Comprehensive Organometallic Chemistry IV [Volume 9. Groups 11 to 13] 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 9

GROUPS 11 TO 13 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.

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

CONTENTS OF VOLUME 9 Editor Biographies

vii

Contributors to Volume 9

xiii

Preface 9.01

xv

Organometallic Complexes of Copper

1

Hugo Valdés and Xavi Ribas

9.02

Silver Organometallics

32

Andrea Biffis, Cristina Tubaro, and Marco Baron

9.03

Zinc, Cadmium and Mercury

89

Debabrata Mukherjee

9.04

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

122

Meera Mehta

9.05

Polyhedral Boranes and Carboranes

196

Igor B Sivaev

9.06

Polyhedral Metallaboranes and Metallacarboranes

263

Sourav Kar, Alaka Nanda Pradhan, and Sundargopal Ghosh

9.07

Gallium, Indium, and Thallium

370

Christoph Helling and Stephan Schulz

v

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

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Editor Biographies

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

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

Editor Biographies

ix

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

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Editor Biographies

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

Editor Biographies

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

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

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Editor Biographies

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

CONTRIBUTORS TO VOLUME 9 Marco Baron Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy

Alaka Nanda Pradhan Department of Chemistry, Indian Institute of Technology Madras, Chennai, India

Andrea Biffis Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy

Xavi Ribas Institut de Quí mica Computacional i Catàlisi (IQCC), Universitat de Girona, Girona, Catalonia, Spain; Departament de Quí mica, Universitat de Girona, Girona, Catalonia, Spain

Sundargopal Ghosh Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Christoph Helling Institute for Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Essen, Germany Sourav Kar Department of Chemistry, Indian Institute of Technology Madras, Chennai, India Meera Mehta Department of Chemistry, University of Manchester, Manchester, United Kingdom Debabrata Mukherjee Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, India

Stephan Schulz Institute for Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Essen, Germany Igor B Sivaev A.N.Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia Cristina Tubaro Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy Hugo Valdés Institut de Quí mica Computacional i Catàlisi (IQCC), Universitat de Girona, Girona, Catalonia, Spain; Departament de Quí mica, Universitat de Girona, Girona, Catalonia, Spain

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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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9.01

Organometallic Complexes of Copper

Hugo Valdés and Xavi Ribas, Institut de Química Computacional i Catàlisi (IQCC), Universitat de Girona, Girona, Catalonia, Spain; Departament de Química, Universitat de Girona, Girona, Catalonia, Spain © 2022 Elsevier Ltd. All rights reserved.

9.01.1 Introduction 9.01.2 Organometallic Cu(I) compounds 9.01.2.1 Organocuprate(I) complexes 9.01.2.2 Carbene-Cu(I) complexes 9.01.3 Organometallic Cu(II) compounds 9.01.3.1 Isolated organocopper(II) species 9.01.3.2 Organocopper(II) species in catalysis 9.01.4 Organometallic Cu(III) compounds 9.01.4.1 Trifluoromethyl ligands in Cu(III) complexes 9.01.4.2 N-confused porphyrins (NCPs) and carbaporphyrins 9.01.4.3 Aryl-triazamacrocyclic aryl-X and aryl-H ligands 9.01.4.4 Other aryl-containing scaffolds 9.01.4.5 Cu(III) intermediate species in catalysis 9.01.4.5.1 Ullmann-type C-heteroatom couplings 9.01.4.5.2 Cu(III) species involved in Hurtley, Stephens-Castro and Csp3-Csp3 C-C couplings 9.01.4.5.3 Photocatalyzed trifluromethylation of C-X and CdH bonds 9.01.4.5.4 Organocopper(III) intermediate species in CdH functionalization processes 9.01.5 Conclusions Acknowledgments References

9.01.1

1 2 2 7 14 14 17 19 19 20 20 22 22 22 24 25 25 27 27 27

Introduction

Copper has been involved in human life for very long time. In fact, the first items made from copper date from 5000 to 6000 years ago and were found in the ancient region of Mesopotamia, and were used to make tools that helped to our ancestors to perform their jobs more efficiently, facilitating the progress of humanity. Nowadays, copper remains as one of the most useful metals in several fields, ranging from electronics to chemistry. This metal is also very common in biology, being present in several important metalloproteins. Actually, copper is the only transition metal that forms compounds in oxidation state +1 in living organisms.1 Furthermore, copper is one of the most earth-abundant non-precious metals in the Earth’s crust and is the cheapest metal of the group 11. The coordination chemistry of copper is very rich, since this metal readily forms stable complexes with ligands that contain different donor-atoms such as nitrogen, oxygen, sulfur and phosphorus. Commonly, copper exhibits oxidation states from 0 (metallic copper) to +4, with +1 and +2 being the most common. The d10 electronic configuration renders Cu(I) complexes diamagnetic character2 and colorless, and usually in the form of three-coordinate trigonal planar, and four-coordinate tetrahedral geometries. In contrast, Cu(II) exhibits typical d9 chemistry; Cu(II) complexes are usually paramagnetic, and form Jahn-Teller distorted tetrahedral, square planar and trigonal bipyramidal geometries. Cu(III) complexes exhibit a d8 configuration and typically present square planar geometry, and consequently they are diamagnetic. The interest in Cu(III) species is continuously rising due to its implication in copper-catalyzed transformations. One of the very first descriptions of copper complexes was made by the alchemist Andreas Libavius around the year 1600.3 He demonstrated that certain copper salts can be detected with ammonia, because the formation of the complex [Cu(NH3)4]2+ generates a deep blue color. It is worth mentioning that copper-based pigments were known and used by some ancient cultures. However, the study of organometallic compounds of copper, complexes that contains at least one CdCu bond, began two centuries later. In the mid 1800s, investigation of the explosive dicopper acetylide [Cu2C2] was performed.4 It took one more century to reach some of the major breakthroughs, and in the 1930s and 1940s, Gilman and Kharasch studied the synthesis and reactivity of alkylcopper(I) reagents.5,6 Some years later it was demonstrated that the alkyl fragment can be ‘transferred’, allowing the alkylation of unsaturated carbon atoms. Nowadays it is well recognized that organocopper compounds play a key role in several catalytic transformations.7–11 The development of novel catalytic processes based on copper make clear the high importance of organocopper complexes. In this sense, organocopper species are involved in CdC and CdN bond formation, allowing the synthesis of a large number of elaborated organic molecules and pharmaceuticals.12–14 However, the identification of some organocopper species is difficult because of their typically low thermal stability and high reactivity. This is especially true for organocopper(III) compounds, which have been proposed in several catalytic transformations. There are two pathways for the formation of organocopper(III)

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00124-4

1

2

Organometallic Complexes of Copper

compounds: the two-electron oxidative addition of Cu(I) species, or by oxidation of Cu(II) intermediates. Subsequently, a reductive elimination regenerates the Cu(I) species, completing the usual steps of a cross-coupling catalytic reaction. Nevertheless, well-defined organocopper(III) compounds can be prepared in the laboratory: a well-known strategy to stabilize Cu(III) involves the use of multidentate ligands, or very strong-donor ligands. Thus, in this chapter we travel through the chemistry of organometallic copper complexes and their involvement in catalytic cycles.

9.01.2

Organometallic Cu(I) compounds

9.01.2.1

Organocuprate(I) complexes

The organometallic chemistry of copper(I), and their applications has been extensively studied since the mid-20th century.8 One of the first examples was described by Kharasch in 1941 when the addition of isophorone to methylmagnesium bromide in the presence of copper chloride was reported.6 One decade after, in a similar approach, Munch-Petersen described the conjugate addition of Grignard reagents to a,b-unsaturated esters in the presence of copper.15 Jones and Woods reported the preparation of methylcopper(I) in 1952.5 The synthesis occurs in two consecutive steps; first the reaction of MeLi and CuCl2 produces ethane and CuCl, and the addition of additional MeLi allows the formation of a yellow solid. This compound is stable at 0  C under a nitrogen atmosphere. However, at room temperature and under air this compound explodes violently. This solid is the so-called “Gilman reagent” or “Gilman cuprate” and usually it is formulated as [R2CuLi]. Later in 1966, Costa and coworkers success in the isolation of phenylcopper(I), which is thermally more stable than methylcopper(I).16 The determination of the structure of organocopper(I) complexes is sometimes challenging due to their high tendency to form clusters and other aggregates, which exhibit a dynamic equilibrium between several species and are sensitive to the presence of different solvents and to salt effects.7,17 Experimentally, they have been studied by different techniques such as cryoscopic molecular weight determination, NMR and X-ray analysis.2,18–31 Fig. 1 shows the general structures of organocopper(I) complexes. The [R2CuLi]n aggregates are typically cyclic structures in which the organic fragment bridges both the copper and lithium atoms. In 1985, van Koten described the X-ray molecular structure of the first dimeric organocopper(I) derived from N,N-dimethyl1-phenylmethanamine, where the nitrogen atoms are coordinated to the Li atoms.32,33 The dimeric nature of arylcopper(I) complexes was also reported for the complexes of the type [Ph4Cu2Li2(Y)2],34–39 where Y ¼ Et2O or Me2S. In all these dimeric complexes the CdCudC angle is 165 , which is close to a linear geometry. On the other hand, if crystals of [R2CuLi] complexes are grown in more polar solvents such as THF or DME, the solvent is capable of interacting with the lithium atom, and consequently leads to the formation of separated ion pair structures of the type [R2Cu] Li
L. A similar behavior is observed by the presence of a strong lithium-complexing agent such as 12-crown-4. In these complexes the CdCudC angle is almost linear, reaching 180o. Interestingly, the complexation of the lithium atom has an important effect in their reactivity40,41; for instance, the conjugate addition of organocopper reagents to enones is highly affected by the polarity of the solvent. The reaction is faster in solvents such as toluene, hexane, and diethyl ether, while is slower or does not occur in coordinating solvents such as THF, pyridine, DMF and DMSO. Other interesting structures such as higher-order cuprates,42 magnesium derivatives43,44 and clusters27,45–51 have been described and proposed. Organocopper(I) complexes are used as “soft nucleophiles” that can react stoichiometrically or catalytically with substrates such as organohalides, enones and epoxides, among others.52–60 In fact, this is a powerful strategy to form CdC bonds. The reaction mechanism is similar in all cases consisting in three elementary steps (Fig. 2). First, the copper(I) salt reacts with a main-group organometallic reagent, i.e. an organolithium, organozinc, organoaluminum or Grignard reagent. The complex resulting from this transmetalation reaction is either a mono- or diorganocopper(I) complex. Subsequently, an oxidative addition reaction takes place through nucleophilic attack of the d-orbital of the copper(I) center to an electrophile, forming a copper(III) species. Finally, the organic product (R-E) is formed by a reductive elimination. These three steps are the most common sequences found in stoichiometric and catalytic reactions mediated by organocopper reagents. The use of copper(I) species for the alkylation of allylic substrates have been tremendously studied. In fact, there are some reviews devoted to this topic that have been recently published.61–64 In the mid 1990s, Bäckvall and van Koten described the first example of the alkylation of allylic systems bearing a leaving group (RC(O)O and phosphate). For this purpose, they employed Grignard reagents and a well-defined arenethiolatocopper(I) complex.65,66 In this pioneering work they reached a moderate enantioselectivity up to 42% ee. Later in 2001, Bäckvall and co-workers improved the ee up to 64% by changing the ligand.67 These works opened the door to the development of a plethora of novel catalytic systems in which the use of new chiral ligands and organometallic transfer agents were explored.68–99

Fig. 1 General Structure of Lithium organocopper(I) complexes.

Organometallic Complexes of Copper

3

Fig. 2 General reaction mechanism of the CdC coupling mediated by Cu(I).

Fig. 3 Reaction mechanism of the alkylation of a,b-unsaturated carbonyl compounds with Grignard reagents and copper(I).

The alkylation of allylic compounds follows the next steps: (i) coordination of the ligand to copper(I), (ii) transmetalation of the Grignard to copper(I), (iii) oxidative addition and (iv) reductive elimination. The magnesium ion plays an important role to stabilize some of the intermediates in the case of a,b-unsaturated carbonyl substrates (Fig. 3).100 In the CudMg intermediate species, the enone double bond coordinates to Cu and the O to Mg, forming a p-complex. The formation of this intermediate is very important mechanistically, since the magnesium atom plays the Lewis acid role, activating the enone, and links the Cu-complex through the bridging halogen. The size and electronegativity of the bromide atom helps in achieving a balance between stabilization of the p-complex and the subsequent s-complex. This observation was confirmed experimentally by carrying out the reaction in a coordinating solvent (THF), and in the absence of halide. Alexakis and coworkers developed a process for the copper-catalyzed asymmetric allylic alkylation, in which enantio-enriched products (> 99% ee) are obtained from racemic substrates.64,101–103 As in the previous cases, the alkylcopper(I) species is formed after the coordination of the chiral phosphine ligand and transmetalation reaction with the Grignard reagent. The resulting species is then coordinated to the double bond of the alkene. According to theoretical studies, the (R)-enantiomer undergoes an anti-SN2’ oxidative addition, while the (S)-enantiomer performs anti-SN2, forming a Cu(III) intermediate that rapidly undergoes reductive elimination to form the product and regenerate the catalyst. This regio-divergent oxidative addition fits well with the steric bias of the chiral ligand. The anti-SN2’ process is generally proposed for asymmetric allylic alkylation reactions catalyzed by copper.61–63,104–107 Lithium organocuprates R2Cu(I)Li
LiX are widely used for CdC bond forming processes due to their ability to deliver alkyl, vinyl, alkynyl or aryl nucleophiles that react with electrophilic carbon centers in a regio- and stereoselective manner.67,68 According to theoretical (DFT) and experimental mechanistic studies, during the reaction mechanism an organometallic copper(III) species is

4

Organometallic Complexes of Copper

Fig. 4 Square-planar organometallic copper intermediates characterized by RI-NMR at −100  C.

formed. In fact, the development of the Rapid Injection(RI)-NMR technique allowed the characterization of highly unstable copper(III) intermediates in stoichiometric reactions of a range of organocuprates with electrophilic substrates.34–36,69 In this context, Bertz, Ogle and coworkers reported in 2007 the first observation of a copper(III) intermediate species in a conjugate addition reaction between a lithium organocuprate reagent and an a,b-unsaturated ketone that afforded the corresponding 1,4-addition product (Fig. 4).108 They also studied the reaction between Gilman reagent (CH3)2CuLi
LiX (X ¼ I, CN) and 2-cyclohexenone in THF at −100  C. A p-complex between the organocuprate and the cyclic a,b-unsaturated carbonyl compound was initially formed, and upon addition of trimethylsilyl cyanide, a square-planar tetra-coordinated copper(III) intermediate was obtained. The latter was characterized by RI-NMR at −100  C, and the proposed structure was further supported by DFT calculations. Strikingly, the conjugated 1,4-addition product was obtained when the copper(III) intermediate was warmed up to −80  C, which illustrates the transient and highly reactive nature of these species. In 2011 Harutyunyan and Feringa described the first example of the enantioselective allylic alkylation using organolithium compounds and CuBr as catalyst.109 Moreover, the addition of a chiral phosphorus-based ligand was necessary to form a Cu complex in situ that allowed control of the reactivity of the organolithium reagent, and the regio-selectivity of the catalytic reaction. According to 31P NMR spectroscopic studies, the phosphine ligand coordinates to CuBr, then a transmetalation reaction with the organolithium reagent takes place. If diethyl ether is present, a dimethylcopper(I) species is formed, while in the absence of ether a monomethylcopper(I) species is observed. The latter complex results relevant for the catalytic cycle, as was demonstrated by a stoichiometric reaction with cinnamyl bromide; the product observed in this case resulted in more than 98% ee. Interestingly, the reaction was also very sensitive to the presence of different solvents and co-solvents in the nBuLi; for instance, when the co-solvent was hexane, the ee reached up to 99%, while in the presence of ethereal solvents the ee dramatically decreased to 28%. This is probably due to the reactivity, structure and aggregation of alkyllithium reagents that are highly dependent of the nature of the solvent.110 In a similar approach, the allylic substitution of meso-1,4-dibromocycloalk-2-enes was described, representing a powerful strategy for the synthesis of bioactive scaffolds.111 Another noteworthy catalytic reaction that involves organocopper(I) species is the hydroamination of alkenes and functionalized alkenes.112–136 This reaction was originally described using precious metals and high temperatures. Frequently, styrene or alkenes bearing boron, nitrogen or oxygen atoms can be used, as well as protected secondary amines, and a hydride source such as R3Si-H or R2B-H is employed. In the copper catalyzed version, the reaction can be carried out at room temperature in hours. The copper starting materials are Cu(I) or Cu(II) salts (CuX, Cu(OAc)2), which can be easily handled (Fig. 5). In the reaction media, Cu(II) species are reduced to Cu(I). Subsequently, a transmetalation reaction with the hydrosilane promotes the formation of a copper(I) hydride species. It is worth mentioning that this last intermediate is key in a number of other transformations, allowing the development of new catalytic strategies.137–147 Subsequently, the alkene is inserted in the CudH bond, forming an organocopper(I) species. The hydroaminated product is formed through an electrophilic amination reaction with an O-benzoylhydroxylamine. Finally, the catalytic species is regenerated by a transmetalation reaction with the tert-butoxide salt (Fig. 5).

Fig. 5 Hydroamination of alkenes by copper(I).

Organometallic Complexes of Copper

5

Hirano and Miura developed the aminoboration of alkenes.148–152 This strategy allows the transfer of amine and boron moieties to an alkene in a single step. The reaction operates through a similar mechanism to that described for the hydroamination of alkenes, but instead of the formation of a Cu(I)dH species, a Cu(I)-Bpin fragment is formed by the reaction of the Cu-alkoxide and pinB-Bpin. Thus, the alkene is inserted into the Cu(I)-Bpin bond, forming a complex with carbon-Cu(I) and C-Bpin bonds, which subsequently forms the aminoborylated product and reforms the catalytically active species L-Cu-OR. The reaction can be made highly regio- and stereo-specific with the right choice of chiral phosphine ligand. On the other hand, alkynes show unique reactivity toward hydroamination reaction conditions catalyzed by Cu. For example, when the catalytic reaction is carried out in the presence of ethanol, the product obtained is a tertiary aliphatic amine, while in the absence of the alcohol the major product is an enamine (Fig. 6).153 This reactivity can be explained by the fact that the alcohol protonates the vinylcopper intermediate faster than the electrophilic attack of the amine, thus the generation of the alkene is preferred. The alkene can then follow a typical hydroamination pathway, and consequently generate the alkylamine. In previous work, Tsuji154 and Lalic155 described the semi-hydrogenation of CdC multiple bonds using silanes and alcohols. Interestingly, the dearomatization and functionalization of pyridines can be carried under copper-catalyzed hydroamination conditions (Fig. 7). Thus, the reaction is carried out using styrene, pyridine, hydrosilane, copper salt (Cu(OAc)2) and a co-ligand to provide a suitable enantio- and regioselectivity. Interestingly, the product is the 4-substituted pyridine, but the reaction mechanism

Fig. 6 Hydroamination and partial hydrogenation of alkynes catalyzed by Cu(I). [156]

[157]

Fig. 7 (A) Original and (B) updated reaction mechanism for the dearomatization/functionalization of pyridines catalyzed by Cu(I).

6

Organometallic Complexes of Copper

has remained controversial. Originally, Buchwald and co-workers proposed a mechanism that starts with the formation of a Cu(I)dH moiety, followed by the insertion of the alkene, forming an organocopper nucleophile (Fig. 7A). Simultaneously, in the reaction mixture the pyridine coordinates to another Cu(I)dH species through the nitrogen atom, and then reacts with the organocopper complex, forming a N-cuprated dihydropyridine intermediate, which reacts with the silane via s-bond metathesis to release the product and regenerate the catalyst.156 Subsequent to this proposal, the same group reported an extensive study of the mechanism, that included computational modeling, kinetics and spectroscopic studies.157 In this new report, they proposed that the reaction operates with a single copper atom (Fig. 7B). After the insertion of the olefin, the pyridine coordinates to the organocopper species, then a dearomative 1,5-Cu-migration occurs via a imidoyl-Cu-ene/retro-(imidoyl-Cu-)ene/5,5-sigmatropic rearrangement sequence. At the same time Lin and Sheong reported another mechanistic proposal based on DFT calculations.158 According to these calculations, the two-metal center mechanism was disfavored by a significant steric repulsive interaction among the ligand substituents of the two complexes, and consequently avoids the dearomatization process. Therefore, the mechanism is thought to operate via single-metal center. As in the previous proposal, the reaction starts with the formation of Cu(I)dH species, followed by the insertion of the alkene. Subsequently, the Cu atom migrates to the para-position, followed by a dearomatization and a s-bond metathesis that leads to the 1,4-dearomatized product. Fig. 8 shows the complete computed calculation performed by Buchwald and co-workers vs that of Lin and Sheong. Both reports confirm that the reaction mechanism is carried out by a single copper atom, and starts with the reduction of the Cu(II) salt to a Cu(I)dH species. Also, both proposals explain the lack of reactivity of para- or ortho-substituted styrenes. In the doubledearomatization pathway, the presence of such groups increases the formation energy of the intermediates by steric effects, and in the case of para-substituted styrenes, the formation of the Cpara-metalated intermediate is unavailable. By contrast, in the Cu-migration pathway the migration of the copper atom and the dearomatization steps are impeded. Furthermore, if a substituent is present at the C(2) position of the pyridine, the reaction is simply inhibited since the dearomatization step is not favorable. However, when the substituent is at C(4) position the dearomatization may occur.

a. Double de-aromatization pathway by Buchwald [157]

[158]

Fig. 8 Free energy profiles for the dearomatization/functionalization of pyridine catalyzed by Cu(I).

Organometallic Complexes of Copper

7

Kinetic studies on both proposals revealed a first-order dependance on the Cu(I) catalyst, which is consistent with the singlecopper mechanism. In the proposal by Lin and Sheong, the determination of the rate-determining transition state is not very clear, since after the insertion of the olefin (resting state) it offers three transition states which are similar in energy. As such, the kinetic order of the species could not be assigned by theoretical calculations. However, Buchwald and co-workers were able to experimentally determine the kinetic orders, being first-order for the heterocycle and zero-order for the other two reactants. Thus, in the double-dearomatization pathway the rate-determining step involves the formation of the bond between the pyridine and the styrene, with loss of aromaticity. Similarly, if the experimental determination of the kinetic-order is considered for the Cu-migration mechanism, the rate-determining step also corresponds to bond formation between pyridine and styrene, and dearomatization. The difference of free energy between the resting state and the rate determining state is quite similar, (approximately 23 kcal/mol in both mechanisms), which suggests that both are plausible, and probably that both operate simultaneously.

9.01.2.2

Carbene-Cu(I) complexes

Carbenes are very attractive ligands since they can play different roles in a catalytic reactions; depending on the nature of the carbene, they can stabilize highly catalytically active species in high oxidation states, acting as spectator ligands, or they can act as substrates in the catalytic reaction. This latter role has been shown to be very useful for the development of CdC coupling processes catalyzed by copper. Usually, carbenes can be “uncovered” as diazo compounds, which may react with copper, generating a carbenecopper species. The carbene fragment can then be “transferred” to another functional group through a migratory insertion. In 2004, Fu and Suárez described the coupling of alkynes with diazo compounds to generate 3-alkynoates.159 The reaction was carried out under base- and auxiliary ligand-free conditions. These simple conditions allowed for excellent functional-group tolerance and high yields. This pioneering work opened the door to more challenging transformations.160–169 Wang,170–175 Fox,176 Sun,173 Ley,177,178 Liu,179 described the synthesis of allenes from N-tosylhydrazones (or diazo reagents) and terminal alkynes. The reaction mechanism proposed by Wang and co-workers starts with the formation of a copper acetylide species (Fig. 9), which reacts with the diazo compound to produce a carbene-copper species. The allene product is then formed by a migratory insertion/protonation pathway.170 However, Fox176 and co-workers demonstrated that the reaction of a copper acetylide and a diazo compound did not afford the desired product, instead they observed the formation of a hexadeca-7,9-diyne from the homocoupling of the alkyne, and other nitrogenated by-products, due to the decomposition of the diazo compound (Fig. 9). Based on this result, they proposed that the reaction begins with the formation of the carbene-copper species that reacts with acetylene, forming a carbene-propargyl-copper species. The latter performs a reductive elimination to regenerate the catalyst, and consequently, releases an alkyl compound, which reacts with the base to form the desired product. [170]

[176]

Fig. 9 Synthesis of allenes from N-tosylhydrazones (or diazo reagents) and terminal alkynes catalyzed by Cu.

8

Organometallic Complexes of Copper

Fig. 10 Synthesis of allenoates from terminal alkynes and diazo compounds.

The synthesis of the allenoates can be carried out under base-free conditions in the presence of a chiral cationic guanidium salt, as reported by Liu and co-workers.179 The mechanism of the reaction was studied by X-band EPR measurements, corroborating both the reduction of Cu(II) to Cu(I) under the reaction conditions, and the presence of the carbene-copper species. In addition, it was proposed that the carbene-propargyl-copper species may isomerize to a carbene-allenyl-copper intermediate, which can be trapped by protonation to afford the desired substituted-allenyl compound (Fig. 10). Very recently in 2019, Lan and Bai studied the reaction mechanism for the synthesis of allenes from alkynes and diazo compounds catalyzed by copper.180 The theoretical (DFT) calculations were performed using the M11-L density functional in chloroform. Fig. 11A shows the free-energy profile of the most plausible calculated pathway. According to this proposal, the [180]

[181]

Fig. 11 Free energy profile for the synthesis of allenes from alkynes and diazo compounds catalyzed by copper(I).

Organometallic Complexes of Copper

9

carbene-copper species undergoes nucleophilic attack on the alkyne, generating a vinyl cation species. This last step is considered to be rate-determining in the catalytic cycle. Then by subsequent deprotonation/protonation steps, the allene is formed and the catalyst is regenerated. Surprisingly, in this proposal the acetylide-copper species, proposed by Wang, is not present because in Wang’s calculations the total activation energy is higher than in Lan and Bai’s studies (40.8 kcal/mol vs 26.9 kcal/mol, respectively). Interestingly, the presence of a nucleophile allows the synthesis of tri- and tetra-substituted allyl allenes, widening the scope of this methodology.181,182 Wang and co-workers reported that the nucleophile (allyl halide) acts in the last step of the reaction mechanism (Fig. 11B), where it reacts with the vinyl cation species or allenyl copper intermediate, instead of a proton, affording the highly substituted allyl allene.181 Because the protonation step is faster than the nucleophilic attack of the allyl halide, an excess of a strong base (NaH) is required to act as a proton trap. Following a similar strategy, Hu and Xing described the functionalization of isatin ketimine with a-diazo amide and terminal alkynes,183 where the nucleophile is trapped via a Mannich reaction due to the presence of the isatin ketimine. So far, we only have described examples involving a carbene transfer step. However, in the literature there are several reports of stable carbene-copper species. In particular, when it is derived from N-heterocyclic carbene (NHC), the resulting copper complexes are relatively stable.10,11,184–189 The first example was described by Arduengo and co-workers in 1993.190 They prepared and characterized a bis(NHC)dCu(I) complex through the direct reaction of copper(I) triflate with two equivalents of IMes. One year later, Raubenheimer and co-workers described the synthesis of dimeric species of the type [{Cu(m-Cl)(NHC)2}], where the NHC was a thiazole derivative.191,192 The isolation of the first monomeric NHCdCu(I) species was reported by Danopoulos and co-workers.193 They compared the structural conformation in the solid state of two NHCdCu(I) derivatives with different N-substituents. One contained a 2-picolyl moiety, while the second featured a 2-pyridyl fragment. In principle, the first one is able to form a six-membered metallocyclic ring, and the second may form a more stable five-membered metallocyclic ring. In fact, their X-ray molecular structures revealed that the 2-picolyl derivative tends to form polymeric species bridged by the pyridine fragment; in contrast, the 2-pyridyl derivative crystallizes as a monomer, where the 2-pyridyl-imidazolylidene ligand is coordinated in a chelate fashion. The strategies to synthesize NHCdCu(I) complexes are common to those used to obtain other NHC-transition metal complexes.185,194 As such, the synthetic route starts with the corresponding azolium salt, then a base promotes the formation of the NHC ligand. Here, there are three possible pathways. The first one is the in situ generation of the free NHC, although this typically requires anhydrous conditions and strong bases such as Na+ or K+ tert-butoxide, or KHMDS (HMDS ¼ HexaMethylDiSilazide). Secondly, the presence of basic ligands in the Cu precursors can be sufficient to deprotonate the azolium salt and generate the desired metal complex. Probably the most convenient system of this type is Cu2O, due to its stability toward air. Finally, the addition of an external base and a Cu salt may generate the NHCdCu(I) complex. Furthermore, NHCdCu(I) complexes may be obtained by a transmetalation reaction using the related NHCdAg(I) complex. In this case, the labile NHCdAg bond, the smaller bond dissociation energy for the Ag-carbene bond and the insolubility of the silver halide salt (formed in situ) favors the formation of the NHCdCu(I) complex. Typically, NHCdCu(I) complexes are relatively sensitive to air, producing copper salts and the corresponding urea derivative or the hydrolysis product (Fig. 12).195 The presence of H2O favors the formation of the hydrolysis product, while in the absence of water the observed decomposition product is the urea derivative. The stability of the complex strongly depends on the NHC backbone and the N-substituents. Aromatic NHC ligand backbones favor the stability of the Cu(I) complexes probably due to their lower affinity toward oxygen in comparison with saturated ones. On the other hand, bulky N-substituents also enhance the stability of the related complex, for example the 1,3-dimethylimidazol-2-ylidene copper-derivative decomposes in 30 h at 25  C, while the decomposition of the more crowded 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene derivative was achieved in 155 h at 150  C. In the mid 2000s, the exploration of the reactivity of complexes of the type [(NHC)Cu(X)] yielded a range of very interesting results. For instance, the preparation of alkyl,196–198 aryl,199 cyclopentadienyl,200 alkoxide,201,202 dibenzoylmethanoate (DBM),203 boryl,204 acetylide,201,205 thiolate206 and hydride201,207,208 analogs were reported. The proof of concept that these species were

Fig. 12 Decomposition routes of NHCdCu(I) complexes.

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Organometallic Complexes of Copper

[201]

[208]

[212]

[207]

[213]

Fig. 13 Selected examples of NHCdCu(I) hydride species.

readily available widened the perspective of copper chemistry, and consequently, allowed the development of new catalytic protocols.9,139,142,185,209 For example, copper hydrides of the type [(NHC)CuH] have been widely proposed in some catalytic cycles, but their isolation and characterization remained difficult due to their relatively instability. Sadighi and co-workers obtained the first NHCdCu(I) hydride species (Fig. 13) by reacting a [(NHC)Cu(OtBu)] complex with triethoxysilane at low temperature (−45  C).201 The obtained hydride complex decomposes rapidly after 1 h in solution at room temperature, while in the solid state it can be stored for several days under an inert atmosphere. The stability of the NHCdCu(I)dH species can be improved by changing the nature of the NHC ring. In this sense, the use of six- and seven-membered NHC ligands allows the stabilization of CudH species. Furthermore, by employing cyclic(alkyl)(amino)carbenes (CAACs), the isolation, and full characterization of the related CudH complex by X-ray diffraction analyses can be performed.207 This is probably due to the greater nucleophilic character (s donating) of these carbenes that allows the stabilization of highly reactive molecules.210,211 Another strategy to stabilize CudH species consists in the use of NHC ligands with very bulky N-substituents that “protects” the CudH fragment. In this sense, Bertrand and co-workers employed the CAAC ligand 1,3-bis[2,6-bis[di(4-tert-butylphenyl)methyl]4-methylphenyl]imidazol-2-ylidene to report the first evidence of a monomeric NHCdCu(I)dH complex (Fig. 13).212 In the 2D HMBC experiment they observed two signals in the proton spectrum at 4.26 and 2.14 ppm, which were assigned to the dimeric and monomeric species, respectively. This observation suggested an equilibrium between these two complexes. More recently, Sollogoub, Riant and Leyssens functionalized a cyclodextrin with an NHC fragment. The cavity of the cyclodextrin allows the stabilization of the monomeric Cu(I)dH species.213 The role of the NHCdCu(I) hydride complexes in catalytic reactions is similar to that exhibited by their phosphine-based analogs.9,139,142,214–217 In principle, the CudH bond can undergo the insertion of p-bonds, and in a subsequent step the copper center is replaced by an electrophile. In a similar fashion to phosphines, the different substituents attached to the NHC ligands allow control of the enantioselectivity, regioselectivity and chemoselectivity of the catalytic reaction. The catalytic activity of NHCdCu (I) hydrides is typically enhanced by the presence of bulky N-substituents.218–220 Bullrock and Tran studied the effect of different para-functionalized aryl N-substituents in the insertion of ketones, aldimines, alkynes and unactivated a-olefins.221 They found that bulky or electron-donating groups promoted the insertion reaction. This behavior is probably due to a steric destabilization of the dimeric hydride, favoring the formation of the mono-hydride species, which is actually the catalytic active species.220 More stable CudNHC complexes have been used for the cycloaddition of azides and alkynes.222–229 The first example was described in 2006 by Nolan and co-workers.230 They employed the very well-known 1,3-bis-(2,4,6-trimethylphenyl)imidazol2-ylidene (IMes) ligand to prepare the complex [(IMes)CuBr]. The s-donor properties of this NHC-type ligand were suitable for

Organometallic Complexes of Copper

11

Fig. 14 Reaction mechanism of CuAAC using internal alkynes.

enhancing the catalytic activity of the copper center, which was able to catalyze cycloadditions involving terminal- and internal-alkynes in very good yields. Fig. 14 shows the proposed reaction mechanism for internal alkynes. The reaction starts with a ligand exchange reaction between bromide and the alkyne, to produce a p-alkyne complex. Subsequently, the [3 + 2] cycloaddition reaction with the azide occurs, followed by the regeneration of the catalyst and the release of the heterocyclic product. The reaction mechanism of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) is slightly different when terminal alkynes are employed. In this sense, the use of CAAC ligands has allowed the stabilization, isolation and characterization of some key intermediates.231,232 The reaction of terminal alkynes with a CAACdCu(I) complex affords a s-acetylide Cu(I) species, from which two possible pathways can occur (Fig. 15): a) the Cu(I) species under a [3 + 2] cycloaddition with the azide substrate, followed by the formation of both the product and the catalytic active species; b) the second pathway involves a dimetallic cycle, where the s-acetylide Cu(I) complex reacts with a second CAACdCu(I) complex, forming a p,s-bis(copper) acetylide species. The suitable electronic properties of the CAAC ligands allowed for the characterization of this unstable dimetallic species (which is the catalytic active species) by X-ray diffraction analysis. Subsequently, cycloaddition occurs, forming a bis(copper) triazole compound (isolated and also characterized by X-ray analysis), which can be protonated by an alkyne, affording the desired product and regenerating the catalyst. According to kinetic studies, both pathways operate simultaneously, but the dimetallic route is faster than the monometallic one. The advancement of NHCdCu chemistry has also allowed the activation of NdO bonds, opening the door to the development of novel strategies to synthesize ethers, amines, benzofurans and benzoxazoles.233,234 Nakamura and co-workers have described an

Fig. 15 Reaction mechanism of CuAAC with terminal alkynes catalyzed by CAACdCu(I) complexes.

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Organometallic Complexes of Copper

Fig. 16 Reaction mechanism of the[1,3]-alkoxy rearrangement of N-alkoxyanilines catalyzed by Cu.

elegant procedure for the synthesis of 1,2-alkoxyanilines via a [1,3]-alkoxy rearrangement of N-alkoxyanilines (Fig. 16).235 The reaction proceeds at 80  C in the presence of a [(IPr)CuBr] complex and a silver salt (IPr ¼ 1,3-(2,6-diisopropylphenyl)imidazol-2ylidene). The role of the silver salt in this reaction is to abstract the halide from the copper species, generating a cationic [(IPr)Cu(I)]+ complex with a vacant coordination site. The N-alkoxyaniline then occupies the vacant site to form a chelate complex. The ionic cleavage of the NdO bond leads to a Cu alkoxide species, which promotes the introduction of a positive charge in the aromatic ring. Subsequently, the oxygen atom of the alkoxy group attacks the ortho-position to form the ether, and by a tautomerization the product is formed. In a subsequent report, Nakurama and co-workers described the domino [1,3]/[1,2]-rearrangement of 2-substituted-N-alkylanilines to obtain multi-substituted ortho-anisidines under similar reaction conditions (IPrdCu(I)/[Ag salt].236 This reaction is strongly influenced by the Lewis acidic character of the (IPr)Cu(I)catalyst and the electron-withdrawing nature of the protecting group on the nitrogen. NHC ligands with strong electron-donating properties reduce the catalytic performance, whereas the presence of an electron-withdrawing group on the nitrogen accelerates the reaction. On this basis, the reaction mechanism was proposed (Fig. 17). First, the activation of the NdO bond follows a similar route to that proposed for the [1,3]-alkoxy rearrangement. As we mentioned above, the NdO bond is ionically cleaved, thus the oxidation state of copper is +1 throughout the reaction mechanism. The -OMe fragment then ‘migrates’ to the sterically hindered ortho position, followed by a semipinacol-type [1,2]-rearrangement of substituent R1 from the ortho to the meta position. In this last step, a proton transfer leads to the product, and the regeneration of the cationic NHCdCu catalyst. This proposal was partially supported by experimental studies. In this sense, the authors reacted 2,6-dimethyl-N-methoxyaniline in the presence of the catalyst and [AgNTf2]. These conditions were selected since both compounds decelerate the reaction, allowing the capture of reactive intermediates. A dimeric species was isolated, which on heating under metal-free conditions afforded a di-substituted compound; by contrast, heating it in the presence of more catalyst afforded the domino product. Dang and Fang studied via theoretical methods the domino [1,3]/[1,2]-rearrangement of 2-substituted-N-alkylanilines catalyzed by NHCdCu complexes.237 According to their DFT calculations, the cleavage of the NdO bond takes place by an oxidative addition, forming a Cu(III) species. This species can exist as a dearomatized isomer, which contains a more stable Cu(I) metal center. Thus, the metal fragment is responsible for introducing a positive charge to the aromatic ring, favoring the formation of a tertiary carbon cation, and consequently this carbon cation is more susceptible to undergo a nucleophilic attack. After the [1,3]-migration of the methoxy group, the domino rearrangement and the proton transfer, the aromaticity of the aromatic ring is restored. The rate-determining step for the entire reaction mechanism is the proton transfer from the phenyl ring to the amine. On the other hand, copper salts are very well known for their ability to catalyze CdN bond formation in classical Ullmann cross-coupling reactions. However, in the literature there are few examples of catalysts based on NHCdCu(I) species being used for such transformation (Fig. 18). For instance, Zeng and Liu described a highly efficient catalytic system for the coupling of (hetero)aryl chlorides and N-heterocycles.238 The reactions were carried out using Cs2CO3, DMF, and 1 mol% of a catalyst based on 1,10phenantroline. Under such conditions, the yields ranged from moderate (52%) to excellent (98%). Moreover, in a very recent report Shi and co-workers reported the cross-coupling reaction of arylboronic acids with amines and azoles under air and base free conditions.239 Despite the relative stability of the NHCdCu(I) bond, such species have been successfully applied in NHC-ligand transfer reactions, allowing the synthesis of NHC-transition metal complexes, and representing an alternative to the silver NHC-transfer strategy. In 2009, Nolan and co-workers described the reaction of [Cu(NHC)2]X type complexes with S8, affording the related thiourea in high yields.240 They also explored the possibility to transfer the NHC to a Ru-containing fragment. For this purpose they reacted [Cu(NHC)2]X with the dimer [RuCl2(p-cymene)]2 over 4 h at 40  C. The expected ruthenium complex was isolated in 96% yield. This strategy has been extrapolated to other transition metals, allowing the synthesis of NHC-complexes of Au(I), Pd(II), Ni(II), Ir(I), Rh(I), Cr(III), as well a p-block complexes of Al(III), Ga(III), among others.187,241

Organometallic Complexes of Copper

Fig. 17 Domino [1,3]/[1,2]-rearrangement of 2-substituted-N-alkylanilines catalyzed by (IPr)Cu(I).d. [238]

[239]

Fig. 18 Examples of CdN bond formation catalyzed by NHCdCu(I) complexes.

13

14

Organometallic Complexes of Copper

9.01.3

Organometallic Cu(II) compounds

The oxidation state +2 is probably the most prevalent for copper in inorganic and coordination chemistry. However, organocopper(II) species are very rare due to the fact that the Cu(II)-carbon s-bond is unstable and undergoes spontaneous decomposition. Nevertheless, organocopper(II) species have been proposed in some catalytic cycles. Hence, we have divided this section into two: the first one describes the strategies employed to stabilize such species, and the second covers their role in catalysis.

9.01.3.1

Isolated organocopper(II) species

The most widely used strategy to stabilize organocopper(II) species consists of the use of multidentate and macrocyclic ligands.242–246 Fig. 19 shows some representative examples of well-defined organocopper(II) complexes.247–258 As can be noted in many of these examples, the combination of a strong s-donor NHC ligand with additional nitrogen donors provides a suitable environment for the stabilization of the organocopper(II) species. The coordination number of the metal fragment ranges from 4 to 6. Among the first examples were those derived from N- and O-confused porphyrins, which were synthesized by the direct metalation using a Cu(II) salt.253,259–264 The catalytic activity of some organocopper(II) complexes has been evaluated in CdN cross-couplings. For instance, a dinucleating py-NHC-NHC-py ligand was tested in the Cu-catalyzed cross-coupling of anilines with boronic acids (Fig. 20).251 After optimization of the reaction conditions, it was concluded that the best results are achieved using 8 mol% of catalyst, K2CO3 and methanol as the solvent, at room temperature over 24 h. Under these conditions the yields with nitro- and methoxy -anilines reached 88%, while using crowded anilines or heterocycles the yields decreased to 3%. The proposed reaction mechanism

[248]

[247]

[251]

[256]

[254]

[252]

[253]

Fig. 19 Representative examples of isolated organocopper(II) complexes combining NHC and nitrogen ligands.

[255]

[249]

[250]

Organometallic Complexes of Copper

15

Fig. 20 Reaction mechanism proposal for CdN cross-coupling catalyzed by Cu(II) described by Emerson and coworkers.251

starts with an equilibrium between the 4-coodinated and the 5-coordinated organocopper(II) species; next a transmetalation step with the boronic acid occurs (Fig. 20). The lability of the pyridine ligand may create a vacant coordination site, allowing the coordination of the aniline, which can be deprotonated by the base. Thus, the potential of the metal center is shifted to where it can readily be oxidized, forming a organocopper(III) species. The product is formed by reductive elimination, followed by the re-oxidation of the metal center (Cu(I) ➔ Cu(II)). In addition, bidentate ligands with NHC and oxygen donors are suitable to prepare organocopper(II) species (Fig. 21).265–268 In the same way, the strong s-donor NHC ligand and the hard character of the alkoxy ligand provides an electronically suitable environment to stabilize organocopper(II) species. Hoveyda and co-workers described the synthesis and characterization of an air-stable dimeric chiral Cu(II) species.268,269 With the help of the chiral N-substituent, the complex affords allylic alkylation products in good enantioselectivities using organozinc reagents and allylic phosphates.

[268]

[267]

Fig. 21 Representative examples of organocopper(II) complexes with NHC and oxygen ligands.

[266]

[265]

16

Organometallic Complexes of Copper

[271]

[270]

[272]

Fig. 22 Selected examples of organocopper(II) complexes featuring a mono-dentated NHC ligand.

Organocopper(II) complexes with monodentate NHC ligands are by far the least studied systems in this class (Fig. 22). Usually, strategies analogous to those used to successfully obtain NHCdCu(I) complexes, such as the direct metalation of Cu(II) dihalides with monodentate NHC ligands or transmetalation from NHCdAg(I), fail in the case of NHCdCu(II) complexes.270 The products of such reactions are typically the corresponding NHCdCu(I) complex and the 2-bromine-azolium. However, by reacting the free NHC ligand with [Cu(OAc)2] the corresponding NHCdCu(II) complex can be formed.270,271 The coordination of two acetate ligands promotes the stabilization of the organocopper(II) complex, but the compound decomposes in the presence of water in a few hours. In particular, the complex with the IPr ligand was very active in the 1,2- and 1,4-hydrosilylation (more active than Stryker’s reagent). Liu and coworkers described a rare methodology to prepare polymeric Cu(II) systems by reacting copper(I) iodide with a bis(azolium) salt and potassium tert-butoxide in air.272 In the monomeric unit, two iodide atoms bridge two metal fragments, and a NHC and a iodide ligand complete the coordination sphere of the Cu(II). In the literature there are some examples that employ copper(II) salts and an azolium salt as a catalytic system. For instance, the mixture of CuCl2 and IPr or IMes catalyzes the borylation of unactivated alkyl halides. It is worth mentioning that the real catalytic species corresponds to a copper(I) complex due to in situ reduction.273 In 2019 Albrecht and co-workers described a new methodology for the synthesis of NHCdCu(II) complexes.274 The reaction of imidazolium carboxylates with different Cu(II) salts affords the corresponding organocopper(II) species by a decarboxylative reaction (Fig. 23). Interestingly, the decomposition pathway of these complexes is influenced by the nature of the anion X coordinated to Cu(II), and the stability of the NHC. The dissociation of the NHCdCu(II) bond occurs in complexes bearing acetate and N-alkylated NHC ligands, whereas disproportionation is predominant in the case of X ¼ halogen, which induces a reductive elimination route; in the case of X ¼ acetate ligand exchange is favored.

Fig. 23 Synthesis of NHCdCu(II) complexes through a decarboxylative reaction, and their decomposition pathways.

Organometallic Complexes of Copper

9.01.3.2

17

Organocopper(II) species in catalysis

Despite the few examples of isolated organocopper(II) described in the literature, such species have been proposed in a number of catalytic cycles.246,275 Very recently, Warren and co-workers reported the first crystallographic characterization of a mononuclear alkynyl copper(II) complex (Fig. 24), which is a key intermediate species in the Csp-Csp Glaser coupling.276 The choice of the arylacetylene was crucial. They observed that 2,6-dichlorophenylacetylene possess the suitable combination of steric protection and CdH acidity of the alkyl moiety to produce a clean reaction. As is typical, the organocopper(II) compound is unstable in solution at room temperature, decomposing over hours to minutes to the corresponding bis-acetylene compound and a Cu(I) complex. This observation is similar to the Bohlmann mechanism of the dimerization of Cu(II) alkynyl intermediate species.277 According to DFT calculations, the acetylide-Cu(II) complex undergoes a redox disproportionation, forming Cu(III) and Cu(I) acetylide complexes. The anionic acetylide of the Cu(I) species attacks the cationic Cu(III) species, affording a bis-acetylide Cu(III) complex. Subsequently, a reductive elimination forms the homo-coupled product. The use of polar solvents accelerates the decomposition pathway due to the stabilization of the charged species. Furthermore, the addition of nucleophiles accelerates the redox disproportionation, and the acetylide-Cu(II) complex is reactive toward radicals. Based on these findings, the alkylation of terminal alkynes with 1-chloro-2-ethynylbenzene in the presence of a radical initiator (tBuOOtBu) was explored. The catalytic reaction tolerates heterocycles, silanes, aryl halides, among other functional groups. The determination of the oxidation state of copper along some reaction mechanisms is found to be challenging due to the relative thermodynamic stabilities of Cu(0), Cu(I), and Cu(II) that promote the disproportionation of Cu(I) into Cu(0) and Cu(II). To circumvent this drawback, the use of some models based on macrocyclic and pincer-like cyclic ligands have been applied to understand elementary steps promoted by copper in a catalytic reaction.278–280 In this sense, CdH activation promoted by copper is probably one of the most important catalytic steps, because it represents a viable alternative to expensive noble metals. In this sense, Wang and co-workers reacted a series of azacalix[1]arene[3]pyridine compounds with a Cu(II) salt, [Cu(ClO4)2] (Fig. 25).281–283 In the resulting complex, the metal center has an oxidation state of +2. This organocopper(II) species undergoes an oxidation reaction with free Cu(II) ions or oxone as an oxidant, to produce the organocopper(III) analog. A detailed experimental and theoretical approach concluded that the CdH bond activation of the aryl fragment occurs through an electrophilic aromatic metallation after the coordination of the pyridine fragments. This result is similar to that described previously with macrocyclic ligands.284–286 The azacalix[1]arene[3]pyridine/Cu(II) system was then evaluated in a three-component radical reaction for the arene CdH bond functionalization (Fig. 26).287 The key for this catalytic reaction was to find an oxidant compatible with a radical initiator. After optimization, the combination of Ag2O or MnO2 and a Michael receptor and AIBN was found to be suitable for this catalytic reaction. The corresponding functionalized azacalix[1]arene[3]pyridines were obtained in 42–72% yield. The reaction of the Michael receptor and AIBN affords the corresponding radical, which can react with the organocopper(II) complex derived from azacalix[1]arene[3]pyridine, forming a transient organocopper(III)-alkyl species that undergoes reductive elimination to form the product and a [Cu(I)] species (which in turn can be re-oxidized with silver or manganese). Interestingly, other radical sources (Zhdankin reagent or azobis(2-phenylethane) can be used, but the reaction is completely quenched if radical traps are used. To this point, organocopper(II) complexes involved in CdC bond formation have been described. However, there are also examples related to C-heteroatom formation. Copper is widely used for Ullmann- and Chan, Lam, Evans-type reaction, which consist in the coupling between aryl-halides and amines, and boronic acids and amines or alcohols. The reaction mechanism for the latter coupling was studied by Stahl and co-workers.288 They proposed a “oxidase”-style mechanism that starts with a

Fig. 24 Reactivity of (Cl2NN)Cu(II)-alkynyl complex.

18

Organometallic Complexes of Copper

Fig. 25 Synthesis of organocopper(II) and (III) complexes with a azacalix[1]arene[3]pyridine ligand.

Fig. 26 CdH functionalization of azacalix[1]arene[3]pyridine catalyzed by Cu.

transmetalation of the Cu(II) species with boronic acid, to afford an aryl-Cu(II) complex (Fig. 27); subsequently, by a disproportionation reaction an aryl-Cu(III) species is formed. From this last complex, the formation of the CdO in the product is easily obtained. As noted above, the heteroatom coordinates to the metal center, prior to reductive elimination. Finally, the Cu(I) species is oxidized by molecular O2. Accordingly, to kinetic and spectroscopic studies the transmetalation step is the turnover-limiting step, while the reoxidation is rapid. In order to shed more light on the C-heteroatom couplings, Warren and co-workers studied the reactivity of a b-diketiminate-based (N,N)dCu(II)-OtBu complex toward tris(aryl)boranes (Fig. 28).289 The reaction with B(C6F5)3 results in the transmetalation product (N,N)dCu(II)-C6F5, while changing the electronic nature of the aryl fragment, leads to the aryl-aryl coupling product and Cu(I) species. In addition, the reactivity of fluorinated aryl-copper(II) species toward radicals was

Organometallic Complexes of Copper

19

Fig. 27 C-heteroatom bond formation catalyzed by Cu(II).

Fig. 28 Reactivity of b-diketiminate-based (N,N)dCu(II)-OtBu complexes.

explored.290 They found that the Cu(II) aryl complex can be reacted with the free radical NO(g), resulting in aryl-N bond formation. Similarly, when a phenolate is used, CdO bond formation is observed. The proposed mechanism follows a transmetalation/ disproportionation/reductive elimination pathway.

9.01.4

Organometallic Cu(III) compounds

The implication of organometallic Cu(III) complexes has become evident in the reactivity of organometallic Cu(I) and Cu(II) compounds disclosed so far. The intrinsic instability of organometallic Cu(II) intermediate species has posed a tremendous challenge to trap and study such complexes. Nevertheless, a selected number of these compounds have been isolated and it is worth noting that organometallic ligand scaffolds afford much stable Cu(III) complexes than those featuring exclusively N- or O-donor ligands. For instance, a significant number of examples of Cu(III) complexes bear the trifluoromethyl (CF−3) ligand,291–296 the vast majority exhibiting a square-planar geometry. Indeed, over 20 new [alkyl-Cu(III)-(CF3)3]− compounds have been recently reported by Liu and coworkers.297 On the other hand, embedding the metal in a square-planar macrocylic environment is a reliable strategy to stabilize Cu(III) species, taking advantage of its d8 electronic configuration. Moreover, macrocyclic aryl or confused pyrrole groups, in combination with N-donors, can also stabilize Cu(III). Researchers have sought to find direct implication of organometallic Cu(III) species as key intermediates in the field of CdC and C-heteroatom coupling reactions, in nucleophilic organocopper(I) chemistry for CdC bond formation and in the reactivity of organocopper(II) compounds.

9.01.4.1

Trifluoromethyl ligands in Cu(III) complexes

The Cu(III) oxidation state can be successfully stabilized by the trifluoromethyl anionic ligand, even at room temperature, and it is a common strategy for this goal. In most of the examples, a minimum of two CF−3 ligands are used (and often three),291 completing the square-planar geometry with other anionic ligands (Fig. 29).291,292,297 The strong Cu(III)dCF3 s bond is behind the ability of CF−3 groups to confer high thermal stability.295,297 In 1989, Burton reported the [Cu(III)(CF3)2(SC(S)NEt2)] complex, providing the first crystal structure of an organometallic copper(III) complex.291 Soon after, Naumann reported the crystal structure of a [Cu(III) (CF3)4]− complex.292 Later in 1996, Eujen reported the analogous [Cu(III)(CF2H)4]− complex, proving that the difluoromethyl group (CF2H−) also serves as good stabilizing ligand for Cu(III) (Fig. 29).293 More recently in 2015, Grushin demonstrated that

20

Organometallic Complexes of Copper

[291]

[292]

[293]

[297]

Fig. 29 Polyfluorinated alkyl-containing Cu(III) complexes with reported crystal structures.

Cu(III) could also adopt square-pyramidal geometries by reporting the penta-coordinate and diamagnetic complex [Cu(III) (CF3)3(bpy)].295 Zhang reported in 2018 the square-planar complex [Cu(III)(CF3)3(py)], featuring a Cu(III)-C3N coordination environment.296 Very remarkably, Liu and coworkers reported in 2019 the reaction of [Cu(III)(CF3)3(py)] with alkyl bromozincates, affording a series of 20 new Cu(III) compounds of the general formula [Cu(III)(CF3)3(alkyl)]−, three of them being crystallographically characterized.297 In addition, the authors pioneered detailed studies of the Csp3-Csp3 reductive elimination process upon heating. If CF−3 groups are replaced for CH−3 groups, the stabilizing effect is completely lost and metastable Cu(III) compounds such as Li [(CH3)4Cu(III)], Li[(CH3)3Cu(III)(s-allyl)] and [(CH3)2Cu(III)(p-allyl)] have been characterized only when using low temperature rapid injection NMR techniques at −100  C (Fig. 30).108 Moreover, this technique allowed the NMR characterization of Z1 s-allyl and Z3 p-allyl Cu(III) species in the allylic alkylations with organocopper(I) reagents and allylic electrophiles.298

9.01.4.2

N-confused porphyrins (NCPs) and carbaporphyrins

Furuta and coworkers reported the crystal structure of a Cu(III) complex featuring a cis-double-NCP (Cu(III)-N2C2 coordination environment, square-planar geometry).299 Later in 2003, the same authors reported the crystal structure of a Cu(III)-NCP complex (Cu(III)-N3C coordination environment) (Fig. 31).264,300 Indeed, they also reported a trans-double-NCP (Cu(III)N2C2 coordination environment) in 2003301 and 2014302 using different ligand derivatives. Moreover, hetero-bimetallic Pd(II)dCu(III) hexaphyrin complexes (Fig. 32),303,304 featuring a square-planar geometry for both metals were reported in 2010 by Osuka and coworkers. Other organometallic square-planar Cu(III) compounds have been reported using analogous ligand environments, i.e. oxacarbaporphyrin-Cu(III),260 carbacorrole-Cu(III,305 dibenzicorrole-Cu(III).306 phenanthriporphyrin-Cu(III) and phenanthribilinone-Cu(III).307

9.01.4.3

Aryl-triazamacrocyclic aryl-X and aryl-H ligands

As described in previous sections, Cu(III) is highly stabilized, even at room temperature, if embedded in a square-planar geometry imposed by macrocyclic ligand scaffolds. Aryl-triazamacrocyclic ligands are ideal to synthesize the corresponding Cu(III)

Fig. 30 Reactive alkyl-Cu(III) complexes detected at low temperature (below −80  C) by rapid injection NMR, reported by Bertz and Ogle.108

Organometallic Complexes of Copper

21

[299]

[264,300]

[302]

[301]

Fig. 31 Crystallographically characterized N-confused porphyrin-based Cu(III) compounds reported by Furuta’s group.

[303,304]

[260]

[306]

[307]

[305]

[307]

Fig. 32 Crystallographically characterized compounds based on bimetallic hexaphyrin-Pd(II)Cu(III), oxacarbaporphyrin-Cu(III), carbacorrole-Cu(III), dibenzicorroleCu(III) phenanthriporphyrin-Cu(III) and phenanthribilinone-Cu(III).

complexes.279,308 Ribas and coworkers pioneered the use of these ligand scaffolds for stabilizing Cu(III) compounds, with synthesis achieved either via aryl-halide oxidative addition or via CdH activation.284–286,309–312 These macrocyclic ligand scaffolds are tunable in size, and via the amine substituents and electronic properties of the aryl moiety aryl (Fig. 33). Nevertheless, these stable compounds were found to be highly reactive toward nucleophiles, becoming valid model intermediate species for the study of Ullmann-type cross-coupling reactions. Indeed, the aryl–Cu(III) model platform exhibits clean reactivity with a variety of nucleophiles, affording the coupling products quantitatively (see below).

22

Organometallic Complexes of Copper

Fig. 33 Aryl-triazamacrocyclic Cu(III) complexes crystallized at room temperature reported by Ribas,284–286,309–312 exhibiting either square-planar or square-pyramidal geometry around the metal.

9.01.4.4

Other aryl-containing scaffolds

In the context of macrocyclic arene models, Wang and coworkers showed the ability of azacalix-[1]arene[3]pyridine scaffolds to stabilize the corresponding square-planar aryl-Cu(III) species.313–315 These Cu(III) complexes also reacted cleanly toward nucleophiles to form either C-heteroatom and CdC bonds, being also good models for copper-catalyzed cross-coupling reactions (Fig. 34). A single example of a non-macrocyclic crystal structure of a mononuclear cyclometalated bis-aryl-Cu(III) complex was reported by Doerrer, featuring a trifluoromethylated phenylmethoxide ligand (Fig. 8).316 Nonetheless, the non-macrocyclic environment makes the latter very sensitive to air, and a low (10%) yield was obtained.

9.01.4.5 9.01.4.5.1

Cu(III) intermediate species in catalysis Ullmann-type C-heteroatom couplings

The stoichiometric reactions of well-defined aryl-Cu(III) species with heteroatom nucleophiles form the corresponding C-heteroatom coupling products via reductive elimination under mild conditions. These stoichiometric reactions can be upgraded to catalytic conditions by aryl-halide triazamacrocyclic substrates, as models of the halobenzene electrophiles in typical cross-coupling reactions, using catalytic amounts of Cu(I) salts.308,309,312,317 The effectivity of the catalysis clearly suggests a facile oxidative addition step at Cu(I). Definitive proof for the reversible 2-electron redox Cu(I)/Cu(III) fundamental step was provided by the reactivity of a well-defined triazamacrocyclic aryl-Cu(III) species with triflic acid. The addition of acid triggers the reductive elimination to form the protonated aryl-Br substrate and Cu(I); basification of the solution triggers a reversible oxidative addition step to finally recover the aryl-Cu(III) complex intact (Fig. 35).312 The robustness of the Cu(I)/Cu(III) process allowed the study of model catalytic Ullmann-type catalytic cross couplings. [314]

[316]

Fig. 34 Aryl-Cu(III) complexes within azacalix-[1]arene[3]pyridine scaffolds (left) and partially fluorinated phenylmethoxide ligand (right).

Fig. 35 Triazamacrocyclic aryl-Cu(III) model compounds exhibiting fully reversible reductive elimination and oxidative addition fundamental steps, reported by Ribas.312

Organometallic Complexes of Copper

23

Fig. 36 Triazamacrocyclic aryl-Cu(III) compound showing stoichiometric C-heteroatom reductive elimination under mild conditions, reported by Ribas.309,310,312,317–319

Firstly, control experiments were performed to account for a mild reductive elimination at aryl-Cu(III) upon reaction with stoichiometric amounts of N-, O- S-, Se-, halides and P-nucleophiles. This was demonstrated with a variety of heteroatom nucleophiles with aryl-triazamacrocyclic scaffolds (Fig. 36)309,310,312,317–319 as well as with azacalix-[1]arene[3]pyridine scaffolds.313–315 Once stoichiometric reactions were proven to be viable, triazamacrocyclic aryl halide models were used to conduct Ullmann-type C-heteroatom coupling catalysis. Using 3–10 mol% of a Cu(I) catalyst, the corresponding coupling products with a large range of nucleophiles were obtained in very good yields. Definitive proof of aryl-Cu(III) intermediacy after the oxidative addition was obtained,312 thus confirming a Cu(I)/Cu(III) redox couple as the operating catalytic cycle (Fig. 37).279,308–310,317–319

Fig. 37 Proposed Cu(I)/Cu(III) catalytic cycle for Ullmann-type catalysis using triazamacrocyclic aryl-halide model substrates (< 10 mol% Cu(I)), reported by Ribas.279,308–310,317–319

24

Organometallic Complexes of Copper

The success of the model platform in demonstrating the feasibility of the Cu(I)/Cu(III) catalytic cycle under very mild conditions clearly suggests that the mechanism is valid provided the right coordination environment for the catalyst can be achieved, although it cannot be taken as a definitive proof for the actual mechanism in standard Ullmann couplings. The mildness of the experimental conditions and the dilution of the reaction medium strongly point toward the appropriate direction for catalyst design in standard Ullmann couplings. In this sense, Ribas and coworkers reported a non-cyclic tridentate N-based auxiliary ligand (1,10 -(pyridine2,6-diyl)bis(N-methylmethanamine), effective in the arylation of phenols,320,321 inspired by an aryl-triazamacrocyclic scaffold, in an attempt to trap aryl-Cu(III) in a standard Ullmann-type coupling. After exhaustive experimental (radical clocks and helium-tagging infrared photodissociation (IRPD) spectroscopy) and computational mechanistic studies, the existence of an aryl–Cu(III) species in Ullmann couplings using this specially designed tridentate scaffold was proposed, albeit not directly detected.320 This highlights the importance of the macrocyclic effect in stabilizing well-defined aryl-Cu(III) species.

9.01.4.5.2

Cu(III) species involved in Hurtley, Stephens-Castro and Csp3-Csp3 C-C couplings

Both azacalix-[1]arene[3]pyridine Cu(III) and aryl-triazamacrocyclic have also shown good stoichiometric reactivity in Csp2-Csp couplings318,322 using acetylenes and Csp2-Csp3 couplings using either alkyl lithium compounds323 or activated methylene substrates.319 The catalytic version was effectively achieved for the reaction of triazamacrocyclic aryl-halide model substrates with activated methylenes, obtaining the Csp2-Csp3 coupling species in the form of intramolecular cyclized 1,2-dihydroisoquinoline compounds (Fig. 38).319 In 2019, the stoichiometric reaction of CF3-containing Cu(III) species with alkyl-ZnBr was pioneered by Liu and coworkers, who obtained a large number of [(alkyl)Cu(III)(CF3)3]− compounds.297 A Csp3-(Csp3F3) reductive elimination took place upon heating under mild conditions (Fig. 39), with no radical-based interference, thus establishing the canonical two-electron redox step to afford the corresponding coupling product and Cu(I).

Fig. 38 Hurtley-type catalysis using triazamacrocyclic aryl-halide model substrates and activated methylenes, reported by Ribas.319

Fig. 39 Stoichiometric Csp3-(Csp3F3) reductive elimination from well-defined [(alkyl)Cu(III)(CF3)3]− complexes, reported by Liu.297

Organometallic Complexes of Copper

9.01.4.5.3

25

Photocatalyzed trifluromethylation of C-X and CdH bonds

In 2018, MacMillan and coworkers developed a very remarkable photocatalytic trifluoromethylation of aryl bromide substrates,324 overcoming the reluctant oxidative addition step at Cu(I). The strategy consisted in supplying at once a CF3
radical and an aryl-radical species to the Cu(I) reagent to generate the aryl-Cu(III)-CF3 intermediate, which afforded the expected aryl-CF3 coupling product by reductive elimination. Both radical species were obtained in situ by coupling the Ir-based photocatalyst irradiated with blue-LED light with a silyl radical precursor and dMesSCF3 as the trifluoromethylating reagent (Fig. 40). The same strategy was successfully applied to bromoalkane substrates in 2019 by the same authors.325 In 2021, MacMillan extended the photocatalyzed trifluoromethylation strategy to pyrrolidine via CdH activation, in this case choosing deca-tungstate as the photocatalyst and the Togni reagent as the trifluoromethyl source.326 Very recently, Hong and coworkers also reported a novel Csp3dH trifluoromethylation, in this case using a photo-induced high-valent bench-stable [(bpy)Cu(III)(CF3)3] complex.327 Diverse unactivated alkanes including bioactive molecules were trifluoromethylated under mild reaction conditions, with preference for the less sterically hindered Csp3dH bonds. The mechanism of the catalysis is proposed to undergo via the CF3
radical-mediated HAT reaction to activate Csp3dH bonds, where [(bpy)Cu(III) (CF3)3] performs multiple roles as the photoinduced reaction initiator, precursor of the CF3
radical as a unique HAT reagent, and trifluoromethylating source. The catalysis required the use of Oxone® as oxidant, which is thought to oxidize the intermediate species [(bpy)Cu(II)(CF3)2] to [(bpy)Cu(III)(CF3)2(bisulfate)], which finally reacts with the alkane carbocation by ionic coupling.

9.01.4.5.4

Organocopper(III) intermediate species in CdH functionalization processes

Copper-catalyzed CdH functionalization strategies have also been developed using the Directing Group (DG) approach. Ge and coworkers reported the intramolecular amidation via Csp3dH activation of substrates bearing the aminoquinoline DG (Fig. 41), obtaining the cyclic 4-membered ring azetidin-2-one derivatives.328,329 The same group developed the acetoxylation of similar substrates.330 Regarding the use of triazamacrocyclic scaffolds for CdH activation processes, arene containing substrates were used for studying in detail the Csp2dH activation by Cu(II) salts. The global reaction is defined as a disproportionation of Cu(II) to form equimolar amounts of Cu(I) and an organometallic aryl-Cu(III) compound.284,285 An efficient catalytic process using amides, aliphatic alcohols and carbamates was designed by Ribas and coworkers via the Wacker-type aerobic oxidation of Cu(I) to Cu(II), as depicted in Fig. 42.311,331

Fig. 40 Photocatalytic trifluoromethylation of aryl bromides designed by MacMillan.324

26

Organometallic Complexes of Copper

Fig. 41 Synthesis of cyclic 4-membered ring azetidin-2-one derivatives via copper-catalyzed amidation of Csp3dH bonds in substrates bearing 8-amonoquinoline DG, reported by Ge.328,329

Fig. 42 The first examples of Cu(II)-catalyzed CdO and CdN couplings via arene CdH activation through Cu(II) disproportionation in model triazamacrocyclic arene substrates, reported by Stahl and Ribas.311,331

Organometallic Complexes of Copper

9.01.5

27

Conclusions

The extense field of copper catalyzed CdC and C-heteroatom couplings is based on the rich redox chemistry of copper, with a large variety of transformations involving organometallic Cu(I), Cu(II) and Cu(III) species. Many efforts have been devoted to trap and characterize the actual intermediate species of the processes, and organometallic Cu(III) species stand as the key actors of many of those transformations. In this chapter we have reviewed the most relevant organometallic Cu(I), Cu(II) and Cu(III) species reported, and their implication in the mechanisms of organocuprate(I) reactions, CdC and C-heteroatom cross coupling, CdH functionalization and photocatalytic processes.

Acknowledgments We acknowledge financial support from MINECO of Spain for projects CTQ2016-77989dP and PID2019-104498GB-I00. We also thank the Generalitat de Catalunya for project 2017SGR264 to X.R. and for a Beatriu de Pinós contract to H
V (Beatriu de Pinós H2O2 MSCA-Cofund 2019-BP-0080). We thank ICREA for an ICREA Acadèmia award to X. R.

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Soc. 2019, 141, 6853–6858. Sarver, P. J.; Bacauanu, V.; Schultz, D. M.; DiRocco, D. A.; Lam, Y.-H.; Sherer, E. C.; MacMillan, D. W. C. Nat. Chem. 2020, 12, 459–467. Choi, G.; Lee, G. S.; Park, B.; Kim, D.; Hong, S. H. Angew. Chem. Int. Ed. 2021, 60, 5467–5474. Wu, X.; Zhao, Y.; Zhang, G.; Ge, H. Angew. Chem. Int. Ed. 2014, 53, 3706–3710. Yang, Y.; Gao, W.; Wang, Y.; Wang, X.; Cao, F.; Shi, T.; Wang, Z. ACS Catal. 2021, 11, 967–984. Wu, X.; Zhao, Y.; Ge, H. Chem. Asian J. 2014, 9, 2736–2739. Bernoud, E.; Company, A.; Ribas, X. J. Organomet. Chem. 2017, 845, 44–48.

9.02

Silver Organometallics

Andrea Biffis, Cristina Tubaro, and Marco Baron, Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Padova, Italy © 2022 Elsevier Ltd. All rights reserved.

9.02.1 9.02.2 9.02.2.1 9.02.2.1.1 9.02.2.1.2 9.02.2.1.3 9.02.2.1.4 9.02.2.2 9.02.2.2.1 9.02.2.2.2 9.02.2.2.3 9.02.2.2.4 9.02.2.3 9.02.2.3.1 9.02.2.3.2 9.02.2.4 9.02.2.4.1 9.02.2.4.2 9.02.2.4.3 9.02.3 9.02.3.1 9.02.3.2 9.02.3.3 9.02.4 References

9.02.1

Introduction Silver(I) organometallics Silver complexes with alkyl, alkene, arene, aryl and related ligands Introduction Silver arene and aryl complexes Silver alkene complexes Silver alkyl complexes Silver-carbene complexes Introduction Novel carbenes as ligands toward silver Polynuclear complex featuring halide bridges and/or argentophilic interactions Macrocyclic/cage complexes Silver complexes with other neutral carbon ligands Silver isocyanide complexes Silver carbonyl complexes Alkynyl complexes of silver Extended polymeric structures High-nuclearity clusters Miscellaneous coordination of alkynyl moieties Silver(III) organometallics Introduction Silver(III) complexes with porphyrinoid ligands Silver(III) complexes with other organometallic ligands Summary and outlook

32 33 33 33 33 38 39 40 40 42 47 54 63 63 64 65 65 77 80 81 81 81 83 84 84

Introduction

The organometallic chemistry of silver has undergone several very significant developments in the 21st century, but it is still overshadowed by the more popular organometallic chemistry of the two other group 11 elements. Historically seen, the organometallic chemistry of copper has always been at the forefront compared to that of silver, and the tremendous development of the organometallic chemistry of gold in the course of the last 20 years has brought also this element to surpass silver in the interest of organometallic chemists. The main reason why silver was and remains the most neglected element in the group from the organometallic point of view can be easily traced back to the lability of silver organometallic compounds.1 Silver is both a weak s-acceptor and a poor p-donor, and as a result AgdC bonds are generally the weakest metal-carbon bonds within the group 11 elements; the light sensitivity of silver compounds further complicates the picture. Still, modern organometallic chemists have learned to take advantage of the lability of organosilver compounds and exploit it for some highly successful applications, most notably in the context of silver complexes with N-heterocyclic carbene and related ligands. Indeed, it can be frankly stated that this latter class of compounds currently dominates the field of the organometallic chemistry of silver. Although more limited compared to copper and gold, the organometallic chemistry of silver is still very extensive, and the length of a single chapter is largely insufficient to provide an exhaustive account of all facets of this chemistry. Consequently, this chapter will build on the contents of chapters on the same topic published in the previous editions of “Comprehensive Organometallic Chemistry.” In the first two editions, the chapters dedicated to silver by Van Koten et al. were mainly dealing with preparation methodologies and with the structure and bonding situations in organosilver compounds.2,3 In the third edition, the chapter on silver by Yam and Chen was organized in a different way and addressed the latest achievements in the different classes of silver organometallic compounds.4 In this chapter, we maintain this approach and focus consequently on the different classes of organosilver compounds, highlighting for each class the most notable developments in the course of the last 15 years, i.e. from the publication of the previous edition of Comprehensive Organometallic Chemistry. Our intent is not to exhaustively cover all published examples of all compound classes, but rather to inform the reader about the current trends in this chemistry and about the classes of compounds that currently enjoy the greatest interest, in the organometallic community and beyond, in view of their peculiar properties.

32

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00161-X

Silver Organometallics

9.02.2

Silver(I) organometallics

9.02.2.1

Silver complexes with alkyl, alkene, arene, aryl and related ligands

9.02.2.1.1

33

Introduction

Silver organometallics with alkyl, aryl and alkenyl ligands have been less investigated compared to the corresponding compounds of the other coinage metals. The low thermal stability and photosensitivity make these organometallics challenging to be synthesized and characterized. In agreement with this, it has been demonstrated at the theoretical level that the MdC bond strengths of the phenyl and methyl compounds follow the order Au > Cu > Ag for the group 11 elements.5 Homoleptic alkyl and alkenyl complexes of silver(I) are known to be very unstable under ambient temperature and light. Pioneering studies in this field were performed by Semerano and Riccoboni in 1941, who reported on the observation of alkylsilver compounds in the reaction between silver nitrate and tetraalkyllead reagents, producing alkyl dimers and elemental silver.6–8 Nowadays, it is known that successful isolation of this class of compounds is fairly limited and mainly restricted to those involving perfluoroorganics, which show significantly increased thermal stabilities compared with those of the non-fluorinated analogs.9 On the other side, many organosilver compounds that are stable at room temperature, such as alkynylsilver compounds and phenylsilver, form coordination polymers and low solubility limits their utility. Under the right conditions, however, organosilver compounds have been effectively used as sources of carbon-based radicals or carbanions.10 They are intermediates in both stoichiometric11 and catalytic12 silver-mediated C − C coupling processes, and have been identified in studies of CdC coupling in the gas phase.13 In general, the syntheses of these complexes are achieved through: (i) nucleophilic addition/substitution reactions of silver(I) fluoride or (ii) transmetallation reactions with other metal alkyl, alkenyl, and aryl complexes.4,14

9.02.2.1.2

Silver arene and aryl complexes

The first report on the synthesis of the insoluble phenylsilver is dated back to 1919 by Krause and Schmitz.15 Subsequently Hofstee et al. in 1979 speculated on its polymeric chain-like structure on the basis of cryoscopic molecular weight determinations in benzene.16 Finally, more than two decades later, the first example of related infinite chains was confirmed via X-ray crystallography, showing the typical bridging coordination mode of the phenyl groups between two silver atoms in an ipso fashion.17,18 The common PhAg2 subunit involves a two-electron three-center bonding.19,20 A few mono- and polynuclear arylsilver(I) complexes of the type AgR (e.g., R ¼ C6H5, C6F5, C6Cl5, MeC6H4, Me2C6H3, (MeO)2C6H3), AgC6F5
L (L ¼ EtCN), and Ag2R2
L have been reported.21–23 However, examples of structurally characterized organosilver(I) complexes are still rare, appearing mostly as monomeric or tetrameric species in the solid state.24–26 Diarylargentate complexes are composed of separated ion pairs.27 The synthesis of pentafluorophenyl silver has been deeply studied by Schulz and Villinger. They introduced a new synthetic protocol for its preparation, formally in a catalytic way (Scheme 1).18

Scheme 1 Catalytic synthesis of pentafluorophenyl silver (AgC6F5).

They discovered this synthetic route by studying the stability of silver and lithium salts with weakly coordinating anions. The salt Li[B(C6F5)4] is obtained by reacting C6F5Br with n-butyllithium in the presence of B(C6F5)3 in n-pentane, and subsequently treated with AgF in CH2Cl2 at 30–60  C under ultrasonic irradiation for 8 h, affording solvent free pentafluorophenylsilver, AgC6F5, in high yield (90%) together with B(C6F5)3 and LiF. Almost three-quarters of the B(C6F5)3 can be recovered in high purity by sublimation at 110  C. The driving force of the AgC6F5 formation is the instability of the solvent free Ag[B(C6F5)4] intermediate. The solvento complex AgC6F5
CH3CN (1) was successfully characterized by single crystal X-ray diffraction (Fig. 1) and is isotypic to the related AgC6F5
C2H5CN compound. In the unit cell two crystallographically different Ag ions, Ag1 and Ag2, are present, with Ag2 in linear coordination. The acetonitrile molecule coordinates only to Ag1 which results in a distorted tetrahedral coordination. While solvent free Ag[B(C6F5)4] is unstable, the solvated analog with toluene [Ag(toluene)3][B(C6F5)4] (2) can be isolated and characterized (Fig. 2). [Ag(toluene)3][B(C6F5)4] also crystallizes in the triclinic space group P1 with two formula units per cell. The asymmetric unit consists of one independent [Ag(toluene)3]+ and one [B(C6F5)4]− ion. The silver cation is coordinated by two carbon atoms (d(AgdC) < 2.89 A˚ ) of each of the three toluene molecules in an Z2 fashion resulting in a coordination number of six. Each toluene molecule forms a short and a slightly longer bond. The local silver coordination environment is slightly distorted pseudo trigonal planar with Cshort-Ag-Cshort angles of 111.91, 118.67, and 128.81 (angle sum 359.39 ). The distortion from perfect planarity can be attributed to packing effects.

34

Silver Organometallics

Fig. 1 Structure of AgC6F5
CH3CN (1) showing a different coordination environment for Ag1 and Ag2.

Fig. 2 Cationic part of [Ag(toluene)3][B(C6F5)4] (2).

Moreover, Schulz and Villinger were able to solve the solid state structure of pure AgC6F5 (Fig. 3) and to find a straightforward protocol to form and obtain crystals of this species.28 A one-dimensional infinite AgdC zigzag chain is formed in the crystal with bent AgdCdAg0 (79.4 ) moieties and equal AgdC bond lengths of 2.195(3) A˚ . Each Ag+ ion is surrounded by two bridging C6F5 ligands with m2-bound carbon atoms in a linear arrangement as displayed by the C1dAg1dC10 angle of 180.0 . All C6F5 rings are found eclipsed to each other, and a straight chain of equidistantly arranged Ag+ ions with Ag ⋯ Ag distances of 2.8058(2) A˚ is recognized, indicating the presence of argentophilic interactions.

Fig. 3 Linear polymeric structure of AgC6F5.

Silver Organometallics

35

Fig. 4 Molecular structures of (AgC6F5)4
narene adducts with molecular units (Ag violet, C gray). 12 dm]1,2-dimethylbenzene, 13 dm ¼ 1,3-dimethylbenzene, 123tm ¼ 1,2,3-trimethylbenzene, 124tm ¼ 1,2,4-trimethylbenzene, 135tm ¼ 1,3,5-trimethylbenzene, 1235tem ¼ 1,2,3,5-tetramethylbenzene, 12345 pm ¼ 1,2,3,4,5-pentamethylbenzene. Reprinted (adapted) with permission from Ref. Ibad, M. F.; Schulz, A.; Villinger, A. Organometallics 2015, 34, 3893–3901. Copyright 2015 American Chemical Society.

By dissolving AgC6F5 at 80  C in different arenes, and allowing the solution to cool down to room temperature, it was possible to isolate different tetranuclear (AgC6F5)4
n(arene) adducts (n ¼ 1, 2 or 4). Upon coordination of arene molecules, no one-dimensional AgdC zigzag chains are formed but rather tetrameric (AgC6F5)4
n(arene) (n ¼ 1, 2 or 4) molecular units (Fig. 4). The coordination of arene molecules converts the infinite zig-zag chain into tetrameric units by means of weak interactions

36

Silver Organometallics

between the arenes and AgC6F5 moieties. AgdC p-complexation, p-stacking between the arene and C6F5 rings, and weak F ⋯ H hydrogen bonding interactions, which are observed in all considered, structurally highly flexible arene adducts, considerably stabilize the four-membered ring system over a chain-like structure as found in neat 1{AgC6F5}. A different type of AgC6F5 aggregation has been reported by Zhu and Fu, who described the preparation of two b-diketiminate germylene-supported pentafluorophenylsilver(I) complexes (Scheme 2).29

Scheme 2 Synthesis of b-diketiminate germylene-supported pentafluorophenylsilver(I) complexes 3 and 4.

Complexes 3 and 4 are sensitive to air but only a little to light. They are both thermally unstable. 3 decomposes at 156  C and 4 at 145  C. The X-ray structural analysis shows 3 as a monomeric compound with AgC6F5 terminally bound to LGeC(SiMe3) N2 (L ¼ HC[C(Me)N-2,6-iPr2C6H3]2). The structure of 4 shows an array of four silver centers bridged by C6F5 units and terminally bound to LGeC(SiMe3)N2. It is interesting to note that the two central silver centers of the array are not bridged by a C6F5 unit but simply involved in an unsupported argentophilic interactions (Ag ⋯ Ag 3.0627(7) A˚ ). The Ag − arene p-complexation plays an important role in silver(I) organometallic chemistry. In this regard, Shionoya et al. reported the preparation of a receptor for ditopic aromatic guests, based on a dinuclear silver(I) complex with a macrocyclic ligand with two phenanthroline metal binding sites (5, Scheme 3).30 The dinuclear Ag(I) complex forms a highly stable inclusion complex with ditopic guests, [2.2]paracyclophane (pCp) and ferrocene, through multipoint Ag–p interactions in the cavity. An interesting control experiment has been performed to estimate the contribution of Ag–p interactions to the stability of the inclusion complex. Dinuclear complexes with the same ligand and different metal centers have been prepared (M ¼ Hg(II), Zn(II) and Cu(I)) and tested as receptors for pCp. The results indicate very weak interactions between the non-silver hosts and the guest.

Scheme 3 Dinuclear silver(I) receptor 5 for aromatic guests, based on Ag-p interactions.

Another interesting system based on Ag–p interactions was reported by Tamaoki et al., who prepared an azobenzenonaphthalenophane ligand that undergoes Z-E photochemical isomerization, and its corresponding Ag(I) complexes E-6 and Z-6 (Scheme 4).31 Interestingly, in E-6, the Ag(I) ion has a rare coordination environment, which contains a nitrogen atom of an azobenzene and an Z2-naphthalene donor. Photo-isomerization of E-6 into Z-6 leads to the cleavage of the p-cation interaction, and vice versa.

Silver Organometallics

37

Scheme 4 Photochemical isomerization of Ag(I) complexes E-6 and Z-6 that switch on and off an Ag-Z2-naphtalene interaction.

An interesting synthetic route to silver(I) aryl complexes has been reported by Hoover et al., who described the synthesis of complex 7, by reacting AgF with 5,5-dimethyl-2-(2-nitrophenyl)-[1,3,2]dioxaborinane in CH3CN at room temperature (Scheme 5).32 The complex has been characterized by 1H, 19F and 13C NMR, IR spectroscopy and elemental analysis, but its structure has not been elucidated. Noticeably, this represents the first reported synthesis of a silver(I) aryl species that employs transmetallation from an arylboronic ester.

Scheme 5 Synthesis of complex 7 via a transmetallation of the arene ligand from an arylboronic ester.

More recently, Sadighi et al. have reported on the synthesis of dinuclear silver(I) complexes with terminal NHC (N-heterocyclic carbene) ligands and a bridging phenyl ring.33 In the structure of complex 8, the distance between the Ag(I) centers is of 2.8169(2) A˚ , indicating the presence of an argentophilic interaction. The complex has been prepared by treatment of the alkoxide-bridged precursor with a carbanion source (Scheme 6).

Scheme 6 Synthesis of the dinuclear Ag(I) complex 8, presenting a bridging phenyl group and two NHC ligands.

Phenyl rings, by acting as bridging ligands, are also useful in the preparation of heterodinuclear complexes, featuring Ag(I) and other metal centers, such as Au(I) (complex 9)34 and Pt(II) (complex 10).35 In the case of the Ag(I)⋯ Pt(II) analog, the interaction between the Ag(I) center and the other metal can be described as a Lewis acid-Lewis base interaction, with Ag(I) acting as an electron pair acceptor and the other metal as an electron pair donor (Scheme 7).

Scheme 7 PtdAg 9 and AudAg 10 heterobimetallic complexes, supported by bridging arene rings.

38

Silver Organometallics

9.02.2.1.3

Silver alkene complexes

Whereas silver aryl compounds can possess sufficient stability to be isolated and characterized, provided the aryl group coordinates in bridging position between two silver(I) centers, silver alkenyl compounds are instead less stable and very difficult to isolate. On the other hand, p complexes of silver(I) with alkenes are quite well known. The bonding of ethene and other olefins to silver(I) metal centers can be explained by the Dewar–Chatt–Duncanson model.36,37 In this model the ligand donates electron density from its HOMO p orbital to the metal and accepts electron density from the metal into its p LUMO. This back-bonding results in a lengthening of the C C bond distance and pyramidalization of the carbon atoms. In this context, the first silver–ethene complexes were prepared by Quinn and Glew in 1962,38 who reacted solid silver(I) tetrafluoroborate with gaseous ethene to obtain Ag(C2H4)+n salts (n ¼ 1–3). Experimentally, the coordination of an olefin to a silver(I) center can be observed by IR studies, with a shift in the C]C stretching vibration to lower frequencies compared to the uncoordinated olefin and by 1H NMR spectra, with characteristic downfield chemical shift of the coordinated alkene compared to the parent alkene. The synthesis of silver(I) complexes with alkene ligands has been extensively reviewed in 2011 by Burgess and Steel,39 hence we will address in the following section only the developments that have occurred during the last 10 years. In this period, the silver(I)-alkene interaction has emerged as particularly useful to assemble supramolecular systems. In particular, the rational use of the silver–alkene interaction can be used to prepare 1-, 2- and 3D supramolecular assemblies using readily synthesized bridging ligands containing multiple alkene subunits.39 Sundermeyer et al. reported the synthesis of [Ag(cod)2]NTf2 (11) and [Ag(C2H4)2(NTf2)] (12) by reacting AgNTf2 and the corresponding olefin.40 In the case of the 1,5-cod complex, the [NTf2]− ion acts as a weakly coordinating anion; by contrast, in the case of the ethylene complex, the [NTf2]− group serves as ligand, strongly bound through the nitrogen atom. In [Ag(cod)2]NTf2, silver(I) adopts tetrahedral coordination, while in [Ag(C2H4)2(NTf2)] it is essentially trigonal planar (Fig. 5). The same synthetic approach can be used to prepare [Ag2(Z2:Z2isoprene)(NTf2)2]1, that presents a two-dimensional polymeric solid state structure. Another heteroleptic silver(I) complex bearing an Z2-olefin ligand has been reported by Mao, Dias et al.41 They successfully isolated the silver(I) complex [CH2(3,5-(CH3)2Pz)2]Ag(CH2]CHBF3) (13) featuring a vinyltrifluoroborate. The X-ray crystal structure shows that the vinyltrifluoroborate moiety is bonded somewhat asymmetrically to the metal site with relatively shorter AgdC(H2) distance (Fig. 6). Ivanova et al. reported the synthesis and characterization of [Ag(Z2-C9H7NO2)3(NO3)] (C9H7NO2 ¼ 1H-indole-5-carboxylic acid) (14), obtained by mixing the indole derivative and AgNO3 (Fig. 7).42 Li and Zhang reported on the use of 5,11,17,23-tetra-allylcalix[4]arene as a ligand (L) to prepare two new Ag(I) organometallic complexes.43 The macrocyclic ligand has four allyl moieties and the obtained complexes differ in the observed Ag(I) to L ratio. Crystallographic studies showed that in L
2AgClO4
2(CH3)2CO
H2O (15) (Fig. 8), the calix[4]arene molecules are linked by Ag(I) ions to form wave-like organometallic chains via silver −alkene interactions, which are further bridged by acetone molecules to afford a 2D polymer. In L
4AgNO3
(CH3)2CO (16), the calix[4]arene molecules act as bridging ligands connecting inorganic AgNO3 layers directly to construct a porous 3D organometallic polymer.

Fig. 5 Structure of [Ag(cod)2]NTf2 (11) and [Ag(C2H4)2(NTf2)] (12) obtained by reacting AgNTf2 and the corresponding olefin.

Fig. 6 Structure of the silver(I) complex [CH2(3,5-(CH3)2Pz)2]Ag(CH2]CHBF3) (13) featuring a vinyltrifluoroborate ligand.

Silver Organometallics

39

Fig. 7 Structure of [Ag(Z2-C9H7NO2)3(NO3)] (C9H7NO2 ¼ 1H-indole-5-carboxylic acid) (14), obtained by mixing the indole derivative and AgNO3.

Fig. 8 The one-dimensional wave-like organometallic chain in L
2AgClO4
2-(CH3)2CO
H2O (15) (Ag violet, C gray). Reprinted (adapted) with permission from Ref. Shi, Q.; Luo, W.; Li, B.; Xie, Y.; Zhang, T. Cryst. Growth Des. 2016, 16, 493–498. Copyright 2016 American Chemical Society.

9.02.2.1.4

Silver alkyl complexes

The organometallic chemistry of silver(I) akyl complexes was, and still is, dominated by perfluoroalkyl derivatives, which provide more stable complexes with respect to conventional alkyl groups. Furthermore, perfluoroorganosilver compounds are also appealing reagents to be used in the development of new perfluoroalkylation protocols.44 Weng, Feng, Huang et al. synthesized the (bathophenanthroline)AgCF3 complex (17, Fig. 9),45 and demonstrated its involvement, via a cooperative effect, in the copper-catalyzed trifluoromethylation of activated and unactivated aryl iodides to trifluoromethylated arenes using Me3SiCF3. Shen and co-workers have reported on the synthesis of the NHC complexes (N-heterocyclic carbene) (SIPr)AgCHF2 (18) and (IPr)AgCHF2 (19), and used them as CHF2 source in a similar copper promoted difluoromethylation of diaryliodonium salts (Fig. 9).46 The authors extended their studies by using the (SIPr)AgCHF2 complex to transmetallate to a Pd(II) center both the NHC ligand and the CHF2 group, obtaining the air stable complex (SIPr)Pd(OAc)(CHF2).47 A silver(I) heptafluoroisopropyl complex with 2,2,6,6-tetramethylpiperidine (Htmp) ligand, has been reported by Baker et al., together with its Cu(I) and Ni(II) analogs (20, Fig. 9).48

Fig. 9 Structure of the Ag(I) perfluoroalkyl complexes 17–20.

40

Silver Organometallics

An alternative and intriguing silver(I) trifuoromethylating reagent has been reported by Temprano and coworkers.48 It consists of Cs[Ag(CF3)2] (21), but it can also be prepared with the [NBu4]+ cation. Combined with a Pd(dba)2/Xantphos (dba ¼ dibenzylideneacetone, Xantphos ¼ 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) catalytic system, the trifluoromethylation of iodobenzene can be achieved both in a stoichiometric and catalytic fashion. The synthesis of well defined and characterized [Ag(CF3)2]− anion has been reported also by Menjón et al., who described the synthesis of [PPh4][Ag(CF3)2] (22) by low-temperature treatment of Ag(OAcF) (AcF ¼ trifluoroacetyl) with CF3SiMe3 in the presence of CsF, followed by addition of [PPh4]Br in a subsequent step.49

9.02.2.2 9.02.2.2.1

Silver-carbene complexes Introduction

Silver complexes, in which at least one of the ligands is formally a nucleophilic carbene, are arguably the class of organometallic silver compounds which has experienced the most tumultuous development in the 21st century. The reason is essentially two-fold. First of all, the great success enjoyed by the use of these complexes as carbene transfer agents toward other metal centers, originally described in 1998,50 has led to the preparation of a plethora of silver-carbene compounds to be used as intermediates en route to the preparation of carbene complexes of several other metals. Further to this, the recognition, around the mid-2000s, of the antimicrobial properties of silver N-heterocyclic carbene (NHC) complexes,51,52 has promoted extensive investigations on the bioactivity of these compounds, testified by a very high number of original publications as well as several reviews, devoted completely or in part to this topic, that appeared in the course of the last 15 years.53–64 Silver-NHC complexes have been already treated in a section of the chapter devoted to silver organometallic compounds in Comprehensive Organometallic Chemistry III.4 Furthermore, the topic was quite comprehensively covered by extensive review articles appearing in the mid-2000s or shortly thereafter.65–68 Since these reviews were published, a very high number of new silver-carbene complexes have been reported for the reasons listed above. Literature search (September 2021) in the Web of Science with keywords “silver” and “carbene” returns 1679 entries from the year 2004 onwards, and search with “silver” and “NHC” still 1035 entries. However, in a number of cases the reported silver complexes have not been investigated and fully characterized (and sometimes not even isolated), since they were intended only for use as intermediates toward the targeted carbene complexes of other metal centers. Consequently, after presenting in general terms the topic we will focus in this section on the most notable developments of silver-carbene chemistry of the last 15 years, without attempting to produce a comprehensive listing of all complexes in this category that have been reported in this timeframe. Rather, we aim at informing the reader about the state of the art in the field, presenting the main types of compounds of this kind that have been developed, the employed synthetic methodologies, the structural peculiarities of the compounds and their fields of application. The first stable silver-carbene complex was a silver complex with the stable N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene originally prepared by Arduengo in 1993.69 The synthetic route involved the preparation of the free carbene and its subsequent coordination to a silver(I) center to yield a monocationic bis-NHC complex 23 (Scheme 8).

Scheme 8 First synthesis of a stable silver complex with a nucleophilic carbene.

This route, which implies the intermediacy of the reactive, air-sensitive and occasionally also unstable free carbene, has subsequently been largely supplanted by the metallation route (silver base route), in which the protonated carbene precursor is treated with a silver(I) precursor bearing an anion exhibiting basic character.50 The method is very general: it avoids the generation of the free carbene, hence it does not involve the use of strictly anhydrous, inert conditions, and does not require the use of external strong bases, which could cause deprotonation of the carbene precursors at sites different from the procarbenic carbon. Consequently, it has become the standard method for producing organometallic silver complexes of this kind, replacing not only the free carbene method, but also all other alternative methods proposed in the early 2000s, e.g. use of an external base in the presence of a silver salt, or metathesis from another metal-carbene complex.70 Originally, AgOAc, Ag2O or Ag2CO3 were all proposed as reagents for this purpose, but later on the oxide clearly stood out as the most convenient reagent for this synthesis, being also the most basic compound.71 The reactivity of silver(I) oxide in this context has been also computationally investigated;71 each formula unit of the oxide is able to deprotonate two carbene precursors, though the first deprotonation is thermodynamically more favored, and excess reagent is easily separated by simple filtration. The method fails only in rare cases, in which a) the carbene precursor features very low acidity; b) the carbene precursor bears other functional groups that can strongly coordinate silver(I); or c) the carbene precursor can be oxidized by silver(I) (e.g. in the case of a benzylic carbon as backbone substituent in the heterocycle).

Silver Organometallics

41

The metallation route potentially allows to access silver carbene complexes with two different stoichiometries, namely monocarbene neutral complexes of general formula (carbene)-Ag-X or bis-carbene, monocationic complexes of general formula [(carbene)2Ag]X.65–68 The preference for either kind of complex depends in first instance on the stoichiometric ratio between silver and carbene precursor, but also on the steric bulk of the carbene (bulky ligands obviously favoring monocarbene complexes) and on the nature of the azolium counteranion, or more generally of the counteranions present in the reaction mixture. More coordinating counteranions favor in principle the neutral monocarbene complexes, although there are exceptions, since in several instances the initially formed neutral complex undergoes a tautomerization to a monocationic one (Scheme 9).

Scheme 9 Neutral-cationic complex solution equilibrium in mono-NHC silver complexes.

Such a tautomerism is favored whenever an argentophilic interaction72 can be established between the two silver(I) centers, hence particularly with carbene ligands featuring a low steric bulk. Argentophilic interactions keep the silver centers at close distance and facilitate ligand metathesis between the two centers. The kinetics of this equilibration process have been followed by NMR spectroscopy and were indeed found to strongly depend on the steric bulk of the carbene ligand.73,74 In the case of halides as coordinating counteranions, additional and more complex species can form with respect to the two products presented in Scheme 9, particularly in the case of soft, heavier halides which have a greater affinity toward silver. For example, polynuclear anionic Ag-halogeno complexes of general formula [AgmXn](n-m)- can form as counteranion, halide bridged oligomeric or polymeric complexes or cluster compounds can be generated, and in some instances even the carbene can act as bridging ligand between two silver centers, potentially leading to cluster formation (Fig. 10). Generation of these complex species can be further supported by argentophilic interactions taking place intra- or intermolecularly or between the cationic and anionic part of the complex. The presence in the carbene ligand of pendant functional groups capable of coordination to silver may provide additional stabilization to these polynuclear compounds; this possibility has been extensively exploited to prepare selectively in one-step even very complex polynuclear compounds (see below). The possible existence of equilibria involving the generation of complex anions, polymers, and/or intermolecular interactions between cation and anion, often complicates the isolation, purification and characterization of these complexes. Consequently, halides or halide-containing anions are often exchanged with non-coordinating anions to facilitate these steps. Moreover, in order to avoid these complications, halide-free azolium salts are often employed as precursors, since they are also much less hygroscopic than azolium salts with halide as counteranion. However, avoiding coordinating anions results in complexes of general stoichiometry (carbene)AgX being poorly stable, hence under these conditions only the stoichiometry [(carbene)2Ag]+ X− is accessible in most instances. The fast equilibration between different species of silver complexes with stable carbenes in solution was recognized very early, and it illustrates very well the most relevant characteristic of these complexes, namely the relative lability of the silver-carbene bond. Lability is in turn the key feature for most applications of silver compounds of this kind: 1) it allows rapid transfer of the carbene to other metal centers capable of forming more stable bonds with the carbene;50 2) it ensures a slow but continuous release of ligand-free silver species in vivo for a long-lasting antimicrobial action;51–64 3) it grants the reversibility in the silver-carbene bond formation, which is necessary for the selective production of complex polynuclear architectures such as macrocycles and cage complexes (see below); 75 4) it provides the possibility of having different species in solution (silver carbene complexes, free silver ions, free carbenes) which are capable to act as catalysts, potentially even in concert, for given reactions.76 The equilibration between various forms of Ag-carbene complexes obviously influences the behavior of these complexes in all respects listed above. For example, carbene transfer capability appears to be dependent on the stoichiometry of the complexes: it was reported that complexes of stoichiometry (carbene)AgX with X ¼ halide could only transfer the carbene, whereas those with stoichiometry [(carbene)2Ag]X with X ¼ non-coordinating anion could also effect carbene-halide exchange.77 In the following subsections, we will highlight some novel classes of silver carbene complexes that have emerged over the last 15 years. We will not however perform a comprehensive listing of the very large number of mono- or dinuclear silver complexes with (bis)imidazole- or (bis)benzimidazol-2-ylidenes that have been prepared e.g. for medicinal applications, since the underlying organometallic chemistry is well established and is the same for all compounds. Virtually all such compounds are prepared from the corresponding azolium precursor by metalation with Ag2O, and share the same properties of the AgdC bond. Apart from the fact that they can be obtained as described above as neutral (NHC)AgX or cationic [(NHC)2Ag]+ complexes, they differ only in the nature of the substituents at the carbene heterocycle (wingtip nitrogen substituents and/or backbone substituents), and in the nature of the anionic ligands (for complexes of general stoichiometry ((NHC)AgX). The interested reader is referred for further details to the large body of review articles on this specific application of silver carbene complexes.53–64

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Silver Organometallics

Fig. 10 Possible speciation of silver complexes of general stoichiometry (carbene)AgX. Reproduced with permission from Ref. Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978–4008. Copyright 2005 American Chemical Society.

9.02.2.2.2

Novel carbenes as ligands toward silver

As it has been stated in the previous section, it is neither possible nor useful to present a comprehensive listing of all Ag complexes with stable carbenes that have been prepared over the last 15 years. In this sub-section, attention of the reader will be focused on the preparation of silver complexes with carbene ligands (isolated and characterized) beyond the more conventional imidazol2-ylidenes and benzimidazol-2-ylidenes. Ring-expanded NHC ligands have been very extensively investigated in view of their greater steric bulk and electron-donating properties. While NHC ligands with a 5- or 6-membered ring size and a saturated backbone were reported already in the very beginnings of the NHC era, extension to larger ring sizes proceeded more slowly. Silver complexes with cyclic carbenes featuring up to 7 carbon atoms have been reported already at the end of the 2000s independently by two groups, using the conventional synthetic strategy based on the metalation of azolium cations (Scheme 10).78,79

Scheme 10 Preparation of Ag complexes with ring-expanded N-heterocyclic carbenes.

Neutral mono- or cationic biscarbene complexes were prepared, depending on the steric bulk of the wingtip substituents at the carbene, larger substituents favoring as usual the formation of the monocarbene complex. Ring size also had its influence in determining the stoichiometry of the reaction product: increased ring size leads to an increase in steric bulk around the silver center, because the wingtip substituents stay closer to the metal. Consequently, starting from seven-membered rings the neutral monocarbene complex starts to predominate.

Silver Organometallics

43

In 2011 the group of Cavell extended this chemistry by preparing silver NHC complexes 24 and 25 with an eight-membered ring saturated heterocycle (diazocanylidene, Scheme 11), prepared in turn through conventional synthetic patterns.80 As expected, only the neutral carbene complex with stoichiometry (NHC)AgX was prepared. In 2020, the group of Hashmi added 9- and 10-membered rings to this family of ligands. In order to avoid ring folding, increased rigidity was enforced by inserting o-phenylene units into the ring structure. Again, only neutral carbene complexes with stoichiometry (NHC)AgX could be prepared (complexes 26–31, Fig. 11).81

Scheme 11 Formation of NHCdAg complexes with an eight-membered cyclic carbene.

Fig. 11 Ag complexes with 9- and 10-membered macrocyclic NHCs.

Another subclass of NHC ligands which was duly investigated during the last years regards compounds in which the carbene structure is part of a fused polycyclic ring system (Fig. 12). To this class belong also the so-called “Janus” type dicarbene ligands, i.e. compounds in which two carbene moieties are part of the same fused polycyclic ring, which allows electronic communications between metal centers bound to them.82 The group of Bielawski was a pioneer in this field; although the work of the group was mainly focused on the preparation of complexes of other transition metals, silver complexes such as 32 were prepared through the usual metalation route, isolated and briefly characterized, before being employed as usual as reagents in transmetallation reactions.83

Fig. 12 Ag complexes with carbenes derived from polycyclic structures.

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Silver Organometallics

More recent research on silver complexes with Janus-type dicarbene ligands has specifically targeted novel carbene moieties. For example, the group of Tapu reported the preparation of dinuclear silver complexes of type 34, exhibiting one to three 1,4-phenylenes as bridging units and six-membered NHC ligands featuring a remote anionic malonic functionality.84,85 These NHC ligands, originally disclosed by Lavigne and Cesar,86 are prepared as free carbenes by treatment of the azolium precursor with KHDMS and can substitute anionic ligands in metal complexes with relative ease, due to their negative charge. Consequently, they substitute chloride in a (Ph3P)AgCl substrate forming rather unusual heteroleptic, formally monocationic silver complexes at each end of the phenylene backbone, as in structure 34. The corresponding mononuclear silver complexes were also prepared, with a ligand system in which the bridging phenylene is replaced by a phenyl group.84 Additional recently reported examples of Janus-type dicarbene ligands include polymetallated heterocycles, as in complexes 35 and 36, developed by Huang and Zhang.87,88 In these structures, typically prepared upon deprotonation with KHMDS of the corresponding gold precursor, the carbon bound to silver can be alternatively viewed as a formal carbanion or as a carbene; experimental data such as the AgdC bond length, the chemical shift in the 13C NMR spectrum, as well as theoretical calculations, all point toward the latter as the most proper description of this ligand. Interestingly, some of the authors demonstrated previously that these peculiar NHC ligands can be generated upon silver-promoted intramolecular amidiniumation of alkynes; this reaction, when performed with a stoichiometric quantity of a silver salt, leads directly to the production of the silver NHC complexes 37 and 38 (Scheme 12); upon addition of triphenylphosphine before reaction, the corresponding mono-NHC silver complex can be also obtained, bearing the phosphine as second ligand.89

Scheme 12 Silver- promoted cyclization reaction directly yielding NHC-silver complexes.

Finally, simpler monodentate NHC ligands derived from a polycyclic ring system were also very recently investigated as ligands toward silver; structure 33 reports an example (Fig. 12).90 This peculiar complex represents a relatively rare case of a polycyclic NHC ligand featuring an unsaturated six-membered ring, the first altogether with a d10 metal center. Another family of carbene ligands enjoying great success in the course of the last 20 years are the so-called abnormal or mesoionic carbenes (i.e. carbenes for which no resonance structure without formal charge separations can be drawn).91 The most common examples include 1,2,3-substituted imidazol-4-ylidenes, and especially 1,3,4-substituted 1,2,3-triazol5-ylidenes, since the 1,4-substituted 1,2,3-triazole skeleton can be conveniently prepared by employing the popular Huisgens reaction (Cu-catalyzed azide-alkyne 3 + 2 cycloaddition).92 Several silver complexes with ligands of this kind have been reported in the course of the last 20 years, starting from the corresponding azolium salts and using the usual metalation route with Ag2O, but in most instances they have been directly employed for the preparation of complexes of other metals, without prior crystallization and structural characterization, and often even without isolation.93,94 The reason for this has been attributed to the relatively low stability of these silver complexes,92 although it has been reported that some complexes of this kind can be stable for months in the solid state.95 Nevertheless, the few silver complexes with these ligands that have indeed been fully characterized are rather peculiar and generally involve polycarbene ligands. In particular, in 2011 the structure of a tetranuclear silver complex with two chelating tetradentate ligands, constructed from two mesoionic carbenes and two pyrrolate moieties was reported (Fig. 13).96 The rather complex, toroidal structure of this compound was formed spontaneously in solution and represents a notable example of a silver-based supramolecular organometallic complex, a class of compound that will be dealt with more extensively in Section 9.02.2.2.4.

Silver Organometallics

45

Me2SO4 3N

2 N 1 N 4 NH

N

CH3CN

N N

+N N N

4 days, reflux

N

N +

2CH3SO4–

Ag2O / TBACI

N N

N

N

N

N

5 CH2Cl2 / CH3CN HN

NH

HN

Ag N

Ag Ag

Ag

N

39 X1H

C68

C44

X1G Ag8

Ag7 X1B

Ag1

N25 Ag4 Ag3

Ag5

Ag6

X1D N12 X1C

X1A N10

Ag2 X1E X1F

N11

Fig. 13 Structure of the first structurally characterized silver complex with a mesoionic carbene ligand. Adapted from Ref. Cai, J.; Yang, X.; Arumugam, K.; Bielawski, C. W.; Sessler, J. L. Organometallics 2011, 30, 5033–5037. Copyright 2011 American Chemical Society.

One year later, mono- and dinuclear biscarbene complexes of silver with mesoionic carbenes were reported and structurally characterized by the group of Crudden (Scheme 13).97 Steric shielding provided by the substituents evidently imparts stability to these compounds, which however can still be proficiently employed as carbene transfer agents toward ruthenium(II) metal centers. Although the preparation of silver complexes with mesoionic 1,3,4-substituted 1,2,3-triazol-5-ylidenes through the usual metalation route appears facile, limitations may impair the preparation of analogous complexes with other mesoionic carbenes. For example, not all precursors of these ligands are acidic enough to be deprotonated by silver oxide, and the instability of the free carbene renders it difficult to employ stronger bases for this purpose. Furthermore, imidazolium precursors substituted in the 2-position with alkyl groups tend to be oxidized by silver at the a carbon, leading to CdC bond cleavage and formation of a “normal” carbene ligand.98 On the other hand, the success enjoyed by complexes with 1,3,4-substituted 1,2,3-triazol-5-ylidenes triggered interest also in the isomeric, “normal” (i.e. non-mesoionic) 1,2,4-substituted 1,2,3-triazol-5-ylidenes, although azolium precursors for the latter carbenes are more difficult to prepare - the procedure implying a ring-closing reaction between isocyanides and hydrazonoyl chlorides. Silver complexes, among others, were prepared with these ligands and structurally characterized in 2013.99 Again, they were employed as carbene transfer agents to prepare other complexes of late transition metals to be employed as catalysts.

Scheme 13 Silver NHC complexes 40 and 41 with mesoionic carbenes prepared by Crudden. Adapted and reproduced with permission from Ref. Keske, E. C.; Zenkina, O. V.; Wang, R.; Crudden, C. M. Organometallics 2012, 31, 456–461. Copyright 2012 American Chemical Society.

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Silver Organometallics

Cyclic Amino Alkyl Carbenes (CAACs) are another class of NHCs which enjoyed considerable success in the course of the last years. Use of this class of ligands was pioneered by the group of Bertrand from 2015 onwards. Their use in connection with silver is however still very limited, possibly because of the stronger bond between these compounds and silver centers, which prevents their use as carbene transfer agents since transmetallation reactions are slowed down. Nevertheless, silver complexes with this type of ligands were reported in 2017 by the group of Bochmann (Scheme 14).100 The preparation of these compounds involved the generation of the free carbene in solution and its subsequent coordination to a silver(I) precursor. The fact that the usual metalation route with Ag2O is not efficient for the preparation of these compounds probably also contributes to the lack of examples of silver complexes with these ligands. Similarly to observations made in the case of the more conventional NHC ligands, both neutral monocarbene complexes of stoichiometry (CAAC)AgX and cationic biscarbene complexes of formula [(CAAC)2Ag]+ X− are accessible, although with bulky ligands the neutral monocarbene stoichiometry is preferred for steric reasons.

Scheme 14 Silver complexes 42–48 with CAAC ligands prepared by Bochmann. Adapted from Romanov, A. S.; Bochmann, M. J. Organomet. Chem. 2017, 847, 114–120, with permission.

Finally, it should be stressed that the stability imparted to silver complexes by carbene ligands also allows the preparation of robust NHC complexes bearing other reactive fragments bound to silver. For example, in 2013 the group of Sadighi reported on the isolation of a dinuclear NHC-silver complex with a bridging hydride ligand, the simplest silver hydride complex known at that time.101 Following this report, silver carbenes bearing additional reactive organic groups were prepared, such as, for example, dinuclear complexes with bridging phenyl groups or NHC silver difluoromethyl compounds that were discussed in Section 9.02.2.1.33,46 Another silver NHC complex with sterically very bulky wingtip substituents, complex 49, was able to coordinate an additional electrophilic carbene ligand to silver, to the point that it was possible to structurally characterize it (complex 50, Scheme 15).102 The product is nevertheless very reactive; analysis of the NMR and UV–vis data in solution for the compound clearly point toward a silver-singlet carbene complex with negligible back donation.

Scheme 15 Preparation of a silver NHC complex with an electrophilic carbene as ligand.

Remarkably, a similar non-heteroatom stabilized diarylcarbene coordinated to silver centers not featuring an NHC was more recently isolated and characterized by the group of Fürstner (complexes 51 and 52, Scheme 16).103 These compounds are stable only at low temperature, but could nevertheless be structurally characterized. The most interesting feature is that they are cluster compounds featuring three or four silver centers and bridging trifluoroacetate and carbene moieties.

Silver Organometallics

47

Scheme 16 Polynuclear silver carbene complexes prepared by Fürstner. Adapted and reproduced from Ref. Tskhovrebov, A. G.; Goddard, R.; Fürstner, A. Angew. Chem. Int. Ed. 2018, 57, 8089–8094. Copyright 2018 Wiley-VCH.

9.02.2.2.3

Polynuclear complex featuring halide bridges and/or argentophilic interactions

As has been stated in the general section, the reaction leading to the formation of silver carbene complexes may also give rise to polynuclear products, since two or more silver centers can become linked to one another through halide bridges and/or argentophilic interactions. Formation of such polynuclear compounds can be further aided by the use of polydentate carbene ligands, homo- and especially heteroleptic. Investigations of this synthetic possibility led to the design of ligands able to selectively promote the direct formation of complex polynuclear adducts, which apart from their conventional use as transmetallating reagents have been often screened in view of their intrinsic luminescence properties, since it is expected that an extensive network of argentophilic interactions could give rise to high quantum efficiencies, and to the possibility of fine-tuning the emission wavelength by changing the ligand properties. Nevertheless, even in cases in which emissive properties were observed, trasmetallation to other metal centers was still investigated, particularly toward the preparation of gold(I) complexes that share the same structure while being on average significantly more emissive. The most common complexes of this kind reported in the literature are dinuclear complexes featuring at least one NHC ligand with an additional coordinating group bound to one of the nitrogens, either directly or through a C1 spacer. These ligands, generally featuring a phosphorus or, more rarely, a nitrogen atom as additional coordinating group, form dinuclear silver complexes in which two heteroleptic ligands bridge the silver centers in anti fashion (i.e. each silver center binds to one carbene moiety) and in a fully extended conformation; in this way, the complex is further stabilized by an argentophilic interaction between silver centers (Fig. 14).104–115 The second coordinating group can also be placed more than three chemical bonds away from the carbene carbon, if the rigidity of the ligand forces proximity to the carbene coordination site, or if (on the other hand) the ligand backbone is flexible enough to fold and bring the silver centers at close distance, enabling the occurrence of an argentophilic interaction. In the latter case, this interaction, and the consequent emissive properties, can often be detected in the solid state but are lost in solution, when the complex unfolds and assumes a more extended conformation without argentophilic interactions. To this latter class belong several macrocyclic dinuclear complexes obtained with bis-NHC ligands, which will be discussed in the subsequent section. Finally, it should be remarked that in very rare cases homoleptic dicarbene complexes can also be formed, when condensed polycyclic dicarbene ligands such as in 58 are employed,113 as well as heteroleptic complexes such as 59 and 60, in which two carbene moieties belonging to two different ligands coordinate to the same silver atoms, while the side groups coordinate the second silver atom.114,115

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Silver Organometallics

Fig. 14 Examples of dinuclear complexes featuring at least one NHC ligand with an additional coordinating group.

Along with dinuclear structures, linear trinuclear structures were also reported to form by these means with symmetric heteroleptic tridentate ligands, in which the central NHC moiety coordinates one silver center and the peripheral phosphine ligands coordinate the other two silver centers; interestingly, transmetallation reactions employing these trinuclear complexes as reagents may lead to the formation of heteronuclear trinuclear complexes (Fig. 15).116,117 Trinuclear complexes with a triangular arrangement of silver centers were instead reported for the first time by Catalano in the early 2000s (Scheme 17)118,119; use of functional carbene ligands bearing two other potentially coordinating moieties, such as pyridyl groups, as wingtip substituents led to the isolation of trinuclear complexes besides mono- and dinuclear ones; the preferential triangular arrangement of silver centers was further supported by argentophilic interactions. Catalano subsequently extended his studies on these trinuclear complexes and investigated the relationships between the structure and substitution pattern of the ligand and the silver-silver distance, which is a key parameter with respect to the luminescence properties of the compound.120

Silver Organometallics

49

Fig. 15 Linear trinuclear Ag complex 61 stabilized by functional NHC ligands, used as a transmetallating agent to give heterometallic complexes.

Scheme 17 Preparation of trinuclear AgdNHC complexes with a triangular arrangement of the silver centers.

Similar trinuclear complexes with the same structure but different substituents on the imidazole backbone or at the pyridyl rings were subsequently reported by several other research groups.121–123 The groups of Chen and Meyer in the following years considerably extended this chemistry by involving more complex polycarbene ligands also bearing other coordinating moieties such as pyridyl, naphthyridyl and pyrazolate. In the context of silver organometallic chemistry, these ligands enabled the preparation of tri-, tetra- and hexanuclear complexes such as those reported in Fig. 16, which were all structurally characterized.124–127 The preferential formation of these complexes is clearly the consequence of the polydentate nature of the ligand but also of the extensive network of argentophilic interactions that can be established between the involved silver centers. Similarly, the group of Kühn investigated silver complexes with related tetradentate ligands of the type shown in Fig. 17, bearing bridges of different length, and were able to isolate and characterize both dinuclear silver complexes with two bridging ligand units or polymeric structures made of infinite strains of Ag3 units.128

50

Silver Organometallics

Fig. 16 Polynuclear silver complexes prepared in the groups of Chen and Meyer.

Fig. 17 Tetradentate ligands bearing bridges of different length, investigated by Kühn.

Silver Organometallics

51

Perhaps the best example of the degree of complexity that can be achieved by the use of polydentate ligand of this kind together with silver centers was provided by the group of Pöthig. They employed a macrocyclic bispyrazolato tetracarbene ligand to selectively prepare an octanuclear silver complex with each silver ion coordinated by a NHC and a pyrazolate unit (Scheme 18). The compound was then able to transmetallate to gold, while maintaining the overall structure.129

Scheme 18 Preparation of an octanuclear silver complex with a toroidal structure. Adapted and reproduced from Ref. Altmann, P. J.; Pöthig, A. J. Am. Chem. Soc. 2016, 138, 13171–13174. Copyright 2016 American Chemical Society.

Pöthig pursued further this chemistry by employing his octanuclear silver complex as the “wheel” in the construction of an organometallic rotaxane (Scheme 19).130 Remarkably, silver cations could be liberated from this supramolecular complex by addition of acid, resulting in a purely organic rotaxane with two “wheels” containing azolium cations. By neutralizing the acid with a suitable base, the silver cations could be brought back to deprotonate the azolium groups, restoring the complex and giving rise to a fully reversible system which was quite unprecedented in the context of NHC ligands.

Scheme 19 A rotaxane macroligand able to reversibly bind silver centers. Reproduced from Ref. Altmann, P. J.; Pöthig, A. Angew. Chem. Int. Ed. 2017, 56, 15733–15736. Copyright 2017 Wiley VCH.

Very recently, Chinese researchers reported on additional polynuclear complexes of this kind, prepared starting from polydentate ligands bearing pyridyl or pyrazole groups beside NHC ones (Scheme 20).131 Tri-tetra- and hexanuclear silver complexes could be prepared, in which however the argentophilic interactions involve either Ag2 diads or Ag3 triads. The novelty of this contribution is that these polynuclear complexes seem to show improved anticancer properties compared to simpler mono- and dinuclear complexes employed in the prior art. Thus, these complex assemblies may result in a further advance on the application of silver compounds in the medicinal field.

52

Silver Organometallics

Scheme 20 Polynuclear complexes developed by the group of Wang.

Another strategy for producing silver cluster compounds stabilized by carbene ligands is to exploit halide bridges instead or in addition to argentophilic interactions. As mentioned above, this possibility was recognized quite early, and indeed it was soon realized that a particularly preferred arrangement was the tetranuclear Ag4X4 structure. This structure was first reported by Braunstein using NHC ligands with a side arm bearing a phosphinite group (Scheme 21).132 The resulting complex exhibited a distorted heterocubane structure with the silver centers occupying opposing vertexes. The structure is stabilized by halide bridges and by two ligand units, each coordinating two silver centers in a bridging fashion.

Silver Organometallics

53

Scheme 21 NHC-stabilized heterocubane silver halide structure reported by Braunstein. Reproduced with permission from Ref. Raynal, M.; Liu, X.; Pattacini, R.; Vallée, C.; Olivier-Bourbigou, H.; Braunstein, P. Dalton Trans. 2009, 7288. Copyright 2009 Royal Society of Chemistry.

Shortly thereafter, Braunstein reported a stoichiometrically analogous compound exhibiting an alternative structure (Scheme 22).133 Using this time an NHC ligand bearing a thioether side group, the production of a tetranuclear complex with a less compact, square planar arrangement of the silver centers was favored. In this case, the two NHC ligands were still found in bridging positions between silver centers, whereas the halide ligands were inequivalent, two of these bridging two silver centers, with the other two interacting instead with three.

Scheme 22 NHC-stabilized square planar arrangement of an Ag4X4 unit.

In subsequent years, several other structures of this kind were reported by the group of Braunstein as well as by other groups, employing NHC ligands with side arms bearing phosphinite or phosphine groups, or even bis-carbene ligands featuring rigid bridges such as the m-phenylene unit.133–137 Remarkably, the group of Braunstein was also able to demonstrate that silver bromide heterocubane structures stabilized by NHC ligands with a side arm containing a phosphine group can be proficiently employed as substrates in partial transmetallation reactions, in which the NHC ligand was transferred to another metal center such as copper(I) or iridium(I), whereas the phosphine moiety remained bound to silver; in the former case, the heterocubane structure was maintained.138 Finally, another kind of tetranuclear, ladder like structure with NHC silver complexes bridged by thiolate or selenolate ligands was reported by the group of Corrigan (complexes 77 and 78). The tetranuclear structure was formed upon reaction of the corresponding mononuclear (NHC)AgOAc complex with PhESiMe3 (E ¼ S, Se) (Fig. 18).139

Fig. 18 Silver complexes bridged by thiolate or selenolate ligands.

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Silver Organometallics

9.02.2.2.4

Macrocyclic/cage complexes

A third class of compounds into which silver carbene complexes can be categorized involves polynuclear species that do not invariably feature interactions between silver centers (direct argentophilic interactions or halide bridges), but in which nevertheless the silver centers furnish the linkers that allow to form supramolecular structures. There is a certain overlap between this class of compounds and the previous one, as in particular some flexible organometallic macrocyclic complexes featuring two silver centers can fold and bring the silver centers at closer distances with one another. The ligands employed for this purpose are almost invariably homoleptic poly-NHC ligands; some examples of complexes of this kind were already provided in the context of the description of silver compounds with mesoionic carbene ligands in Section 9.02.2.2.2. The early recognition of the ease of synthesis and great variety of attainable silver complexes with nucleophilic carbenes, together with the quickly growing interest in polycarbene ligands, already led in the early 2000s to the preparation of numerous silver-carbene complexes having peculiar supramolecular structures. Early examples involved metalla-macrocyclic, dinuclear complexes such as 79, prepared through the free carbene route, which folds into a double helical structure with an AgdAg argentophilic interaction,140,141 as well as the bis-cyclophane and mono-macrocyclic dinuclear structures 80 and 81 reported by Tessier and Youngs using the metalation route (Fig. 19).142–144 Following these pioneering examples, several other complexes have been reported, differing in the stereoelectronic properties, the coordinating ability and the polarity (to ensure e.g. water solubility) of the substituents appended at the carbene moieties, and in the length, rigidity and functionality of the bridge between them.75 Furthermore, applications were envisaged beyond the use of these complexes as transmetallating reagents. Dinuclear complexes undergoing folding and establishment of argentophilic interactions were investigated for their emissive properties,145–149 whereas functionalized, open metallamacrocyclic compounds have been used as receptors for selected target molecules. An early example is represented by the dinuclear metallacalixarene structure 82 proposed by Zeng, Xu and Zhang, which was able to efficiently bind [60]fullerene.150 Metallamacrocyclic complexes containing one or two silver centers and featuring ether moieties and aryl groups as part of a sort of ‘metalla-crown ether’ structure (complexes 83–86) were later prepared and investigated by the group of Liu; these systems acted as efficient receptors for the selective detection by UV–Vis spectroscopy of neutral molecules, such as p-phenylenediamine, or anions, such as dihydrogen phosphate or hydrogen sulfate (Fig. 20).151–153 The complexity of this kind of compounds was considerably increased in the course of the last 15 years, taking advantage of the lability of the silver-carbene bond (enabling the equilibration of the reaction mixture favoring the most stable complex) and also of the possibility to exchange the silver(I) center in the final complex with other metal centers, most notably gold(I). The resulting products exhibit a degree of organization that justifies their categorization as supramolecular organometallic complexes (SOCs), i.e. organometallic analogs of the broader category of supramolecular coordination compounds, in which metal-carbon bonds constitute the linking unit between the complex constituents.75,154,155 The field has been very extensively reviewed in recent years,75,155,156 hence only the most significant examples of silver(I) complexes of this kind will be discussed herein.

Fig. 19 Examples of metallamacrocyclic dinuclear complexes.

Silver Organometallics

55

Fig. 20 Metallamacrocyclic complexes with carbene ligands featuring ether moieties and aryl groups in a sort of metallacrown ether structure.

Formation of metalla-macrocyclic dinuclear dicarbene silver complexes was employed for example by Han en route to the formation of more complex ligand structures (Scheme 23).157 The key feature of this strategy is the use of dicarbene ligands featuring wingtip substituents able to undergo a photoactivated 2 + 2 cyclization reaction. Formation of dinuclear dicarbene complexes of silver through the usual metalation route enables wingtip substituents belonging to different ligand units to come in close proximity and consequently to react, giving rise to a covalently linked dimer. Remarkably, the same strategy has been employed by Han also to produce tetracarbene ligand precursors with a cyclobutane core and four side arms bearing the azolium precursors (Scheme 24).158 Removal of the silver centers liberates the new macrocyclic pro-ligand bearing four pro-carbene groups as the product.

56

Silver Organometallics

Scheme 23 A strategy for the formation of macrocyclic tetracarbene ligands exploited by Han. Adapted and reproduced with permission from Ref. Zhang, L.; Han, Y.-F. Dalton Trans. 2018, 47, 4267–4272. Copyright 2018 Royal Society of Chemistry.

Scheme 24 Alternative strategy for the preparation of tetracarbene ligand precursors with the carbene in a peripheral position. Reproduced with permission from Ref. Han, Y.-F.; Jin, G.-X.; Hahn, F. E. J. Am. Chem. Soc. 2013, 135, 9263–9266. Copyright 2013 American Chemical Society.

Silver Organometallics

57

As will be apparent below, this synthetic strategy has been utilized by Hahn to produce even more complex ligand systems. Macrocyclic ligand precursors featuring four pro-carbenic units were also synthesized using more traditional means and investigated for their silver binding capabilities. Mono-, di- and even tetranuclear silver complexes, sometimes exhibiting argentophilic interactions could be prepared, depending on the ligand to silver stoichiometric ratio and also on the size of the macrocyclic ring, in particular on the length and rigidity of the spacers between the NHC units. For example, the ligand precursor shown in Fig. 21 exhibiting short bridges between the azolium units is unable to form dicarbene silver complexes with two consecutive NHC units (linear coordination would be not ensured), and is also unable to fold and produce a dinuclear complex with each silver center coordinating opposing NHC groups in the ring; consequently, it forms a tetranuclear complex with the macrocycle acting as a bridging ligand (complex 91, Fig. 21).159

Fig. 21 Tetranuclear complexes 91 and 92, formed with tetracarbene ligands featuring short bridges between the heterocyclic units.

Due to the constraints imposed by the ligand, the tetranuclear structure is distorted: the four silver centers are arranged in a rhomboidal rather than square planar shape and the coordination at each silver center significantly deviates from linearity (average CdAgdC angle 166 ). Interestingly, the group of Kühn reported analogous silver complexes with an open chain tetracarbene ligand having the same structure (N-methyl terminated, complex 92 in Fig. 21). Despite the lack of a macrocyclic structure, the complex was still able to organize itself by folding into a ladder-like structure stabilized by argentophilic interactions.160 A ligand precursor, which is slightly more flexible thanks to the substitution of two methylene bridges with ethylene ones, can instead act as a chelating ligand to form a mononuclear complex, or as a chelating and bridging ligand to produce a tetranuclear system (Scheme 25).161 In the tetranuclear complex, the chelating and bridging carbene groups in each ligand are inequivalent, as it can be determined by e.g. 13C NMR spectroscopy, since two signals with different chemical shifts are recorded for the carbene carbon.

58

Silver Organometallics

Scheme 25 Mono- and tetranuclear silver complexes derived from a macrocyclic tetracarbene precursor.

Interestingly, the complexes were produced in this case not through the classical metallation route with Ag2O but rather using a simple silver salt and an external base such as triethylamine. A further increase of the flexibility of the ring, for example by employing 1,3-propylene bridges, yields ligand precursors able to fold and produce dinuclear silver complexes with stoichiometry [LAg2]2+ in which each silver center coordinates two opposing NHC groups in the ring (complex 96, Fig. 22).162,163 By further increasing the length of some of the alkyl linkers, additional polynuclear structures can be obtained, in which one silver center is still coordinated in a chelating fashion by two opposite NHC moieties, whereas the other two coordinate to additional silver centers resulting in structures of stoichiometry [LAg3]3+ or even [L3Ag6]6+.163 Lower nuclearity structures are instead obtained if the linker contains additional coordinating groups such as amine nitrogens, since in this case the amine competes for coordination at silver.164 Using even longer, yet more rigid linkers based on phenylene, xylylene or pyridyl groups, tetranuclear cage complexes can be obtained selectively (complexes 97 and 98, Fig. 23).165,166 However, in cases in which the macrocycle contains o-xylylene,

Fig. 22 A dinuclear complex with tetracarbene ligands having 1,3-propylene bridges.

Silver Organometallics

59

Fig. 23 Tetranuclear complexes with tetracarbene ligands having rigid linkers.

m-xylylene or 2,6-lutidinyl linkers, chelating coordination to silver by the two NHC moieties adjacent to the linker becomes competitive, and consequently complexes with different nuclearity can again be obtained, depending on the ligand to silver ratio, on the reaction conditions and on the degree of flexibility of the ligand (Scheme 26).167,168

Scheme 26 Formation of complexes of different nuclearity from the same macrocyclic poly-NHC ligand. Reproduced with permission from Refs. Hahn, F. E.; Radloff, C.; Pape, T.; Hepp, A. Chem. - Eur. J. 2008, 14, 10900–10904 and Schulte to Brinke, C.; Pape, T.; Hahn, F. E. Dalton Trans. 2013, 42, 7330–7337. Copyright 2008 Wiley-VCH; copyright 2013 Royal Sciety of Chemistry.

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Silver Organometallics

Finally, a tetrazolium ligand precursor with the azolium groups incorporated in a bicyclic system (Fig. 24), exclusively yielded a dinuclear silver complex by folding into a highly symmetric boat conformation, complex 104. Interestingly, this complex generated much higher antimicrobial activity compared to a mononuclear NHCdAg complex, a symmetric dinuclear Ag complex with dicarbene ligands incorporated in a related monocyclic system and also a twisted dinuclear complex such as 105, deriving from flexible, open chain dicarbene ligands.169 Apparently, both the symmetrical chelating structure and the presence of argentophilic interactions impart exceptional stability to complex 104, which consequently may exert its microbial action for a longer time by slowly releasing silver species. Di- as well as tetranuclear silver complexes could be prepared also starting from a tetrazolium precursor in which a central benzene ring is decorated with azolium groups in 1,2,4,5-position (Fig. 25). Ligand precursors featuring a methylene spacer between the benzene ring and the azolium units were more flexible and furnished both the tetranuclear complex 106 and the corresponding dinuclear complex in which a single ligand unit coordinates two silver centers with the NHC groups in 1,3- and 2,5-positions, respectively; the selectivity toward the di- or tetranuclear complex was found to depend on the nature of the employed wingtip substituents.170 Ligands featuring a direct bond between the benzene ring and the azolium precursors are instead more rigid and furnished the tetranuclear complex 107 exclusively; in these complexes, the four silver centers are arranged in a rectangular fashion with Ag ⋯ Ag distances of about 3.4 and 5.9 A˚ .171 Similar chemistry was possible with ligand precursors containing three pro-carbenic units. Early examples of trinuclear cage-like compounds were reported by the group of Mayer as well as by our group using different kinds of tripodal tricarbenes (Fig. 26).172,173 Subsequent research employed ligands sharing the basic tripodal structure of the tricarbenes outlined above, but with a trivalent central group (1,3,5-substituted benzene, adamantyl etc.) imparting to the ligand a planar or slightly bent shape; in the former case, cylindrical adducts were produced when two ligand units were complexed by three Ag centers bridging the ligands, whereas more

Fig. 24 Folding of poly-NHC ligand precursors to form dinuclear Ag complexes.

Fig. 25 Tetranuclear cage complexes formed from tetrasubstituted benzene ligands.

Silver Organometallics

61

Fig. 26 Trinuclear cage compound prepared from tricarbene ligand precursors.

complex, twisted structures were produced in the latter case.171,174–178 Furthermore, spacers were often introduced between the central group and the peripheral pro-carbene units, in order to further tailor the size of the resulting cage. Homoleptic trinuclear complexes with two tricarbene ligands of the same type were formed even if mixtures of 2–3 different proligands were placed in the reaction mixture; thus, a self-sorting mechanism seems to be operative and to direct complex formation.176 Additional variation on this structural theme was provided by ligands in which the NHC units were substituted with mesoionic carbenes,179 or by the use of a different ligand structure, in which the three coordinating NHC moieties are part of a single macro-heterocycle180; in the latter case, the structure was also chiral, since the NHC skeleton was prepared from enantiomerically pure cis-diaminocyclohexane. Finally, the group of Han succeeded in extending to trinuclear complexes their 2 + 2 cycloaddition approach to the preparation of

62

Silver Organometallics

more complex macrocyclic/cage like poly-NHC ligands (Scheme 27).181 Post-reaction, the new poly-cyclic pro-ligand could be liberated by acid hydrolysis, giving rise to a surprisingly simple reaction sequence for the preparation of these compounds. Additional examples of this synthetic strategy for the preparation of complex pro-ligands were very recently published by Hahn, Peris et al.182

Scheme 27 Strategy for preparing poly-cyclic trinuclear cage compounds by the covalent linking of tris-NHC ligands. Reprinted (adapted) with permission from Ref. Sun, L.-Y.; Sinha, N.; Yan, T.; Wang, Y.-S.; Tan, T. T. Y.; Yu, L.; Han, Y.-F.; Hahn, F. E. Angew. Chem. Int. Ed. 2018, 57, 5161–5165. Copyright 2018 Wiley-VCH.

Similar synthetic strategies were employed to prepare hexanuclear cage compounds of silver starting from a hexasubstituted benzene precursor183 or from macrocyclic compounds incorporating six azolium groups (Fig. 27).167

Silver Organometallics

63

Fig. 27 Strategies for the preparation of hexanuclear cage compounds of silver.

9.02.2.3 9.02.2.3.1

Silver complexes with other neutral carbon ligands Silver isocyanide complexes

The coordination chemistry of silver with isonitrile ligands has not enjoyed in the course of the last 15 years a development comparable to silver carbene complexes. Some new compounds were reported, however, in which simple isonitriles have been employed as ancillary ligands without exerting other specific functions.184,185 Homoleptic silver complexes with novel isocyanide ligands such as 1,10 -ferrocenyl diisocyanide were also disclosed but no further investigation concerning their properties was reported.186 The most relevant contributions came from the group of Espinet. For example, this group prepared both neutral and cationic silver complexes with a novel isonitrile ligand including a crown ether (Scheme 28) and preliminarily studied their behavior in the recognition of alkaline metal cations in solution.187

Scheme 28 Synthesis of silver complexes with a novel isonitrile ligand including a crown ether, used in recognition of alkaline metal cations in solution.

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Silver Organometallics

Fig. 28 Cationic silver(I) complexes with isocyanides bearing a hexasubstituted triphenylene group.

Further to this, the group of Espinet also extensively investigated silver complexes with functionalized arylisocyanides in view of their ability to build ordered aggregates leading to the formation of mesomorphic materials. In particular, the preparation of neutral or monocationic silver complexes with arylisocyanides bearing a long chain n-nonylanilido group as the para substituent enabled the production of smectic liquid crystalline phases.188 On the other hand, homoleptic, cationic silver(I) complexes with isocyanides bearing a hexasubstituted triphenylene group did not exhibit liquid crystalline properties since the steric repulsions between the substituted triphenylene units caused a distortion from planarity of the resulting complex (Fig. 28).189 Finally, Espinet also reported on the formation of supramolecular coordination polymers of silver with o-phenylene diisocyanide and with 2-isocyanopyridine. In both cases the resulting polymers were crystallographically characterized but their properties were not further investigated.190

9.02.2.3.2

Silver carbonyl complexes

Carbon monoxide does not form many complexes with silver(I) metal centers, since the silver-carbonyl interaction is notoriously weak due to the low tendency of silver to act as a s-acceptor and as a p-donor. In fact, the few silver carbonyl complexes known before 2006 all exhibited carbonyl stretching frequencies that were higher than in free CO, and are consequently to be categorized as non-classical metal carbonyl compounds. Several of these complexes present tris-pyrazolyl- or tris-triazolylborates as supporting ligands,191 extensively functionalized with electron-withdrawing groups, such as fluorides or perfluoroalkyl moieties.192,193 In 2007, though, the group of Dias succeeded in isolating the first classic silver carbonyl compound 129 (Fig. 29). The strategy employed was to prepare first the corresponding ethylene adduct, one of the rather few isolated examples of stable silver complexes with this ligand (Section 9.02.2.1.3),191 and then perform the substitution in hexane solution under a carbon monoxide atmosphere. The carbonyl compound was stable for several weeks when kept in a sealed dark vial at room temperature and exhibited a carbonyl stretching frequency of 2125 cm−1, significantly below the stretching frequency of free carbon monoxide (2143 cm−1). The factor conferring sufficient stability to both complexes is probably the protection imparted by the bulky mesityl substituents, which allow to avoid the use of strongly electron-withdrawing substituents on the tris-pyrazolate rings (Fig. 30).194 More recently, a novel kind of polynuclear heteroleptic silver carbonyl compound was serendipitously synthesized. In an attempt to oxidize Fe(CO)5 with a silver salt having a tetrakis-pefluoroalkoxyaluminate counteranion, the trinuclear monocationic compound 130 was instead obtained.195 The complex features two intermetallic bonds and also multicentric interactions involving to some extent silver, the iron centers and some of the equatorial carbonyl groups; indeed, the latter are slightly bent toward silver in the reported structure of the compound. The carbonyls give rise to several IR signals, one of which falls at slightly higher wavenumber (2150 cm−1) than the stretching of free carbon monoxide; the complex could consequently be regarded as a non-classical metal carbonyl, as well. The authors subsequently prepared and characterized complexes of similar stoichiometry and characteristics bearing as metal carbonyl metallo-ligand toward silver both M(CO)6 (M ¼ group 6 metal) or [M(CO)5]− (M ¼ group 5 metal) moieties.196

Fig. 29 First classic silver carbonyl compound 129 isolated by Dias in 2007.

Silver Organometallics

65

Fig. 30 Crystal structure of complex 130.

9.02.2.4

Alkynyl complexes of silver

Homoleptic silver alkynyl species of general formula [Ag(C^CdR)]1 are reported to be polymeric and insoluble species, which simply form by reaction of the alkyne and a silver salt (such as silver(I) nitrate, acetate, trifluoroacetate, tosylate, triflate) in a solvent (as acetonitrile, dimethylsulfoxide) in the presence of a base, usually triethylamine. In most cases, the insolubility of these species prevents the formation of single crystals suitable for X-ray crystallographic studies.

9.02.2.4.1

Extended polymeric structures

Crystalline alkynylsilver(I) complexes can be accessed by a crystallization reaction of the polymeric material in the presence of ancillary strong neutral s-donor ligands such as amines or phosphines or in the presence of weakly coordinating anions (WCAs), such as nitrate, phosphates, acetate, trifluoroacetate, triflate etc. The obtained silver(I)-organic architectures can be characterized by X-ray diffraction analysis and range from discrete molecules to large clusters as well as to 1D, 2D and 3D metal organic frameworks. These structures are mainly stabilized by argentophilic interactions and by the coordination of the alkynyl moiety, through s, p or mixed s,p interactions. The silver-ethynide bonds can be in fact be classified into the above cited three types: s, p or mixed (s,p) interactions, whose overall crystallographic features are reported in Fig. 31. The s interaction between a silver ion and the terminal and negatively charged carbon atom C1 is characterized by an approximately flat C2dC1dAg angle and by a short C1dAg bond distance. The p interaction involves a silver center and the electron density of the triple bond; consequently, the C2dC1dAg angle is lower than 90 and the AgdC distances are longer than those observed in the s coordination. The mixed (s,p) interactions lie in between these extreme situations and are associated with C2dC1dAg angles y in the range 90–160 . Looking at the crystal structures, each RdC^C− group can simultaneously interact with 3–5 silver(I) centers and these units are often symbolized as RdC^C Agn (n ¼ 3–5, R ¼ alkyl, aryl); these units are the building blocks of the extended architectures cited above, since they can be connected in the crystal by bridging ligands, and for this reason the RdC^C Agn subunits have been defined as multinuclear metal-ligand supramolecular synthons. Other interactions can further contribute to the stability of the extended structures, although they strongly depend on the type of R substituent of the (C^CdR) moiety, since they involve for example p interaction of the aromatic substituents of the alkyne with the silver atoms or halogen bonds between the halogen atoms present on the organic fragments. The group of T. Mak has intensively and systematically studied this class of alkynylsilver(I) species and published reviews in this regard in 2014197 and 2015.198 The monodentate terminal alkynes reported in Figs. 32 and 34, as well as the bidentate ones reported in Fig. 36 have been studied. In Table 1, the structures reported in the last 15 years are summarized with particular emphasis on the silver(I) coordination mode(s) in the different RdC^C  Agn units present and the supramolecular architecture is briefly described. The most common RdC^C Agn units involve n ¼ 3, 4 and 5, and the possible coordination modes of the ethynide moieties are depicted in Fig. 33; such baskets can be then mutually connected in a higher dimensional framework, via edge or vertex sharing, and/or by bridging anions or weak interactions.

Fig. 31 General crystallographic features of the interactions between a silver center and an ethynide moiety.

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Silver Organometallics

Fig. 32 Monodentate terminal alkynes studied in the reaction between the corresponding [Ag(C^C-R)]1 systems and different WCAs and neutral ligands. Table 1 Summary of the alkynyl silver(I) structures with particular emphasis on the RdC^C− coordination to the silver(I) centers and brief description of the supramolecular architecture. Structure formula

Ethynide-silver(I) coordination 1

2

2

2

AgL1 ∙3AgCF3COO (131)

L1  Ag4 m4-Z , Z , Z , Z

AgL2 ∙6AgCF3COO ∙8H2O (132)

L2  Ag5 m5-Z1, Z1, Z2, Z2, Z2

2AgL2∙ 4AgCF3COO ∙H2O ∙CH3CN (133)

L2  Ag5 m5-Z1, Z1, Z2, Z2, Z2 and m5-Z1, Z1, Z1, Z2, Z2

2AgL2∙ 4AgCF3COO ∙H2O (134)

L2  Ag4 m4-Z1, Z2, Z2, Z2

Brief description of the structure with emphasis on silver

References

The butterfly-shaped Ag4 baskets are connected by bridging trifluoroacetate groups, giving an infinite chain. The chains are interconnected by trifluoracetate groups and silver(I)-aromatic interactions. Two square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups and via silver(I)-aromatic interactions to give an infinite ladder type chain. The two square-pyramidal Ag5 baskets are fused in an Ag8 aggregate via edge sharing and stabilized also by argentophilic interactions. The Ag8 aggregates are further connected by vertex sharing to give an infinite silver(I) column. A 3D supramolecular structure is formed from the columns via silver(I)-aromatic, p-p and lone pair-p interactions. The two Ag4 baskets are fused in an Ag7 aggregate via vertex sharing and stabilized also by argentophilic interactions. The Ag7 aggregates are further connected by another vertex sharing to give a silver(I) column. The columns are linked via trifluoroacetate bridging groups and p-p interactions.

199

199

200

200

Silver Organometallics

Table 1

67

(Continued)

Structure formula

Ethynide-silver(I) coordination 1

1

1

2

2

AgL2 ∙8AgC2F5COO∙ 2H2O∙ 2CH3CN (135)

L2  Ag5 m5-Z , Z , Z , Z , Z

AgL2 ∙6AgC2F5COO∙ 4H2O (136)

L2  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL3 ∙5AgCF3COO ∙H2O (137)

L3  Ag5 m5-Z1, Z1, Z2, Z2, Z2

AgL4 ∙5AgCF3COO ∙2H2O (138)

L4  Ag5 m5-Z1, Z1, Z2, Z2, Z2

AgL4 ∙7AgCF3COO ∙3H2O ∙CH3CN (139)

L4  Ag5 m5-Z1, Z1, Z2, Z2, Z2

AgL5 ∙6AgCF3COO ∙3.25H2O (140)

L5  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL6 ∙2AgCF3COO (141)

L6  Ag4 m4-Z1, Z1, Z2, Z2

AgL6 ∙5AgCF3COO ∙H2O ∙CH3CN (142)

L6  Ag5 m5-Z1, Z2, Z2, Z2, Z2

4AgL6∙ 2AgNO3 (143)

L6  Ag4 m4-Z1, Z1, Z2, Z2 and L6  Ag3 m3-Z1, Z2, Z2

AgL7 ∙4AgCF3COO (144)

L7  Ag5 m5-Z1, Z1, Z2, Z2, Z2

2AgL7∙ 3AgCF3COO (145)

L7  Ag4 m4-Z1, Z1, Z2, Z2 and L7  Ag3 m3-Z1, Z2, Z2

AgL8 ∙5AgCF3COO ∙3H2O ∙(146)

L8  Ag5 m5-Z1, Z1, Z2, Z2, Z2

2AgL8∙ 11AgCF3COO∙ 6H2O∙(147)

L8  Ag5 m5-Z1, Z1, Z2, Z2, Z2

AgL9 ∙3AgCF3COO ∙H2O ∙(148)

L9  Ag4 m4-Z1, Z1, Z1, Z2

AgL10∙ 4AgCF3COO ∙2CH3CN∙(149)

L10  Ag5 m5-Z1, Z1, Z2, Z2, Z2

Brief description of the structure with emphasis on silver

References

The square-pyramidal Ag5 baskets are connected by bridging pentafluoropropionate groups giving a 2D network. The layers are connected by silver(I)-aromatic interactions. The square-pyramidal Ag5 baskets are connected by bridging pentafluoropropionate groups giving a 2D network. The layers are connected by silver(I)-aromatic interactions. Two square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups to give an infinite chain. The chains are interconnected by trifluoracetate groups, argentophilic and silver(I)-aromatic interactions. The square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups to give a zig-zag infinite chain. A 3D supramolecular structure forms by Ag(I)-p interaction and by p-p stacking between adjacent naphthyl rings. The square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups to give a zig-zag infinite chain. A 3D supramolecular structure forms by Ag(I)-p interaction and by p-p stacking between adjacent naphthyl rings. The square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups to give a zig-zag infinite chain. A 3D supramolecular structure forms by Ag(I)-p aromatic interaction and by p-p stacking between adjacent naphthyl rings. Two Ag4 are fused in an Ag6 aggregate via edge sharing. The Ag6 aggregates are further connected by argentophilic interactions and bridging trifluoroacetate groups to give a silver(I) column. The columns are interconnected via p-p interactions. The square-pyramidal Ag5 baskets are connected to an external silver(I) center by bridging trifluoroacetate groups and argentophilic interactions; two adjacent aggregates of this type are connected in a sandwich-type structure by silver(I)-aromatic interactions involving the two aromatic rings of the ligand and the external silver centers. Two butterfly-shaped Ag4 baskets and three trigonal Ag3 give aggregates which are further connected by Ag sharing to give infinite silver columns. The columns are connected by silver centers and symmetry related operations, to give chains, linked together by argentophilic interactions. The two square-pyramidal Ag5 baskets are fused in an Ag8 aggregate via edge sharing. The Ag8 aggregates are further connected by trifluoroacetate groups and via silver(I)-aromatic interactions. The Ag4 basket is fused with the Ag3 plane via vertex sharing, giving an Ag6 aggregate. Edge sharing and symmetry operation give a silver(I) column, further associated via p stacking. The square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups to give infinite columns, which are interconnected in a layer by trifluoroacetate groups. A 3D structure is formed by p-p stacking between phenyl groups of different layers. The square-pyramidal Ag5 baskets are connected by bridging trifluoroacetate groups to give infinite columns, which are interconnected in a layer by trifluoroacetate groups. A 3D structure is formed by argentophilic interactions and trifluoroacetate coordination. The Ag4 baskets are connected by bridging trifluoroacetate groups to give infinite columns, which are interconnected in a layer by silver(I)-chloro interactions. A 3D structure is formed by p-p stacking between phenyl groups of different layers. The Ag5 baskets share and edge giving an Ag8 aggregate, which are linked together by bridging trifluoroacetate groups to give infinite columns. The columns are interconnected in a 2D structure by p-p stacking.

200

200

199

201

201

201

202

202

202

202

202

203

203

203

203

(Continued )

68

Table 1

Silver Organometallics

(Continued)

Structure formula

Ethynide-silver(I) coordination 1

1

2

2

2

AgL10∙ 7AgCF3COO ∙2H2O ∙2CH3CN∙ (150)

L10  Ag5 m5-Z , Z , Z , Z , Z

AgL11∙ 6AgCF3COO ∙4H2O (151)

L11  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL11∙ 4AgCF3COO ∙H2O CH3CN∙ (152)

L11  Ag4 m4-Z1, Z1, Z1, Z2

AgL11∙ 3AgC2F5COO∙ CH3CN∙(153)

L11  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL12∙ 5AgC2F5COO∙ 3.5H2O (154)

L12  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL12∙ 4AgCF3COO ∙H2O ∙C2H5CN∙ (155)

L12  Ag4 m4-Z1, Z1, Z1, Z2

AgL13∙ AgNO3 (156)

L13  Ag4 m4-Z1, Z1, Z2, Z2 and m4-Z1, Z2, Z2, Z2

2AgL13∙ AgNO3 (157)

L13  Ag3 m3-Z1, Z1, Z2 and m3-Z1, Z2, Z2 L13  Ag3 m3-Z1, Z1, Z1 and m3-Z1, Z2, Z2 and L13  Ag4 m4-Z1, Z1, Z1, Z2

[Ag5(L13)4(DMSO)2]X (X ¼ BF4 (158), ClO4 (159), PF6 (160), AsF6 (161), SbF6 (162)) 2AgL13∙ 5AgCF3COO∙ 4DMSO (163)

L13  Ag4 m4-Z1, Z1, Z1, Z2 and L13  Ag5 m5-Z1, Z1, Z1, Z1, Z2

10AgL13∙ 2AgOTf∙ AgNO3 ∙ 3DMSO (164)

L13  Ag3 m3-Z1, Z1, Z1 and m3-Z1, Z1, Z2 and m3-Z1, Z2, Z2 L14  Ag5 m5-Z1, Z1, Z1, Z1, Z2

AgL14∙ 3AgCF3COO ∙3H2O (165)

AgL14∙ 2.5AgCF3COO ∙1.5 DMSO (166)

L14  Ag4 m4-Z1, Z1, Z2, Z2 and m4-Z1, Z2, Z2, Z2 L1  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL15∙ 3AgCF3COO ∙2H2O∙ MeCN (167)

L15  Ag5 m5-Z1, Z1, Z1, Z2, Z2

AgL15∙ 2.5AgCF3COO ∙1.5DMSO (168)

L15  Ag4 m4-Z1, Z1, Z2, Z2 and m4-Z1, Z2, Z2, Z2 and L15  Ag5 m5-Z1, Z1, Z1, Z1, Z2 L16  Ag4 m4-Z1, Z1, Z2, Z2

2(AgL16)∙6AgCF3COO (169) 2AgL17∙ 6AgCF3COO∙ 4H2O∙ 2MeOH (170)

L17  Ag5 m5-Z1, Z1, Z2, Z2

2AgL18∙ 3AgNO3 (171)

L18  Ag4 m4-Z1, Z1, Z2, Z2 and L18  Ag3 m3-Z1, Z1, Z2

Brief description of the structure with emphasis on silver

References

The Ag5 baskets are connected to three external silver(I) centers by bridging trifluoroacetate groups and water molecules, producing zig-zag chains. A 3D structure is formed hydrogen bonds involving the water molecules and the fluoro atoms. The Ag5 baskets are connected by a symmetry operation related silver(I)-bromo interactions. A 3D structure is formed by hydrogen bonding involving the water molecules and by bridging trifluoroacetate ligands. The Ag4 baskets are connected by a symmetry operation related silver(I)-bromo interactions. A 3D structure is formed by hydrogen bonding involving the water molecules and by bridging trifluoroacetate ligands. Two adjacent Ag5 baskets share one edge giving an Ag8 aggregate, characterized also by argentophilic interactions. The aggregates are linked by bridging pentafluoropropionate ligands, giving an infinite chain. Adjacent chains are associated by silver(I)-bromo interactions. The Ag5 baskets are consolidated pentafluoropropionate bridging ligands and connected together by a symmetry operation related silver(I)-iodo interactions. A 3D structure is formed by bridging carboxylate ligands, by hydrogen bonding involving the water molecules and by p-p stacking. The Ag4 baskets are connected by a symmetry operation related silver(I)-iodo interactions. A 3D structure is formed by hydrogen bonding involving the water molecules and by bridging trifluoroacetate ligands. Three Ag4 baskets sharing edge and vertex to form an Ag9 aggregate, further connected to form a layer, stabilized also by argentophilic interactions. Layers are linked by p-p stacking. The silver atoms are linked via argentophilic interactions to form layers, linked by bridging nitrate anions and p-p stacking. The silver atoms form a Ag8 rhombic prisms, which form an infinite column by sharing a vertex. The 3D structure consists in hexagonal packing of columns and the type of the interactions involved in the packing depends on the anion X. Two Ag4 baskets and two Ag5 ones are connected to form higher nuclearity aggregates, which are connected in the 3D structure by trifluoroacetate and DMSO bridging groups, p-p stacking and hydrogen bonds. Silver(I) coordination column are stabilized by triflate and nitrate bridging groups and p-p stacking. The columns are interconnected in a hexagonal array by hydrogen bonding. Two square-pyramidal Ag5 baskets are fused though edge sharing. The Ag8 aggregates are interconnected via silver-aromatic interaction to give an infinite silver-organic chain. The two different butterfly-shaped Ag4 baskets and the two square-pyramidal Ag5 baskets are fused in an Ag15 aggregate via vertex and edge sharing. The Ag15 aggregates form an infinite silver(I) chain via argentophilic interactions. Two Ag5 baskets fused though edge sharing. The Ag8 aggregates are interconnected via silver-aromatic interaction to give an infinite silver-organic chain. The two different butterfly-shaped Ag4 baskets and the two square-pyramidal Ag5 baskets are fused in a Ag15 aggregate via vertex and edge sharing. The Ag15 aggregates form an infinite silver(I) chain via sharing a silver atom. Two Ag4 baskets fused though vertex sharing. The Ag7 aggregates are linked together to give an infinite silver ribbon. Two Ag5 baskets fused though edge sharing. The Ag8 aggregates are interconnected via silver-aromatic interactions, p-p stacking and hydrogen bonding to give a 3D structure. The aggregates generate an infinite silver(I) chain through argentophilic interactions. Adjacent chains are further associated in a wavy layer structure. 3D network is generated by p-p stacking.

203

204

204

204

204

204

205

205 205

205

205

206

206

206

206

206 206

207

Table 1

(Continued)

Structure formula

Ethynide-silver(I) coordination

Brief description of the structure with emphasis on silver

References

2AgL19∙ AgNO3 (172)

Isomorphous with compound 171.

207

AgL20∙ 2AgCF3COO (175)

L20  Ag4 m4-Z1, Z1, Z2, Z2

AgL20∙ 3AgNO3 (176)

L20  Ag4 m4-Z1, Z1, Z2, Z2

2AgL21∙ 5AgCF3COO∙ 2CH3CN∙ H2O (177)

L21  Ag4 m4-Z1, Z2, Z2, Z2 and L21  Ag5 m5-Z1, Z1, Z1, Z2, Z2

4AgL22∙ 6AgC2F5COO∙ 5CH3CN (178)

L22  Ag5 m5-Z1, Z1, Z1, Z1, Z2 and L22  Ag5 m5-Z1, Z1, Z1, Z2, Z2 and L22  Ag5 m5-Z1, Z1, Z2, Z2, Z2 L22  Ag4 m4-Z1, Z1, Z2, Z2 and m3-Z1, Z1, Z2

The units are fused together through argentophilic interaction to yield a silver(I) layer. Layers are interconnected by p-p stacking. Infinite silver(I) chain consolidated by bridging nitrate anions. The chains are associated via silver-halogen bonding and by p-p stacking. Two Ag4 baskets are fused in an Ag6 aggregate though edge sharing. The Ag6 aggregates are linked together via trifluoroacetate anions to give a 2D network. The iodophenyl ethynide ligands connect the layers via AgdI interactions and p-p stacking to generate a 3D network. The Ag4 aggregates are linked together via nitrate anions, AgdI interactions and p-p stacking to generate a 3D network. The Ag4 and Ag5 baskets share an edge forming higher nuclearity aggregates, which further develop in a chain, stabilized also by p-p stacking. The chains are linked via an external silver atom, through bridging trifluoroacetate anions and silver-halogen bond. The Ag5 baskets share an edge forming columns, stabilized also by p-p stacking. The columns are linked via hydrogen and halogen-halogen bonds.

207

2AgL20∙ AgNO3 (174)

L19  Ag4 m4-Z1, Z1, Z2, Z2 and L19  Ag3 m3-Z1, Z1, Z2 L20  Ag3 m3-Z1, Z2, Z2 and m3-Z1, Z1, Z2 L20  Ag3 m3-Z1, Z1, Z2

4AgL20∙ 2AgNO3 (173)

2AgL22∙ AgNO3 (179)

4AgL23∙ 6AgC2F5CO O ∙5CH3CN (180)

2AgL23∙ AgNO3 (181) 2AgL24∙ AgNO3 ∙ H2O (182)

L23  Ag5 m5-Z1, Z1, Z1, Z1, Z2 nd L23  Ag5 m5-Z1, Z1, Z1, Z2, Z2 and L23  Ag5 m5-Z1, Z1, Z2, Z2, Z2 L23  Ag4 m4-Z1, Z1, Z2, Z2 and L23  Ag3 m3-Z1, Z1, Z2 L24  Ag4 m4-Z1, Z1, Z2 and L24  Ag3 m3-Z1, Z2, Z2

2AgL25∙ AgNO3 (183)

L25  Ag3 m3-Z1, Z2, Z2

2AgL25∙ 4AgCF3COO∙ NC(CH2)4CN (184)

L25  Ag5 m5-Z1, Z1, Z1, Z1, Z2 and L25  Ag5 m5-Z1, Z1, Z1, Z2, Z2

2AgL25∙ 4AgCF3COO∙ 2CH3CN (185)

L25  Ag5 m5-Z1, Z1, Z1, Z1, Z2 and L25  Ag5 m5-Z1, Z1, Z2, Z2, Z2 L26  Ag5 m4-Z1, Z1, Z2, Z2

AgL26∙ 2CF2(CF2COOAg)2 ∙ 2CH3CN (186) 3AgL26∙ AgNO3 (187)

2AgL27∙ AgNO3 ∙ DMSO (188)

L26  Ag3 m3-Z1, Z2, Z2 and m3-Z1, Z1, Z2 and m3-Z1, Z1, Z1 L27  Ag3 m3-Z1, Z1, Z2

Ag3(L29)2 ∙ AgNO3 (189)

L29  Ag3 m3-Z1, Z1, Z1 and m3-Z1, Z1, Z2

Ag4L30∙ 3AgNO3 ∙H2O (190)

L30  Ag4 m4-Z1, Z1, Z1, Z1 and L30  Ag4 m3-Z1, Z1, Z1, Z2

Ag3L31∙ AgNO3 ∙(191)

L31  Ag4 m4-Z1, Z1, Z1, Z1

AgL32∙ 3AgNO3 (192)

L32  Ag4 m4-Z1, Z1, Z1, Z1

Infinite silver chains associated together by argentophilic interactions. The layers are consolidated by nitrate anions and p-p stacking. The Ag5 baskets share an edge forming columns, stabilized also by p-p stacking. The columns are linked via hydrogen and halogen-halogen bonds.

207

208

208 208

208

207

208

Isomorphous with compound 179.

207

The Ag5 units are fused through atom sharing to generate an infinite chain. A layer structure is formed by p-p stacking and silver-halogen interactions. Hydrogen bonds and bridging nitrate anions connect the layers in 3D architecture. The Ag3 units are fused via vertex sharing in Ag5 aggregates, which coalesce in a column. Columns are linked by hydrogen bonds and silver-halide interactions. The Ag5 baskets are connected by p-p stacking, hydrogen bonding, silver-halogen and halogen-halogen interactions, giving columnar arrangements. Bridging adiponitrile molecules between columns form the 3D structure. The Ag5 baskets are connected by p-p stacking, silver-halogen and halogen-halogen interactions.

207

The Ag5 baskets and a lone silver center form a 3D network via p-p stacking, silver-halogen and halogen-halogen interactions. Infinite silver(I) chain of Ag8 polyhedra. Chains are linked by hydrogen bonds and silver-halide interactions.

208

The Ag3 units are fused via vertex sharing in Ag5 aggregates, which then form a ribbon stabilized coordination of dmso and nitrate anions. Columns are linked by hydrogen bonds and p-p stacking. The Ag3 aggregates form a column via vertex sharing and argentophilic interactions. Columns are further linked via unsupported argentophilic interactions to give a 2D network. The Ag4 units are linked through carboxylate coordination giving a wave-like network. The layers are further connected by bridging carboxylate, nitrate anions and water molecules. The Ag4 units are linked through silver-carboxylate interactions and argentophilic interactions. The ethynide moiety is bound in a butterfly-shaped Ag4 basket and the pyridyl nitrogen atom of substituent is coordinated to a silver(I) of another basket. The Ag4 units are also linked by bridging nitrate anions giving columns, further stabilized by pyridyl p-p stacking.

207

207

208

208

207

209

209

209 210

(Continued )

Table 1

(Continued)

Structure formula

Ethynide-silver(I) coordination 1

1

1

2

AgL33∙ 3AgCF3COO ∙3H2O∙(193)

L33  Ag4 m4-Z , Z , Z , Z

2AgL33∙ 4AgCF3COO∙ 2CH3CN∙(194)

L33  Ag4 m4-Z1, Z1, Z1, Z2

AgL34∙ 7AgCF3COO ∙2THF ∙2H2O ∙ (195)

L34  Ag4 m4-Z1, Z1, Z1, Z2

AgL35∙ 3AgCF3COO ∙2H2O∙(196)

L35  Ag5

2AgL36∙ 6AgNO3 ∙3H2O (197)

L36  Ag4 m4-Z1, Z1, Z1, Z1

AgL37∙ 4AgCF3COO ∙(198)

L37  Ag5 m5-Z1, Z1, Z2, Z2, Z2

AgL37∙ 4AgCF3COO ∙H2O ∙(199)

L37  Ag5 m5-Z1, Z1, Z2, Z2, Z2

2AgL38∙ 5AgCF3COO∙ 1.5H2O ∙(200)

L38  Ag4 m4-Z1, Z1, Z2, Z2 and m4-Z1, Z1, Z1, Z2

4AgL38∙ 5AgNO3 ∙3DMSO∙(201)

L38  Ag3 m3-Z1, Z2, Z2 and L38  Ag4 m4-Z1, Z2, Z2, Z2

Brief description of the structure with emphasis on silver

References

Two Ag5 baskets form a Ag8 aggregate via vertex sharing. The Ag8 aggregates are linked by trifluoroacetate groups giving a chain. Chains are linked by p-p stacking and hydrogen bonds. Two Ag4 baskets form a Ag6 aggregate via vertex sharing. The Ag6 aggregates are linked by trifluoroacetate groups giving a chain. Chains are linked by p-p stacking and hydrogen bonds. Ag4 baskets are connected to a lone single atom via argentophilic interactions to form a Ag5 segment. The Ag5 segments give chains through argentophilic interactions, hydrogen bonding and trifluoroacetate groups. The cross-linking of the chains is assisted by coordination of the quinolinyl nitrogen atom to silver atom. Two Ag5 baskets are fused in an Ag8 fragment. Such a fragment are further connected via p-p stacking and quinolinyl nitrogen coordination. Each of the two nitrogen atoms of the pyrazinyl substituent are coordinated to a silver vertex of another two adjacent Ag4 butterfly-shaped basket: in this way five neighboring Ag4 baskets are linked through two ethynide anions. The Ag4 units are also linked by bridging nitrate anions giving columns, further stabilized by p-p stacking. The Ag5 baskets share an edge giving Ag8 aggregates, connected by trifluoroacetate anions giving columns. The 3D network is produced by bridging trifluoroacetate groups, AgdN coordination and argentophilic interactions. The Ag5 baskets share an edge giving Ag8 aggregates, connected by trifluoroacetate anions giving columns. The 3D network is produced by bridging trifluoroacetate groups, AgdN coordination and argentophilic interactions. The Ag4 baskets share a vertex giving Ag7 aggregates, further connected by trifluoroacetate anions giving columns. The 3D network is produced by bridging trifluoroacetate groups, AgdN coordination and argentophilic interactions. The Ag3 and Ag4 baskets share vertexes giving Ag9 aggregates, further connected by AgdN coordination to give a 2D network. The 3D structure is produced by bridging nitrate groups and dmso molecules.

211

211

211

211

210

212

212

212

212

The alkynes involved are those shown in Fig. 32.

Fig. 33 Coordination modes of the ethynide moieties in the RdC^C Agn subunits: butterfly-shaped Ag4 baskets and square-pyramidal Ag5 baskets.

Silver Organometallics

71

From the data shown in Table 1, it is clearly evident that even when using the same RdC^C moiety, different structures can be generated, and the type of 3D network is the result of a balance between all the possible strong (bridging ligands) or weak interactions (hydrogen bonds, p-p, lone pair-p, silver(I)-aromatic, silver(I)-halogen and argentophilic interactions). This is particularly apparent from the five different architectures characterized starting from the alkyne HL2. Initially, the metastable structure 133 is isolated in the form of needle-shaped crystals, which in the mother liquor slowly converts in situ into block-like crystals containing structure 134. A silver(I) column is formed in both cases, but the interactions involved in the self-assembly of the columns are different.200 Another example in this regard are the 11 structures characterized starting from the alkyne HL28.213 With the aim of increasing the possibility for p interactions in the construction of the 3D architecture, alkynes have been employed which are functionalized with terminal double bonds in the pendant arms attached to an aromatic skeleton. The investigated alkynes are those depicted in Fig. 34.214,215 In Table 2 the stoichiometry of the isolated species and the coordination of the ethynide moiety are reported. The coordination modes of the anionic ligands L39-L48 in the structures are depicted in Fig. 35. Interestingly, structure 207 presents all the possible silver(I)-carbon interactions: silver(I)-ethynide, –ethynyl, –ethenyl and aromatic contacts. Finally, bidentate bis(alkyne) pro-ligands have also been studied and their structures are depicted in Fig. 36.

Fig. 34 Monodentate terminal alkynes studied in the reaction between the corresponding [Ag(C^CdR)]1 systems and different WCAs and neutral ligands: alkynes are functionalized with double and triple bonds with the aim of increasing the possibility of silver-aromatic and p interactions in the building of 3D networks.

Table 2

Summary of the alkynylsilver(I) structures isolated with the alkynes reported in Fig. 34.

Stoichiometry of the structure

Silver ethynide coordination

References

AgL39∙ 4AgCF3COO ∙3H2O∙ 2CH3OH (202) 2AgL40∙ 6AgCF3COO∙ 0.5H2O ∙ 2CH3CN (203) AgL41∙ 6AgCF3COO ∙5H2O (204) AgL42∙ 6AgCF3COO ∙2H2O∙ CH3OH (205) AgL43∙ 5AgCF3COO ∙2H2O (206) AgL44∙ 6AgCF3COO ∙H2O ∙CH3OH (207) AgL45∙ 6AgCF3COO ∙2H2O∙ CH3OH (208) AgL46∙ 6AgCF3COO ∙H2O ∙2CH3OH (209) AgL47∙ 5AgCF3COO ∙5.5H2O (210) AgL48∙ 6AgCF3COO ∙2H2O∙ CH3OH (211) AgL49∙ 6AgCF3COO ∙3H2O (212) AgL50∙ 7AgCF3COO ∙2H2O∙ CH3CN (213) AgL51∙ 6AgCF3COO ∙5H2O (214) AgL51∙ 3AgCF3COO ∙H2O ∙CH3CN (215) AgL52∙ 4AgCF3COO ∙H2O (216) AgL53∙ 3AgCF3COO (217) AgL54∙ 6AgCF3COO ∙H2O (218) AgL55∙ 7AgCF3COO ∙4H2O (219)

L39  Ag4 m4-Z1, Z1, Z2, Z2 L40  Ag4 m4-Z1, Z1, Z1, Z2 L41  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L42  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L43  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L44  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L45  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L46  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L47  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L48  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L49  Ag4 m4-Z1, Z1, Z1, Z2 L50  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L51  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L51  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L52  Ag4 m4-Z1, Z1, Z2, Z2 L53  Ag4 m4-Z1, Z1, Z2, Z2 L54  Ag5 m5-Z1, Z1, Z1, Z2, Z2 L55  Ag5 m5-Z1, Z1, Z1, Z2, Z2

214 214 214 214 214 214 214 214 214 214 215 215 215 215 215 215 215 215

72

Silver Organometallics

Fig. 35 Coordination modes of the ethynide ligands L39 (b), L40 (a), L41 (d), L42 (e), L43 (c), L44 (j), L45 (f ), L46 (h), L47 (i) and L48 (g). Figure reproduced from Ref. Hau, S. C. K.; Mak, T. C. W. Chem. – Eur. J. 2013, 19, 5387–5400. Copyright 2013 Wiley-VCH.

Fig. 36 Bidentate terminal alkynes studied in the reaction between the corresponding [Ag(C^C-R)]1 systems and different WCAs and neutral ligands.

The structures of compounds derived from L62-L73 and L80-L85 have already been reviewed in 2015,198 so only the structures derived from other ligands will be briefly discussed here (Table 3). The observed coordination of ligands L58-L60 is summarized in Fig. 37.201

Silver Organometallics

73

Table 3 Summary of the alkynylsilver(I) structures with particular emphasis on the RdC^C− coordination to the silver(I) centers and brief description of the supramolecular architecture. Stoichiometry of the structure

Silver ethynide coordination

Brief description of the structure

References

Ag2L58∙ 7CF3COO∙ 5.5H2O (220)

L58  2Ag4 m4-Z1, Z1, Z1, Z2 and m4-Z1, Z1, Z2, Z2

201

Ag2L59∙ 8CF3COO∙ 4H2O (221)

L59  2Ag4 m4-Z1, Z2, Z2, Z2

Ag2L59∙ 5CF3COO∙ 5CH3CN (222)

L59  Ag4 + Ag5 m4-Z1, Z1, Z1, Z2 and m5-Z1, Z1, Z2, Z2, Z2

Ag(L60)0.5 ∙ 6CF3COO ∙2H2O ∙ CH3CN (223)

L60  Ag5 m5-Z1, Z1, Z1, Z2, Z2

Ag(L60)0.5 ∙ 5CF3COO ∙3H2O (224)

L60  Ag5 m5-Z1, Z1, Z2, Z2,Z2

Ag2L61∙ 6AgNO3 ∙H2O (225)

L61  Ag4 m4-Z1, Z1, Z2, Z2 and m4-Z1, Z2, Z2, Z2 L61  Ag5 m5-Z1, Z1, Z2, Z2, Z2

A butterfly-shaped Ag4 basket and a nearly planar Ag4 square, connected by a vertex to give a Ag7 aggregate. The Ag7 units are linked by bridging trifluoracetate anions giving columns, which are further connected by silver(I)-aromatic interactions, other trifluoracetate anions and p-p stacking. Two Ag4 basket linked by bridging trifluoracetate anions giving ribbons, which are further connected by other bridging trifluoracetate anions to give a double-layer network. The layers are stacked through silver(I)-aromatic interactions. A butterfly-shaped Ag4 basket and a square-pyramidal Ag5 basket, connected by an edge sharing, giving a 1D staircase structure. Staircases are further connected by bridging trifluoroacetate groups to give a 2D network. The square-pyramidal Ag5 basket is linked to other two silver(I) centers via bridged trifluoroacetate anion and a water molecule. The formed Ag7 aggregates are connected by silver(I)-aromatic interactions and trifluoroacetate anions, to give a layer-type architecture. Two square-pyramidal Ag5 basket are linked by bridging trifluoroacetate anions to give a staircase structure. The formed staircases are associated by other trifluoroacetate anions, to produce 2D layers, further connected by trifluoroacetate anions and silver(I)-aromatic interactions. The bis(ethynide) ligand coordinated an Ag7 cluster, which are further linked together by bridging nitrate ions and p-p stacking. The bis(ethynide) ligand coordinated an Ag8 cluster, which are further linked together by bridging trifluoroacetate groups, giving a 2D polymer. Ag16 aggregates linked together by bridging trifluoroacetate groups and argentophilic interactions, giving an infinite chain.

(Ag2L61)0.5 ∙4CF3COO∙ 2H2O (226)

Ag2L61∙ 7CF3COO∙ 3CH3CN∙ H2O (227)

Ag2L74∙ 5AgNO3 ∙3H2O (228)

3(AgL75)∙13AgCF3COO∙ 7H2O∙ 2MeOH (229)

Ag2L76∙ 6AgCF3COO∙ 5H2O (230)

Ag2L77∙ 10AgNO3 (231)

L61  Ag5 m5-Z1, Z1, Z2, Z2, Z2 and m5-Z1, Z1, Z1, Z2, Z2 L74  2Ag4 m4-Z1, Z1, Z1, Z1

L75  Ag4 2 x m4-Z1, Z1, Z2, Z2 and m4-Z1, Z1, Z1, Z1 L75  Ag5 m5-Z1, Z1, Z1, Z2, Z2 and m5-Z1, Z1, Z1, Z1, Z2 L76  Ag5 m5-Z1, Z1, Z1, Z2, Z2 and m5-Z1, Z1, Z1, Z1, Z2 L77  2Ag4 m4-Z1, Z1, Z1, Z1

Ag2L78∙ 4AgCF3COO∙ 4H2O (232)

L78  2Ag4 m4-Z1, Z2, Z2, Z2

Ag2L78∙ 3AgNO3 ∙H2O (233)

L78  Ag4 m4-Z1, Z1, Z2, Z2 and L78  Ag3 m3-Z1, Z2, Z2

The described alkynes are those reported in Fig. 36.

201

201

201

201

216 216

216

Each ethynide group of the m-phenylenediethynide moiety is coordinated in a Ag4 butterfly-shaped basket. Sharing of two Ag vertex of this m8 unit gives Ag14 aggregates, which are linked together in an infinite columnar tube by bridging nitrate anions and p-p stacking. The five baskets give an Ag18 aggregate via sharing of a silver vertex, argentophilic interaction and p stacking. The Ag18 aggregates form an infinite silver(I) ribbon via argentophilic interaction.

210

Two symmetry-related Ag8 aggregates coalesce to form a Ag14 fragment. The Ag14 segments are connected via bridging trifluoroacetate and aqua ligands. Each ethynide group of the bidentate moiety is coordinated in a Ag4 butterfly-shaped basket. Sharing of one Ag vertex of these units gives Ag7 aggregates, which are linked together in an infinite ribbon by bridging nitrate anions. The adjacent ribbons are linked by p-p stacking between the thiophene rings. Each ethynide group of the bidentate moiety is coordinated in a Ag4 butterfly-shaped basket and the nitrogen atom of the pyridyl group is bound to another silver atom. (see Fig. 38 and a more detailed description in the text and in Fig. 39). One ethynide group of the bidentate moiety is coordinated in a Ag4 butterfly-shaped basket and one to an Ag3 plane; the nitrogen atom of the pyridyl group is bound to a silver atom of another Ag4 basket. (see Fig. 38 and a more detailed description in the text and in Fig. 40).

206

206

210

217

217

74

Silver Organometallics

Fig. 37 Coordination modes of ligands L58-L60.

Fig. 38 (A) Coordination mode of the 3,5-pyridyldiethynide ligand in 232. (B) Coordination mode of the 3,5-pyridyldiethynide ligand in 233. Reproduced (adapted) from Ref. Zhang, T.; Kong, J.; Hu, Y.; Meng, X.; Yin, H.; Hu, D.; Ji, C. Inorg. Chem. 2008, 47, 3144–3149. Copyright 2008 American Chemical Society.

As reported for ligand L2, and with bidentate ligands, different structures can be obtained, depending on the anion and crystallization conditions. For example, in 232 the pyridyldiethynide groups connects Ag11 cluster units to generate 1D supramolecular chains; the nitrogen atoms of the pyridyl groups are coordinated to silver ions to form wavelike layers, which are further connected by trifluoroacetate ligands to afford a 3D coordination network (Figs. 38 and 39). In 233 a silver double chain is formed through silver-ethynide and argentophilic interactions; the silver double chains are further connected by bridging pyridyldiethynide groups in a 2D network; the 3D architecture forms by interaction between silver atoms and the pyridyl nitrogen atom (Figs. 38 and 40). Finally, tridentate propargyl-functionalized trihydroxybenzene has also been employed for the generation of 3D architectures (Fig. 41).218 In the same crystal, the ethynide groups are coordinated to an Ag11 aggregate, in a m5-Z1, Z1, Z1, Z2, Z2 and two m4-Z1, Z1, Z2, Z2 fashions (Fig. 41A) or to a Ag12 aggregate, in a m5-Z1, Z1, Z1, Z2, Z2, a m5-Z1, Z1, Z2, Z2, Z2 and a m4-Z1, Z1, Z2, Z2 fashion (Fig. 41B). The possibility to obtain 3D structures is not a prerogative of alkynes bearing aromatic substituents or featuring additional unsaturated bonds, as those described above, but can also be observed starting from simpler alkynes like tBuC^CH in the presence of additional added ligands.219–221 For example, the tBuC^C Ag4 synthon in combination with 9-hydroxy-9fluorenecarboxylate (9-fluorenCOO), generates the coordination polymer with stoichiometry Ag14(tBuC^C)8(9-fluorenCOO)2 (CF3COO)4 ∙2CH3OH.219 Solvo-thermal reaction between the polymeric material [Ag(C^CtBu)]1 and nicotinic/isonicotinic acid, affords 3D structures in which the linking of the tBuC^C  Agn (n ¼ 4, 5) synthons is driven by carboxylate anion coordination and also by the coordination of the nitrogen atom present in the (iso)nicotinate ligand.220 The types of isolated structures available to these RdC^C Agn subunits can be further expanded if a neutral ligand is added to the crystallization mixture, such as a phosphine, bipyridine,222 heteroaromatic N-donor (i.e. pyrazole, 1,2,4-triazole, benzimidazole),222 substituted pyrazinyl ligand,223 bis(imidazole) ligand.224 For example, addition of bipyridine to [AgL6]n in methanol and in the

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75

Fig. 39 (A) Supramolecular chain in 232 showing the connection between Ag11 clusters. (B) 2D coordination network. (C) 2D wavelike layers. The bridged trifluoroacetate groups are omitted for clarity. Reproduced from Ref. Zhang, T.; Kong, J.; Hu, Y.; Meng, X.; Yin, H.; Hu, D.; Ji, C. Inorg. Chem. 2008, 47, 3144–3149. Copyright 2008 American Chemical Society.

Fig. 40 (A) Silver double chain and two coordination modes of nitrate groups (marked in gold/green) in 233. Partial nitrate groups are omitted for clarity. (B) 2D coordination network in 233 constructed from the linkage of silver double chains by pyridyldiethynide groups. (C) 3D coordination network in 233 composed of 2D coordination networks linked through the interaction between silver atoms and pyridyl groups, which are marked with gold color. All nitrate groups are omitted for clarity. Reproduced (adapted) from Ref. Zhang, T.; Kong, J.; Hu, Y.; Meng, X.; Yin, H.; Hu, D.; Ji, C. Inorg. Chem. 2008, 47, 3144–3149. Copyright 2008 American Chemical Society.

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Fig. 41 Coordination of the tridentate propargyl-functionalized trihydroxybenzene in the Ag11 and Ag12 aggregates present in the crystal structure.

presence of silver(I) trifluoroacetate, affords product AgL6∙3AgCF3COO∙2bpy, in which two bpy ligands are coordinated in chelating fashion to two silver centers, belonging to an Ag4 butterfly-shaped basket (coordination of the ethynide moiety m4-Z1, Z1, Z2, Z2).202 The same approach has been also investigated with the bidentate ligand L76, leading to isolation of the species Ag2L76∙5AgCF3COO∙3bpy. Also in this case, the bpy ligand chelates two silver centers, belonging to an Ag4 butterfly-shaped basket (coordination of the ethynide moiety m4-Z1, Z1, Z1, Z2).206 In 2017, the effect of the presence of the macrocyclic ligand azacalix[6]pyridine (abbreviated as Py6) was studied with the bis(ethynide) ligand L87 and the tetraethynide donor L88.225 The aim was to observe differences in the properties and behavior of the two silver(I) aggregates (Fig. 42), one free (type II) and the other encapsulated in the cavity of the macrocyclic ligand (type I). With L87 the complex [Ag8(L87](Py6)(CF3COO)6](CH3OH)2.5 was isolated: the ethynide moieties are coordinated to two Ag4 baskets in a m4-Z1, Z1, Z2, Z2 (type I) and m4-Z1, Z1, Z1, Z2 (type II) mode. In the two aggregates the silver-silver distances are very similar, thus suggesting the presence of similar argentophilic interactions. The main difference relates to the interactions between the silver centers and the ethynide function: the steric bulk of the Py6 ligand moves the silver centers in the type I cluster closer to the ethynide group, thus strengthening the corresponding Ag-ethynide interactions. The same approach was also adopted starting from the tetraethynide ligand L88, giving complex [Ag12(L88)Py6)2(CF3COO)8] (CH3OH)4(H2O)3: in this case, the ethynide moieties coordinate to Ag3 aggregates in a m3-Z1, Z2, Z2 (type I) and m3-Z1, Z1, Z1 (type II) mode. In this case, the two aggregates can be differentiated by the argentophilic interactions, which are much weaker in type I clusters (AgdAg distances around 3.3 A˚ , compared to 2.9 A˚ in type II cluster). Despite the different structures, the emission spectra of the complexes do not differ significantly in CH2Cl2 at room temperature, when excited at ca. 275 nm. However at 77 K, a red-shift in the emission is observed only for complex [Ag12(L88)Py6)2(CF3COO)8](CH3OH)4(H2O)3, likely related to the different features of the two clusters and derived from LMCT and intra-ligand transitions. The interest in these types of architecture lies in their potential applications as luminescent materials or as non-linear optical materials. For example, compounds 141–145 display weak emission properties at room temperature around 370–440 nm, originating from ligand-centered excited states and Ag-p interaction withing the 3D structure.202 Compounds 200 and 201 also display weak luminescence in the solid state at room temperature; the emission maximum is blue-shifted compared with the emission of the free alkyne HL38. This shift may originate from the coordination of the ligand to the silver centers.212 This further supports the idea that in this type of silver metal organic framework, the emissions mainly originate from intra-ligand transitions, perturbed by the interaction of the organic species with the silver centers. The same conclusions have been also reached with the alkynes HL29-HL31: the corresponding silver species 189–191 present very broad emission bands in the range 470–700 nm, which is the same range of emission of the ethynide precursors.209 Interesting photoluminescence properties have been observed with the framework [Cl@Ag18(cyclopropyl-C^C)17(BF4)]n226; the 3D structure involves [Cl@Ag16(cyclopropyl-C^C)16] clusters, each linked to four adjacent neighbors via tetrahedral Ag atoms through argentophilic interactions (Fig. 43). At room temperature the network exhibits weak orange-red emission, while its emission becomes stronger upon cooling and presents three emission maxima at 524, 647 and 1036 nm, covering all the visible and NIR region. The visible emissions could involve 3LMCT mixed with a cluster-centered state, whereas the participation of the ethynide moiety in the frontier orbitals of the cluster may be responsible for the NIR emission.

Fig. 42 Differentiation of two silver(I) aggregates through coordination of the Py6 macrocyclic ligand.

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Fig. 43 3D structure of the framework [Cl@Ag18(cyclopropylC]C)17(BF4)]n. Reproduced with permission from Ref. Zhang, S.-S.; Su, H.-F.; Zhuang, G.-L.; Wang, X.-P.; Tung, C.-H.; Sun, D.; Zheng, L.-S. Chem. Commun. 2018, 54, 11905–11908. Copyright 2018 Royal Society of Chemistry.

9.02.2.4.2

High-nuclearity clusters

In addition to the above described extended structures, the organo-substituted ethynide moieties can also form discrete high nuclearity clusters, which have been reviewed in 2014 by T. Mak,197 in 2015 by Wang227 and recently also by Li and Zheng, who focused on cluster-containing polyoxometalate anions (POMs) as templating agents.228,229 In fact, in order to control the synthesis of a high-nuclearity metal cluster and obtain a cluster characterized by unique composition, size and shape, it is helpful to introduce a directing agent during the formation process. It has been verified in several cases that the structural unit Cl@Agn(C^CR)m could be employed as building block for the construction of clusters characterized by a higher nuclearity. Two ethynide-functionalized silver sulfide clusters [Ag9S6@Ag36(C^CtBu)32(H2O)2][Ag(imidazole)(CH3OH)(H2O)](BF4)2 and [Ag120S24(C^CPh)52Cl4 (2-pyridone)10(H2O)8](H3O)4(SiF6)8(BF4)4 have been assembled from Cl@Agn(C^CR)m and the organic sulfide precursors 1,10 -thiocarbonyldiimidazole and di(2-pyridyl) thionocarbonate.230 The Cl@Agm(C^CPh)n entity has been used in the construction of a gigantic silver(I) cluster compound, [Ag216S56Cl7(C^CPh)98(H2O)12][Ag3(imidazole)(H2O)4](SbF6)2 (234), which features an unprecedented five-shell arrangement Cl@Ag12@S12@Ag32@[email protected] The adopted strategy is depicted in Fig. 44. Interestingly, cluster 234 shows intense red emission (ca. 630 nm) in dichloromethane solution when excited at 250 nm, and the large Stokes shift suggests emission from a triplet state. In the second step described in Fig. 44, i.e. the assembly of the supramolecular assembly starting from the Cl@Agn(C^CR)m building blocks, other anions can be used as well, like for example t-butyl phosphonate ligands in the presence of polyoxotungstate salts or Yb(hfac)3 (hfac ¼ hexafluoroacetilacetonate).232 Three silver ethynide clusters, namely [Ag5@(CyC^CdC^C)6Ag3(PPh3)8](NO3)2
8MeOH
4H2O, [Ag5@(CyC^CdC^C)6 Ag3(PPh3)8](ClO4)2
2Et2O
2CH2Cl2 and [Ag5@(IPrC^CdC^C)6Ag3(PPh3)8](NO3)2
MeOH
H2O, have been assembled starting from the supramolecular synthon RC^CdC^C  Agn (R ¼ Cy or iPr).233 In particular, it has been proposed that the formation of the trigonal bipyramidal Ag5 cluster-core presumably forms in solution by the core-transformation described in Fig. 45, which implies the addition of free Ag(I) cations to the precursor [(RC^CdC^C)Ag(PPh3)]4.

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Fig. 44 A two-step synthesis for high nuclearity clusters.

Fig. 45 Proposed assembly mechanism for clusters with alkyl-1,3-diynyl ligands. Reproduced from Ref. Hau, S. C. K.; Mak, T. C. W. Dalton Trans. 2017, 46, 14098–14101. Copyright 2017 Royal Society of Chemistry.

The tetranuclear precursors [(RC^CdC^C)Ag(PPh3)]4 (R ¼ iPr, tBu, and Cy) can also be used to isolate a series of silver(I)copper(I) heteropolynuclear clusters: by reaction with [Cu(MeCN)4]+, the Ag4 unit is transformed into a trigonal-planar CuAg3 unit, which then is assembled in the crystal structure into medium-size clusters stabilized by argentophilic and cupro-argentophilic interactions (Fig. 46).234 Silver(I) clusters can be synthesized also with more exotic ethynide functionalities, like for example the carba-closododecarboranylethynyl ligand (Fig. 47); these clusters are also stabilized by coordination of functionalized pyridine moieties.235–237 Several structures have been described featuring from Ag7 to Ag14 units and some of these clusters give rise to room temperature phosphorescence.

Fig. 46 Proposed assembly mechanism for silver(I)-copper(I) clusters with alkyl-1,3-diynyl ligands. Reproduced from Ref. Hau, S. C. K.; Yeung, M. C.-L.; Yam, V. W.-W.; Mak, T. C. W. J. Am. Chem. Soc. 2016, 138, 13732–13739. Copyright 2016 American Chemical Society.

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Fig. 47 The dianionic carba-closo-dodecarboranyl ligand and the [Ag8(12-C^C-closo-1-CB11H11)4(4-MePy)11] (4-MePy ¼ 4-CH3C5H4N) cluster 235. Reproduced from Ref. Hailmann, M.; Radacki, K.; Finze, M. Z. Für Anorg. Allg. Chem. 2020, 646, 777–783. Copyright 2020 Wiley-VCH.

Using the 1,8-diethynyl-9H-carbazole substrate H2LH79 (Fig. 36) in the presence of additional silver(I) centers and dpppy ligands (dpppy ¼ 2,6-bis(diphenylphoshino)pyridine) it proved possible to isolate three clusters, namely, [Ag8(dpppy)4(L79)2] (ClO4)2 (236), [Ag18(dpppy)4(HL79)4(L79)2](ClO4)2 (237) and [Ag29(dpppy)6(HL79)2(L79)8](ClO4) (238), differing for the number of silver centers.238 The notation HL79 indicates the deprotonation of the two alkyne moieties only, while with L79 the carbazole nitrogen is deprotonated as well. The UV–vis spectra measured in CH2Cl2 show high energy bands below 350 nm, associated with ligand-centered transitions, and low energy bands (around 400–500 nm) which are metal-centered transitions, whose energy correlates well with the HOMO-LUMO gap calculated via TD-DFT methods. Furthermore, a red-shift of these bands is observed with increasing cluster size. When irradiated at 260 nm, the clusters are characterized by a long-lived phosphorescence, both in the solid state and in dichloromethane solution, and also in this case a red-shift of the emission is observed with increasing nuclearity. The use of chiral alkynyl ligands, namely N-((R/S)-1-(naphthalen-4-yl)ethyl)prop-2-yn-1-amine (NYA), allowed for the isolation of enantiomeric silver clusters R/S-[Ag17(NYA)12](NO3)3 (R/S-239, Fig. 48),239 in which the alkynyl ligands adopt m3 coordination. The clusters in the crystalline solid state show a broad emission peak centered at 745 nm under UV light. CPL responses were also achieved, demonstrating that with these clusters chiral superatomic states can be induced by chiral organic ligands. In 2017, Li and Zhang reported the first silver nanocluster protected only by alkynyl ligands.240 The Ag74(C^CPh)44 cluster was isolated by reduction of Ag(I) with NaBH4 in the presence of phenylacetylene and the bidentate phosphine 1,3-diphenylphosphinopropane. The structure of the cluster involves Ag atoms arranged in a Ag4@Ag22@Ag48 three-shell structure, protected by 44 phenylethynyl ligands coordinated to Ag in a m3 mode. The phosphine ligand is not present in the crystal

Fig. 48 NYA structure and crystal structures of the enantiomers of the R/S-[Ag17(NYA)12]3+ clusters. Reproduced (adapted) from Ref. Zhang, M.-M.; Dong, X.-Y.; Wang, Z.-Y.; Luo, X.-M.; Huang, J.-H.; Zang, S.-Q.; Mak, T. C. W. J. Am. Chem. Soc. 2021, 143, 6048–6053. Copyright 2021 American Chemical Society.

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structure: its role is to slow down the reduction rate of Ag(I) by coordinating to it (formation of a dinuclear complex with bridging diphosphine ligand has been observed in NMR experiments); the controlled release of silver centers allows a very narrow dispersion of the nanocluster size to be obtained. Similar results were also observed with other diphosphino ligands.

9.02.2.4.3

Miscellaneous coordination of alkynyl moieties

There are some examples of M/Ag heterometallic complexes or clusters generated by p-coordination of an alkynyl moiety to silver(I) centers. The alkynyl moiety primarily can either act as a s-donor ligand toward M, a group 8 or 11 metal center, or be a functional group in a ligand coordinated to M via a Y donor atom241–248 (Fig. 49). In Fig. 50 selected examples of the first type of structures are shown.242–245 These complexes are characterized by interesting photoluminescence properties that possess a MLCT character.

Fig. 49 Schematic representation of coordination motifs in organosilver bimetallic architectures based on alkynyl ligands.

Fig. 50 Examples of p-bonding alkynyl complexes of silver(I): the alkynyl ligand is s-coordinated to a metal center of group 8 or 11. Reproduced (adapted) from Refs. Fresta, E.; Fernández-Cestau, J.; Gil, B.; Montaño, P.; Berenguer, J. R.; Moreno, M. T.; Coto, P. B.; Lalinde, E.; Costa, R. D. Adv. Opt. Mater. 2020, 8, 1901126; Johnson, A.; Marzo, I.; Gimeno, M. C. Dalton Trans. 2020, 49, 11736–11742; Hailmann, M.; Hupp, B.; Himmelspach, A.; Keppner, F.; Hennig, P. T.; Bertermann, R.; Steffen, A.; Finze, M. Chem. Commun. 2019, 55, 9351–9354; and Kritchenkov, I. S.; Gitlina, A. Y.; Koshevoy, I. O.; Melnikov, A. S.; Tunik, S. P. Eur. J. Inorg. Chem. 2018, 3822–3828. Copyright 2018 and 2020 Wiley-VCH; copyright 2019 and 2020 Royal Society of Chemistry.

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9.02.3

Silver(III) organometallics

9.02.3.1

Introduction

81

The organometallic chemistry of silver in the oxidation state +3 has experienced relatively little development in the course of the last 15 years. As will be apparent in the following section, silver(III) complexes with porphyrinoids continue to dominate the scene. Other organometallic complexes of silver(III) have been postulated as reactive intermediates in several organometallic reactions in which silver centers are used as catalysts, but apart from one exception listed below they could not be isolated and characterized. Interestingly, though, the relative stability of some of these species allowed at least the study of their stability and reactivity by using advanced mass spectrometric techniques.

9.02.3.2

Silver(III) complexes with porphyrinoid ligands

The main class of stable silver(III) complexes that have been isolated and characterized to date is the one involving porphyrinoids ligands in which one or even two of the coordinating nitrogens has been substituted by a carbon atom. To this class of ligands belong singly and doubly N-confused porphyrins, carbaporphyrinoids of various kinds and O-confused oxaporphyrins. Research on this peculiar class of complexes started at the turn of the century and its initial development has been already reviewed in the previous edition of COMC.4 In the course of the last 15 years, work on this topic has rather focused on broadening the variety of accessible ligand systems rather than on the study of the structural, physico-chemical and catalytic properties of the corresponding silver complexes. For example, the group of Furuta, one of the pioneers in this field, reported on the possibility to prepare a silver(III) complex 250 with N-confused porphyrins in which the C-coordinated pyrrole ring is additionally substituted with a bromide; this complex could be subsequently used as reagent in a Pd-catalyzed cross coupling reactions (Suzuki, Sonogashira or Stille), replacing the bromide with an aryl, heteroaryl or alkynyl group. The reaction worked effectively, although silver was partially substituted with palladium during the process.249 The group of Lash also reported quite extensively on these systems. For example, they prepared and used as ligands N-confused porphyrins in which the confused pyrrole ring is also N-substituted with a methyl or a phenyl group. Formation of the silver(III) complex with this proligand is possible, but is accompanied by oxidation by the silver(I) reagent of the N-substituted pyrrole ring, thus providing complexes 251 exhibiting an oxo-substituted, C-coordinated pyrrole.250 The same group later reported on the preparation of silver(III) complexes with alternative N-confused porphyrins bearing exocyclic oxo groups such as oxybenziporphyrins and oxynaphthiporphyrins (complexes 252 and 253) (Fig. 51).251

Fig. 51 Examples of silver(III) complexes with porphyrinoid ligands.

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The same group reported on the reactivity of azuliporphyrins with silver. In this case, reaction with silver(I) acetate proceeded but also caused ring contration to form silver(III) benzocarbaporphyrin complexes in low yield (Scheme 29).252

Scheme 29 Reactivity of azuliporphyrins with silver to form silver(III) benzocarbaporphyrin complexes.

Alternative examples of silver(III) complexes of this kind were published by the same group in 2017 and include compounds 259–261 with various degrees of functionalization at the ligand (Fig. 52).250 Still in 2017, the group of Furuta provided the first example of a silver(III) complex with a doubly N-confused porphyrine ligand. The ligand required thiophenyl moieties as protecting groups for the confused rings. The resulting compound was extensively characterized from the point of view of its physico-chemical properties; aromaticity of the macrocyclic ligand was confirmed, its redox behavior was determined by cyclic voltammetry and the emissive properties of the complex in solution in the NIR range were demonstrated (maxima at 939 and 906 nm) (Fig. 53).253

Fig. 52 Other examples of silver(III) complexes.

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83

Fig. 53 A silver(III) complex with a doubly N-confused porphyrine ligand.

Finally, the preparation of dimers of N-confused porphyrin bridged by a dipyrrin bridge was reported and their coordination chemistry with silver centers was investigated. Reaction with two equivalents of silver(I) acetate allowed the loading of both porphyrin rings with a silver(III) center. Additionally, the use of excess silver(I) centers enabled the formation of octanuclear complexes in which two diporphyrin moieties, each loaded with their competent silver(III) center, are bridged by four silver(I) centers, each coordinating to the free pyrrolic nitrogen of a porphyrin ring and to a pyrrolic nitrogen of the dipyrrin bridge of the other molecule. The assembly is further stabilized by argentophilic interactions and by additional weak p interactions with pyrrole rings. The overall assembly is labile, but it could be nevertheless structurally characterized and identified also by ESI-MS spectroscopy (Scheme 30).254

Scheme 30 Coordination chemistry with silver centers of N-confused porphyrin bridged by a dipyrrin bridge.

9.02.3.3

Silver(III) complexes with other organometallic ligands

It has proved very difficult to isolate and characterize silver(III) organometallic species apart from the porphyrinoid complexes mentioned above, in spite of the interest presented by these compounds, since they are nowadays considered key intermediates in silver-catalyzed cross coupling reactions. Indeed, convincing proof of the intermediacy of such species in several catalytic processes has been gathered, deriving from kinetic data, spectroscopic monitoring and computation,255,256 but attempts to isolate these species has, in the vast majority of cases, been frustrated by the poor stability of these compounds. Nevertheless, isolation was successful at least in one case, in which a triaza macrocyclic compound was used as the reagent in a cross coupling process.257 Treatment with silver perchlorate cleanly afforded the tetracoordinated, mononuclear silver(III) complex 265 which is clearly stabilized by coordination to the macrocyclic nitrogens and can be conveniently isolated and structurally characterized (Scheme 31). The complex reacts readily with nucleophiles delivering the free macrocyclic ligand with a functionalized aryl

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Scheme 31 Preparation of a stable phenylsilver(III) complex by oxidative addition.

group and liberating again a silver(I) center (Scheme 32). Thus, the silver(I) species is restored at the end of the process, and indeed the authors showed that it is possible to carry out such a coupling reaction with a catalytic quantity of silver(I), which also provided direct proof of the intermediacy of the silver(III) complex.

Scheme 32 Reactivity of a phenylsilver(III) complex with nucleophiles.

9.02.4

Summary and outlook

Although less intensively investigated compared to the other group 11 metals, the organometallic chemistry of silver has been significantly expanded in the course of the last 15 years. The field that has enjoyed the greatest attention by the organometallic community is certainly the chemistry of silver complexes with stable carbenes, which is testified both by the great variety of structures that have been disclosed and by the extensive applications involving these compounds as transmetallating reagents and as antimicrobial agents. Silver complexes with standard NHC ligands have been also occasionally employed as catalysts,258 although such application has been not so extensive, given the lability of the silver-carbene bond and the consequent limited stability of the complexes. However, it should be remarked that such lability can also be exploited for the in situ generation and stabilization of free carbenes that can serve as catalytically competent species in their own right in organocatalytic processes.76 Numerous investigations have also regarded discrete polynuclear organosilver complexes, mainly stabilized by carbenes or by alkynyl ligands, whose luminescence properties have raised considerable interest; research in this subfield will be expectedly extended in the next future to involve also molecular or atomically precise silver clusters. In general, it can be expected that future developments in this field of organometallic chemistry will concern more the optimization of existing compound classes for given applications rather than the discovery of novel structural motifs in silver organometallics. Apart from the applications already mentioned above, the construction of silver-based organometallic sensors and receptors is also worth mentioning and further examples of supramolecular organometallic silver complexes exhibiting these properties are very likely to appear in the next future.

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Chem. 2008, 47, 7094–7105. Li, B.; Huang, R.-W.; Zang, S.-Q.; Mak, T. C. W. CrystEngComm 2013, 15, 4087–4093. Li, B.; Zang, S.-Q.; Ji, C.; Mak, T. C. W. J. Organomet. Chem. 2011, 696, 2820–2828. Zang, S.-Q.; Cheng, P.-S.; Mak, W.; T.C., CrystEngComm 2009, 11, 1061–1067. Cheng, P.-S.; Marivel, S.; Zang, S.-Q.; Gao, G.-G.; Mak, T. C. W. Cryst. Growth Des. 2012, 12, 4519–4529. Hau, S. C. K.; Mak, T. C. W. J. Organomet. Chem. 2015, 792, 123–133. Cheng, P.-S.; Hau, S. C. K.; Mak, T. C. W.; Cheng, P.-S.; Hau, S. C. K.; Mak, T. C. W. Aust. J. Chem. 2014, 67, 1849–1859. Wang, S. M. J.; Mak, T. C. W. Polyhedron 2009, 28, 2684–2692. Zhao, Y.; Zhang, P.; Li, B.; Meng, X.; Zhang, T. Inorg. Chem. 2011, 50, 9097–9105. Zhao, L.; Chen, X.-D.; Mak, T. C. W. Organometallics 2008, 27, 2483–2489. Cheng, P.-S.; Hau, S. C. K.; Mak, T. C. W.; Cheng, P.-S.; Hau, S. C. K.; Mak, T. C. W. Aust. J. Chem. 2013, 66, 419–428. Li, B.; Huang, R.-W.; Yao, H.-C.; Zang, S.-Q.; Mak, T. C. W. CrystEngComm 2013, 16, 723–729. Wang, S. M. J.; Zhao, L.; Mak, T. C. W. Dalton Trans. 2010, 39, 2108–2121. Hau, S. C. K.; Mak, T. C. W. Chem. – Eur. J. 2013, 19, 5387–5400. Hau, S. C. K.; Cheng, P.-S.; Mak, T. C. W. Polyhedron 2013, 52, 992–1008. Zang, S.-Q.; Zhao, L.; Mak, T. C. W. Organometallics 2008, 27, 2396–2398. Zhang, T.; Kong, J.; Hu, Y.; Meng, X.; Yin, H.; Hu, D.; Ji, C. Inorg. Chem. 2008, 47, 3144–3149. Li, B.; Zang, S.-Q.; Liang, R.; Wu, Y.-J.; Mak, T. C. W. Organometallics 2011, 30, 1710–1718. Xie, Y.-P.; Al-Thabaiti, S. A.; Mokhtar, M.; Mak, T. C. W. Inorg. Chem. Commun. 2013, 31, 54–57. Xie, Y.-M.; Fan, Y.-Y.; Lin, F.-L.; Hu, T.; Liu, J.; Lu, C.-Z. J. Mol. Struct. 2017, 1150, 335–339. Zhao, L.; Wan, C.-Q.; Han, J.; Chen, X.-D.; Mak, T. C. W. Chem. – Eur. J. 2008, 14, 10437–10444. Hu, T.; Mak, T. C. W. Cryst. Growth Des. 2013, 13, 4957–4967. Wen, L.-L.; Wang, H.; Wan, C.-Q.; Mak, T. C. W. Organometallics 2013, 32, 5144–5152. Yang, J.; Hu, T.; Mak, T. C. W. Cryst. Growth Des. 2014, 14, 2990–3001. Ji, G.; Zhang, S.; Hau, S. C. K.; Zhao, L. Chin. J. Chem. 2017, 35, 1824–1828. Zhang, S.-S.; Su, H.-F.; Zhuang, G.-L.; Wang, X.-P.; Tung, C.-H.; Sun, D.; Zheng, L.-S. Chem. Commun. 2018, 54, 11905–11908. Wang, Q.-M.; Lin, Y.-M.; Liu, K.-G. Acc. Chem. Res. 2015, 48, 1570–1579. Ge, R.; Li, X.-X.; Zheng, S.-T. Coord. Chem. Rev. 2021, 435, 213787. Hu, T.; Hu, C.; Li, Y.; Meng, L.; Xie, Y.; Liao, M.; Zhong, G.; Lu, C.-Z. Nanoscale 2020, 12, 11847–11857. Chen, Z.-Y.; Tam, D. Y. S.; Mak, T. C. W. Chem. Commun. 2016, 52, 6119–6122. Chen, Z.-Y.; Tam, D. Y. S.; Mak, T. C. W. Nanoscale 2017, 9, 8930–8937. Duan, G.-X.; Xie, Y.-P.; Jin, J.-L.; Bao, L.-P.; Lu, X.; Mak, T. C. W. Chem. – Eur. J. 2018, 24, 6762–6768. Hau, S. C. K.; Mak, T. C. W. Dalton Trans. 2017, 46, 14098–14101.

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Hau, S. C. K.; Yeung, M. C.-L.; Yam, V. W.-W.; Mak, T. C. W. J. Am. Chem. Soc. 2016, 138, 13732–13739. Hailmann, M.; Radacki, K.; Finze, M. Z. Für Anorg. Allg. Chem. 2020, 646, 777–783. Hailmann, M.; Wolf, N.; Renner, R.; Hupp, B.; Steffen, A.; Finze, M. Chem. – Eur. J. 2017, 23, 11684–11693. Hailmann, M.; Wolf, N.; Renner, R.; Schäfer, T. C.; Hupp, B.; Steffen, A.; Finze, M. Angew. Chem. Int. Ed. 2016, 55, 10507–10511. Zhang, X.; Wang, J.-Y.; Huang, Y.-Z.; Yang, M.; Chen, Z.-N. Chem. Commun. 2019, 55, 6281–6284. Zhang, M.-M.; Dong, X.-Y.; Wang, Z.-Y.; Luo, X.-M.; Huang, J.-H.; Zang, S.-Q.; Mak, T. C. W. J. Am. Chem. Soc. 2021, 143, 6048–6053. Qu, M.; Li, H.; Xie, L.-H.; Yan, S.-T.; Li, J.-R.; Wang, J.-H.; Wei, C.-Y.; Wu, Y.-W.; Zhang, X.-M. J. Am. Chem. Soc. 2017, 139, 12346–12349. Kiefer, C.; Bestgen, S.; Gamer, M. T.; Kühn, M.; Lebedkin, S.; Weigend, F.; Kappes, M. M.; Roesky, P. W. Chem. – Eur. J. 2017, 23, 1591–1603. Fresta, E.; Fernández-Cestau, J.; Gil, B.; Montaño, P.; Berenguer, J. R.; Moreno, M. T.; Coto, P. B.; Lalinde, E.; Costa, R. D. Adv. Opt. Mater. 2020, 8, 1901126. Johnson, A.; Marzo, I.; Gimeno, M. C. Dalton Trans. 2020, 49, 11736–11742. Hailmann, M.; Hupp, B.; Himmelspach, A.; Keppner, F.; Hennig, P. T.; Bertermann, R.; Steffen, A.; Finze, M. Chem. Commun. 2019, 55, 9351–9354. Kritchenkov, I. S.; Gitlina, A. Y.; Koshevoy, I. O.; Melnikov, A. S.; Tunik, S. P. Eur. J. Inorg. Chem. 2018, 3822–3828. Han, L.-J.; Wu, X.-X.; Ma, Z.-G.; Li, Y.; Wei, Q.-H. Dalton Trans. 2020, 49, 8347–8353. Ibáñez, S.; Peris, E. Chem. – Eur. J. 2018, 24, 8424–8431. Ibáñez, S.; Poyatos, M.; Peris, E. Angew. Chem. Int. Ed. 2018, 57, 16816–16820. Ishizuka, T.; Yamasaki, H.; Osuka, A.; Furuta, H. Tetrahedron 2007, 63, 5137–5147. Lash, T. D.; Von Ruden, A. L. J. Org. Chem. 2008, 73, 9417–9425. Lash, T. D.; Young, A. M.; Rasmussen, J. M.; Ferrence, G. M. J. Org. Chem. 2011, 76, 5636–5651. Adiraju, V. A. K.; Ferrence, G. M.; Lash, T. D. Org. Biomol. Chem. 2016, 14, 10523–10533. Yan, J.; Yang, Y.; Ishida, M.; Mori, S.; Zhang, B.; Feng, Y.; Furuta, H. Chem. – Eur. J. 2017, 23, 11375–11384. Wojaczynski, J.; Maciołek, J.; Chmielewski, P. J. Chem. – Asian J. 2017, 12, 643–647. Capdevila, L.; Andris, E.; Briš, A.; Tarrés, M.; Roldán-Gómez, S.; Roithová, J.; Ribas, X. ACS Catal. 2018, 8, 10430–10436. Deuker, M.; Yang, Y.; O’Hair, R. A. J.; Koszinowski, K. Organometallics 2021, 40, 2354–2363. Font, M.; Acuña-Parés, F.; Parella, T.; Serra, J.; Luis, J. M.; Lloret-Fillol, J.; Costas, M.; Ribas, X. Nat. Commun. 2014, 5, 4373. Wang, Z.; Tzouras, N. V.; Nolan, S. P.; Bi, X. Trends Chem. 2021, 3, 674–685.

9.03

Zinc, Cadmium and Mercury

Debabrata Mukherjee, Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, India © 2022 Elsevier Ltd. All rights reserved.

9.03.1 Introduction and scope 9.03.2 Cp0 -M(II) complexes (M ¼ group 12 metals) 9.03.3 NHC-M(II) complexes (M ¼ group 12 metals) 9.03.4 Group 12 metal hydride complexes 9.03.5 Group 12 metal alkyl peroxides by O2 insertion into MdC bonds 9.03.6 Low-oxidation state group 12 metal-metal bonded compounds 9.03.7 Summary Acknowledgments References

90 90 94 104 105 113 117 117 117

Nomenclature Cp Cp CpCl5 CptBu3 CpMe4 CpiPr4 CpMe4Et CpMe4tBu CpMe4(SiMe3) CpMe4(SiMe2tBu) CpMe CpSiMe3 CpMe4(CH2)3NMe2 CpMe4(CH2)2NMe(CH2)2NMe2 Cp(Ph-4-tBu)5 Ind Flu Dipp Mes Xyl HMDS IPr IMes IMe Me2 IMe ItBu sIMes bpzMe2Cp Dipp nacnacMe ToM (bdmap)H (azol)H (tbo)H Me2 CAACDipp DMAP py4-Me py4-pyr PMDTA TMEDA

C5H5 C5Me5 C5Cl5 C5(1,2,4-tBu3)H2 C5Me4H C5iPr4H C5Me4Et C5Me4tBu C5Me4(SiMe3) C5Me4(SiMe2tBu) C5H4Me C5(SiMe3)H4 C5Me4{(CH2)3NMe2} C5Me4{N(Me)(CH2)2NMe2} C5(4-tBu-C6H4)5 C9H7 (indenyl) C13H9 (fluorenyl) 2,6-iPr2-C6H3 2,4,6-Me3-C6H2 2,6-Me2-C6H3 Hexamethyldisilylamide; N(SiMe3)2 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene 1,3-Bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene 1,3-Dimethylimidazol-2-ylidene 1,3,4,5-Tetramethylimidazol-2-ylidene 1,3-Di-tert-butylimidazol-2-ylidene 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene 2,2-Bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl [(DippNCMe)2CH]− Tris(4,4-dimethyl-2-oxazolinyl)phenylborate 1,3-Bis(dimethylamino)propan-2-ol 1-Aziridineethanol 1,4,6-Triazabicyclo[3.3.0]oct-4-ene 1-(2,6-Diisopropylphenyl)-3,3,5,5-tetramethyl-2-pyrrolidinylidene 4-Dimethylaminopyridine 4-Methylpyridine 4-Pyrrolidinopyridine N,N,N0 ,N00 ,N00 -Pentamethyldiethylenediamine N,N,N0 ,N0 -Tetramethylethylenediamine

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00162-1

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TEEDA Me6TREN TEMPO TBHP [BArF4]−

9.03.1

N,N,N0 ,N0 -Tetraethylethylenediamine Tris[2-(dimethylamino)ethyl]amine (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl tBuOOH [B{3,5-(CF3)2-C6H3}4]−

Introduction and scope

Zinc alkyls being one of the earliest known metal-organic compounds, organozinc chemistry has been pursued quite intensely ever since the inception of organometallics.1 Despite the emergence of more reactive magnesium organometallics, organozinc reagents continue to thrive today by virtue of their milder nature, superior selectivity and functional group tolerance and very importantly, their easier transmetallating ability.2 Reagents like RZnX (R ¼ alkyl,3 aryl,4 allyl,5 alkenyl,6 2-pyridyl7 etc.), organozinc pivaltes,8 and multi-metallic ArZnX.MgX2 ∙ LiX9 have demonstrated a commendable synthetic aptitude in CdC bond forming methodologies in recent times. Despite their toxicity, organomercury chemistry is also well established, with considerable synthetic importance although the nature of HgdC bond is significantly different from the ZndC bond.10 Organocadmium chemistry in comparison has found much less synthetic importance likely due to their reduced activity, thermal and photochemical instability, and toxicity.2d,10c,d,11 Apart from this reagent based research, there is another vast segment of group 12 organometallics that deals with well-defined molecular complexes containing metal-carbon bonds, either at the supporting or at the reactive ligand part. With assured spectroscopic and crystallographic handles, compounds of this sort are useful in understanding catalysis and reaction mechanism, isolating reactive intermediates, and finding unprecedented bonding motifs and reactivity. With the aid of modern tools and techniques and smart ligand design, this area has flourished significantly over the last few decades.2k A few befitting examples could include the dizincocene [Cp 2Zn2],12 the first ever Zn(I) compound featuring a ZndZn bond; [(NHC)ZnH2]2,13 wherein the otherwise unstable molecular ZnH2 is trapped for the first time by NHC coordination; and the scorpionate supported [ToMZnOOR]14 (R ¼ Et, nPr, iPr, tBu), that are not only the first isolated monomeric and terminal zinc alkylperoxides but are also with remarkable thermal stability. Cadmium and mercury have also afforded relevant and interesting compounds along these lines, but of course to a lesser extent than zinc. This chapter intends to highlight five key areas from the wealth of such group 12 organometallic complexes. The first two sections are dedicated to the two most ubiquitous C-donor ligand platforms in organometallic chemistry: the monoanionic cyclopentadienyl and analogous indenyl and fluorenyl donors (hereby denoted as Cp0 ) that can provide a flexible (both s- and p-type) binding pattern, and singlet carbenes, mostly N-heterocyclic, that are strong s-donors and weak p-acceptors. This is followed by a section on heteroleptic molecular hydrides that are highly relevant in functionalizing small unsaturated molecules including CO2. Subsequently, the fundamental reactivity of metal alkyls (mostly of zinc) that is towards the molecular oxygen is discussed in the light of molecular metal alkylperoxides that are long thought to be the reactive intermediates in exothermic oxidation processes. The chapter finally ends with low-oxidation state metal-metal bonded compounds. In that section, reactivity of the much acclaimed dizincocene [Cp 2Zn2] is highlighted for the cases where the Zn-Zn motif is retained. In addition, a common bulky aryl ligand is noted which affords the homologous series of ZndZn, CddCd, and HgdHg bonded compounds. It goes without saying that zinc makes the maximum contribution in all of these areas compared to cadmium and mercury, but the latter two also chip in with some intriguing examples. The total number of compounds falling into these categories are admittedly too many, yet, effort has been made to make the list as comprehensive as possible. Synthesis, reactivity and catalytic activities are categorically mentioned as appropriate. ORTEP diagrams are provided for selected key compounds to allow structural impressions. It is worth noting the significant usage of sterically bulky and often chelating ancillary ligands in most of the cases for better kinetic control and stability.

9.03.2

Cp0 -M(II) complexes (M ¼ group 12 metals)

The cyclopentadienyl ligand class (Cp0 ) bears a special importance in zinc chemistry for molecules like the dizincocene [Cp 2Zn2], the first Zn(I) complex with a covalent ZndZn bond (see later for more details).12 Zn(II) also affords a variety of homo- and heteroleptic derivatives with noticeable structural variation. The Zn(II)-Cp0 interaction exhibits intermediate covalency with both s and p type bonding being envisaged. The hapticity can vary from Z1 to Z5. Assigning the bonding and hapticity thus may sometimes be ambiguous. The homoleptic zincocenes are typically made via salt metathesis between Zn(II) halides and MCp0 (M ¼ alkali metals) and sometimes via protonolysis.15 The parent zincocene [Cp2Zn]1 (1) is a coordination polymer16 whereas the substituted derivatives may vary from coordination polymers to monomeric slipped sandwiches.15d,17 In some cases, the zinc atom is found to be disordered in the solid state between two equivalent sites as shown by [CpMe4tBu Zn] (2) in Fig. 1.17b Zincocenes in solution 2

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Fig. 1 Zincocenes 2–4 and their ORTEP diagrams.

Fig. 2 H2 activation and imine hydrogenation by NHC-zincocene combination and the ORTEP diagram of 7.

also exhibit dynamic coordination behavior of the Cp0 rings, most of the time rapidly on the NMR timescale.15d Interestingly, 2 Me rings are [CpMe 2 Zn(k -teeda)] (3; Fig. 1) gives enantiomerically pure crystals by spontaneous resolution where both the Cp 1 18 Z -bonded with chirogenic a-carbon atoms. It is however stereo-labile in solution as suggested by the NMR spectroscopy. Reacting 1 and [Zn(HMDS)2] in 3:1 ratio gives the mixed-ligand dinuclear complex 4 (Fig. 1) which shows for the first time that a Cp ring can bridge between two nonbonded metal sites on its same face.19 The Cp moieties in 4 exchange position (between terminal and bridging) in solution at elevated temperatures. The combination of NHC and zincocene leads to an interesting H2 activation chemistry, a rare process in general among main group systems. As it turns out, sterically demanding NHCs like IPr or sIMes do not form adducts with [Cp 2Zn] (5) but the smaller Me2 IMe and ItBu do.20 Both the Cp rings in [Cp 2Zn(ItBu)] (6) are s-bonded to the trigonal planar zinc center in solid state (Fig. 2). The said combination of NHC and zincocenes, forming adduct or not, shows catalytic activity in imine and ketone hydrogenation at high pressures (68–100 bar) (Fig. 2). An incipient zinc hydride is likely the active species as suggested by the isolation of [Cp ZnH(sIMes)] (7; Fig. 2).20a 5 alone under hydrogenolysis (PH2: 68 bar) is reduced to the dizincocene [Cp 2Zn2] (8), offering Zn]1 (9) and [CpSiMe3 Zn]1 (10) also form a cleaner synthetic route for the latter (Fig. 2). Other zincocenes such as [CpMe4 2 2 monomeric adducts with NHCs and some of them also catalyze imine hydrogenation.21 The Cp-derived monoanionic hybrid scorpionate [bpzMe2Cp] supports a series of heteroleptic zinc alkyl, chloride, amide, and alkoxide derivatives.22 The Zn center as shown in the alkyl complex 11 (Fig. 3) has a distorted tetrahedral geometry where the Cp ring is Z1(p)-bonded. The alkyl and alkoxide complexes are active catalysts for the ring-opening polymerization of cyclic esters.

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Fig. 3 Hybrid scorpionate supported zinc ethyls 11 and 12 and the ORTEP diagram of 12.

Fig. 4 Zinc bis(fluorenyl) and ansa-bis(indenyl) complexes 14 ∙2THF and 15∙ 2THF and the ORTEP diagram of 14∙ 2THF.

An enantiopure version of this ligand framework featuring a myrtenyl moiety is also available which shows a similar binding pattern in [(R,S-bpzMe2myCp)ZnEt] (12; Fig. 3), that happens to be the first metal complex bearing an enantiopure hybrid scorpionate/Cp ligand.23 The latter gives isotactic-enriched poly(lactides) by catalyzing the ring-opening polymerization of rac-LA. [(Ind)2Zn] (13) and [(Flu)2Zn] (14) are isolated from ZnCl2 via salt metathesis with [K(Ind)] and [Na(Flu)], respectively. The solvent-free derivatives are likely coordination polymers as is 1.24 Their solution NMR data in THF-d8 indicate a different level of charge delocalization. While the indenyls appear to be s-bonded through the a-carbon (high sp3 character), the fluorenyls are more aromatic with complete delocalization of the negative charge. A crystallized sample of the bis(THF)-solvated [(Flu)2Zn(thf )2] (14 ∙2THF) however exhibits s-bonded Flu groups in solid state (Fig. 4). The 1,8-naphthylene-bridged ansa-bis(indenyl)zinc (15 ∙2THF) is also obtained as a bis(THF) adduct from the K2-salt and ZnCl2.25 The Zn center is pseudo-tetrahedral in the solid state where one indenyl is s-bonded, while the other one shows a Z2(s,p)-coordination (Fig. 4). Here too, the solution NMR data suggest a temperature-dependent dynamic behavior. Donor-functionalized Cp0 s offering additional intramolecular stabilization by chelation can be advantageous. A few zincocenes such as 16–19 are made of such Cp0 s with varying length of pendant amine or thio donors (Fig. 5).26 A few heteroleptic derivatives including alkyl (20), chloride (21), and acetate derivatives (22) can also be made via ligand dismutation from the homoleptic compounds (Fig. 5). The Cp0 rings in the solid state are typically Z1-bonded, either through the linker-head carbon or the one immediately next to it, but exhibit dynamic behavior in solution. The CpMe4(CH2)3NMe2 with a –(CH2)3-tethered donor amine in 18 and 19 exhibits hemilabile coordination.26c The CpMe4(CH2)2NMe(CH2)2NMe2 ligand, with a pendant ethylenediamine moiety, functions as a tridentate chelator to give the mononuclear zinc chloride 23.26c The donor arms also help to stabilize cationic zinc centers that result from partial protonolysis of those zincocenes in the presence of pyridine as shown by 24 (Fig. 6).26c Partial protonolysis of [Cp 2Zn] by 0.5 equiv. of the Brookhart’s acid, [H(OEt2)2] [BArF4] in the absence of an additional donor gives the cationic triple-decker sandwich complex [Cp 3Zn2][BArF4] (25; Fig. 6) which catalyzes the intra- and intermolecular hydroamination of alkenes and alkynes, respectively.27 The Cp0 cadmium and mercury compounds are relatively less well known. Reductive elimination from cadmocenes and mercurocenes can sometimes lead to decomposition into metallic cadmium and mercury.28 Cp0 to Cd/Hg bonding in the majority of cases is fluxional in nature in solution and tends to be monohapto- in the solid state with significant diene character.

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Fig. 5 Donor-functionalized zincocenes 16–23 and the ORTEP diagram of 18.

Fig. 6 Zinc cations 24 and 25 and the ORTEP diagram of 25 (showing only the cationic part).

Like [Cp2Zn]1 (1), [Cp2Cd]1 (26) is also polymeric in the solid state.29 It crystallizes from pyridine as [Cp2Cd(py)2] (26∙ 2py) in which both Cp rings are Z1(s)-bonded. The Cp-Cd interactions become Z1 in [Cp2Cd(k3-pmdta)] (27) from Z2 in [Cp2Cd(k2-tmeda)] (28) with the increase in denticity of the amine base.30 LiCp reduces CdCl2 into Cd(0) but reacts with Cd(acac)2 to give tBu3 Cd] (31) are made from NaCpiPr4 and CptBu3Na, respectively, [Cp 2Cd] (29) which is light-sensitive.28b [CpiPr4 2 Cd] (30) and [Cp2 by reacting with CdCl2.31 31 is a linear cadmocene with Z1/Z1-hapticities (Fig. 7). The mixture of 29 and [Cd(HMDS)2] undergoes a Schlenk-type equilibrium favoring the heteroleptic system [Cp Cd(HMDS)]2 (32), which is a silazide-bridged dimer with s-bonded Cp rings (Fig. 7).28b CdCl2 also reacts with one equiv. each of CptBu3Na and MesMgBr to give the heteroleptic compound [CptBu3Cd(Mes)] (33), in which the CptBu3 ring is Z5-bonded.31 Mercurocenes exhibit the highest synthetic variability among the three metals. [Cp2Hg] (34), made via protonolysis from [Hg(HMDS)2] and CpH, is molecular, unlike polymeric [Cp2Zn]1 (1) and [Cp2Cd]1 (26).15b,32 It is air-stable but light sensitive. [CptBu3 Hg] (35) is obtained by reducing [CptBu3 BiCl] by sodium amalgam.33 [Cp2Me4(SiMe2tBu)Hg] (36) is made via salt 2 2 Me4(SiMe2tBu) 34 (SiMe3) ]. [Cp2 Hg] (37) and [CpMe4(SiMe3) Hg] (38) can be obtained from HgCl2, either metathesis from HgCl2 and [LiCp 2

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Fig. 7 Cadmocene 31 and half-sandwich amido system 32, and its ORTEP diagram.

by salt metathesis with KCp0 or by transmetallation with ZnCp0 2.35 The [Cp(Ph-4-tBu)5]% radical reacts with metallic mercury to give [Cp(Ph-4-tBu)5 Hg] (39; Fig. 8).36 All of these mercurocenes are linear or nearly linear in the solid state with both the Cp0 rings being 2 1 Z (s)-bonded. Mercury also forms a series of linear or nearly linear Z1(s)-bonded half-sandwich chlorides [Cp0 HgCl]n featuring various kinds of intermolecular association by means of secondary Hg ⋯ Cl interactions. For example, [Cp HgCl]28a (40), [CpMe4HgCl]35 (41), and [CpMe4(SiMe3)HgCl]35 (42) feature ladderlike double-chain pseudo-polymeric networks, [CpMe4tBuHgCl]35 (43) and [CpMe4(SiMe2tBu)HgCl]434 (44) are pseudo-tetrameric, while [Cp(SiMe3)HgCl]35 (45) forms a pseudo-infinite quadruple chain. [CpCl5HgPh] (46) is monomeric and linear in the solid state where the CpCl5 ring is s-bonded.37 Nearly half of the homoand heteroleptic Cp0 -Hg compounds listed above appear to be fluxional in solution and also in the solid state in some cases.

9.03.3

NHC-M(II) complexes (M ¼ group 12 metals)

N-heterocyclic carbenes (NHCs) have become a ubiquitous ligand class for metals across the Periodic Table. The coordination chemistry of zinc is well established on this ligand platform.38 A host of NHC
ZnR2 adducts (R ¼ alkyl39 (47), aryl39b,40 (48) alkynyl40b (49), cyclopentadienyl20b (50), silyl41 (51), halide40b,42 (52), amide43 (53), hydride13,44 (54), boroydride42c (55), alkoxide45 (56), aryloxide46 (57), thiolate47 (58), o-benzosemiquinolate48 (59), carboxylate42a,49 (60), and triflate42a (61) have been isolated and structurally characterized as either monomers or dimers (Fig. 9). Polymeric networks of NHCs are also known to form adducts with zinc dihalides.50 The hydride dimer 54 deserves a special mention since [ZnH2]1 is otherwise polymeric, thermally unstable, and has never been crystallographically characterized. This highlights the prowess of NHCs in stabilizing reactive main group fragments. NHCs also stabilize cationic zinc alkyl39b,51 (62), aryl40b (63), thiolate52 (64), and hydride40b,44,52 moieties (Fig. 10). The cationic zinc hydrides are further elaborated in a separate section below. The zinc centers in 62 and 63 are two coordinate, with a nearly linear geometry in the solid state showing no close contacts with the [B(C6F5)4]− anion. Dual carbene coordination to a cationic zinc alkyl is also possible.53 These cationic complexes have been shown to act as strong Lewis acid catalysts. A wide variety of heteroleptic RZnR0 species have also been stabilized by NHCs in molecular form (Fig. 11). The list includes molecules like [(IPr)ZnEtCl]42e,54 (65), [(IMes)Zn(SiPh3)Cl(thf )]41 (66), [(IMes)Zn(m-OCH2Ph)Cl]242e,54 (67), [(IMes)Zn(m-O2CH) Me]255 (68; the two formate groups are differently bridged), [(IMes)Zn(m-H)Et]255 (69), [(IPr)Zn(m-H)I]242f,44 (70), [(IMes)Zn(m-OH) (OMes)]246 (71), [(IPr)Zn(HMDS)H]56 (72), and [(ItBu)ZnCp (C6F5)]57 (73). [(ItBu)Cp Zn(C6F5)] readily undergoes CO2 insertion into the Zn–Cp bond to give the carboxylate 74 (Fig. 11) and not the thermodynamically stable NHC
CO2 adduct, which again

Fig. 8 Mercurocene 39 and its ORTEP diagram.

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95

Fig. 9 Examples of NHC
Zn(II) adducts and the ORTEP diagrams of 47 and 54.

emphasizes the strength of NHC
Zn(II) interaction.57 A NHC-supported mixed hydrido silazide cluster 75 (Fig. 11) has also been synthesized.56 Reacting [(IPr)ZnCl2(thf )] with [K(Fp)] (Fp ¼ (Cp)Fe(CO)2) gives the heterobimetallic system [(IPr)ZnCl(Fp)] (76) with an unsupported ZndFe bond (Fig. 11).58 An abnormal carbene is also known to support EtZn(HMDS) (77; Fig. 11).59 NHCs can also be a part of chelating ligands complexing with zinc centers in multidentate fashion (Fig. 12). For example, a neutral hybrid NHC-py chelator gives the EtZnI complex 78.54a NHC-based monoanionic ancillary ligands giving heteroleptic zinc complexes are more versatile. The ligand types include an NHC-alkoxide60 (79), a bis(NHC)-methanide61 (80), an oxazolinylborato hybrid scorpionate62 (81), a chiral NHC-sulfonate63 (82), and a series of CNN and CNP pincers64 (83–85).

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Fig. 10 NHC-stabilized cationic zinc complexes 62–64 and the ORTEP diagrams of 62 and 63 (showing only the cationic parts).

The more strongly electrophilic nature of the cyclic alkyl(amino)carbenes (CAACs) compared to imidazolylidene-based NHCs leads to a different behavior towards nucleophilic zinc dialkyls. The adduct [(Me2CAACDipp)ZnMe2] (86) is unstable even at −40  C and readily undergoes methyl migration from Zn to the “carbene” carbon which decomposes further at room temperature (Fig. 13).65 Diamidocarbenes (DACs) also behave similarly with ZnR2 (R ¼ Me, Et) and even with CdMe2 (see below) for the same reason.66 Cationic zinc alkyl and aryl moieties [(Me2CAACDipp)ZnR][B(C6F5)4] (R ¼ Me (87), C6F5 (88); Fig. 13) are however CAAC-stabilized, in which the pseudo two-coordinate zinc centers have additional close contacts from the counter anion [B(C6F5)4]−.65 Bis(CAAC) stabilized ZnMe+ (89; Fig. 14) has also been reported.65 These cationic species exhibit high Lewis acidic behavior in common with NHC-bonded zinc cations. The [(Me2CAACDipp)ZnCl2] (90) adduct is reduced by KC8 into [(Me2CAACDipp)2Zn] (91) that has a singlet biradicaloid character as shown in Fig. 14.67 The zinc center resides at the center of inversion and has a linear geometry while the “carbene” carbon atom is trigonal planar. A BICAAC-supported (BICAAC ¼ bicyclic (alkyl)(amino)carbene) ZnCl2 adduct can similarly be reduced to give another example of a singlet biradicaloid.68 The catalytic activity of these carbene-bound zinc compounds has been well explored. Cationic complexes with enhanced Lewis acidity are sometimes more effective. Polymerization of cyclic esters39b,d,42e,45,54,59,60,64 and the hydrosilylation of small organic molecules such as carbonyls,44 nitriles,52 alkenes and alkynes,51,52,61,65 and CO251–53,65 have been the prime catalytic processes investigated. Other transformations include CO2 cycloaddition to epoxides,50b,c,69 hydrosilylative N-methylation of amine by CO2,70 alcohol/silane dehydrocoupling,61 enantioselective allylic alkylation,63 hydroboration of internal alkynes,40b borylation of alkyl halides by B2(pin)2,71 C-H borylation of terminal alkynes40b and heteroarenes,72 cycloisomerization,73 intramolecular hydroarylation and hydroalkoxylation of terminal alkynes,73 and transfer hydrogenation of diphenylethylene.73 Carbene-bonded cadmium complexes are much rarer. The few well-characterized examples include the homo- and heteroleptic adducts [(IPr)CdCl2(thf )]74 (92), [(IPr)Cd(Mes)2]40a (93), [(IPr)Cd(m-OTf )2]275 (94), and [(IPr)Cd(m-I)(OTf )]275 (95) (Fig. 15). 95 crystallizes in two different forms with the triflates being terminal in one and bridging in the other. 94 shows catalytic activity in hydrosilylation/hydroboration of sterically demanding carbonyls.75 Similar to zinc dialkyls, diamidocarbene (DAC) bound CdMe2 (96; Fig. 15) is unstable and undergoes methyl group migration from Cd to the “carbene” carbon which further decomposes into metallic cadmium.66 Unlike cadmium, mercury shows a rich carbene chemistry by holding a diverse range of NHCs including multidentate and chelating and giving a variety of neutral and ionic complexes. The synthesis involves either direct coordination of free carbenes to HgX2 salts or preparing the carbene in situ from the corresponding imidazolium salt using Hg(OAc)2 that acts both as a base as well as the metallating agent. External bases such as KOtBu, K2CO3 or NaOAc are however necessary with non-basic mercury precursors. The composition and structure of these systems depends on several factors like carbene/Hg precursor types and ratio, reagent concentration, linker length and flexibility, the presence of additional donors, choice of solvent, and also the anion type (coordinating vs. weakly coordinating or non-coordinating). These studies of Hg-carbenes have, however, been focused more on synthesis and structural elucidation and less on reactivity. They have been used as transmetallating agents on some occasions76; catalytic activity of only a few examples has been registered.

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97

Fig. 11 NHC-supported heteroleptic RZnR0 species with a variety of R and R0 groups, and the ORTEP diagrams of 74 and 76.

1:1 carbene-Hg(II) adducts are relatively rare, but include the dihalides75,77 (97–99), the bis(triflate)75 (100), and the mixed iodo triflate75 (101) (Fig. 16). IPr disproportionates Hg2Cl2 into 97 and Hg.78 Cationic complexes such as 102 have also been accessed (Fig. 16).79 In situ deprotonation of imidazolium salts by KOtBu in acetonitrile may result in the deprotonation of a solvent molecule to various extents as shown by the formation of compounds 103–105 (Fig. 16).80 Bis(NHC) coordination to a single Hg(II) center is more common and with a wider variation of ligand. The two carbene moieites can be tethered together by various linkers, either as a open chain or in the form of a macrocycle. There can also be additional donors, either on the linker or as a side arm which may or may not coordinate depending on the overall flexibility. Selected examples can be categorized based on whether the two NHCs are not connected (106–110)81 (Fig. 17) or are tethered by various linkers (111–118) (Fig. 18).82 107 features metallocene-fused NHCs bonded to a Hg2+ center.81b Arene ⋯ Hg(II) interactions are envisaged in 115 and 116.82e,f

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Fig. 12 Zinc complexes (78–85) supported by NHC-derived ancillary ligands and the ORTEP diagrams of 82 and 85.

Fig. 13 CAAC-bound zinc alkyl and aryl complexes (86–89) and the ORTEP diagram of 87.

Zinc, Cadmium and Mercury

Fig. 14 The bis(CAAC)-supported 91 with a singlet biradicaloid character and its ORTEP diagram.

Fig. 15 Cadmium-carbene complexes 92–96 and the ORTEP diagram of 92.

99

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Fig. 16 NHC-Hg(II) complexes 97–105 with 1:1 carbene/Hg ratio and the ORTEP diagram of 105.

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Fig. 17 non-tethered bis(NHC)-Hg(II) complexes 106–110 and the ORTEP diagram of 107.

101

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Fig. 18 Tethered bis(NHC)-Hg(II) complexes 111–118 and the ORTEP diagram of 115 (showing only the cationic part).

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103

Fig. 19 Multinuclear Hg(II)-NHC complexes 119–122 with tethered multi(NHC) chelating ligands and the ORTEP diagrams of 119 and 122 (showing only the cationic parts).

Tethered bis(NHC) ligands can sometimes bridge between two Hg(II) centers as seen in compounds 119–122 (Fig. 19).82g,83 A tethered tris-carbene ligand supports the trinuclear hexacationic mercury complex 122.84 Mercury-CAAC complexes are much rarer in comparison but have been implicated in catalysis.85 For example, the dimeric HgBr2 adduct 123 catalyzes the intermolecular hydroamination of PhC^CH by aryl amines (Scheme 1).85a

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Scheme 1 CAAC-HgBr2 adduct 123 that catalyzes intermolecular hydroamination.

9.03.4

Group 12 metal hydride complexes

Among the group 12 metals, zinc dihydride ([ZnH2]1) is a polymeric solid but unlike [MgH2]1 it is thermally unstable and slowly decomposes into metallic zinc and H2 at room temperature even under an inert atmosphere.86 [CdH2]1 is even more unstable and rapidly decomposes above −20  C.86 Unlike Zn and Cd, solid HgH2 is a covalently bonded molecular solid but the decomposition temperature is even lower (−125  C).87 A dimeric motif “[HZn(m-H)]2” has been identified recently by the aid of strong NHC coordination for a small number of examples, exemplified by complex 54.13,44 By contrast, a host of heteroleptic zinc hydrides of the type “LZnH” have been isolated featuring a variety of mononionic ancillary ligands (L) featuring C, N, O, and S donors sites. The synthetic routes summarized in Scheme 2 are commonly employed.

Scheme 2 Common synthetic routes towards heteroleptic zinc hydrides.

The wide variety of ancillary ligand types that has been employed includes scorpionates with pyrazolyl (124),88 oxazolinyl (125),89 and 2-mercapto-imidazolyl (126)90 donors, a bis(pyrazolyl)phosphiniminomethanide heteroscopionate (127),91 various other methanides with pyridyl (128),92 pyridazinyl (129),93 benzimidazolyl (130),94 and NHC (131)61 donors, b-diketiminates

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105

without (132 and 133)95 and with (134–135)96 additional donors, dipyrromethenes (136),97 an ethylenediamine phenolate pincer (137),98 a conjugated bis-guanidinate (138),99 an aminoethylamide (139),100 an acenaphthene-derived radical anion (140),101 bulky aryls (141),102 and a bulky amide (142) as summarized in Fig. 20.103 Depending on the steric bulk and the coordinating nature of L, these zinc hydrides are mostly monomers or hydride-bridged dimers while the coordination number of zinc varies from 2 to 4. The ancillary ligand L can also be bridging in nature as seen in compound 139. The amido zinc hydride 142 is essentially two-coordinate with a nearly linear geometry at the zinc center. The b-diketiminato zinc hydride with a pendant phosphine donor arm (135) undergoes TM-mediated (TM ¼ transition metal) dehydrogenation forming TM-bridged ZndZn bonded species.96c Several Zn-TM heterobimetallic hydrides have been made by reacting the b-diketiminato zinc hydride 132 with a variety of TM fragments, showing the gradual transition defined by breaking the polar ZndH bond and forming the increasingly covalent TMdH and TMdZn bonds.104 A few zinc hydride clusters (143–149) have also been reported in which both terminal and bridging hydride moieties can be found (Fig. 21).105 The N-N bridged bis-b-diketiminato tetranuclear zinc hydride cluster 146 has been investigated in the context of hydrogen storage.105e The hexanuclear system [Zn(m-H)(m2-Z1-Z1-TEMPO)]6 (148) features a planar [Zn6H6] ring in the solid state.105g Its synthesis involves a rare case of H2 activation by a zinc alkoxide [Zn(TEMPO)2]2 following a radical pathway. The overall trianionic tris(b-diketiminato)cyclophane ligand provides a highly effective steric encapsulation to the Zn3H3 core in 149 which makes it air and moisture stable.105f It also resists nitriles, carbonyls, terminal acetylenes, and CS2 but reacts with CO2 to give 150 (Fig. 21) by insertion into one of the three Zn-H moieties. A number of structurally well-defined hydridozincates are also known.106 In recent times cationic zinc hydrides supported by neutral donors have also been isolated and explored in reduction catalysis. Strongly s-donating NHCs were the first candidate ligands to stabilize such moieties in both mononuclear40b,44 (151 and 152) and cluster52 (153) forms (Fig. 22). The cationic moieties can be generated by hydride abstraction/protonolysis from “ZnH2” precursors. Later it was shown that simple multidentate amines such as bidentate TMEDA,107 TEEDA,107 and TMPDA,108 tridentate PMDETA,109 and tetradentate Me6TREN109,110 can stabilize cationic zinc hydrides in both mononuclear (154–156) and cluster (157–159) forms (Fig. 22). Zinc hydrides are synthetically important for their reducing capabilities.111 A number of these molecular hydrides including the cationic ones find catalytic activity primarily in alcohol/silane dehydrocoupling and in the hydrosilylation or hydroboration of polar unsaturated molecules including CO2.89b,91,93,94,96b,97a,99,112 The active zinc hydrides are sometimes generated in situ.113,114 Zinc hydride-mediated enantioselective hydrosilylation has also been achieved by using chiral ligands and Zn(OAc)2 or ZnEt2 as the precatalyst.115 Abiotic carbonyl hydrosilylation with high enantioselectivity has even been achieved through an enzymatic zinc hydride in a protic environment.116 A heteroleptic zinc hydride supported by a chiral oxazoline-based scorpionate ligand leads to enantioselective alcohol/silane dehydrocoupling.117 Molecular hydrides of the heavier cadmium and mercury are much rarer compared to the lighter zinc due to their inherent instability. Only a few heteroleptic cadmium hydrides have been isolated and structurally characterized with monoanionic ancillary ligands such as b-diketiminate118 (160), bulky aryl102b,119 (161 and 162), and sila-linked tris(benzimidazolyl)methanide120 (163) (Fig. 23). Interestingly, 160 is reduced to [(DippnacnacMe)Cd-Cd(DippnacnacMe)] accompanied by H2 evolution in reactions with carbodiimides, even with a catalytic amount, instead of C]N bond insertion.118 161 is a loosely associated dimer while the use of even bulkier aryls at the flanks render 162 monomeric.102b,119 162 is also thermally more resilient than 161. The methanide in 163 shows pseudo k3-coordination in the solid state leaving a single benzimidazolyl group uncoordinated.120 A few examples of stoichiometric reactivity of 163 are known, including heterocumulene (CE2; E ¼ O, S) insertion and hydride abstraction by B(C6F5)3.120 The aryl mercury hydride [ArDippHgH] (164; ArDipp ¼ 2,6-(Dipp)2-C6H3) has been spectroscopically characterized but not crystallographically authenticated.119

9.03.5

Group 12 metal alkyl peroxides by O2 insertion into MdC bonds

Oxygen insertion into MdC bonds is one of the oldest known reactions in organometallic chemistry. The reactivity of zinc alkyls towards molecular O2 has been investigated ever since the inception of organozinc chemistry in Frankland’s era.1a The reactions are normally spontaneous and fast with little control, giving zinc alkoxides and oxide as end products, and typically following a radical pathway. Some zinc alkyls are pyrophoric in nature. While zinc alkylperoxides are long thought to be the reactive intermediates, it is not until the early 21st century when a [Zn]OOR complex was isolated and structurally characterized.121 Since then, a handful of zinc alkyl peroxides in molecular or in cluster forms have been isolated by carefully controlled oxidation by employing sterically demanding and chelating ancillary ligands for kinetic protection. The basic [Zn]dH bond can also be protonated by ROOH to generate zinc alkylperoxides.14 The mechanistic insights of the O2 insertion have also been looked at rigorously.

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Fig. 20 Heteroleptic zinc hydrides 124–142 and the ORTEP diagrams of 132 and 142.

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107

Fig. 21 Zinc hydride clusters 143–149, the reactivity of 149 towards CO2, and the ORTEP diagrams of 148 (only the N-O linkages of the TEMPO moieties are shown) and 149.

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Fig. 22 Cationic zinc hydrides 151–159 and the ORTEP diagrams of 152 and 156 (showing only the cationic parts).

Fig. 23 Heteroleptic cadmium hydrides 160–163 and the ORTEP diagram of 162.

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Among the dialkyls, [Zn(tBu)2] is shown to exhibit temperature and co-ligand dependent outcomes under oxidation conditions.122 The reaction with O2 at −78  C in THF without any additional donor other than the coordinating solvent itself selectively leads to the oxidation of a single ZndC bond, giving the mixed alkyl zinc alkoxide 165 as a dimer (Scheme 3). The same reaction in the presence of a strong Lewis base like py4-Me gives the mixed alkyl zinc alkylperoxide 166 at a slightly elevated temperature of −45  C (Scheme 3). Oxygenation of a Me2Zn/a-diimine (tBuDAB) system results in the methylperoxide cluster 167 (Scheme 3).123

Scheme 3 Controlled oxidation of zinc dialkyls to obtain alkoxide 165 and alkylperoxides 166 and 167, and the ORTEP diagram of 167.

The b-diketiminato zinc ethylperoxide dimer 168 with bridging OOEt groups is the first among the block to be structurally characterized which can be cleaved into a monomer unit (169) by py4-Me as an additional donor (Scheme 4).121,124 The N-N bridged bis-b-diketiminate ligated dinuclear zinc system similarly affords the dinuclear zinc ethylperoxide 170 with terminal OOEt groups, but only in the presence of an additional equivalent of py4-Me base per zinc center (Scheme 4).125 In the absence of py4-Me, the oxidation reaction under the same conditions gives a tetranuclear zinc ethoxide cluster.125 168 epoxidizes electron-deficient olefins with high regioselectively (Scheme 4).121 The epoxidation of a,b-unsaturated ketones is made catalytic by using TBHP.126 Enantioselectivity of up to 91% can be achieved in epoxidation by employing chiral ancillary ligands such as aminotroponiminate,126a bisoxazolinate,126b and enaminooxazolinate126b 171 (Scheme 4).

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Scheme 4 b-Diketiminate and chiral enaminooxazolinate supported zinc alkyl peroxides 168–171 in mediating epoxidation and the ORTEP diagram of 169 and 171.

The oxazoline-based “scorpionate” supported zinc alkyls [(ToM)ZnR] (R ¼ Et, nPr, iPr, tBu) yield a series of alkylperoxides 172–175 by O2 insertion (Scheme 5).14 All of them are mononuclear for the first time, featuring terminal [Zn]OOR moieties. Nonetheless, these systems exhibit unusually high thermal robustness, so much so that the compounds are unchanged after heating at 120  C in solution for several days. [(ToM)ZnOOR] oxidizes PPh3 into OPPh3 and undergoes Zn-OOR/Si-H s-bond metathesis with PhCH2SiMe2H, generating [(ToM)ZnOR] and [(ToM)ZnH] (125), respectively. The oxazoline-NHC heteroscorpionate supported zinc ethyl 81 also undergoes O2 insertion to give a similar terminal monomeric zinc alkyl peroxide 176 (Scheme 5).

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111

Scheme 5 Oxazoline-based scorpionate and heteroscopionate supported zinc alkyl peroxides 172–176, and the ORTEP diagram of 172.

Other related examples include the aminoalcoholates 177,127 178,128 and 179–180,129 pyrrolylketiminate130 (181), guanidinate131 (182), and fluorinated triazapentadienyl132 (183), all formed under controlled oxidation conditions (Scheme 6). Aromatic hydrocarbons such as toluene are mostly used as the solvent. Reaction temperature seems critical as on many occasions different products arise at different temperatures. Apart from the ToMZnOOR series, the majority of other zinc alkylperoxides are thermally fragile.

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Scheme 6 Zinc alkylperoxides 177–183 and the ORTEP diagrams of 177 and 180.

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113

The chemistry of cadmium organoperoxides is much less explored than zinc. Only a small number of methylperoxide clusters have been structurally characterized so far. Controlled oxidation of the in situ generated methylcadmium cluster [{MeCd(bdmap)}3 ∙CdMe2] at −78  C gives the double-peroxide species [{MeCd(bdmap)}4Cd(OOMe)2] (184; Scheme 7).133 The same reaction at room temperature gives the triple-peroxide [(MeCd)5(bdmap)2(OOMe)3] (185; Scheme 7).133 Both the OOMe groups in the former are m2-bridging, while in the latter, two of the three OOMe groups are m3-bridging while the third one exhibits m3- and m2-bridging modes, respectively from its -OOMe and -OOMe atoms. With the comparatively higher electronegativity and lower oxophilicity of Hg(II), organomercury compounds are much less sensitive towards air and moisture compared to organozinc and cadmium. For example, HgMe2 survives the exposure to both air and water.

Scheme 7 Cadmium methylperoxide clusters 184 and 185, and the ORTEP diagram of 184.

9.03.6

Low-oxidation state group 12 metal-metal bonded compounds

Hg2+ 2 salts of mercury are well known, while zinc and cadmium typically assume +2 oxidation state in their salts as well as in molecular coordination compounds. Against this perception, the isolation of the dizincocene [Cp 2Zn2]12 (8) with a ZndZn bond in 2004 was a remarkable feat. This discovery has served as a prelude to many other interesting developments in low-oxidation state main group chemistry, especially of divalent group 2 and 12 elements.134 The reactivity of 8 has been the subject of intense investigation since its initial discovery. The original synthesis from [Cp 2Zn] (5) and ZnEt2 was rather accidental and low yielding. A few different and improved synthetic routes have also been established as shown in Scheme 8.135 The hydrogenolysis route from 5 to 8 in Fig. 2 is also advantageous for relatively easier workup.

Scheme 8 Improved synthetic routes for the dizincocene 8.

8 is pyrophoric in nature but unreactive towards H2, CO, and CO2.12,135a Water, alcohol (tBuOH), arylcyanide (XylCN), and ZnR2 (R ¼ Me, Mes) all lead to disproportion into Zn(II) and elemental zinc,135a whereas iodine results in simple oxidation.135a However, in some other controlled modes of reactivity, 8 may not sacrifice the ZndZn bond which is the focus of discussion in this section. Lewis bases such as NMe3, TMEDA, pyridine, bipyridine, PMe3, and PPh3 remain mostly unreactive towards 8, likely due the reduced Lewis acidity of Zn(I).135a However, stronger Lewis bases such as DMAP and NHCs (ItBu, sIMes) do form adducts while preserving the ZndZn bond (Scheme 9). An NHC adduct 6 was already noted in Fig. 2. In [Cp 2Zn2(dmap)2] (186), both DMAP ligands are geminally bonded to one Zn site.136 The two Cp coordination modes are dynamic in solution, making them indistinguishable, and both appearing to be Z5-bonded. Another NHC adduct 187 shows similar solid and solution state behavior.21 The Cp ligands in fact appear to behave like capping agents to the [Zn2]2+ motif that are removable partially or fully under appropriate protonolytic or oxidative conditions in a controlled manner. Two equiv. of a Brønsted acid cleaves off both the Cp groups to give [Zn2(dmap)6][Al{O(CF3)3}4]2 (188; Scheme 9), the first structurally characterized solvated zinc analog of the calomel-dication [Zn2]2+.137 Strong coordination from DMAP helps enhance the stability, but it was later shown that even THF is able to support the [Zn2]2+ ion (see below). Like [Cp 2Zn], [Cp 2Zn2] also undergoes partial protonolysis by 0.5 equiv. of the

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Brookhart’s acid to give another cationic triple-decker sandwich [Cp 3Zn4(OEt2)2][BArF4] (189) with two intact Zn-Zn units sharing one central Z3-Cp ligand (Scheme 9).135b Although tBuOH leads to disproportionation, bulkier alcohols such as ArMesOH (ArMes ¼ 2,6-(Mes)2-C6H3) in the presence of another strong Lewis base like py4-pyr can precisely protonate off one Cp group to give the mixed-ligand half-sandwich system 190 (Scheme 9).138 Although CO and CO2 do not react with 8, the isocyanate (Dipp) N]C]O gives the amidate 191 by insertion while preserving the ZndZn bond (Scheme 9).139 Interestingly, unlike the typical N,O-coordination, the amidate in this case p-chelates through the tethered Cp in Z4-fashion giving the first olefin-bound Zn(I) complex. On a comparative note, intra- and intermolecular Zn(II)-p complexes are relatively more abundant, likely due to its stronger Lewis acidic nature.140

Scheme 9 Reactivity of dizincocene 8 towards strong Lewis donors, Brønsted acids, and isocyanate and the ORTEP diagrams of 186 and 191.

8 acts as a precursor to various other ZndZn bonded compounds 192–196 simply by transferring the [Zn2]2+ motif to other mono-anionic and chelating ligand types by partial or complete protonolysis or by salt exchange (Scheme 10).141

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Scheme 10 Zn(I) dimers 192–196 synthesized from 8 by transferring the Zn-Zn moiety and the ORTEP diagram of 195.

The ferrocenium cation [Fc]+, instead of oxidizing the Zn(I) centers, removes the Cp ligands from 8 by oxidative coupling giving decamethylfulvene (Cp )2 and 197/198 depending on the reaction stoichiometry (Scheme 11).135b,142 The THF ligands in 198 are labile and can be replaced by phenyl isocyanide.142 198 remains unperturbed in the presence of CO but transfers the ZndZn moiety to Cp or DippnacnacMe via salt exchange.142

Scheme 11 Oxidative elimination of Cp 2 from 8 by ferrocenium cation and the ORTEP diagram of 198 (showing only the cationic part).

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In situ generated and isolobal [Cp Zn]+ and [Cp Cu] add to 8 to form the triangular clusters [Cp 3Zn3]+ (199) and [Cp 3Zn2Cu] (200), respectively, with high degree of s-aromaticity (Scheme 12).143 8 is inert towards the metallo-ligand [Cp Ga(I)] but reacts with [Cp Ga2][BArF4] to give the hetero-bimetallic cluster 201 (Scheme 12) where a [Zn2]2+ motif is surrounded by six [Cp Ga] units resembling the DMAP and THF bound cations 188 and 198, respectively.144 The reaction also produces both metallic gallium and zinc, although the mechanism behind this redox process is not well understood. 201 exhibits two different coordination modes (Scheme 12). In the solid state, two of the six [Cp Ga] ligands are bridging between the two Zn(I) centers, but variable-temperature 1 H NMR studies indicate the coexistence of a second isomer in solution featuring six terminal GaCp ligands, three apiece at each Zn(I) center, resembling 188 and 198. 1, 8 and other ZnR2 (R ¼ Me, Et, Bn etc.) species are established sources of one-electron organozinc ligands in transition metal complexes and related clusters and in many cases the Zn-Zn linkage persists.145

Scheme 12 Cationic Zn3 cluster 199 and the heterobimetallic ZndCu and ZndGa clusters 200 and 201 that retain the ZndZn bond and the ORTEP diagram of 199 (showing only the cationic part).

A few other organometallic,102a,135a non-organometallic,146 and metalla-ligated147 ZndZn bonded compounds have been reported following the discovery of 8. Some of them are strong enough reducing agents to convert Cd2+ and Hg2+ into their elemental states. The dizincocene 202 (Scheme 13) is made by reducing a mixture of [CpMe4Et Zn] and ZnCl2 by KH.135a The 2 Me4Et rings being eclipsed and the ringcent-Zn-Zn-ringcent motif being slightly structure of 202 is similar to that of 8, with the two Cp deviated from linearity. It has a lower thermal stability than 8 as a solid: 202 decomposes slowly at 0–20  C. Compared to zinc, molecular coordination/organometallic compounds of cadmium and mercury in the +1 oxidation state featuring CddCd118,146h,148 and HgdHg146h,i,149 bonds are rarer. This is particularly surprising for mercury since there exist several inorganic Dipp salts of Hg2+ gives a rare opportunity to compare all the three metal-metal bonds of group 12 by 2 . The bulky aryl ligand Ar affording the homologous series of low-oxidation state organometallic compounds 203–205.102a,119,150 Their syntheses are achieved by reducing the divalent iodide precursors by alkali metal or alkali metal hydride reagents. Notably, an attempted synthesis of 205 from Hg2I2 and ArDippMgBr led to disproportionation into ArDippHgI and metallic mercury.119 203–205 are structurally similar with the two aryl ligands aligned nearly orthogonal to one another, thereby providing effective steric protection to the M-M core. The Cipso-M-M-Cipso motif is linear and the M-M bond distance varies in the order of Zn < Hg < Cd. The slight contraction in Hg-Hg distance compared to Cd-Cd arises from the relativistic effect of mercury, a phenomenon which is also supported by theoretical studies.119

Zinc, Cadmium and Mercury

117

Scheme 13 Low-oxidation state group 12 organometallic complexes 202–205 with MdM bonds and the ORTEP diagrams of 202 and 203.

9.03.7

Summary

The advances in five key areas of molecular group 12 organometallics have been highlighted in this chapter. Down the group, the periodic variation from zinc to mercury is prominent as the compounds of similar classes and their subsequent chemistries differ noticeably. The zinc organometallics in particular has flourished significantly, especially with the advent of carbene ligands and the low-oxidation state Zn(I)dZn(I) bonding motif. The catalytic applications are also promising which include small molecule activation like the functionalization of CO2.

Acknowledgments The author acknowledges SERB, India for the research funding through a start-up grant and the Ramanujan Fellowship while writing this chapter. The author additionally acknowledges IISER Kolkata and the two graduate students Mr. Sudip Baguli and Ms. Sumana Mondal for helping with the literature survey.

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9.04 Recent Development in the Solution-State Chemistry of Boranes and Diboranes Meera Mehta, Department of Chemistry, University of Manchester, Manchester, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

9.04.1 9.04.2 9.04.3 9.04.3.1 9.04.3.2 9.04.4 9.04.4.1 9.04.4.2 9.04.5 9.04.5.1 9.04.5.2 9.04.6 9.04.6.1 9.04.6.2 9.04.7 9.04.7.1 9.04.7.2 9.04.7.3 9.04.8 9.04.9 9.04.10 9.04.10.1 9.04.10.2 9.04.10.3 9.04.11 9.04.11.1 9.04.11.2 9.04.12 References

Introduction Synthetic methods for the preparation of triaryl boranes Organic diboranes(4) [(alkyl/aryl)4B2] Historical background Synthesis of organodiboranes(4) Boron-boron multiple bonds Diborenes Diborynes Hydroboration reactions Main-group catalyzed hydroborations Uncatalyzed hydroborations using bis(pentafluorophenyl)borane [HB(C6F5)2] Carboboration reactions 1,1-Carboboration 1,2-Carboboration Cationic boron containing compounds Borinium cations Borenium cations Boronium cations Boryl anions Borate weakly coordinating anions Boron radicals Anionic boron radicals Neutral boron radicals Cationic boron radicals Frontiers Main-group catalysis Functional polymers Conclusions

123 123 130 130 130 134 135 136 138 138 147 149 152 159 160 160 162 165 167 172 173 173 178 181 183 183 185 188 189

Abbreviations CAAC Cat Cp DABCO DAC DFT Dipp Dur FLP Mes MIC NacNac NHC NHO NIH NMR − NTf2 − OTf Pin Py

122

Cyclic(alkyl)(amino)carbene Catechol Pentamethylcyclopentadiene 1,4-Diazabicyclo[2.2.2]octane Diamidocarbene Density functional theory 2,6-Diisopropylphenyl 1,2,4,5-Tetramethylphenyl Frustrated Lewis pair 2,4,6-Trimethylphenyl Mesoionic carbene b-Diketiminato liagands N-heterocyclic carbenes N-heterocyclic olefin N-heterocyclic imines Nuclear Magnetic Resonance Trifluoro-N-((trifluoromethyl)sulfonyl)methanesulfonamide anion Trifluoromethanesulfonate anion Pinacol Pyrdine

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00125-6

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

THF Tipp TMP TMS TOF Tol XRD

9.04.1

123

Tetrahydrofuran 2,4,6-Tri-isopropylphenyl 2,2,6,6-Tetramethylpiperidine Trimethylsilyl Turnover frequency Toluene X-ray diffraction

Introduction

Elemental boron was first isolated in 1808 independently by French chemists Joseph Louis Gay-Lussac and Louis Jaques Thénard, and British chemist Sir Humphry Davy.1,2 In terms of crustal abundance, boron is second in Group 13 behind aluminum, and the 38th most abundant element overall at approximately 10 ppm.3 Boron is primarily sourced as mineral borax, Na2[B4O5(OH)4]
8H2O, and kernite, Na2[B4O6(OH)2]
3H2O with large reserves in Kern County, California, and Turkey.4 Boron has three valence electrons and ground state electron configuration of 1s22s22p1. As a trivalent neutral compound, for instance BMe3, boron has six valence electrons, with sp2-hybridization, trigonal planar geometry, and an empty p-orbital making it naturally electron deficient. These properties make boranes isoelectronic with carbocations. The Lewis acidic nature of boranes has been widely investigated in both stoichiometric and catalytic transformations. Boron can also form anionic tetracoordinate compounds with a complete valence shell, these adopt a tetrahedral geometry. Carbon–boron bonds range from 1.55 to 1.59 A˚ in tricoordinate compounds, longer than a typical carbon–carbon bond.5 While, boron–oxygen bonds in tricoordinate compounds range from 1.31 to 1.38 A˚ , shorter than ethereal carbon–oxygen bonds. This shorter bond length is the result of p-backdonation from the oxygen lone pair into the empty p-orbital on boron resulting in partial double bond character. Lone pair p-backdonation is also the reason why the Lewis acidity of haloboranes increases in the order BF3 < BCl3 < BBr3 < BI3. Over the last century, organoboron compounds have come to the forefront of synthetic chemistry. Synthetic transformations where boranes are central include hydroboration reactions, carboboration reactions, frustrated Lewis pair (FLP) reactions, Lewis acid catalysis, hydroboration followed by oxidation to access alcohols, Suzuki-Miyaura reactions, and many others. Given their electron deficient nature and central position in synthetic manipulations, organoboranes have found applications in the pharmaceutical industry, as neutron capture therapy agents, in materials science, and in molecular imaging. Organoborane compounds feature at least one carbon–boron bond and can commonly be divided into the categories: boranes; borinic acids; borinic esters; boronic acids; boronic esters; boronamides; boryl anions; borohydrides; and trihaloborates with general formulas shown in Fig. 1.4 Although there is variation within the literature on the classification of boranes, here the term borohydride is used to describe boron compounds which contain at least one hydrogen–boron bond and triorganoboranes contain three boron–carbon bonds. Borinic acids contain two boron–carbon bonds and a hydroxyl group and borinic esters are related to borinic acids but feature an alkoxy group rather than a hydroxyl group. While, boronic acids and esters feature one organic group and either two hydroxyl groups or two alkoxy groups, respectively. Boroamides contain one organic group and two boron–nitrogen bonds, and boryl anions are isoelectronic with N-heterocyclic carbenes and are often generated from reduction of the haloborane. In this chapter the following areas are discussed: Synthetic methods for the preparation of triaryl boranes; Organic biboranes [(alkyl/aryl)4B2]; Boron-boron multiple bonds; Hydroboration reactions, Carboboration reactions; Boron cations; Boryl anions; Borate weakly coordinating anions; Boron radicals; and Frontiers.

Fig. 1 Common categories of organoboranes.

9.04.2

Synthetic methods for the preparation of triaryl boranes

Three-coordinate boron compounds have found applications in many fields, including optoelectronics,6–8 anion sensors,9–11 small molecule activation and catalysis,12–16 and as bioimaging agents.17–20 Many of these reagents and their applications have been previously reviewed,21–25 while developments in synthetic methodology to prepare aryl boranes have been discussed to a lesser extent.26 Here the discussion is focused on triaryl boranes of the nature BAr13, BAr12Ar2, and BAr1Ar2Ar3, where all three substituents are the same (BAr13), two are the same (BAr12Ar2), and none are the same (BAr1Ar2Ar3).

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The advent of Grignard reagents in 1900 opened the door to a powerful alkyl/aryl transfer tool.27 Early investigations of Grignard reagents in arylborane chemistry were by Khotinsky and Melamed.28 Here, best results were observed when isobutylborate ester was employed as the boron source. In König and Scharrnbeck’s investigations of Grignard reagents with isobutylborate ester they were able to prepare numerous arylboronic acids and diarylborinic acids.29 Over 70 years later, several research groups demonstrated that triaryl boranes could be obtained from boronic esters using more reactive organometallic reagents.30–35 In 1921, Krause employed BF3 as the boron source in combination with Grignard reagents to give trialkylboranes and alkylboronic acids.36 It was found that this method could also be expanded to triarylboranes with the preparation of triphenylborane and tri-p-tolylborane,37 opening the door to reproducible access to triarylboranes. Later, in 1930 and 1931, tri-p-anisylborane, tri-p-xylylborane and tri-a-naphthylborane were also prepared in a similar manner.38,39 These boranes could be subsequently isolated as the carbon disulfide (CS2), diethyl ether (OEt2), or ammonia (NH3) adducts. Brown and co-workers re-synthesized tri-a-naphthylborane as a Lewis acid reference with which to assess the Lewis basicity of primary, secondary, and tertiary amines.40 At this time, Brown et al. moved to BF3(OEt2) as their boron precursor as it is easier to handle and control stoichiometry compared to gaseous BF3. Further, Hawkins found that formation of the Grignard in tetrahydrofuran (THF) rather than diethyl ether gave trimesitylborane with shorter reaction times.41 Wittig and co-workers found that triarylboranes could be accessed from the more reactive lithiated aryl reagent instead of the Grignard reagent.42 In the synthesis of tri(o-diphenylyl)borane and tri(4-(N,N-dimethylamino)phenyl)borane it was found that the lithation route was necessary, whereas the analogous reaction with the corresponding Grignard reagent did not afford the triarylborane. Even today when constructing boron containing compounds, these considerations into the boron source, solvent, and whether to employ a lithium or Grignard reagent are crucial. The most widely employed methods for the preparation of triarylboranes uses BF3(OEt2) as the boron source and either the aryllithium or aryl Grignard reagent.26 However, mercury, zinc, copper, silicon, and tin reagents have also been employed as aryl transfer reagents with varying reactivities and solvent compatibilities. In the case of mercury and tin reagents, their use is often avoided due to their high toxicity and associated safety considerations. Arylmercury reagents were some of the earliest investigated in the synthesis of arylboranes.43,44 In 2001, Piers and co-workers prepared the diborylated ferrocene compound (1,10 -Fc)B(C6F5)2 (2.2) from the chloroborane ClB(C6F5)2 and mercury reagent 1,10 -Fc(HgCl)2 (2.1; Fig. 2).45 The Piers group also prepared diborylated compound 2.4 using [(C6F4)Hg]3 (2.3) and excess BBr3 (Fig. 3).46 Compound 2.4 was subsequently reacted with Zn(C6F5)2 as a C6F5 transfer agent to afford 2.5. In 1998, the same group had reported Zn(C6F5)2 as a powerful reagent to prepare the perfluoroaryl mono-, bis-, and tris-substituted boranes.47 In 2003, Jäkle and co-workers employed aryl copper complexes in Cu/B exchange reactions.48 Reaction of mesitylcopper and trihaloborane were found to yield Mes2BX (X ¼ Br, Cl), where only two arenes could be installed even at elevated temperatures. In a similar fashion, when dichlorophenylborane was employed as the boron precursor the lower steric demands allowed access to Mes2BPh. Further, when C6F5Cu and BX3 (X ¼ Br, Cl) were allowed to react B(C6F5)3 was preferentially generated irrespective of the stoichiometry. Ashley and O’Hare also employed C6F5Cu as a C6F5 transfer reagent to prepare boranes (C6F5)B(C6Cl5)2 and (C6F5)2B(C6Cl5).49 Jäkle and co-workers later showed that 2,4,6-tri-isopropylphenylcopper (CuTipp) could also be used in Cu/B exchange reactions to prepare (2,4,6-tri-isopropylphenyl)diarylboranes, these triarylboranes were subsequently employed in the preparation of organoborane macrocycles and borazine oligomers.50,51 Beyond Grignard and lithium reagents, silicon and tin reagents are the most widely used to transfer aryl units to boron. Since the 1960s, aryltin reagents have been used in Sn/B exchange reactions. In 2005, Jäkle et al. reacted distannylated bithiophene precursors with dibromoarylboranes to give the triarylborane containing polymers shown in Fig. 4 by Sn/B exchange reactions.52 Since this development, many other triarylboranes and polymers featuring triarylborane have been prepared.53,54

Fig. 2 Synthesis of ferrocendiyl bisborane 2.2.

Fig. 3 Synthesis of phenylene bisborane 2.5.

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Fig. 4 Synthesis of polymeric thiophene derived polymers 2.7.

Fig. 5 Synthesis of compounds 2.9 and 2.11 from B/Si exchange.

In 1986, the preparation of mono- and diarylboranes was reported by Haubold and co-workers using Si/B exchange reactions.55 This methodology tolerated numerous functional groups, as shown in Fig. 5, with the BBr3 precursor allowing access to the diarylborane and BCl3 precursor preferring the formation of the monoarylborane product. Under harsh reaction conditions and starting from BBr3, triphenylborane could be access in a 35% yield. A year later, Kaufmann,56,57 Sniechnus,58 and Jäkle59,60 extended Si/B exchange reactions as an efficient method to install arylborane moieties to polystyrene (Fig. 6). First a styrene derivative featuring trimethylsilyl (Me3Si-; TMS) unit is reacted with BBr3 to install a BBr2 unit, followed by addition of aryltin or arylcopper reagents to install the additional aryl groups to boron. Recently, Helten and co-workers significantly improved Si/B exchange reactions by using a catalytic amount of Me3SiNTf for the preparation of triarylborane-containing macromolecules and polymers.61

Fig. 6 Synthesis of polymer 2.13 from Si/B exchange and subsequent synthesis of polymers 2.14 and 2.15.

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In 1995, Schrimpf et al. found that in-situ activation of potassium aryltrifluoroborates using trimethylsilyl chloride gave the corresponding aryldifluorboranes.62 They also found that the potassium aryltrifluoroborates can be accessed from the corresponding boronic acids and KHF2. Initially, potassium fluoroborate salts were used in transformations where the boron moiety is lost, such as coupling reactions. In 2004, Piers and co-workers treated potassium aryltrifluoroborate and the appropriate Grignard reagent to yield triarylboranes.63 Potassium aryltrifluoroborate and boronic acids are attractive precursors to triarylboranes given their greater bench stability. Recent advancements in boron-based catalysts and frustrated Lewis pair (FLP) chemistry, prompted the Soós and Hoshimoto groups to exploit the potassium aryltrifluoroborates and Grignard reagent synthetic route to prepare triarylboranes featuring fluorinated and chlorinated arenes.64–67 To prepare a push-pull system with a pyrene core, Marder and co-workers prepared borane 2.17 also in a similar fashion (Fig. 7).68 While, Wagner and co-workers prepared triarylboranes 2.18–2.20 and 2.23 from the potassium aryltrifluoroborates and an appropriate aryl lithiating reagent (Fig. 8), as precursors to polycyclic aromatic hydrocarbons or quadruply annulated borepins (2.24).69,70 In 2016, Ito and co-workers treated (diphenylmethylsilyl)dimesitylborane (2.25) with arylhalides as a direct method to install dimesitylborane and eliminate the silylhalide (Fig. 9).71 As a minor impurity, the aryl(diphenylmethyl)silane could also be detected. Later in 2019, the same group activated C–H bonds of benzofuranes and installed a dimestitylborane using (diphenylmethylsilyl)dimesitylborane (2.25) and an iridium catalyst, as shown in Fig. 10.72 In these transformations the silylated by-products were formed in approximately 30% yield.

Fig. 7 Synthesis of compound with pyrene core 2.17.

Fig. 8 Polycyclic aromatic boranes 2.18–2.20 and 2.24.

Fig. 9 Direct installation of dimesitylborane during the synthesis of compound 2.26.

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Fig. 10 C–H bond activation and synthesis of compound 2.27 using iridium.

In 1958, Mikhailov and co-workers reported the synthesis of a BAr1Ar2Ar3 borane by step-wise functionalization.73 Starting from an isobutyl borinic ester (2.28) featuring a phenyl group and a chlorine atom, Mikhailov replaced the chlorine unit with an o-tolyl group using the appropriate Grignard reagent. Next, the isobutyloxide group was converted to a chloride using PCl5. This halogen was subsequently replaced by a third different arene again using a Grignard reagent, as shown in Fig. 11. This general process could be used to make other triarylboranes functionalized with three different aryl groups. In 2017, Liu et al. prepared BAr1Ar2Ar3 boranes starting from a BAr12Ar2 type borane using a palladium catalyst, as shown in Fig. 12.74 Compound 2.33 was first prepared from the boronic ester TippB(OMe)2, which was itself prepared from B(OMe)3 and TippMgBr. Once compound 2.33 was prepared, sequential palladium cross-coupling was employed to form the BAr1Ar2Ar3 boranes 2.34a and 2.34b. In the same year, Liu and co-workers prepared BAr3 borane 2.35 from BF3
OEt2 and the respective aryl lithium reagent, and through subsequent cross-coupling reactions prepared BAr1Ar2Ar3 type borane 2.36 (Fig. 13).75

Fig. 11 Synthesis of BAr1Ar2Ar3 borane 2.31.

Fig. 12 Synthesis of BAr1Ar2Ar3 borane 2.34.

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Recent Development in the Solution-State Chemistry of Boranes and Diboranes

Fig. 13 Synthesis of BAr1Ar2Ar3 borane 2.36.

Fig. 14 Synthesis of BAr1Ar2Ar3 borane 2.40 using a linked boronic ester precursor.

Yamaguchi and co-workers prepared numerous BAr1Ar2Ar3 boranes from a boronic ester precursor (2.38) where the two oxygens on boron are linked (Fig. 14).76 This linkage promotes formation of the dimeric species 2.39 upon addition of two equivalents of an aryl lithium species. The dimeric intermediate 2.39 was next reacted with a different aryl lithium reagent to install the final arene and give BAr1Ar2Ar3 type boranes 2.40a–d. In 2016, Blagg and co-workers reported hetero-tri(aryl)borane (2.45) from BH3(SMe2) where the B–H bonds are stepwise substituted for aryl groups (Fig. 15).77 First, BH3(SMe2) was treated with in-situ generated C6F5Li to form (C6F5)BH2 (2.41). This primary borane was next reacted with in-situ generated {3,5-(CF3)2C6F3}Li to install the second aryl group. Secondary borane

Fig. 15 Synthesis of BAr1Ar2Ar3 borane 2.45.

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Fig. 16 Synthesis of BAr1Ar2Ar3 borane 2.48.

(2.42) was next reacted with methanol to give methoxyborane 2.43 which was further reacted with BBr3 to give bromoborane 2.44. Finally, bromoborane 2.44 was reacted with Zn(C6Cl5)2 as an aryl transfer reagent to yield the BAr1Ar2Ar3 type borane 2.45. Kelly et al. prepared a ferrocene-containing hetero-triarylborane 2.48 by stepwise reaction of dibromoferrocenylborane with two different lithium reagents (Fig. 16).78 As molecular models for polymers, Jäkle and co-workers prepared hetero-diaryl-bromoboranes from aryldibromoborane and organotin reagents. A third different aryl group was subsequently installed through the use of an organotin reagent,52 copper reagent,79–81 or Grignard reagent.82,83 Hetero-triaryl-boranes 2.49–2.54 shown in Fig. 17 were all prepared using this method.

Fig. 17 Synthesis of BAr1Ar2Ar3 boranes 2.49–2.54.

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9.04.3

Organic diboranes(4) [(alkyl/aryl)4B2]

9.04.3.1

Historical background

In recognition for their contribution to the field of boron chemistry, in 1976 and 1979 William Lipscomb and Herbert Brown were awarded the Nobel Prizes, respectively. In addition to his contributions to nuclear magnetic resonance (NMR) spectroscopy and in the chemistry of large biomolecules, Lipscomb’s key contribution to boron chemistry came from deducing the nature of chemical bonding in boranes, such as B2H6, diborane(6), and clusters. Lipscomb and others employed X-ray Diffraction (XRD) crystallography and found that the two “BH3” fragments were connected through two bridging B–H–B interactions with 2-electrons shared between the three atoms; known as a 2-electron 3-center bond (Fig. 18).84–87 No significant boron–boron interaction was observed, and subsequent reactivity studies focused on homolytic or heterolytic cleavage of these bridging interactions, often by addition of a nucleophile. Brown developed a range of alkyl- and dialkylboranes by addition of H3B•LB (where the Lewis base (LB) is a weak donor such as THF or SMe2) to alkenes. Addition of the borane to cyclooctadiene affords 9-borabicyclo[3.3.1]nonane (9-BBN),88 which has been extensively employed as a hydroboration reagent and is discussed in greater detail in Section 9.04.5 of this chapter. Generally, in the absence of Lewis bases, dialkylboranes exist as dimers with the two boron units connect through 2-electron 3-center B–H–B bridges (Fig. 18). Diboranes(4), often prepared by hydride abstraction reactions from diboranes(6), feature boron–boron single bonds. A large catalog of B2X4 (X ¼ hydride and halogens), B2(NR2)4 (R ¼ alkyl and aryl), B2(OR)4 (R ¼ alkyl and aryl), as well as some mixed functionalized diboranes(4) have been prepared and studied.89,90 Here the discussion is focused on key R4B2 (R ¼ alkyl, aryl) organodiboranes featuring B–C bonds.

9.04.3.2

Synthesis of organodiboranes(4)

In 1980, the first stable tetraorganodiborane(4) derivatives were reported, (t-Bu)2B–B(t-Bu)Me and (t-Bu)2B–B(t-Bu)(CH2t-Bu), at this time it was demonstrated that incorporation of bulky alkyl groups was essential to stability.91,92 Later, in 1992 Power and co-workers synthesized the range of organodiboranes(4) MeO(Mes)BB(Mes)OMe, Mes2BB(Mes)OMe, Mes2BB(Mes)Ph, and Mes2BB(Mes)CH2SiMe3, via reactions of the stable diborane precursor B2(OMe)4 and the appropriate organolithium reagent.93 In addition, S¸ ahin reacted (Dur)ClB–BCl(Dur) (Dur ¼ 1,2,4,5-tetramethylphenyl), with lithium amides to prepare the stable diboranes (RHN)DurB–BDur(NHR).94 In 1998, Nöth and co-workers prepared dimesityldiboranes(4) B2Mes2X2 (X ¼ Cl, Br, OR, SR, NR2, NHR); subsequent NMR spectroscopy studies and X-ray crystallographic investigations revealed that the B2Mes2X2 derivatives display greater stability than their B2(t-Bu)2X2 counterparts due to the mesityl group increasing the electronic stabilization of the B–B bond.95 In 2009, Wakamiya and Yamaguchi prepared boracycle dithieno-1,2-dihydro-1,2-diboron 3.4 and its dianion 3.5 by reduction with KC8 (Fig. 19).96 p-Conjugation in these systems was studied for potential applications as building blocks for optoelectronic materials. Later, two related diaryl diborane dianions were synthesized and their structural features and photophysical properties studied.97 In 2017, Piers and Yamaguchi contributed to the family of thiophene functionalized diboranes with the preparation of diboranes(4) 3.6 and 3.7 (Fig. 20).98 In addition to investigations into the electronic and photophysical properties of these compounds, compound 3.6 was found to activate hydrogen gas to afford diborane(6) 3.8.

Fig. 18 Examples of B–H–B bonding in diboranes(6).

Fig. 19 Synthesis of diborane 3.5.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

131

Fig. 20 Diboranes 3.6 and 3.7, and reaction of 3.6 with H2 gas.

In 2010, Matsuo and Tamao isolated the doubly-bridged butterfly-shaped diborane 3.11 by allowing diborene(4) dianion 3.10 to react with 1,2-dibromoethane (Fig. 21).99 Upon prolonged heating in benzene, compound 3.11 was found to undergo ligand C–H activation to give the borane 3.13 and eliminate hydrogen gas. In addition, pressuring compound 3.11 with hydrogen gas led to cleavage of the B–B bond and formation of diborane 3.12. Later, in 2011 Matsuo and Tamao reported the two-electron reduction of diborane 3.12 with lithium naphthalenide which led to reformation of a B–B s-bond and isolation of the diborane dianion 3.14 (Fig. 22).100 In 2014, Matsua and Tamao employed bulkier ligands into their diborane systems and found that formation of a H-terminal staggered diborane(4) was preferred rather than the related doubly-bridged butterfly-shaped diborane species (Fig. 23).101 Recently in 2019, Yamashita prepared the hydrogen-bridging tetraborane(4) 3.21 using a boron nucleophile, boryllithium 3.18, as shown in Fig. 24.102 In 2012, Wagner and co-workers prepared 9,10-dihydroanthracene-9,10-diyl-bridged B2(NMe2)2 system 3.24 (Fig. 25) by treating 9,10-dihydroanthracene (3.22) with n-BuLi to form donor-free 9,10-dilithio-9,10-dihydroanthracene, followed by addition of B2(NMe2)2Cl2.103 XRD studies revealed that the B–B bond remains intact, despite a significant increase in the distance between the carbons featuring the boryl moieties. Interestingly when 3.24 was treated with two equivalents of 1-isopropyl-2mercaptoimidazole, transamination reactions occurred together with intramolecular B–S adduct formation to produce compound 3.25 (Fig. 25).

Fig. 21 Synthesis of diborane 3.11 and subsequent reactivity.

Fig. 22 Synthesis of diborane 3.14.

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Recent Development in the Solution-State Chemistry of Boranes and Diboranes

Fig. 23 Synthesis of diboranes 3.16 and 3.17.

Fig. 24 Synthesis of diborane 3.20 and subsequent reduction.

Fig. 25 Synthesis of diborane 3.24 and subsequent reactivity with 1-iso-propyl-2-mercaptoimidazole.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

133

In 2018, Erker reported the synthesis of the tetraaryldiboranes(4) 3.29a and 3.29b by reacting borole 3.28 with terminal alkynes RCCH (R ¼ Ph, n-Pr) at 60  C for 4 days (Fig. 26).104 Tetraaryldiborane(4) 3.29b was characterized by X-ray crystallography as its acetonitrile (3.30) and 4-phenyl-3-butyn-2-one (3.31) adducts. The B–B bond of diboranes of 3.29 is readily cleaved upon reaction with dihydrogen gas under ambient conditions. Subsequent reaction with t-butylacetylene afforded the 1,2-hydroborated products 3.33 and 3.34 (Fig. 27).

Fig. 26 Synthesis of diboranes 3.29 and adducts 3.30 and 3.31.

Fig. 27 Reactivity of diboranes 3.29.

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Recent Development in the Solution-State Chemistry of Boranes and Diboranes

In 2017, Lin and Yamashita prepared tetra(o-tolyl)diborane(4) 3.35 in 30% isolated yield by reacting B2cat2 (cat ¼ catechol) with o-tolMgBr (Fig. 28).105 Reaction of 3.35 with dihydrogen gas under ambient conditions gave the corresponding diarylhydroborane by insertion into the B–B bond (Fig. 29). Later in 2018 and 2019, Lin and Yamashita activated CO, isocyanides, and diazo compounds with diborane(4) 3.35.106,107 In 2019, Yamashita also reported on the reduction of tetra(o-tolyl)diborane(4) 3.35 with an excess of lithium or magnesium metal to give the diborene dianion [(o-tolyl)4B2]2−, with a B]B double bond.108 Most recently, in 2021, Stephan and co-workers reacted tetra(o-tolyl)diborane(4) (3.35) with secondary boranes to give an inseparable mixture of products including B(o-tolyl)3 and (o-tolyl)BR2.109 In a similar fashion, when 3.35 was reacted with BH3 sources, among other products, pentaborane 3.39 (Fig. 30) was identified, the first polyaryl pentaborane(9) and the first to be assembled from reaction of borane and diborane(4).

Fig. 28 Synthesis of diborane 3.35.

Fig. 29 Subsequent reactivity of diborane 3.35.

Fig. 30 Synthesis of pentaborane(9) 3.39.

9.04.4

Boron-boron multiple bonds

Based on the observed absence of multiple bonding in the heavier main-group elements, approximately half a century ago, the classical double bond rule arose as an informal assumption. In 1975, Peter Jutzi formalized this rule as “. . .elements having a principal quantum number greater than 2 should not be able to form (p-p)p bonds with themselves or with other elements”.110 Often understood by considering the difference between the bonding of CO2, N2 and O2, all multiply bound species, compared to the heavier analogous SiO2, P4 and S8 which all present as singly bound species. Nevertheless, increasing documentation of exceptions to the classical double bond rule quickly resulted in it being discredited.

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135

In line with its second period neighbors, carbon, nitrogen and oxygen, boron forms strong stable multiple bonds in many cases. The electron deficient nature and empty p-orbital of sp2-hybridizied boron primes it to form multiple bonds with elements capable of electron-pair p donation. For instance, aminoboranes, with the general formula R2B ¼ NR2, are well known.111,112 Iminoboranes,113 RB  NR, and iminoboryl complexes,114 LnMB  NR, are also known but are rarer. In the case of oxygen-derived substituents, alkoxyboranes, R2BOR, oxoboryl complexes, MB  O,114 are also known. Although boron seems willing to participate in heteroatomic multiple bonding, it is reluctant to form homoatomic multiple bonds. Interestingly, between 1997 and 2006, the Robinson and Power groups reported doubly and triply bonded aluminum and gallium compounds featuring dicoordinate group 13 atoms.115–118 These findings suggested that homoatomic multiple bonding is easier with heavier group 13 elements, a direct contradiction of the classical double bond rule.

9.04.4.1

Diborenes

Initial efforts toward B–B multiple bond formation were focused on controlled reductions. In 1981, Berndt et al. chemically reduced a diborane(4) by transfer of a single electron to form a radical anion that occupies the p(BB) orbital to a formal B–B bond order of 1.5.119 In a similar fashion, Power and Nöth chemically reduced diboranes(4) to form the corresponding dianions with a formal B–B bond order of 2.120–122 In 2007, Robinson and co-workers reduced an N-heterocyclic carbene (NHC) adduct of boron tribromide followed by adventitious hydrogen abstraction from (presumably) the solvent to generate the neutral diborene (IDipp)HB ¼ BH(IDipp) (4.1); IDipp ¼ (1.3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), as shown in Fig. 31 Route A.123 This finding revealed the incredible stabilizing ability of persistent carbenes and reignited the study of carbene-stabilized low-valent compounds.124–126 Since the seminal report by Robinson,123 Braunschweig and co-workers have reported four independent synthetic methodologies to diborene compounds (Fig. 31). Route B involves the reduction of base-stabilized aryldihaloboranes and has been the most broadly applied.127,128 This route offers the advantage of proceeding without the need for advantageous hydrogen atom abstraction. Routes C and D are more closely related to Robinson’s approach, and employ diborane B2Br4(IDipp)2 as a precursor. In the case of Route C, the diborane undergoes a 2-electron reduction with sodium naphthalenide, while in the case of Route D diborane B2Br4(IDipp)2 undergoes a comproportionation reaction with a diboryne.129 In Route E an unusual monophosphine-stabilized diborane(4) is reduced in the presence of excess of a phosphine ligand.130

Fig. 31 General synthetic routes to diborene compounds.

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Recent Development in the Solution-State Chemistry of Boranes and Diboranes

The harsh reduction conditions required to form diborenes serves as a clue that they are highly energetic and ought to be reactive; DFT investigations confirmed that HOMO in each case is the p(BB) orbital. In 2012, Braunschweig et al. reacted diborene B2Dur2IMe2 (4.2b) with silver(I) chloride to give the p-complex [AgCl(Z2-B2Dur2IMe2)] (4.6; Fig. 32).127 Superficially, this complex resembles a conventional p-olefin complex, and DFT investigations revealed that both s donation (B2 ! Ag) and p back-donation (Ag ! B2) are present, in line with a Dewar-Chatt-Duncanson model. The differences in bonding descriptions between p olefin and p diborene complexes are postulated to be due to large negative charges on the boron atoms and the orbital energy mismatch between the accessible diborene HOMO and the silver-based frontier orbitals. Later, in 2014, Braunschweig and co-workers investigated the NHC- and phosphine-stabilized diborenes 4.2b and 4.4 in mono-oxidation reactions with one equivalent of the tropylium salt [C7H7][BArF4] (ArF ¼ 3,5-bis(trifluoromethyl)phenyl) to form the radical cations 4.5a and 4.5b (Fig. 32).130 These radical cations were further characterized by X-ray crystallography and EPR studies. The same diborenes were also studied by cyclic voltammetry, with reversible oxidation waves showing that these diborenes are strong reductants with the B–B p bond acting as a viable electron reserve. Also in 2014, in an effort to effect the hydroboration of diborenes, the Braunschweig group reacted bis(NHC) bis(heterocyclyl) diborenes 4.2c–e with catecholborane (Fig. 32).128 These reactions were found to afford diastereoselectively the hydroborated products 4.7c–e, leading to the generation of the first doubly base stabilized triborane.

9.04.4.2

Diborynes

In 2012, and in a similar fashion to the preparation of diborene 4.3 by reduction of the doubly base-stabilized tetrabromodiborane(4) with two equivalents of sodium naphthalenide (Route C, Fig. 31), the use of four equivalents of sodium naphthalenide yielded instead diboryne B2IDipp2 (4.9) as a dark green solid (Fig. 33).129 Investigation of this diboryne by X-ray L axis, consistent crystallography revealed a very short B–B bond length of 1.449(3) A˚ and near-linear alignment of the L ! B2 (A)

(B)

(C)

Fig. 32 Further reactivity of diborenes.

Fig. 33 Synthesis of diboryne 4.9 and subsequent reactivity.

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137

with a B^B triple bond. The strong reducing potential of the diboryne was revealed in its comproportionation reaction with B2Br2IDipp2 as shown in Route D, and is further evidenced in investigations with carbon monoxide. As reported in 2013, when diboryne 4.9 was exposed to CO, the doubly base stabilized bis(boralactone) 4.11 was formed as a red-orange compound (Fig. 33).131 Compound 4.11 features 4 equivalents of CO coupled in both a head-to-head and head-to-tail fashion. This was the first example of metal-free binding and coupling of multiple CO molecules. Interestingly, intermediate 4.10 could be isolated by controlled treatment of diboryne 4.9 with two equivalents of CO at low temperatures. The solid-state structure of intermediate 4.10 revealed a B–B bond distance of 1.549(3) A˚ , in line with a bond order of two. The analogous 4-electron reduction of the cyclic(alkyl)(amino)carbene (CAAC) supported compound B2Br4CAAC2 (4.12) (CAAC ¼ 3,3,5,5-tetramethyl-1-(20 ,60 -diisopropylphenyl)-pyrrolidine-2-ylidene) afforded B2CAAC2 (4.13) as a deep purple compound (Fig. 34).132 In contrast to B2IDipp2 (4.9), X-ray crystallographic investigations of 4.13 revealed a significantly longer B–B bond length of 1.489(2) A˚ . With 10 electrons distributed along the C–B–B–C chain 4.13 resembles an electron deficient (4-pelectron) cumulene. The structural differences between 4.9 and 4.13 reflect the increased p-acceptor properties of the CAAC donor compared to IDipp. Since the development of isolable diborenes and diborynes many addition reactions with polar and non-polar substrates across B–B multiple bonds have been developed.128,133–142 Selected examples of addition reactions to NHC-stabilized diboryne are shown in Fig. 35. In 2018, Braunschweig and co-workers reacted CAAC-ligated diboryne B2CAAC2 (4.13) and NHC-ligated diboryne B2IDipp2 (4.9) with dibutyldisulfide, diphenyldisulfide, or diphenyldiselenide in a slight excess (Fig. 36).143 In the case of the NHC-ligated diboryne 4.9, the dichalogen was found to add across the B–B multiple bond to give the doubly NHC-stabilized diborenes 4.14a–c. However, in the case of the CAAC-ligated diboryne 4.13 addition of the dichalogen resulted in a dark-colored NMR silent species. X-ray crystallography studies allowed the structures of diradicals 4.15a–c to be elucidated, where in stark contrast to the diborenes 14a–c, the solid-state structures revealed orthogonal C–B–S/Se planes, S/Se–B–B–S/Se torsion angles near 90 , and B–B bond lengths in line with a single bond. EPR and magnetic measurements were consistent with triplet diradical formation with p-donation from the chalcogen to boron and p-back donation into the carbon atom of the carbene. The isolation of these twisted diboranes marked the first examples of the isolation and structural characterization of diradical products from the complete homolysis and 90 twisting of double bonds.

Fig. 34 Synthesis of compound 4.13.

Fig. 35 Select examples of addition reactions across diboryne B–B multiple bonds; NHCs differ.

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Fig. 36 Dichalcogen addition to diborynes 4.9 and 4.13.

9.04.5

Hydroboration reactions

In the 1960s Brown et al. discovered hydroboration reactions, i.e., addition of a H–B bond across an unsaturated functional group.144–146 Early examples of hydroboration reactions employed diborane B2H6 as a source of H–B bonds, and this reagent showed good reactivity toward unsaturated bonds but selective mono-hydroboration remained an issue.147,148 In the presence of a Lewis base, diborane B2H6 can form borane BH3, its monomeric form. In the hydroboration of polar unsaturated groups like carbonyls, it is thought that the carbonyl oxygen coordinates to the borane then the hydride is transferred to the electrophilic carbon to afford alkoxy boranes.148 By contrast, in the case of olefins, H–B addition takes place through a concerted mechanism in a synfashion.149,150 Interestingly, these transformations were also found to prefer the anti-Markovnikov addition. Fig. 37 shows boranes commonly employed in hydroboration reactions. Diborane is highly reactive and effective in hydroboration chemistry, however it is gaseous at room temperature and requires skilled technical handling and the use of specialized equipment. Together with the issues around its selectivity, other hydroborating agents such as borane-THF complex, Me2S-borane complex, thexylborane, disiamylborane, 9-BBN, and HB(C6F5)2 (Fig. 37) were developed. In an effort to prepare boronic esters, which are generally more stable than alkylboranes, hydroboration reactions with pinacolborane and catecolborane were also developed (Fig. 37). Following the discovery of the Suzuki cross-coupling reaction, much attention has been paid to transition-metal catalyzed hydroboration reactions and much of this area has been discussed elsewhere.151–153 Here, the discussion is focused on non-catalyzed and main-group catalyzed hydroboration methods.

9.04.5.1

Main-group catalyzed hydroborations

In 1979, Noyori et al. found that a 1,10 -(bi-2-napthol) (BINAL) derivatized LiAlH4 reagent could stoichiometrically hydroborate alkyl and aryl prochiral ketones using BH3 with up to 100% ee (Fig. 38).154 Inspired by this work, in 2000 Woodward and co-workers prepared catalysts using BINAL-H ligands, such as BINOL, MTBH, and DTB, via reactions with LiMH4 (M ¼ Al and Ga) precursors (Fig. 39).155 It was found that catecholborane could hydroborate substrates with phenyl/alkyl motifs in good yield with

Fig. 37 Common boranes in hydroboration reactions.

Fig. 38 Stoichiometric enantioselective hydroboration of prochiral carbonyls.

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Fig. 39 Enantioselective hydroboration of acetophenone using BINAL derivatized LiMH4 catalysts.

moderate to high enantiomeric excess, between 60% and 93% ee, at room temperature with 5 mol% catalyst loading. However, when the reduction of more sterically encumbered ketones was investigated, there was a significant reduction in reaction rates and enantioselectivities. Later, in 2012, Hill reported a magnesium alkyl complex [CH{C(Me)NAr}2-Mgn-Bu] (Ar ¼ 2,6-i-Pr2C6H3) as a highly efficient pre-catalyst for the hydroboration of aldehydes and ketones using pinacolborane.156 The same catalyst could also be used for the hydroboration-dearomatization of pyridine derivatives, and for the hydroboration of imines and nitriles.157–159 The principal step in each of these reactions is the formation of a Mg–H bond by transfer of the hydride from pinacol borane to the magnesium center. In 2014, Sadow et al. reported a magnesium bisoxazoline complex (5.1), which was found to affect the catalytic hydroboration of esters using pinacolborane (Fig. 40).160 These transformations proceed with low catalyst loading (1–5 mol%), good functional group tolerance, and under ambient conditions. During mechanistic investigations, X-ray crystallography studies revealed that pinacolborane coordinates to the magnesium center through the oxygen. Although the mechanism is not well understood, kinetic analysis ruled out a concerted s-bond metathesis mechanism between LMg-OR and H-Bpin to regenerate a Mg-H containing species. Relative to Group 2 hydroboration catalysts, Group 1 derivatives have only recently been reported. In 2017, Okuda reported on the synthesis of [(L)M][HBPh3], where L ¼ Me6TREN and M ¼ Li, Na, K (Fig. 41), salts and their ability to catalyze the hydroboration of benzophenone with pinacolborane, where the order of activity followed the trend Li  Na  K.161 Furthermore, mechanistic investigations with [(L)Li][HBPh3] showed that the ligand L enhances the rate of the reaction compared to the ligand-free catalyst [Li][HBPh3]. The ligand is presumed to increase activity by preventing aggregation of the lithium centers. Later that year, Okuda also reported on variation in the N,N,N,N-type tripodal polyamine ligand, and its effects on activity in catalytic

Fig. 40 Catalytic hydroboration of esters using 5.1.

Fig. 41 Synthesis of group 1 hydroboration catalysts of the type 5.2.

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Fig. 42 Catalytic hydroboration of ketones and aldehydes using 5.3–5.5.

Fig. 43 Proposed mechanism for hydroboration of benzaldehyde using 5.3.

hydroboration reactions.162 In 2018, Sen and co-workers employed catalysts 5.3–5.5 to affect the hydroboration of ketones and aldehydes using pinacolborane (Fig. 42).163 This transformation proceeds under mild conditions, with good functional group tolerance, and high turnover frequencies (TOFs), for instance a TOF ¼ 66,000 h−1 could be obtained in the case of benzophenone. Stoichiometric experimental investigations were inconclusive in determining a mechanism, although the mechanism shown in Fig. 43 was supported by DFT investigations. Later in the same year, Mulvey and co-workers reported on cooperativity between the Li and Al centers of lithium diamidodihydridoaluminates to affect the hydroboration of ketones and aldehydes, again using pinacolborane.164 Catalytic carbonyl hydroboration using aluminum hydrides, precluding participation from an s-block metal, has also been reported. In 2015, Roesky and co-workers described the first example with the preparation of an Al-NacNac monohydride complex (5.6; Fig. 44).165 DFT investigations suggest that catalyst 5.6 acts as the initial hydride donor, and attack from the carbonyl yields the aluminum alkoxide. Then, the aluminum alkoxide complex apparently undergoes s-bond metathesis with the B–H bond of pinacolborane to regenerate catalyst 5.6. Nembenna et al. prepared a well-defined aluminum monohydride [{(2,4,6-Me3-C6H2)NC(Me)}2(Me)(H)]AlH
(NMe2Et) (5.7) which catalyzes the hydroboration of a wide range of ketones and aldehydes using pinacolborane (Fig. 44).166 The mechanism for this hydroboration is postulated to be similar to that reported by Roesky.165 Inoue prepared aluminum hydride dimers supported with N-heterocyclic imines (NIH) to affect the hydroboration of carbonyls and alkynes with pinacol borane (Fig. 45).167 Thomas and Cowley have also reported on the hydroboration of alkynes using pinacolborane mediated by easily accessible commercially available aluminum reagents iBu2AlH, AlMe3, AlEt3, (AlMe3)2•DABCO (DABCO ¼ 1,4-diazabicyclo[2.2.2]octane), and AlEt3•(DABCO).168 Mechanistic investigations revealed that this transformation proceeds through hydroalumination followed by Al/B exchange to regenerate the aluminum catalyst, as shown in Fig. 46. Thomas and Cowley further exploited hydroalumination followed by B/Al exchange to effect the hydroboration of polar bonds, such as ketones, esters, and nitriles, and non-polar bonds, such as alkenes, all using LiAlH4.169

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Fig. 44 Catalytic hydroboration of ketones and aldehydes using 5.6 and mechanism.

Fig. 45 Catalytic hydroboration of alkynes and carbonyls using 5.8 and 5.9 and mechanism.

In 2012, Crudden and co-workers reported on the isolation of borenium salts stabilized with DABCO, and established their role in the catalytic hydroboration of imines.170 In this report, when B(C6F5)3 reacts with DABCO to generate a Lewis acid-base adduct, which upon addition of pinacolborane forms borenium cation 5.10 (Fig. 47). It was found that the hydroboration mechanism did not follow that of previously reported FLP-mediated hydroborations, and B–H bond activation of pinacolborane was the rate-determining step. Mechanistic investigations showed that the borenium cation is transferred to the imine, and the catalyst regenerated after hydride transfer from another molecule of pinacolborane, as shown in Fig. 48. Later, in 2016, Ingelson and co-workers reported the trans-hydroboration of terminal alkynes using borenium salts stabilized by N-heterocyclic carbenes (Fig. 49).171 Borenium cations of the general formula [NHC(9-BBN)]+ were found to exclusively give the Z-vinylboranes, and deuterium labeling studies showed that the mechanism involved addition of the electrophilic borane to the alkyne, followed by intermolecular transfer of a hydride to the opposite face. Utilization of catalytic B(C6F5)3 and stoichiometric [HB(C6F5)3]− was also found to give trans-hydroboration products of terminal alkynes. In the same year, Stephan reported on the cis-hydroboration of terminal and internal alkynes mediated by Piers’ borane, HB(C6H5)2.172 HB(C6F5)2 acts as a pre-catalyst, first both HBpin and

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Fig. 46 Proposed mechanism for hydroboration of alkyne using commercially available aluminum hydride reagents.

Fig. 47 Synthesis of borenium cation 5.10.

Fig. 48 Proposed mechanism for catalytic hydroboration of imines using 5.10.

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Fig. 49 Trans-Hydroboration of alkynes using borenium cation 5.11.

HB(C6F5)3 undergo uncatalyzed hydroboration with the alkyne to give a mixed 1,1-diborylated alkane species (5.14) as the catalytically active species (Fig. 50). The 1,1-diborylated alkane 5.14 can then activate alkyne to give a zwitterion that reacts with a second equivalent of pinacolborane to undergo a concerted hydroboration reaction and selectively release the E-alkenyl pinacol boronic ester. Oestreich and co-workers reported on the catalytic cis-hydroboration of alkenes using the electron-deficient borane tris[3,5-bis (trifluoromethyl)phenyl]borane, under conditions in which B(C6F5)3 was found to be inactive.173 Tris[3,5-bis(trifluoromethyl) phenyl]borane was reported to be a pre-catalyst in this transformation, in which reaction with pinacolborane is thought to give hydroboranes as the catalytically active species (Fig. 51). The hydroboranes then react with alkynes in a syn-fashion to give hydroboration products which then engage in substituent redistribution with pinacolborane. B(C6F5) is thought to be inactive in

Fig. 50 Proposed mechanism for catalytic hydroboration of alkynes using 5.14.

Fig. 51 Proposed mechanism for catalytic cis-hydroboration of alkynes using tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF3).

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Recent Development in the Solution-State Chemistry of Boranes and Diboranes

this transformation as it reacts with pinacol borane to cleave the 1,3,2-dioxaborolane ring. Later, in 2017, Melen reported the same Lewis acidic borane, tris[3,5-bis(trifluoromethyl)phenyl]borane, to affect the hydroboration of imines and carbonyls.174 Melen and Oestreich extended the borane-catalyzed hydroboration of imines to 2,6-didefluoro-B(C6F5)3 [B(C6F3H2)3], concluding that the ortho-fluorine atoms of B(C6F5)3 account for the observed inactivity of B(C6F5)3 in these hydroboration reactions.175 Recently, in 2020, Thomas and co-workers reported that 9-BBN dimer can act as a catalyst to effect the double hydroboration of alkynes using pinacolborane, yielding gem-diboryl alkanes (Fig. 52).176 Again, this reaction is thought to proceed by hydroboration of 9-BBN with the alkyne, followed by a boron-boron exchange reaction with pinacol borane, proceeding through a s-bond metathesis mechanism, and re-entry of the resulting alkene into the catalytic cycle. In 2021, Cowley and Thomas reported that B(C6F5)3 can be employed as a pre-catalyst to affect the hydroboration of alkynes and styrene with pinacolborane.177 In this transformation, B(C6F5)3 is thought to form a zwitterionic intermediate 5.15 that acts as the active catalyst (Fig. 53). In 2014, Frenking, Jones and co-workers prepared well defined low-valent Ge(II) and Sn(II) hydrides and catalytically affected the hydroboration of carbonyls.178 In this work, highly reactive two-coordinate Ge(II) and Sn(II) hydrides 5.16 and 5.17 featuring the bulky ligand N(C6H2{C-(H)Ph2}2i-Pr-2,6,4)(Sii-Pr3) were found to add pinacolborane to a wide range of carbonyls with incredibly high efficiencies. Catalyst loadings as low as 0.05% gave complete conversion and, with some substrates, turnover frequencies (TOF) greater than 13,300 h−1 could be obtained. The proposed mechanism for this transformation initially involves attack of the metal hydride on the carbonyl functional group, whereupon hydroelementation gives a low-valent metal alkoxide species (Fig. 54). The two-coordinate metal alkoxide intermediate then undergoes a s-bond metathesis with pinacolborane to regenerate the catalyst and eliminate the hydroborated product. This mechanism is reminiscent of the reaction mechanism proposed by Hill with catalyst [CH{C(Me)NAr}2Mgn-Bu] (Ar ¼ Dipp).156 In 2015, Hirao and Kinjo reported the first example of a 1,3,2-diazaphospholene (5.18) to promote the catalytic hydroboration of carbonyls.179 Compound 5.18 was originally prepared by Gudat, and the hydridic nature of the P–H bond established.180 The measured kinetic isotope effect indicated a mechanism which proceeds through addition of the P–H bond of 5.18 to the carbonyl to give an alkoxo phosphine compound, followed by s-bond metathesis with the B–H bond of pinacolborane to regenerate the catalyst (Fig. 55).179 In 2017, Speed and co-workers extended the catalytic applications of 5.18 to the related alkoxydiazaphospholene derivatives 5.19–5.22 as pre-catalysts to effect the hydroboration of imines (Fig. 56).181 In 2017, Speed effected the

Fig. 52 Catalytic double hydroboration of alkynes using 9-BBN.

Fig. 53 Catalytic hydroboration of alkynes using B(C6F5)3.

Fig. 54 Proposed mechanism for the hydroboration of carbonyls using 5.16 and 5.17.

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Fig. 55 Proposed mechanism for catalytic hydroboration of benzaldehyde using 5.18.

Fig. 56 Phosphorus alkoxide precatalysts 5.19–5.24.

asymmetric hydroboration of imines by preparing and employing chiral 1,3,2-diazaphospholene pre-catalysts 5.23 and 5.24.182 In 2018, Cramer added to the family of chiral 1,3,2-diazaphospholenes with the preparation of 5.25–5.28, and exploited 5.25–5.28 to promote the catalytic enantioselective 1,4-reductions of a,b-unsaturated carbonyl compounds (Fig. 57).183 Based on the 1,3,2-diazaphospholene framework, Chong and Kinjo prepared phosphenium cations 5.29 and 5.30 with a range of coordinating and weakly coordinating counterions (Fig. 58).184 It was found that the triflate salts (5.29 and 5.30a–c) mediated the hydroboration and dearomatization of pyridine under ambient conditions with high regio- and chemoselectivity. Mechanistic studies are consistent with the phosphenium cation first abstracting a hydride from pinacolborane to generate a borocation which forms an adduct with pyridine; subsequent hydride transfer from the phosphine regenerates the catalyst. In the same year, the Speed group reported similar reactivity with phosphorus alkoxide precatalysts 5.19–5.22.185 Also in the same year, the Melen group prepared related arsenic derivatives of these precatalysts (5.31 and 5.32; Fig. 59) to effect the hydroboration of aldehydes with pinacolborane.186 In 2017, Liu and Zhao found that commercially available NaOH powder could promote the hydroboration of aldehydes, ketones, imines, terminal alkynes, and alkenes using 9-BBN or pinacolborane.187 This hydroboration was found to be more efficient for polar bonds, such as the carbonyls and imines, while the alkyne and alkene hydroboration reactions revealed a more limited substrate scope. Later in 2019, An and co-workers employed K2CO3 as a catalyst to mediate the hydroboration of aldehydes, ketones, and alkenes with pinacolborane.188 In this report, larger substrates scopes were established, and it was found that aldehydes could be chemoselectively hydroborated in the presence of ketones. Recently, in 2021, Yang and Ma have reported n-BuLi as a catalyst to affect the hydroboration of nitriles and carbodiimides.189 In the case of nitriles, double hydroboration occurs to give the bis(boryl)amines, while in the case of carbodiimides the singly reduced amidinates were isolated.

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Fig. 57 Catalytic hydroboration of a,b-unsaturated carbonyl compounds using 5.25–5.28.

Fig. 58 Catalytic hydroboration of pyridines using 5.29 and 5.30.

Fig. 59 Arsenic cations and alkoxides 5.31 and 5.32.

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9.04.5.2

147

Uncatalyzed hydroborations using bis(pentafluorophenyl)borane [HB(C6F5)2]

Bis(pentafluorophenyl)borane [HB(C6F5)2; Piers’ borane] has been extensively employed as a hydroboration agent without the need for an additional catalyst. Such reactivity is due to the electron withdrawing nature of the pentafluorphenyl groups resulting in a highly electron deficient and Lewis acidic boron center. Furthermore, the presence of only one B–H bond allows for selective mono-hydroboration. Originally HB(C6F5)2 was prepared by Piers to generate soluble self-activating Ziegler-Natta-type olefin polymerization catalysts,190 but its incredible hydroborating potential has since been exploited to install -B(C6F5)2 units on transition metals and the Lewis acidic component in frustrated Lewis pair (FLP) chemistry (with the resulting compounds finding further applications in small molecule activation and catalysis), and to prepare Lewis acid functionalized materials.191 FLP systems feature a Lewis acidic and Lewis basic component that work cooperatively to activate small molecules and affect key organic transformations catalytically, these systems can be intermolecular or intramolecular.12,13,15 Originally, it was believed that FLPs required the use of steric encumbrance between the electron-rich and electron-poor site to induce “frustration,” this definition is now evolving. In 2007, the first intramolecular FLP 5.33 was described by Erker and Stephan by hydroboration of Mes2 PCH]CH2 with HB(C6F5)2.192 FLP 5.33 was found to activate a range of small molecules, as shown in Fig. 60.193–200 Although these products resemble those isolable with intermolecular FLPs, it was found that incorporation of the Lewis acidic and Lewis basic groups in one molecule increased the rate of these activation reactions. It was also found that the H2 activation with this 1,2-vicinal P/B FLP (shown in Fig. 60) is facile under ambient conditions, and 5.33 could be employed to mediate the catalytic hydrogenation of imines.193 It was also noted that substrates like azide,198 CO,199 and NO200 coordinated to 5.33 in 1,1-fashion; analogous 1,1-binding is less prevalent with intermolecular FLPs. Intramolecular FLPs offer additional structural rigidity, which is lacking in their intermolecular counterparts, that can be exploited to affect catalytic transformations in an enantioselective manner. Hydroboration reagent HB(C6F5) proved to be a useful reagent in the preparation of chiral FLPs 5.34–5.40 (Fig. 61),201–204 all of which found applications in the enantioselective hydrogenation of imines. In 1995, Piers and co-workers showed the a-pinene could be diastereoselectively hydroborated, and further isomerized to the thermodynamic isomer 5.34.201 Over a decade later, in 2008, the Klankermeyer group used 5.34 to obtain asymmetric hydrogenation of the N-phenyl ketimine in 13% ee through the addition of an intermolecular phosphine FLP partner.205 Even through this transformation showed disappointing enantioselectivity, it demonstrated the potential for asymmetric induction using FLPs. Hydroboration of phenyl substituted camphor-derivatives with HB(C6F5)2 gave 5.35 and 5.36, and gave % ees of up to 79% for the hydrogenation of the N-phenyl ketimine.202 Preparation of catalysts 5.34–5.37 has the disadvantage of producing diastereomeric mixtures requiring either difficult separations or, in some cases, inseparable racemic mixtures due to retrohydroboration/hydroboration processes. In response to this problem, in 2018, Du and co-workers developed a library of catalysts featuring binaphthyl frameworks (5.38), by rapid hydroboration of divinyl binaphthyl with HB(C6F5)2.203 In the same year, Wang used HB(C6F5)2 to hydroborate C2-symmetric bicyclic[3.3.0]dienes with increasingly bulky aromatic groups to give the kinetic product, catalyst 5.39, and the thermodynamic product, catalyst 5.40.204 To date, the catalyst family prepared by the Wang group is the best performing in the metal-free asymmetric hydrogenation of imines via a FLP-type mechanism.

Fig. 60 Synthesis of compound 5.33, and its action as an intramolecular FLP in the activation of small molecules.

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Fig. 61 Catalytic enantioselective hydrogenations of an N-phenyl ketimine using 5.34–5.40.

In 2017, Erker et al. reacted allene and cyclohexylallene with 10 mol% HB(C6F5)2, to give the 1,3,5-trimethylenecyclohexane products 5.41–5.43 shown in Fig. 62.206 Mechanistic investigations showed that the first step is hydroboration of the allene, then allylboration of two other allenes. The catalyst is regenerated and product released by retrohydroboration. Interestingly, the cyclohexane-based trialkene 5.41 could be further converted to the triborane 5.44 by reaction with additional HB(C6F5)2. Compound 5.44 was further found to act as an FLP and promote the hydrogenation of imines, ligate three equivalents of t-butylnitrile (5.45), and undergo 1,1-carboboration with PhCCSiMe3 (5.46). HB(C6F5)3 is a powerful synthon for the preparation of intramolecular FLPs, and since its application to make the original intramolecular FLP 5.33 many other related FLP systems have been prepared in a similar fashion.207–212 In an effort to prepare intramolecular FLPs with a boron acceptor and nitrogen donor, in 2010 the Stephan group hydroborated carbodiimides with HB(C6F5)2 to give boron amidinates 5.47 (Fig. 63).213 These boron-nitrogen intramolecular FLPs were found to exist as 4-membered rings, which in the presence of small molecules, such as carbon dioxide or a second equivalent of carbodiimide, can ring-open to capture the small molecule and form a 6-membered heterocycle. Compounds of the type 5.47 were also reported to activate benzaldehyde, tBuNC, CO, phenyl acetylene, and MeCN (Fig. 63). One intriguing example from the Erker group involves the capture of carbon monoxide with a series of intramolecular FLPs 5.48, 5.50, and 5.52 which in the presence of an additional equivalent of HB(C6F5)2 were found to hydroborate carbon monoxide to products 5.40, 5.51, and 5.53, respectively, reducing the CO to a formyl group (Fig. 64).199,214,215 Interestingly, when the norbornene-based compound 5.51 was allowed to react with an additional Lewis base such as pyridine, the B–O bond in the FLP-stabilized formato-borane was cleaved to give compound 5.54 (Fig. 65).215 The norbornene-based compound 5.51 was also found to react with high pressures of H2 to cleave the original C–O bond of carbon monoxide and give product 5.55. Recently, in 2021, Erker and co-workers reacted 2,4,6-tri(tert-butyl)phenyl vinyl phosphine with HB(C6F5)2 to produce the ethylene-bridged P/B frustrated Lewis pair system 5.56 (Fig. 66).216 Interestingly, 5.56 was found to exist as the 12-membered macrocyclic trimer 5.57 below 273 K. FLP 5.56 was further employed to activate H2, CO2, CO and phenyl acetylene, and found to promote the catalytic hydrogenation of N-tert-butyl-1-phenylmethanimine.

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149

Fig. 62 Hydroboration of 5.41 to give 5.44 and subsequent reactivity.

In 2016, Stephan et al. reacted HB(C6F5)2 with t-butylphosphaalkyne and 1-adamantylphosphaalkyne to give the corresponding phosphaalkenylboranes 5.58a and 5.58b, respectively.217 The resulting phosphaalkenylboranes were found to be dimeric in both the solution and solid-state. However, the dimers could be easily cleaved in the presence of bases, such as pyridine and t-BuNC, to give the coordinated monomers 5.59 and 5.60, correspondingly (Fig. 67). In 2018, Gagne and co-workers prepared a series of borane catalysts from HB(C6F5)2 and alkenes.218 By employing (allyl)B(pin) as their source of alkene they were able to prepare the heteroleptic diborane (C6F5)2B(CH2CH2CH2)B(pin) (5.62) and found that it mediated the reduction of alkyl-substituted tertiary amides and benzamides in high yield with high functional group tolerance in functionally rich molecules (Fig. 68). Also in the same year, the Stephan group reacted HB(C6F5)2, catecolborane, and 9-BBN stoichiometrically with a series of alkyl fluorides, to activate sp3 C–F bonds.219 In addition, stoichiometric reactions of HB(C6F5)2 with THF and epoxides resulted in hydroboration across a C–O bond and ring-opening to give alkoxy boranes.220,221

9.04.6

Carboboration reactions

When treated with an appropriate reagent, B–C bonds display almost chameleon-like reactivity, allowing chemists to substitute the borane for many other functional groups. As such, organoboranes have been turned into indispensable and omnipresent moieties in organic synthesis and materials science. The utility of organoboranes was further elevated by the advent of carbon-carbon bond formation through the Suzuki-Miyaura cross-coupling reaction,222 a transformation that was award the Nobel Prize in 2010. The air and moisture stability of boronic acids, together with the versatility of the Suzuki-Miyaura coupling reactions makes these transformations one of the true powerhouses of modern synthetic chemistry. Many boronic acids are still prepared by reacting alkyl borates with organomagnesium or organolithium reagents, a method dating back 100 years by Khotinsky.28 Carboboration reactions cleave the p-bond of doubly or triply bonded moieties to install a carbon and boron unit. In the case of alkenes and

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Fig. 63 FLP small molecule activation using 5.47.

Fig. 64 Reactivity of compounds 5.48, 5.50, and 5.52 with CO.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

Fig. 65 Further reactivity of compound 5.51.

Fig. 66 Hydroboration of 2,4,6-tri(tert-butyl)phenyl vinyl phosphine to give 5.56.

Fig. 67 Hydroboration of t-butylphosphaalkyne to give 5.58 and subsequent cleavage of the dimer.

Fig. 68 Synthesis of 5.62 and catalytic reduction of amides using 5.62.

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alkynes, carboboration reactions afford C–B bonds while circumventing the use of pyrophoric magnesium and lithium reagents. Carboboration reactions also rapidly increase molecular complexity by installing two units in one step. Moreover, carboboration reactions can be catalyzed or uncatalyzed, with catalyzed transformations discussed elsewhere,223 here the discussion is focused on uncatalyzed carboboration reactions across unsaturated functional groups in a 1,1- and 1,2-fashion.

9.04.6.1

1,1-Carboboration

Some of the earliest examples of 1,1-carboboration are the reactions of Et3B with trimethylstannyl acetylenes to give the tetra-substituted alkenylboranes by the Wrackmeyer group.224 In these reactions an alkyl group from the boron is shifted to the same carbon where the boron unit is installed, as shown in Fig. 69. Hence, these transformations require a migrating group at the alkyne, and could be further extended to the R3Ge- and R3Pb-functionalized alkyne derivatives. Silyl-functionalized alkynes were also established in the Wrackmeyer reaction, but often required more forcing conditions. Mechanistically, this reaction is a two-step process requiring an alkynyl abstraction and rearrangement. 1,1-Carboboration reactions between dialkynyl Group 14 reagents and boron reagents, such as BEt3 as shown in Fig. 70, provide a useful route to important heterocycles. The reaction sequence is thought to proceed via a traditional 1,1-carboration pathway, but can then follow three possible competing pathways. 1,2-carboboration would give product 6.6, and migration of the second alkyne to boron can lead to products 6.7 and 6.8.224–226 Incorporation of perfluoroarene groups at boron and moving to more Lewis acidic boranes, such as B(C6F5)3, allowed access to “Wrackmeyer-like products” under milder conditions.226 For instance, treatment of bis(phenylethynyl)dimethylsilane with B(C6F5)3 afforded the borylsubstituted silole product 6.10 under ambient conditions.227 Similarly, treatment of the bulkier dialkynsilane 6.11 with B(C6F5)3 also at room temperature gave product 6.12, which could be irradiated to give the isomeric compound 6.13 (Fig. 71). The traditional Wrackmeyer reaction required the use of “activated” alkynes that contain a migrating group, often in the form of main-group metals. By employing more reactive B(C6F5)3 and RB(C6F5)2 boron reagents, the scope of 1,1-carboborations could be expanded to include many more “unactivated” alkynes. In 2010, Erker and co-workers simply stirred phenylacetylene with B(C6F5)3 and found that it quickly resulted in an equimolar mixture of the cis- and trans-1,1-carboborated products E-6.14a and Z-6.15a.228 Subsequent photoloysis was found to isomerize the E-isomer to the Z-isomer. By exchanging the phenyl unit of

Fig. 69 Carboboration of trimethylstannyl acetylenes with Et3B.

Fig. 70 Carboboration of dialkynyl group 14 reagents.

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153

Fig. 71 Synthesis of borylsubstituted silole compounds 6.10, 6.12 and 6.13.

Fig. 72 Carboboration of akynes with B(C6F5)3 and subsequent irradiation.

phenyl acetylene with other alkyl groups, the corresponding 1,1-carboborated products could also be obtained in a similar fashion. In each case, the E- and Z-isomeric mixtures were quickly generated, and subsequent photolysis produced the Z-enriched sample (Fig. 72).229 When the Erker group investigated the reactions of alkylbis(pentafluorophenyl)borane reagents RB(C6F5)2 (R ¼ CH3, CH2CH2Ph) with 1-pentyne, the 1,1-carboboration reactions required longer reaction times (on the scale of hours) but proceeded with high selectivity. In both cases, the alkyl group on boron was transferred to the carbon rather than a C6F5 group. In the same report, the Erker group expanded their investigation of 1,1-carboboration reactions to terminal alkynes featuring ether substituents, revealing facile reactions to produce the respective E- and Z-carboborated isomers. However, upon photolysis the E-isomer was preferred, with the pendant ether acting as a directing group by coordination to the boron center, as shown in Figs. 73 and 74. The broad implications of this work were highlighted by finding that the alkenylborane products were suitable borane reagents for subsequent Pd-catalyzed Suzuki-Miyaura type carbon-carbon coupling reactions.

Fig. 73 Carboboration of alkyne featuring a pendant donor with B(C6F5)3 and subsequent irradiation.

Fig. 74 Carboboration of alkyne featuring a pendant donor with MeB(C6F5)2 and subsequent irradiation.

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1,1-Carboborations have also been observed in FLP activations of alkynes. In 2011, Erker and co-workers reacted 1,2-diethynylbenzene (6.17) with intermolecular FLPs B(C6F5)3/PR3 (R ¼ t-butyl, aryl, C6F5).230 When the FLP combination included PtBu3 or PAr3, either the alkyne deprotonated salt 6.18 or the 1,2-zwitterion 6.19 was formed (Fig. 75). However, when the FLP combination B(C6F5)3/P(C6F5)3 was employed, the reaction afforded product 6.20, formed from a 1,1-carboboration reaction followed by cooperative 1,2-FLP addition. A similar product could be isolated from treatment of 1,6-heptadiyne with the B(C6F5)3/P(o-tolyl)3 pair (Fig. 75).231 1,1-Carboboration reactions have also proved to be useful transformations for the preparation of intramolecular FLPs. In the same year, the Erker group reacted phosphinyl substituted alkynes (6.23) with B(C6F5)3, and showed that the 1,1-carboborated products 6.24 can be used as alkenylen-bridged intramolecular FLPs (Fig. 76).232 When internal alkynes were treated with RB(C6F5)2 (R ¼ CH3, C6F5), 1,1-carboboration reactions produced alkenylborane products 6.25–6.27 (Fig. 77).233 In these reactions, harsh reaction conditions were required as migration via cleavage of a strong C–C single bonds occurs. Piers reported on a related acetylene C–C bond activation when diphenylacetylene was allowed to react with borole 6.28 to give the 1,1-carboborated insertion product 6.29 (Fig. 78).234 In 2010, Erker et al. reacted amino dihydropentalene 6.30 with alkyldi(pentafluorophenyl)boranes RB(C6F5)2 (R ¼ CH3, CH2CH2Ph) and obtained products 6.34a and 6.34b (Fig. 79).235 The strong C]C bond of the five-membered carbocyclic ring is cleaved, and the boron moiety inserted; mechanistic investigations are consistent with the mechanism shown in Fig. 79. The product isolated (6.34) is consistent with formal 1,1-carborboration of an alkene. In 2012, Erker and co-workers prepared naphthalene derivatives 6.37 though benzannulation reactions of 1,2-bis(alkynyl) benzenes with RB(C6F5)2 (R ¼ Me, Ph, C6F5)236 (Fig. 80). Products 6.37 were further employed in cross-coupling reactions, and it was found that the trimethylsilyl groups could easily be displaced. On the other hand, reaction between o-bis(Mes2P-ethynyl) benzene (6.38) and B(C6F5)3 gave a different product, 6.41 (Fig. 81).237 In the case of the indole dialkyne 6.42 shown in Fig. 82, 1,1-carboboration with B(C6F5)3 was found to be reversible, with benzannulated heterocycle 6.44 being isolated as the thermodynamic product.238 Sulfur-containing functional groups were also established to be suitable migrating groups in 1,1-carboboration reactions (Figs. 82 and 83).239 Exploiting this migrating group, in 2015 Stephan and Erker prepared thiophenes 6.51 by reacting

Fig. 75 Carboboration and FLP activation of alkynes.

Fig. 76 Alkenylen-bridged intramolecular FLPs 6.24.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

Fig. 77 Carboboration of internal alkynes.

Fig. 78 Reaction of borole 6.28 with diphenylacetylene to give 6.29.

Fig. 79 Synthesis of compounds of type 6.34.

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Fig. 80 Benzannulation reactions of 1,2-bis(alkynyl)benzenes with RB(C6F5)2 to give 6.37.

Fig. 81 Synthesis of compound 6.41.

Fig. 82 Carboboration of dialkynes featuring heterocycles.

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157

Fig. 83 Carboboration of alkynyl sulfides.

bis(tert-butylethynyl)sulfide with B(C6F5)3, this work was expanded to include the borane Lewis acids RB(C6F5)2 (R ¼ CH3, Cl).240 While reaction of the related tellurium compound 6.52 with B(C6F5)3 led to a series of consecutive 1,1-carboborations to give the 6-membered heterocycle 6.53.241 Benzyl alkynyl telluride was also found to undergo a 1,1-carboboration in the presence of B(C6F5)3 and was employed as a B/Te FLP (6.55) to capture alkynes (6.56; Fig. 84).242 The related sulfur analog of 6.55 (6.49) could also be accessed through 1,1-carboboration with B(C6F5)3. A series of arylbis(alkynyl)phosphanes were reacted with B(C6F5)3 to give the corresponding boryl-substituted phospholes (6.60) at elevated temperatures (Fig. 85).243,244 During these transformations, the 1,1-carboborated intermediates (6.59) could be detected by 31P NMR spectroscopy. The B(C6F5)2 unit of 6.60 could later be replaced with an aryl group through a palladium catalyzed cross-coupling reaction.

Fig. 84 Carboboration of alkynyl tellurides, and subsequent reactivity of 6.55.

Fig. 85 Carboboration of bis(alkynyl)phosphanes with B(C6F5)3.

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In 2014, when phenylbis(alkynyl)borane 6.64 was reacted with B(C6F5)3 clean formation of the borole product 6.65 was observed, accessed through a 1,1-carboboration (Fig. 86).245 Product 6.65 was found to undergo [4 + 2] cycloaddition with 3-hexyne (6.66) and undergo an unusual rearrangement with carbon monoxide (6.67).246 Boron-contained borole 6.62, isolated as its pyridine adduct, could be obtained in a similar fashion from the 1,1-carboboration of 6.61 (Fig. 87). The related silole derivative 6.69 could also be prepared by reaction of the bis(alkynyl)silane shown in Fig. 88 and B(C6F5)3.247 Compound 6.69 was also found to undergo rearrangement to 6.70 upon UV irradiation. Recently, in 2020, Duarte, Cowley, and Thomas isolated and characterized zwitterionic intermediate 6.72 (Fig. 89) generated during the 1,1-carboboration of ethynylferrocene with B(C6F5)3,248 thus providing further evidence for the 1,1-carboboration mechanism first proposed in the 1990s.

Fig. 86 Carboboration of a bis(alkynyl)borane 6.64 with B(C6F5)3 and subsequent reactivity.

Fig. 87 Carboboration of a bis(alkynyl)borane 6.61 with B(C6F5)3.

Fig. 88 Carboboration of a bis(alkynyl)silane and subsequent irradiation.

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Fig. 89 Synthesis of compound 6.72.

9.04.6.2

1,2-Carboboration

In contrast to 1,1-carboboration reactions of alkynes, examples of 1,2-carboborations are much rarer, with these transformations either requiring the use of transition-metal catalysts or a boron cation as the boron source. In 2014, Ingelson and co-workers noted the limited number of examples of 1,2-carboboration of alkynes, and surmised that this limitation was due to the low migratory aptitude of the R groups of BR3 (R ¼ alkyl, C6F5) reagents.249 In an effort to prepare boron reagents with migratory groups they prepared boron cations 6.74a and 6.74b and investigated them toward the carboboration of alkynes (Fig. 90). Terminal alkynes afforded the 2 + 2 + 2 cyclotrimerization product. In contrast, 3-hexylalkyne instead formed the syn-1,2-carboborated products 6.75. The rate of 1,2-carboboration was found to be heavily dependent on the migratory group, with the thienyl-substituted boron reagent 6.74b displaying greater reactivity than penyl-substituted 6.74a. Later in 2015, Bourissou prepared phosphine coordinated borenium cation 6.76, and investigated its reactivity with the internal alkyne 3-hexyne (Fig. 91).250 Similar to the findings of Ingelson, the syn-1,2-carboborated product 6.77 could be isolated. However, the rate of reaction was found to be significantly greater than that observed by Ingelson. In 2015, Stephan and co-workers found that four-membered boron amidinate heterocycles could be accessed by reacting B(C6F5)3 with carbodiimides (Fig. 92).251 This heterocycle is formed by a 1,2-carboboration across a C]N bond of the carbodiimide, followed by coordination of the resulting imine to the -B(C6F5)2 unit. In addition, in 2019, Mehta and Goicoechea reacted B(C6F5)3 with isocyanates and isolated 6-membered heterocycles.252 In these transformations it is believed that the isocyanate undergoes a 1,2-carboboration reaction across the C]O bond to form an imine-boron FLP that captures a second equivalent of isocyanate. Interestingly, these systems were reported as masked-FLPs that—in the presence of a different isocyanate—would undergo exchange reactions, as shown in Fig. 93.

Fig. 90 1,2-Carboboration of alkynes.

Fig. 91 Phosphine coordinated borenium cation in 1,2-carboboration of an alkyne.

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Fig. 92 1,2-Carboboration of carbodiimides with B(C6F5)3.

Fig. 93 1,2-Carboboration of isocyanates with B(C6F5)3 and subsequent heteroallene exchange reactions.

Fig. 94 Carboboration of a phosphathynolate and a phosphaketene.

Recently, in 2020, Goicoechea and co-workers reacted B(C6F5)3 with boron-functionalized phosphaethynolate 6.82 and boron-functionalized phosphaketene 6.85 (Fig. 94).253 In the case of the phosphathynolate 6.82 the triple bond between the phosphorus and carbon is inversely polarized in comparison to nitriles, i.e., d–C  Pd+ rather than d+ C  Nd–. The regioselectivity of the carboboration of 6.82 with B(C6F5)3 to product 6.83 reflects this polarization, with the boron unit transferred to carbon, and the C6F5 unit to phosphorus. Prolonged reaction times led to isomerization of the 1,2-carboborated product 6.83 to the 1,1-carboborated product 6.84; efforts were made to investigate this mechanism by DFT but it is not yet fully understood. In the case of phosphaketene 6.85 the bonding between phosphorus and carbon is phosphalkene-like, and reaction with B(C6F5)3 gives the formal 1,3-carboborated product 6.86 with the boron unit transferred to the oxygen and the C6F5 unit transferred to the phosphorus.

9.04.7

Cationic boron containing compounds

First prepared over 30 years ago, boron cations have garnered increasing interest in main-group chemistry due to their exceptionally high Lewis acidity without the need for electron withdrawing substituents at the boron center. Boron cations are often defined by their coordination number: dicoordinate monocations are labeled boriniums, tricoordinate monocations are labeled boreniums, and tetracoordinate monocations are labeled boroniums (shown in Fig. 95).254

9.04.7.1

Borinium cations

Within the family of boron cations, two-coordinate borinium cations are the most reactive and their low coordination number makes them prone to nucleophilic attack.254 This instability has prohibited their broader application. Initial efforts to prepare a borinium cation were focused on Ph2B+. NMR investigations revealed solvent coordination to afford a mixture of the three- and

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161

Fig. 95 Categories of boron cations.

four-coordinate borenium and boronium cations.255 The first crystallographic structure of a borinium cation was reported in 1982, where the boron center was coordinated to dimethylamido and tetramethylpiperidino ligands.256 Later that year, Parry and co-workers reported another bis(amido)borinium cation in the condensed phase.257 In 2002, Stephan reported on the synthesis of the extended borinium cation [(tBu3PN)2B]+ (7.2; Fig. 96) which was found to be linear at the P ¼ N–B–N ¼ P core supported by crystallographic and DFT evidence.258 At this time, it was largely believed that to form isolable borinium cations the two pendant substituents ought to feature lone pairs that can p donate and relieve some of the electron deficiency at boron.254 In 2014, seminal work in this field was published by Shoji with the successful isolation of a dimesityl borinium cation (Mes2B+) with high thermal stability, afforded by the steric encumbrance and electronic stabilization granted by the mesityl groups (Fig. 97).259 The high Lewis acidity of this borinium cation was exploited in its reactivity with CO2 and CS2, where it was found to cause unusual C ¼ X (X ¼ O, S) bond cleavage to yield [MesCX]+ salts, as shown in Fig. 97.259,260 Later in 2017, Shoji and co-worked reported on the 1,2-carboboration of their diaryl borinium cation and diphenyl acetylene to form the divinylborinium ion (7.6; Fig. 98).261 This finding suggests that borinium cations could be stabilized, without additional coordinating ligands or substituents capable of p back-donation, despite their highly electron-deficient nature.

Fig. 96 Synthesis of boron salt 7.2.

Fig. 97 Synthesis of boron salts 7.3 and 7.4, and subsequent reactivity of 7.4.

Fig. 98 Reactivity of 7.4 with diphenylacetylene.

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Borenium cations

Of the boron cations, perhaps it is the three-coordinate borenium cations that are of the greatest interest as they offer a happy compromise with more tapered reactivity and synthetic versatility compared to borinium cations, and a vacant coordination site when compared to boronium cations. Borenium cations are often prepared in a similar fashion. First, a neutral ligand is coordinated to a either a secondary haloborane or a secondary hydridoborane. Next, the halogen or hydride is abstracted to afford a positively charged boron cation. To facilitate borenium cation generation, the ligand employed must coordinate to the boron center strongly enough that it does not disassociate during the abstraction step, further the B–X (X ¼ halogen, hydride) bond should be significantly weakened by this ligation that X can be abstracted. The highly modular nature of this approach has allowed for the preparation of borenium cations featuring a large variety of substituents. As such, the reactivity of borenium cations is highly tuneable, with the incorporation of electron-donating groups stabilizing the positive charge and electron-withdrawing groups increasing reactivity. Early examples of borenium cations featured nitrogen donor ligands,254 while recent investigations have been focussed on carbene ligands.262 Here, we will focus on recent developments on carbene stabilized borenium cations. In 1997, Weber and co-workers reported the first well-characterized NHC-borenium cations, here the boron center featured two amido substituents within a five-membered chelate ring (Fig. 99).263 X-ray diffraction studies of these compounds 7.8 revealed that the heterocyclic rings are orthogonal and the p-orbital on boron is perpendicular to the p-system of the NHC. Later, Matsumoto and Gabbaï described the synthesis of borenium cation [IMe2B(Mes)2][OTf] (7.9; IMe2 ¼ C3H2(NMe)2; OTf ¼ triflate), where the boron bears two large aryl groups.264 This cation was prepared from bis(mesityl)boron fluoride and the silver complex of the carbene, followed by abstraction of the fluoride using TMSOTf (trimethylsilyl triflate), as shown in Fig. 100. Here it was found that the boron p-orbital is in the correct orientation to conjugate with the p-framework of the carbene. Meanwhile, Stephan and co-workers prepared an NHC-stabilized borenium cation of the formula [(IiPr2)BC8H14][RB(C6F5)3] (7.11, IiPr2 ¼ C3H2(Ni-Pr)2; R ¼ H, C6F5), via the reaction of the carbene with 9-borabicyclo[3.3.1]nonane (9-BBN) dimer, and subsequent hydride abstraction with either trityl borate [Ph3C][B(C6F5)4] or B(C6F5)3.265 They also reported on the intermolecular FLP activation of H2(g) with their borenium cation and a phosphine, as well as the metal-free hydrogenation of imines and enamines to amines (Fig. 101). Crudden reported the related MIC-stabilized borenium cations 7.12–7.16 (MIC ¼ mesoionic carbene) shown in Fig. 102 and found these cations to be effective catalysts in the hydrogenation of imines and quinoline derivatives.266 When investigating carbenes with benzyl substituents at the nitrogen atoms, Stephan found that the boron cations of the formula (NHC)BH3 under hydride abstraction conditions (rather than yielding [(NHC)BH2]+) would undergo dehydrogenation reactions with the ortho-aryl C–H bonds to afford planar singly- or doubly-ring closed borenium cations 7.18 and 7.19 (Fig. 103).267,268 Exploiting this robust synthetic method, Stephan, Crudden, and Melen, prepared a range of chiral borenium cations by introducing chirality through the carbene and substituents at the boron, to effect the asymmetric hydrogenation of N-1-diphenylethan-1-imine; however enantioselectivities did not exceed 20%.269 Gilliard Jr. and co-workers synthesized the first examples of NHC- and CAAC-borepinium and borafluoreniumheterocycles (7.20–7.22; Fig. 104) and reported on their photophysical properties.270 The optical properties of these highly conjugated heterocyclic borenium cations could be tuned by manipulation of the donor ligand, changing the size of the heterocycle ring, and functionalization of the ring. Remarkably, these cationic boron ring systems showed thermosensitive reversible colorimetric properties in the solution-state, allowing them to behave as chemical switches.

Fig. 99 Synthesis of boron cation 7.8.

Fig. 100 Synthesis of boron salt 7.9.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

Fig. 101 Synthesis of boron cation 7.11 and catalytic hydrogenation of imines.

Fig. 102 MIC stabilized boron cations 7.12–7.16.

Fig. 103 Synthesis of boron salts 7.18 and 7.19.

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Fig. 104 Synthesis of boron cations 7.20–7.22.

Fig. 105 NHO stabilized boron salt 7.24.

In 2013, Robinson et al. stabilized borenium cations using N-heterocyclic olefin (NHO) ligands.271 In this work the NHO was directly reacted with BBr3 to afford the adduct 7.23 (Fig. 105), which in the presence of THF, ring-opened two equivalents of solvent to give 7.24. In addition, Gessner’s efforts to prepare C–B–C containing boron cations employed ylide ligands with the formula [Ph3PCSO2Tol]− as strong electron donors.272 Although stabilization by the p-delocalisation with the ylides is most important in the observed thermal stability of the [C–B–C]+ unit, the X-ray diffraction structure and DFT investigations revealed that the oxygen of one tosyl group displays a short contact distance with the boron center, and thus acts as a donor ligand, as shown in Fig. 106.

Fig. 106 Synthesis of boron salts 7.26.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

9.04.7.3

165

Boronium cations

Boronium cations, the four-coordinate derivative within the boron cation family, are generally the least reactive as they have a saturated coordination sphere. In most cases boronium cations feature two neutral ligands and two covalently bound ligands. Owing to the greater stability of this family of compounds, many examples have been reported in the literature and this area has been previously reviewed.254,273,274 Here, only a selection of examples are discussed. Hodgkins and co-workers found that slow addition of bromodimethylborane and 2-lithiopyridine did not exclusively form the expected monomeric or dimeric products 7.27 and 7.28, but also included the zwitterionic boronium–borate complex 7.30 (Fig. 107).275 This transformation is thought to proceed through the dimethyl borate intermediate (7.29) formed with excess 2-lithiopyridine. In 1997, Wagner reported on a family of ferrocene-based 2,20 -bipyridylboronium salts and investigated them as electron acceptors in charge-transfer complexes (Fig. 108). Initially, complexes with a ferrocene core functionalized with one (7.31), two (7.32), and four (7.33) boron substituents were prepared.276 A series of nitrogen donor ligands at boron were also investigated. These complexes demonstrated that despite raising electrostatic barriers, multiple boron centers can be ionized by employing 2,20 -bipyridine ligands affording a polycation. Electrochemical studies into these boronium cations revealed one chemically reversible oxidation and two consecutive chemically reversible reductions in DMF. Later, Wagner also reported the synthesis of the tetrapyridyl diboronium dication 7.34 and employed it in the synthesis of the heterotrimetallic complex 7.35.277 The reversible redox behavior of complexes 7.31–7.35 suggests that application of these materials and their related derivatives as redox catalysts, in optical materials, or in electronic devices may be possible. Jutzi and Cowley have developed a family of Cp (Cp ¼ pentamethylcyclopentadiene)-substituted borocations,278–282 which can be described as two-coordinate borinium ions or alternatively (as 5-coordination of the cyclopentadienyl ring is approached) as four-coordinate boronium ions (Fig. 109). The characterization of these boron cations is not clear: in terms of structure and reactivity they resemble borinium cations, but electronically can be viewed as boronium cations. In 2013, Chiu et al. prepared boron dication 7.39 featuring an 5-Cp fragment and an axial IMes carbene ligand, by stepwise abstraction of chloride anions from [Cp BCl2(IMes)] (7.37), as shown in Fig. 110.283 It was found that when this boron dication was reacted with superhydride the pentagonal pyramidal [C5B]2+ core opened to incorporate the boron atom and yield the NHC-stabilized borabenzene 7.40.283,284 Chiu later expanded their investigations from IMes stabilized boron dications to NHO IDipp]CH2 stabilized boron dications.285 Preliminary studies showed that the NHO stabilized boron dication was more reactive than the NHC stabilized system and would readily decompose in nitromethane. In addition to NHC and NHO donors, CAAC ligands were also explored and this axial carbene ligand was found to experimentally and computationally play an important role in the hydride addition site of the boron dication with [BH4]−.286 Recently, in 2019 Chiu prepared the [Z5-Cp BMes]+ monocation 7.42 (Fig. 111) and employed it to mediate the hydrosilylation and deoxygenation of ketones.287

Fig. 107 Synthesis of compounds 7.27, 7.28 and 7.30.

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Fig. 108 Ferrocene-based 2,20 -bipyridylboronium salts 7.31–7.35.

Fig. 109 Z5-Cp -substituted borocations.

Fig. 110 Synthesis of boron cation 7.39 and subsequent reactivity with superhydride.

Fig. 111 Synthesis of boron salt 7.42.

Recent Development in the Solution-State Chemistry of Boranes and Diboranes

9.04.8

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Boryl anions

Polar carbon–metal bonds are highly important in synthetic chemistry as they allow access to reactive carbanions, where the reactivity and properties of the carbonanion are highly dependent on the metal counterpart, such as lithium, magnesium, copper, and zinc.288 Contrary to the rich chemistry of alkyl metal carbanions, boryl anions have been investigated to a much lesser extent. In general, boron containing reagents are electrophilic and their electron-deficient nature has been widely investigated and exploited. Boryl anions are interesting reagents because the boron-centers in these reagents are nucleophilic. Boryl anions where the boron center features six valence electrons are highly unstable as the octet rule is violated, the boron features a lone-pair, and the boron features a vacant p-orbital. Addition of a Lewis base fills the vacant p-orbital, fulfills the octet rule at boron, and makes the boron center sp3 hybridized and isoelectronic with carbanions, these reasons can make base-stabilized boryl anions more accessible. In 1967, Parson and co-workers reacted (n-Bu)2BCl with Na/K alloy in triethylamine to give (n-Bu)2BK•NEt3 as the first transient boryl anion, which was subsequently reacted with CF3I to yield (n-Bu)2BCF3•NEt3, as shown in Fig. 112.289 Twenty-six years later, Schmidbaur, and Imamoto and Hikosaka independently treated phosphine coordinated iodoborane (Cy3P)•BH2I with lithium 4,40 -di-tert-butylbiphenylide to give (Cy3P)•BH2Li (8.4; Fig. 113).290,291 This phosphine-stabilized boryl anion was further reacted with chlorosilane ClSiMe3 and other organic electrophiles to afford the corresponding substitution products 8.5 and 8.6. The parent boryl anion −:BH2 was computed to possess a triplet ground state.292 In order to increase the stability of boryl anions a number of strategies have been evoked.292 First, complexation of the boryl anion with alkali metal cation increases stability. Substitution at boron with electronegative heteroatoms rather than hydrogen also increases stability through inductive effects, orbital-orbital overlap between the lone-pair of the heteroatoms and empty p-orbital on boron in LiB(NH2)2 and LiB(OH)2 allows for p-interactions. Considering related carbanion chemistry, two synthetic methods to boryl anions can be envisioned. First, the deprotonation of a B–H bond with a base, however the hydrogen atom of a B–H bond is hydridic due to the higher electronegativity of hydrogen compared to boron.293 Also, addition of base to secondary boranes often leads to simple coordination of the base into the vacant p-orbital to give the Lewis acid-base adduct rather than hydrogen abstraction. The second method is reductive dehalogenation of a haloborane using an alkali metal reductant. However, reduction of haloboranes using an alkali metal reductant often leads to formation of B–B bonds through dimerization of a boron radical intermediate.89 In the case of gallium, analogous gallyl anions can be attained from the alkali metal reduction of Ga–Ga single bonds.294–296 However, further reduction of B–B bonds leads to the formation of singly or doubly reduced mono- or di-anionic diborane(4) species without cleavage of the B–B bond.297 Thus, the subsequent reduction of B–B bond of diboranes(4) is also not a viable route to boryl anions. In 2006, Yamashita and Nozaki prepared boryl lithium species 8.8 by reduction of the corresponding bromoborane 8.7 featuring bulky Dipp substituents with lithium powder and naphthalene in THF (Fig. 114).298 Reaction of 8.8 with water afforded the hydroborane (8.9a) in quantitative yield, and reaction with D2O afforded the deuterioborane (8.9b) in 84% yield. Replacing the THF solvent with DME allowed for characterization of 8.8 by X-ray crystallography. The solid-state structure of 8.8-DME showed a B–Li bond length of 2.291(6) A˚ , which is longer than the sum of the covalent radii by 8.5%.293 Structural analysis also revealed that the two B–N bond lengths (1.465(4) and 1.467(4) A˚ ) of 8.8–DME are longer than those in the related hydroborane (1.418(3) and 1.423(3) A˚ ), while the N–B–N bond angle (99.2(2) ) is narrower in 8.8–DME. These structural parameters are close to those computed for the free boryl anion,299 and consistent with high polarization of the B–Li bond with anionic character at the boron. The 1H NMR spectrum of 8.8–DME in d8-THF revealed disassociation of the boron- and lithium-containing fragments in solution. The 11B NMR spectrum revealed a resonance at 45.4 ppm with a large half-width of 535 Hz. The lower field shift and larger half-width of the observed signal compared to the precursor is consistent with paramagnetic contribution to nuclear shielding by a

Fig. 112 Synthesis of boryl anion 8.1 and subsequent reactivity.

Fig. 113 Synthesis of boryl anion 8.4 and subsequent reactivity.

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Fig. 114 Synthesis of boryl anion 8.8 and subsequent reactivity.

low-lying transition from the sp2 lone pair of the boryl anion to the antibonding orbital on the dihydrodiazaborole ring. Interestingly, when the boryl anion 8.8 was reacted with organic electrophiles, such as MeOTf, 1-chlorobutane, and benzaldehyde, it was found to yield the expected products from boron nucleophilic attack (8.10–8.11). Transmetallation of boryl lithium compound 8.8 with one equivalent of MgBr2•OEt2 was found to give borylmagnesium compounds 8.12 an 8.13 (Fig. 115).300 While the analogous reaction with half an equivalent of MgBr2•OEt2 gave 8.14 as the only magnesium containing product. Solid-state X-ray crystallographic studies of all three magnesium compounds revealed a near ideal sp2 boron center and four-coordinate magnesium centers, these compounds represented the first compounds featuring boron– magnesium single bonds. Similar to boryl lithium compound 8.8, the Mg–B bond lengths were longer than the sum of the covalent radii for both atoms. Moreover, B–N bond lengths and N–B–N bond angles for all the boryl magnesium compounds 8.12–8.14

Fig. 115 Transmetallation as a route to 8.12–8.14, and further reactivity of 8.8 and 8.12.

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were more similar to those observed with boryl lithium species 8.8 than hydroborane 8.17. These structural characteristics are consistent with the highly polarized nature of the Mg–B bonds and anionic character at boron. Also similar to the boryl lithium species 8.8, the boryl magnesium compounds 8.12–8.14 showed 11B NMR signals at around 37 ppm. Treatment of in-situ generated boryl magnesium bromide 8.12 with 1–3 equivalents of benzaldehyde afforded a mixture of products consisting of benzoylborane 8.15, boron-substituted ester 8.16, hydroborane 8.17, and benzyl alcohol. The formation of these products contrasts with the reactivity of the boryl lithium species which gave 8.11 in high yield upon reaction with benzaldehyde (Fig. 115). Thus, the counter s-block cation influences the reactivity of the boryl anion. In addition to coordination with s-block counterions, boryl anions have been investigated as ligands on d-block metals. In 2000, Ito and Hosomi reported on the b-borylation of a,b-unsaturated ketones mediated by copper, as shown in Fig. 116.301 Also in 2000, Miyaura and Ishiyama reported on the preparation of boryl copper species 8.19 by reacting the same bis(pinacolato)diborane(4) with copper chloride, lithium chloride, and potassium acetate (Fig. 117). Compound 8.19 was found to subsequently react with an a,b-unsaturated carbonyl, a terminal alkyne, and an allyl chloride to give products 8.20–8.23.302,303 Miyaura and Ishiyama later reported that addition of methanol accelerated this transformation,304 while introduction of a chiral phosphine in place of n-Bu3P allowed access to these b-borylations with moderate to high enantioselectivity.305,306 In 2005, borylcopper species IDippCuBpin 8.24 was generated from IDippCu(Ot-Bu) and bis(pinacolato)diborane(4) (Fig. 118).307 Compound 8.24 was further applied to catalyze the reduction of CO2 and aldehydes with an excess of bis(pinacolato)diborane(4).307,308 Borylcuprate 8.26 and borylzincate 8.27 could be accessed by transmetallation of the boryl lithium compound 8.25 with one equivalent of either CuBr or ZnBr2 (Fig. 119).309 The 11B NMR spectrum for the lithium borylbromocuprate 8.26 showed a signal at 45 ppm, while the lithium borylbromozincate 8.27 showed a signal at 41 ppm. Interestingly, by changing the stoichiometry from one equivalent to two equivalents of CuBr the tetranuclear copper(I) complex 8.28 could be obtained in 32% yield (Fig. 120). In the case of zinc the solvent-free diborylzinc compounds 8.29a and 8.29b could be obtained by changing the stoichiometry of the zinc halide from one equivalent to half an equivalent (Fig. 121). To probe the nucleophilic nature at boron, boryl copper species 8.28 and boryl zinc species 8.27 were reacted with an a,b-unsaturated ketone, as shown in Fig. 122. Reaction of 2-cyclohexen-1-one with 8.28 or 8.27 gave the 3-borylcyclohexan-1-one addition product (8.31) after hydrolysis of the respective 8.30a and 8.30b intermediates in moderate yield.

Fig. 116 Synthesis of compound 8.18.

Fig. 117 Synthesis of boryl anion 8.19 and subsequent reactivity.

Fig. 118 Synthesis of boryl anion 8.24.

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Fig. 119 Transmetallation as a route to 8.26 and 8.27.

Fig. 120 Transmetallation as a route to copper tetranuclear complex 8.28.

Fig. 121 Transmetallation as a route to boryl anions 8.29.

Fig. 122 Reactivity of 8.27 and 8.28 with 2-cyclohexen-1-one.

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Since the discovery of direct transmetallation reactions between the isolable boryl lithium species (8.8) and metal precursors, complexes with K, Be, Ca, Sr, Ag, Au, Y, Gd, Lu, Ti, Hf, Mn, Re, Fe, Co, Zn, Cd, Hg, Al, Ga, In, Tl, Ge, Sn, and Pb have been prepared.292,310–312 Given the importance of the original boryl lithium species (8.8) first reported by Nozaki and Yamashita, in 2019 Wilson, Martin, and Dutton computationally investigated the s-donating and p-accepting properties of naked bis-amino boryl anions compared to their carbene analogs.313 It was found, in general, NHCs are better p-acceptors while boryl anions are better s-donors. Recently, in 2021, inspired by the structure of CAACs, Iwamoto and Yamashita reported on the synthesis and reduction of a cyclic (alkyl)(amino)bromoborane (8.32) to cyclic(alkyl)(amino)boryl anion (8.33; CAAB−; Fig. 123).314 Compound 8.33 was found to be thermally unstable but could be observed spectroscopically revealing a 11B NMR resonance at 75 ppm. Although compound 8.33 could not be isolated, in situ quenching with MeOD and MeOTf to give the corresponding deuterated and methylated products confirmed nucleophilicity at the boron. In 2013, Bertrand and co-workers reacted a CAAC as a Lewis base with BH3 to give the (CAAC)BH3 adduct (8.34; Fig. 124).315 Compound 8.34 was subsequently reacted with two equivalents of triflic acid followed by two equivalents of sodium cyanide to give (CAAC)BH(CN)2 (8.36). Interestingly, it was found that compound 8.36 could be reversibly deprotonated with KHMDS and re-protonated with HCl. The resulting CAAC stabilized boryl anion 8.37 was further reacted with isopropyl iodine to give 8.39 or Me3PAuCl to give the boryl gold complex 8.38. In 2014, Kinjo and co-workers reported the synthesis of bis-ligated chloroborane

Fig. 123 Synthesis of boryl anion 8.33.

Fig. 124 Synthesis of boryl anion 8.37 and subsequent reactivity.

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Fig. 125 Synthesis of the bis-ligated boryl anion 8.42 and subsequent reactivity.

8.41, by first reacting 2-lithio-4,40 -dimethyl-2-oxazolide with dichlorophenylborane to give 8.40 followed by in-situ methylation of the nitrogens (Fig. 125).316 Next, the chloroborane 8.41 was reduced with KC8 to yield the bis-ligated boryl anion 8.42. This boryl anion was subsequently reacted with triflic acid to protonate the boron to give hydroborate 8.43, or instead treated with [(THF)Cr(CO)5] to afford the chromium boryl complex 8.44 in 65% yield. Subsequently, Kinjo and co-workers reacted the bis-ligated boryl anion 8.42 with Li, Rh, Ir, and Au complexes.311

9.04.9

Borate weakly coordinating anions

Weakly coordinating anions (WCAs) display minimal interactions with their cation pairs and are increasingly important as they can support the formation, isolation and structural characterization of reactive cations,317–320 can stabilize vacant coordination sites that improve catalytic performance,321–323 and are important components in room-temperature ionic liquids and electrolytes.324–327 Incorporation of electron-withdrawing groups, and highly symmetric and large hydrodynamic radii are often considered in the design of WCAs as these features diffuse and remove directionality from the anionic charge.328 Here, the discussion is centered on key borate anions, while carboranes and boron cluster based WCAs are discussed elsewhere.329–331 In the 1960s it was found that conventional “non-coordinating” anions, such as BF4 − , PF6 − , ClO4 − , and SbF6 − , displayed varying interactions with their cation counterpart, determined by X-ray crystallography, IR and NMR studies, in the absence of aqueous solvents.332,333 Another limitation of BF4 − , PF6 − , SbF6 − and even [B(OTeF5)4]− is the propensity of strongly electrophilic cations to abstract a fluoride anion.333 These factors drove the need for less coordinating and more stable anions.334 Sodium tetraphenylborate (Na[9.1]; Fig. 126) was first prepared by Wittig in 1947 by reacting triphenylborane with phenyl lithium.335 Shortly afterwards, a preparation involving the use of phenylmagnesium halide and boron trifluoride was reported, and is still widely employed.336 Although originally a promising “non-coordinating” anion, it was not long before phenyl ring coordination to metal cations was observed,337–341 making the label “weakly coordinating” anion more appropriate. In addition to this potential for aryl coordination, other unwanted reactivity of this anion included tetraphenylborate metalation at the phenyl ring,342 phenyl transfer to a metal or an organic substrate,343 photolytic instability,344 and electrochemical oxidation.329 In an effort to reduce the coordination and increase the stability of [BPh4]− ([9.1]−), derivatives featuring fluoride functionalization became increasingly investigated.345 Conceivably, [B(3,5-(CF3)2C6H3)4]− ([9.2]−) and [B(C6F5)4]− ([9.3]−) are the most commonly employed fluoroarylborate anions.341,346–348

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Fig. 126 Weakly coordinating anions [9.1]−–[9.3]−.

Kobayashi and co-workers developed the tetrakis{3,5-bis(trifluoromethyl)phenyl}borate anion, [B(3,5-(CF3)2C6H3)4]− ([BArF4]−; [9.2]−; Fig. 126), to increase the solubility of ion pairs in weakly polar solvents.349,350 Originally [9.2]− was prepared akin to [9.1]− from the iodo-Grignard reagent and boron trifluoride, and although the method was later refined, the explosive nature of the Grignard reagent remained a concern.351 An alternative protocol was later descried by Bergman that circumvented the use of the Grignard/magnesium metal mixture,352 but required prolonged drying under reduced pressure with P2O5 to isolate the anhydrous Na[9.2] salt.351 Synthetic methods to the preparation of hydrated salts Li+ and K+ salts of [9.2]−, or where the water content is unknown, have also been previously reported.353–355 While only recently, in 2019, did Weller and Martínez-Martínez report on a protocol for the solvent-free anhydrous preparation of Li[9.2], Na[9.2] and K[9.2] on a multigram scale.356 The tetrakis(pentafluorophenyl) borate anion ([9.3]−; Fig. 126) is among the least coordinating counterions and compared to [9.2]− has the advantage of lacking alkyl C–F bonds which can be activated,348,357–362 making it particularly useful when isolating main-group cations. Early syntheses of [9.3]− date back to 1964, where Massey and co-workers treated tris(pentafluorophenyl) borane with pentafluorophenyl lithium at low temperatures to afford Li[9.3].363 At this time, the potassium salt of [9.3]− was also reported, formed by reacting Li[9.3] with either potassium chloride or potassium nitrate. The preparation of Li[9.3] requires extreme caution as the in situ generated LiC6F5 intermediate can produce benzyne and detonate at temperatures about −40  C.364 By conducting the reaction in pentane, anhydrous solvent-free Li[9.3] can be obtained.365,366 Alternatively, if diethyl ether and petroleum are employed as solvents the etherate [Li(OEt2)4][9.3] can be isolated which can lose diethyl ether under vacuum to give the salts [Li(OEt2)2.5][9.3] and [Li(OEt2)3][9.3].363,365,367

9.04.10

Boron radicals

Radicals are compounds featuring unpaired electron(s), allowing them to access unique chemical and physical properties and play an important role in chemical and biological reactions, and functional materials.125,368–373 Stabilizing main-group radicals often requires invoking thermodynamic stabilization, involving delocalization of the unpaired electron, and/or kinetic stabilization, involving the use of bulky substituents to prevent subsequent oligomerization. Depending on their thermal stability, radicals can be either labeled “persistent” or “stable.” The discussion below is focused on key metal-free anionic, neutral, and cationic borane radicals that are stable, radicals that can be isolated and stored, and are often characterized by X-ray crystallography. Polyhedral boron cage radicals, a growing and important field, are discussed elsewhere.373–375

9.04.10.1 Anionic boron radicals A century ago, early efforts to prepare boron radicals were focused on the preparation of triarylborane radical anions, given their isoelectronic relationship with the neutral triarylmethyl radicals (Fig. 127). In 1986, Power reported on the first structurally characterized boron-centered radical, [BMes3]•− ([10.5]•−; Fig. 128).376 [10.5]•− is remarkably stable and was found to decompose at 240  C. Interestingly, X-ray crystallographic studies revealed that compared to the BMes3 precursor, there is only slight elongation of the B–C bond lengths, and the BC3 planes for both compounds is very similar. Later, in 2013, Yamaguchi et al. isolated deep blue crystals of the planarized triphenylborane radical anion [10.7]•−,377 which, in similar manner to [10.5]•− displayed high thermal stability (Fig. 129). The EPR spectra of [10.7]•− showed that there was significant delocalization of the unpaired electron density over the methylene bridged phenyl rings. X-ray crystallography showed that [10.7]•− adopts a shallow bowl-shaped structure with slight pyramidalization at boron. In 2015, Marder chemically reduced a 2-(Mes2B)pyrene with potassium anthracenide to afford blue crystals of the respective potassium salt of the radical anion [10.10]•− (Fig. 130).378 In similar fashion to [10.7]•−, the EPR data for [10.10]•− is consistent with significant delocalization of the radical spin density. In addition to the monoB(Mes)2 functionalized pyrene derivative, Marder and co-workers prepared 2,7-bis(Mes2B)pyrene. 2,7-Bis(Mes2B)pyrene, which showed two reversible one-electron reduction waves (E1/2 ¼ −2.17 V and −2.45 V vs Fc/Fc+), allowing for stepwise reduction, first to the monoradical anion [10.12]•− followed by the dianion [10.13]2− (Fig. 131). Meanwhile, Pan and Wu explored the redox behavior of 9,10-bis(dimesitylboryl)anthracene, which displayed two quasi-reversible reduction waves.379 Upon reduction with potassium metal the dianion [10.16]2− was the primary product and only trace amounts of radical anion [10.15]•− was generated (Fig. 132).

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Fig. 127 Structural relationship between boron radical anion [10.1]•− and carbon radical 10.3.

Fig. 128 Synthesis of radical anion [10.5]•−.

Fig. 129 Synthesis of radical anion [10.7]•−.

Fig. 130 Synthesis of radical anion [10.10]•−.

In 2017, Wang and Tan moved to incorporating more bulky mesityl groups and prepared diboranes 2,6-(Mes2B)mesitylene (10.17) and 3,30 -bis(Mes2B)bismesitylene (10.20; Fig. 133).380 In the case of 2,6-(Mes2B)mesitylene (10.17), reduction with two equivalents of potassium metal afforded the C–H activated product 10.19, thought to proceed through a diradical dianion ([10.18]••2−). Whereas, when 3,30 -bis(Mes2B)bismesitylene (10.20) was reduced under similar conditions the targeted diradical dianion [10.21]••2− could be isolated as a blue crystalline material. XRD investigations into [10.21]••2− showed that both boron centers display trigonal planar geometries, and that they are linked by two nearly perpendicular mesityl groups, suggesting minimal p-conjugation between the boron centers. Further reaction of [10.21]••2− with two equivalents of n-Bu3SnH gave dianion [10.22]2−. Later, Wang and Tan also reported on two more diradical dianions where the two Mes2B units are connected via a pyrene moiety in a 1,6- and 1,3-fashion ([10.24]••2− and [10.26]••2−; Fig. 134).381

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Fig. 132 Synthesis of radical anion [10.15]•− and dianion [10.16]2−.

Fig. 131 Synthesis of radical anion [10.12]•− and dianion [10.13]2−.

In contrast to the well-investigated boron radical anions R3B•−, examples of diborane radical anions featuring a one-electron boron-boron s-bond are sparse. In 2014, Wagner and Holthausen prepared diborane 10.27, where both boron pz orbitals are preorganized to allow for interaction (Fig. 135).382 Treatment of 10.27 with lithium napthalenide led to a dark solution, from which black crystals of [10.28]•− were isolated, making this the first crystallographically characterized diborane radical anion containing a B•B one-electron s-bond. Later, in 2015, Wagner and Holthausen also reported on a bis(9-borafluorenyl)methane (10.29; Fig. 136) from which, albeit under more forcing conditions, another B•B one-electron s-bond radical anion ([10.31]•−) could be generated.383

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Fig. 133 Synthesis of anions [10.19]2−, [10.21]••2− and [10.22]2−.

Fig. 134 Synthesis of radical dianions [10.24]••− and [10.26]••−.

Fig. 135 Synthesis of radical anion [10.28]•−.

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Fig. 136 Synthesis of radical anion [10.31]•−.

Recognizing that bulky aryl substituents greatly enhance stability, Power and co-workers were able to isolate and structurally characterize the first diborane radical featuring a B•B one-electron p-bond [10.32]•− (Fig. 137).93 Here, [(MeO)MesB]2 was reduced with lithium to give [10.32]•− as dark blue crystals. Interestingly, the reduction of asymmetric Mes2BB(Ph)Mes with potassium yielded the solvent-separated ion pair [K(18-crown-6)(THF)2][10.33]•− as a dark purple crystalline material (Fig. 138). Meanwhile, Yamashita and Lin reported on the isolation and characterization of [10.34]•− (Fig. 139), a related radical anion from the asymmetric diborane Mes2B–Bpin.384 [10.34]•− was found to activate alkynes, carbon monoxide, and isonitriles under ambient conditions. p-Conjugated boracycles, where the pz orbital of the boron is conjugated, are potential one-electron acceptor precursors to the corresponding radical anions. In 1995, Siebert et al. synthesized 9,10-dihydro-9,10-diboraanthracene derivative 10.35 and reduced it with potassium to give radical anion [10.36]•− (Fig. 140).385 While, in 2008, Yamaguchi et al. generated stable radical anions [10.38]•− from dibenzoborole derivatives 10.37 (Fig. 141).386 Later, in 2012, the Braunschweig group reduced 10.39 with Cp 2Co to give reddish brown compound [10.40]•− (Fig. 142).387 The X-ray structure of [10.40]•− revealed a planar C4B framework with propeller-like orientation of the phenyl groups. Subsequent reaction of [10.40]•− with dibenzoyl peroxide gave the borate salt [10.41]−, indicative of boron-centered radical reactivity despite its steric encumbrance. Related 2,5-bis(borolyl)thiophene radical anion derivative [10.43]•− (Fig. 143) was also reported by Braunschweig and co-workers.388 Recently, the Pammer group has described boron radical anions [10.45]•− and [10.47]•− that feature N–B coordination (Fig. 144).389

Fig. 137 Synthesis of radical anion [10.32]•−.

Fig. 138 Synthesis of radical anion [10.33]•−.

Fig. 139 Synthesis of radical anion [10.34]•−.

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Fig. 140 Synthesis of radical anion [10.36]•−.

Fig. 141 Synthesis of radical anions [10.38]•−.

Fig. 142 Synthesis of radical anion [10.40]•− and borate [10.41]−.

Fig. 143 Synthesis of radical anion [10.43]•−.

9.04.10.2 Neutral boron radicals Compared to anionic boron radicals, neutral boron radicals have been investigated to a much lesser extent. The electron deficient nature of boranes decreases the stability of naked species of the type R2B•. Thus, many examples of neutral boron radicals require donation from a Lewis base, to afford more stable L➔R2B• type species. Typically, synthetic protocols to make neutral boron radicals include adduct formation with R2BH to give L➔R2BH followed by hydrogen atom abstraction. Alternatively, by a one-electron reduction from the corresponding borenium cation can also allow access to the neutral boron radical. Here, the discussion is focused on carbene Lewis base stabilized radicals. High delocalization of the spin density gives a trigonal-planar geometry at the three-coordinate boron center and can be considered a p-type radical. Otherwise, the geometry at boron is pyramidal and the radical displays s-type character.

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Fig. 144 Synthesis of radical anions [10.45]•− and [10.47]•−.

Fig. 145 Synthesis of neutral radicals [10.49]• and [10.51]•.

In 2007, Gabbaï and co-workers prepared the first example of an isolable carbene stabilized neutral boron radical (10.49), by reduction of borenium cation [10.48]+ with magnesium metal (Fig. 145).390 EPR investigation of 10.49 showed that the unpaired electron is largely delocalized on the acridinyl moiety with a non-negligible contribution from the boron atom.390,391 By injection of an additional electron, it was found that the corresponding borata-alkene could be generated.391 Later, the same group also reported the synthesis of the related boron radical 10.51 through a similar protocol.264 In an effort to increase spin density at the boron center, Bertrand and Roesky focused their investigations on CAAC stabilized neutral boron radicals.392–394 In 2014, Braunschweig, and Stephan and Bertrand individually synthesized and isolated neutral boryl radicals 10.53 and 10.55 (Figs. 146 and 147).395,396 For compound 10.53, the boron center was found to adopt a trigonal-planar geometry, and to feature a shortened B–Ccarbene bond,395 suggesting significant delocalization of the unpaired electron into the p-acceptor orbital of the carbene. Compound 10.55 displayed structural parameters in good agreement with 10.53.396 In the same year, Braunschweig and co-workers also reported on the first boryl based radical 10.57, prepared by reacting 10.56 with Ph3ECl (E ¼ Sn and Pb; Fig. 148).397 Interestingly, the alternative reaction of the precursor (10.56) with Me3ECl gave products 10.58a and 10.58b.

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Fig. 146 Synthesis of neutral radical [10.53]•.

Fig. 147 Synthesis of neutral radical [10.55]•.

Fig. 148 Synthesis of neutral radical [10.57]• and compound 10.58.

Fig. 149 Synthesis of neutral radical [10.60]•.

In 2016, Hudall reduced a borenium cation featuring a diamidocarbene (DAC) ligand to afford radical 10.60 (Fig. 149).398 In contrast to 10.53 and 10.55, X-ray crystallographic investigation of 10.60 showed an orthogonal orientation of the carbene ligand compared to the boryl moiety. This positioning suggests minimal delocalization of the spin-density between the two groups. It was determined that this radical is best described as a boryl-substituted DAC-centered radical.

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Recently, Tamm et al. prepared a family of carbene-supported neutral boron radicals (10.62, Fig. 150).399 Contrary to the other carbene-supported neutral boryl radicals discussed, EPR studies showed large hyperfine coupling constants to the boron isotopes and minimal delocalization to the carbene ligand. This electronic situation is thought to be the result of the steric demands on the carbene, enforcing a nearly perpendicular orientation to the boryl substituents. DFT investigations further revealed a 9:1 distribution ratio of the spin density between the boryl fragment and carbene ligand. Hence, compounds 10.62 are best described as carbene stabilized boryl radicals. Subsequent reaction of 10.62 with 1,4-benzoquinone formed compounds 10.63.

9.04.10.3 Cationic boron radicals Typically, in tricoordinate organoboranes the boron center is in the formal oxidation state 3+ and electron deficient from the perspective of the octet rule. This electron deficiency significantly destabilizes boron radicals that are cationic. Hence, there are even fewer examples of cationic boron radicals, compared to neutral and anionic derivatives. In 2011, Bertrand and co-workers prepared bis(CAAC)borylene adduct 10.64 (Fig. 151), where the formal oxidation state of the boron is +1 and was found to act as an electron donor.400 Compound 10.64 is isoelectronic with amines and confirmed by cyclic voltammetry experiments could be oxidized to its radical cation [10.65]•+. Compound 10.65 was crystallographically characterized: upon oxidation it was found that the B–Ccarbene bonds elongated, consistent with less electron density at the boron available for p-donation. Further NBO analysis showed that the SOMO is primarily centered on boron but does partially delocalize into the p-orbitals on the carbenes. As a result of their relative electron richness, Lewis-base stabilized diborenes can act as neutral reductants. The Braunschweig group prepared a series of diborenes (10.66, 10.68, and 10.70) stabilized with phosphine and NHC ligands (Fig. 152), and investigated their electrochemical properties.130,401 The CV experiments demonstrated that the strong s-donating ability of the carbenes raised the HOMO energies of the diborenes, allowing for facile oxidation. In good agreement with their oxidation potentials, the diborenes 10.66, 10.68, and 10.70 could be chemically oxidized with the relatively weak oxidizing agent [C7H7] [BArF4] at room temperature to yield the stable radical cations [10.67]•+, [10.69]•+, and [10.71]•+, respectively. In the case of diborene 10.70, reaction with the anti-aromatic borole MesBC4Ph4 afforded the first fully boron-centered radical cation and radical anion pair (10.72) as a pink crystalline material.401 The structural parameters for this system showed elongation of the B–B bond upon oxidation from the diborene precursor, in line with a decrease in bond order from 2 to 1.5. In 2017, Harman and co-workers reported a comproportionation reaction which yielded the radical cation [10.75]•+ (Fig. 153).402 X-ray crystallographic studies showed a planar diboraanthacene core with a trigonal-planar geometry at the boron centers. EPR studies were consistent with the spin-density primarily being centered on the boron atoms.

Fig. 150 Synthesis of neutral radicals [10.62]• and subsequent reactivity.

Fig. 151 Synthesis of radical cation [10.65]•+.

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Fig. 152 Synthesis of radical cations [10.67]•+, [10.69]•+, [10.71]•+ and radical ion pair 10.72.

Fig. 153 Synthesis of radical cation [10.75]•+.

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9.04.11

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Frontiers

Within the field of inorganic chemistry, research into boron chemistry emerged from humble beginnings. Research in the first half of the 20th century elucidated the structure of diboranes and focussed on investigations into boron hydride chemistry, including into polyhedral boron clusters, hydroboration reactions, and subsequently organoborane chemistry. The development of organoborane chemistry ultimately led to significant advances in organic manipulations and cross-couplings reactions, highlighted by three Nobel Prizes in the field. In addition to these successes, significant advancements in boron chemistry are still being made, including but not limited to using boranes as reagents and catalysts, new small molecule fixations and activations promoted by boranes, new boron-containing materials allowing access to exciting physical and photophysical properties, and new applications for boron-containing clusters. Boron chemistry is studied globally by researchers in the fields of biomedical research, organic synthesis, materials science, and fundamental chemistry. In the section below recent developments of boranes in main-group catalysis and as components in functional polymers are highlighted.

9.04.11.1 Main-group catalysis Approximately 90% of chemical products produced require a catalytic step at some point in their preparation, many of these catalysts are based on expensive precious metals.403 Mining and purification of these precious metals can often be accompanied with environmental and geopolitical issues, which are further exacerbated by their diminishing reserves. Motivated by developing more sustainable catalysts based on earth-abundant elements increasing attention has been given to the reactivity of compounds featuring s- and p-block elements. One limitation with mapping the wealth of transformations possible with d-block metals to main-group elements is the simpler frontier orbitals at main-group elements. In 2006, Stephan and co-workers found that Mes2P(C6F4)B(C6F5)2 (11.1; Fig. 154) reversibly activated H2 gas, representing the first example of reversible metal-free H2 activation.404 Compound 11.1 was thought to heterolytically activate H2 as a result of steric encumbrance at the phosphine and borane preventing Lewis adduct formation. Based on this reactivity the term “frustrated Lewis pair” (FLP) was coined, where the unquenched reactivity at the phosphine and borane allows for donor-acceptor type activations of small molecules in a way that was previously only the purview of transition metals.14 The advent of FLPs sparked a renaissance for the study of main-group catalysts. As demonstrated by the original FLP system 11.1, boranes are an attractive Lewis acidic counterpart in FLP systems. In 2007, Stephan and co-workers expanded their finding of metal-free reversible H2 activation to affect the catalytic hydrogenation of imines (Fig. 155).405 Since this discovery, it has been found that the substrate itself can act as a component in the FLP system, as shown in Fig. 155.406,407 Initial FLP catalytic hydrogenations focused on B(C6F5)3 as the Lewis acidic compound and reduced imines, however the high tunability of both of the boron Lewis acid and Lewis base employed has allowed for the scope of substrates to include olefins, alkynes, enamines, enones, ynones, polyaromatics, imines, anilines, N-heterocycles, ketones, aldehydes and acetals; the list continues to grow.408 B(C6F5)3 and related haloarylboranes have also been employed to effect transfer hydrogenation reactions using amino-borane and cyclohexadiene-derivatives as hydrogen gas surrogates.407,409 Furthermore, incorporation of either a chiral Lewis acidic or Lewis basic component in FLP chemistry has allowed access to enantioselective catalytic reductions with pro-chiral unsaturated functional groups.410–413 Although not identified as an FLP at the time, in 1996 Piers and co-workers reported that B(C6F5)3 could catalytically reduce carbonyls through a hydrosilylation reaction (Fig. 156).414 The mechanism for this hydrosilylation process was further elucidated by Piers through kinetic investigations, and by Oestreich through the use of a chiral silane that showed inversion upon Si–O bond formation, consistent with 11.3 being the transition state for this reduction.414,415 Hydrosilylation reactions of olefins and imines mediated by boranes have also been previously reported.416–419 In 2010, Piers and co-workers expanded their hydrosilylation of carbonyls to the reduction of CO2 and produced methane using B(C6F5)3, 2,2,6,6-tetramethylpiperidine (TMP), and Et3SiH.420 This finding builds the groundwork to recycle a greenhouse gas into value-added products and possibly sustainable fuel. Another interesting development was the use of silane cyclohexadiene derivatives by the Oesterich group to promote transfer hydrosilylation akin to transfer hydrogenation using borane catalysts.421 Related reductions of unsaturated groups via hydroboration have also previously been mediated with boranes, with some key examples outlined in Section 9.04.5 of this chapter. In addition to reduction chemistry, in the form of catalytic hydrogenation, hydrosilylation and hydroboration, FLP mediated C–H bond activation to form C–E bonds allows for efficient access to high-value products. In 2015, seminal work was reported by Fontaine and co-workers with the preparation of FLP 11.4 and the finding that it activated C–H bonds of heteroarenes to catalyze

Fig. 154 Reversible H2 activation with FLP 11.1.

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Fig. 155 Catalytic hydrogenation of imines with FLPs.

Fig. 156 Catalytic hydrosilylation of carbonyls using B(C6F5)3.

the borylation of furans, pyrroles, and thiophenes (Fig. 157).422 In this unprecedented Lewis acid-base cooperative activation of C–H bonds, the base abstracts a proton while the sp2 carbon binds to the borane. This process differs from previous examples of Friedel-Crafts type mechanisms that have been reported with Lewis acidic boranes in that it goes beyond bond polarization and is reminiscent of oxidative addition at transition metals. FLP chemistry is discussed in more detail in another chapter of this book.

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Fig. 157 Catalytic borylation of furans, pyrroles, and thiophenes using 11.4.

9.04.11.2 Functional polymers Polymers containing alternating P–B and N–B bonds have arguably been the most widely investigated boron-containing polymers as there are isoelectronic with C–C bonds and offer exciting new possibilities in sustainable fuel storage.423–430 As such, they have been previously reviewed and are further discussed elsewhere in this book. The application of conjugated polymers in optoelectronic, sensory, and imaging applications has stimulated research efforts into polymers featuring electron-deficient Group 13 elements. The empty p-orbital of these elements dramatically lowers LUMO levels and gives rise to bathochromic shifts in absorption and emission spectra compared to the well-studied Group 14 systems.21,431 Homo-catenation of group 13 elements in to form linear chains greater than two atoms is much more difficult to achieve than for group 14 chains due to weaker E–E bonds.432 Instead, there is a tendency to form small cyclic systems or clusters.433 In 2012, Braunschweig and co-workers reported on the homocoupling of borylene (:BR2) ligands on a transition metal to give a B4 chain under mild conditions (11.5; Fig. 158).434 Later in 2018, Vargas, Ingelson, and Braunschweig found that a transition-metal free B4 chain stabilized with NHC ligands (11.6) was formed in the reaction of biscatechol diborane with a range of NHCs, followed by halogen exchange and reduction with KC8.435 Interestingly, the four sp2 hybridized boron atoms showed 2p-electron delocalization making them attractive models for all boron conjugated oligomers and polymers. Bifunctionalization of boranes through hydroboration/addition across two unsaturated groups connected by an organic linker was first pioneered by Chujo and Matsumi.436 By employing sterically encumbered boranes, such as MesBH2 or TippBH2 (where Mes ¼ 2,4,6-trimethylphenyl and Tipp ¼ 2,4,6-triisopropylphenyl) vinylborane polymers (11.7 and 11.8) were obtained with Mn ¼ 3.0–6.5 kDa (Fig. 159).437,438 The highly fluorescent nature and emission wavelengths of these polymers could be tuned

Fig. 158 Compounds featuring boron chains 11.5 and 11.6.

Fig. 159 Synthesis of polymers 11.7 and 11.8.

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by varying the aromatic organic link of the dialkyne. Recently, in 2017, Qin and co-workers reported on the preparation of boron “doped” polyacetylenes (11.8), representing the first example of boron incorporation in the main chain without aromatic moieties.439 Polymers 11.8 were found to be bathochromically shifted compared to 11.7 as a result of more extensive delocalization across the polymer from the vinylene rather than aromatic units. The hydroboration method does not always allow access to the desired polymer, thus alternative metal-boron exchange reactions can be employed using organolithium, organotin, organosilane, and Grignard reagents, consider Fig. 160.61,440–442 Additionally, metal catalyzed C–C cross coupling reactions can also be employed, consider Fig. 161.54,443,444 Theoretical calculations have predicted that homopolymers derived of only borole heterocycles would display very small band gaps,445,446 as a consequence of the empty p-orbital on boron and the anti-aromatic nature of boryl heterocycles, containing 4p electrons. In 2015, Rupar and co-workers employed Yamamoto’s coupling polymerization procedure to prepare the first poly(9borafluorene) 11.14 (Fig. 162).447 They also prepared the related co-polymer 11.15 through Stille coupling in an effort to prevent

Fig. 160 Synthesis of polymers 11.10.

Fig. 161 Synthesis of polymers 11.12.

Fig. 162 Synthesis of polymers 11.14 and 11.15, and structure of polymers 11.16 and 11.17.

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twisting of the main chain. It was found that when polymer 11.14 was exposed to an atmosphere of NH3 it displayed a change in luminescence from yellow to blue. In addition, co-polymer 11.15 was found to display a small optical band gap (2.12 eV) and significant acceptor character with a low-lying LUMO at approximately −4.0 eV, revealing potential applications of this material in organic electronics. Luminescent diboraanthracene-based polymers 11.16 and 11.17 had also been previously prepared by Wagner and Jäkle using hydroboration and metal-catalyzed cross-coupling reactions.448,449 Over the last decade there has been a surge in interest devoted to polymers that contain tricoordinate arylboranes, as their electron deficient nature can be exploited to form stimuli responsive polymers and sensors.450 The primary synthetic routes to these polymers are the polymerization of monomers featuring the borane unit, usually using styrene or norbornene derivatives on the backbone, and post-polymerization modification to exchange silicon functional groups for boron units (Fig. 163).451–453 The direct polymerization method is synthetically more facile, while the polymer modification method offers greater control over polymer chain length. In 2006, Jäkle and co-workers prepared a highly fluorescent triarylborane polymer (11.18; Fig. 164) and found that it could form adducts with pyridine, cyanide and fluoride.79,454 Later in 2013, the same group built on this work to prepare ambiphilic block copolymers containing pendant triarylboranes (11.19), which allowed for the visual detection of fluoride anions at ppm levels.455 In 2017, Shaver took the innovative approach of applying FLP chemistry to polymer chemistry, and used blends of diarylborane (11.20) and dimesitylphosphine (11.21) bearing linear polymers which do not directly react, but in the presence of diethyl azodicarboxylate cross-link to give 11.22 which displays incredible self-healing properties (Fig. 165).456 In 2018, Yan and co-workers showed that block copolymers with dimesitylphosphine Lewis basic units and bis(pentafluorophenyl)borane Lewis acidic units captured CO2 gas and in doing so reversibly assembled into core-crosslinked micelles.457 In 2018, Kalow reported on the synthesis and responsive properties of poly(ethyleneglycol) (PEG)-based hydrogels (11.23; Fig. 166), which were found to undergo an E/Z configuration isomerization of the diazo unit in the presence of UV and visible light.458 Thus, using an external light stimulus the rheological properties of these polymers could be controlled like an ON/OFF switch.

Fig. 163 Synthesis of boron containing polymers via poly modification vs direct polymerization methods.

Fig. 164 Structure of polymers 11.18 and 11.19.

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Fig. 165 FLP activation diethyl azodicarboxylate to give crosslinked polymer 11.22.

Fig. 166 E to Z isomerization of 11.23.

9.04.12

Conclusions

Boron’s unique position in the Periodic Table, at the boundary of metals and non-metals, coupled with its natural electron-deficient nature allows access to a wide range of chemical and physical properties. Investigations into borane chemistry ranges from fundamental interests into B–B multiple bonds and boryl anions, to applications in organic synthesis and catalysis, to the development of functional materials and switches. The chemistry of boranes and diboranes is internationally investigated and appears to be boundless as it continues to grow. In this chapter recent developments in the preparation of triarylboranes, organic diboranes(4), compounds featuring boron–boron multiple bonds, hydroboration reactions, carboboration reactions, boron cations, boryl anions, borate weakly coordinating anions, main-group catalysis, and boron-containing functional polymers are summarized.

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Commun. 2017, 53, 10390–10393. Lam, J.; Szkop, K. M.; Mosaferi, E.; Stephan, D. W. Chem. Soc. Rev. 2019, 48, 3592–3612. Boom, D. H. A.; Jupp, A. R.; Slootweg, J. C. Chem. Eur. J. 2019, 25, 9133–9152. Lam, J.; Sampaolesi, S.; LaFortune, J. H. W.; Coe, J. W.; Stephan, D. W. Dalton Trans. 2019, 48, 133–141. Gao, B.; Feng, X.; Meng, W.; Du, H. Angew. Chem. Int. Ed. 2020, 59, 4498–4504. Hamza, A.; Sorochkina, K.; Kótai, B.; Chernichenko, K.; Berta, D.; Bolte, M.; Nieger, M.; Repo, T.; Pápai, I. ACS Catal. 2020, 10, 14290–14301. Paradies, J. Chiral Borane-Based Lewis Acids for Metal Free Hydrogenations; Springer: Cham, 2017. Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440–9441. Rendler, S.; Oestreich, M. Angew. Chem. Int. Ed. 2008, 47, 5997–6000. Nakajima, Y.; Shimada, S. RSC Adv. 2015, 5, 20603–20616. Oestreich, M.; Hermeke, J.; Mohr, J. Chem. Soc. Rev. 2015, 44, 2202–2220. Carden, J. L.; Dasgupta, A.; Melen, R. L. Chem. Soc. Rev. 2020, 49, 1706–1725.

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9.05

Polyhedral Boranes and Carboranes

Igor B Sivaev, A.N.Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia © 2022 Elsevier Ltd. All rights reserved.

9.05.1 9.05.2 9.05.2.1 9.05.2.2 9.05.2.3 9.05.2.4 9.05.2.5 9.05.2.5.1 9.05.2.5.2 9.05.2.5.3 9.05.2.5.4 9.05.3 9.05.3.1 9.05.3.1.1 9.05.3.1.2 9.05.3.1.3 9.05.3.1.4 9.05.3.1.5 9.05.3.1.6 9.05.3.2 9.05.3.3 9.05.3.3.1 9.05.3.3.2 9.05.3.3.3 9.05.3.3.4 9.05.3.3.5 9.05.3.4 9.05.3.5 9.05.3.6 9.05.3.7 9.05.4 References

9.05.1

Introduction Polyhedral carboranes Literature reviews on chemistry of some carborane derivatives Applications of carboranes in medicine Carborane based luminescent materials Carboranes as weakly coordinating anions Functional group assisted B-H activation of carboranes by transition metal complexes B-H activation assisted by phosphorus-containing substituents B-H activation assisted by nitrogen-containing substituents B-H activation assisted by oxygen-containing substituents B-H activation assisted by sulfur-containing substituents Polyhedral boranes closo-Dodecaborate anion [B12H12]2− General aspects. Halogen derivatives Derivatives with BdO bond Derivatives with BdS bonds Derivatives with BdN bonds Derivatives with BdP bonds Derivatives with BdC bonds closo-Undecaborate anion [B11H11]2− closo-Decaborate anion [B10H10]2− General aspects. Halogen derivatives Derivatives with BdO bonds Derivatives with BdS bonds Derivatives with BdN bonds Derivatives with BdC bonds closo-Nonaborate anion [B9H9]2− closo-Octaborate anion [B8H8]2− closo-Heptaborate anion [B7H8]2− closo-Hexaborate anion [B6H6]2− Conclusions

196 197 197 197 197 198 198 198 209 223 236 244 244 244 244 245 247 250 250 251 251 251 251 252 253 254 255 255 255 255 256 256

Introduction

This chapter concerns polyhedral carboranes (carbaboranes) and boranes. Published studies on carboranes before 1981 were reviewed in COMC-I (1982), between 1982 and 1992 in COMC-II (1995) and between 1992 and 2005 in COMC-III (2006). Later, published studies on carboranes from their first synthesis to the present were summarized by Russell Grimes in his seminal book Carboranes, two editions of which were published in 2011 and 2016.1,2 The latest edition covers the published literature on carborane up to 2015, and the literature following the publication of this book is annually updated on the website http://booksite. elsevier.com/9780128018941/. In general, according to Web of Science, since 2013, when 50 years have passed since the first publication on the synthesis of ortho-carborane, the annual number of publications concerning various aspects of chemistry and applications of carboranes has exceeded 200 and tends to increase. Therefore, we believe that the traditional exhaustive reviews of the literature in this area for a specific period have lost their original significance, whereas exhaustive reviews concerning various directions in the development of the chemistry of carboranes are gaining in importance. Here will be given the most relevant and high-quality reviews concerning both some individual types of carborane, and the most actively developing areas of the chemistry of carboranes in general. Some of the most interesting and actively developing areas, which have not yet been reviewed, will be discussed in more detail. As for the chemistry of polyhedral boranes, the situation in this area is fundamentally different. Compared to carboranes, their chemistry is much less studied. Due to the presence of carbon atoms in boron clusters, carboranes have received considerable attention from researchers working in the field of organic chemistry, while boron clusters containing no carbon atoms have traditionally been considered the fiefdom of inorganic chemists. Therefore, polyhedral boranes were not considered earlier in the COMC framework. However, the similarity of the structures of polyhedral boranes and carboranes as well as the great progress in

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the chemistry of boron-substituted carboranes, achieved recently, makes it expedient to include polyhedral boranes in this edition. However, it should be noted that simple and complex salts of polyhedral borane anions, which are subjects of inorganic and coordination chemistry, respectively, are outside the scope of this edition. To the best of our knowledge, the last attempts to consider the chemistry of polyhedral boranes in general date back to the last century.3,4 In this edition, published studies on polyhedral boranes after 2000 are reviewed or after the publication of the latest exhaustive reviews, if available. The present review covers the literature up to mid-2021.

9.05.2

Polyhedral carboranes

9.05.2.1

Literature reviews on chemistry of some carborane derivatives

The chemistry of some types of carborane clusters and their derivatives has been the subject of special reviews. First of all, this concerns anionic monocarbaboranes [CB11H12]− and [CB9H10]−,5–10 as well as bis(carboranes), in which two carborane cages are directly connected to each other.11,12 Special reviews were devoted to the chemistry of some specific derivatives of carboranes, such as mercury derivatives of carboranes,13 iodocarboranes,14 carboranyl thioethers,15 carboranyl thiols16–19 and carboranyl phosphines20–23 including complexes thereof. In addition, a number of reviews devoted to individual classes of carborane-containing compounds are directly related to their potential application in various fields and will be considered separately.

9.05.2.2

Applications of carboranes in medicine

The possibility of using carboranes in medicine has been one of the driving forces behind the development of carboranes chemistry over the past several decades.24,25 One of the main directions of the use of carboranes in medicine is boron neutron capture therapy (BNCT) for cancer, which is based on the nuclear reaction of two essentially nontoxic species, nonradioactive 10B and low-energy thermal neutrons. Selective delivery of boron into the tumor tissue and it high accumulation there (>20 mg of 10B per gram of tumor tissue) are the most important requirements to boron compounds to achieve efficient neutron capture therapy of cancer. Therefore, carboranes containing 10 boron atoms in molecule and possessing high stability, along with polyhedral boranes, are good candidates for the design of BNCT agents. To date, a variety of carborane-containing derivatives of various biologically active compounds are capable of selectively delivering boron to the tumor.26–29 The chemistry of many carborane-containing biologically active compounds has also been the subject of special reviews. They includes synthesis and chemistry of boronated aminoacids,30 nucleosides,31 sugars,32 porphyrins and phthalocyanines.33–37 Another important area of medical use of carboranes is due to the unique properties of the carborane cluster. The van der Waals volumes of carboranes (148, 143, and 141 A˚ 3 for ortho-, meta- and para-carborane, respectively) are comparable to that of adamantane (136 A˚ 3). The presence of 10 hydride-like hydrogens at the boron atoms makes carborane clusters extremely hydrophobic. The hydrophobicity of the carboranyl moiety is comparable to the hydrophobicity of the adamantyl group and can vary appreciably depending on the carborane isomer and the place of substituent attachment. In addition, the electronic effects of the carboranyl moiety depend on both the carborane isomer and the attachment position of the substituent, and varies from strongly electron-withdrawing for C-substituted derivatives to moderately electron-donating for B-substituted ones. This makes it possible to consider carboranes as pharmacophores and to use them as surrogates for adamantane and other bulky hydrophobic groups in the design of pharmaceutics. To date, a large number of such compounds have been obtained and described in a number of reviews.38–42 Several illustrative examples of comparing the effect of different pharmacophore groups on the properties of anti-tuberculosis and anti-malarial drugs are presented in the papers.43,44

9.05.2.3

Carborane based luminescent materials

Another driving force behind the development of the chemistry of carboranes in the last decade is the synthesis of luminescent materials based on them for potential use in (opto)electronic devices, such as organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), solar cells, biological sensors and imaging devices, etc.45,46 Since the early 2000s, the main focus has been on the synthesis and study of the properties of transition metal complexes of with various carborane-based ligands.47–57 Later, the remarkable effect of the ortho-carborane cluster on the photophysical properties of luminescent materials in solution and the solid state was demonstrated. When p-conjugated organic moieties are bound to the carbon atoms of the ortho-carborane cage, it acts as an electron-withdrawing group. However, in solution, the photoinduced process of intramolecular charge transfer (ICT) from p-conjugated fragments to the antibonding orbital located at the CdC bond of carborane leads to fluorescence quenching. Nevertheless, these materials can recover the emission capacity in the solid state, due to suppression of the intramolecular motion and the CdC bond vibration. Therefore, ortho-carborane can be considered as a functional unit able to induce highly-efficient solid-state emissions in carborane-containing conjugated materials in the aggregated state. To date, more than a hundred such derivatives of ortho-carborane have been synthesized and their optical properties in solution and in the solid state have been studied.58–65

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9.05.2.4

Carboranes as weakly coordinating anions

One of the most interesting fields of application of anionic carboranes and boranes is their use as weakly coordinating anions to stabilize highly reactive complexes and intermediates. Until recently, evidence for the existence of many highly reactive organometallic complexes and highly electrophilic p-block cations was based mainly on results of ab initio calculations; some of them can also be observed in the gas phase in a mass spectrometer or in a solid argon matrix. Therefore, the challenge posed to synthetic chemists, theoreticians and spectroscopists is to reproduce their results in laboratory conditions, thereby checking whether their proposed models are possible. This problem is not only of academic interest, but also makes possible the synthesis of still inaccessible reactive particles that can find practical application, for example, in catalysis. It is clear that such reactive cations must be partnered with suitable anions, that are ideally non-coordinating and do not distort the cation geometry. Unfortunately, true non-coordination is not possible, because any finite system must be charge neutral and for each positive charge, an equal negative charge should exist somewhere, with the opposite charges interacting with each other. However, it is possible to find anions which help replace a few strong interactions between the reactive cation and anion by many very weak ones, that cancel each other out. These are called “weakly coordinating anions.”66 Ideally, weakly coordinating anions should have the following qualities: (1) absence of basic or nucleophilic sites, such as lone pairs, hydridic hydrogens and easily ionized single or multiple bonds; (2) resistance to oxidation; (3) the largest possible size to minimize electrostatic attractions.66 Around 50 years ago, the term “non-coordinating anion” was commonly used for such anions as [BF4]−, [ClO4]−, [PF6]−, [AsF6]−, [SbF6]−, and [OTf]−.67 However, with the advent of routine X-ray crystallography, it became evident that in many cases these anions can easily be coordinated. The exchange of fluorine atoms in [BF4]− for fluorinated phenyl groups leads to [B(C6F5)4]− and [B(C6H3-3,5-(CF3)2)4]− anions, which are widely used as weakly coordinating anions in homogenous catalysis,68 however both they were found to be amenable to Z-coordination of the aryl ring,69–71 or even cleavage of the CdB bond.72–74 An alternative to these weakly coordinating anions is use of highly stable polyhedral moieties, such as the carbacloso-dodecaborate anion [CB11H12]− and its analogs.75,76 The [CB11H12]− anion is very stable and rather large (the average diameter without hydrogen atoms 3.4 A˚ ), has spheric shape and no lone pairs. Replacing the relatively nucleophilic B-H vertices with more inert B-halogen ones produces a series of halogen derivatives which are more stable and less coordinating than the parent anion. The use of the halogen derivatives of the carba-closo-dodecaborate anion as weakly coordinating anions greatly stimulated rapid progress in its chemistry during the last 25 years.5,6 To date, the synthesis and structure of a large number of highly reactive transition metal complexes and highly electrophilic p-block cations stabilized by weakly coordinating anions based on halogen derivatives of the carba-closo-dodecaborate anion, has been described5,6,77–81 and their use in various catalytic processes has been demonstrated.82–84

9.05.2.5

Functional group assisted B-H activation of carboranes by transition metal complexes

One of the most interesting and actively developing areas of recent years in the synthetic chemistry of carboranes is their B-H activation using transition metal complexes.85 Several examples of direct Pd- and Ir-catalyzed B-H functionalization of ortho-carborane at B(3) and B(6) vertices have recently been reported.86–89 However, the functional group-directed B-H-activation of carboranes with transition metal complexes has received much more attention. A huge impetus to the development of this area was given in 2014 when Jin described the synthesis of metallocycles with a IrdB bond based on carboranyl carboxylic acids,90 and Xie reported the Ir-catalyzed preparation of previously inaccessible B(4)-derivatives of ortho-carborane starting from ortho-carboranyl carboxylic acid.91 It should be noted that the formation of cyclometallated carboranylphosphines with an IrdB bond was first reported by Hawthorne in the mid-1970s,92 and somewhat later, in the 1980s, the phosphine-directed BH-activation of carboranes in the presence of palladium, ruthenium, and osmium complexes was widely studied by Kalinin and Zakharkin.93 As a rule, ortho-carborane based C,C0 -diphosphine ligands form P,P0 -chelate complexes,23 while for C-monophosphine ligands, the formation of P,B-chelate complexes with activation of the BdH bond is more typical. The directing action of the substituent attached to the C(1) carbon atom of ortho-carborane makes boron atoms available for BH-activation in the equivalent positions B(3) and B(6), as well as B(4) and B(5). It should be noted that in this early period (1970s–80s), due to the limitations of NMR instruments of that time, it was exceedingly difficult to establish the exact position of the substitution, and only in a few cases this was done using single crystal X-ray diffraction study. Here we tried to give the most complete picture of the B-H activation reactions of carboranes with transition metal complexes using substituents containing various donor atoms as auxiliary ligands, including the formation of complexes with BdH⋯ M and BdM bonds, as well as their use in the synthesis of various derivatives of carboranes. The B-H activation reactions are considered in the order of decreasing donor properties of the primary substituent in the series P(As) > N > O(S). This makes it possible, on the one hand, to consider them in their historical development, and on the other hand, to draw a line between more stable and more reactive cyclometallated complexes, which are used to introduce various organic substituents into the carbon cage.

9.05.2.5.1

B-H activation assisted by phosphorus-containing substituents

Refluxing carboranyl phosphine palladium complex trans-[PdCl2(1-Ph2PCH2-1,2-C2B10H11)2] in toluene leads to the elimination of HCl with the formation of a mixture of monomeric [PdCl(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3/4))(1-Ph2PCH2-1,2-C2B10H11k1-P)] and dimeric [PdCl(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3/4))]2 five-membered palladacycle complexes,94,95 while refluxing trans-[PdCl2(1-Ph2PCH2-2-R-1,2-C2B10H10)2] (R ¼ H, Me) in isopropanol leads exclusively to dimeric complexes

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[PdCl(1-Ph2PCH2-2-R-1,2-C2B10H9-k2-P,B(3/4))]2.95 The latter can also be obtained by exchange reactions of the cyclopalladated N,N-dibenzylamino complex [PdCl(2-Me2NCH2C6H-k2-N,C)]2 with the corresponding carboranylphosphine ligands in boiling chloroform.94,95 Cyclometallated dimeric complexes [PdX(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3/4))]2 (X ¼ Cl, Br, I) were obtained by the reaction of [PdCl(Z3-Allyl)]2 with 1-Ph2PCH2-1,2-C2B10H11 in acetic acid in the presence of the corresponding lithium halides.94 When treated with pyridine, they are readily cleaved with the formation of the corresponding monomeric complexes [(Py)PdX(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3/4))]2.94,95 Reactions of carboranylphosphines 1-R2P-2-R0 -1,2-C2B10H10 (R ¼ t-Bu, pH; R0 ¼ H, Me, i-Pr) with Na2[PdCl4] in methanol or with [(PhCN)2PdCl2] in benzene or tetrahydrofuran lead to four-membered cyclopalladated monomeric [PdCl(1-Ph2P1,2-C2B10H10-k2-P,B)(1-Ph2P-1,2-C2B10H11-k1-P)] and dimeric [PdCl(1-R2P-2-R0 -1,2-C2B10H9-k2-P,B)]2 (R ¼ t-Bu, R0 ¼ H; R ¼ Ph, R0 ¼ H, Me, i-Pr) complexes.94,96 Dimeric complexes [PdX(1-Ph2P-2-R-1,2-C2B10H9-k2-P,B)]2 (R ¼ H, Me, i-Pr) were also obtained by exchange reactions of cyclopalladated N,N-dibenzylamino complex [PdCl(2-Me2NCH2C6H-k2-N,C)]2 with the corresponding carboranylphosphine ligands in boiling chloroform.94,95 According to the NMR spectroscopy data, the reactions with 1-Ph2P-1,2-C2B10H11 and 1-Ph2P-2-Me-1,2-C2B10H10 lead to a mixture of B(3)H- and B(4)H-activation products, while the reaction with 1-Ph2P-2-i-Pr-1,2-C2B10H10, containing a bulky substituent at the neighboring carbon atom, produces exclusively the B(4)H-activation product.94 The reaction with 1-t-Bu2P-1,2-C2B10H11 results in the selective formation of the dimeric B(3) H-activation product trans-[PdCl(1-t-Bu2P-1,2-C2B10H10-k2-P,B(3))]2, which on the treatment with PEt3 in THF gives trans-[(Et3P) PdCl(1-t-Bu2P-1,2-C2B10H10-k2-P,B(3))] (Scheme 1).96

Scheme 1

It is worth mentioning that the reaction of [PdCl(1-Ph2P-2-i-Pr-1,2-C2B10H9-k2-P,B(4))]2 with 1-Ph2PCH2-1,2-C2B10H11 in a mixture of chloroform and acetic acid leads to the transformation of the four-membered palladacycle into more stable five-membered one [PdCl(1-Ph2PCH2-1,2-C2B10H10-k2-P,B)]2.94,97 Even though the attachment of a phosphine to the carbon atom of the carborane backbone leads to a significant decrease in its donor character due to the strong electron-withdrawing effect of the C-carboranyl group,98 the B-cyclometallated complexes of C-carboranylphosphines are more stable than the complexes formed with the participation of nitrogen- and oxygen-containing directing substituents. Therefore, the cyclopalladated carboranylphosphine complexes are rather well studied, however their use for the synthesis of B-substituted ortho-carborane derivatives was not described until recently, when the preparation of 3-halocarboranes 3-X-1-Ph2P-1,2-C2B10H10 (X ¼ Cl, Br, I) in the Pd-catalyzed reaction of 1-diphenylphosphino-ortho-carborane 1-Ph2P-1,2-C2B10H11 with aryl halides the presence of Li2CO3 was reported (Scheme 2). Interestingly, the reaction with aryl bromides bearing electron-donating groups in the presence of a stronger base t-BuOLi leads to the CH-activation products 1-Ph2P-2-Ar-1,2-C2B10H10.99

Scheme 2

Platinum can also form cyclometallated complexes with ortho-carboranyl phosphines; the structure of trans-[PtCl(1-Ph2P-1, 2-C2B10H10-k2-P,B(3))(1-Ph2P-1,2-C2B10H11-k1-P)] was determined by single crystal X-ray diffraction.94,100 A series of cyclometallated carboranylphosphine complexes of rhodium and iridium were obtained on the base of 1-diphenylphosphinomethyl-ortho-carborane. The reactions of 1-Ph2PCH2-1,2-C2B10H11 with rhodium(I) complexes [(CO)2RhCl]2, [(C8H14)2RhCl]2 or [(COD)RhCl]2 in boiling hexane in the presence of pyridine or its derivatives lead to the corresponding

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Polyhedral Boranes and Carboranes

B(3)-metallated rhodium(III) complexes[RhHCl(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(NC5H4R)2] (R ¼ H, 3-Me, 4-Me, 4-OMe), while the reactions of 1-Ph2PCH2-1,2-C2B10H11 with [(COD)MCl]2 (M ¼ Rh, Ir) in ethanol result in the corresponding rhodium(III) and iridium(III) complexes [(COD)MHCl(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))] (Scheme 3).101,102

Scheme 3

The reactions of 1-Ph2PCH2-1,2-C2B10H11 with the iridium(I) hydride complexes [(Ph3P)3IrH(L)] (L ¼ CO, PPh3) lead to oxidative B(3)-cyclometallation with the formation of the corresponding iridium(III) dihydride complexes of fac-[(Ph3P)(L) IrH2(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))], while the reaction of [(Ph3P)3IrH(CO)] with an excess of the carboranylphosphine produces iridium(III) monohydride complex trans-[(CO)IrH(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))2]. The reactions of the dihydride complexes fac-[(Ph3P)(L)IrH2(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))] (L ¼ CO, PPh3) with fullerene C60 in the presence of t-BuNC in boiling toluene leads to the dihydrogen elimination with the formation of the complex [(Z2-C60)Ir(1-Ph2PCH2-1,2-C2B10H10k2-P,B(3))(t-BuNC)2] containing carboranylphosphine and fullerene ligands in the coordination sphere of the metal (Fig. 1).103

Fig. 1 X-ray structures of [(Z2-C60)Ir(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(t-BuNC)2] (left) and [s-C60OO-Ir(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(t-BuNC)2] (right). Hydrogen atoms of organic substituents and ligands are removed for clarity. Reproduced with permission from Usatov, A. V.; Martynova, E. V.; Dolgushin, F. M.; Peregudov, A. S.; Antipin, M. Yu.; Novikov, Yu. N. Eur. J. Inorg. Chem. 2002, 2565–2567, Usatov, A. V.; Martynova, E. V.; Dolgushin, F. M.; Peregudov, A. S.; Antipin, M. Yu.; Novikov, Yu. N. Eur. J. Inorg. Chem. 2003, 29–33. Copyright © (2002,2003) John Wiley & Sons, Inc.

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201

Oxidation of the last one with oxygen in chloroform leads to the selective insertion of an oxygen molecule into the IrdC bond in the trans-position to the boron atom of carborane with the formation of [s-C60OO-Ir(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3)) (t-BuNC)2] (Fig. 1).104 The reaction of 1-dimethylphosphino-ortho-carborane 1-Me2P-1,2-C2B10H11 with the iridium(I) complex [Ir(C8H14)2IrCl]2 in boiling cyclohexane leads to the oxidative insertion of the metal into the B(3)–H bond with the formation of the iridium(III) complex [IrHCl(1-Me2P-1,2-C2B10H11-k1-P)2(1-Me2P-1,2-C2B10H10-k2-P,B)].92 The reaction of 1-diisopropylphosphino-orthocarborane 1-i-Pr2P-1,2-C2B10H11 with с [(COD)IrCl]2 in hexane results in the cyclometallated complex [(COD)IrHCl(1-i-Pr2P-1, 2-C2B10H10-k2-P,B(3))] (Fig. 2).105

Fig. 2 X-ray structures of [(COD)IrHCl(1-i-Pr2P-1,2-C2B10H10-k2-P,B(3))] (left) and [Rh(m-Cl)(CO)(1-t-Bu2P-1,2-C2B10H10-k2-P,B(3))]2 (right). Here and below hydrogen atoms of organic substituents and ligands are removed for clarity.

The reaction of 1-di-tert-butylphosphino-ortho-carborane with the rhodium(I) complex [(CO)2RhCl]2 unexpectedly resulted in the formation of the cyclometallated dirhodium(III) complex [Rh(m-Cl)(CO)(1-t-Bu2P-1,2-C2B10H10-k2-P,B(3))]2 (Fig. 2).96 It is assumed that the formation of cyclometallated complexes is a key step in the Rh-catalyzed 3-mono- and 3,6-diarylation of 1-diphenylphosphino-ortho-carborane with aryl bromides, as well as 3,6-dilkylation with vinyl and allyl benzenes (Scheme 4). The reactions with aryl bromides bearing electron-donating substituents (4-Me, 3,5-Me2, 3,5-t-Bu2) result in the formation of exclusively 3,6-diarylation products, while the reactions with aryl bromides bearing strong electron-withdrawing substituents and sterically hindered aryl bromides do not occur at all.99

Scheme 4

The BH-activation of carborane can be prevented by introducing bulky substituents at the neighbor carbon atom. Thus, the reaction of 1-i-Pr2P-2-(30 ,50 -(200 ,400 ,600 -(i-Pr)3C6H2)2C6H3)-1,2-C2B10H10 with [(CO)2RhCl]2 in benzene at 60  C slowly (within 6 days) leads to the rhodium dicarbonyl complex [(CO)2RhCl(1-i-Pr2P-2-(30 ,50 -(200 ,400 ,600 -(i-Pr)3C6H2)2C6H3)-1,2-C2B10H10k1-P)], which reversibly loses CO to form the chloride-bridged dimeric complex trans-[(CO)Rh(m-Cl)(1-i-Pr2P-2-(30 ,50 -(200 ,400 ,600 (i-Pr)3C6H2)2C6H3)-1,2-C2B10H10-k1-P)]2 rather than a cylometallation product (Scheme 5 and Fig. 3).106

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Polyhedral Boranes and Carboranes

Scheme 5

Fig. 3 X-ray structure of trans-[(CO)Rh(m-Cl)(1-i-Pr2P-2-(30 ,50 -(200 ,400 ,600 -(i-Pr)3C6H2)2C6H3)-1,2-C2B10H10-k1-P)]2.

The reaction of 1-Ph2PCH2-1,2-C2B10H11 with RuCl3 in 2-methoxyethanol in the presence of formaldehyde leads to the diphosphine complex [RuCl(CO)(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(1-Ph2PCH2-1,2-C2B10H11-k2-P,H)] in which one carboranylphosphine ligand is coordinated according to the k2-P,B-fashion, and the second one in k2-P,H-fashion. The reaction of this complex with CO in benzene results in the replacement of the BH group of carborane in the coordination sphere of the metal by a carbonyl with the formation of the complex [RuCl(CO)2(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(1-Ph2PCH2-1,2-C2B10H11-k1-P)], in which both phosphine ligands are in the trans-position and which slowly isomerized to the corresponding cis-isomer. Reactions of [RuCl(CO)(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(1-Ph2PCH2-1,2-C2B10H11-k2-P,H)] with stronger ligands, such as 2,20 -bipyridine or 4-picoline lead to the complete loss of one of the carboranylphosphine ligands with the formation of complexes [RuCl(CO) (1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(NC5H4-4-Me)2] and [RuCl(CO)(1-Ph2PCH2-1,2-C2B10H10-k2-P,B(3))(2,20 -Bipy)], respectively (Scheme 6).107,108

Polyhedral Boranes and Carboranes

203

Scheme 6

The reaction of 1-Ph2PCH2-1,2-C2B10H11 with H3OsCl6 proceeds in a similar way, leading to [OsCl(CO)(1-Ph2PCH2-1,2C2B10H10-k2-P,B(3))(1-Ph2PCH2-1,2-C2B10H11-k2-P,H)] (Scheme 6), while in the case of sterically hindered carboranylphosphine 1-Ph2PCH2-2-Me-1,2-C2B10H10 the reaction gives a mixture of products B(3)- and B(4)-activation.107 Of particular interest are meta-carborane-based C,C0 -diphosphines: in contrast to ortho-carborane-based C,C0 -diphosphines, which usually form P,P0 -chelate complexes, the activation of positions B(2) and B(3) in meta-carborane-based diphosphines can occur with the participation of both phosphine groups, which makes it possible to obtain carborane analogs of C-pincer complexes of transition metals. The reaction of 1,7-bis(phosphinite)-meta-carborane 1,7-(i-Pr2PO)2-1,7-C2B10H10 with NiCl2 in a mixture of THF and toluene at 90  C leads to selective B(2)-H activation of the carborane cage with the formation of the (POBOP)NiCl complex [NiCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )]. An attempt to oxidize this complex with 1 equiv. of I2 in THF, along with substitution of iodide for chloride, leads to the activation of the B(3)–H bond with the formation of [NiI(1,7-(i-Pr2PO)2-3-I-1,7-C2B10H8-k3-P,B(2),P0 )] (Scheme 7, Fig. 4). The reaction with excess iodine leads to the oxidation of phosphinite substituents with the destruction of the complex and the formation of [1,7-(i-Pr2P(I)O)2-2,3-I2-1,7-C2B10H8](I3)2.109

Scheme 7

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Polyhedral Boranes and Carboranes

Fig. 4 X-ray structures of [NiCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )] (left) and [NiI(1,7-(i-Pr2PO)2-3-I-1,7-C2B10H8-k3-P,B(2),P0 )] (right).

The reaction of 1,7-(i-Pr2PO)2-1,7-C2B10H10 with [(Ph3P)3RhCl] in THF at room temperature gives the POBOP pincer rhodium(III) complex [RhHCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(3),P0 )(PPh3)], which, upon treatment with triethylamine, undergoes reductive elimination of HCl with the formation of a 16-electron rhodium(I) complex [RhHCl(1,7-(i-Pr2PO)21,7-C2B10H9-k3-P,B(2),P0 )(PPh3)]. The latter compound, when treated with iodobenzene in THF, undergoes an oxidative addition reaction with the formation of [Rh(Ph)I(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )], which, in turn, upon heating with acetonitrile in benzene, undergoes B(3)-H activation with migration of the phenyl group and the formation of the hydride complex [RhHI(1,7-(i-Pr2PO)2-3-Ph-1,7-C2B10H8-k3-P,B(2),P0 )(MeCN)] (Scheme 8, Fig. 5).110

Scheme 8

Polyhedral Boranes and Carboranes

205

Fig. 5 X-ray structures of [RhHCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )(PPh3)] (top left), [Rh(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )(PPh3)] (top right), [Rh(Ph)I(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )] (bottom left) and [RhHI(1,7-(i-Pr2PO)2-3-Ph-1,7-C2B10H8-k3-P,B(2),P0 )(MeCN)] (bottom right).

The reaction of 1,7-(i-Pr2PO)2-1,7-C2B10H10 with [Ru(CO)3Cl2]2 in boiling benzene leads to the formation of the ruthenium(II) POBOP complex [RuCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )(CO)2], the treatment of which with Et3N results in the (BB)-carboryne ruthenium(0) complex [Ru(1,7-(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2),B(3),P0 )(CO)2] as a product of double BH-activation (Fig. 6). The resulting BBRu metallacycle can be considered as an inorganic analog of cyclopropane with two highly strained 2c-2e BdRu s-bonds. A significant distortion of the exo-polyhedral bonds of the carborane leads to increased reactivity of these bent BdRu bonds, which can act as nucleophilic centers. Therefore, the reaction of the (BB)-carboryne complex formation is reversible and the addition of HCl leads to the cleavage of one of the RudB bonds with the formation of the ruthenium(II) chloride complex (Scheme 9). In a similar way, the reaction of the (BB)-carboryne ruthenium(0) complex with terminal alkyne leads to

Fig. 6 X-ray structures of [RuCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )(CO)2] (left) and [Ru(1,7-(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2),B(3),P0 )(CO)2] (right).

206

Polyhedral Boranes and Carboranes

Scheme 9

formal oxidative addition with the formation of the B-carboranyl acetylide ruthenium(II) complex [Ru(C^CCO2Et)(1,7(i-Pr2PO)2-1,7-C2B10H9-k3-P,B(2),P0 )(CO)2], while the reaction with 3-hexyne under UV irradiation leads to cycloaddition of the latter with the formation of a ruthenium(II) complex with B-C]C-Ru bridge [Ru(1,7-(i-Pr2PO)2-3-C(Et)]C(Et)-1,7-C2B10H9-k4-P, B(2),C,P0 )(CO)2] (Scheme 9). The reaction of the (BB)-carboryne ruthenium(0) complex with iodine leads to the ruthenium(II) complex [RuI(1,7-(i-Pr2PO)2-3-I-1,7-C2B10H8-k3-P,B(2),P0 )(CO)], in which the iodine atom bound to the neighboring boron atom complements the coordination sphere of ruthenium to an octahedron (Scheme 9, Fig. 7). The reaction of the (BB)-carboryne complex with an excess of H3B
SMe2 in a mixture of tetrahydrofuran and benzene at 80  C leads to the complex [RuI(1,7-(i-Pr2PO)2-3-BH3-1,7-C2B10H8-k4-P,B(2),H,P0 )(CO)2], containing the exo-polyhedral BH3 group, which forms a B(3)BH2–H⋯ Ru bond with the metal atom (Scheme 9 and Fig. 7).111

Fig. 7 X-ray structures of [RuI(1,7-(i-Pr2PO)2-3-I-1,7-C2B10H8-k3-P,B(2),P0 )(CO)] (left) and [RuI(1,7-(i-Pr2PO)2-3-BH3-1,7-C2B10H8-k4-P,B(2),H,P0 )(CO)2] (right).

Polyhedral Boranes and Carboranes

207

Fig. 8 X-ray structures of [(CO)2Ru-Cu(m-Cl)(1,7-(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2),B(3),P0 )]2 (top left), [(CO)2Ru-Au(m-Cl)(1,7-(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2), B(3),P0 )]2 (top right) and [(CO)2Ru-Ag(MeCN)(Z2-NO3)(1,7-(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2)B(3),P0 )] (bottom).

This ruthenium carboryne complex can also react with inorganic electrophiles such as coinage metal cations. The reaction with CuCl in a boiling mixture of tetrahydrofuran and benzene leads to the dimeric complex [(CO)2Ru-Cu(m-Cl)(1,7(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2),B(3),P0 )]2 featuring a four-membered B-Cu-Ru-B metallacycle (Fig. 8). A similar complex featuring a B-Au-Ru-B metallacycle [(CO)2Ru-Au(m-Cl)(1,7-(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2),B(3),P0 )]2 (Fig. 8) was prepared by reaction with [(Me2S)AuCl] in dichloromethane at room temperature. In both complexes, the Cu+ and Au+ cations are in an unusual square planar environment. The reaction of the carboryne complex with AgNO3 in a mixture of dichloromethane and benzene after crystallization from acetonitrile gives the bimetallic complex [(CO)2Ru-Ag(MeCN)(Z2-NO3)(1,7(i-Pr2PO)2-1,7-C2B10H8-k4-P,B(2),B(3),P0 )] (Fig. 8).112 The reaction of 1,7-(i-Pr2PO)2-1,7-C2B10H10 with [RuCl2(PPh3)3] in boiling THF leads to the formation of the POBOP pincer ruthenium(II) complex [RuCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k4-P,B(2),P,H0 )(PPh3)], in which the coordination sphere of the metal is augmented to an octahedron due to the BH⋯ Ru interaction with the neighboring BH group. Its treatment with NaH in THF results in the corresponding hydride complex [RuH(1,7-(i-Pr2PO)2-1,7-C2B10H9-k4-P,B(2),P0 ,H)(PPh3)] (Scheme 10 and Fig. 9). In the hydride complex, there is a rapid exchange between the carborane and terminal hydride hydrogen atoms due to the migration

Scheme 10

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Polyhedral Boranes and Carboranes

Fig. 9 X-ray structures of [RuCl(1,7-(i-Pr2PO)2-1,7-C2B10H9-k4-P,B(2),P,H0 )(PPh3)] (left) and [RuH(1,7-(i-Pr2PO)2-1,7-C2B10H9-k4-P,B(2),P0 ,H)(PPh3)] (right).

Fig. 10 X-ray structures of trans-[Pd(Ph)(1-i-Pr2P-1-CB11H11-k1-P)(1-i-Pr2P-1-CB11H11-k2-P,H)]− (left) and trans-[Pd(1-i-Pr2P-1-CB11H11-k2-P,H)(1-i-Pr2P-1CB11H10-k2-P,B)]− (right).

of a metal atom between the B(2) and B(3) positions of the carborane cage with an activation barrier of 12.2 kcal mol−1; on this basis it serves as a good catalyst for dehydrogenation of cyclooctane.113 The properties of phosphines based on the carba-closo-dodecaborate anion in many respects resemble the properties of ortho-carboranyl phosphines; however, the enhanced hydridic character of the BH groups in the [CB11H12]− anion leads to greater stability of complexes with M ⋯ HdB bonds. The reaction [Li(THF)3][1-i-Pr2P-1-CB11H11] with [(COD)Pd(CH2SiMe3)2] in benzene leads to a 14-electron palladium(0) complex [Li(THF)4]2[Pd(1-i-Pr2P-1-CB11H11)2], which at room temperature reacts with chlorobenzene to form the palladium(II) complex [Li(THF)(Et2O)3][trans-Pd(Ph)(1i-Pr2P-1-CB11H11-k1-P)(1-i-Pr2P-1-CB11H11-k2-P,H)], in which the BH group acts a ligand in the square-planar coordination environment of palladium (Fig. 10). The reaction with chlorobenzene at 80  C leads to the elimination of benzene with the formation of the cyclometallated complex [Li(THF)4][trans-Pd(1-i-Pr2P-1-CB11H11-k2-P,H)(1-i-Pr2P-1-CB11H10-k2-P,B)] (Fig. 10).114

Polyhedral Boranes and Carboranes

209

The reaction of [Li(THF)3][1-i-Pr2P-1-CB11H11] with [Rh(CO)2Cl]2 and AgBF4 in dichloromethane leads to an unstable complex Li[Rh(1-i-Pr2P-1-CB11H11)2(CO)2], which readily loses one CO ligand, with the formation of the complex Li[trans-Rh(1-i-Pr2P-1CB11H11-k1-P)(1-i-Pr2P-1-CB11H11-k2-P,H)(CO)], in which one of the BH groups of carborane plays the role of a ligand in the square-planar coordination environment of rhodium (Scheme 11).115

Scheme 11

Fig. 11 X-ray structure of [(Z2:Z2-COD)Ir(1-i-Pr2P-1-CB11H11-k3-P,H,H)].

The reaction of [Li(THF)3][1-i-Pr2P-1-CB11H11] with [Ir(COD)Cl]2 in benzene results in the complex [(Z2:Z2-COD)Ir(1-i-Pr2P1-CB11H11-k3-P,H,H)], in which the iridium atom is additionally coordinated by two BH groups of the carborane cage (Fig. 11).116

9.05.2.5.2

B-H activation assisted by nitrogen-containing substituents

Due to the variety of nitrogen-containing substituents that can act as directing ligands, BH-activation reactions with their participation are among the most studied. The first described were the reactions of 1-Me2NCH2-2-Ph-1,2-C2B10H10 with [(CO)5MnMe] and [(CO)5ReMe] leading to cyclometallation products with a MdB bond [M(1-Me2NCH2-2-Ph-1,2C2B10H9-k2-N,B)(CO)4] (M ¼ Mn, Re); however the metalation position was not determined.117,118 The reactions of 1-dimethylaminomethyl-7-phenyl-meta-carborane 1-Me2NCH2-7-Ph-1,7-C2B10H10 with [(CO)5MMe] (M ¼ Mn, Re) proceed similarly, resulting in the cyclometallated complexes [(CO)4M(1-Me2NCH2-7-Ph-1,7-C2B10H9-k2-N,B)] (M ¼ Mn, Re).117,118 According to the X-ray diffraction data of [(CO)4Re(1-Me2NCH2-7-Ph-1,7-C2B10H9-k2-N,B(4))], the BH-activation proceeds at position 4, which is explained by the difficulty of substitution at position 2 caused by a phenyl substituent at the adjacent carbon atom.119 Reactions of carboranylamines 1-R2NCH2-2-R0 -1,2-C2B10H10 (R ¼ Me, R0 ¼ Ph; R ¼ Et, R0 ¼ H) with [(PhCN)2PdCl2] in benzene at room temperature lead to the formation of the corresponding cyclopalladated dimeric complexes [PdCl(1R2NCH2-2-R0 -1,2-C2B10H9-k2-N,B)]2, the treatment of which with 4-methylpyridine proceeds with cleavage of the dichloride bridge leading to monomeric complexes [PdCl(1-R2NCH2-2-R0 -1,2-C2B10H9-k2-N,B)(NC5H4-4-Me)]. The PddB bond in these complexes is easily broken by the action of acids. The reactions of both [PdCl(1-Et2NCH2-1,2-C2B10H10-k2-N,B)]2 and [PdCl (1-Et2NCH2-1,2-C2B10H10-k2-N,B)(NC5H4-4-Me)] with diphenylacetylene result in the insertion of two molecules into the boron-metal bond with the formation of a five-membered metallocycle in which the palladium atom is coordinated by the

210

Polyhedral Boranes and Carboranes

double bond of the butadiene fragment in [PdCl(1-Et2NCH2-X-C(Ph)]C(Ph)-C(Ph)]C(Ph)-1,2-C2B10H10-Z2-(C(10 )]C(20 ))k2-N,C(40 ))].120 Similarly, the reaction of 1-Me2NCH2-7-Ph-1,7-C2B10H10 with [(PhCN)2PdCl2] gives the corresponding cyclopalladated dimer [PdCl(1-Me2NCH2-7-Ph-1,7-C2B10H9-k2-N,B)]2, the treatment of which with 4-methylpyridine gives [PdCl(1-Me2NCH2-7-Ph-1,7-C2B10H9-k2-N,B)(NC5H4-4-Me)].120 The reaction of 1-aminomethyl-2-phenyl-ortho-carborane 1-NH2CH2-2-Ph-1,2-C2B10H10 with aryl iodides in the presence of 10 mol% Pd(OAc)2, 20 mol% glyoxylic acid, CF3COOAg and acetic acid in hexafluoroisopropanol at 60  C leads to the formation of B(4,5)-diaryl derivatives 1-NH2CH2-2-Ph-4,5-Ar2-1,2-C2B10H8 (Scheme 12). The presence of electron-withdrawing or electron-donating substituents does not have a large effect on the yield of the aryl derivatives, which ranges from 50% to 87%. Instead of a phenyl group, the second carbon in the carborane can also carry substituted aryl or alkyl groups. The 1-NH2CHMe- or 1-NH2CMe2- groups can also be used as directing groups, and salicylaldehyde, 2-ketopropionic or phenylglyoxalic acids can be used instead of glyoxalic acid. The reaction of 1-NH2CH2-2-Ph-1,2-C2B10H10 with salicylaldehyde at room temperature proceeds through the formation of a Schiff base which in the presence of Pd(OAc)2 forms the B(4)-cyclometallated chelate complex [Pd (1-NH2CH2-2-Ph-1,2-C2B10H10-k-N)(1-(O-20 -C6H4CH]NCH2)-2-Ph-1,2-C2B10H9-k3-O,N,B(4))] (Fig. 12). The latter reacts with iodobenzene in hexafluoroisopropanol in the presence of CF3COOAg at 80  C giving a mixture of 1-(HO-20 -C6H4CH]NCH2)2,4,5-Ph3-1,2-C2B10H8 and 1-CF3CONHCH2-2,4,5-Ph3-1,2-C2B10H8. In addition, this complex, as well as its analog [Pd(PPh3) (1-(O-20 -C6H4CH]NCH2)-2-Ph-1,2-C2B10H9-k3-O,N,B(4))] (Fig. 13), formed upon the substitution of triphenylphosphine for carboranylamine, can act as arylation catalysts.121

Scheme 12

Fig. 12 X-ray structures of [Pd(1-NH2CH2-2-Ph-1,2-C2B10H10-k-N)(1-(O-20 -C6H4CH]NCH2)-2-Ph-1,2-C2B10H9-k3-O,N,B(4))] (left) and [Pd(PPh3)(1-(O-20 -C6H4CH]NCH2)-2-Ph-1,2-C2B10H9-k3-O,N,B(4))] (right).

Reactions of 1-arylimino-ortho-carboranes 1-ArN]CH-1,2-C2B10H11 (Ar ¼ C6H4-4-F, C6H4-4-Cl, C6H2-2,4,6-Me3) with various N-acyl-glutarimides and N-acylsuccinimides RC(O)N(CO)2(CH2)n (n ¼ 2, 3) in the presence of 5 mol% [(COD)RhCl2]2 in toluene at 150 C produce mixtures of the corresponding B(3)-mono- and B(3,6)-disubstituted aryl derivatives ArN]CH-3-R-1,2-C2B10H10 and 1-ArN]CH-3,6-R2-1,2-C2B10H9 (Scheme 13). The reaction can be used to introduce into the carborane cage aryl substituents

Polyhedral Boranes and Carboranes

211

Fig. 13 X-ray structure of [(Ph3P)Pd(1-(20 -OC6H4N]CH)-2-Ph-1,2-C2B10H9-k3-O,N,B(4))].

Scheme 13

containing various functional groups, as well as heteroaryl, vinyl and alkyl substituents. The directing arylimino group can be removed in two stages, including acid hydrolysis to an aldehyde followed by removal of the formyl group upon treatment with KMnO4, which makes it attractive to use in the synthesis of ortho-carborane derivatives.122 The directing arylimino group can also be generated in situ by the reaction of carboranyl aldehydes with amines. The reaction of 1-formyl-2-phenyl-ortho-carborane 1-H(O)C-2-Ph-1,2-C2B10H10 with ortho-aminophenol and Pd(OAc)2 in toluene in the presence of PPh3 gives the B(4)-cyclometallated complex [(Ph3P)Pd(1-(20 -OC6H4N]CH)-2-Ph-1,2-C2B10H9-k3-O,N,B(4))] (Fig. 13). The reaction of the latter with methyl para-iodobenzoate 4-IC6H4COOMe, CF3COOAg and CF3COOH in hexafluoro-isopropanol followed by acid hydrolysis produces a mixture of the corresponding B(4)-mono- and B(4,5)-diaryl carboranes 1-H(O)C-2Ph-4-(40 -MeO(O)CC6H4)-1,2-C2B10H9 and 1-H(O)C-2-Ph-4,5-(40 -MeO(O)CC6H4)2-1,2-C2B10H8. ortho-Aminophenol in this reaction can be replaced with glycine. An excess of aryl iodide leads to the products of B(4,5)-diarylation (Scheme 14). The reaction proceeds well with aryl iodides containing both electron-donating and electron-withdrawing substituents, with the product yield varying from 55% to 86%. The phenyl group at the second carbon atom of the carborane can be replaced by other aryl (C6H4-p-Me, C6H4-p-OMe), benzyl (CH2Ph, CHPh2) or alkyl (Me, i-Pr) groups, whereas in the absence of a substituent the reaction leads to the formation of an inseparable mixture of products.123

Scheme 14

212

Polyhedral Boranes and Carboranes

The arylazo group can also play the role of the directing ligand. The reactions of the phenylazo derivative 1-PhN]N-2i-Pr-1,2-C2B10H10 with [(CO)5MnMe] and [(CO)5ReMe] in alkanes lead to the formation of the corresponding B(4)cyclometallation products [(CO)4M(1-PhN]N-2-i-Pr-1,2-C2B10H9-k2-N(2),B(4))].117,118,124 The reactions of arylazo derivatives 1-(40 -RC6H4N]N)-1,2-C2B10H11 with [Cp IrCl2]2 in dichloromethane in the presence of NaOAc result in the corresponding B(3)-cyclometallated complexes [Cp IrCl(1-(40 -RC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] (R ¼ F, MeO) in 90–94% yields (Scheme 15 and Fig. 14). A similar reaction with [Cp RhCl2]2 requires the use of n-BuLi as a base, leading to [Cp RhCl(1-(40 -FC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] (Fig. 14) in moderate yield.125

Scheme 15

Fig. 14 X-ray structures of [Cp IrCl(1-(40 -FC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] (left) and [Cp RhCl(1-(40 -FC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] (right).

Upon treatment with PhXLi (X ¼ S, Se), the chloride ligand in [Cp IrCl(1-(40 -MeOC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] can easily be replaced with phenylthiolate or phenylselenolate to form the corresponding complexes [Cp Ir(XPh) (1-(40 -MeOC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] in near quantitative yields. The reaction of [Cp IrCl(1-(40 -MeOC6H4N]N) -1,2-C2B10H10-k2-N(2),B(3))] with AgOTf followed by the addition of ortho-carboranyl carboxylate leads to [Cp Ir(100 -OOC-100 , 200 -C2B10H11-k1-O)(1-(40 -MeOC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] (Scheme 15).125

Polyhedral Boranes and Carboranes

213

Fig. 15 X-ray structure of [(Cp IrCl)2(1-(40 -MeOC6H3N]N)-1,2-C2B10H10-k4-N(2),B(3); N(1),C(20 ))].

The reaction of 1-(40 -MeOC6H4N]N)-1,2-C2B10H11 with 1 equiv. [Cp IrCl2]2 and 2 equiv. AgOTf in the presence of NaOAc in dichloromethane leads to the formation of the product of simultaneous CH-and BH-activation—blue complex [(Cp IrCl)2(1-(40 -MeOC6H3N]N)-1,2-C2B10H10-k4-N(2),B(3);N(1),C(20 ))] (Fig. 15) in 80% yield, whereas a similar reaction with 2 equiv. [Cp IrCl2]2 and 4 equiv. AgOTf gives the product of CH- and double BH-activation in 85% yield—brown cationic complex [(Cp Ir)3(m-Cl)2(1-(40 -MeOC6H3N]N)-1,2-C2B10H9-k5-N(2),B(3);N(1),C(20 );B(4))]OTf, in which the third Cp Ir fragment is coordinated by a boron atom in position 4 and two bridging chloride ligands (Scheme 15).126 The reactions of [Cp IrCl(1-(40 -MeOC6H4N]N)-1,2-C2B10H10-k2-N(2),B(3))] with 0.5 equiv. [Cp IrCl2]2 or [Cp RhCl2]2 in methanol in the presence of Et3N at 65  C lead to the corresponding metallacarboranes [Cp IrCl(3-Cp -1-(40 -MeOC6H4N]N)3,1,2-MC2B9H9-k2-N(2),B(6))] (M ¼ Ir, Rh).127 Unlike 1-(40 -MeOC6H4N]N)-1,2-C2B10H11, the reaction of 1,2-(40 -MeOC6H4N]N)2-1,2-C2B10H10 with 0.5 equiv. [Cp IrCl2]2 in the presence of Et3N in dichloromethane leads to the B(4)H-activation product [Cp IrCl(1,2-(40 -MeOC6H4N] N)2-1,2-C2B10H9-k2-N(2),B(4))], along with a minor amount of iridacarborane [3-Cp -1,2-(40 -MeOC6H4N]N)2-3,1,2-IrC2B9H9], while the reaction with 1 equiv. [Cp IrCl2]2 results in the double BH-activation product [(Cp IrCl)2(1,2-(40 -MeOC6H4N]N)21,2-C2B10H9-k2-N(2),B(4);N(20 ),B(7))] (Scheme 16 and Fig. 16). It should be noted that the reaction of 1,2-(40 -MeOC6H4N] N)2-1,2-C2B10H10 with 0.5 equiv. [Cp IrCl2]2 in the presence of Et3N in methanol gives iridacarborane [3-Cp -1, 2-(40 -MeOC6H4N]N)2-3,1,2-IrC2B9H9] as the main product.127

Scheme 16

214

Polyhedral Boranes and Carboranes

Fig. 16 X-ray structures of [Cp IrCl(1,2-(40 -MeOC6H4N]N)2-1,2-C2B10H9-k2-N(2),B(4))] (left) and [(Cp IrCl)2(1,2-(40 -MeOC6H4N]N)2-1,2-C2B10H9-k2-N(2),B(4); N(20 ),B(7))] (right).

The phenylazo derivative of meta-carborane 1-PhN]N-7-Me-1,7-C2B10H10 reacts with [(CO)5MnMe] and [(CO)5ReMe] in similar fashion to the phenylazo derivative of ortho-carborane to form the corresponding cyclometallated complexes [(CO)4M (1-PhN]N-7-Me-1,7-C2B10H9-k2-N,B)] (M ¼ Mn, Re).117,118 The reaction of the arylazo derivative of para-carborane 1-(40 -MeOC6H4N]N)-1,12-C2B10H11 with [Cp IrCl2]2 in dichloromethane in the presence of NaOAc leads to the formation of the corresponding B(2)-cyclometallated complex [Cp IrCl (1-(40 -MeOC6H4N]N)-1,12-C2B10H10-k2-N(2),B(2))] (Fig. 17) in 92% yield, whereas the reaction with an excess of [Cp IrCl2]2 in the presence of AgOTf gives the product of CH- and double BH-activation in 90% yield—the cationic complex [(Cp Ir)3(m-Cl)2 (1-(40 -MeOC6H3N]N)-1,12-C2B10H9-k5-N(2),B(2);N(1),C(20 );B(3))]OTf (Fig. 17 and Scheme 17).126

Fig. 17 X-ray structures of [Cp IrCl(1-(40 -MeOC6H4N]N)-1,12-C2B10H10-k2-N(2),B(2))] (left) and [(Cp Ir)3(m-Cl)2(1-(40 -MeOC6H3N]N)-1,12-C2B10H9-k5-N(2), B(2);N(1),C(20 );B(3))]− (right).

Polyhedral Boranes and Carboranes

215

Scheme 17

The reaction of 1,12-bis(arylazo)-para-carborane 1,12-(40 -MeOC6H4N]N)2-1,12-C2B10H10 with 1 equiv. [Cp IrCl2]2 in the presence of NaOAc in dichloromethane gives blue complex [Cp IrCl(1-(40 -MeOC6H4N]N)2-1,12-C2B10H9-k2-N(2),B(2))], while the reaction with 3 equiv. [Cp IrCl2]2 and 8 equiv. AgOTf leads to dark brown cationic complex [(Cp Ir)4(m-Cl)2Cl(1,12-(40 -MeOC6H3N]N)2-1,12-C2B10H8-k7-N(2),B(2);N(1),C(20 );B(3);N(10 ),C(200 ))]OTf as a product of CH- and double BH-activation of one arylazo group and CH-activation of the second one (Fig. 18). The latter can also be obtained by treating the blue complex with 2.5 equiv. [Cp IrCl2]2 and 7 equiv. AgOTf (Scheme 18).126

Fig. 18 X-ray structures of [Cp IrCl(1-(40 -MeOC6H4N]N)2-1,12-C2B10H9-k2-N(2),B(2))] (top) and [(Cp Ir)4(m-Cl)2Cl(1,12-(40 -MeOC6H3N]N)2-1,12C2B10H8-k7-N(2),B(2);N(1),C(20 );B(3);N(10 ),C(200 ))]− (bottom).

216

Polyhedral Boranes and Carboranes

Scheme 18

It was previously demonstrated that the 2-pyridyl group attached to the carbon atom of ortho-carborane can participate in the formation of various transition metal containing metallacycles with the adjacent carbon atom.128,129 Later, it was shown that the 2-pyridyl group can also play the role of a directing ligand in the transition metal catalyzed BH-activation reactions. The reactions of 1-(20 -pyridyl)-2-methyl-ortho-carborane 1-(20 -NC5H4)-2-Me-1,2-C2B10H10 with aromatic N-acyl-glutarimides ArC(O)N(CO)2(CH2)3 in the presence of 5 mol% [(COD)RhCl2]2 in toluene at 150 C lead to the corresponding B(3,5)-diaryl derivatives 1-(2-NC5H4)-2-Me-3,5-Ar2-1,2-C2B10H8. Similar reactions with tertiary aliphatic N-acyl-glutarimides MeR2C(O)N (CO)2(CH2)3 proceed with rearrangement of the alkyl substituent and result in the products of B(4)-alkylation 1-(20 -NC5H4)-2Me-4-R2CHCH2-1,2-C2B10H9 (Scheme 19).122

Scheme 19

Reactions of 1-(20 -pyridyl)-ortho-carborane 1-(20 -NC5H4)-1,2-C2B10H11 with aryl boronic acids ArB(OH)2 in the presence of Ag2O and 10 mol% Pd(OAc)2 in dimethylformamide at room temperature give the corresponding 3-aryl derivatives 1-(2-NC5H4)3-Ar-1,2-C2B10H10 (Scheme 20). The reactions proceed well both with boronic acids containing electron-withdrawing (Ar ¼ C6H4-4-F, C6H4-3-F, C6H3-3,5-F2, C6H4-4-Cl, C6H4-4-Br, C6H4-4-CF3, C6H4-3-NO2, C6H4-4-CHO, C6H4-4-COMe, C6H4-4-COOMe, C6H4-4-CN), and electron-donating (C6H4-4-Me, C6H4-3-Me, C6H4-2-Me, C6H3-3,4-Me2, C6H4-4-Et,

Polyhedral Boranes and Carboranes

217

C6H4-4-i-Pr, C6H4-4-t-Bu, C6H4-4-OCH2Ph) substituents, as well as with 4-biphenyl-, 2-naphthyl- and 1-pyrenylboronic acids. The arylation at position 3 is not hindered by the presence of n-Bu and Ph substituents at the adjacent carbon atom. This approach can also be used for sequential introduction of aryl groups with various substituents at positions 3 and 6 of the ortho-carborane cage.130

Scheme 20

Reactions of 1-(20 -pyridyl)-2-methyl-ortho-carborane 1-(20 -NC5H4)-2-Me-1,2-C2B10H10 with carboxylic acids RCOOH in the presence of Cu(OH)2, oxone, 5 mol% [Cp RhCl2]2 and 20 mol% AgOAc in 1,2-dichloroethane at 120 C lead to the corresponding B(3)-acyloxy and B-(3,6)-diacyloxy derivatives 1-(2-NC5H4)-2-Me-3-RCOO-1,2-C2B10H9 and 1-(2-NC5H4)-2-Me-3, 6-(RCOO)2-1,2-C2B10H9 (Scheme 21). These reactions proceed well with both aliphatic (primary, secondary, or tertiary) and aromatic or heteroaromatic carboxylic acids. The methyl group on the second carbon atom may be replaced by the benzyl group or may not be present at all. The 2-pyridyl group as the directing ligand can be replaced by a 2-pyrimidyl group.131

Scheme 21

It should be noted that the 2-pyridyl group can also be used for directed oxidative coupling of two ortho-carborane cages. The reaction of 1-(20 -pyridyl)-ortho-carborane 1-(20 -NC5H4)-1,2-C2B10H11 with AgNO3 and 10 mol% of [(MeCN)4Pd](BF4)2 in toluene at 60 C leads to the B(3)H-activation of ortho-carborane with the formation of a mixture of diastereomeric 3,30 - and 3, 60 -bis(1-(200 -pyridyl)-ortho-carboranes), which can be separated by column chromatography.132 The reaction of 1-(20 -pyridylsulphenyl)-ortho-carborane 1-(20 -NC5H4S)-1,2-C2B10H11 with [Cp IrCl2]2 in dichloromethane in the presence of KOAc and AgOTf results in the B(3)-cyclometallated complex [Cp IrCl(1-(20 -NC5H4S)-1,2-C2B10H10-k2-N,B(3))], while a similar reaction with 1-(20 -pyridylsulphenyl)-2-methyl-ortho-carborane 1-(20 -NC5H4S)-2-Me-1,2-C2B10H10 leads to the B(4)-cyclometallated complex [Cp IrCl(1-(20 -NC5H4S)-2-Me-1,2-C2B10H9-k2-N,B(4))] (Scheme 22 and Fig. 19). Interestingly, the reaction of 1-(20 -NC5H4S)-2-Me-1,2-C2B10H10 with [Cp RhCl2]2 under similar conditions also gives the B(4)-cyclometallated complex [Cp RhCl(1-(20 -NC5H4S)-2-Me-1,2-C2B10H9-k2-N,B(4))] (Fig. 19), while the reaction of the methyl-free carborane 1-(20 NC5H4S)-1,2-C2B10H11 proceeds with the incorporation of the metal into the carborane cage resulting in rhodacarborane [3-Cp -1(20 -NC5H4)-3,1,2-RhC2B9H10] in (Scheme 22).133

218

Polyhedral Boranes and Carboranes

Scheme 22

Fig. 19 X-ray structures of [Cp IrCl(1-(20 -NC5H4S)-1,2-C2B10H10-k2-N,B(3))] (left) and [Cp IrCl(1-(20 -NC5H4S)-2-Me-1,2-C2B10H9-k2-N,B(4))] (right).

The treatment of the rhodium complex [Cp RhCl(1-(20 -NC5H4S)-2-Me-1,2-C2B10H9-k2-N,B(4))] with N-bromo- and N-iodosuccinimides in 1,2-dichloroethane at 65  C leads to cleavage of the RhdB bond, with the formation of the corresponding B(4)-halogen derivatives 1-(20 -NC5H4S)-2-Me-4-X-1,2-C2B10H9 (X ¼ Br, I) in near quantitative yields, while the reaction with oxygen in dichloromethane leads to the corresponding hydroxy derivative 1-(20 -NC5H4S)-2-Me-4-HO-1,2-C2B10H9. It should be noted that 4-substituted derivatives 1-(20 -NC5H4S)-2-R-4-X-1,2-C2B10H9 (R ¼ Me, Bn, SiMe3; X ¼ Br, OH) can be obtained by carrying out the reaction in the presence of [Cp RhCl2]2 without isolation of the intermediate cyclometallated complexes.133 The reaction of (ortho-carboran-1-yl)(20 -pyridyl)methanol 1-(2-NC5H4C(OH)H)-1,2-C2B10H11 with [Cp IrCl2]2 in the presence of AgOTf in THF results in the B(3)-cyclometallated complex [Cp Ir(1-(20 -NC5H4C(O)H)-1,2-C2B10H10-k3-N,O,B(3))], while similar reactions with [Cp MCl2]2 (M ¼ Ir, Rh) in the presence of n-BuLi lead to the corresponding C-cyclometallated complexes [Cp M(1-(20 -NC5H4C(O)H)-1,2-C2B10H10-k3-N,O,C(2))] (Fig. 20).134 Another type of directing ligand is the 1,2,3-triazole group. Reactions of 1-(10 ,20 ,30 -triazol-10 -ylmethyl)-ortho-carboranes 1-(40 R-10 ,20 ,30 -N3C2H-10 -CH2)-1,2-C2B10H11 (R ¼ Ph, Bu) with [Cp IrCl2]2 in dichloromethane in the presence of NaOAc result in the corresponding B(3)-cyclometallated complexes [Cp IrCl(1-(40 -R-10 ,20 ,30 -N3C2H-10 -CH2)-1,2-C2B10H10-k2-N(2),B(3))]. In the case

Polyhedral Boranes and Carboranes

219

Fig. 20 X-ray structures of [Cp Ir(1-(20 -NC5H4C(O)H)-1,2-C2B10H10-k3-N,O,B(3))] (left) and [Cp Ir(1-(20 -NC5H4C(O)H)-1,2-C2B10H10-k3-N,O,C(2))] (right).

of carboranyl triazoles containing metallocene substituents (ferrocene or ruthenocene), the reaction requires stronger conditions—the presence of Cs2CO3 as a base and heating in acetonitrile (Scheme 23 and Fig. 21). Analogous reactions with [Cp RhCl2]2 lead to cyclometallated complexes with metallocene substituents (Scheme 23), while complexes with phenyl and butyl substituents could not be obtained.135

Scheme 23

Fig. 21 X-ray structure of [Cp IrCl(1-(40 -Fc-10 ,20 ,30 -N3C2H-10 -CH2)-1,2-C2B10H10-k2-N(2),B(3))].

The treatment of carboranyl triazolium salts [1-(30 -Me-40 -R-10 ,20 ,30 -N3C2H-10 -CH2)-1,2-C2B10H11][BF4] (R ¼ Ph, Bu) with Ag2O/Me4NCl in dichloromethane followed by addition of [Cp MCl2]2 (M ¼ Ir, Rh) leads to the C(2)-cyclometallated carbene complexes [Cp IrCl(1-(30 -Me-40 -R-10 ,20 ,30 -N3C2H-10 -CH2)-1,2-C2B10H10-k2-C(2),C(50 ))] (Scheme 24). The C(2)-cyclometallated

220

Polyhedral Boranes and Carboranes

complexes also can be obtained via deprotonation of the CH group of carborane. Thus, the treatment of 1-(40 -Fc-10 ,20 ,30 -N3C2H-10 CH2)-1,2-C2B10H11 with n-BuLi in THF followed by addition of [Cp IrCl2]2 leads to [Cp IrCl(1-(40 -Fc-10 ,20 ,30 -N3C2H-10 -CH2)1,2-C2B10H10-k2-N(2),C(2))] (Scheme 24).135

Scheme 24

Fig. 22 X-ray structures of [PtCl(1-(20 ,200 -N2C10H5-(50 -p-Tol)-60 -)-2-Ph-1,2-C2B10H9-k3-N,N,B(3))] (left) and [Pt(C^CPh)(1-(20 ,200 -N2C10H5-(50 -(4000 -BrC6H4)-60 -) -2-Me-1,2-C2B10H9-k3-N,N,B(3))] (right).

Reactions of 60 -(ortho-carboran-1-yl)-20 ,200 -bipyridines with K2[PtCl4] in boiling acetonitrile result in B(3)-cycloplatinized carboranes [PtCl(1-(20 ,200 -N2C10H5-(30 -Ar)-60 -)-2-R-1,2-C2B10H9-k3-N,N,B(3))] (Fig. 22). The chloride ligand can be substituted for acetylide by the treatment with phenylacetylene in the presence of K2CO3 in boiling acetonitrile (Scheme 25).136

Polyhedral Boranes and Carboranes

221

Scheme 25

Copper salts can also be used as a catalyst in the BH-activation reactions of ortho-carborane, however, this requires the oxidation of copper in the catalytic cycle. The reactions of 1-(80 -quinolylaminocarbonyl)-ortho-carborane 1-(80 -NC9H6NH(O)C)2-R-1,2-C2B10H10 with terminal aryl-acetylenes in the presence of Cu(OTf )2, 2-phenylpyridine, Ag2CO3 and K2HPO4 in 1,2-dichloroethane at 130  C followed by treatment with HCl in chloroform lead to the corresponding 4-arylacetylenes 1-(80 -NC9H6NH(O)C)-4-ArC^C-2-R-1,2-C2B10H9 (Scheme 26). Ag2CO3 plays the role of an oxidizing agent in this process. The reaction includes the formation of copper(I) acetylene complexes [Cu(1-(8-NC9H6N(O)C)-4-ArC^C-2-R-1,2-C2B10H9-Z2-C, C-k2-N,N)], the structure of one of which was determined by X-ray diffraction.137

Scheme 26

Copper-catalyzed electrochemical reactions of amides of ortho-carboranyl carboxylic acids and 8-aminoquinoline 1-(80 -NC9H6NH(O)C)-2-R-1,2-C2B10H10 (R ¼ Me, Et, Pr, i-Pr, Bu, Ph) with t-BuOLi in the presence lead to the corresponding 4-tert-butoxy derivatives, whereas the reaction of 1-(20 -NC5H4CMe2NH(O)C)-2-Me-1,2-C2B10H10 under similar conditions leads to the corresponding 4,5-di(tert-butoxy) derivative. Hydrolysis of the obtained tert-butoxy derivatives in 12 M HCl at 120  C leads to the corresponding hydroxy derivatives, and heating with K2CO3 in THF leads to the amide hydrolysis and decarboxylation, i.e., complete removal of the directing ligand. Reactions of 1-(80 -NC9H6NH(O)C)-2-i-Pr-1,2-C2B10H10 with lithium phenolates ArOLi under similar conditions lead to the corresponding 4,5-di(aryloxy) derivatives 1-(80 -NC9H6NH(O)C)4,5-(ArO)2-2-i-Pr-1,2-C2B10H8.138 The reactions of 1-(20 -NC5H4CMe2NH(O)C)-2-Me-1,2-C2B10H10 with diaryl disulfides ArSSAr in the presence of Cu(OTf )2, 2-phenylpyridine and t-BuOLi in 1,2-dichloroethane at 130  C proceed via directing group removal, resulting in the corresponding 7,11-di(arylsulfenyl) derivatives 7,11-(ArS)2-1-Me-1,2-C2B10H9. The reaction is tolerant of various substituent in the aryl ring including F, Cl, Br, CF3, OMe and SMe, as well as to substrates bearing naphthalenyl and heteroaryl groups such as thiophenyl and furanyl.139 The reactions of carboranyl tosyl amide (Et4N)[1-(p-TolSO2NH(O)C)-1-CB11H11] with [Cp M(OAc)2] (M ¼ Ir, Rh) in acetonitrile result in the corresponding B(2)-metallated complexes (Et4N)[Cp M(MeCN)(1-(p-TolSO2N(O)C)-1-CB11H10-k2-N,B(2))]

222

Polyhedral Boranes and Carboranes

(Scheme 27, Fig. 23).140,141 The treatment of (Et4N)[Cp Ir(MeCN)(1-(p-TolSO2N(O)C)-1-CB11H10-k2-N,B(2))] with N-chlorosuccinimide in acetonitrile leads to the cleavage of the IrdB bond with the formation of the 2-chloro derivative (Ph4P)[1-(p-TolSO2NH(O)C)-2-Cl-1-CB11H10].140 The reaction of (Et4N)[1-(p-TolSO2NH(O)C)-1-CB11H11] with Pd(OAc)2 in acetonitrile leads to dinuclear palladium complex (Et4N)2[{Pd(MeCN)(1-(p-TolSO2N(O)C)-1-CB11H10-k2-N,B(2))}2] (Scheme 27 and Fig. 24).141

Scheme 27

Fig. 23 X-ray structures of [Cp Ir(MeCN)(1-(p-TolSO2N(O)C)-1-CB11H10-k2-N,B(2))]− (left) and [Cp Rh(MeCN)(1-(p-TolSO2N(O)C)-1-CB11H10-k2-N,B(2))]− (right).

Fig. 24 X-ray structure of [{Pd(MeCN)(1-(p-TolSO2N(O)C)-1-CB11H10-k2-N,B(2))}2]2+.

Polyhedral Boranes and Carboranes

9.05.2.5.3

223

B-H activation assisted by oxygen-containing substituents

ortho-Carborane derivatives with various oxygen-containing substituents, primarily carboxylic acids and amides, have received the greatest attention for the directed introduction of various substituents into the carborane cage. This is largely due to the low stability of the formed intermediate complexes of transition metals with the metal-oxygen bond, as well as the possibility of easy removal of the directing carboxyl group using thermal decarboxylation, which is largely facilitated by the presence of substituents in neighboring positions of the cage. It should be noted that in all cases (even in the absence of a substituent at the adjacent carbon atom of carborane), the carboxyl group promotes BH-activation of positions 4 and 5, which, in the presence of an additional substituent at the second carbon atom, after removal of the carboxyl group, become positions 7 and 11, respectively. The first described example of the use of functional group-directed BH-activation reactions for the synthesis of ortho-carborane derivatives were the reactions of ortho-carboranyl carboxylic acid 1-HOOC-1,2-C2B10H11 with diarylacetylenes Ar-C^C-Ar in the presence of 2.5 mol% [Cp IrCl2]2, AgOAc and Cu(OAc)2 in toluene at 180  C. The reactions proceed with the loss of the directing carboxylic group and lead to the B(4)-substituted alkenes 4-ArHC]C(Ar)-1,2-C2B10H11 (Scheme 28). Similar reactions with C(2)-substituted carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 (R ¼ Me, Bn) lead to the corresponding B(7)-substituted derivatives 7-PhHC]C(Ph)-1-R-1,2-C2B10H10. It is assumed that the reaction proceeds through the formation of a cyclometallated complex, similar to the complex [Cp Ir(1-OOC-2-Me-1,2-C2B10H9-k2-O,B(4))(DMSO-k1-S)] (Fig. 25), which was obtained by the reaction of 1-HOOC-2-Me-1,2-C2B10H10 with [Cp Ir(OAc)2(DMSO)] in toluene.91

Scheme 28

Fig. 25 X-ray structure of [Cp Ir(1-OOC-2-Me-1,2-C2B10H9-k2-O,B(4))(DMSO-k1-S)].

It should be noted that BH-activation reactions with the participation of directing carboxyl groups are sensitive to the choice of the base, which can deprotonate the free CH group of the carborane. The reaction of ortho-carboranyl carboxylic acid 1-HOOC-1,2-C2B10H11 with the iridium(III) complex [(Cp IrCl2)2(m-pz)] (pz ¼ pyrazine) in the presence of AgOTf and Et3N in dichloromethane results in the CH-activation product [(Cp Ir(1-OOC-1,2-C2B10H10-k2-O,C))2(m-pz)], while the analogous reaction of para-carboranyl carboxylic acid 1-HOOC-12-HOBu2C-1,12-C2B10H10, which has no free CH group in the ortho-position, gives the BH-activation product [(Cp Ir(1-OOC-12-HOBu2C-1,12-C2B10H9-k2-O,B(2)))2(m-pz)] (Fig. 26).90

224

Polyhedral Boranes and Carboranes

Fig. 26 X-ray structures of [(Cp Ir(1-OOC-1,2-C2B10H10-k2-O,C))2(m-pz)] (left) and [(Cp Ir(1-OOC-12-HOBu2C-1,12-C2B10H9-k2-O,B(2)))2(m-pz)] (right).

Reactions of ortho-carboranyl carboxylic acid 1-HOOC-2-Me-1,2-C2B10H10 with terminal acetylenes ArC^CH and R3SiC^CH in the presence of 5 mol% Pd(OAc)2, AgOAc and K2HPO4 in toluene at 80  C lead to the corresponding B(7)-substituted acetylenes 7-ArC^C-1-Me-1,2-C2B10H10 and 7-R3SiC^C-1-Me-1,2-C2B10H10, respectively, with yields varying from 44% to 86% (Scheme 29). The replacement of the methyl group at the adjacent carbon atom in the ortho-carboranyl carboxylic acid with other alkyl groups (i-Pr, Bn) practically does not affect the yield of alkynylation products; however, in the absence of a substituent, the yield of 4-i-Pr3SiC^C-1,2-C2B10H11 drops to 35% (when using 1-HOOC-2-Me3Si-1,2-C2B10H10 as the starting material, the yield of 4-i-Pr3SiC^C-1,2-C2B10H11 is 74%). Removal of the silyl protection by treatment with a fluoride ion leads to terminal carboranyl acetylenes, which participate in various reactions characteristic of other terminal acetylenes, such as the [3 + 2] cycloaddition of azides to acetylenes (“click reaction”), Sonogashira coupling and Glaser-Hay homocoupling.142

Scheme 29

The reaction of ortho-carboranyl carboxylic acid 1-HOOC-1,2-C2B10H11 with styrene in the presence of 10 mol% Pd(OAc)2 and AgOAc in 1,2-dichloroethane at 60  C leads to the 4,5-distyryl derivative 4,5-(PhCH]CH)2-1,2-C2B10H10. A series of 7,11-distyryl derivatives 7,11-(ArCH]CH)2-1-R-1,2-C2B10H9 (R ¼ alkyl or aryl) was obtained by the reaction of ortho-carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 with substituted styrenes under similar conditions (Scheme 30).143

Scheme 30

Polyhedral Boranes and Carboranes

225

In a similar way, the reactions of ortho-carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 (R ¼ Alk, Bn, Ar) with 2 equiv. of alkyl and aryl acrylates CH2]CHC(O)OR0 in the presence of 10 mol% Pd(O(O)CCF3)2 and AgOAc in 1,2-dichloroethane at 70  C lead to the corresponding 7,11-disubstituted carboranyl alkyl and aryl acrylates 7,11-(R0 O(O)CCH]CH)2-1-R-1,2-C2B10H9 in 68–88% yields (Scheme 31). The reaction of 1-HOOC-2-Hex-1,2-C2B10H10 with pentyl vinyl ketone gives 7,11-disubstituted a,b-unsaturated ketone 7,11-(C5H11(O)CCH]CH)2-1-C6H13-1,2-C2B10H9, while the reactions with tert-butyl and aryl vinyl ketones lead to the corresponding 7-substituted a,b-unsaturated ketones. The reactions of 1-HOOC-2-Hex-1,2-C2B10H10 with acrylamides CH2]CHC(O)NR0 R00 (R0 ¼ R00 ¼ Me; R0 ¼ t-Bu, R00 ¼ H) result in the corresponding 7-substituted carboranyl acryl amides 7-R0 R00 N(O)CCH]CH-1-Hex-1,2-C2B10H10.144

Scheme 31

Reactions of ortho-carboranyl carboxylic acid 1-HOOC-2-Hx-1,2-C2B10H10 with allyl alcohols HOCH(R)CH]CH2 (R ¼ Alk, Ar) in the presence of 5 mol% [Cp RhCl2]2 and AgOAc in 1,4-dioxane at 70  C lead to the corresponding 7,11-dialkyl derivatives 7,11-(RC(O)CH2CH2)2-1-Hex-1,2-C2B10H9 (Scheme 31). The hexyl group in ortho-carboranyl carboxylic acid can be replaced by butyl or phenyl; however, in the absence of a substituent, the reaction leads to only trace amounts of the target product.145 Reactions of ortho-carboranyl carboxylic acid 1-HOOC-2-Me-1,2-C2B10H10 with substituted propargyl alcohols Ar-C^C-CR1R2OH in the presence of catalytic amounts of [Cp IrCl2]2, AgSbF6 and NaOAc, in trifluoroethanol at 60  C proceed through the formation of B(4)-alkenyl derivatives, which undergo cyclization to the corresponding indenes. Subsequent decarboxylation in toluene at 130  C leads to the removal of the directing group. The methyl group in the ortho-carboranyl carboxylic acid can be replaced by butyl or aryl group. It is interesting that the reaction in mesitylene proceeds without cyclization and leads to the corresponding diene derivatives (Scheme 32).146

Scheme 32

226

Polyhedral Boranes and Carboranes

Reactions of ortho-carboranyl carboxylic acid 1-HOOC-2-Me-1,2-C2B10H10 with aryl iodides in the presence of 10 mol% Pd(OAc)2, AgOAc and acetic acid in toluene at 70  C lead to the corresponding 7,11-diaryl derivatives 7,11Ar2-1-Me-1,2-C2B10H9 (Scheme 33). The reaction is applicable to aryl iodides containing both electron-donating (3-Me, 4-Me, 3,5-Me2, 4-t-Bu, 4-OMe) and electron-withdrawing (4-Ph, 3-Cl, 4-Cl, 3-Br, 4-Br, 4-F, 4-COMe, 3-COOMe) substituents, with the diaryl derivative yields varying from 50% to 82%. The methyl group in the ortho-carboranyl carboxylic acid can be replaced by a phenyl or benzyl group without noticeably affecting the yield of the aryl derivatives, while the reaction of 1-HOOC-1,2-C2B10H11 gives 4,5-4,5-Ph2-1,2-C2B10H10 in 41% yield (in comparison with 69% starting from 1-HOOC-2-Me3Si-1,2-C2B10H10).147

Scheme 33

Reactions of 1-HOOC-2-R-1,2-C2B10H10 with thiophene in the presence of 2.5 mol% [Cp IrCl2]2, 10 mol% AgNTf2, 10 mol% AgOAc, Cu(OAc)2 and Li2CO3 in toluene at 130  C give the corresponding 7-(20 -thienyl)-derivatives of ortho-carborane. The reactions proceed in good yields with various substituted thiophene derivatives, benzothiophenes, thienothiophenes, and oligothiophenes.148 Reactions of ortho-carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 (R ¼ H, Me, Et, i-Pr, Bu) with 2-phenylpyridine in the presence of Cu(OAc)2 and catalytic amounts of [Cp IrCl2]2, AgOTf and AgOAc in toluene at 130  C lead to aryl derivatives containing a 2-pyridyl group in the ortho-position of aryl ring. The substitution proceeds at the 4-position with respect to the directing carboxyl group, however, considering decarboxylation, the aryl substituent appears in position 7 with respect to the substituent at the second carbon atom 7-(200 -NC5H4-20 -C6H4)-1-R-1,2-C2B10H10 (Scheme 34). The reaction proceeds through the formation of a cyclometallated iridium(III) complex with 2-phenylpyridine and ortho-carboranyl carboxylate (Fig. 27), the oxidation of which (with Cu(OAc)2 in the presence of AgOTf ) leads to the formation of a boron-metallated complex followed by ligand insertion into the IrdB bond. The reactions with C-methyl, C-ethyl and C-butyl substituted ortho-carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 proceed in high yields (73–93%), while in the case of the parent ortho-carboranyl carboxylic acid 1-HOOC-1,2-C2B10H11 and the C-isopropyl substituted derivative, the yields are 51% and 41%, respectively. The reaction is tolerant of a wide range of substituents in both the benzene and pyridine rings (F, Cl, Br, Me, t-Bu, CF3, OMe, CHO, CN), as well as the replacement of pyridine with other nitrogen-containing groups, such as quinoline, pyrimidine, thiazole, pyrazole, indole, –C(Me)]NOMe, –N(O)]NPh or –C(OEt)]NH. In the last case, the directing group loses an ethanol molecule, resulting in 7-(20 -cyanophenyl) derivative 7-(20 -NCC6H4)-1-Me-1,2-C2B10H10. The reaction is also applicable to heteroaromatic compounds (thiophene, pyrrole, indole) with nitrogen-containing directing groups.149

Scheme 34

Polyhedral Boranes and Carboranes

227

Fig. 27 X-ray structure of [Cp Ir{20 -(200 -NC5H4)C6H3-50 -CF3-k2-N,C(20 )}(1-OOC-2-Me-1,2-C2B10H10-k1-O)].

Reactions of ortho-carboranyl carboxylic acid 1-HOOC-2-Me-1,2-C2B10H10 with benzoic acids in the presence of Li2CO3 and (t-BuCOO)2Cu and catalytic amounts of [Cp IrCl2]2, AgNTf and AgOAc in toluene at 160  C lead to 7,11-substituted carborane analogs of coumarin (Scheme 35).150

Scheme 35

Reactions of 1-HOOC-2-Me-1,2-C2B10H10 with amides of benzoic acids in the presence of Li2CO3 and Cu(OPiv)2 and catalytic amounts of [Cp IrCl2]2 and AgOAc in toluene at 140  C lead to 7,11-substituted carborane analogs of phenanthridone (Scheme 36). 2-Thiophene and 2-naphthalene carboxamides react in a similar way. Decreasing the reaction temperature to 90  C, replacing AgOAc with AgNTf2, and adding KOAc, suppress the second dehydrocoupling reaction, leading to the formation of 7-aryl derivatives, which, under the initial BH-activation conditions, are transformed quantitatively to the corresponding phenanthridone analogs (Scheme 36). The reaction of 7-(20 -NH2(O)CC6H4)-1-Me-1,2-C2B10H10 with [Cp Ir(OAc)2] results in the dinuclear iridium complex [Cp Ir(7-(20 -NH(O)C)-1-Me-1,2-C2B10H10-k2-N,C(30 ))]2 (Fig. 28), which on heating with Cu(OPiv)2 in toluene at 60  C gives the corresponding carboranyl phenanthridone.151

228

Polyhedral Boranes and Carboranes

Scheme 36

Fig. 28 X-ray structure of [Cp Ir(7-(20 -NH(O)C)-1-Me-1,2-C2B10H10-k2-N,C(30 ))]2.

The carboxyl group can also be used as a directing substituent in the synthesis of derivatives of the carba-closo-dodecaborate anion. Reactions of 1-carboxy-1-carba-closo-dodecaborane (Et4N)[1-HOOC-1-CB11H11] with an excess of aryl iodides in the presence of 2.5 mol% Pd(OAc)2, AgOAc and acetic acid in DMF result in the corresponding 2,3,4,5,6-pentaaryl derivatives (Et4N)[1-HOOC-2,3,4,5,6-Ar5-1-CB11H6]. The reaction proceeds in high yields (70–94%) both with aryl iodides containing electron-donating (3-Me, 4-Me, 3,5-Me2, 4-Et, 4-Hx, 4-t-Bu, 4-Bn, 4-OMe, 4-CH2OH, 4-NHAc) and especially electron-withdrawing (4-Ph, 3-F, 4-F, 3,4-F2, 3-Cl, 4-Cl, 3,5-Cl2, 4-Br, 4-COOMe, 4-COOEt, 4-CHO) substituents. The reaction at 60  C proceeds with decarboxylation resulting in (Et4N)[2,3,4,5,6-Ar5-1-CB11H7]. It is assumed that the reaction proceeds through

Polyhedral Boranes and Carboranes

229

Fig. 29 X-ray structure of [(MeCN)2Pd(1-OOC-1-CB11H10-k2-O,B(2))]−.

the formation of a five-membered palladacycle complex analogous to the complex (Me3NH)[(MeCN)2Pd(1-OOC-1-CB11H10k2-O,B(2))] (Fig. 29), which was obtained from the reaction (Et4N)[1-HOOC-1-CB11H11] with Pd(OAc)2 in acetonitrile and reacts with 4-FC6H4I to form [2,3,4,5,6-(40 -FC6H4)5-1-CB11H7]−.152 Reactions of 1-carboxy-1-carba-closo-dodecaborane (Et4N)[1-HOOC-1-CB11H11] with an excess of substituted styrenes RCH] CH2 (R ¼ Ph, C6H4-4-F, C6H4-4-CF3, C6H4-4-CN, C6F5), allylbenzenes RCH2CH]CH2 (R ¼ Ph, C6F5) or terminal alkenes R-CH]CH2 (R ¼ Bu, i-Bu, CH2CHMeEt, CH2CMe3, CH2CH2Ph, CH2CH2COOEt, CH2OPh) in the presence of AgOAc and 10 mol% Pd(OAc)2 in acetonitrile gives the corresponding 2,3,4,5,6-pentavinyl derivatives (Et4N)[1-HOOC-2,3,4,5,6-(R CH] CH)5-1-CB11H6]. Carborane derivatives containing additional substituents at position 12 of 1-carba-closo-dodecaborane (Et4N)[1-HOOC-2,3,4,5,6-(ArCH]CH)5-12-X-1-CB11H5] (X ¼ Cl, Ar ¼ Ph; X ¼ Br, Ar ¼ C6H4-4-F; X ¼ Me, Ar ¼ C6H4-4-F; X ¼ Cl, Ar ¼ Ph, C6H4-4-F; X ¼ CN, Ar ¼ C6H4-4-F) were obtained in a similar manner, starting from the corresponding acids (Et4N)[1-HOOC-12-X-1-CB11H10]. Reduction of the alkenes with dihydrogen in the presence of 10% Pd/C leads to the corresponding alkyl derivatives (Et4N)[1-HOOC-2,3,4,5,6-(RCH2CH2)5-1-CB11H6] (R ¼ C6H4-4-F, CH2CMe3). The carboxyl group can be removed by microwave irradiation in DMA in the presence of NaOAc at 150  C.153 The carboxyl group can also be used for synthesis of carborane derivatives with BdN bond. Reactions of ortho-carboranyl carboxylic acid 1-HOOC-2-Me-1,2-C2B10H10 with O-benzoyl hydroxylamines PhC(O)ONR2 in the presence of K2HPO4 and 10 mol% Pd(OAc)2 in toluene at 100  C result in the corresponding 7-substituted amines 7-R2N-1,2-C2B10H10 (Scheme 37).

Scheme 37

230

Polyhedral Boranes and Carboranes

Reduction of 7-Bn2N-1-Me-1,2-C2B10H10 with H2 in a solution of HCl in THF in the presence of 10% Pd/C leads to the corresponding amine 7-H2N-1-Me-1,2-C2B10H10 in quantitative yield.154 Reactions of ortho-carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 (R ¼ Me, Et, i-Pr, CH2Ph, CH2C6H4-4-Me, CH2C6H4-4-Cl, CH2C6H4-4-OMe, CH2CH2OMe, SiMe3) with tosylazide p-MeC6H4SO2N3 in the presence of NaOAc and 2.5 mol % [(p-cymene)RuCl2]2 in toluene at 100  C lead to the corresponding 7-tosylamines 7-TsNH-1-R-1,2-C2B10H10 in 84–94% yields (67% for R ¼ SiMe3) (Scheme 38). In a similar way, the reactions of 1-HOOC-2-Me-1,2-C2B10H10 with PhSO2N3, p-MeOC6H4SO2N3, p-CF3C6H4SO2N3, p-ClC6H4SO2N3, p-O2NC6H4SO2N3, 1-NaphSO2N3, BnSO2N3 and BuSO2N3 lead to the corresponding sulfonamides.154

Scheme 38

Reactions of 1-HOOC-2-Me-1,2-C2B10H10 with 3-aryl- and alkyl-1,4,2-dioxazolones in the presence of NaOAc and catalytic amounts of [(p-cymene)RuCl2]2 and AgNTf2 in 1,2-dichloroethane at 80  C lead to the corresponding amides 7-R(O)CNH-1Me-1,2-C2B10H10 (Scheme 39). It has been shown that dioxazolones are much more reactive in amidation reactions than sulfonyl azides, and 3-alkyl-1,4,2-dioxazolones are more reactive than 3-aryl-1,4,2-dioxazolones; among the latter, those containing electron-withdrawing substituents are somewhat more reactive. 2-Furyl- and 2-thiophenylamides, as well as 2-naphthyl- and 2-azulenylamides, were prepared in a similar way. The methyl group in the ortho-carboranyl carboxylic acid can be replaced by other alkyl (butyl, cyclohexyl) or aryl groups.155

Scheme 39

7-Aryl- and alkylamides 7-R(O)CNH-1-Me-1,2-C2B10H10 can also be prepared by the reaction of 1-HOOC-2-R-1,2-C2B10H10 with the corresponding 3-aryl- and alkyl-1,4,2-dioxazolones in the presence of catalytic amounts of [Cp IrCl2]2 and AgSbF6 and NaOAc in 1,2-dichloroethane at room temperature, followed by decarboxylation at 80  C (Scheme 40).156

Scheme 40

Polyhedral Boranes and Carboranes

231

The carboxyl group can also be used to introduce simple substituents at the 4 or 7 position such as hydroxy group or halogen atoms (depending on the presence of a substituent on the second carbon atom of the ortho-carborane). The reaction of ortho-carboranyl carboxylic acids 1-HOOC-2-R-1,2-C2B10H10 with dioxygen in the presence of 2.5 mol% [Cp RhCl2]2 and KOAc in toluene at 95  C leads to 7-hydroxy derivatives 7-HO-1-R-1,2-C2B10H10 in 40–80% yields (Scheme 41). The reaction with 1-HOOC-2-Me3Si-1,2-C2B10H10 results in 4-hydroxy-ortho-carborane 4-HO-1,2-C2B10H11 in 30% yield.157 Reaction of 1-HOOC-2-R-1,2-C2B10H10 with various halogenating agents (FeCl3/Cu(OAc)2, N-bromo- and N-iodosuccinimide) in the presence of 2.5 mol% [Cp IrCl2]2, 10 mol% AgNTf2 and NaOAc in 1,2-dichloroethane at 100–120  C leads to the corresponding 7-halogen derivatives 7-X-1-R-1,2-C2B10H10 (X ¼ Cl, Br, I) in 75–95% yields (Scheme 41). In the case of unsubstituted ortho-carboranyl carboxylic acid 1-HOOC-1,2-C2B10H11, the yield of the 4-iodo derivative 4-I-1,2-C2B10H11 was 62%. Reactions of 1-HOOC2-R-1,2-C2B10H10 with IOAc (generated in situ from I2 and PhI(OAc)2) in the presence of 15 mol% Pd(OAc)2 and NaOAc in toluene at 60  C leads to the corresponding 7,11-diiodo derivatives 7,11-I2-1-R-1,2-C2B10H9 in 68–86% yield (Scheme 41). The reaction of 1-HOOC-1,2-C2B10H11 leads to 4,5-diiodo-ortho-carborane 4,5-I2-1,2-C2B10H10 in 62% yield.158

Scheme 41

Because of the possibility of the formation of different isomers, BH-activation reactions of carboranes containing two directing carboxyl groups are of particular interest. It was found that the reaction of ortho-carborane dicarboxylic acid 1,2-(HOOC)2-1,2-C2B10H10 with the dimeric iridium(III) complex [(Cp IrCl2)2(m-pz)] (pz ¼ pyrazine) in the presence of AgOTf and Et3N in dichloromethane at room temperature results in selective BH-activation at positions 4 and 7 with the formation of the highly symmetrical tetranuclear metallacycle {[(Cp Ir)2(m-1,2-(OOC)2-1,2-C2B10H8-k4-O,B(4); O,B(7))](m-pz)}2 (Fig. 30).90

Fig. 30 X-ray structure of {[(Cp Ir)2(m-1,2-(OOC)2-1,2-C2B10H8-k4-O,B(4);O,B(7))](m-pz)}2.

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Polyhedral Boranes and Carboranes

The similar reaction of meta-carborane dicarboxylic acid 1,7-(HOOC)2-1,7-C2B10H10 proceeds with loss of pyrazine ligands resulting in dinuclear chloride-bridged iridium complex (Et3NH)[(Cp Ir)2(m-Cl)(m-1,7-(OOC)2-1,7-C2B10H8-k4-O,B(2);O,B(3))] (Fig. 31).90

Fig. 31 X-ray structure of [(Cp Ir)2(m-Cl)(m-1,7-(OOC)2-1,7-C2B10H8-k4-O,B(2);O,B(3))]−.

The reaction of para-carborane dicarboxylic acid 1,2-(HOOC)2-1,2-C2B10H10 with the dimeric iridium(III) complex [(Cp IrCl2)2(m-pz)] (pz ¼ pyrazine) proceeds in similar fashion to those of ortho-carborane dicarboxylic acid resulting in selective BH-activation at the 2 and 10 positions with the formation of the tetranuclear metallacycle {[(Cp Ir)2(m-1,12-(OOC)2-1, 12-C2B10H8-k4-O,B(2);O,B(10))](m-pz)}2 (Fig. 32).90

Fig. 32 X-ray structure of {[(Cp Ir)2(m-1,12-(OOC)2-1,12-C2B10H8-k4-O,B(2);O,B(10))](m-pz)}2.

Polyhedral Boranes and Carboranes

233

The amide group can also act as the directing ligand in the BH-activation reactions. The reactions of amides of ortho-carboranyl carboxylic acids 1-R2N(O)C-2-Ph-1,2-C2B10H10 (R2 ¼ Me2, Et2, i-Pr2, (CH2)4, (CH2CH2)2O) with aryl iodides in the presence of catalytic amounts of Pd(OAc)2 and Ag2CO3 in hexafluoroisopropanol at room temperature lead to the corresponding 3-aryl derivatives 1-R2N(O)C-3-Ar-2-Ph-1,2-C2B10H9, while similar reactions in the presence of Pd(OAc)2 and Ag(OOCCF3) lead to the products of 3,4-diarylation of 1-R2N(O)C-3,4-Ar-2-Ph-1,2-C2B10H8 (Scheme 42). The reactions are tolerant to various functional groups, including chloride, bromide, methyl, formyl, acetyl, and nitro groups, and the yield of both arylation reactions is practically independent of the electronic effects of substituents in the benzene ring.159

Scheme 42

The amide group was also used as the directing ligand in the synthesis of derivatives of the carba-closo-dodecaborate and 1-carba-closo-decaborate anions. The reactions of (Et4N)[1-(CH2)4N(O)C-1-CB11H11] with an excess of ethyl- and phenyl acrylates and styrenes in the presence of 5 mol% [Cp RhCl2]2, 20 mol% AgSbF6 and Cu(OAc)2
H2O in dimethylacetamide at 130  C lead to the corresponding tetra-B-substituted derivatives (Et4N)[1-(CH2)4N(O)C-2,3,4-(XCH2CH2)3-5-XCH]CH-1CB11H7] (X ¼ COOEt, COOPh, Ph, C6H4-4-F). The similar reaction with 4-fluorostyrene in acetonitrile at 100  C results in the disubstituted product (Et4N)[1-(CH2)4N(O)C-2-(40 -FC6H4CH2CH2)-4-(400 -FC6H4CH]CH)-1-CB11H9]. The reaction of (Et4N)[1(CH2)4N(O)C-1-CB11H11] with an excess of diphenyl-acetylene in the presence of 5 mol% [Cp RhCl2]2, 20 mol% AgSbF6 and Cu(OAc)2
H2O in dimethyl-acetamide at 130  C lead to (Et4N)[1-(CH2)4N(O)C-2,3,4,5-(PhC]PhC)4-1-CB11H7], while the similar reaction in the presence of 10 mol% [Cp IrCl2]2 and 20 mol% AgSbF6 in 1,2-dichloroethane at 100  C produces (Et4N)[1-(CH2)4N(O)C-2-PhC]PhC-1-CB11H10].160 The amide group can also be used for the synthesis of derivatives with boron-nitrogen and boron-halogen bonds. The reaction of (Et4N)[1-(CH2)4N(O)C-1-CB11H11] with an excess of tosyl azide in the presence of 10 mol% [Cp IrCl2]2 and 20 mol% AgSbF6 in 1,2-dichloroethane at 80  C leads to the 2,4-di(tosylamide) (Et4N)[1-(CH2)4N(O)C-2,4-(p-MeC6H4SO2NH)2-1-CB11H9], whereas the reaction of (Et4N)[1-(CH2)4N(O)C-1-CB11H11] with an excess of N-chlorosuccinimide the presence of 2.5 mol% [Cp RhCl2]2, 10 mol% AgBF4 and Cu(OAc)2
H2O in 1,2-dichloroethane at 80  C results in the 2-chloro derivative (Et4N)[1-(CH2)4N(O)C-2Cl-1-CB11H10].160 It was assumed that the reaction proceeds through the formation of a five-membered iridacycle complex analogous to the complex [Cp Ir(MeCN)(1-(CH2)4N(O)C-1-CB11H10-k2-O,B(2))] (Fig. 33), which was obtained by the reaction of (Et4N)[1-(CH2)4N(O)C-1-CB11H11] with [Cp Ir(OAc)2] in acetonitrile.160

Fig. 33 X-ray structure of [Cp Ir(MeCN)(1-(CH2)4N(O)C-1-CB11H10-k2-O,B(2))].

234

Polyhedral Boranes and Carboranes

The reactions of (Et4N)[1-i-PrHN(O)C-1-CB9H9] with an excess of alkyl-, benzyl- and aryl acrylates, vinylsulfones and styrenes in the presence of 10 mol% [Cp RhCl2]2, 40 mol% AgSbF6 and Cu(OAc)2
H2O in DMA at 20–45  C lead to the corresponding 2,3-divinyl derivatives (Et4N)[1-iPrHN(O)C-2,3-(XCH]CH)5-1-CB9H7] (X ¼ COOR (R ¼ Alk, Bn, Ar); SO2R (R ¼ Me, Ph); Ar).161 The phosphine oxide group can also be used as the directing ligand for the modification of carboranes via transition metal catalyzed BH-activation. Reactions of ortho-carboranylphosphine oxides 1-Ph2P(O)-2-Ar-1,2-C2B10H10 with arylboronic acids Ar0 B(OH)2 in the presence of 10 mol% Pd(OAc)2, hydrogen peroxide and trifluoroacetic acid in toluene lead in moderate yields (44–73%) to the corresponding 3-aryl derivatives 1-Ph2P(O)-3-Ar0 -2-Ar-1,2-C2B10H9 (Scheme 43). The reaction of 1-Ph2P(O)1,2-C2B10H11 with 4-МеC6H4B(OH)2 gives 1-Ph2P(O)-3-(40 -MeC6H4)-1,2-C2B10H10 in 43% yield.162

Scheme 43

Another type of directing group with an oxygen donor atom is the acetamido group, which, unlike carbonylamide groups, are bonded not to the carbon atoms, but to boron atoms of carboranes. Reactions of 3-acetamido-ortho-carboranes 3-MeC(O)NH1,2-R2-1,2-C2B10H9 (R ¼ H, Me) with aryl iodides in the presence of 5 mol% Pd(OAc)2, Ag2CO3 and Cu(OTf )2 in 1,2-dichloroethane at 60  C lead to the corresponding 8-aryl derivatives 3-MeC(O)NH-8-Ar-1,2-R2-1,2-C2B10H8 (R ¼ H, Me) (Scheme 44). The yield of the aryl derivatives varies from 62% to 85%, and only weakly depends on the presence of electron-donating or electron-withdrawing substituents. The benzamido group PhC(O)NH- and its substituted derivatives can be used as the directing group instead of the acetamido group. It is interesting that in the intermediate cyclometallated complex [(Phen)Pd(3-MeC(O)NH-1,2-Me2-1,2-C2B10H8-k2-O,B(4))][OTf], the palladium atom is bonded not to the boron atom in position 8, but to the boron atom in the adjacent position 4 (Fig. 34). Probably, this is due to the migration of the palladium atom at the next stage of the reaction. According to the data of quantum chemical calculations, the geometry of the B(4)-metallated complex [(Phen)Pd(II)(3-MeC(O)NH-1,2-Me2-1,2-C2B10H8-k2-O,B(4))]+ is 0.7 kcal mol−1 more favorable than the B(8)metallated analog [(Phen)Pd(II) (3-MeC(O)NH-1,2-Me2-1,2-C2B10H8-k2-O,B(8))]+; however, after the addition of iodobenzene, the geometry of the B(8)-metallated complex [(Phen)(Ph)IPd(IV)(3-MeC(O)NH-1,2-Me2-1,2-C2B10H8-k2-O,B(8))]+, on the contrary, becomes by 5.1 kcal mol−1 more favorable than the corresponding B(4)-metallated complex. It should be noted that the reaction of the B(4)-metallated complex [(Phen)Pd(3-MeC(O)NH-1,2-Me2-1,2-C2B10H8-k2-O,B(4))]+[OTf]− with iodobenzene leads to the B(8)-arylation product 3-MeC(O)NH-8-Ph-1,2-Me2-1,2-C2B10H8.163

Scheme 44

The reactions of 3-acetamido- and benzamido-ortho-carboranes 3-RC(O)NH-1,2-C2B10H11 (R ¼ Me, Ph) with an excess of aryl iodides under similar conditions lead to the corresponding 4,7,8-triaryl derivatives 3-RC(O)NH-4,7,8-Ar3-1,2-C2B10H8 (Scheme 44). It should be noted that the reaction of 3-acetamido-ortho-carborane with N-fluorocollidinium tetrafluoroborate [1-F-2,4,6-Me3-NC6H2][BF4] in the presence of [(MeCN)4Pd](OTf )2 in 1,2-dichloroethane leads to the corresponding trifluoro derivative 3-MeC(O)NH-4,7,8-F3-1,2-C2B10H8.163

Polyhedral Boranes and Carboranes

235

Fig. 34 X-ray structure of [(Phen)Pd(3-MeC(O)NH-1,2-Me2-1,2-C2B10H8-k2-O,B(4))]+.

Reactions of 7-benzamido-1-methyl-ortho-carborane 7-Ph(O)CNH-1-Me-1,2-C2B10H10 with an excess aryl iodides in the presence of 5 mol% Pd(OAc)2, Ag2CO3 and Cu(OTf )2 in 1,2-dichloroethane at 60  C lead to the corresponding 8,11,12-triaryl derivatives 7-PhC(O)NH-8,11,12-Ar3-1-Me-1,2-C2B10H7 (Scheme 45). The reaction yield varies from 53% to 71% and does not depend on the presence of both electron-donating and electron-withdrawing substituents; however, it is overly sensitive to position of the substituent due to steric hindrance. The reaction at room temperature selectively results in 12-aryl derivatives 7-PhC(O)NH12-Ar-1-Me-1,2-C2B10H9. Using different aryl iodides, different aryl substituents can be sequentially introduced at positions 12, 8 and 11 of the carborane cage. The acetamido and substituted benzamido groups can be used as the directing group as well. The methyl group on the carbon atom can be substituted with other alkyl and aryl groups.164

Scheme 45

Reactions of 9-benzamido-ortho-carborane 9-PhC(O)NH-1,2-C2B10H11 with arylboronic acids in the presence of 10 mol% [(MeCN)4Pd][BF4], Cu(OAc)2 and cyclohexyl carboxylic acid in THF lead to the corresponding 4-aryl derivatives of 9-PhC(O) NH-4-Ar-1,2-C2B10H10 (Scheme 46). The yields of the aryl derivatives vary from 51% to 69% for arylboronic acids containing electron-withdrawing substituents and from 64% to 74% for arylboronic acids containing electron-donating substituents. Instead of the benzamido group, various substituted arylamido or alkylamido groups can be used as directing ligands.165

Scheme 46

236

Polyhedral Boranes and Carboranes

Reactions of 9-arylamido-ortho-carboranes 9-ArC(O)NH-1,2-C2B10H11 with MeI in the presence of 10 mol% [(MeCN)2PdCl2] and AgOAc in toluene lead to the 4-methyl derivatives of 9-ArC(O)NH-4-Me-1,2-C2B10H10, whereas the reactions in the presence of AgOTf result in the 12-methyl derivatives of 9-ArC(O)NH-12-Me-1,2-C2B10H10 (Scheme 47). The 4,12-dimethyl derivatives 9-ArC(O)NH-4,12-Me2-1,2-C2B10H9 can be prepared by the selective stepwise methylation.166

Scheme 47

It should be noted that the 9-benzamido group can also be used for directed oxidative coupling of two ortho-carborane cages. The reaction of 9-benzamido-ortho-carborane 9-PhCOHN-1,2-C2B10H11 with 10 mol% Pd(OAc)2, 10 mol% NiCl2, AgF and AgOAc in toluene at 60 C leads to B(4)H-activation of ortho-carborane with the formation of a mixture of diastereomeric 4,40 - and 4,50 -bis(1(900 -benzamido)-ortho-carboranes), which can be separated by column chromatography.167 The hydroxymethyl group can also act as a directing ligand. The reaction of 1-hydroxymethyl-ortho-carborane with [Cp IrCl2]2 in the presence of AgOTf and triethylamine in dichloromethane proceeds with dual CH- and BH-activation resulting in the binuclear complex [(Cp Ir)2(m-1-OCH2-1,2-C2B10H9-k3-O,C(2),B(3))] (Fig. 35).134

Fig. 35 X-ray structure of [(Cp Ir)2(m-1-OCH2-1,2-C2B10H9-k3-O,C(2),B(3))].

9.05.2.5.4

B-H activation assisted by sulfur-containing substituents

Sulfur-containing groups, such as thioamides and dithiocarboxylate esters, are also able to act as directing ligands in BH-activation reactions. The reaction of phenyl ortho-carboranyl thioamide 1-PhNH(S)C-1,2-C2B10H11 with [Cp MCl2]2 (M ¼ Ir, Rh) in the presence of NaOAc in dichloromethane unexpectedly resulted in CH-activation of the carborane, leading to the formation of the corresponding cyclometallated complexes [Cp MCl(1-PhNH(S)C-1,2-C2B10H10-k2-S,C(2))] (Scheme 48).168

Polyhedral Boranes and Carboranes

237

Scheme 48

The reaction of 1-PhNH(S)C-1,2-C2B10H11 with [Cp IrCl2]2 in the presence of AgOTf and Et3N in THF results in the formation of the C-cyclometallated 16-electron iridium(III) complex [Cp Ir(1-S(Ph]N)C-1,2-C2B10H10-k2-S,C(2))]. The reaction of this coordinatively unsaturated complex with t-BuCN in dichloromethane gives [Cp Ir(t-BuNC)(1-S(Ph]N)C-1,2-C2B10H10-k2-S, C(2))], while the reaction with dimethyl acetylenedicarboxylate leads to its insertion in the IrdS bond with formation of [Cp Ir(1-(MeO(O)C)C](MeO(O)C)CS(Ph]N)C-1,2-C2B10H10-k3-C,S,C(2))]. The reaction with HCl in dichloromethane results in [Cp IrCl(1-PhNH(S)C-1,2-C2B10H10-k2-S,C(2))]. The removal of chloride on the treatment with AgOTf and Et3N regenerates the 16-electron complex [Cp Ir(1-S(Ph]N)C-1,2-C2B10H10-k2-S,C(2))] (Scheme 48).169 The reaction of [Cp Ir(1-S(Ph]N)C-1,2-C2B10H10-k2-S,C(2))] with [Cp IrCl2]2 in the presence of AgOTf and Et3N in THF results in the B(3)H-activation with formation of dinuclear iridium complex [(Cp Ir)2 (m-1-S(Ph]N)C-1,2-C2B10H9-k3-S,C(2), B(3))]. Dinuclear rhodium complex [(Cp Rh)2(m-1-S(Ph]N)C-1,2-C2B10H9-k3-S,C(2),B(3))] was obtained in a similar way without isolation of the 16-electron rhodium complex. The treatment of [Cp Ir(1-S(Ph]N)C-1,2-C2B10H10-k2-S,C(2))] with HCl in dichloromethane leads to the cleavage of the CdIr bond resulting in [Cp IrCl(1-PhNH(S)C-1,2-C2B10H10-k2-S,B(3))]. Thus, an unusual transformation of the C-carboranyl- into the B-carboranyl metallacycle takes place here (Scheme 49).169

Scheme 49

Similar B(3)-cyclometallated complexes [Cp M(CNPh)(1-S(PhN])C-1,2-C2B10H10-k2-S,B(3))] (M ¼ Ir, Rh) were prepared by the treatment of 1-PhNH(S)C-1,2-C2B10H11 with n-BuLi in THF followed by the addition of [Cp MCl2]2 (Scheme 48).168 The reaction of phenyl ortho-carboranyl thioamide 1-PhNH(S)C-1,2-C2B10H11 with [(MeCN)2PdCl2] in the presence of NaH in dichloromethane results in B(4)H-activation of the carborane with the formation of the tetranuclear palladium complex [Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))]4 (Scheme 50). Four palladium atoms are located in approximately the same

238

Polyhedral Boranes and Carboranes

Scheme 50

plane and are connected by four bridging sulfur atoms. Each carboranyl thioamide ligand chelates a Pd atom according to the k2-S,B(4)-fashion, while its coordination sphere is complemented by the agostic Pd ⋯ HB(5) contact from the other carboranyl thioamide ligand (Fig. 36).170

Fig. 36 X-ray structures of [Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))]4 (left) and [(CO)Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))]3 (right).

The treatment of the tetranuclear complex [Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))]4 with CO in dichloromethane leads to the trinuclear palladium complex [(CO)Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))]3 (Fig. 36), which easily loses CO reverting back to the tetranuclear complex.170 Reactions of [Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))]4 with other donor ligands (pyridine, triphenylphosphine, tert-butyl isonitrile) lead to the corresponding mononuclear complexes [L2Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))] (L ¼ py, PPh3, t-BuNC) (Scheme 50, Fig. 37).170

Polyhedral Boranes and Carboranes

239

Fig. 37 X-ray structures of [(py)2Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))] (left) and [(t-BuNC)2Pd(m-1-S(PhN])C-1,2-C2B10H10)-k2-S,B(4))] (right).

The reaction of methyl ortho-carboranyl dithiocarboxylate 1-MeS(S)C-1,2-C2B10H11 with [Cp IrCl2]2 in dichloromethane in the presence of AgOTf leads to the formation of the B(3)-cyclometallated complex [Cp IrCl(1-MeS(S)C-1,2-C2B10H10-k2-S,B(3))] (Scheme 51).171

Scheme 51

On the treatment with S-nucleophiles in the presence of AgOTf, the chloride ligand in [Cp IrCl(1-MeS(S)C-1,2-C2B10H10-k2-S, B(3))] can be easily replaced by phenylthiolate or ortho-carboranyl dithiocarboxylate with the formation of the corresponding complexes [Cp Ir(SR)(1-MeS(S)C-1,2-C2B10H10-k2-S,B(3))] (R ¼ Ph, 1-(S)C-1,2-C2B10H11, 1-(NPh)C-1,2-C2B10H11) (Scheme 51).171 The reaction of [Cp IrCl(1-MeS(S)C-1,2-C2B10H10-k2-S,B(3))] with decaborane and triethylamine in the presence of AgOTf in dichloromethane leads to the formation of a complex {(Cp Ir(1-MeS(S)C-1,2-C2B10H10-k2-S,B(3)))2[m-1,10-B10H10-k2-H,H]}, in which the closo-decaborate anion plays the role of a bridging ligand between two iridium atoms (Fig. 38).171

240

Polyhedral Boranes and Carboranes

Fig. 38 X-ray structure of {(Cp Ir(1-MeS(S)C-1,2-C2B10H10-k2-S,B(3 )))2[m-1,10-B10H10-k2-H,H ]}.

Fig. 39 X-ray structures of [{(Cp Ir)2(m-Cl)}2(1,12-S(NPh)C)2-1,12-C2B10H10-k6-S(1),B(2);B(6); S(12),B(8);B(9))] (left) and [{(Cp Ir(t-BuNC)}2(m-1,12-(S(NPh) C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))] (right).

The reaction of 1,12-(PhNH(S)C)2-1,12-C2B10H10 with excess of [Cp IrCl2]2 in the presence of AgOTf and Et3N in THF leads to fourfold BH-activation at the positions 2, 6, 8, and 9 with the formation of the highly symmetrical tetranuclear complex [{(Cp Ir)2(m-Cl)}2((1,12-S(NPh)C)2-1,12-C2B10H10-k6-S(1),B(2); B(6);S(12),B(8);B(9))] (Fig. 39).172 Reactions of 1,12-(PhHN(S)C)2-1,12-C2B10H10 with dimeric complexes [(Cp IrCl2)2(m-L)] (L ¼ pyrazine (pz), 4,40 -bipyridine (bpy), 1,2-di(40 -pyridyl)ethylene (bpe), 1,4-di(40 -pyridyl)-benzene (dpb)) in the presence of Et3N in dichloromethane lead to the formation of the corresponding tetranuclear metallamacrocycles in which BH-activation of the para-carborane cage occurs at the 2 and 9 positions {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))](m-L)}2 (Fig. 40),172,173 whereas similar reactions with [(Cp IrCl2)2(m-L)] (L ¼ 1,2-di(40 -pyridyl)acetylene (bpa), 2,6-di(40 -pyridyl)naphthalene (dpn) and 2,6-di(40 -pyridyl)anthracene (dpan)) result in the tetranuclear metallamacrocycles in which BH-activation of the para-carborane cage occurs at the 2 and 8 positions {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))](m-L)}2.172 The treatment of the latter complexes with the strong donor tert-butylisonitrile in dichloromethane leads to the dinuclear complex [{Cp Ir(t-BuNC)}2(m-1,12-(S(NPh) C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))] (Fig. 39). The underlying reasons for BH-activation at different positions in the para-carborane cage in the above-described metallamacrocycles is the absence/presence of p-p interactions between the bridging bifunctional pyridine ligands.172 It was demonstrated that the metallamacrocycle {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2); S,B(9))](m-dpb)}2 can be used for effective separation of hexane isomers.173

Polyhedral Boranes and Carboranes

241

Fig. 40 X-ray structures of {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))](m-pz)}2 (top left), {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S, B(2);S,B(9))](m-dpe)}2 (top right) and {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))](m-dpb)}2 (bottom).

Reactions of 1,12-(MeHN(S)C)2-1,12-C2B10H10 with dimeric complexes [(Cp IrCl2)2(m-L)] (L ¼ bpy, dpe, dpb) in the presence of Et3N in dichloromethane lead to the corresponding cationic tetranuclear metallamacrocycles {[(Cp Ir)2(m-1,12-(MeNH(S) C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))](m-L)}2(OTf )4 (Fig. 41). The difference between the methyl- and phenylthioamides is caused by the donor effect of the methyl group; as a consequence the methylthioamide, unlike the phenylthioamide, cannot tautomerize into the corresponding iminothiol and deprotonate.173

Fig. 41 X-ray structures of {[(Cp Ir)2(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))](m-bpy)}24+ (left) and {[(Cp Ir)2(m-1,12-(MeNH(S)C)2-1,12C2B10H8-k4-S,B(2);S,B(9))](m-bpe)}24+ (right).

242

Polyhedral Boranes and Carboranes

B(2,8)H-activation of the para-carborane cage can also be influenced by the steric properties of the bridging ligand used. Reactions of 1,2,3,4-tetrakis(40 -pyridyl)cyclobutane (L) with 1,12-(RHN(S)C)2-1,12-C2B10H10 (R ¼ Me, Ph) in the presence of AgOTf and Et3N in dichloromethane result in the corresponding cationic {[(Cp Ir)2(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S, B(2);S,B(8))]2(m-L)}(OTf )4 and neutral {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))]2(m-L)} macrometallacycles (Fig. 42).172

Fig. 42 X-ray structure of {[(Cp Ir)2(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))]2(m-1,2,3,4-tetrakis(40 -pyridyl)cyclobutane)}4+.

The reaction of 1,12-(PhHN(S)C)2-1,12-C2B10H10 with [Cp IrCl2]2 and 2,4,6-tri(40 -pyridyl)-1,3,5-triazine (tpt) in the presence of AgOTf and Et3N in dichloromethane results in the hexanuclear metalla-macrocycle in which the BH-activation of the para-carborane cage occurs at the 2 and 8 positions of {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))]3(m3-tpt)2}, while the similar reactions with excess of 2,4,6-tri(40 -pyridyl)-1,3,5-triazine leads to the formation of the tpt inclusion complex tpt {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))]3(m3-tpt)2} in which the BH-activation of the para-carborane cage occurs at positions 2 and 9 (Fig. 43).172

Fig. 43 X-ray structure of {[(Cp Ir)2(m-1,12-(S(NPh)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))]3(m3-tpt)2}.

Polyhedral Boranes and Carboranes

243

The reaction of 1,12-(MeHN(S)C)2-1,12-C2B10H10 with [Cp IrCl2]2 and tetrakis(40 -pyridyl)-tetrathiafulvalene (TTF-Py) in the presence of AgOTf and Et3N in dichloromethane results in the octanuclear metallamacrocycle in which BH-activation of the two para-carborane cages occurs at the 2 and 9 positions, whereas the BH-activation of other two para-carborane cages at positions 2 and 8 to give {[(Cp Ir)4(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S,B(2);S, B(8))]2(m4-TTF-Py)2}(OTf )8 (Fig. 44).172

Fig. 44 X-ray structure of {[(Cp Ir)4(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S,B(2);S,B(9))(m-1,12-(MeNH(S)C)2-1,12-C2B10H8-k4-S,B(2);S,B(8))]2(m4-TTF-Py)2}8+.

The reaction of 1,12-(PhHN(S)C)2-1,12-C2B10H10 with [(bpy)PdCl2] in the presence of NaOAc in dichloromethane leads to a mixture of products of B(2,7)H- and B(2,8)H-activation of the para-carborane cage [{(bpy)Pd}2(1,12-(S(NPh)C)21,12-C2B12H8-k4-S,B(2);S,B(7))] and [{(bpy)Pd}2(1,12-(S(NPh)C)2-1,12-C2B12H8-k4-S,B(2);S,B(8))] (Fig. 45). It is assumed that the formation of the B(2,7)H-activation product is caused by p-p interactions between the 2,20 -bipydine ligands.172

Fig. 45 X-ray structures of [{(bpy)Pd}2(1,12-(S(NPh)C)2-1,12-C2B12H8-k4-S,B(2);S,B(7))] (left) and [{(bpy)Pd}2(1,12-(S(NPh)C)2-1,12-C2B12H8-k4-S,B(2);S, B(8))] (right).

244

Polyhedral Boranes and Carboranes

9.05.3

Polyhedral boranes

Polyhedral boranes were not previously considered in the framework of previous editions of Comprehensive Organometallic Chemistry, however, the similarity of the structures of polyhedral boranes and carboranes and the great progress in the chemistry of boron-substituted carboranes, achieved recently, makes it expedient to include polyhedral boranes in this edition. However, it should be noted that the simple and complex salts of the polyhedral anions of borane are outside the scope of this publication. Compared to the chemistry of carboranes, the chemistry of polyhedral boranes has been much less studied, which is due to the relatively low stability of most neutral arachno- and nido-boranes and the ionic nature of closo-boranes, which greatly complicates the work with them. As in the case of carboranes, different areas of the chemistry of polyhedral boranes have been studied very unequally due to their different availability.

9.05.3.1 9.05.3.1.1

closo-Dodecaborate anion [B12H12]2− General aspects. Halogen derivatives

Due to its availability, the dodecahydro-closo-dodecaborate anion is the most studied of the polyhedral boranes. An early stage in the development of the chemistry of the closo-dodecaborate anion [B12H12]2− was reviewed by Sivaev et al.174 Despite the fact that the synthesis and properties of simple and complex salts of the closo-dodecaborate anion are outside the scope of this publication, we find it appropriate to list here some recent reviews in this area.175–179 Also, despite the fact that the main methods for the synthesis of closo-dodecaborate anion were developed earlier, the recently published convenient laboratory procedure for its preparation from readily available NaBH4 deserves attention.180 Recently, the synthesis of monohalogen derivatives of the closo-dodecaborate anion [B12H11X]2− (X ¼ Cl, Br, I) was reported by the reaction of the parent closo-dodecaborate with the corresponding N-halosuccinimides in acetonitrile.181 A convenient method for the synthesis of the perfluorinated closo-dodecaborate anion [B12F12]2− with gaseous fluorine in wet acetonitrile was described.182 The perchlorinated closo-dodecaborate anion [B12Cl12]2− can be obtained by the chlorination of the parent closo-dodecaborate with gaseous Cl2 in aqueous solution180 or by the reaction with SOCl2 in refluxing acetonitrile.183 Synthesis of the periodinated closo-dodecaborate anion [B12I12]2− via microwave-assisted iodination of the parent closo-dodecaborate with I2 in acetic acid at 230  C was reported.184 The perfluoro and perchloro derivatives undergo one-electron reversible oxidation with AsF5 in liquid SO2 leading to the hypercloso-[B12X12]− radical anions (X ¼ F, Cl).185,186 Two sequential electrochemical oxidations of [B12Cl12]2− result in the neutral hypercloso-[B12Cl12] species.186 The electrochemical stability of the perhalogenated closo-dodecaborates is increased in the series [B12F12]2− < [B12Cl12]2− < [B12Br12]2− < [B12I12]2−.186 The reaction of the closo-dodecaborate anion with aryliodonium diacetates ArI(OAc)2 (Ar ¼ C6H5, C6H4-4-OMe) in aqueous acetic acid at 0  C leads to the corresponding zwitterionic aryliodonium derivatives [B12H11IAr]−.187,188 whereas the reaction in aqueous trifluoracetic acid at 70  C results in a mixture of neutral 1,7- and 1,12-di(aryliodonium) derivatives.187,189 The same approach can be used to introduce an aryliodonium group into some derivatives of closo-dodecaborate anion.187,189 The aryliodonium group can be substituted with various nucleophiles, such as pyridines, hydroxylamine or N,N-dimethylthioformamide.187–190

9.05.3.1.2

Derivatives with BdO bond

An efficient method for functionalization of the [B12H12]2− anion through the formation of its cyclic oxonium derivatives (Scheme 52) with their subsequent disclosure by various nucleophiles has been proposed.191 More recently, efficient methods for the synthesis of oxonium derivatives have been developed181,192–194 and a series of various practically important functional derivatives have been synthesized.185,192,195–200 This approach proved to be very convenient and effective for attaching the closo-dodecaborate moiety to various biomolecules, including amino acids,197,201 sugars,202,203 nucleosides,199,204–209 porphyrins and phthalocyanines,210–214 cholesterol,215,216 coumarins,193,217,218 and others194,219–228 of interest for use in BNCT, as well as for preparation of boronated polymers229 and nanoparticles.230,231 Cyclic oxonium derivatives containing other substituents in the closo-dodecaborate cage also were synthesized and their ring-opening reactions were studied.232–234 The advantages of this approach are a reasonable length of the spacer between the boron cage and the biologically active part of the molecule and the ability to regulate its hydrophilic-lipophilic properties by choosing a suitable cyclic ether. Recently, a convenient method for shortening the

Scheme 52

Polyhedral Boranes and Carboranes

245

side chain formed during the opening of the dioxane ring with cyanide ion was proposed, and a series of functional derivatives [B12H11OCH2CH2X]2− (X ¼ OH, Br, I, OMs, N3) were obtained and applied for the synthesis of boron-containing nucleosides.235 Several examples of the alkylation of the hydroxy derivative of the closo-dodecaborate anion [B12H11OH]2− have been described earlier.236,237 Reactions of [B12H11OH]2− with o-, m- and p-nitrophenethyl bromides238 as well as with propargyl bromide239 in DMF in the presence of KOH at room temperature result in the corresponding alkoxy derivatives [B12H11OR]2− (R ¼ CH2CH2C6H4-2-NO2, CH2CH2C6H4-3-NO2, CH2CH2C6H4-4-NO2, CH2C^CH). The latter reacts with various functionalized azides containing aryl, carboranyl and lipid groups to form the corresponding triazoles.239 The boron-containing esters of meso-tetra(4-carboxyphenyl)porphyrin240 and protoporphyrin IX241 and were prepared by acylation of the hydroxy derivative of the closo-dodecaborate anion. The perchlorinated hydroxy derivative [B12Cl11OH]2− was obtained bubbling chlorine gas through an aqueous solution of [B12H11OH]2−.242 Alternatively, the perchlorinated hydroxy derivative can be prepared by the reaction of the parent hydroxy derivative with SO2Cl2 in refluxing acetonitrile.243 Alkylation of [B12Cl11OH]2− with alkyl bromides in DMSO in the presence of KOH leads to the corresponding alkoxy derivatives [B12Cl11OR]2− (R ¼ C3H7, C8H17, C12H25).242 The reaction of [B12Cl11OH]2− with tosyl chloride in pyridine results in [B12Cl11OS(O)2-p-Tol]2−, while the reaction with triflic anhydride gives [B12Cl11OS(O) CF3]2−.243 The perbrominated hydroxy derivative [B12Br11OH]2− was obtained by heating [B12H11OH]2− with bromine in aqueous methanol.242 Alkylation of [B12Br11OH]2− with alkyl bromides in DMSO in the presence of KOH leads to the corresponding alkoxy derivatives [B12Br11OR]2− (R ¼ C3H7, C8H17, C12H25).242 An improved method was developed for the synthesis of the perhydroxy derivative of closo-dodecaborate anion Cs2[B12(OH)12] by the reaction of the cesium salt of the parent anion with 30% hydrogen peroxide at 105–110  C.244 Alternatively, the perhydroxy derivative can be obtained by heating Cs2[B12H12] in fuming sulfuric acid at 195  C in the presence of 10 mol% PdCl2 followed by acid hydrolysis of the resulting (H3O)2[B12(OSO3H)12].245 It has earlier been demonstrated that alkylation of [B12(OH)12]2− with benzyl bromide leads to the dodeca(benzyloxy) derivative of the closo-dodecaborate anion [B12(OBn)12]2−.246 Later, synthesis of a series of substituted benzyloxy, alkoxy and allyloxy derivatives [B12(OR)12]2− was described.247–251 The peralkoxy derivatives undergo two reversible sequential chemical or electrochemical oxidations giving the hypercloso-[B12(OR)12]− radical anions and the hypercloso-[B12(OR)12] neutral species.247,248,250,252,253 It was earlier reported that acylation of [B12(OH)12]2− with acetic anhydride or benzoyl chloride results in the corresponding dodeca(acetate) [B12(OAc)12]2− and dodeca(benzoate) [B12(OBz)12]2− derivatives.254 Later, synthesis of a series of various acyloxy derivatives [B12(OC(O)R)12]2− (R ¼ alkyl, aryl) was described.255 The acylation of [B12(OH)12]2− with chloroacetic or bromoacetic anhydride results in the dodeca(chloro/bromoacetate) derivatives [B12(OC(O)CH2X)12]2− (X ¼ Cl, Br), which upon treatment with NaN3 converts into the corresponding azide [B12(OC(O)CH2N3)12]2−, which in turn can be used to obtain the triazole derivatives containing various functional groups.256–259 A series of dodeca(carbonate) derivatives [B12(OC(O)OAr)12]2− were prepared by the reaction of [B12(OH)12]2− with various aryl chloroformates. The reactions of these carbonates with primary amines lead to the corresponding carbamates [B12(OC(O)NHR)12]2−, including those containing various functional groups (–C^CH, –N3, –NHBoc, etc.).260,261 Several methods for the synthesis of vertex-differentiated derivatives of the closo-dodecaborate anion via monoalkylation of [B12(OH)12]2− followed by acylation of the resulting [B12(OR)(OH)11]2− with chloroacetic anhydride or p-nitrophenyl chloroformate and corresponding transformations were developed.261–263 The chemistry of the dodeca(hydroxy)-closo-dodecaborate anion [B12(OH)12]2− and its derivatives was discussed in detail in several mini-reviews.264,265

9.05.3.1.3

Derivatives with BdS bonds

The syntheses of numerous alkylthio derivatives of the closo-dodecaborate anion [B12H11SR]2− via the alkylation of the mercapto derivative [B12H11SH]2− have been earlier described.174 To avoid the formation of dialkylsulfonium derivatives [B12H11SR2]−, an approach based on the use of an easily removable cyanoethyl group (Scheme 53) was developed.266 More recently this approach

Scheme 53

246

Polyhedral Boranes and Carboranes

was used for the synthesis of some important simple functional derivatives of the closo-dodecaborate anion,267,268 as well as boron-containing amino acids,269–271 peptides,272 sugars,273 lipids,274–278 porphyrins,279 coumarin,218 kojic acid,280 and cholesterol.281 The alkylthio derivatives also can be prepared via the Michael addition of the mercapto derivative [B12H11SH]2− to an a,b-unsaturated carbonyl compound containing an electron-withdrawing group. Thus, the reaction of [B12H11SH]2− with methyl acrylate in alkaline solution in one step leads to the corresponding boronated propionic acid [B12H11SCH2CH2COOH]2−.282 A series of dialkylsulfonium derivatives [B12H11SR2]− was prepared by direct alkylation of the mercapto derivative of the closo-dodecaborate anion with arylethyl bromides,238 propargyl bromide267 and o-bromalkyl carboxylic acids.268 Alkylation of [B12H11SH]2− with bifunctional alkylating agents such as 1,5-dibromopntane or bis(2-chloroethyl)amine leads to the cyclic sulfonium derivatives [B12H11S(CH2)5]−266 and [B12H11S(CH2CH2)2NH]−.283 Treatment of the di(phenethyl)sulfonium derivatives with [Me4N]OH results in styrene elimination with the formation of the corresponding alkylthio derivatives [B12H11SCH2CH2C6H4X]2−.238 The reactions of [B12H11S((CH2CH2COOH)2]− with alcohols in the presence of N, N0 -dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) proceed with the loss of one alkyl group leading to esters [B12H11SCH2CH2COOR]2−.268 The reactions of [B12H11S((CH2)nCOOH)2]− (n ¼ 1–3) with propargyl amine also proceed with the loss of one alkyl group resulting in the corresponding amides [B12H11S(CH2)nCONHCH2C^CH]2−, which, in turn, react with various organic azides to form the corresponding triazoles.268 A series of boronated thioesters of protoporphyrin IX241 and meso-tetra(4-carboxyphenyl)porphyrin240 were prepared by acylation of the mercapto derivative of the closo-dodecaborate anion. Addition of the mercapto derivative of the closo-dodecaborate anion [B12H11SH]2− to the double bond of the maleimide group was used earlier to obtain boron-containing streptavidin.284 Subsequently this approach was successfully used for the synthesis of other boronated peptides,285–289 dendrimers290 and water-soluble BODIPY dyes.291 Another approach to boron-containing polypeptides has been developed based on the reaction of [B12H11SH]2− with activated terminal cysteines via the formation of a disulfide bond.292 The mercapto derivative of closo-dodecaborate anion [B12H11SH]2− can be arylated with electron-deficient chloroaromatics and alkylsulfonyl heteroaromatics. The reaction of [B12H11SH]2− with 4-chloro-7-nitrobenzo[c][1,2,5]oxadiazole in acetonitrile in the presence of K2CO3 leads to the corresponding arylthiol derivative,283 while the reactions with 3-alkylsulfonyl-5-phenyl-, 5-(40 -bromophenyl)- and 5-methylamino-6-methylcarboxamido-1,2,4-triazines in acetonitrile in the presence of lutidine result in the corresponding boron-containing 1,2,4-triazines.293,294 Surprisingly, that the similar reaction with 3-ethylsulfonyl-5,7-dimethyl5,6,7,8-tetrahydropyrimido-[4,5-e][1,2,4]-triazine-6,8-dione results in the thiosulfinate [B12H11SS(O)Et]2− (Scheme 54).293

Scheme 54

Substitution in 1,7- and 1,12-bis(dimethylsulfonium) derivatives of the closo-dodecaborate anion [1,7-(Me2S)2B12H10] and [1,12-(Me2S)2B12H10] has been studied. The reaction of [1,7-(Me2S)2B12H10] with N-chlorosuccinimide in refluxing acetonitrile results in the monochloro derivative [9-Cl-1,7-(Me2S)2B12H9], while the dichloro derivative [9,10-Cl2-1,7-(Me2S)2B12H8] was isolated from the reaction of [1,7-(Me2S)2B12H10] with MeSO2Cl in dichloromethane in the presence of AlCl3. The bromination of [1,7-(Me2S)2B12H10] with Br2 in dichloromethane at room temperature leads to the monobromide [9-Br-1,7-(Me2S)2B12H9] or the dibromide [9,10-Br2-1,7-(Me2S)2B12H8] depending on the reagent ratio. The reaction of [1,7-(Me2S)2B12H10] with ICl in refluxing dichloromethane results in the monoiodo [9-I-1,7-(Me2S)2B12H9] or the dibromo [9,10-I2-1,7-(Me2S)2B12H8] derivatives depending on the reagent ratio. The same results were obtained using I2 in dichloromethane in the presence of AlCl3. The cross-coupling of [9-I-1,7-(Me2S)2B12H9] and [9,10-I2-1,7-(Me2S)2B12H8] with Grignard reagents in the presence of 5 mol% [(Ph3P)2PdCl2] in THF result in the corresponding alkyl/aryl derivatives [9-R-1,7-(Me2S)2B12H9] and [9,10-R2-1,7-(Me2S)2B12H8]

Polyhedral Boranes and Carboranes

247

(R ¼ Me, Bn, Ph). Reactions of [1,7-(Me2S)2B12H10] with 2,4-(NO2)2C6H3SCl and PhSeBr in refluxing acetonitrile lead to [9-(20 ,40 -(NO2)2C6H3S)-1,7-(Me2S)2B12H9] and [9,10-(PhSe)2-1,7-(Me2S)2B12H8], respectively.295 Reaction of [1,7-(Me2S)2B12H10] with DMSO upon heating in acidic solution produces [1,7,9-(Me2S)3B12H9]+, which losses the methyl cation on aqueous work-up resulting in [1,7-(Me2S)2-9-MeS-B12H9].296 The reaction of [1,12-(Me2S)2B12H10] with Me2S
Br2 in dichloromethane results in the monobromo derivative [2-Br-1,12(Me2S)2B12H9], whereas the monoiodo derivative [2-I-1,12-(Me2S)2B12H9] was prepared by the treatment of [1,12-(Me2S)2B12H10] with ICl in refluxing acetonitrile. The cross-coupling of [2-I-1,12-(Me2S)2B12H9] with Grignard reagents in the presence of 5 mol% [(Ph3P)2PdCl2] in THF gives [2-R-1,12-(Me2S)2B12H9] (R ¼ Me, Bn, Ph).295 The reaction of the closo-dodecaborate anion [B12H12]2− with thiocyanogen (SCN)2 generated in situ from NaSCN and CuCl2 in aqueous solution results in mono- and disubstituted thiocyano derivatives [B12H11SCN]2− and [1,7-B12H10(SCN)2]2−, depending on the reagent ratio.297

9.05.3.1.4

Derivatives with BdN bonds

The synthesis of the ammonium derivative of the closo-dodecaborate anion [B12H11NH3]− by the reaction of the patent closo-dodecaborate with hydroxylamine-O-sulfonic acid was described previously.298,299 Recently, an alternative method for the synthesis of the ammonium derivative was proposed through the hydrolysis of the nitrilium derivative of the closo-dodecaborate anion, itself formed by the reaction of the parent closo-dodecaborate with acetonitrile in the presence of trifluoroacetic acid.190,300 The ease of protonation of the amino group is due to the strong electron-donating character of the closo-dodecaborate moiety.301,302 Therefore, the amino derivative of the closo-dodecaborate anion [B12H11NH2]2− can act as a good ligand in complexes with various transition metals. The reaction of Na2[B12H11NH2] with [(Ph3P)AuCl] in THF in the presence of 18-crown-6 results in [Na(18-crown-6)]{(Ph3P)Au[B12H11NH2-k1-N]}. The reaction with [(THF)2NiBr2] gives the square planar nickel complex {Na6(THF)16(Ni[B12H11NH2-k1-N]4)}. The reaction of Na2[B12H11NH2] with [(Ph3P)3RhCl] in THF leads to the square planar rhodium complex (Ph3PMe){(Ph3P)2Rh[B12H11NH2-k2-N,H]} where the amino derivative acts as bidentate ligand (Fig. 46).303

Fig. 46 X-ray structures of anionic complexes {(Ph3P)Au[B12H11NH2-k1-N]}− (top), {Ni[B12H11NH2-k1-N]4}6− (bottom left) and {(Ph3P)2Rh[B12H11NH2-k2-N,H]}− (bottom right).

The reaction of Na2[B12H11NH2] with [(Ph3P)3RuCl2] in THF results in the octahedral anionic ruthenium complex (Bu3NMe)[(Ph3P)2RuCl{B12H11NH2-k3-N,H,H}] where the amino derivative acts as a bidentate ligand. The neutral ruthenium complex [(Ph3P)2(CO)Ru{B12H11NH2-k3-N,H,H}] was obtained by bubbling CO through a solution of (Bu3NMe)[(Ph3P)2RuCl {B12H11NH2-k3-N,H,H}] in THF (Fig. 47). Another ruthenium complex (Bu4N)[(dppb)RuCl{B12H11NH2-k3-N,H,H}] was prepared in a similar way using [(Ph3P)(dppb)RuCl2].304

248

Polyhedral Boranes and Carboranes

Fig. 47 X-ray structures of {(Ph3P)2RuCl[B12H11NH2-k3-N,H,H]}− (left) and {(Ph3P)2(CO)Ru[B12H11NH2-k3-N,H,H]} (right).

A series of the trialkylammonium derivatives [B12H11NR3]− (R ¼ Me, Et, Pr, Bu, n-C5H11, i-C5H11, C6H13, C12H15, CH2C^CH, (CH2)3CH]CH2, (CH2)3Ph) was prepared by alkylation of [B12H11NH3]− with the corresponding alkyl bromides in acetonitrile or DMF in the presence of KOH.239,305–307 The dialkylammonium derivatives [B12H11NHR2]− (R ¼ i-Pr, Bn, CMe2C^CH, (CH2)2C6H4NO2, (CH2)3C^CH) were obtained using more sterically hindered alkyl bromides or less active alkyl chlorides.238,239,306 The methylammonium derivative [B12H11NH2Me]− was prepared by the reaction of the parent closo-dodecaborate with N-methylhydroxylamine-O-sulfonic acid.307 A series of the nitrophenethyl-ammonium derivatives [B12H11NH2CH2CH2 C6H4-x-NO2]− (x ¼ o, m, p) was obtained via treatment of the corresponding di(nitrophenethyl)ammonium derivatives with [Me4N]OH in methanol.238 The heterosubstituted trialkylammonium derivatives [B12H11NR2R0 ]− (R0 ¼ Me, R ¼ Et, C12H25; R0 ¼ Bn, R ¼ Et) were obtained by alkylation of the methyl- and benzylammonium derivatives.306 The reactions of the closo-dodecaborate anion with chlorinated iminium salts (Vilsmeier reagents) lead to mixtures of mono- and disubstituted alkyl/arylammonium derivatives [B12H11NRR0 R00 ]− and [1,7-B12H10(NRR0 R00 )2] (R ¼ R0 ¼ Me, R00 ¼ CH2Cl; R ¼ Me, R0 ¼ Ph, R00 ¼ CH2Cl; R ¼ R0 ¼ (CH2CH2)2O, R00 ¼ CH2Cl; R ¼ R0 ¼ Me, R00 ¼ CHCl2).308 The ammonium derivative of the closo-dodecaborate anion can be arylated with electron-deficient chloroaromatics: the reaction of [B12H11NH3]− with 1-chloro-2,4-dinitrobenzene in ethanol in the presence of EtONa leads to the corresponding arylamino derivative [B12H11NHC6H3-2,4-(NO2)2]2−.309 The phenyldimethylammonium derivative (Bu4N)[B12H11NMe2Ph] was obtained by heating (Bu4N)2[B12H12] in N,N-dimethylaniline at 200  C. Interestingly, that the similar reaction with 4-dimethylamino pyridine leads to the pyridinium derivative [B12H11NC5H4-4-NMe2]−.309 Acylation of [B12H11NH2]2− with aryl chlorides ArCOCl leads to amides [B12H11NHC(O)Ar]2− (C6H5, C6H4-4-F, C6H4-4-I, C6H4-4-OMe), which can be reversibly protonated with the formation of the corresponding iminols [B12H11NH]C(OH)Ar]−. In the case of the picolinic acid amide, protonation takes place at the pyridine ring resulting in [B12H11NHC(O)C5H4NH]−.310,311 Boronated amides of meso-tetra(4-carboxyphenyl)porphyrin240 and protoporphyrin IX241 were prepared by acylation of the amino derivative of the closo-dodecaborate anion. The reaction of [B12H11NH2]2− with p-nitrophenyl(diphenylphosphoryl)acetate leads to the amide [B12H11NH2C(O)CH2P(O)Ph2]−, whereas the reaction with chlorodiphenylphosphine gives diphenylphosphinamide [B12H11NH(P(O)Ph2)2]−.232 The combination of N,N-dimethylformamide with 2,4,6-trimethylbenzoyl chloride produces the chlorinated iminium salt, which upon attack by [B12H11NH2]2− gives amidine [B12H11NH]CHNMe2]−.311 Amidines [B12H11NH]C(Ar)NHR]− can be prepared by activation of the arylamides [B12H11NHC(O)Ar]2− by pentafluorobenzoyl chloride followed by the treatment with amines RNH2.311 Reactions of [B12H11NH2]2− with aryl isocyanates ArNCO lead to boron-containing ureas [B12H11NHC(O) NHAr]2− (Ar ¼ C6H5, C6H4-4-Cl).311 The reaction of [B12H11NH2]2− with dimethylcarbamoyl chloride Me2NCOCl in DMF leads to the urea [B12H11NHC(O)NMe2]2−.312 which, when heated in water, gives the isocyanate derivative of the closo-dodecaborate anion [B12H11NCO]2−.311 The reaction of the boron-containing urea [B12H11NHC(O)NMe2]2− with diarylacetylenes in the presence of Cu(OAc)2
H2O and 2.5 mol% [Cp RhCl2]2 in acetonitrile at room temperature leads to B-H activation of the boron cage with the insertion of an olefin, as well as to the formation of the boron–oxygen bond with the closure of the five-membered BNCOB diboraoxazole ring, leading to [1,2-m-NH]C(NMe2)O-B12H9-3-C(Ar)]CH(Ar)]− (Scheme 55).312 The reaction proceeds through the formation of the rhodium complex [Cp Rh(B12H11NHC(O)NMe2-k3-O,H,H)], in which the metal atom is coordinated by two BH groups of the closo-dodecaborate anion (Fig. 48).312 The reaction of this complex with diphenylacetylene in the presence of Cu(OAc)2 leads to [1,2-m-NH]C(NMe2)O-B12H9-3-C(Ph)]CH(Ph)]−. The closure of the five-membered BNCOB diboraoxazole ring with the formation of [1,2-m-NH]C(NMe2)O-B12H10]− also takes place when [B12H11NHC(O)NMe2]2− reacts with AgOAc in acetonitrile.312

Polyhedral Boranes and Carboranes

249

Scheme 55

Fig. 48 X-ray structure of [Cp Rh(B12H11NHC(O)NMe2-k3-O,H,H)].

It was previously shown that the reaction of the amino derivative of the closo-dodecaborate anion with aldehydes leads to the formation of imines [B12H11NH]CHR]−.313 Later, this approach was used to synthesize of a series of boron-containing polyarylimines (Scheme 56).314–316

Scheme 56

250

Polyhedral Boranes and Carboranes

The reaction of the amino derivative of the closo-dodecaborate anion [B12H11NH2]2− with carbodiimides RN]C]NR0 (R ¼ Et, R ¼ (CH2)3NMe2; R ¼ R0 ¼ (CH2)3NMe2; R ¼ R0 ¼ cycloC6H11) leads to the corresponding guanidinium derivatives [B12H11NHC(]NHR)NHR0 ]−.310 The perfluorinated ammonium derivative [B12F11NH3]− was prepared by the reaction of [B12H11NH3]− with gaseous fluorine in liquid hydrogen fluoride317 or acetonitrile.318 Alkylation of [B12F11NH3]− with Me2SO4 or C12H25Br in aqueous KOH result in the corresponding perfluorinated trialkylammonium derivatives [B12H11NR3]− (R ¼ CH3, C12H25).317 The perchlorinated ammonium derivative [B12Cl11NH3]− was prepared by heating [B12H11NH3]− in SbCl5 at 190  C319 or by the reaction of [B12H11NH3]− with gaseous chlorine in aqueous solution at 300  C.320 Alternatively, the perchlorinated ammonium derivative can be prepared by the reaction of the parent ammonium derivative with SO2Cl2 in refluxing acetonitrile.243,321 Methylation of [B12Cl11NH3]− with MeI or Me2SO4 in alkaline aqueous solution results in the trimethylammonium derivative [B12Cl11NMe3]−.243,319,321 Surprisingly, the alkylation of [B12Cl11NH3]− with butyl iodide in acetonitrile in the presence of KOH alkaline aqueous solution results in the monoalkylamino derivative [B12Cl11NH2Bu]−, which on the treatment with MeI gives [B12Cl11NHBuMe]−.307 The hexachlorinated tripropylammonium closo-dodecaborate [B12H5Cl6NPr3]− was prepared by the chlorination of [B12H11NPr3]− with Cl2307 or SOCl2321 in acetonitrile. The perbrominated ammonium derivative [B12Br11NH3]− was prepared by the reaction of [B12H11NH3]− with bromine in aqueous solution at 300  C.320 The reaction of [B12Br11NH3]− with epibromohydrin in THF in the presence of NaH leads to the corresponding alkylammonium derivative [B12Br11NH2CH2CH(Me)OH]−, while the reaction with ethylene oxide unexpectedly results in the hydroxylammonium derivative [B12Br11NH2OH]−.320 The similar hydroxylammonium derivative [B12H11NH2OH]− was obtained by the reaction of [B12H11NH3]− with t-BuOOH.190 The hexabrominated trialkyammonium closo-dodecaborates [B12H5Br6NR3]− (R ¼ C2H5, C3H7, C4H9, C5H11) were prepared by the treatment of [B12H11NR3]− with N-bromo succinimide in acetonitrile.307 The periodinated ammonium derivative [B12I11NH3]− was prepared by the reaction of [B12H11NH3]− with ICl in 1,2-dichloroethane at 300  C.320 The perchlorinated ammonium derivative [B12Cl11NMe3]− undergoes one-electron reversible oxidation with AsF5 in liquid SO2 leading to the formation of the hypercloso-[B12Cl11NMe3]− radical anion, which is a strong oxidizing agent capable of oxidizing hexabromobenzene to the corresponding cation and elemental iodine to the [I5]+ cation.322 The oxidation of [B12X11NH3]− (X ¼ F, Cl, Br, I) with hydrogen peroxide in an alkaline solution results in the corresponding perhalogenated nitro closo-dodecaborates [B12X11NO2]2−.323 Heating the ammonium derivative [B12H11NH3]− in 30% hydrogen peroxide leads to [B12(OH)11NH3]−, which, when treated with H2O2 in an alkaline solution, gives the corresponding nitro derivative [B12(OH)11NO2]2−. The latter compound is a useful synthon for the synthesis of vertex-differentiated derivatives of the closo-dodecaborate anion by acylation of [B12(OH)11NO2]2−, followed by the reduction of the nitro group to the amine one and its acylation to form [B12(OC(O) R)11NHC(O)R0 ]2−.324 A complete set of disubstituted ammonium derivatives of the closo-dodecaborate anion 1,2-, 1,7- and 1,12B12H10(NH3)2 and their perfluorinated analogs 1,2-, 1,7- and 1,12-B12F10(NH3)2 was obtained by the reaction in of the parent closo-dodecaborate [B12H12]2− with hydroxylamine-O-sulfonic acid followed by fluorination with F2 in acetonitrile. The isomers were separated by column chromatography and characterized by multi-nuclear NMR and single crystal X-ray diffraction.325 0

9.05.3.1.5

Derivatives with BdP bonds

The triphenylphosphonium derivative of the closo-dodecaborate anion [B12H11PPh3]− was prepared by the reaction of the iodo derivative [B12H11I]2− with [(Ph3P)4Pd] in THF in the presence of Na2CO3.326 The reaction of the parent closo-dodecaborate [B12H12]2− with [(PhMe2P)2PdCl2] in THF in the presence of NaBH4 lead to a mixture of [B12H11PMe2Ph]−, 1,7- and 1,12B12H10(PMe2Ph)2.327 A series of phosphonium derivatives [B12H11PPhxRy]− (R ¼ Me, x ¼ 2, y ¼ 1; R ¼ Me, x ¼ 1, y ¼ 2; R ¼ Et, x ¼ 2, y ¼ 1) was obtained by microwave heating of the corresponding alkyl triphenyl- and dialkyldiphenylphosphonium salts of the closo-dodecaborate anion in acetonitrile at 180  C.328

9.05.3.1.6

Derivatives with BdC bonds

A series of boronated esters [B12H11COOR]2− (R ¼ Me, i-Pr, t-Bu) and amides [B12H11CONR0 R00 ]2− (R0 ¼ H, R00 ¼ Bu; R0 ¼ R00 ¼ Bu) were obtained by reactions of the carbonyl derivative [B12H11CO]− with alcohols and amines, respectively.329 The ethynyl derivative of the closo-dodecaborate anion [B12H11C^CH]2− was prepared by the microwave-assisted cross-coupling reaction of the iodo derivative [B12H11I]2− with Me3SiC^CMgBr in the presence of 5 mol% [(Ph3P)2PdCl2] followed by removal of the Me3Si protecting group by alkaline hydrolysis.330 The percyano derivative [B12(CN)12]2− was prepared by the microwaveassisted reaction of the periodo derivative [B12I12]2− with CuCN in the presence of catalytic amount of PdCl2.331 Due to the strong electron-donating effect of the closo-dodecaborate cage, the cyano groups in [B12(CN)12]2− can act as donors in complexes with transition metal atoms (Fig. 49).331

Polyhedral Boranes and Carboranes

251

Fig. 49 X-ray structure of {1,12-((MeCN)3Cu)2[m-B12(CN)12]}.

9.05.3.2

closo-Undecaborate anion [B11H11]2−

Despite the relatively easy accessibility of the undecahydro-closo-undecaborate anion, its chemistry is rather poorly studied. The development of the chemistry of the closo-undecaborate anion [B11H11]2− was recently reviewed.332

9.05.3.3 9.05.3.3.1

closo-Decaborate anion [B10H10]2− General aspects. Halogen derivatives

In general, the chemistry of the closo-decaborate anion is comparable in terms of its knowledge with the chemistry of the closo-dodecaborate anion. However, unlike the [B12H12]2− anion, which has an icosahedral structure, the [B10H10]2− anion has a bicapped antiprism structure. This results in a large difference in the reactivity of the apical (B(1) and B(10)) and equatorial (B(2)B(9)) vertices, which leads to the use of different types of reaction for their functionalization. In this case, the reactivity of the equatorial positions resembles the reactivity of the closo-dodecaborate anion. An early stage in the development of the chemistry of the closo-decaborate anion [B10H10]2− was reviewed by Sivaev et al.333 and Zhizhin et al.334 Despite the fact that the synthesis and properties of simple and complex salts of the closo-decaborate anion are outside the scope of this publication, we found it appropriate to give here some recent reviews in this area.175–179 The convenient method for the synthesis of apically substituted iodo derivatives of the closo-decaborate anion has been proposed. The reaction of [B10H10]2− with phenyliodonium diacetate PhI(OAc)2 in aqueous acetic acid at 0  C leads to the mono- and di(phenyliodonium) derivatives [1-B10H9IPh]− and [1,10-B10H8(IPh)2] depending on the reagent ratio.187 The same approach can be applied to some substituted derivatives of the closo-decaborate anion.187,335 The treatment of the phenyliodonium derivatives with n-BuLi in THF at 0  C results in [1-B10H9I]2− and [1,10-B10H8I2]2−, respectively.336 Synthesis of the perhalogenated derivatives of the closo-decaborate anion [B10X10]2− (X ¼ Cl, Br, I) by reactions of the parent closo-decaborate with Cl2, Br2 and I2, respectively, in aqueous solution at 270  C has been described.320 Alternatively, the periodinated closo-decaborate anion [B10I10]2− can be obtained via microwave-assisted iodination of the parent closo-decaborate with I2 in acetic acid at 220  C.184 It was found that the perchlorinated closo-decaborate anion [B10Cl10]2− can be obtained by chlorination of the closo-undecaborate anion [B11H11]2− anion with N-chlorosuccinimide or molecular chlorine in dichloromethane, that opens the way to its synthesis bypassing the preparation of the parent closo-decaborate anion from decaborane.337

9.05.3.3.2

Derivatives with BdO bonds

In addition to the previously reported methods for the preparation of cyclic oxonium derivatives of the closo-decaborate anion, the syntheses of mono- and disubstituted 1,4-dioxane [2-B10H9O(CH2CH2)2O]− and [2,7(8)-B10H8(O(CH2CH2)2O)2] and tetrahydropyran [2-B10H9O(CH2)5]− and [2,7(8)-B10H8(O(CH2)5)2] oxonium derivatives by reactions of the parent closo-decaborate with the corresponding cyclic ethers in the presence of triflic acid have been described.338 The similar 2-methyltetrahydrafuran derivatives of the closo-decaborate anion were also prepared.338 The mono- and disubstituted cyclic oxonium derivatives of the closo-decaborate anion can be also obtained using various Lewis acids as initiators.339 The ring-opening of the oxonium derivatives with azide340 and cyanide341 anions has been described. The carboxylic acid [2-B10H9O(CH2)4COOH]2− was obtained by alkaline hydrolysis of the corresponding nitrile.341 The azido derivative [2-B10H9O(CH2)4N3]2− was used for the synthesis of boron-containing 1,2,3-triazoles through Cu-catalyzed [3 + 2] azide-alkyne cycloaddition reactions.342 The amino derivatives [2-B10H9(OCH2CH2)2NH3]− and [2-B10H9O(CH2)4NH3]− were prepared by the ring-opening of the corresponding oxonium derivatives with potassium phthalimide in DMF followed by deprotection with hydrazine hydrate.340 Alternatively, [2-B10H9(OCH2CH2)2NH3]− was obtained by the reaction of the 1,4-dioxane derivative of the closo-decaborate with ammonia in refluxing ethanol.343 The ring-opening of the 1,4-dioxane derivative with various amines was also described.340,343,344

252

Polyhedral Boranes and Carboranes

A series of boron containing carboxylic acids [2-B10H9(OCH2CH2)2OC6H4-x-COOH]− and [2-B10H9O(CH2)4OC6H4-x-COOH]− (x ¼ m and p) were prepared by ring-opening of the corresponding oxonium derivatives with methyl esters of hydroxybenzoic acids in acetonitrile in the presence of K2CO3, followed by acidic hydrolysis of the ester formed.341 The use of unprotected hydroxybenzoic acids leads to the products of the simultaneous ring opening with hydroxy and carboxy groups.341 The reactions of the cyclic oxonium derivatives with aminoacids (glycine, cysteine, serine, p-aminobenzoic acid) result in the corresponding products of ring-opening with carboxy group.345 The boronated tyrosine derivatives [2-B10H9(OCH2CH2)2OC6H4-p-CH2CH(NH3)COOH]− and [2-B10H9O(CH2)4OC6H4-p-CH2CH(NH3)COOH]− were prepared by the ring-opening of the corresponding cyclic oxonium derivatives with ethyl N-trifluoroacetyl-L-tyrosinate in the presence of K2CO3, followed by acid hydrolysis of protecting groups.201 The ring-opening reactions of the 1,4-dioxane derivative of the closo-dodecaborate anion by various nucleophiles derived from organic CH-acids (acetylenes, diethyl malonate, ethyl acetoacetate, triethyl orthoformate, acetylacetone, malononitrile) were described.346 Acyclic oxonium derivatives (Bu4N)[2-B10H9OR2] (R ¼ Et, i-Pr, Bu) were prepared by heating the protonated closo-decaborate anion (Bu4N)[B10H11] in mixtures of the corresponding ethers and dichloromethane at 80  C. The treatment of this oxonium derivatives with hydrazine hydrate leads to the corresponding alkoxy derivatives [2-B10H9OR]2−.347 Reaction of the closo-decaborate [B10H10]2− with refluxing formic acid results in the mono- and di(formyloxy) derivatives [2-B10H9OC(O)H]2− and [2,7(8)-B10H8(OC(O)H)2]2−.348 The formyloxy derivative [Co(phen)3][2-B10H9OC(O)H] was also obtained from the reaction of (Et3NH)2[B10H10] with CoCl2 in N,N-dimethylformamide in the presence of 1,10-phenanthroline (phen).349 The dimethylformamide derivative [2-B10H9OCHNMe2]− was prepared by heating the parent closo-decaborate with DMF in the presence of trifluoroacetic acid.349 Heating the parent closo-decaborate in acetic acid at 70  C results in the formation of the acetoxy derivative [2-B10H9OAc]2−.349 while the reaction at 120  C leads to a mixture of [2,7(8)-B10H8(OAc)2]2− and [2,6(9)-mMeCO2-B10H8]−.348,350,351 It was found that [2,6(9)-m-MeCO2-B10H8]− is formed by heating the acetoxy derivative [2-B10H9OAc]2− in boiling 1,4-dioxane.352 (Ph4P)[2,6(9)-m-MeCO2-B10H8] was also obtained by the reaction of (Ph4P)2[B10H10] with Pb(NO3)2 in acetic acid.353 The hydroxy derivatives [2-B10H9OH]2− and [2,7(8)-B10H8(OH)2]2− were obtained by treating the corresponding acetoxy derivatives with hydrazine hydrate,350 whereas [2,7(8)-B10H8(OH)(OAc)]2− was obtained by alkaline hydrolysis of [2,7(8)B10H8(OAc)2]2− in aqueous ethanol.348 In a similar way, [2,6(9)-B10H8(OH)(OAc)]2− was prepared by alkaline hydrolysis of [2,6(9)-m-MeCO2B10H8]− in aqueous ethanol.348 The lead complex {(bipy)2Pb[2,6(9)-B10H8(OH)(OAc)]} was obtained from the reaction of (Ph4P)[2,6(9)-m-MeCO2B10H8] with Pb(NO3)2 and 2,20 -bipyridine (bipy) in acetonitrile.348 Alkylation of [2-B10H9OH]2− and [2,7(8)-B10H8(OH)2]2− with dihalosilanes R2SiCl2 (R ¼ Me, t-Bu, Ph) was described.354 The apically substituted dimethylformamide derivative [1,10-B10H8(OCHNMe2)2] was prepared by heating [1,10-B10H8(IPh)2] in N,N0 -dimethylformamide at 90  C. The treatment of [1,10-B10H8(OCHNMe2)2] with [Bu4N]OH in refluxing acetonitrile results in the formation of the diformate (Bu4N)2[1,10-B10H8(OC(O)H)2], which in turn is hydrolyzed with HCl in methanol giving the 1,10-dihydroxy derivative (Bu4N)2[1,10-B10H8(OH)2]. In a similar way, the heating [1,10-B10H8(IPh)2] in N,N0 -dimethylacetamide at 100  C produces [1,10-B10H8(OC(Me)NMe2)2] which on the treatment with [Bu4N]OH in acetonitrile gives the diacetate (Bu4N)2[1,10-B10H8(OAc)2]. The acylation of the 1,10-dihydroxy derivative with benzoyl chloride in THF in the presence of NaH leads to the corresponding diester (Bu4N)2[1,10-B10H8(OC(O)Ph)2].355 The reaction of [1,10-B10H8(IPh)2] with 1 equiv. (Et4N)OAc in acetonitrile at 60  C leads to (Et4N)[1,10-B10H8(IPh)(OAc)],187 which reacts with pyridine at 90  C giving (Et4N) [1,10-B10H8(Py)(OAc)].335 (Me4N)[1,10-B10H8(Py)(OEt)] was obtained by heating of [1,10-B10H8(Py)(IPh)] in ethanol at 110  C.335

9.05.3.3.3

Derivatives with BdS bonds

Reactions of the protonated form of closo-dodecaborate, [B10H11]−, with various thiocarbonyl compounds were studied. The reaction of [B10H11]− with N,N0 -dimethylthioformamide leads to [2-B10H9SCHNMe2]−356; the reactions with N,N,N0 ,N0 -tetramethyl- and N,N0 -diphenylthioureas produce the corresponding thiourea derivatives [2-B10H9SC(NRR0 )2]− (R ¼ R0 ¼ Me; R ¼ H, R0 ¼ Ph),356,357 which upon treatment with hydrazine in refluxing ethanol give the 2-mercapto derivative [2-B10H9SH]2−.356 The reaction of the protonated closo-dodecaborate [B10H11]− with N,N0 -ethylenethiourea unexpectedly leads directly to the mercapto derivative [2-B10H9SH]2−.357 Acylation of the mercapto derivative with acyl chlorides RCOCl in acetonitrile in the presence of triethylamine leads to the corresponding thioesters [2-B10H9SC(O)R]2− (R ¼ M, Ph).356 Alkylation of the mercapto derivative [2-B10H9SH]2− with various alkyl halides in acetonitrile or N,N0 -dimethylformamide in the presence of K2CO3 at 80–90  C results in the corresponding dialkylsulfonium derivatives [2-B10H9SR2]− (R ¼ C3H7, i-C3H7, C4H9, C8H17, C12H25, C18H37, CH2CH]CH2, CH2C6H5).356,358,359 This approach was used for synthesis of dialkylsulfonium derivatives bearing various functional groups [2-B10H9SR2]− (R ¼ CH2COOEt, CH2CONH2, (CH2)nN(CO)2C6H4 (n ¼ 1–3)).358,360 Removal of the phthalimide protecting group with hydrazine in ethanol gives the corresponding amines [2-B10H9S((CH2)nNH2)2]− (n ¼ 1–3).360 The cyclic sulfonium derivatives [2-B10H9S(CH2)4]− and [2-B10H9S(CH2CH2)2O]− were prepared by the alkylation of [2-B10H9SH]2− with 1,4-dibromobutane and 2-chloroethyl ether, respectively, in DMF in the presence of Cs2CO3 at 80  C or, alternatively, by the direct reaction of the closo-decaborate anion with tetrahydrothiophene and 1,4-thioxane in the presence of AlCl3.361 The reaction of [2-B10H9SH]2− with 1,2-dibromoethane results in the formation of the 1,4-dithiane derivative [m-S(CH2CH2)2S-(2-B10H9)2]2−.362 The akylthio derivatives [2-B10H9SR]2− (R ¼ i-Pr, CH2N(CO)2C6H4) were obtained by alkylation of [2-B10H9SH]2− with sterically hindered alkyl halides under milder conditions (DMF, 40  C).356,360

Polyhedral Boranes and Carboranes

253

Chlorination of the dialkylsulfonium derivatives of the closo-dodecaborate anion [2-B10H9SR2]− with SO2Cl2 in acetonitrile leads to the corresponding perchloro derivatives [2-B10Cl9SR2]− (R ¼ C3H7, i-C3H7, C4H9, C8H17, C12H25, C18H37, CH2C6H5, (CH2)nN(CO)2C6H4 (n ¼ 1–3); R2 ¼ (CH2)4, (CH2CH2)2O).360,361,363 The perbromo derivatives [2-B10Br9SR2]− (R ¼ (CH2)nN (CO)2C6H4 (n ¼ 1–3); R2 ¼ (CH2)4, (CH2CH2)2O) were prepared by bromination of the corresponding dialkylsulfonium derivatives [2-B10H9SR2]− with bromine in acetonitrile.361,364 The brominated amines [2-B10Br9S((CH2)nNH2)2]− (n ¼ 1–3) were obtained by the treatment of the corresponding phthalimide derivatives with methylamine in refluxing ethanol.364 The apically substituted thiocyanate derivative (Et4N)[1,10-B10H8(Py)(SCN)] was prepared by the reaction of [1,10B10H8(IPh)2] with 1 equiv. of (Et4N)SCN acetonitrile at 60  C followed by heating the formed (Et4N)[1,10-B10H8(IPh)(SCN)] in pyridine at 60  C or, alternatively, by the reaction of [1,10-B10H8(Py)(IPh)] with (Et4N)SCN in refluxing acetonitrile.335

9.05.3.3.4

Derivatives with BdN bonds

It was demonstrated previously that nitrilium derivatives of the closo-decaborate anion [2-B10H9N^CR]−, formed by the reaction of the [B10H10]2− anion with nitriles in the presence of strong acids, can be considered as useful precursors for the synthesis of various functional derivatives through the addition of nucleophiles to the activated triple bond (Scheme 57).333 More recently, the synthesis of a series of nitrilium derivatives [2-B10H9N^CR]− (R ¼ Me, Et, Pr, t-Bu, Ad, (CH2)nCN (n ¼ 2–4), Ph, Naph), by heating the protonated closo-decaborate [B10H11]− with the corresponding nitriles was described.300,365,366 The addition of water to the activated triple bond proceeds smoothly resulting in the boronated iminols [2-B10H9NH]C(OH)R]−.365,367 The latter species are also formed when trying to chromatographic purification of the nitrilium derivatives.368 The reactions of the iminols [2-B10H9NH]C(OH)R]− with PhI(OAc)2 in THF at 90  C lead to closure of the five-membered cycle, with formation of diboraoxazoles [2,1-m-NH]C(R)O-B10H8]− (R ¼ Me, Et, Pr, i-Pr, t-Bu, Ph, C6H4-4-Cl).367 The treatment of the diboraoxazoles with hydrazine hydride in refluxing ethanol results in [1-OH-2-NH3-B10H8]− (Scheme 58).367

Scheme 57

Scheme 58

The addition of alcohols R0 OH to the nitrilium derivatives [2-B10H9N^CR]− leads to the imidates [2-B10H9NH]C(OR0 )R]−.369 One of the most studied reactions is the addition of amines to the activated triple bond of the nitrilium derivatives of the closo-decaborate anion leading to the formation of the boronated amidines [2-B10H9NH]C(NR0 R00 )R]−. The reaction is applicable to both primary370,371 and secondary372,373 amines. This approach was used for synthesis of boron-containing aminoacids374–377 and porphyrins.378–380 The nitrilium derivatives of the closo-decaborate anion also react with other nucleophiles, such as hydrazines and hydrazones,381 oximes,382–384 as well various carbanions derived from azomethine ylides,385 malononitrile,386 benzoylacetonitrile386 and ethyl benzoylacetate.386 The nitrilium derivatives of the closo-decaborate anion, like organic nitrilium salts, are able to participate in 1,3-dipolar cycloaddition reactions. The reactions of [2-B10H9N^CR]− with sodium azide result in the corresponding boronated tetrazoles [2-B10H9N5C-50 -R]2−.387 whereas the reactions with nitrones ArCH ¼ N+(Me)O− lead to the corresponding boronated 2,3-dihydro-1,2,4-oxadiazoles [2-B10H9{N]CRON(Me)CHAr}]−.388,389

254

Polyhedral Boranes and Carboranes

The reaction of the isocyanate derivative [2-B10H9NCO]2−390 with 4-aminophenethylamine in DMF in the presence of triethylamine leads to the closo-decaborate based urea [2-B10H9NHC(O)NHCH2CH2C6H4-p-NH2]2−, treatment of which with thiocarbonyl diimidazole gives the corresponding thiocyanate [2-B10H9NHC(O)NHCH2CH2C6H4-p-NCS]2−.391 Reduction of the nitrilium derivatives of the closo-decaborate anion [2-B10H9N^CR]− with LiAlH4 in THF results in the alkylammonium derivatives [2-B10H9NH2CH2R]−.300 Treatment of the nitrilium derivative [2-B10H9N^CMe]− with hydrazine hydrate in refluxing ethanol results in the ammonium derivative [2-B10H9NH3]−.392 The perhalogenated ammonium derivatives [2-B10X9NH3]− (X ¼ Cl, Br, I) were prepared by the reaction of the parent closo-decaborate with the corresponding halogens in 1,2-dichloroethane at 80  C.392 The trimethylammonium derivatives [2-B10X9NMe3]− (X ¼ Cl, Br, I) were prepared by the reaction of the corresponding ammonium derivatives with methyl iodide in acetonitrile in the presence of K2CO3 at room temperature.392 The reactions with benzyl bromide under the same conditions produce the benzylammonium perbromo and periodo [2-B10X9NH2Bn]− (X ¼ Br, I) and dibenzylammonium perchloro [2-B10Cl9NHBn2]− derivatives, whereas the synthesis of the dibenzylammonium perbromo and periodo derivatives [2-B10X9NHBn2]− (X ¼ Br, I) requires heating at 80 and 100  C, respectively.392 The alkylation of the perhalogenated ammonium derivatives [2-B10X9NH3]− with epibromohydrin in acetonitrile in the presence of K2CO3 leads to the corresponding boronated oxypropanes [2-B10X9NH2CH2C2H3O]− (X ¼ Cl, Br, I) which react with amines and phenolates giving the corresponding derivatives [2-B10X9NH2CH2CH(OH)CH2X]− (X ¼ NHBu, NHBn, NEt2, OC6H4-2-OMe).393 The pyridinium-substituted derivatives {(MeCN)2Cu[2-B10H9-20 ,200 -Bipy-k2-N,H]} (Fig. 50) and [(HNPy2)Cu(MeCN)2][2B10H9NC5H4-20 -NH-200 -C5H4N] were obtained by reactions of Cu2[B10H10] with 2,20 -bipyridine and 2,20 -bi(pyridyl)amine in acetonitrile.394 The 1,2-disubstituted phenanthroline derivative [1,2-m-N2C12H8-B10H8] was prepared by the CoCl2-mediated reaction of the parent closo-decaborate with 1,10-phenanthroline in acetonitrile or N,N0 -dimethylformamide.363,395

Fig. 50 X-ray structure of {(MeCN)2Cu[2-B10H9-20 ,200 -Bipy-k2-N,H]}.

The apically substituted pyridinium derivatives [1-B10H9Py]− and [1,10-B10H8(Py)2] were prepared by heating the corresponding phenyliodonium derivatives [1-B10H9IPh]− and [1,10-B10H8(IPh)2] in pyridine at 80  C.335 In a similar way, a series of the para-substituted pyridinium derivatives [1-B10H9NC5H4-p-X]− (X ¼ Me, OMe, CN, COOEt)335 and [1,10-B10H8(NC5H4-p-OR)2] (R ¼ C8H17, C10H21, C12H25, C14H29, C16H33, C18H37)396 were prepared by heating the phenyliodonium derivatives with the corresponding para-substituted pyridines. The cyclic dialkylammonium derivative [1-B10H9NH(CH2CH2)2O]− was prepared by heating [1-B10H9IPh]− in morpholine at 80  C. Its reaction with PhI(OAc)2 in acetic acid followed by heating formed [1,10B10H8(IPh)(NH(CH2CH2)2O)] in pyridine at 80  C leads to [1,10-B10H8(Py)(NH(CH2CH2)2O)].335 A series of apically substituted amines [1-B10H9NH2(CH2)nNH2]2− (n ¼ 2–4, 6, 9) were prepared by heating the diazonium derivative [1-B10H9N2]− in the corresponding amines at 120  C.397 Some of them were used for the subsequent modification and preparation of boron-containing silica nanoparticles.398,399 The apically substituted azido derivative (Bu4N)2[1-B10H9N3] was prepared by the reaction of (Bu4N)[1-B10H9IPh] with (Bu4N)N3 in acetonitrile at 55  C.335 The apically substituted nitro derivatives of the closo-decaborate anion (Ph4P)2[1-B10H9NO2] and (Ph4P)2[1,10-B10H8(NO2)2] were prepared by UV irradiation of solutions (Ph4P)2[B10H10] in nitroalkanes RNO2 (R ¼ Et, Pr, i-Pr, t-Bu) at 450 and 350 nm, respectively.400

9.05.3.3.5

Derivatives with BdC bonds

A series of the closo-decaborate derived esters [2-B10H9C(O)OR]2− (R ¼ Me, Et, Pr, i-Pr, Bu, t-Bu) was prepared by the reaction of the carbonyl derivative [2-B10H9CO]− with the corresponding alcohols.401 In a similar way, amides [2-B10H9CONR0 R00 ]2− (R0 ¼ H, R00 ¼ Bu, (CH2)3Si(OEt)3; R0 ¼ R00 ¼ Bu) were obtained by the reaction of [2-B10H9CO]− with amines.398,402

Polyhedral Boranes and Carboranes

255

The cross-coupling reaction of [1-B10H9I]2− with MeO-p-C6H4MgBr in the presence of 1 mol% PEPPSI-IPr in refluxing THF leads to the corresponding apically substituted aryl derivative [1-B10H9C6H4-p-OMe]2−.336 The apically substituted cyano derivatives (Bu4N)2[1-B10H9CN] and (Et4N)2[1,10-B10H8(CN)2] were prepared by the reactions of the corresponding phenyliodonium derivatives (Bu4N)[1-B10H9IPh] and [1,10-B10H8(IPh)2] with (R4N)CN in acetonitrile at 55–65  C.335,355 Due to the electron-donating effect of the closo-decaborate cage, the cyano groups in [1,10-B10H8(CN)2]2− can act as donors in complexes with transition metal atoms (Fig. 51).403

Fig. 51 X-ray of {1,10-(Cp(dppe)Fe)2[m-1,10-B10H8(CN)2]}.

9.05.3.4

closo-Nonaborate anion [B9H9]2−

The closo-nonaborate anion [B9H9]2− belongs to the group of relatively inaccessible polyhedral boron hydrides and therefore its chemistry is poorly studied. It was found that the reaction of [B9H9]2− with HCl in dichloromethane results in the cage opening with formation of the 4,8-dichloro substituted arachno-nonaborate [4,8-B9H12Cl2]−, which reacts with liquid ammonia with the loss of the chlorine atoms and the cage-reclosing giving back the closo-nonaborate anion [B9H9]2−. The reaction with acetic acid in dichloromethane resulted in the complete cage destruction with the formation of the [B2O(OAc)5]− anion.404

9.05.3.5

closo-Octaborate anion [B8H8]2−

Since the closo-octaborate anion [B8H8]2− is formed on oxidation of [B9H9]2− in an alkaline medium, its chemistry is even less studied. It was found that protonation of [B8H8]2− with HCl in an aqueous solution or with Et3N
HCl in acetonitrile leads to the protonated form [B8H9]−. The crystal structures of (Bu4N)[B8H9] and (Ph4P)[B8H9] were determined by single crystal X-ray diffraction.405

9.05.3.6

closo-Heptaborate anion [B7H8]2−

Due to its inaccessibility, the closo-heptaborate anion [B7H7]2− is the least studied of the series of polyhedral borane anions [BnHn]2− (n ¼ 6–12). The first convenient synthesis of the [B7H7]2− anion by oxidation of (Bu4N)2[B9H9] with oxygen in a mixture of 1,2-dimethoxyethane and dichloromethane was described.406 The protonation of [B7H7]2− with Et3N
HCl in acetonitrile leads to the protonated form [B7H8]−. The crystal structures of (Bu4N)2[B7H7], (Ph4P)2[B7H7] and (PNP)2[B7H7] and the protonated form (Bu4N)[B7H8] and (Ph4P)[B7H8] were determined by single crystal X-ray diffraction.406

9.05.3.7

closo-Hexaborate anion [B6H6]2−

An early stage in the development of the chemistry of the closo-hexaborate anion [B6H6]2− and its protonated form [B6H7]− was described in an excellent review by Preetz and Peters.407 Subsequently, the synthesis of perbenzylated derivatives (Bu4N)[B6R6H] (R ¼ CH2C6H5, CH2C6H4-4-Br) by the reaction of (Bu4N)[B6H7] with an excess of the corresponding benzyl bromide in the presence of Et3N in toluene at 140  C408 or K3PO4 in acetonitrile at 120  C409 was reported. The reaction of (Ph3PMe)2[B6H6] with Ph2PCl in THF at −30  C leads to phosphine [B6H6PPh2]−, which is easily alkylated with allyl bromide or methyl iodide giving the corresponding phosphonium salts [B6H6PPh2R] (R ¼ allyl, methyl).409 The iodination of [B6H6PPh2Me] with I2 in acetonitrile in the presence of KI and K3PO4 leads to the pentaiodo derivative Na[B6I5PPh2Me].409 The reaction of (Ph3PMe)2[B6H6] with PhSeCl in THF leads to [B6H6SePh]−, which upon treatment with methyl iodide gives the protonated selenonium salt [B6H6SePhMe].409 Oxidation of the perbenzylated derivatives [B6(CH2Ar)6H]− with tetracyano-quinodimethane (TCNQ) in THF at 0  C results in complete destruction of the closo-hexaborate cluster; upon oxidation in the presence of pinacol, the pinacolates of the corresponding benzyl boronic acids, ArCH2Bpin, were obtained. Oxidation of monoalkylated derivatives [B6H6R]− proceeds in a similar way.409

256

Polyhedral Boranes and Carboranes

The reaction of (Bu4N)2[B6H6] with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in chloroform under reflux conditions results in the insertion of a carbene unit into the closo-hexaborate cage to form a mixture of B-substituted 2-carba-closo-heptaborane derivatives (Ph4P)[3-Cl-2-CB6H6] and [3-DBU-2-CB6H6].410

9.05.4

Conclusions

Probably the chemistry of carboranes and polyhedral boranes is not developing as rapidly as some other growing areas of organometallic chemistry. At the same time, more than 200 new publications appearing every year concerning the chemistry of carboranes and boranes and their various applications make it difficult to navigate in them even for chemists working in this field for many years, not to mention young researchers or researchers coming from others areas. We hope this contribution will be useful for new generation of researchers and help them navigate this interesting and challenging area of research.

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190. 191. 192. 193. 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211. 212. 213. 214. 215. 216. 217. 218. 219. 220. 221. 222. 223. 224. 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235. 236. 237. 238. 239. 240. 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. 252. 253. 254.

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Polyhedral Boranes and Carboranes

325. 326. 327. 328. 329. 330. 331. 332. 333. 334. 335. 336. 337. 338. 339. 340. 341. 342. 343. 344. 345. 346. 347. 348. 349. 350. 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. 379. 380. 381. 382. 383. 384. 385. 386.

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9.06

Polyhedral Metallaboranes and Metallacarboranes

Sourav Kar, Alaka Nanda Pradhan, and Sundargopal Ghosh, Department of Chemistry, Indian Institute of Technology Madras, Chennai, India © 2022 Elsevier Ltd. All rights reserved.

9.06.1 Introduction 9.06.2 Metallaborane clusters 9.06.2.1 Metallaborane single cage clusters 9.06.2.1.1 Metallaborane clusters of group 4 9.06.2.1.2 Metallaborane clusters of group 5 9.06.2.1.3 Metallaborane clusters of group 6 9.06.2.1.4 Metallaborane clusters of group 7 9.06.2.1.5 Metallaborane clusters of group 8 9.06.2.1.6 Metallaborane clusters of group 9 9.06.2.1.7 Metallaborane clusters of group 10 9.06.2.2 Metallaborane fused clusters 9.06.3 Metallacarborane clusters 9.06.3.1 Metallacarborane clusters of group 4 9.06.3.2 Metallacarborane clusters of group 5 9.06.3.3 Metallacarborane clusters of group 6 9.06.3.4 Metallacarborane clusters of group 7 9.06.3.5 Metallacarborane clusters of group 8 9.06.3.6 Metallacarborane clusters of group 9 9.06.3.7 Metallacarborane clusters of group 10 9.06.3.8 Metallacarborane clusters of f-block elements 9.06.4 Supra-icosahedral metallaborane and metallacarborane clusters Acknowledgments References

264 264 265 265 265 268 275 276 281 294 296 308 308 314 317 318 319 332 348 352 355 363 363

Nomenclature Ad Av. BEDT-TTF BMDT-TTF BPDT-TTF bpy CAd CAp COD Cp Cp Cp’’ CVE cym DBTTF DME DMEDA DMPDA dppe dppen dppm DSD HTMP IMe NBD phen

Adamantyl Average Bis(ethylenedithio)tetrathiafulvalene Bis(methylenedithio)tetrathiafulvalene Bis(propylenedithio)tetrathiafulvalene Bi-pyridine Carbon atoms adjacent Carbon atoms apart 1,5-cyclooctadiene Cyclopentadienyl Pentamethylcyclopentadienyl 1,3-(Me3Si)2C5H3 Cluster valance electrons Cymene Dibenzotetrathiafulvalene Dimethoxyethane 1,2-Dimethylethylenediamine N,N0 -Dimethylpropane-1,3-diamine 1,2-Bis(diphenylphosphino)ethane Cis-1,2-bis(diphenylphosphino)ethylene Bis(diphenylphosphino)methane Diamond-square-diamond 2,2,6,6-Tetramethylpiperidine 1,3-Dimethylimidazol-2-ylidene Norbornadiene Phenanthroline

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00169-4

263

264

Polyhedral Metallaboranes and Metallacarboranes

PNP py SEP TBAF TEAF tmeda tmnd TMTSF TMTTF TTF Xyl

9.06.1

[N(PPh3)2]+ Pyridine Skeleton electron pairs Tetrabutylammonium fluoride Tetraethylammonium fluoride Tetramethylethylenediamine N,N,N0 ,N0 -Tetramethylnaphthalene-1,8-diamine Tetramethyl tetraselena fulvalene Tetramethyltetrathiafulvalene Tetrathiafulvalene 2,6-Dimethylphenyl

Introduction

Boron can form self-connected three-dimensional molecular networks owing to its unique multicenter bonding abilities.1–11 During the last six decades, many boron-containing complexes, clusters, materials were synthesized and utilized as catalysts, polymers, nanomaterials, functional building blocks in supramolecular design, pharmaceuticals, ceramics, redox shuttles in dye-sensitized solar cells, etc.3,12–18 In this connection, the chemistry of polyhedral boranes and their derivatives has advanced dramatically owing to their unique structures, bonding and widespread applications.12,19,20 Although single cage borane clusters are known up to icosahedral,1,2,21,22 many condensed borane clusters having more than 12-vertices are known.23,24 On the other hand, the introduction of metal(s) to boron-containing polyhedra has led to the isolation of single cage metallaborane and metallaheteroborane polyhedra of up to 16-vertices, along with many condensed polyhedra.20,25–33 Metallaheteroborane polyhedra are largely dominated by metallacarboranes owing to their diverse applications and structural features. Apart from metallacarboranes, many metallaheteroborane clusters are known with group 16 elements (O, S, Se and Te). The structures of these polyhedral metallaboranes, metallacarboranes and metallaheteroboranes are rationalized using a series of electron counting rules, such as Wade’s rules, Mingos’s fusion formalism and Jemmis’s MNO rule.34–37 Metallaborane and metallacarborane clusters were reviewed earlier in COMC I (1982),38 COMC II (1995),14,39 and COMC III (2007).6,40 In addition, many reviews and book chapters by us and others have been published over the years 2005–2021 both on metallaborane and metallacarborane clusters.

9.06.2

Metallaborane clusters

Metallaborane chemistry has grown in a pleasing fashion based on its structural, bonding and reaction chemistry. The early approach for the synthesis of metallaborane clusters was based on the reaction of preformed anionic boranes with metal precursors.41,42 Also, a few interesting metallaboranes were synthesized from neutral boranes, albeit in poor yields.43 Despite the positive results from the above reactions, the approach is restricted to smaller metallaboranes due to the limited number of available borate dianions. Thus, the search for the new synthetic routes and appropriate synthons received a stimulus. During the last few decades, many new synthetic strategies developed, which led to the synthesis and characterization of many single cage and condensed metallaborane clusters having unique geometries. Most of the metallaborane polyhedra in the last two decades are synthesized utilizing a methodology established by Fehlner.44 The addition of monoboron sources to cyclopentadienyl metal halides led to the isolation of many metallaborane polyhedra (Scheme 1). The reaction of a cyclopentadienyl metal chloride with [BH4]− starts with the rapid metathesis of Cl− by [BH4]− to form a metal borohydride complex. Then, the reaction can go in two directions; it can either form metallaborane by releasing hydrogen or form metal hydride by eliminating borane. Hence, the formation of metallaboranes is decided by the competition between H2 elimination and borane elimination. In the case of the reaction with BH3 ∙ THF, BH2Cl elimination occurs to form a metal hydride, which again reacts with borane to form metal polyborohydride. Subsequent steps occur as in the case of [BH4]− reaction. Using this synthetic strategy, many metallaboranes

Scheme 1 Synthetic strategy for the synthesis of metallaboranes.

Polyhedral Metallaboranes and Metallacarboranes

265

have been synthesized by others and us with diverse geometries. On the other hand, Kennedy and co-workers have developed another synthetic strategy, where a preformed borane, metallaborane or heteroborane has been utilized to synthesize single cage and fused metallaborane clusters.45,46 Some years ago, Housecroft and co-workers established the synthetic methodology of metal-rich metallaboranes having boride boron.47,48 In this section, single cage and condensed metallaborane clusters of d-block transition metals are discussed. In addition, a few interesting metallaheteroboranes of group 16 elements are also discussed.

9.06.2.1 9.06.2.1.1

Metallaborane single cage clusters Metallaborane clusters of group 4 (Table 1)

The number of known structurally characterized group 4 metallaborane clusters is very few. Although no single cage metallaborane with only a group 4 transition metal was isolated in the last two decades, a few interesting examples of group 4/group 9 mixed metal single cage metallaboranes were synthesized (Table 1). For the synthesis of these mixed-metal metallaboranes, preformed metallaboranes were utilized. For example, pyrolysis of in situ generated group 4 metal bis-borohydrates [Cp2M(BH4)2] (M ¼ ]Zr or Hf ) and [Cp IrB3H9] in the presence of excess BH3
THF led to the isolation of arachno-[(Cp2M)(Cp Ir)B3H9] (M ¼ ]Zr (1)49 and Hf (2)50) and arachno-[(Cp2M)(Cp Ir)B4H10] (M ¼ ]Zr (3)51 and Hf (4)49). As shown in Fig. 1, the molecular structure of 1 and 2 can be viewed as an arachno structure based on a pentagonal bipyramid with two missing vertices. Clusters 1 and 2 are the metal analogs of pentaborane(11). As {Cp Ir} and {Cp2Zr} are 2e fragments, as is {BH}, clusters 1 and 2 possess 8 SEP, consistent with pentaborane(11). On the other hand, clusters 3 and 4 are metal analogs of hexaborane(12) and have 9 SEP as in hexaborane(12). Utilizing a similar strategy, Rh analogs of 3 and 4 were isolated. Pyrolysis of [Cp2M(BH4)2] (M ¼ ]Zr or Hf ) and [(Cp Rh)2B2H6] in the presence of excess BH3
THF led to the isolation of arachno-[(Cp2M)(Cp Rh)B4H10] (M ¼ ]Zr (5) and Hf (6)).49

9.06.2.1.2

Metallaborane clusters of group 5 (Table 2)

Group 5 metallaborane clusters are mostly dominated by small size clusters. Clusters 752 and 853 are the smallest clusters among the group 5 metallaboranes synthesized in the last two decades. Both 7 and 8 have a tetrahedral core geometry (Fig. 2). Cluster 7 was isolated from the reaction of a preformed tantallaborane [(Cp Ta)2B4H8(m-BH4)] (13) at ambient temperature with [Fe2(CO)9], whereas the room-temperature reaction of [Cp TaCl4] with LiBH4
THF followed by the addition of S2CPPh3 afforded cluster 8. Although tetrahedral clusters contain 6 SEP, clusters 7 and 8 have 3 and 4 SEP, respectively. Note that compound 8 is the first example of classical [B2H5]− ion stabilized by the binuclear tantalum template. Reactions of 8 with [M(CO)5
THF] (M ¼ ]Mo or W) led to further cluster growth and yielded [{(Cp Ta)(CH2S2)}2(B2H5)(m-H){M(CO)3}] (9: M ¼ ]Mo and 10: M ¼ ]W).53 The core geometry of compounds 9 and 10 can be viewed as trigonal bipyramid (Fig. 2). Compounds 9 and 10 possess 6 SEP, which is in accord with their trigonal bipyramidal geometry. A series of group 5 bimetallic metallaboranes is known which have triple-decker sandwich structures, where the middle deck is non-cyclic. They are either M2B4 or M2B5 types, in which the middle decks are either open 4-membered or 5-membered rings, respectively. For example, clusters 11–14 have non-cyclic 4-membered middle deck (Fig. 3). The thermolysis of an in situ generated intermediate from the reaction of [CpNbCl4] and LiBH4
THF with Ph2Se2 afforded [(CpNb)2B4H9(m-SePh)] (11).54 By contrast, the reaction of [Cp TaCl4] with LiBH4
THF, followed by treatment of BH3
THF at elevated temperature afforded clusters 1255 and 1456. Table 1 No. 1 2 3 4 5 6

Metallaboranes of group 4. Compounds 

[(Cp2Zr)(Cp Ir)B3H9] [(Cp2Hf )(Cp Ir)B3H9] [(Cp2Zr)(Cp Ir)B4H10] [(Cp2Hf )(Cp Ir)B4H10] [(Cp2Zr)(Cp Rh)B4H10] [(Cp2Hf )(Cp Rh)B4H10]

11

Av. MdB (Å)

Av. MdM (Å)

Ref.

8.2, −3.2, −15.7 −15.4, −5.4, 6.6 11.4, −11.1 9.0, −12.7 18.1, −3.1 16.4, −3.4

2.179 (IrdB), 2.619 (ZrdB) 2.192 (IrdB), 2.5855 (HfdB) 2.159 (IrdB), 2.587 (ZrdB) 2.148 (IrdB), 2.566 (HfdB) 2.165 (RhdB), 2.602 (ZrdB) −

2.953(6) (IrdZr) 2.9317(5) (IrdHf ) − 2.927(1) (IrdHf ) 2.964(5) (RhdZr) −

49 50 51 49 49 49

B NMR (ppm)

Fig. 1 Molecular structures of metallaboranes 1–6.

266

Table 2

Polyhedral Metallaboranes and Metallacarboranes

Metallaboranes of group 5.

No.

Compounds

11

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

[{(Cp Ta(CO))}2(CO){Fe(CO)3}(BH)] [(Cp Ta)2(B2H5)(m-H)(m-S2CH2)2] [{(Cp Ta)(CH2S2)}2B2H5(H)Mo(CO)3] [{(Cp Ta)(CH2S2)}2B2H5(H)W(CO)3] [(CpNb)2B4H9(m-SePh)] [(Cp Ta)2B4H10] [(Cp Ta)2B4H8(m-BH4)] [(Cp Ta)2B4H8(m-Z2dCOCH3)] [(CpV)2B5H11] [(Cp Ta)2B5H11] [(Cp Ta)2B5H10(C6H4CH3)] [(Cp Ta)2B5H10(C6H5)] [(Cp TaCl)2B5H11] [(Cp TaBr)2B5H11] [(Cp TaI)2B5H11] [(Cp TaCl)2B5H10Cl] [(Cp Ta)2B5H12I] [(Cp Ta)(Cp TaCl)B9H16] [{CpV(m-SPh)}2{BH3S}] [{CpV(m-SePh)}2{BH3Se}] [{CpV(m-SePh)}2{BH(OC4H8)Se}] [{CpNb(m-SPh)}2{B2H4S}] [{CpNb(m-SePh)}2{B2H4Se}] [(Cp Ta)2(m-Se)B3H6Se(C6H5)] [(Cp Nb)3(m-S)3(m3-S)3(m-S)BH] [(Cp Ta)3(m-S)3(m3-S)3BH] [(Cp Ta)3(m-S)3(m3-S)3B(SH)] [(Cp Ta)3(m-S)3(m3-S)3BCl] [(Cp Ta)3(m-Se)3(m3-Se)3B(H)] [(Cp Ta)3(m-Se)3(m3-Se)3B(OBuCl)] [{CpV(m-TePh)}2{BH(OC4H8)Te}]

119.5 35.0, −11.7 36.4, 16.9 36.4, 18.0 18.2, 15.1, 3.0, −0.2 16.6, 0.3 15.7, −0.1, −20.8 38.7, −5.8 21.9, 3.4, −1.8 44.8, 23.9, 3.7 − 51.0, 20.2, 3.5 77.7, 18.8, 15.1, −10.0 81.1, 23.8, 22.4, −8.3 78.9, 25.8, 24.7, −11.9 79.2, 23.2, 19.8, 16.9, −10.0 21.4, 18.1, 13.9, −1.4, −19.9 74.0, 39.6, 26.4, 18.7, 17.0, 15.3, −33.3 33.4 33.7 33.6 15.6, −36.1 18.9, −26.0 46.5, −4.7, −18.0 −0.2 0.3 1.2 5.6 −11.2 −5.5 21.7

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref

− 2.395a 2.56(3)b, 2.41a 2.515(13)b, 2.385a 2.386a 2.381a 2.363a 2.393a 2.217a 2.326a 2.330a 2.335a 2.384a 2.381a 2.382a 2.372a 2.362a 2.389a 2.335a 2.43a 2.46(5)a 2.432a 2.428a 2.415a − − − − − − 2.471a

− 3.2903(8)c 3.281(2)c 3.2765(6)c 2.8618(5)c 2.8909(4)c 2.8946(2)c 2.8738(6)c 2.7604(10)c 2.9261(4)c 2.9099(12)c 2.9367(3)c 3.2219(3)c 3.2280(4)c 3.2376(4)c 3.2304(6)c 2.8773(10)c 3.2556(16)c 2.6816(12)c 2.751(2)c 2.7562(10)c 2.7726(12)c 2.7698(12)c 2.815(1)c 3.163c 3.098c 3.106c 3.094c 3.229c 3.2402(5)c 2.8587(9)c

52 53 53 53 54 55 57 56 66 55 55 376 55 58 58 59 58 61 62 62 62 62 62 62 63 64 64 64 64 65 66

a

Av. MdB distance between group 5 metal and boron Av. MdB distance between boron and group 6 metal c Homometallic Av. MdM distance between group 5 metals b

Fig. 2 Molecular structures of metallaboranes 7–10.

Interestingly, an acyl group bridged the Ta2B4 core in cluster 14. On the other hand, cluster 13 has a bridging borohydride (BH4)− ligand attached to Ta2B4 core, and it was synthesized by thermolysis of [(Cp Ta)2(B2H6)2] in the presence of BH3
THF.57 Clusters 11–14 can also be viewed as a bicapped tetrahedron, and according to the capping principle, they have an electronic requirement of 6 SEP. Although clusters 11 and 14 are 6 SEP species, clusters 12 and 13 (5 SEP) are electrons deficient. Note that the reaction of 13 with an excess amount of [Fe2(CO)9] under mild reaction conditions yielded cluster 7. On the other hand, there are two types of M2B5 metallaborane clusters. One type of M2B5 has triple-decker geometry with an acyclic 5-membered deck, whereas the other type of M2B5 has a nido structure based on a closo-dodecahedron geometry. For example, clusters 16 and 17 have triple-decker geometry with an acyclic 5-membered deck.55 Cluster 16 was synthesized along with

Polyhedral Metallaboranes and Metallacarboranes

267

Fig. 3 Molecular structures of metallaboranes 11–14.

clusters 12 and 14 from the thermolysis reaction of an in situ generated intermediate of [Cp TaCl4] and LiBH4
THF, with BH3
THF. Further treatment of arene or heteroarene with cluster 16 led to the CdH activation of the arene/heteroarene and the formation of BdC bonds (Scheme 2). In total, sixteen CdH activated arene/heteroarene were isolated in good yields; cluster 17 is one of them. Clusters 16 and 17 can also be viewed as bicapped trigonal bipyramids and have the expected 6 SEP.

Scheme 2 Synthesis of metallaborane 17.

Clusters 19–22 are examples of a second type of Ta2B5 core (Fig. 4).55,58,59 Reactions of cluster 16 with CH2Cl2 or CH2Br2 or I2 led to isolation of [(Cp TaX)2B5H11] (X ¼ ]Cl (19), Br (20), I (21)), respectively. One of the interesting features of these reactions is the reorganization of the cluster core geometry. The core geometry of clusters 19–21 can be viewed as a nido-cluster formally derived from an eight-vertex dodecahedron simply by removing one five-connected vertex of it. From a different viewpoint, King and Ghosh described clusters 19–20 as oblatonido.55,58,60 In cluster 22, one of the terminal hydrogens of 19 is replaced by chlorine.59 Clusters 19–22 have 9 SEP. Along with clusters 12, 14, 16, and 22, cluster 24 was also isolated from the same reaction. The molecular structure of 24 can be viewed as a nido-11-vertex cluster (Fig. 4).61 The core geometry of cluster 24 can be achieved from an icosahedron by removing one degree-five vertex and performing one DSD rearrangement, followed by removing two edges. As a result, cluster 24 has an unusual nido geometry with 11 SEP rather than the prescribed 13 SEP. On the other hand, with the modification of Fehlner’s strategy by altering the borate reagent, a series of chalcogen derivatives of metallaborane clusters, i.e., metallaheteroboranes of group 5 (25–37), has been synthesized. For example, chalcogen-incorporated

Fig. 4 Synthesis of metallaboranes 19–21 (left) and molecular structures of clusters 22 (middle) and 24 (right).

268

Polyhedral Metallaboranes and Metallacarboranes

borate reagents Li[BH3(EPh)] and Li[BH2E3] have been utilized for the synthesis of metallaheteroborane clusters 25–36. The thermolysis reaction of [Cp2VCl2] with Li[BH3(EPh)] resulted in the formation of [{CpV(m-EPh)}2{BHmRnE}] (25: E ¼ ]S, m ¼ 3 and n ¼ 0; 26: E ¼ ]Se, m ¼ 3 and n ¼ 0, and 27: E ¼ ]Se, R ¼ ]OC4H8, m ¼ 1 and n ¼ 1) (Scheme 3).62 Clusters 25–27 have tetrahedral geometries. Note that clusters 25–27 are structurally and electronically analogous to diborane(6). On the other hand, the treatment of [CpNbCl4] with Li[BH3(EPh)] afforded [{CpNb(m-EPh)}2{B2H4E}] (28: E ¼ ]S and 29: E ¼ ]Se) (Scheme 3).62 Clusters 28 and 29 have trigonal bipyramidal Nb2B2E core geometries and possess 6 SEP, as expected. In the case of Ta, the reaction of [Cp TaCl4] with Li[BH3(SePh)] led to the formation of [(Cp Ta)2(m-Se)B3H6Se(C6H5)] (30).62 The cluster core of 30 is the same as that of clusters 11–14.

Scheme 3 Syntheses of metallaheteroboranes 25–27 (left) and 28–29 (right).

A series of cubane and homocubane type chalcogen rich group 5 metallaheteroborane clusters (31–36) have been isolated utilizing trichalcogenatoborate ligand Li[BH2E3]. The treatment of [Cp NbCl4] with four equivalents of Li[BH2S3] at elevated temperature resulted in the formation of trimetallic polysulfide metallaheteroborane [(Cp Nb)3(m-S)3(m3-S)3(m-S)BH] (31) (Scheme 4).63 The solid-state X-ray structure analysis of 31 revealed the geometry as a homocubane-type cluster, where one of the vertices of the homocubane is missing. Cluster 31 consists of an Nb3 triangular core; the edges of the Nb3-triangle are bridged by six monosulfide atoms, two of which are directly coordinated to the B atom, whereas the ‘omitted’ sulfur atom is bridged between B and S atoms. Cluster 31 has 64 CVE, which is 4 electrons lower than its carbon analog. On the other hand, the treatment of [Cp TaCl4] with four equivalents of Li[BH2E3] (E ¼ ]S or Se) at elevated temperature afforded [(Cp Ta)3(m-E)3(m3-E)3B(R)] (32: E ¼ ]S, R ¼ ]H; 33: E ¼ ]S, R ¼ ]SH; 34: E ¼ ]S, R ¼ ]Cl; 35: E ¼ ]Se, R ¼ ]H and 36: E ¼ ]Se, R ¼ ]OBuCl) (Scheme 4).64,65 Clusters 32–36 are examples of trimetallic cubane-type metallaheteroborane clusters, where one of the vertices of cubane is missing. Clusters 32–36 consist of an equilateral Ta3 triangular core, and the edges of this Ta3-triangle are bridged by six monosulfide/monoselenide. Three of these sulfide/selenide units are capped by {BR} fragments. These cubane-type clusters feature 50 CVE, which is 6 electrons short of its organic analog. The only Te incorporated group 5 metallaheteroborane [{CpV(m-TePh)}2{BH(OC4H8)Te}] (37) was synthesized from the reaction of [Cp2VCl2] with excess [LiBH4
THF], followed by thermolysis with diphenyl ditelluride.66 It has a similar structure to cluster 27.

Scheme 4 Syntheses of metallaheteroboranes 31–36. (m-E attached to MdM bonds are not shown for clarity; Nb]NbCp and Ta]TaCp )

9.06.2.1.3

Metallaborane clusters of group 6 ( Table 3)

Metallaborane clusters of group 6 show rich chemistry, many of them are synthesized either utilizing preformed metallaboranes or via Fehlner’s strategy. Many group 6 metallaborane clusters are synthesized with one boron atom; they are therefore known as metal-rich metallaboranes. Transition metal boride clusters are one of the subclasses of metal-rich metallaboranes. Clusters 38–44 are unique examples of boride clusters, where boride borons are in semi-interstitial positions. Clusters 38 and 39 were synthesized from the thermolysis of [Ru3(CO)12] with an in situ generated intermediate obtained from the treatment of [Cp MoCl4] with

269

Polyhedral Metallaboranes and Metallacarboranes

Table 3 No. 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95

Metallaboranes of group 6.

Compounds [{Cp Mo(CO)2}2Ru2(CO)6(m-H)B] [Cp Mo(CO)2Ru3(CO)9(m-H)2B] [{Cp W(CO)2}2Ru2(CO)6(m-H)B] [{Cp W(CO)2}2Ru3(CO)8(m-H)B] [Cp W(CO)2{Ru(CO)3}4B] [Cp Mo(CO)2{Ru(CO)3}4B] [(Cp W)2(CO)4{Co2(CO)5}B] [{Cp W(CO)2}2{Fe(CO)2}(BH)2H2] [{Cp W(CO)2}2{Fe(CO)3}(BH)2] [{WCp (CO)2}2{Ru(CO)3}(BH)2] [Cp Mo(CO)2B3H8] [Cp Mo(CO)2B3H6Fe(CO)3] [{Cp Mo(CO)2}2{B2H4}] [{Cp Mo(CO)2}2B2H2W(CO)4] [{Cp W(CO)2}2B2H2Mo(CO)4] [{Cp W(CO)2}2B2H2W(CO)4] [{(Cp Mo(CO))}2B4H6] [{Cp W(CO)}2B4H6] [(Cp Mo)2B4H7(m-PPh2)] [(Cp Mo)2(CO)(Cl)B3H4W(CO)4] [{(Cp W(CO))}2B3H5W(CO)4] [(Cp W)2B4H8Cr(CO)4] [(Cp W)2B4H8Mo(CO)4] [(Cp W)2B4H8W(CO)4] [(Cp Mo)2B4H8Cr(CO)4] [(Cp Mo)3(m-CO)2B4H7] [(Cp Mo)2B4H8W(CO)4] [(Cp Mo)2B4H6W(CO)5] [(Cp W)2(m-H)2(m3-H)B4H4Co(CO)3] [(Cp W)2B4H8Fe(CO)3] [(Cp W)2B4H8Ru(CO)3] [{(Cp Mo)2B4H4Ru2(CO)6}] [(Cp W)2B4H4Co2(CO)5(H)2] [(Cp W)2B5H5Fe(CO)3(H)2] [(CpMo)3(m-H)B8H8] [(Cp Mo)3(m-H)B8H8] [(Cp Mo)3(m-H)B8H6Cl2] [(Cp Mo)2B4H3I5] [(Cp Mo)2B4H5(SR)2(m-SR)], (R ¼ 2,6-(tBu)2dC6H2OH) [(Cp Mo)2B4H6(SR)(m-SR)], (R ¼ 2,6-(tBu)2dC6H2OH) [(Cp Mo)2B4H5(SePh)2(m-SePh)] [(Cp Mo)2{1-(SPh)B5H8}] [(Cp Mo)2{1-(SePh)B5H8}] [(Cp Mo)2{1-(Cl)B5H8}] [(Cp Mo)2{1,4-(Cl)2B5H7}] [(Cp Mo)2{2,4-(Cl)2B5H7}] [(Cp Mo)2{1,2-(Cl)2B5H7}] [(Cp Mo)2{1,2,4,5-(Cl)4B5H5}] [(Cp Mo)2{2-(I)B5H8}] [(Cp Mo)2{2,4-(I)2B5H7}] [(Cp Mo)2{1,2,4-(I)3B5H6}] [(Cp Mo)2{1,5-(Me)2B5H7}] [(Cp Mo)2{1,3-(Me)2B5H7}] [(Cp Mo)2{1,2,3-(Me)3B5H6}] [(Cp Mo)2{1,2,4-(Me)3B5H6}] [(Cp Mo)2{5-(nBu)B5H8}] [(Cp Mo)2{1,5-(nBu)2B5H7}] [(Cp Mo)2{1,3-(nBu)2B5H7}]

11

B NMR (ppm)

Av. MdB (Å) d

Av. MdB (Å) a

i

Ref. h

128.4 123.1 128.8 189.8 182.0 201.8 132.6 80.0 102.8 102.4 1.0, −39.1 28.7, 4.2, 0.9 27.3, −31.1 98.4, 48.1 92.0, 32.4 92.1, 29.2 15.8, 63.7 60.4, 7.6 47.4, 34.5, 4.6 75.4, 99.2, 106.6 60.5, 62.0, 75.9 70.6, 62.2, 36.5, 25 70.1, 60.1, 39.1, 25.6 70.2, 60.1, 35.5, 26.1 83.5, 81.3, 41.3, 27.1 88.3, 83.2, 46.3, 23.5 81.2, 41.9, 27.9 103.0, 77.9, 77.0 72.5, 53.8, 45.4, 24.2 79, 76.2, 41, 35.9 77.7, 40.2, 34.0 91.3 85.2, 78.8, 38.4 85.2, 78.8, 38.4 97.4, 87.2, 29.4, −34.8 101.7, 88.7, 32.2, −30.1 103.5, 90.4, 35.1, −29.4 53.3, 36.1, 31.3, 13.9 84.4, 49.8, 30.4

2.184 , 2.122 2.160d, 2.122(6)a 2.120b, 2.198d 2.183b, 2.181d 2.156(4)b, 2.136d 2.137d, 2.163(3)a 2.153b 2.298b, 2.043d 2.288b, 2.06d 2.287b, 2.125d 2.546a 2.407a, 2.702d 2.339a 2.274a, 2.341b 2.36a, 2.26b 2.290b 2.22a − 2.206a 2.21a, 2.340(7)b 2.271b 2.32(9)c, 2.277b 2.475(7)a, 2.257b 2.261b 2.243a, 2.31c 2.263a 2.4356b, 2.246a 2.315(3)b, 2.235a 2.253b, 2.131(15)d 2.263b, 2.086d 2.267b, 2.16d 2.180a, 2.179d 2.2515b, 2.125d 2.235d, 2.266b 2.258a 2.236a 2.262a 2.274a 2.284a

2.8293(7) , 3.009 2.8311i, 3.042h 2.8333(5)i, 2.774h 2.799i, 3.013h 2.837i, 3.019h 2.8388i, 3.026h 2.456(2)i,2.80h 3.0491(4)f, 2.783h 3.0516(7)f, 2.799h 3.0585(15)f, 2.91h − 2.7394(3)h 3.0617(8)e 3.133g 3.143g 3.128f 2.9262(11)e − 2.7374(4)e 2.9611(7)e, 2.971g 2.981f 2.824(3)f, 2.882g 2.8420(4)f, 3.014g 2.935f 2.821(9)e, 2.91g 2.931e 2.8275(18)e, 3.016g 2.8308(3)e, 2.985g 2.8325(7)f, 2.671h 2.816(4)f, 2.684h 2.8192(9)f, 2.797h 2.7510(5)e, 2.8822(5)i, 2.807h 2.790f, 2.515i, 2.733h 2.8121(4)f, 2.722h 2.918e 2.962e 2.7801e 2.729e 2.6941(6)e

67 67 68 68 68 69 78 70 70 70 73 73 74 74 74 74 76 77 377 76 77 79 79 79 79 80 80 80 78 79 68 76,81 78 78 86 86 86 87 88

80.4, 48.9, 13.2

2.286a

2.6944(3)e

89

78.8, 54.0, 19.2 62.1, 61.5, 34.2, 24.2 64.1, 62.4, 59.8, 30.7, 24.7 65.7, 57.7, 37.9, 22.6 68.6, 62.6, 59.5, 38.5, 26.2 70.3, 60.4, 21.7 72.3, 5.2, 56.6, 36.7, 22.5 66.0, 54.9, 36.6 65.0, 62.3, 51.9, 29.3, 27.5 62.6, 53.2, 32.9 68.3, 54.9, 54.9, 38.3, 23.2 60.7, 51.8, 41.3 72.8, 63.5, 51.6, 38.6, 23.5 66.5, 55.9, 41.7, 22.8 68.2, 56.9, 54.9, 39.4, 22.4 66.4, 57.2, 55.8, 41.5, 22.6 61.2, 52.1, 41.2 73.1, 56.2, 43.4, 2

2.267a 2.253a 2.255a 2.258a 2.262a − 2.261a 2.274a 2.251a 2.261a 2.252a − − − − 2.254a − −

2.7081(3)e 2.8106(10)e 2.8165(3)e 2.8144(7)e 2.8276(4)e − 2.8262(5)e 2.8441(3)e 2.8134(5)e 2.8314(15)e 2.8317(13)e − − − − 2.7988(8)e − −

88 88 88 90 90 90 90 90 91 91 91 90 90 90 90 92 92 92 (Continued )

270

Polyhedral Metallaboranes and Metallacarboranes

Table 3

(Continued)

No.

Compounds

11

96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125

[(Cp Mo)2{5-(Ph)B5H8}] [(Cp Mo)2{1,5-(Ph)2B5H7}] [(Cp Mo)2{1,3-(Ph)2B5H7}] [(Cp W)2{1-(Cl)B5H8}] [(Cp W)2{1,5-(Cl)2B5H7}] [(Cp W)2{2,3-(Cl)2B5H7}] [(Cp W)2{1,2-(Cl)2B5H7}] [(Cp W)2{1,4-(Cl)2B5H7}] [(Cp Mo)2(m-SPh)2SH2B(SPh)] [(Cp W)2(m-SPh)(B3H3(SPh)S)] [(Cp W)2(m-SePh)(B3H3(SePh)Se)] [(Cp Mo)2B2S2H2(m-S)] [(Cp W)2B2S2H2(m-S)] [(Cp ∗Mo)2B4SH6] [(Cp ∗Mo)2B4SeH6] [(Cp Mo)2B4(Ph)H5Se] [(Cp Mo)2{1-(Cl)B4TeH5}] [(Cp Mo)2{2-(Cl)B4TeH5}] [(Cp Mo)2B5H7O(Et)] [(Cp Mo)2B5H6(nBuO)O(Et)] [(Cp Mo)2B4H4S2] [(Cp Mo)2B4H4Se2] [(Cp Mo)2B4H4Te2] [(CpW)2B4H4S2] [(Cp W)2B4H4Se2] [(Cp Mo)2B4(SeBn)H3Se2] [(Cp Mo)2B4H3(Cl)TeO(Et)] [(Cp Mo)2B4S2H2I2] [(Cp Mo)2B4S2HI3] [(Cp Mo)2{B3H3TeCo2(CO)5}]

126 127 128 129 130

[(Cp Mo)2{B4H4SRu(CO)3}] [(Cp Mo)2{B4H4SeRu(CO)3}] [(Cp Mo)2{B4H4TeRu(CO)3}] [(Cp Mo)2B4H4S(m3-CO)Co2(CO)4] [(Cp Mo)2B4H4Se(CO)Co2(CO)4]

B NMR (ppm)

Av. MdB (Å)

Av. MdB (Å)

Ref.

66.6, 56.9, 55.8, 41.7, 22.7 60.7, 50.7, 40.8 72.5, 55.6, 42.6, 23.6 50.6, 45.1, 44.1, 42.6, 24.3 50.3, 46.2, 46.2, 42.6, 42.6 64.4, 42.6, 39.5, 28.6, 25.2, 6.5 65.4, 43.2, 43.2, 40.3, 23.4 60.5, 46.1, 44.4, 38.5, 26.4 −17.7 91.8, 73.3, −12.2 108.3, 77.2, 1.3 18.8 13.8 101.1, 76.7, 40.3, 10.1 100.7, 76.7, 41.8, 16.8 98.7, 77.5, 42, 28.2 95.5, 73.2, 40.7, 26.7 98.8, 71.8, 51.6, 24.7 68.9, 60.2, 34.8, −4.5 103.6, 89.2, 87.5, 33.2 81.8, −4.1 81.8, 4.2 81.3, 11.6 74.9, −10.6 70.7, −0.4 81.2, 78.3, 13.9, 5.6 87.2, 77.5, 44.1, −12.0 83.6, −0.2 86.9, 69.9, 0.5 84.4, 53.6, 25.9

− − − 2.250b 2.250b − − 2.258b 2.435a 2.25b − 2.487a 2.33b − 2.288a 2.256a 2.176a − 2.295a 2.302a 2.314a 2.335a 2.368a 2.29b − 2.366a 2.281a 2.129a 2.318a 2.314a, 2.166(16)d

92 92 92 93 93 93 93 93 88 94 94 88 378 95 96,97 89 98 98 99 99 100,101 100 100,101 102 100 89 98 91 91 96,97

107.5, 103.4, 54.5 104.5, 101.8, 63.5 102.6, 99.4, 72.4 102.6, 77.2, 35.4 101.8, 77.4, 38.6

2.260(4)d, 2.239a 2.282(8)d, 2.247a 2.243(6)d, 2.246a 2.25a, 2.127d −

− − − 2.8215(3)f 2.8313(3)f − − 2.8301(3)f 2.6962(5)e 2.6930(3)f − 2.6375(12)e 2.647(8)f − 2.777e 2.8103(8)e 2.845(2)e − 2.6380(8)e 2.6615(7)e 2.6333(8)e 2.665(2)e 2.6921(6)e 2.6527(13)f − 2.6531(13)e 2.660(7)e 2.655(9)e 2.6694(5)e 2.7521(6)e, 2.4741(10)i, 2.726h 2.7693(3)e, 2.799h 2.7733(6)e, 2.819h 2.7787(7)e, 2.853h 2.922e, 2.589(11)i, 2.69h −

81 81 81 97 97

a

Av. ModB Av. WdB c Av. CrdB d Av. MdB distance between boron and other than group 6 metals e Av. ModMo f Av. WdW g Heterometallic Av. MdM distance of group 6 metals h Heterometallic Av. MdM distance of group 6 metals and other than group 6 metals i Homometallic Av. MdM distance of other than group 6 metals b

LiBH4
THF.67 Compounds 38 and 39 can be defined as 62 CVE and 7 SEP heterometallic boride clusters with a semi-interstitial m4boron in a butterfly geometry (Fig. 5, type I). Although the boride boron of cluster 38 is completely naked, a bridging hydrogen is attached to the boride boron in 39. In the case of W, under the same reaction conditions, boride clusters 40, 41, and 42 were obtained.68 Cluster 40 is the tungsten analog of boride cluster 38, but the bridging hydrogen is attached to the boride boron. By contrast, clusters 41 and 42 can be defined as 74 CVE and 7 SEP heterometallic boride clusters with a semi-interstitial m5-boron in the square pyramid geometry (Fig. 5, type II). In 42, the boride boron is naked, whereas a bridging hydrogen is attached to m5-boron in 41. On the other hand, the thermolysis reaction of a preformed metallaborane arachno-[Cp Mo(CO)2B3H8] with [Ru3(CO)12] yielded an analog of 42, i.e., metallaborane 43.69 Reacting [Co2(CO)9] with a similar type of tungsten intermediate, one more boride type II (Fig. 5) cluster (44)63 was synthesized. Other than m4- and m5-borides, there are examples of mn-borides (n ¼ 6–9), which are discussed in different sections. All of these boride clusters 38–44 show 11B chemical shifts typically in the downfield region (d ¼ 120–205 ppm).

Polyhedral Metallaboranes and Metallacarboranes

271

Fig. 5 Molecular structures of boride clusters having m4-B (I) and m5-B (II).

Apart from transition metal boride clusters, another subclass of metal-rich metallaboranes feature borylene ligands in the coordination sphere of transition metals. The reaction of [Cp WCl4] and LiBH4
THF, followed by treatment of metal carbonyls of Fe and Ru yielded heterometallic clusters featuring triply-bridging bis-hydrido(borylene) unit (45) and bis-borylene unit (46 and 47) (Scheme 5).70 Clusters 45–47 have a trigonal bipyramid {W2MB2} core, in which two B atoms occupy the apical vertices. Their electron count of 6 SEP is also in accord with closo-trigonal bipyramidal geometry. Note that cluster 45 is an example of a new class of clusters known as hydridoborylene. Hydridoborylene was first reported by Braunschweig and co-workers in 2013.71 Later, Tobita et al. reported the synthesis of hydrido(hydroborylene)tungsten complexes, [Cp W(CO)2(H)(BH
NHC)] (NHC ¼ MeIMe or Me i I Pr).72 These metal-rich metallaboranes 45–47 also showed 11B chemical shifts at a downfield region in common with boride clusters. Furthermore, theoretical calculations indicated that borylene units are bridged between two W and one Fe or Ru atom in m3-fashion.

Scheme 5 Syntheses of metallaheteroboranes 45–47.

Apart from metal-rich metallaborane clusters, many metallaboranes of group 6 have been synthesized utilizing [Cp MCl4], [Cp M(CO)3Me], metal carbonyls, preformed metallaboranes and different types of borane or borate reagents. For example, the photolysis reaction of [Cp Mo(CO)3Me] with BH3
THF afforded metallaborane [Cp Mo(CO)2B3H8] (48) (Scheme 6).73 Cluster 48 can be described as the analog of arachno-[B4H10]. It has butterfly geometry and 7 SEP, which is the same as that of arachno-[B4H10]. Upon treatment with [Fe2(CO)9], arachno-48 underwent cluster growth reaction and yielded nido-[Cp Mo(CO)2B3H6Fe(CO)3] (49) (Scheme 6).73 Nido-49 has a square pyramid geometry and features 7 SEP.

Scheme 6 Syntheses of metallaheteroboranes 48 and 49.

On the other hand, the reaction of a preformed metallaborane [(Cp Mo)2(m-Cl)2B2H6] with CO at room-temperature yielded [{Cp Mo(CO)2}2{m-Z2:Z2-B2H4}] (50) (Scheme 7).74 Compound 50 is the first example of bimetallic diborane(4) conforming to

272

Polyhedral Metallaboranes and Metallacarboranes

Scheme 7 Syntheses of metallaheteroboranes 50 and 51.

a singly bridged Cs structure. Theoretical studies show that 50 mimics the Cotton dimolybdenum-alkyne complex [{CpMo (CO)2}2C2H2].75 On the other hand, compound 50 can be considered as a tetrahedral cluster and has 6 SEP as expected. Further reaction of 50 with [W(CO)5
thf] leads to the replacement of two hydrogen atoms with a 2e [W(CO)4] fragment and yielded [{Cp Mo(CO)2}2B2H2W(CO)4] (51) (Scheme 7).74 Compound 51 can be described as transition metal-stabilized diborene(2) where the BdB bond distance of 1.624(4) Å is very short. The core geometry of 51 is trigonal bipyramidal. However, the tungsten analogs of 51, [{Cp W(CO)2}2B2H2W(CO)4] (52) and [{Cp W(CO)2}2B2H2Mo(CO)4] (53), were isolated from the reaction of an intermediate generated in situ in the low-temperature reaction of [Cp WCl4] with excess [LiBH4
thf], followed by thermolysis with [M(CO)5
thf] (M ¼ ]Mo and W).74 As with group 5 metallaboranes, many group 6 metallaboranes (54–68) have a triple-decker sandwich structure, where the middle decks are acyclic. Except for 56 and 63, all of these clusters are derived from reactions of metal carbonyls, such as [M’(CO)5
THF] (M’ ¼ ]Cr or Mo or W), [Fe2(CO)9], [Ru3(CO)12] and [Co2(CO)9] with in situ intermediates generated via the reaction of [Cp MCl4] (M ¼ ]Mo or W) with LiBH4
THF. Clusters 54 and 55 have an M2B4 type triple-decker sandwich structure with a middle four-membered open deck like clusters 11–14.76,77 By contrast, clusters 57 and 58 have the same structure as those of 54 and 55 with a M2B3M’ type core.76,77 All of these clusters (54–58) have 6 SEP, which is in accord with their bicapped tetrahedral cores. On the other hand, clusters 59–68 have an M2B4M’ type triple-decker sandwich structure with a middle five-membered (B4M’) open deck resembling tantallaboranes 16 and 17. The position of the metal (M’) in the open decks varies. For example, metals are located in the deck’s open faces of clusters 59–66.78–80 By contrast, in 67 and 68, metals (M’) are located in the inner vertices of the middle deck.68,79 The geometries of these M2B4M’ type clusters (59–68) can also be described as a bicapped trigonal prisms, and they have expected 6 SEP. In an attempt to close these 4-membered or 5-membered middle decks, many synthetic methodologies have been employed. The strategy utilizing metal carbonyls has afforded triple-decker sandwich complexes with closed middle decks (Fig. 6). For example, the treatment of [Ru3(CO)12] with in situ generated intermediate [(Cp Mo)2B4H8] (from the reaction of [Cp MoCl4] and LiBH4
THF), afforded [{(Cp Mo)2B4H4Ru2(CO)6}] (69).76,81 Compound 69 has a puckered six-membered hexahapto [B4H4Ru2(CO)6] ring, sandwiched between two {Cp Mo} fragments. By contrast, in a similar type of reaction utilizing an in situ generated intermediate from tungsten and cobalt carbonyl or iron carbonyl, two more triple-decker complexes 70 and 71 have been isolated.78 Although compound 71 has a puckered six-membered hexahapto middle deck, as found in 69, the middle deck is almost planar in 70. Clusters 69–71 can also be viewed as oblatocloso hexagonal bipyramids. Wang and co-workers reported experimental (photo electron spectroscopy) and theoretical studies on [Ta2B6]0/− clusters (A) having a planar six-membered B6 middle deck (Fig. 7) as in 70.82 Later they also reported clusters (C-E) having 7–9 membered middle decks (Fig. 7).83,84 These types of clusters (A-E) are known as inverse sandwich clusters.85 In addition, these inverse clusters show 3D-aromaticity. Wang, Boldyrev and co-workers also incorporated transition metals inside 8–10 membered monocyclic boron wheels (FdH, Fig. 8), which also shown aromaticity.7

Fig. 6 Molecular structures of triple-decker complexes 69–71.

Polyhedral Metallaboranes and Metallacarboranes

273

Fig. 7 Inverse sandwich complexes (A-E) having middle 6–9-membered planar decks.

Fig. 8 Transition-metal-centered monocyclic boron wheel clusters (FdH). F: [CoB8]−, G: [MB9]− (M]Ru, Rh or Ir), H: [MB10]− (M]Nb or Ta).

In this connection, ‘borophenes’—boron analogs of graphene, have emerged during the last two decades.8–10 Photoelectron spectroscopy and theoretical studies have revealed that boron clusters also have planar or quasi-planar (2D) structures up to relatively large sizes, for example- B36. The B36 cluster has extended boron monolayers with hexagonal vacancies.10 Further, metal-doping to these borophenes form metalloborophene, which can expand the range of potential nanostructures based on boron. For example, the [CoB18]− cluster possesses an unprecedented 2D structure, in which the dopant metal atom is part of the 2D boron network (Fig. 9).11 These results proved that boron clusters having transition metals can also construct 2D cluster frameworks, in addition to well-known 3D polyhedral cores. Three 11-vertex trimolybdaborane clusters 72–74 (Fig. 10) were synthesized along with other chalcogen containing metallaheteroboranes from the reaction of an intermediate (generated from the reaction of [Cp’MoCl4] (Cp’ ¼ ]Cp or Cp ) and LiBH4
THF), with different chalcogen ligands, such as 1,2-ethanedithiol, 2-mercaptobenzothiazole and tellurium powder.86 Compounds 72–74 are examples of an unsaturated (n-4) SEP cluster having closed 11-vertex geometry and contain 66 CVE. A similar type tungstaborane (A) was synthesized by Weller et al. in 1998.26

Fig. 9 Planar CoB18 unit, a building block of metalloborophene. Reproduced with permission from Li, W.-L.; Jian, T.; Chen, X.; Chen, T.-T.; Lopez, G. V.; Li, J.; Wang, L.-S. Angew. Chem. Int. Ed. 2016, 55, 7358–7363.

Fig. 10 Molecular structures of metallaboranes 72–74.

274

Polyhedral Metallaboranes and Metallacarboranes

A series of BdH functionalized M2B4 and M2B5 types metallaboranes (75–103) were synthesized utilizing different types of functionalization reagents, such as BHCl2
SMe2, E2R2 (E ¼ ]S or Se; R ¼ ]Ph or 2,6-(tBu)2-C6H2OH), NaI and RI (R ¼ ]Me, nBu or Ph). In most cases, an in situ generated intermediate from the reaction of [Cp MoCl4] and LiBH4
THF was utilized as starting material for the syntheses of these functionalized metallaboranes (75–88) (Scheme 8). When MeI was heated with this intermediate, functionalized M2B4 type metallaborane [(Cp Mo)2B4H3I5] (75) was isolated.87 Interestingly, cluster 75 contains 4 terminal BdI and one bridging ModIdMo units. Treatment with excess S2{2,6-(tBu)2-C6H2OH}2 yielded two Mo2B4 type clusters 76–77.88,89 In cluster 76, two terminal BdH units are functionalized by a S{2,6-(tBu)2-C6H2OH} group and the ModMo bond is bridged by S{2,6-(tBu)2-C6H2OH}. Only one terminal BdH is functionalized in the case of 77. On the other hand, when the intermediate is treated with excess E2Ph2 (E ¼ ]S or Se), both types of functionalized metallaborane are formed. In the case of S2Ph2, the M2B5 type cluster [(Cp Mo)2{1-(SPh)B5H8}] (79) was formed.88 In cluster 79, only one terminal BdH is functionalized by an {SPh} group, whereas in the case of Se2Ph2, both M2B4 and M2B5 type clusters (78 and 80, respectively) are achieved.88 Although 78 has one two terminal BdSePh unit along with one ModMo bond bridged by {SePh}, in the case of 80, only one terminal BdH of Mo2B5 is functionalized by {SePh}. Clusters 81–103 are all terminal BdH functionalized M2B5 type clusters. The reaction of the molybdenum intermediate with BHCl2 ∙ SMe2 yielded clusters 81–85, in which the terminal BdH hydrogen(s) of Mo2B5 is/are substituted by Cl.90 When the molybdenum intermediate was heated with NaI, iodine-functionalized metallaboranes 86–88 are formed.91 In another strategy, preformed metallaboranes [(Cp Mo)2{B5H9}] and [(Cp WH3){B4H8}] have been utilized as starting materials. The reaction of [(Cp Mo)2{B5H9}] with nBuLi, followed by treatment with RI (R ¼ ]Me, nBu and Ph) led to the formation of various BdH functionalized Mo2B5 clusters (89–98), where the terminal BdH hydrogen(s) is/are functionalized by different alkyl or aryl groups, such as Me, nBu or Ph.90,92 On the other hand, when [(Cp WH3){B4H8}] was treated with BHCl2
SMe2, five chlorine(s)-substituted W2B5 type metallaborane clusters (99–103) have been isolated.93 All of these M2B4 and M2B5 types functionalized metallaboranes have 6 SEP, which is in accord with their geometries.

Scheme 8 Syntheses of B-functionalized metallaheteroboranes 75–88. Core geometries of 75–88 with boron labeling are shown in the box.

A series of dimetallaheteroboranes of group 6 metals (Mo and W) have been synthesized and structurally characterized with different structural motifs. In general, they have triple-decker structures with 2–6-membered open middle decks. Most of these group 6 dimetallaheteroborane clusters are synthesized from the reactions of in situ intermediates generated in the reactions of [Cp MCl4] and LiBH4
THF, with different types of chalcogen sources, such as E-powder, E2R2 (E ¼ ]S, Se or Te), CS2, (CH2)2(SH)2, etc. In few cases, preformed metallaboranes are also utilized instead of the in situ generated intermediates. Cluster 104 has a similar type M2BE core (E ¼ ]S) as that of group 5 metallaheteroboranes 25–27, and it electronically and structurally resembles diborane(6).88 Cluster 104 has 6 SEP, which is in accord with its tetrahedral core. On the other hand, clusters 105–107 have triple-decker M2B3E and M2B2E2 cores having acyclic 4-membered middle decks.88,94 By contrast, metallaheteroboranes 109–11389,95–98 and 114–12289,98–102 have open five- and six-membered middle decks, respectively. The geometries of clusters 109–113 are of the M2B4E type (E ¼ ]O, S, Se, Te) and can be described as a bicapped trigonal bipyramid or pentagonal bipyramid with one missing BdE edge, whereas the geometries of clusters 114–122 are M2B5E and M2B4E2 types (E ¼ ]O, S, Se, Te) and can be described as a hexagonal bipyramid with one missing EdE edge. The group 16 elements are located at the open faces of clusters 114–122. All of these metallaheteroboranes obey electron counting rules except the metallaheteroboranes (114, 115 and 122) incorporating oxygen. For example, 105–113 have 6 SEP, and 116–121 have 7 SEP as expected. Metallaheteroborane 116 was also functionalized in the manner of the metallaboranes discussed in the previous paragraph. The reaction of cluster 116 with LiBH4
THF, followed by treatment with NaI led to the iodination of terminal BdH bonds and afforded [(Cp Mo)2B4S2H2I2] (123) and [(Cp Mo)2B4S2HI3] (124).91

Polyhedral Metallaboranes and Metallacarboranes

275

Fig. 11 Molecular structures of metallaboranes 125–130.

Like metallaboranes 69–71, metallaheteroboranes having triple-decker structures with closed 6-membered middle decks have also been synthesized. For example, the thermolysis reaction of [(Cp Mo)2{1-(Cl)B4TeH5}] (112) with three equivalents of [Co2(CO)8] led to the formation of mixed-metal metallaheteroborane [(Cp Mo)2{B3H3TeCo2(CO)5}] (125) having a hexagonal bipyramidal geometry (Fig. 11).96,97 The middle planar hexagonal ring consists of three boron, one tellurium, and two cobalt atoms. Although a closo-hexagonal bipyramidal cluster should have 9 SEP, cluster 125 has only 6 SEP. Therefore, cluster 125 is better described as an unsaturated 24-valence electron (VE) triple-decker complex, and as such does not obey the “30 and 34-electron rule”. On the other hand, thermolysis of [(Cp ∗Mo)2B4EH6] (E ¼ ]S (109) or Se (110)) or [(Cp Mo)2{1-(Cl)B4TeH5}] (112) with [Ru3(CO)12] afforded metallaheteroboranes [(Cp Mo)2{B4H4ERu(CO)3}] (E ¼ ]S (126) or Se (127) or Te (128)), respectively.81 In triple-decker clusters 126–128, the central six-membered rings are puckered and consist of four B, one chalcogen, and one Ru atom (Fig. 11). Triple-decker clusters 126–128 are also unsaturated 24-VE clusters. By contrast, the room-temperature reactions of [(Cp∗Mo)2B4EH6] [E ¼ ]S (109) or Se (110)] with [Co2(CO)8] afforded unusual 11-vertex metallaheteroboranes [(Cp Mo)2B4H4E(m3-CO)Co2(CO)4] (E ¼ ]S (129) or Se (130)), respectively.97 Clusters 129 and 130 do not display the structural motif expected for a closo-9-vertex cluster, i.e., tricapped trigonal prism. Instead, they are isoelectronic and isostructural with dimetallaborane clusters [(Cp M)2B7Hn], [M ¼ ]W, n ¼ 9; M ¼ ]Re, n ¼ 7] (Fig. 11).

9.06.2.1.4

Metallaborane clusters of group 7 ( Table 4)

During the period 2005–2021, metallaborane clusters of group 7 met with very little success. The photolysis reactions of group 7 metal carbonyls [M2(CO)10] (M ¼ ]Mn or Re) with monoborane BH3
THF under different conditions led to the isolation of metallaboranes [M2(CO)8B2H6] (M ¼ ]Mn (131) or Re (132)) or [{Re(CO)4}2(B2H6)] (133) or [{Mn(CO)3}2(B4H8
THF)] (134) (Scheme 9).103–105 Clusters 131–133 have a butterfly M2B2 core. In 131 and 132, the wingtips of the butterfly core are Table 4

Metallaboranes of group 7.

No.

Compounds

11

131 132 133 134 135 136

[Mn2(CO)8B2H6] [Re2(CO)8B2H6] [{Re(OC)4}2(m-Z2:Z2-B2H6)] [{Mn(OC)3}2(B4H8
THF)] [{Mn2(CO)5(PCy3)}(B2H5
PCy3)(m-H)] [(Cp Re)2B10H16]

31.0, 11.4 6.4, −1.1 −50.1 34.5, 25.4, 2.2 −40.1, −57.9 68.6, 13.2, 4.1, 2.9, 0.2, −5.7, −8.9, −15.3, −19.8

a

Av. RedB Av. MndB c Av. RedRe d Av. MndMn distances b

Scheme 9 Synthesis of metallaboranes 131–134.

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

− 2.43a 2.457a 2.215b 2.279b 2.194a

− 3.1104(9)c − − 2.8757(7)d −

103 103 104 105 104 106

276

Polyhedral Metallaboranes and Metallacarboranes

occupied by one metal, and one boron and an MdM bond is present. By contrast, both of the metal atoms are located at the wingtips of the butterfly core of 133. Further, 133 can be viewed as diborane(6) stabilized in the coordination sphere of two rhenium atoms. Clusters 131–133 have 7 SEP, which is in accord with their arachno-butterfly geometry. On the other hand, cluster 134 has a pentagonal pyramidal structure and consists of expected 8 SEP. Interestingly, a THF is attached to the apical boron atom of cluster 134. The reaction of a preformed metallaborane [{(OC)4Mn}{Mn(CO)3}2(B2H6)(m-H)] with PCy3 led to the formation of [{Mn2(CO)5(PCy3)}(B2H5
PCy3)(m-H)] (135) (Fig. 12), along with two fused metallaboranes and a monometallic species having a ĸ3-borohydride with a terminal phosphine-stabilized boryl substituent.104 The formation of 135 is postulated to have occurred from the starting material by the removal of one {Mn(CO)4} fragment by a phosphine induced cleavage reaction. It can be described as diborane(5) having a base-stabilized boryl unit. From a cluster viewpoint, 135 has a tetrahedral Mn2B2 core with 6 SEP. On the other hand, a unique open 12-vertex rhenaborane cluster was synthesized from the thermolysis reaction of [(Cp Re)2B4H8] (136) with 10 equivalents of BH3
THF.106 Cluster 136 has an exo-polyhedral boron atom. The principal cluster framework of 136 (Fig. 13) contains one degree-six ruthenium vertex; hence, it can be derived from a 13-vertex docosahedron.

Fig. 12 Synthesis of metallaborane 135 (note that other products are not shown in the scheme).

Fig. 13 Core geometries of metallaboranes 136, 169–173. Note that bridging hydrogens and metal-attached ancillary ligands are not shown for clarity.

9.06.2.1.5

Metallaborane clusters of group 8 ( Table 5)

Metallaboranes of group 8 metals are dominated by ruthenaboranes. Nevertheless, there are few examples of ferraboranes. For example, the thermolysis reaction of [Cp Fe(CO)2I] with monoborane BH3
THF led to the isolation of arachno-[Cp Fe(CO)B3H8]

Polyhedral Metallaboranes and Metallacarboranes

Table 5 No. 137 138 139 140 141 142 143 144 145 146 149 150 151 152 153 154 155 156 157 158 164 166 167 168 169 170 171 172 173 174 175 176 177 181 182 183 184 186 187 188 189 190

277

Metallaboranes of group 8. Compounds [Cp Fe(CO)B3H8] [CpFe(CO)B2H4Se(Ph)] [CpFe(CO)B2H4Se(CH2Ph)] [CpFe(CO)B2H4S(2,6-tBu2-C6H2OH)] [{Cp Ru(CO)}2B2H6] [{Cp Ru(CO)}2B2H5(C6H4O2H)] [{Cp Ru(CO)}2BH4(Bcat)] [{Cp Ru(CO)}2B3H7] [{Cp Ru(CO)}2H(CO)Mn(CO)3BH] [{Cp Ru(CO)}2(CO)Fe(CO)3BH] [{Cp Ru(CO)}3(m3-H)BH] [(Cp Ru)2(H){Ru2(CO)8}B] [Cp Ru(CO){Ru(CO)3}4B] [(Cp Ru)2(H){Ru4(CO)12}B] [(Z6-C6Me6)HRuB4H9] [(Z6-C6Me6)RuB4H8] [(Z6-C6Me6)RuB5H9] [(Z6-C6Me6)RuB8H14] [(Cp Ru)B5H10] [(Cp Ru)2(m-H){B4H7Cl2}] [Cp 3IrRu2B5H9] [(Cp Ru)2B6H10Cl2] [(Cp Ru)2B6H10(OH)2] [(Cp Ru)2B6H11(OH)] [(Cp Ru)2B8H11Cl] [Cp 2Ru2B10H16] [Cp 2Ru2B10H16] [(Z6-p-cym)OsB10H10] [1,1,3-(dppe)2-1,2-FeSB9H9] [(Cp Ru)2B3H6(m-SPh)] [(Cp Ru)2B3H6(m-SePh)] [(Cp Ru)2B3H6(m-TePh)] [(Cp Ru)2B3H8(SPh)] [(Cp Ru)2B4H9(SePh)] [(Cp Ru)2B2H6S2] [(Cp Ru)2(m-S)2(m3-S)4B2H2] [(Cp Ru)2(m-Se)2(m3-Se)4B2H2] [(Cp Ru)2(B3H8)(CS2H)] [(Cp Ru)2(Me)(S2B2H3)] [CpRu(PPh3)S2B6H9] [(Z6-p-cym)RuS2B6H8] [(Z6-p-cym)Ru(PMePh2)S2B6H6]

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

− − − − 2.9258(10)e 2.926(5)e − 2.9386(8)e 2.7789(5)e, 2.787g 2.7584(2)e, 2.708f 2.75e 2.844e 2.865e − – – − – − 2.8644(2)e 3.0065(4)e − − − 2.9012(10)e − − − − − 2.8259(3)e 2.8927(4)e 2.9750(6)e 2.8633(4)e 2.8709(4)e − 2.8766(9)e 2.9738(3)e 2.6388(10)e − – −

107 108 108 108 52 109 109 110 52 52 111 111 111 111 112 112 112 112 113 114 115 114 105 105 114 106 106 116 117 118 118 118 119 119 118 120 120 121 379 122 126 126

a

−0.5, −41.3 −25.1 −24.1 −28.4 21.3, −3.0 11.6, 13.5 23.6, 8.5 54.5, −9.7 89.0 119.3 91.3 207.7 174.6 191.8 0.2, −8.4 2.9, −12.2, −18.4 40.1, 20.2, −17.7 13.9, 9.0, 6.4, 1.9, 0.6, −6.8, −6.8, −49.0 −0.6 30.43, 0.08 104.5, 82.6, 5.5 38.1, 11.2 37.8, 24.1 38.7, 38.0, 25.7, −0.5 106.1, 63.1, 30.3, 17.2, 14.9, 13.7, 3.2, −24.1 24.9, 18.1, 14.7, 14.9, 11.5, 6.5, 4.6, −0.8, −4.9, −6.1 24.8, 14.2, 11.2 93.0, 17.5, 11.8, 4.8 25.8, −1.9, −23.9, −28.9 64.7, 20.9 64.6, 20.2 64.8, 21.9 30.2, 4.9, −1.7 20.3, 19.5, 4.9 18.6, 16.8 8.8, −4.0 1.7, −10.2 35.6, −3.1, −13.0 13.5 8.4, −22.7, −23.9, −50.4 3.7, −13.9, −33.8, −49.2 10.8, −12.4, −20.8, −47.9

2.249 − − − 2.283b 2.255b − 2.211b 2.175b, 2.115(5)d 2.122b, 2.031(3)a 2.13b 2.095b 2.104b − – – 2.209b – − 2.182b 2.189b, 2.179d 2.261b 2.252b − 2.177b 2.201b 2.189b 2.232c − − 2.185b 2.180b 2.203b 2.202b 2.235b − − 2.207b 2.154b − – 2.288b

a

Av. FedB Av. RudB c Av. OsdB d Av. MdB distance between boron and other than group 8 metal e Av. RudRu f Av. RudFe g Av. RudMn b

(137) (Scheme 10a).107 Cluster 137 has an arachno-butterfly geometry. The iron atom is located at one of the wingtips of the butterfly core. Three chalcogen analogs of cluster 137, [CpFe(CO)B2H4E(R)] (138: E ¼ ]Se, R ¼ ]Ph; 139: E ¼ ]Se, R ¼ ] CH2Ph; 140: E ¼ ]S, R ¼ ](tBu)2C6H2OH) were isolated from the reactions of [Cp Fe(CO)2I] with LiBH4
THF and E2R2 (Scheme 10a).108 The wingtips of the butterfly cores in 138–140 are occupied by iron and chalcogen atoms. These butterfly clusters (137–140) have 7 SEP. On the other hand, the reaction of ruthenaborane [1,2-(Cp RuH)2B3H7] with [Mo(CO)3(CH3CN)3] resulted in cluster degradation, followed by carbonylation at the metal centers, and afforded the diruthena analog of cluster 137, [{Cp Ru(CO)}2B2H6] (141).52 Further reaction of arachno-141 with HBcat (cat ¼ 1,2-O2C6H4) yielded [{Cp Ru (CO)}2B2H5(C6H4O2H)] (142) and [{Cp Ru(CO)}2BH4(Bcat)] (143) (Scheme 10b).109 Although clusters 142 and 143 are electronically and structurally analogous to 141, cluster 143 contains a boryl unit. The pathway for the formation of 142 and 143 is not clear. However, the molecular entity and composition of these compounds show that compound 142 is transformed into 143 with the release of hydrogen by the reaction of the phenolic hydroxyl group with the BdH bond.

278

Polyhedral Metallaboranes and Metallacarboranes

Scheme 10 Synthesis of arachno-butterfly clusters (137–143).

Utilizing arachno-[{Cp Ru(CO)}2B2H6] (141) and various metal carbonyl, a series of metal-rich metallaborane were synthesized (Scheme 11). For example, the reaction of arachno-141 with [Mn2(CO)10] yielded a bimetallic [{Cp Ru(CO)}2B3H7] (144)110 and trimetallic [{Cp Ru(CO)}2(H)(CO)Mn(CO)3BH] (145)52. The core structure of 145 is a five-vertex arachno cluster, which can be generated by opening a basal-apical edge of a nido-square pyramidal cluster. In accord with its arachno geometry, it has 8 SEP. On the

Scheme 11 Synthesis of metal-rich metallaborane clusters (144–152).

Polyhedral Metallaboranes and Metallacarboranes

279

other hand, trimetallic 145 has a tetrahedral core and contains 6 SEP. In cluster 145, a borylene (BH) unit bridges between two Ru and one Mn in m3-fashion. The iron analog of 145, [{Cp Ru(CO)}2(CO)Fe(CO)3BH] (146) was isolated under similar reaction conditions utilizing [Fe2(CO)9].52 Along with 146, two metallaborane clusters (147 and 148) having bis-borylene units were also isolated (vide infra).52 When arachno-141 was treated with [Ru3(CO)12] under thermolytic conditions, the reaction afforded four metal-rich metallaboranes (149–152).111 In 149, a borylene {BH} unit is triply bridging between three Ru atoms. Cluster 149 is also an analog of 145, and has 6 SEP as expected for the tetrahedral core. On the other hand, clusters 150–152 are examples of metal-rich boride clusters. In 150 and 151, naked boride boron atoms are located at the semi-interstitial positions of tetrametallic butterfly and pentametallic square pyramid cores, respectively. By contrast, the naked boride boron atom is located at the interstitial position of a hexametallic octahedral core in 152. Both boride clusters 151 and 152 have 6 SEP, which is in accord with their nido 5-vertex and closo 6-vertex geometries, respectively. On the other hand, cluster 150 can be considered as a 6 SEP closo-trigonal bipyramidal cluster. Kennedy and co-workers isolated several ruthenaborane clusters utilizing preformed borane and ruthenium precursors. The reaction of arachno-[B6H11]− with [RuCl2(Z6-C6Me6)2] afforded boron-rich ruthenaboranes 153–156 (Scheme 12).112 Cluster 153 has a square pyramid structure with a missing basal edge and analogous to arachno-144. Cluster 153 has 8 SEP, which is consistent with the 5-vertex arachno-cluster. By contrast, clusters 154 and 155 have square pyramid and pentagonal pyramid structures, respectively. Both the nido-clusters 154 (7 SEP) and 155 (8 SEP) obey Wade’s electron counting rule. On the other hand, ruthenaborane 156 has an unusual open 9-vertex structure. Fehlner and co-workers isolated the analogs of cluster 155, monometallic nido-[(Cp Ru)B5H10] (157)113 and bimetallic nido-[(Cp Ru)2(m-H){B4H7Cl2}] (158)114 from the reaction of nido-[1,2-(Cp RuH)2B3H7] with BH3
THF and BHCl2
SMe2, respectively (Scheme 13). The reaction with BHCl2
SMe2 also afforded few chlorine-functionalized fused ruthenaboranes 159–163 (vide infra).114 Nido-157 and 158 also obey electron counting rules.

Scheme 12 Synthesis of metallaborane clusters (153–156).

Scheme 13 Synthesis of metallaborane clusters (157 and 158) and fused clusters (159–163).

280

Polyhedral Metallaboranes and Metallacarboranes

Fehlner and co-workers have established one more new synthetic methodology, where two preformed metallaboranes are exploited in the reaction. For example, the reaction of nido-[1,2-(Cp RuH)2B3H7] with arachno-[Cp IrB3H9] in hexane under reflux condition afforded nido-[Cp 3IrRu2B5H9] (164) along with one fused metallaborane 165 (vide infra) (Scheme 14).115 Cluster 164 exhibits an 8-vertex cluster shape that can be derived from a tricapped trigonal prism by removing one degree-five vertex. But this nido cluster has only 9 SEP rather than the prescribed 10 SEP. On the other hand, when a preformed metallaborane nido[(Cp Ru)2B6H12] was treated with BHCl2
SMe2 under thermolytic condition, the reaction led to the chlorination of two of the terminal BdH bonds of nido-[(Cp Ru)2B6H12] and afforded [(Cp Ru)2B6H10Cl2] (166).114 Cluster 166 has a similar nido-8-vertex geometry as that of 164, but it obeys the electron counting rule, unlike cluster 164. Two similar mono and bis hydroxy-substituted nido-8-vertex clusters (167 and 168) were also isolated from the thermolysis reaction of nido-[1,2-(Cp Ru)2(m-H)B4H9] and BH3
THF.105

Scheme 14 Reaction between two metallaborane clusters.

A good number of higher vertex group 8 metallaboranes was isolated during the period 2005–2021. The thermolysis reaction of [(Cp Ru)2B8H12] with BHCl2
SMe2 afforded an unusual 10-vertex ruthenaborane 169.114 The 10-vertex ruthenaborane 169 has a structure (Fig. 13) similar to that of nido-B10H14 but with a DSD rearrangement and can be considered as an isonido cluster with 11 SEP. On the other hand, the thermolysis reaction of [(Cp Ru)2B4H8] with BH3
THF afforded open 12-vertex ruthenaborane 170.106 Cluster 170 is a structural analog of rhenaborane 136 and contains 13 SEP. Further thermolysis of cluster 170 in [D6]-benzene for 12 days afforded a different isomer of 170, ruthenaborane 171.106 Cluster 171 has a 12-vertex open framework (C2v symmetry) with six bridging BdHdB units on the open face. The framework of 171 can be derived from a capped truncated tetrahedron. Two Ru atoms occupy two of the four hexagonal faces; the other two hexagonal faces and the two adjacent degree-three vertices are unoccupied. On the other hand, an 11-vertex osmaborane 172 was synthesized from the reaction of [Os(p-cym)Cl2]2 with [NEt3H]2[B10H10].116 Cluster 172 has an octadecahedral geometry with 11 SEP; thus, it can be described as isocloso. A structural analog of 172, ferraheteroborane 173 was synthesized from the reaction of [SB9H11]2− and [FeCl2(dppe)].117 Cluster 173 has 12 SEP, which is consistent with the closo-bonding topology. A series of ruthenaheteroborane clusters containing chalcogen atoms have been synthesized utilizing nido-[1,2(Cp RuH)2B3H7] and different types of chalcogen sources, such as E powder, E2Ph2, CS2, and Li[BH3(EPh)] (E ¼ ]S or Se). For example, the reaction with E2Ph2 afforded clusters 174–176, which have the same square pyramidal core as that of nido-[1,2(Cp RuH)2B3H7].118 The primary difference is the presence of bridging EPh instead of three bridging H atoms. Here m-EPh provides the same numbers of electrons (3e) as three bridging hydrogens, to the total 7 SEP count of this type of nido cluster. When nido-[1,2-(Cp RuH)2B3H7] was treated with Li[BH3(SPh)], a five-vertex arachno-ruthenaheteroborane 177 along with three monocapped square pyramid clusters 178–180 (vide infra) were obtained (Scheme 15).119 Cluster 177 is the structural analog and isoelectronic (8 SEP) with arachno-153. When the same reaction was performed with Li[BH3(SePh)], a diruthena analog of nido-158, [(Cp Ru)2B4H9(SePh)] (181) was accessed (Scheme 15).119 Cluster 181 has a terminal B-(SePh). When the same starting material was reacted with chalcogen powders, clusters 182–185 were isolated (Scheme 15). Although clusters 181 and 182 have the same structural shape, two of the boron vertices of 181 are substituted by sulfur in 182.118 On the other hand, clusters 183 and 184 are examples of bis-homocubane analogs and have 70 CVE as expected.120 The reaction with CS2 also afforded a diruthenium analog of pentaborane(11), 186.121 The core structure of 186 is attached to a dithioformato ligand. Cluster 186 also obeys the electron counting rule with 8 SEP.

Polyhedral Metallaboranes and Metallacarboranes

281

Scheme 15 Syntheses of metallaheteroboranes (177 and 181–185) and fused clusters (178–180).

Ferguson et al. isolated a unique ruthenaheteroborane [CpRu(PPh3)RuS2B6H9] (188) from the reaction between [CpRuCl (PPh3)2] and [tmndH][hypho-1,2-S2B6H9] at room-temperature.122 Compound 188 can be described as either a hypho-9-vertex {RuS2B6} cluster or a coordination compound of ruthenium which contains a bidentate Z2-dithiaborate cluster ligand (Fig. 14). The reaction with a different ruthenium precursor, [(p-cym)RuCl2]2 afforded arachno-189.122 Further reaction of arachno-189 with PMePh2 afforded another arachno-{RhS2B6} cluster, 190.122 Structural analysis revealed that clusters 189 and 190 have a typical arachno-9-vertex structure in which the Ru atom occupies a position adjacent to both S atoms (Fig. 14).

Fig. 14 Molecular structures of metallaboranes 188–189.

9.06.2.1.6

Metallaborane clusters of group 9 ( Table 6)

A large number of group 9 metallaboranes (191–233) were synthesized utilizing group 9 metal precursors, such as [Cp CoCl]2, [Cp MCl2]2 (M ¼ ]Rh and Ir) and monoborane reagents. These reactions afforded different types of closo, nido or arachno-clusters with smaller nuclearity to higher nuclearity. A few of these metallaboranes have also been synthesized by utilizing preformed metallaboranes and monoboranes. Cluster 191 has a nido-pentagonal pyramid shape as that of B6H10 and has 8 SEP.123 By contrast, 6-vertex clusters 192–194 have octahedral core and 7 SEP.124 When an analog of 192–194, [(Cp Co)2B4H6] was treated with PtBr2, the reaction afforded BdBr inserted [(Cp Co)2B4H2Br4] (195).125 Eight-vertex closed iridaboranes 196–198 have unique electronic structures (Fig. 15). Clusters 196 and 197 have a dodecahedral core as [B8H8]2− (D2d).126,127 Interestingly, clusters 196 and 197 have 8 SEP; thus, they can be described as isocloso. By contrast, cluster 198 was found to exhibit a cluster shape that can be derived from a 9-vertex tricapped trigonal prism by removing one of the capped vertices. Cluster 198 is also hypoelectronic, as it has 8 SEP instead of 10 SEP.126 On the other hand, 8-vertex clusters 199–201 can be considered as nido structures based on a closo-tricapped trigonal prism with one vertex missing (Fig. 15).127,128 Although both clusters 199 and 200 have four bridging hydrogens, cluster 201 has

282

Polyhedral Metallaboranes and Metallacarboranes

Table 6 No.

Metallaboranes of group 9.

Compounds

191 192 193 194 195 196 197 198 199 200 201 202

[(Cp Ir)B5H9] [(Cp Co)2B4H7(Me)] [(Cp Co)2B4H7(Me)] [(Cp Co)2B4H6(Me)2] [(Cp Co)2B4H2Br4] [(Cp Ir)2B6H6] [(Cp Ir)3B5H4Cl] [(Cp Ir)2B6H6] [(Cp Co)2B6H10] [(Cp Rh)2B6H10] [(Cp Ir)2B6H10] [(Cp Co)2B7H6(OMe)]

203 204 205 206 207 208 209

[(Cp Ir)2B7H7] [1,7-(Cp Ir)2B8H8] [1,4-(Cp Ir)2B8H8] [1,4-(Cp Ir)2B8H7Cl] [(Cp Rh)3B7H7] [(Cp Rh)4B6H6] [(Cp Co)B9H13]

210 211 212 213 214

[((Cp Rh)B9H13)] [(Cp Rh)B9H13] [(Cp Ir)B9H13] [(Cp Co)2B8H11Me] [(Cp Co)2B8H12]

215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250

[(Cp Rh)2B8H12] [(Cp Rh)2B8H12] [(Cp Ir)2B8H12] [(Cp Ir)2B8H13(m-BH2)] [(Cp Ir)2B8H12] [(Cp Rh)3B7H11] [(Cp Ir)3B7H11] [(Cp Co)3B6H8O] [(Cp Co)3B8H8] [(Cp Co)3B8H7(Me)] [(Cp Rh)2B9H9] [(Cp Ir)2B9H9] [(Cp Rh)3B8H9(OH)3] [(Cp Rh)2B10H12] [(Cp Rh)2B10H9(OH)] [(Cp Rh)2B10H8(OH)2] [(Cp Ir)2B10H12] [(Cp Ir)2B10H8(OH)2] [(Cp Co)3B8H8S] [(Cp Co)2(m-CO)Cr(CO)5BH] [(Cp Co)2(m-CO)Mo(CO)5BH] [(Cp Co)2(m-CO)W(CO)5BH] [(Cp Rh)2(m-CO)Mo(CO)5BH] [(Cp Rh)2(m-CO)W(CO)5BH] [(Cp Co)2(CO)2HCo(CO)3BH] [(Cp Co)2(CO)H2MnH(CO)3BH] [(Cp Co)2(CO)H2ReH(CO)3BH] [(Cp Rh)2(CO)3{Co(CO)}(m-H)BH] [(Cp Rh)2(CO)3{MnH(CO)3}BH] [(Cp Rh)2(CO)2{Co2(CO)4}BH] [(Cp Co)(m-H)3Ru3(CO)9B] [(Cp Rh){Ru(CO)3}4RuH(CO)2B] [(Cp Rh)2{HOs4(CO)12}B] [(Cp Rh)2Mo(CO)5(CN-tBu)BH] [(Cp Rh)2W(CO)5(CN-tBu)BH] [(Cp Rh)2Mo(CO)5(dppm)BH]

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

2.172 2.102a 2.104a – 2.088a 2.161c 2.149c 2.15c 2.071a 2.166b 2.185c 2.055a

– 2.5878(4)e 2.5773(7)e – 2.5909(18)e − 2.79g − – − − –

123 124 124 124 125 126 127 126 127 128 127 124

2.151c 2.20c 2.169c 2.206c 2.138b 2.138b 2.10a

− − 2.8025(8)g 2.8172(8)g 2.814f 2.792f −

126 130 130 131 132 133 134

2.219b 2.138b 2.14c 2.11a 2.066a

− − – – −

130 128 123 135 134

2.146b 2.132b 2.137c 2.146c − 2.138b 2.163c 2.085a 2.103a 2.103a 2.184b 2.192c 2.169b 2.22b 2.23b 2.218b 2.222c 2.228c 2.10a – 1.947a, 2.406(5)d 1.951a, 2.392(6)d 2.037b, 2.381(3)d 2.045b, 2.396(5)d 2.029a 1.998a, 2.096(15)d − 2.153b, 1.984(16)a 2.122(14)d, 2.066b 1.970(18)a, 2.082b 1.917(4)a, 2.138d 1.979(8)b, 2.091d 2.02b, 2.086d 2.31(2)d, 2.015b 2.279(4)d, 2.081b 2.298(13)b, 2.063b

– − – – − 2.138f − – – – − − − 2.9547(7)f 2.8040(6)f 2.811(2)f 2.9123(3)g 2.8183(5)g – – 2.4264(7)e, 2.832i 2.4238(8)e, 2.821i 2.6339(3)f, 2.91i 2.6342(4)f, 2.904i 2.5646e 2.4643(18)e, 2.683i − 2.6980(15)f, 2.598i 2.6669(9)f, 2.783i 2.6499(14)f, 2.431(3)e, 2.581i 2.794j, 2.74i 2.931i, 2.93j 2.95j, 2.904i 2.6356(18)f, 2.86i 2.6446(5)f, 2.859i 2.6388(10)f, 2.864i

135 128 123 123 130 135 131 135 124 124 130 130 128 132 132 132 127 127 124 136 136 136 137 137 138 138 138 139 139 139 138 67 206 204 204 204

c

−1.79 71.4, 61.0, 14.9 61.5, 27.0, 17.1 69.7, 58.9, 24.5, 14.8 43.0, 11.8 92.1, 64.6, −23.1 87.1, 68.9, 65.0, −17.4 73.1, 51.8 38.1, 9.3 37.1, 5.3 34.4, −8.27 151.3, 110.3, 103.8, 45.0, 31.1, 15.5, 10.0 102.6, 63.2, −4.9 88.9, 72.2, 34.8, −16.4, −19.4 81.9, 71.1, 18.5, 13.4, −10.6 81.2, 72.9, 17.7,13.4, −10.6 35.4, 18.8, 11.2, −2.5, −18.9 38.7, 21.4 29.4, 27.1, 12.5, 4.1, 3.5, 1.7, −1.1, −14.5, −36.9 20.4, 13.4, 5.3, −0.6, −10.2, −29.8 15.3, 10.7, 3.2, −1.5, −3.5, −41.9 12.9, 9.1, −2.5, −3.9, −10.9, −43.4 54.0, 29.7, 22.2, 8.7, 3.3, −40.1 42.4, 30.1, 25.3, 20.1, 9.1, 6.3, −1.2, −12.9 38.1, 32.6, 15.3, 6.8, 3.2, 0.6, −1.2, −14.3 21.1, 9.4, 1.9 1.8, −0.2, −12.9 0.4, −2.6, −4.1, −6.1, −8.2, −9.3, −10.5 34.9, 32.4, 12.4, 11.7, −10.0 60.3, 30.2, 14.9, 1.5, −3.3 65.2, 10.1, −8.8, −10.4, −12.6 62.5, 34.5, 27.0, 22.6 138.2, 57.1, 31.7 141.0, 55.9, 42.4, 31.3 128.3, 88.9, 33.6, 29.8, 28.0, 18.5, 17.8 98.6, 82.7, 52.4, 19.0, 14.2, 7.0, 3.4 36.2, 35.1, 21.9, −0.8, −5.1 82.1, 21.7, 9.0, −10.1 87.7, 29.4, 12.2, 2.4, −8.4, −12.1, −19.9 87.4, 26.9, 4.7, −13.3 64.3, 12.3, 0.9, −23.8 74.7, 14.7, −3.4, −18.4 41.3, 29.7, 7.0, 1.5, −2.9 138.3 139.3 141.3 126.1 127.9 139.4 101.1 91.7 89.0 126.6 121.4 89.5 177.6 178.6 124.8 123.1 124.9

283

Polyhedral Metallaboranes and Metallacarboranes

Table 6 No.

(Continued)

Compounds

251 252 253 254 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301

[(Cp Rh)2W(CO)5(dppm)BH] [(Cp Rh)2Mo(CO)5(dppe)BH] [(Cp Rh)2W(CO)5(dppe)BH] [(Cp Co)2B3H3(m-CO)Fe(CO)3] [(Cp Co)2B2H5Mo2(CO)6I] [(Cp Co)3B6H7Co(CO)2] [(Cp Rh)2Fe(CO)3(m-CO)B3H2Cl] [(Cp Rh)2Fe2(CO)5(m-CO)2B2H2] [(Cp Rh)3Fe(CO)2(m3-CO)2B2H2] [(Cp Rh)3Fe(CO)2(m3-CO)2B2HCl] [(Cp Co)2(AuPPh3)B6H9] [(Cp Rh)2(AuPPh3)2B6H8] [(Cp Rh)2(AuPPh3)2B6H8] [(Cp Rh)2(AuPPh3)2B8H10] [(Cp Co)2B2H2S2] [(Cp Co)2B2H2Se2] [(Cp Rh)2B2H2Se2] [(Cp Ir)2B2H2Se2] [(Cp Co)2B5H5Te2] [{Cp Co}Fe(CO)3B2H2Se2] [(Cp Rh)Fe(CO)3B2H2Se2] [(Cp Co)2{m-B2H2S2Pd(Cl)2}] [(Cp Co)2{m-B2H2Se2Pd(Cl)2}] [(Cp Rh)2{m-B2H2Se2Pd(Cl)2}] [(Cp Ir)2{m-B2H2Se2Pd(Cl)2}] [(Cp Co)2(m-Se)(m3-Se)4B2H2] [(Cp Co)2(m-S)2(m3-S)4(BH)2] [(Cp Co)2(m-Se)2(m3-Se)4(BH)2] [(Cp Rh)2(m-S)2(m3-S)4B2H2] [(Cp Rh)2(m-Se)2(m3-Se)4B2H2] [(Cp Ir)2(m-S)2(m3-S)4(BH)2] [(Cp Ir)2(m-Se)2(m3-Se)4(BH)2] [(Cp Ir)2(m-S)2(m3-S)4(m-BH2)2] [(Cp Co)2(m-S)3(m3-S)4B2H2] [(Cp Co)2(m-S)3(m3-S)4B2H2] [Cp Co(m-S)3(m3-S)4B3H3] [Cp Co(m-Se)3(m3-Se)4B3H3] [(Cp Rh)2(m-Se)3(m3-Se)4(BH)2] [(Cp Ir)2(m-S)3(m3-S)4(BH)2] [(Cp Co)2(m-S)4(m3-S)4B2H2] [(Cp Co)2B6S2H6] [(Cp Co)2B6Se2H6] [(Cp Co)2B6Te2H6] [(Cp Rh)2B6Se2H6] [(Cp Co)2B6H9(SPh)] [(Cp Co)2B6H9(SePh)] [(Cp Co)2B6H9(TePh)] [(Cp Co)2B6Se2H6{Fe(CO)3}] [9-Cp -6,9-NRhB8H11] [8-Cp -6,8-NRhB8H11]

302

[8-Cp -6,8-NIrB8H11]

303 304 305

[9,9-(PPh3)2-9-H-6,9-NIrB8H11] [8,8-(PPh3)2-8-H-6,8-NIrB8H11] [9,9,9,9-(CO)(H) (PPh3)2-9,6-RhSB8H10] [9,9,9,9-(CO)(H) (PPh3)2-9,6-IrSB8H10] [6,6,9-(PPh3)3-6,5-RhSB8H9] [2,2,2-(H)(PPh3)2-2,1-RhSB8H8] [2,2,2-(Cl)(H)(PPh3)-6-(PPh3)2,1-RhSB8H7]

306 307 308 309

11

B NMR (ppm)

Av. MdB (Å) d

Av. MdM (Å) b

f

Ref. i

123.7 124.2 123.1 71.9, 46.1 75.7 125.4, 33.6 76.2, 65.5 111.9 116.6, 110.3 111.8, 94.7 45.2, 38.1, 10.9 43.7, 38.4, 6.5 43.7, 38.4, 6.5 41.6, 33.6, 12.8, 5.2 22.7 33.5 23.9 7.3 −3.6, −8.5 31.2 22.2 32.0 38.5 27.9 13.4 −5.6, −12.9 3.5, 0.9 −4.7, −10.1 5.4, −4.9 4.1, −6.8 1.4, −0.0 −7.9, −8.7 −2.6 5.2, 3.6 7.1, 5.2 6.3 0.2 −2.0, −4.3 4.3, 2.2 −4.6, −13.9 31.8, 22.1 30.8, 29.8 37.6, 30.7 21.8, 21.3 29.6, 21.5, 13.8, 7.9 29.2, 16.4, 12.0, 8.1 30.8, 25.0, 17.1, 7.6 36.9, 27.5, 23.7, 13.6 8.7, 3.2, 2.4, 1.7, −26.1 46.2, 15.1, 10.4, 6.0, −4.2, −5.3, −6.7, −31.6 52.3, 14.7, 0.4, −1.7, −6.0, −8.9, −11.5, −34.7 8.2, 1.4, −3.2, −21.1, −25.5 34.7, 16.1, 11.3, −10.4, −12.2, −33.4 30.9, 14.4, −7.8, −18.1, −27.8, −29.1

2.267 , 2.063 − − 2.043a, 2.106d 2.042a, 2.472d 2.079a 2.135b, 2.121d 2.114b, 2.128d 2.097b, 2.141(11)d 2.10b, 2.140(7)d 2.073a 2.169b 2.169b 2.148b 2.12a 2.161a 2.231b 2.222c 2.151a 2.215d, 2.123a 2.231b, 2.202d 2.123a 2.134a 2.241b − − − − − − − − − − − − − − − − 2.099a 2.106a 2.106a 2.203b 2.087a 2.087a − 2.105a, 2.278d − −

2.6382(5) , 2.862 − − 2.4995(7)e, 2.564i 2.922i, 2.9559(4)j 2.61e 2.6890(4)f, 2.67i 2.581(3)j, 2.668i 2.752f, 2.613i 2.757f, 2.614i − − − − 3.072e − − − − − − 3.207e, 2.803i 3.22e, 2.893i 3.519f, 2.989i − − − − − − − − − − − − − − − − − − − − − − − − − −

204 204 204 124 125 125 140 140 140 140 127 141 141 141 125 142 143 143 143 142 143 144 144 144 144 145 146 146 120 120 146 146 146 145 145 145 145 146 146 145 147 147 147 147 147 147 147 147 148 148

−

−

148

− − 2.277b

− − −

148 148 149

23.2, 11.9, −8.7, −26.2, −31.2, −32.8

−

−

149

14.3, −4.6, −3.6, −26.2, −36.1 71.9, 6.5, 1.5, −5.3, −26.8 63.6, 6.1, 2.2, 1.2, −3.3, −6.5, −23.3, −25.8

2. 247b 2.295b 2.273b

− − −

150 150 150 (Continued )

284

Polyhedral Metallaboranes and Metallacarboranes

Table 6 No.

(Continued)

Compounds

322

[6,6-(PPh3)2-9-(PCy3)-6,5-RhSB8H9] [6,6-(PPh3)2-9-(2-Mepy)6,5-RhSB8H9] [6,6-(PPh3)2-9-(2-Etpy)6,5-RhSB8H9] [6,6-(PPh3)2-9-(py)-6,5-RhSB8H9] [6,6-(PPh3)2-9-(3-Mepy)6,5-RhSB8H9] [6,6-(PPh3)2-9-(4-Mepy)6,5-RhSB8H9] [8,9-m-(H)-9-(PPh3)2-8-(py)9,6-RhSB8H8] [8,9-m-(H)-9-(PPh3)2-8-(3-Mepy)9,6-RhSB8H8] [8,9-m-(H)-9-(PPh3)2-8-(4-Mepy)9,6-RhSB8H8] [2,2-(Z2-C2H4)(PPh3)-6-(PPh3)2,1-RhSB8H7] [2,2,2-(H)2(PPh3)-6-(PPh3)2,1-RhSB8H7] [9,9,9,9-(CO)(H) (PMe3)2-9,6-IrSB8H10] [8,8,8-(CO)(PMe3)2-8,7-IrSB9H10]

323 324

[1,1,1-(H)(PMe3)2-1,2-IrSB9H9] [8,8,8-(EtNC)(PMe3)2-8,7-IrSB9H10]

325 326

[8,8,8-(PMe3)2(NH3)-8,7-IrSB9H10]
NH3 [8,8,8-(PMe2Ph)2(NH3)-8,7-IrSB9H10]

327

[8,8-(bpy)-8,7-RhSB9H10]

328

[8,8-(Me2bpy)-8,7-RhSB9H10]

329

[8,8-(phen)-8,7-RhSB9H10]

330

[8,8-(py)2-8,7-SB9H10]

331 332 333 334

[8,8-bpy-8-PPh3-8,7-RhSB9H10] [8,8-Me2bpy-8-PPh3-8,7-RhSB9H10] [8,8-phen-8-PPh3-8,7-RhSB9H10] [8,8-bpy-8-NCCH3-8,7-RhSB9H10]

335 336

[8,8-Me2bpy-8NCCH3-8,7-RhSB9H10] [8,8-phen-8-NCCH3-8,7-RhSB9H10]

337 338

[8,8-(PPh3)2-8,7-RhNB9H11] [8,8,8-(PPh3)2H-9-py-8,7-RhSB9H9]

339

[1,1-(PPh3)(Z2-C2H4)-3py-1,2-RhSB9H8] [1,1-(PPh3)(CO)-3-py-1,2-RhSB9H8]

310 311 312 313 314 315 316 317 318 319 320 321

340 341 342 343 344

[1,1-(PPh3)2-3-py-1,2-RhSB9H8] [1-PPh3-1-(2-C2(CO2Me)2)8,7-RhSB9H8-3-py] [1-(PPh3)-1-(2-C2(COt2Bu)2)8,7-RhSB9H8-3-Py] [8,8-(PPh3)2-8-(C^CPh)8,7-RhSB9H9-9-py]

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

b

15.5, −5.3, −38.9, −26.0, −36.2 11.9, −2.7, −5.2, −17.6, −28.1, −36.9

2.25 2.259b

− −

151 151

12.3, −3.1, −5.2, −17.8, −27.7, −36.6

−

−

151

9.1, −6.3, −14.1, −28.9, −37.1 8.7, −6.5, −18.5, −28.9, −37.1

− −

− −

151 151

9.1, −3.6, −7.0, −15.6, −27.6, −37.4

−

−

151

17.7, −0.19, −2.6, −3.6, −15.2, −18.3, −37.4 17.9, −0.21, −2.5, −3.5, −15.6, −18.5, −37.4 17.8, −0.1, −2.6, −3.6, −15.6, −18.3 −37.4 43.5, −4.7, −8.6, −14.4, −22.2, −23.4

−

−

151

55.1, −2.5, −5.2, −6.7, −7.7, −23.6, −26.4 23.6, 10.5, 8.1, −29.1, −29.8, −31.5, −33.2 15.1, 0.8, −1.4, −2.4, −14.4, −17.0, −26.9, −29.6 44.9, 24.6, 7.6, −13.5, −25.9, −28.4 12.6, 0.7, −3.7, −5.7, −16.0, −18.4, −27.3, −30.6 7.8, 3.8, −1.2, −7.9, −9.4, −17.6, −21.2, −30.9 8.4, 3.7, −1.3, −7.4, −8.8, −17.9, −20.4, −30.0, −31.1, 17.7, 11.7, 6.8, 2.7, −15.2, −19.9, −22.0, −32.3 18.8, 12.3, 7.1, 2.3, −15.2, −19.3, −21.7, −32.2 18.8, 12.8, 7.2, 4.4, 2.6, −15.1, −19.2, −21.8, −33.0 17.7, 12.5, 6.9, 2.0, −15.3, −20.8, −22.2, −33.1 10.8, 5.9, −3.8, −18.7, −23.0, −30.9 10.5, 5.7, 6.4, −3.5, −19.1, −23.1, −31.2 10.5, 5.6, −4.3, −19.3, −23.7, −31.2 12.0, 9.1, 8.3, 3.1, −4.6, −19.6, −23.7, −31.9 17.3, 14.3, 8.3, 0.7, −14.1, −18.5, −26.5 12.2, 9.6, 8.5, 3.5, −4.2, −19.8, −23.1, −31.9 18.2, 9.3, −7.6, −9.9, −14.0, −25.5 11.8, 7.9, 3.2, 0.3, −3.7, −10.0, −18.1, −25.6, −28.5 55.7, 26.4, 1.1, −14.5, −22.2, −24.6, −30.3 55.4, 28.1, 1.1, −0.7, −14.4, −26.1, −24.4, −31.9 54.6, 27.3, −0.5, −15.2, −24.2, −30.4 58.3, 26.4, 5.2, 2.0, −15.2, −20.0, −22.5, −27.2 58.2, 26.6, 1.5, 1.3, −14.5, −20.0, −23.3, −27.5 11.7, 4.2, −1.4, −11.5, −14.6, −22.3, −29.3

2.283b

151

2.291b

−

151

2.374b

−

152

2.364b

−

152

2.279c

−

149

2.262b

−

153

2.412b 2.285b

− −

153 153

2.207c

−

117

2.208c

−

117

−

−

154

−

−

154

−

−

154

−

−

154

− − − −

− − − −

154 154 154 154

−

−

154

−

−

154

2.214b 2.212b

− –

155 156

2.35b

−

156

−

−

157

2.356b 2.366b

− −

157,159 159

−

−

159

−

−

159

285

Polyhedral Metallaboranes and Metallacarboranes

Table 6 No. 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376

(Continued)

Compounds [8,8,8-(PPh3)2(H)-9-py-RhSB9H10] [OTf] [8,8,8-(PPh3)(CO)(Cl)-9py-8,7-RhSB9H9] [8,8-(PPh3)(Cl)-9-py-8,7-RhSB9H9] [8,8,8-(Cl)(Py)(PPh3)-9-(Py)8,7-RhSB9H9] [8,8,8-(H)(PPh3)(PMe2Ph)-9-(Py)8,7-RhSB9H9] [8,8,8-(H)(PPh3)(PMe3)-9-(Py)8,7-RhSB9H9] [8,8,8-(H)(PMe2Ph)2-9-(Py)8,7-RhSB9H9] [8,8,8-(H)(PMePh2)2-9-(Py)8,7-RhSB9H9] [1,1-(PMe2Ph)(PPh3)-3-(Py)1,2-RhSB9H8] [1,1-(PMe3)(PPh3)-3-(Py)1,2-RhSB9H8] [1,1-(PMe2Ph)2-3-(Py)-1,2-RhSB9H8] [1,1-(PMe3)2-3-(Py)-1,2-RhSB9H8] [1,1-(PMe3)(Z2-C2H4)-3-(Py)1,2-RhSB9H8] [1,1-(CO)(PMe2Ph)-3(Py)-1,2-RhSB9H8] [1,1-(CO)(PMe3)-3(Py)-1,2-RhSB9H8] [8,8-(Cl)(PMe3)-9-(Py)-8,7-RhSB9H9] [8,8-(Cl)(PMe2Ph)-9(Py)-8,7-RhSB9H9] [8,8-(Cl)(PMePh2)-9(Py)-8,7-RhSB9H9] [8,8,8-(Cl)(PMe3)2-9-(Py)-8, 7- RhSB9H9] [8,8,8-(Cl)(PMe2Ph)2-9-(Py)-8, 7- RhSB9H9] [1,1-(Z2-dppe)-3-(Py)-1,2-RhSB9H8] [8,8-(Z2-dppe)-9-(Py)-8, 7-RhSB9H9]+ [8,8,8-(H)(Z2-dppe)-m-8, 9-(H)-9-(Py)-8,7-RhSB9H10]+ [8,8,8-(H)(PMePh2)2-9-(Py)8,7-RhSB9H10]+ [8,8,8-(H)(PPh3)(PMePh2)9-(Py)-8,7-RhSB9H10]+ [8,8,8-(H)(PPh3)(PMe3)-9(Py)-8,7-RhSB9H10]+ [1,3-m-(H)-1,1-(PPh3)2-3-(Py)1,2-RhSB9H8]+ [1,3-m-(H)-1,1-(PPh3)(PMe3)-3(Py)-1,2-RhSB9H8]+ [1,3-m-(H)-1,1-(PMe2Ph)2-3-(Py)1,2-RhSB9H8]+ [1,3-m-(H)-1,1-(PMe3)2-3-(Py)1,2-RhSB9H8]+ [1,3-m-(H)-1,1-(PMePh2)2-3-(Py)1,2-RhSB9H8]+ [1,3-m-(H)-1,1-(PPh3)(PMe2Ph)3-(Py)-1,2-RhSB9H8]+

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

−

158

2.202b

−

158

2.166b

−

158

−

−

160

−

−

161

−

−

161

−

−

161

2.217b

−

161

−

−

161

2.338b

−

161

2.331b − −

− − −

161 161 161

−

−

161

−

−

161

−

−

160

b

15.7, 8.9, −8.2, −10.1, −11.5, −12.4, −20.3, −22.2 17.3, 13.6, 10.8, 2.6, 0.3, −14.8, −15.6, −19.3, −28.6 16.3, 4.0, −2.6, −11.9, −20.6, −22.7, −28.5 11.6, 8.1, −0.3, −1.8, −14.8, −16.9, −22.3, −29.5 12.1, 4.3, −0.9, −4.9, −9.6, −21.4, −25.9, −27.6 11.1, 6.0, 2.8, −1.1, −4.0, −10.1, −18.4, −26.3, −28.4 6.3, 2.21, −1.0, −3.6, −5.9, −9.0, −18.7, −26.6, −27.7 12.2, 6.0, 1.9, −0.5, −4.5, −10.1, −19.7, −27.1 52.4, 25.6, 0.6, −2.1, −15.0, −25.6, −32.2, −33.3 51.8, 25.1, −0.1, −2.7, −15.2, −26.2, −32.1, −33.0 51.2, 24.7, −1.8, −14.2, −26.2, −33.1 50.9, 23.9, −2.3, −14.4, −25.9, −33.7 54.5, 24.9, 0.9, −1.1, −20.5, −27.6, 30.5, −31.7 54.7, 27.7, 0.5, −1.3, −13.4, −25.1, −32.1, −33.2 55.0, 27.4, 0.7, −1.8, −14.1, −25.8, −32.3 15.4, 4.1, 1.4, −5.1, −11.7, −22.5, −25.4, −28.7 14.9, 1.9, −11.8, −22.8, −24.9, −28.5

2.357

−

−

160

15.2, 2.6, −4.1, −12.2, −22.3, −23.9, −29.2 12.9, 11.7, 5.8, −1.1, −2.1, −15.3, −21.6, −24.4, −30.1 12.4, 8.3, 2.7, −3.0, −11.5, −13.3, −18.1, −22.0, −29.5 53.2, 26.6, −1.1, −12.5, −24.2, −32.9 38.2, 15.1, 11.6, −17.2

−

−

160

2.215b

−

160

2.209b

−

160

2.363b 2.364b

− −

162 162

16.6, 11.1, 4.8, 3.7, −8.8, −12.1, −13.1, −15.9, −17.1, −19.8, −23.4, −24.8. 15.6, 12.8, 6.8, 4.5, −11.2, −14.9, −21.0, −23.6 −

−

−

162

−

−

163

−

−

163

15.3, 7.9, −11.5, −15.4, −21.9, −23.3, −25.9 −

−

−

163

−

−

163

57.9, 29.6, 10.8, 8.9, −13.6, −23.8, −21.8, −19.9, −26.2 58.1, 30.0, 9.6, −13.1, −23.4, −24.3

2.406b

−

164

−

−

163

58.3, 29.7, 9.0, −13.2, −23.6, −24.9

−

−

164

58.3, 29.8, 10.9, −12.9, −21.8, −22.5

−

−

163

−

−

−

163 (Continued )

286

Polyhedral Metallaboranes and Metallacarboranes

Table 6

(Continued)

No.

Compounds

11

377 378 379

[8,8-(PMePh2)2-8,7-RhSB9H10] [8,8,8-(PMePh2)2(H)8,7-RhSB9H9-9-(PMePh2)] [8,8-(PPh3)(PMe2Ph)-8,7-RhSB9H10]

380

[8,8-(PPh3)(PMe3)-8,7-RhSB9H10]

381

[8,8,8-(PMe2Ph)3-8,7-RhSB9H10]

382

[8,8,8-(PMe3)3-8,7-RhSB9H10]

383 384

[8,8,8-(H)(PMePh2)2-9-(PMePh2)8,7-RhSB9H10]+ [8,8-(IMe)(PPh3)-8,7-RhSB9H10]

385

[8,8-(IMe)2-8,7-RhSB9H10]

386 387

[1,1-(IMe)(PPh3)-3-(py)1,2-RhSB9H8] [{Rh(IMe)(PPh3)}{SB9H9(py)}]

388

[8,8,8-(NH3)(PPh3)2-8,7- RhSB9H10]

389

[8,8,8-(H)(PPh3)2-9-(NH3)8,7-RhSB9H9] [8,8,8-(PPh3)2H-9-(3-Py-CH2CN)8,7-RhSB9H9] [1,1-(PPh3)(Z2-C2H4)-3-(3Py-CH2CN)-1,2-RhSB9H8] [2,2-(PMe2Ph)2-2-Cl-2,1-RhSB10H10] [2,2-(PPh3)2-2-H-2,1-IrSB10H10] [2-Cp -2,1-RhSB10H10] [1,2-(m-CO)-1,1,2-(PMe3)3-2-(PPh3)1,2-IrRhSB9H9]

390 391 392 393 394 395

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

10.6, −1.8, −9.1, −27.6 4.2, −0.8, −2.8, −4.0, −7.5, −21.2, −25.1

− 2.226b

− −

165 165

15.5, 10.6, 2.6, −1.3, −8.5, −18.5, −24.4, −28.2 15.8, 10.3, 2.2, −1.3, −8.2, −23.2, −25.6, −27.7 13.5, 7.1, 5.3, −1.1, −14.1, −15.5, −21.2, −27.0 13.0, 6.1, −2.3, −15.4, −15.5, −22.9, −27.1 14.8, 7.8, −8.5, −7.1, −5.9, −9.8, −13.2, −21.9, −20.9 16.8, 12.8, 6.2, −1.7, −9.1, −19.0, −22.4, −29.4 15.9, 10.5, 3.7, −4.3, −7.7, −23.6, −25.7, −28.5 52.5, 24.2, −2.6, −15,5, −27.7, −32.6

−

−

165

−

−

165

−

−

165

2.258b

−

165

−

−

163

2.183b

−

166

−

−

166

2.315b

−

166

−

−

166

−

−

167

2.227b

−

167

−

−

168

2.352b

−

168

2.250b − − 2.286c, 2.327b

− − − 2.7583(4)h

169 169 169 149

9.5, 4.1, −9.2, −12.1, −19.0, −27.5, −31.2 32.0, 9.4, 4.04, −2.8, −16.8, −23.0, −32.5. 7.2, 8.5, −0.3, −3.5, −13.1, −18.4, −25.9, −30.8 12.1, 7.8, 3.5, 0.3, −3.8, −9.9, −18.5, −25.6, −29.6 55.4, 26.2, 1.1, −14.2, −21.9, −24.5, −29.8 14.9, 13.4, 13.0, 9.3, −10.3, −14.2 12.0, 13.2, 4.6, 1.6, −16.0, −19.4 14.1, 9.1, 5.3, 5.3, −10.5, −18.1 5.6, 2.9, −0.4, −2.0, −10.4, −37.0

a

Av. CodB Av. RhdB c Av. IrdB d Av. MdB between boron and other than group 8 metal e Av. CodCo f Av. RhdRh g Av. IrdIr h Av. RhdIr i Heterometallic Av. MdM of group 9 metal and other than group 9 metal j Homometallic Av. MdM of other than group 9 metal b

two bridging hydrogens, and the other two are replaced by two terminal IrdH bonds. Clusters 199–201 possess 10 SEP as that of nido-[B8H12]. King, Lupan and co-worker theoretically studied the structural variations of dimetallaborane analogs of the octaboranes of the type Cp2M2B6H10 with changes in the skeletal electron count and gave insight into their bonding.129 Interestingly, experimentally achieved cores of 199–201 are same as that of theoretically proposed ones. There are also examples of 9 and 10-vertex closed and open clusters. Cluster 202 is a 9-vertex hypoelectronic cluster (9 SEP) having tricapped trigonal pyramidal geometry (Fig. 16).124 By contrast, 9-vertex cluster 203 has monocapped square antiprism geometry.126 Cluster 203 is also a hypoelectronic cluster (9 SEP) and has an isonido-bonding topology. Clusters 204–208130–133 and 209–221123,128,130,134,135 have 10-vertex closed and opened geometries, respectively (Fig. 16). Clusters 204–208 can be described as isocloso based on a 10-vertex closo-bicapped square antiprism by one DSD rearrangement (Fig. 17).34,130–133 The six-membered rings of 204–208 are distorted to some extent from a regular chair conformation. The degree-six vertices of 204–208 are occupied by Rh or Ir. Clusters 209–221 are isoelectronic and isostructural with decaborane(14). The only difference among nido-209-221 clusters is the position of the metal centers.

Polyhedral Metallaboranes and Metallacarboranes

287

Fig. 15 Molecular structures of 8-vertex metallaboranes 196–201.

Fig. 16 Molecular structures of 9 and 10-vertex metallaboranes 202–221. Note that positions of metals and boron atoms are not shown in 209–221.

Fig. 17 DSD rearrangement to generate 10-vertex isocloso from 10-vertex closo.

Clusters 223–227 are examples of group 9 metallaborane clusters having 11-vertices (Fig. 18);124,128,130 clusters 223–226 have octadecahedral cores. These octadecahedral cores have one degree-six vertex, which is occupied by Co, Rh or Ir owing to their diffuse orbitals. Interestingly, clusters 223–226 have 11 SEP, which suggests isocloso-bonding topology. On the other hand, 11-vertex 227 has a nido-shape that can be derived from a 12-vertex icosahedron by removing one of the vertices. The nido-geometry of 227 is in

288

Polyhedral Metallaboranes and Metallacarboranes

Fig. 18 Molecular structures of 11 and 12-vertex metallaboranes 223–233.

accord with its 13 SEP. Clusters 228–233 have 12-vertex closed geometries (Fig. 18).124,127,132 Although cluster 233 has an icosahedral core, the observed shape of clusters 229–233 can be related to the canonical deltahedra by one DSD rearrangement keeping the total vertex connectivities same. Although the connectivity pattern of 229–233 is consistent with hypercloso geometry, it has (n + 1) SEP, obeying the Wade−Williams relationship.127,132 As with group 6 and 8 metal-rich metallaboranes, group 9 metal-rich metallaboranes were also synthesized in good numbers in 2005–2021. For example, clusters 234–244 are examples of metal-rich metallaborane clusters having triply bridging borylene units (Fig. 19).136–139 Clusters 234–243 have a tetrahedral core, in which trigonal metal frameworks are bridged by a borylene {BH} unit in m3-fashion. In general, tetrahedral geometries imply 6 SEP. Interestingly, clusters 239 and 243 contain 7 SEP.138,139 On the other hand, cluster 244 has a trigonal bipyramidal core, where one of the apical vertices is occupied by boron, and all of the other four vertices are occupied by transition metals.139 These clusters are mostly synthesized utilizing metal carbonyls and preformed metallaboranes or in situ generated intermediates from the reactions of pentamethyl-cyclopentadienyl metal chloride dimers and LiBH4
THF. Utilizing a similar synthetic strategy, metal-rich metallaborane clusters 245–247 were also synthesized.67,138,139 Clusters 245–247 have boride boron atoms in the semi-interstitial position of a butterfly core, or the interstitial positions of an octahedral core (Fig. 20).

Fig. 19 Molecular structures of metallaborane clusters (234–244) having borylene unit.

Polyhedral Metallaboranes and Metallacarboranes

289

Fig. 20 Molecular structures of metallaborane clusters (245–247) having boride boron.

Apart from metal-rich metallaboranes having borylene or boride units, the reactions of metal carbonyls and preformed metallaboranes (or in situ generated intermediates from the reactions of pentamethylcyclopentadienyl metal chloride dimers and LiBH4
THF) also allowed access to mixed-metal metallaborane clusters 254–261.124,125,140 For example, the reaction of cobaltaborane [(Cp Co)2B4H6] with [Fe2(CO)9] and S powder afforded [(Cp Co)2B3H3(m-CO)Fe(CO)3] (254) and a condensed cluster 255 (vide infra).124 Cluster 254 is the trimetallic derivative of the starting material [(Cp Co)2B4H6]. Compound 254 can be derived from [(Cp Co)2B4H6] by the replacement of two bridging hydrogens and one BH by its isoelectronic carbonyl and {Fe(CO)3} fragments, respectively. The reaction of the in situ generated cobaltaborane intermediate with [Mo(CO)3(CH3CN)3] afforded mixed-metal cluster 256, and tetracobaltaborane 257.125 Cluster 256 has 7 SEP, which is consistent with its octahedral core (Fig. 21). By contrast, cluster 257 has a 10-vertex closed geometry (Fig. 21), which is an analog of isocloso-208. The reaction of nido-[(Cp Rh)2B3H7] with [Fe2(CO)9] afforded mixed-metal octahedral clusters 258–261 (Fig. 21).140 All of these octahedral clusters obey the SEP counting rule with 7 SEP. As m-H is isolobal with {AuPPh3}, many reactions have been carried out with open metallaboranes to substitute a m-H with an LAu unit and expand the cluster. For example, the reaction of nido-[(Cp M)2B6H10] (M ¼ ]Co (199) or Rh (200)) or nido[(Cp Rh)2B8H12] with sources of {AuPPh3} afforded [(Cp Co)2(AuPPh3)B6H9] (262)127, two isomers of [(Cp Rh)2(AuPPh3)2B6H8] (263 and 264) and [(Cp Rh)2(AuPPh3)2B8H10] (265)141. In 262, one Au center exhibits a regular m3-bonding mode, while in 263 and 264, there are two m3-bonded Au atoms (Fig. 22). On the other hand, two Au units are bonded m2-fashion in 265 (Fig. 22).

Fig. 21 Molecular structures of metallaboranes 254–261.

Fig. 22 Molecular structures of Au-incorporated metallaboranes 262–265.

290

Polyhedral Metallaboranes and Metallacarboranes

The reactions of group 9 pentamethylcyclopentadienyl metal chloride dimers with LiBH4
THF, followed by treatment with chalcogen powders, afforded a series of unique metallaheteroboranes 266–270 (Scheme 16).125,142,143 For example, in the case of S/Se powder, bimetallic triple-decker sandwich complexes 266–269 were formed, where the middle four-membered B2E2 deck is open. In the case of Te, a monocapped square antiprism cluster 270 was formed.143 Cluster 270 has 11 SEP, which is in accord with its 9-vertex nido-shape. When clusters 267 and 268 were treated with [Fe2(CO)9], one of the {MCp } units was substituted by an isolobal {Fe(CO)3} fragment and afforded similar clusters 271 and 272, respectively.142,143 To close the middle four-membered B2E2 ring of 266–269, these clusters were treated with [PdCl2(COD)], which afforded the triple-decker sandwich complexes 273–276, in which the middle decks are five-membered B2E2Pd rings (Scheme 16).144,145

Scheme 16 Syntheses of metallaheteroboranes (266–270) and triple-decker complexes (273–276).

A series of mono/bis/tris/tetra-homocubane type clusters 277–291 (Fig. 23) were synthesized utilizing group 9 metal precursors and chalcogen powder or chalcogenatoborate ligands Li[BH2E3]. In cluster 277, one cubane edge is substituted with an extra chalcogen vertex.145 Cluster 277 has 66 CVE, which is as expected for the bimetallic homocubane analog cluster. In clusters 278–283, two cubane edges are substituted with two extra chalcogen vertices.120,146 These bimetallic bis-homocubane analogs have the expected 72 CVE. Interestingly, two edges of cubane are substituted by two {BH2} units, and one of the edges of cubane is missing in 284.146 As one of the bonds is missing in bishomocubane 284, it has two extra CVE than usual. There are different types of mono/bi-metallic trishomocubane analogs 285–290, where three of the cubane edges are substituted by three chalcogen vertices. The monometallic (287 and 288) and bimetallic (285, 286, 289 and 290) trishomocubane analogs have expected 78 and 68 CVE, respectively. On the other hand, the only known example of tetrahomocubane analog is cluster 291, which has expected 84 CVE.145

Fig. 23 Molecular structures of homocubane-type metallaheteroboranes 277–291.

Polyhedral Metallaboranes and Metallacarboranes

291

The reactions of dimetallaoctaboranes(12) [(Cp M)2B6H10] [M ¼ ]Co (199) or Rh (201)] with different chalcogen sources, such as Li[BH2E3] and Li[BH3EPh] (E ¼ ]S, Se, or Te), led to two unique reaction outcomes (Scheme 17). For example, the reactions with Li[BH2E3] led to the cluster growth reactions and yielded 10-vertex nido-[(Cp M)2B6E2H6] (292, M ¼ ]Co, E ¼ ]S; 293, M ¼ ]Co, E ¼ ]Se; 294, M ¼ ]Co, E ¼ ]Te; 295, M ¼ ]Rh, E]Se).147 By contrast, the reactions with Li[BH3EPh] afforded arachno-[(Cp Co)2B6H9(EPh)] (E ¼ ]S (296), Se (297), or Te (298)), which are the product of a unique reaction methodology that yielded arachno clusters, keeping the core geometry identical.147 In addition, the formation of arachno-296-298 is a unique method that converts ‘disobedient’ clusters to ‘obedient’ clusters. Further, the reactivity of nido-293 with [Fe2(CO)9] led to additional cluster growth and afforded 11-vertex nido-299.147 The core geometry of nido-299 is very similar to that of [C2B9H11]2− with a unique open pentahapto-coordinating five-membered face.

Scheme 17 Syntheses of nido-292–298. (a): RT, 16 h (for 292, 293 and 295), (b): 80  C, 16 h (for 294).

Macías, Kennedy, and co-workers utilized preformed heteroboranes and various group 9 metal precursors to synthesize a large number of metallaheteroboranes. The reaction of 9-vertex arachno-azanonaborane [4-NB8H12]− with [Cp RhCl2]2 in CH2Cl2 at room-temperature yielded nido-[9-Cp -6,9-NRhB8H11] (300) and its 6,8-isomer, nido-[8-Cp -6,8-NRhB8H11] (301) (Scheme 18a).148

Scheme 18 Syntheses of 10-vertex metallaheteroboranes (300, 301 and 305–309).

292

Polyhedral Metallaboranes and Metallacarboranes

Clusters 300 and 301 are structural and electronic analogs of decaborane(14). Under the same conditions, the reaction of arachno-[4-NB8H12]− with [Cp IrCl2]2 yielded the Ir analog of nido-301, nido-302.148 By contrast, a different iridium precursor [IrCl(PPh3)3] afforded the 6,9-isomer nido-[9,9-(PPh3)2–9-H-6,9-NIrB8H11] (303). At elevated temperature, compound 303 converted to its 6,8-isomer 304.148 On the other hand, another 9-vertex arachno-heteroborane [4-SB8H12]− in reaction with Rh and Ir-precursors afforded 10-vertex open clusters 305–307 (Scheme 18b), having a similar type of core structure as those of 300–304.149,150 Interestingly, according to SEP counting, clusters 305–307 have been considered as arachno-clusters. Along with cluster 307, two 10-vertex closo-clusters 308 and 309 were isolated from the same reaction (Scheme 18b).150 Clusters 308 and 309 have bicapped square antiprismatic cores with 11 SEP. When the 10-vertex hydridorhodathiaborane closo-308 was treated with various Lewis bases, such as PCy3, 2-MePy, 2-EtPy, Py, 3-MePy, the reaction yielded 10-vertex arachno-rhodathiaborane clusters 310–318, which have a similar type of core to that of arachno-307.151 The differences among arachno-310-318 are the presence of various Lewis bases and the different positions of the S atoms. On the other hand, when arachno-307 was treated with an excess of ethylene in dichloromethane, the reaction yielded closo-10-vertex cluster 319, which has an identical bicapped square antiprismatic geometry to that of closo-308 and 309.152 This ethylene-ligated rhodathiaborane 319 is the result of the substitution at the metal center of a PPh3 ligand by ethylene, the loss of two hydrogen atoms from the cage and a consequent arachno-to-closo structural transformation. The formation of ethane in this reaction suggested that the H2 loss occurs via hydrogenation of the double bond of the entering olefin. Furthermore, the treatment of 319 with H2 led to the replacement of the Z2-ethylene ligand with two hydride ligands affording closo-320.152 In a similar approach to that employed for the synthesis of cluster 306, arachno-[4-SB8H12]− was treated with [IrCl(CO)(PMe3)2], which afforded an analog of 306, nido-321149 along with a nido-11-vertex cluster 322153 (Scheme 19). Cluster 322 has an 11-vertex geometry which can be generated by removing one vertex of the icosahedron. Further reaction of nido-322 with the decarbonylating agent Me3NO in dichloromethane removed the CO ligand and yielded an isonido-11-vertex cluster 323.153 The isonido shape of 323 can be generated from the octadecahedron by removing an edge that is connected to the degree-six vertex of it. Further reaction of isonido-323 with EtNC afforded nido-324, an analog of nido-322 (Scheme 19).153 The only difference between 322 and 323 is the presence of EtNC in 324 instead of CO. The addition of excess ammonia to the dichloromethane solutions of isonido-[1,1,1-(H) (L)2–1,2-IrSB9H9] (L ¼ ]PMe3 (323) or PMe2Ph) led to the formation of analogs of 324, nido-325, and 326, where Rh centers are ligated by NH3.117

Scheme 19 Syntheses of metallaheteroboranes 321–323.

A series of 11-vertex nido-clusters (327–336) was isolated from the reaction of [Rh(Z4-diene)(L2)]+ (L2 ¼ ]Py2, bpy, Me2bpy or phen; diene ¼ COD or NBD) with [SB9H12]− in CH2Cl2 (Scheme 20).154 The primary differences among 327–336 are the substituents at the rhodium center. On the other hand, the reaction between [RhCl(PPh3)3] and nido-[6-NB9H11]− yielded a nitrogen analog of 327–336, nido-337,155 where one N vertex is present at the 7-position instead of S. When a similar type 11-vertex nido-rhodathiaborane [8,8-(PPh3)2-nido-8,7-RhSB9H10] was treated with a 4-fold excess of pyridine, a hydrido-ligated 11-vertex nido-[8,8,8-(PPh3)2H-9-py-8,7-RhSB9H9] (338) was formed (Scheme 21).156 Further room-temperature reaction of nido-338 with excess ethene or CO in dichloromethane led to a nido to closo transformation to afford closo-rhodathiaboranes 339 or 340, respectively.156,157 In CH2Cl2 under reflux conditions also nido-338 underwent a nido to closo transformation to afford analog of 339 and 340, closo-341.157,158 Clusters 339–341 have closo-octadecahedral geometries with 12 SEP. The Rh centers of closo-339-341 are ligated by PPh3 and ethylene ligands, PPh3 and CO ligands, and two PPh3 ligands, respectively. On the other hand, the reactions of nido-338 with different types of alkynes gave interesting results. For example, when nido-338 was treated with activated alkyne reagents C2(CO2R)2 (R ¼ ]Me or tBu), H2 elimination was observed and partial hydrogenation of the triple bond was observed, affording Z2-alkyne-ligated 11-vertex closo analogs of 339, closo-342, and 343.159 By contrast, the similar reaction with PhC^CH afforded an analog of 338, nido-344, in which the Rh center is attached to a terminal C^CPh unit.159 On the other hand, the reaction of nido-338 with triflic acid yielded the salt [8,8,8-(PPh3)2(H)-9-py-RhSB9H10][OTf] (345), which have a similar core to that of nido-338.158 When closo-340 or 341 were treated with HCl, the reactions afforded 11-vertex nido-rhodathioboranes 346 and 347, respectively, in which rhodium centers are chlorinated.158 The treatment of nido-347 with pyridine afforded new rhodathiaborane nido-[8,8,8-(Cl)(Py)(PPh3)-9-(Py)nido-8,7-RhSB9H9] (348).160

Polyhedral Metallaboranes and Metallacarboranes

293

Scheme 20 Syntheses of metallaheteroboranes 328–337.

Scheme 21 Syntheses of metallaheteroboranes 338 and 339.

Utilizing different types of phosphines, the substitution chemistry of nido-338 was evaluated. For example, treatment of nido-338 with PMePh2, PMe2Ph, and PMe3 led to mono/bis-substitution at the Rh center and afforded nido-349-352.161 The thermal dehydrogenation of nido-349 − 351 afforded the corresponding closo-derivatives 353–356.161 On the other hand, the reactions of closo-349 and 350 with C2H4 promoted a nido to closo cluster formation and afforded C2H4-ligated closo-339 and 357, respectively.156,161 In these reactions ethane is detected in situ, which indicates that the olefin is hydrogenated. By contrast, the reactions of closo-349 and 350 with CO afforded the CO-ligated 11-vertex species closo-358 and 359.161 Clusters 353–359 have octadecahedral cores. On the other hand, the reactions of nido-338, 350 or 352; or closo-354-356 with HCl yielded monophosphine species nido360-362 and bisphosphine species, closo-363 and 364.160 The reaction of nido-338 with the bidentate bisphosphine, dppe afforded closo-365, having an octadecahedral geometry.162 The 11-vertex rhodathiaborane 365 reacted readily with triflic acid to give cationic [8,8-(Z2-dppe)-9-(Py)-8,7-RhSB9H9]+ (366), which has a 10-vertex nido core geometry similar to that of 338.162 The proton enhances the stereochemical non-rigidity and Lewis acidity of 366 versus neutral 365, which allow for the efficient heterolytic splitting of the HdH bond, leading to the formation of new hydridorhodathiaborane nido-367.162 The treatment of the nido-hydridorhodathiaboranes, 351, 349 and 350 with triflic acid afforded the corresponding cations 368–370.163 Similarly, the 11-vertex rhodathiaboranes, closo-341, 354–356 reacted with TfOH to afford the corresponding cations, isonido-371-374.163,164 On the other hand, nido-368 and 369 lost H2 to yield isonido-375 and 376.163

294

Polyhedral Metallaboranes and Metallacarboranes

The reaction of nido-[8,8-(PPh3)2–8,7-RhSB9H10] with different neutral donors led to the formation of various types of phosphine-substituted nido-11-vertex rhodathiaboranes. For example, the reaction with different equivalents of PMePh2 afforded diphosphine substituted nido-rhodathiaborane 377, and B-ligated nido-rhodathiaborane 378.165 The reaction with PMe2Ph or PMe3 afforded the mono-substituted bis-PR3-ligated rhodathiaboranes nido-[8,8-(PPh3)(L)-8,7-RhSB9H10] (L ¼ ]PMe2Ph (379), PMe3 (380)) and the corresponding tris-PR3-ligated compounds nido-[8,8,8-(L)3–8,7-RhSB9H10] (L ¼ ]PMe2Ph (381), PMe3 (382)).165 The reaction of nido-378 with triflic acid afforded the corresponding cation nido-383.163 On the other hand, the reaction of nido-[8,8-(PPh3)2–8,7-RhSB9H10] with the 1,3-dimethylimidazol-2-ylidene (IMe) yielded mono and bis-IMe functionalized rhodathiaboranes, nido-384, and 385.166 Furthermore, the treatment of nido-385 with pyridine afforded the pyridine adduct isonido-386 with loss of H2.166 Isonido-386 can be converted to nido-387 utilizing H2, which can be presented as a dihydrogenassisted isonido-nido opening: a truly reversible activation of H2 by a rhodathiaborane system.166 The reaction of nido-[8,8(PPh3)2–8,7-RhSB9H10] with NH3 at different conditions afforded B-ligated ammonium adduct, nido-388 or Rh-ligated ammonium adduct, nido-389.167 The reaction between the nido-[8,8-(PPh3)2–8,7-RhSB9H10] and 3-pyridylacetonitrile afforded the hydrorhodathiaborane nido-390.168 Treatment of this cluster with ethylene led to the formation of ethylene-ligated metallaheteroborane closo-391.168 The reaction of closo-[2,2-(PPh3)2-2-H-2,1-RhSB10H10] with PMe2Ph led to phosphine exchange and hydride substitution, affording the chloro analog, closo-392.169 In this context, the reaction between [IrCl(PPh3)3] and nido-[7-SB10H12] with tmnd afforded 12-vertex closo-iridathiaborane 393.169 By contrast, the reaction of [Cp RhCl2]2 with nido-[7-SB10H12] under the same conditions afforded 12-vertex closo-394.169 On the other hand, treatment of the 11-vertex rhodathiaborane, nido-[8,8-(PPh3)28,7-RhSB9H10] with nBuLi, followed by addition of [IrCl(CO)(PMe3)2] afforded the 12-vertex iridarhodathiaborane, closo-395.149 All these closo-12 vertex metallathiaboranes have icosahedral geometries (Fig. 24).

9.06.2.1.7

Metallaborane clusters of group 10 ( Table 7)

Beyond group 9, examples of single cage metallaboranes are very limited. Bould and co-workers carried out the reaction of K [arachno-B9H14] with [NiCl2(dppe)], which led to the formation of two 10-vertex single-cage clusters, [1-(dppe)-1-NiB9H7Cl2] (396) and [1-(dppe)-1-NiB9H7Cl(OH)] (397) along with four more condensed metallaboranes (vide infra).170 Clusters 396 and 397 have bicapped square antiprism geometries, in which one of the capping vertices is occupied by Ni (Fig. 25). On the other hand, the reaction between [(tmndH)[SB8H11]], Li[BHEt3] and [NiCl2(dppe)] at low-temperature afforded 10-vertex arachno-nickellathiadecaborane [6,6-(dppe)-6,9-NiSB8H10] (398).117 Cluster 398 took up CO to give carbonyl incorporated arachno-[6,6,6-(CO) (dppe)-6,9-NiSB8H10] (399).117 On the other hand, the reaction between Cs[arachno-SB9H12], Li[BHEt3] and [NiCl2(dppe)] at low-temperature afforded nido-nickellathiaboranes 400–402 along with a closo-nickellathiaborane 403.117 Nido-400-402 have 11-vertex geometries (Fig. 25), analogous to the nido-11-vertex rhodathiaboranes discussed in the previous section. By contrast, closo-403 has a similar type bicapped square antiprism geometry to that of closo-396 and 397. In closo-403, however, one of the capping vertices is occupied by S, and Ni is located at a vertex of the square base. The reaction of the same nickel precursor [NiCl2(dppe)] with decaborane afforded 11-vertex nido-nickelaborane 404, which showed interesting reactivity and functionality toward carbon monoxide and ethylisonitrile.171 Upon treatment of nido-404 with EtCN or CO, the Ni center assimilates EtCN (405) or CO (406).171 A similar type 11-vertex nido-palladaborane 407 was isolated from the thermolysis reaction of [cis-PdCl2(PPh3)2] and [NHEt3][B10H10] in ethanol.172 When a similar type of 11-vertex nido-platinaborane [(PMe2Ph)2PtB10H12] was treated with PMe2Ph, the reaction afforded nido-[(PMe2Ph)3PtB10H12] (408).173 On the other hand, boron-functionalized 11-vertex nido-[1(HS)-7,7-(PMe2Ph)2–7-PtB10H11] (409) and nido-[4-(HS)-7,7-(PMe2Ph)2–7-PtB10H11] (410) were prepared by the simple addition of dichloromethane solutions of [PtMe2(PMe2Ph)2] to nido-decaborane thiol clusters [1-(HS)-B10H13] or [2-(HS)-B10H13], respectively.174 Beyond group 9, icosahedral clusters have been synthesized utilizing Pt and Pd precursors. For example, bimetallic metallaboranes closo-[(PMe2Ph)4M2B10H10] (411: M2 ¼ ]Pt2; 412: M2 ¼ ]PdPt; 413: M2 ¼ ]Pd2) were synthesized by the addition of 2 equiv. K[HBEt3] to suspensions of nido-[(PMe2Ph)2MB10H12] (M ¼ ]Pt or Pd) in toluene at low-temperature, followed by the addition of [MCl2(PMe2Ph)2] (M ¼ ]Pt or Pd) and slow warming to room-temperature.175,176 Further treatment of O2, CO or SO2 with closo-411-413 afforded closo-414-420, which have the same icosahedral core as that of 411–413, but the MdM bonds are bridged by O2 or CO or SO2 (Fig. 26).175,176 On the other hand, the reaction of closo-411 with EtNC at room-temperature affords

Fig. 24 Molecular structures of icosahedral metallaheteroboranes 392–395.

Table 7

Metallaboranes of Group 10.

No.

Compounds

11

Av. MdB (Å)

396 397 398 399 400

[1-dppe-1-NiB9H7Cl2] [1-dppe-1-NiB9H7Cl(OH)] [6,6-dppe-6,9-NiSB8H10] [6,6,6-(CO)(dppe)-6,9-NiSB8H10] [8,8-dppe-8,7-NiSB9H11]

401 402 403 404 405 406 407

[9-Et-8,8-dppe-8,7-NiSB9H10] [7-Et-8,8-dppe-8,7-NiSB9H10] [2,2-dppe-2,1-NiSB8H8] [7,7-dppe-7-NiB10H12] [(EtNC)(dppe)NiB10H12] [(CO)(dppe)NiB10H12] [7,8-(PPh3)2-7,8-(m-PPh2)-9,11(OEt)2-7,8-Pd2B9H8] [(PMe2Ph)3PtB10H12] [1-(HS)-7,7-(PMe2Ph)2-7-PtB10H11] [4-(HS)-7,7-(PMe2Ph)2-7-PtB10H11] [(PMe2Ph)4Pt2B10H10] [(PMe2Ph)4PtPdB10H10] [(PMe2Ph)4Pd2B10H10] [(PMe2Ph)4(O2)Pt2B10H10] [(PMe2Ph)4(CO)Pt2B10H10] [(PMe2Ph)4(CO)PtPdB10H10] [(PMe2Ph)4(CO)Pd2B10H10] [(PMe2Ph)4(SO2)Pt2B10H10] [(PMe2Ph)4(SO2)PtPdB10H10] [(PMe2Ph)4(SO2)Pd2B10H10] [{(EtNC)(PMe2Ph)3}Pt2(m-EtNC)B10H10] [{(EtNC)(PMe2Ph)3}Pt2(m-CO)B10H10] [{(EtNC)(PMe2Ph)3}Pt2(m-SO2)B10H10] [{(EtNC)2(PMe2Ph)2}(m-SO2)Pt2B10H10] [{(EtNC)2(PMe2Ph)2}Pd2B10H10] [(PMe2Ph)2PtPd(phen)B10H10] [(PMe2Ph)2Pt(SO2)Pd(phen)-B10H10] [(EtNC)3(PMe2Ph)2Pt2B10H10]

50.3, 22.9, 3.9, 90.9 86.0, 51.6, 49.9, 19.3, 4.9, −0.1 36.6, 0.46, −2.1, −20.3, −23.7 5.51, 3.35, 2.57, 1.21, 1.47, −1.26 9.9, 6.6, 2.9, −6.5, −10.9, −11.9, −16.2, −17.6, −23.4 18.3, 5.7, −4.1, −8.7, −13.1, −18.6, −24.0 3.2, 2.1, 1.9, −4.5, −12.6, −18.0, −20.9, −29.1 54.9, −6.4, −7.5, −12.1, −20.9 13.4, 17.1, 11.9, 5.9, −1.2, −21.1 19.6, 14.5, 10.7, −2.1, −5.3, −22.8 27.0, 20.3, 16.0, 5.3, 0.5, −22.0 57.3, 48.5, 7.2, −14.2, −19.2, −21.2 19.4, 9.5, 0.7, −11.6, −12.0, −27.7 18.1, 14.2, 10.9, 6.1, −1.0, −25.8 17.3, 15.1, 12.9, 9.5, 7.6, 3.2, −2.9, −17.7, −26.0 24.6, 22.1, 18.6, −26.8 31.9, 25.3, −23.0 29.7, 32.6, 32.6, −20.1 18.2, 16.0, 1.9, −3.6 17.5, 5.4, 3.4, −16.0 23.9, 20.7, 16.6, 14.6, 7.3, −13.9 28.5, 28.5, 19.6, −11.3 20.8, 11.7, 4.8, −10.4 27.4, 24.3, 18.0, 9.8, −7.8 31.4, 34.7, 21.3, −6.2 15.3, 14.9, 5.0, 2.2, 1.0, −16.2 17.6, 3.6, 2.8, −16.5, −17.5 18.5, 10.9, 5.1, 1.1, −14.3, −15.2 19.6, 8.3, 3.6, −13.3 23.8, 18.1, −17.9 29.4, 26.7, 21.2, 19.7, −24.3 25.1, 23.8, 22.2, 21.7, 8.7, −10.5 14.5, 2.7, 0.3, −17.7

408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428

B NMR (ppm)

a

Av. NidB Av. PddB c Av. PtdB d Av. PddPd e Av. PtdPt f Av. PddPt b

Fig. 25 Molecular structures of 10 and 11-vertex metallaheteroboranes (396–407).

Av. MdM (Å)

Ref.

2.048a − 2.091a − 2.176a

− − − − −

170 170 117 117 117

2.195a 2.11a 2.254a 2.161a 2.165a − 2.274b

− − − − − − 2.6666(4)d

117 117 117 171 171 171 172

2.259c 2.296c 2.282c 2.274c − 2.296b 2.252c 2.291c 2.295c, 2.286b 2.301b 2.286c 2.27c, 2.277b 2.282b 2.278c 2.29c 2.281c 2.312c 2.281b 2.289c, 2.232b 2.311c, 2.253b 2.263c

− − − 2.9552(4)e − 2.9163(3)d 2.7143(3)e 2.7487(3)e 2.749(3)f 2.7491(6)d 2.8194(4)e 2.810(7)f 2.8041(6)d 2.7354(3)e 2.7481(8)e 2.7435(2)e 2.7519(13)e 2.8275(6)d 2.8744(3)f 2.7880(5)f 2.717(7)e

173 174 174 175 176 176 175 175 176 176 175 176 176 177 177 177 177 177 178 178 179

296

Polyhedral Metallaboranes and Metallacarboranes

Fig. 26 Molecular structures of icosahedral metallaheteroboranes 411–420.

closo-421, in which one molecule of EtNC has substituted a terminal PMe2Ph ligand, and a second EtNC bridges the PtdPt edge.177 The bridging EtNC could be substituted by CO to form closo-422,177 and the CO sequentially replaced by SO2 to give closo-423. By contrast, the treatment of closo-421 with SO2 afforded closo-424, in which SO2 takes up a bridging position, and the bridging EtNC substituted by a terminal metal-bonded phosphine.177 The reaction of closo-413 with EtNC resulted in the displacement of two PMe2Ph ligands to form closo-425.177 When a solution of K[HBEt3] was added to toluene suspension of [(phen)PdB10H12], followed by treatment with [PtCl2(PMe2Ph)2], the reaction afforded phen-ligated icosahedron [(PMe2Ph)2PtPd(phen)B10H10] (426).178 As with previous examples, the treatment of closo-426 with SO2 afforded SO2-bridged closo-427.178 Surprisingly, when a metallaborane conglomerate [(PMe2Ph)8Pt8B40H40] was treated with ethyl isonitrile, the cluster assembly was effectively reversed, affording icosahedral [(EtNC)3(PMe2Ph)2Pt2B10H10] (428).179

9.06.2.2

Metallaborane fused clusters (Table 8)

In addition to single cage metallaboranes, a large number of condensed clusters were synthesized during the last two decades. These macropolyhedral clusters are fused through the vertex, edge, triangle or square faces. According to the Mingos fusion formalism, the total CVE of such clusters is equal to the sum of the CVE for the parent polyhedra minus the electron count of the shared unit (atom, pair of atoms, etc.). For example, clusters 429–432 are examples of edge-fused clusters.180,181 Clusters 429–432 were isolated from the reaction of [Cp2M(BH4)2] (M ¼ ]Zr or Hf ) with a large excess of [BH3
THF] under vigorous conditions. Clusters 429–430 and 431–432 contain a triangular B3 core and a pentagonal bipyramidal B7 core, respectively, which are coordinated by two {Cp2M} and two {BH2} units (Fig. 27). Alternatively, clusters 429 and 430 can be viewed as fusion of two arachno-MB3 butterfly cores (2{(14 + 4  3) + 6} ¼ 64e) with one triangular B3 core (6  3 ¼ 18e) through two BdB edges {2(6  2 + 2) ¼ 28e}. The requirement of 54e of 429 or 430 is achieved from their CVE count (CVE of [(Cp2M)2B5H11] ¼ 2  14 + 5  3 + 11  1 ¼ 54). On the other hand, clusters 431 and 432 can be described as the fusion of two arachno-MB3 butterfly cores (2{(14 + 12) + 6} ¼ 64e) and a closo-B7 pentagonal bipyramidal core {(4  7 + 2) ¼ 30e} moieties through two BdB edges {2(6  2 + 2) ¼ 28e}. Therefore, clusters 431 and 432 require 66e, which is achieved from their CVE (CVE of [(Cp2M)2B9H11] ¼ 2  14 + 9  3 + 11  1 ¼ 66e). A series of group 5 edge-fused clusters synthesized and structurally characterized by us and others. Most of them (433–437 and 439–456) have condensed M2B4 cores (M ¼ ]V, Nb or Ta), where two tetrahedral M2B2 cores are fused by MdM edge (Fig. 28). Clusters 433–437 are isolated from the reactions of cyclopentadienyl-based metal chlorides of group 5 with an excess of LiBH4
THF at −70  C, followed by pyrolysis with BH3
THF.57,59,182 Furthermore, a range of substitution reactions of 433–437 were carried out, which led to the mono/bis/tris/tetra-terminal BdH functionalization. For example, the reaction of chlorinating reagent CCl4 with clusters 434, 436, or 437 led to the terminal BdH chlorination and afforded bis/tris/tetra-chlorinated M2B4 (439–445).59,87,182 By contrast, the treatment with chalcogen containing reagents E2R2 (E ¼ ]S, Se or Te; R ¼ ]Ph, Bn, or C6H2(tBu)2OH) afforded mono/bis/tris/tetra BdH terminal functionalized M2B4 core (446–454).54,66,182–184 Thermolysis of 436 with 2-mercaptobenzothiazole or 2-mercaptobenzoxazole led to functionalization of one of the terminal BdH and afforded 455 and 456, respectively.184 Clusters 433–437 and 439–456 have two tetrahedral M2B2 cores (2(15  2 + 5  2) ¼ 80e), which are fused by an MdM edge (16  2 + 2 ¼ 34e). Thus, they have an electronic requirement of 46e, which is achieved by their CVE. By substituting one or two BH3 with isoelectronic chalcogen atom(s), their structural analogs 457 and 458 were achieved. Clusters 457 and 458 were isolated from the reaction of Cp2VCl2 with excess LiBH4
THF, followed by thermolysis with 2-mercaptobenzothiazole or Ph2Te2, respectively.66,185 The reaction of [CpNbCl4] with excess LiBH4
THF, followed by thermolysis with [(2,6-(tBu)2-C6H2OH)2S2] yielded [(CpNb)2B4H10S] (459) along with fused 454.66 Cluster 459 is an edge fused cluster, where trigonal bipyramidal M2B2S core and tetrahedral M2B2 core are fused by NbdNb edge (Fig. 28). A similar type of cluster, 460, having three more chalcogen atoms instead of (isoelectronic) BH3, was isolated from the reaction of [CpNbCl4] with excess LiBH4
THF, followed by thermolysis with Ph2Se2.185 The oxo analogs of 459, clusters 461 and 462 were synthesized by purging O2 gas into fused 437 or 436, respectively.185,186 As clusters 459–462 have both trigonal bipyramidal (14  2 + 4  3 + 2 ¼ 42e) and tetrahedral cores (15  2 + 5  2 ¼ 40e), which are fused by NbdNb edge (16  2 + 2 ¼ 34e), it has a requirement of 48e. The required 48e is achieved from the CVE of 459–462.

Polyhedral Metallaboranes and Metallacarboranes

Table 8 No.

297

Condensed Metallaboranes.

Compounds

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref. 180 181 181 181 57 182 57 59 57 66 182 182 182 59 59 59 87 183 183 182 183 182 184 66 54 66 184 184 66 185 66 185 186 187 186

a

429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463

[(Cp2Zr)2B5H11] [(Cp2Hf )2B5H11] [(Cp2Zr)2B9H11] [(Cp2Hf )2B9H11] [(CpV)2(B2H6)2] [(Cp V)2(B2H6)2] [(CpNb)2(B2H6)2] [(Cp Nb)2(B2H6)2] [(Cp Ta)2(B2H6)2] [(CpV)2B3H8(m3-OEt)] [(Cp V)2B4H10Cl2] [(Cp V)2B4H9Cl3] [(Cp V)2B4H8Cl4] [(Cp Nb)2B4H10Cl2] [(Cp Nb)2B4H8Cl4] [(Cp Ta)2(B2H5Cl)2] [(Cp Ta)2(B2H6)(B2H4Cl2)] [(CpV)2B4H10(SPh)2] [(CpV)2B4H9(SPh)3] [(Cp V)2B4H11(SePh)] [(CpV)2B4H11(SeBn)] [(Cp V)2B4H8(SePh)4] [(Cp Nb)2B4H11(SePh)] [(CpNb)2B4H11(TePh)] [(Cp Ta)2B4H11(SePh)] [(CpNb)2B4H11(SC6Ht2Bu2OH)] [(Cp Nb)2B4H11(C6H4NSCS)] [(Cp Nb)2B4H11(C6H4NOCS)] [(CpV)2(BH3S)2] [(CpV)2B3H9(m3-Te)] [(CpNb)2B4H10S] [(CpNb)2BH(Se)4] [(Cp Ta)2B4H10O] [(Cp Nb)2B4H10O] [(Cp Ta)2B2H4OH2Fe2(CO)6BH]

8.1, 2.5, −4.1 4.6, 2.0, −3.8 29.1, 21.6, 16.9, 13.2, 6.8, 6.3, 0.8, −3.6 28.7, 21.5, 17.8, 16.2, 6.5, 5.0, 2.4, −3.4 1.7 −2.9 1.7 −1.3 −4.6 4.5, −5.9, −25.8 9.0, −0.5 10.3, −1.6 10.1 10.7, −0.3 9.6 11.3, −6.8 12.5, −5.7 10.1, −0.3 7.9, −0.4 0.3, −6.5 6.7, 2.9, −1.2 1.9 1.2, −3.5 25.9, 11.6, −5.7 0.1, −3.3, −8.0 14.2, 2.8, −2.5 3.3, −3.9 2.9, −3.6 −24.6 9.5, −11.9 24.0, −1.4, −17.5, −31.3 −3.9 23.9, −0.8, −4.3, −34.1 21.0, 5.7, −4.7, −27.0 153.3, 27.9, −10.0

2.517 2.50a 2.496a 2.457a 2.291b 2.304b 2.416&b − 2.372b 2.31b 2.297b 2.297b 2.304b 2.396b 2.406b 2.372b – 2.320b 2.313b 2.306b 2.306b 2.325b 2.411b − 2.379b 2.403b 2.404b 2.371b 2.362b − 2.45b 2.467b 2.40b 2.396b 2.322b, 1.986e

464

[(Cp Nb)2B2H4OH2Fe2(CO)6BH]

148.5, 31.3, −4.3

2.305b, 1.921e

465

60.9, 33.9, 14.1, 11.2

2.317e, 2.569b

129.2, 124.6, 31.9, −2.4

2.16e, 2.35b

2.9373(10)h, 2.889k

187

467 468 469 470 471

[(Cp Ta)2(B3H4O){Ru(CO)2}3{m7-B}{m-CO}2{mH}4] [(Cp Nb)2(B2H4O){Ru(CO)2}2(B2H4){Ru(CO)3}2{mH}4] [Cp TaCl(m-Cl)B2H4Ru3(CO)8] [(Cp Ta)2B5H7{Fe(CO)3}2] [(Cp Ta)2B5H9{Fe(CO)3}4] [(Cp Mo)3MoB9H18] [(Cp W)3WB9H18]

− − − − 2.787(2)h 2.7820(9)h 2.9477(16)h − 2.9325(4)h 2.7586(10)h 2.7946(7)h 2.8105(15)h 2.8389(6)h 2.9687(16)h 2.9918(4)h 2.9587(4)h – 2.827(2)h 2.8318(5)h 2.7996(5)h 2.8054(9)h 2.8595(10)h 2.9563(3)h − 2.9454(5)h 2.9481(3)h 2.9597(5)h 2.9566(14)h 2.777(2)h − 2.8043(5)h 2.811h 2.7554(2)h 2.7458(8)h 2.9142(6)h, 2.7201(13)k, 2.963m 2.9290(4)h, 2.7248(7)k, 2.977m 2.8648(6)h, 2.84k, 3.053m

2.215e, 2.269b 2.304b, 2.203e 2.387b, 2.109e 2.255c 2.24c

2.834k, 2.8151(6)m 2.9207(7)k, 2.771m 3.280h, 2.666k, 2.898m 2.934i 2.938i

87 188 188 189 80

472 473 474 475 476 477 478 479 480 481 482

[(Cp W)2(m-TePh)B5H5(m-H)3] [{Cp W}2B6H10] [(Cp W)2B5H8]2 [(Cp W)2B5H9Fe(CO)3] [(Cp Mo)2B5H8(m-H){Ru3(CO)9}S] [(Cp ∗Mo)2B4H6SFe(CO)3] [(Cp ∗Mo)2B4H6SeFe(CO)3] [(Cp Mo)2B5SeH7] [(Cp Mo)2B6SeH8] [(CpW)2B5Te2H5] [(Cp Mo)2B3H3SCo2(CO)3(m-CO)3]

112.9, 57.9 84.7, 53.9, 36.5 87.8, 72.3, 48.7, 9.4, −3.9 62.7, 59.1, 56.3, 51.1, 25.1, 23.5 55.3, 51.0, 47.8, 45.8, 44.6, 40.7, 27.6, 24.5, 21.9 64.9, 52.7, 9.4, −14.1 83.9, 47.9, −12.7 50.6, 48.3, 40.6, 26.1 84.2, 46.2, 39.1, −6.6 73.4, 65.2, 56.1, 33.7, 29.2 86.7, 44.7, −5.3 85.9, 44.9, −4.9 86.4, 33.7, 32.0, 20.3, −3.2 95.4, 78.2, 14.1, 6.5, −10.9 87.2, 77.5, 44.1, −12.0 75.7, 71.7

2.318c 2.284c 2.27c 2.28c, 2.171e 2.257c 2.233c, 2.228e 2.209e, 2.279c 2.331c 2.328c 2.294c 2.284c, 2.137f

94 190 190 190 191 95 192 97 97 97 96,97

483

[(Cp Mo)2B3H3SeCo2(CO)3(m-CO)3]

77.4, 73.4

2.238c, 2.133f

2.848i 2.95850(12)i 2.821i 2.9392(7)i, 2.759m 2.8427(8)i, 2.789k 2.8846(4)i, 2.757m 2.9201(14)i, 2.794m 2.777i 2.779i 2.772i 3.0087(4)i, 2.4888(6)l, 2.705m 3.0464(5)i, 2.4827(10)l, 2.730m

466

50 187

96,97 (Continued )

298

Polyhedral Metallaboranes and Metallacarboranes

Table 8

(Continued)

No.

Compounds

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

484 485

[(Cp Mo)2B4H4S2Fe2(CO)5] [(Cp Mo)2B4H4Se2Fe2(CO)5]

75.1, 61.4, 5.2 74.2, 58.5, 11.6

− 2.352c, 2.107e

110 110

486 487

[(Cp Mo)2(m3-S)2B2H(m-H){Fe(CO)2}2Fe(CO)3] [(Cp Mo)2(m3-Se)2B2H(m-H){Fe(CO)2}2Fe(CO)3]

143.4, 107.2 144.1, 108.1

– 2.089e, 2.088c

488 489

[(Cp Mo)4B4H4(m4-BH)3] [(Cp MoSe)2Fe6(CO)13B2(BH)2]

131.5, 94.5, 58.8 162.5, 158.2, 106.2, 67.8

2.241c 2.081c, 2.116e

490

[(Cp Mo)2B3H3(m3-Se)2{Fe(CO)2}2{Fe(CO)3}2]

169.8, 89.4

2.139c, 2.133e

491 492 493 494

[(Cp Mo)2(BH)4(m3-Te){Fe(CO)3}] [(Cp MoCo)2B3H2(m3-Te)(m-CO){Co3(CO)6}] [(Cp MoCO)2B3H2(m3-Te)(m-CO)4{Co6(CO)8}] [(Cp Mo)2B4H2S2Fe4(CO)9(m-CO)]

85.6, 45.6, −4.1 110.0, 91.3, 76.2 166.6, 102.1 75.9, 75.0, 63.5, 56.9

2.244c, 2.229e 2.40c, 2.076f 2.258c, 2.109f 2.314c, 2.162e

495

[(Cp Mo)2B3H2Se2Co(CO)(m-CO)3{Fe4(CO)7}]

93.3, 90.7

496 497 159 160 161 162 163 178 179 180 498 147

[Re(CO)4B2H6{Re(CO)3}2(m-H)] [{Re(CO)4}2Re(CO)3B2H6(m-H)] [(Cp Ru)2(m-H){2,3-(Cl)2B4H5}] [(Cp Ru)2(m-H){1,2-(Cl)2B4H5}] [(Cp Ru)2(m-H){2,4-(Cl)2B4H5}] [(Cp Ru)2H{2,3,4-(Cl)3B4H4}] [(Cp Ru)2H{1,2,3-(Cl)3B4H4}] [(Cp Ru)2B4H7(Ph)] [(Cp Ru)2B4H7(Cl)] [(Cp Ru)2B4H6(SPh)(Cl)] [(Cp Ru)3(m3-BCl)(B2H7)] [{(BH)Cp Ru(m-CO)}2Fe2(CO)5]

−47.7 −47 132.08, 22.96, 18.56, −9.3 125.1, 23.8, −33.3 115.3, 26.0, 18.6, −33.0 131.7, 118.6, 23.7 21.4, 18.5, −11.3 124.8, 33.9, 21.6, −31.6 128.8, 15.7, −10.8 133.8, 23.0, 19.9, −7.9 114.5, 32.6, −0.9 125.4

2.331c, 2.134e, 2.501(7)f 2.405d 2.459d 2.163e − − − 2.162e 2.178e 2.162e 2.165e 2.187e 2.085e, 2.201e

148 499

[{(BH)Cp RuFe(CO)3}2(m-CO)] [3,4-dppm-1,2,3,4-Ru2Fe2(BH)2]

121.3, 110.6 127.7, 120.2

2.093e, 2.274e 2.072e, 2.06e

500

[3,4-dppe-1,2,3,4-Ru2Fe2(BH)2]

127.6, 119.6

2.077e, 2.042e

501 502

127.3, 119.1 132.5, 100.5

2.747k, 2.837m

137

2.751k, 2.841m 2.759k, 2.819m

137 137

11.2, −0.7, −3.1 21.5, 14.2, 11.5, 9.5, −1.9, −30.6 80.3, 23.1, 17.5, −17.5 34.3, 12.9, 0.1 115.9, 51.7, 50.5, 28.5, −10.6 130.3, 47.9, 25.5, −12.3 119.9, 99.0, 68.3, 66.5, 14.8, −7.7 141.7, 129.5 151.7, 128.9, 95.1

− 2.101e, 2.20 (2)c 2.10e, 2.399 (12)c 2.113e, 2.302c 2.108e, 2.30 (3)c 2.125e 2.228e 2.267e 2.23e 2.161e 2.156e 2.155e 2.128e 2.215e

− 2.283k 2.8556(5)k 2.8973(10)k 2.7933(4)k 2.7890(2)k 2.7155(5)k 2.841k 2.827k

106 197 119 120 114 114 114 198 198

514 515 516

[3,4-dppp-1,2,3,4-Ru2Fe2(BH)2] [(Cp Ru)3(m3-CO)2(m3-BH)2(m3-CO) (m-H){Cr(CO)3}] [(Cp Ru)3(m3-CO)2(m3-BH)2(m3-CO) (m-H){Mo(CO)3}] [(Cp Ru)3(m3-CO)2(m3-BH)3(m-H){Mo(CO)3}] [(Cp Ru)3(m3-CO)2(m3-BH)2(m3-CO) (m-H){W(CO)3}] [(Cp Ru)B8H14(RuCp )] [(Cp Ru)2{(Cp Ru)2B6H14}] [(Cp Ru)2B4H6Te] [(Cp Ru)2(m3-Se)(m4-Se)B3H5] [(Cp Ru)2(m3-H)B5HCl3(SMe2)] [(Cp Ru)2(m3-H)B5HCl3(SMe2)] [(Cp Ru)2B6H3Cl3] [(Cp Ru)3{Ru(CO)2}2BH(m3-CO)(m-H)B(m-H)3] [(Cp Ru)2{Ru(CO)2}2BH(m3-BH) (m-H)B(m-H)3Ru(Cp Ru)2(m-CO)3(m-H)BH] [(Cp Ru)3(m3-CO)(BH)3(m3-H)3] [(Cp Ru)2(m3-CO){Ru(CO)3}2(BH)2(m-H)B] [(Cp Ru)2(m3-CO)2B2H(m-H){Fe(CO)2}2Fe(CO)3]

− 2.7437(16)i, 2.634(3)k, 2.810m – 2.9216(3)i, 2.595k, 2.792m 2.840i 2.8645(8)i, 2.686k, 2.812m 2.7603(4)i, 2.6857(7)k, 2.853m 2.9518(11)i, 2.806m 2.809m, 2.535l 2.838m, 2.586l 2.9433(4)i, 2.695k, 2.815m 2.9514(8)i, 2.645k, 2.494m, 2.8964m, 2.969m 3.0511(5)j 3.1405(8)j 2.8690(2)k − − − 2.8846(2)k 2.8580(3)k 2.8899(3)k 2.8744(10)k 2.832k 2.7954(4)k, 2.651m, 2.6216(7)k 2.703k, 2.634m, 2.556(7)k 2.7202(6)k, 2.615(11)k, 2.661m 2.7145(10)k, 2.6282(17)k, 2.666m − 2.744k, 2.761m

91.4, 82.3 94.2, 50.9, 43.8 158.5, 128.5

2.303e 2.172e 2.067e, 2.029e

198 198 193

517 518

[Fe2(CO)6(Cp RuCO)(Cp Ru)B6H10] [(p-cym)RuS2B16H16]

47.1, 37.1, 29.9, 1.37, −2.7 27.8, 27.8, 22.7, 16.6, 13.5, 7.7, 6.2, 5.5, 4.7, −0.5, −12.6, −13.2, −17.1, −29.2, −32.6

2.203e, 2.247e 2.244e

2.809k 2.807k 2.7167(4)k, 2.612k, 2.652m 2.5694(9)k –

503 504 505 506 507 508 185 509 510 511 512 513

134.5, 98.4 130.8, 110.3 135.2, 102.2

193 193 194 186 195 98 98 98 196 196

104 104 114 114 114 114 114 119 119 119 137 52 52 136 136 136 137

199 200

Polyhedral Metallaboranes and Metallacarboranes

Table 8 No.

(Continued)

Compounds

519

[(p-cym)RuS2B15H15]

520

[(PPh3)RuB9H9{RuCl2(PPh3)2}2]

521

anti-[8-(p-cym)-8-RuB17H21]

522

[(p-cym)RuB18H20]

523

[(p-cym)2Ru2B18H17Cl]

524

[(PPh3)2RuB16H20]

525 526 527 528

[(Cp Rh)2Cr(CO)3(m-CO)(m3-BH)(B2H4)] [(Cp Rh)2Mo(CO)3(m-CO)(m3-BH)(B2H4)] [(Cp Rh)2W(CO)3(m-CO)(m3-BH)(B2H4)] [(Cp Rh)2(m3-BH)2Co2(CO)5(m-CO)]

529 530 531 532 533 534 535 536 537 538 539 165 255 540 541 542 543 544

554

[(Cp RhB2H3Se2)2(m4-Zn)] [(Cp IrB2H3Se2)2(m4-Zn)] [(Cp Ir)3B4H4] [(Cp Rh)3(BH)3(BBr)] [(Cp Rh)3(BH)(BBr)3] [(Cp Co)2(m-H)(BH)4{Co(CO)2}] [(Cp Rh)2{Fe(CO)3}2B3H3] [(Cp Rh)2B6H6{Fe(CO)2}2{Fe(CO)3}2] [(Cp Rh)2B3H3{Ru(CO)3}2] [(Cp Rh)2B4H4Rh{Cp RhB3H8}] [(Cp Rh)B3H7{Ru(CO)2}(Cp RhCO)2] [Cp Ir(Cp Ru)2B5H5] [(Cp Co)2B4H5SFe3(CO)9] [(Cp Ir){Ru3(CO)8}B4H10] [Ru4(CO)11(Cp Co)2B3H3] [(Cp Co)2{Fe(CO)3}2B6H10] [(Cp Rh)2{Fe(CO)3}2B6H10] [(Cp Co){m-Z5:Z5-B2H2S2-Ru(CO)2} Ru(CO)2{m-Ru(CO)4}] [(Cp Co){m-Z5:Z5-B2H2Se2-Ru(CO)2} Ru(CO)2{m-Ru(CO)4}] [(Cp Co){m-Z5:Z5-B2H2Se2-Ru(CO)2} Ru(CO)2{m-Fe(CO)4}] [(Cp Co)2B6Se2H6{Ru3(CO)8}] [(IrCp )3{Ir(CO)2}3(m-CO)(m3-CO)B] [(RhCp )3{Rh(CO)}3(m-CO)3{MnH(CO)3}B3H2] [(Cp Rh)3Rh(CO)2Mn2(CO)6B4H3] [(Cp Rh)3(RhCO)3Fe(CO)3(m-CO)3B3H2] [(Cp Rh)2{Co6(CO)12}((m-H)(BH)B)] [{(Cp Co)2(m-H)(m3-Se)2} Co(m-Se)3(m3-Se)4B3H3] [(Cp Co)3B15H10{Cp CoH}]

555

[(Cp Co)3CoB16H17]

556

[(Cp )2Ir2B16H17Cl]

557 558

[(Cp )2Ir2B16H15Cl] [(Cp )2Rh2B11H15]

545 546 547 548 549 550 551 552 553

299

11

B NMR (ppm)

Av. MdB (Å)

Av. MdM (Å)

Ref.

e

–

200

2.314e

–

201

2.226e

–

202

–

–

202

–

–

202

2.392e

−

203

137 204 204 70

2.282

12.9, 11.8, 4.7, 2.2, 0.1, −1.4, −4.7, −5.4, −8.4, −16.4, −17.9, −23.7, −32.4, −33.3, −40.5, 85.0, 77.4, 27.5, 20.9, 27.5, 8.7, −14.6, −29.9 25.7, 16.4, 15.7, 13.0, 10.4, 2.6, −2.7, −3.0, −6.6, −7.6, −13.9, −21.4, −30.0, −32.4, −40.0 16.9, 12.1, 8.8, 6.2, 6.1, 6.0, 3.6, 0.1, −0.1, −1.6, −1.7, −1.8, −7.0, −16.6, −16.5, −26.7, −37.3, −39.1 22.0, 20.5, 10.0, 9.4, 7.6, 3.5, 2.5, 1.5, 0.5, 0.1, −2.6, −4.2, −4.8, −23.0, −24.7, −31.0, −32.0 28.8, 22.2, 18.3, 7.6, 1.7, 0.5, −0.1, −2.0. −5.9, −1.5, −22.1, −27.9, −34.2, −39.6, −42.2 121.3, 59.9, 23.5 124.8, 60.7, 20.1 126.0, 59.9, 22.5 125.5

2.095f, 2.24c 2.104f, 2.353c 2.102f, 2.361c 2.06f, 1.995f

19.2 11.7 102.4, 50.1 128.3, 75.8, 69.6 128.3, 89.3, 62.6 109.8, 95.3, 81.9 138.3, 114.1 139.2, 106.8, 28.8, 6.3 118.1, 104.3 118.8, 75.4, 65.5, 7.5, 1.4 4.7, 1.3 118.0, 100.4, 71.4, −1.5 64.3, 49.1, 10.3 116.8, 21.8, 5.2 122.1, 99.3, 85.1 45.5, 32.0, 26.9, 25.3, 9.1, −5.5 34.7, 28.1, 25.2, 16.4, 1.6, −12.3 19.9

2.160f 2.166f 2.125f – 2.089f 2.029f 2.09f, 2.092e 2.131e, 2.121f 2.216e, 2.09f 2.159f 2.406e, 2.099f 2.149e, 2.113f 2.10f − 2.026f, 2.257e 2.102f, 2.191e 2.194f, 2.184e −

2.6848(17)l, 2.68m 2.6965 (11)l, 2.791m 2.6944 (9)l, 2.786m 2.6285(16)l, 2.372(3)l, 2.582m − − 2.703l – 2.691l 2.451l 2.596(2)k, 2.655m 2.646k, 2.734m 2.7968(8)k, 2.754m 2.701l 2.6214(6)l, 2.777m 2.716k, 2.722m 2.5909(12)l, 2.627k − 2.827k, 2.749m 2.5779(5)k 2.5795(10)k −

22.3

2.308e, 2.148f

2.817k, 2.9356(7)m

143

23.0

2.297e, 2.149f

143

39.4, 3.9 51.9 96.2, 88.9 93.3, 79.4, 71.1, 64.0 93.2, 96.1 101.2, 90.3 −44.6

2.134f, 2.379e 2.153f 2.244f, 2.208d 2.187f, 2.206d 2.206f, 2.217e 2.137f, 2.149f –

2.8837(6)k, 2.9469(7)m, 2.696m 2.815k 2.239l 2.845l, 2.739m 2.875l, 2.738m 2.845l, 2.666m 2.50l, 2.406m −

56.3, 37.8, 29.0, 25.4, 16.1, 14.2, 10.6, 3.8, −10.6, −12.0 158.9, 86.7, 72.3, 36.9, 35.0, 29.8, 25.1, 22.8, 18.7, 15.6, 13.6, 3.9, −4.4, −18.7, −29.4, −45.9 24.6, 14.9, 14.9, 13.0, 9.0, 8.1, 5.8, 0.9, −2.1, −4.0, −8.0, −9.2, −23.0, −42.1 – 15.6, 13.9, 5.5, 2.1, −1.9, −4.0, −10.9, −13.0, −25.1, −42.9

2.067f

−

208

2.082f

2.512l

208

2.215f

–

209

2.203f 2.216f

– –

209 210

205 205 131 87 87 136 141 141 206 206 206 115 124 180 134 134 128 143

147 139 139 140 140 207 146

(Continued )

300

Polyhedral Metallaboranes and Metallacarboranes

Table 8 No.

(Continued)

Compounds

559

[90 -(dppe)-90 -Ni-anti-B18H20]

560

[50 -Cl-90 -(dppe)-90 -Ni-anti-B18H19]

561

[110 -(dppe)-110 -Ni-syn-B18H20]

562

[70 -(dppe)-70 -anti-NiB17H21]

563

[(PMe2Ph)2PtB16H17Me]

564

[(PMe2Ph)2PtB14H16]

565

[(PMe2Ph)2Pd2B16H20(PMe2Ph)2]

566

[(PMe2Ph)3Pt2B16H18(PMe2Ph)]

11

B NMR (ppm)

19.5, 11.2, 9.7, 5.4, 4.2, 1.1, −2.7, −5.5, −9.9, −12.8, −17.2, −29.4, −30.5, −39.6 33.0, 13.1, 12.6, 11.3, 9.6, 6.1, 2.1, −1.3, −4.9, −9.1, −17.3, −27.2, −30.1, −38.7 20.8, 20.4, 11.3, 8.8, 8.4, 6.5, 2.7, 2.1, −0.3, −2.7, −5.1, −10.5, −19.0, −25.8, −26.6, −39.9 25.5, 13.4, 9.7, 7.2, 4.9, 2.1, −0.1, −1.7, −7.0, −10.1, −26.9, −35.2, −37.7 19.4, 17.3, 16.2, 14.4, 13.8, 3.2, 2.7, −0.8, −2.8, −14.5, −18.2, −21.8, −28.9, −36.3 23.8, 22.0, 18.1, 15.7, 2.7, 2.5, 0.0, −0.6, −19.4, −27.3, −31.1, −41.9 28.0. 24.7, 20.2, 8.0, 3.1, 1.0, 212.4, 213.5, 218.1, 220.0, 227.0 233.0, 245.8, two other resonances within 10 to 25 ppm 31.0, 30.3, 7.5, 6.1, 2.0, 1.2, 0.5, −5.5, −7.0, −13.5, −14.0, −14.3, −15.8, −23.0, −34.5

Av. MdB (Å)

Av. MdM (Å)

Ref.

g

–

170

2.302g

–

170

2.191g

–

170

2.486g

–

170

2.259g

–

211

–

–

211

2.245g

–

212

2.251g

–

212

2.178

a

Av. MGr.4-B Av. MGr.5-B c Av. MGr.6-B d Av. MGr.7-B e Av. MGr.8-B f Av. MGr.9-B g Av. MGr.10-B h Homometallic Av. MGr.5-MGr.5 i Homometallic Av. MGr.6-MGr.6 j Homometallic Av. MGr.7-MGr.7 k Homometallic Av. MGr.8-MGr.8 l Homometallic Av. MGr.9-MGr.9 m Heterometallic Av. MdM b

Fig. 27 Molecular structures of fused metallaboranes 429–432.

Fig. 28 Molecular structures of edge-fused metallaboranes 433–437, 439–457, 459, 461 and 462 (note that Cp/Cp attached to metal centers of 433–437 and 439–457 are not shown).

Polyhedral Metallaboranes and Metallacarboranes

301

Fig. 29 Molecular structures of fused metallaboranes 463–469. Note that only core structures are shown for the fused clusters starting from this figure to all figures related to fused clusters.

The reactions of fused clusters 461 and 462 with [Fe2(CO)9] led to further fusion and afforded clusters 463 and 464, respectively.50,186 In 463 and 464, a trigonal bipyramidal fragment [M2B2O] (14  2 + 4  3 + 2 ¼ 42e) was fused with a tetrahedral core {M2FeB} (15  3 + 5  2 ¼ 50e) via a common TadTa edge (16  2 + 2 ¼ 34e) and the {M2Fe2B} tetrahedral is further fused with another tetrahedral [MFe2B] core (15  3 + 5  2 ¼ 50e) through a triangular {MBFe} face (16  2 + 6 ¼ 38e) (Fig. 29). The requirement of this 70e is achieved from their CVE (CVE of [(Cp M)2B2H4O{H2Fe2(CO)6BH}] ¼ 2  10 + 2  3 + 4  1 + 1  6 + 2  1 + 2  8 + 6  2 + 1  3 + 1  1 ¼ 70e). Further, the thermolysis reaction of 463 and 464 with [Ru3(CO)12] led to the isolation of fused metallaborane clusters 465 and 466, respectively.187 Interestingly, cluster 465 has a m7-boride atom that shares three cluster units (one monocapped trigonal prism and two tetrahedral) (Fig. 29). The structure of 466 can be considered as a fusion of five subunits: a {Nb2B2O} trigonal bipyramid is fused with a {Nb2RuB} tetrahedral unit through an NbdNb edge that is further face fused with a square pyramid {NbRu2B2} through a triangular {NbRuB} face (Fig. 29). The overall unit is further fused with another square pyramid {Ru3B2} unit through a Ru2B2 butterfly face followed by a face fusion with a tetrahedron {Ru3B}. Clusters 465 and 466 do not follow any of the cluster-counting rules. Thermolysis of [(Cp Ta)2(BH3)2Cl2] in the presence of [Ru3(CO)12] yielded pileo-[Cp TaCl(m-Cl)-B2H4Ru3(CO)8] (467) having 1 SEP less than the requirement for the observed capped square pyramidal geometry.87 Cluster 467 is the first example of an unsaturated cluster comprising early and late transition metals in a square pyramid core. The reaction of [(Cp Ta)2B4H9(m-BH4)] (13) with [Fe2(CO)9] yielded two fused clusters 468 and 469.188 Cluster 468 represented the first example of a bicapped pentagonal bipyramidal metallaborane. By contrast, two pentagonal bipyramidal units are fused through a triangular face in 469. Group 6 metals give rise to plenty of vertex, edge, and face-fused metallaborane clusters. The thermolysis reaction of [Cp MoCl4] with LiBH4
THF afforded vertex fused cluster 470, in which two open seven vertex clusters are fused at a Mo-vertex (Fig. 30).189 The

Fig. 30 Molecular structures of fused metallaboranes 470–474 and 476.

302

Polyhedral Metallaboranes and Metallacarboranes

reaction of [Cp WCl4] with LiBH4
THF, followed by treatment of CO led to the isolation of W analog of 470, cluster 471 along with other products.80 Thermolysis of Ph2Te2 with an in situ generated intermediate, obtained from the reaction of [Cp WCl4] with LiBH4
THF, afforded a fused cluster 472, which has a capped octahedral geometry that is unique, due to the presence of three WdHdB bridging hydrogens in a closo cluster.94 The thermolysis of [{Cp W}H3B4H8] with BH3
THF led to the isolation of bicapped octahedron 473, conjuncto-474, and [(Cp W)2B5H9].190 The reaction of [{Cp W}2B5H9] with a 3-fold excess of Fe2(CO)9 in hexane afforded an iron analog of 473, bicapped octahedron 475.190 Bicapped octahedron clusters 473 and 475 possess 7 SEP, which is in accord with their structures. By contrast, cluster 474 is a coupled-cage structure in which two bicapped trigonal bipyramidal W2B5 cores are joined by an exopolyhedral borondboron bond. One more conjuncto-cluster 476 was isolated from the reaction of [(Cp Mo)2B5H9] with [Ru3(CO)12] and 2-methylthiophene. Conjuncto-476 is comprised of [Mo2B5] and tetrahedral {Ru3S} cores, which are attached through BdS bond.191 On the other hand, the reaction of [(Cp∗Mo)2B4EH6] (109: E ¼ ]S or 110: E ¼ ]Se) with Fe2(CO)9 yielded 47795 and 478192, respectively, which have a similar type bicapped octahedron core as that of 473 and 475 and possessed regular 7 SEP. The thermolysis reaction of in situ generated intermediates, obtained from the reaction of [Cp MoCl4] or [CpWCl4] with LiBH4
thf, in the presence of chalcogen powders (Se or Te) led to the formation of fused clusters 479–481 (Fig. 31).97 Cluster 479 can be viewed as the fusion of octahedral [Mo2B4] core (14  2 + 4  4 + 2 ¼ 46e) and trigonal bipyramidal [Mo2B2Se] core (14  2 + 4  3 + 2 ¼ 42e) through a triangular [Mo2B] face (16  2 + 6 ¼ 38e). The requirement of this 50e is fulfilled by the CVE of 479 (CVE of [(Cp Mo)2B5SeH7] ¼ 11  2 + 3  5 + 6  1 + 1  7 ¼ 50e). On the other hand, cluster 480 can be viewed as a fusion of trigonal bipyramid and capped octahedron cores. Cluster 481 is the fusion of octahedron, tetrahedron, and trigonal bipyramid through a butterfly face. The reaction of [(Cp Mo)2B4EH6] (109: E ¼ ]S or 110: E ¼ ]Se) with [Co2(CO)8] yielded fused clusters 482 and 483, respectively.96,97 482 and 483 have a similar core and fusion pattern to cluster 479. The thermolysis reaction of [(Cp Mo)2B4H4E2] (116: E ¼ ]S; 117: E ¼ ]Se) in presence of [Fe2(CO)9] yielded fused clusters 484 and 485, respectively.110 The geometry of both clusters 484 and 485 consists of a bicapped octahedron [Mo2Fe2B3E] and a trigonal bipyramidal [Mo2B2E] core, fused through a common triangular [Mo2B] face (Fig. 31). Mild pyrolysis reactions of [(Cp Mo)2B4EH6] (109: E ¼ ]S or 110: E ¼ ]Se) with [Fe2(CO)9] led to the isolation of cubane type clusters 486 and 487, respectively.193 Clusters 486 and 487 can be viewed as cubane shaped systems made of two Mo, two Fe, two chalcogen atoms (S or Se), and two B atoms, capped by a third Fe atom attached to one of the triangular Fe2B faces of the cube (Fig. 32). Alternatively, clusters 486 and 487 can also be described as tetracapped tetrahedron, and one triangular face of this

Fig. 31 Molecular structures of fused metallaboranes 479–481, 484 and 485.

Fig. 32 Molecular structures of fused metallaboranes 486–490 and 492–495 (Interstitial boride boron of 493 is not shown for clarity).

Polyhedral Metallaboranes and Metallacarboranes

303

tetracapped tetrahedron was further capped by Fe atom. Clusters 486 and 487 have 60 CVE, which is in accord with their cubane-type structures having 6 MdM bonds. The reaction of [Cp MoCl4] with an excess of LiBH4
THF, followed by thermolysis with Te powder afforded a tricapped cubane cluster 488, where four square faces of the cubane are capped by three boron atoms.194 Cluster 488 has 58 CVE. The reaction of [(Cp Mo)2B4H4Se2] (117) with [Fe2(CO)9] led to the isolation of a fused m8-boride cluster 489186 and tetracapped pentagonal bipyramidal cluster 490,195 along with fused cluster 485110. Boride 489 is a face-fused cluster generated from the fusion of a cubane (Mo2Fe2B2Se2) and a tricapped trigonal prism (Fe6B3). Cluster 489 is the first example of this type, where the m8-boride B atom is encapsulated inside the trigonal prismatic skeleton and connected to six Fe and two B vertices. Cluster 490 has 8 SEP and 98 CVE, appropriate for its tetracapped pentagonal bipyramidal structure. Mild pyrolysis of [(Cp Mo)2B4TeH5Cl] (112) with [Fe2(CO)9] yielded distorted cubane cluster 491 and with [Co2(CO)8] produced the bicapped pentagonal bipyramid cluster 492 and pentacapped trigonal prism cluster 493.98 Cluster 491 can also be viewed as a bicapped octahedron. Interestingly, one of the boron atoms of cluster 492 occupies the semi-interstitial position; thus, it can also be considered as a boride cluster. On the other hand, a boride boron atom is encapsulated inside cluster 493, which is connected to all the vertices of trigonal prism and three of the square faces capping vertices. Fused cluster 493 has 118 CVE. A complicated fused 11-vertex cluster 494 was isolated from the thermolysis of [(Cp Mo)2B4H4S2] (116) with [Fe2(CO)9] along with fused clusters 484 and 486.196 The structure of cluster 494 can be derived from 11-vertex closo-tricapped square antiprism by performing DSD rearrangement and removing two edges. A similar type fused structure 495 was isolated from the thermolysis of fused 485 with [Co2(CO)8].196 The photolysis reaction of [Re2(CO)10] with BH3
THF yielded [Re(CO)4(m3-Z2:Z2:Z2-B2H6){Re(CO)3}2(m-H)] (496) and [{Re(CO)4}2Re(CO)3(m3-Z2:Z2:Z1-B2H6)(m-H)] (497).104 Compounds 496 and 497 contain [B2H6]2− ligand in diverse coordination modes. Alternatively, they can be viewed as edge fused clusters. One tetrahedral {Re2B2} core (15  2 + 5  2 ¼ 40e) and one triangular {ReB2} core (16  1 + 6  2 ¼ 28e) are fused through a BdB edge (6  2 + 2 ¼ 14e) in 496 (Fig. 33). By contrast, one arachno-butterfly {Re2B2} core (14  2 + 4  2 + 6 ¼ 42e) and one triangular {Re2B} core (16  2 + 6  1 ¼ 38e) are fused through a RedB edge (16  1 + 6  1 + 2 ¼ 24e) in 497 (Fig. 33). The requirement of 54e and 56e are fulfilled by the CVE of 496 and 497, respectively.

Fig. 33 Molecular structures of fused metallaboranes 496 and 497.

A series of different types of group 8 condensed clusters has been synthesized and characterized. The reaction of a preformed ruthenaborane, nido-[1,2-(Cp RuH)2B3H7] with BHCl2
SMe2 afforded Cl-functionalized fused ruthenaboranes 159–163 along with nido-158.114 Clusters 159–163 have capped square pyramidal core structures. A similar type of capped square pyramid cluster (178–180) with different types of substituents at the B centers were isolated from the thermolysis of nido-[1,2-(Cp RuH)2B3H7] with Li[BH3(SPh)]. The reaction of the same nido-ruthenaborane with [Re2(CO)10] afforded another capped square pyramidal cluster 498. Clusters 159–163,114 178–180119 and 498137 have the expected 7 SEP. The thermolysis of arachno-[{Cp Ru(CO)}2B2H6] (141) with [Fe2(CO)9] afforded clusters 147 and 148 having bis-(borylene) units.52 Clusters 147 and 148 have tetrahedral {Fe2Ru2} cores, and two triangular faces of these tetrahedral cores are triply bridged by two borylene units. The geometry of 147 and 148 can be viewed as bicapped tetrahedral, and these systems have 6 SEP as expected. Photolysis of cluster 148 with dppm, dppe, and dppp led to the coordination of these ligands to both Fe atoms and afforded clusters 499–501, respectively.136 The pyrolysis reactions of nido-[1,2-(Cp RuH)2B3H7] with [M(CO)5
THF] (M ¼ ]Cr, Mo, W) afforded tetrametallic borylene species 502–505 (Scheme 22).137 Clusters 502, 503, and 505 can be described as a tetrahedral (Ru3M), in which two BH and two CO ligands cap

Scheme 22 Syntheses of fused metallaboranes 502–505.

304

Polyhedral Metallaboranes and Metallacarboranes

Fig. 34 Molecular structures of fused metallaboranes 506 and 507.

all the four triangular faces of the tetrahedron. By contrast, three BH units and one CO ligand cap the faces of Ru3M tetrahedron in 504. Clusters 502–505 have 6 SEP and obey the capping principle. The reaction of nido-[1,2-(Cp RuH)2B3H7] with BH3
THF afforded [(Cp Ru)B8H14(RuCp )] (506) along with ruthenaborane 157.106 Cluster 506 can be viewed as the fusion of two ruthenaborane cages of 157 through BdB edge, i.e., two pentagonal pyramid cores are fused by a common BdB bond (Fig. 34). Cluster 506 can also be described as pentalene analog. When the same nido-[1,2(Cp RuH)2B3H7] was heated with Te powder, it afforded an analog of 506, [(Cp Ru)2{(Cp Ru)2B6H14}] (507).197 In cluster 507, two of the BH vertices of 506 are substituted by two isoelectronic {Cp RuH} vertices. On the other hand, the thermolysis of nido-[1,2-(Cp RuH)2B3H7] with Li[BH3(TePh)] afforded a capped pentagonal pyramidal cluster 508, which has expected 8 SEP.119 By contrast, when the same starting material is reacted with Se powder, a unique fused cluster 185 was isolated along with bis-homocubane type cluster 184.120 The structure of 185 can also be described as a condensed cluster, in which a pentagonal pyramidal unit is fused with a butterfly core through two edges (BdSe and SedRu). Alternatively, cluster 185 can be derived from a tricapped trigonal prism by the successive removal of two degree-four vertices. The reaction of nido-[1,2-(Cp RuH)2B3H7] with BHCl2
SMe2 at 95  C afforded two monocapped-octahedral clusters (509 and 510), and a bicapped-octahedral cluster (511).114 Clusters 509–511 have 7 SEP as expected for these types of capped clusters. A series of cubane-type clusters (512–515) was isolated from a thermolysis reaction of nido-[1,2-(Cp RuH)2B3H7], and [Ru3(CO)12].198 Clusters 512 has a similar type of capped cubane core as those of 486 and 487, but interestingly a triply bridged hydrogen (m3-H) atom participates as a vertex in the cubane core formation. The capped cubane core is further fused with a tetrahedron through a vertex in 513 (Fig. 35). The fused cubane clusters 512 and 513 possess 60 CVE with six metaldmetal bonds. Clusters 514 and 515 also have a simple cubane core. Also, the mild pyrolysis of arachno-[{Cp Ru(CO)}2B2H6] (141) with [Fe2(CO)9] afforded mono capped cubane 516, which has a similar structure as that of 486 and 487.193 On the other hand, mild pyrolysis of [(Cp Ru)2B6H12] with Fe2(CO)9 led to the fusion of one BdB edge of the starting material with the diiron carbonyl unit to form fused cluster 517.199 The core geometry of 517 represented a novel class of hybrid cluster in which a Fe2B2 tetrahedron has been fused to a ruthenaborane substrate (Fig. 35). Cluster 517 has 12 SEP, which is consistent with its composition and also obeys the Jemmis MNO rule. Kennedy and co-workers have isolated many unique fused metallaborane clusters of group 8 metals. Deprotonation of a fused thiaborane [S2B17H17] with NaH and subsequent reaction with [RuCl2(p-cym)]2 afforded two edge-fused clusters, 19-vertex 518 and 18-vertex 519 (Fig. 36).200 One nido-{SB9} unit and an arachno-type {RuSB9} unit condensed through a BdB edge in 518. By contrast, in 519, a nido-{RuSB9} unit and a nido-{B8} unit are fused via a common BdB edge that is additionally linked exo to the {RuSB9} unit by a bridging S atom that is held endo to the {B8} unit. The reaction of [RuCl2(PPh3)2] with arachno-[B9H14]− afforded cluster 520, which has an isocloso 10-vertex {RuB9} core geometry that is fused with two {Ru2B} triangles via two BdB edges.201 On the other hand, the reaction of [{RuCl2(p-cym)}2] with syn-[B18H22] and a non-nucleophilic base afforded fused clusters 521–523.202 In cluster 521, two 10-vertex nido-clusters fused through a BdB in anti-configuration. By contrast, clusters 522 and 523 result from the simple addition of one and two Ru centers, respectively, to the syn-[B18H22] skeleton. The reaction of fused borane cluster [B16H20] with [RuCl2(PPh3)3] afforded a face-fused cluster 524, which consists of a nido-{B10} unit and a ten-vertex neonido-{RuB9} unit.203

Fig. 35 Molecular structures of fused metallaboranes 513 and 517.

Polyhedral Metallaboranes and Metallacarboranes

305

Fig. 36 Molecular structures of fused metallaboranes 518–524.

Like group 8, group 9 has given rise to a large number of condensed metallaborane clusters. The thermolysis of nido[(Cp Rh)2B3H7] with [M(CO)5
THF] (M ¼ ]Cr, Mo or W) at 80  C led to the isolation of trimetallic rhodaboranes [(RhCp )2M(CO)3(B2H4)(m-CO)(m3-BH)] (M ¼ ]Cr (525), Mo (526), W (527)).137,204 Clusters 525–527 can be viewed as a capped square pyramid (pileo) as that of cluster 467 and contain 56 CVE and 6 SEP. The thermolysis reaction of the same starting material with [Co2(CO)8] afforded a tetrametallic triply bridging bis-borylene species 528.70 In 528, borylene {BH} units symmetrically coordinate to both the RhdCodRh deltahedral faces of the tetrahedron [Rh2Co2] in m3-fashion. The bicapped tetrahedral core of 528 is the similar to that of 147. The thermolysis of an in situ generated intermediate, obtained from the reaction of [Cp MCl2]2 (M ¼ ]Rh, Ir) and LiBH4
thf, with Se powder and [Zn(BH4)2] afforded the two unusual fused clusters, 529 and 530.205 Clusters 529 and 530 belong to a rare class of metal-centered commo-bis(metallaheteroborane) species (Fig. 37). The molecular structures of 529 and 530 can be viewed as two arachno-metallaheteroborane units conjoined at the common zinc atom that obey Mingos’ fusion formalism with 82 CVE. The reaction of [Cp IrCl2]2 with BH3
THF at high temperature led to the isolation of a trimetallic capped octahedral cluster 531, where the Ir3 face of the octahedron is capped by BH unit.131 When the Rh analog of 531, [(Cp Rh)3B4H4], is thermolyzed in the presence of PtBr2, the reaction led to bromination and afforded mono- and tribrominated rhodaboranes 532 and 533 having capped octahedral cores.87 A similar type of capped octahedral core 534 was isolated, along with borylene species 234–236, from the reactions of {Cp CoCl}2 and LiBH4
THF with [M(CO)3(MeCN)3] (M ¼ ]W, Mo, Cr).136 The room-temperature reaction of nido-[(Cp Rh)2B6H10] (200) with [Fe2(CO)9] also yielded a 7-vertex metallaborane 535 having a monocapped octahedral core, along with another 12-vertex fused cluster 536.141 All these mono capped octahedral clusters (531–535) obey the capping principle with 7 SEP. In 536, two faces of a 10-vertex isocloso are capped by {Fe(CO)3} unit (Fig. 37). The thermolysis of [(Cp Rh)2B2H6] with [Ru3(CO)12] yielded fused clusters 537–539.206 Cluster 537 has capped octahedral geometry and has 7 SEP. Cluster 538 is a vertex-fused cluster in which a monocapped octahedron and a square pyramidal core are fused. By contrast, a square pyramid and a triangle are fused through a vertex in 539. Both 538 (CVE ¼ 84) and 539 (CVE ¼ 74) follow Mingos fusion formalism for fused clusters. On the other hand, the reaction of nido-[1,2-(Cp RuH)2B3H7] with arachno[Cp IrB3H9] in hexane under reflux condition afforded a bicapped octahedral metallaborane 165 (7 SEP) along with nido-164.115 The reaction of [(Cp Co)2B4H6] with [Fe2(CO)9] and S powder afforded conjuncto-255 along with octahedral 254.124 In cluster 255, the boron vertex of octahedral Co2B4 core is linked with S vertex of tetrahedral {Fe3S} core.

Fig. 37 Molecular structures of fused metallaboranes 529, 530, 536, and 538.

306

Polyhedral Metallaboranes and Metallacarboranes

Fig. 38 Molecular structures of fused metallaboranes 540–543.

The thermolysis reaction of arachno-[(Cp2Zr)(Cp Ir)B4H10] with [Ru3(CO)12] afforded a unique fused cluster 540, in which a pentagonal pyramid IrRuB4 core was fused with a tetrahedral Ru3B core via a common RudB edge and further one of the Ru atoms of tetrahedral Ru3B core is linked with one of the B atoms of the pentagonal pyramid {IrRuB4} core (Fig. 38).180 The reaction of nido-[(Cp Co)2B6H10] (199) with [Ru3(CO)12] at elevated temperature yielded fused cluster 541, in which four sub-cluster units (one octahedron, one trigonal bipyramid, and two tetrahedra) are fused.134 Interestingly, fused cluster 541 has a semi-interstitial boron atom in m7-bonding mode. The reaction of nido-[(Cp M)2B6H10] (199: M ¼ ]Co or 200: M ¼ ]Rh) with [Fe2(CO)9] yielded edge fused clusters 542 and 543, respectively.128,134 The core geometries of 542 and 543 are the same as cluster 517 and have 12 SEP (Fig. 38). Thermolysis of [Ru3(CO)12] with open triple-decker complexes (266, 267, or 270) afforded fused clusters 544–546.143 Clusters 544–546 have pentagonal pyramid core as that of clusters 273–276, in addition, the M-M bond of these pentagonal pyramid cores are fused with the M-M bond of an M3 triangle. Clusters 544–546 have 74 CVE and obey Mingos fusion formalism. On the other hand, the reaction of nido-[(Cp Co)2B6Se2H6] (293) with [Ru3(CO)12] at elevated temperature afforded edge fused cluster 547 (104 CVE), in which an icosahedral core is fused with a Ru3 triangle through a common RudRu bond.147 The reaction of arachno[Cp IrH2(B3H7)] with [Mn2(CO)10] at 90  C yielded a fused iridaborane cluster 548.139 Cluster 548 can be viewed as a fusion of trigonal bipyramid {Ir4B} (14  4 + 4  1 + 2 ¼ 62e) and square pyramid {Ir4B} (14  4 + 4  1 + 4 ¼ 64e) through a triangular face {Ir2B} (16  2 + 6 ¼ 38e) (Fig. 39). Therefore, cluster 548 has a requirement of 88 electrons, which is satisfied from the available cluster valence electrons of 548, i.e., [6Ir  9 + 3Cp  5 + 8(CO)  2 + 1B  3 ¼ 88e]. Cluster 548 has a semi-interstitial boride boron atom bonded to five metals. The thermolytic reactions of nido-rhodaborane [(Cp Rh)2(B3H7)] with [Mn2(CO)10] at 80  C yielded m9-heptametallic boride (549)139 and m9-hexametallic boride (550)140 with other products. The same reaction with [Fe2(CO)9] led to the isolation of another m9-heptametallic boride (551).140 Also, the reaction of nido-[(Cp Rh)2B6H10] (200) with [Co2(CO)8] afforded m9-octametallic 552.207 The core geometries of 549–552 can be viewed as tricapped trigonal prism. One of the striking features of 549–552 is the presence of a naked boron atom at the interstitial point, which is coordinated to all nine vertices of the tricapped trigonal prism. The room-temperature reactions between [Cp CoCl]2 and excess of Li[BH2E3] afforded a fused homocubane type cluster (553) along with other homocubane analogs (277 and 288).146 Cluster 553 is a unique vertex-fused trishomocubane derivative, in which a trishomocubane moiety is fused with an exopolyhedral trigonal bipyramid (Fig. 40). The pyrolysis reaction of an in situ generated intermediate, obtained from the fast metathesis of [Cp CoCl]2 and LiBH4
THF, with an excess amount of [BH3
THF] yielded two fused clusters 554 and 555 (Fig. 40).208 Cluster 554 is a closo-19-vertex face-fused cluster, in which an icosahedron {Co3B9}, a tetrahedral {B4} and a 10 vertex {CoB9} units are fused by one triangular {B3} face and one butterfly {B4} face. Cluster 554 obeys the Mingos fusion formalism with 112 CVE and Jemmis MNO rule with 54 SEP. By contrast, cluster 555 is a 20-vertex face-fused cluster, in which an icosahedron {Co4B8}, a square pyramid {CoB4}, a tetrahedron {Co2B2} and a nido-{CoB7} unit are fused through three triangular faces. Cluster 555 follows the Mingos fusion formalism with 116 CVE and Jemmis MNO rule with 48 SEP. The reaction of B16H20 with [Cp IrCl2] and tmnd afforded fused clusters 556 and 557 (Fig. 41).209 Cluster 556 is an edge-fused cluster in which two 10-vertex nido-{MB9} units were fused via a common BdB edge. By contrast, cluster 557 is a face-fused cluster, in which an 11-vertex nido-{MB10} and a 10-vertex nido-{MB9} are fused via a common triangular {B3} face. On the other hand, the

Fig. 39 Molecular structures of fused metallaboranes 548 and 549.

Polyhedral Metallaboranes and Metallacarboranes

307

Fig. 40 Molecular structures of fused metallaboranes 553–554.

Fig. 41 Molecular structures of fused metallaboranes 556–558.

reaction of the same borane cluster with [Cp RhCl2] and tmnd afforded an edge fused cluster 558.210 Cluster 558 has a thirteen-vertex macropolyhedral cluster core based on a nido ten-vertex {MB9} subcluster and a nido five-vertex {MB4} subcluster, which are fused through common BdB edge. The reaction of K[arachno-B9H14] with [NiCl2(dppe)] under reflux conditions yielded four new 19-vertex edge-fused metallaboranes 559–562 (Fig. 42).170 Clusters 559 and 560 each exhibit an 18-boron unit with the same configuration as anti-B18H22. However, the equivalent unit in cluster 561 mimics the configuration of syn-B18H22. Cluster 562 has lost a boron atom, presumably either after or during the fusion process, and the resulting 18-atom {NiB17} unit has the same 18-vertex configuration as anti[B18H22]. The reaction of 16-vertex fused borane cluster [B16H20] with [PtMe2(PMe2Ph)2] under mild conditions led to the incorporation of Pt to the cluster and afforded a 17-vertex fused cluster 563.211 The platination occurred on the {B10} subcluster of B16H20. For a similar type of platination, the reaction of the same platinum precursor with another fused borane cluster B14H18 was carried out. The reaction afforded a 15-vertex edge fused cluster 564,211 in which one 11-vertex nido-{PtB10} and pentagonal

Fig. 42 Molecular structures of fused metallaboranes 559–566.

308

Polyhedral Metallaboranes and Metallacarboranes

pyramid nido-{B5} units are fused via BdB edge. On the other hand, thermolysis of [(PMe2Ph)2MB8H12] (M ¼ ]Pd or Pt) led to thermal fusion and afforded 18-vertex fused clusters 565 and 566, respectively (Fig. 42).212 The formation of vertex fused cluster 565 can be envisaged as the simple confluence of two starting-material arachno-{B8Pd} geometries. By contrast, the formation of edge-fused cluster 566 can be rationalized from 565, in which one of the boron vertices of a sub-cluster is bonded with two boron vertices of another sub-cluster to form 566.

9.06.3

Metallacarborane clusters

During the last two decades, metallacarborane chemistry has progressed impressively, with the synthesis of small cage clusters up to 16-vertices.6,8,9,16,23,24 These metallacarboranes were synthesized in many ways utilizing carboranes, metallaboranes, or preformed metallacarboranes as starting materials. In particular, the reduction-metalation methodology was used broadly. In this process, various carboranes were reduced by Na/Li metal followed by metalation with transition metal precursors to yield a series of metallacarboranes. In another strategy, metallaboranes can be treated with alkynes to produce metallacarboranes. Furthermore, these metallacarboranes underwent various reactions at metal/boron/carbon centers with a range of substrates. In this section, the syntheses, structures, and chemistry of metallacarboranes of d- and f-block are discussed.

9.06.3.1

Metallacarborane clusters of group 4 (Table 9)

Metallacarboranes of group 4 were mainly synthesized by Xie and co-workers. The reaction of 11-vertex nido-carborane [(C6H5CH2)2C2B9H9]Na2(THF)x with [MCl4(THF)2] (M ¼ ]Zr or Hf ) in THF gave the bis(carboranyl) complexes 567 and 568, Table 9

Metallacarborane clusters of group 4.

No.

Compounds

567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585

[Z4:Z2-{(C6H5CH2)2C2B9H9}2ZrCl(THF)]Na(THF)3 [Z4:Z2-{(C6H5CH2)2C2B9H9}2HfCl(THF)]Na(THF)3 [Z3:Z2-{(C6H5CH2)2C2B9H9}2ZrCl(THF)]Li(THF)4 [Z2-(C6H5CH2)2C2B9H9]Ti(NEt2)2(NHEt2)
CH2Cl2 [Z2-(C6H5CH2)2C2B9H9]Zr(NEt2)2(NHEt2) [{o-C6H4(CH2)2}C2B9H9]2Zr(THF)2 [{o-C6H4(CH2)2}C2B9H9]2Hf(THF)2 [Z1:Z5-(iPr2C6H3N]CH)C2B9H10]Ti(NMe2)2 [Z1:Z5-(iPr2C6H3N]CH)C2B9H10]Ti(NMe2)20.5C7H8 [Z1:Z5-(Me2N)CH(NMe2)(C2B9H10)]Ti(NMe2)2
0.5C7H8 [Z1:Z5-(C13H9)(iPr2N)P(O)(C2B9H10)]Zr(NMe2)2 [Z1:Z5-(C13H9)(iPr2N)P(O)(C2B9H10)]Ti(NMe2)2
CH3CN [Z1:Z5-(C13H9)(iPr2N)P(O)(C2B9H10)]Ti[(NMe2)2]{(Z6-C6D6)2K}
C6D6 [s:Z5-(C9H6)C2B9H10]Zr(NMe2)(DME) [s:Z5-(C9H6)C2B9H10]Zr(NMe2)(Py)2 [s:Z5-(C9H6)C2B9H10]Zr(NMe2)(THF)2
THF [Z5-(CH2OCH2)C2B9H9]Zr(NMe2)2(NHMe2) [Z5-(CH2OCH2)C2B9H9]Hf(NMe2)2(NHMe2) [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti(NMe2)

586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601

[s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti(NEt2) [Z5-(C9H7)C2B9H10]Ti(NMe2)2(HNMe2) [Z5-(C9H7)C2B9H10]Zr(NMe2)2(HNMe2) [Z5-(C9H7)C2B9H10]Hf(NMe2)2(HNMe2) [s:Z5-(C9H6)C2B9H10]Ti(NMe2)(DME) [Z5:Z6-H2C(C5Me4)(C2B9H10)]Zr(NMe2)(NHMe2) [Z5:Z6-H2C(C5Me4)(C2B9H10)]Zr(m-Cl)2Li(thf )2 [Z5:Z6-H2C(C5Me4)(C2B9H10)]Hf(m-Cl)2Li(thf )2 [{Z5:Z5-Me2C(C5H4)(C2B9H10)}ZrCl2][Na(DME)3] [{Z5:Z5-Me2C(C5H4)(C2B9H10)}HfCl2][Na(DME)3] [{Z5:Z5-Me2C(C5H4)(C2B9H10)}ZrCl2][Li(DME)3] [Z5:Z5-Me2C(C9H6)(C2B9H10)]Zr[(NMe2)(NHMe2)] [Z5:Z5-Me2C(C9H6)(C2B9H10)]ZrCl2[Na(DME)3] [Z5:Z5-Me2C(C9H6)(C2B9H10)]HfCl2[Na(DME)3] [Na(tmeda)][(Z7-{C7H7})Zr(Z5-C2B9H11)] [(thf )5Ba(Z7-C7H7)Zr(Z5-C2B9H11)][(Z7-C7H7)Zr(Z5-C2B9H11)]

11

B NMR (ppm)

−2.48, −15.97, −19.73, −25.57, −34.26 −2.18, −15.70, −19.62, −25.32, −33.86 −8.37, −11.88, −19.49, −26.60, −37.10 1.15, −7.03, −23.08, −25.67 0.24, −8.41, −24.56, −27.14 −5.86, −9.50, −12.06, −32.51 −5.71, −9.43, −12.00, −32.23 8.8, 1.1, −2.5, −4.4, −5.5, −15.2, −17.8 8.8, 1.1, −2.5, −4.4, −5.5, −15.2, −17.8 − 7.4, −1.3, −12.5, −17.7 7.9, −1.6, −12.7, −17.9 3.9, −4.4, −6.6, −15.2, −20.8 4.0, −4.5, −6.2, −14.4 4.0, −4.5, −6.0, −14.3 4.3, −4.3, −6.0, −14.1 1.2, −8.9, −11.9, −14.3, −16.2, −33.4 1.5, −11.4, −13.1, −14.4, −16.7, −35.3 12.9, 1.0, −0.8, −3.4, −4.9, −9.7, −11.5, −13.9, −17.8 9.9, 0.9, 0.0, −3.4, −5.1, −11.0, −15.2, −18.6 6.8, −1.4, −2.9, −4.2, −10.7, −13.3, −17.3 −1.3, −2.7, −6.5, −13.8, −19.7 1.2, −5.9, −10.3, −16.0, −26.8 12.0, −2.7, −4.1, −11.1 2.0, −3.2, −5.8, −7.3, −12.1, −15.3, −20.6 1.9, −2.5, −4.8, −7.9, −9.7, −12.7, −17.9 0.7, −3.5, −5.6, −6.4, −8.4, −10.7, −13.4, −19.7 4.9, −3.1, −5.9, −12.2, −19.3 3.2, −4.2, −6.3, −7.4, −13.3, −14.3, −21.3 1.9, −3.4, −5.6, −6.9, −13.1, −15.0, −21.1 0.9, −5.3, −8.6, −12.7, −21.5 4.7, −2.1, −5.7, −11.3, −18.8 3.1, −3.0, −5.8, −13.2, −20.7 −5.06, −10.42, −12.84, −20.11, −28.26 −

Av. MdB (Å)

Ref.

2.493b − 2.449b 2.331a − − − 2.406a 2.395a 2.416a 2.537b 2.423a 2.453a − − − − 2.413a

213 213 213 213 213 213 213 214 214 214 215 215 215 216 216 216 217 217 217

2.421a

217 216 216 216 216 218 218 218 219 219 219 220 220 220 221 221

2.560b 2.589c 2.416a 2.485b 2.548b − 2.525b − 2.527b 2.535b − − 2.523b 2.504b

5

7

606

[Z1:s:Z5-{MeN(CH2)CH2CH2}C2B9H10]ZrCp

607

[Z1:s:Z5-{MeN[CH2(Et)C]C(Et)]CH2CH2}C2B9H10]Zr-{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(nPr)C]C(nPr)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(Ph)C]C(Ph)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(Ph)C]C(Me)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(Ph)C]C(TMS)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(nBu)C]C(TMS)]CH2CH2}-C2B9H10]Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(nBu)C]C(H)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(Ph)C]C(H)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} [Z1:s:Z5-{MeN[CH2(H)C]C(TMS)]CH2CH2}C2B9H10]-Zr{Z5-1,3(Me3Si)2C5H3} (Z5-1,3-(Me3Si)2C5H3)[Z1:Z5-(Me2NCH2CH2)C2B9H10]-Zr[C^C(tBu)] [Z1:s:Z5-{MeN[CH2(Et)C]C(Et)]CH2CH2}C2B9H10]-ZrCp [Z1:s:Z5-{MeN[CH2(nPr)C]C(nPr)]CH2CH2}C2B9H10]-ZrCp [Z1:s:Z5-{MeN[CH2(nBu)C]C(nBu)]CH2CH2}C2B9H10]-ZrCp [Z1:s:Z5-{MeN[CH2(Ph)C]C(Me)]CH2CH2}C2B9H10]-ZrCp [Z1:s:Z5-{MeN[CH2(2-Py)C]C(nBu)]CH2CH2}C2B9H10]-ZrCp [Z1:s:Z5-{MeN[CH2(Ph)C]C(TMS)]CH2CH2}C2B9H10]-ZrCp [Z1:s:Z5-{MeN[CH2(nBu)C]C(TMS)]CH2CH2}C2B9H10]-ZrCp

610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640

Av. MdB (Å)

Ref.

d

5

[(Z -C5Me5)Ru(tmeda)][(Z -C7H7)Zr(Z -C2B9H11)] [N(CH3)4][(Z7-C7H7)Zr(Z5-C2B9H11)] [Z1:s:Z5-{MeN(CH2)CH2CH2}C2B9H10]Zr(Z5-1,3-(Me3Si)2C5H3) [Z1:Z5-{Me2NCH2CH2}C2B9H10]ZrCp (Me)

609

B NMR (ppm)

11

Compounds

602 603 604 605

608

309

(Continued)

Table 9 No.

Polyhedral Metallaboranes and Metallacarboranes

[Z1:s:Z5-{MeN[CH2(CH2)]CHCH2N[[((Ts)CH2)C]C(H)]-CH2CH2}C2B9H10] ZrCp [Z1:Z5-(Me2NCH2CH2)C2B9H10]ZrCp (C^CtBu) [Z1:Z5-(Me2NCH2CH2)C2B9H10]ZrCp (N]CMe2) [Z1:Z5-(Me2NCH2CH2)C2B9H10]ZrCp (N]C(Me)-{CH2CH(CH2)2}) [Z1:s:Z5-{MeN[HC]C(Ph)N(H)]CH2CH2}C2B9H10]-ZrCp (PhCN) (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[CH2CPh2O]-(CH2CH2)}C2B9H10]Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[CH2C(]S)NnBu]-(CH2CH2)} C2B9H10]Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[CH2C((]S)NCy)-NCy]CH2CH2} C2B9H10]Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[HC]C(CH3)NH]-(CH2CH2)} C2B9H10]Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[HC]C(Ph)NH]-(CH2CH2)}C2B9H10] Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[CH2C(]NXyl)-C(] NXyl)](CH2CH2)}C2B9H10]Zr(CNXyl) (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[CH2C(]NCy)-NCy](CH2CH2)} C2B9H10]Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-{MeN[CH2C(]NiPr)-NiPr](CH2CH2)} C2B9H10]Zr (Z5-1,3-(Me3Si)2C5H3)[Z1:s:Z5-(Me2NCH2CH2)-C2B9H10]Zr(OMe) [Z1:s:Z5-(MeN(CH2)CH2CH2)C2B9H10]Zr-(CH2SiMe3)(THF) [Z1:s:Z5-(MeN(CH2)CH2CH2)C2B9H10]Hf-(CH2SiMe3)(THF) [s:s:Z1:Z5-{(CH2)[(CH2)PhC]CPh]N(CH2CH2)-C2B9H10}]Hf(THF)

− −4.90, −10.32, −12.60, −19.83, −28.16 2.1, 0.9, −2.2, −4.9, −7.9, −13.9, −18.1 1.5, 0.4, −1.9, −3.1, −4.3, −7.8, −9.6, −10.9, −18.2 3.5, 0.4, −2.4, −4.0, −5.1, −6.3, −7.2, −13.7, −19.0 2.4, 0.9, −1.6, −3.3, −5.2, −8.5, −12.5, −16.4

2.535 − 2.493b 2.550b

221 221 222 223

2.503b

223

2.526b

222

2.8, 1.0, −1.6, −3.3, −5.7, −9.5, −12.3, −16.5

−

222

5.2, −0.2, −1.6, −11.2, −14.1

2.519b

222

2.3, 0.1, −1.5, −3.3, −5.8, −9.9, −12.4, −16.3

2.521b

222

1.9, −2.3, −4.0, −9.8, −11.6, −16.5

2.521b

222

1.4, −2.1, −3.7, −7.4, −10.4, −15.2

2.532b

222

−0.4, −2.8, −6.7, −9.8, −11.5, −16.5

−

222

−0.2, −2.4, −6.3, −9.5, −15.9

2.517b

222

2.2, −2.7, −3.7, −6.5, −9.6, −13.4, −16.7

2.524b

222

2.2, −0.5, −1.9, −3.8, −6.4, −9.5, −12.6, −16.7 5.6, 4.7, 1.0, −1.9, −5.3, −6.67, −12.0 −0.3, −4.8, −7.4, −10.5, −12.9, −18.1 2.0, −0.0, −1.6, −3.1, −9.6, −16.7 −0.6, −4.4, −7.5, −10.0, −17.2 −0.2, −4.8, −10.9, −17.8 1.2, −1.6, −5.0, −7.6, −10.1, −17.4 2.7, −2.4, −5.1, −6.8, −8.6, −9.7, −16.9, −18.0, −20.1 0.3, −2.7, −7.9, −11.5, −13.8, −17.2, −20.8

2.532b 2.524b 2.532b 2.542b − 2.537b 2.536b 2.53b

222 223 223 223 223 223 223 223

−

223

3.1, 1.0, −0.4, −1.4, −4.8, −6.5, −8.0, −15.3 −10.9, −14.0, −15.6, −19.1, −21.4, −32.8, −36.9 −0.4, −2.9, −5.2, −8.0, −10.0, −18.4, −20.7 2.5, 0.8, −1.5, −3.4, −5.9, −9.4, −12.4, −16.4 2.9, −0.5, −5.9, −13.3, −15.2, −20.6 5.1, 0.9, −1.7, −8.4, −16.2

2.536b 2.575b 2.571b 2.618b 2.542b 2.521b

223 223 223 223 224 224

4.8, 0.8, −2.7, −8.9, −17.2

2.528b

224

1.6, −1.8, −6.2, −12.8, −17.0

2.524b

224 224

−2.0, −6.5, −12.3, −17.6 1.6, −1.9, −6.0, −9.5, −17.4

2.549b

224

1.2, −0.4, −3.7, −7.5, −10.1, −15.3, −21.2 2.8, 1.5, −1.4, −2.9, −5.4, −8.4, −10.4, −14.5, −19.0 2.5, 0.9, −1.5, −3.5, −5.8, −9.3, −12.5, −16.4 4.9, −3.3, −5.0, −6.6, −10.0, −13.4, −21.5 2.9, −5.2, −7.0, −8.7, −12.0, −15.4, −23.4 7.4, −5.1, −6.7, −10.4, −21.4

224

2.549b

224

2.565b 2.483b 2.471c 2.50c

224 225 225 225 (Continued )

310

Polyhedral Metallaboranes and Metallacarboranes

(Continued)

Table 9 No.

11

Compounds 1

5

B NMR (ppm)

1

641 642

[{Z :s:Z -(MeN(CH2)CH2CH2)C2B9H10}Zr(m:Z -OCH2CH2OCH3)]2 [{Z5-(C9H7)C2B9H10}Hf(NMe2)(m:Z1-OCH2-CH2OCH3)]2
2THF

643 644 645 646

[Z5:Z5-Me2C(C5H4)(C2B9H10)]Zr[Z2-N(Me)(CH2)2NH(Me)] [Z5:Z5-Me2C(C5H4)(C2B9H10)]Zr[Z2-N(Me)(CH2)3NH(Me)] {[Z5:Z5-Me2C(C5H4)(C2B9H10)]Zr[Z2-N(Me)(CH2)3N(Me)-Li]}2 [Z5:Z5-Me2C(C9H6)(C2B9H10)]Zr[Z2-N(Me)(CH2)3NH(Me)]

647 648 649 650

[s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[Z3-CyNC(NMe2)NCy] [[s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti(Z3-S2CNMe2)] [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[Z2-C(NMe2)N(C6H3-2,6-Me2)] [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[N]C(NMe2)Ph]

651 652 653

[s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[Z3-SC-(NMe2)NnBu] [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[Z3-SC-(NEt2)NnBu] [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[s:Z1-OC(NMe2)]CPh2]

654 655 656

[s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[Z3-OC-(NMe2)NPh] [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[s:Z1-OCH(Ph)N(Ph)C(NMe2)]O] [s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[NH(C6H4-4-OMe)]

657 658

[s:Z1:Z5-(OCH2)(Me2NCH2)C2B9H9]Ti[s:Z1-(2-NH-3-CH3-C5H3N)] [Z1-C5H3N-2-NH2-3-CH3] [{s:Z1:Z5-(OCH2)(Et2NCH2)C2B9H9}Ti-(OMe)]2
C7H8

659 660 661 662 663 664 665 666 667 668 669 670

[s:Z5-(C9H6)C2B9H10]Zr(NMe2)(DME) [s:Z5-(C9H6)C2B9H10]Zr[Z2-(iPrN)2C(NMe2)](THF) [s:Z5-(C9H6)C2B9H10]Zr[Z2-(CyN)2C(NMe2)](THF) [s:Z5-{3-[C(]NiPr)NiPr]-1-C9H6}C2B9H10]Zr[Z2-(iPrN)2C(NMe2)] [s:Z5-{3-[C(]CyN)CyN]-1-C9H6}C2B9H10]Zr[Z2-(CyN)2C(NMe2)] [Z5-(C9H7)C2B9H10]Zr[Z2-(iPrN)2C(NMe2)]2 [Z1:s:Z5-{2-[C]NiPr(NHiPr)]-C9H5}C2B9H10]Zr[Z2-(iPrN)2C(NMe2)] [Z1:s:Z5-{2-[C]NiPr(NHiPr)]-C9H5}C2B9H10]Zr[Z2-(iPrN)2C(NEt2)] [Z1:s:Z5-{2-[C]NCy(NHCy)]-C9H5}C2B9H10]Zr[Z2-(CyN)2C(NMe2)] [Z5-(C9H7)C2B9H10]Zr(NMe2)[Z2-(PriN)2-C(NMe2)]
0.5C7H8 [s:Z5-{[3-C(]CPh2)-O]C9H6}C2B9H10]Zr(NMe2)(THF)2
THF [{Z5:Z2-Me2C(C5H4)(C2B9H10)}Zr(Z5-C5H5)(m-Cl){Na(DME)2}]

671 672 673 674 675 676 677 678 679 680

[{Z5:Z5-Me2C(C5H4)(C2B9H10)}ZrCl(CH2C6H5)]-[Na(DME)3] [{Z5:Z5-Me2C(C5H4)(C2B9H10)}Zr(NHC6H3Pri2)-(THF)]
C7H8 trans-[Z5:Z5-Me2C(C9H6)(C2B9H10)]ZrCl(Z5-C5H5)[Na(DME)3] trans-[Z5:Z5-Me2C(C9H6)(C2B9H10)]ZrCl(CH2C6H5)-[Na(DME)3] trans-[Z5:Z5-Me2C(C9H6)(C2B9H10)]Zr(NHC6H3Me2-2,6)(THF) trans-[Z5:Z5-Me2C(C9H6)(C2B9H10)]Zr(OCH2CH2CH2-CH3)(THF) [Z5:Z6-H2C(C5Me4)(C2B9H10)]Zr(OCH2CH2CH2CH2)2-N(CH3)2
THF [Z5:Z6-H2C(C5Me4)(C2B9H10)]ZrCl2[Li(DME)2] [Z5:Z6-H2C(C5Me4)(C2B9H10)]Zr(CH2TMS)2{Li(thf )3} [Z5:Z6-H2C(C5Me4)(C2B9H10)]Zr[s:s-CH2(NMe2)-o-C6H4]
THF

Av. MdB (Å)

Ref.

b

−8.7, −10.7, −13.7, −19.1, −32.2, −36.4 −8.6, −9.5, −12.5, −15.6, −16.4, −19.0, −20.8, −32.2, −36.0 −0.2, −3.8, −5.6, −10.3, −11.3, −15.6, −22.2 0.2, −1.8, −3.6, −6.4, −9.5, −11.7, −22.7 −7.6, −10.1, −12.5, −13.8, −16.5, −30.1 1.1, −0.1, −2.8, −5.2, −6.8, −9.0, −10.1, −13.3, −21.8 12.3, −2.1, −9.6, −16.8 17.9, 4.9, 2.0, −1.9, −2.8, −5.8, −10.4, −14.7 7.3, −0.56, −3.5, −10.0, −17.6 9.2, −0.7, −3.0, −4.2, −5.5, −9.2, −12.7, −14.7, −19.8 15.0, 3.6, 0.7, −2.4, −4.0, −6.9, −11.3, −16.2 13.7, 4.2, 1.0, −2.8, −8.2, −10.1, −16.0 16.3, 3.6, 1.3, −1.3, −3.0, −7.2, −10.3, −13.5, −15.9 18.0, 5.4, 3.1, −2.7, −6.8, −10.2, −15.6 17.5, 5.0, 2.7, −3.0, −7.1, −10.4, −16.0 13.5, 2.0, 0.2, −2.9, −4.6, −8.9, −11.2, −13.9, −17.1 16.2, 3.9, 0.8, −3.8, −7.5, −11.1, −16.0

2.522 2.511c

225 216

2.537b − 2.612b 2.548b

226 226 226 226

2.446a − − −

217 217 217 217

− 2.43a 2.410a

217 217 217

2.449a 2.430a −

217 217 217

2.472a

217

14.3, 2.1, 0.3, −2.0, −3.8, −8.1, −10.6, −13.4, −16.8 4.0, −4.5, −6.2, −14.4 12.6, 5.5, −3.4, −5.9, −9.2, −11.5, −13.6 12.9, 6.0, −3.4, −6.1, −9.9, −13.7 1.8, −1.2, −2.7, −4.9, −13.6, −16.8 2.3, −0.4, −2.2, −4.4, −13.8, −16.5 1.4, −6.1, −8.7, −16.3, −29.3 6.2, 3.1, −2.7, −5.2, −6.5, −12.9, −16.3 6.3, 3.4, −2.1, −4.7, −6.5, −13.0, −15.8 3.7, −2.4, −5.1, −12.3, −15.9 3.5, −3.6, −6.0, −11.4, −16.0, −26.6 1.2, −3.5, −6.1, −15.0, −18.7 −8.5, −9.3, −12.9, −13.5, −15.1, −18.7, −19.7, −34.5 3.6, −0.2, −2.7, −5.7, −7.7, −11.3, −12.5, −18.3 2.4, −3.5, −7.1, −13.2, −20.6 3.8, −3.6, −4.9, −6.4, −10.6, −12.4, −16.2 0.9, −0.8, −2.7, −5.5, −7.0, −11.3, −13.3, −20.1 4.5, −3.3, −4.6, −6.7, −11.5, −18.6 2.0, −1.9, −6.7, −10.0, −12.9, −15.0, −20.0 −3.9, −6.1, −11.5, −15.3, −25.6 2.3, −2.9, −5.4, −8.6, −10.1, −12.7, −18.6 0.1, −6.2, −10.1, −13.7, −15.9, −18.9, −22.9 1.7, −4.0, −6.6, −9.0, −11.4, −16.4, −22.0

2.401a

217

2.531b − − 2.571b 2.582b 2.506b 2.514b 2.532b 2.536b 2.876b 2.604b 2.972b

227 227 227 227 227 227 227 227 227 216 216 219

2.5535b 3.551b 2.560b 2.527b 2.608b 2.525b 2.626b 2.576b 2.621b 2.526b

219 219 220 220 220 220 218 218 218 218

a

Av. TidB Av. ZrdB c Av. HfdB d Av. RudB b

respectively (Scheme 23).213 One of the C2B9 cores of 567 and 568 are connected with group metal (Zr or Hf ) through two MdHdB, whereas the other C2B9 core is connected with metal in such a manner that it formed an icosahedral metallacarborane with one edge missing. Further treatment of 586 with 1 equiv. of Li[N(SiMe3)2] resulted in the replacement of Na+ by Li+ to yield ionic metallacarborane complex 569.213 Although the structures of 567 and 569 are almost same, one of ZrdC connections is missing in 569. An equimolar reaction between [M(NEt2)4] (M ¼ ]Ti or Zr) with another 11-vertex nido-carborane [(C6H5CH2)2C2B9H11] afforded monocarboranyl complexes 570 and 571, in which the C2B9 core is connected with Ti or Zr

Polyhedral Metallaboranes and Metallacarboranes

311

Scheme 23 Syntheses of metallacarboranes 567, 568, 572, and 573.

through two MdHdB units. By contrast, the reaction of [MCl4(THF)2] (M ¼ ]Zr or Hf ) with 11-vertex nido-carborane having less bulky yet rigid substituents, Na2(THF)x[{o-C6H4(CH2)2}C2B9H9] generated bent-metallocene type complexes, i.e., metallacarboranes 572 and 573, respectively (Scheme 23).213 Many metallacarboranes of group 4 were synthesized utilizing various closo-12 vertex or nido-11 vertex carboranes and group 4 metal precursors. For example, the reactions of closo-12-vertex imido-carboranes [1-(CH]NC6H3R2-2,6)-1,2-C2B10H11] (R ¼ ]iPr or Me) with [Ti(NMe2)4] afforded metallacarboranes 574 or 575 and 576, respectively.214 In 574–576, the Ti centers are coordinated by the dicarbollyl group in Z5-fashion. Similarly, the reactions of closo-[(C13H9)(iPr2N)P(]O)(C2B10H11)] with [M(NMe2)4] (M ¼ ]Zr or Ti) in toluene under reflux conditions afforded metallacarboranes 577 or 578, respectively, which have a similar type of core as those of 574–576.214 Further reaction of 578 with KH in C6D6 yielded deprotonated product 579, which has the same metallacarborane core as that of 578.215 On the other hand, dissolving [Z5-(C2B10H11)C9H6]Zr(NMe2)3 in polar solvents (DME, Py or THF) led to the isolation of metallacarboranes 580–582 having a half-sandwich core.216 On the other hand, the reaction of nido-11 vertex carborane [Me3NH][7,8-CH2OCH2-7,8-C2B9H10] with [M(NR2)4] (M ¼ ]Zr, Hf or Ti; R ¼ ]Me or Et) afforded simple amine elimination products 583 and 584, or the CdO bond cleavage products 585 and 586 (Scheme 24).217 Metallacarboranes 583–586 have similar type half-sandwich cores as that of 574–582. Similar reactions with another nido-11 vertex carborane [Me3NH][7-C9H7-7,8-C2B9H11] with [M(NMe2)4] (M ¼ ]Ti, Zr, Hf ) in toluene at room-temperature afforded metallacarboranes 587–589, having similar half-sandwich cores to that of 574–586.217 Dissolving metallacarborane 587 in DME led to amine elimination and afforded metallacarborane 590.216 The amine-elimination reaction of nido-11 vertex carborane [Me3NH][H2C(C5Me4H)(C2B9H11)] with [Zr(NMe2)4] produced metallacarborane 591, having a similar type of structure to that of the a metallocene (Scheme 24).218 Reaction of the same nido-carborane with nBuLi and [MCl4(thf )2] (M ¼ ]Zr or Hf ) afforded two more bent metallocene-type metallacarboranes 592 and 593.218 Likewise, a few more bent metallocene-type metallacarboranes 594–599 were isolated utilizing different types of 11-vertex nido-carboranes, and metal-precursors of group 4, such as [MCl4(THF)2] and [M(NMe2)4].219,220 The reaction of [(Z7-C7H7)ZrCl(tmeda)] with Na2[C2B9H11] afforded metallocene-type metallacarborane 600, where Zr is coordinated in Z7-mode with cycloheptatrienyl ligand and in Z5-mode by the dicarbollyl (Scheme 24).221 Furthermore, the reaction of 600 with BaI2 resulted in the formation of multi-decker metallacarborane 601, in which the Ba(THF)5 fragment is coordinated with the cycloheptatrienyl group of 600 in Z7-mode. A similar reaction between 600 and [Cp Ru(m3-Cl)]4 afforded 602, in which the Cp Ru unit is coordinated by tmeda

312

Polyhedral Metallaboranes and Metallacarboranes

Scheme 24 Syntheses of metallacarboranes 583–587, 591, and 600.

instead of the metallacarborane core of 600. The tetramethylammonium salt of 600, metallacarborane 603, was prepared via an acid-base reaction between the amido complex [(Z7-C7H7)Zr{N(SiMe3)2}(thf )] and [NMe4][C2B9H12].221 On the other hand, treatment of [(Z5-Cp”)ZrMe3] with 1 equiv. of the zwitterionic salt [7-Me2N(H)CH2CH2-7,8-C2B9H11] in toluene afforded a mixed-sandwich zirconacarborane alkyl 604.222 The same reaction with [Cp ZrMe3] in THF medium afforded another neutral mixed-sandwich zirconacarborane methyl complex 605.223 Zirconacarborane methyl complex 605 underwent intramolecular CdH activation at 70  C to afford the Cp analog of 604, zirconacarborane 606.223 These group 4 metallacarboranes undergo various reactions with unsaturated organic substrates such as alkynes, alkyl nitriles, diarylketones, carbodiimides, etc. In the main, these reactions take place at the metal center(s). For example, the reactions of zirconacarborane methyl complex 604 with various internal alkynes and trimethylsilylacetylene afforded the ZrdC s-bonded mono-insertion products 607–612 (Scheme 25).222 Similarly, terminal alkynes also afforded the ZrdC s bonded insertion products 613–615.222 However, the terminal alkyne 3,3-dimethyl-1-butyne underwent an acid-base reaction to produce a zirconium alkynyl complex 616.222 When zirconacarborane methyl complex 605 was treated with various alkynes, a similar reactivity trend was observed as for alkyne insertion reactions of 604, affording 617–625.223 On the other hand, 605 reacted with alkyl nitriles at room-temperature to give the mono-inserted zirconacarborane imide complexes, 626 and 627 (Scheme 25).223 By contrast, the insertion reaction with aryl nitriles at elevated temperature generated a different type of insertion product, zirconacarborane amide 628.223 Similarly, zirconacarborane 604 reacted with diphenylketone, nBuNCS, carbodiimide, acetonitrile, and phenyl nitrile to give various mono-insertion products, in which the unsaturated bond inserted into the ZrdC bond, resulting in the formation of new CdC/C-heteroatom bonds and afforded 629–633.224 The same reaction with aryl isonitrile XylNC afforded double-insertion into the ZrdC bond to yield compound 634.224 Similarly, zirconacarborane methyl complex 605 reacted with carbodiimides (CyN] C]NCy or iPrN]C]NiPr) after a methane elimination to afford mono-inserted products 635 and 636, respectively.224 When 605 was treated with methanol at room-temperature, an OMe group substituted the Me group in 605 to afford 637.224

Polyhedral Metallaboranes and Metallacarboranes

313

Scheme 25 Syntheses of metallacarboranes 607–616 and 626–628.

Treatment of [7-Me2N(H)CH2CH2-7,8-C2B9H11] with [M(CH2SiMe3)4] (M ¼ ]Zr or Hf ) in toluene/thf gave the CdH activated metallacarborane products 638 and 639, respectively.225 As with zirconacarboranes 604 and 605, hafnacarborane 639 undergoes an insertion reaction with diphenylacetylene, which led to the elimination of SiMe4 and afforded alkyne inserted hafnacarborane 640.225 Reaction of [Zr(CH2Ph)4] with the same carborane [7-Me2N(H)CH2CH2-7,8-C2B9H11] in DME yielded a CdH/CdO activated product 641, in which two zirconacarboranes are dimerized, being connected through a bridging oxygen (Fig. 43).225 A similar type metallacarborane dimer 643 was synthesized by directly dissolving metallacarborane [Z5-(C9H7) C2B9H10]Hf(NMe2)2(HNMe2) (589) in DME at room temperature.226 On the other hand, the reaction of zirconacarborane [Z5:s-Me2C(C5H4)(C2B10H10)]Zr(NMe2)2 with DMEDA and DMPDA led to the substitution of two NMe2 groups and yielded zirconacarboranes 643 and 644, respectively.226 Deprotonation of 644 by nBuLi in toluene afforded dimerized zirconacarborane 645.226 Similarly, the indenyl analog 646 was isolated by refluxing a toluene solution of [Z5:s-Me2C(C9H6)(C2B10H10)]Zr (NMe2)2 with an excess amount of DMPDA.226 The reactions of titanacarborane 585 with unsaturated molecules, such as CyN]C]NCy, CS2, XyldNC, PhCN, nBuNCS, Ph2C]C]O, and PhN]C]O yielded mono-inserted products 647–654, 653, and 654, respectively.217 Further, the reaction of 654 with benzaldehyde led to the isolation of titanacarborane 655, where PhCHO inserted into the TidN bond of 654 to form a

Fig. 43 Molecular structures of metallacarboranes 641, 642, and 645.

314

Polyhedral Metallaboranes and Metallacarboranes

six-membered metallacycle.217 On the other hand, titanacarborane 585 reacted with 4-methoxyaniline or 2-amino-3-picoline in toluene at room-temperature to afford titanacarboranes 656 and 657, respectively.217 In 656, the NMe2 group of 585 was substituted by NH(C6H4)OMe group. By contrast, in 657, along with the substitution of the NMe2 group by NH(C5H3N) CH3 group, one more 2-amino-3-picoline donor is coordinated to the Ti center. The reactions of the ethyl analog of 585, [s:Z1:Z5-(OCH2)(Et2NCH2)C2B9H9]Ti(NEt2) (588) with methyl esters such as methyl methacrylate, methyl propiolate, or dimethyl acetylenedicarboxylate afforded the same dimeric complex 658.217 The reactions of zirconacarborane 659 with 2 equiv. guanidines iPrNHC(NR2)]NiPr (R]Me or Et) in refluxing toluene afforded zirconacarboranes 665 and 666 (Scheme 26), in which the interactions between the Zr atom and the indenyl-dicarbollyl unit are very similar to those observed in 659.227 An equimolar reaction of 659 with R’N]C]NR’ (R’ ¼ ]iPr or Cy) in THF at room-temperature afforded mono-inserted products [s:Z5-(C9H6)C2B9H10]Zr[Z2-(R’N)2C(NMe2)] (THF) (R’ ¼ ]iPr (660), Cy (661)).227 Furthermore, treatment of 660 and 661 with 1 equiv. of R’N]C]NR’ in THF at room-temperature generated another type of insertion product, [Z5-(C9H7)C2B9H10]Zr[Z2-(R’N)2C(NMe2)]2 (R’ ¼ ]iPr (662), Cy (663)).227 Interestingly, zirconacarboranes 660–663 can readily be converted to 665 and 667 by simply refluxing in toluene.227 Also, the reaction of 660 with 1 equiv. guanidine iPrNHC(NMe2)]NiPr in toluene at room-temperature gave proton exchanged product 664, in which the Zr atom has no bonding interactions with the neutral indenyl ring.227 Complex 664 was also converted to 665 upon refluxing in toluene. On the other hand, reactions of zirconacarborane 588 with 1 or 2 equiv. diisopropylcarbodiimide in THF at room-temperature afforded monoguanidinate zirconacarborane complex 668216 and dinguanidinate zirconacarborane complex 664. Monoguanidinate zirconacarborane 668 was also converted to 665 under reflux conditions. The room-temperature reaction of 659 with an equimolar amount of diphenylketene in THF afforded 669, which may be generated through the coordination of ketene to the Zr atom, followed by the nucleophilic attack of the indenyl on ketene.216

Scheme 26 Synthesis of metallacarboranes 665 and 666.

These metallacarboranes undergo many substitution reactions at the metal centers. For example, when bent metallocene-type metallacarborane 594 was treated with NaCp, one of the Cl atoms of 594 is substituted by Z5-Cp and yielded 670.219 The same reaction with KCH2Ph in THF led to the substitution of one Cl atom by CH2Ph group and generated 671.219 By contrast, the treatment of NaNHC6Hi3Pr2 in THF led to the substitution of both Cl atoms of 594 by NHC6Hi3Pr2 and THF to form 672.219 Likewise, another bent metallocene-type metallacarborane, 598, underwent substitution reactions with NaCp, KCH2Ph, or NaNHC6Hi3Pr2 to afford the substituted products 673–675, respectively.220 By contrast, the reaction of 598 with excess NaH in THF led to the substitution of both Cl atoms by THF, and one ring-opened alkoxy group generated from THF to afford compound 676.220 Another bent metallocene-type metallacarborane 591 in hot THF undergoes THF ring-opening to afford zwitterionic complex 677.218 Zirconacarborane 592 in DME is converted to the corresponding ionic complex 678.218 The same zirconacarborane 592 reacts with LiCH2TMS in THF at room-temperature to yield a doubly-alkylated product 679.218 By contrast, the same reaction with [KCH2(NMe2)-o-C6H4] afforded neutral product 680, where [CH2(NMe2)-o-C6H4] is bonded with Zr center through N atom and benzyl CH2 group.218

9.06.3.2

Metallacarborane clusters of group 5 (Table 10)

Group 5 metallacarboranes are not very widespread. Nevertheless, Xie and co-workers were able to isolate a few exciting examples of such systems. The reaction of 11-vertex nido-carborane [(Me2NHCH2CH2)C2B9H11] with excess NaH in refluxing THF, followed by

Polyhedral Metallaboranes and Metallacarboranes

Metallacarboranes of Group 5.

Table 10 No.

Compounds 1

5

681

[Z :Z -(Me2NCH2CH2)C2B9H10]TaMe3

682

[Z5-(Me2NCH2CH2)C2B9H10]Ta(NMe2)3

683

[Z1:Z5-{(CH2)5NCH2CH2}C2B9H10]TaMe3

684

[s:Z1:Z5-{MeN(CH2)CH2CH2}(CHMe2)C2B9H9]-Ta[]N(2,6-Me2C6H3)](THF)

685

693

[s:Z1:Z5-(MeNCH2CH2CH2)(CHMe2)C2B9H9]-Ta[]N(2,6-Me2C6H3)][Z2-C, N-MeC]N(2,6-Me2C6H3)] {Z1:Z5-(Me2NCH2CH2)C2B9H10}Ta[]N(2,6-iPr2C6H3)][Z2-C, N-MeC]N(2,6-iPr2C6H3)] [Z1:Z5-(Me2NCH2CH2)C2B9H10]Ta(]NCy)[N-(CMe]CMe2)(2,6-Me2C6H3)] [Z1:Z5-(Me2NCH2CH2)C2B9H10]Ta(]NAd)[N-(CMe]CMe2)(Cy)] [Z1:Z5-(Me2NCH2CH2)C2B9H9]Ta(]NCy)[N-(2,6-Me2C6H3) {CHCMe2CMe]N(2,6-Me2C6H3)}] [Z1:Z5-(Me2NCH2CH2)C2B9H9]Ta(]NAd)[N-(2,6-Me2C6H3) {CHCMe2CMe]N(2,6-Me2C6H3)}] [Z1:Z5-(Me2NCH2CH2)(CHMe2)C2B9H9]Ta[]N-(2,6-Me2C6H3)][Z2-C, N-MeC]N(iPr)] [Z1:Z5-(Me2NCH2CH2)(CHMe2)C2B9H9]Ta[]N-(2,6-Me2C6H3)][Z2-C, N-MeC]N(Cy)] [s:Z5-(MeNCH2CH2)C2B9H10]Ta(]NAd)(THF)

694

[s:Z1:Z5-{((CH2)5NCHCH2)(CHMe2)}C2B9H9]Ta(]NAd)(THF)

695 696 697 698

[Z1:Z6-(Me2NCH2CH2)C2B9H10]Ta(NMe2)(Py) (Z6-C2B9H11)Ta[Z3-C,C,N-CH2C(CH3)NAd](DME) ansa-(2’,4-(CH2)4-)-commo-V-(1’-V-2’,3’,5’-C3B7H9)(1-V-2,3,4-C3B7H9) ansa-(2’,4-(CH2)4-)-commo-V-(1’-V-2’,3’,4’-C3B7H9)(1-V-2,3,4-C3B7H9)

686 687 688 689 690 691 692

315

11

B NMR (ppm)

Av. MdB (Å)

Ref.

a

2.411

228

2.493a

229

2.417a

230

15.6, 7.9, 0.5, −3.0, −5.2, −6.2, −10.0, −13.4 3.9, −0.2, −3.2, 6.1, −7.6, −8.5, −9.9, −11.2, −17.3 15.0, 7.0, −1.4, −4.1, −5.2, −6.7, −11.0, −14.2 3.7, 2.2, −5.1, −6.9, −8.4, −10.7, −14.0, −17.7 2.6, 0.8, −7.6, −9.5, −11.4, −18.1

2.456a

228

2.488a

228

1.7, 0.6, −6.5, −9.1, −9,7, −16.3

2.490a

228

2.2, −2.1, −5.6, −6.8, −7.6, −12.6, −18.0 −1.2, −7.4, −10.6, −20.1 0.8, −3.5, −10.9, −17.3

2.471a 2.465a 2.413a

230 230 230

−0.4, −3.3, −5.0, −11.8, −18.9

2.404a

230

7.3, 0.1, −7.1, −8.9, −12.9, −17.1

2.516a

230

5.8, −1.5, −8.6, −10.2, −14.3, −18.7

2.511a

230

2.462a

230

2.491a

230

2.476a 2.459a 2.380b 2.390b

229 229 231 231

−1.8, −5.8, −6.5, −8.2, −10.8, −13.2, −18.9 11.9, 4.8, 0.1, −3.4, −5.0, −8.4, −11.4, −14.7, −18.2 11.8, 5.0, 1.5, −0.1, −19.4, −35.2 20.2, 6.2, 2.5, 0.4, −21.3, −33.1 – –

a

Av. TadB Av. VdB

b

reaction with TaMe3Cl2 in toluene at room temperature afforded tantallacarborane 681 (Scheme 27), in which the Ta center is ligated by a dicarbollyl group in Z5-mode.228 The same carborane reacts with 1 equiv. of Ta(NMe2)5 in THF at room-temperature to afford another tantallacarborane, 682, in which Ta is attached to the C2B9 core in Z5-mode.229 To replace the {Me2NCH2CH2} unit in 681 by a piperidinyl sidearm {(CH2)5NCH2CH2}, the reaction of a modified carborane [7-(CH2)5NHCH2CH2-7,8-C2B9H11] was carried out with excess NaH in refluxing THF, followed by reaction with Me3TaCl2 to yield tantallacarborane 683.230

Scheme 27 Syntheses of fused metallacarboranes 681 and 682.

These metallacarboranes of group 5 also undergo various insertion reactions with unsaturated organic species. For example, the equimolar reaction of tantallacarborane 681 with 2,6-dimethylphenyl isonitrile (XylNC) in THF yielded a mixture of three metallacarborane regioisomers {s:Z1:Z5-[MeN(CH2)CH2CH2](CHMe2)C2B9H9}Ta[]N(2,6-Me2C6H3)](THF) (684a, 684b, 684c).228

316

Polyhedral Metallaboranes and Metallacarboranes

Fig. 44 Molecular structures of metallacarboranes 684–694.

The Ta atom of 684a adopts a four-legged piano stool geometry in which the Ta atom is Z5-bound to the dicarbollyl unit, s-bound to a methylene carbon and the imido group, and coordinated to the nitrogen atom from the sidearm and one THF molecule (Fig. 44). In addition, an isopropyl group is bonded to the boron atom of the dicarbollyl cage. By contrast, treatment of tantallacarborane 681 with 2 or 3 equiv. XylNC afforded two regioisomers 685a and 685b, which have the same tantallacarborane core, but feature the Ta atom attached to two imido groups.228 When sterically demanding 2,6-diisopropylphenyl isonitrile (DippNC) was used, a similar reaction produced tantallacarborane 686, in which no isopropyl group was linked to the boron of the dicarbollyl cage.228 On the other hand, the reaction of 681 with 1 equiv. R1NC (R1 ¼ ]Cy, Ad) at −30  C for 2 h, followed by reaction with 1 equiv. of R2NC (R2]Xyl, Cy), afforded imido alkenylamido tantallacarborane complexes 687 and 688.230 Under similar conditions, the reaction of 681 with 1 equiv. of R1NC (R1 ¼ ]Cy, Ad) and 2 equiv. of XylNC yielded four tantallacarborane: regioisomers 689a and 689b; and regioisomers 690a and 690b.230 Similarly, utilizing tantallacarborane 681 and different types of isonitrile, a range of tantallacarboranes (691a, 691b, 692a, 692b, 693, and 694a-694c) having various imido groups could be isolated (Fig. 44).230 On the other hand, heating a benzene solution of tantallacarborane 682 in the presence of pyridine gave a tantallacarborane [Z1:Z6-(Me2NCH2CH2)C2B9H10]Ta(NMe2)(NC5H5) (695).229 When a similar type of tantallacarborane [(Z5-C2B9H11)TaMe3] was treated with 1 equiv. AdNC in DME another unexpected tantallacarborane (Z6-C2B9H11)Ta[Z3-C,C,N-CH2C(CH3)NAd](DME) (696) was produced.229 Interestingly, both tantallacarboranes 695 and 696 adopt a distorted three-legged piano stool geometry in which the Ta atom is Z6-bound to the arachno-[C2B9]4− ligand (Fig. 45). Sneddon and co-workers isolated two isomeric ansa-vanadabis(tricarbadecaboranyl) complexes (697 and 698) from the reaction of Li2[6,6-(CH2)4-nido-(5,6,9-C3B7H9)2] with VCl3
THF.231 In both 697 and 698, a formal V2+ ion is sandwiched between two tricarbadecaboranyl cages that are linked by the ansa-(CH2)4 group, but the points of linker-attachment on the two cages are

Fig. 45 Molecular structures of metallacarboranes 695 and 696.

Polyhedral Metallaboranes and Metallacarboranes

317

Fig. 46 Molecular structures of metallacarboranes 697 and 698.

different (Fig. 46). A cage-carbon rearrangement along with the long flexible (CH2)4 linker enabled the two cages in these complexes to rotate into a perpendicular interlocking configuration that maximizes cage-metal bonding, reduces unfavorable steric interactions between the two linked cages, and encapsulates the vanadium inhibiting its interactions with other potential reactants.

9.06.3.3

Metallacarborane clusters of group 6 (Table 11)

Group 6 metallacarboranes have also met with little development in the period since 2005. Welch and co-workers carried out the reaction of nido-[7,8-Ph2-7,8-C2B9H9]2− and [(Z7-C7H7)Mo(MeCN)3]+ in THF, which afforded 12-vertex molybdacarboranes 699–702 (Fig. 47).232 Cluster 699 has a pseudocloso shape as the CdC bond stretches to 2.594(3) Å to relieve the steric hindrance between the phenyl rings. Molybdacarborane 700 has a closed but non-icosahedral cage geometry (C2 symmetry). The C atoms are located in degree-four vertices, Mo and one of the B atoms are positioned in degree-six vertices, and all other B atoms are located at degree-five vertices in 700. On the other hand, clusters 701 and 702 have icosahedral geometry. As in both 701 and 702, the carbon atoms are not in adjacent vertices; these metallacarboranes can also be viewed as CAp closo-metallacarboranes. The same group has isolated several more icosahedral molybdacarboranes (704, 706–708) from the reaction of 703 and PPh3 or PEt3 with one equivalent of HBF4
OEt2.233 By contrast, the cage C-methylated analog of 704, molybdacarborane 705, was prepared by protonation of [PNP][1,2-Me2-3,3-(CO)2-3-(Z-C3H5)-3,1,2-closo-MoC2B9H9] in the presence of one equiv. of PPh3. Clusters 704–708 have icosahedral geometries, in which the carbon atoms are located at adjacent vertices. The differences between 704 and 706–708 are the phosphine ligands and their orientations. Hey-Hawkins and co-workers also isolated two CAd icosahedral molybdacarboranes 709 and 710 incorporating well-known potentially non-innocent ligands (CO, 2,20 -bpy, or 1,10-phen) at the Mo center.234

Table 11 No. 699 700 701 702 703 704 705 706 707 708 709 710

Metallacarboranes of Group 6. Compounds

11

B NMR (ppm)

7

1,2-Ph2-3-( -C7H7)-3,1,2-MoC2B9H9 1,2-Ph2-5-(7-C7H7)-5,1,2-MoC2B9H9 2,4-Ph2-1-(7-C7H7)-1,2,4-MoC2B9H9 1,8-Ph2-2-(7-C7H7)-2,1,8-MoC2B9H9 [PNP][3,3-(CO)2-3-(Z-C3H5)-3,1,2-MoC2B9H11] [3,3,3-(CO)3-3-PPh3-3,1,2-MoC2B9H11] [1,2-Me2-3,3,3-(CO)3-3-PPh3-3,1,2-MoC2B9H9] trans-[3,3-(CO)2-3,3-(PPh3)2-3,1,2-MoC2B9H11] cis-[3,3-(CO)2-3,3-(PEt3)2-3,1,2-MoC2B9H11] trans-[3,3-(CO)2-3,3-(PEt3)2-3,1,2-MoC2B9H11] [3,3-{2,2-bpy-k2-N,N}-3-(CO)2-3,1,2-MoC2B9H11] [3,3-{1,10-phen-k2-N,N}-3-(CO)2-3,1,2-MoC2B9H11]

Fig. 47 Molecular structures of metallacarboranes 699–702 and 704.

− − − − −4.2, −7.3, −12.2, −13.9, −19.3 −0.2, −3.2, −11.3, −14.9, −17.9 0.0, −3.9, −5.1, −6.3, −7.6 0.2, −2.5, −7.1, −11.6, −15.8, −22.1 −0.2, −3.0, −4.6, −12.5, −15.9, −19.4 −1.4, −4.6, −6.5, −12.5, −16.9, −20.2 −17.2, −12.3, −5.4, −2.4, 7.7 −17.6, −12.4, −5.3, −2.5, 7.8

Av. M-B (Å)

Ref.

2.373 2.445 2.335 2.345 − 2.389 2.377 2.422 2.385 2.431 2.354 2.359

232 232 232 232 233 233 233 233 233 233 234 234

318

Polyhedral Metallaboranes and Metallacarboranes

9.06.3.4

Metallacarborane clusters of group 7 (Table 12)

Sneddon and co-workers have synthesized many group 7 metallacarboranes. For example, the reaction of the tricarbadecaboranyl anion, nido-[6-Ph-5,6,9-C3B7H9]− with [M(CO)5Br] (M ¼ ]Mn or Re) or [(Z6-C10H8)Mn(CO)3]BF4 yielded the half-sandwich metallatricarbadecaboranyl 711 and 712, respectively.235 Metallacarborane 711 and 712 are the analogs of CpM(CO)3 (M ¼ ]Mn, Re). Clusters 711 and 712 have octadecahedron geometry, in which the metal centers are located at degree-six vertex and connected to three CO ligands. Further, reactions of 711 and 712 with isocyanide at room-temperature produced metallacarboranes 713 and 714, respectively.235 Clusters 713 and 714 have an 11-vertex nido-geometry. The metal centers in 713 and 714 are located at degree-four vertices and are ligated by three CO and one isocyanide ligands. Further, photolysis of 714 resulted in the loss of one CO ligand and the formation of 715, which has the same core structure as those of 711 and 712.235 On the other hand, reactions of 711 and 712 with 1 equiv. of phosphine at room-temperature led to the substitution of one CO ligand by a phosphine, keeping the core unchanged, and affording monosubstituted 716–719.235 Reaction of 718 with an additional equivalent of PMe3 led to the addition of one more PMe3 at the metal center and afforded 720, which has same nido core as that of 713 and 714. Photolysis of nido-720 resulted in the loss of CO and the formation of the disubstituted closo-721.235 On the other hand, deboronation of the 11-vertex closo-712 utilizing TBAF produced 10-vertex nido-722, which has a 10-vertex nido core and is consistent with its 12 SEP (Fig. 48).236 Table 12

Metallacarboranes of group 7.

No.

Compounds

11

Av. MdB (Å)

Ref.

711 712 713 714 715 716 717 718 719 720 721 722 723 724 725

[1,1,1-(CO)3-2-Ph-1,2,3,4-MnC3B7H9] [1,1,1-(CO)3-2-Ph-1,2,3,4-ReC3B7H9] [8-(CNtBu)-8,8,8-(CO)3-9-Ph-8,7,9,10-MnC3B7H9] [8-(CNtBu)-8,8,8-(CO)3-9-Ph-8,7,9,10-ReC3B7H9] [1-(CNtBu)-1,1-(CO)2-2-Ph-1,2,3,4-ReC3B7H9] [1,1-(CO)2-1-PMe3-2-Ph-1,2,3,4-MnC3B7H9] [1,1-(CO)2-1-PPh3-2-Ph-1,2,3,4-MnC3B7H9] [1,1-(CO)2-1-PMe3-2-Ph-1,2,3,4-ReC3B7H9] [1,1-(CO)2-1-PPh3-2-Ph-1,2,3,4-ReC3B7H9] [8,8-(CO)2-8,8-(PMe3)2-9-Ph-8,7,9,10-ReC3B7H9] [1-CO-1,1-(PMe3)2-2-Ph-1,2,3,4-ReC3B7H9] Bu4N+[5,5,5-(CO)3-10-Ph-5,6,9,10-Re-C3B6H9]– [Z5-Re(CO)3-1,2-C2B9H11] [Z5-Re(CO)3-7-CH2CH2COOH-C2B9H10] [Z5-Re(CO)3-7-CH2CH2CH2NMe2-C2B9H10]

2.365 2.475 2.599 2.681 2.447 2.332 2.344 2.453 2.453 2.654 2.458 2.367 − − −

235 235 235 235 235 235 235 235 235 235 235 236 237 237 237

726 727 728

[Z5-99mTc(CO)3-7-CH2CH2COOH-C2B9H10] NEt4[Z5-Re(CO)3-7-CH2C5H4NH-7,8-C2B9H10] NEt4[Z5-Re(CO)3-8-CH2CH2CH2NHMe2-7,8-C2B9H10]

− − 2.322

237 238 238

729 730 731 732 733 734 735 736 737 738 739 740 741

Na[Z5-Re(CO)3-7-C6H5-8-C6H4OH-7,8-C2B9H10] [rac-1-Ph-3-Re(CO)3-1,2-(C2B9H10)] [rac-1-CH2Ph-3-Re(CO)3-1,2-(C2B9H10)] Na[3,3,3-(CO)3-1-CH2C5H4N-3,1,2-ReC2B9H10] K[(3,3,3-(CO)3-1-CH2Cy)-ReC2B9H10] [rac-8-(4-p-PhO(CH2)2NCCH3)2-2-Re(CO)3-2,1,8-C2B9H10]Na [(Z5-Re(CO)3HCB9H9C)CH2NHC(NH2)NH2] [(Z5-Re(CO)3HCB9H9C)CH2NHC(NH2)NHEt] [(Z5-Tc(CO)3HCB9H9C)CH2NHC(NH2)NH2] [(Z5-Tc(CO)3HCB9H9C)CH2NHC(NH2)NHEt] [rac-8-(4-PhOH)-2-Re(CO)3-2,1,8-C2B9H10]Na [rac-1-(4-PhOH)2-2-Re(CO)3-2,1,8-C2B9H9] [rac-8-Ph-2-Re(CO)3-2,1,8-C2B9H10]Na

− − − − − − − − − − 2.315 − −

238 239 239 240 241 241 242 242 242 242 239 239 241

742 743 744 745 746

[rac-8-(4-PhOH)-1-Ph-2-Re(CO)3-2,1,8-C2B9H9]Na [rac-8-(4-p-PhO(CH2)2NCCH3)2-1-Ph-2-Re(CO)3-2,1,8-C2B9H9]Na [rac-1,8-(4-PhOH)2-2-Re(CO)3-2,1,8-C2B9H9]Na [rac-1,8-((4- p-PhO(CH2)2NCCH3)2)2-2-Re(CO)3-2,1,8-C2B9H9]Na K[2,2,2-(CO)3-2,1,8-closo-ReC2B9H11]

− − 2.317 − −

241 241 241 241 241

747 748

2,2,2-(CO)3-8-[(N-Me)CH2C5H4N]-2,1,8-ReC2B9H10 2,2,2-(CO)3-8-[(NH)CH2C5H4N]-2,1,8-ReC2B9H10

2.210 2.318

241 241

749 750 751 752

K[2,2,2-(CO)3-8-C5H4N-2,1,8-ReC2B9H10] K[2,2,2-(CO)3-1,8-Bn2-2,1,8-ReC2B9H9] NEt4[Z5-Re(CO)3-7-CH2CH2COOH-7,9-C2B9H10] [rac-8-(4-PhOH)-2-Re(NO)(CO)2-2,1,8-C2B9H10]Na

11.4, 5.5, 4.6, 0.3, −14.4, −18.2, −27.2 4.7, 1.8, −0.4, −4.1, −18.7, −20.5, −31.1 6.8, 4.8, −1.4, −9.7, −13.7, −20.5, −21.2 6.4, 2.1, −3.3, −11.9, −15.6, −23.2, −24.1 4.0, 1.4, −2.5, −8.0, −24.5, −25.8, −33.8 4.6, 1.3, −0.2, −3.0, −20.6, −24.7, −32.9 5.2, 1.8, 0.4, −18.6, −22.5, −30.8 4.9, 1.7, −4.1, −7.4, −25.3, −26.3, −31.4 4.9, 1.7, −4.1, −7.4, −25.3, −26.3, −31.4 −0.4, −6.9, −9.0, −13.7, −15.4, −22.1, −31.4 7.5, −1.5, −8.8, −18.8, −26.0, −31.6, −32.6 20.1, −0.2, −3.0, −7.4, −23.6, −33.1 −6.86, −10.50, −14.71, −21.73, −23.12 −10.89, −13.79, −16.87, −18.24, −21.82, −33.151, −37.04 −10.22, −10.75, −13.37, −15.26, −18.69, −21.04, −32.53, −36.43 − −7.88, −11.44, −14.11, −16.77, −18.20, −19.96 −10.22, −10.75, −13.37, −15.26, −18.69, −21.04, −32.53, −36.43 −9.04, −11.61, −15.80, −19.57, −20.46, −21.93 − −5.53, −7.57, −10.18, −11.71, −18.25, −20.06 − −5.2, −7.7, −10.4, −11.9, −14.8, −20.0 −4.87, −7.81, −11.70, −18.31, −19.85 −6.0, −9.9, −10.4, −14.6, −20.6, −22.1 −6.1, −9.7, −10.1, −14.7, −20.8, −22.5 − − −4.90, −7.22, −7.95, −9.31, −11.95, −18.45, −19.92 − −4.92, −6.97, −7.90, −8.88, −11.69, −12.56, −18.30, −19.69 −4.89, −7.58, −8.25, −11.61, −16.26, −17.75 −4.34, −6.63, −10.78, −14.77, −15.81, −16.99 −4.59, −7.36, −11.52, −15.07, −17.56 −4.76, −7.49, −11.62, −15.16, −17.61 −7.4, −8.1, −10.7, −11.2, −11.6, −14.7, −18.8, −20.2, −21.6 −5.4, −8.5, −9.0, −11.0, −11.7, −18.0, −19.7, −20.0 −5.72, −8.38, −9.35, −11.2, −11.86, −18.06, −19.61, −19.93 −4.2, −6.4, −7.0, −10.8, −11.6, −17.4, −18.7 −5.5, −8.1, −9.1, −11.8, −15.7, −17.5 −6.68, −10.81, −13.06, −16.51, −18.44, −22.05 2.50, −3.26, −6.02, −6.61, −10.06, −10.69, −15.24.

− − − 2.311

241 241 238 241

B NMR (ppm)

Polyhedral Metallaboranes and Metallacarboranes

319

Fig. 48 Molecular structures of metallacarboranes 711–722.

Valliant and co-workers have isolated a handful of examples of metallacarboranes of group 7 metals. The thermolysis reactions of nido-carboranes [C2B9H11R]− (R ¼ ]H, CH2CH2OH, CH2CH2CH2NMe2) with [Re(CO)3Br3]2− in water and in the presence of aqueous potassium fluoride led to the capitation of the open five-membered face by Re(CO)3 and afforded the corresponding rhenacarboranes 723–725 (Scheme 28).237 By contrast, the thermolysis reaction of [99mTc(CO)3(OH2)3]+ with the nido-carborane [C2B9H11(CH2CH2OH)]− under the same reaction conditions afforded the first technetacarborane 726.237 All of these metallacarboranes (723–726) are anionic and have icosahedral core geometries.

Scheme 28 Synthesis of metallacarboranes 723–726.

Fig. 49 Molecular structures of metallacarboranes 727–751.

In a similar manner to the syntheses of 723–726, many more 12-vertex metallacarboranes of group 7 (727–751) were synthesized utilizing the corresponding 11-vertex nido or 12-vertex closo-carboranes, metal precursors [Re(CO)3Br]2− or [M(CO)3 (H2O)3]+ (M ¼ ]Re, 99mTc) and KF/NaF/TEAF salt. All of these metallacarboranes have icosahedral geometries with various substituents at the carbon vertices. Clusters 727–738 are examples of CAd-metallacarboranes, By contrast, clusters 739–751 are examples of CAp-metallacarboranes (Fig. 49).238–242 The metal centers of 727–751 are coordinated by three exopolyhedral carbonyl ligands. The low-temperature reaction of rhenacarborane 739 with [NO][BF4] in THF at −55  C led to the substitution of one of the exopolyhedral COs by an NO+ ligand and afforded neutral rhenacarborane 752.241

9.06.3.5

Metallacarborane clusters of group 8 (Table 13)

There are many examples of Group 8 metallacarboranes. In 2012, the Ghosh group isolated four novel heterometallic metallacarboranes 753–756 by photolysis of bis-borylene species 148 in the presence of a series of alkynes such as 1,2-diphenylethyne,

320

Polyhedral Metallaboranes and Metallacarboranes

Table 13 No. 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804

Metallacarboranes of group 8.

Compounds [1,1,1-(CO)3-m-2,3-(CO)-2,3(Cp )2-4,6-Ph2-1,2,3,4,6-FeRu2C2BH] [1,8-(Cp )2-2,2,7,7-(CO)4-m-2,8-(CO)-m-7,8-(CO)-4Me-5-Ph-1,2,7,4,5-RuFe2C2(BH)2] [1,8-(Cp )2-2,2,7,7-(CO)4-m-2,8-(CO)-m-7,8-(CO)4,5-Me2-1,2,7,4,5-RuFe2C2(BH)2] [1,2-(Cp )2-6,6,7,7-(CO)4-m-2,7-(CO)-m-5,6-(PPh2)m3-1,2,6-(BH)-4-Ph-1,2,6,7,4,5-Ru2Fe2C2BH] [1,2-(Cp Ru)(m-CO)2{Fe2(CO)5}-4-Ph-4,5-C2BH2] [1,2-(Cp Ru)(m-CO)2{Fe2(CO)5}-4-Ph-4,5-C2BHD] [1,1,7,7,7-(CO)5-2,3-(Cp )2-m-2,3-(CO)-m3-1,2,3-(CO)5-(SiMe3)-1,7,2,3,4,5-Fe2Ru2C2BH] [(Cp Ru)2B2H6C6H3(CH3)] [(Cp Ru)3(m3-BH)(m3-CPh)(m-H)(m3-H)] [(Cp Ru)3(m-H)3(m3-Z3-BHCHCPh)] [(Cp Ru)3(m-H)3(m3-Z3-B(OH)CHCPh)] [(Cp Ru)3(m3-BOH)(m3-CPh)(m-H)2(m3-H)]+ [(Cp Ru)3(m3-BO)(m3-CPh)(m-H)2(m3-H)] [1,2-(Cp Ru)2(1,5-m-C{Ph}Me)B3H7] [1,2-(Cp Ru)2(1,5-m-C{CH2Ph}H)B3H7] [1,2-(Cp Ru)2(m-H)(m-BH2)-4-or-5-Ph-4,5-C2B2H5] [1,2-(Cp Ru)2(m-H)(m-BH2)-3-CH2CH2Ph-5Ph-4,5-C2B2H4] [1,2-(Cp Ru)2-5-Ph-4,5-C2B2H7] [1,2-(Cp Ru)2-3-CH2CH2Ph-4-Ph-C2B2H6] [1,2-(Cp Ru)2-3-CH2CH2Ph-5-Ph-C2B2H6] [1,2-(Cp Ru)2-3-(trans-CH]CHPh)-5-Ph-4,5-C2B2H6] [4-Ph-1,2-(Cp RuH)2-4,6-C2B2H3] [1,2-(Cp RuH)2-3-CH2CH2Ph-5-Ph-7-CH2CH2Ph4,5-C2B3H2] [1,2-(Cp RuH)2-5-Ph-4,5-C2B3H4] [1,2-(Cp RuH)2-3-CH2CH2Ph-5-Ph-4,5-C2B3H3] [1,2-(Cp RuH)2-7-CH2CH2Ph-5-Ph-4,5-C2B3H3] [1,2-(Cp Ru)2(1,5-m-C{Fc}Me)B3H7] [1,2-(Cp Ru)2(1,5-m-C{Fc}Me)B3H7] [4-Fc-1,2-(Cp RuH)2-4,6-C2B2H3] [2-Cp-10-Ph-2,1,6,10-RuC3B6H8] [2-Cp-10-Ph-2,1,6,10-FeC3B6H8] [1-Cp -1,2,3,10-RuC3B7H10] [1-Cp -10-tBuNH-1,2,3,10-RuC3B7H9] [1-Cp-10-tBuHN-1,2,3,10-FeC3B7H9] [1-Cp-2-NC(CH2)4-1,2,3,4-FeC3B7H9] [1-Cp-2-{(p-BrC6H4)(Me3SiO)CH}-1,2,3,4-FeC3B7H9] [1-Cp-2-C14H11-1,2,3,4-FeC3B7H9] [1-Cp-2-H3BNMe2(CH2)2-1,2,3,4-FeC3B7H9] [1-Cp-2-NMe2(CH2)2-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-((CH3)3SiC-C)C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-((CH3)3SiC-C)C6H4)-1,2,3,4-RuC3B7H9] [1-Cp-2-(p-(HC-C)C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(HC-C)C6H4)-1,2,3,4-RuC3B7H9] [1-Cp-2-(p-IC6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-BrC6H4)-1,2,3,4-FeC3B7H9] [1-Cp-4-(p-ClC6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-IC6H4)-1,2,3,4-RuC3B7H9] [1-Cp-2-(p-BrC6H4)-1,2,3,4-RuC3B7H9] [1-Cp-2-(p-ClC6H4)-1,2,3,4-RuC3B7H9] [1-Cp-2-(p-(PhC^C)-C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(CH3CH2C(O)OCH2C^C)-C6H4)1,2,3,4-FeC3B7H9]

11

B NMR (ppm)

Av. MdB (Å) a

Ref. b

53.7

2.132(10) , 2.051(10)

243

139.7, 26.2

2.188a, 2.126b

243

145.2, 23.5

2.199a, 2.135b

243

121.7, 69.3

2.137a, 2.15b

243

74.8 73.3 71.8

2.193a, 2.206(10)b 2.202a, 2.213(3)b 2.179(12)a, 2.178b

244 244 111

23.64, 20.99, 13.94 28.66, 23.64, 13.94

2.375a 2.121a 2.161a 2.128a − 2.307a 2.223a − − 2.302a

197 245 245 246 246 246 115 115 115 115

−13.04, −16.67 −1.24, −17.11 −7.02, −13.58 −3.50, −16.52 −19.80 16.21, 9.79

2.367a 2.373a 2.38a 2.376a 2.306a 2.265a

115 115 115 115 115 115

10.12 16.73, 9.09 16.72, 10.30 19.83, 12.26, 9.70 18.22, 13.74, 11.42 −19.32 8.4, −2.4, −7.6, −23.4, −27.2 4.6, −4.9, −8.2, −21.7, −28.9, −30.8 1.16, −17.2, −19.6, −35.1 −0.5, −16.2, −24.9 −1.1, −18.1, −22.3, −9.1, −15.4, −17.8, −24.3 2.2, −0.5, −8.9, −11.4, −25.4, −28.3, −32.5 4.6, −1.2, −8.0, −11.0, −23.8, −27.6, −32.1 7.5, 0.8, −3.9, −7.5, −19.8, −23.9, −28.8 3.4, −0.4, −9.3, −9.8, −10.9, −24.5, −27.6, −32.2 2.3, −0.3, −8.8, −11.3, −25.3, −28.1, −32.2 3.0, 0.0, −10.9, −11.8, −25.9, −28.9, −33.9 4.5, 2.4, −10.5, −11.6, −29.4, −30.2, −30.9 3.8, 0.5, −10.3, −11.1, −25.3, −28.2, −33.2 5.0, 2.3, −10.3, −11.4, −29.2, −29.9, −30.8 3.5, 0.4, −10.5, −11.2, −25.5, −28.3, −33.3 2.6, −0.5, −11.5, −12.3, −26.4, −29.3, −34.4 3.8, 0.7, −10.2, −11.1, −25.2, −28.2, −33.1 5.2, 2.7, −10.1, −10.9, −28.8, −29.8, −30.4 5.7, 3.0, −9.6, −10.7, −28.4, −29.2, −30.0 5.2, 2.7, −9.9, −11.1, −28.9, −29.6, −30.4 3.8, 0.7, −10.2, −10.9, −25.2, −28.1, −33.2 3.8, 0.4, −10.3, −25.4, −28.2, −33.2

− − − 2.225a 2.221a − 2.216a − − 2.242b 2.249b 2.247b 2.259b 2.255b − 2.247b − 2.247b 2.351a 2.253b 2.251b − 2.341a 2.346a 2.345a 2.247b −

115 115 115 380 380 380 236 236 247 247 248 249 249 249 249 249 250 250 250 250 251 251 251 251 251 251 251 251

3.8, 0.4, −10.3, −11.0, −25.2, −28.2, −33.1

2.263b

251

−16.8, −19.2 138.9 59.1 57.6 66.8 42.7 19.17, 15.58, 9.62

321

Polyhedral Metallaboranes and Metallacarboranes

Table 13 No.

(Continued) 11

Compounds

B NMR (ppm)

Av. MdB (Å)

Ref.

3.4, 1.1, −10.5, −11.0, −25.4, −28.2, −33.1 3.2, 0.7, −10.5, −11.3, −25.6, −28.5, −33.2

− −

251 251

3.5, 0.9, −10.3, −11.1, −25.3, −28.2, −33.0 3.5, 0.7, −10.4, −11.2, −25.4, −28.3, −33.1 3.8, 0.7, −10.3, −11.1, −25.3, −28.2, −33.1 3.1, 0.4, −10.8, −11.4, −25.8, −28.7, −33.5

2.251b 2.244b − −

251 251 251 250

5.1, 3.1, −10.0, −10.9, −29.0, −29.7

2.351a

250

1.8, −0.8, −11.9, −26.9, −29.8, −34.6

−

250

3.4, 0.7, −10.6, −25.5, −28.4, −33.1

−

250

3.1, 0.5, −10.7, −25.6, −28.4, −33.1

−

250

3.9, 1.2, −9.8, −24.8, −27.8, −32.7

−

250

3.5, 0.8, −10.3, −25.3, −28.1, −33.1

−

250

3.6, 0.8, −10.3, −25.3, −28.2, −32.9

−

250

6.3, 1.3, −6.2, −8.8, −24.3, −26.7, −34.6 3.5, 0.7, −10.3, −11.0, −25.1, −27.9, −33.4 3.4, 1.0, −10.9, −12.1, −25.7, −28.9, −32.5 2.9, 1.6, −10.4, −11.2, −25.6, −28.4, −33.4 3.6, 0.5, −10.1, −11.7, −25.3, −28.6, −32.7 3.4, 1.2, −10.2, −11.1, −25.3, −28.1, −33.4 3.8, 0.0, −10.7, −11.7, −25.3, −29.2, −33.3 3.2, 1.6, −9.7, −11.5, −25.1, −28.1, −32.4 2.8, 0.6, −9.9, −25.3, −28.2, −32.7

2.242b 2.248b 2.249b – 2.253b 2.239b 2.252b 2.257b 2.258b

252 252 252 252 252 252 252 252 252

3.0, 1.0, −11.4, −25.6, −28.7, −32.1

2.253b

252

4.3, −1.0, −10.6, −25.5, −27.3, −36.4 4.4, −0.2, −6.9, −10.1, −24.9, −26.9, −35.3 3.9, 0.8, −9.7, −21.7, −24.3, −27.1, −33.0 4.6, 1.4, −9.4, −10.2, −26.4, −29.4, −31.7 5.4, 1.6, −8.5, −20.2, −25.3, −27.9, −31.3 3.9, 1.7, −11.1, −12.3, −30.0, −30.8, −31.5 5.4, 2.6, −8.9, −22.9, −27.9, −28.5, −30.5 3.8, 1.9, −9.5, −20.9, −22.6, −28.0, −28.7 5.1, 2.6, −8.4, −19.4, −26.2, −27.4, −29.1 5.4, 2.9, −9.6, −13.3, −29.2, −32 1.6, 1.7, −10.0, −13.5, −29.2, −31.9

2.244b 2.243b 2.251b 2.262b 2.253b 2.349a 2.337a 2.353a 2.356a 2.359a 2.336a

253 253 253 253 253 254 254 254 254 254 254

4.5, 0.4, −10.6, −13.4, −30.2, −33.3

2.343a

254

5.3, 1.6, −9.5, −13.3, −29.2, −32.1 3.9, −0.5, −10.2, −12.9, −25.6, −28.0, −34.7 3.7, −0.5, −10.0, −13.5, −25.8, −28.0, −34.9 3.8, −0.4, −10.3, −13.6, −25.6, −28.0, −34.6 3.7, −0.4, −10.3, −13.5, −25.7, −28.0, −34.6

2.344a 2.259b 2.253b 2.254b 2.25b

254 253 253 253 253

4.0, −0.5, −9.9, −12.4, −25.5, −28.1, −35.0 −1.2, −3.1, −8.7, −13.9, −26.4, −29.5, −33.1. 11.0, 0.5, −1.0, −2.6, −18.1, −18.4, −32.8 8.1, 5.4, 2.2, 0.5, −0.4, −1.5, −9.6, −10.6, −21.5, −22.3, −23.8, −25.4 9.1, 6.7, −0.4, −2.3, −4.3, −7.0, −9.9, −20.2, −21.2, −23.4, −25.0, −31.7

2.253b 2.240b 2.301b 2.307b

253 255 255 255

2.324b

255

5

845 846 847 848

[1-Cp-2-(p-((Cp)Fe(Z -C5H4C^C))-C6H4)1,2,3,4-FeC3B7H9] [1-Cp-2-((p-PhCH2CH^CH)-C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(CH3(CH2)2CH^CH)-C6H4)1,2,3,4-FeC3B7H9] [1-Cp-2-(p-Ph-C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(CH2^CH)-C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(CH2^CHCH2)-C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(C6H5CH2)-1H-1,2,3-N3C2-4-)C6H4)1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(C6H5CH2)-1H-1,2,3-N3C2-4-)C6H4)1,2,3,4-RuC3B7H9] [1-Cp-2-(p-(1-(N-PhOMe-C(O)CH2)-1H-1,2,3-N3C2-4-) C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(N-LeuOMe-C(O)CH2)-1H-1,2,3-N3C2-4-) C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(N-ValOMe-C(O)CH2)-1H-1,2,3-N3C2-4-) C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(N-MetOMe-C(O)CH2)-1H-1,2,3-N3C2-4-) C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(N-AlaOMe-C(O)CH2)-1H-1,2,3-N3C2-4-) C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-(p-(1-(N-TrypOMe-C(O)CH2)-1H-1,2,3-N3C2-4-) C6H4)-1,2,3,4-FeC3B7H9] [1-Cp-2-C6F5-1,2,3,4-FeC3B7H9] [1-Cp-2-C6H4F-1,2,3,4-FeC3B7H9] [1-Cp-2-C4H5N2-1,2,3,4-FeC3B7H9] [1-Cp-2-C9H8N-1,2,3,4-FeC3B7H9] [1-Cp-2-C4H5N2-1,2,3,4-FeC3B7H9] [1-Cp-2-C8H7N2-1,2,3,4-FeC3B7H9] [1-Cp-2-C5H6N-1,2,3,4-FeC3B7H9] [1-Cp-2-C4H11N-1,2,3,4-FeC3B7H+9 ]Cl[1-(Z5-C5H4-CH2-(p-OCH3-C6H4))-2Ph-1,2,3,4-FeC3B7H9] [1-(Z5-C5H4-CH2-(p-OCH3-C6H4))-2C4H5N2-1,2,3,4-FeC3B7H9] [1-Cp-2-Ph-6-Cl-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-Br-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-I-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-11-I-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-I-11-I-1,2,3,4-FeC3B7H7] [1-Cp-2-Ph-1,2,3,4-RuC3B7H9] [1-Cp-2-Ph-6-I-1,2,3,4-RuC3B7H8] [1-Cp -2-Ph-6-I-1,2,3,4-RuC3B7H8] [1-Cp -2-Ph-6,11-I2-1,2,3,4-RuC3B7H7] [1-Cp-2-Ph-6-[C6H5-C^C]-1,2,3,4-RuC3B7H8] [1-Cp-2-Ph-6-[CH3CH2C(O)OCH2-C^C]1,2,3,4-RuC3B7H8] [1-Cp-2-Ph-6-[(Z5-C5H5)Fe(Z5-C5H4)-C^C]1,2,3,4-RuC3B7H8] [1-Cp-2-Ph-6-[(CH3)3Si-C^C]-1,2,3,4-RuC3B7H8] [1-Cp-2-Ph-6-(PhC-C)-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-((CH3)3SiC-C)-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-(HC-C)-1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-[CH3CH2C(O)OCH2C-C]1,2,3,4-FeC3B7H8] [1-Cp-2-Ph-6-[CpFeCp-C-C]-1,2,3,4-FeC3B7H8] [ansa-(2-(CH2)2)-(1-Z5-C5H4)-1,2,3,4-FeC3B7H9] [ansa-(CH2)2-(C3B7H9)2Fe] [ansa-(CH2)2-(C3B7H9)2Fe]

849

[ansa-(CH2)2-(C3B7H9)2Fe]

805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844

(Continued )

322

Polyhedral Metallaboranes and Metallacarboranes

Table 13 No.

(Continued)

Compounds

858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874

[3-(p-cym)-3,1,2-RuC2B9H11] [3-(-C10H8)-3,1,2-RuC2B9H11] [3,3-(PPh3)2-3-Cl-3,1,2-RuC2B9H11]Li [1,2-Ph2-3,3-(PPh3)2-3-Cl-3,1,2-RuC2B9H9]Li [1,2-Ph2-3,3-(tBuNC)2-3-PPh3-3,1,2-RuC2B9H9] [1,2-Ph2-3,3-(CO)2-3-PPh3-3,1,2-RuC2B9H9] [{1,2-Ph2-3,1,2-RuC2B9H9}{1’,8’-Ph2-2’,1’,8’RuC2B9H9}] [(1,2-Ph2-3,1,2-RuC2B9H9)(1’,8’-Ph2-2’,1’,8’-RuC2B9H9) (1”,8”-Ph2-2”,1”,8”-RuC2B9H9)] [3-(Z6-biphenyl)-3,1,2-RuC2B9H11] [3-(Z6-(1-Me-4-COOEt-C6H4))-3,1,2-RuC2B9H11] [3-(6-biphenyl)-1,2-Me2-3,1,2-RuC2B9H9] [3-(6-(1-Me-4-CO2Et-C6H4))-1,2-Me2-3,1,2-RuC2B9H9] [3-(6-p-cym)-1,2-(CO2Me)2-3,1,2-RuC2B9H9] [1,2-m-(C4H4)-3,3,3-(CO)3-3,1,2-RuC2B9H9] [1,2-m-(C4H6)-3,3,3-(CO)3-3,1,2-RuC2B9H9] [Z-{1,2-m-(C4H6)}-3,3-(CO)2-3,1,2-RuC2B9H9] [1,2-m-(C4H6)-3,3-(CO)2-3-PMe3-3,1,2-RuC2B9H9] [1,2-m-(C4H6)-3,3-(CO)2-3-P(OMe)3-3,1,2-RuC2B9H9] [1,2-m-(C4H6)-3,3-(CO)2-3-tBuNC-3,1,2-RuC2B9H9] [1,2-m-(C6H4)2-3,3,3-(CO)3-3,1,2-RuC2B9H9] [2-(p-cym)-2,1,7-RuC2B9H11] [2-(p-cym)-2,1,12-RuC2B9H11] [1,2-Ph2-8-(p-cym)-8,1,2-RuC2B9H9] [2-Cp -8-SMe2-2,1,8-RuC2B9H10] [(-9-Me2S-7,8-C2B9H10)Ru(-C6H6)]Cl

875

[(-9-Me2S-7,8-C2B9H10)Ru(-C6H6)]Br

876

[(-9-Me2S-7,8-C2B9H10)Ru(-C6H6)]I

877 878 879

[(-9-MeS-7,8-C2B9H10)Ru(-C6H6)] [(-10-MeS-7,8-C2B9H10)Ru(-C6H6)] [(Z-9-SMe-7,8-C2B9H10)Fe(Z-C6H6)]

880 881 882

[(Z-10-SMe-7,8-C2B9H10)Fe(Z-C6H6)] [(Z-9-SMe-7,8-C2B9H10)Fe(tBuNC)3] (Z-9-SMe2-7,8-C2B9H10)Fe(Z5-C6H7)

883 884

[(Z-9-SMe2-7,8-C2B9Me2H8)Fe(Z5-C6H7)] (Z-9-NMe3-7,8-C2B9H10)Fe(Z5-C6H7)

885

[(Z-9-SMe2-7,8-C2B9H10)Fe(Z5-C6H6)]PF6

886 887 888

[(Z-9-SMe2-7,8-C2B9Me2H8)Fe(Z5-C6H6)]PF6 [(Z-9-NMe3-7,8-C2B9H10)Fe(Z5-C6H6)]PF6 [(Z-9-SMe2-7,8-C2B9H10)Fe(tBuNC)3]PF6

889

[(Z-9-SMe2-7,8-C2B9H10)Fe(P(OMe3))3]-PF6

890

[(-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(-C6H5OMe)]PF6

891

[(-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(-C6H5Me)]PF6

892

[(-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(-1,3-C6H4Me2)]PF6

893

[(-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(-1,3,5-C6H3Me3)] PF6 [(-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(-1,2,4,5-C6H2Me4)] PF6 [(-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(-C6Me6)]PF6 [(5-Cp){5-[C6H4(CH2)2]C2B9H9}]Fe [1-(1-1,2-C2B10H11)-3-(p-cym)-3,1,2-RuC2B9H10]

850 851 852 853 854 855 856 857

894 895 896 897

11

B NMR (ppm)

1.87, 0.34, −7.68, −9.17, −19.54, −24.23 2.03, 0.83, −7.25, −8.61, −19.48, −24.50 −1.5, −5.8, −8.4, −10.9, −12.3, −23.3, −24.5 − 23.6, 15.1, 12.7, 4.5, −0.6, −19.9 17.8, 7.1, 3.7, 0.1, −18.0 29.4, 18.0, 16.8, 12.6, 0.8, −0.8, −3.0, −4.5, −8.5, −14.8, −15.8, −17.3, −19.9 29.8, 17.9, 13.2, 1.4, −0.6, −2.1, −8.1, −12.7, −15.1, −16.7, −19.3, −22.9 −24.7, −20.1, −9.9, −8.1, −1.1, −0.7 −24.3, −19.5, −9.4, −7.3, −0.5, 1.3 2.4, 0.5, −2.9, −9.4, −14.1 2.7, 1.6, −2.3, −8.9, −13.5 27.7, 11.1, 8.7, 0.11, −1.6, −21.8 5.4, −2.4, −6.3, −10.6 8.3, −3.8, −5.1, −8.5, −9.9 8.1, 0.9, −4.1, −7.1, −10.1, −17.7 2.2, −5.2, −7.2, −9.4, −12.4 2.6, −4.5, −7.1, −11.0 4.2, −4.6, −5.9, −6.8, −10.1, −11.6 8.2, −1.2, −4.7, −6.3, −6.9, −9.0 −0.62, −5.50, −11.16, −14.50, −15.82, −20.19 −3.92, −7.48, −8.82, −19.35, −21.39 0.7, −3.9, −5.5, −14.7 −1.8, −7.5, −12.6, −17.2, −23.5, −25.8 −0.08, −0.59, −1.47, −7.57, −8.47, −12.56, −19.60, −21.20, −24.31 −0.12, −0.54, −1.54, −7.70, −8.45, −12.53, −19.58, −21.19, −24.35 −0.05, −0.06, −1.53, −7.70, −8.35, −12.58, −19.52, −21.25, −24.37 − 11.95, −1.93, −7.83, −10.23, −19.79, −27.56 2.8, 1.8, −2.9, −8.1, −9.7, −10.6, −18.9, −22.3, −25.1 11.6, −3.7, −9.1, −20.1, −27.9 2.1, −3.4, −7.9, −9.8, −11.6, −19.8, −25.4 −3.4, −11.2, −13.0, −14.3, −15.7, −23.7, −25.3, −28.7 6.2, −4.4, −12.0, −13.7, −14.2, −24.4, −25.9, −28.7 −1.2, −1.7, −4.1, −7.1, −8.8, −11.5, −19.1, −21.1, −24.7 −1.2, −5.0, −7.5, −12.4, −13.4, −14.4, −15.9 5.4, −1.5, −8.8, −10.1, −20.1, −21.2, −25.9 −26.0, −20.1, −17.5, −12.7, −11.3, −8.7, −8.0, −2.6, −0.3 −3.0, −4.5, −8.7, −10.9, −15.2, −16.6, −20.9, −26.6 −1.4, −2.6, −4.5, −7.4, −8.1, −11.7, −19.4, −21.3, −24.7 −0.7, −2.2, −4.1, −7.3, −8.2, −11.5, −19.2, −21.0, −24.6 −0.2, −2.5, −4.3, −7.4, −8.0, −11.5, −19.2, −20.9, −24.6 1.4, −2.9, −4.6, −7.6, −9.2, −11.6, −19.0, −20.6, −25.0 0.9, −3.5, −4.4, −6.8, −7.3, −11.4, −19.1, −21.3, −24.6 0.1, −4.3, −6.4, −7.7, −11.7, −19.4, −21.3, −25.1 − 2.8, 0.5, −2.8, −3.9, −7.2, −8.8, −10.7, −12.6, −14.4, −17.2

Av. MdB (Å) a

Ref.

2.22 2.209a − − 2.255a 2.267a 2.192a

256 257 258 258 258 258 258

2.192a

259

2.214a 2.213a − 2.194a 2.216a 2.276a 2.271a 2.278a 2.280a 2.280a 2.264a 2.267a 2.191a 2.178a 2.169a 2.194a −

260 260 261 261 261 262 262 262 262 262 262 262 263 263 264 265 266

−

266

−

266

− − 2.118b

266 266 267

− − 2.107b

267 267 268, 269

−

268 268

2.121b

268, 269

− − 2.15b

268 268 269

−

268

−

270

−

270

−

270

−

270

−

270

2.113b 2.089b 2.208a

270 271 318

323

Polyhedral Metallaboranes and Metallacarboranes

Table 13 No.

(Continued)

Compounds

898

[8-(1-1,2-C2B10H11)-2-(p-cym)-2,1,8-RuC2B9H10]

899

904 905

a-[1-(8’-2’-(p-cym)-2’,1’,8’-RuC2B9H10)-3-(p-cym)3,1,2-RuC2B9H10] b-[1’-(8’-2’-(p-cym)-2’,1’,8’-RuC2B9H10)-3-(p-cym)3,1,2-RuC2B9H10] [HNMe3][8-(7’-7’,8’-C2B9H11)-2-(p-cym)2,1,8-RuC2B9H10] a-[8-(1’-3’-Cp-3’,1’,2’-CoC2B9H10)-2-(p-cym)2,1,8-RuC2B9H10] b-[8-(1’-3’-Cp-3’,1’,2’-CoC2B9H10)-2-(p-cym)2,1,8-RuC2B9H10] [1-(1’-1’,2’-C2B10H11)-3-(C6H3Me3)-3,1,2-RuC2B9H10] [1-(1’-1’,2’-C2B10H11)-3-(C6Me6)-3,1,2-RuC2B9H10]

906 907 908

commo-[3,3’-Ru(8-SMe2-1,2-C2B9H10)2] K[8,8’-(MeO)2-3,3’-Fe(1,2-C2B9H10)2] (Bu4N)[4,7’-(MeO)2-3,3’-Fe(1,2-C2B9H10)2]

909 910 911 912 913 914 915 916

(Me4N)[8,8’-Cl2-3,3’-Fe(1,2-C2B9H10)2] (BEDT-TTF)2[8,8’-Cl2-3,3’-Fe(1,2-C2B9H10)2] (ЕТ)2[8,8’-Br2-3,3’-Fe(1,2-C2B9H10)2] (ЕТ)2[8,8’-I2-3,3’-Fe(1,2-C2B9H10)2] [Me4N][8,8’-Br2-3,3’-Fe(1,2-C2B9H10)2] [Me4N][8,8’-I2-3,3’-Fe(1,2-C2B9H10)2] [8,8’-(Me2S)2-3,3’-Fe(1,2-C2B9H10)2] [4,4’-(Me2S)2-3,3’-Fe(1,2-C2B9H10)2]

917

[4,7’-(Me2S)2-3,3’-Fe(1,2-C2B9H10)2]

918 919 920 921 922

(Bu4N)2[8,8’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2] (Bu4N)2[4,4’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2] (Bu4N)2[4,7’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2] (Me4N)2[8,8’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2] (Me4N)[4,4’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2]

923

(Bu4N)[4,4’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2]

924

(Me4N)[4,7’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2]

925 926

(Bu4N)[4,7’-(MeS)2-3,3’-Fe(1,2-C2B9H10)2] K[1,1’-(2’)-(MeS)2-3,3’-Fe(1,2-C2B9H10)2]

927 928 929 930

(TMTSF)2[3,3’-Fe(1,2-C2B9H11)2] [BEDT-TTF]2[3,3’-Fe(1,2-C2B9H11)2] (DBTTF)[3,3’-Fe(1,2-C2B9H11)2] [3,3’-Fe(8-(OCH2CH2)2-1,2-C2B9H10)(1’,2’-C2B9H11)]

931

[3,3’-Fe(8-(OCH2CH2)2OMe-1,2-C2B9H10)(1’,2’C2B9H11)] [3,3’-Fe(8-(OCH2CH2)2OEt-1,2-C2B9H10)(1’,2’-C2B9H11)]

900 901 902 903

932

934

[3,3’-Fe(8-(OCH2CH2)2OCH2CH2OCH3-1,2-C2B9H10) (1’,2’-C2B9H11)] [3,3’-Fe(8-(OCH2CH2)2Cl-1,2-C2B9H10)(1’,2’-C2B9H11)]

935

[3,3’-Fe(8-(OCH2CH2)2Br-1,2-C2B9H10)(1’,2’-C2B9H11)]

936

[3,3’-Fe(8-(OCH2CH2)2I-1,2-C2B9H10)(1’,2’-C2B9H11)]

933

11

B NMR (ppm)

Av. MdB (Å)

Ref.

a

318

−1.0, −2.8, −4.1, −4.9, −8.0, −10.1, −13.4, −16.2, −19.2, −20.4 1.7, 0.1, −1.3, −7.7, −8.2, −12.9, −16.8, −21.5

2.179

2.189a

272

1.7, 0.0, −1.2, −3.8, −4.9, −7.9, −13.1, −16.9, −21.1 −1.1, −2.8, −4.9, −7.9, −10.2, −13.8, −16.7, −19.5, −21.0, −25.5, −33.5, −35.9 5.2, 2.3, −0.9, −3.8, −5.2, −6.8, −8.5, −10.0, −14.6, −16.2, −17.9, −20.7 5.0, 2.2, −0.9, −3.9, −5.6, −7.5, −8.7, −10.4, −14.6, −16.5, −17.4, −20.7 3.3, 0.4, −3.3, −7.3, −10.6, −12.7, −17.4 5.6, −0.9, −2.1, −4.0, −5.3, −7.3, −8.1, −10.5, −12.8, −15.0, −17.6 1.9, −5.4, −9.5, −15.0, −22.7, −24.7 114.6, −6.2, −8.0, −69.1, −443.2 109.5, 9.7, 7.5, 1.1, −21.8, −40.7, −403.4, −431.7, −461.1 119.9, 21.4, 0.6, −58.1, −365.7, −538.2 − − − − 121.8, 25.3, 7.1, −48.4, −337.8, −573.3 0.8, −9.6, −12.0, −14.9, −23.1, −26.2 −3.8., −5.3, −8.9, −11.0, −14.5, −23.6, −24.3, −26.7 3.8, −5.5, −8.9, −9.6, −11.8, −15.5, −22.9, −24.9, −26.4 1.5, −13.1, −13.7, −24.6 −10.7, −12.5, −15.0, −23.2 −11.7, −13.0, −13.6, −23.0, −24.3 118.8, 41.8, −4.0, −52.8, −383.7, −442.5 100.7, 55.2, 1.0, −0.9, −3.2, −61.1, −402.6, −486.2, −528.9 101.5, 55.7, 1.0, −0.9, −3.3, −61.7, −406.1, −490.6, −534.1 98.8, 50.3, 18.8, 13.4, −11.2, −56.0, −417.3, −434.5, −475.7 − 104.2, 29.4, 9.9, −4.2, −11.5, −31.8, −36.1, −384.2, −556.5 − − − 109.14, 104.32, 8.50, 4.56, −4.41, −25.18, −37.53, −325.67, −410.22, −477.92, −620.99 115.98, 100.83, 36.25, 29.65, −0.26, −4.64, −37.22, −39.23, −362.17, −424.92, −465.75 116.31, 100.82, 33.50, 28.85, −0.14, −4.37, −36.25, −39.10, −362.07, −370.26, −428.52, −466.84 115.47, 102.01, 24.99, 0.4, −3.55, −32.70, −38.20, −372.58, −394.49, −428.62, −480.14 116.22, 99.91, 40.67, 30.14, −1.22, −5.99, −17.15, −23.83, −40.24, −359.53, −428.37, −463.87 116.55, 100.39, 41.51, 30.83, −0.62, −5.45, −39.62, −357.58, −426.26, −461.50 117.29, 100.87, 41.37, 30.62, −1.24, −5.98, −40.65, −362.07, −431.06, −469.17

2.184a

272

−

273

2.184a, 2.066c

273

2.185a, 2.082c

273

2.146a 2.189a

274 274

2.215a − −

275 276 276

2.145b − 2.158b 2.138b − − − −

277 277 278 278 278 278 279 279

−

279

2.122b − − − −

279 279 279 279 279

2.145b

279

2.137b

279

2.138b −

279 280

2.146b 2.134b 2.131b −

281 282 283 284

−

284

−

284

−

284

−

284

−

284

−

284 (Continued )

324

Polyhedral Metallaboranes and Metallacarboranes

Table 13

(Continued)

No.

Compounds

11

937

[3,3’-Fe(8-(OCH2CH2)2SH-1,2-C2B9H10)(1’,2’-C2B9H11)]

938 939

[N(CH3)4][2,2’-Fe(1,7-closo-C2B9H11)2] [8-EtC^N-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]

940

[8-EtC(OH)]HN-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]

941

Et3NH[8-EtC(O)HN-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]

942

[8-EtC(OEt)]HN-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]

943

(Z-1-tBuNH-1,7,9-C3B8H10)Fe(Z-9-SMe2-7,8-C2B9H10)

944

(Z-1-tBuNH-1,7,9-C3B8H10)Fe(Z-9-NMe3-7,8-C2B9H10)

945

[(Z-9-SMe2-7,8-C2B9H10)Fe(Z-9-NMe3-7,8-C2B9H10)]

946

949

[8-(CH2CH2CH2CH2O)-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]− [8-(CH2CH2CH2CH2CH2O)-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]− Cs[8-(OCH2CH2CH2CH2OCH2)-3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)] Cs[8-(O(CH2)4N3)-3,3’-Fe(1,2-C2B9H10)-(1’,2’-C2B9H11)]

950

Cs[8-(O(CH2)5N3)-3,3’-Fe(1,2-C2B9H10)-(1’,2’-C2B9H11)]

951

[8-(O(CH2)4N+ H(CH2CH2)2O)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(O(CH2)5N+ H(CH2CH2)2O)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(OCH2CH2)2N+ H(CH2CH2)2O-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(O(CH2)4N+ Me2)(CH2CH2OH)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(O(CH2)5N+ Me2)(CH2CH2OH)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(OCH2CH2)2N+ Me2(CH2CH2OH)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(O(CH2)4N+ Me2)(CH2CH^CH)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(O(CH2)5)N+ Me2(CH2CH^CH)-3,3’-Fe(1,2-C2B9H10) (1’,2’-C2B9H11)]− [8-(OCH2CH2)2N+ Me2(CH2CH^CH)3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]− [8-(O(CH2)4)P+ Ph2(CH2CH2(C6H5)2)3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]− [8-(O(CH2)5)P+ Ph2(CH2CH2(C6H5)2)3,3’-Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]− [8-(OCH2CH2)2P+ Ph2(CH2CH2(C6H5)2)3,30 -Fe(1,2-C2B9H10)(1’,2’-C2B9H11)]− [8-(O(CH2)5P+ Ph2)(CH2CH2(C6H5)2)-8I-3,3’-(1,2-C2B9H10)(1’,2’-C2B9H11)3,30 -Fe(1,2-C2B9H10)-(1’,2’-C2B9H11)]− [1-Cp -12-tBuNH-1,2,4,12-RuC3B8H10] [1-Cp -10-tBuNH-1,2,4,10-RuC3B8H10] [2-Cp-8-tBuHN-2,1,8,10-FeC3B8H10] [2-Cp-4-tBuHN-2,1,4,12-FeC3B8H10]

117.29, 101.39, 37.23, 30.19, −0.22, −4.69, −37.62, −39.31, −360.00, −366.56, −427.77, −465.63 −400.9, −321.9, −22.4, 10.4, 26.6, 33.4 111.7, 101.5, 10.7, 6.9, 4.3, 1.8, −34.7, −324.7, −411.3, −421.8, −488.0 114.0, 104.3, 12.3, 2.5, 3.7, 1.2, −33.2, −36.7, −360.0, −411.6, −464.4, −541.3 117.2, 104.9, 25.1, 12.8, 4.2, 0.6, −38.1, −377.9, −388.3, −407.2, −448.8 170.0, 161.3, 65.8, 62.2, 60.3, 29.0, 23.4, −299.5, −351.2, −413.9 −3.8, −4.4, −8.0, −8.7, −9.3, −10.5, −12.6, −14.6, −15.5, −16.8, −21.9, −22.7, −24.6, −26.0 4.3, −5.9, −7.9, −10.0, −10.9, −12.2, −13.1, −14.9, −15.6, −16.8, −21.9, −22.7, −24.6, −26.0 5.0, −4.4, −6.3, −8.9, −11.0, −11.7, −12.4, −14.3, −23.0, −24.4, −25.7, −27.3 110.9, 105.2, 7.2, 5.5, 3.9, −5.7, −14.9, −26.7, −38.4, −339.6, −418.6, −487.8, −625.8 110.9, 105.2, 7.2, 5.5, 3.9, −5.8, −26.7, −38.4, −339.6, −418.6, −487.8, −625.8 118.4, 101.0, 43.5, 30.4, −1.8, −6.7, −40.8, −362.2, −365.5, −441.6, −475.2 117.9, 100.7, 42.9, 29.8, −1.7, −14.1, −6.9, −40.5, −53.0, −362.7, −366.7, −424.0, −439.5, −476.5 119.0, 101.6, 43.3, 30.3, −1.7, −6.6, −40.8, −367.5, −379.0, −448.3, −479.8 112.8, 104.6, 14.5, 0.2, −1.1, −2.8, −3.9, −29.4, −39.9, −400.5, −440.9, −452.3, −525.8 118.4, 100.9, 38.0, 27.8, −1.8, −6.4, −39.2, −369.7, −380.0, −446.3, −484.6 116.5, 102.4, 22.4, 20.3, −0.9, −4.1, −34.1, −38.8, −384.4, −411.1, −443.7, −499.3 117.8, 99.6, 35.6, 24.8, −1.9, −7.2, −38.5, −373.2, −389.3, −446.9, −494.1 118.7, 100.6, 38.8, 27.8, −1.85, −6.6, −39.5, −368.1, −378.0, −446.1, −483.8 117.6, 101.1, 31.4, 25.5, −1.5, −5.8, −37.9, −39.1, −374.6, −390.3, −441.3, −489.5 117.9, 99.7, 35.6, 24.8, −1.9, −7.2, −38.6, −373.6, −389.8, −446.9, −495.4 118.7, 100.6, 38.8, 27.8, −1.85, −6.6, −39.5, −368.1, −378.0, −446.1, 483.8 117.2, 101.1, 30.5, 25.0, −1.3, −5.7, −36.7, −38.8, −374.9, −392.2, −440.7, −489.8 118.0, 101.0, 35.0, 25.5, −1.52, −6.7, −39.0, −375.5, −390.4, −447.8, −494.3 119.0, 100.9, 39.8, 28.6, −1.6, −6.4, −39.5, −368.3, −376.5, −446.2, −483.0 117.9, 101.3, 31.6, 27.1, −1.03, −5.4, −36.5, −38.9, −369.2, −382.5, −436.4, −478.4 119.2, 101.6, 39.7, 28.7, 3.1, −1.8, −3.7, −6.4, −15.2, −22.9, −39.5, −368.1, −376.6, −446.8, −483.4 −9.3, −12.9, −18.6, −21.8, −25.7 −6.5, −11.6, −12.8, −18.3, −27.7 −3.1, −4.0, −11.3, −14.6, −22.9, −25.7 −6.5, −8.7, −12.7, −15.2, −16.0, −19.7, −23.1, −23.4

947 948

952 953 954 955 956 957 958 959 960 961 962 963

964 965 966 967

B NMR (ppm)

Av. MdB (Å)

Ref.

−

284

2.114b −

285 286

−

286

−

286

−

286

2.093b

287

2.108b

287

2.136b

268

2.144b

288

−

288

−

288

−

288

−

288

2.144b

288

2.142b

288

−

288

−

288

−

288

−

288

−

288

−

288

−

288

2.144b

288

−

288

−

288

−

288

2.160a − 2.028b 2.036b

289 289 248 248

325

Polyhedral Metallaboranes and Metallacarboranes

Table 13 No.

(Continued)

Compounds

B NMR (ppm)

t

968

[2-Cp-1- BuHN-2,1,7,10-FeC3B8H10]

969 970 971

[1-(Z-C6H6)-12-tBuNH-1,2,4,12-FeC3B8H10]PF6 [1-(Z-C6H5Me)-12-tBuNH-1,2,4,12-FeC3B8H10]PF6 [1-(Z-1,3,5-C6H3Me3)-12-tBuNH-1,2,4,12-FeC3B8H10] PF6 [1-(Z-C6Me6)-12-tBuNH-1,2,4,12-FeC3B8H10]PF6 [1-(Z-C6H5OMe)-12-tBuNH-1,2,4,12-FeC3B8H10]PF6 [1-Cp-2-(C6H5)-1,2,4,10-FeC3B8H10] [1-Cp-12-(C6H5)-1,2,4,12-FeC3B8H10] [1-Cp-12-(1’-C10H7)-1,2,4,12-FeC3B8H10] [1-Cp-12-(2’-C10H7)-1,2,4,12-FeC3B8H10] [1-Cp-10-(C6H5)-1,2,4,10-FeC3B8H10] [1-Cp-10-(1’-C10H7)-1,2,4,10-FeC3B8H10] [1-Cp-10-(2’-C10H7)-1,2,4,10-FeC3B8H10]

972 973 974 975 976 977 978 979 980

11

Av. MdB (Å)

Ref.

b

248

2.028

−7.4, −9.0, −14.0, −16.2, −18.6, −21.7, −24.8, −25.1 −9.1, −13.3, −14.9, −19.5 −9.1, −13.3, −14.8, −19.7 −8.1, −12.7, −14.7, −19.5

− − 2.028b

290 290 290

−6.7, −11.1, −14.2, −19.6 −9.1, −13.3, −14.9, −19.5 −10.2, −11.3, −12.9, −17.1, −20.8, −25.4 −9.7, −13.9, −16.9, −19.6, −24.6 −10.0, −14.3, −15.7, −19.1, −23.6 −9.7, −13.9, −16.7, −19.5, −24.4 −8.0, −10.1, −16.1, −24.7 −8.0, −10.1, −16.1, −24.7 −8.1, −10.9, −11.3, −17.3, −24.3

− − 2.024b − 2.041b − − − 2.022b

290 290 291 291 291 291 291 291 291

a

Av. RudB Av. FedB c Av. CodB b

1-phenyl-1-propyne, 2-butyne, and 1-(diphenylphosphino)-2-phenylacetylene (Scheme 29).243 Compound 753 has an octahedral core and consists of 7 SEP and 56 CVE. Compounds 754–756 have a monocapped pentagonal bipyramidal core (8 SEP). Similarly, the photolysis of another heterometallic borylene species 146 with phenylacetylene yielded a closo-metallacarborane 757, and its deuterated analog 758 was generated by using 1-deuterio-2-phenylacetylene.244 Both metallacarboranes 757 and 758 have a pentagonal bipyramidal {Fe2Ru2C2B} core. Thermolysis of the same borylene species 146 with trimethylsilylethylene afforded diruthenacarborane 759.111 Cluster 759 has 8 SEP, which is consistent with its pentagonal bipyramidal geometry. Ghosh and co-workers isolated another unique metallacarborane 760 from the reaction of 5 equiv. of Te powder and nido-[1,2(Cp Ru)2(m-H)2B3H7] (along with fused cluster 507).197 In 760, a toluene ring is fused to a pentagonal pyramidal {Ru2B2C2} core through a common CdC bond, and the RuB2C6 fragment is an analog of the Z5-indenyl ligand.

Scheme 29 Syntheses of metallacarboranes 753–756.

326

Polyhedral Metallaboranes and Metallacarboranes

Suzuki and co-workers have also synthesized many metallacarboranes utilizing borylene species. For example, photolysis of a m3-borylene m3-Z2(k)-phenylacetylene triruthenium complex [(Cp Ru)3(m3-BH)(m3-Z2(k)-PhCCH)(m-H)3] at 313 nm afforded a ruthenacarborane 761 (Scheme 30), having a capped trigonal bipyramidal geometry.245 By contrast, the irradiation of the same borylene complex at 436 nm yielded another ruthenacarborane 762.245 Cluster 762 has an unusual geometry, in which one Ru3 triangle and a C2B triangle are held in a mutually anti position to give a square antiprism. When ruthenacarborane 761 was heated at 50  C in the presence of water, the BH vertex is replaced by a {BO} unit to form another capped trigonal bipyramidal ruthenacarborane 763.246 The Lewis basic nature of the m3-BO ligand of 763 was proved by protonation at the oxygen atom to form its cation 764.233 On the other hand, the hydroxy derivative of 762, [(Cp Ru)3(m3-BO)(m3-CPh)(m-H)2(m3-H)] (765) was isolated by reaction of 761 with H2O in a closed vessel via a hydroxyborylene intermediate [(Cp Ru)3(m3-BOH)(m3-CPh)(m-H)(m3-H)] which can be trapped by the accumulated hydrogen.246

Scheme 30 Syntheses of metallacarboranes 761–762.

The Fehlner group in 2005 explored the reactivity of nido-[1,2-(Cp RuH)2B3H7] with phenylacetylene under different conditions (Scheme 31).115 At ambient temperature, eight products were isolated by reduction to m-alkylidene RudB bridges (766 and 767), reduction to exo-cluster alkyl substituents on boron, cluster insertion with extrusion of a {BH2} fragment into an exo-cluster

Scheme 31 Syntheses of metallacarboranes 766–778.

Polyhedral Metallaboranes and Metallacarboranes

327

bridge (768), combined insertion with BH2 extrusion and reduction (769), insertion and loss of borane with and without reduction (770–772), or insertion and borane loss and reduction (773).115 Clusters 766 and 767 have an edge-fused geometry in which a square pyramidal {Ru2B3} core and a triangular RuCB core are fused through a common RudB bond. Clusters 768 and 769 are also examples of edge-fused clusters, in which a pentagonal pyramidal {Ru2B2C2} core and a triangular RuB2 core are fused through a common RudB bond. By contrast, clusters 770–773 have pentagonal pyramidal Ru2B2C2 cores. At 90  C the same reaction produced insertion products 774–778 along with ruthena-carboranes 770, 771, and 773.115 Ruthenacarborane 774 has an octahedral core, whereas ruthenacarboranes 775–778 have pentagonal bipyramidal cores. The same nido-[1,2-(Cp RuH)2B3H7] cluster reacted with ethynylferrocene at room-temperature to yield two geometric isomers 779 and 780 (having a chiral C-center which arises from Markovnikov addition) and an octahedral ruthenacarborane closo-781. Clusters 779 and 780 have a similar type of core to those of 766 and 767. Sneddon and co-workers isolated the first 10-vertex closo-metallatricarbanonaboranes [2-Cp-10-Ph-2,1,6,10-MC3B6H8] (M ¼ ]Ru (782), Fe (783)) by the deboronation of the 11-vertex closo-[1-Cp-2-Ph-1,2,3,4-MC3B7H9] (M ¼ ]Ru, Fe) with TBAF.236 Both the closometallatricarbanonaborane 782 and 783 have bicapped square antiprism geometry with 11 SEP. Many metallacarboranes (784–845) of group 8 with octadecahedron core were isolated in two isomeric forms based on the carbon atom positions (Fig. 50). In the first isomeric form, the carbon centers are located at the 2, 3, and 10-positions, as observed in clusters 784–786.247,248 By contrast, in the second isomeric form, the carbon centers are located at 2, 3, and 4-positions, as observed in clusters 787–845. In the octahedron core of 784–845, the metal center occupies the degree-six vertex. Closo ruthenacarboranes 784 and 785 were isolated from the reaction of nido-[7-tBuNH-7,8,9-C3B8H10]− anion with [Cp RuCl2]2 in xylene under refluxing conditions along with two more icosahedral ruthenacarboranes (vide infra). The Fe analog of 785, ferracarborane 786 was isolated via a cluster opening reaction of closo-[2-Cp-9-tBuNH-2,1,7,9-FeC3B8H10], followed by oxidation.248 On the other hand, the second type of octadecahedral isomer of metallatricarbaboranes has many examples. For example, the treatment of nido-[6-R-5,6,9-C3B7H9]− (R ¼ ]NC(CH2)4, [(p-BrC6H4)(Me3SiO)CH], C14H11 or H3BNMe2(CH2)2) with [CpFe(CO)2I] in THF at room-temperature yielded 11-vertex ferracarborane clusters closo-[1-Cp-2-R-1,2,3,4-FeC3B7H9] (R ¼ ]NC(CH2)4 (787), [(p-BrC6H4)(Me3SiO)CH] (788), C14H11 (789) and H3BNMe2(CH2)2 (790)).249 In all of these clusters, the carbon atom at 2-position of the octadecahedral core is functionalized. Further reaction of BH3-coordinated amine compound 790 with triethylenediamine (DABCO) for 12 h in glyme led to the removal of the exo-BH3 group and afforded closo-791.249 Similarly, the reaction of tricarbadecaboranyl anion nido-[6-(p-((CH3)3SiC^C)C6H4)-5,6,9-C3B7H9]− with [CpFe(CO)2I] or [CpRu(CH3CN)3PF6] in glyme led to the isolation of metallacarboranes 792 and 793, respectively, in which the carbon at 2-position of the octadecahedral core is attached to a {p-((CH3)3SiCdC)C6H4} group.250 Further treatment of 792 and 793 with the deprotecting agent K2CO3 in MeOH/THF produced desilylated products 794 and 795.250 Similarly, the reactions of nido-[6-(p-XC6H4)-5,6,9-C3B7H9]− (X ¼ ]I, Br, Cl) with [CpFe(CO)2I] and [CpRu(CH3CN)3PF6] in glyme yielded closo-ferracarboranes [1-(Z5-Cp)-2-(p-XC6H4)-closo-1,2,3,4-FeC3B7H9] (X ¼ ]I (796), Br (797), Cl (798)) and closo-ruthenacarboranes [1-(Z5-Cp)-2-(p-XC6H4)-closo-1,2,3,4-RuC3B7H9] (X ¼ ]I (799), Br (800), Cl (801)).251 Furthermore, the functionalizations at the cage-carbon iodoaryl substituent of ferracarborane 796 by palladium-catalyzed Sonogashira, Heck, and Stille cross-coupling reactions were carried out. From the sonication-promoted Sonogashira coupling reactions of 796 with a set of terminal alkyne in the presence of Pd(dppf )Cl2/CuI alkynyl-linked derivatives closo-[1-Cp-2(p-RC6H4)-1,2,3,4-FeC3B7H9] (R ¼ ]PhC^C (802), CH3CH2C(O)OCH2C^C (803), (Cp)Fe(Z5-C5H4C^C) (804)) were isolated.251 Heck reaction of 796 with terminal alkenes catalyzed by Pd(OAc)2 produced alkene-functionalized products closo-[1-Cp-2p-RC6H4-1,2,3,4-FeC3B7H9] (R ¼ ]PhCH2CH^CH (805), CH3(CH2)2CH^CH (806)).251 The Stille cross-coupling reactions of 796 with organotin compounds Bu3SnR catalyzed by [Pd(PPh3)2Cl2] yielded functionalized metallatricarbadecaboranes closo[1-Cp-2-(p-RC6H4)-1,2,3,4-FeC3B7H9] (R ¼ ]Ph (807), CH2^CH (808), CH2^CHCH2 (809)).251 Similarly, at room-temperature ‘Click’ addition of benzyl azide to 794 and 795 led to functionalization of the carbon atom at the 2-position and afforded the triazole complexes closo-[1-(Z5-C5H5)-2-(p-(1-(C6H5CH2)-1H-1,2,3-N3C2–4-)C6H4)-1,2,3,4-MC3B7H9] (M ¼ ]Fe (810), Ru (811)).250 Another Click addition reaction of 794 with N-azidoacetyl amino acid methyl esters in the presence of copper sulfate and sodium ascorbate yielded a series of amino acid-functionalized ferratricarbadecaboranyl complexes 812–817.250 Metallacarboranes 812–817 having cytotoxic properties have been investigated as anticancer agents and with low cytotoxicity are potential agents to deliver high boron concentration to cells for use in BNCT. Reactions of nido-[6-R-5,6,9-C3B7H9]− (R ¼ ]C6F5, C6H4F, C4H5N2, C9H8N, C4H5N2, C8H7N2, C5H6N, C4H10N) with [CpFe(CO)2I] under reflux conditions in glyme resulted in 2-position carbon functionalized ferratricarbadecaboranes 818–825, respectively, which contain potential anticancer and solubilizing groups.252

Fig. 50 Molecular structures of metallacarboranes 784–845.

328

Polyhedral Metallaboranes and Metallacarboranes

By contrast, ferracarboranes 826 and 827 were synthesized in a different way. Nido-[6-R-5,6,9-C3B7H9]− (R ¼ ]Ph, C4H5N) was reacted with FeCl2 in glyme and subsequently treated with p-methoxybenzyl cyclopentadienide Li[p-OCH3C6H4CH2C5H4] for 24 h under reflux to afford 826 and 827.252 On the other hand, this type of octadecahedral metallacarborane was also synthesized with B-functionalization. For example, the reactions of N-chlorosuccinimide or N-bromosuccinimide with a preformed ferracarborane closo-[1-Cp-2-Ph-1,2,3,4-FeC3B7H9] at room-temperature resulted in selective halogenation at the 6-position boron of the octadecahedral core and afforded closo-828 and 829, respectively.253 The AlCl3-catalyzed reaction of the same nido-carborane with ICl led to halogenation at boron vertices, and afforded two mono-substituted isomers 830 and 831, along with the dihalogenated product 832.253 Treatment of nido-[6-Ph-5,6,9-C3B7H9]− with [CpRu(CH3CN)3][PF6] in glyme produced a ruthenatricarbadecaboranyl complex 833.254 Further reaction of 833 and its Cp analog with ICl produced their mono-iodo derivatives 834 and 835 in good yields, but the di-iodo product 836 was isolated in a trace amount. Sonogashira cross-coupling of 834 with different terminal alkynes catalyzed by Pd(dppf )Cl2/CuI led to B-functionalization and yielded alkynyl-linked derivatives 837–840.254 The reaction of 830 with different terminal acetylenes in the presence of (PPh3)2PdCl2/CuI in Et2NH formed a series of acetylene-functionalized metallatricarbadecaboranyl compounds closo-[1-Cp-2-Ph-6-(RC-C)-1,2,3,4-FeC3B7H8] (R ¼ ]Ph (841), (CH3)3Si (842), H (843), CH3CH2C(O) OCH2 (844), (Z5-C5H4)Fe(Z5-C5H5) (845)).253 When 842 was treated with tetrabutylammonium fluoride hydrate, an unsubstituted ethynyl ferracarborane 843 is formed by removal of the SiMe3 group. All of these metallacarboranes (784–845) have 12 SEP which is in accord with their 11-vertex closo geometry. On the other hand, treatment of a ferrabistricarbadecaboranyl anion Li2[6-C5H4-(CH2)2-nido-5,6,9-C3B7H9] with FeCl2 yielded a hybrid ansa-sandwich complex 846 (Scheme 32), in which the ansa-CH2CH2-group is connected to C-atoms of the Cp ring and tricarbadecaborane cage.255 Compound 846 has 12 SEP and a closo-octadecahedron structure. The reaction of a linked-cage bistricarbadecaboranyl dianion Li2[6,60 -(CH2)2-nido-(5,6,9-C3B7H9)2] with FeCl2 afforded three isomeric ansa-ferrabistricarbadecaboranyl sandwich complexes 847–849 (Scheme 32).255 In 847, two cages are linked at the C2 and C20 positions, but two cages are connected by C2 and C40 in 848 and 849. Two isomers (848 and 849) are quite different from each other in terms of the relative positions of C40 atoms. At 200  C 847 is completely converted to 848 in 8 h.

Scheme 32 Syntheses of fused metallacarboranes 846–849.

Metallacarboranes of group 8 having an icosahedral core have been synthesized in huge numbers. Icosahedral metalladicarboranes can be isolated in a total of nine isomeric forms (Fig. 51). Most of these metallacarboranes were synthesized utilizing the well-known reduction-capitation methodology. For example, 2e reductions of nido-[7,8-C2B9H11] and further reaction with [(p-cym)RuCl2]2 at room-temperature afforded icosahedral ruthenacarborane 850 (I, Fig. 51).256 Deprotonation of nido[7,8-C2B9H12]− with nBuLi in Et2O and subsequent metalation with [RuCl2(COD)]x in the presence of naphthalene in THF afforded naphthalene ruthenacarborane 851 (I, Fig. 51).257 The Welch group reported the formation of two closo anionic ruthenacarboranes 852 and 853 from the deprotonation of zwitterions [exo-5,6,10-{RuCl(PPh3)2}-5,6,10-(m-H)3–7,8-Ph2-nido-7,8-R2-C2B9H7] (R ¼ ]H, Ph) (Scheme 33).258 Dehalogenation of 853 in the presence of tBuNC and CO yielded 854 and 855 by replacing PPh3 and Cl from Ru center in 854 (Scheme 33).258 But dehalogenation of 854 in the absence of any donor ligands and non-donor solvent afforded a pseudocloso-ruthenacarborane 856 (Scheme 33).258 Compound 856 contains 2 units: one is pseudocloso[3,1,2-RuC2B9H9], and another is distorted icosahedral [20 ,10 ,80 -RuC2B9H9]. Along with the formation of ruthenacarborane 856, a triple ruthenacarborane cluster 857 was also isolated.259 In 857, three RuC2B9H9 ruthenacarborane units are linked by Z6-ligation of Ph rings.

Polyhedral Metallaboranes and Metallacarboranes

329

Fig. 51 Isomers of MC2B9-type icosahedral metallacarboranes.

Scheme 33 Syntheses of metallacarboranes 852–856.

Hey-Hawkins and co-workers carried out the reactions of one equivalent of each Ru(arene) dimer, [{(Z6-biphenyl)2RuCl (m-Cl)}2] and [{(Z6–1-Me-4-CO2Et-C6H4)2RuCl(m-Cl)}2] with Tl[3-Tl-1,2-C2B9H11] and it’s C,C-dimethyl analog Tl[3-Tl-1,2Me2-3,1,2-C2B9H9] which afforded icosahedral ruthenacarboranes 858–861, respectively, in which the metal centers are coordinated with different arene groups in Z6-fashion.260,261 On the other hand, to avoid base-promoted cleavage of methoxy esters, the deprotonation of closo-[1,2-(CO2Me)2-m-H-1,2-C2B9H9] was carried out by TlOEt at low-temperature followed by reaction with [(p-cym)RuCl2]2 in CH2Cl2, which afforded closo-862.261 This is unexpected that 862 has a closo structure instead of pseudocloso— a ruthenacarborane having C-bound substituents provides extra electron density to the CdC bond, leading to the deformation of closo to pseudocloso. Deprotonation of nido-[7,8-m-(C4H4)-7,8-C2B9H10][HNMe3] followed by addition of [Ru(CO)3Cl2]2 afforded 12-vertex benzocarborane derivative, ruthenacarborane 863 (I, Fig. 51), in which exopolyhedral diene {C4H4} units are connected to two carborane C atoms.262 From nido-[7,8-m-(C4H6)-7,8-C2B9H10][HNMe3] the analog of 863, ruthenacarborane 864 was isolated along with a 12-vertex dicarbonyl Z-ene ruthenacarborane 865 in which a C]C unit has displaced one CO ligand.262 Compounds 864 and 865 can be interconverted by heating 864 at reflux in THF or via reaction with Me3NO which afforded 865, which on treatment with CO converts back to 864. When 865 was treated with PMe3, P(OMe)3 and tBuNC, the reaction led to the displacement of the Z-ene unit and afforded 866–868, respectively.262 On the other hand, deboronation of [1,2-m-(C6H4)2-1, 2-closo-C2B10H10] with EtOH/KOH led to the isolation of [HNMe3][7,8-m-(C6H4)2-7,8-nido-C2B9H10], which further reacted

330

Polyhedral Metallaboranes and Metallacarboranes

with [Ru(CO)3Cl2]2 and yielded the first examples of metal derivatives of biphenylcarborane [1,2-m-(C6H4)2-3,3,3-(CO)3-3,1, 2-closo-RuC2B9H9] (869).262 Clar’s rule for polycyclic aromatic hydrocarbons (PAC) can be applied to PACs fused onto carborane cages like in 869. In compound 869, the intramolecular crowding between the biphenyl substituent and {Ru(CO)3} fragment is less due to the unusual orientation of CO, which lies opposite to Ccage–Ccage connectivity. When double lithiated THF solution of nido-[7,9-C2B9H11]2− or nido-[2,9-C2B9H11]2− was treated with [(p-cym)RuCl2]2 in room-temperature, the reaction yielded icosahedral 870 (II, Fig. 51) and its 2,1,12 analog 871 (III, Fig. 51), respectively.263 On the other hand, naphthalene-assisted sodium reduction of [1,8-Ph2-4-(p-cym)-4,1,8-closo-RuC2B10H10] (vide infra) followed by the addition of 0.5 equiv. of [Ru(Z-C6H6)Cl2]2 led to the formation of a series of 12, 13, and 14 vertex supra-icosahedral metallacarboranes (vide infra) along with icosahedral 872 (IV, Fig. 51).264 By contrast, treatment of Tl[7-SMe2-7,8-C2B9H10] with [Cp RuCl]4 generated icosahedral 2,1,8-MC2B9 isomer 873 (V, Fig. 51) by polyhedral rearrangement of the carborane ligand and the migration of the substituted C-atom.265 Treatment of previously reported ruthenacarborane [(Z-9-Me2S-7,8-C2B9H10)Ru(Z-C6H6)]PF6 with [BnNEt3]Cl, [Bu4N]Br or [Bu4N]I in DCM resulted in ion-exchange with halogen and afforded icosahedral 874–876, respectively.266 The demethylation of [(Z-9-Me2S-7,8-C2B9H10)Ru(Z-C6H6)]+ by halide anions in CH2Cl2 afforded a neutral complex, 877.266 The reaction of the 10-substituted isomer [(Z-10-MeS-7,8-C2B9H10)Ru(Z-C6H6)] with iodide anion in CH2Cl2 resulted in an isomer of 877, [(Z-10-MeS-7,8-C2B9H10)Ru(Z-C6H6)] (878).266 The demethylation reactions of the cyclohexadienyl complexes [(Z-X-SMe2–7,8-C2B9H10)Fe(Z5-C6H7)] (X ¼ ]9, 10 positions) with Na[PhCH2S] in DMF and subsequent treatment with acetic acid afforded thioether-benzene-iron complexes 879 and 880.267 Further reaction of icosahedral 879 under visible-light in the presence of tBuNC resulted in the substitution of the benzene ring by three tBuNC ligands and yielded ferracarborane 881.267 Possessing a similar type of ferracarborane core to that of 879, ferracarboranes (Z-9-L-7,8-C2B9H8R2)Fe(Z5-C6H7) (882: L ¼ ]SMe2, R ¼ ]H; 883: L ¼ ]SMe2, R ¼ ]Me; 884: L ¼ ]NMe3, R ¼ ]H) were isolated from the visible light irradiation of [(Z5-C6H7)Fe(Z-C6H6)]+ with the [9-L-7,8-C2B9R2H8]− anions.268 In ferracarboranes 882–884, the C6H7 ring is connected with the Fe center of the icosahedron core in Z5-fashion. Further reactions of 882–884 with hydrochloric or acetic acids yielded benzene complexes 885–887.268,269 Visible light irradiation of cationic carborane 885 with tBuNC or P(OMe)3 in acetonitrile yielded tris(ligand) derivatives 888 and 889 by the replacement of the benzene ligand.268,269 Visible light irradiation of the same dicarbollide benzene complex 885 with a library of benzene derivatives in CH2Cl2/MeNO2 led to the generation of arene exchange products [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)Fe(Z-C6R6)]PF6 (C6R6 ¼ ]anisole (890), toluene (891), m-xylene (892), mesitylene (893), durene (894), hexamethylbenzene (895)).270 Another CAd ferracarborane 896 was isolated from the salt metathesis reaction of FeCl2 with {Z5-(C6H4(CH2)2)C2B9H9}Na2(THF)3 in THF followed by oxidation with O2. In icosahedral 896, the carbon vertices are linked by C6H4(CH2)2.271 Metalation of [7-(10 -10 ,20 -closo-C2B10H11)-7,8-nido-C2B9H10]2− with [(p-cym)RuCl2]2 yielded unisomerized ruthenacarborane 897 and isomerized ruthenacarborane 898 (Fig. 52).259 Conjuncto-897 was slowly converted to conjuncto-898 when heated to reflux for 2 h in THF. By contrast, deprotonation of the doubly-deboronated [HNMe3]+ salt of [7-(70 -70 ,80 -nido-C2B9H11)-7,8-nidoC2B9H11]2− with nBuLi in THF followed by the reaction with [(p-cym)RuCl2]2 afforded two isomeric conjuncto-diruthenacarborane 899 and 900.272 Compounds 899 and 900 have one 2,1,8-RuC2B9 cage, and 3,1,2-RuC2B9 cage. On the other hand, deboronation of conjuncto-[8-(10 -closo-10 ,20 -C2B10H11)-closo-2,1,8-RuC2B9H10] with KF and followed by reaction with an aqueous solution of excess [HNMe3]Cl afforded another conjuncto-metallacarborane 901.273 Further deprotonation of 901 and reaction with CoCl2/ NaCp followed by aerial oxidation yielded two diastereoisomers a (902) and b (903) form of conjuncto-[8-(10 -30 -Cp-closo-30 ,10 , 20 -CoC2B9H10)-2-(p-cym)-closo-2,1,8-RuC2B9H10].273 Clusters 902 and 903 are the first examples of heterometalated derivatives of 1,10 -bis(ortho-carborane). Deprotonation of 1,10 -bis(ortho-carborane) with nBuLi followed by metalation with [(Z-mes)RuCl2]2 or [(Z-C6Me6)RuCl2]2 afforded two more conjuncto-metallacarboranes 904 and 905, respectively.274

Fig. 52 Molecular structures of metallacarboranes 897–903.

Polyhedral Metallaboranes and Metallacarboranes

331

A series of commo-metallacarboranes, having two icosahedral units fused by a common vertex, were isolated and functionalized. They were synthesized either from 11-vertex nido-carboranes and metal precursors or from preformed commo-metallacarborane. For example, the reaction of K[10-SMe2-7,8-nido-C2B9H10] with [RuCl2(dmso)4] under reflux condition afforded dicarbollide sandwich of ruthenium, commo-[3,30 -Ru(8-SMe2-1,2-C2B9H10)2] (906) (Scheme 34).275 Compound 906 has two quasi-enantiomeric independent entities having a pseudo-gauche conformation. The reaction of symmetrical K[10-OMe-7,8-C2B9H11] with anhydrous FeCl2 in the presence of tBuOK yielded a paramagnetic 8,80 -dimethoxy bis(dicarbollide) 907.276 The reaction of asymmetric K [10-OMe-m-H-7,8-C2B9H10] with anhydrous FeCl2 in the presence of tBuOK, followed by the addition of [Bu4N]Br afforded a 4,70 -isomer of the dimethoxy derivatives of iron bis(dicarbollide) (908).276

Scheme 34 Synthesis of fused metallacarboranes 906.

On the other hand, (Me4N)[8,8´-Cl2-3,3´-Fe(1,2-C2B9H10)2] (909) was isolated by the reaction of Cs[3,30 -Fe(1,2-C2B9H11)2] with two-fold excess of N-chlorosuccinimide and subsequent treatment with an aqueous solution of Me4NBr.277 The same isomer of 909 with the salt of BEDT-TTF (910) was obtained by anodic oxidation of BEDT-TTF in the presence of supporting electrolyte 909. Like 910, the bromine (911) and iodine (912) derivatives were grown by anodic oxidation of bis(ethylenedithio) tetrathiafulvalene, in the presence of [Me4N][8,80 -X2-3,30 -Fe(1,2-C2B9H10)2] (X ¼ ]Br or I, respectively).278 Compound 911 and 912 are the first radical-cation salts partnered with the [8,80 -X2-3,30 -Fe(1,2-C2B9H10)2]− anion and the first molecular conductors. When N-bromosuccinimide and N-iodosuccinimide are treated with Cs[3,30 -Fe(1,2-C2B9H11)2] in THF at the room-temperature it afforded [Me4N][8,80 -X2-3,30 -Fe(1,2-C2B9H10)2] (X]Br (913), I (914)).278 By the non-aqueous method, the symmetrically substituted [8,80 -(Me2S)2-3,30 -Fe(1,2-C2B9H10)2] (915) was isolated from the reaction of anhydrous FeCl2 and base tBuOK with [10-Me2S-7,8-C2B9H11], and in this case, the yield is more than the aqueous method previously reported by Kennedy group in 1998.279 With the similar approach using 9-Me2S-7,8-C2B9H11, two asymmetrical diastereomers 916 and 917 having cisoid and transoid conformation of dicarbollide ligands, respectively, were isolated.279 Furthermore, treatment of 915–917 with nBuSK yields bis(methylsulfanyl) derivatives of iron(II) bis(dicarbollide) 918–920.279 The transoid configuration in 918 is stabilized by four intramolecular CH⋯S contacts between the dicarbollide ligands. The iron(II) bis(dicarbolides) 918–920 were easily oxidized by air in aqueous solution to the corresponding paramagnetic iron(III) complexes 921, 922, and 924 having tetramethylammonium salts, 923 and 925 as tetrabutylammonium salts.279 In 923, the dicarbollide ligands are in a gauche conformation stabilized by four intramolecular CH⋯S contacts between the dicarbollide ligands with somewhat different CH⋯S distances. But in 925, the dicarbollide ligands with gauche conformation stabilized by one pair of intramolecular CH⋯S hydrogen bonds with one MeS group and one short BH


S contact with another one. Paramagnetic iron(bis(dicarbolides)) (926) isolated using anhydrous FeCl2 and base tBuOK with Cs[7-Me2S-7,8-C2B9H11].280 The anodic oxidation of TMTSF, BEDT-TTF and DBTTF with metallacarborane anions [Me4N][3,30 -Fe(1,2-C2B9H11)2] under galvanostatic conditions yielded iron bis(dicarbollide) complexes of the corresponding salts (927–929).281–283 The reaction of NMe4[3,30 -Fe(1,2-C2B9H11)2] with Me2SO4/H2SO4 or BF3.Et2O in dioxane yielded zwitterion 930, which reacted with different alkoxides to afford sandwich compounds 931–933.284 Reaction of 930 with halides and hydrosulfide ions led to the formation of [3,30 -Fe(8-(OCH2CH2)2X-1,2-C2B9H10)(10 ,20 -C2B9H11)] (X ¼ ]Cl (934), Br (935), I (936), SH (937)).284 On the other hand, [HN(CH3)3][7,9-nido-C2B9H12] in THF treated with nBuLi, then reacted with FeCl2, Which yielded commo938.285 Reaction of Cs[3,30 -Fe(1,2-C2B9H11)2] with ethylnitrile in the presence of tBuBr produced paramagnetic 8-propionitrilium derivative 939.286 By alkaline hydrolysis of 939 in acetonitrile, iminol 940 is isolated which with triethylamine afforded amide 941 and reaction with triethylamine yielded mixtures of E (942a) and Z (942b) isomers of imidates.286

332

Polyhedral Metallaboranes and Metallacarboranes

Fig. 53 Isomers of MC3B8-type icosahedral metallacarboranes.

The photochemical reaction of tricarbollide complex [(Z-1-tBuNH-1,7,9-C3B8H10)Fe(Z-C6H6)]+ with sodium salt Na[9L-7,8-C2B9H10] led to the formation of non-symmetrical ferracarboranes 943 and 944.287 Compounds 943 and 944 are the first examples of the iron bis(carborane) complexes containing both the tricarbollide and charge compensated dicarbollide ligands. On the other hand, the photochemical reaction of [9-NMe3-7,8-C2B9H10]− and 885 yielded an unsymmetrical iron(bis-dicarbollide) complex 945.256 Semioshkin and co-workers isolated 8-tetrahydrofuronium and 8-tetrahydropyronium iron bis(dicarbollide) anions (946 and 947) from iron-bis(1,2-dicarbollide) anions.288 Furthermore, they had carried out the ring cleavage reactions using different reagents, such as MeOH, NaN3, amines, and 1,2-bis(diphenylphosphino)ethane, which afforded their derivatives 948–966.248,288,289 Apart from a large number of icosahedral metallacarboranes with two carbon vertices, a significant number of icosahedral metallacarboranes with three carbon vertices have also been synthesized. A room-temperature reaction of [7-tBuNH-nido7,8,9-C3B8H10]− and [Cp RuCl]4 yielded ruthenatricarbollide 964 (II, Fig. 53), which can also be obtained by complexation of [8-tBuNH-nido-7,8,9-C3B8H10]− anion.289 Upon refluxing at 145  C in xylene, 964 rearranged to its isomer 965 (I, Fig. 53).289 On the other hand, the tricarbollide complex [2-Cp-9-tBuNH-closo-2,1,7,9-FeC3B8H10] upon reduction with Na/naphthalene yielded an air-sensitive metallatricarbaborane nido-[Cp-tBuNH-FeC3B8H10]2− which on oxidation with CuCl2 produced an icosahedron 966 (III, Fig. 53).258 Compound 966 can also be synthesized by the air oxidation of dianion [nido-Cp-tBuNH-FeC3B8H10]2− along with the formation of two more isomers 967 (II, Fig. 53) and 968 (IV, Fig. 53) and 11-vertex metallacarborane 786.248 The visible light irradiation of [(Z5-C6H7)Fe(Z-C6H6)]+ with [7-tBuNH-7,8,9-C3B8H10]− generated [1-(Z5-C6H7)-12-tBuNH-1,2,4,12FeC3B8H10] which, upon treatment with hydrochloric acid, afforded ferracarborane 969 (II, Fig. 53).290 Under visible light induced arene exchange in 969, a series of compounds having Z-C6H5Me (970), Z-1,3,5-C6H3Me3 (971), Z-C6Me6 (972), Z-C6H5OMe (973) were generated.290 The Štíbr group reported the high-temperature reactions of [8-Ar-nido-7,8,9-C3B8H11] (Ar ¼ ]C6H5, 1’-C10H7, 2’-C10H7) and [CpFe(CO)2] at 200  C which generated a series of different isomers of monoaryl-substituted ferratricarbollide complexes [1-Cp-2-(C6H5)-1,2,4,10-FeC3B8H10] (974; II, Fig. 51), [1-Cp-12-Ar-1,2,4,12-FeC3B8H10] (II, Fig. 53; Ar ¼ ]C6H5 (975), 10 -C10H7 (976), 20 -C10H7 (977)), [1-Cp-10-Ar-1,2,4,10-FeC3B8H10] (I, Fig. 53; Ar ¼ ] C6H5 (978), 10 -C10H7 (979), 20 -C10H7 (980)).291 Compounds of types 974–980 represent the first examples of monoaryl-substituted ferratricarbollides. In the case of 978–980, Cs symmetry is present, whereas 974 is unsymmetrical with no Cs symmetry.

9.06.3.6

Metallacarborane clusters of group 9 (Table 14)

As with group 8, a vast number of metallacarboranes of group 9 have been synthesized. Kennedy and co-workers isolated a closo-7 vertex metallacarborane 981 by cage closure, together with the loss of two boron vertices from nido-[2,5-(Cp )2–10-Mel-2,5-H-2,5,1-Rh2CB6H8], using an approximately molar equivalent of elemental iodine and excess triethylamine.292 Ruthenacarborane 981 has 8 SEP, which is consistent with its pentagonal bipyramidal core. Fehlner and co-workers have utilized preformed metallaboranes and alkynes to synthesize metallacarboranes. For example, the reaction of nido-[1-Cp -2,2,2(CO)3-2-THF-1,2-IrMoB4H8] with an excess of 2-butyne yielded mixed-metal metallacarborane 986 (Scheme 35).293,294 Cluster 986 has a well-known 8-vertex nido-core and 10 SEP. The intermediates (982, 983, and 985) in the synthesis of 986 were also isolated. Intermediates 982 and 983 have 7-vertex nido-cluster cores.293 By contrast, intermediate-985 has a 9-vertex closo-geometry, which can be generated from tricapped trigonal prism by two DSD rearrangements.293,294 The degree-six vertex of 985 is occupied by the Mo atom. Under alkyne deficient conditions, a 7-vertex closo-metallacarborane 984 was isolated in low yield.

333

Polyhedral Metallaboranes and Metallacarboranes

Table 14 No.

Metallacarboranes of group 9.

Compounds

11

B NMR (ppm)

Av. MdB (Å) a

Ref.

47.3, 22.2, 10.7, 8.3 32.5, 1.1

2.18 −

292 293

33.9, −6.1

−

293

33.4, 1.9

986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005

[1-Cp -5,6,7,8-Me4-1,5,6,7,8-IrC4B3H3] [1-Cp -6,7,8,8-Ph5-1,6,7,8,8-IrC5B4H4] [1,1-(CO)2-2-Ph-1,2,3,4-CoC3B7H9] [1,1-(CO)2-2-Ph-1,2,3,4-RhC3B7H9] [8,8,8-(CO)3-9-Ph-8,7,9,10-IrC3B7H9] [1,1-COD-2-Ph-1,2,3,4-RhC3B7H9] [1,1-COD-2-Ph-1,2,3,4-IrC3B7H9] [1-(Z4-C4Me4)-2-Ph-1,2,3,4-CoC3B7H9] [1,1-dppe-2-Ph-1,2,3,4-CoC3B7H9] [1,1-dppe-2-Ph-1,2,3,4-RhC3B7H9] [8-CO-8,8-dppe-9-Ph-8,7,9,10-IrC3B7H9] [8,8,8-(CNtBu)3-9-Ph-8,7,9,10-CoC3B7H9] [8,8,8-(CNtBu)3-9-Ph-8,7,9,10-IrC3B7H9] [8,8-COD-8-CNtBu-9-Ph-8,7,9,10-IrC3B7H9] [1-(Z4-C4Ph4)-2-Ph-1,2,3,4-CoC3B7H9] [8,8-COD-8-CNtBu-9-Ph-11-I-8,7,9,10-IrC3B7H8] [2,2-COD-10-Ph-2,1,6,10-IrC3B6H8] [3-Cp -1,2-(PPh2)2-IrC2B9H9] [3-Cp -1,2-(PPh2)2-RhC2B9H9] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)RhCl2]

1006 1007 1008 1009

[(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)RhBr2] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)RhI2] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)IrCl2] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)IrBr2]

2.164a − − 2.173b

297 297 297 297

1010

[(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)IrI2]

2.173b

297

1011 1012 1013 1014 1015

[3-Cp -C+(NHiPr)2-3,1,2-IrC2B9H10]OTf [3-Cp -C+(NHCy)2-3,1,2-IrC2B9H10]OTf [3-Cp -(N]N-C6H4OMe)2-3,1,2-IrC2B9H10] [3-Cp -1-(N]N-C6H4OMe)-3,1,2-IrC2B9H10] [3-Cp -1-(N]N-C6H4OMe)-3,1,2-RhC2B9H10]

2.201b 2.193b 2.202b 2.196b 2.195a

298 298 299 299 299

1016

[3-Cp -1-(C5H4N)S-3,1,2-RhC2B9H10]

2.186a

300

1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030

[3-Cp-1,2-m-(C4H6)-1,2-CoC2B9H9] [3-Cp-1,2-m-(C4H4)-1,2-CoC2B9H9] [1,2-m-(C6H4)2-3-Cp-3,1,2-CoC2B9H9] [(Z5-Cp){Z5-[C6H4(CH2)2]C2B9H9}]Co [(Z-7,8-C2B9H11)Ir(Z5-indenyl)] [1,2-Me2-3-Cp-3,1,2-CoC2B9H9] [1,2-Me2-3-Cp -3,1,2-CoC2B9H9] [3-(Z-Cp )-3,1,2-CoC2B9H11] [3-(Z-Cp )-3,1,2-CoC2B9H11] [3-(Z-Cp)-3,1,2-RhC2B9H11] [3-(Z-Cp )-3,1,2-RhC2B9H11] [3-(Z-Cp)-3,1,2-IrC2B9H11] [3-(Z-Cp )-3,1,2-IrC2B9H11] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)Rh(SMe2)Br2]

11.6, −10.4 9.6, −9.1 5.5, −1.5, −11.3, −14.1, −27.5, −29.6, −32.5 10.5, 1.3, −4.9, −10.9, −12.8, −19.9, −26.6 −1.1, −4.1, −10.3, −12.3, −16.5, −37.6 7.8, 3.8, −4.2, −7.3, −9.8, −16.9, −20.4 1.5, 0.2, −7.1, −10.1, −25.0, −26.9, −28.9 −2.0, −2.9, −5.7, −7.6, −22.1, −22.6, −25.9 2.3, −0.2, −15.0, −16.1, −28.0, −31.0, −32.5 8.3, −0.1, −1.2, −10.0, −16.4, −17.4, −21.1 −8.9, −13.1, −15.8, −19.0, −40.2 −3.4, −8.7, −11.1, −13.1, −17.7, −33.5 −4.9, −10.3, −13.2, −15.6, −20.8, −43.4 −2.6, −5.3, −11, −13.9, −16.4, −38.5 6.7, 0.0, −2.0, −5.1, −19.1, −24.3 −0.7, −6.4, −10.8, −12.2, −14.9. −20.0, −38.4 3.3, −4.2, −9.6, −16.5, −19.1, −21.1 20.7, 13.9, 6.1, 4.6, 2.2, −2.4, −6.2, −18.7 18.7, 12.3, 11.6, −0.9, −3.1, −6.7, −17.9 15.30, 13.49, 5.37, 4.08, −3.00, −5.50, −6.24, −7.88, −16.40 15.09, 10.14, 4.14, 3.19, −5.41, −7.93 −8.58, −15.76 14.26, 3.13, 2.11, 0.41, −5.52, −8.51, −10.36, −14.73 13.93, 6.49, 3.57, −0.52, −1.87, −6.37, −9.31, −18.50 14.48, 13.21, 5.74, 3.01, −1.02, −2.39, −6.40, −9.51, −18.67 14.89, 10.37, 3.49, 1.30, −2.41, −3.89, −6.53, −10.46, −18.96 25.1, 18.8, −9.0, −6.3, −22.5 25.2, 18.5, −9.1, −6.4, −22.5 10.60, 2.16–1.29, −6.65 to −10.32, −23.74 −0.89, −8.62, −11.61, −13.80, −19.67 to −26.52 8.38, –7.15, −1.23 to −3.31, −8.58 to −10.70, −16.65 to −20.43 9.40, 0.31, −1.00, −3.02, −7.68, −10.62, −16.59, −18.16, −21.19 5.8, 3.3, −3.0, −6.8, −10.8, −14.9 2.5, 0.6, −2.8, −7.8, −11.9, −13.8 4.7, 3.3, −2.5, −6.4, −11.5, −13.4 −0.42, −2.41, −7.6, −12.75, −17.35, −19.30 −27.0, −21.6, −11.3, −9.4, −2.2, 1.3 5.06, 2.12, −1.59, −7.22, −12.09, −13.06 7.78, −1.27, −2.02, −8.91, −11.82, −13.72 − − 5.5, −2.8, −5.3, −7.9, −18.0, −23.7 − 2.4, −3.9, −11.1, −11.6, −21.5, −26.7 − 15.02, 10.10, 4.18, 3.2, −5.4, −7.92, −8.61, −15.73

2.18b, 2.461 (2)d 2.18b, 2.5716(16)d 2.227b 2.209b − − − 2.469a 2.432b 2.244c 2.223c 2.404a 2.551b 2.406c 2.519b 2.548b 2.532b 2.311b − 2.192a 2.154a

293,294

985

[1,2-Cp 2-l-1,2-H-1,2,3-Rh2CB4H4-3-I] [1-Cp -7,7,7-(CO)3-7-THF-2,3(CH3)2-1,7,2,3-IrMoC2B3H5] [5-Cp -7,7,7-(CO)3-7-THF-2,3(CH3)2-5,7,2,3-IrMoC2B3H5] [1-Cp -2,2,2-(CO)3-(m-CO)-3,4(CH3)2-1,2,3,4-IrMoC2B3H3] [1-Cp -5,6,7,8-Me4-1,5,6,7,8-IrC4B3H3Mo(CO)3]

2.065c 2.086c 2.080c 2.066c 2.183b 2.047c 2.074c − − 2.186a − 2.093b − 2.168a

301 301 301 257 302 303 303 304 304 304 304 304 304 297

981 982 983 984

33.1, −10.6

293,294 293,294 293 295 295 295 295 295 295 295 295 295 295 295 295 295 295 295 296 296 297

(Continued )

334

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

No.

Compounds

11

1031 1032

[(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)Ir(SMe2)Br2] [(Z-9-SMe-7,8-Me2-7,8-C2B9H8)RhCp]

1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053

[(Z-9-SMe-7,8-Me2-7,8-C2B9H8)IrCp] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)RhCp]PF6 [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)IrCp]PF6 [3-(C6H6)-3,1,2-IrC2B9H11]PF6 [3-(C6Me6)-3,1,2-IrC2B9H11]PF6 [3-(1,2-C6H4Me2)-3,1,2-IrC2B9H11]PF6 [3-(1,3-C6H4Me2)-3,1,2-IrC2B9H11]PF6 [3-(1,2,4,5-C6H2Me4)-3,1,2-IrC2B9H11]PF6 [3-(MeCN)3-3,1,2-IrC2B9H11]PF6 [3-(1,3,5-C6H3Me3)-3,1,2-IrC2B9H11]PF6 [3-[2,2]paracyclophane-3,1,2-IrC2B9H11]PF6 [3-(Z-Cp)-3,1,2-IrC2B9H11] [3-(Z-C5H4C(O)Me)-3,1,2-IrC2B9H11] [(Z-9-SMe2-7,8-C2B9H10)Rh(C6H6)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Rh(1,3-C6H4Me2)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Rh(1,3,5-C6H3Me3)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Rh(1,3,4,5-C6H2Me4)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Rh(C6Me6)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Rh(C6H5OMe)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Ir(C6H6)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Ir(1,3-C6H4Me2)](BF4)2

1054 1055 1056 1057 1058 1059 1060 1061 1062

[(Z-9-SMe2-7,8-C2B9H10)Ir(1,3,5-C6H3Me3)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Ir(1,3,4,5-C6H2Me4)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Ir(C6Me6)](BF4)2 [(Z-9-SMe2-7,8-C2B9H10)Ir(C6H5OMe)](BF4)2 [(Z-7,8-C2B9H11)Rh(Z6-C6Me6)][BF4] [(Z-7,8-Me2-7,8-C2B9H9)Rh(Z6-C6Me6)][BF4] [(Z-7,8-C2B9H11)Rh(Z6-[2,2]paracyclophane)]BF4 [(Z-7,8-Me2-7,8-C2B9H9)Rh(Z6-[2,2]paracyclophane)] BF4 [Cp Rh(Z-9-SMe-7,8-C2B9H10)]

15.88, 15.24, 11.38, 7.73, 2.83, 0.38, −4.78, −8.55, −17.1 11.68, 6.27, 1.35, −1.9, −7.28, −10.12, −13.37, −14.15, −15.32 4.85, 1.05, −2.08, −8.06, −10.50, −13.31, −16.15, −18.07 3.58, 2.58, 0.19, −7.05, −11.44, −12.0, −14.1 3.1, −1.85, −3.57, −6.2, −10.26, −14.54, −16.46, −18.03 11.05, 2.1, −6.03, −6.68, −15.91, −22.50 10.4, 2.84, −5.46, −6.79, −16.02, −22.40 9.80, 4.06, −4.82, −6.87, −16.08, −22.27 9.80, 3.54, −4.92, −6.82, −16.06, −22.23 8.48, 4.02, −3.82, −6.89, −16.20, −21.94 16.69, 6.20, 0.48, −4.99, −6.79, −22.11 9.21, 3.93, −4.96, −6.92, −16.07, −22.14 10.0, 1.65, −6.48, −7.45, −16.50, −23.40 − − 16.5, 13.7, 7.9, 4.4, 0.2, −1.5, −3.7, −9, −11.9, −18.1 14.9, 6.7, 5.5, −0.5, −1.4, −3.9, −10.3, −12.1, −18.6 16.2, 14.1, 5.9, 4.3, −0.9, −1.4, −4.1, −10.4, −12.0, −18.7 15.8, 13, 6.6, 5.8, −1.5, −2, −4.3, −10.7, −12.2, −18.7 15.5, 11.5, 7.5, 3.7, −1.5, −3, −4.5, −10.6, −12.1, −20 14.5, 13.0, 6.4, 4.3, −0.7, −1.5, −4.4, −10.7, −12.7, −18.8 11.7, 1.6, 0.1, −1.5, −4.6, −8.3, −15.2, −16.7, −21.6 10.5, 3.1, −0.6, −1.5, −3.7, −5.0, −8.4, −15.4, −16.7, −21.5 10.0, 4.4, −1.0, −1.5, −5.3, −8.5, −15.5, −16.5, −21.6 9.3, 4.0, −0.9, −1.5, −2.2, −5.2, −8.4, −15.5, −16.9, −21.6 8.1, 3.8, −1.5, −2.5, −5.8, −7.1, −8.6, −15.7, −17, −21.1 10.4, 1.2, −0.9, −1.5, −5.2, −8.8, −15.7, −17, −21.8 18.2, 10.5, 4.87, −1.06, −2.88, −11.98, −18.89 17.62, 12.66, 8.27, −1.09, −4.46, −8.89 13.56, 1.79, −0.96, −3.13, −11.90, −20.09 14.55, 12.73, 5.69, −0.77, −4.07, −8.08, −11.15

1063

[Cp Ir(Z-9-SMe-7,8-C2B9H10)]

1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082

[3-Co(Z5-Cp)-8-Cl-1,2-C2B9H10] [3-Co(Z5-Cp)-8-Br-1,2-C2B9H10] [3-Co(Z5-Cp)-8-I-1,2-C2B9H10] [3-Co(Z5-NC4H4)-1,2-(CH3)2-1,2-C2B9H9] [3-Co(Z5-NC4H4)-1,2-(m-CH2)3-1,2-C2B9H9] [2-(Z-Cp)-2,1,7-CoC2B9H11] [2-(Z-Cp)-2,1,12-CoC2B9H11] [1-(4’-F3CC6F4)-2,8-Cp 2-12-THF-2,8,1-Co2CB9H8] [1-(4’-F3CC6F4)-2-Ph-8-(Z-C5H5)-8,1,2-CoC2B9H9] [(Z-Cp)CoC2B9H12][C6H5CH2NEt3] [HNMe3][(Z-Cp)CoC2B9H12] [8-(Z-Cp)-8,1,2-CoC2B9H11] [2-(Z-Cp)-2,1,8-CoC2B9H11] [1-COD-8-SMe2-1,2,8-IrC2B9H10] [1-COD-8-SMe2-1,2,8-RhC2B9H10] [1,1-Cl2-8-SMe2-1,2,8-IrC2B9H10]2 [1,1-Br2-8-SMe2-1,2,8-IrC2B9H10]2 [1,1-I2-8-SMe2-1,2,8-IrC2B9H10]2 [1-Cp-8-SMe-1,2,8-IrC2B9H10]

1083 1084 1085

[(Z-1-tBuNH-1,7,9-C3B8H10)Ir(COD)] [(Z-1-tBuNH-1,7,9-C3B8H10)IrI2]2 [(Z-1-tBuNH-1,7,9-C3B8H10)IrCl2]2

B NMR (ppm)

−24.19, −21.60, −17.48, −9.06, −8.34, −3.57, −2.60, 5.12, 8.50 −26.75, −25.35, −20.97, −12.74, −11.78, −9.33, −3.03, −0.86, 0.53 18.8, 0.2, −3.6, −6.4, −17.4, −26.2 12.5, 2.3, −2.0, −4.3, −15.4, −23.9 3.6, −0.5, −2.2, −4.3, −14.6, −22.2 5.5, 4.1, −0.4, −5.8, −11.6 8.2, 7.0, −3.2, −6.5, −11.6 −2.15, −2.93, −9.49, −12.26, −13.95, −17.44 −3.39, −4.69, −6.66, −15.93, −19.17 27.15, 19.17, 17.32, 4.04, 0.57, −18.74 8.6, 4.1, −0.3, −9.0, −11.3 15.0, 11.7, 7.7, 4.7, −6.6, −24.4, −26.2 13.8, 10.8, 7.8, 5.3, −7.0, −25.3, −27.1 5.1, −3.6, −5.6, −10.8, −16.3 0.9, −1.5, −3.1, −8.2, −9.1, −12.0, −17.9, −19.3 −7.47, −10.03, −12.61, −13.33, −22.12, −23.43, −25.52 −6.3, −8.2, −11.1, −14.9, −16.2, −21.7, −22.9, −24.9 −0.55, −11.79, −21.50 1.42, −10.21, −19.87 1.29, –9.13, –19.06 0.94, −7.59, −8.59, −9.7, −11.36, −11.74, −18.4, −22.56, −21.92 −22.8, −17.4, −15.3, −11.8 −21.3, −17.2, −6.6 −20.7, −18.8, −16.9, −4.8, −2.0

Av. MdB (Å)

Ref.

− 2.177a

297 266

− 2.18a − − − − − 2.193b 2.182b − − − − − − − − − − − −

266 266 266 305 305 305 305 305 305 305 305 305 305 306 306 306 306 306 306 306 306

− − − 2.187b 2.163a − 2.176a −

306 306 306 306 307 307 307 307

2.196a

381

2.198b

381

2.062c − 2.081c 2.059c 2.069c 2.018c 2.043c 2.064c 2.047c 2.179c − 2.053c 2.054c 2.193b 2.183a − − − 2.157b

308 308 308 309 309 263 263 310 264 264 264 264 264 311 265 265 265 265 265

− 2.145b −

312 312 312

335

Polyhedral Metallaboranes and Metallacarboranes

Table 14 No.

(Continued)

Compounds [(Z-1- BuNH-1,7,9-C3B8H10)IrBr2]2 [(Z-1-tBuNH-1,7,9-C3B8H10)RhBr2]2 [(Z-1-tBuNH-1,7,9-C3B8H10)RhI2]2 [(Z-1-tBuNH-1,7,9-C3B8H10)2Rh2(m-I)3]PF6 [Cp Ir(Z-9-SMe2-7,8-C2B9H10)]+ PF–6 [(Z1,Z3-cyclooctenediyl)Co(Z-9-SMe2-7,8-C2B9H10)] [(Z1,Z3-cyclooctenediyl)Co(Z-1-tBuHN-1,7,9-C3B8H10)] [(Z-7,8-C2B9H11)Rh(Z-C5H5BMe)] [(9-SMe2-7,8-C2B9H10)Rh(Z5-C4H4BPh)]

1095

[(9-SMe2-7,8-C2B9H10)Rh(m-Z5:Z6-C4H4BPh) RhCp ](BF4)2 [(9-SMe2-7,8-C2B9H10)Rh(m-Z5:Z5-C4H4BPh) IrCp ](BF4)2 [(9-SMe2-7,8-C2B9H10)Rh(m-Z5:Z5-C4H4BPh)(9SMe2-7,8-C2B9H10)Rh]2+ [(9-SMe2-7,8-C2B9H10)Rh(m-Z5:Z6-C4H4BPh)(9SMe2-7,8-C2B9H10)Ir]2+ [1-(1’-1’,2’-C2B10H11)-8-(Z-C5H5)-8,1,2-CoC2B9H10] [1-(1-1,2-C2B10H11)-3-Cp-3,1,2-CoC2B9H10] [8-(1-1,2-C2B10H11)-2-Cp-2,1,8-CoC2B9H10]

1097 1098 1099 1100 1101

1106

a-[1-(8’-2’-Cp-2’,1’,8’-CoC2B9H10)-3Cp-3,1,2-CoC2B9H10] b-[1-(8’-2’-Cp-2’,1’,8’-CoC2B9H10)-3Cp-3,1,2-CoC2B9H10] [8-(8’-2’-Cp-2’,1’,8’-CoC2B9H10)-2Cp-2,1,8-CoC2B9H10] rac-[1-(1’-3’-Cp-3’,1’,2’-CoC2B9H10)-3Cp-3,1,2-CoC2B9H10] [HNMe3][8-(7’-7’,8’-C2B9H11)-2-Cp-2,1,8-CoC2B9H10]

1107 1108

[1-(1’-1’,2’-C2B10H11)-3-Cp -3,1,2-CoC2B9H10] [8-(1’-1’,2’-C2B10H11)-2-Cp -2,1,8-CoC2B9H10]

1109

[HNMe3][8-(7’-7’,8’-C2B9H11)-2-Cp -2,1,8-CoC2B9H10]

1110

[8-(1’-3’-(p-cym)-3’,1’,2’-RuC2B9H10)-2-Cp 2,1,8-CoC2B9H10] [8-(8’-2’-(p-cym)-2’,1’,8’-RuC2B9H10)-2-Cp 2,1,8-CoC2B9H10] [8-(1’-1’,2’-C2B10H11)-2-H-2,2(PPh3)2-2,1,8-RhC2B9H10] a-[8-{8’-2-(p-cym)-2’,1’,8’-RuC2B9H10}-2-H-2,2(PPh3)2-2,1,8-RhC2B9H10] b-[8-{8’-2-(p-cym)-2’,1’,8’-RuC2B9H10}-2-H-2,2(PPh3)2-2,1,8-RhC2B9H10] a-[8-(8’-2-Cp -2’,1’,8’-CoC2B9H10)-2-H-2,2(PPh3)2-2,1,8-RhC2B9H10] b-[8-(8’-2-Cp -2,1’,8’-CoC2B9H10)-2-H-2,2(PPh3)2-2,1,8-RhC2B9H10] 8,9’-[{3-Co(Z5-C5H5)-1,2-C2B9H10}]2 [(Z-C5H5)2Co][2,2’-Co(1,12-C2B9H11)2]– [3,3’-Co(8-SMe2-1,2-C2B9H10)2]Cl [3,3’-Co(8-SMe2-1,2-C2B9H10)2] [NMe4][3,3-Co(1,2-C2B9H11)2] [NMe4][8-C8H7-3,3’-(1,2-C2B9H10)(1’,2’-C2B9H11)]

1102 1103 1104 1105

1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125

Av. MdB (Å)

Ref.

−20.7, −18, −16.9, −5.2, −3 − − − −25.2, −21.9, −21, −13.3, −10.1, −6.8, −6, −2.1, −1.3 −2.34, −4.28, −8.32, −12.4, −15.79, −23.28, −26.12 −10.15, −12.75, −16.93, −17.42, −21.94 −22.4, −15.7, −5.4, −2.7, 8.0, 9.9, 28.8 −25.5, −23.0, −21.1, −14.0, −10.2, −9.0, −6.9, −4.3, 0.4, 20.9 −25.1, −22.4, −21.2, −14.2, −9.7, −8, −4.1, −1.4, 1, 12.5

− 2.135a 2.154a − 2.196b − 2.123c 2.241a −

312 313 313 313 314 315 315 316 317

−

317

−20.8, −15.9, −14.0, −7.2, −3.5, −1.4, 2.9, 8.5, 13.3

317

−21.2, −15.2, −13.4, −6.7, −3.0, −1.4, 3.4, 9.7, 13.9

2.261(4)b, 2.294a −

−25.1, −16.7, −12.2, −6.9, −5.1, −2.2, −1.4 , 6.0, 14.2

−

317

6.2, 4.5, −2.3, −5.3, −9.9, −12.9, −14.7 6.5, 2.5, −2.6, −4.4, −8.0, −9.7, −12.3, −14.2, −15.9 1.7, 0.0, −0.9, −2.5, −3.8, −6.1, −9.9, −11.8, −13.2, −16.8, −17.7 5.5, 2.5, 1.7, −1.4, −3.7, −8.5, −10.2, −12.0, −14.6, −15.8, −18.2 5.4, 2.4, 1.7, −1.7, −3.8, −8.5, −10.4, −12.3, −14.6, −18.2

2.050c 2.084c 2.049c

264 318 318

2.062c

272

2.063c

272

1.1, −0.9, −5.7, −12, −16.8, −18.0

2.036c

272

5.2, 1.2, −2.5, −3.3, −7.6, −11.7, −13.2, −14.2

2.078c

272

−0.4, −1.2, −5.9, −9.6, −10.1, −12.7, −14.1, −17.3, −18.3, −20.4, −24.3, −33.3, −35.7 10.1, −1.9, −3.8, −6.0, −9.6, −12.4, −15.0 3.2, 1.1, −2.6, −4.1, −4.9, −7.2, −9.9, −13.3, −13.9, −18.0, −18.6 1.3, −4.2, −7.1, −9.4, −10.1, −13.9, −17.2, −18.0, −19.1, −20.5, −24.3, −33.4, −35.7 2.7, 1.9, 0.3, −3.8, −6.1, −7.6, −13.1, −14.8, −17.2, −19.6

−

273

2.082c 2.065c

273 273

−

273

−

273

2.17d, 2.038c

273

2.198a

319

2.215a, 2.182d 2.216a, 2.185d 2.236a, 2.049c 2.228a, 2.057c 2.092c 2.073c 2.107c 2.146c 2.097c 2.099c

319

2.109c − 2.112c

322 322 323

B NMR (ppm)

t

1086 1087 1088 1089 1090 1091 1092 1093 1094

1096

11

[NMe4][3,3’-Co(8-nBu-1,2-C2B9H10)(1’,2’-C2B9H11)] [NMe4][3,3’-Co(8-Ph-1,2-C2B9H10)(1’,2’-C2B9H11)]

2.4, 1.0, −1.7, −4.9, −6.6, −7.9, −14.3, −16.2, −19.1, −20.5 −0.8, −3.3, −4.4, −6.1, −10.5, −13.1, −17.4, −19.1 0.2, −1.7, −5.2, −8.6, −15.7, −16.5, −21.0 0.0, −5.1, −8.5, −15.8, −16.6, −21.1 0.9, −6.0, −8.5, −14.7, −15.5, −19.6, −22.0 0.8, −6.7, −8.4, −14.9, −15.5, −19.0, −21.9 18.0, 8.7, 4.1, −1.9 to −3.6, −14.3 to −16.1, −22.0 −3.38, −5.02, −6.41, −16.84, −19.79 9.7, 4.3, −5.4, −13.8, −19.8 62.8, 51.2, 11.8, −30.6, −34.4 − 13.09, 7.13, 1.83, −4.14, −4.74, −5.71, −16.23, −17.46, −21.48, −23.04 18.13, 7.74, 1.18, −4.69, −5.84, −16.89, −21.84, −24.41 −

317

319 319 319 320 263 275 275 382 321

(Continued )

336

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

Compounds

11

1128 1129 1130

[NMe4][3,3’-Co(8-(p-C6H4C2H3)-1,2-C2B9H10)-(1’,2’C2B9H11)] [NMe4][3,3’-Co(8-(p-C6H4CHO)-1,2-C2B9H10)-(1’,2’C2B9H11)] [NMe4][3,3’-Co(8-(m-C6H4CHO)-1,2-C2B9H10)-(1’,2’C2B9H11)] [NMe4][Co(C2B9H11)(8-C10H7-C2B9H10)] [NMe4][Co(C2B9H11)(8-C10H7-8’-C19H15-C2B9H10)] [NMe4][Co(C2B9H11)(8’-o-C19H15-C2B9H10)]

1131 1132 1133 1134 1135

[NMe4][Co(C2B9H11)(8’-m-C19H15-C2B9H10)] [NMe4][Co(C2B9H11)(8-o-C19H15-8’-o-C19H15-C2B9H10)] [NMe4][Co(C2B9H11)(8-m-C19H15-8’-m-C19H15-C2B9H10)] [NMe4][Co(C2B9H11)(8-o-C19H15-8’-m-C19H15-C2B9H10)] Cs[Co(C2B9H11)(8’-m-C13H11-C2B9H10)]

1136 1137 1138 1139 1140

Cs[Co(C2B9H11)(8’-o-C13H11-C2B9H10)] Cs[Co(C2B9H11)(8’-o-C13H11-8’-o-C13H11-C2B9H10)] Cs[Co(C2B9H11)(8’-m-C13H11-8’-m-C13H11-C2B9H10)] Cs[Co(C2B9H11)(8’-o-C13H11-8’-m-C13H11-C2B9H10)] [Li2(DME)][1”,2”-{3,3’-Co(8-O(CH2CH2O)2-1,2-C2B9H10) (1’,2’-C2B9H11)}2-C6H4] [NMe4]2[1”,3”-{3,3’-Co(8-O(CH2CH2O)2-1,2-C2B9H10) (1’,2’-C2B9H11)}2-C6H4] [NMe4]2[1”,4”-{3,3’-Co(8-O(CH2CH2O)2-1,2-C2B9H10) (1’,2’-C2B9H11)}2-C6H4] [Li(DME)]3[1”,3”,5”-{3,3’-Co(8O(CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)}3-C6H3] Na3[1”,3”,5”-{3,3’-Co(8-O(CH2CH2O)2-1,2-C2B9H10) (1’,2’-C2B9H11)}3-C6H3] [NMe4][3,3’-Co(8-O(CH2CH2O)2CH2C6H5-1,2-C2B9H10) (1’,2’-C2B9H11)] [Li(DME)]2[1”,4”-{3,3’-Co(8O(CH2CH2O)2CH2-1,2-C2B9H10)(1’,2’-C2B9H11)}2-C6H4] [NMe4][3,3’-Co(8-O(CH2CH2O)2C(O)C6H5-1,2-C2B9H10) (1’,2’-C2B9H11)] Na(H2O)][3,3’-Co(8-O(CH2CH2O)2C(O)CH3-1,2-C2B9H10) (1’,2’-C2B9H11)] Na[1”-{3,3’-Co(8-O(CH2CH2O)2C(O)-1,2-C2B9H10)(1’,2’C2B9H11)}-2”-OH-C6H4] Na3[C12H24O6][1”,3”,5”-{3,3’-Co(8-O(CH2CH2O)2CO1,2-C2B9H10)(1’,2’-C2B9H11)}3-C6H3] Cs[3,3’-Co(8-(OCH2CH2)2CH3-1,2-C2B9H10)(1’,2’C2B9H11)] [NEt3H+][3,3’-Co(8-(OCH2CH2)2C3H5-1,2-C2B9H10)(1’,2’C2B9H11)] Li(THF)2[1”-{3,3’-Co(8-(OCH2CH2)2S-1,2-C2B9H10)(1’,2’C2B9H11)}-2”-CH3-1”,2”-C2B10H10)] [NMe4]2[1”,2”-{3,3’-Co(8-(OCH2CH2)2S-1,2-C2B9H10) (1’,2’-C2B9H11)}2-(1”,2”-C2B10H10)] [1”-3,3’-Co-8-(CH2CH2O)2-(1,2-C2B9H10)-1’,2’(C2B9H11)-1”,2”-C2B10H11]− [1”-3,3’-Co-8-(CH2CH2O)2-(1,2-C2B9H10)-1’,2’(C2B9H11)-1”,7”-C2B10H11]− [1”-3,3’-Co-8-(CH2CH2O)2-(1,2-C2B9H10)-1’,2’(C2B9H11)-1”,12”-C2B10H11]− [1”,12”-(8-CH2CH2OCH2-CH2O-1,2-C2B9H10-3,3’Co-1’,2’-C2B9H11)2-1”,12”-C2B10H10]−

13.19, 6.07, 2.44, −2.86, −4.50, −5.90, −16.46, −17.70, −21.10, −22.23 13.48, 2.86, −2.74, −3.73, −5.21, −15.86, −17.14, −21.16, −22.21 14.01, 8.26, 2.94, −2.71, −3.37, −5.1, −15.73, −17.04, −20.87, −22.09 − 11.1, 2.0, −5.2, −19.1, −22.48 12.2, 4.5, 1.7, −3.4, −5.6, −6.9, −17.4, −18.7, −21.8, −23.0 − 10.9, 2, −5.3, −19.2, −22.6 − − 12.5, 4.7, 1.9, 1.4, −3.4, −5.5, −7.0, −17.4, −18.8, −21.8, −23.2 − 10.9, 1.9, −5.3, −19.1, −22.8 − − 25.5, 6.4, 2.8, −0.2, −2, −5, −5.9, −15.1, −18.2, −19.5, −26.3 25.3, 6.3, 2.8, −0.1, −1.8, −5.1, −5.8, −14.9, −18, −19.5, −26 25.2, 6.2, 2.8, −0.1, −1.7, −5.1, −5.8, −14.9, −18.0, −19.5, −26.0 25.3, 6.3, 2.8, −0.1, −1.9, −5.1, −5.8, −14.9, −18.1, −19.6, −26.1 23.2, 4.3, 0.45, −2.4, −4.4, −8.0, −17.2, −20.4, −22.3, −28.5 25.0, 5.9, 2.7, −0.2, −1.9, −5.2, −6, −15, −18.2, −19.5, −26.2 25.2, 6.2, 2.7, −0.2, −1.9, −5.1, −5.9, −15.0, −18.2, −19.5, −26.2 30.3, 11.3, 7.9, 5.0, 3.3, 0.0, −0.8, −9.8, −12.9, −14.4, −21.0 30.5, 11.5, 7.9, 5.1, 3.3, −0.1, −0.7, −9.7, −12.8, −14.3, −20.9 25.3, 6.3, 2.6, −0.2, −2.1, −5.2, −6.1, −15.1, −18.3, −19.8, −26.5 23.6, 4.7, 1.1, −1.8, −3.5, −6.7, −7.5, −16.5, −19.7, −21.2, −27.6 25.2, 6.3, 2.7, −0.2, −1.9, −5.1, −5.9, −14.9, −18.0, −19.5, −26.1 22.8, 3.7, −0.4, −2.5, −4.1, −7.5, −8.3, −17.3, −20.5, −21.9, −28.4 25.1, 6.3, 2.7, −0.2, −1.9, −2.9, −5.1, −6.1, −7.6, −14.9, −18.1, −19.6, −26.1 25.0, 6.1, 2.7, −0.2, −1.8, −5.2, −6.0, −7.8, −10.9, −14.9, −18.1, −19.5, −26.2 23.2, 4.5, 0.4, −2.7, −2.8, −4.5, −5.9, −7.3, −7.9, −9.9, −10.7, −11.9, −12.7, −17.3, −20.4, −22.1, −28.4 22.7, 3.7, 0.5, −2.4, −4.1, −7.4, −8.2, −9.2, −10.9, −13.5, −14.9, −16.6, −17.3, −20.4, −21.8, −28.3 22.5, 3.5, 0.5, −2.4, −4.1, −7.5, −8.3, −12.4, −15.1, −17.3, −20.5, −21.8, −28.4 23.5, 3.5, 0.4, −2.4, −4.1, −7.5, −8.3, −12.7, −17.3, −20.5, −22.0, −28.4

No.

1126 1127

1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158

B NMR (ppm)

Av. MdB (Å)

Ref.

2.111c

323

−

323

− − −

324 324 324

− − − − −

324 324 324 324 324

− − − − −

324 324 324 324 325

−

325

−

325

−

325

2.092c

325

−

325

−

325

2.101c

325

2.105c

325

−

325

−

325

−

325

−

325

−

325

−

325

−

326

−

326

−

326

−

326

337

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

No.

Compounds

11

1159

Na[3,3’-Co(8-(OCH2CH2)2F-1,2-C2B9H10)(1’,2’-C2B9H11)]

1160

Cs[3,3’-Co(8-(OCH2CH2)2Cl-1,2-C2B9H10)(1’,2’C2B9H11)] [NMe4][3,3’-Co(8-(OCH2CH2)2Br-1,2-C2B9H10)(1’,2’C2B9H11)] [NMe4][3,3’-Co(8-(OCH2CH2)2I-1,2-C2B9H10)(1’,2’C2B9H11)] [NMe4][3,3’-Co(8-(OCH2CH2)2Br-1,2-C2B9H10)-(8’I-1’,2’-C2B9H10)] [NMe4][3,3’-Co(8-(OCH2CH2)2I-1,2-C2B9H10)-(8’-I-1’,2’C2B9H10)] [NMe4][3,3’-Co((8-(OCH2CH2)2SH)-1,2-C2B9H10)-(1’,2’C2B9H11)] [NMe4][3,3’-Co(8-(OCH2CH2OH)-1,2-C2B9H10)-(1’,2’C2B9H11)] [NMe4][1,1’-m-PPh-3,3’-Co(1,2-C2B9H10)2]

24.5, 5.9, 1.3, −1.6, −3.7, −6.3, −7.4, −16.4, −19.5, −21.2, −27.7 25.4, 6.6, 2.7, −0.2, −2, −5.0, −5.9, −15, −18.1, −19.7, −26.2 23.7, 4.8, 1.3, −1.6, −3.4, −6.6, −7.5, −16.4, −19.6, −21.1, −27.6 23.7, 4.7, 1.3, −1.6, −3.4, −6.6, −7.5, −16.4, −19.6, −21.1, −27.6 22.4, 0.3, −3.8, −4.9, −6.5, −6.5, −17.1, −19.2, −22.4, −26.6 22.4, 0.4, −3.9, −4.9, −6.4, −6.4, −17.1, −19.2, −22.4, −26.5 22.8, 3.7, 0.5, −2.4, −4.1, −7.5, −8.2, −17.2, −20.4, −21.8, −28.4 24.31, 5.45, 1.07, −1.84, −3.69, −6.57, −7.30, −16.57, −19.57, −21.40, −27.94 8.3, 6.3, 2.2, −1.0, −2.5, −3.1, −4.6, −6.3, −14.6, −16.3, −20.4, −22.7 8.7, 2.6, −0.2, −1.3, −3.2, −4.7, −15.2, −17.1, −22.5 8.2, 2.1, −0.4, −1.7, −3.2, −4.8, −15.6, −17.5, −22.7 8.1, 2.1, −0.3, −1.8, −3.4, −4.8, −15.6, −17.3, −22.3 26.3, 23.9, 1.9, 0.2, −1.3, −5.4, −7.0, −12.8, −13.8, −15.5, −26.3 26.7, 0.7, −3.9, −5.7, −13.7, −26.6

1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175

[NMe4][1,1’-m-OPPh-3,3’-Co(1,2-C2B9H10)2] [NMe4][1,1’-m-SPPh-3,3’-Co(1,2-C2B9H10)2] [NMe4][1,1’-m-SePPh-3,3’-Co(1,2-C2B9H10)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-PPh-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-OPPh-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-C6H4)-1,1’-m-SPPh-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-SePPh-3,3’Co(1,2-C2B9H9)2] [NMe4][1,1’-m-PtBu-3,3’-Co(1,2-C2B9H10)2]

1183 1184

[NMe4][1,1’-m-OPtBu-3,3’-Co(1,2-C2B9H10)2] [NMe4][1,1’-m-SPtBu-3,3’-Co(1,2-C2B9H10)2] [NMe4][1,1’-m-SePtBu-3,3’-Co(1,2-C2B9H10)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-PtBu-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-OPtBu-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-SPtBu-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-Ph)-1,1’-m-SePtBu-3,3’Co(1,2-C2B9H9)2] [NHMe3][3,3’-Co-(9-I-1,2-C2B9H10)2] [NMe4][3,3’-Co-(4-I-1,2-C2B9H10)2]

1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197

[NMe4][3,3’-Co-(1-Me-9,12-I2-1,2-C2B9H8)2] [NMe4][3,3’-Co-(1-Ph-9,12-I2-1,2-C2B9H8)2] [NMe4][3,3’-Co-(4,7-I2-1,2-C2B9H9)2] [NMe4][3,3’-Co-(1-Me-8,9,10,12-I4-1,2-C2B9H6)2] [NMe4][3,3’-Co-(1-Ph-8,9,10,12-I4-1,2-C2B9H6)2] [NMe4][3,3’-Co-(4,9,12-I3-1,2-C2B9H8)2] [NMe4][3,3’-Co-(9,12-Me2-1,2-C2B9H9)2] [NMe4][3,3’-Co-(8,9,10,12-Me4-1,2-C2B9H7)2] Na[NMe4][3,3’-Co-(8,9,10,12-I4-1,2-C2B9H7)2]
2.5H2O H[3,3’-Co-(1,2-C2B9H11)2] Li[3,3’-Co-(1,2-C2B9H11)2] Na[3,3’-Co-(1,2-C2B9H11)2] K[3,3’-Co-(1,2-C2B9H11)2]

1176 1177 1178 1179 1180 1181 1182

B NMR (ppm)

Av. MdB (Å)

Ref.

−

327

−

327

2.093c

327

2.108c

327

−

327

−

327

−

327

−

327

−

328

2.110c 2.109c 2.110c 2.089c

328 328 328 328

−

328

26.2, 0.2, −4.4, −5.9, −14.2, −26.5

−

328

26.3, 0.2, −4.4, −5.9, −14.1, −26.3

−

328

8.2, 5.2, 2.1, 0.5, −1.1, −2.6, −3.4, −5.1, −6.6, −14.8, −16.6, −20.8, −22.8 8.6, 2.6, −1.2, −3.1, −5.5, −15.5, −16.6, −17.5, −22.2 8.5, 2.5, −1.5, −5.5, −15.6, −17.7, −22.5 7.4, 2.1, −1.6, −5.7, −6.5, −15.8, −17.7, −22.4 26.1, 23.4, 3.6, 0.1, −2.4, −3.5, −5.6, −13.0, −13.9, −14.45, −15.4, −26.7 26.3, 23.7, 0.6, −2.3, −4.1, −6.0, −14.4, −26.6

−

328

− − − −

328 328 328 328

−

328

26.0, 23.5, 0.4, −4.5, −5.9, −14.3, −26.2

−

328

25.7, 23.2, 0.2, −4.7, −6.0, −14.4, −26.3

−

328

7.3, 4.0, −4.4, −16.9, −21.9 11.2, 7.8, 4.1, −0.6, −1.8, −2.7, −3.9, −11.1, −13.2, −13.8, −15.1, −16.2, −21.1 8.9, 5.0, −2.7, −4.2, −8.8, −12.9, −14.9 9.4, 5.6, −3.7, −9.4, −14.6 14.9, 4.7, 3.4, −0.8, −9.1, −11.4, −12.9, −20.6 −0.5, −1.3, −3.2, −4.9, −7.4, −9.4, −11.7, −13.2 −1.89, −6.7, −11.3, −14.02 12.0, 7.5, −1.1, −10.0, −10.7, −13.4, −14.1, −19.2 10.9, 4.1, −3.7, −16.9, −22.1 12.8, 9.9, 1.9, −5.0, −17.2, −24.0 28.4, 20.7, −55.1, −95.7 − − − −

− −

383 383

− − − − − − − − − − − − −

383 383 383 383 383 383 383 383 383 384 384 384 384 (Continued )

338

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

No.

Compounds

1198 1199 1200

Na[3,3’-Co(8-I-1,2-C2B9H10)(1,2-C2B9H11)] Na[3,3’-Co(8-I-1,2-C2B9H10)2] [NMe4][3,3’-Co(1,2-C2B9H11)2]

1201

[NMe4][3,3’-Co(1-Me-1,2-C2B9H10)2]

1202

[NMe4][3,3’-Co(1-Ph-1,2-C2B9H10)2]

1203

[NMe4][3,3’-Co(1-Et-2-Me-1,2-C2B9H10)2]

1204

[NMe4][3,3’-Co(1,2-Et-1,2-C2B9H10)2]

1205 1206

[NMe4][3,3’-Co(1-SEt-2-Me-1,2-C2B9H10)2] [NMe4][1-SiMe2H-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1207 1208

Cs[1,1’-m-SiMe2-3,3’-Co(1,2-C2B9H10)2] Cs[1-SiMe3-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1209

Cs[1,1’-(SiMe3)2-3,3’-Co(1,2-C2B9H10)2]

1210

1219

[NMe4][8,8’-m-(1”,2’-C6H4)-1,1’-m-SiMe2-3,3’Co(1,2-C2B9H9)2] Cs[8,8’-m-(1”,2”-C6H4)-1,1’-m-SiMeH-3,3’Co(1,2-C2B9H9)2] [NMe4][8,8’-m-(1”,2”-C6H4)-1-SiMe3-3,3’Co(1,2-C2B9H9)(1’,2’-C2B9H10)] Cs[1,1’-m-(HO)(O)P(1,2-C2B9H10)2-3,3’-Co] (Et4N)[8-(4”-MeOC6H4)-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] (Et4N)[8-Et-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] (Et4N)[8-(2”-C4H3S)-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] 8-[HC^C-CH2-N+ Me2-(CH2CH2O)2]-3,3’Co(1,2-C2B9H10)(1’,2’-C2B9H11)] 8-[HC^C-CH2-N+ Me2-(CH2)5O]-3,3’-Co(1,2-C2B9H10) (1’,20 -C2B9H11)] [80 -SMe2-1,1’-Ir-(2,3-C2B9H11)(2’,8’-C2B9H10)]

1220 1221

[8’-I-8-(S+ CH2CH2O)-3,3’-Co(1,2-C2B9H10)2] [8’-I-8-(ONC5H5)-3,3’-Co(1,2-C2B9H10)2]

1222 1223 1224 1225 1226 1227 1228 1229

[8’-I-8-(+ OC5H10)-3,3’-Co(1,2-C2B9H10)2] Cs[8-I-8’-OH-3,3’-Co(1,2-C2B9H10)2] Cs[8-I-8’-OMe-3,3’-Co(1,2-C2B9H10)2] Cs[8-OBu-8’-I-3,3’-Co(1,2-C2B9H10)2] Cs[8-OH-8’-OBu-3,3’-Co(1,2-C2B9H10)2] Cs[3,3’-Co(8-OBu-1,2-C2B9H10)2] Cs[8-I-8’-OEt-3,3’-Co(1,2-C2B9H10)2] Cs[8-OCH2C^CH-8’-I-3,3’-Co(1,2-C2B9H10)2]

1230 1231 1232

Cs[8-O(CH2)2C^CH-8’-I-3,3’-Co(1,2-C2B9H10)2] [8,8´-m-Br+-3,3’-Co(1,2-C2B9H10)2]− Cs[8-Br-8’-C6H5-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H10)]

1233

Cs[8-Br-8’-m-CH3C6H4-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H10)] Cs[8-Br-8’-3,4-(CH3)2-C6H3-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H10)] [HTMP][8-Br-8’-2,4,6-(CH3)3-C6H2-3,3’-Co(1,2-C2B9H10) (1’,2’-C2B9H10)] [8-Br-8’-S+(CH2)2O-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H10)]−

1211 1212 1213 1214 1215 1216 1217 1218

1234 1235 1236

11

Av. MdB (Å)

Ref.

− − 6.92, 5.86, 1.68, 0.62, −4.97, −5.76, −6.82, −16.91, −18.08, −22.36, −23.67 9.49, 8.03, 2.36, 0.90, −3.51, −7.98, −9.46, −11.59, −14.99, −15.61, −16.42 8.84, 3.27, 1.71, −3.14, −4.80, −6.46, −9.48, −11.04, −15.45, −17.01 7.23, 4.73, 3.74, 0.18, −1.08, −2.23, −4.46, −5.54, −6.48, −7.57, −8.39, −10.15, −11.39, −14.2, −1543, −17.46 7.6, 6.67, 1.5, 0.37, −5.71, −6.67, −10.34, −11.39, −12.44, −13.4, −14.92, −16.31 − 9.1, 7.29, 3.56, 2.5, −1.7, −4.65, −5.0, −14.25, −16.79, −21.15, −22.26 8.07, 2.7, −1.83, −3.78, −4.7, −14.29, −15.61, −22.11 8.2, 6.1, 3.53, 1.19, −1.95, −4.04, −5.6, −6.32, −7.32, −14.26, −15.15, −17.58, −18.4, −21.4, −23.06 8.25, 3.71, −2.45, −5.44, −6.52, −11.59, −13.42, −15.09, −19.66 25.93, 2.41, 1.69, −2.98, −11.76, −24.51

− − −

384 384 385

−

385

−

385

−

385

−

385

− 2.103c

385 329

2.095c −

329 329

2.103c

329

−

329

27.47, 2.56, −2.73, −11.55, −24.04

−

329

26.5, 24.5, 1.06, −1.46, −2.58, −4.64, −5.67, −10.8, −11.96, −13.67, −23.64, −25 7.67, 0.07, −3.58, −4.13, −5.72, −9.9, −11.54, −16.21 12.8, 4.8, 2.0, 1.3, −3.6, −5.2, −5.8, −7.0, −17.4, −18.8, −21.8, −23.2 18.1, 7.0, 0.4, −5.6, −6.6, −17.4, −17.9, −22.7, −25.24 8.7, 4.8, 1.8, −3.2, −5.41, −6.7, −17.2, −18.9, −21.5, −22.6 24.2, 5.9, 0.3, −2.8, −4.9, −7.5, −17.3, −20.2, −21.9, −28.6 23.7, 4.8, 0.0, −2.7, −4.8, −8.0, −17.5, −20.3, −22.4, −28.9 2.23, −2.16, −2.68, −3.43, −7.81, −8.93, −9.70, −11.35, −12.46, −18.33, −19.42, −20.01, −22.47, −24.15 6, 4.9, 1.2, −1.5, −1.6, −4, −14.9, −15.8, −20.7, −20.7 21.4, 3.1, −1.6, −3.6, −5.4, −8.9, −16.1, −18.0, −22.5, −26.5 23.2, 3.9, −4.4, −5.6, −15.3, −17.6, −22.7, −25.0 − 22.3, −0.4, −4.6, −5.8, −7.3, −17.9, −19.9, −23.4, −27.2 21.8, −0.6, −4.8, −5.8, −7.2, −18.0, −19.9, −23.4, −27.4 26.6, 25.2, −4.6, −5.7, −7.1, −9, −20.2, −29.3 20.5, −3.5, −7.3, −9.0, −20.5, −28.4 21.8, −0.5, −4.8, −5.8, −7.1, −18.0, −20.0, −23.5, −27.3 21.4, 0.1, −0.6, −4.4, −5.5, −7.3, −17.8, −19.8, −23.3, −27.1 21.7, −0.4, −4.6, −5.7, −7.1, −17.9, −20, −23.1, −27.2 2.3, −6.5, −13.9, −23.6 12.0, 6.4, 3.4, −0.2, −4.3, −5.6, −18.0, −19.1, −22.1, −24.5 12.1, 6.4, 3.4, −0.3, −4.3, −5.6, −18.1, −19.1, −22.2, −24.5 12.3, 6.4, 3.5, −0.4, −4.3, −5.6, −18.3, −19.1, −22.2, −24.6 11.0, 6.1, 1.8, −0.3, −4.6, −17.0, −19.2, −20.6, −24.2

−

329

− −

386 387

− −

387 387

2.116c

388

2.109c

388

2.204b

265

− 2.112c

389 389

− 2.118c − − − − − −

389 330 330 330 330 330 330 330

− − −

330 390 390

−

390

−

390

−

390

12.9, 6.2, 3.4, −0.8, −2.3, −5.1, −8.4, −15.9, −20.7, −24

2.119c

390

B NMR (ppm)

339

Polyhedral Metallaboranes and Metallacarboranes

Table 14 No.

(Continued) 11

Compounds

B NMR (ppm)

+

1238

[8-Br-8’-HN (CH2)2O-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H10)]− [NMe4][8-OCH3-3,3’-Co(1,2-C2B9H10)(1,2-C2B9H11)]

1239

[NMe4][8-OCH2CH3-3,3’-Co(1,2-C2B9H10)(1,2-C2B9H11)]

1240

1269 1270 1271 1272 1273

[NMe4][8-OCH2CH2CH3-3,3’-Co(1,2-C2B9H10) (1,2-C2B9H11)] [NMe4][8-OCH3-8’-OH-3,3’-Co(1,2-C2B9H10) (1,2-C2B9H11)] [NMe4][8,8’-(OCH3)2-3,3’-Co(1,2-C2B9H10)2] [NMe4][8-OCH2CH3-8’-OH-3,3’-Co(1,2-C2B9H10) (1,2-C2B9H11)] [NMe4][8,8’-(OCH2CH3)2-3,3’-Co(1,2-C2B9H10)2] [NMe4][8-OCH2CH2CH3-8’-OH-3,3’-Co(1,2-C2B9H10) (1,2-C2B9H11)]− [NMe4][8,8’-(OCH2CH2CH3)2-3,3’-Co(1,2-C2B9H10)2] (Bu4N)[4,7’-(MeO)2-3,3’-Co(1,2-C2B9H10)2] (BPDT-TTF)[3,3’-Сo(1,2-C2B9H11)2] (BEDT-TTF)2[3,3’-Co(1,2-C2B9H11)2] (BEDT-TTF)2[8-I-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] (BMDT-TTF)[3,3’-Co(1,2-C2B9H11)2] (TTF)[8,8’-I2-3,3’-Co(1,2-C2B9H10)2] (BMDT-TTF)4[8,8’-I2-3,3’-Co(1,2-C2B9H10)2] (BEDT-TTF)2[8,8’-I2-3,3’-Co(1,2-C2B9H10)2] [8-C5H5N-8’-I-3,3’-Co(1,2-C2B9H10)2] [8-O(CH2CH2)2NH-8’-I-3,3’-Co(1,2-C2B9H10)2] (Me3NH)[8-Ph-8’-I-3,3’-Co(1,2-C2B9H10)2] Cs[8-Ph-8’-I-3,3’-Co(1,2-C2B9H10)2] Cs[8-(4-MeOC6H4)-8’-I-3,3’-Co(1,2-C2B9H10)2] [TMP][8-(2,4,6-Me3C6H2)-8’-I-3,3’-Co(1,2-C2B9H10)2] [8-Ph3P-8’-I-3,3’-Co(1,2-C2B9H10)2] [8-Ph3PO-8’-I-3,3’-Co(1,2-C2B9H10)2] (TMTTF)[8-HO-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] (BMDT-TTF)[8-HO-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] (BEDT-TTF)[8-HO-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] (BEDT-TTF)[8,8’,(7)-Cl2(Cl0.09)-3,3’-Co(1,2-C2B9H9.91) (1’,2’-C2B9H10)] (BEDT-TTF)[8-Br0.75Cl0.25-8’-Cl-3,3’-Co(1,2-C2B9H10)2] (BMDT-TTF)4[8,8’-Br1.16(OH)0.72-3,3’Co(1,2-C2B9H10.06)2] Cs[8-I-8’-(3,4-Me2C6H3)-3,3’-Co(1,2-C2B9H10)2] Cs[8-I-8’-(3,5-Me2C6H3)-3,3’-Co(1,2-C2B9H10)2] Cs[8-I-8’-(2,5-Me2C6H3)-3,3’-Co(1,2-C2B9H10)2] Cs[8-I-8’-PhO-3,3’-Co(1,2-C2B9H10)2] Cs[8,8’-m-(o-C6H4O)-3,3’-Co(1’,2’-C2B9H10)2]

1274 1275

Cs[8-I-8’-(4-Me2NC6H4)-3,3’-Co(1,2-C2B9H10)2] [8-I-8’-Et3PO-3,3’-Co(1,2-C2B9H10)2]

1276

1279

[8-Me2S(CH2CH2O)2-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] (Et3NMe)[8-MeS(CH2CH2O)2-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] Na[8-MeS(CH2CH2O)2-3,3’-Co(1,2-C2B9H10)-(1’,2’C2B9H11)] K[8-HOCH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1280

Na[8-HOCH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1237

1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268

1277 1278

Av. MdB (Å)

Ref.

c

14.5, 11.7, 2.2, −0.1, −1.8, −5.7, −7.6, −16.5

2.114

390

23.8, 4.3, 0.2, −2.6, −4.6, −7.4, −8.3, −17.4, −20.4, −22.2, −28.7 23.1, 3.9, 0.1, −2.4, −4.5, −7.5, −8.1, −17.4, −20.5, −22.3, −28.5 22.9, 3.66, 0.2, −2.5, −4.3, −7.6, −8.3, −17.5, −20.4, −22.2, −28.6 27.2, 25.4, −4.8, −5.5, −7.5, −9.1, −20.1, −20.7, −29.1, −30.2 21.1, −3.4, −7.5, −9.1, −20.6, −28.4 26.6, 24.9, −4.5, −5.5, −7.2, −8.9, −20.0, −20.7, −29.1, −30.1 20.4, −3.5, −7.3, −8.9, −20.6, −28.5 26.7, 24.8, −4.8, −5.5, −7.4, −8.86, −20.0, −20.6, −29.0, −29.8 20.4, −3.5, −7.3, −9.0, −20.6, −28.5 13.9, 5.2, −0.8, −7.9, −9.0, −19.8, −24.6 − − − − − − − − 14.1, 3.5, 0.2, −0.9, −2.9, −4.6, −7.4, −15.5, −16.4, −22.4 12.3, 3.8, 0.9, −3.9, −4.8, −6.1, −18.1, −22.2, −22.9 12.7, 4.2, 1.3, −3.5, −4.4, −5.7, −17.7, −21.8, −22.6 12.6, 3.9, 0.6, −3.9, −4.9, −18.3, −22.9 11.3, 2.1, 0.7, −4.2, −17.1, −18.4, −20.7, −22.7 4.2, −1.2, −2.0, −4.5, −7.0, −14.5, −15.6, −18.0, −22.9 19.9, 2.3, −0.6, −3.8, −7.4, −16.2, −18.2, −23.0, −26.4 − − − −

−

289

−

289

−

289

−

289

2.120c −

289 289

− −

289 289

− − 2.064c 2.099c 2.115c 2.087c 2.148c 2.142c 2.158c − 2.094c 2.127c − − − 2.137c 2.109c 2.097c 2.096c 2.076c 2.122c

289 276 391 281 281 281 392 392 392 393 393 393 393 393 393 393 393 394 394 394 395

− −

2.116c 2.048c

395 395

− − 2.129c − −

396 396 396 396 396

− 2.114c

396 396

−

397

−

397

−

397

−

397

−

397

12.6, 3.8, 0.6, −4.1, −6.3, −18.4, −22.8, −23.1 12.5, 3.7, 0.7, −4.2, −6.3, −18.2, −22.7, −23.1 12.4, 3.1, 0.7, −3.7, −4.7, −6.4, −18.1, −22.1, −23.0 20, 0.2, −0.2, −5.3, −17.7, −19.8, −23.1, −26.4 26.7, 14.7, −2.8, −3.7, −4.8, −7.7, −10.7, −18.2, −20.4, −25.2, −28.9 11.7, 3.8, 0.9, −4.7, −5.9, −18.1, −22.4, −23.1 18.8, 2.2, −0.9, −3.6, −5.8, −7.4, −16.4, −18.4, −22.9, −26.7 24.2, 5.9, 0.3, −2.8, −4.8, −7.0, −7.6, −8.8, −17.4, −20.2, −22.4, −28.8 27.9, 8.7, 5.6, 2.8, 1.0, −2.3, −3.2, −12.1, −15.3, −16.7, −23.3 23.0, 4.1, 0.4, −2.4, −4.3, −7.3, −8.2, −17.3, −20.5, −21.9, −28.4 23.4, 4.4, 0.1, −2.5, −4.6, −7.4, −8.1, −17.4, −20.3, −22.3, −28.7 23.6, 4.7, 0.3, −2.6, −4.5, −7.2, −7.9, −17.3, −20.3, −22.2, −28.7

(Continued )

340

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

No.

Compounds

11

1281

[8-O(CH2CH2)2S(CH2CH2O)2-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] H[8MeOOCCH(NHBoc)CH2CH2S(CH2CH2O)2-3,3’Co(1,2-C2B9H10)(1’,2’-C2B9H11)] Cs[8-(7”,8”-C2B9H11-9”-)S(Me)CH2CH2OCH2CH2O-3,3’Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

24.2, 5.9, 0.2, −2.8, −4.8, −7.63, −17.4, −20.3, −22.5, −28.9 −

1282 1283

1284 1285 1286 1287 1288 1289 1290

[8-Ph3P(CH2CH2O)2-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] K[8-MeSO3CH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] K[8-N3CH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] K[8-C6H4(CO)2NCH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] [8-H3NCH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)] [8-(H2N)2CSCH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’C2B9H11)] Na[8-HSCH2CH2O-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1303 1304 1305 1306 1307 1308 1309 1310 1311

K[8-(1”-Ph-1”,2”,3”-triazol-4”-yl)-CH2CH2O-3,3’Co(1,2-C2B9H10)(1’,2’-C2B9H11)] K[8,8’-m-(OCH2CH2OCH2CH2S)2-o-C6H4-3,3’Co(1,2-C2B9H9)2] [Et3NH][8,8’-m-(OCH2CH2OCH2CH2S)2-o-C6H4-3,3’Co(1,2-C2B9H9)2] Na[8,8’-m-(OCH2CH2OCH2CH2S)2-o-C6H4-3,3’Co(1,2-C2B9H9)2] K[8,8’-m-(OCH2CH2OCH2CH2)2S-3,3’-Co(1,2-C2B9H9)2] [Et3NH][8,8’-m-(OCH2CH2OCH2CH2)2S-3,3’Co(1,2-C2B9H9)2] (Bu4N)[8,80 -(MeS)2-3,3’-Co(1,2-C2B9H10)2] (Me4N)[8,80 -(MeS)2-3,3’-Co(1,2-C2B9H10)2] (Bu4N)[4,40 -(MeS)2-3,3’-Co(1,2-C2B9H10)2] (Bu4N)[4,70 -(MeS)2-3,3’-Co(1,2-C2B9H10)2] Bu4N{(CO)5W[к-8,8’-(MeS)2-3,3’-Co(1,2-C2B9H10)2]} Bu4{[N{(CO)5}W]2[к2-8,8’-(MeS)2-3,3’Co(1,2-C2B9H10)2]} (Bu4N)[1,1’-(2’)-(MeS)2-3,3’-Co(1,2-C2B9H10)2] (Me4N[1,1’-(2’)-(MeS)2-3,3’-Co(1,2-C2B9H10)2]) (BEDT-TTF)[1,1’-(MeS)2-3,3’-Co(1,2-C2B9H10)2] L2Cu[8,8’-(MeS)2-3,3’-Co(1,2- C2B9H10)2-к2-S,S’] LAg[8,8’-(MeS)2-3,3’-Co(1,2-C2B9H10)2-к2-S,S’] (Ph3P)Ag[8,8’-(MeS)2-3,3’-Co(1,2-C2B9H10)2-к2-S,S’] (Ph3P)ClPd[8,8’-(MeS)2-3,3’-Co(1,2-C2B9H10)2-к2-S,S’] (COD)Rh[8,8’-(MeS)2-3,3’-Co(1,2-C2B9H10)2-к2-S,S’] (Z-9-SMe2-7,8-C2B9H10)Ir(Z-7,8-C2B9H11)

1312 1313

[(Z-1-tBuNH-1,7,9-C3B8H10)Rh(Z-7,8-C2B9H11)] [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)Rh(Z-7,8-C2B9H11)]

1314

[(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)Ir(Z-7,8-C2B9H11)]

1315

[(8-t-C8H17NH2-(CH2CH2O)2-1,2-C2B9H10) (1’,2’-C2B9H11)-3,3’-Co] [(8-NH3-(CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’Co] [(8-C6H5NH2-(CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co]

1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302

1316 1317 1318

B NMR (ppm)

Av. MdB (Å)

Ref.

−

397

−

397

23.2, 4.6, 0.4, −2.6, −4.5, −6.6, −7.3, −8.2, 12.5, 16.7, −17.3, 18.3, −20.3, −22.2, −23.3, −26.4, −28.6, −30.1, −36.9 23.7, 5.2, 0.2, −2.6, −4.6, −7.1, −7.8, 8.6, −17.3, −20.3, −22.3, −28.5 23.3, 5.2, 0.6, −2.5, −4.4, −7.1, −8.1, −17.2, −20.2, −22.1, −28.5 22.8, 4.2, 0.5, −2.6, −4.3, −7.4, −8.4, −17.3, −20.4, −22.0, −28.5 22.7, 4.4, 0.6, −2.5, −4.5, −7.4, −8.4, −17.2, −20.4, −22.2, −28.6 22.7, 4.4, 0.6, −2.5, −4.4, −7.4, −8.3, −17.2, −20.3, −22.2, −28.6 24.5, 7.6, 0.8, −2.7, −6.1, −9.1, −17.1, −19.9, −22.4, −28.5 22.5, 3.6, 0.5, −2.5, −4.2, −7.5, −8.4, −17.2, −20.4, −21.8, −28.7 23.8, 5.8, 0.4, −3.0, −4.6, −6.9, −8.3, −17.3, −20.1, −22.3, −28.9 25.9, −3.7, −6.7, −8.9, −20.1, −28.5

−

397

−

397

−

397

−

397

−

397

−

397

−

397

−

397

−

397

−

331

23, −4.1, −8.1, −20, −29

−

331

−

2.121c

331

26.2, −3.9, −6.9, −9.0, −20.3, −29.0 −

− −

331 331

11.3, 0.1, −4.7, −6.8, −18.0, −24.0 11.3, 0.1, −4.6, −6.7, −18.0, −24.0 6.5, 2.2, 0.4, −4.8, −5.9, −8.1, −16.9, −18.0, −23.3 7.0, 5.2, −0.3, −4.3, −6.9, −15.6, −19.1, −22.2 13.2, 9.8, 2.7, 1, −5.9, −17.2, −22.2 12.4, 2, −5.5, −16.4, −22.6

2.131c − 2.116c 2.115c − −

398 398 398 398 399 399

− − − 15.8, −0.7, −4.8, −5.6, −15.7, −17.1, −23.5 13.9, −0.3, −4.4, −6.0, −16.9, −23.5 12.8, 0.1, −4.5, −6.1, −17.3, −23.7 15.1, 11.2, 2.1, −0.1, −4.5, −15.1, −22.3 13.4, 0.5, −5.3, −15.8, −22.7 −24.3, −23, −20.4, −18.5, −17.7, −12.0, −8.7, −7.7, −4.6, −1.9, 0.2, 1.4, 4.5 − 5.76, 3.08, 0.69, −1.80, −4.42, −6.45, −7.38, −11.11, −12.98, −14.47, −15.21, −16.07, −20.91 2.46, 1.05, −1.95, −3.06, −5.59, −7.4, −9.41, −10.55, −13.9, −15.73, −16.8, −18, −18.76, −23.23 24.41, 6.8, 0.34, −2.26, −4.83, −6.60, −8.92, −17.30, −20.20, −22.31, −28.69 −

− − 2.107c − − − 2.106c 2.108c −

280 280 280 400 400 400 400 400 314

− −

313 297

−

297

−

401

−

401

−

401

2.112c

401

24.15, 5.67, 0.37, −2.44, −4.86, −7.05, −8.95, −17.20, −20.18, −22.2, −28.48

341

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

Compounds

11

23.3, 4.57, 0.34, −2.59, −4.44, −7.3, −8.16, −17.41, −20.38, −22.16, −28.73 23.62, 4.95, 0.37, −2.51, −4.55, −7.15, −7.67, −17.25, −20.32, −22.08, −28.47

1323

[(8-(C6H5)2(PO)(CH2)C(O)-N(t-C8H17)(CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]Na [(8-(C6H5)2(PO)(CH2)C(O)NH-(CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co] Na [(8-(C6H5)2(PO)(CH2)C(O)-N(C6H5)(CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]Na [(8-(n-C8H17)(C6H5)(PO)(CH2)C(O)-N(t-C8H17) (CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]Na [(8-(C6H5)2(PO)(CH2)2C(O)-N(t-C8H17) (CH2CH2O)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]Na [((8-CH3CN)-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1324

[((8-C6H5CN)-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1325

1330

[((8-CH3C(O)]NH)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-C6H5C(O)]NH)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-C4H10NH-CH3C]N)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-(C2H5)2NH-CH3C]N)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-(C2H5)2NH-C6H5C]N)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-H3N)-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1331

[((8-(C2H5)H2N)-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1332

1334

[((8-(CH2C6H5)H2N)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-(CH2C6H5)2HN)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] [((8-O3SC3H6NH2)-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1335

[8,8’-m-(CH2O(CH3))-(1,2-C2B9H10)2-3-Co]

1336

[8-((CH3)2O-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1337 1338

[8,8’-(CH3O)2-(1,2-C2B9H10)2-3,3’-Co]Me4N [(8-C5H5N-CH2-1,2-C2B9H10)(8’-CH3O-1’,2’-C2B9H10)3,3’-Co] [(8-n-C6H13NH2-CH2-1,2-C2B9H10)(8’-CH3O-1’,2’C2B9H10)-3,3’-Co] [(8-(2-OH-C2H4-1-NH2)-CH2-1,2-C2B9H10)(8’-CH3O1’,2’-C2B9H10)-3,3’-Co] [((8-(C6H5)3P)-CH2-1,2-C2B9H10)(8’-CH3O-1’,2C2B9H10)-3,3’-Co] Na[(8-(4-tBu-C6H4-1-O)-CH2-1,2-C2B9H10)(8’-CH3O1’,2-C2B9H10)-3,3’-Co] [8,8’-m-(CH2-O)-(1,2-C2B9H10)2,3-Co](Et3NH)]

No.

1319

1320 1321 1322

1326 1327 1328 1329

1333

1339 1340 1341 1342 1343 1344

1345 1346 1347

1,3-Dicyanomethyl-2-(8-methylene-8’methoxycobaltbis(dicarbollide))-4-methyl-tert-butylcalix[4]arene [3,3’-Co-(8-t-C8H17NH2-(CH2-CH2O)2-1,2-C2B9H10)(8’Cl-1’,2’-C2B9H10)] [3,3’-Co-(8-t-C8H17NH2-(CH2-CH2O)2-1,2-C2B9H10)(8’Br-1’,2’-C2B9H10)]

B NMR (ppm)

23.42, 4.41, 0.21, −2.51, −4.58, −7.65, −7.88, −17.32, −20.37, −21.9, −28.48 24.22, 5.74, 0.39, −2.58, −4.6, −7.05, −17.3, −20.25, −22.0, −28.31 23.33, 4.62, 0.41, −2.50, −4.30, −7.25, −7.97, −17.24, −20.21, −21.8, −28.23 9.26, 4.60, 3.53, −1.89, −3.67, −4.77, −7.31, −15.52, −18.18, −21.39, −24.03 9.34, 4.63, 3.44, −1.82, −3.55, −4.72, −7.31, −15.42, −17.87, −21.18, −23.63 12.64, 6.96, 1.46, −0.67, −4.10, −5.79, −6.27, −16.85, −19.39, −21.91, −25.24 13.62, 7.50, 1.54, −1.25, −4.29, −6.00, −7.19, −16.85, −18.97, −22.06, −25.81 12.83, 8.57, 2.87, −1.3, −4.29, −5.17, −6.05, −7.98, −16.16, −18.63, −21.6, −25.05 13.24, 8.60, 2.68, −1.2, −4.32, −5.36, −7.81, −16.23, −18.56, −21.63, −25.17 13.33, 8.27, 2.32, −1.51, −4.24, −5.36, −6.86, −16.25, −17.94, −21.46, −25.77 10.31, 7.20, 3.22, −1.3, −5.27, −6.65, −7.69, −16.06, −18.04, −21.63, −25.65 12.19, 7.17, 3.87, −1.34, −4.79, −7.57, −15.59, −17.61, −21.01, −25.1 12.07, 7.1, 3.98, −1.2, −4.84, −7.45, −15.44, −17.35, −20.84, −24.93 15.13, 6.74, 4.15, −1.56, −4.55, −8.0, −15.09, −17.13, −20.55, −24.79 12.18, 7.26, 3.31, −1.28, −5.25, −6.61, −7.63, −16.12, −18.07, −21.62, −25.75 30.70, 13.01, 0.67, −3.64, −5.44, −8.82, −14.43, −16.77, −23.78, −28.53 24.55, 8.05, 4.39, −3.31, −4.31, −8.03, −9.45, −15.28, −18.77, −21.23, −26.98 −3.34, −7.35, −9.14, −20.51, −28.38 27.19, 12.46, −2.09, −6.43, −6.81, −8.86, −17.61, −18.85, −24.01, −28.52 26.55, 10.02, −0.13, −2.53, −5.15, −6.46, −8.21, −18.32, −19.23, −23.17, −28.31 26.81, 10.12, −0.32, −2.51, −5.3, −6.55, −8.17, −18.2, −19.11, −23.32, −28.36 27.06, 10.15, −1.2, −2.55, −5.24, −6.47, −7.9, −17.87, −18.82, −23.72, −28.26 23.29, 8.46, −0.25, −2.63, −5.33, −6.79, −8.69, −18.82, −20.39, −23.63, −28.1 32.74, 25.34, −4.22, −6.62, −8.82, −11.88, −16.68, −17.92, −27.14, −31.78 24.13, 9.5, 0.04, −2.27, −5.03, −6.47 to −7.95, −17.68, −20.06, −21.6, −28.36 21.49, 11.67, −1.50, −6.26, −7.71, −19.15, −19.79, −25.65, −27.28 21.49, 5.65, −1.53, −6.07, −7.54, −18.75, −19.77, −24.92, −27.24

Av. MdB (Å)

Ref.

−

401

−

401

−

401

−

401

2.102c

332

−

332

−

332

−

332

2.100c

332

2.104c

332

−

332

−

332

−

332

−

332

2.106c

332

−

332

2.079c

402

−

402

− 2.107c

402 402

−

402

2.112c

402

2.113c

402

−

402

2.093c

402

2.121c

402

−

403

−

403

−

403 (Continued )

342

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

Compounds

11

21.61, −0.10, −0.55, −4.48, −5.50, −7.24, −17.82, −19.77, −23.29, −27.09 21.37, 11.46, −1.53, −6.28, −7.69, −19.32, −25.85, −27.58

1351

[3,3’-Co-(8-t-C8H17NH2-(CH2-CH2O)2-1,2-C2B9H10)(8’I-1’,2’-C2B9H10)] [(3,3’-Co-8-Ph2P(O)-CH2C(O)(t-C8H17)N(CH2-CH2O)2-1,2-C2B9H10)(8’-Cl-1’,2’-C2B9H10)] [(3,3’-Co-8-Ph2P(O)-CH2C(O)(t-C8H17)N(CH2-CH2O)2-1,2-C2B9H10)(8’-Br-1’,2’-C2B9H10)] [(3,3’-Co-8-Ph2P(O)-CH2C(O)(t-C8H17)N(CH2-CH2O)2-1,2-C2B9H10)(8’-I-1’,2’-C2B9H10)] Cs[(1-HO(CH2)-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1352

Cs[(1-HO(CH2)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1353 1354

Me4N[(1-HO(CH2)2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co] Cs[(1-HO(CH2)3-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co]

1355 1356

Cs[(HO(CH2)-1,2-C2B9H10)2-3,3’-Co] (Me3NH)2[(HO(CH2)2-1,2-C2B9H10)2-3,3’-Co]

1357

Cs[syn-(HO(CH2)2-1,2-C2B9H10)2-3,3’-Co]

1358

Me3NH[(HO(CH2)3-1,2-C2B9H10)2-3,3’-Co]

1359 1360

Cs[syn-(HO(CH2)3-1,2-C2B9H10)2-3,3’-Co] Cs[vicinal-(HO(CH2)3-1,2-C2B9H10)2-3,3’-Co]

1361

1363

[(1-(HO)2P(O)-OC3H6-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’Co](Me3NH) [(1’,2’-C2B9H11)-3,3’-Co(1,2-C2B9H10-1-(C3H6O)(HO)P(O)-(OC3H6-1”)-1”,2”-C2B9H10) (1”’,2”’-C2B9H11)-3”,3”’-Co](Me3NH)2 [m-(HOP(O)(OC3H6)2)-(1,2-C2B9H10)2-3,3’-Co]2Ca

1364

[((1-HOOC-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’-Co)]

1365 1366 1367

1,1’-anti-[(HOOC)2-(1,2-C2B9H10)2–3,3’-Co] 1,2’-syn-[(HOOC)2-(1,2-C2B9H10)2–3,3’-Co] Me4N[1-(EtOOC-CH2-1,2-C2B9H10)(1’,2’-C2B9H11)-3,3’Co] Me4N[(1-HOOC-CH2-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] Me4N[(1-HOOC-(CH2)2-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] Me4N[(1-HOOC-CH2-O-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] Me4N[1-(1,4-NO2C6H4OOC-CH2-1,2-C2B9H10) (1’,2’-C2B9H11)-3,3’-Co] Me4N[1-(1,4-NO2C6H4OOC-(CH2)2-1,2-C2B9H10)(1’,2’C2B9H11)-3,3’-Co] Me4N[8-(1,4-NO2C6H4OOC-(CH2O)-1,2-C2B9H10)(1’,2’C2B9H11)-3,3’-Co] Me4N[(1-BuNHOC-(CH2)-1,2-C2B9H10) (1’,2’-C2B9H11)-3,3’-Co] Me4N[(1-BuNHOC-(CH2)2-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] Me4N[(1-BnNHOC-(CH2)-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co] Me4N[(1-BnNHOC-(CH2)2-1,2-C2B9H10)(1’,2’-C2B9H11)3,3’-Co]

No.

1348 1349 1350

1362

1368 1369 1370 1371 1372 1373 1374 1375 1376 1377

B NMR (ppm)

Av. MdB (Å)

Ref.

−

403

−

403

−

403

−

333

−

333

2.101c −

333 333

2.101c 2.097c

333 333

−

333

−

333

− −

333 333

−

333

−

333

−

333

−

334

− − −

334 334 334

7.01, 1.11, −5.64, −11.51, −17.58, −19.92, −23.09

−

334

6.56, 0.92, −5.79, −15.23, −17.72, −19.53, −23.05

−

334

25.20, 6.77, −0.63, −4.22, −5.81, −7.05, −9.12, −17.73, −19.51, −22.56, −30.36 7.37, 1.37, 0.85, −5.38, −12.73, −17.39, −19.6, −22.95

−

334

−

334

6.63, 0.95, −5.81, −15.39, −17.7, −19.5, −23.07

−

334

23.39, 7.1, 0.37, −3.24, −5.17, −7.43, −8.33, −17.46, −20.17, −22.5, −28.95 6.77, 0.95, 0.35, −5.86, −11.4, −16.49, −17.87, −20.58, −23.43 6.27, 0.66, −5.98, −15.23, −18.08, −19.65, −23.73

−

334

−

334

−

334

6.75, 0.95, −5.79, −11.45, −17.65, −20.17, −23.1

−

334

6.41, 0.78, −5.86, −15.11, −17.91, −19.72, −23.19

−

334

21.44, 5.54, −1.05, −6.07, −7.54, −18.72, −19.74, −24.81, −27.12 21.56, −0.17, −0.57, −4.45, −5.60, −7.24, −17.84, −19.86, −23.29, −27.31 6.94, 6.34, 1.64, 0.85, −5.29, −6.33, −7.85, −13.87, −17.46, −20.7, −23.07 6.44, 0.71, −4.98, −5.9, −6.95, −14.49, −16.3, −17.94, −19.77, −23.19 − 6.67, 0.49, −5.84, −6.62, −7.45, −14.80, −16.54, −18.04, −19.39, −23.05 8.22, 0.95, −4.08, −5.81, −8.16, −13.65, −16.98, −19.98 8.15, −0.29, −3.84, −4.95, −6.62, −8.71, −13.20, −16.01, −20.22 7.56, 0.22, −3.81, −5.64, −7.92, −8.73, −14.11, −16.01, −18.72 8.15, −0.13, −3.96, −5.12, −6.03, −8.88, −13.75, −16.01, −18.04 7.147, −0.13, −6.03, −5.24, −7.83, −14.92, −19.53 6.077, 5.197, 0.13, −3.93, −6.03, −7.83, −13.92, −16.02, −18.04, −23.26 6.53, 0.77, −4.91, −5.93, −7.12, −14.87, −16.28, −17.78, −19.32, −23.0 6.39, 0.78, −5.0, −5.98, −7.01, −14.94, −16.32, −17.94, −19.24, −23.0 7.96, −0.10, −3.88, −6.17, −8.82, −15.34, −16.18, −17.51, −19.86, −22.97 6.06, 0.85, −5.45, −6.26, −7.55, −15.25, −17.53, −19.62, −22.95 8.51, 1.54, −4.31, −5.9, −13.2, −16.99, −19.81 7.32e 7.2, 1.33, 0.8, −5.55, −11.83, −16.11, −19.98, −22.98

343

Polyhedral Metallaboranes and Metallacarboranes

Table 14

(Continued)

No.

Compounds

11

1378 1379

Me4N[1,1’-CH2CONHCH2CH2-{(1,2-C2B9H10) (1’,2’-C2B9H11)-3,3’-Co}2] [8-EtC^N-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1380

[8-EtC(OH)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1381

Et3NH[8-EtC(O)HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1382

[8-EtC(OMe)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1383

[8-EtC(OMe)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1384

[8-EtC(OEt)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1385

[8-EtC(OEt)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1386

[8-EtC(OiPr)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1387

[8-EtC(OiPr)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1388

[8-EtC(OBu)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1389

[8-EtC(OBu)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1390

[8-EtC(SEt)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1391

[8-EtC(SEt)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1392

[8-EtC(SBu)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1393

[8-EtC(SBu)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1394

[8-EtC(SHx)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

1395

[8-EtC(SHx)]HN-3,3’-Co(1,2-C2B9H10)(1’,2’-C2B9H11)]

B NMR (ppm)

Av. MdB (Å)

Ref.

6.51, 1.06, −5.86, −17.53, −19.80, −23.03

−

334

9.9, 5.0, 3.6, −1.9, −3.3, −4.5, −7.2, −15.4, −18.1, −21.2, −24.0 10.6, 8.3, 2.7, −1.4, −4.6, −5.5, −6.1, −7.6, −16.4, −18.6, −21.8, −24.7 12.7, 6.9, 1.2, −0.8, −4.2, −5.8, −7.6, −17.3, −19.5, −22.0, −25.5 11.0, 8.7, 3.6, −1.3, −4.0, −5.1, −6.5, −7.9, −16.0, −18.3, −21.5, −24.7 10.2, 8.4, 3.5, −1.4, −4.3, −5.3, −6.1, −7.8, −16.1, −17.2, −21.6, −24.6 10.8, 8.8, 4.5, −0.6, −4.6, −6.4, −7.4, −15.5, −17.6, −21.2, −24.6 10, 8.7, 4, −0.5, −4.8, −6, −7.4, −15.8, −17.9, −21.4, −24.5 11.2, 8.4, 3.3, −1.4, −5.1, −6.2, −7.8, −16.1, −18.4, −21.7, −24.7 10.2, 8.5, 3.1, −1.4, −5.2, −6.2, −7.9, −16.4, −18.7, −21.8, −25.0 11.0, 8.6, 3.5, −1.3, −5.1, −6.4, −7.8, −16.1, −18.4, −21.7, −24.7 10.2, 8.5, 3.0, −1.4, −5.3, −6.2, −7.9, −16.5, −18.8, −22.2, −25.5 11.1, 8.2, 3.9, −1.5, −4.2, −5.0, −6.8, −7.8, −15.9, −18.2, −21.4, −24.2 10.4, 8, 3.4, −1.5, −5, −6.7, −7.8, −15.9, −18.2, −21.6, −24.2 11.0, 8.6, 3.5, −1.3, −5.1, −6.4, −7.8, −16.1, −18.4, −21.7, −24.8 10.6, 8.5, 3.8, −1.3, −4.9, −6.8, −7.8, −16.0, −8.3, −21.8, −24.7 11.2, 8.7, 3.6, −1.3, −5.0, −6.4, −7.7, −16.0, −18.3, −21.7, −24.7 10.5, 8.3, 3.6, −1.5, −4.9, −6.8, −7.8, −15.9, −18.2, −21.6, −24.3

−

286

−

286

−

286

−

286

−

286

−

286

−

286

−

286

2.102c

286

−

286

−

286

−

286

−

286

−

286

−

286

−

286

−

286

a

Av. RhdB Av. IrdB c Av. CodB d Av. MdB between boron and metal other than group 9 metal e Other boron signals of 1366 overlap with 1365 b

Sneddon and co-workers isolated a series of Group 9 (Co, Rh, and Ir) 11-vertex closo and nido-metallacarboranes having three carbon vertices. The reactions of nido-carborane Li[(6-Ph-5,6,9-C3B7H9)] with [Co(CO)4I], [Rh(CO)2Cl]2, and [Ir(CO)3Cl] yielded metallacarboranes 988–990, respectively (Scheme 36).295 Metallacarboranes 988 and 989 have an octadecahedral core similar to that of manganacarborane 711, but the metal centers are coordinated with two CO ligands in 988 and 989 to meet the electronic requirement of 11 SEP. By contrast, iridacarborane 990 has an 11-vertex nido-shape. The reaction of the same nido-carborane with [Rh(COD)Cl]2 and [Ir(COD)Cl]2 produced the COD-coordinated rhodacarborane 991 and iridacarborane 992, which have similar octahedral cores to those of 988 and 989.295 Their tetramethylcyclobutadienyl derivative cobaltacarborane 993 was synthesized from the reaction of the same nido-carborane with [(Z4-C4Me4)Co(CO)2I]. These clusters undergo further reactions with various reagents at the metal center. For example, the reactions of 988–990 with dppe lead to the substitution of two CO ligands and yielded dppe-ligated closo-994, closo-995, and nido-996, respectively.295 On the other hand, the reactions of 988, 990, or 992 with the different equiv. of the stronger donor tBuNC yielded tBuNC-coordinated nido-997-999, respectively, which have the same cluster core as that of 990.295 Upon the reaction of closo-988 with diphenylacetylene, both carbonyls were displaced with subsequent alkyne cyclization to form the tetraphenylcyclobutadienyl-ligated metallacarborane closo-1000.295 When nido-999 was reacted with I2 and perfluoro-1-iodohexane, iodination of a cage-boron occurred to produce nido-1001.295 The reaction of 11-vertex closo-992

344

Polyhedral Metallaboranes and Metallacarboranes

Scheme 35 Synthesis of metallacarboranes 982–986.

Scheme 36 Synthesis of 11-vertex metallacarboranes 988–990.

Polyhedral Metallaboranes and Metallacarboranes

345

with 3-hexyne resulted in cage deboronation to produce closo-[2,2-COD-10-Ph-2,1,6,10-C3B6H8] (1002), which has a bicapped square antiprismatic core.295 A series of 12-vertex metallacarboranes of group 9 were isolated by several groups around the world. For example, when zwitterionic complexes [Cp M(7,8-(PPh2)2-7,8-C2B9H9)] (M ¼ ]Ir or Rh) were refluxed in MeOH, 12-vertex metallacarboranes 1003 and 1004 were synthesized (Scheme 37).296 Interestingly, clusters 1003 and 1004 have a 12-vertex pseudocloso geometry, in which the CdC bond distance is long. Reactions of preformed metallacarboranes [(Z-9-SMe2-7,8-dMe2-d7,8-C2B9H8)M(COD)] (M ¼ ]Rh or Ir) with hydrohalic acids afforded 12-vertex pseudocloso-[(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)MX2] (M ¼ ]Rh, X ¼ ]Cl (1005), Br (1006), I (1007); M ¼ ]Ir, X ¼ ]Cl (1008), Br (1009), I (1010)).297 Jin and co-workers also isolated several more pseudocloso 12-vertex metallacarboranes from carboranes. For example, 12-vertex pseudocloso-metallacarboranes [3-Cp C+(NHR)2-3,1,2-IrC2B9H10]OTf (R ¼ ]iPr (1011) and Cy (1012)) were isolated from the reactions of [Cp IrCl2]2 with AgOTf in MeOH for 6 h, followed by the addition of nido-DcabNH at room-temperature.298 Treatment of the diazo-substituted o-carborane

Scheme 37 Synthesis of 12-vertex pseudocloso-metallacarboranes 1003 and 1004.

[1,2-(N]N-C6H4OMe)2-1,2-C2B10H10] with [Cp IrCl2]2 and Et3N in methanol afforded another diazo-substituted pseudoclosoiridacarborane 1013.299 On the other hand, treatment of the mono diazo-substituted o-carborane [1-(N]N-C6H4OMe)-1,2-C2B10H11] with [Cp MCl2]2 (M ¼ ]Ir or Rh) and Et3N in methanol afforded mono diazo-substituted metallacarboranes 1014 and 1015, respectively.299 Clusters 1014 and 1015 have CAd icosahedral geometry (I, Fig. 51). A similar type mono-substituted rhodacarborane 1016, having a 2-pyridylsulfenyl ligand, was isolated from the reaction of mono 2-pyridylsulfenyl substituted o-carborane with [Cp RhCl2]2 (0.5 equiv.) and AgOTf in the presence of CH3COOK in CH2Cl2 at room-temperature.300 The reactions of nido-carborane [HNMe3][7,8-m-R-7,8-C2B9H10] (R ¼ ]C4H6, C4H4 or (C6H4)2) with nBuLi in hexane, followed by treatment of NaCp and CoCl2 afforded CAd metallacarboranes 1017, 1018 and 1019, respectively (Scheme 38).301 Icosahedral 1017, 1018, and 1019 (I, Fig. 51) are the analogs of dihydrobenzocarborane, benzocarborane, and biphenylcarborane, respectively. A similar type of CAd cobaltacarborane 1020 (I, Fig. 51) was isolated along with three supra-icosahedral species from the reaction of anhydrous

Scheme 38 Synthesis of 12-vertex metallacarboranes 1017 and 1018.

346

Polyhedral Metallaboranes and Metallacarboranes

CoCl2 with [{m-7,8-[o-C6H4(CH2)2]-7,8-C2B10H10}2Na4(THF)6]n and NaCp in THF in the temperature range −78  C to 0  C, followed by oxidation with O2.257 The carbon vertices of 1020 are linked by -CH2(C6H4)CH2- group. On the other hand, the reactions of thallium dicarbollide Tl[Tl(Z-7,8-R2-7,8-C2B9H9)] (R ¼ ]H or Me) with various metal precursors, such as [(Z5-indenyl)IrI2]n, [(C5R’5)Co(CO)I2] (R0 ¼ ]H or Me), [(Z5-C5R0 5)MX2]2 (X ¼ ]Cl, I) or [CpM(MeCN)3]2+ (M ¼ ]Rh or Ir) afforded icosahedral metallacarboranes 1021–1029 (I, Fig. 51).302–304 In icosahedral 1021–1029, the metal centers are coordinated with indenyl, Cp or Cp ligand in Z5-mode. Reactions of pseudocloso-metallacarboranes [(Z-9-SMe2-7,8-Me2-7,8-C2B9H8)MBr2] (M ¼ ]Rh (1006) or Ir (1009)) with SMe2 led to the addition of SMe2 to the metal centers and converted 12-vertex pseudocloso-core to closo-core to afford metallacarboranes 1030 and 1031, respectively.297 Similar cage transformations from pseudocloso to closo were accompanied by room-temperature reactions of same pseudocloso-metallacarboranes 1006 or 1009 with CpTl. These reactions afforded closo-[(Z-9-SMe-7,8-Me2-7,8-C2B9H8)MCp] (M ¼ ]Rh (1032) and Ir (1033)).266 A series of (arene)iridacarboranes closo-[3-(arene)-3,1,2-IrC2B9H11]+ (arene ¼ benzene (1036), toluene (1037), o-xylene (1038), m-xylene (1039), durene (1040)) were synthesized from the reactions of [(COD)IrCl]2 with Tl[Tl(Z-7,8-C2B9H11)], followed treatment with arenes in refluxing trifluoroacetic acid.305 Metallacarborane 1036 reacted with acetonitrile, which led to the substitution of benzene by three acetonitrile ligands to afford 1037. Further reactions of 1037 with arenes and Cp− anions afforded the arene derivatives [3-(arene)-3,1,2-IrC2B9H11]+ (arene ¼ mesitylene (1042) and [2,2]paracyclophane (1043)) and cyclopentadienyl derivatives [3-(Z-C5H4R)-3,1,2-IrC2B9H11] (R ¼ ]H (1044) and C(O)Me (1045)), respectively.305 Similar types of (arene) metallacarboranes 1046–1057 were obtained by bromide abstraction from [(Z-9-SMe2-7,8-C2B9H10)MBr2]2 (M ¼ ]Rh or Ir) with Ag+ in the presence of the corresponding groups.306 By contrast, (hexamethylbenzene)rhodacarboranes (1058 and 1059) and ([2.2] paracyclophane)rhodacarboranes (1060 and 1061) were obtained by substituting the benzene ligand of [(Z-7,8-R2-7,8-C2B9H9)Rh(Z-C6H6)]+ (R ¼ ]H, Me) in refluxing nitromethane.307 On the other hand, the reactions of closo-[3-CoCp-1,2-C2B9H11] in benzene with an excess of AlX3 (X ¼ ]Cl, Br, I) led to the monohalogenation and afforded closo-1064-1066.308 The reactions of 12-vertex carboranes closo-[1,2-(CH3)2-1,2-C2B10H10] or closo-[1,2-(m-CH3)2-1,2-C2B10H10] with K[NC4H4] in THF led to the deboronation; further treatment with CoCl2 under reflux conditions afforded icosahedral cobaltacarboranes 1067 and 1068 (I, Fig. 51), respectively.309 On the other hand, the reactions of nido-11-vertex carboranes Li2[7,9-C2B9H11] or Li2[2,9-C2B9H11] with NaCp and CoCl2 in THF afforded cobaltacarboranes 1069 and 1070, respectively.263 Cobaltacarboranes 1069 and 1070 are the 2,1,7-isomer (II, Fig. 51) and 2,1,12-isomer (III, Fig. 51) of a MC2B9 icosahedron. When closo-[1,2(40 -F3CC6F4)2–1,2- C2B10H10] was reduced and treated with NaCp and CoCl2, the icosahedral metallacarborane 1071 was isolated along with two more supra-icosahedra.310 Metallacarborane 1071 has only one carbon vertex and two Co vertices. On the other hand, reduction of closo-[1-(40 -F3CC6F4)-2-Ph-1,2-C2B10H10], followed by treatment with NaCp and CoCl2 afforded icosahedral metallacarborane 1072 (IV, Fig. 51) along with two more supra-icosahedral clusters.264 The reactions of closo-[4-(Z-Cp)4,1,8-CoC2B10H12] with one equivalent of KOH in EtOH, followed by treatment with [PhCH2NEt3]Cl or [HNMe3]Cl led to the isolation of 11-vertex nido-cobaltacarboranes 1073 and 1074, respectively.264 Further treatment of nido-1074 with excess NaH in THF and aerial oxidation afforded icosahedral cobaltacarboranes 1075 (IV, Fig. 51) and 1076 (V, Fig. 51).264 The reactions of Tl[7-SMe2-7,8-C2B9H10] with [(COD)MCl]2 (M ¼ ]Ir or Rh) at room-temperature led to capitation and afforded icosahedral metallacarboranes 1077 and 1078 (V, Fig. 51), respectively.265,311 Metal insertion is accompanied by polyhedral rearrangement of the carborane ligand with the migration of the substituted carbon atom from the metal-bonded

Scheme 39 Synthesis of metallacarboranes 1079–1082.

Polyhedral Metallaboranes and Metallacarboranes

347

pentagonal face. Further reactions of iridacarborane 1077 with anhydrous hydrohalic acids HX (X ¼ ]Cl, Br, I) afforded the dimeric halide iridacarboranes [1,1-X2–8-SMe2-1,2,8-IrC2B9H10]2 (1079–1081) (Scheme 39).265 The reaction of the dimeric iridacarborane 1080 with TlCp afforded iridacarborane 1082 (V, Fig. 51).265 On the other hand, 12-vetrex closo-[(Z-1-tBuNH-1,7,9-C3B8H10)Ir (COD)] (1083) having three carbon vertices was obtained from the reaction of Tl[(7-tBuNH-7,8,9-C3B8H10)] with [(COD)IrCl]2 in the presence of TlPF6.312 Further, reactions of 1083 with I2, Cl2 or Br2 afforded dimeric halide iridacarboranes [(Z-1-tBuNH1,7,9-C3B8H10)IrX2]2 (X ¼ ]I (1084), Cl (1085) and Br (1086)), respectively.312 Similarly, reactions of (Z-1-tBuNH1,7,9-C3B8H10)Rh(COD) with Br2 or I2 afforded dimeric halide rhodacarboranes 1087 and 1088, respectively.313 The reaction of 1088 with Tl[BF4] afforded dimeric halide rhodacarborane cation [(Z-1-tBuNH-1,7,9-C3B8H10)2Rh2(m-I)3]+ (1089), in which three I atoms bridge between two Rh centers. On the other hand, the reaction of Na[9-SMe2-7,8-C2B9H10] with [Cp IrCl2]2 led to capitation and afforded iridacarborane 1090 (I, Fig. 51).314 Similarly, the reactions of [(Z1,Z3-cyclooctenediyl)Co(Z-1,4-C6H4Me2)]BF4 with Tl [9-SMe2-7,8-C2B9H10] or Tl[7-tBuHN-7,8,9-C3B8H10] led to capitation, and afforded icosahedral cobaltacarboranes 1091 and 1092, respectively, in which the Co vertex is attached with cyclooctenediyl ligand in Z1,Z3-fashion.315 The reaction of bromide complex [(Z-C5H5BMe)RhBr2]2 with Tl[Tl(Z-7,8-C2B9H11)] afforded (boratabenzene)rhodacarborane 1093.316 Similarly, a (borole)rhodacarborane (9-SMe2-7,8-C2B9H10)Rh(Z5-C4H4BPh) (1094) was isolated from the reaction of iodide complex [(Z5-C4H4BPh)RhI]4 and carborane anion [Carb0 ]− (Carb0 ¼ 9-SMe2-7,8-C2B9H10).317 In addition, the reactions of 1094 with [Cp MCl2]2 or [(Carb0 )MBr2]2 (M ¼ ]Rh or Ir) afforded the m-borole triple-decker complexes [(Carb0 )Rh

Scheme 40 Synthesis of metallacarboranes 1095–1098.

(m-Z5:Z5-C4H4BPh)ML]2+ [LM ¼ ]Cp Ir (1096), (Carb’)Rh (1097)] or the arene-type complexes [(Carb0 )Rh(m-Z5:Z6-C4H4-BPh) ML]2+ [LM ¼ ]Cp Rh (1095), (Carb0 )Ir (1098)] (Scheme 40).317 Welch and co-workers have isolated a series of conjuncto-metallacarboranes. For example, conjuncto-[1-(10 -10 ,20 -closoC2B10H11)-8-(Z-Cp)-8,1,2-closo-CoC2B9H10] (1099), was isolated in trace amounts during the 2e reduction and metalation (NaCp/CoCl2) of [1-(10 -10 ,20 -closo-C2B10H11)-1,2-closo-C2B10H11] along with two conjuncto-carboranes and one known conjunctometallacarborane.264 One of the icosahedra of 1099 is an 8,1,2-closo-MC2B9 metallacarborane. Deprotonation of [HNMe3] [7-(10 -10 ,20 - closo-C2B10H11)-7,8-nido-C2B9H11] followed by addition of [CpCo(CO)I2] led to metalation and afforded

348

Polyhedral Metallaboranes and Metallacarboranes

conjuncto-[1-(10 -10 ,20 -closo-C2B10H11)-3-Cp-3,1,2-closo-CoC2B9H10] (1100), in which one of the icosahedra is 3,1,2-closoMC2B9 isomer.318 Interestingly, the treatment of same deprotonated conjuncto-carborane with CoCl2/NaCp followed by oxidation yielded conjuncto-[8-(10 -10 ,20 -closo-C2B10H11)-2-Cp-2,1,8-closo-CoC2B9H10] (1101), in which one of the icosahedra is 2,1,8-closoMC2B9isomer.318 Double deboronation of 1,10 -bis(ortho-carborane) results in a mixture of racemic and meso diastereoisomers which are sources of the [7-(70 -70 ,80 -nido-C2B9H10)-7,8-nido-C2B9H10]4− tetranion. Metalation of the mixture with CoCl2/NaCp followed by oxidation yielded cobaltacarboranes a-[1-(80 -2’-Cp-20 ,10 ,80 -closo-CoC2B9H10)-3-Cp-3,1,2-closo-CoC2B9H10] (1102) and b-[1-(80 -20 -Cp-20 ,10 ,80 -closo-CoC2B9H10)-3-Cp-3,1,2-closo-CoC2B9H10] (1103), along with a small amount of the unique species [8-(80 -2’-Cp-20 ,10 ,80 -closo-CoC2B9H10)-2-Cp-2,1,8-closo-CoC2B9H10] (1104).272 The reaction between [Tl]2[1-(10 -30 ,10 ,20 -closoTlC2B9H10)-3,1, 2-closo-TlC2B9H10] and [CoCpI2(CO)] led to the isolation of a further isomer of [(CpCoC2B9H11)2], rac-[1-(10 -30 -Cp-30 ,10 ,20 -closoCoC2B9H10)-3-Cp-3,1,2-closo-CoC2B9H10] (1105), which displays intramolecular dihydrogen bonding.272 On the other hand, deboronation of 1101 afforded diastereoisomeric mixtures of [8-(70 -nido-70 ,80 -C2B9H11)-2-Cp-closo-2,1,8-CoC2B9H10]− anion (1106).273 Deprotonation of [7-(10 -closo-10 ,20 -C2B10H11)-nido-7,8-C2B9H11]−, metalation with CoCl2/NaCp and oxidation afforded the isomers [1-(10 -closo-10 ,20 -C2B10H11)-3-Cp -closo-3,1,2-CoC2B9H10] (1107) and [8-(10 -closo-10 ,20 -C2B10H11)-2Cp -closo-2,1,8-CoC2B9H10] (1108).273 Reduction then reoxidation of 1107 converted it to 1108. Deboronation of either 1107 or 1108 yielded a diastereoisomeric mixture of [8-(70 -nido-70 ,80 -C2B9H11)-2-Cp -closo-2,1,8-CoC2B9H10]− (1109).273 Deprotonation of this species followed by treatment with [RuCl2(p-cym)]2 produced [8-(10 -30 -(p-cym)-closo-30 ,10 ,20 -RuC2B9H10)-2-Cp -closo2,1,8-CoC2B9H10] (1110) as a mixture of two diastereoisomers in a 2:1 ratio.273 Finally, thermolysis of mixture 1110 in refluxing dimethoxyethane yielded [8-(80 -20 -(p-cym)-closo-20 ,10 ,80 -RuC2B9H10)-2-Cp -closo-2,1,8-CoC2B9H10] (1111), again as a 2:1 diastereoisomeric mixture.273 Deprotonation of [7-(10 -closo-10 ,20 -C2B10H11)-nido-7,8-C2B9H11]− and reaction with [Rh(PPh3)3Cl] resulted in isomerization of the metalated cage and the formation of [8-(10 -closo-10 ,20 -C2B10H11)-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (1112).319 Similarly, deprotonation/metalation of [80 -(7-nido-7,8-C2B9H11)-20 -(p-cym)-closo-20 ,10 ,80 -RuC2B9H10]− and [80 -(7nido-7,8-C2B9H11)-20 -Cp -closo-20 ,10 ,80 -CoC2B9H10]− afforded [8-{80 -20 -(p-cym)-closo-20 ,10 ,80 -RuC2B9H10}-2-H-2,2-(PPh3)2-closo2,1,8-RhC2B9H10] (1113 and 1114) and [8-(80 -20 -Cp -closo-20 ,10 ,80 -CoC2B9H10)-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (1115 and 1116), respectively, as diastereoisomeric mixtures.319 Teixidor and co-workers isolated another conjuncto-metallacarborane, 8,90 -[closo-{3-Co(Z5-Cp)-1,2-C2B9H10}]2 (1117), from the reaction of closo-[3-Co(Z5-Cp)-1,2-C2B9H11] with sulfur in the presence of aluminium chloride under reflux conditions.320 In 1117, two large closo-CoC2B9 sub-clusters are connected by a BdB bond. A series of metallacarborane vertex fused clusters (1118–1395), in which two icosahedra are fused through a common vertex, have been synthesized by several groups. Most of these were synthesized from nido-11-vertex carboranes and exchanging ions of preformed vertex fused icosahedral metallacarboranes. In addition, many of these vertex-fused icosahedral metallacarboranes were functionalized at C/B vertices with different types of substituents, such as phosphines, phosphates, alkoxy, nitrile, alkyl, aryl, etc. For example, fused clusters 1122–1139 are examples of BdC bond formation between the boron vertex of fused clusters and different types of alkyl/aryl groups.321–324 Metallacarboranes 1140–1154 are utilized as building blocks for polyanionic polyarmed aryl-ether materials.325 Clusters 1155–1166 were functionalized with various polyethoxy groups.326,327 By contrast, in metallacarboranes 1167–1182, two icosahedra units are bridged through various phosphine ligands.328 Clusters 1206–1212 are examples of C-mono- and C-disubstituted derivatives of [3,30 -Co(1,2-C2B9H11)2]− having organosilane groups.329 Clusters 1224–1230330 and 1238–1247276,289 are the alkoxy derivatives of cobalt bis(dicarbollide). Compounds 1292–1296 are examples of crown ethers with the incorporated cobalt bis(dicarbollide) fragment.331 Cobaltacarboranes 1323–1334 are resulted by substitution at boron vertex with different types of nitrile/amine groups.332 Metallacarboranes 1351–1360 are examples of carbon substituted alkylhydroxy derivatives of cobalt bis(1,2-dicarbollide).333 Metallacarboranes 1364–1372 have various carboxylic groups at B/C-vertex.334

9.06.3.7

Metallacarborane clusters of group 10 (Table 15)

Although examples of group 10 metallacarboranes are not as well-known as group 8 and 9 systems, the limited examples available include a number with interesting structures. The reaction of a preformed arachno-platinacarborane [4,4-(PMe2Ph)2-4-PtB8H12] and phenylacetylene led to the addition of carbon vertices in the preformed metallaborane and afforded a CAp platinacarborane 1396.335 Platinacarborane 1396 has an open 4-membered PtB3 face and has a 9-vertex isonido-geometry (Fig. 54). On the other hand, stirring of another preformed nido-platinamonocarbaborane [8,8-(PMe2Ph)2-8,7-PtCB9H11] in methanol at roomtemperature led to the formation of the B-methoxy derivative of the starting material and afforded 11-vertex nido-platinamonocarbaborane 1397 (Fig. 54).155 Welch and co-workers isolated several icosahedral nickelacarborane and platinacarboranes. For example, the reaction of a preformed nido-carborane [5-I-7,8-Ph2-7,8-C2B9H8]2− and NiCl2(dppe) afforded nickelacarboranes 1398 and 1399.336 By contrast, the reaction between the same nido-carborane and cis-PtCl2(PMe2Ph)2 generated three platinacarboranes 1400–1402.336 Metallacarboranes 1398–1402 have icosahedral geometries and one of the boron vertices of 1398–1402 was functionalized with iodine (Fig. 55). Cluster 1398 is an example of a CAd metallacarborane, whereas 1399–1402 are examples of CAp metallacarboranes. When a similar type of diiodide nido-carborane [7,8-Ph2-9,11-I2-7,8-C2B9H8]2− was treated with the same platinum precursor

Table 15

Polyhedral Metallaboranes and Metallacarboranes

349

Metallacarboranes of group 10.

No.

Compounds

11

Av. MdB (Å)

Ref.

1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406

[7,7-(PMe2Ph)2-7,6,8-PtC2B6H7-6-Ph] [9-(O Me)-8,8-(PMe2Ph)2-8,7-PtCB9H10] [1,2-Ph2-4,4-dppe-12-I-4,1,2-NiC2B9H8] [1,8-Ph2-2,2-dppe-10-I-2,1,8-NiC2B9H8] [1,8-Ph2-2,2-(PMe2Ph)2-10-I-2,1,8-PtC2B9H8] [1,8-Ph2-2,2-(PMe2Ph)2-12-I-2,1,8-PtC2B9H8] [1,8-Ph2-2,2-(PMe2Ph)2-7-I-2,1,8-PtC2B9H8] [1,8-Ph2-2,2-(PMe2Ph)2-6,7-I2-2,1,8-PtC2B9H7] [1,8-Ph2-2,2-(PMe2Ph)2-6,12-I2-2,1,8-PtC2B9H7] [1,8-Ph2-2,2-(PMe2Ph)2-10,12-I2-2,1,8-PtC2B9H7] [3-Ph3P-3-(8-MeOCH2CH2N]C(Et)NH)3,1,2-NiC2B9H10] [3-Ph3P-3-(8-MeOCH2CH2N]C(Et)NH)3,1,2-PdC2B9H10] [3,3-(8-Me2NCH2CH2N]C(Et)NH)-3,1,2-NiC2B9H10] [3,3-(8-Me2NCH2CH2N]C(Et)NH)-3,1,2-PdC2B9H10] [3-Ph3P-3-(8-PrN]C(Et)NH)-3,1,2-NiC2B9H10] [3-PhMe2P-3-(8-PrN]C(Et)NH)-3,1,2-NiC2B9H10] [3-Bu3P-3-(8-PrN]C(Et)NH)-3,1,2-NiC2B9H10] [3-Cl-3-Ph3P-8-PrN]C(Et)NH-3,1,2-NiC2B9H10] [3-Cl-3-PhMe2P-8-PrN]C(Et)NH-3,1,2-NiC2B9H10] [3-Cl-3-Bu3P-8-PrN]C(Et)NH-3,1,2-NiC2B9H10] [3,3-dppe-3,1,2-NiC2B9H11] 7-[C(NHiPr)2]+[(Z5-7,8-C2B9H10)Ni-(PPh3)Cl]− 7-[C(NHCy)2]+[(Z5-7,8-C2B9H10)Ni-(PPh3)Cl]− [Ni(C2{C(NHCy)2}B9H10)2] [3,3’-Ni(8-SMe2-1,2-C2B9H10)2] [1-(1’-1’,2’-C2B10H11)-3-dppe-3,1,2-NiC2B9H10]

9.2, 4.8, −12.6, −19.6, −22.8 28.7, 11.7, 10.0, −8.5, −11.4, −13.5, −15.1, −27.4, −29.2 11.14, −4.73, −7.30, −12.26, −16.22 −1.81, −5.52, −9.91, −12.97, −16.40, −27.24 −2.07, −4.88, −10.88, −11.67, −16.75, −29.15 −1.63, −6.62, −9.83, −12.15, −15.73, −18.77 −1.32, −5.11, −9.32, −12.70, −14.99, −16.86, −21.91 −3.69, −8.61, −11.70, −15.44, −19.75, −23.93 −3.80, −11.38, −15.39, −18.09, −23.76 −6.74, −13.28, −14.91, −31.11 4.0, −11.2, −13.8, −23.6, −26.5

2.48 2.289 2.116 2.09 2.247 2.242 2.239 2.257 2.256 2.245 2.124

335 155 336 336 336 336 336 337 337 337 338

1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429

[2-(1’-1’,2’-C2B10H11)-4-dppe-4,1,2-NiC2B9H10] [8-(1’-1’,2’-C2B10H11)-2-dmpe-2,1,8-NiC2B9H10] [1-(1’-1’,2’-C2B10H11)-3,3-(PMe3)2-3,1,2-NiC2B9H10] [1-(1-1,2-C2B10H11)-3Cl-3-PMe3-8-PMe3-3,1,2-NiC2B9H9] [1-(1’-1’,2’-C2B10H11)-3,3(PMe2Ph)2-3,1,2-NiC2B9H10] [2-(1’-1’,2’-C2B10H11)-4,4(PMePh2)2-4,1,2-NiC2B9H10] [1-(1’-1’,2’-C2B10H11)-3,3{P(OMe)3}2-3,1,2-NiC2B9H10] [1-(1’-1’,2’-C2B10H11)-2,2{P(OMe)3}2-2,1,8-NiC2B9H10]

B NMR (ppm)

338

11.7, −10.2, −12.4, −15.3, −20.5, −24.5 −8.0, −14.7, −20.1, −25.0, −29.6 −4.4, −12.2, −16.4, −21.8, −26.8 4.0, −11.2, −13.6, −23.5, −26.7 4.2, −12.0, −14.0, −15.7, −23.8, −27.4 3.8, −11.8, −14.3, −15.1, −23.6, −26.8 1.9, −10.2, −12.3, −20.8, −24.2 3.5, −10.5, −13.4, −19.2, −22.9, −26.3 3.5, −10.6, −13.9, −20.3, −23.7, −26.3 −3.70, −5.51, −8.84, −14.67, −16.27, −24.11 −8.4, −16.3, −21.7, −25.5, −31.2, −36.3 − − 139.2, 72.6, 71.5, −26.2, −116.3 5.9, 2.6, −2.8, −5.3, −8.3, −10.2, −11.5, −12.9, −14.3, −15.8, −18.3 4.7, −3.4, −4.1, −6.9, −10.6, −13.2, −15.8 −3.6, −4.7, −6.0, −7.6, −10.6, −13.5, −16.9, −18.7, −20.3 0.5, −1.4, −2.6, −4.6, −10.3, −12.4, −15.4, −16.4, −21.8 −1.9, −3.3, −4.0, −5.3, −6.5, −9.6, −10.6, −12.6, −15.9, −17.7, −25.4 1.6, −0.7, −2.4, −4.7, −8.7, −9.7, −12.1, −16.2, −21.6

2.098 2.231 − − − − − − 2.108 2.109 − 2.188 2.195 2.146

338 338 339 339 339 339 339 339 340 341 341 298 275 342

2.093 2.077 2.116 2.129

342 342 342 342

2.112

342

5.1, −1.6, −3.2, −6.9, −10.4, −13.1, −15.0, −17.3

−

342

1.3, −0.6, −2.2, −4.6, −6.3, −9.7, −10.2, −13.1, −14.0, −18.7

2.102

342

−2.5, −5.6, −8.2, −9.8, −13.2, −17.6, −18.4

2.107

342

cis-PtCl2(PMe2Ph)2, it afforded bis-B-iodide platinacarboranes 1403–1405, which have the same CAp icosahedral core as that of

Fig. 54 Molecular structures of metallacarboranes 1396 and 1397.

350

Polyhedral Metallaboranes and Metallacarboranes

Fig. 55 Molecular structures of icosahedral metallacarboranes 1398–1402.

1400.337 The reactions of nido-carboranyl amidines [10-RNHC(Et)]HN-7,8-C2B9H11] (R ¼ ]CH2CH2OMe or CH2CH2NMe2) with [(Ph3P)2MCl2] (M ¼ ]Ni, Pd) in THF in the presence of tBuOK as a base led to the formation of icosahedral CAd metallacarboranes 1406–1409 (Scheme 41).338 In 1406 and 1407, one of the triphenylphosphine ligands was retained, and the amidine sidearm is coordinated at the metal center in k1(N)-fashion to form a five-membered MdBdNdCdN ring. By contrast, in 1408 and 1409, both triphenylphosphine ligands are not retained, and the metal center is coordinated by an amidine ligand in k2(N,N0 )-fashion. Similarly, the reactions of nickel(II) phosphine complexes [(PR2R’)2NiCl2] (R ¼ ]R0 ¼ ]Ph, Bu; R ¼ ]Me, R0 ¼ ]Ph) with another nido-carboranyl amidine [10-PrNHC(Et)]HN-7,8-C2B9H11] in THF at ambient temperature yielded icosahedral CAd nickelacarboranes 1410–1412, which are analogs of nickelacarborane 1406.339 Furthermore, the addition of HCl to acetonitrile solutions of nickelacarboranes 1410–1412 led to the opening of the five-membered {NiN2CB} ring and afforded nickelacarboranes 1413–1415.339 On the other hand, closo-o-carborane in the presence of ethanolic potassium fluoride yielded a deboronated product, nido-carborane K[7,8-C2B9H12], which underoes a capitation reaction in the presence of [Ni(dppe)Cl2] in refluxing benzene to form CAd nickelacarborane 1416.340 Cluster 1416 is a recyclable catalyst for selective carbene transfer reactions with low catalyst loading

Scheme 41 Synthesis of metallacarboranes 1406–1409.

under mild conditions. The reactions of nido-[7-{C(NHR)2}(7,8-C2B9H11)] (R ¼ ]iPr, Cy) with nBuLi in THF, followed by treatment with [NiCl2(PPh3)2] afforded zwitterionic nickel dicarbollide complexes 1417 and 1418, respectively.341 Compounds 1417 and 1418 have CAd icosahedral core geometries, and one of the carbon vertices of these clusters is attached to an amidinate ligand. By contrast, the treatment of nido-[7-{C(NHCy)2}(7,8-C2B9H11)] with nBuLi in THF, followed by treatment with [Ni(acac)2] afforded bis(3-1,2-carbollyamidinate)-nickel(II) complex 1419.298 Nickelacarborane 1419 is a vertex fused cluster in which two icosahedra is fused through a common Ni vertex (Fig. 56). Teixidor and co-workers isolated a similar type sandwich nickellacarborane 1420 from the reaction of nido-[10-SMe2-7,8-C2B9H10]− with [NiCl2
6H2O].275 In the vertex fused cluster 1420, two of the

Polyhedral Metallaboranes and Metallacarboranes

351

Fig. 56 Molecular structures of metallacarboranes 1419, 1421–1423, and 1429.

boron vertices are ligated by SMe2 groups. Spokoyny and co-workers utilized a similar type of fused nickelacarborane as a non-corrosive redox shuttle for dye-sensitized solar cells (Scheme 42).11,12 They also incorporated nickel(IV) bis(dicarbollide) in a Zr-based metal-organic framework, NU-1000, to create an electrically conductive MOF with mesoporosity.13 Capitations of the conjuncto-carborane [7-(10 -10 ,20 -closo-C2B10H11)-7,8-nido-C2B9H10]2− with various Ni(II)-precursors, such as [NiCl2(dppe)], [NiCl2(dmpe)], [NiCl2(PMe3)2], [NiCl2(PMe2Ph)2], [NiCl2(PMePh2)2], [NiBr2{P(OMe)3}2], or [Ni3Cl5(tmeda)3]Cl led to the isolation a series of conjuncto-nickelacarboranes 1421–1429.342 Two icosahedra cores are connected through carbon vertices in 1421–1429. They can only be differentiated by their carbon positions and ligands at the metal vertex. Clusters 1421, 1424–1426, and 1428 have the same core, in which carbon atoms are at adjacent vertices in both the icosahedral cores. Clusters 1422 and 1427 also have the same core with two CAd icosahedral. By contrast, 1423 and 1429 have one CAd icosahedron and one CAp icosahedron.

Scheme 42 Oxidation-reduction of Ni-bis(dicarbollide) complexes. Oxidation: in situ aq. FeCl3, 5 min; reduction: MeOH, NaBH4.

352

Polyhedral Metallaboranes and Metallacarboranes

Metallacarboranes of f-block.

Table 16 No.

Compounds 1

[Z :Z -(Me2NCH2CH2)C2B9H10]Sm[N-(SiHMe2)2](THF)2 [Z1:Z5-(Me2NCH2CH2)C2B9H10]Er[N-(SiHMe2)2](THF)2 [Z1:Z5-(Me2NCH2CH2)C2B9H10]YCl(THF)2

1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448

[Z1:Z1:Z5-(Me2NCH2CH2)(MeOCH2CH2)C2B9H9]Y(m-Cl)2K(THF)2 ([Z1:Z1:Z5-(Me2NCH2CH2)(MeOCH2CH2)C2B9H9]Y-(Me3Si)2C5H3) [Z1:Z1:Z5-(Me2NCH2CH2)(MeOCH2CH2)C2B9H9]Er-((Me3Si)2C5H3) [(ImArN)Sc(Z5-C2B9H11)(THF)2] [(ImArN)Y(Z5-C2B9H11)(THF)2] [(ImArN)Lu(Z5-C2B9H11)(THF)2] [(ImArN)Sc{s:Z5-(Me2NHCH2CH2)C2B9H10}(THF)] [(ImArN)Y{s:Z5-(Me2NHCH2CH2)C2B9H10}(THF)] [(ImArN)Lu{s:Z5-(Me2NHCH2CH2)C2B9H10}(THF)] [(ImArN)Y{s:s:Z5-(Me2NCH2)2C2B9H9}(THF)] [(ImArN)Y{s:s:Z5-(Me2NCH2CH2)(MeOCH2CH2)-C2B9H9}] [{Z1:Z5-O(CH2)2C2B9H9}Gd(THF)(m-Cl)2Na(THF)2]2 [Z1:Z5-O(CH2)2C2B9H9]Y(s:Z1-CH2C6H4-o-NMe2)(THF)2 [Z1:Z5-O(CH2)2C2B9H9]Gd(s:Z1-CH2C6H4-o-NMe2)(THF)2 [Z1:Z5-O(CH2)2C2B9H9]Er(s:Z1-CH2C6H4-o-NMe2)(THF)2 [Z1:Z5-O(CH2)2C2B9H9]Y[Z2-(CyN)2C(CH2C6H4-o-NMe2)](DME)

1449 1450 1451 1452

[Z1:Z5-O(CH2)2C2B9H9]Y[Z2-(iPrN)2C(CH2C6H4-o-NMe2)](DME) [Z1:Z5-O(CH2)2C2B9H9]Y[C5H4NC(Ph)(CH2C6H4-o-NMe2)O](THF)2 [Z1:Z5-O(CH2)2C2B9H9]Er[C5H4NC(Ph)(CH2C6H4-o-NMe2)O](THF)2 [Z1:Z5-O(CH2)2C2B9H9]Y[OC(]NC6H3Me2)N(C6H3Me2)C(CH2C6H4-o-NMe2)O](THF)2 [Z1:Z5-O(CH2)2C2B9H9]Y[C(]NC6Hi3Pr2)C(vNC6Hi3Pr2)CH2C6H4-o-NMe2](DME) [Z1:Z5-O(CH2)2C2B9H9]Y(CH2C6H4-o-NMe2)(CNC6Ht2Bu3) [s:Z5-(C9H6)C2B9H10]Y(DME)2 [s:Z5-{OC(Py)(Ph)C9H6}C2B9H10]Y(THF)2 [s:Z5-{OC(Ph2)C9H6}C2B9H10]Y(THF)3 [s:Z5-{OC(Py2)C9H6}C2B9H10]Y(DME) [s:Z5-{OC(]CPh2)C9H6}C2B9H10]Y(THF)3

1454 1455 1456 1457 1458 1459

9.06.3.8

B NMR (ppm)

5

1430 1431 1432

1453

11

− − −2.2, −4.5, −8.7, −10.0, −13.0, −17.2, −28.0, −32.7, −36.3 −2.7, −9.3, −16.8, −27.4, −32.9, −35.9 −5.9, −8.4, −17.0, −27.2, −32.7, −35.8 − −34.2, −29.44, −18.6, −13.8, −13.0, −7.12 −37.9, −33.2, −22.2, −17.2, −16.7, −10.64 −37.6, −32.9, −21.9, 17.1, −16.4, −10.5 −33.9, −27.6, −8.0, −1.6 −34.7, −32.9, −29.3, −9.8, −5.9, −4.4 −34.7, −29.0, −9.3, −4.1 −7.0, −10.3, −12.7, −32.5 −5.2, −8.5, −10.8, −31.3 − −3.4, −6.7, −10.9, −20.8, −28.2 − − −12.0, −14.8, −16.0, −18.0, −18.9, −30.9, −35.5, −39.6 −10.7, −11.6, −15.5, −17.3, −18.5, −19.9, −38.9 −9.1, −11.6, −18.5, −21.1, −30.5 − 6.7, −10.0, −12.9, −16.9, −18.1, −29.0, −33.6 −7.0, −11.0, −11.6, −14.3, −18.6, −20.6, −18.1, −29.0, −30.3, −35.2 −7.7, −11.5, −18.5, −20.8, −30.5 − −4.0, −8.3, −11.0, −15.3, −26.5, −32.1, −.9 −4.2, −8.9, −11.1, −13.5, −17.2, −26.6 − −3.8, −8.4, −10.1, −12.6, −16.1, −25.8

Av. MdB (Å)

Ref.

2.705 2.656 2.585

343 343 343

− 2.621 2.606 2.486 2.618 − 2.489 2.611 2.565 2.686 2.576 2.646 2.666 − − 2.680

343 343 343 344 344 344 344 344 344 344 344 345 345 345 345 345

− 2.692 − 2.668

345 345 345 345

2.656

345

− 2.652 2.751 2.726 − −

345 346 346 346 346 346

Metallacarborane clusters of f-block elements (Table 16)

As with d-block metallacarboranes, f-block polyhedral metallacarboranes were also pioneered by Xie and co-workers. In most of the syntheses of metallacarboranes of f-block, the open face of nido-carboranes was capped. For example, the reaction of 11-vertex nido-carborane [7-Me2NHCH2CH2-7,8-C2B9H11] with Ln[N(SiHMe2)2]3(THF)2 (Ln ¼ ]Sm or Er) afforded icosahedral metallacarboranes 1430 and 1431, respectively (Scheme 43).343 In 1430 and 1431, the lanthanide centers are s-bonded to N(SiHMe2)2 ligand and coordinated to the N atom of the sidearm and two THF ligands. By contrast, the reaction of the same nido-carborane with ClY[N(SiHMe2)2]2(THF)x yielded icosahedral 1432.343 Clusters 1430–1432 have a similar core, but in place of N(SiHMe2)2 ligand one Cl atom is bonded to Y center in 1432. On the other hand, the reaction of another nido-carborane [(Me2NCH2CH2)(MeOCH2CH2)C2B9H9], having two substituents at the carbon vertices, with KH and followed by treatment of YCl3 afforded another icosahedral metallacarborane 1433.343 The Y center of 1433 is coordinated with the O and N atoms of the sidearm and bonded with two Cl atoms. Reactions of the same nido-carborane with KH and followed by treatment of 1 equiv. Cp”2LnI (Ln]Y or Er) in THF afforded mixed-sandwich lanthanacarboranes 1434 and 1435, respectively.343 Compound 1434 was also prepared from an equimolar reaction of 1433 with Cp”Li in THF. The lanthanide center of 1434 and 1435 is Z5-bound to the Cp” ligand and coordinated to one O atom and one N atom of the two sidearms. On the other hand, several metallacarboranes (1436–1448) of rare-earth-metals having both imidazolin-2-iminato and dicarbollide ligands were prepared by the reaction of imidazolin-2-iminato rare-earth-metal dichlorides [(ImArN)MCl2(THF)3] with 1 equiv. Na2[C2B9H11].344 On the other hand, reactions of [(ImArN)M(CH2SiMe3)2(THF)n] with 1 equiv. of another nido-carboranes [(Me2NHCH2CH2)C2B9H11] in THF afforded the metallacarboranes 1439–1441.344 In a similar manner, two more metallacarboranes 1442 and 1443 were prepared from the reactions of [(ImArN)Y(CH2SiMe3)2(THF)2] and nido-carboranes [7-Me2NHCH2-8-Me2NCH2-7,8-C2B9H10] or [7-Me2NHCH2CH2-8-MeOCH2CH2-7,8-C2B9H10], respectively.344 In clusters 1436–1443, the metals are connected with nido-carboranes in Z5-fashion to form icosahedron. Further, the imidazolin-2-iminato ligands are coordinated with the metals in exopolyhedral manner.

Polyhedral Metallaboranes and Metallacarboranes

353

Scheme 43 Synthesis of f-block metallacarboranes 1430, 1431, and 1433–1438.

On the other hand, the reactions of nido-carborane [Me3NH][7,8-O(CH2)2-7,8-C2B9H10] with one equiv. of lanthanide precursors [Ln(CH2C6H4-o-NMe2)3] (Ln ¼ ]Y, Gd or Er) afforded the first examples of rare-earth metallacarborane alkyl species 1445–1447 via alkane elimination.345 The Ln center of 1445–1447 is coordinated by an O atom from the sidearm, one N and one C atom from the CH2C6H4-o-NMe2 group, and two THF molecules. By contrast, another dimeric metallacarborane 1444 was synthesized from the reaction of the same nido-carborane and NaH in THF followed by treatment with GdCl3. Apart from the

354

Polyhedral Metallaboranes and Metallacarboranes

cluster part of 1444, each Gd is s-bound to two doubly bridging Cl atoms and coordinated to two oxygen atoms from the sidearm and THF molecule, respectively, to form a dimeric structure via two Na(THF)+2 moieties. Interestingly, compounds 1444, 1446, and 1447 are paramagnetic species. Like d-block metallacarboranes, f-block metallacarboranes were also taken part in reactions at lanthanide centers. For example, metallacarboranes 1445 and 1447 undergo multiple reactions with DCC (DCC ¼ N,N0 -dicyclohexylcarbodiimide), DIC (DIC ¼ N,N0 -diisopropylcarbodiimide), 2-benzoylpyridine, 2,6-dimethylphenyl isocyanate, 2,6-diisopropylphenyl isocyanide, or 2,4,6-tri-t-butylphenyl isocyanide to afford metallacarboranes 1448–1454 (Scheme 44).345 All of these products are the result of insertion at the pendant CH2C6H4-o-NMe2 group except 1454. Metallacarborane 1454 was generated by replacing two THF molecules and coordinating 2,4,6-tri-t-butylphenyl isocyanide to the Y center. These results showed that the reactivity pattern of the LndC s bond in rare-earth metallacarborane alkyls was dependent on the nature of the unsaturated organic molecules.

Scheme 44 Synthesis of f-block metallacarboranes 1448 and 1450–1454.

Xie and co-workers also isolated a rare-earth metallacarborane alkyl 1455 without any appended donor functionalities in the ligands.346 Yttracarborane 1455 was prepared from the reaction of nido-carborane [7-indenyl-7,8-C2B9H11][Me3NH] with 1 equiv. Y(CH2C6H4-o-NMe2)3 in DME. Yttracarborane 1455 is a highly constrained-geometry molecule with a very long YdC bond and an extremely small CdCdY angle. Yttracarborane 1455 also underwent further insertion reaction at the YdC bond with 1 equiv. 2-benzoylpyridine, diphenylketone, di(2-pyridyl)ketone, or diphenylketene to afford yttracarboranes 1456–1459 (Scheme 45).346

Polyhedral Metallaboranes and Metallacarboranes

355

Scheme 45 Synthesis of f-block metallacarboranes 1456–1459.

9.06.4

Supra-icosahedral metallaborane and metallacarborane clusters (Table 17)

Utilizing a reduction-capitation methodology, a series of metallacarboranes having supra-icosahedral clusters were synthesized. For example, reduction of [1,12-closo-C2B10H12] and its C,C-dimethyl analog [1,12-Me2-closo-C2B10H12] with Na in liquid NH3 followed by metalation with different {(arene)Ru}2+ fragments led to the isolation of 13-vertex ruthenacarborane 4,1,10-MC2B10 isomers (1460–1464 and 1466) (Scheme 46).257,347,348 Along with the formation of 1464, another 13-vertex ruthenacarborane 4,1,11-MC2B10H10 isomer (1465) was isolated by low-temperature reduction and metalation of [1,12Ph2-1,12-closo-C2B10H10].348 The 4,1,10-MC2B10 isomers (1460–1464 and 1466) isomerize to their respective 4,1,12-MC2B10 isomers (1467–1471) in toluene under reflux conditions (Scheme 46).257,347 On the other hand, another 13-vertex 4,1,12MC2B10 isomeric ruthenacarborane 1472 was synthesized directly from the reduction of [1,12-Me2-1,12-closo-C2B10H10] by Na metal in liq. NH3 and metalation with [(p-cym)RuCl2]2.349 All of these 13-vertex metallacarboranes (1460–1472) have a docosahedral core, in which the metal center occupies the degree-six vertex, and one of the degree-four vertices is occupied by a carbon atom. All of the 13 vertex ruthenacarboranes (1460–1472) have 14 SEP. Furthermore, reduction of [4-(p-cym)-4,1,12closo-RuC2B10H12] (1467) followed by metalation with CoCl2/NaCp, [(Z-C6H6)RuCl2]2, [(p-cym)RuCl2]2 and (dppe) NiCl2 afforded 14-vertex bimetallic metallacarboranes 1577–1580, respectively (Scheme 46).350 These 14-vertex metallacarboranes (1577–1580) have a bicapped hexagonal antiprismatic geometry, in which both the degree-six vertices are occupied by metal center owing to their more diffuse orbitals.

356

Polyhedral Metallaboranes and Metallacarboranes

Table 17 No.

Supra-icosahedral metallaboranes and metallacarboranes.

Compound

13-vertex clusters 1460 [4-(p-cym)-4,1,10-RuC2B10H12] 1461 [4-(Z-C6H6)-4,1,10-RuC2B10H12] 1462 [4-(Z-C6Me6)-4,1,10-RuC2B10H12] 1463 [1,10-Me2-4-(Z-C6H6)-4,1,10-RuC2B10H10] 1464 [1,10-Ph2-4-(p-cym)-4,1,10-RuC2B10H10] 1465 [1,11-Ph2-4-(p-cym)-4,1,11-RuC2B10H10] 1466 [4-(Z-C10H8)-4,1,10-RuC2B10H12] 1467 [4-(p-cym)-4,1,12-RuC2B10H12] 1468 [4-(Z-C6H6)-4,1,12-RuC2B10H12] 1469

[4-(Z-C6Me6)-4,1,12-RuC2B10H12]

1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484

[1,12-Me2-4-(Z-C6H6)-4,1,12-RuC2B10H10] [4-(Z-C10H8)-4,1,12-RuC2B10H12] [1,12-Me2-4-(p-cym)-4,1,12-RuC2B10H10] [4-(p-cym)-4,1,6-RuC2B10H12] [1,6-Ph2-4-(Z-C6H6)-4,1,6-RuC2B10H10] [1,6-Ph2-4-(p-cym)-4,1,6-RuC2B10H10] [4-(Z-C10H8)-4,1,6-RuC2B10H12] [BTMA][4-Cp -4,1,6-RuC2B10H12] [1,6-Me2-4-(p-cym)-4,1,6-RuC2B10H10] [1,2-Me2-4-(p-cym)-4,1,2-RuC2B10H10] [4-(p-cym)-1,2-Me2Si(CH2)2-4,1,2-RuC2B10H10] [4-(p-cym)-1-TMSCH2-2-Me-4,1,2-RuC2B10H10] [1,8-Ph2-4-(p-cym)-4,1,8-RuC2B10H10] [1,12-Ph2-4-(p-cym)-4,1,12-RuC2B10H10] [4-(Z-C10H8)-4,1,8-RuC2B10H12]

1485 1486 1487 1488 1489 1490

[1,2-(CH2)3-4-(p-cym)-4,1,2-RuC2B10H10] [4,5-(p-cym)2-4,5,2,3-Ru2C2B9H11] [4,5-(p-cym)2-4,5,1,6-Ru2C2B9H11] [1,6-Ph2-4-(Z-C6H6)-5-(p-cym)-4,5,1,6-Ru2C2B9H9] [4,5-Cp 2-4,5,1,6-Ru2C2B9H11] [(Z5-C5H5){Z6-[C6H4(CH2)2]-4-(THF)-C2B10H10}]Fe

1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513

[1-Cp-1,2,7,9-FeC3B9H11] [2-Cp-1-tBuHN-4,1,6,8-FeC3B9H11] [Cp-tBuNH-FeC3B9H12] [Cp-tBuNH-FeC3B9H12] [4,5-Cp2-4,5,1,6,7-Fe2C3B8H11] [4,5-Cp2-4,5,1,7,12-Fe2C3B8H11] [7-tBuNH-4,5-Cp2-4,5,1,7,12-Fe2C3B8H11] [4-Cp-4,1,10-CoC2B10H12] [N(PPh3)2][4,4-COD-4,1,10-RhC2B10H12] [1,2-(CH2)3-4-Cp-4,1,2-CoC2B10H10] [1,2-(CH3)2-4-(Z-C9H7)-4,1,2-CoC2B10H10] [1,2-Me2-4-Cp-4,1,2-CoC2B10H10] [1,6-Me2-4-Cp-4,1,6-CoC2B10H10] [1,8-Me2-4-Cp-4,1,8-CoC2B10H10] [1,6-Ph2-4-Cp-4,1,6-CoC2B10H10] [1,8-Ph2-4-Cp-4,1,8-CoC2B10H10] [1,12-Ph2-4-Cp-4,1,12-CoC2B10H10] [4-(Z-C9H7)-4,1,6-CoC2B10H12] [1,6-Me2-4-(Z-C9H7)-4,1,6-CoC2B10H10] [1(or6)-Me-4-(Z-C9H7)-4,1,6-CoC2B10H11] [4-(Z-C9H7)-4,1,10-CoC2B10H12] [1,2-l-(CH2)3-4-(Z-C9H7)-4,1,2-CoC2B10H10] [4-(Z-C9H7)-4,1,8-CoC2B10H12]

1514

[1,8-Me2-4-(Z-C9H7)-4,1,8-CoC2B10H10]

1515

[4-(Z-C9H7)-4,1,12-CoC2B10H12]

11

B NMR (ppm)

−2.23, −4.43, −8.69, −10.61, −14.83, −16.72 −3.27, −5.42, −9.05, −11.02, −15.74, −17.34 −0.58, −4.07, −10.44, −15.33, −16.88 1.60, −2.05, −6.22, −7.08, −10.61, −14.74 0.1, −5.0, −9.8, −13.7 10.8, −4.6, −8.0, −12.4, −24.3 −0.12, −4.14, −9.08, −14.67, −1216.36 2.84, −0.61, −2.37, −4.70, −10.19, −13.73, −14.89, −23.22 1.79, −1.52, −3.25, −5.50, −11.07, −14.07, −15.42, −23.67, −24.14 4.43, 0.51, −2.61, −4.66, −9.04, −13.36, −14.28, −15.81, −24.16, −24.71 2.30, −0.10, −2.99, −5.81, −6.95, −7.78, −10.73, −19.76, −21.03 3.41, 0.00, −3.17, −9.29, −10.20, −14.08, −22.73 3.69, −5.54, −6.71, −8.42, −9.78, −11.92, −19.84, −21.04 11.06, 1.40, −1.47, −6.69, −7.77, −10.56, −19.69 11.60, 4.75, −0.50, −5.03, −15.43 14.15, 4.18, −0.92, −5.16, −16.67 9.06, −1.07, −3.76, −8.26, −9.46, −13.31, −23.36 4.9, −9.5, −11.5, −15.2, −18.4, −19.4, −32.1 13.97, 3.67, 2.13, −0.64, −3.49, −5.94, −14.02 14.19, 11.52, 5.21, 3.21, −5.98, −11.96 6.9, 6.2, −0.2, −1.4, −9.4, −16.6 10.2, 7.7, 4.9, 0.7, −1.6, −8.8, −12.5, −15.7, −18.6 16.3, 5.3, −4.3, −7.0, −9.8, −20.4 3.7, 2.7, −8.3, −9.5, −19.8, −21.2 15.68, 3.41, 1.78, −1.98, −6.32, −7.68, −10.00, −10.21, −13.43, −19.36 5.78, 4.34, 1.09, −1.35, −8.17, −17.66 12.54, −2.00, −5.26 6.55, 0.74, −6.14, −12.07, −16.78 9.3, 6.5, 0.8, −1.4, −9.0, −11.2 31.1, 28.3, 23.2, 16.8, 0.7, −8.0, −14.3 23.63, 4.19, 0.43, −2.11, −5.84, −12.22, −17.78, −18.73, −21.39, −22.14 −7.0, −9.1, −15.4, −17.8, −24.3 0.6, −11.0, −12.7, −13.9, −15.6, −21.0, −27.8 −5.2, −7.1, −8.5, −13.5, −16.1, −18.5, −22.3, −23.3, −24.9 −4.9, −7.2, −10.2, −18.9, −21.6, −44.0 − − − 3.71, 1.84, −4.09, −6.89, −9.40, −12.62 −4.5, −6.0, −12.3, −13.5, −16.2, −17.2 13.29, 6.18, 4.19, 3.66, −4.09, −10.93 17.02, 11.93, 4.58, 2.28, −6.72, −9.41 14.52, 10.83, 3.79, −6.09, −8.89 13.59, 3.00, 1.85, −1.63, −4.44, −5.78, −11.18 18.77, 10.56, 9.83, 2.69, 2.26, −1.04, −2.11, −5.71, −12.49 15.1, 2.8, −2.9, −5.5, −12.9 21.6, 7.9, 3.9, −1.3, −4.1, −13.0 7.7, 5.4, −1.6, −5.0, −7.4, −13.8 14.54, 1.95, −0.52, −3.80, −11.90, −16.89 15.08, 4.87, 0.33, −0.47, −4.79, −7.28, −12.05 14.32, 8.20, −0.10, −2.38, −3.43, −4.59, −7.59, −10.21, −16.46 4.92, 3.86, −7.26, −9.79, −12.77 16.24, 7.64, 5.54, 2.25, −4.67, −10.45 22.61, 9.32, 7.43, 6.34, −0.92, −5.80, −7.23, −8.69, −9.72, −12.52 21.23, 13.23, 10.79, 5.22, 3.34, −2.11, −2.57, −2.97, −5.67, −13.19 8.78, 6.67, 3.99, 3.15, −2.78, −10.02, −14.32, −15.85, −17.32

Av. M-B (Å)

Ref.

2.294 − − − 2.289 2.282 2.277 − −

347 347 347 347 348 348 257 347 347

−

347

2.255 2.247 2.275 2.284 2.286 2.29 2.269 2.266 2.29 2.273 2.286 2.247 2.281 2.273 2.268

347 257 349 351 351 351 257 352 349 349 353 353 354 354 257

2.283 2.227 2.286 2.317 2.244 2.17

355 256 256 264 352 271

− 2.137 − − 2.171 2.169 2.159 2.19 2.283 2.182 2.182 2.182 2.172 2.169 2.174 2.167 2.169 2.180 2.172 − 2.176 2.185 −

248 248 248 248 356 356 356 347 357 355 358 349 349 349 354 354 354 358 358 358 358 358 358

2.168

358

2.154

358

Polyhedral Metallaboranes and Metallacarboranes

Table 17

357

(Continued)

No.

Compound

11

Av. M-B (Å)

Ref.

1516 1517 1518 1519 1520 1521 1522 1523

[1,12-Me2-4-(Z-C9H7)-4,1,12-CoC2B10H10] [1-Me-4-(Z-C9H7)-4,1,8-CoC2B10H11] [8-Me-4-(Z-C9H7)-4,1,8-CoC2B10H11] [1,12-(4’-F3CC6F4)2-4-Cp-4,1,12-CoC2B10H10] [1,12-(4’-F3CC6F4)2-4-Cp -4,1,12-CoC2B10H10] [1,12-Me2-4-Cp-4,1,12-CoC2B10H10] [1-Ph-4,5-(Z-Cp)2-6-(4’-F3CC6F4)-4,5,1,6-Co2C2B9H9] [1,4-m-[2’-(C5H4)-4’-F3CC6F3]-6-Ph-4,1,6-closoCoC2B10H10] [Cp{Z6-[C6H4(CH2)2]C2B10H10}]Co [Cp{Z6-[C6H4(CH2)2]C2B10H10}]Co [Cp{Z6-[C6H4(CO)(CH2)]C2B10H10}]Co [Cp{Z6-[C6H4(CHOH)(CH)2]C2B10H10}]Co [4-Cp-2,3,5,8,9,11,12,13-Me8-4,1,6-CoC2B10Me8H4] [4-Cp-2,3,5,9,10,11,12,13-Me8-4,1,6-CoC2B10Me8H4] [4-Cp-1,9-Et2-2,5,6,7,8,11,12,13Me8-4,1,9-CoC2B10Me8H4] [4-Cp-2,3,5,6,8,11,12,13-Me8-4,1,9-CoC2B10Me8H4] [4-Cp-2,3,5,6,7,11,12,13-Me8-4,1,9-CoC2B10Me8H4] [4-nBuCp-2,3,5,8,9,11,12,13-Me8-4,1,6-CoC2B10Me8H4] [4-nBuCp-2,3,5,9,10,11,12,13Me8-4,1,6-CoC2B10Me8H4] [4-MeCp-2,3,5,9,10,11,12,13Me8-4,1,6-CoC2B10Me8H4] [1,6-PPh2-4-MesBIAN-4,1,6-CoC2B10H10] [4,5-Cp2-4,5,2,3-Co2C2B9H11] [Z1:Z6-(Me2NCH2CH2)C2B10H11]Hf-(CH2SiMe3)2 [4-(dppe)-4,1,10-NiC2B10H12] [4-(dppe)-4,1,12-NiC2B10H12] [1,2-(CH2)3-4,4-(dppe)-4,1,2-NiC2B10H10] [1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-PtC2B10H10] [4,4-(PMe2Ph)2-4,1,6-PtC2B10H12] [4,4-(PMe2Ph)2-4,1,10-PtC2B10H12] [Z1:Z6-(Me2NCH2CH2)C2B10H11]Y-Cl(THF)3 [Z1:Z6-(Me2NCH2CH2)C2B10H11]Y-Cl(MeCN)3
MeCN [Z1:Z6-(Me2NCH2CH2)C2B10H11]Y-((Me3Si)2C5H3) (m-Cl)Li(THF)2 [{Z5:Z6-(2-C9H6)(C2B10H11)Y(THF)}2(m-Cl)][Na(THF)6] [{Z5:Z6-(2-C9H6)(C2B10H11)Er(THF)}2(m-Cl)][Na(THF)6] [{Z5:Z6-(1-C9H6)(C2B10H11)Y(THF)}2(m-Cl)][Na(THF)3] [{Z5:Z6-(1-C9H6)(C2B10H11)Nd-(THF)}2(m-Cl)][Na(THF)3] [{Z5:Z6-(1-C9H6)(C2B10H11)Er(THF)}2(m-Cl)][Na(THF)3] {[Z5:Z6-(1-C9H6)(C2B10H11)Er(THF)]2(m-BH4)}{Na(THF)2} [Z5:Z7-(1-C9H6)(C2B10H11)Er(THF)]2-Na4(THF)8 [K(18-crown-6)]2[4,4’-Fe(1,10-C2B10H12)2] [BTMA]2[4,4’-Fe(1,10-C2B10H12)2] [K(18-crown-6)][4,4’-Fe-(1,10-C2B10H12)2] [BTMA][4,4’-Fe-(1,10-C2B10H12)2] [BTMA][4,4’-Co-(1,10-C2B10H12)2] [K(18-crown-6)][4,4’-Co-(1,10-C2B10H12)2] [K(18-crown-6)]2[4,4’-Ti-(1,10-C2B10H12)2] [K(18-crown-6)]2[4,4’-Ni-(1,10-C2B10H12)2] [1-(1’,2’-C2B10H11)-4-{C10H14Ru(p-cym)}4,1,6-RuC2B10H11] [1-(1’-1’,2’-C2B10H11)-4-(Z-C6H6)-4,1,6-RuC2B10H11] [1-(1’-1’,2’-C2B10H11)-4-(Z-p-cym)-4,1,6-RuC2B10H11]

10.18, 8.97, 8.21, 4.23, 1.86, −5.19, −5.75, −8.02, −14.36 23.16, 11.23, 8.88, 6.77, 0.26, −5.20, −6.26, −7.30, −13.22 20.44, 11.66, 8.71, 4.62, 2.09, −2.74, −3.52, −7.07, −12.63 7.43, 4.65, 3.15, 0.08, −4.10, −5.76, −9.00, −13.60 8.78, 4.73, 1.93, 0.12, −2.21, −7.98, −9.76, −14.00 7.52, 6.39, 2.70, −0.81, −3.69, −5.91, −7.21, −13.69 20.0, 14.0, 12.1, 8.3, 4.2, 0.6, −1.6, −5.7, −9.0 −

2.164 − 2.173 2.165 − 2.166 2.191 2.178

358 358 359 310 310 349 264 264

12.50, 8.46, 2.94, 2.24, −6.45, −10.27 16.81, 15.55, 10.94, 6.12, 5.14, −3.53, −8.48, −9.28, −10.43 11.58, 10.86, 7.41, 4.22, 1.88, 0.57, −8.84, −9.87, −11.06 15.6, 14.5, 13.5, 5.3, 0.63, −4.1, −5.0, −6.2 13.0, 10.8, 5.0, 0.6, −1.6, −2.7, −4.1, −8.3 30.5, 16.0, 13.4, 7.8, 5.1, 3.0, −0.6, −5.2

2.176 2.176 2.185 2.183 2.176 2.187 2.182

271 271 271 271 359 359 359

23.2, 17.1, 13.0, 9.1, 6.6, 5.8, 3.2, 1.2, −4.7 28.6, 14.1, 11.5, 9.7, 3.8, 0.9, −9.9 15.4, 13.9, 12.6, 5.0, 0.2, −4.5, −6.6 13.5, 12.7, 5.8, 1.7, 0.1, −2.3, −3.2, −7.0

2.185 2.210 2.182 2.183

359 359 359 359

12.0, 11.0, 4.2, 0.2, −1.4, −3.8, −4.8, −8.5

2.176

359

13.3, 0.4, −9.6, −20.9 25.30, 8.75, 2.99 7.9, 3.5, 0.4, −3.2, −5.4, −21.3 −0.57, −4.38, −8.42 1.88, −2.75, −8.72, −11.58, −13.59, −16.86 9.77, 4.36, 0.68, −3.10, −9.87 3.60, 0.85, −1.34, −6.64, −10.99 −0.98, −2.6, −4.54, −9.46, −13.85, −15.0 15.91, 11.04, 0.17, −8.38, −16.3, −22.83 16.2, 10.6, 0.3, −7.7, −12.4, −20.4, −29.5 15.9, 10.2, 1.0, −8.2, −12.9, −19.8, −30.1 14.5, −1.5, −0.8, −5.6, −10.3, −23.7

2.172 2.103 2.581 2.209 2.191 2.171 2.30 2.367 2.336 − 2.821 2.816

360 256 361 347 347 355 355 357 357 343 343 343

9.1, 2.0, −4.7, −8.3, −12.9 − 2, −0.8, −4.7, −7, −9.7, −12.9, −15.6, −17.9 − − − − −2.83, −13.64, −15.66, −16.61 −2.84, −13.62, −15.64, −16.61 − − 6.55, −0.72, −5.43, −9.79 7.62, 0.50, −4.68, −8.95 7.42, 3.58, −12.88, −14.43, −28.31 −6.29, −12.10, −14.83 5.57, 2.60, −4.03, −5.61, −7.44, −8.37, −10.61, −13.55, −24.44

− 2.331 2.712 − 2.706 2.724 2.703 2.234 − 2.257 2.263 2.241 − 2.405 2.269 2.318

362 362 362 362 362 362 362 363 363 363 363 363 363 363 363 364

8.0, 3.3, 2.0, −3.5, −4.1, −6.6, −8.1, −9.9, −11.2, −12.8, −23.7 10.2, 3.3, 1.8, −3.5, −4.0, −6.7, −8.3, −9.9, −11.1, −12.0, −12.9, −24.1 19.8, 8.4, 1.7, −1.8, −2.7, −5.1, −7.7, −8.7, −11.7, −18.1 11.2, 4.0, 1.3, −0.8, −4.3, −4.9, −9.2, −11.3, −13.8

2.279 2.286

365 365

2.269 −

365 365

17.1, 8.5, 6.2, 4.1, 0.9, 0.2, −3.9, −6.1, −8.7

2.303

365

1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568

[8-(1’-1’,2’-C2B10H11)-4-(Z-mes)-4,1,8-RuC2B10H11] [4-(Z-C6H6)-5-(Z-mes)-6-(1’-1’-2’-C2B10H11)4,5,1,6-Ru2C2B9H10] [4-(Z-p-cym)-5-(Z-mes)-6-(1’-1’,2’-C2B10H11)4,5,1,6-Ru2C2B9H10]

B NMR (ppm)

(Continued )

358

Polyhedral Metallaboranes and Metallacarboranes

Table 17 No.

(Continued)

Compound

[1,13-(Z-mes)2-9-(1’-1’,2’-C2B10H11)1,13,2,9-Ru2C2B10H11] 1570 [1-(4’-Cp-4’,1’,6’-CoC2B10H11)-4-Cp-4,1,6-CoC2B10H11] 1571 [1-(4’-Cp-4’,1’,6’-CoC2B10H11)-4-Cp-4,1,6-CoC2B10H11] 1572 rac-[1-(1’-4’-Cp-4’,1’,12’-CoC2B10H11)-4-Cp-4,1,12CoC2B10H11].2THF 1573 meso-[1-(1’-4’-Cp-4’,1’,12’-CoC2B10H11)-4-Cp-4,1,12CoC2B10H11] 1574 [1-(1’-1’,2’-C2B10H11)-4-(Z-Cp)-4,1,8-CoC2B10H11] 1575 [1-(1’-1’,2’-C2B10H11)-4-(Z-Cp)-4,1,12-CoC2B10H11] 1576 [12-(1’-1’,2’-C2B10H11)-4,5-(Cp )2-4,5,1,12Co2C2B9H10] 14-vertex clusters 1577 [1-(p-cym)-14-Cp-1,14,2,10-RuCoC2B10H12] 1578 [1-(p-cym)-14-(Z-C6H6)-1,14,2,10-Ru2C2B10H12] 1579 [1,14-(p-cym)2-1,14,2,10-Ru2C2B10H12] 1580 [1-(p-cym)-14-(dppe)-1,14,2,10-RuNiC2B10H12] 1569

1581 1582

[1-(p-cym)-2,3-m-(CH2)3-1,2,3-RuC2B11H11] [1-(p-cym)-2,8-m-(CH2)3-1,2,8-RuC2B11H11]

1583 1584

[2,3-(CH2)3-1-(p-cym)-1,2,3-RuC2B11H11] [2,13-(CH2)3-1-(p-cym)-1,2,13-RuC2B11H11]

[1-(p-cym)-2,3-Me2Si(CH2)2-1,2,3-RuC2B11H11] [1-(p-cym)-2,8-Me2Si(CH2)2-1,2,8-RuC2B11H11] [1-(p-cym)-2,10-Ph2-14-(Z-C6H6)-1,14,2,10Ru2C2B10H10] 1588 [2,7-Cp -2,7,1,13-Ru2C2B10H12] 1589 [2,7-Cp -2,7,1,12-Ru2C2B10H12] 1590 [2,7-Cp -2,7,1,6-Ru2C2B10H12] 1591 [1,14-(Cp)2-2,10-Me2-1,14,2,10-Co2C2B10H10] 1592 [1,14-(Z-C9H7)2-1,14,2,10-Co2C2B10H12] 1593 [1,14-(Cp)2-1,14,2,9-Co2C2B10H12] 1594 [1,14-(Cp)2-2,9-Me2-1,14,2,9-Co2C2B10H10] 1595 [1,14-(Cp)2-1,14,2,9-Co2C2B10H12] 1596 [1,13-(Cp)2-1,13,2,10-Co2C2B10H12] 1597 [1,8-(Cp)2-3-OEt-1,8,2,10-Co2C2B10H11] 1598 [1,13-(Cp)2-1,13,2,9-Co2C2B10H12] 1599 [1,8-(Cp)2-1,8,2,4-Co2C2B10H12] 1600 [1,13-(Cp)2-1,13,2,4-Co2C2B10H12] 1601 [1,8-(Cp)2-1,8,2,5-Co2C2B10H12] 1602 [Z5-(CH2)3C2B11H11]Ni(dppe) 1603 [Z5-(CH2)3C2B11H11]Ni(dppen) 1604 [1-(Z-mes)-9-(10 -10 ,20 -C2B10H11)-13-(Z-C6H6)1,13,2,9-Ru2C2B10H11] 1605 [1-(Z-mes)-9-(10 -10 ,20 -C2B10H11)-13-(Z-p-cym)1,13,2,9-Ru2C2B10H11] 1606 [4,5-(Z-mes)2-6-(10 -10 ,20 -C2B10H11)4,5,1,6-Ru2C2B9H10] 15-vertex clusters 1607 [1-(p-cym)-8,14-m-(CH2)3-1,8,14-RuC2B12H12] 1585 1586 1587

1608

[1,4-(CH2)3-7-(p-cym)-7,1,4-RuC2B12H12]

1609 [7,8-(Cp )2-7,8-Rh2B13H13] 16-vertex clusters 1610 [9,11-(Me3Si)2-1-(p-cym)-1,9,11-RuC2B13H13] 1611 [1-{Cp RhB4H9}-11,12,13-(Cp )3-1,11,12,13-Rh4B12H12]

11

Av. M-B (Å)

Ref.

10.6, 7.5, 0.6, −3.7, −4.9, −9.4, −10.8, −13.8, −18.7, −25.6

2.214

365

14.54, 11.30, 2.75, −3.25, −6.32, −16.43 14.63, 9.66, 3.21, −1.63, −3.46, −4.97, −16.26 8.1, 3.0, −3.8, −7.1, −8.3, −9.4, −15.2, −16.8

2.179 2.183 2.169

366 366 367

8.0, 3.2, −3.8, −7.0, −7.9, −9.4, −15.2, −16.6

2.195

367

24.5, 9.4, 3.5, 0.4, −2.8, −6.6, −9.8, −11.6, −13.1 24.5, 9.4, 3.5, 0.4, −2.8, −6.6, −9.8, −11.6, −13.1 7.6, 3.0, −3.1, −4.7, −5.9, −10.6, −13.4

2.113 2.161 2.189, 2.181

264 264 273

−11.69, −15.31, −17.67, −20.17, −23.18 −16.49, −20.68, −21.27, −22.90, −25.94 −16.14, −20.69, −23.01, −25.60 −9.34, −13.40, −16.72, −20.34, −24.80, −27.78

2.214 − 2.249 2.266, 2.194 2.261 2.218

350 350 350 350 368 368

2.269 2.254

369 369

2.267 − 2.260

353 353 264

76.0, 45.8, 34.9, 29.2, 26.1, 15.5, 10.1, −3.5, −13.6 58.4, 54.2, 36.2, 29.7, 28.3, 19.1, 12.1, 3.6, −7.9, −22.9 72.1, 63.3, 44.9, 37.4, 22.5, 18.5, 11.2, 7.3, 0.4, −12.1 −4.89, −8.54, −12.54 −6.17, −11.70, −13.19, −14.67, −18.74 −1.18, −2.30, −16.15, −21.24, −25.99 1.20, −1.45, −10.01, −14.17, −18.10 −0.76, −1.60, −14.23, −20.28, −23.83 13.9, 5.2, −2.1, −5.7, −6.7, −15.8, −26.8, −28.2 − 18.2, 8.8, 6.6, 2.8, −2.5, −5.1, −14.9, −30.1 7.0, 3.6, 2.0, −4.2, −5.5, −10.2, −18.3, −19.9, −27.7 13.7, 11.5, 2.0, −0.7, −5.3, −8.2, −13.8, −18.0, −21.2, −26.4 8.4, 3.0, −0.7, −2.4, −9.9, −14.8, −17.7, −21.2, −28.3 − − 15.6, 12.1, 3.9, 0.8, −0.6, −3.8, −5.1, −6.2, −9.4, −11.0, −13.9, −18.7, −25.9 17.6, 11.9, 4.6, 2.1, 0.1, −3.9, −5.2, −9.1, −11.0, −14.0, −18.8, −25.4 12.2, 2.6, 1.7, −1.0, −4.2, −5.2, −9.0, −11.2, −14.1

2.203 2.215 2.195 − 2.162 − 2.151 2.157 2.114 2.116 2.115 2.129 2.108 2.115 2.197 2.186 2.215

352 352 352 370 370 370 370 370 371 371 371 371 371 371 372 372 365

2.215

365

2.319

365

19.44, 14.89, −0.98, −3.20, −11.92, −15.10, −18.49, −28.48, −31.34 14.83, 2.46, −0.45, −6.17, −9.87, −12.99, −17.78, −19.81, −26.83, −31.12 58.6, 22.0, 19.9, 4.7, −42.0

2.248

368

2.249

369

2.206

132

0.1, −10.3, −13.0, −16.8, −21.3, −28.2 37.1, 35.0, 24.3, 4.9

2.278 2.215

373 133

B NMR (ppm)

−5.86, −13.58, −14.73, −17.39, −20.69 14.73, 3.38, 2.62, −0.70, −6.30, −10.11, −13.19, −18.03, −20.00, −27.03, −31.38 −3.84, −12.17, −15.22, −18.82 1.89, −1.67, −6.62, −9.23, −11.01, −20.39, −23.62, −24.81, −27.28 −6.4, −12.1, −14.2, −14.8, −17.1, −22.0, −23.4 0.0, −2.5, −6.5, −7.1, −10.7, −13.3, −20.5, −22.9, −24.0 −12.9, −13.7, −16.8, −19.9, −21.6

Polyhedral Metallaboranes and Metallacarboranes

359

Scheme 46 Synthesis of 13-vertex (1460–1464 and 1466–1471) and 14-vertex (1577–1580) metallacarboranes.

Via the same reduction-capitation approach, the 13-vertex docosahedron ruthenacarboranes having 4,1,6-MC2B10 (1473–1478)257,349,351,352 and 4,1,2-MC2B10 isomeric structures (1479–1481)349,353 were synthesized utilizing various icosahedral carboranes C2B10H12, excess Na and ruthenium precursors. Furthermore, thermolysis of ruthencarborane 1475 at 180  C led to the formation of an equilibrium mixture of two new docosahedral metallacarboranes 4,1,8-MC2B10 isomer (1482) and 4,1,8-MC2B10 isomer (1483).354 Another docosahedral 4,1,8-MC2B10H10 ruthenacarborane 1484 was isolated from the overnight reflux of 1476 in toluene.257 On the other hand, reduction of [1,2-(CH2)3-1,2-closo-C2B10H10] with Na/naphthalene in THF followed by treatment with [(p-cym)RuCl2]2 synthesized a 13-vertex ruthenacarborane 1485.355 The internuclear distances within the polyhedral framework and the exo-polyhedral atom show that 1485 has a henicosahedral structure, in which the degree-six vertex is occupied by Ru and both the degree-four vertices are occupied by C atoms. Reduction of 12-vertex ruthenacarborane, [3-(p-cym)-3,1,2-closo-RuC2B9H11] (850) by Na in THF for overnight and then metalation with 5 equiv. [(p-cym)RuCl2]2 yielded dimetallacarborane [4,5-(p-cym)2-4,5,2,3-closo-Ru2C2B9H11] (1486).256 Another isomer [4,5-(p-cym)2-4,5,1,6-closo-Ru2C2B9H11] (1487) was obtained by heating 1486 at 150  C in oxygen-free tetra(ethyleneglycol)dimethylether.256 On the other hand, reaction of [Z5-(C6H4(CH2)2)C2B9H9]Na2(THF)3 with anhydrous FeCl2 and NaCp in THF followed by oxidation with O2 yielded a CAd zwitterionic salt 1490.271 The ferracarboranes 1490 is a diamagnetic 18-electron Fe(II) complex. From a cluster point of view, 1490 can be considered as a 13-vertex cluster. Reaction of dianion nido-[Cp-tBuNH-FeC3B8H10]2− with Me2S
BH3 in THF generated a series of 13-vertex ferratricarboranes 1491–1494.248 The reduction of tricarbollide complex [2-Cp-9-tBuNH-closo-2,1,7,9-FeC3B8H10] with Na/naphthalene afforded the 12-vertex metallatricarbaborane dianion [nido-Cp-tBuNH-FeC3B8H10]2−, which further reacted with [CpFe(CO)2I] or [CpFe(CO)2]2 in situ for 16 h in room-temperature then reflux for 4 h to afford 13-vertex paramagnetic diferratricarbaboranes 1495–1497.356 Utilizing the reduction-capitation methodology, many group 9 supra-icosahedral metallacarboranes were also synthesized. For example, Na reduction of [1,12-closo-CoC2B10H12] followed by reaction with NaCp/CoCl2 or [(COD)RhCl]2 afforded docosahedral 4,1,10-MC2B10 type cobaltacarborane 1498 and rhodacarborane 1499, respectively.347,357 Similarly, reduction of carboranes [1,2-(CH2)3-1,2-closo-C2B10H10] or [1,2-m-(CH2SiMe2CH2)-1,2-closo-C2B10H10] with Na/naphthalene or Li in THF and followed by treatment with NaCp/CoCl2 or Li[C9H7]/CoCl2 afforded henicosahedral 4,1,2-MC2B10 cobaltacarboranes 1500–1502349,355,358 and docosahedral 4,1,6-MC2B10 cobaltacarborane 1503349. The 4,1,6-isomer 1503 was further heated to get its 4,1,8-isomer 1504.349 Metalation of reduced carborane [1,7-Ph2-1,7-closo-C2B10H10] with NaCp/CoCl2 in THF yielded another docosahedral 4,1,6-MC2B10 cobaltacarborane 1505, which further isomerized by heating to produced first 4,1,8-isomer 1506 and then to 4,1,12-isomer 1507.354 13-vertex indenyl cobaltacarboranes 1508–1510 (4,1,2-MC2B10 isomer), 1511 (4,1,10MC2B10 isomer) and 1512 (4,1,2-MC2B10 isomer) were synthesized by Na or Li reduction of 12-vertex carboranes (1,2-closoC2B10H12) (for 1508), 1,2-Me2-1,2-closo-C2B10H10 (for 1509), 1-Me-1,2-closo-C2B10H11 (for 1510), 1,12-closo-C2B10H12 (for 1511), 1,2-m-(CH2)3-1,2-closo-C2B10H10 (for 1512) and subsequent treatment with CoCl2 and air oxidation. Thermolysis of 4,1,6-isomers 1508 and 1509 yielded the corresponding 4,1,8-isomers 1513 and 1514 and thermolysis at higher temperature these 4,1,8-isomers converted to 4,1,12-isomers 1515 and 1516.358 Also, the 4,1,6-isomers 1510 heated to form two

360

Polyhedral Metallaboranes and Metallacarboranes

4,1,8-isomeric cobaltacarboranes 1517 and 1518.358 All the 4,1,6-; 4,1,8-; 4,1,10- and 4,1,12-isomeric species have docosahedral geometry, whereas 4,1,2-isomeric species 1512 has henicosahedral geometry. Similarly, many more 13-vertex docosahedral metallacarboranes of group 9 (1519–1530) were synthesized utilizing reduction-capitation methodology. The thermal isomerization of 1528 and 1529 led to formation of two new isomers [4-Cp-2,3,5,6,8,11,12,13-Me8-4,1,9-CoC2B10Me8H4] (1531) and [4-Cp-2,3,5,6,8,11,12,13-Me8-4,1,9-CoC2B10Me8H4] (1532).359 The formation of 1531 and 1532 from 1528 and 1529 can be rationalized by the DSD mechanism. Further reaction of 1528 and 1529 with strong base like nBuLi and MeLi led to the nucleophilic substitution of Cp rings and afforded cobaltacarboranes 1533–1535.359 By one-pot procedure from 1,2-bis(diphenylphosphino)-closo-carborane and a highly reduced cobaltate anion [K(thf ){(MesBIAN)Co(Z4-COD)}] a 13-vertex cobaltacarborane, 1536 with a redoxactive {MesBIAN ¼ bis(mesityliminoacenaphthene)diimine} bis(iminoacenapthene) (BIAN) ligand at Co and two phosphanyl moieties attached to the cluster C atoms was isolated.360 On the other hand, when a 12-vertex cobaltacarborane [3-Cp-3,1,2-closo-CoC2B9H11] reduced and treated with NaCp/CoCl2, a 14 SEP docosahedral [4,5-Cp2-4,5,2,3-closo-Co2C2B9H11] (1537) was isolated.256 Other than group 8 and 9 metallacarboranes, there are very few metallacarboranes of other transition metals (1538–1544)347,355,357,361 and lanthanides (1545–1554)343,362. 13-vertex docosahedral type metallacarboranes 1538–1545 were also synthesized utilizing reduction-metalation methodology. For metalation, Hf(CH2SiMe3)4 (for 1538),361 NiCl2(dppe) (for 1539–1541),347,355 PtCl2(PMe2Ph)2 (for 1542–1544)355,357 and YCl3 (for 1557)363 were utilized. On the other hand, directly attached carboranyl-indenyl compound, 2-(o-carboranyl)indene was treated with excess Na metal afforded [{Z5:Z6-(2-C9H6) (C2B10H11)}Na3(THF)n], which further reacted with LnCl3 to produce dinuclear complexes [{Z5:Z6-(2-C9H6)(C2B10H11)Ln (THF)}2(m-Cl)][Na(THF)6] (Ln ¼ ]Y (1548), Er (1549)).362 Similarly, (1-(o-carboranyl)indene) was reacted with excess sodium metal, yielding [{Z5:Z6-(1-C9H6)(C2B10H11)}{Na3(THF)5}]2, which reacted with LnCl3 or Ln(BH4)3(THF)3 to afford [{Z5:Z6-(1C9H6)(C2B10H11)Ln(THF)}2(m-Cl)][Na(THF)3] (Ln ¼ ]Y (1550), Nd (1551), Er (1552)) or {[Z5:Z6-(1-C9H6)(C2B10H11)Er (THF)]2(m-BH4)}{Na(THF)2} (1553), respectively.362 From cluster view-point, 1548–1553 have two docosahedral cores. Further, reaction of 1553 with excess sodium metal generated dinuclear complex {[Z5:Z7-(1-C9H6)(C2B10H11)Er(THF)]2}{Na4(THF)8} (1554), in which each Er atom is Z7-bound to the arachno-carboranyl.362 On the other hand, many fused and conjuncto-metallacarboranes having 13-vertex cluster unit(s) were also synthesized. For example, reduction of [1,2-closo-C2B10H12] and subsequent treatment with FeCl2 and cation metathesis with either [K(18-crown-6)] Br or [BTMA]Cl formed supraicosahedral sandwich compounds [K(18-crown-6)]2[4,40 -Fe-(1,10-closo-C2B10H12)2] (1555) and [BTMA]2[4,40 -Fe-(1,10-closo-C2B10H12)2] (1556).363 Further oxidation of 1555 and 1556 with FeCl3 in THF led to the formation of oxidized fused feracarboranes 1557 and 1558, respectively.363 Similarly, reduction of [1,12-closo-C2B10H12] followed by reaction with CoCl2 or TiCl4 or NiCl2 and cation metathesis with [BTMA]Cl or [K(18-crown-6)]Br yielded salts of the 13-vertex sandwich anions X+[4,40 -Co-(1,10-closo-C2B10H12)2]− (X ¼ ][BTMA] (1559), [K(18-crown-6)] (1560)), [K(18-crown-6)]2[4,40 -M-(1,10-closoC2B10H12)2] (M ¼ ]Ti (1561) and Ni (1562)), respectively.363 On the other hand, the reaction of 1,10 -bis(o-carborane) with Li/naphthalene in THF followed by the reaction with [(p-cym) RuCl2]2 in room-temperature for 18 h afforded [1-(10 ,20 -closo-C2B10H11)-4-{C10H14Ru(p-cym)}-4,1,6-closo-RuC2B10H11] (1563).364 Conjuncto-1563 has a 13-vertex docosahedral 4,1,6-RuC2B10 ruthenacarborane unit, which is connected to a 1,2-C2B10 icosahedron. Similarly, the reduction of 1,10 -bis(o-carborane) with Na/naphthalene followed by metalation with 0.5 equiv. [Ru(Z-C6H6)Cl2]2 or [(p-cym)RuCl2]2 yielded conjuncto-1564 and 1565, respectively, in which a 13-vertex ruthenacarborane is connected with a 12-vertex carborane.365 Metalation of same 2e reduced 1,10 -bis(o-carborane) with 0.5 equiv. [Ru(Z-mes)Cl2]2 afforded conjuncto-1566 having a 13-vertex 4,1,8-RuC2B10 ruthenacarborane and icosahedral carborane units365 Further, reduction of 1566 by Na/naphthalene in THF followed by treatment with {Ru(arene)}2+ (arene ¼ (Z-C6H6) or (p-cym) or (Z-mes)) afforded conjuncto-1604-1606 as major products having 14-vertex bicapped hexagonal antiprism ruthenacarborane and 12-vertex carborane cores.365 From the same reaction, minor products conjuncto-1567-1569 were also isolated. Conjuncto-1567-1569 have 13-vertex docosahedron ruthenacarborane (Ru2C2B10) and 12-vertex carborane cores.365 Conjuncto-supraicosahedral bis(heteroborane)s ras-1570, and meso-1571 were synthesized by reducing 1,10-bis(o-carborane) using Li/naphthalene, subsequent treatment with NaCp/CoCl2 and then aerial oxidation (Scheme 47).366 Compounds ras-1570 (RR/SS) and meso-1571 (RS) are the first examples of supraicosahedral heteroboranes in which the 13-vertex cages are directly linked by the bond between C1–C10 , the two degree-four C atoms. Heating of 1:1 rac (1570) and meso (1571) mixture of 1-(10 -4’-Cp-40 ,10 ,60 -closo-CoC2B10H11)-4-Cp-4,1,6-closo-CoC2B10H11 afforded rac (1572) and meso (1573) mixture of [1-(10 -40 -Cp-40 ,10 ,120 -closo-CoC2B10H11)-4-Cp-4,1,12-closo-CoC2B10H11].367 2e-reduction and metalation of 1-(10 -10 ,20 -closo-C2B10H11)-1,2-closo-C2B10H11 yielded conjuncto-1574 and 1575 having 13-vertex metallacarborane and 12-vertex carborane cores.264

Polyhedral Metallaboranes and Metallacarboranes

361

Scheme 47 Synthesis of supraicosahedral conjuncto-metallacarboranes 1570–1573.

The Na/naphthalene reduction of 13-vertex carborane [1,2-m-(CH2)3-1,2-closo-C2B11H11] and followed by reaction with [(p-cym)RuCl2]2 led to the formation of two 14-vertex ruthenacarborane isomers [1-(p-cym)-2,3-m-(CH2)3-1,2,3-closoRuC2B11H11] (1581) and [1-(p-cym)-2,8-m-(CH2)3-1,2,8-closo-RuC2B11H11] (1582).368 The compound 1581 and 1582 have bicapped hexagonal antiprismatic structure. Similarly, capitation of 13-vertex nido-carborane salt [(CH2)3C2B11H11{Na2(thf )4}]n with [(p-cym)RuCl2]2 in THF yielded bicapped hexagonal antiprismatic 1,2,3-MC2B11H11 isomeric metallacarborane 1583.369 When the disodium salt of 1583 reacted with 2 equiv. HBBr2
SMe2 in toluene, the reaction afforded 1583 and its isomer 1,2,13-MC2B11 isomer (1584).369 Na metal reduction of [1,2-Me2Si(CH2)2-1,2-C2B11H11] and followed by metalation with [(p-cym)RuCl2]2 afforded two 14-vertex ruthenacarboranes 1585 and 1586.353 On the other hand, naphthalene-assisted sodium reduction of 13-vertex metallacarborane [1,8-Ph2-4-(p-cym)-4,1,8-closo-RuC2B10H10] (1482) followed by addition of 0.5 equiv. [Ru(Z-C6H6)Cl2]2 afforded 14 vertex metallacarborane 1587 and 13-vertex metallacarborane 1488 along with few more 12-vertex metallacarboranes.264 Reaction of [BTMA][4-Cp -4,1,6-RuC2B10H12] (1477) with [Cp RuCl]4 in THF under reflux condition for 16 h yielded a 13-vertex hypercloso diruthenacarborane [4,5-Cp 2–4,5,1,6-Ru2C2B9H11] (1489) and 14-vertex hypercloso metallacarboranes 1588–1590 (Scheme 48).352 Hypercloso1489 has a docosahedral structure and 13 SEP. By contrast, 14-vertex hypercloso-1588-1590 (14 SEP) have two degree-six vertices occupied by Ru atoms, 10 degree-five vertices and two degree-four vertices.

362

Polyhedral Metallaboranes and Metallacarboranes

Scheme 48 Synthesis of supraicosahedral metallacarboranes 1489 and 1588–1590.

Group 9 also has a series of 14 vertex metallacarboranes. Treatment of [1,12-Me2-4-Cp-4,1,12-closo-CoC2B10H10] with Na/ naphthalene subsequent metalation with NaCp/CoCl2 afforded a 14-vertex dicobaltacarborane 1591.370 Similarly, reduction of various 13-vertex metallacarboranes, followed by metalation afforded 14-vertex dicobaltacarboranes 1592–1601.370,371 These 14-vertex dicobaltacarboranes (1592–1601) have different isomeric forms of M2C2B10 bicapped hexagonal antiprismatic core (Fig. 57). Although both the degree-six vertices are occupied by Co atoms in 1591–1595, the Co atoms occupied one degree-six and one degree-five vertex in 1596–1601. On the other hand, two 14-vertex nickelacarboranes [m-2,3-(CH2)3-8-L-8,2,3-NiC2B11H11] (L ¼ dppe (1602) or dppen (1603)) were achieved by capping the 5-membered open face of [{nido-1,2-(CH2)3-1,2-C2B11H11} {Na2(THF)4}]n utilizing NiCl2L.372 In 1602 and 1603, both the degree-six vertices of bicapped hexagonal antiprism are occupied by boron atoms, and the Ni atom occupied a degree-five vertex.

Fig. 57 Molecular structures of 14-vertex metallacarboranes 1591–1601.

Only two 15-vertex metallacarboranes and one 15-vertex metallaborane are known to date (Fig. 58). 14-vertex metallacarborane 1582 afforded a new 15-vertex ruthenacarborane 1607 upon refluxing in toluene for 48 h.368 The formation of 1607 might have occurred due to the adventitious capture of a {BH} unit from the reaction system. Carborane 1607 has one trapezoidal face along with 24 triangular faces. On the other hand, treatment of a 14-vertex nido-carborane salt [{(CH2)3C2B12H12}{Na2(thf )4}]n with 0.5 equiv. [(p-cym)RuCl2]2 in THF afforded 15-vertex ruthenacarborane 1,4-(CH2)3-7-(p-cym)-7,1,4-RuC2B12H12 (1608) which has a

Fig. 58 Molecular structures of 15-vertex clusters (1607–1609).

Polyhedral Metallaboranes and Metallacarboranes

363

Fig. 59 Molecular structures of 16-vertex metallacarborane (1610) and metallaborane (1611).

hexacosahedron core (D3h symmetry) with 26 triangular faces.369 The Ru atom is situated at one of the degree-six vertices in both clusters 1607 and 1608. Although in cluster 1607, the carbon atoms are placed at the degree-four and degree-five vertices, both the carbon atoms are placed in degree-five vertices in cluster 1608. Xie and co-workers have recently synthesized the 16-vertex metallacarborane [9,11-(Me3Si)2-1-(p-cym)-1,9,11-RuC2B13H13] (1610) by reduction of 15-vertex carborane [1,14-(SiMe3)2-1,14C2B13H13] with an excess of sodium, followed by reaction with 0.5 equiv. [Ru(p-cym)Cl2]2 in THF.373 16-vertex 1610 has a closo geometry, which has 26 triangular and one rhombus face (Fig. 59). Interestingly, the boron atoms occupied all four vertices of the open rhombus face. By contrast, two carbon centers are located at degree-five vertices, and Ru atom occupied degree-six vertex of 1610. On the other hand, the reaction of [Cp RhCl2]2 and excess of LiBH4
THF followed by thermolysis with excess BH3
thf at 105  C for prolonged time afforded a 15-vertex rhodaborane [7,8-(Cp )2-7,8-Rh2B13H13] (1609)132 and a 16-vertex rhodaborane [1-{Cp RhB4H9}-11,12,13-(Cp )3-1,11,12,13-Rh4B12H12] (1611)133. The molecular structure of 1609 can be viewed as a closo 15-vertex 26-face icosihexahedron or hexacosahedron. It can also be viewed as a tricapped truncated trigonal prism. Out of three degree-six vertices of 1609, two are occupied by rhodium atoms as expected. By contrast, cluster 1611 has a 16-vertex tetra capped truncated tetrahedron core with an external {Cp Rh(B4H9)} unit. The exo-{Cp Rh(B4H9)} unit has a pentagonal pyramid core and is operating as a trihapto ligand to one of the Rh vertices of the 16- vertex rhodaborane 1611. All the boron atoms of the tetra capped truncated tetrahedron core are five-connected to three boron, and all four degree-six vertices are occupied by rhodium atoms. This cluster can also be called an icosioctahedron as it has 28 triangular faces. Later King, Lupan and co-workers described the isocloso-bonding topology of 1611 and its spherical aromaticity.374 They also proposed many 14–16-vertex cationic/anionic/neutral metallaboranes and metallacarboranes with different structural motifs and shown how distortions from spherical closo- deltahedral geometries arise in such metalloboranes and metallocarbaboranes.374,375 As earlier proposed by Jemmis, transition metals utilized their diffused d-orbitals in the stabilization of these supraicosahedral clusters.5

Acknowledgments We acknowledge the support of SERB, New Delhi, India, Grant No. CRG/2019/001280. S.K. thanks IIT Madras and A.N.P. thanks UGC for research fellowships. This review is a tribute to the excellent experimental chemistry done by the current and past members of our laboratory. The scientific discussions and collaboration with Prof. Thomas P. Fehlner, Prof. Eluvathingal D. Jemmis, Prof. Jean-François Halet, Prof. R. Bruce King, Prof. Lai-Sheng Wang, Prof. Jean-Yves Saillard and Prof. Samia Kahlal are gratefully acknowledged.

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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. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211. 212. 213. 214. 215. 216. 217. 218. 219. 220. 221. 222. 223. 224. 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235.

Polyhedral Metallaboranes and Metallacarboranes

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Polyhedral Metallaboranes and Metallacarboranes

236. 237. 238. 239. 240. 241. 242. 243. 244. 245. 246. 247. 248. 249. 250. 251. 252. 253. 254. 255. 256. 257. 258. 259. 260. 261. 262. 263. 264. 265. 266. 267. 268. 269. 270. 271. 272. 273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295. 296. 297. 298.

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368

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. 343. 344. 345. 346. 347. 348. 349. 350. 351. 352. 353. 354. 355. 356. 357. 358. 359. 360. 361. 362. 363. 364. 365. 366. 367. 368.

Polyhedral Metallaboranes and Metallacarboranes

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9.07

Gallium, Indium, and Thallium

Christoph Helling and Stephan Schulz, Institute for Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Essen, Germany © 2022 Elsevier Ltd. All rights reserved.

9.07.1 Introduction and scope 9.07.2 Boron-based ligands 9.07.3 Carbon-based ligands 9.07.3.1 s Ligands 9.07.3.1.1 Carbenes 9.07.3.1.2 Alkyl substituents 9.07.3.1.3 Aryl substituents 9.07.3.2 p Ligands 9.07.3.2.1 Arene ligands 9.07.3.2.2 Cyclopentadienyl ligands 9.07.3.3 Unsaturated group 13 metal-containing heterocycles 9.07.4 Heavier group 14 element-based ligands 9.07.5 Nitrogen-based ligands 9.07.5.1 Neutral donor ligands 9.07.5.2 Hydrazide ligands 9.07.5.3 Amide ligands 9.07.5.4 Amidinate and guanidinate ligands 9.07.5.5 a-Diimine ligands 9.07.5.6 b-Diketiminate ligands 9.07.5.7 Other nitrogen-based ligands 9.07.6 Heavier group 15 element-based ligands 9.07.7 Chalcogen-based ligands 9.07.8 Conclusions Acknowledgments References

9.07.1

370 371 372 372 372 375 377 379 379 379 380 382 383 383 384 385 386 387 390 395 397 400 401 402 402

Introduction and scope

Since the last edition of Comprehensive Organometallic Chemistry (COMC-III) in 2007, heavier group 13 element chemistry has continued to receive considerable attention, resulting in rapid developments in this field, including novel synthetic approaches for the preparation of group 13 compounds and a steadily increasing knowledge of their structural features and bonding properties. Apart from the synthesis and structural characterization, the reactivity of various types of heavier group 13 compounds was also intensively studied. These developments were driven by, for instance, the extensive and systematic use of N-heterocyclic carbenes and cyclic (alkyl)(amino)carbenes for the stabilization of unforeseen structural motifs as well as the design of novel, or improvement of known, ligand systems with tunable steric and electronic properties. Thus, unprecedented coordination complexes with s and p donor ligands, metal-centered radicals, metalloid clusters, and compounds containing group 13 metals incorporated in unsaturated p-bonded systems were prepared, isolated, and characterized, frequently supported by quantum chemical analyses of their bonding and electronic structures. Most remarkably, the chemistry of low-valent and low-oxidation state Ga, In, and Tl compounds moved further into the focus of research, particularly regarding their reactivity in bond activation reactions, a key step in many catalytic transformations, and their use for the construction and stabilization of unusual structural motifs of main group compounds, such as silicon carbonyl complexes, multiple-bonded compounds, and radicals to name only a few. This chapter aims to summarize the most significant progresses and focuses on new developments in the synthesis and reactivity of metalorganic compounds of the heavy group 13 elements Ga, In, and Tl achieved since 2007. The discussed compounds are not restricted to such containing metal-carbon bonds, since, for instance, the capability of sterically demanding N,N0 chelating ligands and of boryl ligands for the stabilization of new structural and binding motifs has been demonstrated in the last decade, proving their enormous potential in this research field. Therefore, these compounds are explicitly considered, and this chapter is structured according to the types of ligands employed. However, pure coordination complexes as well as compounds containing transition and rare earth metals are generally excluded. While it is not feasible to examine and discuss every reported compound and all facets of this chemistry in detail, this chapter provides an overview of this research field and the reader is referred to previous editions of COMC for very fundamental aspects of group 13 organometallic chemistry and more specialized review articles wherever possible.1

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https://doi.org/10.1016/B978-0-12-820206-7.00126-8

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9.07.2

371

Boron-based ligands

In 2006, Yamashita and co-workers reported the isolation of a sterically demanding boryllithium complex, which was shown to act as a strong nucleophilic boron source.2,3 The value of this compound was promptly recognized and it was exploited for the synthesis of compounds containing boron-element bonds across the entire periodic table.4,5 The first report on its use in the chemistry of Ga appeared in 2014, when Anwander et al. described the synthesis of (HCNDip)2 BGaMe2 (1, Dip ¼ 2,6-i-Pr2C6H3) by reaction of the boryllithium salt (HCNDip)2BLi(THF)2 with an excess of GaMe3.6 The borylsubstituted gallane 1 is monomeric in solution and in the solid-state and represents the first compound with an electron-sharing s-bond between three-coordinate B and Ga centers. It features a short B–Ga bond (2.067(3) A˚ ) that shows a high covalent character according to quantum chemical calculations using density functional theory (DFT). In THF solution, 1 forms the Lewis acid-base adduct (HCNDip)2BGaMe2(THF). The addition of MeLi to solutions of 1 in non-coordinating solvents resulted in the formation of tetrameric [(HCNDip)2BGaMe3Li]4 featuring a 16-membered ring structure, whereas the reaction of MeLi with 1 in THF solution yielded the cyclic dimer [(HCNDip)2BGaMe3Li(THF)]2, featuring an 8-membered ring structure.7 Analogous reactions of (HCNDip)2BLi(THF)2 with AlMe3 revealed similar reactivity, however, these were affected by the higher Lewis acidity of the Al center in comparison to the Ga center.8 Aldridge and co-workers reported on the synthesis of a homologous series of divalent boryl-substituted radicals, [(HCNDip)2B]2M (M ¼ Ga 2, In 3, Tl 4), which were formed in consecutive metathesis and disproportionation reaction sequences starting with the corresponding M(I) precursors.9 Thus, the formation of 2–4 was accompanied by the formation of mixed-valence by-products such as M2[B(NDipCH)2]3 (M ¼ Ga, In) and Tl8[B(NDipCH)2]4. Monomeric radicals 2–4 possess bent to almost linear structures in the solid-state with B–M–B bond angles of 156.0(1) (2), 145.4(1) (3), and 177.6(2) (4), respectively, reflecting the extent of ns orbital contribution to the singly occupied molecular orbital (SOMO). EPR spectroscopic studies on 2–4 furthermore confirmed the metal-centered character of these species with about 70% of the spin density located at Ga, In, and Tl. Moreover, Tl radical 4 was shown to undergo one-electron oxidation and reduction reactions to the corresponding cation [(HCNDip)2B]2Tl+ (4+) and anion [(HCNDip)2B]2Tl− (4−), respectively. Reactivity studies of radicals 2–4 with 2,3-dimethylbutadiene revealed oxidative M–C bond formation processes, leading to the formation of {(MeCCH2)2}[M{B(NDipCH)2}2]2 (M ¼ Ga 5, In 6).10 In remarkable contrast to the boryl-substituted gallane 5, the corresponding indane 6 exists in an equilibrium with the starting reagents in benzene solution, whereas dissolution of 6 in hexanes gave rise to formation of the metalloid In19 cluster, In19[B(NDipCH)2]6 (7). Finally, the reaction of thallium radical 4 with 2,3-dimethylbutadiene yielded {(MeCCH2)2}[Tl{B(NDipCH)2}2][B(NDipCH)2] (8), which subsequently decomposed to {(MeCCH2)2}[B(NDipCH)2]2 (9) with successive B–C bond formation steps. This reaction shed some light on potential reaction pathways resulting in formation of 7. Reduction of the trivalent boryl-substituted In complex [(HCNDip)2B]2InCl with potassium metal also yielded radical the indium 3 together with the metalloid monoanionic indium cluster In68[B(NDipCH)2]−12 (10) as side-product.11 Very large metalloid indium clusters such as 7 and 10 are extremely scarce, whereas even larger metalloid clusters have been previously prepared for Al and Ga, including [Al77R20]2− and [Ga84R20]4−.12,13 A series of group 13 metal boryl complexes {PhC(NiPr)2}M[B(NDipCH)2]X (M ¼ Al, Ga, In; X ¼ Cl, Br) were also successfully prepared by use of ancillary amidinate ligands. Subsequent reduction of the indium complex {PhC(NiPr)2}In[B(NDipCH)2]Br with potassium metal led to the formation of the highly reduced complex K2[In4{B(NDipCH)2}4] (11) containing a nearly planar In4 array. Tetraindane 11 formally contains a delocalized 2p electron system, according to which it can be described as potential Hückel aromatic, and DFT calculations indeed indicated a weak p aromatic character of 11 based on the highest occupied molecular orbital (HOMO) and nucleus-independent chemical shift (NICS) values11 (Scheme 1).

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Scheme 1 Heavy group 13 metal compounds containing boryl substituents.

9.07.3

Carbon-based ligands

9.07.3.1

s Ligands

9.07.3.1.1

Carbenes

Over the past three decades, singlet carbenes emerged as one of the most versatile and important neutral s donor ligands not only in transition metal chemistry but also increasingly in main group chemistry.14,15 Trivalent group 13 compounds typically exhibit Lewis-acidic character, rendering them obvious targets for carbene coordination, and hence completion of their coordination sphere. Accordingly, a vast number of carbene-group 13 metal adducts/complexes has been reported in the literature. Among a large variety of singlet carbenes, Arduengo-type N-heterocyclic carbenes (NHC) and Bertrand-type cyclic (alkyl)(amino)carbenes (CAAC) are the most prominent representatives of this class. Since 2007, several N-heterocyclic carbene complexes of group 13 trihalides (NHC)EX3 have been synthesized. For instance, Nolan and co-workers prepared a series of analogous 1:1 imidazol-2-ylidene-GaCl3 adducts, [(MeCNiPr)2C]GaCl3 (12), [(HCNMes)2C] GaCl3 (13, Mes ¼ 2,4,6-Me3C6H2), and [(HCNDip)2C]GaCl3 (14), with N-substituents of varying steric demand,16 while the corresponding imidazolin-2-ylidene-GaCl3 complexes containing a saturated backbone, [(H2CNMes)2C]GaCl3 (15) and [(H2CNDip)2C]GaCl3 (16), were synthesized by Gandon and coworkers.17 In contrast, using the chelating methylene-bridged carbene H2C[{N(C2H4)NDip}C]218 or bisoxazolin-inspired carbenes,19 ion-separated complexes of the type [L2GaCl2][X] (X ¼ Cl, GaCl4) were obtained. Interestingly, Gandon et al. found a direct relationship between the pyramidalization of the Ga center in the solid-state as determined from single crystal X-ray structure studies of LGaCl3 complexes containing various donor ligands L (L ¼ THF, R3P, NHC, CAAC etc.) and the donor properties of the ligand L as evaluated by the Tolman electronic parameter (TEP). Hence, the sum of the Cl–Ga–Cl bond angles in LGaCl3 complexes can be used as an alternative scale to assess ligand donor properties.20 Moreover, Tamm and co-workers employed anionic carbenes for the preparation of adducts of the type [{(F5C6)3BCCH(NDip)2}C]MCl3Li(solv) (M ¼ Ga 17, In 18).21

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In addition to these carbene complexes of metal trihalides, adducts of binary group 13 metal hydrides EH3 have also been studied. Among these, GaH3 and InH3 were of particular interest due to their thermal instability. In case of InH3, stabilization by coordination of a neutral donor to maintain room temperature stability was achieved only in very few cases using PCy3 (Cy ¼ c-C6H11)22 and [(HCNMes)2C].23 More recently, such adducts featuring a ring-expanded carbene, [(H4C2NDip)2C] InH3 (19),24 and imidazol-2-ylidenes possessing sterically more demanding N-substituents, [(HCNDip)2C]InH3 (20) and [(HCNAr )2C]InH3 (21, Ar ¼ 2,6-(CHPh2)2-4-Me-C6H2),25 were synthesized from either [Cy3P]InH3 or LiInH4. The reduced thermal stability of compounds 19 and 20 compared to 21 and [(HCNMes)2C]InH3 (22) was attributed to the close proximity of alkyl groups in 19 and 20 to the In–H functions, which has a promoting effect toward their decomposition. The NHC-adducts of GaH3 generally possess superior thermal stability and several examples were prepared from carbenes of various steric demands and LiGaH4, including [(MeCNMe)2C]GaH3 (23), [(HCNiPr)2C]GaH3 (24), [(MeCNiPr)2C]GaH3 (25), [(H2CNDip)2C]GaH3 (26),26 [(HCNMes)2C]GaH3 (27),23 and [(HCNDip)2C]GaH3 (28),27 respectively. In contrast, the use of CAACs typically resulted in the formation of Ga–H insertion products such as (MeCAACH)GaH2(NHC) and (MeCAACH)2GaH (29), which were obtained by reaction of MeCAAC with (NHC)GaH3.26 Similar adducts were prepared in reactions of mixed chloridohydridogallanes, HGaCl2 and H2GaCl, which were synthesized by dismutation reactions between the corresponding GaH3 and GaCl3 adducts.26 In addition, Rivard and co-workers reported on the synthesis of mixed pseudohalidohydridogallane adducts including [(HCNMes)2C] GaH2(N3) (30), [(HCNMes)2C]GaH(OTf )2 (31, OTf ¼ OSO2CF3), and [(HCNMes)2C]GaH2[N(SiMe3)2] (32)28 (Scheme 2).

Scheme 2 Carbene-coordinated (and related) group 13 metal complexes.

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Falivene, García and co-workers prepared a series of trialkylgallane and -indane adducts of the type [NHC]MMe3 (NHC ¼ [(HCNMes)2C], [(H2CNMes)2C], [(HCNDip)2C], [(H2CNDip)2C]; M ¼ Ga, In) and analyzed their thermal stability and bonding.29 Based on a close comparison of their structural parameters, i.e. the M–C bond length, and their 13C NMR chemical shifts, decreasing stability of the complexes with increasing steric bulk of the carbene ([(HCNMes)2C] < [(HCNDip)2C] < [(H2CNMes)2C] < [(H2CNDip)2C]) was concluded, which is caused by increasing steric repulsion with increasing steric demands of the NHC. Moreover, the thermal stability in group 13 metal carbene adducts is largely influenced by the Lewis acidity of the group 13 center, leading to shorter M–C bonds and thus stronger interactions for MH3 and MX3 complexes in the order: MMe3 < MH3 < MX3 (X ¼ halide). These observations are in accordance with reactivity studies by Horeglad and co-workers, which demonstrated the influence of the M–NHC interaction strength on the dismutation behavior of [NHC]MMe2OR complexes. According to these studies, Ga–NHC adducts exhibit stronger interactions compared to analogous In–NHC adducts.30,31 While NHCs typically coordinate to the Lewis acidic group 13 metal center in a normal (C2-coordinated; nNHC) binding mode, combinations of sterically demanding carbenes with group 13 organyls with bulky substituents frequently resulted in formation of structural isomers, in which the NHCs adopt an abnormal/mesoionic (C4-coordinated; aNHC) binding mode. This was reported for reactions of [(HCNtBu)2C] and MMe3, which initially gave the nNHC adducts [nNHC]MMe3 (M ¼ Ga n33, In n34), which isomerized in solution with subsequent formation of the corresponding aNHC adducts [aNHC]MMe3 (M ¼ Ga a33, In a34).32 The normal-to-abnormal isomerization is typically attributed to the release of steric pressure, and even though normal carbene adducts are in the focus of most researchers, rational syntheses of abnormal aNHC–GaR3 complexes have been also developed33,34 (Scheme 3).

Scheme 3 Normal-to-abnormal isomerization of NHC-group 13 trimethyl complexes.

In addition to simple monoadducts, several double carbene adducts of low-oxidation state group 13 compounds were reported. For instance, Ga[GaCl4], Ga2Cl4(1,4-dioxane)2, as well as “GaI” were found to react with two equivalents of the carbene L (L ¼ CAAC, NHC) with formation of the bis-carbene tetrahalidodigallane adducts [L2Ga2Cl4] (L ¼ MeCAAC 35, CyCAAC 36, [(H2CNDep)2C] 37, [(HCNDip)2C] 38)35 and [{(HCNMes)2C}2Ga2X4] (X ¼ Cl 39, I 40, Dep ¼ 2,6-Et2C6H3),36 in which the Ga centers adopt the oxidation state +II. The bromo- and iodo-substituted analogs of 35 and 36 were synthesized by treatment of chloro-substituted compounds 35 and 36 with BX3 (X ¼ Br, I), respectively,35 whereas the corresponding hydrido-substituted species [L2Ga2H4] have only been studied computationally.37,38 In addition, Robinson and co-workers synthesized the carbene-stabilized digallane {[(MeCNiPr)2C]GaClMes}2 by reduction reaction of [(MeCNiPr)2C]GaCl2Mes with KC8, whereas the reduction with potassium metal led to the formation of [(MeCNiPr)2C]Ga[Ga4Mes4]Ga[C(NiPrCMe)2] (41), a neutral 14 skeletal electron Ga6 octahedron consistent with the Wade-Mingos Rules.39 The use of a chelating amidinate ligand in conjunction with a CAAC ligand enabled the isolation of the CAAC-stabilized Ga radical [PhC(NtBu)2]GaCl(CAAC) (42), which was formed by reduction of [PhC(NtBu)2]GaCl2 with KC8 in the presence of CAAC. EPR spectroscopy and DFT studies demonstrated the localization of unpaired spin density on the carbene fragment, mainly on the carbene carbon atom, as is usually observed for CAAC-substituted main group radicals40 (Scheme 4).

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Scheme 4 Carbene-coordinated low-oxidation state group 13 metal complexes.

NHCs have also been used to stabilize low-valent group 13 metal cations. The extremely bulky trityl-substituted carbene [(HCNCPh3)2C] forms the quasi one-coordinate Tl(I) carbene complex [{(HCNCPh3)2C}Tl][B(3,5-(CF3)2C6H3)4].41 Most remarkably, Krossing and co-workers prepared two-coordinate M(I) cations (M ¼ Ga, In) of the type [L2M][Al(OC(CF3)3)4] (L ¼ [(HCNDip)2C], [(HCNMes)2C]) by ligand exchange reactions. The solid-state structures of the cations revealed that the metal centers are coordinated by two sterically demanding NHC ligands, which adopt an unusually tilted coordination geometry. The observed tilting of the carbene planes was attributed not only to steric repulsion between the bulky ligands but also to a s back-bonding interaction from the M s-orbital to the carbene carbon p-orbital, which compensates for the high electron density at the group 13 center imposed by the strongly s-donating NHC ligands.42

9.07.3.1.2

Alkyl substituents

The class of group 13 alkyl compounds is well-known for a long time and deeply anchored in main group element chemistry. However, within the past decade some new trends in this field have emerged, and novel structures, properties, and reactivity patterns were observed. Among known heavier group 13 alkyls, simple metal triallyls are interesting compounds due to the various bonding modes that allyl ligands can adopt. Unfortunately, such allyl complexes are still very rare. The first complex of the desired type was reported by Hanusa and coworkers in 2006, describing the synthesis of Ga[1,3-(SiMe3)2C3H3]3 (43) containing bis-silylated allyl ligands.43 Compound 43 shows a fluxional behavior in solution due to [1,3]-sigmatropic shifts with a low activation barrier, whereas in the solid-state structure, the s-bound/Z1 coordination mode of the allyl ligands at the trigonal-planar Ga center was proven. Mixed allyl indium bromides, RInBr2 and R2InBr (R ¼ C3H5, 3-PhC3H4, 3,3-(4-tBuC6H4)2C3H3) were obtained from oxidative addition reactions of indium metal and the corresponding allylbromides by Baba and co-workers.44–46 Solid-state structures of these compounds were obtained upon coordination of the In center with N-based donors, again revealing the s/Z1 coordination of the allyl ligands. The parent compounds M(C3H5)3 (M ¼ Ga 44, In 45) were obtained by Okuda and co-workers from salt metathesis reactions and crystallographically characterized in form of their Ph3PO and 1,4-dioxane adducts, (Ph3PO) Ga(C3H5)3 (46)47 and [(1,4-dioxane)In(C3H5)3]1 (47),48 respectively. Moreover, the closely related 2-methylallyl compound In(C4H7)3 (48), was synthesized in an analogous manner. Allyl complexes 46–48 show highly fluxional behavior in solution, which most likely originates from ligand exchange or sigmatropic rearrangement reactions. While the solid-state structures of the

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adducts 46 and 47 reveal the Z1 coordination mode, a single crystal X-ray diffraction study of donor-free compound 48 proved the presence of two terminal Z1-C4H7 and one bridging m,Z1:Z1 ligand linking two In centers.47,48 In addition, cationic bisallylgallium complexes [(THF)2Ga(C3H5)2][B(C6H3Cl2)4] (49) and anionic tetrakisallylgallates, [Cat][Ga(C3H5)4] (50, Cat ¼ K, [K(dibenzo-18c-6)], PPh4) were synthesized, and their reactivity toward benzophenone and isoquinoline was investigated47 (Scheme 5).

Scheme 5 Ga and In allyl compounds.

Metal methylene compounds are also of great interest and importance, i.e. for carbonyl olefination reactions in organic syntheses. Anwander and co-workers obtained the homoleptic gallium methylene species [Ga8(m-CH2)12] (51) as a molecular compound via a rare-earth metallocene-mediated C–H bond activation cascade of GaMe3.49 51 features a cage-type structure with three methylene bridges at each Ga center, and undergoes a reversible de-aggregation reaction in THF solution with subsequent formation of the smaller gallium methylene oligomer [Ga6(m-CH2)9(THF)6] (52). The suitability of compound 51 as a methylene transfer reagent was proven in the carbonyl olefination reaction with 9-fluorenone, which yielded 9-methylene-fluorenone (Scheme 6).

Scheme 6 Gallium methylene compounds.

Donor-free dialkyl metal cations of the group 13 elements, particularly Al and Ga, hold the potential of a highly electrophilic metal center, which is typically connected to unique properties and reactivities and could be used in catalysis. For their stabilization, however, weakly-coordinating anions are necessary to minimize cation-anion interactions, which would otherwise result in a decreased reactivity. Knapp and co-workers used the weakly-coordinating and robust perchlorinated closo-dodecaborate dianion [B12Cl12]2− to synthesize (Et2M)2[B12Cl12] (M ¼ Al, Ga, In) salts by ethyl abstraction with the tritylium cation.50 Vibrational spectroscopy indicated a linear structure for the Me2In+ cation, while the aluminum counterpart features a bent structure due to interactions with the anion. Unfortunately, single crystals of the Ga and In compounds suitable for single crystal structure determinations could not be obtained. Wehmschulte and co-workers successfully employed the chlorinated carborane anion [CHB11Cl11]− for the synthesis of [Et2Ga][CHB11Cl11] (53), which adopts a polymeric structure in the solid-state featuring four close Ga–Cl contacts at each Ga center. This was proposed as the main reason for the low catalytic activity of 53 in the Lewis acid catalyzed reduction of CO2 with Et3SiH.51 In contrast, the mixed alkyl/aryl Ga cation [(2,6-Dip2C6H3)GaEt]+ showed increased catalytic activity in this reaction with the selective, yet unexpected, formation of Et3SiOMe as the main product.51 Group 13 trialkyls tend to form bimetallic ‘ate complexes when combined with metal alkyls due to their Lewis acidic nature. As-formed ‘ate complexes possess stronger polarized M–C bonds with enhanced chemical reactivity compared to the respective

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metal alkyls. However, such complexes are typically formed in situ as reaction intermediates, hence their structures have only rarely been determined. Hevia and co-workers synthesized unsolvated complexes M[Ga(CH2SiMe3)4] (M ¼ Li 54, Na 55) and benzenecoordinated complexes [(C6H6)2K][Ga(CH2SiMe3)4] (56), respectively, which all showed polymeric arrangements in their solid-state structures arising from methylene-alkali metal contacts.52 An analogous structure to those of 54 and 55 was observed for Li[GaEt4].53 Moreover, heterobimetallic alkaline-earth metal tetraalkylgallate complexes M[GaMe4]2 (M ¼ Ca 57,54 Ba 5855) and [(tol)Ba{GaEt4}2]2 (59, tol ¼ toluene)55 were reported. 57 shows a polymeric arrangement in the solid-state with hexacoordinated Ca centers, while 59 is dimeric with each Ba center featuring five methylene contacts in addition to a Z6 toluene contact. Additionally, the heteroleptic compound [TptBu,MeCa][GaMe4] (TptBu,Me ¼ tris(3-tBu-5-Me-pyrazolyl)borato) was prepared.56 An important class of compounds are weakly coordinating anions (WCA), which are mainly based on group 13 elements as the central atom. Hoge and co-workers reported the novel WCA [Ga(C2F5)4]− (60) containing perfluorinated ethyl groups by treatment of GaCl3(DMAP) (DMAP ¼ N,N-dimethylaminopyridine) with LiC2F5. The versatility of gallate 60 was demonstrated by the synthesis of various salts containing useful cations, such as [PPh4]+, [CPh3]+, [(O2H5)2(OH2)2]+, Li(OEt2)+ and [Li(dec)2]+ (dec ¼ diethylcarbonate), and their applications as conducting salts or additives in batteries was preliminarily investigated.57 Moreover, anionic gallates [Ga(C2F5)3X]− (X ¼ F, Cl, Br, I) and neutral gallane [Ga(C2F5)3Y] (Y ¼ (OH2)2, OH2(OEt2), OEt2, DMAP) adducts of the parent Lewis acid Ga(C2F5)3 were also reported58 (Scheme 7).

Scheme 7 Tetraalkylgallates.

9.07.3.1.3

Aryl substituents

Aryl substituents possess a similar importance in group 13 organometallic chemistry to alkyl substituents. Fast progress was observed by the introduction of sterically demanding terphenyl substituents as already emphasized in COMC-III, however, they retain their high significance until today. Jutzi and co-workers prepared a series of mono-, bis-, and tris-dimethylgallylated benzenes Me2GaPh, 1,3-(Me2Ga)2C6H4, 1,4-(Me2Ga)2C6H4, and 1,3,5-(Me2Ga)3C6H3 by transmetallation reactions of the corresponding arylmercury compounds with GaMe3. In the solid-state, these compounds form coordination polymers, in which the monomers are connected by either Ga-parene, Ga-parene/Ga-methyl, or ipso-bridging arene contacts, respectively.59,60 Aryl groups containing pendant arms with dimethylamino donor functions, 2,6-(Me2NCH2)2C6H3, have also been applied and the dimeric, divalent dichlorogallane [2,6-(Me2NCH2)2C6H3)GaCl]2 (61) was reported. Substitution of the chloride substituents by hydrides yielded the corresponding Ga(II) hydride [2,6-(Me2NCH2)2C6H3)GaH]2, whereas reduction of 61 resulted in activation of the toluene solvent with formation of [2,6-(Me2NCH2)2C6H3)Ga(CH2Ph)]2.61 This N,C,N ligand was further utilized for the synthesis of a series of Ga chalcogenides of the types [2,6-(Me2NCH2)2C6H3)ME]2, [2,6-(Me2NCH2)2C6H3)M]E4, and [2,6(Me2NCH2)2C6H3)M](EPh)2 (M ¼ Ga, In; E ¼ S, Se, Te), which were used as single-source precursors in the preparation of ME thin films62–64 (Scheme 8).

Scheme 8 Reactivity of compound 61.

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Gallium, Indium, and Thallium

Two-coordinate diaryl and diterphenyl group 13 cations (which have been known since the pioneering synthesis of the [2,6-Mes2C6H3]2Ga+ cation partnered with a weakly coordinating counterion),65 have also been prepared. In the related compound [{2,6-(Me5C6)2C6H3}2Ga][GaI4] the Ga center is shielded by the sterically demanding aryl ligands, which prohibited close contacts to the [GaI4]− anion.66 The importance of steric bulk is further shown by comparing [Mes2Tl][BF4] (62)67 and [{2,6-Mes2C6H3}2Tl] [TlCl4] (63)68: While in 62 each Tl center shows four Tl–F contacts, no significant cation-anion interaction is observed in 63. Since the landmark discoveries of monomeric and mono-coordinate terphenyl indanediyls69 and thallanediyls,70 as well as weakly associated terphenyl gallanediyl dimers71 by Power and co-workers, the synthesis and reactivity of low-valent and low-oxidation state compounds [ArM] stabilized by terphenyl ligands was further investigated. Systematic studies on the reactions of the isolated and freshly prepared lithium salt 2,6-Mes2C6H3Li with various low-oxidation state In sources such as InCl and In [InI]4 in different solvents led to a number of mixed-valent terphenylindium subhalides, i.e. [{(2,6-Mes2C6H3)InCl}2]2, [{(2,6-Mes2C6H3)InI}2]2, [(2,6-Mes2C6H3)4In4Cl2I2], [(2,6-Mes2C6H3)4In8], [{(2,6-Mes2C6H3)2In2I}2], [(2,6-Mes2C6H3)3In3I2], and [(2,6-Mes2C6H3)3In3ClI], respectively, revealing the strong dependency of the reaction outcome on the reaction conditions.72,73 The use of even bulkier terphenyl ligands 2,6-Dip2-3,5-iPr2C6H and 2,6-(2,4,6-iPr2C6H3)2-3,5-iPr2C6H, containing iPr groups on the central aryl ring, allowed for the synthesis of the monomeric, one-coordinate gallanediyls [(2,6-Dip2-3,5-iPr2C6H) Ga] (64) and [{2,6-(2,4,6-iPr2C6H3)2-3,5-iPr2C6H}Ga] (65).74 Structural, spectroscopic, and computational investigations proved that the Ga–Ga interactions,75 which were observed in the solid-state structures of several terphenyl [ArGa]2 dimers, are rather weak, allowing for the dissociation of these complexes in solution with formation of monomeric units.74 Moreover, the reactivity of [(2,6-Dip2C6H3)Ga]2 (66)71 was intensively studied. Simple oxidation reactions of 66 with N2O and S8 afforded [(2,6-Dip2C6H3)GaO]2 (67) and [(2,6-Dip2C6H3)GaS]2 (68) with the Ga centers oxidized to the +III oxidation state. Analogous reactions were observed for the corresponding diindene [(2,6-Dip2C6H3)In]2 (69), yielding [(2,6-Dip2C6H3)InO]2 (70) and [(2,6-Dip2C6H3)InS]2 (71), respectively.76 In addition, digallene 66 was found to readily and selectively activate H2 and NH3 under ambient conditions yielding [(2,6-Dip2C6H3)Ga(m-H)H]2 (72) and [(2,6-Dip2C6H3)Ga(m-NH2)H]2 (73). These reactions are consistent with oxidative addition of the H–H and N–H bonds to the low-valent Ga centers.77 With simple olefins such as ethylene, propene, 1-hexene, and styrene, 66 reacted with subsequent formation of 1,4-digallacyclohexanes of the type [(2,6-Dip2C6H3)Ga(CH2CHR)]2 (R ¼ H 74, Me 75, nBu 76, Ph 77), in which the Ga2C4 six-membered ring either adopts a twist-boat (74) or chair-type (75, 77) conformation. However, no reactions with larger and branched alkenes were observed due to the sterically shielded Ga2 moiety in 66.78 A variety of polyolefins, norbornadiene, quadricyclane, cyclopentadiene, cyclooctatetraene, and 1,3,5-cycloheptratriene also readily reacted with 66 in higher-order cycloaddition reactions ([2p + 2p + 2p], [6p + 2p], [4p + 2p]), some of which are reversible, to give higher-order ring structures, which are interconvertible in part. According to DFT orbital calculations and in contrast to all-carbon cycloadditions, the digallane systems are less restricted by the Woodward-Hoffmann rules, rendering these various types of cycloaddition reactions symmetry-allowed.79 These types of cycloaddition reaction of group 13 dimetallenes were independently investigated computationally by Su and co-workers.80 Digallene 66 also reacted with the alkyne phenylacetylene to afford the 1,4-digallacyclohexadiene [(2,6-Dip2C6H3)Ga(CHCPh)]2 (78), which was further reduced with potassium metal to the corresponding dianionic 1,4-digallatabenzene (79). The aromatic character of the planar six-membered [Ga2C4]2− 6p electron system was disclosed from DFT calculations and an aromatic ring current was observed in 1H NMR spectrum.81 Further experimental and computational investigations on the mechanism of the Ga-mediated bond activation reactions indicated that the reactions indeed proceed via the Ga–Ga bonded digallane 66, despite being mainly dissociated into [(2,6-Dip2C6H3)Ga] monomers in solution. This was evidenced by attempted reactions of the monomeric gallanediyl 65 with H2 and ethylene under ambient conditions, from which only the starting material was recovered82 (Scheme 9).

Scheme 9 Structures and reactivity of terphenyl-substituted Ga(I) compounds.

Gallium, Indium, and Thallium

9.07.3.2 9.07.3.2.1

379

p Ligands Arene ligands

Arene complexes of M(I) salts of Ga, In, and Tl, mainly [M(arene)2][MX4] (X ¼ Cl, Br), were studied extensively by Schmidbauer almost 40 years ago.83 However, new developments in this field were achieved only recently through the use of WCAs. Krossing and co-workers synthesized a number of Ga(I) arene complexes [Ga(arene)n][Al(OC(CF3)3)4] (arene ¼ tol (n ¼ 2), C6H5F (n ¼ 2, 3), o-C6H4F2 (n ¼ 2), p-C6H4Me2 (n ¼ 2, 3), C6Me6 (n ¼ 2), 1,3,5-Me3C6H3 (n ¼ 2), PhC2H4Ph (n ¼ 1)) by oxidation of Ga metal with the silver salt of the WCA, Ag[Al(OC(CF3)3)4], followed by subsequent arene exchange reactions.84–86 Use of 1,3-Ph2C6H4 afforded the bimetallic dication [(C6H5F)Ga(m-1,3-Ph2C6H4)2Ga(C6H5F)]2+, in which two Ga+ cations are bridged by the triaryl ligand.86 The Ga centers show weak contacts to the fluorine atoms of the WCA depending on the number, the electron richness, and the steric bulk of the arene ligands coordinating to the Ga cation. For instance, [Ga(C6Me6)2]+ and [Ga(p-C6H4Me2)3]+ show no further Ga–F contacts, whereas [Ga(p-C6H4Me2)2]+ and [Ga(o-C6H4F2)2]+ exhibit four and two contacts, respectively.86 The corresponding In(I) complexes [In(C6H5F)n][Al(OC(CF3)3)4] (n ¼ 2, 3) and [In(o-C6H4F2)2][Al(OC(CF3)3)4] were prepared by the same oxidation approach as was used for the synthesis of the corresponding Ga(I) complexes87 as well as by salt metathesis reactions between InCl and Li[Al(OC(CF3)3)4],88 respectively. Moreover, the Tl(I) arene complexes [Tl(C6Me6)2][H2N{B(C6F5)3}2] and [Tl(tol)3] [H2N{B(C6F5)3}2] were obtained by the protonation reaction of TlOEt with [H(OEt2)2][H2N{B(C6F5)3}2].89 Remarkably, the M-centroid distances in the Tl complexes were found to be shorter than those in the In complexes, which is due to the relativistic contraction of the Tl+ ion and the stronger bonding interactions of the more electron-rich arenes, i.e. tol and C6Me6, to the M+ ions.88,89 Wehmschulte and co-workers exploited the weakly-coordinating carborate anion [CHB11Cl11]− for the synthesis of the corresponding Ga(I) and In(I) arene complexes, [M(arene)n][CHB11Cl11] (M ¼ Ga, arene ¼ C6H5Br, n ¼ 1; M ¼ In, arene ¼ C6H5Br, n ¼ 1.5; M ¼ In, arene ¼ tol, n ¼ 3), according to the oxidation protocol with Ag+. Due to the more basic nature of the carborate anion [CHB11Cl11]− compared to the aluminate anion [Al(OC(CF3)3)4]−, stronger M-anion interactions were observed in these complexes.90,91 Remarkably, mono-coordinated monoarene complexes [M(C6Me6)][Al(OC(CF3)3)4] (M ¼ Ga, In) and [In(1,3,5-Me3C6H3][Al(OC(CF3)3)4] were formed in oxidation reactions of the corresponding metals with in situ prepared solutions of the respective arene radical cation, which was found to act both as oxidizing reagent and as stabilizing ligand. In comparison with the two-coordinate complex [Ga(C6Me6)2][Al(OC(CF3)3)4], [Ga(C6Me6)][Al(OC(CF3)3)4] shows shorter Ga-centroid distances indicative of a stronger interaction due to the presence of only one arene ligand.92 Even more interestingly, the ligand exchange reaction of [Ga(C6H5F)2][Al(OC(CF3)3)4] with free COD results in the formation of [Ga(COD)2][Al(OC (CF3)3)4] (COD ¼ 1,5-cyclooctadiene), which represents the first homoleptic olefin complex of a main group metal. The ligand-metal bonding interaction in this complex was found to be mainly electrostatically dominated, which is in remarkable contrast to the orbital-based bonding interactions between olefins and transition metals.93 Moreover, the [Ga(arene)]+ salts were employed as catalysts in alkene polymerization reactions,85,86,90 in dihydroarylation reactions of arenynes, and in transfer hydrogenation reactions,94 whereas the monoarene complexes [M(C6Me6)][Al(OC(CF3)3)4] (M ¼ Ga, In) were utilized for the synthesis of heterobimetallic main group element sandwich complexes and coordination polymers.95

9.07.3.2.2

Cyclopentadienyl ligands

The synthesis of monovalent metallanediyls such as CpM (Cp ¼ C5H5) and Cp M (Cp ¼ C5Me5) has drawn much attention to cyclopentadienyl group 13 chemistry. However, much of the work on these systems was performed during the 1990s and early 2000s.96,97 Nevertheless, some new cyclopentadienyl heavy group 13 metal compounds have been synthesized and studied since 2007. Cyclopentadienyl compounds with Ga in the +III oxidation state were prepared by Shapiro and co-workers with the aim of designing potential single-source precursors for the synthesis of GaN.98 These include [(C5Me4H)2Ga(m2-NH2)]2, [Cp 2Ga(m-NH2)]2, [Cp Ga(Cl)(m-NH2)]3, and [Cp 2Ga(m-NHEt)]2, which were synthesized by ammonolysis of (C5Me4H)3Ga, Cp 3Ga, and Cp 2GaCl, respectively. Cowley and co-workers investigated the synthesis of decamethylgallocenium cations with [BF4]− and [AlCl4]− counterions, [Cp 2Ga][BF4] and [Cp 2Ga][AlCl4], by protonation of Cp 3Ga with HBF4 or chloride abstraction from Cp 2GaCl with AlCl3, respectively. However, rather short Ga-halide contacts were observed in these salt-like compounds, altering the structure of the [Cp 2Ga]+ moiety. The use of the strong Lewis acid B(C6F5)3 for chloride abstraction of Cp 2GaCl, however, resulted in the formation of decomposition products, such as Cp 2GaC6F5.99 Power and co-workers obtained the unprecedented In(II) compound [{In(N(Dip)SiMe3)}2(m2-Z1:Z1-C5H5)(m2-Z1:Z2-C5H5) (80) by reaction of CpIn with lithium amide LiN(Dip)SiMe3. The reaction proceeds via disproportionation of the In(I) compounds with formation of indium metal and diindane 80, in which the two In centers are bridged by two p-bound Cp ligands, revealing a new bonding mode of the Cp substituent in main group element chemistry.100 CpGa was the first organometallic Ga(I) compound reported in the literature and was prepared by treatment of metastable GaX solutions with LiCp.101 Schnepf and co-workers presented a more convenient synthesis of CpGa starting from “GaI” and NaCp. Attempts to crystallize the compound resulted in formation of crystals of the composition CpGaGaCp2I, while the in situ prepared solution of GaCp was used to synthesize the borane-adduct CpGaB(C6F5)3.102 In addition, the reactivity of Cp Ga was investigated. Fischer and co-workers reported on the protonation of Cp Ga using half an equivalent of [H(OEt2)2][B(3,5-(CF3)2C6H3)4] producing the [Ga(m-Cp )Ga]+ cation, which adopts a bipyramidal double-cone structure. This salt was shown to provide a source of the “naked” Ga+ ion in reactions with K[(NDipCMe)2CH] affording the known b-diketiminate Ga[(NDipCMe)2CH] (cf. Section 9.07.5.6), thus being comparable to the [Ga(arene)n]+ complexes

380

Gallium, Indium, and Thallium

(Section 9.07.3.2.1).103 With heavier alkaline-earth metallocenes [Cp 2Ae] (Ae ¼ Ca, Sr, Ba), Cp Ga forms donor-acceptor complexes of the type [{Cp 2Ae}{GaCp }n] (Ae ¼ Ca, n ¼ 1 (81); Ae ¼ Sr(THF), n ¼ 1 (82); Ae ¼ Ba, n ¼ 2 (83)) with Ga–Ae donor-acceptor bonds. The interactions were shown to rely mainly on dispersion forces, whereas the size of the alkaline-earth metal determines the number of coordinated Cp Ga moieties.104 Moreover, Cp Ga forms a Lewis acid-base adduct through coordination to the boron center of pentaarylboroles, which contrasts with the results from the analogous reaction with Cp Al, which resulted in the formation of a neutral aluminocene sandwich complex due to reduction of the pentaarylborole unit105

Scheme 10 Group 13 cyclopentadienyl compounds and Lewis acid-base adducts with group 2 metallocenes.

(Scheme 10). Aside from Cp and Cp ligands, bulkier C5H2tBu3 ligands have been used in group 13 metal chemistry. Trivalent Ga complexes [(C5H2tBu3)Ga(X)(m-X)]2 (X ¼ Cl, I) were synthesized by metathesis reactions of (C5H2tBu3)2Mg and GaX3, and subsequently reduced upon treatment with KC8 in THF, yielding the Ga(I) compound (C5H2tBu3)Ga (84) together with the THF-adduct (C5H2tBu3)GaCl2(THF). While 84 is a liquid at ambient conditions and no structural information could be gathered, its reaction with GaI3 yielded the Lewis acid-base adduct (C5H2tBu3)GaGaI3, which was structurally characterized, proving the low-valent and hence typically Lewis basic nature of 84. The corresponding In counterparts could not be obtained, however; [(C5H2tBu3)In(nBu) (m-Br)]2 was formed by alkylation of the proposed intermediate [(C5H2tBu3)In(Br)(m-Br)]2.106 Furthermore, bulky pentaarylcyclopentadienyl complexes of low-valent Ga, In, and Tl, [(C5(4-tBuC6H4)5]M (M ¼ Ga 85, In 86, Tl 87), were introduced by reactions of the corresponding stable [(C5(4-tBuC6H4)5] radicals with the respective metal amalgams, allowing direct access to low-oxidation state group 13 complexes without the need for low-valent group 13 salts as starting materials, or subsequent reduction reactions.107 Thiel and co-workers reported the Tl(I) salt of a dibenzocycloheptene-based cyclopentadienyl ligand possessing intrinsic helical chirality, which was used as a cyclopentadienyl transfer reagent.108

9.07.3.3

Unsaturated group 13 metal-containing heterocycles

Aromaticity is one of the most important concepts in chemistry with benzene being the parent compound of aromatic molecules. Therefore, the study of compounds containing heteroatoms, especially heavy main group elements, incorporated in a cyclic system containing potentially delocalized p electrons is of fundamental interest. In addition to the dianionic 1,4-digallatabenzene 79 obtained by Power and co-workers (Section 9.07.3.1.3),81 only a very limited number of heavy group 13 metal-containing six-membered heterocycles exhibiting potential aromaticity have been reported. These include the anionic gallabenzene [{HC(CH)2}2Ga(2,4,6-tBu3C6H2)]− reported by Ashe and co-workers, which was unfortunately neither isolated nor structurally characterized.109 In 2015, Yamashita and co-workers reported on the isolation and structural characterization of the lithium salt of an anionic gallabenzene, [Li(solv)][{HC(CHCSiiPr3)2}2GaMes] (88, solv ¼ Et2O, 1,2-dimethoxyethane (DME)), which was synthesized by addition of MesLi to the gallacyclohexadiene pyridine adduct [{HC(CHCSiiPr3)2}2GaCl(py)] (py ¼ pyridine).110 A similar anionic indabenzene, [Li(DME)][{HC(CHCSiiPr3)2}2InMe] (89), which was prepared via a similar route, was also studied by the same group.111 The cyclic C5M moieties of the anions of 88 and 89 are planar and thus satisfy the structural criteria for aromaticity. Results on both systems from NMR spectroscopy as well as from DFT calculations (such as slightly negative NICS values) suggest some degree of aromaticity. However, the electronic structure in

Scheme 11 Anionic metallabenzenes.

Gallium, Indium, and Thallium

381

these anionic metallabenzenes may be best described by contributions of two resonance structures: one aromatic, C5M-delocalized and one ambiphilic, C5-delocalized structure110,111 (Scheme 11). Another class of unsaturated metal-containing heterocycles are group 13 metallacyclopentadienes, i.e. compounds in which one CH unit of the unsaturated C5 cycle is replaced by a group 13 metal, thus rendering it a 4p electron system with potential antiaromatic character. The first group 13 cyclopentadienes, gallacyclopentadiene and indacyclopentadiene [(MeCCMe)2M(2,4,6tBu3C6H2)] (M ¼ Ga, In), were reported by Cowley and co-workers in 1994.112 Since then, Tokitoh and co-workers synthesized the derivative [(EtCCEt)2Ga(2,4,6-tBu3C6H2)] (90) and its doubly-reduced dianion [Li(THF)]2[(EtCCEt)2Ga(2,4,6-tBu3C6H2)] (91). While 90 is non-aromatic rather than antiaromatic, contact-ion pair 91 exhibits aromatic character as shown by the calculated NICS values.113 In addition, Ga–Ga bonded bis(dilithiogallacyclopentadienes) {[Li(THF)]2[(RCCSiMe3)2Ga]}2 (R ¼ Me 92, (CH2)4 93) were obtained from reduction reactions of lithium spirogallate complexes [Li(solv)][{(RCCSiMe3)2}2Ga] (R ¼ Me, solv ¼ Et2O; R ¼ (CH2)4, solv ¼ THF). Calculations of NICS values and the isomerization stabilization energy indicated aromaticity of the [C4Ga]2− moieties in 92, as observed for compound 91.114 Dianionic diindanes {[Li(THF)]2[(RCCSiMe3)2In]}2 (R ¼ Me 94, (CH2)4 95) are similar to 92 and 93, and both compounds contain In–In bonds. In addition, the spiroindates [Li(THF)4] [{(RCCSiMe3)2}2In] (R ¼ Me, (CH2)4, Ph) were also reported, as well as the further reduced dilithioindacyclopentadienyllithium

Scheme 12 Group 13 metallacyclopentadienes.

species [Li(THF)]2[(PhCCSiMe3)2InLi(THF)3] (96), containing an In–Li bond. As was reported for the Ga analogs, 94–96 feature aromatic character of the [C4In]2− rings according to DFT calculations115 (Scheme 12). A class of compounds related to metallacyclopentadienes are metallafluorenes or dibenzometalacyclopentadienes. The first representatives, 9-galla- and 9-indafluorene containing bulky 2,4,6-tBu3C6H2 groups as kinetically stabilizing ligands at the group 13 metals (M ¼ Ga 97, In 98) were synthesized by Cowley and co-workers in 1995.116 Chujo and co-workers prepared a series of 9-metallafluorenes containing 2-(Me2NCH2)-4,6-tBu2C6H2 (M ¼ Ga 99, In 100) and 2,6-(Me2NCH2)2C6H3 (M ¼ Ga 101) substituents. These complexes feature four-coordinate, trigonal pyramidal and five-coordinate, trigonal bipyramidal coordination modes at the group 13 center, and their optical and photophysical properties were studied.117–119 Linti and co-workers utilized 9zincafluorenes for the synthesis of a related intramolecularly donor-stabilized 9-gallafluorene containing a 2-(Me2NCH2) C6H4 substituent at the Ga center. Moreover, external base-stabilized, bridged and unbridged, 9-gallafluorenes featuring Ph and 2-PhC6H4 substituents were obtained using the strong-coordinating base N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA).120 Neutral, unsaturated seven-membered heterocycles including a group 13 element (heteropins) are formally derived from the aromatic tropylium (cycloheptatrienyl) cation, [C7H7]+, possessing a 6p electron system. The first of such compounds containing a Ga center in the seven-membered framework reported in the literature was the donor-stabilized, TMEDA-linked bis(dibenzogallepin) 102, that was synthesized by reaction of 2,20 -dilithio-Z-stilbene with GaCl3 in the presence of TMEDA. Due to the base-coordination, the gallacycles in 102 adopt a boat-type conformation. Calculations on model systems including the base-free gallepin suggest aromatic character for both the base-coordinated and base-free gallepins, while the benzannulation reduces the aromaticity of the central seven-membered C6Ga ring.121 Chujo and co-workers prepared a series of dibenzogallepins containing three-coordinate, four-coordinate, and five-coordinate Ga centers using 2,4,6-tBu3C6H2 (103), 2-(Me2NCH2)-4,6tBu2C6H2 (104), and 2,6-(Me2NCH2)2C6H3 (105) groups as supporting ligands at the dibenzogallepin core structures. Gallepins 103 and 105 feature remarkable planar C6Ga rings due to the trigonal planar and trigonal bipyramidal coordination spheres at the Ga centers, while the trigonal pyramidal coordinate Ga center in 104 induces a boat-type conformation, as was previously observed122 (Scheme 13).

382

Gallium, Indium, and Thallium

Scheme 13 Group 13 metallafluorenes and gallepins.

9.07.4

Heavier group 14 element-based ligands

Aside from alkyl ligands, heavier group 14 element-based ligand systems, particularly bulky silyl groups, have also been used for the synthesis and stabilization of heavier group 13 organometallic compounds. Sekiguchi and co-workers reported the one-electron reduction reaction of Ga trisilyl compound Ga(SiMetBu2)3 with alkali metals, leading to the formation of the corresponding radical anion [Ga(SiMetBu2)3]− (106), which was isolated in form of the K([2.2.2]cryptand) salt. From EPR spectroscopy a planar conformation of 106 was evident, however, a crystal structure determination revealed the virtually planar arrangement in the solid-state was observed only for the Al counterpart.123 Recently, Wang and co-workers synthesized the NHC-stabilized and silyl-substituted digallane {[(MeCNiPr)2C](SiMetBu2) Ga}2 (107) by KC8 reduction of [(MeCNiPr)2C](SiMetBu2)GaCl2. Digallene 107 exhibits a planar conformation with trans oriented silyl and NHC ligands.124 The Ga–Ga distance in the solid-state molecular structure of 107 is much shorter compared to those observed in Power’s weakly associated gallanediyl dimers such as 66,71 indicating a pronounced double bond character in 107. One- and two-electron oxidation reactions of 107 produced radical cation salt [107][B(3,5-(CF3)2C6H3)4] and dication salt [107] [B(3,5-(CF3)2C6H3)4]2, respectively. The oxidation reactions result in a successive Ga–Ga bond elongation, consistent with a reduced formal bond order due to removal of the electrons from the Ga–Ga p bond, which was calculated to be the HOMO of 107 by DFT calculations. The localization of the unpaired electron in the p orbital of the Ga–Ga bond, which is the SOMO of radical cation [107]+, was proven by EPR spectroscopy in conjunction with DFT calculations. Moreover, the reactivity of [107][B(3,5(CF3)2C6H3)4] toward nBu3SnH and S8 was investigated, yielding the cationic Ga hydride [{[(MeCNiPr)2C](SiMetBu2)Ga}2H] [B(3,5-(CF3)2C6H3)4] ([107H][B(3,5-(CF3)2C6H3)4]) and the Ga4S4 ladder-like dication [[(MeCNiPr)2C]4(SiMetBu2)2Ga4S4] [B(3,5-(CF3)2C6H3)4]2 ([108][B(3,5-(CF3)2C6H3)4]2), respectively124 (Scheme 14).

Scheme 14 One- and two-electron oxidation reactions of digallane 107.

Wagner and co-workers introduced trichlorosilylated tetrylide groups, [(Cl3Si)3E] (E ¼ Si, Ge), as ligands at gallium centers. Reactions of the anions [(Cl3Si)3E]− with GaCl3 yielded the adducts [(Cl3Si)3EGaCl3]− (E ¼ Si 109, Ge 110) together with the [NEt4]+ counterion. The reaction of [NEt4][110] with a second equivalent of GaCl3 yielded the neutral dimer {[(Cl3Si)3Ge] GaCl2}2 (111) by chloride abstraction.125

Gallium, Indium, and Thallium

383

Reactions of InCl3 and Cp In with bulky silanides [M(THF)3SiPh2R] (M ¼ Li, Na; R ¼ Me, tBu, Ph) were studied by Linti and co-workers, resulting in the formation of several indium species with different structures and oxidation states, which were formed by a range of reaction mechanisms including substitution, disproportionation, and redox reactions. Trivalent indium trisilyl In(SiPh2tBu)3 (112) was formed in the reaction of [Li(THF)3SiPh2tBu] with InCl3. In contrast, a number of products was isolated from the reaction of [M(THF)3SiPh3] (M ¼ Li, Na) and Cp In, including In(SiPh3)(THF) (113), [Na(THF)6][In3(SiPh3)6] (114), [In8(SiPh3)8] (115), and [Li(THF)n]2[In8(SiPh3)8] (116), respectively. The anionic part of 114 contains a linear In3 chain, which can be formally regarded as an basic In− center stabilized by two acidic In(SiPh3)3 units. The In− center formally represents the smallest building block of higher anionic metalloid In clusters. By close examination the structures of the two In8 clusters, dodecahedral (neutral) 115 and antiprismatic (dianionic) 116, it was found that the structural features strongly depend on the number of skeletal electrons.126 Moreover, reactions of in situ prepared solutions of GaOTf127 with the bulky silanide [Li(THF)3SitBuPh2] were reported to yield the octahedral closo-cluster ion [Ga6(SitBuPh2)6]2− (117), which was also obtained starting from “GaI” together with disphenoidal closo-cluster ion [Ga8I4(SitBuPh2)4]2− (118), digallane Ga2(SitBuPh2)4 (119), and gallate [IGa(SitBuPh2)3]− (120).128 In contrast, reaction of “GaI” with [Na(THF)2SitBu3] resulted in formation of [Ga13(SitBu3)6] (121) and [Ga10(SitBu)6]n − (122, n ¼ 0, 1) among other products,129 reflecting the unpredictability of these disproportionation reactions. Tetrel-based ligands have also been used to stabilize low-coordinate group 13 metal ions. The [B(C6F5)4]− salt of the twocoordinate Ga cation [(tBu2MeSi)Ga[Si(tBu)2(SiMetBu2)]+ (123) was obtained by methyl abstraction reaction from Ga(SiMetBu2)3 and subsequent 1,2-migration of one (SiMetBu2) group. Cationic gallium complex 123 reacted with acetonitrile with formation of the corresponding bis(acetonitrile) adduct [(tBu2MeSi)Ga(NCMe)2{Si(tBu)2(SiMetBu2)}][B(C6F5)4],130 whereas a similar — yet homoleptic — cationic Ga salt [(tBu3Si)2Ga][Al(OC(CF3)3)4] (124), which features a linear Si-Ga-Si moiety according to X-ray diffraction studies, was obtained via chloride abstraction from (tBu3Si)2GaCl with Ag[Al(OC(CF3)3)4].131 Heteroallenic anions [(tBu2MeSi)2EME(SiMetBu2)2]− (125, M ¼ Ga, In, E ¼ Si, Ge) were synthesized by reactions of (tBu2MeSi)2ELi2 with MCl3 and isolated as the respective [Li(THF)4]+ salts. In the solid-state, 125 features short M–E bonds, which are substantially shortened by 9% (M–Si) and 7% (M–Ge) in comparison to typical M–Si and M–Ge single bonds, respectively. The E–M–E moieties are slightly bent with E–M–E bond angles of about 160 , suggesting contributions from both an allenic and an allylic resonance structure, which was supported by DFT calculations showing rather negatively charged E centers and reactivity studies with MeI132,133 (Scheme 15).

Scheme 15 Allenic and allylic resonance structures of compounds of type 125.

9.07.5

Nitrogen-based ligands

9.07.5.1

Neutral donor ligands

Neutral N-donor ligands have been used to stabilize novel subhalide clusters of Ga and In. Schnöckel and co-workers obtained triethylamine-stabilized octameric aggregate Ga8Br8(NEt3)6 as well as the tetrameric compounds Ga4Br4L4 (L ¼ NEt3, py, NH3) from metastable monovalent gallium(I)bromide solutions.134 Dissolution of indium(I)iodide in TMEDA afforded the moderately stable cluster complex In6I8(TMEDA)4, which allowed insights into the structures of low-oxidation state In halides in donor-solvent solutions.135 In contrast to Al(I) and Ga(I) halides, disproportionation reactions of In(I) halides were investigated to a far lesser extent. Moreover, pyridine-based ligands have been coordinated to M(I) (M ¼ In, Tl) cations. Aldridge and co-workers employed a 2,6-diarylpyridine ligand, the neutral counterpart to Power’s anionic terphenyl ligands, to isolate the formally analogous cationic indane- and thallanediyls, [(2,6-Mes2C5H3N)M(solv)n][B(3,5-(CF3)2C6H3)4] (M ¼ In, solv ¼ C6H5F, n ¼ 1 (126); M ¼ Tl, solv ¼ C6H5F, n ¼ 2 (127); M ¼ Tl, solv ¼ tol, n ¼ 2 (128)), which were found to exhibit weak interactions with solvent molecules in the solid-state. The two-coordinate dipyridine In complex [(2,6-Mes2C5H5N)2In][B(3,5-(CF3)2C6H3)4] (129) was also obtained.136 Richeson and co-workers used bis(imino)pyridine ligands for the stabilization of low-coordinate In(I) and Tl(I) cations including [[{ArNCPh}2(NC5H3)]M][OSO2CF3] (M ¼ In, Tl; Ar ¼ 2,5-tBu2C6H3, Dip, 2,6-Et2C6H3) and [{[{ArNCPh}2(NC5H3)]Tl}2(m-solv)] [OSO2CF3] (solv ¼ C6H6, tol). Structural findings such as the long M–N bonds in these pyridine complexes in combination with results from DFT calculations indicated only weak orbital/covalent interactions due to a discrepancy of the donor and acceptor orbital energies, while noncovalent interactions were determined to be important, particularly in the Tl complexes.137–139 Krossing and co-workers employed aromatic N-bases, 2,6-tBu-4-MeC5H2N and pyrazine, as ligands for Ga+ cations, which feature the option for s and p coordination, and found that the s coordination via the nitrogen lone pairs is preferred.140 Moreover, they isolated polycationic cluster-type complexes of donor-stabilized M(I) cations (M ¼ Ga, In) of the type [MnLm]n+, which were

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formed upon addition of donor solvents such as 2,20 -bipyridine (bipy), 1,10-phenanthroline (phen), tBuNC, and DMAP, to solutions of [M(C6H5F)2][Al(OC(CF3)3)4]. In case of indium, different triangular clusters such as [In3(bipy)6]3+ (130), [In3(bipy)5]3+ (131) as well as rhombic clusters, i.e. [In4(bipy)6]4+ (132), [In4(phen)6]4+ (133), were isolated, depending on the reaction conditions. The strongly-coordinating bipy ligand was used for the stabilization of the monometallic paramagnetic dication [Ga(bipy)3]2+ (134) containing a Ga(III) center, which was formed via disproportionation reaction of the starting low-valent gallium species. Dication 134 was disclosed as a ligand-centered radical by EPR spectroscopy.141 Disproportionation reactions also occurred upon treatment of Ga(I) with the phen ligand, resulting in the formation of the dimeric Ga–Ga bonded Ga(II) tetracation [Ga2(phen)4]4+ (135). Reaction of 135 with isocyanide tBuNC yielded the tetracation [Ga4(tBuNC)8]4+ (136) containing a square-planar Ga4 ring,142 whereas the analogous reaction with DMAP gave the pentagonal-planar pentacation [Ga5(DMAP)10]5+ (137), respectively.143 According to quantum chemical calculations, the cluster formation proceeds stepwise via the triplet state fragments, while the driving force for the formation of these unusual aggregates was assumed to rely on the strong M–M bonds and the high lattice energy of the salts, which outperform the Coulombic repulsion141–143 (Scheme 16).

Scheme 16 Donor-stabilized M(I) polycations.

9.07.5.2

Hydrazide ligands

Group 13 element hydrazides are of interest due to their potential application in the synthesis of the respective metal nitrides, as well as the coordination behavior of the directly connected N2 moiety in hydrazide ligands.144 Uhl and co-workers reacted methylhydrazine H2NN(H)Me with group 13 trimethyls MMe3 (M ¼ Ga, In). At ambient temperature, GaMe3 forms a simple adduct with H2NN(H)Me yielding [Me3Ga{N(H)MeNH2}] (138), which upon thermal treatment in solution reacted with elimination of methane and formation of [(Me2Ga)4{N(H)N(H)Me}2{N(H)NMe}] (139). 139 exhibits a bicyclic structure with two five-membered Ga2M3 heterocycles fused via a dianionic [HNNMe]2− moiety. In contrast, the analogous reaction of H2NN(H) Me with InMe3 at ambient temperature directly proceeded with formation of the isostructural indium congener of 139, [(Me2In)4{N(H)N(H)Me}2{N(H)NMe}] (140), whereas an adduct as was formed with GaMe3 could not be obtained.145 Further thermal treatment of neat 139 resulted in gas evolution and formation of [(MeGa)4(Me2Ga)4{N(H)NMe}4(N2)] (141), which features a polycyclic Ga8N10 core consisting of ten five-membered Ga2N3 and one four-membered Ga2N2 heterocycles, in which the Ga centers are bridged by [HNNMe]2− and a tetraanionic hydrazinetetraide [N2]4− ligands.146 In contrast, thermolysis of the adduct [tBu3Ga{N(H)MeNH2}] (142) initially yielded [(tBu2Ga)2(m-NH2)(m-N(Me)NCH2)] (143) at 170  C and finally the gallium cyanide complex [tBu2GaCN]4 (144) at 400  C, whereas thermolysis of the cyclic gallium hydrazide [(tBu2Ga)2{N(H)N(H) tBu}2] (145) yielded the polycyclic gallium imide [tBuGaNH]8 (146) via the trimeric intermediate [tBu2GaNH2]3 (147). Both compounds are formed via cleavage of the N–N bonds of the hydrazine moieties Depending on the nature of the substituents in GaR3 (R ¼ iPr, Me) and the hydrazine H2NN(H)R0 (R0 ¼ Ph, tBu), various Ga4-hydrazido cage compounds were obtained, in which the N–N bonds of the hydrazine moiety are retained, showing the structural versatility of hydrazido ligands.147 Moreover, hydrogallation reactions of R2GaH (R ¼ Me, tBu) with the hydrazone (H10C5N)N]C(C9H14) (C(C9H14) ¼ 2-adamantdiyl) resulted in formation of [R2GaN(NC5H10)(Ad)] (R ¼ Me 148, tBu 149; Ad ¼ 2-adamantyl), which adopt either dimeric structures with central Ga2N2 heterocycles (148) or form monomers with strained GaN2 heterocycles (149) in the solid-state. Both Ga hydrazides react as active Lewis pairs in reactions with terminal acetylenes, yielding the hydrazone-coordinated gallium trialkynyl (PhCC)3Ga[N]C(C9H14)(NC5H10)] (150) in the reaction of compound 148 with PhCCH, whereas alkynyl-bridged compounds

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[tBu2Ga(m-CCR)]2 (R ¼ Ph 151, tBu 152) are formed in the reaction of 149 and PhCCH (151) and tBuCCH (152), respectively. Remarkably, the reaction of 148 with C6F5H proceeds via C–H activation and liberation of H2 and CH4, yielding (C6F5)2(Me)Ga [N]C(C9H14)(NC5H10)] (153) which is coordinated by the hydrazone.148

9.07.5.3

Amide ligands

Simple gallium amides such as dimeric [PhNHGaMe2]2 have been shown to react with benzonitrile, PhCN, with formation of tetracyclic triazagallane {[PhNC(Ph)N]3[PhNC(Ph)NH]Ga[GaMe][GaMe2]2} (154) as well as the bowl-shaped Ga6N9C3 cluster {[PhNC(Ph)N][PhN][GaMe]2}3 (155). In contrast, the reaction of sterically more hindered [DipNHGaMe2]2 and PhCN yielded the tetrameric amidinate {[DipNC(Ph)N]GaMe}4 (156). Similar products were also obtained from reactions of the corresponding benzamidines with GaMe3, while slight changes of the amidine also yielded bicyclic triazagallane {[DipNC(Me)NH]2[DipNC(Me) N][GaMe]2} (157). The isolation of reaction intermediates gave insights into the reaction mechanism leading to compound 156.149 Sundermeyer and co-workers reported a novel perfluorinated gallium trisamide, Ga[N(C6F5)2]3 (158), which was found to act as a strong Lewis acid. 158 was prepared by a simple salt metathesis approach, and the Lewis acidic nature of the Ga center is already shown by six secondary ortho-fluorine contacts of the C6F5 moieties to the trigonal-planar Ga center. The almost Lewis superacidic character was shown by the calculated fluoride ion affinity (FIA) of 472 kJ mol−1 in the gas-phase and the chloride abstraction reaction with [AsPh4][ClB(C6F5)3], producing the weakly coordinating anion {ClGa[N(C6F5)2]3]−. In contrast, the Lewis acidity of the lighter Al congener Al[N(C6F5)2]3 was shown to exceed that of the benchmark Lewis acid SbF5.150 Bulky amide ligands have also been used to stabilize low-valent and low-oxidation state group 13 complexes. Treatment of a metastable GaCl solution with an excess of LiN(SiMe3)2 afforded the neutral metalloid cluster [Ga23{N(SiMe3)2}11] (159) by simultaneous metathesis and disproportionation, which contains a Ga12 core of naked Ga atoms.151 Using “GaI” as the starting reagent, low yields of the unusual cluster [Li(THF)][Ga6IO{N(SiMe3)2}6] (160) were obtained, in which oxygen and iodine atoms were incorporated. In contrast, reactions of “GaI” with LiN(Dip)SiMe3 and LiN[(Me2CCH2)2CH2] gave the tetrameric Ga(I) compounds [GaN(Dip)SiMe3]4 (161) and [GaN[(Me2CCH2)2CH2]]4 (162), which showed distorted tetrahedral arrangements of the four Ga atoms. The DFT calculated tetramerization energies for the monomeric gallium amides GaN(Dip)SiMe3 (157 kJ mol−1) and GaN[(Me2CCH2)2CH2] (173 kJ mol−1) are lower than those of corresponding silyl- and alkyl-substituted gallanediyls (200–400 kJ mol−1), thus indicating the higher stability of the Ga(I) amide monomers.152 Monomeric and essentially one-coordinate amido-substituted gallanediyls were finally obtained by introduction of extremely bulky amido ligands, leading to the isolation of [GaN(2,6-Mes2C6H3)SiMe3] (163)153 and [GaN{2,6-(Ph2HC)2-4-MeC6H2}SiR3] (R ¼ Me 164, Ph 165).154 The corresponding monomeric In(I) and Tl(I) amides, [InN{2,6-(Ph2HC)2-4-MeC6H2}SiMe3] (166),154 [TlN(2,6-Mes2C6H3)(Me)] (167),155 and [TlN{2,6-(Ph2HC)2-4-MeC6H2}SiR3] (R ¼ Me 168, Ph 169),154 were also obtained. Compounds 163–169 exhibit weak interactions with the p systems of the flanking aryl groups in the solid-state, and DFT calculations on model systems suggested a partial Ga–N double bond character due to p donation from the nitrogen electron lone pair to a vacant p orbital at gallium.153 In contrast, quantum chemical calculations of compound 164 did not reveal any substantial N–Ga p bonding interaction.154 Amide 163 was shown to react with N3(2,6-Mes2C6H3) with elimination of N2 and formation of monomeric gallium imide [(2,6-Mes2C6H3)(SiMe3)N]GaN(2,6-Mes2C6H3) (170), which features a very short Ga–N(imide) bond, thus indicating Ga–N double bond character. In contrast to linear iminoboranes, 170 adopts a trans-bent conformation, and DFT calculations emphasize strongly polarized Ga–N bonds.153 Moreover, a combined experimental and theoretical electron density analysis of 164 revealed a diffuse Ga-centered lone pair opposite to the amide substituent and a Ga–N bond that is mainly polar-covalent in nature156 (Scheme 17).

Scheme 17 Amido-substituted low-valent group 13 metal complexes.

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Gallium, Indium, and Thallium

Aldridge and co-workers recently reported the Ga(I) compound (PON)Ga (171) containing a mixed amide and phosphino ligand anchored to a xanthene backbone, thus possessing neutral hemilabile P and O donor groups in addition to the anionic amide donor. 171 was synthesized by reaction of “GaI” with (PON)K, and its solid-state structure revealed weak Ga–O and Ga–P bonding interactions in addition to a strong Ga–N bond. Oxidation reactions of 171 with N2O lead to an intramolecular benzylic C–H activation mediated by the Ga–O double bond of a putative gallanone intermediate, which could also not be trapped in the presence of a Lewis acid.157 Moreover, 1,8-substituted carbazolyl substituents have been used in low-valent group 13 chemistry. Reaction of the bulky carbazolyl bearing tBu groups in the 1- and 8-positions (tBu4carb) with MCl (M ¼ In, Tl) yielded the corresponding In(I) and Tl(I) complexes, [M(tBu4carb)]n (M ¼ In 172, Tl 173), in which the ligands coordinate in a Z3(p) fashion to the group 13 centers via the central pyrrolyl ring, while further contacts of the arene p systems lead to a coordination polymer in the solid-state. In contrast, the less bulky 1,8-H-substituted carbazolyl ligand (tBu2carb) leads to a disproportionation of In(I) and subsequent formation of the mixed-valent and In–In bonded compound [In2{In2(tBu2carb)6}] (174), which features N-s coordination of the carbazolyl ligand to indium enabled by the lower steric demand of (tBu2carb) in comparison to (tBu4carb).158 Monomeric Tl(I) complexes [Tl{N[C(C3F7)NAr]2}] (Ar ¼ Mes 175, Dip 176) containing Tl–N bonded triazapentadienyl ligands with additional weak secondary Tl–arene contacts were synthesized by Cundari and co-workers.159

9.07.5.4

Amidinate and guanidinate ligands

Amidinates, R0 C(NR)2, and guanidinates, R0 2NC(NR)2, are monoanionic N,N0 chelating ligand systems possessing C(CN2) and N(CN2) ligand backbones, respectively. Due to the lone pair and its associated p donor function of the backbone nitrogen atom, guanidinates are usually anticipated to be more electron donating than amidinates. In addition, the steric demands of both systems can be conveniently tuned by choice of the R and R0 substituents. Homoleptic indium tris-amidinates and tris-guanidinates [HC(NiPr)2]3In (177),160 [MeC(NiPr)2]3In (178),161 [Me2NC (NiPr)2]3In (179),162 and [Et2NC(NiPr)2]3In (180)163 were synthesized by straight-forward salt metathesis reactions of the corresponding amidinate and guanidiante lithium salt with InCl3. Moreover, the heteroleptic amidinate complexes [MeC(NiPr)2]2InCl (181) and [MeC(NiPr)2]2InMe (182) were synthesized analogously.161 Compounds 177–180 and 182 were shown to possess appropriate properties such as volatility and thermal stability, which render them suitable candidates for the use as precursors in metal-organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD) processes for the fabrication of transparent and conductive In2O3 thin films, which can be suitable for optoelectronic applications.160–163 Moreover, fluorinated formamidinate complexes [HC(NC6F5)2]3M (M ¼ Ga 183, In 184) were synthesized and tested as co-catalysts with B(C6F5)3 in the hydrosilylation of CO2 with Et3SiH.164 Heteroleptic bis-amidinate complexes, [MeC(NiPr)2]2GaCl (185)165 and [PhC(NtBu)2]2InX (X ¼ F 186, Br 187, I 188),166 which are similar to 181, as well as the bis-guanidinate [iPr2NC(NiPr)2]2GaCl (189),167 were also synthesized. In addition, mono-formamidinate compounds [HC(NDip)2]GaMe2 (190)168 and [HC{N(2,6-Et2C6H3)}2]InCl2[[HC{N(2,6-Et2C6H3)}2H] (191)169 were prepared by alkane elimination and salt metathesis, respectively. The mono-guanidinate complex [iPr2NC(NiPr)2] GaCl2 (192) was shown to react with carbodiimide C(NiPr)2 with insertion into the Ga–N bond and formation of the usual bis(diguanidinate)GaCl2 complex [iPrN{C(NiPr)NiPr}{C(NiPr)NiPr2}]GaCl2 (193).167 Moreover, the stabilization of group 13 metal centers in low oxidation states using amidinate and guanidinate ligands was of particular interest. However, the reaction outcome was shown to strongly depend on the nature of the ligand and the steric demand of the substituents. Since “GaI” was established as convenient source of Ga+ for the synthesis of low-valent Ga complexes, reactions of various lithium amidinates and lithium guanidinates with “GaI” were investigated. The reactions frequently proceeded under disproportionation of Ga(I), yielding digallanes {[HC(NDip)2]GaI}2 (194), {[tBuC(NDip)2]GaI}2 (195),170 {[MeC(NDip)2] GaI}2 (196),171 and {[iPr2NC(NiPr)2]GaI}2 (197),172 respectively, in which the Ga centers adopt the oxidation state +II. In addition, the formation of Ga(III) compounds such as [HC(NDip)2]2GaI (198) was reported.170 An analogous compound, {[tBuC(NiPr)2]GaI}2 (199), was obtained by reduction of trivalent [tBuC(NiPr)2]GaI2 with potassium metal.173 Interestingly, neutral tri- and tetragallanes [tBuC(NiPr)2]3Ga3I2 (200),173 [tBuC(NCy)2]3Ga3I2 (201),171 and [MeC(NiPr)2]4Ga4I2 (202)173 were isolated by reactions of the corresponding lithium amidinate with “GaI.” Compounds 200–202 consist of Gan chains (n ¼ 3, 4) of [LGa] (L ¼ amidinate) units capped by iodine atoms, hence resulting in different oxidation states of the internal (+I) and terminal Ga atoms (+II), respectively. The terminal iodine atoms allow for further functionalization of the oligogallanes, as was shown in metathesis reactions of 201 with LiNCMe2 and NaBEt3H. Moreover, oxidation of 201 with I2 quantitatively gave the Ga(III) compound [tBuC(NCy)2]GaI2 (203).171 The only monometallic and monomeric Ga(I) compound stabilized by a guanidinate (or amidinate) ligand, [Cy2NC(NDip)2]Ga (204), was obtained by Jones and co-workers by salt elimination reaction of “GaI” and the corresponding lithium guanidinate. The increased stability of 204 toward disproportionation reaction most likely results from the larger steric demand of the [Cy2NC(NDip)2] ligand due to the bulky cyclohexyl groups in the ligand backbone.174 The bonding in 204 was investigated by a combined experimental and theoretical charge density analysis, revealing significant directional lone pair density at the Ga center.175 Moreover, 204 was shown to react with I2 and ISiMe3 with formation of [Cy2NC(NDip)2]GaI2 and [Cy2NC(NDip)2]Ga(I)SiMe3,176 respectively, while reversible temperature-dependent E–E bond insertion reactions were observed with E2R4 (E ¼ Sb, Bi; R ¼ Et, Ph) yielding [Cy2NC(NDip)2]Ga(ER2)2.177 Similar disproportionation reactions were observed by reacting In(I) chloride with lithium amidinates and guanidinates, resulting in the formation of [HC(NDip)2]2InCl (205),170 {[iPr2NC(NDip)2]InCl}2 (206),176 and {[H2C(CH2CHMe)2NC(NDip)2]InCl}2 (207),176 in which the In atoms adopt the oxidation states +II and + III, respectively. Monomeric guanidinate and (phospha)guanidinate In(I) complexes [H2C(CH2CHMe)2NC(NDip)2]In (208),176 [Cy2PC(NDip)2]In (209),176 and [Cy2NC(NDip)2]In (210)174 were also isolated and structurally

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characterized. As was observed for the isolation of compound 204, the size of the organic substituents on the guanidinate ligand plays a major role in the stabilization of these monomeric compounds. In accordance with the higher stability of Tl in the +I oxidation state, no disproportionation reactions were observed using Tl(I)Br, and the monomeric Tl(I) complexes [H2C(CH2CHMe)2NC(NDip)2]Tl (211),176 [iPr2NC(NDip)2]Tl (212),176 [Cy2PC(NDip)2]Tl (213),176 and [Cy2NC(NDip)2]Tl (214)174 were formed in reactions with the respective guanidinates and phosphaguanidinates, respectively. However, in contrast to the related Ga(I) and In(I) complexes, Tl(I) compounds 211–214 exhibit N,arene chelation in the solid-state instead of the typically observed N,N0 chelating mode of the guanidinate ligands, which is proposed to be due to the larger size and softer character of the Tl+ ion, which generally prefers higher coordination numbers than the lighter congeners, Ga and In (Scheme 18).

Scheme 18 Group 13 metal amidinate and guanidinate complexes.

9.07.5.5

a-Diimine ligands

1,4-Diaza-1,3-butadiene-based ligands, typically referred to as a-diimine ligands, represent a versatile ligand class due to the facile variation of the nature of the ligand backbone and the steric bulk of the N-substituents, allowing for precise modulation of steric and electronic properties. Moreover, due to their redox non-innocent character, a-diimine ligands can exist in three redox states: a neutral a-diimine form, a radical monoanionic form, and a dianionic diamido form, rendering them interesting ligands for systems, which involve electron transfer processes. These redox non-innocent properties have been exploited to synthesize the Ga(III) complex Ga[(3,5-Me2C6H3)2-BIAN]3 (215; BIAN ¼ bis(arylimino)acenaphthene), which carries three singly-reduced, radical anionic a-diimine ligands. 215 can undergo threefold oxidation to the corresponding trication [Ga{(3,5-Me2C6H3)2-BIAN}3][PF6]3 (2153 +), featuring the three ligands in the neutral diimine state.178 Reactions of a-diimines RNC(H)C(H)NR (R ¼ Mes, Dip, tBu, Cy) with MexGaCl3− x (x ¼ 0–3) resulted in the formation of various products, including simple adducts, cation/anion pairs, and Ga–C insertion products.179 After the seminal discoveries of low-valent, NHC-valence isoelectronic Ga(I) anions, [(HCNR)2Ga]− (R ¼ tBu 216, Dip 217), by Schmidbaur et al.180 and Jones et al.,181 the low-oxidation state chemistry of gallium using 1,4-diaza-1,3-butadiene-based ligands received significant interest. Derivatives of 216 and 217, [(HCN{2,6-(CHPh2)2-4-MeC6H2})2Ga][NaLn] (Ln ¼ (TMEDA), (TMEDA) (tol), (TMEDA)(OEt2), (THF)4) (218),182 and [(MeCNDip)2Ga][MLn] (MLn ¼ Li(THF)3, Na(THF)3, K(THF)5, K(THF), K(TMEDA)) (219)183,184 were synthesized by reduction of the corresponding Ga(III) complexes (R0 CNR)2GaX2 (X ¼ Cl, I), in which the ligand formally adopts the radical monoanionic state, with alkali metals. Moreover, anionic gallium complexes stabilized by a Dip-substituted BIAN ligand [(Dip-BIAN)Ga][MLn] (MLn ¼ Li(OEt2)3, Na(OEt2)3, Na(DME)2, Na(THF)3(OEt2), K(THF)5, K(18crown-6)(THF)2, K(Ph3PO)3(THF)) (220),185,186 in which the Dip-BIAN ligands serve as dianionic ligands, were synthesized by reduction of the digallane [(Dip-BIAN)Ga]2. The reactivity of the anions 219 and 220 toward unsaturated organic molecules such as isocyanates and carbodiimides was investigated. Reactions of the Na salt of 219 with isocyanates RNCO (R ¼ Et, p-tol, Cy) yielded different products depending on the nature of the group R. The reaction with EtNCO occurred via two-electron reduction of the NCO unit with formation of a 2,5-diaza-1,4-digallacyclohexane containing a Ga2C2N2 heterocycle, [Na2(THF)5] [{(MeCNDip)2Ga}2{EtNC(O)}2] (221), whereas the analogous reaction with p-tolNCO proceeded via reductive coupling of three NCO units under loss of one molecule of CO and formation of [Na(THF)2]2[(MeCNDip)2Ga{p-tolNC(O)}2N(p-tol)]2 (222),

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respectively. The reaction with CyNCO proceeded stepwise, initially forming the m-oxo digallium complex [Na(THF)3]2 [{(MeCNDip)2Ga}{m-O}]2 (223), which was subsequently transferred into complex [Na2(THF)5][{(MeCNDip)2Ga}{CyNCO2}]2 (224) containing a dianionic azacarbonate ligand.187 In addition, Na salts of 219 and 220 were reported to react with carbodiimides such as C(NCy)2 and C(NiPr)2with reductive coupling to yield guanidinate derivatives [Na(solv)][(Dip-BIAN)Ga{(CyN)2CNCy}] (225) and [Na(solv)][LGa{(RN)2CN(R)C(NR)}] (226; L ¼ Dip-BIAN, (MeCNDip)2; R ¼ Cy, iPr; solv ¼ THF, DME). In contrast, the analogous reaction with the sterically more demanding carbodiimide C(NDip)2 only occurred via addition of the nucleophilic Ga center to the electrophilic C center of the carbodiimide and formation of [Na(THF)2][(MeCNDip)2GaC(NDip)2] (227).188 The redox chemistry observed in these reactions with 219 and 220 thus solely occurs at the Ga atom, despite the potential contribution of the redox-active ligand backbones (Scheme 19).

Scheme 19 Reactivity of 219 toward isocyanates and carbodiimides.

Due to this non-innocent nature and depending on the redox state of the a-diimine ligand, the Ga centers can adopt oxidation states ranging from +I to +III. Hence, either the metal center or the ligand can be reduced in reactions of (MeCNR)2GaCl2 (228) with reducing agents, and a number of (paramagnetic) complexes including [Na(THF)4][(MeCNDip)2GaCl2] (229), [(MeCNAr)2 GaCl]2 (230, Ar ¼ 2,6-Et2C6H3, 2,6-Me2C6H3), [Na(OEt)2]2[(MeCNDip)2GaCl]2 (231), and [Na(THF)6][(MeCNDip)2Ga]2 (232), in which both the Ga atom and the ligand adopt various oxidation states, have been prepared.189,190 In addition to Ga(I) anions, related digallanes featuring Ga(II) centers were also intensively investigated. Reaction of the doubly-deprotonated ligand K2[2,6-(DipN)2C6H4] with GaCl3 in the presence of KC8 afforded the bis(amido) Ga(II) dimer [Ga {2,6-(DipN)2C6H4}]2 (233).191 Fedushkin and co-workers comprehensively studied the reactivity of digallane [(Dip-BIAN) Ga]2 (234), which was synthesized from the triply-reduced bis(arylimino)acenaphthenetriide K3[Dip-BIAN] and GaCl3192 or, more conveniently, directly from the bis(arylimino)acenaphthene Dip-BIAN and gallium metal.186 Digallane 234 was found to react with alkaline earth metals with insertion of the group 2 metal into the Ga–Ga bond, yielding [{(Dip-BIAN)Ga}2M(THF)n] (235, M ¼ Mg, n ¼ 3; M ¼ Ca, Sr, n ¼ 4; M ¼ Ba, n ¼ 5), which exhibit a slightly bent arrangement of the Ga–M–Ga moiety.186 Moreover, the chemistry of 234 is dominated by Ga-to-ligand and ligand-to-Ga electron transfer processes due to the redox-active nature of the BIAN ligand system. Thus, single-electron oxidation (per Ga center) occurs preferably at the ligand backbone with preservation of the Ga–Ga bond, as was reported for reactions with I2, yielding [(Dip-BIAN)GaI]2 (236), as well as SO2 and RCHO (R ¼ Ph, CHCHPh),193–195 respectively. In contrast, the reaction of 236 with pyridine induced Ga–Ga bond cleavage concomitant with Ga-to-ligand electron transfer, resulting in the oxidation of the gallium center and formation of [(Dip-BIAN)GaI(py)] (237).193 As observed in the reaction with SO2, only a second consecutive oxidation reaction of 234 leads to Ga–Ga bond cleavage.194 Due to the redox participation of the ligand, 234 also acts as a disguised Ga(I) complex, allowing for concerted two-electron oxidation processes (per Ga center), which were observed in reactions with PhNNPh and o-benzoquinone and which result in Ga–Ga bond cleavage.193–195 The potential capability of digallane 234 for two-electron processes is emphasized by the oxidative addition of allylchloride to each Ga center in 234, yielding [(Dip-BIAN)Ga(Cl)C3H5] (238),196 and the formation of a Lewis acid-base adduct with B(C6F5)3, affording [(Dip-BIAN)GaB(C6F5)3] (239).197 Most remarkably, 234 undergoes reversible addition of two equivalents of terminal and internal alkynes such as HCCH, PhCCH, H3CCCCO2Me, and H3CCCCO2Et, consistent with a ligand-assisted

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concerted 1,3-dipolar cycloaddition. The reversible fixation of alkynes, moreover, allowed for the use of 234 as a catalyst in hydroamination and hydroarylation reactions of PhCCH with anilines.198,199 Similar reversible 1,3-dipolar cycloadditions were observed in reactions of 234 with isocyanates187 and isothiocyanates.200 In addition, mononuclear Ga-BIAN complex [(Dip-BIAN)Ga(S2CNMe2)] (240),201 which was synthesized by reaction of 234 with Me2NC(S)SS(S)NMe2, exhibit an analogous yet irreversible reactivity with terminal and internal alkynes as was observed for 234; however, their catalytic activity in hydroamination and hydroarylation reactions of PhCCH with anilines is low.202 The broad chemical reactivity of 234 can thus be attributed to its ambiphilic nature due to the presence of the Lewis acidic Ga centers and the nucleophilic BIAN ligand backbone, in conjunction with the ready redox participation of the BIAN ligand in electron transfer processes193–202 (Scheme 20).

Scheme 20 Reactivity of digallane 234.

In contrast to the large number of Ga compounds containing 1,4-diaza-1,3-butadiene ligands, the chemistry of analogous complexes comprising the heavier congeners In and Tl is far less developed. After the first reports on the indium complexes [(HCNDip)2InCl]2203 and (HCNDip)2InCl2204 by Jones and co-workers, several coordination complexes of indium trihalides, (HCNR)2InCl3 (241, R ¼ (S)-CH(Me)Ph, (R)-CH(Et)CH2Cl, (S,S)-CH(Me)CH(Ph)Cl, tBu)205 and (MeCNPh)2InCl3(MeCN) (242),206 as well as (Ar-BIAN)InCl3 (243, Ar ¼ Mes,207 4-BrC6H4, 4-OMeC6H4,208 Dip, 2,6-Me2C6H3, 4-(NO2)C6H4, 4-(NMe2) C6H4)209 were synthesized, in which the a-diimine ligand simply coordinates to the In(III) center in a N,N0 chelating fashion. Compounds 243 were prepared by mechanochemical synthesis,208 and their electronic properties were investigated by UV/Vis spectroscopy, cyclic voltammetry, and DFT calculations.209 Moreover, using an asymmetric-substituted 1,4-diaza-1,3-butadiene ligand, indium dimethyl complex [DipNC(Me)C(Me)N(3,5-tBu2-2-OC6H2]InMe2 was obtained.210 Macdonald and co-workers investigated reactions of InOTf with a-diimines and isolated several complexes including (HCNR)2InOTf (R ¼ Mes 244, Dip 245), (MeCNR)2InOTf (R ¼ Mes 246, Dip 247), and (Ar-BIAN)InOTf (Ar ¼ Mes 248, Dip 249), respectively. Use of the electron-rich ligand (MeCNMes)2 allowed for the formation of the expected monomeric In(I) adduct 246, which was characterized in the solid-state, whereas in the case of (HCNMes)2, electron transfer from the In center to the ligand occurred, resulting in the formation of a polymeric chain linked by In(II)–In(II) and C–C bonds in the solid-state structure of 244. The metal-to-ligand electron transfer was again found to depend on the electronic properties of the a-diimine ligand. Electron-poor and p-conjugated backbones were found to facilitate this process, as was demonstrated for complexes 248 and 249, in which complete In-to-ligand electron transfer and an entirely ligand-based radical were concluded from EPR spectroscopic studies.211 The only reported Tl-a-diimine complex to date is the cationic bis-coordinated [(Mes-BIAN)2Tl][PF6] (250), in which the Tl atom adopts the oxidation state +I.207

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Gallium, Indium, and Thallium

b-Diketiminate ligands

N,N0 chelating b-diketiminate ligands incorporate the metal center into a six-membered C3N2M heterocycle. Due to the flexible modification of the nitrogen and backbone substituents, the steric and electronic properties of b-diketiminate ligands can be adjusted over a very broad range, rendering them versatile and important ligands in main group metal chemistry. Since the initial reports on the synthesis and solid-state structures of the monomeric two-coordinate M(I) carbenoids [HC(CMeNDip)2]M (M ¼ Ga 251,212 In 252,213 Tl 253214) by Power et al. and Hill et al. almost 20 years ago, the synthesis of further b-diketiminate M(I) complexes and the reactivity of these compounds has been intensively studied. In particular the promising potential of [HC(CMeNDip)2]Ga 251 to serve a ligand in transition metal chemistry, which is not subject to this review, and in bond activation reactions has been demonstrated. Kretschmer and co-workers reported a novel approach for the synthesis of widely used compound 251 starting from the sodium salt of the b-diketiminate ligand, Na[(NDipCMe)2CH], and Cp Ga as the Ga(I) source giving compound 251 in higher yields compared to the original synthesis, which uses Li[(NDipCMe)2CH] and “GaI.”215 Replacement of the backbone Me groups in Li [(NDipCMe)2CH] by sterically more demanding tBu groups, Li[(NDipCtBu)2CH], and subsequent reaction with “GaI” afforded Ga [(NDipCtBu)2CH] (254)216; reduction of the steric demands of the flanking aryl groups from Dip to Mes gave the desired Ga(I) compound Ga[(NMesCtBu)2CH] (255) together with b-diketiminate gallium subiodide cluster [Ga5I4{(NMesCtBu)2CH}] (256) as a by-product.182 However, the reaction of the considerably less bulky lithium b-diketiminate Li[(N(2,6-Me2C6H3) CMe)2CH] with “GaI” resulted in disproportionation and subsequent formation of the Ga–Ga bonded Ga(II) compound [IGa {(N(2,6-Me2C6H3)CMe)2CH}]2 (257),217 indicating the great influence of the steric profile of the b-diketiminate ligand on the stability of the gallium +I oxidation state. Due to the higher stability of indium in the +I oxidation state, the nature of the b-diketiminate ligand has less influence on the oxidation state but on the solid-state structures of the resulting complexes. In contrast to the purely monomeric In(I) complexes 252 and In[(DipNC(Me)C(H)C(Me)N(2-OMeC6H4)] (258),218 the compounds In[(NArCMe)2CH] (Ar ¼ Mes 259,218 2,6-Me2C6H3 260219) containing less bulky b-diketiminates form weakly-bound dimers in the solid-state. However, this weak aggregation is reversible upon dissolution, as only monomers were detected in the solution-state. The use of an even smaller b-diketiminate ligand resulted in partial disproportionation of indium, with formation of the mixed-valence compound [In6I2{(N(3,5-Me2C6H3)CMe)2CH}6] (261), which forms an In6 chain capped by iodine atoms in the solid state,220 resembling the Ga compounds 200–202. The corresponding Tl(I) derivative [Tl{(N(3,5-Me2C6H3) CMe)2CH}]3 (262) was shown to exhibit a trimeric, chain-like arrangement in the solid-state,221 indicating the dispersive nature of the M–M (M ¼ In, Tl) bonding interactions. Kretschmer and co-workers also synthesized dinuclear Ga(I), In(I), and Tl(I) bis(carbenoid) complexes {M[DipNC(Me)C(H)C(Me)N]-(R)-[NC(Me)C(H)C(Me)NDip]M}, in which two b-diketiminate M(I) fragments are linked by an organic bridging group. Due to the greater accessibility of Tl(I) compounds, a series of bis(thallanediyls) (R){[NC(Me)C(H)C(Me)NDip]Tl}2 (263, (R) ¼ C2H4, 1,2-trans-C6H10, o-C6H4, m-C6H4, p-C6H4, 2,6-NC5H3, C6H4OC6H4, 1,5-(CH2)2C6H4) with various linkers was obtained by one-pot salt metathesis procedures starting from the protio-ligand, KN(SiMe3)2 and TlI. Compounds 263 partially exhibit intramolecular Tl–Tl interactions in the solid-state.222,223 The bis(indanediyl) (C2H4){[NC(Me)C(H)C(Me)NDip]In}2 (264) was prepared via a similar synthetic route using InI, and shows a dimeric structure in the solid-state featuring a diamond-shaped four-membered In4 array, due to intra- and intermolecular In–In interactions, which were found to be dominated by dispersion forces.222 In contrast, the use of InCl as the In(I) source resulted solely in disproportionation, and the In(III) compound [DipNC(Me)CH2C(Me)N]-(C2H4)-[NC(Me)C(H)C(Me)NDip]InCl2 (265) was isolated.223 Compound 264 reacts with O2 yielding (C2H4){[NC(Me)C(H)C(Me)NDip]In}2O2 (266) featuring an almost planar six-membered In4O2 ring with the In centers in the +II oxidation state, whereas the reaction with S8 gave (C2H4){[NC(Me)C(H)C(Me)NDip]In}2S4 (267), which contains an eight-membered In4S4 heterocycle in a deck-chair conformation and In centers in the +III oxidation state.224 The bis(gallanediyl) (1,2-trans-C6H10){[NC(Me)C(H)C(Me)NDip]Ga}2 (268) was synthesized by a metathesis reaction of the potassium salt of the bis(b-diketiminate) ligand with Cp Ga. In the solid-state, no interaction between the two Ga(I) centers is observed, however, quantum chemical calculations indicated the presence of a conformer featuring a dative Ga–Ga interaction that is only slightly higher in energy. This conformer was found to be responsible for the facile C–F bond activation of fluoroarenes, C6F6, C6F5H, C6F4H2, following a concerted mechanistic pathway with formation of a F–Ga–Ga–ArF (ArF ¼ C6F5, C6F4H, C6F3H2) moiety in the products. In contrast, C–F bond activation reactions with the mono(gallanediyl) 251 was shown to require much longer reaction times, thus the enhanced reactivity of 268 was attributed to metal–metal cooperativity of the two proximate Ga(I) centers225 (Scheme 21).

Gallium, Indium, and Thallium

391

Scheme 21 Low-oxidation state group 13 metal b-diketiminate compounds.

Major developments have been achieved through reactivity studies of diyls 251 and 252. Most of these results have been recently summarized in a comprehensive review article.226 However, the most significant findings as well as recently reported studies since its publication will be discussed. The presence of an electron lone pair at the metal center in 251 and 252 is associated with a Lewis-basic reactivity, yielding Lewis acid-base adducts with Lewis-acidic centers. Moreover, due to the presence of a vacant p orbital, the diyls readily undergo bond insertion/oxidative addition reaction with E–X bonds,227 while they can also act as selective two-electron reducing agents. These properties and the targeted combination of these reactivity modes render the compounds 251 and 252 versatile starting reagents for a large variety of reactions, including bond activation reactions. Linti and co-workers studied oxidative addition reactions of E–H bonds to the Ga(I) center in 251. A range of E–H bonds was activated by 251 including those in HSnPh3, HOEt, HNEt2, HPPh2, H2, and H2O, which yielded Ga hydrides of the composition [HC(CMeNDip)2]Ga(H)X (269, X ¼ SnPh3, OEt, NEt2, PPh2, H, OH).228 This E–H bond insertion approach was extended to the synthesis of similar Ga hydrides [HC(CMeNDip)2]Ga(H)OR (R ¼ CH2CCH, SiPh2OH, P(O)(OnBu)2, SO2p-tol) by Jancik and co-workers,229 while Nikonov and co-workers studied reactions of 251 with multiple-bonded organic functional groups. The reaction with methacrolein occurred via [4 + 2] cycloaddition and subsequent formation of [HC(CMeNDip)2]Ga(OCHC(Me)CH2) (270) containing a GaOC3 heterocycle, whereas the reaction with isothiocyanate PhNCS resulted in C–S bond cleavage and C–S bond formation, yielding a mixture of [HC(CMeNDip)2]Ga(S2CNPh) (271) and {[HC(CMeNDip)2]Ga}2(m-S)(m-CNPh) (272), respectively. Isocyanates RNCO (R ¼ Ph, 3,5-Me2C6H3) and carbodiimide C(Np-tol)2 were reductively coupled, affording [HC(CMeNDip)2]Ga[C(O)NRC(NR)O] (273, R ¼ Ph, 3,5-Me2C6H3) and [HC(CMeNDip)2]Ga[C(Np-tol)N(p-tol)C(Np-tol)N (p-tol)] (274), respectively, which contain five-membered gallacycles.230 The in situ oxidation of 251 with N2O in the presence of aliphatic and aromatic substrates such as pyridine, cyclohexanone, OSMe2, and OPEt3, resulted in C–H bond activation across the Ga–O double bond of an elusive monomeric Ga oxide [{HC(CMeNDip)2}Ga]O] yielding [HC(CMeNDip)2]Ga(OH)(2-NC6H4) (275), [HC(CMeNDip)2]Ga(OH)(OC6H9) (276), [HC(CMeNDip)2]Ga(OH)(CH2S(O)Me) (277), and [HC(CMeNDip)2]Ga(OH) (CH(Me)P(O)Et2) (278), respectively.231 In contrast, the reaction in the presence of benzophenone afforded the formal [2 + 2] cycloaddition product, [HC(CMeNDip)2]Ga(O2CPh2) (279),231 whereas dimeric {[HC(CMeNDip)2]Ga(m-O)}2 was formed in the reaction of 251 with N2O.232 Moreover, the unique reactivity of 251 was demonstrated in the successful synthesis of interesting compounds from across the p-block of the periodic table. Gallanediyl 251 was found to form Lewis acid-base adducts [HC(CMeNDip)2]GaM(C6F5)3 (280, M ¼ B, Al, Ga) with Lewis acids M(C6F5)3, which subsequently reacted with benzaldehyde under M–C bond cleavage.233 Reactions of 251 with GaCl3, GaMe3, and InEt3 proceed with one- or two-fold M–X bond insertion, yielding bi- and trimetallic inter-group 13 compounds {[HC(CMeNDip)2](X)Ga}MX2 (281; M ¼ Ga, X ¼ Me; M ¼ In, X ¼ Et) and {[HC(CMeNDip)2](X)Ga}2MX (282; M ¼ Ga, X ¼ Cl, Me; M ¼ In, X ¼ Et).234,235 The indium counterparts were found to decompose in solution via further sequential In–C bond activation reactions, resulting in the formation of elemental In and [HC(CMeNDip)2]GaEt2.235 In addition, 251 was found to react with silicon tetrahalides SiX4. While the reaction with SiCl4 exclusively yielded the oxidative addition product {[HC(CMeNDip)2](Cl)Ga}SiCl3 (283),234 reactions with SiBr4 proceeded with formation of {[HC(CMeNDip)2](Br)Ga}SiBr3 (284) and {[HC(CMeNDip)2](Br)Ga}2SiBr2 (285). The formation of 284 and 285 resulted from single and double oxidative addition of the Si–Br bonds at the Ga center, respectively. Treatment of 285 with another equivalent of 251 led to the elimination of [HC(CMeNDip)2]GaBr2 and formation of the elusive

392

Gallium, Indium, and Thallium

digallylsilylene {[HC(CMeNDip)2](Br)Ga}2Si (286), which degrades by insertion of the Si(II) center into a C–C bond of the b-diketiminate ligand backbone and subsequent formation of a bicyclic compound containing SiGaNC2 and SiGaNC heterocycles (287). Most remarkably, the short-lived silylene intermediate 286 could be trapped with CO affording the stable Si-carbonyl complex {[HC(CMeNDip)2](Br)Ga}2SiCO (288). The exceptional stability of 288 arises from electronic effects imposed by the Ga-based ligands resulting in an efficient synergistic s(OC-to-Si) donation and p(Si-to-CO) backdonation, of which the backdonation term was found to dominate as is typically observed in transition metal–CO complexes. Si–CO complex 288 was furthermore shown to act as a masked silylene in reactions with PBr3 and H2, whereas CO–isocyanide ligand exchange was observed in reactions with CyNC yielding {[HC(CMeNDip)2](Br)Ga}2SiCNCy (289).236 In regard of the heavier group 14 halides, 251 reacts with phosphine- or carbene-stabilized GeCl2 in the presence of KC8 to yield the unusual Ga–Ge compounds {[HC(CMeNDip)2]Ga}2Ge2 (290) and {[HC(CMeNDip)2]Ga}2Ge4 (291). Compound 291 features a puckered Ge4 ring derived from a formal [Ge4]4− polyanion, whereas the bonding in 290 was described to exhibit a transannular p-only single-bond interaction.237 The reaction of 251 with SnCl2 afforded the metalloid clusters {[HC(CMeNDip)2](Cl)Ga}2Sn7 (292) and {[HC(CMeNDip)2](Cl) Ga}4Sn17 (293) via reduction and disproportionation reactions, which are stabilized by electropositive gallyl fragments and can thus be regarded as Zintl-type cluster anions.238 These reactions already show the potential of 251 to act as both a reducing agent and subsequent stabilization agent allowing the generation and isolation of unprecedented structural motifs in main group chemistry. Oxidative addition reactions between 251 and Me3PbCl, Pb(OTf )2, and Pb(OTf )2(H2O)2 gave rise to the first molecular compounds containing Ga–Pb bonds, {[HC(CMeNDip)2](Cl)Ga}PbMe3 (294), as well as the Ga-substituted plumbylenes {[HC (CMeNDip)2](OTf )Ga}2Pb(THF) (295) and {[HC(CMeNDip)2](OTf )Ga}2Pb(H2O) (296)239 (Scheme 22).

Scheme 22 Reactivity of gallanediyl 251.

Transformation reactions of group 15 elements compounds also received great attention. Fischer and co-workers reported on the reaction of 251 with white phosphorus (P4), which proceeded with oxidative addition of one of the P–P bonds to the Ga center in 251 and formation of {[HC(CMeNDip)2]Ga}P4 (297).240 Weigand and co-workers employed 297 in further halophosphination reactions with R2PX (R ¼ Cy, Ph, Mes, C6F5; X ¼ Cl, Br) and reported on the synthesis of new polyphosphanes such as {[HC (CMeNDip)2](X)Ga}P5R2 (298), which in case of X ¼ Br and R ¼ C6F5 initially dimerizes to yield the decaphosphane {[HC (CMeNDip)2](Br)Ga}2P10R4 (299) and finally eliminates P2(C6F5)4 to give a tetracyclic octaphosphane {[HC(CMeNDip)2](Br)

Gallium, Indium, and Thallium

393

Ga}2P8 (300).241 Inspired by these findings, a detailed reinvestigation of the reaction of 251 with P4 resulted in the isolation of a number of polyphosphanes {[HC(CMeNDip)2]Ga}nPm (n ¼ 1, m ¼ 4; n ¼ 2, m ¼ 4, 6, 8, 12, 14, 16) with up to 16 phosphorus atoms, of which the hexaphosphane {[HC(CMeNDip)2]Ga}2P6 (301) was subsequently derivatized with various electrophilic and nucleophilic reagents.242 Twofold P–X bond insertion of 251 was observed in reactions with PX3 with formation of {[HC (CMeNDip)2](X)Ga}2PX (302, X ¼ Cl, Br),243 and P–X bond insertion reactions were also used for the synthesis of Ga-substituted phosphinidenes {[HC(CMeNDip)2](X)Ga}P(MeCAAC) (303, X ¼ Cl, Br), which were transformed by halide abstraction into Ga–P–C heteroallyl cations [{[HC(CMeNDip)2]Ga}P(MeCAAC)][An] (304, An ¼ B(C6F5)4, B(3,5-(CF3)2C6H3)4, Al(OC(CF3)3)4) featuring two delocalized p electrons over the GaPC moiety. The implied electrophilic character of the Ga center was shown by coordination with DMAP yielding [{[HC(CMeNDip)2](DMAP)Ga}P(MeCAAC)][B(C6F5)4] (305).244 The decarbonylation of phosphaketene complexes by 251 afforded gallaphosphenes {[HC(CMeNDip)2]Ga}PP(NDipCH)2 (306), {[HC(CMeNDip)2] Ga}PP(NDipCH2)2 (307),245 and {[HC(CMeNDip)2]Ga}P{Ga(Cl)[(NDipCMe)2CH]} (308)246 containing Ga–P double bonds. The strongly polarized nature of the Ga–P p bond was shown by reactions of 307 with H2 and CO2 revealing its frustrated Lewis Pair (FLP)-like character due to the presence of a nucleophilic P-based ligand and an electrophilic Ga-based fragment at the central P atom,245 and isolation of the pyridine adduct of a related gallaphosphene, [Dip-BIAN]GaP{Ga(py)[(NDipCMe)2CH]} (309).247 Compound 308 was converted to a Ga–P–Ga heteroallyl cation [{[HC(CMeNDip)2]Ga}P{Ga[(NDipCMe)2CH]}][B(C6F5)4] (310) by halide abstraction, and was furthermore shown to reversibly bind two molecules of CO2 yielding {[HC(CMeNDip)2]Ga} (CO2)2P{Ga(Cl)[(NDipCMe)2CH]} (311), as well as to react with acetone and acetophenone under C–H activation to give {[HC(CMeNDip)2]Ga}PH{Ga(OC(CH2)(R))[(NDipCMe)2CH]} (312, R ¼ Me, Ph).246 Diyl 251 was also found to undergo one- and twofold oxidative addition reactions with E–H bonds of EH3 (E ¼ P, As, Sb) giving rise to compounds {[HC(CMeNDip)2](H)Ga}EH2 (313, E ¼ P, As, Sb) and {[HC(CMeNDip)2](H)Ga}2EH (314, E ¼ As, Sb), respectively.248 Reactions of 251 and 252 with BiEt3 proceeded under oxidative addition yielding Bi–C insertion products {[HC (CMeNDip)2](Et)M}BiEt2 (315, M ¼ Ga, In), which decompose via subsequent Bi–C activation to {[HC(CMeNDip)2]MEt2 and Bi metal.249 Surprisingly, oxidative addition reactions of 251 with dipnictanes E2R4 (E ¼ Sb, Bi; R ¼ Et, Ph), which resulted in the formation of compounds of the type {[HC(CMeNDip)2]Ga(ER2)2 (316), were found to be fully reversible upon thermal treatment, resulting in reductive elimination of the dipnictanes.177,250 In reactions of two equivalents of 251 with EX3, consecutive two-electron reduction and E–X bond insertion steps resulted in the formation of a series of Ga-substituted dipnictenes {[HC (CMeNDip)2](X)Ga}2E2 (317, E ¼ As, Sb, Bi; X ¼ Cl, Br, I, NMe2, NMeEt, NEt2, OEt, OSO2CF3, OC6F5) containing E–E double bonds,251–255 some of which could be converted by thermal treatment to {[HC(CMeNDip)2](X)Ga}2Sb4 (318, X ¼ Cl, NMe2) featuring a tetrastibabicyclo[1.1.0]butane core.252,253 The corresponding tetrabismabicyclo[1.1.0]butane {[HC(CMeNDip)2](Cl) Ga}2Bi4 (319) was formed in the reaction of 251 with Cp 2BiCl.256 In contrast, the use of three equivalents of 251 in reactions with EX3 gave rise to gallapnictenes {[HC(CMeNDip)2]Ga}E{Ga(X)[(NDipCMe)2CH]} (320, E ¼ As, Sb; X ¼ F, Cl, Br, I) containing Ga–E double bonds.257,258 The reactions of 251 with SbX3 (X ¼ Cl, Br) were shown to proceed via several intermediates including {[HC(CMeNDip)2](X)Ga}2SbX (321) and stibinidenes {[HC(CMeNDip)2](X)Ga}Sb, which were isolated as the corresponding carbene adducts, {[HC(CMeNDip)2](X)Ga}Sb(L) (322, L ¼ (HCNDip)2C, MeCAAC).258,259 Gallapnictenes 320 were furthermore found to react with molecules containing polarized s bonds such as HCl and MeI with addition of the positively polarized part to pnictogen center E, and the negatively polarized part to the Ga center, respectively, showing the polarized nature of the Ga–E p bond. Moreover, addition of a chloride anion to 320 yielded pnictinide anions [(HCNiPr)2CH][{[HC(CMeNDip)2](Cl) Ga}2E] (323, E ¼ As, Sb), which feature partial Ga–E double bond character due to negative hyperconjugation.260 The facile homolytic cleavage of Sb–C bonds in Sb–Cp compounds enabled the synthesis of {[HC(CMeNDip)2]Ga}2Sb4 (324) which formally contains a [Sb4]4− polyanion, by reaction of 251 with [SbCp ]4.261 This approach was further exploited to the synthesis of pnictinyl radicals262 of the type {[HC(CMeNDip)2](X)Ga}2E (325, E ¼ As, Sb, Bi; X ¼ Cl, Br, I) by reactions of [C5(4-tBuC6H4)5] AsCl2 and Cp EX2 (E ¼ Sb, Bi, X ¼ Cl, Br, I) with two equivalents of 251, respectively, which proceeded via liberation of the corresponding cyclopentadienyl radical. Compounds 325 are pnictogen-centered radicals that exhibit partial delocalization of unpaired electron density onto the Ga centers due to the p-accepting properties of the gallyl ligands.263,264 Interestingly, using 252 instead of 251 in reactions with Cp SbX2, led to the formation only of In-substituted Sb hydrides, {[HC(CMeNDip)2](X) In}2SbH (326, X ¼ Cl, Br, I), due to the lower steric demand of the indyl compared to gallyl substituents, and thus the lower kinetic stabilization of the reactive Sb(II) center.265 Depending on the E–C bond strength, the formation of gallapnictenes [HC(CMeNDip)2](Cl)GaER (327, E ¼ As, R ¼ Cp ; E ¼ Sb, R ¼ 2,6-Mes2C6H3) was observed. These systems possess very short Ga–E double bonds, however, the p bonding contribution was estimated to amount to about 10 kcal mol−1 by variable-temperature NMR spectroscopic studies.263,266 As a by-product, the unusual heterotetrametallic compound {[HC(CMeNDip)2](Cp )Ga}{[HC (CMeNDip)2]Ga}(m,Z1:2-As3) (328) containing a central butterfly triarsagalla[1.1.0]butane core was obtained.266 The use of unsymmetrically substituted precursors, Cp (R)SbCl, enabled the synthesis of heteroleptic Ga-substituted stibinyl radicals, {[HC (CMeNDip)2](Cl)Ga}(R)Sb (329, R ¼ B(NDipCH)2, 2,6-Mes2C6H3, N(Dip)SiMe3),267 whereas for co-ligands R with lower steric demands, the formation of Ga-substituted Sb hydrides {[HC(CMeNDip)2](Cl)Ga}(R)SbH (330, R ¼ Dip, N(SiMe3)2, OB(NDipCH)2) was observed.268 By oxidative addition reactions with Cp (Ph)ECl (E ¼ As, Sb) a homologous series of fully unsymmetrically substituted group 13 metalylpnictanes {[HC(CMeNDip)2](Cl)M}E(Ph)Cp (331, M ¼ Al, Ga, In; E ¼ As, Sb) was obtained, which allowed structural trends to be derived depending on the group 13 metal. Metalylpnictanes 331 were converted to the corresponding dimetallyldipnictines [{[HC(CMeNDip)2](Cl)M}(Ph)E]2 (332, E ¼ As, Sb; M ¼ Al, Ga, In) by thermal elimination of the Cp moiety.269 Moreover, gallanediyl 251 was shown to reductively cleave the Te–Te and Te–C bonds of Ph2Te2 and iPr2Te affording [HC(CMeNDip)2]Ga(TePh)2 (333) and {[HC(CMeNDip)2](iPr)Ga}TeiPr (334), respectively, whereas oxidation of the Ga(I) center with elemental tellurium yielded dimeric {[HC(CMeNDip)2]Ga(m-Te)}2 (335)270 analogously to the corresponding m-oxo and m-sulfido complexes232 (Scheme 23).

394 Gallium, Indium, and Thallium

Scheme 23

Reactivity of gallanediyl 251 toward group 15 element compounds.

Gallium, Indium, and Thallium

395

Aldridge and co-workers synthesized Ga(III) complexes containing a b-diketiminate-derived dianionic diamido ligand, [DipNC(CH2)CHCMeNDip]GaR (R ¼ tBu 336, CMe2Et 337), by deprotonation of [HC(CMeNDip)2]Ga(Br)tBu at the b-Me group. Reactions of 336 with Lewis acidic B(C6F5)3 and Lewis basic DMAP yielded [DipNC(CH2B(C6F5)3)CHCMeNDip]GatBu and [DipNC(CH2)CHCMeNDip]Ga(DMAP)tBu, respectively. 336 and 337 were shown to act as single-component ambiphilic systems, which are capable of Ga-ligand cooperative activation of protic, apolar, and hydridic H–X bonds, such as those in NH3, H2S, H2, and SiH4, affording [HC(CMeNDip)2]Ga(X)tBu (X ¼ NH2 338, SH 339, H 340), as well as [DipNC(CH2SiH3)CHCMeNDip]Ga(H)tBu (341). Gallium hydride 340 was found to react with CO2 to the corresponding formate complex [HC(CMeNDip)2] Ga(OCHO)tBu (342), which was converted back to 340 upon treatment with HB(OCMe2)2 under formation of MeOB(OCMe2)2. Thus, 340 was applied as a catalyst for the selective hydroboration of CO2 to the methanol oxidation state using HB(OCMe2)2.271,272 Moreover, the mechanism of the cooperative H–X bond activation by 336 was investigated computationally by means of DFT calculation by Kretschmer and co-workers, which revealed bi- and termolecular pathways depending on the substrate.273 A similar Ga(III) hydride, [HC(CMeNDip)2]Ga(H)Ad (343), was converted to the corresponding cationic three-coordinate Ga complex, [[HC(CMeNDip)2]GaAd][HB(C6F5)3] (344) by hydride abstraction with B(C6F5)3, and shown to act as a moderately active catalyst in the hydrosilylation of CO2 with Et3SiH to the formaldehyde state. The presence of additional B(C6F5)3 as co-catalyst was found to significantly enhance the catalytic activity and effects the CO2 reduction to the methane oxidation level.274

9.07.5.7

Other nitrogen-based ligands

While ligand systems such as amidinates, guanidinates, a-diimines, and b-diketminates are the most prominent N,N0 chelating ligands, several other nitrogen-based ligands have also been successfully introduced in the chemistry of Ga, In, and Tl. These include iminophosphonamide ligands of the type [R2P(NR0 )2], which incorporate the metal center in a four-membered ring. Singh and co-workers presented a bulky and asymmetrically N-substituted iminophosphonamide ligand [Ph2P(NtBu)(NDip)], which was used for the synthesis of the corresponding monomeric gallium and dimeric indium dichloride complexes [Ph2P(NtBu)(NDip)] GaCl2 (345) and {[Ph2P(NtBu)(NDip)]InCl2}2 (346), respectively.275,276 Whereas in situ lithiation of the pro-ligand [Ph2P(NtBu) (NDip)]H and subsequent reaction with GaCl3 was found to be a convenient and straightforward method for the synthesis of 345,275 the same procedure only provided the LiCl-adduct of 346, [Ph2P(NtBu)(NDip)]In(Cl)(m-Cl)2Li(OEt2)2 (347), in the reaction with InCl3. 346 was obtained from the potassium salt of the ligand in a non-coordinating solvent. Addition of Lewis bases to 346 was shown to induce the dissociation of the dimer into the corresponding base-stabilized adducts [Ph2P(NtBu) (NDip)]InCl2(L) (348, L ¼ THF, OPPh2NHtBu).276 Roesky and co-workers synthesized enantiopure neutral and cationic bis(iminophosphonamide) complexes [Ph2P(N-(R)-CHMePh)2]2GaCl (349) and [{Ph2P(N-(R)-CHMePh)2}2Ga][AlCl4] (350) by salt metathesis and subsequent chloride abstraction with AlCl3, respectively.277 Stasch and co-workers employed alkali metal salts of the bulky diiminophosphinate ligand [Ph2P(NDip)2] in the synthesis of monomeric low-valent Ga(I), In(I), and Tl(I) complexes [Ph2P(NDip)2]M (M ¼ Ga 351, In 352, Tl 353) via salt metathesis using “GaI,” InBr, and TlBr, respectively. Interestingly, a second modification of 351 containing dimeric (351)2 was obtained, however, the weak association was evident by long Ga–Ga separations of 2.79 A˚ , the influence of packing effects, and the monomeric nature in solution. Gallanediyl 351 reacted with O2 with formation of the m-oxo bridged species {[Ph2P(NDip)2]Ga(m-O)}2 (354), that is comparable to compound 251. The important influence of steric demand was shown by the use of the less bulky diiminophosphinate ligand [Ph2P(NMes)2], which resulted in partial disproportionation of Ga(I) and formation of the mixed-valent diiminophosphinate gallium subiodide cluster [Ga4I3{Ph2P(NDip)2}]278 (Scheme 24).

Scheme 24 Group 13 metal diiminophosphinate complexes.

396

Gallium, Indium, and Thallium

Moreover, bis(iminophosphoranyl)methanide ligands of the type [HC(R2PNR0 )2] have been used in group 13 chemistry. These ligands typically involve the metal center being constrained into a puckered six-membered CP2N2M heterocycle, while in more general terms they resemble b-diketiminate ligands. Ga(III) and In(III) complexes [HC(Ph2PNSiMe3)2]MX2 (355, M ¼ Ga, X ¼ Br; M ¼ In, X ¼ Cl, Br) were synthesized by salt metathesis and structurally characterized.279 Stasch and co-workers prepared monomeric M(I) complexes [HC(Ph2PNDip)2]M (M ¼ Ga 356, In 357, Tl 358) using alkali metal salts of the bulky ligand [HC(Ph2PNDip)2] and the corresponding M(I) halides. Structural characterization and DFT calculations confirmed the resemblance of 356–358 to their b-diketiminate counterparts 251–253, yet with puckered heterocycles, particularly for the heavier congeners. 356 was shown to readily react with I2 affording the corresponding Ga(III) compound [HC(Ph2PNDip)2]GaI2 (359).280 Double deprotonation of a bis(iminophosphoranyl)methane ligand and reaction with TlCl afforded a dithalliumbis(iminophosphoranyl) methandiide complex, {[C(Ph2PNSiMe3)2]Tl2}2 (360), in which the two Tl centers are coordinated by the same carbon center, and which dimerizes via Tl(I)–Tl(I) interaction in the solid-state to give a planar Tl4 array.281 Furthermore, low-oxidation state group 13 complexes were obtained using an aminoimidazoline-2-imine (AmIm ¼ o-C6H4{NC (NiPrCMe)2}(NDip)) ligand, [AmIm]M (M ¼ Ga 361, In 362, Tl 363), representing the first neutral five-membered carbenoids, which thus complete the series of neutral four- to six- membered M(I) heterocycles. Compounds 361–363 were synthesized by reactions of [AmIm]K and [AmIm]Li with GaCp , InCp, and TlBF4, respectively, and were shown to exhibit comparable properties to the corresponding four-membered guanidinate and six-membered b-diketiminate counterparts.282 In addition, related triazenide ligands [R2N3] containing bulky aryl substituents and a N3 backbone were also employed in group 13 chemistry. The moderately bulky triazenide ligand [Dip2N3] was used to synthesize M(III) hydride complexes [Dip2N3]2MH (364, M ¼ Ga, In) by reaction of the pro-ligand [Dip2N3]H with LiMH4, whereas hydride-chloride exchange at the Tl(III) complex [Dip2N3]2TlCl (365) failed. Attempts to synthesize low-oxidation state group 13 complexes using [Dip2N3] by salt elimination reactions starting from “GaI,” InI, and M[MCl4] (M ¼ Ga, In) remained without success, resulting only in the formation of [Dip2N3]2MX (M ¼ Ga, In; X ¼ Cl, I) and [(Dip2N3) GaI]2 by disproportionation reactions. In contrast, the Tl(I) compound [Dip2N3]Tl was obtained from [Dip2N3]H and TlOEt, and was found to exhibit a dimeric structure in the solid-state.283 The more bulky terphenyl-substituted triazenide ligand [(2,6-Mes2C6H3)2N3] is capable of effectively stabilizing the +I oxidation state also of In and Ga resulting in the isolation of monomeric M(I) complexes [(2,6-Mes2C6H3)2N3]M (M ¼ Ga 366, In 367, Tl 368) containing a four-membered N3M heterocycle, which feature a narrower HOMO-LUMO gap than related M(I) species according to DFT calculations. In addition, M(III) halide, methyl, and hydride complexes [(2,6-Mes2C6H3)2N3]MX2 (369, MX2 ¼ GaCl2, InBr2), [(2,6-Mes2C6H3)2N3]MMe2 (370, M ¼ Ga, In, Tl), and [(2,6-Mes2C6H3)2N3] GaH2 (371) were obtained using this ligand.284 The Tl(I) complex of an even more bulky variant, [(2,6-(CHPh2)2-4-MeC6H3)2N3]Tl (372), was also synthesized.285 Moreover, formazanido complexes [PhC(NNPh)2]MMe2 (373, M ¼ Ga, In) containing CN4M heterocycles were synthesized by CH4 elimination. The indium congener forms an adduct with DMAP resulting in a five-coordinate In center as disclosed from crystallographic studies286 (Scheme 25).

Scheme 25 Group 13 metal triazenide compounds.

By use of chelating dianionic diamido ligands, Ga(I) and In(I) anions similar to compound 217 were isolated. A xanthene-based ligand system containing sterically demanding NDip donors in the 4,5 positions (NON) in conjunction with the O donor of the backbone enabled the synthesis of gallylpotassium salt [(NON)GaK]2 (374) by KC8 reduction of (NON)GaI, which exists as a potassium-bridged dimer in the solid-state and in solution.287 Coles and co-workers exploited a dianionic ligand containing bis(amido)disiloxane backbone [O(Me2SiNDip)2] for the synthesis of In complexes in the oxidation states +I, +II, and + III. The +III compounds [O(Me2SiNDip)2]In(m-Cl)2Li(OEt2)2 (375) and [O(Me2SiNDip)2]In(m-Cl)2K (376) were obtained from salt metathesis reaction of the dilithium and dipotassium salts of the ligand with InCl3, respectively.288,289 Reduction of 375 with one equivalent of Na afforded an In(II) compound, diindane {[O(Me2SiNDip)2]In}2 (377), containing an In–In bond,288 which isomerizes at elevated temperatures in the presence of an organic azide to {[O(Me2SiNDip)2]In2[(DipNSiMe2)2O]} (378) featuring a bridging mode of the diamido ligands.290 Reduction of 375 with two equivalents of Na furnished the indyllithium compound [O(Me2SiNDip)2]InLi(THF)3 (379), whereas with potassium dimeric {[O(Me2SiNDip)2]InK}2 (380) and sequestered [K([2.2.2] crypt)][[O(Me2SiNDip)2]In] (381) were obtained, all of which feature +I oxidation state In centers formally possessing an electron

Gallium, Indium, and Thallium

397

lone pair.288,289 The nucleophilic indyl anion 380 was reacted with MesN3 under N2 liberation to give anionic In imides featuring In–N multiple bond character {[O(Me2SiNDip)2]InN(Mes)K}2 (382) and [K([2.2.2]crypt)][[O(Me2SiNDip)2]InNMes] (383), which further react with organic azides in a (2 + 3) cycloaddition yielding In compounds with a tetrazenide ligand and a planar InN4 heterocycle, {[O(Me2SiNDip)2]In[RNNNNMes]2K}2 (384, R ¼ Mes, SiMe3)289 (Scheme 26).

Scheme 26 Low-oxidation state gallium and indium compounds featuring dianionic diamido ligands.

In addition to the various N,N0 chelating ligands, some N,N0 ,N00 chelating ligands have been used in group 13 chemistry, particularly for the complexation of Tl+ ions. These include redox-active tris(pyrazolyl)borates, which contain ferrocenyl-substituents at the pyrazolyl moieties,291 highly fluorinated aryl-substituted tris(indazolyl)borates,292 and a tris(pyrazolyl)methanide ligand,293 as well as neutral b-triketimines.294

9.07.6

Heavier group 15 element-based ligands

Heavier group 15 element-based ligands have been employed in the chemistry of the group 13 elements for the synthesis of novel group 13/15 compounds, which are potential single source precursors for the synthesis of semiconductor materials. However, also the inherent properties of M–E bonds (M ¼ Ga, In, Tl; E ¼ P, As, Sb, Bi) are of interest considering further reactivity. Krossing and co-workers used arene-coordinated M(I) salts [M(arene)n][Al(OC(CF3)3)4] (M ¼ Ga, arene ¼ tol, n ¼ 2; M ¼ In, arene ¼ C6H5F, n ¼ 2, 3) (cf. Section 9.07.3.2.1) in ligand exchange reactions with PPh3 to obtain cationic M(I) tris(phosphine) complexes [M(PPh3)3]+ (M ¼ Ga 385, In 386), which feature trigonal pyramidal coordination geometries due to the donation of the P-centered lone pairs into the empty p-orbitals at the M+ centers. Due to the M-centered lone pair and the associated potential viability of switching between the +I and + III oxidation states, 385 and 386 resemble classical transition metal phosphine complexes used in catalytic applications. Increase of the steric bulk of the phosphine by using PtBu3, afforded cationic M(I) bis(phosphine) complexes [M(PtBu3)2]+ (M ¼ Ga 387, In 388), which exhibit bent structures with P–M–P angles of about 117 , while the availability of a mainly s-type lone pair as well as a vacant p-orbital at the metal center renders 387 and 388 isolobal to singlet carbenes.84,87 Beckmann and co-workers investigated donor-acceptor interactions of Ph2P and MCl2 (M ¼ Ga, In, Tl) fragments anchored to the peri-positions of a rigid acenaphthyl (Ace) backbone in compounds 6-Ph2P-Ace-5-MCl2 (389). Despite the conformational constrain, formation of regular Lewis acid-base adducts with short dative P–E interactions of polar covalent character was observed.295 Geometrically and sterically more constrained arrangements of Lewis-basic R2P and Lewis-acidic MR2 moieties typically result in frustration of the acid-base interaction and formation of frustrated Lewis pairs (FLP). Uhl and co-workers utilized a phosphinylvinyl magnesium compound for the transmetallation of the vinyl moiety yielding Mes2P(C(CHPh))GatBu2 (390) containing a geminal arrangement of the Mes2P and GatBu2 functions. 390 acts as a frustrated Ga/P Lewis pair in reactions with phenylacetylene and azide p-ClC6H4N3 affording the five-membered GaPC3heterocyclic adduct 390-PhCCH (391) and four-membered GaPCN heterocycle 390-N3(p-C6H4Cl) (392).296 Moreover, 390 was found to irreversibly bind CS2 (393), whereas the formation of the CO2 adduct (394) is reversible and only observed at low temperatures. With benzaldehyde and OPEt3 only Ga–O bonded adducts (395 and 396) were obtained. The latter characterized 390 as a relatively weak Lewis acid, particularly compared to the corresponding Al/P Lewis pair Mes2P(C(CHPh))AltBu2 (397). Ga/P Lewis pair 390 promotes the dehydrocoupling of H3BNHMe2 to [H2BNMe2]2, whereas dehydrogenation and formation of the five-membered PCGaNB aminoborane adduct of

398

Gallium, Indium, and Thallium

390 (398) was observed with H3BNH3. This reactivity was used in the transfer hydrogenation of imine Ph(H)CNtBu with H3BNH3 catalyzed by 390 yielding PhCH2NHtBu and [HBNH]n.297 390 reacted with diphenylazirine under cleavage of the three-membered C2N ring of the azirine affording 399, which rearranged in solution yielding 400. The reaction of 390 with diphenylcyclopropenone proceeded via formation of the corresponding Ga–O adduct and subsequently with mesityl group transfer from the P center to the cyclopropane moiety, that induces ring-opening of the C3 ring with formation of 401. The reactions with diphenylthiocyclopropenone proceeded analogously, with formation of the S-analogue of 401 (402).298 The frequently different and increased reactivity observed in reactions with Al/P FLP 397 were attributed to the different hardness in terms of the HSAB concept and Lewis acidity of the Al center in comparison to the Ga center in 390.299 Moreover, two In/P2-based FLPs, RIn[C(PMes2) CHPh]2 (403, R ¼ Cl, Ph), were obtained and shown react with (Me2S)AuCl under Au–Cl cleavage to [Cl2In[C(PMes2)CHPh]2]Au (404) due to the cooperative Lewis acid activation and P,P0 chelation of the Au center.296 Zhu and co-workers reported the synthesis of a novel Ga-based FLP possessing a tetracoordinated yet Lewis-acidic Ga center with three pendant Lewis-basic PtBu2 groups, N(C2H4NPtBu2)3Ga (405), which was reported to activate a variety of heterocumulenes including tBuNCO, PhNCS, C(NiPr)2, and (SiMe3)HCN2, as well as phenylacetylene, in a 1,3 fashion across the Ga center and one of the three N-PtBu2 functions300 (Scheme 27).

Scheme 27 Reactivity of Ga/P FLP 390.

Scheer and co-workers obtained the monomeric base-stabilized parent phosphanyl- and arsanylgallanes [(HCNDip)2C] GaH2EH2 (406, E ¼ P, As), which are valence isoelectronic to ethylene but typically prone to oligomerization. Stabilization was thus only achieved through the use of a single sterically demanding and strongly donating carbene ligand at the Ga center, and compounds 406 were synthesized by salt metathesis reactions starting from [(HCNDip)2C]GaH2X (X ¼ Cl, I) and LiPH2/KAsH2 or H2 evolution reactions from [(HCNDip)2C]GaH3 and EH3.301,302 In addition, monomeric carbene-stabilized phosphanylgallane [(HCNDip)2C]GaH2PCy2301 and arsanylgallane [(HCNDip)2C]GaH2AsPh2302 were obtained using the salt metathesis approach. Starting from [(HCNDip)2C]GaHCl2, branched parent compounds [(HCNDip)2C]GaH(EH2)2 (407, E ¼ P, As) were additionally obtained via salt metathesis using NaPH2 and KAsH2.302 Compounds 406 and 407 represent potential precursors for group 13/15 materials via CVD processes avoiding the use of gaseous EH3. Similar to compounds of type 407, related b-diketiminate-stabilized group 13 dipnictogenide complexes [HC(CMeNDip)2]M(EH2)2 (408, M ¼ Ga, E ¼ P, As; M ¼ In, E ¼ P) were synthesized using a similar approach. Moreover, the corresponding monopnictogenide complexes [HC(CMeNDip)2]M(Cl)E(SiMe3)2 (409, M ¼ Ga, In; E ¼ P, As) using bulkier E(SiMe3)2 substituents were obtained from salt metathesis reactions.303

Gallium, Indium, and Thallium

399

Von Hänisch and co-workers synthesized a number of phosphanylgallanes containing bulky alkyl and silyl groups such as [R2GaP(H)SitBuPh2]2 (R ¼ Et 410, iPr 411, tBu 412) by alkane elimination, which typically form cyclic dimers in solution and in the solid-state. While the sterically most crowded system 412 only reacted with small bases such as pyridine, DMAP and [(HCNiPr)2C] to form the corresponding monomerized adducts [(L)tBu2GaP(H)SitBuPh2] (413, L ¼ py, DMAP, [(HCNiPr)2C]),304 the less crowded analogs 410 and 411 reacted with the more sterically demanding carbene [(HCNMes)2C] to yield the analogous monomers [{(HCNMes)2C}R2GaP(H)SitBuPh2] (414, R ¼ Et, iPr), which were subsequently converted to the Ga4P4 heterocubanes [RGaPSitBuPh2]4 (415, R ¼ Et, iPr) by thermal treatment under loss of RH and [(HCNMes)2C].305 In contrast to 412, the sterically even more encumbered phosphanylgallane [tBu2GaP(H)SitBu2Ph]2 (416) also reacted with the sterically demanding carbenes [(HCNMes)2C] and [(HCNDip)2C] to yield the corresponding monomeric adducts [{(HCNR)2C}tBu2GaP(H)SitBu2Ph] (417, R ¼ Mes, Dip), in which the carbenes adopt abnormal coordination modes. The increased reactivity of 416 compared to 412 was attributed to the dynamic behavior of 416 in solution. Moreover, 416 was found to react with two molecules of PhNCO by insertion into the Ga–P bond and subsequent coupling of the isocyanides, yielding a GaC2N2O heterocycle (418).306 Cyclic base-stabilized phosphagallene dimer [(Me3N)HGaPSitBuPh2]2 (419) was synthesized by a similar alkane elimination approach, and subsequently subjected to base exchange reactions with carbenes.307 The potassium salts of N-heterocyclic carbene-stabilized phosphinidenides [(H2CNR)2CPK] (R ¼ Mes, Dip) were used to construct novel group 13/15 compounds. Salt metathesis reactions with GaCl3 and tBu2GaCl afforded Ga2PCl and Ga2P2 heterocyclic compounds [{(H2CNMes)2CP}(GatBu2)2(m-Cl)] (420) and [{(H2CNR)2CP} GaR0 2]2 (421, R ¼ Mes, Dip; R0 ¼ Cl, tBu)308,309 (Scheme 28).

Scheme 28 Group 13 metal compounds containing group 15 element-based ligands.

Kemp and co-workers synthesized a heteroleptic In(III) complex [N(PiPr2)2]2InCl (422) containing two P,P0 chelating bis(phosphino)amido ligands, with strained four-membered rings, by salt metathesis using InCl3 and (iPr2P)2NLi. Compound 422 reacts in the solid-state or in solution with CO2 by insertion of CO2 into two of the four In–P bonds affording [O2CPiPr2NPiPr2]2InCl (423). This process was found to be reversible with CO2 release and regeneration of 422 occurring upon heating to 75  C. In contrast, analogous reactions using CS2 proceeded with C–S bond cleavage and resulted in a mixture of products.310 The facile access to the 2-phosphaethynolate anion allowed to study its interaction with Ga centers supported by salen and b-diketiminate ligands resulting in the formation of Ga phosphaketene complexes [(H2CNCH-3,5-tBu2-2-OC6H3)2GaPCO] (424)311 and [HC(MeCNDip)2]Ga(Cl)PCO (425)246 with the anion bound to Ga via the P atom, and which were synthesized by salt metathesis. Replacement of the chloride in [H2C(CH2CMe2)2N]2GaCl by PCO− resulted in transfer of the amide substituent to the C atom of the PCO moiety via the putative intermediate in [H2C(CH2CMe2)2N]2GaPCO and ultimately in formation of dimeric tricyclic ladder-type compound {[H2C(CH2CMe2)2N]GaPC(O)[N(CMe2CH2)2CH2]}2 (426) containing a central Ga2P2 heterocycle.312

400

9.07.7

Gallium, Indium, and Thallium

Chalcogen-based ligands

Preparation of GaOTf by oxidation of Ga metal with AgOTf in toluene led to crystallization of mixed-valent salt [Ga][Ga (tol)2]2[Ga3(OTf )8] (427), which exhibits two arene-coordinated [Ga(tol)2]+ cations (cf. Section 9.07.3.2.1), a “naked” Ga+ cation, and a [Ga3(OTf )8] trianion, containing a [Ga3]5+ chain. Conducting the synthesis in the presence of C6Me6 lead to formation of [Ga(tol)(C6Me6]2[Ga2(OTf )6] (428) containing Ga(II) centers in the anionic moiety. Addition of 18-crown-6 to 427 resulted in formation of [Ga(18-crown-6)(OTf )] (428), which shows a strong Ga–O interaction with the triflate anion.313 The analogous In-crown ether complexes [In(18-crown-6)(OR)] (429, R ¼ SO2CF3, COCF3),314,315 as well as an In “crown ether sandwich” complex, [In(15-crown-5)2][OTf] (430) were also obtained.316 The use of a weakly coordinating aluminate anion enabled the synthesis of ion-separated complexes [M(18-crown-6)(C6H5F)2][Al(OC(CF3)3)4] (431, M ¼ Ga, In), which feature two additional M-arene contacts in the solid-state structures.317 The reaction of Ga(II) compound Ga2Cl4(THF)2 with [2.2.2.]cryptand affords the mixed-valence compound [Ga3Cl4([2.2.2]crypt)][GaCl4] (432) and [Ga2Cl2([2.2.2]crypt)][OTf]2 (433) in the presence of Me3SiOTf.318 Krautscheid and co-workers prepared a complete series of dimethyl group 13 phenylchalcogenolates Me2MEPh (434, M ¼ Ga, In, Tl; E ¼ S, Se, Te) by methane elimination reactions of the group 13 trimethyls with the corresponding phenylchalcogenols. As formed compounds exhibited dimeric and polymeric structures in the solid-state. Due to additional M–E contacts, the lighter group 13 and group 16 compounds form polymeric chains, which are shielded by the organic substituents, whereas for the larger group 13 and 16 elements polymeric chains and layer-like dimeric and polymeric structures were observed.319 Additional indium dimethyl chalcogenolates Me2InER (435, ER ¼ OPh, O(2,6-Me2C6H3), O(2,4,6-tBu3C6H3), SPh, S(2,4,6-tBu3C6H3), SePh, Se(2,4,6-Me3C6H3), Se(2,4,6-tBu3C6H3))320 and thallium dimethyl phenolates Me2TlOR (436, R ¼ Ph, 2,6-Me2C6H3, Dip, 2,6-Ph2C6H3, 2,4,6-tBu3C6H2)321 were synthesized in a similar manner and also structurally characterized. They typically exhibit monomeric, dimeric, or polymeric structural motifs depending on the steric bulk of the organic group R and the size of the group 16 element. However, the structural type proved to be unpredictable320,321 (Scheme 29).

Scheme 29 Group 13 metal dimethyl chalcogenolates.

Reactions of gallium amides Me(Cl)Ga[N(SiMe3)2] and Cl2Ga[N(SiMe3)2] with alcohols containing pendant donor functions resulted in amine HN(SiMe3)2 elimination and the formation of compounds of the type [Me(Cl)GaOR]2 (437, R ¼ CH2CH2OMe, CH(CH3)CH2NMe2, OtBu), [MeGa(OCH(CH3)CH2NMe2)2] (438), and [Cl2GaOCH2CH2NMe2] (439), which exhibited penta-coordinated Ga atoms due to both coordination of the pendant sidearms and dimer formation.322 Compounds containing thiophenol-based substituents with phosphine and arsine donor functions in the ortho-position (2-PPh2-C6H4S)GaR2 (440, R ¼ Me, tBu), [(2-AsPh2-C6H4S)GaMe2]2 (441), (2-PPh2-C6H4S)2GatBu (442, E ¼ P, As) were formed in alkane elimination reactions of the corresponding thiophenol with GaR3 (R ¼ Me, tBu). In case of 440 and 441, the stronger Ga–P interaction in 440 effectively inhibits dimerization, whereas for 441 dimer formation is observed due to the weak Ga–As interaction.323 A potentially tridentate S,P,S0 ligand was used to synthesize tetracoordinate Ga complexes PhP(2-SC6H4)2GaR (443, R ¼ Me, tBu) by alkane elimination reactions starting from the protio-ligand and GaR3, in which the P-donor saturates the Ga vacancy. Irradiation of 443 led to homolytic Ga–C bond cleavage with formation of several products including digallane [PhP(2–SC6H4)2Ga]2 (444) and the sulfido-bridged digallium compound [PhP(2-SC6H4)2Ga]2(m-S) (445).324 Structural/oxidation state isomers of compounds 70 and 71, [(2,6-Dip2C6H3)OIn]2 (446) and [(2,6-Dip2C6H3)SIn]2 (447), were obtained from salt metathesis reactions between (2,6-Dip2C6H3)ELi (E ¼ O, S) and InCl. As-formed compounds show central In2E2 moieties, which are shielded by flanking terphenyl groups.76 Moreover, the corresponding Tl(I) compounds [(2,6-Dip2C6H3) ETl]2 (448, E ¼ O, S)76 and [(2,6-(2,4,6-iPr3C6H3)2C6H3)ETl]2 (449, E ¼ S, Se)325 were prepared via acid-base reactions between (2,6-Dip2C6H3)EH (E ¼ O, S) and TlCp as well as (2,6-(2,4,6-iPr3C6H3)2C6H3)EH (E ¼ S, Se) and TlOEt, respectively (Scheme 30). Ga and In b-diketonates, HC(CtBuO)2MtBu2 (M ¼ Ga 450, In 451) and HC(CPhO)2IntBu2 (452) were synthesized by reactions of MtBu3 and HC(CRO)2H. 451 reacts with O2 with insertion of O and O2 units into the In–C bonds and formation of indium peroxo complexes [HC(CtBuO)2]2In2(OtBu)2(tBu)(OOtBu) (453)326 and [HC(CtBuO)2]2In3(OOtBu)6(OtBu) (454)327 depending on the reaction conditions, whereas 452 was found to form doubly peroxo-bridged compound {[HC(CPhO)2]2InOOtBu}2 (455).326 Bis-

Gallium, Indium, and Thallium

401

Scheme 30 Low-oxidation state group 13 compounds containing chalcogen-based ligands.

([BoR]) and tris(2-oxobenzimidazolyl)borato ([ToR]) complexes with varying steric bulk were introduced as a new class of bi- and tridentate O-based ligands for Tl(I) ions (Scheme 31). Thus, a series of monomeric complexes [BoR]Tl (456, R ¼ Me, tBu, Ad) and [ToR]Tl (457, R ¼ Me, tBu, Ad) were prepared by salt metathesis using the sodium salts of the respective ligands and TlOAc. Compounds 456 show secondary Tl–H interactions with the backbone BH2 moiety in the solid-state.328 Related tris(2-mercapto-1-tbutylimidazolyl)borato [TmtBu] complexes containing three soft S donor functions were employed for the synthesis of compounds with low-oxidation state Ga, In, and Tl centers in sulfur-rich environments. The Tl and In +I oxidation state compounds {[TmtBu] Tl}2 (458) and [TmtBu]In (459) were obtained as dimeric and monomeric structures in the solid-state.329 The corresponding Ga counterpart was not obtained due to disproportionation reactions yielding several compounds including mixed-valence complexes [TmtBu]GaGaX3 (X ¼ Cl, I), {[TmtBu]Ga}2X 2 (X ¼ I, GaCl4), and [{[TmtBu]Ga}2(m-GaI2)]I, however, the B(C6F5)3 adduct [TmtBu] GaB(C6F5)3 (460) was isolated.330

Scheme 31 Bis- and tris(2-oxobenzimidazolyl)borato Tl(I) complexes.

The mixed alkyl-teflate Ga compound Et2Ga(m-OTeF5)2Ga(Et)(OTeF5) (461) was obtained from the reaction of GaEt3 and excess HOTeF5. In contrast to the corresponding Al compound no further reaction of 461 with HOTeF5 was observed, and 461 was used for the synthesis of the salts [PPh4][Ga(Et)(OTeF5)3] and [Ph3C][Ga(Et)(OTeF5)3], containing the weakly coordinating anion [Ga(Et)(OTeF5)3]− (462), by reactions of 461 with PPh4Cl and Ph3CCl, respectively. Moreover, the related weakly coordinating anion [Ga(OTeF5)4]− (463) was synthesized as the Ag+ salt by reaction of GaCl3 with Ag(OTeF5), and further converted to the [PPh4]+ and [Ph3C]+ salts by salt metathesis.331

9.07.8

Conclusions

Since 2007, the chemistry of metalorganic compounds of the heavy group 13 elements gallium, indium, and thallium has experienced considerable developments, which significantly contributed to an enhanced understanding of group 13 compounds containing organic substituents. Progress was achieved through novel synthetic approaches resulting in the isolation of unprecedented compounds in various oxidation states with unforeseen structural features, electronic structures, and bonding properties. Such species include for instance monomeric divalent radicals and large metalloid clusters, while ligands such as carbenes were found to enable the stabilization of a wide range of oxidation states and structures. Weakly coordinating anions allowed stabilization of simple group 13 metal-arene complexes, and novel approaches led to the generation of formally “simple” yet previously unknown compounds. Moreover, the reactivity particularly of low-valent group 13 compounds established new synthetic directions and opportunities in this field. For instance, weakly associated dimers of terphenyl-substituted gallanediyls showed a rich chemistry in the activation of small molecules, while monomeric b-diketiminate Ga(I) complexes were found to be

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exploitable for the synthesis and stabilization of unusual main group compounds such as silicon-carbonyl complexes and group 15 element-centered radicals, as well as for the construction of systems with double bonds between heavy group 13 and 15 elements. In the future, discoveries of novel unprecedented bonding types and the targeted utilization of particularly low-valent group 13 compounds for selective and catalytic bond transformations are expected.

Acknowledgments Financial support by the University of Duisburg-Essen is gratefully acknowledged.

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