exposing [{(PPOiPrP)Ni}2(m-N2)] to CO at ambient pressure. Compound 48 (Fig. 8) was in turn obtained by treatment of the Ni(II) compound, [{(2-PiPr2-C6H4)2P} Ni(OiPr)], with excess of tert-butyl isocyanide.90 Alternatively, reaction of the Ni(0) compound, [{(2-PiPr2-C6H4)2P(OiPr)}Ni}2

Fig. 9 Nickel(0) bis-, tris-, and tetrakis-isocyanide complexes.103,104

688

Zerovalent Nickel Organometallic Complexes

(m-N2)], with tert-butyl isocyanide also yielded complex 48 as a dark red solid in 98% yield.90 Complexes 47 and 48 showed pseudo tetrahedral geometry at the Ni(0)-center in the solid state. IR spectroscopy of both compounds showed significant back bonding toward the corresponding p-acidic ligands. Addition of 1 equiv. of tert-butyl isocyanide to Ni(II) compound, [{(TripNH)SiP2}Ni(Ph)] ({TripNH}SiP2 ¼ [(2,4,6iPr3C6H2NH)Si(2-PiPr2C6H4)2]−), at room temperature, produced complex 49 in 97% yield (Fig. 8).101 IR spectroscopy supported p-acceptor character of the isocyanide ligand. In addition, complex 49 exhibited a p-interaction of a phenyl group with the Ni(0) center, as revealed by X-ray crystallography. When an excess of tert-butyl isocyanide was added to the starting Ni(II) source, [(TripNH)SiP2]Ni(Ph), demetallation of the latter occurred. Such reactivity was also observed after treatment of [(TripNH)SiP2] Ni(Ph) with CO. Ta-containing Ni(0) isocyanide complex [Ta({2-Ph2P}C4H3N)3Cl2Ni(CN(Xylyl)] 50 (Fig. 8) was obtained by addition of 2,6-xylyl isocyanide to a THF and benzene solution of parent Ni(0) compound [Ta({2-Ph2P}C4H3N)3Cl2Ni], synthesized from [Ni(COD)2] and Ta({2-Ph2P}C4H3N)3Cl2.102 Complex 50 showed a Ni center in a distorted trigonal bipyramidal geometry bound to two phosphines, a bridging chloride, and a seven-coordinate Ta-containing moiety along with an isocyanide ligand in the axial position. In solution, 50 existed as a mixture of three isomers.

8.10.3

Nickel(0) s-adducts with E-H bonds (E ¼ B, Si, Mg) and hydride-bridging ligands

s-Adduct formation with zerovalent nickel centers can be described as a three-center two-electron type of interaction. This section covers a selection of s-adducts with hydroboranes and hydrosilanes, through their corresponding B-H and Si-H groups, as well as the interaction of Ni(0) with Mg-H bonds and Mg-alkynyl groups. Additionally, examples of Ni(0) ate complexes featuring hydride-bridging ligands, are presented in this subchapter. As in the case of carbonyl and isocyanide Ni(0) complexes, access to Ni(0) s-adducts was achieved by both, ligand replacement at Ni(0) sources, or by reduction of Ni(I) precursors in the presence of the corresponding ligands. Methods of characterization included NMR solution analysis, elucidation of the molecular structure by X-ray diffraction, CV analysis, and DFT calculations.

8.10.3.1

s-Adducts with E-H bonds (E ¼ B, Si, and Mg), and hydride-bridging ligands

Addition of the nickel(I) hydrides 51a–c, at room temperature, to a mixture of Super-Hydride (LiHBEt3) and BEt3, produced the Ni(0) s-alkylboranes 52a–c and the Ni(II) trihydride complexes 53a–c as revealed by NMR spectroscopy in each case for every pair of complexes (Scheme 2).105 In the 31P{1H} NMR spectra, all the s-alkylboranes exhibited two asymmetric doublets with scalar coupling constants typical of Ni(0) compounds.106–108 Compounds 52a–c were also prepared in 10–15% isolated yield, at room temperature, by adding stoichiometric amounts of 51a–c to (HBEt2)2 solutions independently prepared in THF.105 In such case, none of the Ni(II) trihydride complexes 53a–c was formed.

Scheme 2 Formation of Ni(0) s-alkylboranes and Ni(II) trihydride complexes.105

Reaction of pentane solutions of [{(dtbpe)Ni}2(m-2:2-C6H6)] with silanes Mes2SiH2, Ph2SiH2, Ph2MeSiH, and Ph2SiHCl at room temperature afforded [(dtbpe)Ni(m-H)SiH(Mes)2] (54a), [(dtbpe)Ni(m-H)SiH(Ph)2] (54b), [(dtbpe)Ni(m-H)SiMePh] (54c), and [(dtbpe)Ni(m-H)SiCl(Ph)2] (54d), respectively.109 In the solid state, compounds 54a and 54d were best described as s-adducts, since the position of the hydride and the silyl fragments was consistent with arrested oxidative addition of the silane, as indicated by a Si-H bond distance involving the bridging hydrogen longer than the one of the terminal Si-H bond. In solution, compounds 54a–d showed fluxionality due to reversible oxidative addition via Ni(0)-2 silane intermediates. The reaction between [Ni(PMe)4] and {2-Ph2PC6H4}2SiHMe in THF at room temperature afforded complex 55a (Fig. 10), which was isolated at 0  C, from diethyl ether, as yellow crystals in 75% yield.80 Complex 55a featured an 2-(Si-H) moiety according to spectroscopic characterization and X-ray crystallography. Compound 55a readily formed the carbonyl complex 55b (Fig. 10) in THF solution under an atmosphere of CO at room temperature without losing the 2-(Si-H) moiety.80

Zerovalent Nickel Organometallic Complexes

689

Fig. 10 Intramolecular nickel(0) s-adduct with a Si-H moiety.80

Fig. 11 Miscellaneous nickel(0) ate and neutral complexes with the trop2NH ligand.110,111

The dark red complex [Ni(PPh3)(trop2NH)] (56a) (trop2NH ¼ bis(5H- dibenzo[a,d]cyclohepten-5-yl)amine) (Fig. 11) was prepared by reduction of the Ni(I) complex [Ni(TFA)(trop2NH)] (TFA ¼ trifluoroacetate) with Zn powder in THF and in the presence of PPh3.110 According to CV analysis, complex 56a exhibited a reversible redox wave at −1.28 V for the Ni(I)/Ni(0) couple. Molecular structure of 56a showed the Ni center in a distorted pseudo-tetrahedral geometry. The C ¼ C bond distances in 56a were comparatively slightly longer than the ones in the Ni(I) precursor, [Ni(TFA)(trop2NH)], suggesting more nickel-to-ligand back donation of the d10-nickel(0) center into the p-accepting olefinic units, which was also supported by NMR data. Reaction of Ni(I) complex, [Ni(TFA)(trop2NH)], with a slight excess of amido borane K[Me2NBH3] formed hydride-bridged compound 56b (Fig. 11) in situ.110 On the other hand, when K[Me2NBH3] was formed in situ from Me2HN-BH3 with KOtBu, the reaction produced nickel(0) hydride 56c (Fig. 11) as the main product.110 Reaction of amidoborane salt K[Me2NBH3] with monovalent nickel complex [Ni(TFA)(trop2NMe)] (trop2NH ¼ bis(5Hdibenzo[a,d]cyclohepten-5-yl)amine ; TFA ¼ trifluoroacetate) selectively yielded the potassium salt of dinuclear bridging hydride complex 57a (Fig. 11).111 Compound 57b (Fig. 11) was obtained as a stable Li salt in 56% yield after treating [Ni(TFA)(trop2NMe)] with LiAlH4, which reacted as a reductant and hydride transfer agent. DFT and atoms-in-molecules theory analysis of 57b indicated metallacyclopropane character of the nickel and C ¼ C moiety interaction as a result of significant ligand to metal donation,

690

Zerovalent Nickel Organometallic Complexes

p(C ¼ C)! Ni, and metal to ligand back donation, Ni !p (C¼ C).111 To probe the activity of complex 57a in the dehydrogenation of silanes, this was treated with 3 equiv. of Ph2SiH2 to form a 1:1 mixture of the complexes 57c and 58a (Fig. 11).111 Over time, the reaction mixture containing 57c, 58a, and Ph2SiH2, at room temperature, slowly produced disilane complex 58b (Fig. 11), indicating suitability of the low-valent nickel hydride or silane species for catalytic dehydrogenation of silanes. Isolated d10 2-(Si-H)Ni0 complex 58a showed that back donation from the Ni(0) center to the C ¼ C moiety was sufficiently strong to prevent oxidative addition of the Si-H bond. In the solid state, compounds 57b and 58a displayed trigonal pyramidal geometry around the nickel(0) center. The bridging complex 57a featured a short Ni-Ni distance attributed to a weak non-covalent interaction between the d10-valence-electron configured nickel centers. Reaction of a Mg monoalkyl complex with [Ni(COD)2] in toluene at a molar ratio 1:1, produced hetero-bimetallic hydride-bridged compound 59 (Scheme 3) as an orange solid in 53% isolated yield.112 Simultaneous formation of cyclohexene during production of 59 indicated b-hydride elimination as an intermediate step, which occurred at the Ni-center according to DFT calculations.112 NBO analysis of 59 indicated the formation of a three-center two-electron bond for the bridging hydride. Addition of a second Mg monoalkyl unit to 59, led to complete substitution of the COD ligand forming 60.112 Reactivity of 60 toward isocyanates did not lead to insertion reactions but to coordination to the nickel center, along with hydride dissociation from nickel and bridge formation between two magnesium atoms, overall yielding complexes 61a and 61b as orange solids in 87% and 71% isolated yields (Scheme 3).112 When treated with phenylacetylene, complex 60 generated product 62 (Scheme 3) in 93% isolated yield, with concomitant formation of hydrogen.112 In the solid state, 62 featured a heterotrimetallic [Mg-Ni-Mg] framework containing two acetylide ligands, one forming a m2-bridge between Mg and Ni, and the other forming a m3-bridge between two Mg atoms and the Ni center.

Scheme 3 Nickel(0) complexes with hydride- and acetylide-bridging ligands.112

8.10.4

Nickel(0) complexes with olefin ligands

8.10.4.1

Complexes with the COD (1,5-cyclooctadiene) ligand

The Ni(0) complex, [Ni(COD)2] (COD ¼ 1,5-cyclooctadiene), is currently one of the most employed Ni(0) sources for the synthesis of other Ni(0)-complexes, as well as for conducting research in catalysis.3,5,11,13,20,21,24,25,28,37–39,74,77,104,112–161 Extended use and popularity of [Ni(COD)2] is in part due to its high versatility and reasonable shelf stability under appropriate storage conditions. Main drawback on the use of [Ni(COD)2] is air- and moisture sensitivity, requiring the use of glovebox and Schlenk techniques for appropriate manipulation. Currently, an alternative to the use of [Ni(COD)2], is the use of recently introduced Ni(0) complexes, [(DQ)Ni(COD)] (84) (Fig. 12),161 and [Ni(Fstb)3] (113b) (vide infra Fig. 17),159 which featured air-, moisture-, and thermal stability, as well as competitive performance in proof-of-concept practical applications. Synthetic versatility of [Ni(COD)2] mostly relies in the rather common, straightforward replacement of one COD ligand in the presence of a wide variety of ligands; and usually, complete substitution of both COD ligands is also possible under appropriate reaction conditions. This section covers some examples of Ni(0)-complexes isolated after substitution of one COD ligand from [Ni(COD)2]. Also covered are some DFT calculations featuring haptotropic rearrangements of the COD ligand. In general, COD

Zerovalent Nickel Organometallic Complexes

691

Fig. 12 Nickel(0) complexes with the COD ligand.120,160–165

ligand coordinates in an 4-, also denoted as an 2:2-fashion, therefore occupying two coordination sites at the Ni(0) centers. Common characterization techniques for this family of compounds include NMR spectroscopy and X-ray crystal structure determinations. Equimolar amounts of methyl iodide or 1-fluorochlorobenzene were added to [Ni(COD)2], followed by the addition of 1 equiv. of the DABMes (RN ¼ C(Me)-C(Me)¼NR; R ¼ Mes) ligand to produce [Ni(DABMes)()COD] (63) (Fig. 12) in 72% isolated yield.120 The UV/Vis spectrum of a 10−4 M solution of 63 in hexane showed absorptions at l ¼ 280 and 496 nm. In the solid state, the nickel atom in compound 63 featured distorted tetrahedral coordination with two nitrogen atoms of the DABMes ligand and two alkene entities of the COD ligand. The C ¼ N distances of the diazadiene ligand differed significantly from the corresponding bond lengths in free DABMes. Also, a shorter C-C distance and an elongated C-N of the diazadiene ligand was observed as a result of back bonding to the p orbital of the diazadiene ligand in 63. The reaction of 1 equiv. of 1,8-bis(diisopropylphosphino)naphthalene (dippnapht) with [Ni(COD)2] in a THF solution resulted in the formation of complex [(dippnapht)Ni(COD) (64) (Scheme 4).166 NMR analysis of 64 indicated that a dynamic process involving the COD ligand occurred at room temperature, however, its low solubility at lower temperatures limited further studies by VT-NMR. The addition of 1 equiv. of S8 to a toluene solution of 64 resulted in the oxidation of the nickel center yielding the monomeric 2-disulfide complex 65 in 76% isolated yield (Scheme 4).166 Reaction between complexes 64 and 65 in a 1:1 stoichiometry, produced the dimeric Ni(II) complex, [(dippnapht)Ni(m-S)]2, where the nickel(0) compound, 64, formally reduced the S2− 2 unit in 65 by two electrons giving the Ni(II)-Ni(II) product.

Scheme 4 Sulfur reduction by a Ni(0) complex.166

The red complex [Ni(COD)(GaAr0 )2] (66) (Scheme 5) was synthesized by the reaction of the digallane Ar0 GaGaAr0 (Ar ¼ C6H3-2,6-(C6H3-2,6-i-Pr2)2) and [Ni(COD)2].135 The molecular structure of 66 showed a distorted tetrahedral coordination geometry. The Ga-Ni bond distances indicated good s-donor character of the GaAr0 ligand. Reaction of 66 with excess ethylene in hexane of benzene at room temperature and atmospheric pressure produced complex 67 in moderate yield (Scheme 5).135 For complex 67, the solid-state structure showed complete substitution of the COD ligand and activation of a molecule of ethylene by the two GaAr0 entities, forming a Ni2Ga2C2 bicyclic core structure. The nickel atoms in complex 67 displayed nearly symmetric 6-arene coordination with the rings of the Ar0 ligands. 0

692

Zerovalent Nickel Organometallic Complexes

Scheme 5 Nickel(0) digallane-derived complexes.135

Fig. 13 Nickel(0) alkene- and alkyne-complexes with bis-imino-pyridine ligands.136

Treatment of 2 equiv. of [Ni(COD)2] with ligand 68a formed complex 69 in 57% yield (Fig. 13).136 NMR spectroscopy revealed the presence of the COD ligand in the structure of 69. This compound displayed limited stability under vacuum or in solution for prolonged periods of time. DFT calculations for 69 suggested Ni(I)-character with a radical anion localized on one iminopyridine fragment. The reactions of 68a or 68b (Fig. 13) with 1 equiv. of [Ni(COD)2] led to complete substitution of the COD ligands, yielding mixtures of syn and anti-isomers of the formula Ni2(L)2 (L ¼ 68a–b) in 98% and 63% isolated yields.136 Species Ni2(L)2 (L ¼ 68b) exhibited reversible binding of diphenylacetylene, since treatment of its mixture of syn and anti-isomers with excess diphenylacetylene generated a mixture of compounds 70a–b (Fig. 13),136 identified by NMR and mass spectrometry. Upon removal of excess diphenylacetylene, most of the material reverted back to the Ni2(L)2 (L ¼ 68b) form. The pyridyl-phosphine ligand, 2-((di-t-butylphosphino)methyl)pyridine (PN) was treated with a toluene solution of [Ni(COD)2] to afford [(PN)Ni(COD)] (71) (Fig. 12) in modest yield.162 The solid-state structure of 71 revealed a slightly distorted tetrahedral coordination sphere. Complex 71 was useful for the activation of light olefins such as ethylene, propylene, and acrylonitrile. Addition of excess ethylene to a benzene-d6 solution of 71 resulted in formation of 2-ethylene complex 72a (Fig. 16).162 Analogous complexes 72b and 72c (Fig. 26) were obtained in a similar manner.162 Species 72a and 72b featured exchange between free and bound alkene at room temperature. The solid-state structure of complex 72c revealed an approximate square planar geometry around the nickel center with the CN substituent oriented away from the large PtBu2 ligand. In addition, the C¼ C bond was substantially elongated from a typical double bond indicating a strong metallacyclopropane resonance contribution. Compounds 72a and 72b reacted in the presence of 1 atm CO2 at ambient temperature to give the oxidative coupling products [(PN)Ni(kC,kO-CH2CH2COO)] and [(PN)Ni(kC,kO-CH2CH(CH3)COO)] in good yields.162 Complex 73 (Fig. 12) was obtained by treatment of [Ni(COD)2] with the corresponding bis-phosphane ligand in THF.163 NMR analysis revealed P-coordination of the bis-phosphene ligand in 73, and it slowly decomposed in air at ambient conditions, although it was less sensitive in comparison to [Ni(COD)2]. The molecular structure of 73 featured angle values of the coordination sphere close to an ideal tetrahedral geometry. Complexes [(dcpe)Ni(COD)] (74) (dcpe ¼ 1,2-bis(dicyclohexylphosphino)ethane),119 and [(dippe)Ni(COD)] (75) (dippe ¼ 1,2-bis(diisopropilphosphino)ethane),120 were obtained in 86% and 89% yield by treatment of [Ni(COD)2] in the presence of equivalent amounts of diphosphines, dcpe and dippe, respectively. Complex 75 served as precursor of carbonyl complexes when reacting in the presence of ketones (vide infra Fig. 44). Complex [{Si(Xant)Si}Ni(2-1,3-COD)] (76) (1,3-COD ¼ 1,3-cyclooctadiene) (Fig. 12) was synthesized by the reaction of the corresponding bis(NHSi)xanthene chelating ligand with [Ni(COD)2] in Et2O at room temperature.164 This compound featured a coordinatively unsaturated 16-electron Ni(0) center, where isomerization of 1,5-cycloocadiene occurred, through initial formation of intermediate [{Si(Xant)Si}Ni(4-1,5-COD)]. In the molecular structure of 76, the nickel center adopted a trigonal-planar geometry. The 2-coordinated C¼ C bond of the 1,3-COD ligand was longer than the uncoordinated C ¼ C bond, indicating back donation from the Ni center to the p orbital of the 2-coordinated C¼ C bond. Substitution of the 1,3-COD ligand was observed

Zerovalent Nickel Organometallic Complexes

693

Fig. 14 Calculated nickel(0) complexes bearing Z2- and Z4-COD ligands.155,167,168

upon treatment of complex 76 with PMe3 in Et2O, at room temperature, forming compound [{SiII(Xant)SiII}Ni(PMe3)2] in 62% isolated yield.164 [{SiII(Xant)SiII}Ni(PMe3)2] featured symmetric tetrahedral geometry around the nickel center. Complex 76 activated H2 at the stoichiometric scale, and catalyzed the hydrogenation of benchmark norbornene and several substituted olefins in quantitative yields at the 2 mol% loading, under 1 atm H2 at room temperature.164 Structures of the Ni(0)-intermediates 77a–e (Fig. 14), featuring 2-COD ligands and 2-bound arenes, were analyzed during DFT calculations relevant to the [Ni(COD)2]-catalyzed ipso-silylation of 2-methoxynaphthalene with silyl boronate, Et3SiBpin, in the presence of KOtBu.155 Species [(2-COD)2NiSiEt3]K (77a) (Fig. 14) was computed to have formed by the reaction of [Et3SiBpin(OtBu)] with [Ni(COD)2], and it acted as the substrate-catalyst complex. Ate complex 77b (Fig. 14) showed 2-coordination with a double bond of an activated aromatic ring of 2-methoxynaphthalene, and 2-coordination with a COD ligand, thus bearing a vacant site in the coordination sphere. Intermediate 77c (Fig. 14) was formed after transfer of the SiEt3 group to the aromatic moiety, featuring simultaneous partial loss of aromaticity. Species 77d (Fig. 14) was formed and recovered its aromaticity after methoxy loss from 77c. In 77d, a methoxy ligand coordinated to nickel, which resulted in a tetrahedral geometry around the metal center. Finally, 77e (Fig. 14) formed from 77d via dissociation of the arene moiety, therefore featuring 4-coordination of the COD ligand to Ni(0). Structures of the type 78a–b (Fig. 14) were calculated by DFT during mechanistic investigations of the Ni(0)-catalyzed anionic cross-coupling reaction (ACCR) of lithium sulfonimidoyl alkylidene carbenoids with organolithium reagents.167 Computational studies revealed two pathways of the ACCR depending on whether a phosphine or 1,5-cyclooctadiene was the ligand of the Ni atom. Intermediates 78a–b were comparable since they only differed in the L ligand. Electron-rich center in Ni(0)-ate complex 78a (Fig. 14) was stabilized by p-back donation to the antibonding orbital of the C ¼ C moiety in the lithioalkenyl sulfoxime, leading to notable lengthening of the C ¼ C bond. In the case of complex 78b (Fig. 14), stabilization of such an electron-rich center was instead provided by two olefin ligands, one from the lithioalkenyl sulfoxime and one from the 2-bound 1,5-cyclooctadiene. Addition of dppf (1,10 -bis(diphenylphosphino)ferrocene) to [Ni(COD)2] in a 1:1 metal to ligand ratio in toluene, exclusively produced [(dppf )Ni(COD)] (79), as revealed by NMR spectroscopy.158,169 X-ray crystallography featured 4-coordination of the COD ligand to a nearly tetrahedral nickel center. Complex [(dppf )Ni(COD)] (79) catalyzed the trifluoromethyl thiolation of several substituted aryl and heteroaryl chlorides with yields ranging 40–98%, with the use of the coupling reagent (Me4N)SCF3. Experimental data and DFT calculations on the in situ formation of the side-on complex, [(dppf )Ni(2-C,N-MeCN)] (80), from [(dppf )Ni(COD)] (79) in the presence of MeCN, indicated that the addition of such an additive decreased the activation energy barrier of the trifluoromethyl thiolation reaction, therefore suggesting that MeCN coordinated more weakly than COD to the [(dppf )Ni0] fragment.123 Complex [(dtbbpy)Ni(COD)] (81) (Fig. 12), was prepared from [Ni(COD)2] and 4,40 -di-tert-butyl-2,20 -bipyridine.160 Complex 81 catalyzed the coupling reaction of alkenyl chlorosilanes, such as dimethylvinylchlorosilane, with either aryl or alkenyl electrophilic reagents in the presence of Mn or Zn as reducing agents to yield tetrasubstituted silanes in 43–93% yields. Complexes 82 and 83 (Fig. 15) were obtained in quantitative yields by treatment of [Ni(COD)2] with strongly electron donating phosphines, bearing imidazoline-2-ylidenamino-substituted phosphorus atoms, linked by the ligand backbones 1,10 -ferrocene in the case of complex 82, and 1,2-benzene in the case of complex 83.170 Complex 82 showed two inequivalent phosphorus atoms in

694

Zerovalent Nickel Organometallic Complexes

Fig. 15 Nickel(0) complexes with strongly electron-donating phosphines featuring Z4- and m-Z2:Z2-COD ligands.170

accordance to NMR analysis. In the solid state, 82 and 83 featured bisphosphine-chelating ligands with an additional cyclooctadiene ligand, coordinated either in a 4- or a m-2:2-fashion, respectively. The formation of unsaturated dinuclear 16-electron complex, 83, occurred by the increased steric bulk around the Ni atom. The C ¼ C bond lengths of the cyclooctadiene ligand in 82 and 83 were elongated compared to those of [(L2)Ni(2:2-cod) (L ¼ MeDuphos, dppf )152,171 in agreement with electron back bonding from the electron rich Ni atom into antibonding p -orbitals of the C¼ C bonds. Reaction of [Ni(COD)2] with duroquinone (DQ) produced the red-colored solid compound [(DQ)Ni(COD)] (84) (Fig. 12) in 79% yield in gram scale.161 Alternatively, [(DQ)Ni(COD)] (84) was obtained more conveniently from air stable Ni(II) precursor, [Ni(acac)2] in 60% yield upon treatment with the reducing agent DIBAL-H or from [NiCl2(pyridine)4] in 28% yield in the presence of sodium as the reducing agent.161 Complex 84 was originally described in 1962, however less conveniently synthesized from highly toxic Ni(CO)4.172 Molecular structure of 84 featured boat-conformation of the COD ligand, as in Ni(COD)2, and the two pairs of olefin moieties in DQ and COD showed orthogonal to one another.161 The nickel center exhibited tetrahedral geometry and featured p-back donation to the DQ ligand as further confirmed by IR analysis of the C ¼ O stretching bands. [(DQ)Ni(COD)] (84) (Fig. 12), isoelectronic to 18-electron Ni(0) complex, [Ni(COD)2], featured stability toward air and moisture, in both solution and the solid state, and showed high thermal stability which enabled it to be stored at room temperature. [(DQ)Ni(COD)] (84) proved to be a competent catalytic precursor for several Ni-catalyzed synthetic methods such as Suzuki-Miyaura cross coupling of aryl and heteroaryl coupling partners, amination of aryl chlorides, borylation of aryl halides, C-H activation/alkyne annulation, directed hydroarylation of unactivated alkenes, and formation of quinazolinediones through decarboxylative cycloaddition. In all these reactions, due to its stability, complex 84 was conveniently manipulated without the use of glovebox or Schlenk techniques.161 During mechanistic investigations on the nickel-catalyzed enantioselective C-H cyclization of imidazoles, the Ni(0) intermediates 85 and 86 (Fig. 14) were studied by DFT calculations.168 [(JoSPOphos)Ni(COD)] (85) (Fig. 14) was computed to have formed by ligand exchange of [Ni(COD)2] with the corresponding JoSPOphos ligand. In complex 85, the bidentate JoSPOphos ligand coordinated with the metal through the lone pair of the P(III) atom and the P(V)-H bond, respectively, thus featuring a P-H agostic interaction with Ni(0), confirmed by atoms-in-molecules analysis. Complex 85 underwent facile P-H oxidative addition and migratory insertion to participate in a chain walking process. Complex [(JoSPOphos)Ni{(2-C,N:2-C,C)-N-[CH2CH2C(CH3)¼ CH2]-benzoimidazolyl}] 86 (Fig. 14), calculated amongst other similar benzoimidazolyl-alkene-Ni(0) intermediates, showed a three-coordinate Ni-center forming an agostic interaction with the C(sp2)-H bond, and the absence of a P-H agostic interaction, such as the one predicted for 85, due to steric repulsion between the JoSPOphos ligand and the benzoimidazolyl moiety.

8.10.4.2

Ethylene complexes

Zerovalent nickel ethylene complexes are the simplest p-olefin organometallics, bearing at least one p-coordinated ethylene ligand. Coordination of ethylene to Ni(0) usually is accompanied by partial loss of bond-order of the C-C double bond, as a consequence of p-back bonding into the p orbitals of the C¼ C moiety. Three-coordinate Ni(0) monoethylene complexes, usually bearing up to two additional s-donor ligands, tend to be diamagnetic and exhibit trigonal-planar coordination geometry. As described in this section, some methods to access nickel(0) ethylene complexes include ligand replacement from a Ni(0) source upon exposure to an ethylene atmosphere, or reductive pathways starting from Ni(II) sources, for instance reductive decoupling reactions in the absence of ethylene or reductive elimination in the presence of ethylene. Common characterization techniques for ethylene complexes include molecular structure determination by X-ray diffraction, and NMR spectroscopy in solution.

Zerovalent Nickel Organometallic Complexes

695

Reductive decoupling of the NiCH2CH2COO − unit in Ni(II) complex, [(py)2Ni(CH2CH2COO)] (87), in the presence of bulky dtbpf (1,10 -bis(di-tert-butylphosphino)ferrocene), resulted in the formation of the nickel(0) ethylene complex 88 and CO2 (Scheme 6).122 Complex 88 was also obtained after heating the Ni(II) metallacycle, [(dtbpf )Ni(CH2CH2COO)], at 80  C. The nickel center in 88 was in the distorted tetrahedral environment of the two P donor groups and the carbons of the ethylene ligand. The double bond of the ethylene ligand was significantly elongated compared to free ethylene and the angle between the planes P-Ni-P and plane C-Ni-C was 15.71 .

Scheme 6 Synthesis of ethylene complex 88 by reductive decoupling of a nickela(II)lactone.122

Fig. 16 Nickel(0) ethylene complexes.79,100,122,162,165,173–175

Crystals of complex 89 (Fig. 16) were obtained from toluene at −40  C after the reduction of Ni(acac)2 with AlEt3 in the presence of dppm (dppm ¼ 1,1-bis(diphenylphosphino)methane) and an atmosphere of ethylene, performed at 0  C.173 The crystal structure of 89 showed a nickel(0) dimer with two m-dppm bridging ligands connecting the two Ni atoms. The nickel centers were in a distorted trigonal planar environment formed by two phosphorus atoms of two dppm ligands and the centroid of the coordinated C-C double bond of the ethylene ligand. The angles between the planes defined by P-Ni-P and C-Ni-C were 12.27 and 10.21 . The C-C bonds in the ethylene ligand were lengthened compared with the uncoordinated olefin. Complex [(iPr3)2Ni(2-C2H4)] (90) was conveniently prepared in 83% yield through the reaction of (iPr3P)2Ni(X)2 (X ¼ Cl, Br, I) with magnesium in the presence of ethylene. Solid-state structure for 90 was typical for Ni(0) bisphosphine adducts, featuring a C-C bond length for the p-coordinated ethylene slightly elongated when compared with the free ligand.94 Deprotonation of Ni(II)-complex 91 (Scheme 7) by the sterically hindered phosphazene base, BTPP, led to the formation of the 2-acrylate complex, 92, over 2 days at room temperature.174 Addition of DBU to complex 91, also promoted acrylate formation,

Scheme 7 Synthesis of acrylate complex 92 and ethylene complex 93.174

696

Zerovalent Nickel Organometallic Complexes

although unselectively. Treatment of complex 92 with 2 atm each of ethylene and CO2 resulted in immediate formation of complex 93 with no evidence of any activation of CO2. Independent preparation of 93 (Fig. 16) was performed by addition of ethylene to a mixture of [Ni(COD)2] and the dppf (1,10 -bis(diphenylphosphino)ferrocene) ligand.174 Solutions of 93 were moderately stable in arene solvents under inert N2 or Ar atmospheres. Exposure of a toluene solution of a mixture of [Ni(COD)2] and 2 equiv. of PCy3 to an atmosphere of tetrafluoroethylene at room temperature led to the clean formation of complex 94a (Fig. 16) in 64% isolated yield.175 When PPh3 was used instead of PCy3 a metallacycle formed by the oxidative cyclization of tetrafluoroethylene was exclusively observed. The use of bulky phosphines thus allowed for the synthesis of (
2-CF2 ¼ CF2)Ni(PR3)2 complexes, such as (
2-CF2 ¼ CF2)Ni(PCy3)2 (94a) or (
2-CF2 ¼ CF2)Ni(PiPr3)2 (94b) (Fig. 16).175 VT NMR studies of complex 94a revealed a fluxional process occurring through either rotation or a twisting motion of the coordinated tetrafluoroethylene ligand. In the solid-state, 94a featured a Y-shaped trigonal-planar coordination geometry of the metal center, in which the midpoint of the carbon-carbon double bond was assumed to be the coordination center. The C¼ C bond length was significantly longer than that of a free tetrafluoroethylene molecule. Additionally, the C ¼ C bond was twisted by 23.5 with respect to the P-Ni-P plane. Formation of
2-CF2 ¼ CF2 nickel-compounds was also observed with the use of bidentate phosphines; for instance, treatment of [Ni(COD)2] with equimolar amounts of 1,2-bis(dicyclohexylphosphino)ethane (dcpe) or 1,4-bis(dicyclohexylphosphino)butane (dcpb) under a gas atmosphere of tetrafluoroethylene produced complexes 95a and 95b (Fig. 16).175 Compounds 94a and 95a showed intermediacy in the C-F bond activation of tetrafluoroethylene. Ethylene complexes [[{iPr2PCH2)2NR}Ni(C2H4)] (R ¼ Me, iPr, 2-FuCH2) 96a-c (Fig. 16) were prepared in 81–92% yield from the corresponding Ni(II) precursors [[{iPr2PCH2)2NR}NiCl2] (R ¼ Me, iPr, 2-FuCH2) upon reaction with methyllithium at low temperatures, followed by saturation of solution and headspace with ethylene, and heating to 65  C.79 The solid-state structures for 96a and 96c exhibited trigonal-planar coordination geometries with the ethylene ligands nearly coplanar to the P-Ni-P plane. In situ formation of complex [{iPr2PCH2)2NMe}Ni(CO)2] (19) (vide supra, Fig. 3) was observed during the decarbonylative arylation of ethylene complex 96a (Fig. 16) in the presence of phenyl-2-thiophenecarboxylate, in C6D6, at 65  C.79 The iPrBenzP ligand coordinated to Ni(0) after stirring a THF solution with [Ni(COD)2] at ambient temperature to afford [(iPrBenzP)Ni(COD)] (97) (Fig. 12), isolated as a yellow solid.165 Treatment of 97 with ethylene produced complex 98 (Fig. 16), which was also obtained when exposing 97 to 2 atm of a 1:1 mixture of CO2 and ethylene.165 Complex 98 proved to be unstable toward removal of the ethylene atmosphere, reverting to 97. Additionally, 98 exhibited dynamic exchange between free and coordinated alkene in solution. Prolonged exposure of 98 to a mixture of ethylene and CO2 at 60  C afforded nickel(II)-lactone species, [(iPrBenzP)Ni(kC,kO-CH2CH2COO)] in ca. 20%. The latter nickel(II)-lactone complex converted into
2-sodium acrylate Ni(0) complex, 99 (Fig. 23), upon reaction with sodium hexamethyldisilazide (NaHMDS) in THF solution.165 The molecular structure of 99 showed a tetramer in the solid state, with each acrylate bound in an
2-fashion to the [iPrBenzP)Ni] moiety. The geometry around the nickel center was best described as square planar with moderate distortion. The C ¼ C bond distance in 99 was longer than in free acrylic acid, suggesting bond order reduction due to p-back bonding with the nickel and from deprotonation of the carboxylic acid. Compound 99 (Fig. 16) was a key intermediate in nickel catalyzed coupling between ethylene and CO2.165 Treatment of 99 with 0.25 atm of ethylene afforded dissociation of sodium acrylate and generation of ethylene-complex 98, which provided insight of catalytic turnover for this reacting system. Further incorporation of Zn as a reducing agent and 3-fluorophenoxide as a base, in the presence of 97 as catalytic precursor, allowed for the catalytic coupling of CO2 and ethylene with TON values up to 84.

8.10.4.3

Styrene and stilbene complexes

Zerovalent nickel styrene and stilbene complexes comprise a class of compounds bearing at least one p-coordinated styrene or stilbene-derivative. Coordination of styrene or stilbene to Ni(0) usually leads to partial loss of bond-order of the C-C double bond, as a consequence of p-back bonding into the C¼ C moiety. Three-coordinate Ni(0) styrene or stilbene complexes, bearing up to three olefin ligands, are usually diamagnetic and exhibit trigonal-planar coordination geometry. As detailed in this section, preparation methods include ligand substitution from [Ni(COD)2], [Ni(PEt3)4], or related Ni(0) sources, or the reaction of Ni(I) or Ni(II) precursors under reducing conditions with borohydrides, lithium or aluminum reagents, among others. Common analysis techniques for styrene and stilbene complexes include X-ray crystal diffraction, NMR spectroscopy, and in some cases, DFT calculations. [{(R,R)-MeDuphos}Ni(trans-stilbene)] (100), which was highly air sensitive in solution but stable as a solid, was obtained as orange needles in 59% yield upon treatment of a mixture of [{(R,R)-MeDuphos}NiCl2] and trans-stilbene with NaBH(OMe)3 in THF. Alternatively, the reaction of [Ni(COD)2] with MeDuphos and stilbene produced instead [{(R,R)-MeDuphos}Ni(COD)] (101), along with a small amount of [{(R,R)-MeDuphos}2Ni]. Complex 101 was independently synthesized in 72% yield by the direct reaction between [Ni(COD)2] and (R,R)-MeDuphos in hexane. In the solid state, [{(R,R)-MeDuphos}Ni(trans-stilbene)] (100) exhibited distorted trigonal-planar geometry with the nickel center bound in an
2-fashion to the C¼ C bond of stilbene and to two phosphorus atoms of the chelating phosphine. The angle between the P-Ni-P and the C-Ni-C planes was 4.6 . Compound [{(R,R)-MeDuphos}Ni(COD)] (101) featured a tetracoordinated nickel atom with a 1,5-cyclooctadiene ligand coordinated in an
2:
2-fashion. Complex 100 reacted in the presence of PhI to give the oxidative addition product, Ni(II)-complex [{(R,R)MeDuphos}Ni(Ph)(I)].171 The addition of triphenylphosphine to [Ni(COD)2] in a 2:1 ratio led to the synthesis of [Ni(PPh3)2(COD)] (102) with high yield. The excess of PPh3 or the presence of styrene or ethylene caused the substitution of the second molecule of 1,5-cyclooctadiene

Zerovalent Nickel Organometallic Complexes

697

yielding [Ni(PPh3)2(
2-CH2 ¼ CHC6H5) (103) and [Ni(PPh3)2(C2H4)] (104), respectively. Styrene-compound 103 was also synthesized upon addition of styrene to ethylene-compound 104, and in the direct reaction from [Ni(COD)2] and PPh3. The replacement of PPh3 by PCy3 allowed for the synthesis of [Ni(PCy3)2(
2-CH2 ¼ CHC6H5)] (105), in an analogous reaction.118 Compounds 102-104 catalyzed the reaction between styrene and triethoxysilane favoring dehydrogenative silylation over hydrosilylation processes. Complexes 102–105 also catalyzed the reaction of vinyltris(trimethylsiloxy)silane with heptamethyltrisiloxane, which was a model for cross-linking of polysiloxane via hydrosilylation. The reaction of an equimolar amount of [Ni(PEt3)4] with (4-NC5H4)CH ¼ CH(4-XC6H4) (X ¼ Br, I) in dry toluene-d8 at 271 K for 80 min yielded quantitative formation of complexes 106a–b (Fig. 17).176 These species were not isolated, but characterized in situ in the presence of 2 equiv. of PEt3 by low temperature NMR spectroscopy. Prolonged reaction times of about 14 h or increasing the reaction temperature led to C(sp2)-X activation of the ligands (4-NC5H4)CH-CH(4-XC6H4), producing the Ni(II) complexes [(PMe3)2Ni{(4-NC5H4)CH ¼ CH(C4H4)}(X)] (X ¼ Br, I) The trans-stilbene complex [{(iPr2PCH2)2NCH2Ph}Ni(trans-PhCH ¼ CHPh)] (107) (Fig. 17) was synthesized by treatment of [{(iPr2PCH2)2NCH2Ph}NiCl2] with MeLi at −78  C and subsequent heating to 65  C in the presence of trans-stilbene.79 Complex 107 was isolated in 78% yield. The molecular structure of 107 showed a trigonal-planar geometry with the olefinic carbon atoms being nearly coplanar with the P-Ni-P plane. The bite angle of the chelating bisphosphine was significantly greater than those exhibited by the starting Ni(II) complex, [{(iPr2PCH2)2NCH2Ph}NiCl2]. Ni(0) intermediates 108a–b and 109a–b (Fig. 18) were calculated by DFT during computational studies on the nickel(0)-mediated coupling of CO2 and benzylidene cyclopropane using DBU and MTBD as ligands.178 The coordination of the C¼ C bond of benzylidene cyclopropane to [Ni(MTBD)2] afforded the p-complex [(MTBD)2Ni(
2-C2H4C¼ CHPh)] (108a) (Fig. 18). In 108a, coordinated benzylidene cyclopropane was not planar and the C ¼ C bond was elongated compared to the free ligand. 108a featured a 16-electron nickel center in a distorted trigonal-planar structure. Barrierless dissociation of one MTBD ligand from 108a was predicted to form [(MTBD)Ni(
2-C2H4C¼ CHPh)] (108b) (Fig. 18), which was the key intermediate prior to reaction with CO2 yielding five- and six-membered nickela(II)carboxylates. Similarly, complex 109a (Fig. 18) was formed from

Fig. 17 Nickel(0) complexes with styrene and stilbene-derivatives.79,149,159,176,177

Fig. 18 Calculated benzylidene cyclopropane nickel(0) complexes bearing amine ligands.178

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Zerovalent Nickel Organometallic Complexes

[DBU2Ni] and easily formed [(DBU)Ni(
2-C2H4C¼ CHPh)] (109b) after ligand dissociation. In this case, both 109a–b were plausible intermediates in nickela(II)carboxylate formation through further activation of CO2. A 1:1 mixture of [{Ti(
5-C5Me5)(m-NH)}3(m3-N)] and [Ni(COD)2] heated in toluene at 80  C and further treated with 1 equi. of diphenylacetylene or trans-stilbene, yielded compounds 110 and 111 (Fig. 17), as air sensitive red-brown solids in 43–80% yield.149 IR spectra for 110 and 111 revealed bands for NH groups, and NMR analysis featured signals for both equivalent NH and
5-C5Me5 ligands. Additionally, according to 13C NMR and IR analyses, complexes 110 and 111 exhibited a low degree of p-back donation to the alkyne or the alkene ligands. The dinickel(I) compound [(NiIL)2] (L ¼ [2,6-iPr2C6H3)NC(Me)]2) readily reacted with ethylene to afford the complex [LNi(
2-CH2 ¼ CH2)] (112a) (Fig. 17).177 The nickel atom in complex 112a showed a formal oxidation state of 0. Since 112a was obtained from the Ni(I) species [(NiIL)2], electron transfer from the monoanionic ligand L to Ni occurred during the reaction, resulting in a zerovalent nickel which facilitated coordination of ethylene. The ethylene ligand was almost coplanar with the NiN2C2 five-membered ring of the coordinated ligand L. NMR studies revealed considerable metallacyclopropane character of complex 112a. In a similar way, dinickel(I) precursor, [(NiIL)2] (L ¼ [2,6-iPr2C6H3)NC(Me)]2), reacted with styrene, allylbenzene, stilbene, and 1,4-diphenyl-1,3-butadiene, respectively, to form the alkene complexes 112b–e (Fig. 17).177 These products were thermally stable, with melting points up to 182  C, but air- and moisture sensitive. Solid-state structures for complexes 112b–e featured three-coordinated Ni centers with the bidentate L ligand and the olefinic C ¼ C bond in the coordination sphere. In the case of 112b, the phenyl substituent in the olefin caused increased p-back bonding compared with 112a, whereas complex 112c, bearing a phenyl and a methyl substituents, exhibited moderate p-acceptance. Complex 112d featured the highest p-accepting character of the series, whereas complex 112e featured only the coordination of one of the two double bonds of the diene moiety with moderate p-back bonding. Evidence of p-back bonding was provided by the C ¼ C bond lengths and NMR data in all cases. DFT calculations for compounds 112a–e ruled out the possibility of a nickel(I) bound to a radical monoanionic supporting ligand L, as in precursor [(NiIL)2] (L ¼ [2,6-iPr2C6H3)NC(Me)]2), therefore these complexes were better described as formed by Ni(0) and a neutral ligand L.177 The synthesis [Ni(stilbene)3] (113a) (Fig. 17),159 was performed by the reaction of the highly reactive Ni(0) precursor, [trans, trans,trans-(CDT)Ni] (CDT ¼ cyclododeca-1,5,9-triene) with slight excess of stilbene (stb) in THF at −5  C. Deep red 113a was obtained in 46% yield. Similarly, [(CDT)Ni] reacted with slight excess of trans-1,2-bis(4-trifluoromethyl)phenyl)ethene (Fstb) producing red [Ni(Fstb)3] (113b) (Fig. 17) in 70% yield.159 Compound 113a was air-stable but temperature sensitive, showing slow dissociation of stilbene. Complex 113b, featuring electron-withdrawing groups, exhibited air-stability for months when stored in the freezer, and was conveniently manipulated at ambient conditions, for several days, before any decomposition was noted. In the solid state, complexes 113a–b revealed the coordination of three olefins around the nickel center in a propeller-like arrangement rendering a slightly distorted trigonal planar geometry. In accordance to the Dewar-Chatt-Duncanson model, olefinic bonds revealed that partial electron density of the Ni(0) was delocalized by back-donation to the empty p -orbitals. The phenyl rings on the trans-stilbene units were arranged in a staggered situation. Space-filled representation of 113a indicated that phenyl rings protected the axis perpendicular to the coordination plane of the olefins, likely preventing oxidation of the nickel-center. In an alternative and more convenient synthesis, complex 113b (Fig. 17) was obtained at the gram scale from the Ni(II) source, [Ni(acac)2], and the reducing agent, AlEt3, in the presence of trans-1,2-bis(4-trifluoromethyl)phenyl)ethene in one single run.159 Complex 113b stranded in contrast to almost all known Ni(0)-olefin catalyst precursors, which suffer from instability and decomposition when exposed to air. Such is the case of [Ni(COD)2], which has vastly dominated research in nickel catalysis, thus becoming the main source of Ni(0) for exploring new catalytic reactivity. In this context, envisioned as a breakthrough in the area was the synthesis of the general and air-stable 16-electron Ni(0)-catalytic precursor [Ni(Fstb)3] (113b) (Fig. 17), which is highly modular, thus allowing ligand exchange with a variety of commonly used ligands in catalysis such as diamines, phosphines or N-heterocyclic carbenes providing access to well-defined Ni(0) species.

8.10.4.4

Polyene and polyenyne complexes

Addition of a cyclohexane solution of (5Z,11E)-dibenzo[a,e]cyclooctatetraene to a cyclohexane solution of [Ni(COD)(PtBu3)], prepared from [Ni(COD)2] and PtBu3, yielded pale yellow crystals of compound 114 (Fig. 19). 179,180 The structure of 114 featured exclusive coordination of trans C¼ C bonds, chirality of the complex exhibiting a nickel-centered three-bladed propeller, and steric protection of the Ni center by the bulky alkene ligands leading to air-stability. Coordination of the C ¼ C bonds led to lengthening, which relieved strain in the ligand, (5Z,11E)-dibenzo[a,e]cyclooctatetraene. According to NMR, complex 114 was better described as a metallacyclopropane. In the solid state, the unit cell for complex 114 showed that the crystalline material existed as a racemate. The reaction of [bipy)Ni(COD)] (bipy ¼ 2,20 -bipyiridine) with 1 equiv. of (5Z,11E)-dibenzo[a,e]cyclooctatetraene produced complex 115 (Fig. 19).140 Both complexes 114 and 115 produced cyclobutene derivatives upon heating. [Ni(COD)2] reacted in diethyl ether with 1 equiv. of 1,4-bis(allylmethylamino)-trans-2-butene or 1,4-bis(diallylamino)-trans2-butene at ambient temperature to yield air-sensitive complexes 116a-b (Fig. 19).121 Theses complexes dissolved readily in diethyl ether, pentane, and benzene; they showed melting ranges between 50 and 90  C (116a) and 30 and 60  C (116b), and both decomposed above 160  C. Compounds 116a–b also sublimed under vacuum above 50 and 75  C, respectively. In the IR spectrum for 116a, bands for the free ligand at 1675 and 1643 cm−1 shifted to 1520–1465 cm−1, which were attributed to the coordinated C¼ C bonds. NMR spectra for 116a–b were in agreement with C2 symmetry, with the C2 axis passing through the metal atom and the midpoint of the trans-disubstituted C¼ C bond. In the solid state, compound 116b showed that the nickel atoms were

Zerovalent Nickel Organometallic Complexes

699

Fig. 19 Nickel(0) complexes with triene and tetraene ligands.121,179–182

coordinated by C¼ C bonds of 1,4-bis(diallylamino)-trans-2-butene, leaving two N-allyl groups uncoordinated. This arrangement formed two Ni-1,6-diene chelate rings, which assumed chair-like geometry ensuring little conformational strain. The geometry around the Ni(0) center was nearly trigonal-planar. The three coordinated C ¼ C bonds were of about the same length and longer than the two uncoordinated C¼ C bonds. Replacement of the COD ligands by the reaction of equimolar amounts of [Ni(COD)2] and (E,E,E)-1,6,11-tris(4-tolylsulfonyl)1,6,11-triazacyclopentadeca-3,8,13-triene in THF solution at room temperature, produced complex 117 (Fig. 19) in 78% yield.181 Complex 3 was soluble in THF, insoluble in diethyl ether and pentane, and decomposed in dichloromethane. In a similar reaction, [Ni(COD)2] reacted with 1 equiv. of (E,E)-1,6,11-tris(4,tolylsulfonyl)-1,6,11-triazacyclopentadeca-3,8-diene-13-yne forming a mixture of 118 and 119 (Fig. 20), which precipitated at 20  C.181 Formation of 119 was avoided by using an excess of the dienyne ligand. Complex 119 was also obtained in 91% yield from the reaction between 118 and [Ni(COD)2].181 In the solid state, complexes 117–119 (Figs. 19 and 20) exhibited Ni atoms with trigonal-planar geometry, coordinated by the C-C triple bond and two trans-substituted C-C double bonds, with both carbon atoms of the C-C triple bond and the midpoints of the C ¼ C bond lying on the same plane. In the case of 119 (Fig. 20), the additional Ni atom displayed trigonal-planar coordination geometry with the two C¼ C bonds of the COD ligand lying perpendicularly to the coordination plane. The angle between the coordination planes of

Fig. 20 Mono- and dinuclear nickel(0) complexes with cyclic dienyne ligands.181

700

Zerovalent Nickel Organometallic Complexes

Fig. 21 Mono- and dinuclear nickel(0) complexes with acyclic dienyne ligands.142,182

the two metals in 119 was 92 . Complexes 117-119 were thermally stable and did not react with butadiene or neat 1,3,5,7-cyclooxtatetraene. In the presence of CO, all complexes formed [Ni(CO)4]. Additionally, 117-119 showed catalytic activity at the 5 mol% loading for Suzuki-Miyaura cross-coupling of aryl/heteroaryl boronic acids and aryl bromides with yields ranging 47–76%.181 THF solutions of [Ni(COD)2] reacted with an equimolar amounts of 4,9-bis(tosyl)-4,9-diazadodeca-1,11-dien-6-yne or 4,9-bis(tosyl)-4,9-diazatrideca-1,(E)-6-dien-11-yne, forming complexes 120 and 122 (Fig. 21) by complete COD substitution.142 When 2 equiv. of [Ni(COD)2] reacted with 4,9-bis(tosyl)-4,9-diazadodeca-1,11-dien-6-yne or 4,9-bis(tosyl)-4,9-diazatrideca-1,(E)6-dien-11-yne, dinuclear complexes 121 and 123 (Fig. 21) were obtained in 60% and 50% yield, respectively, through partial COD substitution.142 Formation of 121 and 123 occurred through intermediacy of 120 and 122, respectively. NMR analysis of 120 in THF-d8 showed the presence of two isomers, exhibiting C2 and Cs symmetry undergoing slow exchange, which required dissociation of one Ni-alkene bond and coordination of the same alkene by its reverse side. In the solid state, 120 and 122 showed the nickel atom in a trigonal-planar coordination, with the metal, the two terminal C ¼ C bonds and the midpoint of the C-C triple bond lying in the coordination plane. Ligand exchange between complexes 120 and 122 in the presence of ligands 4,9-bis(tosyl)-4,9-diazadodeca-1,11-dien-6-yne and 4,9-bis(tosyl)-4,9-diazatrideca-1,(E)-6-dien-11-yne, revealed the presence of an equilibrium favoring complex 120, thus indicating that the nickel atom was able to migrate from one acyclic ligand to the other. Treatment of several dienynes with [Ni(COD)2] and PPh3 provided isolable complexes 124a–c (Fig. 21),182 which were further converted to 1,4-cyclohexadiene derivatives through intramolecular [4 + 2] cycloaddition. Compound 124a reacted in the presence of cyclopropylideneacetate to afford complex 125 (Fig. 19), featuring a nine-membered ring, in 25% yield.182 In the solid state, compounds 124c and 125 exhibited nickel centers with trigonal geometry.

8.10.4.5

Vinyl complexes

The complexes [{(R2SiCH¼ CH2)2O}3Ni2] [R ¼ Me (126a), Ph (126b)] (Fig. 22) were prepared from [Ni(COD)2] and either divinyl tetramethyl- or divinyl tetraphenyldisilioxane at room temperature.147 NMR of 126a featured signals for vinylic protons as a multiplet at dH 2.94–3.9, and carbon resonances at dC 65.5 and 66.6. Similarly, divinyl tetramethyl cyclotetrasiloxane reacted with Ni(COD)2 and trisubstituted phosphines to form complexes 127a–c (Fig. 22).183 Compounds 127a–c, also featured d values for the vinylic protons and carbons high-field shifted. Additionally, all resonances for vinylic protons in complexes 127a–c exhibited fine structure patterns due to scalar coupling between hydrogen and phosphorous nuclei. The reaction of [Ni(COD)2] with 1 equiv. of NQA (NQA ¼ MeNC5H4 ¼ NiPr) and 2 equiv. of H2C¼ CHSnPh3 produced compound [(NQA)Ni(
2-H2C¼ CHSnPh3)2 (128) (Fig. 22) as a yellow precipitate in 69% isolated yield.143 Molecular structure of 128 featured a C1 symmetric complex with
2-H2C¼ CHSnPh3 groups and a NQA s-bound to the nickel center. The two SnPh3 substituents pointed away from the NQA ligand, on opposite sides of the Ni coordination plane. The corresponding IR spectrum displayed a C¼ N stretching frequency comparable to that of free NQA, which supported the N-bound nature of the NQA donor in solution. In addition, rotation around the Ni-N bond did not occur in the NMR time scale. At the 5 mol% loading, complex 128 catalyzed the reaction of H2C¼ CHSnPh3 with C6F5H in a sealed tube, giving the C-H bond alkylation product, C6F5CH2CH2SnPh3 in 95% yield.143

Zerovalent Nickel Organometallic Complexes

701

Fig. 22 Nickel(0) complexes with vinyl siloxane and vinylstannane ligands.143,157,183–185

Preparation of vinyl-siloxane Ni(0)-complexes 129-132 (Fig. 22) in 24–77% isolated yields, was performed by the reaction of either THF solutions of PCy3, PPh3, PiPr3, or PiPr2tBu with [NiCl2(DME)] (DME ¼ dimethoxyethane) in the presence of both the ligand 1,3-divinyltetramethyldisiloxane and the organic reducing agent 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (TMP) at 20  C.157,185 Likewise, ethylene-complex [(PCy3)Ni(
2-H2C¼ CH2)2] (133) was obtained in quantitative yield from the reaction of a THF solution of PCy3 with [NiCl2(DME)] and the reducing agent TMP under 1 atm of ethylene at 20  C. The structure of complexes 132 and 133 showed three-coordinate nickel atoms with slightly distorted trigonal-planar geometry. The C¼ C bond lengths of the olefin ligands attested back donation from the nickel center into the corresponding p -orbitals. Following an analogous methodology, the highly relevant Ni(0) precursor, [Ni(COD)2] was synthesized in 54% yield after reaction between a THF solution of 1,5-cyclooctadiene (COD) with [NiCl2(DME)] in the presence of 60 equiv. of TMP at −20  C.157,185 [Ni(COD)2] had been mostly synthesized by reduction of Ni(acac)2 with alkylaluminium compounds, such as AlEt3 or DIBAL-H, in the presence of COD and butadiene at low temperature.

8.10.4.6

Alkene complexes with miscellaneous p-coordinating groups

Alkene ligands bearing additional p-coordinating groups introduced in this section, mostly include non-conjugated alkene-substituted aldehydes and nitriles, a,b-unsaturated ketones, and acrylates. Coordination of Ni(0) to these ligands occurred preferentially at the C¼ C bond and, in the case of non-conjugated alkene-substituted aldehydes or nitriles, the extra coordinating functionality acted as a chelating source. Zerovalent nickel species covered in this section were mainly synthesized by ligand substitution from [Ni(COD)2]; reductive elimination of H2 from the Ni(I) species, [(dippe)Ni(H)]2 (dippe ¼ 1,2-bis(diisopropylphosphino)ethane), upon treatment with the corresponding alkene-containing ligands; or reduction of Ni(II) precursors with borohydride species. Characterization primarily included NMR analyses and molecular structure determinations. Also included in this section are DFT calculations of some Ni(0) intermediates bearing alkene ligands with additional functionalities. The reaction of 2-allylacetophenone or 2-allylbenzophenone with [Ni(COD)2] and PCy3 gave
2:
2-1,5-enonickel complexes 133a–b (Scheme 8).186 NMR spectra indicated that both C¼ C and C ¼ O bonds coordinated to Ni(0) in an
2-fashion; for instance,

Scheme 8 AlMe3 promoted oxidative cyclization of alkene and ketone.186

702

Zerovalent Nickel Organometallic Complexes

Fig. 23 Nickel(0) complexes with a,b-unsaturated carbonyl and carboxylate ligands.122,128,133,165

for 133a, resonances for the three vinylic protons showed at dH 2.31, 2.55, and 3.36, whereas the 13C NMR spectrum featured doublets at 49.5 and 72.9 ppm with JCP values of 3.1 and 9.1 Hz, respectively, for the vinylic carbons and at 112.1 ppm with JCP ¼ 9.2 Hz for the carbonyl carbon. The molecular structure of 133b showed that the Ni-center, the C atoms in the C ¼ C double bond, and the C and O atoms in the C ¼ O bond, were on the same plane. Compounds 133a–b were relevant intermediates in the oxidative cyclization of alkene and ketone, thus the reaction of 133a–b with AlMe3 in C6D6 proceeded to give cyclized compounds 134a–b quantitatively.186 Molecular structure for 134b showed a nickela(II)cycle structure having a bridging methyl group, which exchanged with the other two methyl groups at room temperature according to NMR analysis. The acrylic acid complexes 135a–b (Fig. 23) were obtained by reaction of a mixture of Ni(COD)2, dppf or dippf (dppf ¼ 1,10 bis(diphenylphosphino)ferrocene; dippf ¼ 1,10 -bis(diisopropylphosphino)ferrocene) and acrylic acid in a solution of THF at 0  C.122 NMR spectroscopy indicated olefin-like coordination of the acrylic acid according to the corresponding signals for the olefinic carbons and protons. Both 135a–b showed essentially the same solid-state structure with
2-coordination of the acrylic acid ligand, and hydrogen bonds between two acrylic acid units forming a dimer. The geometry around the metal center was planar, surrounded by two phosphorus atoms and the C-C double bond of the acrylic acid. The corresponding C-C bond was slightly enlarged compared with the uncoordinated state of the acrylic acid. Complex 135a was thermally stable in THF and in the solid state, indicating that a possible isomerization into a nickela(II)cyclic carboxylate was energetically unfavorable. The reaction of cyclopropanecarboxaldehyde, cyclopropyl methyl ketone, or cyclopropyl phenyl ketone with [Ni(COD)2] and PBu3 at 100  C in toluene-d8 quantitatively produced the
2-enonenickel complexes 136a–c (Fig. 23).128 Formation of these compounds indicated ring-opening of the cyclopropyl substituent attached to the carbonyl group. Dissociation of the coordinated enones, along with the formation of [Ni(CO)2(PBu3)2] was observed after treatment of compounds 136a-c with carbon monoxide. In the presence of PCy3, also the ring-opening reaction occurred but produced dimeric complexes 138a-c in good to excellent yield.128 The reaction between 2 equiv. of cyclopropyl phenyl ketone and 1 equiv. of [Ni(COD)2] in the presence of PCy3 in C6D6 at 40  C showed rapid formation of
2-ketone nickel complex 137 in 32% yield (Fig. 23),128 which further reacted through cyclopropyl ring-opening forming a (
1,k1)-Ni(II) metallacycle. In the presence of 2 equiv. of PCy3, the reaction of ethyl cyclopropylideneacetate (ECPA) with [Ni(COD)2] in toluene-d8 at −15  C led to the quantitative formation of the
2-ECPA complex 139 (Fig. 23).133 Monitoring of the reaction at room temperature by NMR indicated gradual decomposition of 139 to yield a 1:1 mixture of [Ni(COD)2] and Ni(0) complex 140 (Fig. 23).133 X-ray crystallography revealed that an (E,E)-1,2-bis(exo-alkylidene)cyclohexane unit, which formed after [3 +3] cyclodimerization of ECPA, coordinated to the nickel atom in a
2:
2 fashion, with tetrahedral coordination geometry for the four-coordinated Ni(0) compound. A PPh3-containing analog of complex 139, [(
2-ECPA)Ni(PPh3)2] (139a) (Fig. 23), was quantitatively obtained after treatment of [Ni(COD)2] with PPh3 in the presence of ECPA.133 In the solid state, species 139a displayed a three-coordinated Ni(0) structure. In the presence of PCy3, the reaction of 2-(2-methylallyl)benzonitrile or 2-allylbenzonitrile with [Ni(COD)2] led to quantitative formation of the
2:
2-5-ene-nitrile Ni(0) complexes 141a–b (Scheme 9).187 Molecular structures of these species showed a Y-shaped trigonal-planar coordination geometry of the nickel center, in which the midpoints of the C-C double and C-N triple bonds acted as coordination centers. The C-C and C-N bond distances in complexes 141a–b were slightly longer than those observed in uncoordinated PhCN and ethylene, due to the contribution of back-donation from the Ni(0) center. Complexes 141a–b underwent oxidative cyclization in the presence of Me2AlCl or TfOH to give nickela(II)dihydropyrroles 142a–f.

Zerovalent Nickel Organometallic Complexes

703

Scheme 9 Lewis acid-assisted oxidative cyclization of alkene and nitrile.187

Fig. 24 Calculated nickel(0) complexes with ethyl acrylate ligands.188

Optimized geometries for olefin intermediates 143, 144, and 145, relevant to catalytic [2 + 2+2] cycloaddition of acrylic acid esters to norbornadiene in the presence of Ni(0) complexes, were calculated by means of DFT (Fig. 24).188 Complex 143 was predicted to be the catalytically active species. Intermediate 144 was formed by coordination of an ethyl acrylate unit with 143 and showed a DG323 value 6.7 kcal mol−1 higher than active species 143. Intermediate 145, bearing an acrylate ligand and a C ¼ O coordinated equivalent of the cycloaddition product, showed a DG323 value 1.7 kcal mol−1 lower than starting complex 143. The reaction of the hydride dimer [(dippe)Ni(H)]2 (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) with symmetrical dienones in a 1:1 molar ratio, in THF-d8, produced dinuclear complexes 146a–e (Fig. 25).156,157 NMR analysis of 146a–e indicated
2-Ca ¼ Cb coordination of enones to Ni(0). Slow and little decomposition of complexes 146a–e was observed upon thermolysis in solution by heating from 25 to 120  C for 7–10 days. Molecular structure of complexes 146b and 146d showed that each nickel center was coordinated to two P atoms of the dippe ligand, and to one of the two
2-Ca¼ Cb entities in the dienone, which was elongated compared to the one in the free ligand. Each Ni-center showed a slightly distorted trigonal-planar geometry, and the two Ni(dippe) units exhibited a mutual trans arrangement relative to the plane of the dienone ligand. Reaction of [(dippe)Ni(H)]2 with dienones in a 1:2 molar ratio, at 80–130  C in THF-d8 during 15–24 h, yielded mononuclear Ni(0) complexes 147a–e (Fig. 25).

Fig. 25 Mono- and dinuclear nickel(0) complexes with dienone ligands.189,190

704

Zerovalent Nickel Organometallic Complexes

NMR analysis indicated
2-Ca ¼ Cb coordination in these complexes. Solid-state structures of 147a and 147d consisted of the corresponding
2-dibenzylidene acetone coordinated through an Ca¼ Cb bond, which showed lengthening compared to the free ligand. The Ni-centers exhibited distorted trigonal-planar geometry. Complexes 147a–c showed catalytic activity toward the transfer hydrogenation of dienones using methanol as the hydrogen source,189 while complexes 147d–e were suitable catalytic precursors for the tandem transfer hydrogenation and condensation of fluorinated a,b-unsaturated ketones with primary amines as the hydrogen source.190 The synthesis of olefin-complexes relevant to the carboxylation of alkenes was performed with the use of 1-hexene, styrene, butadiene, methyl-2,4-pentadienoate, 1,3-pentadiene, and cis-3-hexene, in the presence of mixtures of [Ni(COD)2] and the ligands dtbpe (1,2-bis(di-tert-butylphosphino)ethane) (148a–f) and dcpe (1,2-bis(dicyclohexylphosphino)ethane) (149a–f) (Fig. 26).191 Complexes 149a and 149e were obtained at elevated temperature with low yields, whereas high conversions were obtained for 149b–c. Complexes of 3-hexene, 148f and 149f, were synthesized through the reaction between [(L)NiCl2] (L ¼ dtbpe or dcpe), excess of cis-3-hexene, and NaBHEt3 or N-selectride for dtbpe and dcpe, respectively.191 VT NMR analysis of [(dcpe)Ni (1,3-butadiene)] (149c) revealed fluxional behavior, where likely a
2-trans butadiene molecule adopted a single cis-configuration via an
4-bound butadiene. Reactions between Ni(II)-bisphenoxide complex, [(dcpe)Ni(OC6H4F)2], with excess styrene in the presence of Zn, in THF at 100  C, yielded the Ni(0)-complex 149b in 65% yield.191 Complexes 150a–b were slowly formed along with the five-membered nickela(II)lactone-complexes [(dcpe)Ni{(C4H9)C-CC(¼O)O}] and [(dcpe)Ni{(Ph)C-CC(¼O)O}], by treatment of [Ni(COD)2] with dcpe and 2-heptenoic or cinnamic acid in THF-d8. Structures 151a–b, 152a–b, and 153 (Fig. 27) were calculated by means of DFT during the study of the mechanism of Lewis acid (LA) assisted cyano esterification of alkynes with cyanoformates using a mixture of [Ni(COD)2] and BPh3(LA) in the presence of benchmark ethyl cyanoformate and but-2-yne as the substrates, and PMe3 as the ancillary ligand.192 Calculations indicated that species 151a was more stable than 151b, which did not include the interaction with the Lewis acid. Both 151a–b featured side-on coordinated C-N triple bonds to Ni(0). Additionally, Ni(0)-intermediates 152a–b and 153 displayed
2-coordinated C-C double and triple bonds, and were relevant for the preferred reaction pathway in the presence of the Lewis acid, whose role was to enhance reactivity with high regioselectivity.

Fig. 26 Nickel(0)-olefin complexes with miscellaneous p-coordinating ligands.162,191

Fig. 27 Calculated nickel(0) intermediates with miscellaneous p-coordinating ligands.192

Zerovalent Nickel Organometallic Complexes

705

Fig. 28 Calculated nickel(0) intermediates with allylcarboxylate ligands.193

Structures 154a–b (Fig. 28) were calculated during quantum chemical investigation of the oxidative addition reaction of allyl carboxylates to Ni(0) complexes.193 The reaction occurred by activation of a C-O bond and the formation of a Ni-O bond, either through a three-center or a five-center transition state, involving either the participation of one or both oxygen atoms of the carboxylate moiety. Complexes 154a–b (Fig. 28) were computed to have formed by ligand substitution from [Ni(OiPr3)3] in the presence of allyl acetate. Complex 154b was higher in energy than 154a, but 154b reacted through a five-center transition state with a lower activation energy than the three-center transition state originating from 154a, indicating that additional interaction with an extra O-atom in the carboxylate ligand was a key factor during the bond activation.

8.10.5

Nickel(0) complexes with alkyne ligands

8.10.5.1

Complexes with alkyne ligands

Zerovalent nickel alkyne complexes include compounds bearing at least one p-coordinated C-C triple bond of an alkyne ligand. Coordination of alkynes to Ni(0) usually leads to deviation from linearity and partial loss of bond-order of the C-C triple bond, as a consequence of p-back bonding into the p-antibonding orbitals of the C  C unit. More drastic decreasing of bond-order is observed for dinuclear complexes where alkynes act as bridging ligands. Three-coordinate Ni(0) alkyne complexes are usually diamagnetic and exhibit trigonal-planar coordination geometry. As described in this section, preparation methods include ligand substitution from [Ni(COD)2], [L2Ni(
2-C2H4)], or related Ni(0) sources, or reductive elimination of H2 from the Ni(I) complex, [(dippe)Ni(H)]2, in the presence of alkynes. Common analysis techniques for alkyne complexes include X-ray crystal diffraction and NMR spectroscopy. The unsubstituted 1-zirconacyclopent-3-yne Cp2Zr(
4-H2C4H2) reacted with equimolar amounts of the nickel(0) complexes [L2Ni(
2-C2H4)] (L ¼ PPh3 or PCy3) in THF at room temperature to give binuclear complexes [Cp2Zr{m(
4-H2C4H2)}NiL2] (115a, L ¼ PPh3; 155b, PCy3) (Fig. 29).194 Complexes 155a–b were obtained as golden-yellow crystals, stable at room temperature.

Fig. 29 Mononuclear nickel(0) complexes with alkyne ligands and dinuclear Ni(0) complexes with alkyne-bridging ligands.77,120,194–197

706

Zerovalent Nickel Organometallic Complexes

External complexation of the triple bond of ansa-dimethylsilanediyl-dicyclopentadienyl-1-zirconacyclopent-3-yne, Me2Si (
5C5H4)2Zr(
4-H2C4H2), to [(PPh3)2Ni(
2-C2H4)], yielded the complex [Me2Si(
5C5H4)2Zr[m(
4-H2C4H2)]Ni(PPh3)2] (156) (Fig. 29).195 The IR spectrum of 156 showed n(C  C) at 1645 cm−1, which was shifted by 368 cm−1 compared to the uncoordinated C C bond of 7. The 1H NMR room temperature spectrum displayed equivalent
5-C5H4 and a-CH2 protons with C2v symmetry. The 13C NMR signals of the coordinated triple bond appeared at 116.3 ppm, downfield from the respective signals in free 7 (101.2 ppm). In the solid state, complex 156 showed a bent zirconocene together with an additional cis-bridging but-2-yne-1,4-diyl ligand, whose C  C bond was coordinated to nickel in a slightly distorted trigonal planar arrangement. The longer bond distance for the coordinated triple bond in 156 compared to the 1-zirconacyclopent-3-yne, indicated coordination with the Ni(0) moiety. The reaction between [Ni(COD)2] and [C5H4CH¼ NC(H)(CH3)(Ph)]Fe[
-C5H4PCy2] under argon and excess diphenylacetylene yielded the Ni(0) alkyne complex [C5H4CH¼ NC(H)(CH3)(Ph)]Fe[
-C5H4PCy2]Ni(
2-Ph-C  C-Ph) (157) (Fig. 29).77 Compound 157 was extremely air- and moisture-sensitive and decomposed easily in solution in the presence of trace oxygen or water. X-ray crystal structure of 157 featured a combination of a chelating iminophosphine with a side-on alkyne at nickel. The two sterically demanding ligands were bound at a distorted planar Ni(0) center. The alkyne ligand showed a weaker C  C bond and bent C C-C angles compared to free diphenylacetylene, which was in agreement with p-backdonation. Compound [(DABMes)Ni(COD)] (DABMes ¼ {RN ¼ C(Me)-C(Me)¼NR}; R ¼ Mes) (63) (vide supra, Fig. 12) reacted with diphenylacetylene to form [Ni(DABMes)(Ph-C  C-Ph)] (158) (Fig. 29).120 Compound 158 was also obtained from the reaction between equimolar amounts of [Ni(COD)2], DABMes and diphenylacetylene. In the UV/vis spectrum of 158, an absorption at 666 nm was observed. According to DFT calculations on the electronic structure of 158, this band was assigned to a mixture of metal-to-ligand and ligand-to-ligand charge transfer. In the solid state, the nickel atom of 158 was essentially planar, coordinated with both the DABMes ligand nitrogen atoms and the carbon atoms of the alkyne entity. The C-C bond length in 158 indicated the presence of a C¼ C rather than a C C bond. IR analysis of complex 158 displayed a sharp absorption shifted to lower frequencies than the C C stretching vibration of unsymmetrically substituted acetylenes. Therefore, a nickelacyclopropene structure seemed to be a more likely resonance structure contributor to the molecular structure of 158. DFT calculations on the formation of five-membered metalacyclic intermediates during the Ni(0)-mediated coupling reactions of both terminal and internal alkynes with CO2 with the use of DBU (DBU ¼ 1,8-Diazabicyclo[5.4.0]undec-7-ene) as ancillary ligand, featured intermediacy of the Ni(0)-complex, [Ni(DBU)2(
2-HC  CMe)] (159a), which formed the side-on CO2-Ni(0) adduct [Ni(DBU)(
2-HC  CX)(
2-CO2)] (160a) prior to oxidative coupling yielding five-membered cyclic Ni(II) complex, [Ni(DBU)2{OC(¼O)C(Me)¼CH}] (160b). Further study with other acetylene-derivatives also showed intermediacy of the corresponding Ni(0)-complexes bearing
2-coordinated alkynes, namely, [Ni(DBU)2(
2-HC  CX)] [X ¼ CN (159b), OMe (159c), TMS (159d)], in the nickela(II)cycle formation.198 Reaction of the Ni(II) complex, [(PiPr3)2Ni(H)(C6F4H), with 3-hexyne in toluene, generated Ni(0)-complex [Ni(PiPr3)2Ni (
2-EtC  CEt)] (161) in >95% yield. Compound 161 was also synthesized by the reaction of [(PiPr3)2Ni(
2-anthracene)] with 3-hexyne in pentane, or by treatment of [Ni(COD)2] with 2 equiv. of PiPr3 and 3-hexyne in pentane. In the solid state, [Ni(PiPr3)2Ni(
2-EtC  CEt)] (161) displayed the Ni center residing in a C2 axis with pseudo planar geometry and the alkyne ligand
2-coordinated with a longer C-C bond distance than the one in the free alkyne, indicating back bonding from the Ni(PiPr3)2 moiety. Complex [Ni(PiPr3)2Ni(
2-EtC CEt)] (161) reacted with 1,2,4,5-tetrafluorobenzene at 50  C to yield the product of both C-H activation and alkyne insertion into the Ni-H bond, namely the Ni(II) complex, [(PiPr3)2Ni(C6F4H){(Et)C¼ CH(Et)].130 Reaction of Ni(I) hydride dimer, [(dippe)Ni(H)]2 (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) with 2 equiv. of partially fluorinated diphenylacetylene derivatives yielded complexes 169a–c (Fig. 29).197 When only 1 equiv. of alkyne was used, the reaction produced dinuclear alkyne-bridged complexes 170a–c (Fig. 29).197 Complexes 170a–c were also observed as intermediates during the synthesis of corresponding mononuclear compounds 169a–c. In the solid state, complex 169a exhibited a distorted trigonal-planar geometry featuring
2-coordination of the C-C triple bond. 13C{1H} NMR comparative analysis of complexes 169a–c and 170a–c revealed significant shielding for the alkyne-bridged complexes. Molecular structures of compounds 170a–c featured doubly-coordinated alkyne bridges in the meridional plane with pseudo C2 symmetry. The
2-bound C-C bond lengths were longer in complex 170a than in complex 169a, as a result of coordination of the second [(dippe)Ni0] unit, indicative of bond orders closer to a single-bond around the coordinated C-C bond in the bridging structure. In the presence of water, complexes 169a–c performed tandem stoichiometric semi hydrogenation of the C-C bond and hydro defluorination of the aromatic substituents. Such reactivity further improved in the presence of Et3SiH. Hexafluorobutyne reacted with aqueous-generated [(dippe)2Ni] (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) yielding complex [(dippe)Ni(
2-F3CC  CCF3)] (162), which in the solid state exhibited a distorted trigonal-planar geometry with a coordinated C-C triple bond length of 1.301 A˚ .199 Ni(0) complex, [(dippe)Ni(
2-F3CC CCF3)] (162), reacted with excess water promoting alkyne semi reduction to yield the corresponding trans alkene selectively through intermediacy of the cis isomer. Complex 162 also promoted partial hydro defluorination of hexafluorobutyne in the presence of a stoichiometric amount of water. Ni(0)-complexes 163a–c, 164, 165a–b, and 166a–b (Fig. 30) were calculated by DFT during the elucidation of the reaction mechanisms of the Ni(0)-catalyzed cross-dimerization and trimerization of trimethylsilyl acetylene and diphenylacetylene. Electron-donating ability of the ligand played a crucial role in the regioselectivity of the tandem process, since strongly electron-donating ligands favored the formation of cross-dimer intermediates, whereas cross-trimer products were synthesized with weakly electron-donating ligands. Ni(0) ate complexes 167a and 167b (Fig. 31) were calculated by DFT during mechanistic investigation on the nickel-catalyzed C-H/C-N oxidative annulation of aromatic amides with alkynes in the presence of KOtBu.201 DFT calculations indicated that

Zerovalent Nickel Organometallic Complexes

707

Fig. 30 Calculated nickel(0) complexes with alkyne and enyne ligands.200

Fig. 31 Nickel(0) ate intermediates in LLHT between an alkyne and a C-H moiety.201,202

reaction involved the anionic Ni(0) ate complex 167b (Fig. 31) as key intermediate prior to ligand-to-ligand hydrogen transfer (LLHT), which was the rate-determining step. Complex 167b formed by PPh3 substitution from 167a (Fig. 31), and featured an agostic interaction between the Ni(0)-center and an ortho-C-H moiety in the aromatic amide anionic ligand. LLHT involving such an agostic interaction is also known as ambiphilic metal-ligand activation.167,168 Complex [(
2-PhC  CPh)Ni{(iPr2PCH2NPh)2Y(k2-iPr2PCH2NPh)}] (168) (Fig. 29) was obtained by addition of 5 equiv. of diphenylacetylene to complex [{(iPr2PCH2NPh)3Y}Ni].196 Formation of 168 indicated hemilability of the (iPr2PCH2NPh)3Y ligand, allowing for alkyne binding at Ni. Hemilability involved P-dissociation from the Ni(0)-center and coordination to the Y-atom, as revealed by 31P-89Y scalar coupling in NMR analysis, and further confirmed by X-ray crystallography. The solid-state structure of 168 revealed p-back bonding into the alkyne as evidenced by an elongated C-C distance and the loss of linearity with a bent Ph-C-C angle of 142.5(2) . Complex 168 was found to be the resting state in the semi hydrogenation of diphenylacetylene catalyzed by [{(iPr2PCH2NPh)3Y}Ni], yielding E-stilbene selectively in quantitative yield. Similarly, complexes [(
2-PhC  CPh)Ni {(iPr2PCH2NPh)2M(k2-iPr2PCH2NPh)}] (M ¼ Lu, La) were observed as resting states when [{(iPr2PCH2NPh)3M}Ni] (M ¼ Lu, La) were used as catalytic precursors in the same reaction.196

8.10.6

Nickel(0) complexes with p-arene ligands

Zerovalent nickel arene complexes comprise a general group of either mononuclear or dinuclear compounds having in common at least one p-coordinated arene ligand, showing either
2-,
4-, or
6-coordination. Coordination of arenes to Ni(0) usually is accompanied by deviation from planarity of the aromatic system and distorted C-C lengths, as a consequence of p-back bonding. Ni(0)-arene complexes usually exhibit fluxionality and offer the possibility of haptotropic rearrangements. As detailed in this section, methods to obtain nickel(0) arenes can include the use of Ni(II) or Ni(I) sources under appropriate reducing conditions in the presence of aromatic ligands, or the use of an already formed Ni(0) source by ligand exchange with an aromatic ligand.

708

Zerovalent Nickel Organometallic Complexes

Characterization of p-arene complexes typically included molecular structure determination by X-ray crystal diffraction, and solution characterization by NMR spectroscopy; however, an important number of p-arene complexes was studied instead by means of DFT calculations. Additionally, this section features the special cases of p-coordinated fullerene complexes and the formation of mononuclear and dinuclear Ni(0)-aryne species from Ni(0)-arene precursors.

8.10.6.1

Arene complexes with benzene-derived ligands

The reaction of [(PiPr3)2NiCl2] with an excess of (THF)3Mg(
2-C14H10) in THF afforded the deep red, 16-electron complex 171 (Fig. 32) in 71% isolated yield.130 Complex 171 was soluble in nonpolar organic solvents. In the molecular structure of 171, the Ni(PiPr3)2 fragment appeared
2-coordinated to the 1,2-position of anthracene, with an approximately planar geometry for the Ni center. Anthracene exhibited loss of planarity and lengthening of the
2-bound C-C bond due to back-bonding from the Ni(0) atom, which was comparable to analogous anthracene adducts [(PR3)2Ni(
2-C14H10)] (R ¼ Et or Cy).204,205 VT NMR analysis of 171 in nonaromatic solvents indicated fluxional behavior with exchange by haptotropic rearrangement between the 16-electron complex [(PiPr3)2Ni(
2-C14H10)] (171) and the 18-electron complex [(PiPr3)2Ni(
4-C14H10)] (171a).130 In aromatic solvents such as benzene, toluene, and mesitylene, VT NMR analysis of 171 revealed a thermal equilibrium favoring solvent-adduct formation upon dissociation of one PiPr3 ligand, forming complexes 172a–c (Fig. 32).130 Anthracene-complex 171 bound preferentially to the more electron-donating mesitylene ligand over toluene or benzene. Complexes 172a–c decomposed in the presence of Lewis acids, such as BEt3, B(C6F5)3 or anhydrous NiCl2. Complex 171 reacted with partially substituted fluorobenzene derivatives performing C-H bond activation and producing the corresponding Ni(II)-oxidative addition products. Reaction of p-terphenyl phosphine, 1,4-(2-iPr2PC6H4)2C6H4, with 1 equiv. of [Ni(COD)2] generated red species [{k2-P,P-
2-C, C-1,4-(2-iPr2PC6H4)2C6H4}Ni] (173) (Fig. 32).203 The solid-state structure of 173 showed a Ni(0) center coordinated to the two phosphine donors and stabilized by additional
2-coordination of a C-C double bond of the aromatic system. Complex 173 reacted with [Ni(DME)Cl2] (DME ¼ 1,2-dimethoxyethane) via comproportionation to produce the dark green dimeric NiI-NiI species, [(m{k-P-
2-C,C: k-P-
2-C,C-1,4-(2-iPr2PC6H4)2C6H4})Ni2(m-Cl)2], whose solid-state structure revealed a Ni2 moiety coordinated by phosphines in a nearly linear PNiNiP arrangement.203 The metal centers were bridged by chlorides and interacted with a vicinal diene moiety of the central ring of the terphenyl framework. The reaction of the anthracene adduct [(PEt3)2Ni(
2-C14H10)] with partially substituted-fluorobenzene derivatives produced equilibria between dinuclear adducts [{(PEt3)2Ni}2(m-
2:
2-C6FnH6-n)] (n ¼ 4: 174a–c; n ¼ 3: 174d) and mononuclear adducts [(PEt3)2Ni(
2-C6F4H2)] (175a–c) (Fig. 33).206 When studying 1,2,4,5- and 1,2,3,5-tetrafluorobenzene, additional C-H activation

Fig. 32 Nickel(0)-arene complexes with benzene-derived ligands.94,130,203

Fig. 33 Mono- and dinuclear nickel(0)-arene complexes with fluorobenzene ligands.206,207

Zerovalent Nickel Organometallic Complexes

709

Fig. 34 Calculated nickel(0)-arene intermediates in the hydrofluoroarylation of alkynes and alkyne-intermediate performing key LLHT.208

Ni(II)-products, [(PEt3)2NiH(C6F4H)], were found to be in equilibrium with species 174a and 175a, and with species 174c and 175c, respectively. Equilibrium mixtures were also generated from the reaction of in situ generated [(PEt3)2Ni(
2-H2C¼ CMe2)] with tetrafluoro benzenes. Molecular structure of complexes 174a and 174b (Fig. 33) exhibited extensive back-bonding, with one localized C-C bond with a double bond-like distance, and the remaining C-C bonds with bond lengths longer than those expected for aromatic C-C bonds. Ni(0)-pentafluorobenzene complexes 176a–b and 177 (Fig. 34) were calculated by DFT during computational mechanistic studies on the hydrofluoroarylation of alkynes by nickel diphosphine complexes.208 Such mechanism was previously studied by experimental means for the hydrofluoroarylation of several alkyne derivatives with partially fluorinated benzene and pyridine-derivatives catalyzed by [Ni(COD)2] and either tricyclopentylphosphine or tricyclohexylphosphine.131 Complexes 176a–b (Fig. 34) were computed to have formed favorably by
2-coordination of C6F5H to [Ni(PMe3)2] via a CH ¼ CF or a CF ¼ CF bond, which differed in 3.2 kJ mol−1, and were key intermediates prior to C-F bond activation by irreversible oxidative addition to generate thermodynamically favored trans-[(PMe3)2Ni(F)(C6F4H)].208 Mechanistic pathway leading to C-H bond activation predicted instead intermediacy of
2-alkyne-s-C-H adduct 177 (Fig. 34). In 177, the C-H bond of C6F5H was coordinated in the molecular plane defined by Ni, the alkyne, and the phosphine. The 2-butyne ligand was
2-coordinated and closer to the phosphine than in precursor species [(PMe3)Ni(
2-MeC  CMe)]. The C-H bond in 177 was elongated indicating significant activation with the H atom close to the one carbon of the alkyne. Intermediate 177 performed Ligand-to-Ligand H-Transfer (LLHT) to generate the Ni(II) complex, [(PMe3)Ni(C6F5){(Me)C¼ CH(Me)}]. Reductive elimination from [(PMe3)Ni(C6F5) {(Me)C ¼ CH(Me)}] generated the corresponding arylalkene. The reaction of distannene 178a with [Ni(COD)2] in hexane yielded complex 178b (Scheme 10).137 In contrast to a related stannene-diyl complex, with two tin(II) atoms coordinated separately to Ni(0),209 complex 178b exhibited side-on coordination of the trans-bent distannene 178a to the nickel atom. Molecular structure of 178b showed the nickel atom coordinated almost perpendicularly to the plane containing the two Sn atoms and the two ipso C atoms of the xanthene backbone. Both Sn atoms displayed different coordination geometries as indicated by their respective Sn-Ni bond lengths. Additionally, complex 178b featured Ni-arene adduct-formation with one mesityl substituent of the distannene ligand.137

Scheme 10 Side-on distannene nickel(0) complex featuring additional p-arene-coordination.137

Complex [{(iPr3P)2Ni}2N2] participated in an equilibrium reaction with benzene or toluene under a N2 atmosphere, in situ forming complexes 179a–b (Fig. 32), respectively. At low temperature, equilibrium shifted toward formation of [{(iPr3P)2Ni}2N2], as revealed by NMR.94 The reaction of a solution of [(dippe)Ni(H)]2 (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) with either pentafluoro- or hexafluoro-benzene in THF resulted in immediate formation of complexes [{(dippe)Ni}2(m-
2:
2-C6F6)] (180a) and [{(dippe)

710

Zerovalent Nickel Organometallic Complexes

Ni}2(m-
2:
2-C6F5H)] (180b) (Fig. 33).207 NMR analysis of both 180a–b revealed fluxionality of the C6F6 and C6F5H moieties. Molecular structure of [{(dippe)Ni}2(m-
2:
2-C6F6)] (180a) displayed C2 symmetry with the two [(dippe)Ni] moieties anti facially coordinated to adjacent C¼ C bonds of a bridging C6F6 ligand. The arene ring showed significant elongation of the coordinated C¼ C bonds from typical aromatic values, consistent with back bonding from the Ni(0) centers. Additionally, the fluorine substituents showed considerable deviation from planarity. Complex [{(dippe)Ni}2(m-
2:
2-C6F5H)] (180b) performed C-F bond activation by oxidative addition upon heating at 80  C for 4 h, yielding the Ni(II) complex, [(dippe)Ni(F)(1,2,3,4-C6F4H)]. Structures 181 and 182 (Fig. 35) were calculated by DFT during mechanistic studies on the Ni-dcype-catalyzed C-H/C-O coupling of benzoxazole and naphthalen-2-yl pivalate (dcype ¼ 1,2-bis(diphenylphosphino)ethane), involving both C-O oxidative addition and C-H bond activation.146 Arene-complex 181 was calculated to have formed from NaphOPiv substitution in [Ni(COD) (dcype)]. Given the structure of the NaphOPiv ligand, complex 181, featuring
2-C ¼ C and k-O chelation, was one of several possible isomers, and it also was a key intermediate prior to C-O oxidative addition to Ni(0) to form the Ni(II)-intermediate, [(dcype)Ni(OPiv)(Naphthyl)]. C-H activation of benzoxazole generated Ni(II) intermediate [(dcype)Ni(Naphthyl)(Benzoxazolyl)], which after reductive elimination was computed to favorably form Ni(0)-arene 182 (Fig. 35). Arene-intermediates 183a–b, 184a–b, and 185a–b (Fig. 36) were calculated by DFT during mechanistic studies on the nickel(0)-catalyzed cross coupling of methoxyarenes through C-O bond activation.210 In the Ni/PCy3-catalyzed reactions the oxidative addition of the C(aryl)-OMe bond became energetically feasible when CsF, used as an additive, and an arylboronic ester interacted with a Ni(PCy3)2/methoxyarene moiety forming quaternary complex 183b (Fig. 36). Further CsF-assisted C-O activation, transmetallation and reductive elimination led to formation of arene-complex 184b, still bearing a quaternary boroncenter. In contrast, complex 183a formed by p-coordination of Ni(PCy3)2 to methoxy naphthalene, was computed to react through an energetically unfavorable pathway to form the Ni(0)-arene intermediate 184a. Complexes 184a–b featured
2-coordination of phenyl naphthalene, the corresponding cross-coupling product. Competing b-hydride elimination, producing formaldehyde, was also calculated either in or without the presence of CsF. Complexes 185a–b thus formed by
2-coordination of formaldehyde to Ni(0)-arene species. Reaction of 1-naphthyl pivalate with a 1:1 mixture of [Ni(COD)2] and PCy3 in toluene-d8 at 50  C led to the formation of dinuclear Ni(I) complex 186 (Fig. 37) in 50% yield. Air-sensitive complex 186 was also obtained from the reaction of [{(PCy3)2Ni}2(N2)] at room temperature in full conversion.211 Solid-state structure of 186 featured a s-bond interaction and a
2-arene interaction between naphthyl moiety and the Ni-Ni core, along with a bridging pivalate ligand with nearly identical Ni-O bond lengths. Upon heating at 100  C, complex 186 released dinaphthalene while forming Ni(II) species, [(PCy3)2Ni2(m-OPiv)4].

Fig. 35 Calculated nickel(0)-arene intermediates with naphthalene-substituted ligands.146

Fig. 36 Nickel(0)-arene intermediates in C-O activation with CsF and boronicesters.210

Zerovalent Nickel Organometallic Complexes

711

Fig. 37 Nickel(I)-arene complex and calculated nickel(0)-arene intermediates with naphthalene ligands.211

Fig. 38 Nickel(0)-fluorenyl ate complexes with olefin and arene ligands.212

Formation of 186 was suggested to occur through Ni(II)/Ni(0)-comproportionation, and was found to be an off-cycle species during the Ni(0)/PCy3-catalyzed silylation of aryl pivalates. Structures 187 and 188 (Fig. 37) were calculated by DFT during mechanistic studies of the silylation of naphthyl pivalate with Me3Si-Bpin with CuF2/CsF additives.211 Ni(0)-arene intermediate 187 was formed by initial p-coordination of the arene to [Ni(PCy3)(toluene)] and led to facile C-O oxidative addition generating [Ni(PCy3)(s-1naphthyl)(k2-OPiv)], which after transmetallation with Me3Si-Bpin, dissociation of PivO-Bpin, and reductive elimination, formed Ni(0)-arene intermediate 188, prior to regeneration of 187 by ligand exchange. Gas phase low-valent Ni complex 189a (Fig. 38), relevant as a model for a pristine graphene-supported single-atom catalyst, was obtained using a modified ion-trap mass spectrometer by subjecting the precursor ion, fluoren-9-carboxylate, and nickel(II) oxalate to collision-induced dissociation (CID). During CID, two-electron metal reduction caused both oxalate breakdown and fluoren9-carboxylate decarboxylation, releasing CO2 and generating the Ni(0)-fluorenyl ate complex 189a (Fig. 38).212 Complex 189a underwent dehydrogenation reactions with several alkanes. With cyclohexane, complex 189a formed H2 and cyclohexene-adduct 189b (Fig. 38). DFT calculations on the reaction mechanism suggested insertion of complex 189a into a C-H bond followed by b-hydride elimination, leading to complex 189b and H2. Complex 189a also performed sequential dehydrogenation steps under CID conditions resulting in formation of hexadiene and benzene-adducts, 189c and 189d. The whole series of arene- and side-on-intermediates 191 and 192 (Scheme 11) were calculated by DFT including THF as solvent during studies on the effect of coordinating functional groups on the performance of Suzuki-Miyaura reactions catalyzed by

Scheme 11 Calculated nickel(0)-arene and nickel(0)-side-on complexes with bromobenzene-derived ligands and their oxidative addition products.213

712

Zerovalent Nickel Organometallic Complexes

nickel.213 Both families of compounds were considered to have formed from common precursor 190 (Scheme 11), obtained by transmetallation of [(dppf )Ni(Cl)(o-Tol)] with p-tolylboronic acid and subsequent reductive elimination. Ligand substitution in 190 generated
2-arene complexes 192, which underwent oxidative addition to form the Ni(II) compounds [(dppf )Ni(Br) (p-YC6H4] (193) (Scheme 11). Alternatively, some substrates could coordinate to Ni(0) through their functional groups forming side-on complexes 191. If the energy of 191 was lower in than that of 192, then the rate of oxidative addition would decrease. Besides suggesting that oxidative addition was unlikely to be rate-determining for these Ni(0)-complexes, calculations indicated that side-on complexes 191 were typically lower in energy than the corresponding complexes 192. Arene-intermediate [(PMe3)2Ni(
2-p-OTs-C6H4Cl)] (194), was calculated by DFT to have formed by the reaction of Ni(PMe3)2 with 4-chlorophenyltosylate, during studies of the selective oxidative addition of aryl-oxygen over aryl-chloride bonds at Ni(0). DFT and stoichiometric oxidative addition studies demonstrated that PMe3 promoted preferential reaction with aryl tosylates in the presence of aryl chlorides. Such reactivity allowed for performing Ni-catalyzed C-O selective Suzuki-Miyaura coupling of chlorinated phenol derivatives. The role of PMe3 in selectivity was predicted to involve a close interaction between nickel and a sulfonyl oxygen of tosylate during oxidative addition.214

8.10.6.2

Arene complexes with pyridine, thiophene and pyrrole ligands

2


-pyridine Ni(0) complexes 195a–b (Fig. 39) were calculated by DFT during studies on the activation of C-F bonds in pentafluoropyridine with [Ni(PR3)2] species.215 Formation of 195a–b from pentafluoropyridine and [Ni(PMe3)2] was computed to be exothermic and such products were approximately trigonal-planar
2-arene complexes with coordinated C ¼ C or C¼ N bonds longer than those found in pentafluoropyridine, consistent with the Dewar-Chatt-Duncanson model. Coordination at C3 ¼ C4 in 195b was preferred to coordination at C2 ¼ N in 195a by 5.3 kcal mol−1. Complexes 195a–b were intermediates further undergoing Ni migration away from the aromatic p-system along a C-F bond, forming three-center transition states, which evolved to squareplanar Ni(II) products cis-[(PMe3)2Ni(2-C6F4)(F)] and cis-[(PMe3)2Ni(4-C6F4)(F)]. Ni(0) complexes 196 and 197a–b (Fig. 40) were obtained during mechanistic studies on the C-F activation reactions of 2,3,5,6-tetrafluoropyridine and pentafluoropyridine with the phenanthrene adduct [(PEt3)2Ni(
2-C14H10)] and [Ni(PEt3)].218 Activation of 2,3,5,6-pentafluoropyridine proceeded through intermediacy of mononuclear complex [(PEt3)2Ni(
2-2,3,5,6-C5F4HN)] (196) (Fig. 40), which established an equilibrium with the C-H activation product [trans-(PEt3)2Ni(H)(2,3,5,6-C5F4N)]. Complexes 197a–b (Fig. 40) were obtained as a mixture after treatment of [(PEt3)2Ni(
2-C14H10)] with pentafluoropyridine. Ni(0) arene-intermediates 198 and 199 (Fig. 39) were predicted by DFT computations during mechanistic investigation of the oxidative addition (OA) of haloarenes to zerovalent nickel catalysts.216 Modelling the OA reaction of Ni(dppp) (dppp ¼ 1,3-bis (diphenylphosphino)propane) with o-bromotoluene revealed favorable formation of
2-arene complexes with neighboring sp2 carbons followed by ring-walking, thus complex 198 (Fig. 39) was one of six possible isomers. Further nickel insertion from 198 formed the Ni(II) complex, [(dppp)Ni(Br)(o-tolyl)]. To study heteroatom effects on such OA mechanism, thiophene was modelled to react with Ni(dppp) showing favorable formation of
2-thiophene adducts featuring ring-walking, with 199 (Fig. 39) being one of two possible isomers, prior to formation of the corresponding Ni(II) complex, [(dppp)Ni(Br)(2-Me-thiophenyl)].

Fig. 39 Calculated nickel(0)-arene complexes with substituted benzene, pyridine, thiophene, and pyrrole ligands.215–217

Zerovalent Nickel Organometallic Complexes

713

Fig. 40 Mono- and dinuclear nickel(0)-arene complexes with pyridine and thiophene ligands.218,219

Theoretical structures and energetics for ring-walking and oxidative addition of zerovalent nickel with 1-bromo-2methylbenzene, 2-bromopyridine, 2-bromo-3-methylthiophene, and 2-bromopyrrole were computed by DFT, featuring intermediacy of the
2-arene Ni(0) adducts 200- 203 (Fig. 39), which represent one of several possible isomers in each case.217 Intermediate 200 (Fig. 39) featured a Ni(0) center coordinated to the C ¼ C bond adjacent to the halogen in a non-symmetrical fashion. Formation of other
2-arene isomers of 200 and 201 involved a ring-walking process above the plane of the arene moiety through pseudo
3-transition states. p-Complexation of 2-bromo-3-methylthiophene with Ni(dppp) in 202 (Fig. 39) caused elongation of the C4¼ C5 bond in the thiophene ring, revealing substantial weakening of the double bond. Oxidative addition of the C(sp2)-Br bond in all cases occurred through a three-coordinate C-C-Br transition state, prior coordination of the Ni(0) moiety to the double bond adjacent to the halogen substituent. Complexes 204 and 205 (Fig. 40), relevant to nickel-catalyzed catalyst transfer polycondensation of thiophenes, were synthesized by the reaction between [{(dtbpe)Ni}2(m-
2:
2-C6H6)] (dtbpe ¼ 1,2-bis(di-tert-butylphosphino)ethane) and either excess thiophene or bisthiophene.219 Complex [{(dtbpe)Ni}2(trans-m-
2:
2-thiophene)] (204) was obtained in pentane at low temperature and both, NMR and solid-state structure analysis indicated formation of a Ni(0) dimer with a bridging thiophene ligand. Treatment of 204 with further excess of thiophene generated an equilibrium mixture of mononuclear [(dtbpe)Ni(
2-thiophene)] and the C-S insertion product [(dtbpe)Ni(S,C-k2-thiophene)]. Molecular structure of complex 205 exhibited an exo-C,C-
2-binding geometry. The metal center displayed a square planar geometry, suggesting stabilization by p-back bonding. Formation of the corresponding C-S activation product, derived from the bis thiophene complex 205, was less favored when compared with the C-S insertion reaction of complex 204.

8.10.6.3

p-Fullerene complexes

Excess of zinc dust and stoichiometric mixtures of [(dppe)NiCl2] (dppe ¼ 1,2-bis(diphenylphosphino)ethane) or [(dppf )NiBr2] (dppf ¼ 1,10 -bis(diphenylphosphino)ferrocene) and C60 were stirred in o-dichlorobenzene at 160  C during 20 min, then stirred at room temperature during 4 h. Diffusion of hexanes allowed for the isolation crystals of [Ni(dppe)(
2-C60)] (206) and [Ni(dppf ) (
2-C60)] (207) (Fig. 41).220 Crystals of [Ni(dppp)(
2-C60)] (208) (Fig. 41) (dppp ¼ 1,3-bis(diphenylphosphanyl)propane) were obtained by hexane diffusion into the reaction mixture formed after reduction of a stoichiometric mixture of [Ni(dppp)Cl2] and C60 in o-chlorobenzene by sodium fluorenone.221 Solid-state structures of complexes 206–208 displayed nearly planar coordination geometry, since the five atoms (Ni, two carbon, and two phosphorus bound to nickel) were nearly coplanar. Ni(0) coordinated to the 6-6 bond of C60 in a
2-fashion. In all three cases, due to
2-coordination, elongation of the C-C bonds was observed, indicating metal-to-C60 p-back-donation. When comparing complexes 206–208, the P-Ni-P angle increased in the order 206 < 208 < 207. Further IR, UV-vis and EPR studies performed with complexes 206–208 revealed the molecular character of such compounds, which were best described as neutral fullerene C60 bound to zerovalent nickel atoms.220,221 Nickel-fullerene polymeric complex 209 (Fig. 41) was obtained by the reduction of a stoichiometric mixture of [(Me3P)2NiCl2] and C60 with excess zinc dust in o-dichlorobenzene for 10 min at 160  C. Further addition of either DMF or benzonitrile and diffusion of hexane yielded crystals of 209.222 Solid-state structure of 209 exhibited each nickel atom coordinated to two fullerene

714

Zerovalent Nickel Organometallic Complexes

Fig. 41 Nickel(0) complexes with fullerene ligands.220–222

Fig. 42 Mono- and dinuclear nickel(0)-aryne complexes.206,223

units in a
2-Ni-C(C60) fashion. Coordination to the Ni(0) centers occurred by oppositely located 6-6 bonds of C60. Such bonds elongated in comparison with other 6-6 bonds in C60, which was attributed to p-back donation. Ni(0)-centers also coordinated two PMe3 ligands forming a four-coordinated arrangement exhibiting distorted tetrahedral geometry. SQUID and EPR analysis showed that 209 was diamagnetic and EPR-silent at room temperature. The latter plus further characterization by IR and UV-vis indicated d10 electron configuration for the Ni(0) atoms.

8.10.6.4

Aryne complexes

Dinuclear adduct [{(PEt3)2Ni}2(m-
2:
2-1,2,3,5-C6F4H2)] (174c) (vide supra, Fig. 33) underwent rapid C-F activation forming the aryne adduct [{(PEt3)2Ni}2(m-
2:
2-3,4,6-C6F3H)] (213) (Fig. 42), whose production formally involved the dehydrohalogenation of an ortho-disposed hydrogen and fluorine from 1,2,3,5-tetrafluorobenzene.206 Solid-state structure of dinuclear aryne complex 213 featured considerable back-bonding from the Ni centers to the aryne moieties aromatic C-C bonds, and suggested no significant bonding interaction between the Ni centers. The reaction of the aryne complex [(PEt3)2Ni(
2-C6H2-4,5-F2)] (210a) (Fig. 42) with a catalytic amount of [(PEt3)2NiBr2] over Na/Hg cleanly produced the dark brown dinuclear Ni(I) complex (211), as shown in Scheme 12.223 The reduction of [Br2Ni(PEt3)2] produced transient Ni(PEt3)2, which catalyzed the reaction. Complex 211 was the product of the putative intermediate [(PEt3)2Ni(
2-C6H2-5,6-F2)] (210b), which was an isomer of 210a, formed by aromatic C-H bond activation followed by the formation of an alternate aromatic C-H bond. The conversion of aryne complex 210a to dinuclear nickel complex 211 occurred through intermediacy of the long-lived species 212 (Scheme 12 and Fig. 42). Intermediate 212 was independently isolated by the addition of 1 equiv. of [(PEt3)2NiBr2] to a pentane solution of 210a stirred over Na/Hg. In the solid state, complex 212 showed a Ni(1)-Ni(2) distance of 2.7242(3) A˚ . The presence or absence of a Ni-Ni bonding interaction was not readily established from the structural data alone. The C-C bond lengths of the aryne fragment indicated a slight disruption of aromaticity. These structural features indicated that the coordination of Ni(PEt3)2 to complex 210a required back-donation into both p-systems of the aryne. Complex 212 cleanly catalyzed the conversion of solutions of 210a–211 in the absence of Na/Hg. The driving force of this reaction appeared to be the formation of a more stable isomer of 210a. DFT calculations on the model complexes (PMe3)2Ni(
2-C6H2-4,5-F2) (210a) and (PMe3)2Ni
2-C6H2-3,4-F2) (210b) demonstrated that the latter was more stable by 9 kcal mol−1.

Zerovalent Nickel Organometallic Complexes

715

Scheme 12 Conversion of aryne complex 210a to dinuclear Ni(I) compound 211 through intermediacy of Ni(0) species 212.223

8.10.7

Side-on nickel(0) complexes with C ¼ E (E ¼ O, S, N) and C  N moieties

Zerovalent nickel complexes with side-on coordinated C ¼ O, C¼ S, C¼ N, and C N moieties comprise a diverse family of compounds bearing ligands such as aldehydes and thioaldehydes, ketones and thioketones, esters and thioesters, aldimines, nitriles, CO2 and CS2, all having in common p-coordination of at least one of the C-E (E ¼ O, S, N) double or triple bonds. Such side-on bonding interactions are usually accompanied by an important loss of bond-order of the C-E (E ¼ O, S, N) double or triple bonds, as a consequence of p-back bonding. The presence of heteroatoms in these complexes offer the possibility of forming Lewis acid-adducts, which in some cases promoted reactivity patterns involving C ¼ N and C¼ O participation in oxidative coupling or C-CN bond activation, as described in this section. Three-coordinate Ni(0)-side-on complexes are usually diamagnetic and exhibit coordination geometries best described as square planar. Detailed herein, methods of synthesis of nickel(0)-side-on complexes can include usual ligand substitution from a Ni(0) source, namely but not restricted to [Ni(COD)2], [L2Ni(COD)], [LnNi2(m-arene)], or [(LnNi2(m-N2)]; and reduction of Ni(II) or Ni(I) sources either by reaction with borohydride reagents or by reductive elimination, in the presence of the p-coordinating ligands. Characterization of side-on complexes typically included determination of the C¼ O, C¼ S, C¼ N, or C N stretching frequencies by FTIR, molecular structure determination by X-ray crystal diffraction, solution characterization by means of NMR spectroscopy, and in some cases, DFT calculations. Briefly included in this section is also the synthesis and characterization of side-on borane-containing complexes, featuring either
3-C,C,B or
2-C¼ B-bonding interactions.

8.10.7.1 2

Side-on carbonyl and thiocarbonyl complexes


-Arylaldehydenickel complexes (214a–d) were prepared by the reaction of the corresponding arylaldehyde with Ni(COD)2 and PCy3 or dppf (1,10 -bis(diphenylphosphino)ferrocene) (Scheme 13).224 The formyl hydrogen and carbon in arylaldehyde coordinated to nickel were found upfield compared to those of free aldehyde indicating
2-coordination of the arylaldehyde to nickel(0), and strong back bonding from the metal into the carbonyl group. The reaction of (
2-PhCHO)Ni(PCy3)2 (214a) with Me3SiX (X ¼ OTf or Cl) gave the corresponding
1:
1-siloxybenzylnickel complexes 215a and 216a (Scheme 13). In contrast, the reaction of [{
2-(1-NaphCHO)}Ni(PCy3)2] (214b) with Me3SiCl produced (
3-1-Me3SiOCHC10H7)Ni(PCy3)(Cl) (215b) quantitatively. The treatment of 214c or 214d with Me3SiOTf led to cationic
3-siloxymethylarylnickel complexes 215c and 215d, featuring coordination of the aromatic ring (Scheme 13). The addition of PCy3 to solutions of 215a or 216a let to the formation of 1,2-bis(trimethylsiloxy)-1,2-diphenylethane in 78% or 48% yield. Similarly, addition of 5 atm CO to 216a led to the formation of 1,2-bis(trimethylsiloxy)-1,2-diphenylethane in 63% yield. In contrast, 215c regenerated PhCHO and Me3SiOTf along with the formation of [Ni(CO)2(DPPF)] in the presence of CO. Complex 215c also reacted with nBu4NCl to yield (
2-PhCHO)Ni(DPPF) (214c) and Me3SiCl. In the latter case, 214c further reacted with Me3SiCl to give 1,2-bis(trimethylsiloxy)-1,2-diphenylethane in 51% yield.

716

Zerovalent Nickel Organometallic Complexes

Scheme 13 Reactivity of arylaldehyde-containing Ni(0) compounds.224

The addition of excess 2,3-dimethyl-1,3-butadiene to a solution of benzaldehyde, PCy3, and [Ni(COD)2] gave the
3:
-allylalkoxy-nickel complex (217a). Reversible formation of 217a was also observed by the reaction of [(
2-PhCHO)Ni(PCy3)2] (218a) with 2,3-dimethyl-1,3-butadiene (Scheme 14).119 The addition of PCy3 to a solution of 217a led to the formation of an equilibrium mixture with the regeneration of [(
2-PhCHO)Ni(PCy3)2] (218a) and 2,3-dimethyl-1,3-butadiene. Similarly, the reaction of 2,3-dimethyl-1,3-butadiene with acetone generated the corresponding
3:
1-allylalkoxy-nickel complex (217b) in 94% yield. In this case, quantitative regeneration of [Ni(COD)2] and PCy3 occurred during concentration of the reaction mixture under reduced pressure. Reaction of 2,3-dimethyl-1,3-butadiene with styrene oxide occurred at 60  C to give complex 217c, derived from 2,3-mehtyl1,3-butadiene and phenylacetaldehyde. This indicated that styrene oxide isomerized into phenylacetaldehyde, which was confirmed by treatment of [Ni(COD)2] and PCy3 with styrene oxide producing [(
2-PhCH2CHO)Ni(PCy3)2] (218b) in 60% yield.119 1

Scheme 14 Reactions of 2,3-dimethyl-1,3-butadiene-derived nickel complexes with carbonyl compounds and styrene oxide.119

Zerovalent Nickel Organometallic Complexes

717

The use of the Lewis acid, Me2AlOTf, allowed the oxidative cyclization of either pivalaldehyde or benzaldehyde and diphenylacetylene with Ni(0) in the presence of PCy3 to form oxanickela(II)cyclopentenes 220 and 221 (Scheme 15). Ni(II) complex 220 was also prepared in 87% yield by the treatment of the
2-PhCHO complex 218a with Me2AlOTf through intermediacy of the Lewis acid-adduct 219 (Scheme 15), which further reacted with diphenylacetylene (Scheme 15).225 Analog Ni(II) complex 221 was prepared in 92% yield. Treatment of 220 and 221 with carbon monoxide resulted in quantitative formation of five-membered lactones 220a and 221a, and the Ni(0) complex [Ni(CO)3(PCy3)] (Scheme 15). The molecular structure of the side-on Ni(0) complex 219 featured adduct formation between the oxygen atom in the
2-coordinated carbonyl group and a Me2AlOTf unit. The C-O bond distance indicated a resonance structure of an oxanickela(II)cyclopropane, rather than that of an
2-aldehyde Ni(0) species.225

Scheme 15 Lewis-acid-promoted reactions of side-on nickel(0)-aldehyde complexes with alkynes.225

DFT calculations were performed for geometry optimization of species 222a and 223a (Fig. 43), and their Zn adducts, 222b and 223b (Fig. 43).226 These compounds were relevant for the reaction mechanisms of the nickel(0)/zinc-catalyzed decarbonylative addition of anhydrides to alkynes. Compound 222a (Fig. 43) resulted from the interaction between phthalic anhydride and Ni(PMe4)4, and featured a synergic pp-dp back donation bond between nickel and a side-on coordinated C ¼ O bond. Formation of such a bond weakened and activated the C-O bond, which resulted in the oxidative addition of phthalic anhydride. The corresponding Ni(0)-Zn(II)-anhydride complex, 222b (Fig. 43) resulted from the interaction between phthalic anhydride, Ni(PMe4)4, and ZnCl2, and underwent oxidative addition through a pathway significantly lower in energy than in the case where Zn was not present. Species 223a and 223b resulted from the interaction between isocumarine, obtained by decarbonylative addition of phthalic anhydride to dimethyl acetylene, and the Ni(0) moiety [Ni(CO)(PMe3)], where the carbonyl ligand originated

Fig. 43 Calculated side-on nickel(0) complexes with phthalic anhydride and isocumarine ligands.226

718

Zerovalent Nickel Organometallic Complexes

Fig. 44 Side-on nickel(0) complexes with ketone, thioketone, ester, and thioester ligands. 150,227–229

from anhydride decarbonylation, either in the presence or absence of the Lewis acid ZnCl2. All the four species 222a–b and 223a–b showed optimized geometries with trigonal-planar coordination geometry around the Ni(0) centers. Dimeric Ni(I) complex [(dippe)Ni(H)]2 (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) reacted with alkyl and aryl ketones at room temperature to form complexes 224a–e (Fig. 44) in 85–90% yields, along with the evolution of hydrogen, whereas compound 224f (Fig. 44) was obtained in 70% yield together with 30% of the corresponding N,N0 -coordinated Ni(0) compound with the same ligand.199 In a similar system, reaction of [(dippe)Ni(COD)] (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) with acetophenone derivatives in THF at 100  C formed complexes 224c, 224g, and 224h (Fig. 44) in 72–75% yields.200 Compounds 224a–224h displayed side-on coordination to the Ni(0) center as revealed by 31P{1H} NMR analysis. In the solid state, complexes 224a and 224e showed square planar geometry around the Ni center, with considerable lengthening of the
2-coordinated carbonyl group compared to the corresponding free ligands. When [(dippe)Ni(H)]2 reacted with the vicinal diketone, benzil ([PhC(O)]2), two complexes were observed, namely, [(dippe)Ni(k2-O,O-benzil)] (225a) and [{(dippe)Ni}2(
2-C,O-benzil)] (225b).199 The structure for 225b featured Ni centers with square planar coordination, with each carbonyl group coordinated to a [(dippe)Ni0] fragment exhibiting a mutually anti conformation. Observed lengthening of the side-on coordinated carbonyl groups indicated back donation from the Ni(0) center. Complexes 224a–f and 225b were relevant for the catalytic hydrogenation of the carbonyl groups to the corresponding alcohols.199 Complexes 224c and 224g–h were relevant intermediates in the catalytic transfer hydrogenation of ketones with ethanol as the hydrogen source.200 Side-on carbonyl Ni(0) complex 226 (Fig. 44) was isolated in 44% yield after the reaction of [{(dtbpe)Ni}2(m-
2:
2-C6H6)] (dtbpe ¼ 1,2-bis(di-tert-butylphosphino)ethane) with trifluoromethyl-p-tolyl ketone. Complex 226 was stable upon thermolysis at 70  C for 3 days.228 Treatment of [{(dtbpe)Ni}2(m-
2:
2-C6H6)] with trifluoromethyl thioesters CF3C(O)SR (R ¼ Et, Ph), produced
2-carbonyl complexes 227a–b (Fig. 44) in 71 and 70% yield, respectively. When the same Ni(0) source reacted with acetyl thioesters, Cacyl-S bond cleavage occurred followed by decarbonylation to generate methyl nickel(II) complexes, [(dtbpe)Ni(CH3) (SR)] (R ¼ Et, Ph) in 58–67% yield.228 Carbonylation was reverted in the presence of CO, and thioester complexes were able to perform stoichiometric cross-coupling with phenylboronic acid to yield sulfides (Ph-S-R). Side-on complex 229 (Fig. 45) was formed in 54% yield upon heating Ni(II) disulfide, [(dtbpe)Ni(SEt)2], at 60  C, with concomitant formation of EtSH.228 Reaction of [{(dtbpe)Ni}2(m-
2:
2-C6H6)] with ethyl trifluoroacetate yielded
2-carbonyl complex 228 (Fig. 44) in 59% yield, while phenyl esters were found to undergo Caryl-O bond cleavage forming aryl nickel(II) complexes, [(dtbpe)Ni(Ph)(OC(O)CF3)] or [(dtbpe)Ni(Ph)(OC(O)CH3)] in 59% and 58% yield. These performed transmetallation with phenyl boronic acids to yield biaryls.228 Solid-state structures of
2-carbonyl complexes 226, 227a and 228 exhibited elongated C-O bond lengths in the three cases. Reaction of thiobenzophenone derivatives with basic [Ni(CH3)2(PMe3)3] in diethyl ether at −70  C led to reductive elimination of ethane with formation of zerovalent
2-C ¼ S complexes 230a–b (Fig. 44) in 71% and 56% yield, respectively.229 The molecular structure of 230b showed planar coordination geometry with two P-ligand atoms and the
2-C ¼ S moiety. Elongation of such C ¼ S bond occurred due to side-on coordination. Addition of 3-phenyl-2-tosyl-1,2-oxaziridine to a C6D6 solution of [{(dtbpe)Ni}2(m-
2:
2-C6H6)] (dtbpe ¼ 1,2-bis(di-tertbutylphosphino)ethane) resulted in the formation of side-on Ni(0) complexes 231a and 231b (Fig. 45) in 54% and 13% yield as determined by NMR. Complex 231a was also prepared directly by addition of the imine TsN ¼ CHPh to [{(dtbpe)Ni}2 (m-
2:
2-C6H6)]. In the solid state, the C-N bond length in complex 231a was appreciably elongated.230

Zerovalent Nickel Organometallic Complexes

719

Fig. 45 Side-on nickel(0) complexes with aldehyde, thioaldehyde and aldimine-ligands. 119,224,225,228,230

8.10.7.2

Side-on borane-containing complexes

Complex [(TXPB)Ni] (232) (Fig. 46) was synthesized in 70% yield by the reaction of [Ni(COD)2] with the TXPB ligand in toluene at room temperature.231 In the solid state, complex 232 featured an
3-bound aryl borane with the Cipso–Cortho unit closer to the metal compared to boron, thus leading to a non-symmetrical
3-binding. Such binding was also characterized by approximate trigonal planarity at boron.
2-vinyl borane complex 233 (Fig. 46) was isolated in 89% yield after treatment of [Ni(COD)2] with (E)-PhHC ¼ CHB(C6F5)2 (VBPh) in the presence of PPh3 in benzene.132 NMR spectroscopy of 233 indicated a strong metal-carbon interaction and hindered rotation of the VBPh ligand. According to the molecular structure, the B-C distance in 233 was more similar to that found in free R2B¼ CR−2 or R2B¼ C¼ BR22− anions than in vinyl boranes, indicating alkyl/borataalkene-like coordination. DFT calculations suggested both donor and acceptor character of the VBPh ligand, and effective back-donation from the Ni-center into the VBPh ligand consistent with the short B-C distance experimentally observed. Complex 234 (Fig. 46) was isolated in 78% yield after treatment of [Ni(COD)2] with the ambiphilic bisphosphine-borane ligand, [Fe(
5-C5H4PPh2)(
5-C5H4PtBu{C6H4(BPh2)-o})] (FcPPB).147 The solid-state structure of 234 showed coordination of the FcPPB ligand to the Ni(0) center via both phosphine donors as well as an
3-B,C,C interaction with boron and the ipso- and ortho-carbon atoms of a B-phenyl group. The Ni center displayed a highly distorted square planar geometry, forming a plane together with both Cipso and Cortho, and the two P atoms; and showing a planar B atom out of the plane. NMR spectroscopy revealed Ni-borane- and cis-k2-P,P-coordination and indicated that the
3-B,C,C interaction was not maintained in solution. Reaction of [Ni(COD)2] with the triphosphine-analog of FcPPB, [Fe(
5-C5H4PPh2)(
5-C5H4PtBu{C6H4(PPh2)-o})] (FcPPP) in the presence of an atmosphere of N2 produced dinuclear Ni(0) compound, [{Ni(FcPPP)}2(m-N2)] (234), featuring a bridging N2 ligand and pseudo tetrahedral coordination geometry around the Ni(0) centers bound to one N atom and the three P donors of the FcPPP ligand.

Fig. 46 Side-on nickel(0)-complexes with borane-containing ligands.132,147,231

720

8.10.7.3

Zerovalent Nickel Organometallic Complexes

Side-on imine complexes

Complexes 235a–n were formed immediately in good yields upon reductive elimination of H2 after treatment of dimeric Ni(I) complex, [(dippe)Ni(H)]2, (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) with 2 equiv. of N-benzylidene benzylamine and its fluorinated derivatives (Scheme 16).232 The resulting compounds were isolated from toluene/benzene solutions at low temperature, or by vapor phase diffusion of THF/hexane and exhibited thermal stability between 40 and 150  C. All Ni(0) compounds, 235a–n, exhibited two types of phosphorus environments with scalar coupling constant values ranging 58.5–50.9 Hz, with the most electron poor complexes showing the smallest coupling constants. NMR analysis also suggested substantial decreasing of the sp2 character of the CH ¼ N bond in the coordinated aldimines due to back donation from the metal center. Molecular structures of [(dippe)Ni(
2-C,N-(Ph)HC ¼ NPh)] (225a), [(dippe)Ni{
2-C,N-(Ph)HC ¼ N(4-C6H4F)}] (235d), [(dippe) Ni{
2-C,N-(Ph)HC ¼ N(2,4-C6H3F2)}] (235f), [(dippe)Ni{
2-C,N-(Ph)HC ¼ N(2,4,6-C6H2F3)}] (235j), and [(dippe)Ni{
2-C,N(Ph)HC¼ N(2,3,5,6-C6HF4)}] (235m) revealed a distorted tetrahedral geometry around the Ni center and lengthening of the
2-coordinated imines due to side-on coordination and to Ni(0) back bonding into the C ¼ N p orbitals. Such lengthening increased with an increment in fluorination of the substituents, therefore in the p-accepting character of the imine ligands. In compounds 235d, 235f, and 235j, additional shortening of the C-F bonds was observed upon side-on coordination, whereas in complex 235m, the opposite was observed, with elongation of C-F bonds.

Scheme 16 Synthesis of side-on nickel(0) complexes with fluorinated imine ligands.232

Heterovalent bimetallic NiI-Ni0 complex [{[
-C5H4CH¼ N(C6F5)]Fe[
-C5H4PPh2]}2Ni2Cl] (236) (Fig. 47) was isolated in 57% from the reaction between the hybrid ligand [
-C5H4CH ¼ N(C6F5)]Fe[
-C5H4PPh2] and [Ni(COD)2] in the presence of AlEtCl2 or

Fig. 47 Dinuclear nickel complexes with imine-bridging ligands.83,123,144,233

Zerovalent Nickel Organometallic Complexes

721

AlCl3.123 Paramagnetic complex 236 was air- and moisture sensitive, and showed stability in the solid state and in solution under an inert atmosphere. Molecular structure of 236 displayed a dinickel core with one terminal chloride and two pC¼ N,s P-iminophosphines chelating the nickel atoms. The two metals were bridged by an m-s,
2-coordinated iminophosphine acting both as a side-on and an end-on ligand. Both Ni centers exhibited distorted-square-planar geometries without any Ni-Ni bonding. DFT calculations predicted favored formation of the NiINi0 dimer over their mononuclear Ni(I) and Ni(0) forms, and predicted destabilization upon either monoelectronic reduction or oxidation. Complex 236 showed catalytic activity toward the selective formation of C4-oligomers from ethylene (300 psi) at 30  C with TOF values of up to 90,500 h−1 and selectivity of 91%. The reaction of N-benzenesulfonyl benzaldimine (PhCH¼ NSO2Ph) with [Ni(COD)2] and PCy3 produced [(
2-PhCH¼ NSO2Ph)Ni(PCy3)2] (237) (Scheme 17) quantitatively. Solid-state structure of 237 featured side-on coordination of the C ¼ N bond in its E-geometry to the Ni(0) center. NMR analysis revealed significant upfield-shifting of C(sp2) and C(sp2)-H signals of the HC¼ N moiety, consistent with
2-coordination and strong back-donation from the Ni(0) to the imine ligand. Complex 237 reacted with diphenylacetylene to give five-membered aza-nickela(II)cycle 238 quantitatively. Further reactivity of such Ni(II) complex featured formation of lactam 238a in 78% yield, along with the co-production of Ni(0)-carbonyl species [Ni(CO)3(PCy3)]. Additionally, complex 3 performed alkyne insertion forming aza-nickela(II)cycles 239a–b, which upon reductive elimination, yielded 1,2-dihydropyridine derivatives.125

Scheme 17 Nickel(0) mediated oxidative coupling of imines and alkynes.125

Treatment of [Ni(COD)2] with Me2C(CH2N ¼ CHpy)2 (dmp(PI)2) in toluene afforded pseudo-square planar, diamagnetic II 144 This Ni(II) complex contained radical anion pyridine-imine (PI) units bound to Ni(II). [{dmp(PI)2− 2 }Ni ] in 81% yield. II Oxidation of [{dmp(PI)2− }Ni ] with AgOTf generated [{dmp(PI)2}Ni](OTf ), which upon recrystallization formed the asymmetric 2 dimer, [{dmp(PI)2}2NiIINi0](OTf )2 (240) (Fig. 47).144 In the solid state, dinuclear 240 consisted of pseudo-octahedral Ni(II) and pseudo-tetrahedral Ni(0), comprised of two
2-C ¼ N units and a k2-N,N-pyridine-imine (PI) moiety. The Ni(II) was coordinated by a PI, two pyridines, and the nitrogen-atoms of two C ¼ N imine bonds, the ones which bound to the neighboring Ni(0)-center, thus exhibiting substantial p-back bonding. Further SQUID analysis and cyclic voltammetry measurements indicated the presence of both, Ni(II) and Ni(0) oxidation states in complex 240. The reaction of N-benzylidene-P,P-diphenylphosphinic amide with 2-butyne, [Ni(COD)2], and PCy3, was completed in 24 h to afford aza-nickela(II)cycle 241 in 87% yield and side-on dimeric Ni(0)-complex 242 (Fig. 48) in 13% yield. Addition of excess of 2-butyne to complex 242 did not generate any product. In contrast, direct reaction of N-benzylidene-P,P-diphenylphosphinic amide with [Ni(COD)2] and PCy3 in benzene-d6 at room temperature was completed in 24 h to produce complex 242 in 97% yield.51 Complex 242 existed as a mixture of syn/anti isomers in solution. In the solid state, 242 featured
2-coordination of the C¼ N moiety in the imine ligand and coordination of one oxygen atom in the same ligand to one neighboring Ni(0) center forming the dimeric arrangement. Reaction of imine ligands (2-R2PC6H4)N ¼ CH(2-R0 2PC6H4) (R ¼ Ph, R0 ¼ Ph, oTol) with [Ni(COD)2] in the presence of PPh3 in THF afforded the diamagnetic Ni(0) complexes 243a–b (Fig. 49).83 Low temperature NMR showed signals for 243a indicating that the three phosphorus nuclei coupled to each other. In the case of 243b, broad signals in the corresponding NMR suggested fluxionality. Coordination of the imine moiety was demonstrated by the absence of signals both for distinctive imine-CH peak in the 1H NMR and the C¼ N band in IR analysis. 13C NMR for 243a indicated strong rehybridization toward sp3, consistent with p-backdonation to the antibonding p orbital of the imine C¼ N bond. Crystallographic analysis of 243a–b showed a distorted tetrahedral geometry around the nickel centers, bound to PPh3 and to ligands (2-Ph2PC6H4)N ¼ CH(2-Ph2PC6H4) and (2-Ph2PC6H4)N¼ CH(2-oTol2PC6H4) in a k4(P,P,C,N)-fashion with
2(C,N)-coordination of the imine moiety. Dimeric compound 245 (Fig. 47) was found to be an intermediate in the synthesis of 243a, formed after mixing [Ni(COD)2] and the imine ligand

722

Zerovalent Nickel Organometallic Complexes

Fig. 48 Synthesis of dimeric side-on nickel(0)-imine complex 242.51

Fig. 49 Side-on nickel(0) complexes with imine ligands.51,83,125

(2-Ph2PC6H4)N¼ CH(2-Ph2PC6H4), prior to addition of the ligand PPh3.83 X-ray crystallography of 245 featured each imine moiety acting as a bridge, m-
1(N)
2(C,N), binding side-on to one metal and end-on to the other, with the two P-donor sites of one ligand binding each to a different Ni-center. Reaction of [Ni(COD)2] with the bulkier ligand (2-oTol2PC6H4)N¼ CH(2-oTol2PC6H4) yielded compound 244 (Fig. 49) featuring a three-coordinated Ni-center with an uncoordinated arm of the phosphine-containing imine ligand.83 According to crystallography, nickel was bound to such ligand in a k3(P,C,N)-fashion with
2(C,N)-coordination of the imine moiety, and a similar degree of p-backbonding was observed upon comparison with 243a–b despite the lower coordination number in 244. Cyclic trinuclear a-diimine nickel(0) complex [{Ni(m-LMe-2,4)}3] (246) (LMe-2,4 ¼ [(2,4-Me2C6H3)NC(Me)]2) (Fig. 47) was synthesized in 65% yield by reduction of the trimerized trigonal Ni(II) precursor, [Ni3(m2-Br)3(m3-Br)2(LMe-2,4)3]∙ Br, in the presence of sodium metal in diethyl ether at room temperature.233 In complex 246, imines exhibited N,N-chelating mode and they also acted as bridging ligands through p-coordination the C¼ N bond to Ni. Such
2-C ¼ N bonds were significantly longer than those in the neutral ligand. Each Ni atom was three-coordinated showing trigonal geometry and the Ni-Ni distances indicated the lack of Ni-Ni bonding. In addition, both zerovalent character of the Ni-centers and p-back donation to the side-on C ¼ N moieties were also supported by DFT calculations. Complex 246 catalyzed the cyclotrimerization of alkynes, with a Ni loading of 9 mol%, to give substituted benzenes in good yield (up to 96%) and good regio-selectivity for the 1,3,5-isomers, which varied with the nature of the alkyne employed.

Zerovalent Nickel Organometallic Complexes

8.10.7.4

723

Side-on nitrile complexes

The compound [Ni(dippe)(
2-CH3CN–BEt3)] (247) (Scheme 18 and Fig. 50) (dippe ¼ 1,2-bis(diisopropylphosphino)ethane) was obtained from the reduction of [Ni(dippe)(CH2CN)(Cl)] with LiHBEt3. Below −40  C, also the Ni(II) hydride [Ni(dippe)(CH2CN) (H)] (247a) was observed in the same reaction mixture. Compound 247 was independently synthesized from [(dippe)Ni(H)]2 with acetonitrile in the presence of BEt3 in hexanes (Scheme 18).234 The corresponding 31P{1H} NMR spectrum showed two doublets, indicating two chemically inequivalent phosphines with a scalar coupling constant of 47 Hz. The structure of 247 was confirmed by X-ray structure determination. The C-N bond lengthened upon side-on coordination in 247, and the N-C-CH3 angle deviated from linearity to an angle of 137.9(2) . Complex 247 was thermally stable. After heating a solution of 247 at 100  C for 2 days, no C-CN bond cleavage was observed. In contrast, upon UV irradiation for 2 h, half of 247 was converted to the BEt3 adduct of the Ni(II) complex, [Ni(dippe)(CH3)(CN–BEt3)] (248). Complex 248 featured two doublets in the 31P{1H} NMR spectrum with a coupling constant of 17 Hz. Based on the observed Lewis acid inhibition of the thermal C-CN bond cleavage, a Lewis base, NEt3, was added to the LiHBEt3 reduction of [Ni(dippe)(CH2CN)(Cl)]. In this case, a larger amount of 247a relative to 247 was formed below −40  C. When a mixture of [(dippe)Ni(H)]2 and CH3CN was irradiated with UV light at −65  C, only the C-CN cleavage product [Ni(dippe)(CH3)(CN)] was observed, indicating that the C-CN bond activation pathway was kinetically more facile than the C-H bond activation pathway.234

Scheme 18 Synthetic pathways toward side-on nickel(0) nitrile complex 247.234

Fig. 50 Lewis acid-N-adducts of side-on nickel(0) nitrile complexes.124,234

Side-on nitrile complex 248, and
2-arene species 249a–c (Fig. 51) were key intermediates calculated by DFT in the C-CN bond activation of benzonitrile with zerovalent nickel.235 The reaction of [(dippe)Ni(H)]2 with benzonitrile in THF-d8 led to rapid formation [(dippe)Ni(
2-C,N-PhCN)], analogous to 248 (Fig. 51), which equilibrated with the C-CN oxidative addition product [(dippe)Ni(Ph)(CN)] at room temperature after several days.107 Low temperature NMR analysis of this reaction suggested intermediacy of fluxional Ni(0)-arene complexes, three of which were further calculated by DFT as analogous intermediates

724

Zerovalent Nickel Organometallic Complexes

Fig. 51 Calculated side-on nickel(0)-nitrile and p-arene intermediates occurring in C-CN activation.235

249a–c (Fig. 51).235 The key transition state for the C-CN cleavage was found to connect with the
2-arene complex 249a, which featured the cyano group attached to the p-coordinated C¼ C bond. In situ formation of side-on nitrile complexes of the type [(dippe)Ni(
2-C,N-RCN)] (dippe ¼ 1,2-bis(diisopropylphosphino) ethane) (R ¼ Ph, pyridyl, Me) from [(dippe)Ni(H)]2 was exploited in several catalytic systems performing Ni(0)-catalyzed hydration of nitriles to produce amides,236,237 Ni(0)-catalyzed addition of alcohols to cyanopyridines to yield carboximidates,238 and Ni(0)-catalyzed partial hydrogenation of nitriles to yield both secondary aldimines,239 and triaryl imidazoles.240 AlMe2Cl adduct of
2-nitrile complex 250 (Scheme 19 and Fig. 50), relevant to intramolecular aryl cyanation of alkenes catalyzed cooperatively by nickel and AlMe2Cl, was synthesized by the reaction between [Ni(COD)2], P(nBu)3, AlMe2Cl, and 2-(3-methyl-3-buten-1-yl)benzonitrile. No
2-nitrile complex was observed in the absence of AlMe2Cl, thus the Lewis acid promoted nitrile side-on coordination. Complex 250 performed oxidative addition of the aryl-CN bond at room temperature within 6 h yielding Ni(II) complex 251 (Scheme 19). Upon heating at 60  C for 46 h, complex 251 was further converted to Ni(0) species 252, which upon treatment with 2-(3-methyl-3-buten-1-yl)benzonitrile, regenerated complex 250 (Fig. 50).124

Scheme 19 Intramolecular aryl cyanation of alkenes.124

Nickel(0)-catalyzed alkyne phenylcyanation was investigated by means of DFT. Mechanistic studies using the catalyst Ni(PMe3)2 featured initial end-on coordination of PhCN, forming [Ni(PMe3)2(PhCN)], followed by thermodynamically favored isomerization to the side on Ni(0) complex [(PMe3)2Ni(
2-N,C-PhCN)] (253). Complex 253 performed C-CN oxidative addition of PhCN to the Ni(0) center forming Ni(II) complex [(PMe3)2Ni(Ph)(CN)] through intermediacy of the Ni(0)-
2-arene species, [(PMe3)2Ni(
2-C,C-PhCN)] (254). Further alkyne coordination and PMe3 dissociation formed the Ni(II) complex [Ni(CN)(Ph) (PMe3)(alkyne)], which underwent alkyne insertion and reductive elimination of the CN and vinyl groups producing cyano-olefin derivatives.127 Side-on nitrile Ni(0) complexes 255a–c, and Lewis acid-adducts 256a–c (Fig. 52) were calculated by DFT during theoretical investigation on transfer hydrocyanation of simple olefins with Ni(0) catalysts.241 Calculations showed that the mechanism consisted on oxidative addition of the nitrile, b-hydride elimination, ligand exchange, alkene insertion, and reductive elimination, during which Ni(0) intermediates 255a–c (Fig. 52) were predicted to act as intermediates. Computational studies also revealed that the effect of using a Lewis acid, Me2AlCl, was mainly reflected in the interaction with the N atom of the nitrile moiety, forming key

Zerovalent Nickel Organometallic Complexes

725

Fig. 52 Calculated side-on nickel(0) nitrile complexes and Lewis acid-adducts.241

intermediate 256a (Fig. 52), which weakened the C(sp3)-C(sp) s bond, thus lowering the barrier of the rate-determining oxidative addition step. Intermediates 256b–c (Fig. 52) featuring O-Al and N,O-Al adduct formation reacted through less energetically favored pathways.

8.10.7.5

CO2 and CS2 complexes

Reaction of CS2 with [{(dtbpe)Ni}2(
2,m-C6H6)] (dtbpe ¼ 1,2-bis(di-tert-butylphosphino)ethane) in toluene produced the side-on carbon disulfide complex [(dtbpe)Ni(
2-CS2)] (257) (Fig. 53) in 88% yield.242 Molecular structure of 257 featured square-planar geometry at nickel. Uncoordinated C ¼ S bond remained essentially as a double bond, while lengthening of the
2-C ¼ S bond indicated a large degree of back bonding from nickel into the corresponding p orbital. Significant differences in the character of the two C-S bonds in
2-CS2 were also observed by IR analysis. Reaction of complex 257 with the Ni(I) complex [(dtbpe)Ni(OTf )], yielded the diamagnetic, trimetallic cluster [{(dtbpe)Ni(k1,
2-CS2)}2(dtbpe)Ni](OTf )2 (258a) (Fig. 53) in 60% yield.242 The solid-state structure of 258a revealed two CS2 ligands
2-C,S bound to two (dtbpe)Ni moieties, and k1-S bound to the third (dtbpe)Ni unit in the dicationic complex. The geometries of all three nickel atoms were approximately square planar. NMR analysis of 258a indicated that the same structure remained in solution. Hexafluorophosphate analog of the same cationic complex, 258b (Fig. 53) was obtained in 95% yield, by oxidation of complex 257 with ferrocenium hexafluorophosphate, through intermediacy of Ni(I) species [(dtbpe)Ni(CS2)+], which was unstable toward disproportionation.242 Reaction of 1 equiv. of CO2 with a cold toluene

Fig. 53 CO2 and CS2 nickel(0) complexes.89,94,242

726

Zerovalent Nickel Organometallic Complexes

solution of [{(dtbpe)Ni}2(
2,m-C6H6)] provided complex [(dtbpe)Ni(
2-CO2)] (259) (Fig. 53) in 91% yield.242 In the solid state, complex 259 featured strong back bonding and a nickel center slightly distorted from planarity. Thermolysis of complex 259 in benzene at 80  C resulted in the reduction of CO2 to CO, forming [(dtbpe)Ni(CO)2] along with mono oxidation of the ancillary ligand to give O¼ P(tBu)2CH2CH2P(tBu)2. Stirring a solution of [{(iPr3P)2Ni}2N2] in the presence of CO2 at −80  C resulted in the formation of side-on complex 260 (Fig. 53).94 IR spectrum for 260 featured a strong nCO band at 1721 cm−1. VT NMR experiments were consistent with a fluxional complex with decoalescence occurring below 243 K, with an estimated free activation energy of 9.5 kcal mol−1 for the dynamic process. In the solid state, complex 260 featured a typical distance for a C-O double bond for the uncoordinated C ¼ O moiety of CO2, whereas the length for the side-on coordinated C ¼ O bond indicated a large degree of back bonding from the nickel center to the corresponding p orbital. Thermal deoxygenation of 260 afforded small amounts of complex [(PiPr3)2Ni(CO)2] and iPr3P ¼ O. Reaction between [{(iPr3P)2Ni}2N2] and CS2 produced dinuclear complex 261 (Fig. 53), as revealed by NMR spectroscopy and crystallography.94 In the solid state, complex 261 showed two (iPr3P)2Ni fragments related by a crystallographic inversion center held together by two CS2 molecules forming a six-membered ring. Each CS2 featured side-on coordination through a C ¼ S moiety to one nickel atom and a s-bond to the other nickel center through the second S atom, and the chemically different C-S bonds featured similar lengths, suggesting extensive p-delocalization. The coordination environment around each nickel center was nearly planar. The IR spectrum of 261 showed stretching bands at 1124 and 662 cm−1. Ni-CO2 adduct, 262 (Fig. 53), was observed to occur as an intermediate during the quantitative conversion of the dinuclear Ni(0) species, [{(2-iPr2PC6H4)2P(OMe)}2Ni2(m-N2)], into the Ni(II)-carbonate, [{(2-iPr2PC6H4)2P}Ni(OCO2Me)], in the presence of CO2.89 Complex 262 (Fig. 53) was not stable in acetonitrile/toluene at room temperature; consequently it was converted to [{(2-iPr2PC6H4)2P}Ni(OCO2Me)] and the reaction reached completion within 5 h. Recent DFT calculations for a whole series of side-on Ni(0)-CO2 complexes with both monodentate phsophines and bisphosphines of the type, [(PR3)2Ni(
2-C,O-CO2)] (R ¼ Me, Et, Ph, OMe, OPh, Cl, F) (263a–g), [(PR0 2R00 )2Ni(
2-C,O-CO2)] [R0 (R00 ) ¼ Et(Ph), Me(Ph), Cl(OEt), CF3(F2)] (264a–d), and [(P-P)Ni(
2-C,O-CO2)] (P-P ¼ dppm, dppe, dppp, dpm, dpe, dpp, dpb, dmpe, dcpe, dtbpe, dtfmpe, CHIRAPHOS, BDPP, DIOP, BINAP, BIPHEP, SEGPHOS, xantphos, dppf ) (265a–s), has shown that p-back donation from the Ni(0)-center to
2-coordinated CO2 is the dominant part of the orbital interaction component. For complexes [(PR3)2Ni(
2-CO2)] (263a–g) and [(PR0 2R00 )2Ni(
2-CO2)] (264a–d), linear correlation was found between the asymmetric stretching frequency of the bound CO2 ligand and the Tolman’s electronic parameters. For Ni(0)-diphosphine complexes [(P-P)Ni(
2-C, O-CO2)] (265a–s), calculations showed that the size of the chelate ring had a marginal effect in the coordination properties of CO2. Overall, calculations revealed that the coordination strength, as well as the C ¼ O bond order in
2-coordinated CO2 can be tuned by varying the substituents on phosphorus; for instance, in the presence of electron withdrawing groups the C ¼ O bond remained stronger and the Ni-C interaction was weaker. In contrast, for more basic phosphines the Ni-C bond order was higher and the coordinated CO2 exhibited a weaker C¼ O bond.243

Acknowledgment We thank DGAPA-UNAM and CONACYT for supporting our research in the area.

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V.; Pape, T.; Hepp, A.; Hahn, F. E. Homoleptic complexes of bisstannylenes with nickel(0): Synthesis, X-ray diffraction studies, and 119Sn NMR investigations. Organometallics 2008, 27 (12), 2756–2760. 210. Schwarzer, M. C.; Konno, R.; Hojo, T.; Ohtsuki, A.; Nakamura, K.; Yasutome, A.; Takahashi, H.; Shimasaki, T.; Tobisu, M.; Chatani, N.; Mori, S. Combined theoretical and experimental studies of nickel-catalyzed cross-coupling of methoxyarenes with arylboronic esters via C-O bond cleavage. J. Am. Chem. Soc. 2017, 139 (30), 10347–10358. 211. Somerville, R. J.; Hale, L. V. A.; Gomez-Bengoa, E.; Bures, J.; Martin, R. Intermediacy of Ni-Ni species in sp2 C-O bond cleavage of aryl esters: Relevance in catalytic C-Si bond formation. J. Am. Chem. Soc. 2018, 140 (28), 8771–8780. 212. Borrome, M.; Gronert, S. Gas-phase dehydrogenation of alkanes: C-H activation by a graphene-supported nickel single-atom catalyst model. Angew. Chem. Int. Ed. 2019, 58 (42), 14906–14910. 213. Cooper, A. K.; Burton, P. M.; Nelson, D. J. Nickel versus palladium in cross-coupling catalysis: On the role of substrate coordination to zerovalent metal complexes. Synthesis 2020, 52 (4), 565–573. 214. Entz, E. D.; Russell, J. E. A.; Hooker, L. V.; Neufeldt, S. R. Small phosphine ligands enable selective oxidative addition of Ar-O over Ar-Cl bonds at nickel(0). J. Am. Chem. Soc. 2020, 142 (36), 15454–15463. 215. Nova, A.; Reinhold, M.; Perutz, R. N.; MacGregor, S. A.; McGrady, J. E. Selective activation of the ortho C-F bond in pentafluoropyridine by zerovalent nickel: Reaction via a metallophosphorane intermediate stabilized by neighboring group assistance from the pyridyl nitrogen. Organometallics 2010, 29 (7), 1824–1831. 216. Sontag, S. K.; Bilbrey, J. A.; Huddleston, N. E.; Sheppard, G. R.; Allen, W. D.; Locklin, J. p-Complexation in nickel-catalyzed cross-coupling reactions. J. Org. Chem. 2014, 79 (4), 1836–1841. 217. Bilbrey, J. A.; Bootsma, A. N.; Bartlett, M. A.; Locklin, J.; Wheeler, S. E.; Allen, W. D. Ring-walking of zerovalent nickel on aryl halides. J. Chem. Theory Comput. 2017, 13 (4), 1706–1711.

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(solvent) obtained by reduction. Eur. J. Inorg. Chem. 2011, 2011 (6), 816–820. 222. Konarev, D. V.; Khasanov, S. S.; Nakano, Y.; Otsuka, A.; Yamochi, H.; Saito, G.; Lyubovskaya, R. N. Linear coordination fullerene C60 polymer [{Ni(Me3P)2}(m-Z2,Z2-C60)]1 bridged by zerovalent nickel atoms. Inorg. Chem. 2014, 53 (22), 11960–11965. 223. Keen, A. L.; Johnson, S. A. Nickel(0)-catalyzed isomerization of an aryne complex: Formation of a dinuclear Ni(I) complex via C-H rather than C-F bond activation. J. Am. Chem. Soc. 2006, 128 (6), 1806–1807. 224. Ogoshi, S.; Kamada, H.; Kurosawa, H. Reaction of (Z2-arylaldehyde)nickel(0) complexes with Me3SiX (X ¼ OTf, Cl). Application to catalytic reductive homocoupling reaction of arylaldehyde. Tetrahedron 2006, 62 (32), 7583–7588. 225. Ohashi, M.; Saijo, H.; Arai, T.; Ogoshi, S. Nickel(0)-catalyzed formation of oxaaluminacyclopentenes via an oxanickelacyclopentene key intermediate: Me2AlOTf-assisted oxidative cyclization of an aldehyde and an alkyne with nickel(0). Organometallics 2010, 29 (23), 6534–6540. 226. Meng, Q.; Li, M. Nickel/zinc-catalyzed decarbonylative addition of anhydrides to alkynes: A DFT study. J. Mol. Model. 2013, 19 (10), 4545–4554. 227. Flores-Gaspar, A.; Pinedo-González, P.; Crestani, M. G.; Muñoz-Hernández, M.; Morales-Morales, D.; Warsop, B. A.; Jones, W. D.; García, J. J. Selective hydrogenation of the CO bond of ketones using Ni(0) complexes with a chelating bisphosphine. J. Mol. Catal. A Chem. 2009, 309 (1), 1–11. 228. Desnoyer, A. N.; Friese, F. W.; Chiu, W.; Drover, M. W.; Patrick, B. O.; Love, J. A. Exploring regioselective bond cleavage and cross-coupling reactions using a low-valent nickel complex. Chem. Eur. J. 2016, 22 (12), 4070–4077. 229. Beck, R.; Sun, H.; Li, X.; Camadanli, S.; Klein, H.-F. Cyclometalation of thiobenzophenones with mononuclear methyliron and -cobalt complexes. Eur. J. Inorg. Chem. 2008, 21, 3253–3257. 230. Desnoyer, A. N.; Chiu, W.; Cheung, C.; Patrick, B. O.; Love, J. A. Oxaziridine cleavage with a low-valent nickel complex: Competing C-O and C-N fragmentation from oxazanickela(II)cyclobutanes. Chem. Commun. (Cambridge, U. K.) 2017, 53 (92), 12442–12445. 231. Emslie, D. J. H.; Harrington, L. E.; Jenkins, H. A.; Robertson, C. M.; Britten, J. F. Group 10 transition-metal complexes of an ambiphilic PSB-ligand: Investigations into Z3 (BCC)-triarylborane coordination. Organometallics 2008, 27 (20), 5317–5325. 232. Iglesias, A. L.; Munoz-Hernandez, M.; Garcia, J. J. Fluoro aromatic imine nickel(0) complexes: Synthesis and structural studies. J. Organomet. Chem. 2007, 692 (16), 3498–3507. 233. Shen, L.; Zhao, Y.; Luo, Q.; Li, Q.-S.; Liu, B.; Redshaw, C.; Wu, B.; Yang, X.-J. Cyclotrimerization of alkynes catalyzed by a self-supported cyclic tri-nuclear nickel(0) complex with a-diimine ligands. Dalton Trans. 2019, 48 (14), 4643–4649. 234. Atesin, T. A.; Li, T.; Lachaize, S.; Brennessel, W. W.; Garcia, J. J.; Jones, W. D. Experimental and theoretical examination of C-CN and C-H bond activations of acetonitrile using zerovalent nickel. J. Am. Chem. Soc. 2007, 129 (24), 7562–7569. 235. Atesin, T. A.; Li, T.; Lachaize, S.; Garcia, J. J.; Jones, W. D. Experimental and theoretical examination of C-CN bond activation of benzonitrile using zerovalent nickel. Organometallics 2008, 27 (15), 3811–3817. 236. Crestani, M. G.; Arevalo, A.; Garcia, J. J. Catalytic hydration of benzonitrile and acetonitrile using nickel(0). Adv. Synth. Catal. 2006, 348 (6), 732–742. 237. Crisostomo, C.; Crestani, M. G.; Garcia, J. J. The catalytic hydration of 1,2-, 1,3- and 1,4-dicyanobenzenes using nickel(0) catalysts. J. Mol. Catal. A: Chem. 2007, 266 (1–2), 139–148. 238. Garduño, J. A.; Garcı´a, J. J. Synthesis of amidines and benzoxazoles from activated nitriles with Ni(0) catalysts. ACS Catal. 2015, 5 (6), 3470–3477. 239. Zerecero-Silva, P.; Jimenez-Solar, I.; Crestani, M. G.; Arevalo, A.; Barrios-Francisco, R.; Garcia, J. J. Catalytic hydrogenation of aromatic nitriles and dinitriles with nickel compounds. Appl. Catal. A 2009, 363 (1–2), 230–234. 240. García, J. J.; Zerecero-Silva, P.; Reyes-Rios, G.; Crestani, M. G.; Arévalo, A.; Barrios-Francisco, R. One-pot synthesis of imidazoles from aromatic nitriles with nickel catalysts. Chem. Commun. 2011, 47 (36), 10121–10123. 241. Ni, S.-F.; Yang, T.-L.; Dang, L. Transfer hydrocyanation by nickel(0)/Lewis acid cooperative catalysis, mechanism investigation, and computational prediction of shuttle catalysts. Organometallics 2017, 36 (15), 2746–2754. 242. Anderson, J. S.; Iluc, V. M.; Hillhouse, G. L. Reactions of CO2 and CS2 with 1,2-Bis(di-tert-butylphosphino)ethane complexes of nickel(0) and nickel(I). Inorg. Chem. 2010, 49 (21), 10203–10207. 243. Kegl, T. R.; Carrilho, R. M. B.; Kegl, T. Theoretical insights into the electronic structure of nickel(0)-diphosphine-carbon dioxide complexes. J. Organomet. Chem. 2020, 924, 121462.

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K Matsubara, Fukuoka University, Fukuoka, Japan © 2022 Elsevier Ltd. All rights reserved.

8.11.1 8.11.2 8.11.2.1 8.11.2.2 8.11.2.2.1 8.11.2.2.2 8.11.2.2.3 8.11.2.3 8.11.2.3.1 8.11.2.4 8.11.2.5 8.11.2.6 8.11.2.6.1 8.11.2.6.2 8.11.2.7 8.11.2.8 8.11.2.9 8.11.3 8.11.3.1 8.11.3.2 8.11.3.2.1 8.11.3.2.2 8.11.3.3 8.11.3.3.1 8.11.3.4 8.11.4 8.11.4.1 8.11.4.2 8.11.4.3 8.11.5 References

8.11.1

Introduction Monovalent nickel complexes Mononuclear nickel(I) carbonyl, isocyanide and related complexes Mononuclear nickel(I)-carbon s-bonded complexes Mononuclear complexes with s-alkyl and aryl ligands Mononuclear complexes with cyclopentadienyl and related ligands Mononuclear complexes with p-allyl ligands Mononuclear nickel(I)-carbon p-bonded complexes Mononuclear complexes with p-olefin and arene ligands Mononuclear N-heterocyclic carbene complexes Dinuclear nickel(I) carbonyl, isocyanide and related complexes Dinuclear nickel(I)-carbon s-bonded complexes Dinuclear complexes with bridging s-aryl ligands Dinuclear complexes with bridging Cp and related ligands Dinuclear nickel(I)-carbon p-bonded complexes Dinuclear carbene complexes Dinuclear N-heterocyclic carbene complexes Monovalent palladium complexes Carbonyl, isocyanide and related complexes Palladium(I)-carbon s-bonded complexes Palladium(I) complexes with allyl ligands Palladium(I) complexes with cyclopentadienyl ligands Palladium(I)-carbon p-bonded complexes Palladium(I) complexes with arene ligands N-heterocyclic carbene complexes Monovalent platinum complexes Carbonyl and related ligands Allyl ligands Alkene ligands Conclusion

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Introduction

COMC-III has not specifically taken up and summarized monovalent complexes of Group 10 metals. Over the past 70 years, many stable Group 10 monovalent organometallic complexes have been synthesized and their structures have been determined, especially centering on nickel and palladium. Therefore, the chemistry of palladium(I) and nickel(I) is occasionally appeared and included in the latest version of COMC-III (2006).1–6 During the period 2006–20, significant progress has been made in the organometallic chemistry of nickel(I), and palladium(I) complexes have sometimes emerged as precursors for catalysts, catalytic intermediates, and deactivated products, in various catalytic organic transformations.7–9 Detailed discussions were made on the difference in synthesis method and structure due to the difference in oxidation number. The widespread use of theoretical calculation methods has aided the detailed understanding of the structure and reactivity of Group 10 organometallic complexes. Recent mechanistic studies supported by the theoretical studies have implicated significant positions of monovalent metal species in catalysis. As a result, it has become necessary to independently summarize the properties that characterize monovalent complexes of Group 10 metals. This chapter covers all new findings reported after COMC-III (2006) on the synthesis and structure of monovalent organometallic complexes of nickel, palladium, and platinum and the roles and properties of monovalent complexes in catalytic reactions. Although the fields of application to catalytic reactions are diverse, detail discussion are made in the range of applications of monovalent organometallic complexes and their roles in various catalytic processes. Recent reviews will also help understand the chemistry of nickel(I)10–14 and palladium(I).15–20

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8.11.2

Monovalent Group 10 Organometallic Complexes

Monovalent nickel complexes

Compared to the analogous palladium chemistry, thermodynamic stability of paramagnetic, “monomeric” nickel(I) complexes are comparable to that of diamagnetic “dimeric” nickel(I) complexes. Most of these complexes are catalytically relevant, and their catalytic applications have been developed in the chemistries of mononuclear and dinuclear nickel(I) complexes, during the period 2006–20.10–14 Rare example of development of functional materials is also known. In the chemistry of dinuclear nickel(I) complexes has been initially promoted by the great interest in unusual interactions of conjugated p-ligands with the dinickel framework. Preparative methods to access the dinickel(I) complexes are quite similar to those of the palladium complexes, as might be expected. Dinickel(I) complexes can be prepared by simply mixing unsaturated zerovalent and divalent nickel complexes, by 1e-oxidation of nickel(0) complexes, and by 1e-reduction of nickel(II) complexes. Additionally, in these processes, bridging ligands and/or a coordinatively unsaturated site in one of the metal centers lead to the facile formation of the dimeric structures, similar to procedures using bridging aryl, allyl, and the related groups in palladium chemistry.

8.11.2.1

Mononuclear nickel(I) carbonyl, isocyanide and related complexes

Carbonyl complexes represent a significant series of compounds to evaluate the electronic properties of the metal centers, because carbonyl ligand is a strong p-acceptor and easy to measure the CO stretching band by IR spectroscopy. Furthermore, understanding nickel(I) carbonyl species is important, because it is biologically relevant to CO dehydrogenase/acetyl-CoA synthase (CODH/ACS), a bifunctional enzyme which enables archaea and bacteria to grow autotrophically on CO and hydrogen/carbon dioxide using the Wood −Ljungdahl pathway.21–23 The active site of ACS contains a binuclear “proximal” and “distal” Ni centers, and the reaction of ACS with CO occurs at the cysteine-surrounded “proximal” Ni center to produce a single nickel(I)-carbonyl species.

季 A series of nickel(I) b-diketiminate complexes, where the b-diketiminate has tert-butyl substituents at the C]N carbon atoms, have been prepared. On exposure of a hexane solution of the dinuclear nickel(I) dinitrogen complex [Ni(tBu-diketiminate)]2(N2) (1) (tBu-diketiminate ¼ HC(C(tBu)N(2,6-diisopropylphenyl)2)) to a carbon monoxide atmosphere, an immediate color change occurs to afford a three-coordinate nickel(I) carbonyl complex Ni(tBu-diketiminate)(CO) (2) (Scheme 1).24 The crystal structure of the carbonyl complex is similar, T-shape planar geometry, to the analogous methyl-substituted b-diketiminate complex, as Holland et al. have reported in 2005.25 The CO stretching band in the IR spectrum (2020 cm−1) is approximately the same as the methyl analogue (2022 cm−1). Thereafter, they have reported that the same carbonyl complex is also formed by the reaction a dianionic dinitrogen complex K2[Ni(tBu-diketiminate)]2(N2)2− (3) with CO2 at room temperature, along with a formation of the hexanuclear nickel(II) carbonate compound K6[Ni(tBu-diketiminate)CO3]6.26 The dinitrogen complex effectively mediates the reductive disproportionation of CO2 to form the CO and CO2− 3 containing the nickel(I) complexes.

Monovalent Group 10 Organometallic Complexes

735

Scheme 1 Synthetic routes to (b-diketiminato)nickel(I) carbonyl complex 2.

As above noted, the three-coordinate nickel(I) complexes have planar, T- or Y-shape structures; however, four-coordinate nickel(I) complexes might not be similar. According to the study by Caulton et al., reduction of a pincer-type nickel(II) chloride complex [Ni(PNP)Cl] (PNP ¼ (tBu2PCH2SiMe2)2N) with magnesium affords three-coordinate T-shape nickel(I) complex [Ni(PNP)] (4).27,28 Interestingly, CO reversibly binds to the nickel center to give a CO adduct [Ni(PNP)(CO)] (5) (Eq. 1), whose nCO value is 1940 cm−1, indicating moderate back-donation from nickel(I). Removal of volatiles (including dissolved CO) from a solution of 5 results in complete reformation of [Ni(PNP)]. DFT calculation revealed that the reaction energy for binding CO to 4 is only −8.1 kcal mol−1, fully consistent with a DG of binding of near zero, in agreement with experiment. X-ray crystallography demonstrated that the CO complex does not have a planar skeleton around the Ni center but has a flattened tetrahedral geometry (PdNidP ¼ 149.59(4) ). Consistent with its structure, the EPR spectrum of 4 shows three g values at 77 K with no ligand hyperfine structure, while 5 shows coupling (triplet structure) at low temperature, due to two equivalent P. Theoretical studies demonstrated that there is negligible P character in the SOMO of 4 (nickel spin density ¼ 0.93), while the CO complex has contributions which give a spin density at P totaling 0.11 (at Ni, 0.78). Both have amide N participation (0.05) in the SOMO, and the SOMO of 4 is a mainly Ni orbital directed into the open site trans to N. This forces CO binding away from that trans site.

ð1Þ

The similar electronic structures of four-coordinate nickel(I) carbonyl complex are independently observed in the literature. An o-arylene bridged PNP ligand (PNP ¼ (2-PiPr2-4-Me-C6H3)2N) stabilizes nickel(I) carbonyl complex Ni(PNP)CO (7) (Eq. 2).29 This nickel(I) carbonyl 7 is obtained by 1e-reduction of cationic nickel(II) carbonyl [Ni(PNP)CO]+ (6) with sodium naphthalenide. The IR resonance of the CO stretching of 7 appears at 1927 cm−1, which is significantly lower (144 cm−1) than that of 6. The CdO bond distance in the crystal structure of 7 also increases from 1.133(2) (a) to 1.149(2) A˚ (7) because of p-back donation, but the NidC bond in nickel(I) complex 7 is slightly longer 1.776(2) than that of 6, 1.746(2) A˚ . This weak NidC s-bonding may be ascribed by the unpaired electron in the metal-orbitals with antibonding character; however, CO elimination does not occur under vacuum, in contrast to the Caulton’s complex 5.27 The geometry around the nickel center is also changed: while the nickel(II) complex 6 is nearly planar, the nickel(I) complex 7 has a pyramidal geometry with the metal ion out of the ligand plane.

736

Monovalent Group 10 Organometallic Complexes

+ PiPr2 N Ni

NaC10 H8

CO

PiPr2 Square Planar 6

PiPr2 N Ni

THF

CO PiPr2

ð2Þ

Pyramidal 7

A 9,10-dihydroacridine-based acriPNP tridentate ligand (acriPNP, 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin10-ide) can also be introduced to the analogous T-shape nickel(I) complex Ni(acriPNP) (8), which affords a nickel(I) carbonyl complex Ni(acriPNP)(CO) (9) in the reaction with CO (Scheme 2).30 The electronic nature of the CO complex is quite resemble to the PNP Ni(I)dCO complex 7 without bridging methylene group between the aromatic rings, and the optimized SOMO-orbital energies of three complexes, the T-shape three-coordinate complex 8, THF-coordinated complex 10 (not detected), and the CO complex 9, have been calculated (Scheme 2). The DFT calculations with Mulliken population analysis carried out on 8 suggests the approximately 73% of the unpaired spin is located on nickel. Thus, a T-shaped nickel(I) center has a half-filled dx2-y2 orbital as the singly occupied molecular orbital (SOMO). This makes the addition of a fourth ligand energetically difficult due to the s-antibonding character of the SOMO. Thus, the SOMO energy of the THF complex 10 becomes higher than that of 8. Conversely, p-acceptor ligand can coordinate to produce stable four-coordinate complex due to strong p-back-donation. In the pyramidal geometry of the CO complex 9, the MdL antibonding interaction in the SOMO is reduced.

Scheme 2 Reaction of dihydroacridine-based PNP nickel(I) complex 8 with CO and THF. The reaction with THF does not afford any adduct like 10.

季 Significantly, Lee’s nickel(I) carbonyl complex [Ni(PNP)CO] (7) react with MeI to form a nickel(II) acyl product [Ni(PNP) COCH3] (11) (37% conv.) as a mixture containing nickel(II) iodide [Ni(PNP)I] (12) (50%) and nickel(II) methyl [Ni(PNP)CH3] (13) (13%) (Scheme 3).29 In ACS (acetyl-CoA synthase) chemistry, a methyl cation is transferred from methylcobalamine to a carbonyl ligand on the proximal nickel(I) center, resulting in the CdC bond formation to form nickel(II) acyl species.21–23 The cationic nickel(II) carbonyl does not react with MeI, whereas an anionic nickel(0) carbonyl K+[Ni(PNP)CO]−, which is synthesized by reduction of the corresponding nickel(I) complex with KC8, reacts with MeI to form nickel(II) methyl 13. They proposed that a

Monovalent Group 10 Organometallic Complexes

737

Scheme 3 Addition of iodomethane to nickel(I) carbonyl complex 7.

radical process in the formation of the acyl complex 11 is likely to occur: the reaction of nickel(I) species with MeI proceeds to produce a nickel(II)-iodide 12 and a methyl radical, which can recombine with 7 to form a five-coordinate species [Ni(PNP)(CO) (CH3)]. If the nickel center is sterically hindered, insertion proceeds to form the acyl product 11. If not, CO elimination dominantly occurs to form the methyl complex 13. NNN pincer-type nickel(I) complexes Ni(iso-PyrrMeBox) (14) (iso-PyrrMeBox ¼ bis(oxazolinylmethylidene)pyrrolidinido) with methyl, isopropyl and tert-butyl wing-tip substituents of the oxazoline moieties are synthesized and characterized.31 The isopropyl-substituted complex reversibly reacts with CO resulting in a CO-pressure dependent equilibrium between the three-coordinate complex and the CO adduct Ni(iso-PyrrMeBox)(CO) (15) (Eq. 3). The characteristic band of the CO vibration of the CO complex is found at 1955 cm−1 in the IR spectrum. Although there is no x-ray diffraction data, DFT calculations revealed the similar SOMO distribution and spin density to the Caulton’s PNP nickel(I) carbonyl complex 5.

N O

N

N

CO I

Ni N

O

O - CO

i

i

Pr

Pr

14

N

NiI N

O

ð3Þ

C i Pr O i Pr 15

Reactions of CO and CNR (R ¼ 2,6-xylyl) with the IPr-ligated nickel(I) N-isopropyl-pivaloyl amidate complex Ni(IPr)(k3-N, bis(H2CMe)-N(iPr)C(O)tBu) (16) reveal that an agostic interaction with the two CdH hydrogens of the tert-butyl group stabilizes the two-coordinate complex.32 On exposure of solution of the complex to excess CO gas, a quantitative disproportionation reaction proceeds within minutes to form a mixture of nickel(0) tricarbonyl Ni(IPr)(CO)3 (17) and five-coordinate nickel(II) bis(k2-N,O-amidate) complex Ni(IPr)(k2-N,O-N(iPr)CO(tBu))2 (18) (Scheme 4A). When crystalline nickel(I) amidate complex is subjected to CO gas over a period of 3 days, the color of the crystals turns from yellow to bright green, which is caused by formation of an intermediary mono-carbonyl nickel(I) complex Ni(IPr)(amidate)(CO) (19) in the solid state. The nickel(I) CO complex 19 is robust in the solid state but eliminate CO in solution under vacuum to readily revert to the starting nickel(I) amidate 16. Though the crystal studies are unsuccessful, the CO stretching bands of 19 (nCO ¼ 1997, 1992, 1978, 1970 cm−1, derived from geometric isomers) appeared in IR spectrum. When isocyanide (2 equiv) is added to 16 at 25  C, a three-coordinate nickel(I) isocyanide complex Ni(IPr)(amidate)(CNR) (20) (R ¼ 2,6-xylyl) is afforded in low yields.33 The reactions with higher loading of isocyanide (>4 equiv) for more hours (2 weeks) or at elevated temperature (70  C for 4 h) result in disproportionation to produce a nickel(0) tri-isocyanide Ni(IPr)(CNR)3 and 18 (Scheme 4B).32 The disproportionation reaction rate is depend on the stability of the nickel(0) species and the reaction mechanisms of the reactions of CO, CNR, and alkenes are similar to the above studies.

738

Monovalent Group 10 Organometallic Complexes

Scheme 4 Reactions of IPr-ligated amidato nickel(I) complex 16 with (A) CO and (B) 2,6-xylyl isocyanide.

Nickel(I) bis-isocyanide complexes of nickel(I) b-Me-ketiminate, Ni(Me-ketiminato)(CNR)2 (R ¼ 2,6-xylyl (22a) and tBu (22b)) (Me-ketiminato ¼ HC(C(Me)N(mesityl)2)) serve as intermediates in catalytic nitrene transfer reaction with aryl and alkyl azide for carbodiimide formation.34 The complexes are produced from the reaction of the nickel(I) catalyst precursor, Ni(Meketiminato)(2-picoline) (21) with isocyanide (2 equiv) (Scheme 5). In solutions, these complexes release isocyanide partially to form mixtures of mono- and bis(isocyanide) complexes, which are detected in the EPR glass spectra, and the NMR spectra (showing the presence of free isocyanide). The X-ray structure of [Ni(Me-ketiminato)(CNAr)2] (Ar ¼ 2,6-xylyl) shows a distorted tetrahedral

N NiI N

O C C O

N

CO

N

N

Et2O

N

NiI

NiI

CNR

N

Et2O

N

NiI

N

R = 2,6-xylyl t Bu 21

Scheme 5 Reaction of b-ketiminato nickel(I) picoline complex 21 with carbonyl and 2,6-xylyl isocyanide.

C C

N

N

R

R

22a (R = 2,6-xylyl) 22b (R = tBu)

Monovalent Group 10 Organometallic Complexes

739

coordination at the Ni center. While the nitrene transfer reaction with isocyanide using the nickel(I) b-Me-ketiminato complex is performed efficiently, the similar reaction with CO to form isocyanates RNCO poorly occur. They described that is due to the poor reactivity of a rather stable dinuclear nickel carbonyl complex, which can be obtained by the exposure of the solution of the nickel(I) 2-picoline complex to a CO atmosphere (see Section 8.11.2.5.1).

8.11.2.2

Mononuclear nickel(I)-carbon s-bonded complexes

The chemistry of mononuclear organonickel(I) complexes, in which hydrocarbon is directly bound to nickel(I) center, is new, compared with the dinuclear organonickel(I) and organopalladium(I) chemistry. The first s-bonded organonickel(I) complex was reported in 2000 by Smith et al.35 Hillhouse et al. synthesized stable three-coordinate nickel(I) alkyl complexes by transmetallation of bulky bisphosphine-ligated nickel(I) chloride dimer with alkyllithium in 2004.36 Although the 15e-alkyl complex has no additional interaction between nickel and the alkyl CdH bonds in the crystal structure, the nickel(II) complex after 1e-oxidation with Cp2Fe+ contains an agostic interaction of CdH bond with nickel(II). That demonstrates specific stability of planar, threecoordinate geometry in nickel(I) complexes. Vicic et al. shortly reported that addition of tridentate terpyridine to [Ni(TMEDA) (CH3)2] (TMEDA ¼ N,N,N0 ,N0 ,-tetramethylethylene-diamine) afforded the nickel(I)-methyl complex [Ni(tpy)(CH3)], while addition of bidentate 2,20 -bipyridine yielded nickel(II)-dimethyl complex [Ni(bipy)(CH3)2].37,38 Thereafter, along with the development of catalytic processes involving nickel(I) complexes, the studies of alkyl and aryl nickel(I) complexes as intermediary compounds have increasingly reported to elucidate the reaction mechanism. Me Me Si SiMe N Ni

t

Bu

Bu

P

3

R

Ni

SiMe 3

PPh3

8.11.2.2.1

t

P t

Bu

t

R = CH2 CMe3 CH2SiMe 3 CH2CMe 2Ph

N N

Bu

N Ni Me

Mononuclear complexes with s-alkyl and aryl ligands

Terpyridine-ligated nickel(I) methyl, Ni(tpy)Me (tpy ¼ terpyridine (24a), 4,40 ,400 -tri-tert-butylterpyridine (24b)), as an intermediate in alkyl-alkyl Negishi coupling can be synthesized by the reaction of Ni(TMEDA)(CH3)2 (23) (TMEDA ¼ N,N,N0 , N0 -tetramethylethylenediamine) with terpyridine and 4,40 ,400 -tri-tert-butylterpyridine (Eq. 4).39 In this reaction, addition of terpyridine can slowly lead to a ligand-induced loss of ethane, in which the resulting Ni(0) fragment then undergoes a comproportionation reaction with a remaining Ni(II)-dimethyl species to afford 24. Because the experiment using a mixture of normal 23 and deuterium-labelled precursor Ni(TMEDA)(CD3)2 (23-d6) and 4,40 ,400 -tri-tert-butylterpyridine provides only a mixture of CH3dCH3 and CD3dCD3, radical pathway involving NidC bond homolysis to form methyl radical is ruled out. The complex 24 has a square planar geometry, which is determined by X-ray crystallography. In the EPR spectra, complex 24a provides a strong unresolved EPR signal with isotropic g ¼ 2.021  0.002 at room temperature in THF solution. The same solution when frozen at 77 K exhibits a rhombic signal with g1 ¼ 2.056, g2 ¼ 2.021, and g3 ¼ 1.999. The g values suggest a more organic-based radical rather than a metal-centered one, implying that the charge-transfer state consisting of a Ni(II)-methyl cation and a reduced ligand (Chart 1) is the ground-state structure; g values for radicals in extended organic p-systems typically fall in the range of 2.003–2.005, whereas a four-coordinate Ni(I) complex was previously reported to have a g value close to 2.18. DFT calculations support this electronic structure that features a singly reduced terpyridine ligand. Because highly reduced terpyridine ligand can reduce alkyl halides, oxidative addition is promoted via formation of ionic pair of nickel(II)-methyl cation and anionic halide and alkyl radical, which are proposed as a part of catalytic pathway in the Negishi coupling of alkyl halides. These aspects can be supported in the Negishi coupling of 1-halo-3-phenylpropane with pentyl-ZnBr. Use of electron-withdrawing groups at 4,40 ,400 -positions of terpyridine ligand decrease the yields of the cross-coupling products.

N N

Ni

N

Me

Chart 1

R Me Me N Me Ni Me N Me Me 23

R

terpyridine ligand N - TMEDA R

N

N Ni Me

+ 1/2 ethane 24a: R = H 24b: R = t Bu

ð4Þ

740

Monovalent Group 10 Organometallic Complexes

Nickel(I)-phenyl complex 26 stabilized by tridentate (−)-i-Pr-pybox (2,20 -(Pyridine-2,6-diyl)bis[(−)-4-isopropyl-2-oxazoline]) ligand is prepared by a reduction of the corresponding cationic nickel(II)-phenyl complex, [((−)-i-Pr-pybox)NiPh]BArF4 (25) (BArF4 ¼ tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) (−1.37 V vs. Fc/Fc+, 0.10 M TBAPF6 in THF (TBA ¼ tetra(n-butyl)ammonium)), with Cp 2Co (Cp ¼ pentamethylcyclopentadienyl) (−1.94 V vs. Fc/Fc+) (Eq. 5).40 Reduction of the analogous (−)-indanyl pybox-ligated nickel(II) phenyl is unsuccessful. EPR analysis of the paramagnetic complex 26 revealed existence of ligand-centered radical, which is like the nickel(I) terphenyl complex 24. Mechanistic studies to elucidate its reactivity as an intermediate in nickel-catalyzed Negishi arylations of propargylic bromides have been conducted. The carbon-carbon bond forming product ((rac)-3-bromo-1-trimethylsilyl-heptyne) is afforded in 35% yield (59%ee) in reaction of the phenylnickel(I) complex (26) with the racemic propargylic bromide ((rac)-3-bromo-1-trimethylsilyl-heptyne) (1.05 equiv) (Eq. 6). Because the yield and enantioselectivity are modest compared to the catalytic yield and selectivity (72% yield (81%ee)), the nickel(I) transmetallation product may not be the intermediate in the catalytic cycle. That is also indicated by that the 1e-oxidized Ni(II) phenyl complex 25 provided the desired yield and selectivity (77% yield (82%ee)) under the similar conditions.

ð5Þ

ð6Þ

Two-coordinate IPr-ligated nickel(I) alkyl and aryl complexes are synthesized in the reactions of the IPr-nickel(I) chloride dimer [Ni(IPr)Cl]241 (27) with alkylmagnesium and aryllithium reagents.42 Incorporation of small alkyl groups is ineffective. When dibenzylmagnesium Mg(CH2Ph)2 was added, homocoupling product, 1,2-diphenylethane, and known nickel(0) complex [Ni(IPr)]243 were obtained. Similar salt metathesis reaction of bis(trimethylsilyl)methylmagnesium chloride, ClMg(CH(SiMe3)2), serving more bulky alkyl group, affords the two-coordinate alkyl complex, Ni(IPr)(CH(SiMe3)2) (28). The similar two-coordinate IPr-nickel(I) complex having s-terphenyl, Ni(IPr)(Ar) (Ar ¼ 2,6-dimesitylphenyl) (29) can be synthesized when 2,6dimesitylphenyllithium is added (Scheme 6). X-ray diffraction studies of these products showed that CdNidC angles of both complexes are nearly linear (174.81(10) (28) and 175.97(8) (29)) and there is no agostic interaction between Ni and a-CH in the alkyl complex. They noted that the complexes are electronically stabilized by localization of the unpaired electron on nickel composed of Ni 3d(2z ) mixed with Ni 4s. No other atom has an unpaired spin density, determined by DFT studies. That is also a general property in T-shape three-coordinate nickel(I) complexes.25 Although the nickel(I) alkyl complex is sterically protected from dimerization, it reacts with the small Lewis base pivaloisocyanide to afford the three-coordinate Ni(I) alkyl complex, Ni(IPr)(CNtBu){CH(SiMe3)2} (30), in 62% isolated yield (Eq. 7). The crystal structure of 30 revealed a distorted trigonal planar geometry. The NidCNHC bond to 1.950(2) A˚ and of the NidCalkyl bond to 2.008(3) A˚ is slightly elongated when compared with those of 28 (1.910(2) and 1.968(3) A˚ , respectively).

Scheme 6 Salt metathesis reactions of the Sigman’s dimer 27 with 2,6-dimesitylphenyllithium, bis(trimethylsilyl)methylmagnesium chloride, and dibenzylmagnesium.

Monovalent Group 10 Organometallic Complexes

741

ð7Þ

Reactions of 28 with alkyl halides have been investigated. Upon addition of 1-bromo-1-phenylethane, no cross-coupling product is formed. Instead, homocoupled product, 2,3-diphenylbutane and a three-coordinate nickel(II) alkyl bromide Ni(IPr)Br(CH (SiMe3)2) (31) are produced (Scheme 7A).42 The formation of 31 suggests a radical mechanism in oxidative addition of alkyl halides. Conversely, when the similar reactions with benzyl bromide is conducted, the major formation of the heterocoupled product 1,1-bis(trimethylsilyl)-2-phenylethane is revealed, along with formation of byproducts, dibenzyl and 1,1,2,2-tetra (trimethylsilyl)ethane (4:2:1 relative ratio). In addition, the nickel(II) bromide 31 and the nickel(I) bromide dimer [Ni(IPr) Br]2 are obtained in a 4:1 ratio. Benzyl bromide would form a less stable radical than (1-bromoethyl)benzene as it forms a primary instead of a secondary radical. Additionally, the smaller steric profile of a benzyl radical would make formation of a four-coordinate Ni intermediate more facile. Therefore, a mixture of products is reasonable from the reaction of benzyl bromide. The nickel(I) alkyl complex 28 reacts with (Me3Si)2CHBr to afford only the nickel(II) bromide 31 and the homocoupled organic product (Scheme 7B). The possibility of alkyl group scrambling during the course of reaction of 28 with alkyl halides can be investigated in the reaction of (Me3Si)2CDBr, providing only 31-d0 and 1,1,2,2-tetrakis(trimethylsilyl)ethane-d2. No alkyl scrambling is observed, ruling out the transient formation of a dialkyl Ni(III). Reactions of other unactivated alkyl bromides, such as 1-bromobutane, 2-bromobutane, and menthyl bromide, occur slowly to afford (Me3Si)2CH2 and/or alkenes, such as butene isomers, along with formation of the nickel(I) bromide dimer, [Ni(IPr)(m-Br)]2 (32),42 and the nickel(II) alkyl bromide 31.

Scheme 7 Reactions of bis(trimethylsilyl)methyl nickel(I) complex 28 with (A) 1-bromo-1-phenylethane and (B) benzyl bromide.

Simple, bulky monodentate-phosphine and pentafluorophenyl group can stabilize three-coordinate nickel(I)-aryl complex, according to the report by Johnson et al.44 They accidentally isolated the nickel(I)-pentafluorophenyl complex, Ni(PiPr3)2(C6F5) (35), from the reaction of anthracene adduct of bis(tri-isopropylphosphine)nickel(0) complex 33 with hexafluorobenzene at room temperature, accompanied with the formation of a normal oxidative addition nickel(II) product, Ni(PiPr3)2F(C6F5) (34) (Eq. 8). Although it is hypothesized that F− should liberate in the radical addition of the CdF bond, attempts to identify the fluorine-containing species such as Ni(PiPr3)2F failed. Alternative synthetic routes to 35 by reaction of nickel(I) chloride Ni(PiPr3)2Cl with either C6F5MgI or Bu3SnC6F5 and reduction of 34 with Na/Hg are not successful. Only comproportionation of Ni(PiPr3)2(C6F5)2 and [Ni(PiPr3)2]N2 affords the complex 35 (Scheme 8). The nickel(I)-aryl analogues have not been isolated in the reactions with other fluorine-substituted benzenes, pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,2,3,4-tetrafluorobenzene, and 1,2,3,5-tetrafluorobenzene. Instead, CdH oxidative addition occurs slowly to form nickel(II) aryl hydride complex Ni(PiPr3)2H(C6F5).

742

Monovalent Group 10 Organometallic Complexes

+ C6 F5MgI

F

(iPr3 P)2NiCl

F

F

+ Bu 3SnC6F5 (iPr3 P)2NiCl 34

F

F Ni

1% Na/Hg i

Pr3P

35

P iPr3

i

0.5 [( Pr3 P)2Ni] 2N2 (iPr3P)2 Ni(C6F5 )2

comproportionation

Scheme 8 Synthetic approaches to pentafluorophenyl nickel(I) 35.

ð8Þ

Thereafter, it has been demonstrated in detail how the CdF bond activation of hexafluorobenzene occurs on the Ni center of nickel(0) IMes complex, Ni(IMes)2 (36) (IMes ¼ 1,3-bis(mesityl)imidazole-2-ylidene).45 Three different compounds are isolated and determined from the reaction mixture, an oxidative addition product Ni(IMes)2F(C6F5) (37), nickel(I)-aryl complex Ni(IMes)2(C6F5) (38), and nickel(II) difluoride Ni(IMes)2F2 (39). In addition, the EPR spectra of the frozen reaction mixture revealed the formation of a new nickel(I) fluoride Ni(IMes)2F (40), which is independently formed as a mixture with 39 by reaction of cationic nickel(I) complex [Ni(IMes)2](BF4) with CsF. Detailed DFT calculation studies revealed the overall mechanism of the reaction process: (1) homolytic cleavage of a CdF bond with interaction between Ni and F occurs to form nickel(I) fluoride 40 and the aryl radical, (2) the nickel(I) fluoride 40 again cleave a CdF bond of the second hexafluorobenzene, (3) the C6F5 radical can recombine with 40 to form the normal oxidative addition product 37 in part, whereas it mainly add to the nickel(0) precursor 36 to afford nickel(I)-aryl complex (Scheme 9). Because of the steric bulkiness of the IMes ligands, the recombination reaction is restricted, resulted in only poor-yield ( [Ni](C6H6)+ (77) > [Ni](Mes)+ (76) > [Ni](C6Me6)+ (75) ([Ni] ¼ [Ni(cod)]). The remaining spin density is located partly on the aromatic ligand and on the olefinic carbons of the COD ring.

Scheme 17 Reactions of cationic bis(1,5-cyclooctadiene)nickel(I) 74 with arenes.

8.11.2.4

Mononuclear N-heterocyclic carbene complexes

There are several studies on N-heterocyclic carbene (NHC) complexes of nickel(I) published after COMC-III. Chart 2 illustrates the structures of these complexes. They have sterically hindered NHC ligands, such as IPr, IMes, and SIPr (IPr ¼ 1,3-bis

750

Monovalent Group 10 Organometallic Complexes

Two-Coordinate NHC-Ni(I) Ar N

N Ar

Ar N

Ar N

N Ar

Ar N

N Ar

N Ar

Ar

Ni

Ni

Ni

R

NR2

N

O(2,6- tBu2Ph)

COR'

N

Ar

Ar

N

N

i

Pr

i

Pr

Dipp :

Ni

Ni

R

N

Ar Mes :

Ar = Dipp (80)

Ar = Dipp

Ar = Dipp

Ar = Dipp

R = CH(SiMe3 )2 (28) 2,6-(Mes) 2Ph (29)

R, R' = iPr, tBu (16) R = N(SiMe3 )2 (78a) i NH(Dipp) (78b) Pr, iPr (79) N(SiMe 3)(Dipp) (78c) NPh2 (78d)

Ar = Mes (81)

季

Three-Coordinate NHC-Ni(I) N

Ar

N Ar

N

Ar

Ni Cl Ar

N Ar

N

Ar = Dipp (82)

N Ar Ni

Ar

N

X N Ar

N

Ar

Ar = Mes

N Ar

Ar N

Ni C6F5 Ar

N

Ni

Ar = Mes (38)

O iPrNi t

Bu

N

Ar

N

N Ar

N

Ni O

Ni

O

PPh 3

i

Pr

Ar = Dipp (72)

Ar

N

Ar = Dipp (87)

X

Ar

Ar = Dipp Ar X = Br L = P(ArF)3 (85)

N

iPr

ArF: CF3

Ar = Mes L = PPh 3 X = Cl (86a) Br (86b)

N Ar Ni

CF3

Ar = Dipp X = Cl L = PPh 3 (84a) L = P(OPh)3 (84b) pyridine (84c) CNtBu (84d) n

N Ar

X

L

N Ar

X = F (40) Cl (83a) Br (83b) I (83c)

Ar N

N Ar

N

O n = 1, X = Br Ar = Mes (88a) NHC : N N Ar Ar 2-MeOPh (88b) 2-MeOPh/Mes (88c) X = Cl/Br (92a/92b)

COt Bu

Ar = Dipp L = CO (19) Ar = Dipp L = CN(2,6-Me2Ph) (20)

n = 2, X = Br Ar = o-Tol (89) n = 3, X = Cl/Br Ar = Mes (90a/90b) o-Tol (91a/90b)

L

i

Pr

i

Pr

Dipp :

Mes :

季

季 Chart 2

Monovalent Group 10 Organometallic Complexes

751

(2,6-diisopropylphenyl)imidazol-2-ylidene, IMes ¼ 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene, SIPr ¼ 1,3-bis(2,6diisopropylphenyl)-4,5-dihydro-imidazol-2-ylidene), which have bulky wing-tip substituents. Some of them directly bound to carbon are picked up in the other sections. The first mononuclear NHC-ligated nickel(I) complexes had been reported in 2008.67 Two-coordinate (13e) complexes, most of which have IPr ligands, can be synthesized by salt-metathesis of the IPr-nickel(I) halide dimer 2741 or 3242 with anionic groups, and should be kinetically stabilized by bulky ligands especially in the alkyl and aryl complexes 28 and 29.67 If not, it is thermally less stable than its dimer complex. Amide complexes, Ni(IPr)(NR2) (NR2 ¼ N(SiMe3)2 (78a), NH(Dipp) (78b), N(SiMe3)(Dipp) (78c), NPh2 (78d) (Dipp ¼ 2,6-diisopropylphenyl)), does not need such bulky substituents.67–69 The lone pair electrons on the amide nitrogen donate to the metal center, which enhances the back-donation from Ni to carbene C and enhances the stability of the two-coordinate nickel(I) complex.68 A rare two-coordinate phenoxide complex Ni(IPr)(O(2,6-tBu2Ph)) (80) can be also synthesized.68 A cationic two-coordinate bis-NHC complex [Ni(NHC)2]+ Br− (81) (NHC ¼ 1,3-bis(2,4,6-trimethylphenyl)3,4,5,6-tetrahydropyrimidin-2-ylidene) has a characteristic magnetic anisotropy derived from the orbital degeneracy, and behave as a single-ion magnet.70 This behavior arises as a direct result of the 2:1:2 splitting of the occupied metal-based orbitals determined computationally. Three-coordinate (15e) complexes are the most common in the mononuclear NHC-nickel(I) chemistry. Various synthetic routes have been studied: comproportionation of nickel(0) and nickel(II) precursors in the presence of NHC, reduction of NHC-nickel(II) halides with strong reducing agents, and addition of a 2e-donor ligand to the two-coordinate complex or nickel(I) dimer. A reaction of bis-NHC nickel(0) complex with aryl halide efficiently affords bis-NHC nickel(I) halide complex, NiCl(IPr)2 (82) and NiX(IMes)2 (X ¼ Cl (83a), Br (83b), I (83c)) as a result of radical addition reaction, independently found by Matsubara et al.71 and Louie et al.72 The pair of the bulky NHC ligands efficiently prevent recombination of aryl radical to the nickel center, as Kochi et al. unveiled early.49 A series of NHC ligands, which are composed of saturated 6-, 7- and 8-membered rings containing the carbene carbon are also prepared and used as ligands in triphenylphosphine-nickel(I) halides.73,74 As the number of rings increases, the NHC ligand becomes sterically bulkier, even if the NHCs have the same wing-tip substituents. This is because the NdCdN angle containing carbene carbon is increased, and thus the wing-tip substituents are placed closer to nickel. In a reaction of Ni(acac)2 (1 equiv.), IPr (1 equiv.), PhB(OH)2 (2 equiv.) and tBuOK (2 equiv.), a red crystalline compound Ni(IPr)(acac) (87) is afforded in 62% yield in addition to a biphenyl product.75 The X-ray crystal analysis of 87 clearly shows a rare, three-coordinate distorted T-shaped Ni(I) structure. Comproportionation of Ni(cod)2 and Ni(acac)2 in the presence of IPr can also give the same complex 87 in moderate yield. Because the complex 87 is catalytically active in the addition reaction of aryl boronic acid to alkynes, some nickel(I) species would be involved in the catalytic system. Only limited examples of the four-coordinate nickel(I) complexes bearing monodentate NHC ligand are known.69,76 Among bidentate ligands, only 2,20 -bipyridine and its derivative such as 2,20 -biquinoline can provide the stable mononuclear nickel(I) complexes, Ni(IPr)Br(bipy) (bipy ¼ 2,20 -bipyridine) (93a), while bisphosphines and diamines provide disproportionation products, nickel(0) and nickel(II) complexes.77 Generally, 2,20 -bipyridine coordinates to a late-transition metal to make a rigid structure because of the chelate effect and the strong p-acceptor property; thus difficult to eliminate from the metal center. However, in the four-coordinate nickel(I) complexes, 2,20 -bipyridine can be immediately replaced with 2,20 -biquinoline to form the complex 93b at room temperature in solution.69 The complex 93a reacts with diarylamines, such as HNPh2 and HN(o-tolyl)2, in the presence of a base to yield the four-coordinate amide complexes, Ni(IPr)(bipy)NR2 (NR2 ¼ NPh2 (94a), HN(o-tolyl)2 (94b)) via transmetallation. A rare nickel(I)-mediated catalytic process has been experimentally proposed using IPr-ligated nickel(I) halides in Buchwald-Hartwig amination (Scheme 18). The four-coordinate nickel(I) complex 93a eliminates bipyridine to form active two-coordinate nickel(I) bromide intermediate, which reacts with diphenylamide anion to afford the two-coordinate nickel(I) amide 78d via transmetallation. The complex 78d has an equilibrium with a four-coordinate nickel(I) amide complex 94 and react with aryl bromides to serve the coupling products via formation of nickel(III) species, which can be detected in EPR spectroscopy.

Scheme 18 Proposed mechanism in nickel(I)-catalyzed Buchwald-Hartwig Amination of aryl bromide with diarylamines, using 93a or 32 as a catalyst precursor.

752

Monovalent Group 10 Organometallic Complexes

A four-coordinate, cationic nickel(I) complex, [Ni(IPr)(CNtBu)3][BArF4] (ArF ¼ 3,5-bis(trifluoromethyl)phenyl) (95) can be obtained in the reaction of an excess of tert-butyl isocyanide with a cationic, dinuclear nickel(I) m-diphenylcarbene complex [{Ni(IPr)}2(m-Cl)(m-CPh2)][B(ArF)4], along with the formation of the carbene-transfer product Ph2C]C]NtBu and a threecoordinate nickel(I) isocyanide complex Ni(IPr)Cl(CNtBu) (84d) (see Section 8.11.2.5.4).76 The X-ray structure of 95 shows a distorted tetrahedral Ni(I) center with three linear isocyanide ligands.

8.11.2.5

Dinuclear nickel(I) carbonyl, isocyanide and related complexes

Only a limited example is known for dinuclear nickel(I) carbonyl complexes. Treatment of mesitylene-substituted b-ketiminato nickel(I), (Me-ketiminato)Ni(2-picoline) (96) with excess CO(g) to readily afford the dinuclear nickel(I) complex {(Meketiminato)Ni}2(m-CO)2 (97) (Scheme 19).34 This complex 97 is alternatively obtained by a reaction of CO with a nickel-imide complex, (Me-ketiminate)Ni(NAd) (Ad ¼ adamantyl), accompanied with the formation of O]C]NdAd.78 The X-ray structure of this carbonyl complex 97 shows a rather short NidNi distance of 2.401(2) A˚ , and the nickel center has distorted trigonal pyramidal geometry. The two carbonyl groups are not symmetrically bound to the metal center, with NidC bond distances of 1.759(4) and 2.078(4) A˚ . Both CO ligands are coordinated in a bent fashion with MdCdO angles of 160.8(3) and 121.8(3) , one being significantly more obtuse than the other. In solution, the 1H NMR spectrum at −90  C is consistent with its solid-state structure, whereas at −75  C the coalescence of the signals from p-methyl groups at the mesityl substituent at d 2.37 and 2.13 ppm is observed, indicating a twisting motion of each (Me-ketiminate)Ni fragment about the NidNi vector, corresponding to an activation barrier DG{ ¼ 9.4(4) kcal mol−1 at this temperature. All o-Me resonances coalesce at −23  C, likely a result of reversible dissociation/ reassociation of a (Me-ketiminato)Ni moiety from the {(Me-ketiminato)Ni(m-CO)}2 dimer with an activation barrier DG{ ¼ 11.3 (3) kcal mol−1 at this temperature. Stoichiometric reaction of 96 with 3,5-dimethylphenylazide gave a 13% yield of the corresponding carbodiimide after 3 h at RT. However, a catalytic attempt employing 3,5-dimethylphenylazide and CO with 5 mol% of 95 results in no yield of the anticipated isocyanate O]C]NAd (see Section 8.11.2.1).

N NiI

CO

N

- picoline

N

NiI

N

N

O C C O

N3 N NiI

O=C=N N

- N2 13% after 3 h, r.t.

96

97

Scheme 19 Sequential reactions of diketiminato nickel(I) picoline complex 96 with CO and then arylazide. Only 13% of the product isocyanate is obtainable, indicating that the dinuclear carbonyl complex 97 is not the key intermediate in the catalytic nitrene transfer.

The similar CO-bridging dimer is obtainable using guanidinato-ligated nickel(I) dimer, where one of the guanidinate aryl substituents coordinate to two nickel centers.79 The complex can react with CO(g) to form dimeric nickel(I) carbonyl complex 98 (Scheme 20). The presence of bridging CO ligand is confirmed by IR spectroscopy (nCO ¼ 1847 cm−1), and the Ni(I)dNi(I) bond distance is 2.437(1) A˚ , indicating the presence of the NidNi single bond. In contrast to the b-ketiminato complex 97,34 two CO ligand of the guanidinato complex 98 symmetrically bridge between the nickel centers, and the NidC bond distances are approximately the same, 1.857(2) and 1.8616(18) A˚ .

Scheme 20 Reaction of CO with dinuclear guanidinato nickel(I) complex to yield dinuclear nickel(I) carbonyl complex 98.

Monovalent Group 10 Organometallic Complexes

753

The reaction of mononuclear nickel(I) thiolate, Ni(SDmp)(PPh3) (68) with tBuNC or CO affords the corresponding isocyanide or carbonyl complexes.59 Treatment of 3 with 1 equiv of tBuNC at room temperature provides the thiolato-bridged dinuclear Ni(I) complex [Ni(CNtBu)(m-SDmp)]2 (99) after ligand exchange of PPh3 with tBuNC (Scheme 21). Interestingly, when the toluene solution of 68 is charged with 1 equiv of CO gas, only a mixture of nickel(0) and nickel(II) complexes, Ni(PPh3)2(CO)2 and Ni(CO) (SDmp)2(PPh3) are formed. The formation of nickel(0) and nickel(II) species indicates that disproportionation proceeds followed by addition of CO to nickel(I) and dimerization. A low-temperature NMR observation of the reaction mixture at −80  C reveals the presence of CO-ligated Ni(I) species [68-CO] (nCO band at 2000 cm−1). However, a significant amount of Ni(PPh3)2(CO)2 and Ni(CO)(SDmp)2(PPh3) is found within 15 min even at −80  C, and thus the characterization of the CO adduct is unsuccessful.

t

S BuNC

1/2

t

Ni CNtBu

BuNC Ni S

S 68

Ni PPh 3

99 CO CO

[68-CO]

Ni0(PPh3 )2 (CO)2 +

S

NiII

S

PPh 3

Scheme 21 Reactions of nickel(I) arylthiolate complex with tert-butyl isocyanide and CO.

8.11.2.6 8.11.2.6.1

Dinuclear nickel(I)-carbon s-bonded complexes Dinuclear complexes with bridging s-aryl ligands

Treatment of biphenylene with Ni(cod)2 and PiPr3 results in oxidative addition of two biphenylene CdC bonds to form dinuclear bis(biphenyl) complex 100 containing a Ni(III)dNi(I) bond.80 One of the biphenyldiyl moiety in 100 adopts a bridging m-Z1,Z1-bonding mode whereas the other biphenyldiyl moiety is coplanar with nickel(III) center. The other nickel(I) center engages in an additional asymmetric p-bonding interaction with a short NidC distance (1.941(3) A˚ ). The nickel(I) center does not take square planar geometry, not indicating a highly polarized Ni(II) anion and Ni(II) cation pair but formal Ni(I)dNi(III) moieties. Subsequent reductive coupling of the CdC bond provided a dinuclear tetraphenylene nickel(I) complex 101 over the course of 6 h at 25  C. The complex 101 catalytically produced tetraphenylene in the presence of biphenylene upon heating at 90  C in toluene, accompanied by regeneration of the dinuclear complex 100 (Scheme 22). A deuterium labeling study proves that the CdC bond-forming step that provides 101 proceeds through dinuclear 100, rather than via initial cleavage of the NidNi bond to

Scheme 22 Catalytic cycle for tetraphenylene formation involving dinuclear biaryl and tetraaryl nickel(I) complexes 100 and 101.

754

Monovalent Group 10 Organometallic Complexes

provide 2 equiv of mononuclear complex. These results indicated that the unsaturated dinickel(I) system can also be effective for catalytic reactions involving concerted activation of substrates by a pair of active metal centers. Reaction of aryl chloride (p-chlorotoluene or p-chloroanisole) with 1 equiv of IPr and Ni(cod)2 efficiently affords dinuclear nickel(I) m-s-aryl complex bearing the IPr ligand, [Ni(IPr)]2(m-Cl)(m-Z1,Z2-aryl) (aryl ¼ p-tolyl (102a) or p-anisyl (102b)), rather than the monomeric nickel(II) oxidative addition product (Eq. 19).81 This dinickel(I) complex is alternatively formed in the reaction of Sigman’s dimer 27 with 1 equiv of p-tolyl- or p-anisylmagnesium chloride via transmetallation. Subsequent reaction of 102a with p-chloroanisole yields a mixture of biaryls, and regenerated complex 27, via oxidative addition of aryl chloride and reductive elimination of the biaryl (Eq. 20). Thus, these complexes could potentially be active as catalysts in the Kumada-TamaoCorriu coupling of aryl halides. On the other hand, further reaction of 102a with p-tolylmagnesium chloride affords bis-s-p-tolyl nickel(I) dimer 103, which reacts with chlorobenzene to afford biaryls and 27 (Scheme 23). However, heterocoupled product is not included in the product mixture, indicating that reductive elimination of 4,40 -dimethylbiphenyl from 103 occurs before oxidative addition of chlorobenzene. The similar attempt to prepare the bromide analogue of 102, upon treatment using bromoarenes with Ni(cod)2 and IPr, provides only the 2-arylated imidazolium salt rather than the dinuclear compound.82

Scheme 23 Stepwise reactions of dinuclear nickel(I) aryl complex 102a with p-tolyl Grignard and then chlorobenzene.

ð19Þ

ð20Þ

The rate of decrease in complex 102a in the reaction with p-chlorotoluene performed at −81, −70, −55, and −43  C has been monitored by proton (1H) NMR measurements.81 The activation parameters, determined from the Arrhenius plot using the obtained reaction rate constants, are DH{ ¼ +10.1  0.5 kJ mol−1, DS{ ¼ −237.4  2.2 J K−1 mol−1, and DG{298 ¼ +80.9  1.1 kJ mol−1 (Scheme 24). A large negative activation entropy is characteristic in this reaction, suggesting that the coordination and addition of 4-chlorotoluene is the rate-determining step, among each elementary step in the catalytic cycle. DFT calculations of this reaction pathway supports the

Scheme 24 Reaction of dinuclear nickel(I) aryl complex 102a with p-chlorotoluene and kinetic parameters of the reaction.

Monovalent Group 10 Organometallic Complexes

755

experimental results. In the structure of the dinuclear complex, the steric environment sandwiched between large IPr ligands can be a determinant in slowing the reaction rate. Thus, this is significant evidence that the catalytic cycle proceeds while retaining the dinuclear geometry. An alternative pathway involving the oxidative addition of aryl halide to complex 27 is that the m-s-aryl complex 102a can act as a precursor in catalysis.81 The addition of chlorobenzene to the complex 27 occurs without loss of the dinuclear framework, resulting in a semi-stable dinuclear Ni(II) complex. Subsequent transmetallation with phenylmagnesium chloride forms the biaryl and 27 again (Scheme 25). Notably, in the DFT calculations, the Ni(II)dNi(0) complex with a dative bond from Ni(0) to Ni(II) is temporally formed just before the CdCl bond cleavage to one of the nickel centers to give the Ni(II)dNi(II) complex. This catalytic reaction affords no homo-coupling product, probably because the equilibrium of homolysis into mononuclear intermediates is unfavorable. That prevents the intermolecular aryl exchange process, consistent with the existence of the dinuclear system during catalysis.

Scheme 25 Proposed catalytic cycle of Kumada-Tamao-Corriu coupling using dinuclear complexes 102a or 27.

8.11.2.6.2

Dinuclear complexes with bridging Cp and related ligands

Treatment of nickel(I) triisopropylphosphine chloride 104 with a suspension of cyclopentadienyllithium or indenyllithium in n-pentane at 25  C caused a color change to form the dinuclear nickel(I) complex 105a or 105b having bridging m-Z3cyclopentadienyl or indenyl ligand (Scheme 26).83 Comproportionation to the dinuclear nickel(I) complex 105a or 105b is possible when using mononuclear nickel(II) chloride bearing Z5-cyclopentadienyl or Z3-indenyl, Ni(Cp)(PiPr3)Cl (106a) or Ni(Ind)(PiPr3)Cl (106b), and the nickel(0) source, {Ni(PiPr3)}2(m-N2) (107). Conversely, the reaction of mononuclear nickel(II) chloride bearing Z5-(tetramethyl)cyclopentadienyl with 107 resulted in no NMR spectroscopic evidence for the formation of a dinuclear analogue. Use of a nickel(II) complex, where chloride ligand is replaced to methyl via transmetallation, to obtain a dinuclear complex via comproportionation is not successful to result in any conversion, even under reflux conditions in benzene. The 1H NMR spectrum of 105a features a cyclopentadienyl resonance at d 5.07 as a broad singlet. Cooling solutions of 105a in

107 (0.5 equiv)

LiCp -LiCl -2 Pi Pr3

i

Pr3P

2

Ni

Pr3P Ni

Ni

i

Cl

PiPr3

105a

104

Ni Pr3P Cl 106a

i

Cl

i

Pr3 P

-1/2 N2 -PiPr3

107 (0.5 equiv)

LiInd -LiCl -2 PiPr3

i

Pr3P Ni

Ni Cl

105b

Pi Pr3

-1/2 N2 -PiPr3

i

PiPr3

Pr3P Ni N2 Ni

Ni i

Pr3P

Cl

106b

Scheme 26 Synthetic procedures for dinuclear nickel(I) m-Z3-cyclopentadienyl or indenyl complex 105a or 105b.

i

Pr3 P

107

PiPr3

756

Monovalent Group 10 Organometallic Complexes

toluene-d8 as low as 193 K did not result in decoalescence of this signal. Solutions of 105a decompose over days at room temperature, with the deposition of a nickel mirror. Additionally, (iPr3P)2NiCl2 and (Z5-C5H5)2Ni were both isolated as decomposition products from solutions of 105a after extended periods in C6D6, presumably from a disproportionation reaction that liberates nickel metal. Complex 105b decomposes in a similar manner in solution over time, as judged by the observation of (iPr3P)2NiCl2, Ni(C9H7)2, iPr3P, and a Ni mirror. A series of dinuclear NHC-ligated nickel(I) complexes bearing m-Z3-cyclopentadienyl and indenyl ligands, [Ni(NHC)]2 (m-Z3-Cp) (NHC ¼ IPr (108) or SIPr (109)) and [Ni(NHC)]2(m-Z3-Ind) (NHC ¼ IPr (110) or SIPr (111)), have been synthesized, when 1 equiv of cyclopentadienylsodium or indenyllithium are added to the nickel chloride dimer 27 or 47 (see Section 8.11.2.2.2).51 A dinuclear nickel(I) complex bearing two cyclopentadienyl or indenyl ligands does not generate; instead, mononuclear nickel(I) Z5-cyclopentadienyl complex 48 or 49 or indenyl complex 50 or 51 are yielded in the reaction with 2 equiv of the corresponding anionic reagents. In contrast to the nickel(I) phosphine analogue or the palladium(I) congener, decomposition via heterolytic cleavage of the monovalent metal-metal bond to form a pair of zerovalent and divalent metal complexes is disfavored. There is an equilibrium in solution between the dinuclear Cp nickel(I) complex 108 or 109 and mononuclear Cp nickel(I) complexes 48 or 49 as well as 0.5 equiv of the corresponding NHC-nickel(I) chloride dimer 27 or 47, along with a change between Z5- and Z3-coordination of the Cp ligand, respectively (Scheme 27).51 Variable temperature 1H NMR spectroscopy reveals that, at −50  C, the recrystallized sample of the dinuclear complex only contains signals associated with the dimer, whereas, at 70  C, the monomeric nickel(I) Cp complex 48 or 49 and the NHC-nickel(I) chloride dimer 27 or 47 are the major species present in solution. The van’t Hoff analysis is consistent with an equilibrium, and indicates that the formation of the Cp monomer is enthalpically more favorable for SIPr (DH0 ¼ 41.2(7) kJ mol−1) than for IPr (DH0 ¼ 46.0(10)− kJ mol−1). In both cases the dissociation of the dimer is entropically driven (DS0 ¼ +112(3) (IPr) and +111(2) (SIPr) J K−1 mol−1) (Scheme 27). In a solution of the analogous dinuclear triisopropylphosphine-nickel(I) m-Z3-Cp complex 106a,83 the similar equilibrium is not observed. In contrast to the Cp chemistry, the dinuclear m-Z3-indenyl complexes 110 or 111 do not dissociate into the monomer.51 As is observed in the palladium chemistry (see Section 8.11.3.2.2), dinuclear indenyl complex is more stable than dinuclear cyclopentadienyl complex. The reaction of the bridging Cp dimer 108 with the indenyl monomer 50 results in complete conversion to the bridging indenyl dimer 110 and the Cp monomer 48 (Eq. 21). Moreover, the treatment of 108 with 0.5 equiv of LiInd, results in the formation of 48 and 110 (Eq. 22). Interestingly, the cyclopentadienyl complex 108 or 109 mediated the Suzuki-Miyaura cross coupling of 4-chlorotoluene, but the dinuclear indenyl complex 110 or 111 does not, strongly suggesting that only the mononuclear nickel(I) complex has activity toward this catalytic transformation.

Scheme 27 Dinuclear nickel(I) m-Z3-cyclopentadienyl complex 108 or 109 having an equilibrium with mononuclear complex 48 or 49 and nickel(I) chloride dimer 27 or 47. However, the indenyl analogue 110 or 111.

ð21Þ

Monovalent Group 10 Organometallic Complexes

757

ð22Þ

8.11.2.7

Dinuclear nickel(I)-carbon p-bonded complexes

Triphenylphosphine nickel(II) chloride complexes bearing the 1-trimethylsilyl-3-R-indenyl ligand (R ¼ H (112) and SiMe3 (113)) decompose, followed by anion exchange with a non-coordinating tetraphenylborate anion.84 From 113 dinuclear nickel(I) complex 114 forms in which one of the phenyl groups of tetraphenylborate bridges between the two nickel centers in a m-Z2,Z2-coordination mode as a stable and isolable compound (Scheme 28). Two nickel centers are also bridged by a m-phosphide ligand. In the decomposition of complex 112 the mononuclear cationic nickel(II) complex [Ni(1-SiMe3-Ind)(PPh3)2][BPh4] can be detected by NMR spectra as a transient species, although the ready decomposition of this intermediate prevented its isolation and full characterization. In the case of 113, the two trimethylsilyl substituents of the indenyl ligand prevent the formation of bis(phosphine) complex, as is favored from 112. GC-MS and 1H NMR analysis revealed the formation of 3-phenyl-1,1-bis (trimethylsilyl)indene in the decomposition from the reaction mixture of 113 and NaBPh4. Thus, the mechanism of formation of this dinuclear nickel(I) complex is proposed that halogen abstraction from complex 113 forms unsaturated cationic species, following PdC bond oxidative addition and reductive elimination occurs to form the silylated indene and 114; however, the source of nickel(0) phosphine species and the mechanism of the silyl migration pathway in the indene molecule are unclear.

Scheme 28 Formation of dinuclear nickel(I) phosphide complex 114 bearing bridging tetraphenylborate anion and proposed mechanism of the reaction.

The similar m-Z2,Z2-phenyl coordination of tetraphenyborate anion can be achieved more simply. The reaction of nickel(I) bis(triisopropylphosphine) chloride Ni(PiPr3)2Cl (105) with NaBPh4 affords the dinuclear nickel(I) complex [Ni(PiPr3)2](m-Cl) (m-Z2,Z2-C6H5BPh3) (114) (Eq. 23).83

ð23Þ

An interesting finding reported by Agapie et al. is the well-defined dinuclear nickel(I) complex 115 stabilized by a terphenylene-bridged diphosphine ligand.85 The reaction of the diphosphine with Ni(cod)2 affords mononuclear nickel(0) complex 116, in which one C]C double bond of the arene coordinates to the nickel(0) center (NidC bond distances of 1.992 (1) and 2.002(1) A˚ by X-ray crystallography) (Scheme 29). Comproportionation of 116 and NiCl2(dme) (dme ¼ 1,2-dimethoxyethane) occurs to form a chloride-bridged dinuclear nickel(I) complex 115, where the Ni2 moiety is coordinated by the phosphines in a nearly linear PNiNiP arrangement. The dinickel centers interact with a vicinal diene moiety of the central ring of the terphenyl

758

Monovalent Group 10 Organometallic Complexes

Scheme 29 Formation of dinuclear nickel(I) complexes 115 and 117 bearing terphenylene-bridged bisphosphine ligand, and reactions of 117 with CRHCl2 (R ¼ H or Me) or CO.

framework (NidC bond distances of 2.05–2.10 A˚ ). Reaction of 3 with o,o0 -biphenyldiyl Grignard affords a transmetallated dinuclear m-Z1,Z1-biphenyldiyl complex of nickel(I) 117. Strong interaction between the metal centers and the central aryl ring of the terphenyl unit is observed in the interaction with two non-vicinal double bonds and the nickel centers (NidC bond distance of 1.98–2.06 A˚ by X-ray crystallography) and with a third carbon at a longer distance (2.20–2.30 A˚ ) in 117. Thus, the complex 117 shows a significant distortion of the plane of the central ring toward a boat conformation. In contrast to the solid-state structures, solution NMR spectroscopic data for 115–117 are consistent with species that have C2v symmetry on average, which are pseudoCs (115, 116) or pseudo-C2 (117). These observations suggest that fluxional processes occur in solution that allow for the exchange of metal-arene bonds. Treatment with excess CO at room temperature led to the generation of fluorenone (Scheme 29). A possible mechanism involves the formal reduction of the dinickel fragment by two electrons. In agreement, dinuclear Ni(0) carbonyl complexes, are observed in the resulting mixture. Reaction of 117 with geminal dichloroalkanes affords fluorene derivatives and 115 as the major products, indicating potential activity of dinuclear Ni(I) species in CdC bond coupling reaction. A nickel(II) bromide dimer 118 bearing chelating guanidinato ligand with bulky 2,6-diisopropylphenyl group is a good precursor to nickel(I) complexes. The reduction of 7 with potassium in toluene or benzene results in moderate to good isolated yields of red, arene-bridged systems 119a or 119b, respectively (Scheme 30).79 These are analogous to the b-diketiminato–nickel

NiPr2 Ar

N

N

Ar

Ni 119a R = CH3 119b R = H

R K, C6 H5R Ar N i

Pr2N

Ni N Ar

Ni Br 118

Ar

N

Br II

N

i

II

N Pr2 N Ar

N

Ar

i

i

Pr

-C6 H5 R

NiPr2 K, cyclohexane

Pr

Ar:

Ni

Ar

i

NiPr2 N Pr2N

Ar

Ar +

N Ni Ni N NiPr2

Ar N 120

Ar

N

N

Ni

Ni

N

N

Ar

Ar

i

N Pr2 121

季

Scheme 30 Reaction of dinuclear nickel(II) bromide with potassium in toluene or benzene to form dinuclear complex 119a or 119b or in cyclohexane to form 120 and 121.

Monovalent Group 10 Organometallic Complexes

759

(I) complex bridged by toluene molecule.57 In contrast, a reduction of 118 in the non-coordinating solvent cyclohexane affords the isomeric nickel(I) complexes 120 (brown) and 121 (orange). In the complex 120, one of the aromatic substituents bridged the two nickel(I) centers to stabilize the nickel(I) dimer, whereas in complex 121, two guanidinato ligands bridges between two nickel(I) centers though the nitrogen atoms. Upon standing the hexane solution of 119a or 119b at ambient temperature for less than 24 h, these complexes eliminate their arenes to generate mixtures of 120 and 121. No dinitrogen-bridging dinuclear nickel(I) complex is formed under nitrogen atmosphere, in contrast to the b-ketiminato nickel(I) system.24 The elimination of benzene is faster than toluene, presumably as benzene is a weaker p-donor than toluene. When crystals of 120 are dissolved in hexane, the complex quantitatively rearranges to form 121 over one month, indicating that complex 120 is the kinetic product of the arene elimination and isomerizes into thermodynamic product 121.

8.11.2.8

Dinuclear carbene complexes

Dinuclear nickel(I) complex with bridging m-arylimido imido ligand [Ni(IPr)]2(m-Cl)(m-NAr) (NAr ¼ N(mesityl)) can be produced through the reaction of Sigman’s dimer 27 with the arylazide.86 The result intrigues that a similar synthetic strategy using diazoalkanes affords dimeric complexes with bridging carbene ligands. The reaction of 27 with 1 equiv of diphenyldiazomethane in the presence of NaB(ArF)4 (ArF ¼ 3,5-bis(trifluoromethyl)phenyl) resulted in N2 evolution and clean formation of cationic dinuclear nickel(I) diphenylcarbene complex, [{Ni(IPr)}2(m-Cl)(m-CPh2)][B(ArF)4] (122) (Scheme 31).76 Without the free anion B(ArF)4, a complex mixture including neutral carbene complex is obtained. The similar reaction of (trimethylsilyl)methylazide also affords the bridging carbene derivative, [{Ni(IPr)}2(m-Cl)(m-CHSiMe3)][B(ArF)4] (123). The bridging carbene carbon of 122 or 123 resonates at d 274 or 294 in the 13C NMR spectrum, demonstrating the presence of the carbene carbon. The solid-state structure of 122, in contrast to the solution structure, shows an unsymmetrical dimer in which the carbene functionality adopts Z3-coordination involving the ipso and one ortho-carbon of a phenyl ring along with the carbene carbon. One of the NidC bond lengths (1.831(5) A˚ ) is close to the value expected for a Ni]C double bond (cf., 1.836(2) A˚ in Ni(0)(dtpe)(]CPh2) (dtbe ¼ 1,2-bis(di(tert-butyl) phosphino)ethane)),87 while the other NidC bond, where the phenyl ring also coordinates in a Z2-fasion, is significantly longer at 1.958(3) A˚ , closer to a single bond. In contrast, the X-ray structure of trimethylsilylmethyl analogue 123 has Cs symmetry.

Scheme 31 Synthesis of dinuclear nickel(I) m-carbene complex 122 or 123 from nickel(I) chloride dimer 27.

Reactions of 122 with mesityl azide, carbon monoxide, and tert-butyl isocyanide has been explored to elucidate its carbene group-transfer capabilities.76 Reaction of 122 with N3Mes results in its conversion to cationic, dinuclear nickel(I) imido complex 124 with elimination of the ketimine Ph2C]NMes, isolated in 80% yield (Eq. 24). The reaction is slow, occurring over a period of 3 weeks at room temperature. Diphenylketene is readily formed by carbene-group transfer in the reaction of 122 with carbon monoxide, resulting in clean conversion to free Ph2C]C]O and the dinuclear nickel(I) dicarbonyl complex [{Ni(IPr) (CO)}2(m-Cl)][B(ArF)4] (125) (Eq. 25). Complex salt 125 is stable in the presence of 1 atm CO, and disproportionation to Ni(0) and Ni(II) is not observed. This is in contrast to the reaction of excess CO with 27 that results in disproportionation to the Ni(0) carbonyl complex Ni(IPr)(CO)3 and the Ni(II) dimer [Ni(IPr)Cl]2(m-Cl)2.76 Addition of an excess of tert-butyl isocyanide to a solution of 122 gives the carbene-coupling product Ph2C]C]NtBu in 76% isolated yield (Eq. 26). Although alkyl isocyanides often give metal products isostructural with the CO analogues, the fate of the Ni(I) fragment in this reaction differs from 124, undergoing dimer dissociation to give the mononuclear Ni(I) salt [Ni(IPr)(CNtBu)3][B(ArF)4] (95) and three-coordinate Ni(I) complex Ni(IPr)(CNtBu)(Cl) (84d) (see Section 8.11.2.4).76

ð24Þ

760

Monovalent Group 10 Organometallic Complexes

ð25Þ

ð26Þ

8.11.2.9

Dinuclear N-heterocyclic carbene complexes

Monodentate, bulky N-heterocyclic carbene, such as IPr and SIPr is significant in the chemistry of dinuclear nickel(I) complexes having anionic bridging ligands, such as halides, alkoxides, thiolates, and selenolates (vide infra). The chloride dimer 2741 and the bromide analogue 3242 have been used as precursors for making various complexes and catalysis. These complexes are good precursor for preparing various nickel(I) complexes as described in the above sections. Catalytic nitrene transfer reaction occurs using a dinuclear Ni(I) complex 126 bearing a bridging imide ligand, which is a product of the reaction of Sigman’s dimer 27 with mesityl azide (N3Mes).76 The coupling reaction of the bridging imide ligand with isocyanide gave the isocyanate derivative accompanied by the regeneration of the starting complex 27 (Scheme 32). The complex 126 is diamagnetic, has a NidNi single bond (NidNi distance is 2.5765(15) A˚ ) and short NidN multiple bonds (NidN distances are 1.762(2), 1.757(2) A˚ ), and is quite similar to the Warren’s b-diketiminato complex [Ni(b-ketiminate)]2(m-NAd) (b-diketiminate ¼HC(C(Me)N(mesityl)2)), where the NidNi bond distance is 2.5057(12) A˚ and the NidN bond distances are 1.732(4), 1.752 (4) A˚ .34 Reaction of complex 126 with NaBArF4 affords the cationic m-chloro nickel(I) dimer 124.

Scheme 32 Nitrene-transfer reaction using nickel(I) chloride dimer 27 and stepwise formation of 126 and 124.

Sigman’s dimer 27 is in equilibrium with the stable mononuclear Ni(I) complex in the presence of an appropriate ligand, such as phosphine, phosphite, pyridine, and 2,20 -bipyridine (Eq. 27).77,88 Catalytic reactions using these mononuclear complexes in the presence of an excess amount of the ligands, such as Kumada-Tamao-Corriu cross coupling, Suzuki-Miyaura cross coupling, and Buchwald-Hartwig amination of aryl halide. Notably, the mononuclear Ni(I) complexes bearing a bulky NHC were thermally stable even in the absence of a redox-active ligand.

ð27Þ

The bridging hydrosulfide complex of nickel(I) (128a) is synthesized by the reaction of nickel(II) bridging sulfide dimer 127 with 1 atm of H2 gas over 24 h at 70  C, in 85% yield (Scheme 33). The production of 128a can be monitored by 1H NMR resonance at d −4.81 (s, 2 H), corresponding to the m-SH units in 128a. Complex 128a can be alternatively synthesized by the salt metathesis

Monovalent Group 10 Organometallic Complexes

761

Scheme 33 Reactions of nickel(II) sulfide dimer 127 with H2 or pinacolborane.

reaction of 27 with 2 equiv of KSH in 92% yield.89 Kinetic studies and DFT calculations revealed that the HdH bond coordinates to a nickel center and is cleaved heterolytically.90 The nickel(II) disulfide dimer 127 also reacts with pinacolborane to afford the S-borylated analogue [Ni(IPr)]2(m-SH)(m-S(pinacol)) (128b), having the similar dinickel framework to 128a.89 Reduction of a three-coordinate nickel(II) complex Ni(IPr)(I)(NO) (129) with Na/Hg affords dinuclear nickel(I) complex bearing bridging iodide and nitrosyl ligands [Ni(IPr)]2(m-I)2(m-NO) (130) (Eq. 28).91 The complex 130 is diamagnetic, and the NidNi bond distance in the X-ray structure is relatively short (2.314(1) A˚ ), compared to that of complex 27 (2.5194(5) A˚ ),41 despite that the large iodide ion bridges.

ð28Þ

The SIPr-ligated nickel(I) bis(phenoxide) dimer (132) is synthesized by the reaction of the nickel(0) Z6-benzene complex Ni(SIPr) (Z6-benzene) (131) with phenol at room temperature, which is a good catalyst for hydrogenolysis of diaryl ether to form phenol and arene derivatives (Scheme 34).92 Oxidative addition of the phenol OdH bond to 131 generates Ni(SIPr)(OPh)(H) and then would provide Ni(SIPr)(OPh)2 and a nickel(II) dihydride. Reductive elimination of H2 from the latter complex would afford 131, which could react with Ni(SIPr)(OPh)2 to form 132. The complex 132 can independently synthesized by the reaction of the Sigman’s dimer 27 with NaOPh. Interestingly, the complex 132 has 2 unpaired electrons and the magnetic moment is 2.89 mB. Thus, there is no bonding interaction between two nickel centers (NidNi distance ¼ 2.9710(7) A˚ ).

Scheme 34 Synthesis of dinuclear nickel(I) m-phenoxide complex 132 via comproportionation reaction.

762

Monovalent Group 10 Organometallic Complexes

In the mechanistic studies of the nickel-catalyzed trifluoromethylselenolation of aryl halides, dinuclear nickel(I) trifluoromethylselenolate-bridged complex is synthesized as a possible intermediate in the catalytic reaction.93 The SIPr-ligated iodide-bridged nickel(I) dimer, [Ni(SIPr)](m-I)2 (133) reacts with (Me4N)SeCF3 to afford the SeCF3-bridged nickel(I) dimer [Ni(SIPr)](m-SeCF3)2 (134) (Scheme 35). The reaction of 134 with 4-fluoro-iodobenzene at 45  C affords (4-fluorophenyl) trifluoromethylselenide in 80% yield relative to total SeCF3 group in 134. If (Me4N)SeCF3 is used as the anionic reagent, a range of aryl iodides are successfully transformed into the corresponding ArSeCF3 products in the presence of complex 133 (10 mol%), avoiding the formation of undesired biaryl side products. This reaction occurred via a pathway like Matsubara’s process,81 involving the efficient catalytic substitution of aryl iodide with (Me4N)SeCF3 to yield trifluoromethylselenoarene. Similarly, the biaryl homo-coupling byproduct is not obtained in this transformation using 133 as catalyst, in contrast to the selectivity of mononuclear Ni(0) catalysts, giving the biaryl homo-coupling byproduct in 11% yield. DFT calculations supported the chemoselectivity of the oxidative addition of aryl iodide rather than aryl selenide to dinuclear nickel(I) species (Scheme 36).93 While Ni(0) clearly preferred addition to the product ArSeCF3 by DDG{ ¼ 6.3 kcal mol−1 (with M06L), for Ni(I)dNi(I), a different reactivity pattern is seen, substantially favoring addition to the aryl halide over the product (by DDG{ ¼ 9.0 kcal mol−1 at M06L), supporting the chemoselectivity of the catalysis.

Scheme 35 Stepwise reactions of nickel(I) iodide dimer 133 with (Me4N)SeCF3 and then 4-fluoroiodobenzene.

Scheme 36 Comparison between nickel(0) and nickel(I) systems in catalytic trifluoroselenation of aryl iodide. The Ni(0) system can activate SedC(Ph) bond rather than IdC(Ph) bond, leading to formation of byproducts from selenoarene, whereas nickel(I) system favored activation of IdC(Ph) bond rather than SedC(Ph) bond.

8.11.3

Monovalent palladium complexes

There are numerous examples of dinuclear palladium(I) complexes reported in these decades. One of the most accessible routes is that treatment of palladium(II) complexes with those corresponding zerovalent metal compounds in the presence of bridging bidentate or tridentate ligands via comproportionation (Method A in Chart 3). Other methods to obtain such dinuclear complexes include 1e-reduction of mononuclear divalent complexes (Method B), the oxidation of zerovalent complexes (Method C), and the oxidative addition to zerovalent complexes to form unsaturated divalent complexes, which are stabilized by forming a metal-metal bonding interaction with a second zerovalent complex (Method D). Rare examples of the photoirradiation of alkylmetal complexes, in which the carbon-metal bond was homolytically broken and smoothly dimerized from the unstable monomeric metal radical to form a metal-metal bond (Method E), have also been reported.

Monovalent Group 10 Organometallic Complexes

Method A

M(II)

+

L

M(0)

Method B

M(II)

L

Method C

M(0)

L

Method D

M(0)

L

red.

L

M(I)

L

M(0)

M(I)

L

M(I)

L

M(I)

L

oxd. R

R

X L

M(I) X

photo-irrad. Method E

763

R

-R

R

L

M(I)

Chart 3

There are significant number of studies exploring the mechanism of catalysis involving dinuclear palladium(I) species which have been isolated and studied in detail. Several possible mechanisms for catalytic transformation by dinuclear palladium(I) complexes can be proposed: (1) Activation of substrates by the intact bimetallic complex, (2) disproportionation of the complex into palladium(II) and palladium(0), which is involved in the catalytic cycle, and (3) homolytic splitting of the complex into two palladium(I) molecules, which are smoothly reduced to palladium(0) or oxidized to palladium(II).94 Most catalytic systems have the case of (2) or (3). When monomeric complexes bearing more than two auxiliary ligands are used as catalyst precursors, liberation of these ligands requires harsh conditions. On the other hand, the Pd(I)dPd(I) bond in dinuclear complexes can be broken quite easily to form the active mono-ligated complex without breaking the stronger metal-ligand bonds. This is probably the mechanism by which dinuclear palladium(I) complexes are active in these catalytic systems. Given that comproportionation occurs from the palladium(0) and palladium(II), the dinuclear complex product can exist as a deactivated intermediate. If the dinuclear complex reversibly disproportionates into the mononuclear complexes, it can be the resting-state product outside the catalytic cycle. However, if there is no equilibrium between them, the formation of the dinuclear palladium(I) complex can be a deactivation process. The activation of substrates by unsaturated palladium(I) dimers that do not undergo splitting into mononuclear complexes in the case of (1) is rare in palladium chemistry.

8.11.3.1

Carbonyl, isocyanide and related complexes

An especially large number of transition metal carbonyl complexes have been reported so far. This is because carbonyl is an excellent p-acceptor and it is possible to stabilize low valence electron-rich metal centers by back donation from metal d-orbital to CO p orbital. However, because of the easiness with which metallic palladium is generated from palladium(II) complexes under a CO atmosphere, the chemistry development of palladium carbonyl complexes, which can be involved in catalytic processes such as carbonylation and formylation reactions, is limited. Synthesis and structures of tetranuclear palladium(I) carboxylate complex bearing bridging carbonyl ligands [Pd2(m-CO)2(m-OCOCH3)2]2 had been studied in the early report.95 Formation of the palladium(I)-CO complex in catalysis is especially preferable for redox processes where easiness of palladium(I) transformation into palladium(0) or palladium(II) is very important feature.96 Since 2006, several significant studies of stabilized carbonyl complexes of palladium(I) have also been reported. How metallic palladium is formed from palladium(II) under a CO atmosphere is of great interest in the chemistry of palladium carbonyl chemistry. In the studies on the palladium/phenanthroline-catalyzed carbonylation of nitroarenes to carbamates and ureas, reversible formation of Pd(RPhen)X2(CO), where RPhen is 1,10-phenanthroline and its derivatives, under CO from Pd(RPhen)X2 is revealed to be an essential step.97 However, this complex is easily decomposed into metallic palladium at room temperature. When using 6,60 -dimethyl-2,20 -bipyridine (Me2Bipy) or 2,9-dimethylphenanthroline (neocuproine), which is replaced to the RPhen, reaction of palladium(II) diiodide Pd(Me2Bipy)X2 (X ¼ Cl (135a), Br (135b), I (135c)) with CO gave the mononuclear carbonyl complex Pd(Me2Bipy)X2(CO) (X ¼ Cl (136a), Br (136b), I (136c)), and by prolonging reaction time, insoluble crystals are precipitated as a stable dinuclear palladium(I) carbonyl complex Pd2(Me2Bipy)2I2(m-CO)2 (X ¼ Cl (137a), Br (137b), I (137c)) (Scheme 37).98 The complex 3 is alternatively synthesized by a slow addition of Pd(dba)2 to a solution of 2, indicating that the in situ formation of 3 is resulted by decomposition of the mononuclear palladium(II) carbonyl complex in part. The complex 2 would be initially reduced to a palladium(0) complex by reaction with CO and trace amounts of water. The so-formed complex would be trapped by still unreacted 2 to afford the dimeric 3. Formation of a small amount of CO2 could indeed be observed by IR. Further reaction of 3 with CO forms metallic palladium. Because the original 2,9-dimethylphenanthroline palladium(I) complex, which can be detected by IR spectra, further decomposed with CO into metallic palladium under the identical conditions, the dimer complex can be an intermediate of the decomposition process in the catalytic carbonylation reactions.

764

Monovalent Group 10 Organometallic Complexes

Scheme 37 Reaction of palladium(II) dihalides 135a–c with carbonyl to form dinuclear palladium(I) carbonyl complexes 137a–c.

Ligand exchange reaction of the acetate with various carboxylate, OCOR where R ¼ CF3, CCl3, CH2Cl, C(Me)]CHMe, CH] CHMe, iPr, nBu, iBu, tBu and nPent, by use of tetranuclear cluster [Pd2(m-CO)2(m-OCOCH3)2]2 (138) and the corresponding carboxylic acids occurs to form polynuclear carbonyl clusters.99,100 Carboxylates where R ¼ CF3, CCl3, CH2Cl, and CH]CHMe yield tetranuclear complexes 139, whereas those containing alkyl substituents, iPr, tBu and nPent, afford hexanuclear complexes 140 (Scheme 38). The origin of the difference could be derived by electronic factors, but the details are unknown. The structure consists of two alternating Pd2(m-CO)2 and two Pd2(m-O2CR)2 units. The coordination mode of all carbonyl ligands is symmetric while the differences between PddC distances are less than 0.02 A˚ and PddCdO angles are equal within 2.8 . Both Pd2(m-CO)2 fragments are planar up to 0.1910(6) A˚ . Comparison of IR analysis among these complexes demonstrated that the increasing acceptor ability of the R group in the carboxylate ligands is accompanied with increasing of nCO value of the bridging CO ligand100: the complex where R ¼ CF3 is the highest among them, and the alkyl substituents make them lower than those containing halogen substituents (Table 1).

Scheme 38 Reactions of tetranuclear carbonyl complex 138 with several carboxylic acids.

Table 1

CO stretching bands of carbonyl ligand in poly(R-carboxylato)palladium(I) clusters.

R

CF3

CCl3

CH2Cl

MeCH]CMe

Me

i

Pr

Bu

t

Bu

C5H11

nCO (cm−1)

2000 1968

1999 1967

1979 1934

1975 1954

1976 1934

1974 1944

1972 1936

1976 1932

1968 1936

Monovalent Group 10 Organometallic Complexes

8.11.3.2

765

Palladium(I)-carbon s-bonded complexes

There were many examples of dinuclear palladium(I) complexes bearing hydrocarbon ligands before 2006.7–9 The first dinuclear palladium(I) allyl and cyclopentadienyl complexes had been studied by Yamazaki101 in 1972 and Werner102 in 1975. After these studies, various attractive studies concerning the ligand variation, reactivity, and theoretical calculations had been conducted. Most of the complexes are obtained in the reaction of palladium(II) allyl or cyclopentadienyl complexes with a palladium(0) source. After the last COMC, a great effort to uncover the chemistry of palladium(I) m-allyl complex in detail have also been made.

Ph3P

Pd

Pd PPh 3

Ph 3P

Pd

I

8.11.3.2.1

Pd PPh 3

季

Palladium(I) complexes with allyl ligands

The dinuclear palladium(I) m-allyl complex 141 using a chelating NP ligand, phosphanyl-oxazoline (phox) is synthesized as an intermediate in catalytic allylic substitution reaction of allyl-benzoyl.103 This dinuclear palladium(I) is alternatively formed in the reaction of palladium(0)-phox 142 with mononuclear palladium(II) p-allyl phox complex 143 or reduction of the half amount of the palladium(II) p-allyl 143 with malonate. The stable configuration of the 1,3-diphenylallyl ligand in the dinuclear complex 141 is syn-anti, whereas that in the mononuclear complex 143 is syn-syn. Therefore, a dinuclear formation pathway, such as intermolecular nucleophilic attack of palladium(0) to the allyl moiety and subsequent PddPd bond formation along with the CdC bond rotation, is proposed. However, because mononuclear cyclic p-allyl complex also forms the analogous dinuclear m-allyl complex, direct PddPd bond formation (path A) in the reaction of palladium(0) and palladium(II) is preferred in this case (Scheme 39).

Scheme 39 Possible pathways A and B to form dinuclear palladium(I) m-Z3-allyl complex 141.

Nucleophilic substitution of allylbenzoate with malonate catalyzed by 143 is an example of the formation of a dinuclear structure proposed to be an off-cycle product that stabilized the active mononuclear intermediates.103 A reversible routes from the palladium(0) complex 142 and palladium(II) p-allyl complex 143 to the dinuclear palladium(I) m-allyl complex 141 is possible. However, further reaction of 141 with malonate did not yield the substitution product, indicating that 141 is not active in the catalytic transformation. This dinuclear palladium(I) complex 141 reversibly produces the mononuclear complex 143 and 142 in the presence of allylbenzoate (Scheme 40).

766

Monovalent Group 10 Organometallic Complexes

Scheme 40 Proposed cycle of catalytic allylic substitution and a comproportionation path to the palladium(I) dimer 141 as a resting state.

Reaction of mononuclear bis-p-allyl palladium(II) with free ligand or reaction of Z3-allyl palladium(II) chloride with free ligand and the sequential addition of an allyl Grignard reagent affords dinuclear palladium(I) bis(m-Z3-allyl) complexes, [Pd(L)]2(m-Z3-allyl)2 (L ¼ PMe3 (144a), PEt3 (144b), PPh3 (144c), SIPr (144d)), or [Pd(L)]2(m-Z3-2-methallyl)2 (L ¼ PMe3 (144e), PEt3 (144f), PPh3 (144g), SIPr (144h)) (Scheme 41).104 Interestingly, CO2 inserts into the palladium-carbon bond of dinuclear palladium(I) bis(m-Z3-allyl) complexes as an electrophile to form bridging carboxylate complexes 145, which led to further catalytic processes as described below (Eq. 29). There is a significant back-donation from the ds −ds and dp−dp orbitals of the PddPd bond to the p orbital of the bridging allyl ligand and proposed that the allyl ligand is negatively charged as reported by Kurosawa et al.105 However, other type of m-Z3-allyl complexes, such as that one of the bridging ligands is m-chloride or m-carboxylate, are not favorable for the CO2 insertion (Eq. 30).

Scheme 41 Synthetic routes to dinuclear palladium(I) bis(m-Z3-allyl) complexes 144a–h.

ð29Þ

Monovalent Group 10 Organometallic Complexes

767

ð30Þ

Photoelectron spectroscopy of the analogous dinuclear palladium(I) m-Z3-allyl complexes 146, 147, 148 having an anionic phosphine PPh2(3-SO−3-Ph) instead of PPh3 reveals that that a low-energy electron detachment feature is present in the species with two bridging allyl ligands. Density functional theory calculations indicate that the HOMO in complex 146 with two bridging allyl ligands is localized on the terminal carbon atoms of the bridging allyl ligands, which make the out-of-phase combination of p2 orbitals, leading to the low-energy band.106 Contrarily in complexes 147 and 148 with one bridging allyl and one bridging chloride ligand, and one bridging allyl and one bridging carboxylate ligand, respectively, the HOMO is a PddPd bonding orbital. Therefore, relative weakness of the palladium-allyl bond in doubly bridged allyl dimers means that CO2 insertion into the bridging allyl ligand is thermodynamically favorable for these species, whereas it is unfavorable for species that contain a bridging carboxylate or chloride ligand.

The reactive doubly bridged allyl dimers 144b–d can react with some protic reagents such as diphenylacetic acid, thiophenol, and hydrochloride, to form dinuclear palladium(I) mono-m-allyl complexes having bridging carboxylate 149b–d (Eq. 31), thiolate 150b–d (Eq. 32), and chloride 151b–d (Eq. 33), respectively.104 These reactions and properties indicated that the bridging allyl ligand in the dinuclear palladium(I) behaves like Z1-allyl ligand in mononuclear palladium(II) complexes.

ð31Þ

768

Monovalent Group 10 Organometallic Complexes

ð32Þ

ð33Þ

The dinuclearpalladium(I) bis(m-Z3-allyl) complexes 144 can act as catalysts in the allylic carboxylation of allylstannane and allylboronate with CO2.104 Scheme 42 represents the proposed mechanism of this reaction. Stoichiometric reaction of 144 with CO2 afforded a bridged mono-carboxylate dinuclear palladium(I) complex 145 via facile insertion of CO2 into the palladiumm-allyl bond. Subsequent addition of allylstannane and allylboronate gave the starting m-allyl complex accompanied by liberation of allylcarboxylate. Detailed theoretical calculations also indicated that the dinuclear framework of the dinuclear palladium(I) catalyst remains intact during the catalytic allylic carboxylation.107 However, the addition of weakly coordinating ligands, such as acetonitrile and 1-octene, accelerated the catalytic reaction. Moreover, the further addition of these ligands or the addition of other ligands yielded mononuclear palladium(0) complexes and palladium black.108 Therefore, another catalytic pathway involving insertion of CO2 into a mononuclear palladium(II) complex has been proposed. In this pathway, coordination of the ligand L0 stabilizes the disproportionated mononuclear palladium(0) complex in an equilibrium with the dinuclear complex, and the accompanying formation of the s-allyl palladium(II) complex enables the insertion of CO2 into the Pd-carbon bond (Scheme 42).

Scheme 42 Proposed mechanisms in the catalytic allylic carboxylation of allylstannane with CO2.

Buchwald-Hartwig amination of aryl bromides with primary and secondary amines has been mediated by an in-situ generated catalyst, containing di(tert-butyl)neopentylphosphine and p-allyl palladium(II) chloride dimer.109 The intermediary complexes,

Monovalent Group 10 Organometallic Complexes

769

Pd(PtBu2Np)2 (152), PdCl(PtBu2Np)(m-Z3-allyl) (153), and Pd2(PR3)2(m-Z3-allyl)(m-Cl) (154) are isolated and determined. When the coupling reaction of 4-bromotoluene with aniline is conducted with high loading (7.5 mol%) of the palladium (II) catalyst precursor 153, the complex 154 can be detected in the 31P{1H} NMR spectrum of the reaction mixture, indicating that, in the presence of an alkoxide nucleophile, the complex 153 is rapidly converted to the Pd(I) dimer 154. Presumably, the reaction of tert-butoxide with 153 results in the generation of a 12e active species, Pd(PtBu2Np) (155), which is then trapped by unreacted 153 to give 154. The complex 154 is a significantly less effective precatalyst in the coupling of 4-bromoanisole and N-methylaniline, resulting in only 9% conversion after 1 h, while the identical reaction with 153 provides 93% conversion. Thus, the dinuclear palladium(I) complex 154 is an off-cycle product, and disproportionation to form mononuclear palladium(0) and palladium(II) is proposed by Hazari et al. to generate as active species in the catalytic cycle (Scheme 43).110

Scheme 43 Formation of dinuclear palladium(I) m-Z3-allyl complex 154 in the palladium-catalyzed Buchwald-Hartwig amination of aryl bromide.

Amidinate-based tetradentate chelate-bridging ligands, such as N,N0 -bis[2-(diphenylphosphino)phenyl]formamidinate (dpfam), have been designed to stabilize dinuclear palladium(I) m-Z3-allyl complex 156.111 The complex can be synthesized by a reaction of bisp-allyl palladium(II) dimer, [Pd(p-allyl)]2(m-X)2 (X ¼ Cl (157a) or OAc (157b)) with the protonated chelating ligand precursor in the presence of TMEDA (TMEDA ¼ N,N,N,N-tetramethylethylenediamine) via transmetallation (Scheme 44). Mechanistic studies of the

Scheme 44 Synthetic routes to dinuclear palladium(I) m-Z3-allyl complex 156 bearing tetradentate N,N0 -bis[2-(diphenylphosphino)phenyl]formamidinate ligand.

770

Monovalent Group 10 Organometallic Complexes

formation of the palladium(I) complex indicates that a palladium(0) intermediate forms by reductive elimination of allyl-X (X ¼ Cl or OAc) from 157a or 157b and subsequent reaction between the palladium(0) and Pd(dpfam)(Z3-allyl), which is generated from dpfamH and 157a or 158b. Terminal substituents of the allyl moiety, such as methyl and phenyl groups, makes a kinetic mixture of syn/anti stereoisomers. Upon addition of tert-butylisocyanide, the stereoisomers isomerize to afford the thermodynamic mixture in equilibrium. Palladium(I) m-allyl dimer formation is a general phenomenon in reaction mixtures that use precatalysts of the type Pd(L) (Z3-allyl)Cl, which had been previously thought to only involve palladium(0) and palladium(II) complexes.110 Furthermore, the palladium(I) m-allyl dimer formation can be a deleterious process that removes an active palladium(0) species from the reaction mixture. The IPr-supported m-Z3-allyl palladium(I) dimers (allyl ¼ allyl (158a), butenyl (158b), 1-phenylallyl (158c)), which are rare examples with 1-substituted m-allyl ligands, are synthesized by the reaction of Nolan’s Pd(IPr)Cl(Z3-1-R-allyl) (R ¼ H (159a), Me (159b), phenyl (159c))112,113 with K2CO3 in the presence of ethanol.110 When methyl (Z3-butenyl) and phenyl (Z3-1-phenylallyl) groups are substituted at the terminal carbon of the m-Z3-allyl moiety, steric hindrance between these substituents and the bulky IPr ligand makes the dinuclear framework more unstable to enhance the kinetic barrier of comproportionation from the palladium(II) Z3-1-R-allyl 159 and IPr-palladium(0) complex, which can be easily formed as the active species in catalysis (Scheme 45). By increasing the barrier to comproportionation between palladium(0) and palladium(II) or by developing systems that activate sufficiently quickly so that all of the palladium(II) is converted to palladium(0) before comproportionation can occur, it should be possible to develop precatalysts that are even more active.

Scheme 45 Proposed pathway for activation of palladium(II) Z3-allyl precatalysts 159a–c, involving an equilibrium between dinuclear palladium(I) m-Z3-allyl complex 158a–c and palladium(0) active species with 159a–c, which can be sterically controlled by 1-substituents of the allyl moiety.

Five dinuclear palladium(I) complexes, [Pd(IPr)]2(m-Z3-2-R-allyl)(m-Cl) (R ¼ Me (160a), Ph (160b), tBu (160c), OMe (160d), and CN (160e)), are synthesized by reactions of palladium(II) Z3-2-R-allyl complex, Pd(IPr)Cl(Z3-2-R-allyl) (R ¼ Me (161a), Ph (161b), tBu (161c), OMe (161d), and CN (161e)) with K2CO3 (Eq. 34). Electronic effects of the substituent R on the stability of these dimeric structures are systematically investigated.114 Structural features of 160a–e are similar to those discussed previously by Kurosawa et al.115 The central carbon atom of the m-Z3-allyl ligand is canted in toward the PddPddCl bonding plane. This orientation in 160 is in contrast to the orientation of the central carbon in p-allyl ligands of 161, which is canted away from the coordination plane of the Pd(II) center. This orientation of the m-allyl ligand results from a back-bonding interaction from the PddPd bond into the p orbital of the m-allyl ligand, according to Kurosawa et al.115 Consistent with the proposal, y deviates from 88.3 in 160d to 71.9 in 160e. Presumably, for example, the electron-donating methoxy group raises the energy of the p orbital of the m-allyl ligand and weakens the back-bonding, which results in a higher value of y.

ð34Þ

Comparison of equilibrium constants Keq obtained for crossover experiments between the monomeric IPr-palladium(II) Z3-allyl 161 and the dimeric IPr-palladium(I) m-Z3-allyl 160 reveals that electron-withdrawing substituents in the 2-position of the allyl ligand thermodynamically stabilize the dimeric form, with the following trend observed for dimer stability: dCN (160e) >> H (158a) > Ph (160b) > Me (160a) > tBu (160c)  OMe (160d).114 The similar results had been obtained in the early study by Kurosawa et al.115 There is an inverse correlation of the catalytic performance of the corresponding palladium(II) congener 161 in Suzuki-Miyaura reaction. Therefore, if the electron-withdrawing group, such as dCN, is substituted at the 2-position of the allyl

Monovalent Group 10 Organometallic Complexes

771

ligand in 160e, palladium(II) Z3-2-R-allyl complex 161e can be rapidly activated to form the IPr-palladium(0) species, which, readily comproportionates with the starting palladium(II) Z3-allyl 161e into the poorly reactive palladium(I) m-Z3-allyl dimer 160e (Scheme 46). This phenomenon indicates dinuclear palladium(I) complex can play the crucial role in catalytic reaction that only involve palladium species in the 0 and II oxidation states.

Scheme 46 Proposed deactivation pathway in a catalytic reaction using 2-CN-substituted complex 161e, because of fast comproportionation into the unreactive dimer 160e.

The mechanism of comproportionation of palladium(0) and palladium(II) has been investigated using DFT calculations.114 In order to simplify the reaction pathways, IPh (IPh ¼ 1,3-diphenyl-imidazol-2-ylidene) was employed as the auxiliary ligand for calculations. In the first step, which is rate determining, the iPrOH-stabilized palladium(0) species interacts with the allyl ligand of Pd(IPh)Cl(Z3-2-R-allyl) (161-IPh) at TS1. This generates an intermediate (INT) in which the m-Z3-2-R-allyl ligand and the Pd − Pd bond have been formed. Subsequently, in a relatively low energy process, the chloride ligand displaces the iPrOH molecule and bridges the Pd centers to form the palladium(I) dimer 160-IPh (Scheme 47).

Scheme 47 Calculated mechanism for comproportionation of palladium(0) and (II) monomers into the corresponding dimer 160-IPh.

8.11.3.2.2

Palladium(I) complexes with cyclopentadienyl ligands

A reaction of palladium(II) Z3-indenyl chloride dimer 162 with n-butylamine forms mononuclear palladium(II) Z1-indenyl (Ind) intermediate, (Z1-Ind)Pd(PR3)(BnNH2)Cl (R ¼ Cy (163a), Ph (163b)), which rapidly decomposed into two complexes including dinuclear palladium(I) m-Z3-indenyl m-chloride, (m-Z3-Ind)(m-Cl)Pd2(PR3)2 (R ¼ Cy (164a), Ph (164b)), and mononuclear palladium(II) complex (BnNH2)(PCy3)PdCl2 (R ¼ Cy, Ph) (Method A in Scheme 48).116 Various other reaction pathways to produce the dinuclear palladium(I) complex 164 are possible: (1) reaction of palladium(II) Z3-indenyl complex (R ¼ Cy (165a), Ph (165b)) with n-butylamine giving 164 via formation of 163 (Method B), (2) partial reduction of 165 with 0.5 equiv of n-BuLi, via formation of the half amount of palladium(0) intermediate (Method C), and (3) direct reaction of palladium(II) complex 162 or 165 with Pd(PPh)4, occurring comproportionation similar to the method C (Method D). The bonding mode of the bridging indenyl ligand is as same as that of the dinuclear m-Z3-allyl complexes, as summarized above.

772

Monovalent Group 10 Organometallic Complexes

Scheme 48 Reaction paths to dinuclear palladium(I) m-Z3-indenyl complexes 164a–b.

Three mixed-ligand palladium(I) dimers, [Pd(PEt3)]2(m-Z3-Cp)(m-Z3-allyl) (166), [Pd(PEt3)]2(m-Z3-Ind)(m-Z3-allyl) (167), [Pd(PEt3)]2(m-Z3-Cp)(m-Z3-Ind) (168) are synthesized, and the electronic structures of those containing a combination of bridging allyl, Cp and indenyl ligands with those of the bis(m-allyl) (144b),104 bis(m-cyclopentadienyl) (169),117 and bis(m-indenyl) (170) complexes are discussed (Chart 4).118 Preparation of bis-Ind complex 170 is unsuccessful. The complex 166 can be made from mononuclear palladium(II) Z3-allyl-Z5-cyclopentadienyl complex with phosphine via formation of intermediary palladium(0) in part by reductive elimination of allylcyclopentadiene, or transmetallation of dinuclear palladium(I) m-allyl m-chloride with CpNa, for example. Detailed studies on the properties of the complexes revealed significant electronic characteristics of m-Z3-Cp ligand compared to the other allyl and indenyl ligands. The primary bonding interaction between the Pd centers and the bridging allyl, Cp,

L Pd

L Pd

Pd L

L Pd

Pd L

L = PEt3

144b

169

170 (not obtained)

L = IPr

171

172

173

L Pd

L = PEt3 L = IPr Chart 4

Pd L

Pd L

166 174

L Pd

Pd L

167 175

L Pd

Pd L

168 176

Monovalent Group 10 Organometallic Complexes

773

or indenyl ligands occurs through the in-phase combination of the closely related allyl p2 orbitals, the Cp p2bCp orbitals, and a mixture of the indenyl p2bIn(1) and p2bIn(2) orbitals. Cp ligands are more likely to promote an anti configuration of the bridging ligands than either allyl or indenyl ligands. Complexes containing a bridging Cp ligand, where the HOMO is greatly contributed from the Pd centers, are proposed to be less nucleophilic than those containing bridging allyl or indenyl ligands, where the HOMO is centered on unstable ligand orbitals. In the case of Cp, there is some additional donation from the p2aCp orbital; the bridging Cp ligand donates more than three electrons. The bridging fragments bind with different strengths to the Pd2 core, with allyl binding the tightest and Cp the weakest. N-Heterocyclic carbene can stabilize the palladium(I) complexes having bridging allyl, Cp, and/or indenyl ligands (Chart 4), providing the complete series of six mixed-ligand IPr-analogues of palladium(I) dimer containing bis(m-allyl) (171),104 bis(m-Cp) (172), bis(m-indenyl) (173), (m-allyl)(m-Cp) (174), (m-allyl)(m-indenyl) (175), or (m-Cp)(m-indenyl) (176) in the similar ways to those of the triethylphosphine complexes.119 In contrast to the related triethylphosphine-supported complexes, the IPr-supported species are thermally stable at room temperature. Although synthesis of bis(m-Z3-indenyl) palladium dimer is unsuccessful using triethylphosphine as the auxiliary ligand,118 that having IPr, 173, is isolable and characterized in detail.119 The complexes also have the similar dinuclear palladium core structures and the similar trend in electronic properties to them.

8.11.3.3

Palladium(I)-carbon p-bonded complexes

Compared with the dinuclear palladium(I) allyl complexes, there have been very few reports of arene palladium(I) dimers, where the arene bridges between both palladium centers. In contrast to allyl ligand, arene ligand is weakly coordinated and sometimes thermodynamically unstable to easily eliminate from the metal center. However, the ligand can bridge over multi metal centers and has a potential to stabilize the dinuclear framework. One early report of a complex described by Allegra in 1965 and 1970 is that of (PhdPd)2(AlCl4)2 (A) prepared by refluxing PdCl2, Al, and AlCl3 in benzene.120,121 After these studies, there have been a trinuclear Pd “sandwich” complex (B)122 and a palladium(I) dimer with sulfur based ligands (C).123 Other palladium(I) dimers include a 124 (dppp-Pd)2+ triphenylphosphine complex (E),125 and a biarylphosphine palladium(I) dimer (F)126 bearing a 2 complex (D), bridging bromide have also been studied (Chart 5). Until now, several palladium(I)-carbon p-bonded complexes have been emerged from the last COMC. The bridging-arene complex can be a good precursor for catalysis in these reports because of its labile character to stabilize a coordinatively unsaturated active palladium(0) species.

2+

Ph

Ph

Ph

Cl3Al Cl Pd

2+

Ph

Pd Cl

Ph Ph P Ph P Pd Pd P Ph Ph P Ph

AlCl 3 A

Me Pd S

S Pd

D

Me

Ph 3P Pd

Pd B

Pd PPh 3 PPh 2

F 2+

2+

Ph 2P

t Br Bu Bu P Pd Pd Br

t

PPh 2 Ph

Ph Ph

C

Ph

MeCN Pd

Pd NCMe

Ph 2P E

季

Chart 5

8.11.3.3.1

Palladium(I) complexes with arene ligands

Dicyclohexyl(biaryl)phosphines, which are well-known as excellent ligands for catalyst precursors in various palladium-catalyzed cross-coupling reactions, can bridge between two palladium(I) centers to form air-stable palladium(I) dimer 178,127 similar to the Vilar’s complex.126 The complex 178 can be obtained by the reduction of palladium(II) dicyclohexyl(biaryl)phosphine complex 177 with silver tetrafluoroborate salt, leading to the coordination of the m:Z2:Z2-aryl moiety of the biarylphosphine ligand as a bridge between the two unsaturated metals (Eq. 35). The existence of 20 ,60 -dimethoxy groups on the biaryl substituent is essential to stabilize the dimeric structure; spectral and theoretical electron topographical analyses shows that the PddCipso bond has s-bonding nature, indicating existence of arenium ion, which is stabilized by p-donation from the ortho-methoxy oxygen atoms. On the other hand, the PddCpara bond was revealed to possess p-character.

774

Monovalent Group 10 Organometallic Complexes

2 BF4 -

OMe 2 equiv. AgBF 4 P OMe

MeO

2

P Pd MeO

PdCl 2

OMe Pd P

ð35Þ

MeO 177

178

With use of the cationic palladium(I) dimer 178 bearing the biarylphosphine ligand, efficient Suzuki-Miyaura coupling of aryl chloride can be accomplished.127 The complex 178 cannot activate aryl chloride at low temperature, and formation of the active complex at a temperature above 60  C may be necessary. Disproportionation of complex 178 gives not only palladium(0) complex but also palladium(II) complex, which would be easily reduced to palladium(0) by transmetallation with arylboronic acid to form biphenyl as a by-product (Scheme 49).

2

2 BF4 -

OMe

P Pd MeO MeO

B(OH)2

2 BF 4-

OMe Pd P

P PdII MeO

disproportionation

P Pd 0 MeO

+

MeO

MeO 178

Scheme 49 Proposed activation pathway in the catalytic Suzuki-Miyaura coupling using palladium(I) biarylphosphine dimer 178 via disproportionation.

A series of the similar palladium(I) dimers 179–184 having biarylphosphines, where the biarylphosphines are JohnPhos (179), DavePhos (180), tBuPhos (181), XPhos (182), SPhos (183), and RuPhos (184), are directly synthesized from Kurosawa’s dicationic palladium(I) dimer [Pd2(MeCN)6]2(SbF6)128 with the corresponding biarylphosphines L1–L6 efficiently (Scheme 50).129 Only when SPhos L5 is added to the palladium(I) dimer, Pd-black forms within minutes and the high-yield formation of the corresponding biarylphosphine dimer 183 requires shorter reaction time (1 min). In contrast, RuPhos L6 provides quantitative formation of the dimer complex 184 without Pd-black precipitation. The obtained dimer complexes are stable as solids indefinitely when stored in air at 5  C, while only the complexes having SPhos 183 and RuPhos 184 are air-stable even in solutions on the bench top under atmospheric conditions. In the catalytic application to the Buchwald-Hartwig amination of aryl halides with morpholine, all the synthesized dimers are active, and the RuPhos dimer 184 is the best precatalyst for the cross-coupling. 2+

L Pd2 (dba)3 + [Pd(MeCN)4][BF4]2

1:1 MeCN/CH 2 Cl2 1 h, r.t.

L

2 BF 4

Rn R R P Pd

Pd L L

L Pd

2(BF 4-)

L L = MeCN

2 equiv. L1-L6 CH2Cl2 5-30 min, r.t.

Pd P R R Air stable Pd(I) dimer precatalysts Rn

179 (L1, JohnPhos) 180 (L2, DavePhos) 181 (L3, tBuXPhos) 182 (L4, XPhos) 183 (L5, SPhos) 184 (L6, RuPhos)

Scheme 50 Synthesis of palladium(I) dimers 179–184 using various biarylphosphines L1–L6.

PR2 R'

i

Pr

i

PR2 Pr R

i Pr L3: R = tBu L1: R = tBu; R' = H (JohnPhos) (tBuXPhos) L2: R = Cy; R' = NMe2 L4: R = Cy (XPhos) (DavePhos)

PCy 2 R

L5: R = OMe (SPhos) L6: R = OiPr (RuPhos)

Monovalent Group 10 Organometallic Complexes

775

Phosphide- and arene-bridged palladium(I) complexes 185a–c having biaryldicyclohexylphosphine, PCy2(2-(2,6-R-Ph)Ph) (R ¼ OMe (185a), OiPr (185b), Me and H (185c)) can be simply synthesized from palladium acetate and the biarylphosphines in methanol solution at 80  C and subsequent reaction with Na+[PF6]− via PdC bond cleavage (Eq. 36).130 Depending on the reaction solvent, the formation of 185 fell in the order MeOH > EtOH > MeOCH2CH2OH > n-butanol, while no product was observed with t-amyl alcohol or water. When the reaction is conducted in toluene at 60  C without addition of the salt, a divalent 4-membered palladacycle complex is produced via ortho-metalation of the PdAr group. However, the correlation between these complexes and the mechanism of formation of the dinuclear palladium(I) complex is unclear.

R2

Pd(OAc)2 + o-biarylphosphine (1.5 equiv)

(i) MeOH 85 C, 2 h

+ PF 6-

R1 Pd PCy 2

R2

Cy2 P Pd R1 P Cy Cy

(ii) excess NaPF 6

ð36Þ

185a R1 = R2 = OMe, 39% 185b R1 = R2 = OiPr, 61% 185c R1 = Me; R2 = H, 10% Reaction of palladium(0) bis(tert-butyl)phenylphosphine complex with Pd(cod)Br2 affords a palladium(I) dimer 186 where the phosphine ligands bridge between two palladium atoms (Scheme 51), as reported by Vilar et al.131 The phenyl group of phosphine coordinates to the remote palladium center, while bromine atoms locate at the terminal position, in contrast to the halogen-bridging palladium(I) dimer using bulky trialkylphosphine ligands.126,132,133 Density functional theory calculations reveals that the interaction between the bridging phosphine and the remote palladium center contains contributions from both the C]C p and PdC s bonds. The analogous arene-bridging structure in arylphosphine-ligated palladium(I) dimer has also been seen in early studies by van Leeuwen et al.124 in 1992 and Kurosawa et al. in 2000.125 Interestingly, the iodide analogue of 186 cannot be obtained. Addition of CH3I, used as a good source of iodide,132 to the in situ-generated palladium(0) complex, Pd(PtBu2Ph)2, affords palladium(I) m-iodide dimer, [Pd(PtBu2Ph)(m-I)]2 (187) (Scheme 51). DFT calculations reveals that the critical feature is that while the m-halide structure is marginally less stable than its semibridged phosphine counterpart for the bromide complex (DE ¼ 2.1 kcal mol−1), the opposite is true for the iodide analogue (DE ¼ −4.4 kcal mol−1), consistent with experiment. Vilar et al. noted that the precise nature of the bonding within the semibridging architecture is highly sensitive to steric factors. Given the very small energy differences between the two isomeric forms in each case, subtle differences in the steric interactions between the phosphine and the halide might cause the switch in structure.

t

Bu

tBu

P

Br Pd

Pd Br P

Pd(cod)Br2

t

t

1/2 Pd2(dba)3

P Bu2Ph

186

Pd(P tBu2 Ph)2 CHI3 Bu

P Pd tBu

Bu

Bu

t

Bu

I t

t

t

Pd P

Bu

I 187

Scheme 51 Synthesis of different palladium(I) dimers 186 and 187 using bis(tert-butyl)phenylphosphine and palladium(0) precursor.

Air-stable dinuclear complex Pd2(PPh3)2(OdSO2CF3)2 (188) can be synthesized by exposure of a solution of Pd(PPh3)2 (OAc)2 with CF3SO3H in methanol (Eq. 37).134 The structure of 188 is similar to the previously reported dinuclear arylphosphine derivatives.124,125,131 One of three phenyl groups of triphenylphosphine binds to the remote palladium center via the C]C p-bond. Linear-type trinuclear complex 189 containing two terminal palladium(I) and one central palladium(0) is also afforded as crystals from the solution (Eq. 37). These complexes can mediate catalytic hydromethoxycarbonylation of alkenes, which uses the similar

776

Monovalent Group 10 Organometallic Complexes

conditions for the in-situ generated catalyst in the previous study by Williams et al.135 However, mechanism of the formation of the complexes and the precatalyst activation pathway in the catalysis are unclear.

Ph Ph P

Ph Ph P

CF 3SO3 H (3 eq) (PPh3 )2 Pd(OAc)2

(F3C)O2 SO

Pd

Pd P

188

OSO2(CF 3)

Ph Ph

Ph3P

+

Pd

Pd

Pd P

2

1

PPh3

Ph Ph

189 ð37Þ

8.11.3.4

N-heterocyclic carbene complexes

Bulky monodentate NHC can kinetically stabilize coordinatively unsaturated iodide-bridging palladium(I) dimer [Pd(IPr) (m-I)]2 (191).136 The complex can be synthesized by reaction of IPr-palladium(II) iodide dimer [Pd(IPr)I]2(m-I)2 (190) with KOH/methanol (Eq. 38), while mononuclear palladium(II) hydride [Pd(IPr)2BrH] (193) is afforded from the bromide dimer [Pd(IPr)Br2]2 (192) under the identical conditions (Eq. 39). Excellent catalytic activities of 191 in cross coupling reactions of aryl halide can be performed, in which higher efficiency is observed compared to that of the tert-butylphosphine palladium(I) dimer [Pd(PtBu3)(m-I)]2.132,133,137

ð38Þ

ð39Þ

A cationic dinuclear palladium(I) hydride complex 195 can be stabilized by a bidentate methylene-bridged NHC ligand equipped with bulky N-mesitylene substituents.138 The complex can be synthesized by base-assisted reduction of a dicationic, mononuclear bis(NHC) palladium(II) complex 194 (Eq. 40). A pair of the bis(NHC) ligands bridge between two palladium(I) centers and the hydride ligand locates at the terminal position in 195 detected in the neutron crystal structure. Attempts to obtain N-alkyl analogs are in fail, giving only palladium black, suggesting the bulky mesitylene groups have a critical role to stabilize the dinuclear palladium form.

ð40Þ

A silver complex bearing a tridentate P^C^P type phosphine/NHC mixed ligand reacts with Pd2(dba)3 in acetonitrile led to the formation of homotrinuclear dicationic palladium complex (196), accompanied by the dinuclear dicationic palladium complex (197) via transmetallation and metal-oxidation (Scheme 52).139 While 196 has one central palladium(II) and two terminal palladium(0) atoms, 197 contains two palladium(I) atoms, which make a PddPd covalent bond. Oxidation of the palladium(0) center occurs with Ag+, accompanied with transmetallation and an Ag mirror forms after the reaction. However, the mechanism in the formation of these two isolated complexes is unknown.

Monovalent Group 10 Organometallic Complexes

tBu P 2

N

2(OTf-)

Pt Bu2 SOCF3

O Ag

2(OTf-)

N

O Ag

Bu 2P N

N PtBu 2

Pd II

PdII

t

Pd2(dba)3

Ag

Pd 0

Bu2 P

N

N

Bu 2P N

Pt Bu2

2(OTf-) Bu 2P N

N PtBu 2

t

PdI

+

t N P Bu 2

t t

777

PdI

t

t N P Bu 2

Bu 2P N

196

197

Scheme 52 Synthesis of palladium complexes 196 and 197 using a tridentate P^C^P type phosphine/NHC mixed ligand.

8.11.4

Monovalent platinum complexes

There are limited numbers of organoplatinum(I) complexes, characterized and investigated in detail. All of them are dinuclear complexes, as same as the organopalladium(I) complexes. Dinuclear platinum(I) m-Z3-allyl m-Z3-cyclopentadienyl complexes had been synthesized and studied in detail in the early studies since 1977.140 Other bridging ligands have also been revealed to be effective to stabilize the dinuclear platinum(I) framework: for instance, platinum(I) dimers having m-phosphido ligands have been investigated in detail before 2006.141 Ethylene, norbornadiene, carbonyl, isocyanide can coordinate to the platinum(I) center of the Pt2(m-phosphido)2 framework as terminal ligands to afford diamagnetic organoplatinum(I) complexes. A few complexes have been synthesized and studied since 2006.

8.11.4.1

Carbonyl and related ligands

Phosphide derivatives are generally protonated to form hydrophosphines, whereas terminal metal hydride or PdH agostic metal is frequently formed via protonation if basic metal site is present in bridging metal phosphide. Thus, a series of bis(phosphido)bridged dinuclear platinum(I) complexes with a common Pt2(m-PtBu2)2 core and variable terminal ligand (CO, CH2]CH2, or phosphines), bonded to each platinum center is prepared by the ligand exchange reactions of Pt2(m-PtBu2)2(PtBu3)2.142 Dicarbonyl complex 199 can be obtained when the dinuclear platinum precursor Pt2(PHtBu2)(CO)(m-PtBu2)2 (198) is pressurized (8 MPa) under a CO atmosphere at 25  C for 12 h (Eq. 41). Complex 199 is thermally stable and is not oxidized in the solid state or in solution, but is moisture sensitive, readily leading to transform into trinuclear hydride complex Pt3(m-PtBu2)3(CO)2(H).143 In the temperature-controlled experiment with acid, complex 199 is protonated at phosphorus, because the basicity of the platinum center is reduced by the CO ligands. The CD2Cl2 solution obtained by treating 199 with an equimolar amount of CF3SO3H at −60  C contains unique product 200 (Scheme 53), which contains one bridging phosphide and one terminally bonded secondary phosphine in mutual trans position. In contrast, when the similar complex bearing terminal tert-phosphine, Pt2(m-PtBu2)2(PR3)2 ((PR3)2 ¼ (PPh3)2 or (PtBu2H)(PPh3)), protonation occurs at one of two platinum centers. Due to its unsaturation, complex 200

Bu tBu

t

t

Bu tBu P

P OC Pt

OTf-

OC

H+

Pt CO

Pt

-60 o C P

tBu

P

t

Bu

Bu tBu

t

Pt

CO

H 200

199 20 oC CO Bu tBu

t

OTf-

P

H

OC

CO Pt

Pt

t

Bu P t

Bu

CO H 201a

+

OTf-

Bu tBu

t

t

P

P

Bu Pt t Bu OC

CO Pt

201b

Scheme 53 Reactions of dinuclear platinum(I) carbonyl complex 199 with carbonyl in the presence of acid.

CO

778

Monovalent Group 10 Organometallic Complexes

decomposes rapidly on warming to 20  C to give a complex mixture of unidentified products; however, it can be intercepted by operating under a carbon monoxide atmosphere. Under these conditions, the stable tricarbonyl derivative, [Pt2(m-PtBu2)(PtBu2H) (CO)3]CF3SO3 (201) is quantitatively formed as a mixture of two isomers 201a and 201b (Scheme 53).

Bu tBu

t

tBu P Bu P Pt Pt CO

t

H

Bu tBu

t

CO, O2 80 atm, 12 h r.t., 80%

P OC Pt

PtBu

P

2H

ð41Þ

P

Bu tBu

Bu tBu

t

t

198 8.11.4.2

Pt CO

199

Allyl ligands

A new dinuclear platinum(I) m-Z3-allyl complex can be synthesized employing an anionic tetradentate chelating ligand, N,N0 -bis[2(diphenylphosphino)phenyl]formamidinate (dpfam), where the amidinate moiety can bridge between two metal centers.112 The reaction of tetranuclear platinum(II) Z3-allyl complex, [Pt(Z3-C3H5)Cl]4, with dpfamH affords the desired dinuclear complex, Pt2(m-Z3-C3H5)(dpfm) (202). This is a first example that dinuclear platinum(I) m-Z3-allyl complex is crystallographically characterized. The structure of 202 is similar to that of the corresponding palladium complex 157. The PtdPt distance of 202 (2.6411 A˚ ) is slightly longer than the PddPd distance of 157 (2.6073 A˚ ). In contrast, the PtdP distances are slightly shorter than the PddP distances. There are no significant differences between the Pt-allyl carbon distances and the Pd-allyl distances.

N Ph2P

Pt

N PPh 2

Pt

Ph2P

N

Pd

Pd

PPh 2

157

202 8.11.4.3

N

Alkene ligands

Intramolecular exchange by alkene rotation observed for the first time in a dinuclear d9-Pt(I) olefin complex was evidenced for [Pt2(m-dppm)(m-PPh2)(Z2-bicyclo[2.2.1]hept-2-ene)2](O3SCF3) (203) (dppm ¼ bis(diphenylphosphino)methane).144 In platinum(0) complexes the p-back-donation from the electron-rich metal center is the most important contribution for the metal-alkene bond, whereas this contribution is usually much smaller for divalent platinum complexes,145 leading to different dynamic properties of the coordinated alkene ligand. The energy barrier to rotation of the olefin about the platinum-alkene bond is generally low for Pt(II), whereas in Pt(0) complexes the olefin appears to be rigidly bound. However, it has not been clear how the alkene-rotation behavior is in a complex having a d9-electron configuration, such as complex 203, being intermediate between the well-studied d8- and d10-complexes. The complex had been previously prepared and characterized.146 The dynamic behavior leads to the occurrence of two diastereomers in solution with relative syn (Cs) and anti (C2) orientation of the two in-plane coordinated bicyclo[2.2.1]hept-2-ene ligands (Scheme 54). The low activation energy △G{rot of 43 kJ mol−1 found for this exchange process is Ph

Ph

OTf

-

P Pt Ph

P

Pt P

Ph

Ph Ph

203 Cs

C2

OTf -

P Pt

Pt P

Pt P

syn

OTf -

P Pt

P

P anti

Scheme 54 Dynamic behavior between syn- and anti-orientation of two norbornene ligands bound to the dinuclear platinum(I) complex 203.

Monovalent Group 10 Organometallic Complexes

779

attributed (a) to a favorable geometry due to the ample PdPtdP angle of the phosphorus ligands in mutual trans position promoting flexibility and reducing steric hindrance in the intermediate or transition state with upright alkene orientation and (b) to the positive charge of the complex cation, which reduces the ability of platinum for metal-to-ligand p-back-donation compared to neutral or anionic complexes.

8.11.5

Conclusion

This Chapter has covered the chemistry of well-defined monovalent organometallic complexes of nickel, palladium, and platinum. The chemical characteristics of these complexes are summarized by considering facets such as their synthesis, structure, electronic properties, reactivity, catalytic function, and physical properties. Highlighted is the recent great development of organometallic chemistry of mononuclear nickel(I) complexes which behave as metal radicals. Low coordinate nickel(I) complexes typically possess a two-coordinate linear geometry or a three-coordinate planar T- or Y-shape geometry sterically stabilized through use of various ligands incorporating bulky substituents. The unpaired electron (SOMO) is mainly localized in the metal d-orbital and less distributed in the metal-ligand anti-bonding orbital to provide stable complexes. Thus, a four-coordinate complex cannot have a square-planar geometry like a low-spin nickel(II) complex, but has an unstable and distorted tetrahedral geometry, leading to easy elimination of the fourth ligand to regenerate the three-coordinate complex. In contrast, in a nickel(I) complex bearing large p-conjugated ligand, the unpaired electron is delocalized to the ligand p orbital, increasing the oxidation number of nickel to nearly divalent. This stabilizes a square-planar geometry of nickel. Since two-coordinate complexes are relatively stable in the presence of a bulky ligand, catalytically relevant two-coordinate complexes may be easily generated from a mononuclear three-coordinate complex. Alternately, such two-coordinate complexes may be generated from the corresponding dimers which has an entropy advantage. While supported by theoretical calculations, the presence of such species in reactions warrants further elucidation. Because the nickel(I) center can behave as an electron donor like the nickel(0) species, the unpaired electron of nickel(I) readily migrate to the p-orbital of the organic substrate, which promotes the addition of radical to the metal center. For instance, electrophilic aryl halide become anionic radical by a single-electron transfer and promotes elimination of the anion to form nickel(II) halide and aryl radical. This aryl radical occasionally recombines with the nickel(II) halide, but predominantly adds to unreacted nickel(I) to generate nickel(II) aryl, resulting in the mixture of nickel(II) halide and nickel(II) aryl complexes. Transmetallation, that is salt metathesis, can occur in nickel(I) complexes to form stable s-bonded organonickel(I) complexes. It is still somewhat unpredictable, however, to determine whether oxidative addition or transmetallation is favored in cross-coupling reactions, given collections of substrates in catalysis. The most important feature of dinuclear nickel(I) complexes is that homolytic decomposition into mononuclear nickel(I) complexes occurs easily, often resulting in an equilibrium mixture. This is in contrast to the case of dinuclear palladium(I) complexes, which readily disproportionate into palladium(0) and palladium(II) species. However, nickel(I) complex can undergo disproportionation to produce stable nickel(0) and (II) complexes, depending on the reaction conditions. The opposite reaction, comproportionation, can also occur, as is performed in the palladium(I) chemistry. Although it is necessary to elucidate in the future, the disproportionation in dinuclear nickel(I) complex seems to be thermodynamically controlled, likely promoted by the addition of a specific ligand that stabilizes the corresponding nickel(0) complex. Nickel(I) complexes can play various roles in catalytic reactions. Unlike the case of palladium complexes, reactions utilizing the above characteristic features of metal radicals has been reported. Theoretical calculations and advanced analytical techniques have supported the structural chemistry of many dinuclear palladium(I) complexes that have been experimentally indicated. For instance, the lower stability and higher reactivity of the bism-allyl complex compared to the mono-m-allyl complex has been clearly demonstrated using the molecular orbitals of the palladium(I) complex. One of the most important features of dinuclear palladium(I) complex is that the disproportionation reaction to palladium(0) and (II) can occur in various cases. Conversely, in a conventional catalytic cycle via zero- and divalent-palladium, a stable dinuclear palladium(I) complex can be generated. Because the dinuclear palladium(I) complex is generally an off-cycle product, comproportionation can be a deactivation pathway, in which the palladium(I) dimer is particularly stabilized by bridging acceptor ligand. Thus, the catalyst concentration and activity can be potentially controlled by formation of the palladium(I) dimer. The chemistry of platinum(I) complexes is somewhat less developed but includes reactions with carbonyls and the chemistry of m-allyl complexes. Most chemical properties may be similar to those of palladium(I); however, the examples are still limited and thus, are not fully characterized. Whether it has unique properties might be revealed in the development of future catalytic protocols comparing both palladium(I) and platinum(I) species.

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M.; Horner, A.; Elorriaga, D.; Sparkes, H. A.; Bedford, R. B. Dalton Trans. 2019, 48, 3539–3542. 131. Christmann, U.; Pantazis, D. A.; Benet-Buchholz, J.; McGrady, J. E.; Maseras, F.; Vilar, R. Organometallics 2006, 25, 5990–5995. 132. Dura-Vila, V.; Mingos, D. M. P.; Vilar, R.; White, A. J. P.; Williams, D. J. J. Organomet. Chem. 2000, 600, 198–205. 133. Vilar, R.; Mingos, D. M. P.; Cardin, C. J. J. Chem. Soc. Dalton Trans. 1996, 4313–4314. 134. Omondi, B.; Shaw, M. L.; Holzapfel, C. W. J. Organomet. Chem. 2011, 696, 3091–3096. 135. Williams, D. B. G.; Shaw, M. L.; Green, M. J.; Holzapfel, C. W. Angew. Chem. Int. Ed. 2008, 47, 560–563. 136. Pirkl, N.; Del Grosso, A.; Mallick, B.; Doppiu, A.; Gooßen, L. J. Chem. Commun. 2019, 55, 5275–5278. 137. Johansson Seechurn, C. C. C.; Sperger, T.; Scrase, T. G.; Schoenebeck, F.; Colacot, T. J. J. Am. Chem. Soc. 2017, 139, 5194–5200.

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138. Boyd, P. D. W.; Edwards, A. J.; Gardiner, M. G.; Ho, C. C.; Lemée-Cailleau, M.-H.; McGuinness, D. S.; Riapanitra, A.; Steed, J. W.; Stringer, D. N.; Yates, B. F. Angew. Chem. Int. Ed. 2010, 49, 6315–6318. 139. Ai, P.; Gourlaouen, C.; Danopoulos, A. A.; Braunstein, P. Inorg. Chem. 2016, 55, 1219–1229. 140. Werner, H.; Kϋhn, A. Angew. Chem. Int. Ed. Engl. 1977, 16, 412–413. 141. Mastrorilli, P. Eur. J. Inorg. Chem. 2008, 4835–4850. 142. Albinati, A.; Leoni, P.; Marchetti, F.; Marchetti, L.; Pasquali, M.; Rizzato, S. Eur. J. Inorg. Chem. 2008, 4092–4100. 143. Leoni, P.; Manetti, S.; Pasquali, M.; Albinati, A. Inorg. Chem. 1996, 35, 6045–6052. 144. Wachtler, H.; Schuh, W.; Augner, S.; Hägele, G.; Wurst, K.; Peringer, P. Organometallics 2008, 27, 1797–1803. 145. van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L. Inorg. Chem. 1994, 33, 1521–1531. 146. Wachtler, H.; Schuh, W.; Ongania, K.-H.; Wurst, K.; Peringer, P. Organometallics 1998, 17, 5640–5646.

8.12 Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes Andrei N Vedernikov, Department of Chemistry and Biochemistry, University of Maryland, College Park, MD, United States © 2022 Elsevier Ltd. All rights reserved.

8.12.1 Palladium(III) complexes 8.12.1.1 Mononuclear organopalladium(III) complexes 8.12.1.2 Dinuclear and polynuclear organopalladium(III) complexes 8.12.1.2.1 Dinuclear bridged complexes with a PddPd bond 8.12.1.2.2 Dinuclear bridged complexes without a PddPd bond 8.12.1.2.3 Polynuclear bridged complexes 8.12.2 Palladium(IV) complexes 8.12.2.1 Palladium(IV) trihydrocarbyls 8.12.2.1.1 Bidentate chelating ligands 8.12.2.1.2 fac-Chelating ligands 8.12.2.2 Palladium(IV) dihydrocarbyls 8.12.2.2.1 Bidentate chelating ligands 8.12.2.2.2 fac-Chelating ligands 8.12.2.2.3 Pincer ligands 8.12.2.3 Palladium(IV) monohydrocarbyls 8.12.2.3.1 Bidentate chelating ligands 8.12.2.3.2 fac-Chelating ligands 8.12.2.3.3 Pincer ligands 8.12.2.4 Palladium(IV) complexes supported by N-heterocyclic carbene ligands 8.12.3 Platinum(III) complexes 8.12.3.1 Mononuclear organoplatinum(III) complexes 8.12.3.2 Dinuclear and polynuclear organoplatinum(III) complexes 8.12.3.2.1 Unsupported dinuclear Pt(III) complexes 8.12.3.2.2 Monobridged dinuclear Pt(III) complexes 8.12.3.2.3 Doubly bridged dinuclear Pt(III) complexes 8.12.3.2.4 Polynuclear Pt(III) complexes 8.12.4 Platinum(IV) complexes 8.12.4.1 Five-coordinate organoplatinum(IV) complexes 8.12.4.2 Six-coordinate organoplatinum(IV) complexes 8.12.4.2.1 Complexes with six hydrocarbyl ligands 8.12.4.2.2 Complexes with five hydrocarbyl ligands 8.12.4.2.3 Complexes with four hydrocarbyl ligands 8.12.4.2.4 Complexes with three hydrocarbyl ligands 8.12.4.2.5 Complexes with two hydrocarbyl ligands 8.12.4.2.6 Complexes with one hydrocarbyl ligand 8.12.4.2.7 Organoplatinum(IV) complexes with no hydrocarbyl ligands 8.12.5 Conclusions Acknowledgments References

8.12.1

783 784 787 787 789 789 790 791 791 792 793 793 795 798 798 799 800 802 803 803 803 805 805 806 806 808 809 810 811 811 811 812 813 820 826 830 831 832 832

Palladium(III) complexes

The chemistry of organopalladium(III) complexes is young. Although the first Pd(III) compound, Na[PdF4], was prepared and characterized in 1982,1 the area of research dealing with organopalladium(III) compounds received increased attention much later, in the 2000s, thanks to the development of oxidative CdH bond functionalization chemistry utilizing palladium(II) complexes as catalysts.2 There are few reviews covering the progress in the field of organopalladium(III) chemistry.3,4 The first isolated compounds of this class were dinuclear paddlewheel (or lantern) complexes 1–3 which were prepared and characterized in 2006.5 A few years later, in 2010, the first stable mononuclear organopalladium(III) complexes 4–6 were reported.6 Finally, first dinuclear organopalladium(III) complexes without a PddPd bond, 7 and 8, were prepared in 2012.7

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Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.1.1

Mononuclear organopalladium(III) complexes

A controlled-potential electrolysis of neutral d8 metal complexes (tBuN4)PdR0 (Cl) (R0 ¼ Me, Ph; tBuN4 ¼ N,N0 -di-tert-butyl-2,1 1-diaza-[3.3]-(2,6)-pyridinophane) in dichloromethane solutions as well as chemical oxidation of (tBuN4)PdMe2 complex with 1 equivalent of ferrocenium hexafluorophosphate (FcPF6) allowed to produce the first mononuclear organopalladium(III) complexes 4, 5 and 6, respectively.6 Single crystal X-ray diffraction analysis of the derived salts, 4(PF6), 5(ClO4) and 6(ClO4), revealed a Jahn-Teller tetragonally-distorted octahedral coordination geometry at the metal center with long axial amine nitrogen-metal distances of 2.41–2.48 A˚ and shorter equatorial pyridine nitrogen-metal distances of 2.02–2.11 A˚ . As expected for odd-electron species, these d7 complexes are paramagnetic. The bulky tetradentate pyridinophane ligand tBuN4 appears to sterically protect the metal center in 4–6. Complexes 4–6 are stable in solutions and in the solid state in the dark but decompose under light. A photo-induced elimination of ethane as a major product (50% yield) is observed for the methyl chloro complex 4, along with small quantities of MeCl and methane (Scheme 1). The reaction is homolytic and is fully inhibited by 1 equivalent of TEMPO that forms a

Scheme 1

Me-TEMPO adduct (84% yield). Similarly, the analogous phenyl complex 5 photo-eliminates biphenyl (42% yield) as a major product, whereas the dimethyl complex 6 produces mostly ethane (82% yield). Notably, when mixtures of methyl and phenyl complexes 4 and 5 are used in photochemical decomposition reactions, toluene is formed along with ethane and biphenyl. The proposed reaction mechanism involves light-induced homolysis of PddC bonds with subsequent formation of transient Pd(IV) dihydrocarbyl species which are responsible for the observed elimination of R0 2 (R0 ¼ Me, Ph) and Me-Cl products.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

(

785

The oxidation potential of the (tBuN4)PdMe2 complex is low enough to allow for the use of O2 in the oxidative CdC coupling of N4)PdMe2 (Scheme 2).8 The organopalladium(III) cation 6 is formed as an intermediate in this reaction. The overall reaction

tBu

Scheme 2

produces ethane and a methyl hydroxo complex (tBuN4)PdMe(OH). The precursor (tBuN4)PdMe2 is stable under O2 atmosphere in aprotic solvents such as benzene, fluorobenzene or THF, but it reacts with O2 in the presence of protic solvents such as MeOH. In particular, the reaction carried out in 1:3 benzene–MeOH mixtures under 1 bar O2 produced the methyl hydroxo Pd(II) derivative (tBuN4)PdMe(OH) and ethane as the final products in a virtually quantitative yield with respect to (tBuN4)PdMe2. The complex 6 appeared only in 26% yield at its maximum concentration and was eventually consumed after 15 h. The proposed mechanism of the reaction involves slow formation of a cationic hydroperoxo dimethyl Pd(IV) intermediate which is reduced with the available (tBuN4)PdMe2 to form the cationic 6 along with hydroxide anions. Notably, while 6 is stable in the absence of light and strongly coordinating ligands, it reacts with hydroxide anions to form ethane and (tBuN4)PdMe(OH). Hydroxide anions were proposed to drive a reversible disproportionation of 6 into (tBuN4)PdMe2 and hydroxo dimethyl Pd(IV) intermediates which, eventually, generate trimethyl Pd(IV) species responsible for clean elimination of ethane. Two less bulky analogs of the pyridinophane tBuN4 that bear methyl or isopropyl groups at the amine nitrogen atoms, MeN4 and iPr N4, respectively, were used to prepare a series of derived mononuclear cationic Pd(III) complexes [(L)PdMe(Cl)]+ and [(L) PdMe2]+, 9–12 (L ¼ MeN4 and iPrN4).9 A controlled-potential electrolysis of the methyl chloro Pd(II) precursors (L)PdMe(Cl) was utilized to synthesize cationic Pd(III) methyl chloro complexes 9 and 11 whereas FcPF6 was effective in the oxidation of (L) PdMe2 species and allowed for the preparation of dimethyl complexes 10 and 12. Notably, when second equivalent of FcPF6 was employed in the oxidation of dimethyl complexes 10 and 12, dicationic dimethyl Pd(IV) derivatives were produced. In the case of methyl chloro Pd(III) complexes (L)PdMe(Cl), NOBF4 was efficient at converting 9 and 11 to the derived dicationic Pd(IV) products. An electron-poor dicationic Pd(III) monomethyl complex 13 was synthesized using a controlled-potential electrolysis of a cationic Pd(II) precursor [(tBuN4)PdMe(NCMe)]+. The product was isolated as a bis-triflate salt and characterized by single crystal X-ray diffraction.10 As a consequence of increased cation charge, the metal-nitrogen atom distances in dicationic 13 are about 0.3 A˚ shorter than in its monocationic Pd(III) analog [(tBuN4)PdMe(Cl)]+.

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Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

An analysis of the oxidation potentials of the Pd(II) precursors (L)PdR0 (R00 ) supported by ligands MeN4, iPrN4 and tBuN4 showed that the potentials increase with the increasing steric bulk of the alkyl substituents at the amine nitrogen atoms. It was proposed that this change in the reactivity may be related to the increasing difficulty for the pyridinophane ligands in these complexes to attain a boat-boat conformation (Scheme 3). The boat-boat conformation and k4-RN4 ligand coordination are required for the stabilization

Scheme 3

of the emerging Pd(III) center by two pendent tertiary amine nitrogen donor atoms.10 According to existing estimates, each axial tertiary amine nitrogen donor in a boat conformation lowers the oxidation potential of (tBuN4)PdR0 (R00 ) by 0.6 V, so that a total 1.2 V oxidation potential reduction is observed when a boat-boat conformer, (k4-tBuN4)PdR0 (R00 ), is kinetically accessible. The kinetic accessibility of the more reactive k3- and k4-RN4 coordinated conformers decreases with decreasing temperature.10 In particular, at −70  C in EtCN solutions of a (tBuN4)PdMe(EtCN)+ complex only two oxidation waves were observed that were assigned to two major conformers, with k2- and k3-coordinated ligand tBuN4, whereas at 10  C a third current wave was present, at a lower potential, associated with the oxidation of the most reactive conformer with a k4-coordinated ligand tBuN4. Cationic Pd(III) complexes 9–12 were isolated as perchlorate salts and characterized by single crystal X-ray diffraction. When comparing the Pd-Naxial distances in 9–12 and those observed in 4, 6,6 it was noted that the distances increase in the direction from L ¼ MeN4 to iPrN4 and tBuN4,10 so supporting the hypothesis of an increasing steric interference between the alkyl substituents at the axial amine nitrogen atoms and the ligands occupying the equatorial positions at a Pd(III) center. Finally, similar to the chloro methyl complex 4 derived from a bulkier ligand tBuN4, the less sterically encumbered complexes 9 and 11 also react under light to produce mixtures of products, C2H6 (up 28–34% yield), CH3Cl (5–21% yield) and CH4, also unselectively and in even lower yields. In the case of the photochemical decomposition of Pd(III) dimethyl complexes 10 and 12 ethane (56–60%) and methane (9–13%) were produced in low yields. A stable mononuclear cationic organopalladium(III) complex 14 featuring a metal–carbon bond with one of the atoms of a macrocyclic pyridinophane ligand was prepared using tBuN3CMe, an analog of tBuN4, in which one of the pyridine fragments was replaced by toluene residue (Scheme 4).11 The reaction involved PhICl2 as an oxidant and a Pd(II) precursor

Scheme 4

(tBuN3CHMe)Pd(OAc)2 which had no PddC bonds. Hence, the reaction involved a pyridinophane’s CdH bond activation. In turn, a stable dicationic derivative of 14, complex 15, was generated using halide abstraction from 14 with TlPF6 in MeCN solution. The derived salts, 14(ClO4) and 15(PF6)2, were isolated and characterized by X-ray crystallography. Both complexes 14 and 15 were also characterized by EPR. Neutral organopalladium(III) complexes 16 and 17 derived from the same family of pyridinophane ligands, tBuN3CR, (R ¼ H, Me; Scheme 5) were identified as catalysts’ resting states in radical addition of CCl3Br across a C]C bond of methyl methacrylate.11 The reaction was catalyzed by (tBuN3CHR)Pd(OAc)2. One of the products, 17, was characterized crystallographically. In summary, RN4 and RN3C tetradentate macrocyclic ligands proved to be an effective and tunable scaffold allowing to stabilize a variety of mononuclear organopalladium(III) complexes, neutral, mono- and dicationic. A controlled potential electrolysis or the use of stoichiometric amounts of an appropriate oxidant served as a gentle means for the oxidation of Pd(II) precursors to their Pd(III) derivatives allowing to avoid “overoxidation” to Pd(IV) complexes (see Section 8.12.2). The drawback of using tetradentate ligands such as N4 and N3C is a potentially heavy stabilization of a Pd(III) center resulting in a lack of reactivity of the derived organopalladium(III) complexes under thermal conditions. At the same time, coordination-induced disproportionation of Pd(III) complexes to their Pd(IV) and Pd(II) derivatives may pave the way to faster and cleaner CdX coupling reactions. The N4

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

787

Scheme 5

pyridinophane-supported Pd(III) complexes become reactive in CdX elimination reactions under photochemical conditions but demonstrate poor selectivity in these photo-induced transformations.

8.12.1.2 8.12.1.2.1

Dinuclear and polynuclear organopalladium(III) complexes Dinuclear bridged complexes with a PddPd bond

8.12.1.2.1.1 Paddlewheel (lantern) complexes The dinuclear paddlewheel (or lantern) organopalladium(III) complexes 1–35 are diamagnetic which implies the presence of a PddPd bond between two d7 metal centers. Two of the four bridging ligands in lantern complexes 1–3 are derived from ortho-metallated triphenylphosphine and two other bridging ligands are k2-O,O-coordinated carboxylates RCO2 (R ¼ Me, CF3, Me3C). The less electron-donating carboxylate, CF3CO2, imparts lower stability of 2; the latter decomposes slowly in solutions or in the solid state at 20  C.5 A similar structural motif including two ortho-metallated triphenylphosphine residues and two N,N-, O,O-, O,S- or N,S-bridging ligands is found in a number of related complexes prepared later in 2010s.12–16 The most stable and well-characterized of these complexes are 18, 19,12 20, 21,13 22, 23,14 24, 25,15 26–28,16 that are shown below. The nature of bridging ligands is essential for enhancing the stability of these paddlewheel complexes.17

The preparation of complexes 1–3, 18–28 involves reaction of their dinuclear lantern-type Pd(II) precursors with PhICl2 at −50 to −70  C. Except formamidinate-bridged compounds 18 and 19,12 most of the products are relatively stable at 20  C. The available single crystal X-ray diffraction data allow to observe some trends in the PddPd distances in these complexes. The pyrazolato-bridged 22 and 23 feature the shortest PddPd bonds among all known dinuclear Pd(III) complexes, 2.505–2.507 A˚ .14 The PddPd distance is slightly longer in benzoate-bridged 24, 2.521 A˚ , and is noticeably longer in tetrazolylsulfido-bridged complex 26, 2.633 A˚ ,16 thanks to the presence in the bridging ligand of a sulfur donor atom which is larger than donor atoms derived from the elements of the 2nd period, N and O. Paddlewheel complexes 24 and 25 were shown to catalyze ligand-directed oxidative acetoxylation of 2-phenylpyridine with PhI(OAc)2 and C2-selective oxidative arylation of indoles with (Ph2I)PF6.15

788

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.1.2.1.2 Half-lantern complexes A series of dinuclear half-lantern bis-carboxylato-bridged organopalladium(III) complexes 29–36 derived from cyclometallated benzo[h]quinoline and 2-phenylpyridine and bearing two axial chloro or two acetato ligands were prepared by reacting their dinuclear bis-carboxylato-bridged Pd(II) precursors with PhICl2 or PhI(OAc)2, respectively.18–20 Several representative complexes were characterized by single crystal X-ray diffraction to confirm the presence of a short PddPd bond (e.g., the PddPd distance is 2.568 A˚ in 29, 2.567 A˚ in 30 and 2.555 A˚ in 31). The reactivity of complexes 29–36 in CdX bond reductive elimination was studied in the relationship to the mechanism of Pd(OAc)2-catalyzed ligand-directed oxidative CdH chlorination and acetoxylation of the respective substrates, benzo[h]quinoline and 2-phenylpyridine.2 Complexes 29–36 have a relatively low thermal stability. In particular, while 29 is stable in solutions and in solid state below −30  C, it eliminated 10-chloro benzo[h]quinoline in 94% yield on warming to 23  C.18 Similarly, complex 31 is stable below −10  C but it eliminated 2-(2-acetoxyphenyl)pyridine in 91% yield when warmed in the presence of excess 2-phenylpyridine at 40  C.19

Analysis of the reaction kinetics, mechanistic tests and computational modeling of catalytic chlorination and acetoxylation of the respective organic substrates, benzo[h]quinoline and 2-phenylpyridine, with PhIX2 (X ¼ Cl, OAc, O2CAr) suggested the intermediacy of the corresponding carboxylato-bridged organopalladium(III) complexes 29–36. Although these complexes were not directly observed in catalytic reaction mixtures, they were shown to be kinetically competent reaction intermediates that can be involved in CdX bond elimination, the product-forming step of the catalytic reaction.18–20 Interestingly, when oxidation of dinuclear bis-acetato-bridged cyclopalladated benzo[h]quinoline was performed with Togni’s reagent capable of installing an axial CF3 group at a Pd center, the reaction resulted in the formation of a mononuclear Pd(IV) product (Scheme 6). The kinetics study, experimental observations and a computational (DFT) study suggested the involvement of dinuclear half-lantern bis-carboxylato-bridged organopalladium(III) transients which underwent disproportionation into organopalladium(II) and organopalladium(IV) species.21

Scheme 6

Hence, in general, organopalladium intermediates in both oxidation states, Pd(III) and Pd(IV), may be present simultaneously in reaction mixtures when carrying out Pd(II)-catalyzed oxidative functionalizations of organic substrates. Finally, concluding the characterization of half-lantern dinuclear organopalladium(III) complexes, an unusual half-lantern complex 37 was prepared using PhICl2 from a Pd(II) acyclic diaminocarbene precursor (Scheme 7). The complex decomposed at 20  C but could be characterized at lower temperatures by means of NMR spectroscopy, as well as by elemental analysis and crystallographically.22 No reactivity was reported for 37.

Scheme 7

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.1.2.2

789

Dinuclear bridged complexes without a PddPd bond

Dinuclear organopalladium(III) complexes without a PddPd bond are expected to be paramagnetic. A few known representative of this group of organopalladium(III) compounds are the complexes 7 and 8. These complexes were produced by reacting the corresponding dinuclear choro-bridged Pd(II) complexes with 4 equivalents of AgX (X ¼ CF3CO2 or NO3, respectively) acting as a source of the incoming ligands X, halide abstractors and oxidants (Scheme 8).7 According to X-ray absorption spectroscopy

Scheme 8

(EXAFS) and Mössbauer study, the oxidation state of the iron center in the ferrocene moieties of 7 and 8 remained unchanged after the reaction, but, according to EXAFS, the oxidation state of palladium atoms was increased. The latter conclusion was also supported by analysis of EPR spectra of 7 and 8 recorded at 110 K in the solid state and in frozen solutions. Oxidation of the parent dinuclear choro-bridged Pd(II) complexes with thianthrenium tetrafluoroborate produced cationic paramagnetic organopalladium(III) derivatives 38–40 (Scheme 8). The latter complex 40 which resulted from a partial hydrolysis of 39 was characterized crystallographically as a tetrafluoroborate salt 40(BF4)2. The crystal structure demonstrated a PddPd distance of 3.574 A˚ that supported the notion of a negligible metal–metal bonding in the complexes of this series.7 The so-produced dinuclear organopalladium(III) complexes were used as highly active catalysts in aza-Claisen rearrangement.

8.12.1.2.3

Polynuclear bridged complexes

Oxidation with 1 equivalent of XeF2 of a dinuclear bis-acetato-bridged cyclopalladated benzo[h]quinoline complex resulted in the formation of a polynuclear polycationic organopalladium(III) complex 41 with fluoride counterions that can be viewed as a 1D molecular palladium wire and has properties of a semiconductor (Scheme 9).23 The complex is stable in solutions. In a crystalline solid state, the bridged PddPd distances of 2.721 A˚ inside each monomeric unit and unbridged PddPd distances 2.972–2.982 A˚ between adjacent units were found using X-ray crystallography. Both the number of monomeric Pd2 units in 41, the interunit PddPd distance and the semiconductor bandgap can be tuned by replacing fluoride with tetrafluoroborate or hexafluorophosphate anions. The structure of the solids and their conductivity properties were also dependent on the identity of the alkyl group in the bridging carboxylate ligand, Me or n-C5H11, which affected the packing in crystals.23,24 Notably, polynuclear complex 41 was easily destroyed by the addition of suitable strongly coordinating ligands such as chloride to produce bis-nucleophile-capped dinuclear Pd(III) species such as 29. The electronic properties of the organopalladium chelate fragments affected significantly the ability of Pd(II) precursors to produce PddPd-bound molecular wires. In particular, when a 7-chloro-substituted benzo[h]quinoline was used in the attempted preparation of polynuclear analogs of 41 in reaction with 1 equivalent of XeF2, only a dinuclear half-lantern organopalladium(III) difluoride 43 was produced.24 Notably, when 0.5 equivalent of XeF2 was used for oxidation of a dinuclear bis-acetato-bridged cyclopalladated benzo[h] quinoline complex, a Pd(II)/Pd(III) mixed-valence molecular wire 42 was produced that had a metallic conductivity above 200 K. Interestingly, the PddPd distances in 42 are about the same as in 41, of 2.727 A˚ inside each monomeric unit and 2.980 A˚ between adjacent units. In summary, polynuclear organopalladium(III) complexes and, especially, their mixed-valence analogs, can act as molecular wires possessing interesting solid-state structure and electrical conductivity properties.

790

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 9

8.12.2

Palladium(IV) complexes

The beginning of the chemistry of organopalladium(IV) compounds dates back to 1970s25 and its progress since then was reviewed extensively in the previous editions of Comprehensive Organometallic Chemistry. Similar to organopalladium(III) chemistry, the interest to the chemistry of organopalladium(IV) compounds increased dramatically in the last two decades. This trend is related to the development of organic synthetic methods utilizing oxidative palladium catalysis such as functionalization of CdH bonds or olefin C]C bonds in the presence of strongly oxidizing agents such as PhICl2, PhI(O2CR)2, N-halogenoimides, N-fluoropyridinium and N-fluoroammonium salts.2,26–30 There are a number of reviews discussing the progress of organopalladium(IV) chemistry in the past 20 years that focus on the selectivity of CdC coupling at a Pd(IV) center,26 the pincer-type ligands in organopalladium(IV) chemistry,27 CdX reductive elimination reactivity of organopalladium(IV) complexes (X ¼ O),28,31 (X ¼ S, Se, F, Cl, Br, I),28 the intermediacy of organopalladium(IV) complexes in cascade reactions producing complex carbo- and heterocycles,29 and the emergence of organopalladium(IV) chemistry in organic synthesis and catalysis.30 In spite of a great attention to and a high utility of organopalladium(IV) chemistry in modern organic synthesis, currently, the number of well-characterized organopalladium(IV) complexes remains small. There are many reports in the literature providing indirect evidence for the formation of organopalladium(IV) intermediates but, at best, they include only limited spectroscopic characterization of such species without proven structural details.27,29,30 This situation is a reflection of a typically low stability and high reactivity of most organopalladium(IV) complexes. The number of hydrocarbyl ligands, along with their identity (e.g., alkyl, aryl, vinyl, alkynyl), the type of supporting ligands (e.g., monodentate, chelating bidentate, fac-chelating) and their identity (e.g., N-donors, P-donors, C-donors, etc.) affect dramatically the reactivity and stability of organopalladium(IV) compounds. When comparing structurally similar organopalladium(IV) compounds, as a rule, the more persistent and less reactive of them contain three or two hydrocarbyl ligands whereas their most reactive counterparts are their monohydrocarbyl analogs. The use of bidentate and, especially, tridentate fac-chelating ligands of matching geometry, especially those with “hard” donor atoms, such as N and O,32–34 allows to enhance the kinetic stability of organopalladium(IV) complexes. The decreased reactivity of so-derived complexes stems from the decreased ability of rigid polydentate ligands to change their coordination mode at a metal center that would allow for an opening in the metal coordination sphere. A coordination vacancy is often needed for faster CdC or CdX bond reductive elimination from a Pd(IV) atom.31 The fac-chelating ligands that can adapt a coordination geometry matching the metal atom size may be especially efficient at stabilizing octahedral d6 metal complexes.35 As the rigidity of fac-chelating ligands decreases, so decreases their ability to stabilize derived Pd(IV) complexes, as it was demonstrated computationally (DFT) by analyzing MedH bond reductive elimination from octahedral d6 metal complexes (L)M(Me)2H as a model reaction.36

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

791

This section will focus, for the most part, on the structure and reactivity of isolable and well-characterized organopalladium(IV) compounds reported in the period of 2005–mid-2021. The discussion will be organized according to the number of hydrocarbyl ligands at a Pd(IV) center and the type of supporting ligands (bidentate chelating, fac-chelating, pincer ligands). Separately will be considered organopalladium(IV) compounds where the only available PddC bonds are due to coordination of N-heterocyclic carbene ligands.

8.12.2.1

Palladium(IV) trihydrocarbyls

The well-characterized palladium(IV) trihydrocarbyl complexes contain at least one chelating ligand and the most stable of them are supported by fac-chelating ligands, as described below.

8.12.2.1.1

Bidentate chelating ligands

A palladium(IV) trimethyl complex supported by 2,20 -bipyridyl (bpy), (bpy)PdMe3(I), 44, (Scheme 10) was the first of the alkylpalladium(IV) compounds that could be reliably characterized back in 1980s, including single crystal X-ray diffraction.37 The complex is stable at −20  C but, owing to the presence of an easily ionizable iodo ligand trans- to the axial methyl ligand, it readily decomposes in polar enough solvents. The reaction involves a reversible ionization of iodide ligand with concomitant formation of cationic 5-coordinate transient [(bpy)PdMe3]+ and, accordingly, is inhibited by additives of ionic iodides. The decomposition leads to ethane and (bpy)PdMe(I) and is complete after 30–40 min at 25  C when carried out in acetone solutions.

Scheme 10

For mixed trihydrocarbyl Pd(IV) complexes including one aryl and two alkyl ligands two reaction directions may be anticipated, one leading to alkyl-alkyl CdC coupling and one leading to alkyl-aryl CdC coupling. Typically, such complexes demonstrate a high preference for alkyl-aryl CdC coupling.25 In a recent study of palladium-mediated synthesis of highly substituted heterocycles,38 an aryl dialkyl Pd(IV) complex 45 (Scheme 11) was produced using oxidative addition of methyl ester of 4-bromo-2-butenoic acid and a corresponding bpy-supported pallada(II)cycle. Complex 45 was sufficiently stable to allow for its isolation and handling at 20  C for brief periods of time or at −10  C in chloroform solutions for the time needed for acquisition of its 13C NMR spectra. The complex was also characterized crystallographically. A slow conversion of 45 to an aryl-alkyl CdC coupled product was observed at room temperature. The CdC coupling reaction was very selective and the resulting organopalladium(II) product was successfully converted into a single benzoxepine.

Scheme 11

A reactive Pd(IV) dihydrocarbyl, a 2-biphenylyl trifluoromethyl complex 46, supported by 4,40 -di-tert-butyl-2,20 -bipyridyl ligand (t-Bu2-bpy) was generated in MeCN solutions at −30  C by oxidation of its Pd(II) precursor with PhICl2 (Scheme 12).39 The Pd(IV) complex was characterized by multinuclear NMR spectroscopy and ESI-MS. Upon warming of the reaction mixtures to 25  C intermediate 46 reacted cleanly via intermolecular CdH activation of the 2-biphenylyl ligand to form a stable trifluoromethyl diaryl complex 47. When the oxidation reaction was conducted at 25  C by using PhICl2 or similarly strong oxidants, PhI(O2CCF3)2 or N-fluoro-2,4,6-trimethylpyridinium triflate (NFTPT), stable trifluoromethyl biphenyldiyl Pd(IV) complexes 47–49 formed in high yield. Complex 48 was characterized crystallographically. This and other observations were used to conclude that CdH bond activation at a Pd(IV) center is operational and it can proceed under very mild conditions. The success of this study was, in part, due to the choice of trifluoromethyl ligand as a part of model Pd(IV) complexes. Compared to methyl ligand, trifluoromethyl is much more reluctant to undergo C(sp3)dC(aryl) coupling at a Pd(IV) center so helping enhance the stability of the model Pd(IV) trihydrocarbyls 46–49.

792

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 12

8.12.2.1.2

fac-Chelating ligands

A very stable trifluoromethyl diaryl Pd(IV) complex 50 supported by a fac-chelating ligand derived from 1,1-di(2-pyridyl)ethylbenzene was prepared by reacting its p-tolyl trifluoromethyl Pd(II) precursor with NFTPT and subsequent substitution of coordinated triflate with chloride (Scheme 13).34 Thanks to a strong trans-effect of the axial aryl group the chloride ligand in complex 50 was very labile and could be readily substituted with a variety of other ligands, triflate, by the action of AgOTf, tert-butyl isocyanide, PMe3, p-nitrophenoxide or phthalimide, to afford stable trihydrocarbyl Pd(IV) complexes 51–55, respectively.

Scheme 13

The fac-chelating ligands 1,4,7-triazacyclononane (tacn) and N,N0 ,N00 -trimethyl-1,4,7-triazacyclononane (Me3-tacn) were known as the ligands especially strongly enhancing the stability of octahedral d6 metal complexes, and, in particular, the stability of Pd(IV) trialkyl complexes [(tacn)PdMe3]+, 56, and [(tacn)PdMe2Et]+, 57 (Scheme 14).40 These complexes were prepared by reacting (tacn)PdMe2 with MeI or EtI, respectively, and characterized by NMR spectroscopy, ESI-MS and elemental analysis. The trimethyl complex 56 in DMSO solutions eliminated cleanly ethane at 140  C, whereas its ethyl homolog 57 produced a mixture of propane, ethane, ethylene and methane in 38:11:9:2 ratio under the same reaction conditions.

Scheme 14

The use of N,N0 ,N00 -trimethyl-1,4,7-triazacyclononane ligand (Me3-tacn) in the oxidation with O2 of a derived Pd(II) dimethyl complex, (Me3-tacn)PdMe2, in MeCN solutions at 25  C resulted in a fast formation of a stable cationic Pd(IV) complex [(Me3-tacn) PdMe3]+, 58 (Scheme 14).41 The complex was characterized crystallographically as an iodide salt (58)I. The proposed mechanism of the formation of 58 involves the generation of a cationic highly electrophilic [(Me3-tacn)PdMe2(OOH)]+ transient that transfers one of its methyl ligands, formally, as a Me+, to the precursor Pd(II) complex, (Me3-tacn)PdMe2, to form 58. Complex 58 is thermally stable; it reductively eliminated ethane in high yield only when heated at 110  C in DMSO solutions for 60 h.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.2.2 8.12.2.2.1

793

Palladium(IV) dihydrocarbyls Bidentate chelating ligands

Reaction of PhICl2 with a palladium(II) diaryl complex, a formal product of cyclopalladation of two 2-phenylpyridine molecules, resulted in the formation and isolation of its Pd(IV) dichloro derivative 59 (Scheme 15).42 In turn, the reaction of the same palladium(II) diaryl complex with N-chlorosuccinimide (NCS) resulted in the formation of Pd(IV) dihydrocarbyl 60, the first reported product of oxidative addition of NdCl bond of NCS to a Pd(II) center. Complex 60 was characterized crystallographically. Both 59 and 60 reacted in AcOH solutions at 80  C to reductively eliminate a CdCl bond and form the corresponding arylchloride as a major reaction product (Scheme 15). If pyridine was used as a solvent, the major reaction product in both cases was the CdC coupled biaryl. This work was the first report of a selective CdCl bond forming reductive elimination from isolated Pd(IV) complexes.

Scheme 15

When oxidation of a bis-palladacyclic 2-phenylpyridine-derived diaryl Pd(II) complex was done with PhI(OAc)2, a diaryl diacetoxo Pd(IV) compound 61 was produced. The complex was fully characterized, including single crystal X-ray diffraction (Scheme 16).43 Complex 61 was stable as a solid for an extended period of time at −35  C and eliminated the corresponding aryl acetate in high yield when heated in MeCN solutions at 80  C for 30 min. By using as oxidants phenyliodinanes PhI(O2CR)2, p-X-substituted benzoate analogs of PhI(OAc)2, complexes 62–72 were produced (X ¼ MeO, Me, H, Ph, F, Cl, Br, Ac, CF3, CN, NO2). Complex 72 was characterized crystallographically.44 The series of compounds were used to study the kinetics and elucidate the mechanism of CdO reductive elimination at a Pd(IV) center.43 According to the authors’ conclusion, the CdO elimination reaction begins with a fast and reversible dissociation of the carboxylate ligand located trans- to an aryl group. The resulting 5-coordinate Pd(IV) intermediate undergoes fast CdO coupling involving the metal-bond aryl carbon atom and an oxygen atom of the carboxylate ligand occupying a cis-position with respect to the aryl.

Scheme 16

794

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

A stable trifluoromethyl p-fluorophenyl Pd(IV) complex 73 supported by t-Bu2-bpy ligand was prepared by oxidation of its Pd(II) precursor with NFTPT at 25  C in 1,2-dichloroethane or nitrobenzene solutions (Scheme 17).45 The product was fully characterized including X-ray crystallography. Heating 73 in nitrobenzene at 80  C for 3 h led to a clean formation of 1-fluoro-4-trifluoromethylbenzene, the product of a C(aryl)-CF3 reductive elimination at a Pd(IV) center. This report was the first of its king that disclosed an ArdCF3 bond forming reaction from an isolated organopalladium(IV) complex.

Scheme 17

Reaction of bis-palladacyclic 2-phenylpyridine-derived diaryl Pd(II) complex with various alkane and arenesulfonyl chlorides RSO2Cl (R ¼ CF3, PhCH2, p-FC6H4, p-MeOC6H4, p-MeC6H4) in dichloromethane solutions at 25  C resulted in the formation of the corresponding sulfinato diaryl Pd(IV) compounds 74–78, the first reported Pd(IV) sulfinato complexes (Scheme 18).46 The solid state structure of complexes 77 and 78 was established by X-ray crystallography. A benzo[h]quinoline analog of 78, complex 79, was also prepared using the same reaction protocol. Complex 78 was stable in the solid state and in dichloromethane solutions for more than 5 days but it decomposed slowly at 25  C when dissolved in DMSO. Subjecting solutions of 78 in DMSO, pyridine, AcOH or DCE to 120  C for 6–18 h resulted in the formation of the corresponding arylsulfones as major products (52–71% yield), two biaryls (8–31% combined yield) and an aryl chloride (2–10% yield) as the minor products. The formation of the chloride byproduct was minimized when 1 equivalent of AgBF4 or AfOTf was used in the reaction. The proposed reaction mechanism involves a slow dissociation of the chloride ligand from a Pd(IV) center with subsequent fast C(aryl)-S coupling at the metal atom.

Scheme 18

The formation of a Pd(IV) dihydrocarbyl, 2-biphenylyl trifluoromethyl complex 46, supported by 4,40 -di-tertbutyl-2,20 -bipyridyl ligand (t-Bu2-bpy), which served as an intermediate en route to a trihydrocarbyl derivative 47 was already presented in Scheme 12.39 The chemistry of cycloneophyl Pd(IV) complexes such as 80 (Scheme 19)47 attracted recently a significant attention.32,47–49 The presence of a metallacyclic fragment enhances stability of such compounds whereas the presence of both C(sp2)dPd and C(sp3) dPd bonds in them allows to explore the selectivity of CdX coupling reactions at a Pd(IV) center. Complex 80 was produced by the oxidation with NFTPT of a cycloneophyl Pd(II) precursor supported by a 2,20 -bipyridyl ligand (Scheme 19).47 The high lability of the triflate ligand present in 80 was used to prepare a cationic aqua complex 81 which was characterized crystallographically as a triflate salt (81) OTf. In a similar way, using ligand exchange, were produced a cationic pyridine adduct 82 and a Pd(IV) difluoride 83. Both (82) OTf, (82) BF4 and 83 reacted selectively at 80  C in dichloromethane solutions to form the derived alkyl fluorides, the C(sp3)dF coupled products. No intermediates were detected in the course of these reactions by means of 19F NMR spectroscopy. No derived aryl fluorides, the C(sp2)dF coupled products, formed in these reactions either. The reaction of 82 was inhibited by pyridine additives. It was proposed that the C(sp3)dF coupling proceeds via 5-coordinate intermediate resulting from the loss of pyridine ligand with a subsequent direct C(sp3)dF bond elimination at a Pd(IV) center.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

795

Scheme 19

A cycloneophyl Pd(IV) complex 84 was used in a similar study targeting a C(sp3)dN coupling at a Pd(IV) atom (Scheme 20).48,50 Complex 84, along with its less stable isomer 85, was prepared by a triflate ligand exchange using 80 (Scheme 19) and tosylamide/Cs2CO3 combination in MeCN (Scheme 20). As compared to the major product 84 having a trans-arrangement of its alkyl fragment and the sulfonylamido ligand, the minor isomer 85 had its aryl fragment arranged transto the sulfonylamide. The mixture of isomers reacted with 1 equivalent of NMe4NHTs in the presence of free bpy ligand to produce a target C(sp3)dN coupled product in 98% yield. In the absence of NMe4NHTs a competing reaction led to the predominant formation of a C(sp3)dF coupled product.50 The proposed mechanism of the C(sp3)dN coupling of 84 involves a slow and reversible dissociation of NHTs− from the metal center followed by a fast exogenous nucleophilic attack of NHTs− at the Pd(IV)-bound carbon atom. This reaction mechanism predicts the overall reaction order in [NHTs−] being zero. By contrast, the order in [NHTs−] for the competing C(sp3)dF coupling process is −1, so that, by contracts to C(sp3)dN coupling, this reaction is suppressed at high enough concentration of the exogenous nucleophile NHTs−. The reaction mechanism was later explored using a computational-combinatorial method.48

Scheme 20

8.12.2.2.2

fac-Chelating ligands

A series of chemically robust trifluoromethyl p-tolyl Pd(IV) complexes 86–88 supported by fac-chelating (pre-)ligands, hydridotris(pyrazolyl)borate (Tp) (86), N-[di(2-pyridyl)methyl]acetamide (87), and N-[di(2-pyridyl)methyl]-4-tert-butylbenzenesulfonylamide (88), were prepared by reacting a p-tolyl trifluoromethyl Pd(II) precursor with a suitable (pre-)ligand and PhICl2 or NFTPT (Scheme 21).34 These dihydrocarbyl Pd(IV) complexes supported by fac-chelating ligands were essentially inert toward reductive elimination when kept in solutions for several days at 25  C.

796

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 21

A detailed study of an acetate-assisted CdH activation at a Pd(IV) center was performed using a cationic 2-biphenylyl Pd(IV) complex 89 supported by a fac-chelating tris(2-pyridyl)methane ligand (Tpm) (Scheme 22).51 Three salts derived from this cation were prepared that contained dichloroiodate (ICl−2), chloride or triflate counterions. The preparation was carried out by reacting a 2-biphenylyl iodo or triflato Pd(II) precursors with 2 equivalents of PhICl2. When preparing (89)OTf, a subsequent exchange of a halide anion with triflate was done using AgOTf. Heating the product (89)Cl containing a Cl− counterion for 2 h at 90  C resulted in a CdCl bond elimination only. No CdH bond activation reactivity was observed for other two salts, (89)ICl2 and (89)OTf, either. Also unsuccessful were the attempts of using pyridine, Et3N or NaOH additives at promoting CdH activation at a Pd(IV) center. In turn, the use of 4 equivalents of AgOAc in a reaction with (89)Cl in chloroform solutions resulted in a fast CdH activation and the formation of the derived stable diaryl complex 90 (Scheme 22). Similar was the reactivity toward AgOAc/CHCl3 of a dibromo analog of 89. The proposed mechanism of CdH activation at a Pd(IV) center includes dissociation of one of the pyridine donor fragments from the metal atom with following acetate ligand coordination, chloride ligand loss and the subsequent carboxylate ligand-assisted CdH activation. Interestingly, neither the acetate for chloride ligand substitution nor the CdH activation step was rate-limiting. The authors proposed that either conformational or configurational isomerization likely limited the rate of the overall transformation.

Scheme 22

The use of anionic fac-chelating ligands such as hydridotris(pyrazolyl)borate (Tp) or hydridotris(3,5-dimethylpyrazolyl)borate (TpMe2) allowed to use mild oxidizing agents such as ferrocenium32 or arenediazonium52 salts for oxidation of derived anionic dihydrocarbyl Pd(II) complexes and the preparation of dihydrocarbyl Pd(IV) compounds. In an early work32 two anionic cycloneophylpalladium(II) complexes supported by Tp and TpMe2 ligands were oxidized using 2 equivalents of ferrocenium tetrafluoroborate, FcBF4, in the presence of various 4-R-substituted pyridines (R ¼ NMe2, H, CN) to form a series of stable cycloneophylpalladium(IV) products 91–96. These complexes were isolated and fully characterized, including elemental analysis (Scheme 23). The reactivity of these products was not discussed.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

797

Scheme 23

In a more recent work, a TpMe2 ligand-supported anionic dimethylpalladium(II) complex [(TpMe2)PdMe2]− was converted to a relatively stable dimethyl aryldiazenido Pd(IV) compound 97 upon its reaction with an aryldiazonium salt (p-MeOC6H4N2) BF4 (Scheme 24).52 The complex 97 was isolated in 94% yield and characterized crystallographically. Thermolysis of 97 in benzene solutions at 70  C for 1 h resulted in a complex mixture of products with 4,40 -dimethoxybiphenyl (36% yield) and a trimethyl Pd(IV) complex (TpMe2)PdMe3, 98, (34% yield) being the major components of the mixture. A radical decomposition mechanism involving an aryldiazenyl radical (p-MeOC6H4N2)• and dimethylpalladium(III) transient species was proposed for the decomposition reaction.

Scheme 24

Two polydentate dipyridine ligands that can adapt fac-chelating coordination mode were used for the preparation of bromo and iodo cycloneophylpalladium(IV) complexes 99–102 (Scheme 25).49 The derived Pd(IV) complexes were isolated in 78–90% yield and two of them, 99 and 100, were characterized crystallographically. Decomposition of complexes 99–102 in chloroform solutions at 50  C with subsequent acidic work up to protonolyze any remaining Pd(II)dC bonds resulted in complex mixtures of organic compounds. The reaction products included (2-methylprop-1-eneyl)benzene as the major component (up to 61% yield), C(sp3)dX coupled products, 2-methyl-2-phenylethyl halides (up to 50% yield), and 2,2-dimethylbenzocyclobutene (up to 30% yield). The reactions were proposed to include initial ionization of a Pd(IV)dX bond with a subsequent C(sp2)dC(sp3) coupling resulting in 2,2-dimethylbenzocyclobutene and a competitive C(sp3)dX coupling operating an SN2 mechanism leading to 2-methyl-2-phenylethyl halides. In turn, the formation of the neophyl rearrangement products such as (2-methylprop-1-eneyl) benzene was proposed to result from secondary transformations of the initially formed 2-methyl-2-phenylethyl halides. A computational (DFT) analysis was carried out in support to this conclusion.

Scheme 25

798

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

The pyridinophane ligands RN4 that were used for kinetic stabilization of mononuclear organopalladium(III) complexes described in Section 8.12.1.1 were also characterized in terms of their ability to stabilize organopalladium(IV) species. In particular, the oxidation of dimethylpalladium(II) complexes (RN4)PdMe2 (R ¼ Me, iPr, tBu) with 2 equivalents of FcPF6 resulted in the formation of stable dicationic dimethylpalladium(IV) derivatives 103–105.6,9 These complexes were characterized crystallographically as hexafluorophosphate salts. Their reactivity toward CdC reductive elimination and the formation of ethane was explored under both thermal and photochemical conditions. No ethane was observed upon attempted thermolysis of complex 1039 featuring the least bulky pyridinophane ligand MeN4 at 70  C. In turn, a derivative of the isopropyl-substituted ligand, iPrN4, complex 104,9 produced ethane in a virtually quantitative yield under the same reaction conditions. The reaction of 104 was not inhibited by TEMPO and was proposed to include a thermal dissociation of one of the axial amine donors to form a reactive 5-coordnate intermediate. Photolysis of both 103, 1049 and 1056 led to a photo-induced axial amine donor dissociation and clean ethane elimination. The use of N2S2, a dithia analog of the pyridinophanes RN4, led to similar results.53 A stable dicationic dimethylpalladium(IV) complex 106 was produced by chemical oxidation of its Pd(II) precursor, (N2S2)PdMe2, using 2 equivalents of FcPF6. The Pd(IV) product was isolated as a hexafluorophosphate salt (106)(PF6)2 and characterized crystallographically. Similar to 103, ethane was not produced upon attempted thermolysis of 106. In contrast to 103–105, attempted photolysis of 106 did not lead to ethane formation either. In turn, addition of 1.1 equivalents of strongly reducing CoCp2 allowed to observe ethane elimination (76% yield), so suggesting that this reaction of 106, most likely, involves Pd(III) species.

8.12.2.2.3

Pincer ligands

A dihydrocarbyl alkyl aryl Pd(IV) complex 107 supported by a pincer ligand derived from 2,6-diacetylpyridine was prepared by reacting the corresponding Pd(II) precursor pincer complex and o-iodobenzoic acid (Scheme 26).54 Complex 107 is indefinitely stable in the solid state at 25  C. It was fully characterized, including single crystal X-ray diffraction. In solutions 107 exists in equilibrium with its diastereomer 108 and a Pd(II) redox isomer having a coordinated o-iodobenzoate ligand. It was shown that a combination of 107 and 1 equivalent of AgClO4 results in an active catalyst for Heck reaction involving o-iodobenzoic acid and methyl acrylate.55

Scheme 26

8.12.2.3

Palladium(IV) monohydrocarbyls

The group of well-characterized palladium(IV) monohydrocarbyl complexes is smaller than that of Pd(IV) dihydrocarbyl complexes. Here the role of stabilizing chelating ligands is even more important for the success of their isolation.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.2.3.1

799

Bidentate chelating ligands

A series of model monoaryl Pd(IV) fluoro complexes 109–117 were prepared with the goal of a detailed characterization of the C(sp2)dF bond elimination reactivity at a Pd(IV) center.56–58 In the first report of this series,56 a reaction with Selectfluor of a bis-chelated monoaryl Pd(II) precursor in MeCN solutions at 50  C afforded 10-fluorobenzo[h]quinoline in 94% yield (Scheme 27). A well-defined single intermediate 109 with an assumed coordinated solvent molecule (L ¼ MeCN) occupying one of the metal coordination sites was observed at 23  C. The complex was characterized by 1H, 13C and 19F NMR spectroscopy at lower temperatures. At 23  C the complex decomposed with a half-life of about 70 min and produced the CdF coupled product, 10-fluorobenzo[h]quinoline; this decomposition reaction of 109 was high-yielding at 50  C. When 109 was treated with Me4NF, a difluoride complex 110 formed. This product was more stable than 109 and was isolated and fully characterized including single crystal X-ray diffraction. When heated at 150  C in DMSO solutions, 110 produced 10-fluorobenzo[h]quinoline in high yield. The difluoro Pd(IV) complex 110 was also prepared independently from the bis-chelated aryl Pd(II) precursor using XeF2 as fluorinating agent.

Scheme 27

A subsequent extended experimental study of the C(sp2)dF reductive elimination at a Pd(IV) center57 included complex 111, a pyridine analog of 109 (L ¼ py), as well as a series of complexes 112–117 derived from 7-R-substituted benzo[h]quinolines (Scheme 28). A combination of dynamic NMR experiments allowed to propose the structure for complexes 112–117 where one of the metal coordination sites was occupied by an oxygen atom of the sulfonyl fragment. A kinetics study of CdF coupling reactions of complexes 112–117 was carried out. The effect of substituents R at the C7 position of the benzo[h]quinoline moiety was characterized by a Hammett constant r ¼ +1.41. The effect of para-substitution in the arenesulfonyl fragment on the reaction rate was studied as well (r ¼ −0.45). The proposed mechanism of the CdF bond elimination from complexes 112–117 involved the formation of 5-coordinate Pd(IV) intermediates resulting from dissociation of the sulfonyl group oxygen from the metal with a subsequent rate-determining CdF coupling of the 5-coordinate transient species.

Scheme 28

An exotic monoaryl Pd(IV) trifluoride complex 118 was prepared by fluorination with XeF2 of a tBu2-bpy-supported Pd(II) p-fluorophenyl fluoro complex (Scheme 29).58 The product was fully characterized including single crystal X-ray diffraction. The fluoride ligand trans- to the aryl was sufficiently basic to bind readily HF present in reaction mixtures. When 118 was heated in PhNO2 solutions at 80  C, only trace amounts of the CdF coupled product, p-difluorobenzene, were observed, whereas the major reaction product was 4,40 -difluorobiphenyl. In turn, the formation of p-difluorobenzene proceeded with high yields when the reaction was performed in the presence of XeF2 (92% yield) or similarly strong oxidants such as (PhSO2)2NF (83% yield).

800

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 29

A chelating carbene alkoxide ligand was used to support a monohydrocarbyl Pd(IV) chloride 119 which demonstrated a C(sp2) dCl bond reductive elimination reactivity at a Pd(IV) center (Scheme 30).59 The Pd(IV) compound was produced by chlorination with PhICl2 of the carbene-supported cyclopalladated benzo[h]quinoline Pd(II) precursor at −35  C in MeCN solutions. Complex 119 is stable in the solid state for at least a week and was fully characterized, including X-ray crystallography. Warming dilute 0.5 mM solutions of 119 in MeCN at 33  C resulted in a slow elimination of Pd(II)-coordinated 10-chlorobenzo[h]quinoline in high yield. No products of functionalization of the carbene ligand were observed. The yield of the CdCl coupled 10-chlorobenzo[h] quinoline decreased when the concentration of 119 was increased to 11 mM, presumably, due to some secondary transformations of the CdCl elimination product. In conclusion, complex 119 was demonstrated to serve as an effective catalyst for the bromination of benzo[h]quinoline with N-bromosuccinimide.

Scheme 30

8.12.2.3.2

fac-Chelating ligands

A series of relatively stable alkoxo and hydroxo monohydrocarbyl (aryl) Pd(IV) complexes 120–127 were prepared by oxidation with dilute aqueous hydrogen peroxide of their Pd(II) metallacyclic precursors supported by di(2-pyridyl)ketone (dpk) ligand (Scheme 31).33,60 The dpk ligand was responsible for H2O2 activation and was transformed into a fac-chelating

Scheme 31

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

801

1-hydroxy-1,1-bis(2-pyridyl)methoxide under the reaction conditions. The so-produced fac-chelating ligand is acidic and the supported cationic complexes 120, 121 were converted to neutral zwitterionic compounds 122, 123 by the action of dilute aqueous NaOH. The deprotonation reaction was fully reversible. A gentle warming of 120 and 121 in the presence of excess HCl or HBr produced the corresponding C(sp2)dX coupled products in high yield. In turn, in alkaline aqueous solution the zwitterionic complexes 122 and 123 produced the derived phenols. Under the action of excess aqueous HX (X ¼ Cl, Br, CF3CO2) on aqueous solutions of hydroxo Pd(IV) complex 125 at 0  C, dicationic aqua complexes (128)X2 formed and were isolated. Complexes (128) X2 were converted to the corresponding halido derivatives 129 (X ¼ Cl) and 130 (X ¼ Br) upon their gentle drying under high vacuum. Both 129 and 130 eliminated the corresponding C(sp2)dX coupled compounds in high yield. In turn, the hydroxo complexes 124–127 in aqueous solutions produced cleanly at 23  C the derived oxapalladacycles which, upon addition of aqueous HCl, liberated the corresponding phenols. All the Pd(IV) complexes were fully characterized including single crystal X-ray diffraction for 120(OAc), 123 and (128)(CF3CO2)2. The rates of the CdO coupling of complexes 124–127 were measured60 and found to be very weakly dependent on the identity of the substituent R. It was proposed that these reactions involve the same rate determining step, the dissociation of one of the nitrogen atoms of the fac-chelating ligand from the metal, which made the reaction rates of these substrates almost undistinguishable. Two stable N-sulfonylamido monohydrocarbyl (aryl) Pd(IV) complexes, 131 having an N-methanesulfonylamido moiety and 132 having an N-trifluoromethanesulfonylamido moiety, supported by fac-chelating 1-hydroxy-1,1-bis(2-pyridyl)methoxide were prepared by oxidation with dilute aqueous H2O2 of the corresponding N-sulfonylamido aryl Pd(II) precursor complexes having a coordinated di(2-pyridyl)ketone ligand (Scheme 32).61 When the oxidation with H2O2 of the N-methanesulfonylamido Pd(II) precursor was carried out in MeOH or AcOH at 23  C, the corresponding N-methanesulfonyl carbazole was produced in 88–92% yield after a few hours of reaction. Two (MeOH solutions) or three (AcOH) reaction intermediates were observed by 1H NMR spectroscopy. In the case of N-trifluoromethanesulfonylamido Pd(II) precursor the formation of the derived carbazole occurred in 91–98% yield but was only possible in AcOH solutions at 60  C. The Pd(IV) complexes 131 and 132 and the corresponding intermediates were isolated and fully characterized. Single crystal X-ray diffraction was used to solve the structure of Pd(IV) complex 132 as well as that of the intermediate resulting from H2O2 addition across the C]O bond of a Pd(II)-coordinated dpk ligand in the N-methanesulfonylamido precursor complex. The reactivity of the respective isolated H2O2 adduct toward the formation of Pd(IV) amido aryls 131 and 132, including the reaction kinetics for 132, was characterized, and the kinetics of the C(sp2)dN bond reductive elimination leading to the corresponding N-methanesulfonyl carbazole from the Pd(IV) amido aryl complex 131 was studied. Both reactions followed clean first-order kinetics. Overall, the electron-richer N-sulfonylamido aryl Pd(IV) complex 131 was more reactive than its electron-poorer analog 132. A cationic Pd(IV) aqua derivative 133 was produced from 132 in the presence of HBF4 in MeCN or dichloromethane solutions and proved to be more reactive in C(sp2)dN bond reductive elimination than 132.

Scheme 32

The preparation and CdCl bond elimination reactivity of a stable monoaryl dichloro Pd(IV) complex 89 supported by fac-chelating tris(2-pyridyl)methane were already presented (Scheme 22).51 A chemical oxidation of RN4 pyridinophane (R ¼ Me, iPr, tBu)-supported Pd(II) methyl chloro complexes (RN4)PdMe(Cl) with 1 equivalent of FcPF6 followed by 1 equivalent of NOBF4 resulted in the formation of dicationic complexes [(RN4)PdMe(Cl)]2+, 134–136, which could be isolated and were fully characterized (Scheme 33).9 A perchlorate salt (134)(ClO4)2 was characterized crystallographically. Upon the action of 2 equivalents Et4NCl on these complexes MeCl was produced in high yield. The reaction was not sensitive to the presence of additives of TEMPO, consistent with the realization of an SN2 mechanism of C(sp3)dCl coupling.

802

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 33

The use of a modified pyridinophane ligand, tBuN3CMe, allowed to prepare a moderately unstable cationic monohydrocarbyl (aryl) Pd(IV) complex [(tBuN3CMe)PdBr2]+, 137 (Scheme 33).11 The preparation involved the oxidation with thianthrenium tetrafluoroborate of arylpalladium(III) complex 17. Complex 137 was isolated as a tetrafluoroborate salt (137)BF4 and characterized by means of 1H NMR spectroscopy. No reactivity was reported for this compound.

8.12.2.3.3

Pincer ligands

The use of pincer-type ligands may have a profound effect on the stability of otherwise highly reactive Pd(IV) monohydrocarbyl complexes. Two stable monohydrocarbyl (alkyl) trihalido Pd(IV) pincer complexes, 138 and 139, were prepared by reacting excess free halogens X2 (X ¼ Cl, Br) with the corresponding halido Pd(II) precursors in dichloromethane solutions at 0  C (Scheme 34).62 Both complexes were characterized crystallographically. The compounds can be stored at 4  C for an indefinitely long time without decomposition. In chloroform solutions their decomposition was complete after 24 h at 23  C. The reaction products liberated upon aqueous work up of the reaction mixtures are the corresponding a-halogenodiketones.

Scheme 34

A reaction of an iodo Pd(II) pincer precursor with iodine at −40  C resulted in an equilibrium mixture of reactant and a Pd(IV) alkyl triiodo complex 140.63 This reactive complex could be isolated and characterized crystallographically. The reaction equilibrium constant of 142 was calculated for −33  C. A study of a temperature dependence of the reaction equilibrium constant allowed to find the following reaction parameters: DH ¼ −24 kJ/mol, DS ¼ −59 J/K mol. At room temperature 140 rapidly decomposed to produce the corresponding a-iododiketone. Another monohydrocarbyl (alkyl) Pd(IV) dihalo complex 141 was prepared by reacting elemental bromine with a metallacyclic Pd(II) precursor at −78  C (Scheme 35).64 The Pd(IV) complex was isolated at a low temperature and characterized crystallographically. Complex 141 decomposes in solutions at 0  C in the course of a few hours and within a few days in the solid state. No detailed report of reactivity of 141 was provided.

Scheme 35

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

803

Two stable monohydrocarbyl (aryl) Pd(IV) trihalo complexes 142 and 143 supported by a monoanionic bis-carbene CC0 C pincer ligand were prepared by reacting their Pd(II) pincer precursors with various halogenating agents (Scheme 36).65 The best oxidant for the preparation of the trichloride 142 was PhICl2 (5 min, >99% yield), but CuCl2 (24 h, 87% yield) also worked well. In turn, the free halogen, chlorine gas, was not as efficient (30 min, 51% yield). In contrast to the preparation of the trichloride 142, for the preparation of the analogous tribromide 143 a free halogen, elemental bromine, was the best oxidant (5 min, >99% yield), although CuBr2 (2 h, 90% yield) was also efficient. Complexes 142–143 were fully characterized including single crystal X-ray diffraction. A thermal gravimetric analysis of 142 demonstrated its remarkable thermal stability characterized by a decomposition temperature above 200  C. In turn, the tribromide 143 decomposed slowly by losing Br2 in chloroform solutions at 50  C. Accordingly, while 142 was chemically very inert, complex 143 reacted with styrene at 80  C to form products of styrene bromination.

Scheme 36

8.12.2.4

Palladium(IV) complexes supported by N-heterocyclic carbene ligands

The ability of N-heterocyclic carbene ligands to support high-valent Pd(IV) complexes was further demonstrated by the synthesis of stable tetrachloro Pd(IV) complexes 144 and 145 containing chelating bis-N-heterocyclic carbene ligands.66 The complexes were prepared by reacting their Pd(II) dichloro precursors with Cl2 at 0  C in DMF solutions. These Pd(IV) compounds have no s-bonded hydrocarbyl ligands at the metal and are stabilized only by the carbene. Complex 144 is poorly soluble in common organic solvents; it was characterized crystallographically. The more lipophilic complex 145 with longer alkyl substituents was much more soluble in common organic solvents and was used as stoichiometric reagent for chlorination of various olefins, alkynes and alkylarenes at 25  C.

8.12.3

Platinum(III) complexes

In the past two decades the interest to the chemistry of organoplatinum(III) compounds, especially those having PtdPt bonds, was driven mostly by the development of new anticancer agents,67 new catalysts for organic oxidation reactions68 and new photoluminescent materials.69 Dinuclear and polynuclear Pt(III) complexes are relatively common and, typically, feature a PtdPt bond between two adjacent odd-electron d7metal centers. Dinuclear and polynuclear Pt(III) complexes having no PtdC bond were reviewed in a corresponding chapter of a recent edition of Comprehensive Coordination Chemistry III.70 In turn, mononuclear organoplatinum(III) complexes, similar to mononuclear organopalladium(III) compounds, are very rare. The mononuclear complexes will be considered first, followed by di- and polynuclear complexes.

8.12.3.1

Mononuclear organoplatinum(III) complexes

The structural factors that can disfavor the formation of PtdPt bond between two odd-electron d7 metal centers are the presence of bulky surrounding ligands and/or a significant delocalization of the unpaired electron between the metal center and the coordinated ligands. The first organoplatinum(III) complex, anionic [Pt(C6Cl5)4]−, 146, was reported in 1984.71 The complex was produced in a high yield by the action of free halogens X2 (X ¼ Cl or Br) on the anionic tetraarylplatinate(II) complex used as a tetra-n-butylammonium salt (nBu4N)2[Pt(C6Cl5)4] and isolated as a paramagnetic dark blue-colored solid, (nBu4N)(146). The product is indefinitely stable under aerobic conditions at 25  C both in the solid state and in solutions and decomposes only at 210  C. The metal atom in 146 has a square planar coordination geometry which, in combination with the metal electron

804

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

configuration d7, could allow for a PtdPt bonding but the presence of four bulky pentachlorophenyl ligands prevented the complex from dimerization. When reacted with HCl or PPh3, 146 is reduced to form Pt(II) compounds. The complex reacts reversibly with gaseous nitrogen monoxide to form a moderately stable adduct (nBu4N)2[Pt(C6Cl5)4(NO)] with a linear nitrosyl ligand occupying the axial site of a slightly distorted tetragonal pyramidal coordination sphere of the metal.72 A structurally very similar anionic tetrakis-2,3,5,6-tetrachlorophenyl complex [Pt(C6Cl4H)4]−, 147, was also prepared and characterized.73

Reaction of an electron-deficient cyclometallated Pt(II) complex supported by a N-heterocyclic carbene ligand with 0.5 equivalent of Br2 at −78  C in dichloromethane solutions produced a unique cationic alkyl Pt(III) complex 148 that could be isolated at temperatures below −20  C as a deep blue-colored BArF4 salt (148)(BArF4) (BArF4 ¼ tetrakis[3,5-bis(trifluoromethyl) phenyl]borate) (Scheme 37).74 The compound was characterized by single crystal X-ray diffraction. Its magnetic moment was determined to be 1.75 mB, consistent with the presence of one unpaired electron. Intriguingly, the solid state structure of 148 showed a 4-coordinate see-saw geometry at the metal center, with the C(alkyl)-Pt-Br angle of 149.5 . Reaction of 148 with another half equivalent of Br2 at −78  C resulted in the formation of a reactive dibromo Pt(IV) complex which, in turn, eliminated cleanly a C(sp3)dBr bond when the solution was warmed to 23  C. Complex 148 was proposed to be an intermediate in the halogenolysis of a PtdC(sp3) bond of the original Pt(II) alkyl complex.

Scheme 37

Oxidation with 0.5 equivalent of I2 of a diiodo Pt(II) complex supported by two bulky N-heterocyclic carbene ligands IPr in the presence of NaBArF4 resulted in the formation of a stable cationic mononuclear complex 149 that was isolated as a deep-blue salt (149)(BArF4) (Scheme 38).75 The compound is indefinitely stable in the solid state at 5  C but decomposes at 23  C under air in the course of a few weeks. The salt (149)(BArF4) was characterized by X-ray crystallography, according to which the metal atom in 149 has a square planar coordination geometry. According to EPR characterization, the unpaired electron in the cation 149 resides predominantly on the metal atom.

Scheme 38

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.3.2

805

Dinuclear and polynuclear organoplatinum(III) complexes

Di- and polynuclear organoplatinum(III) complexes are much more common than their mononuclear organoplatinum(III) counterparts. The least stable of dinuclear complexes have an unsupported PtdPt bond that can be broken relatively readily under thermal or photochemical conditions. In turn, one or, most typically, two bridging ligands can make the PtdPt fragment more robust so enhancing the stability of di- and polynuclear Pt(III) species. The following presentation is organized according to the number of bridging ligands that hold together adjacent metal atoms in a complex, as well as the number of platinum atoms in the coordination compound.

8.12.3.2.1

Unsupported dinuclear Pt(III) complexes

Complexes with unsupported PtdPt bond are extremely rare. The first reported dinuclear organoplatinum(III) complex 150 without bridging ligands between two Pt(III) centers was isolated in 2004. The complex was originally prepared in a low yield upon oxidation of bis-metallacyclic Pt(II) derivative of 2-phenylpyridine with equimolar amount of [AuCl(SMe2)] (Scheme 39).76 The solid state structure of complex 150 which was characterized crystallographically exhibited a 2.726 A˚ long PtdPt bond, that is about 0.2–0.3 A˚ longer than in the reported unbridged non-organometallic Pt(III) complexes,77–79 and a staggered conformation along PtdPt bond with the CldPtdPtdCl dihedral angle of 141 . The complex 150 was stable in the dark for at least a few days. Later N-chlorosuccinimide was found to be a much more efficient oxidant to produce 150 in a similar reaction where it formed as the major reaction product (56–63% yield) after 5 min at 25  C (Scheme 39).80 Notably, if the reaction was carried out under ambient light the yield of the product was noticeably higher when higher concentrations of the Pt(II) precursor were employed (10 vs 1.25 mM). In the dark, the reaction efficiency was not affected by concentration of the reactant. Heating 150 in MeOH or DMSO solutions at 100  C for 24 h resulted in isomerization of 150 to form a more symmetrical complex 151.

Scheme 39

A reaction of a Pt(II) dimethyl complex (bpy)PtMe2 with an iodonium triflate I(C^CSiMe3)Ph(OTf ) at −80  C resulted in the formation of an unstable product that was assigned the structure of an unsupported dinuclear Pt(III) complex 152 (Scheme 40).81 The complex was readily transformed to a Pt(IV) trihydrocarbyl complex (bpy)PtMe2(C^CSiMe3)(I) upon its reaction with NaI in acetone at a higher, −20  C, temperature. When the reaction with NaI was carried out at −80  C, a crystalline dinuclear Pt(III) complex 153 formed which was isolated and characterized crystallographically as an iodo-analog of 152. At −20  C the complex disproportionated into the starting (bpy)PtMe2 and the Pt(IV) product (bpy)PtMe2(C^CSiMe3)(I). The solid state structure of 153 demonstrated a relatively short PtdPt bond of 2.764 A˚ and a very long distance between iodine and an adjacent Pt atom, 3.411 A˚ , which is much longer than 2.763 A˚ found in a related mononuclear Pt(IV) complex (bpy)PtMe2Ph(I), and only slightly shorter than the sum of van der Waals radii of Pt and I atoms, of 3.7 A˚ . It was proposed that the formation of complex 153 points to a general mechanism by which 2-electrons oxidants such as I(X)+2 react with Pt(II) complexes. The mechanism may include the initial transfer of an electrophile “X+” to a Pt(II) center and the formation of 5-coordinate Pt(IV) cations that are next intercepted by excess of Pt(II) reagent with concomitant formation of PtdPt bond. If sufficiently stabilized by ligands in the axial positions, the resulting dinuclear Pt(III) may then be isolated. If their stability is not high enough, the Pt(III) dinuclear species disproportionate into Pt(II) and Pt(IV) products.

Scheme 40

806

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.3.2.2

Monobridged dinuclear Pt(III) complexes

Non-organometallic dinuclear Pt(III) complexes, most typically, are doubly bridged or quadruply bridged (lantern complexes).70 Triply bridged lantern complexes are also known but are rare. There are only a few reported singly bridged heterodinuclear organoplatinum(III) compounds.82,83 Singly acetato-bridged homodinuclear organoplatinum(III) complex 154 was prepared by oxidation with PhICl2 of a benzo[h] quinoline-derived metallacyclic Pt(II) precursor dissolved in acetic acid (Scheme 41).84 The use of MeCN or MeOH as a solvent led to different reaction products. Complex 154 was isolated as a tetrafluoroborate salt (154)BF4, characterized crystallographically and had a PtdPt distance of 2.565 A˚ .

Scheme 41

8.12.3.2.3

Doubly bridged dinuclear Pt(III) complexes

Carboxylate ligands and acetate ligand, in particular, as well as carboxamidate ligands are the most common bridging motifs in polynuclear Pt(III) complexes. Reaction of cis-[PtCl2(DMSO)2] with 2,5-bis(4-ethoxyphenyl)pyridine in refluxing AcOH resulted in the formation of dinuclear bis-acetato-bridged diplatinum(III) complex 155 (Scheme 42).85 The coordinated DMSO ligand served as oxidant in this reaction. The product was characterized crystallographically and had a PtdPt bond length of 2.573 A˚ .

Scheme 42

Several monoaryl complexes 156–159 with aryl ligands in the axial position of a Pt(III)–Pt(III) core bridged by two pivalamidate ligands were prepared using a tetracationic diaqua complex and its direct reaction with phenol (complex 156) or with suitable phenylboronic acids (complexes 156–159) (Scheme 43).86 The reaction with phenol was highly selective, not affected by an

Scheme 43

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

807

additive of 100 equivalents of 2,4,6-tri-tert-butylphenol and produced a single o-hydroxyphenyl isomer. The high selectivity was attributed to the involvement of a hydrogen bond between the reactants in the course of the metallation reaction. Complex 156 was isolated as two salts, (156)(NO3)(SO4) and (156)(NO3)3, and characterized crystallographically as well by means of multinuclear NMR spectroscopy. Crystallographic characterization was also performed for the phenyl complex 157 isolated as a nitrate salt (157) (NO3)3. The PtdPt bond lengths in the phenol derivative 156 and its phenyl analog 157, were determined as 2.684 and 2.754 A˚ , respectively. Complex 157 was stable in the presence of aqueous acids in solution. Upon irradiation with light from a low-pressure mercury lamp 157 decomposed with the formation of phenol (28% yield) and benzene (11% yield). The use of EPR spectroscopy for the characterization of the reaction intermediates produced in frozen solutions of 157 pointed to the formation of an organic radical (g-factor ¼ 2.0032) and a Pt(III)–Pt(II) mixed-valence complex. The preparation of a series of dinuclear organoplatinum(III) complexes 160–165 derived from cyclometallated 2-phenylpyridine and containing two axial chloride ligands and various N,S-bridging ligands (Scheme 44) was carried out with a goal to explore their solid state structure and the crystal packing.87,88 The so-produced complexes were stable in the solid state and in solutions. The complexes were fully characterized including X-ray crystallography for 160–163. The PtdPt bond length varied in a narrow range from 2.619 A˚ for 161 to 2.643 A˚ for 160.

Scheme 44

A series of half-lantern organoplatinum(III) complexes 166–169 with two 2-pyridinethiolate N,S-bridging ligands were prepared and fully characterized.89 Although their Pt(II) precursors exhibited red luminescence in the solid state and solution, complexes 166–169 were not photoemissive.

The lack of photoluminescence of organoplatinum(III) compounds is very common and was related to a short lifetime of their lowest energy excited triplet states. To overcome this limitation, new N,S-bridging ligand, 5-phenyl-1,3,4-oxadiazole-2-thiolate, was designed that incorporated both a donor and an acceptor fragments with a hope to alter the nature of the electronic excited state of the supported diplatinum(III) complexes. Three half lantern organoplatinum(III) complexes, 170–172, were prepared and fully characterized, including single crystal X-ray diffraction for 170.69 The compounds were found to exhibit intense phosphorescence

808

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

under ambient conditions. Using 170 as an emitter, new organic light-emitting diodes were fabricated with wavelength of 716 nm (non-doped), 614 nm (doped) and white-light emission in a hybrid device.

A diphenylphosphido-bridged dinuclear complex 173, [Ar2Pt(m-PPh2)2PtAr2] (Ar ¼ C6F5), was prepared and its reactivity toward nucleophiles, OH−, N−3 and NCO−, was studied.90 Complex 173 had two Pt(III) centers in a square planar configuration and features a PtdPt bond, so that both metal centers are readily available to accept a nucleophilic attack. As a result of the action of [Bu4N]OH in dichloromethane solution the Pt(III) centers in 173 were reduced to Pt(II) by acting cooperatively, formally, as one-electron oxidant each. In turn, one of the phosphido ligands was oxidized to form a diphenylphosphinite ligand featuring a Ph2PdO bond. The reaction resulted in (Bu4N)2[Ar2Pt(m-PPh2)(m-OPPh2)PtAr2] as the main product. An action of NaN3 or KOCN upon 173 in acetone solutions resulted in the reduction of Pt(III) to Pt(II), coordination of N3 or OCN as m2-bridging ligands and an oxidative Ph2P-Ar coupling.

8.12.3.2.4

Polynuclear Pt(III) complexes

The use of diphenylphosphido bridging ligands allowed to link together three platinum centers in a Pt(III)/Pt(II) mixed-valence complex 174, [Ar2Pt(m-PPh2)2Pt(m-PPh2)2PtAr2] (Ar ¼ C6F5) (Scheme 45).91 The complex was fully characterized. Reaction of 174 with a source of halide anions, Bu4NBr or Bu4NI, resulted in the halide coordination to two Pt centers as a bridging ligand, the reduction of both Pt(III) centers to Pt(II) and the oxidative ArdPPh2 coupling of one of the diphenylphosphido ligands with an aryl.

Scheme 45

A diiodo diaryl analog of 174, complex 175, was also prepared.92 The substitution of two iodo ligands in 175 with acetylacetonate led to a cationic complex 176.92 Both 175 and 176 were characterized crystallographically. Two PtdPt distances in both complexes are substantially different. In 176, a short 2.763 A˚ distance between the Ar2Pt and the central Pt atoms suggested the existence of a metal-metal bond between these Pt(III) centers, whereas a 3.612 A˚ distance between the central Pt atom and the acac-coordinated Pt atom argued against the presence of a metal-metal bond. Similarly, a short 2.778 A˚ distance between the Ar2Pt and the central Pt atoms in 175 pointed to a metal-metal bonding between these Pt(III) centers while there was no significant bonding interaction between the central Pt atom and that in the PtI2 fragment, consistent with a long metal-metal distance of 3.636 A˚ between these two centers. Similar to the reactivity found for 174, the polynuclear Pt(III) complexes 175 and 176 demonstrated a cooperative 2-electron redox behavior when combined with a source of halide ligands, Br− or I−.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

809

A series of pentanuclear bis-acetonyl Pt(III)/Pt(II) mixed-valence complexes 179–182 containing a chain of five bonded metal atoms were prepared by reacting two equivalents of dinuclear pivalamidato-bridged Pt(III) acetonyl complexes 177 or 178 and 1 equivalent of a Pt(II) species [PtX4]2− (X ¼ Cl, Br) (Scheme 46).93 The four pentanuclear complexes were characetrzied crystallographically. The bond distances in 179 were as follows: 2.699 A˚ in the pivalamidato-bridged units and 3.014 A˚ between the central and two adjacent Pt atoms. The distances varied slightly from 179 to 182. It is noteworthy that the bonds between the central Pt atom in the [PtX4] fragment and two Pt atoms in the half lantern-type fragments are unsupported and, therefore, are relatively strong. The strong unsupported PtdPt bonding was proposed to result from a Pt(II)dPt(IV) polarization in the bridged fragments and an electron transfer from the central Pt(II) atom to the adjacent Pt(IV)-like centers. The terminal Pt(II)-like atoms have, therefore, an increased electron density on them which, in turn, makes the attached acetonyl ligand more basic. Accordingly, a slow decomposition of 179–182 in water results in the formation of mixtures of acetone and hydroxyacetone.

Scheme 46

8.12.4

Platinum(IV) complexes

Organoplatinum(IV) complexes constitute a very large group of organometallic compounds. The metal center in organoplatinum(IV) complexes is typically six-coordinate, although a growing number of 5-coordinate platinum(IV) complexes are now characterized. A high kinetic inertness and a relatively high strength of the metal-ligand bonds make organoplatinum(IV) complexes to a good model of their more reactive and less stable analogs derived from Pd(IV),80,84 as well as a convenient object for the study of the mechanisms of organometallic reactions. Accordingly, in the past two decades a significant number of works involving organoplatinum(IV) compounds were carried out with the goal to explore the mechanisms of the fundamental organometallic reactions such as CdH activation,94–98 oxidative addition of various CdX bonds to organoplatinum(II) precursors,99–102 O2 activation,103–107 CdC26,99,108–111 and CdX bond reductive elimination from a Pt(IV) center (X ¼ F,96,112 Cl,113,114 Br,115 I,116 O,103,104,117–120 N120,121), etc. Much attention was paid to the study of model 5-coordinate organoplatinum(IV) complexes98,122–131 since 5-coordinate Pt(IV) species can play the role of the key intermediates in a number of reactions involving organoplatinum(IV) compounds. Synthesis of new organoplatinum(IV) complexes was also pursued that targeted the development of new potent anticancer drugs,132–134 as well as the development of new photoluminescent materials.135–137 Several reviews were published that covered some aspects of the synthesis100,138–140 and reactivity26,94,100,138,139 of organoplatinum(IV) compounds. In the subsequent presentation the chemistry of five-coordinate organoplatinum(IV) complexes will be discussed first. The chemistry of 6-coordinate complexes will be given next, organized according to the number of hydrocarbyl ligands at a Pt(IV) center, from six to one. Finally, organoplatinum(IV) complexes without hydrocarbyl ligands will be presented where the only Pt(IV)dC bonds available are those between a carbene ligand and the metal.

810

8.12.4.1

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Five-coordinate organoplatinum(IV) complexes

The 18-electron count at a metal center is typical for stable organoplatinum(IV) complexes and is usually attained in 6-coordinate compounds of Pt(IV). Given a relatively high strength of Pt(IV)-ligand bonds, a loss of a 2-electron donor from the coordination sphere of an octahedral 18-electron Pt(IV) complex is usually associated with an significant energy penalty. Nevertheless, if there is a strong steric interference involving bulky ligands and/or there are ligands with a very strong trans-influence in the metal coordination sphere, such as silyl, hydride or a hydrocarbyl, a ligand loss from such a 6-coordinate Pt(IV) species can be facile. The resulting electron-deficient five-coordinate Pt(IV) species can become persistent and even stable enough to be isolated and fully characterized. A weak CdH agostic stabilization of a 5-coordnate metal center by a nearby alkyl group CdH bond can be an additional stabilizing factor in some cases. Some recently prepared and fully characterized 5-coordinate organoplatinum(IV) complexes 183,124 184, 185,125 186,126 187,129 188,123 189,128 190,127 191, 192,131 19398 demonstrate the effect of all these factors. In the solid state these isolated 5-coordinate organoplatinum(IV) compounds exhibit a square pyramidal geometry around the metal atom. The complexes feature an alkyl, an aryl or a silyl ligand trans- to an empty coordination site. The strong trans-influence of these ligands weakens significantly any metal-ligand bond trans- to these ligands so discouraging coordination of an additional electron donor. The electronic effects of other ligands at a Pt(IV) center, cis- to the empty site, are not so critical, as it is demonstrated by the example of an aryl trichloro Pt(IV) species 187 or dibromo- and diiodo complexes 191 and 192. A steric protection of an empty coordination site at a Pt(IV) center is achieved in 189 with the help of a phenylene fragment and with the help of an agostic alkyl group in the remaining complexes except 183–185.

Complexes 183–186 and 189 were readily prepared by reacting [PtMe3OTf]4 with the corresponding ligands,124–126 whereas complexes 190127 and 19398 resulted from oxidative addition of methyl iodide to their dimethylplatinum(II) precursors. Preparation of 188 involved abstraction of triflato ligand from its 6-coordinate triflato precursor with Li(Et2O)3[B(C6F5)4] in dichloromethane solutions.123 Complexes 187,129 191 and 192 resulted from oxidation with PhICl2, Br2 of I2, respectively, of their CdH agostic Pt(II) precursors. The weakness of CdH agostic interaction with a Pt(IV) center in 187 was manifested by the facile exchange of three methyl groups in the tert-butyl fragment whose CdH bonds can be involved in the agostic interaction with the metal, as evidenced by the appearance of only one signal for all nine tert-butyl group protons at 23  C.129 The signal was somewhat broadened at −40  C but still remained unresolved at −95  C. There was no such facile methyl group exchange in the Me2B fragment in complex 190 where the methyl groups produced two distinct signals because they were “locked” in a relatively rigid conformation dictated by the geometry of the dimethyldi(2-pyridyl)borate ligand.127

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

811

Interestingly, the three methyl ligands at a Pt(IV) center in trimethyl Pt(IV) complexes 183–186 produce only one signal in their H NMR spectra at 23  C, so suggesting a rapid exchange of the methyl ligands on the NMR time scale.124–126 The facile exchange is enabled by the presence of an open coordination site at the metal. The presence of sterically interfering groups near the open coordination site in other trimethyl Pt(IV) complexes, 189,128 190127 and 193,98 slows down the methyl ligands exchange, so that two separate signals are observed for the equatorial and axial methyl ligands in these complexes. Still, in 189 the methyl ligands signals are broad at 23  C and they coalesce at 45  C, so suggesting that the activation barrier for the ligand exchange is not high.128 Complexes 183–186 are thermally stable at 23  C but reductively eliminated ethane when heated at 60–75  C.124–126 Methane formation was also observed and was a result of a subsequent intramolecular CdH bond activation of the Pt(II) product. The analogs of complexes 183–186 with a somewhat sterically protected open coordination site, the electron-poor cationic complexes 191 and 192, were even more reactive. The derived BArF4 salts, 191(BArF4) and 192(BArF4), were stable in the solid state under air at 23  C but decomposed after several hours in dichloromethane solutions to form mixtures of unidentified products.131 The more electron-rich analog of 191 and 192, complex 189, was more chemically inert. The derived triflate salt, (189)OTf, showed no sign of decomposition after heating at 60–70  C in MeNO2 solutions for 5 days. Still, perhaps, due to its cationic nature, complex 189 was degraded rapidly by nBu4NX additives (X ¼ Cl, Br, I) to form [Me3PtX]4.128 Finally, the cationic complex 188123 was the most robust. The derived salt (188)[B(C6F5)4] showed no change in dichloromethane solutions after heating at 90  C for 3 days. The complex did not react with various hydride sources such as LiAlH4 or LiBEt3H either. 1

8.12.4.2 8.12.4.2.1

Six-coordinate organoplatinum(IV) complexes Complexes with six hydrocarbyl ligands

A few anionic organoplatinum(IV) complexes with six hydrocarbyl ligands at the metal atom were reported and partially characterized. The first reported complex was [PtMe6]2− that was prepared and isolated as a salt (Li-PMDETA)2[PtMe6] with a lithium counterion coordinated to N,N,N0 N00 ,N00 -pentamethyldiethylenetriamine (PMDETA).141 Although the low solubility of this product in diethyl ether simplified its isolation, no satisfactory elemental analysis data were produced. The anionic nature and the presence of methyl ligands mutually trans- to each other makes [PtMe6]2− complex highly reactive toward moisture, oxygen and various electrophiles. In support to this statement, an ether-soluble lithium salt Li2[PtMe6] could only be prepared in solutions and characterized by 1H and 195Pt NMR spectroscopy. The compound was stable in these solutions as long as a free MeLi was present.141,142 In the absence of added MeLi the complex decomposed in a few minutes at 23  C to form Pt black.142 Ether solutions of Li2[PtMe6] reacted rapidly with MeI to produce ethane and Li2[PtMe3I3].141 In a similar vein, a tetra-n-butylammonium salt (nBu4N)2[PtMe6] was reported to be stable as a solid under N2 atmosphere at 23  C but decomposed in contact with such common solvents as dichloromethane, MeCN or acetone.142 A more recent work reported the preparation of (nBu4N)2[Pt(CF3)6] although it was not isolated in a pure form.143

8.12.4.2.2

Complexes with five hydrocarbyl ligands

A series of anionic pentakis(trifluoromethyl) Pt(IV) complexes [Pt(CF3)5X]2− (194, X ¼ I; 195, X ¼ Br; 196, X ¼ Cl) and [Pt(CF3)5L]− (197, L ¼ H2O; 198, L ¼ MeCN; 199, L ¼ py; 200, L ¼ CO; 201, L ¼ tetrahydrothiophene) were prepared as thermally stable tetra-n-butylammonium salts and fully characterized, including X-ray diffraction analysis for 194, 198, 199 and 200 (as a Ph4P+ salt (Ph4P)(200)).144 The complexes 197 and 198 contain very labile solvento ligands L that could be readily displaced by other ligands, neutral (py, CO, tetrahydrothiophene) or anionic (Br, Cl), to produce 199–201 or 195, 196, respectively. Considering the chemical robustness of complexes 194–201, an especially remarkable is the observation of the thermal stability of the carbonyl derivative 200 which showed the lack of any noticeable M-to-CO p-backbonding. The lack of the p-backbonding was supported, in particular, by a high frequency of the CO stretching vibration of the coordinated carbonyl ligand, 2194 cm−1, which is well above the value for a free CO, 2143 cm−1. The thermal stability and the chemical robustness of the [Pt(CF3)5]− core are in a stark contrast with the high reactivity of the related pentamethylplatinate complexes [PtMe5X]n− which were only produced in solutions.145 This difference in reactivity and stability stems from a much higher electronegativity of the trifluoromethyl ligand, as compared to the methyl, with trifluoromethyl behaving as a “pseudo-halogen.”

812

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.4.2.3

Complexes with four hydrocarbyl ligands

Neutral tetramethylplatinum(IV) complexes are chemically much more robust than their anionic penta- and hexamethylplatinate(IV) analogs. A series of bis-phosphine-chelated tetramethyl Pt(IV) complexes 202–204 were prepared using a ligand exchange reaction between Pt2Me8(SMe2)2 and the corresponding diphosphines and fully characterized (Scheme 47).109 The phosphines chosen for this work featured different substituents on the C2 atom of the propylene backbone that would affect the energy needed to dissociate one of the phosphine donor atoms and open the corresponding chelate ring. The slow chelate ring openings precedes a subsequent facile CdC coupling to form ethane and dimethyl Pt(II) products. The observed rates of ethane elimination measured at 165  C in benzene solutions decreased in the order, 202 > 204 > 203, which demonstrated the existence of the gem-dialkyl effect in this reductive elimination reaction.

Scheme 47

A series of trimethyl aryl Pt(IV) complexes 205–208 (Scheme 48), which were considered as the intermediates in a Pt(II)-catalyzed CdC coupling of o-fluorobenzaldimines and dimethylzinc, were prepared and characterized, including X-ray crystallography for 208.99 The rates of the C(sp2)dC(sp3) reductive elimination reactions decreased in the order 205 > 206 > 207, which corresponded to a transition from more electron-rich to less electron-rich metallacycles. Specifically, the reaction half-lives measured at 60  C in MeCN solutions were 0.2, 0.6 and 3 h, respectively.

Scheme 48

An irreversible electrochemical oxidation of a tetramethyl Pt(IV) complex 209 supported by an a-diimine ligand was studied with a goal to explore plausible mechanisms of transformations of electron-poor Pt(IV) methyl complexes.146 It was proposed that a homolytic Pt(IV)dMe bond cleavage was involved in the observed electrochemical oxidation reaction. A cyclometallated tris-alkynyl aryl Pt(IV) complex 210 and its enantiomer supported by two enantiomeric NNC-pincer ligands were prepared and fully characterized.147 A cyclometallated alkynyl aryl dimethyl Pt(IV) complex 211 was prepared by displacing a reactive anilide ligand situated trans- to the aryl carbon with phenylacetylene.148

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

813

A series of anionic tetrakis-trifluoromethyl Pt(IV) complexes 212–214 were prepared while exploring the reactivity of the corresponding tetrakis-trifluoromethyl Pt(II) precursor, [Pt(CF3)4]2− (Scheme 49).149 The latter complex used as a tetra-n-butylammonium salt reacted with thionyl chloride, SOCl2, in acetone at −78  C to produce a dianionic species 212 resulting from a trans-oxidative addition of a SdCl bond to a Pt(II) center. The complex was isolated as a salt with bis(triphenylphosphine) iminium cation, [N(PPh3)2]2 (212), and characterized crystallographically. When warmed to room temperature, the complex lost the “SO” fragment and produced the cis-dichloroplatinate(IV) 213. The trans-isomer of the latter complex, 214, was prepared independently by oxidative addition of Cl2 to the Pt(II) precursor, [Pt(CF3)4]2−.

Scheme 49

8.12.4.2.4

Complexes with three hydrocarbyl ligands

Complexes with three hydrocarbyl ligands at a Pt(IV) center are very common thanks to their stability and structural versatility. Such complexes were prepared to develop new anticancer drugs,150,151 to develop new photoluminescent135,136,152–157 and supramolecular materials,158–162 to study mechanisms of CdN121 and CdC coupling reactions,108,110,111 as well as CdH activation at a Pt center,97 among others. 8.12.4.2.4.1 Development of new anticancer drugs A series of cationic trimethylplatinum(IV) complexes supported by 4,40 -R2-2,20 -bipyridyl ligands (R ¼ H, tBu) of a general structure 215150 containing S-coordinated ethyl- or thiazol-2-yl thioglycoside ligands R0 S-(cbh) were prepared via a ligand exchange of the parent cationic trimethylplatinum(IV) acetone complex fac-[PtMe3(Me2CO)3]+ with various thioglycosides. The products were fully characterized including X-ray diffraction for a few representative compounds isolated as tetrafluoroborate salts. The complexes have potential applications as bioactive materials. A large series of cyclometallated triarylplatinum(IV) complexes supported by CNN-pincer ligands and having a general structure 216151 were prepared using intramolecular C(sp2)dX (X ¼ Cl, Br) oxidative addition of the respective diarylplatinum(II) precursors. The compounds were tested as new antitumor drugs and showed high anticancer activity in cisplatin-resistant cancer cells. The related metallacyclic diaryl methyl Pt(IV) complexes 217–219 were prepared using oxidative addition of methyl iodide to the respective metallacyclic diarylplatinum(II) precursors.132 The pincer-ligated complex 217 showed a promising bioactivity whereas its analogs 218 and 219 were considerably less potent than 217.

8.12.4.2.4.2 Development of photoluminescent materials Cyclometallated dimethyl aryl Pt(IV) complexes 220–223 derived from 2-phenylpyridine or benzo[h]quinoline that contained different coordinated phosphines and a perrhenate ligand were synthesized by oxidative addition of MeI to the corresponding Pt(II) precursors with subsequent perrhenate for iodo ligand exchange using AgReO4 for this purpose.152 The compounds were fully characterized including X-ray diffraction analysis and were shown to exhibit an uncommon for Pt(IV) complexes phosphorescent

814

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

emission both in the solid state and in solutions at 25  C. The analogs of complexes 220–223 containing an iodo ligand instead of a perrhenate exhibited a red shift in wavelength of the emitted light and had a lower quantum yield of phosphorescence. Two dimethyl aryl Pt(IV) halides 224 and 225 (X ¼ Br, I) supported by a naphthalene-derived CNN pincer ligand and having a relatively rare six-membered metallacycle were synthesized using intramolecular C(sp2)dX bond oxidative addition (X ¼ Br, I) of the respective dimethylplatinum(II) precursors.136 Complex 225 was characterized crystallographically. Both 224 and 225 were chemically robust and did not change after heating for several hours at 110  C in toluene. The complexes exhibited both phosphorescence and fluorescence when excited with a 415 nm wavelength light. A triarylplatinum(IV) chloro complex 226 supported by a 2,6-diphenylpyridine-derived CNC-pincer ligand and a cyclometallated mesoionic aryl-substituted carbene was prepared in an effort to develop new photoluminescent materials.135 A weak photoemission of this complex and its diaryl dichloro analogs was demonstrated.

A highly efficient photoemitter, a dimethyl aryl iodo Pt(IV) complex 227 was designed by employing a perylene-based ligand.154 The complex was prepared by oxidative addition of iodomethane to the corresponding methyl aryl platinum(II) precursor. A few dihydrocarbyl Pt(IV) analogs of 227 containing one methyl ligand were also characterized.154 Another efficient photoemitter, a diaryl methyl Pt(IV) complex 228 supported by bi-2,20 -quinolyl ligand was prepared using oxidative addition of methyl iodide to its diarylplatinum(II) precursor.155

A simple preparation of highly efficient luminescent bis-cyclometallated chloro methyl Pt(IV) complexes 229–237 was reported which included the respective mono-cyclometallated methyl Pt(II) precursor complexes containing coordinated 2-arylpyridine ligands. The Pt(II) precursors were reacted with PhICl2 in the presence of iPr2EtN to form 229–237.153 These new Pt(IV) complexes were fully characterized including X-ray crystallography. Complexes 229–237 featured a photoemission with high quantum yields and long excited state lifetimes.

fac-Isomers of tris-cyclometallated triarylplatinum(IV) complexes 238–243 were reported to display long-lived luminescence at 23  C.156,157 These compounds were synthesized starting from bis-cyclometallated Pt(IV) dichloro precursors that were reacted with a suitable 2-arylpyridine or 1-arylpyrazol ligand in the presence of AgOTf. The resulting tris-cyclometallated mer-isomers were converted to the fac-isomers 238–243 under UV-light. Complexes 238–243 were shown to be chemically and photochemically robust.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

815

8.12.4.2.4.3 New structural motifs Two remarkably stable trialkyl Pt(IV) complexes 244 and 245 containing very bulky 2-adamantyl ligand (2-Ad) were prepared by reacting (cod)PtMe(2-Ad) complex with 2,20 -bipyridyl ligand and methyl iodide with a subsequent exchange of the iodo ligand with a triflate by an action of AgOTf. The complexes were used to characterize the steric and electronic properties of 2-adamantyl ligand which remained to be very uncommon in organoplatinum chemistry.163 It was shown that 2-adamantyl ligand has a very strong trans-influence resulting in very long PtdI (2.826 A˚ in 244 vs 2.763 in its phenyl analog) and PtdO (2.389 A˚ in 245) bonds situated trans- to 2-adamantyl ligand. Heating 244 at 100  C in benzene for 3 days did not show any sign of decomposition that would be detectable by NMR spectroscopy. A series of alkyl diacetyl Pt(IV) complexes 246–251 supported by fac-chelating tris(3,5-dimethylpyrazolyl)hydridoborate (Tp∗) or tris(pyrazolyl)hydridoborate (Tp) ligands were prepared by reacting a diacetylplatinum(II) precursor with K(Tp∗) or K(Tp) and subsequent oxidative addition of methyl iodide, ethyl iodide or benzyl bromide.164 The solid state structure of the products was established by means of X-ray diffraction crystallography. A reaction of an acetyl chloro Pt(II) precursor with quinoline-8-carbaldehyde resulted in an unusual diacyl 1-hydroxyalkyl Pt(IV) chloro complex 252.165 The solid state structure of 252 was found using single crystal X-ray diffraction. The complex included an intramolecular hydrogen bond between an acyl oxygen atom and the hydroxo group of the 1-hydroxyalkyl ligand.

An unusual dinuclear anionic acetonyl Pt(IV) complex 253 was produced by reaction of Zeise’s salt, K[PtCl3(C2H4)], with diacetonylmercury(II), acting as a transmetallating agent and as an oxidant.166 A subsequent reaction of 253 with [(Ph3P)2N]Cl in dichloromethane resulted in a mononuclear anionic tris-acetonyl Pt(IV) complex 254. A neutral tris-acetonyl Pt(IV) chloro complex 255 supported by phenanthroline was isolated from a reaction of 253 with phenanthroline. All compounds were fully characterized including X-ray crystallography for 253 and 255.

Cycloneophyl complexes, metallacycles derived from tert-butylbenzene and containing two metal-carbon bonds, one with an aryl carbon atom, and one with an alkyl carbon atom, attracted recently some attention when studying mechanisms of organometallic reactions.47,48 Cycloneophyl alkyl Pt(IV) complex 256 was produced by oxidative addition of 4-nitrobenzyl bromide to a respective cycloneophyl Pt(II) precursor supported by a phenanthroline ligand.167 The original planar arrangement of two phenanthroline nitrogen atoms and two Pt-bound carbon atoms in the Pt(II) precursor has changed in the course of the reaction to accommodate the neophyl’s alkyl carbon atom in an axial position. A reaction of another cycloneophylplatinum(II) complex with methyl iodide produced complex 257 with a similarly rearranged donor atoms of the corresponding Pt(II) precursor.168

816

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Complex 258, an isomer of 257 resulting from a trans-addition of methyl iodide to the same Pt(II) precursor, could be produced cleanly at −20  C in dichloromethane solutions but it isomerized quickly above 0  C to form a less soluble complex 257. A neutral trimethylplatinum(IV) complex 259 supported by weakly coordinating closo-hexabromocarborane ligand was prepared and fully characterized.169 Its solid state structure showed a fac-coordination of the carborane to the metal center through three bromine atoms. Attempted displacement of the carborane ligand by coordinating solvents (solv) to form the derived cationic fac-[PtMe3(solv)3]+ complexes did not result in any reaction when acetone was used. At the same time, a more coordinating tetrahydrofuran (THF) was able to displace the carborane to form fac-[PtMe3(THF)3]+.

A series of cyclometallated aryl dimethyl Pt(IV) complexes of the structural types 260 and 261 were prepared from cyclometallated aryl methyl Pt(II) precursors and methyl iodide.170 The enantiopure cyclometallated benzaldimines ligands were derived from methyl esters of natural aminoacids. A number of these complexes were characterized crystallographically. It was found that the chiral aminoacid-derived benzaldimines ligands underwent an epimerization under reaction conditions. In spite of this fact, two diastereomers, 260 and 261, were produced with a high degree of stereoselectivity when starting from the corresponding enantiopure aryl methyl Pt(II) precursors and methyl iodide.

A cationic trimethylplatinum(IV) complex 262 supported by three N,O-heterocyclic carbene ligands, 3-methyloxazol-2-ylidenes, was prepared from fac-[PtMe3(acetone)3](BF4) and the free carbene.171 The product was thermally stable as a solid and decomposed only above 218  C. By contrast, the reaction of fac-[PtMe3(acetone)3](BF4) and N,N-heterocyclic carbene 1,3-dimethylimidazol2-ylidene resulted in a rapid reductive elimination of ethane and formation of a Pt(II) byproduct. It was proposed that the reason behind the fast ethane elimination during the attempted synthesis of an N,N-heterocyclic carbene trimethylplatinum(IV) complex was the stronger trans-influence of the N,N-heterocyclic carbene which facilitated the formation of 5-coordinate Pt(IV) transients needed for a fast ethane elimination of a PtMe3 fragment. The trans-influence of the N,O-heterocyclic carbene was lower, so, the stability of the derived reaction intermediates and the final trimethylplatinum(IV) complex 262 was higher. Reaction of methyl iodide or allyl chloride with an exotic 21-platina(II)-23-telluraporphyrin containing a platinacyclopentadiene unit in the porphyrin skeleton in place of one pyrrole ring resulted in complexes 263 and 264, respectively.172 The product 263 was characterized by single crystal X-ray diffraction. The solid state structure showed a strongly distorted equatorial plane around the metal center with a relatively long PtdTe bond (2.72–2.74 A˚ ) and a significant deformation of the whole porphyrin skeleton. According to 1H NMR spectroscopy, complex 263 is fluxional with several isomers involved in a fast equilibrium at room temperature. In particular, the tellurium atom can be positioned above or below the average plane of the porphyrin macrocycle, cisor trans- to the methyl ligand. In turn, only one diastereomer was predominant in solutions of 264.

8.12.4.2.4.4 Preparation of trihydrocarbylplatinum(IV) complexes Oxidation with O2 of dimethyl- or diphenylplatinum(II) complexes supported by dimethyldi(2-pyridyl)borate ligand in ethanol or in aqueous solutions resulted in the oxidation of Pt(II) to Pt(IV), migration of a methyl group from a boron atom to a Pt(IV) center

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

817

and the formation of trihydrocarbyl complexes 265 and 266, respectively, in high yield (Scheme 50).127 A similar clean oxidatively induced migration of a phenyl group from a boron atom to a Pt(IV) center was observed when a dimethylplatinum(II) complex supported by diphenyldi(2-pyridyl)borate ligand was involved in the aerobic oxidation reaction in isopropanol solutions to form a dimethyl phenyl Pt(IV) complex 267.173 Finally, a reaction with methanol of a 5-coordinate diphenyl methyl Pt(IV) complex 268 supported with dimethyldi(2-pyridyl)borate ligand in methanol solutions leads to the formation of benzene and a methyl group migration from the boron atom to the Pt(IV) center to produce phenyl dimethyl Pt(IV) complex 269.127 All the products were fully characterized, including X-ray crystallography.

Scheme 50

The methyl group migration between a boron atom and a Pt(IV) center in 5-coordinate Pt(IV) dimethyldi(2-pyridyl)borato complexes was found to be reversible and accelerated by aprotic Lewis basic additives or solvents such as DMSO.173 In particular, when one of the methyl groups of the PtMe3 fragment in complex 190 was 13C-labeled, the label could be found on the boron-bound methyl groups after heating solutions of 190 in DMSO at 60  C for a few days. No decomposition of 190 was observed in these experiments. A similar oxidatively induced methyl group migration from a silicon atom in dimethyldi(2-pyridyl)silane ligand (Me2SiPy2) to a coordinated platinum center was reported later.174 Oxidation of (Me2SiPy2)PtMe2 with O2 in methanol solutions resulted in the formation of a trimethylplatinum(IV) complex 270. An analog of the fac-chelating dipyridine ligands that supported Pt(IV) center in complexes 265–270 was derived from di(2-pyridyl)ketone (dpk) whose carbonyl group reacted with various protic nucleophiles by adding them across the C]O bond. In particular, oxidation of a dimethylplatinum(II) complex (dpk)PtMe2 with MeOTf in dichloromethane solutions with a subsequent alkaline aqueous work up resulted in the formation of a cationic trimethylplatinum(IV) complex 271.175

A facile intramolecular oxidative addition of an ArdS bond to a dimethylplatinum(II) fragment was reported to produce a mixture of stereoisomeric dinuclear Pt(IV) complexes having one aryl and two methyl ligands at each metal center (Scheme 51).101 A representative isomer 272 is shown in Scheme 51. The remaining three diastereoisomers included other possible combinations of mutual cis- or trans-arrangements of the aryl ligands and the nitrogen donor atoms.

Scheme 51

818

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.4.2.4.5 Reactivity of trihydrocarbylplatinum(IV) complexes Trimethyl sulfonylamido Pt(IV) complexes 273–275 supported by a cis-chelating 1,2-bis(diphenylphosphino)benzene ligand were used to explore a C(sp3)dN reductive elimination at a Pt(IV) center (Scheme 52).121 These complexes underwent a competitive CdC and CdN elimination when heated at 100  C in benzene solutions. The formation of ethane was suppressed by additives of ionic sulfonamides to the reaction solutions. The proposed reaction mechanism involves a slow dissociation of a sulfonamide anion to form a 5-coordinate Pt(IV) intermediate with a subsequent fast nucleophilic attack of the anion at the apical methyl group.

Scheme 52

A reductive elimination of a quinone methide and a bromide ligand from a trialkylplatinum(IV) complex 276 occurred when the complex was reacted with a desilylating agent, nBu4NF, in acetone solutions (Scheme 53).176 An unstable zwitterionic trialkylplatinum(IV) intermediate 277 was characterized by NMR spectroscopy at −30  C. The quinone methide product was trapped with methanol to form a methyl aryl ether.

Scheme 53

Cationic trimethyl Pt(IV) complexes of a general formula 278 supported by cis-chelating 1,2-bis(diphenylphosphino)ethane and 1,2-bis(diphenylphosphino)benzene ligands were used to explore a C(sp3)dC(sp3) reductive elimination at a Pt(IV) center.108 Similar to C(sp3)dN reductive elimination in Scheme 52, it was proposed that the reaction is a two-step process. In the first slow step a pyridine ligand trans- to the axial methyl group dissociated to produce a 5-coordinate Pt(IV) intermediate which then rapidly eliminated ethane.

A triply cyclometallated triarylplatinum(IV) complex 279 was produced by reacting PhICl2 with a bis-metallacyclic Pt(II) complex derived from a 2,6-diarylpyridine and a coordinated tribenzylphosphine ligand. Complex 279 was transformed to a reactive 5-coordinate Pt(IV) species 280 by the action of AgBF4 in acetone solution (Scheme 54).177 The intermediate 280 underwent CdC reductive elimination to produce two isomeric CdC coupled products, A and B, both featuring highly strained 9-memebered metallacyclic fragments. The high degree of ring strain in A and B made these CdC coupling steps reversible. As a result, A and B could interconvert already at 23  C. The ratio of A and B changed over time until an equilibrium was reached after about 180 h at 23  C.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

819

Scheme 54

Metal-catalyzed cross-coupling reactions resulting in products with new CdC bonds constitute one of the key tools of synthetic organic chemistry. Controlling the selectivity of such reactions involving di- and trihydrocarbyl metal intermediates is an important problem. A dimethyl aryl complex 281 underwent a highly selective C(sp3)dC(sp3) coupling upon its reaction with AgBF4 in dichloromethane solutions and heating at 50  C.178 By contrast, two other dimethyl aryl complexes, 282, featuring a more flexible tridentate chelating ligand, as well as complex 283 having a more ridging chelate structure, demonstrated a clean C(sp2)dC(sp3) reductive elimination reactivity under the same reaction conditions. It was concluded that the CdC coupling selectivity was governed by the specific geometry of supporting chelating ligands. A detailed computational analysis of CdC reductive elimination at a Pt(IV) center in a series of aryl alkyl Pt(IV) complexes was carried out.26 A large series of dimethyl alkyl and dimethyl phenyl Pt(IV)–Mn(I) heterodinuclear compounds of a general structure 284 were prepared by exchanging with NaMn(CO)5 a nitrato ligand present in (4,40 -R2-bpy)PtMe2(R0 )(NO3) complexes.179 Some of the complexes were characterized crystallographically. The complexes underwent a Me-Mn(CO)5 reductive elimination from a Pt(IV) center at 20–70  C in benzene solutions. Remarkably, the rate of the elimination reaction increased more than 10-fold when the reaction was run under visible light. The estimated quantum yield of the photochemical reaction was 0.50.

Two dinuclear allyl diphenyl platinum(IV) complexes of a general structure 285 were produced when their dinuclear diphenylplatinum(II) precursors reacted with N-chlorosuccinimide.180 Both complexes were characterized crystallographically. The proposed reaction mechanism included the formation of Pt(III)–Pt(III) species that were involved in the dearomatization of the benzene residue present in the ligands.

820

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

8.12.4.2.5

Complexes with two hydrocarbyl ligands

Organoplatinum(IV) complexes with two hydrocarbyl ligands are about as common and structurally versatile as trihydrocarbyl Pt(IV) derivatives. Similar to the latter, the newly reported dihydrocarbyl Pt(IV) complexes were prepared to design new anticancer drugs,132,133 photoluminescent137,181 and supramolecular182 materials, to develop new organometallic reactions102,103,105 and study their mechanisms,114–116,119,183–185 among others. As a result, a substantial progress was achieved at understanding the suitable pathways for and the mechanisms of transformation of Pt(IV)dH complexes and O2 to Pt(IV)-OOH derivatives,183 preparation of Pt(IV) aryl complexes from arenes, O2 and Pt(II) precursors,107 reductive elimination of Pt(IV) dihydrocarbyls to form C(sp2)dI bonds,116,185 C(sp2)dBr bonds,115 C(sp3)dCl bonds,113,114 C(sp3)dO bonds,117,119 C(sp3)dN bonds119 and C(sp3)dP bonds.186

8.12.4.2.5.1 Development of new anticancer drugs Aryl methyl Pt(IV) complexes 286–288 were prepared by oxidative addition of methyl iodide to arylplatinum(II) precursors supported by a CNN-pincer ligand or by oxidative addition of elemental iodine to their aryl methyl platinum(II) analogs.133 All complexes exhibited a remarkably high cytotoxicity and the methyl complexes 286–287 featuring an axial methyl ligand were the most potent. Cyclometallated diarylplatinum(IV) complexes 288–291 were prepared by oxidative addition of elemental iodine to the corresponding metallacyclic diarylplatinum(II) precursors.132 Complex 288 was shown to suppress growth of colon cancer cells. The low solubility of 290 and 291 prevented their study as antitumor agents.

8.12.4.2.5.2 Development of photoluminescent materials A large series of bis-cyclometallated cationic Pt(IV) complexes of a general structure 292 were prepared by oxidation with PhICl2 of their mono-cyclometallated precursors with a subsequent coordination of (substituted) 2,20 -bipyridyl ligands.181 The second metallacycle was produced during the oxidation step. The complexes were fully characterized and exhibited blue emission, long excited state lifetimes albeit with low quantum yields reaching 1%.

Three cyclometallated aryl methyl Pt(IV) complexes 293–295 were produced by oxidation of a metallacyclic aryl methyl Pt(II) precursor with PhICl2, Br2 or I2, respectively.137 The compounds were fully characterized including X-ray crystal structure analysis for 294. The chloro and bromo complexes, 293 and 294, were emissive when excited with a UV light. The iodo complex 295 was not photoluminescent.

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

821

8.12.4.2.5.3 New structural motifs A bis-cyclometallated Pt(IV)-bridged cofacial Ni(II) diporphyrin complex 296 was prepared by reacting (nBu4N)2[PtCl6] with 2-pyridyl-substituted Ni(II) porphyrin precursor. Complex 296 was fully characterized.187 According to X-ray diffraction analysis, two porphyrin macrocycles have a ruffled geometry. Their mean planes are almost parallel with a dihedral angle of 4 and the Ni-Ni distance of 5.15 A˚ . The compound is chiral and the racemic mixture produced in the reaction was separated into enantiomers using HPLC with a chiral sorbent. Upon changing the metallating reagent to K2[PtCl4] and switching to a N,N-dimethylformamide (DMF) - containing solvent, the same 2-pyridyl-substituted Ni(II) porphyrin precursor produced a stable bis-metallacyclic aryl alkyl Pt(IV) complex 297 containing a cyclometallated DMF molecule.188 The complex was isolated in 8% yield. The solid state structure of 297 showed a highly distorted porphyrin macrocycle. The metallated DMF fragment was out of the plane of the porphyrin ring.

A dimethyl Pt(IV) complex 298 with a chloromercuryl ligand was produced by reacting a dimethylplatinum(II) precursor and HgCl2.189 The complex was characterized crystallographically. Two isomeric diarylplatinum(IV) dichloro complexes, 299 and 300, containing two 3,4,5-trimethoxy-2,6-dinitrophenyl ligands were produced by the oxidation of a corresponding diarylplatinum(II) precursor with PhICl2 or Cl2, respectively.190 Both complexes featured a bidentate coordination of the nitroaryl ligands to the metal with an oxygen atom of one of their nitro groups involved in metal bonding.

8.12.4.2.5.4 Preparation of dihydrocarbylplatinum(IV) complexes A bis-cyclometallated aryl alkyl Pt(IV) complex 301 containing a fragment of cycloplatinated tri-n-propylphosphine was produced by oxidation with PhICl2 of a metallacyclic Pt(II) precursor.191 The second cyclometallation involving the phosphine alkyl group was proposed to result from CdH activation of its methyl group by a 5-coordinate Pt(IV) transient that was produced upon reaction of the metallacyclic Pt(II) precursor with PhICl2. Complex 301 was characterized crystallographically. A peroxo-bridged dinuclear Pt(IV) complex 302 resulted from photolysis of a bis-p-tolyl allyl Pt(IV) precursor under air.192 The reaction was proposed to include a photo-induced homolysis of a Pt(IV)-allyl ligand bond with a subsequent bonding of the resulting Pt(III) transient species with O2

A dimethylplatinum(IV) hydroxo methoxo complex 303 supported by phenanthroline ligand resulted from a reaction of a dimethylplatinum(II) complex supported by N,O-coordinated phenanthroline N-oxide and methanol (Scheme 55).102 Phenanthroline N-oxide acted as an oxidant in this reaction and complex 303 formed when the reaction was run under inert atmosphere. If the reaction was run under air, a different Pt(IV) complex formed with the ligand NdO bond remaining intact and the ligand N, O-coordination mode retained.

822

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 55

A metallacyclic aryl phenyl hydroxo complex 304 was produced as a result of benzene CdH bond activation by a Pt(II) aqua complex supported by a sulfonated CNN-pincer ligand and a subsequent oxidation with O2 of the resulting phenylplatinum(II) intermediate (Scheme 56).105 The reaction was carried out in 2,2,2-trifluoroethanol (TFE) solution at 23  C. The product was fully characterized including X-ray crystallography. A number of electron-rich and electron-poor arenes, o- and m-xylenes, o-dichlorobenzene and o-difluorobenzene, were also involved in a similar transformation.

Scheme 56

A dimethylplatinum(IV) complex 305 incorporating an unusual “self-assembled” fac-chelating N,N,O-donor ligand was produced in a reaction with O2 of a dimethylplatinum(II) precursor supported by a bidentate pyridaldimine with an appended o-hydroxyphenyl group (Scheme 57).107 The product was characterized crystallographically. The reaction was proposed to involve a phenol group-assisted oxidation of Pt(II) to Pt(IV) with a subsequent condensation of the resulting coordinated pyridaldimine and acetone.

Scheme 57

Oxidation of a dimethylplatinum(II) complex supported by 4,40 -di-tert-butyl-2,20 -bipyridyl with XeF2 led to either of two different products, 306 or 307, depending on the solvent used for the reaction, dichloromethane or MeCN, respectively (Scheme 58).193 Both compounds were characterized crystallographically. It was proposed that the reaction produced initially a cationic Pt(IV) fluoro solvento complex, along with xenon and a fluoride anion. The latter reacted with dichloromethane to produce a chloride anion which then coordinated to the cationic Pt(IV) center. In the case of MeCN solvent the Pt(IV)-coordinated nitrile ligand underwent addition of adventitious water molecule and deprotonation with fluoride anion to produce an acetamido ligand.

Scheme 58

A series of chemically robust methyl alkynyl hydrido Pt(IV) complexes (Tp )PtMe(C^CR)H, 308, (R ¼ Me, tBu, n-Pr, CH2Ph) were produced by oxidative addition of alkyne CdH bonds to a Pt(II) center upon heating the respective Pt(II) precursor Z2-alkyne complexes at 40  C in dichloromethane solutions.97 The analogous stable diacetyl hydrido Pt(IV) complexes 309 and 310 supported by Tp and Tp ligands, respectively, resulted from a reaction of K(Tp) or K(Tp ) and a dimeric Pt(II) acetyl chloro complex supported by 1-hydroxymethylcarbene, {Pt(Ac)[C(OH)Me]}2(m-Cl)2, when the reaction was run in THF solutions.164 In chloroform solutions the Pt(IV) hydrido complexes exchanged their hydride ligand for chloride. The Tp-supported complex 309, as well its chloro derivative 311 were characterized by

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

823

single crystal X-ray diffraction analysis. In contrast to complexes 309 and 310, their cationic analogs, tris-pyrazolylmethane complexes 312, were thermally labile and could only be characterized by low-temperature NMR spectroscopy at −80  C.194

A relatively stable dimethyl hydrido Pt(IV) complex 313 supported by di(2-pyridyl)ketone (dpk) was prepared by reacting HCl with its dimethylplatinum(II) precursor, (dpk)PtMe2, dissolved in dichloromethane.195 Complex 313 was isolated in a pure form at −30  C and characterized by single crystal X-ray diffraction. At 0  C a slow partial isomerization of 313 occurred to form an isomer having its hydride ligand trans- to a pyridyl fragment. At 20  C a mixture of both isomers reductively eliminated methane to form (dpk)PtMe(Cl).

8.12.4.2.5.5 Reactivity of dihydrocarbylplatinum(IV) complexes Dimethyl- and diphenyl Pt(IV) hydrido complexes, (Tp )PtMe2(H) and (Tp )PtPh2H, reacted with O2 at 50  C in the presence of a radical reactions initiator, 2,20 -azobis(isobutyronitrile), to form the corresponding dihydrocarbyl Pt(IV) hydroperoxides, 314 and 315, respectively.183 It was concluded that the reaction operated a chain radical mechanism.

A trans-diiodo diaryl Pt(IV) complex 316 supported by 1,2-bis(dimethylphosphino)ethane eliminated mixtures of the derived aryl iodide and the biaryl upon heating at 60  C in benzene or DMF solutions, both in the dark (slower) and under light (faster) (Scheme 59).116,185 The cis-isomer of 316, complex 317, in turn, underwent an exclusive and much slower biaryl reductive elimination under thermal conditions. The isomerization but not the C(sp2)dI reductive elimination reaction was inhibited in the presence of (nBu4N)I. Accordingly, the selectivity of aryl iodide formation from 316 increased in the presence of (nBu4N)I. The reaction selectivity in aryl iodide also increased as a result of the use of a more rigid analog of 1,2-bis(dimethylphosphino)ethane ligand, 1,2-bis(dimethylphosphino)benzene.116 The derived trans-diiodo diaryl Pt(IV) complex 318 did not isomerize and

Scheme 59

824

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

produced predominantly the aryl iodide upon heating at 60  C. Complexes 316–318 were characterized crystallographically. According to available observations and a computational (DFT) analysis, the isomerization reactions required the presence of an open coordination site at the metal. The formation of 5-coordinate Pt(IV) intermediates, in turn, resulted from dissociation of iodide ligand that could be suppresses using additives of (nBu4N)I, or from a thermally or photochemically induced chelate ring opening. In turn, the concerted C(sp2)dI bond reductive elimination did not require the formation of 5-coordinate Pt(IV) species and proceeded through an ion-pair intermediate and/or ion-pair-like transition state and, hence, its rate was not affected by(nBu4N) I additives. A trans-dibromo pentafluorophenyl p-fluorophenyl Pt(IV) complex 319 supported by an electron-poor rigid (R,R)-2,3-bis(tertbutylmethylphosphino)quinoxaline ligand was prepared by oxidative addition of Br2 to the respective diarylplatinum(II) precursor (Scheme 60).115 Heating complex 319 in MeCN solutions at 80  C produced p-bromofluorobenzene in 80–90% yield, along with a pentafluorophenyl Pt(II) byproduct. Complex 319 was characterized crystallographically. The reaction was only slightly affected by light. The proposed reaction mechanism included dissociation of a bromide anion and a subsequent concerted C(sp2)dBr reductive elimination from the resulting 5-coordinate Pt(IV) intermediate.

Scheme 60

Chlorination with PhICl2 of a bis-metallacyclic aryl alkyl Pt(II) DMSO complex at −40  C produced two derived dihydrocarbyl Pt(IV) dichlorides, 320 and 321 (Scheme 61).114 The trans-isomer 320 was chemically robust and was isolated and fully characterized. By contrast, the cis-isomer 321 was unstable and reacted above −40  C in chloroform solutions to isomerize to 320 and, concurrently, eliminate a C(sp3)dCl bond. The rate of the latter reaction was not dependent on the concentration of added (nBu4N)Cl. Complex 322, a triphenylphosphine analog of complex 321, underwent a similar C(sp3)dCl coupling and reacted at a faster rate than 321 (Scheme 61).113 The Pt(II) complex resulting from the CdCl bond elimination was isolated in 48% yield and characterized spectroscopically. Based on the existing literature examples of C(sp3)dX bond reductive elimination at a Pt(IV) center (X ¼ I, O, N) and the associated reactions rate laws,121 one can propose that the C(sp3)dCl reductive elimination in Scheme 61 operates a similar two-step mechanism involving (i) a chloride dissociation from a reactive 6-coordinate Pt(IV) complex to form a 5-coordinate Pt(IV) transient and (ii) a rate-determining SN2-type nucleophilic substitution at the axial methyl ligand of the 5-coordinate species. This reaction sequence resulted in an overall zero order in added chloride anion concentration.

Scheme 61

Two metallacyclic aryl methyl Pt(IV) diacetato complexes 323 and 324 were prepared by oxidation with PhI(OAc)2 of their metallacyclic Pt(II) precursors (Scheme 62).117 Complex 324 was characterized crystallographically. Upon heating at 60  C in various organic solvents, benzene, acetone, chloroform and MeCN, reductive elimination of a C(sp3)dO bond from both complexes occurred that led to the formation of methyl acetate in 16% (acetone)–88% (MeCN) yield, depending on the substrate and the solvent. The rates of both reactions were first order in [323] or [324] and zero order in concentration of (nBu4N)OAc additives. The benzo[h]quinone derivative 324 was a slightly more reactive. Based on the results of the kinetics study and a computational modeling (DFT) of the reaction, it was proposed that the reaction mechanism includes three key steps, (i) dissociation of an acetate ligand to form a 5-coordinate Pt(IV) transient, (ii) isomerization of the 5-coordinate Pt(IV) intermediate that places the methyl ligand in an axial position and (iii) a nucleophilic abstraction of the axial methyl ligand with the acetate anion to form the C(sp3)dO coupled product (SN2-type-like reaction).

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

825

Scheme 62

The C(sp3)dX reductive elimination reactivity (X ¼ N, O, S) of metallacyclic methyl Pt(IV) complexes 325–327 supported by a sulfonated CNN-pincer ligand was studied in 2,2,2-trifluoroethanol (TFE) and DMSO solutions (Scheme 63).119 When PhNMe2 was used as a nucleophile, a quantitative C(sp3)dN coupling to form PhNMe+3 salts was observed in both solvents at 22  C. The kinetics study of this reaction showed that the reaction was 1st order in each [PhNMe2] and a metal complex concentration and the reactivity decreased from chloro (325) to iodo (326) and to trifluoroethoxo (327) complex. Based on the results of the kinetics study and a computational (DFT) analysis, an SN2 mechanism for the C(sp3)dN coupling reaction was proposed. The use of aqueous DMSO acidified with CF3CO2H resulted in MeOH elimination from 326 in 87% yield after 3 h at 80  C, with Me3SO+ (10%) being the major organic byproduct. In turn, the use of NaO2CCF3 additives in wet DMSO solutions allowed to raise the yield of MeO2CCF3 to 80% with MeOH being the main byproduct (15%) after 1.5 h at 80  C.

Scheme 63

A moderately stable cationic dimethylplatinum(IV) PPh2Me complex 329 supported by a N,N0 -bis-(p-tolyl)aminotroponiminate ligand was prepared by displacing with PPh2Me a triflato ligand in a parent dimethylplatinum(IV) triflato complex 328 (Scheme 64).186 Complex 329 underwent a slow C(sp3)dP elimination in solutions at 23  C to form 0.5 equivalent of Ph2PMe+2, 0.5 equivalent of 328 and a 0.5 equivalent Pt(II) byproduct. Addition of 2 equivalents of an even bulkier phosphine, PPh3, to 328 resulted in a clean formation of MePPh+3. A two-step mechanism was proposed for the C(sp3)dP coupling reaction: (i) formation of a 5-coordinate Pt(IV) intermediate and free PPh2Me and (ii) an SN2 attack of the liberated phosphine at the axial methyl group of the Pt(IV) intermediate.

Scheme 64

A series of cyclometallated diaryl Pt(IV) difluoride complexes 330 derived from 1-(bis(tert-butyl)phosphino)2-methylnaphthalene were prepared by low temperature fluorination with XeF2 of their diarylplatinum(II) precursors (Scheme 65).184 Some of the Pt(IV) difluorides were characterized crystallographically. A gentle heating of 330 at 40–60  C in MeCN or dichloromethane solutions resulted in a C(sp2)dC(sp2) reductive elimination reaction with an overall 1,3-migration of an axial aryl ligand to a naphthalene carbon atom and the formation of a second metallacycle. A computational (DFT) analysis suggested that the initial CdC coupling product is highly strained and undergoes a rate-determining 1,2-migration to form a transient Pt(II) carbene complex. The rate-determining step was followed by HF elimination/aromatization of the carbene and the formation of the second metallacycle.

826

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 65

8.12.4.2.6

Complexes with one hydrocarbyl ligand

Complexes with one hydrocarbyl ligand at a Pt(IV) center are more rare, as compared to their di- and trihydrocarbyl analogs. The monohydrocarbyl Pt(IV) complexes are, in general, also more reactive. Investigation of the pathways leading to their formation, such as the use of O2 as an oxidant for the generation of monohydrocarbyl Pt(IV) species from their Pt(II) precursors,103,104,196–198 and their reactivity continued to remain in the focus of current research. Most of the works done with monohydrocarbyl Pt(IV) complexes were centered on the study of various bond-making and bond-breaking processes at a Pt(IV) center, such as CdH activation/cyclometallation,95 reductive elimination of C(sp3)dO bonds,103,104,118,197,199 C(sp3)dCl bonds,113 C(sp2)dF bonds,200 C(sp2)dN bonds,120 photo-induced transformations of Pt(IV)dO201,202 and Pt(IV)dBr203 bonds, among other research directions.

8.12.4.2.6.1 Development of new anticancer drugs Metallacyclic monoaryl Pt(IV) trihalides 331 and 332 were prepared by oxidative addition of I2 to their respective cyclometallated Pt(II) precursors. Complex 331 was characterized crystallographically.133 Both complexes exhibited remarkable cytotoxicity. A cyclometallated monoaryl Pt(IV) trichloro complex 333 was prepared by reacting H2PtCl6 with 6,60 -dimethyl-2,20 -bipyridyl in a mixture of DMSO, DMF and water at 60  C.204 The solid state structure of 333 was solved using X-ray crystallography. The Pt(IV) compound demonstrated an antitumor activity exceed that of cisplatin.

8.12.4.2.6.2 Preparation of monohydrocarbylplatinum(IV) complexes Considering a versatile reactivity of monohydrocarbyl Pt(IV) complexes, and, in particular, their promise and a historical role in alkane functionalization catalysis,205 assuming also that the standard methods for the preparation of Pt(IV) compounds include oxidation of their Pt(II) precursors, it would be desirable to develop routes to Pt(IV) monohydrocarbyl complexes relying on the use of O2 as oxidant. A series of monohydrocarbyl Pt(II) complexes were converted to their hydroxo Pt(IV) derivatives of structural types 334,103,196,198 335,197 336,206 337,104 338196 using O2 as oxidant in protic media (Scheme 66). The oxidation was enabled by a tripod fac-chelating di(2-pyridine)methanesulfonate ligand (dpms) and did not work for analogous dipyridine complexes devoid the sulfonate group. A number of the complexes 334–338 were characterized crystallographically. Most of the reactions are high-yielding, highly selective and fast when run in water of MeOH solutions at 23  C. Methyl Pt(II) complexes reacted at about 20–40 times faster rate than their phenyl analogs under otherwise identical conditions. Amine-supported complexes reacted at a much slower rate, as compared to analogous aqua complexes.196 For the parent Pt(II) complex, (dpms)PtMe(H2O), a dependence of the reaction rate on solution pH was studied.198 The reaction was the fastest and 100% selective at pH 8.0. At pH 10 and higher formation of a C1-symmetric dimethyl hydroxo Pt(IV) complex, (dpms)PtMe2(OH), was noticeable; this product was predominant at pH > 12. A detailed analysis of a plausible reaction mechanism was presented that was supported computationally.207

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

827

Scheme 66

A similar ligand, bis(3,5-dimethylpyrazolyl)acetate, dpza, also enabled a similar aerobic transformation of a methyl Pt(II) aqua complex (dpza)PtMe(H2O) to a monomethyl Pt(IV) dihydroxo derivative 339, (dpza)PtMe(OH)2, that was produced in 89% yield under 8 atm pressure of O2 after 2 weeks at 22  C (Scheme 67).208

Scheme 67

8.12.4.2.6.3 Reactivity of monohydrocarbylplatinum(IV) complexes Monoalkyl Pt(IV) complexes 334–337 prepared using dpms ligand-enabled oxidation with O2 of their Pt(II) precursors (Scheme 66) reacted at elevated temperature in the presence of aqueous HBF4 to form the corresponding C(sp3)dO coupled products (Scheme 68). In particular, complex 334 (R ¼ Me) in dilute acidic aqueous solutions eliminated MeOH at 70  C in >97% yield, the balance being Me2O.103,118 At high concentrations of the complex formation of Me2O could reach 20%.118 The reaction was explored mechanistically in detail.118 The key reaction intermediate, a Cs-symmetric isomer (dpms)PtMe(OH)2, 340, was produced by thermal isomerization of 334 (R ¼ Me) in neutral solutions. Complex 340 eliminated MeOH in acidic aqueous solutions already at 23  C. Based on the results of 18O-labeling experiments and detailed kinetics studies, it was proposed that the C(sp3)dO coupling reaction operates an SN2 mechanism where a protonated complex 340 accepts a nucleophilic attack by either a water molecule, a hydroxo ligand of another species of 340 or a methoxo ligand of a Pt(IV) methoxo complex resulting from the previous reaction. Consistent with this mechanism, the phenyl analog, 334 (R ¼ Ph), when exposed to 100  C in acidic aqueous solutions isomerized to form a Cs-symmetric complex, a phenyl analog of 340, but did not eliminate phenol.196 A subsequent detailed computational (DFT) analysis of methanol elimination from complex 340 was carried out.207

828

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

Scheme 68

When heated at 80  C in aqueous solutions, complex 335 eliminated mixtures of ethylene oxide and ethylene glycol,197 whereas complexes 337 eliminated the corresponding olefin oxides104 when heated in DMSO solutions or in the solid state in high yields. Finally, complex 336 only reacted in acidic aqueous solutions at 100  C and formed the corresponding ethanolamine in a high yield.206 Two metallacyclic mer-trichloro aryl Pt(IV) complexes 341 and 342 supported by triphenylphosphine ligand were prepared by oxidation of their metallacyclic Pt(II) precursors with PhICl2 in chloroform solutions at 23  C (Scheme 69).113 Compounds 341 and 342 were isolated and fully characterized. A slow C(sp2)dCl elimination and a competitive isomerization to form fac-trichloro Pt(IV) isomers were observed for both 341 and 342 in chloroform solutions at 23  C. The reactions took about 4 weeks at 23  C. The C(sp2)dCl bond elimination products were isolated and fully characterized. The reaction was proposed to operate a concerted C(sp2)dCl reductive elimination at a Pt(IV) center.

Scheme 69

A cis-difluoro mesityl Pt(IV) pyridine complex 343 supported by a bulky P,O-chelating 2-[bis(1-adamantyl)phosphine]phenoxide ligand was prepared by fluorination with XeF2 in dichloromethane–MeCN mixtures at −30  C of the respective arylplatinum(II) precursor (Scheme 70).200 Complex 343 was fully characterized including single crystal X-ray crystallography. Heating complex 343 in toluene solution at 60–100  C resulted in a C(sp2)dF bond elimination from the complex and the formation of mesityl fluoride in a quantitative yield, along with a Pt(II) fluoro complex. The reaction followed a clean first order

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

829

Scheme 70

kinetic and was not inhibited by additives of pyridine. According to a computational (DFT) analysis of the reaction mechanism, the reaction involved a rate-determining dissociation of the pyridine ligand with a subsequent fast C(sp2)dF coupling. The steric interference between the ligands in the metal coordination sphere of 343 appeared to be the key factor that allowed to outcompete a potential ArdO coupling reaction. Analogs of 343 having less bulky aryl ligands at a Pt(IV) center, such as 4-C6H4F or 3,5-C6H3F2, failed to eliminate products with a C(sp2)dF bond. The use of the same ligand platform, a bulky P,O-chelating 2-[bis(1-adamantyl)phosphine]phenoxide ligand, in a combination with aryl Pt(IV) fluoride fragment and a ligand that can serve as a source of nucleophile X, led to the development of a series of Pt(IV) complexes 344–346 that demonstrated facile C(sp2)dX coupling at a Pt(IV) center with different heteroatoms X (X ¼ Cl, O, N) (Scheme 71).120 The complexes were fully characterized including single crystal X-ray crystallography. In the case of Pt(IV) amine complexes of a general structure 346, the outcome of a C(sp2)dN coupling reaction depended on the presence of an external base, such as Et3N. In the presence of 1 equivalent of the base, a high-yielding formation of N-aryl-N-alkylamines was observed. In the absence of a Et3N additive, a second fast C(sp2)dN coupling reaction occurred to produce the corresponding N,N-diarylN-alkylamines. Both electron-rich 4-methoxyphenyl and electron-poor 3,5-difluorophenyl complexes also reacted by producing C(sp2)dN bonds. The mechanism of the C(sp2)dN coupling reaction was analyzed using means of kinetics and computational modeling (DFT). A concerted C(sp2)dN bond formation mechanism was proposed that involved a 5-coordinate Pt(IV) transient resulting from an axial fluoride ligand dissociation. Remarkably, in the external base-free systems the second CdN coupling occurred at a faster rate than the first CdN coupling so resulting in N,N-diaryl-N-alkylamines. The second CdN coupling was preceded by coordination of a singly CdN coupled product to a Pt(IV) center and a displacement of the initially coordinated primary alkylamine R0 NH2 in 346.

Scheme 71

830

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

A series of aryl tribromo Pt(IV) complexes of a general structure 347 reacted under a 500 W halogen lamp light within a few minutes to eliminate molecular bromine in a high, up to 82%, quantum yield to form the derived aryl Pt(II) bromo complexes.203 In the absence of added bromine traps, the released reactive bromine species were involved in bromination of the aryl ligands in complexes 347. In the presence of bromine traps, such as cis-2-hexene, the corresponding dibromoalkanes were produced in a high yield. The reaction with cis-2-hexene proceeded as a trans-addition. A 4-trifluorophenyl Pt(IV) chloro complex 348 having a hydroxo and a hydroperoxo ligands reacted under 380 nm UV-light at −78  C to produce dihydrogen trioxide, HOOOH, in 16–20% yield, along with the corresponding aryl Pt(II) chloro complex.201 The formation of dihydrogen trioxide was confirmed by means of 1H NMR spectroscopy. When carried out in the presence of 2,5-dimethylfuran, a singlet oxygen traps, a room temperature photolysis of 348 resulted in the formation of the corresponding 2,5-dimethylfuran-derived peroxide, so suggesting the formation in the photolysis reaction of singlet oxygen as another product arising from 348.

A series of 4-trifluoromethylphenyl Pt(IV) trans-bis(triethylphosphine) complexes 349 having a hydroxo and a carboxyloxo ligands (CH3CO2, CF3CO2, 2-BrC6H4CO2) in a cis-position with respect to each other reacted under 380 nm light at 23  C to produce the derived phosphaplatinacyclic compounds 350 in up to 50% yield (Scheme 72).202 The proposed reaction mechanism involved a homolytic cleavage of a Pt(IV)-OH bond to produce a OH radical that remained hydrogen-bonded to an adjacent carboxyloxo ligand. While held by a hydrogen bond in a close proximity to the methyl group of the PEt3 ligand, the OH radical abstracted a hydrogen atom from the methyl group. The resulting alkyl radical and a Pt(III) center recombined to form a Pt(IV) metallacycle. Compound 350 had low stability and was characterized in solutions by means of NMR spectroscopy.

Scheme 72

8.12.4.2.7

Organoplatinum(IV) complexes with no hydrocarbyl ligands

Organoplatinum(IV) complexes featuring no hydrocarbyl ligands but having coordinated N-heterocyclic carbenes attracted some attention as potential catalysts of organic reactions209,210 and potential anticancer drugs.134,211 Stable Pt(IV) tetrahalide (X ¼ Cl, Br) complexes 351–354 containing abnormal C4-coordinated diimidazolylidene carbene ligands were prepared to justify the expectation of a high donicity of these ligands, as compared to standard C2-coordinated imidazolylidene carbenes.212 The oxidation of the respective Pt(II) carbene precursors was carried out using Br2 of PhICl2. Complexes 351 and 352 were characterized crystallographically. Another series of stable Pt(IV) tetrahalides 355–358 (X ¼ Cl, Br) supported by chelating C2-coordinated diimidazolylidene carbenes were reported that contained one or two chelating bis-carbenes.210 The complexes containing two bis-carbene ligands were

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

831

dicationic. The compounds 356, (358)(PF6)2 and (358)(PF6)2 were characterized crystallographically. The use of a more rigid CNC-pincer bis-carbene ligand resulted in an unstable cationic Pt(IV) triiodo complex 359. The solid state structure of the derived triiodide salt (359)I3 was determined using X-ray crystallography.209 The corresponding Pt(II) precursors were used as catalysts of hydrovinylation of 2-methyl-2-butene. A large series of stable Pt(IV) tetrahalides (X ¼ Cl, Br) including representative complexes 360–363 supported by one imidazolylidene carbene ligand (R ¼ PhCH2, n-C5H11, n-C18H37, Me, cycloC5H11, p-O2NC6H4CH2, among others) and one amine ligand (L ¼ pyridine, cycloC6H11NH2, morpholine, n-BuNH2, n-C12H25NH2, among others) were prepared and their cytotoxicity properties were investigated.134,211

8.12.5

Conclusions

The impressive progress in the field of the organometallic chemistry of high-valent palladium and platinum complexes achieved in the past 16 years included a number of significant discoveries and findings, a few of which are mentioned below. In the area of high-valent organopalladium chemistry, first of all, it is the development of a wide range of oxidative functionalization reactions, such as CdH bond or olefin C]C bond functionalization in the presence of strongly oxidizing agents, such as organic and inorganic peroxides, N-halogenated compounds or hypervalent iodine compounds, catalyzed by soluble palladium compounds, and the discovery of the key role that organopalladium(IV) complexes and their organopalladium(III) counterparts can play in these transformations. The important achievements in this area of organopalladium chemistry included also the development and study of novel synthetic routes to various high-valent organopalladium complexes that employed cyclometallated palladium(II) precursors, bidentate chelating and fac-chelating ligands, the use of such oxidants as molecular oxygen and hydrogen peroxide, as well as such tools as electrosynthesis, to name a few. Still, in spite of the impressive progress, the number of well-characterized/ isolated high-valent organopalladium complexes remains low, owing to their generally high reactivity. Of the key importance for the progress achieved in the high-valent organopalladium catalysis was also the study of the reactivity in various C(spn)dC(spm) (n, m ¼ 1, 2, 3) and C(spn)-heteroatom (n ¼ 2, 3) bond-forming, as well as C(sp2)dH bond cleavage reactions of isolable model organopalladium(IV) complexes and their organopalladium(III) counterparts. Another significant development in the field of high-valent organopalladium chemistry was the discovery of efficient synthetic routes to isolable mononuclear organopalladium(III) complexes and characterization of their structure and reactivity. Some further developments in these directions may continue into the foreseeable future, especially in the areas related to the synthesis and study of still rare monohydrocarbyl high-valent palladium complexes, the design of new palladium catalysts for the selective oxidation of organic substrates, including reactions utilizing molecular oxygen as the oxidant, as well as the development of solid state materials for possible applications in molecular electronics. In the area of a much more mature high-valent organoplatinum chemistry, the most important achievements included the detailed mechanistic understanding of the fundamental organometallic reactions, such as C(spn)dC(spm) (n, m ¼ 1, 2, 3) and C(spn)-heteroatom (n ¼ 2, 3) bond-forming reactions occurring at a high-valent metal center, as well as the understanding of the role that 5-coordinate d6 metal complexes may play in these transformations. New synthetic routes for the preparation of high-valent organoplatinum complexes were also developed, including the use of such diverse reagents as molecular oxygen and xenon difluoride employed for the oxidation of platinum(II) precursors. The traditional areas of application of platinum complexes as anti-cancer drugs and photoluminescent materials continued to develop at an accelerated pace owing to the involvement of

832

Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes

high-valent organoplatinum complexes. It is expected that the progress in the area of high-valent organoplatinum chemistry may lead to the discovery of novel platinum catalysts for the selective functionalization of organic substrates, including aerobic routes employing molecular oxygen as the oxidant. The development of new organoplatinum compounds for the application as drugs and materials for electronics is another foreseeable trend in high-valent organoplatinum chemistry. The use of high-valent organoplatinum compounds as models in studies of structure and bonding in organometallic compounds, as well as the mechanisms of various organometallic reaction can be expected to retain its importance. In addition, with the emergence of the chemistry of mononuclear organopalladium(III) complexes, some attention to their organoplatinum(III) analogs may also be anticipated in the future.

Acknowledgments The author is grateful to the National Science Foundation for the continuing support of his work in the field of aerobic CdH functionalization by late transition metal complexes (CHE-1800089).

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Trivalent and Tetravalent Palladium and Platinum Organometallic Complexes 192. 193. 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211. 212.

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