Comprehensive Organometallic Chemistry IV. Volume 10: Groups 14 to 15, Frustrated Lewis Pairs [10] 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 10

GROUPS 14 AND 15, FRUSTRATED LEWIS PAIRS VOLUME EDITOR

SIMON ALDRIDGE Department of Chemistry, Oxford University, Oxford, United Kingdom

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

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

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

CONTENTS OF VOLUME 10 Editor Biographies

vii

Contributors to Volume 10

xiii

Preface 10.01

xv

Low-Valent Silicon Compounds

1

Shiori Fujimori and Shigeyoshi Inoue

10.02

Compounds With Bonds Between Silicon and d-Block Metal Atoms

52

Terrance J Hadlington

10.03

Organometallic Compounds of Germanium

92

Selvarajan Nagendran, Jyoti Shukla, Pratima Shukla, and Pritam Mahawar

10.04

Organometallic Compounds of Tin and Lead

422

Keith Izod

10.05

Organometallic Compounds of Arsenic, Antimony and Bismuth

478

Josep Cornella and Yue Pang

10.06

Frustrated Lewis Pair Systems

523

Miquel Navarro, Juan José Moreno, and Jesús Campos

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

vii

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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.

x

Editor Biographies

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

Editor Biographies

xi

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

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

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Editor Biographies

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

CONTRIBUTORS TO VOLUME 10 Jesús Campos Instituto de Investigaciones Quí micas (IIQ), Departamento de Quí mica Inorgánica and Centro de Innovación en Quí mica Avanzada (ORFEO-CINQA), Consejo Superior de Investigaciones Cientí ficas (CSIC) and University of Sevilla, Sevilla, Spain Josep Cornella Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany Shiori Fujimori Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Garching bei München, Germany Terrance J Hadlington Department of Chemistry, Technical University Munich, Munich, Germany Shigeyoshi Inoue Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Garching bei München, Germany Keith Izod School of Chemistry, Newcastle University, Newcastle upon Tyne, United Kingdom

Pritam Mahawar Department of Chemistry, IIT Delhi, New Delhi, India Juan José Moreno Instituto de Investigaciones Quí micas (IIQ), Departamento de Quí mica Inorgánica and Centro de Innovación en Quí mica Avanzada (ORFEO-CINQA), Consejo Superior de Investigaciones Cientí ficas (CSIC) and University of Sevilla, Sevilla, Spain Selvarajan Nagendran Department of Chemistry, IIT Delhi, New Delhi, India Miquel Navarro Instituto de Investigaciones Quí micas (IIQ), Departamento de Quí mica Inorgánica and Centro de Innovación en Quí mica Avanzada (ORFEO-CINQA), Consejo Superior de Investigaciones Cientí ficas (CSIC) and University of Sevilla, Sevilla, Spain Yue Pang Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany Jyoti Shukla Department of Chemistry, IIT Delhi, New Delhi, India Pratima Shukla Department of Chemistry, IIT Delhi, New Delhi, India

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

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10.01

Low-Valent Silicon Compounds

Shiori Fujimori and Shigeyoshi Inoue, Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Garching bei München, Germany © 2022 Elsevier Ltd. All rights reserved.

10.01.1 Introduction 10.01.1.1 Recent developments 10.01.1.2 Literature 10.01.2 Silylenes 10.01.2.1 Introduction 10.01.2.2 Two-coordinated silylenes 10.01.2.2.1 Cyclic silylenes 10.01.2.3 Higher coordinated silylenes 10.01.2.3.1 Silylenes stabilized by chelating ligands 10.01.2.3.2 Silylenes stabilized by NHCs 10.01.2.3.3 Silylenes stabilized by cyclopentadienyl, arene, and related ligands 10.01.2.3.4 Bis-Silylenes 10.01.2.4 Acyclic silylenes 10.01.2.4.1 Isolable acyclic silylenes 10.01.2.4.2 Masked acyclic silylenes 10.01.2.5 Reactivity towards small molecules 10.01.2.5.1 Activation of H2 10.01.2.5.2 Activation of NH3 10.01.2.5.3 CdO bond activation 10.01.2.5.4 C]C and C^C bond activation 10.01.2.5.5 Activation of P4 29 Si NMR chemical shifts of silylenes 10.01.2.6 10.01.3 Disilenes 10.01.3.1 Introduction 10.01.3.2 Aryl group substituted disilenes 10.01.3.3 Alkyl group substituted disilenes 10.01.3.4 Silyl group substituted disilenes 10.01.3.5 Heteroatom substituted disilenes 10.01.3.6 Small molecule activation with disilenes 10.01.3.6.1 Activation of H2 10.01.3.6.2 Activation of NH3 10.01.3.6.3 CdO bond activation 10.01.3.6.4 CdC bond activation 10.01.3.6.5 Activation of P4 29 10.01.3.7 Si NMR chemical shifts of disilenes 10.01.4 Conclusion Acknowledgment References

1 1 2 2 2 2 2 4 4 8 10 11 11 12 13 13 14 15 16 22 24 26 28 28 29 31 32 34 36 36 37 37 39 41 42 44 45 45

10.01.1 Introduction 10.01.1.1 Recent developments In recent decades, the chemistry of low-valent/oxidation state main-group compounds has developed rapidly. In 2005, Power and co-workers achieved dihydrogen splitting using a digermyne (RGe^GeR) under ambient conditions. This was the first example of H2 activation by a main-group center without any added catalyst. While the cleavage of such inert s-bonds was considered to be limited to transition-metals for a long time, Power’s groundbreaking discovery has led to a plethora of novel low-valent main-group species which exhibit fascinating reactivity. Such compounds and their reactivity have been the subject of many recent reviews.1–8 Silicon, the second most abundant element in the Earth’s crust, has been attracted attention due to its high natural abundance and low-toxicity. Such compounds containing environmentally benign silicon can be good alternative carbon-based nucleophiles in cross-coupling reactions compared to classical Suzuki and Stille cross-coupling reagents. In addition, biologically active organosilanes have been utilized for the development of unnatural amino acids, protease inhibitors, and fragrances. Thus, organosilicon compounds have been widely used in both academia and industry, and related organosilicon chemistry has been one of the most important research areas for a century. Numerous examples of the synthetic use of organosilicon compounds have

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00174-8

1

2

Low-Valent Silicon Compounds

been described in earlier reviews in COMC (1982), COMC (1995), and COMC (2007). For the last decade, organosilicon compounds, especially low-valent silicon species have also attracted attention as such species have potential applications in bond activations and related catalytic chemical transformations. This chapter highlights recent advances in the chemistry of low-valent silicon compounds, mainly focusing on silylenes (R2Si:) and disilenes (R2Si]SiR2) along with their reactivity towards small molecules published from 2007 to 2021.

10.01.1.2 Literature Silicon chemistry has evolved significantly and a number of monographs, books, and reviews have been published. Various examples of synthetic use of organosilicon compounds were described in the earlier reviews in COMC (1982), COMC (1995), and COMC (2007).9–12 Edited books highlighting the progress of silylenes13–15 and multiple bonds in silicon compounds16–18 were published. Some reviews have been published as journal papers which will be referred to in the relevant sections of this chapter.

10.01.2 Silylenes 10.01.2.1 Introduction Silylenes (R2Si:), that is, the silicon analogue of carbenes (R2C:), are important reactive intermediates in silicon chemistry. In order to isolate silylenes as stable compounds, sterically demanding ligands (kinetic stabilization) and/or electronically stabilizing ligands based on heteroatom substituents (thermodynamic stabilization) are required due to the high reactivity and short lifetimes of silylenes. Of particular importance in the development of low-valent silicon chemistry was the successful isolation of the dodecamethylsilicocene (Cp 2Si:) stabilized by the Cp (Cp ¼ Z5-C5Me5) ligand by Jutzi and co-workers in 1986.19 Since this groundbreaking seminal work, a variety of isolable cyclic silylenes20–25 and higher coordinated silylenes26–31 have been isolated and studied. More recently, several isolable two-coordinate acyclic silylenes have been disclosed while such silylenes had previously been considered to be transient compounds and non-isolable compounds. Singlet silylenes have been shown to possess a high energy lone pair [Highest Occupied Molecular Orbital (HOMO)] and an available vacant p-orbital [Lowest Unoccupied Molecular Orbital (LUMO)]. This dual donor/acceptor character of silylenes can mimic the electronic situation observed in transition-metals (d-orbitals). Therefore, silylenes show possibilities for the activation of inert small molecules such as dihydrogen, which has been considered to be the exclusive to transition-metals in bygone days. The typical synthetic pathways for silylenes (R2Si:) are shown in Scheme 1. Several possible reductions of Si(IV) species are utilized as follows: (i) the reductive dehalogenation of the corresponding dihalides [R2SiX2 (X ¼ Cl, Br, I)] using reducing reagents such as KC8 and metal naphthalenides, (ii) the photochemical reductive elimination of a disilane (R0 3Si–SiR0 3) from R2Si(SiR0 3)2, and (iii) the thermal and/or photochemical reductive elimination of an olefin (R0 2C]CR0 2) or alkyne (R0 C^CR0 ) from three-membered ring systems. (i) R

X Si

R

(ii) hv

R

– R’3Si–SiR’3

R

R

reduction

Si R

X

Si

SiR’3 SiR’3

X = Cl, Br, I, etc. (iii)

– R’

R’

R’

R Si R

R’

Scheme 1 Synthetic methods for silylenes.

10.01.2.2 Two-coordinated silylenes 10.01.2.2.1

Cyclic silylenes

A series of stable cyclic silylenes with carefully designed substituents/ligands have been isolated since the first synthesis of a stable two-coordinate N-heterocyclic silylene (NHSi),20–25 which is the silicon analogue of stable N-heterocyclic carbenes.32 NHSis are stabilized by the effects of cyclicization together with the interaction from the lone pairs on the directly bonded nitrogen atoms to the vacant 3p orbital on the silicon atom. Besides NHSis, a wide range of silylenes bonded to other elements such as carbon and phosphorus with various ring sizes (4-, 5-, or 6-membered rings) have been described. In carbon-substituted cyclic silylenes, the empty 3p orbital on the Si center is little effected by electronic perturbation from the substituents, and remains as a low-lying accessible LUMO, which leads to high reactivity towards various small molecules.

Low-Valent Silicon Compounds

3

10.01.2.2.1.1 N-Heterocyclic silylenes (NHSis) Since the first report of an isolable two-coordinate N-heterocyclic silylene (NHSi) by Denk and co-workers,33 a variety of stable NHSis have been synthesized to date.20–25 Synthesis of bis-N-aryl substituted heterocyclic silylenes 1a-1c by metal-reduction of the appropriate silicon(IV) heterocycles was reported (Fig. 1).34–36 The reactivity of NHSis 1a-1c towards such reagents as chalcogenides, azides, and transition-metal complexes were revealed. It was found that these NHSis 1a-1c show no significant differences in structure features, spectroscopic properties or reactivities when compared to those of previously reported N-alkyl substituted silylenes, such as Denk’s NHSi. In addition, the Cui group disclosed an NHSi bearing bulky boryl ligands 2.37 The first base-free bis-silylene 3 was prepared by the Roesky group.38 Bis-silylene 3 can react with N2O to give the mono-silylene which is the first four-membered mono-silylene with a di-coordinate silicon atom.39 Recently, the Iwamoto group succeeded in the synthesis of an isolable cyclic alkyl(amino)silylene 4 which undergoes C–H insertion and dehydrogenation reactions together with typical silylene reactions such as cyclizations, Si–H insertions, and photochemical reactions.40–43 The group of Siemeling and Holthausen isolated ferrocenium bridged NHSis 5a and 5b which exhibit a good combination of high thermal stability and high reactivity towards small molecules such as CO2, N2O and NH3 (vide infra).44 An isolable heterocyclic silylene 6 bearing two different p-donating substituents, an amino group and a phosphonium bora-ylide function, was reported by Kato, Baceiredo, and co-workers.45 The same group also prepared the isolable heterocyclic silylene 7 bearing an amino group and a carbon-based p-donating phosphonium ylide.46 The introduction of these two different p-donating ligands to the Si center of silylenes 6 and 7 provides high thermal stability and unusual nucleophilic character.

Dipp N B N

Ar N N

N

Si

N

Ar Dipp N 1a: Ar = 2,4,6-Me3C6H2 1b: Ar = 2,6-iPr2C6H3 1c: Ar = 2,6-Me2C6H3

Dipp

N

Si

B

NHC

Dipp

Ar

Si

Si

B N

Dipp

Ar

N N

Ar

N

Si

3

PR2

NiPr2 PR2

6

7

4

PR3

NHC = iPr

N

MeO

tBu tBu

tBu

OMe tBu

Si

tBu

N

N

Ar 5a: Ar = 2,4,6-Me3C6H2 5b: Ar = 2,6-iPr2C6H3

tBu N PR2 = P Si N

iPr

Si

Fe

Ad =

2

Si

N

Ar

Ar = 2,4,6-iPr3C6H2

N

Ar

Si

Ad

Dipp = 2,6-iPr2C6H3

Dipp

Me3Si SiMe 3

Si

PR3 R = Ph or m-tol 8

tBu

MeO

tBu tBu

tBu OMe

9

Fig. 1 Examples of isolable cyclic silylenes.

10.01.2.2.1.2 Dialkyl cyclic silylenes In 1999, the first isolable cyclic dialkylsilylene stabilized by the sterically bulky dialkyl substituents was reported by the Kira group.47–49 Since then, several isolable cyclic dialkylsilylenes have been isolated including the carbocyclic silylenes 8 isolated by Driess and co-workers.50 These silylenes possess two phosphonium ylides that show comparable aromatic character. In addition, the isolable cyclic dialkylsilylene 9 bearing a sterically bulky alkyl ligand was prepared by Iwamoto and co-workers.51 The reaction of dialkylsilylene 9 and N2O afforded the first isolable silicon analogue of a ketone (silanone) that contains an unperturbed Si]O double bond. The same group also described the synthesis of isolable cyclic dialkylsilylene 10 bearing a bidentate alkyl substituent with 1,3-disilaindane moieties.52 Recrystallization of silylene 10 afforded two types of crystals, yellow crystals of 10 and orange-red crystals of tetraalkyldisilene 11 which is the dimer of 10. Investigation by NMR and UV-vis spectroscopies revealed that silylene 10 is in equilibrium with tetraalkyldisilene 11 in solution (Scheme 2).

4

Low-Valent Silicon Compounds

Me2 Si Si Si Me2

Me2 Si Si Me2

in solution

Me2 Si Si Me2 Si Me2 Si Si Si Me2

1/2

10

Me2 Si Si Me2 Me2 Si Si Me2

11

Scheme 2 Dynamic equilibrium between silylene 10 and disilene 11.

Iwamoto and co-workers also reported the formation of silylene 12 bearing a new alkyl substituent, isopropyldimethylsilyl groups (Fig. 2).53 Silylene 12 is thermally instable and gradually undergoes a facile 1,2-silyl migration at room temperature. The Oestreich group demonstrated the generation of the transient silylene 13, bearing an a-cyclopropyl substituent, which undergoes insertion and addition reactions with tert-butyl alcohol and 2,3-dimethyl-1,3-butadiene, respectively.54 Generation of the disilyl cyclic silylene 14 that underwent 1,2-silyl migration to furnish endocyclic disilene was reported.55,56 Me3Si

R3Si SiR 3 Si

Si

R3Si SiR3 SiR3 = SiMe2iPr 12

Me2Si Me2Si

Si

Si Si

Me3Si 13

SiMe3

SiMe3

14

Fig. 2 Examples of non-isolable silylenes.

10.01.2.3 Higher coordinated silylenes Besides the aforementioned cyclic silylenes, a large number of higher coordinated silylenes, that is, silicon-(II) compounds with coordination number greater than two have been studied.26–31 Higher coordinated silylenes are considered as donor-stabilized silylenes and the silicon atom persists in +II formal oxidation state. The first major discovery in the chemistry of stable silylenes was the isolation of a Cp -stabilized silicocene Cp 2Si: (Cp ¼ Z5-C5Me5) as a Si(II) compound with higher coordination number reported by the Jutzi group in 1986.19 In recent decades, many functionalized silylenes with higher coordinate Si(II) centers have been prepared by using various donor ligands such as amidinate ligands, b-diketiminato ligands, N-heterocyclic carbenes (NHCs). The reactivity of such silylenes have been investigated as well, which revealed that these species show similar reactivity as two-coordinated silylenes, whereas they are less reactive than two-coordinate silylenes as the vacant orbital is occupied by a donor ligand. Moreover, halo-silylenes [R(X)Si:] (X ¼ halogen) and hydrosilylenes [R(H)Si:] have been isolated as stable compounds by taking advantage of the higher coordinated system, as the isolation of these species in the absence of a supporting Lewis base has not yet been reported.

10.01.2.3.1

Silylenes stabilized by chelating ligands

Monoanionic bidentate ligands have been utilized to synthesize many types of unusual low-valent silicon compounds. Seminal work in this area was the synthesis of a silylene stabilized by two diphosphinomethanide ligands, reported by Karsch and co-workers in 1990.57 Since then, a variety of chelating ligands have been developed and, particularly, systems bearing N atom-donors have been widely used in silylene chemistry due to the tunability of the steric and/or electronic nature by changing the substituents on the nitrogen atoms.22,24 The Driess group reported the synthesis of an isolable siloxysilylene 45 stabilized by a b-diketiminato ligand by the reaction of two-coordinate cyclic silylene58 with H2O (Fig. 6).59 One of remarkable achievements in this field was the preparation of halogen substituted three-coordinate silylenes [(L)X2Si:] (X ¼ halogen) that are utilized as precursors for synthesis of new silicon-based compounds. The Roesky group reported the first isolable monomeric chlorosilylene ([PhC(NtBu)2]ClSi:) 15 stabilized by an amidinate ligand in 2006 (Fig. 3).60,61 By using chlorosilylene 15 as a precursor, a variety of novel functionalized silylenes such as alkyl-(23a, 23b),62 silyl-(23c),63 amido-(23d,64–66 24a,67 24b,68 25,69 2670), aryl-(27,71 28a,72 28b73), phosphino-(29),74 phosphinidene-(30),75 Cp -(31),76 carborane-fused-(32),77 and N-heterocyclic carbene-(33)78 substituted silylenes have been prepared via metathesis reaction (Fig. 4). It is worth mentioning that chlorosilylene 15 can be served as an excellent precursor for novel low-valent silicon compounds.79–81 Similarly, a salt metathesis reaction of chlorosilylene 15 with a lithium germylidenide gave the first isolable silylene–germylene complex 34.82 The versatile reactivity of chlorosilylene 15 has also been intensively investigated.83–89

Low-Valent Silicon Compounds

N

Ph

N

Si

Dipp

H

tBu

N

Si

Me2N

Dipp 2,6-iPr 16

2C6H3

Ph Ph

Cl

N

P

N

Si

18

Dipp

tBu

Cl

Dipp

N Si Cl

E1

Dipp

tBu

Cl

17

Dipp

tBu

P

N

Cl

H Dipp =

Ph

Si

N

15

N

N

Si

Cl

tBu

Ph

Dipp

Me2N

N

19

N

PR2 = P

N

PR2

E2

Si

tBu

20: E1 = (CH2)2, E2 = CH2 21: E1 = E2 = CH2 22: E1 = E2 = (CH2)2

Fig. 3 Isolable chlorosilylenes stabilized by chelating ligands.

N

Ph

N

Si

tBu

tBu

tBu

Ph

R

N N

Si

tBu

tBu

23a: R = C(SiMe3)3 23b: R = tBu 23c: R = Si(SiMe3)3 23d: R = NR’2, R’ = Ph, Cy, iPr, Me, or SiMe3

N

Ph N

N

Dipp

Si

tBu

R

Ph N

Me

N Si

N

N

tBu

Ph Ph 27

25

24a: R = PPh2 24b: R = SiMe3

N

O N Si

Ph

N

26

tBu

R

H

Si

tBu

N Si

N

N N

Si

tBu

P

SiMe3

tBu

Ph

N

Si

tBu

tBu

Si

N

SiMe3

Dipp 30

Dipp

Ph

N N

tBu

N N Si

tBu

Ph Ge

Dipp

Dipp 32

Fig. 4 Selected examples of silylenes stabilized by an amidinate ligand.

33

tBu

P

29

N

N

N

tBu

tBu

Cp*

31

Ph

28a: R = BMes2 28b: R = SiHMes2 Mes = 2,4,6-Me3C6H2

tBu

PtBu2

Dipp = 2,6-iPr2C6H3

Ph

tBu

Ph

tBu

tBu

Si N

Ph

P

N

N

tBu

Ph tBu

N

34

Ph

N Ph

5

6

Low-Valent Silicon Compounds

Ph

N N

Si

Ph

tBu

tBu

Ph R

tBu

35a: R = 35b: R = OiPr 35c: R = PiPr2 OtBu

N N

Si

tBu

iPr

tBu

X

Si

N

H Ph

N

iPr

N

Ph Ph

N

tBu

N iPr

Si

N

R’ Dipp N N

NMe2

Dipp

iPr

36a: X = Cl 36b: X = H

Si

R2N

N

Si

NR’’2

R’ 39a: R = iPr, R’ = iPr 39b: R = Me, R’ = Dipp, R’’ = SiMe3

38

37

N

NiPr2 NR’’2 = N

N

iPr

iPr for 39a

Fig. 5 Selected examples of silylenes stabilized by an amidinate or guanidinato ligand.

Besides that the amidinate-stabilized silylenes 35a-35c were synthesized by the reaction of trichlorosilane [{PhC(NtBu)2}SiCl3] with lithium reagents LiR (R ¼ OtBu, OiPr, PiPr2), followed by the reduction with potassium (Fig. 5).90 A monomeric silylsilylene 36a was prepared by the reduction of dichlorosilane [{PhC(NtBu)2}SiHCl2] with 2 equivalents of potassium graphite.91 Alternatively, silylsilylene 36a was also obtained by the reaction of chlorosilylene 15 with [K{HB(iBu)3}]. The results suggest that intermediate [{PhC(NtBu)2}(H)Si:] is formed in situ which reacts with the amidinate ligand of chlorosilylene 15 to form silylsilylene 36a. Moreover, the reaction of dichlorosilane [{PhC(NtBu)2}SiHCl2] with 4 equivalents of KC8 afforded the silylene 36b. An amidinato-stabilized imidosilylene 37 was synthesized by the treatment of the six-coordinate bis(amidinato)silicon(IV) complex with one molar equivalent of potassium bis(trimethylsilyl)amide.92–94 Additionally, isolation of silylene 38 bearing the bulky bidentate amidinato ligand [DippNC(Ph)NDipp]− (Dipp ¼ 2,6-iPr2C6H3) and a dimethylamido substituent was reported along with reactivity towards small molecules such as N2O, chalcogenides, and azides.95 Similarly, silylenes 39a and 39b supported by guanidinato ligands, [iPrNC(NiPr2)NiPr]− and [DippNC(NMe2)NDipp]−, were also reported.96 The electron-rich b-diketiminate ligands bearing electron-donating dimethyl amide substituents at the backbone (so-called N-nacnac ligands) are capable of stabilizing single-site metal centers of various highly reactive low-valent species. In the N-nacnac complexes of silylenes, the highly reactive Si center is stabilized by bulky substituents at the N atoms and intramolecular N–Si donor-acceptor interaction. A chlorosilylene stabilized by the N-nacnac ligand 16 was isolated by the Driess group (Fig. 3).97 Likewise, the Aldridge group reported the synthesis of a N-nacnac supported chlorosilylene 1798 which undergoes salt metathesis reactions with K[Si(SiMe3)3], Na[N(SiMe3)2], and Li[P(SiMe3)2](dme) (dme ¼ 1,2-dimethoxyethane) to form the corresponding functionalized silylenes.99 Synthesis and reactivity of N-heterocyclic silylenes supported by monoanionic bidentate ligands has been the subject of reviews.100,101 Nakata and co-workers have developed a series of iminophosphonamide ligands, in which a phosphorus atom takes place of a central C atom of the amidinate ligand. The electropositive P atom induces a large polarization of the NdP bond which leads to the higher s-donating ability of the Si(II) center. They succeeded in the isolation of chlorosilylenes 18102 and 19103 by using these iminophosphonamide ligands. Chlorosilylenes 18 and 19 show a stronger s-donating nature compared to other NHSis and typical NHCs and 18 can serve as a ligand for a rhodium complex. Additionally, 18 reacts with KN(SiMe3)2 to form iminophosphonamide-supported silaimine 40, that is in equilibrium with the corresponding aminosilylene 41 (Scheme 3).104 The groups of Kato and Baceiredo reported the isolation of chlorosilylenes 20–22 supported by phosphine-based ligands, which can be good precursors to get access to novel low-valent silicon species such as the first isolable silyne (–Si^C–) and the silavinylidene phosphorane (Si]C]P).105,106 tBu

tBu

Ph Ph

P

N N

Si

tBu

40

SiMe3

Ph

N SiMe3

Ph

P

N N

Si

tBu

N

SiMe3

SiMe3

41

Scheme 3 Thermal silaimine/silylene equilibrium.

In such three-coordinate complexes, another significant discovery was the isolation of hydrosilylenes [R(H)Si:] which are notable species for application in catalytic transformations such as the hydrosilylation of alkenes, alkynes and carbonyl compounds. Only a few isolable hydrosilylenes without stabilization by a Lewis acid have been reported. By utilizing phosphine-based ligands, the groups of Kato and Baceiredo achieved isolation of a three-coordinate hydrosilylene 42 stabilized by an intramolecular coordination of phosphine (Fig. 6).107,108 Phenyl- and trimethylstannyl-substituted silylenes 43109 and 44110 were isolated using a

Low-Valent Silicon Compounds

Ar

R N Si PR2

X

tBu iPr N N PR2 = P Si or P N N iPr tBu (for 43) (for 42,44)

N N Si R

2Si

H

Si

Si

Me2EtN

SiDsi2

N

iPr

NMe2 Dsi = CH(SiMe3)2 46

Cl Cl

R H O

N Si

N R

R = Dipp 45

42: R = H, X = CH2 or CH2CH2, Ar = Dipp = 2,6-iPr2C6H3 43: R = Ph, X = CH2, Ar = Dipp 44: R = SnMe3, X = CH2, Ar = Dipp or 2,4,6-Me3C6H2 iPrDsi

R

LB

Si

Si

SiCl3 SiCl3

Si

H R

Ar* = 2,6-{2,4,6-Me3C6H2}2C6H3 Si

NHC =

SiMe3 SiMe3

48a: LB = NHC, R = Cp* 48b: LB = NHC, R = Ar* 48c: LB = 4-PPy, R = Ar*

47

7

N

N

4-PPy = N

N

Fig. 6 Examples of functionalized silylenes stabilized by chelating ligands.

structurally related phosphine-based ligand. The same group also found that the phosphine supported hydro- and chloro-silylenes (49a-49b) undergo an unusual dimerization via insertion into SidX s-bonds (X ¼ H, Cl), that is a reversible reaction at ambient temperature (Scheme 4).111 The group of Sekiguchi reported the synthesis of the intramolecularly N-coordinated silylene 46 obtained by the addition of 4-dimethylaminopyridine (DMAP) to disilyne (Dsi2iPrSi)Si^Si(SiiPrDsi2) [Dsi ¼ CH(SiMe3)2].112 Holthausen, Schneider, and co-workers reported base stabilized disilene 47 which has a dative SidSi single bond between two silylene moieties [Me2EtN ! SiCl2 ! Si(SiCl3)2].113 Similarly, Cowley, Holthausen, and co-workers described the synthesis of base-coordinated silylenes 48a-48c with hydride substituents.114 It was found that the equilibrium between silylsilylenes 48a-48c and their disilenes can be controlled by the steric bulk of the coordinating base and the substituent.

R2P

N Si X

iPr

Ar + Ar

X Si N

R2P PR2

RT

49a: X = H or Cl, Ar = Dipp = 2,6-iPr2C6H3 49b: X = Cl, Ar = Mes = 2,4,6-Me3C6H2

N Si X Ar

Ar

PR2 =

X Si N

PR2

tBu

N

or

P

N

Si N N iPr tBu (for 49a,50a) (for 49b,50b) P

50a: X = H or Cl, Ar = Dipp 50b: X = Cl, Ar = Mes

Scheme 4 Reversible dimerization of hydro- and chlorosilylenes 49a and 49b.

N-Heterocyclic silylenes (NHSis), with a singlet electronic ground state, show strong s-donor and p-acceptor character and are able to serve as effective stabilizing ligands in transition-metal complexes.115,116 The Driess group has developed a variety of amidinate-stabilized chelating bis-NHSi ligands (Fig. 7).117–124 The first example of isolable oxygen-bridged bis-silylene [LSi–O–SiL] [L ¼ (NtBu)2CPh] 51 can act as a ligand for Ni(0) to form the first bis-silylene oxide Ni complex.117 In addition, bis-Si(II)-based SiCSi pincer arene 55, bearing amidinate ligands, was prepared.118 The Si(II) centers in the arene-derived pincer 55 shows tremendous electron-rich Lewis donor character, which leads to facile access to bis-silylene-silyl(phenyl)-palladium(II) complex with potential application in catalytic chemical transformations. Thus, these NHSis can be utilized as ligands for transition-metal complexes to give transition-metal NHSi complexes, pre-catalysts active in the C–H borylation and Sonogashira cross-coupling reaction.125–134

8

Low-Valent Silicon Compounds

Ph tBu

N

N

Si

Ph tBu

N Si

Si

tBu

N

N

tBu

N

N

tBu

N Si

tBu

N

tBu

N

N Si

N

N

N

N

Si

O tBu

N

N

tBu

N tBu

Si

Ph 55

N Ph

56

tBu

Si N N tBu

57

N

tBu

N

tBu

tBu

N Ph

54

Ph tBu

N

Si

tBu

Si N

N tBu

Si O

Si

tBu tBu

N

tBu

53 Ph

tBu

N

Ph

tBu

Ph

52 Ph

O

tBu

Si

Ph

51

tBu

N C C

Si

Ph

tBu

N

Ph tBu

Fe

O tBu

tBu

tBu

Ph

Ph tBu

N

N

Si

N

Si

tBu

N

N

N

tBu

tBu

Ph 58

Fig. 7 Examples of chelating bis-NHSi ligands.

10.01.2.3.2

Silylenes stabilized by NHCs

Another important class of donor ligand to stabilize highly reactive silylenes are N-heterocyclic carbenes (NHCs). In recent decades, the use of NHCs in silicon chemistry has provided many unexpected low-valent silicon species bearing unusual structure and electronic nature. In addition to the strong s-donating nature of the NHC, the tunable steric bulk, accessed by varying the substituents on the NHC moieties, gives the advantage of altering the kinetic influence of the NHC. A large number of NHC-stabilized silylenes including halo-silylenes [(NHC)X2Si:] (X ¼ halogen) and hydrosilylenes [(NHC)R(H)Si:] have been synthesized.135,136 “NHCs in Main Group Chemistry” by our group is a comprehensive review highlighting the progress of main-group chemistry by utilizing the ability of NHCs.135 Dihalosilylenes stabilized by NHC 59a-59d have been synthesized and used as building blocks in synthetic chemistry (Fig. 8).28,137–140 aNHCs are of stronger s-donor compared to common NHCs and undergo a Lewis base exchange reaction with dichlorosilylene 59a selectively to form aNHC-stabilized dichlorosilylene 60.141 Besides dihalosilylenes, a variety of functionalized halo-silylenes [(NHC)R(X)Si:] (X ¼ halogen) have been prepared to date. The first examples of isolable aryl(chloro)silylenes 61a-61b stabilized by an NHC were reported by the Filippou group.142 By using 61b as a precursor, the first transition-metal–silicon triple bond (“silylidyne complex”) was synthesized.143 Similarly, NHC-supported amido-(61c),144–147 amino-(61d),148 silyl(61e)149substituted chloro-silylenes were isolated. The groups of Tokitoh, Sasamori, and Matsuo reported the synthesis of NHC-stabilized aryl(bromo)silylenes 62a-62b obtained by the reaction of dibromodisilenes [Ar(Br)Si]Si(Br)Ar] (Ar ¼ Bbt or EMind; Bbt ¼ 2,6-[bis(bistrimethylsilyl)methyl]-4-tris(trimethylsilyl)methylphenyl, EMind ¼ 1,1,7,7-tetraethyl-3,3,5,5-tetramethyls-hydrindacen-4-yl) with NHCs.150 Additionally, a bromo(silyl)silylene stabilized by NHC 63 was reported by the Filippou group.151 Using such a carbene stabilization strategy, several examples of hydrosilylenes [(NHC)R(H)Si:], important precursors for applications in catalytic transformations, have been isolated. Our group prepared NHC-stabilized hydro(silyl)silylene 64a by taking advantage of a sterically bulky silyl group as a protecting and electron-donating substituent and an NHC ligand as a strong s-donor.152–154 Similarly, the Müller group reported isolation of NHC-stabilized hydro(aryl)silylenes 64b-64c.155 Furthermore, NHCs are used to stabilized trisilacyclopropylidenes 65a-65b, which may be viewed as a heavy, all-silicon analogue of the carbon-based three-membered cyclopropylidene ring.156,157 Other cyclic silylenes stabilized by NHC such as silole silylenes (66a-66b),158,159 cyclic amino(carboranyl)silylene (67),160 and bis(phosphanyl)silylene (68)161 were also reported. In 2005, the Bertrand group reported cyclic alkyl(amino)carbenes (cAACs), which exhibit stronger s-donor and p-acceptor properties when compared to NHCs.162 Since their discovery, the chemistry of low-valent main-group compounds supported by cAAC ligands has developed rapidly.163–166 Such species stabilized by cAACs tend to exhibit remarkably different structural and electronic character in comparison with their NHC-stabilized counterparts. The first isolable cAAC-silylene, cAAC-diiodosilylene 69, was reported by the group of So and Parameswaran (Fig. 9).167 In general, silylenes bearing strong s-donor and p-acceptor ligands like cAACs can easily lead to disilenes or diradicals during the reduction of silicon precursors. Roesky, Stalke, Yang, and co-workers

Low-Valent Silicon Compounds

R

R Dipp N

N Dipp

Ph

Si

Cl

61a: R1 = 2,6-(2,4,6-Me3C6H2)2C6H3, R2 = R3 = Me 61b: R1 = 2,6-(2,4,6-iPr3C6H2)2C6H3, R2 = R3 = Me 61c: R1 = N(SiMe3)(Dipp), R2 = iPr, R3 = Me 61d: R1 = N(H)(Dipp), R2 = Dipp, R3 = H 61e: R1 = SitBu3, R2 = Et, R3 = Me

R3

N

N

Dipp

N

R2

N Si

Si

Br

Br

R2

R1

Cl

60

59a: X = Cl, R = H 59b: X = Br, R = H 59c: X = I, R = H 59d: X = Br, R = Me

N Si

Cl

Dipp = 2,6-iPr2C6H3

R2

N

R2

Si

X

R3

R3

N

N Dipp

X

R3

Ph

Dipp

Ar

R1 R1

R2 R2

R1 R1

R2 R2

R R

Me3Si

Tbb

63

62a: Ar = Bbt, R2 = R3 = Me 62b: Ar = EMind, R2 = iPr, R3 = Me

R

Br Si

Br

SiMe3

Me3Si

Dipp

SiMe3

EMind: R1 = Et, R2 = Me

Bbt: R = SiMe3 Tbt: R = Me

R3 R3 N

N

N R2

Si H

64a: R = SitBu3 64b: R = 2,6-(2,4,6-Me3C6H2)2C6H3 64c: R = 2,6-(2,4,6-iPr3C6H2)2C6H3

R1

Ph

N R Ph

Ph

iPr

Ph

Si

X

R1

65a: X = Br, R1 = 2,4,6-iPr3C6H2, R2 = iPr, R3 = Me 65b: X = R1 = 2,4,6-Me3C6H2, R2 = Me or iPr, R3 = Me iPr

N

Si

Si

R1 Si

R

R N

2 N R

66a: R = Me 66b: R = iPr

tBu

N iPr iPr Si N N 67

P

N Si

Fe

N

P tBu

68

Fig. 8 Examples of silylenes stabilized by NHCs.

Dipp N

Si

Si I

I 69

Fig. 9 Examples of silylenes stabilized by cAACs.

N

X

N Dipp

Dipp = 2,6-iPr2C6H3 70a: X = Cl 70b: X = Me

N Dipp

P

Si

P

Dipp N

N Dipp

71

9

10

Low-Valent Silicon Compounds

were succeeded in the isolation of monomeric cAAC-stabilized silylenes (70a-70b) by introduction of a suitable N-donor side-arm which provides lone pair on the nitrogen atom to the electronically depleted Si(II) center.168 cAAC-Stabilized silylenes (70a-70b) are stable both in the solid state and in solution at ambient temperature under an inert atmosphere. The SidC(cAAC) bond lengths in silylenes (70a-70b) are similar to SidC single bonds reported for other cAAC-stabilized silicon compounds. Furthermore, Roesky, Frenking, Dittrich, and co-workers synthesized the cAAC anchored silylene 71 with two phosphinidenes, by the reduction of Cl2Si{P(cAAC)}2 with 2 equivalents of KC8.169 A computational study revealed that the P–C(cAAC) bonds in silylene 71 can be described with a resonance between dative bonds and electron-sharing double bonds while the Si–C(cAAC) bond in 71 is described as a Si]C(cAAC) double bond.

10.01.2.3.3

Silylenes stabilized by cyclopentadienyl, arene, and related ligands

One of the significant advances in silylene chemistry was the isolation of Jutzi’s silicocene Cp 2Si: (72) (Cp ¼ Z5-C5Me5) as mentioned above (Fig. 10).19 Thus, Cp is also able to serve as an effective electron donor ligand for the Si(II) center in silylenes. The same group achieved the isolation of the first aryl-substituted silylene (Cp )(Ar)Si: (Ar ¼ 2,6-(2,4,6-iPr3C6H2)2C6H3) (73) stabilized by coordination of Cp and a sterically bulky terphenyl group.170 Additionally, the amide-substituted silylene (Cp ){(Me3Si)2N}Si: (78) coordinated by Cp was found to be in equilibrium with the dimer (disilene 79) (Scheme 5).171 Our group reported the isolation of iminosilylene (Cp )(IPrN)Si: (74) (NIPr ¼ [N{C(NDippCH)2}]−) bearing electron donating Cp ligand and a sterically bulky NHI (N-heterocyclic imine) substituent.172 Leszczy nska et al. reported the isolation of the silyl-substituted silylene (Cp ){(SiMe3)3Si}Si: (75) along with the reactivity with various small molecules such as H2 and ethylene (vide infra).173 The first isolable donor-free halosilylenes 76 supported by a carbazole scaffold and intramolecular interaction from arene were reported by Hinz.174 The Müller group reported an isolable silylene 77 bearing a s2, p-butadiene ligand in which silylene 77 is stabilized by homoconjugation with the remote C]C double bond.175 Tip Si

Si

*Cp 72

tBu

tBu tBu

X

Tip

Cp*

N

Si

N

Dipp

(Me3Si)3Si

74

tBu

Si

Si

Ph Ph tBu

X = Br, I

Dipp

N

Si

73

N tBu

Cp*

75

SiMe3 HfCp2

Me3Si Cp = η5-C5H5

76

77

Fig. 10 Examples of silylenes coordinated by Cp (72–75), arene (76), and butadiene (77).

SiMe3

SiMe3 Me3Si N

Si Cp*

78 Scheme 5 Dynamic equilibrium between silylene 78 and disilene 79.

1/2

Me3Si N

Si

Cp* Si N SiMe3

Cp* 79

SiMe3

Low-Valent Silicon Compounds

10.01.2.3.4

11

Bis-Silylenes

As mentioned in the Section 10.01.2.3.1, spacer separated bis-silylenes (:Si–LU–Si:, LU ¼ linking unit), in which two silylenes are connected by a linker such an arene or heteroatom, show unique and characteristic reactivity owing to the effect of two silylenes in close proximity. Within the bis-silylene class, interconnected bis-silylenes (:Si–Si:), in which two silylene units bridged by a SidSi bond, are known as well. Interconnected bis-silylenes (:Si–Si:) are isoelectronic species of disilynes (RSi^SiR) and can be isolated by using additional Lewis donors such as NHCs, whereas the carbon analogues are not observed. On the other hand, the heavier analogues [bis-germylenes (:Ge–Ge:),176–178 bis-stannylenes (:Sn–Sn:)176,179 can be isolated as stable compounds. While the chemistry of disilynes (RSi^SiR) have been also developed significantly and subject to many recent reviews,180–184 this section highlights the interconnected bis-silylenes (:Si–Si:). The first isolable interconnected bis-silylene, the NHC-stabilized bis-silylene 80a [NHC(Cl)Si–Si(Cl)NHC; NHC ¼ :C(NDippCH)2], was described by Robinson, Schleyer, and co-workers (Fig. 11).185 Supporting DFT calculations revealed that the bonding within bis-silylene 80a is best described as a SidSi s-bonding orbital and two non-bonding lone-pair orbitals, one at each Si atom. Additionally, natural bonding orbital (NBO) analysis showed the SidSi s-single bond [Wiberg bond index (WBI) ¼ 0.94] in 80a. Later, the bromo- and iodo-analogues 80b-80c were successfully prepared by the Filippou group.186 Bis-silylene 80a was found to react with thiolate [S]C{N(Dipp)CHC(SLi(thf )3)N(Dipp)}] to form the sulfur-containing cyclic silylene isomers via CdH and CdN bond cleavage.187 The Roesky group reported the interconnected bis-silylene 81a stabilized by bulky amidinate ligands.188 Similarly, a bulkier amidinate ligand with Dipp substituents on the N atoms was utilized to stabilize the interconnected bis-silylene 81b along with the germanium and tin analogues.176,189 The reactivity of bis-silylene 81a towards various molecules such as N2O,190 Ph2CO,190 alkynes,191–193 Br2,194 benzil,195 cyclooctatetraene,196 azides,197 carbodiimides,198 PhBCl2,68 and [Cp Fe(Z5-P5)]199 have been investigated. Additionally, bis-silylene 81a can be used as a precursor for a variety of novel silicon compounds such as the silyliumylidene cation,200 tetrasilacyclobutadiene dication,201 aromatic Si4 ring,202 Si3Ge ring,203 Al2Si4 ring,204 and as a ligand within the coordination sphere of transition-metals such as Ir, Rh,205 Fe,206 and Co.207 Computational investigations showed that the electronic structure of the amidinate-stabilized bis-silylenes 81a-81b are similar to those of 80a which has SidSi single bond with negligible p character. Kato, Baceiredo, and co-workers reported the synthesis of the interconnected bis-silylene 82 supported by phosphine-based ligands.208 The Wiberg bond index for the SidSi bond (1.262) in bis-silylene 82 is larger than that for typical SidSi single bonds, which suggests a certain degree of multiple bonding character, caused by enhanced negative hyperconjugative p interactions between the two Si centers (nSi–s Si) due to the weak P–Si p interaction. Furthermore, the interconnected bis-silylenes 83a209 and 83b,210 together with the germanium analogue were synthesized by using cyclic alkyl(amino) carbenes (cAACs). Both bond lengths of SidSi bond and SidC bond in cAAC-stabilized bis-silylene 83a are significantly shorter than those in NHC-stabilized bis-silylene 80a. This difference can be explained by the bonding situation of the molecules: bis-silylene 83a exhibits cAAC-(SiCl)2-cAAC electron-sharing bonds, NHC donor-acceptor bonds. whereas bis-silylene 80a possesses NHC ! (SiCl)2 Moreover, a triatomic Si(0) cluster (cAAC)3Si3 (84), supported by p-accepting cAAC ligands, was synthesized by the reduction of (cAAC)SiCl4 with KC8 (Fig. 12).211 Compound 84 has a triangular Si3 unit with a lone pair at each Si atom, which are polarized towards C atom of the cAAC. One interpretation of this Si(0) cluster 84 is as a triangular trisilylene.

10.01.2.4 Acyclic silylenes While silylene chemistry has advanced significantly in recent decades, the activation of inert molecules including rigid s-bond, e.g., H–H, with silylenes still remains scare. In computational studies, it has been suggested that acyclic silylenes which have a wider angle at the Si(II) center would exhibit a smaller singlet–triplet energy gap and allow to activate inert molecules.212,213 Thus, the study of acyclic silylenes could provide new bond breaking and formation reactions using main-group compounds as mimics for transition-metals. In fact, there have been substantial developments in this field of acyclic silylenes (vide infra). Dipp

Dipp N

N

X Si Dipp

N

Si

R’

N

X

N

N Dipp

R

Dipp = 2,6-iPr2C6H3 80a: X = Cl 80b: X = Br 80c: X = I Fig. 11 Interconnected bis-silylenes.

N

R

R

Dipp

Si

Si

N N

PR2 R’

R

81a: R = tBu, R’ = Ph 81b: R = Dipp, R’ = 4-tBuC6H4

N Si Si Dipp R2P R = tBu 82

R

Dipp N

Si

Si R

N Dipp 83a: R = Cl 83b: R = Me

12

Low-Valent Silicon Compounds

Dipp

N

Si Si

N

Si

Dipp

N

Dipp

84 Fig. 12 Triatomic Si(0) cluster (cAAC)3Si3 (84).

10.01.2.4.1

Isolable acyclic silylenes

Simple two-coordinate acyclic silylenes are highly reactive species and thus the isolation of such silylenes as stable compounds is synthetically challenging. Only a few examples of two-coordinate acyclic silylenes have been reported to date in contrast to a large number of isolable cyclic silylenes20–25 and Lewis based stabilized silylenes.26–31 In 2012, the first isolable two-coordinate acyclic silylenes 85214 and 87a215 were reported (Fig. 13).216 Amido(boryl)silylene 85214 and amido(silyl)silylene 86217 were synthesized by the reactions of a tribromo(amino)silane with the boryllithium reagent and hypersilyl potassium [K(THF)2][Si(SiMe3)3], respectively. An extensive series of bis(arylthiolato)silylenes 87a-87c were prepared by Power, Tuononen, Herber, and co-workers.215,218 The groups of Jones and Aldridge reported the isolation of an amido substituted silylene 88 with two extremely bulky boryl-amido ligands, [N(SiMe3){B(DAB)}]− [TBoN ¼ N(SiMe3)(DAB); DAB ¼ (DippNCH)2, Dipp ¼ 2,6-iPr2C6H3].219 With regard to this field, our group found that the facile thermal rearrangement of the silanone O]Si(NIPr)(SitBu3) (NIPr ¼ [N{C(NDippCH)2}]−) at room temperature after 2 days affords imino(siloxy)silylene 89.220 The isolation of the bis(boryloxy) silylene 90 with two bulky boryloxy protecting groups was reported by the Aldridge group.221 Additionally, Rivard and co-workers employed a bulky vinylic ligand [(MeIPr)CH]− (MeIPr ¼ (MeCNDipp)2C), a carbon-based donor, to prepare the vinyl(silyl)silylene 91222 and bis(vinyl)silylene 92.223

TBoN R

SiMe3 Dipp N Dipp Si N B N Dipp

SiMe3 Dipp N Si (Me3Si)3Si

Dipp = 2,6-iPr2C6H3

86

TBoN 88

R

S Si S

R

R

Dipp 87a: R = 2,4,6-Me3C6H2 87b: R = 2,6-iPr2C6H3 87c: R = 2,4,6-iPr3C6H2

Me3Si

Dipp N

N

N

Dipp

Si O SitBu3 89 Fig. 13 Isolable acyclic silylenes.

Dipp N B N Dipp O Si O Dipp N B N Dipp 90

Dipp N Dipp N H

N Dipp C Si

(Me3Si)3Si 91

H H

N B N N Dipp

TBoN

85

N Dipp C C

Dipp N

Si N Dipp

92

Si

Low-Valent Silicon Compounds

10.01.2.4.2

13

Masked acyclic silylenes

Some disilenes (R2Si]SiR2) are known to exist in equilibrium with the corresponding monomeric silylene (R2Si:) in solution and act as synthetically equivalent to silylenes. The nature of substituents at the silicon atom play a crucial role in the properties of disilenes, because the dissociation energy of the Si]Si double bond relies on the singlet–triplet energy gap of the monomer (R2Si:). Electronegative and p-donating and substituents stabilize the singlet state and promote the dissociation of disilenes (R2Si]SiR2) into the corresponding monomeric silylenes (R2Si:).224–226 On the other hand, p-accepting and electropositive substituents stabilize the triplet state and the Si]Si double bond in the disilenes. In addition, the steric repulsion between the sterically bulky substituents destabilizes the Si]Si double bond, which leads to the dynamic disilene-silylene equilibria in some disilenes.227–231 The dynamic equilibria between dibromodisilenes 93a-93b bearing sterically overcrowded 1,1,3,3,5,5,7,7-octa-R-s-hydrindacen4-yl (Rind) groups and bromosilylenes 94a-94b at room temperature are revealed by experimental and computational studies (Scheme 6).232 While bromosilylenes 94a-94b cannot directly be observed by spectroscopic methods, the equilibria become observable by the addition of Lewis base, e.g., 4-pyrrolidinopyridine (PPy) to give isolable PPy-stabilized bromosilylenes 95a-95b. The isolation of a disilenyl silylene stabilized by an NHC (98) was reported by the groups of Scheschkewitz and Rzepa (Scheme 7).233,234 Disilenyl silylene 98 was found to coexist in equilibrium with the isomeric cyclotrisilene 96 and the free NHC 97 in solution. Our group synthesized DMAP-stabilized silylenes, (DMAP)(R0 3Si)(R3Si)Si: [100a: SiR3 ¼ SitBu3, SiR0 3 ¼ Si(SiMe3)3, 100b: SiR3 ¼ SiR0 3 ¼ SiMetBu2, 100c: SiR3 ¼ SiR0 3 ¼ Si (SiMe3)3] (DMAP ¼ 4-N,N-dimethylaminopyridine), which act as synthetic alternative to acyclic silylenes in solution and undergo facile oxidative addition with small molecules such as H2 and ethylene under mild conditions (1 atm, 65  C) (Fig. 14).235 Besides that our group reported isolable silepins 102a-102b and revealed that silepins 102a-102b are in equilibrium with their corresponding imino(silyl)silylenes (R3Si)(IPrN)Si: [R ¼ SiMe3 (101a) or t-Bu (101b)] in solution by both experimental and computational studies.220,236 Silepins 102a-102b are formed via the intramolecular insertion reaction of the silylenes 101a-101b, which are generated in situ, into the C]C bond of the aromatic ligand framework. Our group also reported the dynamic equilibrium between a bis(silyl)silylene 103 and the corresponding tetrasilyldisilene 104.237 Reactivity studies revealed that these silicon compounds 102a-102b and 104 can be behave as masked acyclic silylenes sources (vide infra).

N Rind Br

Br Si Si Rind

Rind in solution

2

Si Br

N

N

(PPy)

Br

R2 R2

R1 R1

R2 R2

Rind =

Si

94a 94b

93a: EMind: R1 = Et, R2 = Me 93b: Eind: R1 = R2 = Et

N

2 Rind

benzene RT

R1 R1

95a 95b

Scheme 6 Dynamic equilibrium between disilenes (93a and 93b) and silylenes (94a and 94b).

R R

Si

Si Si

R

iPr

R +

N

N R iPr 96 97 R = 2,4,6-iPr3C6H2

R Si Si R R Si iPr

N

N

iPr

–97 +97

R

R Si Si R Si R

98

99

Scheme 7 Equilibrium of cyclotrisilene 96 and N-heterocyclic carbene 97 with NHC-coordinated disilenyl silylene 98.

10.01.2.5 Reactivity towards small molecules The activation of inert small molecules such as H2, alkenes, alkynes and CO is a key step in many important catalytic processes and these bond activations have been generally mediated by transition-metals. In recent decades, the chemistry of main-group compounds has shown tremendous growth with a variety of isolable main group compounds including heavy analogues of alkenes, alkynes, and carbenes. These species possess high-energy lone pairs and available vacant orbitals, which enable to activate relatively unreactive small molecules akin to transition-metals. In 2005, Power and co-workers achieved H2 activation by a digermyne (–Ge^Ge–), as the first example of catalyst-free addition reaction of H2 to a main-group compounds.238 Since the breaking discovery, related studies on the bond activations using low-valent main-group compounds have been intensively focused by the community. More recently, some examples of catalytic cycles by a low-valent main-group center were achieved. In 2014, the

14

Low-Valent Silicon Compounds

N N

R’3 Si

100a: R3Si = tBu3Si, R’3Si = (SiMe3)3Si 100b: R3Si = R’3Si = tBu2MeSi 100c: R3Si = R’3Si = (SiMe3)3Si

Si

R3Si

Dipp

N

N N

C C

Si R3Si

N C Si C N N Dipp R3Si

101a: R = SiMe3 101b: R = t-Bu

tBu

3 Si

(Me3Si)3Si 103

SiMe3 Si

1/2 Me3Si

Si Si

SitBu3

SiMe3 104

102a 102b

Fig. 14 Selected examples of masked acyclic silylenes.

Jones group reported the catalytic hydroboration of carbonyl compounds by hydro-metallylenes L{(H)E: [E ¼ Ge or Sn, L{ ¼ -N(Ar{)(SiiPr3), Ar{ ¼ 2,6,4-{C(H)Ph2}i2Pr-C6H2].239 In 2018, Sasamori and co-workers reported the use of digermyne Tbb–Ge^Ge–Tbb {Tbt ¼ 2,6-[CH(SiMe3)2]2-4-tBu-C6H2} for the catalytic formation of CdC bonds.240 Our group also demonstrated a dialumene-catalyzed CO2 hydroboration, in which the NHC-dialumene [(NHC)(tBu2MeSi)Al]Al(SiMetBu2)(NHC)] [NHC ¼ :C{N(iPr)C(Me)}2] was used as a precatalyst.241 The activation of small molecule by silylenes (R2Si:) have also been investigated significantly. Recent some reviews highlighted the small molecule activation with silylenes.3,242–245 The focus of the following paragraphs is the activation of classic small molecules such as H2, NH3, CO2, CO ethylene, and P4 (vide infra).

10.01.2.5.1

Activation of H2

The splitting of H2 is a key step in a various catalytic process such as the hydroformylation and hydrogenation of unsaturated organic compounds.246–249 While this desirable reactivity towards dihydrogen has been long thought to be limited to transition-metals, some examples of H2 activation with main-group centers in the past several decades have altered this viewpoint. In 2012, the first example of the activation of H2 with a silylene was reported. Amido(boryl)silylene 85 is able to activate H2 at mild condition (1 atm, 20  C) to give the dihydrosilane 105 (Scheme 8).214 Similarly, amido(silyl)silylene 86 undergo facile H2 activation at room temperature to afford the corresponding dihydrosilane 106.217 Our group demonstrated the H2 splitting with the silepin 102a, which provides the in situ generated imino(silyl)silylene 101a in solution.236 Additionally, it was found that the disilene 104/silylene 103 equilibrium mixture reacts with H2 at very mild conditions (1 atm, −40  C) to yield the corresponding dihydrosilane 108.237 More recently, the activation of H2 with an intermediary silylene [L(Br)Ga]2Si: (L ¼ HC[C(Me)N(2,6i Pr2C6H3)]2) (109) was reported by the groups of Schulz and Schreiner.250 The reaction of [L(Br)Ga]2SiBr2 with an equivalent of LGa at 60  C under an H2 atmosphere generates the dihydrosilane [L(Br)Ga]2SiH2 110. Silylene 109 shows the narrowest singlet–triplet gap (5.9 kJ mol−1) and lowest HOMO–LUMO gap energy (2.7 eV) owing to its strongly s-donating, electropositive gallyl substituents. The Cp -supported silylsilylene 75 can activate H2 under mild condition (1 atm, room temperature) to form the corresponding dihydrosilane 111.173 When an NHC was added to dihydrosilane 111, the reductive elimination of Cp H occurred to give an NHC-stabilized hydrosilylene (NHC){(Me3Si)3Si}(H)Si: (112). This is a rare example of reductive elimination from a silane (tetravalent silicon compound, namely, Si(IV) compound) to form a low-valent silicon compound, which is a key process in establishing Si-based catalytic cycle.

Low-Valent Silicon Compounds

15

Scheme 8 Activation of H2 by silylenes.

10.01.2.5.2

Activation of NH3

The activation of the NdH bonds of ammonia is key step in catalytic transformation such as hydroamination. Many low-valent main-group species are known to activate the NdH bonds in NH3, whereas the activation by transition-metals is challenging since Werner-type complexes are easily formed in the reaction with Lewis basic amines.251–253 The bis(amido)silylene 88 can activate NH3 to yield triaminosilane 114 along with the secondary amine TBoNH (Scheme 9).219 It is plausible that diaminosilylene 113 is generated via s-bond metathesis reaction of 88 with NH3, and 113 undergoes an oxidative addition reaction with NH3 to give the triaminosilane 114. Amido(silyl)silylene 86, imino(siloxy)silylene 89, NHSi 117, and NHSi with the ferrocenediyl backbone 5b can also activate NH3 to form the hydro-amination products 115254 116,255 118,256

16

Low-Valent Silicon Compounds

Scheme 9 Activation of NH3 by silylenes.

and 119,44 respectively. Furthermore, compound 116 reacts with excess amounts of NH3 and provide unidentified mixture. The result suggests that the elimination of the imino ligand occurred and IPrNH and (H2N)(tBu3SiO)Si(H)(NH2) were formed in the reaction, which is reminiscent of s-bond metathesis reaction observed for the bis(amido)silylene 88.

10.01.2.5.3

CdO bond activation

Carbon dioxide (CO2) is a crucial greenhouse gas and a universal chemical feedstock. Over the last few decades, many studies have been made on the chemical activation and utilization of CO2 as a sustainable C1 source.257–259 Although most CO2 activation studies have been conducted using transition-metals to date, the development of eco-friendly systems without the use of transition-metals is desirable. It has currently been demonstrated that low-valent main-group molecules are also capable to activate CO2.

Low-Valent Silicon Compounds

17

The first example of CO2 activation with silylenes was described by the Jutzi group.260,261 Kira and co-workers investigated CO2 activation by the dialkylsilylene 120. The reaction of 120 with CO2, followed by the work up with MeOH afforded a carbonate-bridged disilanol 121 (Scheme 10).262 DFT calculations revealed the reaction mechanism: a plausible pathway involves the formation of the transient silanone 123 via the oxidative addition of CO2 and the elimination of CO, followed by the cycloaddition of second equivalent of CO2 to form cyclic carbonate 124 (Scheme 11). Then the spiro compound 125 was formed by the addition of silanone 123 to carbonate 124. Tacke and co-workers reported the deoxygenation of both CO2 and N2O using NHSis 37 and 39a (Scheme 12).263,264 When NHSis 37 and 39a were treated with CO2, carbonates 128a-128b were formed. In the reaction with CO2, silanones 126a-126b are generated in situ and subsequently undergo a cycloaddition reaction with an additional equivalent of CO2, generating products 128a-128b. On the other hand, the reactions of NHSis 37 and 39a with N2O afforded cyclic disiloxanes 127a-127b formed by dimerization of putative silanone intermediates 126a-126b. Likewise, the ferrocene-bridged NHSi 5b reacts with CO2 under mild conditions to give the orthocarbonate 131, formed by cycloaddition of silanone 129 with CO2, while a cyclic disiloxane 130 was obtained by dimerization of the silanone 129 in the reaction with N2O.44 Similarly, silepin 102a, which reacts as a masked silylene and forms the silylene 101a in situ, can activates CO2 to furnish the silicon carbonate 132 (Scheme 13).236 While such carbonates tend to undergo addition reactions or dimerization, carbonate 132 was isolated as the first four-coordinate monomeric silicon carbonate. Furthermore, siloxysilylene 45 can activate CO2 to form a silanoic silyl ester 133 together with the liberation of carbon monoxide.265 The reaction of bis-NHSi 82 with 4 equivalents of CO2 affords the striking carboxylato bis-silicate 134.208 The Aldridge group reported activation of CO2 by using amido(boryl)silylene 85 to give the (trimethylsiloxy)iminosilane {(HCDippN)2B}Si(NDipp)(OSiMe3) (135).266 Hydrosilylene 64a also reacts with an excess amount of CO2 to form cis/trans-cyclotrisiloxane 136.267 The Driess group investigated the reactivity of the ortho-dicarborane-based bis-NHSi 53121 and the ortho-dicarborane-based phosphanyl-silylene 138,268 where one silylenyl moiety in 53 is replaced by a phosphanyl group, towards small molecules such as CO2, N2O, O2 (Scheme 14).269 Bis-NHSi 53 activates CO2, N2O, and O2 to furnish the same dioxygenation product 137, whereas silanone-phosphine oxide 141 with mono-oxygenated P and Si atoms was obtained in the reaction of phosphanyl-silylene 138 with O2. Moreover, the reaction of 138 with N2O yielded the silanonephosphine 142, in which only the silicon atom is mono-oxygenated, and with CO2 afforded the silicon carbonate-phosphine 139 formed by cycloaddition of silanone-phosphine 142 with another molecule of CO2. Silicon carbonate-phosphine 139 undergoes isomerization in Et2O at room temperature to give 140 quantitatively within 24 h, whereas 139 is stable at ambient temperature in the solid state.

Me3Si SiMe 3 Si

2

Me3Si

3 CO2 (1 atm) 1) n-hexane, –78 °C 2) MeOH, RT

Me3Si SiMe3

Me3Si SiMe 3

SiMe3 Si

O

O

Si OH O HO Me3Si SiMe Me3Si SiMe3 3

120

121

Scheme 10 Activation of CO2 by dialkyl cyclic silylene 120.

120

CO2

Me3Si SiMe 3

Me3Si SiMe 3 O Si O Me3Si SiMe3

123

Me3Si SiMe3

O

O

O

O

Me3Si SiMe3

Si

Me3Si

H2O

SiMe3

125 Scheme 11 Plausible pathway for the reaction of dialkyl cyclic silylene 120 with CO2.

Me3Si SiMe 3 O Si O O Me3Si SiMe3

123

SiMe3 Si

O

Me3Si SiMe3

122

Me3Si

Si

– CO

CO2

124

121

18

Low-Valent Silicon Compounds

R 1/2

iPr

dimerization R N

iPr

N iPr

N

R iPr

Si

R CO2 or N2O

iPr

N

toluene

iPr

N

iPr

N

N

R

R

N

CO2

N

O O iPr iPr

N

R

Si

N

N

i R Pr

N

iPr

127a: R = Ph 127b: R = NiPr2 R

iPr

126a: R = Ph 126b: R = NiPr2

37: R = Ph 39a: R = NiPr2

Si

O

Si N

N

iPr

iPr

iPr

iPr

N

iPr

iPr

N N

N

R

iPr

O

Si

N

O

C

O

iPr

128a: R = Ph 128b: R = NiPr2 Dipp

Dipp N

dimerization 1/2 Dipp N Fe

Si N

N

Dipp CO2 or N2O C6D6, RT

Dipp 5b

Si

Fe

Si

O

N Si

Fe N

Dipp

Dipp

N Fe

O

130

O

N Dipp 129

Dipp

Dipp CO2

N 1/2

Fe

Si N Dipp

O O

C

131

O O

Si

N Fe N

Dipp

Scheme 12 Activation of CO2 by silylenes.

Silylene 85 can also activate CO at room temperature to give the homologation product 143 which was characterized by NMR spectroscopy and X-ray diffraction analysis (Scheme 15).266 Computational study revealed the reaction mechanism for the reactions of the acyclic silylene 85 with CO, CO2, and N2O.270 Carbon monoxide activation with silylenes was achieved by Driess and co-workers as well. Bis-NHSis 52 and 54 have been shown to undergo the selective deoxygenative homocoupling of CO to furnish disilaketenes 144.271 Carborane-supported bis-NHSi 53 showed higher reactivity towards CO when compared to bis-NHSis 52 and 54, presumably owing to the rigidity and steric hindrance of the ortho-carborane backbone. Bis-silylene 53 reacts with CO to yield the novel bis-silylene-mediated polycyclic CO coupling product 145.272 Computational studies revealed that the reaction mechanism for the formation of 145 involves similar reaction steps to the reaction of bis-silylenes 52 and 54 leading to disilaketenes 144. Furthermore, compound 144 reacts with further small molecules such as ammonia and amines to form acetamides.273 In the reaction of 53, the disilaketene analogue is a reactive intermediate and undergoes migration of a C moiety of the carborane from the Si center to the C center of the ketene moiety to form another intermediate containing a O]CdC]Si unit, followed by the dimerization gave the final product 145.

Low-Valent Silicon Compounds

Dipp N

N

N

Dipp

2 CO2 (1 atm) n-hexane, RT

Si (Me3Si)3Si

Dipp N

N

Dipp O Si C O O (Me3Si)3Si N

101a

H

N Si R

R

R

N

132

N Si

O

N Si Si Dipp R2P

N R

R = Dipp 45

PR2

R R N H O N Si N N Si O R R

CO2

133

Dipp N

Dipp C O O N Si O Si N O P Dipp R R P

C6D6, RT

R = tBu 82 SiMe3 Dipp N Dipp Si N B N Dipp 85

O

R R

4 CO2 (3 atm)

134 Dipp

CO2 (1 atm)

Dipp

N

Dipp 135

H

64a

CO2 toluene, RT

Si SitBu3

Scheme 13 Activation of CO2 by silylenes.

Si O

N B N

C6D6, RT

tBu

N

SiMe3

N

1/3

3 Si

O H Si tBu

3Si

H

Si O 136

O Si H SitBu3

19

20

Low-Valent Silicon Compounds

N

Ph

Si

N

Si

C

tBu

N

Ph

N

C

tBu

tBu

tBu

tBu

N

Ph

2 CO2, 2 N2O, or O2

N

Et2O, –20 °C

tBu

N O Ph Si O N C C tBu

Si

tBu

53

137 tBu

tBu

tBu

Ph

N N tBu

Si

P

C

C

tBu

N 2 CO2

N tBu

N Ph

Et2O, –20 °C

N

tBu

N2O Et2O, –20 °C

138

O C Si O C

N Ph

N

tBu

Si

O O

C

C

tBu

N N tBu

141

C

tBu

N N tBu

N isomerization Et2O, RT

Ph

N

tBu

O Si C

P O

N C

tBu

O

C

140 CO2 –20 °C

tBu

P

P

tBu

139

O2 Et2O, –20 °C tBu

O

N

N Ph

N

tBu

Si

tBu

O

C

P C

N N tBu

142

Scheme 14 Activation of CO2 by silylenes.

Although it is well known that transition-metal complexes react with CO to form the stable carbonyl complexes under mild condition, such reactivity is virtually unknown for main-group compounds. The first example of isolable carbonyl complex of heavier Group 14 element, silicon carbonyl complex [L(Br)Ga]2Si⇆CO (146), reported by the groups of Schulz and Schreiner.250,274 Compound 146 behaves as a masked silylene and react with H2 to yield the dihydrosilane H2Si[Ga(Br)L]2 (110). Our group also achieved the isolation of room temperature stable silyl-substituted silicon carbonyl complex [(Me3Si)3Si](tBu3Si)Si⇆CO (147).275 The reaction of an equilibrium mixture of tetra(silyl)disilene 104 and bis(silyl)silylene 103 with CO affords complex 147. Alternatively, 147 is also obtained by the reaction of DMAP-stabilized silylene 100a with CO. Furthermore, photolytic decarbonylation of 147 generates an equilibrium mixture of 103 and 104. The chemical bonding in silicon carbonyl complexes 146 and 147 are investigated by computational study.276

Low-Valent Silicon Compounds

N

Dipp SiMe3 Dipp N Si 2 Dipp N B N Dipp

N Dipp

B

Dipp

C O

4 CO Me3Si

benzene, RT

C O

N

Si

O

O C

Dipp

85

N SiMe 3

Si C

Dipp

B

N

N Dipp

143

tBu

Ph N Si

tBu

N

tBu

Si tBu

N 52 54

Et2O, RT tBu

N

2

N N tBu

X

=

Fe

O

54

52

Ph

Ph

144

Si

Si

C

C

C

LSi

tBu

tBu

Ph

N

N t Bu Si X O C C O Si t N Bu N tBu

2 CO (1 atm)

X

Ph

N

O

Ph

N tBu

4 CO

C

C

C

O

toluene, –20 °C

C

C C

53

tBu

O

LSi

C

SiL O SiL

L=

N N

Ph

tBu

145 O L(Br)Ga Si L(Br)Ga

CO (1 atm) benzene, 60 °C

L(Br)Ga C Si L(Br)Ga 146

109

N

O tBu Si 3

Si

(Me3Si)3Si 103 Scheme 15 Activation of CO by silylenes.

CO (1 atm) n-hexane, RT

tBu

3 Si

(Me3Si)3Si 147

C Si

CO (1 atm)

tBu Si 3

toluene, RT – DMAP

(Me3Si)3Si

N Si 100a

21

22

Low-Valent Silicon Compounds

10.01.2.5.4

C]C and C^C bond activation

The activation of small organic molecules and CdC bond formation is a crucial step for transformation of simple molecules into important chemical compounds including synthetic products such as pharmaceuticals and plastics.277–279 These processes have been conducted by using transition-metals and various transition-metal catalysts have been developed to date. Transition-metals are able to bind reversibly with a variety of organic molecules and serve as effective catalysts. On the other hand, oxidation states in main-group compounds vary in a much narrower range and the bond activation and formation of new CdC bonds catalytically is challenging for main-group species. Meanwhile, some examples of main-group compounds have been reported which can activate CdC bonds in neutral organic compounds and in some cases show dynamic equilibria, a key step in catalytic processes. Furthermore, it has been demonstrated that several main-group molecules enabled the catalytic bond activation of alkynes, followed by CdC bond formation reactions.240 It is well known that silylenes undergo cycloaddition reactions with substrates containing unsaturated CdC bonds.20–22,24 Silylenes 89, 101a, 103 and 75 can react with the parent alkene (ethylene) under ambient conditions to yield the corresponding [2 + 1] cycloaddition products 148–151 (Scheme 16).173,220,236,237 Silylene 85 also reacts with ethylene at room temperature to yield the silirane 152 (Scheme 17).280 Interestingly, 152 undergoes further reaction with another molecule of ethylene at 60  C, to form the alkylated silirane 153. By an NMR experiment with deuterated ethylene, it was found that a migratory insertion of the coordinated ethylene into the SidSi bond of 152 occurs, followed by the formation of the silirane with C2D4.

Dipp N

N

Dipp

N Si

O SitBu3 89

Dipp N

N

N

C6D6, RT

O

(1 atm)

N

N

Si

(Me3Si)3Si

(Me3Si)3Si 101a

tBu

Si

Dipp N

C6D6, RT

Si

N

(1 atm)

Dipp CH2 CH2

SitBu3 148

Dipp

N

Dipp N

Dipp CH2 CH2

149

3 Si

Si

(Me3Si)3Si

(1 atm) n-hexane, RT

tBu

3Si

Si

(Me3Si)3Si

CH2 CH2

150

103

Cp* Si

Cp*

(1 atm) C6D6

(Me3Si)3Si

Si (Me3Si)3Si

75

CH2 CH2

151

Scheme 16 Activation of ethylene by silylenes.

SiMe3 Dipp N Si (Me3Si)3Si

(0.5 atm) n-hexane RT

85 Scheme 17 Activation of ethylene by silylenes.

SiMe3

SiMe3 Dipp N Si (Me3Si)3Si 152

CH2 CH2

(0.5 atm) benzene 60 °C

Dipp N Si H2C CH2 (Me3Si)3Si 153

Low-Valent Silicon Compounds

23

The groups of Kato and Baceiredo reported the first example of a reversible alkene-insertion into a Si(II)dSn bond. Silylenes 44a-44b react with ethylene at room temperature to form their corresponding alkyl substituted silylenes 154, respectively (Scheme 18).110 These reactions of 44a-44b with ethylene are reversible and the equilibria are observed at 85  C for 44a and room temperature for 44b, which is a key step in catalytic processes. Similarly, the reversible reaction of silylene 43 with ethylene at room temperature was demonstrated.281 While some germanes [R4Ge] and stannanes [R4Sn] are known to undergo reductive elimination to their germylenes [R2Ge:] and stannylenes [R2Sn:],6,282–284 examples of silylenes [R2Si:] remain scare due to strong SidC covalent bonds and the relatively high energy of the Sill oxidation state. Power, Tuononen, and co-workers also reported the reactions of silylenes 87a-87b with ethylene, afforded the corresponding cycloaddition products 156a-156b which underwent reversible reactions with ethylene at room temperature (Scheme 18).285 The same groups also investigated the reactivity of bis(arylthiolato)silylenes 87a-87b towards alkynes.286 Similarly, a reversible insertion of hydrosilylene 42 with olefins was reported by the groups of Kato and Baceiredo (Scheme 19).107 Hydrosilylene 42 reacts with mono-substituted olefins such as vinyltrimethylsilane (H2C]CHSiMe3) to yield the corresponding cycloaddition product 157, which is a reversible reaction. Furthermore, when 157 was heated at 70  C in the presence of vinyltrimethylsilane, olefin insertion products 158a and 158b were obtained, which suggests that the cycloaddition product 157 is an intermediate in the hydrosilylation reaction. This was the first example of catalyst-free hydrosilylation of an alkene by a stable silicon hydride. Additionally, the hydrosilylene 42 reacts with diphenylacetylene at room temperature through a [2 + 1] cycloaddition reaction to afford the corresponding silirene 159, which undergoes an isomerization at 80  C to give the silacyclopropylidene 160 (Scheme 20).287–289

Sn

Sn

Ar

Ar

N Si PR2 44a: Ar = Dipp = 2,6-iPr2C6H3 44b: Ar = 2,4,6-Me3C6H2

Ph

Dipp

N Si

THF-d8 85 °C (44a) 25 °C (44b)

N Si THF-d8, RT

PR2

tBu N PR2 = P Si N

CH2 CH2

PR2

tBu

154

Dipp Ph CH2 N Si CH2 PR2

43

155

tBu iPr N N PR2 = P Si or P N N iPr tBu

R

R S

(1 atm)

R

toluene, RT

Si S

S

R

R

R Si

S

R

CH2 CH2

R 156a 156b

87a: R = 2,4,6-Me3C6H2 87b: R = Dipp Scheme 18 Reversible reactions of silylenes with ethylene.

H

Dipp

N Si PR2 42

SiMe3 (excess) RT

Dipp

H

SiMe3

N Si PR2 157

Scheme 19 Hydrosilylation of an alkene by hydrosilylene 42.

70 °C

Dipp

H N Si PR2 158a

Dipp SiMe3 +

H

SiMe3 tBu

N Si PR2 158b

PR2 = P

N N

Si

tBu

24

Low-Valent Silicon Compounds H

Dipp

N Si PR2

Ph

Ph

THF, RT

42

Dipp

H

Ph 80 °C

N Si PR2

Dipp

Ph

159

N

Si

Ph H

PR2 Ph 160

tBu

PR2 = P

N N

Si

tBu

Scheme 20 Activation of diphenylacetylene by silylene 42 and isomerization of silirene 159.

10.01.2.5.5

Activation of P4

White phosphorus (P4) is readily obtained by the reduction of phosphate rock and is an important chemical feedstock for organophosphorus compounds in industry. The chlorination or oxychlorination of P4 provides phosphorus chloride (PCl3) and phosphoryl chloride (POCl3) which can be used as precursors for organo-phosphorus products.290,291 The development of atom-efficient and eco-friendly systems for the direct transformation of P4 into phosphine-containing products have long been desired.292 Some main-group compounds are known to activate P4 under mild conditions.293,294 Additionally, a recent study showed that a transition-metal complex can be employed as a catalyst for direct conversion of P4 to organophosphorus compounds.295 However, for both transition-metals and main-group compounds, the stoichiometric and catalytic reaction of P4 under ambient condition is still challenging. The first example of an insertion reaction of an isolable silylene into the PdP bond in P4 was reported by the Driess group.296 N-heterocyclic silylene (NHSi) 117 activates P4 at ambient temperature to form activated product 161 which further reacts slowly with the second silylene 117 to yield double insertion product 162 (Scheme 21). In the reaction of the amidinato stabilized chlorosilylene 15 with white phosphorus, further fragmentation of the P4 tetrahedron occurred to give the Si-P-Si-P four-membered ring product 163.297 Alternatively, the reaction of the bis-NHSi 81a with P4 also gave compound 163. NHSi 23d reacts with P4 to form compound 164, which is the first example of an acyclic Si2P4 chain bearing 6p electrons.298 Vinyl(silyl)silylene 91 reacts with P4 and an oxidative addition into the PdP bond of P4, followed by 1,2-silyl migration occurs to give compound 165.222 In this reaction, two PdP bonds of P4 are cleaved and four new SidP bonds are formed regioselectively. In the reaction of imino(siloxy) silylene 89 with P4, the oxidative addition of 1 equivalent of P4 undergoes to yield compound 166.255 Additionally, Driess and co-workers reported the bis-silylene-stabilized diphosphorus compound 167 obtained by the reaction of bis-silylene 54 with 0.5 equivalents of P4 at room temperature.299 Compound 167 is a silylene-stabilized zero-valent P2 complex and possess two lone pairs on each P atom which exhibits high reactivity towards further small molecules such as CO2 and borane.

25

Low-Valent Silicon Compounds P

Dipp

P

N Si N

P (1 eq.)

N

toluene, RT

N

Dipp

P P

Si

N

P (0.5 eq.)

Ph

N N

N

Si

P

P

P

P P

Si

P

tBu

P

Si

P

N(SiMe3)2

P (1 eq.)

toluene, RT

23d

N

P

P Dipp N

N Dipp C

P

P

P (1 eq.)

toluene, RT

Si

THF, RT

Ph

Dipp N

N

N O

P Dipp

Si

P

P P (1 eq.)

benzene

tBu (TMS) N N(TMS) 2 2 N N Ph P Si Si P N N P P tBu tBu TMS = SiMe3 tBu

Ph

N

P Si P Si(SiMe3)3

C H

N

N

tBu

O

P

P P P (0.5 eq.) Et2O, RT

Si N

Dipp N

N

N

Dipp P P Si P P

166

Ph Si

tBu

N

P

P

Dipp

tBu 3SiO

SitBu3 89

N

tBu

54 Scheme 21 Activation of P4 by silylenes.

Ph N Si

N

tBu

O P P Si

tBu

Ph

N

Si

Si

tBu

N

N Ph

167

tBu

N N tBu

81a

165

91

N

tBu

Dipp

(Me3Si)3Si

tBu

tBu

P (0.5 eq.)

Ph

N

P

164

tBu

N Dipp

tBu

P

tBu

H

N Si

163

tBu

N

toluene, RT

P

162

tBu

15

N

Si Dipp

tBu

P

toluene, RT – LSiCl3

Cl

tBu

2 Ph

P

117

161

tBu

N

P P

N

Dipp

117

3 Ph

P

Si

Dipp

Dipp

Dipp

P

Ph

26

Low-Valent Silicon Compounds

10.01.2.6 29

29

Si NMR chemical shifts of silylenes

Si NMR Chemical shifts for Si(II) center in selected silylenes are shown in Table 1.

Table 1

29

Si chemical shifts (in C6D6) for silylenes. 29

References

+77.8 +76.7 +76.5 +137.8 +183.3 +274.7 +121.5 +115.7 +295.4a +202.9 +213.3 +212.4 +258.4b +542.0 +123.5c (Si]Si) +567.3 +14.6 −10.7 +1.8 +59.8 +62.6 −50.9 −1.8 +72.7 +61.5 +76.9 −20.5 −5.8 −6.5 −2.7 −8.1 +10.7 +1.2 −12.0 −18.8 +18.5 +13.3 +13.9 +44.0 +56.0 +52.5 +27.4d +10.6 +30.9 −5.2 −13.4 +56.2 +26.8 +45.6 −31.4 +12.1 −25.6 +14.9 +47.4a −44.8, −38.0e −63.7 −17.7, −14.0e −26.0a −29.4

34 34 35 37 38 40 44 44 45 46 50 50 51 52 52 53 60 97 98 102 103 105 106 62 62 63 64 64 64 64 64 67 68 69 70 71 72 73 74 75 76 77 78 82 90 90 90 91 91 92 95 96 96 104 107 107 109 110 110

Si chemical shifts (ppm)

:Si{N(2,4,6-Me3C6H2)CH}2 (1a) :Si{N(2,6-iPr2C6H3)CH}2 (1b) :Si{N(2,6-Me2C6H3)CH}2 (1c) :Si{N{B(DAB)}CH2}2 (2) {:SiN(2,6-Ar2C6H3)}2 (3) :Si{N(Ad)CH2CH2C(SiMe3)2} (4) :Si{N(Mes){Fe(C5H5)2}N(Mes)} (5a) :Si{N(Dipp){Fe(C5H5)2}N(Dipp)} (5b) 6 7 8 (R ¼ Ph) 8 (R ¼ m-tol) 9 10 11 :Si{C(SiMei2Pr)2CH2}2(12) :SiCl[(NtBu)2CPh] (15) :SiCl[(NDipp)2(CH)2CH] (16) :SiCl[(NDipp)2(CNMe2)2CH] (17) :SiCl[(NtBu)2PPh2] (18) :SiCl[(NDipp)2PPh2] (19) 20 21 :Si{C(SiMe3)3}[(NtBu)2CPh] (23a) :Si(tBu)[(NtBu)2CPh] (23b) :Si{Si(SiMe3)3}[(NtBu)2CPh] (23c) :Si(NR0 2)[(NtBu)2CPh] (23d) R0 ¼ Ph :Si(NR0 2)[(NtBu)2CPh] (23d) R0 ¼ Cy :Si(NR0 2)[(NtBu)2CPh] (23d) R0 ¼ iPr :Si(NR0 2)[(NtBu)2CPh] (23d) R0 ¼ Me :Si(NR0 2)[(NtBu)2CPh] (23d) R0 ¼ SiMe3 :Si{N(Dipp)(PPh2)}[(NtBu)2CPh] (24a) :Si{N(Dipp)(SiMe3)}[(NtBu)2CPh] (24b) :Si{NMe(C5NH4)}[(NtBu)2CPh] (25) :Si{NC(CH2PtBu2)CHCHCH}[(NtBu)2CPh] (26) :Si(2-PPh2C6H4)[(NtBu)2CPh] (27) :Si(xanthene)[(NtBu)2CPh] (R ¼ BMes2) (28a) :Si(xanthene)[(NtBu)2CPh] (R ¼ SiHMes2) (28b) :Si{P(SiMe3)2}[(NtBu)2CPh] (29) :Si{P(CNDippCMe2CH2CMe2)}[(NtBu)2CPh] (30) :Si(Cp )[(NtBu)2CPh] (31) :Si(carborane)[(NtBu)2CPh] (32) :Si{CN(Dipp)CN(Dipp)CH}[(NtBu)2CPh] (33) 34 :Si(OtBu)[(NtBu)2CPh] (35a) :Si(OiPr)[(NtBu)2CPh] (35b) :Si(PiPr2)[(NtBu)2CPh] (35c) 36a 36b 37 :Si(NMe2)[(NDipp)2CPh] (38) :Si(NR00 2)[(NiPr)2CNiPr2] (39a) :Si{N(SiMe3)2}[(NDipp)2CNMe2] (39b) :Si{N(SiMe3)2)[(NtBu)2PPh2] (41) 42 (X ¼ CH2) 42 (X ¼ CH2CH2) 43 44 (Ar ¼ Dipp) 44 (Ar ¼ Mes)

Low-Valent Silicon Compounds

Table 1

(Continued) 29

Si chemical shifts (ppm)

45 46 Me2EtN ! SiCl2 ! Si(SiCl3)2 (47) NHC ! Si(H)Cp ! Si(SiMe3)2 (48a) (4-PPy) ! Si(H)Ar ! Si(SiMe3)2 (48c) 49a (X ¼ H) 49a (X ¼ Cl) 49b (X ¼ Cl) 50a (X ¼ H) 50b (X ¼ Cl) :Si[(NtBu)2CPh]–O–[PhC(NtBu)2]Si: (51) :Si[(NtBu)2CPh]–Fe(C5H5)2–[PhC(NtBu)2]Si: (52) :Si[(NtBu)2CPh]–(carborane)–[PhC(NtBu)2]Si: (53) :Si[(NtBu)2CPh]–(xanthene)–[PhC(NtBu)2]Si: (54) 55 56 57 58 :SiCl2(IDipp) (59a) :SiBr2(IDipp) (59b) :SiI2(IDipp) (59c) :SiBr2(IDipp2Me2) (59d) :SiCl2(aNHC) (60) :SiCl{2,6-(2,4,6-Me3C6H2)2C6H3}(IMe4) (61a) :SiCl{2,6-(2,4,6-iPr3C6H2)2C6H3}(IMe4) (61b) :SiCl{N(SiMe3)(Dipp)}(IiPr2Me2) (61c) :SiCl{N(H)(Dipp)}(IDipp) (61d) :SiCl(SitBu3)(IEt2Me2) (61e) :SiBr(Bbt)(IMe4) (62a) :SiBr(EMind)(IiPr2Me2) (62b) :SiBr(SiBr2Tbb)(SIDipp) (63) :SiH(SitBu3)(IMe4) (64a) :SiH{2,6-(2,4,6-Me3C6H2)2C6H3}(IMe4) (64b) :SiH{2,6-(2,4,6-iPr3C6H2)2C6H3}(IMe4) (64c) 65a 65b (R2 ¼ Me) 65b (R2 ¼ iPr) :SiC4Ph4(IMe4) (66a) :SiC4Ph4(IiPr2Me2) (66b) 67 68 :SiI2(Me2cAAC) (69) 70a 70b (Me2cAAC)Si{P(Me2cAAC)}2 (71) :SiCp 2 (72) :Si(2,6-Tip2C6H3)Cp (73) :Si(NIPr)Cp (74) :Si{Si(SiMe3)3}Cp (75) 76 (X ¼ Br) 76 (X ¼ I) 77 :Si{N(SiMe3)2}Cp (78) (IDipp)(Cl)Si–Si(Cl)(IDipp) (80a) (IDipp)(Br)Si–Si(Br)(IDipp) (80b) (IDipp)(I)Si–Si(I)(IDipp) (80c) [PhC(NtBu)2]Si–Si[(NtBu)2CPh] (81a) {:Si[(NDipp)2C(4-tBuC6H4)]}2 (81b) 82

f

f

−9.6, –7.9 +59.9 +43.7c (Me2EtN–SiCl2) −155.6c (Si(SiCl3)2) −5.1 (Si-H), −202.2 (Si(SiMe3)2) +34.6 (Si-H) −184.0 (Si(SiMe3)2) −66.9,a –61.9a,e −8.0, +2.6e −8.4, −1.4e −69.3 −42.8, −41.7e −16.1 +43.3 +18.9 +17.3 −24.0 −14.9,g –13.8,h –17.1h +16.8 −16.4 +19.1 +10.9 −9.7 +14.0 +24.2a +1.3 +0.8 +3.1 −6.0 +18.3 +10.9 +13.1 −1.9 −137.8 −87.6 −80.5 −110.5c −136.6 −132.3 −48.6 −43.6 −4.3a −72.5 −2.1 +27.3 +21.1 +6.8 −577 +51.6 −43.8 +207.2 +129.2 +152.8 −155.2 −10.2 +38.4 +34.9 +18.7 +76.3 +96.9 −18.5

References 59 112 113 114 114 111 111 111 111 111 117 119 121 122 118 120 123 124 137 138 139 140 141 142 142 144 148 149 150 150 151 152 155 155 156 157 157 158 158 160 161 167 168 168 169 19 170 172 173 174 174 175 171 185 186 186 188 176 208 (Continued )

27

28

Low-Valent Silicon Compounds

Table 1

(Continued) 29

References

+25.6 +0.7a +7.2 +439.7 +438.2,f +467.5f +285.5 +270.4 +270.9 +204.6 +58.9 +35.5 +432.9 +271.9 +59.3 +63.1 +100.9c (Tip2Si), +51.5c (Tip2Si]Si(Tip)Si(Tip)), −62.9c (Tip2Si]Si(Tip)Si(Tip)(NHC)) +68.8 +61.5 +72.5 +300.0c −145.3 +88.4

209 210 211 214 217 215 218 218 219 220 221 222 223 232 232 233

Si chemical shifts (ppm)

Me2

Me2

( cAAC)(Cl)Si–Si(Cl)( cAAC) (83a) (Me2cAAC)(Me)Si–Si(Me)(Me2cAAC)(Me) (83b) {:Si(Me2cAAC)}3 (84) :Si{B(NDippCH)2}{N(SiMe3)-Dipp} (85) :Si{Si(SiMe3)3}{N(SiMe3)-Dipp} (86) :Si(SArMe6)2 (87a) :Si(SAriPr4)2 (87b) :Si(SAriPr6)2 (87c) :Si(TBoN)2 (88) :Si(OSitBu3)(NIPr) (89) :Si[OB(NDippCH)2]2 (90) :Si(MeIPrCH){Si(SiMe3)3} (91) :Si(MeIPrCH)2 (92) :Si(Br)(EMind)(PPy) (95a) :Si(Br)(Eind)(PPy) (95b) Tip2Si]Si(Tip)Si(Tip)(NHC) (98)

:Si{Si(SiMe3)3}(SitBu3)(DMAP) (100a) :Si(SiMetBu2)2(DMAP) (100b) :Si{Si(SiMe3)3}2(DMAP) (100c) :Si(NIPr){Si(SiMe3)3} (101a) :Si(H){Si(SiMe3)3}(NHC) (112) :Si{[N(Dipp)CMe]2CH} (117)

237 235 235 236 173 58

a

THF-d8. Cyclohexane-d12. c Toluene-d8. d Dichloromethane-d2. e Minor diastereomer. f A mixture of two rotational isomers. g Symmetric conformer. h Asymmetric conformer. b

10.01.3 Disilenes 10.01.3.1 Introduction As described in the Section 10.01.2, a variety of silylenes (R2Si:) have been isolated by taking advantage of sterically demanding ligands (kinetic stabilization) and/or electronically stabilizing ligands (thermodynamic stabilization). On the other hand, it is known that silylenes (R2Si:) undergo dimerization to form their corresponding disilene (R2Si]SiR2), or oligomerization when the kinetic or thermodynamic stabilization is not sufficient to stabilize highly reactive silylenes. While alkenes (R2C]CR2) such as ethylene (H2C]CH2) exhibit an unambiguously planar geometry, disilenes (R2Si]SiR2) are known to possess a trans-bent geometry. In comparison with carbon, silicon has less tendency to form hybrid orbitals (inert pair effect). Thus, the stability of the divalent state of Group 14 elements relative to the tetravalent state increases upon descending the periodic table (Si, Ge, Sn, and Pb) as the (ns)2(np)2 valence electron configuration is preferred by the heavier homologs of Group 14. Therefore, silylenes generally exhibit a singlet ground state where two electrons remain as a singlet pair in the 3s orbital whereas the ground electronic state in the parent carbene, H2C: is a triplet. In the case of alkenes (R2C]CR2), the parallel 2p orbitals of each triplet CH2 fragment enable to overlap efficiently on the planer structure. On the other hand, disilenes (R2Si]SiR2) have trans-bent structure to avoid the repulsion between lone pairs of each singlet silylenes (R2Si:) as shown in Fig. 15. In 1981, West and co-workers described the first disilene Mes2Si]SiMes2 (Mes]2,4,6dMe3C6H2) (168), which was formed by the dimerization of the transient silylene Mes2Si: at 77 K.300 Since the groundbreaking discovery of West’s disilene, a number of isolable disilenes have been reported over the past four decades. While the study of the low-valent main-group chemistry had long focused on the synthesis and characterization of novel main group species, this trend has been changed after the Power’s report on the H2 activation by a digermyne (RGe^GeR) in 2005. Subsequently, Power wrote the review ‘Main-Group Elements as Transition Metals’ which highlighted how heavier main-group elements mimic transition-metals, as shown by their reactivity towards small molecules.1 For the last decade, disilene chemistry have been brought back into the spotlight due to the potential for activation of small molecules.

Low-Valent Silicon Compounds

Ethylene (C=C): Planar H H

H C

C

H

H H

C

29

Disilene (Si=Si): Trans-bent

C

H H

H H

H Si

Si

H

H H H

Si

Si

H

Fig. 15 Multiply bonded systems.

A variety of recent reviews have already highlighted functional disilenes,301 disila analogues of vinyl anions,302,303 and their reactivity.304 Disilenes as a ligand for transition-metals305 have also been reported. In addition, multiple bonds of heavier Group 14 elements including silicon have been the subject of numerous reviews.306–312

10.01.3.2 Aryl group substituted disilenes Introduction of sterically bulky alkyl, aryl, or other hydrocarbon substituents on the silicon center in disilenes is an effective method to protect highly reactive silicon centers and stabilize the disilenes. In addition to the steric protection offered by bulky hydrocarbon substituents, London dispersion forces (LDF) between the C–H units of the adjacent groups, provide a significant stabilizing influence.313,314 In general, LDF effects are considered to be a minor influence within hydrocarbon-substituted molecules due to the comparatively small C–H–H–C interactions (340  C) and in solution, as neither decomposition nor dissociation products were observed upon refluxing for 1 week in toluene.

Si Si 183a

Rs R R

Rs Si Si

183b

Rs

Rs

Rs Si

N

Si Si

Ad Rs

R = SiMe3 Ad = 1-adamantyl

Rs

Si

Si

Rs

Si Rs

Si

Rs = Si

Rs

185

184

R Bbt

Me3Si

Si Si Bbt

Me3Si Me3Si

SiMe3 SiMe3

Bbt = Me3Si

R

SiMe3

SiMe3

Trp

Trp 188

Fig. 17 Alkyl group substituted disilenes.

187b: R =

Trp =

SiMe3 SiMe3

187a: R = SiMe3

SiMe3

186a: R = SiMe3 186b: R = Ph

Trp* Trp* H H C C H H Si Si

R R Si Si

Trp* =

tBu SiMe3 SiMe3

32

Low-Valent Silicon Compounds

10.01.3.4 Silyl group substituted disilenes As mentioned in the Section 10.01.3.1, disilenes (R2Si]SiR2) usually adopt a trans-bent structure (Carter-GoddardMalrieu-Trinquier model). In general, the silylene fragments exhibit a singlet ground state and need to rotate each other to form bonding interaction between the doubly-occupied s-type orbitals and the vacant p orbitals. The flexibility of the structure in disilenes is quite fascinating as the geometry of disilenes can be easily designed by changing the substituents. The choice of substituents on the Si center in R2Si]SiR2 makes a great impact on the structure features which affects their reactivity. Strongly s-donating silyl ligands such as SitBu3 and Si(SiMe3)3 destabilize the ground singlet states in the silylene fragments of disilenes which leads to a planarization of the resulting disilenes with shortened Si]Si bond.

R R R R Si R R

Si

R

R R

Si R

Si

benzene 80 °C

Si

R R

R R

R

Si

Si R

R R 195

194

Si

R

R Si

Si R

+

Si

R R 196

R

R R

R R

hv

R = SiMe3 193

195

194

benzene RT

+

Si R R 197

Scheme 22 Thermolysis and photolysis of tetrasila-1,3-diene 193.

Selected examples of silyl-substituted disilenes are shown in Fig. 18. A variety of silyl groups such as SiMeiPr2, SiMet2Bu, SiiPr3, and SiMetBu2 have been employed to stabilize disilenes 189a-189d.355–357 On the basis of electronegativity the opposite trend might be expected, silyl-substituted disilenes 189a-189d have slightly longer Si]Si double bonds (2.20–2.26 A˚ ) compared with those in Mes2Si]SiMes2 (2.143 A˚ )358 and Tip2Si]SiTip2 (2.144 A˚ )317 together with a less trans-bent (more planar) geometry around the silicon centers. This elongation can be ascribed to substantial steric repulsion among the trialkylsilyl groups could lead to the planarity with concomitant Si]Si bond elongation in the disilenes 189a-189d. Some conjugated disilenes, the silicon congeners of polylenes, have been synthesized. Kira, Iwamoto, and co-workers reported the isolation of tetrasila-1,3-diene 193 (Scheme 22).359 Tetrasila-1,3-diene 193 was obtained by the reduction of tribromodisilane, which is prepared by the reaction of dialkylsilylene with (Me3Si)SiBr3. When 193 was heated up to 80  C in benzene, cleavage of the Si]Si bond occurred to form cyclotrisilene 195 along with silene 196. On the other hand, irradiation (l > 390 nm) of 193 in benzene gave cyclotrisilene 195 and

RSi

RSi Si Si

Me2Si

RSi

RSi

Si

Tip

Tip

Si

Si

Tip

189a: RSi = SiMeiPr2 189b: RSi = SiMe2tBu 189c: RSi = SiiPr3 189d: RSi = SiMetBu2

Me

191

SiiBu2 SiMe3

Si Si iPr

Me3Si

Tip

iPr

Me3Si Si Si

R

190a: R = SiMe3 190b: R = SiEt3

SiMe3 Me

Si

192

Fig. 18 Selected examples of silyl group substituted disilenes.

SiMe3

= Si,

SiiBu

iBu

Si Si SiMe3

2 Si

SiiBu2

2

bicyclo[1.1.1]pentasilanyl group

Low-Valent Silicon Compounds

33

silepin 197. It is known that silene 196 and silepin 197 are obtained by the thermolysis47 and photolysis360 of the dialkylsilylene, respectively. A plausible mechanism for the formation of cyclotrisilene 195 involves the facial intramolecular insertion of disilenylsilylene 194, which is generated via a Si]Si bond dissociation, into the Si]Si bond in 194. Iwamoto and co-workers prepared the 1,4-bis(trimethylsilyl)tetrasila-1,3-dienes 190a and 190b through a selective Si(sp2)dSi(sp3) cleavage reaction for silyl-substituted disilenes (Fig. 18).361 The same group reported the formation of fused polycyclic disilenes 191 and (E),(Z)192 by the reduction of the 1,2ddibromodisilanes (BPS)(Br)RSidSiR(Br)(BPS) (R]Me or iPr) bearing bicyclo[1.1.1]pentasilanyl (BPS) groups.362 Experimental and computational studies suggested that the initial products of the reduction of 1,2ddibromodisilanes (BPS)(Br)RSidSiR(Br)(BPS) were the disilenes (BPS)RSi]SiR(BPS), which subsequently undergo silyldisilene–disilanylsilylene rearrangement reactions, and insertion of a silylene into the SidSi bond to form the disilenes 191 and (E),(Z)-192. The Iwamoto group also disclosed the first examples of stable silicon analogues of bicyclo[1.1.0]but-1(3)-ene (198a,363 198b364,365), which contain a formal double bond between bridgehead Si atoms in an inverted geometry (Scheme 23). The bond lengths between the bridged silicon atoms of 198a and 198b are comparatively longer than those of typical Si]Si bonds. The bridgehead disilene 198b can be used as a precursor for the novel silicon compound 199.366 Compound 199 is a planar silicon analogue of bicyclo[1.1.0]butane which contains p-type single bonding between the bridgehead silicon atoms. Investigation by NMR spectroscopy revealed that the chloro-analogue 200 is in equilibrium with 1-chloro-2-(chlorosilyl)cyclotrisilene 201 in solution, which is an unprecedented interconversion mode among Si4R6 isomers.367

R R R R

Si Si Si Si

R R

I

R = SiMe3

R R

R R

R R R Si R X Si S Si X R Si R R R

I or CCl4

198a: R = SiMe2iPr 198b: R = SiMe3

R

R

199 (X = I) 200 (X = Cl)

Si

Cl Si Si Si

R R R = SiMe3 201

R R Cl

Scheme 23 Synthesis of 199–200 and dynamic equilibrium between 200 and 201.

Haas, Jones, and co-workers prepared the endocyclic disilene 203 formed by a 1,2-trimethylsilyl migration of bis(silyl) cyclicsilylene 14 (Scheme 24).56 Disilene 203 undergoes a slow dimerization process at room temperature to form the corresponding dimer. Similarly, the silylenecyclotetrasilane 204 undergoes a ring expansion reaction to form cyclopentasilane 205 through stepwise 1,2-disilyl migrations (Scheme 25).368 Thus, trialkylsilyl groups are more prone to undergo migratory insertion compared with alkyl groups, which makes the isomerization kinetically feasible, meaning that it can be used as the driving force to form novel low-valent silicon compounds.

Me3Si Me2Si Me2Si

Si

Si Si

Me3Si

Me3Si

SiMe3

Mg(I)

Me2Si

benzene, RT

Me2Si

Br Br

SiMe3

Si

Si Si

Me3Si

202

Me3Si

SiMe3 1,2-silyl migration

Me2Si Me2Si

SiMe3

Me2Si

Si R2

Si Si

1,2-silyl migration with ring expansion

SiMe2tBu

RT R = SiMe3

204 Scheme 25 Formation of disilene 205 by 1,2-silyl migration of disilene 204.

Si Si 203

Scheme 24 Formation of disilene 203 via transient silylene 14.

SiMe2tBu

SiMe3 SiMe3

SiMe3

14

R2 Si

Si

R2 Si Me2Si Si R2

Si Si

205

SiMe2tBu

SiMe2tBu

34

Low-Valent Silicon Compounds

10.01.3.5 Heteroatom substituted disilenes In a previous theoretical study, Apeloig and Karni investigated the substituent effects on the geometry and energy of H2Si]SiHX systems, and it was found that the substituent on the Si center has a great effect on the structures of the substituted disilenes.369 Electronegative substituents (e.g., X ¼ NH2, OH, F) increase both DEST (the singlet–triplet energy gap in the silylene components XHSi:) and the degree of p–s orbital mixing to cause remarkable trans-bending at the doubly bonded silicon atoms. This leads to an elongation and overall weakening of the Si]Si double bond in disilenes bearing electronegative substituents. In contrast, electropositive groups (e.g., X ¼ Li, BeH, BH2, SiH3) reduced DEST and the degree of p–s interaction, which provides considerable planarization at the doubly bonded Si atoms with shortened Si]Si bonds in the disilenes.369–372 In fact, many experimental studies have been proved that nearly planar structures with the short Si]Si bond length for the disilenes bearing electropositive substituents such as silyl groups,355 lithium,373–376 sodium,375,376 potassium,375,376 and boranes.377–379 On the other hands, disilenes bearing electronegative substituents such as amino groups reveal pyramidal trans-bent geometry at the center silicon atoms with the elongated Si]Si bond.226,378,380 Nitrogen substituents (amines, amides, or imines) stabilize silylenes (R2Si:) by the strong p-interaction between the lone pair on the nitrogen atom with the vacant p orbital of the silicon center. Introduction of p-donating substituents on the silicon center in R2Si]SiR2 induces trans-bent geometry with elongated Si]Si bond while silyl groups lead to the planar disilenes with short Si]Si bond as described above. Computational studies revealed that diaminosilylenes (R2N)2Si: prefer the formation of m-amino-bridged dimers (R2N)Si{m-(R2N)}2Si(R2N) to tetraaminodisilene (R2N)2Si]Si(NR2)2.381–383 The diaminodisilene 79 supported by the Cp substituent is known to show dynamic equilibrium with the corresponding silylene in solution,171,384 and the reactivity of diaminodisilene 79 towards small molecules such as S8 and N2O was investigated (Fig. 19).385 It should also be mentioned that the Sekiguchi group reported the synthesis of a series of amino-substituted disilenes 206a-206d obtained by the reaction of disilyne (Dsi2iPrSi)Si^Si(SiiPrDsi2) with amines.378,386 While disilenes bearing small amino groups (206a-206b) possess planar structure suggesting p-conjugation between the Si]Si double bond and the lone pair on the nitrogen atom of the amino group, disilenes bearing bulky amino groups (206d) tend to exhibit large twist angles between the amino-group plane and the Si]Si double-bond plane, which reduces p-conjugation. The Scheschkewitz group synthesized three Tip- and one dimethylamino-substituted disilene 207 by the reaction of the disilenide Tip2Si¼ Si(Tip)Li with phosphasilene {(Me2N)Tip2Si}(Tip)Si]PNMe2.387 Driess and Kostenko disclosed the isolation of hyper-coordinated disilene 208 with four-coordinate Si(II) atoms by using amidinate ligands.388 X-ray crystallographic analysis revealed an expectedly elongated SidSi distance in disilene 208. The computational studies revealed that the HOMO–2 and HOMO–11 in 208 exhibit interactions between the two silicon centers that originate from the mixing of lone pairs on the silicon atoms and the 3p orbitals. Additionally, natural bonding orbital (NBO) analysis showed significant n(Si) ! 3p(Si0 ) and n(Si0 ) ! 3p(Si) interactions, which suggests that the Si]Si bond in 208 consists of a double SidSi donordacceptor bond.388,389 Our group prepared a series of asymmetrically substituted disilenes [ABSi]SiAB; 209, 210, and 211] bearing N-heterocyclic imino (A ¼ NItBu ¼ [N{C(NtBuCH)2}]−) and trialkylsilyl [B ¼ Si(SiMe3)3, SiMetBu2, SitBu3] groups.390,391 Imino(silyl)disilene [209-(Z)] exhibits an extremely trans-bent and twisted structure and the significant elongated Si]Si double bond.390 Theoretical calculations of imino(silyl)disilene 209 suggested that the Si]Si bond in 209 can be considered as double donor–acceptor bond between two silylene fragments. It is of note that highly twisted and trans-bent structure of iminodisilene 209 enables it to activate various small molecules including the first hydrogen activation by a multiply bonded silicon compound under ambient conditions (vide infra). Iminodisilenes 210 and 211 show increased stability when compared to 209 (m.p. 129  C; gradually decomposes at room temperature in benzene or n-hexane solution); disilene 211 is stable in the solid state (m.p. 161  C) as well as in benzene solution at room temperature.391 Moreover, disilene 210 shows remarkably stability both in the solid state (m.p. 211  C) and in solution (benzene, toluene, n-hexane, THF), even at 90  C. Additionally, disilene 210, which exhibits dynamic equilibrium between the (E/Z)-isomers [210-(E) and 210-(Z)], was found to undergo an irreversible rearrangement upon heating at 115  C for 4 days to form the A2Si]SiB2 type disilene 212.

Fig. 19 Selected examples of N-substituted disilenes.

Low-Valent Silicon Compounds

tBu

2MeSi

BR2 Si

tBu

SiMetBu2 or B

O 213a

Ar

N N B

O

O O

Si

Si Mg

N B N

SiiPrDsi2

Dsi = CH(SiMe3)2 BR2 = B

213b

Ar Ar

Si

H

2Me Si

BR2 = B

Si

Si

O

BR2

Dsi2iPrSi

Ar

O

O Ar = Dipp 216

or B 214a

Me3Si Me3Si

Si Si

Ph Si

Dipp

Si B N N

Dipp

O

Dipp = 2,6-iPr2C6H3 215

214b

R R SiMe3 SiMe3

217a: R = SiMe3 217b: R =

Ph

Dipp

O

B

R

Dipp

N N B

B

35

Si

B Si

N

Tip

Ad R = SiMe3 Ad = 1-adamantyl Tip = 2,4,6-iPr3C6H2 218

Fig. 20 Selected examples of B-substituted disilenes.

Boryl ligands exhibit both p-acceptor and s-donor character due to the vacant p orbital on the B center and the comparatively low electronegativity (2.04, Pauling). In 2008, the Sekiguchi group reported the first boryl-substituted disilenes 213a-213b, obtained by the reactions of disilenyllithium (tBu2MeSi)2Si]Si(SiMetBu2)Li with boranes (Fig. 20).377 While the boryl-substituted disilene is also expected to exhibit p-conjugation between the Si]Si bond and the vacant 2p orbital on the B atom, disilenes 213a-213b have no p-conjugation due to the almost perpendicular orientation of the boryl group to the Si]Si bond plane, arising from steric hindrance of the bulky substituents. Additionally, boryl-substituted disilenes 214a, showing p-conjugation between the Si]Si bond and boryl group, was synthesized by the same group, whereas the catecholboryldisilene 214b shows no p-conjugation.378,379 Cui and co-workers reported the synthesis of a boryl-substituted disilene 215, which is obtained by the reduction of dibromosilane {(DAB)B}Si(Ph)Br2 [DAB ¼ (DippNCH)2] with KC8.392 Additionally, the same group isolated the 1-magnesium2,3-disilacyclopropene [(boryl)Si]2Mg(THF)3 216 with an Si]Si double bond including Mg–Si–Si three-membered ring, by using boryl ligands.393 Additionally, dialkyl(boryl)disilenes 217a and 217b were prepared by the selective desilylation-borylation of the silyl-substituted disilene 187a.394 As can be seen, various isolable disilenes containing either p-donor groups (amino, imino, etc.) or p-acceptor substituents (boryl, anthryl, etc.) have been reported, however the examples of push–pull disilenes containing both a p-donor and a p-acceptor in one molecule remain rare. The Iwamoto group reported the first synthesis of a push–pull disilene, 1-amino-2-boryldisilene 218.395,396 The structural features of 218 indicated a significant contribution of canonical structures which contain both N+]Si and Si]B− bonds. In 2010, Scheschkewitz and co-workers reported the first phosphino disilenes 219a by the reaction of disilenide Tip2Si]Si(Tip) Li with Pdchloro phosphines (Fig. 21).397 When the phosphino disilene (219a; R¼Ph) was heated to 140  C in the solid state, a 1-phospha-2,3-disilaindane was formed which can be explained by insertion of the transient silylene into one of the ortho–CH bonds of the phenyl substituents. In the reactions of disilenide Tip2Si]Si(Tip)Li with ClP(NR2)2, either the amino(phosphino) disilene Tip2Si]Si(Tip)P(NR0 2)2 (219b; R0 ¼ iPr) or the isomeric phosphasilenes Tip2(R0 2N)Si–Si(Tip)]P(NR0 2) (R ¼ Me, Et), were obtained depending on the steric hindrance of phosphino ligands.398 Disilenes (219b; R ¼ Me, Et) are intermediates for the phosphasilenes formed via the 1,3-migration of an amino group from phosphorus to the remote silicon atom. Izod and co-workers reported the fully phosphide-substituted disilene 220 {(Mes)2P}2Si]Si{P(Mes)2}2 which obtained by the reduction of a simple Si(IV) precursor SiBr4 with 4 equivalents of lithium phosphanide Li[P(Mes)2].399 Additionally, a series of phosphino disilenes 221a-221d with different coordination numbers around the silicon atoms were synthesized.400 Amidinate-stabilized silylsilylene ([PhC(NtBu)2][Si(SiMe3)3]Si:) (23c) was utilized as a precursor and treated with the corresponding chlorophosphines to afford unsymmetrical disilenes 221a-221d via elimination of Me3SiCl. Only a limited number of examples of disilenes with strong p-accepting groups such as cyano, carbonyl, or nitro groups have been reported, due to the lack of suitable synthetic methodology. In 2012, Sekiguchi and co-workers reported the 1,2-dicyanodisilene 222, the first example of a disilene with a p-accepting group, obtained by the reaction of disilyne (Dsi2iPrSi) Si^Si(SiiPrDsi2) with C^N–R (R ¼ tBu or CMe2CHt2Bu).401 Below −30  C, this reaction affords disilyne–isocyanide adducts [(RNC)(Dsi2iPrSi)SidSi(SiiPrDsi2)(CNR)] (R]tBu or CMe2CH2tBu), which undergo thermal decomposition at room temperature to form the 1,2ddicyanodisilene 222. Disilene 222 shows a slightly pyramidalized geometry with an Si]Si double bond of typical length. Additionally, the first stannyl-substituted disilenes 223a-223b obtained the reaction of disilenide Tip2Si]Si(Tip)Li with chlorostannanes were reported by the Scheschkewitz group.402

36

Low-Valent Silicon Compounds

Tip

(Mes)2P

PR2

Si

Si

Si

Tip

Tip

Dsi2iPrSi Si

Si

tBu

PR2 Si

R’’

R’

221a: PR2 = R’ = PCy2, R’’ = SiMe3 221b: PR2 = PCy2, R’ = R’’ = SiMe3 221c: PR2 = R’ = R’’ = PiPr2 221d: PR2 = PtBu2, R’ = R’’ = SiMe3

Tip Si

Si

Tip

SiiPrDsi2

N

P(Mes)2

Tip

Si

N

Ph

Mes = 2,4,6-Me3C6H2 220

CN

NC

Si

(Mes)2P

219a: R = Ph, iPr, cyclohexyl, tBu 219b: PR2 = P(NR’2)2, R’ = Me, Et, iPr

tBu

P(Mes)2

SnR2R’

223a: R = R’ = Me, Bu, or Ph 223b: R = tBu, R’ = Cl

222

Fig. 21 Selected examples of heteroatom-substituted disilenes.

10.01.3.6 Small molecule activation with disilenes Since the discovery of West’s disilene Mes2Si]SiMes2 in 1981, the chemistry of disilenes has developed significantly and a number of disilenes have been reported along with the fascinating properties. Over the last decade, disilenes have again become a hot topic due to newly discovered reactivity towards small molecules such as H2, CO, and alkenes. A high energy p-HOMO and low-lying p -LUMO in disilenes mimic the electronic situation observed in many transition-metal compounds. The next challenge for disilene chemistry is the development of catalytic reactions in which the Si]Si double bond is regenerated. In the following sections, the stoichiometric reactivity of classic small molecules (e.g., H2, NH3, CO, CO2, P4) with disilenes is discussed.

10.01.3.6.1

Activation of H2

Only a few examples of H2 activation with multiply bonded silicon compounds have been reported and the first example was achieved in 2017 by our group.390 Imino(silyl)disilene 209, bearing a highly trans-bent and twisted structure and the significantly elongated Si]Si double bond, is capable of activating H2 under mild conditions (1 atm, RT) to form 1,2-disilane 224 (Scheme 26). Associated computational studies revealed that imino(silyl)disilene 209 possesses a very weak Si]Si double bond described as (Me3Si)3Si

Si(SiMe3)3 Si

Si

NHIt-Bu

NHIt-Bu

H2 (1 atm)

H

NHIt-Bu (Me3Si)3Si

Si

n-hexane, RT

SiMetBu2 Si NHIt-Bu

NHIt-Bu

H2 (1 atm)

Si

NHIt-Bu =

Si

toluene, RT

Si

H

NHIt-Bu SiMetBu2

225

B Si Tip

Ad R = SiMe3 Ad = 1-adamantyl Tip = 2,4,6-iPr3C6H2 218 Scheme 26 H2 activation by disilenes.

R R 2 H2 (1 atm) C6D6, RT

H

Si

H Si

Ad 226

Tip

H

N

N N tBu

H

NHIt-Bu 2Me Si

tBu

210

R R

NHIt-Bu Si(SiMe3)3

224

2Me Si

Si

Si

H

209

tBu

tBu

+

1/2

B

H H

B

227

Low-Valent Silicon Compounds

37

donor-acceptor bond. Likewise, imino(silyl)disilene 210 can also react with H2 under ambient conditions to yield the corresponding disilane 225.391 Another example of H2 cleavage by a disilene was reported by the Iwamoto group.395 The push-pull disilene, 1-amino-2-boryldisilene 218, activates two molecules of H2 at ambient temperature to give the trihydridodisilane 226 and the dimer of 9-borabicyclo[3.3.1]nonane [(9-BBN)2] (227) via hydrogenation of the Si]Si bond together with cleavage of the SidB bond.395,396

10.01.3.6.2

Activation of NH3

In 2001, Weidenbruch and co-workers reported the first example of NH3 activation by a disilene. Whilst Tip substituted disilene Tip2Si]SiTip2 shows no reactivity towards ammonia, the two Si]Si double bonds in tetrasilabuta-1,3-diene 228 activate NH3 to afford the six-membered ring product 229 (Scheme 27).403 This suggested increased reactivity of the conjugated double bonds. Similarly, the Scheschkewitz group achieved NH3 activation with disilene Tip2Si]SiTip(TMOP) (230) in which the reactivity of Si]Si bond is enhanced by substituting one ligand with a TMOP moiety.404 When disilene 230 was treated with NH3, a regioselective 1,2-addition occurred to yield the corresponding hydroamination product 231. Imino(silyl)disilene 209 also can split the NdH bond of ammonia. The 1,2-addition reaction of disilene 209 with NH3 occurs even at low temperature (−78  C) to form the trans-1,2-adduct 232.405 A theoretical study confirmed a much smaller activation energy for the addition of NH3 (7.7 kcal mol−1) compared to that of H2 (15.6 kcal mol−1). On the other hand, when disilene 209 is treated with NH3 at room temperature, the oxidative addition product 233 of the corresponding imino(silyl)silylene (NItBu){(Me3Si)3Si}Si: is obtained. Tip

Tip Si Tip

Si

Si

Si

Tip

Tip Tip

Tip

2 NH3

H

Tip

Si

Si

H2N

228

Si Si Tip

HNH

229

Tip

Tip

SiTip2

Tip2Si

RT

TMOP

Tip

Tip

NH3

Tip

RT

Tip = 2,4,6-iPr3C6H2 TMOP = 2,4,6-OMe3C6H2 230

Si

Si

H

NH2

TMOP

231

NH3

NHIt-Bu (Me3Si)3Si

–78 °C (16 h) to RT (Me3Si)3Si

NH2 Si

Si

H

Si(SiMe3)3 Si

H

NHIt-Bu Si(SiMe3)3

232

Si

tBu

NHIt-Bu

NHIt-Bu 209

2 NH3 –78 °C (10 min) to RT

(Me3Si)3Si

NH2 Si

2 NHIt-Bu

H

NHIt-Bu =

N

N N tBu

233 Scheme 27 Activation of NH3 by disilenes.

10.01.3.6.3

CdO bond activation

As discussed in the Section 10.01.2.5.3, stoichiometric and catalytic CO2 reduction by earth abundant elements such as silicon has recently attracted much attention. In the reaction of diphenyl(disilyl)disilene 234 with CO2, a [2 + 2] cycloaddition reaction occurs at ambient conditions to furnish the CO2 addition product 235 (Scheme 28).406 Similarly, the imino(silyl)disilene 209 activates CO2 at low temperature (−78  C) to form the [2 + 2] cycloaddition product 236.405 Furthermore, when the same reaction is performed at room temperature, a mixture of products, including the five-membered silacycle 237, is obtained. Driess and co-workers reported that disilene 208 activates both CO2 and O2 at room temperature to yield the same cyclic disiloxane 238, which was formed by dimerization of an intermediary silanone.388

38

Low-Valent Silicon Compounds

Scheme 28 Activation of CO2 by disilenes.

Carbon monoxide (CO) activation by low-valent main-group species has been a recent subject of renewed interest.274,407 Pioneering work in the realm of CO activation by multiple bonds of heavier Group 14 was reported by the Scheschkewitz group in 2015. The reductive coupling of CO was observed in the reaction of a Tip substituted disilenide 239 with CO to give compound 240 (Scheme 29).408 The Si centers of the Si]Si double bond in disilenide 239 act in a cooperative manner in the reduction of CO. A plausible mechanism involves the formation of the intermediate 244 in which two CO units form a ketenylidene moiety, followed by a ligand metathesis to form 2 equivalents of silylenes 245 and ethynolate 246 (Scheme 30). A subsequent [1 + 2] cycloaddition reaction of the silylene 245 to the C^C triple bond and oxidative addition of another molecule of silylene 245 to the CdOLi bond then yields the final product 240. The Scheschkewitz group also reported direct carbonylation with CO under ambient condition (1 atm, room temperature) by using cyclotrisilene 96, which affords bis-silene 241.409 The formation of bis-silene 241 can be explained by the generation of a transient bicyclobutanone. The monomer 242 of bicyclobutanone was trapped using Et2O
B (C6F5)3.410 Additionally, the Lewis base adduct 243 was synthesized by the reaction of CO with the NHC-stabilized disilenyl silylene 98 generated in solution of 96 and 97.

39

Low-Valent Silicon Compounds

Scheme 29 Activation of CO by disilenes.

2 239

4 CO

Si 2

Tip

O

Si

Tip

Li

2

Tip

245

Tip C

Tip

+

C

Tip

O ketenylidene 244

Tip

Si

Tip

LiO

C

C

Si

O O

Tip Si

Si

Si O

O O Li

Si

Si O

Tip

Li (dme)

C

240

C

Tip Tip Tip

OLi

ethynolate 246 Scheme 30 Plausible pathway for the formation of CO coupling product 240.

10.01.3.6.4

CdC bond activation

Disilenes are well known to undergo cycloadditions with unsaturated CdC bonds.411,412 The putative reaction mechanism for the reactions of disilenes with terminal alkenes (H2C]CR2) and alkynes (HC^CR) is generally described as a radical pathway.413 Baines and co-workers demonstrated that the reaction of tetramesityldisilene Mes2Si]SiMes2 (168) with trans-d-styrene H(D)C] C(Ph)H (247) gave a diastereomeric mixture of cis- and trans-[2 + 2] cycloaddition products 249 in a ratio of 7:3 (Scheme 31).414,415 This result indicated a bond rotation of the intermediate 248 which led to the cis isomer.

40

Low-Valent Silicon Compounds Ph Mes

D

Mes

247

Mes Mes

Si

Si

Si

D

Mes

Mes

Si

Mes Mes

Mes Mes

Ph

168

Si Si

D

Mes Mes Ph

249

248

Scheme 31 [2 + 2] cycloaddition reaction of disilene 168 and trans-d-styrene 247.

As with typical disilenes, a formal [2 + 2] cycloaddition reaction of 230 and 208 with alkynes occurs to form the corresponding cycloaddition products 250 and 252, respectively (Scheme 32).388,404 By using a similar unsymmetrically-substituted disilene Tip2Si]Si(Tip)Ph, a mechanistic investigation for the addition of a cyclopropyl-substituted alkyne was conducted.416 Additionally, disilene 208 can activate ethylene under ambient conditions to give compound 251. Unsaturated compounds such as alkenes and alkynes are utilized as prototypical trapping reagents for reactive intermediates. In the reaction of imino(silyl)disilene 210 with diphenylacetylene at 90  C, the [1 + 2] cycloaddition product 253 was obtained (Scheme 33).391 This result suggested the generation of the monomeric silylene. Similarly, dibromodisilene 170 reacts with phenylacetylene, diphenylacetylene, and cyclohexene to form the corresponding [1 + 2] cycloaddition products 254 and 255.319,417

Tip

Tip Si Si Tip

H

Ph

benzene, RT

TMOP

Tip = 2,4,6-iPr3C6H2 TMOP = 2,4,6-OMe3C6H2

Tip Tip

Tip Si Si TMOP

Ph

H 250

230 R N

Ph H2C

Ph

R

R

N

N

N

Si

Si

N

CH2

R = tBu

R N

Ph NR

n-hexane, RT Ph

251

Ph 208

Si

NR

R

R

Si

Ph

Ph R N

Ph

Et2O, RT

Ph Si

Si

NR

Ph NR

252 Scheme 32 [2 + 2] cycloaddition reactions of disilenes (230, 208) and unsaturated CdC bonds.

R N

Low-Valent Silicon Compounds

NHIt-Bu

NHIt-Bu

1/2

Si tBu

Ph

Si

2MeSi

tBu Me Si 2

Ph

Br

Br

Si R

254

Me3Si

Me3Si Si

benzene, RT, 24 h

Br

SiMe3 R R

Bbt

170

N

Ph

Br

Bbt

N

N

tBu

Bbt

R

benzene, RT R = H, 3 h R = Ph, 3 d

Si Si

1/2

Ph

NHIt-Bu =

253

210

Bbt

tBu

Si

C6D6, 90 °C, 20 min

SiMetBu2

Ph

NHIt-Bu

Ph

41

R SiMe3

Bbt: R = SiMe3

255

Scheme 33 [1 + 2] cycloaddition reactions of disilenes.

10.01.3.6.5

Activation of P4

Earlier studies by West and co-workers revealed that the disilene 168 activated white phosphorus P4 at 40  C to yield the Si2P2 heterobicyclo[1.1.0]butane 256 (Scheme 34).418 Additionally, disilene 79, which is in equilibrium with the corresponding silylene in solution, reacts with P4 to form the silicon-phosphorus cage 257 via double insertion of the silylene into the phosphorus tetrahedron.298 Imino(silyl)disilene 210 can also activate P4 under mild conditions to form compound 258, which suggested the insertion of the P4 tetrahedron into the Si]Si double bond.391 When either excess (4 eq.) or 0.5 equivalents of P4 was added to the imino(silyl)disilene 210, selective formation of 258 was observed.

P Mes

Mes Si

2

P

Si

Mes

P

P

P (1 eq.)

2 Mes Si

toluene 40°C

Mes

Mes

168 P Cp*

(Me3Si)2N Si

P

Si

Cp*

P

P (1 eq.)

*Cp P

toluene RT

N(SiMe3)2

(Me3Si)2N

79 P NHIt-Bu

NHIt-Bu Si

tBu

Si

2Me Si

SiMetBu2 210

Scheme 34 Activation of P4 by disilenes.

P

P

P (1 eq.) toluene RT

Si

P

P

Mes

Cp* N(SiMe3)2

257

tBu

P Si

Mes

Si

2Me Si

NHIt-Bu

256

P

P tBu

Si

SiMetBu2

P P

P 258

Si NHIt-Bu

NHIt-Bu =

N

N N tBu

42

Low-Valent Silicon Compounds

10.01.3.7 29

29

Si NMR chemical shifts of disilenes

Si NMR Chemical shifts for the Si centers in selected disilenes are shown in Table 2.

Table 2

29

Si chemical shifts (in C6D6) for disilenes. 29

˚) Si]Si bond lengths (A

References

Mes2Si]SiMes2 (168) Tip2Si]SiTip2 (169) Bbt(Br)Si]Si(Br)Bbt (170) Tbb(Br)Si]Si(Br)Tbb (171) EMind(Br)Si]Si(Br)EMind (93a) Eind(Br)Si]Si(Br)Eind (93b) {H2C(Me3Si)2C}2Si]Si(tBu)(Ar) (Ar ¼ 1-naphthyl) (172a) {H2C(Me3Si)2C}2Si]Si(tBu)(Ar) (Ar ¼ 9-phenanthryl) (172b) {H2C(Me3Si)2C}2Si]Si(tBu)(Ar) (Ar ¼ 9-anthracenyl) (172c) Eind(Ar)Si]Si(Ar)Eind (Ar ¼ 1-naphthyl) (173a) Eind(Ar)Si]Si(Ar)Eind (Ar ¼ 2-naphthyl) (173b)

+63.6a +53.4 +79.4 +84.1 +74.6 +73.2 +97.7 (Si]Si(tBu)Ar) +131.8 (Si]Si(tBu)Ar) +99.6 (Si]Si(tBu)Ar) +132.4 (Si]Si(tBu)Ar) +85.6b (Si]Si(tBu)Ar) +123.2b (Si]Si(tBu)Ar) +55.0c +60.2,c,d +64.8c,d

2.143(2) 2.144 2.2264(8) 2.2220(16) – 2.1795(9) 2.1943(14)

300,358 317 318 150 232 232 322

2.209(2)

322

2.1754(12)

322 327 328

Eind(Ar)Si]Si(Ar)Eind (Ar ¼ 2-fluorenyl) (173c)

+60.4c

Eind(Ar)Si]Si(Ar)Eind (Ar ¼ thiophen-2-yl) (173d) Eind(Ar)Si]Si(Ar)Eind (Ar ¼ 2,20 -bithiophen-5-yl) (173e) Eind(Ar)Si]Si(Eind)Ar (Ar ¼ 1-pyrenyl) (174) 175 (n ¼ 0) 175 (n ¼ 1) Tip2Si]Si(Tip)(SiMe2Cl) (176a)

+51.5 +51.7 +58.7 +63.2 – +39.9 (]SiTip) +103.0 (]SiTip2) +35.5 (]SiTip) +109.1 (]SiTip2) +48.9 (]SiTip) +100.2 (]SiTip2) +47.2 (]SiTip) +105.7 (]SiTip2) +71.2 (]SiTip) +55.1 (]SiTip2) +70.9 (]SiTip) +56.1 (]SiTip2) +70.8 (]SiTip) +56.3 (]SiTip2) +71.8 (]SiTip) +55.2 (]SiTip2) +70.6 (]SiTip) +55.7 (]SiTip2) +69.3 (]SiTip) +57.3 (]SiTip2) +69.2 (]SiTip) +57.4 (]SiTip2) +69.7 (]SiTip) +57.9 (]SiTip2) +70.7 (]SiTip) +56.8 (]SiTip2) +42.5 +112.6, +95.6 +122.3, +107.3, +106.9, +93.3d +60.4a (]SiMes) +61.6a (]SiEMind) +72.6 +110.3 +105.0

2.1688(7) 2.1623(18)d 2.1667(12)d 2.149(5)d 2.1531(13)d 2.1712(11) 2.1584(9) 2.1718(6) 2.1593(16) 2.156(2) – 2.1723(8)–2.1900(8)e

333

–

334

2.1860(6)

334

2.1563(4)

335

–

335

–

335

2.1754(11)

336,337

2.147(1)

336

2.1735(4)

336

2.1707(5)

336

–

336

2.1674(8)

337

2.118(1) 2.170(1) 2.175(1) 2.1283(6)

341 343 343 344

2.1733(15) 2.1920(9) 2.1886(5)

345 349 349

Si chemical shifts (ppm)

Tip2Si]Si(Tip)(SiPh2Cl) (176b) Tip2Si]Si(Tip)(SiMes2Cl) (176c) Tip2Si]Si(Tip)(SiTip2Cl) (176d) Tip2Si]Si(Tip)(4-SMeC6H4) (177: n ¼ 1) Tip2Si]Si(Tip){4-(4-SMeC6H4)C6H4} (177: n ¼ 2) Tip2Si]Si(Tip)[4-{4-(4-SMeC6H4)C6H4}]C6H4 (177: n ¼ 3) Tip2Si]Si(Tip)(C6H4X) (X ¼ H) (178) Tip2Si]Si(Tip)(C6H4X) (X ¼ F) (178) Tip2Si]Si(Tip)(C6H4X) (X ¼ Cl) (178) Tip2Si]Si(Tip)(C6H4X) (X ¼ Br) (178) Tip2Si]Si(Tip)(C6H4X) (X ¼ I) (178) Tip2Si]Si(Tip)–C6H4–(Tip)Si]SiTip2 (179) Si3Tip4 (96) 180 (R ¼ H) 180 (R ¼ Me) {H2C(Me3Si)2C}2Si(Mes)Si]Si(EMind) (181) Tip{Fe(C5H5)2}Si]Si{Fe(C5H5)2}Tip (182) 183a 183b

328 331 331 332 324 324 333

Low-Valent Silicon Compounds

Table 2

43

(Continued) 29

˚) Si]Si bond lengths (A

References

+92.8 +86.5 +44.6 +42.6 +131.4 +88.2 +115.6 +144.5a +142.1 +154.5 +155.5a +79.4 (]SiTip) +94.4, (]SiTip(SiMe3)) +74.1f (]SiTip), +95.4f (]SiTip(SiMe3)) 74.1, 79.8, 80.2, 85.4, 92.0, 93.9, 94.4, 95.4d

2.1676(6) 2.1666(10) 2.202(2) 2.1871(10) 2.1762(5) 2.1525(7) 2.223(1) 2.228(2) 2.202(1) 2.251(1) 2.2598(18) 2.1650(12), 2.1755(12) (190aapg); 2.1592(7), 2.1617(7) (190asch)

350 351 352 352 353 353 354 355 355 355 356 361

2.1736(10), 2.1688(10)

361

+133.4 +143.4, +146.7 +9.3 (]Si(central)) +210.2 (]Si(terminal)) +217.0 +216.6 +103.8

2.1665(14) 2.2020(6) 2.1980(16), 2.2168(16)

362 362 359

2.4716(11) 2.4873(10) 2.5822(11), 2.5251(10)i 2.581(2) –

363 364 366

– 2.2061(8)

56 368

2.1926(6) 2.1683(5) 2.1647(10)

368 171 378,386

2.1949(7)

386

2.1596(17)

386

2.1790(14)

378,386

–

387

2.623(1) 2.3124(7) 2.2844(7)

388 390 391

+71.4

–

391

+72.5b +30.4 (]SiN2) −176.1 (]SiSi2) +111.5 (]SidSi) +127.0 (]SidB) +111.7 (]SidSi) +127.3 (]SidB) +121.7a (]SiBR2) +150.5a (]SiH) +84.3 (]SiBR2) +123.4 (]SiH)

2.2534(7) 2.219(4)

391 391

–

377

2.192(2)

377

2.1838(12)

379

2.1634(12)

379

Si chemical shifts (ppm)

184 185 (RC^C)BbtSi]SiBbt(C^CR) (R ¼ SiMe3) (186a) (RC^C)BbtSi]SiBbt(C^CR) (R ¼ Ph) (186b) {]Si(R)C(SiMe3)2CH2}2 (R ¼ SiMe3) (187a) {]Si(R)C(SiMe3)2CH2}2 (R ¼ 9-anthracenyl) (187b) (H2Trp C)(Trp)Si]Si(Trp)(CTrp H2) (188) (iPr2MeSi)2Si]Si(SiMeiPr2)2 (189a) (tBuMe2Si)2Si]Si(SiMet2Bu)2 (189b) (iPr3Si)2Si]Si(SiiPr3)2 (189c) (tBu2MeSi)2Si]Si(SiMetBu2)2 (189d) Me2Si(Tip)Si]Si(Tip)–(Tip)Si]Si(Tip)(R) (R ¼ SiMe3) (190a)

Me2Si(Tip)Si]Si(Tip)–(Tip)Si]Si(Tip)(R) (R ¼ SiEt3) (190b) 191 192 193 198a 198b 199 200 201 203 204 205 (Me3Si)2N(Cp )Si]Si(Cp )N(SiMe3)2 (79) Dsii2PrSi(H)Si]Si(NR2)SiiPrDsi2 (R ¼ NEt2) (206a) Dsii2PrSi(H)Si]Si(NR2)SiiPrDsi2 (R ¼ NHtBu) (206b) Dsii2PrSi(H)Si]Si(NR2)SiiPrDsi2 (R ¼ pyrrolidyl) (206c) Dsii2PrSi(H)Si]Si(NR2)SiiPrDsi2 (R ¼ NPh2) (206d) Tip2Si]Si(Tip)NMe2 (207) 208 (NItBu){(Me3Si)3Si}Si]Si{Si(SiMe3)3}(NItBu) (209-(Z)) (NItBu)(tBu2MeSi)Si]Si(SiMetBu2)(NItBu) (210-(Z)) (NItBu)(tBu2MeSi)Si]Si(NItBu)(SiMetBu2) (210-(E)): (NItBu)(tBu3Si)Si]Si(NItBu)(SitBu3) (211-(E)) (NItBu)2Si]Si(SiMetBu2)2 (212) (tBu2MeSi)2Si]Si(SiMetBu2)(BR2) (BR2 ¼ Bpin) (213a) (tBu2MeSi)2Si]Si(SiMetBu2)(BR2) (BR2 ¼ Bcat) (213b) Dsii2PrSi(H)Si]Si(SiiPrDsi2)(BR2) (BR2 ¼ 9-bora[3.3.1]nonan-9-yl) (214a) Dsii2PrSi(H)Si]Si(SiiPrDsi2)(BR2) (BR2 ¼ cat) (214b)

+126.4 +48.7 (Si]SidCl) +104.1 (Si]SidCl) −50.7, −59.0 +136.7 ((tBuMe2Si)2Si]Si) +162.2 ((tBuMe2Si)2Si]Si) +150.2 – −39.3 (]SiH) +170.5 (]SiNR2) −47.2 (]SiH) +136.5 (]SiNR2) −34.1 (]SiH) +150.9 (]SiNR2) +66.6 (]SiH) +136.4 (]SiNR2) +95.5 (]SiTip) +26.2 (]SiTip2) −36.5 +72.0a +67.4

367 367

(Continued )

44

Table 2

Low-Valent Silicon Compounds

(Continued) 29

˚) Si]Si bond lengths (A

References

+200.0 +204.1b +128.7 (]SiBBN) +187.2 (]SiSiMe3) +166.2 −33.0 (]Si(Tip)) +142.4 (]Si(alkyl)(amino)) +54.4 (]SiP) +96.5 (]SiTip2) +53.2 (]SiP) +96.6 (]SiTip2) +52.5 (]SiP) +95.4 (]SiTip2) +53.0 (]SiP) +98.8 (]SiTip2) +71.1 (]SiP) +90.7 (]SiTip2) +111.7c +72.9 (]Si(PCy2)(SiMe3)) −128.4 (]Si(PCy2)L) +72.6 (]Si(SiMe3)2) −128.8 (]Si(PCy2)L) 68.9–66.7 (]Si(PiPr2)2) −94.5 (]Si(PiPr2)L) +59.4 (]Si(SiMe3)2) −148.1 (]Si(PtBu2)L) – +103.4 (]SiTip2) +38.9 (]SiTipSn) +105.3 (]SiTip2) +36.8 (]SiTipSn) +113.3 (]SiTip2) +30.4 (]SiTipSn) +114.5 (]SiTip2) +40.9 (]SiTipSn)

2.170(3) 2.223(17) 2.1990(8)

392 393 394

2.2114(5) 2.2146(6)

394 395

–

397

–

397

2.1542(11)

397

–

397

2.1968(9)

398

2.1901(12) 2.2720(8)

399 400

2.2860(9)

400

2.2901(4)

400

2.2927(3)

400

2.213(2) 2.1618(8)

401 402

–

402

–

402

2.1882(12)

402

Si chemical shifts (ppm)

{(DAB)B}(Ph)Si]Si(Ph){B(DAB)} (215) {(DAB)B}Si]Si{B(DAB)}
Mg(thf )3 (216) 217a 217b 218 Tip2Si]Si(Tip)(PR2) (R ¼ Ph) (219a) Tip2Si]Si(Tip)(PR2) (R ¼ iPr) (219a) Tip2Si]Si(Tip)(PR2) (R ¼ cyclohexyl) (219a) Tip2Si]Si(Tip)(PR2) (R ¼ tBu) (219a) Tip2Si]Si(Tip){P(NR0 2)2} (R0 ¼ iPr) (219b) {(Mes)2P}2Si]Si{P(Mes)2}2 (220) 221a 221b 221c 221d Dsii2PrSi(NC)Si]Si(CN)SiiPrDsi2 (222) Tip2Si]Si(Tip)(SnR3) (R ¼ Me) (223a) Tip2Si]Si(Tip)(SnR3) (R ¼ Bu) (223a) Tip2Si]Si(Tip)(SnR3) (R ¼ Ph) (223a) Tip2Si]Si(Tip)(SntBu2Cl) (223b) a

Toluene-d8. THF-d8. c Solid-state NMR spectroscopy. d A mixture of two rotational isomers. e Four independent molecules in the asymmetric unit. f Minor rotational isomer. g Antiperiplanar conformation. h Synclinal conformation. i Two different conformations. b

10.01.4 Conclusion The chemistry of low-valent main-group compounds has developed significantly over the past decades. Since Power’s groundbreaking discovery of dihydrogen splitting by the digermyne (RGe^GeR), activation of small molecules such as H2, alkenes and alkynes by main-group compounds has been studied actively while the cleavage of such inert s-bonds was considered to be limited to transition-metals for a long time. Low-valent silicon compounds such as silylenes (R2Si:) and disilenes (R2Si]SiR2) have been also found to activate such small molecules. In this chapter, we have summarized recent developments in silylene and disilene chemistry along with their reactivity towards small molecules such as H2, NH3, CO, CO2, alkenes, alkynes, and P4. These highly reactive silicon compounds may offer environmentally friendly and cost-effective alternative to the current transition-metal mediated reactions.

Low-Valent Silicon Compounds

45

Acknowledgment This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 754462 (Fellowship S.F.), as well as the WACKER Chemie AG.

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Low-Valent Silicon Compounds 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199.

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Low-Valent Silicon Compounds 272. 273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295. 296. 297. 298. 299. 300. 301. 302. 303. 304. 305. 306. 307. 308. 309. 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320. 321. 322. 323. 324. 325. 326. 327. 328. 329. 330. 331. 332. 333. 334. 335. 336. 337. 338. 339. 340. 341. 342.

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Büttner, T.; Weisshaar, K.; Willmes, P.; Huch, V.; Morgenstern, B.; Hempelmann, R.; Scheschkewitz, D. Z. Anorg. Allg. Chem. 2021, 647. https://doi.org/10.1002/ zaac.202100161. Jeck, J.; Bejan, I.; White, A. J. P.; Nied, D.; Breher, F.; Scheschkewitz, D. J. Am. Chem. Soc. 2010, 132, 17306–17315. Bejan, I.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2007, 46, 5783–5786. Majumdar, M.; Huch, V.; Bejan, I.; Meltzer, A.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2013, 52, 3516–3520. Majumdar, M.; Bejan, I.; Huch, V.; White, A. J. P.; Whittell, G. R.; Schaefer, A.; Manners, I.; Scheschkewitz, D. Chem. A Eur. J. 2014, 20, 9225–9229. Obeid, N. M.; Klemmer, L.; Maus, D.; Zimmer, M.; Jeck, J.; Bejan, L.; White, A. J. P.; Huch, V.; Jung, G.; Scheschkewitz, D. Dalton Trans. 2017, 46, 8839–8848. Leszczynska, K.; Abersfelder, K.; Mix, A.; Neumann, B.; Stammler, H.-G.; Cowley, M. J.; Jutzi, P.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2012, 51, 6785–6788. Ohmori, Y.; Ichinohe, M.; Sekiguchi, A.; Cowley, M. J.; Huch, V.; Scheschkewitz, D. Organometallics 2013, 32, 1591–1594.

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343. Yildiz, C. B.; Leszczynska, K. I.; González-Gallardo, S.; Zimmer, M.; Azizoglu, A.; Biskup, T.; Kay, C. W. M.; Huch, V.; Rzepa, H. S.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2020, 59, 15087–15092. 344. Kosai, T.; Nishimura, S.; Hayakawa, N.; Matsuo, T.; Iwamoto, T. Chem. Lett. 2019, 48, 1168–1170. 345. Sasamori, T.; Yuasa, A.; Hosoi, Y.; Furukawa, Y.; Tokitoh, N. Organometallics 2008, 27, 3325–3327. 346. Yuasa, A.; Sasamori, T.; Hosoi, Y.; Furukawa, Y.; Tokitoh, N. Bull. Chem. Soc. Jpn. 2009, 82, 793–805. 347. Sasamori, T. Dalton Trans. 2020, 49, 8029–8035. 348. Tokitoh, N.; Yuasa, A.; Sasamori, T. Phosphorus Sulfur Silicon Relat. Elem. 2011, 186, 1217–1219. 349. Ishida, S.; Sugawara, S.; Honda, S.; Iwamoto, T. Chem. Commun. 2020, 56, 15072–15075. 350. Koike, T.; Honda, S.; Ishida, A.; Iwamoto, T. Organometallics 2020, 39, 4149–4152. 351. Honda, S.; Sugawara, R.; Ishida, S.; Iwamoto, T. J. Am. Chem. Soc. 2021, 143, 2649–2653. 352. Sato, T.; Mizuhata, Y.; Tokitoh, N. Chem. Commun. 2010, 46, 4402–4404. 353. Akasaka, N.; Tanaka, K.; Ishida, S.; Iwamoto, T. Inorganics 2018, 6, 21/1–21/15. 354. Yukimoto, M.; Minoura, M. Bull. Chem. Soc. Jpn. 2018, 91, 585–587. 355. Kira, M.; Maruyama, T.; Kabuto, C.; Ebata, K.; Sakurai, H. Angew. Chem. Int. Ed. Engl. 1994, 33, 1489–1491. 356. Sekiguchi, A.; Inoue, S.; Ichinohe, M.; Arai, Y. J. Am. Chem. Soc. 2004, 126, 9626–9629. 357. Molev, G.; Tumanskii, B.; Sheberla, D.; Botoshansky, M.; Bravo-Zhivotovskii, D.; Apeloig, Y. J. Am. Chem. Soc. 2009, 131, 11698–11700. 358. Shepherd, B. D.; Campana, C. F.; West, R. Heteroat. Chem. 1990, 1, 1–7. 359. Uchiyama, K.; Nagendran, S.; Ishida, S.; Iwamoto, T.; Kira, M. J. Am. Chem. Soc. 2007, 129, 10638–10639. 360. Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 2002, 124, 3830–3831. 361. Akasaka, N.; Fujieda, K.; Garoni, E.; Kamada, K.; Matsui, H.; Nakano, M.; Iwamoto, T. Organometallics 2018, 37, 172–175. 362. Yokouchi, Y.; Ishida, S.; Iwamoto, T. Chem. A Eur. J. 2018, 24, 11393–11401. 363. Iwamoto, T.; Abe, T.; Sugimoto, K.; Hashizume, D.; Matsui, H.; Kishi, R.; Nakano, M.; Ishida, S. Angew. Chem. Int. Ed. 2019, 58, 4371–4375. 364. Nukazawa, T.; Kosai, T.; Honda, S.; Ishida, S.; Iwamoto, T. Dalton Trans. 2019, 48, 10874–10880. 365. Fujinami, M.; Seino, J.; Nukazawa, T.; Ishida, S.; Iwamoto, T.; Nakai, H. Chem. Lett. 2019, 48, 961–964. 366. Nukazawa, T.; Iwamoto, T. J. Am. Chem. Soc. 2020, 142, 9920–9924. 367. Nukazawa, T.; Iwamoto, T. Dalton Trans. 2020, 49, 16728–16735. 368. Iwamoto, T.; Furiya, Y.; Kobayashi, H.; Isobe, H.; Kira, M. Organometallics 2010, 29, 1869–1872. 369. Karni, M.; Apeloig, Y. J. Am. Chem. Soc. 1990, 112, 8589–8590. 370. Trinquier, G.; Malrieu, J.-P. J. Am. Chem. Soc. 1987, 109, 5303–5315. 371. Malrieu, J.-P.; Trinquier, G. J. Am. Chem. Soc. 1989, 111, 5916–5921. 372. Trinquier, G. J. J. Am. Chem. Soc. 1990, 112, 1039–1041. 373. Scheschkewitz, D. Angew. Chem. Int. Ed. 2004, 43, 2965–2967. 374. Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A. Organometallics 2004, 23, 3088–3090. 375. Inoue, S.; Ichinohe, M.; Sekiguchi, A. Chem. Lett. 2005, 34, 1564–1565. 376. Ichinohe, M.; Sanuki, K.; Inoue, S.; Sekiguchi, A. Silicon Chem. 2007, 3, 111–116. 377. Inoue, S.; Ichinohe, M.; Sekiguchi, A. Chem. Lett. 2008, 37, 1044–1045. 378. Takeuchi, K.; Ikoshi, M.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2010, 132, 930–931. 379. Takeuchi, K.; Ichinohe, M.; Sekiguchi, A. Organometallics 2011, 30, 2044–2050. 380. Müller, T.; Apeloig, Y. J. Am. Chem. Soc. 2002, 124, 3457–3460. 381. Apeloig, Y.; Müller, T. J. Am. Chem. Soc. 1995, 117, 5363–5364. 382. Apeloig, Y.; Karni, M.; Müller, T. Silylenes and Multiple Bonds to Silicon: Synergism between Theory and Experiment. In Organosilicon Chemistry II; Auner, N., Weis, J., Eds.; VCH: Weinheim, 1996; pp 263–288. Chapter 36. 383. Takahashi, M.; Kira, M.; Sakamoto, K.; Muller, T.; Apeloig, Y. J. Comput. Chem. 2001, 22, 1536–1541. 384. Khan, S.; Sen, S. S.; Roesky, H. W.; Kratzert, D.; Michel, R.; Stalke, D. Inorg. Chem. 2010, 49, 9689–9693. 385. Khan, S.; Michel, R.; Koley, D.; Roesky, H. W.; Stalke, D. Inorg. Chem. 2011, 50, 10878–10883. 386. Takeuchi, K.; Ikoshi, M.; Ichinohe, M.; Sekiguchi, A. J. Organomet. Chem. 2011, 696, 1156–1162. 387. Willmes, P.; Junk, L.; Huch, V.; Yildiz, C. B.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2016, 55, 10913–10917. 388. Kostenko, A.; Driess, M. J. Am. Chem. Soc. 2018, 140, 16962–16966. 389. Yang, M.-C.; Su, M.-D. Dalton Trans. 2020, 49, 12842–12853. 390. Wendel, D.; Szilvási, T.; Jandl, C.; Inoue, S.; Rieger, B. J. Am. Chem. Soc. 2017, 139, 9156–9159. 391. Holzner, R.; Porzelt, A.; Karaca, U. S.; Kiefer, F.; Frisch, P.; Wendel, D.; Holthausen, M. C.; Inoue, S. Dalton Trans. 2021, 50, 8785–8793. 392. Liu, Z.; Zhang, J.; Yang, H.; Cui, C. Organometallics 2020, 39, 4164–4168. 393. Tian, M.; Zhang, J.; Yang, H.; Cui, C. J. Am. Chem. Soc. 2020, 142, 4131–4135. 394. Tanaka, K.; Akasaka, N.; Kosai, T.; Honda, S.; Ushijima, Y.; Ishida, S.; Iwamoto, T. Molecules 2021, 26, 1632. 395. Kosai, T.; Iwamoto, T. J. Am. Chem. Soc. 2017, 139, 18146–18149. 396. Kosai, T.; Iwamoto, T. Chem. A Eur. J. 2018, 24, 7774–7780. 397. Hartmann, M.; Haji-Abdi, A.; Abersfelder, K.; Haycock, P. R.; White, A. J.; Scheschkewitz, D. Dalton Trans. 2010, 39, 9288–9295. 398. Willmes, P.; Cowley, M. J.; Hartmann, M.; Zimmer, M.; Huch, V.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2014, 53, 2216–2220. 399. Izod, K.; Evans, P.; Waddell, P. G. Angew. Chem. Int. Ed. 2017, 56, 5593–5597. 400. Bisai, M.; Das, T.; Vanka, K.; Gonnade, R.; Sen, S. S. Angew. Chem. Int. Ed. 2021. https://doi.org/10.1002/anie.202107847. 401. Takeuchi, K.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2012, 134, 2954–2957. 402. Abersfelder, K.; Nguyen, T.-L.; Scheschkewitz, D. Z. Anorg. Allg. Chem. 2009, 635, 2093–2098. 403. Boomgaarden, S.; Saak, W.; Weidenbruch, M.; Marsmann, H. Z. Anorg. Allg. Chem. 2001, 627, 349–352. 404. Meltzer, A.; Majumdar, M.; White, A. J. P.; Huch, V.; Scheschkewitz, D. Organometallics 2013, 32, 6844–6850. 405. Wendel, D.; Szilvási, T.; Henschel, D.; Altmann, P. J.; Jandl, C.; Inoue, S.; Rieger, B. Angew. Chem. Int. Ed. 2018, 57, 14575–14579. 406. Wiberg, N.; Niedermayer, W.; Polborn, K.; Mayer, P. Chem. A Eur. J. 2002, 8, 2730–2739. 407. Kong, R. Y.; Crimmin, M. R. Dalton Trans. 2020, 49, 16587–16597. 408. Majumdar, M.; Omlor, I.; Yildiz, C. B.; Azizoglu, A.; Huch, V.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2015, 54, 8746–8750. 409. Cowley, M. J.; Ohmori, Y.; Huch, V.; Ichinohe, M.; Sekiguchi, A.; Scheschkewitz, D. Angew. Chem. Int. Ed. 2013, 52, 13247–13250. 410. Cowley, M. J.; Huch, V.; Scheschkewitz, D. Chem. A Eur. J. 2014, 20, 9221–9224. 411. Okazaki, R.; West, R. Chemistry of Stable Disilenes. In Advances in Organometallic Chemistry; Gordon, F., Stone, A., West, R., Eds.; Elsevier: London, 1996; vol. 39; pp 231–273.

Low-Valent Silicon Compounds 412. Kira, M.; Iwamoto, T. Progress in the Chemistry of Stable Disilenes. In Advances in Organometallic Chemistry; West, R., Hill, A. F., Eds.; Elsevier: Oxford, 2006; vol. 54; pp 73–148. 413. Gottschling, S. E.; Milnes, K. K.; Jennings, M. C.; Baines, K. M. Organometallics 2005, 24, 3811–3814. 414. Dixon, C. E.; Baines, K. M. Phosphorus Sulfur Silicon Relat. Elem. 1997, 124, 123–132. 415. Milnes, K. K.; Pavelka, L. C.; Baines, K. M. Chem. Soc. Rev. 2016, 45, 1019–1035. 416. Henry, A. T.; Bourque, J. L.; Vacirca, I.; Scheschkewitz, D.; Baines, K. M. Organometallics 2019, 38, 1622–1626. 417. Han, J. S.; Sasamori, T.; Mizuhata, Y.; Tokitoh, N. Chem. Asian J. 2012, 7, 298–300. 418. Driess, M.; Fanta, A. D.; Powell, D. R.; West, R. Angew. Chem. Int. Ed. Engl. 1989, 28, 1038–1040.

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10.02

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Terrance J Hadlington, Department of Chemistry, Technical University Munich, Munich, Germany © 2022 Elsevier Ltd. All rights reserved.

10.02.1 10.02.2 10.02.2.1 10.02.2.2 10.02.2.3 10.02.3 10.02.3.1 10.02.3.1.1 10.02.3.1.2 10.02.3.1.3 10.02.3.1.4 10.02.3.2 10.02.3.3 10.02.4 10.02.4.1 10.02.4.2 10.02.4.3 10.02.5 10.02.6 References

Introduction Silyl complexes Silyl complexes of V, and Ag Disilane activation Silylborane activation Silylene complexes N-heterocyclic Silylene (NHSi) complexes 4-membered NHSi complexes 5-membered NHSi complexes 6-membered NHSi complexes Further cyclic-Silylene complexes Chelating bis(NHSi) complexes Acyclic silylene complexes Heavier derivatives of p-complexes Disilene, and heteroatomic Silene complexes Silapnictene complexes Disilyne complexes Silicon-transition metal triple bonds (silylidyne complexes) Summary

52 52 53 53 55 55 56 56 62 65 67 68 73 80 80 85 86 86 88 88

10.02.1 Introduction Silicon chemistry is one of the center-points of modern synthesis, spanning from materials chemistry (silica, silicates, silicones, etc.), to numerous catalytic transformations involving activation of Si-X bonds (X ¼ Si, H, B, etc.) for the formation of value-added materials.1 These catalytic transformations have historically inspired the investigation of transition metal-silicon bonds, the first example of which was described by Wilkinson as early as 1956.2 Of course, our fascination with broader molecular silicon chemistry, and indeed related chemistry of all heavier group 14 elements, stems in part from comparisons (and thus, differences) with classical organic chemistry, that is the chemistry of carbon, which is the basis for all known complex life forms.3 Since Wilkinson’s seminal report, well over 1000 structurally characterized examples of transition metal-silyl complexes have been accessed, with a vast number of these reported between editions II and III of the COMC series,4,5 making the previous chapter relating to Silicon-Transition metal bonding largely a detailed summary of the synthesis and structure of such metal-silyl complexes. Extending into further carbon-analogous chemistry, more recent developments have pushed the boundaries of low-valent Silicon-Transition metal chemistry, with complexes akin to long-known carbon derivatives being a central focus. Complexes of silylenes, silylynes, silenes, disilenes, and disilynes are now known, reported almost entirely over the past 15 years, allowing for bonding comparisons with the carbon analogs of these species. Perhaps even more surprising is the now prevalent use of stable silylenes as ligands in catalysis, an area that was all but unknown at the publication of the previous volume. As such, this chapter will only briefly update developments in the chemistry of transition metal-silyl complexes, and more heavily focus on the now well-established domain of low-valent silicon-transition metal chemistry, which encompasses countless novel bonding situations, as well as previously unknown modes of reactivity. As broader synthetic regimes and catalysis will be covered elsewhere in this book, here the direct synthesis, key structural parameters, and reactivity highlights of transition metal-silicon bonds will be discussed, to give the reader a directed overview of the state-of-the-art in this fascinating area of chemistry, and where it may be heading as we move into the future.

10.02.2 Silyl complexes Silyl chemistry, that is complexes bearing the [R3Si]− functional group, is extremely well established, most likely given the importance of these compounds in hydrosilylation catalysis, as intermediates or otherwise. This is perhaps best exemplified by a trio of review articles written by J. Y. Corey dealing (almost) exclusively with transition metal-silyl complexes formed through silane activation, published in 1999 (547 refs.),6 2011 (572 refs.),7 and 2016 (426 refs.).8 At the time of publication of the previous volume of this chapter, well-defined silyl complexes were known for nearly all the transition metals, aside only from V, Ag, and Tc; for the former two elements, silyl complexes are now known, while this is not the case for Tc most likely due to radioactivity of all

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isotopes of this element. It was also known at the time that chelating silyl ligands can be employed to good effect, largely due to the strong trans-influence of the silyl group, which has recently been rather thoroughly summarized by Simon and Breher.9 Generally, an overview of transition metal-silyl complexes published since the previous volume will not be discussed here. A brief update on key aspects of modern metal-silyl chemistry will be given, specifically the synthesis of the first examples of V and Ag silyl complexes, as well as silyl complexes formed through SidSi and SidB bond cleavage, given the importance of these reactions in regards to more recently established catalytic protocols.

10.02.2.1 Silyl complexes of V, and Ag Despite the fervent investigation of d-block metal-silyl complexes, those of V and Ag were only forthcoming in 2008 and 2015, respectively. The V(II) complexes 1 and 2 were synthesized through dealkylation of CpV(dmpe)Me (dmpe ¼ Me2PC2H4PMe2) with silanes Ph2SiH2 and MesSiH3 (Scheme 1). The further reaction of 1 with Ph2SiHCl led to the regeneration of Ph2SiH2, and formation of the novel (chloro)silyl complex 3.10

Scheme 1 The synthesis of Vanadium of silyl complexes, 1–3.

Reaction of NHC-coordinated (NHC ¼ N-Heterocyclic carbene) monomeric silver(I) halides with (THF)2
K[Si(SiMe3)3] in toluene led to good yields of the monomeric silyl complexes 4 and 5. Exchanging the NHC ligands for the neutral N-donor ligands N-methylimdazole, DMAP, and bipyridine instead led to dimeric species with bridging silyl groups and AgdAg bonding interaction (6–8, Scheme 2).11

Scheme 2 The synthesis of Silver silyl complexes, 4–8.

10.02.2.2 Disilane activation Despite the challenges in activating apolar SidSi bonds, the use of disilanes in catalysis remains a useful strategy for the disilylation of unsaturated bonds, as well as related mono-functionalization protocols.12,13 While this is true, isolation of silyl complexes derived from the activation of the SidSi bond is very rare indeed. An initial report toward this end was forthcoming in 1992, through addition of disilanes to a dimeric Pd(I) complex, leading to 9 and 10 (Scheme 3).14 The further reactivity with an alkyne was shown to form the di-silylated alkene product. A number of related cyclic bis-silyl complexes of Pd have been reported, through the oxidative addition of strained cyclic disilanes to tetrakis(triphenylphosphino) or bis(isocyanide) palladium(0) species.15,16 Aside from these, no further examples of such complexes accessed through this methodology had been reported prior to the previous volume of this chapter.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 3 Cleavage of silane by a Pd(I) complex, and subsequent silyl transfer to an alkyne.

In 2009 it was shown that Pt(PEt3)3 readily reacts with halogenated disilanes, (ClMe2Si)2 and (Cl2MeSi)2, with loss of one PEt3 ligand and oxidative cleavage of the SidSi bond to form 11 and 12 (Scheme 4).17 Surprisingly good yields were achieved, with no noted formation of chloro-palladium species. The activation of (Me3Si)2, perhaps the most challenging of the disilanes to cleave, also slowly proceeds to form 13, but reaches only 50% conversion after three weeks due to reversibility. Subsequent reaction of all silyl complexes with H2 yielded the free hydro-silanes and (PEt3)2PdH2, indicating the likely mechanism for the hydrogenation of disilanes.

Scheme 4 Cleavage of silanes by a Pt(0) complex, forming 11–13.

It was later shown that the NHC Palladium(0) complex, (IMe)2Pd (IMe ¼ [(Me)CN(Me)]2C:), reacts readily with (Me3Si)2 to yield the bis-silyl complex 14, with good conversion after 18 h (Scheme 5). Heating this complex to 85  C in the absence of excess disilane led to the reductive elimination of (Me3Si)2 and regeneration of the starting material, while reaction with aryl-acetylenes led to the first examples of the bis-silylation of CdC unsaturated species with this typically unreactive disilane.18

Scheme 5 Cleavage of the challenging (Me3Si)2 by a bis(NHC)dPd(0) complex.

Beyond the group 10 metals, well-defined disilane cleavage is extremely rare. However, it was recently shown that Au(I), isoelectronic to Pt(0), is capable of such a process. Abstraction of the chloride ligand from Ph3PAuCl with GaCl3 in the presence of (PhMe2Si)2 allowed for the isolation of 15, a Au(III) species formed through oxidative addition of the disilane to the Au(I) center (Scheme 6).19 The reaction was carried out at −80  C, and above −60  C the Au(III) product was said to rapidly decompose, highlighting the challenges in achieving oxidative addition at gold complexes.

Scheme 6 Cleavage of the a disilane by cationic Au(I) complex.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

55

10.02.2.3 Silylborane activation Activation of silylboranes, closely related to disilane activation, has been increasingly employed in recent years for the catalytic (bis) functionalization of a range of substrates under catalytic conditions, particularly as access to silylboranes has increased.20 As for disilane activation, despite the vast array of available catalytic transformations involving silylboranes, very few examples of transition metal silyl complexes have been reported through the activation of the SidB single bond. Two (boryl)(silyl)-iridium complexes have been reported, synthesized through either the metathetical or oxidative cleavage of the SidB bond. In the first instance, a modified 1,9-phenanthroline ligated Ir complex was reacted with an excess of Et3SiBpin, in the formation of complex 16.21 Later, pyridine-functionalized silylborane ligands were reacted with [Ir(cod)Cl]2, leading to SidB bond cleavage and formation of (boryl)(silyl)-Iridium complexes 17–19 (Scheme 7).22

Scheme 7 Cleavage of silylboranes by Iridium complexes to form 16–19.

It has also be shown that NHC-stabilized Palladium(0) complexes of diphenylacetylene and azobenzene readily undergo exchange of the unsaturated ligand in the presence of silylboranes leading to oxidative addition, in the formation of (silyl) (boryl)dPd(II) complex 20 (Scheme 8).23,24 The silyl-borylated organic products were also observed, and the Palladium(0) starting compounds shown to be active catalysts for the silyl-borylation of a range of alkynes.

Scheme 8 Cleavage of silylboranes by alkyne- and diazobenzene-Pd(0) complexes, forming 20.

10.02.3 Silylene complexes The first examples of ‘true’ transition metal (TM) silylene complexes, bearing terminal three-coordinate SiII centers, were only accessed in 1990,25 some 25 years after the seminal report of a carbene complex reported by Fischer.26 Following the discovery of the first base-free silylene complex the research field burgeoned, particularly following the isolation of stable, crystalline N-heterocyclic silylenes (NHSis) which can be employed as ligands in the same manner as NHCs and phosphines.27 Ancillary ligands at Si(II) greatly affect the bonding in and the chemistry of silylene-TM complexes, ranging from systems in which the Si(II) center is a spectator to TM-centered reactivity, to synergistic systems whereby the Si(II) center partakes in bond activation processes in conjunction with the TM center. Here, examples of silylene complexes which have been developed as catalysts, to those forming

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

Fig. 1 Examples of 4-, 5-, and 6-membered N-Heterocyclic Silylenes (NHSis), and a schematic representation of donor-acceptor bonding between a Transition metal (M) and a silylene.

reactive Si(II)-TM bonds will be discussed. It is noted that bridging silylene-transition metal complexes have been known since the early 70s, and of the 170 structurally characterized examples, the majority were reported prior to the publication of the previous volume of this chapter. The directed synthesis of such complexes will thus not be a primary focus here.

10.02.3.1 N-heterocyclic Silylene (NHSi) complexes Like NHCs, isolable NHSis have been employed as ligands toward transition metals. These can be placed in two distinct classes: 5-membered NHSis similar in structure to the well-known Arduengo-type NHCs, and derivatives of the Roesky silylene (i.e. 4-membered NHSis), accessed through salt metathesis from the readily accessible N-heterocyclic chloro- silylene 21. The higher coordination number in derivatives of 21, owing to the amidinato ligand, render them typically more stable and thus easier to handle than the aforementioned 5-membered NHSis, leading to a large number of reports involving such ligands over the past 10 years. A small number of complexes of 6-membered NHSi 22 have also been reported (Fig. 1).

10.02.3.1.1

4-membered NHSi complexes

The gram-scale synthesis of the (amidinato)(chloro)silylene 21 is achieved by dehydrochlorination of the respective (amidinato) (dichloro)silane with lithium hexamethyldisilazane (Scheme 9).28 The chloride ligand can be readily exchanged for a plethora of R groups through straightforward salt metatheses processes. Here, transition metal complexes of these ligands will be discussed, beginning from the group 4 metals, and moving toward the group 12 metals. Note that complexes utilizing chelating bis-silylene ligands derived from 21 will be discussed subsequently.

Scheme 9 Synthesis of the broadly applied 4-membered NHSi, 21.

The only reports of silylene complexes for group 4 based upon 21 are titanocene derived Ti(II) complexes. Reaction of 21 with phosphine stabilized Ti(II) complex Cp2Ti(PMe3)2 leads to phosphine-substitution, in the formation of bis-silylene complex 23.29 Subsequent salt-metathesis of the chloride ligands was shown to be possible, leading to methyl- and hydrido-silylene complexes 24 and 25 (Scheme 10).

Scheme 10 Accessing Ti(II) complexes bearing 4-membered NHSi ligands.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

57

Complexes of the group 5 metals are similarly rare, with only one example of a V(I) complex reported, accessed simply through combination of silylene 21 with CpV(CO)3, with loss of a single carbonyl ligand, leading to complex 26 (Scheme 11).30

Scheme 11 Synthesis of the only reported Vanadium complex bearing a 4-membered NHSi ligand, 26.

For group 6, Cr, Mo, and W carbonyl complexes of silylene 21 have been reported, accessed through addition of the silylene ligand to THF∙ M(CO)5 species (M ¼ Cr-W, 27–29; Scheme 12).31 Furthermore, the chloride ligand could be exchanged for fluoride, giving access to the first examples of Si(II) fluoride species 30 and 31. Two additional W(0) complexes of 4-membered NHSi ligands have been reported. The phosphide-substituted silylene 32 was combined with THF
W(CO)5 leading to complex 34, which could also be accessed through thermal rearrangement of phosphasilene complex 33.32 Cationic complex 36 was accessed through a similar combination of the cationic silylene 35 and THF
W(CO)5.33

Scheme 12 Synthesis of group 6 complexes bearing 4-membered NHSi ligands.

Bis- and tris-silylene Mn(0) and Re(0) complexes bearing chloro-silylene 21 or (diphenyl)amido-derivative 37 have also been reported (38–41, Scheme 13).34,35 In all cases products are cationic, featuring the [M(CO)5]− counter ion (M ¼ Mn, Re), arising from the M2(CO)10 starting materials employed. Neutral bis-silylene complexes and halide-bridged mono-silylene complexes have also been accessed through reaction of silylene 21 with MnX2 (X ¼ Cl, Br) or (SiMe3)2NMnCl.36,37

Scheme 13 Examples of group 7 complexes bearing 4-membered NHSi ligands.

A number of 4-membered-silylene complexes of Fe have been forthcoming. The initial report involved the tert-butoxy substituted silylene 42, which, when combined with Fe2(CO)9, led to loss of Fe(CO)5 and formation of complex 43 (Scheme 14).38 It was later

58

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 14 Iron(0) complexes bearing various 4-membered NHSi ligands.

shown the addition of chloro-silylene 21 to the Fe(0) complex [(dmpe)2Fe(PMe3)] led to PMe3-exchange, and formation of complex 44.39 As with Ti(II) complex 23, the chloride ligand in 44 could be exchanged for hydride or methyl ligands, forming 45 and 46. Hydride complex 45 was also shown to be an active catalyst for the hydrosilylation of ketones, operating via an outer-sphere mechanism whereby the Si(II) center coordinates the ketone prior to H-transfer through the Fe(0) center. The phosphine-functionalized silylene 32 was shown to be effective in the exchange of a single CO ligand in Fe(CO)5 to form 47, which could be subsequently hydrolyzed leading to the primary-phosphine complex 48 (Scheme 15).40 Subsequent reactivity of 48 toward Fe2(CO)9 or THF
W(CO)5 led to homo- and hetero-bimetallic complexes 49 and 50 through P ! M donation (M ¼ Fe or W), while reaction with (C2H4)Pt(PPh3)2 yielded the phosphide-Pt(II) complex 51 through insertion into one PdH bond.

Scheme 15 Phosphino-silylene complexes of Fe(0), and related hetero-bimetallic complexes.

The imine-functionalized silylene 52 has also shown the capacity to stabilize Fe(0), in the arene- and nitrogen-coordinated complex 53, accessed through reduction of the silylene-ligated FeBr2 precursor with KC8 (Scheme 16).41 Under an argon atmosphere the N2 ligand is lost, while under an atmosphere of N2 complex 53 was shown to be catalytically active in the reductive silylation of N2 to (Me3Si)3N, with the KC8/Me3SiCl couple. The bis(amido)dFe(II) complex 54 could also be synthesized by initial reaction of the silylene 52 with FeBr2, followed by KN(SiMe3)2.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

59

Scheme 16 Fe(0) and Fe(II) complexes bearing an imine-functionalized 4-membered NHSi.

A number of cobalt complexes bearing silylenes derived from 21 are known as well as a small number of rhodium and iridium species. The first cobalt example, 55, was accessed in a straightforward manner through the addition of 21 to CpCo(CO)2 (Scheme 17).30 It was further shown that the similar reaction involving Co2(CO)8 led to the cationic bis-silylene complex 56, partnered with the [Co(CO)4]− counter anion. It was later reported that addition of 21 to CoCl2 in the ratio of 5:4 led to partial reduction of cobalt in forming the cationic complex 57, in which the central cobalt is in the +1 oxidation state.42 The similar reaction with CoBr2 circumvented the reductive process, leading to bis-silylene complex 21
CoBr2.

Scheme 17 Cobalt complexes bearing the 4-membered NHSi ligand, 21.

The reaction of CoBr2 with disilyne 58 led to a reductive insertion reaction, in the formation of tetrameric 59, which can be viewed as a complex containing two bromo-silylene fragments and two cobalto-silylene fragments (Scheme 18).43 This unique species was shown to be catalytically active in the CdH functionalization of arylpyridines with alkynes. A further example of silylene-coordinated cobalt catalysis was later reported, utilizing complex 62, accessed through addition of the pincer ligand 60 to a silylene-ligated Co(I) complex (Scheme 18).44 Complex 61 contains a high valent Co(III) center, and is capable of catalyzing the Kumada coupling of aryl Grignard reagents with mono- and di-chloroarenes, and bromoarenes, with some degree of functional group tolerance.

60

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 18 Further examples of Cobalt complexes bearing 4-membered NHSi ligands.

A range of iridium complexes were accessed through CdH activation of the mesityl group in mesityl-silylene 62, with the iridium source being (Cp IrCl2)2, [(cod)IrCl]2, or [(coe)2IrCl]2 (cod ¼ 1,5-cyclooctadiene; coe ¼ cyclooctene), leading to complexes 63, 64, or 65, respectively (Scheme 19).45 Silylene-functionalized NHC 66 was shown to undergo selective coordination of Ir(I) at its Si(II) center when reacted with [(cod)IrCl]2, forming 67.46 Similarly, the Cp -substituted silylene 68 (68 ¼ [PhC {N(tBu)}2](C5Me5)Si:) was shown to coordinate to Ir(I) and Rh(I) upon reaction with [(cod)MCl]2 (M ¼ Ir, Rh), forming 68
MCl(cod).47

Scheme 19 Iridium complexes bearing 4-membered NHSi ligands.

A somewhat more complex pathway was observed on reaction of the silane-functionalized silylene 69 with [(coe)2RhCl]2, leading to silylene coordination and SidH bond activation, in the formation of rhodium hydride complex 70 (Scheme 20).48 This species could hydrogenate norbornene through RhdH hydride abstraction and CdH activation of one mesityl-Me group of the

Compounds With Bonds Between Silicon and d-Block Metal Atoms

61

Scheme 20 Rhodium complexes bearing a chelating silylene-silyl ligand incorporating 4-membered NHSi.

silyl ligand, yielding dimeric complex 71. Both complexes 70 and 71 were capable of the CdH alkylation of 2-phenylpyridine with norbornene, with catalytic activity surpassing that of related phosphine, NHC, or bis(NHSi) Rhodium complexes. The majority of silylene complexes of the group 10 metals involve chelating bis(NHSi)s, and so will be discussed subsequently. As with a number of examples discussed above, direct combination of 21 with Ni(CO)4 leads to carbonyl exchange in the formation silylene complex 72 (i.e. 21 ∙ Ni(CO)3).49 The reaction of phosphasilene 73 with the Pt(0) complex (C2H4)Pt(PPh3)2 led to cleavage of the SidP bond, and formation of novel metallo-silylene complex 74, via coordination of the Si-center of 73 to the Pt(0) fragment, a similar reaction being observed for Pd(PPh3)4, yielding 75 (Scheme 21).50 As a comparison, the related reaction with Ni(cod)2 instead led to the loss of (Me3Si)3P and formation of the bis(silylenyl)phosphine complex of Ni(0) (76), which was also formed upon the addition of phosphino-silylene 32 to Ni(cod)2.

Scheme 21 Group 10 element complexes bearing 4-membered NHSi ligands.

The initial report of 4-membered NHSi complexes of coinage metals involved the use of the Cu(I) cation, [(TMEDA)Cu]OTf (TMEDA ¼ N,N,N0 ,N0 -tetramethylethylenediamine; OTf ¼ triflate), and the earlier described silylene ligands 21 and 42, bearing chloride or tert-butoxide ligands at Si(II), as well as 77 bearing the NMe2 ligand at Si(II).51 These were all accessed through direct combination of the silylene ligands with MeCN ∙[{(TMEDA)Cu}OTf], with displacement of the nitrile ligand. Cu(I), Ag(I), and Au(I) complexes supported by the bulky amido-silylene ligand 78 have also been reported (Scheme 22). In the first instance, CuX

Scheme 22 Neutral and cationic coinage-metal complexes bearing 4-membered NHSi ligands.

62

Compounds With Bonds Between Silicon and d-Block Metal Atoms

complexes (X ¼ Cl, Br, I) have been readily accessed through direct addition of 78 to the Cu(I) halides (viz. 79–81).52,53 Related Ag(I) and Au(I) complexes 82–84 were synthesized in a similar fashion, as was the cationic bis-silylene Ag(I) complex 85.52,53 A number of cationic copper-arene complexes (arene ¼ C6H6, MeC6H5, 1,3-Me2C6H4, Me6C6; 86–89) have been accessed through bromide abstraction from 80 by Ag[SbF6] in aromatic solvents,54,55 with toluene derivative 87 demonstrating the capacity to catalyze the ‘click’ reaction of benzyl azides with a range of both conjugated and aliphatic alkynes. Discreet Cu(I)X clusters (X ¼ Cl, Br, I), bearing cubic [Cu4X4] cores (viz. 90–92, Scheme 23), have also been accessed through direct reaction of chloro-silylene 21 with the Cu(I) halides, giving some indication of the effects of silylene bulk on species aggregation.56 Further reaction of these clusters with a pyridine-functionalized secondary amine ligand (viz. (Mes)(2-Py)NH) ligand followed by Li[N(SiMe3)2] yielded [Cu3X3] clusters stabilized by two pyridine-functionalized silylene ligands (93–94, Scheme 23). All complexes were active in ‘click’ catalysis, with the introduction of the pyridine arm leading to a marked improvement in efficiency.

Scheme 23 Copper(I) clusters stabilized by 4-membered NHSi ligands.

Of the group 12 metals, only Zn(II) complexes bearing 4-membered NHSi ligands have been forthcoming. The first examples of such a species involved chloro-silylene 21, and demonstrated that ligand exchange between Si(II) and Zn(II) readily occurs (Scheme 24).57 As such, reaction of 21 with Cp 2Zn, Et2Zn, and Ph2Zn led to cyclopentadienyl-, ethyl-, and phenyl-silylene complexes 96–98, respectively. Further cyclopentadienyl-silylene complexes of various Zn(II) species were later accessed through combination of the independently synthesized cyclopentadienyl-silylene 68, followed by reaction with ZnX2 species (X ¼ Cl, I, Et, Ph, C6F5; compounds 99–103, respectively).58 Complexes of the bulky amido-silylene 78 with ZnI2 have also been synthesized through direct combination of this ligand with ZnI2, and recrystallization from either toluene, giving dimeric [78
ZnI2]2, or THF/ dioxane, forming monomeric 78
ZnI2
THF.59

Scheme 24 Zn(II) complexes bearing a variety of 4-membered NHSis.

10.02.3.1.2

5-membered NHSi complexes

Since the initial isolation of a stable 5-membered NHSi by the group of Denk (viz. 104),60 which is stabilized by N ! Si donation as per NHCs, a number of such compounds have been isolated and employed as ligands toward transition metal centers, albeit to a considerably lesser degree than closely related NHCs (Fig. 2). Here, reported 5-membered NHSi complexes will be described, moving from group 5 to group 12 (no such complexes have been reported for group 4).

Compounds With Bonds Between Silicon and d-Block Metal Atoms

63

Fig. 2 Reported 5-membered NHSis, and a schematic for the frontier orbitals in such species.

One complex has been reported for group 5, the Cp2V complex 112, accessed through direct combination of silylene 108 with the organometallic V(II) fragment (Scheme 25).61 Interestingly, the same reaction does not proceed for the related NHC, IPr (IPr ¼ [(H)CN(Dipp)]2C:; Dipp ¼ 2,6-iPrC6H3), and is viable for the silylene due to the increased p-acceptor character of this ligand class.

Scheme 25 The only reported example of a group 5 element complex containing a 5-membered NHSi.

The majority of group 6 complexes in this category were reported prior to the publication of the previous volume of this book (Scheme 26), with the complete series of bis-silylene M(CO)4 complexes reported with both saturated and unsaturated t Bu-substituted silylenes 104 and 109 (viz. 113–118),62 as well as the related Mo(0) complex 119 bearing the neopentyl substituted silylene 110,63 and the mono-silylene complex of molybdocene (120).64 More recently, it was shown that employing more bulky NHSis, bearing flanking Xyl or Dipp groups, leads to the mono-silylene adducts of M(CO)5 fragments (M ¼ Cr-W), accessed through addition of the silylene ligand to THF
M(CO)5 species, through THF substitution (Scheme 27).65,66

Scheme 26 Examples of group 6 element complexes bearing 5-membered NHSis reported prior to the previous edition of this chapter.

Scheme 27 Mono-NHSi complexes of M(0) element complexes (M ¼ Cr-W).

Reaction of Dipp-substituted 108 with pentacarbonyl Mn(I) halides, (CO)5MnX (X ¼ Cl, Br, I), led to two differing outcomes; for the chloride complex, double carbonyl substitution as well as insertion into the MndCl bond was observed, leading to (silylene)(silyl)manganese complex 125.67 For both the bromide and iodide complex, double carbonyl substitution was observed without Mn-X bond cleavage (viz. 126 and 127, Scheme 28).66,67

64

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 28 5-membered NHSi complexes of Mn(I) halides.

Aside from earlier reported NHSi complexes of iron, it was recently shown that NHSi 108 reacts with dimeric [CpFe(CO)2]2 through mono-carbonyl substitution, yielding silylene-bridged complex 128, while reaction of the same silylene with the Fe(II) complex CpFeI(CO)2 led only to insertion into the FedI bond in the formation of 129 (Scheme 29).66,67 The closely related addition of the same silylene to bis-amido Fe(II) species [(Me3Si)2N]2Fe yielded the expected mono adduct 130.68

Scheme 29 Reactions of Fe(I) and Fe(II) complexes with a 5-membered NHSi.

It has also been shown that 5-membered NHSi complexes can in fact be accessed from silane precursors, in the reaction of silane 131 with bis(amino)pyridine Ru(0) complexes.69 The reaction, when conducted in pentane, leads to the silylene-coordinated Ru(II) complex, 132, proceeding through initial oxidative addition of the silane at Ru(0), followed by chloride migration. Conducting the reaction in THF leads instead to a mixture of THF-coordinated Ru(II) complex (LN3)Ru(H)(Cl)
THF (LN3 ¼ 2,6-CNAr-Py; Ar ¼ Dipp) and the dinitrogen- and silylene-bound Ru(0) complex 133. This chemistry is postulated to occur first through formation of NHSi complex 132, which eliminates the NHSi through THF substitution, with the free NHSi then reacting with the Ru(0) starting material in the formation of 133 (Scheme 30).

Scheme 30 Formation of Ruthenium complexes bearing 5-membered NHSi ligands through silane activation.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

65

Of the group 9 metals, only Rh complexes of 5-membered NHSis are known. Two complexes, bearing the saturated or unsaturated tBu-substituted NHSis (viz. 104 and 109) were accessed through addition of four equivalents of the NHSi ligand to [(cod)2Rh][BArF4] (ArF ¼ 3,5-CF3C6H3), leading to substitution of both cod ligands in formation of 134 and 135 (Scheme 31).70 It was noted that, regardless of the number of added equivalents of the NHSi ligand, the tetrakis-complexes were always formed.

Scheme 31 Formation of cationic Rhodium complexes bearing 5-membered NHSi ligands.

Again, the majority of 5-membered NHSi group 10 metal complexes were reported prior to the previous volume of this book. Since that time, a handful of nickel complexes have been reported. While earlier examples utilizing the less bulky tBu-substituted NHSi 104 led to tris-substituted Ni(0) complexes in the reaction with Ni(cod)2 (e.g. 136, Scheme 32),71 employing the bulkier Dipp-substituted 108 led only to bis-ligation, forming bis(NHSi)Ni(cod) complex 137.72 Addition of the same silylene to Ni(CO)4 led to loss of two equivalents of CO and dimerization, forming NHSi-bridged complex 138.66 Reaction with Cp2Ni, with simultaneous reduction with Li naphthalenide, led to the similarly bridged Ni(I) dimer 139, with loss of LiCp.67

Scheme 32 Synthesis of Ni(0) and Ni(I) complexes supported by 5-membered NHSi ligands.

Since the initial report of a copper NHSi complex in 2003,73 no further reports of coinage metal 5-membered NHSi complexes have been reported, and no such species have been reported for any group 12 metal.

10.02.3.1.3

6-membered NHSi complexes

Essentially all chemistry involving 6-membered silylene 22 has been reported by the group of Driess, with the majority of examples ligating nickel. The first such complex was generated by the addition of 22 to Ni(cod)2 in aromatic solvents, leading to silylene Ni(0) complexes 140–142, with arene ligands at Ni(0) (Scheme 33).74 It was also shown that, given the charge delocalization in the unsaturated ligand backbone of silylene 22, the addition of strongly Lewis acidic borane B(C6F5)3 to toluene-capped complex 140 yielded a cationic species, 143, with considerably increased bonding interactions between the Si(II) and Ni(0) centers.

Scheme 33 Synthesis of Ni(0)-arene complexes supported by 6-membered NHSi ligands.

66

Compounds With Bonds Between Silicon and d-Block Metal Atoms

It was later shown that the Ni(0) arene fragment could be exchanged for the Ni(0) tris-carbonyl fragment, forming 144 under an atmosphere of CO. From this species it was possible to observe the addition of acidic species across the unsaturated ligand backbone/Si(II) center, forming a range of Si-substituted Nickel-silylene complexes (145–148, Scheme 34).75,76 This chemistry was later extended to the synthesis of Ni-stabilized Si(II) hydride complex 149, through the hydrogenation of 140 with ammonia borane (Scheme 35). The hydride complex 149 was shown to be considerably more reactive than classical Si(IV) hydrides, readily undergoing insertion of a range of alkynes into the SidH bond, in forming vinyl-silylene complexes of Ni(0) 150 and 151. This chemistry was shown through computational analyzes to occur via alkyne coordination at Ni(0), indicating the potential for synergistic metal-ligand effects in such systems.77

Scheme 34 Synthesis of various Ni(0) complexes bearing 6-membered NHSi ligands.

Scheme 35 Formation of a 6-membered hydrido-NHSi in the coordination sphere of Ni(0), and subsequent alkyne hydrosilylation.

Iridium di- and tri-hydride complexes supported by the 6-membered NHSi have been accessed through the addition of Cp IrH4 to 22 (Scheme 36).78 In the first instance, silyl-iridium complex 153 is formed, through Si(II) insertion into one IrdH bond. Addition of B(C6F5)3 to this intermediate species leads to hydrido-silylene complex 154 with three hydride ligands at Ir, while after 24 h in solution in the absence of B(C6F5)3, hydride migration from Ir to the silylene ligand backbone occurs, yielding hydrido-silylene complex 155 with two hydride ligands at iridium.

Scheme 36 Generation of Ir complexes of 6-membered NHSi ligands.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

67

Fig. 3 Cyclic bis(alkyl)silylene 156, and cyclic (alkyl)(amino)silylene 157.

10.02.3.1.4

Further cyclic-Silylene complexes

Beyond the aforementioned 4-, 5-, and 6-membered NHSi ligands, a small number of complexes incorporating related cyclic silylenes have been reported, largely involving silylenes 156 and 157 (Fig. 3). The 5-membered cyclic bis(alkyl)silylene 156 has been used to generate mono(silylene) complexes of group 10 metals, 158–164, through direct reaction with M(0) precursors (M ¼ Ni-Pt), followed by ligand exchange reactions (Scheme 37).79 Given the electron deficient nature of the Si(II) center in this silylene ligand when compared with N-heterocyclic derivatives, considerable backbonding from the M(0) centers to Si(II) is observed, borne out by contracted Si-M double bonds.

Scheme 37 Group 10 element complexes bearing the cyclic bis(alkyl)silylene 156.

The 14-electron, bis(silylene)platinum complex 165 was also accessible, through combination of two equivalents of 156 with (Cy3P)2Pt, which was shown to readily cleave H2 at ambient temperature to yield Pd(II) disilane complex 166 (Scheme 38).80 Isoelectronic cationic Cu(I) and Ag(I) complexes could also be synthesized, again through direct combination of silylene 156 with cationic Cu and Ag fragments, leading to 167 and 168.81

Scheme 38 Synthesis of, and H2 activation by a 14-electron Pd(0) complex supported by two cyclic bis(alkyl)silylene ligands, and closely related cationic coinage metal complexes.

68

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Further examples of Ni(0) and Pt(0) complexes were also reported (viz. 169–171, Scheme 39), accessed from the combination of silylene 156 with M2(dvtms)3 (dvtms ¼ 1,3-divinyl-1,1,3,3-tetramethyldisiloxane), followed by ligand exchange with CO (for Ni).82,83

Scheme 39 Synthesis of further examples of group 10 elements complexes supported by cyclic bis(alkyl)silylene ligands.

Regarding the cyclic (alkyl)(amido)silylene 157, only one complex has been forthcoming, namely the Pt(0) bis(alkene) species 172, synthesized through combination of the silylene ligand with Pt2(dvtms)3 (Scheme 40).84

Scheme 40 Synthesis of further examples of a Pt(0) complexes bearing the cyclic (alkyl)(amino)silylene 157.

The unique phospha- and bora-ylidic heterocyclic silylenes 173 and 174 have shown the capacity to act as particularly strong donor ligands, in complexes 175–179 (Scheme 41).85,86 Notably, the CdO stretching frequencies in Ni(CO)3 complexes could be compared with those for known phosphine, carbene, and silylene complexes, and suggested that both 173 and 174 are stronger donors than the majority of these ligands, although this character was not demonstrated through catalyzes or further reactivity.

Scheme 41 Various transition metal complexes incorporating phospha- and bora-ylidic heterocyclic silylenes.

10.02.3.2 Chelating bis(NHSi) complexes The high-yielding synthesis of the amidinate-stabilized N-heterocyclic chloro silylene 21 has led to numerous investigations involving salt-metathesis of the SidCl bond in this species, which are typically expedient and high yielding. This has borne a range of bis(silylene) species, all reported by the group of Driess, bearing a range of backbones, leading to a family of chelating ligands with differing bite-angles and electronic characteristics (viz. 180–185, Fig. 4). Several reviews on these species highlight their

Compounds With Bonds Between Silicon and d-Block Metal Atoms

69

Fig. 4 Chelating bis-silylene ligands derived from the 4-membered NHSi 21.

capacity to act as strong, stable donor ligands in catalytic regimes,27,87,88 and in a handful of cases to actively play a role in bond breaking/forming reactions in a catalytic context, suggesting that this may be a considerable area of growth in coming years. The first example of a chelating bis-silylene ligand derived from silylene 21 was not synthesized using this chloro-silylene, but rather by the dehydrochlorination of disiloxane 186 with Li[N(SiMe3)2], leading to ‘disilylenoxane’ 187, which was shown to readily act as a chelating ligand toward Ni(0) in the reaction with Ni(cod)2, forming 188 (Scheme 42).89

Scheme 42 Synthesis of ‘disilylenoxane’ 187 and its employment as a chelating ligand toward Ni(0).

Soon after this initial example, it was shown that chloro-silylene 21 could be used to readily access chelating bis-silylenes, when this species was reacted with di-lithio-resorcinolate to form 180. Furthermore, the reaction of this chelating ligand with Pd(PPh3)4 led to insertion into the central aryl CdH bond, followed by H-migration to one silylene center, to generate Pd(II) complex 181, further stabilized by one silylene center of a second equivalent of 180 (Scheme 43).90

Scheme 43 Employment of chelating bis-silylene ligand 180 in the formation of a Pd(II) complex.

70

Compounds With Bonds Between Silicon and d-Block Metal Atoms

This methodology was also extended to Ir and Rh complexes, as well as complexes incorporating closely related Ge(II) and P(III) ligands (Scheme 44). In all cases no H-migration was observed, thus maintaining E(II) character (E ¼ Si, Ge) of the donor centers of the ligand.91 The complexes were shown to be efficient catalysts for the dehydrogenative borylation of arenes with pinacol borane, giving the first indication that chelating silylene (and germylene) ligands are markedly stronger s-donors than related phosphines. The same resorcinol-derived ligands were used in the Ni-catalyzed Sonogashira cross-coupling reaction, using the Ni(II) complexes 182 and 183.92 Notably, the heterobimetallic complex 184 was isolated through the addition of a Copper acetylide to 182; this species is thought to be an important intermediate in the catalytic reaction mechanism.

Scheme 44 Chelating NHSi (and NHGe) ligands in the synthesis of well-defined Ni(II) catalytic complexes.

The ferrocene-bridged bis-silylene 181 was also shown to be easily accessible, through the reaction of di(lithio)ferrocene with chloro-silylene 21. In an initial publication on this ligand, it was shown that the reaction with in-situ generated [CpCo] led to the Co(I) complex 185 (Scheme 45).93 The catalytic cyclotrimerization of phenylacetylene and acetonitrile was achieved with 185, expanding the range of possible transformations involving this ligand class. Later, the same bis-silylene ligand was use to stabilize novel Fe(0) arene complexes, 186 and 187, as well as Fe(II) dihalide complexes 188 and 189, the former being successfully employed as catalysts for the hydrogenation of ketones.94

Scheme 45 Employment of ferrocene-derived chelating ligand 181 in the synthesis of Co(I), Fe(II), and Fe(0) complexes.

The pyridine-derived pincer ligand 182, closely related to resorcinol derivative 180, is also readily synthesized through a salt metathesis reaction involving chloro-silylene 21. In an initial publication regarding Fe(II) and Fe(0) complexes, the coordination of the pyridine moiety to iron was shown to be dependent on the oxidation state of the metal center, the ligand being tridentate in Fe(0) complexes 191 and 192, but with no N ! Fe donation in Fe(II) complex 190 (Scheme 46).95 Bis(trimethylphosphine) complex 191 was employed as a catalyst for the hydrosilylation of ketones, with lower catalyst loadings required for comparable transformations catalyzed by ferrocene-derived bis-silylene complex 187.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

71

Scheme 46 Employment of diamino-pyridine-derived chelating ligand 182 in the synthesis of Fe(II), and Fe(0) complexes, the latter of which was utilized as a catalyst for hydrosilylation.

A later publication showed that Fe(0) complex 191 in fact undergoes oxidative addition of silanes to yield octahedral complexes 193–195, which was hypothesized to be the initial step in hydrosilylation catalysis (Scheme 47).96 Following this step, a ‘peripheral’ mechanism was found to be the lowest in energy through DFT calculations, whereby the incoming ketone attacks the silyl-Si center, before a second equivalent of silane forms a 4-membered transition complex at the activated ketone, leading to hydrosilylation.

Scheme 47 Oxidative addition of silanes to the Fe(0) center in 191, and hypothesized outersphere mechanism for ketone hydrosilylation catalyzed by 191.

Cobalt complexes of the same pyridine-derived bis-silylene have also been developed and successfully employed in CdH borylation of arenes and heterocycles. In an initial report, cobalt dibromide complex 196 was employed as the catalyst, with an additional hydride source, Na[BEt3H], used to generate the active cobalt hydride catalyst in-situ (Scheme 48).97 A subsequent

Scheme 48 Synthesis of Cobalt complexes bearing chelating bis-silylene ligand 182.

72

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Fig. 5 Novel chloro-silylene and chelating bis-silylene species synthesized as part of a study regarding Cobalt catalyzed CdH functionalization.

publication showed that the hydride complex 198 could be isolated by reaction of dichloro complex 197 with Na[BEt3H], while the (hydrido)(boryl)cobalt complex 199 could be subsequently generated by reaction with HBpin.98 It should be noted here that the novel chloro-silylene 200, bis-silylene ligand 201, and related cobalt complexes relating to 197–199 were also reported here, bearing the para-methylphenyl (tolyl) group in the amidinato ligand backbone (Fig. 5). A considerable in-depth study was undertaken here to define the possible reaction mechanism, which goes somewhat beyond the scope of this chapter. Nevertheless, it was postulated that a Co(I)dCo(III) redox cycle could be at play, particularly as non-innocent ligand behavior was not observed. The carborane-derived bis-silylene ligand, 183, as with other examples described here, was synthesized through a straightforward salt-metathesis reaction, and was employed in the nickel-catalyzed Buchwald-Hartwig amination of halo-arenes, with the impressive reaction rates attributed to the high donor-strength of the ligand imparted by the electron rich carborane moiety. The manganese dichloride complex of this bis-silylene ligand, as well those of ligands discussed above (Scheme 49), were also reported and utilized in the transfer semi-hydrogenation of alkynes to E-alkenes.37

Scheme 49 Synthesis of MnCl2 complexes of various bis-silylene ligands (see also Fig. 4).

One further catalytically active system involving a chelating bis-silylene ligand was reported, utilizing the novel xanthenederived ligand 184,99 the wide bite-angle of which leads to impressive catalytic activity, similar to the well-known XantPhos ligand. Here, the Ni(0) complex 202 could be synthesized, and shown to catalyze the hydrogenation of unactivated alkenes, including the challenging substrate tetramethylethylene, at ambient temperature and 1 bar H2. In this example, it was suggested that the ligand is in fact non-innocent, with pre-catalyst 202 forming silyl complex 203 on reaction with H2, and bis(trimethylphosphine) complex 204 also reacting with H2, in this case reversibly, to yield mixed-valence silicon complex, incorporating one silyl and one silylene center (viz. 204
H2, Scheme 50).

Scheme 50 Synthesis of, and H2 activation by Ni(0) complexes supported by a Xanthene-derived chelating bis-silylene ligand.

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73

Two further examples of Ni(0) complexes incorporating chelating bis-silylene ligands are known. The first, acenaphthenederivative 205, was accessed through addition of disilene 185 to Ni(cod)2, leading to SidSi bond cleavage (Scheme 51).100 The second was accessed through reaction of the bis-silylene stabilized germylone 206 with Ni(cod)2, leading to a unique complex (viz. 207, Scheme 52) in which two bis-silylene ligands stabilize a [Ge2Ni] three-membered ring.101

Scheme 51 Cleavage of disilene 185 in the formation of chelating bis-silylene complex 205.

Scheme 52 Complexation of Ni(0) by silylene-stabilized ‘germylone’ 206.

The impressive array of complexes, both chemical curiosities and active catalysts, accessible through chelating bis-silylene ligands indicates that this fervent area of research will likely continue to blossom in the coming years, expanding our understanding of the bonding in these complexes, as well as leading to ‘tuned’ catalysts for a range of catalytic processes. This is especially true for complexes in which the low-valent silicon center is non-innocent, a concept which is only just being uncovered.

10.02.3.3 Acyclic silylene complexes While the vast majority of cyclic NHSi transition-metal complexes are derived from isolable NHSis, the opposite is true for most acyclic silylene complexes, given that base-free acyclic silylenes were only recently realized as stable species.102–105 Again, base-free acyclic silylene complexes will be discussed systematically, moving from group 4 to group 12. As a number of such complexes have reactivity pertaining to intermediary species in hydrosilylation catalysis, or indeed show interesting reactivity in their own right, key examples of such scenarios will also be described. Only a single example of a structurally characterized acyclic silylene group 4 complex is known, the hafnocene species 211. This was accessed through reaction of the dilithiosilane 208 with hafnocene dichloride, leading initially to unstable 16-electron complex 209.106 While under these conditions CdH activation was observed in the formation of 210, addition of PMe3 led to the isolation of a stable 18-electron complex, 211, with considerable SidHf double bond character (Scheme 53).

Scheme 53 Formation of 16-electron hafnium silylene complex 209, its CdH activation product, and stabilization with PMe3.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

Given the capacity of the group 6 metals to form alkylidyne complexes, they have also been the most studied candidates for the formation of the heavier analogs, metallo-silylidyne complexes, discussed later in this chapter. A good number of synthetic routes to access these species are via silylene complexes, which are themselves relatively rare. The first example of a base-free silylene complex of a group 6 metal was the tungsten species 212, bearing the Mes2Si ligand (Scheme 54). As one would expect, this complex has a very short SidW double bond (2.3850(1) A˚ ), particularly when compared with the SidW single bond in the same molecule (2.6456(1) A˚ ).107This complex was accessed through photolysis of methyl tungsten complex, Cp WMe(CO)3, in the presence of the disilane HMe2Si-SiMeMes2, leading to initial dealkylation followed by a series of migratory reactions leading to the product silylene complex.

Scheme 54 Formation of 16-electron hafnium silylene complex 209, its CdH activation product, and stabilization with PMe3.

The (mesityl)(chloro)silylene complex 213 was accessed through a similar alkyl elimination strategy, that is reaction of MesSiClH2 with Cp Mo(dmpe)Bz (Scheme 55; Bz ¼ benzyl).108 Initial loss of toluene is followed by H-migration to Mo, forming the target silylene complex. This species was utilized to access the first example of a metal complex with some degree of Si-M triple bond character (viz. 214), but due to the bridging nature of the hydride ligand the triple bond is somewhat perturbed.

Scheme 55 Formation of Molybdenum complexes bearing acyclic silylene ligands.

The same group utilized the same benzyl elimination route to access a range of bis-alkyl and bis-aryl silylene complexes of molybdenum hydrides, 215–218, as well as (aryl)(hydrido)silylene complexes 219–221, with varying degrees of Mo-H


Si interaction.109 Similar chemistry was achieved utilizing tungsten complex 222 in which the Cp ligand is activated, with silane addition leading to protonation and Cp ligand regeneration, and novel base-free silylene complexes 223 and 224 (Schemes 56 and 57).110

Scheme 56 Formation of bis-alkyl, −aryl, and (hydrido)(aryl)silylene Molybdenum complexes through silane activation.

Scheme 57 Formation of further Tungsten complexes bearing acyclic silylene ligands.

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75

An alternative route to related bulky (alkyl)(hydrido)silylene tungsten complexes was later reported, through direct reaction of Cp-bound tungsten methyl complexes with monoalkyl-silanes, leading to methyl elimination and H-migration in the formation of 225.111 Addition of acetone to 225 led to a range of reactive process, including insertion of the C]O bond into the SidH bond (viz. 226, Scheme 58), and C]O bond metathesis (viz. 227, Scheme 58). Furthermore, it was later shown that addition of iPrNHC (iPrNHC ¼ [(Me)CN(iPr)]2C:) to 225 in fact leads to hydride abstraction from this complex, in the formation anionic complex 229, with a contracted SidW bond length given the now ‘formal’ silylene character (i.e. no bridging hydride interaction).112,113

Scheme 58 Formation of an (alkyl)(hydrido)silylene Tungsten complex, and further reactivity toward acetone and a small NHC.

En route to a stable molybdenum silylidyne complex, reaction of the NHC-stabilized (aryl)(chloro)silylene 230 with the anionic molybdenum salt Li[CpMo(CO)3] led to metathesis in the formation of 231, in which the most likely resonance form (i.e. with a positively charged carbene and anionic molybdenum center) is a formal silylene complex (Scheme 59).114 Subsequent NHC abstraction led to the first example of a molybdenum silylidyne complex, 232. Addition of nucleophiles to this species led in the majority of cases to addition at Si, yielding a range of novel molybdenum silylene complexes, 233–235.115 Notably, this indicates the electrophilic nature of the silicon center in such triply-bonded Si-M species. The [2 + 2] cycloaddition of carbodimides and ketones to metal silylidyne complexes can also lead to metal silylene complexes, via the formation of 4-membered metallacycles bearing metal-silicon double bonds. Such reactivity has been forthcoming for complex 236, leading to 237–239, all of which contain a silylene fragment (Scheme 60).116,117

Scheme 59 Formation of a (aryl)(NHC)silylene molybdenum complex, NHC-abstraction to form a silylidyne complex, and subsequent addition reactions forming a range of novel silylene complexes.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 60 Cycloaddition reactions of a SidW triple bond, forming a range of 4-membered cyclic tungsten silylene complexes.

Two group 7 acyclic silylene complex are known, accessed through addition of diethyl or diphenyl silane to the alkene coordinated Mn(I) hydride complex [(dmpe)2MnH(C2H4)], forming 240 and 241, with concomitant ethane elimination.118 While diethyl derivative 240 exists only as the trans isomer, with no Mn-H


Si interaction, the more stable isomer for diphenyl derivative, 241, is the cis isomer. Both complexes react with H2 to yield hydride bridged silyl complexes 242 and 243 (Scheme 61).

Scheme 61 Formation of Manganese-hydride complexes bearing acyclic silylene ligands, and their reaction toward H2.

The first base-free silylene complexes, reported in 1990, were the cationic ruthenium species 244 and 245, synthesized through triflate abstraction from their silyl complexes,119 followed 4 years later by silylene complexes absent of pi-donating substituents at Si(II) (viz. 246 and 247, Scheme 62).120 Since this time, a number of group 8 silylene complexes have been reported.

Scheme 62 Synthesis of the first examples of transition metal complexes containing acyclic, base-free silylene ligands.

The single example of an iron complex bearing an acyclic silylene ligand, 248, was synthesized in a similar manner to tungsten complex 212, that is through irradiation of a mixture of Cp FeMe(CO)2 and disilane HMe2Si-SiMeMes2, leading to initial loss of methane, followed by Me3Si migration.121 Evidence for both 1,2- and 1,3-group migrations in 248 were seen upon addition of tert-butyl isocyanide, which coordinates at Fe and leads to the formation of silyl complex 249, thereby giving insight into the fluxional binding in such complexes (Scheme 63).

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77

Scheme 63 Synthesis of the first examples of transition metal complexes containing acyclic, base-free silylene ligands.

Related osmium complexes have been accessed through initial dealkylation reactions of Os(II) alkyls. Specifically, bis(alkyl) silylene complexes 250 and 251 were formed through this initial dealkylation reaction, followed by conversion of (chloro)silyl complex Cp Os(PMe3)2(SiClR2) to the related triflate, and triflate abstraction (Scheme 64).122 The (hydrido)(aryl)silylene complexes 252 and 253 were formed directly through the dealkylation of Os(II) benzyl species Cp Os(PiPr3)(Bz) with aryl silanes, followed by spontaneous hydride migration (Scheme 64).123

Scheme 64 Synthesis of Osmium complexes bearing acyclic silylene ligands.

Further examples of Ru-silylene complexes have been accessed through alkyl abstraction and hydride/group migration methods (viz. 254–259),124–126 including the scorpionate complex 258, in which ligand activation plays a key role in its synthesis127; the (chloro)(silyl)silylene complex 260 was synthesized through combination of the cationic NHC-stabilized silylene [(IMe)2(tBu3Si) Si]Cl with arene-coordinated [RuCl2(p-cym)]2 (p-cym ¼ 1-Me-4-iPrC6H4), followed by reduction with KC8 (Scheme 65).128

Scheme 65 Synthesis of a range of ruthenium complexes bearing acyclic silylene ligands.

The further chemistry of a handful of the described ruthenium silylene complexes is also rather interesting. The (hydrido)silylene complex 254 was shown to react with nitriles through insertion into the SidRu double bond, leading to silyl-isocyanide complexes 261 and 262 after a series of bond-migration processes (Scheme 66).124 In a more applicable context, closely related hydrido complex 259 was shown to be an effective catalyst for the hydrosilylation of alkenes, the mechanism postulated to occur through alkene insertion in the SidH bond of the silylene ligand (Scheme 67).126 This mechanism had not been observed before, and highlights the potential importance of silylene-metal complexes as intermediates in hydrosilylation, an industrially applicable catalytic process.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 66 Reactivity of hydrido-silylene ruthenium complex 254 toward nitriles.

Scheme 67 Reactivity of cationic hydrido-silylene ruthenium complex 259 toward alkenes and silanes, highlighting a novel potential mechanism in hydrosilylation catalysis.

Base-free, acyclic silylene chemistry of group 9 has been dominated by Iridium, with these complexes being the only stable, structurally characterized examples (aside from a single scorpionate-silylene cobalt example) bearing chelating phosphine arms (Scheme 68). This stand-alone cobalt complex was synthesized by combination of phosphine-functionalized bis(aryl)silane 263 with Co2(CO)8, with loss of H2 and CO. Hydride-abstraction from the resulting silyl complex led to the cationic cobalt silylene complex 264.129 Notably, the Si(II) center in this species was shown to be Lewis acidic, binding nucleophiles and even allowing for synergistic bond activation across the Si]Co bond, following substrate (EtOH, H2O) coordination at silicon, forming cationic silyl complexes 265 and 266.

Scheme 68 Synthetic access to a cationic phosphine-functionalized silylene complex of cobalt, and subsequent synergistic OdH bond activation reactions.

The first examples of iridium species, published prior to the previous volume of this book, involved deallylation of tris (phosphino)phenyl borate-stabilized Ir(III) allyl complex, 267, with bis(mesityl) silane, with concomitant SidH bond migration, leading to silylene complexes 268–271 (Scheme 69).130,131 It was also shown that the (aryl)(hydrido)silylene derivative, 272, could

Scheme 69 Synthesis of acyclic silylene complexes of octahedral Iridium, and subsequent reactivity toward alkenes.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

79

be accessed by employing TripSiH3 as the silane (Trip ¼ 2,4,6-iPr3C6H2), which undergoes insertion of alkenes at the SidH bond in a similar fashion to ruthenium complex 259, which was catalytically active in alkene hydrosilylation. As such, a range of later reported Iridium silylene complexes, 273–275, could be accessed through hydride-abstraction from Iridium silyl complexes (Scheme 70). The hydrogen-substituted silylene complexes 273 and 275 were shown to be active catalysts for alkene hydrosilylation, again postulated to occur through a second sphere mechanism whereby alkene insertion into the SidH bond is a key step.132

Scheme 70 Synthetic access to trigonal bipyramidal Iridium complexes bearing acyclic silylene ligands.

One of the first examples of a transition metal silylene complex was the platinum species 276, bearing the bis(thionyl)silylene ligand.133 Despite this early report, very few further well-defined examples have been forthcoming involving the group 10 metals. Following the described 1993 report, the neutral platinum silylene complex 277 was reported, through in-situ generation of bis(mesityl)silylene in the presence of (R3P)2Pt (R ¼ iPr, Cy).134 The resulting PCy3 stabilized complex was also shown to undergo reactivity across the Pt]Si bond, in the cleavage of H2 to yield silyl complex 278, and the cleavage of alcohols to generate Pt(0) species and alkoxy silanes (Scheme 71).

Scheme 71 Synthesis of platinum complexes bearing acyclic silylene ligands.

A closely related (aryl)(bromo)silylene complex of the (Cy3P)2Pt fragment (viz. 279, Scheme 72) was also accessed through addition of the a stable 1,2-diaryl-1,2-dibromo disilene with (Cy3P)2Pt,135 although further chemistry of this species was not forthcoming.

Scheme 72 Synthesis of a platinum complex bearing an acyclic (aryl)(bromo)silylene ligand.

The acyclic-silylene Ni(0) complex 280 was later reported, generated through combination of NHC-stabilized (amido)(chloro) silylene [(Dipp)(Me3Si)N](Cl)Si
iPrNHC with Ni(cod)2, in the presence of a further equivalent of iPrNHC.136 The resulting 16 electron Ni(0) complex was shown to be highly reactive in synergistic bond activation processes. In the first instance, it was shown to cleave H2 akin to platinum complex 277, leading to silyl complex 281. More remarkably, 280 cleaves the catechol ligand when reacted with catechol borane, in the formation of the unique Ni(II) hydroborylene complex 283 (Scheme 73). It was later shown that the chloride ligand in 280 can also be exchanged through salt metathesis, leading to a ‘half parent’ bis(amido) silylene complex, as well as to sila-phosphene, −arsene, −phosphinidene, and -arsinidene species (vide infra).137 It was also demonstrated that 280 can activate ammonia across the Si]Ni bond forming 282, a particularly rare reaction for transition metal species, as well as undergo cycloaddition reactions with alkenes, alkynes, aldehydes, and imines in the formation of metallacyclic complexes 284–288 (Scheme 74).138 In the case of ethylene the reaction is reversible.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

Scheme 73 Synthesis of and synergistic bond cleaved in acyclic silylene Ni(0) complex 280.

Scheme 74 Cycloaddition chemistry of acyclic silylene Ni(0) complex 280.

To date, there are no base-free acyclic silylene complexes of the group 11 or 12 metals.

10.02.4 Heavier derivatives of p-complexes The now well-understood transition metal complexes of organic p-systems are a cornerstone of organometallic chemistry, being some of the first true organometallic species to be realized. This, of course, raises questions regarding the bonding differences between the classical Dewar-Chatt-Duncanson model for alkene binding versus that for the heavier derivatives such as disilenes, the degree of multiple bonding in which is perturbed by a significant reduction in the degree of orbital hybridization. A number of homo- and hetero-atomic silene complexes are now known, allowing for some degree of understanding of the bonding nature in these heavier derivatives of p-complexes, which will be discussed here.

10.02.4.1 Disilene, and heteroatomic Silene complexes The first examples of disilene-transition metal complexes, reported in 1989 (Fig. 6), utilized platinum as the transition metal, resonating with the fact that the first alkene-transition metal complex, namely Ziess’s salt, was built upon the same metal.139 Prior to

Fig. 6 The first example of alkene and disilene complexes (left), and the extremes of the bonding structure in transition metal complexes of alkenes and the heavier analogs (E ¼ C - Pb).

Compounds With Bonds Between Silicon and d-Block Metal Atoms

81

the previous volume of this book, a handful of further examples of Pt-derived complexes had been reported, as well as Mo and W derivatives. Since that time, a number of further examples of disilene-transition metal complexes, accessed via novel routes, have been forthcoming. A central point in such complexes is their predominant resonance form, the extremes being a pi-complex or a metalladisilacyclopropane (Fig. 6). A strong contender for the former resonance structure, which is common for alkenes, is the 14-electron palladium complex 289, which shows minimal SidSi bond elongation when bound to Pd, relative to the calculated SidSi distance in the free disilene, as well as negligible out-of-plane bending akin to classical alkene complexes (Scheme 75).140 It was further demonstrated that the addition of donor ligands to 289 generated 16-electron complexes that bearing a closer resemblance to the metalla-disilacyclopropane resonance form, giving experimental insight into the electronic nature of these rare complexes.141

Scheme 75 Synthesis of a 14-electron Pd(0) complex, beading a disilene ligand with considerable p-character.

The same group has also shown that Fe(0) complexes of 1,2-dichlorodisilene complexes can be accessed,142 through reaction of dipotassium tetracarbonylferrate with the tetrachlorodislane [(tBu2MeSi)Cl2Si]2, leading in the first instance to the Z-isomer, 292-Z, which slowly isomerizes to the thermodynamically favored E-isomer, 292-E, with a first-order rate constant of 4.66  10−7 s−1 at 323 K (Scheme 76). The preferred initial formation of 292-Z was mechanistically justified through the formation of a bis(silyl) silylene intermediate, followed by b-chloride migration, which is preferred for generation of the Z-isomer. Both 292-E and 292-Z are best described in terms of the metallacycle resonance form.

Scheme 76 Synthesis of E and Z isomers of a 1,2-dichlorodisilene in the coordination sphere of Fe(0).

A closely related 1,2-dihydrodisilene Ni(0) complex has also been reported, synthesized through a novel ligand migration reaction which occurs on combining NHC-stabilized hydridosilylene [(tBu3Si)(H)Si]
IMe with Ni(cod)2 (Scheme 77).143 Here, the addition of two equivalents of [(tBu3Si)(H)Si]
IMe leads to transfer of both NHC ligands to nickel, and dimerization of the hydridosilylene fragments to form the final disilene complex 293. In this case, the E-isomer is exclusively formed, with the SidSi bond length and out-of-plane bending indicative of metallacycle character.

Scheme 77 Synthesis of the 1,2-dihydridodisilene Ni(0) complex, 293.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

A general route to heavier alkene complexes of electron poor group 4 metallocene fragments has been developed, through combination of the 1,2-dipotassiodisilane 294 first with magnesium dibromide, followed by the dichlorometallocenes Cp2MCl2 (M ¼ Ti, Zr, Hf ), leading to the desired disilene complexes 295–297 (Scheme 78).144 Structural insight was only attained for the Ti derivative, while the Zr and Hf derivatives could be structurally characterized as their PMe3-adducts. In all cases a metallacycle resonance form is apparent, largely due to considerable [Si2] ! M donation, although in the PMe3-coordinated complex (in which the metal center is more electron rich) this donation is apparently lessened, leading to contraction of the SidSi bond. A similar synthetic protocol utilizing a Si,Ge-dipotassiosilylgermane gave facile access to the germasilene complex of hafnocene, 298, which was also isolated as the PMe3 adduct, and is also best described as a metallacycle.

Scheme 78 Synthetic access to the series of group 4 metallocene complexes of disilenes, and the hafnium complex of a germasilene.

Mixed-silene complexes akin to 298 are very rare, with the first examples reported prior to the previous volume of this book. Two of those examples, namely silene complexes of Ir and W, highlight potential routes to such species, one example utilizing SidH activation and methane elimination at Ir (route (a), Scheme 79),145 and the second involving the reductive ring-closure of a chlorosila-alkyl tungsten complex (route (b), Scheme 79).146 A similar approach to the former has also been used in the formation Ru silene complexes (Scheme 80).147

Scheme 79 Synthetic routes to transition metal-silene complexes.

Scheme 80 Synthetic access to a Ruthenium silene complex.

It has more recently been shown that the free silene, 299, readily forms a complex with Pt(0) upon reaction with (PCy3)2Pt (viz. 300, Scheme 81).148 The product, closely related to disilene complex 289, shows a considerable degree of p-character as ascertained through geometrical observations, but was rather described as a hybrid between p-complex and metallacycle, given apparent differences in geometry at the Si and C centers. Similarly, a p-stabilized silene, accessed through either thermal or photolytic rearrangement of a silacyclopropane, was shown to readily undergo complexation with Ni(0) and Pt(0) in the formation of 301 and 302, whereby both complexes show similar geometrical parameters to 300 and as such were also described as hybrids of the p-complex and metallacycle resonance forms.149

Compounds With Bonds Between Silicon and d-Block Metal Atoms

83

Scheme 81 Synthetic access to a group 10 complexes of silenes through combination of free silenes with M(0) fragments (M ¼ Ni, Pt).

Beyond the hafnium complex of a germasilene, 298, only two further examples of heteroatomic silene-TM complexes are known, namely stannasilene Pd and Pt complexes 304 and 305. The novel synthesis of these species, which presumably targeted stannylene complexes, involved the addition of the phosphine-coordinated cyclic bis(silyl)stannylene 303 to either PtCl2/K, or Pd(PR3)3 (R ¼ Ph, Et), which led to ring-opening of the stannylene and silyl-migration in the formation of 304 and 305 (Scheme 82).150 Again, the geometrical parameters in these complexes point toward metallacycle character. Similar reactivity was observed for the reaction of [(Me3Si)3Si]2Sn
PEt3 with PtCl2/K in the presence of dppe, forming the acyclic stannasilene complex 306.

Scheme 82 Synthesis of group 10 element complexes of stannasilenes.

Related to the above disilene and silene complexes are TM-complexed unsaturated sila-cyclobutene, cyclopentadiene, and benzene species. The initial examples of 5-silacyclopentadienyl (viz. silolyl) complexes were reported in 1998 by Tilley, accessed via the reaction of lithium siloyl 307 with Cp HfCl3, forming complex 308,151 which was later extended to the related Zr complex 309 (Scheme 83).152 It was later shown by Sekiguchi that trisilacyclopentadiene and germadisilacyclopentadiene ligands, each

Scheme 83 Synthesis of the first examples of ‘heavy’ metallocene complexes, bearing silolyl ligands in place of one Cp ring.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

containing three heavier group 14 atoms in the 5-membered ring, could also be complexed to a transition metal, again through a salt-metathesis mechanism. This led to complexes 310–312, where the ‘heavy’ cyclopentadienyl ligands show an 5-coordation in all cases (Scheme 84).153,154

Scheme 84 Synthesis of transition metal complexes bearing germadisila- and trisila-cyclopentadienide ligands.

A number of silicon-containing cyclobutadiene complexes have been accessed, the first example being the monosilacyclobutadiene cobalt complex, 313, which was surprisingly accessed through rearrangement of a 4-silatriafulvene when combined with CpCo(CO)2, following exchange of both carbonyl ligands (Scheme 85).155 The anionic Cp-free complex 314, bearing a tetrasilacyclobutaidiene ligand, was later accessed through addition of the tetrasilacyclobutadiene dianion [{(tBu2MeSi)Si}4]K2 to CpCo(CO)2, with loss of CpK.156 Closely related Fe and Ru complexes 315 and 316 were subsequently accessed through similar synthetic protocols (Scheme 86).157,158

Scheme 85 Synthesis of transition metal complexes bearing germadisila- and trisila-cyclopentadienide ligands.

Scheme 86 Synthesis of tetrasilacyclobutadiene complexes of Fe(0) and Ru(0).

Finally, two TM-complexes bearing a silabenzene derivative have been reported, namely 317 and 318, resulting from the direct addition of a stable silabenzene analog to M(0) precursors under UV irradiation (Scheme 87). Only the Cr complex 317 was structurally authenticated, and shown to contain the silabenzene ligand engaging in 6-coordination with the TM center.159

Scheme 87 Synthesis of transition metal complexes bearing germadisila- and trisila-cyclopentadienide ligands.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

85

10.02.4.2 Silapnictene complexes TM-complexes of doubly-bonded silicon-pnictogen species are rather rare, with early examples of silaimine complexes of zirconocene, reported by Berry as early as 1991, being two of only a handful of such species. More recently, it was shown that a (hydrido) (silylenyl)tungsten complex reacts with nitriles through concomitant insertion into the WdH bond and SidN coupling, forming tungsten silaimine complexes 319 and 320 (Scheme 88).160

Scheme 88 Synthesis of transition metal complexes bearing germadisila- and trisila-cyclopentadienide ligands.

Fig. 7 Isomeric forms of 1,2-dihydrosilapnictene transition metal complexes, formed through H-migration between E and Si (E ¼ N-Sb).

A range of 1,2-dihydrosilapnictene complexes of nickel were recently accessed, and shown to readily tautomerize through H-migration between silylene, silapnictene, and pnictinidene complexes depending on the identity of the pnictogen atom (Fig. 7, Scheme 89).137 Half-parent bis(amido)silylene complex 321 could be isolated as a stable compound, while attempts to make the P-derivative led rather to the stable 1,2-dihydrosilaphosphene complex 322, which to date has no counterpart for classical carbon derivatives (e.g. 1,2-dihydroimine complexes). This complex was shown to coordinate BPh3 at its P-center, while the free complex undergoes a P ! Si proton migration to form the unstable phosphinidene complex 323, which dimerizes in forming 324. The corresponding arsenic derivative of 323 (viz. 325, Scheme 89) could be isolated as a stable compound through deprotonation at arsenic, preventing H-migration, and yielding the stable complex metallo-silaarsene Ni(0) complex 326. In the absence of deprotonation, the complex disproportionates, forming an interesting nickela-silaarsene complex, 327, with concomitant

Scheme 89 Synthesis of 1,2-dihydrosilapnictenes in the coordination sphere of Ni(0), and various products formed through dynamic migration processes.

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Compounds With Bonds Between Silicon and d-Block Metal Atoms

formation of a diarsene-Ni(0) complex 328. Given the geometrical parameters in these species they were ascertained to have high metallacycle character, although the AsdSi bond in 325 is within the range of known double bonds for these elements, most likely due to the high charge localization on As.

10.02.4.3 Disilyne complexes Despite the vast number of alkyne-TM complexes which are known, very few heavier derivatives have been isolated. In fact, only two disilyne TM-complexes have been accessed (viz. 329 and 330, Scheme 90), formed through combination of a stable disilyne with (Cy3P)2M (M ¼ Pd, Pt).161 Both compounds show metallacycle character, due to considerable M ! (Si2) back-donation, with the SidSi distance in both cases being in keeping with known double bonds.

Scheme 90 Synthesis of and bonding in M(0) complexes of a disilyne (M ¼ Pd, Pt).

10.02.5 Silicon-transition metal triple bonds (silylidyne complexes) The chemistry of alkylidyne, or carbyne complexes has seen considerably attention on both fundamental grounds, and through applications in metathesis chemistry.162,163 In more recent years, related silicon chemistry has been uncovered, albeit somewhat more challenging given the reduced propensity for heavier main group elements to partake in stable multiple bonding interactions.164 The first example of a complex with considerable Si-transition metal triple bond character, reported by Tilley, was generated though chloride abstraction from (aryl)(chloro)silylene-molybdenum hydride complex 213, described earlier in this chapter.108 In the molecular structure of the formed cationic silylidyne complex 214 it was found that the hydride ligand at Mo in fact bridges the Si and Mo centers, presumably perturbing the triple bond character between the elements to some degree (Scheme 91). Later, a hydride-free, neutral molybdenum silylidyne complex 232 was reported, also described earlier in this chapter due to it being accessed from silylene complex 231 (Scheme 91).114 It is interesting to note that this latter complex, in the absence of a ModH fragment, in fact has a longer SidMo bond than in hydride-bridged 214, perhaps due to the charged nature of this complex leading to a contraction of the highly polarized SidMo bond. More recently, the Cp -bound Molybdenum silylidyne complex 332 was reported, synthesized through combination of the cationic Si(II) complex, [Cp Si][BArF4], with the anionic tris(pyrazolyl)borate molybdenum complex 331, in the loss of Na[BArF4] (Scheme 92).165 Again, the SidMo bond length in this complex is longer than the two species described above, indicating a strong effect the cationic charge in complex 214 on the SidMo bond in that system.

Scheme 91 The earlier describe Molybdenum silylidyne complexes 214 and 232.

Scheme 92 Synthesis of the Cp -bound Molybdenum silylidyne 332, utilizing the novel Molybdate complex 331.

Compounds With Bonds Between Silicon and d-Block Metal Atoms

87

A particularly remarkable example of a silicon-metal triple bonded complex was reported in 2019, also involving the anionic complex 331 as a precursor. Here, two equivalents of 331 were reacted with the Si(II) dibromide complex, SIdipp
SIBr2 (SIdipp ¼ [H2CNDipp]2C:), resulting in the loss of sodium bromide and SIdipp, and forming metalla-silylidyne 333 (Scheme 93).166 The central silicon atom in 333 forms a triple bond with one Mo-center, and a single bond with the second. Furthermore, this species could be reduced with potassium metal to yield the dianionic ‘1,3-dimetalla-2-silallene’ 334, featuring a two-coordinate central silicon atom having a double bond to each Mo center. Formally, in both complexes 333 and 334, the central Si atom is in the − 4 oxidation state, the first example of a stable molecular species featuring silicon in this oxidation state. It was later shown that tungsten-derived silylidyne complex 335 could also be readily accessed through similar methods to those described above. Reaction of both Mo and W complexes with various alkynes led to unprecedented planar tricyclic [M2SiC2] complexes (M ¼ Mo, W), 336–341,167 coined as ptSi (planar tetracoordinate Silicon) complexes, which violate established concepts of Le Bel and van’t Hoff stating that such a geometry for carbon (viz. ptC) is highly unstable given the favorable energy of the tetrahedral coordination mode (Scheme 94). While the ptC conformation had been achieved in stable complexes some years ago, that for the remainder of group 14 had not been previously observed.

Scheme 93 Synthesis and reduction of a metalla-silylidyne complex.

Scheme 94 Addition of alkynes to metalla-silylidyne complexes in forming the first examples of planar tetracoordinate Silicon (ptSi) complexes.

Further examples of tungsten silylidyne complexes have also been reported. Complex 342, bearing the bulky [(Me3Si)3C] group at Si, was synthesized by the sequential addition of two equivalents of B(C6F5)3 to the anionic (alkyl)(hydrido)silylene complex 229 described earlier in this chapter, standing as a unique synthetic method for silylidyne complexes.112 Silylidyne complex 236, bearing the bulky aryl group Eind at Si (viz. Ar in Scheme 95), was accessed in a similar fashion by the same group, albeit in considerably higher yields. This silylidyne complex in fact exists as the CO-bridged dimeric form 343 in the solid state, but forms a mono-dimer equilibrium in solution. As discussed earlier, the SidW triple bond in this species undergoes [2 + 2] cycloaddition reactions with carbodimides and ketones in the generation of metalla-sila-cyclobutene species, essentially cyclic silylene complexes.116

Scheme 95 Synthesis of tungsten silylidyne complexes.

88

Compounds With Bonds Between Silicon and d-Block Metal Atoms

Only one silylidyne complex of a first-row transition metal has been accessed, namely the chromium species 344. Although the Si center in 344 is bound by what is classically seen as a dative ligand, the NHC SIdipp (SIdipp ¼ [(CH2)CN(Dipp)]2C:), the cationic charge in this complexes resides largely on the carbene hence leading to an essentially formal covalent bond between the NHC-carbon and Si.168 This complex was synthesized through initial salt metathesis of SIdipp
SiBr2 with the anionic Cr species, [Cp Cr(CO)2]Li, followed by bromide abstraction. Further reaction of 344 with CO led to coordination at Cr, and disruption of the triple bond, in the formation of metallo-silylene 345, the oxidation of which with N2O gave access to the first example of a complex containing the ‘silanone’ moiety (i.e. a SidO double bond) in 346 (Scheme 96).

Scheme 96 Synthesis of Chromium silylidyne, metallasilylene, and metallasilanone complexes.

Finally, one example of a niobium silylidyne complex is known, accessed in a rather straightforward manner through the combination of dimeric [(Br)(Tbb)Si]2 with the anionic Nb(dI) complex [{MeSi(CH2PMe2)3}Nb(CO)4]NMe4, yielding octahedral silylidyne complex 347 (Scheme 97).169 It should be noted here that several further silylidyne complexes, for example of Fe and Ni, are proposed to have been synthesized as part of PhD theses, however as these examples are not reported in peer reviewed journals, and/or the data are not readily accessible, they will not be discussed here.

Scheme 97 Synthesis of a Niobium silylidyne complex.

10.02.6 Summary Observing the contents of this chapter, it is clear that, over the past two decades, there has been tremendous growth in our capacity to access transition metal complexes bearing low-valent silicon ligands. This has allowed for the synthesis of a number of heavier congeners of what we would consider classical transition metals complexes, bearing ligands such as alkenes, alkynes, carbenes, and carbynes, among others. In turn, this has begun a consequent growth in knowledge regarding the reactivity of these fascinating silicon analogs, giving further access to new bonding modes for this earth-abundant element, as well as numerous efficient catalytic regimes with low-valent silicon-derived ligands. Already established silane chemistry continues to be a corner-stone of transition metal catalysis. Silyl complexes, which are presumed as key intermediate in such catalyzes, are now known for almost all transition metals, lacking only for technetium. Moving forward, it is clear that the chemistry of low-valent silicon will continue to grow within the context of transition metal ligation. It is expected that this will focus on reactive processes involving both the transition metal and silicon centers, a handful of examples of which have been discussed herein. One can envisage that this leads us toward a greater understanding of dual-centered reactive synergism, perhaps even producing non-innocent silicon-derived ligands for new catalytic transformations. In any case, the interplay between silicon and the transition metals has always stood at the forefront of inorganic chemistry research, and the previous 15 years have been no exception. We can only look forward to what will come of the next 15!

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C.; Feldman, J. D.; Tilley, T. D. J. Am. Chem. Soc. 1999, 121, 9871–9872. Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4065–4075. Calimano, E.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 11161–11173. Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 7884–7885. Feldman, J. D.; Mitchell, G. P.; Nolte, J.-O.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 11184–11185. Agou, T.; Sasamori, S.; Tokitoh, N. Organometallics 2012, 31, 1150–1154. Hadlington, T. J.; Szilvási, T.; Driess, M. Angew. Chem. Int. Ed. 2017, 56, 7470–7474. Hadlington, T. J.; Szilvási, T.; Driess, M. J. Am. Chem. Soc. 2019, 141, 3304–3314. Hadlington, T. J.; Kostenko, A.; Driess, M. Chem. Eur. J. 2020, 26, 1958–1962. Pham, E. K.; West, R. J. Am. Chem. Soc. 1989, 111, 7668–7670. Kira, M.; Sekiguchi, Y.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 2004, 126, 12778–12779. Iwamoto, T.; Sekiguchi, Y.; Yoshida, N.; Kabuto, C.; Kira, M. Dalton Trans. 2006, 177–182. Hashimoto, H.; Suzuki, K.; Setaka, W.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 2004, 126, 13628–13629. Inoue, S.; Eisenhut, C. J. Am. Chem. Soc. 2013, 135, 18315–18318. Zirngast, M.; Flock, M.; Baumgartner, J.; Marschner, C. J. Am. Chem. Soc. 2009, 131, 15952–15962. Campion, B. K.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112, 4079–4081. Koloski, T. S.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 1990, 112, 6405–6406. Campion, B. K.; Heyn, R. H.; Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 5527–5537. Bravo-Zhivotovskii, D.; Peleg-Vasserman, H.; Kosa, M.; Molev, G.; Botoshanskii, M.; Apeloig, Y. Angew. Chem. Int. Ed. 2004, 43, 745. Nakata, N.; Rodriguez, R.; Troadec, T.; Saffon-Merceron, N.; Sotiropoulos, J.-M.; Baceiredo, A.; Kato, T. Angew. Chem. Int. Ed. 2013, 52, 10840–10844. Arp, H.; Marschner, C.; Baumgartner, J.; Zark, P.; Müller, T. J. Am. Chem. Soc. 2013, 135, 7949–7959. Dysard, J. M.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 8245–8246.

Compounds With Bonds Between Silicon and d-Block Metal Atoms 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169.

Dysard, J. M.; Tilley, T. D. J. Am. Chem. Soc. 2000, 122, 3097–3105. Lee, V. Y.; Kato, R.; Sekiguchi, A.; Krapp, A.; Frenking, G. J. Am. Chem. Soc. 2007, 129, 10340–10341. Yasuda, H.; Lee, V. Y.; Sekiguchi, A. J. Am. Chem. Soc. 2009, 131, 9902–9903. Kon, Y.; Sakamoto, K.; Kabuto, C.; Kira, M. Organometallics 2005, 24, 1407–1409. Takanashi, K.; Lee, V. Y.; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2005, 127, 5768–5769. Takanashi, K.; Lee, V. L.; Ichinohe, M.; Sekiguchi, A. Angew. Chem. Int. Ed. 2006, 45, 3269–3272. Takanashi, K.; Lee, V. L.; Sekiguchi, A. Organometallics 2009, 28, 1248–1251. Shinohara, A.; Takeda, N.; Sasamori, T.; Matsumoto, T.; Tokitoh, N. Organometallics 2005, 24, 6141–6146. Watanabe, T.; Hashimoto, H.; Tobita, H. J. Am. Chem. Soc. 2006, 128, 2176–2177. Ishida, S.; Sugawara, R.; Misawa, Y.; Iwamoto, T. Angew. Chem. Int. Ed. 2013, 52, 12869–12873. Fürstner, A. Angew. Chem. Int. Ed. 2013, 52, 2794–2819. Ehrhorn, H.; Tamm, M. Chem. Eur. J. 2019, 25, 3190–3208. Jutzi, P. Angew. Chem., Int. Ed. 1975, 11, 232–245. Ghana, P.; Arz, M. I.; Schnakenburg, G.; Straßmann, M.; Filippou, A. C. Organometallics 2018, 37, 772–780. Ghana, P.; Arz, M. I.; Chakraborty, U.; Schnakenburg, G.; Filippou, A. C. J. Am. Chem. Soc. 2018, 140, 7187–7198. Ghana, P.; Rump, J.; Schnakenburg, G.; Arz, M. I.; Filippou, A. C. J. Am. Chem. Soc. 2021, 143, 420–432. Filippou, A. C.; Baars, B.; Chernov, O.; Lebedev, Y. N.; Schnakenburg, G. Angew. Chem. Int. Ed. 2014, 53, 565–570. Filippou, A. C.; Hoffmann, D.; Schnakenburg, G. Chem. Sci. 2017, 8, 6290–6299.

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10.03

Organometallic Compounds of Germanium

Selvarajan Nagendran, Jyoti Shukla, Pratima Shukla, and Pritam Mahawar, Department of Chemistry, IIT Delhi, New Delhi, India © 2022 Elsevier Ltd. All rights reserved.

10.03.1 10.03.2 10.03.2.1 10.03.2.1.1 10.03.2.1.2 10.03.2.1.3 10.03.2.2 10.03.2.2.1 10.03.2.2.2 10.03.2.3 10.03.2.3.1 10.03.2.3.2 10.03.3 10.03.3.1 10.03.3.1.1 10.03.3.1.2 10.03.3.1.3 10.03.3.1.4 10.03.3.1.5 10.03.3.2 10.03.4 10.03.4.1 10.03.4.1.1 10.03.4.1.2 10.03.4.1.3 10.03.4.1.4 10.03.4.2 10.03.4.2.1 10.03.4.2.2 10.03.5 10.03.5.1 10.03.5.1.1 10.03.5.1.2 10.03.5.1.3 10.03.5.2 10.03.5.3 10.03.6 10.03.6.1 10.03.6.1.1 10.03.6.1.2 10.03.6.2 10.03.7 10.03.7.1 10.03.7.1.1 10.03.7.1.2 10.03.7.2 10.03.7.3 10.03.7.3.1 10.03.7.3.2 10.03.7.3.3 10.03.7.3.4 10.03.7.4 10.03.7.4.1 10.03.7.4.2

92

Introduction Compounds containing germanium carbon bonds Preparation of compounds with GedC single bonds Tetraalkyl, tetraaryl, and organofunctional germanium compounds Intracyclic germanium–carbon single bonded compounds Organogermanium compounds featuring metals Preparation of compounds featuring Ge]C double bonds Acyclic germanium–carbon double bonded compounds Cyclic germanium–carbon double bonded compounds Reactivity Reactivity of GedC single-bonded compounds Reactivity of Ge]C double-bonded compounds Compounds with germanium group 17 element bonds Synthesis of compounds with germanium group 17 element bonds Synthesis from lithium salts Synthesis from Grignard reagents Synthesis from germanium(II) halides Synthesis from germane hydrides Synthesis via halogen exchange reactions Reactivity of germanium halogen bonded compounds Compounds with germanium–hydrogen bonds Synthesis of compounds with germanium–hydrogen bonds Reduction of halogermanes Addition to unsaturated germanium compounds Oxidative addition to germylenes Other procedures Reactivity of compounds with germanium–hydrogen bonds Germanium cation/anion formation Hydrogermylation reactions Compounds with germanium-group 15 element bonds Synthesis of compounds with GedE single bonds (E ¼ group 15 element) Synthesis from alkali metal salts Synthesis from germylenes Synthesis via other methods Synthesis of compounds with germanium doubly-bonded to group 15 elements Reactivity of germanium group 15 element bonded compounds Compounds with germanium-group 16 element bonds Synthesis Synthesis of compounds with germanium-oxygen bonds Synthesis of compounds with germanium bonded to sulfur, selenium or tellurium Reactivity of germanium-chalcogen bonded compounds Compounds with germanium–metal (or metalloid) bonds Compounds with Ge–alkali metal bonds Synthesis Reactivity of compounds with Ge–alkali metal bonds Compounds with germanium–group 13 metal (metalloid) bonds Germanium compounds with group 14 metal/metalloid bonds Synthesis of compounds with GedSi bonds Synthesis of compounds with GedSn bonds Synthesis of compounds with GedPb bonds Reactivity Compounds with germanium–transition metal bonds Synthesis Reactivity of compounds having germanium transition metal bonds

Comprehensive Organometallic Chemistry IV

95 95 95 95 108 131 136 136 137 139 140 148 152 153 153 154 155 158 159 159 162 162 162 164 164 164 165 165 166 166 167 167 170 172 174 175 178 178 178 186 191 194 194 194 197 199 201 201 203 204 204 205 205 218

https://doi.org/10.1016/B978-0-12-820206-7.00176-1

Organometallic Compounds of Germanium

10.03.8 Germanium containing polymers 10.03.8.1 Synthesis by addition polymerization 10.03.8.2 Synthesis by condensation polymerization 10.03.8.3 Synthesis by Suzuki polycondensation 10.03.8.4 Synthesis by Stille polycondensation 10.03.8.5 Synthesis by Yamamoto coupling 10.03.8.6 Synthesis by Witting reaction 10.03.8.7 Synthesis from germyl halides 10.03.8.8 Synthesis from a germyl hydride 10.03.8.9 Synthesis from germylenes 10.03.8.10 Synthesis using transition metal precursors 10.03.8.11 Polymers with germanium pendants 10.03.9 Germylenes 10.03.9.1 Preparation 10.03.9.1.1 Acyclic germylenes 10.03.9.1.2 N-Heterocyclic germylenes 10.03.9.1.3 Heterocyclic germylenes 10.03.9.1.4 Germylenes with donor arm(s) and their metal complexes 10.03.9.1.5 Pincer ligand stabilized germylenes and their metal complexes 10.03.9.1.6 Bis(germylenes) and their metal complexes 10.03.9.1.7 Carbene stabilized germylenes 10.03.9.1.8 Air and water stable germylenes 10.03.9.1.9 Germylene cations 10.03.9.1.10 Germylene anions 10.03.9.1.11 Germylene radicals 10.03.9.2 Reactivity of germylenes 10.03.9.2.1 Oxidation reactions 10.03.9.2.2 Reduction reactions 10.03.9.2.3 Germylones 10.03.9.2.4 Coordination chemistry of germylenes 10.03.9.2.5 Germylenes and their metal complexes as catalysts 10.03.10 Compounds with germanium-germanium single bonds 10.03.10.1 Synthesis 10.03.10.1.1 Synthesis of digermanes 10.03.10.1.2 Synthesis of linear oligogermanes 10.03.10.1.3 Synthesis of branched oligogermanes 10.03.10.1.4 Synthesis of polygermanes 10.03.10.1.5 Synthesis of metal complex supported oligogermanes 10.03.10.1.6 Synthesis of cyclic oligogermanes 10.03.11 Compounds with germanium-germanium multiple bonds 10.03.11.1 Synthesis 10.03.11.1.1 Synthesis of compounds with Ge]Ge bonds 10.03.11.1.2 Synthesis of compounds with Ge^Ge bonds 10.03.11.1.3 Synthesis of cyclic compounds with Ge]Ge bonds 10.03.11.2 Reactivity of germanium germanium multiply bonded compounds 10.03.11.2.1 Reactivity of compounds with Ge]Ge bonds 10.03.11.2.2 Reactivity of compounds with Ge^Ge bonds 10.03.11.2.3 Reactivity of cyclic compounds with Ge]Ge bonds 10.03.12 Conclusion Acknowledgments References

Nomenclature [PNP]Cl acac ACF AIBN ATI B2Pin2 Bbt

Bis(triphenylphosphoranylidene)ammonium chloride Acetylacetonate Aluminum chlorofluoride Azobisisobutyronitrile Aminotroponimine Bis(pinacolato)diboron 2,6-Bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl

93

226 226 226 229 230 233 233 234 235 235 236 236 237 237 237 246 273 289 297 301 306 311 312 323 325 326 326 338 350 358 377 381 381 381 385 388 390 391 393 394 394 394 397 398 401 401 404 407 412 413 413

94

Organometallic Compounds of Germanium

BINAP Boc BPO Bz or Bn cAAC Cat. COD COT Cp Cp CSA Cy cyp DAB DABCO DBA DBP DCC DCE DCM dcpe DIBAL-H Dip Dippe DMAc DMAP DME DMF DMP DMSO DPA DPM dppe DTBP Dtp Eind EMind FET HBcat HBpin HMPA IPr LDA LiAlH4 LiBH4 LiHMDS Me i I Pr Me IMe ¼ IMe4 Mes Mes MOCVD MOM MW NaBArF4 NaBH4 NBE NBS NCS NHC NIS NMMO NMP

2,20 -Bis(diphenylphosphino)-1,10 -binaphthyl tert-Butyloxycarbonyl Benzoyl peroxide Benzyl Cyclic(alkyl)(amino)carbene Catalyst Cyclooctadiene Cyclooctatetraene Cyclopentadienyl 1,2,3,4,5-Pentamethylcyclopentadienyl Camphorsulfonic acid Cyclohexyl Cyclopropyl Diazabutadiene 1,4-Diazabicyclo[2.2.2]octane Dibenzylideneacetone Dibenzoylperoxide N,N0 -Dicyclohexylcarbodiimide 1,2-Dichloroethane Dichloromethane Bis(dicyclohexylphosphino)ethane Diisobutylaluminum hydride 2,6-Diisopropylphenyl Bis(diisopropylphosphino)ethane Dimethylacetamide 4-Dimethylaminopyridine Dimethoxyethane N,N-Dimethylformamide 2,6-Dimesitylphenyl Dimethylsulfoxide Dipyridylamine Dipyrromethene 1,2-Bis(diphenylphosphino)-ethane Di-tert-butyl peroxide 3,5-Di-tert-butyl-phenyl 1,1,3,3,5,5,7,7-Octaethyl-s-hydrindacen-4-yl 1,1,7,7-Tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl Field-effect transistor Catecholborane Pinacolborane Hexamethylphosphoramide 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene Lithium diisopropylamide Lithium aluminum hydride Lithium tetrahydridoborate Lithium bis(trimethylsilyl)amide 1,3-Diisopropyl-4,5-dimethylimidazol-2-ylidene 1,3,4,5-Tetramethylimidazol-2-ylidene 2,4,6-Trimethylphenyl 2,4,6-Tri-tert-butylphenyl Metal organic chemical vapor deposition Methoxymethyl Microwave reactor Sodium tetrakis(pentafluorophenyl)borate Sodium borohydride Norbornene N-Bromosuccinimide N-Chlorosuccinimide N-Heterocyclic carbene N-Iodosuccinimide N-Methylmorpholine N-oxide N-Methyl-2-pyrrolidone

Organometallic Compounds of Germanium

PCy PDI PMB PMDTA ROP SCXRD TBAF TBAI Tbb TBDPS TBS Tbt TCCA TFA THF THP Tip TMEDA TMS TMSCN TMSN3 Tos or Ts TPP Trip Tsi UV

95

Tricyclohexylphosphine Polydispersity index p-Methoxybenzyl Pentamethyldiethylenetriamine Ring opening polymerization Single-crystal X-ray diffraction Tetra-n-butylammonium fluoride Tetrabutylammonium iodide 2,6-Bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl tert-Butyldiphenylsilyl tert-Butyldimethylsilyl 2,4,6-Tris-[bis(trimethylsilyl)methyl]phenyl Trichloroisocyanuric acid Trifluoroacetic acid Tetrahydrofuran Tetrahydropyran 2,4,6-Triisopropylphenyl Tetramethylethylenediamine Trimethylsilyl Trimethylsilylcyanide Trimethylsilylazide Toluenesulfonyl Triphenyl phosphite 2,4,6-Triisopropylphenyl Tris(trimethylsilyl)methyl Ultraviolet

10.03.1 Introduction The organometallic chemistry of germanium is an ever-burgeoning field of research that occupies a unique position in the chemistry of group 14 elements. Following tradition, this chapter revolves around the organic derivatives of germanium; compounds without GedC bonds demonstrating chemistry comparable to authentic organometallic compounds are also not overlooked. The chemistry of germanium(IV) and germanium(II) compounds have both witnessed prodigious developments; Ge(II) chemistry, in terms of volume, can comfortably match, if not surpass, that of germanium(IV) chemistry. Several novel aspects of low-valent germanium chemistry, such as NHC/cAAC stabilized germylenes, germylones, catalytic applications of germylenes and their metal complexes, and so forth, have bloomed since this chapter was last written by Weinert in 2005. Along with these aspects, progress in all the conventional domains of organogermanium chemistry that happened during the last one and a half decades (from 2006 to 2020) are systematically presented, considering the published reviews judicially. Furthermore, full prominence to synthesis and reactivity is provided; representative structural details are also included.1–6

10.03.2 Compounds containing germanium carbon bonds Conventionally, compounds with Ge(IV)–C bonds are isolated by reacting compounds containing GedX bonds with Grignard/ organolithium reagents (X ¼ halogens). Other routes for making GedC bonded compounds, such as hydrogermylation of unsaturated hydrocarbons using germyl hydrides, reactions of germyl anions with suitable organic substrates, metal-catalyzed coupling reactions, and so forth, are discussed under this heading along with their reactivity.

10.03.2.1 Preparation of compounds with GedC single bonds 10.03.2.1.1

Tetraalkyl, tetraaryl, and organofunctional germanium compounds

10.03.2.1.1.1 Synthesis from germyl hydrides The cyclopropylgermanes 1a-d (Eq. 1) and 2a-b (Eq. 2) were obtained by platinum dichloride catalyzed hydrogermylation reactions of 3,3-disubstituted cyclopropenes with HGeEt3.7 It was found that GeEt3 prefers attach from the less hindered side. The Cu(I) catalyzed reactions of propargyl esters with triethylgermane resulted in trisubstituted furan derivatives 3a-c (Eq. 3).8

96

Organometallic Compounds of Germanium

ð1Þ

ð2Þ

ð3Þ

Regio- and stereoselective hydrogermylation reactions of a-trifluoromethylated alkynes with HGePh3 in the presence of ammonium persulfate as radical initiator afforded Z-alkenes 4a-g. However, if these reactions were carried out in the presence of a Pd(0) catalyst, complementary E-alkenes 5a-f were obtained (Scheme 1).9

Scheme 1

A stereospecific addition of HGeEt3 to pentaphenylborole 6 resulted in syn-1-bora-cyclopent-3-ene 7 (Eq. 4).10

Organometallic Compounds of Germanium

97

ð4Þ

Conversion of the CdF bond of a polyfluorinated olefin into a CdH bond was achieved by treating 8 with HGeR3 (R ¼ Et, tBu, Ph) in the presence of a nanoscale aluminum chlorofluoride catalyst under photolytic conditions (311 nm), which resulted in the formation of 11a-c (Eq. 5). The reaction of HGeR3 with compound 12 was found to be less selective, and a mixture of two products 13a-c and 14a-c, were obtained (Eq. 6).11

ð5Þ

ð6Þ

The regioselective and stereoselective germylzincation reaction of terminal and internal ynamides with HGeR30 (R0 ¼ Ph, 2-furyl, Bu, Et) and Et2Zn resulted in a-zincated b-germylenamides, which were converted to hydrogermylation products 16–24 (Schemes 2 and 3).12 The representative structure of compound 15a that gave compound 16a (Fig. 1 and Table 1) shows the coordination of the carbonyl oxygen with zinc. n

Scheme 2

98

Organometallic Compounds of Germanium

Scheme 3

Fig. 1 Molecular structure of compound 15. Table 1

Selected bond lengths and angles of compound 15.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Zn-C1 Zn-C26 Zn-O1 Zn-O3 C1-C2 C26-C27

1.995 1.985 2.218 2.208 1.346 1.331

C1-Zn-C26 O1-Zn-O3

170.83 97.78

Similarly, the reactions of a-heteroatom substituted alkynes with HGePh3 and Et2Zn afforded hydrogermylated products 25 to 29 (Scheme 4).

Scheme 4

Organometallic Compounds of Germanium

99

Furthermore, the CdZn bond in a-zincated b-germylenamide intermediates remains available to form new CdC/C-heteroatom bonds with retention of the configuration of the double bond, through Cu(I) mediated electrophilic substitution reactions, to afford compounds 30–44 (Schemes 5 and 6).12

Scheme 5

Scheme 6

100

Organometallic Compounds of Germanium

10.03.2.1.1.2 Synthesis from germyl anions The reactions of tri-tert-butylcyclopropenium salt 45 with triarylgermyllithium salts resulted in the corresponding triaryl(tri-tertbutylcyclopropenyl)germanes 46a-c (Eq. 7).13,14 Furthermore, the reaction of compound 46b with HCl and catalytic amounts of AlCl3 afforded p-anisyl(tri-tert-butylcyclopropenyl)dichlorogermane 47 (Eq. 8).13

ð7Þ

ð8Þ

Several tetra-substituted acylgermanes 48a-m were synthesized by reacting tetrakis(trimethylsilyl)germane with KOtBu followed by the reactions of the in situ generated potassium salts with acid fluorides (Scheme 7).15

Scheme 7

10.03.2.1.1.3 Synthesis from germyl halides The reaction of difluorodimesitylgermane with nBuLi and chloroform afforded the corresponding trichloromethylgermane 49, which upon treatment with tBuLi followed by the addition of methanol resulted in methoxygermane 50 (Eq. 9). However, the reaction of compound 49 with tBuLi without methanol resulted in 1,2-dichloro(chlorogermyl)(fluorogermyl)ethene 51. Moreover, the reaction of compound 51 with tBuLi afforded bis(chlorodimesitylgermyl)alkyne 52 (Eq. 10).16

Organometallic Compounds of Germanium

101

ð9Þ

ð10Þ

The first-generation polyacetylenic germanium dendrimer 53a was synthesized by reacting ethynylbis(trimethylsilylethynyl) (methyl)silane with EtMgBr and GeCl4. Analogous compounds 53b and 53c were also obtained by reacting ethynyltris (trimethylsilylethynyl)silane/germane with EtMgBr and GeCl4 (Eq. 11).17

ð11Þ

The reaction of 4-bromobenzophenone 54 with 2,2-dimethylpropane-1,3-diol resulted in 1,3-dioxane derivative 55, which further reacted with nBuLi and GeCl4 to afford tetrahedral germane 56 (Eq. 12).18 Treatments of various aryl bromides with magnesium and tetrahalogermanes selectively afforded tetraarylgermanes 57a-d and triarylgermanium halides 58a-f depending on the substitution pattern in the aryl moieties (Eq. 13).19

ð12Þ

ð13Þ

The reactions of triorganogermanium chlorides with lithium acetonitrile resulted in a-germylated nitriles 59a, 59b, and 59c (Eq. 14).20 Selenocarbamoylgermane 60 was synthesized by reacting N,N-dibenzyl selenoformamide with LDA and trimethylgermanium chloride (Eq. 15).21

ð14Þ

ð15Þ

The reaction of 1,8-dichloroanthracene with elemental bromine afforded 10-bromo-1,8-dichloroanthracene 61, which on treatment with nBuLi and Me3GeCl resulted in dichloroanthracene containing germane 62 (Eq. 16).22 (4-Bromophenyl)

102

Organometallic Compounds of Germanium

trimethylgermane 63 was synthesized by reacting 1,4-dibromobenzene with nBuLi and Me3GeCl. Further reaction of compound 63 with nBuLi and DMF resulted in 4-(trimethylgermyl)benzaldehyde 64, which oxidized in air to 4-(trimethylgermyl)benzoic acid 65 (Eq. 17).23

ð16Þ

ð17Þ

The reaction of 1-methyl-4-bromopyrazole with LDA and trimethylgermanium chloride afforded the corresponding pyrazole 66 with a trimethylgermyl substituent. Lithiation followed by boronation of compound 66 resulted in pyrazolylboronic acid 67 with germyl substituent (Eq. 18).24

ð18Þ

Triethylgermanes 68a and 68b with alkynyl substituents were synthesized through the reactions of terminal alkynes with triethylgermanium chloride; additionally, an alkali metal iodide, amine (to scavenge HI), and iridium(I) catalyst were used (Eq. 19).25 Similarly, the reactions of 1,4-diethynylbenzene with 2.2 and 1 equiv. of trimethylgermanium chloride afforded bis- and mono-germyl products 69 and 70, respectively (Scheme 8).25

ð19Þ

Scheme 8

Organometallic Compounds of Germanium

103

The reaction of 4-bromotoluene with magnesium and dimethylgermanium dichloride afforded diarylgermane 71, which on further reaction with NBS provided the corresponding bromo derivative 72. The nucleophilic substitution reaction of compound 72 with triphenylphosphine resulted in diphosphonium salt 73 (Scheme 9), which was utilized to synthesize various polymers through Wittig reactions (Section 10.03.8, Scheme 91).26

Scheme 9

Fluorous tagged phenylgermane 74 was obtained by reacting bromobenzene with the corresponding bromogermane. Borylation of compound 74 was achieved using B2pin2 and an iridium(I) catalyst to afford a mixture of boronic esters 75. The Suzuki coupling between compound 75 and N-benzyl derivative of piperidinyl vinyl triflate in the presence of TlOH and LiBr resulted in styrene derivative 76. Metal catalyzed hydration of compound 76 afforded the germane functionalized piperidine tert-alcohol 77 (Scheme 10).27

Scheme 10

Organogermane functionalized 1,3-bis(ketenes) 78a and 78b were obtained by treating bis(alkoxyethynyl)silanes with trimethylgermanium iodide (Eq. 20). Further reactions of compound 78a with benzylamine and water resulted in bis(2trimethylgermyl)-acetamide 79 and acetic acid 80 derivatives, respectively (Eq. 21).28

ð20Þ

104

Organometallic Compounds of Germanium

ð21Þ

The reactions of 2-acetylfuran/2-acetylthiophene with germahalides facilitated by LiNMP (to protect the carbonyl group) and nBuLi resulted in the corresponding germyl-substituted acetylfurans 81a-b/acetylthiophenes 81c-d (Eq. 22).29

ð22Þ

10.03.2.1.1.4 Synthesis from organogermyl compounds Diaryldimethoxygermane 82 was obtained by the reaction of an aryllithium salt with tetramethoxygermane. The reaction of compound 82 with AgF in the presence of water resulted in fluorodiarylhydroxygermane 83 (Scheme 11), which underwent lithiation using tBuLi to afford lithiogermanolate 84. Treatment of compound 84 with chlorotrimethylsilane provided fluorosiloxygermane 85 (Scheme 11).30

Scheme 11

The germylation reactions of various terminal alkynes with vinylgermanes 86a-b were achieved using ruthenium catalysts with RudH/RudGe bond (Eq. 23, Table 2).31

ð23Þ

The photochemical (l ¼ 455 nm) decarboxylation of germacarboxylic acid using [Ir(ppy)2(dtbbpy)][PF6] catalyst produced a germyl radical, which was trapped with alkenes to afford the corresponding germylated derivatives 88a and 88b (Eq. 24).32

Organometallic Compounds of Germanium

105

ð24Þ

Table 2 Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Experimental data for Eq. (23). R

GeR0 3

Catalyst

Yield (%)

t Bu Cy n C5H11 SiEt3 Si(iPr)3 Si(tBu)Me2 SiMe2Ph GeEt3 GeEt3

GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeEt3 GeMe2Ph

[RuHCl(CO)(PCy3)2] [RuHCl(CO)(PPr3)2] [RuHCl(CO)(PPh3)3] [Ru(GeEt3)Cl(CO)(PPh3)2] [RuH(CO)(MeCN)2(PCy3)2][BF4] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2] [RuHCl(CO)(PCy3)2]

88% (87a) 80% (87a) 54% (87a) 50% (87a) 26% (87a) 99% (87b) 37% (87c) 69% (87d) 81% (87e) 74% (87f) 50% (87g) 47% (87h) 94% (87i) 90% (87j)

A range of tetraalkyl germanes 89–96 was obtained by the hydrogermylation of alkenes using cyclohexa-2,5-dien-1-ylgermanes (as Et3GeH, Me2GeH2, and MeGeH3 surrogates) in the presence of B(C6F5)3 as a catalyst (Schemes 12 and 13).33

Scheme 12

106

Organometallic Compounds of Germanium

Scheme 13

The hydrogermylation of internal alkynes 97 and 99 selectively afforded cis-(Z-98) and trans-(E-100) vinylic germanes, respectively, subject to the electronic preference of alkynes (Eqs. 25 and 26).33

ð25Þ

ð26Þ

The reaction of dimethylgermyl phenol 101 with HypoGel®-Br resin and 2-chlorodiethylether in the presence of K2CO3 and TBAI resulted in dimethylgermanes with HypoGel® 102a and ethoxyethyl ether 103a functionalizations, respectively. These compounds were converted to their bromide derivatives 102b and 103b by reacting them with HBr (Scheme 14).34

Scheme 14

tert-Butyl ester functionalized pyrazole derivative 104 was reacted with bromogermane 103b to obtain the corresponding germyl pyrazole ester 105, which on further reaction with N-aminomorpholine resulted in the morpholinylamide containing germyl pyrazole 106 (Scheme 15).

Organometallic Compounds of Germanium

107

Scheme 15

Similarly, HypoGel®-bound germyl pyrazole ester 107 was obtained by reacting compound 104 with bromogermane 102b. The amidation reactions of compound 107 with various amines resulted in the corresponding HypoGel®-bound germyl pyrazolyl amides 108a-e (Scheme 16).34

Scheme 16

108

Organometallic Compounds of Germanium

10.03.2.1.2

Intracyclic germanium–carbon single bonded compounds

10.03.2.1.2.1 Small rings 10.03.2.1.2.1.1 Five-membered rings Germanium-selenium containing five-membered heterocycles 110a-c (as a mixture E- and Z-isomers) and 111 (as Z-isomer) with fulvene-based structures were synthesized by reacting SeBr2/SeCl2 with diorganodiethynylgermanes 109a-c (Scheme 17).35 Treatment of diethynylgermanes 109a and 109b with SeBr4 and SeCl4, respectively, yielded a mixture of non-fulvenic germaselena heterocycles (112 and 113), trihalofulvenes (114a and 115), and fulvenic heterocycles (Z-110a and Z-111). However, in the reactions of compounds 109b and 109c with SeBr4, tribromofulvenes 114b and 114c were obtained exclusively (Scheme 17).35

Scheme 17

The intramolecular trans-hydrogermylation of 116 using a ruthenium catalyst [Cp Ru(MeCN)3][PF6] produced 1-germaindene 117 (Eq. 27).36 Using the same catalyst, the double trans-hydrogermylation of 1,3-diynes 118a-o and 120 with diphenylgermane and dibutylgermane were carried out to obtain 2,5-disubstituted germoles 119a-o and 121, respectively (Eqs. 28 and 29, and Table 3). Notably, through the reaction of 1,3,5,7-tetrayne 122 with diphenylgermane, 2,20 -bigermole 123 was obtained (Eq. 30).36

ð27Þ

Organometallic Compounds of Germanium

Table 3

109

Experimental data for 119a-o prepared in Eq. (28).

Entry

Reactant

R

Product

% Yield

1 2 3 4 5 6 7 8 9 10 11 12

118a 118b 118c 118d 118e 118f 118g 118h 118i 118j 118k 118l

Phenyl 2-Naphthyl 4-Methoxyphenyl 4-Fluorophenyl 3-Bromophenyl 3-B(pin)-phenyl 4-(Trimethylsilyl)phenyl 4-Nitrophenyl 3-Thienyl 5-Pyrimidyl Cyclohexen-1-yl

119a 119b 119c 119d 119e 119f 119g 119h 119i 119j 119k 119l

93 66 87 80 93 91 75 40 94 69 70 95

13

118m

119m

71

14

118n

119n

87

15

118o

119o

44

ð28Þ

ð29Þ

ð30Þ

The dehydrogenative germylation of biarylgermanes 124a-e using a catalytic system of [RhCl(COD)]2 and PPh3 resulted in unsymmetrical 9-germafluorenes 125a-e via the activation of CdH and GedH bonds (Eq. 31). This protocol was also utilized for the synthesis of naphthalene 127 and benzothiophene 129 fused tetracyclic germoles (Eqs. 32 and 33).37

110

Organometallic Compounds of Germanium

ð31Þ

ð32Þ

ð33Þ

The reaction of 1,2-diphenylacetylene with lithium metal and germanium tetrachloride led to 1,1-dichloro-tetraphenylgermole 130, which was reacted with various alkynyllithium salts to afford 1,1-disubstituted germoles 131-136 (Scheme 18, Fig. 2, Table 4).38

Scheme 18

Organometallic Compounds of Germanium

111

Fig. 2 Molecular structure of compound 132.

Table 4

Selected bond lengths and angles of compound 132.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge-C1 Ge-C4 Ge-C29 Ge-C38

1.954 1.940 1.890 1.897

C1-Ge-C4 C4-Ge-C29 C29-Ge-C38 C38-Ge-C1

91.83 111.78 105.68 118.57

Dibenzogermole 138 featuring a stereogenic germanium atom was synthesized by a [2 + 2+2] cycloaddition of prochiral triyne 137 with 1,4-dimethoxy-2-butyne in the presence of a rhodium catalyst and a homochiral (R)-ligand (Eq. 34).39

ð34Þ

Various substituted benzogermoles 140a-o were synthesized by the reactions of 2-trimethylgermylphenylboronic ester 139 with alkynes in the presence of [RhCl(COD)]2 as a catalyst (Eq. 35); the reactions involved the activation of a GedMe bond.40 Similarly, dibenzogermole 142 was synthesized from trimethylgermyl substituted biphenylboronic acid 141 in the presence of a rhodium catalyst (Eq. 36).40

112

Organometallic Compounds of Germanium

ð35Þ

ð36Þ

The reactions of dichlorogermafluorene 143 with one and two equiv. of lithiated phosphaalkene afforded monophosphaalkenyl 144 and diphosphaalkenyl 145 derivatives, respectively (Eq. 37).41 Reduction of spirogermabifluorene 146 with two equiv. of cesium or excess sodium resulted in germafluorenyl monoanion 147 and dianion 148 via the cleavage of one and two GedC bonds (Eq. 38).42 Germanium-bridged bipyridyl derivative 150 was prepared by reacting dibromobipyridyl 149 with nBuLi and diphenyldichlorogermane (Eq. 39).43

ð37Þ

ð38Þ

ð39Þ

Benzofuran-fused germoles 152a and 152b were synthesized via the reactions of diiodobi(benzofuran) 151 with nBuLi and diorganodichlorogermane (Scheme 19). Recrystallization of 152a can then be exploited to afford [2 +2] dimer 153 (Fig. 3, Table 5), which upon dissolving in CDCl3, reverts to the monomer 152a (Scheme 19).44

Organometallic Compounds of Germanium

Fig. 3 Molecular structure of compound 153.

Table 5

Scheme 19

Selected bond lengths and angles of compound 153.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C8 Ge1-C10 C9-C10 C9-C100 C9-O2

1.934 2.008 1.571 1.566 1.449

C8-Ge1-C10 Ge1-C10-C9 Ge1-C10-C90 C9-C10-C90 C10-C9-C100

89.93 106.75 119.69 89.03 90.97

113

114

Organometallic Compounds of Germanium

The reactions of dibromobithiophene 154 with nBuLi and diorganodichlorogermanes led to dithienogermoles 155a and 155b. These compounds were converted to the corresponding dibromo derivatives 156a and 156b by reacting them with NBS; the latter compound was also converted to a distannyl derivative 157 using nBuLi and Me3SnCl (Scheme 20).45 Compound 157 was then utilized as a monomer in polymer synthesis (Section 10.03.8, Scheme 90).

Scheme 20

Dithienogermoles 159a-c having two GedCl bonds were obtained by reacting a dibromo derivative of bithiophene 158 with BuLi and tetrachlorogermane. The reaction of compound 159a with LiAlH4, Grignard reagents, and organolithium reagent afforded dithienogermoles 160-162 with two GedX bonds (X ¼ hydrido/alkyl/aryl) (Scheme 21).46

n

Scheme 21

Reactions of 2,20 -bithieno(3,2-b)thiophene 163 (featuring bromo and trimethylsilyl substituents) with nBuLi and diorganodibromogermanes was shown to afford the corresponding germole derivatives 164a-c, featuring trimethylsilyl groups. Treatment of these compounds with NBS gave dibromo derivatives 165a-c, which reacted with nBuLi and trimethyltin chloride to produce distannyl derivatives 166a-c (Scheme 22); the latter compounds were subsequently employed in polymer synthesis (Section 10.03.8, Scheme 89).47–49

Organometallic Compounds of Germanium

115

Scheme 22

Bis(thiophene) derivative 167 was reacted with LDA and trimethylchlorosilane to obtain the corresponding trimethylsilyl derivative 168, which was reacted with nBuLi, diorganodibromogermane, and NBS to afford germaindacenodithiophene 169 (Scheme 23), which was used as a monomer in polymer synthesis (Section 10.03.8, Scheme 88).50

Scheme 23

Trimethylsilyl-substituted dithienogermolocarbazole 171 was synthesized by the reaction of dithienocarbazole 170 with nBuLi and dibutyldichlorogermane. Conversion of the trimethylsilyl derivative 171 to bromo derivative of germanium bridged heptacyclic arene 172 using NBS, followed by its reaction with tBuLi and Me3SnCl afforded stannylated dithienogermolocarbazole 173. When compound 173 was subjected to column chromatography using neutral alumina, destannylated heptacyclic arene 174 (Scheme 24) was obtained.51 Compound 172 was also utilized to synthesize polymers (Section 10.03.8, Eq. 283).

116

Organometallic Compounds of Germanium

Scheme 24

Dibromodiselenogermole 176 was obtained through a sequence of reactions between 2,20 -biselenophene 175 with nBuLi, diorganodibromogermane, and NBS. Stannylated diselenogermole 177 was synthesized by treating compound 176 with nBuLi and Me3SnCl, which was involved in polymer synthesis (Eq. 40) (Section 10.03.8, Eq. 284).52 Similarly, the octyl analog of compound 176 (179) was also isolated starting from compound 175 (Eq. 41).53

ð40Þ

ð41Þ

Dibromo pyridylthiophene derivatives 180a and 180b were treated with nBuLi and diorganodichlorogermanes to obtain pyridinothienogermoles 181a-c.54 Compound 181b was brominated using NBS to generate brominated pyridinothienogermole 182, which was subsequently reacted with nBuLi and trimethyltin chloride to afford stannyl derivative 183. The Stille coupling between compounds 182 and 183 resulted in bi(pyridinothienogermole) 184 (Scheme 25). Similarly, Stille coupling of compound 182 with N-[(trimethylstannyl)phenyl]carbazole and diphenyl[(trimethylstannyl)phenyl]amine afforded donor-acceptor type carbazolylphenyl-185 and diphenylaminophenyl-186 pyridinothienogermoles, respectively.54

Organometallic Compounds of Germanium

117

Scheme 25

Sandmeyer reaction conditions were used to convert triaminotriphenylene 187 to 1,5,9-triiodotriphenylene 188, which was reacted with nBuLi, dimethyldichlorogermane, and LiAlH4 to result in the dimethylhydridogermane substituted triphenylene 189. Treatment of compound 189 with 3,3-dimethyl-1-butene in the presence of Wilkinson’s catalyst gave C3-symmetric germasumanene 190 (Scheme 26, Fig. 4, Table 6).55

Scheme 26

118

Organometallic Compounds of Germanium

Fig. 4 Molecular structure of compound 190.

Table 6

Selected bond lengths and angles of compound 190.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C1 Ge1-C16 Ge2-C4 Ge2-C7 Ge3-C10 Ge3-C13

1.951 1.980 1.951 1.967 1.962 1.978

C1-Ge1-C16 C4-Ge2-C7 C10-Ge3-C13

89.78 90.64 89.65

10.03.2.1.2.1.2 Six-membered rings Various electron-deficient and electron-rich germylated aromatic substrates 191a-j were reacted with nBuLi to obtain germanium-containing binuclear heterocycles 192a-i, 193a-e, and 194a-j (Eq. 42); predominantly [4:4:0] bicyclic isomers were formed, however benzo-oxa/azagermole type [4:3:0] bicyclic products were obtained in a few cases (Table 7).56

ð42Þ

Table 7 Entry

Experimental data for Eq. (42). Starting material

Products

% Yield

Ratio of 192:193:194

1

82

25:6:69

2

99

0:100:0

Organometallic Compounds of Germanium

Table 7 Entry

119

(Continued) Starting material

Products

% Yield

Ratio of 192:193:194

3

57

0:54:46

4

89

0:83:17

5

62

15:85:0

6

55

100:0:0

7

75

100:0:0

8

93

100:0:0

9

84

100:0:0

10

92

0:0:100

120

Organometallic Compounds of Germanium

A range of benzogerminones 196a-o were synthesized by the reactions of dibutyl(2-iodophenyl)germane 195 with terminal aromatic alkynes in the presence of Pd(PPh3)4 as a catalyst and DABCO as a base; these reactions showed good functional group tolerance (Eq. 43, Table 8).57

ð43Þ

Table 8 Entry

Experimental data for compounds 196a-o prepared in Eq. (43). Alkyne

Product

Yield (%)

1

77

2

60

3

62

4

67

5

60

Organometallic Compounds of Germanium

Table 8 Entry

121

(Continued) Alkyne

Product

Yield (%)

6

74

7

69

8

78

9

78

10

84

11

74

(Continued )

122

Table 8 Entry

Organometallic Compounds of Germanium

(Continued) Alkyne

Product

Yield (%)

12

76

13

69

14

65

15

65

Treatment of bis(2-bromo-4-dimethylaminophenyl)methane 197 with sBuLi, dimethyldichlorogermane, and chloranil resulted in germapyronine 198. Insertion of the 2-methylphenyl substituent at the 9-position of germapyronine 198 using (2-methylphenyl) lithium afforded germarhodamine 199 (Eq. 44).58

ð44Þ

Organometallic Compounds of Germanium

123

10.03.2.1.2.1.3 Seven-membered rings The reaction of the bromo derivative 200 of benzo[b]thiophene carboxaldehyde acetal with nBuLi and dimethyldichlorogermane resulted in germane derivative 201 featuring two acetal units. Acetal deprotection by aqueous perchloric acid gave the corresponding dialdehyde 202, which under McMurry coupling conditions afforded 1-germacycloheptatriene 203 (Scheme 27).59

Scheme 27

Repetition of the reaction sequence in Scheme 27 using 2-bromobenzaldehyde acetal 204 afforded another germacycloheptatriene 207 (Scheme 28).59

Scheme 28

10.03.2.1.2.2 Large rings Diorganogermadialkynes 209a and 209b were obtained by the diethynylation of diorganodichlorogermanes 208a and 208b. Compounds 209a-b were reacted with nBuLi to generate dianions, which coupled with compound 208a/b and cyclized to afford a mixture of germa[4]pericyclyne 210 (Fig. 5, Table 9), germa[6]pericyclynes 211a-b, and germa[8]pericyclynes 212a-b (Eq. 45).60

124

Organometallic Compounds of Germanium

ð45Þ

Fig. 5 Molecular structure of compound 210.

Table 9

Selected bond lengths and angles of compound 210.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C1 Ge1-C2 C2-C3 Ge2-C3 Ge2-C4 C4-C10

1.900 1.911 1.209 1.916 1.906 1.217

C1-Ge1-C2 Ge1-C2-C3 C2-C3- Ge2 C3-Ge2-C4 Ge2-C4-C10

103.97 167.70 171.01 103.65 176.19

Organometallic Compounds of Germanium

125

The reaction of tetrachlorogermane with ethynyl Grignard reagent gave tetraethynylgermane 213, which, when silylated using two equiv. of nBuLi and trimethylchlorosilane, afforded disilylated derivative 214. Treatment of compound 214 with nBuLi and dipheynldichlorogermane afforded germa[4]pericyclyne 215, exclusively (Scheme 29).61

Scheme 29

Germapericyclynes 210, 212a-b, 219a-b, and 220a-b were isolated through a stepwise synthetic protocol starting from diorganodiethynylgermanes 209a-b.62 Treatment of compounds 209a-b with nBuLi and the corresponding diorganodichlorogermanes resulted in trigermane 217a-b. Macrocyclization reactions of compounds 217a-b with nBuLi and diorganodichlorogermanes afforded germa[4]- 210a and germa[8]- 212a-b pericyclynes. Compounds 217a-b were converted to tetragermanes 218a-b using a protocol that involved reactions with an ethyl Grignard reagent, diorganodichlorogermanes, and alkynyl Grignard reagents. Subsequent macrocyclization reactions of compounds 218a-b using nBuLi and diorganodichlorogermanes gave a mixture of germa[5]pericyclynes 219a-b and germa[10]pericyclynes 220a-b (Scheme 30).62

126

Organometallic Compounds of Germanium

Scheme 30

Organometallic Compounds of Germanium

127

Treatment of hexachlorobutadiene 221 with nBuLi produced diyne diacetylide 222, which was reacted with diphenyldichlorogermane over 7 days to afford a mixture of germa[4]- 223, germa[5]- 224, germa[6]- 225, germa[7]- 226 and germa[8]- 227 pericyclynes (Scheme 31).63

Scheme 31

128

Organometallic Compounds of Germanium

Bis(trimethylsilylethynyl)benzene 228 was prepared by the Sonogashira coupling of diiodobenzene with trimethylsilylacetylene. The desilylation of compound 228 with KOH resulted in diethynylbenzene 229, which was macrocyclized using nBuLi and diisopropyldichlorogermane to afford a mixture of arene-inserted germa[4]- 230 (Fig. 6, Table 10), germa[5]- 231, germa[6]- 232 and germa[7]- 233 pericyclynes (Scheme 32).64

Fig. 6 Molecular structure of compound 230.

Table 10

Selected bond lengths and angles of compound 230.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C10 Ge1-C11 C11-C12 Ge2-C20 Ge2-C10 C19-C200

1.917 1.917 1.190 1.912 1.904 1.201

C10-Ge1-C11 Ge1-C10-C9 C11-C12- Ge1 C20-Ge2-C10 Ge2-C20-C19 Ge2-C10 -C20

104.15 176.29 173.08 103.15 170.39 177.46

Organometallic Compounds of Germanium

129

Scheme 32

10.03.2.1.2.3 Bridged cyclic rings Diels-Alder reactions of tetraphenyl germole 234 with N-methylmaleimide 235a and maleic anhydride 235b under high pressure afforded 7-germanorbornene derivatives 236a and 236b, respectively (Eq. 46).65 Similarly, the reaction of compound 234 with the dibromo derivative of 1,4-epoxy-1,4-dihydronaphthalene (237) gave the corresponding adduct 238 via a Diels-Alder reaction (Eq. 47).66 Dibenzo-7-germanorbornadiene 239 (Fig. 7 and Table 11) was synthesized by the reaction of magnesium anthracene with dimethyldichlorogermane. Heating of compound 239 in toluene resulted in digerma species 240 through ring expansion (Eq. 48).67

130

Organometallic Compounds of Germanium

ð46Þ

ð47Þ

ð48Þ

Fig. 7 Molecular structure of compound 239.

Table 11

Selected bond lengths and angles of compound 239.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C2 C1-Ge1 Ge1-C3 Ge1-C4

2.028 2.029 1.940 1.947

C3-Ge1-C4 C3-Ge1-C2 C4-Ge1-C1 C1-Ge1-C2

112.26 115.46 115.28 77.72

Organometallic Compounds of Germanium

10.03.2.1.3

131

Organogermanium compounds featuring metals

10.03.2.1.3.1 Organogermanium compounds featuring group 13 metals 10.03.2.1.3.1.1 Synthesis from monoalkynylgermane Alkynylchlorogermane 241 was obtained by reacting diphenyldichlorogermane with tert-butylethynyllithium. The hydroalumination of 241 using tert-butylaluminium hydride afforded a four-membered AlCGeCl heterocycle 242 featuring an intramolecular Cl !Al interaction (Eq. 49).68,69

ð49Þ

10.03.2.1.3.1.2 Synthesis from dialkynylgermanes The reaction of diorganodichlorogermanes with phenylethynyllithium resulted in the dialkynylgermanes 243a-b, which undergo hydroalumination reactions with dialkylaluminium hydrides to afford aluminum containing alkenylalkynylgermanes 244a-c and 245a-b (Scheme 33). Single crystal X-ray diffraction analysis of compounds 244a-c showed intramolecular interactions between aluminum and the alkynyl a-carbon atoms.70

Scheme 33

By contrast, the reaction of compound 243b with two equiv. of di(tert-butyl)aluminum hydride afforded aluminum-containing dialkenylgermane 246, which was further reacted with tetra(nbutyl)ammonium halides to afford six-membered heterocycles 247a and 247b (Eq. 50).71

ð50Þ

132

Organometallic Compounds of Germanium

Hydrogallation of diaminodialkynylgermane 248 using di-tert-butylgallium hydride resulted in a four-membered NGeCGa heterocycle 249, which on treatment with heterocumulenes afforded six-membered heterocycles 250a-b via the cleavage and formation of GadN and GadX bonds, respectively (X ¼ O, S) (Eq. 51).72

ð51Þ

Hydroalumination/hydrogallation of dioragnodialkynylgermanes using di-tert-butyl-aluminum/gallium hydrides afforded compounds 251a-g, analogous to 244a-c (Eq. 52, Table 12).73

ð52Þ

Table 12

Experimental data for Eq. (52).

Entry

Product

M

R1

R2

R3

Yield (%)

1 2 3 4 6 7 8

251a 251b 251c 251d 251e 251f 251g

Al Ga Al Ga Ga Ga Ga

C6H5 C6H5 CH3 CH3 CH3 C6H5 C6H5

CMe3 CMe3 CMe3 CMe3 C6H5 n Bu CH3

CMe3 CMe3 CMe3 CMe3 CMe3 CMe3 CMe3

82 82 99 99 62 99 70

The reaction of dichlorodialkynylgermane 252 with biphenyl-2,2-diyldilithium resulted in an dialkynyl derivative of 9-germafluorene 253, which undergoes hydrometallation to afford the corresponding aluminum and gallium compounds 254a and 254b (Eq. 53).73

ð53Þ

Aminodialkynylgermane 255 was obtained by reacting phenyltrichlorogemane with LiNEt2 and alkynyl lithium. Hydrometallation of compound 255 resulted in the corresponding four-membered aluminum and gallium heterocycles 256a and 256b. However, the reaction of compound 255 with two equivalents of alkylaluminium hydride afforded spiro-germane 257, which further reacted with [nBu4N]Cl to produce a six-membered heterocycle 258 (Scheme 34).74

Organometallic Compounds of Germanium

133

Scheme 34

The reaction of aryltrichlorogermane with alkynyllithium resulted in dialkynylchlorogermanes 259a-c, which upon hydrometallation afforded four-membered heterocycles 260a-d (Eq. 54). Spiro-germane 261 was obtained via the double hydrogallation of 259a with two equiv. of di-tert-butylgallium hydride (Eq. 55).68

ð54Þ

ð55Þ

Thermolysis of compound 260a resulted in alkenylalkynylgermane 263, in which the migration of a bridging chloride from germanium to aluminum, with concomitant transfer of a tert-butyl group from aluminum to germanium atom has occurred (Eq. 56). Single crystal X-ray diffraction analysis of compound 263 revealed its existence as a centrosymmetric dimer in the solid state.68

134

Organometallic Compounds of Germanium

ð56Þ

10.03.2.1.3.1.3 Synthesis from trialkynylgermane Double hydrometallation of aminotrialkynylgermane 264 afforded the corresponding spiro-germanes featuring aluminum 265a and gallium 265b atoms (Scheme 35). The high-temperature reaction of 265b with phenylalkyne resulted in GedN bond cleavage to afford bis(alkenylalkynyl)germane 266 having two different alkynide substituents at the germanium atom. Monohydrometallation of compound 264 was also pursued (Scheme 35).72

Scheme 35

10.03.2.1.3.1.4 Synthesis from tetraalkynylgermane Double hydroalumination of tetraethynylgermane 268 with two equiv. of a bulky dialkylaluminum hydride gave bis(alkenylalkynyl)germane 269, featuring two free alkynyl groups. Upon heating, the latter compound underwent intramolecular double 1,1-carbalumination of the alkynyl groups to afford spiro-germane 270 (Eq. 57, Fig. 8, Table 13).75

ð57Þ

Organometallic Compounds of Germanium

135

Fig. 8 Molecular structure of compound 270.

Table 13

Selected bond lengths and angles of compound 270.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

C1-Ge1 C2-Ge1 C3-Ge1 C4-Ge1

1.975(4) 1.973(5) 1.971(4) 1.980(3)

C1-Ge1-C3 C1-Ge1-C2 C2-Ge1-C4 C4-Ge1-C3

131.1(2) 70.1(2) 134.8(2) 70.5(2)

10.03.2.1.3.2 Organogermanium compounds with transition metals The treatment of heteroleptic dilithio vanadium sandwich compound 271 with dimethyldichorogermane afforded highly strained germanium-bridged [1]trovacenophane 272 (Eq. 58).76 In a similar manner, the chromium analog of compound 272, ansa [1] trochrocenophane 274, was also prepared using the heteroleptic dilithio chromium sandwich compound 273 (Eq. 59). However, the reaction of compound 273 with an excess of trimethylchlorogermane afforded 1,10 -digerma-trochrocene 275 (Eq. 59).77 Germanium-bridged [1]chromoarenophane 277a78 and [1]manganoarenophane 277b79 were prepared by reacting dimethyldichlorogermane with dilithio chromium 276a and manganese 276b sandwich compounds, respectively (Eq. 60).

ð58Þ

ð59Þ

136

Organometallic Compounds of Germanium

ð60Þ

10.03.2.2 Preparation of compounds featuring Ge]C double bonds 10.03.2.2.1

Acyclic germanium–carbon double bonded compounds

The reaction of dimesityldifluorogermane with lithiated fluorene resulted in fluorenyl fluorogermane 278. The dehydrofluorination of compound 278 with tert-butyllithium afforded germene 279 (Eq. 61).80 Fluorovinylgermane 280 upon reaction with tert-butyllithium also gave germene 281 (Eq. 62).81

ð61Þ

ð62Þ

The reaction of trimethoxy-tert-butylgermane 282 with TipLi resulted in dimethoxygermane 283, which reacted with HF to afford difluorogermane 284. The treatment of C-lithiophosphaalkene with compound 284 provided 1-phospha-3-germapropene 285 featuring both GedF and CdCl bonds. The dechlorofluorination of compound 285 using tert-butyllithium afforded phosphagermaallene 286 (Scheme 36).82 Similarly, debromofluorination of 1-arsa-3-germapropene 288 afforded arsagermaallene 289; compound 288 was obtained from arsaalkene 287 featuring two CdBr bonds (Scheme 37).83

Scheme 36

Organometallic Compounds of Germanium

137

Scheme 37

10.03.2.2.2

Cyclic germanium–carbon double bonded compounds

Germanium bismethanediide 291 was obtained by reacting bis(thiophosphoryl)methane 290 with MeLi and tetrachlorogermane (Eq. 63, Fig. 9, and Table 14).84

ð63Þ

Fig. 9 Molecular structure of compound 291.

Table 14

Selected bond lengths and angles of compound 291.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

S1-Ge1 Ge1-C1 S1-P1 C1-P1

2.325 1.882 2.060 1.723

S1-Ge1-C1 Ge1-C1-P2

85.33 131.3

138

Organometallic Compounds of Germanium

Isotellurochromene 292 was treated with nBuLi and a mixture of organotrihalogermanes (as the preparation of pure organotrichlorogermane was not possible) to obtain a mixture of halogermanes 293a and 293b. Consequently, to prepare the pure bromogermane 293b, the mixture of compounds 293a and 293b was reduced by LiAlH4 and brominated using NBS. Treatment of compound 293b with LDA afforded 2-germanaphthalene 295 (Scheme 38).85

Scheme 38

The reactions of bis(2-bromophenyl)methane 296 with nBuLi and aryltrichlorogermanes gave 10-dihydro-9-germaanthracene derivatives 297a-b and 298a-b featuring GedX bonds (X ¼ Cl, Br); due to the halogen exchange reaction with the side product LiBr. These compounds were converted to derivatives 300a and 300b featuring Ge-OTf units through a reaction sequence that involved treatment with LiAlH4, NBS, and AgOTf. Compounds 300a and 300b were then reacted with LDA to afford the 9-germaanthracenes 301a and 301b. When germaanthracene 301b was heated, a [4 +4] head-to-tail dimer 302 was obtained (Scheme 39).86

Scheme 39

Organometallic Compounds of Germanium

139

The reaction of bromobiphenyl 303 with magnesium, organotrichlorogermane, and LiAlH4 resulted in germane substituted biphenyl 304. Bromination of compound 304 with NBS in the presence of AIBN, followed by the cyclization reaction using magnesium, resulted in 10-dihydro-9-germaphenanthrene 306. The dehydrobromination reaction of compound 306 using LDA afforded 9-germaphenanthrene 307 (Scheme 40, Fig. 10, Table 15).86

Scheme 40

Fig. 10 Molecular structure of compound 307.

Table 15

Selected bond lengths and angles of compound 307.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

C1-Ge1 Ge1-C2 Ge1-C3

1.792(3) 1.886(3) 1.930(3)

C1-Ge-C2 C1-Ge1-C3 C2-Ge1-C3

106.0(1) 127.9(1) 125.9(1)

10.03.2.3 Reactivity Saturated compounds featuring GedC bonds have been utilized to synthesize various important compounds, such as halogenated enamides, arenes, and biaryls. Unsaturated systems were used to construct novel organogermanes through cycloaddition reactions.

140

Organometallic Compounds of Germanium

10.03.2.3.1

Reactivity of GedC single-bonded compounds

10.03.2.3.1.1 Reactions involving GedC bond cleavage Halodegermylation of b-triphenylgermylenamides 16a, 20b, and 30 with ICl and Br2 resulted in the cleavage of the germanium-carbon bond to afford iodo- 308a and bromo- 308b-308c enamides (Eq. 64).12

ð64Þ

The reaction of germyl substituted styrene derivative 76 with TFA afforded tetrahydropyridine 309 (Eq. 65). Similarly, germane-functionalized piperidine tert-alcohol 77 reacted with MeCN-H2SO4 (4:1) to yield acetylaminopiperidine 310 (Eq. 66).27

ð65Þ

ð66Þ

Solid-phase synthesis of rimonabant iodo analogs 311a-e was achieved by reacting HypoGel®-bound germyl pyrazolyl amides 108a-e with NaI and NCS (Eq. 67).34

ð67Þ

A range of functionalized electron-rich, electron-deficient, and heterocyclic triethylarylgermanes were reacted with NBS and NIS to afford the corresponding brominated and iodinated products 312-342 by electrophilic substitution reactions (Scheme 41). This halogenation protocol is effective for the orthogonal and chemoselective introduction of halogens with good functional-group tolerance and positional control.87

Organometallic Compounds of Germanium

141

Scheme 41

10.03.2.3.1.2 Coupling reactions The reaction of trimethylalkynylgermane 344 with propargyl chloride 343 and CsF in the presence of CuI resulted in the 1,4-skipped diyne 345 (Eq. 68).88

ð68Þ

142

Organometallic Compounds of Germanium

Irradiation of bis(2-naphthylmethyl)arylgermanes 346a-f along with Cu(BF4)2 yielded the corresponding difluorogermanes 347a-f, which undergo palladium-catalyzed cross-coupling reactions with aryl halides to afford biaryls 348a-v (Eq. 69, Table 16).89

ð69Þ

Table 16

Experimental data for Eq. (69).

Entry

R

Ar

Product

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

4-OMe (346a) 4-OMe (346a) 4-OMe (346a) 4-OMe (346a) 4-Me (346b) 4-Me (346b) 4-Me (346b) 4-Me (346b) H (346c) H (346c) H (346c) H (346c) 4-Cl (346d) 4-Cl (346d) 4-Cl (346d) 4-Cl (346d) 2-OMe (346e) 2-OMe (346e) 2-OMe (346e) 2-OMe (346e) 4-CF3 (346f) 4-CF3 (346f)

3,5-(CF3)2C6H3 4-ClC6H4 4-BnOC6H4 1-Naphthyl 3,5-(CF3)2C6H3 4-ClC6H4 4-BnOC6H4 1-Naphthyl 3,5-(CF3)2C6H3 4-ClC6H4 4-BnOC6H4 1-Naphthyl 3,5-(CF3)2C6H3 4-BnOC6H4 1-Naphthyl 2-NO2C6H4 3,5-(CF3)2C6H3 4-ClC6H4 4-BnOC6H4 1-Naphthyl 3,5-(CF3)2C6H3 4-ClC6H4

348a 348b 348c 348d 348e 348f 348g 348h 348i 348j 348k 348l 348m 348n 348o 348p 348q 348r 348s 348t 348u 348v

96 85 65 75 84 69 48 71 74 63 40 60 71 42 75 61 65 49 11 27 26 11

The palladium-catalyzed cross-coupling of phenylchlorogermanes 349a, 349b, and 349c with three, two, and one GedCl bond(s), respectively, with aryl halides in the presence of TBAF
H2O gave the corresponding heterocoupled products 351a-i, along with homocoupled side-products 352a-i (Table 17). The fluoride additive activates the GedCl bond, and promotes the transfer of up to three phenyl groups from the germanium center (Eq. 70).90

ð70Þ

Organometallic Compounds of Germanium

Table 17 Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

143

Experimental data for Eq. (70). Germane 349a 349a 349a 349a 349a 349a 349a 349a 349a 349a 349a 349b 349b 349b 349b 349b 349b 349b 349b 349b 349b 349b 349c 349c 349c 349c 349c 349c 349c 349c 349c 349c 349c

RX 1-bromonaphthalene 4-CH3OC6H4I 4-CH3OC6H4Br 2-CH3OC6H4I 3-CF3C6H4I 4-CF3C6H4I 4-CH3COC6H4I 4-CH3COC6H4Br PhCH]CHBr 2-Iodo-5-methylthiophene Bromomethylenecyclohexane 1-Bromonaphthalene 4-CH3OC6H4I 4-CH3OC6H4Br 2-CH3OC6H4I 3-CF3C6H4I 4-CF3C6H4I 4-CH3COC6H4I 4-CH3COC6H4Br PhCH]CHBr 2-Iodo-5-methylthiophene Bromomethylenecyclohexane 1-Bromonaphthalene 4-CH3OC6H4I 4-CH3OC6H4Br 2-CH3OC6H4I 3-CF3C6H4I 4-CF3C6H4I 4-CH3COC6H4I 4-CH3COC6H4Br PhCH]CHBr 2-Iodo-5-methylthiophene Bromomethylenecyclohexane

n 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3

Product 351a 351b 351b 351c 351d 351e 351f 351f 351g 351h 351i 351a 351b 351b 351c 351d 351e 351f 351f 351g 351h 351i 351a 351b 351b 351c 351d 351e 351f 351f 351g 351h 351i

Yield (%) a

90 88a 5b 71 93 98 99 91 30a 48a 5 54 86b 4b 49 70 72 12 26b 8 13 24 24 48 14b 1 48 12 3 24b 3 3 2

Ratio (351:352) 99:1 10:1 2:3 15:1 9:1 20:1 99:1 99:1 3:1 3:2 3:1 7.2:1 9.8:1 1:2 199:1 3.4:1 7:2 3:2 99:1 1:3 2:3 2:1 1.4:1 4:1 4:1 1:2 3:2 3:4 1:20 1:1 1:8 2:3 1:6

a

Without water. At 115  C.

b

The coupling reactions of triethylarylgermanes with arenes resulted in the corresponding biaryls 353-372 through gold-catalyzed C-H activation; this methodology showed good functional group tolerance (Scheme 42).91

144

Organometallic Compounds of Germanium

Scheme 42

Organometallic Compounds of Germanium

145

Similarly, the gold-catalyzed chemoselective coupling between polyfluoro-arenes/heteroarenes and triethylarylgermanes afforded the corresponding biaryls 373-388, in which tolerance towards aromatic C–Cl, C–Br, C–I, C–SiMe3, and C–OTf functionalities was reported (Scheme 43).92

Scheme 43

146

Organometallic Compounds of Germanium

10.03.2.3.1.3 Other reactivities Condensation reactions between germyl substituted furyl/thienyl aldehydes and nitromethane in the presence of ammonium acetate catalyst resulted in the corresponding nitroethenes 389a-d (Eq. 71).93 Similarly, condensation reactions between germyl substituted furyl/thienyl aldehydes and pyranone derivative in the presence of piperidine acetate catalyst afforded the corresponding furyl/thienyl derivatives 390a-d (Eq. 72).94

ð71Þ

ð72Þ

2-Germyl-furylhydrosilane 392 was synthesized by reacting germanium-containing furan 391 with nBuLi and 1-chlorosilinane. The hydrosilylation of various N-allylamines using compound 392 in the presence of Speier’s catalyst resulted in the corresponding silacycles 393a-d (Scheme 44).95

Scheme 44

The reaction of trimethylgermyl substituted propionic acid (394) with N-hydroxysuccinimide and DCC resulted in the corresponding germylhydroxysuccinimide derivative 395 (Scheme 45). Water-soluble organogermane 397 was synthesized by treating ascorbic acid derivative 396 with PPh3, 395, and Pd(OH)2 (Scheme 45).96

Organometallic Compounds of Germanium

147

Scheme 45

Germyl substituted propynal 398 was reacted with hydroxylamine hydrochloride and NaHCO3 to afford a mixture of E- and Z-oximes 399 (E:Z ¼ 38:62), which reacted with trimethylsilyl azide to result in germyl substituted triazolecarbaldehyde oxime 401, albeit in low yield (15%) even after 8 d. However, a germyl derivative of triazolocarbaldehyde 400 gave 80% yield of compound 401 when reacted with hydroxylamine hydrochloride and NaHCO3 for 10 d; the same reaction in a microwave reactor can produce 401 (78% yield) in 10 min (Scheme 46).97

Scheme 46

Catalyst and solvent-free reactions of germyl substituted propynals with trimethylsilyl azide and functionalized primary amines gave the corresponding germyl substituted triazoloimines (403–405) under microwave assistance (Scheme 47).98 Fused pyrrole 407 with germyl functionalization was obtained by the cyclo-isomerization of germyl substituted propargyl pyridine 406 in the presence of AuBr3 catalyst; the reaction involved alkyne-vinylidene isomerization with simultaneous 1,2-migration of GeMe3 group (Eq. 73).99

ð73Þ

148

Organometallic Compounds of Germanium

Scheme 47

10.03.2.3.2

Reactivity of Ge]C double-bonded compounds

10.03.2.3.2.1 Addition reactions The reaction of germene 279 with methyl acrylate, dialkyl-fumarates, and -maleates resulted in oxagermacyclohexenes 408 and 409a-b through [2 +4] cycloaddition between the Ge]C double bond and C]CdC]O moieties (Eq. 74). Similarly, [2 +2] cycloaddition between germene 279 and diethyl oxalate afforded oxagermetane 410 (Eq. 75).100

ð74Þ

ð75Þ

Treatment of germene 279 with 1,4-naphthoquinone in a 2:1 molar ratio yielded the o- 411 and p- 412 quinodimethane via [2 +3] and [2 +4] double cycloaddition reactions, respectively (Eq. 76, Fig. 11, Table 18).101 Similarly, [2+ 4] double cycloaddition between germene 279 and 9,10-anthraquinone led to dioxadigermabenzopyrene 413 (Eq. 77).102

ð76Þ

Organometallic Compounds of Germanium

149

ð77Þ

Fig. 11 Molecular structure of compound 412.

Table 18

Selected bond lengths and angles of compound 412.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

C3-Ge1 Ge1-O1 C4-Ge2 O2-Ge2 Ge1-C1 Ge2-C2

2.021(3) 1.808(2) 2.010(4) 1.802(2) 1.976(5) 1.969(3)

C3-Ge1-O1 C3-Ge1-C1 O1-Ge1-C1 C4-Ge2-O2 C4-Ge2-C2 O2-Ge2-C2

98.3(1) 115.6(2) 104.5(1) 98.4(1) 124.4(2) 99.6(1)

By contrast, the [2 +3] cycloaddition reaction between germene 279 and benzoquinone afforded a mixture of double cycloadduct 414 and its double 1,3-H-shifted derivative 415 (Eq. 78). However, the reaction between germene 279 and tetramethylbenzoquinone afforded [2+ 3] cycloaddition product 416 exclusively (Eq. 79).102

ð78Þ

150

Organometallic Compounds of Germanium

ð79Þ

A mixture of germacyclobutene 417 (via [2+ 2] cycloaddition) and vinylgermane 418 were obtained by reacting germene 281 with ethoxyacetylene (Eq. 80). However, treatment of germene 281 with phenylacetylene, ethynyl-4-(trifluoromethyl)benzene, and 4-ethynylanisole afforded a mixture of germacyclobutene 419a-c, vinylgermane 420a-c, and germylacetylene 421a-c (Eq. 81).81

ð80Þ

ð81Þ

Addition reactions of trimethylsilylacetylene and tert-butylacetylene with germene 281 yielded a mixture of compounds (vinylgermane 422 and germylacetylene 423) and exclusively germylacetylene 424, respectively (Eqs. 82 and 83).81

ð82Þ

ð83Þ

Azagermacyclobutane 425 was obtained from the [2 +2] cycloaddition reaction between the Ge]C bond of phosphagermaallene 286 and the C]N bond of N-benzylidenemethylamine. However, treatment of phosphagermaallene 286 with methyl(benzylideneamino)acetate afforded azadienyl(germyl)ether 426 through the ene reaction (Scheme 48).103

Organometallic Compounds of Germanium

151

Scheme 48

The [2 +3] cycloaddition reactions of compound 286 with N-tert-butyl-a-phenylnitrone and nitrile oxide led to the C2NOGe five-membered heterocycles 427 and 428 that are saturated and unsaturated, respectively (Scheme 48). The reaction of compound 286 with pivalonitrile resulted in [2 +2] addition product azagermacyclobutene 429. However, its reactions with acetonitrile and benzonitrile led to germaphosphapropene 430 through the 1,2-addition of a CdH bond (of acetonitrile) to the Ge]C bond, and a tricyclic adduct 431, respectively (Scheme 49).103

Scheme 49

The [2 +2] cycloaddition reaction of compound 286 with E-crotonaldehyde and E-cinnamaldehyde afforded oxagermabutanes 432a and 432b, respectively. By contrast, treatment of compound 286 with methyl vinyl ketone gave a mixture of six-membered C4OGe heterocycle 433 and germyl(butadienyl)ether 434. However, the ethenyl analog of compound 434 (435) was obtained exclusively in the reaction between compound 286 and acetophenone (Scheme 50).82,104

152

Organometallic Compounds of Germanium

Scheme 50

Addition of methanol to the Ge]C bond of arsagermaallene 288 led to the formation of germaarsapropene 436. The [4 +2] cycloaddition reaction between compound 288 and 2,3-dimethyl-1,3-butadiene yielded a six-membered heterocycle 437 (Eq. 84).83

ð84Þ

10.03.2.3.2.2 Ligand exchange reactions Transition metal coordinated
6-germabenzene complexes 439a, 439b, and 440 were obtained by ligand-exchange reactions between group 6 metal carbonyl complexes [M(CO)3(CH3CN)3] and germabenzene 438 (Eq. 85).105

ð85Þ

10.03.3 Compounds with germanium group 17 element bonds The synthesis and reactivity of compounds with bonds between germanium and group 17 elements (X ¼ F, Cl, Br, I) are documented in this section.106 Importantly, these compounds find immense utility as precursors for various other germanium compounds.

Organometallic Compounds of Germanium

153

10.03.3.1 Synthesis of compounds with germanium group 17 element bonds 10.03.3.1.1

Synthesis from lithium salts

Trichlorogermane 442 that is heptacoordinate was prepared by reacting a tetradentate methane ligand 441 (that bears three aryl groups) with an excess of nBuLi and tetrachlorogermane (Eq. 86).107

ð86Þ

Treatment of pentafluoroethyl lithium with diphenyldibromogermane 443 gave bis(pentafluoroethyl)diphenylgermane 444, which was reacted with hydrogen bromide in the presence of aluminum tribromide to afford bis(pentafluoroethyl)dibromogermane 445. The reactions of compound 445 with silver carbonate and with two equiv. of Bu3SnH gave a cyclic trigermoxane 446 and bis(pentafluoroethyl)germane 447, respectively (Scheme 51).108

Scheme 51

Similarly, the reaction of pentafluoroethyl lithium with aminotrichlorogermane afforded tris(pentafluoroethyl)aminogermane 448. Removal of the diethylamino protecting group in compound 448 through the addition of two equiv. of hydrogen chloride and bromide gave tris(pentafluoroethyl)halogermanes 449 and 450 selectively (Eq. 87).109 A single crystal X-ray diffraction study of compound 450 revealed a distorted tetrahedral geometry around the germanium center (Fig. 12, Table 19).

ð87Þ

154

Organometallic Compounds of Germanium

Fig. 12 Molecular structure of compound 450.

Table 19

10.03.3.1.2

Selected bond lengths and angles of compound 450.

Atoms

Bond lengths [pm]

Atoms

Bond angles [  ]

Ge-Br Ge-C1 Ge-C3 Ge-C5

224.46(2) 201.1(1) 201.2(1) 201.1(1)

Br-Ge-C1 Br-Ge-C3 Br-Ge-C5 C1-Ge-C3 C3-Ge-C5

112.80(4) 112.29(4) 111.74(4) 105.65(5) 107.43(6)

Synthesis from Grignard reagents

Triarylbromogermane 451 was synthesized by treating tetrabromogermane with tert-butylphenyl Grignard reagent, which was prepared through the Sandmeyer and Grignard reactions of ortho-tert-butylaniline (Eq. 88).110

ð88Þ

Mixtures of triarylhalogermanes [(454 and 456) and (455 and 457)] were synthesized by the reactions of in situ generated Grignard reagents with tetrachlorogermane in a 3:1 molar ratio (Eq. 89).111

ð89Þ

Organometallic Compounds of Germanium

10.03.3.1.3

155

Synthesis from germanium(II) halides

2-Bromo-2-phenylacetophenone 458 reacted with GeBr2
dioxane to produce C-bound germyl enolate 459 (Eq. 90). 1H NMR studies revealed the initial formation of a mixture of O- and C-bound germyl enolates 459 and 460; however, with over 6 h compound 459 was converted to compound 460. The same reaction, if carried out with the addition of HMPA, allowed the O-bound germyl enolate to be stabilized to afford compound 461 (Eq. 91). The 1,10-phenanthroline analog of compound 461 (462) was obtained by adding 1,10-phenanthroline to the reaction mixture that afforded compound 462; it is anticipated that 1,10 phenanthroline should provide more stabilization to the O-bound germyl enolate (Eq. 92).112

ð90Þ

ð91Þ

ð92Þ

Allyltriiodogermanes 463 and 464 were prepared by reacting iodopropene and a diarylderivative of iodopropene with germanium(II) iodide (Eq. 93). The addition of an NHC (IPr) to compound 464 yielded cationic allyldiiodogermane 465 via carbene stabilization (Eq. 94).

ð93Þ

156

Organometallic Compounds of Germanium

ð94Þ

Allyl halide derivatives 466 and 467 possessing N,N-dimethylcarbamoyl groups were reacted with GeBr2
dioxane and GeCl2
dioxane to afford pentacoordinate allyltrihalogermane 468 and 469, respectively, featuring an intramolecular oxygen to germanium interaction. Treatment of compound 468 with silver perchlorate and 1,10-phenanthroline gave cationic allyldichlorogermane 470, stabilized by the bidentate ligand (Scheme 52). The iodo analog 471 of allyltrihalogermane was obtained through a halide exchange reaction of compound 469 with NaI.113

Scheme 52

Organometallic Compounds of Germanium

157

The reaction of ferrocenyllithium 472 with GeCl2
dioxane afforded chlorogermylenoid 473, which formed separated ion pair 474 upon crystallization from THF. Compound 473 was reacted with butylbromide and HCl in THF to yield organodichlorogermanes 475 and 476, respectively (Scheme 53). By contrast, the reactions of compound 473 with 1,3-dimethylbutadiene and butylbromide in toluene afforded [1 +4] cycloaddition and oxidative addition products 478 and 479, respectively, potentially through the intermediacy of germylene 477 (Scheme 54). Compound 473 on reaction with benzaldehyde afforded the corresponding dichlorogermane 481 most likely through the reaction of 480 with moisture (Eq. 95).114

ð95Þ

Scheme 53

Scheme 54

Carbene complexes of germanium(IV) halides 484-487 were obtained by the oxidative addition reactions of bis(dialkylamino)-difluoromethylenes (acyclic 482 and cyclic 483) with GeCl2
dioxane (Eq. 96). However, the reaction of compound 485 with an excess of compound 483 afforded anionic fluorogermane 488 (Eq. 97, Fig. 13, Table 20).115 A single crystal X-ray diffraction study showed that the germanium center is in a trigonal bipyramidal geometry (Fig. 13, Table 20).

158

Organometallic Compounds of Germanium

ð96Þ

ð97Þ

Fig. 13 Molecular structure of compound 485.

Table 20

10.03.3.1.4

Selected bond lengths and angles of compound 485.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C1 Ge1-F1 Ge1-F2

198.1(4) 178.1(2) 172.9(2)

N1-C1-N10 F1-Ge1-F10 F2-Ge1-F20

111.8(4) 178.87(13) 117.73(16)

Synthesis from germane hydrides

Germanes 454-457 can also be obtained by heat-induced radical halogenation reactions of triarylhydridogermanes 489 and 490 with CCl4 and NBS (Eq. 98).111

ð98Þ

The selective monochlorination of organohydridogermanes using triflic acid-LiCl and trichloroisocyanuric acid afforded organohydridochlorogermanes 491-494 (Eq. 99).116

Organometallic Compounds of Germanium

159

ð99Þ

10.03.3.1.5

Synthesis via halogen exchange reactions

Aryltrichlorogermane 442 undergoes chlorine exchange reactions with silver fluoride, trimethylsilylhalides, and tribromoborane to yield the corresponding trifluoro-495, tribromo-496, and triiodo-497 germanes (Eq. 100). The reactions of compounds 495 and 496 with trichloroborane afford the starting germane 442 (Eq. 101).107

ð100Þ

ð101Þ

Fluorination of dichlorogermanium phthalocyanine 498 using CsF as the fluoride source in the presence of 18-crown-6 afforded difluorogermane 499 (Eq. 102).117

ð102Þ

Triarlybromogermane 451 was converted to triarylchlorogermane 500 by its reaction with LiAlH4 and CuCl2 (Eq. 103).110

ð103Þ

10.03.3.2 Reactivity of germanium halogen bonded compounds The reaction of bis(triphenylphosphoranylidene)ammonium chloride [PNP]Cl with compound 449 afforded the corresponding dichlorogermanate 501 (Eq. 104). The fluorination of compound 450 with two equiv. of metal fluorides yielded difluorogermanates 502-506 with metal cations (Eq. 105). Difluorogermanate 507 supported by an organic counter ion was isolated by reacting

160

Organometallic Compounds of Germanium

aminogermane 448 with HF (Eq. 106). In the reactions of compound 507 with silane and phosphorene, the fluoride donating ability of the germanate was displayed through the formation of tris(pentafluoroethyl)fluorogermane 508 (Eq. 107). Fluorogermanate 511 was isolated by reacting difluorogermanate 509 with two equiv. of LiC2F5; pentakis(pentafluoroethyl)germane 510 was formed as an intermediate, and eliminated tetrafluoroethylene to afford compound 511 (Eq. 108).118

ð104Þ

ð105Þ

ð106Þ

ð107Þ

ð108Þ

The reactions of compounds 449 and 450 with silver carbonate and sodium bicarbonate afforded digermoxane 512 (Eq. 109) and germanol 513 (Eq. 110).

ð109Þ

ð110Þ

The reactions of acetonitrile adduct of germanium(IV) fluoride with pyridine, 2,20 -bipyridyl, 1,2-bisdimethylaminoethane, 1,10-phenanthroline, triethylamine, and cyclam afforded cis-adducts 514-520 that are hexacoordinate (Scheme 55, Eq. 111). However, the addition of 1,4,7-trimethyl-1,4,7-triazacyclononane afforded cationic fac-adduct 521 through the displacement of fluoride ion (Eq. 112).119

Organometallic Compounds of Germanium

161

ð111Þ

ð112Þ

Scheme 55

The reactions of tetrahalogermanes with 1,10-phenanthroline, 1,2-bisdimethylaminoethane, and pyridine gave adducts 522-527 that are hexacoordinate (Scheme 56).119

162

Organometallic Compounds of Germanium

Scheme 56

10.03.4 Compounds with germanium–hydrogen bonds Compounds with germanium hydrogen bonds are commonly isolated by reducing halogermanes and adding small molecules with hydrogen to unsaturated germanium compounds and germylenes.

10.03.4.1 Synthesis of compounds with germanium–hydrogen bonds 10.03.4.1.1

Reduction of halogermanes

The reaction of o-tert-butylbromobenzene with magnesium and tetrabromogermane resulted in triarylbromogermane 526, which can be reduced using lithium aluminum hydride to the triarylhydridogermane 527 (Eq. 113).110

ð113Þ

Phosphine substituted diarylhydridogermanes 530a and 530b were prepared by reacting lithiated arylphosphine 529 with alkyltrichlorogermanes and lithium aluminum hydride (Eq. 114).120

ð114Þ

Reduction of dichlorogermane 531 with lithium aluminum hydride gave the corresponding dihydridogermane 532 (Eq. 115).121

Organometallic Compounds of Germanium

163

ð115Þ

Tris(pentafluoroethyl)hydridogermane 534 was prepared by reacting bromogermane 533 with tributyltin hydride (Eq. 116).122 Similarly, reduction of alkynylgermanes 535a and 535b through bis-tert-butylaluminium hydride led to alkynylhydridogermanes 536a and 536b, respectively; compounds 535a and 535b were obtained by reacting diorganodichlorogermanes with nBuLi and terminal alkyne (Eq. 117).123 ð116Þ

ð117Þ

Hydroalumination reactions of alkynylhydridogermanes 536a and 536b using organoaluminium hydrides afforded the corresponding alkenylhydrogermanes 537a-b and 538 (Eq. 118).123

ð118Þ

Hydroalumination and reduction of dialkynylgermane 259b with two equiv. of di-tert-butylaluminium hydride resulted in alkenylalkynylgermane 539a with GedH bond functionalization; the corresponding Trip analog 539b was obtained by di-tert-butylaluminium hydride reduction of 260c (Eq. 119).68

ð119Þ

164

Organometallic Compounds of Germanium

10.03.4.1.2

Addition to unsaturated germanium compounds

The addition of small protonated molecules to the unsaturated germanium molecules resulted in the hydrido compounds, such as the addition reaction of 2-germadisilaallene 540 with excess water and methanol afforded dihydridogermanes 541a and 541b with two silyl substituents, respectively (Eq. 120).124 Several examples have been given in other sections (Sections 10.03.7.4 and 10.03.9).

ð120Þ

10.03.4.1.3

Oxidative addition to germylenes

The oxidative addition reactions of small protonated molecules such as water and methanol to the germylenes afforded the corresponding hydridogermane compounds. For example, the oxidative addition reactions of m-terphenyl ligand stabilized germylene 542 with water and methanol resulted in diorganohydridogermanes with GedOH 543a and GedOMe 543b bonds, respectively (Eq. 121).125

ð121Þ

10.03.4.1.4

Other procedures

Phosphine substituted triarylhydridogermane 544 was obtained by reacting lithiated arylphosphine 529 with dichlorogermylene, followed by hydrolysis using excess water (Eq. 122).126

ð122Þ

The salt elimination reaction of the in situ generated dilithium salt of 1,8-iodonaphthalene 545 with two equiv. of halohydridogermane yielded naphthyldigermane 546 (Eq. 123). Similarly, naphthylsilylgermane 549 was also prepared through a multi-step synthetic protocol starting from 1,8-bromonaphthalene (Eq. 124).127

ð123Þ

Organometallic Compounds of Germanium

165

ð124Þ

10.03.4.2 Reactivity of compounds with germanium–hydrogen bonds 10.03.4.2.1

Germanium cation/anion formation

Triorganogermylium borate 551 was obtained by treating triorganohydridogermane 550 with trityl borate (Eq. 125).128 The monodeprotonation reaction of terphenyltrihydridogermane 553 using methyllithium afforded the corresponding dihydridogermanium anion 554 (Eq. 126).129 ð125Þ

ð126Þ

Treatment of compounds 546 and 549 with trityl borate produced digermylhydronium 555 (Fig. 14, Table 21) silylgermylhydronium 556 borates (Eq. 127).127

ð127Þ

166

Organometallic Compounds of Germanium

Fig. 14 Molecular structure of compound 555.

Table 21

10.03.4.2.2

Selected bond lengths and angles of compound 555.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-H1 Ge2-H1 Ge1-Ge2 Ge1-C1 Ge2-C7

1.48(3) 1.78(3) 3.048(3) 1.931(2) 1.925(2)

Ge1-H1-Ge2 H1-Ge1-C1 H1-Ge2-C7

137(2) 101(1) 104(1)

Hydrogermylation reactions

Compounds with germanium hydrogen bonds have been utilized for the preparation of organogermanium compounds through hydrogermylation reactions. For example, the hydrogermylation reaction of phenylacetylene using hydridogermane 534 under catalyst-free conditions afforded Z-styrylgermane 557 as a b-product. However, in the presence of palladium catalyst, a mixture of a-product (phenylvinyl)germane 558 and E-(diphenylbutadienyl)germane 559 were observed (Scheme 57).122 Several other examples are included in Sections 10.03.2.1 and 10.03.7.4.

Scheme 57

10.03.5 Compounds with germanium-group 15 element bonds The chemistry of compounds containing single bonds between germanium and group 15 elements is captured in this section. Furthermore, compounds with Ge]E double bonds are discussed (E ¼ group 15 element); these compounds are attractive due to the presence of two reactive sites (Ge]E p-bond and a lone pair on the group 15 element).

Organometallic Compounds of Germanium

167

10.03.5.1 Synthesis of compounds with GedE single bonds (E ¼ group 15 element) 10.03.5.1.1

Synthesis from alkali metal salts

Heteroleptic lithium amide 561 (obtained from heteroleptic amide 560) was reacted with GeCl2
dioxane to yield imido cubane 562, containing divalent and exocube tetravalent germanium atoms bridged by a nitrogen atom; elimination of trimethyltin chloride is thought to be the thermodynamic driving force (Eq. 128).130

ð128Þ

The reaction of the lithium salt of hexamethylcyclotrisilazane with tetrachlorogermane in a 2:1 molar ratio resulted in cyclohexasilazane-1,5,9-triidogermanium derivative 564 featuring one GedCl bond (Eq. 129).131

ð129Þ

Azagermatranes 565a and 565b were synthesized by reacting the in situ generated trilithium salt of tetraamine with tetrachloro- and tetrabromogermanes, respectively.132 Furthermore, a series of azagermocanes (566a-566d, 567a and 567b) was isolated by treating tetrahalogermanes with dilithium salts of triamines (Scheme 58). Unusually, in the reaction of the dilithium salt of triamine with tetrahalogermanes, digermanium compounds 568a and 568b with two GedX bonds were obtained (Eq. 130).133

ð130Þ

168

Organometallic Compounds of Germanium

Scheme 58

The synthesis of germanium N,N,N0 ,N0 -tetramethylguanidinate 569 with two GedCl bonds was achieved by reacting lithium salt of tetramethylguanidinate with tetrachlorogermane. Its Ge-NR2 analogs 570a and 570b were obtained using lithium amides (Eq. 131). Compounds 569, 570a, and 570b were used as germanium(IV) precursors in the MOCVD deposition of germanium and germanium telluride films.134

ð131Þ

By treating 3-aminocrotonitrile and 4-amino-2,6-dimethylaminopyrimidine with nBuLi and triphenylchlorogermane, the corresponding triphenylgermanes 571 and 572 were obtained, respectively (Eqs. 132 and 133).135

ð132Þ

Organometallic Compounds of Germanium

169

ð133Þ

The reaction of tetrachlorogermane with an excess of sodium hexamethyldisilazide under reflux conditions led to the formation of tris(amido)germane 573 featuring a GedCl bond (Eq. 134), which was used as a catalyst for the hydrophosphination of styrene and internal alkynes using HPPh2.136 By contrast, when this reaction was carried out at a low temperature, tetra(amido)germane 574 with two coordinated THF molecules was obtained (Eq. 135).137

ð134Þ

ð135Þ

The reaction of the lithium derivatives of 2-(phenylamidomethyl)phenols 575a, 575b, and 575c with tetrachlorogermane afforded amidoalkoxygermanes 576a, 576b, and 576c, respectively (Eq. 136).138

ð136Þ

170

Organometallic Compounds of Germanium

10.03.5.1.2

Synthesis from germylenes

Insertion of Lappert’s germylene into the PdP bond of hexaphospha-pentaprismane 577 formed the corresponding insertion product 578, where the formal oxidation state of the germanium atom is changed from two to four (Eq. 137).139

ð137Þ

Similarly, Lappert’s germylene undergoes oxidative insertion reactions into the PdCl bonds of trichlorophosphine, tertbutyldichlorophosphine, and diorganochlorophosphine resulting in compounds 579–581b with Ge(IV)–P bonds (Scheme 59).140,141

Scheme 59

Activation of white phosphorus was achieved by diarylgermylene 582 to obtain germanium phosphorus cage compound 583, which showed the reverse reaction upon UV irradiation (Eq. 138).142

ð138Þ

Organometallic Compounds of Germanium

171

The reaction of phenylaminomethylphenol with Lappert’s germylene afforded amidoalkoxygermane 584 (Eq. 139).138

ð139Þ

Veith’s germylene Me2Si(m-NtBu)2Ge underwent oxidative addition reactions with various chlorophosphines, such as bis(dichlorophosphino)methane, organodichlorophosphines, and di-tert-butylchlorophosphine, to afford Ge(IV)–P bonded compounds 585-587 (Scheme 60).140,141

Scheme 60

172

Organometallic Compounds of Germanium

10.03.5.1.3

Synthesis via other methods

Bis(tert-butylamido)cyclodiphospha(III)azane stabilized stannylene 588 reacted with excess GeCl4 to afford the corresponding diazadichlorogermane 589 and -stananne 590 in equimolar ratio (Eq. 140).143

ð140Þ

The reactions of aryl(diphenylphosphanyl)aminotrichlorogermane 591a, 591b, and 591c with dimethyl acetylenedicarboxylate, afforded C2PNGe five-membered heterocycles 592a, 592b, 592c and 592d, which are zwitterionic (Eq. 141). Similarly, zwitterionic heterocycles 593 and 594 with OCPNGe five-membered rings were synthesized by reacting compound 591a with propionaldehyde and terephthalaldehyde, respectively (Scheme 61).144

ð141Þ

Scheme 61

Organometallic Compounds of Germanium

173

The reactions of amino alcohols 595a and 595b with tetrachlorogermane in the presence of triethylamine afforded spirocyclic amidoalkoxygermanes 596a and 596b, respectively (Eq. 142).138

ð142Þ

Tetrafluorogermane-phosphane complexes 597a-599c were prepared by treating tetrafluorogermane-acetonitrile adduct with various phosphines and diphosphines (Scheme 62).

Scheme 62

Similarly, tetrachlorogermane-phosphine 600 and -arsines 601a and 601b were prepared by treating tetrachlorogermane with trimethylphosphine and trialkylarsanes; the reaction with phosphine was carried out under neat conditions. Dissolution of compound 600 in dichloromethane afforded phosphonium salt 602 partnered with a [GeCl3]− counter-anion (Scheme 63).145 Cyclopentadienyltrichlorogermane 603 was reacted with tris(trimethylstannyl)phosphane at elevated temperatures to produce dichlorogermane 604 featuring a GedP bond, through the elimination of chlorotrimethylstannane (Eq. 143).146

ð143Þ

174

Organometallic Compounds of Germanium

Scheme 63

10.03.5.2 Synthesis of compounds with germanium doubly-bonded to group 15 elements Phosphagermene 606 was formed in the coupling reaction of 1,1-dilithogermane 605 with mesityldichlorophosphine; the precursor 605 was obtained by reacting 1-germacyclopropene with excess lithium metal (Eq. 144).147 Likewise, the reaction of compound 605 with mesityldifluoroarsane gave arsagermene 607 featuring a Ge]As bond (Eq. 145).148 A single crystal X-ray diffraction study on compound 607 showed double bond character between As and Ge (Fig. 15, Table 22).

ð144Þ

ð145Þ

Fig. 15 Molecular structure of compound 607.

Organometallic Compounds of Germanium

Table 22

175

Selected bond lengths and bond angles of compound 607.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Atoms

Torsional angles [  ]

As1¼Ge1 As1-C1 Si1-Ge1 Si2-Ge1

2.2727(8) 1.996(5) 2.4174(16) 2.4336(17)

Ge1-As1-C1 Si1-Ge1-As1 Si2-Ge1-As1 Si1-Ge1-Si2

108.33(16) 104.85(5) 127.25(5) 127.90(6)

Si1-Ge1-As1-C1 Si2-Ge1-As1-C1

179.88(17) 179.73(18)

10.03.5.3 Reactivity of germanium group 15 element bonded compounds The reaction of phosphaketene 608 with a potassium phosphide afforded potassium acylphosphine phosphide 609, whose potassium counterion was separated through the use of 18-crown-6. Compound 609 was reacted with methyl iodide and triphenylchlorogermane to afford diphospha-urea derivatives 611a and 611b. Treatment of compound 611a with KOtBu gave the methyl analog 612 of compound 609. Moreover, compound 612 with no germanium atom reacted with methyl iodide to afford germanium-free diphospha-urea 613 (Scheme 64).149

Scheme 64

176

Organometallic Compounds of Germanium

UV irradiation of compound 611a led to decarbonylation, and afforded a mixture of diphosphines 614, 615, 616 with or without PdGe bonds (Eq. 146). By contrast, compound 611b gave a diphosphine featuring a PdGe bond (Eq. 147); however, under thermal conditions, triphenylphosphagermane 618 along with phosphaketene (Eq. 147).150 A single crystal X-ray diffraction study on compound 611a confirmed methylation at the phosphorus center (Fig. 16, Table 23).

ð146Þ

ð147Þ

Fig. 16 Molecular structure of compound 611a.

Organometallic Compounds of Germanium

Table 23

177

Selected bond lengths and angles of compound 611a.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C06 Ge1-C07 Ge1-C05 Ge1-P2 P2-C0P C1-O1

1.950(2) 1.949(2) 1.955(2) 2.3493(7) 1.842(2) 1.219(3)

P2-Ge1-C05 P2-Ge1-C06 P2-Ge1-C07 Ge1-P2-C0P

112.29(6) 113.39(7) 103.47(6) 101.47(8)

The reaction of compound 608 with potassium hydridophenylborate resulted in ionic compound 619 via the formation of CdH and BdO bonds, which gave ion-separated product 620 upon treatment with 18-crown-6. The reaction of compound 619 with two equiv. of methyl iodide and three equiv. of [NBu4][Ph3SiF2] (as a fluoride source) afforded formylphosphine 621 with two PdMe bonds (Scheme 65).149

Scheme 65

178

Organometallic Compounds of Germanium

Silylphosphination of phosphaketene using triphenylsilylphosphine afforded (E)-phosphaalkene derivative 622 (Eq. 148), whose (Z)-isomer was obtained by reacting compound 609 with triphenylchlorosilane (Eq. 149).151

ð148Þ

ð149Þ

The reduction of phosphagermene 606 with KC8 afforded the corresponding anion radical 623 (Eq. 150).147

ð150Þ

10.03.6 Compounds with germanium-group 16 element bonds Developments in the synthesis and reactivity of compounds containing Ge(IV)–E bonds are accommodated in this section (E ¼ chalcogens, such as O, S, Se, Te).

10.03.6.1 Synthesis 10.03.6.1.1

Synthesis of compounds with germanium-oxygen bonds

Bis[N-(dimethylamino)ethoxy]germanium(IV) halides 624a, 624b, and 624c with two N ! Ge bonds were synthesized by the substituent exchange reactions of organotrihalogermanes with [(N-dimethylamino)ethoxy]triethylgermanium in a 1:2 molar ratio (Eq. 151).152 Oxidative addition of methyl iodide to germylene 625 stabilized by a b-(N,N-dimethylamino)ethoxy ligand afforded cationic germanium(IV) compound 626 (Eq. 152).153

ð151Þ

Organometallic Compounds of Germanium

179

ð152Þ

Macrocycle 628 featuring a germanium atom was isolated by reacting coordinated-1,3-diyne-diol 627 with two equiv. of nBuLi and diphenyldichlorogermane (Eq. 153).154 Germanium alkoxide 630 stabilized by a C3 symmetric amine(trisphenolate) ligand was obtained by treating amine(trisphenol) derivative 629 with Ge(OiPr)4. Compound 630 features a H-bonding interaction with isopropanol in the solid state; it was subsequently used as a catalyst for the ROP of rac-lactide under solvent-free conditions (Eq. 154).155

ð153Þ

ð154Þ

Cyclic organogermanium peroxides 632a-632e were synthesized by the reactions of 1,10 -dihydroperoxyperoxide 631 with diorganodihalogermanes (Eq. 155).156 The reaction of (chloromethyl)trichlorogermane with O-TMS substituted glycolic acid-N, N-dimethylamide 633 in a 1:2 molar ratio afforded hexacoordinate germanium compound 634, which reacted with HgCl2 to afford pentacoordinate germanium compound 635 that is cationic (Eq. 156).157

ð155Þ

180

Organometallic Compounds of Germanium

ð156Þ

The reactions of O-TMS derivatives of 2-hydroxycarboxylic acid-N,N-dialkyl/arylamides 636 with organohalogermanes afforded the corresponding pentacoordinate germanium compounds 637a–637k (Eq. 157). When the reactions of compound 638 were carried out with tetrahalogermanes in a 2:1 molar ratio, (O,O)-bischelate complexes 639a–639f, containing hexacoordinate germanium atoms were obtained (Eq. 158). Hexacoordinate germanium complexes 640a and 640b were synthesized by the reactions of O-TMS hydrazide with methyltrichlorogermane and tetrachlorogermane, respectively (Eq. 159). 158–161

ð157Þ

ð158Þ

Organometallic Compounds of Germanium

181

ð159Þ

Oxidative addition of ortho-quinone to GeCl2
dioxane afforded germanium biscatecholates with two solvent (L) molecules 641a (L ¼ THF) and 641b (L ¼ Et2O) (Eq. 160).162

ð160Þ

Dipyrrin 642 with bulky hydroxyphenyl decorations upon deprotonation using an excess sodium hydride and reaction with germanium tetrachloride produced dipyrrinatogermanium(IV) chloride 643 (Eq. 161). Complex 643 was used as a catalyst for the copolymerization of epoxides with CO2.163 The disodium salt (of 9,10-phenanthrenequinone dioxime) reacted with alkyl/ arylgermanium chlorides in 1:1 and 1:2 molar ratios to afford the corresponding organogermanium compounds 644a-644c, 645a645c (Eq. 162).164 The reaction of silanediol 646 with Lappert’s germylene afforded Si3O6Ge2 bicyclic siloxane 647 containing two germanium(IV) hydride units (Eq. 163, Fig. 17, and Table 24); the reaction involves the oxidation of Ge(II) to Ge(IV) and the reduction of hydroxyl protons to hydrides.165

ð161Þ

ð162Þ

182

Organometallic Compounds of Germanium

ð163Þ

Fig. 17 Molecular structure of compound 647.

Table 24

Important bond parameters in the Si3O6Ge2 core of compound 647.

Ge-O (A˚ )

Si-O (A˚ )

O-Ge-O (  )

O-Si-O (  )

1.717(6)–1.738(7)

1.623(9)–1.641(8)

109.5(3)–110.5(4)

106.0(3)–107.2(5)

Sodium salts 649a and 649b (of pyridine-2-carbaldehyde and 2-acetylpyridine oximes 648a, and 648b, respectively) were reacted with organohalogermanes to afford the corresponding organogermanyl(IV) oximates 650a-650d, 651a-651f (Scheme 66).166

Organometallic Compounds of Germanium

183

Scheme 66

Di-tert-butyldiethoxygermane underwent an alkoxide exchange reaction with 5-bromo-2-hydroxybenzyl alcohol to produce dioxagermine 652 (Eq. 164).167 The reaction of germylene 653 with oxygen afforded m-oxo dimer 654 (Eq. 165).168 Synthesis of germatrane 656 with GedOEt bond was achieved using the reaction between amino alcohol 655 and tetraethoxy germane. Bromination of compound 656 using trimethylsilyl bromide gave germatrane 657 with GedBr bond (Eq. 166).169 Furthermore, enantiomeric germatranes (S,D)-659 and (R,L)-660 were synthesized by the reactions of in situ generated ethoxy germatrane 658 with (R)- and (S)-Mosher’s acid, respectively (Eq. 167).170

ð164Þ

ð165Þ

184

Organometallic Compounds of Germanium

ð166Þ

ð167Þ

Exchange of ligands between hexacoordinate silicon(IV) compounds 661a-661b, 662a-662b and 663a-663f with germanium tetrachloride resulted in the formation of the analogous hexacoordinate germanium(IV) compounds 664a-664b, 665a-665b, and 666a-666f (Eqs. 168–170).161,171

ð168Þ

Organometallic Compounds of Germanium

185

ð169Þ

ð170Þ

The reaction of borocane 667 with germanium tetrachloride in the presence of aluminum trichloride gave germocane 668a featuring two GedCl bonds.172 Its ethoxy analog 668b was obtained by treating compound 667 with germanium ethoxide in the presence of Al2(OEt)6 (Eq. 171). The reaction of salicylaldehyde-(2-hydroxyethyl)imine 669 with diphenyldichlorogermane and triethylamine afforded pentacoordinate germanium(IV) compound 670 (Eq. 172).173

ð171Þ

ð172Þ

186

Organometallic Compounds of Germanium

10.03.6.1.2

Synthesis of compounds with germanium bonded to sulfur, selenium or tellurium

Reactions of one equiv. of thioethers 671a and 671b with the acetonitrile adduct of germanium(IV) fluoride afforded thioether germanium(IV) complexes 672a and 672b, respectively (Eq. 173).174 The reaction of organogermanium hydride 673 with an excess of elemental sulfur gave a mixture of trigermasulfide 674a and three digermapolysulfides 674b, 674c, and 674d (Eq. 174). SdS bond cleavage occurred in the reaction of compound 674c with sodium borohydride to afford hydrosulfide 675, which can also be obtained when a mixture of digermapolysulfides 674b, 674c, and 674d was treated with sodium borohydride (Eq. 175).175

ð173Þ

ð174Þ

ð175Þ The reactions of the in situ generated dilithium salt of compound 675 (676) with palladium metal precursors afforded Ge2PdS4 clusters 677 and 678 (Scheme 67).175

Organometallic Compounds of Germanium

187

Scheme 67

The lithium salt 679 (of an aluminum disulfide) reacted with diorganogermanium dichlorides and germanium tetrachloride to afford heterobimetallic disulfides 680a-680b and 681, respectively (Eq. 176).176 The molecular structure of compound 680b showed the planar nature of the AlS2Ge four-membered ring (Fig. 18 and Table 25).

ð176Þ

Fig. 18 Molecular structure of compound 680b.

188

Organometallic Compounds of Germanium

Table 25

Selected bond lengths and angles of compound 680b.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Al-N1 Al-S1 Ge-S1 Al-Ge Ge-C31 Al-N2 Al-S2 Ge-S2 Ge-C30

1.886(3) 2.226(2) 2.225(1) 2.891(1) 1.939(5) 1.877(3) 2.229(2) 2.227(1) 1.933(5)

Al-S1-Ge S2-Ge-S1 S1-Al-S2 Al-S2-Ge C31-Ge-C41

81.01(5) 99.08(4) 99.01(6) 80.88(5) 114.0(2)

The reaction between germanium(IV) isopropoxide and 2,20 -oxybisbenzenethiol in a 1:2 molar ratio afforded spiro-dibenzogermocine 682 (Eq. 177).177 The reaction of phenoxanthin-4,6-dithiol 683 with organochlorogermane and germanium tetrachloride afforded the corresponding germanium(IV) derivatives 684a-684b, 685a-685b, and 686, respectively. Furthermore, compounds 685a-685b and 686, when treated with a mixture of KBr and HBr, yielded the corresponding bromo derivatives 685a-685b and 688, respectively (Scheme 68).178

ð177Þ

Scheme 68

Organometallic Compounds of Germanium

189

Diferrocenyl disulfide underwent homolytic SdS bond cleavage reaction with digermatrisilapropellane 689, affording bicyclo [1.1.1]pentane 690 (Eq. 178).179 Cationic germanium(IV) compound 691 was synthesized by the reaction of methyltribromogermane with two equiv. of S-trimethylsilyl-N,N-dimethyl-2-thioacetamide (Eq. 179). 180

ð178Þ

ð179Þ

The reaction of germanium sulfide cluster 692 with thiosemicarbazide led to cluster splitting and formation of C,N,S-tridentate ligand stabilized germanium(IV) compound 693 featuring a planar Ge2(m-S)2 four-membered ring (Eq. 180).181 Pyridine-2-thiolate ligand stabilized germanium(IV) complex 694 was isolated by reacting chlorotrimethylsilane and silylated 2-mercaptopyridine (Eq. 181).182

ð180Þ

ð181Þ

The reactions of organotrichlorogermanes 695, 696, and 697 (obtained starting from germanium dioxide) with lithium/sodium tellurides afforded germatellurones 695a, 696a, and 697a, respectively; compounds 695a and 696a, 6975a have double-decker and noradamantane type structures (Eq. 182).183 Germanium chalcogenide clusters of adamantane (699a X ¼ S; 699b X ¼ Se) and noradamantane 700 were synthesized by treating nitrile functionalized trichlorogermane 698 with sodium chalcogenides (Scheme 69).184 Oxidative addition of Te(iPr)2 at the germanium center of germylene 701 gave tellurolate 702 (Eq. 183), which was used as a single-source precursor for the generation of GeTe films through MOCVD.185

190

Organometallic Compounds of Germanium

Scheme 69

ð182Þ

ð183Þ

The reactions of organotrichlorogermane 703 with bis(trimethylsilyl)chalcogens afforded germachalcogenide compounds 704a–704c featuring GedCl bonds (Eq. 184).186

Organometallic Compounds of Germanium

191

ð184Þ

Bidentate chalcogen ligands 705a-705b and 706a-706b containing rigid aromatic backbones were reacted with two equiv. of superhydride and organogermanium dichlorides to afford germanium(IV) compounds 707a-707d and 708a-708d (Eqs. 185 and 186).187

ð185Þ

ð186Þ

10.03.6.2 Reactivity of germanium-chalcogen bonded compounds The reaction of quinolin-2-one 709 and oxazin-4-one 712 derivatives with pentacoordinate germanium(IV) complex 710 formed bischelated hexacoordinate germanium(IV) compound 711 and 713 (Eq. 187). One-pot isolation of compound 713 involves the reaction of compound 712 with chloromethyltrichlorogermane and mandelic amide derivative 714 (Scheme 70).188

ð187Þ

Oxidative addition reactions of germylene aryloxides 715 and 716 with alkyl iodides gave the corresponding germanium(IV) compounds 717a–717b, 718a-718b (Eq. 188). Treatment of compound 718b with 2,6-diphenylphenol was shown to yield 719 tri(aryloxy) germanium(IV) species (Eq. 189).189

192

Organometallic Compounds of Germanium

Scheme 70

ð188Þ

ð189Þ

Bis(perchlorocatecholato)germane 720 was synthesized by reacting two equiv. of perchlorocatechol with germanium dioxide (Eq. 190). Single crystal X-ray diffraction studies on compound 720 showed two coordinated water molecules; these water molecules are involved in H-bonding with four more water molecules (Fig. 19, Table 26). The addition of donor molecules such as CH3CN, DMSO, and (nBuO)3PO to compound 720 led to the removal of coordinated water, and the formation of the corresponding Lewis adducts 721a, 721b, and 721c with two donor molecules. Addition of two equiv. of tetraethylammonium chloride and KF/18-crown-6 to 720 gave dichlorido 721d and monofluorido 722 adducts, respectively (Eqs. 191–193).190

Organometallic Compounds of Germanium

193

ð190Þ

ð191Þ

ð192Þ

ð193Þ

Fig. 19 Molecular structure of compound 720.

194

Organometallic Compounds of Germanium

Table 26

Selected bond lengths of compound 720.

Atoms

Bond lengths (pm)

Ge-O1 Ge-O3

184.5(3) 195.7(3)

10.03.7 Compounds with germanium–metal (or metalloid) bonds Compounds with germanium metal/metalloid bonds are commonly synthesized via the substitution reactions of metal/metalloid halides with germyl hydride, germyl anions, and germyl halides.191

10.03.7.1 Compounds with Ge–alkali metal bonds Compounds with germanium alkali metal bonds are usually isolated by the cleavage of germanium-silicon, germanium-halogen, and germanium-carbon bonds by strong alkali metal bases.

10.03.7.1.1

Synthesis

Chlorodemethylation of tetrakis(trimethylsilyl)germane using aluminum chloride and acetyl chloride resulted in chlorinated product 723, which on further reaction with methyl-substituted ethylene glycol in the presence of triethylamine afford tetrasilylgermane 724 featuring ethylene glycol moieties. Treatment of compound 724 with MOtBu (M ¼ Li, Na, K) resulted in the GedSi bond cleavage to afford alkali metal germanides 725a-c (Eq. 194).192

ð194Þ

Treatment of 1,4-digermatetrasilacyclohexane 726 with potassium tert-butoxide was shown to give potassium germanide 727 through GedSi bond cleavage. The reaction of compound 727 with aliphatic/aromatic acyl chlorides afforded acylgermanes 728a-c (Scheme 71).193 Further reaction of compounds 728a-c with potassium tert-butoxide led to exocyclic germenolates 729a-c (Scheme 71, Fig. 20, Table 27).

Organometallic Compounds of Germanium

Scheme 71

Fig. 20 Molecular structure of compound 729c.

Table 27

Selected bond lengths and angles of compound 729c.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C1 C1-O1 K1-O1 K1-Ge1

2.063(2) 1.231(3) 2.740(2) 3.613(9)

Ge1-C1-O1 Si1-Ge1-Si4 Si2-Ge2-Si3

114 100 108

195

196

Organometallic Compounds of Germanium

The reaction of acylgermanes 728a and 728c with two equiv. of potassium tert-butoxide yielded dianionic germenolates 730a and 730b, respectively (Eq. 195).194

ð195Þ

Germabenzene 732 was synthesized by treating chlorogermane 731 with LDA. Compound 732 was reduced with 2.5 equiv. of KC8 to obtain germabenzenylpotassium 733; the aryl group was eliminated as its potassium salt (Eq. 196).195 Similarly, reduction of compound 732 by lithium and sodium naphthalenides gave germabenzenyllithium 734a and germabenzenylsodium 734b, respectively (Eq. 197).196

ð196Þ

ð197Þ

The reduction of organotrichlorogermane using KC8 in the presence of cyclic alkyl(amino) carbene 735 resulted in germanylidene anion 736 (Eq. 198).197

Organometallic Compounds of Germanium

197

ð198Þ

10.03.7.1.2

Reactivity of compounds with Ge–alkali metal bonds

Silagermylpotassium 737 was utilized for the synthesis of oligomeric germanes and germanium containing oligosilanes. Its reaction with dimethyl(phenyl)chlorosilane resulted in tetrasilylgermane 738, which was treated with trifluoromethanesulfonic acid to afford germyl triflate 739 (Eq. 199).198

ð199Þ

Treatment of triflate 739 with compound 737 and (SiMe3)3SiK led to digermyldimethylsilane 740 and germyl(silyl)dimethylsilane 741, respectively (Eq. 200). Similarly, reactions of compound 737 with dimethyldichlorogermane and oligomeric choro-/ triflatosilane gave trigermane 742, and a series of germyl terminated oligosilanes 743a-d, respectively (Eq. 201).

ð200Þ

ð201Þ

Compound 726 and digermatrisilacyclopentane 745 were obtained by treating germyl dianion 744 with dichlorotetramethyldisilane and dichlorodimethylsilane, respectively (Eq. 202).198

198

Organometallic Compounds of Germanium

ð202Þ

The reactions of germenolates 729a and 729b with trimethylchlorosilane led to exocyclic germenes 746a and 746b; however, germenolate 729c gave acylgermane 728c (Eq. 203).193

ð203Þ

Dianionic germenolate 730a upon treatment with tri-iso-propylchlorosilane afforded germene 747. However, the reaction of compounds 730a and 730b with excess methyl iodide resulted in methylated acylgermanes 748a-748b as a mixture of cis/trans isomers (Eq. 204).194

ð204Þ

The reaction of germabenzenylpotassium 733a with trimethylchlorosilane afforded [4 +2] dimers 749a and 749b of germabenzene featuring trimethylsilyl substituents.199 By contrast, utilization of Met2BuSiCl instead of Me3SiCl resulted in coupling between germabenzenylpotassium and two molecules of the resultant germabenzenes (with Met2BuSi substituents) to afford trimerized product 750 (Eq. 205).200

ð205Þ

Organometallic Compounds of Germanium

199

10.03.7.2 Compounds with germanium–group 13 metal (metalloid) bonds The reactions of methylphenylgermyl lithium with triphenylborane afforded the corresponding germylborates 751a-c as lithium salts (Eq. 206). Similarly, treatment of Et3GeM with triphenylborane led to germylborates 752a-c (M ¼ Li, Na, K) (Eq. 207).201

ð206Þ

ð207Þ

Oxidative addition of germylene–phosphine Lewis pair with boron trihalides yielded addition products 753a and 753b with GedX and BdX bonds, whose reduction using magnesium and a catalytic amount of anthracene produced germaborenes 754a, and 754b (Eq. 208, Fig. 21, Table 28; X ¼ Cl, Br).202 Germaborenes 754a and 754b undergo photochemical reversible cycloaddition with a phenyl group of terphenyl ligand to afford [2 +2] cycloadducts 754c and 754d (Eq. 209).202

ð208Þ

ð209Þ

200

Organometallic Compounds of Germanium

Fig. 21 Molecular structure of compound 754a. Table 28

Selected bond lengths and angles of compound 754a.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge-B Ge-C1 Ge-C2 B-P B-Cl C2-C3 C3-P

1.886(2) 1.944(1) 1.948(1) 1.888(2) 1.786(2) 1.411(2) 1.811(1)

C1-Ge-C2 C2-Ge-B C1-Ge-B Ge-B-P B-P-C3 Cl-B-Ge Cl-B-P

112.9 102.0 144.7 103.1 106.4 138.7 118.2

1,3-Digerma-2-gallataallenic anion 756 was obtained by reacting dilithium disilylgermane 755 with gallium trichloride (Eq. 210). Its indium analog, 1,3-digerma-2-indataallenic anion 757, was prepared through the reaction of 755 with indium trichloride (Eq. 211).203

ð210Þ

ð211Þ

Organometallic Compounds of Germanium

201

10.03.7.3 Germanium compounds with group 14 metal/metalloid bonds 10.03.7.3.1

Synthesis of compounds with GedSi bonds

Enantiomerically enriched silagermane (R)-759 was obtained by reacting disilane (R)-758 with lithium metal and trimethylchlorogermane; the reaction occurred with retention of configuration (Eq. 212).204 Reaction of o-bromo-(fluorodimethylsilyl)benzene 760 with tert-butyllithium and half equiv. of GeCl2
1,4-dioxane resulted in benzosilagermacyclobutene 761 (Eq. 213, Fig. 22, Table 29).205

ð212Þ

ð213Þ

Fig. 22 Molecular structure of compound 761 (H atoms are omitted for clarity).

Table 29

Selected bond lengths and angles of compound 761.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge-Si1 Ge-C1 Ge-C9 Ge-C17 Si1-C2 C1-C2 Si2-F1 Si3-F2

2.371(3) 1.986(9) 1.973(9) 1.976(9) 1.890(10) 1.405(12) 1.598(7) 1.592(6)

Ge-C1-C2 C1-C2-Si4 C2-Si1-Ge Si1-Ge-C1 C9-Ge-C17

102.9(6) 106.0(7) 76.9(3) 74.2(3) 111.1(4)

202

Organometallic Compounds of Germanium

Treatment of Si(SiH3)4 762 with methyllithium and triphenylchlorogermane afforded germasilane 763, which was reacted with methyllithium and triphenylchlorogermane to afford digermasilane 764 (Eq. 214).206

ð214Þ

Trimethylgermyl substituted cyclohexasilane 765 was reacted with a catalytic amount of AlCl3 (or FeCl3) to afford a 1:1 mixture of germacyclopentasilanes 766a and 766b, in which the germanium atom has become a part of the five-membered ring (Eq. 215).207

ð215Þ

Trimethylgermyltris(trimethylsilyl)silane 767 was treated with a catalytic amount of AlCl3 (or FeCl3) to yield tetrakis(trimethylsilyl) germane 768 through the migration of the germanium atom from the terminal to the central position (Scheme 72). Similarly, a rearrangement reaction of tetramethyldisilane 769 containing two Si(SiMe3)2(GeMe3) groups in the presence of AlCl3 or FeCl3 catalyst gave tetramethyldisilane 770 containing two Ge(SiMe3)3 groups. Treatment of compound 770 with potassium tert-butoxide and trimethylchlorogermane led to tetramethyldisilane 771, in which two new GeMe3 groups are incorporated at the terminal positions. The AlCl3/FeCl3 catalyzed rearrangement of 771 converted it to a tetragermane 772 (Scheme 72).207

Scheme 72

Organometallic Compounds of Germanium

203

Reduction of trans-dichlorodisilagermetane 773 using two equiv. of KC8 afforded disilagermabicyclo[1.1.0]butane 774, which upon thermolysis undergoes isomerization to afford disilagermirene 775 (Eq. 216).208 Disilagermirene 776 was reacted with NHC-stabilized dichlorosilylene to afford trisilagermetene 777 via ring expansion (Eq. 217).209

ð216Þ

ð217Þ

Treatment of tetrachlorodisilacyclohexadiene 778, with eight equiv. of lithium and trimethylchlorogermane yielded the corresponding tetrakis(trimethylgermyl) derivative 779 (Eq. 218).210

ð218Þ

10.03.7.3.2

Synthesis of compounds with GedSn bonds

Germanium-tin bonded compound 780 was obtained by reacting bromogermane with triphenyltin lithium (Eq. 219).211

ð219Þ Reaction of tritolylgermane with nBuLi and trimethylchlorostannane resulted in tris(p-tolyl)(trimethylstannyl)germane 781 (Eq. 220). A similar compound, tris(trimethylsilyl)(triphenylstannyl)germane 782, was obtained by treating tetrasilylgermane with potassium tert-butoxide and triphenylchlorostannane (Eq. 221).212

ð220Þ

ð221Þ

Germylstannylene 783 was prepared by reacting terphenyldihydridogermanium anion 554 with half an equiv. of dimeric monochlorostannylene (Eq. 222).129

204

Organometallic Compounds of Germanium

ð222Þ

10.03.7.3.3

Synthesis of compounds with GedPb bonds

Treatment of compound 554 with half equiv. of dimeric monochloroplumbylene yielded germylplumbylene 784 (Eq. 223).

ð223Þ

10.03.7.3.4

Reactivity

The reaction of silagermane (R)-759 with two equiv. of lithium and pentamethylchlorodisilane afforded enantiomerically enriched trisilane (R)-785 (Eq. 224).204

ð224Þ

The ring-opening reaction of benzosilagermacyclobutene 761 using KF and [2.2.2]cryptand afforded triarylgermyl anion 786, which on subsequent reaction with Li[BPh4]/BF3 led to the reversible formation of the ring-closed product 761. Treatment of compound 786 with acetic acid afforded triarylhydridogermane 787 (Eq. 225).213

ð225Þ

Germyl-1-germaindene 789 was synthesized when ethynyl-substituted germane 788 on thermolysis at high temperatures (Eq. 226). However, thermolysis in the presence of 2,3-dimethylbutadiene afforded germacyclopentene 790 (Eq. 227).214

Organometallic Compounds of Germanium

205

ð226Þ

ð227Þ

Heating of acyltrisilylgermanes 792a-b gave intermediate germenes, which were trapped with 2,3-alkyl-1,3-butadienes to obtain germacyclohexenes 793a-d through [2 +4] cycloaddition reactions (Eq. 228).215

ð228Þ

10.03.7.4 Compounds with germanium–transition metal bonds Transition metal complexes containing single, double, and triple bonds between metal and germanium are commonly called germyl, germylidene, and germylidyne complexes. These complexes have been widely studied for their interesting bonding and reactivity.

10.03.7.4.1

Synthesis

10.03.7.4.1.1 Synthesis of complexes having Ge–metal single bond 10.03.7.4.1.1.1 Synthesis from spiro-germanium compound A stable anionic iron(0) complex bonded with pentacoordinate germanium 795a was synthesized by reacting a spirogermane 794 with Na[Fe(Cp)(CO)2] in THF (Eq. 229). A reaction of 795a with benzyltrimethylammonium chloride ((PhCH2NMe3)Cl) gave the cation-exchanged product 795b.216 These complexes feature a polar GedFe bond with an electron-rich iron atom.

ð229Þ

206

Organometallic Compounds of Germanium

10.03.7.4.1.1.2 Synthesis from germanium halides/organogermane The reaction of germyl halides with suitable metal precursors have also provided access to organogermanium compounds featuring GedM bonds. The reactions of RMe2GeX (R ¼ NPh2, NMe2, Me; X ¼ Cl, Br) with K[Cp (OC)2Fe] 796 resulted in compounds 797ac, each having a GedFe bond (Eq. 230). The UV irradiation of 797a-c in the presence of pyridine provided the corresponding pyridine substituted complexes 798a-c (Eq. 230).217

ð230Þ

The reactions of a bulky germyl chloride RGeCl (R ¼ C6H3-2,6-Trip2, where Trip ¼ 2,4,6-iPr3C6H2) with ruthenium/osmium precursors MClX(PPh3)3 (M ¼ Ru, Os; X ¼ H, Cl) resulted in the formation of the corresponding chloro/hydrido metal complexes 799a-d featuring dichlorogermyl substituents.218 During these reactions, the germanium(II) center was formally oxidized to germanium(IV), and a Trip group (of R substituent) replaced two PPh3 ligands (of the metal precursors) through an
6-coordination mode (Eq. 231).

ð231Þ

Compounds with iridium-germanium bonds were isolated through iridium-hydride-mediated Ge–X (X ¼ Ph, Cl) bond activation reactions (Eq. 232). The reactions of M(H)(CO)(PPh3)3 (M ¼ Ir, Rh) with germanium halides/organogermanes (800a-d) provided a mixture of mer-(801a-c) and fac-(802a-c) isomers. Interestingly, when X is fluorine, only the trans-product 801d was obtained (Eq. 232).219–221

ð232Þ

Organometallic Compounds of Germanium

207

Compounds 804a, 804b, and 804c containing a single bond between germanium and group 11 metals were synthesized by reacting a tetradentate tripodal germyl ligand 803 with copper(I), silver(I), and gold(I) chlorides, respectively (Eq. 233).222

ð233Þ

10.03.7.4.1.1.3 Synthesis from germyl hydrides Organogermyl hydrides can be utilized for the synthesis of various transition metal complexes with germanium metal bonds. Bis(germyl)hydridoiron complexes 805a and 805b were obtained through the reaction of HGeR3 (R ¼ Et and Ph) with CpFe(CO)2(Me) (Eq. 234).223 The reaction of HGeR3 (R ¼ p-tolyl) with RuCl(Ph)(CO)(PPh3)2 afforded compound 806, featuring a GedRu bond (Eq. 235).224

ð234Þ

ð235Þ

The reaction of dimesitylgermane Mes2GeH2 with a benzyl derivative of rhodium, (dtbpm)Rh(CH2Ph) 807, resulted in the elimination of toluene to afford rhodium compound 808, in which two hydrogen atoms of a mesityl group attached to germanium atom are involved in agostic interactions with the metal center (Eq. 236).225

ð236Þ

Compound 809 with a
2-(Ge–H) moiety bonded to palladium(0) center was synthesized through the reaction of bis[o-(diphenylphosphanyl)phenyl]methylgermane 530a with Pd(PPh3)4 (Scheme 73). In the solid, compound 809 is stable, but in solution, it gradually decomposes to a germyl-bridged palladium dimer 811 (Scheme 73). Hydrometallation of ethylene occurred when compound 809 was reacted with ethylene at atmospheric pressure leading to the formation of a P,Ge,P-germyl ligand stabilized ethylpalladium(II) complex 810 (Scheme 73).226

208

Organometallic Compounds of Germanium

Scheme 73

The chloro analog (812) of compound 810 was obtained from the reaction of compound 530a with half an equiv. of [Pd(C3H5) Cl]2. Furthermore, the reaction of compound 812 with silver triflate resulted in a P,Ge,P-germyl ligand stabilized Pd(II) triflate complex 813 (Scheme 73).120 Similarly, the anisole derivatives (815 and 816) of compounds 812 and 813 were synthesized from compound 814 (Scheme 73).227

ð237Þ

Palladium bonded germyl complexes 818a, 818b, and 818c were synthesized by reacting (PCP)Pd(PMe3) (PCP ¼ bis[2-(di-isopropylphosphino)phenyl]methylene) (817a) with H3GePh, H2GePh2, and HGePh3, respectively (Eq. 237). Compounds 818a, 818b, and 818c can also be obtained by reacting (PCP)Pd(PPh3) 817b with H3GePh, H2GePh2 and HGePh3, respectively, albeit in lower yields (53%, 57%, and 48%, respectively).228 A dirheniumdigermyl complex 820 with GedRe bonds was obtained in moderate yield by refluxing a solution of Ph3GeH and Re2(CO)8[m-
2-C(H)]C(H)Bun](m-H) 819 in heptane containing 0.5% of H2O (by weight) (Eq. 238). The reaction of 820 with [nBu4N][OH] in methanol afforded the salt [nBu4N][820].229

Organometallic Compounds of Germanium

209

ð238Þ

Germylene-bridged heterobimetallic compounds 822a, and 822b were obtained by reacting 821 [RhIr(CO)3(dppm)2] with H3GeR (R ¼ Ph and tBu, respectively; Eq. 239). The reaction of 822a with H2GePh2 resulted in a heterobimetallic compound 823 featuring two germylene bridges (Eq. 240).

ð239Þ

ð240Þ

The low-temperature reaction of 821 with H2GePh2 resulted in diphenyl germylene bridged heterobimetallic compound 824, which was further reacted with H2GePh2 in the presence of CO at room temperature to afford heterobimetallic compound 825 featuring a terminal germyl group and a germylene bridge. Moreover, reaction of 821 with two equiv. of H2GePh2 at room temperature can directly afford compound 825 (Eq. 241).230

ð241Þ

The reaction of Ph3GeH with [Ir(COD)Cl]2 and CO in hexane resulted in iridiumdigermyl complex 826. The thermal decomposition of compound 826 in toluene at 110  C provided the GePh2 bridged iridium dimers 827, 828, 829, and 830 (Scheme 74).231

210

Organometallic Compounds of Germanium

Scheme 74

The reaction of tripodal germyl hydride 544 HGe(2-C6H4PPh2)3 with Pt(nbe)3 (nbe ¼ norbornene) in an excess of norbornene afforded a racemic mixture of germyl platinum complex 831 (Eq. 242).232 Similar compounds 832a,126 832b,233 832c,234 832d232 and 832e232 were synthesized by reacting 544 with related metal precursors (Eq. 242). The reaction of 832a with KH in the presence of dihydrogen resulted in hydrido dihydrogen complex 833. Furthermore, the reaction of 833 with hydrazine, ammonia, and dinitrogen afforded hydrido hydrazine 834a, hydrido ammonium 834b, and hydrido dinitrogen 834c complexes, respectively (Eq. 243).126

ð242Þ

ð243Þ

Reaction of RGeH3 with (Ph3P)2Pt(
2-C2H4) afforded dihydridogermyl platinum complex 835 with monodentate phosphines (R ¼ 9-triptycene). A ligand exchange reaction of compound 835 with dcpe resulted in complex 836 containing a chelating

Organometallic Compounds of Germanium

211

phosphine. The dppe analog of compound 836 (837) was obtained along with bis(dihydridogermyl) platinum complex 838 through the reaction of RGeH3 with (dppe)PtCl2 in the presence of excess NaBH4 (Scheme 75). Compound 838 can be obtained exclusively in high yield by using 0.5 equiv. of (dppe)PtCl2 (instead of one equiv.). Heating compound 838 in toluene at 60  C for 1 day afforded a digermyl platinum complex 839.235 Palladium analog 841 (of 836) was synthesized by reacting the RGeH3 with [(mdcpe)Pd]2 840 (Eq. 244).236

ð244Þ

Scheme 75

Diphenylgermyl bridged dinuclear palladium complex 842 was synthesized by reacting Pd(PCy3)3 with H2GePh2 (Eq. 245). However, the reaction of Pt(PCy3)2 with H2GePh2 afforded a mononuclear (germyl)hydridoplatinum complex cis-843 as the kinetic product, which converted to the thermodynamic product trans-843 with time (Eq. 246).237

212

Organometallic Compounds of Germanium

ð245Þ

ð246Þ

The low-temperature reaction of Pt(PPh3)2(
2-C2H4) with H3GeMes resulted in unsymmetrical (844a), and symmetrical (844b) germyl-bridged dinuclear platinum complexes as minor and major products, respectively. The 1H NMR analysis of complex 844b revealed the presence of cis- and trans-isomers in a 2:3 ratio. Nevertheless, the reaction of Pt(PPh3)2(
2-C2H4) with two equiv. of H3GeMes at low temperature afforded a bis(germyl)mononuclear platinum complex 845. When a toluene suspension of 845 was stored at room temperature overnight, a digermyl complex 846 was obtained (Scheme 76).238

Scheme 76

The reaction of Ph3GeH with Pt(COD)Me2 847 resulted in a mixture of germyl platinum compounds 848 and 849. Compound 849 can also be isolated by reacting 848 with two equivalents of Ph3GeH for 2 days (Scheme 77). The reaction of Ph3GeH with Pt(COD)Me2 in the presence of CO afforded a mixture of digermyl platinum carbonyl compound 850 and GePh2-bridged platinum dimer 851. Alternatively, compound 850 can be synthesized through the reaction of 849 with CO (Scheme 77).239

Organometallic Compounds of Germanium

213

Scheme 77

10.03.7.4.1.1.4 Synthesis from germyl anions The reaction of the lithium salt of triphenyl germyl anion (LiGePh3) with [Rh(CO)2Cl]2 under a CO atmosphere led to the formation of germyl rhodium compound Rh(CO)4(GePh3). The reaction of this rhodium compound with Ph3GeH in the atmosphere of CO afforded a Ph2Ge-bridged dirhodium complex 852. Furthermore, the reaction of 852 with Pt(PtBu3)2 resulted in a trimetallic compound 853 featuring two rhodium and one platinum centers (Eq. 247).240 The reaction of LiGe(m-tolyl)3 with cis-PtCl2(PMe2Ph)2 provided digermyl platinum(II) compound trans-854. Isomerization of trans-854 can be effected by the irradiation of light (hn > 450 nm) to obtain its cis-isomer cis-854 (98% conversion). Thermal conversion of cis-854 back to trans854 with 80% conversion was also possible by heating the former at 50  C for 5 h (Eq. 248).241

ð247Þ

ð248Þ

10.03.7.4.1.1.5 Synthesis from digermanes The reaction of unsymmetrical digermanes H2PhGeGeR3 (R ¼ Me and Et) with Pt(dppe)(
2-C2H4) 855 resulted in bis(m-germylene)diplatinum complexes 856a and 856b. The reaction of another digermane H3GeGeEt3 (with an H atom rather than a more bulky phenyl group) with the same metal precursor in a 2:3 molar ratio afforded a bis(m3-germylyne)triplatinum complex 857 (Scheme 78). When this reaction is carried out in a 1:4 molar ratio at low temperature and allowed to stand at room temperature, a spiro-type tetraplatinum complex 858 with a m4-Ge atom was obtained (Scheme 78).242

214

Organometallic Compounds of Germanium

Scheme 78

10.03.7.4.1.2 Synthesis of compounds containing germanium metal double bonds 10.03.7.4.1.2.1 Synthesis from germylene monochloride The photochemical reaction of a bulky germylene chloride RGeCl (R ¼ 2,6-dimesitylphenyl) with CpNb(CO)4 afforded a niobium germylidene complex 859 with the expulsion of one CO ligand from the metal precursor (Eq. 249, Fig. 23, Table 30).243

ð249Þ

Fig. 23 Molecular structure of compound 859.

Organometallic Compounds of Germanium

Table 30

215

Selected bond lengths and angles of compound 859.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Nb-Ge Ge-C1 Ge-Cl Nb-C17 Nb-C18 C17-O1 C18-O2

2.5178(6) 1.962(4) 2.191(1) 2.074(3) 2.061(4) 1.152(4) 1.145(5)

Nb-Ge-C1 Nb-Ge-Cl Cl-Ge-C1

141.4(1) 118.75(4) 99.8(1)

10.03.7.4.1.2.2 Synthesis from germyl hydrides An iron germyl compound 861, featuring a GedFe bond was synthesized as a precursor to a germylidene compound 862 through the reaction of germanium hydride H3GeC(SiMe3)3 with Cp Fe(CO)(py)(Me) 860. Subsequently, the abstraction of pyridine from 861 with the help of the Lewis acid BPh3 afforded complex 862 featuring a Ge]Fe double bond (Eq. 250).244 A germylidene 864 featuring a Mo]Ge bond was synthesized by reacting Cp (dmpe)Mo(
3-CH2Ph) 863 with Et2GeH2 (Eq. 251). Single crystal X-ray diffraction studies demonstrated the presence of an intramolecular interaction between the hydride (attached to molybdenum) and the germanium atom. The related diphenyl derivative 865 was isolated by reacting the same molybdenum complex with Ph2GeH2 (Eq. 251). Unlike compound 864, compound 865 has no such intramolecular interaction, probably due to steric hindrance.245

ð250Þ

ð251Þ

Reaction of H3Ge(trip) with Cp (iPr2MeP)Ru(
3-CH2Ph) 866 resulted in a germylidene 867 featuring a Ru]Ge double bond (Eq. 252).246 A cationic germylene ruthenium complex 869 was synthesized by reacting germanium hydride H2GeMes2 with Cp (iPr3P)RuOTf (Scheme 79). The cationic germylidene 869 can be converted to a neutral germylidene 870 through its reaction with DMAP (Scheme 79).247 The reaction of H3Ge(trip) with Cp (iPr3P)RuCl gave a ruthenium germyl complex 871. Subsequent reaction with (Et2O)2Li[B(C6F5)4] affords a cationic germylidiene 872 (Scheme 79).247

ð252Þ

216

Organometallic Compounds of Germanium

Scheme 79

The reaction of germanium trihydride H3GeC(SiMe3)3 with Cp (MeCN)(Me)W(CO)2 873 provided complex 874, containing a W]Ge double bond (Eq. 253). IR and single crystal X-ray diffraction analyzes revealed an interaction between germanium and the hydride bound to tungsten (Eq. 253).248 A cationic germylidene 876 featuring a Pt]Ge bond was obtained by reacting H2GeMes2 with [(dippe)PtMe(Et2O)][BArf4] 875 (Eq. 254).249

ð253Þ

ð254Þ

10.03.7.4.1.3 Synthesis of compounds containing germanium metal triple bonds 10.03.7.4.1.3.1 Synthesis from germylene monochlorides An example of a niobium germylidyne 878 was synthesized by reacting (NMe4)[Nb(CO)4(k2-tmps)] 877 (tmps ¼ MeSi (CH2PMe2)3) with a bulky germylene monochloride ArMesGeCl (ArMes ¼ C6H3-2,6-Mes2) (Eq. 255).243 Similarly, the reactions of a range of other germylene monochlorides with molybdenum, tungsten and rhenium metal precursors afforded metal carbynes 880a (Fig. 24, Table 31),250 880b,250 882,251 883,251 and 885252 (Eqs. 256–258).253

Organometallic Compounds of Germanium

217

ð255Þ

ð256Þ

ð257Þ

ð258Þ

Fig. 24 Molecular structure of compound 880a.

218

Organometallic Compounds of Germanium

Table 31

Selected bond lengths and angles of compound 880a.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Mo-Ge Mo-Cl Mo-P1 Mo-P2 Mo-P3 Mo-P4 Ge-C1

2.3041(3) 2.5380(7) 2.4837(7) 2.4732(7) 2.4917(7) 2.4595(7) 1.994(3)

Mo-Ge-C1 Ge-Mo-Cl Ge-Mo-P1 Ge-Mo-P2 Ge-Mo-P3 Ge-Mo-P4 P1-Mo-Cl P2-Mo-Cl P3-Mo-Cl P4-Mo-Cl

176.27(8) 178.25(2) 100.92(2) 88.86(2) 104.10(2) 87.96(2) 77.34(2) 91.32(2) 77.64(2) 91.87(2)

10.03.7.4.1.3.2 Synthesis from germyl anions Group 6 metal germylidyne complexes 887a-b were prepared by reacting the carbonyl metalates of molybdenum and tungsten with lithium germylenoid [Li(THF)3GeCl2C(SiMe3)3] (Eq. 259).254,255

ð259Þ

10.03.7.4.1.3.3 Synthesis from germylidene metal complex The Cp analog (888) of compound 887b was prepared by heating tungsten germylidene 874 with MesNCO.256 Compound 888 can also be obtained by reacting 874 with MeIme (Eq. 260).257

ð260Þ

10.03.7.4.2

Reactivity of compounds having germanium transition metal bonds

10.03.7.4.2.1 Substitution reactions Compounds having bonds between germanium and metal atoms can undergo ligand substitution at either the germanium or metal center; it is also possible that it can occur at both centers. Compound 806 undergoes a substitution reaction at germanium atom when reacted with ethanol and pyridine to afford ethoxygermyl complex 889. The ethoxy group attached to the germanium atom is labile; compound 889 reversibly undergoes a facile exchange reaction with n-propanol to afford propoxygermyl complex 890. Moreover, the reaction of compound 889 with water resulted in hydroxygermyl complex 891 (Scheme 80).

Organometallic Compounds of Germanium

219

Scheme 80

On the other hand, a substitution reaction at the metal center of 806 occurred when it was reacted with the sodium salt of dithiocarbamate, NaS2CNR2 (R ¼ Me and Et), to afford chelate complexes 892 and 894. One of the PPh3 ligands of ruthenium in 892 was replaced by CO during its reaction with carbon monoxide to obtain compound 893 (Scheme 81). However, the reaction of 894 with two equiv. of NaS2CNMe2 resulted in the replacement of one of the PPh3 ligands from the Ru center and one of the p-tolyl groups at germanium, to afford bis-chelate complex 895. Compound 895 can also be obtained directly by treating compound 806 with an excess of NaS2CNMe2 (Scheme 81).224

Scheme 81

A high-temperature reaction of germyl ruthenium complex 799a with PMe3 afforded compound 896, in which a less bulky PMe3 ligand replaced a PPh3 ligand. Similarly, the reaction of compound 799a with DMSO in the presence of CuCl (as a PPh3 scavenger) at high temperature resulted in DMSO-substituted complex 897 (Scheme 82).218 Rhenium germylidyne 885 was converted to diiodido derivative 898 when reacted with two equiv. of LiI (Eq. 261). The diiodido compound 898 was converted to dihydrido derivative 899 through the reaction of compound 898 with two equiv. of Na[BEt3H] (Eq. 261).252

220

Organometallic Compounds of Germanium

ð261Þ

Scheme 82

Treatment of tripodal rhodium carbonyl complex 832b with trimethylphosphite afforded P(OMe)3-substituted complex 900. Compound 900 can be converted back to 832b by reacting it with CO (Eq. 262).233 Similarly, hydrido- and methylplatinum complexes 832d and 832e were reacted with HCl and HOTf to afford the chlorido- and triflatoplatinum complexes 901 and 902, respectively (Eqs. 263 and 264).232

ð262Þ

ð263Þ

Organometallic Compounds of Germanium

221

ð264Þ

10.03.7.4.2.2 Addition reactions Treatment of rhodium germyl complex 808 with diphenylacetylene resulted in a hydrogermylation reaction to afford germaallyl complex 903 (Eq. 265).225 The reactions of iron germylidene complex 862 with organic nitriles RCN (R ¼ Me, Mes) lead to iron germyl complexes 904a and 904b, in which the nitriles are connected to the iron atoms. The reaction of complex 862 with carbonyl compounds [RC(O)R0 ] (R ¼ R0 ¼ Me, R ¼ Ph; R0 ¼ Me, and R ¼ Mes; R0 ¼ H) afforded hydrogermylation products 905a, 905b, and 905c. The reactions of complex 862 with organic isocyanates RCNO (R ¼ Ph and Mes) lead to hydrogermylation of the carbonyl group to afford five-membered metallacycles 906a and 906b. By contrast, the reaction of mesitylisothiocyanate with complex 862 resulted in C]S bond cleavage to afford sulfur bridged iron germyl complex 907 in which mesityisocyanide is bonded to the iron atom (Scheme 83).244

ð265Þ

Scheme 83

222

Organometallic Compounds of Germanium

A water molecule was added to the Ru]Ge bond of ruthenium germylidene 867 during its hydrolysis with water to afford 908 (Eq. 266).246 Treatment of cationic ruthenium germylidene complex 872 with various alkenes H2C]CHR (R ¼ C4H9, Ph, tBu, and CH2Cl) afforded germylidene complexes 909a, 909b, 909c, and 909d, through hydrogermylation reactions (Eq. 267). Similarly, alkynes RCCR0 (R ¼ tBu; R0 ¼ H and R ¼ R0 ¼ Me) were hydrogermylated by compound 872 to afford hydrogermylated products 910a and 910b (Eq. 267).247

ð266Þ

ð267Þ

Treatment of hydrido(hydrogermylene)tungsten complex 874 with excess nitriles resulted in compounds 911a-b featuring three-membered WGeN rings via hydrogermylation chemistry (Scheme 84).248 The reaction of compound 874 with isothiocyanates afforded the five-membered chelate complexes 912a and 912b through the hydrogermylation of the C]N bond. Similarly, the reaction of phenyl isocyanate with compound 874 gave five-membered chelate complex 913.258 Ketones were also hydrogermylated to afford tungsten germylidenes 914a-b (Scheme 84).248 Similar reactivities were observed for the molybdenum analog of compound 874.259

Scheme 84

Organometallic Compounds of Germanium

223

The reaction of tungsten germylidyne 888 with alcohols gave alkoxygermylenes 915a, 915b, and 915c. Its reaction with two molecules of aromatic aldehydes proceeded through CdH bond activation and hydrogermylation to afford alkoxygermylenes 916a-c.260 Treatment of compound 888 with RC(O)CH]CH2 (R ¼ Me, Et) afforded tungsten complexes 917a-b having an
3-allyl ligand. When MeC(O)C(Me)]CH2 was reacted with compound 888, a tungsten complex 918 featuring an
3-oxagermacyclopentenyl ring was obtained (Scheme 85).261

Scheme 85

The molybdenum and tungsten germylidynes (887a and 887b) produced zwitterionic metal germylidenes 919a-b when reacted with 1,3,4,5-tetramethylimidazol-2-ylidene (Eq. 268).254

ð268Þ

10.03.7.4.2.3 Reactions leading to cationic metal germanium complexes The protonation of neutral iridium complex 832c using [Et2OH][BF4] afforded cationic iridium complex 920, which was deprotonated back to compound 832c using triethylamine in quantitative yield (Eq. 269).234 Dihydridogermyl platinum 836 and palladium 841 complexes were treated with BArF3 to obtain cationic dinuclear metal complexes 921a and 921b that bear bridging hydrido and germyl ligands (Eq. 270).262 The reaction of molybdenum and tungsten germylidyne complexes 880a and 880b with [Ph3C][BArF4]) resulted in cationic metal germylidyne complexes 922a and 922b (Eq. 271).250

224

Organometallic Compounds of Germanium

ð269Þ

ð270Þ

ð271Þ

10.03.7.4.2.4 Transition metal-germanium compounds as catalysts in organic transformations The trans-selective hydrogermylation of phenylacetylene using HGeEt3 was catalyzed by bis(germyl)hydridoiron complex 805a leading to (Z)-vinylgermane 923 (Eq. 272).223 The chloropalladium(II) complex 815 and the palladium(II) triflate complexes 813 and 816 function as catalysts in the hydrocarboxylation of allenes derivatives; formate salt HCOONBnMe3 was utilized as the hydride and CO2 source (Eqs. 273 and 274, Table 32).227

ð272Þ

ð273Þ

ð274Þ

Organometallic Compounds of Germanium

Table 32

Experimental data for Eq. (274).

Entry

Allene substrate

Product

225

Yield (%)

1

92 (925)

2

88 (926)

3

86 (927)

4

83 (928)

5

56 (929)a

6

72 (930)a

7

60 (931)a,b

8

65 (932)a,b

9

75 (933)

10

75 (934)

11

59 (935)

12

48 (936)a,b

(Continued )

226

Organometallic Compounds of Germanium

Table 32

(Continued)

Entry

Allene substrate

Product

Yield (%)

13

85 (937)a

14

76 (938)a,b

15

95 (939)a

16

71 (940)a,b

17

52 (941)a,b

a

Isolated as methyl ester after the reaction with TMSCHN2. 5 mol% of catalyst 815 was used.

b

10.03.8 Germanium containing polymers Organometallic polymers containing germanium atoms in the main chain have gained considerable attention due to their potential applications in photovoltaic devices, field-effect transistors, and light-emitting diodes.263 A summary of the synthetic routes employed for these polymers is provided here.

10.03.8.1 Synthesis by addition polymerization Polymerization of 1,1-dimesitylneopentylgermene 282 using tBuLi as an anionic initiator resulted in polymer 942 with an alternating carbon/germanium backbone after quenching the reaction with methanol (Eq. 275).264

ð275Þ

10.03.8.2 Synthesis by condensation polymerization Germanium containing poly(amides) 945a, 945b, 945c, and 945d were synthesized by condensation reactions between the corresponding germanium-based diacid chlorides 943a-d and bis(4-aminophenyl)diphenylgermane 944 (Eq. 276).265,266 The reaction of the diamine 944 with thiophenedicarboxylic acid 946 in the presence of pyridine, CaCl2 and triphenyl phosphite (TPP) afforded germanium-thiophene based poly(amide) 947 (Eq. 277).267

Organometallic Compounds of Germanium

227

ð276Þ

ð277Þ

The synthesis of poly(imide-amide) 950 was achieved through a two-step process. By reacting diamine 944 with trimellitic anhydride 948 in acetic acid, a dicarboxylic acid 949 was obtained; the reaction of compound 949 with diamine 944 in the presence of TPP, pyridine and CaCl2 gave polymer 950 (Scheme 86).268 Polyimides 952a and 952b were obtained by reacting the corresponding germane dianhydride 951a/951b with bis(4-aminophenyl)diphenylgermane 944 (Eq. 278).265

ð278Þ

228

Organometallic Compounds of Germanium

Scheme 86

Polyurethanes 954a and 954b containing germanium atoms in the main chain were synthesized through the reaction between the corresponding germanium-based bis(chloroformates) 953a/953b and bis(4-aminophenyl)diphenylgermane 944 (Eq. 279).269 Polyurethanes 956a and 956b containing sulfone and germanium atoms in the main chain were synthesized by the treatment of 4,40 -sulfonyldianiline 955 with the corresponding germanium based bis-chloroformate 953a/953b in the presence of pyridine (Eq. 280).270

ð279Þ

ð280Þ

Organometallic Compounds of Germanium

229

The reaction of germanium containing fatty methyl ester 957 with poly(tetramethylene oxide) glycols 958a and 958b (having Mn values of 650 and 1000 g mol−1) in the presence of Ti(OBu)4 under reduced pressure afford amphiphilic polyesters 959ab (Eq. 281).271

ð281Þ

10.03.8.3 Synthesis by Suzuki polycondensation Germafluorene based polymers 961-964 were synthesized by palladium-catalyzed Suzuki cross-coupling reactions between Bpin-functionalized germafluorene and the corresponding aryldibromide monomers (Scheme 87).272 Copolymers 962, 963, and 964 were used as active materials in field-effect transistors (FETs) and bulk heterojunction photovoltaic cells (BHJ PCs). Among these polymers, 962 displayed better results in FET (hole mobility of 0.04 cm2 V−1 s−1; Ion/Ioff value of 1.0  106), while 964 fared well in BHJ PCs (2.8% power conversion efficiency (PCE)).272

Scheme 87

230

Organometallic Compounds of Germanium

Similarly, the usage of 2,1,3-benzothiadiazole-4,7-Bpin 965 with various dithienogermole, germaindacenodithiophene, and germanium-bridged heptacyclic arene monomers under suitable Suzuki cross-coupling reaction conditions afforded copolymers 966-968,273,274 969,50 and 970,51 respectively (Scheme 88).45 Polymer 966 was studied for FET applications; it showed a saturated hole mobility of 0.11 cm2 V−1 s−1. The BHJ solar cell fabricated with 966, 967, 968, 969, and 970 showed PCE values of 4.5%, 2.66%, 1.50%, 5.02%, and 3.03%.

Scheme 88

10.03.8.4 Synthesis by Stille polycondensation A range of donor-acceptor type copolymers 971-980 with alternate electron-rich dithienogermolodithiophenes and the corresponding electron-deficient comonomers were synthesized by Stille polycondensation using Pd2(dba)3 and P(o-tolyl)3 (Scheme 89).47–49 Copolymers 971, 972, and 978 showed p-type behavior in FET devices with up to 0.26 cm2 V−1 s−1 field-effect mobility. In the case of copolymer 973, ambipolar behavior was seen with average hole and electron mobilities of 0.7  10−3 cm2 V−1 s−1 and 3  10−3 cm2 V−1 s−1, respectively.47 The polymers 974 (1.16% PCE), 975 (0.87% PCE), 976 (7.2% PCE), 977 (2.06% PCE), 979 (4.66% PCE), and 980 (2.04%) were studied for BHJ solar cell applications, where 976 displayed the highest PCE value.49

Organometallic Compounds of Germanium

231

Scheme 89

Copolymerization of dithienogermoles and bis(trimethylstannyl)-2,20 -bithiophene using Stille polycondensation reaction afforded copolymers 983a and 983b (Eq. 282); these polymers showed saturated hole mobilities of 3.2  10−4 and 4.0  10−4 cm2 V−1 s−1 in FET applications, respectively.275

ð282Þ

Dithienogermolocarbazole 172 was polymerized with dithienylbenzothiadiazoles 984a and 984b using the Stille conditions to obtain copolymers 985a and 985b, respectively (Eq. 283).51 The solar cell device fabricated with fluorinated polymer 985b showed

232

Organometallic Compounds of Germanium

a better PCE of 4.05% compared to 985a (1.69%). Similarly, in the FET device, polymer 985b displayed better charge mobility (1.20  10−3 cm2 V−1 s−1) than the non-fluorinated derivative 985a (3.20  10−4 cm2 V−1 s−1).51

ð283Þ

Germolodithiophene 157 and its selenium analog 177 were copolymerized with various electron acceptors to obtain a series of donor-acceptor type copolymers (986–991) through Stille coupling (Scheme 90, Eq. 284).45,52,276–279 The BHJ PC made using polymer 986 showed better PCE (7.3%)263 than its selenium analog 991 (5.2%). The PCE values of copolymers 987, 988, and 989 are 2.4%, 2.1%, and 4.6%.276

ð284Þ

Organometallic Compounds of Germanium

233

Scheme 90

10.03.8.5 Synthesis by Yamamoto coupling Bis(4-bromophenyl)-phenylacridine germane 992a was polymerized using Ni(COD)2, COD, and 2,20 -bipyridine to obtain s-p conjugated homopolymer 992b having two electron-donating acridine moieties on the germanium atom.280 This system has 4-tert-butylphenyl end groups that are incorporated using 1-bromo-4-tert-butylbenzene during its synthesis (Eq. 285). The polymer 992b was used as a host material in polymer light-emitting diode, where it showed high triplet energy of 2.86 eV.

ð285Þ

10.03.8.6 Synthesis by Witting reaction Witting condensation reactions of germanium-based diphosphonium salt 73 and aromatic aldehydes in the presence of tBuOK afforded corresponding poly(p-phenylenevinylene) derivatives 993, 994, and 995 containing germanium atoms in the main chain (Scheme 91).26 The photoluminescence spectra of these polymers 993, 994, and 995 showed blue, greenish-blue, and green emissions, respectively.

234

Organometallic Compounds of Germanium

Scheme 91

10.03.8.7 Synthesis from germyl halides Poly(diphenylgermylene)ethynylene 996 was obtained by reacting diethynyldiphnylgermane with dichlorodiphenylgermane in the presence of nBuLi (Eq. 286).281 A polyacetylene 997 containing Ge and Si atoms alternatively in the polymer main chain was synthesized by reacting diethynyldimethylsilane with dichlorodimethylgermane using nBuLi (Eq. 287).282

ð286Þ

ð287Þ

Poly(s-indacenylgermanes) 1000a and 1000b were prepared by reacting digermyl-s-indacene dichlorides 998a and 998b with lithium derivative of s-indacenyls 999a and 999b, respectively (Eq. 288).283 Similarly, the reactions of 998a and 998b with dilithium carbodiimide afforded poly(carbodiimide germyl-s-indacene) 1001a and 1001b (Eq. 289).283

ð288Þ

Organometallic Compounds of Germanium

235

ð289Þ

10.03.8.8 Synthesis from a germyl hydride Hydrogermylation polymerization of aliphatic and aromatic diynes with diphenyldihydrogermane using PdCl2(PCy3)2/ Pd2(dba)3 as a catalyst resulted in the corresponding germylene-divinylene polymers 1002a-d (Scheme 92).284

Scheme 92

10.03.8.9 Synthesis from germylenes Polymerization reactions of N-phenyl-p-quinoneimine with bis(trimethylsilyl)amido- or (t-butyl) (trimethylsilyl)amido-germylene afforded the corresponding copolymers 1005a and 1005b with alternating R2Ge and p-aminophenol moieties (Eq. 290).285,286

ð290Þ

236

Organometallic Compounds of Germanium

10.03.8.10

Synthesis using transition metal precursors

Anionic ring-opening polymerization of dimethylgerma[1]ferrocenophane using nBuLi resulted in the formation of poly(ferrocenyldimethylgermane) 1006 (Eq. 291).287 Copolymerization of diethynyldiphenylgermane with trans-PtCl2(PBu3)2 in the presence of a CuI catalyst afforded polymer 1008 containing alternating germanium and platinum atoms in the main chain (Eq. 292).288

ð291Þ

ð292Þ

10.03.8.11

Polymers with germanium pendants

Germanium containing polystyrenes 1009a to 1009c were synthesized by the copolymerization reactions of trimethylgermane functionalized p-ethylstyrene and p-substituted styrenes in the presence of di-tert-butyl peroxide (DTBP) as a radical initiator (Eq. 293, Table 33).289

ð293Þ

Table 33

Experimental data for Eq. (293).

Entry

R

Initiator

Solvent

Temp (  C)

Time (h)

PDI

Mn

Yield (%)

1 2 3 4 5 6 7

H H H H t Bu t Bu Ph

DTBP DTBP DBP DBP DTBP DBP DTBP

Toluene DMF Toluene DMF Toluene Toluene Toluene

110 150 110 150 110 110 110

13 13 17 68 17 16 16

3.9 3.7 2.7 2.1 4.4 2.2 2.0

12800 8400 6300 5400 12900 2000 24800

75% (1009a) 69% (1009a) 55% (1009a) 40% (1009a) 90% (1009b) 82% (1009b) 90% (1009c)

Organometallic Compounds of Germanium

237

10.03.9 Germylenes Recent years have witnessed tremendous growth of germylene chemistry (i.e., the chemistry of germanium in its formal divalent state). These developments have been covered in various reviews;290–302 the advances that happened in this field are captured here, along with an overview of the contents in these reviews.

10.03.9.1 Preparation 10.03.9.1.1

Acyclic germylenes

The stabilization of the germanium(II) center using bulky secondary amines 1010-1018, 1028-1030, and 1033 was studied. Their in situ generated metal salts undergo salt elimination reactions with GeCl2
1,4-dioxane to afford dicoordinate amidogermylenes 1019-1027, 1031-1032, and 1034 featuring a GedCl bond (Eqs. 294–296, Table 34).251,303–306 By contrast, the potassium salt 1035 (derived from an amine with phosphine substituent) gave a mixture of bisgermylene 1036 and dichlorogermylene 1037 (Eq. 297).307

ð294Þ

ð295Þ

ð296Þ

238

Organometallic Compounds of Germanium

ð297Þ

Table 34

Experimental data for 1019–1027 prepared in Eq. (294).

Germylene

t1 (h)

t2 (h)

% Yield

1019 1020 1021 1022 1023 1024 1025 1026 1027

1 1 1 4 4 4 1 2 2

3 3 3 12 12 12 12 16 16

72 66 67 65 72 89 50 55 82

Treatment of germylenes 1019 and 1022 with Na[CpMo(CO)3] gave molybdenum germylene complexes 1038 and 1039. By refluxing a toluene solution of compound 1038, germylyne complex 1038a was obtained by eliminating one equiv. of carbon monoxide; the same transformation can be achieved through photoirradiation (Eq. 298).251 Compound 1024 reacted with L-selectride to yield hydridodigermene 1040, which in solution dissociated and existed in equilibrium with hydridogermylene 1041. The formation of DMAP !hydridogermylene adduct 1042 in the reaction of compound 1040 with DMAP substantiates its dissociation (Scheme 93) (Fig. 25, Table 35).308

ð298Þ

Organometallic Compounds of Germanium

Scheme 93

Fig. 25 Molecular structure of compound 1042.

Table 35

Selected bond lengths and bond angles of compound 1042.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

N1-Ge1 Ge1-N2 Ge1-H1

1.933(3) 2.204(4) 1.490(2)

N1-Ge1-N2 N1-Ge1-H1 H1-Ge1-N2

108.0(1) 100.0(1) 89.0(1)

239

240

Organometallic Compounds of Germanium

Hydridogermylene 1041 undergoes hydrogermylation reactions with various unactivated alkenes (cyclic and acyclic) and alkynes (Scheme 94).309 Hydridogermylenes 1047 and 1048 were isolated by reacting tert-butoxygermylenes 1045 and 1046 with HBcat. One-pot synthesis of compounds 1045 and 1046 was achieved by treating potassium amides 1043 and 1044 with GeCl2
1,4-dioxane and KOtBu (Eq. 299).310

Scheme 94

ð299Þ

The synthesis of bis(amido) and bis(arsinoamido) ligand stabilized germanium(II) complexes has been scrutinized. The salt elimination reactions of lithium amide 1049 and lithium arsinoamide 1052 with half an equiv. of GeCl2
1,4-dioxane afforded bis(amido) 1050-1051 and bis(arsinoamido) 1054 germylenes, respectively (Eqs. 300 and 301).311–313 With one equivalent of GeCl2
1,4-dioxane, compound 1052 gave a Ge(IV) compound 1053, due to the insertion of germylene into the NdAs bond (Eq. 301).312 Similarly, the in situ generated lithium salt of phosphinoamine 1055 reacted with various equivalents of GeCl2
1,4-dioxane to yield bis(phosphinoamido)germylene 1057 and dimeric amidophospinogermylene 1056 featuring two GedCl bonds (Eq. 302).311

Organometallic Compounds of Germanium

241

ð300Þ

ð301Þ

ð302Þ

The stabilization of germylenes using bulky Eind, terphenyl, and phosphide ligands has been investigated. The Eind and 2,6-dimesityl substituted terphenyl ligand stabilized germylenes 1059 and 1063-1065 were synthesized by the salt elimination reactions of lithium salts 1058 and 1060-1062 with GeCl2
1,4-dioxane, respectively (Eqs. 303 and 304).314,315 The reaction of secondary phosphine 1066 with benzyl potassium and GeCl2
1,4-dioxane yielded diphosphagermanium(II) compound 1067 (Eq. 305).316

ð303Þ

242

Organometallic Compounds of Germanium

ð304Þ

ð305Þ

Chloride metathesis reactions of the terphenyl monochlorogermylenes 1068 and 1069 with various alkali metal salts resulted in the corresponding germylenes 1070-1077 with N, C, P, B, and Si functionalization (Eq. 306).317 Germylene 1078 (related to compound 1077) was synthesized by the boryl/amide metathesis reaction between Lappert’s germylene and (THF)2Li[B-(NDippCH)2] (Eq. 307).317

ð306Þ

ð307Þ

The adducts of disilylgermylenes with NHC and phosphine donors (1079-1080) were obtained by reacting tris(trimethylsilyl)silyl potassium with GeCl2
1,4-dioxane and NHC/phosphine (Eq. 308).318 The adduct 1080 was reacted with phenyl and diphenyl acetylenes to yield monovinylgermylene 1081 and germirene 1083 through insertion and [1 + 2] cycloaddition reactions (Scheme 95); treatment of compound 1080 with two equiv. of phenylacetylene afforded divinylgermylene 1082 (Fig. 26, Table 36). Lewis acid assisted phosphine abstraction from compounds 1081 and 1082 resulted in silagermetes 1084 and 1085, respectively, via the insertion of germylene into the SidSiMe3 bond (Scheme 95).318

Organometallic Compounds of Germanium

243

ð308Þ

Scheme 95

Fig. 26 Molecular structure of compound 1082.

244

Organometallic Compounds of Germanium

Table 36

Selected bond lengths and bond angles of compound 1082.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

P1-Ge1 Ge1-C1 Ge1-C2

2.426(1) 2.035(2) 2.043(2)

C1-Ge1-C2 P1-Ge1-C1 P1-Ge1-C2

105.32(9) 91.85(6) 95.16(6)

The N-heterocyclic vinyl and guanidine ligand stabilized acyclic germylenes were isolated and studied. The reduction of bis(vinyl) dichloro-1086 and bis(guanidine)dichlorogermane 1088 gave the corresponding germylenes 1087 and 1089, respectively (Eqs. 309 and 310).319,320 An amino(imino)germylene 1091 was synthesized by the transamination reaction of imidazolin-2-imine 1090 with Lappert’s germylene (Eq. 311).321

ð309Þ

ð310Þ

ð311Þ

Substituted pyridine, phosphine, amido, and ylidic Wittig ligands are also reported to provide the necessary stabilization for the synthesis of acyclic germylenes 1092–1095 (Scheme 96).322,323 Compound 1094 was reacted with pyridine to afford germylene-pyridine adduct 1094a. Furthermore, dihydridogermylene/borane 1096 and dichlorogermylene/tungsten 1097 adducts were isolated from the reactions of compound 1095 with LiBH4 and (THF)2
SnCl2
W(CO)5, respectively (Eq. 312).323

Organometallic Compounds of Germanium

Scheme 96

245

246

Organometallic Compounds of Germanium

ð312Þ

10.03.9.1.2

N-Heterocyclic germylenes

10.03.9.1.2.1 Four membered N-heterocyclic germylenes Stabilization provided by amidinate and guanidinate ligand systems has allowed for the isolation of a number of N-heterocyclic germylenes. The lithium salts of amidinate and guanidinate ligands react with 1 and 0.5 equivs. of GeCl2
1,4-dioxane to afford tricoordinate (1098-1107) and tetracoordinate (1108-1111) germylenes, respectively (Eq. 313).324–330 The tricoordinate germylenes possess a Ge-Cl functionality.

ð313Þ

Monoiodo germylene (1106a) was formed by the reaction of lithium amidinate [(Me3SiNC(Ph)NSiMe3)Li] with GeI2 (Eq. 314).326

ð314Þ

The nucleophilic substitution reactions of monochloro germylene 1101 with various lithium salts afforded a range of other functionalized germylenes 1112–1118. (Scheme 97).312,331–337

Organometallic Compounds of Germanium

247

Scheme 97

Like germylene 1101, germylene 1098 undergoes salt elimination reactions to afford germylene amide 1119 and metallogermylene 1120 (Eq. 315).338 Compound 1099 reacted with a- and b-sulfinyl carbanions to result in heteroleptic organogermylenes 1122, 1123,338 and 1121339 with sulphoxides (Eq. 316). Guanidinatogermylene 1100 reacts with an anionic gallium(I) nucleophile to form gallyl germylene 1124 (Eq. 317).340

ð315Þ

248

Organometallic Compounds of Germanium

ð316Þ

ð317Þ

The reaction of compound 1107 with the lithium phenylacetylide afforded phenylethynyl germylene 1125 (Eq. 318). The reaction of compound 1125 with B(C6F5)3 gave germylene borane Lewis pair 1126. The molecular structure of compound 1126 shows a cis-arrangement of the germylene and B(C6F5)2 moieties (Fig. 27, Table 37). Compound 1126 was reacted with RC(O)Me and t BuNC to afford germanium-centered spirocycles featuring GeC2BO (1127) and GeC2BC (1128) units, respectively. Similarly, its

Fig. 27 Molecular structure of compound 1126.

Organometallic Compounds of Germanium

Table 37

249

Selected bond lengths and angles of compound 1126.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-B1 N2-Ge1 N1-Ge1 B1-C1 Ge1-C2 C2-C1

2.160(2) 1.939(2) 1.933(2) 1.659(3) 1.935(2) 1.359(3)

N2-Ge1-N1 N1-Ge1-B1 N2-Ge1-B1 Ge1-C2-C1 C2-C1-B1 C1-Ge1-B1

67.47(7) 149.09(8) 133.99(8) 93.9(1) 111.9(2) 78.2(1)

reaction with half an equiv. of iPrNCO gave an equimolar mixture of spirocycles featuring GeC2BO (1129a) and GeC2BC (1129b) heterocyclic rings (Scheme 98).324

ð318Þ

Scheme 98

250

Organometallic Compounds of Germanium

The reaction of compounds 1101 and 1107a with the lithium salt of trimethylsilylethyne gave alkynylgermyl substituted germylenes 1130 and 1131 through double and triple catenation, respectively (Eqs. 319 and 320). Furthermore, the reaction of compound 1130 with TMSN3 afforded aminogermylene 1130a (Eq. 319).341

ð319Þ

ð320Þ

The reaction of germylene 1101 with a cAAC and KC8 in a molar ratio of 1:0.5:1.2, afforded a Ge3 compound (cAAC)Ge(GeL)2 (1132) featuring a Ge]C bond [cAAC ¼ cyclic alkyl(amino) carbene; L ¼ PhC(tBuN)2]. Compound 1132 was subsequently converted to germylone (cAAC)2Ge (1133) in solution at room temperature (Eq. 321).342 Compound 1101 was also reacted with a polyphosphorous source, the potassium salt of pentaphosphaferrocene [{K(dme)}2Cp Fe(
5-P5)], to afford a bis(germylene)-polyphosphide complex 1134 that undergoes a unique constitutional isomerization to complex 1134a (Eq. 322).343

ð321Þ

Organometallic Compounds of Germanium

251

ð322Þ

10.03.9.1.2.2 Five-membered N-heterocyclic germylenes Five-membered N-heterocyclic germylenes (NHGe) are more diverse than their four-membered ring counterparts due to the presence of a variety of ligand backbones. This class includes germylenes stabilized by ligands such as aminotroponimine (ATI), substituted diazabutadiene (DAB), N,N0 -disubstituted 1,2-diaminobenzene, substituted oxalamidine, a-iminopyridine, and pyrrolylaldimine. The reaction of GeCl2
1,4-dioxane with the lithium salts of aminotroponimine ligand gave monochloro germylenes 1135 and 1136, featuring bulky iso-butyl and tert-butyl substituents on nitrogen atoms. (Eq. 323).344,345

ð323Þ

Germylenes 1135 and 1136 undergo nucleophilic substitution reactions with different lithium, sodium and potassium salts to afford a range of functionalized aminotroponiminatogermylenes 1137–1144 and 1145–1150 (Schemes 99 and 100 and Eq. 324).346–351

ð324Þ

252

Organometallic Compounds of Germanium

Scheme 99

Scheme 100

The reactions of compounds 1135 and 1136 with CsF gave monofluoro germylenes 1151 and 1152, respectively (Eq. 324).345 The reactions of compounds 1141 and 1137 with trimethylsilyl bromide resulted in monobromo germylene 1153 (Eqs. 325 and 326).349 The syntheses of germylene thiophenoxide 1154 and selenophenoxide 1155 were achieved through the reactions of compound 1137 with thiophenol and selenophenol, respectively (Eq. 326).346,349 Futhermore, the reaction of compound 1137 with trimethylsilyl cyanide 1156 afforded germylene cyanide 1157 (Eq. 326).346,349

Organometallic Compounds of Germanium

253

ð325Þ

ð326Þ

The reaction of monochlorogermylene 1135 with an iso-propyl Grignard reagent afforded alkyl germylene 1158 (Eq. 327).352 The formation of digermylene oxides 1159 and 1160 were achieved by the reaction of germylenes 1135 and 1136 with excess KOH (Eq. 328).353 The molecular structure reveals the presence of an oxide bridged germanium(II) centers (Fig. 28, Table 38). The facile reaction of compound 1160 with reagent 1156 afforded germylene monocyanide 1161 (Eq. 328).354

ð327Þ

ð328Þ

Fig. 28 Molecular structure of compound 1160.

254

Organometallic Compounds of Germanium

Table 38

Selected bond lengths and angles of compound 1160.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

N2-Ge1 N1-Ge1 Ge1-O1 O1-Ge2

2.017(3) 2.021(3) 1.791(2) 1.789(3)

N2-Ge1-N1 N2-Ge1-O1 N1-Ge1-O1 Ge2-O1-Ge1

79.2(1) 93.4(1) 100.3(1) 154.4(2)

The stabilization of germylenes by N-monosubstituted and N,N0 -disubstituted 1,2-diaminobenzenes, has also been investigated. Transamination reactions of Ge{N(SiMe3)2}2 with protio-ligands 1162–1165 and 1170–1173 afforded benzannulated N-heterocyclic germylenes 1166–1169 and 1174–1177 (Eqs. 329 and 330).355,356 The deprotonation of compounds 1174 and 1175 gave anionic germylenes 1178 and 1179, respectively (Eq. 330).356 Chiral benzannulated N-heterocyclic germylene 1181 was also isolated using the reaction of chiral N,N0 -disubstituted o-phenylenediamine ligand 1180 with Ge[N(SiMe3)2]2 (Eq. 331)357

ð329Þ

ð330Þ

ð331Þ

The transamination reaction of diamine 1182 with Ge{N(SiMe3)2}2 gave a tetrameric pyridoannulated germylene 1183, formed through strong intermolecular Npy !Ge(II) interactions (Eq. 332).358 (Fig. 29 and Table 39)

Organometallic Compounds of Germanium

255

ð332Þ

Fig. 29 Molecular structure of compound 1183.

Table 39

Selected bond lengths and angles of compound 1183.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

N6-Ge1 Ge1-N2 Ge1-N1 N3-Ge2 N4-Ge2 Ge2-N5

2.137(2) 1.942(2) 1.865(3) 2.104(2) 1.955(2) 1.928(2)

N6-Ge1-N2 N6-Ge1-N1 N1-Ge1-N2 N3-Ge2-N5 N3-Ge2-N4 N4-Ge2-N5

95.3(1) 92.7(1) 83.9(1) 93.19(9) 97.63(9) 81.6(1)

The addition of GeCl2
1,4-dioxane complex to the dilithium salt of bisborylated diamine resulted in N,N0 -bisborylated N-heterocyclic germylene 1184 (Eq. 333).359 The transmetallation reaction of the hexalithium salt of hexamine with GeCl2
1,4-dioxane afforded tris(N-heterocyclic germylene) 1185 (Eq. 334).360

256

Organometallic Compounds of Germanium

ð333Þ

ð334Þ

The reaction of the lithium salt 1186 of pyrrolylaldimine with GeCl2
1,4-dioxane resulted in pyrrolylaldiminatogermylene 1187. Furthermore, the reactions of germylene 1187 with KOtBu and LiN(H)Dip afforded alkoxy- and aminogermylenes 1188 and 1189 (Eq. 335).361

ð335Þ

The utility of non-innocent ligands for the isolation of N-heterocyclic germylenes has also been investigated. The reaction of GeCl2
1,4-dioxane with the in situ generated potassium salt of a-iminopyridine 1190 gave bis-monochlorogermylene 1191, which upon reduction using KC8 afforded non-functionalized N-heterocyclic germylene 1192 via homolytic CdC bond cleavage (Eq. 336).362

ð336Þ

The transamination reaction of a-aminopyridine 1193 with Ge{N(TMS)2}2 afforded aminogermylene 1194, which upon reaction with one mol% of LiHMDS underwent deamination to yield non-functionalized germylene 1195 (Eq. 337). Furthermore, the reaction of germylene 1195 with HCl and BCl3 resulted in monochloro germylenes 1196 and 1197; compound 1197 has a BCl2 moiety attached to the ligand backbone (Eq. 338).363

Organometallic Compounds of Germanium

257

ð337Þ

ð338Þ

10.03.9.1.2.3 Six-membered N-heterocyclic germylenes Stabilization of the germanium(II) centers using b-diketiminate (BDI), anilido–imine, bis(oxazoline), and 2,2-dipyridylamine (dpa) ligands afforded six-membered N-heterocyclic germylenes. The reactions of the lithium salts of differently substituted b-diketiminate ligands with GeCl2
1,4-dioxane, GeBr2, and GeI2 afforded the corresponding N-heterocyclic germylenes 1198–1209 (Eqs. 339 and 340).364–373

ð339Þ

ð340Þ

Salt elimination reactions of germylene 1198 with different lithium, sodium, and potassium salts afforded a range of functionalized N-heterocyclic germylenes featuring GedX bonds (X ¼ C, O, N, P, Fe) 1210–1220 (Scheme 101).374–377 A similar reaction occurred when compound 1198 was treated with less bulky LiNMe2, leading to amino germylene 1221 (Eq. 341).378 However, the use of bulky amines LiNEt2 and LiNiPr2 resulted in back-bone deprotonated germylene 1222.372

258

Organometallic Compounds of Germanium

ð341Þ

Scheme 101

The NHC-assisted reaction of germylene 1198 with dithienyl phosphine gave Ge-P functionalized germylene 1223 (Eq. 342).377 Silylation of the germylene phosphine 1216 with trimethylsilyl triflate afforded monosilylated product 1224 featuring a PdSi bond. Further reaction of compound 1224 with trimethylsilyl triflate gave triflatogermylene 1225 instead of the anticipated disilylated product. The reaction of 1216 with tBu2Hg gave P,P0 -bis(phosphanylgermylene) 1226 featuring a PdP bond (Eq. 343).376

Organometallic Compounds of Germanium

259

ð342Þ

ð343Þ

Salt metathesis reactions of germylene monochlorides with Na(OCP) gave phosphaketenyl germylenes 1227–1230 (Eq. 344).370,379,380 Similarly, the reaction of germylenes 1207 and 1198 with Na(OCAs) resulted in arsaketenyl germylenes 1231 and 1232, respectively (Eq. 345).381,382

ð344Þ

ð345Þ

The UV irradiation of germylenes 1228, 1227, and 1229 resulted in heavier analogs of cyclobutadiene 1233, 1234, and 1235 with Ge2P2 four-membered ring through the intermediacy of phosphagermyne 1228a, 1227a, and 1229a (Scheme 102).379,380 The dimeric structure of compound 1233 was confirmed by its X-ray crystallography (Fig. 30, Table 40).

260

Organometallic Compounds of Germanium

Scheme 102

Fig. 30 Molecular structure of compound 1233.

Table 40

Selected bond lengths and angles of compound 1233.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-P1 Ge1-P2 Ge1-N2 Ge1-N1

2.2672(6) 2.2550(6) 1.988(2) 1.983(2)

N2-Ge1-N1 N2-Ge1-P2 N1-Ge1-P1 P1-Ge1-P2

89.32(7) 116.44(5) 114.52(5) 105.41(2)

Interestingly, irradiation of phosphaketenyl germylene 1230 stabilized by an electron-rich b-diketiminate framework featuring backbone NMe2 groups, yielded imine-coordinated N,P-heterocyclic germylene 1236 (Eq. 346).370

Organometallic Compounds of Germanium

261

ð346Þ

The light-sensitive germylenes 1231 and 1232 decomposed at room temperature to afford arsenic analogs 1238a and 1238b via arsagermynes 1237a and 1237b (Eq. 347). The formation of compound 1237b was confirmed using trapping reagents PPh3 and Me i I Pr, which resulted in the formation of complexes 1239 and 1240, respectively (Eq. 348).381,382

ð347Þ

ð348Þ

The unsymmetric substituted digermylene 1242 having Ge(I)–Ge(I) s bonds with some ionic character was synthesized by the salt metathesis reaction of the N-heterocyclic germanium(II) nucleophile 1241 with the b-diketiminato Ge(II) chloride (Eq. 349).383

ð349Þ

The reduction of germanium(II) chloride complex 1204 with elemental lithium resulted in the formation of the lithium complex of N-heterocyclic germylidenide 1246 and the germanium(II) amide complex 1247 (Scheme 103).369

262

Organometallic Compounds of Germanium

Scheme 103

The reduction of b-diketiminato ligand stabilized monochlorogermylene 1205 with Jones’ magnesium(I) dimer resulted in ring-contracted product 1248a (Eq. 350).383b

ð350Þ

The reactions of germylene monochlorides 1198, 1206, and 1200 with an appropriate hydride source afforded hydrido germylenes 1249, 1250, and 1251, respectively (Eq. 351).370,373,384 By contrast, the bulky germylene monochlorides 1204 and 1205 gave diamidogermylenes 1252 and 1253 via the unstable hydrido germylene intermediate (Eq. 352).373

Organometallic Compounds of Germanium

263

ð351Þ

ð352Þ

The reactions of hydrido germylene 1249 with N2O or three equiv. of trimethylsilyl azide afforded germylene hydroxide 1254 and germylene azide 1255, respectively; germanium(IV) diamide 1256 was also formed along with compound 1255 (Eq. 353).385

ð353Þ

Hydrogermylation of various unsaturated substrates, such as ketones, alkyne, and phosphaalkyne, has been investigated using compounds 1249 and 1251 to obtain the corresponding hydrogermylated products 1257–1260 (Eqs. 354 and 355).373,386 Heating compound 1259 to high temperatures (>30  C) regenerated hydrido germylene 1251. Furthermore, hydridogermylenes 1249 and 1251 mediated the reduction of CO2 to formic acid and methanol using amino borane as the hydrogen source (Eq. 356).386

ð354Þ

264

Organometallic Compounds of Germanium

ð355Þ

ð356Þ

The reaction of hydridogermylene 1249 with an NHC/borane frustrated Lewis pair afforded germylene 1222 (Eq. 357).387

ð357Þ

The reactions of non-functionalized germylene 1222 were also investigated; the NdH bond activation of ammonia, hydrazine, secondary amine (HN(C6F5)2), and the CdH bond activation of trimethylsilyl diazomethane were reported to afford the corresponding aminogermylenes 1263, 1263a, 1264387–389 and diazogermylene 1265 (Scheme 104).387

Organometallic Compounds of Germanium

265

Scheme 104

Rearrangement of germylene 1265 to isonitriletrimethylsilyl germanium(II) amide 1268 occurred over a period of 3 months (Eq. 358).387 Reaction of germylene 1222 with the parent alkyne gave [4 +2] cycloadduct 1267. However, phenyl acetylene in its reaction with germylene 1222 gave a mixture of [4+ 2] cycloadduct 1266 and alkynyl germylene 1211 (Scheme 104).390

ð358Þ

The reaction of germylene 1222 with B(C6F5)3 gave zwitterionic germylene 1269 via CdB bond formation, which on further reaction with 1,3-di-tert-butylimidazol-2-ylidene (NHC) gave a germylene featuring a pendant borate anion 1270 (Eq. 359).387 The X-ray crystal structure of compound 1269 confirms the presence of the exocyclic B(C6F5)3 group (Fig. 31, Table 41). Additionally, reactions of 1222 with water, alcohols, and carboxylic acid afforded the corresponding nucleophilic addition products (Eq. 360).391

266

Organometallic Compounds of Germanium

ð359Þ

ð360Þ

Fig. 31 Molecular Structure of compound 1269.

Table 41

Selected bond lengths and angles of compound 1269.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

N1-Ge1 N2-Ge1 C2-B1

1.881(2) 1.843(2) 1.656(3)

N2-Ge1-N2 Ge1-N2-C1 Ge1-N1-C2

93.91(9) 127.4(2) 128.5(2)

The syntheses of various germylenes functionalized with phosphorus moieties was achieved by activating a range of organic phosphorous compounds using germylene 1222 (Scheme 105).392

Organometallic Compounds of Germanium

267

Scheme 105

The synthesis of N-heterocyclic germylenes using b-diketiminate ligands decorated with various substituents has also been investigated. The salt elimination reactions of the in situ generated alkali metal salts of bis(2-methoxy)phenyl and bis(2-diphenylphosphine)phenyl functionalized b-diketimines (1277, 1278, and 1283) with GeCl2
1,4-dioxane resulted in the formation of the corresponding monochlorogermylenes 1281, 1282, and 1284 (Eqs. 361 and 362).393,394 Amine elimination reactions of b-diketimines 1277 and 1278 with Ge{N(SiMe3)2}2 gave aminogermylenes 1279 and 1280 (Eq. 361).394

ð361Þ

268

Organometallic Compounds of Germanium

ð362Þ

The b-diketiminatogermylene 1286 featuring a piconyl moiety on the ligand backbone was synthesized by reacting b-diketimine 1285 with GeCl2
1,4-dioxane (Eq. 363).

ð363Þ

The reaction of germylene 1286 with AlCl3 gave b-diketiminatoaluminum dichloride 1289 via transmetallation. When compound 1286 was reacted with GeCl2
1,4-dioxane, a five-membered N-heterocyclic monochloro germylene 1287 and amine/GeCl2 adduct 1288 were obtained (Eq. 364).395

ð364Þ

The decarbonylation of germylene phosphaketene 1290 (obtained via the reaction of monochlorogermylene 1284 with [Na(dioxane)2.5]PCO) using tris(pentafluorophenyl)borane afforded a phosphinidene 1291 with one of the ligand phosphine groups bound to the low-valent phosphorus atom. Alternatively, compound 1291 can be visualized as a phosphine bridged phosphonium borate 1292 (Scheme 106).393

Organometallic Compounds of Germanium

269

Scheme 106

N-Heterocyclic germylenes with benzyl and phosphine groups on the g-C of b-diketiminato scaffolds have also been investigated. Use of the lithium salts 1293 and 1296 afforded the N-heterocyclic germylene 1294396 and a five-membered N,P-heterocyclic germylene 1297397 via salt elimination reactions with GeCl2
1,4-dioxane (Eqs. 365 and 366). Compound 1294 can be converted to a non-functionalized germylene 1295 upon reaction with LiN(TMS)2 (Eq. 365). The nucleophilic addition of phenylacetylene to germylene 1295 gave alkynylgermylene 1298 (Eq. 367).396

ð365Þ

ð366Þ

270

Organometallic Compounds of Germanium

ð367Þ

b-Diketimines featuring a different number of phenyl rings on the b- and g-carbon atoms of the ligand backbone were also studied for stabilizing germanium(II) species 1302-1304 (Eq. 368).398,399 The reductive dehalogenation of germylenes 1302 and 1303 using sodium naphthalene afforded p-type germanium(II) radicals 1305 and 1306, with the N2C3 backbone possessing the majority of the spin density (Eq. 368).398,399 The reaction of cyclic germylene radical 1305 with CoBr2 and NHC 1307 gave Ge(I) dimer 1308 along with various side products that include aminogermylene 1309 (Eq. 369).400a

ð368Þ

ð369Þ

N-Heterocyclic germylene 1311 stabilized by an unsymmetrical b-diketiminate ligand was obtained through the salt elimination reaction between lithium salt 1310 and GeCl2
1,4-dioxane (Eq. 370). Germylene 1311 reacted with 1,3-di-tertbutylimidazol-2-ylidene to afford non-functionalized germylene 1312, which upon ammonolysis and hydrolysis resulted in aminogermylene 1313 and germylene hydroxide 1314, respectively (Scheme 107).401 The X-ray crystal structure of compound 1314 confirmed its monomeric nature (Fig. 32, Table 42).

ð370Þ

Organometallic Compounds of Germanium

271

Scheme 107

Fig. 32 Molecular structure of compound 1314.

Table 42

Selected bond lengths and angles of compound 1314.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

N2-Ge1 Ge1-N1 Ge1-O1 O1-H1

2.005(2) 2.007(1) 1.825(2) 0.840

N2-Ge1-O1 N1-Ge1-N2 N1-Ge1-O1 Ge1-O1-H1

93.87(8) 89.43(7) 95.29(8) 109.5

Akin to the synthesis of germylene hydrides 1249, 1250, and 1251, the reaction of 1311 with K[BH(sBu)3] resulted in unstable hydridogermylene 1315, which undergoes 1,3-hydrogen transfer to afford diamido germylene 1316a that is non-functionalized. However, the labile hydride compound 1315 afforded germylene formate 1316b in the presence of a carbon dioxide atmosphere (Scheme 108).402

272

Organometallic Compounds of Germanium

Scheme 108

The anilido iminate ligand stabilized N-heterocyclic germylenes 1319 and 1320 were obtained through the reactions of the in situ generated lithium salts (of 1317 and 1318) with GeCl2
1,4-dioxane. Germylenes 1319 and 1320 undergo nucleophilic substitution and oxidative cycloaddition reactions with lithium methoxide and o-quinone to produce germylene methoxides (1321a–1321b) and Ge(IV) compounds (1322a–1322b), respectively (Eqs. 371 and 372).403

ð371Þ

ð372Þ

Organometallic Compounds of Germanium

10.03.9.1.3

273

Heterocyclic germylenes

Apart from N-heterocyclic germylenes featuring CN2Ge, C2N2Ge, and C3N2Ge rings, germylenes with various other heterocyclic rings have also been developed.292,299,300 This class of compound mainly includes germylenes with N2PGe, N2SiGe, N2SGe, N2Si2Ge, NC2PGe, NC2PGe, NC3PGe, OC3NGe, and N2CP2Ge heterocyclic rings. 10.03.9.1.3.1 Four-membered heterocyclic germylenes The reactions of the dilithium salts of bis(amido)silanes (1323–1324) with GeCl2
1,4-dioxane gave bis(amido)silyl ligand stabilized germylenes 1325 and 1326 (Eq. 373, Table 43).404 Similarly, the reactions of lithium salts generated in situ from bis(amido)silanes 1327a–1327c with GeCl2
1,4-dioxane also gave the corresponding germylenes 1328a–1328c (Eq. 374, Table 43).405,406

ð373Þ

ð374Þ

Table 43

Experimental data for Eqs. (373) and (374).

Germylenes

t (h)

t1 (h)

t2 (h)

% Yield

1325 1326 1328a 1328b 1328c

48 48 – – –

– – 3 1.5 12

– – 12 12 12

53 58 34 94 84

The iminophosphonamide ligand stabilized monochlorogermylene 1330 was synthesized by treating the in situ generated lithium salt (of compound 1329) with GeCl2
1,4-dioxane. The germylene 1330 reacted with silver triflate and potassium tert-butoxide to produce triflato- and butoxygermylenes 1331 and 1332, respectively (Eq. 375).407

274

Organometallic Compounds of Germanium

ð375Þ The reaction of aminoiminophosphine 1333 with dialkoxydichlorogermanes afforded the corresponding dichlorogermylene  complexes 1334–1336 which are stable at 0 C. However, at room temperatures, these species are converted to the corresponding monochlorogermylenes 1337–1339 by elimination of chlorotrimethylsilane (Eq. 376)408. Germylene 1336 eliminates isopropyl chloride at higher temperatures (50–60  C) to afford monochlorogermylene 1340 (Eq. 377).408 Germylene 1338 undergoes a nucleophilic substitution reaction with the lithium salt of trimethylsilyldiazomethane to produce germylene 1341 with C(N2) SiMe3 functionalization (Eq. 378).408

ð376Þ

ð377Þ

Organometallic Compounds of Germanium

275

ð378Þ

A phosphine-functionalized triimido sulfur ligand was also utilized to stabilize heterocyclic germylene 1343 featuring a N2SGe ring; 1343 was obtained by reacting the lithium salt 1342 with GeCl2
1,4-dioxane (Eq. 379).409

ð379Þ

10.03.9.1.3.2 Five membered heterocyclic germylene Phosphine-stabilized monochlorogermylenes 1344 and 1345 featuring NC2PGe heterocyclic rings were obtained as diastereomeric mixtures through the reactions of the in situ generated lithium salts of the corresponding ligands with GeCl2
1,4-dioxane (Eq. 380). The germylenes 1344 and 1345 were reacted with lithiated phosphinodiazomethane to afford diastereomeric phosphinogermylene diazomethanes 1346 and 1347. Irradiation (l ¼ 300 nm) of compounds 1346 and 1347 resulted in the formation of phosphinogermynes 1348 and 1349 (Scheme 109). Single-crystal X-ray diffraction analysis of compound 1348 indicated a long GedC bond length for a formal triple bond and an extremely short PdC bond (Fig. 33, Table 44).410

ð380Þ

276

Organometallic Compounds of Germanium

Scheme 109

Fig. 33 Molecular structure of compound 1348.

Table 44

Selected bond lengths and angles of compound 1348.

Atoms

Bond lengths (A˚ )

Atoms

Bond Angles (  )

Ge1-C1 C1-P1 P2-Ge1 Ge1-N1 P1-N5 P1-N4

1.888(5) 1.549(4) 2.490(1) 2.005(3) 1.657(4) 1.670(4)

P2-Ge1-C1 N1-Ge1-C1 Ge1-C1-P1 P2-Ge1-N1 C1-P1-N5 C1-P1-N4

113.6(1) 102.4(2) 147.3(3) 82.8(1) 126.8(2) 130.5(2)

Organometallic Compounds of Germanium

277

The reaction of germylene 1344 with NaPCO afforded diastereomeric germylene 1350 with phosphaketene functionalization (Eq. 381).411 Moreover, phosphine-stabilized diastereomeric alkyl/arylgermylenes 1351–1353 were also isolated using germylene 1345 (Eq. 382).412 Compound 1348 isomerized to a mixture of phosphaalkene-functionalized germylene 1354a and a six-membered heterocyclic germylene 1354b featuring a C3NPGe ring, through 1,2-migration of a di-iso-propylamino group from phosphorus to carbon, and phosphine migration from germanium to the carbon atom, respectively.410 By contrast, compound 1349 gave exclusively a six-membered heterocyclic germylene 1355 analogous to compound 1354b (Scheme 110).

ð381Þ

ð382Þ

Scheme 110

278

Organometallic Compounds of Germanium

Compound 1348 is thermally unstable; upon thermolysis, it is converted to a mixture of heterocyclic germylene 1355 with C2NP2Ge ring and an unsymmetrical dimer 1356 that is diastereomeric. At a higher temperature (130  C), compound 1356 is completely converted to compound 1355 (Eq. 383).411

ð383Þ

The diphosphagermylenes 1358 and 1360 featuring NC2PGe and SC2PGe heterocyclic rings were prepared through the salt elimination reactions of the potassium 1357 and lithium 1359 salts with GeCl2
1,4-dioxane (Eqs. 384 and 385).413,414 The reactions of the in situ generated dilithium salts of bis(amido)disilyl ligands 1361 and 1362 with GeCl2
1,4-dioxane gave heterocyclic germylenes 1363 and 1364 with N2Si2Ge rings (Eq. 386).406

ð384Þ

ð385Þ

ð386Þ

Stabilization of germylenes 1365–1366, 1367–1368, and 1369 using O,N-heterocyclic, C,N-chelating, and O,S-chelating ligands, respectively, have also been reported (Eqs. 387–390).406,415–417

Organometallic Compounds of Germanium

279

ð387Þ

ð388Þ

ð389Þ

ð390Þ

Cyclic(alkyl)(amino)germylenes 1372 and 1373 were isolated by the reduction of cyclic dichlorogermanes 1370 and 1371 with three equiv. of KC8 (Eq. 391, Table 45).418

ð391Þ

Table 45

Experimental data for 1372–1373 in Eq. (391).

Compound

S

T

t

1372 1373

Benzene THF

60  C −78  C to 30  C

6h 12 h

Germylene 1373 was reacted with different hydridosilanes in the presence of borane B(C6F5)3 as a catalyst to afford cyclic(alkyl) (amino)germylenes 1374a–1374g through cross-metathesis reactions that involve CdSi and SidH bonds (Scheme 111).419

280

Organometallic Compounds of Germanium

Scheme 111

Upon reaction of compound 1373 with dibromomesitylborane, a six-membered dibromogermane 1375 was formed via ring expansion. Compound 1375 was reacted with two equiv. of KC8 and PMe3/NHC to afford cyclic(alkyl)(boryl)germylenes 1376 and 1377 as their Lewis adducts (Scheme 112, Fig. 34, Table 46).420a Cyclic(alkyl)(germyl)germylene 1378 was obtained upon thermolysis of compound 1376, and a dihydridogermane 1379 formed when compound 1376 was heated in the presence of H2 gas (Eq. 392).420a

ð392Þ

Organometallic Compounds of Germanium

Scheme 112

Fig. 34 Molecular structure of compound 1376.

Table 46

Selected bond lengths and angles of compound 1376.

Atoms

Bond lengths (A˚ )

Atoms

Bond Angles (  )

N1-B1 B1-Ge1 Ge1-P1 C1-Ge1

1.426(2) 2.121(2) 2.4107(6) 2.061(2)

N1-B1-Ge1 B1-Ge1-C1 B1-Ge1-P1 Ge1-B1-C2

119.2(2) 98.92(9) 97.84(6) 115.3(1)

281

282

Organometallic Compounds of Germanium

Insertion of cyclic(alkyl)(boryl)germylene 1376 into a BdCl bond of organodichloroborane afforded insertion product 1380 featuring GedCl and BdCl bonds, which upon reduction using three equiv. of KC8 resulted in the formation of boragermene 1381 (Eq. 393).420a

ð393Þ

Boragermene 1381 was reacted with MeIMe and hydrogen to afford NHC adduct 1382 and hydrogenated product 1383. Upon exposure to a hydrogen atmosphere at high temperature, compound 1383 yields dihydridogermane 1384 through GedB bond cleavage. Deuterium analogs 1383-D and 1384-D can also be isolated using D2 instead of H2 (Scheme 113).

Scheme 113

Organometallic Compounds of Germanium

283

Treatment of 1,4-dipotassiotetrasilane1385 and its 1,4-digerma analog 1386 with GeBr2
(dioxane) and PEt3 resulted in cyclic germylenes 1387-1388 and 1389-1390 as their phosphine adducts, respectively. Compounds 1387 and 1389 reacted with B(C6F5)3 to afford tricyclic compounds 1393 and 1394; the formation of these compounds occurred via the formation of intermediate germylenes 1391a and 1391b. 1,2-Trimethylsilyl migration in these germylenes leading to intermediate silagermene 1392a and digermene 1392b was followed by final [2 + 2] cycloaddition reactions of these multiply bonded species. Compounds 1387 and 1389 reacted with MeIMe to produce NHC-stabilized germylenes 1395 and 1396 (Scheme 114).421

Scheme 114

Treatment of germylene 1388 with H2O, ethyl bromide, and dimethyl glyoxal resulted in germanol 1397 and bromogermane 1398, and [1 +4] cycloadduct 1400. Insertion of germylene 1388 into the GedCl bond of GeCl21,4-dioxane gave an inseparable mixture of germylated chlorogermylene adduct 1399a and dichlorogermane 1399b. Silyl migration and cycloaddition occurred in the reaction of compound 1388 with benzophenone, yielding [2 +2] cycloadduct 1401 featuring SidO and GedC bonds (Scheme 115).422

284

Organometallic Compounds of Germanium

Scheme 115

Treatment of cyclic germylene 1388 with phenyl acetylene, tolane, trimethylsilylacetylene, and di(trimethylsilyl)acetylene yielded a mixture of spirocycles 1402a (unsymmetrical) and 1402b (symmetrical), [1 +2] cycloadduct 1403, trimethylsilyl analog 1404 of compound 1402b, and a mixture of regio-isomeric [2 +2] cycloadducts 1405a and 1405b, respectively (Scheme 116).422,423

Organometallic Compounds of Germanium

285

Scheme 116

Similarly, treatment of digermylated germylene 1390 with phenylacetylene or tolane afforded unsymmetrical spirocycle 1407 and [1 +2] cycloadduct 1408. Compounds 1403 and 1408 undergo thermal rearrangement to yield unsymmetrical spirocycles 1406 and 1409, respectively (Schemes 116 and 117).423

Scheme 117

286

Organometallic Compounds of Germanium

10.03.9.1.3.3 Six-membered heterocyclic germylenes The organochalcogenolato-ligand-stabilized germylenes 1411a and 1411b containing NC3SGe and NC3SeGe heterocyclic rings were isolated via the transamination reactions of diorganodichalcogenides 1410a and 1410b with Ge{N(SiMe3)2}2, respectively (Eq. 394).423 In a similar fashion, bis(pyridylalkenolato) germylene 1412, featuring a C3NOGe ring has also been prepared (Eq. 395).424

ð394Þ

ð395Þ

The salt elimination reaction of lithium salt 1413 of 2-imino-5,6-methylenedioxylphenyl ligand with GeCl2
1,4-dioxane resulted in monochlorogermylene 1414 (Eq. 396).425

ð396Þ

The salt elimination reaction of the in situ generated potassium salt (of ketoimine 1415) with GeCl2
1,4-dioxane afforded monochlorogermylene 1416, which undergoes nucleophilic substitution with LiN(TMS)2 to afford aminogermylene 1417. Compound 1417 was reacted with Rh and Ir precursors to yield cyclometallated products 1418a and 1418b through the CdH bond activation (Eq. 397).426

Organometallic Compounds of Germanium

287

ð397Þ

The reaction of the N,P-heterocyclic germylene 1355 with B(C6F5)3 resulted in germylene/borane Lewis adduct 1419 (Fig. 35, Table 47), which reacted further with a range of silanes, such as Et3SH, Ph2HSiH, and Ph3SiH to afford cationic (amino) (phosphino)germylenes 1420, 1421, and 1422, respectively.427 By contrast, in the activation of PhH2SiH using compound 1419, product 1423 (analogous to compounds 1420–1422) was observed. However, compound 1423 reacted further with another molecule of PhH2SiH, in which the germylene center activated the SidH bond to afford germanium(IV) compound 1424 (Eq. 398).427 The reaction of germylene 1420 with trifluoroacetophenone and CO2 resulted in the regeneration of Lewis adduct 1419, along with the corresponding hydrosilylated products (Eq. 399). Germylene 1421 undergoes [1 +4] cycloaddition with 2,3-dimethylbutadiene to result in the corresponding cycloadduct 1425 (Eq. 400).427

ð398Þ

288

Organometallic Compounds of Germanium

ð399Þ

ð400Þ

Fig. 35 Molecular structure of compound 1419.

Table 47

Selected bond lengths and angles of compound 1419.

Atoms

Bond lengths (A˚ )

Atoms

Bond Angles (  )

N1-Ge1 Ge1-P1 P1-P2 Ge1-B1 B1-C36

1.863(2) 2.1936(8) 2.104(1) 2.223(3) 1.61(1)

N1-Ge1-P1 P2-P1-Ge1 P1-Ge1-B1 N1-Ge1-B1 G1-B1-C36

115.15(7) 102.43(4) 111.04(7) 133.59(9) 109.4(5)

Organometallic Compounds of Germanium

10.03.9.1.4

289

Germylenes with donor arm(s) and their metal complexes

A number of germylene systems with side donor arm(s) were reported; they can act as chelate/pincer ligands for the isolation of transition metal complexes.428–430 Treatment of b-sulfinyl germylene 1121 with tungsten and molybdenum carbonyl precursors yielded complexes 1426 and1427, respectively, in which the metal atoms are chelated by germanium and sulfur donor atoms (Eq. 401).339 Germylene 1121 reacted with Ru(PPh3)3Cl2 to afford Ru(II) complex 1428, which, when crystallized, gave diruthenium complex 1429 with two bridging chlorine atoms (Eq. 402, Fig. 36, Table 48).339 Treatment of germylene 1121 with Ni(COD)2 afforded nickel(0) complex 1430, which underwent COD exchange reaction with CO to afford tricarbonylnickel(0) complex 1431 (Eq. 403).339

ð401Þ

ð402Þ

ð403Þ

290

Organometallic Compounds of Germanium

Fig. 36 Molecular structure of compound 1429.

Table 48

Selected bond lengths and bond angles of compound 1429.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge-Ru1 Ru1-Cl2 Ru2-Cl1 Ru1-Cl3 Ru2-O

2.3671(6) 2.456(1) 2.471(1) 2.402(1) 2.196(2)

Ge1-Ru1-Cl3 Ge1-Ru1-Cl2 S-O-Ru2 O-Ru2-Cl3 Cl1-Ru1-Cl2

96.74(3) 92.40(3) 121.8(1) 84.35(6) 78.93(3)

Treatment of in situ generated lithium salt of diimino ligand 1432 with GeCl2
1,4-dioxane resulted in a chelating monochlorogermylene 1433 (Eq. 404).425 The phosphine functionalized germylene 1114 reacted with M(II) and M(0) metal precursors to yield chelate complexes 1434–1436 stabilized by a bidentate germylene ligand featuring a phosphorus donor atom; complexes 1436a and 1436b contain tricoordinate Pd(0) and Pt(0) atoms (Scheme 118).332

ð404Þ

Organometallic Compounds of Germanium

291

Scheme 118

Similarly, germylene 1115 reacted with Ni(COD)2 to afford tetracoordinate nickel complex 1437, which was reacted with Ad-C^P and nitrosobenzene derivative to afford nickel complexes 1438 and 1439 with 1,3-diphosphacyclobutadiene nitrosoarene ligands, respectively (Scheme 119).331

Scheme 119

292

Organometallic Compounds of Germanium

A germylene 1441 featuring two phosphine donor arms was isolated by reacting the in situ generated lithium salt of diamine 1440 with GeCl2
1,4-dioxane; compound 1441 can act as a GeP chelating or PGeP pincer ligand (Eq. 405).431

ð405Þ

Compound 1441 was exploited to form Ge,P-chelating ruthenium, rhodium, and iridium complexes 1442, 1443, and 1444; in complexes 1443 and 1444, the insertion of the germylene into the RhdCl and IrdCl bonds occurred, respectively.431,432 The reactions of metal carbonyls Mn2(CO)10 and Co2(CO)8 with compound 1441 afforded Ge,P-chelating complexes 1445 and 1446, with bridging germanium centers (Scheme 120).431,432

Scheme 120

Ligand exchange reactions of complexes 1443 and 1444 with CO converted them into P,Ge,P-tripodal pincer metal complexes 1447 and 1449, respectively; dicarbonyl complex 1447 transformed to monocarbonyl derivative 1448 during the vacuum workup (Eqs. 406 and 407).431,432

Organometallic Compounds of Germanium

293

ð406Þ

ð407Þ

Treatment of compound 1441 with Ni(II), Pd(II), and Pt(II) metal precursors yielded P,Ge,P-tripodal pincer complexes 1450, 1451, and 1452, respectively; nickel complex 1450 was not isolated as it was converted to oxygen-bridged dimeric complex 1453 due to hydrolysis by adventitious water. Palladium complex 1451 was also hydrolyzed to the corresponding hydroxy derivative 1454 featuring a GedOH bond; however, platinum complex 1452 was inert to hydrolysis (Scheme 121).433

Scheme 121

The methoxy derivatives 1455 and 1456 featuring GedOMe bonds were obtained by reacting compounds 1451 and 1452 with BuLi and methanol. Similarly, methyl derivative 1457a of complex 1451 can be isolated using methyl lithium (Fig. 37, Table 49). The 1:1 reactions of complexes 1451 and 1452 with PhLi and RLi afforded a mixture of products (R ¼ Me, Ph); however, the use of two equiv. of the alkyl/aryl lithium afforded clean products 1459-1460 through the intermediacy of compounds 1457b and 1458 (Scheme 122).433 n

294

Organometallic Compounds of Germanium

Fig. 37 Molecular structure of compound 1451.

Table 49

Scheme 122

Selected bond lengths and bond angles of compound 1451.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-N1 Ge1-N2 Ge1-Cl1 Ge1-Pd1

1.843(2) 1.834(2) 2.160(9) 2.278(4)

N2-Ge1-N1 N2-Ge1-Pd1 N1-Ge1-Pd1 N1-Ge1-Cl1

92.46(1) 113.09(7) 111.75(7) 112.18(7)

Organometallic Compounds of Germanium

295

The reaction of the in situ generated lithium salt of phosphanylmethylpyrrole 1461 with GeCl2
1,4-dioxane resulted in monochlorogermylene 1462, which upon treatment with the lithium salt of compound 1461, afforded germylene 1463 which can act as a PGeP pincer ligand (Eq. 408).434 Germylene 1463 reacted with group 10 metal dichloride precursors to afford P,Ge, P-pincer complexes 1464–1466, in which insertion of the germylene into the MdCl bond has occurred.434 Similarly, compound 1463 reacted with gold(I) chloride to afford P,Ge,P-pincer complex 1467, which upon hydrolysis, resulted in gold(I) complex 1468 (Scheme 123).434

ð408Þ

Scheme 123

Starting from diphosphine-functionalized diamine 1469, P,Ge,P-pincer germylene 1470 was isolated (Eq. 409).435

ð409Þ

The reactions of germylene 1470 with group 11 metal precursors yielded P,Ge,P-pincer complexes 1471–1473 (Eq. 410); the tetrahedral copper complex 1471 has a CudPPh3 bond, while the T-shaped silver and gold complexes have no MdPPh3 bond (M ¼ Ag 1472, Au 1473).435

296

Organometallic Compounds of Germanium

ð410Þ

Compound 1470 reacted with Pd(PPh3)4 to afford T-shaped palladium(0) complex 1474 with the germylene acting as a Z-type ligand. Oxidation of complex 1474 using AuCl(tht), Ph2S2, or HCl converted it to palladium(II) complexes 1475, 1476, and 1477, respectively. Complex 1475 was synthesized from compounds 1470 and 1477 by treating them with PdCl2(MeCN)2 and HCl, respectively (Scheme 124).436

Scheme 124

Organometallic Compounds of Germanium

10.03.9.1.5

297

Pincer ligand stabilized germylenes and their metal complexes

The reaction of the in situ generated potassium salt of diphosphine functionalized diamines 1478 with GeCl2
1,4-dioxane afforded P,Ge,P-pincer germylene 1479 (Eq. 411).437 Germylene 1479 reacted with group 11 metal(I) halides to afford dimeric complexes 1480–1481; the complexes 1480a–1480b and 1481 have Ge2Cl2M2 and GeAu2 six-membered and triangular cores, respectively. The addition of two equiv. of CuCl to compound 1479 resulted in a copper complex 1482, featuring two GeCu2 triangles (Scheme 125).437

ð411Þ

Scheme 125

298

Organometallic Compounds of Germanium

Bis(sulfonyl) aryl O,C,O-pincer ligand 1483 provided the necessary stabilization for the isolation of monochlorogermylene 1484 (Eq. 412), which acted as a ligand for the synthesis of tungsten (1485) and iron complexes (1486; Eq. 413).438 Compound 1484 was reacted with o-benzoquinone to afford [4 +1] cycloaddition product 1487 (Eq. 414).439

ð412Þ

ð413Þ

ð414Þ

Organometallic Compounds of Germanium

299

An aryl N,C,N-pincer ligand 1488 stabilized monochlorogermylene 1489 was isolated; the reaction of compound 1489 with K [B(sec-Bu)3H] afforded hydridogermylene 1490 (Scheme 126).440

Scheme 126

NHC stabilized P,Ge,P-pincer germylene 1492 was isolated from MeIiPrGeCl2 adduct 1491; the reaction of compound 1492 with Ni(COD)2 involved NHC transfer from the germylene to nickel, to yield nickel complex 1493. Complex 1493 is amphiphilic and gave adducts 1495 (through the intermediacy of compound 1494) and 1496 upon reactions with Lewis base MeIMe and Lewis acid (BH3
SMe2), respectively (Scheme 127).441 The dimeric nature of compound 1495 featuring two MeIMe ligands is reflected in its molecular structure (Fig. 38, Table 50).441

Scheme 127

Fig. 38 Molecular structure of compound 1495.

Organometallic Compounds of Germanium

Table 50

10.03.9.1.6

301

Selected bond lengths and bond angles of compound 1495.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-Ge2 Ge1-Ni1 Ge2-Ni2 Ge1-C1

2.616(7) 2.388(6) 2.388(6) 1.997(2)

Ni1-Ge1-Ge2 Ge1-Ge2-Ni2 Ni1-Ge1-C1 Ni1-Ge1-C2

139.83(2) 139.83(2) 106.36(7) 101.76(7)

Bis(germylenes) and their metal complexes

10.03.9.1.6.1 Spacer separated bis(germylenes) The chemistry of bis(germylenes) is also fast-growing, and various reviews have covered recent developments.442 The transamination reactions of the tetra(alkylamino)-benzene 1496 and -diphenyl 1498 with Lappert’s germylene resulted in rigid ditopic non-functionalized bis(germylenes) 1497 and 1499, respectively (Eqs. 415–417).443,444 The lithium salts 1500 and 1502 of Dip substituted bis(amidines) bridged by 1,4-phenylene and 1,4-cyclohexylene linkers undergo salt elimination reactions with GeCl2
1,4-dioxane to afford the corresponding bis(germylenes) 1501 and 1503, respectively, (Eqs. 417 and 418).445

ð415Þ

ð416Þ

ð417Þ

ð418Þ

302

Organometallic Compounds of Germanium

The 2,6-bis(imino)phenyl ligand stabilized bis(germylene) 1506 was synthesized by reacting monochlorogermylene 1505 with lithium metal, in which both salt elimination and C-C coupling reactions occurred; compound 1505 was obtained from the bromo derivative 1504 via a conventional procedure (Scheme 128). The distorted trigonal pyramidal geometries of the germanium atoms in compound 1506 indicate the presence of lone pair of electrons (Fig. 39, Table 51).446

Scheme 128

Fig. 39 Molecular structure of compound 1506.

Table 51

Selected bond lengths and bond angles of compound 1506.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-N1 Ge2-N1 Ge1-N2 Ge2-N2

2.082(2) 2.119(2) 2.126(2) 2.081(2)

N2-Ge2-N1 N2-Ge1-N1 Ge2-N2-Ge1 Ge2-N1-Ge1

69.34 69.17 86.05 86.21

The lithium salt of 2,3-dimethyl-1,4-diaza-1,3-butadiene (1507) was reacted with GeCl2
1,4-dioxane complex to afford a mixture of functionalized and nonfunctionalized bis(germylenes) 1508 and 1509 (Eq. 419).447 A macrocyclic bis(germylene) 1511 with two bridged lutidine groups was prepared by the transamination reaction of tetraamine 1510 with two equiv. of Lappert’s germylene (Eq. 420).448

Organometallic Compounds of Germanium

303

ð419Þ

ð420Þ

A bis(germylene) 1512 with both amidinate and b-diketiminate ligands were synthesized by the 1,4-nucleophilic addition of the GedCl bond of an amidinatomonochlorogermylene to non-functionalized germylene 1222. The reaction of N,Ge,N pincer germylene 1479 with GeCl21,4-dioxane resulted in bis(monochlorogermylene) 1513 (Eq. 422).437 Bis(germylenes) 1515 with ferrocene spacer was isolated through the salt elimination reactions between the dilithium salt 1514 and two equiv. amidinatomonochlorogermylene 1101 (Eq. 423).448

ð421Þ

304

Organometallic Compounds of Germanium

ð422Þ

ð423Þ

10.03.9.1.6.2 Chelating and pincer bis(germylenes) Bis(germylenes) 1516-1517 and 1518 featuring substituted resorcinol and 2,6-diamino-N,N0 -diethylpyridine spacers were isolated through the salt elimination reactions between the corresponding in situ generated dilithium salts with two equiv. of amidinatomonochlorogermylene 1101 (Eqs. 424–426).449–451 The bromo analog of compound 1516 (1517) was obtained by reacting the substituted resorcinol with compound 1101 and LiHMDS.450

ð424Þ

Organometallic Compounds of Germanium

305

ð425Þ

ð426Þ

Bis(germylenes) 1516–1518 acted as pincer ligands (GeCGe and GeNGe pincers) in order to isolate several transition metal complexes.449–451 Compound 1516 was reacted with NiBr2(dme) and {IrCl(coe)2}2 to yield nickel and iridium bis(germylene) pincer complexes 1519 and 1520; compound 1519 can also be obtained by treating Ni(COD)2 with 1517 (Eq. 427).450 Compound 1518 was employed for the isolation of Fe(II) (1521) and Fe(0) (1523) complexes; for this purpose, 1518 was reacted with FeCl2 and Fe(PMe3)4, respectively. Complex 1523 undergoes ligand substitution reactions with CO to yield a mixture of pincer and chelate complexes 1525, 1524a, and 1524b; heating the mixture for 1 day resulted in compound 1525 being identified as the major product (Scheme 129).449

ð427Þ

306

Organometallic Compounds of Germanium

Scheme 129

10.03.9.1.7

Carbene stabilized germylenes

The NHC ligand class has been employed in recent years for the isolation of a wide range of main group metal complexes. In this regard the chemistry of NHC-stabilized germylene is vast and has been reviewed on a number of occasions.452,453 The reaction of NHC-stabilized dichlorogermylene IPrGeCl2 (1526) with NaOSiMe3 resulted in NHC stabilized chlorosiloxygermylene 1527. Treatment of compound 1527 with Me2SBH3 and Li[BH4] gave BH3 adducts of chlorosiloxygermylene 1528 and hydridosiloxygermylene 1529, respectively. Na[BArF4] reacted differently with compound 1527 depending on the solvent used; in fluorobenzene and dichloromethane Ge(II) 1530 and Ge(IV) 1531 cations were obtained, respectively. The reaction of compound 1527 with HBpin afforded a mixture of compound 1526, siloxyborane, and IPrH2 through the intermediacy of hydridochlorogermylene Scheme 130).454

Organometallic Compounds of Germanium

307

Scheme 130

Dichlorogermylenes 1491 and 1526 were reacted with diiron nonacarbonyl to afford iron complexes 1532 and 1533.325 The dimethyl analog 1534 was isolated by the salt elimination reaction of complex 1532 with two equiv. of methyl lithium. Carbene ligand exchange in compound 1534 to obtain compound 1535 was also feasible (Scheme 131).455

308

Organometallic Compounds of Germanium

Scheme 131

The reduction of compound 1532 using KC8 resulted in digermanium(0) iron complex 1537 featuring two Fe(CO)4 moieties instead of the anticipated symmetrical product [1536] (Scheme 132). The molecular structure of compound 1537 showed both bridging and terminal positions for the Fe(CO)4 unit. Furthermore, it appears that between compounds 1537a and 1537b, there is degenerate equilibrium (Scheme 132).456

Scheme 132

Organometallic Compounds of Germanium

309

Treatment of compound 1537 with a molecule of diironnonacarbonyl resulted in germanium(0) iron complex 1538 featuring three Fe(CO)4 moieties. Scavenging a Fe(CO)4 fragment from complex 1538 using an NHC yielded a mixture of germanium(0) diiron 1539 and NHC iron complexes (1540).457 Compound 1538 can also be synthesized directly from compound 1532 and excess diiron nonacarbonyl. Compound 1537 reacted with propylene sulfide to afford digermathiirane 1541 featuring a Ge2S three-membered ring (Scheme 133).456

Scheme 133

Treatment of MeIiPr stabilized tetrachlorogermane 1542 with Li[BH4] resulted in a BH3 adduct of dihydridogermylene 1543 (Eq. 428).458 The reaction of the in situ generated IMe with GeCl21,4-dioxane afforded dichlorogermylene 1544, which undergoes reduction with Li[BH4] to afford dihydridogermylene borane adduct 1545 (Eq. 429). Compound 1545 was used for synthesizing germanium thin films (20–70 nm) on silicon wafers.458

ð428Þ

310

Organometallic Compounds of Germanium

ð429Þ

The reaction of germylene 1491 with boryllithium afforded (boryl)monochlorogermylene 1546 (Eq. 430).459

ð430Þ

The treatment of cyclic(alkyl)(amino)carbene (cAAC) 1547 with GeCl21,4-dioxane in the presence of 3 mol% of LDA afforded cAAC stabilized dichlorogermylene 1548 (Eq. 431).460

ð431Þ

The reactions of chelating bis(NHCs) 1549 and 1551 with GeCl21,4-dioxane resulted in chlorogermyliumylidenes 1550 and 1552, featuring chloride counter ions (Eq. 432, Scheme 134). Treatment of compound 1552 with Ni(COD)2 produced nickel complex 1553 via transmetallation. Alternatively, complex 1553 could also be obtained from bis(NHC)Ni(COD) complex 1554 and GeCl21,4-dioxane (Scheme 134).461

ð432Þ

Organometallic Compounds of Germanium

311

Scheme 134

The insertion of West’s silylene into the GedCl bond of dichlorogermylene 1491 yielded (chlorosilyl)chlorogermylene 1555 (Eq. 433).462

ð433Þ

10.03.9.1.8

Air and water stable germylenes

The synthesis of air and water-stable germylenes using bulky dipyrromethene ligands has been reported. The reaction of the in situ generated lithium salts of dipyrromethenes 1556 and 1557 resulted in monochlorogermylenes 1558 and 1559; compound 1558 showed halogen exchange reaction with CsF to provide monofluorogermylene 1560 (Eq. 434).463,464

312

Organometallic Compounds of Germanium

ð434Þ

Treatment of compound 1559 with water and excess cesium carbonate yielded hydroxygermylene 1561, which showed antiproliferative effects against some human cancer cell lines. The reactions of compound 1559 with different alcohols afforded alkoxygermylenes 1562–1564, which can be converted back to hydroxygermylene 1561 using water and excess cesium carbonate. Additionally, compound 1559 reacted with MeOTf to afford cationic germanium(IV) compound 1565 (Scheme 135).463

Scheme 135

10.03.9.1.9

Germylene cations

Germylenes bearing uni- and di-positive charges are commonly termed germylene mono- and dications, respectively; other names used for the germylene monocations are germylene cations, germanium(II) cations, and germyliumylidenes. There are also bis(germylene cations) that are derived from bis(germylenes). The diimino, aminotroponiminato, b-diketiminato, and NHC ligands are often used to stabilize such compounds. Several reviews covered the developments concerning cationic germanium(II) species;465,466 the recent aspects are summarized in this chapter.

10.03.9.1.9.1 Germylene monocations 10.03.9.1.9.1.1 Synthesis of germylene monocation by halide abstraction from monohalogermylenes The abstraction of chloride from aminotroponiminatomonochlorogermylenes 1136 and 1135 using AgOTf or GaCl3 afforded germanium(II) cations 1566, 1567 and 1568. Compounds 1566 and 1567 reacted with lithium iodide to afford monoiodogermylenes 1569 and 1570, respectively (Scheme 136).467

Organometallic Compounds of Germanium

313

Scheme 136

Treatment of resonance stabilized b-diketiminatomonochlorogermylene 1206 with Li[Al(ORF)4] gave germanium(II) cation 1571 partnered with an [Al(ORF)4]− counter ion. Compound 1571 reacted with tBuNH2 to yield an adduct 1572 (Scheme 137).468

Scheme 137

Dechlorination of monochlorogermylene 1019 using Ag[Al{OC(CF3)3}4]/Li[Al{OC(CF3)3}4] yielded amidogermylene cation 1573 stabilized by intramolecular
2-arene interaction. Compound 1573 reacted with DMAP to afford donor-stabilized germylene cation 1574 (Eq. 435).469

314

Organometallic Compounds of Germanium

ð435Þ

Treatment of NHC stabilized chlorogermylene 1526 with Na[BArF4] resulted in the formation of germylene cation 1575, which reacted with CH2Cl2 and PhCCH to afford oxidative addition 1576 and [1 +2] cycloaddition 1577 products, respectively. Moreover, compound 1575 reacted with trimethylsilylazide and trimethylsilyldiazomethane to yield cationic germanium(IV) imido 1578a and alkylidene 1578b derivatives (Scheme 138).470

Scheme 138

Treatment of the amino(imino)germylene 1091 with BF3
Et2O gave germylene-germyliumylidene 1580 partnered with a [BF4]− counterion, through the intermediacy of dimeric fluorogermylene 1579 (Eq. 436);471 the anion of compound 1580 can be exchanged to give compound 1584. The fluoride scavenging reaction of compound 1580 using two equiv. of TMSOTf resulted in germylene-germyliumylidene 1581 featuring a triflato group bridging the two germanium centers, and a triflato counterion. Anion exchange reactions of compound 1581 with Na[BArF4] and Ag[Al(ORF)4] yielded derivatives 1582 and 1583 featuring [BArF4]− and [Al(ORF)4]− counterions, respectively (Scheme 139, Table 52, Fig. 40).471

Organometallic Compounds of Germanium

315

ð436Þ

Scheme 139

Table 52

Selected bond lengths and angles of compound 1582.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-N1 Ge1-N2 Ge2-N1 Ge2-N2

1.949(2) 1.959(2) 1.956(3) 1.959(3)

Ge1-N1-Ge2 Ge1-N2-Ge2 N2-Ge1-O1 N1-Ge1-O1

101.32(1) 100.81(1) 88.59(9) 88.24(9)

316

Organometallic Compounds of Germanium

Fig. 40 Molecular structure of compound 1582 ([BArF4]− counterion omitted for clarity).

10.03.9.1.9.1.2 Synthesis of germylene monocations by chloride abstraction from dichlorogermylenes One-pot reactions of diimino ligands 1585a and 1585b (containing donor-arms) with GeCl21,4-dioxane and TMSOTf yielded chlorogermyliumylidenes 1586a and 1586b, featuring [OTf]− counter ions, respectively(Eq. 437).472

ð437Þ

The reaction of 2,20 -bipyridine (bipy) 1587 with GeCl21,4-dioxane gave dichlorogermylene 1588, which upon treatment with TMSOTf yielded germylene monocation 1589. Alternatively, the reaction of compound 1587 with GeCl21,4-dioxane and TMSOTf directly affords compound 1589 (Eq. 438).473

ð438Þ

Organometallic Compounds of Germanium

317

The salt elimination reactions of the thallium bis(phosphino)borate salt 1590 or potassium bis(NHC)borate 1594 with GeCl21,4-dioxane resulted in the formation of chlorogermyliumylidene borates 1591 and 1595, respectively (Scheme 140 and Eq. 439).474,475 Compound 1590 reacted with half equivalent of GeCl2
1,4-dioxane to yield chlorogermyliumylidene borate 1592 featuring a Ge-CH2PPh2 moiety; the germylene center undergoes insertion into the aliphatic BdC bond (Scheme 140). The free phosphine arm of compound 1592 forms an adduct with BH3 to afford compound 1593 (Scheme 140).475 The salt metathesis reaction of compound 1595 with NaPCO resulted in phosphaketenyl germyliumylidene 1596 (Eq. 439).474

Scheme 140

ð439Þ

Exposure of compound 1596 to UV radiation afforded dicationic Ge(I) dimer 1597 as the major product through the reductive elimination reaction; fractional crystallization afforded other products, including bis(germyliumylidenyl)diphosphene 1598 and digermyliumdiphosphacyclobutadiene 1599 (Scheme 141).474 The molecular structure of compound 1597 shows the trans-conformation of the two bis(NHC)borate ligands that stabilizes the [Ge(I)–Ge(I)]2+ fragment (Fig. 41, Table 53). Treatment of compound 1600 with one equiv. each of GeCl21,4-dioxane and BCl3/BF3
OEt2 afforded germylene cations 1601/1602 featuring [BCl4]−/[BClF3]− counterions (Eq. 440).476

318

Organometallic Compounds of Germanium

Scheme 141

Fig. 41 Molecular structure of compound 1597.

Table 53

Selected bond lengths and angles of compound 1597.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-Ge2 Ge1-C1 Ge1-C2 Ge2-C1 Ge2-C2 Ge1-B1

2.673(5) 2.062(4) 2.053(3) 2.062(4) 2.053(3) 3.653(4)

C1-Ge1-C2 C1-Ge1-Ge2 C2-Ge1-Ge2 C3-Ge2-C4 C3-Ge2-Ge1 C4-Ge2-Ge1

89.7(1) 96.73(9) 93.26(7) 89.7(1) 96.73(9) 93.26(7)

Organometallic Compounds of Germanium

319

ð440Þ

10.03.9.1.9.1.3 Synthesis of germylene monocation from autoionization of GeCl21,4-dioxane The N0 ,N,N-pincer ligands, bis(imino)pyridine 1603a–1603c or 2,6-bis(benzimidazol-2-yl)pyridine 1605a–1605c bring about auto-ionization reactions with two equiv. of GeCl21,4-dioxane to yield germyliumylidenes 1604a–1604c and 1606a–1606c, together with the [GeCl3]− anion (Eqs. 441 and 442) 476–478

ð441Þ

ð442Þ

10.03.9.1.9.2 Germylene dications Isocyanide stabilized germanium(II) dication 1608 was formed in an autoionization reaction between four equiv. of 2,6-dimethylphenylisocyanide 1607 and three equiv. of GeCl21,4-dioxane (Eq. 443).479

320

Organometallic Compounds of Germanium

ð443Þ

The reaction of compound 1587 with half and one equiv. of GeCl21,4-dioxane and TMSOTf led to the isolation of germylene dication 1609. Treatment of compound 1588 with one and two equiv. of bipy and TMSOTf, respectively, also afforded the dication 1609 (Eq. 444).473

ð444Þ

Treatment of GeCl21,4-dioxane with a diimine ligand 1610 featuring pyridine donor arms and two equiv. of TMSOTf led to the synthesis of germylene dication 1611, which due to its nucleophilic character reacted with silver and gold reagents to form complexes 1612 and 1613 with formal +4 and +5 charges, respectively (Scheme 142).480

Organometallic Compounds of Germanium

321

Scheme 142

10.03.9.1.9.3 Bis(germylene) cations Treatment of compound 1611 with GeCl21,4-dioxane converted germylene dication to bis(germylene cation) 1614 (Eq. 445).480,481

ð445Þ

Treatment of bis(imino)phenylchlorogermylene with AgOTf in a 2:3 molar ratio afforded bis(germylene) silver(I) complex 1615, which was reacted with two equiv. of MeIMe to give bis(germyliumylidene)silver(I) complex 1616 (Eq. 446).482

322

Organometallic Compounds of Germanium

ð446Þ

10.03.9.1.9.4 Miscellaneous reactions leading to the formation of germanium(II) cations The in situ generated lithium salt of substituted imidazole 1617 reacted with GeCl21,4-dioxane to afford germyliumylidene 1618 featuring two GedCl bonds. Compound 1618 reacted with excess lithium to form digermylenedilithium 1619 which features a GedGe bond (Eq. 447).483a

ð447Þ

Treatment of NHC-stabilized monochlorogermylene 1620 with two equiv. of MeIMe afforded germyliumylidene ion 1621 partnered with a chloride counterion, and which undergoes an anion exchange reaction with Na[BArF4] to yield an analog of compound 1622 featuring a [BArF4]− counterion. Compounds 1621 and 1622 reacted with N2O to give germa-acylium ions 1623 and 1624 (Scheme 143), which can be converted to germa-ester 1625 and heavier germa-acylium ions 1626-1627 upon reactions with Ph3SiOH, Lawesson’s reagent, and Woollins’ reagent (Eq. 448).483b

Organometallic Compounds of Germanium

323

ð448Þ

Scheme 143

10.03.9.1.10

Germylene anions

Treatment of aminogermylene 1114 with HF
pyridine resulted in the formation of anionic trifluoro germanium(II) derivative 1628, which is stabilized via H–F interactions with the amidinium counterion (Eq. 449).337

324

Organometallic Compounds of Germanium

ð449Þ

Treatment of monochlorogermylene 1629 with excess calcium yielded germylidendiide dianion radical 1630, formed through two-electron reduction of a germanium(I) radical intermediate. By contrast, the magnesium analog of compound 1630, being unstable, undergoes dimerization to yield germylidenide anion 1631 (Eq. 450).484

ð450Þ

The bisgermylene 1506 reacts with two equiv. of Li metal to afford lithium germylidenide 1632 via the reduction and homolytic cleavage of the germanium(II) centers and CdC bond, respectively. Direct isolation of compound 1632 from monochlorogermylene 1505 was also achieved using four equiv. of lithium metal (Eq. 451).446

ð451Þ

The reaction of 9-germaanthracene 1633 with two equiv. of KC8 afforded a trimeric germaanthracenyl trianion 1634 which has prominent germylene character. By contrast, the reaction with one equiv. of KC8 gave dimeric dianion 1636 via the intermediacy of radical anion 1635 (Scheme 144).485

Organometallic Compounds of Germanium

325

Scheme 144

10.03.9.1.11

Germylene radicals

The reaction of monochloro germylene 1204 with sodium napthalenide gave neutral germanium-based radical 1637, which converted back to germylene 1204 up on treatment with chlorinating agent C2Cl6.486 Treatment of 1637 with nBu3SnH afforded diamidogermylene 1638 along with other unidentified products (Eq. 452).486

ð452Þ

Compounds 1233 and 1238a showed one-electron oxidation with one molar equivalent of Ag[Al(ORF)4] to afford radical cations 1639 and 1640 (Eq. 453).380,382

ð453Þ

326

Organometallic Compounds of Germanium

10.03.9.2 Reactivity of germylenes 10.03.9.2.1

Oxidation reactions

10.03.9.2.1.1 Oxidation reactions of acyclic germylenes 10.03.9.2.1.1.1 Oxidation reactions with chalcogens The oxidation reactions of germylenes with elemental chalcogens/chalcogen precursors are usually pursued with the aim of isolating germacarbonyl compounds. The bis(Eind) germylene 1059 was reacted with trimethylamine N-oxide to result in kinetically-stabilized tricoordinated germanone 1641 (Eq. 454).315 Germanone 1641 undergoes reduction, methylation, and addition reactions with LiAlH4, MeLi, and H2O to afford hydridogermanol 1642a, methylgermanol 1642b, and germanediol 1642c, respectively. Its reactions with acetone, phenylsilane, and CO2 produced (hydroxyl)germyl enolate 1642d, hydrosilylated product 1642e, and cyclic carbonate 1642f, respectively (Scheme 145).315

ð454Þ

Scheme 145

Diarylgermylenes 1059 and 1643 reacted with elemental sulfur to afford germanethione 1644 and tetrathiagermolane 1645, respectively (Eq. 455).487

ð455Þ

Organometallic Compounds of Germanium

327

10.03.9.2.1.1.2 Oxidative addition and cycloaddition reactions The oxidative addition reactions of bis(ferrocenyl)germylene 1646 with methyl iodide, ferric chloride, and iodine yielded dimethyl1647b, dichloro-1647c, and diiodo-1647d germanes (Scheme 146); The latter compound upon reduction with KC8 regenerated germylene 1646 (Scheme 146).488 (Aryl)silylgermylene 1074 showed oxidative addition reactions with ammonia and hydrogen to afford hydrogermanes 1647e and 1647f. The reactions of (amido)borylgermylene 1078 with SiH4, Me3N
BH3, and [Me3NH]I gave the corresponding oxidative addition products 1647g, 1647h, and 1647i, respectively (Scheme 146).317

Scheme 146

328

Organometallic Compounds of Germanium

10.03.9.2.1.2 Oxidation reactions of cyclic germylenes 10.03.9.2.1.2.1 Oxidation reactions with chalcogens 10.03.9.2.1.2.1.1 Oxidation reactions with oxygen precursors The oxidation reactions of germylenes with elemental chalcogens/chalcogen precursors lead to germanium group 16 element bonded compounds.295,299,300 Bis(amido)silane ligand stabilized germylene 1325 reacted with half equiv. of oxygen to afford germanium m–oxo dimer 1648 (Eq. 456).405 Cyclic(Alkyl)(amino) germylene 1372 reacted with N2O, TEMPO, and S8 to yield germanium m-oxo dimer 1649, TEMPO adduct 1650, and a mixture of diastereomeric germanium m-sulfido dimers 1651a–1651b, respectively (Scheme 147).418

ð456Þ

Scheme 147

The aminotroponiminatogermylenes 1158, 1135, 1138, 1137, 1142, and 1143 were reacted with N2O to yield germanium m-oxo dimers 1652–1657 (Eq. 457).348,352 Treatment of 1652–1655 with Lewis acids afforded germanones 1658–1661, germaacid chloride 1662, germaester 1663, and germaacyl pyrrole 1664 (Scheme 148).348,352 The molecular structure of compound 1659 shows distorted tetrahedral geometry around the germanium center (Fig. 42, Table 54, Scheme 148).

Organometallic Compounds of Germanium

329

ð457Þ

Fig. 42 Molecular structure of compound 1659.

Table 54

Selected bond lengths and angles of compound 1659.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-N1 Ge1-N2 Ge1-O1 O1-B1

2.082(2) 2.119(2) 2.126(2) 2.081(2)

N2-Ge1-N1 N1-Ge1-O1 N2-Ge1-O1 Ge1-O1-B1

69.34 69.17 86.05 86.21

330

Organometallic Compounds of Germanium

Scheme 148

Compound 1662 showed nucleophilic substitution reactions with various lithium salts to yield germaesters 1163, 1665, germanones 1666–1667, germaynone 1668, and N-germacyl pyrrole 1664; by contrast, reaction with LiN(TMS)2 resulted in germaimine 1669 (Scheme 149).348 Germaesters 1663 and 1665 can regenerate germaacid chloride 1662 upon treatment with TMSCl (Scheme 149). The reaction of germaacyl pyrrole 1664 with thiophenol afforded germaester 1670 featuring a Ge-SPh moiety (Eq. 458).348

ð458Þ

Organometallic Compounds of Germanium

331

Scheme 149

The NHC and DMAP adducts of b-diketiminatogermylene 1222 (1671, 1672, and 1673) were reacted with N2O to afford donor-stabilized germanones 1674, 1675, and 1676, respectively (Eq. 459, Table 55).489,490 Compound 1676 reacted with AlMe3 to afford compound 1677 featuring a Ge-O-Al moiety via the insertion of Ge-O unit into the AldCH3 bond, followed by migration of DMAP from Ge to Al (Eq. 460).490

ð459Þ

332

Organometallic Compounds of Germanium

ð460Þ

Table 55

Experimental data for 1671–1676 prepared in Eq. (459).

Compound

S

T1

t1

T2

t2

1671 1672 1673 1674 1675 1676

Toluene Toluene n-Hexane Toluene Toluene Toluene

−30  C −30  C rt – – –

0.5 h 0.5 h 2h – – –

– – – rt rt −78  C-rt

– – – 12 h 12 h 48 h

10.03.9.2.1.2.1.2 Oxidation reactions with elemental sulfur and selenium The guanidinato-1678 and iminophosphonamidogermylenes 1332 and 1330 undergo oxidation with elemental sulfur and selenium to afford germachalcogenides 1679–1680 (Eq. 461)491 and 1681–1684 (Eq. 462)407 featuring Ge]E bonds (E ¼ S, Se). The bis(amido)silyl ligand stabilized germylene 1363 was reacted with elemental sulfur to afford m-sulfido dimer 1685 (Eq. 463).406 O,N-heterocyclic germylene 1365 reacted with dithiodiiron hexacarbonyl to produce inorganic germanium(IV) tetrathiolate 1687 and spirocyclic germanium(IV) derivative 1688 through the intermediacy of compound 1686; compound 1688 was isolated as its pyridine adduct 1689 (Scheme 150).492

ð461Þ

ð462Þ

Organometallic Compounds of Germanium

333

ð463Þ

Scheme 150

The aminotroponiminatogermylenes 1135, 1149, 1137, 1139, 1138, 1153, 1141, 1152, 1150, and 1147 were reacted with elemental sulfur and selenium offering germacarbonyl compounds 1690–1703 (Eq. 464, Tables 56 and 57) and 1704–1709 (Eq. 465, Table 58) featuring Ge]E bonds (E ¼ S, Se).345,346,349,493 The geometry around the germanium atom of compound 1692 is distorted tetrahedral (Fig. 43, Table 57).

334

Organometallic Compounds of Germanium

ð464Þ

ð465Þ

Table 56

Experimental data for 1690–1703 prepared in Eq. (464).

Compound

S (Solvent)

T

t (h)

% Yield

1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703

THF Toluene THF THF THF THF THF THF THF THF THF THF THF THF

rt 50  C rt rt rt rt rt rt rt rt rt rt rt rt

12 48 2 6 4 6 1 3 2 5 3 12 12 36

92 54 98 97 93 86 97 97 97 97 98 83 90 83

Table 57

Selected bond lengths and angles of compound 1706.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-N1 Ge1-N2 Ge1-O1 Ge1-S1

2.082(2) 2.119(2) 2.126(2) 2.081(2)

N2-Ge1-N1 N1-Ge1-S1 N2-Ge1-O1 S1-Ge1-O1

69.34 69.17 86.05 86.21

Organometallic Compounds of Germanium

Table 58

335

Experimental data for 1704–1709 prepared in Eq. (465).

Compound

(S) Solvent

T

t (h)

% Yield

1704 1705 1706 1707 1708 1709

THF THF THF THF THF THF

0  C-rt 40  C rt rt rt rt

4 24 2 6 12 12

86 68 98 98 98 99

Fig. 43 Molecular structure of compound 1706.

The substitution of pyrrole moiety of N-germachalcogenoacylpyrroles 1694-1695 with EPh group resulted in germachalcogenoesters 1710–1713 (E ¼ S, Se) (Eq. 466).346

ð466Þ

Germa seleno ester 1699 displays chalcogen substitution reactions with elemental sulfur and NMMO to afford germathio ester 1698 and germanium m-oxo dimer 1654 (Eq. 467).351 Treatment of compound 1698 with NMMO yielded compound 1654. The OSiMe3 moiety of germaesters 1696 and 1697 can be exchanged with an OMe group to yield germaesters 1714 and 1715 through reactions with excess methanol (Eq. 468).351 Similarly, nucleophilic substitution reactions of heavy carbonyl compounds 1694–1695 and 1702–1703 with Me3SiBr yielded germachalcogeno-acid bromides 1700 and 1701 (Eq. 469).349

ð467Þ

336

Organometallic Compounds of Germanium

ð468Þ

ð469Þ

Digermylene oxide 1160 was reacted with elemental sulfur and selenium to afford germa-thioacid 1716 and germa-selenoacid anhydrides 1717, respectively (Eq. 470).353 Silathiogermylene 1140 was reacted with elemental sulfur and selenium to afford trichalcogenogerma-acid anhydrides 1718 and 1719.350 Compound 1140 was reacted with NMMO to yield germa-thioester 1696 through 1,3-silyl migration in a germa-ester intermediate featuring a GeSSiMe3 moiety (Scheme 151).350

ð470Þ

Scheme 151

Organometallic Compounds of Germanium

337

The b-diketiminatogermylene amide 1221 and hydride 1249 were reacted with elemental sulfur to yield germathioamide 1720 and germadithiocarboxylic acid 1721, respectively (Eq. 471).378,386

ð471Þ

10.03.9.2.1.2.1.3 Oxidation reactions with elemental tellurium The oxidation of germylene centers using tellurium was also studied. Treatment of aminotroponiminatogermylenes 1149, 1141, and digermylene oxide 1159 with elemental tellurium afforded germatelluroester 1722, germatelluroamide 1723, and germatelluroacid anhydride 1724, respectively (Eq. 472).494

ð472Þ

The cyclopentadienyl and alkynyl functionalized b-diketiminato germylenes 1219, 1210, and 1211 yielded germanetellurones 1725–1727 upon treatment with elemental tellurium.374 Compound 1725 additionally undergoes reaction with Lewis acidic GeCl2 to afford dichlorogermylene germanetellurone Lewis acid/base adduct 1728 (Scheme 152).374

Scheme 152

338

Organometallic Compounds of Germanium

10.03.9.2.1.2.2 Cycloaddition reactions Amidinatogermylenes 1106, 1729, 1106a, and 1730 and N,P-heterocyclic germylene 1355 showed cycloaddition reactions with substituted o-quinone to yield [1 +4] cycloadducts 1731–1734,326 and 1735,411 respectively; compound 1735 was obtained as a diastereomeric mixture (Eqs. 473 and 474). [4 +1] Cycloadduct 1736 was also isolated by treating germylenes 1355 with 2,3-dimethylbutadiene (Eq. 474).411

ð473Þ

ð474Þ

10.03.9.2.2

Reduction reactions

10.03.9.2.2.1 Reduction of acyclic germylenes and reduced products’ reactivity Reduction of amidogermylene 1019 with half an equiv. of [(MesNacNac)Mg]2 afforded digermyne 1737 featuring a Ge(I)dGe(I) bond (Fig. 44, Table 59). Compound 1737 was reacted with hydrogen to afford (dihydridogermyl)germylene 1738, which was also obtained through the treatment of amidogermylene 1019 with Li[HBBus3]/K[HBEt3]. The reactions of compound 1737 with bis(trimethylsilyl)butadiyne, COT, and azobenzene led to addition product 1739, inverse sandwich cyclooctateraenediyl complex

Fig. 44 Molecular structure of compound 1737.

Organometallic Compounds of Germanium

Table 59

339

Selected bond lengths and angles of compound 1737.

Atoms

Bond lengths ( Ǻ )

Atoms

Bond angles (  )

Ge(1)-N(1) Ge(1)-Ge(1) Si(1)-N(1) N(1)-C(1)

1.872(2) 2.7093(7) 1.762(2) 1.445(3)

N(1)-Ge(1)-Ge(1) C(1)-N(1)-Si(1) C(1)-N(1)-Ge(1) Si(1)-N(1)-Ge(1)

100.09(6) 118.50(16) 110.12(15) 131.38(11)

1740, and hydrazine spacer-separated bis(germylene) 1741, respectively, through the reductive insertion of unsaturated substrates into the Ge(I)dGe(I) bond. Treatment of compound 1737 with DMAP resulted in an adduct 1742 (Scheme 153).495,496

Scheme 153

Treatment of amidogermylene 1024 with an half equiv. of [(MesNacNac)Mg]2 yielded digermyne 1743, which reacted with hydrogen to form dihydridodigermene 1744. Treatment of compound 1743 with ethylene and COD afforded cycloadducts 1746 and 1748 through [2 +2+ 2] and [2 +2] cycloaddition reactions, respectively. However, in the reactions with propyne, norbornadiene, and 1,3-cyclohexadiene, organic spacer separated bis(germylenes) 1745, 1747, and 1749 were obtained as insertion products. Thermolysis of compound 1749 led to CdH bond activation and furnished compound 1744 and benzene (Scheme 154).496,497

340

Organometallic Compounds of Germanium

Scheme 154

Monochlorogermylenes 1750 and 1751 stabilized by 2,6-diiminophenyl and 2-imino-5,6-methylenedioxyphenyl ligands upon reduction with KC8 yielded Ge(I) dimers 1752 and 1753. The germylidenides 1754 and 1755 were obtained by KC8 mediated reduction of compounds 1752 and 1753; alternatively, these compounds can also be isolated by reducing compounds 1750 and 1751. Treatments of compound 1753 with 1.5 equiv. of metal precursors ([Rh(COD)Cl]2 and [Ir(COD)Cl]2) gave a mixture of metallogermylene-chlorometal(I) (1756 and 1757) and chlorogermylene-chlorometal(I) complexes (1758 and 1759). In the reaction with Mn2(CO)10, dimeric manganogermylene complex 1760 was isolated along with 2-imino-5,6-methylenedioxyphenyl ligand (Scheme 155).425,498

Organometallic Compounds of Germanium

341

Scheme 155

Silylene stabilized dichlorogermylene 1762 and (amido)monochlorogermylene 1094 undergo reduction reactions with two and 1.5 equiv. of KC8 and potassium to afford digermanium(0) derivative 1763 and spiropentagermadiene 1764 with one and two Ge]Ge bonds, respectively (Eqs. 475 and 476).322,499

ð475Þ

342

Organometallic Compounds of Germanium

ð476Þ

Reduction of NHC-stabilized (boryl)monochlorogermylene 1546 using [(MesNacNac)Mg]2 and excess of potassium/KC8 yielded unsymmetrical digermyne 1765 and dianionic digermanium(0) derivative 1766, respectively. Treatment of compound 1766 with two equiv. of an oxidant, such as a ferrocenium or trityl salt, afforded digermavinylidine 1767 through 1,2-boryl migration, which can be converted back to compound 1766 using excess KC8 (Fig. 45, Table 60). Furthermore, the addition of MeIiPr to compound 1767 gave compound 1765. Compound 1767 reacted with hydrogen at four atm pressure offering tetrahydridodigermane 1768 through 1,2-boryl migration (Scheme 156).459

Fig. 45 Molecular structure of compound 1767.

Table 60

Selected bond lengths and angles of compound 1767.

Atoms

Bond lengths ( Ǻ )

Atoms

Bond Angles (  )

Ge(1)–Ge(2) Ge(1)–B(1) Ge(1)–B(2) Ge(2)–C(3) Ge(2)–C(29)

2.312(1) 2.033(2) 2.033(2) 2.904(2) 2.917(2)

B(1)–Ge(1)–Ge(2) B(1)–Ge(1)–B(2) B(2)–Ge(1)–Ge(2) Ge(1)–B(1)–N(1) Ge(1)–B(1)–N(2)

108.5(1) 142.6(1) 108.9(1) 116.3(1) 139.9(1)

Organometallic Compounds of Germanium

343

Scheme 156

10.03.9.2.2.2 Reduction of four-membered germylenes and reactivity of the reduced products Two equiv. of amidinatogermylenes 1101 and 1102 with GedCl bond undergoes reduction with three equiv. of potassium metal and one equiv. of [(MesNacNac)Mg]2 to yield germanium(I) dimers 1769 and 1770, respectively (Eq. 477, Table 61).328,329

344

Organometallic Compounds of Germanium

ð477Þ

Table 61

Experimental data for 1769–1770 prepared in Eq. (477).

Compound

I

S

t (h)

Yield (%)

1769 1770

3K [(MesNacnac)Mg]2

THF Toluene

48 1

35 71

Compound 1769 reacted with Fe2(CO)9 and azobenzene to afford Ge(I)-dimer-stabilized diiron complex 1771 and GedGe bond cleaved oxidative addition product 1772. In the reaction of compound 1769 with Cp Fe(Z5-P5), bis(germylene) stabilized iron polyphosphide 1773 was obtained via reductive GedGe bond cleavage (Scheme 157).329,500

Scheme 157

Treatment of three equiv. of amidinate ligand stabilized Si(I) dimer 1774 with three and one equiv. of Ge(I) dimer 1769 yielded germatrisilacyclobutadiene ylides 1775 and 1777, respectively, along with germylene 1776 (Scheme 158).329,501

Organometallic Compounds of Germanium

345

Scheme 158

10.03.9.2.2.3 Reduction of five-membered germylenes and reactivity of the reduced products Reduction of the C, N-chelating ligand stabilized germylenes 1366 and 1367 with KC8 and potassium metal afforded digermynes 1778 and 1779 featuring three-coordinate germanium atoms, respectively (Eqs. 478 and 479).501

ð478Þ

ð479Þ

346

Organometallic Compounds of Germanium

The reduction of O,N-heterocyclic germylene 1365 with half an equiv. of potassium metal gave conjugated Ge4 chain containing dianionic low-valent germanium compound 1780 (Scheme 159).416

Scheme 159

10.03.9.2.2.4 Reduction of six-membered germylenes and reactivity of the reduced products The reduction of pyridyl-1-azaallyl monochlorogermylene 1781 with lithium metal/KC8 afforded Ge(I) dimer 1782, which upon treatment with excess elemental sulfur afforded trithiogerma-carboxylic acid anhydride 1783 via the insertion of sulfur into the Ge(I)–Ge(I) bond and oxidation of germanium centers by sulfur atoms (Scheme 160).502 The reaction of Ge(I) dimer 1782 with azobenzene afforded hydrazine spacer-separated bis(germylene) 1786, an analog of compound 1772. Treatment of compound 1782 with one and two equiv. of Fe2(CO)9 afforded mono-1784 and di-1785 iron complexes; complex 1784 can also be obtained by reacting monochlorogermylene 1781 with Collman’s reagent (Scheme 161).502,503

Scheme 160

Organometallic Compounds of Germanium

347

Scheme 161

Digermavinylidene 1788 stabilized by phosphine coordination was isolated by reducing germylgermylene 1787 using [(MesNacNac)Mg]2/sodium; compound 1787 was obtained by treating germylene-phosphine Lewis pair 1787a with GeCl2
1,4-dioxane. Compound 1788 was reacted with adamantyl-phosphaalkyne and -azide to yield [2 +2] cycloadduct 1790 and tetrameric organogermanium nitride 1789, respectively (Scheme 162, Fig. 46, Table 62).400b

Scheme 162

Fig. 46 Molecular structure of compound 1788.

Organometallic Compounds of Germanium

Table 62

349

Selected bond lengths and angles of compound 1788.

Atoms

Bond lengths ( Ǻ )

Atoms

Bond Angles (  )

Ge1–Ge2 Ge1–C1 Ge1–C2 Ge2–P C3–P

2.30597(19) 1.9699(12) 1.9563(14) 2.3894(4) 1.8230(14)

C1–Ge1–C2 C2–Ge1–Ge2 C1–Ge1–Ge2 Ge1–Ge2–P Ge2–P–C3

108.3(1) 113.6(1) 136.9(1) 80.210(10) 115.4(1)

Amidogermylenes 1031 and 1032, were reduced with half an equiv. of [(MesNacNac)Mg]2 to yield digermynes 1792 and 1793 as products; however, germylene 1034 afforded tetranuclear Ge(I) cluster 1791 under similar conditions. Compounds 1792 and 1791 were reacted with hydrogen to afford digermene 1794 and cyclotetragermane 1795 (Schemes 163 and 164).305

Scheme 163

Treatment of unsaturated small-molecules, such as 1,3-cyclohexadiene, ethylene, and CO2 with 1791 yielded polycyclic compounds 1796, bicycle 1797, and digermylene oxide 1798, respectively, through tetranuclear core breakage reactions. Compound 1795 further reacted with an NHC leading to the facile cleavage of all GedGe bonds of the tetramer to yield hydridogermylene complex 1799 (Scheme 164).305

350

Organometallic Compounds of Germanium

Scheme 164

10.03.9.2.3

Germylones

Germylones are a class of compounds in which the valence electrons of germanium are retained as two lone pairs, thus offering the germanium atom a formal oxidation state of zero. At least two donor atoms from donor ligand(s) stabilize such Ge(0) atom, and the concept of the donor-acceptor interaction best describes the interactions between the donor ligand(s) and the Ge(0) atom. When two carbenes are used for stabilization, the resulting germylones are also termed germa-dicarbenes. Reducing a mixture of cAAC and GeCl21,4-dioxane or a donor ligand stabilized germyliumylidene using KC8 can lead to germylones.

10.03.9.2.3.1 Acyclic germylones Acyclic germylones 1800 and 1802 were also isolated by one-pot reactions of cyclic alkyl(amino) carbenes (cAAC) 1547 and 1801 with GeCl21,4-dioxane and KC8 in a 2:1:2.1 ratio (Eqs. 480 and 481). A bent C-Ge-C backbone was determined from the X-ray crystal structure of compound 1800 (Fig. 47, Table 63).460

ð480Þ

Organometallic Compounds of Germanium

351

ð481Þ

Fig. 47 Molecular structure of compound 1800.

Table 63

Selected bond lengths and angles of compound 1800.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-C1 Ge1-C2 C1-N1 C2-N2 C1-C3

1.942(2) 1.939(2) 1.367(2) 1.367(2) 1.535(3)

C1-Ge1-C2 Ge1-C1-N1 Ge1-C1-C3 Ge1-C2-N2 Ge1-C2-N2

115.27(7) 113.90(1) 136.27(1) 113.55(1) 136.53(1)

10.03.9.2.3.2 Cyclic germylones 10.03.9.2.3.2.1 Chelate ligand stabilized germylones Reduction of germyliumylidene 1804 with five equiv. of KC8 afforded germylone 1805; compound 1805 can also be considered a mesoionic germylene 1805a (Scheme 165).504 Compound 1804 was obtained from imino-N-heterocyclic carbene 1803 via reduction. Reactions of compound 1805 with one and two equiv. of methyl triflate yielded germanium(II) mono-1806 and germanium(IV) di-1807 cations, respectively (Eq. 482).504,505

ð482Þ

352

Organometallic Compounds of Germanium

Scheme 165

Germylone stabilized monometallic and bimetallic complexes 1808–1810 have been synthesized through the reactions of germylone 1805 with the corresponding metal precursors (Scheme 166)505,506

Scheme 166

Reductive dechlorination of germyliumylidene 1812 with sodium naphthalenide gave bis(carbene) stabilized germylone 1813 (Eq. 483); compound 1812 was obtained from bis(carbene) 1811.507 Compound 1813 on treatment with GaCl3 afforded germylone/GaCl3 adduct 1814, which on oxidation using elemental chalcogens resulted in germanium(II) monochalcogenides 1815-1816 and germanium(IV) dichalcogenides 1817–1818 (Scheme 167).508

Organometallic Compounds of Germanium

353

ð483Þ

Scheme 167

Akin to the synthesis of germylone 1813, the dechlorination of chlorogermyliumylidene 1820 using two equiv. of KC8 afforded bis(NHSi) stabilized germylone 1821 (Eq. 484); compound 1820 was synthesized from bis(NHSi)xanthene 1819. Direct isolation of germylone 1821 from compound 1819 was also possible.509

354

Organometallic Compounds of Germanium

ð484Þ

Treatment of bis(NHSi)pyridine 1822 with GeCl21,4-dioxane gave chlorogermyliumylidene 1823, which was reacted with Collman’s reagent to afford iron carbonyl germylone adduct 1824. In the reaction of compound 1824 with GeCl21,4-dioxane, insertion of GeCl2 into the Ge ! Fe bond resulted in germylone-stabilized germylene iron complex 1825 (Scheme 168, Fig. 48, Table 64).510

Scheme 168

Organometallic Compounds of Germanium

355

Fig. 48 Molecular structure of compound 1825.

Table 64

Selected bond lengths and angles of compound 1825.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-Ge2 Ge1-Si2 Ge1-Si2 Ge2-Fe2

2.4878(7) 2.373(1) 2.388(1) 2.373(9)

Si1-Ge1-Si2 Si1-Ge1-Ge2 Si2-Ge1-Ge2 Ge1-Ge2-Fe2

98.92(5) 118.65(4) 101.11(4) 140.13(3)

Stepwise addition of AlBr3 to germylone 1821 gave first germylone mono-Lewis acid adduct 1826 and then germylone bis-Lewis acid adduct 1827.509 Treatment of germylone 1821 with 9-BBN provided boryl(silyl)germylene 1828 through the transfer of hydrogen from 9-BBN to the carbon atom of the amidinatosilylene (Eq. 485).509 In the reaction of compound 1821 with Ni(COD)2, nickelacycle 1829 featuring a Ge2Ni three-membered ring was obtained. Compound 1821 formed a frustrated Lewis pair (FLP) with BPh3, and activates H2 to yield hydridogermyliumylidene 1830 (Scheme 169).509

ð485Þ

356

Organometallic Compounds of Germanium

Scheme 169

10.03.9.2.3.2.2 Pincer ligand stabilized germylones The usefulness of diiminopyridine and diiminocarbene ligands for synthesizing germanium(0) compounds was investigated via germylene anions and cations. The autoionization reaction of diimino pyridine 1831 with two equiv. of GeCl21,4-dioxane afforded germyliumylidene 1832,511 which upon reduction using two equiv. of potassium graphite yielded germylone 1833 (Eq. 486).476 Similarly, the reduction of diiminocarbene stabilized germyliumylidene 1835 with four equiv. of KC8 afforded germylone 1836; compound 1835 was isolated starting from imidazolium salt 1834 (Scheme 170).512

Organometallic Compounds of Germanium

357

ð486Þ

Scheme 170

The oxidative addition of methyl and phenyl iodides to germylone 1836 afforded iodogermylenes 1838 and 1840, via methyl and phenyl migration from germanium to the carbene carbon in the intermediate germylenes 1837 and 1839, respectively.512 Similar reactions of compound 1836 with HCl and C5F5N yielded monochloro-1841 (major product) and monofluoro-1842 germylenes (Scheme 171). Reactions of compound 1836 with GeCl21,4-dioxane and tetrachloro-o-benzoquinone gave [GeCl3]− analog 1843 (of compound 1835) and germanium(IV) bis(catecholate) 1844, respectively (Eq. 487).512

358

Organometallic Compounds of Germanium

ð487Þ

Scheme 171

10.03.9.2.4

Coordination chemistry of germylenes

The lone pair of electrons on germylenes allows them to stabilize various main-group and transition-metal complexes. Consequently, the coordination chemistry of germylenes is vast, and several reviews have covered its growth.428–430,513

Organometallic Compounds of Germanium

359

10.03.9.2.4.1 Germylene stabilized main group element complexes Treatment of amidinatogermylene 1113b with indium(III) halides, GeCl2
1,4-dioxane or SnCl2 resulted in the amidogermylene stabilized complexes 1845–1848, respectively (Eq. 488).514,515 The reaction of West’s germylene with alkaline-earth metal precursors (Z5-C5Me5)2M yielded adducts 1849–1851 (Eq. 489), which were converted to 1,4-diazabuta-1,3-diene stabilized metal complexes 1852–1854 by thermal decomposition with the elimination of tetragonal elemental germanium (Eq. 490).516

ð488Þ

ð489Þ

ð490Þ

10.03.9.2.4.2 Germylene stabilized transition metal complexes 10.03.9.2.4.2.1 Four-membered cyclic germylene stabilized group 6 metal complexes The a-sulfinyl germylene 1123 was reacted with tungsten and molybdenum carbonyl precursors to afford germylene-stabilized metal carbonyl complexes 1855 and 1856 (Eq. 491).517

360

Organometallic Compounds of Germanium

ð491Þ

Treatment of amidinatogermylene 1857 with a chromium Fischer carbene complex yielded 1858 by substituting the carbene for the germylene (Eq. 492).334

ð492Þ

Alkylgermylene 1859 was reacted with MnBr(CO)5 to afford manganese(I) complex 1860, which undergoes reduction with organolithium salts to produce dimeric manganese(0) complex 1861 (Fig. 49, Table 65). The CO ligand substitution reactions of compound 1860 with neutral donors gave mer-1862–1863 and fac-1864 tricarbonyl complexes. Compound 1860 reacted with H2O molecules to yield germanato(II)-ligand-containing manganese complex 1865 (Scheme 172).518 An alternate route to complex 1861 involved the reaction of Mn2(CO)10 with germylene 1859, in which CO substituted product 1866 was formed; the reaction of the latter with another equiv. of compound 1859 gave complex 1861 (Scheme 173).518

Fig. 49 Molecular structure of compound 1861.

Table 65

Selected bond lengths and angles of compound 1861.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-Mn1 Ge2-Mn2 Mn1-Mn2 Ge1-N1

2.3271(6) 2.3222(6) 2.86921(7) 1.998(2)

N1-Ge1-N2 N3-Ge2-N4 N1-Ge1-Mn1 N2-Ge1-Mn1

65.9(1) 66.1(1) 116.97(8) 121.22(8)

Organometallic Compounds of Germanium

361

Scheme 172

Scheme 173

10.03.9.2.4.2.2 Four membered cyclic germylene stabilized group 8 metal complexes Amidinatogermylenes 1099, 1101 and 1862 were reacted with Fe2(CO)9 to afford germylene-stabilized Fe(0) complexes 1863–1865 (Eq. 493).325,519,520 Complexes 1863 and 1865 subsequently reacted with another molecule of germylene 1099 in the presence of UV light to afford Fe(0) complexes 1866 and 1867, stabilized by two germylene units (Eq. 494).325,519,520

362

Organometallic Compounds of Germanium

ð493Þ

ð494Þ

By contrast, the tetracoordinate amidinatogermylene 1868 reacted with Fe2(CO)9 to yield digermylene oxide stabilized Fe(0) complex 1870 via the intermediacy of Fe(0) complex 1869 (Eq. 495).325

ð495Þ

The iminophosphonamide germylene 1330 also gives access to an iron carbonyl complex 1871 (Eq. 496).407

ð496Þ

The aminogermylene 1872 was reacted with Ru3(CO)12 to afford bidentate ligand stabilized diruthenium(0) complex 1873 with the germylene adopting a bridging coordination mode and the imine donor also binding to one Ru center (Eq. 497).521

ð497Þ

Organometallic Compounds of Germanium

363

The CO substitution reactions of complex 1873 with different donor ligands afforded axially 1874–1875 and equatorially 1876–1879 substituted products at low and high temperatures, respectively (Scheme 174).521

Scheme 174

The oxidative addition of HSiEt3 and HSnPh3 to complex 1873 afforded coordinatively unsaturated complexes 1881 and 1882. These systems can be converted to coordinatively saturated complexes 1880a–1880b and 1883a–1883b by adding donor ligands t BuNC and CO, respectively (Scheme 175). Similarly, complex 1873 reacted with H2 to yield coordinatively unsaturated tetraruthenium complex 1885, which was transformed into the corresponding saturated derivatives 1884 and 1886 by adding tBuNC and CO (Scheme 175). The thermolysis product of compound 1873 (1887) contains germylidyne and bridging benzamidinate ligands (Eq. 498).521

364

Organometallic Compounds of Germanium

Scheme 175

Organometallic Compounds of Germanium

365

ð498Þ

The germylenes 1857 and 1857a reacted with Grubbs’ first-generation catalyst to yield phosphine displaced complexes 1888 and 1889 containing two germylenes in trans- and cis-orientations, respectively (Eq. 499).522 The X-ray crystal structure of compound 1888 corroborates the trans-arrangement of the germylene moieties (Fig. 50, Table 66).

ð499Þ

Fig. 50 Molecular structure of compound 1888. Table 66

Selected bond lengths and angles of compound 1888.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-Ru1 Ge2-Ru1 Ru1-Cl1 Ru1-Cl2

2.4576(8) 2.5042(8) 2.407(2) 2.392(2)

N1-Ge1-N2 N3-Ge2-N4 N1-Ge1-Ru1 N2-Ge1-Ru1

66.8(3) 66.6(3) 116.5(2) 117.7(2)

366

Organometallic Compounds of Germanium

Germylene stabilized iridium complexes 1891 and 1892 were obtained from the reactions of germylene 1890 with iridium precursors; thermolysis of complex 1892 gave cyclometallated derivative 1893 (Scheme 176).523 Similarly, germylene 1857 reacted with group 9 and 10 metal precursors to afford the corresponding metal complexes 1894-1898 (Scheme 177).524

Scheme 176

Scheme 177

Organometallic Compounds of Germanium

367

Germylenes 1118 and 1899 were reacted with half an equiv. of PtMe2(Z4-COD) to produce platinum(II) complexes 1900 and 1901 featuring two germylene ligands in cis-arrangement. Complex 1900 was subsequently reacted with [H(OEt2)2][BArF4] to generate ionic platinum(II) complex 1902 via cyclometallation and the formation of an agostic interaction (Eq. 500).525

ð500Þ

10.03.9.2.4.2.3 Four-membered germylene stabilized group 11 and 12 metal complexes The coinage metal complexes 1903–1907 stabilized by coordination of germylene 1857 were prepared by reacting compound 1857 with suitable precursors (Scheme 178). With CuCl/AgCl, tetrameric complexes 1903 and 1904 containing M4Cl4 cubane-type cores were obtained, while a linear mononuclear gold complex 1905 was obtained in the reaction with AuCl(tht) (Scheme 178).526 In the reactions with half equivalents of [Cu(MeCN)4][BF4] and Ag[BF4], ionic complexes 1906 and 1907 stabilized by two germylene ligands were isolated (Scheme 178).526

Scheme 178

Dimeric zinc complexes 1908a and 1908b with Zn2X2 (X ¼ Br 1908a, I 1908b) cores were isolated from the reactions of amidogermylene 1113b with zinc bromide and zinc iodide, respectively (Eqs. 501).527

368

Organometallic Compounds of Germanium

ð501Þ

10.03.9.2.4.2.4 Five-membered germylene stabilized group 6 and 7 metal complexes The amidinatogermylene 1857 was reacted with Fischer carbene complexes to afford chromium and tungsten complexes 1909a–1909b, and 1910 where the insertion of carbene into the GedN bond of the germylene 1857 took place to form C2N2Ge five-membered rings (Eqs. 502 and 503).334

ð502Þ

ð503Þ

The halogen-substituted aminotroponiminatogermylenes 1135, 1151, 1153 and pseudohalogen substituted 1144, 1148 were reacted with cis-Mo(CO)4(COD) to afford molybdenum complexes 1911-1913 and 1914-1915, in which the two germylene ligands are in cis- and trans-dispositions, respectively (Eqs. 504 and 505). Likewise, the reactions of cis-W(CO)4(COD) with germylenes 1135 and 1148 gave cis-1916 and trans-1917 tungsten complexes, respectively (Eqs. 506 and 507).347

ð504Þ

Organometallic Compounds of Germanium

369

ð505Þ

ð506Þ

ð507Þ

Monochlorogermylene 1367b stabilized W(0) complex 1919 was isolated using [W(CO)5
THF]. However, (diazomethyl)germylene 1918 gave tungsten complex 1920a via the intermediacy of complex 1920; SidC bond hydrolysis led to the decomposition of complex 1920. Complex 1920a can also be formed by the reaction of complex 1919 with lithium diazomethane (Scheme 179).528

Scheme 179

370

Organometallic Compounds of Germanium

10.03.9.2.4.2.5 Five-membered germylene stabilized group 8, 9, and 10 metal complexes The aminotroponiminatogermylenes 1135 and 1137 were reacted with half an equiv. of [RuCl2(p-cymene)]2 to afford germylene-stabilized ruthenium complexes 1921 and 1922 (Eq. 508). Complex 1922 was further reacted with TMSCl, H2O or SnCl2 to afford compound 1923, hydroxygermylene stabilized ruthenium(II) complex 1924, and bimetallic complex 1925, respectively. The chloro analog of complex 1925 (1926) was obtained by treatment with TMSCl (Scheme 180).529

ð508Þ

Scheme 180

Alkylgermylene 1158 was reacted with Pt(COD)Cl2 in a 2:1 molar ratio to afford Pt(II) complex 1927, which on treatment with TMSCN, TMSN3 or AgOTf resulted in dicyano 1928, diazido 1929, and cationic 1930 platinum complexes, respectively (Scheme 181). Monomeric platinum complex 1931 which is also cationic, was isolated by reacting dimeric complex 1930 with DMAP (Scheme 181).530

Organometallic Compounds of Germanium

371

Scheme 181

The reaction of phosphine-stabilized germylene 1344 with half an equiv. of [Rh(m-Cl)(COD)]2 afforded a mixture of three rhodium complexes 1932a, its diastereomer, and rhodacycle 1933; over time, conversion of complex 1932a and its diastereomer to complex 1933 occurred leading to the exclusive isolation of the latter. The geometry around the germanium center in compound 1933 is distorted tetrahedral (Fig. 51, Table 67). By contrast, germylene 1351 produced a diastereomeric mixture of rhodium(I) complexes 1932ba and 1932bb (Scheme 182).412

Fig. 51 Molecular structure of compound 1933.

372

Organometallic Compounds of Germanium

Table 67

Selected bond lengths and angles of compound 1933.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-Rh1 Ge1-N1 Rh1-P1 Ge1-Cl1 Ge1-Cl2

2.357(6) 1.882(3) 2.286(1) 2.312(1) 2.232(1)

N1-Ge1-Rh1 Ge1-Rh1-P1 Cl1-Ge1-Cl2 Cl1-Ge1-Rh1 N1-Ge1-Cl2

119.93(1) 87.46(3) 98.32(6) 113.19(4) 100.65(1)

Scheme 182

10.03.9.2.4.2.6 Five-membered germylene stabilized group 11 metal complexes Monochlorogermylene 1135 was reacted with copper(I) iodide in the presence of one or two equiv. of pyridine to afford germylene-stabilized dimeric and monomeric Cu(I) iodide complexes 1934 and 1935, respectively. When two equiv. of CuI were used in the presence of acetonitrile, tetrameric complex 1936 featuring a cubane-type Cu4I4 core was obtained (Scheme 183).531

Scheme 183

Organometallic Compounds of Germanium

373

Treatment of digermylene oxide 1159 with copper(I) iodide under different reaction conditions afforded complexes with Cu2I2 (1937), Cu3I3 (1938), and Cu4I4 (1939) cores (Scheme 184). The X-ray crystal structure of compound 1939 confirmed the presence of the Cu4I4 core (Fig. 52, Table 68). Similarly, 1159 was reacted with AgI and AuI in the presence of pyridine to afford complexes with Ag4I4 (1940) and Au2I2 (1941) cores, respectively (Scheme 185).532

Scheme 184

Fig. 52 Molecular structure of compound 1939.

374

Organometallic Compounds of Germanium

Table 68

Selected bond lengths and angles of compound 1939.

Atoms

Bond lengths (A˚ )

Atoms

Bond angles (  )

Ge1-Cu4 Ge2-Cu3 Cu1-Cu4 Cu2-Cu3 Ge1-O1

2.3126(9) 2.3035(8) 2.573(1) 2.573(1) 1.812(4)

Ge1-O1-Ge2 Cu4-Ge1-O1 Cu3-Ge2-O1 Ge3-O1-Ge4 Cu1-Ge3-O2

117.4(2) 119.7(1) 121.5(1) 117.4(2) 121.5(1)

Scheme 185

10.03.9.2.4.2.7 Five-membered germylene stabilized group 12 metal complexes Alkylgermylene 1158 was reacted with ZnCl2, ZnI2, CdCl2, and CdI2 to afford the germylene stabilized metal complexes 1942-1945, respectively, featuring M2X4 cores (Eq. 509). Treatment with half an equiv. of ZnCl2 and CdI2 yielded complexes 1946 and 1947 with the MX2 cores stabilized by two germylene units (Eq. 509). The interconversion between 1:1 (1942 and 1945) and 2:1 (1946 and 1947) complexes was also demonstrated (Eq. 510).533

ð509Þ

ð510Þ

Organometallic Compounds of Germanium

375

10.03.9.2.4.2.8 Six-membered germylene stabilized metal complexes The b-diketiminatochlorogermylene 1198 was reacted with Ru3(CO)12 in 3:1 and 6:1 molar ratios to afford digermylene-stabilized ruthenium complex 1948 (Eq. 511). However, treatment of alkenyl germylene 1211 with Ru3(CO)12 in 6:1 and 3:1 molar ratios gave complex 1949 and a mixture of complexes (1950 and 1949), respectively (Eq. 512), implying that the digermylene complexes were formed via the monogermylene complex.534

ð511Þ

ð512Þ

Compound 1948 was reacted with Li[BHEt3], Li[N(TMS)2], and iodine to yield Cl/Et metathesis product 1951, dehydrochlorinated product 1952, and oxidative addition product 1954, respectively. Treatment of germylene-stabilized ruthenium complex 1952 with H2O yielded hydroxygermylene ligated ruthenium complex 1953 (Scheme 186).534

Scheme 186

Methylgermylene 1955 was reacted with different equivalents of [Cu(C6F5)]4 to afford complexes 1956 and 1957 featuring rhombically-bridged [Cu(C6F5)]2 and chain-like [Cu(C6F5)]4 units stabilized by two germylene ligands, respectively (Scheme 187).535

376

Organometallic Compounds of Germanium

Scheme 187

Treatment of diazomethylgermylene 1265 with one or two equivalents of AgC6F5
MeCN yielded complexes 1958 and 1959 with terminal Ag(C6F5) and chain-like [Ag(C6F5)]4 units stabilized by one and two germylenes, respectively (Scheme 188)535. Interconversion reactions among the copper (1956 and 1957) and silver (1958–1959) complexes have been shown to be feasible.

Scheme 188

10.03.9.2.4.2.9 Miscellaneous germylene stabilized metal complexes Germylene 1960 was reacted with half an equiv. of molybdenum or tungsten carbonyl to afford digermylene metal carbonyl complexes 1963 and 1964, respectively (Scheme 189). Reactions of 1960 with rhodium and iridium metal precursors failed to afford stable products in the solid-state (Scheme 189).536

Organometallic Compounds of Germanium

377

Scheme 189

10.03.9.2.5

Germylenes and their metal complexes as catalysts

With massive advancements in the chemistry of germylenes, exploring the applications of germylenes and their metal complexes has witnessed a boom during the last decade. Concerning this, the catalytic applications of germylenes and their complexes have also been studied with interest and a number of reviews covering these advances have appeared.3,290,294 This section provides a summary of catalytic studies performed using germylenes and their metal complexes.

10.03.9.2.5.1 Catalytic hydroboration of aldehydes and ketones Hydroboration of aldehydes/ketones using pinacolatoborane in the presence of germylene catalysts Cat.1-Cat.7 (Chart 1) resulted in the addition of boron and hydrogen atoms across the C]O bond of these carbonyl compounds to afford boronic esters (Eq. 513). The efficiencies of these catalysts (Cat.1-Cat.3 and Cat.5-Cat.7) for the hydroboration of benzaldehyde are shown in Table 69.454,467,537–541 The catalytic efficiency of Cat.4 for hydroboration of pinacolone is shown in Table 70.

ð513Þ

378

Organometallic Compounds of Germanium

Chart 1

Table 69

Catalytic efficiencies of the catalysts Cat.1-Cat.7 for the hydroboration of benzaldehyde.

S. no.

Substrate

Catalyst

mol%

Temperature (  C)

Time ( h)

TOF (h−1)

1

Benzaldehyde

Cat.1 Cat.2 Cat.3 Cat.5 Cat.6 Cat.7

2 1 2 4 1 0.05

rt 20 rt rt rt 20

24 1 4 3 0.96 2.5

2 91 12.5 5.5 456 800

Table 70

Catalytic efficiency of the catalyst Cat.4 for the hydroboration of pinacolone

S. no.

Substrate

Catalyst

mol%

Temperature (  C)

Time ( h)

TOF (h−1)

2

Pinacolone

Cat.4

10

rt

0.42

24.4

10.03.9.2.5.2 Cyanosilylation Aldehydes and ketones were reacted with trimethylsilyl cyanide in the presence of germylene catalysts Cat.3 and Cat.8 to afford cyanosilylated products in which the addition of the nitrile and silyl groups across the C]O bond has occurred (Eq. 514, Chart 2).354,537 Germylene-stabilized platinum(II) complex Cat.9 has also shown catalytic potential for the cyanosilylation of various aldehydes and ketones.530 The efficiencies of catalysts Cat.3, Cat.8, and Cat.9 for the cyanosilylation of propionaldehyde and/or benzaldehyde are shown in Tables 71 and 72.

Organometallic Compounds of Germanium

379

ð514Þ

Chart 2

Table 71

Catalytic efficiency of the catalyst Cat.8 and Cat.3 for the cyanosilylation of aldehydes

S. no.

Substrate

Catalyst

Mol%

Temperature (  C)

Time ( h)

TOF (h−1)

1 2

Propionaldehyde Benzaldehyde

Cat.8 Cat.3

1 1

0 - rt rt

2 2

50 50

Table 72

Catalytic efficiency of the catalyst Cat.9 for the cyanosilylation of aldehydes

S. No.

Substrate

Catalyst

Mol%

Temperature (  C)

Time ( h)

TOF (h−1)

1 2

Propionaldehyde Benzaldehyde

Cat. 9 Cat. 9

0.25 0.25

50 50

2 6

198 64

380

Organometallic Compounds of Germanium

10.03.9.2.5.3 Other catalytic reactions Pincer germylene stabilized nickel (Cat.10), amidinatogermylene stabilized iridium (Cat.11), ruthenium (Cat.12), and cationic Au(I) (Cat. 13) complexes have been used as catalysts for Sonogashira coupling (Eq. 515), transfer hydrogenation (Eq. 516), N-alkylation (Eq. 517, Chart 3) and glycoside synthesis (Eq. 518) reactions.450,524a,b

ð515Þ

ð516Þ

ð517Þ

ð518Þ

Organometallic Compounds of Germanium

381

Chart 3

10.03.10

Compounds with germanium-germanium single bonds

The past decade has experienced tremendous growth in the chemistry of Ge-Ge singly-bonded compounds.542 The isolation and reactivity of a number of linear, branched, and cyclic oligogermanes and polygermanes are discussed here.

10.03.10.1

Synthesis

Hydrogermolysis (GedH bond cleavage) reaction has been proved as a key route for the synthesis of compounds containing GedGe single bonds.

10.03.10.1.1

Synthesis of digermanes

Digermanes 1967–1976 have been isolated by the reaction between a nitrile featuring a labile GedC bond and a germanium hydride; the nitrile precursors themselves were obtained by the reactions of the corresponding germaamides with acetonitrile (Eq. 519, Table 73).543–546

ð519Þ

382

Organometallic Compounds of Germanium

Table 73

Digermanes 1967–1976 and their percentage yields.

Compound

R1

R2

R3

R4

R5

R6

% Yield

1967 1968 1969 1970 1971 1972 1973 1974 1975 1976

n

n

n

Ph Ph Ph Ph Ph Ph Ph Ph Me i Pr

Ph Ph Ph Ph Ph Ph Ph Ph Me i Pr

Ph Ph Ph Ph Ph Ph Ph Ph Me i Pr

83 84 79 43 50 89 81 76 86 32

Bu Et i Bu n Hex C18H37 t BuMe2 s Bu Me Bu i Pr

Bu Et i Bu n Hex C18H37 t BuMe2 s Bu Me Bu i Pr

Bu Et i Bu n Hex C18H37 t BuMe2 s Bu Ph Bu i Pr

The reaction of tris(p-tolyl)germane with nBuLi generated the corresponding lithium salt, which undergoes an in situ reaction with Me3GeBr to give digermane (p-Tol)3Ge-GeMe3 1977 (Eq. 520).

ð520Þ

The reaction of 4,4-dichlorodithienogermole 1978 with ethylmagnesium bromide resulted in 4-chloro-4-ethyl-dithienogermole, which reacted with sodium to afford 4,40 -bis(ethyldithienogermole) 1979 (Eq. 521).547

ð521Þ

The reaction of a dichlorogermole with MesLi afforded mesitylchlorogermole 1980, which upon reduction using KC8 afforded bisgermole 1981 (Eq. 522).548

ð522Þ

Reduction of spirogermane 1982 using lithium metal afforded pentacoordinate digermanate 1983, partnered with lithium counter ions (Eq. 523). The reaction of 1983 with benzyltrimethylammonium chloride led to exchange of counterions to afford another pentacoordinate digermanate 1984 (Eq. 524).

Organometallic Compounds of Germanium

383

ð523Þ

ð524Þ

Protonation of compound 1983 by an excess of HCl afforded digermane 1985; however, the use of one equiv. of HCl resulted in the formation of germylgermanate 1986. Interestingly, deprotonation of compounds 1985 and 1986 using an excess of LiH and nBuLi, respectively, regenerated compound 1983. The reaction of digermane 1985 with nBuLi gave compound 1986, and the latter can be converted back to 1985 using an excess of aqueous HCl (Scheme 190). Reactions of compound 1986 with benzyltrimethylammonium chloride and (Ph3P)2NCl gave germylgermanates 1987 and 1988 (Eq. 525).549

ð525Þ

384

Organometallic Compounds of Germanium

Scheme 190

Addition of the potassium salt of germane 1989 to triphenylchlorogermane exclusively afforded digermane 1990. By contrast, if triphenylchlorogermane was added to the potassium salt 1989, a mixture of digermane 1990 and trigermane 1991 was obtained (Scheme 191).212

Scheme 191

Organometallic Compounds of Germanium

385

1-Germylgermatrane 1992 featuring a hyper-coordinate germanium atom was obtained through the treatment of the in situ generated germanide with 1-germatranyl triflate. Alternatively, the reaction of digermanetrichloride with triethanolamine and Et3N also gave compound 1992 (Eq. 526)550

ð526Þ

Digermane 1996 featuring a GedCl bond can be obtained from digermane 1993. Accordingly, the reaction of hexaphenyldigermane 1993 with trifluoromethanesulfonic acid afforded monogermyltriflate 1994 along with trace amounts of digermane side product 1995.551 Treatment of compound 1994 with ammonium chloride gave digermane 1996 (Scheme 192).

Scheme 192

10.03.10.1.2

Synthesis of linear oligogermanes

The reaction of diphenylgermane 1997 with 2 equiv. of tri(iso-propyl)germaamide afforded trigermane 1998 (Eq. 527).546

ð527Þ

Reactions of germaamides 1999, 2000, and 2001 first with the solvent (acetonitrile) and then with half an equiv. of diphenylgermane gave trigermanes 2002, 2003, and 2004, respectively (Eq. 528).552

ð528Þ

The reaction of tetraphenyldigemane 2005 with two equiv. of iPr3GeNMe2 in acetonitrile afforded tetragermane 2006 (Eq. 529).546

386

Organometallic Compounds of Germanium

ð529Þ

Reaction of trigermane 2007 with triphenylgermaamide in acetonitrile, leads to the formation of hydride terminated tetragermane 2008. The reaction of germane 2008 with a second equiv. of triphenylgermaamide afforded pentagermane 2009, which can also be obtained directly by treating compound 2007 with two equiv. of triphenylgermaamide in acetonitrile (Scheme 193).553

Scheme 193

Digermanes 2013–2015 were obtained by the reactions of germaamides 2010–2012 with triphenylgermane in acetonitrile. Furthermore, the reactions of compounds 2010 and 2011 with DIBAL-H afforded digermanes 2016 and 2017 featuring a GedH bond. Reactions of germaamides 2010 and 2011 with digermanes 2016 and 2017 in acetonitrile gave trigermanes 2018 and 2019, respectively. Trigermanes featuring GedH bonds (2020 and 2021) could be isolated from the reactions of compounds 2018 and 2019 with DIBAL-H. The reactions of trigermahydrides 2020 and 2021 with germaamides 2010 and 2011 in acetonitrile gave tetragermanes 2022 and 2023, respectively (Scheme 194).543

Organometallic Compounds of Germanium

387

Scheme 194

Digermane 2024 was used for the synthesis of trigermanes 2026 and 2027 and tetragermanes 2028, 2029 and 2030. The in situ generated hydridodigermane 2025 (formed through the treatment of compound 2024 with DIBAL-H) has been shown to react with a-germyl nitriles resulting in the formation of the corresponding trigermanes 2026 and 2027 (Eq. 530).

ð530Þ

Treatment of these trigermanes with DIBAL-H resulted in the formation of the related hydrides, which upon reaction with a-germyl nitriles in acetonitrile gave the corresponding tetragermanes 2028–2030 (Eq. 531).552

ð531Þ

Pentagermane 2032 was synthesized by the reaction of trigermane 2031 with two equiv. of tri(iso-propyl)germaamide in acetonitrile (Eq. 532).

ð532Þ

388

Organometallic Compounds of Germanium

Cyclotetragermane 2033 reacts with bromine to give 1,4-dibromotetragermane 2034 through ring-opening. The reduction of tetragermane 2034 with LiAlH4 afforded 1,4-dihydrotetragermane 2035 (Eq. 533). The reaction of compound 2035 with two equiv. of tri(iso-propyl)germaamide in acetonitrile afforded hexagermane 2036 (Eq. 534);554 single crystal X-ray diffraction studies showed that compound 2036 features a chain of six germanium atoms (Fig. 53, Table 74).

ð533Þ

ð534Þ

Fig. 53 Molecular structure of hexagermane 2036.

Table 74

10.03.10.1.3

Selected bond lengths and angles of compound 2036.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-Ge2 Ge2-Ge3 Ge3-Ge30

2.4670(2) 2.4715(3) 2.4745(3)

Ge1-Ge2-Ge3 Ge2-Ge3-Ge30

117.330(8) 114.15(1)

Synthesis of branched oligogermanes

The parent germane, GeH4, reacts with 3.3 equiv. of a-aminonitrile in acetonitrile to afford the branched hydrido tetragermane 2037 (Eq. 535). Repetition of this reaction with phenyl germane afforded branched phenyl tetragermane 2038 (Eq. 536). When four equiv. of a-aminonitrile were added to the parent germane in acetonitrile, branched pentagermane 2039 was obtained (Eq. 537).555

Organometallic Compounds of Germanium

389

ð535Þ

ð536Þ

ð537Þ

The reaction of four equiv. of chlorotrimethylgermane with tetrabromogermane using eight equiv. of lithium at low temperature afforded tetrakis(trimethylgermyl)germane 2040. Furthermore, the branched oligogermane 2040 in the presence of 18-crown-6 ether reacted with KOtBu to afford tris(trimethylgermyl)germanide 2041 (Eq. 538). The germanide 2041 reacted readily with various electrophiles to afford the corresponding oligogermanes 2042–2046 (Scheme 195).556

ð538Þ

Scheme 195

390

Organometallic Compounds of Germanium

The reaction of phenylgermane with three equiv. of germaamide afforded branched tetragermane 2047 which was then reduced to a hydride intermediate using three equiv. of DIBAL-H (Eq. 539). This hydride was reacted with three equiv. of germaamides 2010, 2011, and 2012 to produce branched heptagermanes 2048, 2049, and 2050, respectively (Eq. 540).555 The intermediate hydride was also reacted with three equiv. of trigermaamide to afford dendritic tridecagermane 2051 (Eq. 541).557

ð539Þ

ð540Þ

ð541Þ

10.03.10.1.4

Synthesis of polygermanes

Reductive coupling reactions of 4,4-dichlorodithienogermoles 2052, 2053, and 2054 using sodium afforded poly(dithenogermane4,4-diyl)s 2055, 2056, and 2057, respectively. Furthermore, the oxidation of polymers 2056 and 2057 by trimethylamine N-oxide under reflux conditions gave polymers 2058 and 2059, respectively featuring GedO bonds in the main chain (Eq. 542). Polymerization of 4,4-dichlorodithienogermole 2054 with dibutyldichlorogermane using sodium under reflux conditions afforded copolymer 2060 (Eq. 543).547

ð542Þ

Organometallic Compounds of Germanium

391

ð543Þ

10.03.10.1.5

Synthesis of metal complex supported oligogermanes

Bis(germyl)palladium complex 2061, when heated at 80  C, produced a dipalladium complex with bridging digermene and germylene ligands 2062. By contrast, heating compound 2062 with an excess of H2GePh2 at 90  C afforded tetragermapalladacyclopentane 2063 (Scheme 196).558

Scheme 196

Reaction of platinum(0) complex 2064 with mesitylgermane in a 1:2 molar ratio afforded cis-bis(germyl)platinum 2065, which undergoes 1,2-migration of a germyl group at room temperature to afford digermanylplatinumhydride 2066 (Eq. 544).238

ð544Þ

A mixture of trigermane 2067, digermane 2068, and monogermane 2069 was obtained in the reaction of compound 2061 with dmpe in a 1:3 molar ratio (Eq. 545).559 The reaction of bis(germyl)platinum 2070 with depe resulted in the coupling of GeHPh2 moieties to produce digermane 2068 along with Pd(depe)2 (Eq. 546).

392

Organometallic Compounds of Germanium

ð545Þ

ð546Þ

The reaction of dihydrogermafluorene 2073 with platinum complex 2072 afforded tetragermaplatinacyclopentane 2074 along with trace amounts of bis(germyl)platinum (Eq. 547).560 The reaction of bis(germyl)platinum 2076 with 1.2 equiv. of H2GePh2 gave trigermaplatinacyclobutane 2077, which, when treated with an excess of H2GePh2, resulted in the formation of tetragermaplatinacyclopentane 2078. Compound 2078 can also be obtained directly by reacting compound 2076 with excess H2GePh2 (Scheme 197).561

Scheme 197

Organometallic Compounds of Germanium

393

ð547Þ

10.03.10.1.6

Synthesis of cyclic oligogermanes

Treatment of 1,2-diethynylbenzene 2079 with nBuLi gave dilithium salt 2080, which was further reacted with digermanes to form the corresponding dibenzodehydro[16]annulenes 2082 and 2083 containing two GedGe bonds (Eq. 548).562

ð548Þ

Reduction of the germanium analog of cyclopropane 2083 with six equiv. of lithium followed by the addition of three equiv. of GeCl2
dioxane afforded pentagerma[1.1.1]propellane 2084. Compound 2084 can also be obtained by treating 1.5 equiv. of dichlorogermane 2085 with 3.3 equiv. of lithium naphthalenide and GeCl2
dioxane. Compound 2084 features two ligand-free bridgehead germanium atoms (Fig. 54, Table 75). Its further reaction with Me3SnH gave bicyclo[1.1.1]pentagermane 2086 as the addition product, featuring GedSn and GedH bonds (Scheme 198).563

Fig. 54 Molecular structure compound 2084.

394

Organometallic Compounds of Germanium

Table 75

Selected bond length and bond angles of compound 2084.

Atoms

Bond lengths [pm]

Atoms

Bond angles [  ]

Ge1-Ge2 Ge1-Ge3 Ge1-Ge4 Ge1-Ge5 Ge2-Ge3 Ge2-Ge4 Ge2-Ge5 Ge3-C10 Ge3-C20 Ge4-C30 Ge4-C40 Ge5-C50 Ge5-C60

286.9(2) 248.9(2) 247.6(1) 245.3(1) 246.6(1) 248.5(2) 250.1(1) 197.7(9) 198(1) 196.7(9) 197.0(9) 197.0(9) 197.7(9)

Ge1–Ge3–Ge2 Ge1–Ge4–Ge2 Ge1–Ge5–Ge2 Ge3–Ge1–Ge4 Ge3–Ge1–Ge5 Ge4–Ge1–Ge5 Ge3–Ge2–Ge4 Ge3–Ge2–Ge5 Ge4–Ge2–Ge5 C10–Ge3–C20 C30–Ge4–C40 C50–Ge5–C60

70.75(4) 70.65(4) 70.77(4) 89.85(5) 90.33(5) 90.18(4) 90.17(5) 89.75(5) 88.89(5) 111.6(4) 110.0(4) 111.1(4)

Scheme 198

10.03.11

Compounds with germanium-germanium multiple bonds

Several reviews have covered this aspect of the germanium chemistry.1,564–567 An overview of the topic is presented here through the recent developments.

10.03.11.1 10.03.11.1.1

Synthesis Synthesis of compounds with Ge]Ge bonds

10.03.11.1.1.1 Synthesis from germylene The reaction of GeCl2
dioxane with the Grignard reagent RMgBr (R ¼ 2,5-tBuC6H3) forms tetraaryldigermene 2087 (Eq. 549).568 Reaction of dimeric monochlorogermylene (Ar0 GeCl)2 (Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) 2088 performed with alkynyllithium salts LiC^CR (R ¼ SiMe3 and tBu) resulted exclusively in digermenes 2089 and 2090 (Eq. 550).569

Organometallic Compounds of Germanium

395

ð549Þ

ð550Þ

The 1:1 reaction of NHC carbene (MeIiPr) stabilized silylgermylene 2091 with BPh3 at room temperature resulted in the formation of silyl substituted digermene 2092 (Eq. 551).570 Digermene 2093, featuring Ge-Cl functional groups, was isolated through the reaction of EMind stabilized diarylgermylene ((EMind)2Ge) with GeCl2
dioxane (EMind ¼ 1,1,7,7-tetraethyl-3,3,5,5-tetramethyls-hydrindacen-4-yl) (Eq. 552, Fig. 55, and Table 76).571

ð551Þ

Fig. 55 Molecular structure of compound 2093.

Table 76

Selected bond lengths and angles of compound 2093.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-Ge1 Ge1-Cl1 Ge1-C1

2.3853(7) 2.1980(13) 1.981(3)

C1–Ge1–Ge1 Cl1–Ge1–Ge1 C1–Ge1–Cl1 C1–Ge1–Cl1

124.24(9) 104.89(4) 107.57(10) 44.02(12)

396

Organometallic Compounds of Germanium

ð552Þ

Compounds akin to 2093, i.e., 1,2-dihalodigermenes 2094 and 2095 bearing Eind groups, were obtained by ligand redistribution reactions of the diarylgermylene ((Eind)2Ge) with GeCl2
dioxane and GeBr2
dioxane, respectively (Eq. 553).572

ð553Þ

10.03.11.1.1.2 Synthesis from germyl halides Tetrakis(trialkylsilyl)digermene 2097 was synthesized by the reductive dechlorination of dichloro bis(trialkylsilyl)germane 2096 using potassium graphite (Eq. 554).573 Trigermaallene 2100 was obtained by reducing tetrachlorodigermane 2099 with KC8 at a low temperature. Unlike allene, germaallene 2100 is bent with a Ge–Ge–Ge bond angle of 122.61(6) . Compound 2099 was obtained by reacting germylene 2098 with tetrachlorogermane (Eq. 555).574

ð554Þ

ð555Þ

Dibromogermane 2101 on reduction with KC8 gave 1,2-bis(ferrocenyl)digermene 2102, in which the ferrocenyl systems are connected through the p-system of digermene (Eq. 556).575

ð556Þ

Reduction of diaryldichlorogermane Tip2GeCl2 2103 to afford monoanionic digermenide Tip2Ge ¼ Ge(Tip)Li
DME2 (2104) was carried out at a low temperature using 3.3 equiv. of lithium and 0.4 mol% of naphthalene.576 The salt elimination reactions of compound 2104 with silyl chlorides and acid chlorides afforded various digermenes 2105–2112 (Scheme 199).577

Organometallic Compounds of Germanium

397

Scheme 199

10.03.11.1.2

Synthesis of compounds with Ge^Ge bonds

10.03.11.1.2.1 Synthesis from digermene The reduction of trans-1,2-dibromogermenes 2113 and 2114 using two equiv. of potassium graphite afforded digermynes BbtGe^GeBbt 2115578 and TbbGe^GeTbb 2116,579 respectively (Bbt¼2,6-[CH(SiMe3)2]-4-[C(SiMe3)3]-C6H2, Tbb¼2,6-[CH(SiMe3)2]-4-tBu-C6H2) (Eq. 557). Compound 2116 was used as a precatalyst for the cyclotrimerization of terminal arylacetylenes to afford 1,2,4-triarylbenzenes.580

ð557Þ

10.03.11.1.2.2 Synthesis from germylenes The synthesis of a series of digermynes 2117–2119 stabilized by terphenyl ligands was achieved by reducing the corresponding halogermylenes using KC8 (Eq. 558).581

398

Organometallic Compounds of Germanium

ð558Þ

10.03.11.1.3

Synthesis of cyclic compounds with Ge]Ge bonds

10.03.11.1.3.1 Synthesis of three-membered rings with Ge]Ge bonds The reaction of cyclotrigermenylium ion 2120 with excess KX (X ¼ Cl, Br, and I) led to the formation of trigermacyclopropenes 2121-2123 featuring the respective GedX bonds (Eq. 559).582 The reaction of tetrachlorodigermane 2124 with two equiv. of dilithosilane and dilithogermane resulted in one equiv. of heavy analogs of cyclopropene, i.e., (tBu2MeSi)4SiGe2 2125 and (tBu2MeSi)4Ge3 2126; half equiv. of disilene 2127 and digermene 2128 were also formed, respectively (Eq. 560).583

ð559Þ

ð560Þ

The solid-state thermolysis of compound 2097 afforded cyclotrigermene 2126; hydrosilane tBu2MeSiH and tris(trialkylsilyl)germyl radical (tBu2MeSi)3Ge are the other products (Eq. 561).584

ð561Þ

10.03.11.1.3.2 Synthesis of four-membered rings with Ge]Ge bonds 1,2-Digermacyclobutene 2129 was synthesized through the reaction of digermyne 2115 with ethylene at low-temperature quantitatively (Eq. 562).585

Organometallic Compounds of Germanium

399

ð562Þ

Digermyne 2115 reacted with an excess of styrene and 1-hexene through [2 +2] cycloaddition to form 1,2-digermacyclobutenes 2130 and 2131, respectively. Upon mild heating of compounds 2130 and 2131, reductive elimination of alkenes occurred with the regeneration of digermyne 2115 (Eq. 563). Digermyne 2116 did react with styrene to afford 1,2-digermacyclobutenes 2132; however, the reverse reaction of compound 2132 to digermyne did not occur (Eq. 564).586

ð563Þ

ð564Þ

The reaction of cyclotrigermacyclopropene 2126 with GeCl2
dioxane afforded tetragermacyclobutene 2133 featuring two GedCl bonds (Eq. 565).587 Reaction of disilagermirene with GeCl2
dioxane afforded disiladigermetene 2134 featuring SidCl bonds (Eq. 566).588 Compound 2093 reacts with two equiv. of lithium naphthalide to give tetragermacyclobutadiene, (Ge4(EMind)4) (2135) (Eq. 567).571

ð565Þ

ð566Þ

400

Organometallic Compounds of Germanium

ð567Þ

The reaction of digermyne 2115 with diphenylacetylene afforded 1,2-digermacyclobutadiene 2136 through a (2+2) cycloaddition (Eq. 568).589

ð568Þ

10.03.11.1.3.3 Synthesis of six-membered rings with Ge]Ge bonds When a hexane solution of 2115 was exposed to an acetylenic atmosphere, 1,2-digermabenzene 2137 was obtained via a formal [2 +2 +2] cycloaddition reaction (Eq. 569).579 The X-ray crystal structure of compound 2137 shows that the two germanium atoms are part of the six-membered ring (Fig. 56 and Table 77).

ð569Þ

Fig. 56 Molecular structure of compound 2137.

Organometallic Compounds of Germanium

Table 77

10.03.11.2 10.03.11.2.1

401

Selected bond lengths and angles of compound 2137.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge-Ge Ge-Cl C1-C2 C2-C2

2.3117(6) 1.897(3) 1.359(5) 1.417(7)

C1-Ge-Ge Ge-C1-C2 C1-C2-C2

101.35(10) 129.3(2) 128.07(19)

Reactivity of germanium germanium multiply bonded compounds Reactivity of compounds with Ge]Ge bonds

The reaction of benzyl and tert-butyl isocyanides with digermene 2138 afforded digermane 2139 and trigermacyclopropane 2140 (Eq. 570).590

ð570Þ

Addition reactions of primary and secondary amides to digermene 2138 were performed; addition of primary and N-methyl amides afforded amide adducts 2141–2144, whereas the addition of N-phenyl amides gave imidate adducts 2145–2147 (Eq. 571).591

ð571Þ The addition reaction of substituted cyclopropyl alkyne 2148 to digermene 2138 afforded a mixture of products: digermacyclobutenes 2149a and 2149b, digermane 2150, digermacyclohepta-1,2-diene 2151 and digermylcyclopenta-1,3-diene 2152 (Eq. 572).592

402

Organometallic Compounds of Germanium

ð572Þ The reaction of digermene 2138 with an excess of acetonitrile, propionitrile or acrylonitrile afforded 1,2,3-azadigermitenes 2153, 2154, and 2155, respectively; compounds 2156 and 2157 were formed along with compounds 2153 and 2154 (Eq. 573).593

ð573Þ The reaction of excess benzenesulfonyl chloride with digermene 2138 afforded 2-chloro-tetramesityldigermyl benzenesulfinate 2158 via a 1,2-addition reaction (Eq. 26).594

ð574Þ

Organometallic Compounds of Germanium

403

Compound 2097 reacted with excess tert-butylisocyanide to form germanium cyanide 2159 along with H2C¼ C(CH3)2. The reaction of 2097 with two equiv. of 3,5-di-tert-butyl-1,2-benzoquinone gave 1,3-dioxa-2-germacyclopent-4-ene 2160 through a [1 +4] cycloaddition.595 Compound 2097 can be used to prepare synthetically beneficial 1,1-dilithogermane 2161 (Scheme 200).573

Scheme 200

Digermene 2102 reacted with atmospheric oxygen to give 1,3,2,4-dioxadigermetane 2162. On reacting excess elemental sulfur with digermene 2102, thiadigermirane 2163 was obtained along with dithiadigermetane 2164. The selenium analog 2165 (of 2163) was obtained by the reaction of compound 2102 with an excess of elemental selenium (Scheme 201).575

Scheme 201

The reaction of acyl digermenes 2110-2112 with methanol resulted in acyldigermanes 2166-2168 via an addition reaction (Eq. 575).577

404

Organometallic Compounds of Germanium

ð575Þ

10.03.11.2.2

Reactivity of compounds with Ge^Ge bonds

1,1-Dimethoxydigermane 2169 was obtained by the reaction of an excess of methanol with digermyne 2114. Likewise, treatment of digermyne 2114 with an excess of water afforded 1,1-dihydroxydigermane 2170 (Eq. 576).578

ð576Þ

Digermyne 2114 reacted with 2,3-dimethyl-1,3-butadiene through [1 +4] cycloaddition to give germylene 2171. However, when an excess of 2,3-dimethyl-1,3-butadiene was used, digermane 2173 was obtained. The reaction of germylene 2171 with excess methanol afforded a digermane 2172 (Scheme 202).578.

Scheme 202

The reactions of digermyne 2115 with tolan and 3-hexyne afforded 1,4-digermabenzene derivatives 2174 and 2175 through double cycloaddition reactions and GedGe bond cleavage (Eq. 577).596

Organometallic Compounds of Germanium

405

ð577Þ

The reaction of digermyne Ar0 Ge^GeAr0 2176 (Ar0 ¼ C6H3-2,6-(C6H3-2,6-iPr2)2) with cyclic olefins at room temperature showed CdH bond activation and formation of dehydroaromatized products. Its reaction with two equiv. of cyclopentadiene afforded a cyclopentadienyl anion containing species 2177 along with hydrogen gas. The same species was obtained in the reaction of compound 2176 with two equiv. of cyclopentene along with other products. Furthermore, the reaction of compound 2176 with cyclohexadiene afforded a mixture of germanorbornene 2178, (Ar0 GeH)2, and benzene (Eq. 578).597 The nature of the carbocyclic ring systems attached to the germanium atoms are unambiguously established through the X-ray structure of compound 2178 (Table 78 and Fig. 57). Table 78

Selected bond lengths and angles of compound 2178.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-C1 Ge1-C4 Ge1-C7 Ge1-C13

1.9970(18) 1.9937(19) 1.975(2) 1.9909(17)

C4-Ge1-C1 C11-C12-C7 C12-C11-C10

77.99(8) 119.5(16) 127.9(9)

Fig. 57 Molecular structure of compound 2178.

406

Organometallic Compounds of Germanium

ð578Þ

The reaction of digermyne 2176 with two equiv. of mesitylisocyanide afforded adduct 2179 featuring two Lewis bases. However, in its reaction with tert-butylisocyanide, an adduct (2180) with only one isonitrile coordinated to one of the germanium atoms was isolated (Eq. 579).598

ð579Þ

The reaction of digermyne 2176 with silver hexafluoroantimonate at low temperature resulted in digermyne metal cation adduct 2181. By contrast, the reaction of digermyne with silver hexafluoroantimonate at room temperature afforded adduct 2182 (Eq. 580).599

ð580Þ

Digermyne 2176 reacted with two equiv. of group 6 transition metal carbonyls M(CO)6 (M ¼ Cr, Mo, and W) upon irradiation with UV light, undergoing Ge^Ge bond cleavage to afford metallogermylene complexes 2183, 2184, and 2185, respectively (Eq. 581).600

ð581Þ

The reaction of digermyne 2176 with 2-methyl-nitrosobenzene afforded oxo-imido bridged germanium diradicaloid 2186 that is unsymmetrical and has a singlet ground state. Compound 2186 on atmospheric contact afforded bis-m-oxo-germanium derivative 2187 (Eq. 582). Compound 2176 upon reaction with two equiv. of oxygen gave bis-m-oxo-germanium derivative 2188 containing an Z1,Z1-m-peroxo linkage (Eq. 583).601

Organometallic Compounds of Germanium

407

ð582Þ

ð583Þ

Treatment of digermyne 2176 with COT in diethyl ether afforded the kinetic product, germanium(II) inverse sandwich compound 2189 (Eq. 584); however in toluene, the thermodynamic product, tetracyclic digermane 2190 was obtained (Eq. 585). Interconversion between compounds 2189 and 2190 was also shown to be possible (Eq. 586).602

ð584Þ

ð585Þ

ð586Þ

10.03.11.2.3

Reactivity of cyclic compounds with Ge]Ge bonds

10.03.11.2.3.1 Reactivity of three-membered cyclic compounds with Ge]Ge bonds The reaction of heavy cyclopropenes 2125 and 2126 with dichloromethane afforded heavy cyclobutanes 2191 and 2192 featuring GedCl bonds through ring enlargement reaction (Eq. 587).583

408

Organometallic Compounds of Germanium

ð587Þ

10.03.11.2.3.2 Reactivity of four-membered cyclic compounds with Ge]Ge bonds The reactions of 1,2-digermacyclobutene 2128 with an excess of elemental sulfur or selenium afforded 5,6-dithia- and 5,6-diselena1,4-digermabicyclo[2.1.1]hexanes 2193 and 2194, respectively (Eq. 588).603

ð588Þ

Treatment of 1,2-digermacyclobutene 2128 with excess ethylene at room temperature and high pressure resulted in the formation of bis(germiranyl)ethane 2195 as the kinetic product; compound 2195 can be reconverted back to compound 2128 when the ethylene pressure is released. By contrast, the reaction of compound 2128 with ethylene at low temperature and ambient pressure gave the thermodynamic product 1,4-digermabicyclo[2.2.0]hexane 2196 (Scheme 203).585

Scheme 203

Reduction of tetragermetene 2133, which bears two chlorides, with KC8 gave tetragermacyclobutadiene dianion 2197 as its potassium salt587. Compound 2197 reacted with CpCoI2(PPh3) and half an equiv. of [Ru(CO)3Cl2]2 to yield the cobalt sandwich complex [Z4{(tBu2MeSi)4Ge4}]CoCp 2198 and ruthenium half-sandwich complex [Z4{(tBu2MeSi)4Ge4}]Ru(CO)3 2199,604 respectively. The reaction of compound 2133 with Na2[Fe(CO)4] yielded iron half-sandwich complex [Z4-{(tBu2MeSi)4Ge4}] Fe(CO)3 2200 (Scheme 204). Single crystal X-ray diffraction studies confirmed the Z4 mode of attachment of the Ge4 four-membered ring to the iron atom in compound 2200 (Table 79 and Fig. 58).

Organometallic Compounds of Germanium

Scheme 204

Table 79

Selected bond lengths and angles of compound 2200.

Atoms

Bond lengths [A˚ ]

Atoms

Bond angles [  ]

Ge1-Ge2 Ge2-Ge3 Ge3-Ge4 Ge1-Ge4 Ge1-Fe Ge2-Fe

2.3709(8) 2.3541(8) 2.3614(8) 2.3743(8) 2.6247(10) 2.5390(10)

Ge1-Ge2-Ge3 Ge2-Ge3-Ge4 Ge3-Ge4-Ge1 Ge4-Ge1-Ge2

90.62(3) 89.70(3) 90.36(3) 88.99(3)

Fig. 58 Molecular structure of compound 2200.

409

410

Organometallic Compounds of Germanium

The reaction of 1,2-digermacyclobutadiene 2137 with an excess of elemental sulfur gave a complex mixture of products, which upon reaction with P(NMe2)3 formed germanium containing polysulfides 2201 and 2202 (Eq. 589). The reaction of compound 2137 with excess selenium gave a mixture of polyselenides 2203 and 2204. Heating this mixture in the presence of P(NMe2)3, converted 2203 to 2204 through deselenization (Eq. 590).603 The reaction of compound 2137 with GeCl4 produced 1,2-digermacyclobut-3-ene 2205 with GedCl bonds (Eq. 591).589

ð589Þ

ð590Þ

ð591Þ

The reaction of 1,2-digermacyclobutadiene 2137 with phosphine selenide afforded 2,5-digermaselenophene 2206. Treatment of compounds 2206 and 2137 with excess elemental selenium afforded a mixture of germa-selena heterocycles 2203 and 2204. Small molecule (hydrogen, acetylene, and trimethylsilylacetylene) activation by digermaselenophene 2006 gave the corresponding addition products 2007, 2008, and 2009, respectively (Scheme 205).605

Organometallic Compounds of Germanium

411

Scheme 205

The reactions of 1,2-digermacyclobutadiene 2137 with tolan and 2-hexyne afforded 1,4-digermabenzene derivatives 2174 and 2210, respectively (Eq. 592).596

ð592Þ

10.03.11.2.3.3 Reactivity of six-membered cyclic compounds with Ge]Ge bonds The reaction of 1,2-digermabenzene 2138 with an excess of elemental sulfur and P(NMe2)3 at high temperature gave 7,8-dithia1,6-digermabicyclo[4.1.1]octa-2,4-diene 2211 (Eq. 593).603

412

Organometallic Compounds of Germanium

ð593Þ

1,4-Digermabenzene 2175 has been shown to activate various small molecules, such as acetylene, carbon dioxide, and hydrogen to afford the corresponding addition products 2212, 2213, and 2214, respectively; compound 2213 on heating regenerates compound 2175 (Scheme 206).596

Scheme 206

10.03.12

Conclusion

In summary, this chapter has captured the progress of organogermanium chemistry during the review period mentioned at the outset. The synthesis and reactivity of germanium compounds with Ge-H, Ge-alkali metal, and Ge-group 13/14/15/16/17 element bonds were systematically presented. The chemistry of multiply bonded systems with Ge]C, Ge]Ge, Ge]E, and Ge^Ge bonds was discussed (E ¼ chalcogen). While offering the developments that occurred with compounds containing germanium and transition metals (M), systems with GedM single, Ge]M double, and Ge^M triple bonds were included. The advancements on acyclic and cyclic oligogermanes were described; germanium-containing polymers synthesized through various polymerization methods were discussed along with their applications. Under the low-valent germanium chemistry, almost all its branches, such as acyclic and heterocyclic germylenes, bisgermylenes, pincer ligand stabilized germylenes, germylenes containing donor arms, carbene stabilized germylenes, air and water stable germylenes, germylones, germylenes with anionic/cationic charge, and germylene based radicals have been covered thoroughly. Details about the emerging catalytic applications using germanium compounds for various organic transformations were delineated. Finally, it is almost certain that organogermanium compounds’ synthesis, reactivity, and applications will increase manifold in the years to come; advances in novel ligand design, synthetic methodology, and handling techniques will aid the development process.

Organometallic Compounds of Germanium

413

Acknowledgments The authors thank Vivek Kumar Singh and Prakash Chandra Joshi for their help in drawing the Schemes and Equations. Hemant Kumar is thanked for his help in making Figures. For proofreading, the authors thank Dr. Priyanka Kundu. Smitkriti and Shubham are thanked for their contributions in referencing. S.N. thanks the SERB, DST, New Delhi, India, for funding (EMR/2017/005519).

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Organometallic Compounds of Germanium 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. 448. 449. 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. 475. 476. 477. 478. 479. 480. 481. 482.

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Organometallic Compounds of Germanium 553. 554. 555. 556. 557. 558. 559. 560. 561. 562. 563. 564. 565. 566. 567. 568. 569. 570. 571. 572. 573. 574. 575. 576. 577. 578. 579. 580. 581. 582. 583. 584. 585. 586. 587. 588. 589. 590. 591. 592. 593. 594. 595. 596. 597. 598. 599. 600. 601. 602. 603. 604. 605.

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D.; Sasamori, T.; Nagase, S.; Tokitoh, N. Angew. Chem. Int. Ed. 2018, 57, 3499–3503. Peng, Y.; Fischer, R. C.; Merrill, W. A.; Fischer, J.; Pu, L.; Ellis, B. D.; Fettinger, J. C.; Herber, R. H.; Power, P. P. Chem. Sci. 2010, 1, 461–468. Sekiguchi, A.; Ishida, Y. Phosphorus Sulfur Silicon Relat. Elem. 2011, 186, 1317–1322. Lee, V. Y.; Yasuda, H.; Ichinohe, M.; Sekiguchi, A. J. Organomet. Chem. 2007, 692, 10–19. McNeice, K.; Lee, V. Y.; Sekiguchi, A. Organometallics 2011, 30, 4796–4797. Sasamori, T.; Sugahara, T.; Agou, T.; Sugamata, K.; Guo, J. D.; Nagase, S.; Tokitoh, N. Chem. Sci. 2015, 6, 5526–5530. Sugahara, T.; Guo, J. D.; Sasamori, T.; Nagase, S.; Tokitoh, N. Chem. Commun. 2018, 54, 519–522. Lee, V. Y.; Ito, Y.; Yasuda, H.; Takanashi, K.; Sekiguchi, A. J. Am. Chem. Soc. 2011, 133, 5103–5108. Lee, V. Y.; Takanashi, K.; Kato, R.; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. J. Organomet. Chem. 2007, 692, 2800–2810. Sugahara, T.; Tokitoh, N.; Sasamori, T. Inorganics 2017, 5, 4–11. Tashkandi, N. Y.; Cook, E. E.; Bourque, J. L.; Baines, K. M. Chem. A Eur. J. 2016, 22, 14006–14012. Hardwick, J. A.; Pavelka, L. C.; Baines, K. M. Dalton Trans. 2012, 41, 609–621. Hurni, K. L.; Baines, K. M. Chem. Commun. 2011, 47, 8382–8384. Hardwick, J. A.; Baines, K. M. Angew. Chem. Int. Ed. 2015, 54, 6600–6603. Tashkandi, N. Y.; Bourque, J. L.; Baines, K. M. Dalton Trans. 2017, 46, 15451–15457. Lee, V. Y.; McNiece, K.; Ito, Y.; Sekiguchi, A.; Geinik, N.; Becker, J. Heteroatom Chem. 2014, 25, 313–319. Sugahara, T.; Guo, J.; Hashizume, D.; Sasamori, T.; Tokitoh, N. J. Am. Chem. Soc. 2019, 141, 2263–2267. Summerscales, O. T.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2011, 133, 11960–11963. Spikes, G. H.; Power, P. P. Chem. Commun. 2007, 85–87. Wang, X.; Peng, Y.; Olmstead, M. M.; Hope, H.; Power, P. P. J. Am. Chem. Soc. 2010, 132, 13150–13151. McCrea-Hendrick, M. L.; Caputo, C. A.; Linnera, J.; Vasko, P.; Weinstein, C. M.; Fettinger, J. C.; Tuononen, H. M.; Power, P. P. Organometallics 2016, 35, 2759–2767. Wang, X.; Peng, Y.; Olmstead, M. M.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2009, 131, 14164–14165. Summerscales, O. T.; Jime, J. O. C.; Merino, G.; Power, P. P. J. Am. Chem. Soc. 2011, 133, 180–183. Sugahara, T.; Sasamori, T.; Tokitoh, N. Chem. Lett. 2018, 47, 719–722. Lee, V. Y.; Ito, Y.; Sekiguchi, A. Russ. Chem. Bull., Int. Ed. 2013, 62, 2551–2553. Sugahara, T.; Sasamori, T.; Tokitoh, N. J. Am. Chem. Soc. 2018, 140, 11206–11209.

10.04

Organometallic Compounds of Tin and Lead

Keith Izod, School of Chemistry, Newcastle University, Newcastle upon Tyne, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

10.04.1 10.04.2 10.04.2.1 10.04.2.2 10.04.2.3 10.04.3 10.04.3.1 10.04.3.1.1 10.04.3.1.2 10.04.3.1.3 10.04.3.2 10.04.3.3 10.04.4 10.04.4.1 10.04.4.2 10.04.4.3 10.04.4.4 10.04.5 10.04.5.1 10.04.5.2 10.04.6 10.04.6.1 10.04.6.2 10.04.6.3 References

Introduction and scope Tin(IV) and Pb(IV) compounds Organostannanes and -plumbanes R(4-n)EXn Catenated compounds and clusters Stannylium and plumbylium cations R3E+ Tin(II) and lead(II) compounds Stannylenes and plumbylenes R2E R ¼ alkyl, alkenyl R ¼ aryl R ¼ cyclopentadienyl Methanediides (R2C)E Stannate and plumbate anions R3E− Compounds with E]E and E^E multiple bonds Bonding models and theoretical studies Distannenes and diplumbenes R2E]ER2 Distannynes and diplumbynes RE^ER/RE-ER Stannaethenes R2E]CR2 Unsaturated heterocycles Stannoles and plumboles Stannabenzenes, plumbabenzenes and related compounds Compounds with MdE, M]E and M^E bonds Complexes of stannylenes and plumbylenes R2E Metallostannylenes and plumbylenes Complexes with M^E triple bonds (stannylidynes and plumbylidynes)

422 423 423 432 436 437 437 437 440 448 449 452 452 453 454 457 461 462 463 467 468 468 470 470 471

10.04.1 Introduction and scope This chapter describes recent developments in the organometallic chemistry of tin and lead. Earlier work is described in the previous editions of this volume, COMC-I, COMC-II, and COMC-III, although some seminal early discoveries are included here for context. The organometallic chemistry of tin and lead continues to develop at a rapid rate. Concurrent with a decrease in interest in organolead(IV) compounds, with the use of tetraalkyllead anti-knock fuel additives having fallen into desuetude, there has been a substantial increase in interest in low oxidation state organotin and organolead compounds, due to their unusual structural features and unique reactivities. This change in research emphasis is reflected in this chapter: while significant developments in Sn(IV) and Pb(IV) chemistry are discussed, these are fewer than the numerous advances in low oxidation state organometallic tin and lead chemistry. A comprehensive overview of fundamental Sn(IV) and Pb(IV) organometallic chemistry is given in COMC-III; this is not repeated in depth here, but significant details are included where appropriate. This chapter concentrates on well-characterized complexes, especially where structures have been unambiguously confirmed by single crystal X-ray diffraction; however, important chemistry where this is not the case is included where appropriate. Complexes where the organotin or -lead fragment is merely a non-functional substituent and plays essentially no part in the chemistry of the complex (e.g. Me3Sn- or Ph3Sn-substituted compounds) are not covered. However, selected compounds where tin or lead is exclusively bonded to silicon-based substituents, which are not strictly organometallic compounds, are included for context. The many and varied applications of organotin reagents in organic synthesis (e.g. in the Stille reaction1) also lie without the scope of this chapter. Early developments in organotin chemistry have been recently reviewed.2 While interest in tetravalent organotin and -lead compounds has waned somewhat in the last two decades, two areas that are still receiving considerable attention are the use of organotin(IV) compounds in medicine, particularly as cancer chemotherapeutics and anti-virals, and studies of the toxicity of organotin(IV) pollutants, which are now widespread in the natural environment, due to their historical use as agrochemicals and marine anti-fouling agents. Interested readers are directed to the numerous excellent review articles on these topics.3–19

422

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00149-9

Organometallic Compounds of Tin and Lead

423

10.04.2 Tin(IV) and Pb(IV) compounds 10.04.2.1 Organostannanes and -plumbanes R(4-n)EXn The various synthetic routes to organotin(IV) compounds were comprehensively summarized in COMC-III. The synthesis of organostannanes is also included in a wider review covering both synthetic routes to stannanes and their use in a variety of applications, such as the synthesis of diarylketones and tertiary phosphines.20 Typical routes used for the synthesis of Sn(IV)dC bonds include: (i) metathesis reactions between organometallic reagents such as RLi, RMgX, R3Al or RZnX and a tin halide, triflate, etc. (Eq. 1), (ii) metathesis reactions between alkali metal stannates R3SnM (M ¼ Li, Na, K) and an electrophile (Eq. 2), (iii) hydrostannation (i.e. the addition of a SndH bond across a C]C or C^C bond; Eq. 3), (iv) carbostannation (i.e. the addition of a SndC bond across a C]C or C^C bond; Eq. 4), or (v) oxidative addition of an organic halide to a Sn(II) compound (Eq. 5). Organolead(IV) compounds are accessible by similar routes. +

MR

Sn

R

+

MX

ð1Þ

Sn M +

RX

Sn

R

+

MX

ð2Þ

Sn

Sn

C R2

Sn

C R2

X

CHR2 R2C=CR2

CR′R2 R2C=CR2

Sn

+

Sn

H

Sn

R′

R

R

R

R

Sn

RX

CHR Sn

C R

Sn

C R

ð3Þ

CR′R

ð4Þ

R

ð5Þ

X

An unusual synthetic route involving a combined tin-mediated CdH activation and cross-coupling reaction was recently reported.21 The reaction between {(Me3Si)2N}2Sn and iodobenzene gave the corresponding oxidative addition product {(Me3Si)2N}2Sn(Ph)I. However, when the same reaction was conducted in the presence of a range of organic compounds, including cyclopentane, cyclohexane, THF, and tBuOMe, this resulted in CdH activation of the substrate and formation of the corresponding alkyl derivative {(Me3Si)2N}2Sn(R)I (1; Scheme 1; R ¼ cyclopentyl, cyclohexyl, 2-tetrahydrofuranyl, pentyl).21 The amount of CdH activation product in these reactions was maximized by performing the reaction under high dilution. R R R R

I

R

Sn

R

O

I

R

I

R

Sn

Sn

R R

I Sn

I Sn

O

R + I

Sn R Me

R R

O

tBu

I Sn R R

I Sn

Scheme 1 CdH activation of alkanes by PhI/R2Sn (R ¼ (Me3Si)2N).

R R

I Sn

O

tBu

424

Organometallic Compounds of Tin and Lead

The elimination of Me3SiF, containing the strong SidF bond, from a trisilylstannate anion provides an unusual route to the first examples of tetraacylstannanes. Thus, treatment of Sn(SiMe3)4 with KOtBu yielded the stannate complex K[Sn(SiMe3)3], which, on subsequent reaction with four equivalents of RC(O)F, gave the tetraacyls Sn{C(O)R}4 (2; R ¼ Mes, m-Xyl; Mes ¼ 2,4,6-Me3C6H2; m-Xyl ¼ 2,6-Me2C6H3).22 These compounds have the potential to act as efficient visible light photo-initiators for the polymerization of biocompatible materials and exhibit remarkably low toxicities compared to other organotin compounds.

The preparation of organotin(IV) compounds via oxidative addition is exemplified by the reactions between N-heterocyclic stannylenes and MeI (Scheme 2A).23 Treatment of the iodide products with AgOTf generated the corresponding triflates (3), in which the triflate ligands are strongly coordinated to the tin centers. The corresponding dimethyl derivatives were obtained by the reaction between Me2SnCl2 and the appropriate dilithiated diamine (Scheme 2B).

Dipp N Sn

Dipp N

MeI

Sn

N

N

Dipp

Dipp I

N

AgOTf

Sn

Me

N

Dipp

OTf Me

(a)

Dipp 3

Dipp N N

Li

Dipp N

Me2SnCl2

Sn

Li

N

Dipp

Me

(b)

Me

Dipp

Scheme 2 Synthesis of N-heterocyclic stannylene derivatives.

An unusual reaction has been observed between RSnCl3 (R ¼ Et, nBu, Ph) and two equivalents of R3Sn(OR0 ) (R ¼ Me2NCH2CH2, Me). This unexpectedly led to the unsymmetrical adduct Ph2SnCl2.Ph2Sn(OCH2CH2NMe2)2 (4), or the symmetrical dimers [R2SnCl(OMe)]2 (5; R ¼ Et, nBu).24 0

Cl Cl

Ph

O

Sn

Sn Ph

Ph

O

4

Ph

NMe2 NMe2

Cl

R Me R O Sn Sn O R Me R

Cl

5 (R = Me, nBu)

Diaryltin dichlorides can be troublesome to access by conventional metathesis reactions between aryl Grignard reagents and SnCl4 due to the formation of statistical mixtures of halide-containing products. In this regard, the sterically hindered diaryltin(IV) dichlorides R2SnCl2 (R ¼ Mes, Dep, Dipp, Trip; Dep ¼ 2,6-Et2C6H3, Dipp ¼ 2,6-iPr2C6H3, Trip ¼ 2,4,6-iPr3C6H2) have been selectively synthesized by the reaction of SnCl4 with two equivalents of the corresponding Grignard reagent RMgBr.25 The initial products of this reaction were an approximately 1:2:1 mixture of R2SnCl2, R2SnClBr and R2SnBr2; however, treatment of these mixtures with aqueous HCl led to complete halide exchange and the formation of the corresponding dichlorides in good yield. This provides a simple method for synthesizing diorganotin(IV) dichlorides with sterically demanding aryl groups. The dichlorides were treated with LiAlH4 to obtain the corresponding dihydrides R2SnH2.

Organometallic Compounds of Tin and Lead

425

In an unusual reaction, the tetraferrocenylstannane [{(MeOC(O))C5H4}Fe(C5H4)]4Sn (6) has been synthesized by heating 1-bromoferrocene-10 -carboxylic acid methyl ester over copper bronze at 130  C.26 Cyclic voltammetry indicates that compound 6 is oxidized in three steps to the tetracation.

A series of tin-containing peri-substituted acenaphthene compounds has been isolated. The reaction between 5-bromo-6-lithio-acenaphthene and PhSnCl3 gave the peri-substituted acenaphthene 7, which exhibits a Sn ⋯ Br distance of 3.340(3) A˚ .27 A similar reaction between 5-bromo-6-lithio-acenaphthene and R2SnCl2 (R ¼ Ph, PhCH2, nBu, Cl) gave a mixture of mono- and disubstituted stannanes 7 and 8 for R ¼ Ph, but only the disubstituted compounds 8 for R ¼ PhCH2, nBu, or Cl; each of 7 and 8 exhibits weak Sn ⋯ Br interactions, which span the range 3.1451(15)-3.394(5) A˚ . The reaction between 5-lithio-6-phenylchalcogeno-acenaphthene and Ph3SnCl, Ph2SnCl2, or nBu2SnCl2 gave the complexes 9, 10, or 11, respectively.28 The extent of Sn ⋯ E interaction (E ¼ S, Se, Te) was found to be greater in the more Lewis acidic chloro-substituted compounds 10 than in the fully hydrocarbon substituted 9. The 5-phosphino-6-stannyl peri-acenaphthenes 12 and 13 have also been isolated by straightforward metathesis reactions; the Sn ⋯ P distances range from 2.7032(9)-3.332(2) A˚ .29,30 Density Functional Theory (DFT) calculations suggest that the Sn ⋯ P interactions in these compounds are relatively strong, with SndP Wiberg Bond Indices (WBIs) of up to 0.35; the strength of the Sn ⋯ P interactions is supported by the observation of significant 31Pd119Sn coupling in the NMR spectra of these compounds (JPSn ¼ 740–754 Hz). The transmetallation of Pb(OAc)4 with R2Hg (R ¼6-Ph2P(O)-5-acenapthene), followed by treatment with HCl, gave RPbCl3 (14), the first monoorganolead trihalide that resists reductive elimination under ambient conditions;31 compound 14 reacted with R0 2Hg (R0 ¼ Ph, 4-MeOC6H4, 4-Me2NC6H4) to give the mixed arylplumbanes 15. A similar reaction between R2Hg and SnCl4 yielded the stannane 16.32

A series of organotin(IV) formates has been prepared by the addition of formic acid to nBu2SnH2, Cy3SnOH, (Mes)3SnOH, or (m-Xyl)3SnOH, or by a dehydration reaction between (PhCH2)3SnOSn(CH2Ph)3 and formic acid, to give [Me2Sn(OC(O)

426

Organometallic Compounds of Tin and Lead

H)2]n (17), which crystallizes with a 2-dimensional sheet structure, polymeric [(PhCH2)3Sn(OC(O)H)]n (18), and monomeric, four-coordinate (Mes)3Sn(OC(O)H) (19) and (m-Xyl)3Sn(OC(O)H) (20), along with Cy3Sn(OC(O)H) (21), which was not structurally characterized.33 Attempted thermolysis of 17, 18, and 21 left the starting materials unchanged, whereas thermolysis of 19 and 20 led to CO elimination and the formation of the corresponding tin hydroxides, rather than the expected tin hydrides. The chemistry of organotin(IV) hydrides is long established, in large part due to their application as reagents, often in combination with AIBN, for a number of useful organic transformations.34 However, there has been renewed interest in the reactions of these species, especially in their dehydrogenation. Irradiation of tBu2SnH2 in the presence of CpFeL2Me, CpFeL2Me, or CpMoL2(CO)Me (L2 ¼ (CO)2, (CO)(PPh3)) gave the distannane HtBu2Sn-SntBu2H, rather than a cyclic or polymeric oligostannane.35 It was shown that the active catalyst in this process was in situ-formed CpFe(PPh3)(SntBu2H) or CpMo(CO)3(SntBu2H); both transition metal complexes were structurally characterized. The alkyltin trihydride RSnH3 (22) was prepared by the reaction of RSnCl3 with LiAlH4 (R ¼ (Me3Si)2CH).36 Treatment of 22 with two equivalents of MeNHC in benzene led to dehydrogenation to give the stannylene RSnH(MeNHC) along with the corresponding imidazole, which were characterized by NMR spectroscopy (MeNHC ¼ tetramethylimidazol-2-ylidene). Switching the reaction solvent to n-hexane resulted in the alternative product (MeNHC)RSn-SnR(H)-SnR(H)-SnR(MeNHC) (23), which contains an unusual Sn4 chain. A further reaction between 22 and 2.5 equivalents of MeNHC gave the distannyne derivative R(MeNHC)Sn-SnR(MeNHC). Reductive elimination of hydrogen from 22 was also achieved by treatment with a large excess of Et2MeN, which gave the cyclic (RSnH)4 (24). Similar reactions between the terphenyltin hydride (ArMes)SnH3 (25) and two equivalents of MeNHC gave the corresponding stannylene adduct (ArMes)SnH(MeNHC) (ArMes ¼ 2,6-(Mes)2C6H3).37 In contrast, when the more sterically demanding iPr NHC was used in this reaction, a mixture of (ArMes)SnH(iPrNHC) and the stannylstannylene adduct (ArMes)(iPrNHC)Sn-Sn(ArMes) (H)2 (26) was observed (iPrNHC ¼ 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). Compound 26 and its MeNHC analog could be prepared in essentially quantitative yield by the treatment of 25 with 1.5 equivalents of the corresponding NHC. Reaction of 25 with half an equivalent of NHC gave the distannane (ArMes)(H)2Sn(H2)(ArMes) (27). Similar behavior has been observed in the reactions of (ArTrip)SnH3 with nitrogen bases such as DMAP and tertiary amines (ArTrip ¼ 2,6-(Trip)2C6H3).38 Me3Si SnH3 Me3Si 22

23 (R = (Me3Si)2CH, NHC =

H H ArMes Sn Sn SnH3

R

R H NHC Sn Sn R NHC Sn Sn R R H

NHC

N

ArMes

26 (ArMes = C6H3-2,6-(C6H2-2,4,6-Me3)2 NHC = ) iPr

N

N

H

H Sn

Sn Sn

R

N

)

H Sn R R H

24 (R = (Me3Si)2CH)

H H ArMes Sn Sn ArMes H H 27

iPr

25

Dehydrogenation of the stannane (ArTrip)SnH3 with the frustrated Lewis pair tBu3P/B(C6F5)3 led to a mixture of products, including [(ArTrip)SnH2(PtBu3)]+ and [(ArTrip)Sn(PtBu3)]+, along with some unreacted (ArTrip)SnH3, according to NMR spectroscopy.39 Deprotonation of ArSnH3 with strong bases, including MeLi, iPr2NLi, PhCH2K, gave the stannate complexes [ML][ArSnH2] (28) after crystallization in the presence of the appropriate co-ligand (Ar ¼ ArTrip, 2,6-(Dipp)2C6H3 (ArDipp); ML ¼ Li(THF)3, Li(TMPDA), K(cryptand[2.2.2]; TMPDA ¼ N,N,N0 ,N0 -tetramethyl-1,3-propanediamine).40 In solution, 1Hd7Li HOESY NMR spectroscopy indicated that 28 (L ¼ TMPDA) exhibited LidHdSn contacts, rather than a LidSn contact; this is consistent with the solid-state structure of this compound. The reaction between 28 (ML ¼ Li(THF)3) and [(ArTrip)EX]2 (E ¼ Ge, Pb; X ¼ Cl, Br) gave the mixed-metal compounds (ArDipp)SndGe(ArTrip)(H)2 and (ArDipp)Sn(H)2dPb(ArTrip). The reaction between (ArTrip)SnH3 and AdN^C in the presence of an excess of Et2MeN led to complete hydrogen transfer to give 29.41 In contrast, the reaction of (ArTrip) SnH3 with PhC^N under the same conditions gave the iminato complex 30.

Organometallic Compounds of Tin and Lead

427

Numerous hypervalent organotin(IV) species have been isolated. This hypercoordination may be the result of either the use of chelating alkyl or aryl ligands, or the complexation of organotin species by mono- or polydentate co-ligands. For example, halide abstraction from Me2SnCl2(DMAP) or Me2SnCl2(bipy) by AgOTf gave the six-coordinate complexes Me2Sn(OTf )2(DMAP) and Me2Sn(OTf )2(bipy) (DMAP ¼ 4-N,N-dimethylaminopyridine, bipy ¼ 2,20 -bipyridine), in which the triflate ligands strongly coordinate to the tin centers.42 This behavior contrasts with that of the corresponding Si and Ge congeners, which adopt separated ion-pair structures in the solid state. The reaction between either R2SnCl2 (R ¼ Me, Et, tBu, Ph) or Ph2PbCl2 and glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil) (H2L1) in the presence of Et3N gave the six-coordinate complexes R2EL1 (31).43 Cyclic voltammetry revealed that the redox active ligand in these complexes undergoes four one-electron redox processes. The related complexes R2SnL2 (32; R ¼ Et, Ph; L2 ¼ dianion of 4,6-ditert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzoquinone) reacted with oxygen to give paramagnetic diorganotin(IV) (R ¼ Ph) or monoorganotin(IV) (R ¼ Et) complexes of this radical anion.44 tBu N

tBu

O tBu

N

Sn

E R

O

tBu

R

O

N

tBu

R R

Dipp 32 (R = Ph, Et)

tBu

31 (E = Sn; R = Me, Et, nBu, Ph) (E = Pb, R = Ph)

The carbodithioate complexes Ph3SnC(S)SR (R ¼ CH2Ph, CH(Me)Ph, CH2C6H4-4-NO2, CH2C6H4-4-F, CH2CN, CH(Me)C(O) OMe) and (p-tolyl)3SnC(S)SR (R ¼ CH2Ph, CH(Me)Ph) have been synthesized by the reaction between R3SnNa and CS2, followed by RBr.45 These compounds are efficient chain-transfer agents for reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene and n-butyl acrylate, giving controlled molecular weights and polydispersities. Ortho-functionalized aryl groups have been used extensively as ligands for tin(IV) and lead(IV), especially C6H4-2-CH2NMe2 (ap) and C6H3–2,6-(CH2NMe2)2 (dap). The N,N-dimethylaminomethylphenyl ligand (ap) has been used to support Sn(IV) nitrates and nitrites (e.g. (ap)Ph2Sn(ONO2) and (ap)nBuSn(ONO)2),46 catecholates (e.g. (ap)2Sn(3,5-tBu2C6H2–1,2-(O)2),47 cyclopentadienyls and hydrides (e.g. (ap)Ph2SnH),48,49 halides (e.g. (ap)Ph2SnX (X ¼ F, Cl, Br, I)),49,50 sulfides (e.g. {(ap)nBuSn (m-S)}2),51 and pseudohalides (e.g. (ap)Ph2Sn(NCS)).52 The related homoleptic compound (2-RC6H4)4Sn (33; R ¼ (CH2O)2CH) has been synthesized by the reaction of four equivalents of 2-RC6H4Li with SnCl4. Hydrolysis of 33 yielded (2-RC6H4)4Sn (R ¼ CH(O)), and subsequent condensation of this latter compound with secondary amines gave (2-RC6H4)4Sn (R ¼ R0 N¼CH; R0 ¼ PhCH2, Mes, Me2NCH2CH2).53,54 In each case there are Sn ⋯ N or Sn ⋯ O interactions in the solid state, which increase the coordination number at tin to between six and eight, depending on the nature of the aryl substituent. A series of chalcogen-functionalized Sn(IV) complexes (2-MeEC6H4CH2)SnPhnCl(3-n) (34; E ¼ O, S; n ¼ 0–2) and (2-MeEC6H4CH2)2SnPhnCl(2-n) (35; E ¼ O, S; n ¼ 0–2) has been isolated.55,56 In the solid state compounds 34 and 35 exhibit Sn ⋯ O and Sn ⋯ S contacts, and these appear to persist in solution. The strength of the Sn ⋯ E interactions increased with increasing chlorine-substitution at tin, reflecting the increased Lewis acidity of the tin centers as the number of chlorine substituents increased. The meta and para isomers (3-MeOC6C4CH2)2SnCl2 and (4-MeOC6C4CH2)2SnCl2 adopted polymeric structures in the solid state due to intermolecular Sn ⋯ Cl interactions.56 Ph MeE

Sn

X X

34 (E = O, S; X = Ph, Cl)

MeE

X

Sn

X Me E

35 (E = O, S; X = Ph, Cl)

The phosphine-functionalized compound FSn(C6H4-2-PPh2)3 (36) adopted a structure in the solid state which exhibited three Sn ⋯ P contacts.57 DFT studies suggested that the three phosphorus lone pairs donated into the single SndF s -orbital. The corresponding hydride HSn(C6H4-2-PPh2)3 (37) adopted a different structure in the solid state which precluded interaction

428

Organometallic Compounds of Tin and Lead

between the three P lone pairs and the SndH s -orbital, although the Sn⋯ P distances were well within the sum of the Sn and P van der Waals radii. The chloro-, bromo- and iodo- analogs of 36 also exhibited three short Sn ⋯ P contacts, although, whereas 36 had C3 symmetry in the solid state, the heavier halogen analogs adopted a structure with C1 symmetry.58 The closely related fluorinated compounds MeSn(C6F4–2-PPh2)3 (38), RSn(C6F4-2-PPh2)3 (39; R ¼ Me, Cl) and Me2Sn(C6F4-2-2PPh2)2 (40) have also been isolated.59,60 Compounds 39 (R ¼ Me) and 40 reacted with Pd(PPh3)4 via oxidative addition of a Sn-Aryl bond to the Pd(0) center, whereas 39 (R ¼ Cl) reacted via oxidative addition of the SndCl bond.60 The alkoxy-functionalized dialkyl {Me2(iPrO) SiCH2}2SnPh2 reacted with Br2 to give {Me2(iPrO)SiCH2}2SnBr2 (41), which adopted a distorted octahedral geometry in the solid state, with two SndO contacts.61

F Sn P Ph2

P Ph PPh2 2

36

Since the publication of COMC-III the range of pincer complexes based on 2,6-disubstituted aryl rings has increased significantly. In this regard, numerous Sn(IV) derivatives of the diamino-substituted dap ligand have been isolated. Oxidation of the stannylene (dap)2Sn with air, selenium or tellurium gave the chalcogenides [(dap)2Sn(m-E)]2 (E ¼ O, Se, Te).62 The corresponding sulfide could not be isolated cleanly from the reaction between (dap)2Sn and elemental sulfur, but was successfully prepared by the reaction of (dap)2SnCl2 with Na2S. Reactions between (dap)2Sn and PhCH2SSCH2Ph, PhSeSePh, or PhTeTePh gave the compounds (dap)2Sn(SCH2Ph)2 and (dap)2Sn(EPh)2 (E ¼ Se, Te), while oxidative addition of RBr gave (dap)2SnBrR (R ¼ tBu, CH(SiMe3)2).62 Abstraction of Br from this latter compound gave the separated ion pair [(dap)2Sn{CH(SiMe3)2}][BPh4]. The dap ligand has also been used to support an intramolecularly stabilized stannonic acid derivative [(dap)Sn(OH)O]6 (42).63 Compound 42 was crystallized as an adduct with KO2CCF3 and has a Sn6O12 core. The reaction between (dap)PhSnCl2 and Li2E (E ¼ S, Se, Te) generated dimeric [(dap)PhSn(m-S)]2 and monomeric (dap)PhSn]E (E ¼ Se, Te).64 The reaction between (dap)SnPh3 and I2 gave (dap)SnPh2I, which reacted with AgX to give (dap)SnPh2X (X ¼ CF3CO2, CH3CO2, Et2NCS2).65 NMR spectroscopy indicated that the CH3CO2 fragment was only weakly bound to the tin center in CD2Cl2 solution. The distannoxanes [(dap)SnR(m-O)]2 (R ¼ nBu, Ph) reacted with CO2 to give the carbonates (dap)SnR(CO3) (43); heating 43 at 150  C for 2 h resulted in the release of CO2 to give the starting stannoxanes.66 Compounds 43 reacted with organoboronic acids RB(OH)2 to give the boroxines (dap)SnPh {(OBR)2O}, which possess a SnB2O3 core (R ¼ Ph, 4-CF3C6H4, fc).67 A similar reaction between 43 (R ¼ Ph) and two equivalents of RB(OH)2 gave the zwitterionic stannaoxidoborates (dapH)Sn{(OBR)2O}2 (44; R ¼ Ph, 4-BrC6H4, 3,5-(CF3)2C6H3, 4-CHOC6H4), accompanied by the elimination of CO2, H2O and PhH.68 The controlled hydrolysis of (dap)SnPhX2 and its ether-substituted analogs L1SnPhX2 (L1 ¼ 2,6-(MeOCH2)2C6H3; X ¼ Cl, OTf, 1-CB11H12) gave the cations [LnSnPh(OH2)X]X.69 The similar ether-substituted compounds L3SnPhnCl(3-n) (L3 ¼ 2,6-(EtOCH2)2C6H3; n ¼ 1–3) have been prepared and structurally characterized; solution and solid-state NMR studies, combined with DFT calculations, on these compounds and their OMe, OiPr, and OtBu analogs showed that the strength of any Sn ⋯ O interactions was extremely sensitive to the nature of the OR substituent.70 O

NMe2 O Sn R C O NMe2 43 (R = Ph, nBu)

Me2NH O

RB O

BR O Sn

O

R B

O

B R

NMe2

O

44 (R = Ph, 4-BrC6H4, 3,5-(CF3)2C6H3, 4-CHOC6H4)

The mixed O,C,S-donor complex L4SnPh3 (45; L4 ¼ 2-Ph2P(S)-4-tBu-6-Ph2P(O)C6H2) was prepared by the reaction between Ph3SnCl and the corresponding aryllithium.71 Abstraction of a phenyl group from 45 by [Ph3C][PF6] gave the cationic complex [L4SnPh2]PF6, which, on treatment with [PPh4]Br gave the unusual complex 46, via a CdO cleavage reaction. The triarylmethanide complex {(2-MeO-5-tBuC6H3)3C}SnCl3 (47) was synthesized by a metathesis reaction between SnCl4 and the corresponding alkyllithium; this reaction proceeds best in the presence of hexamethylphosphoramide when 1,2-dimethoxyethane (DME) is used as the solvent.72 In the solid state, the aryl rings align such that the methoxy O atoms lie close to the tin center and so the tin center may be regarded as being heptacoordinate.

Organometallic Compounds of Tin and Lead

Ph

Ph P

Ph

S

Ph P

Ph Sn Ph Ph P iPrO

S Sn

O

P iPrO

OiPr 45

429

Ph MeO

Ph

OMe MeO Sn Cl Cl Cl

O

O

47

46

A number of organotin(IV) complexes with porphyrins and related ligands have been reported since the publication of COMC-III. The N-confused porphyrin complex 48 has been prepared by the reaction of an N-confused tetraarylporphyrin with SnCl2 under reflux in pyridine, in air; the corresponding oxoporphyrin complexes 49 were isolated when the reaction was repeated, with stirring for 3 days at room temperature, rather than under reflux.73 Complex 48 was shown to bind halide ions at the outer NH site, whereas 49 were shown to bind halide ions at the tin(IV) center. The related N-confused porphyrin complex 50 was obtained by a similar method to 48.74 The bis(ferrocenyl) porphyrin complex 51 was prepared by the reaction between the tin(IV) dihalide complex and two equivalents of FcLi.75 O

NH

Ph

N

Cl

Sn

Cl

Ph

N

N Sn H2O N

N Ph

Ph 48

N

Cl

Cl

Ar

N

Ar

Ar

49 (Ar = Ph, 4-CF3C6H4, 4-MeC6H4) N

Ph

NH

Ar

Sn

Me

Cl

Ph

Ph N N

N

N Ph

fc

Sn

fc

N

N Ph

50

Ph

Ph

Ph

51 (fc = 1-ferrocenyl)

The tin(IV) corrole derivatives 52 have been isolated by the oxidative addition of alkyl halides to the corresponding anionic Sn(II) corrole complex; the methylene-bridged bimetallic complex 53 was identified by 1H and 119Sn NMR spectroscopy, but its solid-state structure was not confirmed by X-ray crystallography.76 The reaction between SnCl2Pc and excess Na[BPh4] in the presence of C60 yielded the salt [SnPhPc][BPh4], whereas the same reaction carried out in the presence of C70 gave the complex SnPhPc, which contains the Pc3-• radical trianion (PcH2 ¼ phthalocyanine).77 p-Stacking in SnPhPc led to weak antiferromagnetic coupling of the unpaired spins.

430

Organometallic Compounds of Tin and Lead

C6F5

C6F5 N N

Sn

R

C6F5 N N N Sn N

C6F5

N

H2C

N C6F5

N

C6F5

N

52 (R = Me, CH2Ph, C(O)OMe, C(O)Et)

C6F5

C6F5

Sn

N

C6F5

N

53

The chemistry of perfluroalkyl-substituted tin(IV) compounds has recently seen a surge in interest and trifluoroethyl-substituted tin compounds have recently been reviewed.78 The reactions between PhnSnCl(4-n) and the appropriate number of equivalents of in situ-generated LiC2F5 gave the compounds PhnSn(C2F5)(4-n), which were converted into the corresponding halides XnSn(C2F5)(4-n) (X ¼ Cl, Br) by treatment with gaseous hydrogen halide.79 The methyl-substituted compound Me2Sn(C2F5)2 was isolated in a similar manner from the reaction between Me2SnCl2 and two equivalents of LiC2F5. Six-coordinate complexes XnSn(C2F5)(4-n)(phen) were obtained on treatment of the appropriate perfluoroethyl-substituted stannane with 1,10-phenanthroline. The homoleptic tetraalkyl Sn(C2F5)4 was isolated as a colorless oil from the reaction of the stannate(IV) complex [Li(OEt2)n][Sn(C2F5)5] (see below) with gaseous HCl.80 Although isolated as an oil, this compound was successfully crystallized in situ in a glass capillary and its structure determined by X-ray crystallography. The reaction between BrSn(C2F5)3 and nBu3SnH in the absence of solvent gave the hydride HSn(C2F5)3 (54), along with the side-product nBu3SnBr.81 A simpler synthesis of 54, which does not require the multi-step synthesis of BrSn(C2F5)3 was provided by the protonation of [Ph4P][Sn(C2F5)3] (see Section 10.04.3.3) with triflic acid. Compound 54 was once again isolated as an oil, but was crystallized in a glass capillary. The SndH functionality in 54 is amphiphilic and so may display protic, hydridic or radical behavior; in accordance with this, over time, 54 decomposed in solution to give the distannane (F5C2)3SndSn(C2F5)3. Reactions between PhnSn(C2F5)(4-n) and anhydrous HF gave the compounds FnSn(C2F5)(4-n).HF, which reacted with 1,8-bis(dimethylamino) naphthalene to give the salts [C10H6–1,8-(NMe2)2H][F3Sn(C2F5)2(THF)] and dinuclear [C10H6-1,8-(NMe2)2H][(F5C2)3Sn(m-F)3Sn(C2F5)3].82 The adducts FnSn(C2F5)(4-n)(phen) (n ¼ 1–3) have also been isolated and structurally characterized. The heteroleptic complex (p-tolyl)3Sn(C2F5) has been prepared by the reaction of (p-tolyl)3SnCl with LiC2F5.83 Treatment of this compound with two equivalents of gaseous HCl or HBr gave the dihalides (p-tolyl)SnCl2(C2F5) and (p-tolyl)SnBr2(C2F5). The Frustrated Lewis Pair (FLP) complex (F5C2)3SnCH2PPh2 has been prepared by the metathesis reaction between (F5C2)3SnCl and LiCH2PPh2.84 This compound is unreactive toward H2 and binds CO2 reversibly, but reacted cleanly and irreversibly with PhNCO, CS2 and SO2. The perhalophenyl compounds Ar4Sn and Ar3SnCl (Ar ¼ C6F5, C6Cl5, C6-2,3-Cl2-4,5,6-F3) have been prepared through the reaction between SnCl4 and either four or three equivalents, respectively, of the corresponding aryllithium.85 Treatment of Ar3SnCl with potassium gave the distannanes Ar3Sn-SnAr3, which underwent transmetallation reactions with AuCl(PPh3) to give AuAr(PPh3) species. The tetrafluoropyridyl complexes RnSn(C5F4N)(4-n) (R ¼ nBu, Ph, p-tolyl, Cl; n ¼ 0–3) have also recently been isolated.83 Perfluorophenyl compounds are well explored, but far less is known about complexes possessing the perfluoro-biphenyl group. The perfluoro-biphenyl complex Me3Sn(C12F9) was isolated from the reaction between (C12F9)MgBr and Me3SnCl.86 The reaction between Me3Sn(C12F9) and BBr3 led to bromine-methyl exchange and the formation of Me2Sn(C12F9)Br. The complexes Ar3SnX and Ar4Sn (X ¼ Cl, Br; Ar ¼ 2,5-(CF3)2C6H3) have been prepared by the reactions of SnX4 with the appropriate stoichiometric ratio of ArLi.87 While Ar3SnX adopted straightforward tetrahedral structures in the solid state, Ar4Sn exhibited four additional Sn⋯ F interactions. Organotin fragments have been incorporated as pendant functionalities to crown ethers in order to generate heteroditopic receptors for specific cation/anion combinations. In this regard, the reaction between Ph3SnH and 15-methylene-1,4,7,10,13pentaoxa-cyclohexadecane gave the stannyl-functionalized crown ether 55.88 Treatment of 55 with Br2 or I2 gave the halogen-substituted stannanes 56 (X ¼ Br, I), which further reacted with nBu4NF, AgCl, or AgSCN to give 57 (X ¼ F, Cl, SCN); treatment of 55 with two equivalents of either HCl or I2 gave the dihalides 58 (X ¼ Cl, I). In both the solid state and in solution the tin(IV) atom in 55 is four coordinate. However, for the more Lewis acidic halide-substituted tin centers in 57 and 58 additional SndO contacts render the tin centers five- and six-coordinate, respectively. The 16-crown-5 fragment in 56 is ideal for the complexation of sodium ions, while the stannane fragment may act as a Lewis acid for binding selected anions: preliminary complexation studies showed that 56 (X ¼ Cl) formed ditopic complexes with NaNCS. The closely related trihalides 59 and the corresponding stannate salts 60 have also been shown to complex lithium salts; the selectivity of these complexes for lithium cations was attributed to the increased Lewis acidity of the tin centers, which led to strong SndO interactions.89 O

O

O

O

O

O

O

O

SnPhnX(3-n)

55 (n = 3) 56 (n = 2; X = Br, I) 57 (n = 2; X = F, Cl, SCN) 58 (n = 1; X = Br, I) 59 (n = 0; X = Cl, Br, I)

X Sn

O

O

X

X X

PPh4

60 (X = Cl, Br)

The range of crown ethers used for this application has been increased to include 13-crown-4 (61) and 19-crown-6 (62) and their mono- and dicationic analogs (63) act as receptors for lithium and potassium ions, respectively.90–93 The bis(organostannyl)

Organometallic Compounds of Tin and Lead

431

methane-substituted crown ethers 64 have also been isolated and shown to be effective receptors for alkali metal salts.90,91,94 Similarly, the bis(crown ether)-substituted stannanes 65 have been prepared and shown to be ditopic receptors for alkali metal and silver halide salts.95

Several pentaorganostannate(IV) and hexaorganostannate(IV) compounds have recently been isolated. The first isolated pentaorganostannate(IV) complex bearing five carbon substituents (66) was prepared by the reaction of spirobistannafluorene 67 with MeLi and was crystallized as its DME adduct.96 This compound is stable at room temperature and exhibits a 119Sn chemical shift of −253.0 ppm at 213 K. The pentaalkylstannate(IV) complex [Li(12-crown-4)2][Sn(C2F5)5] (68) has been isolated from the reaction between SnCl4 and five equivalents of LiC2F5; in the absence of 12-crown-4 the lithium stannate(IV) complex is not stable.80 The five-coordinate tin(IV) center in 68 adopts a distorted trigonal bipyramidal geometry in the solid state.

Sn

[Li(DME)3]

Me

Sn

67

66

[Li(12-crown-4)2]

F3CF2C

CF2CF3 CF2CF3 Sn CF2CF3 CF2CF3

68

The first examples of hexaorganostannate(IV) complexes Li2(THF)nSn(2-pyR)6 (69) were prepared by the reaction between Sn(2-pyR)4 and two equivalents of Li(2-pyR) in THF (2-pyR ¼ C5H3N-5-Me, n ¼ 2; C5H3N-6-OtBu, n ¼ 0).97 The tin centers in 69 adopt almost ideal octahedral geometries, while the lithium ions are bound by the nitrogen atoms of two adjacent pyridyl ligands. The 119Sn chemical shifts of 69 (−451 and −555 ppm for R ¼ 5-Me and 6-OtBu, respectively) lie significantly upfield of the chemical

432

Organometallic Compounds of Tin and Lead

shifts of 66 and 68. Similarly, the reaction between SnCl4 and six equivalents of 2-furyllithium yielded the hexaorganostanate(IV) complex Li2(OEt2)2Sn(fu)6 (fu ¼ 2-furyl), which adopted a structure similar to that of 69 in the solid state.98 The reaction between Sn(fu)4 and two equivalents of Li(fu) in the presence of TMEDA gave [Li(TMEDA)2][Sn(fu)5], while the reaction between Sn(fuBz)4 and two equivalents of Li(FuBz) in THF gave [Li(THF)4][Sn(fuBz)5] (fuBz ¼ 2-benzofuryl). All of the furyl and benzofuryl derivatives were unstable in solution. R

THF N

R N

Sn N

R

Li

Li N

N R

N

THF

R

R

69 (R = 5-Me, 6-OtBu)

The application of organotin(IV) compounds as precursors for solid-state materials has received some attention. The reaction between the long chain alkylstannane Me(CH2)21Sn(C^C-nBu)3 and nonporous silica led to surface modified silica via the loss of the three alkynyl groups and the formation of SidOdSndR linkages.99 Diorganotin(IV) complexes of 2-pyridylselenolates have been investigated as precursors to semiconducting thin films or nanocrystals of SnSe or SnS2.100,101 The diorganotin(IV) dithiocarbamates R2Sn(S2CNR0 2)2 (R ¼ nBu, Ph; R0 ¼ Me, nBu, n-hexyl) have been synthesized and structurally characterized.102 These compounds are single-source precursors for the deposition of orthorhombic SnS thin films by Aerosol-Assisted Chemical Vapor Deposition (AACVD). The mixed-metal complexes (TMEDA)Zn(SnR2)E3 (70; R ¼ Me, tBu, Ph; S ¼ S, Se) have been isolated and structurally characterized (TMEDA ¼ N,N,N0 ,N0 -tetramethylethylenediamine).103 Co-thermolysis of 70 with {(iPr3P) Cu}2(ECH2CH2E) (E ¼ S, Se) as a copper source gave up to 78 wt% CuZnSnS4 and up to 43 wt% Cu2ZnSnSe4; both materials are direct band gap semiconductors with potential applications in solar cell technology. The air-stable compounds Ph3SnEH3 (71; E ¼ Si, Ge) have been prepared by the reaction between Ph3SnLi and either H3Si(O3SC4F9) or BrGeH3.104 Theoretical studies indicate that these compounds should disproportionate readily to form the corresponding SnSi and SnGe stoichiometric solids, which may have applications in the synthesis of tin-rich semiconductors. R Me2 E N Zn N Me2 E

R Sn Sn

R

H

Ph E

R

Ph Ph

Sn

E

H

H

71 (E = Si, Ge)

70 (R = Me, tBu, Ph; E = S, Se)

10.04.2.2 Catenated compounds and clusters Distannanes R3Sn-SnR3 and diplumbanes R3Pb-PbR3 may be accessed by a variety of routes (see COMC-III), although the reduction of a chlorostannane or –plumbane is probably the most common. In this regard, the reaction between (ap)R2SnCl (R ¼ nBu, tBu, Ph) and an excess of potassium at −30  C gave the distannanes (ap)R2Sn-SnR2(ap) (72).48 Compound 72 (R ¼ nBu) does not react with O2 or heavier chalcogens and does not form radical species on irradiation with UV light or on treatment with TEMPO. The related reaction between (ap)nBuSnCl2 and one equivalent of potassium gave the distannane (ap)nBuSn(Cl)dSn(Cl)nBu(ap) (73), whereas the reaction between (ap)2SnCl2 and potassium gave the stannylene (ap)2Sn.105 Treatment of (ap)nBuSnCl2 with an excess of potassium (or several other reducing agents) gave the tetranuclear compound (ap)3(nBu)3Sn4Cl2 (74). In contrast to 72, compound 73 reacted with O2, S8, Se or Te to give the corresponding chalcogenides (ap)nBuSn(Cl)-E-Sn(Cl) nBu(ap) (E ¼ O, S, Se, Te).

Organometallic Compounds of Tin and Lead

Me2N R R Sn Sn R R NMe2

Me2N nBu Cl Sn Sn Cl nBu NMe2

nBu Cl NMe2 Sn

Sn Sn Cl

nBu

73

72 (R = nBu, tBu, Ph)

433

NMe2 Me2N Sn nBu

74

The unusual distannate anion salt 75 has been synthesized by reductive coupling of the fluorostannate 76 using lithium naphthalenide.106 Compound 75 represents the first example of a dianion containing a bond between two five-coordinated tin atoms. CF3 F3C

F3C

[nBu4N]

2

F3C

O

O

Sn

Sn

O

O

CF3 F3C

F3C

CF3

CF3 O Sn

[nBu4N]

F

O F3C

CF3

75

CF3

76

It has been observed that HSn(C2F5)3 (54, see above) slowly decomposes to give the distannane (F5C2)3SndSn(C2F5)3 (77) and dihydrogen. However, the difficult synthesis of 54 and the slow nature of its decomposition made this route to 77 unviable. A better route is provided by the reaction of Li[Sn(C2F5)3] with gaseous HCl at low temperature.107 Multinuclear NMR experiments indicated that, in the presence of one equivalent of halide ion, the anionic adduct [X(F5C2)3SndSn(C2F5)3]− (78; X ¼ Cl, Br, I) was formed in solution. F3CF2C F3CF2C F3CF2C

Sn Sn 77

CF2CF3 CF2CF3

CF2CF3

F3CF2C CF2CF3 X CF2CF3 Sn Sn F3CF2C CF2CF3 F3CF2C 78

An unusual route to the distannane (2-C5H4N-6-Me)3SndSn(2-C5H4N-6-Me)3 (79) is provided by the reaction between SnCl2 and Li[EtAl(2-C5H4N-6-Me)3]; this reaction involves both pyridyl transfer and oxidation of the tin center.108 The use of distannanes for the copper-catalyzed distannylation of alkynes has been reported.109 This reaction was efficient across a wide range of terminal alkynes. It was subsequently shown that a similar copper catalyst mediated the hydrostannylation of terminal alkynes in water as a protic solvent to selectively give branched alkenylstannanes.110 The phosphine-functionalized distannane (Ph2P-2-C6H4) Me2Sn-SnMe2(C6H4–2-PPh2) undergoes oxidative addition to a variety of Au(I) and Cu(I) complexes; however, X-ray crystallography suggested a significant Sn⋯ Sn interaction in the complex {(Ph2P-2-C6H4)Me2Sn ⋯ SnMe2(C6H4–2-PPh2)}CuBr.111 Long chain polystannanes have potential applications as intrinsic semiconductors and printable polymeric wires. Recent advances in this field include the synthesis of alternating polystannanes by the deamination of mixtures of dihydrostannanes and diaminostannanes.112 The dehydrocoupling of diarylstannanes (4-RC6H4)2SnH2 with Wilkinson’s catalyst gave the homopolymers [(4-RC6H4)2Sn]n for electron donating substituents (R ¼ MeO), but was unsuccessful for electron-withdrawing substituents (R ¼ CF3), giving only low molecular weight oligomeric products.113 Alternating polymers [(4-RC6H4)2Sn-nBu2Sn]n (R ¼ OMe, CF3, H) have also been prepared by deamination of (4-RC6H4)2SnH2 and nBu2Sn(NEt2)2.113 Readily modified poly(aryl)(alkoxy) stannanes have been synthesized by dehydropolymerization of functionalized dihydrostannanes using Wilkinson’s catalyst.114 119 Sn NMR spectroscopy suggested SndO hypercoordination and high stability for some of these polymers. A range of cyclic organotin oligomers containing alternating SiR2CH2 and SnR2CH2 units (e.g. 80) have been prepared by a variety of routes, including the Wurtz-type coupling of the corresponding chlorostannane compounds using sodium.115 In contrast, the oligostannanes Cl(tBu2Sn)nCl (n ¼ 1–3) were isolated from the reaction between tBuMgCl and SnCl4, depending on the solvent: THF favored the monostannane, whereas toluene and hexane favored the formation of a distannane and tristannane, respectively.116 Irradiation of the tristannane at 350 nm resulted in the formation of the distannane, while irradiation of the

434

Organometallic Compounds of Tin and Lead

distannane led to the formation of the monostannane. The reaction between tBu2SnCl2 and an excess of Mg gave the tetrastannacyclobutane (tBu2Sn)3SntBu(MgCl) (81), which could be derivatized by reaction with alkyl bromides to give the corresponding peralkyl-substituted species.117 Me2 Si

tBu

tBu tBu

Me2Sn

SnMe2 tBu

Si Me2

Sn Sn

tBu

Sn Sn MgCl tBu

tBu 81

80

Cluster compounds containing tin and lead continue to be of interest; the chemistry of tin- and lead-containing Zintl ions and their neutral analogs has recently been reviewed.118–120 The reaction of subvalent, metastable SnCl with Li[Si(SiMe3)3] in the presence of nBu3P yielded the anionic complex [Li(TMEDA)(THF)2][Sn10{Si(SiMe3)3}5], whereas the reaction between metastable SnBr and Li[Si(SiMe3)3] gave the neutral cluster Sn10{Si(SiMe3)3}6 (82).121,122 The monoanionic (THF)Li[Sn{Si(SiMe3)3}3], an oxidation product in the reaction between SnBr and Li[Si(SiMe3)3], has also been isolated.123 Further investigation of this reaction yielded the alternative oxidation product [Sn4Si{Si(SiMe3)3}4(SiMe3)2] (83), which has a butterfly-shaped Sn4 core.124 The anionic cluster species [Li(THF)2(OEt2)2][Sn8E{Si(SiMe3)3}3] (E ¼ Si, Sn) co-crystallize from the same reaction.125 The related reaction between SnBr and M[Si(SiMe3)2(SiPh3)] (M ¼ Li, K) gave the tetranuclear cluster [ClSn{Si(SiMe3)2(SiPh3)}]4.126 In a similar vein, the reaction between SnBr and Li[Ge(SiMe3)3] gave a mixture of neutral Sn10{Ge(SiMe3)3}6 and anionic [Sn10{Ge(SiMe3)3}5]− and [Sn10{Ge(SiMe3)3}4]2− species.127 Sn R3Si

R3Si

Sn Sn Sn

Sn

SiR3

Sn

Sn Sn

Sn SiR3

Sn

R3Si

R2 Si R3Si

R3Si SiR3

Sn

Sn

Sn

Sn

SiR3

SiR3

83 (R = SiMe3)

82 (R = SiMe3)

Thermolysis of [(ArTrip)Sn(m-H)]2 in toluene gave the neutral cluster (ArTrip)3Sn9.4THF (84), which possessed a tricapped trigonal prismatic closo Sn9 core.128 Related cationic clusters [(ArMes)3Sn10][ECl4]
PhMe (85; E ¼ Al, Ga) were isolated from the reaction between [(ArMes)Sn(m-Cl)]2 and KC8 in the presence of AlCl3 or GaCl3. The Sn10 framework of 85 is similar to that of 84, with an additional Sn atom bridging the four Sn atoms in two of the adjacent triangular faces.128 The related cluster [(ArDipp)2Sn7] (86) was synthesized by the reaction of a 2:1 mixture of [(ArDipp)Sn(m-Cl)]2 and SnCl2 with KC8.129 Compound 86 has a rare pentagonal bipyramidal Sn7 core with the aryl ligand bonded to the axial Sn atoms. The cluster [(ArMes)4Sn8] was shown to react with H2 to give the tetramer [(ArMes)Sn(H)]4 (87; ArMes ¼ 2,6-Mes2C6H3), the first time that such a reaction between H2 and a main group cluster had been observed.130

Organometallic Compounds of Tin and Lead

Sn

ArTrip

ArMes

Sn

Sn

Sn

Sn

Sn Sn

ArTrip

Sn Sn

Sn

Sn

Sn

ArMes

Sn ArDipp

Sn

H

ArMes ArMes

Sn

Sn

Sn Sn

ArMes

85

Sn Sn

Sn

Sn Sn

ArTrip

84

ArDipp Sn

Sn

435

Sn

H

Sn

Mes Sn Ar

Sn

Mes Sn Ar

H 87

86

H

Thermolysis of the twisted distannene (tBu2MeSi)2Sn]Sn(SiMetBu2)2 (see below) at 70  C in hexane for 24 h gave a mixture of the stable radical (tBu2MeSi)3Sn•, the cluster (tBu2MeSi)6Sn5 and the octastannacubane (tBu2MeSi)8Sn8 (88) in an approximate 65:35:5 ratio. Compound 88 may be accessed in 70% yield on thermolysis of the distannane (tBu2MeSi)2HSndSn(SiMetBu2)2H at 50  C in hexane.131 The reaction between (Trip)SnH3 and 1.5 equivalents of 1,3-diethyl-4,5-dimethylimidazol-2-ylidene gave the hexastannabenzene modification (Trip)6Sn6 (89) in low yield.132 Reductive dehydrocoupling of Ph2SnH2 with LiAlH4 gave the complexes [Li(12crown-4)2]2[Ph10Sn7] (90) and [Li(12-crown-4)2]2[Ph12Sn8] (91), depending on the stoichiometry of the reaction, after crystallization in the presence of 12-crown-4.133 The tetranuclear cluster (ArMes)4Sn4 (92) was obtained by the reaction of [(ArMes)SnCl]2 with two equivalents of Cp Co(IPr).134 Unexpectedly, in the solid state structure of 92, one of the aryl groups lies significantly out of the Sn4 plane, while the others deviate only slightly from this plane; variable-temperature 1H NMR studies indicate a dynamic exchange between these groups in solution. The high nuclearity cluster (Me3SiCH2)12Sn14Cl6 was isolated in 22% yield from the reaction between Me3SiCH2SnCl3, SnCl4 and LiAlH4.135 This cluster has crystallographic S6 symmetry in the solid state.

RSn RSn

RSn

Trip

SnR

RSn SnR

Trip SnR

Trip

Sn

Sn

Sn

Sn

Sn

SnR

Ph Sn

Trip [Li(12-crown-4)2]2

Trip

Ph Ph [Li(12-crown-4)2]2

91

Ph

Ph

Sn

Sn

Ph Ph

Trip 89

88 (R = SiMe3, SitBu2Me)

Ph Sn

90

Ph ArMes

Ph Sn Ph

Sn Sn Sn Ph Sn Sn Ph Ph Ph Ph

Sn

Sn ArMes

Sn

Sn

Sn

Sn

Ph

Ph Sn

Ph

Ph Ph

ArMes Sn

Sn

Ph Sn

ArMes

92

The neutral cluster Sn10(Trip)8 has been isolated from the reaction between the distannene (Trip)2Sn]Sn(Trip)2 and the Mg(I) compound [{CH(CMeN(2,6-iPr2C6H3))2}Mg]2.136 A similar reaction between [{CH(CMeN(2,6-iPr2C6H3))2}Mg]2 and the ditin compound RSn(Cl)2CMe2Sn(Cl)2R (R ¼ CH(SiMe3)2) gave the tetrastanna-tetrahedrane cluster 93.137 The reaction between 93 and O2 gave organotin suboxides. R

R

Me

Me

Sn

Sn

Sn

Sn

Me

Me

R

R

93 (R = CH(SiMe3)2)

436

Organometallic Compounds of Tin and Lead

10.04.2.3 Stannylium and plumbylium cations R3E+ The chemistry of stannylium and plumbylium ions has been included in several reviews and these species continue to be of interest.138–140 Typically, stannylium cations are accessed by halide abstraction from a triorganotin(IV) halide by a suitable Lewis acid or an alternative halide abstraction reagent. In this regard, the reactions between Me3SnCl or Bu2SnCl2 and either PMe3 or DPME in the presence of AlCl3 or Me3SiOTf gave cationic ([Me3Sn(PMe3)][AlCl4], [Me3Sn(DMPE)][AlCl4], [Bu2SnCl(DMPE)] [AlCl4]) and dicationic ([(Me3Sn)2(DMPE)](OTf )2, [(Bu2Sn)(DMPE)][AlCl4]2) species which may be regarded as phosphine complexes of stannylium cations or dications, or else as stannylphosphonium salts (DMPE ¼ Me2PCH2CH2PMe2).141 An alternative approach to the synthesis of stannylium ions involves the reaction of a stannylene with the source of a cationic substituent. Thus, the reaction between the stable, monomeric dialkylstannylene {CH2C(SiMe3)2}2Sn (see below) and a silylarenium borate salt gave the stannylium salts [{CH2C(SiMe3)2}2Sn(SiR3)][B(C6F5)4] (94; R ¼ Et, iPr).142 These compounds were stable for weeks in the solid state at −18  C, but decomposed rapidly at room temperature. The 119Sn NMR spectra of 94 exhibited signals at 1412 (R ¼ Et) and 1441 (R ¼ iPr). Although two H atoms from adjacent SiMe3 groups lie in close proximity to the tin centers in the solid state structure of 94, DFT calculations and Atoms-in-Molecules (AIM) analysis suggested only a very weak interaction between the tin atoms and the distal H atoms. A similar synthetic strategy has been used to obtain the aryl-substituted stannylium complexes [(2,3,4-Me3-6-tBuC6H)2Sn(SiR3)][B(C6F5)4] (95; R ¼ Et, iPr), which were characterized by NMR spectroscopy.143 In contrast, a similar reaction between (Trip)2Sn and silylarenium borate salts led to decomposition in toluene and formation of the tris(arene) complex [Sn(C7H8)3][B(C6F5)4]2 (96).

Me3Si

SiMe3 Sn

Me3Si

tBu

SiR3 [B(C6F5)4]

Sn

SiMe3

SiR3

[B(C6F5)4]

tBu

94 (R = Et, iPr)

95 (R = Et, iPr)

Sn

[B(C6F5)4]2

96

The alkene-stabilized stannylium salt [nBuSnR2][B(C6F5)4] (97; R ¼ cyclopentenemethyl) has been synthesized by the abstraction of a proton and elimination of butane from nBu2SnR2.144 DFT calculations indicate a significant interaction between the tin center and the C]C bonds in this compound. The hydride-substituted stannylium complex [(ArTrip)SnH2][Al{OC(CF3)3}4] was accessed by hydride abstraction from (ArTrip)SnH3 by [Ph3C][Al{OC(CF3)3}4].145 The 119Sn NMR spectrum of this compound exhibited a triplet centered at 88 ppm (J119SnH ¼ 2636 Hz). This compound was found to be unstable at room temperature, undergoing elimination of H2 in benzene/C6H4F2 to give the tin(II) cation [(ArTrip)Sn(Z2-C6H6)][Al{OC(CF3)3}4]. The stannylium complex [Ph3Sn(OEt2)][H2N {B(C6F5)3}2] has been prepared by protonolysis of Ph3Sn{N(SiMe3)2} with [H(OEt2)2][H2N{B(C6F5)3}2].146 A similar reaction between Me3Sn{N(SiMe3)2} and [H(NMe2H)2][B(C6F5)4] gave the cationic complex [Me3Sn(HNMe2)][B(C6F5)4].

nBu

Sn

[B(C6F5)4]

97

Treatment of {2,6-(MeOCH2)2C6H3}Ph2SnCl with Ag[1-CB11H12] gave the stannylium complexes [{2,6-(MeOCH2)2C6H3} Ph2Sn][1-CB11H12] or [{2,6-(MeOCH2)2C6H3}Ph2Sn][Ag(1-CB11H12)3], depending on the reaction stoichiometry.147 Similar

Organometallic Compounds of Tin and Lead

437

reactions between [{2,6-(ROCH2)2C6H3}Ph2Sn]OTf and Cs[Co(C2B9H11)2] gave the cationic complexes [{2,6-(ROCH2)2C6H3} Ph2Sn][Co(C2B9H11)2] (R ¼ Me, tBu). When the related reaction between {2,6-(MeOCH2)2C6H3}PhSnCl2 and Ag[1-CB11H12] was carried out in wet THF the hydroxide-bridged dication [{2,6-(MeOCH2)2C6H3}PhSn(m-OH)2SnPh{C6H3-2,6-(CH2OMe)2}] [CB11H12]2 was isolated.148 The reaction between [nBu2Sn(OH2)4][2,5-Me2C6H3SO3]2 and 1,10-phenanthroline (phen) resulted in water displacement to give [nBu2Sn(OH2)(phen)(2,5-Me2C6H3SO3)][2,5-Me2C6H3SO3].149 In contrast, treatment of [nBu2Sn(OH2)4][2,5-Me2C6H3SO3] with pyridine was found to give [nBu2Sn(m-OH)(2,5-Me2C6H3SO3)]n. The related reaction between Me2SnCl2 and AgNO3 in the presence of 1,10-phenanthroline gave cationic [Me2Sn(phen)(NO3)(m-OH)SnMe2(phen)(NO3)]NO3.150 The reaction between Ph2SnO and naphthalene-1,5-disulfonic acid led to the formation of the hydrated monoorganotin cationic complex [PhSn(OH2)3(m-OH)SnPh(OH2)3][1,5-C10H6(SO3)2]2.151 An alternative route to cationic complexes containing two tin(IV) centers was provided by the reaction between R2Sn(Cl)CMe2Sn(Cl)R2 and Na[B{C6H3-3,5-(CF3)2}4] (R ¼ CH(SiMe3)2); this gave the cation [R2Sn(m-Cl)CMe2SnR2] [B{C6H3-3,5-(CF3)2}4] (98).152 Hydrolysis of 98 gave [R2Sn(m-OH)CMe2SnR2][B{C6H3-3,5-(CF3)2}4], while treatment of 98 with ethyl acetate/water gave [R2Sn(m-OAc)CMe2SnR2][B{C6H3-3,5-(CF3)2}4]; the reaction between R2Sn(Cl)CMe2Sn(Cl)R2 and NaHSe, in the presence of air unexpectedly gave the neutral distannoxane R2Sn(m-O)CMe2SnR2. Attempts to use 98 and its derivatives as catalysts for the cleavage of phosphate esters were unsuccessful.

R R

Me

Me

Sn

Sn

Cl

R

[B{C6H3-3,5-(CF3)2}4]

R

98 (R = CH(SiMe3)2)

The tricarbastannatrane N(CH2CH2CH2)3SnCl reacted with AgBF4 in 1,2-dichloroethane to give the stannatrane cation [N(CH2CH2CH2)3Sn]BF4 (99).153 In the solid state the [BF4]− ion has a relatively short Sn ⋯ F contact. Similar, although weaker, Sn ⋯ F contacts were seen in the corresponding [SbF6]− salt, while the [B{C6H3-3,5-(CF3)2}4]− salt appeared to consist of a separated cation and anion pair in solution. The stannatrane N(CH2CH2CH2)3SnMe was shown to mediate Me transfer to benzylidene derivatives of Meldrum’s acid in the presence of B(C6F5)3.

N

Sn

[BF4]

99

10.04.3 Tin(II) and lead(II) compounds 10.04.3.1 Stannylenes and plumbylenes R2E The chemistry of diorganotin(II) and -lead(II) compounds (stannylenes and plumbylenes) has seen a significant increase in interest in recent years. The synthesis, structures and reactions of tetrylenes have been reviewed.154 The application of tetrylenes as catalysts for the mediation of a range of organic transformations has also been reviewed.155 The source of the unusually low field chemical shifts of 13C and 29Si nuclei directly bonded to lead in plumbylenes has been investigated computationally.156 The low field resonances of these nuclei were attributed to relativistic spin-orbit effects, which were traced to efficient 6p-6p lead-based orbital magnetic couplings.

10.04.3.1.1

R ¼ alkyl, alkenyl

Alkylstannylenes and dialkylstannylenes are substantially less well represented than their aryl-substituted cousins. This is, perhaps a little surprising given that the distannene {(Me3Si)2CH}2Sn]Sn{CH(SiMe3)2}2 (100) was shown to disaggregate to give the dialkylstannylene {(Me3Si)2CH}2Sn (101) in solution (Scheme 3).157 The first dialkylstannylene to remain monomeric in the solid state, cyclic [CH2C(SiMe3)2]2Sn (102) was isolated by Kira and co-workers in 1991,158 but these compounds remain something of a rarity. Typically, such species are stabilized kinetically by the use of sterically demanding alkyl substituents. However, transient Me2Sn and Ph2Sn have been prepared in solution by laser flash photolysis of unsaturated cyclic stannanes.159 These compounds decayed with second-order rate constants, with the decay accompanied by the growth of new transient absorptions corresponding to the distannene Me2Sn]SnMe2 and the stannylstannylene Ph3SnSnPh. The Lewis acid-base chemistry of Me2Sn and Ph2Sn has also been probed by laser flash photolysis methods. The complexation of these two transient compounds with a series of N- and O-donor ligands, alkenes and an alkyne proceeded rapidly and reversibly to generate the corresponding adducts.160 The stannylenes SnXY, RSnX, R2Sn (X, Y ¼ H, F, Cl, Br, I; R ¼ Me, SiH3, GeH3, SnH3) have been investigated computationally.161 For Me2Sn a neutral C2v symmetry species was not found to be a minimum on the potential energy surface. An alternative method for the stabilization of dialkylstannylenes and plumbylenes is provided by weak agostic-type B-H ⋯E interactions with remote BH3 functionalities. Thus, the reaction between [[{nPr2P(BH3}(Me3Si)CCH2]Li(THF)2]2 and one equivalent of SnCl2 provided the cyclic dialkylstannylene [{nPr2P(BH3}(Me3Si)CCH2]2Sn (103) as an approximate 1:1 mixture of rac and meso diastereomers.162 In the solid state the rac isomer exhibited two close B-H ⋯ Sn contacts, whereas the meso diastereomer

438

Organometallic Compounds of Tin and Lead

(Me3Si)2HC (Me3Si)2HC

Sn Sn

CH(SIMe3)2 CH(SiMe3)2

100

(Me3Si)2HC Sn

2 (Me3Si)2HC 101

Scheme 3 Solution behavior of Lappert’s distannene {(Me3Si)2CH}2Sn]Sn{CH(SiMe3)2}2 (100).

exhibited a single B-H ⋯ Sn contact. Consistent with this, the 119Sn chemical shifts of rac-103 and meso-103 were 587 and 787 ppm, respectively, far upfield of the 119Sn chemical shift of 102 (2323 ppm), reflecting the pseudo-four-coordinate nature of the tin centers in 103. DFT calculations indicated that the B-H ⋯Sn interactions stabilized rac and meso-103 by 40 and 30 kcal mol−1, respectively. The corresponding dialkylplumbylene [{nPr2P(BH3}(Me3Si)CCH2]2Pb (104) was prepared by a metathesis reaction between [[{nPr2P(BH3}(Me3Si)CCH2]Li(THF)2]2 and Cp2Pb and was isolated as a 1.5:1 mixture of rac and meso diastereomers.163 Once again, X-ray crystallography and NMR spectroscopy indicated weak B-H ⋯Pb interactions in both the solid state and in solution. Similar seven-membered cyclic dialkylstannylene and plumbylene compounds rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2) (CH2)}]2E (105; E ¼ Sn, Pb) have been isolated; these compounds were isolated solely as the rac diastereomers and both exhibited two B-H ⋯ E interactions.164 The compounds rac-[(Me2RSi){Me2P(BH3)}CH]2E (106; E ¼ Sn, Pb; R ¼ Me, Ph) are isoelectronic and isosteric analogs of Lappert’s dialkylstannylene 101. However, while 101 dimerized to the corresponding distannene 100 in the solid state, 106 remain monomeric due to the stabilizing effect of the B-H ⋯ E contacts.165 Although 106 are stabilized toward dimerization by the B-H ⋯ E contacts, they behave as typical stannylenes, e.g. undergoing oxidative addition on reaction with alkyl halides.166 Unexpectedly, the reaction between the dicarbanion complex [1,2-C6H4{CHP(BH3)Cy2}2][Li(THF)n]2 and Cp2Sn gave the unusual stannyl–stannylene [[1,2-C6H4{CHP(BH3)Cy2}2]Sn]2.1.5PhMe (107), rather than the expected stannylene.167 In 107 one dicarbanion chelated a tin center, while the second bridged the two tin centers; the stannylene center was stabilized by a single agostic-type B-H ⋯ Sn contact.

Treatment of the dihydride (C2F5)2SnH2 with various donors gave the donor-stabilized stannylenes (C2F5)2SnL (L ¼ (THF)n, PMe3, (4-Me2NC5H4N)2), along with H2.168 The reaction between this compound and (C2F5)Li gave the stannate complex Li [(C2F5)3Sn]. The reactivity of the cyclic stannylene 102 has been investigated in detail. The reaction of 102 with PhI led to the corresponding oxidative addition product (Scheme 4).169 However, when this reaction was carried out in the presence of compounds containing an allylic H atom, CdH activation was observed and the corresponding allylic tin(IV) compound was isolated.169 Propargyltin compounds were also obtained by a similar synthetic strategy by the reaction of 102 with PhI in the presence of internal alkynes (Scheme 4).170 The reaction between 102 and an excess of CS2 gave a mixture of the isomeric products 108 and 109, depending on the reaction conditions. A detailed study found that 108 is the kinetic product and that it isomerized irreversibly to the thermodynamic product 109, with 102 acting as a Lewis acid catalyst.171 Compound 102 also reacted with acyl chlorides RC(O)Cl to give acyl(chloro) stannanes [CH2C(SiMe3)2]2Sn{C(O)R}(Cl) for R ¼ Ph, 4-R0 C6H4 (R0 ¼ Me, CF3), 1-Ad, tBu; in contrast, for the less sterically

Organometallic Compounds of Tin and Lead R R R

R

R Sn

R

R I

Sn

I R

R

R

R

I

R

R

R R

I

R

R

R

R

R

R Sn

R I

R Sn

R

O

R

R

R

R

I

Sn

I

Sn

+ O

R Sn

Sn

R

439

I

R

R

Scheme 4 Activation of allylic and propargylic CH functionalities by 102.

demanding RC(O)Cl (R ¼ Et, Me), this reaction led to the formation of the dichlorostannane [CH2C(SiMe3)2]2SnCl2.172 The reaction between 102 and benzyne gave the rearrangement product 110, while the reaction between 102 and two equivalents of either methyl or ethyl propynoate gave the corresponding 1:2 addition products 111.173 The reversible addition of H2 to Lappert’s stannylene 101 in the presence of a range of amines has recently been observed.174 Experimental and computational studies suggested a frustrated Lewis pair mechanism for this reaction, in which 101 acted as the Lewis acid. Me3Si

SiMe3 Sn

Me3Si

SiMe3

Me3Si

S

S

S

S

Me3Si

Me3Si 108

Me3Si

SiMe3

Me3Si

SiMe3

Sn

S SiMe3

S Me3Si

SiMe3

109 Me3Si

H

C H2

SIMe3 110

SiMe3

RO O

Sn

Sn Me3Si

Me3Si S

Sn

Sn

SiMe3

SiMe3 S

SiMe2

Me3Si

SiMe3

O RO

111 (R = Me, Et)

The addition of two equivalents of Mes P]C(Cl)Li to (NHC)SnCl2 at low temperature gave the NHC-stabilized bis(phosphaalkenyl)tin(II) compound (NHC)Sn{C(Cl)]PMes }2 (NHC ¼ {(Me)CN(iPr)}2C; Mes ¼ 2,4,6-tBu3C6H2).175 The reaction of this compound with elemental sulfur gave the compound 112, containing a Sn2S2 core. The related chlorostannylene (NHC)Sn(Cl){C(SiMe3)]PMes } was isolated from the 1:1 reaction between (NHC)SnCl2 and Mes P]C(SiMe3)Li.176 The reaction of this compound with (Me2S)AuCl led to transfer of the phosphaalkenyl substituent from Sn to Au.

440

Organometallic Compounds of Tin and Lead Cl

S P

Mes* Mes*

Sn P S

Cl S S

Cl

S P

Mes* Mes*

Sn P

S

Cl 112

The first examples of monomeric amido(alkyl)stannylenes have been prepared by the hydrometallation of alkenes by [{Ar (iPr3Si)N}SnH]2 (Ar ¼ 2,6-(Ph2CH)2–4-iPrC6H2). Reaction with terminal alkenes gave the compounds {Ar(iPr3Si)N}SnCH2CH2R (113; R ¼ H, tBu), while the reaction with cyclopentene gave {Ar(iPr3Si)N}SnCH(CH2)4.177 The corresponding reaction between [{Ar(iPr3Si)N}SnH]2 and MeC^CPh gave {Ar(iPr3Si)N}SnC(Ph)]CMe (114). Insertion of acetylenes RC^CPh (R ¼ H, Ph) into the SndB bonds of the diborylstannylene [{HC(Dipp)N}2B]2Sn gave the bis(alkenylstannylene) 115; a similar reaction between the boryl(amino)stannylene [{HC(Dipp)N}2B]{N(Dipp)(SiMe3)}Sn gave the monoalkenylstannylene 116.178 iPr Ph2HC

Ph2HC CHPh2

N

iPr3Si

iPr

CHPh2

N

iPr3Si

Sn

Sn

R

Ph

113 (R = Me, CH2tBu)

114

Me

Dipp N

R Ph

B

Dipp B

R

N

R

N

B

Dipp

Sn

Ph

Dipp

N

N Dipp

Ph Me3Si

Sn

N Dipp

N Dipp

116 (R = H, Ph)

115 (R = H, Ph)

10.04.3.1.2

R ¼ aryl

Like their alkyl-substituted cousins, aryl-substituted stannylenes and plumbylenes have been known for many years.154 Nonetheless, these compounds continue to be of significant interest, not least due to their unusual reactivities. In particular, the group of P. P. Power has made extensive use of terphenyl substituents to stabilize tetrylenes which undergo novel reactions. The flanking rings in terphenyl ligands provide efficient steric protection for low oxidation state tin and lead centers. The influence of ligand substituents on the structures and spectroscopic properties of stannylenes, plumbylenes and germylenes has recently been probed.179 In the series of compounds {2,6-(Mes)2-4-RC6H2}2E (117; E ¼ Ge, Sn, Pb; R ¼ H, SiMe3, Cl) the 4-H and 4-Me3Si derivatives adopt similar structures in the solid state, whereas the 4-Cl compounds exhibit smaller C-E-C angles, while the more hindered compounds {2,6-(Mes)2-3,5-iPr2C6H}2E (118; E ¼ Ge, Sn, Pb) exhibit larger C-E-C angles. The structural changes were found to be in agreement with bonding models based on second-order Jahn-Teller orbital interactions, which predict increased bending and greater HOMO-LUMO gaps for the more electron-withdrawing ligands. A subsequent study of the diaryls (ArTrip)2E (119; E ¼ Ge, Sn, Pb), which were isolated from metathesis reactions between EX2 (X ¼ Cl, Br) and the corresponding lithium aryl, showed that these compounds exhibited narrower C-E-C angles than less hindered diaryltetrylenes.180 These narrow angles were attributed, in part, to increased London dispersion forces between the two bulky aryl ligands. The analogous compounds (ArDipp)2E (120; E ¼ Ge, Sn, Pb) have also been synthesized by a metathesis route,181 while the reaction between PbBr2 and two equivalents of either [2,4,6-Ph3C6H2]Li or [2,6-(1-naphthyl)2C6H3] gave the corresponding plumbylenes [2,4,6-Ph3C6H2]2Pb and [2,6-(1-naphthyl)2C6H3]2Pb.182

Organometallic Compounds of Tin and Lead

441

R iPr iPr Mes Mes

Mes Mes

E

Mes

E

Mes

Mes

Mes iPr iPr

R

118 (E = Ge, Sn, Pb)

117 (E = Ge, Sn, Pb; R = H, SiMe3, Cl)

Trip Trip

Dipp Dipp

E

Trip

E

Dipp Trip

Dipp

119 (E = Ge, Sn, Pb)

120 (E = Ge, Sn, Pb)

The relationship between the frontier orbitals of transition metal complexes and certain low oxidation state main group compounds has been elegantly described by Power and others.183–185 For tetrylenes the donor and acceptor orbitals are the tetrel lone pair and the vacant p-orbital at the tetrel center (Fig. 1). The first observation of the activation of H2 by a tetrylene was reported by Power in 2008.186 The reaction between 120Sn and H2 led to arene elimination and the formation of the hydride [(ArDipp)Sn(m-H)]2 (121). A similar reaction between 120Sn and NH3 gave the amide-bridged dimer [(ArDipp)Sn(m-NH2)]2 (122). The closely related stannylene (ArMes)2Sn does not activate H2, but reacts with ammonia to give the amide [(ArMes)Sn(m-NH2)]2 (123); in contrast, the corresponding germylenes 120Ge and (ArMes)2Ge undergo oxidative addition on treatment with H2 or NH3 to give the hydrides Ar2GeH2 and Ar2Ge(H)(NH2).187 A similar reaction between (ArMes)2Sn and PH3 gave an approximate 7:3 mixture of the oxidative addition product (ArMes)2Sn(H)(PH2) (124) and the arene elimination product [(ArMes)Sn(m-PH2)]2 (125); this represented the first observation of P-H activation by a group 14 element compound.188 The reaction between (ArMes)2Sn and either H2O or MeOH also gave the arene elimination products [(ArMes)Sn(m-OR)]2 (126; R ¼ H, Me), rather than the oxidative addition products observed for the Ge analog.189 In contrast, the reaction between {(Me3Si)2CH}2Sn (101) and H2O or MeOH gave the oxidative addition products {(Me3Si)2CH}2Sn(H)(OH) and {(Me3Si)2CH}2Sn(H)(OMe), respectively,189 while the reaction between (ArMes)2Sn and HBF4 gave the oxidative addition product (ArMes)2Sn(H)(F) and BF3.190 The compound [(ArMes)Sn(m-OMe)]2 (126) and its bulkier analog [(ArDipp)Sn(m-OMe)]2 facilitated the dehydrocoupling of amines with pinacolborane; while the more bulky compound only facilitated the dehydrocoupling of primary amines, the less bulky analog mediated the dehydrocoupling of primary and secondary amines and even ammonia.191 ArDipp Sn ArDipp

H H

ArDipp

Ar

ArDipp

Ar

Sn

Sn

121

ArMes Sn ArMes

124

H PH2

N H2

Ar Sn Ar

122 (Ar = ArDipp) 123 (Ar = ArMes)

ArMes Sn ArMes

H2 N

H2 P P H2 125

ArMes

ArMes

ArMes

ArMes

Sn

Sn

R O O R

ArMes Sn ArMes

126 (R = H, Me)

442

Organometallic Compounds of Tin and Lead

While the reaction between (ArMes)2Sn and Na[PCO] did not proceed at room temperature and, at elevated temperatures, gave inseparable mixtures of compounds, the reaction of (ArMes)2Sn with [Na(18-crown-6)][AsCO] proceeded at room temperature (albeit rather slowly) to give [Na(18-crown-6)][(ArMes)3Sn2As2] (127) the anion of which contains a Sn2As2 butterfly-shaped core.192 DFT studies suggested that the initial decarbonylation reaction formed Na[(ArMes)2Sn]As] (128), which underwent rearrangement and dimerization to form the final product; intermediate 128 was isolated in the solid state as co-crystals with the adduct [Na(18-crown-6)][(ArMes)2Sn(AsCO)] (129). Controlled photolysis of an equimolar mixture of (ArMes)2Sn and [Na(18-crown-6)][AsCO] gave metastable [Na(18-crown-6)][(ArMes)Sn]As(ArMes)] (130).192 While the reaction between (ArMes)2Ge and isonitriles or CO gave adducts or insertion products, no reaction was observed between (ArMes)2Sn and these substrates.193

Fig. 1 Molecular orbitals involved in the activation of H2 by a tetrylene.

The stannylene (ArMes)2Sn inserted into the MedZn bond of dimethylzinc to give (ArMes)2Sn(Me)ZnMe, (131).194 In contrast, the reaction of (ArMes)2Pb with Me2Zn gave the ligand exchange product (ArMes)ZnMe. The reaction of (ArMes)2Sn with Me2Zn was shown to be reversible in hydrocarbon solution at room temperature. A similar reaction between (ArMes)2Sn and Me3Ga gave the insertion product (ArMes)2Sn(Me)(GaMe2) (132).195 In contrast, the reaction between (ArMes)2Sn and Me3Al gave the [1.1.1]-propellane analog Sn2{Sn(Me)(ArMes)}3 (133) in low yield. A similar reaction between the plumbylene (ArMes)2Pb and either Me3Al or Me3Ga gave the diplumbene (ArMes)(Me)Pb]Pb(Me)(ArMes). ArMes Sn ArMes

ArMes

Me Zn

Me

131

Sn ArMes

Me Ga

132

Me Me

ArMes Me

Sn

Sn Sn 133

ArMes Sn Sn

Me ArMes Me

The stannylenes 119Sn or 120Sn underwent reactions with ethylene at 60  C to give the alkyl-aryl stannylenes {(2,6-(Ar)2C6H3} SnCH2CH2C6H3-2,6-Ar2 (134; Ar ¼ Dipp, Trip).196 DFT calculations indicated that a migratory insertion mechanism was energetically favored over a pathway involving homolytic cleavage of a SndC bond. Very recently, it has been shown that 120Sn underwent CdH bond metathesis reactions with toluene, m-xylene, or mesitylene to give the distannenes (ArDipp)(ArCH2)Sn] Sn(CH2Ar)(ArDipp) (135; Ar ¼ Ph, 3-MeC6H4, 3,5-Me2C6H3), along with one equivalent of ArDippH.197 EPR spectroscopic studies of this reaction suggested an intermediate (ArDipp)Sn• radical species. Ar

ArDipp

Organometallic Compounds of Tin and Lead

443

The reaction between Ph3P]Te and the hindered stannylene (Bbt)(Titp)Sn (Bbt ¼ 2,6-{(Me3Si)2CH}2–4-{(Me3Si)3C}C6H2; Titp ¼ 2,6-(2,4-iPr2C6H3)2C6H3) gave the stannanetellone (Bbt)(Titp)Sn]Te, which contains a formal Sn]Te double bond.198 The corresponding stannanethione (Bbt)(Titp)Sn]S has also been reported.199 A series of heteroleptic dimeric terphenyltin hydrides [ArSn(m-H)]2 (136; Ar ¼ 2,6-(Dipp)2–4-RC6H2; R ¼ H, MeO, tBu, SiMe3), was prepared by the reaction between the corresponding aryltin(II) chloride precursors and LiBH4 or tBu2AlH, or by the reaction between the aryltin(II) amides ArSn(NMe2) and BH3.THF.200 Compounds 136 crystallize as orange solids with a centrosymmetric dimeric structure. In contrast, a similar reaction between the more hindered [ArSnCl]2 (Ar ¼ 2,6-(Trip)2–3,5-iPr2C6H) and tBu2AlH gave the unsymmetrical compound ArSnSn(H)2Ar (137). The SndSn distance in 137 is 2.9157(10) A˚ , and the presence of both a divalent and tetravalent tin center in this compound was confirmed by Mössbauer spectroscopy. The symmetrical structures for 136 and the unsymmetrical structure for 137 predominate in solution. In contrast to the foregoing, treatment of the less sterically hindered [(ArMes)SnCl]2 with two equivalents of tBu2AlH gave the tetrameric cluster [(ArMes)SnH]4 (87), which may also be accessed by hydrogenation of the Sn8 cluster [(ArMes)4Sn8] (see Section 10.04.2.3).130 The closely related monomeric organotin(II) chloride (ArMes∗)SnCl reacted with tBu2AlH to give the unsymmetrical dihydride (ArMes∗)SndSn(H)2(ArMes∗) (ArMes∗ ¼ C6H3-2,6-(C6H2-2,4,6-tBu3)2).201

An alternative route to diorganostannylenes and organotin(II) hydrides is provided by the dehydrogenation of organotin(IV) di- or trihydrides by an NHC. Thus, treatment of Ar2SnH2 with two equivalents of 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (NHC) gave the stannylene-NHC adduct Ar2Sn(NHC) (Ar ¼ Mes, ArMes), whereas reactions between ArSnH3 and two equivalents of NHC gave the monomeric adduct (ArTrip)Sn(H)(NHC).132 A similar reaction between ArSnH3 and two equivalents of either 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene or 1,3-dimethyl-4,5-dimethylimidazol-2-ylidene gave the hydrides (ArMes)Sn(H) (NHC).37 The reaction between (ArMes)SnH3 and 1.5 equivalents of either NHC gave the stannylstannylene product (ArMes)Sn(NHC)dSn(H)2(ArMes) (138), while the reaction between (ArMes)SnH3 and 0.5 equivalents of NHC gave the distannane (ArMes)Sn(H)2dSn(H)2(ArMes) (139). Similar dehydrogenations were also observed on treatment of ArSnH3 with 4-Me2NC5H4N (DMAP).37 The reactions between (ArTrip)Sn(H)(NHC) or (ArMes)Sn(H)(NHC) and Lewis acids such as B(C6F5)3 or [CPh3]+ led to hydride abstraction and the formation of the corresponding [ArSn(NHC)]+ cationic species, according to detailed NMR studies.202 Mes NHC Mes Sn

Mes Sn H

H

Mes

Mes

H

H Sn

Sn H

Mes

Mes

138

139

H

Mes

The first organolead(II) hydride [(ArTrip)Pb(m-H)]2 (140) was synthesized by the reaction between the plumbylene (ArTrip)Pb(CH(Ph)PPh2) and catecholborane at −40  C.203 This compound crystallized as a symmetrical dimer, but was unstable at room temperature, decomposing to the diplumbyne (ArTrip)PbdPb(ArTrip) and H2 within 2 h. Treatment of 140 with 1,3-dimethyl-4,5-dimethylimidazol-2-ylidene gave the adduct (ArTrip)Pb(H)(NHC). Subsequently, it has been shown that the hydrides [ArPb(m-H)]2 (140; Ar ¼ ArTrip, ArMes) may be accessed by the reaction between [ArPbBr]2 and two equivalents of tBu2AlH at low temperature.204 The latter compound decomposed at room temperature to give the plumbylene (ArMes)2Pb, with concomitant deposition of metallic lead.

444

Organometallic Compounds of Tin and Lead

The aryllead hydride 140 (Ar ¼ ArTrip) reacted with phenylacetylene or 1,1-dimethylallene to give the arylalkenylplumbylene (Ar )Pb(CH]CHPh) and the arylallylplumbylene (ArTrip)Pb(Z3-CH2CHCMe2), respectively.205 The related reactions between [(ArTrip)Sn(m-H)]2 or [(ArDipp)Sn(m-H)]2 and two equivalents of diphenylacetylene gave the alkenyl stannylenes ArSn{C(Ph)] C(H)Ph}, whereas the reaction between [(ArTrip)Sn(m-H)]2 and phenylacetylene gave the alkenyl-bridged distannane (ArTrip)Sn(H){C(H)]C(Ph)}Sn(H)(ArTrip) (141), while the reaction between [(ArDipp)Sn(m-H)]2 and phenylacetylene gave the alkenyl-bridged stannylene-stannane (ArDipp)Sn{C(H)]C(Ph)}Sn(H)2(ArDipp) (142).206 The unsubstituted allylplumbylenes (Ar)Pb(Z3-allyl) (143; Ar ¼ ArDipp, ArMes) and their Me-substituted analogs (Ar)Pb(Z3-CH2C(Me)CH2) have been prepared by the reaction between [(Ar)PbBr]2 and the corresponding allyl Grignard reagents; similar metathesis reactions gave the tin analogs ArSn(Z3-allyl).207,208 The reactions between (Ar)Pb(Z3-allyl) and PhNH2 gave the aryl/amido plumbylenes (ArTrip)Pb(NHPh) and [(ArMes)Pb(m-NHPh)]2.207 The primary amide derivatives [(Ar)Sn(m-NH2)]2 (122; Ar ¼ ArDipp, ArTrip) may alternatively be accessed by the reaction of [(Ar)SnCl]2 with liquid ammonia in Et2O. In contrast, the corresponding reaction between (ArTrip) PbBr and liquid ammonia gave the complex (ArTrip)PbBr(NH3); the amide derivative [(ArTrip)Pb(m-NH2)]2 (144) was prepared by a metathesis reaction between (ArTrip)PbBr and LiNH2.209 Trip

H

Ph

Sn Sn H ArTrip H ArTrip 141

H

Ph

Sn

ArDipp

HC ArDipp

Sn H 142

CH2 CH2 Pb

Pb ArTrip

Ar

H

H2 N N H2

ArTrip Pb

144

143 (Ar = ArMes, ArTrip)

The phosphanido-substituted stannylene (ArTrip)Sn{P(SiMe3)2} was unexpectedly obtained, along with one equivalent of the phosphanide oxidation product (Me3Si)2PdP(SiMe3)2, from the reaction between the tin(IV) starting material (ArTrip)SnCl3 and three equivalents of [(Me3Si)2P]M (M ¼ Li, K).210 This compound, along with the (ArTrip)Sn{P(H)Trip} analog, may be more simply synthesized by the reaction between (ArTrip)SnCl and the corresponding lithium phosphanide. The stannylene (ArTrip)Sn(Z3-allyl) reacted with an excess of the terminal alkynes RC^CH (R ¼ Ph, CH2Cy) to give the tricyclic compound 145.208 A similar reaction between (ArTrip)Sn(Z3-allyl) and benzonitrile gave the coupled product 146. The related reaction between (ArTrip)Sn(Z3-allyl) and 1-AdC^P gave the phosphaalkenyl-bridged distannane 147.211 Ph

Ph

Ad

R ArTrip

ArTrip

P

NH

HN ArTripSn

Sn

HN

Sn

ArTrip

Ad

NH

145 (R = Ph, CH2Cy)

Ph

146

Sn

ArTrip

P ArTrip 147

Ph

The reaction between [(ArTrip)Sn(m-H)]2 and an excess of ethylene gave a mixture from which the isomeric compounds (ArTrip) (Et)Sn]Sn(Et)(ArTrip) (148) and (ArTrip)SndSn(Et)2(ArTrip) (149) were selectively crystallized.212 The corresponding reaction between the less sterically hindered [(ArDipp)Sn(m-H)]2 and ethylene gave the Sn(IV) compound (ArDipp)(Et)2Sn(CH2CH2)Sn(Et) (CHCH2)(ArDipp) (150), whereas the reactions between either [(ArTrip)Sn(m-H)]2 or [(ArDipp)Sn(m-H)]2 and two equivalents of tert-butylethylene gave the symmetrical distannenes (Ar)(tBuCH2CH2)Sn]Sn(CH2CH2tBu)(Ar), while the reactions between [(ArTrip)Sn(m-H)]2 or [(ArDipp)Sn(m-H)]2 and one equivalent of tert-butylethylene gave (ArTrip)SnSn(H)(CH2CH2tBu)(ArTrip) (151) and (ArDipp)Sn(m-H)Sn(CH2CH2tBu)(ArDipp) (152), respectively. Compound 152 represents the first isolated example of a mono-hydride bridged ditetrelene.

Et ArTrip

Sn Sn

ArTrip Et

Sn Sn ArTrip

148

Et

ArTrip Et

Et Sn

ArDipp

Et 149

ArTrip H

Sn Sn CH2CH2tBu ArTrip 151

Sn 150

H

Et

ArDipp

Sn Sn CH2CH2tBu ArDipp 152

ArDipp

Organometallic Compounds of Tin and Lead

445

Similar reactions between [(ArTrip)Sn(m-H)]2 or [(ArDipp)Sn(m-H)]2 and either norbornene or norbornadiene gave the hydrostannylation products (Ar)Sn(norbornyl) or (Ar)Sn(norbornenyl) (153).213 Heating 153 at reflux in toluene led to a rearrangement of the norbornenyl ligand to give (Ar)Sn(3-tricyclo[2.2.1.02,6]heptane) (154); it was proposed that this rearrangement proceeded via the formation of tin(II) hydride intermediates. The azide [(ArDipp)Sn(m-N3)]2 has been prepared by a metathesis reaction between [(ArDipp)Sn(m-Cl)]2 and two equivalents of NaN3; photolysis of this compound gave the insertion product 155.214 Very recently the heteroleptic terphenyl/silylstannylene (ArMes)Sn(SitBu3) (156) has been isolated and structurally characterized.215 Compound 156 reacted with P4 to give (ArMes)Sn(SitBu3)(P4) (157), but storage of this compound for a few days in the light led to the reverse reaction and reformation of 156. iPr Sn 153

Sn

Ar

154

Ar

ArMes

156

iPr Dipp

P Sn

Sn tBu3Si

157

Sn N

P P P

tBu3Si

iPr

Sn

(Ar = ArDipp, ArTrip) ArMes

Dipp

N

iPr 155

The 2-N,N-dimethylaminomethylphenyl (ap) and 2,6-bis(N,N-dimethylaminomethyl)phenyl (dap) ligands have been used extensively for the synthesis of tetrylenes and the study of their reactions. The homoleptic, four-coordinate, pseudo-trigonal bipyramidal (ap)2Sn (158) was first reported in 1988.216 The reactivity of this compound toward a variety of substrates has been explored. The reaction between 158 and an excess of O2, N2O, or TEMPO gave the dimeric stannoxane [(ap)2Sn(m-O)]2; crystallization of this compound from dioxane gave the cyclic tetramer [(ap)2Sn(m-O)]4 (159), whereas crystallization from n-hexane gave the cyclic trimer [(ap)2Sn(m-O)]3 (160).217 The same compounds were isolated when (ap)2SnX2 was treated with NaOH, followed by azeotropic dehydration. Compound 159 reacted with CO2 to give the carbonate {(ap)2Sn}2(m-O)(m-CO3).218 Similar reactions between 159 and H2O, HOCH2CH2OH, or silicone grease gave the compounds {(ap)2Sn(OH)}2(m-O) (161), (ap)2Sn(OCH2CH2O) (162), and {(ap)2Sn}2(m-OSiMe2O)2 (163), respectively.218 The unusual reactivity of 158 toward siloxanes and silanols was probed in a subsequent study.219 Compound 158 reacted with two equivalents of Li in the presence of 12-crown-4 and silicone grease to give the siloxane product 163, whereas the reaction between 158 and two equivalents of TEMPO in the presence of silicone grease gave the N-oxide complex (ap){C6H4–2-CH2NMe2(O)}Sn{(OSiMe2)2O} (164). The reaction between 158 and one equivalent of Ph3SiOH gave the stannylene complex (Ph3SiO)2Sn(Me2NCH2Ph), leaving half an equivalent of 158 unreacted, whereas the reaction of 158 with two equivalents of Ph3SiOH led to the ion pair [PhCH2NMe2H][(Ph3SiO)3Sn].219 A related lead(II) boroxide complex (ap)Pb[OB{CH(SiMe3)2}2] has been prepared by the reaction of (ap)2Pb with {(Me3Si)2N}2Pb, followed by HOB{CH(SiMe3)2}2; unlike lead(II) alkoxides and siloxides, this compound is stable toward the formation of lead(II) oxo clusters.220

446

Organometallic Compounds of Tin and Lead

C

N

Me2N

Ar

NMe2

Ar

Sn

O Sn

Ar

158

Sn

NO

C

Sn

O

Sn C

C

C N N

N C Ar

O N

Sn O N

N C Sn C N O

N C O C Sn Sn C N N OH OH C N

162

161

O

Me2Si O

N C

Sn

C N

O

Sn

N C C N O Sn

N

C C 160

159 (C-N = bidentate ap Ar = monodentate ap)

O

O

N C C N O

Me2Si

Sn

O

SiMe2

O

Sn

SiMe2 O N

O C NMe2 164

163

The reaction between 158 and azobenzene in diethyl ether gave the cyclic distannane 165, whereas the same reaction in boiling THF gave the cyclometallated compound 166.221 The reaction between 158 and alkyl chlorides led to oxidation to (ap)2SnCl2, whereas the reaction between 158 and benzoyl chloride at low temperature gave the thermally unstable species (ap)2SnCl{C(O) Ph}.222 The corresponding benzoate (ap)2SnCl{OC(O)Ph} was synthesized by the oxidation of 158 with O2, followed by treatment of the product with benzoyl chloride. The germyl and silyl complexes (ap)2SnCl(EPh3) (E ¼ Si, Ge) were prepared by the reaction between 158 and Ph3ECl. Oxidation of the silyl complex with air gave the siloxide (ap)2Sn(Cl)(OSiPh3), whereas oxidation of the germyl complex under the same conditions gave the salt [Ph3Ge]4[Sn6O8].222 Treatment of 158 with BrC^N gave the oxidative addition product (ap)2Sn(Br)(CN), which underwent ligand redistribution in solution to give mixtures of (ap)2SnBr2 and (ap)2Sn(CN)2.223

C N Ar

N C

Sn

Sn

PhN

NPh

Ar

165 (C-N = bidentate ap Ar = monodentate ap)

N C

Sn Ar

N

N

Ar Sn

C N

166

The peri-naphthalene compounds C10H6-1-Sn(dap)-8-X (167; X ¼ BCy2, PPh2, Sn(dap)) have been prepared by the reaction between C10H6-1-Li-8-X and (dap)SnCl.224 A study of the bonding in these compounds concluded that there was a significant SndB interaction, but no SndP or SndSn interaction. Treatment of 167 (X ¼ PPh2) with two equivalents of BH3.SMe2 led to adduct formation with the two dap amino groups to give C10H6-1-[Sn{C6H3-2,6-(CH2NMe2[BH3])2}]-8-PPh2 (168), which exhibited a significant SndP interaction due to the increased Lewis acidity of the tin center in this compound.

The intramolecularly base-stabilized stannylene (ArTrip)Sn{CH(Ph)PPh2} (169Sn), synthesized by the reaction between [(ArTrip)Sn(m-Cl)]2 and LiCH(Ph)PPh2, may be described as a phosphastannirane.225 Compound 169Sn acts as a frustrated Lewis

Organometallic Compounds of Tin and Lead

447

pair (FLP), undergoing addition reactions with alkenes and alkynes to give the heterocycles 170 and 171, respectively. The closely related compounds (ArTrip)Sn(C6H4-2-PPh2) (172) and the peri-acenaphthene derivative 173 have also been prepared.226 While 172 acted as an FLP toward phenylacetylene, generating the corresponding 6-membered heterocycle, 173 exhibited no reaction with this substrate. Similarly, 172 reacted with 1-AdN3 with loss of N2 to give 174, but 173 did not react with this substrate. The lead(II) analog of 169Sn crystallized with a somewhat longer Pb ⋯ P distance, which is not consistent with a significant interaction between these atoms.227 However, the lead analog of 172 had a PbdP distance of 2.8941(7) A˚ , consistent with an interaction between these atoms. Unlike its tin analog, the reaction between 169Pb and 1-AdN3 gave a four-membered heterocycle (174) in which the azide unit remains intact. The reaction between 172 or its analog (ArTrip)Sn(C6H4-2-PCy2) and ECl2 (E ¼ Ge, Sn) gave the stannyl tetrylenes (ArTrip)Sn(Cl)(C6H4-2-PR2)E(Cl) (175; E ¼ Ge, Sn; R ¼ Ph, Cy).228 In contrast, the reaction between 172 and PbCl2 gave the tin(IV) product (ArTrip)Sn(Cl)2(C6H4-2-PCy2), along with elemental lead. Compounds 169Sn reacted rapidly with hexanal to give the heterocycles 176, which underwent hydroboration with pinacolborane.229 Further investigation showed that 169Sn acted as a catalyst in the hydroboration of hexanal by pinacolborane.

Tetrylenes supported by sulfone-substituted pincer ligands have recently been reported. A straightforward metathesis reaction between the bis(sulfone) C6H3-1-Li-2,6-(SO2C6H4-4-Me)2 and SnCl2 gave the chlorostannylene {C6H2-4-R-2,6-(SO2C6H4-4-Me)2} SnCl (177; R ¼ H, tBu), in which one O atom from each sulfonyl group is coordinated to the tin(II) center.230,231 The related sulfone/sulfoxide-supported chlorostannylene {C6H2-4-tBu-2-(SO2C6H4-4-Me)-6-(SOC6H4-4-Me)}SnCl (178) has also been prepared.232 In the solid state this asymmetric ligand coordinates the tin center via the aryl C atom and the sulfoxide O atom to give a five-membered chelate ring (SndO(sulfoxide) 2.269(2) A˚ ), with a much longer SndO(sulfone) distance of 2.656(1) A˚ .

Imine-functionalized ligands have also been used to support organostannylene compounds. The pincer ligand complex [C6H3-2,6-{C(Me)]N(Dipp)}2]SnCl (179) was prepared by a metathesis reaction between SnCl2 and the corresponding lithium aryl.233 The same ligand has been used to support the first example of a monomeric, terminal tin(II) hydride [C6H3-2,6-{C(Me)]N(Dipp)}2]SnH (180), which was prepared by the reaction between 179 and K[sBu3AlH] at 0  C.234 The related monoimine complexes Ar2E (181) and ArSnCl (182) [E ¼ Sn, Pb; Ar ¼ C6H4-2-C(H)]N(Dipp), (OCH2O)C6H2-2-C(H)] N(Dipp)] have also been isolated;235 attempts to reduce ArSnCl to the distannyne ArSnSnAr were unsuccessful.

448

Organometallic Compounds of Tin and Lead

Related P]O substituted pincer ligand complexes [{4-tBu-2,6-{P(O)(OiPr)2}2C6H2]EX (183; E ¼ Sn, Pb; X ¼ Cl, Br, I) were prepared by the reaction of EX2 and the corresponding lithium aryl; treatment of 183Sn (X ¼ Cl) with NaSPh gave the thiolate [{4-tBu-2,6-{P(O)(OiPr)2}2C6H2]Sn(SPh).236 For X ¼ Cl, SPh, the shortest Sn ⋯ Sn distances were in excess of 7 A˚ , whereas for X ¼ Br or I, there were short Sn ⋯ Sn distances [3.6809(4) and 3.5953(4) A˚ , for X ¼ Br or I, respectively], suggesting a significant Sn ⋯ Sn interaction, which DFT calculations indicated involved delocalization of the tin lone pairs into the Sn-X s -orbitals. The reaction between 183Sn (X ¼ Cl) and KC8 in toluene gave the bis(stannylene) 184.237 Compound 184 was found to be unstable at room temperature, disproportionating to the stannylene [{4-tBu-2,6-{P(O)(OiPr)2}2C6H2]2Sn (185), which could not be obtained by alternative reaction pathways, and elemental tin. Treatment of 183Sn (X ¼ Cl) with Na[B{C6H3–3,5-(CF3)2}4] in the presence of a strong donor such as DMAP or an NHC gave the donor stabilized organotin(II) salts [[4-tBu-2,6-{P(O)(OiPr)2}2C6H2]Sn(L)][B {C6H3-3,5-(CF3)2}4] (L ¼ DMAP, C{N(Dipp)CH}2).238 The reaction between two equivalents of the metalated ylide [Ph3PCSO2(C6H4-4-Me)]Na and SnCl2 gave the diylidestannylene [Ph3PCSO2(C6H4-4-Me)]2Sn, which exhibited Sn ⋯ O contacts in the solid state.239

Several b-diketiminate-supported organotin(II) and -lead(II) compounds have been reported. The reaction between [HC{CMeN(Dipp)}2]SnCl and MeLi gave the methylstannylene [HC{CMeN(Dipp)}2]SnMe, while a similar reaction between [HC{CMeN(Dipp)}2]PbCl and MeLi, PhLi or LiC^CPh gave the corresponding organolead(II) compounds [HC {CMeN(Dipp)}2]PbR (R ¼ Me, Ph, C^CPh).240,241 The hydride [HC{CMeN(Dipp)}2]SnH reacted with RC^CCO2R0 (R ¼ H, CO2R0 ; R0 ¼ Me, Et) to give the alkenyltin compounds [HC{CMeN(Dipp)}2]SnC{(CO2R0 )]CH2} via hydrostannylation of the C^C bond.242 The lead(II) chloride [HC{CMeN(Dipp)}2]PbCl underwent metathesis reactions with Grignard or organolithium compounds to give the alkyls [HC{CMeN(Dipp)}2]PbR (R ¼ iPr, tBu, sBu, neopentyl, CH2Ph).243 The correlation between the 207 Pb chemical shifts and the spara Hammett constant was attributed to the paramagnetic shielding contribution.

10.04.3.1.3

R ¼ cyclopentadienyl

Although Cp2Sn and its lead analog have been known for many decades, cyclopentadienyl derivatives of these elements continue to be of interest. The sterically hindered Cp00 2Pb (Cp00 ¼ C5H3–1,3-(SiMe3)2) was prepared by a metathesis reaction between PbI2 and two equivalents of Cp00 K. In the solid state the Cp00 dPbdCp00 angle is 171.0(3) suggesting significant steric repulsion between the BIG cyclopentadienyl groups.244 The Cp rings are parallel in CpBIG ¼ C5(C6H4-4-nBu)5), consistent with the predominant 2 Sn (Cp 245 This prompted a re-appraisal of the bending observed in Cp 2Sn, which was attributed by the s-character of the tin lone pair. authors to van der Waals attraction between the Cp ligands and to some extent to a small p-contribution to the tin lone pair. A series of cyclopentadienyltin(II) cations has recently been accessed using weakly coordinating anions. The reaction between Cp2Sn and two equivalents of RFOH (RF ¼ C(CF3)3) unexpectedly gave the weakly associated dinuclear system [CpSn ⋯(ORF)3Sn] (186); treatment of 186 with Li[Al(ORF)4] in toluene led to isolation of the complex [CpSn(PhMe)][Al(ORF)4] (187).246 The solvent-free compound [CpSn][Al(ORF)4] was isolated when the same reaction was carried out in pentane, but this compound could not be crystallized. The naked [CpSn]+ cation was eventually isolated from the reaction between Cp2Sn and two equivalents of Me3Si-F-Al(ORF)3, which generated [CpSn][(RFO)3AldFdAl(ORF)3] (188), along with Me3SiF and Me3SiCp. In the solid state the tin atom is bonded to the five Cp carbon atoms, but has no additional short contacts with the anion. Unexpectedly, treatment of Cp2Sn with one equivalent of [CpSn][Al(ORF)4] gave the quadruple-decker dication salt [Cp4Sn3][Al(ORF)4]2 (189).

Organometallic Compounds of Tin and Lead

449

The stannocenophane and plumbocenophane {Me2SiC5H4}2E (190; E ¼ Sn, Pb) were recently isolated.247 In the solid state 190Sn crystallized with a relatively short Sn ⋯Sn distance of 4.358 A˚ , which is close to the sum of van der Waals radii for Sn (4.34 A˚ ). The plumbocenophane 190Pb crystallized as a cyclic hexamer in which each lead atom is coordinated by a third cyclopentadienyl ring from an adjacent molecule. In contrast, the permethylated analogs {SiMe2C5Me4}2E (E ¼ Sn, Pb) crystallized as monomeric species with no short intermolecular contacts.248 Somewhat surprisingly, NHC adducts of stannocene were not isolated until 2018. In this regard, the reactions between a variety Me  ¼ C5H4Me) and the stannocenophane 190Sn have been investigated.249 of NHCs and stannocenes (Cp2Sn, CpMe 2 Sn, Cp 2Sn; Cp  It was found that weak adducts were formed for Cp2Sn and CpMe 2 Sn, but that the increased steric demands of Cp 2Sn prevented coordination of the NHC. In solution the adducts were found to be in dynamic equilibrium with their component parts; DFT studies indicated that the bonding between the NHC and the tin center in these adducts was due to attractive dispersion forces. The iminostannylene [(Z1-Cp)Sn[m-N]C{N(Dipp)CH}2]2 has been prepared by elimination of CpLi from Cp2Sn by Li[N]C {N(Dipp)CH}2]. In the solid state the imino ligands bridge the two tin centers, which are further coordinated by an Z1-Cp ligand.250 Treatment of [(Z1-Cp)Sn[m-N]C{N(Dipp)CH}2]2 with CH2Cl2 or BrCH2CH2Br gave the halide complexes [(Cl)Sn [m-N]C{N(Dipp)CH}2]2.

10.04.3.2 Methanediides (R2C)E The double deprotonation of a variety of phosphinimine-substituted alkanes and related compounds provides an excellent route to the synthesis of group 14 methanediide complexes. The metathesis reaction between two equivalents of Li[CH{iPr2P]N(SiMe3)} (2-C5H4N)] and ECl2 gave the methanediide complexes [[C{iPr2P]N(SiMe3)}(2-C5H4N)]E]2 (191; E ¼ Sn, Pb).251 The closely related compounds 192 have been prepared by the reaction between {(Me3Si)2N}2E and the corresponding neutral phosphinimine.252 A similar reaction between {(Me3Si)2N}2Sn and the phosphine-phosphinimine Ph2PCH2P(Ph)2]N(SiMe3) gave the complex [[(Ph2P){(Me3Si)N]P(Ph)2}C]Sn]2 (193).253 Treatment of 193 with Fe2(CO)9 gave the stannavinylidene complex 194; in solution 194 was found to be in dynamic equilibrium with the monomer [(Ph2P){(Me3Si)N]P(Ph)2}C]Sn and its complex [(Ph2P){(Me3Si)N]P(Ph)2}C]Sn[Fe(CO)4].

450

Organometallic Compounds of Tin and Lead

N

N SIMe3 NN E

C

Ph2P N E C

C

E

N

iPr2P

N

PiPr2

E N

SiMe3

N

Me3Si

C P Ph2

SIMe3

N

191 (E = Sn, Pb) 192 (E = Sn, Pb)

N C

Ph2P

Ph2P

PPh2

Sn N

N Fe(CO)4 C Sn

Ph2P

C

Sn

N

SiMe3

SiMe3 Ph2P

N

PPh2 Sn

PPh2

(OC)4Fe

Me3Si

C N

PPh2

Me3Si

193

194

The bis(thiophosphinoyl)methanediide complexes [[{S]P(Ph)2}2C]E]2 (195; E ¼ Sn, Pb) have been prepared by the reaction between the corresponding bis(thiophosphinoyl)methane and {(Me3Si)2N}2E.254 The tin(II) and lead(II) compounds 195 crystallized as dimers; for 195Sn the ligand acted as a C,S-chelate, whereas for 195Pb the ligand bound each lead center via the central C atom and both S atoms. In contrast, the analogous tin(IV) compound [{S]P(Ph)2}2C]2Sn (196), prepared by a metathesis reaction between SnCl4 and [[{S]P(Ph)2}2C]Mg]2, adopted a monomeric structure that may be considered a tin-containing analog of an allene.255 S

S

Ph2P C Ph2P S

PPh2

E

C

E S

PPh2

195 (E = Sn, Pb)

Ph2P Ph2P

S

S C S

PPh2 C

Sn S

PPh2

196

In contrast to the simple dimeric structure of 195, the related compound [[(Ph2P]S){(Me3Si)N]P(Ph)2}C]Sn]2 (197) crystallized as a dimeric stannavinylidene.256 Compound 197 reacted with elemental sulfur to give the mixed Sn(IV)/Sn(II) compound [[(Ph2P]S){(Me3Si)N]P(Ph)2}C]3Sn4(m3dS) (198).257 The mixed valence nature of this compound was confirmed by 119Sn NMR and Mössbauer spectroscopies. Compound 197 also reacted with AdNCO to give the stannylene 199, while the reaction between 197 and DippNCO gave the zwitterionic compound 200, which lacks a CdSn bond.258 The closely related 2-quinolinyl-substituted compounds [{(2-C9H6N)(iPr2P]S)C}E]2 (E ¼ Sn, Pb) have been isolated.259 Somewhat surprisingly, a metathesis reaction between PbCl2 and [(Et2O)Li{(2-C9H6N)(iPr2P]S)CH}]2 gave the trans-methanediide, whereas the reactions between the protio precursor (2-C9H6N)(iPr2P]S)CH2 and {(Me3Si)2N}2E (E ¼ Sn, Pb) gave the corresponding cis isomers.

Organometallic Compounds of Tin and Lead

Ph2 P N C Sn

Ph2P S Sn N

Me3Si

P Ph2

Ph2P

N

C

P Ph2

PPh2 N SiMe3

198

SiMe3

Sn N

N

Dipp

O

Ad

199

Ph2 P N

S

Ph2P

Sn

C

O

Sn

SiMe3 SiMe3

S

C

N

197

Ph2P

PPh2

Sn C N S Sn Me3Si S S Ph2P

S P Ph2

C

S

S Sn S PPh2 S C

Ph2P

SiMe3

451

200

A tin(II) 1,2-benzobis(thiophosphinoyl)methanediide complex [[C6H4-1,2-{P(Ph)]S}2C]Sn]2 (201) has been prepared by the reaction of C6H4-1,2-{P(Ph)]S}2CH2 with {(Me3Si)2N}2Sn; in the solid state 201 adopts a trans configuration.260 The reaction between 201 and one equivalent of AlCl3 led to decomplexation of one SndC bond to give the heterometallic compound 202; the reaction of 201 with two equivalents of AlCl3 gave 203. A similar reaction between 201 and GeCl4 gave the addition product 204, while the reaction between 201 and Et2Zn gave heterometallic 205.261 Ph

Ph P

Ph

P

C

S

S

Sn

P

C

Sn

Ph

S

Ph

Ph

S

P

C P

Ph

AlCl3

P Ph Cl3Ge

Cl S

P

Sn

203

Sn S

CH S

Cl

S

Ph

GeCl3 Ph

HC Sn

P

P

P Ph

204 Ph P

Ph

S

202

S

Ph

Ph P

C

AlCl3

Ph

201

Sn

C

P

P

S

S

P

S

Sn

S

P S

CH

S

S

Zn

Zn Sn Et2

Et2 Sn

S 205

S

S HC

P

Ph

P

Ph

The thiophosphinoyl-substituted lutidine C5H3N-2,6-{CH2P(S)iPr2}2 reacted with one equivalent of {(Me3Si)2N}2E (E ¼ Sn, Pb) to give the complexes [[C5H3N-2-{CP(S)iPr2}-6-{CH2P(S)iPr2}]E]2, in which one side-arm has been doubly deprotonated, while the second side-arm remains intact.262 These compounds adopt a cis configuration in the solid state. In contrast, the metathesis reaction between (Et2O)Li[C5H3N-2-{CHP(S)iPr2}-6-{CH2P(S)iPr2}] and SnCl2 gave the trichlorostannate salt [[C5H3N-2-{CHP(S)iPr2}-6-{CH2P(S)iPr2}]Sn][SnCl3].

452

Organometallic Compounds of Tin and Lead

10.04.3.3 Stannate and plumbate anions R3E− Triorganostannate and -plumbate ions are useful synthons for the synthesis of organotin and -lead compounds and have been known for many decades. Nevertheless, there is still interest in fundamental aspects of structure and bonding in these compounds. Synthetic routes to triorganostannate ions include the addition of an organoalkali metal compound to a stannylene or plumbylene, or cleavage of a suitable leaving group from a triorganostannane. The simplest triorganostannate ion [Me3Sn]− was isolated as its potassium salt by an alkoxide-induced BdSn cleavage reaction. Thus, the reaction between the borylstannane {(iPrN)2C2H4}BdSnMe3 and KOtBu in the presence of a donor ligand gave the trimethylstannate complexes [K(12-crown-4)2][Me3Sn] (206), [K(TMEDA)2(SnMe3)]n (207), [K(16-crown-6)(SnMe3)]n (208), and [K(18-crown-6)(THF)2][K(SnMe3)2] (209).263 In the solid state 207 and 208 exhibit KdSn and K ⋯ Me contacts, while in 209 the potassium cation is coordinated by two SnMe3 anions. A similar B-E cleavage reaction was used to synthesize the triphenylstannate and -plumbate complexes [K(18-crown-6)EPh3] (E ¼ Sn, Pb).264 These compounds crystallize as polymeric chains due to K⋯ Ph interactions. The reaction between SnCl2 and three equivalents of the potassium allyl K[(Me3Si)2C3H3] gave the stannate complex (THF)K {(Me3Si)2C3H3}3Sn (210), in which the potassium cation is coordinated to the three allyl groups and has no short contact with the tin center.265 A mechanochemical reaction between SnCl2 and two equivalents of K[(Me3Si)2C3H3] gave the related solvent-free complex K{(Me3Si)2C3H3}3Sn.266 Longer grinding or use of three equivalents of K[(Me3Si)2C3H3] in this mechanochemical reaction led to disproportionation and the generation of {(Me3Si)2C3H3}4Sn. Me3Si

Sn

SiMe3 SiMe3

K Me3Si

THF 210

SiMe3 SiMe3

The amino-functionalized stannate (ap)3SnLi(THF)2 was prepared by a metathesis reaction between SnCl2 and three equivalents of Li(ap) or by the reaction between (ap)2Sn and Li(ap).267 Selective removal of THF in vacuo gave (ap)3SnLi(THF) and (ap)3SnLi. The deprotonation reaction between (C2F5)3SnH (54) and 1,8-bis(dimethylamino)naphthalene gave the stannate salt [C10H6–1-NMe2–8-HNMe2][(C2F5)3Sn].268 The corresponding lithium salt Li[(C2F5)3Sn] was prepared by the reaction of SnCl2 with three equivalents of LiC2F5. The reaction of Li[(C2F5)3Sn] with benzylic and allylic halides gave the corresponding benzyl or allylstannanes. The coordination chemistry of this stannate ion with transition metals has been investigated. The reaction between 54 and Et2Zn gave the stannate complex {(C2F5)3Sn}2Zn, while the reaction between 54 and Cp(Co)3MMe (M ¼ Mo, W) gave the complexes Cp(Co)3M{Sn(C2F5)3}. Related reactions between 54 and either {Cp(CO)2Fe}2 or Ni(CO)4 gave the complexes Cp(CO)2{Sn(C2F5)3} and (CO)3Ni{Sn(C2F5)3}2, respectively.269 Unexpectedly, whereas the reaction between SnCl2 and the lithium complex of a phosphine-borane-stabilized carbanion gave the corresponding dialkylstannylene (see above), the reaction between [{nPr2P(BH3)}2CH]Li and one equivalent of PbI2 gave the plumbate complex [Li(THF)4][[{nPr2P(BH3)}2CH]PbI2].270 In the solid state the anions of [Li(THF)4][[{nPr2P(BH3)}2CH]PbI2] form weakly associated dimers. Stannate complexes of the lanthanide elements are rare. The reaction between two equivalents of the tris(pyridyl)stannate (THF)Li(2-C5H3N-5-Me)3Sn and Cp 2Ln(OEt2) gave the stannate complexes {Sn(2-C5H3N-5-Me)3}2Ln (Ln ¼ Eu, Yb), with concomitant elimination of Cp Li.271 In contrast, the reactions between (THF)Li(2-C5H3N-5-Me)3Sn and Cp3Ln gave the stannate complexes {(THF)Li(2-C5H3N-5-Me)3Sn}LnCp3 (Ln ¼ La, Yb), which contain a direct Sn-Ln interaction; DFT studies indicated that this interaction is largely ionic in nature.272 Similar reactions between (THF)Li(2-C5H3N-6-OtBu)3Pb and Cp3Ln gave the compounds {Li(2-C5H3N-6-OtBu)3Pb}LnCp3 (Ln ¼ Sm, Eu), which again possess a short Pb-Ln contact.273 The Eu complex was found to be unstable in solution, decomposing to Cp2Eu(THF)n and the diplumbane (2-C5H3N-6-OtBu)3PbdPb((2-C5H3N-6-OtBu)3.

10.04.4 Compounds with E]E and E^E multiple bonds Since the isolation of the distannene {(Me3Si)2CH}2Sn]Sn{CH(SiMe3)2}2 (100) by Lappert and co-workers in 1976,274 ditetrelenes R2E]ER2 (E ¼ Si, Ge, Sn, Pb) have continued to be the subject of intense scrutiny. The more recent isolation of compounds possessing a formal E^E triple bond has served to increase interest further. This is due to the unusual structures adopted by these compounds, their unique reactivities, and their potential use in the activation of small molecules such as H2 and NH3.

Organometallic Compounds of Tin and Lead

453

10.04.4.1 Bonding models and theoretical studies Distannenes R2Sn]SnR2 and diplumbenes R2Pb]PbR2 typically adopt highly distorted structures in which the substituents deviate significantly from the E]E vector, adopting so-called trans-bent (or, more correctly, trans-pyramidalized) structures (Fig. 2A). In almost all cases, distannenes and diplumbenes dissociate into the corresponding stannylenes and plumbylenes in solution, indicating relatively weak EdE interactions. Several theoretical models have been developed to describe the bonding in these compounds and to account for their trans-bent structures and weak EdE bonds. The earliest proposed model invoked the dimerization of two tetrylene units via donation of the lone pair on one unit into the vacant p-orbital on an adjacent unit and vice-versa, i.e. a double donor-acceptor interaction (Fig. 2B).274 This simple model neatly accounts for the observed geometric distortion in ditetrelenes and the generally observed weakness of the E]E bond, although it results in a rather different bonding motif to the more traditional s + p bonding observed in alkenes. An alternative valence-bond approach to the bonding in ditetrelenes invoked a singly-bonded R2Sn-SnR2 fragment in which a single lone pair resonates between the two tetrel atoms (Fig. 2C).275 More recently, the structural distortions observed in heavier ditetrelenes have been rationalized on the basis of a second-order Jahn-Teller (SOJT) effect resulting from interactions between the filled and vacant molecular orbitals.276,277 The HOMO-1, HOMO, LUMO and LUMO + 1 of a planar R2E]ER2 molecule (with local D2h symmetry) have the symmetries a1g, b2u, b1g and b3u, corresponding to the s, p, p , and s -orbitals, respectively (Fig. 3). A deformation (i.e. a bending vibration) to afford a trans-bent molecule with local C2h symmetry results in symmetry designations of s (HOMO-1, ag), n— (HOMO, bu), n+ (LUMO, ag), and s (LUMO +1, bu) for the corresponding MOs. This allows mixing of the s and n+ orbitals (both of ag symmetry) and, more importantly, of the n— and s orbitals (both of bu symmetry). This mixing results in stabilization of the s and n— orbitals and a corresponding destabilization of the n+ and s orbitals, leading to an overall stabilization of the trans-bent D2h symmetry geometry. In other words, the symmetry change caused by the trans-bending vibration enables mixing of orbitals with the same symmetry and consequent stabilization of the trans-bent form. In accordance with this analysis, the direct product of the irreducible representations for the HOMO and LUMO + 1 (b2u x b3u) corresponds to an irreducible representation of b1g symmetry, which is the same symmetry as the trans-bending vibrational deformation.278 R R R

E

E

R

(a)

R

E

E

R

R R

R E R

(b)

R E R

R

R

E R

E R

(c)

Fig. 2 Bonding models for ditetrelenes.

Fig. 3 Changes in symmetry of the frontier orbitals of a ditetrelene on bending from a planar D2h to a trans-pyramidalized C2h symmetry. Reproduced with permission from Wedler, H. B.; Wendelboe, P.; Power, P. P. Organometallics 2018, 37, 2929. Copyright 2018 American Chemical Society.

454

Organometallic Compounds of Tin and Lead R E R

E

R

E

E

R

Fig. 4 Singly and triply bonded forms of a ditetrelyne.

The greater tendency toward trans-bending with increasing atomic number of the group 14 element in ditetrelenes is related to the decreasing energy gap DE between the interacting HOMO and LUMO +1 orbitals. Since the SOJT interaction is inversely proportional to DE, this leads to a greater tendency toward trans-bending for the heavier elements, due to the increasingly weak E-E bonding, which may be ascribed to the poorer overlap of the larger and more diffuse valence orbitals and increased core-core repulsion for the heavier elements, and a consequently smaller gap between the HOMO and LUMO +1. Consistent with this, for the model compounds H2E]EH2 DFT calculations indicate a linear correlation between DE and the barrier to planarization at the tetrel center, such that the values of DE for E ¼ Si, Ge, Sn are 9.78, 9.05 and 7.44 eV, respectively, while the corresponding barriers to planarization are 0.9, 2.7 and 10.1 kcal mol−1, respectively.276 However, while the SOJT interaction is inversely proportional to DE, and so should decrease in importance with increasing DE, recent calculations suggest that an effect is observed up to an upper limit of 12 eV between the two interacting orbitals.276 Similar symmetry-based arguments may be used to rationalize the trans-bent structures of ditetrelynes RE^ER.276,277 However, for these compounds, a second issues arises, since examples of these species have been isolated where the bond between the tetrel elements may be described as a triple bond (i.e. the compound is best considered an analog of an alkyne), or as a single bond (i.e. the compound is best considered a ditetrylene possessing a lone pair at each tetrel center) (Fig. 4); the latter form is particularly favored for diplumbynes RPbPbR. A DFT study of the model compounds MeEEMe concluded that the multiply-bonded forms possess significant diradical character, and that there was only a small difference in energy between the singly and multiply-bonded tin derivatives.279 Subsequent DFT studies on a series of distannynes and diplumbynes ArEEAr (Ar ¼ C6H3–2,6-(C6H2-2,6iPr2-4-R)2, R ¼ H, SiMe3, iPr) concluded that these compounds possess a multiply-bonded structure in solution, although the solid state structure for R ¼ SiMe3 has a trans-bending angle of 99.3 , consistent with a SndSn single bond.280–282 Very recently, the concept of charge-shift bonding has been used to explain the unique properties (relatively weak bond strengths, trans-bent structures) of compounds containing formal triple bonds between heavier group 14 elements.283–285 This concept arises from Valence Bond theoretical calculations from the early 1990s and involves a relatively high resonance stabilization energy due to mixing of the weakly bound (mainly as a result of Pauli repulsion) covalent Heitler − London wave function with that of higher energy ionic configurations. Atoms-in-Molecules (AIM) analysis of the E-E bonding in ditetrelynes reveals that the calculated electron density at the bond critical points drops significantly to relatively low values with increasing atomic number, consistent with a significant charge-shift contribution to the bonding.285 A key concept that had not been fully appreciated until recently is the importance of London dispersion forces (one of the van der Waals forces), resulting from the bulky nature of the substituents, to the stability of ditetrelenes with respect to fragmentation into their corresponding tetrylene components.286 Thus, DFT calculations using the B3PW91 functional, which does not include a correction for dispersion, yielded free energy changes (DG) for dissociation of the “dimers” R2E]ER2 (R ¼ CH(SiMe3)2) into the corresponding tetrylene “monomers” R2E of −17.8, −14.8, and − 9.9 kcal mol−1 for E ¼ Ge, Sn, and Pb, respectively (i.e. dissociation is favored).287 In contrast, when a dispersion correction was included (B3PW91dD3 functional), these values changed to +9.4, +7.1, and − 1.7 kcal mol−1 for E ¼ Ge, Sn, and Pb, respectively, indicating that the dimeric structure is significantly favored for germanium and tin and only slightly disfavored for lead. Further DFT calculations indicated that London dispersion forces are similarly responsible for the stability of a series of terphenyl-substituted diplumbynes.288 The importance of both London dispersion forces and charge-shift effects to multiple bonding between heavier main group elements has recently been nicely summarized by Power.286,289

10.04.4.2 Distannenes and diplumbenes R2E]ER2 The chemistry of multiply-bonded heavier main group compounds, including ditetrelenes R2E]ER2 and ditetrelynes REER, was comprehensively reviewed by Power and Fischer in 2010290 in an update to Power’s previous review on this topic in 1999.291 The chemistry of distannenes and diplumbenes has also been reviewed by Lee,292 while Power and Rivard reviewed the use of terphenyl ligands for the stabilization of multiply-bonded heavier elements in 2007.293 While the first distannene was isolated in 1976, there is continuing interest in these species.274 In the majority of cases, these compounds are stabilized by bulky aromatic or silyl substituents and are subject to disaggregation into their stannylene and plumbylene “monomers” in solution. The cyclic distannenes 211, 212, 213 were synthesized by straightforward metathesis reactions between [ArSnCl]2 and either 4,5-dilithio-9,9-dimethylxanthene or 1,8-dilithionaphthalene (Ar ¼ ArMes, ArTrip). A similar reaction between [(ArTrip)SnCl]2 and 4,5-dilithio-9,9-dimethylxanthene gave the related bis(stannylene) 214.294,295 The SndSn distance in 211 is rather long (3.0009(7) A˚ ), but the corresponding distances in 212 and 213 (2.7299(3) and 2.7688(2) A˚ , respectively) lie at the shorter end of the typical range of Sn]Sn distances. Although 211–213 adopted trans-bent structures in the solid state, the rigid spacer groups imposed a twist such that the cyclic Sn2C2 core was essentially planar, pushing the terphenyl groups out of their typical trans-bent arrangement, such that the angles between the central C atoms of the terphenyl groups were 72, 112 and 76 for 211, 212 and 213, respectively.

Organometallic Compounds of Tin and Lead

455

The formation of a bis(stannylene) for 214 was attributed to either steric repulsion between the terphenyl groups or crystal packing effects. Variable-temperature NMR studies, along with DFT calculations and UV–visible spectra indicated that 211, 212 and 213 remain intact distannenes in d8-toluene solution; a weak Sn ⋯ Sn interaction was evident even for the bis(stannylene) 214. Oxidation of 211 and 214 by air led to the hydroxy-substituted distannoxane 215 and the bis(dihydroxide) 216, respectively.

O Sn ArMes

O Sn

Sn

Ar

ArMes

211

Sn

Sn

Ar

ArTrip

212 (Ar = ArMes) 213 (Ar = ArTrip)

O HO ArMes

Sn

Sn

O

HO

ArMes

Sn ArMes

215

H O

O H O

O

H

Sn 214

ArTrip

Sn

Mes OH Ar

216

The similar acenaphthene-bridged distannene 218 has been prepared by the reaction of in situ-prepared 5,6-dilithioacenaphthene and [(ArMes)SnCl]2. X-ray crystallography revealed a SndSn distance of 2.7838(2) A˚ , trans-bending angles of 43 and 65 , and a twist angle of 65 .296 The reactions between 211, 212, 214 or 218 and the terminal alkynes RC^CH (R ¼ Ph, Me3Si) in n-hexane gave the corresponding [2 + 2] cycloaddition products 219. Unexpectedly, while the reactions between 211 or 212 and PhC^CH were irreversible, the reactions between 212 and Me3SiC^CH, as well as the reaction of 214 with phenylacetylene, and the reactions of 218 with either alkyne were reversible, according to NMR spectroscopy. A van’t Hoff analysis of the variable-temperature 1H NMR spectra of the reaction between 218 and Me3SiC^CH afforded a dissociation enthalpy of 17.1 kcal mol−1, in accord with the value obtained from DFT calculations.

Sn ArMes

Sn ArMes

218

Sn ArMes

Sn

ArMes

R 219 (R = Ph, SiMe3)

Unexpectedly, the reaction of 211 with [Ni(COD)2] led to the formation of a side-on bonded distannene complex (220) rather than a bis(stannylene) complex (COD ¼ 1,5-cyclooctadiene).295 Complexation does not significantly affect the SndSn bond length, which is 3.0592(3) A˚ (cf. 3.0009(7) A˚ in 211). The reaction between (Trip)2Sn(Im-4Me) and half an equivalent of ML2 gave the side-on bonded distannene complexes [(Im-4Me)2M{(Trip)2SnSn(Trip)2}] (Im-4Me ¼ 1,3,4,5-tetramethylimidazol-2-ylidene; M ¼ Ni, Pd, Pt; L ¼ COD, PCy3, PtBu3).297 DFT studies on these compounds suggested that they were best described as p-complexes with strong back-bonding into the (Trip)2SnSn(Trip)2 fragment, rather than distannametallocyclopropanes.

456

Organometallic Compounds of Tin and Lead

Sn

O

Mes

Sn

ArMes

Ni

220

While the cyclic distannenes 211–218 persist in solution, the reaction between 1,1-dilithioferrocene and [ArSnCl]2 (Ar ¼ ArMes, Ar ) gave the corresponding bis(stannylene)s (221).298 However, variable-temperature 119Sn NMR spectroscopy indicated that, in solution, the less bulky compound (Ar ¼ ArMes) adopted a distannene structure (2210 ) and that the SndSn bonding interaction persisted up to 100  C; for the more bulky system (Ar ¼ ArTrip) a dynamic equilibrium operated between the bis(stannylene) and distannene forms. Compound 221 (Ar ¼ ArTrip) reacted with Ni(COD)2 or Pd(nbe)3 to give the corresponding bis(stannylene) complexes [{Fc(SnAr)2}M] (fc ¼ 1,1-ferrocenyl; nbe ¼ norbornene; M ¼ Ni, Pd), while the reaction between 221 and two equivalents of Pd(nbe)3 gave the complex [{Fc(SnAr)2}Pd2]. Trip

The reaction between [(ArTrip)SnCl]2 and two equivalents of the alkynyllithiums RC^CLi gave either a distannene (ArTrip) (RC^C)Sn]Sn(C^CR)(ArTrip) (R ¼ Me3Si) or the isomeric stannyl-stannylene (ArTrip)Sn]Sn(C^CR)2(ArTrip) (R ¼ tBu); these represented the first examples or alkynyl-substituted low oxidation state tin compounds.299 The difference in the structures adopted by these two compounds was attributed to crystal packing effects. According to 1H NMR and UV–visible spectroscopies, both compounds exist as stannylenes in solution at room temperature; at low temperatures UV–visible spectroscopy suggested that the distannene persisted for (ArTrip)(RC^C)Sn]Sn(C^CR)(ArTrip) (R ¼ Me3Si). The remarkable, twisted distannene (tBu2MeSi)2Sn]Sn(SitBu2Me)2 (222) was isolated as dark green crystals from the reaction between two equivalents of [tBu2MeSi]Na and SnCl2(1,4-dioxane) in THF.300 The solid-state structure of this distannene is unique in that (i) the tin atoms are essentially planar (sum of bond angles at Sn ¼ 359.98 ), (ii) the planes of the two SnSi2 fragments are highly twisted (twist angle 44.62(7) ), and (iii) the SndSn bond is exceptionally short (2.6683(10) A˚ ). In spite of the significant twist angle, DFT calculations indicate that the HOMO and LUMO of the twisted geometry of the model compound (Me3Si)2Sn] Sn(SiMe3)2 correspond to the p- and p -orbitals, respectively. Unusually, the structure of 222 appears to persist in solution: the 119 Sn chemical shift of 630.7 ppm lies in the region expected for a distannene, while 222 reacts in the manner expected for a distannene, rather than a stannylene. For example, the reaction between 222 and CCl4 gave the distannane (tBu2MeSi)2(Cl)Sn] Sn(Cl)(SitBu2Me)2, while the reaction with PhC^CH gave the [2 + 2] cycloaddition product 223. Reduction of 222 with K in the presence of cryptand[2.2.2] gave a radical species with an EPR spectrum consistent with the [(tBu2MeSi)2SndSn(SitBu2Me)2](%—) radical anion. Very recently, it has been shown that thermolysis of 222 at 100  C leads to decomposition to the radical species (tBu2MeSi)3Sn• and the octastannacubane cluster [(tBu2MeSi)Sn]8 (see above).131 MetBu2Si Sn Sn MetBu2Si 222

SitBu2Me

MetBu2Si SitBu2Me MetBu2Si SitBu2Me Sn Sn

SitBu2Me H

223

Ph

An unusual rearrangement is observed in the reaction between the dipotassium salt KSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2K and {(Me3Si)2N}2Sn, which did not generate the corresponding stannylene or its distannene dimer, but gave the endocyclic distannene 224.301 The solid-state structure of 224 was only of low quality, but indicated short SndSn distances (2.689(5) and 2.686(2) A˚ for the two independent molecules in the asymmetric unit), a moderate trans-bending (25.8–29.6 ) and a moderate twist (27.0 and 28.6 ). When the same reaction was carried out in the presence of PEt3 the stannylene adduct {Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2}Sn(PEt3) (225Sn) was isolated. Treatment of this compound with B(C6F5)3 gave the

Organometallic Compounds of Tin and Lead R

R R Si

Me2Si Me2Si

Si R

Sn Sn R R

R Si

Si

Me3Si SiMe2

Me2Si

SiMe2

Me2Si

R

224 (R = SiMe3)

Si

R

SiMe3 E

Me2Si

PEt3

Pb

Me2Si

Si

Me3Si

R

Si R

SiMe3

Pb

Pb

Me2Si

Si

Si

R Me2Si

Si SiMe2

Si Me2

Si R

R

Sn Sn R

R

R Si SiMe2 Si R

227 (R = SiMe3)

R Si

Me2Si

Pb

226

225 (E = Sn, Pb)

R

R R

R

R

Si

457

Si R

R

R

Si

Si

R

SiMe2 Si Me2

Fig. 5 A single donor-acceptor interaction in 226.

adduct {Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2}Sn{B(C6F5)3}. In contrast, treatment of the analogous plumbylene complex {Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2}Pb(PEt3) (225Pb) with B(C6F5)3 led to abstraction of the phosphine and formation of the diplumbene {Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2}PbdPb{Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2} (226).302 The solid-state structure of 226 has a PbdPb distance of 3.0640(8) A˚ , longer than a typical PbdPb single bond. The two Pb atoms have substantially different geometries: one is strongly trans-bent (66.0 ), while the other is essentially planar (trans-bending angle 15.1 ). This was rationalized on the basis of a single donor-acceptor interaction between two plumbylene monomers (Fig. 5). Compound 226 is unstable in solution with respect to reductive elimination of a cyclic tetrasilane and the formation of elemental lead, but the 207Pb NMR spectrum contained a signal at +19,516 ppm corresponding to the plumbylene monomer. Similar to the formation of 224, the reaction between KSi(SiMe3)2SiMe2Si(SiMe3)2K and SnCl2 in the presence of tertiary phosphines or an NHC gave the endocyclic distannene 227.303 The distannenes R2Sn]SnR2 (R ¼ (Me3Si)2CH, ArTrip) reacted with MeC^P via consecutive [2 + 1] and [2 + 2] cycloadditions to give bridged 2,3,5,6-tetraphospha-1,4-dimethylidenecyclohexanes.304

10.04.4.3 Distannynes and diplumbynes RE^ER/RE-ER Due to their tendency to oligomerize and their inherent high reactivities, the isolation of a stable heavier group 14 analog of an alkyne presented a significant challenge. This challenge was finally met with the isolation of the diplumbyne (ArTrip)PbPb(ArTrip) (228) by Power and co-workers in 2000.305 Since that seminal report a number of differently-substituted distannynes RSnSnR and diplumbynes RPbPbR have been isolated and several reviews covering the synthesis and reactions of these compounds have been published.290,306–311 It is notable that the vast majority of distannynes and diplumbynes are supported by bulky terphenyl ligands or by aromatic ligands possessing donor functionality in the ortho positions; few alternatively substituted distannynes and diplumbynes have been reported. ArTrip Pb Pb ArTrip

228

For the ditetrelynes REER, as the group is descended from Si to Pb, both the E-E distances and the degree of trans-bending tend to increase, such that the bonding in diplumbynes is typically better represented as a single PbdPb bond (i.e. the compound is best regarded as a diplumbylene), whereas distannynes may be regarded as possessing some SndSn multiple bond character, although this is significantly less than the triple bonds observed in disilynes. An overview of the numerous theoretical studies of this phenomenon is given in Section 10.04.4.1. In a detailed study of the impact of electronic factors on the trans-bending in digermynes and distannynes, a series of para-substituted terphenyl compounds [{2,6-(Dipp)2-4-RC6H2}E]2 (229; E ¼ Ge, Sn; R ¼ tBu, SiMe3, GeMe3, F, Cl, OMe) and [{2,6-(Trip)2-3,5-iPr2C6H}E]2 (230; E ¼ Ge, Sn), along with (ArDipp)SnSn(ArDipp) (231), was prepared.280,312 In the solid state a multiply-bonded form was observed for 231 and for the para-substituted species 229 (R ¼ Cl, OMe, tBu), with relatively short SndSn distances (2.6461(3) to 2.6675(4) A˚ ) and moderate trans-bending angles (121.8(4)-125.24 ). In contrast, for 229 (R ¼ SiMe3, GeMe3) a singly-bonded form was observed, with SndSn distances of 3.0577(2) and 3.077(12) A˚ , respectively, and narrow trans-bending angles of 99.07(3) and 97.779(17) , respectively. The 3,5-iPr2-substituted compound 230 adopted a structure which was intermediate between the multiply and singly bonded forms, with a SndSn distance of 2.7205(12)/2.7360(14) A˚ and a trans-bending angle of 125.1(2)/127.6(2) ; however, 230 was somewhat twisted, such that the CdSndSndC dihedral angle was 151.97/166.22 and this twisting may play a role in the relative stabilities of the singly and multiply-bonded forms. Treatment of

458

Organometallic Compounds of Tin and Lead

ArPbBr with either tBu2AlH or the Mg(I) reducing agent [[CH{C(Me)]N(Mes)}2]Mg]2 gave the dyplumbynes ArPbPbAr (233; Ar ¼ ArDipp, 2,6-(Dipp)2-4-(Me3Si)C6H2, ArMes∗, 2,6-(Trip)2-3,5-iPr2C6H).288 The latter compound exhibited a PbdPb distance of 3.0382(5) A˚ and widened PbdPbdC angles of 114.73(7) and 116.02(6) , consistent with multiple bond character, whereas the remaining diplumbynes in this study adopted more significantly trans-bent structures in the solid state, consistent with a PbdPb single bond. While treatment of (ArMes∗)PbBr with tBu2AlH led directly to the diplumbyne, a similar reaction between (ArMes∗)SnCl and tBu2AlH gave the unsymmetrical stannyl-stannylene (ArMes∗)SnSn(H2)(ArMes∗). However, thermolysis of (ArMes∗)SnSn(H2)(ArMes∗) at 100  C in toluene led to the formation of the distannyne (ArMes∗)SnSn(ArMes∗) (232); compound 232 may also be synthesized by the reaction of (ArMes∗)SnCl with KC8.201 R iPr iPr ArDipp Sn Sn

ArDipp

ArTrip

ArDipp ArTrip

ArDipp

ArTrip

Sn Sn ArTrip

iPr iPr 230

R 229 (R = tBu, SiMe3, GeMe3,F, Cl, OMe)

ArDipp ArDipp

E

ArMes*

ArDipp

E ArDipp

231 (E = Sn) 233 (E = Pb)

ArMes*

Sn Sn

ArMes*

ArMes*

232

Distannenes R2Sn]SnR2 typically undergo disaggregation to their stannylene “monomers” in solution (see above). In contrast, distannynes and diplumbynes usually remain intact under the same conditions; calculations suggest that for (ArDipp)EE(ArDipp) disaggregation into RE• radical units is disfavored by 25.9 and 17.6 kcal mol−1 for E ¼ Sn and Pb, respectively.313 A similar calculation suggested that dissociation of the diplumbyne (ArTrip)PbPb(ArTrip) (228) was disfavored by 12.2 kcal mol−1.282 Nonetheless, it has recently been shown by NMR and EPR spectroscopies that (ArDipp)SnSn(ArDipp) (231) dissociates at elevated temperatures in toluene solution to give just such RE• radical species; a van’t Hoff analysis of the variable-temperature 1H NMR data for 231 indicated an enthalpy of dissociation for this process of 17.2  1.7 kcal mol−1.314 The analogy between the frontier orbitals of transition metals and those of low oxidation state main group compounds has been mentioned already. The HOMO and LUMO in distannynes and diplumbynes correspond to an E-E p-orbital of au symmetry and an unoccupied n+ non-bonding combination of ag symmetry which lie approximately 2 eV apart. These act as the donor and acceptor orbitals which may mediate the activation of small molecules such as H2 (Fig. 6).183,184,309 The activation of H2 by a ditetrelyne was first observed for the digermyne (ArDipp)GeGe(ArDipp) in 2005; this reaction represented the first time that H2 had been observed to react with a main group compound in the absence of a catalyst and gave a mixture of hydrogenolysis products including (ArDipp)Ge(H)2Ge(H)2(ArDipp), (ArDipp)Ge(H)]Ge(H)(ArDipp), and (ArDipp)GeH3.315 A similar activation of hydrogen has been observed for distannynes; however, reactions between the distannynes ArSnSnAr (Ar ¼ ArDipp; 2,6-(Dipp)2-4-R-C6H2, R ¼ F, SiMe3) and H2 gave the single ArSn(m-H)2SnAr product.316 The more bulky distannyne ArSnSnAr (Ar ¼ 2,6-(Dipp)2–3,5-iPr2C6H) gave the unsymmetrical stannylstannylene ArSnSn(H)2Ar under the same conditions. The differing reactions of distannynes and digermynes toward H2 have been explored computationally.317 More recently the addition of H2 to (ArDipp)SnSn(ArDipp) (231) was shown to be reversible.318 Heating a sealed sample of the dihydride (ArDipp)Sn(m-H)2Sn(ArDipp) in d8-toluene led to gradual formation of the distannyne 231 in 22% yield. Repeated removal of H2 from the headspace in the tube and replacement of this with N2, followed by further heating, led to complete conversion to the distannyne. Distannynes have also been shown to react reversibly with ethylene.319 Treatment of ArSnSnAr (Ar ¼ ArDipp, 2,6-(Dipp)2–3, 5-iPr2C6H) with ethylene under ambient conditions gave the cycloadducts ArSn(CH2CH2)2SnAr (234), which were isolated and structurally characterized. However, in hydrocarbon solution, variable-temperature NMR experiments indicated dissociation of

Organometallic Compounds of Tin and Lead

459

Fig. 6 Molecular orbitals involved in the activation of H2 by a ditetrelyne.

both ethylene units. In contrast, the reaction between (ArDipp)SnSn(ArDipp) and cyclooctatetraene resulted in complete cleavage of the SndSn multiple bond and irreversible formation of the inverse sandwich cyclooctatetraenyl complex {(ArDipp) Sn}2(m-Z2:Z3-COT) (235).320 Sn Sn Ar 234 (Ar

= ArDipp,

ArDipp

Sn

ArDipp Sn

Ar 2,6-(Dipp)2-3,5-iPr2C6H)

Sn 235 ArDipp

236

The reactions between (ArDipp)EE(ArDipp) (E ¼ Ge, Sn) and cycloalkenes nicely illustrate the difference in reactivity between these two compounds. While both (ArDipp)EE(ArDipp) (E ¼ Ge, Sn) reacted with cyclopentadiene to give the cyclopentadienyls (ArDipp)E(Z3-Cp) (236; E ¼ Ge, Sn), with the corresponding elimination of H2, only the Ge homolog reacted with cyclopentene or 1,4-cyclohexadiene.321 These reactions differ significantly from the reactions of distannynes and digermynes with ethylene (see above), presumably due to the formation of aromatic products. The reduced reactivity of (ArDipp)SnSn(ArDipp) (231) in comparison to its lighter homolog (ArDipp)GeGe(ArDipp) is also reflected in the fact that, while the latter compound reacted with PhC^CPh, Me3SiC^CH, tBuN^C, or PhC^N, no reaction was observed between 231 and these substrates at room temperature.322 Nonetheless 231 was shown to react with PhN]NPh or Me3SiN3 to give the bis(stannylene)s (ArDipp)Sn{N(Ph)N(Ph)}Sn(ArDipp) (237) and (ArDipp)Sn{N(SiMe3)}Sn(ArDipp) (238), respectively. While 231 was shown to not form a stable adduct with tBuNC at room temperature, addition of an excess of either tBuNC or MesNC to a concentrated solution of (ArDipp)SnSn(ArDipp) gave, after cooling to −18  C, the 1:2 adducts (ArDipp)Sn(CNR)Sn(CNR)(ArDipp); these compounds dissociated to give 231 and the isocyanide on dissolution in hydrocarbon solvent.323 Oxidation of 231 by either TEMPO or N2O gave the tin(II) hydroxide (ArDipp)Sn(m-OH)2Sn(ArDipp).324 In contrast, the reaction between 231 and one equivalent of pyridine-N-oxide gave the distannyldiyl ether (ArDipp)Sn(m-O)Sn(ArDipp), which co-crystallized with (ArDipp)Sn(m-OH)2Sn(ArDipp).325 Ph

Ph N

SiMe3

N

Sn ArDipp

Sn 237

ArDipp

ArDipp N Sn

Sn

ArDipp

238

While the chemistry of distannynes and diplumbynes is dominated by sterically demanding terphenyl ligands, several distannynes have been prepared which are supported by intramolecularly-coordinating amino- or imino-functionalized aryl groups. The reaction between (dap)SnCl and K[sBu3BH] in THF at 0  C gave the distannyne (dap)SnSn(dap) (239).326 In the solid state each tin atom in 239 is coordinated by the aryl C atom and the two N atoms of the amino side-arms of the ligands; DFT calculations indicated a single SndSn bond in this compound. The closely related compounds ArSnSnAr (Ar ¼ C6H2–2,4-tBu2–6d -CH2NMe2 (240), C6H2-2,4-tBu2-6-CH]N(Dipp) (241), C6H3-2,6-{CH]N(Dipp)}2 (242) have been prepared by a similar route (i.e. the reaction between ArSnCl and K[sBu3BH]).327 In 240 and 241 the tin atoms are three-coordinate whereas the tin atoms are four-coordinate in 242; DFT studies suggested that 240 and 241 possess a greater SndSn bond order than is evident in 242. The alternative pincer ligand-stabilized distannyne ArSnSnAr (184; Ar ¼ 4-tBu-2,6-{P(O)(OiPr2)2}2C6H3) was prepared by the reaction of ArSnCl with K[sBu3BH], KC8 or Li[C10H8] (see above).237 DFT calculations indicated the presence of a single SndSn bond in this compound.

460

Organometallic Compounds of Tin and Lead NMe2 Me2N

NEt2 tBu

Sn

Sn

tBu Sn

Sn

NMe2 Me2N

tBu Et2N 240

239 Dipp

Ar N Ar

N

N

tBu Sn

tBu

tBu

Sn

Sn

Sn

tBu N

tBu N

Ar N Ar

Dipp 241

242 (Ar = 2,6-Me2C6H3)

The reaction between 239 and S8 gave the sulfur-bridged (dap)Sn(m-S)Sn(dap); when this reaction was repeated over a longer period (24 h) the alternative polysulfide product {(dap)Sn(m-S)}2(m-S5) was isolated.328 A similar reaction between 239 and elemental Se initially gave the selenium-bridged (dap)Sn(m-Se)Sn(dap), but extended reaction times led to further oxidation of the tin centers and isolation of (dap)Sn(]Se)(m-Se)Sn(]Se)(dap).329 The addition of elemental Te to 239 in hexane initially proceeded in a similar manner, to give (dap)Sn(m-Te)Sn(dap), which underwent further oxidation over extended reaction times to give the unsymmetrical (dap)Sn(]Te)(m-Te)Sn(dap).330 The fully oxidized (dap)Sn(]Te)(m-Te)Sn(]Te)(dap) (243) was obtained by the reaction between 239 and excess Te in toluene over 24 h. Exposure of 243 to visible light or heating to 40  C led to decomposition to give (dap)Sn(m-Te)2Sn(m-Te)2Sn(dap) (244). The reaction between 239 and one equivalent of SnO gave the stannoxane (dap)Sn(m-O)Sn(dap), whereas an excess of SnO gave the cluster species {(dap)Sn}4Sn6O8.331 Unexpectedly, the reaction between 239 and ethylene did not give the expected addition product, but led to the hydride {(dap)Sn}3SnH.331 The oxidation of 239 by one equivalent of the diorganodisulfides REER led to the complexes (dap)Sn(SR) (R ¼ Me2NC(S), C6H4–2NH2, 2-C5H4), while treatment of 239 with three equivalents of REER gave the tin(IV) compounds (dap)Sn(SR)3.332 Similar reactions between either 239 or 184 and one equivalent of the dichalcogenides REER gave ArSn(ER), while reactions with three equivalents of REER gave the tin(IV) compounds ArSn(ER)3 (Ar ¼ dap, 4-tBu-2,6-{P(O)(OiPr2)2}2C6H2; R ¼ Ph, 2-C5H4N; E ¼ S, Se, Te).333 NMe2 Me2N Sn Te

Te

Sn

NMe2 Me2N 243

NMe2 Me2N

Te

Sn

Te Te Sn Sn Te Te

NMe2

244

Me2N

Treatment of [C6H3-2,6-{C(Me)]N(Dipp)}2]SnCl (179) with two equivalents of potassium graphite gave the asymmetric distannyne 245, in which one pincer ligand is tridentate, as expected, but the second acts as a bidentate ligand, leaving one imine function uncoordinated. Treatment of 245 with P4 gave the activation product 246, containing a butterfly-shaped P4 core.233 The reaction between the stannylene [C6H3-2,6-{CH]N(tBu)}2]SnCl and KC8 generated the related distannyne [C6H3-2,6-{CH] N(tBu)}2]SnSn[C6H3-2,6-{CH]N(tBu)}2] (247), in which both tin atoms are four-coordinate.334 Treatment of 247 with a further two equivalents of KC8 gave the stannylidenide [C6H3-2,6-{CH]N(tBu)}2]SnK(THF) (248). A similar reaction between the plumbylene [C6H3-2,6-{CH]N(Dipp)}2]PbBr and excess elemental lithium in THF gave the plumbylidenide [Li(THF)4] [[C6H3-2,6-{CH]N(Dipp)}2]Pb] (249).335 Oxidation of 249 with SnCl2 gave the diplumbyne [C6H3–2,6-{CH]N(Dipp)}2] PbPb[C6H3-2,6-{CH]N(Dipp)}2] and elemental tin; the reaction of this latter compound with two equivalents of Li regenerated 249.

Organometallic Compounds of Tin and Lead Ar N Ar N

N

Sn

Sn

N

N

Dipp Dipp N

Sn

N

Ar Ar 245 (Ar = Dipp)

tBu N tBu N

P

Sn N

(THF)K

Sn

Sn

247

P

246

N

tBu N tBu

P P

Dipp Dipp

Sn N

461

N tBu 248

N

tBu

Dipp

Pb

[Li(THF)4] N

Dipp 249

10.04.4.4 Stannaethenes R2E]CR2 While there are many examples of Sn]Sn and Pb]Pb bonds, compounds with E]C bonds are rather rare.336 The dehydrofluorination of (Trip)2Sn(F)CHR2 with tBuLi gave the stannafulvene (Trip)2Sn]CR2 (250; CR2 ¼ 2,7-di-tert-butylfluorenylidene).337 Compound 250 has a planar C2SnCC2 core and possesses the shortest reported SndC distance (2.003(5) A˚ ). This compound undergoes [2 + 2] cycloaddition with benzaldehyde and [2 + 4] cycloadditions with unsaturated aldehydes and ketones. The reaction between 250 and 1,4-benzoquinone gave the cycloadduct 251 via a double [2 + 3] cycloaddition reaction; similar reactions between 250 and 1,4-naphthoquinone or 9,10-anthraquinone gave the [4 + 2] cycloaddition products 252 and 253, respectively (Scheme 5).338 The reaction between 250 and 9,10-phenanthroquinone gave the [4 + 2] cycloaddition product 254. Treatment of 250 with PhNCO, PhNCS, or Ph2CCO gave the [2 + 2] cycloaddition products 255, while the reaction between 250 and tBuCN gave the [2 + 2] cycloaddition product 256; in contrast, the reaction between 250 and MeCN led to addition of the CdH bond across the Sn]C bond to give 257.339

462

Organometallic Compounds of Tin and Lead

O

Sn(Trip)2 (Trip)2Sn

CR2 (Trip)2Sn

R2 C

O

CR2 C

O

Sn(Trip)2 NPh

255

O CR2 251

O

(Trip)2Sn O

PhNCO

C R2

O

252

O O (Trip)2Sn

CR2

MeCN

NCCH2 H 256

Trip

Trip Sn

Sn

=

CR2

Trip

Trip 250

O

tBuCN O (Trip)2Sn N 257

O

CR2 O

C tBu (Trip)2Sn

CR2

O

(Trip)2Sn

R2 C

O

O

254

(Trip)2Sn

C R2

O

253

Scheme 5 Reactions of stannaethene 250.

Analogs of cumulenes containing tin or lead are extremely rare. In this regard, the reaction between SnCl4 and [[{S]P(Ph)2}2C] Mg]2 gave the stannaallene compound [{S]P(Ph)2}2C]2Sn (196; see above); this represented the first example of a 2-stannallene.255 While compounds containing Sn]C double bonds are rare, stable examples of the corresponding alkyne analogs RSn^CR are, as yet, unknown. Theoretical calculations suggested that a stannaethyne possessing large substituents (e.g. ArTrip, C6H2-2,4,6{CH(SiMe3)2}3, SiMe(SitBu3)2, SiiPr{CH(SiMe3)2}2) at both the Sn and C centers should be accessible and that these compounds would possess a trans-bent geometry.340 Similar calculations of the lead analogs RPb^CR also suggested stability for large substituents and a trans-bent geometry.341 Photolysis of the diazomethylstannylenes {(R3Si)C(]N2)}(ArTrip)Sn (R ¼ Me, iPr) in benzene gave the corresponding stannylstannylenes.342 The proposed mechanism for the formation of these systems involved the initial formation of a carbene-stannylene intermediate (ArTrip)SndC(SiR3), which may be regarded as a stannaacetylene (ArTrip)Sn^C(SiR3) (R ¼ Me, iPr); laser flash photolysis experiments indicated the intermediate formation of just such a stannaacetylene during this reaction, with a very short lifetime of 3.7 ms.

10.04.5 Unsaturated heterocycles The chemistry of unsaturated and aromatic heterocycles containing tin or lead has seen great progress over the last two decades, with the isolation of numerous five-membered stannoles and plumboles (and their mono- and dianions) and the very recent isolation of the first neutral stannanaphthalenes and a stannabenzene. This area has recently been reviewed.343–346

Organometallic Compounds of Tin and Lead

463

10.04.5.1 Stannoles and plumboles Neutral, tetravalent stannoles and plumboles (stanna- and plumbacyclopentadienes) may be considered special cases of dialkenylstannanes and plumbanes, and, as such, are non-aromatic (although s -p conjugation has been proposed for these systems).343 Typically these compounds are synthesized by metathesis reactions between a dichlorotetrelane R2ECl2 and the respective dilithium compound, or by the reaction between a cyclic diorganozirconium compound and a dichlorotetrelane (e.g. Scheme 6A).347 An unusual route to borane-substituted stannoles is the 1,1-carboboration of bis(trimethylsilylethynyl)tin compounds (e.g. Scheme 6B).348 When the highly electrophilic borane B(C6F5)3 was used the intermediate zwitterionic compound 258 was isolated and structurally characterized, lending support to the proposed reaction mechanism. Tin-containing triphenylene derivatives (259, 260), unusual examples of fused ring stannoles, have been prepared by a metathesis route from the lithiated aromatic compound.349

Me3Si

SnMe2

C6F5

Sn

B(C6F5)2

Me3Si

Sn Me2

258

Sn R2

259

260 (R = Me, nBu)

The strong s -p conjugation in non-aromatic stannoles leads to a decrease in the HOMO-LUMO gap, compared to the parent aromatic species, which is potentially useful in the development of semiconducting polymers used in organic electronics.350 Judicious choice of the substituents has been used to manipulate the size of the HOMO-LUMO gap in these compounds and hence their optoelectronic properties.351 In 2014 the first example of a stannole-containing polymer was reported.352 The poly(thiophene)-based polymer was synthesized by a tin-selective Stille coupling reaction and exhibited a narrow band-gap; the same synthetic route has very recently been used to synthesize a series of stannole-containing polymers in essentially quantitative yields.353 An alternative approach to the synthesis of stannole-containing polymers employed a post-polymerization metathesis reaction between a titanacyclopentadienecontaining polymer and R2SnCl2.354 Examples of neutral divalent stannoles and plumboles (which may also be considered unsaturated, conjugated cyclic tetrylenes) are extremely rare. The reaction of a silicon-substituted 1,4-dilithio-1,3-butadiene with {(Me3Si)2N}2Pb in the presence of a Lewis base gave the adducts 261 ((L)n ¼ (THF)2, (py)2, NHC).355 The NMR spectra of the THF adduct suggested reversible dissociation of the coordinated THF in C6D6; recrystallization of this THF-dissociated compound from n-hexane revealed it to be the dimer 262, which has a PbdPb distance of 3.1777(4) A˚ . The solid-state structure of 262 revealed it to consist of a THF-stabilized plumbole coordinating to a second plumbole unit through donation of its lone pair into the vacant Pb 6p-orbital (i.e. a single donor-acceptor PbdPb interaction). Compound 262 was unstable in solution, decomposing to the spiroplumbole 263 (50% conversion after 54 h at room temperature). Displacement of the THF ligands in 261(THF)2 by 1,4-dioxane led to the formation of a coordination polymer (264) in which the dioxane ligands bridge adjacent Pb atoms by donation into their vacant 6p-orbitals, such that the p-skeletons of the plumbole units are aligned.356 In contrast, substitution of THF with pyrazine led to the formation of the oligomer 265. SiMe3

SiMe3 Ph

L Pb

Ph

L

Ph

THF SiMe3

SiMe3 261 (L = THF, py, NHC)

O

O R

Pb

Ph Ph 264 (R = SiMe3)

262

n

Ph

Ph

Ph

Ph

N

Ph

Me3Si

Me3Si

R

Ph

Pb

Pb

Pb

R

R

Ph

Ph

Pb

SiMe3

Me3Si

Me3Si

Ph

263

N R

Pb

Ph

R

Ph

N

SiMe3

N R

Pb

Ph

R

Ph

265 (R = SiMe3)

Anionic, and especially dianionic stannoles and plumboles are much more numerous than their neutral congeners.344 Careful metalation of hexaphenylstannole with 2.5 equivalents of Li gave the monoanion 266.357 Although 266 may be considered a

464

Organometallic Compounds of Tin and Lead

Ph

Ph

Ph

Ph

Ph

Ph2PbCl2

(a) Ph

Ph Li Li

Pb Ph

SiMe3 R′

Ph SiMe3

R2B

BR3

Sn

Ph

R′

Sn

R′

R

(b)

R′

SiMe3

SiMe3 Scheme 6 (A) Synthesis of plumbole via a metathesis route, (B) the carboboration route.

heavier group 14 analog of the cyclopentadienyl anion, X-ray crystallography showed that both 266 and its Me3Si-substituted analog 267 were non-aromatic; the SndR substituent was not coplanar with the five-membered ring and the CdC distances exhibited significant alternation around the ring.358 Stannole anion 266 underwent oxidation on treatment with ClCH2CH2Cl, BrCH2CH2Br, or air to give the non-aromatic bis(stannole) dianion 268 containing a SndSn bond; treatment of 268 with Li led to regeneration of 266.359 Further treatment of the ethyl-substituted analog of 268 with one equivalent of 12-crown-4 generated the unusual lithocene 269, in which one lithium is sandwiched between two Z5-stannole rings.360 X-ray crystallography, NMR spectroscopy and DFT calculations again indicated the non-aromatic nature of the rings in this compound. Ph Li(12-crown-4)2]

Ph

Ph

Ph

Sn

Ph Ph

Li(12-crown-4)2]2

Sn Ph Ph

R 266 (R = Ph) 267 (R = SiMe3)

268

R

Ph Ph

R Li

Sn

R

Sn

R

Sn

269

Ph

R

R

Li(12-crown-4)2]

Ph

R R

Treatment of 268 with an excess of Li in THF under reflux gave the stannole dianion 270.361 This compound was more easily accessible from the reaction between hexaphenylstannole and an excess of Li in diethyl ether under reflux.362 X-ray crystallography revealed that 270 adopted an inverse sandwich structure in the solid state, in which each Li ion was bound in an Z5-manner to either side of the stannole dianion, with its coordination completed by a molecule of diethyl ether. The CdC distances in the ring are nearly equal (1.422(6) to 1.466(6) A˚ ) and the ring is essentially planar, indicating significant aromatic character. This is supported by a NICS(1) value of −5.96, which, however, suggests reduced aromaticity in 270 compared to its Si and Ge analogs. Oxidation of 270 with 1.3 equivalents of O2 gave the tris(stannole) dianion 271 containing a central SndSndSn unit, after crystallization in the presence of 12-crown-4.363

Organometallic Compounds of Tin and Lead

465

The corresponding tetraethyldilithiostannole 272 was accessible via lithium-induced SndPh cleavage; this compound was isolated as a mono-ether adduct and adopted a polymeric structure involving inverse-sandwich [Li-(stannole dianion))dLi(OEt2)] repeat units in the solid state.364 Similarly, the stannaindenyl anion 273 and dianion 274 were obtained by SndPh cleavage by lithium metal; these compounds have not been structurally characterized but their composition has been confirmed by NMR spectroscopy and their reactions with electrophiles.365 The related silicon-substituted stannole dianion complexes 275 have been synthesized in a similar manner and adopt inverse sandwich structures similar to that of 270.366 Theoretical calculations of the 119Sn chemical shifts suggested that the stannylene character of these silyl-substituted species was enhanced compared to the corresponding alkyl and aryl-substituted compounds, due to greater interaction between the tin 5p-orbital and the butadiene p-system.

Et

Li

Et

tBu

Ph Sn

Et

Ph Sn

Li

Et

OEt2 272

tBu

tBu 273

Li

THF SiMe3 Li Sn

Ph Sn Li 274

Li

Ph Li

SiMe3

THF 275

The synthesis of a dilithioplumbole was finally realized in 2010.367 Treatment of hexaphenylplumbole with Li and a catalytic amount of naphthalene in diethyl ether, followed by treatment with DME to remove the PhLi by-product, gave the plumbole dianion 276 in good yield. In the solid state the plumbole dianion was coordinated in an Z5-manner to one lithium ion, which was further coordinated by a molecule of DME; the second lithium ion formed the counter-cation and was coordinated solely by DME ligands. As with 270, structural and NMR spectroscopic data, along with DFT calculations suggested aromatic character in the ring of 276. The reaction between 276 and MesBr gave the mono-anion complex 277, in which the Mes group lies significantly out of the plane of the five-membered ring, and which does not exhibit aromatic character.

The related silicon-substituted dilithioplumbole 278 has been prepared by the reduction of the neutral plumbole 261 (THF)2 with elemental lithium in toluene.368 Compound 278 crystallized with an inverse-sandwich structure; the structural data, combined with the NMR spectra of this compound indicated significant aromatic character, however, the substantial low-field shift of the 207Pb NMR signal (2573 ppm) suggested increased plumbylene character in comparison to 276 (1713 ppm). Treatment of 278 with two equivalents of [Cp2Fe][BF4] regenerated 261 quantitatively, whereas oxidation with BrCH2CH2Br gave the dilithiodiplumbole 279, which had non-aromatic character.

466

Organometallic Compounds of Tin and Lead

Ph

THF SiMe3 Li

R Ph Pb

Pb

Ph

Ph Li

Li THF

SiMe3

R

R

THF Li

Ph

Pb Ph R

THF

279 (R = SiMe3)

278

Anionic and dianionic stannoles and plumboles are analogs of the ubiquitous cyclopentadienyl anion (and its, as yet unknown, dinegative counterpart) and, as such, might be expected to have an extensive chemistry as ligands to transition metal centers. Somewhat surprisingly, the first such compound was not isolated until 2013. The reaction mixture comprising tetraethylstannole dianion complex 272 and half an equivalent of [Cp RuCl]4 in diethyl ether underwent a series of color changes from yellow to deep blue to brown and gave the complex [Cp Ru(m-SnC4Et4)2RuCp ] as dark brown crystals (Cp ¼ C5Me5).369 A subsequent reaction between 272 and 0.2 equivalents of [Cp RuCl]4 showed a color change from yellow to deep blue, but did not go further, and gave the dark blue complex [Li(OEt2)]2[Cp Ru(m-SnC4Et4)2RuCp ]. Interconversion between [Cp Ru(m-SnC4Et4)2RuCp ] and [Li(OEt2)]2[Cp Ru(m-SnC4Et4)2RuCp ] was found to be reversible: treatment of [Li(OEt2)]2[Cp Ru(m-SnC4Et4)2RuCp ] with O2 gave [Cp Ru(m-SnC4Et4)2RuCp ], whereas treatment of [Cp Ru(m-SnC4Et4)2RuCp ] with Li metal gave [Li(OEt2)]2[Cp Ru (m-SnC4Et4)2RuCp ]. X-ray crystallography revealed that in both [Cp Ru(m-SnC4Et4)2RuCp ] and [Li(OEt2)]2[Cp Ru(mSnC4Et4)2RuCp ] the stannole dianion bridges the two Ru Centers in m-Z1:Z1-fashion. A similar reaction between silyl-substituted 275 and two equivalents of Cp2HfCl2 gave the complex {Cp2(Cl)Hf}2{m-2,5-(tBuMe2Si)2-3,4-Ph2C4Sn}, in which the stannole dianion again bridges the two Hf centers in a m-Z1:Z1-fashion.370 The solid-state structure of {Cp2(Cl)Hf}2{m-2,5-(tBuMe2Si)2-3,4-Ph2C4Sn} revealed alternating CdC distances in the ring, consistent with non-aromatic character. Unexpectedly, the reaction between the tetraethylstannole dianion complex 272 and one equivalent of Cp2TiCl2 led to the distannene complex 280.371 R

R

R

Sn R

Cp2Ti

R

Sn R

R R

280 (R = Et)

The first Z5-stannole-transition metal complex was isolated in 2014. The reaction of stannole dianion complex 270 with half an equivalent of [Cp RuCl]4 gave the triple-decker Z5-stannole complex 281, in which the stannole dianion acts as a m-Z5: Z5-bridge between the two Ru centers.372 Unexpectedly, a similar reaction between 270 and ¼ of an equivalent of [Cp RuCl]4 also gave 281 and left half an equivalent of 270 unreacted, suggesting that the second substitution is faster than the first. However, the reaction between the silyl-substituted stannole dianion 275 and ¼ of an equivalent of [Cp RuCl]4 gave the anionic sandwich complex 282.

Treatment of 282 with CCl4, MeI, EtBr, or Me3SiCl gave the sandwich complexes 283, which contain the monoanionic cyclopentadienyl analogs [1-R-2,5-(tBuMe2Si)2-3,4-Ph2C4Sn]− (R ¼ Cl, Me, Et, Me3Si).373 The coordination mode of the stannole anion was found to depend on the nature of the substituent at tin. For the silicon-substituted species an approximately Z5-coordination mode was observed with a near planar stannole ring, whereas for R ¼ Cl, Me, or Et, the stannole ring was bent, such that the SndR fragment lay out of the C4 plane and the coordination mode of the stannole anion may best be described as

Organometallic Compounds of Tin and Lead

467

similar to an Z4-butadiene. DFT studies suggest that the differing coordination modes are associated with different hybridization of the tin lone pair, which has essentially s-character in the chloro-derivative, but has more p-character in the silyl derivative. The first complex of a plumbole dianion was reported in 2017. The reaction between 278 and ¼ of an equivalent of [Cp RuCl]4 gave the plumbole complex 284, which had a planar C4Pb ring in the solid state, in which the CdC distances do not alternate, indicating aromatic character.374 Reaction of 284 with electrophiles gave the corresponding plumbole monoanion complexes 285, in which the PbdR fragment deviates from the C4 plane, indicating a non-aromatic ring which principally acts as an Z4-butadiene ligand to Ru.

10.04.5.2 Stannabenzenes, plumbabenzenes and related compounds The synthesis of a neutral stannaaromatic compound (i.e. a tin-containing analog of benzene or naphthalene) has been a long-standing challenge in main group chemistry. This goal was first achieved in 2006 by Tokitoh and co-workers with the synthesis of the 2-stannanaphthalene 286 using an elegant synthetic strategy (Scheme 7).375 The use of the extremely bulky Tbt substituent was key to the stabilization of this compound (Tbt ¼ {2,4,6-(Me3Si)2CH}3C6H2)). The solid-state structure of 286 revealed a trigonal planar geometry around the tin atom and that the SndC distances within the naphthalene ring (2.029(6) and 2.081(6) A˚ ) are shorter than typical SndC single bonds. The 119Sn chemical shift of 264 ppm was

Scheme 7 Synthesis of the first stannaromatic compound 286.

consistent with a low-coordinate tin center, while the 1H and 13C chemical shifts of the naphthalene ring lay in the typical range for an aromatic compound, indicating delocalization of the p-system around the ring. Consistent with this, nucleus independent chemical shift (NICS) calculations indicated an aromatic 2-stannanaphthalene ring, with approximately 80% of the aromaticity exhibited by the parent naphthalene.376 Treatment of 286 with [Cr(CO)3(NCMe)3] yielded the corresponding 2-stannanaphthalene complex 287.375 Sn

Tbt tBu

Cr

OC OC

CO

287

Attempts to synthesize a stannabenzene were initially frustrated by the formation of dimers via a [4 + 2] homo-cycloaddition reaction.377 Thus, the reaction of a Tbt- or Bbt-substituted 1-bromo-1-stannacyclohexa-2,5-diene with iPr2NLi gave the dimer 288 in essentially quantitative yield (Bbt ¼ {2,6-(Me3Si)2CH}2–4-{(Me3Si)3C}C6H2). The ready dimerization of these stannabenzenes contrasts with the behavior of the corresponding Tbt-substituted sila- and germa-benzenes, which were stable to 100  C in C6D6.378–380 R Sn

Ar

Sn

tBu

[K(THF)] R

Sn Ar

288 (R = H; Ar = Tbt, Bbt; Bbt = 2,6-{(Me3Si)2CH}4-{(Me3Si)3C}C6H2) 289 (R = tBu)

290

The related tBu-substituted compounds 289 have also been isolated from the reaction of the corresponding 1-bromo-1-stannacyclohexa-2,5-diene.381 Variable-temperature 1H NMR spectroscopy revealed that in C6D6 there is a dynamic equilibrium between the dimers 289 and the constituent stannabenzene monomers. The thermodynamic parameters for the equilibrium between the monomeric and dimeric forms were calculated to be DH ¼ −21.9  0.2 kcal mol−1 and DS ¼

468

Organometallic Compounds of Tin and Lead

−50.0  0.5 cal mol−1 K−1 for the Tbt-substituted system and DH ¼ − 20.5  0.3 kcal mol−1 and DS ¼ −56.7  0.8 cal mol−1 K−1 for the Bbt-substituted system. Similar values were obtained in d8-THF, indicating that the solvent had little effect on the dimerization reaction. Treatment of the equilibrium mixture of 289 (R ¼ Bbt) with [Cr(CO)3(NCMe)3] gave the corresponding Z6-stannabenzene complex. Formation of an anionic compound is an interesting strategy to overcome the dimerization of stannabenzenes. In this regard, treatment of the equilibrium mixture of 289 (Ar ¼ Tbt) and its dimer with potassium graphite led to the formation of the stannabenzene anion 290, along with a benzylpotassium side-product.382 DFT and NICS calculations indicated that the stannabenzene ring in 290 possessed some aromatic character, but also some stannylene character. The synthesis of a neutral stannabenzene was finally achieved in 2019.383 The reaction of an aluminacyclohexadiene with SnCl2 gave a stannacyclohexadiene, which was dehydrochlorinated with [(Me3Si)2N]K to generate the stannabenzene 291 in good yield (Scheme 8). Although air-sensitive, 291 was thermally stable, even in solution at 90  C. Compound 291 crystallized as a discrete monomeric species with a planar C5Sn ring, a planar Sn atom, and equivalent SndC distances of 2.0515(19) and 2.052(2) A˚ . The variable-temperature 1H NMR spectra of 291 indicated no propensity for dimer formation, while the 119Sn chemical shift of 291 was 491 ppm; the UV–visible spectrum of 291 had a lmax at 368 nm. DFT, NICS, and Natural Bond Orbital (NBO) calculations were consistent with a delocalized p-system within the C5Sn ring.

SnCl2 R

Al Cl tBu

R Cl

Al

[o-tol]Li R

Sn Cl

R Cl

K[N(SiMe3)2] R

Sn

R

R

Sn

R

Cl

Cl 291 (R = iPr3Si)

Scheme 8 Synthesis of the neutral stannabenzene 291.

10.04.6 Compounds with MdE, M]E and M^E bonds Compounds with Sn-M or Pb-M single or multiple bonds fall into four main categories: (i) simple complexes between a stannylene or plumbylene and a metal center (usually a transition metal), (ii) metallostannylenes and plumbylenes containing an E-M s-bond, (iii) complexes with formal M^E triple bonds, and (iv) complexes of stannate or plumbate anions (see Section 10.04.3.3 above).

10.04.6.1 Complexes of stannylenes and plumbylenes R2E The phosphine-functionalized stannylenes (ArTrip)Sn{CH(Ph)PPh2} (169Sn) and (ArMes)Sn{CH(Ph)PPh2}, reacted with Ni(COD)2 to give the complexes [ArSn{CH(Ph)PPh2}]Ni (292), in which the Ni atom is coordinated by the phosphine donor group and one aromatic ring of the terphenyl ligand; there is also a somewhat long SndNi contact (SndNi 2.677(1) A˚ ).384 A similar reaction between Ni(COD)2 and the closely related stannylenes (ArTrip)Sn(C6H4-2-PR2) (172; R ¼ Ph, Cy) resulted in stepwise loss of the COD ligands and initial formation of {(ArTrip)Sn(C6H4-2-PR2)}Ni(COD) (293), in which the Ni center is coordinated by the P and Sn centers of the stannylene, along with a molecule of COD. Stirring a benzene solution of 293 for extended periods of up to 10 days led to loss of the second COD ligand and formation of {(ArTrip)Sn(C6H4-2-PR2)} Ni (294), which is structurally analogous to 292. Similar reactions between 172 and Pd(PCy3)2 gave the complexes (ArTrip)Sn(C6H4-2-PR2)Pd(PCy3) (295), in which the stannylene binds the Pd center via both its Sn and P atoms. In solution, compounds 295 appeared to undergo loss of PCy3, possibly generating a complex 296 analogous to 294, in which one aromatic ring was Z6-coordinated to the Pd atom; clean access to 296 was subsequently provided by the reaction between 172 and Pd(nbe)3 (nbe ¼ norbornene). The corresponding reaction between Pd(nbe)3 and an excess of 172 gave the bis(stannylene) complex {(ArTrip)Sn(C6H4-2-PPh2)}2Pd (297), although in solution a dynamic equilibrium operated between 296, 297 and 172.385 Similar reactions between 172 and (COD)RhCl or (COD)IrCl resulted in insertion of the stannylene into the M-Cl bond and formation of the stannate complexes 298, while the reaction between 172 and (Me2S)AuCl gave the dimer 299.

Organometallic Compounds of Tin and Lead

H

R

R

469

Ph PPh2 Sn

Ni

R

iPr

PR2

R

iPr Sn iPr

R R

M

iPr

iPr iPr

292 (R = Me, iPr)

294 (M = Ni; R = Ph, Cy) 296 (M = Pd)

PPh2

PR2 Sn ArTrip

Sn

Pd PCy3

ArTrip

Pd

Sn ArTrip M

ArTrip

Ph2P

295 (R = Ph, Cy)

Cl

Sn

297

Cl ArTrip Ph2 P Sn Au PR2 Sn ArTrip

Au Cl

P Ph2

299 298 (M = Rh, Ir; R = Ph, Cy)

Amino- and methoxy-functionalized aryl-supported stannylenes have been used as ligands to transition metal centers. For example, the reactions between [(Z3-C3H5)PdCl]2, [(Z6-C6H6)Ru(Cl)2]2, [(Z6-cymene)Ru(Cl)2]2, [(CO)3Ru(Cl)2]2, or [(CO)2RhCl]2 and (dap)SnCl gave the corresponding stannylene complexes, each exhibiting a single Sn-M bond.386 Similar reactions between (dap)SnCl and Pd(PPh3)4 gave the Pd(II) complex {(dap)SnCl}Pd(Cl)2, regardless of stoichiometry, while the reaction of (dap)SnCl with [CpMo(CO)2(MeCN)2] gave {CpMo(CO)2Cl}Sn(Cl)(dap).387 The tungsten complex {(dap)SnCl} W(CO)5 was isolated from the reaction between (dap)Li and W(CO)5(SnCl2).388 Treatment of this complex with Ag[CB11H12] in the presence of water gave the unusual stannylene complex [{(dapH)Sn(OH)}W(CO)5][CB11H12]. The chromium complex {(dap)SnCl}Cr(CO)5 was isolated in a similar manner from (dap)Li and Cr(CO)5(SnCl2); this compound reacted with NaOMe to give the alkoxide {(dap)Sn(OMe)}Cr(CO)5, which underwent hydrolysis to give [{(dap)Sn(OH)}Cr(CO)5]2.389 The related chromium complex [{C6H3-2,6-(CH2OMe)2}SnCl]Cr(CO)5 has also been isolated. This complex reacted with AgOTf in the presence of water to give [[{C6H3-2,6-(CH2OMe)2}Sn(OH2)]Cr(CO)5][OTf], or with Ag[CB11H12] to give [[{C6H3-2,6(CH2OMe)2}Sn(THF)2]Cr(CO)5][CB11H12].390 Treatment of Pt(PPh3)2Cl2 with two equivalents of (dap)SnCl gave {(dap)SnCl}2PtCl2;391 further treatment of this latter compound with two equivalents of Na[C5H4N-2-S] gave the complex {(Dap)SnCl}Pt(m-C5H4-2-S)2Cl; NBO analysis suggested a strong SndPt interaction in this compound.392 A range of similar complexes {(dap)SnX}2MX2 (M ¼ Pd, Pt; X ¼ Cl, I), (ArSnCl)2MX2 (Ar ¼ 4-tBu-2,6-{P(O)(OiPr2)2}2C6H2; M ¼ Pd, Pt; X ¼ Cl, Br, I), and [(ArSnCl)MI2]2 have also been reported.393 The latter ligand has also been used for the synthesis of the tungsten complexes {[4-tBu-2,6-{P(O)(OiPr2)2}2C6H2]SnX} W(CO)5 (X ¼ F, Cl, PPh2, PPh2W(CO)5).394 Complexes of Fe and W with the stannylene {C6H2-4-tBu-2,6-(SO2C6H4-4-Me)2} SnCl (177) have also been reported.231 The reaction between Cp2Zr(Bu)2 and (ap)2Sn gave the bis(stannylene) complex Cp2Zr{Sn(ap)2}2 as the major product; in contrast, the reaction between Cp2Zr(Bu)2 and (ap)2Pb resulted in the formation of the cyclometalated complex 300.395 The reaction between the stannylene (2,4-tBu2C6H2-6-CH2NEt2)SnCl and W(CO)5(THF) gave the tungsten complex {(2,4tBu2C6H2-6-CH2NEt2)SnCl}W(CO)5, which was reduced to its hydride {(2,4-tBu2C6H2-6-CH2NEt2)SnH}W(CO)5 on treatment with K[Et3BH].396 Subsequent treatment of {(2,4-tBu2C6H2-6-CH2NEt2)SnH}W(CO)5 with (dap)Sn(NEt2) resulted in elimination of Et2NH and formation of the unsymmetrical distannyne complex 301.

470

Organometallic Compounds of Tin and Lead NMe2 Sn

Sn

ZrCp2 NMe2 300

tBu

(OC)5W

NMe2

tBu

N

Dipp 301

10.04.6.2 Metallostannylenes and plumbylenes Metallostannylenes and metalloplumbylenes, in which a metal complex is a direct substituent at the tin or lead center remain rather rare species. A DFT study of the bonding in the model metallostannylenes Cp(L)2Fe(ER) (L ¼ CO, PMe3; E ¼ Si, Ge, Sn, Pb; R ¼ Me, Ph) concluded that the Fe-E bonds in these compounds were single covalent bonds.397 A similar DFT study of the metallostannylenes CpM(PMe3)(H)2(EPh) (M ¼ Fe, Ru, Os; E ¼ Si,Ge, Sn, Pb) also found the M-E bonds to be single in nature, with a covalent contribution ranging from 36% to 48%.398 Tilley and co-workers reported in 2009 that the reaction between Cp Os(PiPr3)(Z3-CH2Ph) and (Trip)SnH3 gave the stannylene complex Cp Os(PiPr3)(H){Sn(H)(Trip)}.399 Thermolysis of this compound at 60  C or photolysis with ambient light gave the metallostannylene Cp Os(PiPr3)(H)2{Sn(Trip)} via a-H migration, according to NMR spectroscopy. A similar synthetic strategy was used for the synthesis of the ruthenostannylene Cp (NHC)Ru(H)2{Sn(Trip)} (302): the reaction between Cp (NHC)Ru(N2) and (Trip)SnH3 gave the stannylene intermediate Cp (NHC)Ru(H){Sn(H)(Trip)}, which rearranged within 2 h at room temperature to give the metallostannylene 302.400 Compound 302 reacted with a range of substrates to give unusual stannylene and stannane complexes of ruthenium, including stannaimine and ketenylstannyl complexes.401 The reaction between the diplumbyne (ArTrip)PbPb(ArTrip) (228) and Mn2(CO)10 or Fe2(CO)9 gave the metalloplumbylenes (CO)5Mn{Pb(ArTrip)} and cis-(CO)4Fe {Pb(ArTrip)}2, respectively. In contrast, the reaction between 228 and Co2(CO)8 gave the cluster Co4(CO)9{Pb(ArTrip)}2.402

A small number of dimetallostannylenes have been reported. The reactions of ArSnCl with K[CpFe(CO)2] gave the metallostannylenes (ArSn)FeCp(CO)2 (Ar ¼ ArDipp, ArTrip).403 Photolysis of this latter compound with UV light resulted in decarbonylation to give the dimetallostannylene dimers [(ArSn){FeCp(CO)}]2. Related dimeric dimetallostannylenes [{(ArDipp)Sn} M(CO)4]2 (303; M ¼ Cr, Mo, W) have been isolated from the reaction between the distannyne (ArDipp)SnSn(ArDipp) (231) and M(CO)6 under UV irradiation.404 In the solid state 303 crystallized as dimers with a planar Sn2M2 core, while DFT studies indicated both a s- and p-component to the Sn-M bonding.

10.04.6.3 Complexes with M^E triple bonds (stannylidynes and plumbylidynes) The first example of a higher homolog of a transition metal alkylidyne, Cp(CO)2Mo^Ge(ArMes) was reported by Power and co-workers in 1996.405 Since then, several further examples of compounds containing M^E (E ¼ Si, Ge, Sn, Pb) triple bonds have been isolated. The unusual plumbylidyne complex trans-(H)(PMe3)4W^Pb(ArTrip) (304) has been prepared by either the reaction between (H)(PMe3)4W(CH2PMe2) and half an equivalent of [(ArTrip)Pb(m-NMe2)]2 or by the reaction of (H)(PMe3)4W (CH2PMe2) with half an equivalent of [(ArTrip)Pb(m-Br)]2 followed by one equivalent of LiNMe2.406 The W^Pb-C unit in 304 was essentially linear and the WdPb distance was 2.5525(3) A˚ . The first complex exhibiting a Mn^Sn triple bond was prepared in a two-step reaction: treatment of H(dppm)2Mn(H2) with (ArMes)SnCl gave the stannylidene complex H(dppm)2Mn{¼Sn(ArMes)Cl}, which was further treated with either Na[BArF4] or Li[Al {OC(CF3)3}4] to give the salt [H(dppm)2Mn^Sn(ArMes)][X] (305; dppm ¼ Me2PCH2CH2PMe2; X ¼ BArF4, Al{OC(CF3)3}4; ArF ¼ 3,5-(CF3)2C6H3).407 Complex 305 featured a very short MndSn distance of 2.3434(5) A˚ and an essentially linear MndSndC unit. Natural Resonance Theory (NRT) analysis indicated a highly polar Mn^Sn triple bond with approximately 65% ionic character. The niobium complex {MeSi(CH2PMe2)3}(CO)2Nb^Sn(ArMes) was prepared by thermolysis/decarbonylation of the metallostannylene {MeSi(CH2PMe2)3}(CO)3Nb{Sn(ArMes)};408 previous attempts to prepare transition metal stannylidyne complexes by the decarbonylation of metallostannylenes had failed.403 Once again, this compound is characterized by an essentially linear Nb^SndC unit and a short NbdSn distance (2.533(1) A˚ ). The reaction between [(ArDipp)SnCl]2 and [K(THF)0.2][Co(1,5-COD)2] gave the bridged stannylidyne complex [(ArDipp) SnCo]2 (306).409 The centrosymmetric Sn2Co2 core of 306 exhibits two single SndCo bonds (SndCo 2.5365(5) A˚ ) and two

Organometallic Compounds of Tin and Lead

471

Sn]Co double bonds (SndCo 2.4071(6) A˚ ); the SndSn distance is similar to that of a typical SndSn bond, but DFT and AIM calculations indicated that there was no SndSn bonding interaction in this compound. Compound 306 reacted with P4 to give the cluster species 307. The rhodium complexes (PMe3)2(PPh3)Rh^E(ArTrip) (E ¼ Sn, Pb) were synthesized by an unusual route involving abstraction of hydride from (Ph3P)2Rh(H)2{E(ArTrip)} by styrene, with concurrent formation of ethylbenzene.410 The Rh^Sn and Rh^Pb distances in (PMe3)2(PPh3)Rh^E(ArTrip) are the shortest so far reported (2.3856(2) and 2.4530(2) A˚ , respectively). Treatment of (PMe3)2(PPh3)Rh^E(ArTrip) with H2 gave the hydrides cis-(Me3P)2(Ph3P)Rh(H)2{E(ArTrip)}.

While alkyne metathesis reactions are well established, similar reactions involving their heavier group 14 analogs were unknown until the recent report by Power and co-workers. The reaction between ArEEAr (E ¼ Sn, Pb; Ar ¼ ArDipp, ArTrip) and (CO)2CpMo^MoCp(CO)2 gave the complexes (CO)2CpMo^EAr (308).411 This represented the first metathesis reaction between two M^M triple bonds.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

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472

Organometallic Compounds of Tin and Lead

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Organometallic Compounds of Tin and Lead

100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172.

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473

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Saito, M.; Sakaguchi, M.; Tajima, T.; Ishimura, K.; Nagase, S. Phos. Sulf. Silicon 2010, 185, 1068–1076. Wrackmeyer, B.; Thoma, P.; Marx, S.; Bauer, T.; Kempe, R. Eur. J. Inorg. Chem. 2014, 2103–2112. Tanikawa, T.; Saito, M.; Guao, J. D.; Nagase, S.; Minoura, M. Eur. J. Org. Chem. 2012, 7135–7142. Parke, S. M.; Boone, M. P.; Rivard, E. Chem. Commun. 2016, 52, 9458–9505. Ramirez y Medina, I.-M.; Rohdenburg, M.; Mostaghimi, F.; Grabowsky, S.; Swiderek, P.; Beckmann, J.; Hoffmann, J.; Dorcet, V.; Hissler, M.; Staubitz, A. Inorg. Chem. 2018, 57, 12562–12575. Linshoeft, J.; Baum, E. J.; Hussain, A.; Gates, P.; Näther, C.; Staubitz, A. Angew. Chem. Int. Ed. 2014, 53, 12916–12920. Ramirez y Medina, I.-M.; Rohdenburg, M.; Rusch, P.; Duvinage, D.; Bigall, N. C.; Staubitz, A. Mater. Adv. 2021, 2, 3282–3293. Matsumara, Y.; Sugihara, M.; Tan, S.-E.; Sato, T.; Hayashi, K.; Nishiyama, H.; Zhou, W.-M.; Inagi, S.; Tomita, I. Macromol. Rapid Commun. 2019, 40, 1800929. Saito, M.; Akiba, T.; Kaneko, M.; Kawamara, T.; Abe, M.; Hada, M.; Minoura, M. Chem. Eur. J. 2013, 19, 16946–16953. Saito, M.; Akiba, T.; Furukawa, S.; Minoura, M.; Hada, M.; Yoshikawa, H. Y. Organometallics 2017, 36, 2487–2490. Haga, R.; Saito, M.; Yoshioka, M. Eur. J. Inorg. Chem. 2007, 1297–1306. Masaichi, S.; Takuya, K.; Kazuya, I.; Shigeru, N. Bull. Chem. Soc. Jpn. 2010, 83, 825–827. Haga, R.; Saito, M.; Yoshioka, M. J. Am. Chem. Soc. 2006, 128, 4934–4935. Saito, M.; Kuwabara, T.; Ishimura, K.; Nagase, S. Chem. Asian J. 2011, 6, 2907–2910. Saito, M.; Haga, R.; Yoshioka, M. Chem. Commun. 2002, 1002–1003. Saito, M.; Haga, R.; Yoshioka, M.; Ishimura, K.; Nagase, S. Angew. Chem. Int. Ed 2005, 44, 6553–6556. Haga, R.; Saito, M.; Yoshioka, M. Chem. Eur. J. 2008, 14, 4068–4073. Saito, M.; Kuwabara, T.; Kambayashi, C.; Yoshioka, M.; Ishimura, K.; Nagase, S. Chem. Lett. 2010, 39, 700–701. Saito, M.; Shimosawa, M.; Yoshioka, M.; Ishimura, K.; Nagase, S. Organometallics 2006, 25, 2967–2971. Kuwabara, T.; Guo, J.-D.; Nagase, S.; Minoura, M.; Herber, R. H.; Saito, M. Organometallics 2014, 33, 2910–2913. Saito, M.; Sakaguchi, M.; Tajima, T.; Ishimura, K.; Nagase, S.; Hada, M. Science 2010, 328, 339–342. Saito, M.; Nakada, M.; Kuwabara, T.; Minoura, M. Chem. Commun. 2015, 51, 4674–4676. Kuwabara, T.; Saito, M.; Guo, J.-D.; Nagase, S. Inorg. Chem. 2013, 52, 3585–3587. Kuwabara, T.; Saito, M. Organometallics 2015, 34, 4202–4204. Kuwabara, T.; Guo, J.-D.; Nagase, S.; Saito, M. Angew. Chem. Int. Ed. 2014, 53, 434–438. Kuwabara, T.; Guo, J.-D.; Nagase, S.; Sasamori, T.; Tokitoh, N.; Saito, M. J. Am. Chem. Soc. 2014, 136, 13059–13064. Kuwabara, T.; Nakada, M.; Guo, J.-D.; Nagase, S.; Saito, M. Dalton Trans. 2015, 44, 16266–16271. Nakada, M.; Kuwabara, T.; Furukawa, S.; Hada, M.; Minoura, M.; Saito, M. Chem. Sci. 2017, 8, 3092–3097. Mizuhata, Y.; Sasamori, T.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc. 2006, 128, 1050–1051. Mizuhata, Y.; Sasamori, T.; Nagahora, N.; Watanabe, Y.; Furukawa, Y.; Tokitoh, N. Dalton Trans. 2008, 4409–4418. Mizuhata, Y.; Noda, N.; Tokitoh, N. Organometallics 2010, 29, 4781–4784. Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S. Angew. Chem., Int. Ed. 2000, 39, 634–636. Wakita, K.; Tokitoh, N.; Okazaki, R.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 5648–5649. Nakata, N.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc. 2002, 124, 6914–6920. Mizuhata, Y.; Fujimori, S.; Noda, N.; Kanesato, S.; Tokitoh, N. Dalton Trans. 2018, 47, 14436–14444. Fujimori, S.; Mizuhata, Y.; Tokitoh, N. Chem. Eur. J. 2018, 24, 17039–17045. Kaiya, C.; Suzuki, K.; Yamashita, M. Angew. Chem. Int. Ed. 2019, 58, 7749–7752. Krebs, K. M.; Freitag, M.; Schubert, H.; Gerke, B.; Pöttgen, R.; Wesemann, L. Chem. Eur. J. 2015, 21, 4628–4638. Krebs, K. M.; Freitag, M.; Maudrich, J.-J.; Schubert, H.; Sirsch, P.; Wesemann, L. Dalton Trans. 2018, 47, 83–95. Martincová, J.; Dostálová, R.; Dostál, L.; Ru˚ žicka, A.; Jambor, R. Organometallics 2009, 28, 4823–4828. Martincová, J.; Jambor, R.; Schürmann, M.; Jurkschat, K.; Honzícek, J.; Paz, F. A. A. Organometallics 2009, 28, 4778–4782. Jambor, R.; Kašná, B.; Koller, S. G.; Strohmann, C.; Schürmann, M.; Jurkschat, K. Eur. J. Inorg. Chem. 2010, 902–908. Dostál, L.; Ru˚ žicka, A.; Jambor, R. Eur. J. Inorg. Chem. 2014, 5266–5270.

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10.05

Organometallic Compounds of Arsenic, Antimony and Bismuth

Josep Cornella and Yue Pang, Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany © 2022 Elsevier Ltd. All rights reserved.

10.05.1 Introduction 10.05.1.1 Preface 10.05.1.2 Organization of the material 10.05.2 Monovalent compounds 10.05.2.1 Carbene-stabilized pnictinidenes 10.05.2.2 Multidentate ligand-stabilized pnictinidenes 10.05.3 Divalent compounds 10.05.4 Trivalent compounds 10.05.4.1 Pnictogen-carbon metal-carbon multiple-bonded compounds 10.05.4.2 Pnictogen-pnictogen bonded compounds 10.05.4.2.1 Diarsines, distibines, and dibismuthines 10.05.4.2.2 Diarsenes, distibenes, and dibismuthenes 10.05.4.2.3 Polypnictogen compounds 10.05.4.3 Pnictogen hydrides 10.05.4.4 Transition-metal-pnictogen bonded compounds 10.05.4.5 Cyclopentadienyl compounds 10.05.4.6 Triorganopnictogen(III) compounds 10.05.4.7 Diorganopnictogen(III) compounds 10.05.4.8 Monoorganopnictogen(III) compounds 10.05.5 Pentavalent compounds 10.05.5.1 Tetraorganopnictogen(V) compounds 10.05.5.2 Triorganopnictogen(V) compounds 10.05.5.3 Diorganopnictogen(V) compounds 10.05.5.4 Monoorganopnictogen(V) compounds 10.05.6 Conclusions Acknowledgment References

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10.05.1 Introduction 10.05.1.1 Preface The history, synthesis, structures and reactivity of organoarsenic, antimony and bismuth compounds have been reviewed in COMC-I (1982),1 COMC-II (1995)2 and COMC-III (2007).3 As a continuation of the previous three editions of COMC, this chapter mainly focuses on the recent developments in this field spanning 2005–2020. An update to Science of Synthesis on the synthesis of organobismuth compounds has been published in 2018.4 Other specialized reviews or book chapters summarized the fields of Pn(I) compounds,5–7 hypervalent organoantimony and bismuth compounds with pendant arm ligands,8 heavier organopnictogen radicals9 and the EPR spectroscopic studies,10 As/Sb/Bi ligands,11–16 interpnictogen cations,17 coordination complexes of Pn(V) cations,18 the organometallic compounds containing BidBi bonds,19 Pn]Pn bonds,20 transition-metalbismuth bonds21 and heavy group 15/16 covalent bonds,22 Sb/Bi ⋯ p-arene interactions,23 pnictogen bonding in coordination chemistry,24 pnictogen redox catalysis,25,26 CdH activation mediated by bismuth compounds,27 organoantimony and bismuth compounds for CO2 fixation,28 organobismuth reagents for organic synthesis,29 and the applications of As/Sb/Bi compounds in material and biological sciences.30–33

10.05.1.2 Organization of the material In this chapter, the term (heavier) pnictogens (Pn) refers to arsenic (As), antimony (Sb) and bismuth (Bi). The material in this chapter is organized by the valency of the pnictogen centers, and then the number of organic ligands attached to the pnictogen centers, following the structure of COMC-II (1995) and COMC-III (2007). The valency is defined as the number of electrons that an atom uses in bonding.34 In this sense, monovalent and divalent organopnictogen compounds are new categories and indeed, the majority of these compounds emerged after COMC-III (2007). However, controversy arises for these two classes of compounds that should be classified further. For monovalent compounds, the trivalent resonance form can be written because the ligand invariably contains certain p-accepting character. For many divalent compounds, the spin-density partially delocalizes from the pnictogen center to the ligands. The general criteria of classification is based on the electronic structures, as revealed by X-ray crystallographic

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and computational analysis in the original literature. The treatment of carbene ligands represents another point of potential conflict because the bonding situations between pnictogen and carbene ligands depend on the pnictogen, the oxidation state of the pnictogen, the types of carbenes (NHC or cAAC) and other supporting ligands. In most cases in this chapter, carbene ligands are presented as dative ligands, however, it should be noted that this is merely out of organizational consideration and might not represent the true electronic structures of these compounds. All compounds discussed in this chapter contain at least one PndC bond. However, numerous “inorganic” ligands have been applied to support a wide variety of pnictogen compounds in different oxidation states in the recent decades. These ligands include Ga-, In- and Ge-based ligands, diamido ligands and N-heterocyclic boryls (NHB), among others. The compounds supported by these ligands are briefly mentioned in this chapter, especially when they represent a class of pnictogen compounds without precedent.

10.05.2 Monovalent compounds Monovalent organopnictogen compounds primarily are termed pnictinidenes (As, arsinidenes; Sb, stibinidenes; Bi, bismuthinidenes). Genuine pnictinidenes (RdPn) are neutral and singly-coordinated compounds, which have an incomplete electron octet (6 electrons) and in principle can possess both singlet and triplet ground states (Fig. 1A). Singlet pnictinidenes exhibit both Lewis acidic and basic properties due to the presence of both empty and occupied p orbitals. As a result, pnictinidenes can be stabilized by coordination of Lewis bases or by coordination to transition metal fragments. According to the canonical counting rule of oxidation states based on the relative electronegativities of the bonding atoms, the majority of pnictogen atoms in these compounds are in the +1 oxidation state. In general, pnictinidenes are highly unstable and tend to undergo dimerization or oligomerization, forming dipnictenes (RdPn]PndR) or linear and cyclic oligomers. The use of sterically hindered ligands is key to the isolation of dipnictenes, but is insufficient for the stabilization of pnictinidenes. In contrast to phosphinidenes, the heavier pnictinidenes have been much less developed and isolated examples of genuine heavier pnictinidenes still remain elusive. Nevertheless, the recent decades have witnessed the successful isolation and characterization of Lewis base-pnictinidene adducts. In these compounds, both sterically demanding ligands and inter- or intra-molecular coordination were applied, providing crucial kinetic and thermodynamic stabilization of the pnictogen centers. As many of these ligands are also p-acceptors, a certain degree of multiple bond character exists between the pnictogen centers and the ligands (Fig. 1B). As a result, it is quite difficult to distinguish pnictinidenes and pnicta-alkenes in an unambiguous way. However, the majority of the compounds discussed in the following section are better described as pnictinidenes instead of pnicta-alkenes because of weak double bond character. Pnictinidenes are mainly synthesized by reduction of the corresponding monoorganopnictogen dihalides. The common reductants consist of metal hydrides (e.g., K[B(sBu)3H]) and single-electron reductants (e.g., KC8, Cp2Co). The field of heavier pnictinidenes has received considerably more attention in the last decade, albeit reported examples still remain rare. An excellent review has recently summarized the state-of-the-art of the field until 2017.6

10.05.2.1 Carbene-stabilized pnictinidenes The arsinidenes 1 stabilized by NHC ligands were synthesized by desilylation of NHC-As-SiMe3.35 An alternative method is the reaction of [Na(dioxane)x][AsCO] with imidazolium chlorides. cAAC-supported chloroarsinidene 2 was synthesized by reduction of corresponding cAAC-AsCl3 system with tetrakis(dimethylamino)ethylene (TDAE) or KC8.36 Bis-NHC- and bis-cAAC-stabilized diarsenic compounds can be regarded as diarsinidines with certain As]C bond character.36,37

Fig. 1 Pnictinidenes (A) and Lewis base-stabilized pnictinidenes (B).

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Stepwise reduction of cAAC-SbCl3 adduct 3 with 2 or 3 equivalents of KC8 affords a cAAC-stabilized chlorostibinidene 4 and a diatomic species (5) with formal oxidation state of 0, respectively (Scheme 1).38 Compound 4 is better described as a cAACstabilized-stibinidene instead of an stiba-alkene due to weak p-back donation. Stibinidenes supported by an electron-deficient diamidocarbene were synthesized by reduction of the corresponding NHC-SbPhCl2 complexes with Mg.39

Scheme 1 Stepwise reduction of cAAC-SbCl3 adduct 3.

cAACdBiPhCl2 6 is reduced by (cAAC)2Be to give cAAC-stabilized bismuthinidene 7 (Scheme 2), which rapidly decomposes to bismuth metal and free cAAC ligand in solution at 0  C, resulting in a very low isolated yield.40 The partial double bond character of the Bi-Ccarbene bond is reflected in the short bond distance of 2.199(2) A˚ .

Scheme 2 The synthesis of cAAC-stabilized bismuthinidene 7.

Treatment of SbF3 and BiCl3 with 2 equivalents of KC8 in the presence of cAAC ligands affords cAAC-stabilized Sb(I) and Bi(I) cations, [(cAAC)2Sb][OTf] 8a and [(cAAC)2Bi][OTf] 8b, respectively.41 Compound 8b is thermally unstable and decomposes upon dilution. X-ray crystallographic and computational analysis revealed that systems of type 8 are heavier valence isoelectronic analogues of carbones, featuring dicoordinated Sb(I) and Bi(I) centers with two lone pairs of electrons.

Germylidenylarsinidene complex 9 was synthesized by trapping arsagermyne intermediate with an NHC ligand.42 (RGaX)2SbX (R]CH[C(Me)N(Dipp)]2; X]Cl, Br) react with cAAC and IDipp to yield the corresponding stibinidenes 10.43

10.05.2.2 Multidentate ligand-stabilized pnictinidenes Intramolecular coordination of bidentate or tridentate ligands also enables the stabilization of pnictinidenes. A geometrically enforced phosphine donor facilitates the facile dehydrocoupling of phosphine-arsine peri-substituted acenaphthene 11, yielding an arsanylidene-phosphorane 12 (Scheme 3).44 The constrained geometry also results in an arsenic center with negligible steric hindrance and high thermal stability of 12. The PdAs bond lengths have been found to be 2.260(1) and 2.262(2) A˚ in 12, and they represent PdAs single bonds with a moderate double-bond contribution. 12 exhibits arsinidene reactivity through the formation of cyclooligoarsines upon oxidation of the phosphine donor.

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Scheme 3 The formation of arsanylidene-phosphorane 12 via dehydrocoupling.

NCN-pincer azaarsoles 13 are masked arsinidenes, showing fluxional structures in solution as depicted in Scheme 4. The structure of 13 in the solid state is desymmetrized, in which the arsenic center only coordinates with one of the imine arms and forms an AsC3N heterocycle.45

Scheme 4 Fluxional structures of NCN-pincer azaarsoles 13.

Application of bis-imine NCN-pincer ligands to Sb and Bi analogues enabled the isolation and characterization of the first monomeric Sb(I) and Bi(I) complexes 14 in 2010, representing a breakthrough in the field of heavier pnictinidenes.46 In these and later reported complexes, the Sb(I) and Bi(I) centers are stabilized by sterically hindered substituents on the imine moieties, strong N ! Pn coordination, the rigidity of the pincer scaffolds and aromaticity of the PnC3N rings.46–49 The monomeric structure of the pnictinidene is maintained when one of the imine arms is replaced with a sterically bulky tBu (15)47 or a weak coordinating amino group (16).48 As revealed by noticeably short Bi/SbdN distances [BidN: 2.230(4) A˚ , SbdN: 2.254(6) A˚ ] and nucleus-independent chemical shift (NICS), the aromaticity of 15 and 16 with one imine donor is significant. With two amine arms, LBi(I) [L ¼ 2,6(Me2NCH2)2C6H3] is highly unstable and can be trapped by diphenyldichalcogenides to yield LBi(EPh2) (E]S, Se, Te).50,51

The coordination chemistry of NCN-chelated pnictinidenes with a variety of transition metals has been investigated. The reported transition metal centers include Cr, Mn, Fe, Co, Mo, Rh, Pd, W, Ir, Pt and Au.52–58 The preferred coordination mode of pnictinidene ligands is the two-electron side-on manner, using the p-type lone pair, while pnictinidenes as four-electron m-bridging donors have also been reported in some cases, such as in [Pd(
3-allyl)X2](m-RSb) [allyl]C3H5 or C3H4Me; X]Cl or CF3CO2; R] C6H3d2,6-(CH]NtBu)2], in which the s and p lone pairs are hybridized.58 Stoichiometric reactivity of NCN-chelated Sb(I) and Bi(I) compounds has been investigated to a limited extent. They react with diorganodichalcogenides (e.g., PhSSPh) and elemental chalcogens (S, Se), leading to the organochalcogenolates [e.g., C6H3d2,6d (CH]NtBu)2Sb(SPh)2] and chalcogenides [e.g., C6H3d2,6d(CH]NtBu)2Sb]S], respectively.50,51,59 Compounds of type 17 behave as masked dienes, undergoing hetero-Diels-Alder reactions with electron-deficient alkenes and alkynes to yield 18 and 19, in a reversible and an irreversible manner (Scheme 5).60,61 Bismuthinidenes have been reported to conduct oxidative addition reactions towards polar CdX bonds of alkyl iodides and triflates.62

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Scheme 5 The reactivity of 17 with activated alkenes and alkynes.

The redox cycling between +1 and +3 oxidation states of bismuthinidenes has been successfully applied in several catalytic redox processes, including a transfer hydrogenation of azo- and nitro-arenes,63 electrocatalytic H2 evolution reactions64 and N2O decomposition.49 In the proposed catalytic cycle of N2O decomposition, activation of N2O by the Bi(I) catalyst 17b affords N2 and oxo-bismuth(III) intermediates, and regeneration of 17b is achieved by O-abstraction from the oxo-bismuth(III) intermediate by pinacolborane (Scheme 6).49

Scheme 6 N2O decomposition via Bi(I)/Bi(III) redox catalysis

Highly reactive methylbismuth (BidMe) is generated via BidC bond homolysis in the gas phase, which can be trapped by phenyl disulfide in the condensed phase to form MeBi(SPh)2.65 Photoelectron spectroscopy and computational studies revealed that this species possesses a triplet ground state. A T-shaped planar bismuth triamide with the Bi(I) electronic structure has also been reported.66

10.05.3 Divalent compounds With an unpaired electron located in the p orbital, divalent organopnictogen compounds possess inherent radical properties (Fig. 2, type A). Other structures of organopnictogen radicals comprise triplet pnictinidenes (type B), the corresponding radical cations and anions of dipnictenes (type C and D), pnicta-alkenes (type E and F) and pnictines (type G and H). The pnictogen-centered radicals are highly prone to dimerization and other quenching pathways. As a result, the long-lived heavier organopnictogen radicals have not been extensively studied until the last decade. The isolation and characterization of long-lived heavier organopnictogen radicals relies on several strategies, including the introduction of sterically bulky ligands, delocalization of spin density among p-systems and utilization of carbene ligands. In the majority of the reported examples, the spin density is mainly localized on the heavier pnictogen atoms, as revealed by EPR and DFT studies. The field has been reviewed in 2020.9 This section summarizes the chemistry of divalent organopnictogen radicals and other types of organopnictogen radicals will be discussed in other sections. The stable arsinyl radical 20a and the first persistent stibinyl and bismuthinyl radicals 20b and 20c were synthesized by the reduction of the parent pnictogen chlorides with KC8.67,68 The constraint of the ligand scaffold enables efficient steric protection of

Organometallic Compounds of Arsenic, Antimony and Bismuth

483

Fig. 2 Different types of pnictogen radical.

the pnictogen radical centers with four trimethylsilyl substituents. In solution, 20b/20c and dimers 21b/21c participate in a temperature-dependent equilibrium, which at 298 K are predominantly in favor of the radical forms (Scheme 7). These species exist in dimeric forms in the solid state, with exceptionally long PndPn distances [21b, SbdSb 3.0297(4) A˚ ; 21c, BidBi 3.1821(3) A˚ ]. Radicals 20 can be trapped by TEMPO and reversible homolytic cleavage of the PndO bonds has been observed in the cases of 20a and 20c (Scheme 7).

Scheme 7 The stable arsinyl radical 20a and the persistent stibinyl and bismuthinyl radicals 20b and 20c.

Reduction of a cAACdSbCl3 adduct with 1 equivalent of KC8 leads to the radical species 22.38 The structure of 22 was not confirmed by X-ray crystallography, however, EPR studies suggest a T-shaped antimony-centered radical instead of a ligand-centered radical with a trigonal pyramidal geometry of Sb. One-electron oxidation of [(cAAC)2As]Cl with NOPF6 results in the dicationic arsenic radical 23 with the spin density mainly located on the arsenic center (Mulliken spin density for As: 0.868).36

The first stable bismuthinyl radical [O{Si(Me)2N(Dipp)}2]Bi ∙ was obtained by the reduction of the corresponding bismuth chloride with Mg,69 LiHBEt3 or LiPPh2.70 The radical is bismuth-centered with >90% spin density in a p-type orbital on bismuth. This radical has found applications in white phosphorus (P4)70 and S8 activation,71 and in the catalytic oxidative coupling reaction of TEMPO and phenylsilane.72 LGa (L]HC[C(Me)NDipp]2) inserts into the SbdCl bonds of Cp SbCl2, followed by extrusion of the Cp ∙ radical, leading to species 24 (Scheme 8).73 A heteroleptic antimony-centered radical 25 is obtained from Cp (R)SbCl (Scheme 8).74 Similarly, reaction of LGa with Cp BiI2 or BiCl3 gives the corresponding Ga-stabilized bismuth radicals.73,75

484

Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 8 The formation of Ga-substituted homoleptic and heteroleptic antimony radicals.

In addition to these well-defined examples, the involvement of transient divalent organopnictogen radicals has been proposed in several processes, such as the ring-opening reaction of tetrahydrofuran,76 photochemically-induced dehydrocoupling of TEMPO and silanes,77 as well as the transition-metal-bismuthine-catalyzed cycloisomerization of iodoolefins.78

10.05.4 Trivalent compounds 10.05.4.1 Pnictogen-carbon metal-carbon multiple-bonded compounds Pnictogen-carbon multiple-bonded compounds have a gradually higher tendency towards oligomerization upon descending group 15. Utilization of sterically demanding ligands or incorporation of Pn]C bonds into delocalized systems are the common strategies for stabilization of these compounds. The base-stabilized pnictinidenes containing certain degrees of Pn]C bond character have been discussed in section 10.05.2. Bis-dehydrochlorination of HCCl2AsH2 with Na2CO3 at high temperatures (110–335  C) yields unsubstituted methylidynearsine (HC^As), which has been studied in the gas phase.79 The arsaethynthiolate (AsCS−) and the arsaethynselenolate (AsCSe−) anions were synthesized by reacting NaAsH2 with (EtO)2C]E (E]S, Se), and have near linear geometries with triple bond character for the As^C bonds.80 A 1,3-diarsaallendiide (AsCAs2−) ligand has been generated in the coordination sphere of an uranium complex.81 A mixed group 14 and 15 heteroallene 26 exclusively exhibits the reactivity of the Ge]C bond except in the [3 + 2] cycloaddition with diphenylketene.82,83 Dechlorofluorination of R0 Sb(F)dC(Cl)]CR2 (CR2]fluorenylidene, R0 ]2,4,6-tBu3C6H2) by tBuLi gives a transient stibaallene, which undergoes head-to-head dimerization of the Sb]C bonds to give 27.84 The elimination of CO from LGeAs]C]O 28 is induced by light or external PPh3 and NHC ligands.42

An arsaalkene (C5Me5)As]CH2 is transformed into the parent 2,3-diarsa-1,3-butadiene (CH2]AsdAs]CH2) in the coordination sphere of W(CO)5 moieties (Scheme 9).85

Scheme 9 The parent 2,3-diarsa-1,3-butadiene stabilized by W(CO)5.

Organometallic Compounds of Arsenic, Antimony and Bismuth

485

2-arsa-1,3-butadienes undergo one-electron oxidation with GaCl3 to afford the radical cations of type 29.86 The unpaired electron is delocalized over the CAsCN p-conjugated system. The arsa-alkene radical anion 30 and the diradical dianion 31 were obtained by reducing the neutral forms with potassium and KC8, respectively.87 Species 31 possesses an open-shell singlet ground state, with a small singlet-triplet gap (1.0 kcal/mol), which can readily be thermally excited. The average spin-spin distance of 3.4 A˚ in 31 estimated from EPR spectroscopy is considerably shorter than the distance between two As atoms (5.8 A˚ ), suggesting spin delocalization over the bridging ligand in 31.

A 2,1-Benzazaarsole 32 undergoes cycloaddition to activated alkynes, followed by CH ! NH migration of 33, affording 1-arsanaphthalenes 34 (Scheme 10).88

Scheme 10 The formation of 1-arsanaphthalenes 34 via cycloaddition and migration.

The stabilities of arsa-, stiba- and bisma-benzenes decrease in order As, Sb, Bi. Unsubstituted stibabenzene and (un)substituted bismabenzenes reversibly dimerize via Diels-Alder reactions. Protection by sterically demanding SiiPr3 groups at the ortho-positions enabled the isolation of the first stable bismabenzene 35 (Scheme 11).89 35 features a planar ring without bond alterations in the X-ray crystal structure. In addition, the 1H NMR signals of 35 are deshielded and a considerably negative nucleus-independent chemical shifts (NICS) is found from computational studies. These data collectively confirm the aromaticity of 35.

Scheme 11 The synthesis of a stable bismabenzene 35.

2− 0 − 0 The arsenic Zintl cluster anions (As3− 7 or HAs7 ) react with acetylenes to yield 1,2,3-triarsolide anions [As3C2RR ] (R, R ]H or 90,91 Ph), which act as ligands to the ruthenium complex 36 and the molybdenum complexes 37. The arsa-alkyne 2,4,6-tBu3C6H2C^As undergoes [3 + 2] cycloaddition with an organic azide to afford a 3H-1,2,3,4-triazaarsole 38.92

486

Organometallic Compounds of Arsenic, Antimony and Bismuth

10.05.4.2 Pnictogen-pnictogen bonded compounds The bonding of As, Sb and Bi via single or double bonds leads to a variety of compounds, including tetraorganodipnictines (R2PndPnR2), dipnictenes (RdPn]PndR) and polypnictogen compounds.

10.05.4.2.1

Diarsines, distibines, and dibismuthines

PndPn bonds can be conventionally forged via reduction of diorganopnictogen halides with Mg68 or KC893 or via decomposition of unstable pnictogen hydrides with H2 extrusion.70 The reduction of BidO bonds with reagents containing PdH or SidH bonds has also been reported for bismuthinidene synthesis.77,94 In addition to the homolytic cleavage of BidH bonds, BidP bond cleavage can also lead to BidBi bond formation with generation of (Ph2P)2.70 A bis(stibahousene) 39 was isolated from the mixture of the reaction of SbF3 and a cyclobutadiene dianion (Scheme 12).95 It was proposed that the SbdSb bond of 39 forms through the coupling of the SbdLi bond of the transient lithium antimonide with the SbdF bond of the stibahousene.

Scheme 12 The synthesis of a bis(stibahousene) 39.

All tetraorganodipnictines are highly air-sensitive compounds. The insertion of dioxygen into PndPn bonds was proposed as the initial step of oxidation and as an evidence, peroxide 41 has been isolated at low temperature by slow reaction of a dibismuthine 40 with air (Scheme 13).93 41 is unstable at room temperature, decomposing to the corresponding oxide (R2Bi)2O with liberation of O2. The reaction of 40 with air at room temperature causes the partial oxidation of an amino arm of each ligand, yielding 42 (Scheme 13).

Scheme 13 The reactions of a dibismuthine 40 with air.

Thermochromic behavior has been observed in distibines and dibismuthines that have short intermolecular Pn


Pn contacts, and this behavior was attributed to the electron delocalization over the intermolecular chains consisting of geometrically close pnictogen centers. Et4Sb2 has a short intermolecular Sb


Sb distance of 3.6883(5) A˚ while the closest Bi


Bi distance of Et4Bi2 [4.4598(4) A˚ ] exceeds the van der Waals radii (4.14 A˚ ) However, both compounds exhibit thermochromic behavior.96 On the other hand, the Sb


Sb distances remain similarly short (ca. 3.7 A˚ ) in the yellow and the red phases of Et4Sb2.97 Therefore, the origin of this phenomena still remains elusive. The dehydrocoupling of HAsPh2, mediated by the bismuth amide Bi(NR2)3 (R ]4dMedC6H4), leads to selective formation of (AsPh2)2.98

10.05.4.2.2

Diarsenes, distibenes, and dibismuthenes

The use of sterically demanding ligands prevents dipnictenes from undergoing dimerization or polymerization. The common choices are highly bulky aryl ligands such as 2,4,6-[(Me3Si)2CH]3C6H2 (Tbt) and m-terphenyls. In recent years, ferrocenyl99 and inorganic ligands such as phosphanyl,100 gallium,101,102 and N-heterocyclic boryl (NHB)103 have been applied. The distibene and dibismuthene 43 were prepared by reduction of the corresponding antimony and bismuth halides with Mg.99 The dibismuthene 44 features weak coordination of the amino arms to Bi [NdBi, 2.922(6) A˚ ] and an elongated Bi]Bi bond [2.8734(5) A˚ ].47 The synthesis of TbbdPn]PndTbb (Pn]Sb, Bi; Tbb]4dtBud2,6d[CH(SiMe3)2]2dC6H2) has been improved by using an inorganic-salt-free reducing agent, enabling the convenient isolation of these barely soluble dipnictenes.104 London dispersion interactions are a crucial factor for stabilization of [Pn]Pn]2+ (Pn]As, Sb) in 45 using an anionic NHC ligand with a weakly coordinating borate.105

Organometallic Compounds of Arsenic, Antimony and Bismuth

487

Radical anions and cations of heavier dipnictenes have been reported in the recent two decades. BbtdPn]PndBbt (Pn]Sb, Bi) undergo reversible one-electron redox processes in cyclic voltammetry. The reduction of BbtdSb]SbdBbt with Li affords the radical anion 46, which is the first Sb-centered radical structurally characterized in the solid state.106 The SbdSb bond distance of 2.7511(4) A˚ in 46 is between the distances of SbdSb single and double bonds. One- and two-electron oxidation of the carbene-stabilized diarsenic with GaCl3 gives the monocationic diarsenic radical 47 and the dicationic diarsene. Compound 47 is the first arsenic radical confirmed by X-ray crystallographic analysis.107 A similar method was used to synthesize the divinyldiarsene radical cations 48 and the corresponding dications.108

An unsymmetrical dipnictene 50 is obtained by chloride abstraction of 49 with GaCl3; the single-electron reduction of 49 affords the radical species 51 (Scheme 14).109

488

Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 14 The formation of a dipnictene 50 and a radical species 51.

Distibenes and dibismuthenes undergo chalcogenation reactions with S8, Se and (nBu)3P]Te.110,111 Selenization and tellurization reactions of dipnictenes give chalcogenadipnictiranes 52 with the structures featuring three-membered heterocycles. Sulfurization reaction of dipnictenes affords five-membered ring compounds, trithiadistibolane and trithiadibismolane 53, as the major products.

Coordination of dipnictenes to transition-metal moieties leads to complexes with electronic structures between three-membered metallacycles and p-complexes. Cp2Zr(BiR)2 54 is better described as a metallacycle,112 while W(CO)5(SbR)2 55 is a p-complex.113 It has also been reported that the HAs]AsH unit can be stabilized by two sterically demanding uranium moieties.114

10.05.4.2.3

Polypnictogen compounds

In polypnictogen compounds, three or more pnictogen atoms are arranged in monocyclic or polycyclic arrays. PEt The reaction of CpPEt ∙ radical and As4 gives a butterfly compound CpPEt ]C5(4dEtC6H4)5] (Scheme 15).115 This 2 As4 56 [Cp reaction is reversible and As4 is released upon exposure of 56 to light or upon heating.

Scheme 15 The reversible reaction of CpPEt ∙ radical and As4.

Organometallic Compounds of Arsenic, Antimony and Bismuth

489

Without a sterically demanding substituent at the 6-position, the reduction of [2d(CH]NDipp)C6H4]SbCl2 with K[B(sBu)3H] yields the cyclotetramer 57 instead of the stibinidene.116 In the reduction of ArSbCl2 [Ar]2,6-(Me2NCH2)2C6H3], the Ar3Sb5 cluster 58 was formed along with the tetrameric Ar4Sb4.117 A series of cyclic As3,118 Sb3,119 Sb4,102,120 Sb8,121,122 Bi4 and Bi875 species have also been reported, stabilized by Ga- and Mg-based ligands. The oxidation of the arsanylidene-phosphorane 12 leads to cyclooligoarsines 59a and 59b.44

10.05.4.3 Pnictogen hydrides The stability of the pnictogen hydrides decreases dramatically on descending group 15 (As > Sb > Bi) and bismuth hydrides are extraordinarily unstable. In the literature, it has been speculated that the instability of heavier pnictogen hydrides results from a kinetic lability caused by coordinative unsaturation.123,124 NHCdAsdH 135 and phosphine-arsine peri-substituted acenaphthene 1144 have been discussed in sections 10.05.2.1 and 10.05.2.2. Antimony hydrides are isolable when protected by sterically demanding substituents, for example, (2,6-Mes2C6H3)Sb(R)H (R] Me, Ph)125 and (2,6-Mes2C6H3)SbH2.126 Inorganic ligands have been widely used in the recent work of SbdH chemistry, such as NN-chelating diamidodisiloxanes127 and diamidonaphthalenes,128 Lewis-base coordinated boryls,129 Ga-124 and In-ligands.130 Arsenic and antimony hydrides can be transformed into arsenide and stibide ligands in the coordination sphere of transitionmetal complexes. For instance, deprotonation of yttrium–arsine complex 60 using nBuLi results in the m-arsenide complex 61 (Scheme 16).131 In the catalytic dehydrocoupling of stibines, the stibide 62 was isolated as the intermediate, which delivers ArdSb]SbdAr (Ar ¼ 2,6-Mes2C6H3) and regenerates the catalyst CpCp Hf(H)Cl (Scheme 17).126

Scheme 16 The formation of a m-arsenide complex 61.

Scheme 17 The stibide 62 involved in a catalytic dehydrocoupling of stibines.

490

Organometallic Compounds of Arsenic, Antimony and Bismuth

Since the seminal report on (2,6-Mes2H3C6)2BiH in 2000,123 no other X-ray crystal structure of a bismuth hydride has been disclosed. In another report, the hydride signal of (2,6-Mes2H3C6)2BiH was located at dramatically deshielded position (19.6 ppm in C6D6),132 which was explained by a spin-orbit heavy-atom effect on the light atom (SO-HALA effect).133 Bismuth hydride intermediates have been suggested in the synthesis of Bi(I) compounds46,47 and catalytic reactions such as transfer hydrogenation of azo- and nitro-arenes,63 electrocatalytic H2 evolution64 and the oxidative coupling of silanes with TEMPO.72,77

10.05.4.4 Transition-metal-pnictogen bonded compounds R2Pn moieties can be linked to transition-metal moieties via covalent bonds. The chemistry of complexes containing transitionmetal-bismuth bonds has been reviewed in 2010.21 Thermolysis of Os3(CO)11(SbMe2Ar) clusters (Ar ¼ Ph, o-tolyl, p-tolyl) leads to bimolecular oxidative addition of the SbdC and CdH bonds, forming Os3(m-SbMe2)(m-H)(m3,
2-C6H3R)(CO)9.134 Oxidative insertion of Pt(PEt3)3 to the BidCl bond of 63 is kinetically favored and reversible (Scheme 18).135 The BidCl oxidative addition intermediate 64 is gradually converted to the BidC oxidative addition complex 65.

Scheme 18 The oxidative insertion of Pt(PEt3)3 to compound 63.

The reaction of [(o-tolyl)2BiOMe]n with Cp2ReH yields (o-tolyl)2BidReCp2 66 which features a BidRe bond, while subsequent deprotonation of the cyclopentadienyl ligand occurs in the reaction of Bi(OtBu)3 with Cp2ReH, resulting in compound 67, featuring a BidCCp bond (Scheme 19).136

Scheme 19 The formation of BidRe bonds.

LiSb(H)Ar 68 reacts with CpCp Hf(Me)OTf to generate thermally unstable CpCp Hf]SbAr, which is trapped by PMe3 or 2-butyne to afford CpCp Hf(PMe3)]SbAr 69 or a metallastibacyclobutene 70, respectively (Scheme 20).137

Scheme 20 The formation of SbdHf bonds.

Organometallic Compounds of Arsenic, Antimony and Bismuth

491

The absence of a S ! Bi interaction in [S(CH2C6H4)2]Bid[TM] ([TM]]Co(PPh3)(CO)3, CpFe(CO)2, Mn(CO)5) indicates the weakly polarized BidTM bonds.78 BiPh2[Mn(CO)5] has found catalytic application in radical cycloisomerizations of d-iodoolefins.

10.05.4.5 Cyclopentadienyl compounds The hapticity of the cyclopentadienyl ligand at pnictogen centers can variously be
1,
2,
3 or
5. More covalent bonding between cyclopentadienyls and Pn favors lower hapticity. Chloride abstraction from Cp AsCl2 with 2 equivalents of [Et3Si][B(C6F5)4] yields the Lewis superacid [(
5-Cp )As(toluene)][B(C6F5)4]2 71.138 Compound 71 abstracts fluoride and chloride from [NBu4][SbF6] and Ph3CCl, respectively, affording [(
2-Cp )AsX][B(C6F5)4] 72. In addition, 71 also activates THF to afford [(
2-Cp )AsO(CH2)4(THF)] [B(C6F5)4]2 73 (Scheme 21).

Scheme 21 The reactivity of a Lewis superacid 71.

CpBIGt-BuPnCl2 [CpBIGt-Bu ¼ C5(4dtBudC6H4)5; Pn]As, Sb, Bi] were obtained by the reaction of CpBIGt-BuK and PnCl3.139 Depending on the halide substituent at Bi, the reactions of BiX3 (X]Cl, Br, I) with 1 equivalent of Cp Li lead to [
5-Cp 5Bi6Cl12] [(THF)2Bi2Cl7], Cp BiBr2 and [(Cp 5Bi5Br9)(BiBr4)]2, and Cp BiI2; the coordination modes of the cyclopentadienyl ligand in Cp BiBr2 and Cp BiI2 change from
1 in solution to
3 and
2 in the solid state and correspondingly, the aggregation states of Cp BiBr2 and Cp BiI2 change from monomeric to polymeric mainly through BidX⋯ Bi intermolecular interaction.140 Due to the rapid haptotropic rearrangement, cyclopentadienyl compounds are often fluxional even at low temperatures. For instance, the Cp methyl groups in [Me2Si(NDipp)2]SbCp are indistinguishable at 193 K.141 Cyclopentadienyl compounds of pnictogens are generally air-, moisture-, light- and thermo-sensitive. Unlike the parent Cp3As and CpSbI2, As(C5Me4H)3 and Sb(C5Me4H)I2 possess considerably higher thermo-stability and show no photo-sensitivity.142 The reactions of Cp (R)SbCl (R¼2,6-Mes2C6H3, Dipp) and LGa (L ¼ HC[C(Me)N(Dipp)]2) lead to the heteroleptic Sb-center radical 2574 or antimony hydride 74,124,130 via elimination of the Cp radical or fulvene (Scheme 22). This strategy was also applied to the Bi analogues, and the reactions of Cp BiI2 and Cp 2BiCl with LGa result in a Bi-centered radical73 and a Ga-substituted tetrabismuthine,75 respectively. The steric bulk of CpAr [CpAr]C5(4dtBuC6H4)5] and the stability of CpAr ∙ leads to AsdC bond cleavage of CpArAsCl2 in the reaction with LGa, resulting in the corresponding As-centered radical.143 Ga]As bonded 75 was obtained from the reaction of Cp AsCl2 with 2 equivalents of LGa (Scheme 23).118 AsdC bond cleavage in CpPEt 2 As4 56 was found to be reversible.115

Scheme 22 The formation of antimony-centered radical 25 and antimony hydride 74.

Scheme 23 The formation of As]Ga bond.

492

Organometallic Compounds of Arsenic, Antimony and Bismuth

10.05.4.6 Triorganopnictogen(III) compounds The well-established methods of preparing triorganopnictogen(III) compounds are the reactions of pnictogen halides (RnPnX3-n) with organometallic reagents, among which organolithium and Grignard compounds are the most common choices. The reactions of pnictogen trihalides with organolithium and Grignard reagents give symmetrical triorganopnictogen(III) compounds. Recent examples include tris(selenophen-2-yl)stibine and bismuthine,144 tris(2-formylphenyl)stibine,145 [(2,4,6-tBu3C6H2) P]C(Cl)]3Sb,146 [2-{E(CH2CH2)2NCH2}C6H4]3Bi (E]O, NMe),147 tris(1,10 -formylferrocenyl)stibine and bismuthine,148 (iPr2P-Ace)3Pn (Ace ¼ acenaphthene-5,6-diyl),149 tris(pyrenyl)pnictines,150 tris[2-(2-pyridyl)phenyl]stibine and bismuthine.151 The reaction of BiCl3 and a triply lithiated reagent affords a bismasilatriptycene.152 Use of much less volatile AsBr3 enables a safer and more convenient synthesis of a variety of triorganoarsenic ligands.153 It has been reported that mechanochemical reactions of PnX3 (Pn]As, X]I; Pn]Sb, Bi, X]Cl) and [1,3-(SiMe3)2C3H3]nM (M]K or Al) generates tris(allyl)pnictines with stereoselectivity that is different from the solution-phase reactions.154 Unsymmetrical triorganopnictogen compounds can be obtained from the reactions of monoorgano- or diorgano-pnictogen halides with organometallic reagents. For example, several open-chain and cyclic stibine and distibine ligands have been synthesized from SbMenCl3-n (n ¼ 1, 2) and dilithio-reagents.155 Successive treatment of PhSbCl2 with [2-(Me2NCH2)C6H4]Li and MesMgBr yields the corresponding stibine with three different aryl substituents.156 The synthesis of unsymmetrical triorganobismuthines can be complicated due to dismutation process, which can be suppressed when Ar2BiX (X]TsO, I) and ArBiX2 (X]TsO) are used in low concentration during the reactions.157 Sodium tetraarylborate salts Na[BAr4] (Ar]Ph, tolyl, 4dFdC6H4) undergo arylation of bismuth(III) carboxylates.158 Transmetallation reactions of arylboronic acids or esters into bismacycles occur in the presence of bases, which is one of the elementary steps of Bi(V)-mediated arylation reactions of phenols/naphthols,159 and Bi(III)/Bi(V)-catalyzed fluorination and triflation/nonaflation reactions.160,161 A representative reaction is given in Scheme 24. The transmetallation reactions of aryland alkyl-boronic acids to halostibines catalyzed by Ni(OAc)2 has also been reported (Scheme 25).162 The redox transmetallation of Ag(C5F4N) to elemental As, Sb and Bi affords As(C5F4N)3 and extremely air- and moisture-sensitive Sb(C5F4N)3 and Bi(C5F4N)3.163

Scheme 24 The transmetallation reactions of arylboronic acids into a bismacycle.

Scheme 25 The transmetallation reactions to halostibines catalyzed by Ni(OAc)2.

An alternative synthetic route is the reaction of alkali metal-pnictogen compounds and organic electrophiles, such as alkyl halides. This method was used in the synthesis of distibine164 and dibismuthine ligands.165 Arsenic homocycles (RAs)n have been recognized as precursors to a variety of triorganoarsines. The radical reactions of (MeAs)5 with alkynes in the presence of AIBN yield cis(e,e)-1,4-dihydro-1,4-diarsinines 76 (Scheme 26).166,167 Treatment of (PhAs)6 with I2 and PhLi gives PhAsI2 and Ph2AsLi intermediates, which react with 8-lithioquinoline and 8-bromoquinoline to afford a NAsN-ligand 77 and a NAs-ligand 78, respectively (Scheme 26).168

Organometallic Compounds of Arsenic, Antimony and Bismuth

493

Scheme 26 The reactivity of arsenic homocycles.

Insertion of benzyne into the BidS bonds of (Ar0 S)nBiAr3−n (n ¼ 1, 2) provides a synthetic route to ortho-arylthio triarylbismuthines 79 (Scheme 27).169

Scheme 27 Insertion of benzyne into the BidS bonds.

Modification of the organo-substituents enables diversification of triarylpnictines. Copper-catalyzed cycloaddition of organobismuth(III) acetylides 80 with organic azides yields a variety of organobismuth(III) triazolides 81 (Scheme 28).170 This method was also applied to the antimony analogues.171 The formyl group of tris(3-formylphenyl)bismuthine can be easily transformed into other functional groups such as alkenyls and hydroxyalkyls.172

Scheme 28 The syntheses of bismuth triazolides 81 via click reactions.

In general, triorganopnictogen(III) compounds are stable at room temperature in air, but some compounds are air- or moisturesensitive. The alkyl-substituted compounds are very air-sensitive, such as [2,6-(Me2NCH2)2C6H3]Bi(
1-CH2Ph)2, Bi(
1-CH2Ph)3173 and Bi(C2F5)3.174 Perfluoroaryl compounds can be very moisture-sensitive, for example, [2-(Me2NCH2)C6H4]Bi(C6F5)2.175 All triorganopnictogen(III) compounds have pyramidal structures, however, hypervalent interactions are prevalent in these compounds. An X-ray crystal structure shows short intermolecular Bi┄Bi interactions in BiMe3 [3.899(1) A˚ ].176 Bismuth-arene p-interaction was observed in [(2dCldC6H4CH2)3Bi]2 where the distance between Bi and the arene centroid is 3.659 A˚ (no ESD reported).177 In all the X-ray crystal structures of 2-[{E(CH2CH2)2NCH2}C6H4]3Bi (E]O, NMe)147 and [2-(Me2NCH2)C6H4] SbArAr0 (Ar/Ar0 ¼ Ph or Mes),156 the N-donors are coordinated to the Bi and Sb centers, and these complexes are chiral and crystallize as racemic mixtures. The first antimony-centered radical cations [Ar3Sb][BArF4] (Ar ¼ 2,6-iPr2-4dOMedC6H2, 82a; 2,4,6-iPr3dC6H2, 82b) were synthesized by one-electron oxidation of the corresponding stibines with Ag[SbF6] and Na[BArF4].178 The average CdSbdC angle in 82a (115.75 degrees) is larger than that of the parent stibine (107.3 degrees). Compound 82b can be oxidized by 1,4-benzoquinone, nBu3SnH and TEMPO, yielding the stibonium compounds 83, 84 and 85 (Scheme 29). The first tricoordinate antimony-centered radical anion has also been reported, bearing a T-shaped tris-amide ligand.179

494

Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 29 The oxidation reactions of the Sb-centered radical cation 82b.

Dithienobismoles 86180 and dipyridinostibole/dipyridinobismole-Cu2I2(PPh3)3 complexes 87181 are phosphorescent materials. A bismuth-containing analogue of rhodamine 88 is a red light-excitable photosensitizer, generating 1O2 in cells.182 The photoluminescence of dibenzo[b, f]arsepins 89 was ascribed to the planarization of the arsepin core upon excitation.183

Triorganopnictogen(III) compounds have been widely used as ligands in transition-metal complexes. Similar s-donating but varied p-accepting properties were observed for a series of cationic arsines carrying imidazolium, cyclopropenium, pyridinium and formamidinium substituents, as exemplified by 90–93.184 The cyclopropenium-arsine platinum complex 94 exhibits excellent catalytic reactivity in cycloisomerization of enynes.

Organometallic Compounds of Arsenic, Antimony and Bismuth

495

The bismuthine donor in the BiP-bidentate ligand 95 is hemilabile and becomes de-coordinated upon O2 activation at the Pd center (Scheme 30).185 95 is transformed into a PBiP-tridentate ligand upon complexation with Cu(I), Ag(I) or Au(I) ions (Scheme 31).186

Scheme 30 De-coordination of Bi-donor in the ligand 95.

Scheme 31 The formation of PBiP-ligand upon complexation of the ligand 95.

Fluoride anion binds to the Sb center of the cationic [(odPh2PdC6H4)3Sb]PdCl[BPh4], resulting in a photophysical change of the complex.187 In [Pn(6dMed2-py)3Cu(MeCN)]PF6 (Pn]As, Sb, Bi), the As bridgehead shows no short contact with [PF6]− while the shortest Sb┄F [3.480(3) and 3.964(4) A˚ ] and Bi┄F [3.343(4) and 3.728(5) A˚ ] contacts are within the van der Waals distances.188 The coordination behavior of the Bi-tripodal ligand is anion-dependent. Treatment of complex 96 with chloride affords a mixture of monomeric complex 97 and dimeric complex 98 that are in equilibrium (Scheme 32). The coordination modes of the tripodal tristibine N(CH2d2dC6H4SbMe2)3 depend on the associated transition-metal center189: it adopts a tridentate bridging coordination to three [CpFe(CO)2] units, but behaves as tridentate chelating ligand in [Mn(CO)3(L)][CF3SO3].

496

Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 32 The copper complexes 96, 97 and 98 with a Bi-tripodal ligand.

Triorganopnictogen(III) compounds can also be used as ligands for main-group elements. Homoleptic arsenic-chalcogen coordination compounds 99 were obtained by a ligand exchange method, as depicted in Scheme 33.190 An arsine-stibine adduct 100,191 an arsine-stabilized arsenium monocation 101,192 a diarsine-stabilized bismuthenium monocation 102191 and a stibinestabilized bismuth dication 103193 are illustrated, which all contain Pn ! Pn0 dative bonds.

Scheme 33 The formation of arsenic-chalcogen compounds 99 via ligand exchange.

10.05.4.7 Diorganopnictogen(III) compounds Partial alkylation and arylation of PnX3 or RPnX2 with R0 MgX or R0 Li can be difficult to control, however, this method is more readily applied when the ligands contain intramolecular coordinating groups. For instance, the arylation reactions of BiCl3 with RLi (R ¼ 2-[E(CH2CH2)2NCH2]C6H4; E]O, NMe) in the molar ratios of 1:1, 1:2 and 1:3 yield RBiCl2, R2BiCl and R3Bi, respectively.147 CEC-chelated (C6H4CH2ECH2C10H6)PnCl (E]O, S; Pn]Sb, Bi) were also obtained from PnCl3 and RLi.194 The reaction of RSbBr2 (R ¼ 2-(Me2NCH2)C6H4) with MesMgBr affords R(Mes)SbBr.156 [(SiMe3)C]4[Li(THF)]2 reacts with excess of SbF3 to yield 5-stibabicyclo[2.1.0]pentene.95 Cleavage of one of the PndC bonds with HX leads to symmetrical species, R2PnX. The reactions of BiPh3 with sulfonic acids [RSO3H; R ¼ p-tolyl, mesityl, S-(+)-10-camphoryl],195 (thio)saccharin,196 and perfluoroalkylphosphinic acid (C2F5)2P(O)OH197 yield diphenylbismuth sulfonates, (thio)saccharinate and perfluoroalkylphosphinate, respectively. Treatment of (C2F5)2BiPh with anhydrous HX (X]Cl, Br) results in the cleavage of the BidC bond to the phenyl group.174 Ligand redistribution reactions between R3Pn and PnX3 afford species of the type R2PnX. This method was applied in the preparation of phosphine complexes of chiral arsenium ions 104 (Scheme 34).198 Halobismepines 106 were obtained from the redistribution reaction of a dinuclear bismepine 105 and BiX3 (Scheme 35).199

Organometallic Compounds of Arsenic, Antimony and Bismuth

497

Scheme 34 The synthesis of chiral arsenium ions 104.

Scheme 35 The formation of halobismepines 106 via redistribution reactions.

R2Pn(III)X compounds are also formed from the reaction of R3Pn(III) with X2, mainly via the mechanism of reductive elimination via R3Pn(V)X2 intermediates. In the halogenation of 9-arsafluorene, the proposed mechanism comprises oxidative addition of 9-Me-arsafluorene with XCl (X]Cl, I) and reductive elimination from the As(V) intermediates to yield 9-X-arsafluorene and MeI.200 A series of compounds, tBu2BiX (X]Br, I, CN, SCN), were obtained by reacting tBu3Bi with X2.201 In the ArdF formation via an oxidative addition-reductive elimination sequence, the oxidative addition of LBi(III)Ar 107 with XeF2 affords LBi(V)(Ar)F2 108, which releases ArdF and diorgano-LBi(III)F 109 via reductive elimination upon heating (Scheme 36).160

Scheme 36 The CdF bond formation via oxidative addition-reductive elimination at a Bi center.

Diorganopnictogen halides are pyramidal compounds and the hypervalent interactions of the pendant arms are generally situated trans to the polarized PndX bonds. Examples include {2-[E(CH2CH2)2NCH2]C6H4}2BiCl (E]O, NMe),147 (C6H4CH2 ECH2C10H6)PnCl (E]O, S; Pn]Sb, Bi),194 [2-(Me2NCH2)C6H4]Sb(Ar)X (Ar]Ph, Mes; X]Br, I),156 [2-(Me2NCH2)C6H4]Bi(Ar)Br (Ar]Ph, C6F5, Mes),175 and [RN(CH2C6H4)2]BiX (R]C6H5CH2, C6H5CH2CH2, CH3OCH2CH2; X]F, Cl, Br, I).202 In [2-{MeN(CH2CH2)2NCH2}C6H4]2BiCl, the coordination of N donors to Bi is strong for the N trans to Cl [2.744(14) A˚ ] and weak for the N trans to C [3.061(14) A˚ ].147 Most compounds in the form of [RN(CH2C6H4)2]BiX are dimeric in the solid state, in which two molecules aggregate via strong Bi⋯ p-arene and/or Bi ⋯ X interactions.202 (ppy)2BiCl [ppy ¼ 2-(20 -pyridyl)phenyl] possesses a cationic (ppy)2Bi+ moiety with a non-coordinating chloride counterion and two ppy groups are in a trans arrangement.203 The halides of R2PnX are readily replaced by other anions such as N−3.204 NCO−, NCS−, SeCN−.205 NO−3, OSO2CF−3.206,207 OSO2C8F−17.208 [B(C6F5)4]−,209 PF−6.207 ClO−.210 The preparation of R2PnF is non-trivial and the failure of conventional methods, 4 such as chloride abstraction with AgF, has been reported. Compounds of the type R2SbF 111 were obtained by treatment of cationic [R2Sb]+ 110 with [Bu4N]F ∙ 3H2O; one such example is given in Scheme 37.211

Scheme 37 The formation of a SbdF bond from a diorganoantimony cation 110.

498

Organometallic Compounds of Arsenic, Antimony and Bismuth

Cationic diorganopnictogen compounds are generally partnered with weakly coordinating anions and form adducts with Lewis bases. [LBi][B(C6F5)4] [L]tBuN(CH2C6H4)2] has a BidN distance of 2.357(2) A˚ , which is much shorter than the BidN distances in related neutral species.209 The cationic Bi center has a weak interaction with [B(C6F5)4]−, as shown by a Bi┄F distance of 2.971(2) A˚ . [LBi][B(C6F5)4] forms dinuclear compounds with LBiX (X]Cl, Br) through X-bridges and forms adducts with MeCHO, MeOH and MeCN. The narrower CdBidN bite angles of [(Me2NC6H4)(Mes)Bi][X] (X]OTf, PF6) result in remarkable enhancement of Lewis acidity compared to [(Me2NCH2C6H4)(Mes)Bi][X].207 Other examples of Lewis base-stabilized R2Pn+ species include [HN(C6H4)2As(L)][GaCl4] (L ¼ phosphines),212 [(Ph3P)nPnPh2][PF6] (Pn]As, Sb, Bi; n ¼ 1, 2),213 as well as [H2C2(C6H4)2Bi (EPMe3)][SbF6] (E]S, Se).214 Nucleophilic addition to chiral phosphine-stabilized arsenium salts allows asymmetric synthesis of tertiary arsines and bis(tertiary arsines).215,216 The bis(allyl)bismuth cation, obtained from protonolysis of tris(allyl)bismuth, readily undergoes addition to aldehydes, imines and ketones and acts as an initiator for the controlled radical polymerization.217 Oxidative addition of RdX (X]I, OTf ) to NCN-chelated bismuthinidenes affords cationic diorganobismuth(III) species.62 Stibenium cation 113b was synthesized via chloride abstraction from the parent 112b with [Et3Si(1,2-F2C6H4)][B(C6F5)4], while hydride abstraction from bismuth hydride 112c with [Ph3C][BArF4] afforded the corresponding bismuthenium cation 113c (Scheme 38).132 Compounds 113b and 113c represent the heavy pnictogen analogues of carbenes. By contrast, the transient arsenium cation undergoes rapid electrophilic substitution on a mesityl group, which yields an arsole, 114, upon treatment with triethylamine (Scheme 38).218

Scheme 38 The formation of diorganopnictinium cations via abstraction reactions.

2-Arsaethynolate anion (AsCO−) undergoes formal [2 + 2] cycloaddition to a ketene (O]C]CPh2) and a carbodiimide (DippN]C]NDipp), affording four-membered anionic heterocycles 115 and 116, respectively. Better delocalization of the negative charge in 115 is evidenced by the considerably shorter AsdC bond distances in 115 [1.925(4) and 1.941(4) A˚ ] compared to 116 [1.961(2) and 1.976(2) A˚ ] (Scheme 39). The five-membered product 117 was isolated from the reaction of AsCO− and an isocyanate (O]C]NDipp) (Scheme 39).219

Scheme 39 The reactivity of 2-arsaethynolate anion towards heteroallenes.

A NHC-stabilized phosphinidene ligand has been employed in compounds 118.220 Compounds of the type (IDipp) E(H)2AsPh2 119 (E]Ga, Al) were obtained from the reactions of (IDipp)E(H2)Cl and KAsPh2, and the parent system (IDipp)E (H)2AsH2 can be synthesized using a similar method.221

Organometallic Compounds of Arsenic, Antimony and Bismuth

499

Heating of (IDipp)SbBr3 in the presence of a second equivalent of IDipp results in isomerization of both IDipp ligands to the mesoionic form, yielding 120.222 Bismuth compound 121 was obtained from (NHC)Bi(Ph)Cl2 through a similar process.223 (cAAC)Bi(Ph)Cl2 systems 122 were obtained from the reactions of PhBiCl2 with the respective cAAC ligands or via deprotonation of [cAACdH]2[Cl2(Ph)Bi(m-Cl2)Bi(Ph)Cl2] with K[N(SiMe3)2].223 The CcarbenedBi bonds in LBi(Ph)Cl2 (L ¼ NHC, cAAC) are of the dative type.

Successive chloride abstraction from the carbodicarbene-stabilized bismuth compound 123 affords the monocation 124 and dication 125, with the CdBi bond distance decreasing in the order of 123 [2.249(6) A˚ ], 124 [2.226(3) A˚ ] and 125 [2.157(11) A˚ ] (Scheme 40).224

Scheme 40 The formation of bismuth monocation 124 and dication 125 via chloride abstraction.

Isonitriles insert into the PndB bonds of the boryldiphenylpnictines 126. In the reaction of boryldiphenylbismuthine with PhNC, insertion compound 127 was isolated, whereas compounds for the type 128 were obtained via subsequent elimination when nBuNC was employed (Scheme 41).225

Scheme 41 The insertion of isonitriles into PndB bonds

The fixation of CO2 by diorganobismuth(III) hydroxides, oxides and alkoxides has been reported. For example, the bismuth hydroxide 129a and (LBi)2O 129b react irreversibly with CO2, yielding (LBi)2CO3 130, whereas the bismuth methyl carbonate 131

500

Organometallic Compounds of Arsenic, Antimony and Bismuth

is obtained from the reversible reaction of the bismuth methoxide 129c with CO2 (Scheme 42).226 Similar reactivity was also observed for [2-(Me2NCH2)C6H4]2BiOH and the corresponding bismuth oxide.227

Scheme 42 CO2 fixation by oxo bismuth compounds 129.

Homolytic cleavage of BidS bonds has found applications in synthetic chemistry. For instance, (2,6dMes2dC6H3S)BiPh2 is a co-catalyst in organobismuthine-mediated living radical polymerization (BIRP).228 And [S(CH2C6H4)2]Bi(SPh) acts as the catalyst in photochemically-induced radical dehydrocoupling of TEMPO with hydrosilanes.77 Two examples involving Bi-mediated deprotonation of CdH bonds have been reported. In the first example, the expected species LBi(OAr)2 [L ¼ 2,6-(Me2NCH2)2C6H3] was not formed from the reaction of 132 with ArOK (Ar]2,6-tBu2C6H3). Instead, the bismuth compound 133 was the product (Scheme 43), presumably formed via sterically-induced homolytic cleavage of the BidO bond, followed by radical recombination and CdH cleavage by the aryloxide anion.229 Compounds 134 were formed through the insertion of OCE (E]O, S) into the BidC bond of 133 (Scheme 43).230 The insertion chemistry of nitric oxide has also been investigated.231

Scheme 43 The formation of an oxyaryl dianion at Bi and its reactivity towards OCE.

In a second example, treatment of Bi(NPh2)3 with TfOH results in the first ortho-CH deprotonation of diphenylamide.232 Driven by releasing ring strain, 135 was transformed into a cationic bisma-heterocycle 136 in pyridine upon heating, through a second ortho-CH deprotonation step (Scheme 44). A cationic bismuth carbamoyl compound 137 was obtained from insertion of CO into the BidN bond of 135 (Scheme 44), and this reaction is also amenable to aryl- and alkyl-isonitriles.233

Organometallic Compounds of Arsenic, Antimony and Bismuth

501

Scheme 44 Double ortho-CH deprotonation of Bi(NPh2)3 and CO insertion reactivity.

The reduction of NC-chelated compound 138 with K[B(sBu)3H] gives a five membered aza-stiba heterocycle 139 (Scheme 45).116 The cleavage of the SbdN bond of 139 with HX (X]Cl, CH3CO2, CF3CO2, CF3SO3, FcCO2) affords LSb(X)[2(CH2NHDipp)C6H4].234

Scheme 45 The formation of an aza-stiba heterocycle 139 via reduction.

1-Chloro-2,3-diphenylstibaindole 140 is Lewis acidic and p-conjugated, which exhibits a colorimetric response towards X− (X] F, Cl, Br) (Scheme 46).235

Scheme 46 The colorimetric response to chloride binding at a stibaindole 140.

The bismacycle 141 has been developed into a synthetic reagent for ortho-arylation of phenols and naphthols with arylboronic acids.159 The bismacycles 142 and 143 act as catalysts for fluorination160 and triflation/nonaflation161 reactions of arylboronic esters and acids.

502

Organometallic Compounds of Arsenic, Antimony and Bismuth

The reaction of [(o-tolyl)2Bi(hmpa)2][SO3CF3] (hmpa ¼ hexamethylphosphoric acid triamide) and [NBu4][Cp MoO3] affords the Mo(VI)-O-Bi(III) complex 144, which contains the first Mo(VI)dOdBi(III) unit and exists as a coordination polymer through Mo]O


Bi linkage.236 Related Mo(VI)dOdBi(V) complex 145 is obtained from Ph3BiBr2 and [NBu4][Cp MoO3]. Both complexes provide functional models for ModOdBi units on the surfaces of the bismuth molybdate catalysts in the SOHIO process.

The s (BidCl) orbital in the chlorobismuthine [o-(Ph2P)C6H4]2BiCl ligand permits a TM ! Bi interaction. The metal centers of gold complex 146 and platinum complex 147 adopt distorted square planar and square pyramidal geometries, respectively, with short AudBi and PtdBi bond distances of 2.9979(3) and 2.9009 (5) A˚ .237,238

10.05.4.8 Monoorganopnictogen(III) compounds Numerous monoorganopnictogen(III) dihalides have been reported, supported by E0 CE- and CE-chelated ligands (E/E0 ]O, S, N). The examples of E0 CE-chelated compounds include OCO-chelated [2,6-(ROCH2)C6H3]PnX2 (R]Me, tBu; Pn]Sb, Bi; X]Cl, I),239 OCN-chelated [2-(Me2NCH2)-6-(tBuOCH2)C6H3]PnCl2 (Pn]Sb, Bi),240 NCN-chelated [2,6-(Me2NCH2)2C6H3]BiX2241 and [2,6-{MeN(CH2CH2)2NCH2}2C6H3]BiX2,242 OCO-chelated [4-tBu-2,6-{(EtO)2P(O)}2C6H2]BiCl2,243 NCN-chelated [HC (PPh2NSiMe3)2]BiI2,244 NCN-chelated [2,6-(CH]NR)2C6H3]PnCl2 (R]tBu, 2,6-Me2C6H3; Pn]Sb, Bi).245 The examples of CE-chelated compounds comprise [2-(RN]CH)C6H4]SbBr2 (R]Mes, Dipp),246 [2-{E(CH2CH2)2NCH2}C6H4]BiX2 (E]O, NMe),147 (Ph2P-Ace)PnCl2 (Ace ¼ acenaphthene-5,6-diyl; Pn]As, Sb, Bi).247 The standard synthetic methods employed for these compounds are the reactions of PnX3 with ArLi or ArMgX. It has been reported that [L2Sb]4[Sb6Cl22] [L ¼ 2,6(MeOCH2)2C6H3] is the primary intermediate in the reaction of LLi and SbCl3, and this compound is transformed into the desired LSbCl2 and SbCl3 in CHCl3 or CH2Cl2.248 The transmetallation of compound 148 to SbCl3 yields (ppy)SbCl2 149; however, the same reaction with BiCl3 affords 150, consisting of a bismuthenium cation and a monoorganobismuth trichloride anion (Scheme 47).249

Organometallic Compounds of Arsenic, Antimony and Bismuth

503

Scheme 47 The transmetallation of compound 148 to SbCl3 and BiCl3.

Monoorganoantimony(III) and bismuth(III) difluorides are rare. RSbF2 was obtained via anion exchange of RSbCl2 with Me3SnF or [(Me2NCH2)C6H4]SnnBu2F over a prolonged time period, but this method failed to yield bismuth analogues.250 [2,6(Me2NCH2)2C6H3]SbF2 was obtained from (LSbO)2 and HBF4.251 The pnictogen centers of E0 CE-chelated compounds generally have distorted square pyramidal geometries, with a T-shaped CPnX2 core and two trans E/E0 ! Pn interactions. For example, in [2,6-(Me2NCH2)2C6H3]BiX2, the N ! Bi coordinate bonds are strong [BidN, 2.561(3)-2.589(4) A˚ ] and the trans BidX bonds [BidCl, 2.701(1) and 2.706(1) A˚ ; BidBr, 2.840(1) A˚ ; BidI, 3.082 (1) A˚ ] are considerably elongated compared to typical BidX bonds.241 However, the O ! Sb coordinate bonds are trans to SbdCl bonds in [2,6-(ROCH2)C6H3]SbCl2 (R]Me, tBu), and [2,6-(CH3OCH2)C6H3]BiCl2 has a dimeric structure with a Bi2Cl2 core.239 The dimeric structures are held together by Pn2X2 cores in many CE-chelated compounds, as exemplified by [2-(RN]CH)C6H4] SbBr2 (R]Mes, Dipp).246 However, this interaction is interrupted by the ortho-tBu group in [2dRd4,6-(tBu)2dC6H2]BiCl2 (R] CH]NdDipp, CH2NEt2), and two Cl atoms are in trans positions.47 The pincer arms attached to Lewis acidic Pn centers can be susceptible to side reactions. CdO bond cleavage has been observed in some OCO-chelated compounds. For instance, compound 152 was obtained from 151 with release of EtCl (Scheme 48).243 RLi (R]Me, nBu, Ph) attacks one imine arm of [2,6-(CH]NR)2C6H3]SbCl2, yielding benzazastiboles 153 (Scheme 49).245 By contrast, triorganobismuth compounds 154 were formed through alkylation and arylation of the corresponding bismuth chlorides.

Scheme 48 The CdO bond cleavage in compound 151.

Scheme 49 Different reactivity of NCN-chelated pnictogen compounds towards RLi.

NCN- and NC-chelated antimony and bismuth dihalides have been applied as precursors to monomeric Sb(I) and Bi(I) compounds.46–49,63 The syntheses of the first monomeric Sb(I) and Bi(I) compounds 14 are illustrated in Scheme 50.46

504

Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 50 The syntheses of the first monomeric Sb(I) and Bi(I) compounds 14.

The heavier pnictogen complexes of pincer methanides and methanediides have been reported. The SCS-chelated complexes {[C(Ph2PS)2]PnCl}2 155b and 155c have a dimeric structure through intermolecular Pn ⋯ S interactions.252 By contrast, [C(Ph2PS)2]AsI 155a is a monomer and the p-character of the AsdC bond is predicted to be considerably stronger.253 The reaction of BiCl3 with mono-lithiated Li[HC(Ph2PNDipp)2] gives CN-chelated compound 156, in which a 1,3dH shift occurs from the central C to the uncoordinated N.254 Treatment of BiCl3 with di-lithiated Li2[C(Ph2PNDipp)2] affords NCN-chelated complex 157. Compounds 155b, 155c and 157 contain formal Sb]C and Bi]C bonds, but PndC p-interactions are negligible as suggested by computational studies. The use of methanide ligand [HC(PPh2NSiMe3)2]− has also been reported.244

Chloride abstraction from E0 CE- and CE-chelated compounds with AgX introduces anions such as OSO2CF−3, CB11H−12,240,255,256 − 258 RCO−2,257 SO2− NCS−, SeCN−, NCO−,205 N−204 . 4 , NO3, 3 The tendency for dimerization of monoorganopnictogen chalcogenides (ArdPn]E; E]O, S, Se, Te) decreases on descending the group 16. The terminal PndE bond is highly polarized towards the chalcogen atom, with partial to negligible double bond character. Systems of the type [2,6-(Me2NCH2)2C6H3]SbE 158 (E]Se, Te) have terminal SbdE bonds in both the solid state and solution.259 The terminal AsdS bond in 159 is supported by the same ligand scaffold.260 However, antimony sulfide 160 is dimeric in the solid state with a central Sb2S2 ring, but exists as a monomer in solution.261 The terminal SbdS bond [2.2929(17) A˚ ] in bis-ketimine pincer compound 161 is remarkably shorter than the SbdS bonds in dimeric systems.262 In dimeric (ArPnE)2 (Pn] Sb, Bi; E]S, O), the organo-substituents can be syn- or anti-arranged relative to the Pn2E2 ring. {[2,6-(ROCH2)2C6H3]SbS}2 can exist as either the syn- or anti-isomer in the solid state, but forms a mixture of both isomers in solution.263 Terminal SbdO or BidO bonded compounds have not been reported to date and the examples of (ArBiO)2 are also rare. Bismuth oxide 162 exists as the synisomer,264 while 163 is the anti-isomer.49 The interconversion between cis- or trans-isomers was not observed for these two compounds in solution. Controlled hydrolysis of [2,6-Mes2C6H3]SbCl2 affords dinuclear [2,6-Mes2C6H3(Cl)Sb]2O.265

Organometallic Compounds of Arsenic, Antimony and Bismuth

505

NCN-chelated compounds (LPnO)2 [Pn]Sb, Bi; L ¼ 2,6-(Me2NCH2)2C6H3] are versatile precursors to a variety of monoorganoantimony and bismuth compounds. Dimeric antimony oxide 164 reacts with CO2 to yield antimony carbonate 165, and CO2 is eliminated from 165 upon heating (Scheme 51).266 The organoantimony selenite was obtained from the reaction with SeO2.267 The reaction with As2O3 and As2O5 afforded [(LPn)3(AsO4)2] and [(LPn)2(As2O5)].268 An eight-membered stibasiloxanes and a six-membered bismutasiloxane were obtained from cyclo-(Me2SiO)3.269 Disaggregation of the Sb2O2 ring is induced by B(C6F5)3, forming the Lewis pair LSb+dOdB−(C6F5)3 166 (Scheme 51).270 The reaction of (LSbS)2 160 with CS2 yields the trithiocarbonate 167 (Scheme 51).271 Cyclic bis(pentasulfide) 168 was obtained from 160 and S8 (Scheme 51).261

Scheme 51 The reactivity of dimeric antimony chalcogenides 160 and 164.

Dehydration of NCN-chelated (LPnO)2 with the compounds containing EdOH units (E]P, Si, B, C) leads to a variety of acyclic and cyclic systems. (LPnO)2 compounds react with organophosphinic acids, organophophonic acids, H3PO3 and H3PO4, affording the corresponding organoantimony and bismuth phosphinates,272 phosphonates,273 phosphites272 and phosphates.274 Six-membered cycloLPn(OSiPh2)2O systems were obtained from the reactions of (LPnO)2 with (HO)SiPh2(O)SiPh2(OH).269 The reactions with arylboronic acids afford stiba- and bisma-heteroboroxines with a PnB2O3 core.275–277 The reactions with catechol gave LPn(O2d1,2dC6H4).264 Some structures of these compounds are shown in Fig. 3. Organoantimony and bismuth hydroxides tend to undergo dehydration. The PndOH moieties can be stabilized by steric hindrance, as exemplified by compounds 169 and 170.49,278 The BidOH moiety is further stabilized by BidOH ⋯ arene interactions in both compounds, and the OH⋯ phenyl centroid distance is 2.622 A˚ (no ESD reported) in the X-ray crystal structure of 170.49 170 was obtained from the oxidation of the bis-ketimine Bi(I) system by N2O, and one ketimine arm is deprotonated by the basic Bi+dO− moiety.49 In the [LSb(OH)(CF3SO3)]2 [L ¼ 2,6-(Me2NCH2)2C6H3], the cationic [LSb(OH)]+ fragment is stabilized by both NCN-chelation and CF3SO−3, and the dimeric structure is connected through SbdOH⋯ O hydrogen bonding.251

Fig. 3 Representative structures obtained from dehydration of (LPnO)2 with EdOH.

506

Organometallic Compounds of Arsenic, Antimony and Bismuth

Antimony analogue of housene 172 was obtained from the reaction of SbCl3 and zirconocene 1,3-diphosphabicyclo[1.1.0] butane 171 (Scheme 52).279

Scheme 52 The synthesis of antimony analogue of housene 172.

Methyl/X exchange reactions (X]Cl, N3, OTf ) between the Si and As centers occur in silylated aminodichloroarsines in the presence of Lewis acids.280 The methyl/triflate exchange processes are illustrated in Scheme 53.

Scheme 53 The methyl/triflate exchange between Si and As centers.

Terminal alkynes undergo regio- and stereo-selective carbobismuthination with BiBr3 and ketene silyl acetals, as exemplified in Scheme 54.281 The BiBr2 moiety in the resulting alkenylbismuth compounds can be used for diverse functionalization reactions, such as substitution by I, Ts, SPh or Pd-catalyzed cross-coupling. The method was also applied to carbobismuthination of alkenes, where BiBr3 and BiCl3 exhibit different regioselectivities based on different mechanisms.282

Scheme 54 Carbobismuthination reactions.

CdH deprotonation at the methylcyclopentadienyl ligand of (MeC5H4)2Mo]O is induced by the coordination of Mo]O to the Bi center (Scheme 55).283

Scheme 55 CdH deprotonation in a molybdenumoxo-bismuth compound.

Organometallic Compounds of Arsenic, Antimony and Bismuth

507

The insertion of activated carbon-carbon double and triple bonds into the BidN bond of bismuth amide 173 has been reported (Scheme 56).284 Additionally, 173 reacts with AlMe3, PhC^CH and Cp H to yield [Bi]dMe, [Bi]dCCPh and [Bi]dCp (Scheme 56).285

Scheme 56 The reactivity of a bismuth amide 173.

A diamidonaphthalene-supported antimony hydride 174 undergoes anti-Markovnikov hydrostibination with phenylacetylene, 3-vinylanisole and acrylonitrile, yielding 175 or 176 (Scheme 57).128 The radical mechanism of the hydrostibination of alkynes was proposed based on mechanistic studies.286

Scheme 57 Hydrostibination of an antimony hydride 174.

Ring opening of THF is enabled by a bismuth pyridine dipyrrolide 177 and organosilicon reducing agents, affording 178, through a Bi(II) radical intermediate (Scheme 58).76

Scheme 58 Ring opening of THF by a transient Bi(II) radical.

Transmetallation of bis(6-diphenylphosphinoxy-acenapth-5-yl)mercury to SbCl3 and BiCl3 has been reported (Scheme 59).287 Dicationic [(Me2NC6H4)Bi(L)3][B(3,5-C6H3Cl2)4]2 (L ¼ aldehyde/ketone) have been applied as the catalysts for hydrosilylation of aldehydes and ketones.288

508

Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 59 Transmetallation of an organomercury compound to SbCl3 and BiCl3.

The first NHC-bismuth compound 179 contains a BidC bond distance of 2.389(8) A˚ (Scheme 60).289 Upon heating, isomerization of the related systems IDipp-PnBr3 180 to the mesoionic complexes 181 has been observed (Scheme 60).222

Scheme 60 NHC-bismuth compound 179 and the formation of mesoionic compounds 181 via isomerization.

PhBiCl2 reacts with reagents of the type [cAACdH]+ Cl− to yield aldiminium-bismuth salts 182, which can be transformed into (cAAC)Bi(Ph)Cl2 upon treatment with a base (Scheme 61).223

Scheme 61 The intermediacy of aldiminium-bismuth salts 182.

Hexaphenyl carbodiphosphorane (CDPPh) has also been applied to ligate pnictogen compounds.290 The reaction of CDPPh with SbCl3 and BiCl3 to yield neutral complexes 183, while the complexation of CDPPh to AsCl3 requires chloride abstraction using GaCl3.

Application of a s- and p-donating carbodicarbene ligand (CDC) has enabled the stabilization of bismuth monocations, dications and a trication.224 Halide abstraction from [(CDC)BiBr3]2 (184) with 2 equivalents of Ag[SbF6] or 4 equivalents of Ag [NTf2] affords the corresponding monocation 185 and dication 186, with THF coordinated to the vacant sites of the Bi center (Scheme 62). The CdBi bond lengths shorten from 2.292(9) A˚ (184) to 2.226(12) A˚ (185) to 2.199(5) A˚ (186). Bismuth trication 187 is stabilized with two CDC ligands, with even shorter CdBi distances of 2.166(2) and 2.197(2) A˚ (Scheme 62). The bonding between Bi and CDC ligands consists of a strong s- and a considerably weaker p-component.

Organometallic Compounds of Arsenic, Antimony and Bismuth

509

Scheme 62 The formation of CDC-bismuth cations 185–187 via bromide abstraction.

Compound 189 containing a Lewis base-stabilized As]B bond [1.914(6) A˚ ] was synthesized by the elimination of HBr from precursor 188 (Scheme 63).291

Scheme 63 The formation of an As]B bond.

The reaction of (2,4,6-tBu3-C6H2)SbX2 with (tBu2MeSi)2SiLi2 affords stibasilene 190 featuring an undistorted Sb]Si bond [2.4146(7) A˚ ].292 Arsagermene 191 was synthesized using a similar strategy.293 Organo-gallastibene 192 was obtained from the reaction of ArSbCl2 with 2 equivalents of LGa, by extrusion of LGaCl2.143

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Organometallic Compounds of Arsenic, Antimony and Bismuth

MeBiCl(S2CNR2) and MeBi(S2CNR2)2 were synthesized from MeBiCl2 and sodium dithiocarbamates, and thermal decomposition of MeBi(S2CNR2)2 afforded pure Bi2S3.294 Treatment of BiPh3 with salicylic acids afforded a series of phenyl bismuth(III) bis(carboxylates) with considerable structural diversity.295 Arsenicin A is the first isolated natural polyarsenical, which has an adamantane structure similar to arsenic trioxide (compound 193 is (S)-(−)-Arsenicin). The synthesis and resolution of Arsenicin A has been reported, and it exhibits strong anti-acute promelocytic leukemia cell line activity.296,297 A series of dithiadiazarsoles 194 were obtained from the reactions of [nBu2Sn(S2N2)]2 and RAsX2.298 Compound 195 containing77As is a potential radiopharmaceutical.299

Alkylhalostibines and bismuthines are both Lewis acidic and basic, allowing coordination of ligands or coordination to transition-metal moieties, as exemplified by 196–198.300,301

10.05.5 Pentavalent compounds 10.05.5.1 Tetraorganopnictogen(V) compounds Onium salts R4M+ X− are tetrahedral compounds such as [Ph4As]+[N3]−, covalently bonded compounds R4MX possess trigonal bipyramidal structures such as Ph4SbN3.302 Pentacoordinate pnictogen compounds show fluxional behavior through either Berry pseudorotation (BPR) or turnstile rotation (TR): the BPR mechanism is more widely accepted than the TR mechanism. Based on a rigid tridentate ligand, the interconversion between the isomers 199a and 199b proceeds at elevated temperature (>40  C), in this case through the mechanism of turnstile rotation (Scheme 64).303

Scheme 64 Interconversion of 199 by turnstile rotation.

Tetraorganoantimony(V) compounds can be synthesized by arylation of R3SbX2 or SbX5 with R0 Li. Examples of synthesis via mono-arylation include ArSbPh3Br (Ar ¼ 9-phenanthryl, 1-pyrenyl, 3-perylenyl).304 Tetra-arylation of SbCl5 with C6F5Li gives (C6F5)4SbCl.305 Oxidation of ArPh2Sb with MeOTf affords [ArPh2SbMe][OTf].306–308 Anion exchange of R4SbX introduces anions such as azide302 and carboxylates.309,310 Lewis acidic antimony centers in stibonium salts show high fluoride affinity, as exemplified by [o-(Ph2MeSb)(Mes2FB)C6H4].306 Incorporation of fluorophores into stibonium cations enables fluorescence sensing of fluoride anion. For instance, fluorescence is turned on upon complexation of the 9-anthryltriphenylstibonium cation by fluoride anion, which allows fluoride sensing in aqueous phase at sub-ppm level (Scheme 65).311 Other fluorophores employed in stibonium sensors include 3-perylenyl 200,304

Organometallic Compounds of Arsenic, Antimony and Bismuth

511

Scheme 65

carbazole 201308 and BODIPY dye 202.307 Sb(V)-substituted cyclometalated ruthenium polypyridyl complex 203 has also been used for spectroscopic and electrochemical sensing of fluoride and cyanide.312

The Lewis acidity of stibonium cations has found other applications. Air-stable [Sb(C6F5)4][B(C6F5)4] (204) was synthesized from [Sb(C6F5)4]Cl by anion exchange.305 The strong Lewis acidity of 204 is reflected in fluoride abstraction from [SbF6]− and [BF(C6F5)3]−, polymerization of THF and promotion of the hydrodefluorination of fluoroalkanes. Compound 205 catalyzes reductive coupling, Aldol condensation and cyclotrimerization of aldehydes.313 Bis-stibonium compound 206 is an excellent catalyst for transfer hydrogenation of quinolines.314 [o-MePhS(C6H4)Sb(p-tolyl)3][BF4]2 can be used as preanionophore for chloride transport that is activated by the sulfonium reduction.315

In T-shaped gold complex 207, a strong Au ! Sb interaction was evidenced by the short averaged AudSb distance of 2.76 A˚ (Scheme 66).316 The complexation of fluoride anion to the stibonium cation results in a geometric change at the Sb center from distorted trigonal bipyramidal in 207 to nearly octahedral in 208, however, the AudSb distance [2.771(2) A˚ ] remains nearly unchanged. A Hg ! Sb interaction was observed in the corresponding mercury complexes, with longer HgdSb distances of 3.0601 (7) and 3.073(1) A˚ .317

Scheme 66 Fluoride complexation to a stibonium cation 207 featuring a Au ! Sb interaction.

Arsenic ylide Ph3As]CH2 was synthesized by deprotonation of (Ph3AsMe)I with sodium bis(trimethylsilyl)amide (NaHMDS).318 Ph3As]CH2 has a reduced Pn]C character, compared to the phosphorus analogue, which is revealed by a longer As-Cylide distance of 1.826(6) A˚ and the increasing pyramidalization of Cylide. This compound has been used as a ligand in uranium complex 209. Keto-stabilized arsenic ylide ligands have been used in complexes of gold (210)319 and palladium.320

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Organometallic Compounds of Arsenic, Antimony and Bismuth

10.05.5.2 Triorganopnictogen(V) compounds Triorganopnictogen(V) compounds can be synthesized by oxidation of R3Pn with halogenating reagents such as SO2Cl2,321–323 PhICl2,324 Br2325,326 and XeF2.160 Compounds 211a and 211b were obtained from oxidation with Br2 and have zwitterionic structures with a tetracoordinate phosphonium/stibonium cation (R3PnBr+) and a hexacoordinate stiborate anion (R3PnBr−3).325,326 The structure of 211a in solution remains unchanged, while fast exchange between stibonium and stiborate moieties proceeds for 211b in solution, and 211c was obtained by recrystallization of 211b from THF/mesitylene. Monomeric bismuth(V) difluoride 212327 and dimeric bismuth(V) difluoride 213160 were prepared by oxidation with XeF2, and both are octahedral compounds.

Triorganoantimony(V) and bismuth(V) carboxylates can be synthesized by reacting R3PnX2 with RCO2H/base or RCO2M (M] Na, K).310,328–330 They can also be directly obtained from oxidation of R3Pn with PhI(OAc)2,331,332 CH3CO3H/CH3CO2H,333 or H2O2/CH3CO2H.334,335 It has been reported that trace amounts of peroxides in isopropanol act as an oxidant for the synthesis of Ph3Bi(O2CR)2.336 The reaction of Ph3Sb with silanols in the presence of tBuOOH gives Ph3Sb(OSiR3)2.337 In Et3SbE (E]S, Se), the terminal SbdE linkage is a highly polarized single bond, with short SbdE distances of 2.381(7) and 2.4062(8) A˚ , respectively.338 Ph3SbS forms adducts with Cu(I) complexes, and subsequent S-transfer proceeds with generation of Ph3Sb.339 The reactions of polymeric (Ph3SbO)n and different acids afford the corresponding antimony(V) phosphinates, phosphonates and seleninates.340 Disaggregation of dimeric (Ph3SbO)2 is induced by B(C6F5)3, forming Ph3Sb−-O-B+(C6F5)3.270 In 214, a triarylstibine oxide moiety is stabilized by the neighboring Sb(V) center in the biphenylene scaffold.341 Most triorganobismuth(V) oxides are polymeric and 215 represents a rare dimeric species.159

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Anti-apicophilic arsoranes 216a isomerize to more stable isomers 216b through Berry pseudorotation (Scheme 67).342 The activation barriers of this process are reported to increase in the order CF3 < C2F5 < nC3F7.

Scheme 67 Berry pseudorotation of arsoranes 216.

The coordination complexes of triorganopnictogen(V) dications exhibit considerable structural diversity. The As centers in [Ph3As(L)][OTf]2 217 (L ¼ DMAP, IDipp) adopt tetrahedral geometries.343 Systems of the type [Ph3Pn(OPPh3)2][OTf]2 218 are trigonal bipyramidal compounds and [Ph3Pn(DMAP)2(OTf )][OTf] species 219 are distorted octahedral compounds with short Pn-OOTf contacts [SbdO, 2.714(2) A˚ ; BidO, 2.888(2) A˚ ].344 Ph3Bi(OTf )2 mediates thioglycoside activation reactions.332 Hexacoordinated [(ppy)3Sb][SbCl6]2 220 adopts the meridional configuration.323

Successive treatment of gold complex 221 with H2O2 and AgNTf2 yields 222 with a gold-bound dicationic antimony(V) center (Scheme 68).345 Compound 222 has found catalytic applications in the polymerization and hydroamination of styrene.

Scheme 68 The formation of 222 featuring a gold-bound dicationic Sb(V) center.

The stibine moiety in Ni complex 223 acts as a neutral donor (L-type ligand) (Scheme 69).324 The oxidation of 223 with PhICl2 gives 224, in which the Sb center is converted to an anionic donor (X-type ligand). Subsequent treatment of 224 with catechol affords 225, with the stiborane group being a neutral acceptor (Z-type ligand).

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Organometallic Compounds of Arsenic, Antimony and Bismuth

Scheme 69 Conversion among L/X/Z ligand functions in a NidSb complex

Neutral triorganoantimony(V) compounds have also been applied for fluoride binding and sensing. Stiborafluorene 226 manifests a photophysical response to fluoride coordination.346 The bidentate distiborane 227 is highly selective to fluoride anion via SbdFdSb chelation in aqueous solution.347 The methine group of the 1,8-triptycenediyl backbone of 228 assists fluoride anion chelation by forming CdH┄F hydrogen bonding.348 In addition, systems of the type [o-C6H4(PPh2)][SbAr2(O2C6Cl4)] (Ar]Ph, C6F5) form adducts with formaldehyde in aqueous solution.349

Oxidation of SbPh3 with o-iminobenzoquinone affords 229, which can be transformed into 230 via reversible binding of O2 (Scheme 70).350 The distinct reactivity of 229 is attributed to the cooperation of the redox-active amidophenolate ligand and the heavy antimony atom. Subsequently, the ligand scaffold was also extended to catecholates. It was found that electron-donating ligands favor oxygen binding and the process is also influenced by steric factors.351–353 Sb(V) amidophenolates and catecholates have shown antiradical activity in oleic acid auto-oxidation354–356 and have been incorporated into polymers for O2 response.357

Scheme 70 Reversible binding of O2 at a Sb center.

The high reactivity of R3Bi(V)X2 systems enables the formation of strong CdX bonds through reductive elimination, which has been successfully exploited in the bismuth-catalyzed fluorination reaction of arylboronic esters.160 The proposed Bi(III)/(V) redox catalytic cycle is depicted in Scheme 71, and features transmetallation of arylboronic esters to 231, oxidative addition of 232 with 1-fluoro-2,6-dichloropyridinium and reductive elimination from 233 to yield aryl fluorides and regenerate 231. The application of organobismuth reagents in the synthesis of aryl tosylates and a-tosyloxy ketones has been reported.358,359 Later, catalytic triflation and nonaflation of arylboronic acids was also achieved using Bi(III)/Bi(V) redox catalysis.161

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Scheme 71 The Bi(III)/Bi(V) redox catalytic cycle of the fluorination reaction of arylboronic esters.

Dimethylarsinoyl fatty acids have been found in cod-liver oil360 and several arsenic-containing hydrocarbons are known to be natural components of fish oil.361 The key synthetic step for the production of a dimethylarsinoyl fatty acid is illustrated in Scheme 72.362

Scheme 72 The key step for synthesizing a dimethylarsinoyl fatty acid.

10.05.5.3 Diorganopnictogen(V) compounds The reactions of R2SbCl3 (R]Me, Ph) with R0 4EX (E]N, P, Sb) give antimonates [R0 4E][R2SbCl4].363 In polar solvents such as CHCl3 and acetone, triorganoantimony(V) compound 234 is transformed to ion pair 235, which contains a diphenyl-biscatecholatoantimony(V) anion (Scheme 73).364

Scheme 73 The formation of a Sb(V) ion pair 235.

Methylphenylarsinic acid MePhAs(O)OH is a contaminant of rice plants and ground water.365 Me2As(S)dSdAsMe2, known as Bunsen0 s cacodyl disulfide, reacts with I2 to yield Me2AsdSdI.366 Transition-metal complexes supported by PSb(V)P-tridentate ligands are electrophilic catalysts. For instance, gold complex 236 catalyzes the hydroamination of terminal alkynes with anilines.367 The catalytic reactivity of 237 is self-activated through chloride migration from Pt to Sb.368 After fluoride abstraction, platinum complex 238 is an active catalyst for enyne cyclization.369 In addition, a PSb(V)P-tridentate ligand also facilitates photoreductive elimination of Cl2 from the Pt and Pd centers.370,371

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Organometallic Compounds of Arsenic, Antimony and Bismuth

10.05.5.4 Monoorganopnictogen(V) compounds The oxidation of RSbCl2 [R]2d(CH]NdDipp)C6H4] with SO2Cl2 yields a stable RSbCl4 compound with a strong N ! Sb interaction [2.291(2) A˚ ], while the bismuth analogue could not be obtained under the same conditions.322 All known arsonic acids RAs(O)(OH)2 are well-defined compounds with tetrahedral arsenic centers, and intermolecular hydrogen bonding through the As]O┄HdO unit is prevalent in these compounds.372 Arsonic acids are readily available precursors to other monoorganoarsenic(V) compounds: for example, a series of spiro-oxyarsoranes 239 were obtained from diols and arsonic acids.373 PhAsCl4 and PhAs(OMe)4 were prepared from PhAs(O)(OH)2 in two steps.374 By contrast, the majority of stibonic acids are polymeric and ill-defined. Hydrolysis of 2,6-Mes2C6H3SbCl4 affords a dimeric stibonic acid 240, in which the sterically demanding m-terphenyl scaffold prevents the Sb2O2 core from undergoing further aggregation.375 Treatment of stibonic acid 240 with aqueous H2SO4 and NaOH yields (RSb)2(O)(OH)4SO4 and Na2(RSb)4O4(OH)10
2H2O, respectively.376 Reactions of (p-halophenyl)stibonic acids with Ph2Si(OH)2 give Sb(V) oxido cubane clusters of the type 241, which contain a Sb4O4 core.377 Under aerobic conditions, basic hydrolysis of 2,6-Mes2C6H3SbCl2 gives (RClSb)2O, which is gradually transformed into mix-valence clusters (RSb)2(ClSb)4O8 and (RSb)4(ClSb)4(HOSb)2O14.265

Sb(O2CCF3)5 mediates CdH functionalization of methane and ethane, in which RdSb(O2CCF3)4 was proposed as intermediate.378

10.05.6 Conclusions In the last three lustrums, the chemistry of heavy organopnictogens has gained tremendous momentum. During this time, this area has rapidly evolved and its compounds have translated from mere structural and electronic curiosities to useful and relevant catalysts for organic synthesis, material sciences, small molecule activation and catalysis. Of importance is the development of organopnictogens with low-valency such as Sb(I) and Bi(I), which have opened a completely new area of expertise, with intriguing bonding and redox properties. Of notice, these last 15 years have also seen the emergence of open-shell organopnictogen compounds, which were completely elusive before, thus demonstrating that the heavy elements of the group 15 can now be considered in the development of elusive radical processes. It is also important to mention that emphasis has been placed also in the synthesis of cationic organopnictogens in various oxidation states, whose unique Lewis acidic properties led to various applications. Together with the past editions of COMC, we believe that the compilation of advances presented here will certainly provide the necessary and most advanced knowledge for the practitioners interested in this exciting and still underexplored field.

Acknowledgment Financial support for this work was provided by the Max-Planck-Gesellschaft, Max-Planck-Institut für Kohlenforschung and Fonds der Chemischen Industrie (FCI-VCI). Y.P. thanks the China Scholarship Council for a Ph.D. scholarship. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under Agreement No. 850496 (ERC Starting Grant, J.C.).

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Organometallic Compounds of Arsenic, Antimony and Bismuth

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351. 352. 353. 354. 355. 356. 357. 358. 359. 360. 361. 362. 363. 364. 365. 366. 367. 368. 369. 370. 371. 372. 373. 374. 375. 376. 377. 378.

Organometallic Compounds of Arsenic, Antimony and Bismuth

Cherkasov, V. K.; Abakumov, G. A.; Grunova, E. V.; Poddel’sky, A. I.; Fukin, G. K.; Baranov, E. V.; Kurskii, Y. V.; Abakumova, L. G. Chem. Eur. J. 2006, 12, 3916–3927. Poddel’sky, A. I.; Smolyaninov, I. V.; Kurskii, Y. A.; Berberova, N. T.; Cherkasov, V. K.; Abakumov, G. A. Russ. Chem. Bull. 2009, 58, 532–537. Fukin, G. K.; Baranov, E. V.; Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. ChemPhysChem 2012, 13, 3773–3776. Smol’yaninov, I. V.; Poddel’skii, A. I.; Smol’yaninova, S. A.; Movchan, N. O. Russ. J. Gen. Chem. 2014, 84, 1761–1766. Smolyaninov, I. V.; Antonova, N. A.; Poddel’sky, A. I.; Smolyaninova, S. A.; Osipova, V. P.; Luzhnova, S. A.; Berberova, N. T.; Pimenov, Y. T. Appl. Organomet. Chem. 2014, 28, 274–279. Smolyaninova, S. A.; Poddel’sky, A. I.; Smolyaninov, I. V.; Berberova, N. T. Russ. J. Coord. Chem. 2014, 40, 273–279. Arsenyev, M. V.; Shurygina, M. P.; Poddel’sky, A. I.; Druzhkov, N. O.; Chesnokov, S. A.; Fukin, G. K.; Cherkasov, V. K.; Abakumov, G. A. J. Polym. Res. 2013, 20, 98. Sakurai, N.; Mukaiyama, T. Chem. Lett. 2007, 36, 928–929. Sakurai, N.; Mukaiyama, T. Chem. Lett. 2008, 37, 388–389. Rumpler, A.; Edmonds, J. S.; Katsu, M.; Jensen, K. B.; Goessler, W.; Raber, G.; Gunnlaugsdottir, H.; Francesconi, K. A. Angew. Chem. Int. Ed. 2008, 47, 2665–2667. Taleshi, M. S.; Jensen, K. B.; Raber, G.; Edmonds, J. S.; Gunnlaugsdottir, H.; Francesconi, K. A. Chem. Commun. 2008, 4706–4707. Taleshi, M. S.; Seidler-Egdal, R. K.; Jensen, K. B.; Schwerdtle, T.; Francesconi, K. A. Organometallics 2014, 33, 1397–1403. Breunig, H. J.; Koehne, T.; Moldovan, O.; Preda, A. M.; Silvestru, A.; Silvestru, C.; Varga, R. A.; Piedra-Garza, L. F.; Kortz, U. J. Organomet. Chem. 2010, 695, 1307–1313. Poddel’sky, A. I.; Smolyaninov, I. V.; Somov, N. V.; Berberova, N. T.; Cherkasov, V. K.; Abakumov, G. A. J. Organomet. Chem. 2010, 695, 530–536. Chan, E. J.; Edmonds, J. S.; Kazawa, K.; Skelton, B. W.; White, A. H. Chem. Lett. 2006, 36, 160–161. Ioannou, P. V.; Vachliotis, D. G.; Chrissanthopoulos, A. Z. Anorg. Allg. Chem. 2015, 641, 1340–1346. Yang, H.; Gabbaı¨, F. P. J. Am. Chem. Soc. 2015, 137, 13425–13432. You, D.; Gabbaï, F. P. J. Am. Chem. Soc. 2017, 139, 6843–6846. You, D.; Yang, H.; Sen, S.; Gabbaï, F. P. J. Am. Chem. Soc. 2018, 140, 9644–9651. Yang, H.; Gabbaï, F. P. J. Am. Chem. Soc. 2014, 136, 10866–10869. Sahu, S.; Gabbaï, F. P. J. Am. Chem. Soc. 2017, 139, 5035–5038. Lloyd, N. C.; Morgan, H. W.; Nicholson, B. K.; Ronimus, R. S. J. Organomet. Chem. 2008, 693, 2443–2450. Betz, R.; Klüfers, P. Inorg. Chem. 2009, 48, 925–935. Betz, R.; Reichvilser, M. M.; Schumi, E.; Miller, C.; Klüfers, P. Z. Anorg. Allg. Chem. 2009, 635, 1204–1208. Beckmann, J.; Finke, P.; Hesse, M.; Wettig, B. Angew. Chem. Int. Ed. 2008, 47, 9982–9984. Beckmann, J.; Hesse, M. Organometallics 2009, 28, 2345–2348. Prabhu, M. S. R.; Jami, A. K.; Baskar, V. Organometallics 2009, 28, 3953–3956. Koppaka, A.; Park, S. H.; Hashiguchi, B. G.; Gunsalus, N. J.; King, C. R.; Konnick, M. M.; Ess, D. H.; Periana, R. A. Angew. Chem. Int. Ed. 2019, 58, 2241–2245.

10.06

Frustrated Lewis Pair Systems

Miquel Navarro, Juan José Moreno, and Jesús Campos, Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgánica and Centro de Innovación en Química Avanzada (ORFEO-CINQA), Consejo Superior de Investigaciones Científicas (CSIC) and University of Sevilla, Sevilla, Spain © 2022 Elsevier Ltd. All rights reserved.

10.06.1 10.06.2 10.06.2.1 10.06.2.2 10.06.2.3 10.06.2.3.1 10.06.2.3.2 10.06.2.3.3 10.06.2.3.4 10.06.2.4 10.06.2.4.1 10.06.2.4.2 10.06.2.4.3 10.06.2.5 10.06.2.5.1 10.06.2.5.2 10.06.2.5.3 10.06.2.5.4 10.06.2.5.5 10.06.2.5.6 10.06.2.6 10.06.2.6.1 10.06.2.6.2 10.06.2.6.3 10.06.2.7 10.06.2.7.1 10.06.2.7.2 10.06.2.7.3 10.06.2.7.4 10.06.2.7.5 10.06.2.7.6 10.06.2.7.7 10.06.2.7.8 10.06.2.7.9 10.06.3 10.06.3.1 10.06.3.2 10.06.3.3 10.06.3.4 10.06.3.5 10.06.3.6 10.06.3.7 10.06.3.8 10.06.4 10.06.4.1 10.06.4.2 10.06.4.2.1 10.06.4.2.2 10.06.4.2.3 10.06.4.2.4 10.06.4.3 10.06.4.3.1

Introduction Bond activation Introduction Dihydrogen p-Systems Alkenes Alkynes Carbonyl compounds Ethers Ring openings and contractions Lactone and lactide Cyclopropanes Epoxides Small molecule oxides Carbon dioxide Nitrous oxide Sulfur dioxide Carbon monoxide Carbon disulfide Nitric oxide Other bond activation processes C–F activation B–H activation S–S activation N-containing species Azides Isocyanates Nitrosobenzene (PhNO) Azo compounds Carbodiimides Mesityl nitrile-N-oxide (MesCNO) N-sulfinylamine (R-NSO) Diazo compounds Nitriles Main group FLPs beyond phosphine-borane pairs Introduction Nitrogen-based FLPs Carbon-based FLPs Silicon-based FLPs Aluminum-based FLPs Gallium-based FLPs Germanium and tin-based FLPs Other non-traditional main group-based FLPs FLPs in catalysis Introduction Hydrogenation catalysis First examples Expanding the hydrogenation scope Asymmetric hydrogenations Water tolerant FLPs Beyond hydrogenation: Other catalytic transformations Hydrosilylation

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00129-3

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10.06.4.3.2 10.06.4.3.3 10.06.4.3.4 10.06.4.3.5 10.06.4.3.6 10.06.4.3.7 10.06.4.3.8 10.06.5 10.06.5.1 10.06.5.2 10.06.5.3 10.06.5.4 10.06.5.5 10.06.5.6 10.06.5.7 10.06.6 10.06.6.1 10.06.6.2 10.06.6.2.1 10.06.6.2.2 10.06.6.2.3 10.06.6.3 10.06.7 10.06.7.1 10.06.7.1.1 10.06.7.1.2 10.06.7.2 10.06.7.2.1 10.06.7.2.2 10.06.7.2.3 10.06.7.2.4 10.06.7.2.5 10.06.7.2.6 10.06.8 References

Transfer hydrogenation Dehydrogenation of aminoboranes Hydroamination CO2 reduction Hydroboration and C–H borylation C–F derivatization Polymerization catalysis Mechanistic considerations Introduction Thermodynamics of H2 splitting by FLPs Frustrated complex Electron transfer (ET) model Electric field (EF) model Coexistence of ET and EF Summary Transition metal Frustrated Lewis Pairs Introduction Transition metal Frustrated Lewis Pairs with one metal Early and mid-transition metals Late transition metals Rare-earth metals Transition metal Frustrated Lewis Pairs with two metals Recent approaches and revising FLP-like systems Solid-state and heterogeneous FLP chemistry Semi-immobilized Frustrated Lewis Pairs Immobilized Frustrated Lewis Pairs Frustrated radical pairs P/B and P/Al based radical FLP systems Carbon and silicon based radical FLP systems Nitrogen and oxygen based radical FLP systems Other radical FLPs and pseudo-FLP systems Photoinduced vs thermal SET in radical FLP systems Applications of radical FLP derivatives for organic synthesis Conclusions

567 568 569 569 570 572 573 576 576 576 578 579 579 579 580 580 580 582 582 584 586 587 591 591 592 594 597 598 600 601 603 605 605 606 607

10.06.1 Introduction This chapter provides an overview of one of the most paradigmatic examples of chemical cooperation, namely “Frustrated Lewis Pairs” (FLPs). The last edition of Comprehensive Organometallic Chemistry (COMC-III) saw the light on 2006, the exact same year in which the group of Douglas Stephan described the seminal discovery that triggered the foundation of this new field of research.1 The term “Frustrated Lewis Pair” was originally coined shortly after, in 2007.2 Since these original reports, the area has rapidly matured and the concept has become ubiquitous in most journal issues devoted to organic or organometallic chemistry, in no small part due to their proven potential in catalysis. This year marks the 15th anniversary of the realization that frustrated systems offer reactivity patterns more similar to that of transition metal organometallic complexes than to the ones exhibited by traditional main group species. At this time, the field could not be in better shape, enjoying a sort of golden age. Beyond cooperative bond activation and catalysis, the concept of frustration has permeated to other areas such as material science, molecular sensors, bioinorganic chemistry, or carbon capture, among others. Considering that the last edition of this series (COMC-III) could not possibly cover the topic, this contribution aims to present a comprehensive overview of the field since its genesis to current trends. However, the extremely rapid expansion of the field reflected in the vast number of publications along the last 15 years (Fig. 1), hampers a fully exhaustive analysis of all FLP designs and their applications. Instead, we have tried to select a wide range of relevant studies that we believe represent a global picture of the area, advising the reader to access primary literature that covers every point at issue, as well as specific reviews and perspectives on aspects of FLPs that are referenced within this chapter. Besides, we also recommend the interested reader to navigate through several books specifically devoted to Frustrated Lewis Pairs,3–5 as well as a number of general reviews.6–15 The theory of bonding developed by Gilbert N. Lewis in the 1920s provided key understanding on how elements bind to each other, offering an important tool for systematizing the reactivity between molecules in terms of Lewis acids and bases. Based on this notion, when a Lewis acid (electron deficient) and a Lewis base (electron rich) species are mixed together a new dative bond is formed in what we call a Lewis adduct (Scheme 1A). However, the formation of the dative bond is quenched in certain cases due to

Frustrated Lewis Pair Systems

525

Publications about FLPs 250

236 207

193

200

no. of publication

216

215

199

176 159

150

114 100

80

78

2010

2011

39

50

18 2 0 2007

2008

2009

2012

2013

2014

2015

2016

2017

2018

2019

2020

Year Fig. 1 Number of publications by year that include the term “Frustrated Lewis Pair” in the title or abstract. Source: Web of Science.

steric or geometric constraints, and the acidic and basic character of the two sites remain. These systems are referred as “Frustrated Lewis Pairs” and the great interest that they attract mainly derives from their cooperative reactivity toward bond activation and catalysis (Scheme 1B). Although rather useful and pedagogic, this description of FLPs finds some limitations. It has been noted that a categorical definition for these species is at the very least challenging.16 As it will be discussed along this chapter, there are, for instance, Lewis adducts that present FLP-type reactivity, as well as fully frustrated systems that remain inactive toward bond activation. Numerous organometallic species based on non-innocent ligands could be considered as FLPs, and the same could be said about the mode of action of some heterogeneous catalysts. The more recent discovery of radical mechanisms in FLP-like designs introduces an additional degree of complexity. Since focusing on the idea of FLP-reactivity rather than on the intrinsic notion of the nature of an FLP seems less questionable, we have used this guiding principle of FLP-type reactivity throughout the chapter. The idea that Lewis adduct formation may be hampered on steric grounds is not new, being first recognized by Brown back at (A)

B

B

A

Lewis Base

Lewis Acid

A

No reaction

Lewis Adduct

(B)

)) ((

B

A

B A

B

A

))(( Frustrated Lewis Pair (F LP)

FLP bond activation

Scheme 1 Schematic diagrams of (A) traditional Lewis acid/base chemistry and (B) Frustrated Lewis Pairs (FLPs).

1942.17,18 In those early studies, Brown realized that while lutidine forms the expected adduct with BF3, it is unreactive toward BMe3, a phenomenon he attributed to steric reasons (Scheme 2A). However, this anomaly from the Lewis axiom was not exploited in terms of reactivity. This had to wait until 1959, when Wittig and Benz demonstrated that benzyne reacts with an equimolar mixture of PPh3 as donor and BPh3 as acceptor to yield the corresponding zwitterionic 1,2-addition product (Scheme 2B).19 A related transformation was identified by Tochtermann early after by combining the trityl anion (Ph3C−) and BPh3 (what he referred as an “antagonistic pair”) with butadiene, which also resulted in a mixture of the corresponding 1,2- and 1,4-addition products (Scheme 2C).20 At about, the same time, the reaction between the related trityl cation (Ph3C+) and amines and pyridine was investigated, providing additional examples of inhibited Lewis adduct formation.21,22

526

Frustrated Lewis Pair Systems

(A)

N BF3

(B)

F

BF3

N

Mg

BMe3

No reaction

BPh3

BPh3 PPh 3

Br

(C)

PPh 3

BPh3

Na+

Ph3C Ph3C– Na+

+

BPh3

Ph3C

BPh3 Na+

Scheme 2 Some early examples that deviate from the traditional vision of Lewis adduct formation upon combining an acid and a base.

These and other early stoichiometric examples that will be later mentioned escaped from the traditional Lewis acid/base paradigm. However, it was not until the 1990s when the same principles were successfully exploited for catalytic purposes by the group of Piers. In particular, they showed that the highly electrophilic tris(pentafluorophenyl)borane (B(C6F5)3) catalyzes the hydrosilylation of aromatic aldehydes, ketones and esters under mild conditions (Scheme 3).23 The Lewis acid was initially thought to activate the carbonyl functionality upon coordination to the oxygen center, thus favoring the nucleophilic attack of the Si–H termini. However, kinetic studies demonstrated that this path was unproductive. Instead, the concerted activation of the Si–H bond by the borane/carbonyl pair was required for the hydrosilylation to take place. Thus, the cycle starts with reversible coordination of the silane to the borane, which becomes activated and attacked by the nucleophilic substrate with concerted hydride abstraction by the boron atom. The hydride is then rapidly transferred from the borohydride anion to the electrophilic silylcarboxonium cation, liberating the hydrosilylated product and regenerating the catalyst. This mechanism was later corroborated by several elegant experiments by Oestreich and co-workers using a chiral silicon center as a stereochemical probe (Scheme 3B).24 In a key experiment, acetophenone was treated with a chiral silane (90% ee) in the presence of B(C6F5)3 (5 mol%) to yield the corresponding silyl-ether, that was reduced with DIBAL-H to recover the chiral silane with complete inverted absolute configuration. This inversion speaks in favor of a concerted activation of the Si–H bond by the borane/ketone pair in a transition state alike A in Scheme 3B. (A)

(B)

R 3SiH B(C 6F 5) 3

O Ph

B(C 6F 5) 3

H Si iPr (90% ee)

B(C 6F 5) 3 (5 mol%) R 3Si H R 3Si R'

B(C 6F 5) 3 H Si

(C 6F 5) 3B

O

Ph

i A Pr

R''

H B(C 6F 5) 3

O

O R'

R''

OH Ph

H Si iPr (84% ee)

Scheme 3 (A) Mechanism of the B(C6F5) catalyzed hydrosilylation of carbonyl compounds; (B) stereochemical test to support the concerted activation of the Si–H bond.

Despite the relevance of these previous findings, it was not until 2006 that the true potential of Frustrated Lewis Pairs was truly recognized by Stephan and co-workers in demonstrating that the phosphino-borane p-(Mes2P)C6F4(B(C6F5)2) (1) promotes the heterolytic cleavage of dihydrogen under mild conditions (25  C, 1 atm H2).1 The resulting zwitterionic species p-(Mes2PH) C6F4(BH(C6F5)2) (2) exhibits stability toward air. Besides, it releases hydrogen to regenerate 1 after heating at 150  C, which

Frustrated Lewis Pair Systems

527

evidences the reversibility of dihydrogen activation. The reactivity contrasted with phosphino-borane adducts explored by Manners, which tend to oligomerize and polymerize upon hydrogen loss.25,26 This discovery triggered a revolution in the field of main group chemistry, since at that time the cleavage of the H–H bond was believed to require a transition metal. Contemporary independent work from Power27 and Bertrand28 on dihydrogen splitting by multi-bonded and sub-valent main group systems, respectively, provided further evidence of the potential of main group elements to mimic the reactivity of transition metals.

Scheme 4 Reversible dihydrogen activation by the phosphine borane FLP 1.

An interesting insight that emanates from the seminal discovery represented in Scheme 4 is the fact that seemingly simple areas, such as the exploitation of Lewis acid/base combinations as highly reactive species, may offer prosperous surprises. The last 15 years of FLP chemistry are a clear example, with abundant reports on this topic (Fig. 1). As aforesaid, it has become impracticable to cover

2006 2007

2008

2009

2010

2011 2012

2013 2014 2015

2016

2017

2018 2020

Reversible heterolytic dihydrogen activation by a phosphine-borane Dihydrogen activation by intermolecular phosphine/borane pairs Dihydrogen activation by vicinal phosphine/borane pairs P/B activation of olefins and the term Frustrated Lewis Pair (FLP) is coined Demonstration of FLPs as metal-free hydrogenation catalysts for polar substrates Carbon based FLP: dihydrogen activation by N-heterocyclic carbene (NHC)/borane Reversible dihydrogen activation and release under mild conditions (25  C) Preliminary results on asymmetric FLP-hydrogenation Activation of alkynes, CO2 (reversible) and N2O Stoichiometric metal-free FLP-reduction of CO2 toward methanol The concept of inert and thermally induced frustration Enantioselective hydrogenation with chiral FLPs Catalytic reduction of CO2 to methane with hydrosilanes Efficient polymerization of methyl methacrylate Introduction of the concept Lewis pair polymerization (LPP) catalysis Capture of nitric oxide (radical FLP chemistry) Transition metal FLPs (TMFLPs) based on Zr/P combinations Computational evidence for the key role of “encounter complexes” C–H and C–F bond activation Exploitation of solid-state NMR to study interactions in FLPs Catalytic hydrogenation of C–C multiple bonds Stereoselective hydrogenation of alkynes to cis-alkenes Late transition metals as bases for TMFLPs Doping metallic surfaces to access heterogeneous FLPs Catalytic C–H borylation Development of water-tolerant FLPs Anchoring FLPs to metal-organic frameworks (MOFs) Bimetallic FLP based on platinum and aluminum Catalytic dehydrocoupling of amino boranes Silica-supported FLPs as heterogeneous catalysts Transition metal-only FLP based on platinum and gold Identification of single-electron transfer (SET) mechanisms by FLPs Dinitrogen activation and functionalization by TMFLPs Identification of multiply-bonded bimetallic complexes as TMFLPs Microporous polymer networks to immobilize FLPs FLP-type activation by metal-only Lewis pairs (MOLPs) Metal-organic framework (MOF) as a heterogeneous FLP Catalytic C–C and C–N bond forming reactions by SET mechanisms

528

Frustrated Lewis Pair Systems

the results contained in the approximately 2000 publications that comprise the whole body of work on frustrated systems. The following timeline of milestones will offer an initial glance at the achievements that have defined the field. Afterwards, the works to be discussed in greater depth have been organized in several sections. The first one presents an overview of the different types of small molecules that have been activated by FLPs, from the archetypal dihydrogen activation to more complex molecules. Subsequently, we analyze the rich assortment of main group FLP designs that go beyond the foremost phosphine-borane pairs. Catalytic applications are the core of the next section, starting with hydrogenation of unsaturated substrates, including enantioselective versions, and moving to more recent and exotic transformations. The following section pays attention to mechanistic considerations, focusing on the activation of dihydrogen as a model substrate. Bearing in mind that Organometallic Chemistry constitutes the heart of the COMC series, the following discussion gives emphasis to organometallic FLPs, particularly those constructed around transition metals. Finally, the last section presents some recent approaches in the area of FLPs. It contains a discussion on merging the concept of frustration with the inherent advantages of heterogeneous catalysis, including surface chemistry and the preparation of immobilized FLPs. Besides, it presents radical chemistry in the context of frustrated systems and the very recent realization of its catalytic potential.

10.06.2 Bond activation 10.06.2.1 Introduction The ability of Lewis basic and Lewis acidic functionalities to promote a variety of chemical processes has been well-known for many decades.29,30 However, the belief that their cooperative participation in bond activation and catalysis was precluded by Lewis adduct formation was not overturned until 2006, when the group of Stephan reported the reversible activation of dihydrogen (H2) by a phosphine/borane pair.1 A plethora of stoichiometric small molecule and bond activations have since been achieved through the use of Frustrated Lewis Pairs (FLPs), paving the way for the development of metal-free catalytic applications, which are discussed in Section 10.06.4. In this section, we provide an overview of what we have considered some of the most relevant breakthroughs regarding stoichiometric small molecule and overall bond activation with FLPs, with an emphasis on those developed with the classic phosphine/borane combinations. Some of these processes have been intensively investigated during the last 15 years and, as such, several reviews covering specific transformations have been reported. For instance, the activation of carbon dioxide has been comprehensively discussed by several authors,31,32 as well as the activation of C–F bonds33 and small molecule oxides.34

10.06.2.2 Dihydrogen In 1942, Brown and co-workers reported that, contrary to the behavior of analogous, less congested systems, BMe3 and 2,6-dimethylpyridine do not form a Lewis adduct.17 In subsequent years, several combinations of sterically congested Lewis acids and bases led to the observation of unexpected reaction products,19,20,35,36 but it was not until 2006 that Stephan and co-workers described the frustrated phosphino-borane 1 that facilely cleaved the H–H bond in molecular hydrogen to form the zwitterionic phosphonium-hydridoborate 2, a reaction that could be reversed above 100  C (Scheme 5A).1 At the time, the activation of H2 was considered to be exclusively mediated by transition and f-block metal species. As such, this seminal discovery constituted a revolution in the area of main group chemistry and set the foundations for the new area of Frustrated Lewis Pairs. Fifteen years later, the activation of dihydrogen remains as a benchmark transformation to evaluate FLP behavior in most newly developed frustrated systems. Readily thereafter the foremost discovery represented in Scheme 5A, the stereoelectronic requirements for the heterolytic splitting of H2 were evaluated by means of intermolecular phosphine-borane pairs comprised of BR3 (R ¼ C6F5, Ph, Mes) and PR3 (R ¼ tBu, Mes, Ph, Me, C6F5) groups.37 The pairing of B(C6F5)3 3 with PMes3 4 and PtBu3 5 readily splits H2 to form [R3PH] [HB(C6F5)] (6 and 7, Scheme 5B). These species do not release H2 even at 150  C. X-ray crystallographic analysis of 7 disclosed long H–H contacts in the solid state. The combination of the less electrophilic borane BPh3 and PtBu3 activates H2, although in lower yields and through longer reaction times. In turn, the less sterically encumbered PPh3 8 and PMe3 9 form a Lewis acid-base adduct with B(C6F5)3, precluding the observation of reactivity toward H2 at 25  C (Scheme 5C). Other pairings, such as PMes3 and BPh3, P(C6F5)3 and B(C6F5)3, or PtBu3 and BMes3 are inert toward H2, indicating that sufficient combined Lewis acidity and basicity is required to activate H2.38 The fine-tuning of the electronic properties of the Lewis acid and base led to the development of phosphine-borane systems for which H2 release could occur under milder conditions (60  C).39 Moreover, the combination of B(p-C6F4H)3 12 and P(o-C6H4Me)3 13 reversibly activates and releases H2 at room temperature (Scheme 5D).40,41

Frustrated Lewis Pair Systems

529

(A)

(B)

(C)

(D)

Scheme 5 Early examples of intermolecular dihydrogen activation by phosphine borane FLPs.

Erker and co-workers reported the four-membered, cyclic intramolecular phosphine-borane adduct Mes2PCH2CH2B(C6F5)2 15, which activates dihydrogen via accessible open-chain conformers (Scheme 6A).42 This species presents a weak P ⋯ B interaction in the ground state and can be regarded as an early example of thermally-induced FLP behavior, which has been successfully exploited since.40,43–53 Following this seminal report, many intramolecular P/B FLP architectures were developed, including vicinal,43,54 geminal55–58 and phosphido-borane59 motifs that were found to be reactive toward H2 (Scheme 6B). (A)

(B)

(C)

(D)

Scheme 6 Dihydrogen activation by intramolecular P/B FLPs.

While a variety of Lewis bases have been employed in the development of FLPs for H2 activation, the use of highly fluorinated, strongly Lewis acidic boranes has been necessary to achieve reactivity, with few exceptions.55 Krempner and co-workers developed

530

Frustrated Lewis Pair Systems

active FLPs by pairing weak Lewis acidic boranes with strong bases, such as a carbanion, a phosphazene, and proazaphosphatrane 24 (Verkade’s superbase, Scheme 7).60,61 An additional advantage of these strategies is the avoidance of fluorinated boranes, whose high reactivity presents some difficulties in their handling, and synthetic limitations due to challenging preparation protocols.62

Scheme 7 FLP dihydrogen activation by a phosphine/borane pair based on a non-fluorinated borane.

As aforesaid, the activation of dihydrogen is without a doubt the most investigated reaction in the area of FLPs. The previous systems represent some selected examples based on the classic phosphine/borane pair combination, though a number of other systems that mediate the same transformation will be discussed along the following sections.

10.06.2.3 p-Systems 10.06.2.3.1

Alkenes

As highlighted in the introductory section, the term “Frustrated Lewis Pair” was coined in a seminal 2007 report by Stephan and co-workers describing the activation of ethylene, propylene and 1-hexene by the combination of tertiary phosphines and B(C6F5)3 (Scheme 8A).2 Subsequent studies employing boranes with pendant olefins supported the formation of weak “van der Waals” complexes between the olefin and the Lewis acid as intermediates in the FLP reactivity. These support the fact that the nucleophilic attack occurs at the b-carbon of the alkene except when exceptionally bulky Lewis bases are employed, which results in the reverse regiochemistry (Scheme 8B).63,64 Dienes also display reactivity toward the intermolecular FLP comprised of B(C6F5)3 and PtBu3: 1,4-addition products were isolated in 50–60% yield, but the reaction mixtures contained unidentified species that could be derived from 1,2-addition (Scheme 8C).65 The activation of olefins by intramolecular FLPs has also been documented. Mes2PCH2CH2B(C6F5)242 reacts with ethyl vinyl ether, forming the six-membered heterocyclic zwitterion in a regioselective manner, and with norbornene, forming the exo-2,3-adduct 33 under kinetic control (Scheme 8D).66 Although zwitterionic addition products are the most common in the FLP activation of alkenes, the disubstituted olefin isobutylene is proposed to disfavor such a pathway due to steric hindrance, reacting with the B(C6F5)3/PtBu3 pair to afford the allylic salt 34 as the result of C–H activation (Scheme 8E).67 H

(A)

B(C 6F 5) 3

PtBu3

+

3

( tBu) 3P

C2H4

H

C B(C 6F 5) 3

C H

5

H 27

(B)

CF 3

O

PtBu3

(C 6F 5) 2B

PtBu3

F 3C (C 6F 5) 2B

CF 3

CF 3

HNC5H6Me4

CF 3

(C 6F 5) 2B

O H

28

N

29 (C)

( tBu) 3P B(C 6F 5) 3

+

PtBu3

3

5

(D)

B(C 6F 5) 3

32 Mes2P

Mes2P

B(C 6F 5) 2

B(C 6F 5) 2 15 33

(E)

B(C 6F 5) 3 3

+

PtBu3

[HPtBu3]

B(C 6F 5) 3

5 34

Scheme 8 FLP-mediated alkene activation.

CF 3

O

30

5

31

Frustrated Lewis Pair Systems

10.06.2.3.2

531

Alkynes

The activation of alkynes by FLPs can proceed via two different pathways, as first reported by Stephan and co-workers.68 PtBu3 reacted with B(C6F5)3 and phenylacetylene (PhC^CH) via alkyne deprotonation to yield the alkynylborate phosphonium salt 35 (Scheme 9A). In turn, under identical conditions the less basic P(o-C6H4Me)3 forms the zwitterionic alkene species 36 stemming from the 1,2-addition reaction (Scheme 9B). The classical Lewis-adduct Ph3P
B(C6F5)3 reacts with PhC^CH via 1,2-addition, an early example of thermally-induced FLP behavior. The FLP comprised of PtBu3 and B(C6F5)3 also displays reactivity toward the internal alkyne Me3SiC^CSiMe3, affording the alkynylborate salt 37 via C–Si bond breaking and P–Si bond formation (Scheme 9C).69 Stephan’s Mes2PC6F4B(C6F5)2 phosphine-borane FLP 1 reacts with PhC^CH to form the macrocycle 38 by means of 1,2-addition reactions (Scheme 9D). In turn, Erker’s vicinal FLP Mes2PCH2CH2B(C6F5)2 15 reacts selectively with 1-pentyne through C–H cleavage to form the corresponding zwitterion,66 and with 2-methyl-1,3-butenyne, a conjugated enyne, to give a mixture of the alkyne deprotonation and 1,4-addition products (39 and 40, respectively), the latter resulting in a cyclic allene (Scheme 9E).70 Conjugated diynes selectively react with Mes2PCH2CH2B(C6F5)2 via 1,4-addition to form eight-membered

(A)

B(C 6F 5) 3

PtBu3

+

PhC CH

[HPtBu3]

B(C 6F 5) 3

5

3

35 H

(o-tol) 3P

(B)

B(C 6F 5) 3

+

P( o-C6H4Me) 3

3

PhC CH

B(C 6F 5) 3

13 36

(C)

B(C 6F 5) 3

PtBu3

+

3

Me3SiC CSiMe3

[Me3SiPtBu3] Me3Si

5

37 F

(D)

Mes2P

B(C 6F 5) 2 F

F

Mes2P

F

F

B(C 6F 5) 3

PhC CH

B(C 6F 5) 2 F

Ph

F

Ph

F F

F PMes2

(C 6F 5) 2B

1

38

F

F

H (E)

H Mes2P

Mes2P

B(C 6F 5) 2

Mes2P +

B(C 6F 5) 2

• H3C 40

15 39

(F)

Mes2P

B(C 6F 5) 2 15

Scheme 9 Alkyne activation by P/B FLPs.

H 3C

CH3

B(C 6F 5) 2

Mes2P H 3C

B(C 6F 5) 2

• • 41

CH3

H

532

Frustrated Lewis Pair Systems

heterocycles comprising 1,2,3-butatriene subunits (41, Scheme 9F),70 while the B(C6F5)3/PtBu3 FLP gives a mixture of the 1,2- and 1,4-addition products.71 In 2018, Vasko, Kamer, Aldridge and co-workers reported a dimethylxanthene-based P/B FLP (42) that could reversibly cleave the C–H bond in phenylacetylene at room temperature, forming species 43 (Scheme 10A).72 After several hours, a new multi-insertion product (44) forms, which presumably starts by 1,2-addition of the alkyne to the free FLP. Reaction with the more electron rich alkyne 4-MeOC6H4C^CH permitted the isolation of the 1,2 adduct 45 (Scheme 10B). Replacing the phenyl groups by mesityl rendered a more basic phosphine fragment, which resulted in irreversible C–H activation of PhC^CH.

(A)

PhC CH

PhC CH

O Ph 2P

H

O

O B(C 6F 5) 2

PPh 2

B(C 6F 5) 2

B

C6F 5

Ph

42

43

Ph 2P

Ph

C6F 5

Ph

44

(B)

Ph

4-MeOC6H4C CH O PPh 2

O B(C 6F 5) 2

Ph2P

B(C 6F 5) 2

42

H MeO

45

Scheme 10 Alkyne activation with a dimethylxanthene FLP.

10.06.2.3.3

Carbonyl compounds

In 1987, Balueva and Erastov reported the facile 1,2-addition of saIicylaldehyde to the vicinal bora-phosphacyclobutene 46 to afford the corresponding six-membered heterocyclic compound 47 (Scheme 11A).73 A subsequent report from the same group disclosed that such reactivity could be extended to other FLPs and aldehydes.74 Two decades later, Erker and co-workers reported that Mes2PCH2CH2B(C6F5)2 reacts preferentially with the carbon-oxygen double bond in trans-cinnamic aldehyde to give compound 48 (Scheme 11B).66 Related 1,2-additions of the C]O bond were later reported for benzaldehyde,75–77 trans-cinnamic aldehyde,76 and p-chlorobenzaldehyde.78 In turn, the diesters dimethylfumarate and dimethylmaleate undergo 1,2-addition of the C]C bond to Mes2PCH2CH2B(C6F5)2 (Scheme 11C).79 Diphenylbutendione reacts with Mes2PCH2CH2B(C6F5)2 to form the cyclic acetal 50 resulting from the formation of P–C and B–O bonds, and with the B(C6F5)3/PtBu3 FLP to give the 1,4-addition product 51 (Scheme 11D). 1,4-P/B addition was also observed for the reaction of chalcone with a cyclic five-membered FLP.80 The reactivity of a series of conjugated ynones with Mes2PCH2CH2B(C6F5)2 was found to be controlled by subtle effects.81 For instance, ynone CH3C^CC(O)Ph undergoes 1,4-addition at low temperature, forming a zwitterionic eight-membered allene (52), which upon heating converts to the 1,2-addition product of the C^C bond (53, Scheme 11E). In turn, the enolizable ynone PhC^CC(O) CH3 forms product 54, derived from 1,4-addition with concomitant enolate tautomerization (Scheme 11F). Other non-enolizable ynones preferentially formed the eight-membered cyclic allenes.

Frustrated Lewis Pair Systems

Bu

O

533

Ph

(A)

H Bu

Ph

Bu2B

Bu2B O

OH

PPh 2 H OH

PPh 2 47

46 H O

(B)

Mes2P Ph

B(C 6F 5) 2

Mes2P

B(C 6F 5) 2 O

H Ph

15 H

48

H

(C)

Mes2P

MeO2C

B(C 6F 5) 2

CO2Me

Mes2P

B(C 6F 5) 2

H MeO2C

H

15

49 O

(D)

Ph

15

O Ph

O (C 6F 5) 3B

CO2Me

B(C 6F 5) 3 3 PtBu3

PtBu3

5

Ph

Ph

Mes2P

B(C 6F 5) 2

Mes2P H

B(C 6F 5) 2 O O

O Ph

51

Ph

50

O

(E)

H3C Mes2P

Ph

B(C 6F 5) 2

-40 ºC

Mes2P

B(C 6F 5) 2

•

H3C

15

O

reflux Ph

52

CH2Cl2

Mes2P H3C

B(C 6F 5) 2 O Ph 53

O (F)

Ph B(C 6F 5) 2

Mes2P

CH3

15

Mes2P

B(C 6F 5) 2 O

Ph H

CH2 54

Scheme 11 Activation of carbonyl compounds with P/B FLPs.

10.06.2.3.4

Ethers

Shortly before the landmark discovery of the reversible activation of H2 by an FLP,1 Stephan and co-workers reported that the combination of the sterically demanding phosphines R2PH (R ¼ Mes (55), tBu (56)) reacted with (THF)
B(C6F5)3 57 reacted to afford the corresponding ring-opened zwitterionic phosphonium borates 58 and 59 (Scheme 12A).82 In turn, the use of the less bulky Ph2PH led to the formation of the Lewis adduct with concomitant THF release, showcasing the FLP nature of this process. Similar ring-opening reactivity was achieved by combination of (tBu)2PH with the less Lewis-acidic borane B(p-C6F4H)3.83 In the same report, the FLP-mediated ring-openings of 1,4-dioxane and 1,4-thioxane were achieved by heating at 50  C in the presence of PtBu3 and B(C6F5)3 (Scheme 12B). The combination of the bis-borane 1,2-C6H4(BCl2)2 64 and PtBu3 also activates THF, affording the five-membered ring 65 resulting from a bridging alkoxide between the B centers (Scheme 12C).84 In 2014, the group of Stephan reported stoichiometric and selective transformations of ethers dependent on the choice of the components of the FLP.85 The combination of PtBu3 and the oxophilic Lewis acid B(C6F5)3 reacted via oxygen coordination to the boron center, triggering heterolytic C–O bond cleavage, yielding the salt 66 (Scheme 12D). In turn, the hydridophilic tritylium cation 67 and (tBu)2PH, which reversibly form a phosphonium-cyclohexadienyl cation, reacted with ethers via a-hydride abstraction, affording P–C bond formation in the corresponding C–H activation product 68 (Scheme 12E).

534

Frustrated Lewis Pair Systems

(A)

(B)

(C)

(D)

(E)

Scheme 12 Activation of ethers by phosphine/borane FPLs.

10.06.2.4 Ring openings and contractions 10.06.2.4.1

Lactone and lactide

The d-valerolactone
B(C6F5)3 Lewis adduct 70 reacts with bulky tertiary phosphines (PCy3 (71) or PtBu3 (5)) to afford the ring-opened zwitterionic boronic esters 72 and 73, respectively (Scheme 13A).86 Similar reactivity was observed with N-bases, albeit with decreased reaction rates. Interestingly, the rac-lactide
B(C6F5)3 Lewis adduct 74 reacts with bulky P and N Lewis bases via a-hydrogen deprotonation instead of nucleophilic attack, triggering the formation of ring-contracted anions such as 75 (Scheme 13B). (A)

B(C 6F 5) 3

O

O PR 3

O R=

tBu

R 3P

(5), Cy (71)

O

B(C 6F 5) 3

72, 73

70 (B)

O

B(C 6F 5) 3 B(C 6F 5) 3 O

PtBu3 5

O

O

O

[HPtBu3] O

O 74 Scheme 13 Lactone ring-opening and lactide ring-contraction mediated by P/B FLPs.

O 75

Frustrated Lewis Pair Systems

10.06.2.4.2

535

Cyclopropanes

Stephan and co-workers reported that phosphine/borane FLPs could react with substituted cyclopropanes to afford the ring-opened zwitterionic phosphonium borates.87 PtBu3 and B(C6F5)3 react with PhC3H5 via regio-specific ring opening (76), with P–C bond formation taking place at the phenyl-substituted carbon (Scheme 14A). Olefin-substituted cyclopropanes displayed additional reaction pathways. For instance, PhHC]CH(C3H5) gave a 1:1.3 mixture of phosphonium borates 77 and 78, stemming from cyclopropane ring opening and from P-attack to the olefinic carbon concomitant with double bond migration and ring opening (Scheme 14B). In turn, steric crowding at the cyclopropane unit in H2C]CH(C3H3Ph2) triggers deprotonation and cascade rearrangement to form the butadiene-borate anion 79 (Scheme 14C). (A)

B(C 6F 5) 3 3

Ph

PtBu3

( tBu) 3P

5

B(C 6F 5) 3 Ph

76 Ph

(B)

Ph

B(C 6F 5) 3 3 PtBu3

Ph

PtBu3

+ PtBu3

5 (C 6F 5) 3B

(C 6F 5) 3B

77 1

(C)

78

:

1.3

B(C 6F 5) 3 Ph Ph

B(C 6F 5) 3 3 PtBu3

[HPtBu3]

5 Ph

Ph 79

Scheme 14 Ring-opening of cyclopropanes mediated by phosphine/borane FLPs.

10.06.2.4.3

Epoxides

In 2018, Slootweg and co-workers reported that intermolecular, geminal, and vicinal P/B FLPs facilely promoted ring-opening of several substituted epoxides to afford the corresponding open-chain (81), six-membered (82), and seven-membered zwitterionic heterocycles (84), respectively (Scheme 15).88 Mechanistic investigations revealed that the reaction proceeds by initial activation of the epoxide by coordination to a Lewis acidic fragment followed by nucleophilic attack of a phosphine. In the case of intramolecular FLPs 19 and 83 the nucleophilic attack is proposed to involve a second molecule of the FLP.

MetBu2P O 80

PtBu2Me ( tBu)

BPh2 O

2P

25 BPh3

19 ( tBu) 2P

BPh2

BPh3

CH3 81

O 83

CH3 82

CH3 Ph2P

B O O

Ph2P

O B OO CH3 84

Scheme 15 Ring-opening of epoxides mediated by intermolecular, vicinal, and geminal P/B FLPs.

536

Frustrated Lewis Pair Systems

10.06.2.5 Small molecule oxides 10.06.2.5.1

Carbon dioxide

Anthropogenic carbon dioxide emissions constitute the largest contributor to global warming, sustaining interest in CO2 capture and chemical conversion strategies. An early example of CO2 capture by a P/B frustrated system was reported by Nöth and co-workers, disclosing that the amino(phosphanyl-imino)borane 85 underwent addition of CO2 to form the five-membered cycloadduct 86 (Scheme 16A).89 Reversible CO2 binding was first achieved by the FLP comprised of B(C6F5)3 and PtBu3, forming (tBu)3PCO2B(C6F5)3 (87, Scheme 16B).90 CO2 release was triggered at temperatures above 70  C or via displacement by THF or H2. Although CO2 binding was found to be irreversible in the geminal FLP reported by Slootweg and Lammertsma,55 the adduct resulting from the vicinal intramolecular FLP Mes2PCH2CH2B(C6F5)2 15 and CO2 (88) presented limited thermal stability above −20  C, regenerating the FLP above that temperature (Scheme 16C). However, the exergonic formation of FLP-CO2 adducts is not a requisite to achieve CO2 reduction with Frustrated Lewis Pairs.91 This finds support in an early report by Ashley and O’Hare, in which CO2 reacts with the product of H–H splitting by an FLP, forming a formatoborate complex that can further evolve to produce methanol in 24% yield, although under rather harsh conditions.92 A subsequent report confirmed that the formation of formyl derivatives can also be attained by FLP-derived phosphonium hydridoborates.93 An alternative strategy for the stoichiometric reduction of FLP-CO2 adducts to methanol employing amine-boranes was reported by Stephan and co-workers, capitalizing on the thermal stability imparted by chelating bis-boranes (Scheme 16D).84 The use of an ammonium borate salt instead of amine-boranes substantially increased both the reaction rate and yield.

(A)

N B N PtBu2

CO2

85 +

O

O

80 ºC, vacuum

5

(C)

B(C 6F 5) 2

( tBu) 3P

CO2, 25 ºC

PtBu3

3

Mes2P

PtBu2

86

(B)

B(C 6F 5) 3

N N B

CO2, 25 ºC

O B(C 6F 5) 3 O 87

Mes2P

B(C 6F 5) 2 O

-20 ºC, CH2Cl2

O

15

88 O

(D)

Cl2 B BCl2

Cl PtBu3

CO2

O

PtBu3

B Cl B Cl2

64

Me2NHBH3; D2O

CH3OD

89

Scheme 16 Activation and stoichiometric reduction of CO2 mediated by P/B FLPs.

10.06.2.5.2

Nitrous oxide

Nitrous oxide (N2O) is a long-lived greenhouse gas and an environmental pollutant. It is much less abundant than CO2 but about 300 times as potent at heating the atmosphere, which makes its chemical capture and conversion a matter of environmental interest. In 2009, Stephan and co-workers reported its activation with the FLP comprised of B(C6F5)3 and P(tBu)3 to give the “W”-shaped trans-trans chain complex 90 featuring intact PNNOB linkages (Scheme 17A).94 The activation product extrudes N2 to form the Lewis acid-base adduct (tBu)3P]O
B(C6F5)3 91 after prolonged heating (135  C for 44 h) or under photolytic conditions (5 min). Successful activation of N2O was only achieved with highly basic phosphines, but the acidity of the borane could be decreased. In fact, the combination of PCy3, B(C6H4-p-F)3 and N2O rapidly forms the phosphine oxide-borane adduct; attempts to detect a transient N2O adduct were unsuccessful.95 More recently, Aldridge and co-workers combined bulky Lewis bases and an electron-deficient ferrocenyl borane, targeting an FLP approach to the colorimetric and electrochemical detection of N2O.96 While the use of PCy3 rendered the corresponding phosphine oxide resulting from N2 extrusion, PtBu3 and FcB(C6F5)2 (92) generate the ambiphilic N2O adduct 93 concomitantly with a maroon-to-amber color change (Scheme 17B). Two years later, Aldridge et al. developed a dimethylxanthene-based P/B system capable of reversibly capturing N2O (Scheme 17C), which led to the development of a reversible colorimetric detection protocol for N2O.97

Frustrated Lewis Pair Systems

(A)

B(C 6F 5) 3

PtBu3

+

3

N2O, 25 ºC

( tBu) 3P N

135 ºC, 44 h N O B(C 6F 5) 3

5 90

(B)

B(C 6F 5) 2Fc

N2O

PtBu3

+

( tBu) 3P O B(C 6F 5) 3

or hν, 5 min -N2

91

( tBu) 3P N N O B(C 6F 5) 2Fc

5

92

537

93

(C)

N2O, 20 ºC O PPh 2

N2O, 50 ºC B(C 6F 5) 2

Ph 2P

42

N

O N

O

B(C 6F 5) 2

94

Scheme 17 N2O activation by P/B FLPs.

10.06.2.5.3

Sulfur dioxide

Although the impact of sulfur dioxide (SO2) as an indirect greenhouse gas is still under study, when combined with water and air it forms sulfuric acid, the main component of acid rain. In 2013, Grimme, Stephan, and Erker reported that inter- (PtBu3/B(C6F5)3) and intramolecular (Mes2PCH2CH2B(C6F5)2) FLPs readily captured SO2 (Scheme 18).98 These reactions take place at room temperature for the intermolecular FLP and at −78  C for the intramolecular systems studied. The resultant SO2 adducts 95 and 96 are structurally similar to those stemming from CO2 addition to FLPs, but in contrast to the trigonal geometry of the carbon atom, the metrical parameters in the SO2 adducts are consistent with a distorted trigonal pyramidal geometry at sulfur.

( tBu) 3P

S

O

B(C 6F 5) 3

25 ºC B(C 6F5) 3 3 PtBu3

O 95

5

15 SO 2

Mes2P

B(C 6F 5) 2 –78 ºC

Mes2P S O

B(C 6F 5) 2 O 96

Scheme 18 SO2 activation by P/B FLPs.

10.06.2.5.4

Carbon monoxide

Carbon monoxide (CO), an indirect greenhouse gas, is also an extremely toxic inhalant that demands efficient capture and detection strategies. In 2010, the group of Stephan reported four-membered boron amidinates (97 and 98) that could sequester CO through transient open-chain structures to form five-membered heterocycles (99 and 100, Scheme 19A).99 Erker and co-workers have reported reversible CO binding by several vicinal P/B FLPs, including Mes2PCH2CH2B(C6F5)2 15 and norbornane-based systems (101); the CO adducts 102 and 103 were isolated below room temperature (Scheme 19B).100,101 Many examples have been reported where CO uptake by FLPs was combined with chemical modifications, including C^O bond cleavage, head-to-tail coupling, and the formation of formyl-containing products (105) (Scheme 19C).78,102–114 Aldridge and co-workers have recently reported the BNB FLP system 106 capable of reversibly trapping CO above room temperature (Scheme 19D).115 For this system, the CO binding event is concomitant with a color change from orange to colorless, resulting from the migration of one of the aryl substituents to the carbon atom of CO (107), a feature that opens the possibility for this FLP system to be applied in CO sensing.

538

Frustrated Lewis Pair Systems

(A)

O

R N

R N

CO

B(C 6F 5) 2 N R R = iPr (97), tBu (98)

B(C 6F 5) 2 N R

99 ( iPr), 100 ( tBu) B(C 6F 5) 2 H H 101 PMes2

15

(B)

Mes2P

B(C 6F 5) 2

Mes2P

C

B(C 6F 5) 2

B(C 6F 5) 2 Mes2P

-35 ºC

-40 ºC

O 102

O

103

(C)

Mes2P

CO

CO

Mes2P

HB(C 6F 5) 2

H

B(C 6F 5) 2 15

104

B(C 6F 5) 2

C O B(C 6F 5) 2 105

(D)

CO N BAr2

N

40-70 ºC BAr2

Ar

106 Ar = C6H3(CF 3) 2

B

C Ar

O BAr2 107

Scheme 19 CO binding and stoichiometric reduction by FLPs.

10.06.2.5.5

Carbon disulfide

Carbon disulfide, though not being a small molecule oxide as referred in this section, deserves some discussion as the sulfur analog of CO2. Besides, this molecule is also a common atmospheric pollutant. In the early 1990s, before the potential concept of frustration was fully recognized, pioneering research by Balueva and co-workers demonstrated that the vicinal P/B FLP 108 could react toward CS2, forming the six-membered heterocycle 109 (Scheme 20A).116 Soon thereafter, Nöth and co-workers reported heterocumulene addition by the amino(phosphanyl-imino)borane 85 to form the corresponding five-membered cycloadduct 110 (Scheme 20B).89 In 2016, Wagner et al. reported a geminal P/B FLP (111) which also formed a zwitterionic 5-membered ring (112) upon exposure to CS2 (Scheme 20C).57 A related, O-bridged geminal FLP was developed in 2019 by Stephan and coworkers, which despite presenting reversible CO2 binding equilibria readily incorporated CS2.117 (A)

Ph

Ph

Bu

Et2P

BBu2

Et2P

Bu

CS2 BBu2 C S

108

S

109

(B)

N B N PtBu2

CS2

N

PtBu2

S

S

N B

85

110

(C)

( tBu) 2P

BAr2 111

Ar = C6H3(CF 3) 2 Scheme 20 CS2 capture by P/B FLPs.

CS2

( tBu) 2P

BAr2 S

S 112

Frustrated Lewis Pair Systems

10.06.2.5.6

539

Nitric oxide

Despite the prowess displayed by FLPs in small molecule activation, reports of substrates with unpaired electrons are comparatively scarce. Nitric oxide (NO), the simplest stable monoradical, is a well-known phosphine oxidant, giving phosphorus(V) oxo species and N2O.118–120 The reaction of NO with the intermolecular FLP comprised of B(C6F5)3 and PtBu3 resulted in the formation of a 1:1 mixture of (tBu)3P]O
B(C6F5)3 and (tBu)3PdN2OdB(C6F5)3 (see Scheme 17A, 90 and 91), stemming from the aforementioned reactivity of NO with the phosphine fragment of the FLP. In turn, the intramolecular FLP Mes2PCH2CH2B(C6F5)2 gave the FLP-NO nitroxide radical 113 in 58% isolated yield, in which both the boron and phosphorus atoms bind to the nitrogen atom of NO, and the radical character shifts to the O-atom upon binding (Scheme 21A).121,122 Contrary to the sluggish participation of NO in Hydrogen Atom Transfer (HAT) reactivity, the FLP-NO adduct readily reacts with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH species 114 (Scheme 21B). It also reacts with stronger C–H bonds in cyclohexene and ethylbenzene, resulting in H-atom abstraction and formation of new O–C bonds, yielding a 1:1 mixture of FLP-NOH 114 and FLP-NOR species 115 (Scheme 21C). These studies were extended to a family of related intramolecular P/B-FLPs, including Hydrogen Atom Abstraction (HAA)/radical capture reactivity toward toluene.123 These processes are related to the recent development of frustrated radical pairs, a topic that will be discussed in more detail in Section 10.06.7.2.

(A)

Mes2P

NO

B(C 6F 5) 2

Mes2P

B(C 6F 5) 2 N

15

O

113

(B)

2 Mes2P

2 Mes2P

B(C 6F 5) 2 N O

B(C 6F 5) 2 N

113

-

OH

114 Mes2P

(C)

B(C 6F 5) 2 N

2 Mes2P

B(C 6F 5) 2 N O

Mes2P

B(C 6F 5) 2 N

113

OH

+

O

114 115

Scheme 21 FLP binding and reactivity of the NO radical.

10.06.2.6 Other bond activation processes 10.06.2.6.1

C–F activation

Seminal work from Olah and co-workers124,125 disclosed that strong Lewis acids could be employed to effect the activation of C–F bonds, a strategy that has been effectively employed since.126–131 In 2012, the group of Stephan reported stoichiometric C–F activation in fluoroalkanes mediated by the PtBu3/B(C6F5)3 FLP.132 Linear fluoroalkanes gave tetraalkyl phosphonium fluoroborate salts (116, Scheme 22A), whereas in the corresponding reaction between fluorocyclohexane and the P/B FLP system cyclohexene formed alongside [PH(tBu)3][BF(C6F5)3] (117, Scheme 22B). Insight into stoichiometric C–F activation in the trifluoromethyl ketone PhC(O)CF3 was gained by means of the FLP comprised of Verkade’s base (P(MeNCH2CH2)3N) 118 and BPh3 (Scheme 22C).133

540

Frustrated Lewis Pair Systems

(A)

B(C 6F 5) 3 3 F

(B)

PtBu3

[tBu3P( CH2) 4CH3] [FB(C 6F 5) 3]

5

116

F B(C 6F 5) 3 3 PtBu3

[tBu3PH] [FB(C 6F 5) 3]

5

117

N P N

(C)

+

N

118

O N CF 3

O BPh3

N P N

25

CF 2

[FBPh3]

N

N 119 Scheme 22 C–F activation mediated by FLPs.

10.06.2.6.2

B–H activation

In an early report by Stephan and co-workers, the activation of the B–H bond of catechol borane was achieved by reaction with the Lewis pairs comprised of B(C6F5)3 and P(tBu)2R (R ¼ tBu, 2-C6H4(C6H5)) (Scheme 23A)134; the activation products (120) are best described as borylphosphonium salts. In 2015, Wang et al. reported a boryl-borate lithium salt (121) that, in combination with B(C6F5)3, forms an FLP capable of activating pinacolborane (Scheme 23B).135 The following year, the group of Fontaine disclosed that a dimeric ansa-aminohydroborane was capable of reversible H2 release involving FLP-mediated B–H bond cleavage.136

(A)

O

O

B(C 6F 5) 3 3

B H

PtBu3

O

O

5

B PtBu3 [HB(C 6F 5) 3] 120

O

(B)

O

O B B

[Li] O

Ph

O

121

B H O B(C 6F 5) 3 3

O

O

O B Ph

O 122

+ Li[HB(C 6F 5) 3] 123

B B

+

O

O 124

Scheme 23 B–H bond activation by FLPs.

10.06.2.6.3

S–S activation

The heterolytic cleavage of disulfides by FLPs was first reported by the group of Stephan in 2009.137 Treatment of (tBu)2P(C6F4)B(C6F5)2 (125) with diphenyl disulfide afforded the zwitterionic phosphonium borate (tBu)2P(SPh)(C6F4)B(SPh) (C6F5)2 126 (Scheme 24A). Similar reactivity was observed for the intermolecular PtBu3/B(C6F5)3 FLP in the reaction with RSSR (R ¼ Ph, p-tolyl, iPr). In contrast, the corresponding reaction with BnSSBn gave an equimolar mixture of (tBu)2P(S) 127, Bn2S, and B(C6F5)3 (Scheme 24B). FLPs comprised of carbon-based Lewis acids, reported by the groups of Alcarazo,138 Stephan,139 and Gianetti140 also mediate heterolytic S–S bond cleavage in disulfides.

Frustrated Lewis Pair Systems

541

(A)

S S F

F ( tBu)

F ( tBu)

B(C 6F 5) 2

2P

F

SPh B(C 6F 5) 2

2P

PhS

F

F

F

125

F 126

S S

(B)

B(C 6F 5) 3

+

3

PtBu3 5

( tBu) 3P

S

+

S

B(C 6F 5) 3

127 128

Scheme 24 Disulfide activation by P/B FLPs.

10.06.2.7 N-containing species 10.06.2.7.1

Azides

In 2007, Bourissou and co-workers reported the reaction between phenylazide and the vicinal phosphine-borane FLP 129, affording the 1,1-addition to the terminal nitrogen that yields the five-membered heterocycle 130 (Scheme 25A).141 This species could be photo-isomerized to the 1,2-addition product 131. Similar 1,1 azide capture was later reported by Erker et al. using Mes2PC2H4B(C6F5)2 (Scheme 25B); the addition product 132 presented dynamic solution behavior.75 Other examples of 1,1-addition were disclosed in subsequent reports.76,142 In contrast, the use of a geminal FLP led to the formation of a sixmembered heterocycle (134) resulting from 1,3-addition to the frustrated P/B Lewis pair (Scheme 25C).143 The 1,3-addition of an azide to an intermolecular FLP was reported by the group of Stephan.139 Interestingly, depending on the azide sterics the geminal P/B based FLP (tBu)2PCH2BPh2 19 can give 1,1-, 1,2-, or 1,3-addition products, forming the corresponding four(135 and 136), five- (137), or six-membered (138) heterocycles, respectively (Scheme 25D).144

542

Frustrated Lewis Pair Systems

(A)

Mes BMes2

Mes

Mes

B

PhN3

P iPr

129

UV (312 nm)

N

N

PiPr 2

P

N

iPr

iPr

Mes2P Mes2P

iPr

B(C 6F 5) 2 N

PhN3

B(C 6F 5) 2

N N N

131

130

(B)

Mes B

N

N 15

132 (C)

H B(C 6F 5) 2

(C 6F 5) 2P

MesN3

H B(C 6F 5) 2

(C 6F 5) 2P N

133

N

N

Mes

134 (D)

( tBu) 2P

PhN3

BPh2

N

19 RN 3 R = tBu or Mes

( tBu) 2P

B(C 6F 5) 2 N

N

Ph

138

BPh2 N ∆

N

(C 6F 5) 2P

N

R 135 ( tBu), 136 (Mes)

R = Mes

BPh2 ( tBu) 2P N N N Mes 137

Scheme 25 Azide capture by P/B FLPs.

10.06.2.7.2

Isocyanates

In 1992, Arbuzov and co-workers reported the reactivity of phenyl isocyanate (PhNCO) toward the vicinal P/B FLP 46, affording an equilibrium mixture of the C,N (138) and C,O (139) addition products (Scheme 26A).145 Soon thereafter, Nöth and co-workers reported the C,O-addition of phenyl isocyanate to the amino(phosphanyl-imino)borane 85 (Scheme 26B).89 Fifteen years later, Maron, Bourissou, and co-workers discovered that iPr2P(o-C6H4)BMes2 (129), related to the FLP reported by Arbuzov et al., regioselectively adds the C]O bond of PhNCO to form the corresponding six-membered heterocycle 141 (Scheme 26C).146 Similar reactivity was later observed by Erker et al. employing Mes2PC2H4B(C6F5)2.75 In turn, the electron-poor, geminal FLPs (C6F5)2Pd(m-CHCH2CH3)dB(C6F5)2143 (133, Scheme 26D) and (C6F5)2Pd(m-C]CH(R))dB(C6F5)276,147 (R ¼ CH3, Ph) regioselectively add the C]N bond of arylisocyanates to produce five-membered heterocycles such as 142. PhNCO gave a mixture of C,N and C,O addition products upon reaction with the geminal FLP (tBu)2PCH2BPh2, but the bulkier tBuNCO selectively reacted to give the C]O adduct.55 Isocyanates can react with boranes via carbo-148 or hydroboration149 to form four-membered imino-boranes (143) that present FLP behavior, further reacting with a second equivalent of isocyanate to yield a six-membered heterocycle (144, Scheme 26E). In 2020, Stephan and co-workers reported a geminal O-bridged FLP that underwent selective C,O-addition of PhNCO.117

Frustrated Lewis Pair Systems

Ph

(A)

Ph

Bu

Ph2P

PhNCO

Ph 2P

BBu2

Ph

Bu BBu2

Bu

Ph2P

+

BBu2 O

N N

Ph

O

46

543

Ph

138

139

(B)

PhNCO

N B N PtBu2 85

PtBu2

O

N Ph

140 Mes

(C)

BMes2 PiPr

N N B

Mes B

PhNCO

O

P

2 iPr

129

N

Ph

iPr

141 H

(D)

(C 6F 5) 2P

H B(C 6F 5) 2

(C 6F 5) 2P

p-tol-NCO

B(C 6F 5) 2 N

O

133 142

O

(E)

B(C 6F 5) 3

CyNCO

N C 6F 5

B O

3

C 6F 5 C6F 5

N

CyNCO C 6F 5

143

O

N B

C6F 5 C 6F 5

144

Scheme 26 Isocyanate activation by FLPs.

10.06.2.7.3

Nitrosobenzene (PhNO)

In 2010, Erker and co-workers reported that nitrosobenzene (PhNO) reacts with the intramolecular FLP Mes2PC2H4B(C6F5)2 15 via 1,2-addition, forming the 6-membered heterocycle 145 resulting from P–N and B–O bond formation steps (Scheme 27).75

NO

Mes2P

B(C 6F 5) 2 15

Mes2P B(C 6F 5) 2 N O

145

Scheme 27 Nitrosobenzene capture by an intramolecular P/B FLP.

10.06.2.7.4

Azo compounds

Bourissou and coworkers reported that the vicinal FLP iPr2P(o-C6H4)BMes2 129 adds diethyl azodicarboxylate (DEAD) to afford the corresponding six-membered zwitterionic heterocycle 146 (Scheme 28),146 paving the way for the development of dynamic self-healing gels based on FLP polymers.150

544

Frustrated Lewis Pair Systems

Mes BMes2

Mes B

DEAD

PiPr2

P iPr

129

N N

CO2Et CO2Et

iPr

146 Scheme 28 Diethyl azodicarboxylate coordination to a vicinal FLP.

10.06.2.7.5

Carbodiimides

After Nöth’s seminal report on N,N0 -dicyclohexylcarbodiimide addition to an amino(phosphanyl-imino)borane (Scheme 29A),89 in 2010 Dureen and Stephan reported the reaction of Piers’ Borane, HB(C6F5)2 148 with N,N0 -diisopropylcarbodiimide, which affords the four-membered boron amidinate 149.99 This species exhibits FLP behavior toward excess carbodiimide, which inserts into one of the B–N bonds via an open-chain intermediate to yield the six-membered heterocycle 150 (Scheme 29B).

(A)

N CyN=C=NCy

PtBu2

N B N

N B N Cy

85

PtBu2 N Cy

147 (B)

iPr

iPr

N

iPrN=C=NiPr

HB(C 6F 5) 2

B

148

C6F 5

N

iPrN=C=NiPr

iPr

C6F 5

N

iPr

N N

B

C6F 5 C6F 5

N iPr

iPr

150

149 Scheme 29 Addition of carbodiimides to FLPs.

10.06.2.7.6

Mesityl nitrile-N-oxide (MesCNO)

Mesityl nitrile-N-oxide (MesCNO) is an oxygen-atom transfer reagent that has been proposed to effect phosphine oxidation commencing via attack to the electrophilic carbon atom of the nitrile oxide.151 Stephan and co-workers provided experimental support to that mechanism by trapping the 1,3-addition product 151 of the reaction between MesCNO, PR3 (R ¼ Ph, p-tol) and B(C6F5)3 in an FLP-fashion (Scheme 30).152 Reaction of an O-bridged geminal FLP with MesCNO proceeded through an analogous 1,3-addition process to afford a six-membered heterocycle.117

B(C 6F 5) 3 3

+

PPh 3 8

PPh 3

MesCNO Mes

N

O

B(C 6F 5) 3

151 Scheme 30 MesCNO interaction with a P/B intermolecular FLP.

10.06.2.7.7

N-sulfinylamine (R-NSO)

Following a seminal report by Erker and co-workers on N-sulfinylamine capture by a Zr+/P FLP,153 Stephan and co-workers communicated the reaction between a N-sulfinylamine and P/B FLPs.154 The intermolecular FLP constituted by B(C6F5)3 and PtBu3 reacted with p-tolyl-NSO to afford the FLP adduct 152 resulting from P–N and O–B bond formation (Scheme 31A). Mes2PCH2CH2B(C6F5)2 displayed analogous reactivity, which resulted in the formation of the seven-membered heterocycle 153, in which six different atom types were linked contiguously (Scheme 31B). These FLP adducts are best described as phosphinimine-borane complexes of SO. The reaction of Ph-NSO with a geminal FLP (154) featuring a secondary P center resulted in the 1,2-addition of the S]O bond concomitant with proton transfer from P to N to yield compound 155 (Scheme 31C).155

Frustrated Lewis Pair Systems

545

(A)

( tBu) 3P B(C 6F 5) 3 3

S

O

( tBu) 3P

B(C 6F 5) 3

N

S

O

B(C 6F 5) 3

p-tol-NSO

PtBu3

+

N

5 152

(B)

Mes2P

Mes2P

p-tol-NSO

B(C 6F 5) 2

N

B(C 6F 5) 2 S

Mes2P

O

N

B(C 6F 5) 2 S

O

15 153 tBu

(C) tBu

Mes

P H

Mes P B(C 6F 5) 2 S O H N Ph 155

Ph-NSO

B(C 6F 5) 2 154

Scheme 31 N-Sulfinylamine capture by P/B FLPs.

10.06.2.7.8

Diazo compounds

In 2007, Bourissou and co-workers reported the addition of ethyl diazoacetate to a vicinal phosphine-borane FLP 129 (Scheme 32A), giving the five-membered heterocyclic compound 156 which could be reversibly photo-isomerized to the ring-expanded 157 product.141 More recently, Stephan and co-workers disclosed the 1,2-addition of Ph2CN2 to an O-bridged geminal FLP (158), yielding the corresponding five-membered heterocycle 159 (Scheme 32B).117 In turn, due to reduced steric demands, EtO2CCHN2 reacted with two equivalents of the FLP 158 to afford the bicyclic structure 160, where the second equivalent of the diazo compound adds in 1,3-fashion to the five-membered ring resulting from initial 1,2-addition (Scheme 32C).

(A)

Mes BMes2

N2

Mes

CO2Et

B

CO2Et

N

PiPr 2 129

P( tBu) 2

Ph

O O B P( tBu) 2 O N N

Ph Ph

158

Ph

(C)

N2 O

2

B O

O

O

P( tBu) 2 O

158

159

O O B P( tBu) 2 O N N O O B O ( tBu) P O O 2

160

Scheme 32 Reactivity of diazo compounds toward P/B FLPs.

N N iPr

157

N2

O

P iPr

(B)

B O

Mes B

25 ºC, 64 h iPr

156

O

Mes

-60 ºC

N

P iPr

UV (312 nm)

CO2Et

546

Frustrated Lewis Pair Systems

10.06.2.7.9

Nitriles

In 1993, Nöth and co-workers reported the 1,2-addition of nitriles R–CN (R ¼ Me, Ph, PhCH2) to the amino(phosphanyl-imino) borane 85 to give imines (R ¼ Me (161), Ph) and enamines stemming from tautomerization (R ¼ Me (162) and PhCH2) (Scheme 33A).89 In 2016, Wagner and co-workers developed the geminal P/B FLP tBu2PCH2B(Fxyl)2 (111, Fxyl ¼ 3,5(CF3)2C6H3), which adds MeCN in 1,2-fashion to form a thermally stable cyclic imine 163 (Scheme 33B).57 Similar results were obtained for the addition of MeCN, PhCN, and tBuCN to tBu2PCH2B(Ph)2; nitrilium triflates displayed comparable addition reactivity (Scheme 33C).156 The group of Erker revealed that, analogously to the reactivity of N-sulfinylaniline, 1,2-addition of benzonitrile to the geminal P/B FLP 154 (featuring a P–H bond) is followed by proton transfer to the N atom (Scheme 33D).155 The presence of both P–H and B–H bonds in geminal FLPs led to the reduction of the C^N bond in p-tosylcyanide (167, Scheme 33E).109 In turn, 1,2-addition of benzonitrile and subsequent proton transfer were followed by several rearrangements to afford a phospha-amidine compound. (A)

N

MeCN

N B N PtBu2

N B

N

PtBu2

N B

N

85

PtBu2

N H

161

162 (B)

( tBu) 2P

MeCN

BAr2

( tBu) 2P

BH2Ar2 N

111

163

Ar = C6H3(CF 3) 2 (C)

( tBu) 2P

[tBuCN-Ph][OTf]

BPh2

( tBu) 2P tBu

19

BPh2 [OTf] N Ph 164

(D)

tBu

tBu

Mes

P H

Mes P B(C 6F 5) 2 C N H Ph 165

PhCN

B(C 6F 5) 2 154

(E) tBu

F 3C

CF 3 tBu

H tBu

P

B H

CF 3

TsCN

tBu

tBu tBu

166

tBu

F 3C

CF 3

P B N C H tBu F 3C H Ts 167

Scheme 33 Reactivity of nitriles toward P/B FLPs.

10.06.3 Main group FLPs beyond phosphine-borane pairs 10.06.3.1 Introduction Since FLP chemistry began about 15 years ago, an intensive broadening of its chemical designs and applications has emerged. A whole range of new stoichiometric and catalytic transformations have been described using different combination of Lewis acids and bases. As discussed in the previous section, a wide range of small molecule activation reactions, including the challenging CO2, CO, nitrogen oxides and the cleavage of different C–X bonds have been investigated. This reactivity has mostly been developed using FLPs containing phosphine/borane combinations. However, in the last years the use of other main group elements to construct different Lewis acids and Lewis bases has steadily grown.157–159 In this section we focus on the broadening range of main group Lewis acids and bases applied in FLP chemistry and aim to describe the implications of their electronic and steric properties in the stoichiometric activation of small molecules and catalysis.

Frustrated Lewis Pair Systems

547

10.06.3.2 Nitrogen-based FLPs Replacement of the phosphine moiety in FLPs by nitrogen-containing Lewis bases represented an obvious direction to take and resulted in the formation of powerful FLP alternatives to the usual B/P architectures. Reminiscent of the early works of Brown,17,18 the group of Stephan described the first intermolecular nitrogen-based system that exhibited FLP reactivity. A combination of 2,6-lutidine and B(C6F5)3, which is found in an equilibrium between the Lewis adduct 168 and the separated species, acts as an FLP system to activate H2 and effect the ring opening of THF.45 In a related report, a detailed study on the steric effects of pyridine derivatives in their combination with B(C6F5)3 was evaluated together with their combined capacity to cleave dihydrogen (Scheme 34A). A systematic increase of the steric bulk of the substituents on the pyridine moiety prompted elongation of the resulting B–N bonds, ranging from the formation of classic Lewis pairs to full frustration in the case of 2-tert-butylpyridine.46 Similarly, the groups of Berke and Rieger evaluated the activation of dihydrogen using a combination of B(C6F5)3 with bulky Lewis bases like tetramethylpiperidine, 2,4,6-tri-tert-butylpyridine (171) and TMS-protected amines (Scheme 34B).160,161 Stephan and Erker have studied the reaction of N,N-dimethylaniline, N-isopropylaniline, 1,4-C6H4(CH2NHtBu)2, and benzyldimethylamine with B(C6F5)3 and the ability of the resulting FLP systems to activate CO2 (175), silanes, alkenes (176), and alkynes (177) (Scheme 34C).162 Alcarazo and co-workers described the use of a combination of DABCO and B(C6F5)3 to generate a FLP and efficiently hydrogenate electron-poor allenes and alkenes.163 Moreover, the group of Cantat has studied the combination of 9-BBN and different nitrogen bases such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MeTBD) and 1,8-dizabicyclo[4.4.0]dec-5-ene (DBU) for the reduction of CO2 to methanol.164

(A)

(B)

(C)

Scheme 34 Selected intermolecular nitrogen-based FLPs used in small molecule activation.

548

Frustrated Lewis Pair Systems

In 2008, Repo, Rieger and co-workers described the first intramolecular N-based FLP system (178) using an ansa aminoborane with a molecular tweezer, in which the N and B centers are located close to each other (Scheme 35A).165 This system was shown to be able to reversibly activate dihydrogen under mild conditions. In addition, it can efficiently catalyze the hydrogenation of imines and enamines. Chiral molecular tweezers based on binaphthyl aminoboranes have shown good activity in the asymmetric hydrogenation of imines and enamines, with exceptional stereoselectivities of up to 99%.166 The same groups have intensively studied the potential of the 2-boryl-N,N-dialkylanilines scaffold (180–182) for the activation of small molecules (Scheme 35B).167–169 These aminoborane units can activate hydrogen at near ambient conditions and effectively catalyze the hydrogenation of imines and of alkynes to cis-alkenes. Similar aminoborane scaffolds have been used by Fontaine and co-workers to C–H activate and catalyze the borylation of different heteroarenes such as furans, pyrroles, and electron-rich thiophenes. This metal-free catalyst showed selectivities that complement those observed with most transition metal catalysts reported for these transformations.170–172 In addition, these authors have described the cleavage of an sp3 C–H bond by a N/B Frustrated Lewis Pair. The ambiphilic molecule (2-NMe2-C6H4)2BH (184) activates the C–H bond of a methyl group upon mild heating (80  C) generating the N–B heterocycle 185 (Scheme 35C).173 In a similar fashion, Pápai, Repo et al. expanded the substrate scope of this reaction to aromatic and olefinic C–H bonds. Mechanistic studies showed that the C–H insertion proceeds via FLP activation involving heterolytic splitting of the C–H bond through cooperative action of the amine and boryl groups. The unique geometry of the aminophenylborane is key in this process resulting in a remarkably low kinetic barrier for the insertion into the sp2-C–H bond and the intramolecular protonation of the sp2-C–B bond.174 Different aminophenylborane derivatives have also been used as catalysts for the hydrogenation of carbon dioxide.175 In addition, the group of Erker has built a series of substituted alkenyl-bridged Frustrated N/B Lewis Pairs (186), which feature an important participation of the mesomeric s-trans-iminium/borata-alkene resonance form 1860 (Scheme 35D).176 This series of intramolecular FLP systems is able to split dihydrogen heterolytically under mild conditions.

(A)

(B)

(C)

(D)

Scheme 35 Intramolecular nitrogen-based FLP systems for the activation of dihydrogen and sp3 C–H bonds.

Frustrated Lewis Pair Systems

549

The use of nitrogen-based compounds has not only been limited to construct the basic partner of the FLP system, but also to develop Lewis acid derivatives.177,178 In 2018, Stephan et al. described a room-temperature-stable cyclic (alkyl)(amino)nitrenium cation (188) featuring a cationic nitrogen atom with a small HOMO–LUMO gap. The low-lying LUMO results in enhanced electrophilicity, allowing the formation of Lewis adducts with several neutral bases, and in the presence of a bulkier Lewis base such as PtBu3, it forms an FLP system that is able to active S–S bonds (Scheme 36A).179,180 More recently, the group of Gandelman has capitalized on the Lewis acidity of a triazinium-based compound (191) to generate an FLP by combining it with the bulky PtBu3 (Scheme 36B).181 This system was used to cleave Si–H bonds in PhSiH3, generating the triazane product 192 in quantitative yield.

(A)

(B)

Scheme 36 Selected nitrenium cations used as Lewis acids for FLP reactivity.

10.06.3.3 Carbon-based FLPs In 2008, the group of Tamm studied for the first time the combination of N-heterocyclic carbenes (NHCs), more precisely 1,3-ditert-butyl-1,3-imidazol-2-ylidene (194), as a Lewis base together with the acidic B(C6F5)3 to generate a carbon-based FLP (Scheme 37A). This FLP system was able to activate dihydrogen and THF.182 A year later, Stephan and co-workers expanded the study of NHC-based FLP systems and compared the use of 1,3-bis[2,6-(di-iso-propyl)phenyl-1,3-imidazol-2-ylidene and 1,3-ditert-butyl-1,3-imidazol-2-ylidene as Lewis bases with B(C6F5)3 to generate a carbon-based FLPs (Scheme 37A).183,184 While the less sterically encumbered NHC forms the classical Lewis acid-base adduct, the use of 1,3-di-tert-butyl-1,3-imidazol-2-ylidene proved to form a fully frustrated system, which was able to activate N–H bonds (196). In subsequent studies, the group of Alcarazo used allenes (197) as Lewis acids in combination with bulky NHCs (198) to produce purely carbogenic FLP systems capable to heterolytically cleave S–S bonds (Scheme 37B).138,185 Recently, Zhu et al. have computationally studied the activation of dinitrogen with an intramolecular NHC–borane FLP system.186 DFT calculations indicate that the activation of N2 is thermodynamically and kinetically favorable, and that aromaticity plays a crucial role in the stabilization of the product.

(A)

(B)

Scheme 37 Selected examples of NHCs in FLP chemistry.

550

Frustrated Lewis Pair Systems

Carbon-based systems have also been used as the acidic partner in FLPs. In 2013, Ingleson described an acridine-based borenium cation as Lewis acid.187 The combination of [(acridine)BCl2][AlCl4] (200) and 2,4,6-tri-tert-butylpyridine (201) can heterolytically cleave dihydrogen at 100  C (Scheme 38A).188 Somewhat surprisingly, the activation of dihydrogen takes place at the C9 position of acridine instead of at the boron site. The hydride ion affinity of the C9 position was calculated to be 14 kcal mol−1 greater than that of the boron center. The same group expanded this methodology to simpler acridinium-based Lewis acid salts (204, Scheme 38B). In the presence of a weak base such as 2,6-lutidine (205), the acridinium salts behave as FLP systems, activating dihydrogen and terminal alkynes. Besides, they were found to be able to efficiently catalyze the reductive transfer hydrogenation and hydrosilylation of aldimines and alcohols, as well as the 1,2-hydrocarbation of alkynes.189,190 The N-methylacridinium cation possesses excellent electrochemical attributes and has been used for electrocatalytic hydrogen oxidation in the presence of a base and a borane that functions as a hydride shuttle.191 N-Methyl-benzothiazolium salts have also been used as carbon-based Lewis acid alternatives for Si–H s-bond activation and the catalytic (de)hydrosilylation of imines.192 (A)

(B)

Scheme 38 Acridinium-based Lewis acids in FLP dihydrogen activation.

Carbon-based Lewis acids have been expanded to carbocationic species as well. Stephan and co-workers reported different C–H bond activation reactions and the cleavage of the S–S bond of diphenyl disulfide using a combination of phosphines as Lewis bases and hydrophilic tritylium ions ([Ph3C]+, 67) as Lewis acids (Scheme 39A).84 The FLP system was also capable of capturing pentafluorophenyl azide as the Staudinger reaction intermediate, a species that further reacts with Ph3SiH to give the silyl analog.139 Berionni et al. performed detailed structure-reactivity investigations on different combination of phosphines and benzhydrilium and tritylium cations.193 The tritylium-ion-derived FLPs were also shown to activate alkynes. Recently, Gianetti has described the trioxatriangulenium cation 209 (TOTA+) as a robust carbon-based Lewis acid that in combination with phosphines or NHCs can mimic typical main group FLP reactivity (Scheme 39B).140

(A)

(B)

Scheme 39 Carbocationic Lewis acids for the FLP activation of C–H and S–S bonds.

Frustrated Lewis Pair Systems

551

10.06.3.4 Silicon-based FLPs A classic alternative to carbon-based compounds are their silicon-based analogs. The group of Müller synthetized a family of triarylsilylium ions (212), which in combination with a phosphine as Lewis base are able to activate dihydrogen and CO2 (Scheme 40A).194,195 Similarly, Ashley and co-workers described the activation of H2 and D2 using a trialkylsilylium cation as Lewis acid and PtBu3.196 The group of Cantat reported a series of intramolecular base-stabilized silylium species (216) that react with CO2 and SO2 to form the corresponding N/Si+ FLP-CO2/SO2 adducts 217 (Scheme 40B).197,198 These FLP systems are also active catalysts in the hydroboration of CO2 to the methoxide level with 9-BBN, catecholborane (catBH) and pinacolborane (pinBH). Recently, the group of Kato has prepared a norbornene-based phosphine-stabilized silylium ion (218) that exhibits FLP-reactivity with carbonyl derivatives that allows the synthesis of seven- (219) to nine-membered ring heterocycles (Scheme 40C).199 Neutral silanes can also act as Lewis acids in FLP-chemistry. Mitzel and co-workers have developed a family of neutral –CH2– linked silicon/phosphorus FLPs (220) that are capable of cleaving dihydrogen and to activate different small molecules such as CO2, SO2, PhCNO and Me3SiCH2N2 to generate ring-type adducts (Scheme 40D).200–202 Stephan has also reacted intermolecular FLP systems based on silyl triflates and 2,2,6,6-tetramethylpiperidine to react with CO2 to afford silyl carbamates.203 The group of Müller has used a silicon carbene analog (silylene) as Lewis base to construct an FLP system. The isolable silylene combined either with a silylium ion or more conventional Lewis acids like BPh3 and BEt3 was able to activate dihydrogen.204,205 (A)

(B)

(C)

(D)

Scheme 40 Selected silicon-based FLP systems.

10.06.3.5 Aluminum-based FLPs Aluminum has been found to be a very powerful alternative to boron to construct different types of Lewis acids in the context of FLP chemistry. The group of Stephan described the activation of CO2 by combining PMes3 (4) as Lewis base and AlX3 (Cl, Br or I (223)) as Lewis acids to form either Mes3P(CO2)(AlX3) when X ¼ Cl or Br,206 or Mes3P(CO2)(AlX3)2 when X ¼ I (224).207 The former CO2 species rapidly react with excess of ammonia borane to give CH3OH, while the latter in the presence of a CO2 atmosphere forms Mes3P(CO2)(O(AlI2)2)(AlI3) 225 and [Mes3PI][AlI4] (226), together with concomitant evolution of CO (Scheme 41A). These FLP systems are also able to activate ethylene and propylene.208 The same research group has used the more sterically encumbered Al(C6F5)3 (227) as Lewis acid. The FLPs derived from its combination with different phosphines showed the ability to activate isobutylene and ethylene,65 to reduce CO2 to CO209 and to perform dihydrogen activation and hydride transfer to olefins (Scheme 41B).210 Zhang and co-workers have intensively studied the use of alane-based FLPs for the polymerization of different polar alkenes, providing access to multiblock copolymers with tailored properties.211–213 The group of Harder has described an FLP based on a cationic aluminum complex (Scheme 41C).214 The highly Lewis acidic 1,3-diketiminate (Nacnac)-based cationic aluminum species 230 reacts in the presence of an external weak base, as triphenylphosphine, with different small molecules like alkenes, alkynes and CO2.

552

Frustrated Lewis Pair Systems

(A)

(B)

(C)

Scheme 41 Selected aluminum-based intermolecular FLP systems.

In 2011, the groups of Lammertsma and Uhl described the first intramolecular aluminum-based FLP system. The geminal phosphorus/aluminum FLP (232) was easily obtained by hydroalumination of alkynylphosphines. These FLP systems showed good activity in the activation of dihydrogen, CO2, isocyanates and terminal acetylenes (Scheme 42A).55,215–217 In addition, the geminal P/Al-based FLP forms stable adducts (235) with alkali metal hydrides (LiH, NaH and KH), which display enhanced reactivity in the transformation of chlorotriphenylsilane to the corresponding hydride. It thus constitutes one of the very few available strategies to access soluble forms of alkali metal hydrides as amenable reagents for fine chemical synthesis (Scheme 42B).218 Fontaine and co-workers also capitalized on the FLP properties of a methylene-bridged phopshino-alane (236) to activate carbon dioxide, forming aluminum carboxylates (Scheme 42C).219 In a similar fashion, P–H or N–functionalized methylene-bridged alanes have also been used as FLP systems to activate phenylacetylene, CO2 and SO2.48,220 (A)

(B)

(C)

Scheme 42 Aluminum-based intramolecular FLP systems.

Frustrated Lewis Pair Systems

553

10.06.3.6 Gallium-based FLPs Frustrated Lewis Pairs based on gallium are also part of the FLP landscape. Schulz et al. have described a Lewis acid-base adduct of the type [LGa
A(C6F5)3] (239, L ¼ NacNac, A ¼ B or Al) that reacts at room temperature with benzaldehyde in FLP-fashion. In particular, it forms the insertion product for the borane adduct (240) and a zwitterionic species (241) in the case of the alane (Scheme 43A).221 The group of Hevia have described the activation of different carbonyl compounds (via C]O insertion) and other molecules with acidic hydrogen atoms, such as those in phenol, phenylacetylene and pinacolborane, by an intermolecular NHC/gallane pair.222–224 The study reports the use of sterically encumbered trisalkylgallium GaR3 (242, R ¼ CH2SiMe3) as the Lewis acid and a bulky NHC as a base (Scheme 43B). In 2017, Uhl and co-workers described the preparation of an intramolecular P/Ga FLP (244) able to activate phenylacetylene forming a five-membered heterocycle (Scheme 43C).225 Detailed studies on the reactivity of this P/Ga FLP system proved that it is influenced by the relatively weak Lewis acidity of the Ga atom, and differs significantly from that of analogous P/Al compound.226 This intramolecular P/Ga FLP system was able to activate CS2, ammonia-borane and diphenylazirine.227 The same group has also described the use of gallium hydrazides as active Lewis Pairs for the cooperative C–H bond activation of phenylacetylene, pentafluorobenzene and different heterocumulenes.228,229 Zhu and co-workers have recently reported a new intramolecular P/Ga system (247) that contained an unprecedented four-coordinated Lewis acidic center (Scheme 43D).230 This P/Ga species performed classical addition reactions of heterocumulenes, alkynes, diazomethane and transition metals.

(A)

(B)

(C)

(D)

Scheme 43 Selected intermolecular and intramolecular gallium-based FLP systems.

10.06.3.7 Germanium and tin-based FLPs In 2011, Kemp et al. reacted a P,P-chelated stannylene with CO2 to form an unusual product in an FLP-manner, in which CO2 binds the Sn and P sites forming a six-membered ring complex.231 Zhu and co-workers described the preparation of a series of N-bridged P/Ge (249) and P/Sn FLPs.232 These FLP systems react with alkynes (dimethyl acetylenedicarboxylate and methyl propiolate) and aldehydes (propyl aldehyde and 1,4-phthalaldehyde) affording zwitterionic C2PNGe-heterocycles (250), OCPNGe-heterocyles,

554

Frustrated Lewis Pair Systems

(Z)-germyl(iminophosphoranyl)alkenes and (Z)-stannyl(iminiphosphoranyl)alkenes (Scheme 44A). In a similar approach, the group of Mitzel independently described different geminal tin (251) and phosphorus/germanium FLPs.233–236 The FLP-type reactivity of these methylene-bridged compounds was proven by reaction with a variety of small molecules such as CO2, SO2, SO, CS2, PhNCO, cis-azobenzene, HCl and dihydrogen (Scheme 44B). Wesemann and co-workers have described several stannylene-based Lewis pairs (254, 256 and 257) that are able to activate unsaturated hydrocarbons, organic azides and aldehydes (Scheme 44C and D).237–239 In a recent work, Fernández et al. have performed a systematic computational study to understand the reactivity of neutral geminal group 14 element/phosphorus FLPs, concluding that they react similarly to more traditional P/B geminal FLPs when the group 14 element is surrounded by strong electron-withdrawing substituents. Besides, these studies revealed that the activation of CO2 and phenyl isocyanate proceeds in a concerted manner leading to the corresponding five-membered transition states, which lead to the experimentally observed zwitterionic reaction products.240 In other studies, Ashley and co-workers have used intermolecular FLP systems based on tin for dihydrogen activation and for the catalytic hydrogenation of several unsaturated functional groups such as C]C, C]N and C]O bonds.241,242

(A)

(B)

(C)

(D)

Scheme 44 Selected germanium and tin-based FLP systems.

10.06.3.8 Other non-traditional main group-based FLPs Other main group elements have been utilized to construct unconventional or non-traditional FLPs. The ability of phosphorus to act as a Lewis acid has been explored in the last decade,243,244 however its use in combination with a Lewis base for FLP chemistry is still underdeveloped. For instance, the group of Stephan described in an early report the use of two different amidophosphoranes containing highly reactive P–N bonds within strained four-membered cycles. These compounds (260) rapidly react with CO2 in an

Frustrated Lewis Pair Systems

555

FLP-manner, in which the P center acts as the Lewis acid (Scheme 45A).245 The group of Zhu further explored this reactivity and studied experimentally and computationally the mechanism of CO2 and CS2 capture by the amidophosphorane FLPs.246–248 These studies revealed that the interplay of the ring strain and the trans influence determines the reactivity, and that furan- and pyrrole-bridged P/N-FLPs can make CO2 capture both thermodynamically and kinetically more favorable due to the aromatic stabilization of the transition states and products. Recently, Uhl and co-workers have prepared a series of P/Ga (262) and P/In (263) FLPs that present two Lewis basic P atoms in their molecular backbones. These FLPs are able to coordinate and solubilize MCl2 compounds (M ¼ Zn, Cd, Hg) by the chelating coordination of the Zn, Cd or Hg atoms with phosphorus (Scheme 45B).249 The interaction of one chloride atom with the Lewis acidic Ga or In centers resulted in the formation of norbornene-type molecular structures with M–Cl–E (E ¼ Ga (264) or In (265)) bridges and relatively weak M–Cl bonds. On the other hand, Stephan has described a dicationic arsenic species [(Z2-Cp )As(tol)] [B(C6F5)4]2 266 that exhibits Lewis super acidity by the Gutmann-Beckett test and reactivity toward fluoride abstraction reactions. This compound can activate THF in an FLP-manner to afford [(Z2-Cp )AsO(CH2)4(THF)][B(C6F5)4]2 (267, Scheme 45C).250 (A)

(B)

(C)

Scheme 45 Examples of other non-traditional FLPs.

10.06.4 FLPs in catalysis 10.06.4.1 Introduction Homogenous catalysis is of utmost importance in synthetic organic chemistry, as it allows the preparation of a wide range of high value molecules through different transformations using molecular catalysts. In the last decades, the field of homogenous catalysis has been directly linked to the study of transition metal complexes, particularly in the activation of relatively inert small molecules or strong chemical bonds. The rich reactivity of transition metals is attributed to their characteristic electronic structures, in which the presence of partly occupied d orbitals with accessible energies enables their participation in elementary reactions such as oxidative addition, reductive elimination or migratory insertion, among many others. In addition, the electronic and steric properties of transition metal catalysts can be easily tuned by a rigorous ligand design, which can also play a synergistic role in the activation and functionalization of chemical bonds. Stable main group compounds do not commonly share these electronic features, however there are some examples in which sub-valent main group compounds (e.g., tetrylenes) accomplished the activation of different E–H bonds (E ¼ N, H).27,28 These early achievements have prompted many research groups to study the replacement of transition metal complexes by more environmentally benign main group alternatives with lower costs and toxicity and higher abundance.

556

Frustrated Lewis Pair Systems

Nevertheless, the adaptation of stoichiometric bond activation by main group compounds into useful catalytic cycles has proven exceedingly challenging. The main reasons are that unsaturated p-block compounds typically present low stability and a tendency toward decomposition, hampering catalyst regeneration, as well as difficulties in their isolation and handling. As it has already been discussed, the discovery by Stephan and co-workers of the reversible activation of dihydrogen by an intramolecular phosphineborane Frustrated Lewis Pair established an important landmark.1 This discovery represented an alternative and much simpler approach to achieve transition metal-like reactivity using main group compounds, and opened the door for the development of transition metal-free catalysts. After 15 years, it has provided an unprecedented strategy for synthetic chemists to develop new avenues for promising homogenous catalytic protocols. This concept has been exploited for a broad range of catalytic applications, such as hydrogenation, asymmetric reductions, C–H bond functionalization, polymerization processes and a wide variety of metal-free organic synthetic protocols.251–259 As such, a number of specific reviews on different topics of FLP catalysis has been reported along the last decade. The following sections will discuss these transformations.

10.06.4.2 Hydrogenation catalysis 10.06.4.2.1

First examples

In 2007, Stephan and co-workers described for the first time the use of an FLP system, the phosphonium-borate (Mes2PH) (C6F4)BH(C6F5)2 (2), to catalytically hydrogenate sterically encumbered imines. This catalyst was effective in relatively low loadings (5 mol%) and at 80–120  C under 1–5 atm of H2, conditions under which it afforded the corresponding amines in high yields (reaction times between 1 and 24 h) (Scheme 46).260 Purification of the resulting amines was easily achieved by filtration through a plug of silica gel to remove the FLP catalyst. In a similar manner, (Mes2PH)(C6F4)BH(C6F5)2 (2) was used for the FLP-catalyzed reduction of aziridines and nitriles.260,261 In subsequent studies, the same group observed that the imine substrate could act as the basic partner of the FLP system, and therefore only a catalytic amount of the Lewis acid B(C6F5)3 was necessary to achieve the reduction of sterically impeded imines. However, in the case of electron-poor imines, addition of a catalytic equivalent of Mes3P notably accelerated the hydrogenation (8 h vs 41 h). Similarly, a year later Erker et al. used the ethylene-linked phosphonium–borate Mes2PH(C2H4)BH(C6F5)2 (16) in 10 mol% as an FLP catalyst for the hydrogenation of imines and enamines (Scheme 46) under mild conditions (25  C under 1.5 atm H2).42,262 In this case, the decreased Lewis acidity of the boron center slows down the heterolytic cleavage of dihydrogen, but also accelerates the hydride transfer step, giving an enhanced catalytic activity. In some cases, depending on the imine/enamine substrate, the catalyst loading can be lowered down to 3 mol%, while in the case of bulkier enamine substrates such as PhC(NC5H10)]CH2 harsher conditions are required (10 mol% catalyst at 70  C under 50 atm H2).263

Scheme 46 FLP-catalyzed hydrogenation of imines, aziridines and nitriles with (Mes2PH)(C6F4)BH(C6F5)2 (2) and Mes2PH(C2H4)BH(C6F5)2 (16).

Frustrated Lewis Pair Systems

557

The hydrogenation of the imine by pre-catalyst 1 is initiated by imine protonation from the phosphonium borate zwitterion 2 to give the iminium salt, followed by a nucleophilic attack by the borohydride anion 268, transferring the hydride and affording the amine. Dissociation of the amine from the boron atom liberates the phosphine-borane 1, which reacts with H2 regenerating the phosphonium borate catalyst 2 (Scheme 47).

R1

F

F

NH

R2

Mes2P

R3

B(C 6F 5) 2 F

amine

F

H2

1

FLP H2 Activation

H R2 R3 F Mes2P F

F

H R1 B(C 6F 5) 2 N

H

F

F

Mes2P

F 269

H B(C 6F 5) 2

F

2

F R1

R1 R2

N

H R2 R3 F

Mes2P

R3

imine

Hydride transfer F

N

Protonation H B(C 6F 5) 2

F

F 268

Scheme 47 Catalytic cycle for the reduction of imines by the FLP 1.

10.06.4.2.2

Expanding the hydrogenation scope

10.06.4.2.2.1 C]N bond hydrogenations Since the first exciting results in FLP-catalyzed hydrogenation of imines by Stephan and Erker, the pursuit of new chemical transformations catalyzed by FLP systems became a hot topic in main group chemistry and homogenous catalysis, as it has remained along the last 15 years. The ability of imines to act as both the substrate and the basic partner of an FLP system was further exploited by several research groups. Soós and co-workers employed the bulky Lewis acid MesB(C6F5)2 (270) for the hydrogenation of imines (Fig. 2). The intrinsic steric properties of the borane catalyst allowed broadening of the substrate scope, and proved especially useful for less sterically encumbered imine substrates.264 Other bulky, air-stable Lewis acids, more precisely B(C6Cl5)2(C6F5) and B(C6Cl5)(C6F5)2 (271, Fig. 2), were used by Ashley and co-workers to reduce electron-deficient imines with N-bound tosylate groups.265 Moreover, Stephan and Erker capitalized on the sterically demanding properties of alkenyl-boranes (R1R2C](C6F5)B(C6F5)2) (272, Fig. 2) to hydrogenate different types of imines.266 Different alternatives to boranes as Lewis acids, like Ga(C6F5)3 and In(C6F5)3, have been employed to effectively hydrogenate PhC(H)]NtBu at 130  C.267 N-Heterocylic carbene-stabilized borenium cations, such as 274, have also been used for the hydrogenation of imines,268,269 including those stabilized by triazolylidene mesoionic NHCs (275) described by Crudden and co-workers, which are able to reduce imines under 1 atm of H2 at room temperature (Fig. 2).270 Ashley et al. have investigated the use of a Sn-based Lewis acid to effect the hydrogenation of different imines. Remarkably, this system is a surprisingly simple and inexpensive moisture-tolerant alternative

558

Frustrated Lewis Pair Systems

to the common P/B systems (see Section 10.06.4.2.4).241,271 Moreover, the group of Ingleson has exploited an FLP derived from the combination of an N-methylacridinium salt (276) as a carbon-based Lewis acid (Fig. 2) and 2,6-lutidine as Lewis base for the hydrogenation of tBuN]CHPh.272 This work proved that organo-FLP catalysts are also active and represent a valuable alternative to main group FLP systems.

Fig. 2 Selected Lewis acids used in FLP hydrogenations of imines.

Mechanistic studies regarding the catalytic hydrogenation of imines catalyzed by different boranes as Lewis acids were reported by the group of Paradies.273,274 These studies revealed that while B(C6F5)3 operates in a concerted fashion with the imine to activate dihydrogen, FLPs derived from weaker Lewis-acidic boranes operate via an auto-induced cycle in which the amine product and the borane are the ones activating dihydrogen (Scheme 48). More sophisticated phosphine-borane FLP systems have been developed as efficient catalysts for the hydrogenation of imines. In this regard, the quest for more stable systems with respect to storage and recyclability has gathered increasing attention in the last years. Zhong et al. described the use of an intramolecular P/B FLP catalyst with a paracyclophane linker, which is able to maintain its catalytic activity over three successive hydrogenation runs.275 Other intramolecular FLPs have shown activity in the reduction of different imine substrates.57,77,155,276–279

Frustrated Lewis Pair Systems

559

Scheme 48 Simple and autoinduced catalytic cycles for imine hydrogenation.

The capacity of FLP systems to reduce imines has been expanded to include the hydrogenation of different imine derivatives, such as diimines and pyridyl diimines in the presence of catalytic amounts of B(C6F5)3 (Scheme 49A).267,280 Furthermore, the group of Oestreich reported the use of B(C6F5)3 as the Lewis acid for the synthesis of N-monosubstituted hydroxylamines through the catalytic hydrogenation of O-alkyl and O-silyl oxime ethers. In the case of silyl-protected substrates, desilylation provided a highly convenient route to unprotected hydroxylamines (Scheme 49B).281 In addition, other related N-functionalized substrates, such as aldoximines ethers and carbonyl-derived hydrazones, can be easily reduced in a similar manner giving the related hydrazine hydrates or acetyl-substituted hydrazines as hydrogenated products.282 Stephan et al. have reported the reduction of a series of pyridines, quinolines and N-heterocyclic compounds (Scheme 49C).268,270,283–285 In the presence of catalytic amounts of B(C6F5)3 the hydrogenation selectively takes place in the arene fragment containing or adjacent to the N atom, while remote positions from the N atom remained untouched. Similarly, the group of Soós and the group of Ashley used bulkier Lewis acids, MesB(C6F5)2 (270) or MesB(C6F4H)2 and B(C6Cl5)(C6F5)2 (271) respectively, to selectively hydrogenate a variety of quinolines under mild conditions. The reduction of N-heterocyclic substrates has been expanded by several research groups using different FLP systems.264,265

560

Frustrated Lewis Pair Systems

(A)

(B)

(C)

Scheme 49 Selected examples of hydrogenation of different N-functionalized substrates.

10.06.4.2.2.2 C]O bond hydrogenations The hydrogenation of carbonyl compounds has been less explored since, in contrast to imines, the carbonyl group cannot easily act as the basic partner in a FLP system.265,286,287 However, initial results from Stephan and co-workers showed that a combination of B(C6F5)3 (3) with aliphatic ketones and hydrogen at elevated temperatures in toluene generated the corresponding borinic ester releasing HC6F5.288 In subsequent parallel works, the groups of Stephan289 and Ashley290 described the efficient hydrogenation of ketones in diethyl (or diisopropyl) ether, tetrahydrofuran or 1,4-dioxane as solvents at relatively mild conditions (70  C and 5 atm of H2). Mechanistic studies revealed the crucial role of the ethereal solvent to activate H2. Thus, the presence of the ether generates a hydrogen bond between the protonated ketone and the solvent (279), enhancing the electrophilicity of the carbonyl carbon. Subsequent hydride delivery generates the alcohol and regenerates the FLP catalyst (Scheme 50). Theoretical calculations reported by Pati et al. support the dual role of the ethereal solvent as an active player in the H2 activation.291 Replacement of the ethereal solvent by toluene in the presence of either a-cyclodextrin or molecular sieves proved remarkably useful for the reduction of aliphatic, benzylic and cyclic ketones in high yields on multigram scale syntheses. Noteworthy, the heterogenized materials are able to stabilize the transient protonated ketone via hydrogen bonding and can be easily separable and recyclable. In recent years, continuous efforts have been made for the successful development of air-stable FLP systems able to efficiently catalyze the hydrogenation of ketones and aldehydes.292 Furthermore, the group of Soós reported the hydrogenation of acetals to ethers, and the tandem reductive etherification of carbonyl compounds using B(2,6-C6Cl2H3)(2,3,5,6-C6F4H)2 and B(2,3,6-C6Cl3H2) (2,3,5,6-C6F4H)2 as catalysts in THF.293

Frustrated Lewis Pair Systems

561

Scheme 50 Mechanism of FLP Hydrogenation of C–O bonds.

10.06.4.2.2.3 C]C and C^C bond hydrogenations In 2012, the groups of Stephan and Paradies used the combination of (C6F5)Ph2P (281) and B(C6F5)3 as an FLP system for the hydrogenation of a series of 1,1-disubstituted olefins (Scheme 51).294 In a first step, this pair heterolytically splits dihydrogen, then the poor basicity of the fluorinated phosphine facilitates the protonation of olefins. Finally, the resulting carbocation captures a hydride from the borate resulting in the overall reduction of the olefin and the regeneration of the FLP catalyst. In related studies, the use of the (Et2O)
B(C6F5)3 adduct 278 as an FLP system was sufficient to successfully hydrogenate 1,1-disubstituted olefins.295 Dissolution of the adduct generates an equilibrium between free Lewis acid and Et2O, which act as an FLP for the heterolytic cleavage of dihydrogen. Paradies has also described the reduction of nitro-olefins and acrylates using 2,6-lutidine (205) as the base and (THF)
B(2,6-C6F2H3)3 (282) as the acid under mild conditions, demonstrating a high functional-group tolerance.296 Similarly, the group of Alcarazo used a mixture of DABCO and B(C6F5)3 or B(2,6-C6F2H3)3 (282) as catalysts (5 mol%) to efficiently hydrogenate electron-poor allenes and alkenes under H2 atmosphere (Scheme 51).163,297 In addition, intramolecular FLPs have been used for olefin hydrogenation. For instance, Erker et al. have described the cyclopentane derivative trans-C5H8(PMes2) (B(C6F5)2), which is capable of hydrogenating vinylferrocene.77 These examples demonstrate that the research for new FLP systems as olefin hydrogenation catalysts is under continuous exploration.276,277 Escaping from the more traditional FLPs, Ashley and co-workers have replaced the borane as the acidic partner by iPr3SnOTf with the aim of reducing n-butyl acrylate,265 while Stephan has explored the ability of highly electrophilic phosphonium cations to act as acids for the hydrogenation of 1,1-disubstituted olefins.298

Scheme 51 Selected Lewis acids and bases for the hydrogenation of olefins.

562

Frustrated Lewis Pair Systems

Fig. 3 Intramolecular FLP catalysts for the hydrogenation of C]C bonds.

The hydrogenation of C–C bonds has been expanded to alkynes, enamines, enones and ynones. The group of Repo described the use of the intramolecular FLP C6H4(NMe2)(B(C6F5)H) to selectively reduce a series of internal alkynes to the corresponding cisolefins.168,172 The reductions occur via hydroboration of the alkyne, with subsequent heterolytic cleavage of H2 prompting intramolecular proto-deborylation, liberating the alkene and regenerating the FLP catalyst. In a similar approach, the Erker group has described the reduction of the alkynyl fragment in the ynone PhCCC(O)tBu using trans-(C5H8)(PMes2)(B(C6F5)2) and the electrophilic alkenylborane tBuCH]C(C6H5)B(C6F5)2 as catalysts.77,81 In the first case, the reduction did not proceed selectively and a mixture of cis- and trans-enones together with the saturated ketones was observed. However, a more selective 50:1 mixture of the trans and cis enones with a small amount of the corresponding ketones was obtained for the latter catalyst. Studies on this reactivity showed that size and electrophilicity of the Lewis acid are crucial for tuning the catalytic activity.264 Stephan and Erker have used a series of alkenyl boranes and DABCO for the selective hydrogenation of enones266 and Ashley and co-workers described that the air-stable borane B(C6Cl5)(C6F5)2 (271) is able to catalytically reduce CH2]CHC(O)OnBu in THF, which acts as the basic partner of the FLP.265 The catalytic reduction of enamines, enones, and ynones has been intensively explored: for instance, the combination of different bisphosphines, such as PhanePhos or GemPhos, with boranes led to a competent intermolecular catalyst.299 On the other hand, several P/B and N/B intramolecular FLPs (286–290) have also shown good activity for this kind of reductions (Fig. 3).77,283,300–303 Remarkably, Ashley has explored the use of tin derivatives as Lewis acids for enamine species,265 and the group of Stephan reported the use of readily accessible air-stable N-heterocyclic carbenestabilized borenium cations as enamine hydrogenation catalysts.268,269 The FLP-catalyzed reduction of aromatic compounds has also been studied. Although the hydrogenation of anilines remains a challenge even for transition metal catalysts, the use of B(C6F5)3 under H2 atmosphere at 110  C successfully provided the corresponding ammonium hydridoborate salts.304 Mechanistic investigations revealed a transient FLP in which the B and the para-C atom of the aniline activate dihydrogen, delivering a proton to the aromatic ring. Similarly, reduction of hydronaphthylamines by B(C6F5)3 occurs selectively at the naphthyl ring with the N-aryl ring remaining aromatic.305 The combination of boranes and phosphines as FLP catalysts has been used for the hydrogenation of polycyclic aromatic hydrocarbons like anthracenes and tetracene derivatives (Scheme 52).306

Scheme 52 FLP-catalyzed hydrogenation of polycyclic aromatic hydrocarbons.

Frustrated Lewis Pair Systems

10.06.4.2.3

563

Asymmetric hydrogenations

As seen in the previous section, FLP-catalyzed metal-free hydrogenation of unsaturated C–N and C–C bonds has been successfully investigated during the last 15 years. Nevertheless, the asymmetric reduction of such bonds is still relatively underdeveloped. In 2008, Klankermayer and co-workers described the first asymmetric hydrogenation of an imine using a chiral borane (292) that was prepared by the reaction between Piers’ borane (HB(C6F5)2) and (+)-a-pinene. The resulting chiral borane catalyst achieved excellent conversions with moderate to low enantioselectivity (Scheme 53).307 Since this first promising accomplishment, different research groups have focused their efforts on the design of both intermolecular and intramolecular chiral FLP systems for asymmetric hydrogenations.308–314

Scheme 53 Asymmetric hydrogenation of imines catalyzed by a chiral borane.

Several years later, the group of Du exploited a related approach. In particular, they explored the in situ generation of chiral boranes via hydroboration of chiral binaphthyl-based dienes (293) with Piers’ borane (294). This strategy led to a family of chiral bulky binaphthyl-based boranes (295) (Scheme 54), which efficiently reduce imines with enantioselectivities up to 89%.315,316 The same group used a chiral binaphthol (296) to in situ generate a chiral boron Lewis acid (298), which once more effectively hydrogenated imines to amines with 45–89% ee’s.317

Scheme 54 Chiral binaphthyl- and binaphthol-based boranes used in the asymmetric hydrogenation of imines.

In 2015, Pápai and Repo also capitalized on the binaphthyl backbone to prepare an intramolecular chiral aminoborane FLP catalyst (299, Fig. 4).166 The asymmetric hydrogenation of unhindered imines and enamines yielded the corresponding amines in high yields with up to 99% ee. In addition, the group of Erker reported the asymmetric reduction of imines (up to 69% ee) with an intramolecular ferrocene-based phosphine/borane FLP catalyst (300, Fig. 4).51 In a recent study, Wang et al. have developed a family of chiral C2-symmetric bisborane catalysts (301) derived from chiral bicylic[3.3.0] dienes (Fig. 4). These novel bisboranes showed excellent activity and enantioselectivities up to 95% for the hydrogenation of imines.318 N-Heterocyclic carbene scaffolds have also been used to construct NHC-boranes as active chiral FLP catalysts. The chirality can be located both at the NHC (302 and 303) or the borane moiety (304) (Fig. 4). These species showed good activity for the asymmetric hydrogenation of imines and N-alkyl ketimines. However, only low to moderate enantioselectivities were achieved.319,320 In contrast, while the hydrogenation of vicinal diimines and cyclic imines has effectively been performed by non-chiral borane catalysts, the asymmetric version is still less developed and only poor enantioselectivities of 10% and 42% have been obtained, respectively.267,321

564

Frustrated Lewis Pair Systems

Fig. 4 Selected chiral FLP systems and chiral boranes used in asymmetric hydrogenation of C–N bonds.

As important building blocks for medicinal chemistry, N-heterocyclic compounds represent a challenging substrate of interest for hydrogenation catalysis, especially in their asymmetric fashion. Pyridines and bipyridines can be hydrogenated in excellent yields. Remarkable diastereoselectivities, from 90/10 to >99/1, have been accomplished using a combination of several chiral dienes with HB(C6F5)2.322 Du and co-workers have studied the hydrogenation of 1,8-naphthylpyridines and 2,3-substituted quinoxalines using a similar strategy to yield the reduced derivatives in excellent yields, with up to 74% and 96% ee’s, respectively (Scheme 55).323 Finally, the use of chiral boranes derived from chiral binaphthyl-based dienes and HB(C6F5)2 has proved effective for the asymmetric hydrogenation of disubstituted and trisubstituted quinolines and 2-quinolinecarboxylates (up to 99% ee).324,325 The group of Wang has recently developed a series of chiral spiro-bicyclic bisborane catalysts and applied them in the asymmetric reduction of two-substituted quinolines reaching 87–99% ee’s.326

Scheme 55 Asymmetric hydrogenation of N-heterocyclic compounds.

Frustrated Lewis Pair Systems

565

The metal-free hydrogenation of ketones, enones, and silyl-enol ethers have also been investigated using FLP catalysts.290,327 The group of Du combined chiral boranes, in situ generated from the reaction between chiral dienes and HB(C6F5)2, with PtBu3 for the enantioselective reduction of silyl enol ethers. Optically active secondary alcohols were afforded in excellent yields with up to 99% ee’s.328 The use of chiral alkenyl boranes, in which the C]C double bonds were conjugated to the binaphthyl moieties making a more rigid and tunable structure, afforded chiral secondary alcohols with excellent enantioselectivities.329 In recent studies, Du and co-workers have described a novel type of FLP system in which a combination of a chiral oxazoline Lewis base in the presence of an achiral borane as Lewis acid effectively hydrogenates ketones, enones and chromones with up to 95% ee.330 In this case, the combination of the oxazoline and the borane splits dihydrogen heterolytically giving a thermodynamically favorable intermediate that undergoes a concerted H-transfer process.

10.06.4.2.4

Water tolerant FLPs

Despite the great advances in FLP hydrogenation catalysis, most of these transition-metal-free reductions require the rigorous exclusion of H2O. Water can be easily activated by most FLP systems, giving the corresponding protonated Lewis base [LB–H]+ and the hydroxy species of the Lewis acid [LA–OH]−. The formation of such species prevents other small molecules from interacting with the FLP systems, thus deactivating them as catalysts. To overcome the water-intolerance in FLP chemistry it is crucial to consider both steps in the water activation process: the initial coordination of water to the Lewis acid, which is often reversible, and the subsequent deprotonation step that is not. The two main strategies postulated are based on avoiding the use of moderate to strong Brønsted bases to prevent the irreversible deprotonation of the LA
OH2 adduct, or altering the stereoelectronic factors of the Lewis acid center to reduce the propensity of water to coordinate to it. Similar behavior is observed in the presence of alcohols due to the comparable nucleophilicity and acidity to water.331 Stephan and Ashley independently reported the FLP-based catalytic reduction of carbonyl groups to the corresponding alcohols, thus overcoming both the water and alcohol-intolerance problem (Scheme 56A).290,327 The use of weakly coordinating solvents (Et2O or 1,4-dioxane) as the base, instead of moderate to strong Brønsted bases was key for an efficient process. Moreover, the presence of water did not poison the catalyst since the solvent is not basic enough to irreversibly deprotonate the B(C6F5)3
OH2 adduct. Similarly, the group of Soós performed the hydrogenation of carbonyls using the borane B(4-HC6F4)2(2,6Cl2C6H3) (307) under ambient conditions using technical grade THF as solvent (Scheme 56B).286 In this case, the more sterically hindered Lewis acid borane, together with the use of a weakly coordinating solvent, was crucial to avoid water complexation but still allow the cleavage of dihydrogen. (A)

(B)

Scheme 56 FLP-catalyzed hydrogenation of carbonyl compounds using weakly coordinating solvents as Lewis bases.

Ingleson and co-workers have also demonstrated the water-tolerant character of B(C6F5)3 in the reductive amination of primary and secondary arylamines with aldehydes and ketones in wet solvents at raised temperatures in the presence of Me2PhSiH as reductant.332 Arylamines are effectively reduced since their basicity does not result in the irreversible deprotonation of the B(C6F5)3
OH2 adduct, allowing for sufficient B(C6F5)3 to be present at elevated temperatures (Scheme 57A). However, when stronger Brønsted basic amines such as tBuNH2 are employed, the formation of [OH–B(C6F5)3]− through the deprotonation of the B(C6F5)3
OH2 adduct is irreversible and no imine reduction is detected. The groups of Soós,333 Ashley and O’Hare265,334 have used another approach to report the reduction of imines using a series of sterically hindered chloro-aryl-substituted boranes (308–310) that display reduced propensity to coordinate water (Scheme 57B). In this case, the water molecule coordinated to the borane can be rapidly removed under vacuum or in solution using molecular sieves, allowing the boranes to be used as catalysts under ambient conditions.

566

Frustrated Lewis Pair Systems

(A)

(B)

Scheme 57 Borane-catalyzed reductive amination of carbonyl compounds under ambient conditions.

Recently, the use of non-boron-based Lewis acids has become an exciting avenue to overcome the water-intolerance challenge in FLP chemistry due to their reduced oxophilicity.335 Carbenium ions with electron-donating groups on the aryl substituents moderate the reactivity toward water. For instance, the air-stable carbocation [(4-MeOC6H4)CPh2]+ (311) has been successfully employed in the hydrothiolation of 1,1-diphenylacetylene under ambient conditions.336 Other air- and water-stable carbon Lewis acids as N-methyl-2-phenylbenzothiazolium (312) and N-methylacridinium (313) cations have shown remarkable activity in FLP reduction catalysis (Fig. 5).189,192 The group of Ashley has also described the use of the water-tolerant tin(IV)-based Lewis acid i Pr3SnOTf (314) in different FLP-catalyzed hydrogenations.241 In addition, water-tolerant phosphonium cations (315)337 and cationic antimony(V)-based (316 and 317)338 Lewis acids have been respectively described by Stephan and Gabbaï for FLP reactivity.

Fig. 5 Examples of air- and water-tolerant electrophilic carbenium, phosphonium and antimony-base cations used as Lewis acids.

Frustrated Lewis Pair Systems

567

10.06.4.3 Beyond hydrogenation: Other catalytic transformations 10.06.4.3.1

Hydrosilylation

The activation of H2 mediated by FLP systems has encouraged many research groups to investigate the possibility of activating other small molecules. In 1996, Parks and Piers reported the hydrosilylation of ketones and aldehydes by B(C6F5)3.23 As stated in the introductory section, this reaction may be considered as the first catalytic application of FLPs, although the field was not recognized as such at that time. Initial studies proposed that the reaction proceeded by the Lewis acid activation of the Si–H bond, prompting attack by the carbonyl. Subsequently, Oestreich and co-workers proved that hydrosilylation of the ketone proceeds with inversion of configuration at the silane, thus proving that a base (carbonyl) and a Lewis acid act on the substrate in a concerted manner.24 After these findings, borane-mediated hydrosilylation has been intensively studied affording a wide range of silicon derivatives.24,339–342 More recently, the group of Stephan has described the use of readily accessible air-stable Lewis acids such as phosphonium cations [(C6F5)3PF]+ (318) or [(terpy)PPh][B(C6F5)4)]2 (terpy ¼ 2,20 ;60 ,200 -terpyridine) (319) to effectively mediate the hydrosilylation of aldehydes, ketones, olefins, imines, nitriles and amides (Scheme 58).343 Asymmetric versions of FLP-catalyzed hydrosilylation have also been described.320,344–350 Klankermayer reported the first examples using a catalyst derived from camphor (320), which was able to effectively reduce imines to amines with up to 87% ee.312 In 2016, the group of Du described the enantioselective hydrosilylation of 1,2-dicarbonyl compounds using a combination of PCy3 and chiral alkenyl boranes, furnishing a-hydroxy ketones and esters with excellent enantioselectivity.345

Scheme 58 Selected examples of FLP-catalyzed (asymmetric) hydrosilylations.

10.06.4.3.2

Transfer hydrogenation

The ability of strong Lewis acids to abstract a hydride from organic compounds and to deliver it to nitrogen containing species has been exploited to develop hydrogen-borrowing strategies (Scheme 59A). As such, Lewis acid-mediated catalytic transfer hydrogenation of imines, enamines, N-heterocycles, and other N-containing compounds using different sources of hydrogen have been reported.351–354 For instance, the group of Ingleson used the N-methylacridinium salt 204 and lutidine (206) to mediate transfer

568

Frustrated Lewis Pair Systems

hydrogenation reactions based on imines and using Me2NHBH3 as the hydrogen source (Scheme 59B).189 On the other hand, Oestreich and co-workers described the reduction of 1,1-diarylolefines via transfer hydrogenation using cyclohexadienes and B(C6F5)3 (Scheme 59C).355 Asymmetric versions of transfer hydrogenation reactions catalyzed by FLP systems have been studied using ammonia boranes as hydrogen sources (Scheme 59D).356–359 (A)

(B)

(C)

(D)

Scheme 59 Selected examples of catalytic (asymmetric) transfer hydrogenation of imines and olefins.

10.06.4.3.3

Dehydrogenation of aminoboranes

The use of ammonia- and amine-borane as a hydrogen source for the FLP-catalyzed reduction of unsaturated substrates has already been mentioned in the previous section. However, the acceptorless dehydrogenation of this and related N ! B adducts catalyzed by FLP systems is yet limited,360 despite its great interest in the context of hydrogen production and storage.361 The stoichiometric dehydrogenation of ammonia- and amine-borane with FLPs has been known for more than a decade.362,363 However, the formation of stable adducts with the dehydrogenated N/B products or the suppression of FLP-type reactivity due to zwitterionic pairs derived from splitting the released dihydrogen have typically hampered catalytic turnover. In a first example, Slootweg and Uhl reported a geometric phosphorus/aluminum-based FLP that stoichiometrically dehydrogenates both NH3
BH3 and N(Me) NH2
BH3, though catalysis was inhibited by the formation of five-membered cycles with the N(R)H]BH2 unit that served as thermodynamic sinks. Increasing the sterics in N(Me)2H
BH3 permitted catalytic dehydrogenation with formation of cyclodiborazane.364 Similar results where later obtained with the analogous gallium-based FLP.226 The first catalytic example of acceptorless ammonia-borane dehydrogenation incorporated a zirconium center as the acidic site of the FLP.365 The strategy has been subsequently exploited with other transition metal systems,366–368 a topic that will be introduced later in the chapter. However, the first example of catalytic dehydrogenation of ammonia-borane and its related methylamine analog by main group FLPs was reported by Aldridge and co-workers several years later (Scheme 60).369 A dimethylxanthene-linked phosphinoborane (42) permitted the release of dihydrogen under mild conditions (55  C, 1 mol%). Mechanistic investigations supported the initial activation of a B–H bond, with subsequent end-growth BN coupling with the terminal N–H bond of the activated fragment and a B–H bond of an incoming substrate molecule. Interestingly, the first

Frustrated Lewis Pair Systems

569

intermediate of the chain-growth mechanism could be isolated. Further computational mechanistic studies were provided by Paul and co-workers.370 The related ortho-phenylene-bridged phosphinoborane 321, which was reported by Bourissou’s group, exhibited enhanced catalytic activity (Scheme 60).371 An akin study by Slootweg and co-workers demonstrated that the geminal t Bu2PCH2BMes2 FLP (322) presented activity toward ammonia-borane and dimethylamine-borane dehydrogenation (Scheme 60).372 These and other recent main group systems and computational designs evidence notable current interest on this topic.373–375

Scheme 60 FLP-catalyzed acceptorless dehydrogenation of ammonia- and amine-borane.

10.06.4.3.4

Hydroamination

The group of Stephan described the use of catalytic amounts of B(C6F5)3 to mediate the hydroamination of terminal alkynes with arylamines to afford the corresponding enamine products (Scheme 61).376 Metal-free FLP-catalyzed hydroamination has also proved useful in tandem with hydrogenation catalysis, in which the initially formed enamine is hydrogenated in a second step in the presence of H2.377 Enantioselective aminations have also been reported. The group of Wasa has studied a-aminations of ketones, as well as the FLP-catalyzed Mannich type reactions of carbonyl and aldimines to yield a-aminocarbonyl compounds and a- and b-amino esters in high enantiomeric purity, respectively.378,379

Scheme 61 Hydroamination of alkynes catalyzed by FLPs.

10.06.4.3.5

CO2 reduction

As it has already been discussed, FLPs present the ability to capture CO2 by push-pull interactions. However, the efficient catalytic reduction of carbon dioxide remains underdeveloped when compared to other unsaturated substrates. In 2009, Ashley and O’Hare demonstrated the selective hydrogenation of CO2 to CH3OH by using an FLP constructed around the tetramethylpiperidine (TMP, 323)/B(C6F5)3 pair. This system was effective at low pressures (1–2 atm) after 6 days at 160  C (Scheme 62A).92 In a related study, Piers and co-workers used Et3SiH to effect the catalytic reduction of CO2, yielding CH4 and (Et3Si)2O.380 In the presence of excess B(C6F5)3 and triethylsilane, the formatoborate is hydrosilylated leading to a formatosilane and [TMPH]+[HB(C6F5)3]− is formed. The formatosilane is rapidly hydrosilylated by the B(C6F5)3/Et3SiH system to CH4, and (Et3Si)2O as the byproduct (Scheme 62B). Interestingly, at low concentrations of Et3SiH, the intermediate CO2 reduction products are observed. The addition of more CO2/ Et3SiH resumed the hydrosilylation, suggesting a living tandem catalytic behavior for the deoxygenative reduction of CO2 to CH4. The groups of Fontaine and Stephan have independently used phosphine/borane FLPs (83, 324 and 325) to reduce CO2 via hydroboration of carbon dioxide to methoxyboranes (Scheme 62C).381,382

570

Frustrated Lewis Pair Systems

(A)

(B)

(C)

Scheme 62 FLP-catalyzed hydroboration of CO2.

10.06.4.3.6

Hydroboration and C–H borylation

Encouraged by the first stoichiometric activation reports of HBcat by FLPs,134 catalytic hydroboration reactions have gathered increasing interest in the last years. Li, Wang and co-workers described the 1,4-hydroboration of a series of pyridines with HBpin catalyzed by the bulky borane ((CF3)3C6H2)2BMe 326 (Scheme 63A).383 Stephan384 and Oestreich385,386 have reported the

Frustrated Lewis Pair Systems

571

hydroboration of alkynes and alkenes using Piers’ borane 294 and B(3,5-(CF3)2C6H3)3, respectively (Scheme 63B). Similarly, Melen et al. used B(3,5-(CF3)2C6H3)3 to mediate the hydroboration of a wide range of alkynes, aldehydes and imines.387 In addition, Ingleson has described the use of a borane-carbene adduct for the trans-hydroboration of alkynes (Scheme 63C).388,389

(A)

(B)

(C)

Scheme 63 Catalytic FLP-type hydroboration.

More recently, Fontaine and co-workers have developed intramolecular N/B FLPs (327) to mediate the C–H borylation of furans, pyrroles, and electron-rich thiophenes showing selectivities that complement those observed with transition metals (Scheme 64).170,390 Experimental studies, together with DFT methods, allowed the authors to propose a reliable mechanism. After dissociation of the dimeric B/N pre-catalyst (328), the C–H activation of the substrate (1-methylpyrrole) generating the corresponding zwitterionic species 329 was calculated to be the rate-determining step. Then, a rapid release of H2 gives 330, which in the presence of HBpin rapidly reacts to form the final product via a four-center sigma-bond metathesis regenerating the precatalyst. The regioselectivity observed, as well as the relatively low kinetic isotopic effect (KIE) values obtained in competition experiments at 80  C, support the transition state calculations, which indicate that the C–H bond activation is directed by the most nucleophilic carbon. Interestingly, the air-stable zwitterionic salt (1-HN(C5H6Me4)-2-BF3C6H4) has proved to be a very efficient and convenient catalyst for such borylation reaction. The same group also developed amino-borane catalysts containing CF3 groups, as well as less sterically hindered amines which displayed improved reactivity.171,391 The group of Repo has expanded the borylation scope to arenes and alkyne C–H bonds using similar amine-boranes as FLP catalysts.174

572

Frustrated Lewis Pair Systems

Scheme 64 Proposed mechanism for the catalytic hydroboration of 1-methylpyrroled using an intramolecular N/B FLP catalyst.

10.06.4.3.7

C–F derivatization

The Lewis acid B(C6F5)3 was shown to mediate the conversion of alkyl fluorides to the corresponding alkanes in the presence of silane, releasing the corresponding silylfluoride and hydrogen as byproducts.132 The mechanism of these reactions is similar to Piers’ mechanism for hydrosilylation, in which the Lewis acid activates the Si–H bond which is subsequently attacked by the alkylfluoride. In addition, performing these reactions in the presence of electron-rich arenes provides an avenue for C–F arylation. Moreover, the use of benzyl fluorides392 or CF3-aryl compounds393 as the fluoro-precursors produces catalytic routes to a wide range of diarylmethanes and alkylated-arenes (Scheme 65). In a more recent study, the compounds [(bipy)PPh][B(C6F5)4]2 and [(terpy)PPh][B(C6F5)4]2 were used to catalyze the hydrodefluorination of nonactivated alkyl C–F bonds in the presence of silane, as well as for the Csp3–Csp3 coupling of benzyl fluorides with allyl silanes.394

Frustrated Lewis Pair Systems

573

Scheme 65 Selected examples of FLP-mediated C–F derivatization.

10.06.4.3.8

Polymerization catalysis

The use of Lewis pairs as catalysts in polymerization chemistry has steadily grown in the last decade. Lewis pair polymerization (LPP) catalysis can be mediated by either classical Lewis adducts or FLPs. In the case of Lewis pairs, the formation/dissociation of the adduct must be reversible to generate the active species that presents weak (Interacting Lewis Pairs, ILPs) or no acid-base interaction (FLPs).253,395 The cooperativity between the Lewis acid and the base is crucial to promote monomer activation and chain initiation, propagation and termination (Scheme 66). Since the concept of LPP was firstly introduced in 2010 by Chen,211 and compared to conventional polymerization techniques, numerous remarkable successes in the polymerization of polar vinyl monomers and ring opening (co) polymerization of cyclic esters and epoxides have been achieved. These advances relied on the demonstrated broad polar monomer scope in this type of polymerization processes, as well as the typical high activity, control, and complete chemo- or regioselectivity. The significance of these results is amplified by the broad applicability of these polymers in a range of areas. In addition, the use of readily available Lewis acids and bases with high electronic and steric tunability opens the door to construct a wide variety of FLP catalysts avoiding time-consuming multistep syntheses typically required for delicate organometallic and classic coordination compounds.396–403

Scheme 66 Chain initiation step and subsequent propagation steps of a general FLP-catalyzed polymerization reactions.

574

Frustrated Lewis Pair Systems

10.06.4.3.8.1 Polymerization of polar vinyl monomers In early studies in 1960 and 1971, the groups of Murahashi and Ikeda described that the combination of a Lewis acid (Et3Al, Et3In and Et2Zn) and a Lewis base (PEt3, PPh3 and a,a0 -dipyridyl) was able to catalyze the polymerization of methyl methacrylate (MMA) and acrylonitrile with only marginal activity.404,405 Years later, Chen and co-workers explored the combination of bulkier pairs. While the control experiments using either the Lewis acid Al(C6F5)3 or the Lewis base (PMes3, PPh3, PtBu3, ItBu and IMes) alone for the polymerization of MMA did not yield polymer formation, rapid polymerization took place when a combination of the two components was used with a specific addition sequence.211 For instance, treatment of the pre-generated Al(C6F5)3
MMA adduct with PtBu3 in toluene generates a zwitterionic species, which rapidly reacts with MMA giving high TOFs (up to 12,000 h−1), consuming the monomer within only 4 min yielding a high-MW polymer (Mn ¼ 397 kg/mol, Ð ¼ 1.52). In subsequent studies, the scope of Lewis acids and bases was intensively investigated for the polymerization of MMA, but also for vinyl methacrylate (VMA), allyl methacrylate (AMA), and 4-vinylmethacrylate (VBMA) (Fig. 6).406,407 Chen and Cavallo have carefully evaluated a full account of combined experimental and theoretical studies on LPP with a large scope of Lewis acids and bases for the polymerization of different monomers.408,409

Fig. 6 Selected common monomers for FLP-polymerization of polar vinyl monomers.

The group of Lu developed a highly active LPP system for the polymerization of MMA using an N-heterocyclic olefin (NHO) (331) as a neutral nucleophilic Lewis base (Scheme 67).410 The strongly polarized C]C double bond renders the terminal carbon atom of the olefin making it more electronegative. Initial premixing of the MMA and the Lewis acid Al(C6F5)3 (227) followed by the addition of the NHO gives high activity (TOF ¼ 15,040–15,840 h−1, Mn  610 kg/mol, Ð  1.54), which is still lower compared to the use of classic NHC systems, IMes/Al(C6F5)3, (TOF ¼ 24,000 h−1). In 2016, Rieger and co-workers described the controlled polymerization of sterically demanding methacrylates using Lewis pairs based on weaker acids (AlMe3, AlEt3, AlPh3) and bases with small steric hindrance (PMe3, PEt3).411

Frustrated Lewis Pair Systems

575

Scheme 67 Proposed mechanism for MMA polymerization using NHO as Lewis base.

Subsequent work from Takasu’s group showed the exclusive 1,4-addition polymerization of methyl sorbate to afford a cyclic polymer by the combination of a sterically encumbered Lewis base (ItBu, 194) and [MeAl(BHT)2] (336) as the acid.412 Xu et al. expanded the use of FLP catalysts by preparing cationic rare-earth (RE ¼ Sc (338), Y (339), Lu (340)) aryl oxide complexes with phosphorus-tethered b-diketiminates as ancillary ligands. These complexes can be considered as non-interacting Lewis pair systems, and have shown promising activity in the MMA polymerization (Fig. 7).413–415 More recently, Zhang and Chen have described the first living LPP of MMA mediated by a truly frustrated FLP catalyst based on [MeAl(BHT)2] and NHO Lewis base.212

Fig. 7 Selected Lewis Pair catalysts for the polymerization of polar vinyl monomers.

10.06.4.3.8.2 Ring open (co)polymerization In the 1990s, Dubois and Jérôme showed the enhanced activity of the Sn(Oct)2 or Al(OiPr3)-catalyzed ring open polymerization (ROP) of lactide in the presence of a Lewis base.416,417 In more recent years, it has been demonstrated that the coordination of the monomer lactide to the Lewis acid enhanced the electrophilic character of the carbonyl carbon and thus facilitates the nucleophilic ring-opening of the monomer (Fig. 8) by the Lewis base itself. This activation may also occur by a Lewis base-activated alcohol via H-bonding or by the Lewis base-activated monomer via proton abstraction. These findings have led to an enhanced ROP activity and improved selectivity. In 2013, Bourissou and co-workers reported the Lewis pair comprised by the Lewis acid Zn(C6F5)2 and the organic base 1,2,2,6,6-pentamethylpiperidine. This pair mediated controlled ROP of L-lactide (L-LA) and e-caprolactone (e-CL) to cyclic poly(co)esters.418 Similarly, Dove and Naumann described the used of metal halides (MgCl2, YCl3, AlCl3, etc.) as Lewis acids which, in combination with an organic base (NHC, DBU, etc.), mediate the ring opening polymerization of o-pentadecalatone, PDL.419 NHOs in combination with Al(C6F5)3 have also been exploited as catalysts to promote the ROP of d-valerolactone (d-VL) and e-CL (Fig. 8).420

576

Frustrated Lewis Pair Systems

Fig. 8 Selected common monomers for FLP-mediated ring opening (co)polymerization.

Yang et al. have described the use of borane/amine LPs for the controlled ROP of N-carboxyanhydrides (NCAs) to synthesize well-defined polypeptides.421 Replacement the borane for zinc acetate [Zn(OAc)2] as the Lewis acid also mediated the controlled ROP of Glu-NCA under mild conditions in relatively short reaction times (TOF  30 h−1, Mn  44 kg/mol, Đ ¼ 1.10–1.30, GluNCA/LA/LB ¼ 50/1/1).422 Besides, Lewis pairs have been employed for the copolymerization of CO2 with epoxides. A combination of AliBu3 as Lewis acid and lithium halide (LiCl or LiBr) or alkoxide (LiOBn) as the Lewis base is able to catalyze the controlled copolymerization of CO2 with cyclohexene oxide, which selectively gives the alternating copolymer with a high carbonate content of 95–99% after 10 h (TOF  2 h−1, Mn ¼ 2.1–5.8 kg/mol, Đ ¼ 1.10–1.30, CHO/LB/AliBu3 ¼ 20/1/0.4, 60–80  C).423 Similarly, the same group reported the controlled copolymerization of CO2 with epoxides using a combination of BEt3 (341) and onium halides or onium alkoxides (342) derived from alcohol deprotonation by organic phosphazene superbases (Schemes 66 and 68).424

Scheme 68 Example of copolymerization of CO2 with epoxides.

10.06.5 Mechanistic considerations 10.06.5.1 Introduction The discovery of the transition metal-free activation of dihydrogen by FLPs prompted extensive experimental and computational research aimed at elucidating their mechanism of action. Despite the widespread consensus on the involvement of preorganized acid/base partners (the so-called “frustrated” or “encounter” complex) and the heterolytic manner in which the splitting of the H–H bond takes place, an intense debate regarding the mode of action of the active centers endures. In this section, thermodynamic aspects regarding H2 activation and FLP preorganization will be discussed. Besides, the two main reactivity models proposed to interpret heterolytic H2 splitting by FLPs will be presented, as well as other recent advances in the field. The alternative homolytic splitting of dihydrogen by radical pathways will not be discussed herein, as it will be introduced later in Section 10.06.7.2. The activation of dihydrogen represents the benchmark reaction by which FLP activity is often gauged and the most convenient manner of discussing the mechanism of action of FLPs in a comprehensive manner. Thus, the routes by which FLPs activate other small molecules will not be covered. The reader is therefore advised to access the relevant primary literature associated to the reactivity already reviewed in the preceding sections, which in multiple cases contain some additional mechanistic information.

10.06.5.2 Thermodynamics of H2 splitting by FLPs As discussed in Section 10.06.2.2, early reports from Stephan and co-workers suggested that sufficient combined Lewis acidity and basicity was a requisite to achieve H2 activation.37,38 However, although the majority of reactive FLPs display strongly Lewis acidic and/or Lewis basic groups, these were proven not to be essential.55 Furthermore, ground-state frustration is also not indispensable to achieve FLP reactivity, as demonstrated early by Balueva and Erastov73 and by Erker and co-workers regarding H2 activation,42 among others. It was also soon proposed that the Coulombic attraction between the ion pairs or zwitterionic species resulting from

Frustrated Lewis Pair Systems

577

the heterolytic splitting of H2 would substantially contribute to make these FLP reactions favorable.165 In 2009, Pápai and co-workers carried out a thorough thermodynamic study of the dihydrogen splitting by FLPs, which included an elegant partitioning of the free energy of the reaction (Fig. 9).425

Fig. 9 Free energy thermodynamic cycle of the dihydrogen splitting reaction by FLPs.

Delving into the chemically meaningful steps of the decomposition scheme proposed by Pápai and co-workers, the major thermodynamic obstacle to make this process exergonic is the heterolytic cleavage of the highly stable, covalent H–H bond into H+ and H− ions, △GHH. An additional thermodynamic penalty, △Gprep, may be necessary to overcome the cleavage of a dative bond between the Lewis acid and basic centers in cases presenting insufficient steric congestion (D–A in Fig. 10). This event is necessary in order for the acid and base to be capable of incorporating the H+ and H− ions; for systems where steric congestion is exceedingly low relative to acid/base strength, the strong dative bond in the Lewis adduct translates into large, often insurmountable △Gprep values. The two main terms contributing to the thermodynamic viability of H2 splitting by FLPs are the attachment of a proton to the Lewis base and a hydride to the Lewis acid. These values, △Gpa and △Gha (Fig. 10), correspond to the energy stored in the [D–H]+ and [H–A]− bonds. As observed experimentally, a threshold of combined Lewis acidity and basicity has to be met in order to observe H2 activation. The stabilization term, △Gstab, corresponds to the binding free energy of the [DH]+[HA]− ion pair for intermolecular FLPs, while for intramolecular pairs it reflects the enhancement of Lewis acidity upon protonation of the basic site and vice versa. △Gstab depends on several factors, including solvation, Coulombic attraction, non-covalent interactions (i.e., dispersion), and H⋯ H contacts296,426 between the polarized DH and HA bonds. Intramolecular FLPs tend to present larger △Gstab values than intermolecular ones, partly because of the entropic cost associated with the formation of the ion pair. Good correlation between △Gstab and the reciprocal of the distance between the donor and acceptor atoms in the products of H2 activation was found only for intramolecular FLPs, showcasing the importance of electrostatic contributions in the corresponding zwitterions. △Gprep and △Gstab permit tuning the thermodynamics of substrate activation by FLPs beyond the modulation of the acidity and basicity of the active centers. Fine control is of paramount importance for the design of hydrogenation catalysts, for which the free energy of H2 activation should be close to zero. This has to be met in order to allow both H2 activation by the catalyst and subsequent transfer to the substrate to readily take place.

578

Frustrated Lewis Pair Systems

Fig. 10 Substeps of the heterolytic dihydrogen splitting reaction for intermolecular FLPs, according to the thermodynamic cycle in Fig. 9.

10.06.5.3 Frustrated complex The combination of B(C6F5)3 and PtBu3 constitutes the prototypical FLP for the study of the mechanism of H2 activation. Due to the lack of experimentally detected reaction intermediates for this reaction,37 insight into this process was sought by theoretical means. In an early report, Soós, Pápai, and co-workers ruled out the intermediacy of side-on (C6F5)3B⋯ H2 and end-on tBu3P ⋯ H2 species.427 In turn, a weakly bound tBu3P ⋯ B(C6F5)3 complex, arising from the association of the basic and acid components of the FLP, was identified as a minimum in the potential energy surface (Fig. 11, left). However, the entropic penalty of formation of frustrated complexes renders their free energy of formation slightly positive.428 This had been anticipated by means of classical Molecular Dynamics simulations including explicit solvation (toluene) for the B(C6F5)3/PtBu3 pair.429 Unequivocal confirmation of the slightly endergonic formation of frustrated complexes was provided by Rocchigiani, Macchioni, and co-workers by means of NMR spectroscopy.430 It is worth highlighting here that solid-state NMR has also been successfully applied to acquire information about P–B interactions and intermolecular aggregation of phosphine/borane units. The strength of those interactions may be estimated based on 11B chemical shifts and variations on its quadrupolar coupling constants.431

Fig. 11 Calculated molecular geometries of the frustrated complex (left) and transition state for H2 cleavage (right) of the B(C6F5)3/PtBu3 FLP as first reported by Pápai and co-workers.427 Structures redrawn from the coordinates files calculated in Ref. 427; distances in A˚ . Note: calculated geometrical parameters of TSs for H2 cleavage by FLPs were further refined in subsequent studies cited in the following discussion.

C–H⋯ F hydrogen bonds and dispersive interactions were proposed to be the driving force of the association of the B(C6F5)3/ PtBu3 pair; no charge transfer was found to take place in this species, as reflected by the planar environment around the boron center.427 NMR spectroscopy430 and high-level calculations432 provided further support to this hypothesis. This flexible frustrated complex is poised to interact with substrates en route to their activation: accessible concerted transition states (TSs) for H–H bond cleavage were located at several levels of theory (Fig. 11, right).427,433,434 At all these levels, despite differences on the modeling of

Frustrated Lewis Pair Systems

579

non-covalent interactions, the TSs share several features: the H2 molecule simultaneously interacts with both Lewis centers of the FLP, the H–H bond is only slightly elongated, and the borane is substantially pyramidalized. Nevertheless, two conceptually different mechanistic scenarios, the Electron Transfer (ET) and the Electric Field (EF) models, discussed in Sections 10.06.5.4 and 10.06.5.5, were outlined with the aim of understanding the observed reactivity.

10.06.5.4 Electron transfer (ET) model Pápai and co-workers proposed that, in the transition state for H–H bond cleavage by the B(C6F5)3/PtBu3 FLP, both Lewis centers simultaneously polarize the H2 molecule, which acts as a bridge enabling a shift of electron density from the base to the acid.427,433,435 H2 polarization was supported by electron density difference maps and population analysis. According to Molecular Orbital Theory, the cooperative electron transfer (ET) processes arise from the donation of the lone pair of the phosphine (HOMO of the Lewis base) to the s (H2) and from the s(H2) to the empty orbital on the boron center (LUMO of the Lewis acid). The depletion of the bonding s(H2) orbital and the population of the antibonding s (H2) synergically contribute to weaken the H–H bond, reminiscent of transition-metal and carbene-based H2 activation; however, in the case of FLPs the donor and acceptor orbitals are not located on the same atom. An analogous mechanism was found for olefin activation by the combination of B(C6F5)3 and PtBu3, the polarization arising from donations to and from the p and p orbitals, respectively (Fig. 12).436,437

Fig. 12 Simplified representation of the ET model.

10.06.5.5 Electric field (EF) model Following Pápai and coworker’s proposal of the ET model, Grimme, Erker, et al. revisited the mechanism of H2 activation by FLPs.434 They proposed that once the H2 molecule is within the FLP, the heterolytic dissociation would be almost or even barrier-less, depending on the strength of the electric field (EF) generated by the P/B atoms. Therefore, they proposed that H2 activation in FLPs can be explained by the polarization induced by the electric field in the cavity, without the need to involve specific FLP/H2 orbitals or donation processes. Computations showed that strong electric fields could induce the heterolytic dissociation of H2 in a barrier-less fashion. In this mechanism, reaction barriers for the FLP activation of H2 would correspond to preparation or entrance steps, and not to the H–H bond cleavage event (Fig. 13).

Fig. 13 Simplified representation of the EF model.

10.06.5.6 Coexistence of ET and EF In 2012, Camaioni and co-workers performed a computational study of H2 activation by NH3/BX3 pairs (X ¼ H, F, Cl).438 Although an adequate description of the distance between the active Lewis centers in FLPs was proven to be important for H2 activation,427,434 these simple models provided insight by means of a detailed analysis of the electronic structure of these species. The localized molecular orbital energy decomposition analysis (LMOEDA)439 approach was used to evaluate the intermolecular interactions in the weakly bound precursor complexes, featuring side-on coordination of H2 toward B and end-on toward N, and in the transition states for H–H cleavage. A similar coordination arrangement was proposed by Szieberth, Bourissou, and co-workers.440 Camaioni et al. found that although the EF created by the pair had a polarizing effect, charge transfer interactions (orbital overlap) were the main stabilizing factor in the TSs. However, whether these conclusions could be extrapolated to real FLPs was not evaluated.

580

Frustrated Lewis Pair Systems

A year later, Pápai and co-workers evaluated the performance of the ET and EF models in a set of six experimentally characterized FLPs.441 These authors emphasized that at the TSs several criteria indicate substantial activation of the H2 molecule, which does not align with the term “entrance step” employed in the EF model.434 The analysis of the electric field showed important differences between systems displaying analogous reactivity, which were not in agreement with the EF being the sole or principal driving force behind the H–H splitting reaction. Also, the magnitude of the EFs in the direction of the H–H bond, while capable of reducing the cleavage barrier, was insufficient to account for the changes in the electronic structure and induce barrier-less (or almost barrier-less) bond cleavage. Furthermore, based on Saenz’s computations, under an EF where H2 would split in near barrier-less fashion, the rate of H2 ionization would be many orders of magnitude faster than the experimentally determined rates of FLP-mediated H2 heterolytic splitting.442 In addition, the characteristic bent geometry of the DHHA unit in the transition states (D ¼ donor, A ¼ acceptor) could only be explained by the ET model, as it stems from optimum orbital overlap. In turn, the electric field was found to be far from parallel to the H–H bond in the TS geometries. More recent studies from the group of Privalov examined the interactions in the TSs of H2 splitting of FLPs by means of Born-Oppenheimer Molecular Dynamics443 and DFT.444 Energy decomposition analysis revealed that the Pauli repulsion was compensated by electrostatic and orbital interactions, both being essential and of comparable importance. The group of Fernández carried out a DFT study of H2 activation by geminal aminoborane FLPs.445 By combining the activation strain model and energy decomposition analysis, a highly orbital-controlled mechanism was proposed, dominated by the charge transfer events encompassed in the ET model. Liu, Ensing, and co-workers reported a DFT-based meta-dynamics study of H2 activation by the B(C6F5)3/ PtBu3 pair, which concluded that the reaction was stepwise, starting with polarization of H2, followed by rate-limiting hydride transfer and final, facile proton transfer.446 Previously, Li, Wang, and co-workers had proposed a stepwise mechanism between a borane and NEt3.447 Liu, Ensing, et al. also found that the ET and EF models were complementary: when the H2 molecule is farther from the Lewis centers it undergoes electrostatic polarization, but at shorter distances electron transfer takes place. In turn, an ab initio molecular dynamics study carried out by Pápai and co-workers did not find evidence supporting a multi-step reaction mechanism, favoring a notably asynchronic, concerted pathway,448 in agreement with previous findings by Grimme et al.434

10.06.5.7 Summary The combination of experimental and computational efforts in the determination of the mechanism of action of FLPs toward H2 activation has undoubtedly been fruitful, and there is general consensus about key aspects encompassing the reaction thermodynamics, the formation of frustrated complexes and the cooperative action of the Lewis acid and base. Energy decomposition analysis studies have revealed that the relative importance of electrostatic and orbital interactions may vary depending on FLP architectures and substituent effects, supporting the notion that the EF and ET models may be regarded as complementary to each other.

10.06.6 Transition metal Frustrated Lewis Pairs 10.06.6.1 Introduction As evidenced in the previous sections, the excellent ability of FLPs as metal-free hydrogenation catalysts has gathered increasing attention in the last years from a range of areas leading to a remarkable development of the field. One of such developments has been the incorporation of transition metals either as the acid or basic partner (or both) in the design of TM-based FLPs. The use of transition metals in the field of FLP chemistry, which stands out for its metal-free catalytic capacity, might be initially considered of reduced interest. However, it also provides a number of chemical opportunities/advantages that fully justify the intensive research in this direction that has appeared recently. In fact, the approach has been discussed in deeper detail in specific review works449,450 as well as within reference textbooks.4,5 Among the benefits of introducing transition metals into FLP designs the following should be considered: – Enormous rise of combinatorial possibilities derived from introducing three series of transition metals and two series of rare-earth elements. – In contrast to main group elements, transition metals present a rich reactivity resulting from the presence of partly occupied d orbitals with accessible energies to participate in elementary reactions (i.e., oxidative addition, reductive elimination, migratory insertion, . . .). – Structural diversity of organometallic and coordination complexes. The coordination numbers typically range from two to six (or even up to nine) for d-block metals giving a broad variety of structures and geometries. – High degree of electronic and structural tunability provided by the ligands surrounding the metal center in organometallic and coordination complexes. Also, a transition metal has the ability to behave as an acid or a base depending on its oxidation state and ligand environment. – Larger diversity of affinities (oxophilicity, carbophilicity, thiophilicity, metallophilicity, . . .) toward specific elements compared to main group elements. All these advantages induce a high potential for catalytic purposes, and therefore the incorporation of transition metals into FLP systems has become an area of intense research in the last years. In fact, several reviews and perspectives have already appeared to cover this topic, including specific book chapters.451,452 A rigorous and inspiring perspective by Wass presented for the first time the potential of TMFLPs.449 Other works have focused on systems in which the transition metal only occupies a structural role453 or in which main group FLPs have been used as ambiphilic ligands for transition metal chemistry.454 A less obvious analogy, such as the

Frustrated Lewis Pair Systems

581

rationalization of multiple metal-ligand bonds as masked FLPs, has also been discussed.455 Additional reports concentrate on specific families of TMFLPs456 or in systems in which the two acid and basic sites are comprised of transition metal fragments.450 Besides, it is important to highlight that many of the examples that will be discussed in this section closely resemble prominent cooperative catalysts that rely on the presence of pendant bases for the heterolytic activation of strong bonds in catalysis (i.e., Noyori or Shvo catalysts, among many others). This analogy has been discussed in detail in prior works that the reader is advised to peruse.449,457 Moreover, parallels to the mode of action of some metallic heterogeneous catalysts458,459 and metalloenzymes460,461 have been drawn in many occasions, but remain out of the scope of the present contribution. In preliminary studies, Stephan described the possibility of developing FLP chemistry using complexes of the type [CpTi (NPR3)Me(PR3)][MeB(C6F5)3], which did not lead to adduct formation with bulky phosphines such as PtBu3.462 However, it was Wass and co-workers who described for the first time a well-characterized transition metal FLP system based on a zirconocene stabilized by an aryloxide phosphine ligand. Bulky tert-butyl groups in the phosphine ligand and pentamethylcyclopentadienyl substituents on the Zr center impeded the existence of a Zr–P bond (Scheme 69A).463 This metallic intramolecular FLP system 343 was able to activate several small molecules such as H2 (344), ethylene (345), CO2 (346), and formaldehyde (347), as well as different C–X bonds (349, X ¼ Cl, F, O) and to efficiently catalyze the hydrogenation of amine boranes.365,366 It is worth noting that the unsubstituted cyclopentadienyl analog was unreactive under identical conditions, highlighting the importance of the steric effects to achieve frustration. In a subsequent work, the group of Erker described a geminal Zr+/P FLP comprised of a cationic zirconocene stabilized by internal dative bonding from the phosphine (350). However, this species was able to activate an alkyl isocyanate and carbon dioxide (351) to form the respective five-membered metallaheterocyclic adducts, as well as azides to form the four-membered FLP cycloadduct 352 (Scheme 69B).464,465 (A)

H [Zr]

R

H PtBu2

O

[Zr]

X PtBu2

O

349

344 R–X

H2

Ph

[Zr]

Zr

C 2H 4

PtBu2

O

H

PhCCH

[Zr]

O

PtBu2

O

PtBu2 345

Ph

CO2

348

O

343

H

O

Ph O

O [Zr]

O

[Zr]

PtBu2

O

H

PtBu2 347

346 (B)

Ph

Ph

CO2 PPh 2

Cp*2Zr O

O

Cp*2Zr

Ph

N3–Mes

PPh 2 350

351

PPh 2

Cp*2Zr N N

N Mes

352 Scheme 69 Reactivity of the first transition metal containing FLP systems described by the groups of Wass and Erker.

582

Frustrated Lewis Pair Systems

10.06.6.2 Transition metal Frustrated Lewis Pairs with one metal 10.06.6.2.1

Early and mid-transition metals

Electron deficient early transition metal compounds have been extensively used as Lewis acid catalysts for numerous transformations, especially complexes with metals in high oxidation states.466 After the first examples described by Wass and Erker, several studies have been performed in order to expand the use of early and mid-transition metals in FLP chemistry. In this regard, both the groups of Erker and Wass have intensively studied different intramolecular Zr+/P and Zr+/N FLP systems able to activate a variety of small molecules such as dihydrogen, carbon dioxide, terminal alkynes, aldehydes, and sulfinylamines, as well as to catalyze the hydrogenation of alkenes or imines.153,368,467–472 For instance, the incorporation of a pendant amine as a Lewis basic site in a ZrCp 2-based FLP (353) system proved remarkably useful, not only for the activation of small molecules, but also for the catalytic hydrogenation of alkenes and internal alkynes (Scheme 70A). Mechanistic investigations suggest that dihydrogen cleavage proceeds through an FLP-like pathway to produce a zirconium hydride and a pendant ammonium group (354), with subsequent hydrozirconation of the olefin followed by protonolysis of the Zr–C bond to release the hydrogenated product. Also, the group of Wass explored intermolecular FLP systems based on zirconocene aryloxide compounds (356) in combination with a range of phosphines.473,474 The reactivity of these intermolecular Zr+/P systems toward H2, CO2 (357), THF and phenylacetylene (358) directly depended on the stereoelectronic properties of the employed phosphine (Scheme 70A). In addition, these FLPs displayed promising activity in the catalytic hydrogenation of imines when phosphines were replaced by imines, which act both as the Lewis basic partner and as the substrate.

(A)

(B)

Scheme 70 Reactivity of selected (A) intra- and (B) intermolecular Zr-based FLP systems.

The group of Wass has investigated an intramolecular titanocene-phosphinoaryloxy complex, analogous to the zirconium-based counterpart, for the heterolytic cleavage of dihydrogen.475 In a similar fashion, the group of Erker has capitalized on the Lewis acidity of titanium to construct a series of cationic titanium complexes with a pendant phosphine that readily react with benzaldehyde to form the corresponding addition product (Scheme 71A).476,477 In addition, Beckhaus et al. have recently described a series of electrophilic cationic d0 titanium complexes with a single ligand framework based on a novel tridentate Cp,N,P system (362).478,479 In this study, the authors explored the competence between the nitrogen and the phosphine ligand to cooperate with the acidic titanium center to activate C–H bonds (Scheme 71B). Martínez and co-workers have recently designed an elegant endohedral functionalized cage comprising a Verkade-type superbase (365) as a Lewis base partner in a molecular cavity. Whereas the superbase displayed no catalytic activity in Morita-Baylis-Hillman (MBH) reaction on its own, the presence of titanium (IV) chloride (366) as a Lewis acid triggers the catalytic activity of this FLP system (Scheme 71C).480 The key role of the functionalized cage was demonstrated by the fact that model superbases with bulky substituents showed remarkably lower catalytic activity.

Frustrated Lewis Pair Systems

583

(A)

(B)

(C)

Scheme 71 Selected titanium based FLP systems.

Bullock and co-workers have described the heterolytic activation of dihydrogen in a series of molybdenum complexes (367) containing a pendant amine (Scheme 72).481,482 Lowering the interaction strength between the amine and the acidic molybdenum center is crucial to the observation of dihydrogen splitting. In this regard, introducing ring strain to weaken the Mo–N interaction permits rapid H–H bond cleavage.

Me

R' OC Mo P P R

N

H2

R

N

OC H

R P

Mo

RP H

N R'

R' 367 Scheme 72 Molybdenum based FLP system 367.

368

N R'

N OC Mo R 2P R 2P

369

unreactive strong N–Mo interaction

584

Frustrated Lewis Pair Systems

10.06.6.2.2

Late transition metals

In contrast to early transition metals, late transition metals can both act as the acidic or the basic component of a (frustrated) Lewis pair system depending on the ligand employed and/or the oxidation state of the metal center.483 Several research groups have found inspiration in the FLP concept to construct different late transition metal complexes with either basic484–494 or acidic495–499 moieties in close proximity for the cooperative activation of small molecules. Despite the large number of active compounds that have been described, these systems cannot always be strictly considered as TMFLPs. Recently, Carmona and Rodríguez described the activation of dihydrogen and the O–H bond of water mediated by a Rh-based thermally-induced FLP. The cationic rhodium complex 370 presents a tridentate guanidine-phosphine ligand with lability in one of the Rh–N bonds due to a constrained Rh–N–C–N four-membered ring, which upon decoordination gives access to a vacant coordination site at the acidic Rh(III) center (Scheme 73).500 Heterolytic dihydrogen splitting takes place across the labile Rh–N fragment yielding a Rh-hydride and a pendant iminium ion (371). Computational investigations supported the notion of an FLP-like activation through cooperation between the acidic rhodium center and the basic pendant imine.

Rh Ph2P

N Ar

H2

N

Ph2P

Rh H H N N NAr

NAr 370

Ar

371

Scheme 73 FLP-like heterolytic hydrogen activation in a labile Rh–N fragment.

On the other hand, Pt(0) complexes have been used as common Lewis basic partners in combination with different boranes for the activation of several small molecules. For instance, the group of Wass described a tricoordinate Pt(0) carbonyl complex bearing a diphosphine ligand (372) that in the presence of B(C6F5)3 does not form a Pt(0)
B adduct, but was able to heterolytically cleave dihydrogen (373) and activate CO2 (374) in an FLP manner (Scheme 74A).501,502 This intermolecular mixture also reacts with ethylene, promoting its coupling with the already bound carbon monoxide to afford a five-membered metallacyle (375). Figueroa has also studied an intramolecular Pt(0)/borane Lewis pair (376) that enables the formation of a chelating (boryl)iminomethane ligand with a highly constrained bite angle.503,504 Such a small bite angle facilitates the activation of several small molecules across the Pt !B bond in an FLP fashion (Scheme 74B).

Frustrated Lewis Pair Systems

585

(A)

(B)

Scheme 74 Pt(0)/borane FLP reactivity.

Activation of dinitrogen is one of the most challenging reactions in the field of organometallic and coordination chemistry, and has remained an elusive target in the context of FLPs. Many stable low-valent transition metals present a bound N2 molecule which offers an excellent platform for the study of new stoichiometric dinitrogen activation reactions. The groups of Szymczak and Simonneau have independently reported in parallel studies the activation and functionalization of dinitrogen in an FLP manner. In their research the protonation, borylation, and silylation of dinitrogen has been achieved by combining a metal–N2 (metal ¼ Mo (380), W (381), Fe(382)) complex with B(C6F5)3 (Scheme 75).505,506

586

Frustrated Lewis Pair Systems

Scheme 75 N2 activation and functionalization with metal containing FLP-like systems.

10.06.6.2.3

Rare-earth metals

Rare-earth metals present a notorious potential to build up TMFLP systems due to their Lewis acidic properties, which have already been explored in Lewis acid catalysis.507 In pioneering work, Piers and Eisenstein described an ionic FLP system that allows the trapping of small molecules such as CO or CO2 in a polarized Sc+/HB– pocket (387), followed by hydride transfer from the borate anion.508,509 This strategy was key for the development of an efficient cooperative method for the deoxygenative hydrosilylation of CO2 (Scheme 76A). Other scandium, yttrium and lutetium-based systems have been used to parallel the scandium-mediated activation of different small molecules. For instance the intermolecular P/Sc+ system (390) reacts with benzaldehyde to form the addition product 391, and also undergoes a double 1,4-addition reaction in the presence of dimethyl acetylenedicarboxylate forming a bicyclo[7.7.0]cetane-derived metallacycle 392 (Scheme 76B).414,510,511

(A)

(B)

Scheme 76 Intermolecular and intramolecular scandium-based FLPs.

Rare-earth metals have been proved as efficient catalysts in polymerization of polar and nonpolar alkenes yielding high molecular weight polymers.512–515 In addition, different scandium, yttrium, and lutetium-based complexes bearing b-diketiminate ligands with weakly coordinating bridging phosphine arms have shown FLP reactivity toward 1,4-addition reactions and polymerization of conjugated polar alkenes. Moreover, several homoleptic rare-earth metal aryloxide complexes in combination with commercial free Lewis bases (PPh3, PCy3, PEt3 or PMe3)415,516 or bulky NHCs517–519 have been intensively studied in the polymerization of polar alkenes and hydrogenation reactions. The activation of CO2 and isocyanates has been described with these systems (394) as well (Scheme 77).520

Frustrated Lewis Pair Systems

tBu

tBu

tBu tBu

R R N

O N

O Ce

O

N

N

N

N

tBu

O

CO2

tBu

N

N

tBu

O

tBu

O

R

587

R R N O N

O O Ce O O

tBu

tBu

O

tBu tBu

N

N

R

393 394 Scheme 77 Cerium NHC-based FLP for the activation of CO2.

10.06.6.3 Transition metal Frustrated Lewis Pairs with two metals The study of bimetallic systems has witnessed intermittent periods of progress521–529 over the last decades since the first discoveries of the existence of M–M bonding by Ishishi and Cotton.530,531 However, a renewed interest on the use of earth-abundant transition metals and the high impact of cooperative chemistry in the field of homogeneous catalysis have encouraged many research groups to focus their attention toward nature for inspiration. Many inorganic cofactors in biological systems are based in multimetallic structures that cooperatively activate chemical substrates, and therefore the interest on mimicking these systems has steadily grown during the last years.532,533 The interaction of a substrate with a bimetallic active site through the cooperative participation of the two metal centers has been intensively studied and reviewed.528,534,535 In a recent study, the group of Sakaki computationally studied the FLP-like reactivity of a quintuply bonded dimolybdenum complex 395 toward the activation of H–H, C–H and O–H bonds.536 The authors suggest a polarization of the M–M multiple bond at the transition state, which parallels the polarization found in a Frustrated Lewis Pair system. This polarization facilitates the charge transfer from the M–M bond to the antibonding s EH orbital, weakening at the same time the exchange repulsion between the M–M multiple bond and the E–H substrate (Scheme 78). The easy polarization of the d-type molecular orbitals is one of the advantages of metal–metal multiple bond, in contrast with mononuclear metal complexes where this kind of effect is unattainable.

Scheme 78 Dihydrogen addition to the quintuply bonded Mo–Mo complex 395 and simplified representation of the polarized d orbitals participating in the H–H bond cleavage reminiscent of FLPs.

588

Frustrated Lewis Pair Systems

In contrast to homobimetallic complexes, heterobimetallic complexes present intrinsic polarization at the M–M bond and therefore may exhibit cooperative reactivity in bond activation that resemble FLP-type reactivity. However, not all of them can be considered as truly FLP systems, since in most of the cases the integrity of the M–M bond remains virtually intact in the key transition state. Pioneering studies by Bergman described the cooperative activation of different small molecules by an early-late heterobimetallic Zr–Ir complex (397, Scheme 79).537,538 Despite the clear FLP-like reactivity, no mechanistic investigations were pursued at the time.

Scheme 79 Cooperative activation of different small molecules by Zr–Ir heterobimetallic complex 397.

Polar heterobimetallic complexes that are only stabilized through metal–metal interactions are particularly appealing in the context of frustrated systems. Those in which the sole interaction holding the two fragments together is a dative bond between a Lewis acidic and a Lewis basic metal fragment, without the need of a bridging ligand, are referred as metal only Lewis pairs (MOLPs).483,539 These systems have been increasingly explored during the last decade. For instance, the group of Braunschweig has described the formation of diverse MOLPs based on the combination of transition metal bases and s- or p-block metal acidic fragments (402–404), and studied the lability and dynamic behavior of the M–M dative bonds through exchange experiments (Scheme 80A).540–543 The group of Mankad took a step forward and described experimentally and computationally the FLP-like reactivity of a variety of unbridged polarized heterobimetallic systems.544,545 The bimetallic [CuFe] complex 410 is able to activate a range of small molecules, including CS2,546 iodomethane,547 benzyl chloride548 and dihydrogen in FLP-like fashion.549 Mechanistic investigations on dihydrogen and borane activation revealed that the key orbital interactions are similar to those in classic FLPs, although the M–M bond is only partially broken at the transition state, ruling out a genuine FLP activation mechanism (Scheme 80B).

Frustrated Lewis Pair Systems

589

(A)

(B)

Scheme 80 Exchange experiments between different MOLPs and reactivity of the bimetallic [CuFe] complex 410 toward dihydrogen and HBpin.

As previously discussed, metal-containing FLPs in which one of the components is a transition metal center and the other partner corresponds to a main group element have shown interesting reactivity. Following this approach, Bourissou and co-workers described the bimetallic platinum/aluminum complex 414 featuring a Pt–Al bond in a constrained four-membered metallacycle.550 This system exhibits a remarkable reactivity toward dihydrogen, CO2, and CS2, which is facilitated by the strain associated to the four-membered metallacycle (Scheme 81). Activation of dihydrogen through irreversible oxidative addition to platinum and formal insertion of one hydride into the Pt–Al bond afforded a trans hydridoaluminohydride Pt(II) complex (416). Theoretical studies support that the H2 molecule is firstly trapped in an FLP-like fashion by the Pt/Al complex. This occurs via insertion into the Pt–Al bond through an end-on coordination to the basic platinum center and side-on binding to the acidic aluminum center, as found in many main group FLP systems.

Scheme 81 Dihydrogen and CO2 activation by FLP system containing a constrained Pt–Al bond.

590

Frustrated Lewis Pair Systems

On this basis, the construction of an FLP system entirely based on transition metals was an obvious target. In a first attempt, the group of Wass used a phosphinoaryloxide zirconocene to coordinate an electron rich Pt(0) center through its pendant phosphine in order to construct a polarized heterobimetallic system with no M–M bonds that could be considered as a genuine transition metal only Frustrated Lewis Pair (TMOFLP).551 However, this system did not act as a Zr(IV)/Pt(0) FLP, and instead a new heterobimetallic compounds was formed by formal insertion of the platinum atom into the Zr–C bond. Shortly after, the first truly genuine TMOFLP was described by Campos, who used a combination of sterically hindered gold(I) (417) and platinum(0) (418) complexes as Lewis acid and Lewis base partners, respectively.552 The modulation of the steric bulk of the phosphine ligands bound to the Au(I) fragment was crucial to achieve either the formation of a metal-only Lewis pair or a TMOFLP (Scheme 82). Thus, a Lewis pair adduct was observed when the least sterically encumbered phosphine was used and complete frustration was attained when using the more sterically demanding phosphine.553 In addition, the solution equilibria were strongly affected by solvent effects, in analogy to “traditional” FLPs.554

Scheme 82 Equilibria between bimetallic adduct formation and full frustration in solution depending on ligand sterics and solvent conditions.

These Au(I)/Pt(0) pairs showed a remarkable capacity to activate dihydrogen (Scheme 83A) and acetylene (Scheme 83B) in an FLP-fashion, yielding unusual heterobimetallic Au(I)/Pt(II) complexes containing hydride (423), acetylide (424) and vinylene bridges (425). Mechanistic investigations highlighted the importance of the balance between bimetallic adduct formation and complete frustration, with systems that tend to form a bimetallic adduct exhibiting the least reactivity, which provided support for genuine metal-only FLP reactivity in solution. Subtle modifications on the substituents of the phosphine ligands proved critical in controlling the regioselectivity of acetylene activation and the product distributions resulting from sp C–H cleavage.555 The reactivity of these bimetallic FLPs toward tetrylenes has also been investigated.556

Frustrated Lewis Pair Systems

(A)

591

R'

NTf2 H

R'

R' Au P R' R R

R'

R'

P( tBu) 3

H

NTf2 R' Au + P R' R R 417

Pt

[Pt(P tBu3) 2] 418

H2

P( tBu)

3

NTf2-

R‘

TS

+

P( tBu) 3

R‘

R‘ R‘ P Au

H

+ H Pt

R R 419

P( tBu) 3 420

(b) R = Me, R’ = iPr (c) R = Cyp, R’ = Me

(a) R = R’ = Me +

Au Ar

R = Me, R’ =

Au

P R

P R

H

+ NTf 2-

H

R

Ar

+

Au

iPr

Ar

[Pt(P tBu3) 2H]+

R

Pt H P( tBu) 3

P R

NTf2-

P( tBu) 3

R

422

423

421

(B)

NTf2 R' NTf2

R'

R' Au P 417 R' R R +

PtBu3

H

PtBu3

C2H2

Ar

C6D6 25 ºC

Au

Pt

P R

R

PtBu3

Ar

+

Pt

Au H

P

H R

424

NTf2

PtBu3

R 425

[Pt(P tBu3) 2] 418 (a) R = Me, R’ = Me (b) R = Me, R’ = iPr (c) R = Cyp R’ = Me

95 80