Comprehensive Organometallic Chemistry IV. Volume 7: Groups 8 to 10 - Part 1 [7] 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 7

GROUPS 8 TO 10 - PART 1 VOLUME EDITOR

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

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

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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 7 Editor Biographies

vii

Contributors to Volume 7

xiii

Preface 7.01

xv

Introduction to Groups 8 to 10

1

Timothy H Warren

7.02

Ferrocenes and Other Sandwich Complexes of Iron

3

Carmen M Casado, Beatriz Alonso, and Mª Pilar Garcí a-Armada

7.03

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

46

Katharina Münster and Marc D Walter

7.04

Recent Advances in Synthesis, Characterization and Reactivities of Iron-Alkyl and Iron-Aryl Complexes

185

Bufan Zhang, Maria Camila Aguilera, Nathalia Cajiao, and Michael L Neidig

7.05

Alkylidyne and Alkylidene Complexes of Iron

210

Fabio Marchetti

7.06

Small Molecule Activation by Organo-iron Complexes

258

Kaiji Shen, Stéphane Ménage, and Carole Duboc

7.07

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

294

Toshiro Takao and Akiko Inagaki

7.08

Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

444

Alexander D Böth, Michael J Sauer, Robert M Reich, and Fritz E Kühn

7.09

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

528

Noy B Nechmad, N Gabriel Lemcoff, and Lionel Delaude

7.10

Ruthenium and Osmium Carbonyl Cluster Complexes

564

Sumit Saha and Burjor Captain

7.11

N-Heterocyclic Carbene Complexes of Cobalt

632

Thomas Simler, Andreas A Danopoulos, and Pierre Braunstein

7.12

Organocobalt Complexes in C–H Bond Activation

759

Naohiko Yoshikai

7.13

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

816

Hugo Valdés, Rebeca Osorio-Yañez, Ernesto Rufino-Felipe, and David Morales-Morales

v

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

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Editor Biographies

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

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

Editor Biographies

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

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Editor Biographies

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

Editor Biographies

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

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

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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 7 Maria Camila Aguilera Department of Chemistry, University of Rochester, Rochester, NY, United States Beatriz Alonso Inorganic Chemistry Department, Faculty of Sciences, Autonomous University of Madrid, Madrid, Spain

Mª Pilar García-Armada Chemical and Environmental Engineering Department, Higher Technical School of Industrial Engineers, Polytechnic University of Madrid, Madrid, Spain Akiko Inagaki Department of Chemistry, Tokyo Metropolitan University, Tokyo, Japan

Alexander D Böth Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, München, Germany

Fritz E Kühn Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, München, Germany

Pierre Braunstein Université de Strasbourg, CNRS, Institut de Chimie UMR 7177, Laboratoire de Chimie de Coordination, Strasbourg, France

N Gabriel Lemcoff Department of Chemistry, Ben-Gurion University of the Neguev, Beer-Sheva, Israel

Nathalia Cajiao Department of Chemistry, University of Rochester, Rochester, NY, United States Burjor Captain Department of Chemistry, University of Miami, Coral Gables, FL, United States Carmen M Casado Inorganic Chemistry Department, Faculty of Sciences, Autonomous University of Madrid, Madrid, Spain Andreas A Danopoulos Université de Strasbourg, CNRS, Institut de Chimie UMR 7177, Laboratoire de Chimie de Coordination, Strasbourg, France; Laboratory of Inorganic Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, Athens, Greece Lionel Delaude Laboratory of Catalysis, MolSys Research Unit, Institut de Chimie Organique (B6a), Université de Liège, Liège, Belgium Carole Duboc Univ Grenoble Alpes, CNRS, DCM, Grenoble, France

Fabio Marchetti Department of Chemistry and Industrial Chemistry, University of Pisa, Pisa, Italy Stéphane Ménage Univ Grenoble Alpes, CEA-Grenoble, CNRS, LCBM, Grenoble, France David Morales-Morales Instituto de Quí mica, Universidad Nacional Autónoma de México, Coyoacán, Ciudad de México, Mexico Katharina Münster Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Braunschweig, Germany Noy B Nechmad Department of Chemistry, Ben-Gurion University of the Neguev, Beer-Sheva, Israel Michael L Neidig Department of Chemistry, University of Rochester, Rochester, NY, United States Rebeca Osorio-Yañez Instituto de Quí mica, Universidad Nacional Autónoma de México, Coyoacán, Ciudad de México, Mexico

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Contributors to Volume 7

Ernesto Rufino-Felipe Instituto de Quí mica, Universidad Nacional Autónoma de México, Coyoacán, Ciudad de México, Mexico Robert M Reich Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, München, Germany

Toshiro Takao Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, Japan Hugo Valdés Instituto de Quí mica, Universidad Nacional Autónoma de México, Coyoacán, Ciudad de México, Mexico

Sumit Saha Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar, Odisha, India

Marc D Walter Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Braunschweig, Germany

Michael J Sauer Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, München, Germany

Timothy H Warren Department of Chemistry, Michigan State University, East Lansing, MI, United States

Kaiji Shen Université Grenoble Alpes, CNRS, DCM, Grenoble, France Thomas Simler Université de Strasbourg, CNRS, Institut de Chimie UMR 7177, Laboratoire de Chimie de Coordination, Strasbourg, France

Naohiko Yoshikai Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai, Japan Bufan Zhang Department of Chemistry, University of Rochester, Rochester, NY, United States

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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7.01

Introduction to Groups 8 to 10

Timothy H Warren, Department of Chemistry, Michigan State University, East Lansing, MI, United States © 2022 Elsevier Ltd. All rights reserved.

Beyond an intriguing range of structures and transformations, the organometallic chemistry of the iron, cobalt, and nickel triads finds important applications in organic synthesis, energy conversion, materials chemistry, and medicinal research. Major advances from COMC III involve the greater use of the first-row members in catalysis, especially cross-coupling reactions, and expansive use of all members across the field of catalysis along with an appreciation of higher oxidation state complexes of these triads. There has been an explosion of N-heterocyclic carbene (NHC) complexes across this area of the periodic table; several chapters focus on the wide-ranging chemistry and applications that this supporting ligand enables. Recognizing the strong connection of these organometallic complexes to allied fields of chemistry, the formatting of chapters is perhaps a bit different than previous versions of COMC. Chapters in this section cover traditional families of organometallic complexes, but often in the context of diverse, compelling applications that make this area of organometallic chemistry attractive to a broader audience. In Chapter 7.02, Casado, Alonso, and García-Armada describe advances in the synthesis of ferrocenes and other sandwich complexes of iron. Due to their well-defined redox behavior, ferrocenes find applications in a number of fields, including catalysis, bioorganometallic chemistry, polymer science, and materials chemistry. In Chapter 7.03, Münster and Walter report on half-sandwich compounds of iron that attract great interest in small molecule activation and catalysis. In contrast to ferrocenes, many half-sandwich species defy the 18-electron rule. These species support metal-ligand multiple bonds as well as heterometallic metal-metal bonds, even with 4f elements. In Chapter 7.04, Zhang, Aguilera, Cajiao, and Neidig survey families of iron-alkyl and -aryl complexes. The last 15 years truly represent an “iron-age” in organometallic catalysis, with these species taking prominent roles in cross-coupling reactions with a radical edge to them. A deeper understanding of these iron-carbon bonds may suggest new classes of CdC bond forming reactions. In Chapter 7.05, Marchetti reviews iron alkylidene and alkylidyne complexes. The diverse structural and reaction chemistry of broad families of mononuclear, binuclear, and polynuclear iron complexes is covered that features alkylidene and alkylidyne ligands, with and without a-heteroatoms. In Chapter 7.06, Shen, Ménage, and Duboc present a compelling overview of the use of organometallic iron complexes in N2 activation and reduction, H2 production and oxidation, and O2 activation for oxidation reactions. The fields of energy chemistry and catalysis greatly benefit from these organometallic species. In Chapter 7.07, Takao and Inagaki describe advances in the chemistry of half-sandwich complexes of ruthenium and osmium that include pogo-stick as well as two-, three-, and four-legged piano stool structures with a wide variety of ligands. Sandwich complexes, such as metallocenes and cationic Z6-arene complexes, also are covered along with a discussion of their anticancer activities. In Chapter 7.08, Böth, Sauer, Reich, and Kühn provide an overview of NHC and p-acid ligands in ruthenium and osmium chemistry. With a strong focus on N-heterocyclic carbenes (NHCs) that find use in myriad applications, the authors also include heavier tetrylenes, such as silylenes, germylenes, stannylenes, and plumbylenes as well as nitrosyls, phosphine, and phosphite ligands. In Chapter 7.09, Nechmad, Lemcoff, and Delaude chronicle the development of ruthenium alkylidene complexes that result in extraordinarily successful contemporary Ru-benzylidene alkene metathesis catalysts. The authors make important connections between catalytic activity and supporting ligands that include a variety of different N-heterocyclic carbenes. The authors also cover the formation of benzylidyne complexes that may represent deactivation pathways in alkene metathesis, yet are interesting species in their own right. In Chapter 7.10, Saha and Captain survey ruthenium and osmium carbonyl cluster complexes, highlighting emerging trends in their design, properties, and catalytic activities. Mixed-metal cluster systems are also included, featuring other transition metals, such as palladium and platinum as well as main group elements germanium and tin. In Chapter 7.11, Simler, Danopoulos, and Braunstein comprehensively cover NHC complexes of cobalt. This incredibly versatile class of supporting ligand enables the coordination of an incredibly wide range of other ligands and functional groups useful in myriad applications at cobalt centers that span formal oxidation states from -I to +V. In Chapter 7.12, Yoshikai describes families of organocobalt complexes used in CdH bond activation and functionalization chemistry. Mechanisms outlined illustrate the intermediacy of cobalt-alkyl, aryl, and hydride complexes that engage with a variety of unsaturated substrates in synthetically useful transformations. In Chapter 7.13, Valdés, Osorio-Yañez, Rufino-Felipe, and Morales-Morales chronicle the use of group 9 organometallic pincer complexes in catalysis. These complexes find widespread application across catalysis including cross-coupling reactions, hydrosilyation, hydrophosphination, hydroboration, hydrogenation/dehydrogenation, and alkene isomerization. In Chapter 8.01, Lee, Yoo, Kwak and Kim present rhodium and iridium complexes with NHC ligands, focusing on their structure, reactivity and catalytic applications. These also include bis-NHC complexes as well as complexes of NHC ligands linked with other chelating coordinating groups.

Comprehensive Organometallic Chemistry IV

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2

Introduction to Groups 8 to 10

In Chapter 8.02, Gao, Cui, Cui, and Jin provide an overview of half-sandwich complexes of rhodium and iridium focusing on Z5-Cp M species. The synthetic accessibility and chemical robustness of this fragment enables a wide variety of co-ligands to form complexes broadly useful in catalysis and as building blocks in supramolecular chemistry. In Chapter 8.03, Yamashita reviews cobalt, rhodium, and iridium boryl complexes, focusing on structurally characterized examples that feature M-B bonding with sp2-hybridized boron. These complexes serve as catalytic intermediates in synthetically versatile metal-catalyzed hydroborylation and C-H borylation reactions. In Chapter 8.04, Femoni, Cesari, Iapalucci, Ruggieri, and Zacchini cover carbonyl clusters of the group 9 and 10 metals, focusing on complexes with at least four metal atoms. This review covers both homo- and heterometallic clusters that contain other supporting ligands, such as phosphines and clusters that host main group elements in the form of carbide, nitride, and phosphides. In Chapter 8.05 Wagner and Diao present important developments in chemistry of nickel-carbon s-bonded complexes. A variety of coordination environments support nickel alkyl and aryl complexes across formal oxidation states that span 0 to +IV. The reactivity patterns of these nickel alkyl and aryl complexes find use in a wide range of catalytic applications, often enhanced through co-ligands that are redox non-innocent or featuring metal-ligand cooperativity. In Chapter 8.06, Włodzimierz recounts major developments in nickelocene chemistry and monocyclopentadienyl nickel complexes. Nickelocene complexes with a variety of substitution patterns are outlined as well as the particularly rich chemistry of monocyclopentadienyl nickel species that participate in CdC and C-heteroatom bond forming reactions. In Chapter 8.07, Ligielli, Danopoulos, Braunstein, and Simler comprehensively cover the synthesis, structure and reactivity of nickel NHC complexes. Covering an extensive range of NHC architectures and nickel oxidation states, the authors outline their applications in catalytic transformations, such as C-C and C-heteroatom cross-coupling and cycloaddition reactions. In Chapter 8.08, Mohr, Gawlik, and Mell review the broad field of palladium and platinum NHC complexes. With extensive use as selective catalysts in organic synthesis, the incredibly broad family of NHC Pd and Pt complexes also contributes to applications in medicine and luminescence. In Chapter 8.09, Fernandes, Kumar, and Chandra outline a wide range of p-allyl complexes of palladium as intermediates in organic synthesis. CdH activation represents an important pathway for their formation that highlights their role in sustainable synthesis of CdC, CdN, CdO, and even CdSi and CdS bonds. In Chapter 8.10, Garduño and García describe recent advances in the synthesis and characterization of zerovalent nickel complexes. Excluding NHC species, the authors focus on p-acceptor ligands, such as carbonyls, isocyanides, alkenes, and alkynes along with s-adducts involving BdH and SidH bonds. In Chapter 8.11, Matsubara summarizes monovalent complexes of nickel, palladium, and platinum. While more common for nickel, monovalent palladium and platinum complexes have become better understood in recent years, especially in metal-metal bonded compounds connected to catalytically active species in a wide variety of synthetically useful organic transformations. In Chapter 8.12, Vedernikov surveys high valent palladium and platinum organometallic complexes. While covering tetravalent complexes that may participate in M(II)/M(IV) oxidative addition/reductive elimination sequences, the last 15 years have seen important developments in strategies to synthesize and study odd-electron M(III) complexes. Chapters in this section convey a strong sense of the ongoing discovery of the organometallic chemistry of the iron, cobalt, and nickel triads. A vital area of chemistry, continued advances in the understanding of the synthesis, structure, reactivity as well as mechanisms of action of these organometallic complexes will continue to deliver chemistry critical to organic synthesis, energy conversion, and materials chemistry.

7.02

Ferrocenes and Other Sandwich Complexes of Iron

Carmen M Casadoa, Beatriz Alonsoa, and Mª Pilar García-Armadab, aInorganic Chemistry Department, Faculty of Sciences, Autonomous University of Madrid, Madrid, Spain; bChemical and Environmental Engineering Department, Higher Technical School of Industrial Engineers, Polytechnic University of Madrid, Madrid, Spain © 2022 Elsevier Ltd. All rights reserved.

7.02.1 7.02.2 7.02.2.1 7.02.2.2 7.02.2.3 7.02.2.4 7.02.2.5 7.02.2.6 7.02.2.7 7.02.3 7.02.4 7.02.5 References

Introduction Synthesis and reactions of ferrocene derivatives Metalated ferrocenes and haloferrocenes Alkyl, acyl, aryl, alkenyls, alkynyls and related ferrocenes Ferrocenylsilanes Nitrogen containing ferrocene derivatives Phosphorus containing ferrocene derivatives Ferrocenophanes Chiral ferrocenes Macromolecules: Polymers and dendrimers Bioorganometallic chemistry of ferrocene derivatives Other applications

3 4 4 6 11 13 19 22 24 26 30 32 33

Abbreviations BArF COD Cp Cp Dba dppf Fc Fv Me Mes MOF NHC NPs ODG Ph ROMP ROP tren

7.02.1

Bis(trifluoromethyl)phenylborate Cyclooctadiene Z5-C5H5 Z5-C5Me5 Dibenzylideneacetone 1,10 -Bis(diphenylphosphino)ferrocene (Z5-C5H5)Fe(Z5-C5H4) Fulvalene CH3 2,4,6-Me3C6H2 Metal-organic framework N-Heterocyclic carbine Nanoparticles ortho-Directing group C6H5 Ring-opening metathesis polymerization Ring-opening polymerization Tris(2-aminoethyl)amine

Introduction

Since the 1950s, the chemistry of ferrocene and its derivatives has remained a leading topic in organometallic chemistry.1,2 Over the last 15 years, research activity with ferrocene derivatives as main subject has afforded more than 12,000 references in major journals, an average of 800 publications per year. This survey chapter cannot provide an exhaustive literature review of such a highly active and fast-moving field of research. A comprehensive overview on the organometallic facet of ferrocene is presented including references of more exhaustive literature reviews, which focus on particular research areas. A touch of the state of knowledge on the application of ferrocene derivatives in other active fields of chemistry and neighboring disciplines such as catalysis, polymer science, bioorganometallic chemistry and material science is also addressed.

Comprehensive Organometallic Chemistry IV

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Ferrocenes and Other Sandwich Complexes of Iron

The book of Stepnicka “Ferrocenes Ligands, Materials and Biomolecules”3 from 2008, and that of Phillips “Ferrocenes: Compounds, Properties and Applications” from 20114 were the starting point for this overview. Different special issues devoted to ferrocene constituted most helpful sources of bibliography. In 2013, Organometallics dedicated an entire issue to ferrocene, entitled Ferrocene—Beauty and Function.5 The occasion of the 65th birthday of ferrocene was celebrated with the publication of a special issue (cluster issue) of European Journal of Inorganic Chemistry, “The Multifaceted Chemistry of Ferrocene.”6 In 2018, Molecules also published a special issue “Ferrocene and Ferrocene-Containing Compounds.” Multiple chapters and reviews of enormous utility have also been referred to in the corresponding sections. Especially useful was the microreview by Astruc “Why is Ferrocene so exceptional.”7

7.02.2

Synthesis and reactions of ferrocene derivatives

New or improved synthetic routes toward ferrocene compounds, which are used as convenient starting materials, as ligands, or directly in the huge variety of applications, are addressed.

7.02.2.1

Metalated ferrocenes and haloferrocenes

By using modified earlier protocols, 1,10 -dilithioferrocene N,N,N0 ,N0 -tetramethylethylenediamine as well as 1,10 -dibromoferrocene, 1,10 -diaminoferrocene, 1,10 -diaminoferrocenium hexafluorophosphate and 1,10 -diaminoferrocenium triflate have been prepared in multigram quatities.8 Metalated ferrocenes, in particular lithium ferrocenes, continue to be of extraordinary importance because of their general synthetic utility.3,4,9–13 The mechanism of protonation and lithiation of ferrocene has been examined using ab initio chemical dynamics simulations.12 Butler has reported the direct synthesis of 1,10 ,2,20 -tetralithioferrocene and 1,10 ,2,20 ,3,30 hexalithioferrocene from 1,10 ,2,20 -tetrabromoferrocene and 1,10 ,2,20 ,3,30 -hexabromoferrocene, respectively. Further reaction with chlorotrimethylsilane, affords the corresponding 1,10 ,2,20 -tetrakis(trimethylsilyl)ferrocene and 1,10 ,2,20 ,3,30 -hexakis(trimethylsilyl) ferrocene (Scheme 1).14

Scheme 1

The synthesis of ortho-lithiated ferrocene derivatives has become an active area15–21 with new different ortho-directing functional groups.9–11,22,23 Some examples of mono- and disubstituted ferrocenes used in ortho-directed metalations are shown as 1–7 (Chart 1).24–31

Ferrocenes and Other Sandwich Complexes of Iron

5

Chart 1

1,2- and 1,3-Disubstituted ferrocene derivatives have been obtained by halide-mediated ortho-deprotonation reactions using lithium tetramethylpiperidide.32 Deprotonative mono- or polymetalation of a range of ferrocenes have been carried out using mixed lithium–zinc and lithium–cadmium combinations,33–35 mixed alkali metal–magnesium,36 –zinc,37 –aluminum38 and –manganese bases.34 There is no doubt that haloferrocenes are still important starting materials for many ferrocenyl derivatives. The syntheses and typical reactions of homo- and heterohaloferrocenes have been reviewed by Butenschön, including lithiation followed by trapping with an electrophile, copper-mediated halogen substitution, coupling with formation of diferrocenyl derivatives, ortho-lithiation followed by trapping with an electrophile, palladium-catalyzed coupling reactions and others.39 Bromoferrocene is the starting product in a novel synthetic approach for trifluoromethylthioferrocene, avoiding the use of toxic mercury(II)-based reagents. Treatment of bromoferrocene with NaSCN in the presence of copper(I) yields thiocyanatoferrocene, which further reacts with the Ruppert-Prakash reagent and tetrabutylammonium fluoride to give the expected ferrocenyl derivative.40 Various ferrocenyl functionalized thiophene derivatives have been synthesized starting from 1,10 -dibromoferrocene using typical Negishi C,C cross-coupling conditions.41 Mono- and 1,10 -difluoro-substituted ferrocene and asymmetrical 1,10 -disubstituted ferrocenes with one substituent being fluorine have been obtained in a one-pot synthesis consisting of lithiation of metallocenes and subsequent addition of the fluorinating agent N-fluorobenzenesulfonimide.42 An efficient procedure has been developed for the conversion of 1-fluoro-2-iodo (or 1-chloro-2-iodo) to 1-fluoro-3-iodo (or 1-chloro-3-iodo) ferrocenes, based on the suitable protection of the free 5-position.43 In an interesting work,44 all of the halo and 1,10 -dihaloferrocenes (X ¼ I, Br, Cl, F) have been obtained in high purity using straightforward, readily available methods such as oxidative purification45 and column chromatography. The improved high-yielding syntheses of a variety of 1,2-dihaloferrocenes such as 1,2-dibromo- and 1,2-diiodoferrocene as well as 1-bromo-2-iodoferrocene, and 1-bromo-2-fluoroferrocene have been described.46 1,10 ,2-Tribromoferrocene and 1,10 ,2,20 -tetrabromoferrocene, key synthons in ferrocene chemistry, have been prepared using a-halide assisted lithiation.47 An efficient synthesis of 1,10 ,2,20 -tetraiodoferrocene that uses 1,10 ,2,20 -tetrakis(tri-n-butylstannyl)ferrocene as a key intermediate has been developed.48

6

Ferrocenes and Other Sandwich Complexes of Iron

Erb et al. have applied the base-catalyzed aromatic halogen “dance” to convert 1,2-disubstituted ferrocenes bearing only one halogen and a fixed N,N-dialkylcarboxamide directing group into the corresponding 3-iodoferrocenecarboxamides.49 These studies were followed by an asymmetric halogen “dance” reaction for the first synthesis of enantioenriched ferrocenes bearing five different substituents on the same cyclopentadienyl ring. Halogens able to act either as directing or as migrating groups in deprotolithiation and halogen “dance” reactions were used.50 A scalable synthesis of ferrocenylazides in flow has been developed. Halogen-lithium exchange of ferrocenyl halides and trapping with tosyl azide has given access to a variety of functionalized ferrocenyl azides such as 8–19. A reduction process afforded the corresponding ferrocenyl amines (Chart 2).51

Chart 2

7.02.2.2

Alkyl, acyl, aryl, alkenyls, alkynyls and related ferrocenes

Transition metal catalyzed CdH bond functionalization has become a useful strategy in the preparation of substituted ferrocenes52,53 including enantioselective synthesis.54–58 Although noble metal complexes have been most used, such as these examples of Ru,58 Rh,59–63 Ir64–66 and Pd,67–71 other more affordable transition metal complexes have also demonstrated of utility. Alkynyl ferrocenes have been synthesized from ferrocenes and terminal alkynes via Cu-promoted 8-aminoquinoline-assisted CdH bond activation (Scheme 2).72 Ackermann reported the first cobalt-catalyzed direct alkenylation of a ferrocene derivative.73 Later, Butenschön published the cobalt or iron-catalyzed ortho-C,H activation of ferrocenes bearing ortho-directing groups.74–76 Kumar developed a methodology for the synthesis of unsymmetrical ferrocene aryl chalcogenides by C-H activation of ferroceneamide using copper(II) catalyst.77,78 CdN bond formation via Cp Co(III)-catalyzed CdH bond functionalization of ferrocenes has also been reported.79–82

Ferrocenes and Other Sandwich Complexes of Iron

7

Scheme 2

The [3 + 2] cycloaddition reactions of bridging C3 ligands in diiron complexes, reported by Zanotti et al., have shown a valuable synthetic approach which can be used for the direct synthesis of polysubstituted ferrocenes in which only one cyclopentadienyl ligand contains different substituents. Bridging vinylalkylidene, vinyliminium, bis-alkylidene and enamine ligands undergo [3 + 2] cycloaddition with alkynes (Scheme 3).83–87

8

Ferrocenes and Other Sandwich Complexes of Iron

Scheme 3

Substituted alkynylferrocenes have been prepared by the cross metathesis of (prop-1-yn-1-yl)ferrocene with alkynes in the presence of a simple catalytic system comprising [Mo(CO)6] and halophenols. The reaction proceeds well with both aryl- and alkylpropynes (Scheme 4).88

Scheme 4

The coupling of ferrocenyl alkynes in a “one-pot” synthetic methodology consisting in the reaction of 1,4-diferrocenylbutadiyne with dicarbonyl(Z5-cyclopentadienyl)cobalt(I) afforded a series of cyclic multiferrocenyl compounds. The product composition was controlled by the applied reaction conditions, such as temperature, molar ratio of the reactants and application of a steady argon or carbon monoxide flow through the reaction vessel.89 A range of piano-stool organo-iron complexes have been synthesized by the visible-light photolysis of the sandwich complexes [FeCp(arene)][PF6].90 In addition, visible-light photolysis of [FeCp(Z6-toluene)][PF6] in the presence of diphenyldiphosphinoethane and terminal alkynes has shown to be a convenient and general route to iron-vinylidene and iron-alkynyl complexes.91 Several synthetic pathways to ferrocene-substituted allenylidene complexes have been developed.92,93 A straightforward strategy for the synthesis of a series of conjugated trienylferrocenes has been reported. It relies in the Ru(0)-catalyzed cross dimerization of an internal alkynylferrocene with conjugated dienes.94 A modified synthetic method of bis(ethynyl)biferrocene with higher yields and ease of purification was reported by Lang et al., consisting in the palladium-copper-catalyzed Sonogashira cross-coupling and further deprotection reactions shown in Scheme 5.95 They have also described the synthesis and properties of a series of complexes containing bis(ethynyl)biferrocene as a bridge between different redox-active group 8 metal fragments.96 Many other examples of compounds in which two ferrocenyl units are joined forming a fulvalenide linker (biferrocenes) have been reported by this group.97–102

Scheme 5

Ferrocenes and Other Sandwich Complexes of Iron

9

Astruc et al. have reported the synthesis of functional biferrocenes by the visible-light-induced exchange of the toluene ligand in the bimetallic precursor [(m2,Z5,Z0 5-Fv)Fe2(Z6-toluene)2][PF6]2 (Fv ¼ fulvalene), in the presence of a substituted cyclopentadienyl salt C5H4RM (R ¼ COCH3, CO2CH3, PPh2, SiMe2CH2Cl; M ¼ Li or Na) (Scheme 6) or the dicyclopentadienyl salt 1,4-C6H4-(CH2C5H4)2Na2.103 Functional and heterodifunctional 1,10 -ferrocenes have also been obtained by this procedure.104

Scheme 6

Various monosubstituted 1,10 -biferrocenylenes were prepared by Breuer and Schmittel, applying an improved Friedel–Crafts acylation protocol (Scheme 7).105

Scheme 7

10

Ferrocenes and Other Sandwich Complexes of Iron

Optimized and reproducible multistep synthetic routes to obtain a series of mercaptoalkylferrocenes, Fc-(CH2)n-SH with n ¼ 1, 2, 3 and 4, have been reported.106 Thermal and photochemically initiated thiol–yne addition of ethynylferrocene107 and thiol–ene reactions of vinylferrocene108 or ferrocenylvinylsilanes,109 with thiols with different functional groups have shown that the thiol structure is a critical factor regarding the efficiency of these transformations. Monothiols exhibited the highest reactivity and ethynylferrocene undergoes notably easier hydrothiolation than vinylferrocene. Some examples are shown in Scheme 8.

Scheme 8

Hydroalkoxylation and hydrothiolation processes of the alkyne groups in the compound FcC^CSC^CH afforded a series of unsymmetrical vinyl-sulfides and ethers that might act as multifunctional ligands.110 Floris et al. reported the isolation of 1,2,4-triferrocenylbenzene and 1,3,5-triferrocenylbenzene from the cyclotrimerization reaction of ferrocenylethyne, catalyzed either by metalloporphyrins or cobaltocene.111 The first Dewar benzene–ferrocene conjugates were synthesized by reaction of tetraalkylcyclobutadiene–AlCl3 complexes with 3-ferrocenylpropynoates. Rearrange to their corresponding benzenes was only achieved upon heating in a microwave reactor.112 Gleiter et al. summarized the investigations and observations that led to the structure of the a-ferrocenylmethylium ion.113 In 2020, McGlinchey has discussed the progress in the area of molecular rearrangements or ferrocenyl migrations facilitated by charge delocalization. He has examined classic reactions involving ferrocenyl migrations, such as the pinacol, Wolff, Beckmann, and Curtius, as well as the influence of the ferrocenyl substituent on the mechanisms of the Nazarov, Meyer-Schuster, benzoin, and Stevens rearrangements.114 The standard molar enthalpies of sublimation of ferrocene, 1,10 -dimethylferrocene, decamethylferrocene, ferrocenecarboxaldehyde and R-methylferrocenemethanol, and the enthalpy of vaporization of N,N-dimethyl-(aminomethyl)ferrocene, at 298.15 K, were determined by Calvet-drop microcalorimetry and/or the Knudsen effusion method.115

Ferrocenes and Other Sandwich Complexes of Iron

7.02.2.3

11

Ferrocenylsilanes

Di- and triferrocenyl triethoxysilane derivatives (20 and 21) have been synthesized via hydrosilylation reaction of allyltriethoxysilane with diferrocenylmethylsilane and triferrocenylsilane respectively. 20 was obtained in toluene, at 65  C and in the presence of Karstedt catalyst, while a solvent-free platinum-catalyzed hydrosilylation reaction at 85  C was necessary to prepare 21. Both have been used as derivatizing reagents for anodized Pt electrode surfaces (Chart 3).116

Chart 3

The hydrosilylation strategy, in dry toluene with Karstedt catalyst, under heating, has been used by Cuadrado et al. to obtain macrocyclic and linear co-oligocarbosiloxanes and octasilsesquioxanes containing ferrocenyl moieties,117,118 and at room temperature by Teimuri-Mofrad and coworkers in the synthesis of binuclear119 and trinuclear120 ferrocenyl based organosilane compounds. Ferrocenylene copolymers containing carbosilylene, carbosiloxy and cyclopentadienyliron dicarbonyl bridges have also been obtained in THF as solvent.121 Cobaltcarbonyl compounds favor hydrosilylation and decomplexation of the alkynes may generate vinylsilanes in the presence of HSiR3. Regio- and stereoselective hydrosilylation reaction of ferrocenylalkyne–dicobalthexacarbonyl complexes with HSiEt3 afforded the ferrocenylvinylsilanes 22–25 (Chart 4).122

Chart 4

Co2(CO)6(m,Z2-HCCFc) has been used as precursor in the synthesis of new linear and cyclic ferrocenyl-functionalized siloxanes by hydrosilylation reactions.123

12

Ferrocenes and Other Sandwich Complexes of Iron

Novel ferrocenyl silyl ethers have been synthesized via dehydrocoupling reactions of ferrocenyl silanes and various alcohols at room temperature under Karstedt catalyst.124,125 A series of organosilylferrocene derivatives have been synthesized by the reaction of 1,10 -bis(dimethylsilyl)ferrocene with aldehydes and ketones in the presence of (C2H4)Pt(PPh3)2 or Ni(PEt3)4 catalysts. The platinum catalyst favored the cyclic double-silylated products and the nickel catalyst produced the linear hydrosilylated ferrocene products (Scheme 9).126

Scheme 9

Diferrocenyl(3,3-dimethylbutyl)silane was unexpectedly formed as a side product from the reaction of ferrocene, t-BuLi and trichlorosilane in the presence of tetrahydrofuran (THF). It was the result of the competitive salt-metathesis reaction of trichlorosilane with FcLi and 3,3-dimethylbutyllithium generated in situ via the carbolithiation reaction of ethene, which was formed by the a-deprotonation and subsequent reverse [3 + 2] cycloaddition of the tetrahydrofuran solvent.127 Vinyl-functionalized polyferrocenyl organosilicon compounds, including triferrocenylvinylsilane, 1,3-divinyl1,1,3,3-tetraferrocenyldisiloxane and 1,3-divinyl-1,3-dimethyl-1,3-diferrocenyldisiloxane, have been synthesized via the low-temperature salt metathesis reaction of monolithioferrocene and the chlorosilanes (CH2]CH)SiCl3 and [(CH2]CH)(Cl) MeSi]2O.128 Almássy et al. have described the retro-Brook rearrangement on O-silylated ferrocenyl alcohols initiated by a Br-Li exchange. The diastereoselective rearrangement of the trimethylsilyl group to the ortho position of the ferrocenyl cyclopentadienyl ring was also observed.129 The synthesis of ferrocenyl-substituted hydrosilanes, their transformation into silicon cations, and their potential as chiral Lewis acid catalysts have been reported by Oestreich and coworkers.130 Tokitoh and coworkers have reported disilene-functionalized ferrocene.131,132 Lang’s group has prepared ferrocenylsiloles such as 26 by reductive cyclization from diethynylsilanes, followed by ferrocenylation using the Negishi C,C cross-coupling protocol.133 Other siloles have been synthesized via 1,2-hydroboration and 1,1-carboboration of alkynyl(ferrocenyl)vinylsilanes by Wrackmeyer et al. (Chart 5).134

Chart 5

Ferrocenes and Other Sandwich Complexes of Iron

13

The synthesis of the first bis(silylenyl)- and bis(germylenyl)-substituted ferrocenes LSi-Fc-SiL (Fc ¼ ferrocendiyl, L ¼ PhC(NtBu)2) and LGe-Fc-GeL, by the reaction of 1,10 -dilithioferrocene with the respective N-donor-stabilized metallene chlorides has been reported. The corresponding CpCo (Cp ¼ Z5-cyclopentadienyl) complexes were probed as catalysts for [2 + 2 + 2] cycloaddition reactions of phenylacetylene and acetonitrile.135 A systematic experimental and theoretical study to understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom has been carried out.136 Ferrocenyl silanes readily available from ferrocene carboxaldehyde in one to two simple chemical steps have shown to rectify the current when incorporated in molecular diode devices, which makes these compounds very attractive for molecular electronics applications.137

7.02.2.4

Nitrogen containing ferrocene derivatives

The broad chemistry of aminoferrocene has been reviewed by Behera et al.138 including synthetic methods,139 reactivity,28,130,131,140–166 and a variety of applications. Amino-functionalized ferrocenes play a key role and are important precursors in many different areas such as in the synthesis of redox-active dendrimers,167,168 NLO active systems,169–172 peptide bioconjugate and prodrugs,173–187 electroactive indicators,188 organometallic based antiparasitic and anticancer drugs,189–191 as photosynthetic model systems,143 etc. Gasser et al. reported an environmentally gentle and cost-effective improved synthesis of aminoferrocene, using CuI/Fe2O3 as cocatalyst, ethanol/water as solvent and aqueous ammonia as the nitrogen source (Scheme 10),139 and purification of iodoferrocene intermediate by the method of Kubiak.192

Scheme 10

A reliable synthesis of 10 -(diphenylphosphino)-1-aminoferrocene, 27, has been developed by Štepnicka et al., with the aim of preparing 10 -(diphenylphosphino)-1-isocyanoferrocene, 28.193 28 combining two specific soft-donor moieties was studied as a ligand for univalent group 11 metal ions. This group has also reported the synthesis of 10 -(diphenylphosphino)-1-cyanoferrocene, 29, and its coordination properties toward group 11 metals (Chart 6).194,195

Chart 6

A series of aminomethyl-substituted ferrocenes and the parent compounds (iminomethyl)ferrocenes, azaferrocenophanes, and diferrocenylamines have been synthesized from reductive amination of 1,10 -diformylferrocene or formylferrocene (Scheme 11). Tertiary (ferrocenylmethyl)amines and azaferrocenophanes are obtained by using NaBH(OAc)3 as a mild reducing agent and (iminomethyl)ferrocenes and secondary (ferrocenylmethyl)amines by using LiAlH4.196 These aminomethyl-substituted ferrocenes are potentially valuable for further ortho-directed functionalization of ferrocene.

14

Ferrocenes and Other Sandwich Complexes of Iron

Scheme 11

An alkylamine linked bis(polymethylatedferrocenyl)complex, N,N-bis((octamethylferrocenyl)methyl)Npropyldiethoxymethylsilylamine, was prepared via nucleophilic attack of 3-aminopropyldiethoxymethylsilane on the BF4 − salt of (octamethylferrocenyl)methyl carbocation (Scheme 12), and its electrochemistry has been investigated.197

Scheme 12

Moiseev and coworkers have reported a-ferrocenylalkylation reactions to access a variety of derivatives198 such as N-(a-ferrocenylalkyl)amines.199 Roberts reported the improved and accelerated synthesis of amino-substituted (Z6-arene)(Z5-cyclopentadienyl) iron (II) complexes using microwave irradiation.200 This technique was also used to introduce oxygen substituents into these complexes. An efficient microwave assisted protocol for condensation reaction of ferrocenecarboxylic acid with carbonyl derivatives to afford ferrocenoyl hydrazones has been developed.201

Ferrocenes and Other Sandwich Complexes of Iron

15

Four-component reactions have been employed in different synthesis. Ferrocenyl amidodiesters and ferrocenyl triamides were obtained from ferrocenecarboxaldehyde, isocyanides, Meldrom’s acid and alcohols or amines in dichloromethane at room temperature.202 Ugi reaction has allowed the preparation of ferrocenyl bis-amides.203,204 The synthesis of novel atropoisomeric ferrocene-containing six-membered cyclic ureas has been reported.205 Cyclic ferrocenylsiloxane-urea, {1,10-ferrocene-diurea-[1,3-bis(propylene)tetramethyldisiloxane]}, has been obtained from the reaction of 1,10-ferrocenediisocyanate with 1,3-bis(aminopropyl)-tetramethyldisiloxane in a chloroform–toluene mixture.206 A simple one-pot route consisting in the grinding of solid cyanomethylferrocene and silica gel has unexpectedly afforded 1-cyanocarbonylferrocene.207 Ferrocene derivatives bearing isoselenocyanato substituents have been reported.208 Saha et al. have synthesized a new ferrocene functionalized Schiff base by refluxing 4-ferrocenyl aniline and 2-hydroxy-1-naphthaldehyde in ethanol. The corresponding copper(II) complex has been successfully used in ppm amounts as catalyst in CuAAC reactions to obtain 1,2,3-triazoles.209 Yuan et al. have accomplished the synthesis of a ferrocene-based pyrrolide-imine by reaction of pyrrole-2-carboxaldehyde with aminoferrocene in the presence of p-toluene sulfonic acid (TsOH) in methanol at 65  C.161 Bisferrocenylimines and ferrocenylacrylonitriles have been obtained by the solvent-free reaction of ferrocenecarboxaldehyde and diaminoalkanes, aromatic amines or phenylacetonitriles.210–212 Some of their rhodium(I) complexes were also prepared.210 The synthesis of a series of novel ferrocene grafted pyrrolidine heterocycles via a one-pot four component ultrasonic assisted [3 + 2]-cycloaddition reaction of azomethine ylides to various ferrocene derived dipolarophiles has been reported.213 The Braga group has reported the solid-solid reactions of 1,10 -di-pyridyl-ferrocene and the solid fumaric, succinic, tridecanedioic, terephthalic, trimesic and thiophene-2,5-dicarboxylic acids to afford a series of hybrid organic-ferrocenyl materials.214 3,4-Ferrocenyl-substituted pyrroles were synthesized from the corresponding dibromo species using either Negishi (using FcZnCl chloride as reagent), or Sonogashira (with FcC^CH) C-C cross-coupling protocols.215 Synthesis of 3-(ferrocenylmethoxy)-2-sulfanyl-1H-pyrroles has been reported.216 Penoni and coworkers have reported an innovative ultrasound multicomponent reactions protocol for the synthesis of new ferrocenyl derivatives, consisting in the domino allylindation and dehydrative alkylation of ferrocenecarboxaldehyde and nucleophiles such as (hetero)aromatics, stabilized enols and azoles.217 Several ferrocene-based triazole and tetrazole ligands have been prepared by standard methods.218 Click chemistry has played a major role in the synthesis of ferrocene-derived triazoles since the versatility of click reactions enables the introduction of a ferrocenyl moiety under very mild conditions. Chandrasekaran et al. have reviewed the synthesis of ferrocenyl triazoles and their applications in various fields.219 Astruc has addressed the importance of using a CuAAC reaction for ferrocene–triazole conjugation. Click reactions between ethynylferrocene and mono-, bis-, and tris-azido aromatic derivatives afforded mono-, bis-, and tris-1,2,3-ferrocenyltriazoles. Coordination to MCl2 in dimethylsulfoxide (M ¼ Pd, Pt) was also studied.220 A click chemistry strategy for the synthesis of a wide range of ferrocenyl-amino acids and other derivatives was reported.221,222 Treatment of chiral alcohols, esters, diols, amines containing azido group with ethynyl ferrocene in acetonitrile with CuI as catalyst and DIPEA as base, at room temperature afforded the corresponding protected ferrocene a-amino acid derivatives in excellent yields (Scheme 13).

Scheme 13

A facile one-pot, four-component click approach that uses 10 -(1-trimethylsilylethynyl)ferroceneboronic acid, alkynes and azides provides access to 1,10 -functionalized ferrocenes.223 Ferrocene–triazole click conjugates have attracted attention in the field of medicinal chemistry.224–236 Metzler-Nolte et al. reported the Cu(I)-catalyzed [3 + 2] cycloaddition of azidoferrocene and 1,10 -diazidoferrocene with alkyne-modified amino acids and peptides to form 1,4-disubstituted 1,2,3-triazole linked bioconjugates.226 The regiospecific copper(I)-catalyzed cycloaddition of propargyl glycoside, azidomethyl and bis-(azidomethyl)ferrocene as well as azidoethyl glycoside and ethynylferrocene led to the synthesis of 1,2,3-triazole-containing glycoconjugates (Scheme 14).237

16

Ferrocenes and Other Sandwich Complexes of Iron

Scheme 14

Astruc and coworkers were the first to functionalize dendrimers with ferrocenyl groups by click chemistry, and examine their ability to recognize, bind, and sense anions and metal cations using the ferrocenyl termini as a redox monitor.220,238–242 Since those seminal studies, multiple examples of ferrocene-derived triazole conjugates have also shown to behave as efficient recognition receptors.243–245

Ferrocenes and Other Sandwich Complexes of Iron

17

A variety of triazole-derived heterocycles and other related nitrogen substituted ferrocenyl derivatives have been synthesized and exploited as binding sites for anion recognition by several research groups, such as those of Beer,246–249 and Tarraga,250–265 and others.266–268 Beer and coworkers have reported a comprehensive study of ferrocene-isophthalamide-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs). The synthesis of amide and (iodo)triazole containing ferrocenes was carried out in a copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) of the bis(iodo)alkyne-appended isophthalamide, obtained from 5-ferrocenylisophthalic acid, with either octyl azide or disulfide-azide.269 In the search of ion colorimetric chemosensors, a synthetic approach to design ferrocene tailed chalcone appended triazole encapped organosilanes from acetylinic ferrocene precursors has been described.270 Ferrocenyl- and biferrocenyl-1,2,3-triazolyl-b-cyclodextrins have been synthesized by the Huisgen-type 1,3-dipolar “click” cycloaddition reaction, between azidomethyl-b-cyclodextrin and ethynylmetallocenes.271 The first annelated bis- and mono-3H-1,3-azaphosphole ferrocene sandwich compounds have been synthesized from aminoferrocenes.272 The reaction of 1,10 -diaminoferrocene with [chloro(trichloromethyl)phosphanyl]dimethylamine, followed by the synthetic rearrangement involving dimethylamine and the trichloromethyl group, and selenide protection of phosphorus atoms affords the phosphanyl(N-ferrocenyl)formamidine, 30 (Scheme 15). Reduction of P(V) to P(III) in 30, cyclization of the resulting compound produced 31.

Scheme 15

López has incorporated ferrocene onto palladacycles using N,S ligands.93,273 A synthetic route to the first representatives of ferrocenyl-substituted benzothiazines and 2-aryl-substituted ferrocenothiazines has been developed starting from directed lithiation/iodination of [(dimethylamino)methyl] ferrocene, further standard functional-group transformations and a final copper-catalyzed cyclization.274 Bis- and tris-ferrocene containing N-methylimide foldamers have been reported.275 Ferrocenylmaleimides 32–34 have been obtained from 2,5-dibromo-N-methyl-1H-pyrrole, bromine shift and oxidation of the pyrrole core with subsequent ferrocenylation using the Negishi C-C cross-coupling protocol.276 Ferrocenyl-substituted 2H-azaphosphirene complexes such as 35 have been the subject of several studies (Chart 7).277–279

Chart 7

18

Ferrocenes and Other Sandwich Complexes of Iron

Ferrocene–histidine conjugates were designed to emulate tetrahedrally coordinated zinc structural sites. The binding of several divalent metal ions to the Fc–peptide conjugates was investigated.280 Siemeling has reviewed the progress made in the area of ferrocene-based singlet carbenes,281 the vast majority of which belong to the family of N-heterocyclic carbenes (NHCs).282 A new class of ferrocene-based chiral NHCs and their coordination to palladium has been described by Labande et al.283 The treatment of nonamethylferrocenylaldiminium salt at 78  C with sodium hexamethyldisilazide allowed for the isolation of the corresponding (amino)(ferrocenyl)carbene as a yellow powder (Scheme 16). Although even in the solid state it is stable for less than 48 h at 20  C, it constitutes the first spectroscopically characterized carbene featuring a ferrocenyl unit directly bonded to the electron-deficient center.284

Scheme 16

Extremely electron poor dicationic 1,10 -bis(diarylmethylium)-substituted ferrocenes [{Z5-C5H4C(C6H4R-4)}2Fe]2+ [R ¼ NMe2; OMe] and [{Z5-C5H4C(C6H3(CH3-2)(OMe4))}2Fe]2+, were obtained by reacting their neutral bis(carbinol) precursors with Brookharts acid H(OEt2) + [B{C6H3(CF3)2-3,5}4]− (Scheme 17).285

Scheme 17

Ferrocenes and Other Sandwich Complexes of Iron

19

The metallacyclic complexes (OC)4M]C(Z2-NHCH2CH]CHX)Fc (X ¼ H, CH2OH) [M ¼ Cr; Mo; W; Fc ¼ ferrocenyl ¼ CpFe(C5H4)] were obtained in good yields upon photo-decarbonylation of the bimetallic allylaminocarbene complexes (OC)5M]C(NHCH2CH]CHX)Fc.286 3,4-Diferrocenyltoluene (7), 1-morpholino- and 1-piperidino-2,3-diferrocenylbicyclo[3.1.0]hex-2-enes, 1-morpholino- and 1-piperidino-7-ferrocenyl-3,4-ferrocenobicyclo[3.2.1]oct-6-enes, 2- and 3-amino(diferrocenyl)-hexa-1,3,5-trienes have been prepared by reactions of amino(diferrocenyl)cyclopropenylium tetrafluoroborates with 1-methylprop-2-enylmagnesium chloride.287 Rhodium(I) ferrocenyl Fischer carbene complexes, [Rh(LL)Cl{C-(XR)Fc}] [LL ¼ cod, (CO)2, (CO, PR3) (R ¼ Ph, Cy or OPh), and (CO, AsPh3); XR ¼ OEt or NHn Pr] were prepared (Scheme 18).288

Scheme 18

Advances in the synthesis and applications of azaferrocenes289–300 have been reviewed by Kowalski.301

7.02.2.5

Phosphorus containing ferrocene derivatives

A new and efficient, scalable synthesis of the two iconic phosphorus synthons ferrocenylphosphine, FcPH2, and dichloroferrocenylphosphine, FcPCl2, has been developed by Kilian et al. in 2016, consisting in the reduction of Fc2P2S4 with LiAlH4, followed by chlorination with triphosgene/phosgene.302 In 2009, Wright and coworkers had synthesized ferrocenylphosphine starting from the monolithiation of ferrocene, followed by reduction of diethyl ferrocenylphosphonate with LiAlH4/Me3SiCl to prepare the first example of an organometallic phosphanediide anion ½FcPŠ3 2 − obtained by the reaction of FcPH2 with nBuLi and As(NMe2)3 in tetramethylethylendiamine.303 Fe(Z5-C5H4PCl2)2 and Fe(Z5-C5H3-1,2-(PCl2)2)(Z5-C5H5) were prepared in a novel straightforward synthetic process from the corresponding phosphonates which were reduced to phosphines and quantitatively converted into dichlorophosphines upon treatment with a toluene phosgene solution.304 Jurkschat and coworkers have prepared and studied different novel phosphonyl-substituted ferrocene derivatives.305–307 A new approach for the synthesis of FcP(O)(OC2H5)2 by electrochemical reduction of a ferrocene and (Me)2C(OH)P(O)(OC2H5)2 mixture at −50  C has been reported.308 The series of mono- and disubstituted ferrocenylphosphinic acids Fc(P(R)(O)(OH))n (n ¼ 1,2; R ¼ H, Me, Et, Ph) were synthesized and it was observed that their conformations depend on the substituents at phosphorus atoms.309 A series of ferrocenyl-substituted thienylphosphines, as well as their use as ligands in Pd-catalyzed Suzuki–Miyaura C,C cross coupling reaction of aryl halides and boronic acids yielding the respective biaryls has been reported.310 Among ferrocenylphosphanes, 1,10 -bis-(diphenylphosphino)ferrocene (dppf ), remains the archetypal ligand in coordination chemistry and catalysis that still plays an essential role in thousands of transition metal-mediated organic transformations.311 It has also been increasingly used in materials science, both as a reagent and as a component.312 The family of functional ligands structurally related to dppf has been widely extended along the time.3,313,314 Multiple articles have been published on the preparation and coordination properties of 10 -functionalized phosphinoferrocene derivatives, with functional groups such as (diphenylphosphino)methyl,315–318 phosphine oxide,317 betaine,319 pyridyl,320–322 ether and thioether,323 N,N-dimethylaminomethyl,324 methoxymethyl,325 ureas,326 etc. The study of the interactions of various silver(I) salts with 10 -(diphenylphosphino)-1-cyanoferrocene shows that the soft phosphine moiety can be regarded as the primary

20

Ferrocenes and Other Sandwich Complexes of Iron

coordination site while the nitrile group and the counteranions have a supportive character, thus increasing the structural diversity of the final compounds.327,328 10 -(Diphenylphosphino)ferrocene-1-phosphonic acid was prepared from the corresponding phosphonate ester and converted into acidic ammonium salts. Coordination with Pd(II) ions was studied.308 Phosphinoferrocene amidosulfonate ligands have been thoroughly investigated by Stepnicka.329–332 The synthesis of triethylammonium 10 -(diphenylphosphino)ferrocene-1-sulfonate (Scheme 19), its coordination toward Pd(II) ions bearing different supporting ligands, and the activity of the prepared complexes in Pd-catalyzed Suzuki–Miyaura-type cross-coupling of acyl chlorides with boronic acids to give ketones has been described.333 The phosphinoferrocene sulfonamide ligand, Ph2PfcNHSO2Me (fc ¼ ferrocene-1,10 -diyl), was prepared by the reaction of Ph2PfcNH2
BH3 and MeSO2Cl followed by deprotection of the Ph2PfcNHSO2Me
BH3 intermediate. The coordination study with Pd(II) was also reported.334

Scheme 19

An excellent review by Stepnicka on the coordination and catalytic chemistry of phosphinoferrocene carboxamides was published in 2017.335 The multifaceted chemistry of phosphinoamides had also been overviewed before.336 1,10 -Bis(dichlorophosphino)ferrocene [Fe(C5H4PCl2)2] has been widely used as precursor of ferrocenyldiphosphine derivatives such as a the thioether-functionalized ferrocenylbis(phosphonite) Fe(C5H4PR)2 (R ¼ dOC10H6d(m-S)C10H6Od),337 the eugenol derived ferrocenylbis(phosphonite), [Fe{C5H4P-(OC6H3(OMe-o)(C3H5-p))2}2]338 and 1,10 -bis(dipyrrolylphosphino)ferrocene.339 The use of dppf and organometallic p-tweezer molecules for the synthesis of heteromultimetallic ferrocene-based compounds featuring between five and eight different transition metals bridged by carbon-rich p-conjugated units such as 36 has been reported. The synthesis strategies make use of homogeneous C,C cross-coupling reactions, metathesis along with classical synthetic methods. In addition, the number of different redox-active transition metals within the heteromultimetallic compounds make these species interesting for electrochemical investigations (Chart 8).340

Chart 8

Highly functionalized polysubstituted ferrocenes have been accounted by Hierso et al. focusing their attention in phosphine derivatives.341,342 Research on the synthesis and applications of (P,P,P,P)-,343 (P,P,P)-,343 (P,P,P0 )-,344 (P,P)-,345,346 (P,P,N,N)-,347 (P,N)-348,349 ferrocene compounds as well as ambiphilic ferrocenes combining phosphino-borane derivatives350,351 has been widely undertaken. An excellent review by Pietschnig and Dey on the chemistry of bulky bisphosphane ligands related to the parent dppf has been published.314 Two bulky mesityl substituted dppf-analogs Fe(C5H4PMes2)2 and Fe(C5H4PMes2)(C5H4PPh2)

Ferrocenes and Other Sandwich Complexes of Iron

21

(Mes ¼ 2,4,6-Me3C6H2, Ph ¼ C6H5) have been synthesized and the corresponding Cu(I) complexes have been used as catalysts for CO2-fixation reaction with terminal alkynes.352 There are countless examples of ferrocene units bonded to organic fragments containing donor atoms such as O, N, S or P that have been incorporated in ligands for transition metal complexes. Butler has described in a short review the research carried out by his group on the design and develop of simple synthetic routes to ferrocene derivatives of general interest.353 A review by Diaconescu examines the chemistry of metal complexes with two types of ferrocene-based chelating ligands: (1) Schiff base which supports yttrium and cerium alkoxides and (2) 1,10 -ferrocenylene diamide ligands used for scandium, yttrium, lutetium, and lanthanum alkyls.354 A ferrocene-based chelating bis(phosphinoamide) ligand has been coordinated to scandium and after reaction with dihydrogen afforded a dinuclear scandium hydride species 37, wherein the scandium centers are bridged by both hydride and phosphinoamide fragments (Chart 9).355

Chart 9

Liu and Yin have reported the synthesis of a diiron propanedithiolate complex, 38,356 and Hogarth and coworkers have shown later that it can be regarded a biomimetic of the diiron hydrogenase as can catalyze both the reduction of protons and H2 oxidation.357 Related complexes such as [m-(SCH2)2NCH2CH2OH]Fe2(CO)4(m-dppf ), and [m-(SCH2)2NCH2CH2SAc] Fe2(CO)4(m-dppf ),358 [(m-SeCH2)2CHC6H5]Fe2(CO)4(m-dppf ) and [(m-SeCH2)2CHC6H5]Fe2(CO)4(m-dppp) have also been reported (Chart 10).359

Chart 10

The reaction of Fe2(CO)9 and Co2(CO)8 with PCl2-functionalized ferrocenes resulted in the formation of ferrocenyl-functionalized clusters with a wide variety of structures, such as nido phosphinidene clusters Fe3(CO)9(m3-PFc)2 and Co4(CO)10(m4-PFc)2,360 or a Fe2(CO)2(m2-PFc)2 butterfly entity.304 [10 -(Diphenylthiophosphanyl)-ferrocen-1-yl]methanol and [2-(diphenylthiophosphanyl)-ferrocen-1-yl]methanol, have been converted in one step into the 1,10 - and 1,2-ferrocenediyl-linked thiophosphane/N-R-imidazolium salts (R ¼ Me; 2,4,6-Me3C6H2 or Mes). After desulfurization of the phosphane group, the ligands reacted with a Rh(I) precursor, in the presence of tBuOK, to give the corresponding cationic complexes.361

22

Ferrocenes and Other Sandwich Complexes of Iron

Khrizanforov and coworkers have reported the synthesis and structural and electrochemical properties of a new triferrocenyltetratiophosphate as well as comparison with trivalent phosphorus derivative and triferrocenyltrithiophosphate, Fc3S3P, Fc3S3P]O and Fc3S3P]S.362 It is worth to mention that of the heteroferrocenes, the phosphaferrocenes have been most widely studied.363–376 Scheer and coworkers reported on the first synthesis and the comprehensive characterization of the so far missing parent pentaphosphaferrocene [CpFe(Z5-P5)]. It was obtained by the thermolysis of [CpFe(CO)2]2 with P4 using diisopropylbenzene as solvent. Its reactivity toward CuI halides was studied.377 The first coordination polymers based on 1,3-diphosphaferrocenes and 1,10 ,2,30 ,4-pentaphosphaferrocenes have been prepared.378

7.02.2.6

Ferrocenophanes

Ferrocenophane chemistry is still a growing area mainly due to the interesting reactivity of these species and their use as precursors to organometallic polymers. Particularly interesting research in the field of strained [n]ferrocenophanes has been overviewed.379–382 [n]Ferrocenophanes are also of interest as structural supports for low-valent main group species, with examples incorporating carbenes,383–387 silylenes,388,389 germylenes390,391 and stannylenes,390 as well as [2]ferrocenophanes containing trivalent diphosphine units.392 Diaminocarbene[3]ferrocenophane 40 and related bimetallic complexes such as 41 have been obtained (Scheme 20).383

Scheme 20

Silicon-bridged [n]ferrocenophanes are widely employed as precursors to poly(ferrocenylsilane)s393–395 which attract much attention as a result of their interesting redox, preceramic, and etch-resistant characteristics and other physical properties.396–398 Interesting reactivity at the silicon bridge in sila[1]ferrocenophanes has been described.399 In an attempt to synthesize a silylium ion through the hydride abstraction of the Si-H moiety of 42 using trityl tetrakis(pentafluorophenyl)borate the dinuclear species 43 was obtained (Scheme 21). The formation of 43 has been proposed to involve abstraction of hydride from the silicon bridge in 42 with subsequent CdH bond cleavage of a cyclopentadienyl group by the resulting electrophilic transient silyl cation intermediate.

Scheme 21

Ferrocenes and Other Sandwich Complexes of Iron

23

Ferrocenophanes bridged with a range of elements other than silicon, such as carbon,400–404 germanium,405 tin,406 nitrogen,407 phosphorus,392,408–413 sulfur,414 boron,415,416 aluminum and gallium,417–422 indium,423,424 zinc,425 lanthanides426,427 group 10 metals,428,429 have been developed. A mononuclear ferrocenophane bearing two carbodiimide units as bridges has been reported.430 Fluorinated ferrocenophanes have been obtained by redox-autocatalytic reaction from bulky-alkyl-substituted bis(trifluorovinyl)ferrocenes.431 Synthetic access to triphospha[3]ferrocenophanes432 and other related phosphorus-rich [3]ferrocenophanes with PPP433 or NPN,434,435 PBP436 and PSiP437 bridge has been reported. The selective syntheses of 1,5-dioxo-3-ferrocenyl[5]ferrocenophanes in high yields have been achieved by the Claisen–Schmidt reaction between 1,10 -diacetylferrocene and ferrocenecarboxaldehyde or other non-enolizable aldehydes under microwave irradiation within 30 min.438 A series of phenoxy[4]ferrocenophanedienes resulted from the unexpected transannular addition of different phenols to 1,10 -dialkynylferrocenes in the presence as well as in the absence of hexacarbonylmolybdenum.439 An efficient synthetic method for the preparation of carbon-rich trinuclear octamethylferrocenophanes has been developed. Spontaneous cyclization reaction of 1,10 -bis(1-ferrocenylvinyl)-octamethylferrocene affords various [3]- and [4]ferrocenophanes under different reaction conditions.440 The barium-catalyzed dehydrocoupling of diaminoferrocene with Ph2SiH2 or Ph(Rc)SiH2 (Rc ¼ (C5H4)Ru(C5H5)) resulted in the formation of the silazane-bridged ansa-[3]ferrocenophanes 44, while the reaction of ferrocenylhydrosilanes and different amines afforded the corresponding ferrocene-containing polycarbosilazanes (Chart 11).441

Chart 11

Planar chiral [3]ferrocenophane aminosulfane and aminophosphane ligands with a methylene group inserted between the Cp ring and the donor atom were prepared and evaluated in Pd-catalyzed allylic alkylation reactions. The phosphane derivative was found to be an efficient catalyst for this reaction, whereas the corresponding aminosulfane derivative was inactive.442 A synthetic strategy consisting in the quantitative lithiation of a new dibromoferrocene and further salt-metathesis reactions gave enantiomerically pure [1]ferrocenophanes with gallium, silicon, tin, and boron in bridging positions (Scheme 22). In these strained sandwich compounds, formally the bulkiness was transferred from the ligand at the bridging element onto the ferrocene moiety.443

Scheme 22

A large number of chiral ferrocenophanes have been synthesized, and used as ligands in a variety of catalyzed reactions.444–449 Chiral 2-phospha[3]ferrocenophanes have been prepared and used as organocatalysts in [3 + 2] cyclization reactions.450 Enantiopure phospha[1]ferrocenophanes,451 azasubstituted [5]ferrocenophanes,452 chiral bora[1]ferrocenophanes,453 and planar chiral phosphoramidites with a [3]ferrocenophane structure454 have been reported. 1,10 -Disubstituted ferrocenes containing two alkynylcarbonyl groups have been used to synthesize different ferrocenophanes. Some examples of ferrocenophanes as ion sensors: tin-containing crown ether substituted ferrocenophanes as redox-active hosts for the ditopic complexation of lithium chloride455; mononuclear ferrocenophanes with two thiourea arms as a dual binding site for anions and cations456; selective sensing of zinc- and lithium by [2.2]ferrocenophanes457; 1,3-diaza [3]ferrocenophane scaffold as molecular probe for anions458; selective recognition of anions, cations, and amino acids by [3.3]ferrocenophanes with guanidine

24

Ferrocenes and Other Sandwich Complexes of Iron

bridging units459; nitrogen-rich multinuclear ferrocenophanes as multichannel chemosensor molecules for transition and heavy-metal cations.460 Ferrocenophanes have also found applications in medicinal chemistry.461–465

7.02.2.7

Chiral ferrocenes

Much effort has been focused in the development of new approaches to chiral ferrocene derivatives. Asymmetric catalysis lies among the plethora of successful applications of ferrocenyl ligands and several books and chapters3,466–471 as well as many useful reviews on this field have been issued.281,348,472–480 In 2013, Lang and Schaarschmidt published an excellent comprehensive review article on the selective syntheses of planar chiral ferrocenes.473 In 2020, Guiry et al. detailed recent advances in the application of ferrocenyl mono-, bi- and tridentate-ligands and ferrocene-derived organocatalysts in asymmetric catalysis, also illustrating the novel methodologies for the synthesis of ferrocenyl compounds.474 Novel methods to synthesize planar chiral ferrocenes via asymmetric CdH bond functionalization have been accounted by You et al.54 Such a highly active and fast-moving field of research cannot be comprehensively reviewed here. Some results are provided as illustrative examples. 1,2-Disubstituted planar-chiral ferrocene derivatives have been accessible through iron-catalyzed ortho-alkylation and -arylation of ferrocenes bearing ortho-directing groups.9,76 Ortho-condensed aromatic ferrocenes have been obtained via Au-catalyzed cycloisomerization of ortho-alkynylaryl ferrocenes.481 Chiral 2-aryl-ferroceneamides have been prepared by the Catellani reaction, using a palladium-catalyzed and norbornene-mediated methodology, from chiral 2-iodo-N,N-diisopropylferrocencarboxamide, iodoarenes, and alkenes.482 Moyano and Rios have reported the synthesis of b-amino-b-ferrocenyl alcohols, a-ferrocenyl epoxides, interannularly cyclopalladated ferrocenyloxazolines, and a-amino-a-ferrocenyl acids.483 Planar chiral ferrocene carboxamides have also been reported.484,485 Planar chiral ferrocene pyrrolinones have been prepared through Pd(0)-catalyzed syn-carbopalladation/asymmetric C-H alkenylation reaction of N-ferrocenyl propiolamides with aryl iodides.486 The asymmetric click reaction has been exploited to induce planar chirality, giving monotriazole-substituted alkynylferrocenes in high enantiomeric excesses.487 Ferrocene-derived triazoles have shown to play an important role in the design and development of efficient ligands for transition metal catalysts to achieve good enantioselectivity. Carmen Carreño et al. have reported the synthesis of enantiopure helical ferrocene–triazole–quinone triads by CuAAC reaction of azido (or azidomethylene) ferrocenes and an enantiopure (P)-[5]-helicenequinone having a terminal alkyne. Using the corresponding 1,10 -diazido (or diazido methylene) ferrocenes, two triazolyl helicenequinones could be incorporated into the ferrocene. An open chain dimeric structure was obtained using CuI/CH3CN and CuSO4/sodium ascorbate/THF conditions afforded a 1,4-diaza-[4]-ferrocenophane resulting from an intramolecular oxidative coupling of the two triazole units.488 Fukuzawa and coworkers have used the click chemistry methodology to synthesize different ClickFerrophos489–492 and ThioClickFerrophos,493–496 chiral triazole-based ferrocenyl phosphine ligands such as those in Scheme 23, and reported their application as catalysts in asymmetric hydrogenation, allylic alkylation reactions, and others.497–499 This same group has also reported the synthesis of planar-chiral ferrocene-based triazolylidene copper500 and palladium501 complexes, with catalytic activity and enantioselectivity for the asymmetric borylation of methyl cinnamate with bis(pinacolato)diboron, and Suzuki–Miyaura cross-coupling, respectively.

Scheme 23

Chiral ferrocenylphosphanes are among the most effective ligands in transition metal complexes employed in asymmetric catalysis, often with high enantioselectivity. A planar-chiral ferrocene-fused phosphole was synthesized by an enantioselective transformation starting with Kagan’s chiral ferrocenyl acetal.502 A simple approach to ferrocene-based P-chiral phosphine ligands opened access to several new families of ferrocene-based phosphine ligands coupling chirality at phosphorus with other, more standard stereogenic features.503 A variety of planar-chiral phosphino alkenylferrocenes have been prepared.504–506 Planar-chiral

Ferrocenes and Other Sandwich Complexes of Iron

25

(diphenylphosphino)[2-(E)-phenylvinyl]ferrocene has been applied in the presence of [Pd2(dba)3] (dba ¼ dibenzylideneacetone) as a ligand in Suzuki–Miyaura C-C coupling reactions for the synthesis of sterically congested biaryls.507 Asymmetric bidentate phosphine–amine and diphosphine ferrocenyl ligands have been employed to synthesize group 11 metal complexes. The study of the interactions of various silver(I) salts with 10 -(diphenylphosphino)-1-cyanoferrocene shows that the soft phosphine moiety can be regarded as the primary coordination site while the nitrile group and the counteranions have a supportive character, thus increasing the structural diversity of the final compounds.508 Ortho-condensed aromatic ferrocenes have been obtained via Au-catalyzed cycloisomerization of ortho-alkynylaryl ferrocenes.509 A new class of chiral ferrocenyl diphosphine ligands with an imidazole ring, ImiFerroPhos, has been developed through a five-step transformation from acylferrocenes, and applied in the Rh-catalyzed asymmetric hydrogenation of various 3-aryl-substituted 2-phosphonomethylpropenoates510 and of b-substituted a,b-unsaturated phosphonates.511 The first examples of planar chiral ferrocenyl phosphane–benzimidazol-2-ylidene ligands and their coordination chemistry with palladium(II) have been reported.512 In a comparative study, the chiral azinylferrocenes 45 and 47 have been obtained by two synthetic pathways: the Negishi reaction and a shorter methodology consisting in the direct C-H functionalization of aromatics by the C-C coupling of halogen-free (hetero)arenes with lithium ferrocenes bearing stereogenic C and S atoms. A synthetic route to the chiral P,N-ligands 46 and 48 by using a straightforward noncatalytic functionalization of the C(sp2)dH-bond in (hetero)arenes (the SNH methodology) is also reported (Chart 12).513

Chart 12

Hey-Hawkins et al. described an efficient way to obtain diastereomerically pure chiral secondary ferrocenylphosphane derivatives derived from Ugi’s amine.514 Ortho lithiation of Ugi’s amine, further reaction with PCl3 and final treatment with Grignard or organolithium reagents have afforded a series of chiral ferrocene-based phosphines with a wide variety of substituents at the phosphorus atom(s).515 Blaser et al. have published a nice article dedicated to Antonio Togni highlighting the impact and significance of Josiphos and related chiral ferrocene based ligands.516 Highly enantioenriched and enantiopure planar chiral 2-phosphino-1-aminoferrocenes where both heteroatoms are directly attached to a ferrocene Cp ring have been prepared. Their Ir(COD)BArF complexes showed good reactivity as catalysts in promoting asymmetric hydrogenation of several prochiral alkenes, with high enantioselectivities.517 Recent developments and applications of ferrocene oxazoline ligands in asymmetric catalysis are described by Guiry et al. in a comprehensive review.518 The synthesis of novel gem-disubstituted ferrocene oxazoline N,O ligands and their application in asymmetric ethyl- and phenylzinc additions to aldehydes have been reported.519 Ferrocenyloxazoline-derived planar chiral palladacycles have been studied.520 Ferrocene phosphinooxazoline ligand is one of the major contributors in the category of P,N-ligands, with a large number of applications such as in the Pd-catalyzed intermolecular asymmetric Heck reaction521 or the Ir-catalyzed asymmetric hydrogenation of ketones.522 Ferrocene bis(oxazoline-phosphine) ligands with different planar chiralities have been obtained by a temperature-controlled switchable o,o0 -dilithiation of chiral bisoxazoline ferrocene procedure.523 In 2009, Wanbin et al. made a review on the development of planar chiral diarylphosphino-oxazoline ligands and their applications to enantioselective catalysis.524 Reactions of phosphinoferrocene oxazolines, such as rac-1-[4,5-dihydro-4,4-dimethyl-2-oxazolyl]-2-(diphenylphosphino)ferrocene, with group 11 metal ions have been described.525 Chiral ferrocene/indole-based diphosphine ligands have been used in Rh-catalyzed asymmetric hydrogenation of functionalized olefins.526 Cu-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides has been achieved with a chiral ferrocene-derived P,N-ligand.527 A synthetic route to ferrocenyl-substituted heterophosphorinanes has been developed.528 Novel planar chiral ferrocene nucleophilic catalysts containing both central and planar chiral elements were designed and synthesized for catalytic enantioselective acyl transfer of secondary alcohols.529 A phosphanyl–ferrocenecarboxylic compound combining planar and central chirality and some derivatives modified at both the phosphanyl and carboxyl groups have been synthesized and used as ligands in rhodium and palladium complexes and as catalyst components in enantioselective rhodium-catalyzed hydrogenation and palladium-catalyzed allylic alkylation reactions.530 Through the use of N,S ligands, ferrocene has been incorporated onto palladacycles.93,273 Lang et al. reported for the first time on a stereoselective Fries rearrangement to establish planar chirality at 1,2-P,O ferrocenes using chiral pool alcohols.531 Chiral P,O ferrocenyl ligands532 and catalytic application to asymmetric Suzuki–Miyaura coupling have been reported.533

26

Ferrocenes and Other Sandwich Complexes of Iron

Chiral ferrocenyl P,S ligands have been used, for example, for Ir-catalyzed hydrogenation of olefins,534 for Cu-catalyzed asymmetric [3 + 2] cycloaddition of azomethine ylides535 and for diastereo-/enantioselective catalytic [3 + 2] cycloaddition of azomethine ylides with cyclic and acyclic and enones.536 An Ag-Fesulfos complex has been used as catalyst in the enantioselective synthesis of 3-silylproline derivatives.537 A series of palladium and platinum complexes with chiral ferrocenyl phosphine-thioether ligands have been synthesized.538,539 Ferrocenylthiophosphine–sulfoxide and phosphine–sulfoxide derivatives540 possessing planar chirality for the ferrocene moiety and central chirality at the sulfur atom have been reported.541 a-Ferrocenyl carbocations obtained by acidic treatment of planar-chiral ferrocenylmethanols can be sulfurized by the thiophosphinyl group by an intermolecular process, resulting in thio ethers.542 The reaction of the enantiopure, planar-chiral a-ferrocenyl carbocation (Sp)-2-(P(]S) Ph2)FcCH+2 (Fc ¼ Fe(Z5-C5H5)(Z5-C5H3)) toward electron-rich arenes has been reported.543 Advances in planar-chiral ferrocene-based N-heterocyclic carbene ligands have been reviewed by Yoshida and Yasue.544,545 Planar chiral cyclic (amino)(ferrocenyl)carbenes have shown of utility in a Cu-catalyzed enantioselective b-boration of an a,b-unsaturated ester.546 These same authors have changed the Cp to the Cp group and confirmed the influence of the steric effect on the enantioselectivity in an Ir-catalyzed asymmetric transfer hydrogenation of cyclic N-sulfonylimine where the modified cyclic (amino)(ferrocenyl)carbenes were used as chiral ligands.547 A planar chiral five-membered cyclic (amino)(ferrocenylene)carbene ligand and its Ir dicarbonyl complex was also developed.548 New palladium(II) complexes bearing planar chiral ferrocenyl phosphine-NHC ligands have been used as catalysts for the asymmetric Suzuki–Miyaura reaction of aryl bromides with arylboronic acids.549 Scheidt et al. have provided a successful strategy for the synthesis of chiral NHC ligands that fuse together a ferrocene framework with an imidazolium-like core. The azolium salts and subsequent NHCs allow for simple modification either through altering the Cp ring or through late-stage diversification of the N-aryl wingtip. These NHCs were employed in various enantioselective transformations to survey their use as organocatalysts as well as ligands for transition-metal-catalyzed processes.550 Ferrocene phosphane-carbene ligands have also been used in Cu-catalyzed enantioselective 1,4-additions of Grignard reagents to a,b-unsaturated carbonyl compounds551 and in Pd-catalyzed borylation of aryl bromides.552

7.02.3

Macromolecules: Polymers and dendrimers

Recent developments in different fields concerning iron sandwich polymers and dendrimers have been the subject of multiple treatises. Examples are the book by Wang and Yu553 and that edited by Abd-El-Aziz and Manners.554 Some other valuable reviews are the following: on polyferrocenylsilanes,396 polymers with pendant ferrocenes,555 metalloblock copolymers,556 side-chain metallocene-containing polymers by living and controlled polymerizations,557 multiferrocenyl metallacycles and metallacages,558 unsaturated main-chain metallopolymers prepared by olefin metathesis polymerization,559 ferrocene-based heterocycles,560 five-membered heterocycles with directly-bonded sandwich and half-sandwich termini as multi-redox systems,561 iron-containing organometallic polymers by ROMP,562 ferrocene-containing polymers by click chemistry,563 synthetic methodologies and applications,564 main- or side-chain ferrocene-based polymers,565 ferrocene-based derivatives and polymers with azobenzene,566,567 main-chain ferrocene-containing polymers from acyclic diene metathesis polymerization,568 recent trends in metallopolymer design,569 ferrocene- and phosphorus-containing polymers,570 redox-switchable ring-opening polymerization,571 ferrocene-containing coordination polymers,572 metathesis-derived polymers with transition metals in the side chain,573 organometallic polymers assembled from cation–p interactions,574 the controllable synthesis of ferrocene-based polymers,575 electrostatic assembly with poly(ferrocenylsilanes),576 poly(ferrocenylsilane-block-methacrylate)s,577 azinyl derivatives of ferrocene and cymantrene,578 ferrocene-based polythiophenes,579 ferrocene-based epoxy derivatives,580 water-soluble metallocene-containing polymers,581 sandwich complex-containing macromolecules,582 1,10 -bis(diphenylphosphino)ferrocene in functional molecular materials,312 ferrocene-containing hydrogels,583 ferrocene-functionalized hyperbranched polyphenylenes,584 poly(alkyl/ arylphosphazenes),585 ferrocene-functionalized polymer brushes,586 dendrimers designed for functions,587 ferrocene-containing polymers and dendrimers,588 ferrocenyl-terminated dendrimers,240 dendrimers derived from 1 ! 3 branching motifs,589 poly(phosphorhydrazone) metallodendrimers,590 phosphorus-containing dendrimers,591–596 organometallic silicon-containing dendrimers,597 click dendrimers,598,599 mixed-valent metallodendrimers,600 molecular recognition in metallocene-containing macromolecules,241,244,601–605 dendrimers as chemical sensors,606 dendritic molecular electrochromic batteries.607 Some examples on the work of Vancso and coworkers on poly(ferrocenylsilane)s are the following: redox responsive gels, hydrogels and other materials,608–613 electrostatic assembly,614 applications in patterning and lithography,398,615 electrochemical sensing,616,617 optical properties of multilayer thin films.618 Selected publications to illustrate the work of Manners on polyferrocenes are the following: macrocycles and cyclic polymers by ring opening of silicon-bridged [1]ferrocenophanes,619,620 water-soluble polyferrocenylsilane polyelectrolytes by living photolytic ROP of amino [1]ferrocenophanes,621 polyferrocenylsilanes with pendant ruthenocenyl groups,622 thermal ROP of a silicon-bridged [1]ferrocenophane in one dimensional nanochannels,623 metalation of the cyclopentadienyl ligands of poly(ferrocenyldimethylsilane) by reaction with the Schlosser’s base,624 poly(ferrocenylsilane-b-polyphosphazene),625 C60-containing polyferrocenylsilanes,626 self-assembly of fluorescent micelles from a triblock copolymer containing poly(ferrocenyldimethylsilane) and poly(2-vinylpyridine) moieties,627 poly(ferrocenylenevinylene) via ring-opening metathesis polymerization,628 photocontrolled ROP of strained dicarba[2]ferrocenophanes,629 polyferrocenylphosphine homopolymers and block copolymers via photocontrolled living anionic polymerization of phosphorus-bridged [l]ferrocenophanes,408 metallopolysilylethers,630 reactivity studies of dicarba[2]ferrocenophanes and their corresponding ring-opened oligomers and polymers.401 Hyperbranched ferrocene-containing poly(boro)carbosilanes631 and linear-hyperbranched block copolymers based on linear poly(ferrocenylsilane)s and hyperbranched poly(carbosilane)s632 have been synthesized. The electrocatalytic properties of hyperbranched poly(carbosilanes) functionalized with interacting ferrocenyl units have been reported.633,634 A two-step approach for the synthesis of ferrocenyl-functionalized long chain branched polydienes has also been presented.635 Bioelectrocatalytical properties of block-copolymers containing interacting ferrocenyl units have been studied.636

Ferrocenes and Other Sandwich Complexes of Iron

27

The effect of enhanced segmental mixing on various physical properties of ferrocenylsilane tethered polybutadiene based polyurethane was addressed.637 Pannell and coworkers have used hydrosilylation/polymerization reactions for the synthesis of ferrocenylene copolymers containing carbosilylene, carbosiloxy and cyclopentadienyliron dicarbonyl bridges121 and ferrocenylenesilylene polymers.638 Polyvinylferrocene-based diblock copolymers with poly(methyl methacrylate) and poly(2-vinylpyridine) have been obtained using sequential monomer addition protocols, of 1,1-dimethylsilacyclobutane as the activating species and 1,1-diphenylethylene as the end-capping agent.639 A similar protocol has been followed to obtain diblock copolymers with vinylferrocene, ferrocenylmethyl methacrylate, and [1]dimethylsilaferrocenophane.640 Hou et al. have reported a series of polymeric ferrocenyl complexes.641–645 They have used a porous bilayered open coordination zinc ferrocenyl sulfonate polymer as a metal ion adsorbent for the removal of toxic metal ions.646 A series of ferrocene oligo- and polyamides with phenylene, cyclohexyl, or lysyl spacer groups have been synthesized.647 Ferrocenyl-containing silicone rubbers have been prepared via platinum-catalyzed Si-H self-cross-linking.648 In a recent feature article, Astruc and coworkers discuss multiple applications of polymers containing electron-reservoir metal-sandwich complexes, disclosed in their group, focusing on iron derivatives.649 The concept of electron reservoir materials extended to metallodendrimers and metallopolymers is addressed.650,651 The living ring-opening metathesis-polymerization synthesis of a series of side chain amidoferrocenyl containing polynorbornene homopolymers and block copolymers and their redox sensing behavior have been reported.652 Astruc and coworkers have prepared the first polymers containing the electron reservoir complex [Fe(Z5-C5H5)(Z6-C6Me6)][PF6] by living ROMP of norbornene derivatives using Grubb’s ruthenium benzylidene catalyst of third generation as shown in Scheme 24.653 Three related living diblock copolymers each containing two distinct redox-stable iron sandwich complexes of the ferrocene, pentamethylferrocene and cyclopentadienyl-iron-arene families in the side chain have also been reported (Scheme 25). The synthesis of mixed-valent FeIIFeIII polymers was accessible through the selective and reversible chemical oxidation of one of the

Scheme 24

Scheme 25

28

Ferrocenes and Other Sandwich Complexes of Iron

blocks.654 ROMP synthesis have also been employed to obtain diblock metallopolymer polyelectrolytes containing two different cationic sandwich complexes, [Co(Z5-C5H4-)(Z5-C5H5)][PF6] and [Fe(Z5-C5H4-)(Z6-arene)][PF6],655 triblock metallocopolymers containing side-chain iron and cobalt sandwich complexes,656,657 pentamethylferrocene polymers, whose redox activity for noble-metal nanoparticle formation has been investigated,658 the first- and second-generation Percec-type dendronized ferrocenyl norbornene polymers659 and a highly-branched amphiphilic organometallic dendronized diblock copolymer.660 Two copolymers synthesized by “click” polycondensation of bis(ethynyl)biferrocene and di(azido) poly(ethylene glycol) show multifunctional properties as polyelectrolytes and electrochromes, redox sensors for transition-metal cations, electrode modifiers, and templates for the synthesis of Pd and Au nanoparticle catalysts.661 Astruc et al. have developed dendrimer synthetic pathways that involve cross olefin metathesis662 and the CuAAC click reaction,598 among others. They have described an efficient catalytic complex, [Cu(hexabenzyltren)]Br, [tren ¼ tris(2-aminoethyl)amine] as well as other analogous Cu-centered dendritic catalysts for the synthesis of relatively large dendrimers, which circumvent the use of stoichiometric amounts of Sharpless CuI catalyst, generally needed for the preparation of relatively large dendrimers. This group has used click chemistry by either introducing the azido groups at the periphery and the ethynyl group at the focal point of incoming dendrons238 or the opposite242 to decorate dendrimers with a large number of sandwich complexes.599,663–665 They have also described applications including molecular recognition,239,242,666,667 and the stabilization of transition metal nanoparticles (Chart 13).668–673

Chart 13

This same group has reported the synthesis, by Sonogashira coupling and homocoupling conditions, of bis-, tris- and tetra-ethynylbiferrocenes linked by rigid ethynylaryl and diynyl spacers. The oxidized complexes have also been examined.674 Ferrocenyl and pentamethylferrocenyl termini have decorated giant dendrimers, lengthening the tethers to overcome the bulk constraint at the dendrimers periphery,675 dendritic and ion-pairing effects in oxo-anion recognition have been observed.676 A procedure for preparing robust ferrocenyl dendrimer interfaces and probing of their transport properties has been described.677 Ferrocenyl dendrimers with ionic tethers and dendrons have been reported.678 Rigid ferrocenyl dendrimers have been prepared from 1,3,5-tribromo- and triiodobenzene through Sonogashira and Negishi reactions with 1,2,3,4,5-pentamethyl10 -ethylnylferrocene and ferricinium and class-II mixed-valence dendrimers have been isolated.679 Dentromers were introduced as a family of super dendrimers constructed according to successive divergent 1 ! 3 branching.680

Ferrocenes and Other Sandwich Complexes of Iron

29

Casado and coworkers have developed a synthetic strategy aimed at obtaining heteroditopic dendritic macromolecules. They synthesized a family of poly(propyleneamine) dendrimers decorated with amidoferrocenyl aza-crown ethers, which simultaneously complex cationic and anionic guest species.681 An electropolymerizable amido ferrocenyl pyrrole-functionalized dendrimer was prepared. Electrodes modified with electropolymerized ferrocenyl dendrimer films have been studied as anion sensors in aqueous solution.682 Poly(propyleneimine) dendrimers with ferrocenylamidoalkyl terminal groups683 or biferrocenes684 have also been applied for the electrochemical recognition of anions. The synthesis of a new family of octamethylferrocenyl-functionalized dendrimers,685 their electrocatalytical properties686 and their successful application in the construction of bienzyme sensors687 has been reported. Hydrosilylation reactions of two octasilsesquioxane dendritic cores containing terminal vinyl groups with bis(ferrocenyl)methylsilane afforded octasilsesquioxane dendrimers decorated with interacting ferrocenyl units, such as 50.688 This dendrimer 50 has been employed in the preparation of a nanostructured electrode surface that has shown to be of utility in the determination of tryptophan.689 Carbosilane based dendritic cores functionalized with interacting ferrocenyl units 51 and 52690 have been analyzed as multioperational oxidase biosensors.691 Heterometallic dendrimers have been readily synthesized by reacting carbosilane dendrimers containing ferrocenylalkynyl moieties with the activated cluster Os3(CO)10(NCMe)2.692 Covalently cross-linked ferrocenyl polyamidoamine-organosilicon dendrimer networks have been prepared by the sol-gel approach (Charts 14 and 15).693

Chart 14

30

Ferrocenes and Other Sandwich Complexes of Iron

Chart 15

Amide dendrons with focal ferrocenyl moiety self-organize in organic and aqueous media.694 Two synthetic routes were developed to prepare ferrocenyl amphiphilic Janus dendrimers and the redox-controlled self-assembly behavior in aqueous solution was investigated.695 The synthesis of two families of water-soluble poly(phosphorhydrazone) dendrimers incorporating ferrocene(s) in their internal structure was reported.696 A dendritic ferrocenylphosphane ligand was used to develop well-defined controllable catalysts with distinct redox states.697 The preparation and catalytic activity of dendritic ferrocenyl phosphane ligands with different spacers between ferrocene and phosphane and the corresponding ruthenium complexes was reported.698 Phosphorus dendrimers decorated with chiral ferrocenyl phosphine-thioether ligands have been proved for the palladium-catalyzed asymmetric allylic substitution reaction.596 A series of dendrimers containing Frechet- and Newkome-type blocks and an aminoferrocene moiety bonded to a central triazine core have been studied.167 The syntheses of ferrocene-aspartate dendrimers was reported.699

7.02.4

Bioorganometallic chemistry of ferrocene derivatives

It is unquestionable the great interest that ferrocenyl derivatives are attracting in the field of medicinal chemistry and chemical biology, with reviews on the subject receiving a large amount of citations.700–714 A comprehensive approach to the research in this area is clearly out of the scope of this work. Some recent feature articles and other works on different topics are highlighted. Gu and coworkers have summarized in a review article, recent progress in drug delivery systems with supramolecular ferrocenyl-containing polymers based on the host–guest interactions of Fc/b-cyclodextrin and pillar[6]arene.715 A ferrocene-modified phospholipid has been used as a drug delivery vessel able to transport chemotherapeutic agents to target cancer cells and redox-trigger their release.716 A mini review highlighted synthetic approaches and the possible biomedical and materials applications of ferrocenyl chalcones.717 Ferrocenyl-nucleobase complexes may have important applications in biology, pharmacy and material sciences. Their syntheses, chemistry and applications have been reviewed.718 The various applications of ferrocene-based peptides/amides for the period 2006–2010, in different fields including medicine, bioorganometallic and biological chemistry, were examined.719 A review article on the developments, between 2014 and 2018, on the synthesis and evaluation of ferrocene-containing bio-active pharmacophores with emphasis on their structure–activity relationship and mechanism of action was published.720 In very recent overview, de la Mata and coworkers have discussed the latest results on metallodendrimers as tools in cancer treatment, drug delivery, gene therapy, medical imaging and against infectious diseases, as antibacterial and antiviral agents.721 In 2013, Braga and Silva published a review focused on medicinal ferrocenes with cytotoxic activity, classified into functionalized ferrocenes, heterobimetallic complexes with ferrocene as an ancillary ligand and supramolecular structures that act as ferrocene delivery systems.706 A review covers articles published between 2010 and 2020 on ferrocene-containing hybrids as promising drug candidates for cancer therapy.722 The key role of ferrocene and its derivatives in cancer research is unarguable.224,461,723–729 N-alkylaminoferrocene-based reactive oxygen species are the focal point in several cancer approaches, such as chimeric antigen receptor T cell (CART) therapy, which is currently one of the most promising treatment approaches in cancer immunotherapy.730 Endoplasmic reticulum-targeted

Ferrocenes and Other Sandwich Complexes of Iron

31

conjugates of cholic acid with N-alkylaminoferrocene-based prodrugs, activated only in cancer cells in the presence of elevated reactive oxygen species amounts, have been developed. Their anticancer efficacy, and cancer cell specificity has been confirmed.731 Mokhir and coworkers demonstrated that N-ferrocenyl-N-(1-alkyl-1H-1,2,3-triazol-4-yl)aminocarbonyloxymethylboronic acid pinacol ester is a mitochondrial carrier moiety, the carrier function of which is turned on under cancer specific conditions. They prepared a conjugate of a prodrug based on an N-alkylaminoferrocene structure with a clinically approved drug carboplatin and confirmed that its accumulation in mitochondria was higher than that of the free drug, which was reflected in the substantially higher anticancer effect of the conjugate.732 A mitochondria-targeted, photo-responsive polymer, which can self-assemble into nanoparticles encapsulated with diphenylcyclopropenone (light-responsive CO prodrugs) and aminoferrocene-based prodrugs via hydrophobic interactions have been evaluated as new anticancer drugs.733 Ferrocene also plays a role in the development of polymeric conjugates and co-conjugates.734 An overview of the in vitro and in vivo studies performed with ferrocifen-loaded lipid nanocapsules on several multidrug resistance cancers has just been provided.735 Some examples on ferrocifen type anticancer drugs702: as precursors to quinone methides,736 aminoferrocene-based prodrugs against prostate cancer,737 against breastcancer.738–742 A review article summarizes the most potent ferrocene derivatives for infectious diseases, classified according to their targets into four main groups: antiparasitic, antibacterial, antifungal and antiviral agents.743 Ferroquine is the most successful antimalarial drug candidate and its derivatives have also shown promising potential as antimalarials of clinical interest. Therefore, much research is being dedicated to the development of ferroquine derivatives as safe alternatives to antimalarial chemotherapy.744–748 Some examples include hydroxyferroquines such as 53,749 aminoferroquines such as 54,749 trioxaferroquines, chloroquine-bridged ferrocenophanes, thiosemicarbazone derivatives, ferrocene dual conjugates, 4-N-substituted derivatives,748 ferrocene-quinoline conjugates (55 and 56),750 etc. (Chart 16).

Chart 16

A rapidly emerging field in the bioorganometallic chemistry of ferrocene is that of electrochemical biosensors. Some reviews241,603,605,751–754 and articles are included as examples. A brief description of synthesis, analytical performance and glucose sensing application of ferrocene-based dendrimers, polythiophenes, polypyrroles, polyethylenimine, chitosan and carbon nanotubes has been given.754 A large number of ferrocene derivatives have been used for sensing numerous species such as glucose,687,691,755–760 dopamine,761 urea,762–764 acetaminophen,765 NADH,766 lipopolysaccharide.767 Anzai has presented an overview of recent progress made in the development of electrochemical biosensors based on phenylboronic acid and its derivatives which can detect glycoproteins, such as glycated hemoglobin (HbA1c), avidin, and serum albumin.760,768 A ratiometric electrochemical immunosensor with Au NPs modified SiO2-Fc-COOH complex as matrix and UiO-66 loaded with Toluidine blue as a marker for quantitative detection of procalcitonin was developed, which also provides a useful method for clinical detection of other disease markers.769 Ferrocene has been used as high effective quencher in a solid-state RuðbpyÞ3 2 + electrochemiluminescence biosensor for the detection of miRNA.770 A dual-potential ferrocene–peptide nucleic acids DNA biosensor was designed by using two electrochemically distinguishable recognition elements with different molecular information at a single electrode.771 A ferrocene-antimicrobial peptide conjugate based biosensor has been used for the detection of E. coli O157:H7.772 Gold

32

Ferrocenes and Other Sandwich Complexes of Iron

nanoparticles-based multifunctional nanoconjugates for highly sensitive and enzyme-free detection of E. coli K12.773 A new molecule, Hemin-aminoferrocene has been designed and synthesized to build an electrochemical biosensor for simultaneous detection and ratiometric quantification of O2 and pH, which has been applied in the brain upon ischemia, as well as in tumor during cancer therapy.774 Synthetic methodologies to ferrocene-peptide conjugates and different sensing applications, such as protease activity,775 have been outlined.225,776,777 Several examples of hydrogel-functionalized immunosensing platforms containing ferrocenes have been gathered by Kandasubramanian et al.778 In the search of new therapeutic and diagnostic tools organometallic amino acids and peptides containing ferrocene or organotin (IV) derivatives play an important role. Different strategies of synthesis are addressed.779

7.02.5

Other applications

A glance to a few of the multiple applications of ferrocene with selected articles as examples is given below. Among the valuable redox properties of ferrocenes7,780 is included the stability of the corresponding oxidized species. Astruc and coworkers reported the fast biphasic reaction between ferrocene in diethyl ether solution and HAuCl4 in water, to afford air-stable ferricinium chloride protected AuNPs.781 In a very recent paper, Walawalkar et al.782 highlighted the recent discoveries of ferrocenium dications and ferrocenate anions and the effect of the substituents on the cyclopentadienyl ring, as well as the choice of redox agents and solvent system in the preparation of synthetic targets.783–786 The redox-switching abilities of ferrocenyl-imidazolylidene ligands and complexes for gold-based catalysis,787 and for the transfer hydrogenation of ketones and imines788 have been reported. A tetraferrocenyl-resorcinarene cavitand has shown as a redox-switchable host of ammonium salts.789 Nanofibers of a ferrocene–tryptophan conjugate are responsive toward oxidation/reduction and show thermo and redox reversibility.790 The advantageous electrochemistry of ferrocene renders polyferrocenyl derivatives a key class of redox materials with extraordinary electronic properties. A large number of references are devoted to electron transfer between bridged ferrocene units or between ferrocene and other redox centers and to applications of such switchable systems. A series of hexa(ferrocenylethynyl) benzenes and other related ferrocenyl-terminated redox stars prepared using the Negishi reaction show an electrochemistry strongly dependent on the nature of the supporting electrolyte791 and substituents on the ferrocenyl group.792,793 The nature of the electronic communication between the ferrocene centers in di- and tetraferrocenylpyrroles and other electron-rich heterocycles has been extensively examined by Lang and coworkers.89,561,794–798 Beer et al. have recently reported an excellent review on electrochemical anion sensing including multiple examples of ferrocene-based derivatives.244 A comprehensive review of the years 2010–2020 on cation recognition with ferrocenyl derivatives has been published.245 Some other references in this field.250–265,430,452,456–460,601–603,605,667,684,799–831 Applications of Ferrocenes/AuNP hybrids in luminescence, electrochemical, and electro-optical sensing are described and their uses and potential in imaging, photodynamic therapy, nonlinear optics, and catalysis are assessed.22 Some other illustrative examples of ferrocene/nanoparticle hybrid materials with different applications are the following832: triazolylbiferrocenyl polymers for network encapsulation of Au and AgNPs and anion sensing,833 arene- and AuNPs-cored triazolylbiferrocene-terminated dendrimers,668 a heterobifunctional ferrocenyl dendrimer in the formation of AuNPs,666 biferrocenium polymer-nanosnakes that encapsulate AuNPs,834 catalytically efficient dendrimer-stabilized PdNPs.242,835 A versatile method for the fabrication of poly(ferrocenylsilane) based microspheres using microfluidics has been reported by Vancso et al.836 They have also reported on a simple and efficient method for the synthesis of stable and redox responsive AuNPs using poly(ferrocenylsilane) polyelectrolytes in aqueous solutions of HAuCl4.837 In 2017 Clays et al. presented a comprehensive compilation on the NLO behavior of chromophores based on ferrocene.838 A review article focused on the syntheses and potential applications of ferrocene-appended porphyrin architectures has been published.839 A review focused on the electrochemical properties of fullerene complexes of transition metals has appeared in 2021.840 Other ferrocene/fullerene hybrids,841–846 ferrocene/porphyrin-fullerene hybrids847–853 and ferrocene-functionalized carbon nanotubes nanohybrids have also attracted much attention in different fields.183,854–867 Astruc et al. have reported a review article focused on the fabrication of various kinds of ferrocene-containing hydrogels and described their gelling mechanisms, characteristic structures and properties, as well as functional applications.583 Other examples of ferrocene-containing hydrogels can illustrate this topic.553,583,608,868–872 A ferrocene–phenylalanine–histidine–OMe conjugate has been synthesized and tested for supramolecular gelation.873 A very recent review on ferrocene-contained MOFs including the role of ferrocene, synthesis methods, applications, challenges and potential research directions has been published.874 In the field of ferrocene-containing liquid crystals, various reviews,570,875–877 a few examples of triazolylferrocene derivatives878–880 and other illustrative articles are provided.881–886 Ferrocene-based materials have gained much attention as burning rate catalysts.887–906 An account article has compiled advances in redox-active ferrocene-terminated monolayers covalently bound to Si-H surfaces, including their charge storage capabilities.907 Ferrocenes have been used in molecular rectifiers and diodes.908,909 A strategy for the preparation of poly(ferrocenylsilane) immobilized on the surface of cross-linked polystyrene nanoparticles has been developed. The resulting architectures are excellent candidates as preceramic polymers yielding spherical ceramic materials.910 A side-chain ferrocene-containing radiation-sensitive hybrid photoresist material and its application in nanopatterning using extreme ultraviolet lithography has been reported.911

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Organomet. Chem. 2014, 755, 16–32. Amin, B. U.; Yu, H.; Wang, L.; Fahad, S.; Nazir, A.; Haq, F.; Mahmood, S.; Uddin, M. A.; Shen, D.; Liang, R. J. Inorg. Organomet. Polym. Mater. 2021, 31, 2511–2520. Usman, M.; Wang, L.; Yu, H.; Haq, F.; Liang, R.; Ullah, R. S.; Khan, A.; Nazir, A.; Elshaarani, T.; Naveed, K.-R. Inorg. Chim. Acta 2019, 495. Zhou, W.; Wang, L.; Yu, H.; Tong, R.; Chen, Q.; Wang, J.; Yang, X.; Zain-ul-Abdin, ; Saleem, M. Appl. Organomet. Chem. 2016, 30, 796–805. Zhou, W.; Wang, L.; Yu, H.; Zain-ul-Abdin, ; Yang, X.; Chen, Q.; Wang, J. RSC Adv. 2016, 6, 53679–53687. Zain-ul-Abdin, ; Yu, H.; Wang, L.; Saleem, M.; Khalid, H.; Abbasi, N. M.; Akram, M. Appl. Organomet. Chem. 2014, 28, 567–575. Deng, Z.; Yu, H.; Wang, L.; Zhai, X.; Chen, Y.; Sun, R. J. Organomet. Chem. 2015, 799–800, 273–280. Rahimpour, K.; Teimuri-Mofrad, R.; Abbasi, H.; Parchehbaf, M.; Abedinpour, S.; Soleimani, S. Polym. Technol. Mater. 2019, 58, 2056–2065. Ul Amin, B.; Yu, H.; Wang, L.; Nazir, A.; Fahad, S.; Haq, F.; Mahmood, S.; Liang, R.; Uddin, M. A.; Lin, T. J. Organomet. Chem. 2020, 921. Usman, M.; Yu, H.; Wang, L.; Zhizhko, P. A.; Lemenovskiy, D. A.; Zarubin, D. N.; Khan, A.; Naveed, K.-R.; Nazir, A.; Fahad, S. J. Organomet. Chem. 2020, 920, 121336. Usman, M.; Yu, H.; Wang, L.; Qian, J.; Li, X.; Khan, A.; Naveed, K.-R.; Nazir, A.; Elshaarani, T.; Fahad, S. J. Organomet. Chem. 2020, 923, 121412. Ul Amin, B.; Yu, H.; Wang, L.; Nazir, A.; Fahad, S.; Haq, F.; Mahmood, S.; Uddin, M. A.; Elshaarani, T.; Shen, D. Z. Anorg. Allg. Chem. 2020, 646, 1671–1678. Zain-ul-Abdin, ; Wang, L.; Yu, H.; Saleem, M.; Abbasi, N. M.; Khan, R. U.; Ullah, R. S.; Haroon, M. RSC Adv. 2016, 6, 97469–97481. Fabre, B. Acc. Chem. Res. 2010, 43, 1509–1518. Welker, M. E. Molecules 2018, 23, 1551–1559. Herrera, M. U.; Ichii, T.; Murase, K.; Sugimura, H. ChemElectroChem 2015, 2, 68–72. Elbert, J.; Didzoleit, H.; Fasel, C.; Ionescu, E.; Riedel, R.; Stuehn, B.; Gallei, M. Macromol. Rapid Commun. 2015, 36, 597–603. Satyanarayana, V. S. V.; Singh, V.; Kalyani, V.; Pradeep, C. P.; Sharma, S.; Ghosh, S.; Gonsalves, K. E. RSC Adv. 2014, 4, 59817–59820.

7.03

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Katharina Münster and Marc D Walter, Technische Universität Braunschweig, Institut für Anorganische und Analytische Chemie, Braunschweig, Germany © 2022 Elsevier Ltd. All rights reserved.

7.03.1 7.03.2 7.03.2.1 7.03.2.2 7.03.2.3 7.03.3 7.03.3.1 7.03.3.2 7.03.4 7.03.4.1 7.03.4.1.1 7.03.4.1.2 7.03.4.1.3 7.03.4.1.4 7.03.4.2 7.03.4.2.1 7.03.4.2.2 7.03.4.2.3 7.03.4.2.4 7.03.4.2.5 7.03.4.3 7.03.4.3.1 7.03.4.3.2 7.03.4.4 7.03.4.5 7.03.5 7.03.5.1 7.03.5.1.1 7.03.5.1.2 7.03.5.2 7.03.5.3 7.03.6 7.03.6.1 7.03.6.1.1 7.03.6.1.2 7.03.6.1.3 7.03.6.1.4 7.03.6.1.5 7.03.6.2 7.03.7 7.03.7.1 7.03.7.2 7.03.8 7.03.9 7.03.10 References

Introduction and historical perspectives Monocyclopentadienyl compounds bearing classic p-acceptor ligands L Carbonyl complexes (L¼CO) Isonitrile complexes (L¼RNC) Dinitrogen complexes (L=N2) Monocyclopentadienyl compounds bearing halide and pseudohalide ligands Halides Pseudohalides Monocyclopentadienyl compounds bearing group 15 donor ligands Monocyclopentadienyl compounds bearing N ligands Amines Amides Imido and Imidazolin-2-iminato Nitrido Monocyclopentadienyl compounds bearing P ligands Phosphines Phosphides, iminophosphoranes Phosphinidenes P ligands derived from P4 activation Pentaphosphaferrocene and its reaction chemistry Monocyclopentadienyl compounds bearing As ligands As ligands derived from As4 activation Pentaarsaferrocene and its reaction chemistry Monocyclopentadienyl compounds bearing Sb ligands Monocyclopentadienyl compounds bearing Bi ligands Monocyclopentadienyl compounds bearing group 16 donor ligands Monocyclopentadienyl compounds bearing O-ligands Hydroxido/alkoxido/phenoxido Oxido Monocyclopentadienyl compounds bearing S-ligands Monocyclopentadienyl compounds bearing Se/Te-ligands Monocyclopentadienyl compounds bearing group 14 donor ligands Monocyclopentadienyl compounds bearing C-ligands FedC single bonds N-heterocyclic carbene (NHC) adducts Fe]C double bonds (Alkylidene) Fe^C triple bonds (Alkylidyne, Carbyne) p-complexes with C-based ligands Monocyclopentadienyl compounds bearing the heavier homologues (E]Si, Ge, Sn, Pb) Monocyclopentadienyl compounds bearing group 13 donor ligands Monocyclopentadienyl compounds bearing boron-based ligands Monocyclopentadienyl compounds bearing Al—, Ga—, In-based ligands Monocyclopentadienyl iron hydride compounds Heterobimetallic complexes featuring iron-metal bonds Conclusion

49 49 49 53 53 55 55 58 59 59 59 59 60 61 63 63 64 64 66 73 80 80 82 85 86 90 90 90 92 92 104 106 106 106 115 126 134 134 148 153 153 164 169 171 178 178

Nomenclature AC acac Ag[FAl]

46

Alternating current Acetylacetonate Ag[FAl{OC6F10(C6F5)}3]

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00116-5

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

AIM Ar Atm [BArF4] [BArCl 4 ] 9-BBN Bcat BDE bdt Bn BNA Bpin t Bu CASSCF Cp Cp0 Me Cp tms Cp 2tms Cp Cp 4iPr Cp 5iPr Cp 2tBu Cp 3tBu Cp BIG Cp Bn Cp CV Cy 2,2,2-crypt dba DBU DFT dme/DME Dipp DIP2pyr dmap/DMAP DIB DMF dmp dmpe dmso/DMSO dppe cis-dppen dppm dtby EC EI-MS en EPR ESI-MS Et equiv. exc. FLP FeMoCo FT-IR h HERFD-XANES HMDS HMPA HOMO IDipp

Atoms-in-molecules Aryl Atmosphere Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [{3,5-(CF3)2C6H3}4B]− Tetrakis[3,5-bis(trichloromethyl)phenyl]borate, [{3,5-(CCl3)2C6H3}4B]− 9-Borabicyclo[3.3.1]nonane Catecholboryl Bond dissociation energy Benzene-1,2-dithiolate Benzyl 1-Benzyl-3-carbamoylpyridinium Pinacolboryl tert-Butyl Complete active space self-consistent field
5-Cyclopentadienyl, (
5-C5H5)
5-Cyclopentadienyl and cyclopentadienyl derivatives
5-Methyl-cyclopentadienyl, (
5-C5H4Me)
5-Trimethylsilyl-cyclopentadienyl, (
5-C5H4SiMe3)
5-1,3-Bis(trimethylsilyl)cyclopentadienyl, (
5-C5H4(SiMe3)2)
5-Pentamethylcyclopentadienyl, (
5-C5Me5)
5-Tetra-iso-propyl-cyclopentadienyl, (
5-C5HiPr4)
5-Penta-iso-propyl-cyclopentadienyl, (
5-Ci5Pr5)
5-1,3-Di-tert-butyl-cyclopentadienyl, (
5-C5Ht3Bu2)
5-1,2,4-Tri-tert-butyl-cyclopentadienyl, (
5-C5Ht2Bu3)
5-Pentakis(4-n-butylphenyl)cyclopentadienyl, (
5-C5(C6Hn4Bu)5)
5-Pentakis(benzyl)cyclopentadienyl, (
5-C5(CH2C6H5)5) Cyclic voltammetry Cyclohexyl [2.2.2]-Cryptand Dibenzylideneacetone 1,8-Diazabicyclo[5.4.0]undec-7-ene Density functional theory Dimethoxyethane 2,6-Di-iso-propylphenyl 2,5-Bis{N-(2,6-di-iso-propylphenyl)iminomethyl}pyrrolyl N,N-Dimethylaminopyridine 1,3-Di-iso-propylbenzene N,N-Dimethylformamide 2,6-Dimesitylphenyl Bis(dimethylphosphino)ethan Dimethylsulfoxide 1,2-Bis(diphenylphosphino)ethane cis-Bis(diphenylphosphino)ethylene Bis(diphenylphosphino)methan 4,4´-tBu2-2,2´-Bipyridine Electron transfer inducing an irreversible chemical reaction Electron ionization mass spectrometry Ethylenediamine Electron paramagnetic resonance Electron spray ionization mass spectrometry Ethyl Equivalent Excess Frustrated Lewis pair Fe-dMo-dcofactor Fourier-transform infrared spectroscopy Planck constant High energy resolution fluorescence detected XANES Bis(trimethylsilyl)amine, bis(trimethylsilyl)amide Hexamethylphosphoramide Highest occupied molecular orbital 1,3-Bis(2,6-di-iso-propylphenyl)-imidazolin-2-ylidene

47

48

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

IiPr2Me2 IMe IMe2Me2 IMes IR l LED LDA LUMO MA Me Mes Mes MMA Mn Mw/Mn NacNacDipp NBO NHC NImdipp NIR NIS NMR n pADC PAMAM PCET p-cym Pdl0 pDOS PEGMA PES Ph [PPN]Cl i Pr py/Py RAFT polymerization rt SiMes SQUID SOMO SMM t0 tpdt thd thf/THF TIP tmeda TMPH Tol tpdt Tsi Ueff VE VtC-XES VT NMR WBI XANES Xyl

1,3-Di-iso-propyl-4,5-dimethylimidazolin-2-ylidene 1,3-Dimethylimidazolin-2-ylidene 1,3,4,5-Tetramethylimidazolin-2-ylidene 1,3-Bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene Infrared Wavelength Light emitting diode Lithium di-iso-propylamide Lowest unoccupied molecular orbital Methacrylate Methyl 2,4,6-Trimethylphenyl (mesityl) 2,4,6-Tri-tert-butylphenyl Methyl methacrylate Molecular weight Molecular weight distribution HC[C(Me)N(C6H3-2,6-iPr2)]2 Natural bond orbital N-Heterocyclic carbene 1,3-Bis(2,6-di-iso-propylphenyl)imidazolin-2-iminato Near infrared Nuclear inelastic scattering Nuclear magnetic resonance Wavenumber Protic acyclic diaminocarbene Poly(amidoamine) Proton-coupled electron transfer p-Cymene
5-Dimethylnopadienyl or
5-2,4-Di-tert-butylpentadienyl Partial density of vibrational states Poly(ethylene glycol)-functionalized methacrylate Potential energy surface Phenyl Bis(triphenylphosphine)iminium chloride iso-Propyl Pyridine, pyridyl Reversible-addition-fragmentation chain-transfer polymerization Room temperature 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene Superconducting quantum interference device Singly occupied molecular orbital Single-molecule magnet Relaxation time 3-Thiapentane-1,5-dithiolate Tetramethylheptanedionate Tetrahydrofuran 1,3,5-Tri-iso-propylbenzene N,N,N0 ,N0 -Tetramethylethane-1,2-diamine 2,2,6,6-Tetramethylpiperidine Toluene, tolyl 3-Thiapentane-1,5-dithiolate C(SiMe3)3 Effective spin-reversal barrier Valence electron Valence-to-core X-ray emission spectroscopy Variable temperature NMR Wiberg bond index X-ray absorption near-edge structure spectroscopy Xylene, xylyl

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

7.03.1

49

Introduction and historical perspectives

Traditionally diamagnetic, closed-shell iron(II) species of the type [(
5[HYPHEN]Cp0 )FeL2X] (Cp0 ¼ cyclopentadienyl and substituted derivatives) or iron(I) carbonyl species such as [{(
5[HYPHEN]Cp0 )Fe(CO)(m-CO)}2] have dominated the field of half-sandwich iron compounds. More recently, however, this original focus on half-sandwich complexes obeying the 18 VE rule has slowly shifted to now include open-shell species. These new players in the field besides the more traditional representatives have now found application in small molecule activation, catalysis, and material science (e.g., as magnetic materials). This development is closely tied to the emerging field of catalytic applications that utilize non-precious 3d transitions metals as environmentally benign, less toxic, and inexpensive alternatives to the more frequently employed 4d and 5d elements. Now confronted with open-shell species, however, physical and spectroscopic techniques must be applied to fully characterize these molecules and to establish the structure-function relationship. Furthermore, the growing popularity of computational chemistry methods such as density functional theory (DFT) has further fostered this development and provides valuable insights into the bonding and inherent properties of these species. From a more fundamental point of view the {(
5-Cp0 )FeL2} fragment is exceptionally well-suited to support low-coordinate main-group element fragments such as boryls and borylenes. In addition, iron-transition metal bonds including those to 4f elements have successfully been realized. The following chapter is organized according to ligand classes and attempts to highlight the most recent developments in the field and to place them into the context of prior pioneering studies.

7.03.2

Monocyclopentadienyl compounds bearing classic p-acceptor ligands L

7.03.2.1

Carbonyl complexes (L¼CO)

Although complexes of the type [{(
5-Cp0 )Fe(CO)(m-CO)}2], [(
5-Cp0 )Fe(CO)2]−, [(
5-Cp0 )Fe(CO)2X] and [(
5-Cp0 )Fe (CO)2(thf )]+ (Cp0 ¼C5H5 and substituted derivatives) are long-known and their synthesis and molecular structures are well-established, the interest in these species has reemerged in recent years spurred by their applications as suitable pre-catalysts in a broad range of catalytic applications. Some of these developments are summarized below, and some of these molecules will also re-appear in different subchapters. Transition-metal-mediated living radical polymerization plays a central role in the construction of well-defined, functional polymers, in which the metal catalyst controls not only the molecular weight and its distribution, along with the polymerization rate, but also tolerates different monomers and reaction conditions.1 Several monocyclopentadienyl iron compounds of the type [Cp Fe(CO)(Lphos)Br] (Lphos¼PPh3, PMePh2, PMe2Ph, P(m-tol)3, and P(p-tol)3) were successfully screened for the utility in living radical polymerizations.2 In contrast to [CpFe(CO)(Lphos)Br], living radical polymerization of methyl methacrylate (MMA) with the initiator [Hd(MMA)2dBr] catalyzed by [Cp Fe(CO)(Lphos)Br] proceeds more controlled which allows for a broader array of molecular weights (Mn ¼ 104 −105), and narrower molecular weight distributions (Mw/Mn  1.2). Based on FT-IR studies the active catalyst in these polymerization reactions represents the 16 VE species [Cp Fe(Lphos)Br], which is formed after photochemically induced CO release from the 18 VE pre-catalyst [Cp Fe(CO)(Lphos)Br]. This more active catalyst can also be employed for the polymerization of a broader range of monomers including methacrylate (MA) and poly(ethylene glycol)-functionalized methacrylate (PEGMA). Moreover, the commercially available starting material [{CpFe(CO)(m-CO)}2] (1) has a broad absorption spectrum ranging from UV to NIR light and serves under NIR radiation as a halide abstractor towards organic halides forming either organic radicals or cations.3 Both radicals and cations can participate in either radical or cationic RAFT polymerizations to yield well-defined poly(vinyl ether)s and polyacrylates. Further applications of [{(
5-C5R5)Fe(CO)(m-CO)}2] (R¼H (1), Me (2)) include the conversion of isocyanates to the corresponding carbodiimides,4 and light-driven catalytic processes such as hydrophosphination, amine-borane dehydrocoupling and siloxane synthesis (Scheme 1).5

Scheme 1 Selected catalytic applications of [{CpFe(CO)(m-CO)}2] (1) after photochemical activation.5

To obtain a better understanding of the hydrophoshination reaction, 1 was irradiated at l > 500 nm in the presence of a stoichiometric amount of Ph2PH to form a mixture of [CpFe(CO)2H] (3), [CpFe(CO)2PPh2] (4), and [CpFe(CO)(PPh2)(H)] (5) (Scheme 2). Whereas the former products [CpFe(CO)2H] (3) and [CpFe(CO)2PPh2] (4) can be readily explained by direct activation (homolytic bond cleavage) of Ph2PH via two {CpFe(CO)2}• radicals, the formation of [CpFe(CO)(PPh2)(H)] (5) requires the reaction of [CpFe(CO)2H] (3) with Ph2PH.

50

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 2 Photochemically induced reactivity of [{CpFe(CO)(m-CO)}2] (1) with Ph2PH.5

In a related problem, much work was devoted by Manners and co-workers to unravel the mechanistic details of iron-catalyzed dehydrocoupling/dehydrogenation of N,N-dimethylamine-borane by several iron compounds.6 For this purpose, the authors probed the reactivity of 1 and [CpFe(CO)2I] (6a) towards Me2NH-BH3 under UV photoirradiation to afford cyclodiborazane [Me2NdBH2]2. Monitoring this reaction by 11B NMR spectroscopy suggests that different mechanisms are followed depending on the identity of the pre-catalyst. For complex 1 dehydrocoupling of Me2NHdBH3 proceeds via the aminoborane Me2N]BH2 which cyclodimerizes without metal involvement to [Me2NdBH2]2. This contrasts the reactivity of [CpFe(CO)2I] (6a) which yields Me2NHdBH2dNMe2dBH3 as the key intermediate before [Me2NdBH2]2 is formed in a metal-assisted pathway. Photochemical exchange of one CO ligand in 1 against a MeCN ligand yields [CpFe(CO)2Fe(CO)(NCMe)Cp] which is an even more active dehydrocoupling catalyst, obviating photoactivation. It reacts similarly to 1; and the catalytic mechanism involving the pre-catalyst [Cp(CO)2FeFe(CO)(NCMe)Cp] was investigated in great detail, and shows that the formation of Me2N]BH2 proceeds heterogeneously on the surface of iron nanoparticles (10 nm). Under photoirradiation the related pre-catalyst 1 might also form catalytically active iron nanoparticles. This, however, sets both Fe(I) systems apart from the Fe(II) pre-catalyst [CpFe(CO)2I] (6a) for which the authors assume a homogeneous catalytic process. A plausible mechanism, that is also supported by DFT computations, is presented in Scheme 3. Furthermore, it also rationalizes the formation of intermediate Me2NHdBH2dNMe2dBH3 during catalysis.

Scheme 3 Mechanistic proposal for the photochemically driven [CpFe(CO)2I] (6a)-catalyzed dehydrocoupling/dehydrogenation of aminoboranes.6

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

51

The combination of [(
5-C5R5)Fe(CO)2Cl] (R¼H, Me) and 1,3-butadiene acts as effective catalyst in cross-coupling reactions of alkyl halides with alkyl Grignard reagents at ambient temperature.7 In addition, unactivated alkyl bromides and fluorides are tolerated. Nevertheless, also the long-known carbonyl metallate [CpFe(CO)2]− can be successfully applied in photochemical Heck benzylation of styrenes.8 Furthermore, cationic derivatives have found application in CdH bond functionalizations.9 For example, the [Cp Fe (CO)2(thf )][BF4] (7) acts in the presence of the base 2,2,6,6-tetramethylpiperidine (TMPH) as a catalyst in the functionalization of propargylic and allylic CdH bonds. In combination with aryl aldehydes and other carbonyl electrophiles a series of unsaturated alcohol coupling products can be accessed under mild reaction conditions and with a broad functional group tolerance. The proposed mechanism is shown in Scheme 4. The intermediacy of a catalytically competent allenyliron intermediate (8) was successfully verified by its independent synthesis.

Scheme 4 Proposed mechanism for the catalytic functionalization of propargylic CH bonds with aldehydes and other carbonyl electrophiles using [Cp Fe(CO)2(thf )][BF4] (7) as the pre-catalyst. The allenyliron species 8 acts as key intermediate in this transformation.9

Based on a transition-metal frustrated Lewis acid/base methodology the cyclopentadienyl iron(II) tricarbonyl 9 was effectively isolated, characterized and applied in hydrogenation reactions.10 Compound 9 constitutes the first complex based on an earth-abundant metal catalyzing the chemoselective reductive alkylation of various functionalized amines using functionalized aldehydes (Scheme 5). Importantly these reactions may also be performed under mild conditions (ambient temperature and moderate H2 pressure (5–20 bar)). Key intermediates are the cationic non-classic Fe(II)
2-H2 complex 10 as well the neutral Fe(II) monohydride 11. In a related study, Beller and co-workers employed the so-called “Knölker iron complex” for the selective catalytic reduction of aromatic, aliphatic, and a,b-unsaturated aldehydes under water-gas shift conditions.11

52 Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 5 Synthesis of 9 and its application in the chemoselective reductive alkylation of amines using aldehydes.10

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

7.03.2.2

53

Isonitrile complexes (L¼RNC)

Thiel and co-workers prepared a neutral (12) and a series of cationic iron isonitrile (13a-c) compounds in excellent yields (Scheme 6).12 Cationic p-substituted phenyl isonitrile derivatives 13a-c are accessible from 7 by THF replacement with an isonitrile ligand. Coordination of the isonitrile ligand to the Fe atom shifts the NC-stretching vibration to higher energies relative to that of the free isonitriles. Interestingly, no reliable correlation between the experimentally observed shifts in the NC-stretching vibration and the Hammett constant sp was established, instead much better agreements were found when the Hammett constant sp was associated to computed parameters such as reaction enthalpies, FedC distances, charges, and CO-stretching frequencies. Isonitrile species such as 13 may then be used for the synthesis of acyclic diaminocarbene ligands (see Scheme 95).

Scheme 6 Synthesis of cationic isonitrile adducts [CpFe(CO)2(L)]+ (L¼ isonitrile, 13a-c).12

7.03.2.3

Dinitrogen complexes (L=N2)

Besides classic p-acceptor ligands such as CO and isonitriles, also the N2 can bind to coordinatively unsaturated monocyclopentadienyl iron species, which weakens the inert NN bond. The degree of “activation” can then be inferred from IR spectroscopic data evaluating the NdN stretching frequency as well as the NdN bond distance as determined by X-ray diffraction analysis.13 Reduction of the 16 VE, high-spin complex [3tBuCpFe(IiPr2Me2)I] (14; IiPr2Me2 ¼ 1,3-di-iso-propyl-4,5-dimethylimidazolin-2ylidene) under an Ar atmosphere yields the 15 VE complex [3tBuCpFe(IiPr2Me2)] (15), which reversibly binds N2 to form complex 16.14 Compound 16 may also be obtained by reduction of 14 under an N2 atmosphere (Scheme 7). Upon N2 coordination to 15, a spin-state change from S ¼ 3/2 to S ¼ 1/2 occurs, which manifests itself in a contraction of the CpdFe distance. The N2 ligand in this species is only moderately activated judging from IR spectroscopical (nNN ¼ 1979 cm−1) and X-ray diffraction measurements (NdN distance: 1.131 A˚ ). DFT calculations indicate that the high- and low-spin states of 15 are energetically close, suggesting that N2 coordination occurs by a spin-induced reaction barrier. Using SQUID magnetometry, the barriers for N2 coordination and elimination were determined.

Scheme 7 Reversible N2-binding upon reduction of Fe high-spin Fe complex 14.14

Another example comprises the iron hydrido N2 complex 18, which can be either obtained from the allyl precursor 17 by stepwise addition of H2 and N2, or is formed in C6D6 solution of the silyl complex 19 in the presence of N2 (Scheme 8).15 The N2 ligand in 18 coordinates almost linearly and is weakly activated as indicated by the NdN bond length of 1.11 A˚ and the NdN stretch of nNN ¼ 2054 cm−1.

54

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 8 Stepwise addition of H2 and N2 to allyl complex 17.15

Catalytically active FedN2 compounds are depicted in Scheme 9. Complex 21 reversibly forms from the CH-activated NHC complex 20 under an N2 atmosphere.16 When dissolved in organic solvents under an Ar atmosphere, N2 is released to yield 20, which is active in the CH bond activation of heteroarenes.16 Similarly, the bridged N2 complex 23 is isolated from a salt-metathesis reaction between [Cp Fe(NCMe)3]PF6 (22) and a thiolate under an N2 atmosphere (Scheme 9).17 It serves as an efficient catalyst in the hydroboration of N-heteroarenes with pinacolborane (see Scheme 69).17,18 The NdN bond in both complexes 21 and 23, however, is only weakly activated (complex 21:16 NdN 1.132 A˚ , n NN ¼ 2126 cm−1; complex 23:17 NdN 1.130 A˚ , nNN ¼ 2016 cm−1) in line with the relatively facile N2 release, when exposed to Ar.

Scheme 9 Formation of complexes 21 and 23 with end-on-bridging N2 ligands.16–18

As shown in Scheme 10, N2 coordination may also be achieved by hydride abstraction in the presence of N2. While studying the half-sandwich complex [Cp Fe(Ph2PNtBuNPPh2)H] (24) in hydride transfer reactions with the [BNA]+ cation (BNA ¼ 1-benzyl-3carbamoylpyridinium) in different solvents, the N2 complex 25 forms selectively, but slowly in THF under an N2 atmosphere.19 However, it is also detected as a side product when the reaction is carried out in CH2Cl2. In the latter case, 25 is also formed and catalyzes the transformation of the initial hydride complex into a chloride complex.19

Scheme 10 N2 coordination by hydride abstraction.19

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

7.03.3

Monocyclopentadienyl compounds bearing halide and pseudohalide ligands

7.03.3.1

Halides

55

The monocyclopentadienyl iron {Cp0 Fe} fragment is a well-known actor since the early days of organometallic chemistry but has gained renewed attention over the last years. Researchers attempt to mimic the reactivity of ruthenium half-sandwich systems, which have found numerous applications in catalysis.20,21 By developing similar compounds based on iron as an inexpensive, environmentally benign and earth-abundant metal, it is indeed possible to mirror or even surpass the reactivity of Ru compounds. As part of this development convenient, large-scale synthetic routes toward organometallic compounds, which can provide access to the {Cp0 Fe} fragment have been developed. Although complexes of the type [Cp0 FeX(PP)] (X¼Cl, Br, I; PP ¼ two monodentate or one bidentate phosphine(s)) have been known for a long time, their synthesis is still challenging.22–24 In most cases, these complexes are isolated from mixtures containing the iron(II) halide FeX2, monodentate or bidentate phosphines (PP) and alkali metal cyclopentadienides (MCp0 ). Alternatively, also isolated iron halide phosphine adducts [FeX2(PP)] can be treated with MCp0 .23,25 However, for both procedures the FeX2 precursor must be thoroughly dried,26 and the reaction with MCp0 usually also produces significant amounts of [Cp0 2Fe] as side product.25 Nevertheless, following this traditional synthetic procedure, improved protocols toward [FeX2(PP)] and subsequent salt-metathesis reactions with alkali cyclopentadienides to yield compounds A were reported (Scheme 11a).27 However, consistent with earlier studies the formation of [(Z5-CpR)2Fe] and free phosphine could not be completely avoided. In the course of these investigations, these complexes along with their cationic derivatives [(Z5-CpR)Fe(dppe)(NCMe)]I and the iron hydrides [(Z5-CpR)Fe(dppe)H] were extensively studied including their electrochemical and spectroscopic properties (NMR, IR, UV/vis, 57 Fe Mössbauer).27

Scheme 11 Formation of halide complexes depending on the presence of different phosphine ligands.27–29

Meanwhile, an alternative route to prepare [CpFe(dppe)I] on large scale by the oxidation of [{CpFe(CO)(m-CO)}2] (1) with stoichiometric amounts of I2 to give [CpFe(CO)2I] (6) was described, followed by replacement of the CO ligand with the bidentate phosphine dppe under thermal conditions (Scheme 11b).28 Although the formation of [CpFe(CO)(dppe)]I cannot be excluded, it can be easily separated from the desired product. This protocol is based on an earlier report regarding the thermal CO replacement by bidentate phosphines and the resulting product distribution between neutral compounds of the type [CpFe(PP)X] (A) and the complexes [CpFe(CO)(PP)]X (B) containing a cationic {CpFe} fragment (Scheme 11c). The ratio results from a competition between halide vs. CO coordination to the Fe atom, and the fraction of [CpFe(PP)X] increases within the series Cl < Br < I. Furthermore, less flexible phosphines favor the formation of this product (Table 1).29 Reaction of [(
5-C5H4R)Fe(CO)2I] (R¼H, Me) with various phosphine ligands PR3 (R]Ph, m-Tol, p-C6H4OMe, p-C6H4Cl, p-C6H4F) under solvent-free conditions, that is, in the melt phase, preferentially forms the ionic products [(
5-C5H4R)Fe(PR3)3]I instead of the neutral CO substitution products [(Z5-C5H4R)Fe(PR3)2I].30 Addition of catalytic quantities of [{CpFe(CO)(m-CO)}2] not only increases the overall yield of the reaction, but also the [(
5-C5H4R)Fe(PR3)3][I]: [(
5-C5H4R)Fe(PR3)2I] ratio. However, when performed in benzene solution the neutral CO substitution products are favored. To account for the different reaction outcome depending on the phase in which the reactions is performed is traced to a change in mechanism. While the formation of

56

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Table 1

Product distribution referring to reaction (c) shown in Scheme 11.29

X

Y

A

B

Cl

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

– – Main product Main product Main product Single product Single producta Main product Main product Single producta Single product

Single product Single product Small amounts Small amounts Small amounts – – Small amounts Small amounts – –

Br

I

a Formed after prolonged reaction time; as a stable intermediate, the complex, in which the phosphine coordinates only in monodentate fashion, is formed.

the ionic complex in the solid state is proposed to proceed via a 19 VE intermediate, in benzene the preferential formation of [(
5-C5H4R)Fe(PR3)2I] is traced to 17 VE species as key intermediates. The 18 VE half-sandwich complexes A contain relatively electron-rich, low-spin (S ¼ 0) Fe(II) atoms, favoring an one-electron oxidation (Scheme 11). As expected, the presence of alkylated Cp ligands like Cp or 2tBuCp increases the electron density at the Fe atom. Interestingly, the silylated Cp ligands, tmsCp and 2tmsCp, do not exert an electron-withdrawing effect, as might be expected from the more electropositive silicon atom compared to a carbon atom.27 Attempts towards halide exchange in the [CpFe(PP)X] complexes usually lead to the formation of cationic complexes [CpFe(PP)L]X, in which one solvent molecule L coordinates.27 The following paragraphs provide an overview on complexes containing formally ionic sub-units of the {Cp0 Fe} fragment (Fig. 1). The syntheses of the cation-anion pairs [3tBuCpFe(
6-C6H5R)][ 3tBuCpFeI2] (R=H (26a), Me (26b)) are achieved from the m-iodo-bridged compound [{3tBuCpFe(m-I)}2].31,32 The anionic [3tBuCpFeI2]− fragment in 27 is successfully accessed from [{3tBuCpFe(m-I)}2] and NEt4I (Fig. 1).31 Nevertheless, the cationic complexes 28,33 29a, 29b and B are known (Fig. 1).28,29 Most cationic complexes obey the 18 VE count, with the exception of 29a and 29b, in which the bidentate phosphine only coordinates with one arm to the iron atom, resulting in 16 VE complexes.29 Within this selection of Fe(II) complexes, only the anionic fragments in 26a, 26b and 27 are paramagnetic compounds featuring a high-spin S ¼ 2 configuration, whereas all other compounds are low-spin and diamagnetic.28–32 Beside cationic and anionic {Cp0 Fe} fragments, also various neutral three- and two-legged piano stool complexes were reported containing halide ligands (Fig. 2).29,33–36 Two especially interesting representatives constitute [Cp Fe(tmeda)Br] (31a)35 and the 15 VE, Fe(III) complex [3tBuCpFe(C6H2-2,4,6-Me3)3Cl] (33).34 Complex 31a is an analogue to the chloride compound [Cp Fe (tmeda)Cl] (31b).37 Both complexes act as versatile starting materials for further functionalization attributed to their thermal

Fig. 1 Ionic halide half-sandwich complexes.28,29,31–33

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

57

Fig. 2 Neutral halide half-sandwich complexes.29,31–36

stability and the presence of the substitutionally labile tmeda ligand. Despite its formal 18 VE electron count 31a features a high-spin configuration (S ¼ 2) as inferred from its paramagnetic NMR spectrum, its solution magnetic susceptibility, and its solid-state molecular structure, which can be rationalized by the steric demand of the Cp as well as that of the tmeda ligand.35 The s-aryl complex 33 can readily be obtained from [3tBuCpFe(C6H2-2,4,6-Me3)] (14 VE) using oxidizing reagents such as PdCl2 and C2Cl6.34 Similar compounds 34a/b may also be prepared with different Cp ligands and aryl substituents.34 Moreover, the two-legged piano stool complexes 35a and 36 were presented (Fig. 2). While 36 is obtained as an undesired product in only low quantity,34 35a is formed by oxidation of [{3tBuCpFe(m-I)}2] with I2 31 or C2H4I2 (Scheme 76).32 Although it is stable in aliphatic and aromatic hydrocarbons, dichloromethane or diethylether, it decomposes in THF to [FeI2(thf )x] and the fulvalene derivative (3tBuCp2).32 Another important group presented dimeric, m-halido-bridged Cp0 Fe compounds, which have found broad application for the synthesis of various iron half-sandwich complexes (Fig. 3). In 2011, the synthesis and molecular structure of [{3tBuCpFe(m-I)}2]

Fig. 3 Halide-bridged cyclopentadienyl Fe complexes.33,38,40,41

58

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

(37b) were reported,33 while the related bromide-bridged species [{3tBuCpFe(m-Br)}2] (37a) has been known since 2005.38 Furthermore, the synthesis of [{4iPrCpFe(m-Br)}2] (38a) and [{5iPrCpFe(m-Br)}2] (39) were introduced into the literature in 1996,39 but details concerning their molecular structures were revealed in 2014.40 In the same contribution also the iodide-bridged species [{4iPrCpFe(mI)}2] (38b) was mentioned.40 Moreover, analogous complexes 40 and 41 employing pentaaryl-substituted Cp ligands can be prepared.41 All complexes feature two high-spin Fe(II) sites with four unpaired electrons. In coordinating solvents such as THF, Et2O or acetone, [{3tBuCpFe(m-I)}2] (37b) undergoes a monomer-dimer equilibrium. In MeCN solution, however, [3tBuCpFe(NCMe)I] and [3tBuCpFe(NCMe)3]I (28) are formed.33 Similarly, [{5iPrCpFe(m-Br)}2] (39) dissociates into [5iPrCpFeBr] monomers in C6D6 solution, and based on NMR studies it is suggested that one hydrogen atom of one iPr group interacts with the Fe atom to stabilize the monomeric species [5iPrCpFeBr].40 This contrasts the observations for [{3tBuCpFe(m-I)}2] (37b), in which the monomeric fragment {3tBuCpFeI} stabilizes itself by solvent coordination. Nevertheless, a closer analysis of the solid-state structures of the Br-bridged derivatives reveals, that complexes 37a, 38a and 39 exhibit a butterfly structure, which manifests itself in fold angles >0 degree of the respective Fe2Br2 planes.33,40 This conformation is likely induced by the steric congestion enforced by the alkylated 3tBuCp, 4iPrCp, and 5iPr Cp ligands. Because of the decreased steric bulk of pentaarylated Cp ligands compared to the alkylated derivatives, complexes 40 and 41 exhibit planar Fe2Br2 units.40,41 Complexes 40 and 41 differ in the relative orientations of their Cp rings, i.e., they are eclipsed for 41, but almost staggered for 40. 1H NMR and UV/vis spectroscopic studies indicate the presence of a monomer-dimer equilibrium in solution, in which the monomeric forms are stabilized by thf coordination. In the solid state, the high-spin Fe(II) atoms in 37b42 and 4041 exhibit antiferromagnetic coupling and/or zero-field splitting at low temperature.

7.03.3.2

Pseudohalides

The reaction of [{3tBuCpFe(m-I)}2] (37b) towards pseudohalides (NCO−, SCN−, SeCN−, and N−3) was studied in detail. However, only for NCO− a straightforward salt-metathesis reaction is realized to yield [{3tBuCpFe(m-NCO)}2] (42), while in all other cases also redox reactions occur to form [{3tBuCpFe(m-S)}2] (43), [{3tBuCpFe(m-Se2)}2] (44) and [{3tBuCpFe(m-N)}2] (45), respectively (Scheme 12).43 Complex 42 features two Fe(II) high-spin centers which are weakly antiferromagnetically coupled at low temperature based on solid-state magnetic susceptibility studies. The products derived from the reaction with SCN−, SeCN−, and N−3 are discussed in the respective subchapters dealing with sulfido, selenido and nitrido ligands.

Scheme 12 Reactivity of [{3tBuCpFe(m-I)}2] (37b) towards pseudohalides.43

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

7.03.4

Monocyclopentadienyl compounds bearing group 15 donor ligands

7.03.4.1

Monocyclopentadienyl compounds bearing N ligands

7.03.4.1.1

59

Amines

In extension to the known tmeda adduct [Cp Fe(tmeda)Cl] (31b),37 the [Cp Fe(tmeda)Br] (31a)35 was reported (Fig. 2). The steric demand of the Cp and tmeda ligands enforces a high-spin configuration (S ¼ 2) despite being a formal 18 VE species.35 The cationic complex [CpFe(CO)2]+[BF4]− (7) forms at low temperature in the presence of diethyl ether a weak adduct [CpFe(CO)2(OEt2)]+[BF4]−, in which the coordinatively labile OEt2 ligand can readily be displaced by 1-aminoalkanes and a,o-diaminoalkanes to yield the amine adducts [CpFe(CO)2NH2(CH2)nCH3]+[BF4]− [BF4] (n ¼ 2–6) and [{CpFe(CO)2}2(m(NH2(CH2)nNH2))]+[BF4]− (n ¼ 2–4), respectively.27

7.03.4.1.2

Amides

Amide ligands became frequently employed ligands in iron half-sandwich chemistry, because of their ability to stabilize lowcoordinate, low-valent Fe atoms.44,45 This development was further spurred by a growing interest to model the active sites in metalloproteins and because of their distinct magnetic properties. The low-valent bis(trimethylsilyl)amido-substituted “pogo-stick” compounds 46,44 47a,46 and 4935 are readily obtained by salt-metathesis reactions starting from FeCl2, 37b, and [5iPrCpFe(tmeda)Br], respectively (Scheme 13). Moreover, in the case of 3tBu Cp, the trimethylsilyl(tert-butyl)amido derivative 47b was reported.35 These highly air- and moisture-sensitive complexes are monomeric; and compounds 46 and 49 form upon careful treatment with water hydroxido-bridged trimers and dimers, respectively (Schemes 50 and 51).35,46 Their paramagnetic behavior was studied in the solid state and in THF solution. As indicated by a color change from yellow to green upon dissolution in THF, compound 46 forms a THF adduct, which does not occur in the case of the other examples 47a and 49, presumably because of the steric bulk of their respective Cp0 moieties.35,44,46 However, in contrast to complexes 47a and 49, the C5Me5 derivative 48 is diamagnetic when dissolved in deuteriobenzene.35,44 A closer inspection of the magnetic behavior of the four complexes reveals Fe(II) high-spin centers with S ¼ 2, which also manifests in the Cpcent-Fe distances of about 1.9 A˚ .35,46 In addition, 49 exhibits slow magnetic relaxation of the magnetization with an effective spin-reversal barrier of Ueff ¼ 113 cm−1 and a relaxation time of t0 ¼ 4.8  10−10 s.35 Comparison of the compounds 47a and 47b shows that the more nucleophilic N(SiMe3)(CMe3) ligand is thermodynamically slightly favored over the N(SiMe3)2 functionality.46

Scheme 13 Synthesis of Fe half-sandwich amide complexes.35,44,46

The amido ligands in 47a and 49 act as Brønsted bases towards more Brønsted acidic substrates such as dimethylpyrrole, phenols or amines to yield [5iPrCpFe(
5-2,5-Me2C4H2N)],35 [3tBuCpFe(OC6H2-2,4,6-tBu3)] (Scheme 50)46 and the mixed iminocyclohexadienyl compound [3tBuCpFe(N,C-k1,
5-C6H5NPh)-Fe(N-k1-NPh2)3tBuCp].46 However, for steric reasons, the sterically encumbered aniline derivative H2N(2,4,6-tBu3-C6H2) is neither deprotonated by 47a nor 47b. Therefore the iron anilido complex [3tBuCpFe(HN(2,4,6-tBu3-C6H2))] (48) must be synthesized by a salt-metathesis (Scheme 13).46 Very detailed studies concerning the reactivity of the pogo-stick complex 46 towards more acidic substrates were performed, e.g., the reaction with N-methylaniline to yield the dimeric methylphenylamido 50 (Scheme 14).47 Compound 50 features a planar Fe2N2 moiety and is paramagnetic in solution, as deduced from its 1H NMR spectra. Furthermore, when treated with pinacolborane, 46 reacts to a dimeric, bridged m-amido-m-hydrido species 51.47

60

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 14 Formation of amido-bridged complexes 50 and 51.47

A different group of compounds was introduced by Rudolf and co-workers using the {(
5-C5R5)Fe(CO)2} (R¼H, Me) moiety as IR-active label for bioconjugation of target proteins in cells. Since biolabeling is often achieved by maleimide-thiol chemistry, the corresponding maleimidato compounds were synthesized (Scheme 15). In these reactions [(
5-C5R5)CpFe(CO)2I] (R¼H (6a), Me (52a)) is treated under UV irradiation with maleimide in the presence of HNiPr2 to give complexes 5348 and 5449 (Scheme 15). Addition of cysteine derivatives such as glutathione affords the Michael addition products 55 and 56. Kinetic studies reveal that the reaction rate decreases with increasing electron-richness of the {(
5-C5R5)Fe(CO)2} fragment, which was also confirmed by comparison to the analogous W and Ru complexes, establishing an intermediate position for Fe with respect to its reaction rate in this process. The iron maleimidato compounds 55 and 56 may also be utilized in the generation 4 (G4) poly(amidoamine) (PAMAM) dendrimers, which can be employed in carbonyl metallo immunoassays.50

Scheme 15 Synthesis of maleimidato compounds 53-56.48,49

7.03.4.1.3

Imido and Imidazolin-2-iminato

Like the related amides, only a few imido iron species can be found in the literature. In principle, the imido ligand may donate up to six electrons to the metal atom, thus acting as a (weak) 2s,4p-donor.51 One example is the dimer 57 (Scheme 16), which is obtained from the mixed amido imido compound 58 formed in the reaction between [{Cp Fe(m-Cl)2}2] and LiNHPh. The second step can either be performed by H atom abstraction with a radical precursor or by oxidation to cationic species 59 followed by deprotonation (PCET ¼ proton-coupled electron transfer). All three compounds were characterized by spectroscopic methods which reveal, that 58 is paramagnetic (S ¼ ½), whereas 57 and 59 are diamagnetic in solution and in solid state. A comparison of the geometry of the central Fe2N2 moieties shows a planar arrangement for 57, whereas 58 is bent with a dihedral angle of 146.5 degrees.52 Complex 57 can also be isolated following NdN bond cleavage from the reaction of 46 with azobenzene in the presence of a proton source.47

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

61

Scheme 16 Dimeric Fe complexes 57-59 containing imido ligands.47,52

Imidazolin-2-iminato ligands, which are derived from N-heterocyclic carbenes (NHCs), are isolobal to imido ligands and, consequently, may also serve as six-electron donors (Scheme 17).53 The pogo-stick compound 60 is obtained by salt-metathesis between 37b and [LiNImdipp]2 (NImdipp ¼ 1,3-bis(2,6-di-iso-propylphenyl)imidazolin-2-iminato; Scheme 17).51 Complex 60 features a slightly bent CpdFedN structure (166 degrees) with the imidazolin-2-iminato function coordinating in an almost linear fashion (FedNdC 171 degrees). The FedN bond length of 1.789(2) A˚ indicates significant multiple-bond character when compared to [Cp FeN(SiMe3)2] (46) (1.900(2) A˚ )44 or [5iPrCpFeN(SiMe3)2] (49) (1.920(2) A˚ ),35 possibly resulting from 2s,4p-electron donation by the NImdipp ligand. Based on X-ray diffraction analysis, 57Fe Mössbauer spectroscopy, magnetic susceptibility measurements and DFT calculations, 60 features a high-spin Fe(II) atom (S ¼ 2) with a large negative zero-field splitting and intermediate paramagnetic relaxation at low temperature.51

Scheme 17 Relationship between imido and imidazolin-2-iminato ligands and synthesis of pogo-stick complex 60.51

7.03.4.1.4

Nitrido

Monocyclopentadienyl Fe nitrido species are, like imido compounds, only scarcely found in the literature. As one example serves the m2-nitrido-bridged [{3tBuCpFe(m-N)}2] 45, which is derived from the halide precursor 37b by reaction with sodium azide. The authors propose a monomeric and a dimeric m-azido intermediate [{3tBuCpFe(m-N3)}2], which cannot be isolated since it readily eliminates N2 to form 45 (Scheme 18).43 The diamagnetic appearance of the 1H and 13C NMR spectra suggests a ground state in solution with a total effective spin of St ¼ 0. Solid-state 57Fe Mössbauer spectra were recorded and magnetic susceptibility

62

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 18 Formation of nitride species 45 and reactivity with CO and XylNC.43

measurements were performed. Interestingly compound 45 behaves a temperature-independent paramagnet, which based on CASSCF studies can be explained by two Fe(IV, d4) atoms with two unpaired electrons that are strongly antiferromagnetically coupled. X-ray diffraction data show a planar {Fe2N2} core featuring nearly equidistant FedN bonds. Therefore, the authors were the first to present a compound comprising a M2N2 core with a formal d4 valence electron count of M. Complex 45 reacts with CO and XylNC (Xyl ¼ 2,6-dimethylphenyl) to [3tBuCpFe(CO)2(NCO)] (61) and [3tBuCpFe(CNXyl)2(NCNXyl)] (62), respectively, but it is inert towards H2 or NHCs. Sitzmann and co-workers showed that the related iron(IV) m2-nitrido complex [{5iPrCpFe(m-N)}2] can be formed upon reduction of [5iPrCpFe(dme)Br] with Na/Hg under N2.54,55 However, a more detailed study concerning its formation has so far not been revealed. Nevertheless, it is of note that in contrast to the 3tBuCp derivative,43 [{5iPrCpFe(m-N)}2] reacts in the presence of H2 to form a mixture of free 5iPrCpH and the ligand amination product amino-iso-propyl-tetra-iso-propylcyclopentadiene.54 Diverging reactivity upon reduction is observed when 37b is reduced with KC8 in THF under N2. Under these conditions the trimetallic m3-bis(nitrido) species [(3tBuCpFe)3(m3-N)2] (63) is isolated (Scheme 19).56 The long NdN bond distance of 2.267(2) A˚ indicates complete reduction of N2 to two bridging m3-nitrido ligands occupying the axial positions of a trigonal bipyramid, whereas the three {3tBuCpFe} fragments form the equatorial plane. The FedFe bond distances are equally spread ranging from 2.4727(4) to 2.4734(4) A˚ indicating three equivalent Fe sites. This structural arrangement of m3-bound N-atoms resembles that of the Fe(110) surface encountered in the Haber-Bosch process.57–59 The electronic structure of 63 was probed by 57Fe Mössbauer spectroscopy and solid-state magnetic susceptibility measurements indicating weak (intramolecular) antiferromagnetic coupling between the three individual low-spin (S ¼ 1/2) Fe(III, d5) centers to form a ground state with a total effective spin of St ¼ 1/2. In contrast to [{3tBuCpFe(m2-N)}2] (45), [(3tBuCpFe)3(m3-N)2] (63) reacts with H2 in solution and solid state. In hexane solution it forms NH3 in ca. 3–7% yield, but the NH3 yield can be further increased to ca. 10%, when this reaction is carried out in THF (Scheme 19). Besides NH3 also the trimetallic iron m3-imido species [(3tBuCpFe)3(m3-NH)2] (64) and the iron m2-nitrido 63 are formed. However, clean conversion of 63 in the presence of H2 to the mixed-valent m3-bis(imido) species 64 is observed in solid state (Scheme 19). In the m3-bis(imido) 64 the FedFe distances can be divided into two shorter bonds of 2.5195(4) and 2.5284(4) A˚ for Fe1dFe2 and Fe2dFe3, respectively, and a longer bond of 2.5408(4) A˚ for Fe1dFe3, implying two different iron sites in a 2:1 ratio. The N1dN2 distance is also significantly increased from 2.267(2) to 2.352(3) A˚ on H2 addition. Nevertheless, the structural motif of a trigonal bipyramid with three {3tBuCpFe} moieties occupying the equatorial plane is conserved. The barrier for H2 addition in solid state was evaluated by 57Fe Mössbauer spectroscopy to be DG{ (295 K) ¼ 99.2(8) kJmol−1. CASSCF computations in combination with 57Fe Mössbauer data suggest the presence of two Fe(II) intermediate-spin (S ¼ 1) sites and one low-spin (S ¼ 1/2) Fe(III) site. DFT computations were performed to understand the H2 addition mechanism in the solid state and revealed that (1) the H2 splitting occurs along an FedN edge; (2) the flexibility of the N-atoms is essential for a m3- to m2-(edge) migration above the {3tBuCpFe}3 moiety, which significantly lowers the barrier of the H2 addition process to form an intermediate with a (m2-NH, m2-H) coordination mode; (3) the 3tBuCp ligand is not an innocent spectator ligand, but is involved in a proton relay mechanism enabling the H-atom transfer to the m3-bound N atom.

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

63

Scheme 19 NH3 formation from N2 and H2 upon reduction of 37b.56

7.03.4.2 7.03.4.2.1

Monocyclopentadienyl compounds bearing P ligands Phosphines

Complexes of the type [(
5-Cp0 )Fe(L)2X] (Cp0 ]C5H5 or substituted derivatives, X ¼ halide) bearing bidentate L2 and monodentate phosphine ligands L have either been prepared by CO substitution reactions or alternatively by a salt-metathesis reaction between [FeX2L2] and MCp0 (M ¼ alkali metal) (Scheme 11).27–29 The coordinated acetonitrile ligand in the cationic complex [Cp Fe(CO)(NCMe){P(NHPh)(OMe)2}][PF6] (65) converts in the presence of catalytic amounts of NaBH4 to yield 68 (Scheme 20).60 To rationalize this product the authors proposed that the intramolecular cyclization reaction originates from the neutral iron iminophosphorane [Cp Fe(CO)(NCMe){P(NPh)(OMe)2}] (66), which is initially formed from 65 in the presence of NaBH4 and from which 67 is obtained by a nucleophilic attack of the imino nitrogen to the carbon atom of the coordinated acetonitrile. Intermediate 67 then abstracts the PN-H proton in 65 to yield 68. However, neither 66 nor 67 could be detected spectroscopically.

Scheme 20 Reactivity of [Cp Fe(CO)(NCMe)Fe{P(NHPh)(OMe)2}][PF6] (65).60

64

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

A rather unusual reaction was presented by the group of Sitzmann when 49 was treated with an excess of Ph2PH, which forms—besides other products—a mixture of the bis(diphenylphosphine) complex 69, the tetraphenyldiphosphine complex 70, tetraphenyldiphosphine and HN(SiMe3)2 (Scheme 21).35 To account for the formation of these products a mechanistic proposal was advanced which comprises of several steps, starting with the initial acid base reaction to form [5iPrCpFePPh2] followed by oxidative addition of Ph2PH to form [5iPrCpFe(PPh2)2H], which either oxidatively couples yielding the tetraphenyldiphosphine 70 or reductively eliminates Ph2PdPPh2 to afford a reactive [5iPrCpFeH] intermediate. Latter intermediate may rapidly coordinate two equivalents of Ph2PH to form the bis(diphenylphosphine) complex 69. Alternatively, the observation of an NMR resonance assigned to H2 formation implies H2 loss from a [5iPrCpFe(PPh2)H2] intermediate originating from the monophosphine adduct [5iPrCpFe(PPh2)H] followed by oxidative addition of HPPh2.

Scheme 21 Reactivity of 49 towards HPPh2.35

7.03.4.2.2

Phosphides, iminophosphoranes

Iron phosphide species are implicated in [{CpFe(CO)(m-CO)}2] (1) catalyzed hydrophosphination reactions and were further confirmed upon irradiation of 1 at l > 500 nm in the presence of Ph2PH to yield a mixture consisting of [CpFe(CO)2H] (3), [CpFe(CO)2PPh2] (4), and [CpFe(CO) (PPh2)(H)] (5) (Scheme 2).5 The high nucleophilicity of the iron-iminophosphorane-carbonyl complex [Cp Fe(CO)2{P(NPh)(OMe)2}] (71) was probed with MeO2CC^CCO2Me and CO2 (Scheme 22).60 With MeO2CC^CCO2Me complex 71 yields the six-membered metallacycle 72, whereas when exposed to CO2 an aza-Wittig type reaction occurs presumably via a four-membered aza-phosphacyclic intermediate 73 to afford PhN]C]NPh and the iron oxophosphorane (phosphonate) [Cp Fe(CO)2{P(O)(OMe)2}] (74).

Scheme 22 Reactivity of [Cp Fe(CO)2{P(NPh)(OMe)2}] (71) towards MeO2CC^CCO2Me and CO2.60

7.03.4.2.3

Phosphinidenes

When the phosphinidene-bridged compound [{CpFe(CO)}2(m-PCy)(m-CO)] is treated with terminal and internal alkynes a broad array of different products 76-83 is formed, whose identity strongly depends on the identity of alkyne and the experimental conditions such as temperature and stoichiometry (Scheme 23).61 In all cases product formation was traced to a common zwitterionic intermediate originating from nucleophilic attack of the P atom of the phosphinidene ligand to the alkyne Ca atom, which is also the least sterically protected site. As a result, the alkyne Cb atom becomes more nucleophilic, and from here the reaction pathways differ depending on the reaction conditions: In the absence of excess alkyne, there is a strong preference for the deprotonation of the Cp ligand which is in close proximity. Therefore the (cyclopentadienylidene)(alkenyl)phosphine derivatives 76 and 79 are isolated as kinetic products. However, at elevated temperatures some cis/trans isomerization may also occur bringing the alkyne Cb atom into the proximity of a CO ligand, resulting in a nucleophilic attack at this position to yield phosphide-acyl derivatives 77 and 80. However, in the presence of an alkyne excess a new reaction pathway is accessible as demonstrated when CO2Me-substituted internal and terminal alkynes are used. For MeO2CC^CCO2Me, nucleophilic attack occurs at the alkyne C atom, inducing a reaction pathway yielding the phosphole-bridged species 78. In contrast for terminal alkyne MeO2CC^CH deprotonation occurs at a second alkyne molecule eventually leading to the (alkynyl)(alkenyl)phosphine complexes 82 and 83.

Reactivity of [{CpFe(CO)}2(m-PCy)(m-CO)] (75) in the presence of terminal and internal alkynes.61

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 23

65

66

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

7.03.4.2.4

P ligands derived from P4 activation

Several reviews have been devoted to the coordination, activation and functionalization of white phosphorus (P4) to various transition metal fragments.62–66 Coordination of an intact P4 cage to cationic iron half-sandwich complexes was reported using either {Cp0 Fe(dppe)}+ (Cp0 ¼Cp,67 Cp )68 or the less electron-rich {CpFe(CO)2}+ and {CpFe(CO)(PPh3)}+ fragments (Fig. 4).69 The broadened resonances in the 31P NMR spectra of the
1-P4 coordinated compounds 84 and 85 suggest tumbling of the P4-ligand in solution attributed to a dynamic
1 !
2 !
1 hapticity change.67–69 However, for the less electron rich derivatives [CpFe (CO)2(
1-P4)]+ (86) and [CpFe(CO)(PPh3)(
1-P4)]+ (87) this exchange process is significantly slower.69 This slower exchange was rationalized by DFT computations suggesting that the electron deficiency at the Fe atom induces an umpolung of the FedP4 bond. Electron density is transferred from the slightly antibonding P4 HOMO to the Fe atom and therefore the PdP distances around the apical P atom decrease by 6–7 pm.

Fig. 4 Adducts of white phosphorus (P4) to cationic [(
5-C5R5)Fe(CO)2(L2)]+ (R=H, Me; L2=dppe, (CO)2, (CO)(PPh3)).67–69

Nevertheless, P4 activation in the coordination sphere of transition metals involves either formation of other P4-isomers or its fragmentation/reaggregation to yield Pn-containing transitional metal compounds. A frequently encountered P4 activation pathway, however, involves the formation of a tetraphosphabicyclo[1.1.0]butane (“butterfly-P4”) unit resulting from the selective cleavage of one PdP bond within the P4 cage (Scheme 24). This includes the reaction of [{CpRFe(CO)(m-CO)}2] with P4 to yield various P4 activation, fragmentation and reaggregation products.70–76 For example, the sterically encumbered derivative [{BIGCpFe(CO)(m-CO)}2] (88) reacts with a broad selection of small molecules such as P4, As4, P4S3, P4Se3 and CS2 already at ambient temperature.72 Key step in this process is the formation of two 17 VE {BIGCpFe(CO)2}• radicals that interact with the tetrahedral P4 molecule and induce its selective homolytic cleavage to yield the dimeric tetraphosphabicyclo[1.1.0]butane-bridged species 90 (Scheme 24). Furthermore, independent of the stoichiometry no further P4 activation products could be isolated when the reaction is performed at room temperature. However, when 90 is heated, conversion to [BIGCpFe(
5-P5)] (92) and [(BIGCpFe)2(m,
4:
4-P4)] (95) is observed. Furthermore, thermolysis of 88 and 89 in the presence of P4 at T > 190  C yields mixtures of 92 and 95,73 and 93 and 96,70 respectively (Scheme 24). The addition of P4 to 90 also forms 92 and 95 in the identical ratios and yields. This further strengthens the hypothesis that the butterfly product 90 indeed constitutes the initial P4 activation product, when {CpRFe(CO)2} fragments are employed.73 In the presence of yellow arsenic (As4) complex 91 forms mixed species 94 and 97 containing
5-PnAsm and m,
4:
4-PnAsm ligands, respectively (Scheme 24).76

Scheme 24 Reactivity of [{(
5-CpR)Fe(CO)(m-CO)}2] (CpR]BIGCp and 3tBuCp) towards P4.70,72,73,76

Under UV irradiation 90 releases two CO ligands exclusively yielding [{BIGCpFe}{BIGCpFe(CO)2}(m,
4:
1-P4)] (98) (see Scheme 25a).75 The iron butterfly species 91 offers a rich playground for further reactivity studies.77,78 DFT computations at the

Reaction chemistry of 90 and 91 under photochemical conditions, after protonation or in the presence of Fe and Cu salts.75,77,78

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 25

67

68

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

BP86/def-SVP level of theory carried out on the model complex [{CpFe(CO)2}2(m,
1:
1-P4)] showed that the HOMO features a significant contribution from the lone pairs at the “wing-tip” P atoms, which should be readily accessible for protonation and/or reactions with Lewis acids.77 Furthermore, from the evaluation of the LUMO, which is located between the “wing-tip” P atoms, backbonding from a Lewis acidic metal atom to the butterfly complex can be expected. Addition of [(Et2O)nH]+[X]− (X¼BF4, n ¼ 1; X ¼ [Al(OC(CF3)3)4], n ¼ 2) yielded the monoprotonation product 99 (see Scheme 25a).77 Although high-quality X-ray data could not be obtained, based on 31P NMR and computational studies it is concluded that protonation occurs selectively at a “wing-tip” P atom. The ability of 91 to act as a bidentate metalloligand was demonstrated on addition of [Cu(NCMe)4][BF4] and depending on the stoichiometry of the reaction either the monoadduct 100 or the spiro compound 101 are isolated (Scheme 25b). Further studies towards transition metal compounds include the reaction with [FeBr2(dme)] and [Fe(NCMe)6][PF6]2 to yield 102 and 103, respectively.78 Interestingly, in the FeBr2 adduct 102 the coordination occurs via the “wing-tip” P atoms of the butterfly-P4 ligand, whereas conversion of 91 with [Fe(NCMe)6][PF6]2 yields homoleptic-like sandwich complex 103 featuring two 6p-electron aromatic (planar) cyclo-P4R2 ligands. The formation of cyclo-P4R2 ligands originates from an isomerization process, in which the PdP bond between the bridgehead P atoms of the butterfly moiety is cleaved. Addition of TlPF6 (4 equiv.) in the presence of 91 to 102 forms a mixture of complexes including 103 as the major component and 104 and 105 as minor components besides other side-products, which have so far defied further characterization (Scheme 25b).78 While complex 104 can be considered a heteroleptic cyclo-P4R2 product, an intact butterfly-P4 scaffold is maintained in compound 105. Independent synthesis of 104 can also be accomplished by the reaction of 91 with [{3tBuCpFe(m-Br)}2] (37a) and TlPF6 (3 equiv.). As pointed out by the authors the steric hindrance and the electronic properties of the Fe complexes determine the fate of the butterfly-P4 motif.78 The versatility and flexibility of 91 in the coordination chemistry towards transition metal salts was also established by its reactivity with ZnBr2 (1 equiv.), [NiBr2(dme)], and CoBr2 to form compounds 106, 108, and 109, as the major products, respectively (Scheme 26).79 In all cases, coordination is realized via the two “wing-tip” P atoms. Furthermore, addition of [Co (NCCH3)6][SbF6]2 to 91, forms a broad product spectrum including the known compounds [{3tBuCpFe(CO)2}2(m3,
4:
1:
1-P4) (3tBuCpFe)][PF6] (104), [3tBuCpFe(CO)3][PF6] (111) and besides [{(3tBuCpFe(CO)2)2(m3,
4:
1:
1-P4)}2Co][SbF6]3 (110) (3 %) a few crystals of [3tBuCpFe(CO)2]4(m5,
4:
1:
1:
1:
1-P8){Co(CO)2}[SbF6] (112). Surprisingly the Co(II) starting material [Co(NCCH3)6][SbF6]2 gets at least partly oxidized to Co(III) which also induces an unselective degradation of 91 and the formation of several byproducts. Complex 112 contains a {P8} moiety which constitutes an unusual example of an all-phosphorus bicylo [3.3.0]octane derivative. In the presence of 0.5 equivalents of [Zn(NCCH3)4][PF6]2 complex 91 forms the spiro complex 107 (similar to the Cu derivative 101) with unperturbed P4 butterfly moieties. Overall, the isomerization of the butterfly-P4 motif apparently requires the presence of weakly coordinated d6 metal atoms to proceed.

Reaction chemistry of 91 towards Co, Ni and Zn salts.79

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 26

69

70

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

The iron hydride complex [{3tBuCpFe(m-H)2}2] (113) behaves as a synthon for Fe(I). When 113 is heated to 100  C for 10 min in the presence of P4, it releases H2 (2 equiv.) to yield 114. Complex 114 is an isomer of the tetraphosphabutadiene compound 96 (Scheme 27).80 Upon further heating of 114 at 75  C for 7 days three P-containing species are observed, 93, 96, and 115. To account for the formation of 93 and 115 the following mechanistical proposal was advanced: Two PdP bonds of the P4-tetrahedron are initially cleaved to yield a square-planar cyclo-P4 motif, which can experience a kite-like distortion as in 114. As in a “snap-shot” two reaction pathways may now arise starting from 114: (a) rearrangement and formation of the tetraphosphabutadiene compound 96 or (b) disproportionation of this unit resulting in P1 and P3 fragments which result in the formation of 93 and the phosphaallyl complex 115, respectively. The kite-like distortion of the P4-middle deck in 114 not only persists in solution, but also in solid state as established by the A2MX-spin system observed in the 31P{1H} NMR spectrum. The solid-state molecular structure of 114 shows two distinct sets of PdP distances, two short ones (2.13 A˚ ) and two long ones (2.53 A˚ ). The latter ones are situated in between those commonly observed for PdP single bonds and PdP van-der-Waals contacts.80

Scheme 27 Reactivity of [{3tBuCpFe(m-H)2}2] (113) with P4.80

A butterfly-P4 scaffold may also be accessed by P4 activation initiated by {3tBuCp}• radicals whose formation is mediated by transition metals.81 Reaction of 37a with 1 equivalent of P4 resulted in the isolation of the carbon-substituted butterfly-P4 compound 117 (albeit in low yield), along with the iron complex 96 and precipitation of FeBr2 (Scheme 28). Based on the product distribution a mechanistic proposal was advanced which includes disproportionation of 37a into the two iron-containing fragments, {3tBuCpFe} (116) and {3tBuCpFeBr2} (35b). Two Fe(I) fragments {3tBuCpFe} can then react with one molecule of P4 to yield 96, whereas the remaining two 15 VE Fe(III) fragments {3tBuCpFeBr2} (35b) coordinate to one molecule of P4 to transfer two {3tBuCp}• radicals to yield 117 concomitant with Fe(III) to Fe(II)-reduction from which FeBr2 is formed. The reactivity of {3tBuCpFeBr2} (35b) is reminiscent to that of the isolated [3tBuCpFeI2] derivative (35a) in the presence of THF, which yields FeI2 and the fulvalene derivative (3tBuCp)2, resulting from the coupling of two {3tBuCp}• radicals.32 Compound 117 can be isolated in a significantly improved yield, when Na3tBuCp and FeBr3 are directly reacted in the presence of P4.

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

71

Scheme 28 P4-induced disproportionation of 37a.81

Lammertsma studied the reaction of P4 towards iron metalates, which allowed for further functionalization of the reaction products (Scheme 29).82 Reaction of the iron metalate [Cp Fe(CO)2]− with P4 in the presence of a Lewis acid such as BAr3 (Ar¼C6F5, Ph) forms the Lewis acid-stabilized iron compounds [Cp Fe(CO)2(m-
1:
1-P4)BAr3]− (118a and 118b) (Scheme 29). The asymmetric substitution of the tetraphosphido fragment was also established by the solid-state molecular structure of 118a. Addition of

Scheme 29 P4 activation and functionalization in the presence of [Cp Fe(CO)2]− and boranes.82

72

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

[Me3NH][BPh4] to the compounds 118a and 118b initially forms the “wing-tip” protonated intermediates 119a and 119b, respectively, which, however, convert to the neutral bicyclo[1.1.0]tetraphosphabutane isomers exo, endo–120a/b and exo,exo121a/b, concomitant with amine-borane formation, Me3N-BAr3 (Ar¼Ph, C6F5). While DFT computations confirm both isomers to be nearly isoenergetic, the phosphanes rapidly decompose within 24 h because of insufficient steric protection. As shown above in most cases P4 activation frequently involves the formation of an initial tetraphosphabicyclo[1.1.0]butane (“butterfly-P4”) intermediate, from which cyclo-P4 ligands are formed as a result of metal-induced isomerization (Scheme 19).78 However, the group of Wolf followed a different synthetic strategy and generated a cyclo-P4 ligand directly from P4 (Scheme 30a).83 Treatment of [(
5-C5(C6H4-4-Et)5)Fe(m-Br)]2 (40) with potassium naphthalenide (K(C10H8), 4 equiv.), followed by P4 addition (2 equiv.) yields [{(
5-C5(C6H4-4-Et)5)Fe(
4-cyclo-P4)}] (123) in low yield (4%).83 This complex features a delocalized planar [
4-P4]2− ligand with PdP distances of 2.1558(6)–2.1722(6) A˚ . Based on DFT computations a d6 configuration at the Fe atom is proposed. Wolf and co-workers also succeeded to functionalize P4 in the Fe coordination sphere (Scheme 30b).83 Reduction of [(
5-C5(C6H4-4-Et)5)Fe(m-Br)]2 (40) with Na/Hg (exc.) in the presence of P4 formed the diiron complex [Na2(thf )5] [{(
5-C5(C6H4-4-Et)5Fe)2(m,
4:
4-P4)}] (124) in moderate yield (67%) (Scheme 30b).83 Complex 124 features two {CpRFe} fragments being coordinated to two P2 dumbbell moieties which interact only weakly with each other. Nevertheless, 124 is exceedingly sensitive and slowly decomposes in THF solutions to [Na(thf )3{((
5-C5(C6H4-4-Et)5)Fe)2(m,
4:
4-P4)(H)}] (125) as the major product. The latter complex may also be accessed by a controlled protonation of 124 in the presence of one equivalent of [Et3NH]Cl or [H(Et2O)2][BArF4], or even trace amounts of water. The proton shows high mobility as revealed by crystallographic and spectroscopic studies. Using DFT studies two isomeric structures of equal energy are located on the potential energy surface (PES), in which the hydride ligands bridge an Fe atom and one P atom of the P4-chain. When an excess of Me3SiCl is added to 124, the phosphorus scaffold is released and a mixture of P7(SiMe3)3, HP(SiMe3)2 and P(SiMe3)3 is formed.

Scheme 30 Synthesis of 123 and 124 from 40 and P4 under reducing conditions. Further functionalization of 124 in the presence of nucleophiles.83

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

73

The reaction of [K(18-crown-6){Cp Fe(
4-C10H8)}] (126) with P4 yields two major products, the mononuclear complex [{K(18-crown-6)}2(Cp Fe(
2:
2-P7))] (127) and the anionic cluster [K(18-crown-6)(thf )2][(Cp Fe)3(m,
2:
2-P3)2] (128), that can be separated by crystallization and were extensively studied by X-ray diffraction, NMR spectroscopy and DFT computations (Scheme 31).84 Compound 127 features a P7 norbornadiene moiety, while 128 contains two cyclo-P3 units.

Scheme 31 P4 fragmentation induced by [K(18-crown-6){Cp Fe(
4-C10H8)}] (126) to yield 127 and 128.84

7.03.4.2.5

Pentaphosphaferrocene and its reaction chemistry

While a P atom is isolobal to a CdH fragment, which renders the cyclo-P5 ligand isolobal to its Cp counterpart, the reactivity of cycloP5 and Cp varies substantially.85,86 This can be attributed to the availability of a lone pair at the P atom, that can engage in coordination and supramolecular chemistry. Scheer and co-workers performed several studies surrounding the chemistry of pentaphosphaferrocene derivatives in general, but [Cp Fe(
5-P5)] (129) in particular. Two major research lines can be identified: Those devoted to supramolecular assemblies originating from [Cp Fe(
5-P5)] (129) which are not covered here, but the interested reader may refer to the original literature,87–94 and those concerning the reactivity of [Cp Fe(
5-P5)] (129) towards nucleophiles,95–97 electrophiles,98 and upon reduction and oxidation, which are presented below.99,100 Pentaphosphaferrocence 129 reacts with various main-group derived nucleophiles (Scheme 32).95 For example, nucleophiles such as Me3SiCH2Li and Me2NLi attack the P5-ring yielding anionic
4-P5 compounds 130a and 130b, respectively (Scheme 32a). Addition of NaNH2 forms the dimeric species 131a, whereas with LiPH2 a mixture of 130c and 131b is formed, in which 131b constitutes the minor component (Scheme 32b). Full characterization of these products by NMR spectroscopy was achieved, while 130a, 131a and 131b were also structurally authenticated. However, the reactivity is not limited to anionic nucleophiles, but also low-valent silicon species react with 129 (Scheme 32c).96 Addition of (NdN)SiCl (NdN¼PhC(NtBu)2 (3 equiv.)) to 129 selectively substitutes a P-atom and inserts the silicon-based fragment into the P-ring to yield [Cp Fe{
4-P4Si(NdN)}] (133), featuring a silatetraphospha-cyclopentadienyl ligand, and the functionalized phosphasilene 134 (Scheme 32c). Although it is assumed that the formation of 133 proceeds via the intermediate [Cp Fe(
4-P5)Si(NdN)(Cl)] (132a), this species has so far defied isolation. However, it can be detected at −70  C by 31P{1H} NMR spectroscopy. Nevertheless, complex 132b was structurally characterized.

74 Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Reactivity of 129 towards various nucleophiles.95–97 Scheme 32

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

75

In the presence of the low-valent Si(I) reagent [(NdN)Si]Si(NdN)] complex 129 forms [Cp Fe{
4dP5[Si(NdN)]2}] (136) as the major product, originating from a double ring expansion of the cyclo-P5 ring to yield the cyclo-Si2P5 moiety (Scheme 32d). The presence of the cyclo-Si2P5 moiety was also verified by X-ray diffraction. Side products in this transformation are 133 and [(NdN)2Si2P2] (135).96 Also, NHCs such as IMe2Me2 (1,3,4,5-tetramethylimidazolin-2-ylidene), IMes (1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene) and IDipp (1,3-bis(2,6-di-iso-propylphenyl)-imidazolin-2-ylidene) may react with the cycloP5 ligand of 129 to furnish the well-defined and isolable neutral adducts 137a-c, also featuring an envelope-like structure of the P5 moiety (Scheme 32e).97 Nevertheless, in solution an equilibrium between 137a-c, 129 and free NHC exists for which the equilibrium constant depends on the steric bulk of the individual NHC ligands. DFT computations were also used to evaluate the stability of these adducts. In line with the experimental data increased steric bulk of the NHC ligands destabilizes the adducts attributed to steric repulsion. Electrophilic functionalization of 129 is also possible and results in the isolation of the first transition-metal compounds featuring pentaphosphole (cyclo-P5R) ligands (Scheme 33).98 Reaction of [(Et3Si)2(m-H)][B(C6F5)4] or [Me3O][BF4]/B(C6F5)3 (alternatively: MeOTf/[Li(OEt2)2][B(C6F5)4]) with 129 yields [Cp Fe(
4-P5SiEt3)][B(C6F5)4] (138) and [Cp Fe(
4-P5Me)][X] ([X]−¼[FB(C6F5)3]− (139a), [B(C6F5)4]− (139b)), respectively. Upon careful hydrolysis the PdSi bond in 138 is cleaved to form [Cp Fe(
4-P5H)][B(C6F5)4] (140), featuring the parent cyclo-P5H ligand. In the molecular structures of these compounds a slight envelope-like distortion of the cyclo-P5R is noted attributed the coordinated {Cp Fe}+ fragment. In solution complexes 138 and 140 are labile and show dynamic behavior, which is attributed to reversible cleavage of the PdSi and PdH bond, respectively. In contrast, for the methylated species 139a/b no dynamic behavior is detected in solution. DFT computations highlight the aromatic character of the cyclo-P5H ligand. Further functionalization of the cyclo-P5R ligand appear feasible and require further attention.

Scheme 33 Reactivity of 129 towards electrophiles.98

Pentaphosphaferrocene 129 also serves as an ideal starting material for reaction with iodine to form P-I cages (Scheme 34).99 Treatment of 129 with I2 yields [Cp Fe(k3-P6I6)]X (X ¼ I (130a); X ¼ (I)0.5(I3)0.5 (130b)) featuring a {FeP6} moiety. Following the addition of I2 in excess (9 equiv.) 130b is isolated besides PI3. Dissolution of 130a in MeOH furnishes [Cp Fe{k3-P6(OMe)6}]I (131) (Scheme 34) and MeI, based on ESI-MS and 31P{1H} NMR spectroscopy. In contrast in the presence of NaOMe the reaction is faster, but also less selective. Nevertheless, further attempts to functionalize the {FeP6} with either nucleophiles or reducing agents have so far proceeded fruitlessly.

76

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 34 Reactivity of 129 towards I2.99

However, 129 shows an exceptionally broad redox chemistry (Scheme 35).100 A closer inspection of the frontier orbitals of pentaphosphaferrocene reveals that the P atoms strongly contribute to the HOMOs, while both, the metal and the P atoms, contribute to the LUMOs.101 This sets it apart from its all-C analogue (Cp), in which the redox processes are mainly metal-centered. Therefore, participation of the P atom framework in redox reactions can be expected. In the presence of various oxidizing and reducing reagents 129 forms the dicationic species [(Cp Fe)2(m,
4:
4-P10)]2+ (143), and the dianionic species [(Cp Fe)2(m,
4:
4-P10)]2− (144) and [Cp Fe(
4-P5)]2− (145), all of which were successfully structurally characterized (Scheme 35). In all cases, the redox reactions involve the {P5} unit which is coupled with a second {P5} moiety to yield a {P10} bridge. Furthermore, the anionic species 144 and 145 may also react with P4 to form norbornadiene-like {P7} units, including the previously reported 12784 and the P14-bridged species 146 (Scheme 35b). Please note that following this new synthetic protocol 127 can now be isolated in nearly quantitative yield.

Scheme 35 Reactivity of 129 upon oxidation and reduction.100

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

77

The thermally very robust, heterobimetallic triple-decker complex [(Cp Fe)(3tBuCpCo)(m,
5:
4-P5)] (147) is formed upon treatment of 129 with [{3tBuCpCo}2(m,
4:
4-C7H8)] in 88% crystallized yield (Scheme 36).102 Alternatively, 147 can be isolated from the reaction mixture of [K2(dme)3][{Cp Fe}2(m,
4:
4-P10)] (144) with [(3tBuCpCoCl)2], but in lower yield. The solid-state molecular structure of 147 confirms the envelope conformation of the cyclo-P5 ligand, which coordinates in
4 fashion to both the {3tBuCpCo} as well as the {3tBuCpFe} fragments. In solution complex 147 shows dynamic behavior related to the cyclo-P5 ligand. While at room temperature only a broad resonance is observed in the 31P{1H} NMR spectrum, a limiting AMM0 XX0 spin system with three sharp multiplets with an integral ratio of 1:2:2 is achieved at 213 K. The values of 1JPP coupling constants follow the PdP bond distances inferred from the solid-state structure. To hinder the dynamic process in solution, [W(CO)5(thf )] (2.2 equiv.) is added to 147 and the mono- and dicoordinated compounds [(Cp Fe)(3tBuCpCo)(m3,
5:
4:
1-P5){W(CO)5}] (148a) and [(Cp Fe) (3tBuCpCo)(m4,
5:
4:
1:
1-P5){W(CO)5}2] (148b) are isolated (Scheme 36).

Scheme 36 Formation of the heterobimetallic triple-decker complexes [(Cp Fe)(3tBuCpCo)(m,
5:
4-P5)] (147) from 129 and [{3tBuCpCo}2(m,
4:
4-C7H8)].102

Further studies concerning the redox chemistry of 147 were detailed (Scheme 37).103 Addition of Ag[FAl] (1 or 2 equiv.) to 147 in CH2Cl2 or o-C6H4F2 forms the mono- and dications 148 and 149, respectively. Most prominent structural change upon oxidation is the planarization of the cyclo-E5 middle-deck. However, on reduction with KC8 in the presence of 2.2.2-cryptand the monoanionic complex 150 is obtained, in which the folding of the cyclo-P5 middle-deck has increased compared to the neutral starting material 147.

78

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 37 Reduction and oxidation of [(Cp Fe)(3tBuCpCo)(m,
5:
4-P5)] (147).103

Reduction of an 1:1 mixture of 129 and [(DIP2pyr)SmI(thf )3] (DIP2pyr ¼ 2,5-bis{N-(2,6-di-iso-propylphenyl)iminomethyl} pyrrolyl) with K/naphthalene in toluene at 60  C yields the triple-decker complexes [{Cp Fe(P5)Sm(DIP2pyr)}2] (151) and [Cp Fe(P5)Sm(DIP2pyr)(thf )2] (152), which can be crystallized in pure form from different solvent mixtures (Scheme 38).104 Structurally related complexes can be isolated using alternative reducing reagents such as [K(18-crown-6)][((
5-1, 3-Me3Si2C5H3)2Ln)2(m-
6:
6-C6H6)] or [CpR2Sm(thf )2] (CpR]Cp ,
5-C5Me5Et) to yield 153105 and 154,106 respectively. After addition of the Al(I) species [(NacNacDipp)Al:] to 129 a related mixed AldFe triple-decker species 155 is formed (Scheme 38).107

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

79

Scheme 38 Reduction of 129 using low-valent rare-earth metal complexes and an Al(I) species.104–107

Nevertheless, a more complex structural motif is realized in the reaction of a 3:4 mixture of [(Cp Al)4] and 129 to assemble the unusual AldFe polyphosphide cluster 156 containing four metal atoms, which results from a regioselective insertion of three [Cp AlIII]2+ moieties into the cyclo-P5 ring of [Cp Fe(
5-P5)] (Scheme 39).107

Scheme 39 Reactivity of 129 towards [(Cp Al)4].107

80

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Although the chemistry and synthesis of 129 has been well-studied the synthesis of the parent pentaphosphaferrocene [CpFe(
5-P5)] (157) remained a challenge until recently.108 Early attempts to prepare [CpFe(
5-P5)] (157) from P4 in decalin exclusively formed the tetranuclear iron cluster 158 (Scheme 40).109 However, by exchanging decalin against a higher boiling solvent such as 1,3-diisopropylbenzene (DIB) or 1,3,5-triisopropylbenzene (TIP) 157 can be isolated in pure form, but low yield (ca. 8%).108 This allowed 157 to be characterized by X-ray diffraction, and cyclic voltammetric studies to be performed. Complex 157 can be reversibly reduced, but oxidation is irreversible. Addition of CuX (X¼Cl, Br, I) to 157 results in unusual 2D polymers and even the first 3D polymer which differ in the coordination mode of the cyclo-P5 ligand.108

Scheme 40 Reactivity of 1 towards P4 and synthesis of the parent pentaphosphaferrocene 157.108,109

7.03.4.3 7.03.4.3.1

Monocyclopentadienyl compounds bearing As ligands As ligands derived from As4 activation

The reactivity of yellow arsenic (As4) has recently been comprehensively reviewed by Scheer and co-workers.110 Nevertheless, the light-sensitivity and metastability of As4 makes investigations concerning its coordination chemistry and reactivity rather challenging. Similar to the activation of P4, also As4 can be activated using monocyclopentadienyl iron carbonyls to form a broad array of different As-containing monocyclopentadienyl iron compounds (Scheme 41). However, the identity of the products strongly depends on the substituents of the cyclopentadienyl ligand and the reaction conditions, i.e., thermal vs. photochemical activation. For the Cp109 and Cp /C5Me4Et derivatives111 As4 activation was performed at 190  C to form a cluster containing a triangulated dodecahedral Fe4As4 (159)109 and pentaarsaferrocene derivatives 160 and 161,111 respectively (Scheme 41). Complex 160 can be further converted to a 30 VE triple-decker complex using [CpFe(
6-C6H6)][PF6] (162) under UV radiation (Scheme 41).112 Nevertheless, when the sterically more encumbered 3tBuCp74 and BIGCp72 iron carbonyl derivatives are employed, As4 activation already proceeds at ambient temperature via cleavage of one AsdAs bond to form the [As4]2-bridged butterfly complexes 163 and 164, respectively (Scheme 41). This reactivity resembles that observed for P4 (Scheme 24). The 3tBuCp derivative 163 releases CO under irradiation to yield 165 which further converts to As8 cuneane compound [{3tBuCpFe(CO)2}2{3tBuCpFe(CO)}2 (m4,
1:
1:
2:
2-As8)] (166) (Scheme 41).74 The reaction products were comprehensively characterized by NMR spectroscopy, single-crystal X-ray diffraction and DFT computations. Diverging reactivity is observed for the BIGCp derivative 164 which completely decarbonylates under UV light irradiation to yield the triple-decker species {BIGCpFe}2(m,
4:
4-As4) (168) as the only product (Scheme 41).75 Furthermore, when 164 is heated in high-boiling solvents a mixture of pentaarsaferrocene 167, triple-decker complex 168, and the cluster [{BIGCpFe}3(m3,
4:
4:
4-As6)] (169) containing an As6 prism, in which each of the three square faces is capped with a {BIGCpFe} fragment, is obtained (Scheme 41).75

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

81

Scheme 41 Reactivity of [{Cp0 Fe(CO)(m-CO)}2] towards As4.72,74,75,109,111,112

[2tBuCpZr(
1:
1-As4)] prepared from As4 and [2tBuCp2Zr(CO)2] serves as an excellent light-stable and storable As4 transfer reagent (Scheme 42).113 It reacts under mild conditions to yield kinetically controlled products: In the presence of [{3tBuCpFe(m-Br)}2] (37a) [2tBuCpZr(
1:
1-As4)] furnishes the “bonding isomers” [(3tBuCpFe)2(m,
4:
4-As4)] (170a/b) with either a central tetraarsabutadiene-like (170a) or a cyclo-As4 moiety (170b). DFT computations predict 170b to be the kinetic product. It is noteworthy, that the P congener [{3tBuCpFe}2(m,
4:
4-P4)] (96) shows dynamic behavior in solution rendering all P atoms equivalent on the NMR time scale.70

Scheme 42 As4 transfer using [2tBuCp2Zr(
1:1-As4)] to [{3tBuCpFe(m-Br)}2] (37a) and [{BnCpFe(m-Br)}2], respectively.113

82

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Nevertheless, when the BnCp derivative [{BnCpFe(m-Br)}2] is added to [2tBuCpZr(
1:
1-As4)], [{BnCpFe}2(m,
4:
4-As4)] (171) is selectively formed and isolated (Scheme 42). Furthermore, 171 can be readily oxidized by Ag[BF4] to form the unexpected ring-expansion product [{BnCpFe}2(m,
5:
5-As5)][BF4]. Mixed PnAsm compounds are accessible by the reaction of [{3tBuCpFe(CO)2}2(m,
1:
1-P4)] (91) with As4 to give 3tBu [{ CpFe}2(m,
4:
4-PnAs4-n)] (94) and [3tBuCpFe(
5dPnAs5-n)] (97), which were characterized by a combination of NMR spectroscopy, mass spectrometry and X-ray diffraction (Scheme 43).76 Moreover, addition of CuCl to [{3tBuCpFe}2 (m,
4:
4-PnAs4-n)] (94) induces a rearrangement of the P/As positions favoring P coordination towards the Cu atoms forming an 1-dimensional chain polymer.76

Scheme 43 Synthesis of mixed PnAsm compounds from 91 and As4.76

7.03.4.3.2

Pentaarsaferrocene and its reaction chemistry

[Cp Fe(
5-As5)] (160), the heavier homologue to [Cp Fe(
5-P5)] (129), was prepared previously,111 but its coordination chemistry has only recently been studied in more details as part of related studies on its lighter congener 129. For example, in the presence of Cu(I) halides 1D-polymeric structures are formed, in which the cyclo-As5 ring p-coordinates to (CuX)n moieties. Nevertheless, an unusual coordination mode featuring a (CuI)n ladder, that is alternately coordinated by 160 was also realized.101 In cyclic voltammetric studies 160 behaves similarly to 129, whereas upon addition of KH a mixture of new Asn-containing species (172-175) is formed, in which n < 18 and the polyarsenide moieties are stabilized by {Cp Fe} fragments (Scheme 44).114 These complexes were structurally authenticated and spectroscopically characterized. DFT computations suggest that the initially formed anion [Cp Fe(
5dAs5)]− dimerizes to [(Cp Fe)2(m,
4:
4-As10)]2− (173). However, after addition of I2 a dicationic FedAs triple-decker complex [{Cp Fe}2(m,
5:
5-As5)][As6I8] (1630 ) is formed besides [Fe(CH3CN)6][As6I8] and [Fe(CH3CN)6][As4I14] (Scheme 44).99 The AsdAs bond distances in 1630 range from 2.380(5) A˚ to 2.392(6) A˚ , which is in between a normal AsdAs single bond (2.42 A˚ ) and a As]As double bond (2.28 A˚ ). Furthermore, the FedFe distance in 1630 with 2.795(4) A˚ implies some FedFe bonding interactions, which are also supported by DFT computations.

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

83

Reduction of 160 with the samarocene [2tBuCp2Sm(thf )] forms two distinct products [(2tBuCp2Sm)(m,
4:
4-As4)(FeCp )] (176) and [{2tBuCp2Sm}2(As7)(FeCp )] (177) (Scheme 44).115 Complex 176 features an As2− 4 middle deck which is isolobal with the 6p-aromatic cyclobutadiene dianion [C4H4]2−, whereas 177 accommodates a [As7]3− cage, exhibiting a norbornadiene-like structure with two short AsdAs bonds. The Sm(III) oxidation state was verified by NIR spectroscopy, and DFT computations confirmed the ionic bonding in these molecules and reproduced the molecular structures very well.

Scheme 44 Reactivity of 160 in the presence of KH, I2, and [2tBuCp2Sm(thf )].99,114,115

Using a similar approach as for [(Cp Fe)(3tBuCpCo)(m,
5:
4-P5)] (147) (Scheme 36),102 the reaction of pentaarsaferrocene 160 with [{3tBuCpCo}2(m,
4:
4-C7H8)] forms the heterobimetallic triple-decker compound [(Cp Fe)(3tBuCpCo)(m,
5:
4-As5)] (178) in excellent isolated yield (91%) (Scheme 45).116 The cyclo-As5 middle deck adopts an envelope conformation. However, in contrast to [(Cp Fe)(3tBuCpCo)(m,
5:
4-P5)] (147) (Scheme 36),102 complex 178 is only metastable and converts in MeCN/CH2Cl2 solutions at ambient temperature or upon heating in toluene (60  C, Ge >> Sn. In contrast, exchanging the para-substituent on [CpFe(CO)2SiMe2(C6H4-p-X)] has virtually no effect on the reaction rate, while successively exchanging the Me-substituents on the Si atom against Cl substituents slows down the reaction. The reaction is also affected by exchanging [CpFe(CO)2Me] (315) against [CpFe(CO)2(CH2SiMe3)]. A reaction mechanism accounting for these observations is shown in Scheme 135, which involves the initial formation of the 16 VE species [CpFe(CO)Me] (315), which then oxidatively adds the FedSi bond.

Scheme 135 Photochemically induced SidC coupling between [CpFe(CO)2SiR3] (527) and [CpFe(CO)2Me] (315).306

A new flexible, one-step approach to monocyclopentadienyl iron silyl compounds of the type [(
5-C5H4R)Fe(CO)2SiR0 3] (527) was presented. It involves the thermal treatment (refluxing p-xylene) of a mixture of cyclopentadiene C5H5R, iron pentacarbonyl [Fe(CO)5] and hydrosilanes R0 3SiH.307 The iron silyl species are isolated in acceptable yields, and the substituents R and R0 on the Cp and the silane, respectively, can be varied to include a wide range of functional groups. A reaction mechanism was also advanced by the authors.

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

151

Using the iron silyl complex [(
5-C5R5)Fe(CO)2(SiEt3)] (R¼H, Me) the NdCN bond in cyanamides (R2NdCN) can photolytically be cleaved at ambient temperature to form Et3SiCN (yield Cl > F as judged by the CO stretching frequencies. With these dihaloboryl compounds in hand their reaction chemistry was thoroughly investigated. The BX2 moiety still Lewis acidic enough to react with various Lewis bases such as 4-methylpyridine, 3,5-lutidine, PMe3 and N-heterocyclic carbenes (Schemes 148 and 149).324,344 With 4-methylpyridine (1 equiv.) [Cp Fe(CO)2BX2] (X¼Cl (547b), Br (574c)) reacts to the corresponding Lewis-base adduct [Cp Fe(CO)2BX2(NC5H4-4-Me)] (575a/b). When an excess of 4-methylpyridine is used, Braunschweig and co-workers also obtained the first metal-substituted boronium cation [Cp Fe(CO)2BBr(NC5H4-4-Me)2]+ (576) (Scheme 148). Compared to [Cp Fe(CO)2BBr2(NC5H4-4-Me)] (575b) (d(FedB) ¼ 2.106(7) A˚ ) the FedB distance in 576 is slightly elongated with 2.1465(19) A˚ , while the BdN distances are marginally smaller.

158

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 148 Synthesis and reactivity of dihaloboryl compounds towards 4-methylpyridine.324,342,343

Lewis-base adducts [Cp Fe(CO)2BCl2(LB)] (577) are also readily isolated, when 3,5-lutidine, PMe3 and the N-heterocyclic carbene IMe are added to [Cp Fe(CO)2BCl2] (574b) (Scheme 149). Following the halide abstraction strategy developed by Aldridge Cl to access borylene complexes,328 upon addition of Na[BArCl 4 ] (Ar ]3,5-Cl2C6H3) to 577 the corresponding borenium cations +  [Cp Fe(CO)2BBr(LB)] (578) are readily isolated, which constitute the first fully characterized cationic iron chloroborylenes.324 The trigonal-planar ligand arrangement at the borylene boron atoms enables FedB dpdpp backbonding, which results in a shortening of the FedB bond (PMe3 (578a) d(FedB) ¼ 1.918(6) A˚ ; IMe (578b): d(FedB) ¼ 1.940(4) A˚ ; 3,5-lutidine (579c): d(FedB) ¼ 1.928(4) A˚ ) along with a shift of the CO stretches to higher wavenumbers compared to the neutral counterparts [Cp Fe (CO)2BCl2(LB)] (577). Attempts were also made to reduce the Lewis-base adducts [Cp Fe(CO)2BCl2(LB)] (577) to generate stable boryl radicals using various reducing reagents (Scheme 149).344 However, while no reaction was observed for the IMe adduct 577b, and decomposition occurred for the PMe3 adduct 577a, the reduction of the 3,5-lutidine adduct 577c forms the dinuclear bis(boryl) product 578, which was structurally authenticated and in which two lutidine bases have undergone CdC coupling in 4-position. The isolated product is consistent with a reactive radical intermediate, in which significant spin density resides on the lutidine ring, an assumption that is also collaborated by computational studies.

Scheme 149 Reactivity of 574b towards Lewis basis to yield the Lewis base adducts 577 and their reactivity upon halide abstraction and reduction.324,344

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

159

Dihaloboryl iron compounds 574 also readily react with isonitriles and the FedB insertion products 580 and 581 are isolated and comprehensively studied including computational studies (Scheme 150).345,346 They were structurally authenticated in some cases and feature a three-membered CBN-ring coordinated to the Fe atom via the former isonitrile carbon atom.345,346 The short CdN bonds (1.284–1.300 A˚ ) and the FedC bonds (1.912–1.924 A˚ ) in combination with computational studies suggest only minor FedC backbonding implying that of two possible resonance structures I and II which can be invoked for 580, I describes the bonding most adequately (Scheme 150). Furthermore, the insertion of tBuNC into the FedB bond is reversible, and upon addition of B(C6F5)3, 574c is reformed concomitant with the Lewis acid-base pair (C6F5)3B-CNtBu, which was identified by NMR spectroscopy.346 Consistent with the experimental observations, DFT computations at the B3LYP level of theory predict that the adduct formation is only slightly exergonic with DG0 ¼ −5.02 kJmol−1, and that the insertion into the Fe-B bond proceeds with a low barrier of activation (DG{ ¼ +17.15 kJmol−1). Nevertheless, when cyclohexylisonitrile is added to the iron difluoroboryl species 574a, the bimetallic complex 581 is isolated featuring a central, six-membered B2C2N2 ring. Therefore, it is structurally related to 580 by ring-opening of the three-membered ring motifs followed by dimerization.346 To probe the intrinsic reactivity of the strained BCN ring, anhydrous HCl gas was added to 580c in hexane solution (Scheme 150). The yellow complex 582 precipitated from solution and was fully characterized. The 1H and 13C {1H} NMR spectra showed characteristic resonances at d ¼ 12.36 ppm and d ¼ 234.6 ppm, respectively, assigned to the added proton and the C atom of the boron-containing ligand coordinated to the Fe atom, respectively. The molecular structure of 582 was determined by single crystal X-ray diffraction and confirmed the positions of the added H+ and Cl− to the C and B atoms, respectively, yielding a C-iminyl ligand.346

Scheme 150 Reactivity of 574 and 579 towards isonitriles.345,346

In addition, the reactivity of 574b and 574c towards transition metal complexes was thoroughly explored and paved the way to new coordination modes in heterobimetallic complexes (Scheme 151). For the construction of these compounds two different strategies were followed: (a) oxidative addition and (b) the more classic salt-metathesis approach.

160

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Scheme 151 Reactivity of 574b and 574c towards unsaturated late transition metal fragments and metal carbonylates.337,345,347–349

Addition of [Pd(PCy3)2] to 574c formed the first heterobimetallic bridging borylene species, [Cp Fe(CO)(m-CO)(m-BBr) PdBr(PCy3)] (583) (Scheme 151).347 Based on spectroscopic data a trans arrangement of the bromoborylene and the bromide ligand at the Pd atom was proposed. Nevertheless, this ligand arrangement is contrary to that observed for the structurally authenticated ferrocenylborylene derivative [Cp Fe(CO)(m-CO)(m-BFc)PdBr(PCy3)] (Fc ¼ (
5¼C5H4)FeCp),347 which can be prepared from [Cp Fe(CO)2BBr(Fc)]348 and [Pd(PCy3)2] and features a cis conformation of the borylene and bromide ligands. In the latter complex the bulky PCy3 ligand avoids the steric demand of an adjacent ferrocenyl group by adopting a trans position to the borylene ligand, which brings the bromide and borylene ligands into a cis configuration. When the analogous reaction is carried out with [Cp Fe(CO)2BCl2] (574b) no oxidative addition takes place, instead the symmetrically bridged boryl compound 588 is isolated (Scheme 151).349 The origin of this reactivity difference may be traced to the inherently stronger BdCl bond disfavoring oxidative addition. Complex 588 can be considered an intermediate in the oxidative

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

161

addition pathway leading to 583. Based on the solid-state molecular structure a synergistic bonding mechanism was proposed: The saturated {Cp Fe(CO)2BCl2} fragment acts as a donor to the {PdPCy3} fragment, while {Cp Fe(CO)2BCl2} also engages in p backbonding. The in-plane p orbitals of the m-CO ligands as well as the vacant p orbital at the boron atom accept electron density from the electronically unsaturated {PdPCy3} fragment, which results in an effective Pd !B donation. Replacing [Pd(PCy3)2] by [Pt(PCy3)2] formed the analogous complex [Cp Fe(CO)(m-CO)Fe(m-BBr)Pt(PCy3)Br] (584) which was subsequently treated with [M(PCy3)2] (M¼Pd, Pt) to yield 585 and 587, respectively, containing a “naked” boron atom (Scheme 151).350 The 11B NMR spectra feature broad singlets at d ¼ 144 ppm and 130 ppm, respectively, which are downfield-shifted with respect to the starting materials. The most notable feature in the solid-state molecular structures of 585 and 587 is the nearly linear FedBM](M ¼ Pd, Pt) arrangement, which is also found in the metallaborylene compounds 589 and 590 (Schemes 151 and 152). The bond parameters imply an sp-hybridized boron atom, which is additionally stabilized by a boron-metal interaction between the boron atom and the {M(PCy3)} (M¼Pd, Pt) fragment. DFT computations support this bonding description. A “naked” boron atom embedded in a metallaborylene complex can also be accessed by the reaction of 574b towards Na2[M(CO)n], from which the hetero- and homobimetallic species, 589 (M¼Cr, n ¼ 5) and 590 (M¼Fe, n ¼ 4), respectively, are successfully isolated (Scheme 151).337 Most prominent is the pronounced downfield shift in the 11B NMR of d ¼ 204.7 and 190.9 ppm for the Cr and Fe derivatives, respectively. Also, the molecular structures of 589 and 590 were determined, and exhibit a linear M0 dBdM unit (M0 ¼Fe, M¼Cr (589) and M0 ¼M¼Fe (590)). The individual M0 dB and BdM bond distances adopt intermediate values between single and double bonds, which is inconsistent with a localized single-/double bond description and more in line with delocalized p bonds within the {M0 dBdM} moiety as expressed by the two resonance structures I and II, see Scheme 151. The reactivity of [Cp (OC)2Fe(m-B)Cr(CO)5] (589) was further probed in reactions with (a) cyclohexylisonitrile345 and (b) alkynes.351 Addition of cyclohexylisonitrile to 589 yielded the yellow triple insertion product 600 as a [2.3]-spiro species whose molecular structure was determined (Scheme 151). Of the three isonitrile ligands coordinated to the boron atom, two have undergone head-to-head coupling. The experimentally observed coupling is best described by resonance structure II: Double insertion occurs into the Cr]B-bond, while only single insertion occurs into the FedB bond, implying distinctively different reactivity of the boryl and borylene functionalities. The reactivity of 600 towards anhydrous HCl gas was probed and the HCl addition product 601 was isolated as a yellow solid. While the 11B NMR chemical shift for 601 is similar to that of 600 with d ¼ 8.8 ppm, NMR spectroscopy did not allow for unambiguous elucidation of the molecular structure, since no 1H NMR resonance could be assigned to the added proton. However, single-crystal X-ray diffraction studies on 601 revealed that the three-membered CBN-ring was cleaved at the BdN bond instead of the BdC bond in 582 (Scheme 150), while Cl− was added to the boron atom. The resulting ligand is best described as an iminium-borate moiety. Under photochemical conditions 589 may serve as synthon for the metalloborylene {Cp (OC)2FedB} fragment as demonstrated by its reaction with Me3SiC^CSiMe3 to form the yellow ferroborirene 602, whose spectroscopic properties and molecular structure were studied in detail (Scheme 152).351 The experimental and computational investigations indicate pronounced p delocalization within the borirene BCC-ring (2p-aromatic stabilization), but only minor FedB dp-pp backbonding. Interestingly, addition of two equivalents of an N-heterocyclic carbene (NHC) heterolytically cleaves the FedB bond, forming the yellow borironium cations 603 and the [Cp Fe(CO)2]− anion.352 A pronounced upfield shift for 603 is observed in the 11B NMR from d ¼ 63.5 ppm (for 602) to d ¼ ca. −33 ppm.

Scheme 152 Preparation of a ferroborirene 602 from 589 under photochemical conditions in the presence of an alkyne; and reactivity studies on 602 in the presence of NHC ligands.351,352

162

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

A thermally induced borylene transfer strategy to a C^C triple bond was also employed to access the aminoborirene complex [Cp Fe(CO)2{m-BN(SiMe3)2C]C}Ph] (605) using [(OC)5Mo]BN(SiMe3)2] as the transfer reagent and [Cp Fe(CO)2(C^CPh)] (604) as the alkyne (Scheme 153).353 Under thermal conditions 605 is formed in 63% yield. Free rotation around the BdN bond at ambient temperature as judged by NMR spectroscopy and a slightly elongated BdN bond of 1.4319(19) A˚ indicate substantial two p-electron delocalization within the borirene moiety. More importantly, upon photochemical treatment a reversible borireneto-boryl transfer occurs to yield [Cp Fe(CO)FeBN(SiMe3)2(
2-CC)Ph] (606). This transformation can be conveniently monitored by 11B NMR as indicated by a dramatic downfield shift from d ¼ 36.1 ppm for 605 to d ¼ 75.5 ppm for 606 consistent with the borirene-to-boryl rearrangement. The molecular structure of the rearranged product was determined, and reveals that besides the coordination of the boron atom also a
2-alkyne moiety coordinates to the {Cp Fe(CO)} fragment. Upon addition of CO or PMe3 the
2-coordinated alkyne is replaced to form 607 and 608, respectively. The molecular structures of both compounds were determined; note that 608 (11B NMR: d ¼ 86.7 ppm) is an isomer of complex 605. The authors also established experimentally that a stepwise conversion of 605–608 upon thermally induced CO extrusion is feasible. Some additional insights into the bonding were extracted from DFT computations including the relative energies of the individual intermediates along the reaction pathway.

Scheme 153 Synthesis and reactivity of the aminoborirene complex 605.353

A cationic metallaborylene complex with an embedded B+ cation was prepared by halide abstraction from the iron borylene complexes 610354 using Na[BArF4] to yield the cationic complexes 611 as pale red crystalline solids (Scheme 154).338 While the molecular structure of the R¼H derivative suffered from disorder, well-behaved structural data were obtained for the R¼Me derivative 611b. The FedBdFe unit is linear, but the {(
5-C5H4Me)Fe(CO)2} units are slightly twisted against each other by 107.2 degrees. The FedB bond distances of 1.828(5) and 1.851(1) A˚ are substantially shorter than in the starting material 610b (2.018(2) and 2.006(2) A˚ ), but comparable to the values found for [CpFe(CO)2(]BNCy2)][BArF4] (565) (1.859(6) A˚ )332 and for the neutral derivative [Cp Fe(CO)2(m-B)Fe(CO)4] (590, 1.867(2) A˚ ).313,337 The 11B NMR spectrum shows a resonance at d ¼ 191.2 and 193.7 ppm for R¼H and R¼Me, respectively. DFT computations suggest significant FedB p backbonding and a significantly delocalized p-bond within the FedBdFe fragment similar to the neutral complex 610b.

Scheme 154 Synthesis of the cationic metallaborylene complexes 611a/b.338

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

163

More recently Aldridge and co-workers also succeeded in the preparation and isolation of [CpFe(CO)(PCy3)(]B]N]CAr2)] (614, Ar¼p-tolyl (a), mesityl (b)), which can be considered as a BN-analogue of an allenylidene ligand (Scheme 155).355,356 While the p-tolyl derivative 614a could only be investigated in situ, the authors succeeded in isolating and completely characterizing the mesityl-derivative 614b. Characteristic NMR chemical shifts are d(11B) ¼ 82 ppm and d(13C) ¼ 180.8 ppm for the ketimino C atom. The downfield shift of 30.6 ppm for the ketimino C resonance upon halide abstraction implies delocalization of the positive charge over the entire {FeBNCAr2} moiety. Also, the molecular structure of 614 could be determined and features an essentially linear FeBNC fragment. The FedB bond distance of 1.835(6) A˚ is very similar to related cationic aminoborylene compounds, and the NdC distance of 1.287(7) A˚ is nearly identical to that of the precursor 613. DFT computations reveal that both the a- and g-positions of the BNC moiety are electrophilic, which motivated further reactivity studies towards nucleophiles.355,356

Scheme 155 Preparation and isolation of [CpFe(CO)(PCy3)(]B]N]CAr2)] (614, Ar¼p-tolyl (a), mesityl (b)).355,356

The reactivity of compounds 614a/b towards anionic nucleophiles (Cl−, PhS−, and CN−) was explored, all of which exclusively attack at the very electrophilic boron atom (Scheme 156).355,356 In contrast no reaction was observed towards 2,3-butadiene, Me3SiCCH and iPrNCO. However, with carbodiimides metathesis-like reactivity is observed (Scheme 156), which is contrary to the insertion chemistry encountered for aminoborylene compounds (Scheme 147).355 Based on detailed NMR studies on the reaction mixture the authors proposed the reaction pathway as outlined in Scheme 156, which involves the initial formation of an unsymmetric [2 + 2] cycloaddition product 618. However, it turned out to be too unstable for structural authentication, but the degradation barriers to the final products 619 and 620, the iron isonitrile complexes and coordinatively trapped hetero-allenes, were determined.355

Scheme 156 Reactivity studies on 614a and 614b towards nucleophiles and carbodiimides.355,356

164

7.03.7.2

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Monocyclopentadienyl compounds bearing Al—, Ga—, In-based ligands

In contrast to the very well-developed boron chemistry, related studies on the heavier group 13 elements have been more tedious and require a judicious choice of suitable auxiliary ligands. A series of compounds 621 was isolated, in which 4-membered amidinato-Group 13 metal(III) heterocycle coordinates to a {CpFe(CO)2} fragment (Scheme 157a).357 Complexes 621 were fully characterized and in some cases also the solid-state molecular structure was elucidated. Moreover, attempts were undertaken to abstract the coordinated halide, and in one case the cationic complex [CpFe(CO)2Ga(OEt2){(CyN)2CtBu}][BArF4] (622) could be isolated and structurally authenticated. Nevertheless, this species is sensitive to hydrolysis which led also to the formation of [{CpFe(CO)2Ga[(CyN)2CtBu]}2(m-OH)][BArF4] (623). More recently, the series of FedAl containing molecules was extended to include also 4-membered guanidinato-Al(III) species (624) (Scheme 157b).358

Scheme 157 Preparation of a series of monocyclopentadienyl iron compounds featuring 4-membered amidinato/guanidinato-Group 13 metal(III) heterocycles.357,358

In 2005 the Aldridge group presented the synthesis of various three-coordinate, halide-functionalized gallium or indium fragments being coordinated to the {Cp Fe(CO)2} moiety as potential precursors for a further halide abstraction to generate cationic complexes (Scheme 158).326 To access these precursors two synthetic strategies have been described: (a) Salt-metathesis reaction between the Na[(
5-C5R5)Fe(CO)2] (R¼H (532), Me (542)) and Mes GaCl2 or Mes InBr2 (Mes ¼ 2,4,6-tBu3C6H2) to generate [(
5dC5R5)Fe(CO)2E(Mes )X] (R¼H, Me; E¼Ga, X¼Cl; E¼In, X¼Br); (b) gallium(I) or indium(I) halide insertion into a metaldhalogen or metaldmetal bond, followed, if necessary, by substitution with a sterically bulky anionic nucleophile to generate the desired compounds.326 Although the insertion route offers access to a broader range of complexes, it is significantly less convenient than the one-pot salt-metathesis approach. Most of these complexes were structurally characterized, but the authors also noted the impact of sterics in these systems to prevent the formation of halide-bridged oligomers.

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

165

Scheme 158 Synthesis of various three-coordinate, halide-functionalized gallium or indium fragments being coordinated to the {Cp Fe(CO)2} moiety using (a) a salt-metathesis or (b) gallium(I)/indium(I) halide insertion strategy.326

In an attempt to prepare [CpFe(CO)2In(Mes )Br] from [{CpFe(CO)2In(m-Br)}2] and Mes Li the LiBr adduct [{CpFe(CO)2}2In (m-Br)2Li(OEt2)2] was isolated in low yield.359 Successful halide substitution can, however, be achieved in the reaction of [Cp (CO)2In(Mes )Br] with NaOC6H4-4-tBu to yield [Cp (CO)2In(Mes )(OC6H4-4-tBu)].360 The group of Tokitoh prepared related iron-bromoalumanyl compounds [Cp (CO)2FeAl(tbb)Br] (tbb ¼ 2,6d((Me3Si)2CH)2t 4- Bu-C6H2) following a salt-metathesis strategy as advanced for the Ga and In derivates employing K[Cp Fe(CO)2] and [(tbb) AlBr2(OEt2)].361 The AldFe bond of 2.326(2) A˚ is only slightly shorter than in the tetra-coordinated aluminyl complex [Cp (CO)2FeAl(CyNC(NiPr2)NCy)Cl] (2.357(1) A˚ ),358 which implies that FedAl p backbonding is negligible as suggested by DFT computations. In addition, the very low CO stretching frequencies for [Cp (CO)2FeAl(tbb)Br] imply that the aluminyl moiety acts as a stronger s donor and/or weaker p acceptor relative to the related boryl and gallyl fragments in [Cp (CO)2FeB(Mes )Br]362 and [Cp (CO)2FeGa(Mes )Cl].326 Furthermore, DFT computations also implied that p donation from the halide lone pair to the group 13 atom increases the stability of these complexes by moderating the Lewis acidity of the group 13 atom. Dichlorogallyliron complexes exhibit interesting reaction chemistry. For example, the dichlorogallyliron compound [Cp (OC)2FeGaCl2] (625) readily reacts with zirconacyclopentadiene [Cp2Zr(C4Me4)] to form the first
1-gallacyclopentadienyl 626, which can be isolated upon addition of 4-(dimethylamino)pyridine as the Lewis-base adduct 627 (Scheme 159a).363 The solid-state molecular structure of 627 features a distinct alternation of the CdC bond distances within the gallacyclopentadienyl moiety.

166

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

Addition of the carbonylmetallates K2[M(CO)5] (M¼Cr, W) to [(
5¼C5R5)(dppe)FeGaCl2] (628, R¼Me,364 H365) forms the Ga-bridged heterobimetallic species 629 in good yield (Scheme 159b). The molecular structure of the 629b features Fe-Ga and Ga-W distances of 2.2687(12) and 2.5861(8) A˚ , respectively, which are significantly shorter than standard FedGa (2.36–2.46 A˚ ) and Ga-W (2.71–2.76 A˚ ) single bonds and are in the range typically observed for metal gallylene complexes.364 Based on a pronounced shift in the CO stretching frequencies of the {M(CO)5} fragment, the authors classified the {Cp (dppe)FeGa} fragment as strong s donor, but relatively weak p acceptor, which can be rationalized by the fact that the {(
5-C5R5)(dppe)Fe} acts as a donor-ligand to the Ga p-orbitals, reducing the p acidity of the Ga atom.364 These heterobimetallic compounds were also subjected to photoirradiation experiments in the presence of [Fe(CO)5] which induced a formal gallylene transfer to the {Fe(CO)4} fragment forming the gallium-bridged diiron complexes [(
5-C5R5)(dppe)FeGaFe(CO)4] (630, R¼H, Me) in moderate yields (Scheme 159b).365 The reverse reaction, i.e., photochemically induced gallylene transfer from [Cp(dppe)FeGaFe(CO)4] to a {M(CO)5} (M]W, Cr) fragment is also feasible. Please note that for the [Cp (dppe)FeGaFe(CO)4] no reaction occurs under photoirradiation in the presence of [M(CO)6] (M¼W, Cr). However, prolonged irradiation of [Cp(dppe)FeGaFe(CO)4] induces CO release and dimerization to yield a gallylene-bridged tetrametallic complex [{(OC)3Fe}2(m-CO){m-Cp(dppe)FeGa}2] (631). Furthermore, 57Fe Mössbauer spectra were recorded on [(
5dC5R5)(dppe)FeGaFe(CO)4] (630; R¼Me, H) in which two distinct Fe sites were identified exhibiting two well-resolved quadrupole split doublets, corresponding to the {(
5-C5R5)(dppe)Fe} and {Fe(CO)4} fragments, respectively.366

Scheme 159 Preparation of [Cp (OC)2FeGa(
1-gallacyclopentadienyl)] (626) and Ga-bridged heterobimetallic species starting from [(
5-C5R5)(dppe)FeGaCl2] (628, R¼H, Me).363–365

Dichlorogallyliron species [(
5-C5R5)(CO)2FeGaCl2] (R¼Me (625), H (633)) also react with LiPPh2 and/or Me3SiPPh2 to form dimeric species 633 and 634 featuring a four-membered Ga2P2 core (Scheme 160).367 In the solid state the Ga2P2 moiety in the dominant cis-isomer cis-633 adopts a slightly puckered arrangement. In solution the Ga2P2 moiety stays intact and the cis and trans isomers (cis-633 and trans-633) can be detected. When toluene or benzene solutions of 633 are irradiated CO is released and the six-membered FedGadFedPdGadP cyclic diiron complex [{Cp (CO)Fe}2(m-Ga)(m-Ph2PGaCl2PPh2)] (635) is formed, whose molecular structure indicates some FedGa multiple bonding along the FedGadFe fragment (Scheme 160).

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

167

Scheme 160 Reaction of [(
5-C5R5)(CO)2FeGaCl2] (R-Me (625), H (633)) with LiPPh2 and/or Me3SiPPh2 to yield the dimers 633 and 634 containing a four-membered Ga2P2 core.367

From a computational point of view group 8 dihalogallyl368 and dimethylgallyl369 species of the type [Cp(L)2M(GaX2)] (M¼Fe, Ru, Os; L]CO, PMe3; X¼Cl, Br, I, Me) attracted some interest. Based on DFT computations the MdGa bonds can be classified as single bonds, which increase in length progressing from M¼Fe to M¼Os. Substitution of CO by PMe3 decreases the MdGa distances, consistent with an increase in the HOMO and LUMO energies of the {Cp(L)2M} fragment, which slightly increases the p backbonding, but decreases the s-donation from the {GaMe2} fragment. Nevertheless, in all complexes the p backbonding is distinctly smaller than the s bonding. The MdGa s bond has a large contribution from the Ga s orbital. Energy decomposition analysis reveals that the electrostatic contribution to the bonding dominates for the dihalogallyl systems,368 but an increased covalent contribution is noted for the dimethylgallyl derivatives.369 Furthermore, for the dimethylgallyl species short GadC(CO) and GadP bond distances are noted which imply weak intramolecular interactions.369 Early on attempts were made to evaluate the suitability of diiron halogallyl species in halide abstraction reactions, and indeed cationic, dimeric two-coordinate complexes [{Cp Fe(CO)2}2(m-E)]+ (E¼Ga (636); E]In (637)) featuring bridging gallium or indium atoms in a linear arrangement can be isolated (Scheme 161).370 While structural, spectroscopic, and computational studies

Scheme 161 Synthesis and reactivity of [{Cp Fe(CO)2}2(m-E)]+ (E¼Ga (636); E¼In (637)) and halide abstraction reactions on [(
5-C5R5)Fe(CO)2E(Mes )X] (E¼Ga, In).370

168

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

for [{Cp Fe(CO)2}2(m-Ga)]+ (636) are in agreement with a significant FedGa p bonding, for the analogous indium-bridged species 637 the FedIn p component to the FedIn bond is significantly reduced. The cationic two-coordinate complexes 636 and 637 can bind Lewis bases such as THF (E¼Ga (638a), In (639))370 or 4-picoline (E¼Ga, 639b).360 When dynamic vacuum is applied the coordinated THF ligand can be removed. Attempts to generate monomeric species using the supermesityl-substituted gallyl or indyl precursors of the type [(
5-C5R5)Fe(CO)2E(Mes )X] (E¼Ga (640/641); E¼In (642)) only formed halide-bridged species of the type [{(
5-C5R5)Fe(CO)2E(Mes )}2(m-X)]+ (E¼Ga (643/644); E¼In (645)), which is presumably a result of the strong electrophilicity of the cationic intermediate [{(
5-C5R5)Fe(CO)2E(Mes )}]+ (Scheme 161).370 In a follow-up paper halide-abstraction from the [Cp Fe(CO)2ER(X)] (E¼Ga, In) species to access monomeric complexes featuring two-coordinated Ga atoms was evaluated (Schemes 161 and 162).371,372 Nevertheless, the stability of the resulting species varies significantly with the electronic and steric characteristics of the utilized metal/ligand combination, which also led to the synthesis of the mesityl iodide derivatives [Cp Fe(CO)2Ga(Mes)(I)] (650) and [Cp Fe(dppe)Ga(Mes)(I)] (651) (Scheme 163).372

Scheme 162 Comparing the reactivity between B328,329 and its heavier homologues, Ga and In, upon halide abstraction.371,372

Scheme 163 Preparation of [Cp Fe(dppe)Ga(I)(Mes)].372

For example, using p accepting CO ligands, the base-free [Cp Fe(CO)2GaMes]+ is only detectable by ESI-MS experiments, and its isolation requires the presence of Lewis bases such as dtby (dtby ¼ 4,4´-(tBu)2-2,2´-bipyridine) (Scheme 164).372 In contrast, replacement of the two CO ligands by the more s donating bisphosphine (dppe) significantly enhances the stability of these two-coordinate Ga species, which allows the structural authentication of [Cp Fe(dppe)(GaI)][BArF4] (653), in which the GaI ligand acts as an isovalence electronic analogue to the more traditional ligands CO and N2 (Scheme 164).372 DFT computation indicate only weak binding of the GaI ligand. This is also experimentally validated by CO addition, which readily replaces the coordinated CO ligand to yield [Cp Fe(dppe)(CO)][BArF4] (654). However, in fluorobenzene solution 653 is stable for weeks. This remarkable stability is attributed to (i) efficient steric protection of the gallium atom by the supporting phosphine and Cp ligands; (ii) hindered dimerization because of the cationic charge of the complex, and (iii) some stabilizing p backbonding from the HOMO and HOMO-2 of the {Cp Fe(dppe)}+ fragment to the LUMOs of the GaI ligand.

Monocyclopentadienyl and Other Half-Sandwich Complexes of Iron

169

Scheme 164 Halide abstraction from [Cp Fe(CO)2Ga(I)(Mes)] (650) and [Cp Fe(dppe)GaI2] (649).372

A comprehensive DFT study followed up on the initial work concerning 654 and extended it to cationic group 8 gallylene compounds of the type [Cp(L)2M(GaX)]+ (M¼Fe: L¼CO, PMe3; X¼Cl, Br, I, NMe2, Mes; M¼Ru, Os: L¼CO, PMe3; X¼I, NMe2, Mes) was presented.373 The most significant conclusions resulting from this study are: 1) All investigated systems feature short MdGa bonds, which have only a small dependence on the X, when X¼Cl, Br and I. This suggests a pronounced Ga s orbital contribution to the MdGa bond and large electrostatic contributions to the bonding. 2) In all investigated complexes the p-bonding component to the GadX bond is significantly smaller than the s bond contribution, which is consistent with the notion that GaX ligands are essentially s donor ligands. 3) Evaluating the Wiberg bond indices (WBIs) it can be concluded that the MdGaX bonds in these cationic gallylene species are weaker than the MdCO bonds. 4) When X¼Mes, the MdGa bond is more ionic and can engage in an enhanced charge transfer to the [Cp(L)2M]+ moiety, which gives rise to larger bond dissociation energies (BDEs) compared to MdGa species in which X¼Cl, Br, I or NMe2. 5) The BDEs of the MdGa bonds in the systems [Cp(L)2M(GaX)]+ show the following dependences: X¼Cl < Br < I1.70 A˚ ).17 On the other hand, the iron-carbyne bond distances [1.840(10) and 1.828(11) A˚ ] are shorter than the iron-mcarbonyl distances [1.923(10) and 1.949(11) A˚ ] in the same cations, highlighting the higher p-acceptor character of the thio-alkylidyne ligand compared to CO. Complex [70a]+ (R ¼ Me) undergoes substitution of one carbon monoxide by P-, C- and N-donors (Scheme 29A).169 In the products [72]+, due to the partial double bond nature of the carbyne-S interaction, the two possible mutual orientations of the S-methyl group and L usually give raise to stereo-isomerism. The lability of CO is enhanced by mono-electronic reduction.182

Scheme 29 Overview of the reactivity of diiron m-thio-alkylidyne bis-cyclopentadienyl complexes. (A) R ¼ Me; L ¼ PEt3, PMe2Ph, PMePh2, P(OMe)3, CNMe, 4-dimethylaminopyridine. (B) R ¼ Me, CH2Ph, CH2CH]CH2; Nu ¼ H, CN, SMe, SPh. (D) R ¼ CH2Ph, Ph, iPr (from RMgCl). (E) R ¼ Me, Et, R0 ¼ Me, Et. (F) From [NBu4]NCO; R ¼ Me, Et. (G) R0 ¼ Me, Et. (H) R ¼ Me. (I) R ¼ Me; R0 ¼ R00 ¼ Me; R0 ¼ CO2Et, R00 H; R0 ¼ SiMe3, R00 ¼ Me. (K) R0 ¼ Me, Ph (from Li2CuCNR0 2).

234

Alkylidyne and Alkylidene Complexes of Iron

Different types of anionic nucleophiles (hydride, cyanide, thiolates) react with [70]+ giving addition to the alkylidyne carbon, to afford neutral thio-alkylidene complexes 73 (Scheme 29B).47,183 The 13C NMR resonance of the alkylidene carbon in 73 is found in the range 164 (Nu ¼ H, R ¼ CH2Ph) to 189 ppm (Nu ¼ SMe, R ¼ Me). The cyano-thio-alkylidene moiety in 73 is methylated by methyl triflate at the S atom (Scheme 29C).49 Also the cyclopentadienyl ring and the carbonyl moiety are possible targets for nucleophilic additions. As a matter of fact, the reactions of [70]+ with Grignard reagents result in the formation of the thio-alkylidene compounds 75, arising from the initial attack to Cp followed by hydrogen migration from the resulting cyclopentadiene to the alkylidyne, thus regenerating the aromaticity on the C5 cycle (Scheme 29D).6 Instead, alkoxides are directed to one terminal CO to give a carboxylato ligand, which then migrates to the bridging carbyne to yield complexes 76; the coordination vacancy consequent to ligand migration is filled by the sulfido group (Scheme 29E).52 The cyanate ion, NCO−, inserts into the carbyne-S bond of [70]+ affording in 60–80% yields complexes 77, bearing a bridging isocyanide functionalized with a S-methyl ethanethiolate substituent (Scheme 29F).48 It should be noted that this class of isocyanides is otherwise unknown. Complexes 77 undergo isocyanide alkylation, leading to a family of amino-alkylidynes analogous to [28]+ (Scheme 29G). The products, [78]+, can be stored for months under N2 atmosphere but slowly decompose in dichloromethane solution. Besides the route to aminocarbyne (Scheme 29G), complexes [70]+ can be also exploited to convert the thiocarbyne function into phosphino-alkylidyne.204 The reaction with a silyl-phosphine, in the presence of DBU, proceeds with CdP bond coupling involving the carbyne, then CO elimination and {SMe} migration take place to give an unique example of phosphino-alkylidyne complex, [79]+ (Scheme 29H). The IR spectrum (KBr disk) displays the CO stretching vibrations at 2013 and 1935 cm−1, while the resonance of the alkylidyne carbon occurs at 420.0 ppm (C6D6 solution). The reactivity of complexes [70]+ can be modified by preliminary removal of a carbonyl ligand using Me3NO. When the resulting vacant coordination site is occupied by an allene, deprotonation with triethylamine and subsequent C-C coupling with the bridging carbyne take place in a high stereoselective manner concerning the R0 and R00 substituents (Scheme 29I).63 In the absence of trimethylamine-N-oxide, no reaction occurs between [70]+ and allenes. The thiocarbyne-allenyl coupling resembles the analogous process described for aminocarbyne complexes (see Scheme 18). Here, the thio-butadienyl products 80 are obtained in good yields and are susceptible to straightforward alkylation at the sulfur. Carbon monoxide-acetonitrile substitution is also viable by means of Me3NO (Scheme 29J). Interestingly, the acetonitrile ligand in [81]+ is not deprotonated by carbanions, in contrast to what observed for the analogous amino-alkylidyne complexes [35]+ (see Scheme 15). Hence, the reactions of [81]+ with alkyl/aryl cuprates, Li2Cu(CN)R2 (R ¼ Me, pH), proceed with selective addition to the carbyne and nitrile extrusion (and not deprotonation) to give 82, bearing the same structural core as that of 76 (Scheme 29K).11 Conversely, the reactions of the tricarbonyl [70]+ with Li2Cu(CN) R2,42 as well as the reaction of [70]+ with thienyllithium,9 are nonselective yielding mixtures of C-C coupling products. The thio-alkylidyne ligand in [71]+ exhibits a chemistry parallel to that of [70]+ with reference to hydride and cyanide addition7 and cyanate insertion (Scheme 29B and F)51; however, the presence of a thio-alkylidene co-ligand provides a double opportunity of derivatization. A view of the X-ray structure of complex 76a, derived from methoxide addition to [70a]+, is shown in Fig. 10; the coordination of the sulfur atom to one iron center determines a marked asymmetry of the bridging ligands, and especially the carbonyl one [Fem-CO bond distances are 1.82(1) and 1.99(1) A˚ ].

Fig. 10 View of the X-ray structure of [Fe2Cp2(CO)(m-CO){m:kCkS-C(CO2Me)SMe}] (76a).52 Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ), average values: Fe1-Fe2 2.520(2), Fe2-C6 1.75(1), Fe2-C2 1.95(1), Fe1-C2 1.90(1), C2-S1 1.77(1), Fe1-S1 2.269(2), Fe1-C5 1.82(1), Fe2-C5 1.99(1), Fe2-Fe1-S1 79.2(1), C2-S1-Fe1 54.7(2).

Alkylidyne and Alkylidene Complexes of Iron

7.05.3.4

235

Alkoxy-alkylidyne complexes

Diiron complexes with bridging alkoxy-alkylidyne ligands have been typically prepared upon alkylation of a bridging carbonyl ligand in suitable precursors. The first example was reported in 1984: the presence of a net negative charge in the diiron carbonyl species [83]− renders the bridging CO sufficiently nucleophilic to undergo straightforward alkylation (Scheme 30A).176 The resulting alkoxy-alkylidyne complex 84a is intrinsically unstable in solution at room temperature, undergoing a smooth isomerization to alkoxy-alkylidene via carbyne-vinyl coupling (Scheme 30B). The high reactivity of the alkylidyne moiety in 84a is also manifested by the reactions with electron-poor alkynes. The latter initially enter the iron coordination sphere by displacing the alkene from Z2-coordination, then alkyne insertion into carbyne-iron bond takes place analogously to what described above for the synthesis of vinyliminium complexes (Scheme 19). In the present case, five-membered dimetallacyclic alkoxy-alkylidene products (85) are afforded, see Scheme 30C.177 This transformation is accompanied by a dramatic shift of the 13C resonance of the bridging O-substituted carbon, from 384 ppm to ca. 225 ppm. A different outcome (double C-C coupling involving the vinyl ligand) was observed upon reaction of the analog of [Fe2{m:Z1:Z2-CH]CH2}(m-COEt)(CO)6] (84b), lacking phenyl substituents, with hexafluorobutyne.177

Scheme 30 Synthesis and reactivity of m-alkoxy-alkylidyne diiron complexes (R ¼ CO2Me, CF3). Ros, J.; Mathieu, R.; Solans, X.; Font-Altaba, M. J. Organomet. Chem. 1984, 260, C40–C42; Ros, J.; Commenges, G.; Mathieu, R.; Solans, X.; Font-Altaba, M. J. Chem. Soc. Dalton Trans. 1985, 1087–1094.

Hexacarbonyl alkoxy-alkylidyne complexes with a sulfido or selenido bridging co-ligand were also prepared using the CO-alkylation strategy (Eq. 8).185,192

 ½Et3 NHŠ Fe2 ðCOÞ6 ðm −ER Þðm −COÞ +½Et3 OŠBF4 ! Fe2 ðCOÞ6 ðm −ER Þðm −COEtÞ ð86Þ +½Et3 NHŠBF4 +Et2 O (8)

E ¼ S; R ¼ Ph, tBu E ¼ Se; R ¼ 2-C6H4Me, 3-C6H4Me, 4-C6H4Me, 2-naphthyl, 4-C6H4Br, 4-C6H4OMe The X-ray structure of [Fe2(CO)6{m-Se(4-C6H4Me)}(m-COEt)], 86a, is shown in Fig. 11A. The carbyne-O distance [1.307(11) A˚ ] is shorter than typical Csp2–O single bonds in organic molecules17 and, accordingly, the carbyne-O-Et angle [118.0(8) ] is suggestive of an almost sp2 hybridization of the oxygen, accounting for a non-negligible p-contribution to the carbyne-heteroatom bond, as outlined to different extents for amino- and thio-alkylidyne bridging ligands (vide infra). These features are also found in [Fe2(CO)6{m:Z1:Z2-CPh]CHMe}(m-COEt)] (84c), which is analogous to 84a (Fig. 11B).191 A similar picture was traced by DFT calculations on a methoxycarbyne group bridging coordinated in a dimolybdenum bis-cyclopentadienyl complex, for which the authors suggested the carbyne-O bond to possess multiple character.108

236

Alkylidyne and Alkylidene Complexes of Iron

Fig. 11 (A) View of the X-ray structure of [Fe2(CO)6{m-Se(4-C6H4Me)}(m-COEt)] (86a).192 Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): Fe1-Fe2 2.513(4), Fe1-C7 1.843(10), Fe2-C7 1.818(10), C7-O7 1.307(11), O7-C8 1.489(13), Fe2-C5 1.774(13), Fe1-C1 1.832(13), Fe2-C4 1.809(13), Fe2-C6 1.780(14), C7-O7-C8 118.0(8), O7-C7-Fe1 131.2(8). (B) View of the X-ray structure of [Fe2(CO)6{m-Se(4-C6H4Me)}(m-COEt)] (84c).191 Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): Fe1-Fe2 2.549(2), Fe1-C1 1.884(12), Fe2-C1 1.821(12), C1-O1 1.290(12), O1-C2 1.493(13), Fe1-C4 2.092(10), Fe2-C4 2.014(10), C1-O1-C2 118.1, O1-C1-Fe2 143.3(10).

The alkoxy-alkylidyne ligand within thiolato complexes of type 86 undergoes clean isomerization either by means of thermal or prolonged photochemical treatment, to afford a bridging acyl group (Scheme 31A).187 On the other hand, the reactions with activated alkynes usually proceed with the insertion into metal-carbyne bond, accompanied by C-C coupling with one carbon monoxide ligand (58–83% yields, Scheme 31B).187

Scheme 31 Reactivity of diiron (m-thiolato)-(m-alkoxy-alkylidyne) complexes (R ¼ tBu, Ph; R1 ¼ H, CH3 or CO2Me; R2 ¼ CO2Me). Seyferth, D.; Ruschke, D.P.; Davis, W.M. Organometallics 1994, 13, 4695–4703.

Chen and co-workers constructed the alkoxy-alkylidyne unit within diiron carbonyl cycloheptatriene and cyclooctatetraene complexes (Scheme 32). Thus, the 1:2 M reactions of [Fe2(CO)6(m:Z4-C7H8)] with lithium aryls, followed by double alkylation with triethyloxonium tetrafluoroborate, led to 87 in moderate yields.211 This process takes place with the coupling between one alkoxy-alkylidene intermediate fragment and the COT moiety. Instead, compounds 88 were obtained in very low yields as byproducts of a main reaction affording related alkoxy-alkylidene species.220

Alkylidyne and Alkylidene Complexes of Iron

237

Scheme 32 Diiron alkoxy-alkylidyne complexes with cycloheptatriene-modified (87) and cyclooctatetraene (88) ligands. Ar ¼ 4-C6H4Me, 4-C6H4OMe, 4-C6H4Cl, 4-C6H4CF3, C6Cl5; Ar0 ¼ Ph, 4-C6H4Me, 4-C6H4CF3. Zhang, S.; Zhang, L.; Xu, Q.; Sun, J.; Chen, J. Organometallics 2006, 25, 191–196.

7.05.3.5

Comparative analysis of structural and spectroscopic features of diiron alkylidyne complexes

The salient structural and spectroscopic data of selected diiron m-alkylidyne complexes are given in Table 2. A comparison concerning those complexes based on the {Fe2Cp2(CO)3} framework, i.e., [11b]+, [28d-e]+ and [70b]+, highlights an average infrared carbonyl stretching wavenumber (cm−1) decreasing along the sequence [11b]+ (1972) > [70b]+ (1968) > [28e]+ (1956) > [28d]+ (1955). This trend reflects the progressively lower backbonding from the two Fe+I centers to the carbyne on going from classical alkylidyne to thio-alkylidyne and then amino-alkylidyne, thus resulting in enhanced iron to carbonyl backdonation. Accordingly, the interaction of the carbyne empty p orbital with the adjacent atom (E ¼ C, S or N) increases along the same series [11b]+ < [70b]+ < [28e]+ < [28d]+. Interestingly, the influence of the amino-alkylidyne substituents is appreciable, and indeed a higher overlap of carbyne-nitrogen p-orbitals is observed with two alkyl substituents on nitrogen ([28d]+) rather than one alkyl and one less donating aryl ([28e]+). This effect is more evident from the IR values related to the carbyne-N bond (1602 vs. 1522 cm−1), indicating a higher bond order, which approaches two in the case of [28d]+. Crystallographic data are coherent with the general trend and, in particular, significantly longer iron-carbyne distances have been found for the amino-alkylidyne complexes [28d-e]+ with respect to the analogous classical alkylidyne and thio-alkylidyne species. Moreover, the 13C NMR resonance related to the carbyne nucleus appears to correlate with the degree of carbyne-E p-interaction, a lower chemical shift value being detected on increasing the CdE bond order (classical alkylidyne < thio-alkylidyne < aminoalkylidyne). The amino-alkylidyne Fe−I-Fe−I complex 69a displays an exceptionally short carbyne-nitrogen bond, due to lack of backdonation from the metal centers to the alkylidyne, the backdonation being preferentially directed to the strongly electron acceptor carbonyl and nitrosyl ligand set. In the hexa-carbonyl complex 26a, featuring a thiolato group bridging two Fe0 atoms, the iron-carbyne distances are longer than in [11b]+, and this fact may be explained based on a lower degree of backbonding directed to the alkylidyne unit in 26a, lacking strongly donor Cp rings. This determines a significant shielding effect on the carbyne nucleus on going from [11b]+ to 26a (d ¼ 360.5 ppm vs. 499 ppm). A comparison between the homologous hexacarbonyl complexes 26a and 86a highlights slightly shorter Fe0-carbyne distances in the latter alkoxy-alkylidyne compound compared to the former classical alkylidyne. On the other hand, carbyne-O bond distance and carbyne-O-Et angle in 86a suggest non-negligible CdO p-bonding (see above). Combined, it might be concluded that the p-bonding contribution to the carbyne-O interaction in alkoxy-alkylidynes is relatively modest and, therefore, the degree of iron to carbyne backbonding is not substantially different compared to classical alkylidynes.

7.05.4

Advances in polyiron alkylidyne complexes (since 2000)

In this section, advances in the chemistry of alkylidyne polyiron complexes will be presented covering the literature since 2000 onwards. The most common situation refers to triply bridging alkylidyne ligands which behave as tri-electron donors in tri- and tetrairon clusters; the related synthetic procedures were reported in the 1970s and 1980s (clusters with classical alkylidyne,41,206 haloalkylidyne,126 alkoxy-alkylidyne119,206 and amino-alkylidyne ligands116). A structural motif based on diiron bis cyclopentadienyl m2-alkylidyne complexes with an additional ferrocene unit, unconnected to the [FeFe] core, was described by Farrell, Manning and co-workers in 2001.98 In addition, Zacchini and co-workers obtained traces of the anion [Fe3(CO)10(m3-CMe)]− by heating solutions of [Fe(CO)4(AuNHC)]− complexes in dichloromethane at 50  C.34 The m3-alkoxy-alkylidyne triiron complex [Fe3(CO)9(m3COSiMe3){m3-COLi(THF)3}] (89) was prepared in a low yield via silylation of a diiron precursor containing two lithiated carbonyl functions.162 Analogously, [Fe3Cp3{m3-COSiH2N(SiMe3)2}{m2-SiHN(SiMe3)2}] (90) was isolated in 22% yield by a photochemical reaction in toluene using [FeCp(CO)2(SiMe3)] as starting material.112 The alkoxy-alkylidyne carbon within 90 resonates at 195.4 ppm in the 13C NMR spectrum. In the X-ray structure of 89, the carbyne-O distance value is 1.344(4) A˚ , that is significantly longer than in the m-alkoxy-alkylidyne diiron complex 86a [1.307(11) A˚ ] and in the m2-alkoxy-alkylidyne triiron [Fe3(m-H) (m-COMe)(CO)] [91; 1.3021(6) A˚ ].99 This set of data evidences that the bonding of the alkoxy-alkylidyne to three iron centers has a substantial weakening effect on the carbyne-O p-overlap.

Table 2

Comparative view of 13C NMR, IR and X-ray data related to the bridging alkylidyne ligand in selected diiron complexes. 13

C NMR (dm-C, ppm)

IR (ῦm-CE, cm−1)

IR (ῦCO, cm−1)

X-ray (dC-E, ˚A)

X-ray (dFe-m-C, ˚A)

References

499

¼¼

2047, 2016, 1853

1.45(3)

1.862(16), 1.828 (16)

114

360.5

¼¼

¼¼

1.416(5)

1.860(3), 1.881 (3)

186

315.5

1602

2022, 1990, 1853

1.297(4)

1.875(2), 1.875 (2)

1,38

331.5

1522

2030, 1998, 1841

1.295(5)

1.871(4), 1.875 (4)

1,38

314.2

¼¼

2078,2020, 2009, 2000, 1985, 1959

1.266(12)

1.874(11), 1.950 (10)

142

404.6

1024

2040, 2009, 1855

1.666(11)

1.840(10), 1.828 (11)

25,90a

383.2

1296

2066, 2026, 1993, 1970

1.307(11)

1.843(10), 1.818 (10)

176

NMR spectra in CDCl3, CD2Cl2 or other common deuterated solvent; IR spectra in CH2Cl2 or KBr; E ¼ C, N or S according to the case. a − BF4 , PF−6 or CF3SO−3 salt.

Alkylidyne and Alkylidene Complexes of Iron

239

The bis-alkylidyne cluster [Fe3(CO)9(m3-CF)2] (92a) is available from the thermal reaction of iron carbonyls with CFBr3.126 The photochemical reaction of 92a with difluoroallene affords two products (93–94) in very modest yields, as a result of carbon-carbon coupling involving one or both alkylidyne ligands, respectively (Scheme 33A).125 Interestingly, the formation of 93–94 implies intramolecular F-migration steps. Similarly, [Fe3(CO)9(m3-CR)2] (92a-c; R ¼ F, Br, Cl) react with the tert-butyl-phosphaalkyne t BuC^P under UV irradiation to give CdP and CdC bond forming products 95 (phosphaferrole clusters), see Scheme 33B.127

Scheme 33 Reactivity of bis-alkylidyne triiron clusters with (A) difluoroallene (R ¼ F); (B) tert-butyl-phosphaalkyne (R ¼ F, Cl, Br, H). Lentz, D.; Willemsen, S. J. Organomet. Chem. 2002, 641, 215–219; Lentz, D.; Michael-Schulz, H.; Reuter, M.; Z. Anorg. Allg. Chem. 2004, 630, 563–572.

A number of m3-coordinated alkylidyne ligands has been installed on tetrairon clusters. Thus, [Fe4(Cp0 )4(m3-CO)3(m3-CH)]+ ([96]+), [Fe4(Cp0 )4(m3-CO)2(m3-CH)2]2+ ([97]2+) and [Fe4(Cp0 )4(m3-CO)(m3-CH)(HCCH)]+ ([98]+) were isolated as hexafluorophosphate salts from the reaction of the cubane [Fe4(Cp0 )4(m3-CO)4] with LiAlH4 in THF solution, followed by air oxidation in the presence of [NH4]PF6.157,158 Despite obtained in very low yields, the identity of these products demonstrates the feasibility of the direct formation of a triply bridging methylidyne upon carbon monoxide reduction. The alkylidyne fragments in [96–98]n+ are featured by typical downfield resonances in the NMR spectra, i.e., at d ¼ 365–392 ppm (13C) and d ¼ 17.3–20.3 ppm (1H). A view of the X-ray structure of [96]+ is shown in Fig. 12, with relevant bonding parameters listed in the caption. The {m-CH} fragment might be viewed as intermediate along the reductive CO coupling to give acetylene, and indeed the straightforward two-electron cobaltocene reduction of [97]2+ reversibly generates [Fe4(Cp0 )4(m3-CO)2(HCCH)] (Eq. 9). " #2 + CoCp2
 Fe4 ðCp0 Þ4 ðm3 -COÞ2 ðHCCHÞ (9) FeðCp0 Þ4 ðm3 -COÞ2 ðm3 -CHÞ2 Ð ½97Š2 +

air, NH4 PF6

Fig. 12 View of the X-ray structure of the cation within [96][PF6].157 Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): Fe1-Fe2 2.507 (1), Fe1-Fe4 2.5064(9), Fe2-Fe4 2.520(2), Fe1-C1 1.875(5), Fe2-C1 1.883(5), Fe4-C1 1.877(4), Fe1-C2 2.006(5), Fe2-C2 2.012(5), Fe1-Fe2-Fe4 59.81(3).

240

Alkylidyne and Alkylidene Complexes of Iron

The oxidative cleavage of alkyne bonds affording m3-alkylidyne ligands (Eq. 9, reverse reaction) has been extended to analogous systems by Okazaki, Ozawa and collaborators.159–161 In particular, the sequential treatment of [Fe4(Cp0 )4(HCCH)(HCCBr)]+ with NHt2Bu and [FeCp2]+, in acetonitrile solution, leads to [Fe4(Cp0 )4(HCCH)(m3-CNHtBu)(m3-CH)]2+ ([99]2+). The classical alkylidyne and the amino-alkylidyne ligands are generated via triple carbon-carbon bond scission.160 The 62-electron complex [99]2+ exhibits two key signals in the downfield region of the 13C NMR spectrum, i.e., at 342.3 (m3-CH) and 333.7 ppm (m3-CN). The secondary aminocarbyne group is susceptible to deprotonation by means of DBU, affording [Fe4(Cp0 )4(HCCH)(m3-CNtBu)(m3:Z1-CH)]+ ([100]+) in a quantitative yield. Complex [99]2+ provides a rare example of triply bridging amino-alkylidyne ligand within a polyiron species. On the other hand, Shi and Fu reported the preparation from Fe3(CO)12 of tetrairon compounds with one or two m2-amino-alkylidyne ligands (101 −102), in moderate yields (Scheme 34).189

Scheme 34 Synthesis of tetrairon carbonyl complexes with doubly-bridging amino-alkylidyne ligand(s). Shi, Y.-C.; Fu, Q. Z. Anorg. Allg. Chem. 2013, 639, 1791–1794.

Several alkylidyne ligands have been implanted on polynuclear hybrid iron-hetero species. Thus, addition of [ClCCo3(CO)9] to a dichloromethane solution of [Fe3(CO)9(CCO)]2−, in the presence of a thallium salt, generates the cobalt-iron carbide anion [{Co3(CO)9}C{Fe3(CO)9(m3-CCO)}]−, which reacts with ethanol to give [{Co3(CO)9}C{Fe3(CO)9(m3-CCO2C2H5)}] (103), containing a carboxylato-substituted alkylidyne ligand bridging the three iron atoms.170 Cluster 103 represents the starting material to access the mixed iron-gold [(AuPPh3)3{Fe3(CO)9(m3-CCO2C2H5)}] (104). The reaction of [RuCp∗(dppe)}2{m-(C^C)x} (x ¼ 3, 4) with Fe2(CO)9 affords hybrid iron-ruthenium structures (105), wherein two Ru+II alkynyl units are connected to a triiron cluster through alkylidyne moieties (Scheme 35A).45 The trinuclear complexes [MFe2Cp(m-H)(m-CO)2(m3-CPh)(CO)6] (M ¼ Mn, 106a; M ¼ Re, 106b) contain the alkylidyne ligands bridging one group 7 and two iron atoms (Scheme 35B).207 An example of m3-alkylidyne bridging two irons and one tungsten was reported,149 while various alkylidyne ligands (m3-CH, m3-CPh and m3-COMe) have been installed on [MoMoFe] clusters by the reactions of dimolybdenum precursors with Fe2(CO)9.18,19,21,22

(A)

(B)

Scheme 35 Structures of heteronuclear complexes bearing triply bridging alkylidyne ligands. Xiao, N.; Xu, Q.; Tsubota, S.; Sun, J.; Chen, J. Organometallics 2002, 21, 2764–2772.

Alkylidyne and Alkylidene Complexes of Iron

7.05.5

241

Phosphino-alkylidene ligands in diiron complexes

Beside the unique example of iron phosphino-alkylidyne (complex [79]+ in Scheme 29), few diiron complexes with P-substituted-alkylidene ligands have been reported, and their structures are drawn in Scheme 36.

Scheme 36 Structures of iron complexes with P-substituted alkylidene ligands.

Knox and co-workers described the thermally-induced intramolecular rearrangement of a dimetallacyclopentenone complex with a diphosphino ligand, affording the phosphonium-alkylidene 107.113 The phosphonium unit was generated from Fe3(CO)12 upon reaction with a tin-derivatized phosphonium ylide,83 and in the product 108 one phenyl ring is ortho-dimetallated. Complex 109 comprises two iron centers which are not directly linked to each other, and the P atom behaves as a donor towards both metals.205 In 2019, Yogendra, De Beer and co-workers reported that the melting of a phosphine-alkyl results in dimerization with formation of a bis-phosphonium alkylidene adduct 110a in a moderate yield, via self-protolysis reaction (Eq. 10). Similarly, the bis-mesityl complex 110b was afforded upon heating the parent compound in fluorobenzene solution at 85  C (Eq. 11).210
  
     (10) 2 FeðCH2 PPh3 Þ NðSiMe3 Þ2 2 ! NðSiMe3 Þ2 Feðm-CHPPh3 Þ2 Fe NðSiMe3 Þ2 2 +2NHðSiMe3 Þ2 110a



 2 FeðCH2 PPh3 ÞðMesÞ2 ! ðMesÞFeðm-CHPPh3 Þ2 FeðMesÞ +2MesH Mes¼2, 4, 6-C6 H2 Me3 110b

(11)

The X-ray structure of 110a is shown in Fig. 13. Several experiments agreed in indicating the high spin Fe+II configuration of the dinuclear complexes 110a-b, with a remarkable anti-ferromagnetic coupling between the iron centers. Reaction of Ph2PCl with a tetrairon carbonyl salt proceeds with {PPh2} insertion into an activated CdS bond and provides the functionalized alkylidene 111.190 The dimeric complex 112, containing two multidentate SCS alkylidene ligands, was obtained from iron(II) dichloride via transmetalation from a scandium complex.107 On the diiron bis-cyclopentadienyl frame, bridging phosphonium-alkylidene adducts 113 are accessible from suitable alkylidyne precursors via nucleophilic addition of phosphines85; analogous cyano-phosphino-alkylidenes have been prepared from [74]+ upon dimethylsulfide replacement by a series of phosphines in the presence of triethylamine (Eq. 12), to give 114 in moderate to high yields.50

242

Alkylidyne and Alkylidene Complexes of Iron

Fig. 13 View of the X-ray structure of [{N(SiMe3)2}Fe(m-CHPPh3)2Fe{N(SiMe3)2}2] (110a).210 Hydrogen atoms (except m-CH) omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): Fe⋯ Fe 2.5939(12), Fe-carbene 2.050(5) and 2.046(4), carbene-P 1.695(4), Fe-carbene-Fe 78.58(16), carbene-Fe-carbene 101.42(16).





 Fe2 Cp2 ðCOÞ3 fm-CðCNÞðSMe2 Þg CF3 SO3 +PR 2 H=NEt3 ! Fe2 Cp2 ðCOÞ3 fm-CðCNÞðPR 2 ÞgŠ +Me2 S +½NHEt3 CF3 SO3 R2 ¼Et2 , CyH, PhH

(12)

114

The 13C NMR resonance of P-substituted alkylidene ligands in the here discussed iron complexes ranges from 110.9 (114, R ¼ Cy and R0 ¼ H) to 135.1 ppm (107). The presence of the additional amino substituent in 109 determines a significant downfield shift, the related carbene nucleus resonating at 246.3 ppm.

7.05.6

Advances in monoiron alkylidene complexes (since 2000)

7.05.6.1

Classical alkylidene complexes

Synthetic iron porphyrin alkylidenes have aroused a considerable interest due to their relevance to the active site of cytochrome P-450 enzymes.29 Li and Che and co-workers synthesized a range of five-coordinated iron porphyrin complexes bearing various axial alkylidene ligands (115) by reaction of the appropriate porphyrin precursor with diazocompounds.128,136 In a number of cases, the +II oxidation state of the iron center was clearly assigned based on solid state XANES and Mossbauer studies (Scheme 37A).

Scheme 37 Structures of porphyrin-alkylidene Fe+II complexes. (A) R, R0 ¼ Ph, Ph; Cl, Cl; Ph, CO2Et; Ph, CO2CH2CH]CH2; Ar ¼ Ph, C6F5, 4-C6H4Me. (B) Ar ¼ C6F5, tolyl; L ¼ pyridine, 4-dimethylaminopyridine, 1-methylimidazole, isocyanide. (C) Im ¼ 1-ethylimidazole, 1-methylimidazole, 1,2-dimethylimidazole. Wang, H.-X.; Wan, Q.; Low,K.-H.; Zhou, C.-Y.; Huang, J.-S.; Zhang, J.-L.; Che, C.-M. Chem. Sci. 2020, 11, 2243–2259; Wang, H.; Schulz, C.E.; Wei, X.; Li, J. Inorg. Chem. 2019, 58, 143− 151.

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243

Related complexes bearing an adamantyl-derived carbene center (116) are remarkably air- and thermally-stable, and were obtained in 2020 in high yields from the parent tetra-coordinated iron porphyrin complexes with azaadamantane at room temperature under UV irradiation.203 The subsequent addition of a carbon or nitrogen ligand completes the hexacoordination sphere (Scheme 37B). In the NMR spectra of diamagnetic 116 with the fluorinated TPFPP ligand, the alkylidene carbon resonates at typical low fields (395–445 ppm), and the +II oxidation state of the metal was confirmed by Raman and Mossbauer analyzes. Complexes of type 115 and 116 work as efficient catalysts for carbene-transfer reactions, e.g., the cyclopropanation of styrene using the diazocompound Ph2CN2 as the stoichiometric carbene source. Another series of six-coordinated iron(II) porphyrin complexes bearing axial diphenyl-alkylidene and imidazole ligands was also reported (117, Scheme 37C)202; the strong p-acceptor nature of the {CPh2} moiety was evaluated to be even higher than those of carbon monoxide and cyanide in analogous systems. A view of the X-ray structure of [Fe(TPP)(CCl2)] (115a; TPP ¼ tetraphenylporphyrin, Ar ¼ Ph, R ¼ R0 ¼ Cl) is shown in Fig. 14. A family of claimed iron(IV) alkylidene cationic complexes (118) was prepared by Wolczanski et al.,132 these compounds being susceptible to nucleophilic addition at the imine function to afford neutral derivatives (119), see Scheme 38.133 However, 118–119 were computed to possess a significant contribution from Fe+II resonance forms.115

Fig. 14 View of the X-ray structure of [Fe(TPP)(CCl2)] (115a).136 Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): Fe1-C25 1.726(3), Fe1-C25-Cl1 124.69(17), Fe1-C25-Cl2 124.68(17).

Scheme 38 Iron(IV) alkylidene complexes (left) and prevalent contribution from iron(II) resonance form (right). P∗ ¼ PMe3; L ¼ PMe3 or N2. Jacobs, B.P.; Agarwal, R.G.; Wolczanski, P.T.; Cundari, T.R.; MacMillan, S.N. Polyhedron 2016, 116, 47–56.

244

Alkylidyne and Alkylidene Complexes of Iron

Reaction of a high-spin (amido-phosphine-amido)iron(II) complex with (4-tolyl)2CN2 proceeds at room temperature to give a high-spin (amido-ylide-amido)iron(II) complex (Scheme 39).137 The highly reactive alkylidene intermediate 120 undergoes migratory insertion with the phosphine ligand to furnish the final ylide species. This latter is able to transfer the carbene moiety to a variety of alkenes to yield cyclopropane derivatives.

Scheme 39 Carbene transfer reaction to (amido-phosphine-amido)iron(II) complex affording a (amido-ylide-amido)iron(II) product via intermediacy of an alkylidene species (120) active as carbene-transfer catalyst. Ar ¼ 4-C6H4; R ¼ alkyl, aryl, CO2Et, CN. Liu, J.; Hu, L.; Wang, L.; Chen, H.; Deng, L. J. Am. Chem. Soc. 2017, 139, 3876−3888.

It should be noted that cyclopropanation is an undesired competitive reaction, when alkene metathesis is the goal. It was computationally found (but not experimentally realized) that the latter reaction may become favored with mononuclear iron(II) complexes bearing carbonyl and NHC ligands.150 The transfer of a carbene moiety from a diazocompound is a strategy which has been also exploited to synthesize bis-imino-pyridine alkylidene complexes (121).179 In 121, the alkylidene moiety exhibits a rich chemistry, being easily replaced by carbon monoxide, imido and alkene ligands, the latter via an intramolecular rearrangement (Scheme 40).

Scheme 40 Reactivity of iron bis-imino-pyridine alkylidene complexes. (A and B): Ar ¼ 2,6-C6H3Me2; (C): Ar ¼ 2,6-C6H3Et2. Russell S.K.; Hoyt, J.M.; Bart, S.C.; Milsmann, C.; Stieber, S.C.E.; Semproni, S.P.; DeBeer, S.; Chirik, P.J. Chem. Sci. 2014, 5, 1168–1174.

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245

Baker and co-workers synthesized an interesting variety of perfluoro-alkylidene iron complexes.110 Under UV irradiation, [Fe {Z -(CF2)4-}(CO)4] reacts with tripodal phosphine affording the iron(II) fluoride metallacycloalkylidene 122 as the final product (64% yield), passing through the formation of a mono-carbonyl species detected by 19F NMR spectroscopy (Scheme 41A). 2

Scheme 41 Synthesis and reactivity of perfluoro-alkylidene iron complexes. Ghostine, K.; Gabidullin, B.M.; Baker, R.T. Polyhedron 2020, 185, 114587.

Complex 122 is a rare example of organoiron fluoride, the fluoride ligand being detected at −400 ppm in the 19F NMR spectrum. This ligand behaves as a labile one, being readily replaced by acetonitrile and triflate (TfO−) to give the related substitution products 123–124 in high yields (Scheme 41B and C). The alkylidene carbon belonging to 122–124 occurs in the range 108–123 ppm, in the respective 13C NMR spectra. A view of the X-ray structure of the cationic part of 123 is shown in Fig. 15 with relevant bonding parameters provided in the caption. A product analogous to 122 but containing a tridentate nitrogen ligand (terpy0 ) instead of triphos was also prepared (125, Scheme 41D): in this case, the photolytic treatment of the starting tetracarbonyl compound stops at the monocarbonyl stage even upon extended irradiation. However, subsequent reaction with trimethylsilyl triflate promotes ready Ca-F abstraction to afford 125 in 63% yield. Whilst the neutral complex 122 tolerates water and is unreactive towards H2, 123–124 undergoes hydrogenation of the ferracycle yielding an iron(II) hydride derivative and the hydrofluorocarbon HFC-347pcc (see Eq. 13 for the reaction in THF involving 123).
  +
 2    + + ðCF2 Þ3 CH2 F (13) + H2 ! FeðHÞðNCMeÞ k3 −triphos Fe Z −ðCF2 Þ3 CF ðNCMeÞ k3 −triphos The proposed reaction pathway for the hydrogenolysis of 123–124 starts with acetonitrile displacement by dihydrogen, followed by protonation of the [Fe]C] bond. Coordination of a second dihydrogen molecule to the resulting iron perfluorocarbene hydride is followed by a-H elimination, {Fe-CHF} protonation and re-coordination of MeCN, accompanied by release of the organic product.

246

Alkylidyne and Alkylidene Complexes of Iron

Fig. 15 View of the X-ray structure of the cation [Fe{Z2-(CF2)3CF]}(NCMe)(k3-triphos)]+ in 123. Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ): Fe1-C38 1.745(3), C35-Fe1 2.011(3), C38-Fe1-C35 81.1(1), C35-Fe1-N1 86.5(1).

7.05.6.2

Alkoxy-alkylidene and thio-alkylidene complexes

Guerchais and co-workers synthesized a series of iron(II) cyclopentadienyl alkoxy-alkylidene cationic complexes (126) from [Fe2Cp2(CO)4] and studied their reactivity.100 Photolytic treatment results in extrusion of one carbonyl ligand, which can be intramolecularly replaced by a methoxy group to give 127 in 71% yield (Scheme 42A). When a chlorine substituent is present on the

Scheme 42 Reactivity of iron(II) cyclopentadienyl complexes (triflate salts, X ¼ Cl or OMe) with alkoxy-alkylidene ligands. Ferré, K.; Poignant, G.; Toupet, L.; Guerchais, V. J. Organomet. Chem. 2001, 629, 19–27.

Alkylidyne and Alkylidene Complexes of Iron

247

aryl ring in the place of {OMe}, the former is not able to coordinate and thus acetonitrile occupies the vacant metal site (Scheme 42B). Cl-coordination is instead enabled upon replacement of the second carbonyl ligand with triphenylphosphine (Scheme 42C). The alkylidene ligand in 128 is susceptible to easy dissociation; for instance, the straightforward reaction with a fourfold molar excess of sodium ethoxide proceeds with nucleophilic attack to the carbene, followed by a rearrangement with CdCl bond activation (Scheme 42D). A similar reactivity regards homologous Cp∗ complexes. The alkylidene carbon resonates in the range 319 to 328 ppm in the 13C NMR spectra of 126–129, these values resembling those detected for related terminal alkoxy-alkylidene ligands in diiron bis-cyclopentadienyl complexes (see also Scheme 17).1 Alkoxy-alkylidene ligands have been constructed from a cyclopentadienyl iron(II) precursor via iodide abstraction in the presence of alkynes in alcoholic solution (Scheme 43).97 Thus, an intermediate vinylidene species (from trimethylsilyl-acetylene) is quantitatively converted into 130 upon prolonged heating. The formation of 130 is the result of nucleophilic addition of one alcohol (solvent) molecule to the central carbon of the vinylidene moiety; the same reactivity is observable on ruthenium vinylidene species albeit usually with more favorable kinetics. Instead, the reaction of the starting iodide complex with 3-butyn-1-ol, in methanol at reflux temperature, directly leads to the 2-oxacyclopentylidene complex 131, which was isolated in 47% yield.

Scheme 43 Synthesis of cyclopentadienyl iron(II) alkoxy-alkylidene complexes. R0 ¼ Me or Et. El-Tarhuni, S.; Manhaes, L.M.; Morrill, C.; Raftery, J.; Randhawa, J.K.K; Whiteley, M.W. J. Organomet. Chem. 2016, 811, 20–25.

Concerning non-half-sandwich systems, the elusive bis-carbamoyl tetracarbonyl complexes [Fe{C(O)NR2}2(CO)4], obtained by different strategies from either [Fe{C(O)NR2}2(CO)4]− or [Fe(CO2R)2(CO)4], rapidly evolves to 132 via CO elimination and carbon-oxygen coupling between the two carbamoyl ligands (Scheme 44).123 The oxygen-coordination of the carbonyl moiety in 132 is labile, and is readily displaced by phosphines. The hybrid alkoxy-amino alkylidene nature of 132 is reflected by a substantial upfield shift of the related 13C NMR resonance (falling at ca. 244 ppm), compared to pure alkoxy-alkylidene complexes (vide infra).

Scheme 44 Formation of alkoxy-(amino)alkylidene by C-O coupling between carbamoyl ligands (R ¼ Me, Et, Pr). Le Gall, N.; Luart, D.; Salaün, J.-Y.; des Abbayes, H.; Toupet, L. J. Organomet. Chem. 2001, 617–618, 483–494.

The unusual formation of an alkoxy-alkylidene moiety was reported in 2019 as result of C-O coupling between two encumbered acyl ligands triggered by CO addition to the iron center (Scheme 45).188 This reaction is accompanied by formal Fe+II to Fe0 reduction leading to complex 133, analogous to 132. The X-ray structures of representative compounds are shown in Fig. 16, and relevant bonding parameters are provided in the caption. The iron-carbene distance is significantly elongated in the alkoxy-(amino)alkylidene complex 132a compared to the other alkoxy-alkylidene species, due to significant carbene-nitrogen overlap which has a weakening effect on the metal to carbene backbonding.

248

Alkylidyne and Alkylidene Complexes of Iron

Scheme 45 Alkoxy-alkylidene formation from the coupling of acyl ligands; Ar ¼ 2,6-Naph2C6H3. Sharpe, H.R.; Geer, A.M.; Taylor, L.J.; Gridley, B.M.; Blundell, T.J.; Blake, A.J.; Davies, E.S.; Lewis, W.; McMaster, J.; Robinson, D.; Kays, D.L. Nat. Commun. 2019, DOI 10.1038/s41467-018-06242-w.

Fig. 16 View of the X-ray structures of [FeCp{k2-Ph2P(CH2)2PPh2}{]C(Me)OMe}]+ (130a),97 [FeCp{k2-Ph2P(CH2)2PPh2}{kC,O-C(CH2)3O}]+ (131),97 [Fe(CO)3{kC,OC(NEt2)OC(NEt2)O}] (132a)123 and [Fe(CO)3{kC,O-C(Ar)OC(Ar)O}] (133, Ar ¼ 2,6-Naph2C6H3).188 Hydrogen atoms omitted for clarity. Selected bond lengths (A˚ ) and angles ( ) as follows. 130a: Fe1-C32 1.851(5), C32-C33 1.495(7), C32-O1 1.333(6), O1-C34 1.438(6), Fe1-C32-O1 119.0(3), C32-O1-C34 123.2(4). 131: Fe1-C35 1.834(2), C35-C34 1.518(3), C34-C33 1.520(3), C33-C32 1.502(3), C32-O1 1.469(2), C35-O1 1.328(2), Fe1-C35-O1 123.72(14). 132a: Fe1-C4 1.911(5), C4-N1 1.314(6), C4-O5 1.394(5), O5-C5 1.364(6), C5-O4 1.237(6), Fe1-O4 2.066(3), Fe1-C1 1.756(4), Fe1-C2 1.801(5), Fe1-C3 1.745(5), Fe1-C4-O5 115.0(3), O4-Fe1-C4 79.3(2). 133: Fe1-C1 1.840(3), C1-C6 1.484(4), C1-O1 1.427(3), C2-O1 1.317(3), C2-O2 1.252(3), Fe1-O2 1.9572(18), Fe1-C5 1.849(3), Fe1-C3 1.830 (3), Fe1-C4 1.753(3), Fe1-C1-O1 114.12(16), Fe1-C1-C6 140.1(2), C1-O1-C2 112.0(2).

Despite the co-existence of Fischer alkylidene and thiolato ligands on the same metal center is difficult to achieve due to favorable intramolecular coupling, Kawaguchi and co-workers realized this situation using a multidentate thiolato framework on a Fe+II bis-carbonyl complex.213 A dialkoxy-alkylidene iron(I) complex with the two oxygen atoms coordinating a zirconium center, [{Fe}]CO2{Zr}], was synthesized upon insertion into iron-zirconium bond of the carboxylato fragment, generated in situ from a CO ligand and a carbonyl reagent (acting as oxygen transferor).139 The alkylidene carbon occurs at 224.1 ppm in the NMR spectrum of the product. A similar compound, i.e., dialkoxy-alkylidene iron(0) complex with the two oxygens binding a silicon atom, has been obtained by reaction of Fe2(CO)9 with a silanone (13C NMR chemical shift of the alkylidene carbon: 262.0 ppm).178 A cyclic alkoxy-alkylidene iron(I) complex was obtained in low yield upon migration of a methyl ligand induced by a boron Lewis acid in the presence of water (Eq. 14).32

Alkylidyne and Alkylidene Complexes of Iron
 
 FeðIÞðMeÞðCOÞ2 ðPMe3 Þ2 +BðC6 F5 Þ3 +H2 O ! FeðIÞðCOÞðPMe3 Þ2 CðMeÞOBðC6 F5 Þ2 OH −

249

(14)

Iron thio-alkylidene complexes are relatively rare. A thio-alkylidene moiety has been exploited to connect dithienylethenes with an iron cyclopentadienyl structure, resulting in an organometallic assembly with notable electronic properties.154 Tris(2-pyridylthio)methane (tptmH) has been employed as a reactant to install di-thioalkylidene ligands on iron(II) complexes (Scheme 46). Thus, the reaction of iron(II) triflate with tptmH, in dichloromethane at room temperature, proceeds to afford the ionic [134][CF3SO3], resulting from CdS bond cleavage.111 Analogously, the reaction of iron iodide with tptmH was conducted in acetonitrile at 80  C in the presence of triethylamine, to give 135 in 75% yield.122 Interestingly, the carbene belonging to 135 undergoes a reversible conversion to an alkyl form (intact tptm fragment) in acetonitrile upon iodide abstraction.

Scheme 46 Structures of tris(2-pyridylthio)methane (tpmH) and of its iron(II) di-thioalkylidene derivatives, and reversible di-thioalkylidene to tris-alkylthio-methyl conversion. Kuwamura, N.; Kato, R.; Kitano, K.; Hirotsu, M.; Nishioka, T.; Hashimoto, H.; Kinoshita, I. Dalton Trans. 2010, 39, 9988–9993.

7.05.6.3

Amino-alkylidene complexes

The construction of unusual ferracyclic amino-alkylidene compounds has been described as achieved from the clean fragmentation of suitable diiron bis-cyclopentadienyl precursors (Schemes 26 and 27). Here, we will overview the remaining, recent literature on the chemistry of amino-alkylidene monoiron complexes. We previously discussed the formation of mixed alkoxy-amino-alkylidene ligands by coupling of carbamoyl fragments (Scheme 44).123 Diamino-alkynes constitute another possible source for aminocarbene units. In particular, Filippou reported the reaction of Fe(CO)5 with Me2NC^CNMe2 in THF at low temperature, which proceeds with initial formation of a labile ferracyclobutenone intermediate, followed by decarbonylation to give the isolable ferrabicyclobutenone 136, containing an amino-alkylidene moiety (Scheme 47).101 According to 1H NMR spectroscopy, this complex is fluxional because of hindered rotation of the Cb-bonded amino group, while rotation of the Ca-bonded amino group is frozen even at ambient temperature giving rise to two distinct methyl resonances. Complex 136 loses CO either in refluxing toluene or upon melting above 120  C to afford selectively an Z2-alkyne complex in 95% yield. Furthermore, the alkylidene in 136 exhibits a rich chemistry towards nucleophiles, for instance giving raise to cycloaddition reactions with Me2NC^CNMe2 or methyl isocyanide. Treatment of 136 with triflic acid proceeds with CdC bond cleavage leading to the noncyclic amino-alkylidene 137 (85% yield). The diagnostic alkylidene carbon was NMR-detected at 223.1 and 239.6 ppm, respectively in 136 and 137.

250

Alkylidyne and Alkylidene Complexes of Iron

Scheme 47 Synthesis and reactivity of iron(II) ferrabicyclobutenone-aminoalkylidene complex. Filippou, A.C.; Rosenauer, T. Angew. Chem. Int. Ed. 2002, 41, 2393–2396.

A variety of iron compounds bearing cyclic (alkyl)amino carbene ligands (cAAC) has been reported. The stability of the two-coordinate complex 138 arises from the strong s-donating and p-accepting properties of the cAACs, in addition to steric protection.196 Complex 138 binds dinitrogen at low temperature, which is activated towards silylation following monoelectron reduction; the diazenido product 138b was finally isolated in 52% yield (Scheme 48). Formation of ammonia was detected upon stepwise addition to 138 of an excess of both KC8 and HBF4 at −95  C in diethyl ether.195

Scheme 48 Dinitrogen coordination and activation on a two-coordinate iron amino-alkylidene complex. R ¼ Me, Et; Dipp ¼ 2,6-C6H3(iPr)2 (2,6-diisopropylphenyl). Ung, G.; Peters, J.C. Angew. Chem. Int. Ed. 2015, 54, 532–535.

Alkylidyne and Alkylidene Complexes of Iron

251

The Fe0 complex [Fe(CO)4(cAAC)] (139, cAAC ¼ C(NDipp)CMe2CH2CMe2) is readily available from Fe2(CO)9 and operates as an efficient catalyst in promoting the head-to-head dimerization of terminal arylalkynes to conjugated enynes with high E selectivity.36 According to the proposed mechanistic pathway, two alkyne units enter the metal coordination sphere by replacing two carbonyls. Concerning piano-stool systems, when the vinylidene complex shown in Scheme 43 is allowed to react with ammonia (rather than alcohols), formation of the iron(II) amino-alkylidene derivative 140 occurs in 77% yield (Eq. 15).95 Calculations highlighted that the iron-carbene bond is substantially a s donation with scarce p-backbonding contribution. 1H NMR features indicate restricted rotation about the carbene-nitrogen bond, supporting a {C]NH2} valence description, while in the 13C NMR spectrum the alkylidene carbon resonates at 274.1 ppm. ½FeCpðdppeÞð¼ C ¼ CH2 ފ½PF6 Š +NH3 ! ½FeCpðdppeÞfCðMeÞNH2 gŠ½PF6 Š

(15)

140

The phosphine-tethered isocyanide iron(II) complex [FeCp(CO)(PCN)]I smoothly reacts with primary and secondary amines to yield diamino-alkylidene products, 141 (Scheme 49A).212 Related cyclic diaminocarbene species were obtained by a two-step procedure (Scheme 49B): the addition of 2-chloroethylamine or 3-chloropropylamine in dichloromethane solution is straightforward, followed by dehydrohalogenation to afford air-stable complexes 142 in greater than 80% yields. Analogous mixed alkoxy-aminoalkylidene neutral complexes are accessible using a parallel strategy involving alkoxide addition.

(A)

(B)

Scheme 49 Formation of acyclic and cyclic di-aminoalkylidene ligands by amine addition to functionalized isocyanide piano-stool iron(II) complexes (iodide salts). R ¼ Ph, tBu; R0 , R00 ¼ H, alkyl; n ¼ 2,3. Yu, I.; Wallis, C.J.; Patrick, B.O.; Diaconescu, P.L.; Mehrkhodavandi, P. Organometallics 2010, 29, 6065–6076.

The X-ray structures of the five-membered alkylidene complexes are comparatively shown in Fig. 17. The Fe2-C2(carbene) and C2–N1 bond distances are similar in the two structures, suggesting that the additional amino group in 142a and the alkoxy group in 143 exert comparable p-overlapping with the alkylidene carbon. Consistently, the 13C NMR chemical shift of the carbene is not dramatically different in 142a (218.8 ppm) compared to 143 (229.0 ppm); moreover, a minor change in chemical shift value is generally observed on going from the non-cyclic structures 141 to the cyclic ones 142. The double insertion of dicyclohexylcarbodiimide into the iron-boron and boron-nitrogen bonds within cationic aminoborylene complexes, [FeCp(CO)2(]B]NR2)] (R ¼ iPr, Cy), affords metalla-amidinate derivatives with a minor di-aminoalkylidene contribution to the structure.163 Complex [FeCp(CO)2Me] has been employed as a catalyst for some organic transformations involving the formation of intermediate amino-alkylidene complexes. Thus, N,N-dimethylthioformamide, Me2NC(S)H, and secondary thioamides, RNHC(S)R0 , are desulfurized by means of Et3SiH, to give amines or imines as the prevalent products according to the cases. Amino-alkylidene complexes were identified as key species in the catalytic cycle.105,106 In particular, [FeCp(SSiEt3)(CO){]C(Ph) NHMe}] (144) was also obtained in a preparative scale and X-ray characterized; relevant to the catalytic activity, heating a C6D6 solution of 144 at reflux for 1 day led to the formation of some MeN]CHPh which was recognized by NMR. The synthesis of six-membered cyclic germoxanes (R2GeO)3 from (alkyl)2GeH2 in dimethylformamide, catalyzed by [FeCp(CO)2Me], was proved to proceed via the intermediacy of the amino-alkylidene [FeCp(OGeHR2)(CHNMe2)] (145).118

252

Alkylidyne and Alkylidene Complexes of Iron

Fig. 17 View of the X-ray structures of [FeCp(CO){kC,P-PPh2(CH2)3N(CH2)3NHC]}]+ (142a, iodide salt) and [FeCp(CO){kC,P-PPh2(CH2)3N(CH2)3OC]}] (143).212 Hydrogen atoms omitted for clarity, except NH in 142a. Selected bond lengths (A˚ ) and angles ( ) as follows. 142a: Fe1-C2 1.972(7), C2-N1 1.336(9), C2-N2 1.336 (10), N1-C2-N2 116.7(6), N1-C2-Fe1 127.2(6). 143: Fe1-C2 1.976(5), C2-N1 1.322(9), C2-O2 1.330(11), N1-C2-O2 122.8(7), N1-C2-Fe1 127.6(5).

Table 3

Comparative view of 13C NMR and X-ray data related to alkylidene ligands (in red) coordinated in selected monoiron complexes. 13

X-ray (dFe-C, ˚A)

[Fe(TPP)(CCl2)] (115a) [Fe(TPFPP)(Ad)(Py)] (116a) [Fe(F){Z2-(CF2)3CF]}(k3-triphos)] (122) [Fe{Z2-(CF2)3CF]}(NCMe)(k3-triphos)][BPh4] (123) [FeCp(CO){]C(OMe)C6H4OMe}][CF3SO3] (127)

224.7 433.6 108.3

1.726(3) 1.829(9)

316.8

1.745(3) 1.859(6)

[FeCp{k2-Ph2P(CH2)2PPh2}{]C(Me)OMe}][I] (130[I])

321.6

1.851(5)

[FeCp{k2-Ph2P(CH2)2PPh2}{kC,O-C(CH2)3O}][PF6] (131[PF6])

313.0

1.834(2)

C NMR (dFeC, ppm)

[Fe(CO)3{kC,O-C(Ar)OC(Ar)O}] (133)

1.840(3)

[Fe{]C(SPy)2}(kN,S-PyS)(kS-SPyH)][CF3SO3] [134][CF3SO3]

1.7762(17)

[Fe(CO)3{Z2-C(NMe2)C(NMe2)C(]O)}] (136)

216.4

1.896(3)

[FeCp(CO){kC,P-PPh2(CH2)3N(CH2)3NHC]}][I] (142a[I])

218.3

1.972(7)

[FeCp(CO){kC,P-PPh2(CH2)3N(CH2)3OC]}] (143)

231.3

1.976(5)

[FeCp(SSiEt3)(CO){]C(Ph)NHMe}] (144)

264.8

1.9036(17)

X-ray (dC-E, ˚A)

1.299(7) (E ¼ O) 1.333(6) (E ¼ O) 1.328(2) (E ¼ O) 1.427(3) (E ¼ O) 1.7319(18) (E ¼ S) 1.306(4) (E ¼ N) 1.336(9), 1.336(10) (E ¼ N) 1.322(9) (C-N) 1.330(11) (C-O) 1.310(2) (E ¼ N)

References 50 136 150 150 179 110 110 97 32 111 36 36 212

NMR spectra in CDCl3, CD2Cl2 or other common deuterated solvent.

The 13C NMR and X-ray data collected for a series of selected monoiron alkylidene complexes are compiled in Table 3. This set of data outlines a considerable variability, which is mainly dependent on the nature of the alkylidene ligand. Thus, ligands without heteroatoms (“classical ligands”) lack the p-interaction involving the carbene center which is otherwise encountered in those species comprising one or two heteroatom substituent(s); in the former cases, relatively short iron-carbon distances are usually found, although steric factors and other features (e.g., iron oxidation state, co-ligands) may lead to significant exceptions. The very broad interval of 13C NMR chemical shifts reflects the overall variability.

Alkylidyne and Alkylidene Complexes of Iron

7.05.6.4

253

Overview of NHC complexes

Five-membered NHC (N-heterocyclic carbene) ligands have become ubiquitous in organometallic chemistry, due to their robustness, structural variability and versatility. The carbene center is bound to two nitrogen atoms which compete for the empty p orbital on carbon, thus weakening the metal to NHC backdonation and rendering the NHC moiety as a prevalently donor and inert (innocent) ligand. The chemistry of iron-NHC complexes has seen a tremendous progress in the last 15 years, therefore only fundamental aspects will be briefly outlined in this section, and relevant comprehensive reviews will be referenced. A variety of piano-stool iron-NHC complexes, including pincer systems, are available by means of several synthetic routes which have been reviewed in 2017117: (1) thermal activation of enetetramines (NHC dimers); (2) transmetallation (relatively rare); (3) addition of free carbene; (4) NHC formation within the iron coordination sphere; (5) deprotonation of imidazolium precursor by an internal base; (6) C-H oxidative addition of imidazolium. NHC coordination is compatible with different iron oxidation states, including iron(IV) for which a homoleptic organometallic complex, stable both in solution and in the solid state at ambient conditions, was synthesized and isolated in 2020.166 The last decade has witnessed an explosion in the application of the iron-NHC combination in homogeneous catalysis, especially in CdC bond forming reactions, and various reduction and oxidation processes.35,130,138 The structures of a selection of iron-NHC complexes recently reported and investigated for their catalytic potential are drawn in Scheme 50. On the other hand, the interest in the biological activity of iron-NHC compounds has also grown up; for instance, cyclopentadienone-carbonyl iron(0) NHC complexes revealed strongly cytotoxic against pancreatic cancer cell lines,87 whereas iron(II) carbonyl complexes with the NHC moiety connected to a Cp∗-modified ring evidenced bactericidal properties.197 Moreover, Fe+II-based photosensitizers bearing a NHC unit have aroused interest for their applicability in nanostructured dye-sensitized solar cells.92

Scheme 50 Selected iron-NHC complexes with catalytic applications from the recent literature.33,40,67,109,129,165,168

Despite NHC ligands have been conventionally regarded as ancillary ligands, few examples of NHC dislocation/modification from/on iron complexes have been described. Thus, the bis(dithiolene) complexes 146 release NHC (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene or 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) upon electrochemical reduction, and efficient trapping of the dissociated NHC moiety has been achieved with 1-naphthylisothiocyanate (Scheme 51A).184 Song et al. reported the unexpected slippage from N,C-chelate to N,N-chelate of a NHC-based bidentate ligand thermally induced in a three-legged piano-stool iron(II) complex (Scheme 51B).131 A highly active iron(III) tetra-NHC epoxidation catalyst has been shown to degrade under oxidation conditions via single carbene oxygenation, which triggers the protonation of the other NHC moieties.94

254

Alkylidyne and Alkylidene Complexes of Iron

(A)

(B)

Scheme 51 Iron complexes undergoing displacement (A) or modification (B) of the NHC ligand. Selvakumar, J.; Simpson, S.M.; Zurek, E.; Arumugam, K. Inorg. Chem. Front. 2021, 8, 59–71.

7.05.7

Concluding remarks

Alkylidyne and alkylidene iron complexes represent a vast field of organometallic chemistry embracing different types of ligands, nuclearities and metal oxidation states.90 Some of the structural motifs have been known for a longtime, and for instance the reactivity of various monoiron amino-alkylidyne and diiron alkylidyne complexes was the subject of pioneeristic studies in the last century, providing a substantial contribution to the basic knowledge on organometallic reactions and mechanistic pathways. Besides, new classes of compounds have emerged even in very recent times, revealing suitable platforms e.g., for the activation of small molecules and the comprehension of the functioning of the inorganic active centers within natural enzymes. The outstanding properties of the element iron among transition metals (earth abundance, cheapness, low toxicity in many forms, biological activity, redox chemistry) has rendered all the plethora of alkylidyne/alkylidene complexes strongly attractive for future developments. Such unsaturated ligands might manifest enhanced reactivity and be less firmly anchored when coordinated in monoiron complexes, which therefore may become useful in metal-mediated syntheses. In this regard, monoiron NHC complexes are exceptional, in that the NHC ligand usually behaves as an ancillary one but supplies appealing characteristics to the complexes in view of several applications. On the other hand, diiron carbonyl complexes with bridging hetero-alkylidyne ligands (especially amino-alkylidyne) are generally robust species, air-stable and water tolerant, and are susceptible to regio- and stereo-selective reactions controlled by the cooperativity of the diiron framework. This approach makes available a wide range of complexes with functionalized alkylidyne and/or alkylidene ligands, e.g., providing suitable physico-chemical properties for potential medicinal applications.

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7.06

Small Molecule Activation by Organo-iron Complexes

Kaiji Shen , Stéphane Ménageb, and Carole Duboca, aUniversité Grenoble Alpes, CNRS, DCM, Grenoble, France; bUniversité Grenoble Alpes, CEA-Grenoble, CNRS, LCBM, Grenoble, France a

© 2022 Elsevier Ltd. All rights reserved.

7.06.1 7.06.1.1 7.06.1.1.1 7.06.1.1.2 7.06.1.2 7.06.2 7.06.2.1 7.06.2.1.1 7.06.2.1.2 7.06.2.1.3 7.06.2.2 7.06.2.2.1 7.06.2.2.2 7.06.3 7.06.3.1 7.06.3.2 7.06.3.2.1 7.06.3.2.2 7.06.3.2.3 7.06.3.3 7.06.3.4 7.06.4 7.06.4.1 7.06.4.1.1 7.06.4.1.2 7.06.4.1.3 7.06.4.1.4 7.06.4.1.5 7.06.4.2 7.06.4.2.1 7.06.4.2.2 7.06.4.2.3 7.06.4.2.4 7.06.4.3 7.06.4.3.1 7.06.4.3.2 7.06.5 References

N2 activation and reduction N2 activation and key intermediates Formation of N2 complexes N2-derived adducts N2 reduction under catalytic conditions H2 production FeFe catalysts With only CO ligands With phosphine ligands With N-based ligands and metallocene unit NiFe catalysts Stabilization of reduced NiIFeII species Stabilization of NiFe hydride species H2 oxidation Dinuclear FeFe complexes Mononuclear [CpFe((RN1–2)PR12)] complexes Characterization of the initial FeII complexes Characterization of the H2 adducts Characterization of the [FeH(NH)+] intermediates Stoichiometric H2 oxidation Catalytic H2 oxidation O2 activation Synthesis and characterization of the organo iron complexes Synthesis of the NHC-based ligands Structure of the mononuclear ferrous NHC-based complexes Spectroscopic and redox properties of the mononuclear ferrous NHC-based complexes Structure of diiron complexes with NHC-based ligands Structure of the mononuclear ferric complexes with NHC-based ligands Intermediates generated from O2 activation Characterization of a superoxo complex Characterization of a peroxo complex Characterization of mononuclear high valent iron oxo species Characterization of the diferric oxo species as final oxidation products Oxidation properties Under stoichiometric conditions Under catalytic conditions Conclusions

259 260 260 262 266 267 267 267 269 271 273 273 274 276 278 279 279 280 280 281 282 282 283 283 283 286 286 287 287 287 287 288 289 289 289 290 291 291

Small molecules activation is critical for a number of key natural and industrial processes as these gases are involved in natural cycles and metabolic pathways but also, in climate change and other environmentally sensitive matters. We have then brought our attention to the activation of O2, H2 and N2, for which major achievements in the design of organoiron catalysts have been made. The conversion of these abundant molecules into fuels and/or high-value chemical feedstocks remains a major challenge due to their inertness and/or the high thermodynamic barrier to overcome for their chemical transformations. Besides, the mechanisms are in most of the cases very complex since they imply a fine control of multielectron redox processes coupled with proton transfers. Nature has been a source of inspiration for the organometallic chemists with the discovery of organometallic active center with unprecedented complex structures in several metalloenzymes, including nitrogenases and hydrogenases. The three first sections are then focused, through a bio-inspired approach to N2 activation and reduction, H2 production and H2 oxidation. The last paragraph will be devoted to O2 activation implicated in oxidation reactions, for which chemists, inspired by oxygenases, have developed organoiron complexes with the aim of comparing their reactivity with that of heme and non-heme models. In each case, we will describe in details the process at a molecular level, emphasizing the deciphering of each mechanism, and describing the structural characterization of unique reaction intermediates. The reader will find a brief description of the structural and spectroscopic properties of the complexes of interest, followed by their potential reactivity under stoichiometric and catalytic conditions. This chapter covers the last 15 years and compares, where possible, the advances with those of inorganic chemistry.

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Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00102-5

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259

Fig. 1 Structure of the active site of the nitrogenase.

We would also like to point out that we have taken particular care in naming the ligands, highlighting the ligand N, C, P or S donor atoms with their substituents and the connections between the different moieties of the polydentate ligands, where relevant.

7.06.1

N2 activation and reduction

Dinitrogen (N2) is vital for all living organisms. Even if it corresponds up to 78% of the Earth’s atmosphere, N2 is not directly usable to plants and animals because of its chemical inertness. N2 is indisputably one of the most inert molecules, due to several factors, including the thermodynamic strength of the N^N triple bond (bond-dissociation enthalpy of 947 kJ mol−1), the lack of a dipole moment, its low proton affinity and its high ionization potential. The reduction of N2 into ammonia (NH3), also called nitrogen fixation, then becomes one of the most important reactions in chemistry for life (Eq. 1). N2 + 3H2 ! 2NH3

(1)

To meet the needs of the global population, the natural production of ammonia is currently insufficient because it only covers half of human needs. The second half, concerning mostly the mass production of nitrogen fertilizer, is produced industrially by the Haber-Bosch process on the surface of a K+ promoted Fe (or Ru) catalyst doped with Ca and Al. This process operates under high temperatures (300–550  C) and pressures (100–300 atm) using H2 as co-substrate. Its high-energy cost (1–2% of annual global energy consumption) mainly comes from the production of H2 via a “steam reforming” process, which also generates greenhouse gas emissions (about 0.5 gigatons of CO2 released annually). In Nature, nitrogenases are the only enzymes that can convert nitrogen into ammonia. The process occurs under ambient pressure and temperature, uses electron carriers (ferredoxins or flavodoxins) as reducing reagents and water as a proton source, and concomitantly hydrolyses ATP molecules (as a biological fuel)1 (Eq. 2) N2 + 8H + + 8e − + 16ATP ! 2NH3 + H2 + 16ADP + 16Pi

(2)

Their active site is constituted of a sulfur-rich heteronuclear organometallic cluster [7Fe-9S-C-M-homocitrate] (Fe7M, with M being either a Mo, Fe or V ion depending on the type of nitrogenases) (Fig. 1). It has been proposed that the catalytic activity occurs mainly between pairs of Fe ions involved in the formation of a dihydride species, the pivotal intermediate in dihydrogen release, and in the N2 activation via its direct coordination.2 Two important processes could be distinguished: N2 activation and N2 reduction. Indeed, because of the inertness of N2, its coordination to an Fe organometallic compound remains one of the most challenging steps to produce NH3. N2 can be activated through two distinct processes: under very strong reducing conditions or in exchange with a H2 molecule generated from hydride species, as it occurs in the nitrogenase. In all cases, the resulting [N2dFe]-based adducts display different degrees of activation, estimated from the NdN bond distance, and depending on the oxidation state of the bound-Fe and the electronic structure of the ligand(s). N2 reduction has been for long limited to two alternating pathways going through either the formation of nitrido or hydrazine intermediate, called distal and alternating path, respectively (Scheme 1). Recently, it has been shown that a hybrid path can lead to the formation of hydrazine, while the first step is common with the distal pathway.

Scheme 1 Representation of the distal, alternating and hybrid pathways.

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Small Molecule Activation by Organo-iron Complexes

Fig. 2 Structure of the [Fe(iPrP3B)(N2)], [Fe0(iPrP3X)(N2)]− (X ¼ C, Si) and [(C)2Fe0(N2)] (with Dipp ¼ 2,6-diisopropylphenyl).

There are mainly three classes of organometallic Fe-complexes that have been developed in the context of N2 activation and reduction, identified by the nature of their ligands: P-based ligands (tris- and di-phosphine), a combination of P and N (PNP), and C-based ligands with CAAC (cyclic alkyl amino carbene) derivatives. In this part, we will first describe these processes with a special focus on the characterization and reactivity of the key intermediates. Finally, the performance of the Fe-based organometallic catalytic systems will be detailed and compared.

7.06.1.1 7.06.1.1.1

N2 activation and key intermediates Formation of N2 complexes

7.06.1.1.1.1 Under reducing conditions Depending on the reduction conditions, N2 adducts with various Fe-oxidation states can be isolated. Interestingly, two series of isostructural complexes have been described, in which only the oxidation state of the Fe ion varies. These series have been obtained with the use of the P3C3 and P3Si4 ligands (Fig. 2) leading to Fe(0)-, Fe(I)- and Fe(II)-N2 adducts (with S ¼ 0, ½ and 1, respectively). Regarding the synthesis of such compounds, the FeI and Fe0 complexes have been obtained from direct N2 activation under reduced conditions, with Na/Hg and KC8, respectively, while FeII is generated from the oxidation of the FeI complexes. In both series, a shift of the nNN vibration to higher energy is observed with the increase of the Fe oxidation state, (in the case of the P3C complexes, nNN ¼ 1870, 1992 and 2128 cm−1 for [Fe0(iPrP3C)(N2)]−, [FeI(iPrP3C)(N2)] and [FeII(iPrP3C)(N2)]+, respectively).3 Interestingly, it has been shown that the FedC bond distance contracts upon oxidation in the (iPrP3C) series (d(FedC) ¼ 2.1646(17) A˚ , 2.152(3) A˚ , 2.081(3) A˚ for [Fe0(iPrP3C)(N2)]−, [FeI(iPrP3C)(N2)] and [FeII(iPrP3C)(N2)]+, respectively), as opposite to the FedSi bond distance in the (iPrP3Si) series (d(FedSi) ¼ 2.2526(9) A˚ , 2.2713(6) A˚ , 2.298(7) A˚ for [Fe0(iPrP3Si)(N2)]−, [FeI(iPrP3Si)(N2)] and [FeII(iPrP3Si)(N2)]+, respectively).4 This observation has been rationalized in terms of electronic effect: the more electropositive atom (Si vs C) is expected to coordinate more strongly to the more electron-rich iron (reduced Fe ion). In the same family of complexes, the N2-adducts, [Fe(iPrP3B)(N2)]0, 1−, 2−, have been characterized with the P3B derivative ligand (nNN ¼ 1836, 1877 and 2011 cm−1 for [Fe(iPrP3B)(N2)]2−, [Fe(iPrP3B)(N2)]− and [Fe(iPrP3B)(N2)], respectively). The [Fe(iPrP3B) (N2)] has been obtained under reduced conditions (KC8 or Na/Hg) from [Fe(iPrP3B)] with N2, and the other species by the further reductions [Fe(iPrP3B)(N2)] with an excess of KC8.5,6 With an analog of (iPrP3C), (iPrP3Si), the [Fe0(iPrP3Si)(N2)]− complex was isolated (Fig. 2).7 In this N2-adduct, an unusually long FedC bond (2.254(5) A˚ ) has been measured reflecting an ionic character for the FedC bond, suggesting that the modulation of the FedC interaction can assist the N2 coordination. N2-adducts prepared with PNP-type pincer ligands have been also isolated under reduced conditions in the presence of N2. Only complexes with the FeI oxidation state (S ¼ ½) have been isolated with two different total charge (0 or +1) depending on the protonation state of the PNP ligand.8,9 With the lithium 2,5-bis(di-tert-butylphosphinomethyl)pyrrolide derivative, the nNN vibration is of 1964 cm−1, while it shifts to 2034 cm−1 when the ligand becomes protonated in the presence of 1 equiv. of [H(OEt2)2]BArF4 in Et2O (Scheme 2).

Scheme 2 Acid/base chemistry of the [(tBuP2N)FeI(N2)]/[(HtBuP2N)FeI(N2)]+ complexes.

Small Molecule Activation by Organo-iron Complexes

261

From the two-coordinate C2-based Fe0 complex, a N2-adduct (Fig. 2) has been isolated in the presence of KC8, N2 and 18-crown-6.10 The corresponding [(C)2Fe-I(N2)]− complex displays a low frequency nNN vibration at 1850 cm−1, in agreement with a strong N2 activation. The Mössbauer parameters (d ¼ 0.56 mm s−1 and DEQ ¼ 1.67 mm s−1) of [(C)2Fe-I(N2)]− and the cw-EPR spectrum agree with a formal oxidation state of -I and a ground spin state S ¼ 1/2. 7.06.1.1.1.2 From hydride species Inspired by the nitrogenase mechanism, N2 coordination can be generated from hydride species. A N2-adduct [Fe0(N2)(MeP2)2] has been isolated from the bis-diphosphine dihydride Fe complex [FeII(MeP2)2(H)2] under irradiation in the presence of N2 (Scheme 3). It has been shown that a concomitant release of H2 is observed during this photosubstitution process.11–13 The [Fe0(N2)(EtP2)2] complex can be obtained from the deprotonation of the hydride [FeIIH(N2)(EtP2)2]+ species in the presence of a strong base. The shift of nNN vibration from 2093 cm−1 in the hydride species to 1967 cm−2 in the Fe0 N2-adduct is consistent with a stronger N2 activation in this latter species. The [Fe0(N2)(MeP2)2] complex can be also generated under reducing conditions (KC8) and N2 from an FeII precursor.14–16

Scheme 3 Synthesis of [Fe0(MeP2)2N2] and [Fe0(EtP2)2N2] via different pathways.

The irradiation of the dihydride FeII complex, [FeII(iprP2P)(H)2(N2)] leads to the generation of a N2-adduct in the presence of N2 with concomitant release of H2 (Scheme 4).17 The corresponding [Fe0(iprP2P)(N2)2] species displays two nNN vibrations at 2065

Scheme 4 Synthesis of [Fe0(iprP2P)(N2)2], [Fe-I(iprP2P)(N2)][K(18-crown-6)] and [FeI2(iprP2P)(H)2m-(N2)].

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Small Molecule Activation by Organo-iron Complexes

and 2009 cm−1, in agreement with a weak activation for both N2 ligands. This species can be further reduced in the presence of potassium naphthalenide followed by the addition of 18-crown-6 to generate the [Fe-I(iprP2P)(N2)][K(18-crown-6)] complex (S ¼ ½) with n(NN) ¼ 1872 cm−1 (Scheme 4). With the same ligand, the diamagnetic [FeI2(iprP2P)2(H)2(mN2)] (NdN bond length of 1.15 A˚ ), precursor of [FeII(iprP2P)(H)2(N2)], can be isolated from the reaction of a mononuclear dibromide FeII complex in the presence of NaHBEt3 and N2.18 It has been shown that only [Fe0(iprP2P)(N2)2] is capable to release NH3 in the presence of reducing agents and protons.

7.06.1.1.2

N2-derived adducts

7.06.1.1.2.1 Diazenido adduct The diazenido species, HNN-adduct, is not only the first intermediate proposed to be involved in the N2 fixation process, but also the last common of the distal and alternating pathways. Only one such species has been yet described in the literature because of its expected high degree of instability. However, several more stable silylated diazenido analogs (Fe-NNSiR3) have been described as models of this first catalytic intermediate.19,20 The N2-adduct isolated with the (ArP3B) ligand, [Fe(ArP3B)(N2)]− has been used.21 With (ArP3B), the corresponding complexes are not intended to be active to generate NH3 but are expected to be less reactive because of the sterically encumbered trisphosphine, in order to observe unstable and very reactive species. Indeed, they isolated [Fe(ArP3B)(NNH)] from the reaction of [Fe(ArP3B)(N2)]− and of HBArF24
2Et2O (BArF24 ¼ tetrakis(3,5-bistrifluoromethylphenyl)borate) (Scheme 5). The FedNNH adduct displays a rhombic S ¼ ½ EPR spectrum (g ¼ [2.021, 2.023, 2.177]). Complementary ENDOR and HYSCORE measurements (including aiso ¼ 16.5 MHz for the H atom of diazenido ligand) allow to evidence the loss of the NdN sigma bonding (decrease of 40% in e2Qq/h of 2 MHz) and of the FedNdN linearity (increase of electric field gradient asymmetry) in this unique FedNNH adduct. This diazenido ligand can be further protonated to generate an FedNNH2 adduct.

N N

Ar2P Fe Ar2P B

H N H

NH PAr2

H+ -135°C

N Ar2P Fe Ar2P B

PAr2

+

H

-78°C

N Ar2P Fe Ar2P B

PAr2

iPr

OMe

Ar= i

Pr

Ar

Scheme 5 Synthesis of [Fe( P3B)(NNH)] and [Fe(ArP3B)(NNH2)]+.

7.06.1.1.2.2 Hydrazido adduct The hydrazido intermediate is proposed to be the first distinct intermediate of the distal pathway. Several have been characterized including the one generating from the protonation of the diazenido species [Fe(ArP3B)(NNH)] using HOTf in excess (Scheme 6). The corresponding [Fe(ArP3B)(NNH2)]+ complex has been also subject of a detailed investigation by cw and pulsed EPR techniques (g ¼ [2.004, 2.087, 2.167]; aiso(H) ¼ 21.3 and 12.8 MHz; e2Qq/h(14N) ¼ 1.7 MHz), but was not sufficiently stable to get further characterization.21

Scheme 6 Synthesis of [Fe(iPrP3Si)(NNH2)]+, [Fe(iPrP3Si)(NNH2)] and [Fe(iPrP3Si)(H2NNH2)]+.

Small Molecule Activation by Organo-iron Complexes

263

Other hydrazido intermediates with the same family of ligands have been isolated and characterized. The [Fe(iPrP3X)(NNH2)]+ (X ¼ B or Si) complexes have been obtained from reaction between [Fe(iPrP3X)(N2)]− and an excess of HOTf (Scheme 6). Similar EPR parameters have been measured for [Fe(iPrP3B)(NNH2)]+ with respect to the ArP3B derivative (g ¼ [2.006, 2.091, 2.222]; aiso(H) ¼ 16 and 10 MHz; e2Qq/h(14N) ¼ 1.74 MHz).22 In the case of the Si adduct, the structure of the diamagnetic [Fe(iPrP3Si) (NNH2)]+ has been resolved by single-crystals XRD experiments evidencing a short FedN distance (about 1.67 A˚ ) consistent with a significant FedN multiple bond character and a NdN distance of about 1.27 A˚ , which well agrees with a nNN vibration at 1443 cm−1.23 Interestingly, the reduced forms of both [Fe(iPrP3X)(NNH2)]+ (X ¼ B or Si) species have been also generated. In the case of the Si derivative, [Fe(iPrP3Si)(NNH2)] has been synthesized from the one-electron reduction of [Fe(iPrP3Si)(NNH2)]+ using Cp 2Co as reducing agent (Scheme 6). An EPR investigation of [Fe(iPrP3Si)(NNH2)] (g ¼ [2.004, 2.027, 2.070]) has evidenced notable differences with the g-values of [Fe(ArP3Br)(NNH2)]+.23 The diamagnetic [Fe(iPrP3B)(NNH2)] adduct has been obtained from the reaction between [Fe(iPrP3B)(N2)]2− with an excess of TfOH or HBArF4.6 The Mössbauer data of this very unstable adduct (d ¼ 0.14 mm s−1 and |DEQ | ¼ 1.63 mm s−1) are similar to that of the [Fe(iPrP3Si)(NNH2)]+ complex (d ¼ 0.13 mm s−1 and |DEQ | ¼ 1.48 mm s−1). EXAFS data estimate an FedN distance of 1.65 (2) A˚ . As for the Si derivative, the authors confirmed such assignment based on the data acquired on the isoelectronic methylhydrazido adducts, [Fe(iPrP3X)(NN(Me)2)], that are much more stable than the hydrazido ones.6,23 Such hydrazido intermediates are expected to generate nitrido species with the concomitant release of NH3 in the presence of acid, as proposed for the distal pathway and observed in the case of [Fe(iPrP3B)(NNH2)]. However, upon warming a solution containing a mixture of [Fe(iPrP3Si)(NNH2)] and [Fe(iPrP3Si)(NNH2)]+, two main products have been characterized corresponding to [Fe(iPrP3Si)(N2)] and [Fe(iPrP3Si)(H2NNH2)]+ arising from a spontaneous disproportionation o the two initial adducts (Scheme 6).23 This example implies that a hybrid pathway connecting the distal and alternating pathways can be involved (Scheme 1). 7.06.1.1.2.3 Nitrido adduct As mentioned above, the nitrido-adduct [(iPrP3B)Fe  N] can be generated through the protonation of the hydrazido adduct [Fe(iPrP3B)(NNH2)]. It is interesting to note that the oxidized hydrazido adduct, [Fe(iPrP3B)(NNH2)]+, is not basic enough to be protonated. The nitrido [(iPrP3B)Fe^N]+ species has been assigned to an FeIV complex based on its Mössbauer properties (d ¼ − 0.15 mm s−1 and |DEQ | ¼ 6.20 mm s−1) with an S ¼ 0 spin state. EXAFS data provided some structural properties including the FedN distance of 1.54(2) A˚ . The flexibility of the Fe. . .B interaction has been proposed to explain the reactivity of these species. Indeed, the Fe. . .B distance increases gradually from 2.34 A˚ in [Fe(iPrP3B)(N2)]2− to 2.59 A˚ in [Fe(iPrP3B)(NNH2)] and to 2.95 A˚ in [(iPrP3B)Fe^N]+ while conserving the Fe covalency.6 Other reported nitrido Fe-based organometallic complexes have been generated from different synthetic pathways. The nitrido adduct [(iPrP3)FeIV^N] was isolated from the decomposition at room temperature of the [FeI(iPrP3)(dbabh)] (dbabh ¼ 2,3: 5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene) complex (Scheme 7).24 The Mössbauer properties (d ¼ −0.34(1) mm s−1 and |DEQ | ¼ 6.01(1) mm s−1) evidenced an FeIV species with a S ¼ 0 spin state.25 From 15N NMR experiments carried on the labeled [(iPrP3)FeIV^15N] species, the sharp resonance at 952 ppm confirmed the assignment to a terminally bound Fe^N coordination.

Scheme 7 Synthesis of [(iPrP3)FeIV^15N].

Two nitrido adducts were isolated with tris-(carbene)borate ligand. Both complexes have been synthesized by irradiation of the corresponding azide adduct, a common synthetic path in inorganic and organometallic chemistry used to generate nitrido complexes (Scheme 8). The X-ray structures of [(MesC3B)Fe^N]26 and [(tBuC3B)Fe^N]27 revealed short FedN bond lengths of 1.499(5) A˚ and 1.512(1) A˚ , respectively. The nitrido in [(tBuC3B)Fe^15N] has been identified by 15N NMR, with a sharp peak at 1019 ppm. A theoretical investigation has shown that the LUMO of this species, formerly the d2z orbital, is stabilizing by spd mixing leading to a nitrido ligand with an electrophilic character.

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Small Molecule Activation by Organo-iron Complexes

Scheme 8 Synthesis of [(MesC3B)Fe^N].

Interestingly, the oxidized form of [(tBuC3B)Fe^N] was also isolated (Scheme 9).28 The corresponding [(tBuC3B)Fe^N]+ species has been fully characterized with a FedN bond length of 1.506(2) A˚ (XRD data recorded at 35 K), similar to the initial complex. The Mössbauer (d ¼ −0.45 mm s−1 and |DEQ | ¼ 4.78 mm s−1) and cw-EPR data (gperp ¼ 1.971, gpara ¼ 2.299) evidenced a low spin FeV species (S ¼ ½).

Scheme 9 Synthesis of [(tBuC3B)Fe^N]+.

The last examples of organometallic nitrido Fe-based complexes have been reported with N-anchored tris(carbene) ligands, also generated from irradiation of the corresponding azide adducts (Scheme 10).29 In [(mesC3N)FeIV^N]+ and [(xylC3N)FeIV^N]+, the FedN bond length is of 1.526(2) A˚ and 1.527(3) A˚ , respectively. The 15N NMR spectrum of [(mesC3N)FeIV^15N]+ displays the typical signature of the nitrido ligand at 1121 ppm, the IR spectrum a vibration at 1008 cm−1 assigned to a short FedN bond, and the Mössbauer spectrum a quadrupole doublet (d ¼ −0.27(1) mm s−1 and DEQ ¼ 6.04(1) mm s−1) characteristic of a low spin FeIV species.

Scheme 10 Synthesis of [(mesC3N)FeIV^15N]+.

Only two families of nitrido adducts can generate NH3. In the presence of hydrogen atom donors, including TEMPO-H, the reduced complex, [(MesC3N)Fe^N], can lead to ammonia.26 Its oxidized [(tBuC3N)Fe^N]+ form is capable to produce ammonia in the presence of water (15 equiv.) under reducing conditions (3 equiv. of cobaltocene) in high yield (89%).28 In the case of [(iPrP3) Fe^N]+, it has been shown that during its formation, which involves an excess of acid, NH3 is also produced (36  0.5% isolated yield), suggesting that a distal pathway for N2 to NH3 conversion can ocur.6

Small Molecule Activation by Organo-iron Complexes

265

7.06.1.1.2.4 Diazene adduct The diazene intermediate is proposed to be the first distinct intermediate of the alternating pathway. The unique report of such species has been made with a bis-diphosphine Fe-based complex. [Fe0(MeP2)2(N2H2)] can be synthesized in hexane from the reduction of [FeII(MeP2)2Cl2] with KC8 in the presence of an excess of hydrazine30 or from the deprotonation of the cis-[Fe (MeP2)2(N2H4)]2+ complex with KOBut.31 Note that this reaction is reversible and that the hydrazine complex can be reformed in the presence of a weak acid such as 2,6-lutidinium triflate. The X-ray structure revealed a side-on bound diazene complex with NdN bond lengths of 1.427(7) A˚ and 1.398(8) A˚ (two independent molecules in the unit cell). The 15N NMR spectrum of the 15N labeled complex displays a broad resonance at 310.7 ppm. DFT calculations demonstrated that [Fe(MeP2)2(N2H2)] should be described as a donor-acceptor p complex between HN]NH and Fe0 rather than (HNdNH)2− and FeII.31 Interestingly, the hydrazine adduct can produce small amount of ammonia indicating that the diazene adduct can be considered as an active intermediate for N2 to NH3 conversion. 7.06.1.1.2.5 Hydrazine adduct The hydrazine adduct is the last intermediate of the alternating pathway before cleavage of the NdN bond and formation of an FedNH2 adduct, which is common with the distal mechanism. Several hydrazine adducts have been characterized. In the series of iPrP3X (X ¼ Si and B), the hydrazine adduct has been isolated from the reaction of [(iPrP3B)Fe]+ with an excess of N2H4 (Scheme 11).32 The resulting complex is paramagnetic (S ¼ 3/2) and displays a distorted trigonal bipyramidal geometry with an uncommon long FedN bond length of 2.205(2) A˚ consistent with a S ¼ 3/2 ground spin state for [(iPrP3B)Fe(N2H4)]+.

Scheme 11 Synthesis of [(iPrP3B)Fe(N2H4)]+.

For the (iPrP3Si) derivative, the hydrazine adduct has been first isolated from the reaction between [(iPrP3Si)FeII(CH3)] and the acid N2H5CF3SO3, leading to the [(iPrP3Si)FeII(N2H4)]+ adduct (S ¼ 1) with an unusual Z1-binding mode for a five-coordinated complex.4 It has also been shown that this hydrazine adduct can be generated from the disproportionation of two hydrazido-adducts through the hybrid pathway (see above Scheme 6).23 Regarding the bis-diphosphine Fe complexes, hydrazine adducts have been isolated and characterized (Scheme 12). Reaction between diamagnetic trans-[FeIICl2(MeP2)2]30 (or DMeOPrPE ¼ 1,2-bis(dimethoxypropylphosphino)ethane instead of (MeP2))33 with an excess of hydrazine generates the side-on bound hydrazine adduct cis-[FeII(MeP2)2(N2H4)] (or DMeOPrPE) with NdN bond length of 1.444(3) A˚ (or 1.442(3) A˚ ). The 15N NMR spectrum displays a signal at 387.9 ppm for the (MeP2) analog consistent with two equivalent N-donors and thus to a side-on coordination. The IR spectrum highlights the presence of NdH vibrations at 3313, 3227 and 2932 cm−1 for the DMeOPrPE adduct.

Scheme 12 Synthesis of [FeII(L)2(N2H4)] with L ¼ MeP2 or DMeOPrPE.

7.06.1.1.2.6 NH2 and NH3 adducts The formation of the FedNH2 and FedNH3 adducts are the remaining steps before the release of the second ammonia molecule. Even if such adducts are obviously formed at the end of the catalytic process, the number of well characterized species are very limited. The two adducts, [Fe(iPrP3B)(NH2)] and [Fe(iPrP3B)(NH3)]+, have been isolated with the same ligand. The [Fe(iPrP3B) (NH2)] complex has been synthesized from the reaction between [Fe(iPrP3B)]+ and an excess of NaNH2, while [Fe(iPrP3B)(NH3)]+ was generated from the decomposition of the hydrazine adduct [Fe(iPrP3B)(N2H4)]+ or the protonation of [Fe(iPrP3B)(NH2)] with HBArF4 (Scheme 13).32 Both species have been assigned to FeI species with an S ¼ 3/2 spin state based on their cw-EPR spectra. The FedN bond length increases from 1.918(3) A˚ to 2.280(3) A˚ for the FedNH2 and FedNH3 adducts, respectively with long FedB distances (of 2.449(4) A˚ and 2.433(3) A˚ , respectively).

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Small Molecule Activation by Organo-iron Complexes

Scheme 13 Synthesis of [Fe(iPrP3B)(NH2)] and [Fe(iPrP3B)(NH3)]+.

7.06.1.2

N2 reduction under catalytic conditions iPr

The [Fe( P3B)(N2)]− complex catalyzes the reduction of N2 into NH3 selectively. The best catalytic conditions have been investigated with two mixtures of acid and reducing agent with good efficiency and selectivity in both cases: (i) with HBArF4 and KC8: 7.0  1 equiv. of NH3 per Fe, 44  6% under optimized conditions (−78  C, 46 equiv. of HBArF4, 50 equiv. of KC8, in diethyl ether after 40 min)34 and (ii) with [Ph2NH2]OTf and Cp 2Co: 12.8  0.5 equiv. of NH3 per Fe, 72  3% under optimized conditions (−78  C, 108 equiv. of [Ph2NH2]OTf, 54 equiv. of Cp 2Co, in diethyl ether after 3 h).35 The key was to decrease the temperature to prevent the reaction between the acid and the reducing agent, while maintaining reasonable kinetics (initial rate of 1.2  0.1 mol of NH3 of Fe−1 min−1 in the case of the HBArF4 and KC8 mixture). The limitation in reactivity was attributed to the fact that the product of reaction, NH3, inhibits the catalysis. In the case of the HBArF4 and KC8 mixture, the catalytic resting state was characterized to be [(iPrP3Bm-H)Fe(H)(N2)], which contains a hydride terminally bound to the Fe, and a borohydride bridging between the B and Fe.36 Since the same species cannot be observed with [Ph2NH2]OTf and Cp 2Co and other experimental evidences, it has been shown that the mechanism is likely different.32 Using the [Fe(iPrP3C)(N2)]− analog in similar conditions with HBArF4 and KC8, a lower efficiency is observed: 4.6  0.8 equiv. of NH3 per Fe, 36  6% under optimized conditions (−78  C, 37 equiv. of HBArF4, 40 equiv. of KC8, in diethyl ether).37 The origin of this decrease has been proposed to be the formation of a hydride species that cannot liberate H2 and then becomes poisoned. Regarding the [Fe(iPrP3Si)(N2)]− derivative, it is only under high loading of acid (1500 equiv. of HBArF4) and reducing agent (1800 equiv. of KC8), that it displays catalytic properties: 3.8  0.8 equiv. of NH3 per Fe, 0.8  0.2% (−78  C, in diethyl ether). Conversely, it displays selective reactivity toward the production of H2.37 The origin of the difference in reactivity between these three catalysts is still an open question since multiple factors can be affected the mechanism, including the stability of imido like species as intermediate, and the reversibility of hydride binding in key intermediates. With the triphosphine ligand (iprP2P), it has been proposed that one of the key complexes involves in the catalysis is [Fe0(iprP2P) (N2)2]. Under optimized conditions (3600 equiv. of KC8, 3000 equiv. of HBArF4), 24.5  1.2 equiv. of NH3 per Fe are produced.18 Interestingly, under irradiation the efficiency is significantly improved (66.7  4.4 equiv. of NH3 per Fe) evidencing the importance of the photo-assisted H2 release to generate active N2-adducts. The PNP complexes also display catalytic properties. In particular, the catalytic properties of [FeI(tBuP2N)(N2)] has been tested under low and high loading conditions of reducing agent (40 and 200 equiv. of KC8) and acid (38 and 184 equiv. of HBArF4). Under these conditions (−78  C, in diethyl ether, after 1 h), 4.4  0.2 and 14.3  0.4 equiv. of NH3 per Fe have been produced, respectively, together with a small amount of hydrazine (0.2  0.2 and 1.8  0.2 equiv. of N2H4 per Fe have been measured, respectively).8 During mechanistic investigation, it was evidenced that under such conditions, the (tBuP2N) ligand was protonated on the pyrrole-backbone, leading to a less efficient catalyst. Therefore, analogs of (tBuP2N) with substitutions at the 3- and 4-positions of the pyrrole have been synthesized and the corresponding complexes [FeI(tBuPR2 N)(N2)] (R ¼ Ph or Me) display a F slightly better efficiency (in the case of [FeI(tBuPMe 2 N)(N2)], 200 equiv. of KC8, 184 equiv. of HBAr4, 22.7  1.7 equiv. of NH3 per Fe, and 1.7  0.3 equiv. of N2H4 per Fe). However, the protonation of the pyrrole backbone is still observed at the 2-position.9 The pyrrole ring was also replaced by a carbazole. However, the corresponding complex display a decrease of reactivity due to the formation dinuclear complex with N2 bridging the two Fe units, leading to a slowing down of the reaction.38 The bis-diphosphine Fe-based complexes also revealed activity toward N2 reduction, but generally with hydrazine as the main product. As an example, the [Fe0(EtP2)2N2] complex (270 equiv. of Cp 2Co, 360 equiv. of [Ph2NH2]OTf, −78  C, in diethyl ether) produces 24.5  0.5 equiv. of N2H4 per Fe, and only 0.95 equiv.  0.2 of NH3 per Fe.39 The [(C)2Fe(N2)] complex is the unique complex to display activity toward N2 reduction without phosphine-based ligand in its coordination sphere. However, the temperature needs to be decrease to −95  C for the system to become catalytic (50 equiv. of KC8, 50 equiv. of HBArF4, in diethyl ether, 3.3  1.7 equiv. of NH3 per Fe).

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Fig. 3 Structure of the active sites of the [NiFe] and [FeFe] hydrogenases.

7.06.2

H2 production

H2 represents a promising green alternative to fossil fuels, considering the potential energy of the HdH bond and its high energy density. The [NiFe] and [FeFe] hydrogenases are great sources of inspiration for the design of electrocatalysts for efficient H2 oxidation and/or production. Their active sites (Fig. 3) share several structural properties including a dinuclear [M2S2] core (Ni and Fe, or two Fe), a sulfur rich environment with bound cysteine(s), and biologically uncommon CO and CN− ligands. The notable performance of these natural catalysts to produce (oxidize) H2 from (to) H+ in a reversible manner arises from a fine control of how the two electrons and two protons are transferred during the catalytic cycle. In the case of H2 production (HER for hydrogen evolving reaction), inspired by the active site of the FeFe hydrogenase, chemists focused on mimics that reproduce the [Fe2S2]40 core characterized by its bridging dithiolate ligand. Several reviews list the synthesis and redox properties of bio-inspired FeFe and NiFe complexes in an exhaustive manner.41–43 Therefore, we choose to discuss particular examples, which display specific HER activity in terms of mechanism and/or lead to key intermediates species, i.e. hydride species. Even if only two protons and two electrons are involved during the catalytic cycle, their spatial and temporal involvement in the catalytic cycle is still under debate, and depends on numerous factors including the structure of the complex (geometry) and its redox properties (first coordination sphere), the strength of the acids and the presence or not of a proton relay (second coordination sphere). However, there is a common feature to all mechanisms, namely the formation of hydride intermediates. In the following paragraph, different mechanisms will be discussed in relation with the more significant families of FeFe and NiFe complexes, and the relevant hydride species described, when data are available. Their performance as HER electrocatalysts will be also discussed.

7.06.2.1 7.06.2.1.1

FeFe catalysts With only CO ligands

The most simple bio-inspired complexes of the FeFe hydrogenase are the FeIFeI [(CO)3FeI(X)FeI(CO)3] (X ¼ a bridging dithiolate (RS2)2− or two bridging thiolate (RS)− ligands). Even if no hydride intermediate relevant to HER catalysis has been isolated, a bridging hydride FeIIFeII complex, [(CO)3FeII(PrS2)(m-H)FeII(CO)3]+ (PrS2 ¼ 1,3-propanedithiolate) has been characterized, isolated from the reaction between [(CO)3FeI(PrS2)FeI(CO)3] and the acid generated from [SiEt3]B(C6F5)4 and HCl (Scheme 14).44

Scheme 14 Synthesis of [(CO)3FeII(PrS2)(m-H)FeII(CO)3]+.

The presence of the hydride has been confirmed by 1H NMR with a sharp singlet at d −15.9 ppm and X-ray data revealed an Fe. . .Fe distance of 2.5540(4) A˚ , slightly elongated with respect to the initial complex (2.5103(11) A˚ ). The electrocatalytic properties of [(CO)3FeI(PrS2)FeI(CO)3] have been investigated in details via digital simulations.45,46 With HOTfs acid, the initial FeIFeI is not protonated and needs to be reduced first (E1/2 ¼ −1.55 V vs Fc+/0 in MeCN) before to undergo a protonation leading to the proposed [(CO)3FeI(PrS2)(m-H)FeII(CO)3] complex. This hydride species is readily reduced (E1/2 ¼ − 1.59 V vs Fc+/0) to form the [(CO)3FeI(PrS2)(m-H)FeI(CO)3]− hydride. Its further protonation generates two species in equilibrium: (i) a dihydride species with two terminal hydrides, one coordinated to each FeII ion, and (ii) a dinuclear FeI species with a bridging hydride and a protonated thiolate, the corresponding thiol being coordinated to one Fe site. The dihydride species releases H2 and regenerates the initial [(CO)3FeI(PrS2)FeI(CO)3] in a global [ECEC] mechanism (Scheme 15), while the monohydride intermediate undergoes another reduction (E1/2 ¼ −1.84 V vs Fc+/0), before releasing H2. In this latter case, the reduced [(CO)3FeI(PrS2)Fe0(CO)3]− complex is the entry in the catalytic cycle for a general [E(CECE)] mechanism.

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Scheme 15 Catalytic cycle for H2 production with [(CO)3FeI(PrS2)FeI(CO)3].

When the bridging dithiolate (PrS2) ligand is replaced by the 1,2-benzenedithiolate (BzS2), the resulting electron-depleted FeIFeI complex needs to be doubly reduced (E1/2 ¼ −1.31 V and −1.33 vs Fc+/0) before reacting with protons (Scheme 16).47 Depending on the strength of the acid, the resulting [(CO)3FeI(BzS2)(m-H)FeI(CO)3]− hydride species is further reduced (E1/2 ¼ −1.87 V vs Fc+/0) before the fast release of H2 with an additional proton (case of weak acid, pKa > 23, e.g., HOAc), or is directly protonated to slowly release H2 (case of strong acid, pKa < 23, e.g., HOTfs). In the first case, the mechanism follows a [E(ECEC)] mechanism with [(CO)3FeI(BzS2)Fe0(CO)3]− as the entry complex in the electrocatalytic cycle, while no pre-activation step is requested in the second case with a global [EECC] mechanism.

Scheme 16 Catalytic cycle for H2 production with [(CO)3FeI(BzS2)FeI(CO)3].

Another important modification regarding the dithiolate bridge was the use of azadithiolate bridges with various substitutions of the amine function ((NRS2) with R]H, alkyl or ether groups).48–50 In such a case, the basic amine can be readily protonated in the

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Scheme 17 Catalytic cycle for H2 production with [(CO)3FeI(NRS2)FeI(CO)3].

corresponding [(CO)3FeI(NRS2)FeI(CO)3] complexes (Scheme 17). The protonated [(CO)3FeI(H-NRS2)FeI(CO)3]+ complex can be thus reduced at a much more positive potential (about 0.30 V in THF, small variations of the potential as a function of the nature of R) than the initial complex.48 The resulting mixed valent species [(CO)3FeI(H-NRS2)Fe0(CO)3] can then react with an additional proton to generate a hydride species [(CO)3FeII(H-NRS2)(m-H)FeI(CO)3]+, which is immediately reduced. A slow H2 release thus occurs in a global [CECE] mechanism. When a strong acid is used, a third protonation occurs before the second reduction, leading to a fast H2 release in a general [EC(CECE)] mechanism.

7.06.2.1.2

With phosphine ligands

With the aim of mimicking the FeFe hydrogenases, which produce H2 at a much more positive potential and involve terminal hydride during the catalytic cycle, these very simple hexacarbonyls complexes were modified by replacing two or more CO ligands. Even if the substituted complexes with phosphine or carbene based ligands are reduced at a more negative potential than the hexacarbonyl complexes, the Fe-center basicity is increased leading to an immediate protonation followed by a facile reduction at the origin of a catalytic potential notably anodically shifted. Depending on the nature of the (di)thiolate bridge, the protonation of these complexes leads to terminal or bridging hydrides. When 2 CO ligands of one Fe site in [(CO)3FeI(PrS2)FeI(CO)3] are replaced by a diphosphine (enPhP2 ¼ 1,2-bis (diphenylphosphino)ethylene), a terminal hydride has been isolated from the reaction between [(CO)(enPhP2)FeI(PrS2)FeI(CO)3] and 1 equiv. of [H(OEt2)2]((BArF4) at −90  C in MeCN (Scheme 18). Two isomers are observed with characteristic low field 1H NMR features at d ¼ −4.4 and −3.3 ppm (the latter being a triplet with JPH ¼ 70 Hz). The 31P NMR signals at d ¼ 97 and 71 ppm are consistent with the coordination of the terminal hydride on the [Fe(CO)3] site. Upon warming a unique bridging hydride is formed with 31P NMR signals at d ¼ 95 and 92 ppm and 1H NMR ones at d ¼ − 14.4 ppm).51 The unexpected observation of the terminal coordination of the hydride at the [Fe(CO)3] site instead of the [Fe(CO)(enPhP2)] site can be explained by the presence of the bulky diphosphine ligand at this latter Fe site and by the position of a vacant site created trans to a CO that became bridging.52

Scheme 18 Formation of hydride species from [(CO)(enPhP2)FeI(PrS2)FeI(CO)3].

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In the case of the bis(diphosphine) complex, [(CO)(enPhP2)FeI(PrS2)FeI(enPP2)(CO)], a terminal hydride can be isolated in the presence of a strong acid (d(1Η) ¼ − 3.5 ppm and nCO ¼ 1965 and 1905 cm−1) that can be converted at room temperature in a bridging hydride (two isomers: d(1Η) ¼ − 14.5 ppm (with nCO ¼ 1968 and 1951 cm−1) and −15.6 ppm (nCO not observed)).53 Interestingly, it has been shown that the coordination mode of the hydride impacts the redox properties of the corresponding species, the terminal hydride complex being reduced at a potential less negative (about 0.1 V) than the bridging hydride one. Note that in the case of [(CO)(MeP)2FeI(PrS2)FeI(MeP)2(CO)] under similar low temperature conditions, an intermediate protonated species with one coordinated bridging thiol is proposed to be formed evidencing the potential role of the S-atom as proton relay (Scheme 19).54 The proton of the thiol function is converted into a terminal and bridging hydride through a first-order and second-order processes, respectively. The 1H NMR spectra of both species are characteristic for a terminal (d(1Η) ¼ −2.2 ppm) and semi-bridging (d(1Η) ¼ − 18.8 ppm) hydride species.

Scheme 19 Formation of hydride species from [(CO)(MeP)2FeI(PrS2)FeI(MeP)2(CO)].

With the azadithiolate derivatives, several hydride species have been isolated from the reaction between substituted complexes with phosphine ligands and acids of various strengths. Two terminal hydrides have been crystallography characterized, namely [(CO)(MeP)2FeII(NRS2)FeII(MeP)2(CO)(H)]+ and [(CO)(enPhP2)FeII(NRS2)FeII(enPhP2)(CO)(H)]2+, corresponding to the mono- and di-protonation of the initial neutral complexes, respectively (Fig. 4).54,55 The [(CO)(MeP)2FeII(NRS2)FeII(MeP)2(CO)(H)]+ complex displays an FedH bond distance of 1.487 A˚ , with the hydride in trans position with respect to the CO ligand. Besides, a N–H. . .H–Fe interaction (dihydrogen bonding distance of 2.042 A˚ ) has been evidencing both in solid state and solution. In the case of [(CO)(enPhP2)FeII(H-NRS2)FeII(enPhP2)(CO)(H)]2+, the FedH bond is shorter (1.438 A˚ ) as well as the dihydrogen bonding distance of 1.88(7) A˚ (d(1Η) ¼ − 4.95 ppm and nCO ¼ 1986 and 1935 cm−1). The [(CO)(enPhP2)FeII(H-NRS2)FeII(enPhP2)(CO)(H)]+ complex, which contains a terminal hydride and a protonated (NRS2) ligand, has been also isolated and characterized (d(1Η) ¼ −4.2 ppm and nCO ¼ 1965 and 1915 cm−1).55 All these terminal hydride species isolated at low temperature do not correspond to the thermodynamic products, and upon warming, bridging hydride species are formed (two isomers for [(CO)(enPhP2)FeII(m-H)(NRS2)FeII(enPhP2)(CO)]+: d(1Η) ¼ −13.7 ppm (with nCO ¼ 1969 and 1948 cm−1) and −14.8 ppm (nCO not observed) and two isomers for [(CO)(enphP2)FeII(m-H) (H-NRS2)FeII(enphP2)(CO)]2+: d(1Η) ¼ −15.6 ppm (with nCO ¼ 1985 and 1967 cm−1) and −14.5 ppm (nCO not observed)). It has been shown that these above species are involved as active intermediates in the catalytic cycle for H2 production.55 [(CO) enPh ( P2)FeI(NRS2)FeI(enphP2)(CO)] represents the entry of the catalytic cycle whatever the strength of the acid is. The catalysis is initiated with a protonation step that generates the terminal hydride [(CO)(enPhP2)FeII(NRS2)FeII(enphP2)(CO)(H)]+ through an intermolecular proton transfer from the bridge head amininium. Then, two mechanisms are proposed based on the nature of the

Fig. 4 Structure of the [(CO)(MeP)2FeII(NRS2)FeII(MeP)2(CO)(H)]+ and [(CO)(enPhP2)FeII(H-NRS2)FeII(enPhP2)(CO)(H)]2+ complexes.

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acid (Scheme 20). In the case of a strong acid, a second protonation occurs to form [(CO)(enPhP2)FeII(m-H)(H-NRS2)FeII(enPhP2) (CO)]2+, followed by two one-electron reductions (Ecat ¼ −1.72 V vs Fc+/0 in CH2Cl2) leading to the release of H2 and regeneration of the initial FeIFeI species ([CCEE] mechanism). When a weak acid is used, the terminal hydride species is reduced (Ecat ¼ − 1.49 V vs Fc+/0 in CH2Cl2) to generate another proposed terminal hydride [(CO)(enPhP2)FeI(NRS2)FeII(enPhP2)(CO)(H)], which is then protonated and reduced to produce H2 ([CECE] mechanism).

Scheme 20 Catalytic cycle for H2 production with [(CO)(enPhP2)FeI(NRS2)FeI(enPhP2)(CO)].

7.06.2.1.3

With N-based ligands and metallocene unit

Only scarce examples report on the use of other ligands than phosphine and CO ligands in the coordination of the Fe sites. A bipyridine (bpyN2) ligand has been introduced in [(CO)3FeI(PrS2)FeI(CO)3].56 The substitution two CO ligands by (bpyN2) leads to a negative shift of the reduction of the initial FeIFeI complex by 0.4 V, accompanied by an average shift of the nCO vibration of about 70 cm−1 to lower energy because of the strong s-donating propensity of (bpyN2). Electrocatalysis occurs at two different potentials corresponding to two different proposed mechanisms. Thanks to the (bpyN2) ligand that increases the electron density at the FeFe center, the first common step corresponds to the direct protonation of the FeIFeI species in the presence of a strong acid (HBF4
OEt2) in MeCN to generate a proposed bridging hydride species (not observed). The first catalytic wave observed at Ecat ¼ −1.33 V vs Fc+/0 is thus assigned to a [C(ECEC)] mechanism with the [(CO)3Fe(PrS2)(m-H)Fe(bpyN2)(CO)]+ being the electrocatalytic species. For the second catalytic wave (at Ecat ¼ −1.70 V vs Fc+/0), a [CE(CECE)] or [CE(CEEC)] pathway is proposed to occur with the reduced hydride species, [(CO)3Fe(PrS2)(m-H)Fe(bpyN2)(CO)], corresponding to the entry of the catalytic cycle. Diiron complexes with higher oxidations states have been stabilized with a bipyridine substituted by two alkyl thiolates.43,57 The two [(bpyN2S2)FeII(CO)FeIICp]+ and [(bpyN2S2)FeII(m-CO)FeIICp(CO)] complexes can be obtained quantitatively in solution through the reversible (de)coordination of one CO ligand. The [(bpyN2S2)FeII(CO)FeIICp]+ complex displays an electrocatalytic activity in the presence of a mild proton source (Et3NHBF4) in MeCN at Ecat ¼ −1.65 V vs Fc+/0 (Z ¼ 730 mV, TON ¼ 15 in 20 min, FY ¼ 60%) through a proposed [E(ECEC)] mechanism (Scheme 21).

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Scheme 21 Catalytic cycle for H2 production with [(bpyN2S2)FeIIFeIICp(CO)]+.

The electrocatalytic species, [(bpyN2S2)Fe(CO)FeCp], has been fully characterized as a type II–I mixed-valence species with the CO terminally bound to the [(bpyN2S2)Fe] unit (gpara ¼ 2.262 and gperp ¼ 1.947 at 22 K, giso ¼ 2.064 at 100 K; lmax(IV) ¼ 1015 nm; nCO ¼ 1896 cm−1; FeA: d ¼ 0.51 mm s−1, DEQ ¼ 2.19 mm s−1, FeB: d ¼ 0.33 mm s−1, DEQ ¼ 0.44 mm s−1 at 80 K). The presence of the non-innocent redox character of the bipyridine moiety has been proposed to dominate the catalytic process and to be at the origin of the remarkable activity of this complex that occurs with FeFe species displaying redox oxidation states relevant to those of the [FeFe]hydrogenase. Interestingly, the [(bpyN2S2)FeII(CO)FeIICp(CO)] complex has been shown to be electrocatalytically active for efficient H2 production in acidic aqueous solution after its physiadsorption onto a carbon based electrodes (TON ¼ 7.2.106 TON in 10 h, TOF ¼ 200 s−1, at −0.85 V vs SHE, pH 3).58 The reactivity of a similar complex with a N2S2 ligand, [(alkylN2S2)Fe(NO)FeCp(CO)]+ has been also reported.59 Electrocatalytic production of H2 occurs at Ecat ¼ − 1.66 V vs Fc+/0 in the presence of CF3COOH in MeCN (TON ¼ 0.33(2) in 30 min, FY ¼ 77.2(8) %). An extensive theoretical investigation based on DFT calculations highlighted the importance of the hemilability character of the Fe-bound thiolate crucial in such a system: one of them is protonated during the catalytic process followed by its decoordination to allow the formation of proton/hydride pair (Scheme 22).

Scheme 22 Catalytic cycle for H2 production with [(alkylN2S2)Fe(NO)FeCp(CO)]+.

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7.06.2.2

273

NiFe catalysts

The number of bioinspired [NiFe] hydrogenase complexes active for H2 production is much more limited than that of [FeFe] hydrogenase mimics, due to difficult synthesis issues. Apart from the aim of developing active electrocatalysts, chemists focused their attention on the characterization of intermediates species to contribute to the understanding of the enzymatic mechanism. In this context, not only the stabilization and characterization of hydride species were targeted, but also those of reduced species generated by the selective reduction of the Ni ion as in [NiFe]hydrogenase. These potential active catalytic species will be described, together with the reactivity and mechanism of the corresponding NiFe electrocatalysts if applicable.

7.06.2.2.1

Stabilization of reduced NiIFeII species I

The first Ni FeII complex, [CpNiI(PrS2)FeII(PhP2)(CO)] (nCO ¼ 1901 cm−1), has been afforded by reducing [CpNiII(PrS2)FeII(PhP2) (CO)]+ (PhP2 ¼ 1,2-bis(diphenylphosphino)ethane) (E1/2 ¼ −1.16 V vs Fc+/0) with cobaltocene.60 This reduced species was isolated as crystals revealing a NidFe distance of 2.4593(6) A˚ indicating the presence of a NidFe bond. The g values of 1.991, 2.042, and 2.138 are consistent with a S ¼ ½ NiI species and the Mössbauer parameters (d ¼ 0.11 mm s−1, DEQ ¼ 1.15 mm s−1) with a low spin FeII ion. Even if no other intermediate could be observed with this system, DFT calculations predicted that a NiIIIFeII semi-bridging hydride is proposed to be involved in the electrocatalytic cycle, when [CpNiI(PrS2)FeII(PhP2)(CO)] produces H2 in the presence of CF3CO2H as the proton source (Ecat ¼ − 1.16 V vs Fc+/0, TOF ¼ 4 s−1) Scheme 23.42

Scheme 23 Catalytic cycle for H2 production with [CpNiII(PrS2)FeII(PhP2)(CO)]+.

Using the thiolate substituted bpy ligand, (bpyN2S2), the [(bpyN2S2)NiI(CO)FeIICp] complex with a bridging CO (nCO ¼ 1770 cm−1) has been obtained by reducing [(bpyN2S2)NiIIFeIICp(CO)]+ (E1/2 ¼ −1.29 V vs Fc+/0) with cobaltocene (Scheme 24).61 The axial S ¼ 1/2 EPR spectrum (g-values of 2.060 and 2.168) and the Mössbauer parameters (d ¼ 0.44 mm s−1, DEQ ¼ 1.79 mm s−1) are consistent with a NiIFeII species. Interestingly, the hydride [(bpyN2S2)NiII(H)FeIICp(CO)] species could be also generated from [(bpyN2S2)NiIIFeIICp(CO)]+ in the presence of NaBH4 (nCO ¼ 1838 cm−1 and 1H NMR d ¼ −6.80 ppm). The optimized structure of this hydride species indicates that the hydride is semi-bridging between the Ni and Fe ions (NidH and FedH bond lengths of ¼ 1.72 and 1.58 A˚ , respectively).62 In the presence of Et3NHBF4 in MeCN, [(bpyN2S2)NiIIFeIICp(CO)]+ produces H2 electrocatalytically at Ecat ¼ −1.85 V vs Fc+/0 (FY ¼ 70%, kcat of 2.5  0.3  104 M−1 s−1, 16 TON in 4 h) through a proposed [E(ECEC)] mechanism, the reduced NiIFeII being the entry of the catalytic cycle (Scheme 24). However, when a stronger acid is used (HBF4), [(bpyN2S2)NiI(CO)FeIICp] can be directly protonated and H2 is produced in an electrocatalytic manner at Ecat ¼ −1.13 V vs Fc+/0 (FY ¼ 91%, 104 TON in 4 h).63 To investigate the role of the Fe unit on the activity of this catalyst, Cp has been used instead of Cp. The corresponding [(bpyN2S2)NiIIFeIICp (CO)]+ complex is still active (FY ¼ 70%, kcat of 6.8  0.3  103 M−1 s−1) but the mechanism is modified with an [E(CEEC)] cycle.64 This observation has been explained by the fact that the CO ligand is terminally bound to the reduced NiIFeII species instead of semi-bridging with the Cp-based complex, and is thus more prone to be protonated.

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Scheme 24 Catalytic cycle for H2 production with [(bpyN2S2)NiIIFeIICp(CO)]+.

A similar complex [(alkylN2S2)NiIIFeIICp(CO)]+ has been described with a Ni unit containing the redox innocent ligand N2S2) that differs from the redox active (bpyN2S2) one.59 H2 is produced electrocatalytically in MeCN in the presence of ( trifluoroacetic acid (Ecat ¼ −1.64 V vs Fc+/0, TON ¼ 0.26(1) in 30 min, FY ¼ 96(3) %). No intermediate species has been observed but the theoretical investigation indicated that the electrocatalytic species of an [E(CECE)] cycle is a NiIIFeI species, illustrating the important role of the bpy moiety in [(bpyN2S2)NiIIFeIICp(CO)]+ to control the reduction process at the Ni site (Scheme 25). alkyl

Scheme 25 Catalytic cycle for H2 production with [(alkylN2S2)NiIIFeIICp(CO)]+.

7.06.2.2.2

Stabilization of NiFe hydride species

Regarding NiFe-hydride species, besides [(bpyN2S2)NiII(H)FeIICp(CO)] described above, other examples are available with the Ni in P2S2 environment. The [(PhP2)NiI(PrS2)FeI(CO)3] can undergo a redox isomerization in a formal NiIIFe0, which is about eight order of magnitude more basic than the NiIFeI state, allowing its rapid protonation to generate the [(PhP2)NiII(PrS2)(m-H)FeII(CO)3]+

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species (Scheme 26).65 The starting NiIFeI complex is characterized by an unusual metal-metal bond (Ni–Fe: 2.467 A˚ ), with nCO features at 2028 and 1952 cm−1.66 In the presence of HBF4, [(PhP2)NiII(PrS2)(m-H)FeII(CO)3]+ is formed (nCO ¼ 2082 and 2024 cm−1, and 1H NMR d ¼ −3.53 ppm), and the X-ray structure evidenced that the hydride ligand is more bound to the Fe ion (Fe–H: 1.46(6) A˚ ) than to the Ni ion (Ni–H: 1.64(6) A˚ ) with a Ni. . .Fe distance of 2.6131(14) A˚ .67

Scheme 26 Formation of the [(PhP2)NiII(PrS2)(m-H)FeII(CO)3]+ hydride species.

Modifications of the Fe site by replacing one or two CO ligands by a mono- or di-phosphine lead in all cases to bridging hydride NiFe species, in which the hydride ligand is more or less bound to the Fe site with respect to the Ni site. While the [(PhP2)NiII(PrS2) (m-H)FeII(PhP)(CO)2]+ complex (nCO ¼ 2016 and 1964 cm−1, and 1H NMR d ¼ − 3.08 ppm),68 displays a difference dNi-H-dFe-H of 0.403 A˚ (to be compared to 0.177 A˚ in [(PhP2)NiII(PrS2)(m-H)FeII(CO)3]+), this difference decreases to 0.292 A˚ when two CO are exchanged in [(PhP2)NiII(PrS2)(m-H)FeII(PhP2)(CO)]+ (nCO ¼ 1954 and 1938 cm−1, and 1H NMR d ¼ −3.01 ppm).42 Therefore, no trend can be defined because electronic effect and steric interaction can affect the binding of the hydride with opposite weights. With respect to [(PhP2)NiII(PrS2)(m-H)FeII(CO)3]+, the (PhP2) diphosphine ligand on the Ni site has been exchanged with 1,2-bis(dicyclohexylphosphino)ethane (hexP2). In the corresponding [(hexP2)NiII(PrS2)(m-H)FeII(CO)3]+ complex (nCO ¼ 2078 and 2017 cm−1, and 1H NMR d ¼ −3.00 ppm), the hydride ligand is more tightly bound to the Fe center (dNi-H-dFe-H of 0.370 A˚ ) but still bound to the electron rich Ni center.69 The nature of the bridging dithiolate ligand has been also explored. The (PrS2)2− ligand has been replaced by the (EtS2) (1,2-ethanedithiolate) leading to another hydride species [(PhP2)NiII(EtS2)(m-H) FeII(CO)3]+ (nCO ¼ 2084 and 2025 cm−1, and 1H NMR d ¼ − 5.07 ppm), which displays similar structural properties (dNi-H-dFe-H of 0.265 A˚ ).69 It has been proposed that all the complexes of this family cited above displays HER electrocatalytic properties with a [CECE] mechanism (Scheme 27). The hydride NiII(m-H)FeII species are proposed to be the first intermediates of the cycle, followed by their reduction to allow a second protonation to occur and the release of H2. The resulting NiIIFeI species are then reduced to regenerate the initial NiIFeI complexes. The electrocatalytic performance of these complexes are comparable and operate at high rate (TOF values around 50 s−1) with negative Ecat/2 values (between −1.20 and 1.45 V vs Fc+/0) and relatively small overpotentials (Z between 0.49 and 0.60 V), if the weakest acids (such as CF3CO2H or ClCH2CO2H) adjusted to the pKa of the NiII(m-H)FeII complex (comprised between 10.7 for [(PhP2)NiII(EtS2)(m-H)FeII(CO)3]+ and 14.9 for [(PhP2)NiII(EtS2)(m-H)FeII(PhP)(CO)2]+), are used.42

Scheme 27 Catalytic cycle for H2 production with (PhP2)NiI(EtS2)FeICp(CO)3]+.

276

Small Molecule Activation by Organo-iron Complexes

Fig. 5 Representation of the two [(PhP2)NiII(PrS2)FeIICp (CO)]+ isomers and [CpNiII(NRS2)FeII(CO)(PhP2)]+.

The Fe site has also been modified by replacing to CO by a Cp ligand in [(PhP2)NiII(PrS2)FeIICp (CO)]+, in which the CO is terminally bound to the Fe site (two isomers are present with nCO ¼ 1917 and 1880 cm−1 characterized by Ni. . .Fe distance of 3.085 A˚ and 2.7365(6) A˚ , respectively) (Fig. 5).70 This complex displays HER electrocatalytic properties using HOAc in MeCN (TOF ¼ 761 s−1) with a proposed [EECC] mechanism. More recently, the addition of a potential proton relay in the dithiolate bridge has been evaluated on the HER catalytic activity, for example for with [CpNiII(NRS2)FeII(CO)(PhP2)]2+ (Fig. 5). An overpotential of Z ¼ 0.54 V (Ecat/2 ¼ −1.67 V vs Fc+/0) has been measured using TFA as proton source in MeCN.71 A heterodinuclear NiFe complex with only one bridging thiolate has been synthesized, [(S4)NiI(m-CO)FeI(CO)2] (S4H2 ¼ 1,2bis(4-mercapto-3,3-dimeth-yl-2-thiabutyl)benzene) (Scheme 28).72 The V-shape of the [Ni(m-CO)(m-S)Fe]-core displays a distance between the two metallic ion of 2.4262(2) A˚ , a CO vibration in agreement with a bridging coordination (nCO ¼ 1854 cm−1) and Mössbauer parameters consistent with a (d ¼ 0.06 mm−1 s−1, DEQ ¼ 1.39 mm−1 s−1) with a low spin FeI ion. This diamagnetic complex can be reversibly protonated with HBF4
Et2O to generate two DFT-predicted tautomers (free energy difference of  5 kcal mol−1) (Scheme 28). In one complex the protonation occurs on the thiolate terminally bound to the Ni site, while a second corresponds to a bridging hydride species. The spectroscopic properties are more consistent with the thiol-based NiIFeI species. The Mössbauer parameters (d ¼ 0.05 mm s−1, DEQ ¼ 1.20 mm s−1) are similar to those of the initial complex consistently with an FeI ion. The slight DEQ shift is proposed to originate from the changing in the coordination mode of the bridging CO (nCO ¼ 1932 cm−1), with an elongation of the NidCCO bond in favor of a shortening of the FedCCO bond, leading to an increase of the p-backdonation from iron to this CO. Besides, the signal of the thiol proton was identified at 4.79 ppm in the diamagnetic 1H NMR spectrum of this protonated species. The [(S4)NiI(m-CO)FeI(CO)2] complex displays HER electrocatalytic activity in the presence of CF3CO2H in MeCN (Ecat/2 ¼ −1.43 V vs Fc+/0, Z ¼ 0.540 V, TOF ¼ 5 h−1). However, since it has been shown that CF3CO2H cannot protonate [(S4)NiI(m-CO)FeI(CO)2], its protonated form [(HS4)NiI(m-CO)FeI(CO)2]+ is not a catalytic intermediate.

Scheme 28 Equilibrium between the two tautomers [(HS4)NiI(m-CO)FeI(CO)2]+ and [(S4)Ni(m-CO)(m-H)Fe(CO)2]+.

7.06.3

H2 oxidation

Even if H2 represents a promising green alternative to fossil fuels, it is still challenging to achieve its oxidation in an efficient way, because of the poor binding properties of H2 and the requirement of the control of the sequential removal of the two protons and the two electrons. If the proton reduction catalyzed process is well documented (see Section 7.06.2), this is not the case regarding the inverse reaction. So far, only the [FeFe] hydrogenase has been modeled in the context of H2 oxidation. The crucial enzymatic reaction step, i.e. the heterolytic cleavage of H2, into H− and H+, is assisted by an intrinsic base, a pendant amine that captures the proton and also acts as a proton relay. The iron sulfur cluster bound to the active site not only controls the electron flux but also plays a role on the electrophilicity of the proximal iron center, and thus to its reactivity. In contrast, low valent Pt complexes excel at homolytic cleavage activation, in which H2 is reduced into two equivalents of oxidized Pt hydride species. While the strict reproduction of the structure of the active site was unsuccessful regarding reactivity, the bio-inspired approach allows to develop active catalysts for H2 oxidation. These complexes belong mainly to two structural families, the diiron aza dithiolate series and the mononuclear [CpFe(NxP2)] series (with X ¼ 1 or 2, N ¼ pendant amine, P2 ¼ diphosphine ligand). This molecular approach allows to elucidate the mechanism, in particular through the isolation and characterization of catalytic intermediates, including dihydrogen and hydride adducts. Up to now, the mechanism depends on the nature of the catalyst family (Scheme 29). They differ by the sequence of the oxidation and chemical steps, while displaying common intermediates.

Small Molecule Activation by Organo-iron Complexes

Scheme 29 H2 oxidation catalytic cycles for [(CO)3FeI(PrS2)(CO)FeI (RN2P2)] and [FeII(Cp-R)(NR2P2)]+ and complexes.

277

278

Small Molecule Activation by Organo-iron Complexes

The diiron [Fe2(NRS2)(L)(CO)3(RN2P2]+ complexes (L ¼ monodentate phosphine), models of the Hox state of the [FeFe] hydrogenases,73,74 are mixed valent FeIFeII species that need to be one-electron oxidized before to react with dihydrogen. The resulting Z2-H2-FeIIFeII complex evolves via the heterolytic cleavage of the binding H2 into an FeIIFeII hydride species, in which the proton is trapped by the pendant amine. After the displacement of this proton to an external base, a second intramolecular proton transfer occurs from the iron to the Namine, followed by an intermolecular proton transfer to a base in solution. A second one-electron oxidation step regenerates the unsaturated intermediate FeIFeI species into its ferrous state in a final [ECCE]. The reaction pathway for the [Fe(Cp-R)(NR2P2)]+ system (with R ¼ alkyl group) starts also with the unsaturated FeII species. After the H2 cleavage of the Z2-H2− FeII intermediate, followed by the removal of a proton by an external base, a one-electron oxidation step is occurring, with the formation of a ferric hydride intermediate. The subsequent intramolecular proton transfer generates an FeI unsaturated species that needs to be oxidized to regenerate the initial FeII complex in a global [CECE] mechanism.

7.06.3.1

Dinuclear FeFe complexes

Tertiary phosphine ligands have been introduced in the structure of the diiron unit shifting from a rich CO/− CN environment, while the conserved dithiolate bridge was subject to change by the addition a pendant amine. The use of phosphine ligands was dictated by their easy adjustability in terms of steric and donor properties, the fact that the − CN and CO ligands are prone to form bridging ligands during redox processes, and the − CN to protonate readily. The modulation of the redox properties of the initial FeIFeI complex to generate the mixed valence state was then easily achieved through the variation on the CO/phosphine ligand ratio or/and on the length of the bridging dithiolate. A series of complexes with the general formula [(CO)3−x(L)xFeI(PrS2)(CO)FeI(RN2P2)] (RN2P2 ¼ cis − 1, 2 bis(diphenylphosphino) ethylene; L ¼ MeP, PrP),4 highlights that the replacement of CO by phosphine ligands lowers the FeIFeI/FeIFeII redox potential by 625 mV, making it possible to prepare chemically the mixed valent species with Fc+ as an oxidant (Scheme 30). The initial FeIFeI complexes were synthesized by photochemical decarbonylation of the [(CO)3FeI(PrS2)FeI(CO)3] precursor in the presence of (di)phosphine ligands.75

Scheme 30 Representative model of the [(CO)3FeI(PrS2)(CO)FeI(enPh2P2)] series and its oxidation to the mixed valent state.

The general structure of the FeIFeI complexes displays an open coordination site at the Fe(enPh2P2) moiety. After their one electron oxidation (Scheme 30), while the Fe(CO)3 unit is unchanged, the CO on the second iron shifts from a terminal to a semi-bridging coordination mode in an asymmetric way (FedC bond lengths of 2.678 and 1.786 A˚ in [(CO)3 FeII(PrS2)(CO) FeI(enPh2P2)](BF4).76 An axial S ¼ ½ EPR signal with phosphorus hyperfine coupling observed as a triplet pattern on each component was observed for the mixed valence species. The magnitude and the multiplicity of the hyperfine couplings indicate that the unpaired electron is localized on the unsaturated [Fe(CO)(MeP)] moiety. The X ray structure of two adducts with different L monophosphine ligands replacing one CO in [(CO)2(L)FeII(PrS2)(CO) FeI(enPh2P2)] (L ¼ iprP and MeP), crystallized at low temperature, reveals that the two metal centers are in a quasi-octahedral environment with an Fe. . .Fe distance close to 2.58 A˚ in the both complexes. Interestingly, the CH of the bridge points toward the metal but too far for an agostic interaction (2.49 A˚ ). The FedPenPh2P2 bond length is 2.2369 A˚ , while it decreases from MeP to iPr P (FedPMeP ¼ 2.2830(15) A˚ and FedPiPrP ¼ 2.2273(15) A˚ ) and the FedS distances averaged to 2.25 A˚ . The stability of [(CO2) (MeP)FeII(PrS2)(CO)FeI(enPh2P2)](BF4) was enhanced via counteranion exchange, leading to storage up to days at room temperature for the BArF4 analog (BAr−F4 ¼ B(C6H3-3,5-(CF3)2)−4).77 The introduction of the amine on the dithiolate bridge (NRS2; R ¼ Bn or H) did not affect the spectroscopic and structural properties of the mixed valence dinuclear Fe species, confirming the presence of a semi bridging CO and the vacancy on the apical position of the [Fe(enPh2P2)] moiety. Its nearly axial S ¼ ½ EPR signal corroborated also its mixed valent state (with [g] ¼ 2.010, 2.022, 2.118 and Aav P ¼ 73.46, 71.70 and 82.74 MHz). A better mimic of the active site of FeFe hydrogenase introduces an artificial redox center instead of the iron sulfur cluster to the dinuclear unit.78 The [(FcP )(CO)2FeI(Bn-NS2)FeI(CO)(enPh2P2)] complex, where FcP ¼ Cp FeII(C5Me4CH2PEt2), is an evolution of the precedent complex with MeP replaced by a phosphine substituted by ferrocenyl moiety and is obtained from the substitution of one CO ligand of the [(CO)3FeI(Bn-NS2)(CO)FeI(enPh2P2)] precursor (Fig. 6). It possesses spectroscopic parameters similar to the PMe3 derivatives. Three different oxidation states were accessible via the titration of a solution of [(FcP )(CO)2FeI(Bn-NS2)(CO)FeI (enPh2P2)] with Fc(BArF4). The addition of one equivalent of Fc(BArF4) leads to a shift of the nCO of 60 cm−1. A resulting axial S ¼ 1/2 EPR spectrum

Small Molecule Activation by Organo-iron Complexes

Bn N

FeIII Et2P

279

2+

S S FeI

OC OC

Ph2 FeII P C Ph P 2 O

Fig. 6 The two electron oxidized state of [(FcP )(CO)2FeI(Bn-NS2)(CO)FeI (enPh2P2)].

Fig. 7 Structures of [(CO)3FeI(PrS2)FeI(CO)(nPrNPh2P2)] and [(CO)2(CN)2FeII(PrS2)NiII(anePhP2)].

with large hyperfine coupling to a pair of 31P (A (31P) ¼ 79.9 MHz) revealed a mixed valence state and indicates that the unpaired electron is mainly localized on the Fe atom close to FcP . The addition of a second Fc(BArF4) causes the disappearance of the previous S ¼ 1/2 EPR signal. At very low temperature (4.5 K) a broad signal appears at g ¼ 1.65 resulting of a fast-relaxing S ¼ 1 state. The presence of two unpaired electrons was confirmed by magnetic susceptibility measurements. Meanwhile, the nCO values were only slightly affected. Altogether, the two-electron oxidized species was characterized as an FeIIIFeIIFeI spin system (Fig. 6). The position of the pendant amine inside the complex was also assessed for H2 oxidation.79,80 The amine was introduced not on the bridging dithiolate ligand but on a diphosphine ligand. The complex [(CO)3FeI(PrS2)FeI(CO)(nPrNPh2P2)] [nPrNPh2P ¼ Ph2PCH2N(nPr) CH2PPh2] (Fig. 7) was prepared by the exchange of two CO ligands from [(PrP)Fe2(CO)6] with nPrNPh2P2 ligand. The corresponding [Fe2S2] core (Fe. . .Fe distance of 2.5564(7) A˚ ) displays a butterfly structure with each iron coordinated in a pseudo square-pyramidal geometry. The nPrNPh2P2 ligand lies in the apical positions to one iron (average FedP bond lengths of 2.20 A˚ ), with a bridging CO more strongly coordinated toward this iron (FedCCO bond lengths of 1.74 vs 1.81 A˚ ). Its one-electron oxidized species using Fc+ leads to a mixed valent species, [(CO)3FeII(PrS2)FeI(CO)(nPrNPh2P2)]+, as proven by a shift of the nCO bands and a quasi-axial S ¼ ½ EPR spectrum.79 The structure reproduces the typical characteristics of Hox models with an oxidized [Fe(CO)3] unit and a rotated [Fe(CO) (nPrNPh2P2)] unit that contains an open coordination, with a semi bridging CO (FedCCO bond lengths of 1.779(4)A˚ and 2.755(6)A˚ ). The Fe bearing the nPrNPh2P2 ligand is located at 3.8 A˚ from the N atom of the nPrNPh2P2. As stated earlier, the − CN ligands have been shown to be unstable because of their propensity to protonation. One way to circumvent this problem was to decrease the basicity of the cyanide ligands by using a Lewis acid, while retaining their charge. In addition to protecting the nitrogen atom, it also modulates the activation of dihydrogen. This aspect has been emphasized in [(CO)2(CN)2FeII(PrS2)NiII(anePhP2)] [anePhP2 ¼ C2H4(PPh2)2] (Fig. 7).81 Its synthesis relies on combining NiCl2, (anePhP2) and [Fe(CN)2(PrS2)(CO)2] to pre-form the dinuclear unit. Addition of 2 equiv. of B(ArF3) allows the formation of the 1:2 complex: Lewis acid adduct.82 The resulting IR spectrum was very informative since the binding of the Lewis acid leads to a shift of the nCN and nCO of about 100 and 50 cm−1, respectively. The X-ray structure highlights the influence of B(ArF3): the FedNi distance increases from 2.809 to 3.218 A˚ and the NiSFe angle opens from 75.98 to 86.6 . Conversely, the CdN distances are only subtly modified (D ¼ + 0.03 A˚ ) as the CdO bonds that are slightly shortened (D ¼ −0.03 A˚ ).

7.06.3.2 7.06.3.2.1

Mononuclear [CpFe((RN1–2)PR1 2 )] complexes Characterization of the initial FeII complexes

This series was designed in order to develop molecular electrocatalysts for the oxidation of hydrogen and were inspired by the [Ni((R2N)2PR1 2 )2] complexes. They present the advantage to be easily modified structurally in order to modulate the hydricity properties of the iron and the amine-metal distances.50 The initial complexes are chloro species obtained by mixing [CpFeII((R2N)2 R1 P2 )FeCl2]. CO)2Cl] with the (R2N)2PR1 2 ligand under irradiation or the addition of the [CpFe]Na to a solution of [( (R2N)2 R1 The complex [CpFe( P2 )Cl] (R1 ¼ R2 ¼ Bn or Ph) represents the prototype of this series (Fig. 8). Its structure adopts a typical three-legged piano stool geometry.83 The particularity of the ligand lies on its conformation, modulated by the bulkiness of the sixth ligand, evidenced by X ray crystallography. One of the two six-membered ring of the ((R2N)2PR1 2 ) adopts a boat conformation with the lone pair of the nitrogen atom directed to the Cp ring, while the second six-membered ring has a chair conformation to avoid contact with the chloride anion. Ligands with several substitutions on the P or N atoms with mixed tbutyl,

280

Small Molecule Activation by Organo-iron Complexes

Fig. 8 Structure of [CpFeII((R2N)2PPh 2 )Cl)].

phenyl or benzyl groups were prepared to potentially exclude the coordination of an external base during the reactivity toward dihydrogen.84,85 The addition of a bulky substituent on the Cp ring did not change the ligand conformation.86 In the case of the addition of an electron withdrawing pentafluorophenyl substituent, the pKa of the putative dihydrogen iron complex is lowered.84 Typically, the Fe-Ccp (centroid) distances for these complexes averaged around 1.70 A˚ and the PFeCp angles around 125 . The FedCl distances averaged around 2.31 A˚ , while the FedP distances vary as a function of the substituent from 2.15 A˚ for R1 ¼ Ph to 2.21 A˚ for the R1 ¼ Bz analog. The removal of an amine on the ligand to form [CpFeII((R2N)PR1 2 )Cl] (R1 ¼ Ph, Et and R2 ¼ Me, Ph, Bn, tBu provides a greater degree of steric protection to the metal center as the result of larger PFeP bite angles and the bulkiness of the additional substituent. Hence, the average distance of the FedP bonds reaches 2.20 A˚ , while the PFeP angle increases to 92.2 A˚ , as compared to the  87 previous [CpFeII((R2N)2PPh 2 )Cl)] complex (PFeP angle of 81.37 ).

7.06.3.2.2

Characterization of the H2 adducts

The dihydrogen adducts with the [Fe2(PrS2)(CO)4-x(L)x(enPh2P2)] series were not detected and only products arising from the heterolytic cleavage of H2 were observed. On the other hand, the complex [CpFeII((BnN)2PPh 2 )Cl] reacts with H2 (1 atm) in + 83 fluorobenzene leading to a yellow solution attributed to the formation of dihydrogen adduct [CpFeII((BnN)2PPh 2 )(H2)] (Fig. 9). 1 It was characterized by H NMR with a feature observed −12.64 ppm, that shifted and formed a triplet 1:1:1 signal (J ¼ 30 Hz) under D2 atmosphere assigned to the coordination of HD instead of H2. The HD coupling constant value attests that the HdH distance of the dihydrogen ligand is 0.94 A˚ (based on the equation of Heinekey).88 The overall X-ray structure displays bond metrics similar to its chloro precursor, with a slight increase of the FedP bonds by 0.03 A˚ . The presence of the dihydrogen coordination was modeled from the residual electron density at 1.60 A˚ . The substitutions of the Cp by a C5F5 moiety and the replacement of Ph by tBu on the diphosphine lead to slight changes on the HdH bond as evidencing by the 1H NMR resonance at −13.63 ppm for + C5F4N 14,86 CpFeII((BnN)2PtBu In [C5F5CpFeII((BnN)2PtBu 2 )(H2)] . However, DFT calculations predicted a longer HdH bond in [ 2 )(H2)]. MeN Et the latter case, the replacement of an amine arm in the diphosphine ligand by ethyl group ( P2 ) did not affect the structural metrics87(Fig. 9).

7.06.3.2.3

Characterization of the [FeH(NH)+] intermediates

The dihydrogen oxidation is driven by the ability of the iron complexes to stabilize an intermediate containing a hydride and a protonated amine, resulting of the heterolytic cleavage of H2. In the case of the dinuclear species, such intermediate was only observed from the reaction between reduced species and proton and not in the case of H2 activation, as described in the eponymous chapter. In the [CpFeII((RN1–2)PR1 2 )Cl] series, the proton-hydride intermediate was directly isolated from the addition of H2 to C5F4N 13 [ CpFeII((tbuN)2Ptbu thanks to the Lewis acidity of the iron and a pendant 2 )]BArF4 in fluorobenzene at room temperature, C5F4 II (tbuN)2H tbu + Cp(H)Fe ( P2 )] was determined by single crystal neutron amine at the diphosphine ligand. The structure of [ diffraction, revealing an FedH bond length of 1.544(7) A˚ and a NdH one of 1.079(10) A˚ (Fig. 10). The H. . .H bond distance between the protic H and the hydritic FedH is 1.489 A˚ , significantly longer than the bond length of H2, can still be considered as a HdH bond as predicted by DFT. The 1H NMR spectrum displays only a singlet integrating for two protons (at −5.3 ppm) instead of the two expected proton resonances. This is consistent with a rapid intermolecular hybride proton exchange, highlighting the rapid and reversible heterolytic cleavage of the HdH bond (rate of approximatively 2.2 107 s−1 at 22  C). Such an equilibrium between a Z-H2 adduct and its proton-hydride tautomer was also demonstrated by the observation of HD adducts during H2/D2 scrambling experiments.

+ C5F4N Fig. 9 Structures of the [CpFeII((BnN)2PPh CpFeII((MeN)PEt 2 2)(H2)] and [ 2 )(H2)] complexes.

Small Molecule Activation by Organo-iron Complexes

281

F N F

F

+

F t

Bu

FeII

P P

tBu

N

H

tBu

H

N t

Bu

+ C5F4N + Fig. 10 Structures of [C5F4Cp(H)FeII((tbuN)2HPtbu Cp(H)FeII((Me2N-N)HPEt 2 )] and the double protonated [ 2 )] .

When one arm of the previous diphosphine ligand is replaced by ethyl groups and the amine was substituted by an additional proton relay, a dimethylamino propyl group,89 the complex resulting from H2 activation presents a different proton localization. The complex adopts a geometry, in which the diphosphine ligand and the metal display a chair conformation. Interestingly, the proton located on the amine in the outer coordination sphere forms a six members ring via a hydrogen bond with the second pendant base (NH+. . .N ¼ 1.94 A˚ ) (Fig. 10). The distance between the two hydrogen atoms extracted from crystallographic data is shorter than 2 A˚ (NH. . .H–Fe 1.91 A˚ ), revealing a possible interconversion between a H2 adduct and a double protonated species. However, at low temperature two proton resonances were observed at −17.9 and 10.4 ppm, attesting of the H2 cleavage.

7.06.3.3

Stoichiometric H2 oxidation

To activate H2, the catalyst must compensate for the thermodynamic barrier (76 kcal mol−1 in MeCN)90 required for its heterolytic cleavage. This is managed by the combination of two energetic ways: the protonation of an amine contained in the second coordination sphere of the metal(s) via the fine-tuning of its pKa, and the coordination of a hydride to the metal(s) via the modulation of its hydricity. Note that the generation of a hydride is unlikely with a weakly electrophilic FeI center, while becoming favorable with an FeII center. Accordingly, the mixed valent complex [(CO)2(MeP)FeII(BnNS2)(CO)FeI(enPh2P2)]+ reacts with H2 (1800 psi, 26 h) to produce the hydride [(CO)2(MeP)FeII(BnNS2)(H)(CO)FeII(enPh2P2)]+ species.77 Interestingly, the replacement of the N atom of the dithiolate bridge by an O atom reduces the reaction yield, while the presence of a C atom prohibits H2 activation, highlighting the requirement of a pendant amine again for H2 oxidation. It has been also shown that the reaction is speeded up by a 104 fold in the presence of an extra equivalent of Fc+ leading to the [(CO)2(MeP)FeII(BnNP2)(CO)(H) FeII(enPh2P2)]+ species as the final product. The isotopic effect measured from a mixture of D2 and H2 is 0.78 in accordance to a heterolytic cleavage of H2 with H2 binding as the proposed rate determining step.91 The incorporation of an extra redox center, FcP , within the mixed valent species [(FcP )(CO)2FeII(Bn-NS2)(CO)FeI(enPh2P2)]+ affords the formation of the proton-hydride complex without the help of an extra oxidant (Scheme 31).78 In the presence of an external base, P(o-tolyl)3, the hydride complex was formed concomitantly with the protonated external phosphine. Interestingly, [(FcP )(CO)2FeII(Bn-NS2)(CO)FeI (enPh2P2)]+ was faster than a mixture of the corresponding mixed valent diiron species in the presence of external Fc+.

Scheme 31 Stoichiometric activation of H2 by [(FcoxP )(CO)2FeII(Bn-NS2)(CO)FeI (enPh2P2)]2+.

Regarding the heterodinuclear NiFe complex [(CO)2(CN. . .BArF3))2FeII(PrS2)NiII(anePhP2)] (Fig. 7), it reacts rapidly with H2 in the presence of Me3NO, used as a decarbonylation agent. The X-ray structure of the isolated complex [(CO)2(CN. . .BArF3)2(H)FeII(PrS2) NiII(anePhP2)]− revealed the presence of a bridging hydride (the FedH bond length of 1.51 A˚ being shorter by 0.2 A˚ than the NidH bond). In the presence of a weak acid such as (HNMe3)(BArF24) (BArF24 ¼ B(C6H3-3,5-(CF3)2)4]), a H. . .H bond between the proton and the bridging hydride was detected, as demonstrated by the 2 ppm shift of the hydride NMR resonance.82

282

7.06.3.4

Small Molecule Activation by Organo-iron Complexes

Catalytic H2 oxidation

Catalytic conditions require the use of an excess of external base and oxidant (chemically though the use of Fc+ or electrochemically). The first pitfall to avoid concerns the potential binding of the external base, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (BDU), but also of the pendant amine of the ligands. This point was solved by using bulky substituents on the ligands. Up to 6 TON are accessible (6 equiv. P(o-tolyl)3 and 4 equiv. F+C, 1 atm H2) in the presence of [(FcP )(CO)2FeII(Bn-NS2)(CO)FeI(enPh2P2)]+ as the catalyst after 5 h, while [(MeP)(CO)2FeII(Bn-NS2)(CO)FeI(enPh2P2)]+ remains inert.78 The presence of the extra redox center greatly accelerates the reaction. The low efficiency raises questions about the inertness of the hydride intermediate even in the presence of the pendant amine.79 + Higher catalytic efficiency was obtained with [CpFeII((tbuN)2Ptbu 2 )] (25 TON with a 2% catalyst loading with D2 as substrate, KH/ KD of 1.3).86 Furthermore, electrocatalytic H2 oxidation was conducted within the [CpFeII(R2N2PR1 2 )Cl] series. In the presence of an excess base, the cyclic voltammogram shows the build-up of an electrocatalytic curve with a plateau shape at the neighborhood of the FeIII-H/FeII-H potential. The absence of a pendant amine significantly slows down H2 consumption,89 while the presence of an extra pendant amine greatly accelerates the reaction as shown by the remarkable increase of the TOF value from 34 to 290 s−1 in the C5F4N CpFeII((CH2)3NMe2NPEt case of [C5F4NCpFeII(MeNPEt 2 )Cl] and [ 2 )Cl], respectively. The nature of the external base also impacts the C5F4N −1 oxidation rate. As an example, with [ CpFeII((CH2)3NMe2NPEt 2 )Cl], the TOF value decreases from 290 to 8.6 s , when N89 methylpyrrolidine (pKa ¼ 18.5) is replaced by 1,4-diazabicyclo[2.2.2]octane (DABCO) (pKa ¼ 18.2). This trend is related to + the steric bulkiness of the exogeneous base. A similar behavior is observed with the [CpFeII(R2N2PR1 2 )] catalyst but to a lesser extent, since the rates are one or two orders of magnitude lower (2.5 s−1 at its best).14 The overpotential is higher in the case of + II (tbuN)2 tbu + P2 )] from 560 to 95 mV, respectively. [C5F4NCpFeII(MeNPEt 2 )] than for [CpFe ( The thermodynamic of the reaction is driven by the key deprotonation steps involving a H atom transfer from the iron to the pendant amine and then to an external base. This H atom transfer allows to reduce the free energy of the overall direct protonation, often unfavorable due to the large difference in acidity between an FeIII hydride species and a protonated secondary amine. This is well illustrated by the pseudo first order rate constant of the deprotonation with N-methylpyrrolidine, estimated at more than 25 s−1 −1 87 for [C5F4NCpFeII(MeNPph for [C5F4NCFeII(BnNPEt 2 )(H)] and less than 2 s 2 )(H)].

7.06.4

O2 activation

Dioxygen activation represents one of the major processes essential for life. In addition to respiration, O2 is the preferred oxidant in most of enzymatic oxygen transfer reactions. Its subsequent controlled four electrons reduction to water represents also an important stake for the development of new renewable/sustainable energies. Consequently, there is a need to control this process via deciphering the reaction mechanism and define a rationale for the different reaction steps. Molecular oxygen in its fundamental state has a triplet spin state 3O2, S ¼ 1. Although the reactions of O2 with organic molecules, whose electrons are paired (S ¼ 0), are thermodynamically favorable, they are very slow due to the rule of spin restriction. To lift this interdiction, dioxygen must be activated by its coordination to a metal such as iron, allowing its reduction. Oxygen 2p electrons interact with the d-orbitals of iron leading to the stabilization of a singlet state favorable to the oxidation of organic molecules. This activation often leads to the reduction of the dioxygen into more reactive species by lifting the spin interdiction, mostly oxygen centered radicals, e.g. O2 -, H2O2, OH , that play a central role in chemistry of life. These molecules are substrates and products of numerous enzymatic processes catalyze by specific iron-dependent metalloenzymes with mononuclear and polynuclear active sites, namely dioxygenases and (peroxy)oxygenases, catalases and dismutases.92–94 Their selectivity and efficiency have always fascinated the chemists and have been a great inspiration in the last 30 years. This bioinspired approach relies on the design of synthetic complexes that model key structural and/or electronic properties of the environment of the active sites to reproduce complex natural processes such as alkane oxidation or O2 reduction. In the particular field of bioinspired oxygen activation, two main families of ligands have been developed: (i) the porphyrins, mimics of the cofactor of heme proteins and (ii) polynuclear nitrogen-based molecules, models of the non-heme iron dependent enzymes.95,96 An analogy with NHC-based ligands was then explored over the past 15 years, with the expectation that their chelating properties could be beneficial in solving stability issues under harsh catalytic conditions, in stabilizing intermediates in a wide range of oxidation states, or even in preventing dissociation after photoexcitation. The soft carbon donors establish highly robust bonds with soft noble metals, which, combined with easily accessible and tunable structures, makes NHC ligands widely used in homogeneous catalysis, albeit key advances with iron are very recent. In addition, other organometallic Fe complexes have been challenged using metallocene units and/or carbonyl ligand(s). The mechanism of O2 activation, mainly based on coordination chemistry and bioinspired approaches, is decomposed in several steps in the case of a mononuclear iron center without the help of a co-substrate, as in Scheme 32.

Small Molecule Activation by Organo-iron Complexes

283

Scheme 32 O2 activation by mononuclear iron complexes.

The first step corresponds to the activation of the metal. It consists in the bonding of dioxygen with the iron atom in a low oxidation state, often ferrous state in Nature. The efficiency of this reaction depends on the high Lewis acidity of the iron, i.e. when the ligands are electron deficient, and on the accessibility of the iron center (a five-coordination is preferred). The Fe-O bond leads to an increase of the oxidation state of the metal to ferric and a superoxo state for the dioxygen (step 1, Scheme 32). Here, several pathways may take place: (i) the release of superoxide anion in the medium that could generate H2O2 and O2 via a dismutation process, or (ii) the reaction of the superoxo adduct with another ferrous complex to form the second key intermediate, a diferric peroxo complex (step 2). Other pathways may be involved in the presence of an excess of electrons, including the formation of a mononuclear peroxo species (step 2bis). Depending on the electronic and geometric properties of the diferric peroxo adduct, it can evolve either by releasing H2O2 or by breaking the O-O bond homolytically to form an FeIV]O species (step 3). Regarding the monoferric peroxo species, the heterolytic cleavage of the O-O bond can afford a putative FeV]O adduct (step 3bis). Both high valent FeIV and FeV species are capable of oxygen or electron transfer either to substrates or to metallic species leading to the formation of a m-oxo diferric complex as final product (step 4). In the present section, we will depict the characterization of key O2 iron intermediates after the description of different organo iron complexes capable of activating O2.

7.06.4.1 7.06.4.1.1

Synthesis and characterization of the organo iron complexes Synthesis of the NHC-based ligands

A set of the remarkable NHC-based ligands are displayed in Fig. 11, varying in terms of carbene involved, denticity, N/C donor sequence. The NHC ligands are synthesized from the reaction between N-substituted or alkylated imidazoles with bromo substituted imidazoles (or pyridines), usually at high temperature in THF or CH3CN. The ligands are purified by anion exchange with ammonium hexafluorophosphate. The complexation methods include different approaches. The transmetallation route using a silver complex is efficient but often leads to a different stoichiometry between iron and Ag (three for two ligands) altering the yield. Direct metallation using [Fe(N(SiMe3)2)2(THF)] is very attractive but requires the presence of an internal base that can be problematic. For diaryl carbene complexes, direct reaction of the lithium salt of the arene is preferred.

7.06.4.1.2

Structure of the mononuclear ferrous NHC-based complexes

Hexacoordinated ferrous complexes are stabilized by two bidentate or one tetradentate NHC-based ligand(s), and the coordination sphere is fulfilled by two solvent molecules. The molecular structure of [FeII(TMeCNCMeN)(MeCN)2(OTf )2] reveals an octahedral environment with a macrocycle adopting a saddle like shape with an approximate D2 symmetry (Fig. 12). An Fe-C bond distance of 1.936(4) A˚ , Fe-Npy ones between 2.022 and 2.032 A˚ and an Fe-NMeCN one of 1.933(3) A˚ are observed, while the octahedra is nearly perfect as reflected by the almost 90 angles in the coordination sphere.97 The loss of the ring by the use of the (LNMeC2N) ligand affords a nearly square planar environment around the metal with two MeCN molecules located in trans-positions at the origin of a distorted octahedral geometry around the iron center. The bond distances are in the range of the previous complex but the pyridines are further apart: Fe-C distances of 1.837(2) A˚ , Fe-Npy of 2.096 (2) A˚ and Fe-NMeCN of 1.915(2) A˚ . The use of an elongated alkylene bridge in [FeII (LNEtC2N)(CH3CN)2]2+ causes an elongation of the Fe-C (1.905(2) A˚ ) and Fe-NMeCN (1.927(2) A˚ ) bonds, while that of the Fe-Npy remains the same. The increase of the alkylene bridge by one more carbon unit leads to a rearrangement of the coordination sphere with a cis configuration for the MeCN ligands

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Small Molecule Activation by Organo-iron Complexes

Fig. 11 Various carbene ligands used for iron complexation (T for macrocyclic ligand and L for linear ligand).

Fig. 12 Structure of [FeII(TMeCNCMeN)(MeCN)2]2+.

(Scheme 33). The steric encumbrance originating from the higher number of carbon atoms induces a distortion of the planar coordination of the (LNEtC2N) ligand. The Fe-C (1.897(2) A˚ and 1.193(2) A˚ ) and Fe-NMeCN (1.969(2) A˚ and 1.952(2) A˚ ) bond distances are longer and the Fe-Npy ones shorter (2.037 A˚ and 1.993(2) A˚ ) compared to the ferrous (LNMeC2N) derivative.98

Scheme 33 Conformation of the complexes with the (LNnC2N) ligand with respect to the size of the alkene bridge (n ¼ Me, Et or Pr).

Small Molecule Activation by Organo-iron Complexes

285

2+ II Me 2+ Me 2+ Fig. 13 Structure of the [FeII(LMeNMeCMe and [FeII(BpyNMe complexes. 2 N)(MeCN)2] , [Fe (L C3N)(MeCN)2] 2 C N)(MeCN)2]

On the contrary, the presence of a methylene bridge between the NHC moiety and the pyridine provides a greater freedom for the pyridine (ligand (LMeNMeCMe 2 N), Fig. 13) as attested by the observation that the pyridine is uncoordinated in solid state and replaces by an MeCN ligand. The resulting complex reveals a k3 coordination for the polydentate ligand with two cis MeCN molecules (bond distances: Fe-C ¼ 1.883(4) A˚ and 1.953(4) A˚ , Fe-Npy ¼ 2.049(4) A˚ , Fe-NMeCN ¼ 1.991(3) and 1.928(3)A˚ ). Even if similar bond lengths have been found with other derivates, the ligand flexibility tends to generate a cis arrangement for the two MeCN ligands. The increase of the number of NHC units in (LMeC3N) leads to its coordination in a sawhorse type with the two MeCN molecules situated in cis positions and metal ligand bond distances in the range of the above values: the Fe-Npy bond (2.040(2) A˚ ) is longer than the Fe-NMeCN bonds (1.977(2) A˚ ) and the Fe-C distances vary between 1.843(3) A˚ and 1.973(2) A˚ (Fig. 13). The inclusion of four NHC moieties into the (LMeCMe 4 ) ligand in an open chain affords a mixture of three complexes distinguished by the coordination of the MeCN ligands, two resulting from an equilibrium between a cis and trans coordination of the MeCN ligands (Fig. 14). The third species, resulting of the hydrolysis of the two others, displays a structure with a k3 arrangement, the external NHC being uncoordinated and replaced by a third MeCN molecule. The Fe-L distances fall into the range of the above cited complexes. Analogs of the cyclic heme ligand including four NHC ligands were synthesized.99 The ferrous complex with the (TMe,MeC4) ligand shows a distorted octahedral coordination mode with the cyclic ligand ligated in a square planar fashion, the two remaining coordination sites being filled by two MeCN molecules in a trans arrangement (Fig. 15). The FedC bonds have an average length of 1.907 A˚ , close the Fe-NMeCN ones. Lengthening the bridge between the NHC units in (TEt,EtC4) affords a similar structure with a slight elongation of the bonds (FedC average 2.0 A˚ ; Fe-MeCN ¼ 2.01 A˚ ).99,100 Finally, the (TMe,EtCPh 4 ) ligand adopts a planar conformation with a minimal distortion in the iron complex, the octahedron being completed by two trans MeCN molecules.101 The FedC bonds (average distance of 2.01 A˚ ) are slightly longer than those observed in the other tetracarbene complexes (1.96 A˚ ). Me A lower number of carbene moieties in the (BpyNMe N) ligand leads to its positioning in an equatorial geometry with two 2 C ˚ axial ligands. The Fe–C distance of 1.892(2) A is similar to those of the above complexes, while the Fe-Npy bond lengths vary from 1.9743(17) A˚ to 2.0487(17) A˚ , the shortest one for the nitrogen atom of the bipyridine trans to the pyridine moiety (Fig. 13).102

2+ Fig. 14 structure of the three isomers of the [FeII(LMeCMe complex. 4 )(MeCN)2]

Fig. 15 Structure of ferrous complexes with the (TMe,MeC4) and (TMe,EtCPh 4 ) ligands.

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7.06.4.1.3

Spectroscopic and redox properties of the mononuclear ferrous NHC-based complexes

All the [FeIIL(MeCN)2] powders (L for all the above ligands represented in Fig. 11) are brownish to red. The UV visible spectra of the ferrous complexes with ligands displaying a mixture of NHC and pyridine moieties show one prominent feature around 400 nm, attributed to FeII ! Npy MLCT transitions (e around 3000 L mol−1 cm−1) based on TD-DFT calculations. A similar feature with higher extinction coefficient is observed with the complexes displaying a full NHC environment, slightly affected by the alkylene bridge lengths (e around 9100 L mol−1 cm−1 for [Fe(TEt,EtC4)(MeCN)2]2+), corresponding to FeII ! NHC MLCT. These diamagnetic (S ¼ 0) complexes have been characterized in solution by 1H NMR. Their zero-field Mössbauer spectra displayed a quadrupole doublet with isomer shift values in accordance with a low spin state (d ¼ 0.32 mm s−1 and DEq ¼ 3.12 mm s−1 in the case of [FeII(TMeCNCMeN)(MeCN)2](OTf )2, d ¼ 0.23 mm s−1 and D Eq ¼ 2.10 mm s−1 in [Fe(TEt,EtC4)(MeCN)2]2+). The unusual large quadrupole splitting observed with this formally low spin d6 configuration reflects the very different sigma donor character of the NHC, pyridine and MeCN ligands. Their electrochemical properties are characterized by a quasi-reversible FeII/FeIII redox system in MeCN modulated by the ratio between the NHC and pyridine moieties per ligand. It has been shown that increasing the number of NHC decreases the FeII/FeIII Me redox potential from 0.68 V vs Fc/Fc+ with only one NHC unit (BpyNMe N) to 0.42 V with two units (LNMeC2N), 0.25 V with 2 C Me three (L C3N) and 0.08 V with four. A linear correlation between the number of NHC units and the FeII/FeIII redox potential is observed with a decrease of 0.2 V per NHC moiety. While the nature of the alkylene bridges and the ligand conformation have only a slight impact on this redox potential, the presence of substituents on the NHC moiety notably affects the potential, e.g., the 2+ presence of electron donating methyl groups in [FeII(TMe,EtCMe causes its drastic decrease to −0.16 V from +0.15 V vs 4 )(MeCN)2] + Me,Me Fc/Fc in (T C4). Interestingly, this correlation is also applicable to the pure pyridine Fe-Npy4 (no NHC) analog with a value of 0.86 V.103

7.06.4.1.4

Structure of diiron complexes with NHC-based ligands

While NHC ligands have also been used to modulate the activity of models of the [FeFe] hydrogenase, their oxidation catalytic properties have been evaluated in parallel.104 These complexes display a diiron unit including a rare metal-metal bond and a coordination sphere for each Fe atom filled by two or three CO ligands and two sulfur donors arising from a bridging dithiolate. The synthesis of complexes relies on the facile reaction between [(CO)3FeI(2,3BuS2)Fe(CO)3] (2,3BuS2 ¼ 2,3-butanedithiol) and the corresponding imidazolium salt, MesC and EtCMe 2 , in the presence of a strong base. The resulting complexes are air and moisture I I 2,3Bu S2)FeI(CO)2(MesC)] show a common relatively stable. The structures of [(CO)2FeI(2,3BuS2)(EtCMe 2 )Fe (CO)2 and [(CO)3Fe ( Fe2S2 skeleton, with a butterfly conformation, in which each center adopts a distorted square pyramidal coordination geometry I 2,3Bu I Mes (Scheme 34). The two iron atoms are bridged by the (EtCMe S2)(EtCMe C) binds in a 2 ) ligand in [(CO)2Fe ( 2 )Fe (CO)2], while ( I I 2,3Bu )Fe (CO) ] than in [(CO) S2) monodentate fashion. The Fe. . .Fe bond distance is longer in [(CO)2FeI(2,3BuS2)(EtCMe 2 2 3Fe ( I Mes Fe (CO)2( C)] (2.592(6) A˚ vs 2.5870(12) A˚ ) and the Fe-CCO bonds are significantly shorter than in the parent complex I [(CO)3FeI(2,3BuS2)Fe(CO)3] (1.7490(3) A˚ vs 1.8090(2) A˚ ). The Fe-CNHC bond length in [(CO)2FeI(2,3BuS2)(EtCMe 2 )Fe (CO)2] I 2,3Bu I Mes ˚ (1.991(3) A ) is slightly shorter than that in complex [(CO)3Fe ( S2)Fe (CO)2( C)], owing to the steric effect of the substituent in (MesC). CO vibrations are also a good sensor for the increase in electron density on the iron when complex [(CO)3FeI(2,3BuS2) I FeI(CO)3] is substituted by NHC ligands: the CO vibration shifts below 1961 cm−1 in [(CO)2FeI(2,3BuS2)(EtCMe 2 )Fe (CO)2], while −1 I 2,3Bu I I 2,3Bu S2)Fe (CO)3], and [(CO)3Fe ( S2)FeI(CO)2(MesC)] the CO vibrations range between 2071 and 1990 cm in [(CO)3Fe (   displaying an expected intermediate range. The change in electron density also impacts the two-electron Fe Fe ! FeIFeI redox I I 2,3Bu S2)FeI(process by decreasing its potential by 860 mV for [(CO)2FeI(2,3BuS2)(EtCMe 2 )Fe (CO)2] and 460 mV for [(CO)3Fe ( Mes I 2,3 Et Me I 0/+ CO)2( C)], when compared to[(CO)2Fe ( BuS2)( C2 )Fe (CO)2]((E1/2 ¼ − 1.73 V vs Fc ).

Mes Scheme 34 Diiron [(CO)3FeI(2,3BuS2)FeI(CO)3] adducts with (EtCMe C) ligands. 2 ) and (

Small Molecule Activation by Organo-iron Complexes

7.06.4.1.5

287

Structure of the mononuclear ferric complexes with NHC-based ligands

The ferric counterparts of the above mononuclear NHC-based complexes (Section 7.06.4.1.3) have been characterized. They have been synthesized either chemically from the oxidation of the ferrous complexes using one-electron oxidants, such as thianthrenyl hexaflurophosphate, SbCl−6 and AgOTf, or in an electrochemical manner. The structure of these complexes is characterized by an octahedral coordination close to that found in the ferrous analogs but with Fe-L bond lengths that differ from those in the reduced form of the corresponding complex. The Fe-C bond lengths increase by 0.05 A˚ in the case of [FeII(TEt,EtC4)(MeCN)2]2+, [FeII(TEt, Me C4)(MeCN)2]2+, and [FeII(TMe,MeC4)(MeCN)]2+, reflecting the reduced Fe ! NHC P-back-bonding in the case of the oxidized 2 form, while the Fe-NMeCN bonds remain the same.105,106 Conversely, the bonds lengths around the Fe ion in [FeIII(TMeCNCMeN) (MeCN)2]3+ are quasi unaffected by the oxidation state of the iron: the Fe-C bonds are slightly shortened by 0.01 A˚ and the Fe-Npy bonds increase by 0.02 A˚ .97 The ferric complexes are also colored (purple in the case of [FeIII(TEt,EtC4)(MeCN)2]3+) and their UV visible spectra show the disappearance of the ferrous MLCT transition in the favor of a new broad transition at lower energy and lower extinction coefficient (lmax ¼ 540 nm; e ¼ 2170 L mol−1 cm−1 for [FeIII(TEt,EtC4)(MeCN)2]3+ and lmax ¼ 505 nm; e ¼ 5927 L mol−1 cm−1 for [FeIII(TMe, Me C4)(MeCN)2]3+). When the ratio between the carbene units and pyridine ligands decreases, this band is red shifted to 660 nm (e ¼ 1350 L mol−1 cm−1 in [FeIII(TMeCNCMeN)(MeCN)2]3+) attributed to an FeIII ! LMCT transition. This shift underlines the stronger ligand field of macrocyclic ligands with only NHC units ([FeII(TEt,EtC4)(MeCN)2]2+, [FeII(TEt,MeC4)(MeCN)2]2+, and [FeII(TMe,MeC4)(MeCN)2]2+). The Mössbauer parameters of these ferric complexes display similar isomer shifts (0.13 mm s−1 for [FeIII(TEt,EtC4)(MeCN)2]3+) and 0.11 mm s−1 for [FeIII(TEt,EtC4)(MeCN)2]3+) but the values of their quadrupole splitting are notably different (DEq ¼ 2.47 mm s−1 for [FeIII(TEt,EtC4)(MeCN)2]3+ and DEq ¼ 0.63 mm s−1 for [FeIII(TEt,EtC4)(MeCN)2]3+), with asymmetric broadening due to paramagnetic relaxation of these S ¼ ½ spin systems.97,105 Their X-band EPR spectra are in agreement with S ¼ ½ spin systems as shown by features centered at around g ¼ 2. Remarkably, the EPR spectra of [FeIII(TEt,EtC4)(MeCN)2]3+ and its analog [FeIII(TMe,MeC4)(MeCN)2]3+ display an almost cubic signal consistent with a mostly spherically symmetric electron distribution around the metal.105

7.06.4.2 7.06.4.2.1

Intermediates generated from O2 activation Characterization of a superoxo complex

The reaction of [FeII(TMe,MeC4)(MeCN)2]2+ with O2 in an aerated MeCN solution leads to a color change corresponding to the formation of [FeIII(TMe,MeC4)(MeCN)2]3+.107 Lowering the temperature reaction in acetone at −40  C allows to the stabilization of a new chromophore characterized by a transition at lmax ¼ 452 nm (e ¼ 2700 L mol−1 cm−1). Its identification as a ferric superoxo species, [FeIII(TMe,MeC4)(O2)]2+ relies on a detailed EPR and 1H NMR investigation combined with a study of its reactivity (Scheme 35). This species is EPR silent and its diamagnetic spin state is proposed to arise from a magnetic spin coupling between the ferric ion and the superoxo ligand. The bound superoxo was trapped by DMPO (N-5,50 dimethyl-1-pyrroline N-oxide), a nitroxide known to react with oxygen radicals. The corresponding characteristic EPR triplet signal centered at g ¼ 1.95 is observed either with the ferrous complex in the presence of O2, with [FeII(TMe,MeC4)(MeCN)2]2+ and KO2, or with the isolated ferric superoxo species, strongly suggesting the presence of a bound oxygen centered radical. The 1H NMR spectrum of [FeIII(TMe,MeC4)(O2)]2+ reveals the absence of an MeCN molecule in the coordination sphere of the Fe ion, and the splitting of the signal attributed to the methylene bridge of (TMe,MeC4) suggests a square pyramidal geometry. A DFT optimized structure of this species displays a side-on coordination of the superoxo ion. This superoxo species releases O2 and regenerates the initial ferrous complex in the presence of more nucleophilic ligands such as DMSO and PPh3, while CH3CN leads to the displacement of the superoxo anion.

Scheme 35 Formation of the superoxo [FeIII(TMe,MeC4)(O2)]2+ complex through O2 activation.

7.06.4.2.2

Characterization of a peroxo complex

To date, no stabilization of peroxo adducts with NHC-based complexes ligands has been observed during O2 activation.108 In contrast, this was achieved during the study of the O2 tolerance of a bio-inspired complex of the [NiFe] hydrogenase. The initial II Me complex, [NiII(PrNEt Cp)(MeCN)]+ is a heterodinuclear dinuclear complex, in which the NiII and FeII ions are bridged by 2 S2)Fe ( Me Cp, and an two thiolates arising from the (PrNEt 2 S2) ligand of the Ni unit. The iron is stabilized by a Z5-C5Me5 cyclopentadiene, MeCN ligand. The Fe. . .Ni distance determined by X-ray crystallography is of 3.2407(7) A˚ and the Ni-S-Fe angle of 93.35(3). The low spin ferrous ion (proven by its Mössbauer parameters: d ¼ 0.55 mm s−1 and D Eq ¼ 2.1 mm s−1) coupled to a low spin NiII ion confers a diamagnetic ground state to this species as confirmed by its 1H NMR and featureless EPR spectra.

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Small Molecule Activation by Organo-iron Complexes

IV Me + Scheme 36 Formation of the peroxo [NiII(PrNEt 2 S2)Fe ( Cp)(O2)] complex.

II Me Bubbling O2 at −80  C in an MeCN solution of [NiII(PrNEt Cp)(MeCN)]+ provokes a color change from purple to 2 S2)Fe ( brown (Scheme 36). The UV-visible spectrum of the resulting complex is characterized by charge transfer bands at 410 nm (e ¼ 3000 L mol−1 cm−1) and 520 nm (1500 L mol−1 cm−1). The X-ray structure reveals that the dinuclear unit is conserved and that the MeCN molecule has been replaced by a side-on coordinated diatomic molecule, which metrics attest to a peroxo ligand (Fe-O bond of 1.890(2) A˚ and 1.904(2) A˚ and O-O bond of 1.381(3) A˚ ). The peroxo nature was definitively assigned via 18O labeling experiments by mass spectrometry and IR spectroscopy, with which a O-O stretching vibration observed at 940 cm−1. It was further confirmed by exchanging the peroxo unit with labeled H18 2 O2. In the O2 adduct, the Fe. . .Ni distance is shortened to 3.0354 (7) A˚ with a more acute Ni-S-Fe angle (85.84 ), while the Fe-S bonds are unaffected. Based on the Mössbauer parameter (d ¼ 0.38 mm s−1 and DEq ¼ 0.33 mm s−1) and the diamagnetic state of the adduct, the oxidation state of the iron ion was II II proposed to be +4. Besides, it has been shown that this NiIIFeIVO2− 2 species can regenerate the starting Ni Fe complex in the presence of a reductant with the concomitant release of water molecules.

7.06.4.2.3

Characterization of mononuclear high valent iron oxo species

The FeIV]O is a pivotal intermediate in numerous enzymatic processes for chemical transformation especially with heme enzymes.109,110 The core of the porphyrin ring was an inspiration for the design of the (TEt,EtC4) ligand in order to compare its ability to stabilize such highly reactive species.101 [FeII(TEt,EtC4)(MeCN)2]2+ reacts with a PhIO derivative (2-(BuSO2)-C6H4IO) at −40  C to form a green solution, resulting from the slow loss of the 339 nm CT band characteristic of the ferrous complex and the concomitant appearance of a broad band at around 400 nm (e ¼ 200 L mol−1 cm−1). The X-ray structure of crystals grown at low temperature from the solution unambiguously identified this species as an Fe]O species (Scheme 37), in which the ligand adopts a planar fourfold carbene coordination with the oxo trans to an MeCN molecule in axial positions. The Fe-O bond length of 1.661(3) A˚ is compatible with an FeIV]O species. Fe-C distances vary between 1.979(5) A˚ and 2.045(5) A˚ , with short and long bonds trans to each other. Interestingly, one hydrogen from each ethylene bridge points toward the oxo ligand with a C-H. . .O distance of 2.31 and 2.35 A˚ , suggesting the presence of two weak hydrogen bonds. The Mössbauer spectrum recorded in a frozen MeCN solution displays a doublet with very low isomer shift (d ¼ −0.13 mm s−1), while the quadrupole splitting is rather large (D Eq ¼ 3.08 mm s−1). Mössbauer under applied magnetic field and magnetization experiments have evidenced a S ¼ 1 spin state for [FeIVO(TEt,EtC4) (MeCN)]2+ characterized by an almost axial zero field splitting (D ¼ + 16.4 cm−1, with giso ¼ 1.87). This D magnitude is the lowest with respect to other FeIV]O species already described.

Scheme 37 Formation of the high valent [FeIVO(TEt,EtC4)(MeCN)]2+ and [(TEt,EtC4)FeIIIOFeIII(TEt,EtC4)]4+ complexes.

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289

Fig. 16 Crystalline adduct obtained at −35  C from a mixture of [FeIVO(TEt,EtC4)(MeCN)]2+ and [(TEt,EtC4)FeIIIOFeIII(TEt,EtC4)]4+.

The electronic properties of [FeIVO(TEt,EtC4)(MeCN)]2+ is quite reminiscent to those of the [Fe(TAML)O] analog with the tetraamido macrocyclic ligand (TAML),111,112 reflecting the oblate electronic charge distribution around the iron nucleus, donated by the four equatorial ligand into the dx2–y2 orbital. This high valent intermediate has been shown to be involved during O2 activation with [FeII(TEt,EtC4)(MeCN)2]2+. Indeed, the same stable m-oxo diferric species [(TEt,EtC4)FeIIIOFeIII(TEt,EtC4)]4+ is generated from either the reaction between [FeII(TEt,EtC4) (MeCN)2]2+ and O2, or from the decomposition of the high valent [FeIVO(TEt,EtC4)(CH3CN)]2+ species. The connection between the high valent oxo and m-oxo diferric forms was also evidenced in the reverse reaction. Strikingly, the m-oxo diferric complex was capable of unusual disproportionation in solution into the [FeII(TEt,EtC4)(MeCN)2]2+ and [FeIVO(TEt,EtC4)(MeCN)]2+, as a scenario for its C-H activation of dihydroanthracene. The oxo bridge cleavage was also evidenced by the presence of a scrambled m-oxo species during the mixing of two close m-oxo bridge with different NHC ligands. The crucial step of this equilibrium was visualized by the X-ray analysis of a crystalline adduct [FeIV]O. . .FeIIIdOdFeIII. . .O]FeIV], obtained via a co-crystallization experiment of a mixture containing [FeIVO(TEt,EtC4)(MeCN)]2+ and [(TEt,EtC4)FeIIIOFeIII(TEt,EtC4)]4+ in MeCN at −35  C (Fig. 16). The central m-oxo diferric unit is flanked by two monomeric oxo FeIV]O units ion in a linear array, which are in close proximity to the dinuclear unit via their oxo ligand, mimicking the scrambling scenario.113

7.06.4.2.4

Characterization of the diferric oxo species as final oxidation products

The O2 oxidation of the ferrous [FeII(TMe,MeC4)(MeCN)2]2+ at −40  C leads to a m-oxo diferric species. Both FeIII centers are formally 15 valence electrons but lead to a diamagnetic complex due to an antiferromagnetic coupling between the two metal centers. The molecular structure shows a slightly distorted square pyramidal geometry around each Fe ion, with the tetradentate ligand arranged in a saddle fashion (average Fe-CNHC bond length of 1.949 A˚ ). The ferric ions are bridged by an oxo atom (FedO distance of 1.732 A˚ and an FeOFe angle of 162.7 ). The bridge can be broken by the use of strong acid (HPF6).107 In the case of the (TEt,EtC4) analog, the X-ray diffraction analysis of the corresponding [(TEt,EtC4)FeIIIOFeIII(TEt,EtC4)]4+ complex reveals the presence of a linear oxo-bridge diferric unit with each iron ion in a square pyramidal geometry (FedO distance of 1.752 A˚ and FeOFe angle of 178.79 ). The FedC bond lengths vary between 1.947(4) A˚ and 2.045(5) A˚ . The UV visible spectrum of [(TEt,EtC4)FeIIIOFeIII(TEt,EtC4)]4+ displays one transition at 367 nm (e ¼ 11,000 L mol−1 cm−1). It is characterized by a low isomer shift (d ¼ 0.04 mm s−1) and a high quadrupole splitting (D Eq ¼ 2.58 mm s−1) consistent with an unusual low spin (S ¼ 1/2) state for the ferric ion. The strong in-plane-s-donor character of the tetracarbene ligand stabilizes low-spin systems by raising the iron dx2–y2 orbital. The strong antiferromagnetic coupling (J ¼ −606 cm−1 with H ¼ −2JS1S2) measured between the two FeIII ions, leading to a S ¼ 0 ground spin state, is compatible with the sharp features observed in its 1H NMR spectrum.

7.06.4.3 7.06.4.3.1

Oxidation properties Under stoichiometric conditions

The reactivity of the di-m-xo FeIII complexes with (TMe,MeC4) and (TEt,EtC4) has been investigated. The [(TMe,MeC4)FeIIIOFeIII(TMe, Met C4)]4+ complex is capable of fast oxidation of PPh3 into its oxide and slow oxidation of MeCN into cyanic acid by reforming the ferrous state, while the (TEt,EtC4) analog oxidizes dihydroanthracene with a k2 value of 0.76 M−1 s−1 at −40  C.101,113 The reaction of O2 with low-coordinate iron carbene complexes has been poorly studied despite their unusual properties. The two-coordinate orange [FeII(iPr2ArAr)2] complex is formed by the reaction between FeBr2 and two equivalents of (LiiPr2ArAr) under strictly anhydrous and anaerobic conditions. The resulting mononuclear [FeII(iPr2ArAr)2] complex displays Fe-C bond lengths of 2.059(1) A˚ and a CFeC angle of 159.34(6) (Scheme 38). Bubbling dry dioxygen in a hexane solution at −100  C leads to an

Scheme 38 O2 activation by a two coordinate FeII complex.

290

Small Molecule Activation by Organo-iron Complexes

immediate color change from pale yellow to deep red, according to the appearance of two UV–visible transitions at 370 nm (e ¼ 1300 M−1 cm−1) and 417 nm (e ¼ 200 M−1 cm−1). The structure of this resulting species is a homoleptic two-coordinate FeII aryloxide resulting from the oxygen insertion into an Fe-H bond, [FeII(iPr2ArArO)2]. The metrics of the structure are an Fe-O distance of 18,742(9) A˚ , a O-Fe-O angle of 180  C and a short distance between the iron and the ipso carbon of the flanking aryl substituent of 2.765(3)A˚ , suggesting a weak secondary interaction.114 A DFT predicted mechanism for O2 activation has been proposed for these two above reactions based on a [FeII(Me2)] model.115 The first step corresponds to the formation of the superoxo [Me2FeIII(O2)] species followed by the O-O breaking to reach the [(Me)2FeVI(O)2] adduct. This reactive species rearranges through an oxo insertion to generate [Me(MeO)FeIV(O)], followed by a second oxygen insertion to generate the final product. The two coordinate FeI complex, in which a terphenyl carbene AriPr8 (AriPr8 ¼ C6H-2,6-(C6H2–2,4,6-iPr3)2–3,5-iPr2) is bound into a Z6-benzene half sandwich, reacts differently with dioxygen in hexane (Scheme 39).116 The color change from orange to red (lmax ¼ 488 nm; e ¼ 5600 M−1 cm−1) was the signature of the build-up of a dinuclear ferric unit, in which the arene groups have been replaced by two oxo bridges, while the carbene has been oxidized into a phenolate, as observed in the previous diaryl [FeII(iPr2ArArO)2] complex. This reaction involves a two-electron oxidation of the metal and the addition of two oxygen atoms per metal, one of which is inserted into a metal carbon s-bond. The X-ray structure displays a short FeIII. . .FeIII distance of 2.4817(7) A˚ , suggesting a potential Fe-Fe bond, Fe-Oph bonds of 1.864(2) A˚ and 1.874(2) A˚ , and Fe-Obridge bonds of 1.818(2) A˚ and 1.824(2) A˚ . The metal presents a relatively close interaction with the meta carbon from a flanking aryl rin (Fe. . .C distances of 2.457(3) A˚ and 2.427(3) A˚ ). The coordination environment of the metal can be considered as a very distorted tetrahedron. A remarkable feature consists in the planar diamond core structure of the di m-oxo bridge with an acute Fe-O-Fe angle of around 86 that resembles to that observed in the methane monooxygenase intermediate Q.117 The EPR silent diferric unit is characterized by magnetic properties in agreement with a diamagnetic ground spin state. An antiferromagnetic coupling displays a moderate value (J ¼ −240 cm−1), but larger than that measured for its N-based analogs.118The Mössbauer spectrum shows two doublets corresponding to two crystallographically distinct high spin ferric cations, whose parameters (d1 ¼ 0.378(1) mm s−1 and D Eq1 ¼ 2.643(2) mm s−1; d2 ¼ 0.291 (1) mm s−1 and DEq2 ¼ 2.840(2) mm s−1) corroborate the highly tetrahedral distortion of the metal environment.

Scheme 39 O2 reduction by the [Fe(AriPr8)(Z6-C6H6)] complex.

7.06.4.3.2

Under catalytic conditions

The stabilization of high valent oxo Fe complexes from polydentate NHC-based species pushed the chemists to evaluate their homogeneous catalytic properties toward C-H activation and oxo transfer processes such as epoxidation. In particular, the reactivity of the [FeII(TMe,MeC4)(MeCN)2]2+ and [FeIII(TMe,MeC4)(MeCN)2]3+ complexes was compared toward the epoxidation of cis-cycloctene in MeCN using H2O2 as oxidant. The performance of the ferric complex (TOF of 3060 min−1) is far better than its ferrous congener. This difference relies on the fact that the ferrous complex requires a pre-oxidation step, and on its propensity to decompose efficiently H2O2. By lowering temperature, the decomposition is substantially slowed down allowing the ferric catalyst to achieve 4300 TON at −30  C. The use of alkylperoxides increases the yield of the epoxidation, underlining the competitive H2O2 dismutation reaction. This system is one of the most performing among homogeneous epoxidation catalysts.106 The reactivity of FeIV]O tetracarbenes has been also scrutinized in particular for comparison with other heme analogs. The reactivity of [FeIVO(TEt,EtC4)(MeCN)]2+ was investigated in a combined experimental and theoretical study toward a series of substrates spanning a range of C-H bond dissociation energies. Kinetic traces using substrates with BDE up to 80 kcal mol−1 show pseudo first order behavior and large temperature dependent isotopic effect (KIE H/D ¼ 32 at −40  C), evidencing that it is more reactive than its based TMC analog13 (TMC ¼ 1,4,8, 11-tetramethyl 1,4,8, 11-tetraazazcyclotetradecane). A cryomass spectrometry experiment demonstrates the existence of a transient hydroxo ferric complex, [FeIIIOH(TEt,EtC4)]2+, resulting of a H atom transfer in the first step of the reaction, while no oxygen transfer has been observed. Theoretical calculations demonstrate that only the triplet

Small Molecule Activation by Organo-iron Complexes

291

state of the iron is involved all along the catalytic pathway mainly because the computed barrier of the HAT process is lower than the quintet-triplet separation, in contrast to the two spin states reactivity of the TMC complexes.119 Moreover, the performance of transferring an O atom to propene and styrene was compared between the cyclic tetracarbene-based [FeIVO(TMe,MeC4)(MeCN)]2+ complex and the cytochrome P450 compound I, modeling by [FeIVO(porph) (SH)]. As expected, the triplet state is more stable than the quintet state by 15 kcal mol−1 in the case of [FeIVO(TMe,MeC4) (MeCN)]2+. As with the above example, no spin state crossing from the triplet to the quintet state occurs during the full reaction mechanism. Thus, the FeIV]O species attacks the terminal carbon via an electrophilic addition transition state to form a radical intermediate (first energy gap of 13.1 kcal for the styrene). The subsequent reaction consists of a ring closure to form the epoxide ring with an energy gap of 9.5 kcal similar to [FeIVO(porph)(SH)]. The calculated intrinsic properties of these species show that the BDE O-H values are comparable, but the electron affinity is larger for the porphyrinic complex.120 Hydroxylation of benzene and toluene has been performed with the ferrous [FeII(LNMeC2N) MeCN)2]2+ using H2O2 as the oxidant in MeCN. In the case of benzene oxidation, the selectivity for phenol was above 90% with the presence of a small amount of p-quinone, but with a quite modest conversion (7.4%). A small intramolecular KIE determined through competition experiments between deuterated and protic benzene reflects the possible involvement of an electrophilic attack. Toluene reactivity affords to test a substrate displaying aromatic and aliphatic C-H with different bond dissociation energies. Higher conversions, up to 15%, are measured but the selectivity dropped with the formation of several cresol products, benzylalcohol and benzaldehyde. The selectivity of the oxidation is directed toward ring oxidation (77.9%), whereas aliphatic oxidation is less likely (17%). Inversely, an average KIE of 0.8 on the aromatic position was measured, while it reaches 4.9 at the aliphatic position, in agreement with a H abstraction step. It was then suggested that the hydroxylation mechanism consists of an electrophilic attack by a high valent oxo species.100 Regarding the dinuclear Fe complexes with NHC ligands, their oxidation catalytic properties have been investigated.104 Catalytic I hydroxylation of benzene withy H2O2 at 60  C in MeCN was found to be more effective with [(CO)2FeI(2,3BuS2)(EtCMe 2 )Fe (CO)2] I 2,3 I Mes I 2,3 I and [(CO)3Fe ( BuS2)Fe (CO)2( C)] than with [(CO)3Fe ( BuS2) Fe (CO)3] (26.7% and 25.9% vs 7.5% yield, respectively), consistently with the presumed first oxidation event assigned to the oxidation of the FeIFeI to FeIIFeI species. Phenol was detected as the major product, while no products were detected when dioxygen is used. The proposed mechanism was inspired from a previous study on [(CO)3FeI(2,3BuS2)FeI(CO)3], based on calculations and experimental results. An electrophilic addition process is proposed to occur between the benzene and a m-oxo dinuclear FeII species generated from H2O2 and the corresponding FeIFeI complex. The oxygen atom insertion leads to a phenoxy radical, followed by a second electron transfer from the metal to generate [(CO)3FeI(2,3BuS2)FeI(CO)3] and a bound phenyl carbocation. A hydrogen migration then provides a phenol as product.

7.06.5

Conclusions

This chapter highlights the difficulty of reproducing the extraordinary reactivity of enzymes by using synthetic bio-inspired complexes in the field of small molecule activation. Such results highlight the importance of the secondary sphere and tertiary structure of proteins that are perfectly designed for optimal reactivity. In the specific case of bio-inspired organo-iron centers, breakthroughs have been reported in recent years, notably in the field of H2 and N2 production and reduction. Key challenges remain in the control of the successive redox and proton transfer steps occurring during the catalytic cycle and the nature of the intermediates. All these works show that behind these reactions, such as the production of H2 which involves only two protons and two electrons, a very complex chemistry occurs. For each synthetic catalytic system, specific mechanism and intermediates are involved that are difficult to control. Thus, modeling an enzyme for optimal activity proves to be a much more ambitious challenge than it may seem. However, spectacular advances have been published in recent years in these fields, with highly active electrocatalysts for the production of H2 and the first promising catalysts for the production of NH3 by N2 reduction. In the case of oxidation reactions by O2 activation with organo-iron complexes, although not competitive with other systems such as heme complexes, original results have been reported in terms of mechanism and reactivity that open avenues for specific applications.

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Liu, T.; Liao, Q.; O’Hagan, M.; Hulley, E. B.; DuBois, D. L.; Bullock, R. M. Organometallics 2015, 34, 2747–2764. Liu, T.; Wang, X.; Hoffmann, C.; DuBois, D. L.; Bullock, R. M. Angew. Chem. Int. Ed. 2014, 53, 5300–5304. Liu, T.; Dubois, D. L.; Bullock, R. M. Nat. Chem. 2013, 5, 228–233. Darmon, J. M.; Raugei, S.; Liu, T.; Hulley, E. B.; Weiss, C. J.; Bullock, R. M.; Helm, M. L. ACS Catal. 2014, 4, 1246–1260. Luther, T. A.; Heinekey, D. M. Inorg. Chem. 1998, 37, 127–132. Darmon, J. M.; Kumar, N.; Hulley, E. B.; Weiss, C. J.; Raugei, S.; Bullock, R. M.; Helm, M. L. Chem. Sci. 2015, 6, 2737–2745. Rakowski DuBois, M.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62–72. Camara, J. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2011, 133, 8098–8101. Kovaleva, E. G.; Lipscomb, J. D. Nat. Chem. Biol. 2008, 4, 186–193. Poulos, T. L. Chem. Rev. 2014, 114, 3919–3962. Meunier, B.; de Visser, S. P.; Shaik, S. Chem. Rev. 2004, 104, 3947–3980. Que, L., Jr.; Tolman, W. B. Nature 2008, 455, 333. Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L. Chem. Rev. 2004, 104, 939–986. Klawitter, I.; Anneser, M. R.; Dechert, S.; Meyer, S.; Demeshko, S.; Haslinger, S.; Pöthig, A.; Kühn, F. E.; Meyer, F. Organometallics 2015, 34, 2819–2825. Raba, A.; Cokoja, M.; Ewald, S.; Riener, K.; Herdtweck, E.; Pöthig, A.; Herrmann, W. A.; Kühn, F. E. Organometallics 2012, 31, 2793–2800. Cramer, S. A.; Jenkins, D. M. J. Am. Chem. Soc. 2011, 133, 19342–19345. Cramer, S. A.; Hernández Sánchez, R.; Brakhage, D. F.; Jenkins, D. M. Chem. Commun. 2014, 50, 13967–13970. Meyer, S.; Klawitter, I.; Demeshko, S.; Bill, E.; Meyer, F. Angew. Chem. Int. Ed. 2013, 52, 901–905. Weiss, D. T.; Anneser, M. R.; Haslinger, S.; Pöthig, A.; Cokoja, M.; Basset, J.-M.; Kühn, F. E. Organometallics 2015, 34, 5155–5166. Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.; Appelman, E. H.; Que, L. J. Am. Chem. Soc. 1997, 119, 4197–4205. Wang, Y.; Zhang, T.; Li, B.; Jiang, S.; Sheng, L. RSC Adv. 2015, 5, 29022–29031. Schremmer, C.; Cordes, C.; Klawitter, I.; Bergner, M.; Schiewer, C. E.; Dechert, S.; Demeshko, S.; John, M.; Meyer, F. Chem. Eur. J. 2019, 25, 3918–3929. Kück, J. W.; Anneser, M. R.; Hofmann, B.; Pöthig, A.; Cokoja, M.; Kühn, F. E. ChemSusChem 2015, 8, 4056–4063. Anneser, M. R.; Haslinger, S.; Pothig, A.; Cokoja, M.; D’Elia, V.; Hogerl, M. P.; Basset, J. M.; Kuhn, F. E. Dalton Trans. 2016, 45, 6449–6455. Kishima, T.; Matsumoto, T.; Nakai, H.; Hayami, S.; Ohta, T.; Ogo, S. Angew. Chem. Int. Ed. 2016, 55, 724–727. Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L. Science 2003, 299, 1037–1039. Lim, M. H.; Rohde, J. U.; Stubna, A.; Bukowski, M. R.; Costas, M.; Ho, R. Y.; Munck, E.; Nam, W.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3665–3670. Tiago de Oliveira, F.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal, S.; Que, L., Jr.; Bominaar, E. L.; Munck, E.; Collins, T. J. Science 2007, 315, 835–838. Collins, T. J.; Ryabov, A. D. Chem. Rev. 2017, 117, 9140–9162. Cordes, C.; Morganti, M.; Klawitter, I.; Schremmer, C.; Dechert, S.; Meyer, F. Angew. Chem. Int. Ed. 2019, 58, 10855–10858. Ni, C.; Power, P. P. Chem. Commun. 2009, 5543–5545. Prince, B. M.; Cundari, T. R.; Tymczak, C. J. J. Phys. Chem. A 2014, 118, 11056–11061. Zhao, P.; Lei, H.; Ni, C.; Guo, J. D.; Kamali, S.; Fettinger, J. C.; Grandjean, F.; Long, G. J.; Nagase, S.; Power, P. P. Inorg. Chem. 2015, 54, 8914–8922. Shu, L. Science 1997, 275, 515–518. Zang, Y.; Dong, Y.; Que, L.; Kauffmann, K.; Muenck, E. J. Am. Chem. Soc. 1995, 117, 1169–1170. Kupper, C.; Mondal, B.; Serrano-Plana, J.; Klawitter, I.; Neese, F.; Costas, M.; Ye, S.; Meyer, F. J. Am. Chem. Soc. 2017, 139, 8939–8949. Cantú Reinhard, F. G.; de Visser, S. P. Chem. Eur. J. 2017, 23, 2935–2944.

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7.07

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Toshiro Takaoa and Akiko Inagakib, aDepartment of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, Japan; bDepartment of Chemistry, Tokyo Metropolitan University, Tokyo, Japan © 2022 Elsevier Ltd. All rights reserved.

7.07.1 7.07.2 7.07.3 7.07.3.1 7.07.3.2 7.07.4 7.07.4.1 7.07.4.1.1 7.07.4.1.2 7.07.4.1.3 7.07.4.1.4 7.07.4.2 7.07.4.3 7.07.4.4 7.07.4.5 7.07.4.5.1 7.07.4.5.2 7.07.4.5.3 7.07.4.5.4 7.07.4.5.5 7.07.4.5.6 7.07.4.5.7 7.07.4.6 7.07.4.6.1 7.07.4.6.2 7.07.4.6.3 7.07.4.6.4 7.07.4.6.5 7.07.4.6.6 7.07.4.6.7 7.07.4.7 7.07.4.7.1 7.07.4.7.2 7.07.4.7.3 7.07.4.8 7.07.5 7.07.5.1 7.07.5.2 7.07.5.3 7.07.5.4 7.07.6 7.07.6.1 7.07.6.1.1 7.07.6.1.2 7.07.6.1.3 7.07.6.1.4 7.07.6.1.5 7.07.6.2 7.07.6.2.1 7.07.6.2.2 7.07.6.2.3 7.07.6.3 7.07.7 References

294

Introduction Pogo stick type (one-legged piano stool) complexes Two-legged piano stool complexes Two-legged piano stool complexes with non-chelate ligands Two-legged piano stool complexes with a chelate ligand Three-legged piano stool complexes Three-legged piano stool complexes supported by a substituted cyclopentadienyl groups Preparations of substituted cyclopentadienyl groups from unsaturated hydrocarbons Half-sandwich complexes supported by a cyclopentadienyl group containing a chiral unit Half-sandwich complexes supported by a cyclopentadienyl group containing a tethered donor group Miscellaneous Bifunctional complexes (three-legged piano stool complexes supported by a non-innocent ligand) Dihydrogen and hydrido complexes Half-sandwich complexes with a Group 13 element Half-sandwich complexes with a Group 14 element Alkoxycarbonyl complexes NHC complexes Carbene complexes Vinylidene and allenylidene complexes Alkynyl and poly-ynyl complexes p-Allyl complexes of Ru(II) Si, Ge, Sn, and Pb complexes Half-sandwich complexes with a Group 15 element Dinitrogen complexes Azido- and organicazido complexes Diazoalkane and diazene complexes N,N-Chelate ligands P4 and As4 complexes Reactions at the phosphorus atom Half-sandwich complexes containing water soluble phosphine ligands Half-sandwich complexes with a Group 16 element Dioxygen complexes Thiocarbonato and thiocarbamato complexes Half-sandwich Ru complexes bearing dithiolene and other sulfur containing chelates Anticancer activities of half-sandwich Ru complexes with a cyclopentadienyl ligand Four-legged piano stool complexes and penta- and hexahydrido complexes of osmium Polyhydrido complexes Hydrido complexes containing Si, Ge, Sn, and Pb p-Allyl complexes of Ru(IV) and Os(IV) Miscellaneous Metallocenes and arene complexes of ruthenium and osmium Ruthenocenes and osmocenes Applications to functional materials Metalloligands Chiral metallocenes Ruthenocenophanes and osmocenophanes Heavy ruthenocenes Z6-Arene complexes Preparations of cationic arene complexes Catalytic SNAr reactions Metalloligands Anticancer activities of ruthenocenes and arene complexes Concluding remarks

Comprehensive Organometallic Chemistry IV

295 296 297 297 304 313 313 313 318 319 326 327 339 343 348 348 349 354 359 363 368 371 377 377 378 382 384 386 388 391 393 393 395 396 400 404 405 407 411 413 416 416 416 418 419 421 424 425 425 429 430 432 435 435

https://doi.org/10.1016/B978-0-12-820206-7.00143-8

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.1

295

Introduction

Cyclopentadienyl groups stabilize ruthenium and osmium generally in oxidation states from +2 to +4. Although the number of examples are limited, cyclopentadienyl complexes of Ru(0), Os(+6), and Os(+8) are also known. These ligands firmly bind to the metal center and are resistant to displacement from the coordination sphere. In addition, they are inert toward nucleophiles or electrophiles, which makes cyclopentadienyl rings reliable spectator ligands. On the other hand, co-ligands in piano stool cyclopentadienyl complexes that form “legs” are generally labile; thus, their exchange allows the tuning of reactivity of the coordination compounds. Consequently, half-sandwich complexes of ruthenium and osmium with a cyclopentadienyl ligand have been widely used in organometallic chemistry not only as catalysts for various chemical transformations but also in the field of supramolecular and medicinal chemistry. Most investigations have focused on C5H5 (Cp) and C5Me5 (Cp ) ligands because of the presence of versatile and easily accessible starting complexes, [CpRu(MeCN)3]+,1 [CpRuCl(PPh3)2],2 [Cp RuCl]4,3 and [Cp RuCl2]2.4 Half-sandwich ruthenium complexes supported by a functionalized cyclopentadienyl ligand, such as C5Ph4OH, have been shown to exhibit excellent reactivity for transfer hydrogenation of polar unsaturated bonds as well as racemization of secondary alcohols, via metal–ligand cooperation.5 In particular, cyclopentadienyls containing a tethered donor group attract considerable attention not only due to their hemilabile nature but also their combined effect of both metal-centered chirality and planar chirality of Cp ligands on diastereoselectivity in reactions. After the publication of Comprehensive Organometallic Chemistry III in 2006, Severin reported the simple method for synthesizing half-sandwich ruthenium complexes with a sterically demanding cyclopentadienyl ligand, [Cp^RuCl2]2 (1; Cp^¼ 2,4-bis-tert-butyl-1-methoxy-3-neopentylcyclopentadienyl) (Scheme 1).6 Complex 1 was directly obtained from the reaction of RuCl3
(solv.)n with tert-butylacetylene in methanol via [2 +2+ 1] cyclotrimerization (Scheme 1). Although a cyclopentadienyl ligand has not been formed from other alkynes yet, complex 1 is a useful synton for constructing sterically demanding half-sandwich ruthenium complexes.

Scheme 1

Cationic tris(acetonitrile) complex, [CpRu(MeCN)3]+, is the most popular synthetic precursor for half-sandwich complexes comprising a [CpRu]+ fragment; however, due to its air- and moisture instability, its use is severely restricted. Hintermann’s and Kündig’s groups demonstrated that air- and moisture-tolerant complex [CpRu(Z6-naphthalene)]+ (2) can be an advantageous precursor for the [CpRu]+ fragment.7 Due to the absence of coordinated nitriles, a triphenylphosphine complex, [CpRu(PPh3)3]+ (3), which was not accessible from [CpRu(MeCN)3]+, can be synthesized (Scheme 2). The Cp congener [Cp Ru(Z6-naphthalene)]+ (4) displayed similar photo-reactivity, leading to the naphthalene replacement.8

Scheme 2

A tetrameric ruthenium complex, [Cp RuCl]4, is also widely used as a starting complex of half-sandwich complex of Ru(II).9 Although molecular structures of analogous ruthenium halides supported by a Cp group, [CpRuCl2]2 and [CpRuI2]2,10,11 has been elucidated, they were obtained only accidentally in a small quantity. Recently, Takemoto et al. reported the preparation of a complex salt, [CpRu(Z6-C10H8)]+[(CpRu)2(m-Cl)3]− [2][5], which acts as a less hindered version of [Cp Ru(II)] fragment (Scheme 3).12 The complex salt [2][5] readily reacts with donor ligands to yield [CpRuCl(L)2] (6 and 7).

296

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 3

For the preparation of half-sandwich complexes of osmium, the usual precursor is osmium(VIII) oxide, OsO4, which has an appreciable vapor pressure at ambient temperatures and can cause severe physiological problems if appropriate care is not taken. Thus, conversion of OsO4 to other starting material has been desired. Bruce et al. reported one-pot procedure, use of nontoxic potassium osmate, K2[OsO2(OH)4], for the synthesis of some cyclopentadienyl-osmium complexes, and obtained [Cp OsCl(dppe)] (9) in 61% yield (Scheme 4).13 Following this, modified procedure by using HBr instead of HCl, which effectively shortened the reaction period yielding K2[OsBr6], was reported by Poli and co-workers.14

Scheme 4

It is expected that the chemistry of half-sandwich ruthenium and osmium complexes will be developed and deepened further by these novel synthons. In this review, the ruthenium and osmium half-sandwich complexes and metallocene complexes reported after publication of COMC III are summarized for each geometry of the piano-stool skeleton. Although these half-sandwich complexes have been often used for constructing multimetallic complexes and supramolecular skeletons, this review will be mainly focused on mononuclear complexes supported by a cyclopentadienyl ligand.

7.07.2

Pogo stick type (one-legged piano stool) complexes

Monomeric imido complex of Ir with a Cp ligand exhibited unique reactivities arising from multiple IrdN bond.15 Although isoelectronic (p-cymene)Ru16 and (p-cymene)Os17 complexes were synthesized by introducing a bulky imido ligands, the reaction of [Cp RuCl]4 with LiNHtBu resulted in the formation of diruthenium m-imido complex.18 Successful preparation of mononuclear Cp Ru “pogo stick” complex 10 has been achieved by employing a 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato (ImDippN) ligand (Scheme 5).19 The imidazolium ring effectively stabilizes and delocalizes a positive charge, hence arises an anionic charge at the nitrogen atom. Consequently, the ImDippN ligand, acting as a strong 2s, 4p electron donor, can stabilize the highly electron deficient metal center, accompanied with its steric demand. The remarkably short RudN distance (1.8608(15) A˚ ) and linear NdRudCp centroid angle (174.21(6) ) suggests multiple interaction between the Ru center and the imido ligand, which was validated by density functional theory (DFT) calculations. The reaction of 5 with tert-butyl isocyanate results in the formation of ureato complex 11 via [2 +2] cycloaddition, which adopts a two-legged piano stool structure with a formally 16-electron configuration. Again, strong p-electron releasing ability of the ImDippN ligand is responsible for the stabilization of the unsaturated metal center in 11.

Scheme 5

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

297

The reaction of [Cp RuCl]4 with NaN(SiMe3)2 was also examined by the same group, which afforded a bis(trimethylsilyl)amido complex, [Cp Ru{N(SiMe3)}2] (12). Complex 12 seemed to be a congener of bis(trimethylsilyl) complex of iron,20 however in contrast to the linear arrangement of the iron complex, 12 displayed a pronounced deviation from the expected linear arrangement, in which the NdRudCp centroid angle is 151.68(6) . This is due to an agostic CdHdRu interaction with one of the methyl group, which should reduce the vacancy at the Ru center. Although complex 12 is stable in n-hexane, it decomposes with the formation of diruthenium complex 13 in THF via double CdH bond cleavage. Complex 13 was also directly synthesized by the reaction of [Cp RuCl]4 with NaN(SiMe3)2 in THF (Scheme 6).18

Scheme 6

7.07.3

Two-legged piano stool complexes

Half-sandwich complexes of group 8 metals with a two-legged piano stool geometry are characteristic structure of coordinatively unsaturated 16-electron species, and, in general, exhibit a characteristic deep blue color. These complexes have been received much attention due to their relevance to catalytic reactions.21 For example, an unsaturated (
6-arene)Ru(II) complex, [(p-cymene)Ru(k2-N,N-HNCHPhCHPhNTs)] (Ts ¼ p-MeC6H4SO2), and its analogues are actually involved in the catalytic cycle, and exhibit high performance in asymmetric hydrogen transfer reactions between alcohol and ketones.22 Although two-legged piano stool complexes are highly reactive and short-lived due to their unsaturation, a number of complexes has been successfully isolated with a proper combination of the steric bulk and the electron-donating ability of the ligands.23 Since Tilley and co-workers reported the first synthesis of a half-sandwich ruthenium complex with a two-legged piano stool geometry, [Cp RuCl(PiPr3)] (14a), in 1988,24 a number of complexes has been synthesized. Sometimes, an unsaturated metal center in two-legged piano stool complexes is stabilized by an additional agostic interaction of a CdH bond in a coligand or the ligation by dinitrogen. Owing to the lability of these interaction, a 16-electroon species can be readily generated in solution. These complexes can be viewed as a masked source of unsaturated species, and are also mentioned in this section.

7.07.3.1

Two-legged piano stool complexes with non-chelate ligands

The chloride ligand in 14a is cleanly replaced by triflate, and the reaction of 14a with Me3SiOTf at 25  C in Bu2O under vacuum resulted in the formation of 15 (Scheme 7).25 Despite its unsaturated nature, no additional inter- or intramolecular contacts are observed between the Ru center and the triflate ligand. Complex 15 readily activates primary and secondary silanes to afford a dihydride-silyl complex 16 containing a triflate group at the silyl group, which is contrasted to the reactions of 14a and [Cp OsBr (PiPr3)] (17)26 with PhSiH3 yielding [Cp RuClH(SiH2Ph)] (18)24 and [Cp OsBrH(SiH2Ph)] (19), respectively. The triflate group in 16 was cleanly abstracted upon treatment with [Et3Si][B(C6F5)4] to yield a series of cationic terminal silylene complexes.

Scheme 7

298

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

The similar two-legged piano stool complex 20 supported by a substituted cyclopentadienyl group was synthesized by the reaction of [RuCl2(H)2(PiPr3)2] with 2-methyl-1-hexen-3-yne via dimerization of enyne (Scheme 8).27 Notably, an alkenylcarbyne complex 21 was obtained by the reaction of an Os congener, [OsCl2(H)2(PiPr3)2], with the same substrate, likely due to the propensity of Os to make a more metal-carbon bonds.

Scheme 8

Unsubstituted cyclopentadienyl analogue of 14a, [CpRuCl(PiPr3)] (22), has been prepared by the reaction of [CpRu (NCMe)2(PiPr3)]+ with LiCl in THF, however it afforded an equilibrated mixture with cis- and trans-dimer 23 in solution, due to the less steric demand arising from the Cp group (Scheme 9).28 The mixture of 22 and 23 was shown to react with silane smoothly to yield a monomeric s-silane complexes, 24.

Scheme 9

Nolan and co-workers synthesized [(Z5-3-phenylindenyl)RuCl(PPh3)2] (25) from a simple one-pot reaction of [RuCl2(PPh3)3] with 1,1-diphenylprop-2-yn-1-ol (Scheme 10).29 In the presence of a base, 25 has shown to catalyze various reactions, such as alcohol racemization, hydrogenation of ketones, alcohol oxidation, alkene isomerization, sulfur-heteroatom bond formation, redox isomerization of allylic alcohols.30 A cationic species generated from the chloride abstraction is proposed for an active species, and they succeeded in isolation of cationic complex 26 with a two-legged piano stool geometry upon treatment of 25 with Na[BArF4] (ArF ¼ 3,5-(CF3)2C6H3).31 Although the molecular structure of 26 was not determined by XRD, 26 was fully characterized by spectroscopic and analytical data.

Scheme 10

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

299

Strong s-donating and weak p-accepting N-heterocyclic carbenes (NHCs) are attractive alternative to the phosphine ligand, and has been shown to effectively stabilize the unsaturated 16-electron Ru species by Nolan and co-workers.32 They demonstrated that [Cp RuCl(ICy)] (27a; ICy ¼ N,N0 -dicyclohexylimidazol-2-ylidene) effectively catalyses the racemization of (S)-1-phenylehanol in the presence of an equimolar amount of NaOtBu at 25  C (Scheme 11).33 Activity decreases along with the bulkiness of the ligand,34 and detailed mechanistic study revealed that the reaction proceeds via the mechanism involving a hydrido intermediate.35 Although intermediate containing a tBuO ligand was not isolated, hydroxide complexes 28b,c were successfully synthesized by the reaction of 27b and 27c with CsOH and structurally determined.36 The ICy analogue, which was thought to be more reactive, could not be isolated under the conditions. Although these complexes were not active at ambient temperature, they perform racemization of secondary alcohols at 50  C under base-free conditions.

Scheme 11

A Cp analogue, [CpRuCl(IPr)] (31), was synthesized from [CpRu(C5H5N)3]+ (29), which was obtained from the substitution of acetonitrile ligands in [CpRu(MeCN)3]+ by pyridine (Scheme 12).37 The RudC(IPr) distance (2.068(2) A˚ ) is considerably smaller than that of Cp analogue, [Cp RuCl(IPr)] (27b; 2.105 A˚ ),38 which reflects the reduced steric demands of the Cp ligand in comparison with the Cp ring.

Scheme 12

Ruthenium sulfonamido complexes have attracted considerable attentions in the development of transfer hydrogenation, as seen in the Noyori’s catalyst, [(p-cymene)Ru(k2(N,N)-HNCHPhCHPhNTs)].22 On the other hand, those supported by a [CpRu] fragment have been still scarce. Takemoto et al. synthesized half-sandwich Ru complexes containing a tosylamido ligand by the reaction of [Cp Ru(m-OEt)]239 with TsNH2 (Scheme 13).40 In the presence of PCy3, the reaction gave 16-electron complex 32 with a two-legged piano stool geometry, while coordinatively saturated bis(phosphine) complex 33 was obtained in the presence of less bulky PMe3. In the absence of phosphines, a complex salt, [Cp Ru(Z6-C7H8)]+[Cp Ru(NHTs)2]− ([34][35]), with a two-legged piano stool geometry was obtained by employing toluene as a solvent. The RudN distance in 35 (2.106(10) A˚ ) is longer than that of 32 (2.046(2) A˚ ), due to the net negative charge in 35. When the same reaction was conducted in THF, zwitterionic dinuclear complex 36 was formed, in which one of the phenyl group was capped by a [Cp Ru]+ fragment. An anionic complex 35 react with CO and tBuNC with extrusion of 1 equiv of a tosylamido ligand to yield a neutral 18-electron complexes 37a and 37b, in which the liberated tosylamido ion was incorporated as a counter anion of 34 (Scheme 14).

300

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 13

Scheme 14

Fürstner and co-workers developed E-selective transformations of internal alkyne catalyzed by [Cp RuCl]4, in which interligand interaction between a protic substituent placed in the vicinity of the triple bond and the polarized RudCl bond plays a crucial role for the regioselective transformation of unsymmetrical alkynes.41 They isolated an Z2-alkyne adduct, [Cp RuCl {Z2-MeC^CdCRR0 dEH}] (38a; RR0 ¼ C5H10, E ¼ O, 38b; R ¼ R0 ¼ CF3, E ¼ O, 38c; RR0 ¼ C5H10, E ¼ NTs), by the reaction of [Cp RuCl]4 with tert-propargyl alcohol and sulfonamide, and elucidated the hydrogen bonding between the chlorine atom and the protic groups by XRD (EH ⋯ Cl, 2.98  3.15 A˚ ) (Scheme 15).42 These results indicate that p-coordination of alkyne is supported by the internal hydrogen bonding, and DFT calculations suggest that the alkyne ligand acts as a four-electron donor, which makes 38 coordinatively saturated. Despite their saturated configuration, 38a readily uptakes another propargyl alcohol to yield an Z4-cyclobuadiene complex 39.43

Scheme 15

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

301

Similar to [Cp RuCl2]2, complex 1, whose Ru(III) center was supported by a bulky Cp^ligand, was shown to be a good starting material for Ru(II) species. Complex 1a reacted with PCy3 in the presence of Zn to afford [Cp^RuCl(PCy3)] (40a) (Scheme 16).6b The RudP and RudCl bond lengths (2.4188(9) and 2.3936(9) A˚ ) are slightly longer than those of the Cp analogue (2.3834(4) and 2.3776(5) A˚ ).44 Notably, triphenylphosphine complex 40b can be synthesized similarly thanks to the sterically demanding Cp^ ligand, which was not accessible by the Cp analogue.

Scheme 16

Treatment of 1a with MeOH in the presence of K2CO3 afforded a dinuclear m-methoxo complex, [Cp^Ru(m-OMe)]2 (41).45 Complex 41 was shown to be an active catalyst for atom transfer radical cyclization (ATRC) reactions; for example, N-allylN-tosyldichloroacetamide was cleanly transformed into g-lactam in 86% by 5 mol% of 41 at 25  C in 20 min. On the other hand, the Cp analogue, [Cp Ru(m-OMe)]2, gave a g-lactam in the same yield after 150 min. Another Ru(II) species with bridging chlorido ligands, [Cp^RuCl]2 (42), was obtained by the reaction of 41 with Me3SiCl in the presence of LiCl. The dimeric structure of 42 was unambiguously confirmed by XRD, which contrasts with the tetrameric structure of the Cp Ru congener. The RudRu distances in 41 (3.294(1) A˚ ) and 42 (3.6023(5) A˚ ) strongly indicate the absence of a direct intermetallic interaction. Similar dimeric structure of the [RuIICl] fragment was also found in [Cp{RuCl]2 (41; Cp{ ¼ 1,2,4-tri-tertbutylcyclopentadienyl), whose Ru center is also supported by a bulky Cp{ group.46 As seen in the formation of 40b, sterically demanding Cp^group effectively stabilizes unsaturated species, and is expected to arrest reactive intermediates, which are not accessible with the complex containing a [Cp Ru] fragment. In fact, 42 reacts with alkynes to yield an Z2-alkyne adduct 43 (Scheme 16).47 While the molecular structures of 43a–d were confirmed by using single crystals obtained at −20  C, they readily regenerate 42 in solution. While the equilibrium shifts toward 43 by using a large excess amount of alkyne, neither alkyne trimerization nor formation of a ruthenacyclopentatriene complex does not take place, unlike the

302

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

reaction of [Cp RuCl(cod)] with terminal alkynes. In particular, for the 1,1-diphenyl-2-propyn-1-ol adduct 43c, hydrogen bonding between the hydroxyl group and the chlorido ligand was clearly elucidated (OH ⋯ Cl 2.27 A˚ ), as in the case of the Cp analogues 38. Unsaturated terminal imido complex 44 was obtained by the reaction of 42 with a bulky azidoadamantane.48 Nearly linear coordination of the imido ligand (RudNdC ¼ 169.0(2) ) was confirmed by XRD, and the RudN distance (1.718(3) A˚ ) was shown to be considerably smaller than that of the pogo stick type complex 10 (1.8608(15) A˚ ).19 Preparation of terminal imido complexes containing a 2,6-diisopropylphenyl or a 2,6-dimethylphenyl group on the nitrogen atom was reported by Park and co-workers, which involves the reaction of [Cp^RuCl(NH3)2] (45) with bulky aryl azides.49 When the reaction was conducted with 3,5-bis(trifluoromethyl)phenylacetylene, ruthenacyclohexatriene complex 46 was formed together with the Z2-alkyne complex (Scheme 17). In the reaction with methyl propionate, cyclotrimerization took place with the excellent regioselectivity for the 1,2,4-isomer (92:8). While the activity was comparable to [Cp RuCl(cod)], selectivity for the 1,2,4-isomer was significantly improved compared to that observed for [Cp RuCl(cod)] (1,2,4/1,3,5 ¼ 60/40), which was ascribed to the steric demand of the Cp^group. The Ru(III) complex, [Cp^RuCl2]2 (1a), was also shown to catalyze the cyclotrimerization of methyl propionate with the same reactivity and selectivity.

Scheme 17

Rosenberg and co-workers synthesized a terminal phosphido complex of ruthenium 47 by the reaction [(Ind)RuCl(PCy2H) (PPh3)] with KOtBu as a mixture with isomeric phosphaalkene-hydrido complex 48 with a ratio of 88:12 (Scheme 18).50 The sp2 nature of the terminal phosphido group in 47 was inferred by XRD analysis (short RudP bond length (2.1589(14) A˚ ) and the sum of the bond angles around the phosphido P atom (358.2 )), as well as an extreme downfield shift of the 31P signal (dP 276.3 ppm). These features strongly indicate the additional p-interaction of the phosphido ligand with the Ru atom, which is strongly supported by DFT calculations. While complex 47 is formally coordinatively saturated, it readily reacts with CO to yield a CO adduct 50 with a three-legged piano stool geometry. In contrast to the phosphido ligand in 47, that in 50 adopts a tetrahedral geometry and the RudP bond is lengthened by 0.3 A˚ compared to that in 47. These facts clearly represent the lack of the RudP p-interaction in 50, and also strongly suggest the equilibrium of 47 with the unsaturated species 49, which contains a pyramidal phosphido ligand. Complex 47 also reacts with H2 to yield a hydrido-bis(phosphine) complex 51, likely via heterolytic HdH bond cleavage. While the reaction with benzonitrile afforded a nitrile adduct, like 50, the reaction with acetonitrile resulted in the formation of a cyanomethyl complex 52 via the CdH bond cleavage.51 Furthermore, this group also reported the syntheses of ruthenaphosphacyclobutane and ruthenaphosphacyclobutene complexes, 53 and 54, by the reaction of 47 with alkene and alkyne, respectively, via the [2 +2] addition to the Ru]P bond.52,53 These results strongly represent the non-innocent nature of the basic phosphido ligand in the pyramidalized form.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

303

Scheme 18

To envisage application to the catalytic hydrophosphination, the authors tried to synthesize the Cp analogue of 47, [Cp Ru (PCy2)(PPh3)] (55), but isolation of 55 was hampered by the following rapid orthometallation, yielding [Cp Ru(HPCy2) {dPPh2(C6H4)d}] (56).54 The orthometallated product was also observed in the chemistry of 47, but it was not formed at ambient temperature. Although 55 was not isolated, similar reactivity with H2, CO, and ethylene to those observed for 47 was confirmed by using in-situ generated 55. These results imply that variable hapticity is not critical to the chemistry of the indenyl system. Interconversion between the linear and bent form of the nitrosyl ligand has also attracted considerable attention in context with the functional unsaturation.55 Kuwata et al. synthesized formally coordinatively saturated Z2-cyclohexene complex [Cp Ru(NO) (Z2-C6H10)] (57) containing a linear nitrosyl ligand by the reaction of [Cp Ru(NO)Ph2] with cyclohexene (Scheme 19).56 Complex 57 catalyzes the hydrogenation of cyclohexene under 1.0 MPa of H2 at 90  C, via the formation of diruthenium m-nitrosyl complex [Cp Ru(m-NO)]2 (58).57 Although involvement of 57 in the catalytic cycle remains controversial, bending of the nitrosyl ligand is required to uptake both hydrogen and cyclohexene substrates in the hydrogenation step.

Scheme 19

304

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

[CpRu(CO)]−2 (Rp−) (59) represents a coordinatively saturated, anionic dicarbonyl half-sandwich Ru complex with a two-legged piano stool geometry.58 Although 59 is a heavier congener of the well-known [CpFe(CO)2]− (Fp−) fragment, literature reports for 59 are much less extensive than for the iron congener. This is due to difficulty for the preparation of 59 in pure form. Meyer and co-workers reported the improved synthetic method of a potassium salt of 59 by the reaction of [CpRu(CO)2]2 with K[HB(sec-Bu)3] (K-selectride) (Scheme 20).59 This protocol provides KRp (59) accompanied by the formation of readily separable black precipitates 60. Complex 59 was obtained in 37–41% yield as yellow crystals after filtration and subsequent crystallization enabling the crystal structure of 59 to be successfully determined. Because of the poor solubility of the black material 60 in common organic solvents, the structure of this byproduct could not be determined.

Scheme 20

7.07.3.2

Two-legged piano stool complexes with a chelate ligand

Successful isolation of cationic 16-electron complexes supported by a tetramethylethylenediamine ligand (tmeda), [Cp Ru(k2(N,N)-Me2NCH2CH2NMe2)]+ and [CpRu(k2(N,N)-Me2NCH2CH2NMe2)]+,60 and neutral amidinato complex, [Cp Ru(k2-iPrNCMeNiPr)],61 demonstrate that the unsaturated metal center can be effectively stabilized by the N,N-chelation. In the later complex, p-donation from the amidinato ligand to the metal center though to be responsible for the stabilization. Similar to the reaction shown in Scheme 13, an anionic bis(amido) complex 61 supported by a bidentate sulfonamido ligand was synthesized by the reaction of [Cp Ru(m-OEt)]2 with N,N0 -ditosylethylenediamine in toluene as a complex salt with cationic [Cp Ru(Z6-C7H8)] (34) (Scheme 21).40 Although the molecular structure of 61 was not determined yet, its unsaturated nature was verified by the immediate uptake of CO to form 62. In contrast to the reaction of non-chelated bis(amido) complex 35, only simple coordination of CO occurred without liberation of the tosylamido moiety.

Scheme 21

Tamm and co-workers synthesized strongly electron-donating 1,2-bis(imidazolin-2-imino)ethane ligands, BLR (R ¼ Me, iPr), due to the ability of the imidazolium moiety to effectively stabilize a positive charge.62 The reaction of [Cp RuCl]4 with BLR proceeded smoothly to afford cationic half-sandwich ruthenium complexes 63a,b with a 16-electron configuration (Scheme 22). XRD analysis reveals that the Ru center in 63a adopts a planar two-legged piano stool geometry without any agostic interaction between the Ru center and the CdH bonds of the diimine ligand in 63a. The short RudN distances (2.060(2) and 2.073 (2) A˚ ) imply a strong electron-donating capability of the BLMe ligand, likely due to the ylidic resonance structure. In contrast to 63a, owing to the weak intramolecular interaction between the CdH bond in the iPr group and the Ru center, the coordination sphere around the Ru center in 63b slightly distorted from the ideal trigonal planar geometry. Notably, the chlorido ion does not ligate to the metal center even in solution; the strong p-electron releasing capability of the diimine ligands is responsible for the unusual stability of 63.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

305

Scheme 22

When the reaction of BLiPr was conducted with an excess amount of [Cp RuCl]4, a complex salt of [63b]+[64]− was obtained, and the molecular structure of the previously unknown anionic diruthenium chlorido complex, [(Cp Ru)2(m-Cl)3]− (64), was determined by XRD.63 A weak propensity of 63 to coordinate Cl− seems to be also responsible for the effective stabilization of the anionic diruthenium complex. Despite its stability, complex 63 readily reacts with CO and 2,6-dimethylphenyl isocyanide at ambient temperature, as other 16-electron complexes. Catalytic activity of 63a,b toward transfer hydrogenation of acetophenone using 2-propanol as a hydrogen source was demonstrated.64 Although these complexes were not active at 25  C, they are capable of completing the reaction within 1.5 6 h at 82  C with 1 mol% of catalyst loadings and 10 mol% of KOH. Although reactivity decreased considerably in the absence of KOH, a triflate salt of 63a has been shown to catalyze the reaction, indicating that the BLMe ligand is basic enough to promote activation and deprotonation of 2-propanol. This group also synthesized the Cp congeners, 66a,b, by the reaction of cationic half-open ruthenocene 65 with BLR (Scheme 23).65 Formation of a trigonal planar two-legged piano stool geometry was also confirmed by XRD. Activity of 66 for the transfer hydrogenation of acetophenone using 2-propanol was similar to the Cp analogue 63.

Scheme 23

Although [Cp Ru(acac)] (acac ¼ acetylacetonato) was initially reported as a neutral unsaturated monomeric complex,66 the dimeric structure through the intermolecular Ru-acac interaction was subsequently revealed.67 In contrast, 2-N-phenylamino-4N-phenylimino-2-pentene (Ph2nacnac) has been shown to form a coordinatively unsaturated half-sandwich ruthenium complex 67a by Hughes and co-workers (Scheme 24).68 Similar to other unsaturated half-sandwich complexes, a trigonal planar geometry around the Ru center was shown by XRD, and significant delocalization within the p-system of the nacnac ligand was also described. Although a related cationic Ar2nacnac complex, [(C6H6)Ru(Ar2nacnac)]+ (68; Ar ¼ 2,6-dimethylphenyl), displayed unique metal-ligand bifunctional character in reactions with alkenes, alkynes, and dihydrogen (Scheme 25),69 complex 67 does not react with H2 and ethylene, despite its coordinatively unsaturated nature. The low reactivity was ascribed to the decreased Lewis acidity of the metal center in a neutral complex compared to the cationic 68.

306

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 24

Scheme 25

Dyson and co-workers prepared a series of coordinatively unsaturated Cp Ru complexes, 67b–f, supported by a nacnac ligand, bearing different electronic property (Scheme 24).70 They found that 67 was rapidly converted to a Ru(III) species, which adopts a three-legged piano stool geometry with a chlorido ligand, upon exposure to chlorinated solvents such as CH2Cl2, CHCl3, and CCl4. The facile Ru(II)/Ru(III) redox interconversion of 67b–f was examined by cyclic voltammogram analysis. Furthermore, the authors performed atom transfer radical addition (ATRA) and cyclization (ATRC) catalyzed by the Ru(III) species in the presence of Mg as a reducing reagent. The diphosphine analogue of the tmeda complex, [CpRu(dppe)]+ (69, dppe ¼ 1,2-diphenylphopshinoethane), was obtained as a tetraiodogallate salt upon chloride abstraction by GaI (Scheme 26).71 A single crystal of dinuclear complex 70 bridged by two dppe ligands were also obtained from the reaction in a very low yield, in which the [GaI4]− coordinated to the Ru atom through an iodine atom to form a Lewis acid/base adduct. In contrast to the dppe complex, the reaction of [CpRuCl(PPh3)2] with GaI exclusively afforded a Lewis acid/base adduct 71.

Scheme 26

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

307

Ph tBu Ph Abstraction of chloride from a saturated half-sandwich complex, [Cp RuCl(k2-PtBu 2 N2 )] (72, P2 N2 ¼ 1,5-di(phenylaza)F 2 tBu Ph +  3,7-di(tert-butylphospha)cyclooctane), with 1.1 equiv of Na[BAr4] rapidly afforded [Cp Ru(k (P,P)-P2 N2 )] (73) (Scheme 27).72 XRD analysis of 73-1 revealed the weak interaction of one of the proximal pendant NPh groups to the Ru center in addition to Ph the k2(P2)-coordination of the PtBu 2 N2 ligand. This weak interaction reduces the unsaturation at the metal center and DFT calculations suggest that complex 73-1 was stabilized by 2.0 kcal mol−1 by the Ru ⋯ Ph interaction. Owing to this interaction, the Ph geometry around the Ru atom underwent slight pyramidalization. In addition, an isomer 73-2, in which the PtBu 2 N2 ligand adopts 3 a k (P,P,N)-coordination mode, was also obtained in the solid state, however spectroscopic data showed that only k2-isomer 73-1 was present in solution. Immediate reaction of the equilibrated mixture of 73-1 and 73-2 with N2 yielding an N2 adduct 74 clearly represents the unsaturated nature of 73-1.

Scheme 27

Due to the effective donation of its nitrogen electron pairs to the phosphorus atom, phosphinoamines display even stronger s-donor character than some trialkylphosphines. Valerga and co-workers synthesized unsaturated half-sandwich Ru complex 76a supported by a bidentate bis(aminophosphine) ligand, via the reaction of [Cp RuCl]4 with 1,2-bis(diisopropylphosphino) aminoethane (dippae), followed by the chloride abstraction by NaBArF4 (Scheme 28).73 Although only preliminary results have been obtained from the diffraction studies, the structure of 76a was fully supported by the spectroscopic data, as well as its reactivity to form N2 and Z2-dioxygen adducts immediately. They also examined the reaction with (R,R)-1,2-bis(diisopropylphosphino) aminocyclohexane (R,R-dippach). In this case, however, complex 76b, in which the R,R-dippach ligand attached to a metal center in a novel k3(P,P0 ,N)-Z2(P,N) mode, was obtained. Although complex 76b adopts an 18-electron configuration in the solid state, variable-temperature NMR studies revealed that 76b displayed a dynamic behavior via the k2(P,P) form as shown in Scheme 28; the activation parameters were estimated to be DH{ ¼ 8.4  0.2 kcal mol−1 and DS{ ¼ − 9.0  0.9 cal mol−1 K−1.

Scheme 28

In contrast to the unsaturated cyclopentadienyl Ru complexes with N,N- and P,P-chelating ligands, there are few examples of O,O- and S,S-chelating ligands. Although several mononuclear [(arene)Ru] complexes supported by O and S atoms were known,74 those of mononuclear [(cyclopentadienyl)Ru] complexes have never been reported; only diruthenium complex containing a m-Z2(O,O):Z6(C6)-coordinated catecholato or a 2,3-naphthlenediolato ligand has been reported.75 Leung and co-workers reported the first synthesis of unsaturated Cp Ru complex 78 supported by a bidentate sulfur ligand by the reaction of [Cp RuCl2]2 with dithioimidophosphinates [N(R2PS)2]− and subsequent treatment with LiBHEt3 (Scheme 29).76 Complex 78 exhibits a characteristic pseudo-trigonal planar structure of 16-electron complexes, indicating the s and p-donation by the sulfur ligand.

308

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 29

The dynamic motion shown in Scheme 28 implies the weak coordination ability of the nitrogen donor compared to the phosphorus group. Such hemilabile nature of the P,N-chelating ligands have been shown to play crucial roles in catalytic reactions, in particular generating a vacant site at a metal center. In this context, several half-sandwich complexes supported by an unsymmetrical chelating ligand have been synthesized so far. Stradiotto and co-workers synthesized 79 supported by a P,N-chelating ligand by the reaction of [Cp RuCl]4 with 2-NMe2-3-PiPr2-indene (Scheme 30).77 Complex 79 was subsequently isomerized to 80 upon treatment with NEt3. Although treatment of 80 with Li(Et2O)2.5B(C6F5)4 resulted in the formation of hydrido complex 81, in which a CdH bond of the NMe2 group was broken, a fluxional behavior in solution indicates the reversible CdH bond scission process via the formation of a 16-electron species. The role of 81 as a masked source of unsaturated species was confirmed by the formation of a nitrile adduct 82, which contains a NMe2 group.

Scheme 30

When 80 was treated with NaN(SiMe3)2 in toluene, hydrido-amino carbene complex 83 was obtained via the decoordination of the amine moiety followed by double CdH bond activation of the methyl group. Formation of 83 suggests the involvement of a zwitterionic intermediate, which contains a delocalized 10p-electron indenide unit. Formation of an indenide intermediate was confirmed in 86, which was synthesized by the dehydrohalogenation of chlorinated complex 84; In 86, the diastereotopic iPr groups undergo site-exchange within the NMR time scale, which indicates the formation of CS symmetrical intermediate.77b To this end, 18-electron indenide complex 86 was obtained upon treatment with 4-dimethylaminopyridine (DMAP), and the formation of the indenide moiety was unambiguously confirmed by XRD (Scheme 31). The molecular structure of 86 also displays the planarity of the amino moiety, which implies a resonance structure with a C]N p-bond.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

309

Scheme 31

Jiménez-Tenorio and Lledós and co-workers isolated 16-electron species 88 via the chloride abstraction from 87 using NaBArF (Scheme 32).78 DFT calculations suggest that the RuNCCP plane adopts a bent structure, unlike usual two-legged piano stool structures. The 1H NMR spectrum of 88 recorded at −35  C displays the inequivalent methylene protons due to the non-symmetrical structure, while it underwent site-exchange at ambient temperature.

Scheme 32

The Stradiotto group also tried to synthesize coordinatively unsaturated complexes supported by a P,O-chelating ligand. Similar to 79, chlorido complex 89 with a k2(P,O)-ligand was obtained from the reaction of [Cp RuCl]4 with 1-diisopropylphosphino2-indanone (Scheme 33).79 Although monomeric 16-electron complex containing a phosphinoenolate ligand was not obtained upon treatment with NaN(SiMe3)2, the reaction carried out under an N2 atmosphere afforded dinuclear complex 90 containing a m-dinitrogen ligand. Complex 90 acts as a source of unsaturated species in solution with decoordination of the N2 ligand. In the presence of H2 and CO, 89 was cleanly converted to 91 and 92, respectively.

310

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 33

Although the above mentioned unsaturated, half-sandwich Cp Ru complexes with an unsymmetrical chelating ligand were not isolated due to their instability, Stradiotto’s and Grotjahn’s groups succeeded in the isolation of 16-electron species using more electron-donating ligands and demonstrated their monomeric two-legged piano stool structures. Stradiotto and co-workers synthesized 93a,b supported by a monoanionic N-phosphinoamidinate ligand by the reaction of [Cp RuCl]4 with phosphinoamidines in the presence of a base (Scheme 34).80 These complexes readily react with CO and isocyanide to yield 18-electron complexes; notably upon coordination of CO, the RudN distances in 93a (2.0458(13) A˚ ) was lengthened by 0.15 A˚ , while other metrical parameters such as RudP bond length were not changed. This indicates that the RudN p bonding in 93 is operative for the stabilization of the unsaturated metal center. They also represented the reaction of 93a with ammonia borane yielding hydrido complex 94, which corresponds to the net transfer of hydrogen from ammonia borane to 93a.

Scheme 34

The reaction of [Cp RuCl]4 with 4-tert-butyl-2-(diisopropylphosphino)imidazole afforded a two-legged piano stool complex 95a, like [Cp RuCl(PiPr3)] (14a), due to the bulkiness of the imidazole group (Scheme 35).81 In contrast, when the tBu group was substituted by proton, three-legged piano stool complex 95b with a P,N-chelation was obtained. The bulkiness of the imidazole group was also shown to be crucial for the formation of unsaturated complex 96a, which protect the metal center effectively from solvation. DFT calculations indicate that the p-donation from the imidazole moiety effectively stabilizes the unsaturation at the metal center.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

311

Scheme 35

NHCs functionalized with additional donors are also attract considerable attentions, and several half-sandwich Ru complexes containing a functionalized NHCs as a chelating ligand have been synthesized. Morris and co-workers reported that Cp Ru complexes supported by a primary amine-tethered NHC are effective catalysts for the hydrogenation of ketones and esters because of the generation of nucleophilic hydride species upon treatment with base and dihydrogen.82 They synthesized half-sandwich Ru precatalyst 97 supported by NHC and NH2 groups connected with a enantiopure 1,2-diphenylethylene linker, S,S-MeNC3H2NCHPhCHPhNH2 (Kaibene) (Scheme 36).82d Complex 97 was shown to hydrogenate ketones with outstanding catalytic activity with TOF up to 48 s−1 and TON up to 10,000 under relatively mild conditions (0.02 mol% 97, 0.16 mol% KOtBu, 25 atm H2, 50  C), while the enantioselectivity were not so significant (0  60%). They proposed the mechanism of the catalysis based on DFT calculations and direct observation of unsaturated species 98, which was characterized by 1H NMR spectroscopy, and its immediate transformation to a pair of hydrido species 99R and 99S upon exposure to dihydrogen.

Scheme 36

Song and Liang synthesized Cp Ru complex 100 supported by 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-2-ylidene. Chloride abstraction from 100 by NaBPh4 resulted in the formation of cationic complex 101, in which an agostic interaction of a CdH bond of the mesityl group was observed in addition to the k2(N,C)-chelation of the picolyl-NHC ligand (Scheme 37);83 the short RudC bond length with one of the methyl group (2.923(3) A˚ ), as well as the 1H signal appearing in the upfield region (dH 0.92 ppm), strongly indicates the interaction between the Ru center and the CdH bond. When 100 was treated with LiN(SiMe3)2, cyclometallated complex 102 was obtained. Complex 101 reacts reversibly with H2 and N2, which strongly indicates the property of 101 as a masked source of 16-electron species.

312

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 37

Tilley and co-workers examined alkylation of 16-electron complex 27b by LiCH2SiMe3, and obtained complex 104 with a direct RudPh bond by the subsequent treatment with MesSiH3 (Scheme 38).84 Molecular structure of the product 104 demonstrated that one of the iPr groups was eliminated from the aryl moiety. As an intermediate of the unusual transformation, they observed a cyclometallated intermediate 103, which adopts a two-legged piano stool structure with a k2 (C,C)-chelation, by 1H NMR spectroscopy. This transformation was proposed to proceed via the subsequent b-hydrogen elimination and insertion, followed by b-carbon elimination leading to the formation of a propene ligand.

Scheme 38

Although thiolate ligands tend to produce multinuclear complexes owing to high affinity of the sulfur atom for transition metals, sterically demanding 2,6-dimesitylphenylthiolate (SDmp) is effective in stabilizing low-coordinate metal center against aggregation. Tatsumi and co-workers synthesized unsaturated Cp Ru complex 105 containing a SDmp ligand (Scheme 39).85 XRD demonstrated the chelation of the SDmp ligand through the sulfur atom and the ipso-carbon atom of the mesityl group; The RudCipso distance (2.278(3) A˚ ) is comparable to those reported for the p-alkene complexes. Unsaturated nature of 105 was verified by the immediate reactions with L type ligands yielding 106, in which the SDmp ligand was coordinated to the metal center in a k1(S)-fashion. The reaction of 105 with terminal alkyne was shown to yield cationic 1,2,4-trisubstituted p-arene complex 107, via regioselective cyclotrimerization of alkyne. Interestingly, the SDmp ligand exists as a discrete counter anion in the crystal structures, despite its high affinity for the Ru atom. This was ascribed to the bulkiness of the SDmp ligand and stabilization arising from the formation of [Cp Ru(arene)]+.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

313

Scheme 39

7.07.4

Three-legged piano stool complexes

Half-sandwich complexes with a three-legged piano stool geometry are the most common structure for group 8 metals because they adopt a coordinatively saturated 18-electron configuration. Most of the three-legged complexes have a formula of [(Cps)MX(L)2] or [(Cps)ML3]+, where Cps represents a substituted cyclopentenyl group, the X is a monoanionic ligand, such as Cl and H, and the L is a two-electron donor, such as PPh3 and CO. While the cyclopentadienyl ligand tightly binds to a metal center, the remaining three ligands are active for substitution reactions. In addition, unlike corresponding three-legged piano stool complex of Ru(II) supported by an Z6-arene ligand, [(Cps)Ru(II)] complexes have been shown to be redox active. Owing to the stability arising from the coordinatively saturated nature, modification and functionalization of cyclopentadienyl group, such as chiral cyclopentadienyls and cyclopentadienyls containing an additional donor group, have been intensively investigated. Facile ligand substitution ability allows introduction of various types of designed co-ligands, in particular directing to the synthesis of bifunctional catalytic systems. In addition, due to the stability and variety of half-sandwich complexes of Ru, their application to the medicinal chemistry has attracted growing interest. The rich chemistry of three-legged piano stool complexes of Ru and Os is summarized in this section.

7.07.4.1 7.07.4.1.1

Three-legged piano stool complexes supported by a substituted cyclopentadienyl groups Preparations of substituted cyclopentadienyl groups from unsaturated hydrocarbons

It has been demonstrated that replacement of one or more of the hydrogens of the cyclopentadienyl (C5H5) ring by other substituents can lead to significant changes in the reactivity and catalytic properties of these complexes due to steric and electronic effects induced by these substituents. Such substituted cyclopentadienyls can be introduced to a metal center by the reaction of metal-halide with their Li, Na, K, and Tl salts. Sometimes, substituted cyclopentadienyls formed on a metal center via [2 +2 +1] cyclotrimerization of terminal alkynes, as shown in Scheme 1. A mixed metallocene compound, [{Z5-C5H2Ph2-CHPh(OMe)}RuTp] (108; Tp ¼ HB(pz)3), was synthesized by the reaction of [TpRuCl(cod)] with phenylacetylene (Scheme 40).86 Formation of the Z5-C5H2Ph2-CHPh(OMe) group was rationalized by the [2 +2 +1] cyclotrimerization of phenylacetylene leading to an Z6-fulvene intermediate, followed by anti-Markovnikov addition of methanol. Fulvene complex 109 was obtained upon treatment of 108 with NH4PF6.

Scheme 40

314

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Cyclopentadienyl ligands can be formed by insertion reactions of fulvenes with metal alkyl or hydride. This type of reactions are often seen in early transition metal complexes. Jia and co-workers showed that treatment of hydrido complexes of Ru and Os, [RuHCl(PPh3)3] and [OsH3Cl(PPh3)3], with substituted fulvenes afford [(Z5-C5H4R)RuCl(PPh3)2] (110a–f) and [(Z5-C5H4R) OsCl(PPh3)2] (111a–d), respectively (Scheme 41A and B).87,88 When the fulvenes bearing sp3-CH protons at the carbon a to the exocyclic carbon were used, formation of a byproduct which contains a vinyl-cyclopentadienyl ligand was observed (Scheme 41C). Complex 110h was not obtained by the thermolysis of 110g, which suggests that 110g and 110h were produced independently. A similar mixture was also obtained from the reaction of [OsH3Cl(PPh3)3], while the ratio with the dehydrogenated compound was slightly different. In addition, they also demonstrated that [CpOsCl(PPh3)2], [(Ind)OsCl(PPh3)2], and [(Z5-C5Me4R)OsCl(PPh3)2] (R ¼ H, Me, Et, nPr) were obtained by the direct reaction of [OsH3Cl(PPh3)3] with cyclopentadiene, indene, and C5Me5RH after reflux in toluene, respectively.

(A)

(B)

(C)

Scheme 41

The reaction of [(C6H6)RuCl2]2 with cyclopentadienylidene phosphorene, followed by irradiation in acetonitrile, was shown to afford 112a–c supported by a cyclopentadienyl ligand containing a phosphonium substituent (Scheme 42A). The cationic PR3 tag is expected to influence the activity and regioselectivity of a [CpRu] catalyst, and also immobilization in ionic liquids. Electronwithdrawing nature of the phosphonium groups was inferred by the downfield shift of the 1H and 13C signals in the cyclopentadienyl groups compared to those of the unsubstituted CpRu complexes. Catalytic activities of the PR3-substituted complexes 112a–c toward the coupling of methyl 10-undecenoate with 1-octyne, was examined, which was shown to be catalyzed by unsubstituted-Cp complex, [CpRu(MeCN)3]+.89 However, they were less active and less regioselective compared to the unsubstituted-Cp complex. The low activity was assumed to be due to the sterically demanding ionic tag, which hinders a coordination of the two coupling components at one catalytic center. Although the corresponding half sandwich complex containing an aminophosphine group was not obtained from the reaction of [(C6H6)RuCl2]2, complex 113 was synthesized from the reaction of [RuCl2(PPh3)3] with cyclopentadienylidene phosphorane containing an amido group (Scheme 42B).90 Deprotonation of the amino group with NaN(SiMe3)2 in the presence of PPh3 leads to the formation of 114 containing a phosphazene moiety. Heating 114 in methanol with KPF6 resulted in the formation of hydrido complex 115 accompanied by protonation of the phosphazene moiety.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

315

(A)

(B)

Scheme 42

One of the fundamental reactions of metallabenzenes is the process whereby the two metal-bound carbon atoms couple to form a cyclopentadienyl ligand. However, examples that involve the well-defined transformation of isolated metallabenzenes into the corresponding half-sandwich complexes have been quite limited. Wright and co-workers reported that the reaction of ruthenabenzofuran 116 (or tethered ruthenacyclohexadiene) with concentrated HCl resulted in the formation of a mixture of tethered ruthenabenzene complex 117 and cyclopentadienyl complex 118 in a ca. 1:1 ratio (Scheme 43).91 Formation of cyclopentadienyl complexes was also observed in the thermolysis of osmabenzene complexes.92

Scheme 43

Jia and co-workers demonstrated that the reaction of an osmabenzyne complex with NaBH4 resulted in the formation of half-sandwich complex via the formation of osmabenzene intermediate.93 They also reported transformation of vinyl-carbyne complex of osmium 119 to 3-phenylindenyl complex 120 upon reduction with Zn via the osmanaphthalyne intermediate (Scheme 44A).94 Zhu and Xia and co-workers demonstrated the transformation of iso-osmabenzene complex 121 to a half-sandwich Os complex 122a–c containing a phosphonium group on the cyclopentadienyl group, which would proceed via the proton migration from the sp3 carbon to the Ca in the Z1-cyclopentadienyl intermediate (Scheme 44B).95

316

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

(A)

(B)

Scheme 44

As shown in Scheme 10, Nolan and co-workers reported the synthesis of 3-phenylindenyl complex 25 by the reaction of [RuCl2(PPh3)2] with 1,1-diphenylprop-2-yn-1-ol.29 While 3-phenylindenilidene complex 123a, which is an important synthon of a number of ruthenium based olefin metathesis catalysts, was obtained in the absence of alcohol, complex 123a was shown to be unstable in alcohol solution and transformed into 25 under reflux conditions (Scheme 45). While the ethanolysis product of the PCy3 complex 123b was [RuCl(H)(CO)(PCy3)2], which is analogous to the decomposition product of Grubbs catalyst, [RuCl2(PCy3)2(]CHPh)], tBu-Phoban complex 123c was transformed into a hydrido complex 124c supported by a 3-phenylindenyl ligand similarly to the reaction of 123a.96 Complex 25 was converted to a hydrido complex 124a in the prolonged reaction, which indicates that 124c was formed via the chlorido complex, like 25.

Scheme 45

A hydrido complex is often invoked as an active species in hydrogenation reactions. As mentioned before, complex 25 catalyzes transfer hydrogenation of benzophenone. In contrast, complex 124a displayed low activity toward hydrogenation (Scheme 46).30b This implies that 124a was not the active species, but a 16-electron hydrido species bearing one phosphine ligand was proposed to be the active species. Notably, complex 25 exhibits superior activity compared to other Ru(II) complexes, such as [CpRuCl(PPh3)2] and [(Ind)RuCl(PPh3)2], which contain unsubstituted cyclopentadienyls. The effect of the phenyl group placed at the 3-position has not been fully elucidated, but this result indicates that activity of half-sandwich complexes can be dramatically improved by the design of the cyclopentadienyl group.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

317

Scheme 46

Yamamoto et al. investigated the effect of the number and positions of methyl groups on the cyclopentadienyl groups in the cycloaddition of diynes in detail, demonstrating excellent catalytic activity of [(Z5-1,2,4-Me3Cp)RuCl(cod)] (125d) among [(Z5-C5HnMe5-n)RuCl(cod)] (125a–h) complexes (Scheme 47).97 Although a less substituted complex exhibits a greater initial rate, the catalytic efficiency in term of TON can be ascribed to the robustness of the half-sandwich ruthenium complexes.

Scheme 47

Transformation of terminal carbene complex to an Z5-cyclopentadienyl complex was also observed in the reaction of [RuCl2 (PPh3)2] with diazocyclopentadiene (Scheme 48).98 By this reaction, Z5-chlorocyclopentadiene complex 126 was exclusively formed, rather than the cyclopentadienylidene complex. In contrast, indenylidene complex 123a shown in Scheme 45 did not provide the corresponding Z5-chloroindenyl complex. DFT calculations suggest that this is due to the thermodynamic stability of the Ru-cyclopentadienyl interaction in 126; while 126 was more stable than the cyclopentadienylidene complex, Z5-chloroindenyl complex was shown to be less stable than the indenylidene complex.

318

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 48

7.07.4.1.2

Half-sandwich complexes supported by a cyclopentadienyl group containing a chiral unit

As illustrated by [CpRu(MeCN)3]+, many reactions catalyzed by half-sandwich Ru complexes require the maximum number of coordination site available on a metal center. Thus, chiral cyclopentadienyl ligands which do not reduce the number of available coordination sites on the Ru center have been desired resulting in the synthesis of various cyclopentadienyl ligands containing a chiral unit.99 Complexation to ruthenium centers have been achieved as shown in Scheme 49.100–105

Scheme 49

Thiel and coworkers synthesized complex 127 supported by a dibenzo[c,g]fluorenide (Dbf-1) ligand, leading to an intrinsic helical chirality.100 However, racemization barrier of the Dbf-1 ligand has been shown to be quite low. Following this, they synthesized complex 128 containing a dibenzo[e,h]dibenzo[3,4:6,7]cyclohept-[1,2-a]-azulenyl ligand, which have a much higher racemization barrier due to the introduction of an additional sp3 hybridized carbon atom between the six-membered aromatic rings.101 Although the activation barrier increased compared to that observed for the Dbf-1 ligand, EXSY spectroscopy proved that the barriers of inversion is still too low to allow the isolation of enantiomerically pure compound. Cramer and coworkers synthesized a series of cationic and neutral complexes supported by a chiral-Cp ligand containing a sterically adjustable biaryl backbone, 130 and 131. A hexafluoroantimonate salt of 130e (R ¼ Ph) was shown to be an efficient catalyst for Trost’s cyclization reaction of yne-enone (Scheme 50A).103 They also examined enantioselective [2+ 2] cycloaddition; the cationic 130e catalyzed the [2 + 2] cycloaddition of norbornene and alkyne in 98% yield in 1 h at 0  C, however it does not exhibit enantioselectivity. Enantioselectivity was shown to be improved to 96.5:3.5 er by the addition of nBu4NCl, which provides a neutral species possessing a RudCl bond (Scheme 50B).104 A chloride occupies a coordination site at the ruthenium center allowing only for one single alkyne molecule to be coordinated, which directs the reaction in a selective manner. Indeed, the neutral complex 131 has been shown to catalyze the reaction without any additional nBu4NCl in identical enantioselectivity. Kang’s and Cramer’s groups reported the syntheses of novel chiral-Cp ligands, which are readily accessible, and synthesized half-sandwich Ru(II) complexes 129102 and 132,105 respectively. By using 8 mol% of nBu4NI, similar to the reaction of 130e, complex 132a displayed catalytic activity toward cyclization of azabenzonorbornadiene with 4-hydroxybut-2-ynoate, and yielded dibenzoindole in 78% with 97.5:2.5 er and >20:1 regioselectivity (Scheme 50C).

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

319

(A)

(B)

(C)

Scheme 50

7.07.4.1.3

Half-sandwich complexes supported by a cyclopentadienyl group containing a tethered donor group

Cyclopentadienyl ligands with a side chain bearing an additional donor ligand have attracted considerable attention for the stabilization of half-sandwich complexes, in particular in the area of olefin polymerization.106 It is also expected that the tethered-cyclopentadienyl ligands generate a vacant site readily owing to the hemilability of the tethered donor. In addition, the tethered Cps ligands can control metal-centered chirality. Moreover, the combined effect with planar chirality of nonsymmetrically substituted cyclopentadienyl ligands can offer other types of optically active systems from those stemmed from chiral diphosphines and diamines. 7.07.4.1.3.1 Chiral systems In 2001, Onitsuka and Takahashi and co-workers demonstrated that planar-chiral ruthenium complex supported by a phosphine-tethered cyclopentadienyl ligand catalyzes asymmetric allylic amination and alkylation of allyl carbonate with high enantioselectivity.107 Subsequently, this group reported several allylic substitution reactions of allyl chloride by using 133 with high regio- and enantioselectivities as shown in Scheme 51.108–111 They obtained p-allyl complex 134 by the reaction of 133 with cinnamyl chloride as a single diastereomer, whose configuration was proven to be (SCp,RRu,Rallyl) by XRD (Scheme 52).108 Although 134 did not react with o-cresol even in the presence of K2CO3, it reacted with lithium o-methylphenoxide leading to the regioselective formation of (R)-1-methyl-2-{(1-phenylallyl)oxy}benzene in 54% yield with >99% ee, which strongly indicates the presence of p-allyl intermediate in these reactions.

Scheme 51

320

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 52

The authors examined substitution reaction of 135a (R ¼ Me) with tert-butyl isocyanide and observed that the stereochemistry of the starting material does not influence the diastereoselectivity of the product, and disclosed the crucial steric effect of the substituent at the 4-position in the cyclopentadienyl ring and the tethered-phosphine ligand for the control of the stereochemistry at the metal center: the tBu analogue 135b was shown to afford a single diastereomer 136b-I, while diastereomerically pure (SCpSRu/ RCpRRu)-135a gave a mixture of diastereomers, 136a-I and 136a-II, in 44% de (Scheme 53).112

Scheme 53

Trost et al. synthesized ruthenium complexes with a chiral sulfoxide-tethered cyclopentadienyl ligand and showed their utility toward branch-selective asymmetric allylic alkylation reactions as an asymmetric variant of [CpRu(MeCN)3]+.113 They demonstrated that complex 137 catalyzes the reaction of cinnamyl chloride with p-(trifluoromethyl)phenol in 83% conversion in 2 h at ambient temperature, and afforded allyl aryl ether in 92% ee with a branched/linear ratio of 16/1 (Scheme 54).

Scheme 54

Dyker and co-workers prepared sterically demanding pentaarylcyclopentadiene containing a donor group on one aryl group, and examined complexation with [Ru3(CO)12] (Scheme 55).114 While a benzonitrile-substituted cyclopentadiene did not afford a chelated complex, formation of chelated complexes 138a–c was observed with dimethylaniline-, quinoline-, and phenyloxazolinesubstituted cyclopentadienes. In particular, oxazolinyl-tethered complex 138c was isolated with a diastereomeric ratio of 95:5, in which a major isomer adopted an (S)-configuration, and it was shown that an epimerization at the metal center did not take place in CDCl3 solution at room temperature. In the presence of KOH, complex 138c was shown to catalyze asymmetric transfer hydrogenation of ketones using 2-propanol as a hydrogen donor, while the ee values were moderate (acetophenone; 1 mol% 138c, 82  C, 3 h, 90 mol% KOH, 95% yield, 38% ee).

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

321

Scheme 55

Substituted cyclopentadiene 139 with a chiral tether group formed a half-sandwich complex of Rh with a k3(Cp,P, N)-coordination mode;115 however, the reaction of [RuCl2(PPh3)3] with 139 resulted in the formation of a mixture of diastereomers of three-legged piano stool complex 140 with a k2(Cp,P)-coordination in a ratio of 85:15 (Scheme 56).116 The major diastereomer was isolated by recrystallization, and the geometry around the Ru center was determined to be R by XRD. However, the (R)-isomer undergoes epimerization in CDCl3 at 60  C, leading to the formation of the equilibrated mixture in 2 h.

Scheme 56

A chiral ruthenium center stemmed from the k3(Cp,N,O)-coordination of a tethered cyclopentadienyl ligand was obtained by the introduction of a-picolinate.117 Although complex 141 was obtained in a racemic form, the synthetic scheme shown in Scheme 57 can be applicable to the synthesis of related complexes with a large variety of anchored coordinating ligands.

Scheme 57

322

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Indenyl ligands substituted at the 3-position are planar-chiral, thus indenyl ligands containing an additional donor group at the 3-position can induce a chirality at the metal center. There have been two examples for the half-sandwich Ru complexes supported by a tethered-indenyl ligands. Although only one diastereomer, (pR,RRu)-142, was obtained from recrystallization, equilibration leading to the mixture of (pR,RRu) and (pR,SRu)-142 in a ratio of 2:1 occurred rapidly upon re-dissolving in CD2Cl2 and C6D6 (Scheme 58A). In contrast, complex 143, which contains a tethered-phosphine group, was obtained as a single diastereomer (Scheme 58B).118 (A)

(B)

Scheme 58

Wang and co-workers synthesized the first half-sandwich ruthenium complex 144 supported by a NHC-tethered indenyl ligand by the reaction of [Ru3(CO)12] with indenyl-functionalized imidazolium salts (Scheme 59).119 Subsequently, Royo and Peris and co-workers synthesized ruthenium complexes supported by a NHC-tethered cyclopentadienyl ligand in a similar manner.120 In contrast to the low yields of 144, complexes 145a and 145b were obtained in 57 and 55% yield, respectively. Despite the presence of the stereogenic centers at the linker and at the metal center, formation of only one diastereomer was observed. XRD studies of 145b displayed that the methyl group on the NHC moiety was located close to the CO group, avoiding the bulkier iodine ligand. By the complexation with chiral amine ((S)-(−)-a-methylbenzylamine), they succeeded in the separation of (SRu,SC,SN)- and (RRu,RC,SN)146a,b by thin-layer chromatography. Owing to the strong donating ability of the NHC group, 146a was shown to be an excellent catalyst for the redox isomerization of a wide variety of allylic alcohols in a base-free conditions.

Scheme 59

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

323

7.07.4.1.3.2 Non-chiral systems Not only by inducing chirality at a metal center from the tethered cyclopentadienyls, the reversible coordination of the pendant donor group is expected to increase the stability of highly reactive metal centers. In addition, hydrogen bonding with the pendant group, steric hindrance, and ring strain of the pendant groups are shown to perform chemistry different from those of non-tethered cyclopentadienyl complexes. Esteruelas and García-Yebra and co-workers synthesized osmium and ruthenium complexes, 147 and 148, containing a cyclopentadienyl ligand with a pendant secondary amine moiety (Scheme 60).121 Rigid coordination of the pendant amine group was confirmed in the range between 25 and 80  C by 1H NMR spectroscopy. They examined redox isomerization of allylic alcohols by these complexes to ketones, and demonstrated that the osmium complex 148 is a more efficient catalyst precursor than its ruthenium counterpart for primary allylic alcohols to aldehydes, while 147 is more efficient than the former for the redox isomerization of secondary allylic alcohols to ketones. In the reaction of 147 with primary alcohols, a mixture of redox isomerization and aldol reactions was produced. It is noteworthy that the non-tethered Os complex, [CpOs(MeCN)3]+, was much less active than 148.

Scheme 60

From the reaction of 148 with 2-methyl-2-propen-1-ol, the hydroxyallyl-hydrido intermediate 149, in which the p-allyl group coordinated to the metal center in the endo form, was obtained (Scheme 61). XRD studies revealed the presence of hydrogen bonding between the OH group and the NH hydrogen atom of the pendant group, which stabilizes the allyl structure effectively, hence would promote the isomerization, in contrast to [CpOs(MeCN)3]+. The high selectivity for the aldehydes was considered to be stemmed from the low reactivity of the osmium complex 148 toward aldol condensation, unlike 147. Although reactivities have not been examined, related cationic ruthenium complexes with a tethered-Cp, [(Cp^L)Ru(MeCN)2]+ (^L ¼ CH2CH2NMe2, CH2CH2NMe2. CH2CH2OMe), have been synthesized and structurally characterized by Leong and co-workers.122

Scheme 61

Yuan and co-workers synthesized cyclopentadienyl benzoyl ruthenium complex 150 by the reaction of [Ru3(CO)12] with pentaphenylcyclopenta-2,4-dienol (Scheme 62).123 Although complex 150 displayed high stability toward air at 285  C, a mixture of 5% 150 and one equivalent of potassium phosphate was sufficient to racemize (S)-1-phenylethanol within 6 h. Owing to the stability, the catalyst 150 can be recovered and reused. In contrast to Shvo’s catalyst, which requires heating above 80  C to generate a mononuclear active species, the mononuclear skeleton of 150 displayed high activity even at ambient temperature. In addition, a strong base was not required, unlike other Ru systems, to generate an unsaturated active species, which suggests hemilability of the benzoyl tether in 150 is crucial for the generation of a vacant site. Compatibility with Candida antarctica lipase B (CALIB) allowed the application to dynamic kinetic resolution, and they demonstrated that an acetate of 1-phenylethanol was obtained in 10 h with 97% yield and 99% ee.

324

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 62

Lin and Xie and co-workers synthesized a half-sandwich Ru complex 151 with a carbon-bridged carboranyl-cyclopentadienyl ligand, and demonstrated diverse reactivity pattern toward alkynes arising from the sterically demanding carboranyl moiety (Scheme 63). While cyclotrimerization of alkynes were commonly observed in the reaction of half-sandwich Ru complexes, unexpected coupling/ cycloaddition of a Cp ring with aromatic alkynes took place, leading to the formation of 152, owing to the congested reaction field.124 The reaction was considered to proceed via the formation of a ruthenacyclopentatriene intermediate, however the subsequent incorporation of the third alkyne molecule was suppressed by the steric demand. Instead, migration of the diene unit to the Cp ring took place before the uptake of the third alkyne molecule. Formation of the ruthenacyclopentatriene intermediate was inferred by the reaction of 151 with internal alkynes yielding Z4-cyclobutadiene complex 153 and with the reaction of 2,7-nonadiyne yielding 154.125 When 151 was treated with bis(trimethylsilyl)acetylene, bis(vinylidene) complex 155, which was not accessible from the Cp and Cp analogues, was obtained.126

Scheme 63

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

325

By the reaction of similar bis(amine) complex 156 with phenyl acetylene, a mixture of allyl(amino)carbene complex 157 and enamine complex 158 was obtained (Scheme 64).127 The molar ratio between 157 and 158 depends on the polarity of the solvent, and the 157/158 ratio of 76/24 in CH2Cl2 changed to 32/68 in toluene. However, the ratio between the two diastereomers of 157 does not change upon heating. The terminal alkynes possessing an electron-donating group were shown to exclusively afford aminocarbene complexes, while those containing an electron-withdrawing group yielded enamine complexes.

Scheme 64

They also synthesized dihydrido complex 160 supported by a carboranyl-tethered Cp (Scheme 65).128 Complex 160 was obtained by the reaction of the cod complex 159 with phosphine in an atmosphere of H2. Interestingly, the RudC(cage) s-bond in 159 was transformed into the RudB(cage) s-bond in 160. This reaction involves reductive CdH bond formation in the transiently formed dihydrido complex via the oxidative addition of H2, followed by the BdH bond cleavage. This mechanism was supported by DFT calculations, and the switching from the RudC to the RudB bond was shown to gain thermodynamic stability by 14 kcal mol−1.

Scheme 65

The reaction of highly strained [2]-ruthenocenophane 161a with 1,2-dimethylphosphinoethane (dmpe) resulted in a quantitative formation of 162 supported by a Z1-cyclopentadienyl-tehtered Cp ligand (Scheme 66).129 While the Fe congener required irradiation for this transformation, 161a was converted smoothly at 25  C without irradiation, due to higher strain arising from the large size of the central metal nucleus.

Scheme 66

The tin-bridged ansa half-sandwich complex of Ru, 164, was synthesized from 1,2-dichlorodistanne via the two-step protocol via 163 (Scheme 67).130 The ring strain at the CpdSndSndRu linkage causes the facile insertion of chalcogenes into the SndSn bond, and the 1,3-distanna-2-chalcogena-ansa compounds 165a–c were produced.131 Related disilanediyl-bridged Ru and Os complexes, 166a and 166b, were synthesized similarly. While the Fe analogue underwent ring-opening polymerization at room temperature, the osmium analogue 166b did not undergo ring opening. Although the Ru analogue 166a was stable at room temperature, it underwent ring opening at 150  C, leading to the formation of polymer in 60% yield (MW ¼ 7.44  105, Mn ¼ 7.44  103) (Scheme 68).132

326

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 67

Scheme 68

7.07.4.1.4

Miscellaneous

Application of designed cyclopentadienyl groups to molecular machines and motors have been intensively studied by Rapenne’s group.133 They synthesized penta(p-halogenophenyl)cyclopentadienyl hydrotris(indazolyl)borate ruthenium(II) precursor 167 as a key intermediate, in which the ruthenium atom is supported by a substituted cyclopentadienyls and a hydrotris(indazolyl)borate (Tp4Bo) ligand (Scheme 69). These ligands act as a rotor and a stator on metal surfaces, and the ruthenium center acts as a ball bearing allowing rotation of the upper ligand. For a direct monitoring of the molecular motion by the scanning tunnelling microscope (STM), it is important to introduce a different substituent onto the cyclopentadiene ligand. Thus, complex 168 incorporating four identical p-ferrocenyl substituents and one tolyl group on the cyclopentadienyl group was synthesized, and deposited on a Au(111) surface. Stepwise directional rotations were performed at 4.6 K on a stationary molecular motor.134 This rotation is based on electronic excitation by means of tunnelling electron energy transfer from the STM tip to the molecule. Complex 167a itself adsorb on the Au(111) surface, however the five bromophenyl groups of the C5(PhBr)5 are preferentially positioned on the surface as a stator in this case, and the propeller blades formed by the three indazole groups are located at the top. Regardless of the direction of adsorption, adsorbed 167a also displayed stepwise unidirectional rotation, according to the chirality of the C5(PhBr)5 moiety, on a surface when energized.135 Following this, they synthesized complex 169, which contains larger planar blades, porphyrin-derived teeth, to construct a molecular gear.136

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

327

Scheme 69

7.07.4.2

Bifunctional complexes (three-legged piano stool complexes supported by a non-innocent ligand)

The stability of half-sandwich Ru complexes with a three-legged piano stool structure allows the introduction of various co-ligands into the coordination sphere that enables the rational design of catalysts that take advantage of the property of co-ligands. Since the discovery of Ru/NH bifunctional catalysis by Noyori and co-workers, various new catalytic systems promoting hydrogenation of polar organic functionalities have been developed based on incorporating protic sites into the ligands.137 Ikariya’s group reported that a half-sandwich Ru complex 171a chelated by 2-(diphenylphosphino)ethylamine displays excellent reactivity toward reductive opening reaction of epoxides with H2 in the presence of KOH, giving secondary alcohols (Scheme 70),138 while the Ru complex with 2-(dimethylamino)ethylamine 170, which is an effective catalyst for the hydrogenation of ketones,139 did not work. The weaker s-donating and stronger p-accepting abilities of the tertiary phosphino group, in comparison with the amino group, renders the Ru center more Lewis acidic and the Brønsted acidity of the NH2 group increases. This ambivalent effect of the P^N chelating ligand would be responsible for the smooth reduction of epoxides.

328

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 70

Following this, they applied the P^N chelating system 171a to redox isomerization of allylic alcohol,140 hydrogenation of imides, N-acylcarbamates, and N-acylsulfonamides,141,142 oxidative lactonization of 1,4-diols,143 and enantioselective dehydrogenative desymmetrization of bicyclic imides by 171b which contains a chiral P^N chelating ligand (Scheme 71).141,144

Scheme 71

Morris and co-workers synthesized half-sandwich complex 172 supported by a NHC ligand containing a pendant NH2 group (Scheme 72).82a They also prepared a P^NH2 analogue 173. The NHC^NH2 complex 172, when treated with KOtBu in THF, is an efficient catalyst for the hydrogenation of acetophenone. In 2-propanol, the reaction completed within 30 min at 25  C with a low catalyst loading, and displayed a TOF value of 17,600 h−1. In contrast, the phosphine analogue 173, as well as 171a, is shown to be a poor precatalyst for the hydrogenation of ketones. The stronger donor-ability of the NHC moiety compared to the PPh2 group was clearly represented by the n(CO) frequencies in the corresponding carbonyl complexes, which may be responsible for the high activity of 172 toward hydrogenation of ketones.82b

Scheme 72

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

329

It is notable that 172 catalyzes the hydrogenation of methyl benzoate to benzyl alcohol and methanol in mild conditions, at 50  C and 2.5 MPa H2 with a TOF of 838 h−1. A series of lactones were also hydrogenated using complex 172 as the catalyst; phthalide was hydrogenated to 1,2-benzenedimethanol in 96% conversion in 4 h with a TOF value of 1,510 h−1.82c A concerted, asynchronous bifunctional mechanism for the homogeneous ester hydrogenation was supported by DFT calculations. They also applied the NHC^NH2 system to the enantioselective hydrogenation of prochiral ketones using complex 99 with a Kaibene ligand (Scheme 36).82d Ikariya and co-workers also demonstrated that an amino-pyridine chelated complex, [Cp RuCl(k2(N,N)-2-C5H4NCH2NH2)] (174), catalyzes hydrogenation of lactones at 100  C under 5 MPa H2 in 2-propanol in the presence of excess KOtBu.145 Following this, they synthesized complex 175, containing an NHC group instead of the pyridine moiety in 174. Owing to the enhanced s-donation by the NHC moiety, 175 is shown to be more effective catalyst for hydrogenation of ethyl benzoate than 174. Furthermore, it was demonstrated that 175 effectively catalyzes hydrogenation of carboxamides (Scheme 73).146

Scheme 73

Kitamura and co-workers developed an efficient catalytic system 176 that consist of [CpRu(II)] and pyridinecarboxylic acid derivatives for the dehydrative allylation of alcohols using half-sandwich complex (Scheme 74).147 Formation of p-allyl intermediate 177, which was isolated and structurally characterized by XRD, would be facilitated by the intramolecular hydrogen bonding interaction between the COOH moiety and the OH group, as well as the electron rich Ru center ligated by the s-donating pyridine moiety. Nucleophilic attack of alcohol onto the p-allyl carbon is also assisted by the carboxylate ligand. Owing to these bifunctional effects, the allylation proceeds smoothly at 70  C with 0.05 mol% of [CpRu(MeCN)3]+ and 2-quinolinecarboxylic acid to give allyl ether in 90% yield after 6 h.

Scheme 74

Kuwata, Ikariya, and co-workers also displayed that the CdO bond cleavage of the allylic alcohol is facilitated by the protic NH group in the NHC group. They synthesized a half-sandwich complex 178 chelated by a N-(2-pyridyl)benzimidazole containing a protic NH moiety, and demonstrated dehydrative condensation of N-(2-pyridyl)benzimidazole and allyl alcohol, leading to the formation of a mixture of trans- and cis-2-(1-propenyl)-N-(2-pyridyl)benzimidazole (Scheme 75).148 At 25  C, the reaction

330

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

exclusively afforded C2 ^N chelated 2-allylbenzimidazole complex 179 via CdC bond formation accompanied by the decoordination of the pyridine moiety. It is considered that the facile dehydration is also promoted by the hydrogen bond between the NH group and the allylic OH group. They also reported an interplay of a protic pyrazole moiety in 180 with propargyl alcohol, and catalytic isomerization of the propargyl alcohol to an enone in methanol (Scheme 76).149

Scheme 75

Scheme 76

Dehydrative CdN bond formation was observed in the reaction of unsaturated 76a with 1,1-diphenyl-2-propyn-1-ol, which afforded a heterocyclic vinylcarbene complex 181 instead of an allenylidene or an aminocarbene derivative (Scheme 77).150 The formation of the k3(P,P,C)-coordinated vinylcarbene ligand was rationalized as follows: A hydroxyalkynyl-hydride is formed by oxidative addition, which lies in an equilibrium with a p-alkynol complex. Subsequent intramolecular nucleophilic attack of a nitrogen atom on the p-alkynol ligand yield a vinyl intermediate. The vinylcarbene complex 181 would be formed via the following dehydration between the NH group in the phosphanylamino moiety and the OH moiety.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

331

Scheme 77

In relation to the protic NHC ligand containing a pyridine moiety in 178, phosphine-substituted benzimidazoles were synthesized and their complexation with [Cp RuCl]4 were investigated by Hahn et al. (Scheme 78A).151 Complex 182 could be formed by initial binding of the phosphine followed by coordination of the imidazole moiety with subsequent oxidative addition of the C2dH bond followed by reductive elimination/tautomerization. As compared to the six-membered chelate ring in 182b, the larger downfield shift of the 31P signal of 182a indicated the superior donor properties of the phosphorous atom in the five-membered chelate ring. Acidity of the NH moiety was suggested by the formation with DMPU adduct 183 (DMPU ¼ 1, 3-dimethyltetrahydropyrimidin-2(1H)-one) via hydrogen bond. Grotjahn and co-workers synthesized half-sandwich hydrido complex 185 chelated by a diphenylphosphine-substituted imidazole-2-ylidene ligand containing a protic NH moiety, and examined transfer hydrogenation of acetophenone using 2-propanol (Scheme 78B).152 Notably, complex 185 (1 mol%) catalyzes the reduction of acetophenone without addition of base, and provides 1-phenylethanol in 97% yield at 70  C after 3 h. (A)

(B)

Scheme 78

332

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

In 2004, Grotjahn et al. reported anti-Markovnikov hydration of terminal alkynes to aldehydes by half-sandwich Ru complex 186 containing pyridylphosphine ligands (Scheme 79).153 In the presence of H2O, complex 187 was ionized in CH2Cl2 to afford an equilibrium mixture of aqua complex 188 and P^N chelated complex 189 (Scheme 80).154 This mixture reacts with acetylene to yield p-acetylene complex 190 at −40  C, then converted into vinylidene complex 191 at 0  C. Conversion to the vinylidene complex was decelerated in the complex containing PMe2Py, which clearly indicates the profound effects and assistance of the pendant bases on these transformations. The presence of CdHdN hydrogen bond in 190 was inferred by NMR studies. When 187b was allowed to react with acetylene and H2O at 0  C for 4 h, formation of acyl complex 192b was observed (91% yield). At higher temperatures, CH3CHO was cleanly released. Although structure of 192b was not determined by XRD, the presence of hydrogen bond between the carbonyl and the NH group was proposed based on the detailed NMR studies. These results demonstrated the crucial role of bifunctional ligands as a proton shuttle.

Scheme 79

Scheme 80

Jalón et al. also showed the crucial role of pyridylphosphine ligands for intramolecular proton transfer reactions (Scheme 81).155 They synthesized cationic P^N chelated complex 193 and obtained hydrido complex 194 upon treatment with KBH4. In contrast to the protonation of [Cp RuH(PPh3)2] yielding trans-dihydride, complex 194 reacted with 2 equiv of HBF4 to afford pyridinium complex 196, adopting a dihydrogen bond between the hydride and the NH group. Upon evacuation, pyridinium complex 197 was obtained via dihydrogen elimination, which suggest ability to heterolytic activation of H2. In fact, both 195 and 196 catalyze deuteration of H2 in CD3OD.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

333

Scheme 81

anti-Markovnikov hydration of alkyne was also achieved by using self-assembled bidentate ligands through hydrogen bonding.156 Breit and co-workers examined various combination of phosphine ligands substituted by aminopyridine and isoquinoline, and found that only the pair of 6-diphenylphosphino-N-pivaloyl-2-aminopyridine and 3-diphenylphosphinoisoquinolone afford 198, which is an active catalyst. XRD studies clearly represents the presence of multiple hydrogen bonding (Scheme 82). DFT calculations showed the crucial role of the pyridylphosphine moiety for the facile formation of a vinylidene intermediate and the incorporation of a water molecule promoting nucleophilic addition at the a-carbon atom of the vinylidene intermediate.157

Scheme 82

Hintermann and co-workers revealed that a single pyridylphosphine is sufficient to achieve maximal reaction rates in alkyne hydration catalysis.158 They showed that complex 199 is more active than 186; 4-phenyl-1-butyne was fully converted to 4-phenylbutanal by 2 mol% of 199 at 50  C in 3 h. Grotjahn et al. examined the reactivity of half-sandwich complexes containing a 4-tert-butyl-2-(diisopropylphosphino)imidazole ligand toward isomerization of alkenes (Scheme 83).159 Due to the strong coordination of acetonitrile to the metal center, pure chelate complex 201 could not be obtained. Thus, a mixture of 200 and 201 with a ca. 1:9 ratio was used for the reaction. They demonstrated that 201 catalyzes the movement of an alkene double bond over 30 positions along an alkyl chain. The mechanism was proposed as shown in Scheme 84, which involves chelate-opening and deprotonation at an allylic position by the basic nitrogen of the imidazole moiety. This is different from the reaction mechanism catalyzed by metal hydrido complexes via the combination of insertion and b-hydrogen elimination. The mechanism shown in Scheme 84 is strongly supported by labeling experiments carried out in the presence of D2O.160 In contrast, the isomerization completely stopped at one-step when the Cp analogue of 201 was employed, which afforded (E)-2-alkenes with a high selectivity, exceeding 95%.161 They also demonstrated that reaction was remarkably accelerated by using nitrile-free catalyst 95a as shown in Scheme 35.81 The high selectivity for (E)2-alkenes is ascribed to the steric effect arising from the Cp group, which promotes the liberation of internal alkenes.

334

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 83

Scheme 84

Vidovic and co-workers prepared PtBu2 analogue of 201 with a series of C5MenH5-n (n ¼ 0  4), while Cp complex was not obtained due to the steric repulsion.162 They examined isomerization of 1-hexene, however the ratio between 2- and 3-alkenes significantly decreases compared to the results obtained by 95a, which means crucial importance of the steric environment around the Ru center for the selective formation of 2-alkenes. Mayer and co-workers synthesized 202 supported by a 1,5-diaza-3,7-diphosphacyclooctane ligand with tert-butyl substituents on the phosphines and benzyl groups on the amines (Scheme 85).163 The P2N2 ligand acts as a bis(phosphine) ligand, which ligates to the metal center in a k2(P,P)-fashion and the nitrogen atoms do not interact with the metal center. However, it is expected that the pendant amine moieties in a close proximity are able to play an important role in the second-coordination sphere, namely proton relays with incoming substrates. Stable Z2-O2 complex 203 was obtained by the chloride abstraction with TlPF6 in air-saturated acetone. Along with protonation, the conformation of the proximal amine is inverted relative to the structures of 202 and 203, bringing the nitrogen atom into the close proximity with the O2 ligand. The short N ⋯ O distances of 2.7–2.8 A˚ in the protonated complex 204 indicate of hydrogen bonding between the O2 ligand and the NH group, which was also confirmed by the red shift of n(OdO) from 935 in 203 to 905 cm−1 in 204. Although reduction of oxygen was not achieved due to instability of the species formed upon reduction of 204, it was confirmed that the reduction of the peroxo complex was promoted by the proton located near the O2 ligand.164

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

335

Scheme 85

Blacquiere and co-workers applied the P2N2 system to cyclization of 2-ethynylbenzyl alcohol and 2-ethynyaniline (Scheme 86).165 The reaction occurred via dissociation of acetonitrile to generate a vacant site for alkyne capture. In this respect, a Cp ligand can promote the liberation of acetonitrile efficiently compared to a Cp ligand. Thus, the Cp complex 205 catalyzes the cyclization at lower temperatures than the Cp analogue 206. In the reaction of 2-ethynylaniline carried out above 50  C, formation of vinyl complex 207 was observed in the reaction of the Cp complex (Scheme 87). Complex 207 was assumed to be formed via the nucleophilic attack of the nitrogen atom at the a-carbon atom of the vinylidene intermediate. Similar transformations were observed for the formation of 181 as shown in Scheme 77.150 The reaction is competitive with the cyclization process, involving deprotonation by the pendant amine. This intramolecular hydrogen transfer process increases the nucleophilicity of the hydroxyl group, hence promotes the cyclization. In contrast, cyclization of 2-ethynyaniline proceeds at 70  C without deactivation of the catalyst, likely due to the nucleophilicity of the amino group. In this case, 206 was shown to be more efficient catalyst than 205.

Scheme 86

336

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 87

Although the role of the pendant amine moiety has not been elucidated yet, Mock and co-workers showed catalytic oxidation of NH3 to N2 by using P2N2 system with an excess amount of 2,4,6-tri-tert-butylphenoxyl radical (tBu3ArO
) as a H atom acceptor (TON ¼ up to 10).72 The coordinatively unsaturated complex 73 readily reacts with 1 atm of NH3 to yield cationic ammonia complex 208 (Scheme 88). The reaction of 208 with 30 equiv tBu3ArO
in THF resulted in the formation of unidentified paramagnetic species, which generated N2 upon treatment NH3. Based on the labelling experiment using 15NH3 and 14NH3, the N⋯ N bond was proposed to be formed by the nucleophilic addition of 14NH3 to the paramagnetic species, rather than a bimolecular mechanism involving dimerization of terminal nitrido species.

Scheme 88

Colbran and co-workers synthesized iminopyridine ligands functionalized by Hantzsch dihydropyridine or Hantzsch pyridinium cation moieties, and demonstrated the importance of hydride transfer from a metal center to the pyridinium ring in transfer hydrogenation of imines catalyzed by Rh complexes.166 Analogous Hantzsch dihydropyridine complexes 209 and 211 of ruthenium, which adopt a coordinatively saturated electron configuration, were synthesized to examine the electrochemical properties of the non-innocent Hantzsch dihydropyridine or pyridinium cation-substituted pyridylimine ligands (Scheme 89).167

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

337

Cyclic voltammogram (CV) analysis showed that the dihydropyridine-substituted Ru(II) complex 211 afforded the anticipated pyridinium cation-substituted products. In fact, these complexes were cleanly oxidized chemically by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to yield corresponding Hantzsch pyridinium-substituted complexes, 210 and 212. However, CV analysis revealed that electrochemical oxidation of 209 followed a different course, likely due to the conjugation with the pyridylimine ligand and the Hantzsch pyridinium ring. In addition, it has been shown that the primary reduction products of the pyridiniumcation-substituted Ru(II) complexes, 210 and 212, were not the corresponding dihydropyridine-substituted Ru(II) complexes, 209 and 211.

Scheme 89

Klankermayer and co-workers synthesized bifunctional ruthenium hydride complexes 213a bearing borane-based Lewis acidic site, via hydroboration of the vinyl(diphenyl)phosphine ligand (Scheme 90).168 In the solid state, a cyclic conformation with the Ru ⋯ B and BdH distances of 2.895(6) and 1.43(6) A˚ , respectively, were confirmed by XRD. NMR studies clearly represented the dependence of the BdH interaction on the Lewis acidity of the BR2 group. The hydrido ligand in 213a was spontaneously substituted by deuterium upon treatment with methanol-d4. This result indicates, while the 9-BBN analogue 213b only showed the reversible binding of methanol, the enhanced acidity of the MeOH molecule, where the binding at the strong Lewis acid site B(C6F5)2 exerts the interaction with the hydridic RudH moiety. These results clearly demonstrated the synergistic interactions between the hydridic and Lewis acid moieties.

Scheme 90

338

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

The examples shown below may differ from the realm of “bifunctional catalyst”, however the synergy of the metal center with a functional ligand has been shown to play an important role for the catalysis. Gladysz and co-workers synthesized a series of cationic Ru complex 214a–c chelated by 2-guanidinobenzimidazole, as metal-templated “organic” hydrogen bond donors (Scheme 91).169 In these complexes, the metal centers act as architectural units that are inert in catalysis. Chelation to the metal center preorganizes the conformationally flexible 2-guanidinobenzimidazole ligand, affording synperiplanar NH linkages. In addition, the NH hydrogen atoms become more acidic upon coordination to the cationic Ru center. Although the chloride salts of them were ineffective due to the hydrogen bonding with the NH moiety, their BArF4 salts showed excellent reactivities for condensations of indoles and trans-b-nitrostyrene. A BArF4 salt of free 1-methy-2-guanidinobenzimidazole also catalyzed the reaction, however shown to be less active.

Scheme 91

Complexation with 2-guanidinobenzimidazole containing a chiral unit afforded a mixture of two diastereomers, (SRu,RC,RC)215 and (RRu,RC,RC)-215, arising from chirality at the metal center (scheme 92).170 These diastereomers were separated by chromatography, but it was shown that they underwent slow epimerization at the Ru center at room temperature. Both diastereomers catalyze additions of malonate esters to nitroalkene in high yields and enantioselectivities. However, it was shown that the chiral ruthenium center has little influence on the enantioselectivity. They isolated (SRu)-216 by recrystallization using chiral phosphite anion. Due to the absence of chiral units at the ligand, (SRu)-216 displayed very low enantioselectivity.171

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

339

Scheme 92

7.07.4.3

Dihydrogen and hydrido complexes

Oxidative addition of dihydrogen to an unsaturated metal center is one of the most fundamental steps for various catalytic reactions leading to intensive reactivity studies of transition metal complex with dihydrogen.172 While the two-legged piano stool Os complex [Cp OsBr(PiPr3)] (17) reacts with dihydrogen immediately to yield the dihydrido complex trans-[Cp OsH2Br(PiPr3)],26 its Ru analogue [Cp RuCl(PiPr3)] (14a)24 has been shown not to react with dihydrogen. Inertness toward dihydrogen was also observed for the unsaturated half-sandwich complexes, [Cp Ru(Ph2nacnac)] (67)68 and [Cp Ru{k2(S,S)-N(Ph2PS)2}] (78).76 Instead, formation of non-classical dihydrogen complexes, such as 91 and 217, has been often observed, as shown in Schemes 33,79 and 93.83 Notably, while the tmeda complex of [Cp Ru(k2(N,N)-Me2NCH2CH2NMe2)]+ does not react with dihydrogen, that of Cp analogue, [CpRu(k2(N,N)-Me2NCH2CH2NMe2)]+, gave an Z2-dihydrogen complex.60 Tilley and co-workers demonstrated that the hydrogenation product changed depending on the property of a chelate ring in the cyclometallated NHC complexes (Scheme 94).84

Scheme 93

340

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 94

Hydrogenation of bifunctional systems resulted in the formation of mono-hydrido complexes accompanied by the hydrogen transfer to the basic nitrogen or phosphorus atom in the supporting ligand, as shown in Schemes 18,50 and 36.82 These heterolytic activation of dihydrogen clearly represent the importance the cooperative interaction of a Lewis acidic transition metal center with a basic atom properly positioned around the coordination sphere. An alternative method to access hydrido complexes is protonation, and a considerable amount of information has been accumulated since the 1980s on proton transfer to transition metal hydrides. It has been recognized that classical polyhydrides is produced indirectly through initial, kinetically favored proton transfer at the hydride site, followed by rearrangement of the dihydrogen intermediate to the thermodynamically more stable polyhydride. Belkova, Shubina and co-workers investigated the interaction of [Cp RuH(dppe)] (222) and [CpRuH(dppe)] (223) with a series of proton donors in detail by the combination of variable temperature IR and NMR spectroscopy with DFT calculations.173,174 These hydrido complexes were transformed to cationic trans-dihydrido complexes, 224 and 225, representing thermodynamic products, respectively (Scheme 95). In both cases, complex 226 in which the hydride ligand interacts with trifluoroethanol (TFE) via a dihydrogen bond has been established at low temperature as a kinetic product. In complex 226, bifurcate interaction of a proton donor with both the hydride and the metal center was suggested based on a theoretical study.175 Then, it equilibrates with Z2-dihydrogen complex 227 via partial hydrogen transfer. The dihydrogen complex 227 was transformed back to 222 along with increasing temperature up to 240 K. Above 240 K, the Z2-dihydrogen complex was irreversibly isomerized to the trans-dihydrido complex 224.

Scheme 95

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

341

Although the equilibrium between the dihydrogen-bonding intermediate and Z2-dihydrogen complex is also established in the Cp analogue 223, formation to trans-dihydride 225 was indicated to be reversible, which means that protonation at the metal center opposite to the hydrido ligand, yielding 228, is operative in the case of the Cp analogue. This is likely due to the less congested environment around the metal center. Krishnamurthy and co-workers examined protonation of 229 chelated by a diphosphazane ligand, also resulted in the formation of a mixture of the Z2-dihydrogen complex 230 and dihydrido complex 231 (Scheme 96).176 The ratio between them varied depending on the temperatures, and the concentration of 231 increased as elevating temperatures. The presence of an Z2-dihydrogen ligand in 230 was confirmed based on the small T1 value (10 ms), while that of 231 was 150  200 ms. Although the molecular structure was not determined by XRD, the fact that the hydride signal appeared as a simple triplet at −40  C suggests 231 adopts a trans geometry, like 224 and 225.

Scheme 96

The transformation from 227 to 224 would proceed via the formation of a cis-dihydrido intermediate, although it was not detected in the case of ruthenium complexes. In contrast, in the reaction of the Os analogue, [Cp OsH(dppe)] (232), with HBF4 carried out at 193 K, formation of cis-dihydrido complex 233, which displayed a relatively large T1 value (186 ms), was observed instead of an Z2-dihydrogen complex (Scheme 97).14 Upon warming above 233 K, 233 was irreversibly transformed to a trans-isomer, 234. The increase of the relative energy of the metal d electrons upon descending the group destabilizes an Z2-dihydrogen structure through an enhanced back-donation to the s orbital of the dihydrogen ligand, hence it promotes the formation of the cis-dihydride.

Scheme 97

Heinekey and co-workers obtained an equilibrated mixture of Z2-dihydrogen complex 236 and trans-dihydride 237, by the chloride abstraction from [Cp OsCl(CO)2] by [Et3Si][BArF4] under an atmosphere of H2 (Scheme 98A).177 The T1 value was accurately measured due to the absence of protic impurities, and the hydrido signal of 236 exhibits a T1(min) value of 24 ms at 180 K. This value indicates substantial interaction between the hydrogen atoms, and the relatively large JHD value (24.5 Hz) also demonstrates the presence of Z2-dihydrogen ligand in 236. While complex 236 was formed initially, it gradually isomerized to trans-dihydrido complex 237 at room temperature and reached at equilibrium with a 236/237 ratio of 3.5/1.0 at 295 K. A significant concentration of the Z2-dihydrogen complex 236 is attributed to the relatively electron-deficient metal center caused by the carbonyl ligands. In fact, the reaction of analogous complex 238, which is supported by a more electron donating dppm ligand, with H2 led to the formation of dihydrido complex 239 (Scheme 98B).

342

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

(A)

(B)

Scheme 98

Jiménez-Tenorio, Puerta, Lledós, and co-workers investigated the reaction of trihydrido complex 240 with TfOH (Scheme 99).78 Protonation took place at the nitrogen atom and 241 was produced. DFT calculations suggest the hydrogen bonding interaction between the NH in the pyridinium group and the central hydrido. Variable temperature NMR studies revealed site exchange among the hydrides and the NH proton. Z2-Dihydrogen complex 242 then formed via hydrogen elimination with XRD studies clearly showing the presence of an Z2-dihydorgen ligand in 242. The Z2-dihydrogen ligand was readily eliminated upon dissolving in solution under an Ar atmosphere, and unsaturated 16-electron species 88 was obtained. In the presence of excess TfOH, 241 underwent further protonation to yield 243. Although 243 was not isolated, small T1 value of 14 ms indicates that at least one dihydrogen ligand was present in 243.

Scheme 99

Tilley and co-workers reported the synthesis of novel anionic hydrido complex 244 (Scheme 100).178 Complex 244 was obtained cleanly upon treatment of unsaturated di(isopropylmethyl)phosphine complex 14b with NaBHEt3. Complex 244 acts as a hydride-based nucleophile and can be a synthon for preparing bimetallic architecture, by reacting with metal halide. For example, they demonstrated the synthesis of RudIr complex 245 by the reaction of 244 with [IrCl(cod)].

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

343

Scheme 100

Meyer and co-workers reported an interesting reaction on hydrogen activation. As shown in Scheme 20, they synthesized a potassium salt of [CpRu(CO)]−2 (59) by the reaction of [Cp2Ru(CO)2] with K[HB(sec-Bu)3].59 In this reaction, insoluble black solids 60 were obtained as by-product. Although formulation of the black material was not identified, it was found that [CpRuH(CO)2] (246) was formed upon treatment with H2O. They found that 60 reacts with dihydrogen only in the presence of 1,3-bis(2,6-difluorophenyl)-2-(4-tolyl)imidazolinium bromide ([TolImF4]+ Br−), yielding 246 and the corresponding imidazolidine HTolImF4 with a conversion of 96% after 1 d (Scheme 101).179 This reaction strongly implied the occurrence of heterolytic cleavage of dihydrogen, as seen in frustrated Lewis pairs (FLP) and [Fe] hydrogenase.

Scheme 101

7.07.4.4

Half-sandwich complexes with a Group 13 element

Owing to the various coordination modes of boron to a metal center, such as s-borane, boryl, and borylene, the chemistry of transition metal complexes possessing a metal-boron linkage has attracted considerable attention.180 In addition, there is widespread interest in their reactivity associated with a number of important synthetic reactions, for example olefin hydroboration, dehydrogenative borylation of hydrocarbons, and dehydrogenation of Lewis adducts including ammonia borane. As for the borane chemistry of half-sandwich ruthenium complexes, one of the most important outcomes is borylation of alkanes elucidated by Hartwig and co-workers in 2006 (Scheme 102).181 This result highlights the importance of understanding the property of a MdB interaction in a coordination sphere. Although it is well known that borane can form higher-nuclearity metallaborane clusters, this section is limited to the mononuclear compounds containing a MdB bond.

Scheme 102

Kawano and Shimoi and co-workers showed a cationic 16-electron fragment [CpRu(PMe3)2]+ formed particularly stable complexes, 247a,b, with an amineborane or a phosphineborane adduct (Scheme 103).182 In these complexes, the borane ligand is coordinated to the Ru center through a BdHdRu interaction. The NMR studies showed rapid site exchange of the BH protons between the bridge and terminal positions. The Z1-BdH bond was highly polarized due to coordination to the cationic metal center, and 247 was hydrolyzed by a trace amount of H2O via heterolytic cleavage of the Z1-BdH bond.

344

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 103

When 14a was treated with NaBArF4 in the presence of BH3
L (L ¼ quinclidine, NMe3, NHMe2, NHiPr2, PMe3), bidentate s-borane complex 248a–e was obtained (Scheme 104).183 The RudB distance in 248c (2.156(2) A˚ ) is much shorter than that of 247b (2.648(3) A˚ ). The borane group in 248 is readily displaced by another ligand, and it was shown that more basic Lewis base-borane adduct forms a stable complex. Thermolysis of 248c at 40  C resulted in the formation of aminoborane complex 249a, along with elimination of dihydrogen. Dehydrogenation of BH3
NHMe2 was catalyzed by 20 mol% of 248c, however formation of cyclic dimer [BH2NMe2]2 proceeded very slowly.

Scheme 104

Stradiotto and co-workers demonstrated that 14a reacts with mesitylborane immediately to yield bidentate Z2-borane complex 250 (Scheme 105).184 In this reaction, MesBH2 itself acts as a chloride abstraction reagent and forms a chloroborate ligand. The following chloride abstraction by using LiB(C6F5)4
2.5OEt2 led to the formation of cationic mesitylborane complex 251 featuring bis(Z2-BdH) ligation. The RudB distance in 251 (1.921(2) A˚ ) was shorter than the borate complex 250 (2.162(3) A˚ ), which was comparable to that of cationic amine borane complex 248c. Atoms-in-Molecule (AIM) analysis revealed that the remarkably short RudB distance in 251 is due to the presence of a direct RudB interaction, which stabilizes the vacant p orbital on boron by p backdonation. In contrast, no bond critical points were found at the RudB vector in 250, while donor-acceptor interaction between the cationic [Cp Ru(PiPr3)]+ fragment and the bis(Z2-BdH) coordinating borate ligand was inferred as observed in 251.

Scheme 105

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

345

Ghosh and co-workers examined thermolysis of nido-[(Cp Ru)2B3H9]185 in the presence of 2-mercaptobenzothiazole, and obtained a mixture of three mononuclear complexes 252, 253, and 254, among which Z2-BdH coordination to the Ru center was clearly represented by XRD in 252 and 253 (Scheme 106).186 Although the mechanism was not fully elucidated, an intermediate, which contains three boron atoms and 2-mercaptobenzothiazolate (2-mbz) ligand, suggests that coordination of the 2-mbz ligand to one of the RudB edge through the sulfur and the nitrogen atoms is the first step. Similar reaction was also observed with 2-mercaptobenzoxazole and 2-mercaptobenzimidazole.187

Scheme 106

Upon incorporation of dppm, the Z2-BdH bond in 252 was dissociated, and zwitterionic thiolate complex 255 containing a borate group was produced. They demonstrated that the reaction of 255 with phenylacetylene led to the formation of Z4-s,p-borataallyl complex 256 and Z2-vinylborane complex 257 through hydroboration (Scheme 107A).188 XRD studies revealed that hydroboration took place through Markovnikov selectivity. They also showed that bis(s-borate) complex 258189 reacts with terminal alkynes to yield a mixture of Z2-vinylborane complex 259, in which the ratio between the Markovnikov and the anti-Markovnikov products was estimated to be 3:1 (Scheme 107B).190

(A)

(B)

Scheme 107

346

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Aldridge and co-workers synthesized terminal borylene complex of ruthenium 261 via halide abstraction from a haloboryl complex 260 (Scheme 108).191 The highly electrophilic borylene group was stabilized by the p-donation from the di(cyclohexyl) amino substituent, which also protects the borylene center sterically. Since displacement of one of the CO ligands in 261a by PMe3 did not succeed due to decomposition, arising from the reaction at the boron center, 261b could be synthesized from 260b. The RudB distance in 261a (1.960(6) A˚ ) was remarkably shortened from that in 260a (2.1412(19) A˚ ).

Scheme 108

Aminoboranes are the subject of significant interest not only as the hydrogen storage materials but also as the monomeric building blocks of inorganic polymers. Although aminoborane is isoelectronic with 1,1-disubstituted alkenes, its coordination mode to a metal center was revealed to differ significantly from a side-on coordination mode observed for alkene complexes. Aldridge and co-workers showed that aminoborane interacts with an unsaturated metal center through a BdH bond as shown in Scheme 109, not with its B]N bond.192 The B]N distances in 262a and 262b (1.376(4) and 1.382(7) A˚ , respectively) are identical despite the different donating abilities of the phosphine ligand, which implies little population of the BN p orbital in the coordination. The RudB distance (2.530(4) A˚ ) in 262a is shorter than that in [CpRu(PMe3)2(k1-H3B
NMe3)] (247b, 2.648 (3) A˚ ),182 which suggests the presence of interaction with the Ru center.

Scheme 109

While aminoborane complex 263 supported by a Z6-arene ligand yielded borylene complex 264 upon treatment with 3,3-dimethyl-1-butene, the Cp analogue, [Cp Ru(PCy3)(k2-H2BNiPr2)] (249b), does not undergo dehydrogenation, instead a hydroboration product was produced gradually (Scheme 110).193 The different reactivity is ascribed to the absence of a metal–hydride in 249b.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

347

Scheme 110

Manners and co-workers synthesized phosphine-borane complex 266 by the reaction of [CpRuI(CO)2] with Li[PPh2
BH3].194 Complex 266 reacts with PPh2H
BH3 at 120  C without solvent to catalyze dehydrogenative coupling of phosphine-borane (Scheme 111). It was also shown that [Ru3(CO)12] or [Ru/Al2O3] showed low activity toward dehydrogenative coupling of PPh2H
BH3 under the same conditions (conv. 15% and 5%, respectively).

Scheme 111

As for half-sandwich Ru complexes containing heavier group 13 elements, Aldridge and co-workers investigated the reaction of [CpRuCl(PPh3)2] with GaI in anticipation of the formation of a gallanediyl species, Ru]GaR, via halide abstraction. Although expected insertion of GaI into the RudCl bond did not take place, novel half-sandwich complex 71 containing a coordinated tetraiodogallate fragment via a single bridging iodine atom was obtained (Scheme 112).71 When the reaction was conducted with [CpRuCl(dppe)], cationic complex 69 adopting a two-legged piano stool geometry was produced as shown in Scheme 26.

Scheme 112

As mentioned above, HdH and BdH bonds, as well as AldH, GadH, CdH, SidH, GedH, SndH, NdH, and PdH bonds, can bind to a metal center with maintaining bonding interaction, forming s-complexes. In 2018, the first s-complex of a period 6 main-group element was reported by Williams, Lin, and Jia and co-workers.195 This group examined the reaction of [CpRuH(P^P)] (P^P ¼ dppm, dppe) with TlPF6, and obtained Z2-TldH complex 267a,b (Scheme 112). The hydrido signal of 267 appeared at ca. −4 ppm as a broad triplet, which is considerably different from that of the starting complex (−14.1 ppm). XRD studies of 267a

348

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

showed that the PF−6 is in contact with the Tl center with a Tl ⋯ F distance of 2.879 A˚ . The RudTl distance (3.1571(4) A˚ ) is substantially shorter than the sum of the van der Waals radii for Ru and Tl, and the RudH (1.76(7) A˚ ) and TldH (2.32(7) A˚ ) distances indicate the 3-center interaction among Ru, Tl, and H, which was supported by DFT calculations. In the 1H NMR spectrum recorded at low temperatures, the hydrido signal appeared as a broad doublet with a 1JTldH value of 936 Hz (267a) or 1128 Hz (267b), which strongly indicates the TldH interaction in 267.

7.07.4.5

Half-sandwich complexes with a Group 14 element

7.07.4.5.1

Alkoxycarbonyl complexes

In 2004, Bäckvall and co-workers developed a highly efficient dynamic kinetic resolution of secondary alcohols at room temperature that provides enantiopure products in high yields in very short reaction time.196 In this system, coordinatively saturated half-sandwich complex [(C5Ph5)RuCl(CO)2] (268) was used as a catalyst precursor of racemization of secondary alcohol. Complex 268 was converted to a tert-butoxido complex via the metathetical reaction with tBuOK, which is subsequently replaced by a secalkoxide. Racemization via b-hydride elimination requires a free coordination site, and this could be formed by either ring slippage of the cyclopentadienyl group or loss of a CO ligand. However, they found that hydrido complex [(C5Ph5)RuH(CO)2] is not an active catalytic intermediate.197 DFT calculations also showed that the ring-slippage path is unfavorable process,198 instead participation of a CO ligand for the ligand exchange process, leading to a tert-butoxycarbonyl intermediate 269, has been suggested (Scheme 113). In fact, they succeeded in the direct observation of 269 by using in-situ IR measurements.199 To the end, they obtained alkoxycarbonyl complex 270, by the reaction of 268 with 5-hexen-2-ol, in which the alkene moiety is coordinated to the Ru center (Scheme 114).200 Due to the coordination of the olefinic moiety, the racemization slowed down considerably.

Scheme 113

Scheme 114

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

349

Formation of an alkoxycarbonyl complex was also observed in the reaction of diruthenium complex 41 supported by bulky Cp^ groups.43 Complex 41 reacted with CO to yield complex 271 quantitatively (Scheme 115). Formation of 271 is in sharp contrast to the reaction of [Cp Ru(m-OMe)]2 with CO, yielding [Cp Ru(CO)(m-CO)]2.201

Scheme 115

7.07.4.5.2

NHC complexes

As seen in the several examples mentioned above, strong s-donating and weak p-accepting abilities of NHCs as a supporting ligand play important roles not only in catalysis but also in stabilizing coordinatively unsaturated metal centers. Mutoh and co-workers synthesized a series of half-sandwich Ru complexes containing a chalcogenocarbonyl ligand, C^E (E ¼ O, S, Se, Te), with a three-legged piano stool geometry. Previously, they reported the preparation of a series of chalcogenocarbonyl complexes 272a–d from the reaction of a carbido complex, [RuCl2(C)(H2IMes)] (H2IMes ¼ 1,3-dimesitylimidazolin-2ylidene), with elemental chalcogen atoms.202 Introduction of a Cp ligand was successfully achieved by the reaction of 272 with CpLi in the presence of Et3B (Scheme 116).203 In complex 273, the CE ligands become more stable owing to the strong back-donation from the [CpRu] unit as compared to those in 272, which was clearly represented by the considerable red shift of n(CE). While the reaction of 272d with PPh3 resulted in the formation of the carbido complex along with the liberation of phosphine telluride, 273d was found to be resistant to degradation even at 100  C. They also synthesized cationic complexes 274a–d upon treatment with NaBArF4.204 From these studies, they disclosed that the p-accepting nature of the CE ligand increases in the order, CO < CS < CSe < CTe.

Scheme 116

Nikonov and co-workers synthesized [CpRu(IPr)(Py)2]+ (30; IPr ¼ 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) by the reaction of [CpRu(Py)3]+ (29) with IPr.205 Related complex [CpRu(IPr)(MeCN)2]+ (275) was obtained by the reaction of [CpRu(MeCN)3]+ with IPr,206 however they noted that the reaction suffered from the formation of a significant amount of an imidazolium salt [HIPr]+, owing to deprotonation of the coordinated acetonitrile by the basic IPr. Therefore they employed pyridine substituted complex 29 for the preparation of 30. In the presence of KOtBu, complex 30 catalyzes transfer hydrogenation of acetophenone by using 2-propanol as a hydrogen source more efficiently than the corresponding phosphine analogue, [CpRu(PiPr3)(MeCN)]+.207 Complex 30 was also shown to catalyze transfer hydrogenation of nitriles, imines, olefins, and conjugated olefins. In the reaction conditions, 30 was converted to trihydrido complex [CpRuH3(IPr)] (276), and it was proved that 276 is the true catalyst. The trihydrido complex 276 was alternatively synthesized from 30 by the reaction with LiAlH4 (Scheme 117).

Scheme 117

350

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

In general, NHCs bind to a metal center much more strongly than phosphines, less likely to be replaced by other ligands. However, participation of the NHC ligands in rearrangement reactions within the coordination sphere have been often observed, unlike the case of tertiary phosphine ligands, likely due to their electronic differences. Kirchner and co-workers showed that the reaction of 275a with acetylene at room temperature resulted in immediate formation of 277 via [2 +2+ 1] cyclotrimerization of acetylene (Scheme 118).206,208 The subsequent thermolysis at 40  C led to the formation of (ruthenocenylmethyl)imidazolium salt 278 as a result of 1,2-hydrogen shift. The mechanism shown in Scheme 118 was supported by DFT calculations, and clearly indicated that acetylene is able to undergo facile migratory insertion into the RudC bond of NHC ligands.

Scheme 118

Tilley and co-workers also demonstrated unusual degradation of an NHC ligand upon addition of an anionic, nucleophilic base to masked silylene complex 279 (280), leading to the formation of anionic carbene complex 281 (Scheme 119).209 Subsequent treatment with B(C6F5)3 gave complex 282, supported by a formally dianionic Z5-C5Me4{CH2B(C6F5)3} ligand and an aminocarbene ligand containing pendant imine and silane groups.

Scheme 119

Reactivity of an NHC ligand within the metal’s coordination sphere has been also reported by Song and co-workers. They synthesized half-sandwich Ru complex 283 bearing a deprotonated picolyl-functionalized NHC ligand (Scheme 120).210 Although complex 283 adopts a coordinatively saturated configuration, 283 undergoes isomerization to 285 at 90  C; the coordination mode of the deprotonated picolyl-NHC ligand changed from a (N,C)- to (N,N)-chelation mode. From the kinetic and computational

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

351

studies for the analogous Fe complex, the plausible reaction mechanism of unusual transformation was postulated as shown in Scheme 120, which involves aromatization of the pyridyl moiety accompanied by CdN bond cleavage. Complex 285 was formed by the subsequent nucleophilic attack of the N-bound imidazolyl moiety to the pyridylcarbene followed by the dearomatization of the picolyl moiety. Formation of the four-membered metallacycle skeleton in the intermediate 284 was confirmed by the XRD studies of a BH3 adduct of the Fe analog.

Scheme 120

To avoid unexpected elimination and degradation of NHC ligands and to form robust catalytic entities, several half-sandwich Ru complexes, 286–293, supported by an NHC ligand containing a pendant donor group have been synthesized (Scheme 121),211–213 as already seen in complexes, 96, 99, 172, 175, 178, 182, and 184. In addition, mixed sandwich complexes, 294214 and 295,215 supported by a tripodal bis(NHC) ligand, which formally act as a 6-electron donor, have been reported. Notably, complex 294 can serve as a metalloligand; two unsubstituted NH groups in the benzimidazole moiety in 294 can uptake a second transition metal upon deprotonation leading to the formation of a bimetallic complex.

Scheme 121

352

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Imidazolydenes bound to a metal center through the C4/C5 atom, viz. abnormal NHCs, has been shown to be a stronger donors compared to the “normal” C2-bound imidazolydene counterparts. Albrecht and Landman and co-workers synthesized halfsandwich Ru(II) complexes bearing a chelating abnormal NHC ligand. By protecting the C2 position of the imidazolium salt with a methyl group, they obtained complex 296, in which the imidazolydene moiety bound to the Ru center with the C4 atom (Scheme 122). However, the normal NHC complex 297 was also formed in the reaction via C2dMe cleavage. Molecular structures of these complexes were revealed by XRD, however they were not separated from each other. In contrast, abnormal NHC complex 298 was exclusively obtained from [(p-cymene)RuCl2]2 without C2dMe bond cleavage. They examined transfer hydrogenation of benzophenone by 1 mol% of 298 in the presence of KOH (5 mol%) using 2-propanol as a hydrogen source under reflux conditions. Complex 298 catalyze the reaction with initial TOF of 60 h−1, which was comparable to similar bidentate NHC Ru(II) analogues. However, this activity is much lower than the other best-performing Ru catalysts.

Scheme 122

Lynam and Slattery and co-workers demonstrated the important role of a pyridylidene ligand in the coupling reaction of pyridine and terminal alkynes, leading to E-selective 2-alkenylpyridine formation originally reported by Murakami et al.216 Cationic pyridine complex 299 catalyzes the reaction in pyridine solution at 50  C, but the reaction does not proceed in CH2Cl2. Instead, they found that pyridylidene complexes, 300 and 301, were formed. Although 300 indicates the deactivation process of the catalysis, the important role of the pyridylidene intermediate for the productive pathway was shown. By the aid of theoretical studies, they elucidated the reaction mechanism as shown in Scheme 123.217 It involves nucleophilic addition of pyridine at the nucleophilic Ca carbon of the vinylidene intermediate. Coordinated pyridinium moiety undergoes CdH bond cleavage, and forms a pyridylidene intermediate accompanied by dearomatization in the pyridine ring. While migration of the hydride to the Ca atom led to the formation of 300, it to the nitrogen atom causes regeneration of a vinylidene ligand. A vinylpyridine ligand was formed via the insertion of the pyridylidene ligand into the RudC bond.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 123

353

354

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.4.5.3

Carbene complexes

Transition-metal carbenes play a fundamental role as reactive intermediates in modern synthetic chemistry. A prevalent method for accessing carbene intermediates is the metal-catalyzed fragmentation of diazo compounds. A variety of ruthenium catalysts have been developed that catalyze the reaction of diazo compounds with simple unsaturated hydrocarbons to produce high-value chemicals.218 For example, Saá and co-workers reported the synthesis of 2-vinyl-3,4-dihydropyrans by the reaction of alkynal with (trimethylsilyl)diazomethane by using [Cp RuCl(cod)] (125a) as a catalyst precursor, in which a carbene intermediate with a coordinatively unsaturated 16-electron configuration was produced in situ (Scheme 124).219

Scheme 124

Albertin et al. demonstrated that half-sandwich ruthenium complex 302 reacts with diphenyldiazomethane in the presence of NaBPh4 to yield diazoalkanes complex 303 (Scheme 125A).220 When the phosphine ligands were replaced by bipyridine, corresponding diazoalkanes complex was not obtained. Instead, carbene complex 305 was exclusively formed via dinitrogen elimination (scheme 125B).221 DFT calculations represent that isomerization to the C-bound isomer from the N-bound isomer, like complex 303, is more facile in the bipyridine complex, which allows the release of N2. Destabilization of the C-bound isomer by the phosphite was ascribed to a consequence of the steric repulsion with bulky phosphite ligands. (A)

(B)

Scheme 125

Fürstner and co-workers developed novel reduction methods of internal alkynes with high E-selectivity, catalyzed by simple half-sandwich ruthenium complexes, such as [Cp RuCl(cod)], [Cp Ru(MeCN)3]+, and [Cp RuCl]4, which involves trans-selective hydrogenation,222 hydroboration,223 hydrostannation,224 hydrogermylation, and hydrosilylation.43 The unusual E-selectivity strongly suggests that the reductions proceed in a different manner from the classical alkyne hydrogenation via insertion/reductive

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

355

elimination mechanism, leading to Z-alkene. By the reaction of [Cp RuCl]4 with propargyl ether, they isolated carbene complex 306a via net geminal hydrogenation (Scheme 126).225 The dOH analogue 306b undergoes further transformation to E-alkene complex 307. Owing to the strong directing effect of the OR group, the metal carbene resides distal to the ligated OR group, which imparts the regio selective transformation of propargyl alcohol. Based on the detailed experimental results and computational studies, the mechanism of the trans-hydrogenation was fully elucidated as shown in Scheme 126, in which involves the metallacyclopropene intermediate as a key intermediate. Subsequently, this group applied this transformation, geminal hydrogenation, to the synthesis of furans (Scheme 127).226 When the aryl group was introduced, cyclization at the final stage could not occur because of the difficulty of dearomatization. Instead, carbene intermediate 308 was isolated. Interaction between the electrophilic carbene carbon and the carbonyl group was clearly shown, as well as the coordination of the OMe group to the Ru center, by XRD analysis.

Scheme 126

Scheme 127

356

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Trost et al. also used propargyl alcohol as a precursor of metal-carbene species, which acts as an alternative of inaccessible b-diazo carbonyl compound. By using [CpRuCl(PPh3)2] as a catalyst, they developed a novel protocol for accessing b-oxo carbenoid species for the efficient cyclopropanation of unactivated olefins (Scheme 128).227

Scheme 128

As seen in Scheme 30, Stradiotto and co-workers reported the synthesis of half-sandwich Ru complex 83 with an aminocarbene ligand via double CdH bond cleavage of a coordinated NMe2 moiety.77 Complex 83 reacts readily with PhSiD2 to afford 309, in which deuterium was incorporated only at the carbene carbon atom (Scheme 129).228 This result clearly indicates that addition of the SidD bond occurs across the Ru]C unit in 83.

Scheme 129

Lynam and Slattery and co-workers synthesized half-sandwich Ru complexes containing a fluorinated carbene ligand 313, by the consecutive treatment of alkynyl complex 310a with F+, F−, and F+, via the formation of fluorinated vinylidene complex 311 and fluoroalkenyl complex 312 (Scheme 130).229 When the alkenyl complex 312 was treated with HCl instead of N-fluorobenzenesulfonimide, [CpRuCl(dppe)] was quantitatively obtained with liberating fluorinated alkene (E)-CFH]CPhF. The fact that the E/Z mixture of 312 gave only E-difluorostyrene upon protonation suggests that the reaction involves the carbene intermediate, like 313. The styrene ligand was formed by the following 1,2-shift of hydrogen.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

357

Scheme 130

Although examples of three-legged piano stool Os complexes containing a carbene ligand are limited, a few carbene complexes have been isolated by the reaction with terminal alkynes, via the formation of vinylidene intermediates. Esteruelas and López and co-workers synthesized allenylcarbene complex 315 by the reaction of hydrido complex 314 with phenylacetylene (Scheme 131A).230 Although allenylcarbene complexes have been often proposed as key intermediates for the metal-catalyzed polymerization of terminal alkynes, 315 is a rare example that was structurally characterized. They also examined the reaction of 316 with 3,3-di(methoxycarbonyl)-5hexyn-1-al and isolated carbene complex 317 containing a four-membered oxo ring ligand (Scheme 131B).231

(A)

(B)

Scheme 131

358

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Lin and co-workers have intensively studied cyclization of alkynes, and reported numbers of crystal structures of cyclic carbene complexes of ruthenium formed via vinylidene intermediates (Scheme 132A).232 Valerga’s group synthesized cyclic carbene complexes of ruthenium by the reaction of vinylcarbyne complex 320, which was obtained by the protonation of allenylidene complex 319, with resorcinol (Scheme 132B).233 Albertin et al. synthesized pyranylidene complex 322 as a mixture with ethoxycarbene complex 323 by the reaction of [CpRuCl(PPh3){P(OMe)3}] with methyl propionate in the presence of ethanol (Scheme 132C).234 (A)

(B)

(C)

Scheme 132

Esteruelas, López, and Mascarenás and co-workers reported unusual ring expansion reaction of a methylenecyclopropane moiety in the coordination sphere of Os, leading to the formation of cyclobutylidene complex 324 (Scheme 133).235 The mechanism of the ring expansion was proposed based on DFT calculations as shown in Scheme 133; the Z2-methylenecyclopropane complex stabilized by pyridine coordination was transformed into the 1-osma-2-azacyclopent-3-ene intermediate via oxidation of the metal center accompanied by the rehybridization of the nitrogen atom from sp2 to sp3. The four-membered ring was then formed by the migration of the CH2 moiety in the cyclopropane moiety to the vinylic carbon atom.

Scheme 133

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.4.5.4

359

Vinylidene and allenylidene complexes

The reaction of [CpsRu(II)] fragment with terminal alkynes or terminal propargyl alcohols followed by dehydration are one of the most reliable methods to obtain vinylidene or allenylidene complexes. Numerous half-sandwich Ru and Os complexes containing a vinylidene or an allenylidene ligand have been reported so far. The reactivity of vinylidene complexes are rationalized by considering electrophilicity of Ca and the nucleophilicity of Cb of the M]Cd+]Cd− R2 moiety, while the electrophilic Cg is additionally involved in the allenylidene M]Cd+]Cd]Cd+ R2 skeleton. The alternate electrophilic and nucleophilic properties of the carbon atoms in the chain allows the regioselective additions, and this propensity has been successfully applied to many catalytic reactions involving CdC, CdO, and CdN bond formations, as well as construction of complex cyclic structures from acyclic unsaturated starting materials. Since these stoichiometric and catalytic reactions have been extensively reviewed,236 this section will be focused on the mechanistic insight of the vinylidene formation from internal alkynes and preparations of vinylidene and allenylidene complexes of new types. In contrast to the well-known vinylidene formation from terminal alkynes, the conversion of internal alkynes to vinylidenes was considered to be an unusual process and it had only been reported for trialkylsilyl, trimethylstannane, alkylthiol, and iodo substituted alkynes.236 In 2000, Knox and co-workers reported that dimethyl acetylenedicarboxylate is transformed into a bridging vinylidene ligand on a diruthenium complex.237 Following this, Shaw et al. reported that [CpRuCl(PPh3)3] react with 1,3-diphenylpropynone to yield Z1-vinylidene complex 326, via the formation of Z1-ketone intermediate 325 (Scheme 134A).238 Ishii and co-workers elucidated that aryl substituents in internal alkynes can migrate to yield a vinylidene ligand (Scheme 134B).239 They noted that the use of NaBArF4 was essential for the selective formation of 327; either AgPF6 or NaBPh4 instead of NaBArF4 resulted in the formation of a complex mixture containing 327. In contrast, only Z2-alkyne complex was obtained by the reaction with 2-pentyne. They elucidated the order of the migratory efficiency of the aryl groups as follows based on the 13C-labelling experiments: CO2Et > p-EtO2CEtC6H4 > p-ClC6H4 > Ph > p-MeC6H4 > p-MeOC6H4, in which electron-withdrawing substituents on the aryl ring enhance the migratory aptitude, and this means that electron donating aryl group at the b-carbon plays an important role in stabilizing the cationic Cb atom in the transition state. This group also demonstrated that the vinylidene rearrangement of P- and S-substituted internal alkynes at a ruthenium center, where the electron-withdrawing dP(]O)(OMe)2. dSPh, and dSO2Ph groups act as migrating groups.240 The 1,2-aryl migration of the internal alkyne ligand was fully supported by DFT calculations.241 Reversibility of the 1,2-aryl migration was demonstrated by the reaction of 327 with various L type donors (Scheme 134C).242 In this reaction, it was also shown that an electron-withdrawing substituent on the vinylidene ligand accelerates the interconversion. This reversibility is important for constructing a catalytic process, and Mutoh and Saito and co-workers successfully developed this 1,2-carbon migration reaction to catalytic reactions yielding indole, naphthalene, and pyrrole derivatives (Scheme 135).243

(A)

(B)

(C)

Scheme 134

360

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 135

Puerta, Valerga, and Lledós and co-workers examined the reaction of half-sandwich complex 328 supported by a k2-picolylphosphine ligand with alkynones, however only the formation of a mixture of diastereomers of Z2-alkyne complex 329 was observed (Scheme 136A). This is in sharp contrast with the reaction of the Tp analogue 330 with alkynones, leading to the exclusive formation of vinylidene complex 332, via the Z1-ketone intermediate 330 (Scheme 136B).244 Based on kinetic and theoretical studies, they disclosed that the strong p-back donation ability of the [Cp Ru] fragment is responsible for the kinetic stabilization of the Z2-alkyne complex, which considerably increases the barrier for the isomerization to the vinylidene complex.

(A)

(B)

Scheme 136

Owing to the importance of mono- and di(halo)vinylidenes complexes for synthetic reactions, their rational synthetic methods have been pursued. Several mono(halo)vinylidene complexes have been synthesized by the addition of electrophiles at the b-carbon atom of alkynyl complexes. Low and co-workers synthesized half-sandwich Ru complexes containing a cyanovinylidene ligand by the reaction of alkynyl complex 333 with a BF−4 salt of 1-cyano-4-dimethylaminopyridinium ([CAP]BF4) (Scheme 137A).245 When the reaction was conducted with acetylido complex 310b, cyanoacetylido complex 335 was formed via deprotonation from the cyanovinylidene intermediate. The subsequent cyanation led to the formation of the di(cyano)vinylidene complex 327i (Scheme 137B). This group also synthesized mono(bromo)vinylidene complexes 337a by the reaction with BCN. When trimethylsilyl complex 336b was used, di(bromo)vinylidene complex 337b was obtained via desilylation (Scheme 137C and D).246 Di(fluoro) vinylidene complex 327k was synthesized by Lynam and Slattery and co-workers via the sequential deprotonation and fluorination by the N-fluorobenzenesulfonimide (NFSI) from the fluorovinylidene complex 327j (Scheme 137D).247

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

361

(A)

(B)

(C)

(D)

(E)

Scheme 137

As an example of an interesting reaction involving a vinylidene intermediate of osmium, 7-endo-heterocyclization has been reported by Esteruelas, Saá, and co-workers. While the activities of ruthenium complex, [CpRu(py)3]+ (29), was moderate, they found that 7-endo-cyclization of aromatic alkynols was effectively catalyzed by [CpOs(py)3]+ (Scheme 138A).248 They also demonstrated that [CpOs(py)3]+ catalyzes the cyclization of o-ethynyl phenethylamine, leading to the formation of 3-benzazepines (Scheme 138B).249 In this case, the ruthenium analogue 29 was shown to be inactive for the formation of 7-membered ring.

362

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

(A)

(B)

Scheme 138

They isolated osamacyclopropane complex 338 by the stoichiometric reaction of [CpOs(PiPr3)(MeCN)2]+ with o-ethynyl phenethylamine in the absence of pyridine (Scheme 139). XRD studies clearly represented the four-legged piano stool geometry of the Os center in 338, which is a typical geometry for Os(IV) species. In the presence of pyridine, complex 338 was shown to catalyze the reaction, in which pyridine was thought to play a crucial role for the liberation of heterocycles as a mediator of proton shuttle. Based on the observations, they proposed the mechanism of the cyclization as shown in Scheme 139.

Scheme 139

An allenylidene ligand is readily formed on a metal center from propargyl alcohols upon protonation. Owing to the electrophilic nature of the Cg atom, it is expected that nucleophilic addition takes place at this position. However, because the Cg atom is positioned far from the metal center, it is difficult to control the enantioselectivity at this position by usual chiral diphosphines. Nishibayashi and co-workers designed a chiral space on a diruthenium complex of the general formula of [Cp RuCl(m-SR )]2 by using chiral bridging thiolate ligands. The bulky terphenyl group on the sulfido ligand effectively blocks one side of the allenylidene ligand, owing to the p-p interaction between the terphenyl group and the aryl group on the allenylidene ligand. They successfully demonstrated that the diruthenium complexes catalyzes the enantioselective alkylation of propargylic alcohols through allenylidene intermediates.250 Nakamura and Matsuo and co-workers synthesized ruthenium complexes supported by a bulky Z5-fullerene ligand, which is expected to provide a congested reaction field. In fact, introduction of an optically active diphosphine ligand, ((R)-1,2-bis (diphenylphosphinanyl)propane), occurred in a diastereoselective manner, and complex 339, which possesses a chiral metal

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

363

center, was obtained (Scheme 140).251 In the presence of AgPF6, 339 reacted with secondary propargyl alcohol to yield allenylidene complex 340 as a single diastereomer. XRD studies showed that the aryl group on the allenylidene moiety points away from the fullerene core. They showed that addition of Grignard reagent to the Cb atom takes place with moderate to high level of diastereoselectivity. Although the stereochemistry of the products was not determined yet, it has been shown that the reaction of 340 with PhMgBr gave 341 with a 95:5 of diastereoselectivity.

339

340 (90%)

341 (95:5 ds)

Scheme 140

Construction of a rigid chiral metal center was also achieved by introducing a (R)-binol-N,N-dibenzylphosphoramidite into a [(Ind)Ru(PPh3)] fragment. Bauer and co-workers obtained 342 from the reaction of [(Ind)RuCl(PPh3)2] with (R)-phosphoramidite in a diastereomeric pure form (Scheme 141).252 Molecular structure of 342 was determined by XRD, which represents the R-configuration of the metal center. The bulky phosphoramidite ligand was shown to be placed opposite to the six-membered ring of the indenyl ligand. The configuration at the metal center was retained in the allenylidene complex 343, in which the allenylidene ligand was place beneath the indenyl ligand. Nucleophiles react with 343 to yield alkynyl complex 344 as a mixture of two diastereomers. The diastereoselectivity depends on the nucleophiles, and the reaction with lithium 1-phenylenolate gave a mixture with a 84:16 ratio, which originated from the chirality of the ligand and the stereogenic ruthenium center.

342

343 (96%)

344 (72%, 84:16 ds)

Scheme 141

7.07.4.5.5

Alkynyl and poly-ynyl complexes

Reactivity of half-sandwich complexes containing a s-alkynyl complexes have been extensively studied so far, and their reactivity are rationalized by the nucleophilicity of the Cb atom. Thus, alkenyl complexes of half-sandwich complex of group 8 metals have been shown to readily react with variety of electron-deficient alkenes, such as tetracyanoethene (TCNE). The products are generally formed by [2+ 2] cycloaddition reactions to give substituted cyclobutenyl-metal complexes, which can then undergo electrocyclic ring opening to give s-1,3-butadienyl or 1,2,3-Z3-butadienyl complexes.253 The resulting products, in which the electron-rich [(Cps)M] fragment is linked with the electron-accepting groups by a conjugated p-system, are strongly polarized and consequently, among other features, show efficient nonlinear optical properties. Bruce at al. have described the reaction of alkynyl complexes with cyanoalkenes, as shown in Scheme 142.254,255 While one of the PPh3 ligands in 333a was liberated to form 345, the dppe ligand remained in the coordination sphere to form s-1,3-butadienyl complex 346. These reactions were considered to proceed via the formation of the cyclobutenyl intermediate (Scheme 142D).

364

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

(A)

345 (32%) (B)

346a (95%) (C)

346b (79%)

(D)

Scheme 142

They also examined the reactions with 2-(bis(methylthio)methylene)malononitrile,256 1,2-dichlorohexafluorocyclopentene,257 and bis(2,4-dinitrophenyl) oxalate (Scheme 143).258 Although the products 347 and 349 were formed via complicated skeletal rearrangement, these reactions also displayed nucleophilic nature of the Cb carbons of the alkynyl complexes.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

365

(A)

347 (71%)

(B)

348 (87%) (C)

349 (21%)

Scheme 143

They also examined the reaction of 350, which is an Os analogue of 333, with TCNE. While analogous 1,3-butadienyl complex 351 was formed, they found vinyl-alkynyl complex 352 was also produced in this reaction (Scheme 144A).259 Exclusive formation of the vinyl-alkynyl complex 353 was achieved by the replacement of the PPh3 groups by a dppe ligand; the reaction of 310b with TCNE exclusively produced 353 (Scheme 144B). The yield of vinyl-alkynyl complex increased upon further replacement of the Cp group to Cp (Scheme 144C). These results strongly indicated the greater electron-donating properties of the [Cp Ru(dppe)] moiety, and hence greater nucleophilicity of the Cb carbon, together with the increased steric congestion of the alkynyl moiety suppress the formation of the intermediate [2 + 2]-cycloadduct en route to the Z1-butadienyl isomer. Thus, nucleophilic addition of the Cb carbon, leading to substitution of CN−, takes place. They also demonstrated that the CN group at the Cg carbon in 354a readily undergoes substitution by various nucleophiles, such as H, Me, iPr, nBu, tBu, OMe, NEt2 and PPh2.260

366

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

(A)

350

351 (30%)

352 (22%)

(B)

353 (79%) (C)

354a

336c Scheme 144

Stephan and co-workers applied the nucleophilicity of the Cb carbon placed on the congested [(Ind)Ru(PPh3)2] fragment to frustrated Lewis pair (FLP) chemistry. While the alkynyl complex 333e does not react with [B(p-C6F4H)3], but forms a FLP. When the solution of 333e and [B(p-C6F4H)3] was exposed to CO2, it turned to a vinylidene complex 355a (Scheme 145).261 The combination of the Lewis basic b-carbon with Lewis acid was also applied to CdC bond formation with aldehydes and alkynes.

355a (74%)

333e

355b (63%)

355c (89%) Scheme 145

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

367

Bruce et al. also investigated the reactivity of butadiynyl complex 336d with TCNE (Scheme 146).255,262 In contrast to the reaction shown in Scheme 144, substitution of the CN group did not occur. Instead, the product 354b, via the [2 +2]-cycloaddition at the distal C^C bond, was obtained. They also performed metalation at the terminal CH group. Treatment with metallated 336d with TCNE afforded 356. The [2 +2] addition of TCNE at the outer C^C bond took place at first. Complex 356a would be formed by the subsequent ring opening yielding a butadienyl intermediate, like 354b, and the second addition of TCNE at the C^C bond followed by cyclization and elimination of AgCN. Complex 336e reacts with 7,7,8,8-tetracyanoquinodimethane (TCNQ) similarly to yield 354c (Scheme 147), which contains electron-withdrawing TCNQ-derived ligand. The alkynyl-metal moiety takes on some vinylidene character (shorter MdC, longer MdP, and C^C bonds), while the quinodimethane portion, of which the ]C(CN)2 group can accept charge, becomes more aromatic, as measured by the endocyclic C]C distances.

336a 354b (48%)

356a (41%) Scheme 146

336e

354c (73%)

Scheme 147

Bruce’s group developed the coupling reactions of poly-ynyl complexes, which contain a [Au(PPh3)] end-group, with (iodo) poly-ynes catalyzed by [Pd(PPh3)4]/CuI,263 which enable to extended carbon chains, and they obtained diruthenium complex linked by a C22 chain. They also synthesized a series of poly-ynyl complexes, [Cp Ru(dppe)(C^C)nR] and [Cp Fe(dppe)(C^C)nR] (n ¼ 1  3; R ¼ H, Ph, SiMe3, Au(PPh3)), and they described the effects of carbon chain lengths, end-groups, and central-metal core precisely based on spectral, electrochemical, and structural analyses combined with DFT calculations.264 As shown in Scheme 137A, Low and co-workers demonstrated that a BF−4 salt of 1-cyano-4-dimethylamino pyridinium ([CAP] BF4) reacts smoothly with alkynyl complex to yield a cyanoalkynyl complex. They examined the reaction of butadiynyl complex 333e with [CAP]BF4 and obtained cyanobutadiynyl complex 357 (Scheme 148).265

368

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

333e

357 (44%)

Scheme 148

Recently, Low and co-workers demonstrated interesting reactivity of (4-ethynyl)phenyl acetylido complex 336f. The reaction of 336f with CN+ resulted in the formation of vinylidene complex 356b via the usual electrophilic addition at the Cb atom (Scheme 149).266 However, bulky trityl cation cannot approach Cb, instead added at the terminal carbon, giving a cumulated intermediate. Although the isolated product from the reaction was 354d, due to the reaction with the adventitious water, the structure strongly represents that the electrophilic attack took place at the remote position, and indicate the presence quinoidal cumulene intermediate.

336f

356b (30%)

354d (11%) Scheme 149

7.07.4.5.6

p-Allyl complexes of Ru(II)

p-Allyl complexes of Ru(IV), such as complex 177 in Scheme 74, are regarded as an important reactive intermediate of allylic transformations. While examples are not so abundant, half-sandwich p-allyl complexes of Ru(II) have been synthesized. Given that the p-allyl ligand occupies the two coordination sites, the structure of Ru(II) complexes can be considered as a three-legged piano stool geometry, while that of Ru(IV) complexes is a four-legged one. It is well established that in d6-CpM(allyl) complexes the exo structural form is more stable, while those of d4 complexes adopt an endo-form.267 Owing to the reduced Lewis acidity at the metal center, nucleophilic addition at the allyl ligand in the Ru(II) p-allyl complexes does not take place in general, unlike well-known reactivity of the Ru(IV) counterpart. exo-p-Allyl complexes, 359 and 362, were synthesized by Nakamura’s268 and Komiya’s269 groups as shown in Scheme 150. Elimination of the p-allyl group in 362 as a mixture of 2-allylphenol and 2-propenylphenol upon addition of PPh3 in the presence of acid was observed. Other reactivity of these complexes was not described.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

369

(A)

(B)

Scheme 150

Valerga and co-workers reported deprotonation of the carbene complex 181, shown in Scheme 77, with KOH causes unusual rearrangement, leading to the formation of k4(P,C,C,C)-p-allyl complex 363 (Scheme 151).150 XRD studies of 363 demonstrated that the p-allyl moiety was coordinated to the Ru center in an endo-form. It is likely stemmed from the chelation structure of the ligand, which inhibits the isomerization to the more stable exo-form.

Scheme 151

Tamm and co-workers synthesized exo-p-allyl complexes via the Z5 ! Z3 hapticity interconversion of pentadienyl complexes. They synthesized half-open ruthenocene 364 by introducing a 2-methyl-1-phenylallyl ligand, which behaves like an indenyl group (Scheme 152).270 Namely, a vacant site is generated at a metal center via the hapticity change of the C5 moiety, which is induced by rearomatization of the annulated benzene moiety. XRD studies of 364 clearly represented the bond alternation of the CdC bonds in the C6 moiety (1.365(5), 1.416(5), and 1.351(5) A˚ ). In fact, it was demonstrated that 364 readily reacts with PMe3, CO, and isocyanide, despite its formally coordinatively saturated configuration. This is in marked contrast with related 2,4-dimethylpentadienyl complex, [Cp Ru(Z5-2,4-Me2C5H5)], which does not react with CO even under reflux conditions.271 They also noted that the exop-allyl group in 365a and 365c gradually isomerized to a syn-form, but does not to an endo-structure. Tamm and co-workers also synthesized a benzannulated cyclohexadienyl ligand and obtained complex 366. Although it also reacts with L-type donors to yield exo-Z3-allyl complex 367, it required more forcing conditions, due to the stability of the Z5-coordination of an edge-bridged open cyclohexadienyl ligand in 366.

370

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 152

Tilley and co-workers synthesized an Z3-benzyl complex of Os (368) to prepare neutral silylene complexes. The reaction of 16-electron complex 17 with [Mg(CH2Ph)2](THF)2 immediately provided 368 (Scheme 153A).272 Complex 368 was thermally unstable, and characterization by NMR was hampered by its fluxional behavior in solution. However, the presence of the benzyl group in 368 was confirmed by the following reaction with CO to yield [Cp Os(Z1-CH2Ph)(CO)(PiPr3)]. They succeeded in synthesizing neutral silylene complex 369 by the reaction of 368 with primary silanes, in which one molecule of toluene was liberated. They also attempted preparation of a Ru analogue of 368. The reaction of [Cp RuCl(PiPr3)] (14a) with benzylic reagents failed, however, likely due to the thermal instability of the product. They employed 14b, which contains less sterically demanding PiPr2Me. Due to the reduction of the steric repulsion, Z3-benzyl complex 370 was stabilized and successfully isolated (Scheme 153B).273 The formation of an exo Z3-benzyl group was unambiguously confirmed by XRD. (A)

(B)

Scheme 153

Following this, Tilley’s group examined the reaction of 27e with KCH2Ph to envisage the replacement of the phosphine ligand in 370 by an NHC. However, analogous Z3-benzyl complex was not obtained. Instead, complex 372, containing an NHC tethered Z3-benzyl ligand, was obtained as a result of CdH bond cleavage (Scheme 154).274 Complex 372 displayed dynamic behavior involving site-exchange between the uncoordinated mesityl group and the benzyl group via the hydrogen transfer, which indicates that 372 possesses a character of a masked 16-electron species. In fact, 372 reacts with N2 and lies in equilibrium with the N2 adduct 373, in which the Z3-benzyl group in 372 was transformed to an Z1-benzyl form.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

371

Scheme 154

7.07.4.5.7

Si, Ge, Sn, and Pb complexes

As mentioned in Scheme 9, Nikonov and co-workers synthesized a series of s-silane complexes 24a–e by the reaction of [CpRuCl(PiPr3)] (22).28 In 2005, this group reported the preparation of Cp analogs, [Cp Ru(Z2-HSiR3)Cl(PiPr3)] (375a–e), by the reaction of 14a,275 and they succeeded in synthesizing a series of Z2-silane complexes 374a–e by the reaction of 31 with silanes, in which the phosphine ligand in 24 was replaced by an NHC ligand (Scheme 155).37 The most characteristic feature of these Z2-silane complexes is the short Si⋯ Cl distances (2.851–3.111 A˚ ). They investigated the effects of ancillary ligands to the SidH and Si⋯ Cl interactions. The JSidH values are often used as an indicator of strength of an Z2-coordinated SidH bond. In this regard, Cp analogue 24 exhibits significantly larger JSidH values, in comparison with their Cp analogues 375. This result is explained by the reduced donating ability of the [CpRu] unit to the s (SidH) orbital. Electronegativity of the substituents on the silicon atom also affect the JSidH values. In silane s-complexes, the JSidH value usually decreases upon introduction of electron-withdrawing groups at the silicon atom, which enhances the back-donation to the s (SidH) orbital. However, this general trend was not seen in a series of NHC complexes 374a–e; 374a exhibited the largest JSidH value of 64 Hz. The interligand interaction between Si and Cl atoms changes according to the strength of the Z2-SidH bond. Namely, the complex with the strong SidH interaction has the short Si⋯ Cl contact. XRD studies of 374a revealed the shortest Si⋯ Cl distance (2.851 A˚ ) among these series. The larger JSidH values was explained by the hypercoordinated geometry around silicon atom brought about by the interligand interaction.

Scheme 155

372

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Fürstner and co-workers isolated s-stannane complex 376 (Scheme 156).43 The JSndH value of 192 Hz, significantly reduced from 1535 Hz of the free nBu3Sn, is in accord with the formation of a s-complex. This is supported by XRD showing the SndH distance of 2.15(6) A˚ . In addition, noticeably short Sn⋯ Cl contact (3.202 A˚ ), compared to the sum of the van der Waals radii (4.00 A˚ ), indicates the presence of interligand interaction, like in the s-silane complexes shown above.

Scheme 156

This result strongly implies that such Sn ⋯ Cl interaction play an important role for the regioselective assembly of substrates in the coordination sphere. As seen in Scheme 15, the crucial role of hydrogen bonding of the chloride ligand with protic substituents for the regioselective transformation of alkynes has been disclosed by them. The interligand interaction is subtle, but may also be responsible for the control of the reaction. Nikonov and co-workers also synthesized cationic s-silane complexes (Scheme 157).37,276 Owing to their instability, their molecular structures have not been determined. However, an Z2-coordination of silanes were confirmed by their characteristic JSidH values. In contrast to the neutral complexes 374, the JSidH values of the cationic NHC complexes 378a–e exhibit a general trend, likely due to the absence of interligand interaction. By using cationic s-silane complex 377, they demonstrated very efficient catalytic hydrosilylation of polarized molecules, such as benzaldehyde,276 nitriles,277 and pyridines.278 In particular, [CpRu(PiPr3) (MeCN)2]+ catalyzes the regio selective hydrosilylation of pyridines to N-silyl-1,4-dihydropyridines (Scheme 158). While substitution in the 3- and 5-positions is tolerated, pyridines with substituents in the 2-, 4-, and 6-positions are not reduced. This result indicates that the reaction proceeds by an ionic hydrosilylation mechanism. The enhanced Lewis acidity of the silyl group upon s-coordination to the cationic metal center is responsible for the facile silylium ion transfer at the pyridine, as well as increased hydridic nature of the intermediate induced by electron-donating phosphine ligand.

Scheme 157

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

373

Scheme 158

Tobita and Hashimoto and co-workers reported synthesis of neutral silylene complexes of ruthenium with a three-legged piano stool geometry by the reaction of [Cp RuR(CO)(py)] (Scheme 159).279 Owing to the less steric hindrance arising from the hydrosilylene ligand, complex 379a exhibits rich chemistry, and various derivatives were obtained by the reaction of 379a with nitriles,279a ketones, aldehydes,280 isocyanates, and isothiocyanates (Scheme 160).281 These reactions are initiated by the coordination of a basic atom, N, O, and S, to the Si atom, thus these result indicate the importance of less congested environment around the Si atom, as well as electrophilicity of the silylene ligand.

Scheme 159

374

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 160

The short Si ⋯Si distance in 380-cis (2.4946(12) A˚ ) implies the presence of direct bonding interaction between them. Although the JSidSi value of 33.7 Hz, determined by INEPT-INADEQUETE 29Si NMR measurement of 380-trans, is much smaller than those of disilanes (80  90 Hz), the value strongly indicates the presence of a weak interaction between the Si atoms. Three center interaction among SidHdSi was inferred by DFT calculations, and Wiberg bond index of 0.38 also support the presence of a weak bonding interaction between the Si atoms. In 2003, Tilly and co-worker disclosed that the cationic Ru silylene complex [Cp Ru(]SiHPh)(H)2(PiPr3)]+ (385a) is an effective catalyst for hydrosilylation of alkenes.282 The distinct feature of this novel catalytic system was proved to be direct addition of the silylene SidH bond to an alkene, and requirement of double geminal SidH bond activation of primary silane to form a silylene complex.283 To examine the nature of bonding and reactivity of silylene complexes, they synthesized neutral hydrosilylene complexes, 369 and 371, as shown in Scheme 153.273,274 However, it was shown that these neutral hydrosilylene complexes does not react with alkenes. DFT calculations suggest that their low reactivity was due to the reduced silylium character in neutral complexes in comparison with the cationic analogue (Scheme 161).

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

375

Scheme 161

Tilley’s group also exploited the synthesis of half-sandwich Ru complexes containing a heavier divalent group 14 element. Hydrogermylene complex 386 was synthesized in similar manner to the preparation of 369, namely by the reaction of Z3-benzyl complex 370 with primary germane, (Trip)GeH3 (Scheme 162).273 Remarkably short Ru]Ge distance (2.2821(6) A˚ ) was represented by XRD. In contrast to silylene complexes of Ru and Os, the germylene complex 386 is shown to be thermally stable.

Scheme 162

Since isolable divalent starting materials are available for Sn and Pb, they designed another route to access the stannylene and plumbylene complexes. They employed novel hydrido complex 387 obtained by mixing anionic hydrido complex 244 and 14b.178 Complex 387 reacted with smoothly with [DMPSnCl]2 and [ArTrip2PbBr]2 (DMP ¼ 2,6-dimesitylphenyl, ArTrip2 ¼ 2,6-bis(2,4, 6-triisopropylphenyl)phenyl) to yield corresponding tetrylene complexes 388 and 389, respectively (Scheme 163).284 The most remarkable features of these complexes is the RudEdC angles (RudSndC, 145.5(2) ; RudPbdC, 156.5(2) ), and the Pb complex 389 adopts a nearly T shaped coordination geometry around the Pb atom. This is explained by the periodic trend for the heavier p-block elements to utilize more p character in forming bonds.

Scheme 163

376

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Albertin’s and Álvarez-Pazos’ groups synthesized a series of trichlorostannyl and germyl complexes via the well-known insertion of SnCl2 or GeCl2 into a RudCl bond (Scheme 164).285 They examined substitution of chlorides by various nucleophiles. They noted the importance of the phosphite ligand to stabilize the SnCl3 derivative 390 for the indenyl complexes. Although [CpRu(SnCl3)(PPh3)2] and [CpOs(SnCl3)(PPh3)2] were obtained,285a a stable SnCl3 complex was not obtained from [(Ind)RuCl(PPh3)2]. Trihydrostannyl complex 391 was cleanly synthesized by the reaction with NaBH4, while LiAlH4 was required for the preparation of GeH3 complex 392.

Scheme 164

Reactivity of the SndH bonds in 391 was examined as shown in Scheme 165.285b The SndH bonds were shown to add to the C^C bond of methyl propionate to yield tri(vinyl)stannyl complex 394. The SndH bonds readily undergoes hydrolysis to afford trihydroxystannyl complex 395. While protonation of 391 with triflic acid afford a very unstable dihydride derivative 396, the Cp analogue was shown to be stable at −30  C. The dSnH2(OTf ) structure was confirmed by the 119Sn NMR spectrum recorded −30  C displaying quartets of triplets, due to coupling with the two hydrides of SnH2(OTf ) and the two nonequivalent P nuclei of the phosphines.

Scheme 165

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.4.6

377

Half-sandwich complexes with a Group 15 element

7.07.4.6.1

Dinitrogen complexes

Several half-sandwich Ru complexes possessing a terminal or a bridging dinitrogen ligand have been synthesized, in general, upon halide abstraction in an atmosphere of N2 (Scheme 166).72,73,78,79,83,178,192,274 These complexes are evidence for the formation of highly unsaturated species in solution. While transformations of the coordinated dinitrogen ligands have been reported for early transition metals, such reactivity, unfortunately, has not been observed in the chemistry half-sandwich complexes of Ru and Os. The NdN distances of these complexes lie in the range from 1.09 to 1.13 A˚ , which is only slightly larger than that of the N2 molecule (1.098 A˚ ). This fact indicates that back-donation to the p (N^N) orbital is not significant in these complexes. In fact, these dinitrogen ligands have been shown to readily dissociate from the metal center. Instead, as noted above, this property is useful for generation of coordinatively unsaturated species in solution.

Scheme 166

Albertin et al. obtained diosmium complex 403 containing a m-N2 ligand from phenylazido complex 402a (Scheme 167).286 Formation of 403 was explained by the elimination of nitrene from 402a, leading to the formation of diazene PhN]NPh. The molecular structure of 403 was determined by XRD, which displayed linear OsdNdNdOs skeleton with the NdN distance of 1.116(6) A˚ .

Scheme 167

378

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.4.6.2

Azido- and organicazido complexes

Ruthenium(II)-catalyzed azide-alkyne cycloaddition (RuAAC) reactions were reported in 2005 by Jia and Fokin and co-workers.287 This catalytic process allows the synthesis of 1,5-disubstituted triazoles from azides and alkynes, which displays different regioselectivity from copper-catalyzed azide-alkyne cycloaddition (CuAAC), yielding 1,4-disubstituted triazoles.288 Nolan and co-workers elucidated that RuAAC is effectively catalyzed by the coordinatively unsaturated half-sandwich complex [Cp RuCl(PiPr3)] (14a) (Scheme 168).289 While the [Cp RuCl]4 precursor showed good reactivity, alkoxo complex [Cp Ru(OCH2CF3)(PiPr3)] produced only trace amount of 1,2,3-triazole. Based on DFT calculations, they proposed the mechanism of RuAAC as shown in Scheme 168. Benzylazide is coordinated to the Ru center of the alkyne intermediate through the inner nitrogen atom. This Z1-coordination mode was also exemplified in the Os complex 402a, which was determined based on IR and 15N NMR data.286

Scheme 168

In the absence of alkynes, k2-phospazide 404 was obtained in 55% yield, whose structure was determined by XRD, together with a small amount of tetraazadiene complex 405a (Scheme 169). Complex 405 was alternatively synthesized by the reaction of [Cp RuCl]4 with benzylazide in 56% yield. Although 404 catalyzes the cyclization with similar activity to 14a, DFT calculations suggest that this complex plays only a fleeting role in the catalytic cycle. On the other hand, 405a was shown not to catalyze the reaction.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

379

Scheme 169

In relation to Ru-AAC, reactivity of half-sandwich complex containing an azido ligand with alkynes has been investigated. Rao and co-workers demonstrated that half-sandwich azido complexes of Ru and Os react with activated alkynes and alkene to yield a series of N(2)-bound 1,2,3-triazolate complex, 406 and 407, via [3+ 2] dipolar cycloaddition (Scheme 170).290,291 while the reaction with fumaronitrile gave a 1,2,-triazolate complex via elimination of HCN, tetracyanoethene was shown to react with [CpOs(N3)(PPh3)2] through its C^N bond to afford tetrazolato complex 408.

Scheme 170

380

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Chang et al. examined the [3 + 2] cycloaddition of the Ru-azido complex [CpRu(N3)(dppe)] with various alkynes and showed that the reaction was accelerated by electron-withdrawing substituents (Scheme 171).292 While these reactions also provided N(2)-bound 1,2,3-triazolate complexes 409, they observed formation of another isomer, likely N(1)-bound isomer, in the initial stage of the reaction. The reason for the exclusive formation of the N(2)-bound isomer was explained by a steric argument. They also examined the reaction of triazolate complex with electrophiles, and represented that alkylation takes place at the nucleophilic N(3) position selectively to yield N(1)-bond alkylated triazolate complex. This structure is similar to that proposed for the intermediate of the Ru-AAC reaction shown in Scheme 168. In fact, in the presence of MeI, the N(1)-bound complex 410a was transformed into [CpRuI(dppe)] with eliminating methylated triazoles in 67% yield.

Scheme 171

As shown in Scheme 16, Severin and co-workers demonstrated that [Cp^RuCl]2 (42) reacts with azidoadamantane to yield imido complex 44 via dinitrogen elimination.48 They also examined the reaction of 1a, which is supported by a bulky Cp^group, with benzyl azide. At ambient temperature, complex 1a afford tetraazadiene complex 405b, which is the similar product by the reaction of [Cp RuCl]4.286 On the other hand, when the reaction was performed at 50  C, they found that 1,3,5-triphenyl-2,4-diazapenta1,4-diene was catalytically produced. The Cp analog, [Cp RuCl]4, also produced hydrobenzamide, but much less active. This implies the important role of the steric congestion around the metal center imparted by the bulky Cp^group. Complex 405b reacted further with benzyl azide at 50  C, and produced a mixture of several complexes. They obtained a single crystal from the reaction solution, and XRD analysis revealed the presence of 411, which possesses an NdH benzaldimine ligand (Scheme 172). This finding suggests that the key role of the catalyst is the conversion of azide to imine. In fact, in the presence of H2O, benzaldehyde was produced in the reaction of 1a with benzyl azide, which strongly support the formation of benzaldimine intermediate.

Scheme 172

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

381

Formation of an imine ligand by the reaction with organic azide has been also demonstrated by Albertin et al. While [CpOsBr(PPh3)2] does not react with organic azide, a phosphine-phosphite mixed-ligand complex, [CpOsBr(PPh3){P(OMe)3}], reacts with phenyl and benzyl azide after treatment with AgOTf to yield Z1-azide complex 402 (Scheme 173).283 While phenyl azide complex 402a was transformed to a dinuclear dinitrogen complex 403 as shown in Scheme 167, when the hydrogen atom was present at the a-carbon atom, it converted to Z1-imine complex 412 at room temperature via dinitrogen elimination. Complex 412 was shown to be stable in air and in solution of polar organic solvents.

Scheme 173

Focusing on the remarkable stability of the NdH benzaldimine ligand generated from benzyl azide at a transition metal center, Rhee and Park and co-workers succeeded in developing a one-pot and one-step synthetic method of enamides and a-amido ketones in ionic liquid (Scheme 174).293 They showed that tetraazadiene complex 405b reacts with n-hexylamine to yield bis(ammonia) complex 45 with liberating N-hexylbenzylideneimine (Scheme 175).47 The NH3 ligands in 45 readily dissociate from the metal center, in this respect 45 could be a good precursor for generating unsaturated species. Although complex 45 was not isolated due to its instability, pure solution of 45 was successfully obtained by the reaction of 405b with polymer-bound benzylamine via simple filtration. Complex 45 was shown to react with phenyl azide to provide 405c, which was not directly obtained from the reaction of 1a with phenyl azide.

Scheme 174

Scheme 175

382

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Zheng and co-workers reported intramolecular 1,3-dipolar cycloaddition of the azido ligand to the isocyanide, yielding a mixture of cyanamido complex 414 and zwitter ionic Z6-arene complex 415 bearing a cyanamido group (Scheme 176).294 The reaction of a vinylidene complex with trimethylsilyl azide, which corresponds to the opposite combination to the reaction of azido complex with alkynes, was examined by Sung et al. (Scheme 177).295 In this case, cyclization does not take place, instead nitrile complex 416 was obtained via nucleophilic addition of N−3 at the Ca atom. Complex 416 would be formed by following N2 extrusion and hydrolysis of the SiMe3 group added at the Cb atom.

Scheme 176

Scheme 177

7.07.4.6.3

Diazoalkane and diazene complexes

As shown in Scheme 125, Albertin et al. demonstrated that diazoalkane complexes were obtained by the reaction of [CpsRuCl(PPh3){P(OR)3}] with diazoalkanes in the presence of NaBPh4. They synthesized a series of diazoalkane complexes of Ru and Os supported by Cp, indenyl, or Cp groups, and investigated their reactivities toward alkynes and alkenes (Scheme 178).220,296 All of the diazoalkane complexes undergo dipolar [3+ 2] cycloaddition with acetylene under mild conditions affording 3H-pyrazole derivatives. However, cycloaddition does not take place by the reaction with phenylacetylene; vinylidene complexes were obtained as a result liberation of diazoalkane.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

383

Scheme 178

While the Cp complex reacts with ethylene to yield a [3 + 2] cycloaddition product, 4,5-dihydro-3H-pyrazole complex 418, cycloaddition with ethylene was not observed for other diazoalkane complexes; only substitution by ethylene occurred. Even if it is a Cp complex, cycloaddition does not take place in the bisphosphine analog, [CpRu{NNC(C12H8)(PPh3)2}]. They also found that the Cp complex reacts with H2O to yield Z2-diazene complex 425 and ketone (Scheme 179).220,297 Formation of 425 was explained as due to the nucleophilic attack of H2O on the carbon atom of the coordinated diazoalkane followed by hydrogen shifts based on DFT calculations. This reactivity is in stark contrast to the hydrolysis of diazoalkanes, yielding alcohol and N2. While diazenes are very unstable species, they can be barely stabilized by coordination on a metal center. It is a molecule of possible importance to the inorganic and bioinorganic N2 reduction process. Z1-Diazene complexes, 427 and 429, were synthesized by the oxidation of hydrazine complexes, 426 and 428, respectively (Scheme 180).298

384

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 179

Scheme 180

Hydrolysis of a coordinated diazoalkane highlights a new method of synthesizing this important species. While oxidation of methyl and phenyl hydrazine complexes provide diazene complexes, an unsubstituted diazene complex could not be obtained by the oxidation of either Cp and Cp complexes due to decomposition.

7.07.4.6.4

N,N-Chelate ligands

Rao’s group synthesized various cationic half-sandwich Ru and Os complexes with chelating N,N-donor ligands by the reaction of [CpsMX(PPh3)2] (Cps ¼ Cp, Cp , Ind; MdX ¼ RudCl, OsdBr) with combined heterocycles, such as pyridine, imidazole, pyrazole, and thiazole, and exemplified their coordination mode as shown Scheme 181.299

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

385

Scheme 181

Among them, complexes 441, 442, and 443 contain a non-symmetrical 3,6-bis(2-pyridyl)-4-phenylpyridazine ligand. Thus, this ligand can bind to a metal center through the different pair of nitrogen atoms, and there would produce a mixture of isomers. Nevertheless, only single isomer which adopts the structure shown in Scheme 181 was obtained. On the other hand, [(Z6-C6Me6) RuCl] and [Cp RuCl] fragments were shown to favor the other site.299e

386

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Goswami and co-workers performed CdN bond formation of aniline using Ru(II) templates.300 It is well known that anilines polymerized upon electrochemical or chemical oxidations, leading to p-C(arom)dN bond formation. In contrast, in the coordination sphere of the [CpRu]+ fragment, CdN bond formation occurred selectively at the ortho position, yielding phenylenediamine complex 449. The fact that the reaction required to be carried out in air strongly indicates that some oxidation process should be involved. This is also supported by the result of the reaction of redox inert [(C6H6)RuCl2]2 with aniline, which gave only bis(aniline) complex 450 (Scheme 182).

Scheme 182

7.07.4.6.5

P4 and As4 complexes

Stoppioni and co-workers synthesized half-sandwich complexes of Ru and Os 451a,b bearing a P4 molecule (Scheme 183).301 The coordinated P4 molecule undergoes hydrolysis under mild conditions to yield phosphine complexes 452–454. This reaction is intriguing in view of the well-known stability of elemental phosphorus in water. In contrast, the P4 ligand in the analogous Cp Ru complex, [Cp Ru(dppe)(Z1-P4)]+ (451c), is stable toward H2O even at higher temperature.302 Notably, only the formation of PH3 complex 452 was observed in the hydrolysis of 451a, while the Os analog 451b yields a mixture PH3 and P(OH)3 complexes, 453 and 454.

Scheme 183 31

P{1H} EXSY experiments revealed that complex 451 exhibits a dynamic motion process consisting in a tumbling movement of the P4 cage while remaining coordinated to the metal center (Scheme 184).303 Activation parameters were estimated as shown in Scheme 184, which shows that the exchange rate was slower in the [Cp Ru] analog 451c than those in the Cp complexes, 451a,b. This is likely due to the higher basicity of the Cp ligand, and the difference may affect the reactivity of these complexes toward hydrolysis.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

387

Scheme 184

The authors also reported the synthesis of dinuclear complexes by the addition of a [CpRu(PPh3)2]+ or other metal fragments.304 In contrast to the hydrolysis of 451a, hydrolysis of diruthenium complex 454a proceeded unselective to produce a mixture of several complexes involving 452. Among the hydrolysis products, they isolated m-diphosphine complex 455a in 9% yield. The selectivity for the m-P2H4 complex significantly improved by replacing PPh3 groups by PMe3 (Scheme 185).305 Bridging tetraphosphorus trisulfido complex 456 was also synthesized similarly by the reaction of [CpRu(PPh3)2]+ fragments with P4S3, and they isolated unprecedented thiophosphinous complex 457 upon hydrolysis of 456.306

Scheme 185

Scheer and co-workers reported the first half-sandwich complex of Ru bearing an As4 molecule.307 Z1-As4 complex 458 was synthesized by the reaction of [Cp RuCl(dppe)] with [Ag(Z2-As4)2][Al{OC(CF3)3}4] (Scheme 186). Since the formation of the As4 adduct is unfavorable at ambient temperature, the reaction should be carried out at −30  C. Due to the insolubility of As4 at low temperatures, direct synthesis by the reaction of [Cp Ru(dppe)]+ with As4 was not possible, thus [Ag(Z2-As4)2][Al{OC(CF3)3}4] was used as a soluble As4 transfer reagent for the reaction. While the structure of 458 is isomorphous to 451c, addition of a second cationic ruthenium fragment does not lead to an expected second end-on coordination but to the insertion into an AsdAs bond, yielding 459. DFT calculations showed that this is due to the low AsdAs bond energy and the energetically preferred side-on coordination mode of As4 unlike P4.

Scheme 186

388

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.4.6.6

Reactions at the phosphorus atom

Phosphines are one of the most common ancillary ligands used in the organometallic chemistry. Sometimes reactions unexpectedly occurs at the phosphine ligand, however, as seen in orthometallation that can cause catalyst poisoning. Conversely, the reactivity of the phosphorus atom can be used to modify the phosphine ligand. In this section, reactions at the phosphorus atom is briefly summarized. Morris and co-workers synthesized half-sandwich Ru complex 460 supported by a tripodal phosphine ligand, 1,1,2-tris(diphenylphosphino)ethane (Scheme 187A).308 Facial coordination of this triphosphine, as well as the resulting four-membered metallacycle, MdPdCdP, structure, was confirmed by XRD. Despite its strained structure, 460 did not show reactivity toward air, moisture, H2, and CO. However, 460 was converted to phosphido-bis(diphenylphosphino)ethene complex 461a upon treatment with KOtBu via PdC bond cleavage.309 The trigonal-pyramidal structure of the phosphido ligand was confirmed by XRD, which is contrasted to the trigonal planar structure in the phosphido complex 47 with a two-legged piano stool geometry (Scheme 18).50 Complex 461a undergoes protonation reversibly to afford phosphine complex 462a. The reaction of 461a with dioxygen yielding 463 was also demonstrated. In this reaction, 461a splits O2 in a ligand-based 4-electron reduction to produce an endo epoxide, as well as a phosphinito ligand.310 Phosphido-phosphinoethane complex 461 was alternatively synthesized from the reaction of [Cp RuCl(cod)] with cis-diphenylphosphinoethane followed by the introduction of secondary phosphine in the presence of a chloride scavenger (Scheme 187B). However, an unexpected ligand rearrangement took place that provided complex 461b bearing an unsymmetrical bis(phosphino)ethene ligand, perhaps to result in a less basic phosphido ligand. By using 1,2-bis(diphenylphosphino)benzene, this rearrangement could be suppressed. (A)

(B)

Scheme 187

Complex 461a was shown to be an efficient catalyst of hydrophosphination of acrylonitrile at room temperature with a catalyst loading of 1% (TOF ¼ 60 h−1). The reaction was considered to proceed via a Michael addition mechanism, and the basic phosphido ligand in 461a is responsible for the reactivity (Scheme 188).

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

389

Scheme 188

Phosphinines, phosphorus analogues of pyridine, are aromatic heterocycles that are of interest due to their unique properties that include their p-accepting ability and either Z1 or Z6 coordination modes. Mansell and co-workers examined the reaction of 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine with [Cp RuCl]4 to prepare a half-sandwich complex 465 supported by a novel P2 chelate ligand (Scheme 189).311 The k2-coordination of diphenylphosphino-3-methyl-6-trimethylsilylphosphinine was inferred by the appearance of two doublets at d ¼ 240.1 and 18.2 ppm in the 31P{1H} NMR spectrum with the signal at d 240.1 ppm characteristic of phosphinines. However, 465 was not isolated due to its instability toward moisture; instead, hydrolyzed complex 466 was obtained in which water reacted in a syn manner. The 31P{1H} NMR spectrum of 466 showed considerable upfield shift of the P atom in the six-membered ring to d 11.6 ppm. Facile nucleophilic attack at the P atom suggests that phosphinines can function as noninnocent ligands. Unfortunately, 466 does not exhibit catalytic activity toward transfer hydrogenation of acetophenone.

Scheme 189

This group also synthesized a SiMe2-linked bis(phosphinine) ligand (bis{3-methyl-6-(trimethylsilyl)phosphinine-2-yl}dimethylsilane) and prepared complex 467.312 In contrast to 465, 467 was isolated without hydrolysis. The bis(phosphinine) ligand in 467 was distorted away from the plane defined by RudPdP. XRD showed that the molecular plane of the bis(phosphinine) ligand is nearly coplanar to the plane of the Cp group. They also reported the synthesis of hydrido complex 468 by the reaction of 467 with NaBHEt3. Again, the phosphorus atoms in these complexes resonated in the lower magnetic field region (467: d 272.5, 468: d 286.3). In contrast to 466, complex 467 displayed high activity toward transfer hydrogenation of acetophenone with 0.1 mol% catalyst loading in the presence of 0.5 mol% KOtBu (94% yield, 25  C, 1 h). It is well known that phosphorous trifluoride (PF3) exhibits similar p-acceptor property to CO, however very few applications of fluorophosphines in catalysis have been described, owing to their instability with respect to the redox disproportionation and its high toxicity. Caporali, Peruzzini, and co-workers developed a new method to synthesize fluorophosphine ligands, using the commercially available XtalFluor-E® ([Et2N]SF2]+ BF−4) as the fluorine source, thus avoiding the use of highly toxic and unstable

390

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

fluorinating agents (Scheme 190).313 A series of fluorophosphine complexes was obtained upon treatment of the phosphorous oxyacid complexes [CpRu(PPh3)2{PR(OH)2}]+ with XtalFluor-E.314 While extrusion of PF3 from the metal center was not achieved, the deoxofluorination of Ru-coordinated P(OH)3 to give metal coordinated PF3 represents an easy and safe method to generate Ru-coordinated PF3. They also succeeded in the synthesis of 472 and 474 from the reaction of [CpRu(MeCN)3]+ with Ph(H)P(O)(OH).

Scheme 190

As shown in Scheme 185, Stoppioni and co-workers synthesized half-sandwich complex 457 bearing a thiophosphinous acid, PH2(SH). Although free PH2(SH) is quite elusive due to its instability, the coordinated PH2(SH) was shown to be stable. They showed that the PH2(SH) ligand readily undergoes deprotonation by the non-nucleophilic bases, such as proton sponge, to yield a neutral thiophosphinite complex, [CpRu{P(]S)H2}(PPh3)2] (475) (Scheme 191).315 The coordination geometry and the overall molecular conformation are closely similar to those found for the parent cationic complex containing the PH2(SH) ligand. However, the RudP distance involving the PH2S− ligand in 475 is 0.047 A˚ longer than that formed by PH2SH in 457. This significant metrical change, which occurs on going from 457 to 475, is clearly related to differences in PdS bonding, the PdS bond length in 475 being 0.098 A˚ shorter than that in 457. Protonation of 475 regenerates 457, and complex 476 containing PH2(SMe) was obtained by the reaction of 475 with methyl trifluoromethanesulfonate.

Scheme 191

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.4.6.7

391

Half-sandwich complexes containing water soluble phosphine ligands

Water-soluble ruthenium complexes have attracted considerable attention due to not only their important roles for catalysis in water and biphasic conditions, but also their bioactivities. Peruzzini, Vizza, Romerosa, and co-workers synthesized a series of water-soluble piano stool ruthenium complexes of general formula [CpRuCl(L)(L0 )]n+ (L ¼ PPh3, L0 ¼ PTA, mPTA; L ¼ L0 ¼ PTA, mPTA; PTA ¼ 1,3,5-triaza-7-phosphaadamantane; mPTA ¼ N-methyl-1,3,5-triaza-7-phosphaadamantane), and examined their catalytic activity, as well as their biological activity as mentioned in later section (Scheme 192).316 The cage-like phosphine, PTA, displays much smaller cone angle (y  103 ) than PPh3 (y ¼ 145.3 ), and largely soluble in water (S25 C ¼ 235 mg/cm3) but also in most of the organic solvents. Additionally, the nitrogen atoms in PTA are readily methylated without changing their coordination ability, which would bring about enhanced solubility of the complexes toward water. The solubility of these complexes to water, S25  C, were reported to be as follows; 477 (1.5 mg/mL), 478 (40 mg/mL), 479-OTf (1.1 mg/mL), 480-(OTf)2 (16 mg/mL), 481-(OTf)(Cl) (0.4 mg/mL), 482-BF4 (40 mg/mL), and 483-(OTf)(Cl) (200 mg/mL). They showed that complex 478 catalyzes redox isomerization of 1-octen-3-ol to octan-3-one in water under an air atmosphere at 80  C with 1 mol% catalyst loading (91%, 2 h).317

Scheme 192

While 482 and 483 exhibit good solubility, cationic complexes derived from mPTA 479 and 480 are not water soluble. They examined complexation of the dicationic ligand, dmPTA (1,3-dimethyl-1,3-bis-(azonia)-5-aza-7-phosphatricycle[3.3.1.13,7] decane), which has a similar ligand angle and electronic property but has a greater water solubility. However complex 484 bearing

392

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

a HdmoPTA ligand (3,7-H-3,7-dimethyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) was unexpectedly obtained via the removal of the methylene group between the two NMe groups (Scheme 193).318 The S25  C value (1.7 mg/mL) was comparable to that of monocationic mPTA complexes.

Scheme 193

Frost and co-workers synthesized a series of ruthenium(II) PTA complexes bearing the Cp , Cp, 1,2-dihydropentalenyl, and indenyl ancillary ligands, and investigated their abilities for transfer hydrogenation of a,b-unsaturated carbonyl compounds in aqueous media using formic acid or sodium formate as the reducing agent (Scheme 194).319 In the reduction of cinnamaldehyde by HCOOH, it was shown that the complexes selectively reduce the C]O bond prior to reduction of the C]C bond.

Scheme 194

By the reactions with sodium formate, hydrido complexes, which are regarded as intermediates of the transfer hydrogenation, were obtained and structurally characterized. They also observed the formation of formato complexes by the reaction of [(Cps)RuH(PTA)2] with CO2, although they were not isolated due to facile elimination of CO2. Instead, they obtained dithioformato complexes upon treatment of the hydrido complexes with CS2 via the insertion of CS2 into the RudH bond (Scheme 195).320

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

393

Scheme 195

Tris(aminomethyl)phosphines are the interesting class of phosphines due to their stability in water and toward oxygen, and easily functionalized by preparing from amino acids or water-soluble aliphatic secondary amines. Thus, tris(aminomethyl) phosphines are expected to be potential conjugates with a wide range of biomolecules. Starosta and co-workers tried to synthesize half-sandwich Ru complexes containing a morpholine—and piperazine-linked phosphine. Although expected phosphine complexes were not obtained, complex 490 bearing a secondary phosphine, PH{CH2(NC4H8X)}2 (X ¼ O, NEt), was obtained via the PdC bond cleavage (Scheme 196).321 The structures were unambiguously confirmed by XRD, and the products indicate that this is not the simple substitution. Although the mechanism has not been elucidated yet, DFT calculations suggest that concomitant coordination of two molecules of PPh3 and one P{CH2(NC4H8X)}3 is very unlikely because of substantial steric hindrance around ruthenium center. In the optimized structure, substantial elongation of every PdCH2 bonds was observed. Although mechanistic details were unclear, such elongation may promote PdC bond cleavage.

Scheme 196

7.07.4.7 7.07.4.7.1

Half-sandwich complexes with a Group 16 element Dioxygen complexes

Half sandwich complexes of Ru and Os containing an Z2-dioxygen ligand have been prepared from the reaction of unsaturated 16-electron complexes, generated by dissociation of labile ligand or by the halide abstraction, with O2. Albertin et al. reported that treatment of bis(phosphite) and PPh3-P(OMe3)3 mixed ligand complexes with NaBPh4 under air resulted in the formation of Z2-dioxygen complex 491 (Scheme 197).322 The OdO bond lengths in 491a (1.4045(13) A˚ ) and 491b (1.4104(13) A˚ ) imply that they are Ru(IV)-peroxo complexes, supported by DFT calculations.

394

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 197

Similarly, several dioxygen complexes have been synthesized and structurally determined as shown in Scheme 198 (except for 497).73,76,83,163,164,323–328 These complexes displayed similar OdO distances, ranging from 1.371 to 1.416 A˚ , which implies that these complexes are Ru(IV)-peroxo complexes, like 491. Notably, complex 505 is the first example of Z2-O2 complex of osmium with pentamethylcyclopentadienyl (Cp ) as a supporting ligand.

Scheme 198

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

395

While those side-on peroxo complexes of [Cp Ru] and [Cp Os] are stable in both solid state and in solution, complex 494,164 497,73 and 498,325 which are supported by a Cp or an indenyl group, were shown to be unstable. These results imply that the Z2-peroxo ligand is effectively stabilized by the strong electron donating Cp group. Albertin et al. investigated the reactivity of the Z2-peroxo complex 491 with alkenes and alkynes with the aim of the transfer of dioxygen to the organic substrate, however only substitution of the Z2-O2 ligand by the unsaturated hydrocarbons took place. In the presence of triflic acid, they found that 491a was converted to triethylphosphate complex 506 via intramolecular oxygen transfer to the phosphite ligand (Scheme 199A).324 (A)

(B)

Scheme 199

Ogo and co-workers demonstrated that the injecting protons and electrons into the peroxide complex 500 initiates CdH bond cleavage to generate the tetramethylfulvene complex 507 (Scheme 199B).327 Although the yield was low (5%), they demonstrated that the peroxo complex 499, which was formed by the reaction of [Cp Ru(bpy)(OH2)]+ with O2, oxidize guanosine monophosphate to 8-oxo-guanosine monophosphate.326

7.07.4.7.2

Thiocarbonato and thiocarbamato complexes

Thiocarbonates are an important class of compounds for the removal of heavy metals from waste water due to the ability of coordinating wide range of transition metals. Although some di- and trithiocarbonate complexes of [CpsRu] fragments have been reported, the monothiocarbonate complexes are scarce. El-khateeb et al. reported the synthesis of monothiocarbonate complex of ruthenium 508 according to Scheme 200, namely by the reaction of the sulfhydryl complex with chloroformate.329 XRD studies showed that the monothiocarbonato ligand attached to the metal center in a k1(S) coordination mode. Upon treatment with chlorinated solvents, the thiocarbonate ligand readily dissociates to yield [CpRuCl(PPh3)2].

Scheme 200

Dithiocarbamates are versatile ligands, capable of forming complexes with all the transition elements and able to stabilize in a most range of oxidation states. While dithiocarbamates usually adopt a chelate coordination mode with the both S atoms in the [S2CNR]− skeleton, a few cases of monodentate dithiocarbamates are known. Lalrempuia et al. examined the reaction of

396

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

[CpOs(PPh3)2(MeCN)]+ with Na[S2CNEt] (Scheme 201A).330 Formation of a k1(S)-dithiocarbamate complex 509 was inferred based on the NMR and IR spectra, 509 could not be isolated due to the complexity of separation from the starting material. In contrast to the reaction of the Os complex, k2(S,S)-dithiocarbamate complex 510a was obtained by the reaction of [CpRuCl(PPh3)2] with Na[S2CNEt] (Scheme 201B), which possesses almost the same structure as [CpRu(PPh3)(k2-S2CNnPr2)] (510b) reported by Hogarth et al.331 Ghosh and co-workers reported the synthesis of trithiocarbonate complex 511 by the reaction of [CpRuCl(PPh3)2] with LiBH4 followed by addition of CS2 (Scheme 201C).332 Although the mechanism was not fully understood, formation of the k2(S,S) trithiocarbonate ligand was confirmed by XRD. (A)

(B)

(C)

Scheme 201

7.07.4.7.3

Half-sandwich Ru complexes bearing dithiolene and other sulfur containing chelates

The reaction of [CpRuCl(PPh3)2] with the bis(dithiolene)Ni complex [Ni(S2C2Ph2)2] in refluxing toluene produces the cationic dithiolene complex [CpRu(S2C2Ph2)(PPh3)]+ (512),via transfer of the dithiolene ligand from Ni to Ru (Scheme 202).333 Due to the redox-active nature of the dithiolene ligand, its complexation can affect the redox property of the metal center. Although 512 is formally a complex of Ru(IV), a considerable contribution of the Ru(II) dithioketone canonical form was implied from the 13C NMR chemical shift of the dithiolene dC]Cd carbon atoms at d 199.0 ppm. Chemical reduction of 512 led to the color change from purple to blue, and exposure of this solution to the air resulted in regeneration of 512. These facts suggest the formation of neutral complex 513 upon reduction, although 513 was not isolated.

Scheme 202

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

397

The authors also examined the dithiolene transfer reaction using [CpRuCl(CO)2]. Unlike the preparation of 512, this reaction requires the addition of ONMe3 to remove CO. Due to instability and its paramagnetic nature, complex 514 was not fully characterized, but its formation was suggested by the intense n(CO) peaks at 1976 cm−1. The isolated product was dinuclear complex 515 containing a bridging dithiolene ligand, whose structure was determined by XRD. El-khateeb et al. examined the reaction of [CpRuCl(PPh3)2] with pyrimidine- and pyridine-2-thiolate, and obtained half-sandwich Ru complexes 516 supported by a N,S-chelating ligand (Scheme 203).334 Due to the k2(N,S)-chelation, one of the PPh3 ligand was eliminated. Complex 516 smoothly reacts with CO to yield k1(S)-sulfido complex 517. When [CpRuCl(dppe)] was used, only k1(S)-sulfido complex was obtained because of the strong coordination of the dppe ligand.

Scheme 203

This group also examined the complexation of 5-membered N-heterocyclic thiolates.335 In contrast to pyridine-2-thiolate, 2-imidazolyl-, 1-methylimidazolyl-, 5-methyl-1,3,5-thiadiazolyl-, and 5-methyl-4H-1,2,4-triazolyl-thiolate attached to a metal center only through the S atom; k2-coordination was not observed in these ligands, likely due to difficulty of the formation of four-membered metallacycles (Scheme 204).

Scheme 204

Leong, Goh, Webster, and co-workers investigated the coordination chemistry of tris- and bis(N-methyl-2-mercaptoimidazol-1-yl)borate, [HB(mt)3]− and [H2B(mt)2]− on the [Cp Ru] fragment. The [HB(mt)3]− acts as a facial S3-donor, and is a soft analogue of tris(pyrazolyl)borates. The tridentate k3(S,S,S) coordination of the [HB(mt)3]− ligand was confirmed in both Ru(III) and Ru(II) complexes, 519A and 520A (Scheme 205).336 In CD2Cl2 solution, another species attributable to 520B, in which the [HB(mt)3]− adopts a k3(H,S,S) coordination through the agostic BdH bond was present. Although the k3(H,S,S) coordination of the [HB(mt)3]− ligand was not directly observed in the Ru(III) complex, its formation was inferred by the CV analysis. An agostic interaction of a BdH bond was unambiguously confirmed in 521, which was prepared by the reaction of [Cp Ru(m-OMe)]2 with Na[H2B(mt)2].337 As mentioned in Scheme 106, Ghosh and co-workers also elucidated k3(H,S,S)-coordination through a BdH bond in complex 253.185

398

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 205

This group also examined the reaction of [Cp Ru(m-OMe)]2 with Li[HB(py)(mt)2], in which one of the mt pendants was substituted by a pyrazolyl group (Scheme 206).328 Ligation with the pyrazolyl group was not observed, instead the k3(H,S,S)coordination through a BdH bond was also elucidated in 522. Although the k3(N,S,S)-coordination was not observed, the Z2-BdH bond is readily dissociate to generate a vacant site, as seen in the isomerization between 520A and 520B. In fact, they observed reversible coordination of dioxygen, leading to the formation of Ru(IV)-peroxo complex 504 with a k2(S,S)-coordinated [HB(pz)(mt)2] ligand. Complex 522 also reacts with CO smoothly to yield a carbonyl complex irreversibly.

Scheme 206

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

399

Goh, Webster, and co-workers reported unique reactivity of half-sandwich Ru(III) complex 523 bearing a dithiolate-thioether ligand (Scheme 207A).338 Alkylation of one of the S atom in 523 induced SdS bond formation between two molecules of 523, yielding dinuclear Ru(II) complex 524. Accompanied by alkylation, the Ru centers were apparently reduced by 1. This reaction is a rare example of alkylation-induced redox processes resulting in SdS bond formation, reminding biological systems. The dinuclear complex was obtained as a mixture of two diastereomers, among which molecular structure of 524-(R,R) (or (S,S)) was determined by XRD. The oxidation of 523 with I2 also resulted in the SdS bond formation, yielding 525 (Scheme 207B). These SdS bond formations were formed via the dimerization of the Ru-thiyl radical. Formation of a thiyl radical was confirmed by the result that the reaction in the presence of acrylonitrile provided 526 bearing a 1,4,7-trithiacyclononane ligand. Similar S-alkylation-induced reactions were also demonstrated by the reaction of half-sandwich Ru complex supported by a k3(N,S,S)-3-azapentane-1, 5-dithiolato ligand.339 (A)

(B)

Scheme 207

Nishibayashi and co-workers synthesized the half-sandwich Ru complex 527 supported by [PhP(C6H4-o-S)]2−. They used 527 as a building block for the construction of bis(m-sulfido) complexes, and they synthesized dinuclear complexes 528 by the reaction with [Cp RuCl2] (Scheme 208).340

Scheme 208

400

7.07.4.8

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Anticancer activities of half-sandwich Ru complexes with a cyclopentadienyl ligand

Following the early success of Ru(III) compounds NAMI-A and KP1019 (Scheme 209), Ru(II) complexes were soon evidenced as stable, suitable anticancer drug candidates.341 Half-sandwich Ru complexes supported by an Z6-arene ligand, such as RAPTA-C, have been intensively studied, and exhibit good anticancer activities both in vitro and in vivo. Generally, ruthenium complexes display lower toxicity toward healthy cells compared to platinum complexes, and possess ability to mimic iron binding to a variety of biological molecules. In contrast, only a small numbers of studies have been reported for the related CpRu family. Meggers and co-workers demonstrated that the [CpRu(L)] fragment (L ¼ CO, PMe3) with a pyridocarbazole ligand act as a strong and selective inhibitors of protein kinase, which suggests the possibilities of [CpRu] derivatives as potential anticancer drugs.342

Scheme 209

Due to their wide range of accessible oxidation states and facile substitution of “legs” in piano-stool complexes of the CpRu fragment, a number of half-sandwich Ru complexes for study as anticancer drugs have been synthesized. The most notable drawback of half-sandwich Ru complexes are their poor water solubility, a condition that has to be fulfilled to allow for both efficient administration and transport through living organisms. As seen earlier, however, this issue can be overcome by introducing water-soluble co-ligands. In this section, results of biological assays of half-sandwich Ru complexes will be briefly summarized. Dyson and Severin and co-workers synthesized water-soluble PTA Ru complexes 529 supported by a bulky Cp^ ligand (Scheme 210).343 Using cisplatin as a benchmark, the in vitro anticancer activities of 529a,b and the reference compound [CpRuCl(PTA)2] (478) were assessed on the human ovarian cancer cell line A2780 and its cisplatin-resistant analogue A2780cisR. 529a,b are shown to be remarkably cytotoxic toward both cell lines, displaying activity similar to cisplatin, whereas the Cp analogue did not show any antiproliferative effects. Increased lipophilicity stemmed from the Cp^ ligand would be responsible for the high activity of 529.

Scheme 210

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

401

Romerosa and co-workers evaluated the antiproliferative activity of the water-soluble Ru complexes bearing thioether-amine (531), thiopurine (530 and 532), adenine (533 and 534), guanine (535 and 536), and theophylline (537 and 538), anticipated to enhance the activity of the parent complexes (Scheme 211).344 The complexes were tested for cell growth inhibition activity on two human cancer cell line: the cisplatin sensitive T2 and the cisplatin resistant SKOV3. Complex 532b shows an activity comparable with cisplatin on T2 and no activity on SKOV3. Complex 534 displays the highest activity against T2 cell line as well as the cisplatin-resistant SKOV3 cell line, better than that for cisplatin. Other complexes showed a poor bioactivity. These results indicate that complexes containing two PTA (530, 533, 535, and 537) are less active, likely related to the difficulty for hydrophilic PTA complexes in crossing the lipophilic membranes of the cell. The notable results with 532b and 534 are probably due to the favorable hydrophilicity/lipophilicity balance. In particular, remarkable antiproliferative activity of 534 is assumed to be its capability of forming strong hydrogen bonds through the adenine-atoms with the DNA-purine bases, making the interaction mechanism different than that for starting complex.

Scheme 211

This group synthesized the cationic half-sandwich Ru complex 539 containing a HdmoPTA ligand via abstraction of chloride by Ag(OTf ). By replacing the proton between the nitrogen atoms to transition metal ions, they prepared bimetallic complexes 540 and 541 (Scheme 212).345 These complexes exhibit excellent cytotoxicity toward six human solid tumor cell lines with the IC50 values in the nanomolar ranges, and the Ru/Zn complex 541 displays a much better activity (26–426 times) than cisplatin. Notably, 541 showed to be 3–8 times more active against the tumor cell lines than against the tested non-tumor cell line, BJ-hTert, indicating its large selectivity vs. tumor cells. The observed larger antiproliferative activity is due to its composition as bimetallic complex: ZnCl2 is not active in any studied cell lines.

402

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 212

High cytotoxicity with the IC50 values in the nanomolar range have been also reported for other types of half-sandwich Ru complexes. Garcia, Moreno, and co-workers reported that a series of cationic half-sandwich Ru complexes containing a N-heteroaromatic ligand cause a significant effect in cell viability (Scheme 213).346 These results demonstrated that Ru(II) three-legged piano stool complexes possessing planar N-heteroaromatic ligands exhibit high potential antitumor activity on several human cancer cell lines with IC50 values generally lower than those of cisplatin. In particular, 546a and 546b are very effective against the highly glycolytic cell lines MCF7 and MDAMB231.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

403

Scheme 213

Florindo, Fernandes, and co-workers synthesized a series of half-sandwich Ru complexes with a bio-derivative carbohydrate, and D-glucose derivative ligands (Scheme 214). Cell viability of colon cancer HCT116 cell line was determined for a total of 23 organometallic compounds, and they found that complexes 548 and 549, which contain the galactose and fructose nitrile derivative ligands, respectively, exhibit high cytotoxicity comparable to oxaliplatin.347

D-ribose, D-xylose, D-galactose,

Scheme 214

A series of CpRu complexes bearing anastrozole or letrozole were synthesized by Castonguay and co-workers.348 They found that the solubility and the stability can be highly affected by their type of backbone. Complex 550, which contains anastrozole, is the only complex found to be stable in cell culture medium. Although complex 550 approximately equally affected the healthy cell line

404

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

(non-cancerous MCF12A cell line), 550 displayed a high cytotoxicity against all the cancer cell lines with IC50 values lower than 1 mM (Scheme 215). Exposure of zebrafish embryos to complex 550 at concentrations around its in vitro cytotoxicity IC50 value (0.1  1 mM) did not lead to noticeable signs of toxicity over 96 h, making it a suitable candidate for further in vivo investigations.

Scheme 215

McGowan and co-workers showed that half-sandwich Ru complexes with a [CpRu(dppm)]+ fragment gave nanomolar IC50 values against normoxic A2780 and HT-29 cell lines.349 They also tested the activity under a 0.1% O2 concentration, and showed that [Cp RuCl(dppm)]+[SbF6]− displayed high activity with an IC50 value of 0.55  0.03 mM against hypoxic HT-29 cells.

7.07.5

Four-legged piano stool complexes and penta- and hexahydrido complexes of osmium

The four-legged piano stool geometry is characteristic of high valent complexes of M(IV). While a number of complexes with a four-legged piano stool geometry are known for cyclopentadienyl complexes, those of Z6-arene complexes have been quite limited. Only four [(arene)M] complexes with a four-legged piano stool geometry have been structurally characterized so far; [(Z6-arene) RuH2(SiR)2],350 [(p-cymene)OsH2(SiPh3)(IPr)]+, and [(p-cymene)OsH2(Bcat)(IPr)]+ (Scheme 216).351 This fact implies the crucial role of cyclopentadienyl ligand to stabilize the metal center in higher oxidation states efficiently and geometrically flexible nature of the [CpsM] fragment. Half-sandwich complexes of a four-legged piano stool geometry are considered to be important intermediate in a number of catalysis. They are generated by the oxidative addition to a M(II) center, and increased Lewis acidity at a metal center promotes reductive bond formation around the coordination sphere or direct nucleophilic addition to a coordinated substrate on a metal center.

Scheme 216

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.5.1

405

Polyhydrido complexes

Apart from their potential synthetic utility, ruthenium trihydrides [Cp RuH3(L)] (L ¼ PR3,78,352 AsPh3, SbPh3,353 N-heterocyclic carbenes84,354) are also of interest because of their propensity to manifest quantum-mechanical exchange coupling (QMEC). In this context, the effect of the co-ligand L on the QMEC has been intensively investigated.355 Preparation of the Cp analogue [CpRuH3(PPh3)] (551a) was originally reported by Davies et al. from the reaction of [CpRuCl(PPh3)2] with LiAlH4.356 Through this reaction, 551a was obtained as a mixture with [CpRuH(PPh3)2]. Nikonov and co-workers reported the improved synthesis using [CpRu(PR3)(MeCN)2]+ and obtained a series of trihydrido complex 551a–d selectively (Scheme 217).357 They also synthesized the NHC supported trihydrido complex [CpRu(H)3(IPr)] (276) as shown in Scheme 117.206

Scheme 217

The Os analogue [Cp OsH3(L)] (553), was synthesized by Girolami and co-workers by the reaction of [Cp OsBr2]2 with 2-electron donors (L ¼ AsPh3, PPh3, PCy3, PEt3), followed by the treatment with NaBH4 (Scheme 218).358 Unlike the hydride ligands in the Ru analogues, those in 553 are rigid. Based on the observed small JHdH values (230 ms) and JHdD (0 Hz) values, complex 555 was described as a classical hydrido complex. Girolami and co-workers examined the protonation of 555 and obtained the Os(VIII) pentahydrido complex 556 (Scheme 219).362 The minimum T1 value of 556 was estimated to be 375 ms which indicates that 556 is a classical hydrido complex. The reaction of 555 with tBuLi in the presence of N,N,N0 ,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (pmdeta) produced a salt of [Cp OsH4]− [Li(pmdeta)]+ (557), which is soluble in aromatic hydrocarbons. They also reported that diosmium tetrahydrido complex 558 was formed by the photolysis of 555 in benzene.

Scheme 219

Oxidative addition of H2 to coordinatively unsaturated species is a versatile method to obtain hydrido complexes. However, it was reported that [Cp RuCl(PR3)] (14) does not react with H2,24 while the Os analogue [Cp OsBr(PiPr3)] (17) reacts with H2 to yield a dihydrido complex.26 Tilley and co-workers synthesized [Cp RuH3(IXy)] (559) by the reaction of cyclometallated complex 373 with H2 with eliminating N2 (Scheme 220).84 As shown in Scheme 65, Xie and co-workers obtained dihydrido complex 160 by the reaction of carboranyl tethered complex 159 with dihydrogen in the presence of phosphine via unusual switching from the RudC(cage) to the RudB(cage) s-bond.128

Scheme 220

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.5.2

407

Hydrido complexes containing Si, Ge, Sn, and Pb

Nikonov and co-workers synthesized half-sandwich Ru complex 560 bearing a silyl group by the reaction of 551 with HSiMe2Cl (Scheme 221).366 Complex 560 is an analogue to [Cp RuH2(SiMe2Cl)(PR3)] (561a, R ¼ Ph; 561b, R3 ¼ iPr2Me), which was synthesized by them previously,367 and they evaluated the influence of the ancillary ligand on the interligand hypervalent interaction (IHI).

Scheme 221

XRD studies of 560a–c revealed that the chloride ligand on the silicon atom is located approximately trans to one of the hydrides, which is a typical feature for all complexes with IHI. The SidCl bond length in 560c (2.164(1) A˚ ) is slightly larger than that in 560a (2.158(5) A˚ ), which contains a less basic PPh3 ligand. In general, the magnitude of IHI increases depending on the basicity of the hydrides due to the overlap between the s(MdH) bonding orbital and s (SidCl) antibonding orbitals placed opposite to a hydride. Thus, replacement of the Cp group by the less electron donating Cp group is thought to decrease IHI. In fact, the SidCl distance in the Cp analogue 561b (2.170(7) A˚ ) is larger than that in 560c (2.164(1) A˚ ), but the difference is rather small. While IHI was observed in 560a which bears a PPh3 ligand, however, the corresponding Cp analogue 561a does not exhibit IHI. XRD studies showed that the silyl group in 561a does not adopt an optimum geometry due to the steric repulsion of the Me groups with the bulkier Cp groups, while this is allowed in 560a. This result indicates that the strength of IHI depends not only on the electronic factors but also on the steric ones, and this suggests that even less basic hydrides can exhibit IHI by tuning its steric environment. They also synthesized NHC supported complexes, 562a–e (Scheme 222).205 XRD data suggest that the replacement of phosphine ligand in the [CpRu(PiPr3)] fragment for the more electron releasing NHC results in the strengthening of the RuH ⋯ Si interactions.

Scheme 222

As seen in Schemes 7 and 129, hydrosilanes readily react with coordinatively unsaturated species with a two-legged piano stool geometry, yielding silyl complexes with a four-legged piano stool geometry via oxidative addition of an SidH bond. Tilly and co-workers planned to synthesize terminal silylene complexes from the reaction of 373, which can act as a masked 16-electron species as seen in Scheme 220. Although reductive elimination of CdH bond occurred in the reaction with H2, CdH bond formation does not take place in the reaction with primary silanes, and silyl-hydrido complex 563 was obtained (Scheme 223).274 DFT calculations suggest that the Mes- and Trip-substituted silylene complexes are likely to be in equilibrium, however, due to the congestion around the metal center which encourages a decrease in the coordination number. In fact, treatment of 563c with acetophenone and phenol resulted in the formation of 564 and 565, respectively, which strongly indicates the presence of the silylene intermediate (Scheme 224). In particular, formation of 564 clearly represents the occurrence of oxidative addition of a CdH bond of acetophenone. While the formal oxidation state of the metal center in 563c is +4, oxidative addition took place at the Ru(+2) center induced by the polarization of the RudSi bond upon coordination of the carbonyl compound. Similar reaction of the neutral silylene complex 379a was reported by Tobita, Hashimoto, and co-workers (Scheme 225).280

408

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 223

Scheme 224

Scheme 225

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

409

The reaction of 563c with an equimolar amount of AgOTf resulted in the formation of a AgOTf adduct 566. The molecular structure of 566 determined by XRD is described as a distorted four-legged piano stool, reminiscent of the structurally characterized cationic silylene complex [Cp Ru(H)2(]SiHMes)(PiPr3)]+ (385b).283 In contrast to the neutral silylene complex [Cp Ru(m-H)2 (]SiHTrip)(PiPr2Me)] (371a), a significant interaction between the silicon atom and the hydride was observed in 566 (JSidH ¼ 36.6 Hz). Addition of Lewis acidic Ag(+1) results in greater electrophilicity at the silicon atom, which causes the strong interaction with the hydride. The JSidH value was smaller than that observed for [Cp Ru(m-H)2(]SiHMes)(PiPr3)]+ (62.3 Hz), which may reflect the weaker acidity of neutral AgOTf compared with proton. Nolan and co-workers synthesized a series of silyl-hydrido complex 568 supported by a indenyl ligand (Scheme 226).368 They examined catalytic ability of 568 toward borylation of 2-phenylpyridines with low catalyst loading (1.5 mol%). While 568b–e displayed lower activity, 568a was shown to catalyze the reaction very efficiently. The reaction seems to be initiated by the elimination of Et3SiH. Thus, the silyl group seems not to contribute to the catalytic cycle, but smooth elimination is crucial to generate an active species.

Scheme 226

Stradiotto and co-workers examined the reaction of the 18-electron complex 81, which can readily generate unsaturated [Cp Ru {k (P,N)-C9H6}]+ species, with primary silanes (Scheme 227).369 A base-stabilized silylene complex 569 was smoothly obtained via initial oxidative addition of an SidH bond followed by a-H transfer from Si to Ru and migration of the N-donor fragment from Ru to Si. The relatively short Ru]Si (2.262(2) A˚ ) and long SidN distances (1.955(4) A˚ ) are characteristic of those of the base-stabilized silylene complexes. The double geminal SidH bond activation (silylene extrusion) is the key step in the proposed Glasser-Tilley hydrosilylation mechanism; this is the first documented example of the direct formation of Ru]Si species from primary silanes. The zwitterionic complex 570 was obtained upon deprotonation of the indene backbone of 569. Unlike [Cp Ru(m-H)2(]SiHMes)(PiPr3)]+, neither 569 nor 570 reacted with 1-hexene and styrene. 2

Scheme 227

410

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Tilley and co-workers synthesized a series of cationic silylene complexes 385, some of which involves an SdH bond, by the abstraction of the triflate in 16 with [Et3Si][B(C6F5)4] (Scheme 228).25 The two hydride ligands in 385b are observed to be equivalent and exhibit a strong coupling to the silicon atom (2JSidH ¼ 58.2 Hz), which implies the increased Lewis acidity at the silicon atom in the cationic complex. In contrast, complex 385f that possesses a secondary silylene ligand does not exhibit significant RudH ⋯ Si interaction, presumably due to unfavorable HdRudSidC torsion angles.

Scheme 228

As mentioned in Scheme 162, the hydrogermylene complex 386 is stable and does not react with alkenes upon heating. However, it reacts with degassed H2O to yield hydroxygermyl complex 571 quantitatively (Scheme 229).273 Labelling experiment utilizing D2O suggests that 571 is formed via concerted addition of an OdH bond across the Ru]Ge bond.

Scheme 229

Similar reactivity was also observed for the Os stannylene complex 572 (Scheme 230).370 The stannylene complex 572 was shown to isomerize to metallostannylene complex 574 at 60  C. Although 119Sn NMR measurements of 574 was unsuccessful, disappearance of the SnH resonance at dH ¼ 19.4 as well as appearance of equivalent hydrido signal at dH ¼ −14.01 (2H) strongly indicate the formation of the metallostannylene complex. While 1,2-hydrogen migration from Si to Ru was elucidated by Stradiotto and co-workers as shown in Scheme 267, complex 574 was proposed to be formed via radical mechanism.

Scheme 230

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

411

Subsequently, this group succeeded in isolation of the ruthenostannylene complex 576 supported by an NHC ligand (Scheme 231).371 XRD studies revealed that 576 features a Ru-bound, two-coordinate Sn center with a RudSn bond of 2.660 (1) A˚ and a RudSndCipso bond angle of 106.1(2) . Kinetic studies and DFT calculations showed that transformation of 575 to 576 proceeds in an intramolecular manner, and unusual a-H migration involves a reversal role of the coordinated stannylene ligand, from that of an electron donor to an acceptor in the transition state, thereby lifting the usual requirement for generation of an unsaturated metal center in migration chemistry. The ruthenostannylene complex 576 exhibits rich chemistry. Complex 576 reacted with dibenzoyl to give 577 while stanene complex 578 that possesses a Sn]C bond was synthesized by the reaction with diazoalkanes. In these complexes, the [Cp Ru(H)2(IXy)] fragment provides steric and electronic stabilization to the Sn centers. Ruthenostannylene and ruthenoplumbylene complexes 579 and 580 was directly obtained from the divalent starting materials by using anionic dihydrido complex 244 with (Scheme 232).284

Scheme 231

Scheme 232

7.07.5.3

p-Allyl complexes of Ru(IV) and Os(IV)

Ru-catalyzed allylic transformations have attracted significant interest due to the recognized regioselectivity in favor of branched products when starting from allylic precursors of the type RdCH]CHdCH2dX (X ¼ chloride, acetate, carbonate). The most commonly used catalyst precursor contains a [Cp Ru] fragment (e.g., [Cp Ru(MeCN)3]+) and various co-ligands, such as chelating nitrogen and phosphine ligands, can improve the reaction rate as well as regioselectivity. In all these reactions, a p-allyl complex of Ru(IV) is thought to be a common intermediate. Since allylation reactions catalyzed by CpsRu(IV) species have been extensively reviewed,372 this section is focused on the property of the p-allyl complexes. Esteruelas, López, and co-workers examined alkylation of a vinylidene complex of Os, 581, and obtained osmacyclopentadiene complex 582 (Scheme 233).373 Complex 582 isomerized to the Os(II) p-allyl complex 583 upon heating. Since 582 and 583 result from competitive CdH bond activation processes in the alkenyl intermediate formed by the nucleophilic addition of Me− at the a-C atom of the vinylidene ligand, the formation of 582 is in agreement with the kinetic preference of the CdH arene activation over the CdH alkyl, while the formation of 583 agrees well with the higher stability of a M(Z3-allyl) bond with regard to a M-aryl bond. As expected from the theoretical studies,267 the p-allyl ligand in 583 adopts an exo form.

412

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 233

Transformation to an endo form was observed upon protonation of 583, which yielded Os(IV) p-allyl complex 584. The exo-endo interconversion is thought to proceed via the Z1-allyl intermediate. Complex 584 undergoes further isomerization to 585a, in which the hydride occupies the cisoid disposition of the CPhH group of the allyl ligand. This thermodynamic preference implies the substantial steric hindrance between the CPhH group and PiPr3 ligand at the cisoid position in 584. The exo-p-allyl complex 585 was alternatively synthesized by the reaction of Os(IV) hydrido complex 314 with internal alkynes (Scheme 234).374 Since the allyl ligand was formed via the insertion of alkyne into an OsdH bond, the kinetic product was an anti-exo-p-allyl complex 586, which converted to syn isomer 585 upon heating. When the reaction was conducted with an excess amount of alkyne, alkenylphosphine complex 587 was obtained via the elimination of the allyl group as alkene accompanied by dehydrogenation of one of the isopropyl group. At 50  C, g-(Z3-allyl)-a-alkenylphosphine complex 588 was formed via the cycloaddition of alkenyl phosphine and alkyne.

Scheme 234

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

413

Pregosin and co-workers synthesized a series of Ru(IV) p-allyl complex 589, and investigated their behavior in solution (Scheme 235).10 While only endo isomer was observed for 589b, both endo and exo isomers were observed for 589a and 589c. The structures of 589b-endo and 589c-endo were confirmed by XRD, the structures of exo-isomers were confirmed by 2D-NOESY measurements. In addition, these isomers were shown to interconvert within the NMR time scale.

Scheme 235

Diffusion NMR studies revealed that 589-PF6 and dicationic complex [Cp Ru(Z3-PhCHCHCH2)(MeCN)2]2+ (590-(PF6)2) behave as well-separated ions in polar solvents. Nevertheless, 1H, 19F HOESY spectroscopy suggests that one of the two PF6 anions is fairly close to the allyl ligand in the dicationic complex 590, which would influence on the selectivity of allylation. In fact, the dicationic complex 590 performs Friedel-Crafts type allylation chemistry, rather than simple O-, N-, or C-allylation (Scheme 236).375

Scheme 236

7.07.5.4

Miscellaneous

Kadish, Fukuzumi, Sessler, and co-workers synthesized porphycene complex 591 containing a [Cp Ru] fragment accommodated in the central N4 core (Scheme 237).376 XRD studies showed that the [Cp Ru] fragment is bound to the macrocycle in an outof-plane coordination mode with the metal centers displaced from the plane described by the N4 core by ca. 0.88 A˚ . The porphycene macrocycle is distorted, displaying a significant deviation from the planarity, with the pyrrolide units tilted up to accommodate the metallic fragment. Nevertheless, pronounced upfield shift of the Cp signal in the 1H NMR spectrum (dH ¼ −1.17 ppm) suggest the strong ring currents in the porphycene core. The formal oxidation state of the Ru center is +4, and this is consistent with the four-legged structure. Notably, analogous reactions with porphyrins does not afford similar half-sandwich complexes, presumably due to the lower flexibility of the porphyrin skeleton. When the [Cp Ru]+ unit was treated with Ni(II) or Cu(II) metalloporphycenes, the complex 592 was obtained in which the [Cp Ru]+ fragment is directly coordinated to one of the pyrrolide subunits.

414

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 237

Introduction of a [Cp Ru]+ fragment into a N-confused porphyrins is performed by Yoshizawa, Ishida, Furuta, and co-workers (Scheme 238).377 While unsubstituted phenyl groups were employed, the [Cp Ru] fragment was coordinated to the phenyl ring to form an Z6-arene complex. In contrast, introduction of bulky 3,5-di-tert-butylphenyl groups at the meso-positions led to complex 593 with a “sitting atop” coordination mode. Similar to 591, the Ru(IV) ion is accommodated at the pyrrolic NNNC core, however, 1,3-positioned methyl groups of the Cp moiety were covalently connected to the meso-5,15-positions of the N-confused porphyrin skeleton, yielding an N-confused calix[4]phyrin Ru(IV) complex.

Scheme 238

Leong, Goh, and co-workers planned to synthesize novel Ru(IV) complexes by using oxidative addition of disulfides to the Ru(III) center. They successfully obtained a series of Ru(IV) Z2-dithiolato complexes 594 and 595 by the reaction of [Cp RuCl2]2 with tetraalkyldithiuram disulfide, isopropylxanthic disulfide, and bis(thiophosphoryl) disulfide, respectively, via oxidative addition of an SdS bond (Scheme 239).378 The chlorides in 594a undergo further substitution with dithiolate anion to yield bis(dithiocarbamato) complexes 596, while the similar reaction with isopropylxanthic disulfide resulted in the formation of mixed dithiocarbamato-dithiocarbonato complex 597a via the loss of isopropyl group (Scheme 240). Similar reactions were also

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

415

observed for isopropyl xanthate complex 594c. While the reaction of 594a with phosphines resulted in the simple substitution of one Cl ligand by phosphine, the reaction of 594c with PPh3 resulted in the displacement of both chlorido ligands and the reduction of the metal center to the Ru(II) oxidation state. This difference in reactivity can be ascribed to the difference in chelating ligands: while the dithiocarbamato ligand has an ability to stabilize high oxidation state via extensive electron delocalization, higher donor ability of the xanthate ligand facilitates a redox reaction at the metal center.

Scheme 239

Scheme 240

416

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.6

Metallocenes and arene complexes of ruthenium and osmium

7.07.6.1

Ruthenocenes and osmocenes

In general, ruthenocenes are synthesized from [RuCl3
nH2O] and cyclopentadiene under reducing conditions. Unsymmetrical ruthenocenes can be synthesized from [CpRu(MeCN)3]+ and [Cp RuCl]4 with cyclopentadienide or fulvenes. Osmocenes are prepared from M2[OsX6] (M ¼ Na, K; X ¼ Cl, Br) under similar conditions for the preparation of ruthenocenes. In particular, the half-sandwich Os complex [Cp OsBr2]2362a has been shown to serve as a good precursor for unsymmetrical osmocenes.379 Owing to their coordinatively saturated 18-electron configuration, these complexes are chemically stable in general. The robust nature of metallocenes allows direct functionalization of cyclopentadienyl groups by way of common electrophilic substitution reactions, such as Friedel-Crafts acylation and lithiation. Thus, a number of functionalized metallocenes have been synthesized so far for various applications. In 2014, López and co-workers reported a novel CdH bond functionalization method of ferrocene and ruthenocene by gold catalysts with viny- and aryldiazo compounds (Scheme 241).380 Although the reaction of ruthenocene required more forcing conditions than ferrocene due to ruthenocene’s lower reactivity toward electrophiles, the cationic gold complex [Au(IPr)(MeCN)] (SbF6) afforded functionalized ruthenocenes in moderate yields with vinyl- and aryl-diazoacetate upon heating at 50  C. While benzylic CdH bonds of the substituted ferrocenes are readily oxidized, substituted ruthenocenes are shown to be stable owing to their less oxidizable nature than ferrocenes.

Scheme 241

It has been shown that ferrocene reacts with hexafluoroacetone in refluxing octane to afford 2-hydoxy(hexafluoro)propyl substituted ferrocene by Woodward and co-workers.381 While this reaction with ferrocene takes five days, Metzler-Nolte and co-workers reported the improved synthetic method by using microwave irradiation enabling the synthesis of the trifluoromethylated ruthenocene 602 (Scheme 242).382

Scheme 242

7.07.6.1.1

Applications to functional materials

Ruthenocenes or half-open ruthenocenes, such as [(Z5-C5H4R1)(Z5-C5H4R2)Ru] (R1 ¼ R2 ¼ H; R1 ¼ R2 ¼ Et, R1 ¼ iPr, R2 ¼ H) and [(Z5-C5H4Et)(Z5-dmpd)Ru] (dmpd ¼ 2,4-dimethylpentadienyl), are used as precursors for the ruthenium deposition by CVD (chemical vapor deposition) and ALD (atomic layer deposition). Lang and co-workers synthesized a series of substituted ruthenocenes 603–609 for the precursors of MOCVD (metal-organic chemical vapor deposition) (Scheme 243).383 All complexes evaporate without decomposition at atmospheric pressure. Deposition experiments onto Si/SiO2 were carried out with a cold wall CVD reactor. Layers grown by complex 608 and 609 showed a very smooth surface morphology.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

417

Scheme 243

Chung and co-workers attempted the synthesis of novel Ru(0) complexes for the precursors of ALD from the reaction of [RuCl3
nH2O] with 6,6-dimethylfulvene; however, substituted ruthenocenes 607 and 608 were unexpectedly obtained by the nucleophilic attack of the solvent molecules (Scheme 244).384 Complexes 610 and 611 displayed excellent volatility and stability in their TG analysis and sublimation studies, proving their potential as precursors in thin-film deposition techniques. Among them, 611 demonstrated the best properties with a clean single-step TGA curve and good volatile character.

Scheme 244

Ruthenocenes 612a and 612b which contain rigid and relatively well conductive biphenyl ethynyl backbone terminated by a sulfur atom were synthesized by Lang, Zharnikov, and coworkers (Scheme 245).385 These complexes act as precursors for the fabrication of active molecular template. Complexes 612a and 612b were assembled on an Au(1 1 1) surface, and the resulting films were characterized by HRXPS and NEXAFS spectroscopy. Both protected and deprotected thiol molecules were shown to construct the corresponding self-assembled monolayers (SAMs).

Scheme 245

Swarts and co-workers synthesized metallocenylaldehydes 613 and metallocenylcarboxylic acids 614 (Scheme 246).386 These functionalized metallocenes were covalently anchored on the amine-capped Si(1 0 0) surface via imine or amide bond. This allowed formation of nano metallocenyl islets with diameters from 40 to 100 nm and heights of up to 12 nm on the planar support surfaces was confirmed by AFM and SEM measurements.

418

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 246

7.07.6.1.2

Metalloligands

Application of metallocenes to metalloligands have attracted considerable attention because of the structural diversity of the introducing Z5-MLn fragments which serve as a control element in tuning the steric and electronic properties. By introduction of two (or more) donating groups to the cyclopentadienyl groups in a proper arrangement, a second metal fragment, which acts as an activation site, is readily incorporated. Several examples of bimetallic complexes containing a metallocene unit are depicted in Scheme 247.387 The reactivity of individual bimetallic complexes will not be described here, but it has been often shown that the metalloligands act as an electro-withdrawing group as compared to the corresponding non-metallated ligand. Metalloligands with different donating groups, as seen in complex 616, are of particular interest because of their planar-chirality.

Scheme 247

Koridze et al. reported unusual transformation of (P,C,P)-pincer complex 621, in which the ruthenocenyl moiety exhibited non-innocent nature (Scheme 248).388 Complex 621 reacts with H2 upon treatment with NaBArF4 to give 622. Notably, dihydrogen adds to the RudC bond in a trans fashion. Complex 622 reacts with CO accompanied by the elimination of dihydrogen and fulvene-like complex 624 was produced. This reaction involves the migration of the Ru-hydride to the cisoid position with respect to the cyclopentadienyl CdH bond, likely via reversible RudC bond cleavage.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

419

Scheme 248

7.07.6.1.3

Chiral metallocenes

Baird, Budzelaar, and co-workers synthesized a series of phosphonium-1-indenylide ligands (PHIN) and the corresponding Ru complexes (Scheme 249).389 DFT calculations suggest that the PHIN-Ru bond strengths are greater than the corresponding benzene-Ru bond strength in [CpRu(Z6-C6H6)]+. This is consistent with the 31P NMR data of 625c, which displayed two sharp signals stemming from the diastereotopic PMe2 groups. This observation indicated that interconversion of enantiomers via interfacial exchange of the Z5-bound ligands does not occur and complex 625 possesses stable planar chirality.

Scheme 249

Kündig and co-workers reported highly enantioselective synthesis of planar chiral ruthenocenes via a Pd-catalyzed asymmetric hydrogenolysis (Scheme 250).390 While a considerable amount of over reduced product 628a was formed, the Cp complex (SP)627a was obtained in 72% yield with 96% ee. The remaining bromide in (SP)-627 can be replaced by various functional groups, such as phosphine, pyridyl, carboxylic acid, and oxazoline which are used as planar chiral metalloligands.391 For example, 5-pyridylindenyl complex (SP)-629 was shown to react with [Pd(OAc)2] to form six- and five-membered palladacycles, (SP)-630 and (SP)-631, in a controlled manner (Scheme 251).

Scheme 250

420

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 251

The three component Betti condensation has been applied to ruthenocenecarboxaldehydes as an aldehyde component together with (S)-phenylethylamine and 2-hydroxynaphthalene (Scheme 252).392 The obtained aminomethylnaphthol of Ru 632 can be readily transformed to the corresponding dihydrooxazine 633. The absolute configuration of the stereogenic center was determined by NOESY experiment as well as XRD analysis. The obtained chiral aminomethylnaphthol 632 was tested as a precatalyst for the enantioselective addition of Et2Zn to aldehydes providing secondary alcohols up to 88% ee.

Scheme 252

Nozaki and co-workers successfully synthesized optically pure mono- and bis-helicene ruthenocenes by using enantiopure 9H-cyclopenta[1,2-c:4,3-c0 ]diphenanthrene (LH), which has a rigid helical shape comprised of seven fused rings (Scheme 253).393 According to the X-ray structures, the helical part was aromatized and worked as an Z5-ligand like Cp. Upon heating at 122  C, epimerization of the [7]helicene ligand occurred gradually, and gave an equilibrated mixture of (M,M)-, (meso)-, and (P,P)-634, with a high racemization barrier (DG{400K ¼ 33.9 kcal mol−1). The obtained helicene complexes showed large optical rotations and strong responses in CD.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

421

Scheme 253

7.07.6.1.4

Ruthenocenophanes and osmocenophanes

It has been shown that strained ring-tilted [1]ferrocenophanes yield well defined organometallic polymers via ring-opening polymerization (ROP).394 The propensity toward ROP is directly related to the inherent ring strain of a [1]metacenophane. The release of ring strain and relaxation back to the coplanar arrangement of the cyclic ligands had been shown to be the driving force for ROP. Müller and coworkers synthesized two new ruthenocenophanes 635a and 635b, bridged by aluminum and gallium, respectively (Scheme 254).395 Strain involved in the metallocenophanes is indicated by the tilt angle a, which is the angle between the two planes of the tilted cyclic ligand. The tilt angles in 635a,b are estimated to be a ¼ 20.3 and 20.9 , respectively, and these values are considerably larger than those observed in the corresponding Fe analogues (14.3 and 15.8 ). Although these complexes displayed large a values, neither 635a and 635b underwent ROP.

Scheme 254

As mentioned in Scheme 66, one of the Cp ligand in dicarba[2]ruthenocenophane 161a undergoes ring slippage from the Z5- to the Z1-coordination mode upon treatment with dmpe due to its strain. The a value of 20.6 indicates that 161a involves considerable strain in its structure. Manners and co-workers attempted ROP of dicarba[2]ruthenocenophane upon heating. Thermolysis of 161b at 240  C, however, led to the formation of 1,10 -disubstituted ruthenocene 636, via homolytic CdC bond cleavage in the bridge (Scheme 255).396 Cyclic voltammogram of 161a implied the formation of dicationic dimer in solution. In fact, dicationic dimer 637 was obtained by chemical oxidation.397 The presence of a RudRu bond (2.969(7) A˚ ) was unambiguously confirmed by XRD, which is a rare example of an unsupported Ru(III)dRu(III) bond.

422

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 255

Prosenc, Heck, and co-workers synthesized naphthalene-bridged ansa-ruthenocene 638 (Scheme 256).398 While the molecular structure of 638 was determined by XRD, the authors mainly discussed the synthesis, structure, electrochemical, and magnetic properties of naphthalene-bridged ansa-nickelocenes.

Scheme 256

Braunschweig et al. prepared the (N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine) (pmdta) adduct of dilithiated ruthenocene and osmocene. They synthesized disila[2]osmocenophane 639 and distanna[2]metallocenophanes 640 (M ¼ Ru) and 641 (M ¼ Os) by the reaction with Me4Si2Cl2 and tBu4Sn2Cl2, respectively (Scheme 257).399 Distanna[2]metallocenophanes 640 and 641 reacted with S8 or Se to afford the chalcogen inserted products 642 and 643, respectively. This insertion of chalcogen into the SndSn bond was accelerated in the presence of pmdta.

Scheme 257

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

423

The use of metallocenes, particularly, ferrocene and ruthenocene, as an electrochemically active “reporter group” has been explored so far. Srinivasan and co-workers reported the first synthesis of ansa-ruthenocene-based cyclic[2]pyrrole 645 and calix[1]Ru [2]phyrin 646, containing metallocene units in the backbone of the calixpyrrole frame work by acid catalyzed condensation (Scheme 258).400 Changing the aryl substituted precursor 644a to methyl substituted 644b at the meso-position has been shown to assist the expanded macrocycle formation.

Scheme 258

Latos-Graży nski and co-workers synthesized ruthenocenothiaporphyrin 648 via [3 +1] macrocyclization reaction of 1,10 -bis [phenyl(2-pyrrolyl)methyl]ruthenocene and 2,5-bis[hydroxy(p-tolyl)methyl]thiophene (Scheme 259).401 Derivatives of 648 wereobtained by the subsequent protonation (648-H+ and 648-H22 +) and reduction (648-H2). They also synthesized ruthenocenoporphyrin 649-H by the reaction with pyrrole. They observed that the chemical shifts of the 1H signals of the cyclopentadienyl moiety shift largely depending on the aromatic and antiaromatic nature of the macrocycles, which means that these ruthenocenoporphyrinoids possess a p-conjugated surface which adopts topologically distinct states because of a metallocene hinge.

Scheme 259

424

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

7.07.6.1.5

Heavy ruthenocenes

The replacement of skeletal carbon atoms in the Cp ligand by heavy group 14 atoms is intriguing in term of aromaticity of these heavy cyclopentadienyl groups. Since the syntheses of heavy ruthenocenes [Cp Ru{Z5-C4Me4GeSi(SiMe3)3}] and [Cp Ru {Z5-C4Me4SiSi(SiMe3)3}] were reported by Tilley’s group (Scheme 260),402 several other unique heavy ruthenocenes have been synthesized.

Scheme 260

Sekiguchi and co-workers synthesized complexes 650 and 651 that feature more than one heavy group 14 element in one of the cyclopentadienyl ligands by the reaction of [Cp RuCl]4 with corresponding lithium cyclopentadienides (Scheme 261).403 The Z5-coordination of these heavy cyclopentadienyl groups was unambiguously confirmed by XRD which implies p-delocalization within the heavy cyclopentadienyl rings. Cyclic voltammogram analysis of these complexes suggests that these ligands exhibit rather strong p-donating ability than the Cp group.

Scheme 261

Saito and co-workers synthesized heavier ruthenocenes 653 supported by a stannacyclopentadienyl group (Scheme 262).404 The reaction of [Cp RuCl]4 with dilithiostannoles led to the formation of anionic complex 652 with ruthenocenes were produced by the following reaction with electrophiles. While the stannacyclopentadienyl group in 653a–c is coordinated to the Ru atom in an Z5-fashion as seen in the sila- and germa-cyclopentadienyl analogues that in 653d adopted a folded structure and the stannacyclopentadienyl group is coordinated to the Ru atom in an Z4-fashion. Complex 653d is regarded as a complex comprise of a [Cp Ru]+ fragment coordinated by a butadiene and anionic tin moiety. DFT calculations of 653a indicate that the lone pair on the tin atom has large p-character that contributes to the HOMO which can efficiently interact with the ruthenium moiety to afford Z5-coordination. In contrast, in the chloro derivative 653d, the negative chlorine atom enhances the s-character of the lone pair on the tin atom which cannot participate in Z5-coordination. This tendency is more pronounced in the lead analogue 655.405 While the deviation from the planarity is smaller in the iPr complex 655a (33.0 ) than that in the chloro complex 655c (44.9 ), all of the complexes possess a folded cyclopentadienyl ligand and adopts an Z4-coordination mode.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

425

Scheme 262

7.07.6.2

h6-Arene complexes

7.07.6.2.1

Preparations of cationic arene complexes

[CpsRu(Z6-arene)]+ complexes are generally air-, water-, and heat-stable organometallic sandwich compounds that have been extensively studied by chemists of all disciplines which serve as precursors to [CpsRu]+ species that are known to catalyze a variety of organic transformations. Owing to the arenophilic nature of the [Cp Ru]+ fragment, several procedures for the synthesis of simple [Cp Ru(Z6-arene)]+ compounds have been known, but requires organic solvents. Holman and co-workers reported improved synthetic methods of [Cp Ru(Z6-arene)]+ complexes from [Cp RuCl]4 proceeding aqueous media (Scheme 263).406 These microwave (MW)-assisted protocols (i) are near quantitative, (ii) involve aqueous routes, and (iii) are applicable to nearly any arenes, including water-soluble aromatic amino acids. MW irradiation shortens reaction time from 1–3 days to 10–30 minutes. This can avoid thermal degradation of [Cp RuCl]4 that can get in the way of high yields.

Scheme 263

Taking advantage of its stability toward water, Kudinov and co-workers used the [CpRu]+ fragment for selective labeling of tyrosine and phenylalanine residues of small peptides (Scheme 264).407 Complex 2 was shown to react selectively with L-phenylalanine to yield p-complex 656a in the presence of most natural amino acids, which indicates high affinity of the [CpRu]+ fragment to the aromatic moiety against amino- and carboxyl groups. In the presence of sulfur-containing amino acids, however, the yields decreased to ca. 30%, presumably due to the concurrent coordination of ruthenium at the sulfur atom. The irradiation of angiotensin II derivative (Sar-Arg-Val-Tyr-Ile-His-Pro-Leu) with 2 (4 equiv) in D2O resulted in near-quantitative formation of complex 656f while reaction with the larger peptide, lysozyme (129 amino acids) gave no p-complexed products. Probably hydrophobic aromatic residues are hidden inside the peptide globule. This simple approach to the Ru p-complexes of aromatic amino acids and small peptides under the conditions in water at room temperature in the presence of oxygen and biological substances would allow further application for the peptide labeling.

426

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Scheme 264

As shown above as well as in the Introduction, [CpRu(Z6-naphthalene)]+ (2) can be an excellent precursor for the generation of the [CpRu]+ fragment because of its stability and lability of the Z6-naphtalene ligand. For the preparation of 2, however, it requires the use of large excess of naphthalene (10 equiv) and long reaction period under harsh conditions (140  C, 3 days). Kündig and co-workers reported the improved synthesis of 2 from ruthenocene, also by using microwave (Scheme 265).408 Addition of Al powder (50 mol%), AlCl3 (2 equiv), and TiCl4 (50 mol%) was required, among which TiCl4 serves to trap Cp to yield titanocene dichloride. MW shortens reaction time from 3 days to 15 minutes. This synthesis can be applicable to various arenes.

Scheme 265

Kudinov and co-workers synthesized [(Z6-C6H6)OsCl2]2, which is used for versatile starting material for the preparation of a variety of organoosmium compounds, by using micro wave irradiation in an excellent yield (97%) (Scheme 266).409 The reaction time was remarkably reduced to 3 h as compared to 100 h by refluxing in ethanol. They also reported that the reaction of [(Z6-C6H6)OsCl2]2 with CpTl in longer period (24 h) afforded [CpOs(Z6-C6H6)]+ in 75% yield which was improved compared to the previous results (17%).

Scheme 266

Williams, Lin, Jia, and co-workers reported the reaction of [(p-cymene)OsX2]2 (X ¼ Cl, Br, I) with 3-tert-butyldiazoindene leading to the formation of indenylidene complex 657 (Scheme 267A).410 The stability of 657 is affected by the halide X. Complex 657-I is stable enough to be isolated and stored in solutions for weeks without decomposition. Complex 657-Cl was readily rearranged to Z5-indenyl complex 658 which is similar to the reaction of Ru indenylidene complex shown in Scheme 45. Substituted Z5-fluorenyl complex [(Z5-9-etoxyfluorenyl)Ru(p-cymene)]+ (659) was obtained similarly by the reaction of [(p-cymene)RuCl2(PiPr3)] with 9-diazofluorene, while the reaction with its phosphite analogue [(p-cymene)RuCl2{PPh2(OMe)}] gave diazoalkane complexes (Scheme 267B).411

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

427

(A)

(B)

Scheme 267

The arenophilic nature of the [CpsRu]+ fragment can be used for investigation of the properties of aromatic moieties. In particular, for polycyclic aromatic molecule, it is possible to clarify the properties of the molecule by investigating the selectivity of where the [CpsRu]+ coordinates. Various unique molecules containing a pendant [CpsRu]+ moiety have been synthesized and structurally determined as shown in Scheme 268.412

Scheme 268

428

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Lapinte and co-workers demonstrated that [Cp Ru(MeCN)3]+ does not react with ethynylnaphthalene in the absence of coordinating solvent, such as MeOH and acetone. In the presence of small amount of methanol (1–2 mol%), however, it began to react to yield a mixture of Z6-naphtalene complexes 665a and 665b in which the ratio was highly influenced by the solvent, temperature, and reaction time (Scheme 269).413 While 665b was the thermodynamically favorable product, with well-defined concentrations of methanol it was possible to prepare specifically complexes 665a. It was proposed that the C^C unit acts as a regio-directing group for the formation of 665a.

Scheme 269

In anthracene, there are two sites for the Z6-coordination of the [CpRu]+ fragment: the terminal and the central ring. Coordination of transition metals at the terminal ring of unsubstituted anthracene is favored both kinetically and thermodynamically. DFT calculations suggest that introduction of methyl groups at the 9,10-positions and methoxy groups at the 2,3,6,7-positions of anthracene increased the energy of the metal interaction with the central ring (E ¼ −95.7 kcal mol−1 for the central ring vs. −92.3 kcal mol−1 for the terminal ring). Karslyan, Perkalin, and co-workers demonstrated that the anthracene complex 666b in which the Ru atom binds to the central ring is obtained exclusively upon thermolysis of the kinetic product 666a in the presence of acetonitrile (Scheme 270).414 In contrast, in the absence of acetonitrile 666a was not converted to 666b which suggests that this interconversion proceeds via the intermediate formation of [CpRu(MeCN)3]+ rather than through intramolecular Ru shift.

Scheme 270

Letelier, Saillard, Vega, Hamon, and co-workers examined the ligation of the [Cp Ru]+ fragment to pyrene, acenaphthylene, and fluoranthene (Scheme 271).415 While complexes 667 and 669 were obtained as a single isomer from the reaction with pyrene and acenaphthylene, respectively, the reaction with fluoranthene gave a mixture of 668a and 668b in a 9:1 ratio. NMR experiments did not show any evidence for intramolecular haptotropic shift even after heating at 90  C in solution or at 120  C in the solid state for very long periods of time.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

429

Scheme 271

In this way, various kinds of cationic arene complexes have been prepared. Mochida and co-workers synthesized a series of cationic arene complexes [CpRu(Z6-arene)]+ and they demonstrated that these complexes are colorless and their melting points change according to the chain length introduced at arene moiety as well as counter anion. By utilizing their stability toward light, air, and heat, they applied these cationic complexes to organometallic ionic liquids and investigated in detail effects of the structure of the arene complexes, such as chain lengths, substitution patterns, and heteroatoms in the chain, on the property of ionic liquids produced.416

7.07.6.2.2

Catalytic SNAr reactions

Nucleophilic aromatic substitution (SNAr) is one of the building blocks of synthetic chemistry. One method by which unactivated aryl halides can undergo SNAr is via Z6-coordination to a transition metal (e.g. Cr(0), Mn(I), Ru(II), Rh(III)), which increases the reactivity toward nucleophiles. Walton et al. demonstrated that [CpRu(p-cymene)]+ catalyzed SNAr reaction of chlorobenzene (Scheme 272).417 However, it required harsh conditions (180  C). While formation of a substituted arene complex is rapid, spectroscopic studies implied that the arene exchange process is the rate limiting step. To promote the transformation of the coordinated arene from the Z6- to Z4-mode, they used tethered cyclopentadienyl ligand. In fact, remarkable acceleration in arene exchange from p-cymene to C6Me6 was observed for 670, which bears a pyridine-tethered Cp. However, the rate of SNAr was not significantly increased in the reaction of chlorobenzene with morpholine.

Scheme 272

430

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

They also demonstrated novel trifluoromethylation of nitro- and cyanobenzene using Me3SiCF3, facilitated by Z6-coordination to the [CpRu]+ fragment which proceeds under mild conditions (Scheme 273).418 While cyanobenzene gave only cyclohexadienyl complex, in which the CF3 group was introduced at the next position to the CN group, like 672, nitrobenzene afforded a mixture of 671 and 672 in a 1:1 ratio. Trifluoromethylated arenes were extruded from each isomer in nearly quantitative yield, and quantitative stepwise reactions to construct a catalytic cycle were elucidated, but catalytic reactions are not performed.

Scheme 273

Grusgin and co-workers reported the first p-coordination-catalyzed nucleophilic fluorination of unactivated aryl halides (Scheme 274).419 Chlorobenzene reacts with CsF in the presence of [Cp Ru(Z6-naphthalene)]+ (2).

Scheme 274

7.07.6.2.3

Metalloligands

As seen in the ruthenocene, arene complexes of ruthenium have been also widely used as metalloligands. Several bimetallic complexes comprise a [CpsRu] unit are shown in Scheme 275.420–422 Owing to the arenophilic nature of the [CpsRu]+ fragment, most of bimetallic complexes have been synthesized by the ligation of the [CpsRu]+ fragment to the aromatic moiety of a monometallic complex. In contrast to the bimetallic complex comprising of ruthenocene, ligation of the [CpsRu]+ fragment brings a net +1 charge to the complex, which increases the Lewis acidity at the metal center, hence impacts on the reactivity. Le Lagadec and co-workers succeeded in obtaining enantiopure complexes, (RP)-688a/(SP)-688a and (RP)-689a/(SP)-689a, by recrystallization using enantiomerically pure D-tris(tetrachloro-1,2-benzenediolato)phosphate(V) chiral anion (D-TRISPHAT).422c

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

431

Scheme 275

Similar to the synthesis of (S)-627 shown in Scheme 251, Kündig and co-workers applied Pd-catalyzed asymmetric hydrogenolysis to the preparation of planar chiral naphthalene complex (S)-691 (Scheme 276).390 Due to the cationic charge of 691, lithiation of the naphthalene ligand did not succeed, however transition-metal catalyzed cross-coupling reactions are shown to be applicable to the functionalization.423 By the Pd-catalyzed PdC coupling reaction, phosphine substituted complexes (S)-693, as well as Au complexes 694 supported by the planar chiral metalloligand 693 were synthesized.

Scheme 276

432

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Amouri and co-workers demonstrated that reaction of (S)-1-(2-chlorophenyl)ethanol with [Cp Ru(CH3CN)3]+ provided the single diastereomer (SP)-[Cp Ru{Z6-(S)-1-(2-chloropheny)ethanol}]+ ((SP,S)-695) (Scheme 277).424 The other diastereomer (RP,R)-695 was synthesized by the reaction with (R)-1-(2-chlorophenyl)ethanol. They also synthesized planar chiral Au complexes, (SP,S)-696 and (RP,R)-696, by way of introduction of a phosphine group into the aromatic ring.

Scheme 277

7.07.6.3

Anticancer activities of ruthenocenes and arene complexes

During the last several decades metallocene derivatives, particularly those based on ferrocene, have attracted considerable attention as chemotherapeutic agents and antibiotics owing to the findings of ferroquine as an antimalarial drug (Scheme 278) which is active against Plasmodium falciparum cell lines resistant to chloroquine. The ferrocifens were shown to inhibit proliferation of both hormone-dependent and hormone-independent forms of breast cancer. In relation to ferrocenes, the biochemical activity of ruthenocenes has also attracted considerable attention due to their stability toward air and moisture, and more importantly due to their tolerance for introduction of various functional groups at the cyclopentadienyl moiety. In this section, antitumor activities of ruthenocenes, osmocenes, and cationic arene complexes are briefly summarized.

Scheme 278

Leong, Top, and co-workers synthesized a series of Fe-, Ru-, and Os-based tamoxifen derivatives and examined their antiproliferative effects against two breast cancer cell lines, hormone-dependent (MDA-MB-231) and hormone-independent (MCF-7) (Scheme 279).425 The results showed Ru and Os complexes were much less active than the Fe analogues. Ru and Os complexes behaved very similar ways and found heightened cytotoxicity for their tamoxifen-like complexes 698c and 699c but mono- and diphenol complexes were without toxicity. The results indicate that the side-chain is the origin of a major part of the toxicity. The metal center may play a role, particularly on the acidity of the phenol moiety in the 1e-oxidized intermediates, formation of the quinone methides, and reversibility of the metallocene moiety.

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

433

Scheme 279

The cytotoxic activity of various ruthenocenes against various cancer cell lines line has been investigated (Scheme 280).382,426–429 Scheme 280 tabulates the IC50 values for selected cell lines. None of these complexes exhibited the IC50 value in the nanomolar range. The activities of ruthenocenes were similar to those of the iron analogues in most cases, but marked differences between Ru and Fe were observed in complexes 702 and 705. While 702-Ru was more active toward the MCF-7 cell line than the iron analogue, 705-Ru was much less active toward MIA-Pa-Ca-2 cell line than 705-Fe.

Scheme 280

434

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

Although this is not the effect only by the ruthenocene, Darkwa, Elmroth, and co-workers reported that Au/Ru bimetallic complexes 706 and 707-Ru showed cytotoxicity against HeLa (Human adenocarcinoma of the cervix) cells comparable to cisplatin (Scheme 281).430 Notably, the observation that the activity of 707-Ru is superior to 707-Fe suggests that the metallocene moiety plays the crucial role for the cytotoxicity. Although the IC50 values of these Au/Ru complexes against W2 chloroquine resistant strain of the malaria parasite Plasmodium falciparum are low compared to chloroquine (CQ), 708-Ru was shown to exhibit antiHIV behavior superior to the well-known antiviral agent AZT (30 -azido-20 ,30 -dideoxythymidine).

706 707 707 708 706

707

708

Scheme 281

Kudinov and co-workers examined cytotoxicity of cationic Z6-naphtalene complexes 710a,b in which phenylalanine and tryptophan residues were linked to the cyclopentadienyl group (Scheme 282).431 The activities were shown to be comparable to that of cisplatin.

709

710 710

710a 710b

Scheme 282

Williams’s group introduced the [Cp Ru]+ fragment to various substituted arenes, and synthesized a series of cationic Z6-arene complexes 711a–r (Scheme 283).432 They examined cytotoxicity of these complexes against MCF-7, MDA-MB-231, and MM96L cell lines. They demonstrated that cytotoxicity depends on the substituent on the arene ligand, as well as counter anion which control the solubility and transport of the cation, and cytotoxicity activity was found to increase with size and lipophilicity of the attached aromatic ligand. It has been also shown that amides and carboxylic acid were less active, presumably due to less lipophilicity arising from hydrogen bonding. Among the series of arene complexes, complexes having hydrophobic polycyclic aromatic groups, naphthalene (711p), phenanthrene (711q), and pyrene (711r), achieve the highest levels of growth inhibition, and exhibit excellent IC50 values in the nanomolar range (0.35 mM).

Mono- and Bis-cyclopentadienyl Complexes of Ruthenium and Osmium

711a 711b 711c 711d 711e 711f 711g

435

711h 711i 711j 711k 711l 711m 711n 711o

711p 711q 711r

Scheme 283

7.07.7

Concluding remarks

Half-sandwich ruthenium and osmium complexes are among the most familiar compounds to organometallic chemists, known for many years as precursors to a wide variety of organometallic compounds and as catalyst precursors. Their common, stable three-legged piano stool structure allows the introduction of a variety of designed co-ligands which enables the synthesis of novel types of half-sandwich complexes. Since the publication of COMC III, NHC and non-innocent ligands have attracted considerable interests as co-ligands. Such structural flexibility and the redox-active nature of the half-sandwich complex may result in unique reactivity of them. In particular, the reaction with unsaturated hydrocarbons leads to facile isomerization to vinylidene and carbene complexes, and various chemical transformation reactions have been reported using this interconversion as a key step. Advances in analytical techniques and computational science have made it possible to outline reaction mechanisms with a high degree of accuracy. Outside the scope of this chapter, many polynuclear and heterometallic complexes comprising of half-sandwich Ru and Os complexes have been synthesized and exhibit different reactivity from conventional carbonyl clusters, due to the structural flexibility and redox properties characteristic of the half-sandwich metal fragments. It has become clear that not only the ligands on a metal center, but also subtle interactions between them, can have a significant effect on reactivity and selectivity. In addition, the introduction of cyclopentadienyl tether groups and substituents to control steric bulk has been shown critical in many reactions. These findings are expected to contribute greatly to the design of more sophisticated catalysts along with the stabilization of short-lived species that cannot be isolated yet. Moreover, careful design of half-sandwich Ru complexes have been increasingly deployed in biological studies that may foreshadow the development of new Ru-based drugs. On the other hand, it is also surprising that elegant new catalytic reactions using [CpRu(MeCN)3] and [Cp RuCl]4, long been known as starting materials for many half-sandwich complexes, continue to be reported one after another. Thus, it is clear that the field and impact the half-sandwich complexes will continue to expand in impact.

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Rioja, M.; Hamon, P.; Roisnel, T.; Sinbandhit, S.; Fuentealba, M.; Letelier, K.; Saillard, J.-Y.; Vega, A.; Hamon, J.-R. Dalton Trans. 2015, 44, 316–329. 416. (a) Ueda, T.; Mochida, T. Organometallics 2015, 34, 1279–1286; (b) Komurasaki, A.; Funasako, Y.; Mochida, T. Dalton Trans. 2015, 44, 7595–7605; (c) Higashi, T.; Ueda, T.; Mochida, T. Phys. Chem. Chem. Phys. 2016, 18, 10041–10048; (d) Kimata, H.; Mochida, T. Chem. A Eur. J. 2019, 25, 10111–10117. 417. Walton, J. W.; Williams, J. M. J. Chem. Commun. 2015, 51, 2786–2789. 418. Pike, J. A.; Walton, J. W. Chem. Commun. 2017, 53, 9858–9861. 419. Konovalov, A. I.; Gorbacheva, E. O.; Miloserdov, F. M.; Grushin, V. V. Chem. Commun. 2015, 51, 13527–13530. 420. (a) Bonnet, S.; Lutz, M.; Spek, A. L.; Koten, G. V.; Klein Gebbink, R. J. M. Organometallics 2008, 27, 159–162; (b) Bonnet, S.; Li, J.; Siegler, M. A.; von Chrzanowski, L. S.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Chem. A Eur. J. 2009, 15, 3340–3343; (c) Bonnet, S.; van Lenthe, J. H.; Siegler, M. A.; Spek, A. L.; van Koten, G.; Gebbink, R. J. M. K. Organometallics 2009, 28, 2325–2333; (d) Bonnet, S.; Siegler, M. A.; van Lenthe, J. H.; Lutz, M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Eur. J. Inorg. Chem. 2010, 2010, 4667–4677; (e) Bonnet, S.; Lutz, M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Organometallics 2010, 29, 1157–1167. 421. (a) Hildebrandt, B.; Frank, W.; Ganter, C. Organometallics 2011, 30, 3483–3486; (b) Hildebrandt, B.; Raub, S.; Frank, W.; Ganter, C. Chem. A Eur. J. 2012, 18, 6670–6678; (c) Verlinden, K.; Ganter, C. J. Organomet. Chem. 2014, 750, 23–29. 422. (a) Espinosa-Jalapa, N.Á.; Hernández-Ortega, S.; Morales-Morales, D.; Le Lagadec, R. J. Organomet. Chem. 2012, 716, 103–109; (b) Espinosa-Jalapa, N.Á.; HernándezOrtega, S.; Le Goff, X.-F.; Morales-Morales, D.; Djukic, J.-P.; Le Lagadec, R. Organometallics 2013, 32, 2661–2673; (c) Espinosa-Jalapa, N. 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7.08

Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Alexander D Böth∗, Michael J Sauer∗, Robert M Reich, and Fritz E Kühn, Molecular Catalysis, Catalysis Research Center and Department of Chemistry, Technische Universität München, München, Germany © 2022 Elsevier Ltd. All rights reserved.

7.08.1 General introduction 7.08.2 Complexes containing NHC ligands 7.08.2.1 Olefin metathesis catalysts 7.08.2.1.1 Complexes applied in metathesis 7.08.2.1.2 Mechanistic studies 7.08.2.2 Hydrogenation reactions 7.08.2.2.1 Transfer hydrogenation catalysts 7.08.2.2.2 Direct hydrogenation catalysts 7.08.2.3 Abnormally coordinated NHC complexes 7.08.2.4 Other recent NHC complexes 7.08.3 Heavier tetrylenes 7.08.3.1 Mononuclear complexes 7.08.3.1.1 Research of Javier A. Cabeza et al. 7.08.3.1.2 Research of Hisako Hashimoto et al. 7.08.3.1.3 Research of T. Don Tilley et al. 7.08.3.1.4 Further work on ruthenium and osmium heavier tetrylene complexes 7.08.3.2 Multinuclear Ru-, Os-species 7.08.3.2.1 Multinuclear ruthenium species of Cabeza et al. 7.08.3.2.2 Further work on multinuclear Ru-, Os-species 7.08.4 Nitrogen coordinating ligands 7.08.4.1 Introduction into RudNO complexes for medicinal applications 7.08.4.2 Recent work on RudNO complexes 7.08.5 Phosphorous coordinating ligands 7.08.5.1 Complexes with monodentate phosphorous ligands 7.08.5.2 Complexes with bidentate phosphorous ligands 7.08.5.3 Complexes with tridentate phosphorous ligands Acknowledgment References Relevant Websites

446 447 447 448 455 459 459 463 466 473 477 480 480 482 484 494 499 499 501 505 505 506 511 512 515 517 519 519 527

Nomenclature [N3] [PhP2SiH] ampy aNHC Ar ArTrip2 ATI B(dan) BArF bdqi BEMP BINAP bpy CAAC carboxy-PTIO CASSCF CCDC CM cod ∗

2,6-((MesN]CMe)2C5H3N) (H)2Si(o-PPh2-C6H4) 2-(Aminomethyl)pyridine Abnormally coordinated N-heterocyclic carbene Aryl 2,6-Bis(2,4,6-triisopropylphenyl)phenyl Aminotroponiminate Naphthalene-1,8-diaminato-boryl B(C6F5)−4 1,2-Benzoquinonediimine 2-(Tert-butylimino)-N,N-diethyl-1,3-dimethyl-1,3,2l5-diazaphosphinan-2-amine 2,20 -Bis(diphenylphosphino)-1,10 -binaphthyl 2,2’-Bipyridine Cyclic alkyl amino carbene (2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide Complete active space self-consistent field Cambridge Crystallographic Data Center Cross metathesis Cyclooctadiene

These authors contributed equally to this work.

444

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00142-6

Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Cp cyclam DAMBO DAMSA DCE DCM DFT DH D-HT Dipp dippe DMAP DMP DMRG dppb dppf dppm E e.e. e.r. edta eq. ES EtbzamtBu FA Flu GADPH H2Me2bpb HALA Hbzim Him Hind HMDS Hpz HSA HT imN isn LUMO Mes MesF miN MLCT MTT N-GQD NHC NIR NMR np NTO PDA PDI PDMS PDT PEG PhC(NiPr)2 PIB PLGA PTE py

Pentamethylcyclopentadienyl 1,4,8,11-Tetraazacyclotetradecane 8-(3,4-Diaminophenyl)-4,4-difluoro-1,3,5,7-tetramethyl-4-bora3a,4a-diaza-s-indacene N,N-diallyl-4-methylbenzenesulfonamide Dichloroethane Dichloromethane Density functional theory Direct hydrogenation Donor-stabilized heavier tetrylene Di-isopropyl-phenyl i Pr2PCH2CH2PiPr2 N,N-p-dimethylaminopyridine 2,6-Mes2-phenyl Density-matrix renormalization group 1,4-Bis-(diphenylphosphino)butane 1,1’-Bis(diphenylphosphino)ferrocene Bis(diphenylphosphino)methane Heavier tetrel element (Si, Ge, Sn, Pb) Enantiomeric excess Enantiomeric ratio Ethylen-diamine-tetraacetic acid Equivalents Excited state N-ethyl-N0 -tert-butyl-benzamidinate Folic acid Fluorenyl Glyceraldehyde 3-phosphate dehydrogenase 1,2-Bis(pyridine-2-carboxamido)-4,5-dimethyl benzene Heavy-atom effect on light atoms 1H-Benzimidazole 1H-Imidazole 1H-Indazole N(SiMe3)2 1H-Pyrazole Human serum albumin Heavier tetrylene Imidazole coordinated by nitrogen Isonicotinamide Lowest unoccupied molecular orbital 2,4,6-Trimethylphenyl 2,4,6-(CF3)3-C6H2 1-Methylimidazole Metal-to-ligand charge transfer 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide N-doped graphene quantum dot N-heterocyclic carbene Near-infrared Nuclear magnetic resonance Neo-pentyl Natural transition orbital Polydopamine Polydispersity index Poly-(dimethylsiloxane) Photodynamic therapy Poly-(ethylene glycol) N,N0 -bis(isopropyl)benzamidinate Polyisobutylene Poly(D,L-lactic-co-glycolic) acid Periodic table of elements Pyridine

445

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

pybox Pyme RCM rds REMP rNHC ROM ROMP rt. SC-XRD S-HT Si(NN) SLN Trigonoscuta cruzi t Bu2bzam TD-DFT TEP terpy TfOH TH TM TOF TON Tpa Tpm TPP Trip TsOH WFT XRD xs Xyl

7.08.1

Pyridine-functionalized bis(oxazoline) 1-(Pyridin-2-yl)methanamine Ring closing metathesis Rate determining step Ring expansion metathesis polymerization Remote NHC Ring opening metathesis Ring opening metathesis polymerization Room temperature Single crystal X-ray diffraction Simple heavier tetrylene Si((NCHt2Bu)2C6H4-1,2) Solid lipid nanoparticle Trypanosoma cruzi N,N0 -bis(tert-butyl)benzamidinate Time dependent-density functional theory Tolman electric parameter Terpyridine Triflic acid Transfer hydrogenation Transition metal Turnover frequency Turnover number Tris(2-pyridylmethyl)amine Tris(1-pyrazolyl)methane Triphenyl-phosphonium 2,4,6-iPr3-phenyl p-Toluenesulfonic acid Wave function theory X-ray diffraction excess 2,6-dimethylphenyl

General introduction

The noble metal d-block element ruthenium (Ru), located below iron in group 8 of the periodic table of elements (PTE) belongs to the platinum group due to similar physical and chemical properties.1 It was the last metal of this group to be discovered, with the discovery attributed to Karl Ernst Claus in 1844 and named after his homeland, Russia (from Latin Ruthenia) and is therefore considered to be one of several patriotic elements in the PTE.2 Although other scientists had previously claimed its discovery, for example Je˛drzej  Sniadecki in 1808,3 or Jöns Berzelius and Gottfried Osann in 1827,4 none were able to confirm their results and relinquished their claims, even though the samples of Berzelius and Osann were later found to indeed contain traces of ruthenium. Ruthenium is a relatively rare element in the Earth’s crust and is generally obtained as a by-product in nickel, copper and platinum ores.2 Similar to other transition metals (TM), a variety of inorganic coordination compounds and organometallic compounds are known, forming upon binding to soft ligands.5 These most importantly include halides, phosphines, carbenes, cyclopentadienyls, arenes, dienes, hydrides and carbon monoxide. Although ruthenium takes up oxidation states from −2 over 0 to +8, in organometallic compounds states usually range from −2 to +6, with oxidation states > +2 being the most common.2 Similar to ruthenium, osmium (Os) is another example of a group 8 noble metal d-block element assigned to the platinum group due to its physical and chemical properties. It was discovered, earlier than ruthenium, namely in 1803 by Smithson Tennant and William Hyde Wollaston together with iridium.2 Initial isolation as volatile OsO4 lead to its name (from Greek osme, meaning smell). In organometallic compounds, the properties of osmium resemble those of ruthenium complexes, albeit (in some cases) with more stable derivatives due to stronger p-backbonding, i.e., in alkene or carbon monoxide complexes. The high cost of osmium, however, caused less research dedicated to its compounds than to the more abundant ruthenium. Nonetheless, osmium complexes are certainly of interest as, in some cases, they exhibit more desirable properties than their ruthenium analogs or show extraordinary and peculiar reactivity. This chapter will discuss organometallic complexes of both ruthenium and osmium with regard to complexes containing p-acid ligands. According to the Dewar-Chatt-Duncanson model,6 ligands can donate electron density to the metal center via their s-bond (s-donors) as well as accept electron density by p-backdonation from the metal d-orbital into an antibonding p -LUMO-orbital (lowest unoccupied molecular orbital) of the ligand, thus creating a synergetic effect (Fig. 1).

Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

447

Fig. 1 s-bonding of carbon monoxide to metal M by donation of electron density (left) and p-backbonding from d-orbital of metal M to the LUMO of carbon monoxide (right).

Fig. 2 General structures of the main types of p-acid ligands discussed in this chapter.

The term p-acid is derived from the ligand’s ability to function as a Lewis acid and therefore describes a specific manifestation of the general p-backbonding term. Common p-acid ligands include dihydrogen, dioxygen, dinitrogen, carbon monoxide, acetonitrile, alkenes, alkines, nitrosonium or nitrosyl, N-heterocyclic carbenes (NHCs), heavier tetrylenes (HT, such as silylenes, germylenes, stannylenes and plumbylenes), phosphines and other nitrogen coordinating ligands such as pyridines or nitrosyls. Due to the large scope, this chapter will set a focus on NHCs, HTs, nitrogen coordinating ligands such as nitrosyl compounds and also on phosphorous containing ligands such as phosphines and phosphites (Fig. 2). Often, organometallic complexes include multiple different ligand types. Therefore, in these cases, some complexes will be discussed in the sections regarding the corresponding main ligand type (e.g., an NHC complex also containing a monodentate PPh3 ligand will be discussed in NHC section 7.08.2 rather than the phosphorous section 7.08.5). The nomenclature for compound references were chosen in accordance to the corresponding section (i.e., NHC compounds in section two are abbreviated as AXX, whilst HTs in section three as BXX, nitrosyls as CXX and phosphines/phosphites as DXX. Herein, XX represents the number within the section). In this chapter, the time period from 2006 to 2020 is covered. Whenever possible, a comparison of activity and selectivity is performed regarding corresponding catalytic reactions or applications. However, it has to be taken into consideration, that different reaction or test conditions can make these comparisons quite challenging.

7.08.2

Complexes containing NHC ligands

N-Heterocyclic carbenes (NHCs) are of great importance in coordination- and in organometallic chemistry due to their strong s-donating and weak p-acceptor capabilities and their steric and electronic adjustability. NHCs have first been reported as ligands by Wanzlick et al. in 1968.7 However, no further intensive research was performed until Arduengo et al. successfully reported the first isolated crystalline NHC in 1991,8 allowing its application in various fields. Since then, albeit still being a relatively young field, research on NHCs ligated compounds, mainly for catalysis has increased strongly. NHC complexes are more stable towards air, moisture and heat in comparison to previously utilized phosphine ligands.9 This vastly growing importance is also represented by the rapidly increasing number of publications between 2000 and 2015, containing NHC as keyword (Fig. 3), as given by Scopus® (Relevant Websites). Interest in research on ruthenium NHC complexes has also increased, as reflected by the number of publications containing both NHC and ruthenium as keywords. The impressive success of NHCs as ligands in catalysis has also been highlighted in the Nobel Prize in Chemistry of 2005, were some of the most useful olefin metathesis catalysts are NHC ligated compounds (Relevant Websites). This section will therefore take a closer look into the development of ruthenium and osmium NHC complexes since 2006, although the latter still remain a curiosity, as mentioned before. Regarding NHC complexes, numerous reviews have been published. These most notably include reviews on olefin metathesis catalysts,10 immobilization of NHC compounds,11 current advances in hydrogenation reactions,7a,12 complexes applied in medicinal chemistry,13 water splitting reactions,14 as well as general reviews regarding structures, reactivity, properties and syntheses.15

7.08.2.1

Olefin metathesis catalysts

One of the major applications of ruthenium and osmium NHC complexes is their utilization as olefin metathesis catalysts. Most well-known may be the so called Grubbs’ second-generation catalysts, described in 1999,16 and similar unsaturated complexes reported by the groups of Nolan,17 Grubbs,18 Fürstner and Herrmann,19 as well as the Hoveyda-Grubbs catalysts.20 Due to their industrial importance, these catalysts have been intensively studied over the course of the last years in regards to catalytic activity, stability and mechanistic aspects.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Fig. 3 Number of publications for the NHC (blue) and both NHC and Ruthenium (red) keywords from 1965 to 2020, as given by ScopusW. No Data was available for the combined keywords NHC and Ruthenium prior to 1992.

7.08.2.1.1

Complexes applied in metathesis

In 2006 Grubbs et al. reported a novel water-stable and soluble catalyst (A1, Fig. 4), by attaching a poly-(ethylene glycol) group (PEG) to the backbone of a saturated 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligand.21 This resulted in an unprecedented activity for the ring opening metathesis polymerization (ROMP, endo-norbornene monomer, 3.33 mol% [cat.], 45  C, 100% conv., 3 min), ring closing metathesis (RCM, 2-allyl-N,N,N-trimethylpent-4-enaminium chloride, 5 mol% [cat.], rt., >95% conv., 12 h) and cross metathesis (CM, prop-2-enol, 5 mol% [cat.], 45  C, >95% conv., E/Z ¼ 15, 12 h) in water. In addition, Grubbs et al. also reported a set of ROMP and RCM catalyst (A2) containing imine donors which are chelated to the ruthenium center as alkylidenes.22 In these, the relative placement of the imine plays a vital role, as non-latent exocyclic imines lead to faster metathesis initiation. In a further publication, a set of metathesis reactions was evaluated for a large variety of second generation Grubbs catalysts (A3–A5).23 NHC ligands with fluorinated aromatic wingtip substituents were found to show increased metathesis activity in RCM, attributed to an unusual Ru-F interaction reducing the rate-limiting phosphine dissociation (A6).24 In 2008 a new family of ruthenium metathesis catalysts with thiazole-2-ylidene ligands was reported and applied in RCM, CM and ROMP with activities comparable to the analogous NHC catalysts (A7, A8).25

Fig. 4 Several first and second generation metathesis catalysts reported by Grubbs et al.

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Fig. 5 Capping agents for Grubbs catalyst A9.

Fig. 6 Varied tether lengths and saturation of Grubbs catalysts as well as bulky substituents for REMP and CM.

The performance of Grubbs catalysts (A5, A9, Fig. 5) was further optimized by introducing vinylene carbonate, 3H-furanone and exo-N-phenyl-norborene-2,3-dicarboximide as capping agents for the ROMP of vinyl lactones.26 This led to the formation of functional polymer end groups without chemical transformation. Grubbs et al. also improved the previously reported saturated NHC ligands featuring a chelating N-to-Ru tether reported by Fürstner et al. in 2001.27 To combat the drawbacks of instability—albeit high activity in the ring-expansion metathesis polymerization (REMP) reaction (x ¼ 3, unsat., cyclooctene, 0.1 mol% [cat.], 40  C, 85% conv., 6 h) of the previously described catalysts, varying tether lengths and a saturation of the NHC backbone was exerted in a systematic study (A10, Fig. 6). In these, longer tethers (x ¼ 5, unsat., cyclooctene, 0.1 mol% [cat.], 40  C, 99% conv., 1 h) as well as saturation (x ¼ 3, sat., cyclooctene, 0.1 mol% [cat.], 40  C, 99% conv., 2 h) showed an increase in activity, demonstrating that a combination of tether length and electronic properties is vital for metathesis catalyst design. Furthermore, the effect of steric bulk of the wingtip substituents on CM was investigated, demonstrating that the reduction thereof allows for the application of sterically challenging disubstituted olefins (A11).28 Trisubstituted olefins were found to prefer larger bulky substituents. The results were discussed according to a proposed mechanistic pathway (Scheme 1).

Scheme 1 Proposed mechanistic pathway regarding tether lengths and electronic properties of Ru NHC metathesis catalysts by Grubbs et al. in 2008.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

In 2011 Grubbs et al. reported a new CdH activated 1-adamantyl catalyst for Z-selective CM (Scheme 2), showing among the best Z-selectivity among ruthenium based catalysts so far and the first demonstration of Z-selective CM of two different olefins by a ruthenium catalyst (A12) followed by further improved analogous catalysts containing an NO−3 ligand (A13).29

Scheme 2 Z-selective CM of two different olefin substrates by ruthenium catalysts including yields and E/Z ratios.

These were also tested in Z-selective ROMP, including a computational study on the mechanism and selectivity of the metathesis reaction.30 It was found that the Z-selectivity is impacted by the steric and electronic environment, leading to the chelation via a side-bound rather than bottom-bound mechanism forcing the substrate in cis position to the NHC ligand (Scheme 3). A further study regarding the decomposition pathway reported insertion of the alkylidene into the chelating RudC bond, leading to hydride elimination and thus deactivation.31 Very recently, similar systems have been reported for efficient Z-selective olefin-acrylamide CM, which is enabled by the sterically demanding cyclometalated catalysts by Grubbs et al.32

Scheme 3 Proposed bottom-bound and side-bound mechanistic pathways for olefin metathesis.

In 2015 the sequential metathesis with a diacrylate species (Scheme 4) utilizing a second generation Grubbs-Hoveyda catalyst (A3, R]PCy3) was reported, combining ring opening metathesis (ROM), RCM and CM. For this, monomers containing two polymerizable cyclopentane moieties were prepared and previously polymerized using a first generation catalyst.33

Scheme 4 Sequential metathesis pathway with diacrylate species, combining ring opening, closing and cross metathesis.

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The ROMP of cyclopentenes was investigated using soluble, phase-separable polyisobutylene-supported second generation catalysts, showing similar activity as their homogeneous counterparts e.g., the second generation Grubbs-Hoveyda catalyst (A14, A15, Scheme 5).34

Scheme 5 ROMP of cyclopentenes (top right) applying second generation Grubbs catalysts (top left). (PIB]polyisobutylene)

In 2017 Grubbs et al. reported a new synthetic approach enabling density and distribution manipulation of grafts in polymers using living ROMP and a second generation catalyst (Scheme 6).35 Living polymerization is a form of chain growth in which the terminating ability of the polymer chain has been inhibited or removed, resulting in a constant growth rate and forming chains of similar lengths (therefore yielding very low polydispersity indices, PDI).36 It is often desirable due to its precise and controlled macromolecular synthesis.

Scheme 6 General approach of controlled grafting via ROMP with both macromonomer and diluent species.

In addition, multiple other research groups have also performed intensive studies regarding Grubbs type catalysts. Grela et al. reported a second generation Grubbs catalyst (Scheme 7). Herein, the dissociation of the oxygen coordinating moiety relates to catalytic activity. This property was enhanced by introduction of an ester moiety as well an additional electron withdrawing NO2 group (A16, R]H, 55% yield, R]NO2, 99% yield).37 The combined independent effects are additive, showing that minor changes can significantly increase the activity in RCM compared to a standard Grubbs catalyst (A4, 10% yield).

Scheme 7 RCM of N,N-diallyl-4-methylbenzenesulfonamide (DAMSA) using Grubbs catalyst with additional coordinating ester moiety.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

In another report, the catalyst was further modified by addition of an amine to the para position of the aryl ring (A17, Scheme 8).38 The complex was subsequently immobilized on sulfonated polystyrene, providing a novel concept for non-covalent immobilization relying on electrostatic binding. RCM (diethyl-2,2-diallylmalonate, 5 mol% [cat.], 45  C, 99% conv., 2 h) and CM (methyl-undec-10-enoate, 5 mol% [cat.], 22  C, 95% conv., E/Z ¼ 1.2, 18 h) reactions are performed with immobilization on Raschig ring type supports. In 2008 Grela et al. investigated the commercially available and water insoluble second generation Grubbs catalyst and a cyclohexyl catMETium® metathesis catalyst described by Grubbs and Herrmann et al., respectively, regarding their activity in water at ambient conditions (A18, A19).39 Instead of co-solvents and surfactants, ultrasonification was applied to water insoluble substrates which was found to form stable emulsions.

Scheme 8 Various catalytic olefin metathesis reactions of modified Grubbs catalysts.

In 2012 a new family of Hoveyda-Grubbs type complexes with halogen chelation was reported by Grela and Barbasiewicz et al. (A20-A22, Fig. 7), active in various, metathesis reactions (Scheme 9) as well as the synthesis of ruthenium amino (A23) and amidobenzylidene (A24) complexes whose activity in RCM can be switched on by addition of a hydrochloric acid solution.40

Fig. 7 New family of Hoveyda-Grubbs type complexes with halogen chelation (A20-A22), amino and amidobenzylidene complexes (A23, A24) and bimetallic Hoveyda-Grubbs catalyst (A25).

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Scheme 9 Catalytic CM of diethyl-2,2-diallylmalonate. All reactions performed in DCM. A20-A22 were performed at 1 mol% [cat.] and 25  C, conversions given at 70 min A24 with R1 ¼ CF3 and R2 ¼ py, performed at 1 mol% [cat.] and 40  C, both without and with HCl addition. A25 was performed at 0.5 mol% [cat.] and 40  C, conversions given after 1 h reaction time.

The preparation of bimetallic Hoveyda-Grubbs catalysts was also reported by Barbasiewicz et al. (A25), showing similar activity to the corresponding monometallic complexes. A mechanistic study gave further insights into the initiation process of such bimetallic catalysts showing no cooperative effect between the metal centers.41 Furthermore, Bowden et al. reported the immobilization of first and second generation Grubbs catalysts into poly-(dimethylsiloxane) (PDMS) membranes which act as active membranes to impact the functional group selectivity on the occluded catalysts, hence demonstrating that the same catalyst can have different functional group selectivity depending on reagent polarity (Scheme 10).42

Scheme 10 Metathesis reaction using PDMS occluded second generation Grubbs catalyst. p-Toluenesulfonic acid is added for increased substrate diffusion, NaOH for deprotonation of substrate acid.

Although not directly utilized for olefin metathesis, Samoc et al. investigated Grubbs-Hoveyda type catalysts regarding their nonlinear optical properties, demonstrating moderately strong two and three photon absorption properties. This represents the first evaluation regarding this specific characteristic.43 Côté et al. report the preparation of strained helicene architectures applying metathesis reactions (Scheme 11) with Grubbs catalysts (A26, 10 mol% [cat.], DCM, 100  C, microwave, 100% conv., 88% yield, 25 min and A27, 10 mol% [cat.], DCM, 40  C, sealed tube, 100% conv., 78–93% yield, 24 h), a novel and simple preparation method potentially interesting for material science and medicinal chemistry.44 In contrast, other methods rely on radical or carbene intermediates.

Scheme 11 Preparation of strained helicenes utilizing olefin metathesis.

Acrylate CM of anethole, an essential-oil phenylpropenoid, into cinnamates (Scheme 12) with a Hoveyda-Grubbs catalysts showing high conversion and enantiomeric excess, was reported by Fogg et al.45 This process is of high important for cosmetic production and for antioxidant treatment. Whilst previous research has been focused on degradation of renewable resources, this procedure allows net augmentation and therein enables the creation of complex structures in a straightforward manner (R]Me, 0.1 mol% [cat.], DCE, 70  C, 99% conv., 78% yield, >99% E, 6 h).

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Scheme 12 CM reaction of anetholes into cinnamates by Fogg et al. in 2007.

The preparation of both saturated and unsaturated, UV-induced, cationic precatalysts (A28) with unprecedented activity in ROMP reactions was reported by Buchmeiser et al., including the elucidation of some key initiation steps (cyclooctene, 0.5 mol% [cat.], CDCl3, 30  C, 1 h, sat. 99% yield at 254 nm with PDI ¼ 1.2 and unsat. >99% yield at 254 nm with PDI ¼ 1.8).46 In 2019, Fernandez et al. described the synthetic preparation of tetrahydroquinoline-4-carboxylic acid esters via intramolecular CdH functionalization of a-diazoesters using standard Grubbs catalysts (Scheme 13).47 In this report, ruthenium catalysts were shown to be more versatile than most palladium complexes, albeit sometimes providing lower yields and/or selectivity. Mechanistic studies were performed using computational methods.

Scheme 13 Preparation of tetrahydroquinoline-4-carboxylic acid esters via CdH functionalization.

Following previous reports of saturated NHC catechothiolate complexes (A29, A30),48 Hoveyda et al. reported olefin metathesis of Z-a-b-unsaturated carbonyl compounds, scarcely described in literature using a catechothiolate based complex (A31).49 Utilization of the unsaturated NHC catalyst allowed an efficient catalytic activity with electron poor substrates such as esters, acids or Weinreb amides in high Z-selectivity for the first time (Scheme 14).

Scheme 14 Olefin metathesis of Z-a-b-unsaturated carbonyl compounds.

The acid assisted direct olefin metathesis of unprotected carbohydrates in water via a water soluble amine functionalized Hoveyda-Grubbs catalyst was reported by Ramström et al.50 Following reports of unsymmetrical NHC metathesis catalysts by Blechert and Verpoort et al. in 2006 (A32, A33, Fig. 8),51 Grubbs et al. in 2007 (A34),52 Mauduit and Coperet et al. in 2016 (A35, A36),53 as well as Grisi et al. in 2017 (A37),54 Grela et al. very recently presented a phenanthrene based, unsymmetrical second generation Grubbs-Hoveyda catalyst in 2020 with similar initiation activity as commercial catalysts, however with the distinct advantage of higher stability and selectivity (A38, diethyl-2,2-diallylmalonate, 0.1 mol% [cat.], DCM, 40  C, 99% conv., 35 min and N,N-diallyl-4-methyl-benzenesulfonamide, 0.1 mol% [cat.], DCM, 40  C, 99% conv., 10 min).55

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Fig. 8 Unsymmetrical NHC metathesis catalysts by Blechert and Verpoort et al., Grubbs et al., Mauduit and Coperet et al., Grisi et al. as well as Grela et al.

A macrocyclic ruthenium carbine catalyst was applied in selective CM by Diver et al. in which the macrocycle can differentiate alkenes based on size, hence enabling size selective reactions (A39, Scheme 15).56

Scheme 15 Size selective reaction using a macrocyclic ruthenium catalyst.

7.08.2.1.2

Mechanistic studies

A lot of attention has been drawn to the evaluation of the metathesis catalysis mechanism. A vital aspect of these has been the comparison of side- vs. bottom bound olefin coordination, as studied by Grubbs et al. in various studies (Scheme 16).57 Evidence of the previously disputed bottom-pathway, as opposed to side-pathway, was observed in form of ruthenium metallacycles (A40), which represent the first examples of ruthenacyclobutanes. It is also suggested, that these are dynamic structures that can undergo cycloreversions and further metallacycle formations prior to olefin exchange.

Scheme 16 Bottom- vs. side-coordination pathway (top left), Ru metallacycle A41 (top right) and cycloreversion mechanism (bottom).

456

Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Cavallo et al. performed a computational study including previously reported Grubbs style catalysts (A41–A44, Fig. 9).58 It showed, that the metathesis reaction pathway with ruthenium catalysts depends on the balance of steric, electronic and solvent effects. Polar solvents lead to a side-coordination-pathway and can easily overrun the effect of electronic influences, which usually lead to the bottom-coordination-pathway.

Fig. 9 Grubbs catalysts utilized in computational study by Cavallo et al. regarding their steric and electronic influence on the mechanistic pathway.

Catalyst decomposition also plays an important role in mechanistic investigations. Well-characterized initial decomposition products were described by Grubbs et al. in 2004 and decomposition hence attributed to isomerization and migration as well as CdH insertion in following studies.59 The latter can however be hindered by ortho substitution. In general, the decomposition process is reportedly initiated by the nucleophilic attack of a dissociated phosphine on the methylidene carbon of the methylidene complex (A45, Scheme 17) finally forming a dinuclear ruthenium hydride (A46) as well as methyltricyclohexyl-phosphonium.

Scheme 17 Decomposition process initiated by nucleophilic phosphine attack, proposed by Grubbs et al.

In further studies, deuterium labelling was performed by Wagener et al. on a second generation Grubbs catalyst to determine the mechanism of olefin isomerization during metathesis.60 Selective deuteration led to the conclusion, that a metalhydride addition-elimination mechanism is taking place (Scheme 18). The role of the NHC ligand is set in correlation with competitive H/D exchange between the designed CD3 group of the NHC wingtip and the substrate CH moiety, as deuterium is observed in the products following the catalytic reaction. A ruthenium dihydride species is suggested, promoting the exchange and is related to decomposition of the catalyst, in accordance to previous reports61; however, no such species was observed in this study.

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Scheme 18 Metal hydride addition-elimination mechanism.

A further computational study by Vinvent et al. in 2012 investigated the initiation steps of Grubbs-Hoveyda catalysts revealing that the rate determining step (rds) has an interchange rather than dissociation character, as both the incoming substrate as well as the outgoing alkoxy ligand are associated with the ruthenium center.62 In 2016 a computational and experimental study regarding the microstructures of ROMP products supports a mechanism in which stereogenic metal control is vital for cis,syndio-selectivity (Scheme 19).63 Microstructural errors could be attributed to alkylidene isomerization.

Scheme 19 ROMP mechanism with stereogenic metal control for cis,syndio-selectivity.

In 2019 Grubbs and Choi et al. also reported the isolation of the propagating species (A49) for living polymerization and confirmed olefin-chelated structures applying Grubbs and Hoveyda-Grubbs catalysts (A47, A48, Scheme 20).64

Scheme 20 Reaction for the isolation of propagating species for living polymerizations.

Mechanistic studies for ROMP reactions utilizing third generation Grubbs catalysts with varying steric properties showed that cyclic olefin monomers can be both of chelating (A51) and non-chelating (A50) nature, depending on their ability to form metallacycles, representing the active ruthenium center resting state (Scheme 21).65 The ROMP rate was found to be inverse to chelate strength.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Scheme 21 Chelating and non-chelating cyclic olefin monomers at a third generation Grubbs catalyst.

A comprehensive computational study combined with an effective oxidation state analysis was performed by Poater et al. The study examined the evolution of the metal oxidation during metathesis for ruthenium and osmium catalysts as well as the effect of the oxidation state on the s-donor and p-backbonding effects between first and second generation Grubbs catalysts, thus evaluating the direct influence of the applied NHC ligands.66 It is concluded, that a + 2 formal oxidation state is energetically preferred over the commonly suggested +4 state. However, the NHC moiety of the second generation catalyst allowed for a better stabilization of the formally higher Ru(IV) character, in comparison to first generation catalysts, thus rationalizing better the catalytic performance thereof. The impact of oxygen on olefin metathesis was described by Fogg et al. of NHC (A52–A56) and their analogous cyclic alkyl amino carbene (CAAC) complexes (A57, A58).67 Whilst the NHC catalysts show similar activity, a larger deviation is observed for their CAAC analogs. The mechanistic study suggests that oxygen [2 + 2] cycloaddition (Scheme 22) favors the attack at the benzylidene ligand.

Scheme 22 Metathesis catalysts (top) and [2 + 2] cycloaddition reaction (bottom).

Grisi et al. reported the investigation of the NHC wingtip and backbone of standard Grubbs-Hoveyda catalysts on their catalytic performance in CM, in which alkyl moieties are found to yield best activity and selectivity (ethyl oleate and Z-but-2-ene-1,4-diyl diacetate, 2.5 mol% [cat.], toluene, 50  C, up to 90% conv. and 96% yield).68 However, no rationalization was found for the dependence of performances to ligand NHC architecture. Although alkylidene loss in Grubbs type metathesis catalysts (A59) by ethylene decomposition (A60) seemed irreversible, Jensen et al. were able to show, that this process is indeed reversible from cymene coordinated complexes (A61), stabilized by cyclohexyl phosphines (A62) in a kinetic and computational mechanistic study (Scheme 23).69

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Scheme 23 Alkylidene loss (left) and reversibility thereof (right) of Grubbs metathesis catalysts.

An interesting alteration of the typical Grubbs metathesis reaction is the hydrogenative metathesis, using gem-hydrogenation as a gateway to hydrogenative ring expansion reactions, heterocycle species and cyclopropanation which is strongly investigated by Fürstner et al. (A63, Scheme 24).70 This germinal delivery of two hydrogen atoms or H2 to the same carbon atom in the alkyne can also be performed in a light-driven manner utilizing NHC ligands.71

Scheme 24 Proposed mechanism for the gem-hydrogenation.

7.08.2.2

Hydrogenation reactions

Hydrogenation reactions, transferring hydrogen from a source onto another compound in a reductive reaction, either via direct hydrogenation (DH) or transfer hydrogenation (TH), have various industrial applications and are commonly applied for pharmaceutical production or petrochemical transformations.72 Following the first hydrogen transfer reaction by Knoevenagel et al. in 1903 and the first asymmetric TH reactions utilizing ruthenium catalysts in the 1980s,73 Noyori and Knowles were awarded the Nobel Prize in chemistry in 2001 (Relevant Websites) for their work on asymmetric TH, further underlining the importance of such reactions.

7.08.2.2.1

Transfer hydrogenation catalysts

Two main possible mechanistic pathways were suggested for transition metal catalyzed TH, the inner-sphere (also hydric route) and the outer-sphere mechanisms (Scheme 25), as proposed by Noyori et al. in 1997.74 Of these, the inner-sphere mechanism is more prominent, as it applies to most complexes, whereas the outer-sphere mechanism is limited to amine containing complexes.75 In course of the inner-sphere mechanism, the substrate coordinates to the metal center as a ligand via ligand exchange, whilst this is not the case for the outer-sphere mechanism in which the previous ligands are critical for activation and creation of the nucleophilic hydride species.

Scheme 25 Comparison of inner- and outer sphere TH mechanisms.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Regarding TH, ruthenium NHC complexes have undergone a broad investigation, whilst only some osmium analogs have been reported. One of the most common monodentate complex types contains an NHC, a cymene and two chloride ligands and is obtained from commercial precursors (A64, Fig. 10). An optimization thereof by variation of the wingtip substituents of the NHC ligand was performed by Yas¸ar et al. (A65, acetophenone, R2 ¼ 3,4,5-trimethoxybenzene, 0.75 mol% [cat.], isopropanol, 80  C, 96% conv., 30 min).76 Furthermore, Günay et al. examined a variety of wingtip substituents and demonstrated the beneficial influence of bulky, electron-donating wingtip substituents for these kind of catalysts (A66, acetophenone, R2 ¼ 2,3,4,5,6-pentamethylbenzene, 0.5 mol% [cat.], isopropanol, 82  C, 93% conv., turnover frequency (TOF) ¼ 46.5 h−1, 4 h).77

Fig. 10 Several monodentate NHC cymene complexes.

The influence of an ethoxyethyl compared to a phenyl wingtip was investigated by Papish et al., albeit only with marginal difference in activity.78 In 2011 Valerga et al. reported a new class of monodentate ruthenium complexes containing a pyridine (py) moiety (A67, Scheme 26), active for a wide range of ketones and imines for low loadings (acetophenone, 0.1 mol% [cat.], isopropanol, 82  C, 93% yield, 1 h).79

Scheme 26 TH of ketones and imines utilizing monodentate Ru complexes.

Next to monodentate complexes, chelating ligands can further be applied, allowing the variation of several properties such as bite angle, steric hindrance and fluxional behavior. However, these need to be chosen wisely, keeping in mind the requirement of the formation of an active site. Albrecht et al. reported a series of substituent-functionalized NHC complexes with varying donating properties (A68–A73, Fig. 11), notably an olefin-tethered catalyst with very high efficiency and also largely versatile substrate scope (benzophenone, 1 mol% [cat.], isopropanol, reflux, 90% conv. After 5 h and 98% conv. After 24 h).80 Following this publication, further variously tethered complexes were described in literature. Morris et al. reported both a mechanistic investigation on ketone hydrogenation with a primary amine tethered NHC complex (A74, acetophenone, 0.5 mol% [cat.], isopropanol, 75  C, 74% conv. After 1 h and 96% conv. After 3 h, TOF ¼ 183 h−1) as well as the effect of chelating ring size thereof.81 A phosphine-tethered complex was utilized by Domski et al. in 2013 (A75).82 This represents the first report of a TH catalyzed by a ruthenium(II) complex supported by an ortho-metalated, phosphine-functionalized NHC ligand, albeit (so far) with low activity (acetophenone, 0.47 mol % [cat.], isopropanol, 82  C, 10% conv., TOF ¼ 4 h−1, 1 h). The analogous non-metalated complex (A76) however showed much higher activities (acetophenone, 0.47 mol% [cat.], isopropanol, 82  C, 94% conv., TOF ¼ 199 h−1, 1 h). A row of hydride chloride carbonyl NHC complexes with mesityl (Mes ¼ 2,4,6-trimethylphenyl) and isopropyl wingtip substituents (A77–A79) was examined for the catalytic hydrogenation of aromatic ketones such as acetophenone (0.4 mol% [cat.], isopropanol, 10 bar H2, 110  C, 36–84% conv., 20 h) under addition of isopropanol and hydrogen gas by Whittlesey et al., showing activity in both DH and TH reactions.83 Similar to the monodentate cymene compounds described above, pyridine bidentate carbenes have also been reported by Ramaiah (A80, acetophenone, 0.5 mol% [cat.], isopropanol, 80  C, 99% conv., 2 h),84 Paz-Sandoval (A81, benzophenone, 5 mol% [cat.], isopropanol, reflux, 97% yield, 2 h),85 and Domski et al. (A82, acetophenone, 1 mol% [cat.], isopropanol, 82  C, 92% conv., TOF ¼ 44 h−1, A83, 96% conv. TOF ¼ 25 h−1, 4 h),86 with varying activity depending on the applied wingtip substituents (Fig. 12). Similarly, Kühn et al. reported novel water-soluble ruthenium and osmium complexes with chelating NHC pyridine ligands (A84).87 Whilst the alkyl tethered complexes were insoluble in water (R]Me, acetophenone, 1 mol% [cat.], isopropanol, 82  C, 92% yield for Ru and 60% yield for Os, 70 min), sulfonated complexes enhanced the solubility dramatically and were subsequently

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Fig. 11 Variety of chelating TH NHC complexes.

Fig. 12 Pyridine bidentate NHC complexes (A80–A83) and water soluble complexes (A84).

examined in the TH of acetophenone (M]Ru, R]SO3, HCOONa/HCOOH buffer in H2O, 1 mol% [cat.], 80  C, 87% yield). Surprisingly low yields were obtained when applying the sulfonated osmium analogs (3% yield, 7 h). A bis-NHC ligand reported by Peris et al. in 2010 was able to increase the stability of the applied catalyst, allowing for reasonable activity in the TH of carbon dioxide (Scheme 27), albeit only when applying harsh reaction conditions (A85, R]Me, 50 bar. CO2, 0.018 mM cat., H2O:isopropanol ¼ 9:1, 200  C, 6.7∙ 10−5 mol HCOOK, turnover number (TON) ¼ 186, 16 h).88

Scheme 27 TH of carbon dioxide under harsh conditions by Peris et al.

Furthermore, tridentate compounds have also been investigated in TH reactions. Sakaguchi et al. described a series of chiral catalysts (Fig. 13) with varying arene NHC backbones and functionalized with secondary donor groups (A86, R ¼ 1,4-di-isopropylbenzene, acetophenone, 4 mol% [cat.], isopropanol, rt., 56% yield, 35% e.e. (enantiomeric excess), 20 h).89 A row of catalysts containing two triazole wingtip ligands by Albrecht and Kühn et al. showed excellent initial activities (A88, acetophenone, 0.5 mol% [cat.], isopropanol, 82  C, 84% conv., TOF ¼ 1100 h−1, 120 min), however, fall short regarding overall

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Fig. 13 Tridentate and chiral NHC TH complexes.

conversion (A87–A89).90 These are the first mixed normally and abnormally coordinated NHC (aNHC) ruthenium complexes (section 7.08.2.3) with triazole and imidazole based ligands as well as the first example of transition metal complexes with chelating mixed imidazolidine triazolydine based tridentate NHC complexes. Tridentate pincer complexes supporting pyridine and diazole wingtips have also been reported by Chen et al. (A90, acetophenone, 0.01 mol% [cat.], isopropanol, 80  C, 96% yield, TOF ¼ 3200 h−1, TON ¼ 9600, 3 h), Messerle et al. (A91, acetophenone, 1.5 mol% [cat.], isopropanol, 82  C, 14% conv., 2 h),91 as well as a tridentate CCC pincer complex of two NHC moieties by Hwang et al. (A92, R ¼ Me, L ¼ NCCH3, acetophenone, 0.01 mol% [cat.], isopropanol, 82  C, 99% yield, TOF ¼ 1980 h−1, 5 h), bridged by a chelating cyclohexyl group (Fig. 14).92 The first Ru(II) complex with an open chain tetradentate NHC ligand was reported by Kühn et al., the coordination geometry whereof could be controlled by variation of the alkyl linker chain lengths (A93, A94, Fig. 15).93 Of these, the sawhorse-type motif resulted in remarkable TH activities (A93, acetophenone, 0.1 mol% [cat.], isopropanol, 82  C, 98% conv., TOF ¼ 110,000 h−1, 1 min). In a recent report, Samuelson et al. demonstrated TH of aromatic ketones using untethered (A95, n ¼ 1, R ¼ 3,5-dimethyl, acetophenone, 1 mol% [cat.], isopropanol, 82  C, 100% conv., 93% yield, 4 h) and tethered (A96, n ¼ 1, R ¼ 3,5-dimethyl, acetophenone, 1 mol% [cat.], isopropanol, 82  C, 100% conv., 96% yield, 4 h) ruthenium cymene NHC complexes.94 Hereby formed Ru(0) nanoparticles act as active species and are stabilized by the applied NHC ligands, generating an efficient TH catalyst in situ.

Fig. 14 Tridentate pyridine, diazole and CCC pincer complexes.

Fig. 15 Tetradentate NHC complexes (A93, A94) and (un-)tethered cymene complexes (A95, A96).

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7.08.2.2.2

463

Direct hydrogenation catalysts

In comparison to TH, utilization of ruthenium NHC catalysts for DH are limited to comparatively few examples. One common area is the application of DH in aqueous environment, in which the avoidance of organic solvents is crucial from an environmental perspective. Morris et al. reported the first cymene ruthenium(II) complex containing a primary amino-functionalized NHC ligand, active in DH as well as its osmium analogs including a mechanistic investigation thereof (A97, Fig. 16, acetophenone, 0.5 mol% [cat.], H2, 25 bar, THF, 50  C, M]Ru, 88% conv., TOF ¼ 213 h−1, 1 h, R]Os, 23% conv., 3 h).81a,95 In the following years, these systems were further investigated by Morris and Cross et al.81b,96 Whilst primary amines resulted in the coordination of the amine to the metal center (A98), secondary amines with sterically demanding groups such as di-isopropyl-phenyl (Dipp) led to cyclometalation (A99). A comparison of chelating rings was conducted between seven-membered ring (A100) and six-membered ring (A101) complexes. These experiments have shown comparable catalytic activity in the DH of ketones (acetophenone, 0.17 mol% [cat.], H2, 25 bar, THF, 50  C, 49% conv., TOF ¼ 595 h−1, TON ¼ 295, 1 h), however the different electronic properties of the tethers need to be considered.

Fig. 16 Amino functionalized ruthenium DH complexes.

An outer-sphere bifunctional mechanism was proposed for ester hydrogenation using these complexes (Scheme 28).

Scheme 28 Outer-sphere bifunctional mechanism proposed for ester hydrogenation.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Elsevier et al. reported further analogs complexes (A102-A104, Fig. 17), exhibiting the (so far) best performance in DH of acetophenone compared to all previous reports (acetophenone, 0.17 mol% [cat.], H2, 25 bar, THF, 50  C, A102, 99% conv., TOF ¼ 3609 h−1, 30 min, A103, 99% conv., TOF ¼ 595 h−1, 2 h, A104, 57% conv., TOF ¼ 122 h−1, 2.5 h).97 Structural investigations showed that a smaller ring size in combination with ring conjugation of the heteroditopic ligand are favorable, promoting an inner-sphere pathway.

Fig. 17 Additional amino functionalized ruthenium DH complexes.

In comparison, asymmetric DH of ketones utilizing Ru NHC catalysts is only scarcely described. One prominent report was given by Morris et al., examining a chiral half-sandwich complex as a catalyst, bearing a pentamethylcyclopentadienyl (Cp ) as well as an NHC (A105).98 Due to the electron rich Cp moiety the catalyst showed high activity (acetophenone, 0.02 mol% [cat.], H2, 25 bar, isopropanol, 50  C, TOF ¼ 522 h−1, 5% e.e., TON ¼ 5000, 15 min). Mechanistic investigations were performed to get a deeper insight into the role of the applied chiral NHC ligand. An unusual feature was the occasional drop in e.e. from as high as 60% to 0% for some products. This was attributed to racemization, induced by either strong base or competitive inhibition of one diastereomer of the catalyst by reaction with the formed product (Scheme 29).

Scheme 29 Proposed catalytic cycle for asymmetric DH by Morris et al.

The DH of C]C double bonds was investigated thoroughly by Glorius et al. in various publications. Herein, the NHC ligands (A106) are usually applied as in situ additives to precursors such as Ru(cod(2-methylallyl))2 (cod ¼ cyclooctadiene) forming the active complexes. The DH of 2(1H)-pyridones (Scheme 30) represents the first example of asymmetric conversion thereof (6methyl-pyridone, 5 mol% [cat.], H2, 120 bar, t-AmOH:n-hexane ¼ 1:1, −10  C, 99% conv., 94:6 e.r. (enantiomeric ratio), 24 h) but was also applied to indolizine substrates (3-butyl-5-methylindolizine, 5 mol% [cat.], H2, 100 bar, n-hexane, rt., 99% conv., 97:3 e. r., 24 h) as well as an enhanced substrate scope of flavones (2-methyl-4H-chromen-4-one, 5 mol% [cat.], H2, 120–150 bar, n-hexane, rt., 100% conv., 98:2 e.r., 36 h), chromones (2-phenyl-4H-chromen-4-one, 5 mol% [cat.], H2, 120–150 bar, n-hexane, rt., 100% conv., 98:2 e.r., 36 h) and vinylthioethers (N-methyl-1,5-benzothiazepinone, 10 mol% [cat.], H2, 100 bar, n-hexane, rt., 99% yield, 93% e.e., 48 h).99 Furthermore, a highly versatile NHC complex containing an unusual doubly deprotonated ligand was isolated as well as multiple precatalysts. Here, ligand hydrogenation is reported as a key activation process.100

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Scheme 30 DH of pyridones, indolizines, flavones (R ¼ alkyl) and chromones (R ¼ aryl).

Based on these reports, Zhou et al. compared the influence of several different wingtips (A107) for the enantioselective DH of quinoxalines (Scheme 31, R1 ¼ naphthalene, R2 ¼ Me, 24% e.e.) benzothiophenes (R3 ¼ Me, 98% e.e.) and thiophenes (R4 ¼ Me, R5 ¼ 4-F-benzene, 94% e.e.).101 A further optimization approach is the addition of a second diamine (such as (R,R)-1,2-di-paratolylethane-1,2-diamine) ligand besides the NHC, as demonstrated by Glorius et al., providing excellent enantiomeric excess (8-methoxy-3-methyl-1H-isochromenone, 5 mol% [cat.], 5.5 mol% diamine, H2, 50 bar, n-hexane:toluene ¼ 3:1, rt., 64% yield, 99% e.e., 16 h).102

Scheme 31 DH of quinoxalines and thiophenes.

In 2012, Kühn et al. reported a water soluble ruthenium NHC cymene complex (A108, Fig. 18) for the DH of acetophenone in aqueous solution (acetophenone, 2.5 mol% [cat.], H2, 40 bar, water, rt., 90% conv., 87% yield, 21 h).103 The hydrophilic property was enabled by introduction of a sulfonated tether. However, mechanistic studies revealed catalyst decomposition and dissociation of one NHC moiety including backbone reduction. This system was further improved, creating a bridged pincer analogue (A109) and also expanded to the use of osmium as metal center.104 These complexes exhibit a high thermal stability, do not show the decomposition observed before and represent the first reported preparation of water soluble ruthenium and osmium complexes by facile reaction of chelating sulfonated imidazolium precursors with silver(I)oxide in water. They are able to perform DH of aromatic compounds (M]Ru, X]CH2, acetophenone, 1 mol% [cat.], H2, 40 bar, water, 60  C, 27% yield, 16 h) via ruthenium NHC hydride complexes, formed in situ.105 Furthermore, a pyrazolato-bridged dinuclear complex (A110) was reported, including

Fig. 18 Water-soluble DH ruthenium complexes by Kühn et al.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

synthesis, characterization and electrochemical properties, with three observable reversible redox processes, supported by computational methods.106 Hereby, the formation of the bis(imidazolium) or the corresponding bis(NHC) complexes (A111) depend strongly on the applied reaction conditions. In 2018 Kühn et al. reported further monodentate ruthenium and osmium NHC complexes with alkyl and sulfonated tethers (A112, Scheme 32), the latter of which were soluble in aqueous media and examined for the complete hydrogenation of acetophenone with H2 in an autoclave, providing high conversions.87

Scheme 32 Complete hydrogenation of acetophenone with H2 in an autoclave.

DH was also reported using NHC oxazoline pincer complexes by Nishiyama et al. (A113).107 These showed activity in DH of aromatic ketones, in particular, hydrogenation of 9-acetylanthracene was achieved not only at the C]O position but surprisingly also at the anthracene moiety (Scheme 33).

Scheme 33 DH of aromatic ketones, in particular 9-acetylanthracene, at C]O and at anthracene moiety.

Chainese et al. observed unexpected switching coordination of CNN pincer NHC complexes (A114, Fig. 19) during ester hydrogenation and evaluated this accordingly in a mechanistic study.108 Under addition of base, the CNN form is dominant whereas a switch to a CC coordinating form (A115) occurs without base.

7.08.2.3

Abnormally coordinated NHC complexes

A noteworthy subgroup of NHC complexes are the so-called abnormally coordinated NHCs (aNHC, also dubbed mesoionic NHC). Normal NHC complexes show metal coordination at the NCN or C2 carbon atom position, as this possesses the lowest pKa value, allowing easy deprotonation and stabilization of the formed carbene by both neighboring heteroatoms. In general, aNHC complexes exhibit stronger s-donor properties compared to their normally coordinated analogs. The first aNHC complex was reported by Crabtree et al. in 2002 (A116, Fig. 20), in situ in an iridium hydride complex with a bridged aNHC pyridine moiety.109 The first isolation of a metal free aNHC was reported by Bertrand et al. in 2009 (A117).110 However, aNHC ruthenium and especially osmium complexes have only been scarcely reported, compared to e.g., their Ir, Rh and Pd counterparts.15f

Fig. 19 Switching coordination of CNN pincer complexes.

Fig. 20 First aNHC complex by Crabtree et al. (A116) and first metal free aNHC by Bertrand et al. (A117).

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In 2007, Whittlesey et al. reported the preparation of a trimetallic ruthenium aNHC (A118, Fig. 21) from [Ru3(CO)12] and subsequent CdH activation at the remaining backbone carbon, yielding a m3-bridging heterocycle (A119).111 Thermolysis in the presence of H2 (1 bar, 50  C, 17 h) afforded elimination of the NHC ligand as an imidazolium salt, representing an unprecedented set of reactions for the generation of free carbenes and a double CdH activation. Clyburne et al. reported a series of abnormal and normal osmium complexes containing a dimesityl-imidazolium ligand. These are the first normal and abnormal tetranuclear NHC cluster compounds utilizing osmium (A120).112 Esteruelas et al. investigated the influence of the anion of the respective metal salts on the coordination mode of NHC osmium complexes (A121, A122, Fig. 22). The abnormal coordination mode is preferred with less coordinating anions.113 Albrecht et al. have studied 1,2,3-triazolylidenes (A123-A126) as abnormal ligands, demonstrating a high versatility for introduction of further substituents and functionalities from readily available precursors.114 These were further evaluated in catalytic base-free oxidation of benzylic alcohols, homocoupling of amines and oxidative coupling of amines and alcohols (Scheme 34) as well as single-site catalysts for efficient water oxidation.115

Fig. 21 Trimetallic ruthenium and osmium aNHC heterocycles.

Fig. 22 Osmium complexes A121 and A122, investigated regarding the influence of the anion.

Scheme 34 alcohols.

Catalysts reported by Albrecht et al. applied in base-free oxidation of benzylic alcohols, homocoupling of amines and oxidative coupling of amines and

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Albrecht et al. also reported the preparation of imidazole[1,2-a]pyridine complexes in which a pyridine was fused to the NdC2 bond of the imidazolium moiety, forcing abnormal coordination (A127, Fig. 23).116 A preliminary catalytic activity study towards the TH of benzophenone (1 mol% [cat.], isopropanol, 82  C, 90% conv., 16 h). Together with Landman et al., a series of pcymene cyclopentadienyl complexes with alkenyl or picolyl tethered aNHC ligands were prepared (A128-A131).117 The abnormal coordination was facilitated by protection of the C2 position with an isopropyl group and applied in TH, showing higher activities in p-cymene and alkenyl containing complexes (A128, R]H, benzophenone, 1 mol% [cat.], isopropanol, 82  C, 93% conv., TOF ¼ 60 h−1, 6 h), in comparison to their cyclopentadienyl and picolyl analogs (A131, 50% conv., TOF ¼ 22 h−1, 6 h).

Fig. 23 Various aNHC complexes by Albrecht et al. and Landman et al.

Bera et al. investigated the steric control of the wingtip substituents of bis-NHC/aNHC ruthenium complexes and the coordination behavior thereof (A132, A133).118 The catalytic activity towards carboxylic acid addition onto terminal alkynes was evaluated (Scheme 35), in which complexes with abnormal coordinating moieties (89% conv., 94% Z product, 6% E product) showed higher conversions than their purely normal analogs (60% conv., 79% Z product, 21% E product).

Scheme 35 Carboxylic acid addition onto terminal alkynes.

Furthermore, a highly efficient, multifaceted, annulated naphthyridine-aNHC based catalyst (A134) was reported for selective oxidative scission of olefins to aldehydes, e.g., of para-chloro-styrene to the corresponding benzaldehyde (1 mol% [cat.], MeCN: EtOAc:H2O ¼ 2:2:1, rt., 94% yield, 30 min).119 Sarkar et al. also reported click-chemistry based abnormal, arene pyridyl- and bis-triazole complexes (A135).120 These were structurally characterized and the catalytic properties compared to their bipyridyl (bpy) analogs regarding the TH of nitrobenzene, in which the latter yielded worse results. Furthermore, reduction of nitrobenzene resulted in either aniline, azobenzene or azoxybenzene as products, presenting a large product distribution. In correlation with a presented mechanism, this distribution depends on the type of NHC ligand (Scheme 36).

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469

Scheme 36 TH of nitrobenzene and proposed mechanism for the reduction of nitrobenzene.

Both a normal (A136, Fig. 24) and abnormal (A137) osmium NHC polyhydride complex, obtained by direct metalation of imidazolium salts, was prepared by Esteruelas and Olivan et al. in 2008.121 Acquiring either the normal or abnormal variant was reported to depend on the imidazolium wingtip. Whilst mesityl led to abnormal coordination, a benzyl wingtip favored normal coordination due to its lesser steric bulk. Lavigne et al. described chelation-assisted reactions of phosphine and olefin tethered NHC and aNHC complexes (A138), however no systematic preference for either was observed.122 These represent the first isolated congeners of Ropers complex incorporating strongly stabilizing NHC ligands. Furthermore, chelation assistance, which is known to be of importance in stoichiometric and catalytic reactions, was found to be vital for the CdH activation of the parent imidazolium/ phosphine/olefin ligands for the formation of the corresponding complexes.

Fig. 24 Normal and abnormal osmium and ruthenium polyhydride and olefin-tethered complexes.

A heteroleptic bis(tridentate) ruthenium complex (A139, Fig. 25) with an aNHC pincer ligand, prepared using click-chemistry, was described by Schubert and Gonzalez et al. in 2011.123 It was utilized as a potential photosensitizer due to its promising photophysical and electrochemical properties given by the superior s-donation of the aNHC ligand. Although a typical metalto-ligand charge-transfer (MLCT) transition was observed, the reduced symmetry enabled comparatively low extinction coefficients and band splitting, with regards to analogous terpyridyl (terpy) and bipyridyl complexes.

Fig. 25 Heteroleptic bis(tridentate) ruthenium photosensitizer.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

In 2013 Kühn et al. reported an aNHC phosphine Ru(II) complex with mesityl wingtps (A140, Fig. 26), utilized in the catalytic TH of acetophenone with high activity (0.05 mol% [cat.], isopropanol, 82  C, 97% conv., TOF ¼ 38,000 h−1, 2 h).124

Fig. 26 Abnormal and bimetallic complexes by Kühn et al.

The rates could further be enhanced by addition of amine (2 eq. ethylenediamine, 97% conv., TOF ¼ 58,000 h−1, 20 min). In situ addition showed the same activity as pre-synthesized amine complexes. This represents the first solid-state structure of an aNHC-phosphine Ru complex. Further optimization thereof yielded the first examples of complexes bearing an anionic dicarbene ligand connected to two different d-block elements (A141).125 These were found to be catalytically active in ketone TH (acetophenone, 0.1 mol% [cat.], isopropanol, 82  C, 97% conv., TOF ¼ 13,000 h−1, 20 min). Via transmetallation, these systems were further expanded to form hetero-bimetallic PddRu complexes (A142). Next to the TH of acetophenone (0.05 mol% [cat.], isopropanol, 100  C, 95% conv., 2 h), these displayed high activities in tandem Suzuki-Miyaura cross-coupling and TH reactions of bromoaryl ketones with boronic acids (Schemes 37, 0.5 mol% [cat.], toluene/isopropanol, 100  C, 95% yield (of b), 30 min).126 The PddRu catalysts are superior to their monometallic species with higher selectivity (only 76% yield (of b)). The reaction was found to be affected strongly by choice of base. Alkoxides resulted in only low yield (30%) of Suzuki-Miyaura product and mostly dehalogenation side-products (60%), whilst the use of bases such as KOH resulted in mostly Suzuki-Miyaura product (85%) and only low amounts of side products (3.

Scheme 85 Contrasting Reactivity of [Ru3(CO)12] with Ge(NCH2CMe3)2C6H4 and Ge(HMDS)2.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

The field of non-carbene-like coordination chemistry of heavier analogs was expanded by Cabeza and García-Álvarez et al. with the transformation of an amidinate germylene with one accessible electron lone pair into a bidentate k2Ge,N-ligand.234 The stabilization effect of a strong Lewis base on a MR2 fragment235 is exploited synthetically as a potential bidentate ligand upon release of the Lewis base from the metalene to access more robust MR2 transition metal complexes (Scheme 86). However, the transformation of a base stabilized MR2 fragment to a bidentate ligand is quite a challenge. Coordination of the metalene to the transition metal increases the Lewis acidity of the metalene and thus increasing the effect of base stabilization. This phenomenon is elucidated in the research of Tacke et al. concerning bis(amidinato)silylene Si(PhC(NiPr)2)2 (PhC(NiPr)2 ¼ N,N0 -bis(isopropyl) benzamidinate).236 Upon the coordination of silicon to [W(CO)6] an amidinate group shifts its terminal, pendant wing-tip to a chelating binding mode, that allows base stabilization of silicon.

Scheme 86 Formation of a bidentate ligand metal complex from an intramolecularly donor-stabilized group-14 metalene.

Cabeza et al. applied Ge(HMDS)(PhC(NiPr)2), which carries a sterically demanding HDMS group and consists of a strained four-membered GeNCN ring to show on the examples of [Co2(CO)8] and [Ru3(CO)12]—for the first time—the hemilabile character of amidinate group-14 metalenes. The bimetallic trinuclear derivative [Ru2(m-k2Ge,N-Ge(HMDS)(PhC(NiPr)2))(CO)7] (B116, Fig. 46) was synthesized by reaction of Ge(HMDS)(PhC(NiPr)2) with [Ru3(CO)12] in a molar ratio of 1 to 2:3 at 90  C in toluene in 63% yield.234 Notably, the Ru2Ge compound is stable under ambient conditions for several weeks, while transition metal complexes that incorporate group-14 metalenes are mostly air-sensitive. The bidentate coordination mode of the amidinatogermylene is ascribed to the steric influence of HDMS that favors ring opening and the option to form a bridging germylene. The transition metal complexes of the aforementioned ligand-class were reviewed by Cabeza and García-Álvarez in 2015,153 where they summarized these findings since the first reported tungsten and iron amidinato germylenes by Jones et al. in 2008.237

Fig. 46 Bimetallic trinuclear derivative [Ru2(m-k2Ge,N-Ge(HMDS)(PhC(NiPr)2))(CO)7] (B116).

Building on these works, Cabeza and García-Álvarez et al. conducted reactivity studies on B116 expecting a hemilabile behavior of the k2Ge,N-amidinatogermylene ligand.238 However, the strong attachment of the ligand to the metal was confirmed by CO substitution reactions with tBuNC and mono- and diphosphines and by bond activation of HdX (X]Si, Sn, H) under mild conditions. Nevertheless, fission of the GedN was observed at elevated temperatures (>110  C), although this might rather be caused by precedent thermally induced decarbonylation, than by intrinsic thermal instability of the ligand. In a further step, Cabeza and García-Álvarez et al. investigated the influence of the volume of the amidinate NdR groups of Ge(R2bzam)(HMDS) on the example of Ge(tBu2bzam)(HMDS).159k Intriguingly, the reaction of Ge(tBu2bzam)(HMDS) with [Ru3(CO)12] gave intractable decomposition products. Substitution of one tBu group of the amidinato germylene with an ethyl group led to [Ru2(m-k2Ge,N-Ge(EtbzamtBu)(HMDS))(CO)6] (B117, Scheme 87) in 64% yield. Interestingly, B117 is isostructural to

Scheme 87 Structure and reactions of [Ru2(m-k2Ge,N-Ge(EtbzamtBu)(HMDS))(CO)6] (B117).

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501

B116, apart from the amidinato-bound ruthenium that is coordinatively unsaturated. The vacant site is partially intramolecularly protected by a hydrogen atom of a tBu group. 1H and 13C NMR spectra indicate sterically impeded rotation axes of HDMS, Et and Ph. These data imply steric crowding and provide an explanation why Ge(tBu2bzam)(HMDS) cannot form an isostructural complex of B116. B117 is remarkable as it is only the third reported coordinatively unsaturated diruthenium(0) complex after the discovery of [Ru2(m-k2P,P0 -(RO)2-PN(Et)P(OR)2)2(m-CO)2(CO)2] (R]Me, iPr).239 Cabeza et al. showed that B117 can reversibly incorporate one CO molecule (B118), and can oxidatively add HSnPh3, while eliminating one CO molecule (B119). The importance of [Ru3(CO)12] in inorganic synthesis and in catalysis led Cabeza and García-Álvarez et al. to investigate the mechanism of two-electron donor reagents with the ruthenium cluster.159h The aim was to generate a tool to rationalize the outcome of reactions of [Ru3(CO)12] with other highly Lewis basic reagents, such as trialkylphosphines or NHCs, as well. Several complexes (B121-B124, Fig. 47) were synthesized in the reaction of Ge(R2bzam)tBu (B120, R2bzam ¼ N,N0 -disubstituted benzamidinate; R]tBu, iPr) with [Ru3(CO)12] and analyzed by elemental analysis, mass spectrometry, IR and NMR spectroscopy, and in some cases by single-crystal X-ray diffraction. A screening of reactant ratios and temperature provide data for a new mechanistic proposal explaining all findings in detail. In fact the mechanism by Cabeza and García-Álvarez et al. is also able to surpasses Poë and Twigg’s mechanistic proposal240 for the reaction of PBu3 with [Ru3(CO)12], as the latter fails to explain certain details. To round off this section the review of Cabeza and García-Álvarez on N-heterocyclic carbene chemistry of transition-metal carbonyl clusters is mentioned, wherein ruthenium and osmium NHC clusters are discussed.161b

Fig. 47 Products of the reaction of Ge(R2bzam)tBu (B120, R2bzam]N,N0 -disubstituted benzamidinate; R]tBu, iPr) with [Ru3(CO)12].

7.08.3.2.2

Further work on multinuclear Ru-, Os-species

Germanium is commonly used as modifier for reactivity and selectivity of heterogeneous transition-metal catalysts.241 This led Adams et al. to prepare several new compounds by addition of phenylgermanium ligands to tetraruthenium and tetraosmium carbonyl clusters.242 Analogous to tin compounds, oxidative addition of excess Ph3GeH to ruthenium and osmium clusters (H4M4(CO)12, M]Ru, Os) yields GePh3 and hydride ligands. Cleavage of a Ph and recombination with a hydride ligand eliminates benzene and generates a bridging GePh2 moiety.243 Among the prepared compounds with the general formula M4(m4-GePh)2(m -GePh2)2 +n(CO)10-n (M]Ru, n ¼ 0, 1, 2 (B125, Fig. 48), M]Os, n ¼ 1, 2 (B126)) are the first examples of quadruply bridging GePh ligands. The RudGe distances of the quadruply bridging GePh are slightly longer than the RudGe distances of an edge-bridging GePh2 ligand which is attributed to the higher coordination number (five) of the GePh moiety compared to a lower coordination number (four) for the GePh2 moieties.

Fig. 48 Germylene bridged clusters M4(m4-GePh)2(m -GePh2)2 + n(CO)10-n (M]Ru, n ¼ 0, 1, 2 (B125), M]Os, n ¼ 1, 2 (B126)).

Superior bi- and multimetallic heterogeneous catalysts can be prepared from mixed metal carbonyl clusters.244 Adams et al. used IrRu3(CO)13(m-H) (B127, Scheme 88) and HGePh3 to synthesize diverse iridium-ruthenium carbonyl clusters with germylene ligands (B128-B132) as potential precursors for new heterogeneous catalysts for the hydrogenation of unsaturated hydrocarbons.245

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Scheme 88 Reaction of IrRu3(CO)13(m-H) with HGePh3.

Gosh et al. attempted to prepare germaborane derivatives of arachno-[(Cp RuCO)2B2H6] by reaction with GeCl2 ∙ dioxane. Interestingly, they found that thermolysis of arachno-[(Cp RuCO)2B2H6] in presence of GeCl2 ∙dioxane yielded di-m-germylene complex [(Cp Ru)2(m-GeCl2)2(CO)2] (B133, Fig. 49) while boron species are eliminated. Each ruthenium carries a terminally bound carbonyl group and a Cp ligand, while the ruthenium atoms are bridged by germylene moieties. [(Cp Ru)2(mGeCl2)2(CO)2] was reacted with [CpFe(CO)B3H8] in an attempt to displace the germanium fragment with a borane fragment; however, the formation of germanocene was observed, resulting probably from the cleavage of the Cp ligand.

Fig. 49 Di-m-germylene complex [(Cp Ru)2(m-GeCl2)2(CO)2] (B133).

Kabir et al. examined the reactivity of ruthenium and osmium carbonyl clusters with Ph3ESPh (E]Ge, Sn). The objective was the creation of precursors to nanoconductors by synthesis of bimetallic complexes with unusual bonding modes. This was ought to be achieved by potentially capping sulfur ligands that would give the first ever bimetallic clusters containing sulfur-metal and tin-metal bonds simultaneously. [Ru3(CO)12] and [Os3(CO)10(MeCN)2] were reacted in benzene at 80  C with Ph3ESPh (E]Ge, Sn) to give new stannylene bridged clusters (B134-B136, Scheme 89). Of particular interest is the very short RudSn bond in B134 with a length of 2.538(1) A˚ , while comparable bonds show a length of 2.60–2.76 A˚ according to a CCDC search (Relevant Websites).

Scheme 89 Reactivity of [Ru3(CO)12] and [Os3(CO)10(MeCN)2] with Ph3MSPh (E]Ge, Sn).

Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

503

Kabir et al. investigated the reactivity of trinuclear osmium carbonyl m-hydride m-diphosphine clusters with Ph3SnH (Scheme 90).246 Two clusters that differ from each other by the length of the chelating m-diphosphine were at the center of attention. Refluxing [(m-H)2Os3(CO)8(m-dppm)] (dppm ¼ bis(diphenylphosphino)methane, B137) and [(m-H)2Os3(CO)8(mdppf )] (dppf ¼ 1,10 -bis(diphenyl-phosphino)ferrocene, B138) with Ph3SnH in toluene results in multiple osmium carbonyl stannylene bridged clusters (B139-B144). The reactivity difference is attributed to the properties of the bridging diphosphine; dppm is relatively rigid, while dppf is highly flexible.

Scheme 90 Reactivity of [(m-H)2Os3(CO)8(m-dppm)] (B137) and [(m-H)2Os3(CO)8(m-dppf )] (B138) with Ph3SnH.

Subsequently, Kabir et al. investigated the ruthenium congener of B137.247 Reaction of Ph3GeH with Ru3(CO)10(m-dppm) (B137) at room temperature substitutes one carbonyl group of the ruthenium opposite of the diphosphine ligand with GePh3 (Scheme 91). This step is easily repeated upon addition of a second equivalent of Ph3GeH, but then GePh3 substitutes a carbonyl group of a ruthenium adjacent to diphosphine (B147). Thermolysis of B145 gives a Ru2 species that is bridged by the diphosphine as well as a diphenylgermylene (B146).

Scheme 91 Reactivity of Ph3GeH with Ru3(CO)10(m-dppm) (B137).

Contrary to their expectations, Saito et al. where not able to isolate the 5-stannole dianion complexes from the reaction of tetraethyldilithiostannole B148 with [Cp RuCl]4, but instead characterized two bis(stannylene)-bridged dinuclear ruthenium complexes (Scheme 92).248 Curiously, Cp Ru(m-SnC4Et4)2RuCp (B149) with a butterfly structure and [Li(Et2O)]2[Cp Ru(mSnC4Et4)2RuCp ] (B150) show a reversible redox behavior. Also, the RudRu bond of B149 is the shortest among dinuclear ruthenium complexes with CpRu or Cp Ru units so far. According to calculations, there is a s-bond between the ruthenium atoms and the two Ru2Sn rings accommodate one two-electron three-center bonds each.

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Scheme 92 Reactivity of tetraethyldilithiostannole B148 with [Cp RuCl]4.

Takao et al. studied the oxidative addition of RSiH3 (R]Ph, tBu) on trinuclear heterometallic clusters of Ru and group 9 metals, (Cp Ru)2(Cp M)(m-H)3(m3-H) (B151, M]Co, Rh, Ir) in the context of hydrosilylation and dehydrogenative coupling of hydrosilanes.249 Reaction of trinuclear clusters B151 with RSiH3 yields m3-silyl complexes B152 (Scheme 93). The electronic situation of B152 involves three-center two-electron MdHdSi interactions, which have not been observed before. Reaction of the iridium phenylsilyl congener of B152 with CO forms cluster B153, that readily eliminates dihydrogen from to generate m3-silylene B154. Further elimination of dihydrogen from B154 to form a m3-silylyne complex was not observed.

Scheme 93 Reactions of (Cp Ru)2(Cp M)(m-H)3(m3-H) (B151, M]Co, Rh, Ir) with RSiH3 and CO.

Takao et al. investigated the reactions of heterobimetallic trinuclear complexes of ruthenium and platinum in a linear alignment.250 Photolysis of Cp Ru(m-H)4RuCp in the presence of Pt(PtBu3)2 gave (Cp Ru(H)2)2(Pt)(m-PtBu2)2(m-H)2 (B155, Scheme 94). The successive oxidative addition of two SidH bonds of Ph2SiH2 formed m2-silylene complexes cis-B156 and trans-B156.

Scheme 94 Reaction of (Cp Ru(H)2)2(Pt)(m-PtBu2)2(m-H)2 (B155) with Ph2SiH2.

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Komuro and Tobita reported the unexpected thermal reaction of a lutidine-based bis(silyl) ruthenium complex (B157) that forms a m-silyl(m-silylene) diruthenium complex (B158) involving an unprecedented three-center-two-electron RudSidC interaction (Scheme 95).251 The understanding of these interactions is crucial for the transformation of organosilicon compounds where SidC bonds are cleaved by transition metal complexes. During this cleavage, a s-complex having a three-center-two-electron MdSidC bond, just like in B158 may be involved.

Scheme 95 Thermal reaction of lutidine-based bis(silyl) ruthenium complex (B157).

7.08.4

Nitrogen coordinating ligands

Ruthenium and osmium complexes that contain N-coordinating ligands span an vast thematic range and comprise a plethora of publications in the recent years. A shallow general overview is avoided in this section for the benefit of a more in-depth review of recent developments of nitrosyl incorporating complexes. Interested readers will find more information on general ruthenium and osmium complexes with N-coordinating ligands in the references and the literature cited therein.252 Ruthenium nitrosyl complexes are of major importance for medicinal applications due to manifold effects on physiological systems, as is evident from the presented research (vide infra). The diverse roles of nitric oxide are outlined, helpful reviews are cited and the most relevant publications are summarized.

7.08.4.1

Introduction into RudNO complexes for medicinal applications

Before 1980, in the Dark Ages of nitric oxide (NO) biochemistry, the molecule had a bad reputation. It was associated with the decomposition of atmospheric ozone, suspected to be carcinogen and a pollutant playing a key role in the causes for acid rain, while only very little was known about biological roles of NO.253 However, the chemical properties of NO have been subject of many investigations for a long period of time.254 NO is a diatomic radical species (NO∙) and displays oxidizing and reducing properties, reactivity as radical initiator and, most relevant in context of this section, as a strong ligand at transition metal centers.253b,255 The importance of NO in a broad variety of physiological processes became apparent in the 1980s.256 Louis J. Ignarro and others linked the endothelium-derived relaxant factor (EDRF) that was once classified as peptide-signaling agonist, to NO.256–257 Subsequent studies revealed the formation of this biological messenger molecule from arginine by heme-containing NO synthase (NOS). Following investigations soon showed that the net concentration of NO in different tissues are subjected to various feedback pathways, involving activation of enzymes eNOS (endothelial) and iNOS (inducible), as well as main target soluble guanylate cyclase (sGC).253b The reputation reversal of NO led to its election as molecule of the year in 1992.253a,258 “For their discoveries concerning nitric oxide as a signaling molecule in the cardiovascular system,” the Nobel Prize in Physiology or Medicine 1998 was awarded jointly to Robert F. Furchgott, Louis J. Ignarro and Ferid Murad (Relevant Websites).257e,259 Today it is well established, that NO is an endogenous intercellular regulator that participates in several mammalian physiological processes and pathologies, including vasodilatation (blood pressure regulation),260 host immune response261 and neurotransmission.256,262 Pathogen killing,263 platelet activation and adhesion,264 wound healing,265 tissue repair,266 redox balance,256,267 cellular apoptosis (programmed cell death) at relatively high concentrations, inhibition of smooth muscle proliferation and migration and cancer biology and is implicated in both tumor growth and suppression267b,268 are also associated with NO.253b,257c,262b,269 These physiological roles of NO are complex and dependent on concentration, kinetic and thermodynamic factors.256,270 Depending on the circumstances, beneficial or harmful effects go along with high or low concentrations of NO.256,270d,271 Based on these characteristics, NO effects have been categorized as direct or indirect.270d At low concentrations, NO directly interacts with its molecular targets with regulatory and protective implications, while at higher concentrations reactions with O2 or O−2, lipid alkoxyls and metalloproteins yield reactive nitrogen oxide species (RNS) among others that are physiologically active.272 The latter case is expected during immune response, and is related to nitrosative and oxidative stress, which cause cell death and tissue injury. Due to the reasons described above, NO-based therapies have been proposed for the treatment of NO dysfunction-associated ailments.264b,268c,273 Limited use of gaseous NO274 led to the use of N-hydroxyguanidines,275 organic nitrates or nitrites,276 nitrosothiols,277 dieazeniumdiolates278 and metal nitrosyls253b,279 among others. Of the metallo nitrosyls, ruthenium nitrosyl complexes have received considerable attention.261c,269e,272a,280 In addition, ruthenium scavengers were developed for the reduction of bioavailable NO.269e,280l,281 Site-specific delivery of metal nitrosyls is possible by incorporation into matrices, such as silica,

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organic inorganic hybrid materials, polyurethane, poly(vinyl alcohol), poly(methylmethacrylate), and dendrimers.282 Photoactive metal nitrosyls have become the center of attention. During photodynamic therapy (PDT), stable and photoactive metal nitrosyls are deposited in close proximity to malignant sites, where NO release is triggered by light excitation (Scheme 96).253b,283 This photochemical strategy allows for control of location, timing and dose of NO that is delivered to the tissue.279k

Scheme 96 Schematic representation of light-induced release of NO from photolabile metal nitrosyls.

However, low absorptivity at the longer visible and near-infrared (NIR) wavelengths is a problematic trait of many of these metal nitrosyl complexes, since that range offers optimal tissue penetration properties.279k Recently, chromophore conjugation strategies are attempted to shift the photosensitive range of metal nitrosyls towards infrared light.253b,279k,280m Ruthenium nitrosyls combine outstanding properties, as they exhibit low cytotoxicity against host cells, water solubility, stability to air oxidation, and allow for tailoring of the complexes.272a,284 Research efforts focus on how to red-shift the photobands of ruthenium nitrosyls and increase the stability of RudNO bonds under physiological conditions, while incorporating polydentate ligands to prevent further speciation of the drug.280m In contrast, osmium-based drugs have not been as well-studied as ruthenium-based drugs, which is likely due to its high toxicity,285 higher cost and the increased synthetic effort.286 Furthermore, 105-fold lower ligand exchange rates of osmium compounds compared to the corresponding ruthenium complexes render them unfavorable on the timescale of cellular processes.287 Recent reviews on ruthenium nitrosyl complexes for medicinal applications252d,253b,272b,280m,283a,283c,284,288 and ruthenium-based nitrosyl scavengers289 are referenced for further reading. Hereafter, the most cited (including non-medicinal) publications are summarized chronologically.

7.08.4.2

Recent work on RudNO complexes

Olabe et al. reported the synthesis of a new ruthenium nitrosyl complex, containing tris(1-pyrazolyl)methane (Tpm) and 2,20 -bipyridine (bpy) coligands (C1, Fig. 50).290 The compound exhibits structural and spectroscopic properties, that are characteristic for the (RuNO)6 configuration. However, C1 is a strongly electrophilic species as indicated by the values of nNO and ENO +/ − NO. The significant nitrosonium character is reinforced by the highest nucleophilic rate constant for OH addition (kOH) observed in this context, until then. Da Silva et al. reported their findings on structurally related complexes (C2).280f The influence of ancillary ligands (L ¼ bpy, quinonediimine and o-phenylenediamine) on the nitric oxide photorelease by tridentate terpyridine (terpy) ruthenium nitrosyl complex in context of vasodilative properties was under investigation. Apparently, quinonedimine promotes p-backbonding of ruthenium to NO, thus enabling high quantum yields in the UV region and thereby inducing vasorelaxation with potential clinical application. Mascharak et al. successfully photosensitized a ruthenium nitrosyl complex by incorporation of tricyclic chromophore resorufin (C3).291 Compared to the hydroxy congener [(Me2bpb)Ru(NO) (OH)] (H2Me2bpb ¼ 1,2-bis(pyridine-2-carboxamido)-4,5-dimethyl benzene) a red-shift form lmax ¼ 395 nm to lmax ¼ 500 nm, while tripling the molar attenuation coefficient e (4500 M−1 ∙ cm−1 to 12,000 M−1 ∙ cm−1), for the dye-containing complex [(Me2bpb)Ru(NO)(Resf )] was achieved. In depth in vitro and in vivo studies on the antiproliferative and trypanocidal activities of trans-[Ru(NO)(NH3)4L]3+ (C4) for the treatment of Chagas’ disease were performed by Franco et al.261c It was found that NO liberation upon reduction of the complexes of trans-[Ru(NO)(NH3)4isn](BF4)3 (isn ¼ isonicotinamide) and trans-[Ru(NO) (NH3)4imN](BF4)3 (imN ¼ imidazole coordinated by nitrogen) is responsible for the physiological activity. The authors suggest administration of the compounds in anticipation of the parasitema peak of Trypanosoma cruzi as preliminary protocol for the chemotherapy of Chagas’ disease. Kuwata and Takao et al. observed the formation of nitrosyl-imidazolyl complex (C5) upon treatment of [Cp RuCl(LH)] (LH ¼ N-(2-pyridyl)benzimidazolin-2-ylidene-k2N,C) with AgNO2.292 Reversible protonation of C5 by TfOH affords NHC complex [Cp Ru(NO)(LH)][OTf]2. The framework of [Ru(terpy)(L)(NO)]n+ (L ¼ 2-phenylimidazo[4,5-f] 1,10-phenanthroline, C6) was investigated by Lahiri et al. in terms of formation, reactivity and photorelease of ruthenium bound nitrosyl.293 Tri- and dicationic complexes with (RudNO)6 and (RudNO)7 configurations were isolated as perchlorate salts, respectively. One-electron transfer reversibly interconverts NO+ and NO• redox states, with results in a 300 cm−1 shift in the nNO stretching frequency. Building on their previously discussed research, Mascharak and Rose experimentally and theoretically studies the photosensitization of ruthenium nitrosyl complexes by a series of heavy atom substituted (O: resorufin, S: thionol, Se: selenophore) coordinated chromophores (C7).294

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507

Fig. 50 Structure of ruthenium nitrosyl complexes (C1–C7) referenced herein.

A systematic red-shift in the absorption spectrum is induced by the presence of heavier atoms in the dye (lmax, resorufin ¼ 500 nm, lmax, thionol ¼ 530 nm and lmax, selenophore ¼ 535 nm). Several ruthenium nitrosyl complexes, incorporating pyridinefunctionalized NHCs (C8, Fig. 51) were synthesized by Chen et al. by transmetalation from the corresponding silver carbene complexes.295 Complexes C8 have been applied in the transfer hydrogenation of ketones, where complexes bearing a tBu (]R) moiety excelled. This was attributed to steric protection of the highly active 16-electron species. Post-catalytic analysis revealed the absence of the nitrosyl group; hence complexes C8 merely act as precursors to the active species. In 2010, Silva et al. published further research on the potential treatment of Chagas’ disease with ruthenium nitrosyl complexes.296 The ability of compounds of − general formula cis-[Ru(NO)(bpy)2L]Xn (L ¼ imN, miN, SO2− 3 ; miN ¼ 1-methylimidazole; X ¼ PF6; C9) to lyse Trypanosoma cruzi (Trigonoscuta cruzi) in vitro and in vivo was investigated. The complexes exhibit inhibitory effects on T. cruzi GADPH (IC50 89–153 mM) and evidently interact with the cys166 site by S-nitrosylation. In vivo studies in infected mice revealed eradication of any amistogotes from the myocardial tissue with survival rates of 80 and 60% at a dose of 385 nmol kg−1 for cis-[Ru(NO) (bpy)2imN](PF6)3 and cis-[Ru(NO)(bpy)2SO3]PF6, respectively. Lopez et al. loaded prominent NO-donor [Ru(terpy)(bdqi)NO] (PF6)3 (bdqi ¼ 1,2-benzoquinonediimine, C10) onto solid lipid nanoparticle (SLN) carriers by microemulsification method for improved stability and for targeted administration in the context of skin cancer treatment.297 In vitro kinetic studies confirmed significantly improved complex stability. Interestingly, the NO release of ruthenium nitrosyl loaded SLN was about twice the amount compared to ruthenium nitrosyl complex in solution. The photocytotoxic activity of nitrosyl phthalocyanine ruthenium complex (C11, Fig. 52) was studied by Silva et al.298 Nitric oxide and singlet oxygen extrusion by C11 was observed upon irradiation. The biological impact of this synergetic effect was investigated using the B16F10 cell line (melanoma, skin cancer), where significant inhibition of cell growth was observed. An increase of activity by 25% was achieved by encapsulation of C11 into liposomes as drug delivery systems. The anti-tumor

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Fig. 51 Structure of ruthenium nitrosyl complexes (C8–C10) referenced herein.

Fig. 52 Structure of ruthenium nitrosyl complexes (C11-C17) referenced herein.

properties of novel ruthenium nitrosyl complex C12 were at the center of attention of Costa-Neto et al.299 C12 demonstrates activity against tumor cell lines HeLa (cervical cancer) and Tm5 (murine tumor cells) in cytotoxic assays (in vitro) with comparable efficacy to cis-platin. Tumor mass reduction was observed in vivo and fragmentation of DNA upon binding to C12, which results in apoptosis, is indicated by obtained data. A series of ruthenium nitrosyl complexes [RuX(NO)py4]Y2
nH2O (X]Cl−, Br−; Y]PF−6, BF−4, Cl−, Br−; C13) were investigated in terms of structural influence on the photochromic response by Malfant et al.300 SC-XRD and

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509

DFT calculations revealed the presence of intermolecular interactions between the nitrosyl ligand and counterions with beneficial impact on the photoswitching efficiency. Lehnert et al. reported the synthesis, spectroscopic analysis and photolabilization of a series of water soluble ruthenium(III) nitrosyl complexes in the (RudNO)6 configuration with the coligand Tpa (tris(2-pyridylmethyl)amine) (C14).301 The quantum yields (f) for NO photorelease upon irradiation with UV light were studied in aqueous systems. It was found that neutral water does not promote photodissociation of NO, which is a major obstacle in the endeavor of designing photolabile (RudNO)6 delivery agents for in vivo applications. Arion et al. reported distinct gap of antiproliverative activities of ruthenium and osmium nitrosyl complexes with azole heterocycles.302 The compounds (X)[cis/trans-MCl4(NO)L] (X] H2ind+, H2pz+, H2bzim+, H2im+, Bu4N+, Na+; M]Ru, Os; L ¼ Hind, Hpz, Hbzim, Him; Hind ¼ 1H-indazole, Hpz ¼ 1H-pyrazole, Hbzim ¼ 1H-benzimidazole, Him ¼ 1H-imidazole; C15) were assayed in the human cancer cell lines A549 (non-small cell lung carcinoma), CH1 (ovarian cancer), and SW480 (colon adenocarcinoma). According to obtained data, the ruthenium homologs are up to 410-fold more active than their osmium counterparts. Ascorbic acid may activate the ruthenium complexes by hydrolysis of one RudCl bond, as revealed by ESI-MS, whereas the osmium complexes remain inert under the applied conditions. Tfouni et al. entrapped trans-[Ru(NO)Cl(cyclam)](PF6)2 (C16) and [Ru(NO)(H-edta)] (C17) in poly(D,L-lactic-co-glycolic) acid (PLGA) nanoparticles by double emulsification method.303 It was found that the molecular structure and properties of the complexes were preserved in the emulsification process, which allowed in vitro studies on B16F10 cells (melanoma, skin cancer). The cytotoxicity of the complexes is consistent with the rate constants of NO release and inhibition of the cytotoxic properties in the presence of NO scavenger carboxy-PTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was observed. González and Freitag investigated ground and excited states (ES) of ruthenium nitrosyl complex C18 (Fig. 53) and gained insight into NO photodissociation.304 Time-dependent DFT (TD-DFT) revealed a significant admixture of RuIII(NO)0 to the previously sole postulated RuII(NO)+ configuration. The lowest singlet and triplet excited states favor NO dissociation by population of orbitals. According to molecular dynamics the intersystem crossing is ultrafast (99% conv., >99% e.e., TOF ¼ up to 35,000 min−1, measured at 2 min).

Fig. 64 Ruthenabicycle complexes for DH.

These even outperformed the previously reported complexes D21, one of the most efficient catalysts to date (4 h, >99% conv., 98% e.e. for Ru, 15 h, 90% conv., TOF ¼ 10,000 h−1, 90% e.e. for Os).363 The corresponding mechanism (Scheme 100) was evaluated in multiple publications.364 Herein, the high activity of the catalysts was attributed to the bifunctional donor-acceptor characteristic of the ternary catalytic species.

Scheme 100 Catalytic cycle for the asymmetric hydrogenation with ruthenabicycle type complexes (M]Ru). Non-relevant atoms and ligands have been removed for clarity.

In 2019 Kühn and Baratta et al. reported the preparation of diphosphine complexes, starting from precursor D22, enhanced by fluoroacetate ligands (D23–D25, Fig. 65) as well as their catalytic activity regarding the TH of aromatic substrates.365 These types of complexes have been known since the 1990s, however, due to difficulties in their isolation, these have only been scarcely examined since then, albeit offering promising catalytic activities.366 The complexes reported by Kühn et al. are stabilized by water, glycol and amine ligands as well as soluble in a variety of organic solvents. Whilst the monodentate complex D23 showed only low activity (0.1 mol% [cat.], acetophenone, isopropanol, 90  C, 48 h, 21% conv.), the corresponding 1,4-bis-(diphenylphosphino)butane (dppb) complexes D24 and D25 led to much more promising results (0.1 mol% [cat.], acetophenone, isopropanol, 90  C, 8 h, 78% conv., TOF ¼ 5000 h−1 for D24, 30 min, 97% conv., TOF ¼ 4200 h−1 for D25). The best performance was observed for the TH of cyclohexane with D24 (0.03 mol% [cat.], isopropanol, 90  C, 4 h, 82% conv., TOF ¼ 22,000 h−1).

Fig. 65 Fluoroacetate diphosphine complexes by Kühn and Baratta et al.

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517

In the following year, the corresponding mixed acetate/acetylacetonate complexes were described (DX4-DX6, Fig. 66).367 Of these, the diacetylacetonate Ru(acac)2(dppb) complex (D28) was inactive (acetophenone, 0.1 mol% [cat.], isopropanol, 90  C, 8 h, 1% conv.), whilst the mixed complex (D27) yielded superior performance (acetophenone, 0.1 mol% [cat.], isopropanol, 90  C, 8 h, 75% conv., TOF]930 h−1) compared to the corresponding diacetate complex (D26, acetophenone, 0.1 mol% [cat.], isopropanol, 90  C, 8 h, 53% conv., TOF ¼ 100 h−1). Much higher activities (acetophenone, 0.1 mol% [cat.], isopropanol, 90  C, 1 h, 97% conv., TOF ¼ 2200 h−1 for D26, 5 min, 98% conv., TOF ¼ 26,000 h−1 for D27) were observed following the addition of 2-(aminomethyl) pyridine (ampy), except for D28 (acetophenone, 0.1 mol% [cat.], isopropanol, 90  C, 8 h, 4% conv.).

Fig. 66 Mixed acetate/acetylacetonate complexes.

7.08.5.3

Complexes with tridentate phosphorous ligands

Similar to bidentate ligands, tridentate phosphorous pincers have undergone a large amount of research, as their design offers numerous possibilities for modifications. One important type of compound are Milstein’s 2006 catalysts (D29, Scheme 101.) for the hydrogenation of esters (Ethyl benzoate, 1 mol% [cat.], H2, 5.3 bar, 140  C, 16 h, 12% conv., 11.5% yield benzyl alcohol and 12% yield ethanol) as well as organic carbonates, carbamates and formats as alternative routes to methanol based on CO2 and CO.368 However, the PNN complexes (115  C, 4 h, 99.2% conv., 96% benzyl alcohol, 99% ethanol) were shown to be more active than their PNP analogs. This was attributed to easier opening of the chelate ring of the amine species, providing an active site.

Scheme 101 Milsteins catalyst and hydrogenative coupling of esters.

In the following years, several other groups reported related catalysts (Fig. 67) such as the Firmenich catalysts by Saudan et al. in 2007 (D30, methyl 3-phenylpropanoate, 0.05 mol% [cat.], H2, 50 bar, 100  C, 2.5 h, 90% yield),369 the Ru-MACHO complex (D31) by Takasago chemists (Kuriyama, Ino, Ogata, Sayo, Saitoa and Matsumoto) in 2011 (0.05 mol% [cat.], methyl lactate and methyl menthoxyacetate),370 and the PNN complexes (D32) by Gusev et al. in 2012 (dehydrogenative coupling of ethanol, 0.01 mol% [cat.], 24 h, 91% conv. for Ru, 13% conv. for Os).371

Fig. 67 Firmenich catalyst (left), Ru-MACHO (center) and PNN pincer complex by Gusev et al. (right).

In 2013, Ikariya et al. reported quantum chemical calculations concerning the mechanism for the asymmetric TH of ketones.372 It was assumed, that reactions proceed via two steps in solution (Scheme 102), (i) enantio-determining hydride transfer and (ii) proton transfer via contact ion-pair intermediate, contrary to previous results in gas phase calculations, in which a concerted three-bond asynchronous process was suggested.

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Ruthenium and Osmium Complexes Containing NHC and p-Acid Ligands

Scheme 102 Mechanism for the asymmetric TH of ketones as proposed by Ikariya et al.

In 2014 Beller et al. reported the generation of hydrogen from ethanol in water utilizing a pincer [RuHCl(CO)(iPR2PEtN(H) EtPiPr2)] complex (D33, Scheme 103).373 Herein, 70% acetic acid was achieved during the dehydrogenation reaction selectively (25 ppm catalyst in 10 mL 9:1 EtOH:H2O, 81  C, TOF ¼ 1246 h−1, TON ¼ 80,000).

Scheme 103 Acceptorless dehydrogenation of ethanol. Side products: Guerbet product (1-butanol) and ethyl acetate by dehydrogenative coupling.

In 2018, deeper mechanistic insights into coupling of alcohols utilizing Ru-pincer complexes were reported by Nguyen and Gauvin et al.374 A Tishchenko-like pathway was proposed as main catalytic cycle as well as a stereoselective proton-hydride exchange process between 1H and EtOH (Scheme 104).

Scheme 104 Proton-hydride exchange as proposed by Nguyen and Gauvin et al.

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In a final note, the previously described Ru-MACHO type complexes have also been utilized by Prakash et al. in 2020 for the first reported example of hydroxide based integrated CO2 capture from air and consecutive conversion to methanol (D34, D35, Scheme 105).375 The formed methanol was isolated via distillation. The high capture efficiency renders this method superior to existing amine-based routes.

Scheme 105 First reported hydroxide based integrated CO2 capture from air and consecutive conversion to methanol by Prakash et al.

Acknowledgment A.D.B. and M.J.S. thank the TUM graduate school for financial support.

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Relevant Websites www.scopus.com. www.nobelprize.org/prizes/chemistry/2005/summary—Nobel Prize in Chemistry. www.nobelprize.org/prizes/chemistry/2001/summary—Nobel Prize in Chemistry. www.nobelprize.org/prizes/medicine/1998/summary—Nobel Prize in Physiology or Medicine. www.ccdc.cam.ac.uk—Cambridge Crystallographic Data Centre (CCDC).

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7.09 Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis ☆ catalysis Noy B Nechmada, N Gabriel Lemcoffa, and Lionel Delaudeb, aDepartment of Chemistry, Ben-Gurion University of the Neguev, Beer-Sheva, Israel; bLaboratory of Catalysis, MolSys Research Unit, Institut de Chimie Organique (B6a), Université de Liège, Liège, Belgium © 2022 Elsevier Ltd. All rights reserved.

7.09.1 7.09.2 7.09.3 7.09.4 7.09.4.1 7.09.4.2 7.09.4.3 7.09.4.3.1 7.09.4.3.2 7.09.4.3.3 7.09.4.3.4 7.09.4.4 7.09.4.4.1 7.09.4.4.2 7.09.4.5 7.09.4.6 7.09.4.7 7.09.5 7.09.5.1 7.09.5.1.1 7.09.5.1.2 7.09.5.1.3 7.09.5.1.4 7.09.5.2 7.09.5.2.1 7.09.5.2.2 7.09.5.3 7.09.5.4 7.09.5.5 7.09.6 7.09.7 References

7.09.1

Introduction The early days: Ruthenium propenylidene complexes The challengers: Variations involving the ruthenium alkylidene moiety The breakthrough: Ruthenium benzylidene complexes Complexes with two phosphine ligands Complexes with two NHC ligands Complexes with mixed NHC/phosphine ligands Variations involving the nitrogen substituents of the NHC Variations involving the backbone substituents of the NHC Variations involving the ring size of the NHC Variations involving the heterocyclic core of the NHC Complexes with mixed NHC/pyridine ligands Complexes with two pyridine ligands Complexes with one pyridine ligand Complexes with mixed NHC/NHCEWG ligands Complexes with mixed NHC/phosphite ligands Complexes with mixed NHC/Schiff base ligands The state of the art: Chelated ruthenium benzylidene complexes Oxygen chelates Variations involving the benzylidene ring substituents Variations involving the oxygen substituents Variations involving the NHC ligand Anionic ligand exchange Sulfur chelates Variations involving the sulfur atom Anionic ligand exchange Selenium chelates Nitrogen chelates Phosphorus chelates The outsiders: Ruthenium benzylidyne complexes Conclusion and outlook

528 530 532 532 532 533 535 539 540 541 541 543 543 544 546 546 547 547 547 549 550 551 554 555 556 556 557 557 558 558 560 560

Introduction

During the late 1950s and the 1960s, several independent research groups reported an unknown chemical reaction that led to the formation of both higher and lower molecular weight olefinic products when alkenes were fed over molybdenum or tungsten catalysts supported on alumina, in an attempt to achieve their Ziegler-Natta polymerization.1,2 Shortly thereafter, a similar disproportionation was also achieved under homogeneous conditions using a mixture of tungsten hexachloride, ethanol, and ethylaluminum dichloride.3 Using similar homogeneous or heterogeneous experimental setups, cyclic olefinic monomers underwent a ring-opening polymerization that afforded amorphous, unsaturated polymers with unexpected structures.4 In 1967, Calderon first realized that the disproportionation of acyclic olefins and the ring-opening polymerization of cycloalkenes were two facets of the same transformation. He dubbed it “olefin metathesis” from the Greek word metayesiz, which means transposition.5 Indeed, the formation of all the products obtained through the various embodiments of olefin metathesis can be explained if one imagines the C]C double bonds of the various alkene substrates being cleaved, followed by a scrambling of the resulting fragments before they are joined together again (Scheme 1).

☆

Dedicated to Robert H. Grubbs (1942–2021), a pioneer and a leader in the development of ruthenium benzylidene catalysts for olefin metathesis.

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Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00177-3

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

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Scheme 1

Once olefin metathesis was identified as being a novel catalytic process, various hypotheses were formulated to rationalize its mechanism. Through careful analysis of the products obtained from different substrates,6,7 combined with deuterium and carbon-14 labeling studies,8–10 a “quasi-cyclobutane” species was suggested to be the key intermediate. While this proposal explained the distributions of products obtained and their isotopic compositions, the existence of such an intermediate was difficult to admit, since it would imply a [2 + 2] cycloaddition of two alkenes, which is forbidden under thermal conditions according to the Woodward–Hoffmann rules. Thus, to make the reaction allowed, the “quasi-cyclobutane” would somehow have to be coordinated to the metal center.8,11 However, a saturated alkane does not have any free valences to act as a ligand. In addition, cyclobutane was not detected in the reaction mixtures of metathesis experiments and was not a suitable substrate either.11 Besides, the product distributions and kinetics observed for the ring-opening metathesis of cycloolefins remained unexplained.12,13 Eventually, in 1971, Hérisson and Chauvin proposed a mechanism for olefin metathesis based on the [2 + 2] cycloaddition of an alkene with a metal alkylidene active species,12 which is still commonly accepted today.14 Four basic steps are involved in the putative catalytic cycle (Scheme 2). First, the [2 + 2] cycloaddition of a methylidene active species with an alkene substrate gives a monosubstituted metallacyclobutane. This intermediate then collapses via a [2 + 2] cycloreversion, forming a metal alkylidene complex and releasing ethene. Next, the cycloaddition of another equivalent of substrate affords a disubstituted metallacyclobutane whose cycloreversion releases the final product and regenerates the initial metal methylidene complex. Because two new C]C bonds are formed at the cost of breaking two other C]C bonds, the whole process is essentially thermoneutral and reversible, thereby leading to statistical mixtures of products and reagents, except when enthalpy is decreased through the relief of ring tension in highly strained cycloalkenes. Alternatively, the increase of entropy obtained through the release of small, gaseous alkenes, such as ethene or propene, may drive the reaction to completion as well.15,16

Scheme 2

The quest for well-defined metal alkylidene catalyst precursors tailored for olefin metathesis is a direct consequence of the mechanism postulated by Hérisson and Chauvin (Fig. 1). Indeed, Katz was one of the first chemists to demonstrate that metal carbene complexes were indeed suitable initiators for metathesis reactions.17 Using [(CO)5W(]CPh2)] (1), one of the most reactive metal-carbene species discovered by Fischer in 1964,18 he was able to promote cross-metathesis and ring-opening metathesis polymerization reactions.19,20 Yet, the catalytic efficiency of this system was very poor. A major breakthrough was achieved by Schrock et al. while studying alkylidene complexes of tantalum, niobium, and tungsten in a high oxidation state.21 They observed the formation of W]CH2 and W]CHCH2CH3 moieties by NMR spectroscopy upon mixing the neopentylidene complex 2 with

Fig. 1 Well-defined alkylidene complexes based on early transition metals.

530

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

1-butene.22 Moreover, the loss of one chlorido or triethylphosphine ligand dramatically improved the catalytic activity of these complexes. It was an important insight into the nature of the actual active species, which explained the low activity displayed thus far by octahedral coordination compounds. This seminal work ultimately led to the successful development of very active and selective precatalysts based on molybdenum and tungsten, known as the “Schrock catalysts” (3).23,24 These tetrahedral complexes feature a bulky imido, an alkylidene, and two alkoxy ligands bearing highly electron withdrawing groups. They are among the most active and selective initiators for a wide range of metathesis reactions. Unfortunately, being early transition metals, molybdenum and tungsten are highly oxophilic. Therefore, sophisticated techniques and equipment are required to synthesize, manipulate, and store the Schrock catalysts in the rigorous absence of air and moisture. Furthermore, their incompatibility towards polar functional groups, such as aldehydes, ketones, carboxylic acids, most alcohols, and primary amines, severely restricts their application field and often requires a thorough purification of the substrates and solvents used.15,16 A wealth of carbene complexes based on late transition metals from groups 8–10 have been investigated as potential metathesis catalysts to benefit from a carbophilic metal center that would display a high affinity toward alkenes and a better compatibility than molybdenum or tungsten with more polar functional groups. Among them, ruthenium derivatives stand out for their versatility and efficiency. Carbene precursors associated with iron, cobalt, and rhodium react stoichiometrically with olefins to afford cyclopropanes.25 Iridium tends to act only as a metathesis initiator in ill-defined systems.26 Moreover, both osmium and iridium complexes are generally less active than their ruthenium counterparts, and much more expensive. In 1992, Noels et al. showed that the [RuCl2(p-cymene)(PCy3)] complex 4 (p-cymene is 4-isopropyltoluene, PCy3 is tricyclohexylphosphine) promoted the ring-opening metathesis polymerization (ROMP) of strained and low-strained cyclic olefins upon activation with ethyl diazoacetate.27 The in situ formation of a ruthenium carbene active species was invoked to justify this metathetical activity (Scheme 3). A year later, Grubbs and coworkers were able to detect the presence of a ruthenium carbene species in solution by 1H NMR spectroscopy when the [RuCl2(PPh3)3] complex was reacted with ethyl diazoacetate.28 These two reports marked the premises for the synthesis and the isolation of well-defined ruthenium alkylidene catalyst precursors for olefin metathesis.

Scheme 3

7.09.2

The early days: Ruthenium propenylidene complexes

The first well-defined ruthenium alkylidene complex was isolated and characterized by Grubbs et al. in 1992.29 Thus, 3,3-diphenylcyclopropene was reacted with either [RuCl2(PPh3)]3 or [RuCl2(PPh3)]4 in a mixture of benzene and dichloromethane at 53  C for 11 h (Scheme 4). After cooling, the solvents were evaporated under vacuum and the residue was taken up with benzene and pentane. The resulting suspension was decanted and the supernatant liquid was removed with a cannula. This washing procedure was repeated two more times to ensure the complete separation of all the free triphenylphosphine byproduct. Pure diphenylpropenylidene complex 5 was obtained in almost quantitative yield and its structure was confirmed by X-ray diffraction (XRD) analysis. In the solid state, this 16-electron species was indefinitely stable under an inert atmosphere and tolerated air for several minutes. It remained stable for weeks in deoxygenated organic solvents. Most remarkably, compound 5 also resisted degradation for several days when dissolved in a benzene/dichloromethane mixture containing water, alcohol, or hydrogen chloride in diethyl ether. This robustness sharply contrasted with the extreme sensitivity toward oxygen and moisture displayed by Schrock-type molybdenum catalysts, and is therefore one of the main reasons why ruthenium-based olefin metathesis precatalysts have become so popular nowadays.30,31

Scheme 4

Although the 3,3-diphenyl-2-propen-1-ylidene moiety was disordered about a twofold axis, thereby preventing an accurate determination of its metrics, the molecular structure of complex 5 clearly showed that the alkylidene fragment occupied the apical position of a square pyramid whose base consisted of two phosphine and two chlorido ligands.29 The neutral and anionic ligands were disposed in a trans configuration to minimize steric repulsions. This arrangement, shown in Fig. 2, serves as the basis for the design of most of the well-defined ruthenium alkylidene catalyst precursors investigated so far.

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

531

Fig. 2 Generic structure of a ruthenium alkylidene catalyst precursor for olefin metathesis.

Whereas the presence of strongly electron-withdrawing ligands enhanced the catalytic activity of the molybdenum-based systems developed by Schrock, Nguyen and Grubbs soon noticed that the ruthenium initiators required powerful electron-donating ligands to display high catalytic efficiencies.32 Thus, the replacement of triphenylphosphine in complex 5 with the bulkier and stronger s-donor tricyclohexylphosphine (PCy3) afforded complex 6, which was isolated as a mixture of cis and trans isomers, the latter being predominant (Scheme 5). Further attempts to modulate the catalytic activity of complex 5 by exchanging its PPh3 and Cl− ligands with other related neutral and anionic species led to the following sequences of activity: PPh3  PiPr2Ph < PCy2 Ph < PiPr3 < PCy3 and I− < Br− < Cl−.33 Other variations involving the [RuX2(PR3)2(]CHdCH]CPh2)] scaffold include a Cl−/CF3CO−2 exchange with silver(I) trifluoroacetate,34 the use of the water-soluble PhP(p-C6H4SO3Na)2 phosphine instead of PPh3,35 or the anchoring of its phosphine ligands on a cross-linked polystyrene resin.36

Scheme 5

Severe practical limitations to synthesize the highly reactive 3,3-diphenylcyclopropene on a large scale stimulated the quest for alternative strategies to introduce an alkylidene fragment onto a ruthenium center. Thus, in 1998, Grubbs et al. showed that the rearrangement of commercially available propargyl halides provided another, more convenient access to ruthenium propenylidene complexes.37 The reactions proceeded via an insertion-elimination of the halogenated alkynes on the air-sensitive hydrido complex [RuCl(H)(H2)(PCy3)2] (7), which was obtained in 94% yield from [RuCl2(cod)]x and PCy3 under hydrogen pressure (1.5 atm) in sec-butyl alcohol containing triethylamine at 80  C for 20 h. Because this procedure utilizes readily available starting materials and does not require a phosphine exchange step, it is easy to implement and atom-efficient. The reaction is currently performed on a multi-kilogram scale with 3-chloro-3-methyl-1-butyne to produce catalyst 8 (Scheme 6). Other benzylic, secondary, or tertiary propargyl chlorides and bromides were less synthetically attractive due to the formation of unwanted byproducts.37 Although slightly less active than the benzylidene species described in Section 7.09.4, complex 8 is also less expensive to produce and often sufficient for initiating ROMP reactions. Thus, it has been used predominantly in commercial applications for the synthesis of polymeric materials.

Scheme 6

Another convenient synthetic route to prepare ruthenium complexes bearing 3,3-dimethylpropenylidene ligands was reported by Hofmann et al. in 2002.38 It relied on the treatment of the readily available and easy to handle Wilkinson’s hydride [RuCl(H) (PPh3)3] with 3-chloro-3-methyl-1-butyne (Scheme 7). When the reaction was carried out in dichloromethane at −15  C, complex 9 was isolated in 75% yield (93% purity), and shown by X-ray crystallography to display the expected square pyramidal geometry around its ruthenium atom. When the reaction was performed in a 3:1 CH2Cl2/CH3CN mixture at room temperature, the hexacoordinate acetonitrile adduct 10 was obtained in 69% yield. NMR experiments confirmed that the addition of an excess of CH3CN to a solution of complex 9 in CD2Cl2 led to the reversible coordination of a sixth monodentate and labile ligand trans to the carbene moiety.

532

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 7

7.09.3

The challengers: Variations involving the ruthenium alkylidene moiety

Several variations involving the ruthenium alkylidene motif have been investigated to avoid the lengthy and hazardous multi-step synthesis of 3,3-diphenylcyclopropene needed to introduce the propenylidene moiety of complex 5 (cf. Scheme 4).39 Among them, compounds featuring a vinylidene,40–43 allenylidene,42–45 or indenylidene43,46,47 moiety have attracted a great deal of attention from the organometallic community (Fig. 3). In many cases, these precatalysts are more easily accessible than their alkylidene counterparts and they are more resilient in terms of temperature and functional group tolerance. Nevertheless, the most spectacular developments in the field of olefin metathesis were achieved with the introduction of a benzylidene unit on the metal center. This milestone was soon followed by the discovery that a chelating ortho-substituent on this fragment led to highly stable, yet remarkably active catalyst precursors. Indeed, the vast majority of recent developments in the catalytic engineering of ruthenium complexes for olefin metathesis revolves around a benzylidene scaffold chelated to the metal center through various heteroatoms, such as oxygen, nitrogen, or sulfur, to name only the most prominent ones. In the following sections, we strove to provide a comprehensive survey of the multiple types of ruthenium benzylidene complexes that have been described in the literature within the years 1995–2021. The emphasis is placed on their synthesis, characterization, and chemical reactivity rather than their catalytic activity because olefin metathesis is covered in a distinct chapter. Nonetheless, we have highlighted some remarkable structure/activity relationships that helped tailoring the coordination sphere of the metal center through ligand design in order to achieve the highest possible activity and selectivity toward various kinds of metathesis reactions. The compounds under scrutiny are classified according to the nature of their benzylidene moiety, first, and the other ancillary ligands next. A brief overview of the few ruthenium benzylidyne complexes that have been investigated thus far in connection with olefin metathesis is also included at the end of this chapter.

Fig. 3 Examples of metathetically active ruthenium alkylidene fragments.

7.09.4

The breakthrough: Ruthenium benzylidene complexes

7.09.4.1

Complexes with two phosphine ligands

In 1995, Grubbs et al. first installed a benzylidene fragment on [RuCl2(PPh3)3] by reacting it with phenyldiazomethane at −78  C.48 A subsequent phosphine exchange with PCy3 and repeated washing with acetone to remove the displaced triphenylphosphine afforded [RuCl2(]CHPh)(PCy3)2] (12) as a purple, microcrystalline powder in 77% overall yield (Scheme 8). In 1996, the two steps were advantageously combined in a one-pot procedure to allow the preparation of complex 12 in nearly quantitative yield within 1 h.49 The procedure was successfully and safely scaled up to prepare multi-kilogram batches and dichloro (benzylidene)bis(tricyclohexylphosphine)ruthenium(II) is now commercially available and often referred to as the “(firstgeneration) Grubbs catalyst”. Its molecular structure was solved in 2009 by Schore and coworkers who co-crystallized it with benzene.50 The benzylidene unit occupied the apical position of a distorted square pyramid and was nearly coplanar with the two trans-chlorido ligands.

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

533

Scheme 8

Alternatively, it was also possible to install a benzylidene fragment on [RuCl2(PPh3)3] from a sulfur ylide generated in situ by deprotonating benzyldiphenylsulfonium tetrafluoroborate with a strong base at −30  C.51,52 After a phosphine exchange with PCy3 carried out in the same pot, complex 12 was isolated in 96% yield on a half-millimolar scale (Scheme 9). The procedure was extended to other sulfonium salts acting as carbenoid precursors, thereby leading to [RuCl2(]CHR)(PCy3)2] products with R ¼ H, Me, CO2Me, or CH]CH2. Recourse to polymer-supported sulfonium salts allowed the anchoring of either the diphenyl sulfide byproduct (for easier recycling) or the benzylidene complex (for heterogeneous catalysis) on a Merrifield resin.

Scheme 9

Additional variations involving the [RuX2(]CHR)(PR3)2] pattern were achieved through the use of various other alkyl-, aryl, or diaryldiazoalkanes instead of phenyldiazomethane to transfer an alkylidene fragment onto [RuCl2(PPh3)3].48,49 Triisopropylphosphine and tricyclopentylphosphine were also employed instead of tricyclohexylphosphine as bulky and basic phosphines.49 Furthermore, a stoichiometric cross-metathesis between the benzylidene complex 12 and various terminal alkenes substituted with donor groups allowed Louie and Grubbs to prepare a series of well-defined ruthenium complexes with the generic formula [RuCl2(]CH(ER))(PCy3)2] (ER ¼ OEt, SEt, SPh, N(carbazole), or N(pyrrolidinone)) in 66–86% yields.53 The crystal structures of several of these Fischer-type carbene complexes were determined. They were similar to the one of the parent alkylidene compound. No methylidene byproducts were observed in any of these reactions. Lastly, the influence of the chlorido ligands on the metathetical activity of complex 12 was investigated by replacing them with bromide or iodide anions.54 The halogen exchange was accomplished with LiBr or NaI in THF, by analogy with similar reactions performed on complex 6.33 However, full experimental details were not disclosed.

7.09.4.2

Complexes with two NHC ligands

Since the first imidazol-2-ylidene derivative was isolated and characterized by Arduengo and coworkers in 1991,55,56 N-heterocyclic carbenes (NHCs) have become ubiquitous ligands for organometallic chemistry and homogeneous catalysis.57–60 These divalent carbon species are neutral, two-electron donors. They behave as phosphine mimics, yet they are better s-donors and p-acceptors than most phosphines.61–63 Accordingly, they form stronger bonds to metal centers. Indeed, the typical RudNHC bond strengths were calculated to be 20–40 kcal/mol stronger than their RudPR3 counterparts.64 Furthermore, the electronic and steric parameters of nucleophilic carbenes are liable to ample modifications simply by varying the nature of their heterocyclic ring and its substituents.65,66 Altogether, these remarkable properties have contributed to the development of hundreds if not thousands of NHCs, which have found a myriad of applications as ancillary ligands to fine-tune the catalytic activity of transition metal complexes,67–71 as well as main group elements,70,72,73 or lanthanides and actinides.74 The group of Herrmann first reported the synthesis of five ruthenium benzylidene complexes bearing two identical NHC ligands in 1998.75 Thus, the bis(triphenylphosphine) complex 11 was reacted with 2.2 equivalents of a 1,3-dialkylimidazol-2-ylidene in toluene at room temperature for 45 minutes to afford compounds 13–16 as air-stable solids in 80–90% yields after three precipitations from toluene/pentane to remove the displaced phosphine (Scheme 10). The same products were also obtained in high yields starting from the first-generation Grubbs catalyst 12, thereby demonstrating the stronger basicity of NHCs vs. trialkylphosphines. Compound 17 was prepared likewise starting from [RuCl2(]CH-p-C6H4Cl)(PPh3)2] and 1,3-diisopropylimidazol2-ylidene. Its molecular structure was solved by X-ray diffraction analysis. Catalytic tests showed that complexes 13–17 did not outperform the benchmark diphosphine complex 12 in terms of metathetical activity. This is most likely due to the strong bonding of the NHCs to the metal center, which prevents their dissociation to release a 14-electron active species.

534

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 10

When Grubbs et al. tried to substitute the two phosphine ligands of [RuCl2(]CHPh)(PCy3)2] (12) with either 1,3-bis (2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) or 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (Enders’ triazolylidene),76 only the monosubstituted [RuCl2(]CHPh)(PCy3)(NHC)] products were obtained, even when a large excess of NHC was employed.77 These results were explained by a significant decrease of the remaining phosphine exchange rate after the first one was replaced by a stronger carbene donor. Nevertheless, disubstitution could be achieved starting from complex 18 bearing two labile pyridine ligands (see Section 7.09.4.4).77 Thus, ruthenium benzylidene complex 19 with two SIMes ligands and the closely related product 20 with mixed IMes/SIMes ligands were obtained by treating complex 18 with either the chloroform adduct of SIMes or free 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (the so-called IMes carbene) (Scheme 11). Both products were stable and could be purified by column chromatography on silica gel. The molecular structure of [RuCl2(]CHPh)(IMes)(SIMes)] (20) was determined by X-ray crystallography.

Scheme 11

Concomitantly, Fogg and coworkers came up with the same strategy to prepare the [RuCl2(]CHPh)(IMes)2] complex (22) from the labile bis(pyridine) precursor 21, in which one IMes ligand was already installed (Scheme 12).78 The crystal structure analysis of compound 22 revealed that unfavorable steric interactions between the two bulky NHC ligands were minimized by p-stacking of the benzylidene and mesityl aromatic rings, which were about 3.2 A˚ apart and within 8 of coplanarity.

Scheme 12

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

535

In 2007, He and coworkers were able to isolate the mono- and disubstituted complexes 23 and 24 in high yields and purities by reacting the diphosphine complex 11 with either 1.5 or 3 equiv. of the free SIMes ligand in hexane or hexane/THF at 60  C (Scheme 13).79 Both products were characterized by single-crystal X-ray diffraction analysis. The choice of a substrate bearing a triphenylphosphine ligand rather than a more strongly bonded trialkylphosphine, such as PCy3, appeared to be favorable for achieving a double PR3/NHC substitution. A report from Verpoort et al. confirmed, however, that the first-generation Grubbs catalyst 12 was also a suitable starting material to prepare bis(NHC) ruthenium benzylidene complexes under the right experimental conditions.80 Indeed, the treatment of [RuCl2(]CHPh)(PCy3)2] (12) with 1.2 equiv. of 1-(2,6-diisopropylphenyl)3-methylimidazolinium chloride and 1.2 equiv. of a base led exclusively to product 25, while the expected mono(NHC) derivative was observed only transiently during the reaction course. Similar observations were made when 1-(2,6-diisopropylphenyl)3-cyclohexylimidazolinium chloride was employed as a carbene precursor. Eventually, the recourse to 2.2 equiv. of in situ generated NHC ligands allowed to fully convert substrate 12 into products 25 and 26 (Scheme 14).

Scheme 13

Scheme 14

Last but not least, optically active, bidentate imidazol-2-ylidene species generated in situ by reducing the corresponding thioureas with a sodium/potassium alloy were reacted with the first-generation Grubbs catalyst 12 in toluene at room temperature to afford chelates 27 and 28 (Scheme 15).81 Although the chiral backbone of these compounds might prove useful to induce olefin metathesis reactions in an asymmetric fashion, their catalytic activity was not investigated.

Scheme 15

7.09.4.3

Complexes with mixed NHC/phosphine ligands

Extensive mechanistic studies on metathesis reactions initiated by diphosphineruthenium alkylidene complexes, such as 8, 11, or 12, strongly suggested that the actual active species derived from these catalyst precursors were 14-electron monophosphine complexes.33,54,82 The coordination of an olefin to these highly unsaturated intermediates would then trigger the full catalytic cycle, depicted in Scheme 2. Based on this dissociative route, the activity of the precatalyst should be dictated by two distinct events: the reversible dissociation of the phosphine or another neutral ligand L0 from the metal center (k1/k−1) and the competition between the olefinic substrate and the free phosphine or another neutral ligand to bind the vacant coordination site (k2/k−1) (Scheme 16).

536

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

High catalytic activities are expected when the initiation occurs readily (i.e., when k1/k−1 is large) and when the coordinatively unsaturated [RuX2(]CHR)L] intermediate reacts preferentially with an alkene substrate rather than the free phosphine or another neutral ligand (i.e., when k2/k−1 is large).

Scheme 16

Three research groups swiftly realized that the combination of a strongly binding NHC ancillary ligand and a more labile phosphine ligand on a ruthenium alkylidene scaffold should afford the right balance between activity and stability to achieve high catalytic efficiencies in olefin metathesis. They independently and almost simultaneously reported the preparation and the catalytic evaluation of such complexes. Thus, on December 9, 1998, Herrmann and coworkers disclosed the synthesis of three ruthenium benzylidene complexes bearing mixed NHC/PCy3 ligands (29–31).64 They were obtained by reacting the first-generation Grubbs catalyst 12 in THF at −78  C with a slight excess (1.2 equiv.) of a free NHC bearing cyclohexyl, (R,R)-1-phenylethyl or, (R,R)1-naphthylethyl substituents on its nitrogen atoms (Scheme 17). Shortly thereafter, the same procedure was also applied to prepare a fourth complex featuring the 1,3-di-tert-butylimidazol-2-ylidene ligand (ItBu) (32) from [RuCl2(]CHPh)(PPh3)2] (11).83

Scheme 17

In articles submitted on September 1, 1998,84 and December 13, 1998,85 respectively, the groups of Nolan and Grubbs reported the synthesis of the [RuCl2(]CHPh)(IMes)(PCy3)] complex 33 featuring the imidazole-based IMes ligand. In both cases, the first-generation complex 12 served as a starting material and the exchange of one of its tricyclohexylphosphine ligand with the NHC was carried out in toluene at room temperature (Scheme 18). Contrasting with the 1,3-dialkylimidazolylidene derivatives used by Herrmann, which led to either mono- (29–32) or disubstitution products (13–17) depending on the ratio of ligand to metal adopted and the temperature, the presence of bulky aryl groups on the nitrogen atoms of IMes prevented any double substitution. The mixed-ligand product was purified by recrystallization from pentane or hexane at −78  C and its molecular structure was determined by Nolan and his coworkers, who also prepared the analogous complex 34 starting from [RuCl2(]CHPh)(PPh3)2] (11).84 From a practical point of view, it should be pointed out that Bantreil and Nolan issued a very detailed, step by step experimental procedure for the synthesis of complex 33 in 2011.86

Scheme 18

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

537

In 1999, Grubbs et al. used the saturated dihydro derivative of IMes, nicknamed SIMes or H2IMes, and two of its congeners to prepare the first ruthenium benzylidene complexes bearing mixed phosphine/imidazolin-2-ylidene ligands.87 They reasoned that NHCs deprived of aromatic stabilization would be better s-donors than their imidazole-based counterparts, which should result in an increased metathetical activity. Unlike imidazolium salts, imidazolinium tetrafluoroborates did not react cleanly with metal hydrides to afford free carbenes. However, it was possible to convert them into alkoxy-protected NHCs via a treatment with potassium tert-butoxide in THF at room temperature (Scheme 19). Heating these adducts to 60–80  C in the presence of complex 12 led to the in situ formation of the desired NHCs and their coordination to the ruthenium center, thereby affording complexes 35–37 in good yields.

Scheme 19

Complex 36 quickly emerged as an air- and water-tolerant catalyst precursor that outperformed its first-generation parent in terms of metathetical activity. It is now commercially available and often referred to as the “second-generation Grubbs catalyst”. An improved one-pot synthesis of this compound using 1,3-dimesitylimidazolinium chloride and potassium tert-pentoxide in hexane at 50  C allowed to isolate it in 77% yield after a simple filtration followed by a methanol wash.88 Alternatively, it was also possible to use the 1,3-dimesitylimidazolinium-2-carboxylate zwitterion (SIMes
CO2) as a thermolabile carbene adduct to obtain [RuCl2(]CHPh)(SIMes)(PCy3)] (36) in 90% yield after flash chromatographic purification.89 Although less common than IMes and SIMes, a third N-heterocyclic carbene, nicknamed SIDip or SIPr, also gained popularity as a suitable ancillary ligand for preparing robust, highly active second-generation metathesis catalysts. Thus, the reaction of 2-tertbutoxy-1,3-bis(2,6-diisopropylphenyl)imidazolidine with [RuCl2(]CHPh)(PCy3)2] (12) in THF at 80  C for 30 min afforded [RuCl2(]CHPh)(SIDip)(PCy3)] (38) as a violet-brownish solid in 67% yield (Scheme 20).90

Scheme 20

The common trait shared by the (S)IMes and (S)IDip carbenes is that they possess bulky aromatic groups on their nitrogen atoms (Fig. 4). Due to a restricted rotation around the exocyclic CdN bonds, these substituents are almost perpendicular to the central heterocycle, thereby providing an effective protective shield around the metal center.91 Moreover, the presence of methyl or

538

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Fig. 4 Common NHC ligands for second-generation ruthenium alkylidene catalyst precursors.

isopropyl groups on both ortho positions of their phenyl rings prevents the activation of CdH aryl bonds located close to the ruthenium center. Failure to do so usually results in unwanted decomposition reactions, as evidenced by experimental observations and computational studies.92–95 Although SIMes and to a lesser extent IMes and SIDip prevail as the most common NHC ligands for second-generation ruthenium alkylidene complexes, comparative screenings carried out by the groups of Fürstner and Grela, among others, highlighted that there is no such a thing as a “universal catalyst” for olefin metathesis reactions.90,96 Accordingly, there have been tremendous research efforts to develop a large and varied portfolio of metathesis catalysts tailored for specific needs. These endeavors have largely benefited from the wealth of stable nucleophilic carbenes that became available since Bertrand and Arduengo demonstrated the experimental reality of these divalent carbon species.55,56,97,98 Listing all the ruthenium benzylidene complexes with mixed NHC/phosphine ligands that were synthesized over the past 20 years is beyond the scope of this chapter. Furthermore, most of these compounds were obtained using the same strategy, which relies on the substitution of one phosphine ligand from a first-generation complex with an NHC ligand either preformed or generated in situ, as outlined in Schemes 17–20. Because NHCs are usually stronger donors than phosphines, many of these reactions proceeded seamlessly and could be conveniently monitored by 31P NMR spectroscopy.89 For the interested readers, it should be pointed out that the subject has been heavily reviewed, with regular progress reports issued by Dragutan et al. in 2005,99 Beligny and Blechert in 2006,100 Despagnet-Ayoub and Ritter in 2007,101 Grela et al. in 2009,102 Vougioukalakis and Grubbs in 2010,103 Verpoort et al. in 2013,104 Grela et al. in 2014,105 or Grisi and coworkers in 2018,106 to name just a few (Table 1). Most recently, the latest advances concerning the use of cyclic (alkyl)(amino)carbenes (CAACs) or unsymmetrical NHCs (uNHCs) as ancillary ligands for ruthenium-based olefin metathesis catalysts were surveyed in 2021 by Morvan, Mauduit, Bertrand, and Jazzar, and by Monsigny, Kajetanowicz, and Grela, respectively.107,108 In the following paragraphs, we shall discuss briefly a few representative examples of structural variations that were applied to the NHC ancillary ligand of ruthenium benzylidene complexes in order to fine-tune their catalytic activity in olefin metathesis reactions. Cyclic diaminocarbenes will be considered first, as they form the vast majority of stable divalent carbon species investigated so far. Other types of N-heterocyclic carbenes, including triazolylidene and thiazolylidene derivatives will be examined next, while cyclic (alkyl)(amino)carbenes (CAACs) are covered in Section 7.09.5.1.3 of this chapter. The schematic representation of a ruthenium benzylidene scaffold with mixed NHC/phosphine ligands reveals that the NHC framework is indeed the most suitable entry point for modulating the steric and electronic properties of the whole compound, because there are many diverse and efficient synthetic paths leading to imidazolium, benzimidazolium, or imidazolinium salts, which serve de facto as NHC precursors for most applications.109 More specifically, a diaminocarbene can be tailored to specific needs by varying the size of its heterocyclic ring and by adjusting the nature of its substituents, whether they are located on the nitrogen atoms flanking the carbene center or on more remote positions of the core heterocycle (Fig. 5). Contrastingly, modifications of the phosphine and anionic ligands of the second-generation ruthenium catalysts for olefin metathesis has been far less studied, with only isolated reports describing the use of hydrophilic phosphines instead of the most common PCy3 or PPh3 ligands,110 and of “pseudo-halide” anionic ligands, such as carboxylates, perfluorocarboxylates, phenoxides, isocyanates, isothiocyanates, nitrates, and trifluoromethanesulfonates, instead of the universal chlorido ligand.111 Table 1

Selected reviews on ruthenium alkylidene complexes bearing NHC ligands.

Year

Title

References

2005 2006 2007 2009 2010 2013 2014 2018 2021 2021

Ruthenium complexes bearing N-heterocyclic carbene (NHC) ligands N-Heterocyclic carbene-ruthenium complexes in olefin metathesis N-Heterocyclic carbenes as ligands for olefin metathesis catalysts Ruthenium-based olefin metathesis catalysts bearing N-heterocyclic carbene ligands Ruthenium-based heterocyclic carbene-coordinated olefin metathesis catalysts Olefin metathesis ruthenium catalysts bearing unsymmetrical heterocyclic carbenes Towards “cleaner” olefin metathesis: tailoring the NHC ligand of second generation ruthenium catalysts to afford auxiliary traits Ruthenium-based olefin metathesis catalysts with monodentate unsymmetrical NHC ligands Cyclic (alkyl)(amino)carbenes (CAACs) in ruthenium olefin metathesis Ruthenium complexes featuring unsymmetrical N-heterocyclic carbene ligands—useful olefin metathesis catalysts for special tasks

99 100 101 102 103 104 105 106 107 108

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Fig. 5 Possible variations to fine-tune the catalytic properties of ruthenium benzylidene complexes.

7.09.4.3.1

Variations involving the nitrogen substituents of the NHC

Varying the nature of the nitrogen substituents adjacent to the carbene center is the most common strategy to alter the steric and electronic properties of second-generation ruthenium benzylidene complexes. Due to their straightforward synthesis, symmetrical NHCs with two identical alkyl or aryl groups on their nitrogen atoms were investigated at first. Subsequently, complexes featuring unsymmetrical N-heterocyclic carbenes (uNHCs) with mixed N-alkyl, N0 -aryl substituents have also attracted a great deal of attention because they exhibit significantly different activities and selectivities in olefin metathesis reactions.104,106,108 Thus, in the early 2000s, Fürstner et al. synthesized a series of ruthenium benzylidene complexes bearing 1-alkyl-3-mesitylimidazol-2ylidene ligands with various functionalities on their N-alkyl group, such as a silyl ether (39), an alcohol (40), a perfluoroalkyl chain (41), or a terminal alkene (42–44).90,112 The uNHCs were generated in situ by deprotonating the corresponding imidazolium salts with potassium tert-butoxide and reacted with the bis(phosphine) complex 12 in toluene at room temperature (Scheme 21). The products were purified by flash chromatography on silica gel and their molecular structures were determined by XRD in several instances.

Scheme 21

It is noteworthy that complexes 42 and 43 could metathesize their own pending C]C double bond, thereby affording compounds 45 and 46 whose NHC and alkylidene ligands end up tethered to each other (Scheme 22). The structures of these two cyclometalated products was confirmed by X-ray crystallography.90

Scheme 22

In 2003, Mol and coworkers introduced 1-adamantyl-3-mesitylimidazolin-2-ylidene on a ruthenium benzylidene scaffold to afford complex 47 (Fig. 6).113 This compound turned out to be a poor metathesis initiator, most likely due the high steric congestion imparted by the bulky adamantyl moiety around the metal center. Conversely, the methyl or ethyl groups selected by Blechert et al. in 2006 to prepare complexes 48 and 49 led to much more active species with improved diastereoselectivities for ring-closing metathesis.114 Compound 50, sporting a benzyl and a mesityl group on its nitrogen atoms, was further obtained by Blechert and Buchmeiser in 2009, and found to be a valuable catalyst precursor for the synthesis of alternating copolymers via a ring-opening metathesis polymerization.115 It should be pointed out that NMR or XRD measurements showed that in all these compounds, the mesityl substituent of the uNHC ligand was always stacked above the benzylidene moiety, due to p-p interactions between these two groups.116,117

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Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Fig. 6 Examples of ruthenium benzylidene complexes with an unsymmetrical N-heterocyclic carbene (uNHC) ligand.

7.09.4.3.2

Variations involving the backbone substituents of the NHC

Positions 4 and 5 of an imidazol-2-ylidene ligand are often called its “backbone”. Even though this portion of the NHC is far from the metal center, it can drastically influence the catalytic activity and the stability of ruthenium benzylidene complexes. In particular, backbone substituents can prevent the CdH activation of neighboring N-aryl groups, which leads to the deactivation of second-generation olefin metathesis catalysts.118 Detailed mechanistic studies from Grubbs et al. showed that the transfer of a hydrogen atom from one of the N-phenyl groups to the benzylidene moiety of complex 51 first led to an ortho-metallated complex (Scheme 23). The resulting hydride then inserts at the a-carbon atom of the benzylidene ligand to generate a second transient intermediate that further undergoes a reductive coupling to afford product 52, whose structure was elucidated by X-ray crystallography. Eventually, another CdH insertion together with the PCy3-mediated elimination of HCl took place after 3 days of reaction in benzene at 60  C to afford complex 53, which precipitated in 58% yield and was also subjected to X-ray diffraction analysis.92

Scheme 23

The decomposition path outlined in Scheme 23 is suppressed or at least reduced thanks to the hindrance of backbone groups, which prevent or hamper the rotation of the N-substituents. Besides these steric effects, electronic factors play an equally important role when considering the influence of the NHC backbone. As already mentioned above when discussing complexes 33 and 36, the replacement of the IMes ligand with its backbone-saturated derivative SIMes led to subtle yet significant changes of catalytic activity.87 This is not trivial, since the backbone and the metathetically active site are spatially distant. The introduction of stereogenic centers on the C4 and C5 positions of imidazolin-2-ylidene species further expands the potentials of these ligands for modulating the activity of second-generation catalysts, especially when asymmetric metathesis transformations are sought.119 A representative study toward this quest was disclosed by Grubbs and coworkers in 2001.120 In this work, several enantiopure complexes were prepared and successfully employed to induce the desymmetrization of achiral trienes with good enantioselectivities. Tailoring the NHC ligand showed that (1R,2R)-diphenylethylenediamine was a more suitable backbone precursor than (1R,2R)-1,2-diaminocyclohexane to induce high enantiomeric excesses and that the replacement of N-mesityl substituents with 2-isopropylphenyl groups was also beneficial (Fig. 7). Increasing the steric bulk of the halide ligands from chloride to iodide, through the addition of an excess of sodium iodide to the reaction mixtures, further improved the enantioselectivity up to 90% ee.

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541

Fig. 7 Examples of ruthenium benzylidene complexes featuring NHC ligands with a chiral backbone.

In 2007, Fournier and Collins disclosed the synthesis of an uNHC that combined chiral centers on its backbone and mixed alkyl/ aryl substituents on its nitrogen atoms.121 This C1-symmetric monodentate ligand was generated in situ by deprotonating its more stable imidazolinium salt parent. It was coordinated to a ruthenium benzylidene scaffold via a phosphine displacement from the first-generation Grubbs catalyst 12 (Scheme 24). Crystallography and NMR experiments based on the nuclear Overhauser effect (NOE) showed that the methyl group of compound 54 was located directly over the benzylidene unit, in sharp contrast with previous observations on complexes 47–50 (cf. Fig. 6). The authors speculated that the neighboring tert-butyl group could induce a significant rotation of the N-aryl substituent and prevent its interaction with the alkylidene moiety. In a subsequent study, the same group also disclosed the synthesis of compounds 55 and 56, which were both isolated as a mixture of rotamers.122

Scheme 24

7.09.4.3.3

Variations involving the ring size of the NHC

In addition to the usual NHCs derived from imidazole, four- and six-membered rings with two nitrogen atoms have also led to stable diaminocarbenes. For instance, 5,50 -dimethyl-1,3-dimesityl-1,4,5,6-tetrahydropyrimidin-2-ylidene was readily obtained by Grubbs et al. and employed to synthesize complex 57 according to standard experimental procedures (Scheme 25).123 X-Ray crystallography showed that the enlarged NHC exerted a significantly greater steric pressure on the benzylidene unit than the SIMes ligand in catalyst 36 due to the wider NdCdN angle of the carbene motif, that pushes the mesityl groups toward the metal center.

Scheme 25

7.09.4.3.4

Variations involving the heterocyclic core of the NHC

Carbenes based on the 1,2,4-triazole ring system with three nitrogen atoms in their heterocyclic core were only sporadically investigated. A first report by Fürstner et al. in 2001 described the synthesis of complex 58 from the first-generation Grubbs catalyst (12) and Enders’ triazolylidene in toluene at room temperature (Scheme 26).90 Although complex 58 could be stored under argon for several weeks and its molecular structure was solved, it decomposed rapidly in solution, thereby limiting its preparative utility.

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Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 26

A subsequent study, published in 2013 by Grela and coworkers, focused on the synthesis of four complexes bearing chiral 1,2,4-triazol-5-ylidene ligands.124 In two instances, the deprotonation of the triazolium salt precursors with KN(SiMe3)2 followed by the addition of the diphosphino substrate 12 did not afford full conversions. More satisfactory results were attained when an intermediate silver complex was employed to perform the ligand exchange (Scheme 27). Complexes 59 and 60 were, however, rather unstable, which led to modest isolated yields. The reduced donicity of triazolylidene species compared to their imidazol(in) ylidene counterparts could ease their dissociation from a metal center, thereby leading to a rapid decomposition in solution.77

Scheme 27

Attempts to coordinate thiazol-2-ylidene ligands to a ruthenium benzylidene scaffold were disclosed by Grubbs et al in 2008.125 Deprotonating four 3-aryl-4,5-dimethylthiazolium chlorides with KN(SiMe3)2 before adding the first-generation Grubbs catalyst (12) did not afford any of the desired products in a satisfactory way. To circumvent the formation of the unstable free carbenes and their concomitant dimerization, the corresponding silver thiazol-2-ylidene complexes were prepared in a distinct, preliminary step. They were obtained quantitatively and then reacted in situ with the ruthenium source. This led to mitigated results, as only two reactions were successful and led to the isolation of complexes 61 and 62 in modest yields (Scheme 28).

Scheme 28

Unlike 1,2,4-triazolylidenes and thiazolylidenes, mesoionic carbenes (MICs) with a 1,2,3-triazolylidene core and cyclic (alkyl) (amino)carbenes (CAACs) strongly bind metal centers due to their enhanced nucleophilic (s-donating) and electrophilic (p-accepting) properties.126–128 Their coordination to ruthenium alkylidene species has been the subject of intense research over the past few

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543

years and has already led to significant advances towards the development of robust catalysts for olefin metathesis, for instance in the industrially relevant ethenolysis of unsaturated fatty acids.107 Most compounds investigated so far feature an oxygen- or sulfur-chelated benzylidene fragment rather than a monodentate benzylidene unit and are therefore discussed in Section 7.09.5 of this chapter.

7.09.4.4 7.09.4.4.1

Complexes with mixed NHC/pyridine ligands Complexes with two pyridine ligands

In 2001, Grubbs and coworkers displaced the tricyclohexylphosphine ligand of complex 36 by reacting this second-generation catalyst with a large excess of pyridine in toluene at room temperature.129 Product 18 precipitated from n-pentane at −10  C and was isolated as an air-stable green solid in 85% yield. X-Ray crystallography showed that the two pyridine ligands were cis to each other and trans to the benzylidene and SIMes ligands. The preferential replacement of one phosphine with two pyridines was presumably due to steric reasons. The procedure was successfully extended to other pyridine derivatives, including 4-phenylpyridine and 3-bromopyridine, to afford saturated complexes 63 and 64 in high yields (Scheme 29).130 In all cases, the reactions were complete within a few minutes, required little or no solvent, and could be easily performed on a large scale with unpurified, commercially available reagents. The incorporation of labile pyridine ligands in the coordination sphere of ruthenium, in conjunction with the robust SIMes ancillary ligand and a metathetically active benzylidene unit, afforded catalyst precursors that combined a high initiation rate with a remarkable activity and a good tolerance to functional groups. In particular, bis(3-bromopyridine) complex 64 is an initiator of choice to perform living metathesis polymerization reactions.131,132 It is commercially available and known as the “third-generation Grubbs catalyst”.

Scheme 29

Several research groups have investigated the recourse to pyridines bearing hydrophilic chains to prepare well-defined ruthenium initiators compatible with water-based polymerization media. Thus, in 2005, Breitenkamp and Emrick designed an amphiphilic ruthenium benzylidene catalyst for olefin metathesis with tetra(ethyleneglycol)-substituted pyridine ligands (65).133 Its synthesis was readily accomplished via a phosphine displacement starting from the second-generation complex 36 (Scheme 30). However, it proved difficult to separate the organometallic product from the excess of pyridine needed to achieve a full conversion. To circumvent this problem, Crudden, Cunningham and their coworkers synthesized another related catalyst (66) that readily dissolved in the aqueous phase of a mini-emulsion system before diffusing into the monomer droplets following the dissociation of its tri(ethyleneglycol)-tagged pyridines during the initiation of the polymerization.134 Alternatively, Emrick et al. adopted a slightly different strategy to obtain additional water-soluble complexes featuring phosphorylcholine- (67) and poly(ethyleneglycol)substituted pyridines (68–72).135 Complex 18 was employed as a starting material in these experiments (Scheme 31). Because pyridine is a more labile ligand than tricyclohexylphosphine, a stoichiometric amount of the incoming ligand was sufficient to achieve the substitution with satisfactory results.

Scheme 30

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Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 31

Further variations involving the pyridine ligands were reported by Schanz et al. who used N,N-dimethylaminopyridine as a pH-responsive ligand,136,137 and by the groups of Bazzi and Gladysz who grafted fluorinated alkyl chains on the meta positions of pyridine in order to perform catalytic reactions under fluorous/organic liquid/liquid biphasic conditions.138 In the former case, the product was obtained via a PCy3 exchange from a second-generation complex, whereas in the latter one, the third-generation complex 18 underwent a replacement of its pyridine ligands. Dual strategies were also applied to replace the benzylidene unit of ruthenium complexes with mixed NHC/pyridine ligands by other alkylidene moieties. Lehman and Wagener first reacted the second-generation Grubbs catalyst 36 with 2-butene (mixture of cis and trans isomers) to perform a stoichiometric metathesis of its alkylidene fragment.139 The ruthenium ethylidene intermediate 73 was then treated with an excess of 3-bromopyridine to afford product 74 in 95% yield after precipitation from n-pentane (Scheme 32). The crystal structures of both compounds were determined. Alternatively, complex 74 was also obtained by Sponsler et al. starting from 64 and cis-2-butene.140 Its propylidene and butylidene analogues 75 and 76 were prepared likewise by using trans-3-hexene or trans-4-octene, respectively (Scheme 33).

Scheme 32

Scheme 33

7.09.4.4.2

Complexes with one pyridine ligand

Although the bis(pyridine) complex 18 was obtained preferentially when the second-generation catalyst 36 was reacted with a large excess of pyridine (cf. Scheme 29), Grubbs et al. evidenced the formation of the 16-electron complex [RuCl2(]CHPh)(SIMes) (C5H5N)] when they recrystallized complex 18 and dried it under vacuum.129 They also reported that the reaction of complex 18 with sodium iodide afforded [RuI2(]CHPh)(SIMes)(C5H5N)], a likely consequence of the relatively large size of the iodo ligands

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and the lower electrophilicity at the metal center.129 Full characterizations of these chlorido and iodo mono(pyridine) ruthenium benzylidene complexes were, however, not disclosed. In 2003, Fogg and coworkers reported that a substitution of the chlorido ligands of complex 21 with two equivalents of thallium pentafluorophenoxide afforded the five-coordinate mono(pyridine) ruthenium product 77 whose molecular structure was determined by X-ray diffraction analysis (Scheme 34).141 Additional compounds with chelating bis(aryloxide) species or aryloxido/halido ligands were subsequently prepared.142–144

Scheme 34

In 2005, Buchmeiser et al. reported that the ligation of in situ generated 1,3-dimesityltetrahydropyrimidin-2-ylidene to the first-generation Grubbs catalyst 12, followed by the displacement of tricyclohexylphosphine with an excess of pyridine gave the mono(pyridine) complex 78 in modest yield (Scheme 35).145 The increased donor ability and steric hindrance of an NHC with a six-membered ring compared to SIMes was invoked to justify a reduction of the coordination number from six in complex 18 to five in compound 78.

Scheme 35

In 2007, He and coworkers examined in detail the reaction of complex 79 with various pyridines in excess.146 Complexes 80 and 81 easily formed within minutes, whereas the synthesis of compounds 82 and 83, which contained more labile ortho-substituted pyridine ligands, took hours and was relatively difficult to perform (Scheme 36). The reaction of complex 79 with 2,6-dimethylpyridine, quinoline, or 2-bromopyridine failed to yield the corresponding products, probably due to the excessively weak coordinating ability of these ligands. The molecular structures of products 80, 82, and 83 exhibited a distorted square pyramidal geometry with the two chlorido ligands and the pyridine and NHC ligands in mutually trans arrangements, while the benzylidene unit occupied the apical position. Interestingly, a crystal of bis(pyridine) complex 84 suitable for X-ray diffraction analysis could be obtained by slow diffusion of pentane into a chloroform/hexane solution of monopyridine compound 80, thereby demonstrating unambiguously that five- and six-coordinated ruthenium benzylidene complexes with mixed NHC/pyridine ligands can interconvert under the right experimental conditions (Scheme 37).

Scheme 36

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Scheme 37

7.09.4.5

Complexes with mixed NHC/NHCEWG ligands

In 2007, Plenio et al. realized that the donor ability of N-heterocyclic carbenes substituted with electron-withdrawing groups (NHCEWG) was significantly reduced compared to common NHCs, such as IMes and SIMes, and was more in line with the donicities of trialkylphosphines, such as PCy3 or PEt3.147 These results prompted them to synthesize ruthenium benzylidene complexes with mixed NHC/NHCEWG ligands and to probe their ability to act as catalyst precursors for olefin metathesis reactions. A tetranitrocarbene derived from SIMes was first generated in situ from the corresponding tetranitroimidazolinium salt and reacted with the labile bis(pyridine) complex 18 to afford product 85 in 49% yield after column chromatography (Scheme 38).148 Additional compounds with NHC ligands bearing electron-withdrawing groups on their backbone instead of their nitrogen substituents were subsequently prepared using [AgI(NHCEWG)] complexes as carbene transfer agents.149 This revised experimental protocol led to higher isolated yields. It was further improved through the use of the second-generation complexes 23 or 34 instead of their third-generation analogue 18 as starting materials (Scheme 39).150 Indeed, the recourse to a triphenylphosphine sacrificial ligand is much more cost- and time-effective than the use of more elaborate substrates bearing tricyclohexylphosphine or pyridine ligands. Of note, the crystal structure of the complex bearing 1,3-dimesityimidazolin-2-ylidene (SIMes) and 1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene ligands was determined and found to be unremarkably similar to those displayed by standard second-generation catalysts with mixed NHC/phosphine ligands.149

Scheme 38

Scheme 39

7.09.4.6

Complexes with mixed NHC/phosphite ligands

In 2011, Cazin et al. synthesized two ruthenium benzylidene complexes sporting the robust SIMes ancillary ligand and either triethyl- or triisopropylphosphite by reacting the labile bis(pyridine) compound 18 with one equivalent of the incoming p-acceptor ligand (Scheme 40).151 X-Ray crystallography confirmed that products 86 and 87 featured a square pyramidal geometry with the NHC and P(OR)3 ligands trans to each other. This structural feature was not trivial, considering that related ruthenium indenylidene complexes with mixed NHC/phosphite ligands exhibited an unexpected cis-dichloro arrangement.

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

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Scheme 40

7.09.4.7

Complexes with mixed NHC/Schiff base ligands

The synthesis of second-generation ruthenium benzylidene complexes bearing O,N-bidentate Schiff base ligands was first investigated by De Clerq and Verpoort in 2003 starting from the corresponding first-generation chelates.152 Hence, the target compounds resulted from the exchange of a PCy3 ligand for the in situ generated SIMes carbene. A more general and reliable access to complexes with mixed NHC/Schiff base ligands was devised in 2006.153 It involved the reaction of the bis(pyridine) precursor 18 with the thallium salt of a Schiff base in THF at room temperature (Scheme 41). Under these conditions, a color change from green to various shades of red occurred within minutes. It was accompanied by the precipitation of thallium chloride. Chelates 88–91 were isolated in modest to satisfactory yields and the molecular structure of compound 90 was solved. This air- and moisture-stable catalyst precursor displayed a negligible tendency to trigger metathesis polymerizations at room temperature, but became active upon heating or through the addition of an acidic cocatalyst thereby behaving as a latent initiator.154

Scheme 41

7.09.5

The state of the art: Chelated ruthenium benzylidene complexes

7.09.5.1

Oxygen chelates

In 1997, Hoveyda et al. serendipitously observed that the [RuCl2(PCy3)2(]CHdCH]CPh2)] complex 6 failed to catalyze the cross-metathesis of 2-isopropoxystyrene with an allylic ether.155,156 To rationalize this lack of activity, they envisioned the formation of a chelated isopropoxybenzylidene species that would sequester the transition metal and reduce its catalytic efficiency. To validate this hypothesis, they carried out a stoichiometric metathesis between 2-isopropoxystyrene and one equivalent of the first-generation Grubbs catalyst 12 in dichloromethane at room temperature for 24 h (Scheme 42).156,157 Compound 92 was very robust and could be purified to homogeneity by simple column chromatography on silica gel to afford a dark brown crystalline solid in 67% yield. Its anticipated molecular structure was confirmed through a single-crystal X-ray diffraction analysis of the analogous naphthyl derivative 93.157 Indeed, this second representative of a new family of O-chelated ruthenium benzylidene complexes was coordinated by a single monodentate phosphine and featured a distorted square pyramidal geometry around the metal center. The RudO distance confirmed the formation of a chelate with a reasonable strength.

Scheme 42

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Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

The synthesis of complex 92 was also accomplished in two steps starting from [RuCl2(PPh3)3] and 1-(diazomethyl)2-isopropoxybenzene (Scheme 43).157 The intermediate compound 94 was initially obtained in 90% yield and subjected to X-ray diffraction analysis. It was further reacted with tricyclohexylphosphine (2 equiv.) to afford the desired final product in 75% yield after chromatographic purification. From a practical point of view, it should be pointed out that a similar yield was also achieved when PCy3 was added shortly after the diazocompound in the same flask, thereby leading to a one-pot procedure that bypassed the isolation of intermediate 94.

Scheme 43

In 2000, the groups of Blechert and Hoveyda independently reported the chelation of an ortho-isopropoxybenzylidene ligand to a ruthenium NHC framework following two different strategies. The German team used the first-generation chelate 92 as a starting material and reacted it with in situ generated SIMes in THF/toluene at 80  C (Scheme 44).158 The reaction led to the formation of a pink intermediate still bearing its phosphine ligand (95) that could be isolated and characterized. Chelation eventually occurred after 2 h of stirring in chloroform at room temperature. The green crystalline product 96 was separated from the released phosphine by flash chromatography on silica gel and isolated in 75% overall yield. The American team treated the second-generation Grubbs catalyst 36 with 2-isopropoxystyrene in the presence of cuprous chloride and obtained directly product 96 in 85% yield after chromatographic purification (Scheme 45).159 It should be stressed that copper(I) salts have often been used thereafter as phosphine scavengers to ease the formation of chelated ruthenium alkylidene complexes. The molecular structure of complex 96 was solved by Hoveyda and coworkers. The geometry around the metal center and most of the bond lengths and angles were analogous to those determined for complex 93.

Scheme 44

Scheme 45

The replacement of a sacrificial phosphine ligand with a chelated ether in complexes 92 and 96 significantly altered the initiation and propagation rates defined in Scheme 16 for nonchelated catalyst precursors, such as 12 and 36. Thus, complex 92 initiates metathesis reactions about 30 times slower than its parent 12, but propagates them nearly 4 times faster, while its second-generation analogue 96 is a fast initiating catalyst, which offers reactivity levels and selectivities surpassing those displayed by 36.160 Furthermore, the chelated complexes turned out to be exceptionally air- and moisture-stable, enough to be used in water or other polar

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solvents.161–163 Another remarkable feature of these catalyst precursors is the so-called “boomerang” or “release-return” mechanism, that is the re-coordination of the isopropoxystyrene to the metal after the catalytic cycle (Scheme 46). This effect, originally postulated by Hoveyda and coworkers in 1999,157 remained speculative for 15 years, until Fogg et al. demonstrated its experimental reality in various metathesis reactions with 13C-labeled olefins.164 The re-bonding of the styrenyl ether moiety has multiple consequences on catalysis, as it provides a stable resting state that contributes to protect the active species from decomposition. Hence, complexes 92 and 96 often display superior catalytic performances compared to their nonchelated parents 12 and 36. They are both commercially available and known, respectively, as the “first and second-generation Hoveyda–Grubbs catalysts”.

Scheme 46

Because a chelated benzylidene fragment and an ancillary NHC ligand were both identified as key structural elements to afford highly stable yet very active catalyst precursors for olefin metathesis, the latest developments in this area have mainly focused on the design and catalytic evaluation of complexes that derive from the second-generation Hoveyda–Grubbs catalyst 96. Thus, in the following paragraphs and sections, we shall describe a few significant advances that were made recently concerning the synthesis and characterization of chelated, phosphine-free ruthenium benzylidene complexes with two anionic ligands to complete their 16-electron coordination sphere (Fig. 8). The most popular heteroatom used for chelation is oxygen, but sulfur, selenium, nitrogen, and phosphorous have also attracted a great deal of attention and will be covered in the next sections. Due to the tremendous research efforts in this field, we shall restrict our survey to reports published in 2020 and 2021. Reviews listed in Table 1 provide a good overview of the literature published prior to these years.

Fig. 8 Possible variations to fine-tune the catalytic properties of chelated ruthenium benzylidene complexes.

7.09.5.1.1

Variations involving the benzylidene ring substituents

Soon after the catalytic potentials of complex 96 were uncovered, intense research efforts aimed at fine-tuning the electronic and steric properties of its isopropoxybenzylidene moiety. Two main strategies were adopted to ease the decoordination of the ether ligand and to generate a 14-electron active species more efficiently. Thus, Grela et al. reduced the electron density on the chelating isopropoxy group by introducing a strong electron-withdrawing nitro substituent on the phenyl ring,165 while Wakamatsu and Blechert grafted bulky phenyl or naphthyl substituents next to it in order to increase the steric pressure around the RudOiPr bond to favor its dissociation.166,167 In both cases, the second-generation Grubbs catalyst 36 served as a starting material and was reacted with an appropriately substituted styrene in the presence of copper(I) chloride to ease the phosphine removal (Scheme 47).

550

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 47

In 2020, Grela et al. prepared five 2-isopropoxystyrene derivatives with strong electron-withdrawing substituents, viz., thioperfluoroalkyl, sulfone, or ketone functionalities, and used them to chelate the second-generation Grubbs catalyst 36 using the same experimental procedure as described above (cf. Scheme 47).168 Most products remained stable for months in the presence of air. However, the introduction of a (perfluorobutyl)sulfonyl group on the benzylidene ring led to an unstable complex that slowly decomposed even when stored at −30  C, presumably because this substituent was too electron-withdrawing. Kajetanowicz, Grela, and their coworkers continued their search for readily available, strongly deactivated Hoveyda–Grubbs initiators by using Sildenafil aldehyde as a ligand precursor.169 This commercially available pharmaceutical building block was converted into the corresponding styrene derivative through a Wittig olefination with ethyl(triphenyl)phosphonium bromide and reacted with a second-generation ruthenium benzylidene or indenylidene complex (Scheme 48). Thus, products 99 and 100 were obtained in just two steps and their molecular structures were determined. Compound 99, sporting a SIMes ligand, displayed a high sensitivity toward air and moisture. Hence, its synthesis and work-up required an inert atmosphere and dry, degassed solvents to achieve a satisfactory yield. On the contrary, the SIDip variant (100) could be simply isolated under a normal atmosphere. A large difference of stability was further observed when the two complexes were dissolved in dry and degassed CD2Cl2 in a sealed NMR tube at 20  C. After 7 days, more than 20% of the former complex 99 decomposed, whereas the latter one was much more resilient (12% decomposition after 14 days). Under argon at room temperature, the two complexes were stable and could be stored for more than 3 months.

Scheme 48

7.09.5.1.2

Variations involving the oxygen substituents

Numerous efforts were devoted to improve the initiation rate and the stability of second-generation Hoveyda–Grubbs catalysts by modifying the original isopropyl substituent on the chelating oxygen atom. For instance, Al-Awadi et al. recently reported the synthesis and the catalytic evaluation of three novel ruthenium complexes with dangling acetamide or sulfonamide functional groups connected via an ether link to the benzylidene moiety.170 Compounds 101–103 were obtained by reacting the second-generation Grubbs catalyst 36 with a suitable vinylbenzene precursor in the presence of a dry acidic Amberlyst resin (Scheme 49). Single crystals of the three products were successfully grown by solvent diffusion and their molecular structures were determined.

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

551

Scheme 49

7.09.5.1.3

Variations involving the NHC ligand

Unlike the alkylidene unit, which is exchanged during olefin metathesis reactions, the NHC ligand of a second-generation ruthenium initiator remains bound to the metal center throughout the catalytic cycle (cf. Schemes 2, 16, and 46). Therefore, while the nature of a benzylidene chelate strongly affects the kinetics of the transformation, variations involving the NHC ligand can be applied to induce chemo-, diastereo-, or enantioselectivities, and to improve turnover numbers and catalyst lifetime. For instance, Chung, Hong, and their coworkers prepared a series of eight ruthenium isopropoxybenzylidene complexes bearing fluorinated imidazo[1,5-a]pyridin-3-ylidene carbenes that exhibited RudF interactions in the crystal phase.171 Compounds 104–111 were obtained in modest to satisfactory yields by ligand exchange from the first-generation Hoveyda–Grubbs catalyst 92 and an NHC ligand generated in situ by deprotonating the corresponding imidazopyridinium triflate with potassium bis(trimethylsilyl)amide in THF at room temperature (Scheme 50). These catalyst precursors were efficient at promoting the ethenolysis of methyl oleate, a notoriously difficult metathesis reaction, presumably because the fluorine atoms contributed to increase their stability.

Scheme 50

In 2020, Carter and Schrodi designed a hemilabile NHC ligand with four methoxyethoxy arms that was simply mixed with the first-generation Hoveyda–Grubbs catalyst 92 to afford complex 112 in 55% yield (Scheme 51).172 An X-ray diffraction analysis revealed the occurrence of a pseudo-octahedral geometry, hinting at the additional interaction between an ether group from the ancillary carbene ligand and the ruthenium center. This coordination is similar to the one displayed above in structures 104–111. In this case, however, the polyether arms did not improve the stability of metathetically active species, as complex 112 did not outperform its SIMes-based analogue 96 in terms of turnover numbers at low catalyst loading.

Scheme 51

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Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Another NHC modification was reported in 2020 by Zhang and Diver who introduced a sophisticated macrocyclic NHC ligand designed to provide an enzyme-like catalytic pocket.173 Its coordination to a chelated ruthenium benzylidene scaffold required some optimization because the bis(imidazolinium tetrafluoroborate) precursor was not soluble in most organic solvents, which complicated its deprotonation. Initial attempts showed the formation of an unidentified intermediate and the presence of unreacted starting materials. Adjusting the reaction time, the amount of base, and adding copper(I) chloride to the reaction medium eventually afforded compound 113 in 51% yield from the first-generation Hoveyda–Grubbs catalyst 92 (Scheme 52). The recourse to a more labile phosphine, Ph2P(piperidine), instead of PCy3 on the organometallic reagent helped to further increase the yield, which reached 68%. A single-crystal X-ray diffraction analysis confirmed the structure of complex 113 and revealed the conformation of its macrocyclic ligand.

Scheme 52

In 2005, Bertrand et al. disclosed the existence of a new type of persistent carbenes dubbed the cyclic (alkyl)(amino)carbenes (CAACs).174 Thanks to their remarkable nucleophilic (s-donating) and electrophilic (p-accepting) properties, these species form strong bonds with metal centers, thereby affording very robust catalyst precursors.128,175,176 The first ruthenium benzylidene complexes bearing CAAC ligands were reported in 2007 by Bertrand, Grubbs and their coworkers.177 Two compounds related to the third-generation Grubbs catalyst were obtained via a PCy3/CAAC substitution starting from the bis(pyridine) complex [RuCl2(]CHPh)(PCy3)(C5H5N)2]. Likewise, products 114–116 resulted from a phosphine exchange with the first-generation Hoveyda–Grubbs catalyst 92 (Scheme 53). These air- and moisture-stable chelates were isolated in variable yields after chromatographic purification. Their molecular structures showed a distorted square-pyramidal geometry around the metal center with the benzylidene moiety occupying the apical position. Shorter RudCcarbene and longer RudO distances compared to their SIMes-based counterpart 96 were consistent with an increased s-donation of CAACs vs. NHCs. In all the crystal structures, the N-aryl substituent was always located above the benzylidene unit, while the quaternary carbon adjacent to the carbene center was positioned over an empty coordination site, maybe due to unfavorable steric interactions between the alkyl groups on this sp3-hybridized atom and the benzylidene proton.

Scheme 53

In 2020, Bertrand, Jazzar, and Mauduit took advantage of the stereogenic quaternary carbon atom present in the heterocyclic core of CAACs to prepare the optically active complex 117 (Scheme 54).178 This compound was isolated as an air-stable racemic mixture and the two enantiomers were resolved by preparative high-performance liquid chromatography (HPLC). Among the various chiral stationary phases tested, a Chiralpak IF column (amylose tris(3-chloro-4-methylphenylcarbamate) immobilized on silica) led to the best separation and allowed to collect the two enantiomers in good yields (46 and 45%) and high optical purities (>99% and >98% ee). Their absolute configurations were assigned by X-ray diffraction and circular dichroism analyses. Catalysts (+)-(R)-117 and (−)-(S)-117 displayed excellent catalytic performances in asymmetric olefin metathesis reactions (up to 92% ee).

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

553

Scheme 54

In order to carry out the olefin metathesis of renewable feedstocks, such as unsaturated phospholipids and vegetable oils, Tuba et al. designed a ruthenium catalyst that combined a good solubility in protic solvents and a high activity and stability.179 To achieve this dual objective, a new cationic CAAC ligand was devised to impart solubility in aqueous media, and a chelated isopropoxybenzylidene unit was chosen to provide the framework that would guarantee a stable resting state. Thus, a pyrrolinium salt featuring a para-dimethylaminoaryl substituent on its nitrogen atom was first synthesized. It was deprotonated with lithium bis(trimethylsilyl)amide and the first-generation Hoveyda–Grubbs catalyst 92 was added to the reaction mixture to afford complex 118 (Scheme 55). A subsequent reaction with methyl triflate in dichloromethane at −30  C led to the desired cationic complex 119 in 52% overall yield. Gratifyingly, this complex exhibited a good stability and a high catalytic activity in protic solvents, such as methanol, isopropanol, and water.

Scheme 55

In 2020, Grubbs et al. prepared four new ruthenium complexes using a family of six-membered ring CAACs reported earlier by Bertrand and coworkers.180 These ligands displayed enhanced s-donating and p-accepting properties compared to their five-membered ring analogues.181 However, they were also more susceptible to decomposition. Hence, the reaction conditions had to be carefully optimized to ensure their successful coordination to complex 92. Eventually, products 120–123 were isolated in modest yields when the right combination of solvent, base, and other experimental parameters was found (Scheme 56). The molecular structures of compounds 121 and 123 were determined.

Scheme 56

In 2021, Hong et al. reported the synthesis of photosensitive ruthenium catalysts for olefin metathesis whose reactivity could be switched on and off by UV irradiation.182 The photoswitch was based on the judicious installation of an azobenzene group on the CAAC ligand of complexes 124 and 125 (Scheme 57). This functionality can undergo a reversible cis-trans photoisomerization that dramatically altered the outcome of various metathesis reactions.

554

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 57

Last but not least, Ramström et al. synthesized a series of olefin metathesis catalysts featuring CAAC ligands with various aryl groups on their nitrogen atom.183 Of practical importance, they did not follow the standard procedure described above in Schemes 50–57, which involved a PCy3/CAAC exchange reaction starting from the first-generation Hoveyda–Grubbs catalyst 92. Instead, they first installed two carbene ligands on the ruthenium indenylidene precursor 126 using three equivalents of pyrrolinium salts and the same excess of lithium bis(trimethylsilyl)amide (Scheme 58). Bis(CAAC) compounds 127–129 were isolated and fully characterized. Then, in a distinct step, they were treated with a stoichiometric amount of 2-isopropoxystyrene to afford the desired products 130–132 in 30–90% overall yield. This protocol resulted in easier purifications and generally higher yields than a related one-pot method combining the three steps (deprotonation, double substitution, and chelation) in the same flask.184 Moreover, it allowed the synthesis of compound 132, which had previously remained elusive.177,185 The molecular structure of this iconic mesityl-based catalyst was determined.183

Scheme 58

7.09.5.1.4

Anionic ligand exchange

In 2020, Houk, Grubbs and their coworkers prepared two 18-electron oxygen-chelated complexes with a bidentate pivalate anionic ligand and an adamantyl-substituted NHC, which cyclometallated the ruthenium atom.186 The synthetic strategy to obtain compounds 135 and 136 involved the use of a small basic anion, the pyrrolide, to replace the chlorides (Scheme 59). The cyclometallation and a further exchange to install the pivalate took place in a second step when pivalic acid was added to remove the basic ligands. As expected, these highly sterically strained catalysts demonstrated a very high Z-selectivity in cross-metathesis reactions. The molecular structure of complex 136 was determined.

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

555

Scheme 59

The groups of Michel and Fogg showed that the addition of two equivalents of tetrabutylammonium hydroxide to the secondgeneration Hoveyda–Grubbs catalyst 96 led to a rapid exchange of its chlorido ligands with hydroxide anions, thereby affording complex 137 in 83% yield (Scheme 60).187 Catalytic tests performed with this product showed that it initiated olefin metathesis very slowly and decomposed quickly. These results shed light on the catalyst deactivation pathways occurring when water and bases are present in the reaction media.

Scheme 60

Although the substitution of chlorido for iodo ligands in second-generation Grubbs or Hoveyda–Grubbs catalysts is often beneficial to their selectivity and activity, several reports highlighted that it was very difficult to avoid the interference of mixed halide species that remained after the synthesis, even when a large excess of the incoming anion was used.188,189 Indeed, the poor lability of the ancillary PCy3 or ether ligand in complexes 36 and 96, respectively, retards the formation of the four-coordinate intermediate required for an efficient halide exchange. In 2021, Fogg et al. disclosed a new method that eased the access to chelated ruthenium benzylidene complexes with two iodo ligands.190 The strategy exploited the lability of the first-generation Grubbs catalyst 12 to obtain cleanly complex 138 upon treatment with sodium iodide, 2-isopropoxystyrene, and a phosphine-scavenging Merrifield iodide resin (Scheme 61). The subsequent installation of SIMes or CAAC ligands afforded a range of second-generation iodide catalysts in good to excellent yields and high purities.

Scheme 61

7.09.5.2

Sulfur chelates

In 2008, Lemcoff et al. hypothesized that the replacement of the chelating oxygen by a sulfur atom in the second-generation Hoveyda–Grubbs catalyst 96 should afford a stable, inert complex that would become an active catalyst only when heated.191 Thus, they prepared the thioether analogue of 2-isopropoxystyrene and reacted it stoichiometrically with the second-generation Grubbs catalyst 36 in the presence of cuprous chloride (Scheme 62). Unexpectedly, the change of chalcogen atom profoundly altered the disposition of the ligands around the metal center. Indeed, X-ray crystallography and NMR spectroscopy evidenced a cis-dichloro arrangement instead of the usual trans-dichloro configuration observed in most ruthenium benzylidene species. Complex 139 was isolated as a teal-colored solid after column chromatography. It was very stable and completely inert towards olefins at room

556

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

temperature. However, when external stimuli, such as heat or light, were applied, it isomerized into the trans-dichloro compound, which triggered its catalytic activity. The Lemcoff group went on with the development of latent initiators for olefin metathesis by tailoring the ancillary NHC ligand, the sulfur substituents, and the anionic ligands of lead structure 139. Two recent accounts have summarized their endeavors,192,193 and the latest research advances from 2020 and 2021 are briefly described in the following subsections.

Scheme 62

7.09.5.2.1

Variations involving the sulfur atom

In 2020, Lemcoff et al. reported the synthesis of an electron-rich sulfoxide chelated to a ruthenium benzylidene scaffold that presented a cis configuration.194 Complex 140 was obtained from the Grubbs third-generation catalyst 18 in 90% yield (Scheme 63). Monitoring the reaction by NMR spectroscopy showed that the trans-dichloro complex was the kinetic product, but it isomerized to the cis geometry in the warm solution, thereby mimicking the behavior observed with analogous thioether complexes. Moreover, as predicted by DFT calculations, an X-ray diffraction analysis confirmed that the heteroatom chelating the ruthenium center was the sulfur atom and not the oxygen. As a latent precatalyst, compound 140 presented an excellent metathesis reactivity when activated by heat or visible light, but not with UV-C, which allowed to use it efficiently in an orthogonal thermochromatic sequence depending on the order of the applied stimuli.

Scheme 63

7.09.5.2.2

Anionic ligand exchange

Given the impact of iodo ligands on the selectivity of ruthenium-catalyzed olefin metathesis reactions, Lemcoff and coworkers performed a halogen exchange on one of their most potent sulfur-chelated latent initiators, namely the trifluoromethyl (2-benzylidene)sulfane compound 141 (Scheme 64).195 The reaction was performed with an excess of sodium iodide in dry acetone and the crystal structure of product 142 showed the typical cis configuration awaited. This new complex was the first diiodo catalyst precursor readily activated with visible green light and was able to promote several types of photoinduced olefin metathesis reactions.

Scheme 64

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

7.09.5.3

557

Selenium chelates

In 2009, the groups of Straub and Lemcoff applied computational and experimental methods to predict and verify the relative stabilities of cis- and trans-dichloro ruthenium benzylidene complexes chelated with various chalcogen and pnictogen atoms.196 This study showed that oxygen- and nitrogen-chelates were more stable in the trans configuration, whereas sulfur, selenium, and phosphorus favored a cis-dichloro geometry. To probe the validity of their prediction, they prepared and characterized the selenium derivative 144 from the second-generation Grubbs catalyst 36 and isopropyl(2-vinylphenyl)selane according to the procedure outlined above for the corresponding thioether (cf. Scheme 62). In 2021, Monsigny, Grela et al. reported a new route to access this complex.197 The main improvements concerned the organic synthesis of the styrenyl selenoether needed to form the desired chelate. A revised strategy was devised to avoid the use of volatile, hazardous organoselenium reagents and to introduce the heteroatom at a late stage of the synthesis. Thus, bromostyrene was mixed with magnesium chips in THF in the presence of lithium chloride. Then, selenium was added in one portion followed by an excess of isopropyl iodide. A simple extraction concluded this one-pot synthesis and afforded the desired selenide in 74% yield (Scheme 65). Mixing this ligand precursor and the second-generation ruthenium indenylidene complex 143 in the presence of cuprous chloride afforded the final product 144 whose geometry was confirmed by X-ray crystallography. This complex could be activated by both light and heat, being otherwise latent for olefin metathesis reactions.

Scheme 65

7.09.5.4

Nitrogen chelates

In 2020, Polyanskii, Zubkov, et al. synthesized six second-generation ruthenium benzylidene complexes featuring a RudN bond in a six-membered ring chelate to study how this pattern affected their stability and catalytic activity.198 Compounds 145–150 were obtained by reacting appropriate aminostyrene derivatives with the ruthenium indenylidene complex 143 (Scheme 66). They all exhibited a trans-dichloro configuration as evidenced by their crystal structures. The RudN distance increased with the steric hindrance of the nitrogen substituents, and the farther it became, the better catalytic performance was achieved.

Scheme 66

558

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

In 2021, Yusubov, Zhdankin, Verpoort, and their coworkers provided an example of a latent ruthenium catalyst bearing a chelated benzylidene ligand with an oxadiazole fragment.199 Complex 151 was obtained from the straightforward reaction of a third-generation propenylidene catalyst with a functionalized styrene (Scheme 67). Its molecular structure was determined. A preliminary catalytic study demonstrated its inactivity for the ring-closing metathesis of a benchmark a,o-diene, but an excellent thermoswitchability for the ring-opening metathesis polymerization of norbornene.

Scheme 67

7.09.5.5

Phosphorus chelates

The robustness of non-chelated ruthenium benzylidene complexes with mixed NHC/phosphite ligands, such as 87 (cf. Scheme 40), inspired Lemcoff et al. to synthesize the phosphite-chelated complex 152 based on the assumption that the additional binding and a cis-dichloro arrangement would enhance the stability and improve the latency of this new metathesis initiator.200 Complex 152 was prepared by heating the second-generation Hoveyda–Grubbs catalyst 96 and 1.5 equivalent of a suitable ligand precursor in a sealed tube for 4 days (Scheme 68). The new complex presented latency and catalyzed olefin metathesis reactions under UV-A light. Its molecular structure was determined.

Scheme 68

7.09.6

The outsiders: Ruthenium benzylidyne complexes

Whereas the organometallic chemistry of ruthenium alkylidene complexes is an extremely vast and diverse research area, that has been intensively investigated over the past 30 years in conjunction with the successful development of well-defined homogeneous catalysts for olefin metathesis, far less attention has been paid to ruthenium alkylidyne complexes. This is most likely due to their hitherto limited potential to catalyze alkene or alkyne metathesis reactions. Yet, the chemistry of ruthenium carbyne species has strongly benefited from the advances made with their ruthenium carbene cousins. Furthermore, it offers valuable insight to understand the mode of action and deactivation of metathesis catalysts. Thus, in the following paragraphs, we shall briefly highlight a few significant studies that emphasize the similarities and differences between ruthenium benzylidene and benzylidyne complexes. In 2000, Eisenstein, Caulton et al. reported that the reaction of the first-generation Grubbs catalyst 12 or its triisopropylphosphine analogue with two equivalents of sodium tert-butoxide led to the expected halide/alkoxide exchange and was accompanied by the dissociation of one phosphine ligand to afford the four-coordinated benzylidene complex 153.201 When sodium phenoxide was employed, the [RuCl(OPh)(CHPh)(PR3)2] and [Ru(OPh)2(CHPh)(PR3)2] species were detected in the reaction media by 1H and 31P NMR spectroscopy, but the transformation went on with the elimination of phenol to afford the corresponding benzylidyne derivatives (Scheme 69). Products 154 and 155 were isolated in 53% and 61% yield, respectively, and the molecular structure of the former compound was determined. It confirmed the square-planar geometry of these compounds. Fogg and coworkers built on this study to prepare the related complex 156 by treating the first-generation Grubbs catalyst 12 with thallium pentafluorophenoxide (Scheme 70).141 The solid-state structure of this product closely resembled the one reported for 154. Modeling studies suggested that the steric crowding within a putative [Ru(OC6F5)2(CHPh)(PCy3)2] five-coordinate intermediate was sufficient to induce an interaction between the benzylidene proton and the pentafluorophenoxide oxygen, which is ultimately relieved by the elimination

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

559

of perfluorophenol. Failure to observe the same reaction with the tert-butoxide system, despite the thermodynamically more favorable liberation of tert-butyl alcohol, was deemed consistent with a prohibition of the alkylidene/alkoxide interaction by the bulk of the tert-butyl substituent. In this case, the relief of steric pressure could only occur via a phosphine loss.

Scheme 69

Scheme 70

In 2005, Johnson et al. showed that the dehydrohalogenation of first-generation ruthenium alkylidene complexes could be achieved either in one step using the dialkylgermylene [Ge(CH(SiMe3)2)2] or in two steps via a treatment with an excess of aryloxide followed by tin(II) chloride (Scheme 71).202 The latter route gave higher yields but its scope was more restricted. Experimental details pertaining to the synthesis of compound 157 were provided, and the molecular structure of the unsubstituted benzylidyne complex 158 was supplied without any further information on its synthesis. A subsequent study showed that the chlorido ligand of complex 158 could be easily exchanged with other halides or pseudohalides (F, Br, I, CF3SO3), while the addition of ethereal HCl regenerated the parent alkylidene complex.203

Scheme 71

In 2012, Wang and coworkers prepared the cationic ruthenium benzylidyne complex 159 by oxidizing the second-generation Grubbs catalyst 36 with an excess of iodide in dichloromethane at room temperature (Scheme 72).204 An X-ray diffraction analysis confirmed its structure. Unlike its bis(phosphine) analogues 154–156 or 158, this new second-generation ruthenium carbyne complex did not lose one of its anionic ligands to form a neutral product. This might be due to the steric hindrance of the SIMes carbene, which prevented the coordination of iodide to the ruthenium center. It is noteworthy that compound 159 was able to initiate olefin metathesis reactions at elevated temperatures without unwanted isomerization side-reactions. A third-generation ruthenium benzylidyne catalyst was also prepared along the same lines starting from the bis(pyridine) substrate 18 (Scheme 73).205 Complex 160 was isolated in 90% yield and displayed a distorted octahedral geometry in the crystal phase. Compared to its predecessor 159, it displayed a slightly increased efficiency in ring-closing and cross-metathesis reactions, but still required high temperatures (100–137  C) to afford satisfactory conversions.

560

Ruthenium benzylidene and benzylidyne complexes: Alkene metathesis catalysis

Scheme 72

Scheme 73

7.09.7

Conclusion and outlook

Since the first well-defined ruthenium propenylidene complex 5 was isolated and characterized by Grubbs and coworkers in 1992 (cf. Scheme 4),29 the chemistry of ruthenium alkylidene species has reached an unprecedented level of maturity, thanks to the advent of olefin metathesis as a powerful and highly versatile synthetic method for the manipulation of C]C double bonds. In particular, the past 30 years have witnessed the successful development of three successive generations of ruthenium benzylidene catalyst precursors, and the emergence of the corresponding chelated benzylidene derivatives, as robust promoters for numerous types of metathesis reactions. Detailed mechanistic investigations and the in-depth characterization of stable 14-, 16-, or 18-electron species based on a ruthenium benzylidene scaffold, as well as diverse fleeting reaction intermediates, have allowed to understand and to fine-tune the catalytic properties of this family of complexes with a degree of refinement rarely achieved in the history of organometallic chemistry and homogeneous catalysis. Many significant advances have already been accomplished by tailoring the NHC ancillary ligand, the benzylidene moiety (chelated or not), or to a lesser extent the other anionic ligands of ruthenium complexes to specific needs, in order to design the most suitable catalyst precursor for a given application. Endeavors to control the chemo-, diastereo-, or enantioselectivity of metathesis reactions have afforded very encouraging results. Progresses were also made to reduce the catalyst loading and to increase its lifetime. The recent introduction of CAAC instead of NHC ligands resulted in a further leap forward. Yet, there is still space for improvements, and many stimulating challenges await the organometallic chemists working with ruthenium benzylidene or benzylidyne complexes.

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7.10

Ruthenium and Osmium Carbonyl Cluster Complexes

Sumit Sahaa and Burjor Captainb, aMaterials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar, Odisha, India; bDepartment of Chemistry, University of Miami, Coral Gables, FL, United States © 2022 Elsevier Ltd. All rights reserved.

7.10.1 Introduction 7.10.2 Ruthenium carbonyl cluster complexes 7.10.2.1 Monometallic ruthenium carbonyl cluster complexes 7.10.2.2 Bimetallic ruthenium-group 10 transition metal carbonyl cluster complexes 7.10.2.2.1 Bimetallic ruthenium-nickel carbonyl cluster complexes 7.10.2.2.2 Bimetallic ruthenium-palladium carbonyl cluster complexes 7.10.2.2.3 Bimetallic ruthenium-platinum carbonyl cluster complexes 7.10.2.3 Bimetallic ruthenium-group 14 metal carbonyl cluster complexes 7.10.2.3.1 Bimetallic ruthenium-tin carbonyl cluster complexes 7.10.2.3.2 Bimetallic ruthenium-germanium carbonyl cluster complexes 7.10.2.4 Trimetallic ruthenium-platinum-palladium carbonyl cluster complexes 7.10.2.5 Trimetallic ruthenium-platinum-tin carbonyl cluster complexes 7.10.2.6 Trimetallic ruthenium-platinum-germanium carbonyl cluster complexes 7.10.3 Osmium carbonyl cluster complexes 7.10.3.1 Monometallic osmium carbonyl cluster complexes 7.10.3.2 Bimetallic osmium-group 10 transition metal carbonyl cluster complexes 7.10.3.2.1 Bimetallic osmium-palladium carbonyl cluster complexes 7.10.3.2.2 Bimetallic osmium-platinum carbonyl cluster complexes 7.10.3.3 Bimetallic osmium-group 14 metal carbonyl cluster complexes 7.10.3.3.1 Bimetallic osmium-tin carbonyl cluster complexes 7.10.3.3.2 Bimetallic osmium-germanium carbonyl cluster complexes 7.10.3.4 Trimetallic osmium-platinum-tin carbonyl cluster complexes 7.10.4 Conclusions Acknowledgments References

7.10.1

564 567 567 571 571 573 576 587 587 594 600 600 602 604 604 609 609 610 618 618 621 624 627 627 627

Introduction

The past five decades have seen steady advancement in the broad field of inorganic and organometallic cluster chemistry which is built on a solid foundation from pioneering work by outstanding scientists.1–9 In the 1970s Muetterties formulated the “cluster-surface analogy” which has inspired study by a host of groups on the similarities and differences between discrete clusters and heterogeneous catalyst.8,10–13 Simple questions resulting from this comparison are: (a) Does the cluster simply serve as a “safe haven” for a more reactive coordinatively unsaturated species ? (b) As the cluster size increases, at what point does continued cluster growth result in stagnation or even decline in catalytic activity? (c) Can interchange of metal sites in heteronuclear clusters by pseudo rotations increase catalytic activity? (d) Do calcined heteronuclear metal clusters retain their precise atomic arrangements? (e) At what point do metal clusters and nano-particles converge in their properties? These and other questions remain open to investigation and elaboration in the 2020s. The principle goal of this review is to provide a broad overview of advances in the area, and where possible to show what insight has been gained in addressing if not fully answering the questions outlined above. Study of clusters has the advantage that a single well-defined discrete structure is known for the potential active catalytic site as well as surrounding atoms which primarily provide support for that structure. A wide range of techniques are available to study cluster systems: IR, NMR, ESR, mass spectrometry, Mossbauer spectroscopy, and single crystal X-ray diffraction. Computational results also have the most meaning for catalysts where the actual structure is known and it is mono-dispersed. The fact that this can be done does not mean it is that much easier now than it was 50 years ago. An example illustrating this is the first step in the Fischer-Tropsch process for production of synthetic fuels is the dissociation of CO into carbido and oxido ligands on a metal surface, see Fig. 1. The formation of carbido species was supported by evidence of carbido ligands in metal carbonyl clusters, and the origin of the carbide ligand is from a CO ligand, which was established by 13C-labeling experiments.14 The dissociation of CO proceeds through an intermediate and this type of bridging CO mode was observed in the cluster W4(Z2,m4-CO)(CO)2(OCH2-i-Pr)12, and as the temperature was raised, carbido ligands were formed.15,16 Another example was the investigation of norbornene coordinated to an Os3 cluster by single crystal X-ray diffraction, which showed similar norbornene-metal interactions as that proposed for the adsorption of norbornene on Pt(111) surface.17

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Fig. 1 Dissociation of carbon monoxide gas to carbido and oxido species on a metal surface.

In the realm of homogeneous catalysis, metal clusters have been shown to be effective catalysts. The understanding resides in multisite interactions between the substrate molecules, more than one metal center to facilitate activation and transformation of the substrate, and high mobility of the ligands could promote reactions between several molecules bonded to the cluster framework.18–34 Metal cluster complexes have been shown to catalyze reactions such as hydrogenation, hydrosilation, carbonylation, and isomerization.35 For example, the layer-segregated platinum-ruthenium mixed-metal cluster complex Pt3Ru6(CO)20(m3-PhC2Ph)(m3-H)(m-H) has been shown to exhibit high catalytic activity for the hydrogenation of PhC2Ph to (Z)-stilbene36, see Fig. 2, and for the hydrosilation of PhC2Ph to (E)-(1,2-diphenylethenyl)triethylsilane37 at 50  C and 30  C, respectively. Transition metal clusters have also been used for applications in heterogeneous catalysis. Ever since Sinfelt et al. showed that Pt-Re, Pt-Ir, and Pt-Sn on Al2O3 were the catalysts of choice for naphtha reforming process, preparation of supported bimetallic catalyst emerged considerably.38 A recent and now widely used approach for preparing bimetallic nanoparticles is from bimetallic molecular clusters.39–47 The main advantages of using bimetallic molecular clusters as precursors to be expected are (i) clustering of

Fig. 2 Proposed catalytic cycle for the hydrogenation of diphenyl acetylene by the layer segregated complex Pt3Ru6(CO)20(m3-PhC2Ph)(m3-H)(m-H). Modified from Adams, R. D.; Barnard, T. S.; Li, Z; Wu, W.; Yamamoto, J. H. J. Am. Chem. Soc. 1994, 116, 9103.

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the metal particles on the surface when strong metal-metal bonds are present in the cluster precursor; (ii) partial retention of the cluster precursor metal framework on the surface; (iii) uniform particle size and distribution on the surface; and (iv) uniform particle ratios that are predetermined from the molecular mixed-metal cluster precursor. The mixed-metal cluster PtRu5(CO)16(m6-C)48 was supported on carbon to form platinum-ruthenium nanoparticles with an average diameter of 1.5 nm.41–43 The bimetallic nanoparticles have Pt:Ru compositions in a 1:5 ratio which were predetermined by the molecular cluster precursor, see Fig. 3. There has also been much success in supporting mixed-metal carbonyl clusters on mesoporous silica (MCM-41).44–47 The mixed metal clusters [Pd6Ru6(CO)24]2− and Ru12Cu4C2Cl2(CO)32]2− supported on MCM-41 have been shown to be effective hydrogenation catalysts.45 Similarly RudSn bimetallic nanoparticles were obtained from the molecular carbonyl cluster, [Ru6(C) (CO)16SnCl3]−. The hydrogenation of cyclic polyenes at low temperatures and without solvent were investigated using these supported bimetallic catalysts, see Fig. 4.49 Furthermore, ruthenium-platinum nanocatalysts were also prepared using the carbonyl cluster precursors, [Ru5Pt(C)(CO)15]2− and [Ru10Pt2(C)2(CO)28]2−, and were shown to be highly active and highly selective catalysts for the hydrogenation of benzoic acid, dimethyl terephthalate and naphthalene.49 Therefore, it is not surprising, that synthetic cluster chemistry has received much attention through the years. The ability to design and characterize discrete clusters with precise control of metal content as precursors to catalysts can provide fundamental background information to guide the search for more active and improved catalysts. This chapter covers some of the recent developments during the past two decades in the design, synthesis and properties of ruthenium and osmium carbonyl cluster complexes, including bi- and tri-metallic cluster frameworks. The mixed-metal cluster systems include those of transition metals with palladium and platinum as well as main group metals germanium and tin.

Pt Ru H2

H2

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PtRu5(CO)16(C)

[PtRu5]/C

Fig. 3 The nucleation and growth process shows core shell inversion of the Pt/Ru nanoparticles and exchange between Pt and Ru particles to give Pt rich and Ru rich segregated regions. Modified from Nashner, M. S.; Frenkel, A. I.; Somerville, D.; Hills, C. W.; Shapley, J. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1998, 120, 8093.

100 90 80 70

conversion

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Fig. 4 Comparison of catalytic performance of different bimetallic supported catalysts for the hydrogenation of 1,5-cyclooctadiene, after 24 h at 353 K. Modified from Hermans, S.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Sankar, G.; Gleeson, D. Angew. Chem. Int. Ed. 2001, 40, 1211.

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7.10.2

Ruthenium carbonyl cluster complexes

7.10.2.1

Monometallic ruthenium carbonyl cluster complexes

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The hexanuclear cluster Ru6(CO)17(m6-C) has been shown to react with phosphines to afford a variety of substituted derivatives.14,50,51 The Ru6 carbide cluster is prepared from the reaction of Ru3(CO)12 with ethylene gas (30 atmospheres) in an autoclave at 165  C. The compound Ru6(CO)17(m6-C) reacts with two equivalents of PMe2Ph at room temperature to yield the bis-phosphine derivative Ru6(CO)15(PMe2Ph)2(m6-C) (1) (72%).52 The molecular structure of 1 is shown in Fig. 5. Compound 1 consists of an octahedral cluster of six ruthenium atoms with a carbido carbon atom in the center. The PMe2Ph ligands are terminally coordinated to two adjacent ruthenium atoms. Interestingly, the 1H and 31P NMR spectrum of 1 at 25  C indicates that it exists in solution as a mixture of two isomers in a 4:1 ratio. The reaction of 1 with Me3NO
2H2O at 25  C affords two compounds Ru6(CO)13(m-PMe2)(m3-Z3-Me2PC6H4)(m6-C) (2) and Ru6(CO)14(PMe2Ph)(m-Z2-MePhPCH2)(m6-C)(m-H) (3) in 11% and 37% yields, respectively. Both compounds are characterized by a combination of IR, NMR, and single crystal X-ray diffraction analyses. The molecular structures of compounds 2 and 3 are shown in Fig. 6. Compound 2 can be considered as a product of decarbonylation and ortho-metallation of a phenyl ring of one of the phosphine ligands. The second phosphine ligand has lost its phenyl ring completely and it has been eliminated from the complex, probably as benzene, formed by combination with the hydrogen atom that is cleaved from the metallated phenyl ring of the other

Fig. 5 The molecular structure of Ru6(CO)15(PMe2Ph)2(m6-C) (1) [ruthenium (green), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem. 2002, 651, 124–131.

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Fig. 6 The molecular structures of Ru6(CO)13(m-PMe2)(m3-Z3-Me2PC6H4)(m6-C) (2) and Ru6(CO)14(PMe2Ph)(m-Z2-MePhPCH2)(m6-C)(m-H) (3) [ruthenium (green), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem. 2002, 651, 124-131.

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phosphine ligand. The phosphine ligand that loses its phenyl ring is transformed into a PMe2 phosphido ligand that bridges the RudRu bond of the cluster, and serves as a 3e− donor. Compound 3 consists of a Ru6 octahedron with one PMe2Ph ligand terminally coordinated to ruthenium atom. The other phosphine ligand undergoes a metallation of one of its methyl groups by cleavage of one of its CdH bonds. The hydrogen atom becomes a hydride ligand that bridges the two ruthenium atoms. This hydride ligand is located and refined crystallographically. When refluxed in octane at 127  C for 12 h, compound 1 is transformed into 2 in a better yield (61%) and two compounds Ru6(CO)14(m-PMe2)(m-Z2-MePhPCH2)(m6-C) (4) (11%), and Ru6(CO)12(PMe2)2(m3-Z2-C6H4)(m6-C) (5) (17%) are also formed in low yields. Compounds 4 and 5 are also characterized by a combination of IR, NMR, elemental and single crystal X-ray diffraction analyses. The molecular structures of 4 and 5 are shown in Fig. 7. Compounds 4 and 5 both consist of an octahedral cluster of six ruthenium atoms with a carbon atom in the center. Compound 4 can be viewed as a benzene elimination product of 3. It has the same methylene coordination as seen in 3, however, now there is a bridging phosphido group formed by cleavage of the phenyl group which probably combined with the hydride to form benzene as in the formation of product 2. Compound 5 has two bridging phosphido group which bridge the RudRu bond. In this case, both phosphine ligands have undergone loss of their phenyl rings to form the phosphido groups. Compound 2 reacts with CO to form a stable compound Ru6(CO)14(m-PMe2)(m-Z2-Me2PC6H4)(m6-C) (6) in 60% yield. The molecular structure of 6 is shown in Fig. 8. The structure of compound 6 is very similar to that of compound 2, except the

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(5)

Fig. 7 The molecular structures of Ru6(CO)14(m-PMe2)(m-Z2-MePhPCH2)(m6-C) (4) and Ru6(CO)12(PMe2)2(m3-Z2-C6H4)(m6-C) (5) [ruthenium (green), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem. 2002, 651, 124-131.

(6) 2

Fig. 8 The molecular structure of Ru6(CO)14(m-PMe2)(m-Z -Me2PC6H4)(m6-C) (6) [ruthenium (green), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem. 2002, 651, 124–131.

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ortho-metallated phenyl ring is bonded to the cluster by only one carbon atom. The s-bonded carbon atom of the phenyl ring is coordinated to ruthenium atom and the metallated phosphine ligand is transformed from a 5e− donor in 2 to a 3e− donor in 6. When compound 6 is heated to 127  C, CO is eliminated and it is converted back to 2 in 77% yield. The transformations 6 to 2 to 5 provide a sequence of steps that shows the cleavage of a phenyl ring from a phosphine ligand and its conversion into a bridging benzyne ligand in unprecedented detail. The trinuclear cluster [Ru3(CO)12] reacts at room temperature with the phosphine-functionalized N-heterocyclic carbene (NHC) ligand 1-[2-(diphenylphophino)ethyl]-3-methylimidazol-2-ylidene (dppeImMe), in 1:1 molar ratio in THF, to give the edge-bridged trinuclear derivative [Ru3(m-k2C,P-dppeImMe)(CO)10] (7).53 The use of a 1:3 [Ru3(CO)12] to dppeImMe molar ratio leads to the mono-nuclear derivative [Ru(k2C,P-dppeImMe)(CO)3] (8), which has a trigonal-bipyramidal ligand arrangement with the NHC and phosphine fragments of the dppeImMe ligand in axial and equatorial positions, respectively. Upon refluxing in THF, compound 7 is transformed into the face-capped trinuclear dihydrido derivative [Ru3(m-H)2(m3-k3C2,P-dppeImCH)(CO)8] (9), which forms due to the oxidative addition of two CdH bonds of the N-methyl group of the dppeImMe ligand. The molecular structures of compounds 7 and 9 are shown in Fig. 9. Five trinuclear monosubstituted complexes of the type Ru3(CO)11L [L ¼ Ph2P(C6H4Me-p) 10a, Ph2PC6F5 10b, P(C6H4Clp)3 10c, P(3,5-CF3-C6H3)3 10d, P(C6H4Me-p)3 10e] are synthesized by the reaction of Ru3(CO)12 with phosphine ligands (L).54 The structures of the resulting clusters are elucidated by means of elemental analyses and spectroscopic methods, including IR, 1H NMR, 13C NMR and 31P NMR spectroscopy. In all these five monosubstituted complexes, the ligand occupies the equatorial position due to steric reasons and coordination of the ligands are only at the phosphorus atom. 1H NMR spectra of 10a–c and 10e shows a multiplet around d ¼ 7.2–7.6 ppm, characteristic of phenyl groups. For the methyl groups, a singlet is observed at d ¼ 2.4 ppm for the Ph2P(C6H4Me-p) substituted cluster 10a, and at d ¼ 2.4 ppm for the P(C6H4Me-p)3 substituted cluster 10e. 13C NMR spectra of the all the monosubstituted complexes shows a single broad peak for CO instead of a multiplet, exhibiting the fluxional behavior of CO in solution. The tetraruthenium tetrahydrido cluster compound [Ru4(m-H)4(CO)12] reacts with 1,3-disubstituted imidazolin-2-ylidenes, R1R2Im, in THF at room temperature, to give the N-heterocyclic carbene tetranuclear derivatives [Ru4(m-H)4(CO)11(R1R2Im)] (R1R2Im ¼ 1,3-dimethylimidazolin-2-ylidene, Me2Im, 11a; 1-phenyl-3-methylimidazolin-2-ylidene, MePhIm, 11b; 1,3-diphenylimidazolin-2-ylidene, Ph2Im, 11c; and 1,3-dimesitylimi-dazolin-2-ylidene, Mes2Im, 11d).55 In solution, compounds 11a-11d are fluxional in the NMR time scale and display the same pattern of n(CO) IR absorptions. The thermal stability of these compounds has also been studied. While the dimethyl derivative 11a is stable in refluxing toluene for 3 h, the dimesityl derivative 11d slowly decomposes in solution at room temperature. The thermolysis of compounds 11b and 11c in toluene at reflux temperature leads to mixtures of products. The major products are two isostructural heptanuclear derivatives, [Ru7(m3-H)(m4-CO)(m-CO)2(CO)14{m(Z1-Z6-C6H4)RIm}] (R ¼ Me, 12a; Ph, 12b). These clusters contain a quadruply bridging CO ligand and an orthometalated phenyl ring of the NHC ligand that is additionally coordinated as an Z6-arene ligand. The reaction of [Ru3(CO)12] with N,N0 -dimethylimidazol-2-ylidene (Me2Im) in THF at room temperature gives the trinuclear NHC derivative [Ru3(Me2Im)(CO)11] (13).56 The structure of compound 13 is determined by X-ray diffraction analysis and shown in Fig. 10. Here the NHC ligand occupies an equatorial position and the large volume of the Me2Im ligand pushes away the adjacent equatorial CO ligand. The 1H NMR spectrum of compound 13 contains only two singlets, with integral ratio 1:3, assigned to the ring and methyl H atoms of the coordinated Me2Im ligand. The trinuclear NHC cluster [Ru3(MeOx)(CO)11] (14) is obtained after treating [Ru3(CO)12] with N-methyloxazol-2-ylidene (MeOx) in THF at room temperature. The molecular structure of 14 is comparable in many aspects to that of 13. The MeOx ligand exerts a smaller steric hindrance over the adjacent CO ligands than the Me2Im ligand. The reaction of [Ru3(CO)12] with N,N0 -Dimesitylimidazol-2-ylidene (Mes2Im) in THF at room temperature gives

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Fig. 9 The molecular structures of [Ru3(m-k2C,P-dppeImMe)(CO)10] (7) and [Ru3(m-H)2(m3-k3C2,P-dppeImCH)(CO)8] (9) [ruthenium (green), nitrogen (light blue), phosphorus (light violet), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Cabeza, J. A.; Damonte, M.; Garcia-Alvarez, P.; Kennedy, A. R.; Perez-Carreno, E. Organometallics 2011, 30, 826–833.

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Fig. 10 The molecular structures of [Ru3(Me2Im)(CO)11] (13) and [Ru3(Mes2Im)(CO)11] (15) [ruthenium (green), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Cabeza, J. A.; Rio, I. D.; Miguel, D.; Perez-Carreno, E.; Sanchez-Vega, M. G. Organometallics 2008, 27, 211–217.

a separable mixture of the trinuclear NHC derivative [Ru3(Mes2Im)(CO)11] (15) and the hexanuclear salt [Mes2ImH]2 [Ru6(m3-CO)2(m- CO)2(CO)14] (16). The X-ray molecular structure of compound 15 is shown in Fig. 10 and it shows that the axial CO ligands are not perpendicular to the Ru3 plane. The 1H NMR spectrum of 15 shows both imidazolic CH atoms and both mesityl groups are equivalent. Compound 16 is characterized by microanalysis and spectroscopic techniques. Compound 16 is a salt that contains N,N0 -dimesitylimidazolium cations and hexanuclear [Ru6(m3-CO)2(m-CO)2(CO)14]2− dianions. [Ru3(CO)12] reacts slowly with 6,60 -dimethyl-2,20 -bi-pyridine (Me2bipy) or 2,9-dimethyl-1,10-phenanthroline (Me2phen) in refluxing THF to give the trinuclear dihydride complexes [Ru3(m-H)2(m3-L1)(CO)8] (L1 ¼ HCbipyMe 17a, HCphenMe 17b), which result from the activation of two CdH bonds of a methyl group.57 The hexa-, hepta-, and pentanuclear derivatives [Ru6(m3-H) (m5-L2)(m-CO)3(CO)13] (L2 ¼ CbipyMe 18a, CphenMe 18b), [Ru7(m3-H)(m5-L2)(m-CO)2(CO)16] (L2 ¼ CbipyMe 19a, CphenMe 19b), and [Ru5(m-H)(m5-C)(m-L3)(CO)13] (L3 ¼ bipyMe 20a, phenMe 20b) are obtained by treating 17a and 17b with [Ru3(CO)12]. Compounds 18a and 18b have a basal edge-bridged square-pyramidal metallic skeleton with a carbyne-type C atom capping the four Ru atoms of the pyramid base. The structures of 19a and 19b are similar to those of 18a and 18b, respectively, but an additional ruthenium atom now caps a tri-angular face of the square-pyramidal fragment of the metallic skeleton. The most interesting feature of 18a, 18b, 19a, and 19b is that they contain a carbyne-type C atom which is originally bound to three hydrogen atoms in Me2bipy or Me2phen. The pentanuclear compounds 20a and 20b contain a carbide ligand surrounded by five ruthenium atoms in a distorted trigonal-bipyramidal environment. They are the products of a series of processes that includes the activation of all bonds (three CdH and one CdC) of organic methyl groups, and are the first examples of complexes having carbide ligands that arise from C-bonded methyl groups. The reaction of 20b with p-tolylacetylene in toluene at reflux temperature affords a mixture of compounds from which complexes [Ru5(m5-C)(m-p- MeC6H4CHCHphenMe)(CO)13] (21) and [Ru5(m-H)(m5-C) (m-p-MeC6H4CHCHphenMe)(p- tolC2)(CO)12] (22) are separated by chromatographic methods in 29% and 34% yield, respectively. Under similar conditions, the reaction of 20b with phenylacetylene gives another alkenyl derivative [Ru5(m-H)(m5-C) (m-PhCHCHphenMe)(PhC2)(CO)12] (23). These complexes are the result of the formal substitution of an alkenyl group for a methyl group of 2,9-dimethyl-1,10-phenanthroline. The reaction of Ru3(CO)12 with mesitylphosphine PH2Mes (Mes ¼ 2,4,6-trimethyl- phenyl ¼ mesityl) in a 1:1 molar ratio in toluene produces a mixture of tri-ruthenium complexes [Ru3(CO)9(m-H)2(m3-PMes)] (24), [Ru3(CO)8(PH2Mes)(m-H)2(m3-PMes)] (25), and [Ru3(CO)9(m3-PMes)2] (26), a tetraruthenium complex [Ru4(CO)10(m-CO)(m4-PMes)2] (27), and a pentaruthenium cluster [Ru5(CO)10H2(m4-PMes)(m3-PMes)2] (28).58 The molecular structures of complexes 24–27 are confirmed by X-ray diffraction analyses and these structures are shown in Fig. 11. Compound 24 is formed by the coordination of mesitylphosphine and subsequent intramolecular oxidative addition of two PdH bonds accompanied by dissociation of overall three CO ligands. After that, cluster 25 is produced under relatively mild conditions by a simple replacement of a CO ligand of 24 by the second mesitylphosphine. In refluxing toluene, 25 undergoes reductive elimination of H2, intramolecular oxidative addition of two PdH bonds of the mesitylphosphine ligand, another reductive elimination of H2, and final recoordination of a CO ligand to afford the cluster 26. Then, 26 slowly reacts with Ru(CO)4 generated from Ru3(CO)12 and subsequent dissociation of two CO ligands gives cluster 27. Cluster 24 adopts a trigonal pyramidal geometry in which a m3-PMes ligand caps a ruthenium triangle. In compound 25, m3-PMes group caps the basal triangle composed of three ruthenium atoms, one of which is coordinated with a PH2Mes ligand. Complex 26 reveals a distorted square pyramidal geometry with the basal plane composed of two ruthenium and two phosphorus atoms, and a Ru(CO)3 fragment at the apical position. Two aromatic rings of the mesityl groups are nearly coplanar to each other and adopt a twisted arrangement with respect to the basal plane. Cluster 27 has an octahedral framework and both sides of the square Ru4 plane are capped with two m4-phosphinidene ligands. Compound 28 has a bicappped-octahedral core and

Ruthenium and Osmium Carbonyl Cluster Complexes

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Fig. 11 The molecular structures of [Ru3(CO)9(m-H)2(m3-PMes)] (24), [Ru3(CO)8(PH2Mes)(m-H)2(m3-PMes)] (25), [Ru3(CO)9(m3-PMes)2] (26) and [Ru4(CO)10(m-CO) (m4-PMes)2] (27) [ruthenium (green), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Kakizawa, T.; Hashimoto, H.; Tobita, H. J. Organomet. Chem. 2006, 691, 726–736.

the five ruthenium atoms constitute a square pyramidal framework, and each ruthenium atom bears two carbonyl ligands. The basal plane of the ruthenium pyramid is capped with a m4-PMes ligand, and two triangular faces are capped with m3-PMes units.

7.10.2.2 7.10.2.2.1

Bimetallic ruthenium-group 10 transition metal carbonyl cluster complexes Bimetallic ruthenium-nickel carbonyl cluster complexes

The bimetallic cluster complex Ru5Ni(NCMe)(CO)15(m6-C) (29) is obtained in 37% yield from the reaction of Ru5(CO)15(m5-C), with Ni(COD)2 in acetonitrile solvent under refluxing conditions.59 Compound 29 is characterized by a combination of IR, 1H NMR, mass spectrometry, and single crystal X-ray diffraction analyses. The molecular structure of 29 is shown in Fig. 12. Compound 29 consists of an octahedron of one nickel atom and five ruthenium atoms. The carbide ligand is encapsulated in the center of the Ru5Ni octahedron and the acetonitrile ligand from the reaction solvent is terminally coordinated to the nickel atom. The Ni-carbide distance is not significantly different from the Ru-carbide distances. There are two bridging CO ligands which bridge the NidRu bonds, and these two NidRu bonds are shorter than all the other metal-metal bonds. As expected compound 29 contains 86 cluster valence electrons which is in accord with the Polyhedral Skeletal Electron Pair theory.60 When carbon monoxide gas is purged through solutions of 29 at 110  C, replacement of the acetonitrile ligand with CO gives the complex Ru5Ni(CO)16(m6-C) (30) in 97% yield. Compound 30 is characterized crystallographically and its molecular structure is shown in Fig. 13. When solutions of 29 and 30 are exposed to ammonia gas at 0  C, the complex Ru5Ni(NH3)(CO)15(m6-C) (31), is formed and isolated in 97% and 50% yields, respectively. Both reactions do proceed at room temperature, however owing to some decomposition the yields at 0  C are better. The solid-state structure of 31 is shown in Fig. 13. The presence of acetonitrile solvent in the reaction medium is essential to form the Ru5Ni octahedral framework. It has been shown that Ru5(CO)15(m5-C) readily adds small molecules such as acetonitrile to yield an open Ru5(m5-C) cluster, Ru5(CO)15(NCMe)(m5-C), where one ruthenium atom bridges a butterfly arrangement of four other ruthenium atoms.61,62 The opening of the Ru5(m5-C) cluster facilitates the reaction

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Ruthenium and Osmium Carbonyl Cluster Complexes

Fig. 12 The molecular structure of Ru5Ni(NCMe)(CO)15(m6-C) (29) [ruthenium (green), nickel (red), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Saha, S.; Zhu, L.; Captain, B. Inorg. Chem. 2013, 52, 2526–2532.

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(31)

Fig. 13 The molecular structures of Ru5Ni(CO)16(m6-C) (30) and Ru5Ni(NH3)(CO)15(m6-C) (31) [ruthenium (green), nickel (red), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Saha, S.; Zhu, L.; Captain, B. Inorg. Chem. 2013, 52, 2526–2532.

with Ni(COD)2 by providing vacant coordination sites that are not accompanied by cluster degradation. Migration of the acetonitrile ligand to the nickel atom with subsequent loss of the labile COD groups in Ni(COD)2 gives the parent complex 29. Photolysis of 29 in benzene solvent results in the formation of the complex Ru5Ni(CO)13(6-C6H6)(m6-C) (32) in 33% yield as the major product. Compound 32 is characterized crystallographically and its molecular structure is shown in Fig. 14. Compound 32 consists of a Ru5Ni(m6-C) cluster where one of the ruthenium vertices is coordinated to a benzene molecule in an 6-fashion. There are many examples of arene ligands coordinated to metal carbonyl cluster complexes,63–71 the first being Ru6(CO)14(6-C6H6)(m6-C) that is reported many years ago. There have also been examples of metal carbonyl clusters to contain bis(arene) ligands and in some/one case where a metal cluster is “sandwiched” between two 6-C6H6 ligands.67,72–75 However, there have not been many examples of mixed-metal clusters containing the 6-C6H6 ligand. The bimetallic complexes that contain the arene ligand are prepared using an already coordinated benzene ligand in the starting reactant as seen in the complexes, Ru5(CO)12(6-C6H6)(m6-C)[PtPBu3t].70,76,77 In the photolysis reaction of 29, the complexes Ru5Ni(CO)16(m6-C), Ru6(CO)17(m6-C), Ru5(CO)15(m5-C), Ru5(CO)12(6-C6H6)(m5-C), and Ru5Ni(NCMe)(CO)15(m6-C) are also obtained in minor/ trace amounts. Photolysis of a toluene solution of 29 furnishes the complex Ru5Ni(CO)13(6-C7H8)(m6-C) (33) in 37% yield, see Fig. 14. Compound 33 is very similar in structure to 32 where in place of the benzene ligand there is now an 6-C7H8 group coordinated to one of the ruthenium vertices. The 6-C7H8 coordination mode has been observed previously in the homometallic carbide cluster Ru6(CO)14(6-C7H8)(m6-C).66,78–80 The improved synthesis of the various arene coordinated ruthenium carbide

Ruthenium and Osmium Carbonyl Cluster Complexes

(32) 6

573

(33) 6

Fig. 14 The molecular structures of Ru5Ni(CO)13( -C6H6)(m6-C) (32) and Ru5Ni(CO)13( -C7H8)(m6-C) (33) [ruthenium (green), nickel (red), carbon (gray), oxygen (orange)]. Modified from Saha, S.; Zhu, L.; Captain, B. Inorg. Chem. 2013, 52, 2526–2532.

clusters is accomplished using cyclohexadiene under thermal conditions or in the presence of trimethylamine N-oxide as a decarbonylating reagent to furnish the cyclohexadiene coordinated carbide cluster first, followed by loss of H2 to yield the benzene coordinated cluster.67,68,72 In this bimetallic system, the arene coordinated complexes 32 and 33 are formed directly from benzene or toluene solvent in reasonable yields under photolytic conditions. Compounds 32 and 33 can also be obtained from the binary cluster 30 under similar conditions. The reaction of the compound CpNiRu3(H)3(CO)9 with 2-(diphenylphosphino)ethyl-triethoxysilane gives the compound CpNiRu3(H)3(CO)7L2 (34) where L ¼ Ph2PCH2CH2Si(OEt)3 in considerable yield.81 The structure of 34 is determined by single-crystal X-ray diffraction analysis. The complex is formed by an equilateral triangle of Ru atoms capped by a Ni atom to the centroid of the Cp ring. Three hydrogen atoms bridge the RudRu bonds. One Ru atom is linked to three CO groups, while other two Ru atoms are each linked to two CO groups and to one axial phosphine ligand. The phosphine ligands are located trans to the Ni atom. This bimetallic complex 34 containing nickel and ruthenium is successfully anchored on the mesoporous SBA-15 silica, in order to obtain a potential heterogeneous catalyst for several catalytic reactions where the activity of both nickel and ruthenium centers are required. The insertion of ruthenium(II) into an azuliporphyrin (TPAP) yields ruthenium(II) azuliporphyrin carbonyl complex [Ru(TPAP)(CO)] featuring an equatorial CNNN set of donors. The azulene moiety in the ruthenium(II) azuliporphyrin provides the suitable p-surface to bind the Ru4(CO)9 cluster. Two conceptually different organometallic motifs are merged in a unique three-dimensional architecture. It is proved that these unique coordination abilities of azulene introduced into a carbaporphyrinoid frame are preserved in other suitable systems producing a Ru-Ni bimetallic complex [Ni(TPAP){Ru4(CO)9}] (35).82

7.10.2.2.2

Bimetallic ruthenium-palladium carbonyl cluster complexes

The reaction of Ru3(CO)12 with an excess of Pd(PBu3t)2 at room temperature affords the tripalladium complex Ru3(CO)12[Pd(PBu3t)]3 (36) in 49% yield.83 Compound 36 is characterized by a combination of IR, 1H and 31P NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 36 is shown in Fig. 15. The compound has a “raft-like” structure with a triangular Ru3 unit in the center. Each edge of the Ru3 group is bridged by a Pd(PBu3t) group. Each ruthenium atom contains three linear terminal CO ligands plus one CO ligand that forms a bridge to a palladium atom. Because there is no loss of a CO ligand from Ru3(CO)12, compound 36 can be viewed most simply as a tris-Pd(PBu3t) adduct of it with the Pd(PBu3t) groups being generated from Pd(PBu3t)2 by the loss of one of its PBu3t ligands. A simple model for the bonding of the palladium atoms to the RudRu bonds can be constructed as follows: the Pd(PBu3t) fragment contains only 12 valence electrons and will be a strong Lewis acid. If two electrons from a RudRu bond are shared with the proximate Pd atom, then a 3-center/2-electron PdRu2 bond would be formed, and the electron count at the palladium atom would be increased formally to 14, as it is in the parent Pd(PBu3t)2. This is conceptually similar to the well-known protonation of the metal-metal bonds of polynuclear metal complexes that occurs in strong protic media.84–86 The dipalladium complex Ru6(CO)17(m6-C)[Pd(PBu3t)]2 (37) is formed in 33% yield from the reaction of Ru6(CO)17(m6-C) with Pd(PBu3t)2 at room temperature.83 Compound 37 is characterized by a combination of IR, 1H and 31P NMR, and single-crystal X-ray diffraction analyses. The structure of 37 consists of an octahedral cluster of six ruthenium metal atoms with a carbon atom in the center, and two Pd(PBu3t) groups coordinated to it. In the solid-state, compound 37 exists as two isomers, and both isomers can be viewed as bis-Pd(PBu3t) adducts of Ru6(CO)17(m6-C) as there is no loss of CO from the Ru6 starting material. In one isomer the Pd(PBu3t) groups bridge two edges of the Ru6 octahedron, see Fig. 16. In the other isomer, Fig. 16, one Pd(PBu3t) group bridges the

574

Ruthenium and Osmium Carbonyl Cluster Complexes

(36) t

Fig. 15 The molecular structure of Ru3(CO)12[Pd(PBu3 )]3 (36) [ruthenium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Manson, J.; Smith, M. D.; Webster, C. E. J. Am. Chem. Soc. 2004, 126, 5253–5267.

(37) isomer 1

(37) isomer 2

Fig. 16 The molecular structures of Ru6(CO)17(m6-C)[Pd(PBu3t)]2 (37), isomer 1 and Ru6(CO)17(m6-C)[Pd(PBu3t)]2 (37), isomer 2 [ruthenium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Manson, J.; Smith, M. D.; Webster, C. E. J. Am. Chem. Soc. 2004, 126, 5253–5267.

RudRu edge of the Ru6 octahedron, while the other Pd(PBu3t) group serves as a triple bridge capping the Ru3 triangle. The RudRu bond distances in the Ru6 cluster are similar to those found in the parent compound.87 Carbonyl ligands bridge from the Ru6 cluster to the palladium atoms in both isomers. The reaction of the benzene-coordinated Ru6 carbonyl cluster, Ru6(CO)14(Z6-C6H6)(m6-C) (38) with Pd(PBu3t)2 at room temperature yields mono- and dipalladium complexes Ru6(CO)14(Z6-C6H6)(m6-C)[Pd(PBu3t)]n where n ¼ 1 (39), n ¼ 2 (40).83 Both compounds are characterized by IR, 1H and 31P NMR and single-crystal X-ray diffraction analyses. The molecular structure of 39 is shown in Fig. 17. Compound 39 consists of an Ru6 octahedron with a carbon atom in the center, a benzene ligand coordinated to one of the Ru atoms, and a Pd(PBu3t) group bridging the RudRu bond. There is no loss of CO from the Ru6 starting material, and thus compound 39 can be viewed as a mono-Pd(PBu3t) adduct of 38. The molecular structure of 40 is shown in Fig. 17. Compound 40 consists of an Ru6 octahedron with a carbon atom in the center, a benzene ligand coordinated to one of the Ru atoms, and two Pd(PBu3t) bridging groups. Like compound 39, the dipalladium adduct of 40, has one Pd(PBu3t) group bridging the RudRu bond with a CO ligand. In the solid-state structure of 40 the two PBu3t ligands are inequivalent, but the 31P NMR spectrum of 40 shows only a single resonance even at −80  C. Although it is possible that the molecule has adopted a different structure in solution having equivalent PBu3t groups, it is also possible that the molecule is dynamically active on the NMR time scale and the Pd(PBu3t) groups are interchanging equivalent sites rapidly on the NMR time scale. The most remarkable demonstration of the potential of the Pd(PBu3t) group to drive new chemistry is found from the reaction of Ru(CO)5 with Pd(PBu3t)2.88 From this reaction the dipalladium-diruthenium complex, Ru2(CO)9[Pd(PBu3t)]2 (41) is obtained in

Ruthenium and Osmium Carbonyl Cluster Complexes

(39)

575

(40)

Fig. 17 The molecular structures of Ru6(CO)14(Z -C6H6)(m6-C)[Pd(PBu3 )] (39) and Ru6(CO)14(Z -C6H6)(m6-C)[Pd(PBu3t)]2 (40) [ruthenium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Manson, J.; Smith, M. D.; Webster, C. E. J. Am. Chem. Soc. 2004, 126, 5253–5267. 6

t

6

(41) Ru2(CO)9[Pd(PBu3t)]2

Fig. 18 The molecular structure of (41) [ruthenium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Am. Chem. Soc. 2002, 124, 5628–5629.

40% yield. Compound 41 is characterized structurally and is shown in Fig. 18. This compound can be viewed as a dipalladium adduct of the compound Ru2(CO)9. Ru2(CO)9 is first obtained by the photodecarbonylation of Ru(CO)5 in 1977 and is reported to be a “very unstable” compound at room temperature.89 This compound contains two ruthenium atoms joined by a RudRu single bond. The Ru2(CO)9 group in 41 is stabilized by the presence of two Pd(PBu3t) groups both of which bridge one single RudRu bond on opposite sides of the molecule. With nine CO ligands, compound 41 can be viewed as a dipalladium adduct of the compound Ru2(CO)9. It is demonstrated that the bis-phosphine compound Pd(PBu3t)2 is an excellent reagent for the transfer of PdPBu3t groups to ruthenium cluster compounds under mild conditions to produce a variety of new bimetallic cluster complexes containing palladium. The reaction of Ru5(CO)15(m5-C) with Pd(PBu3t)2 at 25  C to gives two products, PdRu5(CO)15(PBu3t)(m6-C) (42) in 50% yield and Pd2Ru5(CO)15(PBu3t)2(m6-C) (43) in 6% yield.90,91 Both compounds are characterized by a combination of IR, 1H and 31P NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 42 is shown in Fig. 19. The structure of 42 consists of an octahedral cluster of one palladium atom and five ruthenium atoms with an interstitial carbido ligand in the center. The PBu3t ligand is terminally coordinated to the palladium atom. There is one bridging carbonyl ligand between the palladium atom and one of the ruthenium atoms, and this is the shortest Pd-Ru bond. The molecular structure of 43 is shown in Fig. 19. This compound also consists of an octahedral cluster of one palladium atom and five ruthenium atoms with an interstitial carbido ligand in the center of the octahedron, but in addition it has a second palladium atom that is capping one of the triangular triruthenium faces of the cluster. Each palladium atom contains one tri-tert-butylphosphine ligand. There are two bridging carbonyl ligands. The 1H NMR spectrum for 43 at −90  C shows four doublets with relative intensities 1:1:2:2. These resonances are attributed to the methyl groups of the inequivalent phosphine ligands. The reaction of [PPN][Ru5Co(m6-C)(CO)16] with [Pd(MeCN)4][BF4]2 at room temperature undergoes smooth replacement of cobalt with palladium atom to yield the cluster complex Ru5Pd(C)(CO)16 (44).92 The solid-state structure of 44 is determined by single-crystal X-ray analysis and is identical to that of platinum analog Ru5Pt(m6-C)(CO)16. The molecular structure of 44 is

576

Ruthenium and Osmium Carbonyl Cluster Complexes

(42)

(43) t

t

Fig. 19 The molecular structures of PdRu5(CO)15(PBu3 )(m6-C) (42) and Pd2Ru5(CO)15(PBu3 )2(m6-C) (43) [ruthenium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from (a) Adams, R. D.; Captain, B.; Fu, W.; Pellechia, P. J.; Smith, M. D. Inorg. Chem. 2003, 42, 2094 and (b) Adams, R. D.; Captain, B.; Fu, W.; Pellechia, P. J.; Smith, M. D. Angew. Chem., Int. Ed. 2002, 41, 1951.

(44)

(45)

Fig. 20 The molecular structures of Ru5Pd(C)(CO)16 (44) and [PPh4][Ru5Pd(m6-C)(CO)13(m2-PPh2)(PPh3)] (45) [ruthenium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Nakajima, T.; Konomoto, H.; Ogawa, H.; Wakatsuki, Y. J. Organomet. Chem. 2007, 692, 4886–4894.

shown in Fig. 20. The reaction of 44 with excess PPh3 in hexane at room temperature gives the product [PPh4][Ru5Pd(m6-C) (CO)13(m2-PPh2)(PPh3)] (45) in 56% yield. This compound is resulted from substitution of CO and concomitant disproportionation of PPh3. The molecular structure of 45 is determined by X-ray analysis is shown in Fig. 20.

7.10.2.2.3

Bimetallic ruthenium-platinum carbonyl cluster complexes

Ru6(CO)17(m6-C) reacts with Pt(PBu3t)2 to yield a monoplatinum complex, Ru6(CO)17(m6-C)[Pt(PBu3t)] (46) in 11% yield; in addition a diplatinum complex Ru6(CO)17(m6-C)[Pt(PBu3t)]2 (47) is also obtained in 24% yield.83 These compounds are characterized by IR, 1H and 31P NMR and single-crystal X-ray diffraction analyses. Compound 46 consists of a Ru6 octahedron with a carbon atom in the center. The Pt(PBu3t) group is bonded to three ruthenium atoms, see Fig. 21. There is no loss of CO from Ru6(CO)17(m6-C), and thus the compound can be viewed as a mono-Pt(PBu3t) adduct of Ru6(CO)17(m6-C). Like compound 37, compound 47 also has two independent molecules in the asymmetric unit in its crystal structure. However, in this case both molecules are structurally similar with two Pt(PBu3t) groups bridging two RudRu bonds, making them analogous to the isomer of 37 which has Pd(PBu3t) groups bridging two RudRu bonds, see Fig. 21. Each PtdRu bond has a bridging CO ligand. The reaction of Pt(PPh3)2(PhC2Ph) with Ru3(CO)12 yields two mixed-metal cluster complexes RuPt2(CO)5(PPh3)2(PhC2Ph) (48) and Ru2Pt(CO)7(PPh3)2(PhC2Ph) (49).93 Both compounds are characterized by single-crystal X-ray diffraction analyses.

Ruthenium and Osmium Carbonyl Cluster Complexes

(46) Ru6(CO)17(m6-C)[Pt(PBu3t)]

577

(47) t

Fig. 21 The molecular structures of (46) and Ru6(CO)17(m6-C)[Pt(PBu3 )]2 (47) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Manson, J.; Smith, M. D.; Webster, C. E. J. Am. Chem. Soc. 2004, 126, 5253–5267.

(48)

(49)

Fig. 22 The molecular structures of RuPt2(CO)5(PPh3)2(PhC2Ph) (48) and Ru2Pt(CO)7(PPh3)2(PhC2Ph) (49) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Bunz, U.; Captain, B.; Fu, W.; Steffen W. J. Organomet. Chem. 2000, 614–615, 75–82.

The molecular structure of compound 48 is shown in Fig. 22. Compound 48 consists of a triangular Pt2Ru cluster with a triply bridging diphenylacetylene ligand. The alkyne CdC bond is parallel to the PtdPt bond and the CdC distance is slightly longer than that found for the uncoordinated alkyne, as expected due to the effects of its coordination. Each platinum atom coordinated with one triphenylphosphine ligand and one carbonyl ligand while the ruthenium atom is coordinated with three carbonyl ligands. Assuming that the alkyne serves as a four-electron donor, each cluster contains a total of 46 electrons which is two electrons less than the 48 electrons expected for a triangular cluster where all three metal atoms obey the 18-electron rule. Previous studies have shown that clusters containing diplatinum groups often contain fewer electrons than that predicted by the conventional electron counting theories.94 The structure of compound 49 is shown in Fig. 22. Compound 49 contains a triangular cluster composed of one platinum and two ruthenium atoms. The diphenylacetylene ligand is coordinated as a triply bridging ligand as in 48, but the CdC bond of the alkyne is parallel to one of the RudPt bonds. Each ruthenium atom has three carbonyl ligands while the platinum atom contains the triphenylphosphine and one carbonyl ligand. When a hexane solution of compound 49 is heated to reflux in the presence of a slow purge with H2, the tetranuclear mixed metal cluster compound Ru2Pt2(CO)8(PPh3)2(m-H)2 (50) is formed in 11% yield.93 Compound 50 is characterized by IR, elemental and single crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 23. The molecule contains two ruthenium atoms and two platinum atoms in a tetrahedral arrangement. The two hydride ligands bridge two of the RudPt bonds. The electron

578

Ruthenium and Osmium Carbonyl Cluster Complexes

(50) Fig. 23 The molecular structure of Ru2Pt2(CO)8(PPh3)2(m-H)2 (50) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Bunz, U.; Captain, B.; Fu, W.; Steffen W. J. Organomet. Chem. 2000, 614–615, 75–82.

count for the cluster is 58 which is two less than the expected 60 electron count. However, it has been found that platinum containing tetrahedral clusters often contain 58 electrons.94 Another interesting bimetallic Pt-Ru complex, [PtRu5(CO)15(PBu3t)(C)] (51) is obtained in 52% yield from the reaction of [Ru5(CO)15(C)] with [Pt(PBu3t)2] at 25  C.90,91 The crystal structure analyses reveal two isomeric forms for the molecular structure of 51. One isomer, 51A, is found in both the triclinic form and one of the monoclinic forms obtained by crystallization from solutions in benzene/octane solvent mixtures at −5  C. The molecular structure of the compound is similar in both of these crystal forms and the structure consists of a square-pyramidal cluster of five ruthenium atoms with one platinum atom spanning the square base. The molecule as found in the triclinic form is shown in Fig. 24. A carbido ligand lies inside the cluster of six metal atoms. The single tri-tert-butylphosphine ligand is coordinated to the platinum atom. There is one bridging carbonyl ligand between the platinum atom and one of the ruthenium atoms, and this PtdRu bond distance is the shortest of all of the PtdRu bond distances. The second monoclinic form 51B is obtained by crystallization from solutions of the complex in diethyl ether solvent. In this crystalline form there are two completely independent molecules in the asymmetric unit. Both are structurally similar, and a diagram of the molecular structure of one of these molecules is shown in Fig. 24. In this crystalline state the compound has assumed an isomeric structure in which the platinum atom occupies an edge of the square base of the cluster of ruthenium atoms. The reactions of 51 with hydrogen and phenylacetylene, PhC2H, has been investigated and evidence is found for catalytic hydrogenation activity in solutions.95 The dihydride complex Ru5(CO)14(m6-C)[Pt(PBu3t)](m-H)2 (52) is obtained in 68% yield from the reaction of 51 with hydrogen in a heptane solution at reflux. Compound 52 is characterized crystallographically, and its molecular structure is shown in Fig. 25. The molecule consists of an octahedral cluster of six metal atoms, PtRu5, with a carbido ligand in the center. A PBu3t ligand is coordinated to the platinum atom, and there are two hydride ligands that bridge two of the

(51A)

(51B)

Fig. 24 The molecular structure of [PtRu5(CO)15(PBu3t)(C)] (51) as found in the triclinic crystal form 51A and in the monoclinic crystal form 51B [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from (a) Adams, R. D.; Captain, B.; Fu, W.; Pellechia, P. J.; Smith, M. D. Inorg. Chem. 2003, 42, 2094 and (b) Adams, R. D.; Captain, B.; Fu, W.; Pellechia, P. J.; Smith, M. D. Angew. Chem., Int. Ed. 2002, 41, 1951.

Ruthenium and Osmium Carbonyl Cluster Complexes

(52)

579

(53)

Ru5(CO)14(m6-C)[Pt(PBu3t)](m-H)2

Fig. 25 The molecular structures of (52) and Ru5(CO)13(m5-C)(PhC2H)[Pt(PBu3t)] (53) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Am. Chem. Soc. 2004, 126, 3042–3043.

Pt-Ru bonds. The alkyne complex Ru5(CO)13(m5-C)(PhC2H)[Pt(PBu3t)] (53) is obtained in 41% yield from the reaction of 51 with PhC2H in a CH2Cl2 solution at reflux. Compound 53 is characterized crystallographically, and its molecular structure is shown in Fig. 25. The molecule consists of a square-pyramidal cluster of five ruthenium atoms with an interstitial carbido ligand in the center. There is a platinum atom capping the Ru3 triangle. A PBu3t ligand is coordinated to the platinum atom, and a PhC2H ligand bridges one of the PtRu2 triangles. A PBu3t ligand is coordinated to the platinum atom, and a PhC2H ligand bridges one of the PtRu2 triangles. As expected, hydrogen atom exhibits a very low-field resonance in the 1H NMR spectrum of 53, d ¼ 8.58 ppm, with coupling to the phosphorus atom of the neighboring PBu3t ligand. Overall, compound 53 contains a total of 86 valence electrons, which is precisely the number expected for a metal-capped square-pyramidal cluster of five metal atoms.60 When compound 52 is treated with Pt(PBu3t)2, the complex Pt2Ru5(CO)14(PBu3t)2(m-H)2(m6-C) (54) is formed in 58% yield.96 Compound 54 is characterized by a combination of IR, 1H NMR, 31P NMR, and single-crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 26. Compound 54 can be viewed as a Pt(PBu3t) adduct of 52 formed by adding a Pt(PBu3t) grouping across one of the RudRu bonds in the base of the square pyramidal part of the Ru5 portion of the cluster of 52. The two hydrido ligands in 54 are located in the structural analysis. As in 52, these metal-metal bond distances seem to be related to steric interactions between CO ligands and the PBu3t ligand on the neighboring Pt atom. The hydrido ligands in 54 are inequivalent.

(54)

(55)

Fig. 26 The molecular structure of Pt2Ru5(CO)14(PBu3t)2(m-H)2(m6-C) (54) and Ru5(CO)12(m5-C)[PtPBu3t](PhC2H)(m-H)2 (55) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from (a) Adams, R. D.; Captain, B.; Zhu, L. Inorg. Chem. 2005, 44, 6623−6631 and (b) Adams, R. D.; Captain, B.; Zhu, L. J. Am. Chem. Soc. 2004, 126, 3042–3043.

580

Ruthenium and Osmium Carbonyl Cluster Complexes

When compound 53 is treated with hydrogen (30 psi) in the presence of a 50-fold excess PhC2H at 80  C, styrene is obtained catalytically at a rate of 20(2) turnovers/h. From these solutions, the platinum-ruthenium cluster complex, Ru5(CO)12(m5-C) [PtPBu3t](PhC2H)(m-H)2 (55) is isolated.94 The structure of 55 is shown in Fig. 26. Like 53, compound 55 contains a platinum-capped Ru3 triangle of a Ru5 cluster, but the Ru5 cluster is not a square-pyramidal cluster as in 53. Compounds 53 and 55 have the same number of valence electrons; however, 55 is unsaturated because it has one less RudRu bond than 53. One CO ligand is eliminated in going from 53 to 55, and one equivalent of H2 is added. The hydrogen exists in the form of two hydride ligands, that bridge the PtdRu and RudRu metal-metal bonds. Compound 55 contains a PhC2H that bridges a PtRu2 triangle similarly to that in 53. Interestingly, 55 can be obtained independently in a better yield simply by treating 53 with hydrogen and the decarbonylation agent Me3NO at 40  C. When 55 is treated with CO at 25  C, both hydrides and the PhC2H ligand are eliminated in the form of styrene, and compound 51 is formed. When solutions of 55 are treated with hydrogen and an excess of PhC2H, styrene is formed catalytically. When a solution of 53 containing one equivalent of Pt(PBu3t)2 in CH2Cl2 solvent is heated to reflux for 30 min, two bimetallic cluster complexes, Pt2Ru5(CO)13(PBu3t)2(m5-C)(m3-PhC2H) (56) and Pt3Ru5(CO)13(PBu3t)3(m5-C)(m3-PhC2H) (57) are obtained in 57% and 8% yield, respectively.97 Compounds 56 and 57 are both characterized by a combination of IR, 1H NMR, 31P NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 56 is shown in Fig. 27. Compounds 56 is very similar to 53 except that it contains an additional Pt(PBu3t) group that bridges the RudRu edge of the Ru5 square pyramid on the side opposite the bridging PhC2H ligand. The total valence electron count for 56 is only 98, which is two less than that expected if all of the metal atoms obeyed the skeletal electron pair theory.60 The molecular structure of 57 is shown in Fig. 27. Overall, this molecule is very similar to 56, except that it contains still another Pt(PBu3t) group. This one bridges the RudRu edge of the Ru5 square pyramid on the side adjacent to the bridging PhC2H ligand. Thus, in 57 there are two 16-electron bridging Pt(PBu3t) groups. Carbonyl ligands bridge each PtdRu bond to each of the unsaturated Pt(PBu3t) groups, as also found in 56. As a result, the total valence electron count for 57 is only 110, which is four less than that expected if the cluster obeyed the skeletal electron pair theory. The reaction of platinum-ruthenium cluster PtRu5(CO)16(m6-C) with PMe2Ph yields the mono- and disubstituted derivatives PtRu5(CO)15(PMe2Ph)(m6-C) (58) and PtRu5(CO)14(PMe2Ph)2(m6-C) (59), in yields of 36% and 45%, respectively.98 The molecular structures of 58 and 59 are shown in Fig. 28. Both compounds are structurally similar to PtRu5(CO)16(m6-C)48 and consist of an octahedral cluster containing one Pt and five Ru atoms with a single carbido carbon atom in the center. Compound 58 contains two crystallographically independent molecules in the solid state. Both molecules are structurally similar and have a PMe2Ph ligand terminally coordinated to the Pt atom. There is one carbonyl ligand that bridges one of the PtdRu bonds, which is the shortest PtdRu bond in the molecule. The two PMe2Ph ligands are both coordinated to Ru atoms adjacent to the Pt atom. The bridging carbonyl ligand bridges one of the Pt-Ru bonds, which, similar to that in 58, is the shortest Pt- Ru bond in the molecule. The reaction of PtRu5(CO)16(m6-C) with Me2S yields PtRu5(CO)15(Me2S)(m6-C) (60) in 44% yield.98 Compound 60 is characterized by a combination of IR, NMR, and single-crystal X-ray diffraction analyses. The molecular structure is shown in Fig. 29. The cluster of 60 is similar to that of 58. The Me2S ligand is terminally coordinated to the Pt atom. As in compounds 58 and 59, there is one bridging carbonyl ligand that bridges the shortest Pt-Ru bond in the cluster. Interestingly, the 1H NMR spectrum of compound 60 at −40  C shows three resonances of unequal intensities for the methyl groups, indicating that 60 exists as a mixture of three isomers in solution. The reaction of PtRu5(CO)16(m6-C) with an equimolar amount of Pt(PBu3t)2 at room temperature affords two complexes, PtRu5(CO)16(m6-C)[Pt(PBu3t)] (61) in 28% yield and PtRu5(CO)16(m6-C)[Pt(PBu3t)]2 (62) in 21% yield.99 Both compounds are characterized by IR, 1H and 31P NMR, and single crystal X-ray diffraction analyses. The molecular structures of 61 and 62 are shown in Fig. 30. The structure of 61 consists of an octahedral cluster of six metal atoms; one platinum and five ruthenium atoms with a carbon atom in the center. A Pt(PBu3t) group bridges the RudRu edge of the octahedron and the PBu3t ligand is terminally

(56)

(57) t

Pt3Ru5(CO)13(PBu3t)3(m5-C)(m3-PhC2H)

Fig. 27 The molecular structures of Pt2Ru5(CO)13(PBu3 )2(m5-C)(m3-PhC2H) (56) and (57) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Organometallics 2005, 24, 2419–2423.

Ruthenium and Osmium Carbonyl Cluster Complexes

(58)

581

(59)

Fig. 28 The molecular structures of PtRu5(CO)15(PMe2Ph)(m6-C) (58) and PtRu5(CO)14(PMe2Ph)2(m6-C) (59) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Pellechia P. J. Inorg. Chem. 2003, 42, 3111–3118.

(60) Fig. 29 The molecular structure of PtRu5(CO)15(Me2S)(m6-C) (60) [ruthenium (green), platinum (red), carbon (gray), oxygen (orange), sulfur (yellow)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Pellechia P. J. Inorg. Chem. 2003, 42, 3111–3118.

(61) PtRu5(CO)16(m6-C)[Pt(PBu3t)]

(62) PtRu5(CO)16(m6-C)[Pt(PBu3t)]2

Fig. 30 The molecular structures of (61) and (62) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem. 2003, 682, 113–118.

582

Ruthenium and Osmium Carbonyl Cluster Complexes

coordinated to the platinum atom. The PtdRu and RudRu bond distances in the PtRu5 cluster are similar to those found in PtRu5(CO)16(m6-C).48 In compound 62 two Pt(PBu3t) groups have been added across two different metal-metal bonds of the cluster PtRu5(CO)16(m6-C). One Pt(Bu3t) group bridges the RudRu edge of the Ru4 square plane of the octahedron with two CO ligands from the PtRu5 cluster bridging to the platinum atom. The other Pt(PBu3t) group is a bridge across the PtdRu bond. There is only one bridging CO ligand to this platinum atom, which bridges the PtdRu bond. The PtdPt bond distance is similar to that found in the compound Pt2Ru4(CO)18.100 The reaction of 61 with PhC2H at 40  C gives the compound PtRu5(CO)15(m6-C)(m3-PhC2H)[Pt(PBu3t)] (63) containing one PhC2H ligand in 39% yield.101 Compound 63 is characterized by a combination of IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses. The molecular structure of 63 is shown in Fig. 31. The structure of 63 consists of a central cluster of five ruthenium atoms and one platinum atom arranged in a pseudo-octahedral shape with a carbido ligand in the center. There is a second platinum atom bridging a basal edge of the Ru5 square pyramid. The PhC2H ligand is bonded to the two platinum atoms and one ruthenium atom as a triple bridge. The hydrogen atom exhibits the typical low field resonance in the 1H NMR spectrum, d ¼ 9.54 ppm and shows the expected coupling to the phosphorus atom on the neighboring platinum atom. One CO ligand is lost from 61 upon addition of the PhC2H ligand to form 63. Overall, this complex has a total of 100 cluster valence electrons. This is exactly the number expected for an octahedral cluster of six metal atoms (86 electrons) with one edge bridging metal containing fragment. At 68  C, the reaction of 61 with PhC2H results in the displacement of three CO ligands and the addition of two molecules of PhC2H to yield the compound PtRu5(CO)13(m6-C)(m3-PhC2H)2[Pt(PBu3t)] (64) in 30% yield.101 Compound 64 is characterized by a combination of IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses, and the molecular structure of 64 is shown in Fig. 31. Compound 64 contains five ruthenium atoms in a square pyramidal arrangement. A carbido ligand sits in the center of this group of five ruthenium atoms and is bonded to all five ruthenium atoms. There are two mutually bonded platinum atoms, that are both bonded to the open edge of the Ru5 square pyramid. The triply bridging PhC2H ligand is coordinated to the triangular group of three metal atoms. The alkyne hydrogen atoms exhibit the typical low field resonances in the 1H NMR spectrum, d ¼ 8.81 and 7.56 ppm, and shows the expected coupling to the phosphorus atom on the neighboring platinum atom. Compound 64 has only 100 valence electrons, two short of the expected number. As expected, the mono alkyne compound 63 appears to be an intermediate en route to 64. When 63 is treated with an additional quantity of PhC2H at 68  C, compound 64 is obtained in 44% yield. The reaction of 62 with PhC2H at 40  C gives the compound PtRu5(CO)16(m6-C)(m3-PhC2H)[Pt(PBu3t)]2 (65) containing one PhC2H ligand in 56% yield.101 Compound 65 is characterized by a combination of IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses. The molecular structure of 65 is shown in Fig. 32. The structure of 65 consists of a central cluster of six metal atoms, five ruthenium and one platinum arranged in a pseudo-octahedral shape with a carbido ligand in the center. The PhC2H ligand is coordinated to the group of three platinum atoms in the usual di-s + p bonding mode. The hydrogen atom exhibits low field resonance in the 1H NMR spectrum, d ¼ 8.89 ppm and shows the expected coupling to the phosphorus atom on the neighboring platinum atom. The reaction of 62 with PhC2H at 97  C gives the compound PtRu5(CO)12(m6-C)(m3-PhC2H)2[Pt (PBu3t)]2 (66) in 25% yield that contains two PhC2Ph ligands. Compound 66 is characterized by a combination of IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses. The molecular structure of 66 is shown in Fig. 32. Compound 66 contains eight metal atoms: five of ruthenium and three of platinum. The structure of 66 consists of a central cluster of six metal atoms, five ruthenium and one platinum arranged in a pseudo-octahedral shape with a carbido ligand in the center. The two

(63)

(64) t

Fig. 31 The molecular structures of PtRu5(CO)15(m6-C)(m3-PhC2H)[Pt(PBu3 )] (63) and PtRu5(CO)13(m6-C)(m3-PhC2H)2[Pt(PBu3t)] (64) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Custer Sci. 2005, 16, 397–411.

Ruthenium and Osmium Carbonyl Cluster Complexes

(65)

583

(66) t

Fig. 32 The molecular structures of PtRu5(CO)16(m6-C)(m3-PhC2H)[Pt(PBu3 )]2 (65) and PtRu5(CO)12(m6-C)(m3-PhC2H)2[Pt(PBu3t)]2 (66) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Custer Sci. 2005, 16, 397–411.

remaining platinum atoms occupy face-capping positions on PtRu2 groups on opposite sides of the central cluster, and each one contains one PBu3t ligand. Each PtdPt bond contains one bridging carbonyl ligand and the PtdPt bond distances are slightly longer than those in 65. There are two PhC2Ph ligands and both are coordinated to Ru3 triangles on the central PtRu5 cluster in the usual di-s + p bonding mode. The total valence electron count for 66 is 110 electrons. The reaction of mixed-metal complex Ru4Pt2(CO)1893 with Pt(PBu3t)2 at room temperature affords the complex Pt2Ru4(CO)17[Pt (PBu3t)] (67) in 75% yield.102 The product is characterized by IR, 1H and 31P NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 67 is shown in Fig. 33. The structure of 67 is derived from that of Ru4Pt2(CO)18 by the addition of a Pt(PBu3t) group across the butterfly arrangement of the four metal atoms. There is a bridging carbonyl ligand across the RudPt bond. The bond distances of 67 are very similar to the parent compound Ru4Pt2(CO)18. This reaction involves the loss of one CO ligand from Ru4Pt2(CO)18. The reaction of the benzene coordinated pentaruthenium cluster Ru5(CO)12(Z6-C6H6)(m5-C) (68) with Pt(PBu3t) at room temperature gives two products Ru5(CO)12(Z6-C6H6)(m6-C)[Pt(PBu3t)] (69) in 62% yield and Ru5(CO)12(Z6-C6H6)(m6-C)[Pt (PBu3t)]2 (70) in 13% yield.103 Both products are characterized via IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses. The molecular structure of 69 is shown in Fig. 34. Because there is no loss of CO from the starting material, compound 69 can be viewed as a Pt(PBu3t) adduct of the parent complex 68. The compound consists of a square pyramidal cluster of five ruthenium atoms with a platinum atom located on the square base, but it is not symmetrically bonded to all four ruthenium atoms of the square base. The reason for the unsymmetrical bonding of the platinum atom on the Ru4 square base in 69 is attributed

(67) Pt2Ru4(CO)17[Pt(PBu3t)]

Fig. 33 The molecular structure of (67) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, J. L.; Smith, M. D.; Chen, G.; Wu, W. Organometallics 2004, 23, 589–594.

584

Ruthenium and Osmium Carbonyl Cluster Complexes

(69) 6

(70) 6

-C6H6)(m6-C)[Pt(PBu3t)]

Fig. 34 The molecular structures of Ru5(CO)12(Z (69) and Ru5(CO)12(Z -C6H6)(m6-C)[Pt(PBu3t)]2 (70) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Pellechia, P. J.; Zhu, L. Inorg. Chem. 2004, 43, 7243–7249.

to steric effects introduced by the benzene ligand that is coordinated to ruthenium and the bulky tri-tert-butylphosphine ligand that is coordinated to the platinum atom. The molecular structure of compound 70 is shown in Fig. 34. Like compound 69, the structure of this compound also consists of a Ru5Pt open octahedral-like arrangement of the metal atoms with a carbon atom in the center and a benzene ligand coordinated to Ru. In addition, there is a second Pt(PBu3t) group that bridges the RudRu bond. Each of the RudPt bonds contains one bridging carbonyl ligand. When a heptane solution of compound 69 is heated to reflux in the presence of a hydrogen atmosphere, the dihydrido complex Ru5(CO)11(Z6-C6H6)(m6-C)[Pt(PBu3t)](m-H)2 (71) is formed in 59% yield. Compound 71 is characterized by IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses, and its molecular structure is shown in Fig. 35. One carbonyl ligand is eliminated from 69, and two hydride ligands are added to the cluster to form 71. The structure of 71 is similar to that of the dihydrido complex Ru5(CO)14(m6-C)[Pt(PBu3t)](m-H)2 (52). Compound 71 consists of a Ru5Pt octahedron with a carbon atom in the center. The PBu3t ligand is coordinated to the platinum atom, and the benzene ligand is coordinated to a ruthenium atom. The two hydrido ligands bridge two oppositely positioned PtdRu bonds. Interestingly, the hydride bridged PtdRu bonds are significantly shorter than the unbridged PtdRu bonds. The two hydride ligands in 71 are equivalent and appear as a single resonance, d ¼ −14.93 ppm, in the 1H NMR spectrum that exhibits one-bond coupling to platinum and two-bond coupling to the

(71) 6

-C6H6)(m6-C)[Pt(PBu3t)](m-H)2

Fig. 35 The molecular structure of Ru5(CO)11(Z (71) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Pellechia, P. J.; Zhu, L. Inorg. Chem. 2004, 43, 7243-7249.

Ruthenium and Osmium Carbonyl Cluster Complexes

585

phosphorus atom. When compound 70 is allowed to react with hydrogen (1 atm) at 97  C, compound 71 is obtained in low yield along with a few other minor products that are not characterized. The reaction of triruthenium dodecacarbonyl, Ru3(CO)12, with 2,20 -bis(1,3-dimesitylimidazol-2-ylidene)platinum(0), Pt-(IMes)2, in benzene solvent at room temperature affords two bimetallic cluster complexes, the monoplatinum-triruthenium cluster complex Ru3Pt(IMes)2(CO)11 (72) in 21% yield and the diplatinum-triruthenium cluster complex Ru3Pt2(IMes)2(CO)12 (73) in 26% yield.104 Both compounds 72 and 73 are structurally characterized by a combination of IR, 1H NMR, mass spectrometry, and single-crystal X-ray diffraction analyses. The molecular structure of 72 is shown in Fig. 36. Compound 72 consists of a square plane with three ruthenium atoms and one platinum atom, and can be viewed as two triangles that share an edge formed by a RudRu single bond. The Pt(IMes) group is an edge bridging on the Ru3 triangle. There is also an IMes group that is coordinated to atom Ru3 opposite the Pt atom. There are two bridging carbonyl groups that bridge the ruthenium-platinum bonds. The IMes group on Ru3 lies perpendicularly to the Ru3 triangular plane. The structure of complex 73 in the solid state is given in Fig. 36. Compound 73 has a trigonal bipyramidal geometry of three ruthenium atoms and two platinum atoms. The Ru atoms occupy the equatorial plane, while the Pt atoms occupy the apical positions of the trigonal bipyramid. With no loss of CO ligands, compound 73 can be viewed as an adduct of Ru3(CO)12, where two Pt(IMes) groups cap the Ru3 triangle. The two carbonyl ligands coordinated to each of the Pt atoms are edge bridging and slightly semibridging in nature. Ru(CO)5 reacts with Pt(IMes)2 in benzene solvent at 0  C to afford the bimetallic trinuclear cluster complexes, Ru2Pt(IMes) (CO)9 (74) (15% yield), and RuPt2(IMes)2(CO)6 (75) (21% yield).104 Both compounds 74 and 75 are characterized crystallographically. As shown in Fig. 37, compound 74 contains a triangle of three metal atoms of which two are ruthenium atoms and one

(72)

(73)

Fig. 36 The molecular structures of Ru3Pt(IMes)2(CO)11 (72) and Ru3Pt2(IMes)2(CO)12 (73) [ruthenium (green), platinum (red), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Saha, S.; Captain, B. Inorg. Chem. 2014, 53, 1210−1216.

(74)

(75)

Fig. 37 The molecular structures of Ru2Pt(IMes)(CO)9 (74) and RuPt2(IMes)2(CO)6 (75) [ruthenium (green), platinum (red), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Saha, S.; Captain, B. Inorg. Chem. 2014, 53, 1210−1216.

586

Ruthenium and Osmium Carbonyl Cluster Complexes

is a platinum atom. There are three bridging carbonyl ligands that bridge each of the PtdRu bonds and a RudRu bond. With nine CO ligands, this compound can be viewed as a monoplatinum adduct of Ru2(CO)9. Compound 75 is another trinuclear cluster complex that is furnished in this reaction but contains two platinum atoms and one ruthenium atom. Its structure in the solid state (see Fig. 37) consists of a RuPt2 triangle with the IMes groups located on the platinum atoms. Interestingly, the ruthenium atom just as in 74 has five carbonyl ligands, two of which bridge to the neighboring Pt atoms and the other three carbonyl ligands are terminally coordinated. The sixth carbonyl ligand bridges the two platinum atoms, and the PtdPt bond distance is shorter than the RudPt bond distances. The reaction of compound 73 with H2 affords the tetrahydrido-tetraruthenium complex Ru4(IMes)(CO)11(m-H)4 (76) (12% yield), and the dihydride-diruthenium-diplatinum complex Ru2Pt2(IMes)2(CO)8(m-H)2 (77) (36% yield), at 80  C. Both compounds 76 and 77 are structurally characterized by single-crystal X-ray diffraction analyses. Compound 76 consists of a Ru4 tetrahedron with an IMes ligand on ruthenium. There are four hydride ligands that bridge four of the ruthenium bonds. Compound 77 is obtained as a major product from this reaction. As can be seen in Fig. 38, the structure of this compound has a butterfly geometry, containing two ruthenium and two platinum atoms. Both of the platinum atoms contain IMes groups, which are present at the “wing-tips” of the butterfly. This dihydride-diruthenium-diplatinum compound contains two ruthenium atoms joined by a RudRu single bond. Each ruthenium atom is bonded with two Pt(IMes) groups and contains three terminally coordinated carbonyl ligands. The platinum atoms both have one carbonyl ligand, which is terminally coordinated. There are no bridging carbonyl ligands present in this compound. Appropriately, the complex contains two hydride ligands, which bridge two of the RudPt bonds. The presence of two hydride ligands is not located crystallographically, but they appear as one high-field resonance, in the 1H NMR spectrum of the compound. These two hydride ligands are equivalent and appear at −9.89 ppm, in the 1H NMR spectrum of the compound, showing one and two bond coupling to platinum. Complex 77 may be interpreted as a butterfly rather than a tetrahedron, with two 16-electron Pt atoms, with a total count of 58 electrons and no PtdPt bond. Ru(CO)5 reacts with Pt(PBu3t)2 to yield the monoplatinum-diruthenium complex PtRu2(CO)9(PBu3t) (78) in 20% yield, but in addition has also yielded the diplatinum-diruthenium complex Ru2(CO)9[Pt(PBu3t)]2 (79) in 33% yield.105 Both compounds are characterized by a combination of IR, 1H and 31P NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 78 is shown in Fig. 39. Compound 78 contains a triangle of three metal atoms, two ruthenium and one platinum, with a carbonyl group bridging the RudPt bond. The platinum atom not only is coordinated by the PBu3t group, but also has one terminal carbonyl ligand. Both ruthenium atoms are coordinated by four CO ligands, and these CO ligands prefer terminal coordination. Compound 78 can be viewed as a combination of Ru2(CO)9 and Pt(PBu3t) groupings, but in this case one of the CO ligands has been transferred completely to the Pt(PBu3t) group. Compound 79 is characterized crystallographically, and its molecular structure is shown in Fig. 39. In compound 79, only three of the CO ligands on ruthenium atom bridge to the neighboring platinum atoms and one of the CO ligands on the Ru(CO)4 group forms a bridge to a neighboring platinum atom. This compound can be viewed as a diplatinum adduct of Ru2(CO)9. Compound 79 reacts with hydrogen at 1 atm in hexane at 68  C to afford the tetranuclear metal complex Pt2Ru2(CO)8(PBu3t )2(m-H)2 (80) in 64% yield. Compound 80 is characterized by IR, NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 80 is shown in Fig. 40. The molecule contains two ruthenium atoms and two platinum atoms in a pseudotetrahedral arrangement. There are two hydride ligands that bridge two of the RudPt bonds. The hydride ligands are equivalent and exhibit only one resonance in the 1H NMR spectrum, d ¼ −8.76 ppm. Compound 80 has eight carbonyl ligands: three terminal CO ligands on each ruthenium atom and one on each of the platinum atoms. The valence electron count for 80 is 58, which is two less than the expected 60-electron count for closed tetrahedral clusters; however, it has been found that tetrahedral clusters that contain platinum often contain 58 electrons.93

(77) Fig. 38 The molecular structure of Ru2Pt2(IMes)2(CO)8(m-H)2 (77) [ruthenium (green), platinum (red), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Saha, S.; Captain, B. Inorg. Chem. 2014, 53, 1210−1216.

Ruthenium and Osmium Carbonyl Cluster Complexes

(78)

587

(79)

Fig. 39 The molecular structures of PtRu2(CO)9(PBu3t) (78) and Ru2(CO)9[Pt(PBu3t)]2 (79) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Smith, M. D.; Webster, C. E. Inorg. Chem. 2004, 43, 3921−3929.

(80) Pt2Ru2(CO)8(PBu3t)2(m-H)2

Fig. 40 The molecular structures of (80) [ruthenium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Smith, M. D.; Webster, C. E. Inorg. Chem. 2004, 43, 3921−3929.

The reaction of platinum-alkenyl [PtL2(1-alkenyl)2] precursors (where L2 ¼ dppe, 1,2-bis(diphenylphosphino)ethane or dppp, 1,3-bis(diphenylphosphino)propane; L ¼ PPh3) with [Ru3(CO)12] affords platinum containing mixed bimetallic carbonyl clusters of the type [PtRu2(CO)8L2] in high yield.106 The compounds [Pt(dppp){(CH2)3CH ¼ CH2}2] and [Pt(dppe){(CH2)3CH]CH2}2] react with Ru3(CO)12 at 80  C to give the bimetallic compound [PtRu2(CO)8(dppp)] (81) and [PtRu2(CO)8(dppe)] (82) respectively. When [Pt(PPh3)2 {(CH2)3CH]CH2}2] reacts with Ru3(CO)12 at 50  C, another bimetallic complex [PtRu2(CO)8(PPh3)2] (83) is formed. Compound 83 undergoes a irreversible transformation reaction in air or under UV light at room temperature to give the compound [Pt2Ru(CO)2(m-CO)3(PPh3)3] (84). During this reaction, the {PtRu2} framework of 83 is completely changed into a cluster with a {Pt2Ru} core (84) and Ru3(CO)11(PPh3). It is found that this transformation reaction is selective to the PPh3 derivatives of the {PtRu2} clusters and is not observed with the dppp and dppe analogues 81 and 82. Compound 84 is characterized by a combination of IR, 1H and 31P NMR spectroscopy, and single crystal X-ray diffraction analysis. The structure of 84 consists of a triangular array of two Pt atoms and one Ru atom. There are two terminal carbonyl ligands bonded to each Ru atom and three carbonyls are bridging along the three metal-metal edges.

7.10.2.3 7.10.2.3.1

Bimetallic ruthenium-group 14 metal carbonyl cluster complexes Bimetallic ruthenium-tin carbonyl cluster complexes

The reaction of Ru5(CO)15(m5-C) with Ph3SnH at room temperature in the presence of UV irradiation yields the compound Ru5(CO)15(SnPh3)(m5-C)(m-H) (85) in 22% yield.107,108 Compound 85 is characterized by a combination of IR, NMR, and single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 41. This compound contains an open Ru5(m5-C) cluster where one ruthenium atom bridges the wingtips of the Ru4C butterfly cluster arrangement. The triphenyltin group is coordinated to the bridging ruthenium atom. The hydride ligand bridges the hinge bond of the Ru4 butterfly, and it exhibits the usual high-field resonance, d ¼ −22.31 ppm in the 1H NMR spectrum of the compound. Compound 85 is formed by an oxidative addition of the tin-hydrogen bond to Ru5(CO)15(m5-C) with a cleavage of one of the apical-equatorial RudRu bonds of the square pyramidal cluster. Interestingly, the thermal reaction of Ru5(CO)15(m5-C) with an excess of Ph3SnH at 127  C does not yield 85 but leads to the formation of the high nuclearity compound Ru5(CO)10(SnPh3)(m-SnPh2)4(m5-C)(m-H) (86) in a 6% yield.

588

Ruthenium and Osmium Carbonyl Cluster Complexes

(85)

(86)

Fig. 41 The molecular structures of Ru5(CO)15(SnPh3)(m5-C)(m-H) (85) and Ru5(CO)10(SnPh3)(m-SnPh2)4(m5-C)(m-H) (86) [ruthenium (green), tin (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from (a) Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. Inorg. Chem. 2002, 41, 5593−5601. (b) Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. Inorg. Chem. 2002, 41, 2302–2303.

Compound 86 is characterized by a combination of IR, NMR, and single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 41. Compound 86 contains a square pyramidal cluster of five ruthenium atoms with a carbon atom located in the center of the base of the square pyramid. Surprisingly, compound 86 contains five tin ligands. Four of these are in the form of SnPh2 groups that bridge each of the four RudRu edges of the square base of the cluster. The fifth tin-containing ligand is a SnPh3 group that is terminally bonded to one of the basal Ru atoms. The one bridging hydride ligand is not located crystallographically, but it is clearly indicated by its high-field resonance in the 1H NMR spectrum, at d ¼ −23.31 ppm. The reaction of Ph3SnH with the benzene-substituted pentaruthenium carbido cluster Ru5(CO)12(C6H6)(m5-C) (68) at 68  C yields two products Ru5(CO)11(SnPh3)(C6H6)(m5-C)(m-H) (87) (26% yield) and Ru5(CO)10(SnPh3)2(C6H6)(m5-C)(m-H)2 (88) (8% yield).107,108 Compounds 87 and 88 are both characterized by a combination of IR, NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 87 is shown in Fig. 42. Compound 87 is formed by the oxidative addition of one equivalent of Ph3SnH to 68, and a loss of one CO ligand. The structure of 87 consists of a square pyramidal Ru5C cluster with one SnPh3 ligand bonded terminally to the basal ruthenium atom of the square pyramid. The benzene ligand is coordinated to another basal ruthenium atom. The compound contains one hydride ligand that bridges across an apical-basal ruthenium bond, d ¼ −21.75 ppm. Compound 88 is formed by the oxidative addition of two equivalent of Ph3SnH to 68, and loss of two CO ligands. The molecular structure of 88 is shown in Fig. 42. Like 87, compound 88 also contains of a square pyramidal Ru5C cluster, but it has two

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Fig. 42 The molecular structures of Ru5(CO)11(SnPh3)(C6H6)(m5-C)(m-H) (87) and Ru5(CO)10(SnPh3)2(C6H6)(m5-C)(m-H)2 (88) [ruthenium (green), tin (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from (a) Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. Inorg. Chem. 2002, 41, 5593−5601. (b) Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. Inorg. Chem. 2002, 41, 2302–2303.

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SnPh3 ligands, one bonded terminally to the basal ruthenium atom, and one bonded terminally to the apical ruthenium atom. The benzene ligand in 88 is coordinated to the basal ruthenium atom. There are two hydride ligands that bridge different apical-basal ruthenium bonds. The two hydride ligands are inequivalent. This is confirmed by the observation of two mutually coupled high-field resonances in the 1H NMR spectrum, d ¼ −19.68 and −20.70 ppm. The reaction of 80 with an excess of Ph3SnH at 127  C leads to the formation of two high-nuclearity cluster complexes: Ru5(CO)8(C6H6)(m-SnPh2)4(m5-C) (89) in 2% yield and Ru5(CO)7(SnPh3)(C6H6)(m-SnPh2)4(m5-C)(m-H) (90) in 26% yield.107,108 Compounds 89 and 90 are both characterized by a combination of IR, NMR and single-crystal X-ray diffraction analyses. Compound 89 consists of a square pyramidal cluster of five ruthenium atoms with four bridging SnPh2 groups, one on each edge of the base of the square pyramid. The benzene ligand has been relocated from a basal coordination site to the apical ruthenium atom. This relocation process is not unusual and has also been observed to occur in the parent compound 68.109 As in compound 86, compound 90 has also incorporated five tin ligands into the square pyramidal Ru5 cluster. Four of these tin ligands are bridging SnPh2 groups on each edge of the square base. As in 86, the fifth tin grouping is a SnPh3 ligand that is terminally coordinated to the basal ruthenium atom. Compound 90 contains one bridging hydride ligand at d ¼ −25.63 ppm in the 1H NMR spectrum, that is believed to bridge the long RudRu bond, proximate to the SnPh3 group. From the reactions affording compounds 85, 87, and 88, it is shown that triphenylstannane can oxidatively add to pentaruthenium carbido carbonyl clusters by reaction of its SndH bond to yield stannylpentaruthenium hydride cluster complexes. This would be the first step in the formation of compounds 86, 89, and 90. Indeed, compounds 86 and 90 contain both SnPh3 and hydride ligands. The formation of the SnPh2 groups then occurs by cleavage of a Ph group from an intermediate containing a SnPh3 group. The phenyl group is then combined with the hydride ligand and eliminated as C6H6. Cleavage of phenyl groups from PPh3 ligands in metal clusters is a well-established transformation.110,111 Triruthenium compounds containing multiple SnR2 bridging groups have been obtained by the reaction of Ru3(CO)12 with SnR2 precursors;112,113 however, the formation of the SnPh2 groups by this route is new, and introduction of four such groups is unique. Interestingly, when treated with CO under 45 atm pressure, compound 90 is converted to 86 by replacement of the benzene ligand with three CO ligands. When the compound 87 is heated at 68  C for 45 min, the compound Ru5(CO)11(C6H6)(m4-SnPh)(m3-CPh) (91) is formed in 68% yield.107,108 Compound 91 is characterized crystallographically, and its molecular structure is shown in Fig. 43. Compound 91 contains the usual square pyramidal cluster of five ruthenium atoms but also has a novel quadruply bridging stannylyne group (SnPh) capping the base of this square pyramid. The original benzene ligand is coordinated to ruthenium, in the base of the Ru5 square pyramid. Surprisingly, compound 91 does not contain an interstitial carbido atom but instead contains a benzylidyne ligand (CPh) that bridges the three ruthenium atoms. It is believed that the benzylidene ligand is formed by transfer of a phenyl group from the tin atom to the carbido carbon atom, and the new group then moved out from the interior of the cluster to its surface. In the formation of 91, two phenyl groups are cleaved from the tin atom. One of the phenyl groups is eliminated from the compound as benzene by combination with the hydride ligand in 87. Four compounds [Ru4(m4-SnPh)2(CO)12] (92), [Ru4(m4-SnPh)2(m-SnPh2)2(m-CO)2(CO)8] (93), [Ru4(m4-SnPh)2(m-SnPh2)3(m-CO) (CO)8] (94), and [Ru4(m4-SnPh)2(m-SnPh2)4(CO)8] (95) are obtained from the reaction of [Ru4(CO)12(m-H)4] with Ph3SnH in octane solvent at 125  C.114 These compounds are characterized by a combination of IR and 1H NMR spectroscopy, single-crystal X-ray diffraction, and mass spectrometry. The molecular structures of 92, 93, 94, and 95 are shown in Fig. 44. Each compound contains an approximately square-planar cluster of four ruthenium atoms with two quadruply bridging SnPh ligands, one on each side of the Ru4 square. Compound 92 contains 12 carbonyl ligands like its parent compound, whereas in compounds 93–95, two, three, and four

(91) Fig. 43 The molecular structures of Ru5(CO)11(C6H6)(m4-SnPh)(m3-CPh) (91) [ruthenium (green), tin (purple), carbon (gray), oxygen (orange)]. Modified from (a) Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. Inorg. Chem. 2002, 41, 5593−5601. (b) Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. Inorg. Chem. 2002, 41, 2302–2303.

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Fig. 44 The molecular structures of [Ru4(m4-SnPh)2(CO)12] (92), [Ru4(m4-SnPh)2(m-SnPh2)2(m-CO)2(CO)8] (93), [Ru4(m4-SnPh)2(m-SnPh2)3(m-CO)(CO)8] (94) and [Ru4(m4-SnPh)2(m-SnPh2)4(CO)8] (95) [ruthenium (green), tin (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Boswell, E. M.; Captain, B.; Hungria, A. B.; Midgley, P. A.; Raja, R.; Thomas, J. M. Angew. Chem. Int. Ed. 2007, 46, 8182–8185.

of the CO ligands are replaced by SnPh2 groups that bridge the RudRu edges of the Ru4 square. The m4-SnPh stannylyne ligands present in these clusters are very rare. In fact, there is only one reported example of a m4-SnPh ligand that is observed for the compound Ru5(CO)11(C6H6)(m4-SnPh)(m3-CPh) (91). Two compounds Ru3(CO)9(SnPh3)2(NCMe)(m-H)2 (96) (19% yield) and Ru3(CO)10(SnPh3)2(m-H)2 (97) are obtained from the reaction of Ru3(CO)10(NCMe)2 with HSnPh3 in hexane solvent at room temperature.115 Both compounds are characterized by a combination of IR, 1H NMR, mass spectra and by a single crystal X-ray diffraction analyses. The molecular structure of 96 is shown in Fig. 45. Compound 96 contains a triangular cluster of three ruthenium atoms. There are two SnPh3 ligands and the tin atoms lie essentially in the plane of the Ru3 triangle. There are two bridging hydrido ligands, nine terminal CO ligands and one MeCN ligand. The hydrido ligands bridge the RudRu bonds cis to the SnPh3 ligands. The hydrido ligands are inequivalent and the 1H NMR spectrum of 96 exhibits two high-field resonances at d ¼ −13.89 ppm with appropriate HdH coupling. The one MeCN ligand occupies an axial position on Ru. The one MeCN ligand shows its methyl resonance at d ¼ 1.42 ppm in the 1H NMR spectrum. Compound 97 is structurally similar to 96 except that the MeCN ligand has been replaced by a terminally coordinated CO ligand (see Fig. 45). The RudRu and RudSn bond distances in 97 are similar to those in 96. The two bridging hydrido ligands are inequivalent as in 96, and two high-field resonances are observed for these ligands in the 1H NMR spectrum, d ¼ −15.03 and −16.68 ppm. Compound 96 is also obtained in 19% yield from the reaction of Ru3(CO)10(NCMe)2 with HSnPh3 in a CH2Cl2/MeCN solvent mixture at room temperature. However, a compound Ru3(CO)7(SnPh3)3(NCMe)2(m-H)3 (98) is also obtained in a low yield (3%).115 Compound 98 is characterized by a combination of IR, 1H NMR, elemental and a single crystal X-ray diffraction analyses,

Ruthenium and Osmium Carbonyl Cluster Complexes

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Fig. 45 The molecular structures of Ru3(CO)9(SnPh3)2(NCMe)(m-H)2 (96) and Ru3(CO)10(SnPh3)2(m-H)2 (97) [ruthenium (green), tin (purple), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Trufan, E. J. Organomet. Chem. 2008, 693, 3593–3602.

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Fig. 46 The molecular structures of Ru3(CO)7(SnPh3)3(NCMe)2(m-H)3 (98) and Ru3(CO)9(SnPh3)3(m-H)3 (99) [ruthenium (green), tin (purple), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Trufan, E. J. Organomet. Chem. 2008, 693, 3593–3602.

and its molecular structure is shown in Fig. 46. The molecule contains a triangular cluster of three ruthenium atoms held together by three RudRu bonds. Each RudRu bond also contains a bridging hydrido ligand. Bridging hydrido ligands are known to produce lengthening of the associated metal-metal bonds.116,117 The hydrido ligands lie in the plane of the Ru3 triangle. All three hydrido ligands are inequivalent and high-field resonances with appropriate SndH couplings are observed for them in the 1H NMR spectrum at d ¼ −11.04, −12.55 and −13.02 ppm. Each ruthenium atom contains one SnPh3 ligand. All three SnPh3 ligands lie in the plane of the Ru3 triangle. Compound 98 contains seven linear terminal carbonyl ligands and two MeCN ligands. The MeCN ligands are inequivalent and two separate resonances are observed for their methyl groups in the 1H NMR spectrum: d ¼ 1.47 and 1.43 ppm. Compounds 96 and 98 both react with CO by substitution of their MeCN ligands to yield the fully carbonylated compounds 97 (20% yield) and Ru3(CO)9(SnPh3)3(m-H)3 (99), but the reaction of 98 with CO is slow and the yield of 99 is very low (4%).115 It is found that compound 99 can be prepared in a much better yield (44%) directly from the reaction of Ru3(CO)12 with HSnPh3 under a hydrogen atmosphere in heptane solvent at reflux for 90 min. Compound 99 is characterized by single crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 46. The cluster consists of a Ru3 triangle with three hydride-bridged RudRu bonds. The hydrido are all equivalent and appear as a single resonance in the 1H NMR spectrum, d ¼ −14.60 ppm.

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Ruthenium and Osmium Carbonyl Cluster Complexes

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Fig. 47 The molecular structures of Ru3(CO)10(m-SnPh2)2 (100) and Ru3(CO)9(m-SnPh2)3 (101) [ruthenium (green), tin (purple), carbon (gray), oxygen (orange)]. Modified from (a) Adams, R. D.; Captain, B.; Trufan, E. J. Organomet. Chem. 2008, 693, 3593–3602 and (b) Adams, R. D.; Captain, B.; Hall, M. B.; Trufan, E.; Yang, X. J. Am. Chem. Soc. 2007, 129, 12328–12340.

When compound 97 is heated to reflux in benzene solution for 30 min, it is converted to the compound Ru3(CO)10 (m-SnPh2)2 (100) in 61% yield.115 The molecular structure of 100 is shown in Fig. 47. Compound 100 consists of a triangular cluster of three ruthenium atoms with two SnPh2 ligands bridging adjacent RudRu bonds. The third RudRu bond contains a bridging CO ligand. Each ruthenium atom also contains three linear terminal carbonyl ligands, two occupy axial sites perpendicular to the Ru3 triangle and one lies in the plane of the Ru3 triangle. Compound 100 is formed by the loss of two phenyl groups, one from each SnPh3 ligand, and the two hydrido ligands. These ligands combined to form benzene which is confirmed by 1H NMR spectroscopy. Compound Ru3(CO)9(m-SnPh2)3 (101) is obtained in 20% yield from the reaction of Ru3(CO)12 with Ph3SnH in octane solution at 125  C under hydrogen atmosphere.118 Compound 101 is characterized by IR, 1H and 119Sn NMR, mass spectrometry, and single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 47. The compound consists of a triangular cluster of three ruthenium atoms with three bridging diphenylstannylene ligands (SnPh2), one on each of the three RudRu bonds of the cluster. The tin atoms of the SnPh2 ligands lie in the plane of the Ru3 triangle. Each ruthenium atom contains three linear terminal carbonyl ligands, two lie perpendicular to the plane of the cluster, while one lies in the plane of the cluster. The reaction of Ph3SnH with the hexaruthenium cluster Ru6(CO)14(6-C6H6)(m6-C) at room temperature gives the SnPh2 derivative Ru6(CO)13(m-SnPh2)(6-C6H6)(m6-C) (102) in 10% yield.119 Compound 102 is characterized by a combination of IR, NMR, and single crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 48. Compound 102 consists of a Ru6 octahedron with a carbon atom in the center. There is an 6-benzene ligand coordinated to one of the ruthenium atoms, and an SnPh2 group bridging the RudRu bond. Here, the bridging SnPh2 group has replaced the bridging CO ligand in Ru6(CO)14(6C6H6)(m6-C).

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Fig. 48 The molecular structures of Ru6(CO)13(m-SnPh2)(Z -C6H6)(m6-C) (102) [ruthenium (green), tin (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W. J. Organomet. Chem. 2003, 671, 158–165.

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The reaction of [Ru3(CO)12] with the cyclic stannylene 1,3-bis(neo-pentyl)-2-stannabenzimidazol-2-ylidene, using Sn/ Ru3 ratios  3, in toluene at 110  C, leads to the trisubstitued derivative [Ru3{m-Sn(NCH2CMe3)2C6H4}3(CO)9] (103).120 Compound 103 itself is very air-sensitive and decomposes quickly when it is dissolved in wet solvents. In the case of the bulky stannylene Sn(HMDS)2 [HMDS ¼ N(SiMe3)2], the reaction of 2.5 equivalent of Sn(HMDS)2 with [Ru3(CO)12] in toluene at 80  C affords disubstituted cluster derivative [Ru3{m-Sn(HMDS)2}2(m-CO)(CO)9] (104). But the reaction of 3.5 equivalent of Sn(HMDS)2 with [Ru3(CO)12] in toluene at 110  C affords trisubstituted cluster derivative [Ru3{m-Sn(HMDS)2}3(CO)9] (105). As expected, 104 leads to 105 when it is heated with Sn(HMDS)2 in refluxing toluene. The molecular structures of 104 and 105 has been determined by X-ray diffraction crystallography and are shown in Fig. 49. The cluster 104 comprises an isosceles triangle of ruthenium atoms with three terminal carbonyl ligands attached to each Ru atom, one bridging carbonyl symmetrically spanning an RudRu edge, and two Sn(HMDS)2 ligands that symmetrically bridge the remaining RudRu edges of the cluster. The Sn and Ru atoms are essentially coplanar and the SnN2 plane of each stannylene ligand is roughly perpendicular to the Ru3Sn2 plane. The compound 105 comprises a regular triangle of ruthenium atoms with an Sn(HMDS)2 ligand spanning each RudRu edge. The tin atoms are in the same plane as the Ru3 triangle and have a distorted tetrahedral environment, the SnN2 planes being perpendicular to the Ru3 triangle. Both Sn(HMDS)2 derivatives, 104 and 105, are more stable toward hydrolysis than compound 103. This greater kinetic stability should be due to the rigidity and larger volume of the HMDS SiMe3 groups, which are more efficient at protecting the RudSn and SndN bonds from external attacks than the more flexible neo-pentyl groups of compounds 103. The reaction of Ru3(CO)12 with Ph3SnSPh in refluxing benzene produces the bimetallic cluster [Ru3(CO)8(m-SPh)2(m3-SnPh2) (SnPh3)2] (106) in 18% yield.121 The molecular structure of 106 is shown in Fig. 50. The molecule consists of three ruthenium

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Fig. 49 The molecular structures of [Ru3{m-Sn(HMDS)2}2(m-CO)(CO)9] (104) and [Ru3{m-Sn(HMDS)2}3(CO)9] (105) [ruthenium (green), tin (purple), nitrogen (light blue), silicon (yellow), carbon (gray), oxygen (orange)]. Modified from Cabeza, J. A.; García-Alvarez, P.; Polo, D. Inorg. Chem. 2012, 51, 2569− 2576.

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Fig. 50 The molecular structures of [Ru3(CO)8(m-SPh)2(m3-SnPh2)(SnPh3)2] (106) and [Ru3(CO)6(m-dppm)(m3-S)(m3-SPh)(SnPh3)] (107) [ruthenium (green), tin (purple), phosphorus (light blue), sulfur (yellow), carbon (gray), oxygen (orange)]. Modified from Kabir, S. E.; Raha, A. K.; Hassan, M. R.; Nicholson, B. K.; Rosenberg, E.; Sharmin, A.; Salassa, L. Dalton Trans., 2008, 4212–4219.

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atoms, each bonded to the SnPh2 group to give a planar tetra-metallic core. Each of the RudRu edges is bridged by a m-SPh group, positioned to give shorter distances to the outer ruthenium atoms. There are terminal Ph3Sn groups attached to the outer ruthenium atoms, with remaining coordination sites occupied by CO ligands. Despite the very different chemical environments for the Ph3Sn and Ph2Sn groups, the 119Sn NMR spectrum of 106 shows only a small difference in chemical shift, with signals at d ¼ −25.3 and −29.9 ppm respectively relative to SnMe4. The reaction of [Ru3(CO)10(m-dppm)] (dppm ¼ bis(diphenylphosphino)methane) with Ph3SnSPh in refluxing benzene affords [Ru3(CO)6(m-dppm)(m3-S)(m3-SPh)(SnPh3)] (107) in 54% yield. Compound 107 is characterized by single crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 50. In this compound, the Ph3Sn group remains intact and is coordinated to one ruthenium atom. The three ruthenium atoms form an open triangle bridged on one side by a PhS− ligand, and on the other by an S2− one. An interesting feature is the sulfido and the SPh ligands, formed by cleavage of CdS and SndS bonds of the ligand, respectively, which asymmetrically cap the Ru3 system.

7.10.2.3.2

Bimetallic ruthenium-germanium carbonyl cluster complexes

The reaction of Ru5(CO)15(m5-C) with Ph3GeH in a 1:3 ratio at 150  C yields two pentaruthenium cluster complexes: Ru5(CO)11(m-CO)(m-GePh2)3(m5-C) (108) in 35% yield; Ru5(CO)11(m-GePh2)4(m5-C) (109) in 24% yield.122 Compounds 108 and 109 are characterized by a combination of IR, NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 108 is shown in Fig. 51. Compound 108 consists of a square pyramidal cluster of five ruthenium atoms with three GePh2 groups bridging three of the four edges of the base of the square pyramid. The fourth edge contains a bridging carbonyl ligand. Compound 109 is obtained in 24% yield under the above reaction conditions; however, when Ru5(CO)15(m5-C) is allowed to react with Ph3GeH in a 1:5 ratio at 150  C, the yield of 109 is increased to 80% and no 108 is obtained. It appears that 108 is a precursor to 109, and this is independently confirmed by the formation of 109 in 80% yield from 108 in the reaction with Ph3GeH at 150  C. The molecular structure of 109 is shown in Fig. 51. Compound 109 consists of a square pyramidal cluster of five ruthenium atoms with four bridging GePh2 groups, one on each edge of the base of the square pyramid. The mechanism for the formation of compounds 108 and 109 is believed to be oxidative addition of the GedH bond to the metal atoms of the cluster, followed by formation of GePh2 groups by cleavage of a Ph group from intermediate GePh3 ligands that then combines with a hydride ligand to eliminate as benzene. However, because of the limited number of hydrogen atoms (i.e. there is only one hydrogen/Ph3GeH that can be used for benzene formation) only one benzene molecule is formed/germanium group. Thus, compounds 108 and 109 contain only GePh2 ligands. An additional source of hydrogen is needed to form benzene by cleavage of additional phenyl groups. Reaction of 109 with H2 (1 atm) at 150  C for 30 min produces the compound Ru5(CO)10(m-GePh2)4(m5-C)(m-H)2 (110) in 31% yield.122 Compound 110 is characterized by a combination of IR and 1H NMR spectroscopy and single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 52. Like 109, compound 110 contains of a square pyramidal cluster of five ruthenium atoms with four bridging GePh2 groups, one on each edge of the base of the square pyramid. The compound contains two hydride ligands that bridge two oppositely positioned apical-basal edges of the Ru5 square pyramid. These two hydride ligands are equivalent and appear as one high-field resonance, d ¼ −23.11 ppm, in the 1H NMR spectrum of the compound. One CO ligand is eliminated from 109, and two hydride ligands are added to the cluster to form 110. The reaction of 110 with H2 (1 atm) at 150  C for 90 min provides the compound Ru5(CO)10(m-GePh2)2(m3-GePh)2(m3-H) (m4-CH) (111) in 34% yield.122 Compound 111 is characterized by a combination of IR, 1H NMR, and single-crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 52. Compound 111 consists of a square pyramidal cluster of five ruthenium atoms with two bridging GePh2 groups occupying opposite two edges of the base of the square pyramid. The RudGe bond distances to the bridging GePh2 groups are slightly shorter than those found in 108–110. Interestingly, compound 111

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Fig. 51 The molecular structures of Ru5(CO)11(m-CO)(m-GePh2)3(m5-C) (108) and Ru5(CO)11(m-GePh2)4(m5-C) (109) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W. Inorg. Chem. 2003, 42, 1328−1333.

Ruthenium and Osmium Carbonyl Cluster Complexes

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Fig. 52 The molecular structures of Ru5(CO)10(m-GePh2)4(m5-C)(m-H)2 (110) and Ru5(CO)10(m-GePh2)2(m3-GePh)2(m3-H)(m4-CH) (111) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Fu, W. Inorg. Chem. 2003, 42, 1328− 1333.

contains two triply bridging GePh groups that occupy the Ru3 triangles on opposite sides of the square pyramid. There is also one triply bridging hydride ligand and a quadruply bridging methylidyne ligand (m4-CH) across the base of the Ru5 square pyramid. Four triphenylgermylruthenium carbonyl compounds HRu(CO)4GePh3 (112), Ru(CO)4(GePh3)2 (113), Ru2(CO)8(GePh3)2 (114) and Ru3(CO)9(GePh3)3(m-H)3 (115) are obtained from the reaction of Ru(CO)5 with an excess of Ph3GeH in hexane solvent at 68  C for 30 min.123 All four compounds are characterized by a combination of IR, 1H NMR, single- crystal X-ray diffraction analyses. The molecular structure of 112 is shown in Fig. 53. The molecule contains only one ruthenium atom with four carbonyl ligands, a GePh3 ligand and one hydrido ligand. The hydride resonance is observed at high field in the 1H NMR spectrum, d ¼ −7.32 ppm. The molecular structure of 113 is shown in Fig. 53. Compound 113 contains only one ruthenium atom with two GePh3 ligands and four CO ligands. The RudGe bond distance is longer than that in 112. The molecular structure of 114 is shown in Fig. 54. Compound 114 contains two ruthenium atoms and each ruthenium atom contains one GePh3 ligand that is positioned trans to the RudRu bond. The carbonyl ligands on the ruthenium atoms have a staggered conformation to minimize steric interactions. The molecular structure of 115 is shown in Fig. 54. The molecule consists of a triangular cluster of three ruthenium atoms and each ruthenium atom contains one GePh3 ligand and three linear terminal carbonyl ligands. Each RudRu bond contains one bridging hydrido ligand. The longer RudRu bond lengths found in 115 are probably due to the presence of the bridging hydride ligands. When solutions of compound 112 are heated at 125  C in octane solvent, the compound Ru3(CO)10(m-GePh2)2 (116) is obtained in 42% yield.123 Compound 116 is also characterized crystallographically, and its molecular structure is shown in Fig. 55. The molecule consists of a triangular cluster of three ruthenium atoms joined by RudRu single bonds. Two of the RudRu bonds are bridged by a GePh2 ligand; the third RudRu bond contains a bridging CO ligand. When Ru3(CO)12 is allowed to react with Ph3GeH in heptane solvent at 97  C, the compound Ru3(CO)9(m-GePh2)3 (117) (27%) is obtained. The molecular structure of 117 is shown in Fig. 55. It has three GePh2 groups, one bridging each of the RudRu bonds. The structure is similar to 116, but it has a bridging GePh2 group in place of the bridging CO ligand.

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Fig. 53 The molecular structures of HRu(CO)4GePh3 (112) and Ru(CO)4(GePh3)2 (113) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Trufan, E. J. Custer Sci. 2007, 18, 642–659.

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Fig. 54 The molecular structures of Ru2(CO)8(GePh3)2 (114) and Ru3(CO)9(GePh3)3(m-H)3 (115) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Trufan, E. J. Custer Sci. 2007, 18, 642–659.

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(117)

Fig. 55 The molecular structures of Ru3(CO)10(m-GePh2)2 (116) and Ru3(CO)9(m-GePh2)3 (117) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Trufan, E. J. Custer Sci. 2007, 18, 642–659.

When compound 114 is allowed to react with an additional quantity of Ph3GeH in hexane solution at 68  C, the compound Ru2(CO)6(m -GePh2)2(GePh3)2 (118) is obtained in 15% yield.123 The molecular structure of 118 is shown in Fig. 56. Compound 118 contains two ruthenium atoms joined by a RudRu single bond. There are two GePh2 ligands bridging the RudRu bond. Each Ru atom contains one terminal GePh3 ligand and three linear terminal carbonyl ligands. The RudGe distances to the bridging GePh2 ligand are significantly different. The GePh3 ligand appears to exhibit a strong structural trans effect, that is, the RudGe that lies trans to the GePh3 ligand, is much longer than the RudGe distance that lies trans to the CO ligand. Six bimetallic ruthenium-germanium carbonyl cluster complexes Ru3(CO)9(m3-GeBut)2 (119), Ru2(CO)6(m-GeButH)3 (120), Ru4(CO)10(m4-GeBut)2(m-GeButH)2 (121), Ru4(CO)8(m4-GeBut)2(m-GeButH)2(m3-GeBut)(H) (122), Ru5(CO)12(m3-GeBut)2 (m4-GeBut)(H) (123), Ru6(CO)12(m3-GeBut)4(H)2 (124) are obtained from the reaction of Ru3(CO)12 with tertiary butyl germane, ButGeH3, in heptane solvent at reflux condition for 45 min.124 All six compounds are structurally characterized by a combination of IR, 1H NMR, mass spectrometry and single-crystal X-ray diffraction analyses. Compound 119 shows the addition of GeBut groups to the intact starting Ru3 cluster. The molecular structure of 119 is shown in Fig. 57. The compound contains a Ru3 triangle that is capped on either side by m3-GeBut ligands to afford a trigonal bipyramidal Ru3Ge2 cluster. With three carbonyl ligands on each ruthenium atom, the complex is electron precise having 48 cluster valence electrons. Elucidation of the structure of compound 120 in the solid state as seen in Fig. 57 provides evidence for cluster fragmentation in the reaction system. Compound 120 is a dinuclear ruthenium complex where the RudRu vector is bridged by three m-GeButH groups. The 1H NMR spectrum shows a singlet resonance for the hydride ligand on germanium at 7.92 ppm and for the But protons at 1.25 ppm. In addition to this fragmentation product 120, four complexes as a result of cluster condensation are also obtained. The molecular structure of 121 is shown in Fig. 58. Compound 121 contains an octahedral cluster comprised of four ruthenium atoms

Ruthenium and Osmium Carbonyl Cluster Complexes

597

(118) Fig. 56 The molecular structures of Ru2(CO)6(m -GePh2)2(GePh3)2 (118) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Trufan, E. J. Custer Sci. 2007, 18, 642–659.

(119)

(120) t

t

Fig. 57 The molecular structures of Ru3(CO)9(m3-GeBu )2 (119) and Ru2(CO)6(m-GeBu H)3 (120) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Saha, S.; Isrow, D.; Captain, B. J. Organomet. Chem. 2014, 751, 815-820.

(121)

(122) t

t

t

t

Fig. 58 The molecular structures of Ru4(CO)10(m4-GeBu )2(m-GeBu H)2 (121) and Ru4(CO)8(m4-GeBu )2(m-GeBu H)2(m3-GeBut)(H) (122) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Saha, S.; Isrow, D.; Captain, B. J. Organomet. Chem. 2014, 751, 815–820.

598

Ruthenium and Osmium Carbonyl Cluster Complexes

and two germanium atoms. There are two m-GeButH groups that bridge two of the RudRu bonds in 121. Each of the germanium atoms in the octahedral core contains a terminal But group. With 10 CO ligands, the cluster valence electron count is 66 which is consistent for a closo octahedron based on the Polyhedral Skeletal Electron Pair Theory [10M + 2 + 4n ¼ 10(4) + 2 + 4(6) ¼ 66 e−]. Compound 122, which also contains four ruthenium atoms, is isolated from the reaction mixture. As can be seen in Fig. 58, this complex also consists of a Ru4Ge2 octahedral cluster as in 121. However, unlike in 121 the two germanium atoms are now bonded to each other. Compound 122 in addition to also having two m-GeButH groups bridging two of the RudRu bonds, there is also a triply bridging m3-GeBut group that caps the Ru3 triangular face. Appropriately, the complex contains one hydride ligand which is a triple bridge on the Ru3 face. 1H NMR confirms the presence of the hydride ligand which shows a high-field singlet at d ¼ −20.84 ppm. With 8 CO ligands, just like in 121 the cluster valence electron count remains at 66 in accordance with a closo octahedral structure. Two higher nuclearity ruthenium cluster complexes Ru5(CO)12(m3-GeBut)2(m4-GeBut)(H) (123) and Ru6(CO)12(m3-GeBut)4 (H)2 (124) are also obtained. The molecular structure of 123 is shown in Fig. 59. The compound contains an octahedral cluster comprised of five ruthenium atoms and one germanium atom. Two m3-GeBut groups are present as triply bridging ligands capping two of the Ru3 faces. One of the other Ru3 faces contains a triply bridging hydride ligand. The hydride peak is observed as a high-field resonance in its 1H NMR spectrum, d ¼ −22.77 ppm. The cluster valence electron count is 76 which is consistent for a closo octahedron structure, where five of the vertices are occupied by a transition metal atom [10M + 2 + 4n ¼ 10(5) + 2 + 4(6) ¼ 76 e−]. Compound 124, see Fig. 59, is a Ru6 octahedron with four triply bridging m3-GeBut groups. With 12 CO ligands and two face bridging hydride ligands the cluster valence count is 86 e− which is exactly the number expected for an octahedral cluster of six metal atoms. These two hydride ligands are equivalent and appear as one high-field resonance, at −26.77 ppm, in the 1H NMR spectrum of the compound. The reaction of H4Ru4(CO)12 with an excess of Ph3GeH in octane at 125  C yields two tetraruthenium cluster complexes, Ru4(m4-GePh)2(m-GePh2)2(m-CO)2(CO)8 (125) and Ru4(m4-GePh)2(m-GePh2)3(m-CO)(CO)8 (126), in 53% and 10% yield, respectively.125 Both compounds are characterized by a combination of IR, 1H NMR, single-crystal X-ray diffraction, and mass spectral analyses. The molecular structure of 125 is shown in Fig. 60. The molecule contains a rectangular cluster of four ruthenium atoms bridged by two quadruply bridging GePh ligands that cap each side of the Ru4 rectangle. There are also two GePh2 groups that bridge two opposite edges of the Ru4 rectangle, and there are two trans-positioned edge-bridging CO ligands that lie cis to the bridging GePh2 ligands. Each ruthenium atom contains two terminally coordinated CO ligands. The molecular structure of 126 is shown in Fig. 60. The structure of compound 126 is similar to that of 125 except in place of a bridging CO group there is a bridging GePh2 group. The four ruthenium atoms in 126 have a trapezoidal-like arrangement of four ruthenium atoms. There are two quadruply bridging GePh ligands, three edge-bridging GePh2 ligands, and one bridging CO ligand. Each ruthenium atom has two terminal CO ligands. The RudRu bond that is bridged by the CO ligand, is significantly shorter than the three RudRu bonds that are bridged by GePh2 ligands. At 175  C, the reaction of H4Ru4(CO)12 with excess Ph3GeH yields the compound Ru4(m4-GePh)2(m-GePh2)4(CO)8 (127) in a 76% yield.125 Compound 127 is characterized by a combination of IR, 1H NMR, single-crystal X-ray diffraction, and mass spectral analyses. The molecular structure of 127 is shown in Fig. 61. The molecule contains an approximately square arrangement of four ruthenium atoms with two quadruply bridging GePh ligands and a bridging GePh2 on each edge of the Ru4 square. Each ruthenium atom has two CO ligands that are terminally coordinated. The reaction of amidinatogermylene Ge(iPr2bzam)(HMDS) (iPr2bzam ¼ N,N0 -bis(iso-propyl)benzamidinate; HMDS ¼ N(SiMe3)2) with Ru3(CO)12 leads to the diruthenium(0) derivative [Ru2{m-k2Ge,N-Ge(iPr2bzam)(HMDS)}(CO)7] (128), which

(123)

(124) t

t

t

Fig. 59 The molecular structures of Ru5(CO)12(m3-GeBu )2(m4-GeBu )(H) (123) and Ru6(CO)12(m3-GeBu )4(H)2 (124) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Saha, S.; Isrow, D.; Captain, B. J. Organomet. Chem. 2014, 751, 815–820.

Ruthenium and Osmium Carbonyl Cluster Complexes

(125)

599

(126)

Fig. 60 The molecular structures of Ru4(m4-GePh)2(m-GePh2)2(m-CO)2(CO)8 (125) and Ru4(m4-GePh)2(m-GePh2)3(m-CO)(CO)8 (126) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Boswell, E. M.; Captain, B.; Patel, M. A. Inorg. Chem. 2007, 46, 533−540.

(127) Fig. 61 The molecular structures of Ru4(m4-GePh)2(m-GePh2)4(CO)8 (127) [ruthenium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Boswell, E. M.; Captain, B.; Patel, M. A. Inorg. Chem. 2007, 46, 533−540.

contains a bridging bidentate germylene-imine ligand.126 Complex 128 reacts with tert-butylisonitrile and trimethylphosphine in toluene at room temperature to give the CO-substitution derivatives [Ru2{m-k2Ge,N-Ge(iPr2bzam)(HMDS)}(L)(CO)6] (L ¼ t BuNC, 129a; PMe3, 129b), which contain the ligand in an axial position on the ruthenium atom that is not attached to the amidinato fragment. At 70  C, 128 reacts with PPh3 and PMe3 to give the equatorially substituted derivatives [Ru2{m-k2Ge, N-Ge(iPr2bzam)(HMDS)}(L)(CO)6] (L ¼ PPh3, 130a; PMe3, 130b). The bidentate diphosphines bis(diphenylphosphino)methane (dppm) and 1,2-bis(diphenylphosphino)ethane (dppe) react with complex 128 to give the to the disubstituted derivatives [Ru2{m-k2Ge,N-Ge(iPr2bzam)(HMDS)}(m-k2P,P0 -L2)(CO)5] (L2 ¼ dppm, 131a; dppe, 131b). Triethylsilane and triphenylstannane are oxidatively added to complex 128 at 70  C, leading to the coordinatively unsaturated products [Ru2(ER3)(m-H){m-k2Ge, N-Ge(iPr2bzam)(HMDS)}(CO)5] (ER3 ¼ SiEt3, 132a; SnPh3, 132b). Complexes 132a and 132b easily reacts with tBuNC and CO to give the saturated derivatives [Ru2(ER3)(m-H){m-k2Ge,N-Ge(iPr2bzam)(HMDS)}(tBuNC)(CO)5] (ER3 ¼ SiEt3, 133a; SnPh3, 133b) and [Ru2(ER3)(m-H){m-k2Ge,N-Ge(iPr2bzam)(HMDS)}(CO)6] (ER3 ¼ SiEt3, 134a; SnPh3, 134b), respectively. The reaction of 128 with H2 at 70  C leads to the unsaturated tetranuclear complex [Ru4(m-H)2{m-k2Ge,N-Ge(iPr2bzam)(HMDS)}2(CO)10] (135), which also reacts with tBuNC and CO to give the saturated derivatives [Ru4(m-H)2{m-k2Ge,N-Ge(iPr2bzam) (HMDS)}2(L)2(CO)10] (L ¼ tBuNC, 136a; CO, 136b). All tetraruthenium complexes contain an unbridged metal-metal connecting two germylene-bridged diruthenium units. All of the coordinatively unsaturated products (132a, 132b, and 135) have their unsaturation(s) located on the ruthenium atom(s) that is(are) attached to the amidinato fragment(s).

600

7.10.2.4

Ruthenium and Osmium Carbonyl Cluster Complexes

Trimetallic ruthenium-platinum-palladium carbonyl cluster complexes

The reaction of PtRu5(CO)16(m6-C) with Pd(PBu3t)2 yields the mono- and di-palladium adducts: PtRu5(CO)16(m6-C)[Pd(PBu3t)] (137) in 33% yield and PtRu5(CO)16(m6-C)[Pd(PBu3t)]2 (138) in 35% yield.99 Both compounds are characterized by IR, 1H and 31P NMR, and single crystal X-ray diffraction analyses and their molecular structures are shown in Fig. 62. Compound 137 consists of a PtRu5 octahedral core with a carbon atom in the center and a Pd(PBu3t) group bridging the RudRu edge on the Ru4 square plane. Both PddRu bonds are bridged by a CO ligand from the PtRu5 cluster. The compound 138 has one Pd(PBu3t) group bridging the RudRu edge on the Ru4 square and the other Pd(PBu3t) group bridging the PtdRu edge. CO ligands bridge each of the PddRu bonds. In compound 138 the edge bridging Pd(PBu3t) group on RudRu is displaced away from the Pt atom by 1.5782(6) A˚ .

7.10.2.5

Trimetallic ruthenium-platinum-tin carbonyl cluster complexes

The reaction of PtRu5(CO)16(m6-C) with Ph3SnH at room temperature affords the trimetallic cluster complex PtRu5(CO)15 (m-SnPh2)(m6-C) (139) in 78% yield.119 This compound is characterized by a combination of IR, NMR, and single crystal X-ray diffraction analyses. The molecular structure of 139 is shown in Fig. 63. Compound 139 consists of an octahedral-shaped cluster of six metal atoms, one platinum and five ruthenium, with a carbido carbon atom in the center. There is an SnPh2 group bridging one

(137) PtRu5(CO)16(m6-C)[Pd(PBu3t)]

(138) t

Fig. 62 The molecular structures of (137) and PtRu5(CO)16(m6-C)[Pd(PBu3 )]2 (138) [ruthenium (green), platinum (red), palladium (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D. J. Organomet. Chem. 2003, 682, 113–118.

(139)

(140)

Fig. 63 The molecular structures of PtRu5(CO)15(m-SnPh2)(m6-C) (139) and PtRu5(CO)14(m-SnPh2)(PMe2Ph)(m6-C) (140) [ruthenium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W. J. Organomet. Chem. 2003, 671, 158–165.

Ruthenium and Osmium Carbonyl Cluster Complexes

601

of the PtdRu bonds in the site analogous to that of the bridging carbonyl ligand in PtRu5(CO)16(m6-C). The PtdSn distance is slightly larger than the Ru-Sn distance. It is proposed that the reactions of PtRu5(CO)16(m6-C) with Ph3SnH proceeds by CO elimination from PtRu5(CO)16(m6-C) and an oxidative addition of the SndH bond to the cluster to yield some unobserved intermediate containing an SnPh3 group and a hydride ligand. The phosphine derivative of PtRu5(CO)16(m6-C), PtRu5(CO)15(PMe2Ph)(m6-C) reacts with Ph3SnH at 68  C to afford the cluster complex PtRu5(CO)14(m-SnPh2)(PMe2Ph)(m6-C) (140) in 20% yield which is the PMe2Ph derivative of 139.119 Compound 140 can be obtained in a better yield (41%) simply by the treatment of 139 with PMe2Ph. Compound 140 is also characterized by a combination of IR, NMR, and single crystal X-ray diffraction analyses. The molecular structure of 140 is shown in Fig. 63. Compound 140 consists of an octahedral cluster containing one platinum and five ruthenium atoms, and a single carbon atom in the center. The PMe2Ph ligand is terminally coordinated to the platinum atom and the SnPh2 group has replaced the bridging carbonyl ligand between platinum and ruthenium atoms in PtRu5(CO)15(PMe2Ph)(m6-C). Replacement of the CO ligand in 139 with the poorer acceptor PMe2Ph should lead to an increase in electron density on the platinum atom. This could result in better orbital overlaps between the platinum and tin atoms and result in a stronger and shorter platinum-tin bond. The bimetallic cluster complexes PtRu5(CO)13(PBu3t)(m-H)3(SnPh3)(m5-C) (141) and PtRu5(CO)13(PBu3t)(m-H)2(m-SnPh2) (m6-C) (142) are obtained in low yields, 8% and 10% yield, respectively, from the reaction of PtRu5(CO)14(PBu3t)(m-H)2(m6-C) with HSnPh3 in a hexane solution at reflux for 30 min.96 Compound 141 is obtained in a much better yield (61%) when the reaction is performed at room temperature in CH2Cl2 solvent over 5 h, but no 142 is formed at this temperature. Compound 142 can be obtained directly from 141 in a 24% yield by heating a solution in heptane solvent at reflux for 1 h. Both compounds are characterized by a combination of IR, 1H NMR, and 31P NMR analysis. Attempts to obtain a high quality single-crystal X-ray diffraction analysis of 141 are unsuccessful due to crystallographic disorder problems. Compound 141 contains three hydride ligands, and the 1H NMR spectrum shows that one of these ligands is proximate to the platinum atom, d ¼ −7.49 ppm, and two are remote to platinum, d ¼ −21.31 and −21.69 ppm with no observable couplings to the platinum or phosphorus atoms. Compound 142 is successfully characterized by a single-crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 64. The compound 142 contains an SnPh2 ligand bridging the RudRu bond. Compound 142 also contains two inequivalent hydrido ligands that bridge oppositely positioned PtdRu bonds. These inequivalent hydrido ligands exhibit separate resonances at room temperature in the 1H NMR spectrum, d ¼ −11.81 and −14.09 ppm, and there is no evidence for dynamical exchange between them on the NMR time scale at room temperature. The reaction of Pt(Bu3t)2 with Ru3(CO)9(m-SnPh2)3 (101) yields Ru3(CO)9(m-SnPh2)3[Pt(PBu3t)]x, 143-145, x ¼ 1–3.118 The yields vary depending on the amount of Pt(PBu3t)2 that is supplied. When the ratio of Pt(PBu3t)2/101 is 1.5:1 in the reaction mixture, compound 143 is the major product and the yields of 143, 144, and 145 are 40%, 9%, and 0.2%, respectively. However, when the ratio of Pt(PBu3t)2/101 is increased to 3:1 in the reaction mixture, compound 144 is the major product and the yields of 143, 144, and 145 are 8%, 34%, and 0.6%, respectively. The yield of 145 is always very low even when the ratio of Pt(PBu3t)2/101 is as high as 10:1 in the reaction. This may be due to unfavorable steric effects when three Pt(P PBu3t) groups are added to the Ru3Sn3 cluster. In fact, when 145 is dissolved, it slowly loses a Pt(PBu3t) group and small amounts of compound 144 form spontaneously. All three products are characterized by single-crystal X-ray diffraction analyses, and the molecular structures of 143–145 are shown in Figs. 65 and 66. Compound 143 contains a central unit of Ru3(CO)9(m-SnPh2)3 with a Pt(PBu3t) group bridging its RudSn bond. The platinum atom lies essentially in the plane of the six metal atoms Ru3Sn3. The platinum bridged RudSn bond, is significantly longer than all the RudSn bond distances in 101 and 143 except the RudSn bond adjacent to it. Compound 144 is structurally similar to 101 and 143 except that it contains two Pt(PBu3t) groups bridging RudSn bonds. The RudRu bond distances are similar to those in 101 and 143. The platinum atoms are bonded to the SnPh2 ligands and a CO ligand bridges from a ruthenium atom to each added platinum atom. All nine metal atoms lie in the same plane.

(142) PtRu5(CO)13(PBu3t)(m-H)2(m-SnPh2)(m6-C)

Fig. 64 The molecular structures of (142) [ruthenium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Inorg. Chem. 2005, 44, 6623− 6631.

602

Ruthenium and Osmium Carbonyl Cluster Complexes

(143) Ru3(CO)9(m-SnPh2)3[Pt(PBu3t)]

(144) Ru3(CO)9(m-SnPh2)3[Pt(PBu3t)]2

Fig. 65 The molecular structures of (143) and (144) [ruthenium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Hall, M. B.; Trufan, E.; Yang, X. J. Am. Chem. Soc. 2007, 129, 12328–12340.

(145)

(146)

Fig. 66 The molecular structures of Ru3(CO)9(m-SnPh2)3[Pt(PBu3t)]3 (145) and Pt2Ru2(CO)9(SnBu3t)2(m-H)2 (146) [ruthenium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from (a) Adams, R. D.; Captain, B.; Hall, M. B.; Trufan, E.; Yang, X. J. Am. Chem. Soc. 2007, 129, 12328–12340 and (b) Manzoli, M.; Shetti, V. N.; Blaine, J. A. L.; Zhu, L.; Isrow, D.; Yempally, V.; Captain, B.; Coluccia, S.; Raja, R.; Gianotti, E. Dalton Trans. 2012, 41, 982-989.

The trimetallic mixed metal cluster Pt2Ru2(CO)9(SnBu3t)2(m-H)2 (146) having a Pt:Ru:Sn ratio of 1:1:1 is obtained from the reaction of Pt2Ru4(CO)18 with Bu3tSnH at 68  C.127 The complex is characterized in solution by IR and 1H NMR, and its structure in the solid state is elucidated crystallographically. The molecular structure of 146 is shown in Fig. 66. The cluster contains two platinum atoms and two ruthenium atoms in a tetrahedral arrangement. The SnBu3t groups are terminally coordinated to each of the platinum atoms. There are two hydride ligands that bridge two of the RudPt bonds. The two hydride ligands are equivalent and exhibit only one resonance in the 1H NMR spectrum showing appropriate coupling to both platinum and tin (d ¼ −10.48 ppm). The cluster valence electron count is 58 electrons, which is two less than the expected 60 electron count for closed tetrahedral clusters; however, it has been found that tetrahedral clusters that contain platinum often contain 58 electrons.

7.10.2.6

Trimetallic ruthenium-platinum-germanium carbonyl cluster complexes

The reactions of PtRu5(CO)16(m6-C) with Ph3GeH affords the trimetallic cluster complex PtRu5(CO)15(m-GePh2)(m6-C) (147) in 67% yield.119 In this reaction heating is required to the reflux temperature of the hexane solvent for 2 h. Compound 147 is

Ruthenium and Osmium Carbonyl Cluster Complexes

603

(147) Fig. 67 The molecular structures of PtRu5(CO)15(m-GePh2)(m6-C) (147) [ruthenium (green), platinum (red), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W. J. Organomet. Chem. 2003, 671, 158–165.

characterized by a combination of IR, NMR, and single crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 67. Compound 147 consists of an octahedral-shaped cluster of six metal atoms, one platinum and five ruthenium, with a carbido carbon atom in the center. There is an GePh2 group bridging one of the Pt-Ru bonds in the site analogous to that of the bridging carbonyl ligand in PtRu5(CO)16(m6-C). The Pt-Ge distance is slightly larger than the RudGe distance. The RudGe bond distance in 147 is slightly shorter than those found for the bridging GePh2 ligands in the compounds Ru5(CO)11(m-CO) (m-GePh2)3(m5-C) (108).122 It is proposed that the reactions of PtRu5(CO)16(m6-C) with Ph3GeH proceeds by CO elimination from PtRu5(CO)16(m6-C) and an oxidative addition of the GedH bond to the cluster to yield some unobserved intermediate containing an GePh3 group and a hydride ligand. Two cluster complexes, PtRu5(CO)13(PBu3t)(m-H)3(GePh3)(m5-C) (148) and PtRu5(CO)13(PBu3t)(m-H)2(m-GePh2)(m6-C) (149) are obtained in 29% and 15% yield, respectively, when a solution of PtRu5(CO)14(PBu3t)(m-H)2(m6-C) and HGePh3 in heptane solvent is heated to reflux for 30 min.96 Compound 149 can be converted directly from 148 in 45% yield by heating to reflux in a heptane solution for 110 min. Both products are characterized by a combination of IR, 1H NMR, 31P NMR, and single-crystal X-ray diffraction analyses. The molecular structure of 148 is shown in Fig. 68. The structure of 148 contains an open PtRu5 cluster with the Pt(PBu3t) group bridging one edge of the base of the Ru5 square pyramid. A Ph3Ge ligand is coordinated to the apical ruthenium atom. Compound 148 contains three hydride ligands. Notably, the hydride-bridged PtdRu bond distance is much longer than the unbridged Pt-Ru bond, presumably due to the presence of this bridging hydride ligand. Compound 148 contains 86 valence electrons. This is two less than the number, 88, expected for an edge-bridged square pyramid, 74 + 14. The reason for this is because in 148 the edge-bridging platinum atom has only a 16-electron configuration instead of the usual 18-electron configuration of a typical transition metal atom.

(148) PtRu5(CO)13(PBu3t)(m-H)3(GePh3)(m5-C)

(149) PtRu5(CO)13(PBu3t)(m-H)2(m-GePh2)(m6-C)

Fig. 68 The molecular structures of (148) and (149) [ruthenium (green), platinum (red), germanium (purple), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Inorg. Chem. 2005, 44, 6623− 6631.

604

Ruthenium and Osmium Carbonyl Cluster Complexes

The molecular structure of 149 is shown in Fig. 68. The structure of the cluster of 149 is very similar to that of PtRu5(CO)14 (PBu3t)(m-H)2(m6-C) and contains a closed PtRu5 cluster where the Pt(PBu3t) group bridges the entire base of the Ru5 square pyramid. Compound 149 contains only two hydrido ligands that bridge oppositely positioned PtdRu bonds. The hydride ligands are inequivalent, and accordingly, two resonances are observed for them in the 1H NMR spectrum, d ¼ −12.28 and −14.83 ppm. It is noteworthy that there is no evidence for dynamical exchange between these hydride ligands on the NMR time scale at room temperature. Compound 149 also contains a GePh2 ligand that bridges the RudRu bond. Compound 149 has structural similarities to the compound 147, but compound 147 has no hydrido ligands and the GePh2 ligand in 147 bridges one of the PtdRu bonds. Compound 149 contains 86 valence electrons. This is precisely the number expected for a closo-octahedron.

7.10.3

Osmium carbonyl cluster complexes

7.10.3.1

Monometallic osmium carbonyl cluster complexes

The chemistry of triosmium clusters with a wide variety of small molecules has been studied in detail but there have been problems caused by the relatively low reactivity of the parent complex, Os3(CO)12. In particular, products of the reactions between Os3(CO)12 and arenes have only been obtained in low yield,128 despite the interest in the interactions between arenes and metal surfaces and the analogy with clusters.129 The complex Os3(CO)10(NCMe)2 (150) has been exploited successfully as a much more reactive starting material,130–132 the acetonitrile groups being replaced easily by other ligands. The reactions between compound 150 and a number of arenes have been reported. The cluster Os3H2(CO)9(C6H4) is obtained by the reaction of compound 150 with benzene under reflux. Similarly, when the reaction is carried out in toluene and chlorobenzene, the complexes Os3H2(CO)9(C6H3Me) and Os3H2(CO)9(C6H3Cl) are obtained respectively.133 The reaction of biphenylene and Os3(CO)12 is carried out in refluxing decane yielding Os2(CO)6(m-Z2,Z4-(C6H4)2 and Os4(CO)12(m4-Z2-(C6H3)Ph).134 The compound 150 reacts with an excess of acenaphthylene at room temperature to yield the complex Os3(CO)10(m-H) (m-Z2-C12H7) (151) in 69% yield.135 Compound 151 is converted to the compound Os3(CO)9(m-H)2(m3-Z2-C12H6) (152) in 79% yield, when it is heated to reflux in cyclohexane for 2 h. Compound 152 can also be obtained in one step by the reaction of 150 with an excess of acenaphthylene in refluxing cyclohexane for 2 h. Compounds 151 and 152 are both characterized by IR, NMR, elemental, and single crystal X-ray diffraction analyses. The molecular structure of 151 is shown in Fig. 69. Compound 151 is formed by the activation of one of the CdH bonds of the five membered ring of acenaphthylene by the triangular osmium metal cluster. The two NCMe ligands are displaced from 150, but there is no loss of CO. The molecular structure of 152 is shown in Fig. 69. Compound 152 is formed from 151 by the loss of one CO ligand and the cleavage of the alkenyl CdH bond, and thus requires a higher temperature (80  C) and longer reaction time (2 h). This compound contains a triply bridging acenaphthyne ligand which has adopted the usual di s + p coordination mode. There are two bridging hydride ligands. At room temperature, there is only a single broad resonance at d ¼ −19.0 ppm for the hydride ligands in the 1H NMR spectrum. This is due to a dynamical exchange process that averages the ligands between the two sites. Compound 152 reacts with an additional quantity of acenaphthylene at 160  C over a period of 8 h to give four products that have been identified as: Os4(CO)12(m4-Z2:Z2-C12H6) (153), Os2(CO)6(m-Z4-C24H12) (154); Os3(CO)9(m-H)(m3-Z4-C24H13) (155); Os2(CO)5(m-Z4-C24H12)(Z2-C12H8) (156) in 11%, 13%, 4%, and 3% yields. All four products are characterized by IR, NMR, mass spectra, and single crystal X-ray diffraction analyses. The molecular structures of 153, 154, 155, and 156 are shown in Fig. 70. Compound 153 contains a butterfly cluster of four osmium atoms bridged by an acenaphthyne ligand. Compound 155 contains two acenaphthyl groups that have been coupled by formation of a CdC single bond. Both compound 154 and 156

(151)

(152) 2

2

Fig. 69 The molecular structures of Os3(CO)10(m-H)(m-Z -C12H7) (151) and Os3(CO)9(m-H)2(m3-Z -C12H6) (152) [osmium (green), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Smith, J. L. J. Organomet. Chem. 2003, 683, 421–429.

Ruthenium and Osmium Carbonyl Cluster Complexes

(153)

605

(154)

(155)

(156) 2

2

4

Fig. 70 The molecular structures of Os4(CO)12(m4-Z :Z -C12H6) (153), Os2(CO)6(m-Z -C24H12) (154), Os3(CO)9(m-H)(m3-Z4-C24H13) (155) and Os2(CO)5(m-Z4-C24H12)(Z2-C12H8) (156) [osmium (green), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Smith, J. L. J. Organomet. Chem. 2003, 683, 421–429.

contain two coupled acenaphthyne molecules that bridge the two metal atoms to form a metallacycle similar to the ferrole structures formed by the coupling of alkynes by iron carbonyl.136 Compound 156 differs from 154 by the presence of an acenaphthlene ligand that has been substituted for one of the CO ligands in 154. Compound 155 is a precursor to 154 and it is converted into 154 in 40% yield when it is heated to 160  C for a period of 5 h. Carbon-hydrogen activation by osmium cluster complexes on compounds containing unsaturated 5-membered rings can lead to triply bridging cyclopentyne and related ligands: Os3(CO)9(m-H)2(m3-Z2-C7H8);17 Os4(CO)9(m-H)(Z5-C9H7)(m3-Z2-C9H6);137 Os3(CO)9(m-H)(m3-Z2-C4H2NCH3);138 Os3(CO)9(m-H)2(m3-Z2-C6SH4).139 Metal clusters are also known to facilitate the dimerization and trimerization of alkynes.140 The osmium-acetylene complex Os3(CO)9(m-H)2(PhC2Ph) reacts with PhC2Ph to give the compound Os3(CO)9(C4Ph4) which contains a metallacyclopentadiene ring. Interest in the organometallic chemistry of N-heterocyclic carbenes (NHCs) and their mesoionic analogues (mNHCs) remains as high as ever,141–149 and this interest has naturally been carried over into organometallic cluster chemistry.150–153 Cabeza et al. has reported that the reactivity of NHCs with [M3(CO)12] (M ¼ Ru, Os) depends strongly on the nature of the substituent on the NHC: the more basic and less bulky the NHC ligand, the more facile its reaction with the metal clusters. The reaction of N,N0 -dimethylimidazolin-2-ylidene (IMe) with [Os3(CO)12] at room temperature gives trinuclear derivative [Os3(CO)11(IMe)], but the bulkier carbene N,N0 -dimesitylimidazolin-2-ylidene (IMes) and N,N0 -bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IPr) fails to react even at high temperature as high as 70  C.154 The even bulkier NHC N,N0 -bis(adamantyl)imidazolin-2-ylidene (IAd) also reacts with [Os3(CO)12] at 70  C to generate the mesoionic NHC complex [Os3(CO)11(mIAd)].155 All these reactions can be described as overall carbonyl-substitution reactions, in which a carbonyl ligand has been substituted by an NHC, either in the normal or in the mesoionic coordination mode. The reaction of the free N-heterocyclic carbene N,N0 -dimesitylimidazolin-2-ylidene, IMes (157) with [Os3(CO)12] in THF at room temperature affords [Os3(CO)11{C(]O)mIMes}] (158) as the major product together with [Os3(CO)11(mIMes)] (159),

606

Ruthenium and Osmium Carbonyl Cluster Complexes

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(159)

161) Fig. 71 The molecular structures of [Os3(CO)11{C(]O)mIMes}] (158), [Os3(CO)11(mIMes)] (159), and [Os3(CO)10(IMes)2] (161) [osmium (green), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Liu, Y.; Ganguly, R.; Huynh, H. V.; Leong, W. K. Angew. Chem., Int. Ed. 2013, 52, 12110–12113.

[Os3(CO)11(IMes)] (160), [Os3(CO)10(IMes)2] (161).156 The molecular structures of 158, 159, and 161 are shown in Fig. 71. The analogous products [Os3(CO)11{C(]O)mIPr}] (158b) and [Os3(CO)11{C(]O)mItBu}] (158c) are obtained similarly from IPr (157b) and ItBu (157c). Here, the reaction of a free NHC with [Os3(CO)12] appears to proceed through initial attack of the NHC ligand on the carbon atom of a carbonyl ligand to form an acyl ligand. Several research groups have investigated the use of NHC-silver(I) halide complexes as NHC transfer agents and have successfully prepared NHC complexes of several late transition metals.157,158 The NHC-silver(I) halide transfer method is used to introduce NHCs into clusters as it appears to be the most robust. Silver(I) NHC complexes with a singly coordinating anion are neutral, with silver linearly bound to both the carbene and the halide. The NHC transfer complex used in this study is 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene-silver(I) chloride.159 The reaction of ([IMes]AgCl) with [Os3(m-H)2(CO)10] gives [Os3(m-H)(m-Cl)(CO)9(IMes)], an osmium N-heterocyclic carbene complex.160 The reaction of ([IMes]AgCl) with the activated cluster [Os3(CO)10(CH3CN)2] yields two products, [Os3(m-Cl)(CO)10(m-Ag(IMes)] (162) and [(IMes-H)][Os3(m-Cl) (CO)10](m4-Ag)[Os3(m-Cl)(CO)10] (163). Compound 162 is a heterobimetallic carbene complex resulting from the complete incorporation of ([IMes]AgCl). Transmetalation using [(IMes)AgCl] and the cluster [Os4(m-H)4(CO)12] in the presence of trimethylamine-N-oxide results in the formation of [Os4(m-H)4(CO)11(n
IMes)] (164), where (n denotes normal carbene bonding, i.e., through C2). [Os4(m-H)4(CO)10(IMes)2] (165) is formed using the activated cluster [Os4(m-H)4(CO)10(CH3CN)2] and [(IMes)AgCl]. The molecular structures of 164 and 165 are shown in Fig. 72. Compound 165 is found to be sensitive to silica gel column chromatography in the presence of dichloromethane decomposing to [Os4(m-H)3(m-Cl)(CO)11(IMes)] (166), which possesses a butterfly metal skeleton. Two higher-nuclearity clusters, {Os5(m5-C) (CO)14[Z-C-Z-NC2H2(Mes)]} (167) and [Os6(m-H)4(m5-C)(CO)15(IMes)] (168), both of which contain carbide ligands, are also isolated.160 The molecular structures of 166, 167, and 168 are shown in Fig. 73.

Ruthenium and Osmium Carbonyl Cluster Complexes

(164)

607

(165)

Fig. 72 The molecular structures of [Os4(m-H)4(CO)11(n
IMes)] (164), [Os4(m-H)4(CO)10(IMes)2] (165) [osmium (green), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Cooke, C. E.; Jennings, M. C.; Katz, M. J.; Pomeroy, R. K.; Clyburne, J. A. C. Organometallics 2008, 27, 5777–5799.

(166)

(167)

(168) Fig. 73 The molecular structures of [Os4(m-H)3(m-Cl)(CO)11(IMes)] (166), {Os5(m5-C)(CO)14[Z-C-Z-NC2H2(Mes)]} (167), [Os6(m-H)4(m5-C)(CO)15(IMes)] (168) [osmium (green), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue), Chlorine (yellow)]. Modified from Cooke, C. E.; Jennings, M. C.; Katz, M. J.; Pomeroy, R. K.; Clyburne, J. A. C. Organometallics 2008, 27, 5777–5799.

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Ruthenium and Osmium Carbonyl Cluster Complexes

Three high-nuclearity osmium carbonyl cluster complexes, [Os7(m5-k2-ampy)(m-CO)2(CO)17] (169), [Os8(m3-Η)(m-H) (m5-k2-ampy)(m-CO)(CO)19] (170), and [Os9O(m4-k2-ampy)2(CO)18] (171) (H2ampy ¼ 2-amino-6-methylpyridine), have been prepared by heating [Os3(CO)12] with H2ampy in decane at reflux temperature (Fig. 74).161 The trinuclear complex [Os3(m-Η) (m3-k2-Hampy)(CO)9] is an intermediate in this reaction. The metallic skeletons of 169 and 170 can be described as mono- (169) or bi-face-capped (170), basal-edge-bridged, square pyramids, whereas that of compound 171 consists of a pentagonal bipyramid with two equatorial edges spanned by metal atoms. In the three clusters, the ampy ligands are attached to edge-bridging Os atoms through their pyridine N atoms, while they also cap metallic squares (in 169 and 170) or triangles (in 171) through their imido N atom. These complexes are remarkable because: (a) compounds 169 and 170 are the first examples of osmium carbonyl clusters that contain m4-imido ligands, (b) the metallic skeletons of compounds 170 and 171 have never been observed in osmium carbonyl clusters, and (c) compound 171 contains a terminal oxo ligand that is unprecedented in osmium carbonyl chemistry. An important key feature of the synthesis of clusters 169–171 is the high reaction temperature (174  C). Prior to this work, m4-imido ligands have never been observed in osmium carbonyl chemistry; this may be a consequence of the fact the reactions of osmium carbonyl cluster complexes with precursors of imido ligands have generally been performed at lower temperatures. Thermolysis of [Os3(CO)10(m-OH)(m-H)] with a slight excess of bis-diphenylphosphino methane (dppm) in refluxing cyclohexane affords two compounds. The major product (62% yield) is confirmed as [Os3(CO)8(m-OH)(m-H)(m-dppm)] (172a) based on IR and NMR spectroscopy, and X-ray diffraction analysis.162 The minor product (15% yield) is subsequently established as the isomeric cluster [Os3(CO)8(m-OH)(m-H)(m-dppm)] (172b) based on solution spectroscopic data and X-ray crystallography. The major difference between clusters 172a and 172b resides in the disposition of bridging ligands. In 172a, all three bridging ligands share a common OsdOs edge while in 172b the dppm ligand ligates an adjacent OsdOs edge. Control experiments confirms that 172a slowly transforms into 172b in cyclohexane at 81  C. This transformation likely proceeds through a dissociative release of one of the coordinated phosphine moieties of the dppm ligand. Heating 172b in cyclohexane under comparable conditions returns the starting cluster in quantitative yield. Both isomers undergo oxidative-addition of the OdH bond with concomitant elimination of H2 to produce the oxo-capped [Os3(CO)7(m3-CO)(m3-O)(m-dppm)] (173) in refluxing xylene. It is also observed that 172a furnishes

(169)

(170)

(171) 2

Fig. 74 The molecular structures of [Os7(m5-k -ampy)(m-CO)2(CO)17] (169), [Os8(m3-Η)(m-H)(m5-k2-ampy)(m-CO)(CO)19] (170), [Os9O(m4-k2-ampy)2(CO)18] (171) [osmium (green), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Cabeza, J. A.; del Rio, I.; Garcia-Alvarez, P.; Miguel, D. Organometallics 2007, 26, 3212–3216.

Ruthenium and Osmium Carbonyl Cluster Complexes

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609

(175)

Fig. 75 The molecular structures of [Os3(CO)10(m-H)(m-PHMes)] (174) and [Os3(CO)9(m-H)2(m3-PMes)] (175) [osmium (green), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Kakizawa, T.; Hashimoto, H.; Tobita, H. J. Organomet. Chem. 2006, 691, 726–736.

172b and 173 in 13% and 43% yield, respectively, when it is refluxed in xylene, and 172b furnishes 172a and 173 in 13% and 27% yield, respectively, under comparable conditions. The thermal reaction of [Os3(CO)12] with mesitylphosphine PH2Mes (Mes ¼ 2,4,6-trimethyl-phenyl ¼ mesityl) in refluxing toluene affords triosmium complexes [Os3(CO)10(m-H)(m-PHMes)] (174) and [Os3(CO)9(m-H)2(m3-PMes)] (175) in 19% and 40% yields, respectively.58 The X-ray crystal structures of 174 and 175 are shown in Fig. 75. In complex 174, one OsdOs edge of the osmium triangle is bridged by m-phosphido and m-hydrido ligands. The OsdOs bond bridged by these ligands is slightly longer than the other two OsdOs bonds. In 175, the mesitylphosphinidene ligand caps the basal metal triangle and two hydride ligands bridge over two of the OsdOs edges. The 1H NMR spectrum displays the m-hydrido resonance at d ¼ 20.84 ppm as a doublet coupled with the phosphorus atom.

7.10.3.2 7.10.3.2.1

Bimetallic osmium-group 10 transition metal carbonyl cluster complexes Bimetallic osmium-palladium carbonyl cluster complexes

Palladium has been shown to be of great value as a catalyst for the hydrogenation of unsaturated organic molecules.163 It is reported that improved catalytic activity is obtained when palladium is combined with certain transition metals.164–167 It has also been shown that bimetallic complexes containing palladium can be good precursors to supported bimetallic catalysts. Dahl and co-workers have prepared and structurally characterized a series of remarkable high nuclearity heteronuclear palladium complexes.168–175 Adams and Captain have shown that Pd(PBu3t)2 readily loses one of its PBu3t ligands in solutions and the remaining Pd(PBu3t) group readily adds to the metal-metal bonds of polynuclear metal-carbonyl complexes to yield a variety of mixed metal-carbonyl complexes containing palladium, e.g., Ru3(CO)12[Pd(PBu3t)]3176 and Re2(CO)8(m-SnPh2)[Pd(PBu3t)]2.177 Rh4(CO)12 reacts with Pd(PBu3t)2 to yield the hexarhodium cluster complexes, Rh6(CO)12 [Pd(PBu3t)]3 and Rh6(CO)12[Pd(PBu3t)]4 that contain three and four edge-bridging Pd(PBu3t) groups, respectively.178 Palladium also adds across the OsdOs bonds of Os3(CO)12. The reaction of Pd(PBu3t)2 with Os3(CO)12, yields two compounds Pd2Os3(CO)12(PBu3t)2 (176), and Pd3Os3(CO)12(PBu3t)3 (177), (Fig. 76) at room temperature.179 The products are formed by the addition of two and three Pd(PBu3t) groups across the OsdOs bonds of Os3(CO)12. The reaction proceeds quickly (10–20 min) and the compounds must be isolated in a timely way or no products at all might be obtained. For example, for reaction periods of 2 h or more, neither of these products is obtained. Once they are separated from the reaction mixture, 176 and 177 are stable in air in the solid state for days. However, in solution they interconvert rapidly by intermolecular redistribution of the Pd(PBu3t) groups and then progress to decompose within a few hours in the presence of air. Each compound consists of a triangular cluster of three osmium atoms with bridging Pd(PBut3) groups across two and three of the OsdOs bonds, respectively. For each addition of a Pd(PBu3t) group, two PddOs bonds are formed. An unusual tetrahedral octanuclear 4:4 Os:Pd cluster complex Os4(CO)12[Pd(PBu3t)]4 (178) (Fig. 77) is obtained from the reaction of Os3(CO)12 with Pd(PBu3t)2 in octane solution at reflux.180 This tetraosmiumtetrapalladium complex contains triply bridging Pd(PBu3t) groups on all four triangular faces of a central tetrahedral Os4(CO)12 cluster. The metal atoms in 178 obey neither the 18-electron rule nor the polyhedral skeletal electron pair (PSEP) theory. A tetracapped tetrahedral metal cluster in which all the metal atoms have 18-electron configurations would contain 108 valence electrons.181 Compound 178 contains a total of 104 valence electrons, so it is formally electronically unsaturated by the amount of four electrons by these criteria. It is shown that electronically unsaturated mixed metal cluster complexes readily activate hydrogen under mild conditions.182–185 Accordingly, compound 178 is examined for its reactivity toward hydrogen and it is found that this compound readily reacts with hydrogen to yield the compound Os4(CO)12(m-H)4 in good yield under very mild conditions.

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Ruthenium and Osmium Carbonyl Cluster Complexes

(176) Pd2Os3(CO)12(PBu3t)2

(177) Pd3Os3(CO)12(PBu3t)3

Fig. 76 The molecular structures of (176) and (177) [osmium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Fu, W.; Smith, M. D.; Zhu, L. J. Cluster Sci. 2006, 17, 87–95.

(178) Os4(CO)12[Pd(PBu3t)]4

Fig. 77 The molecular structure of (178) [osmium (green), palladium (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Boswell, E. M.; Captain, B. Organometallics 2008, 27, 1169–1173.

The reaction of Os3(CO)12 with three equivalents of [PdCl(allyl)(IPr)] at room temperature in the presence of a base and 2-propanol as a solvent affords three clusters [PdOs3(CO)12(IPr)] (179), [Pd2Os3(CO)12(IPr)2] (180), and [Pd3Os3(CO)12(IPr)3] (181).186 The molecular structures of 179, 180, and 181 are shown in Fig. 78. The structures of these compounds can be regarded as being built up from the sequential addition of a Pd(IPr) unit across an OsdOs bond. In fact, an increase in the reaction time leads to an increase in the yield of 181 at the expense of 179 and 180. It is believed that the Os3Pdn clusters obtained from these reactions result from the attack of Pd(IPr) fragments. Such fragments are proposed to be formed when [PdCl(allyl)(NHC)] complexes are treated with a strong base.187 Furthermore, it is proposed that the initial site of attack by the Pd(IPr) group is the carbon atom of a carbonyl ligand, which carries a d+ charge.188 This may account for the presence of carbonyl ligands bridging all the OsdPd edges; it is the interaction of the Pd(IPr) fragment with the CO that draws the fragment in toward the OsdOs bond.

7.10.3.2.2

Bimetallic osmium-platinum carbonyl cluster complexes

Adams and Captain have shown that the compounds M(PBu3t)2, M ¼ Pd or Pt are excellent reagents for the addition of M(PBu3t) groups to the metal-metal bonds of transition metal carbonyl cluster complexes.189–194 In most cases, the M(PBu3t) group is added to just two metal atoms to form an edge-bridging group. The reaction of Pt(PBu3t)2 with Os3(CO)12, yields a series of three adducts, PtOs3(CO)12(PBu3t) (182), Pt2Os3(CO)12(PBu3t)2 (183), and Pt3Os3(CO)12(PBu3t)3 (184) (Fig. 79), formed by the sequential

Ruthenium and Osmium Carbonyl Cluster Complexes

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(181) Fig. 78 The molecular structures of [PdOs3(CO)12(IPr)] (179), [Pd2Os3(CO)12(IPr)2] (180), [Pd3Os3(CO)12(IPr)3] (181) [osmium (green), palladium (red), nitrogen (light blue), carbon (gray), oxygen (orange)]. Modified from Liu, Y.; Ganguly, R.; Huynh, H. V.; Leong, W. K. Organometallics 2013, 32, 7559−7563.

addition of one-three edge bridging Pt(PBu3t) groups to the three OsdOs bonds of Os3(CO)12.191 When 182 is treated with PPh3, the mono- and bis(PPh3) derivatives of Os3(CO)12 are formed. Each compound consists of a triangular cluster of three osmium atoms with a Pt(PBu3t) group bridging one-three of the OsdOs bonds, respectively. For each addition of a Pt(PBu3t) group, two PtdOs bonds are formed. In each case, one of the two PtdOs bonds contains a bridging carbonyl ligand that is derived from the original Os3(CO)12 compound. By using Os3(CO)12 and Pt(PBu3t)2, it is shown that Pt(PBu3t) groups can be sequentially added to each of the OsdOs bonds in the triangular Os3 cluster complex. In solution, compounds 182–184 interconvert by inter-molecular exchange of the Pt(PBu3t) groups. As there is no loss of CO from Os3(CO)12, each of the products 182–184 could be viewed as a Pt(PBu3t) adduct of Os3(CO)12 formed by the sequential addition of one, two, and three Pt(PBu3t) groups to the OsdOs bonds, respectively. When heated to reflux in hexane solvent, compound 183 is transformed into the compound Pt2Os3(CO)10(PBu3t)(PBu2t)(CMe2CH2)(m-H) (185) (Fig. 80) by the loss of two CO ligands and a metalation of one of the methyl groups of one of the PBu3t ligands.191 Compound 185 consists of a tetrahedral-shaped group of four metal atoms, one platinum and three osmium atoms. It contains a total of 70 cluster valence electrons, which is four less than the number expected for an edge-bridged tetrahedron. This can be explained by assuming that the two platinum atoms have 16-electron configurations, but there can be some intrinsic electronic unsaturation within the metal-metal bond framework in this compound. It is found that the reaction of the lightly-stabilized triosmium complex Os3(CO)10(NCMe)2 with Pt(PBu3t) provides unsaturated pentanuclear platinum-osmium cluster complex, Pt2Os3(CO)10(PBu3t)2 (186) by the addition of two Pt(PBu3t) groups to the

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(183)

(184) t

Fig. 79 The molecular structures of PtOs3(CO)12(PBu3 ) (182), Pt2Os3(CO)12(PBu3t)2 (183), and Pt3Os3(CO)12(PBu3t)3 (184) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L.; Inorg. Chem. 2006, 45, 430−436.

(185) Pt2Os3(CO)10(PBu3t)(PBu2t)(CMe2CH2)(m-H)

Fig. 80 The molecular structure of (185) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L.; Inorg. Chem. 2006, 45, 430−436.

Os3(CO)10(NCMe)2 and the loss of the two NCMe ligands.195,196 Three other products: PtOs3(CO)10(PBu2t)(CMe2CH2)(m-H) (187), Os3(CO)10(PBu3t)2 and Pt2Os3(CO)10(PBu3t)(PBut2)(CMe2CH2)(m-H) (185) are obtained in lower yields. The X-ray crystallographic analysis of 186 shows that it has a trigonal bipyramidal structure with three osmium atoms in the trigonal plane and two platinum atoms in the axial positions (Fig. 81). Each platinum atom contains one PBu3t ligand. The two Pt(PBu3t) groups in 186 have adopted triply-bridging positions on opposite sides of the Os3 triangle. There is one bridging CO ligand to each

Ruthenium and Osmium Carbonyl Cluster Complexes

(186) Pt2Os3(CO)10(PBu3t)2

613

(187) t

Fig. 81 The molecular structures of (186) and PtOs3(CO)10(PBu2 )(CMe2CH2)(m-H) (187) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Organomet. Chem. 2008, 693, 819–833.

platinum atom and the remainder are terminal ligands on the osmium atoms. Compound 186 contains 68 cluster valence electrons, four less than the 72 electrons expected for a trigonal bipyramidal cluster in which all metal atoms have 18 electron configurations. Compound 187 contains four metal atoms, one of platinum and three of osmium, arranged in a tetrahedral-type structure. The PBu3t ligand is coordinated to the platinum atom and it is metallated on one of its methyl groups to form a four-membered ring with the platinum atom. Compound 187 contains one hydrido ligand, that is derived from the metallated methyl group. The molecular structure of 187 is shown in Fig. 81. When compound 186 is exposed to hydrogen (1 atm) at 0  C in methylene chloride solution, the dihydrido complex Pt2Os3(CO)10(PBu3t)2(m-H)2 (188), is formed within 10 min.196 The molecular structure of 188 is established crystallographically and is shown in Fig. 82. The molecule contains five metal atoms with a platinum atom bridging an OsdOs edge of tetrahedral

(188) Pt2Os3(CO)10(PBu3t)2(m-H)2

(189) Pt2Os3(CO)10(PBu3t)2(m-H)4

Fig. 82 The molecular structure of (188) and (189) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Organomet. Chem. 2008, 693, 819–833.

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Ruthenium and Osmium Carbonyl Cluster Complexes

PtOs3 cluster. The cluster of 188 is formed by cleaving one of the PtdOs bonds in the cluster of 186. Compound 188 contains two hydrido ligands. These hydrido ligands do not exhibit mutual coupling. The two PBu3t groups are chemically inequivalent in the solid-state structure and the 1H NMR shows two tertiary-butyl resonances at d ¼ 1.34 and 1.39 ppm. Compound 188 has a total of 70 valance electrons. This is four electrons less than what would be expected for such a structure with all metal atoms having 18 electron configurations. When solutions of 188 in methylene chloride solvent are exposed to hydrogen at 1 atm at 0  C for 1 h, the tetrahydrido complex Pt2Os3(CO)10(PBu3t)2(m-H)4 (189) is formed in 70% yield.196 Compound 189 can also be obtained directly from 186 in 63% yield in one step under similar conditions. The structure of 189 is shown crystallographically to contain a triangular Os3 cluster with two edge-bridging Pt(CO)(PBu3t) groups, see Fig. 82. The molecule contains four hydrido ligands. One hydrido ligand bridges to each platinum atom while the other two bridge OsdOs bonds. Overall, compound 189 has a total of 72 valance electrons which is four electrons less than is expected for such a structure with all metal atoms having 18-electron configurations according to the polyhedral skeletal electron pair theory for a triangle having two edge-bridging metal atoms. The reaction of 186 with hydrogen at 0  C is performed over a period of 2.5 h, and compound 189 is formed as the major product (47% yield) together with three tetranuclear metal cluster complexes PtOs3(CO)10(PBu3t))(m-H)2 (190), PtOs3(CO)9(PBu3t)(m-H)4 (191), and PtOs3(CO)8(PBu3t)2(m-H)4 (192), which are obtained in low yields, 19%, 6% and 8%, respectively.196 Compound 190 consists of a tetrahedrally-shaped metal cluster containing one platinum and three osmium atoms. The molecular structure of 190 is shown in Fig. 83. The molecule contains two hydrido ligands. One hydrido ligand bridges the PtdOs bond while the other bridges the OsdOs bond. Each osmium has three carbonyl ligands. The platinum atom contains one terminal carbonyl ligand and the one PBu3t ligand. This compound has a total of 58 valence electrons, which is two electrons less than the number expected for a tetrahedral structure of four metal atoms. Compound 191 is structurally similar to that of 190, except that it contains one less CO ligand and two more hydrido ligands than 190. Compound 191 is also obtained directly from 190 in 48% yield by reaction with hydrogen at 1 atm/25  C for 1 h. Compound 191 is also characterized crystallographically and its molecular structure is shown in Fig. 83. The molecule contains a tetrahedrally-shaped PtOs3 cluster with four hydrido ligands. In the solid-state structure, two of the hydrido ligands bridge OsdOs bonds, one bridges one of the PtdOs bonds, and one is terminally coordinated to the platinum atom and is positioned opposite to the PtdOs bond. In the solid state, the four hydrido ligands in 191 are inequivalent. Compound 191 has a total of 58 valence electrons, which is two electrons less than the number expected for a tetrahedron of four metal atoms. Compound 192 is a PBu3t derivative of 191, and it is also obtained in 41% yield directly from 191 by reaction with PBu3t at room temperature. The molecular structure of 192 is shown in Fig. 84. The molecular structure of 192 consists of a tetrahedral PtOs3 cluster with two PBu3t ligands. As in 191, one of the PBu3t ligands is coordinated to the platinum atom. The other one is coordinated to an osmium atom. Compound 192 contains a minor component of disorder (15%) in the solid state. Only the metal atoms of the disordered component are located and refined. As a result, the hydrido ligands of the major component ware not observed in the analysis. However, on the basis of a comparison of the structures of 191 and 192, it is believed that the hydrido ligands occupy positions in 192 that are similar to those in 191. Compound 192 has a total of 58 valence electrons, two electrons less than the number expected for a tetrahedral cluster of four metal atoms.

(190) PtOs3(CO)10(PBu3t)(m-H)2

(191) t

Fig. 83 The molecular structures of (190) and PtOs3(CO)9(PBu3 )(m-H)4 (191) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Organomet. Chem. 2008, 693, 819–833.

Ruthenium and Osmium Carbonyl Cluster Complexes

(192) PtOs3(CO)8(PBu3t)2(m-H)4

615

(193) Pt2Os3(CO)9(PBu3t)(m-PBu2t)(m-H)(m3-CCMe2)

Fig. 84 The molecular structure of (192) and (193) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from (a) Adams, R. D.; Captain, B.; Zhu, L. J. Organomet. Chem. 2008, 693, 819-833 and (b) Adams, R. D.; Boswell, E. Organometallics 2008, 27, 2021–2029.

Two products, Pt2Os3(CO)10(PBu3t)(PBu2t)(CMe2CH2)(m-H) (185) (18% yield) and Pt2Os3(CO)9(PBu3t)(m-PBu2t)(m-H) (m3-CCMe2) (193) (3% yield) are obtained from the reaction of Os3(CO)12 with Pt(PBu3t)2 when heated to reflux in a heptane solution for 5 h.197 Compound 193 has been characterized by a combination of IR and NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 84. The structure of 193 consists of a square-pyramidal cluster of five metal atoms: three of osmium and two of platinum. The two platinum atoms and two of the osmium atoms define the “square” base, and one of the osmium atoms, occupies the apical position of the pyramid. The molecule contains nine carbonyl ligands. There is one PBu3t that is coordinated to platinum, one PBu2t ligand that bridges the OsdPt bond, and a dimethylvinylidene ligand that bridges the triangular group of three osmium atoms. Compound 193 also contains one bridging hydrido ligand. The hydrido ligand exhibits the expected high-field resonance shift in the 1H NMR spectrum: d ¼ −20.26 ppm. Compound 185 is a precursor to 193, as it is converted into 193 in 30% yield by heating it to reflux in octane solvent for 1.5 h. This supports the notion that the vinylidene ligand is derived from a tert-butyl group of a PBu3t ligand by cleavage of the PdC bond and abstraction of three hydrogen atoms from one of its methyl groups. The remnant PBu2t group and one of the hydrogen atoms are present in 193 as bridging ligands. When Os3(CO)12 is allowed to react with Pt(PBu3t)2 at a slightly higher temperature (octane reflux), compound 193 is obtained in a better yield (12%) along with two compounds, PtOs3(CO)7(PBu3t)(m-PBu2t)(m4-CHCMeCH) (194) (4% yield) and Pt4Os3(CO)12(m-PBu2t)2 (195) (3% yield).197 Compounds 194 and 195 have been characterized by a combination of IR and NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction analysis. The molecular structure of 194 is shown in Fig. 85.

(194) PtOs3(CO)7(PBu3t)(m-PBu2t)(m4-CHCMeCH)

(195)

Fig. 85 The molecular structures of (194) and Pt4Os3(CO)12(m-PBu2t)2 (195) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Boswell, E. Organometallics 2008, 27, 2021–2029.

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Ruthenium and Osmium Carbonyl Cluster Complexes

Compound 194 contains only four metal atoms: three of osmium and one of platinum arranged in the form of a butterfly geometry. The platinum atom in a “wingtip” site bridges one edge of a triangular group of the three osmium atoms. The molecule contains one Bu3t ligand that is coordinated to the platinum atom, and there are seven terminal CO ligands that are coordinated to the osmium atoms. The most interesting feature in 194 is a CHCMeCH ligand that could be described as a dimetalla-2-methylallyl group that bridges all four metal atoms. The quadruply bridging CHCMeCH ligand serves a 5-electron donor; thus, compound 194 contains a total of 58 cluster valence electrons and is thus electron-deficient by the amount of four electrons. An electronically saturated butterfly cluster of four metal atoms should have 62 valence electrons. The molecular structure of 195 is shown in Fig. 85. Compound 195 contains seven metal atoms: four of platinum and three of osmium. The arrangement of the metal atoms can be described as a doubly capped square pyramid. The four platinum atoms define the square base, and one of the osmium atoms occupies the apical position. Each platinum atom has only one linear terminal carbonyl ligand. The molecule also contains two bridging di-tert-butylphosphido ligands. These are obviously formed by cleavage of a tert-butyl group from two PBu3t ligands supplied by the Pt(PBu3t)2. These ligands bridge the platinum atoms on opposite sides of the square base of the cluster. Compound 195 contains 94 cluster valence electrons. A doubly capped square pyramidal cluster of metal atoms should have 102 valence electrons. In an independent experiment it is shown that compound 193 can be transformed into 38 and 195 in yields of 23% and 13%, respectively, when solutions of 193 are heated at 174  C in decane solvent. Because of its unusual electronic unsaturation, the reactivity of 194 toward CO is examined. Compound 194 is found to react with CO at room temperature to yield the complex PtOs3(CO)8(PBu3t)(m-PBu2t)(m4-CHCMeCH) (196) in 54% yield in 2 h.197 Compound 196 is characterized by a combination of IR and NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 86. Compound 196 is structurally very similar to 194, except that it contains one additional CO ligand. The added CO ligand is coordinated to the platinum atom. Compound 196 contains 60 cluster valence electrons, two electrons more than in 194. When it is heated to 125  C for 2 h in an octane solution, compound 196 released one equivalent of CO and is converted back to 194 in 38% yield. The reaction of Os3(CO)12 with Pt(IPr)(SnBu3t)(H) in hexane solvent at reflux affords two bimetallic cluster complexes, the triosmium-monoplatinum cluster complex Os3Pt(IPr)(CO)12 (197) in 51% yield and the triosmium-diplatinum cluster complex Os3Pt2(IPr)2(CO)12 (198) in 19% yield.198 Both compounds 197 and 198 are characterized by a combination of IR, 1H NMR, and single-crystal X-ray diffraction analyses. The molecular structures of compounds 197 and 198 are shown in Fig. 87. Both compounds consists of a triangular cluster of three osmium atoms with a Pt(IPr) group edge bridging one and two of the OsdOs bonds, respectively. In both cases, each of the OsdPt bonds contains a bridging carbonyl group that is derived from the starting Os3(CO)12 cluster. The four metal atoms in 197 and the five metal atoms in 198 are coplanar. In case of compound 198 a raft structure is obtained. For both the compounds 197 and 198, the Pt-bridged OsdOs bonds, are significantly longer than the non-Pt-bridged OsdOs bonds. Overall, compound 197 has only 60 cluster valence electrons, which is two less than the amount usually found for butterfly cluster complexes, 62. Compound 198 has 72 cluster valence electrons which is four electrons less, than the number required for each of the metal atoms to achieve 18-electron configurations. The previously reported reaction of Os3(CO)12 with Pt(PBu3t)2 yields three products as a result of the sequential addition of 1–3 Pt(PBu3t) groups to the three OsdOs bonds of Os3(CO)12.191 For Pd(PBu3t)2 the reaction with Os3(CO)12 results in the addition of 2-3 Pd(PBu3t) groups across the OsdOs bonds of Os3(CO)12.179 However, in the reaction with Pt(IPr)(Sn Bu3t)(H) it is not possible to obtain the product where Pt(IPr) groups add across all three OsdOs bonds. This can be explained by the differences in the steric bulk of the IPr ligand when compared to PBu3t.

(196) PtOs3(CO)8(PBu3t)(m-PBu2t)(m4-CHCMeCH)

Fig. 86 The molecular structure of (196) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Boswell, E. Organometallics 2008, 27, 2021–2029.

Ruthenium and Osmium Carbonyl Cluster Complexes

(197)

617

(198)

Fig. 87 The molecular structures of Os3Pt(IPr)(CO)12 (197) and Os3Pt2(IPr)2(CO)12 (198) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Zollo, V.; Etezadi, S.; Gamage, M. M.; Captain, B. J. Cluster Sci. 2019, 30, 1355–1361.

The reaction of Os3(CO)12 with three equivalents of [PtCl(allyl)(NHC)], where NHC ¼ SIPr [N,N0 -bis-(2,6-diisopropylphenyl) imidazolidin-2-ylidene] (a), IPr [N,N0 -bis(2,6-diisopropylphenyl)imidazolin-2-ylidene] (b) or IMes [N,N0 -dimesitylimidazolin-2ylidene] (c), at room temperature in the presence of a base (KOtBu) affords the clusters [PtOs3(CO)12(NHC)] (199), [Pt2Os3 (CO)12(NHC)2] (200) and [Pt3Os3(CO)12(NHC)3] (201), together with a trace amount of the tetraosmium cluster [Os4(CO)10 (m-H)2(NHC)].199 All the clusters are characterized spectroscopically and analytically; for clusters 199a, 199c, 200a, 200c, and 201b they are also characterized by single-crystal X-ray crystallographic studies. The molecular structures of [PtOs3(CO)12(SIPr)] (199a), [Pt2Os3(CO)12(SIPr)2] (200a) and [Pt3Os3(CO)12(IPr)3] (201b) are given in Fig. 88. All the metal cores comprise a triangular

(199a)

(200a)

(201b) Fig. 88 The molecular structures of [PtOs3(CO)12(SIPr)] (199a), [Pt2Os3(CO)12(SIPr)2] (200a), [Pt3Os3(CO)12(IPr)3] (201b) [osmium (green), platinum (red), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Liu, Y.; Ganguly, R.; Huynh, H. V.; Leong, W. K. Eur. J. Inorg. Chem. 2019, 1966–1969.

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arrangement of three Os atoms, with a Pt atom bridging one to three of the OsdOs edges, respectively. In these NHC derivatives, more bridging CO ligands are present which probably reflects the greater electron donor ability of the NHCs. An increase in electron density on the metal core is known to favor carbonyl ligands adopting a bridging mode.200,201 In these clusters, Pt atoms are lying significantly out of the Os3 planes, due to the bulky aromatic substituents on the NHC ligands, which tend to occupy the space above and below the plane of the metal core to reduce steric interactions. The Pt-bridged OsdOs bonds are significantly elongated compared to the unbridged OsdOs bonds.

7.10.3.3 7.10.3.3.1

Bimetallic osmium-group 14 metal carbonyl cluster complexes Bimetallic osmium-tin carbonyl cluster complexes

The reactions of Ph3SnH with metal carbonyl cluster complexes can lead to polynuclear metal carbonyl complexes containing large numbers of SnPh2 ligands by cleavage of a phenyl group that is eliminated as benzene.202–205 Cleavage of phenyl groups from the SnPh3 ligand in Ru5(CO)11(C6H6)(SnPh3)(m-H)(m5-C) yields the complex Ru5(CO)11(C6H6)(m4-SnPh)(m3-CPh), containing the first example of a quadruply bridging SnPh ligand.202 The reaction of Os3(CO)12 with Ph3SnH at 140  C provides the compounds Os3(CO)11(SnPh3)(m-H) (202) and Os3(CO)9(m-SnPh2)3 (203) in 20% and 21% yields, respectively.205 Compound 202 is obtained in a better yield from the reaction of Os3(CO)11(NCMe) with Ph3SnH at room temperature.206 It has been found that all three phenyl groups can be cleaved from the tin atom with the formation of the “bow-tie” cluster complex Os4(CO)16(m4-Sn), which contains a naked tin atom serving as a bridge between two Os2(CO)8 groups. Single crystal X-ray diffraction analysis of 202 is performed, and its molecular structure is shown in Fig. 89. Compound 202 is formed by a loss of one CO ligand from the Os3(CO)12 group and the oxidative addition of one equivalent of Ph3SnH at the SndH bond. The compound consists of an Os3 triangle with one SnPh3 group bonded terminally to one osmium atom. It also contains one hydrido ligand that bridges the OsdOs bond next to the SnPh3 ligand. Further evidence for the hydrido ligand is obtained in its 1 H NMR spectrum, which exhibits a high-field resonance at d ¼ −18.59 ppm, showing appropriate coupling to tin. Compound 202 contains 48 cluster valence electrons; therefore, each osmium atom formally has an 18-electron configuration. Compound 203 is characterized by a combination of IR, NMR, single-crystal X-ray diffraction, and elemental analysis. The molecular structure of 203 is shown in Fig. 89. The structure of 203 consists of a central triosmium triangle with a SnPh2 ligand bridging each of the three OsdOs bonds. Each of the osmium atoms has three terminal carbonyl ligands. All of the Sn atoms lie in the plane defined by the Os3 triangle. Compound 203 is structurally very similar to the compound Os3(CO)9(m-SnMe2)3 reported by Pomeroy.207 Compound 203 also contains 48 cluster valence electrons, with each osmium atom having an 18-electron configuration. The mechanism for the formation of 203 is shown to involve species containing triphenyltin and hydrido ligands (just as in compound 202) formed by oxidative addition of the SndH bond of Ph3SnH to the metal atoms of the cluster. This is followed by the cleavage of a phenyl group from the SnPh3 ligand, combination of the phenyl group with a hydrido ligand, and reductive elimination of benzene and formation of a bridging SnPh2 ligand. Two cluster complexes, Os3(CO)12(Ph)(m3-SnPh), 204 (35% yield), and Os4(CO)16(m4-Sn), 205 (10% yield), are formed when the complex Os3(CO)11(SnPh3)(m-H), 202, is heated to reflux in toluene solvent under a CO atmosphere for 10 h.208 Benzene is observed as a coproduct from this reaction. Compound 204 is characterized by single-crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 90. The molecule contains three Os(CO)4 groups connected by a triply bridging SnPh group. Compound 204 is a precursor to 205, and it is transformed to 205 in 28% yield when a solution in octane solvent is heated to reflux under an atmosphere of CO. Biphenyl is observed as a coproduct from this reaction and is obtained in a yield (34%) that is similar to the yield of 205. Compound 205 is also characterized by a single-crystal X-ray diffraction analysis, and its molecular structure is

(202)

(203)

Fig. 89 The molecular structures of Os3(CO)11(SnPh3)(m-H) (202) and Os3(CO)9(m-SnPh2)3 (203) [osmium (green), tin (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Organometallics 2006, 25, 2049–2054.

Ruthenium and Osmium Carbonyl Cluster Complexes

(204)

619

(205)

Fig. 90 The molecular structures of Os3(CO)12(Ph)(m3-SnPh) (204) and Os4(CO)16(m4-Sn) (205) [osmium (green), tin (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L.; Organometallics 2006, 25, 4183–4187.

shown in Fig. 90. Compound 205 contains two Os2(CO)8 groups held together by a central quadruply bridging tin atom, giving an overall bow-tie structure for the five metal atoms. The four OsdSn bond distances are slightly shorter than those in 204, but are very similar to those in the compound Os3(CO)9(m-H)3(m4-Sn)Os3(CO)10(PEt2Ph)(m-H).209 When compound 202 is treated with Ph3SnH, two compounds, Os2(CO)6(m-SnPh2)2(SnPh3)2, 206, and HOs(CO)4(SnPh3), 207, are formed in 51% and 20% yields, respectively.208 Both compounds are characterized crystallographically. The structure of 206 is shown in Fig. 91. Each osmium atom contains one terminal SnPh3 ligand, and three linear terminal carbonyl ligands. The two SnPh3 ligands have an overall trans geometry with respect to the OsdOs bond. The molecule contains two bridging SnPh2 ligands positioned on opposite sides of the molecule. It is found that metal-metal bonds containing bridging SnPh2 tend to be longer than the corresponding unbridged metal-metal bonds.204,210,211 Theoretical analyses have indicated that this is due to the presence of strong MdSn bonding interactions that compete with the M-M bonding interactions.204 Compound 206 is structurally similar to the related known ruthenium compound Ru2(CO)6(m-SnMe2)2(SnMe3)2.212 The structure of 207 is shown in Fig. 91. Compound 207 contains only one osmium atom. It has a six-coordinate pseudo-octahedral coordination with four carbonyl ligands, and one SnPh3 ligand and a hydrido ligand that are positioned cis to one another. The hydrido ligand is located and refined crystallographically. Its resonance in the 1H NMR spectrum exhibits the characteristic high-field shift, d ¼ −8.94 ppm, with two bond couplings to tin. Compounds 206 and 207 are both obtained also from the reaction of 204 with HSnPh3. Indeed, the opening of the Os3 triangle to form 204 may be the first step in the fragmentation process that leads to 206 and 207 in the reaction of 204 with HSnPh3. These compounds show a sequence of steps for the incorporation of a naked tin into the tetraosmium complex 205. Even though cluster fragmentation does occur in this reaction sequence, the nature of tin-carbon bond cleavages may be relevant to the incorporation of tin atoms into higher nuclearity clusters and transition metal nanoparticles from organotin reagents in general.213–216 The facile

(206)

(207)

Fig. 91 The molecular structures of Os2(CO)6(m-SnPh2)2(SnPh3)2 (206) and HOs(CO)4(SnPh3) (207) [osmium (green), tin (purple), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L.; Organometallics 2006, 25, 4183–4187.

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Ruthenium and Osmium Carbonyl Cluster Complexes

(208) Fig. 92 The molecular structure of [Os3(CO)9(m-SPh)(m-SnPh2)(MeCN)(1-C6H5)] (208) [osmium (green), tin (purple), nitrogen (light blue), carbon (gray), oxygen (orange), sulfur (yellow)]. Modified from Kabir, S. E.; Raha, A. K.; Hassan, M. R.; Nicholson, B. K.; Rosenberg, E.; Sharmin, A.; Salassa, L. Dalton Trans., 2008, 4212–4219.

cleavage of phenyl rings from phenyltin ligands may prove to be an effective route to families of metal carbonyl cluster complexes containing naked bridging tin atoms. Tin-containing cluster complexes are now proving to be useful precursors to supported bi- and trimetallic heterogeneous catalysts.49,217 The reaction of [Os3(CO)10(MeCN)2] with Ph3SnSPh in refluxing benzene yields the bimetallic complex [Os3(CO)9(m-SPh) (m-SnPh2)(MeCN)(1-C6H5)] (208) in 27% yield.121 Compound 208 is characterized by a combination of spectroscopic data and single crystal X-ray diffraction analyses, and its molecular structure is shown in Fig. 92. There is a planar array for the metal atom and one of the OsdOs bonds is bridged by the PhS ligand which is longer than the unbridged OsdOs bond. One of the Os atoms is attached to both terminal phenyl group and MeCN ligand. The closest analogue for this cluster is [Os3(m3-SnCl2)(m-CH2)(CO)11], reported by Geoffroy’s group.218 This has a SnCl2 in place of the Ph2Sn in 208, and a bridging methylene instead of a m-SPh, but is otherwise similar. In addition to the phenyl proton resonances, the 1H NMR spectrum of 208 contains a singlet at d ¼ 1.69 ppm assigned to the methyl protons of the coordinated MeCN ligand. The reaction of [Os3(CO)10(NCMe)2] with pySSnPh3 in refluxing benzene affords the triosmium-tin cluster [Os3(CO)9(SnPh3) (m3-pyS)] (209) in 41% yield.219 Compound 209 is characterized by a combination of IR, NMR, mass spectroscopy and single crystal X-ray diffraction analyses, and its solid-state molecular structure is shown in Fig. 93. The molecule contains a triosmium core with three distinctly different OsdOs interactions ligated by nine carbonyls, a triphenyltin and a pyridine-2-thiolate ligand. The triphenyltin ligand occupies an equatorial position on Os with the OsdSn bond distance very similar to those found in related triosmium clusters. The OsdN and OsdS distances are within the usual ranges.220 Compound 209 is a 48-electron cluster with three metal-metal bonds, considering the pyridine-2-thiolate ligand serves as five-electron donor. The aromatic region of the 1H

(209)

(210)

Fig. 93 The molecular structures of [Os3(CO)9(SnPh3)(m3-pyS)] (209) and [Os3(CO)7(SnPh3){m-Ph2PCH2P(Ph)C6H4}(m-pyS)(m-H)] (210) [osmium (green), tin (purple), nitrogen (light blue), phosphorus (light violet), carbon (gray), oxygen (orange), sulfur (yellow)]. Modified from Raha, A. K.; Ghosh, S.; Hossain, I.; Kabir, S. E.; Nicholson, B. K.; Hogarth, G.; Salassa, L. J. Organomet. Chem. 2011, 696, 2153–2160.

Ruthenium and Osmium Carbonyl Cluster Complexes

621

(211) Fig. 94 The molecular structure of [H2Os3(CO)7(m-SnPh2)2(m-dppm)] (211) [osmium (green), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Sarker, J. C.; Uddin, K. M.; Rahman, M. S.; Ghosh, S.; Siddiquee, T. A.; Tocher, D. A.; Richmond, M. G.; Hogarth, G.; Kabir, S. E. Inorg. Chim. Acta 2014, 409, 320–329.

NMR spectrum shows two doublets and two triplets with a relative intensity of 1:1:1:1 due to the ring protons of pyridine-2-thiolate ligand and two multiplets integrated to 15H for the phenyl protons of the triphenyltin ligand. The reaction of [Os3(CO)8 {m3-PPh2CH2P(Ph)C6H4}(m-H)] with pySSnPh3 in refluxing toluene results in the formation of another Os-Sn cluster [Os3(CO)7(SnPh3){m-Ph2PCH2P(Ph)C6H4}(m-pyS)(m-H)] (210) in 38% yield.219 Compound 210 is characterized by a combination of spectroscopic data and an X-ray crystal structure, and its molecular structure is shown in Fig. 93. It consists of an open triosmium cluster with two quite different OsdOs interactions. The triphenyltin ligand is equatorially bonded to osmium. The deprotonated bis-diphenylphosphino methane (dppm) ligand spans the OsdOs edge and it is equatorially bonded to osmium using one phosphorus atom. The 31P{1H}NMR spectrum shows two resonances at d ¼ −5.5 and −28.5 ppm indicating the presence of two non-equivalent phosphorus atoms in 210. The reaction of unsaturated cluster [(m-H)2Os3(CO)8(m-dppm)] with excess Ph3SnH in refluxing toluene affords the electronprecise distannylene dihydride cluster [H2Os3(CO)7(m-SnPh2)2(m-dppm)] (211) in 35% yield.221 The molecular structure of 211 is shown in Fig. 94. The structure consists of an equilateral triangle of osmium atoms with three elongated and approximately equal OsdOs bond lengths. These are comparable to the OsdOs distances in [Os3(CO)9(m-SnPh2)3]205 but significantly different from those in [Os3(CO)8(m-SnR2)2(m-dppm)] [where R ¼ CH(SiMe3)2].222 One osmium center has three terminal carbonyls, while the other two osmium centers each bind two carbonyls. The two hydrides are found to be terminally bonded to the osmium centers. The two bridging diphenlstannylene groups each ligates one of the two remaining OsdOs edges. Cluster 211 is electron-precise and therefore exhibits a 48-electron count, assuming the two bridging stannylene ligands function as two-electron donor ligands.

7.10.3.3.2

Bimetallic osmium-germanium carbonyl cluster complexes

Multiple addition of germanium-containing ligands to polynuclear metal carbonyl clusters can be achieved upon treatment with organo-germanium hydrides such as Ph3GeH. Thermal reactions between Os3(CO)12 and organo-germanium hydrides often result in the formation of products by cluster degradation. In order to prevent cluster fragmentation during the incorporation of organo-germanium hydrides into triosmium clusters, bis(diphenylphosphino)methane (dppm) substituted triosmium clusters Os3(CO)10(m-dppm) are used. It has been established that the dppm ligand stabilizes the trimetallic core from degradation even under severe conditions.223–226 The reaction of Os3(CO)10(m-dppm) with two equivalents of Ph3GeH at 110  C yields the OsdGe cluster compound Os3(CO)9(GePh3)(m-dppm)(m-H) (212) in 83% yield.227 Cluster 212 results from the oxidative-addition of the GedH bond to the 46-electron cluster Os3(CO)9(m-dppm) generated in situ upon decarbonylation of the parent cluster. Cluster 212 is characterized by a combination of analytical and spectroscopic data together with single crystal X-ray diffraction analysis, and its molecular structure is shown in Fig. 95. The molecule consists of a triosmium core with three distinctly different OsdOs bonds. A hydride atom bridges the longest OsdOs edge, and lying approximately within the triosmium plane and the Ph3Ge ligand occupies an equatorial site on the remote osmium, and lies cis to the bridging hydride. The 1H NMR spectrum shows a high field doublet of doublets at -18.53 ppm confirming that the hydride spans a non-diphosphine bridged OsdOs vector. Compound 212 is transformed into the 46-electron cluster Os3(CO)7(GePh3)(m3-Ph2PCH2P(Ph)C6H4)(m-H)2 (213) in 80% yield upon prolonged thermolysis in boiling toluene. This results from loss of two carbonyl ligands and concomitant CdH activation of one of the phenyl rings of the diphosphine. Cluster 213 is characterized by a combination of analytical and

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(212)

(213)

Fig. 95 The molecular structures of Os3(CO)9(GePh3)(m-dppm)(m-H) (212) and Os3(CO)7(GePh3)(m3-Ph2PCH2P(Ph)C6H4)(m-H)2 (213) [osmium (green), germanium (purple), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Haque, M. R.; Hossain, M. J.; Rahaman, A.; Ghosh, S.; Kabir, S. E.; Hogarth, G.; Tocher, D. A. J. Organomet. Chem. 2016, 812, 240–246.

spectroscopic data together with a single crystal X-ray diffraction analysis and the molecular structure of 213 is shown in Fig. 95. The phosphorus atom bonded to osmium atom now moves to an axial site in order to facilitate CdH activation of a phenyl group which asymmetrically bridges the shortest OsdOs edge using a carbon atom. This edge is also bridged by a hydride lying on the opposite face of the metallic plane with respect to the metallated phenyl ring. The Ph3Ge ligand occupies an axial site on osmium, probably to avoid steric crowding between the phenyl rings. The C-H activated cluster Os3(CO)8(m3-Ph2PCH2P(Ph)C6H4)(m-H) reacts with Ph3GeH in boiling CH2Cl2 to afford 212 and the cluster Os3(CO)8(GePh3)2(m-dppm)(m-H)2 (214) in 16 and 26% yields respectively. Cluster 214 is formed by oxidative-addition of two GedH bonds to Os3(CO)8(m3-Ph2PCH2P(Ph)C6H4)(m-H) with concomitant reductive-elimination of the metallated phenyl ring. A single crystal X-ray diffraction analysis of 214 is carried out and the molecular structure is shown in Fig. 96. The cluster core is comprised of a metal triangle of osmium which is ligated by eight carbonyls, two Ph3Ge ligands, a dppm and two hydrides. The longer edges of the metal triangle is spanned by the hydrides while the dppm bridges the shortest OsdOs edge. Both Ph3Ge ligands occupy equatorial sites of the corresponding osmium and lay mutually trans with respect to OsdOs vector to minimize the steric crowding between their phenyl rings. The reaction of Os3(CO)10(NCMe)2 with HGePh3 yields the cluster compounds Os3(CO)10(NCMe)(GePh3)(m-H) (215) and Os3(CO)10(GePh3)2(m-H)2 (216) by the sequential replacement of the NCMe ligands and the oxidative addition of the GeH bonds of one and two HGePh3 molecules to the osmium atoms of the cluster.228 Compound 215 is converted to 216 by the reaction with

(214) Fig. 96 The molecular structure of Os3(CO)8(GePh3)2(m-dppm)(m-H)2 (214) [osmium (green), germanium (purple), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Haque, M. R.; Hossain, M. J.; Rahaman, A.; Ghosh, S.; Kabir, S. E.; Hogarth, G.; Tocher, D. A. J. Organomet. Chem. 2016, 812, 240–246.

Ruthenium and Osmium Carbonyl Cluster Complexes

(215)

623

(216)

Fig. 97 The molecular structures of Os3(CO)10(NCMe)(GePh3)(m-H) (215) and Os3(CO)10(GePh3)2(m-H)2 (216) [osmium (green), germanium (purple), nitrogen (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Kan, Y.; Zhang, Q. Organometallics 2012, 31, 8639−8646.

an additional quantity of HGePh3. Both compounds are characterized by IR, 1H NMR, mass spectra, and single-crystal X-ray diffraction analyses. The molecular structure of 215 is shown in Fig. 97. The structure of compound 215 consists of a closed triangular cluster of three osmium atoms. There is one GePh3 ligand coordinated to osmium. The GePh3 ligand lies in an equatorial position, in the plane of the Os3 triangle. There is one hydride ligand that bridges the OsdOs bond and one NCMe ligand that occupies an axial coordination site on osmium. The hydride ligand exhibits a high-field shift in the 1H NMR spectrum (d ¼ −16.10 ppm). The molecular structure of 216 is shown in Fig. 97. Like 215, the structure of compound 216 consists of a closed triangular cluster of three osmium atoms, but it has two GePh3 ligands on adjacent osmium atoms and two hydrido ligands that bridge neighboring OsdOs bonds. Both GePh3 ligands occupy equatorial positions, in the plane of the Os3 triangle. The two hydride-bridged OsdOs bonds are significantly longer than the OsdOs bond that does not have a bridging hydride ligand. The 1H NMR spectrum of 216 exhibits a single high-field resonance for the two inequivalent hydride ligands at room temperature at d ¼ −17.05 ppm. When a solution of 215 in hexane solvent is heated to reflux for 4 h, the higher nuclearity pentaosmium complex Os5(CO)17(m-GePh2) (217) is obtained in low yield (9.3%). Compound 217 is characterized by a single-crystal X-ray diffraction analysis, and the molecular structure is shown in Fig. 98. Compound 217 contains five osmium atoms arranged in a planar raft-like structure with one GePh2 ligand that bridges the OsdOs bond. Compound 217 contains a total of 76 valence electrons on the metal atoms, which is in accord with the 18-electron rule for a cluster of five metal atoms having seven metal-metal bonds. The reaction of

(217)

(218)

Fig. 98 The molecular structures of Os5(CO)17(m-GePh2) (217) and Os3(CO)10(m-Z2-O]COGePh3)(m-OMe) (218) [osmium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from (a) Adams, R. D.; Kan, Y.; Zhang, Q. Organometallics 2012, 31, 8639− 8646 and (b) Adams, R. D.; Chen, M.; Trufan, E. J. Organomet. Chem 2011, 696, 2894–2898.

624

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(219)

(220)

Fig. 99 The molecular structures of Os4(m4-GePh)2(m-GePh2)3(m-CO)(CO)8 (219) and Os4(m4-GePh)2(m-GePh2)4(CO)8 (220) [osmium (green), germanium (purple), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Boswell, E. M.; Captain, B.; Patel, M. A. Inorg. Chem. 2007, 46, 533− 540.

Ph3GeOH with Os3(CO)12 in the presence of [Bu4N]OH in methanol solvent gives the compound Os3(CO)10(m-Z2-O]COGePh3) (m-OMe) (218) in 7.3% yield.229 Compound 218 is characterized by a combination of IR, 1H NMR, mass spec. and single-crystal X-ray diffraction analyses and the molecular structure is shown in Fig. 98. The molecule consists of a triangular cluster of three osmium atoms with one bridging methoxyl ligand and one bridging triphenylgermylcarboxylate ligand m-O]COGePh3. There are ten linear terminal carbonyl ligands and two OsdOs bonds. The methoxyl ligand that bridges the same pair of metal atoms as the m-O]COGePh3 ligand in 218 is obviously derived from the MeOH solvent. The reaction of H4Os4(CO)12 with Ph3GeH at 175  C yields two tetraosmium cluster complexes, Os4(m4-GePh)2(m-GePh2)3 (m-CO)(CO)8 (219) and Os4(m4-GePh)2(m-GePh2)4(CO)8 (220) in 28% and 2% yield, respectively.125 Compounds 219 and 220 are characterized by a combination of IR, 1H NMR, single-crystal X-ray diffraction, and mass spectral analyses. The molecular structure of 219 is shown in Fig. 99. Compound 219 consists of a trapezoidal arrangement of four osmium atoms with three bridging GePh2 and one bridging CO. There are also two quadruply bridging GePh ligands. Each osmium atom contains two terminally coordinated CO ligands. The OsdOs bond that is bridged by the CO is significantly shorter than the OsdOs bonds that are bridged by GePh2 ligands. The molecular structure of 220 is shown in Fig. 99. Compound 220 contains four osmium atoms in approximately a square arrangement. There are two quadruply bridging GePh ligands and four bridging GePh2 ligands. Each osmium atom contains two terminal CO ligands. All four OsdOs bond lengths in 220 are similar to each other. Both of these compounds contain four metal atoms and has a total of 62 cluster valence electrons.

7.10.3.4

Trimetallic osmium-platinum-tin carbonyl cluster complexes

Os3(CO)9(m-SnPh2)3 (203) reacts with Pt(PBu3t)2 at 68  C to afford a trimetallic osmium-platinum-tin carbonyl cluster complex Os3(CO)9[Pt(PBu3t)](m-SnPh2)3 (221) in 29% yield by adding a Pt(PBu3t) group across one of the OsdSn bonds.205 Compound 221 is characterized by a combination of IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses, and its molecular structure is shown in Fig. 100. Compound 221 retains the structure of the Os3Sn3 core of the cluster found in 203 but includes a Pt(PBu3t) group that bridges one of the OsdSn bonds. Compound 221 can be viewed as a monoplatinum adduct of compound 203, as there is no loss of a carbonyl group from the starting cluster 203. The 31P{1H} NMR spectrum for 221 appropriately shows one-bond coupling to platinum and two-bond coupling to tin. The osmium and tin atoms all lie in the same plane, while the platinum atom is displaced slightly out of the plane. The OsdPt bond distance is shorter than the similar carbonyl-bridged OsdPt bonds in compounds 182, 183, and 184. This could be because the Pt(PBu3t) group is bonded to only one osmium atom instead of two, as found in 182, 183, and 184, and the PtdSn interaction in 221 is weak. The PtdSn distance in 221 is significantly longer than the PtdSn distance found in the compound HPt(PPh3)2(SnPh3) (2.564(1) A˚ )230 but is still consistent with a significant PtdSn interaction. Compound 203 reacts with Pt(PPh3)4 at 40  C to afford another trimetallic osmium-platinum-tin carbonyl cluster complex Os3(CO)9[Pt(Ph)(PPh3)2](m-SnPh2)2(m3-SnPh) (222), in 21% yield by insertion of a Pt(PPh3)2 into one of the SndC bonds to one of the phenyl groups.205 Compound 222 is characterized by a combination of IR, 1H and 31P NMR, single-crystal X-ray diffraction, and elemental analyses, and its molecular structure is shown in Fig. 100. Compound 222 contains a planar Os3Sn3 cluster with nine carbonyl ligands arranged just as in 203 and 221. A Pt(PPh3)2 group is added to 203 to form 222, but unlike the formation of 221, the platinum atom is inserted into one of the SndC bonds to one of the phenyl groups of one of the bridging SnPh2 ligands. Including the two phosphine ligands, the platinum atom has four substituents. Its coordination geometry is approximately square

Ruthenium and Osmium Carbonyl Cluster Complexes

(221)

625

(222)

Fig. 100 The molecular structures of Os3(CO)9[Pt(PBu3t)](m-SnPh2)3 (221) and Os3(CO)9[Pt(Ph)(PPh3)2](m-SnPh2)2(m3-SnPh) (222) [osmium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Organometallics 2006, 25, 2049–2054.

planar. The two phosphorus ligands are inequivalent, and as expected, the 31P{1H} NMR spectrum shows two resonances at d ¼ 34.8 and 15.5 ppm with appropriate coupling to 195Pt and 31P. Attempts to produce additional transformations of 222 are performed by heating a solution in heptane to reflux for 1 h, but surprisingly the principal product is simply its precursor 203 (52% yield), indicating that the insertion of Pt(PPh3)2 into the SndC bond is largely reversible. Pidcock and Schubert have shown that Pt(phosphine) groups can be readily inserted into SndC bonds, but they do not report on their reversibility.231–233 The main difference between the reactions of Pt(PBu3t)2 and Pt(PPh3)4 with 203 is that Pt(PBu3t)2 loses one PBu3t ligand and adds a Pt(PBu3t) group across one of the OsdSn bonds, but while Pt(PPh3)4 loses two PPh3 ligands, it still retains two PPh3 ligands coordinated to the Pt atom and is thus more sterically crowded than a Pt(PBu3t) group. In the presence of Pt(PBu3t), there is a rapid reaction between HOs(CO)4(SnPh3) (207) and PhC2H at room temperature to yield the trimetallic complex PtOs(CO)4(SnPh3)(PBu3t)[m-HC2(H)Ph] (223).234 The molecular structure of 223 as determined crystallographically is shown in Fig. 101. Compound 223 contains a Pt(PBu3t) group bonded to the osmium atom by the platinum atom. The phenylalkenyl ligand, HC2(H)Ph, is formed by the insertion of the PhC2H into the OsdH bond with transfer of the hydrogen atom to the phenyl-substituted carbon atom of the alkyne. It is s-bonded to the platinum atom and p-bonded to the osmium atom. The E-stereochemistry of the alkenyl ligand is consistent with cis insertion of the alkyne via the classical four-center transition state.235,236 To understand the nature of the promotional effect of the platinum on this reaction, the reaction of 207 with Pt(PBu3t) is investigated in the absence of added PhC2H. In this case, the compound PtOs(CO)4(SnPh3)(PBu3t)(m-H), 224 is formed in 45% yield.234 A diagram of the molecular structure of 224 as determined crystallographically is shown in Fig. 101. Compound 224 is formed by the direct addition of a Pt(PBu3t) group to the OsdH bond of 207. The hydrido ligand has assumed a bridging position between the osmium atom and the platinum atom and one CO ligand from the Os(CO)4 group has been shifted into a bridging

(223)

(224)

Fig. 101 The molecular structures of PtOs(CO)4(SnPh3)(PBu3t)[m-HC2(H)Ph] (223) and PtOs(CO)4(SnPh3)(PBu3t)(m-H) (224) [osmium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange), hydrogen (blue)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. J. Am. Chem. Soc. 2006, 128, 13672–13673.

626

Ruthenium and Osmium Carbonyl Cluster Complexes

position between the two metal atoms. Notably, the OsdH bond is longer and presumably weaker than the OsdH bond in 207.208 Compound 222 formally contains an unusual heteropolar PtdOs metal-metal bond, formed by donation of a pair of electrons from the 18 electron osmium atom in 207 to the 12-electron platinum of the Pt(PBu3t) group. Most importantly, when 224 is treated with PhC2H, it is converted to 223 in 28% yield. In this study, it is shown how the addition of the electronically unsaturated metal-containing molecular fragment, Pt(PBu3t), promotes an important reaction both by activating a selected bond (OsdH) of one metal complex and then by facilitating the addition and transformation of a selected reagent (PhC2H) at that bond. The two metals working together are able to accomplish what the one metal atom, osmium, could not do alone. It should also apply to other types of insertion reactions and quite possibly other related metal-ligand fragments, such as Pd(PBu3t).237–239 Two products PtOs3(CO)12(PBu3t)(Ph)(m3-SnPh) (225) and Pt2Os3(CO)12(PBu3t)2(m2-Ph)(m3-SnPh) (226) are obtained in the yields 44% and 9%, respectively, from the reaction of Os3(CO)12(Ph)(m3-SnPh) (204) with Pt(PBu3t) when combined in a 1:1 ratio at room temperature.240 When the amount of Pt(PBu3t)2 is increased to four times the amount of 204, the yield of 226 is increased to 40% at the expense of 225, yield of 15%. Compounds 225 and 226 are both characterized by single-crystal X-ray diffraction analysis. The molecular structure of 225 is shown in Fig. 102. The molecule is structurally similar to that of 202 except that it contains a Pt(PBu3t) group that has been added across the OsdOs bond that connects the two bonded Os(CO)4 groups. The CO ligands on the two platinum bonded osmium atoms have adopted semibridging coordination to the platinum atom. As in 204, there is a phenyl group s-bonded to the third Os(CO)4 group. The OsdSn distance to the phenyl substituted osmium atom is the longest of all, but is similar to that in 204.208 The molecular structure of 226 is shown in Fig. 102. The molecule is structurally similar to that of 225, except that there is a second Pt(PBu3t) group that has been added across the OsdC bond to the s-bonded phenyl group. As in 225, two of the CO ligands on the osmium atoms have adopted semibridging coordination to the platinum atom. Interestingly, the second platinum atom bridges the OsdC bond to the s-bonded phenyl group. The OsdC bond distance to the s-bonded phenyl group is slightly shorter than that in 225. This may be due to the presence of the bridging platinum atom. When 226 is treated with PBu3t, the platinum phosphine group that bridges the Os-C (phenyl) bond is removed and the compounds 225 and PtOs3(CO)11(PBu3t)2(Ph)(m3-SnPh) (227) are formed in the yields 30% and 15%, respectively.240 The molecular structure of 227 is shown in Fig. 103. Compound 227 is structurally similar to 225 but has a PBu3t ligand trans to the OsdSn bond in place of one of the CO ligands on the osmium atom. The OsdC distance to the s-bonded phenyl group, is slightly longer than that in 225. Os4(CO)16(m4-Sn) (205) reacts with Pt(PBu3t)2 to form a bis-Pt(PBu3t) adduct, Os4(CO)16[Pt(PBu3t )]2(m4-Sn) (228) in a good yield (43%). The molecular structure of 228 is shown in Fig. 103. When 205 is treated with Pt(PBu3t)2, Pt(PBu3t) groups are added to the OsdOs bond of each Os2(CO)8 group to yield 228. The Os4Sn core of 228 is similar to that of its

(225)

(226)

Fig. 102 The molecular structures of PtOs3(CO)12(PBu3t)(Ph)(m3-SnPh) (225) and Pt2Os3(CO)12(PBu3t)2(m2-Ph)(m3-SnPh) (226) [osmium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Inorg. Chem. 2007, 46, 4605− 4611.

(227)

(228) t

t

Fig. 103 The molecular structures of PtOs3(CO)11(PBu3 )2(Ph)(m3-SnPh) (227) and Os4(CO)16[Pt(PBu3 )]2(m4-Sn) (228) [osmium (green), platinum (red), tin (purple), phosphorus (light blue), carbon (gray), oxygen (orange)]. Modified from Adams, R. D.; Captain, B.; Zhu, L. Inorg. Chem. 2007, 46, 4605− 4611.

Ruthenium and Osmium Carbonyl Cluster Complexes

627

precursor 205 and contains a spiro-structure with the Sn atom in the center. A Pt(PBu3t) group is added across each OsdOs bond in 228. The PtdOs bond distances are nearly equal and similar in length to those in compounds 225–227. As found in 225–227, one CO ligand on each neighboring osmium atom has formed a bridging interaction to the platinum atom. The OsdSn bond distances are very similar to those in 205. This study reaffirms the strong affinity of the Pt(PBu3t) group for OsdOs bonds. Any products with PtdSn interactions are not observed. The first Pt(PBu3t) addition occurs at the OsdOs bond of the Os2(CO)8 group to yield 225. Interestingly, the addition of a Pt(PBu3t) group to an OsdC bond is observed, and, in this case to the s-bonded phenyl group in 226. Pt(PBu3t) additions to MdC bonds have not been seen before and can lead to new chemistry. For example, the addition of a Pt(PBu3t) group to the OsdH bond of the compound HOs(CO)4(SnPh3) is observed to yield the adduct PtOs(CO)4(SnPh3)(PBu3t)(m-H), 224.234 It is found that the Pt(PBu3t) group in 224 facilitates the insertion of phenylacetylene in the OsdH bond. It is also found that Pt(PBu3t) groups can facilitate the substitution of CO ligands by phosphine ligands.191 This effect appears to be operative in the chemistry of 226 because compound 227, which contains a PBu3t ligand on the phenyl-substituted carbon atom, is obtained from the reaction of 226 with PBu3t in the course of the removal of the Pt(PBu3t) group. Interestingly, the Pt(PBu3t) group that bridges the OsdOs bond in 226 is not removed in the treatment with PBu3t.

7.10.4

Conclusions

The study of metal carbonyl clusters remains a frontier area in inorganic chemistry. New and exciting results continue to emerge and solidify an area which remains in the forefront of catalytic science. Electronic unsaturation in metal complexes plays a crucial role in their reactivity towards other molecules and the nature and reactivity of vacant sites in high nuclearity metal clusters has expanded in the last fifteen years as can be seen in the original references cited in this review. Transition metal complexes have and always will play a key part in catalytic chemistry. One of the best ways to induce electronic unsaturation in metal complexes is by using sterically bulky ligands which occupy large spatial area around the metal center. Another is the use of weak, or “token” ligands that readily dissociate. The ability to design and characterize discrete clusters with precise control of metal content as precursors to catalysts can provide fundamental background information to guide the search for more active and improved catalysts. Industrial research centers focus on heterogeneous catalysis and have largely abandoned homogeneous and even cluster systems. However, this ignores the fact that principles of reactivity for mononuclear catalysis were learned from mononuclear systems primarily by the hard work of step-by-step elucidation of the now classic understanding of ligand substitution, oxidative addition, migratory insertion and reductive elimination. Understanding of these processes is now taken for granted for mononuclear complexes but still remains largely unexplored for polynuclear cluster systems. The focus of industrial research on finding methods that work is balanced by academic research focused on how and why, exactly, these systems function. It is often the case that synthetic advances and new chemistry are discovered as part of fundamental studies, and subsequent to that can be put to use for applications in catalysis. Thus, numerous avenues of research may be envisioned based upon metal cluster systems and its variants in the years to come. This review is dedicated to the pioneering scientists whose work on clusters is acknowledged with the added hope that researchers interested in this area will be inspired by their studies and go on to add new direction, vision, and discovery in this beautiful area of cluster chemistry.

Acknowledgments Sumit Saha wishes to thank Prof. Suddhasatwa Basu, Director, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar, India, for in-house financial support (Grant number: CSIR-IMMT-OLP-112) and requisite permissions.

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E.; Pervin, S.; Raha, A. K.; Hossain, G. M. G.; Haworth, D. T.; Lindeman, S. V.; Siddiquee, T. A.; Bennett, D. W.; Salassa, L.; Roesky, H. W. J. Chem. Soc., Dalton Trans. 2009, 3510. 221. Sarker, J. C.; Uddin, K. M.; Rahman, M. S.; Ghosh, S.; Siddiquee, T. A.; Tocher, D. A.; Richmond, M. G.; Hogarth, G.; Kabir, S. E. Inorg. Chim. Acta 2014, 409, 320–329. 222. Bartlett, R. A.; Cardin, C. J.; Cardin, D. J.; Lawless, G. A.; Power, J. M. J. Chem. Soc., Chem. Commun. 1988, 312. 223. Hassan, M. R.; Hogarth, G.; Hossain, G. M. G.; Kabir, S. E.; Raha, A. K.; Saha, M. S.; Tocher, D. A. Organometallics 2007, 26, 6473–6480. 224. Raha, A. K.; Ghosh, S.; Hossain, I.; Kabir, S. E.; Nicholson, B. K.; Hogarth, G.; Salassa, L. J. Organomet. Chem. 2011, 696, 2153–2160. 225. Sarker, J. C.; Uddin, K. M.; Rahman, M. S.; Ghosh, S.; Siddiquee, T. A.; Tocher, D. A.; Richmond, M. G.; Hogarth, G.; Kabir, S. E. Inorg. Chim. Acta 2014, 409, 320–329. 226. Kabir, S. E.; Hogarth, G. Coord. Chem. Rev. 2009, 253, 1285–1315. 227. Haque, M. R.; Hossain, M. J.; Rahaman, A.; Ghosh, S.; Kabir, S. E.; Hogarth, G.; Tocher, D. A. J. Organomet. Chem. 2016, 812, 240–246. 228. Adams, R. D.; Kan, Y.; Zhang, Q. Organometallics 2012, 31, 8639–8646. 229. Adams, R. D.; Chen, M.; Trufan, E. J. Organomet. Chem. 2011, 696, 2894–2898. 230. Latif, L. A.; Eaborn, C.; Pidcock, A.; Ng, S. W. J. Organomet. Chem. 1994, 474, 217. 231. Butler, G.; Eaborn, C.; Pidcock, A. J. Organomet. Chem. 1981, 185, 367.

Ruthenium and Osmium Carbonyl Cluster Complexes 232. 233. 234. 235. 236. 237. 238. 239. 240.

Butler, G.; Eaborn, C.; Pidcock, A. J. Organomet. Chem. 1979, 181, 47. Gilges, H.; Schubert, U. E. J. Inorg. Chem. 1998, 897. Adams, R. D.; Captain, B.; Zhu, L. J. Am. Chem. Soc. 2006, 128, 13672–13673. Li, X.; Vogel, T.; Incarvito, C. D.; Crabtree, R. H. Organometallics 2005, 24, 62. Selmeczy, A. D.; Jones, W. D. Inorg. Chim. Acta 2000, 138, 300–302. Adams, R. D.; Captain, B.; Fu, W.; Hall, M. B.; Manson, J.; Smith, M. D.; Webster, C. E. J. Am. Chem. Soc. 2004, 126, 5253. Adams, R. D.; Captain, B.; Smith, M. D. J. Cluster Sci. 2004, 15, 139. Adams, R. D.; Captain, B.; Zhu, L. J. Cluster Sci. 2006, 17, 87. Adams, R. D.; Captain, B.; Zhu, L. Inorg. Chem. 2007, 46, 4605–4611.

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N-Heterocyclic Carbene Complexes of Cobalt

Thomas Simlera,∗, Andreas A Danopoulosa,b, and Pierre Braunsteinb, aLaboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Institut Polytechnique de Paris, Palaiseau, France; bUniversité de Strasbourg, CNRS, Institut de Chimie UMR 7177, Laboratoire de Chimie de Coordination, Strasbourg, France © 2022 Elsevier Ltd. All rights reserved.

7.11.1 General introduction 7.11.2 NHC cobalt complexes 7.11.2.1 Mononuclear Co0 complexes 7.11.2.1.1 Monodentate carbene ligands 7.11.2.2 Mononuclear Co-I complexes 7.11.2.2.1 Monodentate carbene ligands 7.11.2.3 Mononuclear CoII complexes 7.11.2.3.1 Monodentate carbene ligands 7.11.2.3.2 Bidentate bis-carbene ligands 7.11.2.3.3 Tridentate tris-carbene ligands 7.11.2.3.4 Tetradentate tetra-carbene ligands 7.11.2.3.5 Functionalized NHC complexes 7.11.2.4 Mononuclear CoI complexes 7.11.2.4.1 Monodentate carbene ligands 7.11.2.4.2 Functionalized NHCs 7.11.2.5 Mononuclear CoIII complexes 7.11.2.5.1 Monodentate carbene ligands 7.11.2.5.2 Tris-carbenes complexes 7.11.2.5.3 Functionalized NHCs 7.11.2.6 Mononuclear CoIV and CoV complexes 7.11.2.6.1 Monodentate carbene ligands 7.11.2.6.2 Functionalized carbene ligands 7.11.2.7 Polynuclear homometallic complexes 7.11.2.7.1 Binuclear complexes 7.11.2.7.2 Tetranuclear complexes 7.11.2.8 Polynuclear heterometallic complexes 7.11.3 General conclusion Acknowledgments References

634 635 635 635 645 645 652 652 665 665 666 667 693 693 704 721 721 723 724 739 739 740 741 741 746 746 755 755 755

Abbreviations 3c-2e acac ACR Ad aNHC Ar aver. BArF4 [B(cat)]2 BD(F)E BHT-H [B(pin)]2 cAAC Cp Cp CPET Cy ∗

Three-center two-electron Acetylacetonate Amino-carbene Adamantyl Abnormal (mesoionic) N-heterocyclic carbene Aryl Average Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate Bis(catecholato)diboron Bond dissociation (free) energy 2,6-Di-tert-butyl-4-methylphenol Bis(pinacolato)diboron Cyclic (alkyl)(amino)carbene C5H5 (cyclopentadienyl) C5Me5 (pentamethylcyclopentadienyl) Concerted proton-electron transfer Cyclohexyl

Current address: Laboratoire de Chimie Moléculaire, CNRS, Ecole polytechnique, Institut Polytechnique Paris, Palaiseau, France.

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Cy

cAAC IDep Cy IPr dc Dep DFT DiPP DiPP nacnac DMF Dmp DTA/TG dvtms e EPR equiv. Et ET Et2 cAAC Fc+/Fc Fc HB(pin) Hex HOMO HS IAR IAd ICy IDep IiPr Im IMes IMes’ IMesCy IMesI
HI3 iPr IPr IPrPh IR ItBu LDA LS LUMO MCD mCPBA Me Me2 cAAC Me2 IAdMes Me2 IEt Me2 IMe Me2 IiPr Me2 ItBu MeIm Mes Mes nacnac nBu Naph nd NHC Cy

1-(2,6-Diisopropylphenyl)-3-cyclohexyl-5,5-dimethylpyrrolidin-2-ylidene 1,3-Bis(2,6-diethylphenyl)-4,5-(CH2)4-imidazol-2-ylidene 1,3-Bis(2,6-diisopropylphenyl)-4,5-(CH2)4-imidazol-2-ylidene Direct current 2,6-Diethylphenyl Density functional theory 2,6-Diisopropylphenyl HC[C(Me)N(2,6-iPr-C6H3)]2 Dimethylformamide 2,6-Dimesitylphenyl Differential thermal analysis/thermogravimetry Divinyltetramethyldisiloxane Electron Electron paramagnetic resonance Equivalent Ethyl Electron-transfer 3,3-Diethyl-5,5-dimethyl-1-(2’,6’-diisopropylphenyl)pyrrolidin-2-ylidene Ferrocenium/ferrocene Decamethylferrocene Pinacolborane Hexyl Highest occupied molecular orbital High spin Imidazolium-amido 1,3-Diadamantylimidazol-2-ylidene 1,3-Dicyclohexylimidazol-2-ylidene 1,3-Bis(2,6-diethylphenyl)imidazol-2-ylidene 1,3-Diisopropylimidazol-2-ylidene Imidazole 1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene Cyclometallated anionic IMes ligand 1-Mesityl-3-cyclohexylimidazol-2-ylidene 2-Iodo-1,3-dimesitylimizadolium triiodide Isopropyl 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene 1,3-Bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene Infrared 1,3-Di-tert-butylimidazol-2-ylidene Lithium diisopropylamide Low spin Lowest unoccupied molecular orbital Magnetic circular dichroism meta-Chloroperbenzoic acid Methyl 1-(2,6-Diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene 1-Mesityl-3-adamantyl-4,5-dimethylimidazol-2-ylidene 1,3-Diethyl-4,5-dimethylimidazol-2-ylidene 1,3,4,5-Tetramethylimidazol-2-ylidene 1,3-Diisopropyl-4,5-dimethylimidazol-2-ylidene 1,3-Di-tert-butyl-4,5-dimethylimidazol-2-ylidene 1-Methylimidazole Mesityl Supermesityl (2,4,6 tri-tert-butylphenyl) b-Diketiminate n-Butyl 1-Naphthyl Not determined N-Heterocyclic carbene

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NHDC NMR OTf Ph PT py rt SCE SIMes SIPr SMM SQUID tBu temp. TEMPO TEP THF TMEDA TMP Tol UV-vis vtms VT XANES XAS XPS Xyl n meff mB

7.11.1

Anionic N-heterocyclic dicarbene Nuclear magnetic resonance Triflate ¼ trifluoromethanesulfonate Phenyl Proton transfer Pyridine Room temperature Saturated calomel electrode 1,3-Bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene 1,3-Bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene Single-molecule magnet Superconducting quantum interference device tert-Butyl Temperature 2,2,6,6-Tetramethylpiperidinyloxyl Tolman electronic parameter Tetrahydrofuran N,N,N0 ,N0 -tetramethylethane-1,2-diamine 2,2,6,6-Tetramethylpiperidinyl Tolyl Ultraviolet-visible Vinyltetramethyldisiloxane Variable temperature X-ray absorption near edge structure X-ray absorption spectra X-ray photoelectron spectroscopy Dimethylphenyl Frequency Effective magnetic moment Bohr magneton

General introduction

The spectacular developments of N-heterocyclic carbene (NHC) transition metal complexes, that followed their availability,1–4 have a major impact in fundamental and applied chemistry, as documented in the rapidly increasing number of books and review articles dedicated to these topics.5–36 Numerous synthetic methods are now available to access late transition metal NHC complexes, especially the heavier congeners. The latter are usually air stable species comprising strong, rather inert metal-CNHC bonds and can be conveniently obtained using the popular Ag transmetallation reaction.37,38 NHCs constitute a class of versatile donor ligands with unique stereoelectronic properties39,40 that account for their ability to stabilize metal complexes displaying low coordination numbers and/or unusual oxidation states endowed with unique catalytic, physical or medicinal properties.41–43 A range of heterocyclic rings (mainly 5- and 6-membered) that can be used as precursors, and the electron-donor characteristics of NHC ligands can be fine-tuned by switching from normal (C-2) NHC coordination to abnormal and mesoionic NHCs,13,44 by moving to cyclic Alkyl-Amino Carbenes (cAACs)45,46 and taking advantage of the numerous possible access to new poly-NHC and hybrid ligand designs,19,47,48 the latter combining one or more NHC donors with other ’classic’ functional groups such as amide, alcoholate, phosphine or thiolate donors.9,36,49–58 In this Chapter, we focus on NHC cobalt complexes, but early work including studies by Lappert using electron-rich alkenes to generate the first examples of NHC-Co complexes has not been included.59–62 We provide an update, until end 2020, of a recently published comprehensive review on this topic (Chem. Rev. 2019, 119, 3730 −3961).20 To allow the reader to follow the development of this chemistry, some repetition will therefore be inevitable. Successive sections deal with mononuclear and polynuclear complexes with ascending nuclearity and ligand denticity of the NHC containing ligands (monodentate, bidentate, polydentate exclusively with NHC donors, heteroatom functionalized NHC ligands etc.). For the mononuclear complexes featuring monodentate NHC ligands, a more detailed classification is introduced according to the metal formal oxidation state, their homoleptic or heteroleptic nature and the column of the Periodic Table to which the donor atom(s) of the non-NHC ligand(s) belong. Within heteroleptic complexes, all coligands (not the NHCs) are grouped by

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adhering to the LnXm ligand classification, while the notion of the coordination number for classification purposes is relaxed, even though coordination geometries are discussed for the majority of the complexes throughout the text. Separate sections are dedicated to mononuclear complexes featuring ligands with more than one NHC donor and to complexes with ligands featuring NHC donors functionalized with heteroatom donors. Polynuclear complexes are classified according to the number of atoms involved in the bridge. The aim of the ’hybrid’ ligand-based grouping approach that is followed is advantageous in highlighting comparable patterns of structure and reactivity as a function of the type and number of ligands forming the coordination sphere. However, the outline detailed above cannot always be rigidly adhered to in view of common interconversions between complexes belonging to different classes (e.g. mononuclear vs. binuclear, ligand associations-dissociations, redox reactivity etc.); in this case, the relevant sections and Schemes are cross-referenced within the document to avoid repetition. The generic term NHC is used for all types of N-heterocyclic carbenes, irrespective of the nature of the heterocycle in which the divalent C donor is integrated. Heteroatom (B, Si etc.) NHC analogues are treated as co-ligands and therefore appear only in combination with NHCs and not in separate classes. In the description of NHCs, widely used acronyms are maintained and used throughout without assignment of additional numbering e.g. IPr, SIPr, IMes, Me2cAAC etc. (see also list of abbreviations). Sequential numbering is used throughout with Arabic numerals combined (or not) with letters occasionally accompanied by the superscripts s (for imidazolin-2-ylidenes) and u (imidazol-2-ylidenes), featuring saturated and unsaturated backbones, respectively; accordingly, the superscript u/s refers to complexes with both unsaturated or saturated imidazol(in)-2-ylidenes. The generic anion is represented as (A). We will first deal with highly reduced Co0 and Co–I complexes, the latter being generally prepared from the former, then describe complexes with the three ‘easily’ accessible oxidation states CoI, CoII and CoIII, and finally rarer highly oxidized (formally CoIV or CoV) species. Non-innocent ligands sometimes play a role in stabilizing such complexes, rendering the ‘observed oxidation state’ of the metal center ambiguous. In the classification used here, the different complexes are sorted according to their ‘formal’ oxidation state. Selected structural, magnetic and spectroscopic data for the different NHC Co complexes are compiled in Tables at the end of every subsection (Tables 1–7); characteristic IR absorption bands of selected CO, N2 and related complexes are presented in Tables 8 and 9.

7.11.2

NHC cobalt complexes

7.11.2.1

Mononuclear Co0 complexes

The first examples of Co0 complexes with NHC ligands date back to 2014.63,65 In general, Co0 NHC complexes are very reactive species and have been accessed either by ligand exchange from a Co0 precursor,74 by reduction of the corresponding NHC-CoI complexes,63,65,75 or by reduction of in situ generated NHC-CoII complexes in the presence of olefin ligands.66,71,168 The hydrogenolysis of bis(alkyl) NHC-CoII complexes in the presence of alkyne ligands has recently offered an alternative strategy for the synthesis of NHC-Co0 complexes.73 Selected structural and magnetic data of NHC-Co0 complexes are compiled in Table 1.

7.11.2.1.1

Monodentate carbene ligands

7.11.2.1.1.1 Homoleptic complexes of type [Co(NHC)2] The only examples belonging to this category correspond to the bis-cAAC complexes 1a and 1b (Scheme 1).63,65 The synthetic accessibility to the formally Co0 complex 1a was suggested by the cyclic voltammogram of the CoI precursor complex, 2a, which exhibited a quasi-reversible one-electron reduction at E1/2 ¼ –0.57 V in DMF vs. (FeCp 2)+/(FeCp 2). The synthesis of 1a was achieved in 98% yield by reduction of 2a with KC8 in THF or, alternatively, by reaction of 2a with 2.2 equiv. LDA as reducing agent. An X-ray diffraction analysis of 1a revealed a two-coordinate bent structure in the solid state. The two Me2cAAC ligands coordinated to the Co0 center form a CcAAC-Co-CcAAC angle of ca. 170.1 , resulting in a bent ‘2-metallaallene’ structure. Upon reduction of the CoI precursor, a slight shortening of the Co-CcAAC bond distances is observed (from 1.920(2) and 1.932(2) A˚ in 2a to 1.871(2) and 1.877(2) A˚ in 1a), as well as an elongation of the CcAAC-N bond; both changes were attributed to increasing p-backdonation upon reduction. The EPR spectrum of the paramagnetic 1a exhibits a broad unresolved resonance both in the solid state and in THF solution, arising from rapid relaxation in the quasi-linear structure. The results of theoretical calculations favored a doublet S ¼ ½ ground state. The CcAAC-Co-CcAAC bending was shown to lead to more effective overlap between the ligand and the Co 3d orbitals. In an alternative synthesis, 1a can be prepared in high yield and on a multigram scale by reaction of 2a with organometallic reagents such as MeLi (Scheme 1).64 The magnetic moment at room temperature (3.7(1) mB as determined by the Evans’ method) is much higher than the spin-only value of 1.73 mB for a low-spin Co0 metal center (d9, S ¼ 1/2), probably due to unquenched orbital angular momentum. The Et2cAAC analogue 1b was obtained by halide abstraction from 2b using NaBArF4, followed by reduction with one equiv. of Na/Hg (Scheme 1).65 The room-temperature solution magnetic moment of 1b (2.0 mB) is consistent with a low-spin S ¼ ½ center. A well-resolved EPR signal observed at low temperature (77 K) with clear hyperfine coupling to the 59Co nucleus suggests a metal-centered spin character. However, the electron density is partially delocalized over the cAAC ligand, as evidenced by relatively long C-N bonds (1.355(5) A˚ ). DFT calculations on 1b confirmed only partial spin delocalization on the ligand and a predominantly metal-centered spin density (60%).

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Scheme 1 Synthesis and reactivity of Co0 bis-cAAC complexes.

A rich reactivity was observed for 1a (Scheme 1) which is associated with the lability of the cAAC ligand, the coordinative unsaturation and electron-rich nature of the formal Co0 metal center.64 Reactions with organic halides, e.g. ArBr (Ar ¼ Ph, Mes) and n-C8H17X (X ¼ Br, Cl), afforded the corresponding CoI complexes [CoX(Me2cAAC)2] (X ¼ Br, Cl) in high yield through singleelectron-transfer processes involving an inner-sphere mechanism. Oxidation of 1a with [FeCp2][BArF4] cleanly afforded the CoI complex 3, which can also be accessed by treatment of 2a with Na[BArF4]. The solid-state structure of 3 revealed a perfectly linear coordination geometry (CcAAC–Co–CcAAC angle of 180.0(1) ) with a “head-to-tail” arrangement for the two Me2cAAC ligands. The magnetic moment at room temperature (4.80 mB) is consistent with that reported for two-coordinate CoI-NHC complexes (4.0–5.4 mB) and much higher than the spin-only value for an S ¼ 1 metal ion. Reaction of 1a with the diazo compounds (p-Tol)2CN2 and DmpCHN2 (Dmp ¼ 2,6-dimesitylphenyl) led to the paramagnetic complexes 4 and 5, respectively. Analysis of the crystal structure of 4 revealed a four-coordinate Co0 center surrounded by one Me2cAAC ligand and one Z6-arene azine-type ligand, the latter possibly resulting from migratory insertion of one k1-N diazo ligand into a Co-CcAAC bond. In 5, the diazo ligand displayed an end-on coordination mode with a Co-Ndiazo separation of 1.687(2) A˚ ; an analysis of the bond lengths revealed a radical anionic nature for the diazo ligand, which is consistent with the high reducing power of 1a. Reaction of 1a with 2,6-dimethylphenyl isocyanide (XylNC) led to the substitution of one Me2cAAC by three isonitrile ligands and formation of the Co0 complex 6. Formation of a ketenimine by-product may be initiated by the migratory insertion of one isonitrile ligand into a CcAAC-Co bond. In the IR spectrum, the n(C^N) stretching vibrations of the coordinated isonitriles were observed at 2049 and 1939 cm–1. Furthermore, reaction of 1a with 3-hexyne afforded the three-coordinate Co0 complex 7 bearing an Z2-alkyne ligand. The magnetic moment of 2.90 mB in the solid state at room temperature is within the range reported for other NHC-Co0-alkene complexes (2.8–3.3 mB). The X-ray diffraction analysis of 7 revealed a C2 symmetric arrangement and a C^C bond distance of 1.285 (5) A˚ for the coordinated alkyne ligand, which is longer than that in free acetylenes (ca. 1.17 A˚ ), in agreement with p-backdonation from the electron-rich Co0 metal center.64

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7.11.2.1.1.2 Heteroleptic complexes 7.11.2.1.1.2.1 Complexes of type [Co(NHC)L2] The Co0 complexes 8a,b bearing a chelating divinyltetramethyldisiloxane (dvtms) olefin ligand were prepared in good yield (66–75%) by the one-pot reaction between free NHC, CoCl2, divinyltetramethyldisiloxane (dvtms) and KC8 (Scheme 2).66,67 Alternatively, magnesium can be used as reducing agent instead of KC8, as exemplified in the synthesis of 8c.68 Following the same strategy, 9a – 9f were synthesized in moderate to good yield by reaction of the corresponding free carbenes with CoCl2 and Na/Hg or KC8 in the presence of vinyltrimethylsilane (vtms).69–71 An excess of vtms was used to maximize the isolated yields of the Co0 complexes. The magnetic moments of 8a – 8c and 9a – 9f in solution are in the range 2.7–3.3 mB, in agreement with low-spin Co0 centers, and are larger than the spin-only value for S ¼ ½ systems, probably as a result of strong spin-orbit coupling and/or the presence of trace amounts of paramagnetic impurities in the samples. The 1H NMR spectra of these paramagnetic complexes indicate that the idealized C2 symmetry observed in the solid state is retained in solution. The preferred NHC-to-vtms 1:2 ratio in 9a – 9f seems to be mainly governed by the steric properties of the NHCs, since the attempted synthesis of [Co(CyIDep)2(vtms)] only resulted in the isolation of 9e (Scheme 2). The generality of this one-pot procedure for accessing low-coordinate Co0 complexes was further exemplified in the synthesis of the styrene complex 10 (Scheme 2).72 The one-pot reaction of CoCl2, IMes, styrene and KC8 led to the isolation of the formal Co0 complex 10 in 64% yield and its spectroscopic, magnetic and structural features are similar to those of the vinylsilane analog 9a. The p-acceptor character of the olefin ligands in 8 – 10 plays a key role in the stabilization of these three-coordinate Co0 species.168

Scheme 2 One-pot synthesis of Co0 NHC complexes.

The formation of low-valent, low-coordinate Co0 intermediates is thought to occur in the cobalt-catalyzed Suzuki cross-coupling reaction of aryl chlorides and activated arylboronic pinacolate esters.67 The reducing organoborates in the presence of dvtms acting as a trap for low-valent cobalt species may lead to the formation of 8b. Although dvtms acts as a poison in the catalytic reaction, the bis-norbornene complex 11 (Scheme 2) was found to be a suitable pre-catalyst for cobalt-catalyzed cross-coupling reactions.67

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N-Heterocyclic Carbene Complexes of Cobalt

The reactivity of 8a and 9a – 9f was explored towards bulky organic azides and silanes (Scheme 3).66,69–71 The vtms complexes are more reactive towards organic azides than 8a featuring a chelated dvtms. Consistently, reaction of 8a with DiPPN3 (DiPP ¼ 2,6-diisopropylphenyl) resulted in the CoIV complex 12 (see Section 7.11.2.6 on “Mononuclear CoIV complexes”) while the reaction with DmpN3 (Dmp ¼ 2,6-dimesitylphenyl) failed. However, reaction between the vtms analogue 9a and DmpN3 yielded the cyclometallated CoII amido complex 13.69 The nature of the reaction products was governed by the nature of the NHC wingtip substituents as treatment of 9b – 9d with DmpN3 afforded the CoII imido complexes 14a – 14b.69,70 Reaction of the vtms Co0 complexes [Co0(NHC)(vtms)2] with silanes led to the dinuclear cobalt silyl complexes 15 and 16a or 16b, depending on the nature of the NHC ligand and on the stoichiometry of the added SiH2Ph2 reagent (see also below Scheme 89, Section 7.11.2.7.1.1.3).71

Scheme 3 Reactivity of Co0 NHC complexes with silanes and organic azides.

In order to investigate the ability of Co0 NHC complexes to activate P–H bonds in phosphines, the reactivity of 9b and 17b (see below for its synthesis) was examined towards primary aryl phosphines (Scheme 4).78 Reaction with the relatively bulky PH2Dmp at elevated temperature (80  C) led to the diamagnetic CoI phosphido complexes 18a and 18b. The crystal structure determination of 18a revealed a Co–(Z6-arene) interaction between the cobalt center and one flanking mesityl ring. The presence of a phosphido ligand featuring one hydrogen atom on the phosphorous atom was confirmed by 1H and 31P NMR spectroscopy, with clear identification of the P–H resonance. Exposure of 18a to 1 atm of CO led to decoordination of the flanking arene ring and formation of the low-spin CoII (meff ¼ 2.3(1) mB) terminal phosphido complex 19. In the latter, the Co-P and Co-CNHC bond distances are slightly elongated by 0.15 and 0.08 A˚ , respectively, in comparison to 18a. This is consistent with the strong trans-influence of the NHC and phosphido ligands. Oxidation of 18a with [FeCp2]BArF4 afforded cleanly the CoII phosphido complex 20. Both complexes feature structural similarities, suggesting possible reversibility in the one-electron redox event, which was confirmed by cyclic

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639

Scheme 4 Reactivity of the Co0 NHC complexes with primary aryl phosphines.

voltammetry (E1/2 ¼ –0.70 V). Reaction of 17b with the sterically less demanding PH2Mes led to the low-spin CoII phosphido alkyl complex 21 (meff ¼ 2.6(1) mB) featuring two mutually trans ICy ligands. The Co0 alkene complexes 9b and 17b are effective precatalysts for the dehydrocoupling of primary aryl phosphines to form diphosphines. Similar activities were observed for the CoI phosphido complexes 18a,b and the CoII phosphido alkyl complex 21, which suggests the formation of similar species as intermediates in the catalytic cycle.78 Reaction of the three-coordinate Co0 complex 9b with 2,4,6-tri(tert-butyl)-1-nitroso-benzene (Mes NO) at room temperature led to the formation of the bis(nitrosoarene) complex 22 (Scheme 5).79 The latter further evolved at elevated temperature (80  C) into the CoII nitrosyl aryl complex 23 through a stepwise mechanism involving an unusual nitrosoarene C–N bond oxidative addition. The molecular structure of 22 revealed two distinct coordination modes for the nitrosoarene ligands, Z2-ONMes and k1-O-ONMes . The redox non-innocent nitrosoarene in 22 is best described as radical anionic [Mes NO]%– in view of the relatively long N-O bond distances (1.366(3) and 1.314(2) A˚ ). Spectroscopic and theoretical studies support the description of 22 as containing a low spin CoII (SCo ¼ 1/2) center bearing two radical anionic [Mes NO]%– ligands and an overall S ¼ 1/2 ground spin-state as a result of antiferromagnetic coupling between the spin carriers. Magnetic susceptibility measurements point to the presence of low-lying excited states that are populated a room temperature. In contrast, the nitrosyl aryl complex 23 is best described as a high-spin CoII (SCo ¼ 3/2) complex bearing one antiferromagnetically-coupled triplet NO- anion (SNO ¼ 1), leading to a net S ¼ 1/2 system. Magnetic measurements, EPR studies and DFT calculations are consistent with this formulation. The anionic nature of the nitrosyl ligand is further supported by a long N-O distance of 1.198(5) A˚ and a bent Co-N-O angle of 135.9(4) in the solid-state structure of 23.79

Scheme 5 Reactivity of the Co0 NHC complex 9b with nitrosoarenes.

The structurally characterized (Table 1) paramagnetic bis-alkyne Co0 complex 24 (Fig. 1) was obtained by reaction of the tricoordinate NHC CoII complex [Co(CH2SiMe3)2(ItBu)] with diphenylacetylene in the presence of H2 (see below Scheme 22, Section 7.11.2.3.1.2.1).73

640

N-Heterocyclic Carbene Complexes of Cobalt

Fig. 1 The bis-alkyne Co0 complex 24.

7.11.2.1.1.2.2 Complexes of type [Co(NHC)L3] Zerovalent phosphine complexes served as precursors to the paramagnetic Co0 complexes 25a – 25c bearing a bis(olefin)-amino ligand after PPh3 substitution by the NHC ligand (Scheme 6).74 The effective magnetic moments of 25a – 25c (meff ¼ 1.8 – 1.9 mB) in solution were consistent with S ¼ ½ spin ground-states, which was further supported by SQUID magnetic susceptibility measurements. In the three complexes, the Co0 center is in a distorted tetrahedral coordination environment, defined by the NHC, the amine and two olefin ligands. A significant elongation of the coordinated C]C bonds (aver. 1.43 A˚ , in comparison to 1.34 A˚ for an unperturbed C]C bond) is indicative of strong p-backdonation from the metal to the olefins. Room temperature EPR spectra were consistent with a low spin d9 metal center in a distorted tetrahedral coordination geometry. These complexes were evaluated for the selective oxidation of secondary and allylic phosphines with N2O.74

Scheme 6 Paramagnetic Co0 NHC complexes with a bis(olefin)-amino ligand and application in selective phosphine oxidation.

7.11.2.1.1.2.3 Complexes of type [Co(NHC)2L] The bis(NHC) Co0 complexes 17a,b (Schemes 4 and 7) were prepared in moderate yield (34–53%) following the same procedure as for 9a – 9f but using 2 instead of 1 equiv. of the NHC ligand (Scheme 2).71 Only with the moderately bulky IMesCy and ICy ligands was the synthesis successful. Furthermore, attempts to prepare the mono(NHC) complex [Co(IMesCy)(vtms)2] failed, further suggesting that the NHC-to-vtms ratio in the complex is mostly governed by the steric properties of the NHC ligand. The structures of 17a,b were determined by X-ray diffraction and revealed trigonal planar coordination environments with marginally longer Colefin]Colefin bond lengths in comparison with the mono(NHC) complexes 9a – 9f, probably due to enhanced Co-to-olefin backdonation in the bis(NHC) complexes.71

N-Heterocyclic Carbene Complexes of Cobalt

641

Scheme 7 Reactivity of the bis-NHC Co0 complex 17a towards SiH2Ph2.

The reactivity of 17a towards silanes depends on the experimental conditions (Scheme 7).71 At low temperature, 17a reacted with SiH2Ph2 to give in 21% yield the square-planar CoII silyl hydride complex 26. The two NHC ligands are in a cis arrangement and the Co-H and Co-Si distances are rather short (ca. 1.47 and 2.25 A˚ , respectively). The steric congestion of the metal coordination sphere, due to the presence of two NHC ligands, probably prevents the formation of dinuclear complexes, which have been observed in the reaction of the mono(NHC) [Co0(NHC)(vtms)2] complexes with SiH2Ph2 (Scheme 3). Complex 26 is not stable at room temperature and converts to 27, which contains a silyl-functionalized NHC chelate and a cyclometallated [IMes’Cy]- ligand. The CoII center in 27 is in a distorted square-planar coordination environment, with two mutually trans NHC ligands. The synthesis of 27 also proceeds directly by reaction of 17a with SiH2Ph2 at higher temperature.71

7.11.2.1.1.2.4 Complexes of type [Co(NHC)3L] The CoI-Cl complex 28 was reduced with 1 equiv. KC8 under N2 atmosphere to yield the Co0 dinitrogen complex 29 in 87% isolated yield (Scheme 8).75 The magnetic moment of 29 in solution (meff ¼ 2.6(1) mB) is larger than the spin-only value for an S ¼ ½ system, possibly due to contribution from the orbital moment. The presence of a doublet ground state with small g-value and large 59Co nuclear hyperfine constants was deduced from EPR spectroscopy data. The structure of 29 revealed a distorted tetrahedral

Scheme 8 Synthesis and reactivity of the Co0 dinitrogen complex 29.

642

Table 1

N-Heterocyclic Carbene Complexes of Cobalt Selected structural and magnetic data of NHC Co0 complexes. Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)a

Spin state

References

1a

1.871(2), 1.877(2) 1.868(4), 1.872(4)

–

3.7(1)

LS Co0

63,64

1b

1.882(4), 1.875(4)

–

2.0

LS Co0

65

4

Low quality X-ray data

3.2(1)

LS Co0

64

5

1.902(3) 1.875(3)

1.687(2) Co-N

2.8(1)

LS Co0

64

6

1.940(5)

1.820(5) Co-CCN 1.822(5) Co-CCN 1.854(5) Co-CCN

2.0(1)

LS Co0

64

7

1.900(2)

1.902(3) Co-C

2.8(1)

LS Co0

64

Complex number

Formula

N-Heterocyclic Carbene Complexes of Cobalt

Table 1

643

(Continued) Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

8a

1.947(2)

–

2.9(1)

LS Co0

66

8b

1.9333(13)

–

2.9(6)

LS Co0

67

8c

No crystal structure

2.7(1)

LS Co0

68

9a

1.946(2)

–

3.1(1)

LS Co0

69

9b

1.952(2)

–

3.2(1)

LS Co0

69

9c

No crystal structure

–

2.9(1)

LS Co0

70

9d

1.9562(14)

–

3.3(1)

LS Co0

70

9e

No crystal structure

–

2.8(1)

LS Co0

71

9f

1.929(4)

–

2.9(1)

LS Co0

71

Complex number

Formula

(Continued )

644

Table 1

N-Heterocyclic Carbene Complexes of Cobalt

(Continued) Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

10

1.953(3)

–

2.6(1)

LS Co0

72

11

1.950(3)

–

2.6(5)

LS Co0

67

17a

1.922(2), 1.924(2)

–

2.9(1)

LS Co0

71

17b

1.928(2), 1.931(2)

–

2.8(2)

LS Co0

71

24

1.968(2)

1.9470(14), 1.9196(13) Co-C

2.38b

LS Co0

73

25a

1.981(1)

2.129(1) Co-N

1.78

LS Co0

74

25b

1.985(2)

2.135(2) Co-N

1.81

LS Co0

74

25c

1.968(4)

2.126(3) Co-N

1.76

LS Co0

74

Complex number

Formula

N-Heterocyclic Carbene Complexes of Cobalt

Table 1

645

(Continued) Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

29

1.977(5), 1.966(5), 1.970(5)

1.798(5) Co-N2

2.6(1)

LS Co0

75

39c

1.968(3)

–

2.6(1)

LS Co0

68

Complex number

Formula

a

Solution magnetic moment, unless otherwise stated. In the solid state. c This complex is actually an N-aryl imidazolate but it was obtained from the NHC complex 8a after N-Mes bond breaking (Scheme 12). b

coordination environment with an end-on coordination mode for the N2 ligand. The IR absorptions at 1917 and 1921 cm–1 (in KBr and THF, respectively) correspond to the N-N stretching vibration of the dinitrogen ligand and are indicative of substantial backdonation from the electron-rich Co center to the N2 ligand. Analysis of 29 by cyclic voltammetry indicates two quasi-reversible one-electron processes at –2.10 and 0.34 V vs. SCE, likely to correspond to the formation of the anionic [Co(ICy)3(N2)]– and the cationic [Co(ICy)3(N2)]+ species, respectively. Accordingly, the chemical oxidation of 29 with [FeCp2] BF4 afforded the trigonal planar CoI homoleptic complex 30 after loss of the coordinated N2 ligand. Alternatively, 30 may be obtained by chloride abstraction reaction from 28 with NaBF4.

7.11.2.2 7.11.2.2.1

Mononuclear Co-I complexes Monodentate carbene ligands

7.11.2.2.1.1 Complexes of type [CoX(NHC)L2(NO)] The 4-coordinate formal Co–I complexes [Co(NHC)2(CO)(NO)] (31a–h) and [Co(NHC)(CO)2(NO)] (32a–j) bearing linear nitrosyl ligands were prepared by reaction of [Co(CO)3(NO)] with the corresponding free NHCs (Scheme 9).76 With sterically non-demanding NHC ligands, the metal-to-NHC ratio in the complexes is mainly governed by the stoichiometry of the two reagents. In contrast, with bulky NHCs, the mono-NHC-substituted complexes are exclusively obtained, even in the presence of excess NHC ligand. In the bis(carbene) complexes 31a–h, two CO ligands have been displaced by two NHC donors. Further substitution of the remaining CO ligand was not successful, even when the less bulky IMe ligand was used under more forcing conditions. With the bulkier ItBu ligand, only the mono-NHC-substituted [Co(ItBu)(CO)2(NO)] complex could be isolated. The 1 H NMR spectrum of the product revealed the presence of two isomers, 32k and 33, corresponding to a ‘normal’ (C-2) and ‘abnormal’ (C-4) coordination mode of the ItBu ligand, respectively. Formation of the ‘abnormal’ 33 is driven by a release in the steric congestion around the metal center, but only a very small energy difference between the two isomers was established by DFT calculations. With the sterically demanding IPr and IMes ligands, only the mono(NHC) complexes 32i,j were obtained, even in the presence of excess NHC ligand. Similarly, reaction of Me2cAAC with [Co(CO)3(NO)] only resulted in the formation of the mono-substituted 34, irrespective of the metal-to-ligand stoichiometry.76

646

N-Heterocyclic Carbene Complexes of Cobalt

Scheme 9 Synthesis of formally Co–I carbonyl nitrosyl NHC complexes.

In these metal complexes, the nitrosyl ligand is linear at nitrogen and therefore the metal is in a (-I) formal oxidation state. The crystal structures of the bis(NHC) complexes 31a–h invariably featured a tetrahedral coordination geometry around the cobalt center, the latter being surrounded by two NHCs, one carbonyl and one (linear) nitrosyl ligand. A similar coordination environment was also observed in the structures of the mono-substituted complexes 32a–j and 34, which feature one carbene, two carbonyl and one nitrosyl ligands. A slight elongation of the Co-CNHC bond was noticed when the steric demand of the NHC ligands was increased (Table 4). In contrast, no major change of the Co-CNHC bond distance was observed between the mono- and bis-NHC complexes. The Co-CcAAC bond in 34 (1.9585(15) A˚ ) is slightly shorter than the Co-CNHC separation in the corresponding mono-NHC complexes 32a–j (1.961–2.011 A˚ ), which is consistent with the stronger s-donor and p-acceptor capabilities of the cAAC ligand. All complexes are diamagnetic and the CNHC resonance was detected at ca. 200 ppm in the corresponding 13C{1H} NMR spectra (the CcAAC resonance in 34 was not observed). Surprisingly, analysis by IR spectroscopy did not reveal any direct correlation between the n(CO) stretching frequency (Table 8) and the reported Tolman electronic parameter (TEP) value for the corresponding NHC ligands. The totally symmetric A1 vibration of the CO ligands in the mono(NHC) complexes 32a–j was found in the typical range 2007–2011 cm–1, at a considerably higher wavenumber than in the bis(NHC) analogues 31a–h (in the range 1865–1878 cm–1). This blue shift can be explained by a higher electron density at the metal in the latter complexes due to the strong s-donor and weak p-acceptor nature of the NHC ligands. In the Me2cAAC complex 34, the A1 n(CO) vibration mode was detected at 2004 cm–1, at a slightly lower wavenumber than in the mono(NHC) complexes 32a–j. All complexes are stable upon sublimation in vacuo (ca. 10–2 mbar) and were characterized by DTA/TG analysis. The bis(carbene) complexes (31a–h) showed an increased stability towards thermal decomposition compared to the mono(NHC) analogues (32a-j).76

N-Heterocyclic Carbene Complexes of Cobalt

647

The non-symmetrically substituted Co–I complexes 35a–g were obtained by selective substitution of one carbonyl by a phosphine ligand in the corresponding [Co(NHC)(CO)2(NO)] complexes 32 (Scheme 10).77 In some cases, UV irradiation was necessary to achieve quantitative substitution of the ligand. Interestingly, the synthesis of 35d and 35g did not require photochemical activation and occurred smoothly at room temperature. The different complexes are chiral at the metal center but a racemic mixture was obtained. All complexes were characterized spectroscopically by multinuclear NMR and IR spectroscopy (see Table 8 for the CO and NO stretching vibrations). The crystal structures of 35c and 35g revealed a distorted tetrahedral coordination geometry for the metal in both complexes. The thermal properties of the different complexes were investigated by thermogravimetric studies (DTA/TG). The phosphine complexes 35a–g exhibited a higher thermal stability compared to the [Co(NHC) (CO)2(NO)] precursors (32). All complexes are volatile and stable upon sublimation. Successful vapor-phase deposition of thin cobalt films was achieved using 35a as well as 31a, 31h and 32a.77

Scheme 10 Formation of the phosphine complexes 35a–g.

7.11.2.2.1.2 Complexes of type [CoX(NHC)2L(NO) The synthesis and properties of the bis-NHC carbonyl/nitrosyl Co–I complexes 31a–h have been described above in Scheme 9.76

7.11.2.2.1.3 Complexes of type (Cation+)[Co(NHC)2L2]– Cyclic voltammetry studies of 29 (Scheme 8) suggested possible access to complexes of the type [Co(ICy)n(N2)m]– by a one-electron reduction process (Scheme 11).75 The bis(dinitrogen) Co–I complexes 36a – 36c were obtained in moderate to good yield by reduction of 29 with the corresponding alkali metals. These compounds rapidly decomposed in THF and their 1H NMR spectra were consistent with an idealized C2 symmetry in solution. Two IR absorption bands at ca. 1800 and 1900 cm–1 (Table 9) correspond to the symmetric and asymmetric stretching vibrations of the two dinitrogen ligands, these values indicating a stronger N2 activation in the Co–I complexes than in the Co0 complex 29. The solid-state structures of 36a – 36c revealed nearly isostructural complexes with the Co center in a distorted tetrahedral coordination environment defined by two ICy ligands and two end-on N2 ligands. The Co-N bond distances were found in the range 1.747(4)–1.765(4) A˚ (Table 2) and are shorter than in 29 (1.798(5) A˚ ), further implying a stronger activation of the N2 ligand in 36a – 36c.

648

N-Heterocyclic Carbene Complexes of Cobalt

Scheme 11 Synthesis of the Co–I complexes 36a–36c.

The reaction of 36a–36c with triflic acid generated N2H4 in relatively high yields of 24–30% (relative to Co), probably as the result of a strong N2 activation in these complexes. In contrast, only traces of hydrazine were formed when 29 was reacted with triflic acid under similar conditions. In order to gain some insight into the mechanism of the N2 activation and reduction reaction, 36a was reacted with 1 equiv. of SiClR3 (R ¼ Me, Et) to give in moderate yield the low-spin CoII diazene complexes 37a,b (Scheme 11) which were characterized spectroscopically (including EPR) and by X-ray diffraction. The results of combined experimental and theoretical studies supported the formulation of 37a,b as low-spin CoII complexes with a coordinated dianionic [Z2-R3SiNNSiR3]2– ligand. Addition of 18-crown-6 to a solution of 36a yielded the dinuclear complex 38 of which the structure revealed a tetrahedral coordination environment for the Co–I center, constituted by two ICy and two end-on coordinated N2 ligands. One of the N2 ligands is bridging the Co center and the chelated K cation. The spectroscopic data of 38 were found to be very similar to those of its precursor 36a. In attempts to synthesize a three-coordinate Co–I complex, 8a was reduced with KC8 in the presence of 18-crown-6 (Scheme 12).68 However, instead of the desired Co–I complex, the formally Co0 N-aryl imidazolate complex 39 was isolated, its formation resulting from C–N bond cleavage of an N-aryl NHC wingtip substituent. However, when a similar reaction was performed in the presence of 2,2,2-cryptand with the alkyl-substituted NHC Co0 complex 8c, the expected three-coordinate formally Co–I complex 40 was isolated in very good yield. In comparison to the Co0 complex 8a, the Co-CNHC bond distance is much shorter in 40 (1.876(3) A˚ vs. 1.947(2) in 8a). The dearylation reaction leading to 39 sheds light on similar degradation reactions that might occur using N-aryl NHC complexes under strong reducing conditions.68

Scheme 12 Synthesis of the Co–I complex 40.

N-Heterocyclic Carbene Complexes of Cobalt

Table 2

649

Selected structural and spectroscopic data of NHC Co–I complexes. Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

Spin state

References

31a

1.995(2)

nd

199.5

LS Co–I

76

31b

1.977(2), [1.965(3)]a 1.974(2), [1.975(2)]a

1.689(2), [1.710(3)]aCoCCO 1.709(3), [1.699(3)]aCo-N

200.9

LS Co–I

76

31c

1.973(2), 1.987(2)

1.689(2) Co-CCO, 1.698(2) Co-N

200.4

LS Co–I

76

31d

1.962(3), 1.962(2)

1.726(2) Co-CCO, 1.671(3) Co-N

201.9

LS Co–I

76

31e

2.011(2), 1.995(2)

1.717(2) Co-CCO, 1.698(2) Co-N

200.3

LS Co–I

76

31f

1.998(2), 1.984(2)

1.719(2) Co-CCO, 1.688(2) Co-N

199.0

LS Co–I

76

31g

1.972(2) 1.977(2)

1.730(2) Co-CCO, 1.677(2) Co-N

201.0

LS Co–I

76

31h

Heavy disorder in the crystal structure

199.5

LS Co–I

76

32a

1.984(1)

186.4

LS Co–I

76

Complex Number

Formula

1.790(2) Co-CCO 1.778(2) Co-CCO 1.665(2) Co-N

(Continued )

650

N-Heterocyclic Carbene Complexes of Cobalt

Table 2

(Continued)

Complex Number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

Spin state

References

nd

LS Co–I

76

32b

No crystal structure

32d

1.971(3)

1.732(2) Co-CCO 1.754(2) Co-CCO 1.733(2) Co-N

nd

LS Co–I

76

32e

2.002(2)

1.735(1) Co-CCO 1.772(2) Co-CCO 1.728(2) Co-N

nd

LS Co–I

76

32f

1.986(2), [1.980(2), 1.978(2)]a

1.788(2), [1.798(2), 1.805(2)]a Co-CCO 1.776(2), [1.783(2), 1.790(2)]a Co-CCO 1.651(2), [1.654(2), 1.652(2)]a Co-N

nd

LS Co–I

76

32g

2.011(1)

1.789(1) Co-CCO 1.781(2) Co-CCO 1.654(1) Co-N

nd

LS Co–I

76

32h

No crystal structure

nd

LS Co–I

76

32i

1.961(15)

1.786(2) Co-CCO 1.725(2) Co-CCO 1.718(2) Co-N

197.5

LS Co–I

76

32j

1.970(2)

1.789(2) Co-CCO 1.782(3) Co-CCO 1.671(2) Co-N

194.3

LS Co–I

76

N-Heterocyclic Carbene Complexes of Cobalt

Table 2

(Continued)

Complex Number

Formula

651

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

Spin state

References

33

2.018(4)

1.727(4) Co-CCO 1.796(6) Co-N

nd

LS Co–I

76

34

1.959(2)

1.744(2) Co-CCO 1.784(2) Co-CCO 1.710(2) Co-N

nd

LS Co–I

76

35a

No crystal structure

197.1

LS Co–I

77

35b

No crystal structure

187.2

LS Co–I

77

195.8

LS Co–I

77

iPr

N

N iPr Co

ON CO

PEt3

35c

1.976(2)

1.733(2) Co-CCO 1.668(1) Co-N 2.209(1) Co-P

35d

No crystal structure

186.8

LS Co–I

77

35e

No crystal structure

186.8

LS Co–I

77

35f

No crystal structure

198.6

LS Co–I

77

(Continued )

652

N-Heterocyclic Carbene Complexes of Cobalt

Table 2

(Continued)

Complex Number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

Spin state

References

35g

2.001(2)

1.724(2) Co-CCO 1.682(1) Co-N 2.216(1) Co-P

nd

LS Co–I

77

36a

1.984(2), 1.971(2) with Co1; 1.968(2), 1.978(2) with Co2b

1.755(2), 1.758(2) with Co1; 1.758(2), 1.752(2) with Co2

211.1

LS Co–I

75

36b

1.972(5), 1.957(4) with Co1; 1.958(4), 1.984(4) with Co2b

1.747(4), 1.752(4) with Co1; 1.760(4), 1.754(4) with Co2

214.8

LS Co–I

75

36c

1.954(5), 1.966(6) with Co1; 1.963(5), 1.967(5) with Co2b

1.748(5), 1.765(4) with Co1; 1.756(5), 1.749(5) with Co2

214.6

LS Co–I

75

38

1.944(3), 1.956(4) with Co1; 1.961(3), 1.945(3) with Co2a

1.749(3), 1.771(3) with Co1; 1.755(3), 1.760(3) with Co2

213.2

LS Co–I

75

40

1.876(3)

1.946(4) Co-Cdvtms 1.992(3) Co-Cdvtms 1.961(3) Co-Cdvtms 1.992(3) Co-Cdvtms

nd

LS Co–I

68

a

Two crystallographically independent molecules in the asymmetric unit. Polymeric structure with two subunits in the asymmetric unit.

b

7.11.2.3 7.11.2.3.1

Mononuclear CoII complexes Monodentate carbene ligands

7.11.2.3.1.1 Homoleptic complexes of type [Co(NHC)4]2+(A–)2 It appears that 41 is the only example reported in the literature of a homoleptic CoII NHC complex (Scheme 13).80 It has been obtained in 52% yield by reaction of 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (Me2IEt) with [CoCl(PPh3)3] and NaBF4, followed by one-electron oxidation with [FeCp2]BF4. An improved procedure consists of the direct reaction of Me2IEt with CoCl2 in the presence of NaBF4. The square-planar coordination environment of the Co center in 41 was established by X-ray diffraction. Its solution magnetic moment (meff ¼ 2.4 mB) is consistent with a low-spin four-coordinate CoII center and 41 was further characterized by EPR spectroscopy. Complex 41 can engage in one-electron redox reactions, as evidenced by its reaction with aryl Grignard reagents (p-tolylmagnesium bromide), which led to the reduction of the metal center. As a result, the corresponding CoI complex 42a was formed along with biaryl coupling products. Conversely, reaction of 42a with organic halides regenerated 41, suggesting that [Co(Me2IEt)4]1+/2+ is a privileged platform for one-electron redox reactions. Complexes 42a – 42c were also obtained directly by reaction of the corresponding free NHC with [CoCl(PPh3)3] and NaBPh4.80

N-Heterocyclic Carbene Complexes of Cobalt

653

Scheme 13 Complex 41, a rare example of a homoleptic CoII NHC complex.

7.11.2.3.1.2 Heteroleptic complexes 7.11.2.3.1.2.1 Complexes of type [CoX2(NHC)] and related • Imido complexes The two-coordinate CoII imido complexes 14au/s and 14b were obtained in good yields (71–82%) from the reaction of the Co0 complexes 9b – 9d with DmpN3 (Schemes 3 and 14).69,70 These complexes are air-, moisture- and heat-sensitive and slowly decompose in solution at room temperature but they can be stored as solids under inert atmosphere at –30  C for several months without any decomposition. The nearly linear CNHC-Co-N arrangements with short Co-Nimido distances (1.675(3)–1.691(6) A˚ , Table 3) found for 14au/s and 14b is consistent with a multiple bond character. In comparison, the Co–Namido separations reported for two-coordinate CoII amido complexes are much longer and fall in the range 1.84–1.91 A˚ . The Co-CNHC bond is slightly longer in 14as, probably as a result of the different steric properties of the saturated NHC ligand. The magnetic moments of 14au/s and 14b in solution are in the range meff ¼ 4.6–5.1 mB, values which are larger than the spin-only value for a high-spin CoII center (S ¼ 3/2), probably because of spin-orbit coupling. The results of DFT calculations were consistent with a high-spin ground state and indicated a Mayer bond order of 1.55 for the Co–Nimido bond in 14au. A slow relaxation of magnetization with high effective relaxation barriers was observed for 14au/s and 14b under zero applied direct current (dc) field.70 For 14as, an effective relaxation barrier of 413 cm–1 was obtained, which is the second largest value to date for transition-metal based single-molecule magnets (SMMs), close to the value of 450 cm–1 obtained for the CoII dialkyl complex [Co{C(SiMe2ONaph)3}2] (Naph ¼ 1-naphthyl).169 The large magnetic anisotropy in 14as was shown by ab initio calculations to originate mainly from the short Co]Nimido bond.70 The two-coordinate CoII imido complex 14au was reacted with CO, ethylene, silanes and terminal alkynes (Scheme 14).69 Reaction of 14au with ethylene or CO resulted in imido group transfer reactions. In the former case, transfer of DmpN led to the formation of the imine DmpN]CHdCH3 along with the putative Co0 complex 43, identified by 1H NMR spectroscopy. Using excess CO, the dinuclear Co0 carbonyl complex 44a and the organic isocyanate DmpN]C]O were isolated in very good yield and fully characterized.

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N-Heterocyclic Carbene Complexes of Cobalt

Scheme 14 Synthesis and reactivity of two-coordinate CoII imido complexes.

Reaction of 14au with the silanes SiH2Ph2 and SiH3Ph afforded the low-spin CoII complexes 45 and 46, respectively, which display a b-agostic interaction between the Co center and the Si–H moiety. The CoII hydride complex 45 and the cyclometallated 46 result from a Si–H bond activation reaction, accompanied by bond formation between the less electronegative Si atom and the nucleophilic N center. The lack of cyclometallation in 45 may be due to steric reasons.69 Reaction of 14au with the terminal alkyne p-TolC^CH resulted in C–H activation and formation of the amido alkynyl CoII complex 47 in 65% yield. Its crystal structure reveals a pseudo three-coordinate Co center surrounded by one NHC, one alkynyl and one amido ligands, and features secondary metal-ligand interactions with one mesityl group of the Dmp moiety. Accordingly, the Co-Namido bond length (1.918(1) A˚ ) in 47 is significantly longer than the Co–Nimido separation in 14au (1.691(6) A˚ ).69

•

Amido complexes

The cobalt bis(trimethylsilyl)amide complex [Co{N(SiMe3)2}2]170 turned out to be a good precursor for the synthesis of 3-coordinate CoII NHC complexes (Scheme 15).81,83 The aminolysis of [Co{N(SiMe3)2}2] with 1 equiv. of imidazolium salt afforded the corresponding, highly air-sensitive complexes 48au/s and 48b in good yield. Their solution magnetic moments were found to be in the range 4.8–5.0 mB (Table 3), indicating a high-spin CoII center. Although larger than the spin-only moment for an S ¼ 3/2 spin ground-state, these values are in the typical range observed for related trigonal-planar high-spin CoII complexes. The outcome of this reaction depends on the steric bulk of the NHC ligand. Reaction of the bulkier SIPr
HCl imidazolium salt with [Co {N(SiMe3)2}2] did not afford the expected Co NHC complex but instead the ion pair [SIPrH][Co{N(SiMe3)2}2Cl] was isolated along with small amounts of [SIPrH][CoCl3(SIPr)].81 An alternative access to 48b consists of the reaction of the dinuclear complex [{Co(m-Cl)Cl(IPr)}2] (49au) with NaN(SiMe3)2.83

N-Heterocyclic Carbene Complexes of Cobalt

655

Scheme 15 Synthesis of 3-coordinate CoII NHC complexes.

The bis(amido) complexes 50au/s – 50g were prepared in good to excellent yield by reaction of the corresponding free carbenes with [Co{N(SiMe3)2}2] (Scheme 15).84–87 Metallation of the in situ generated CycAAC free carbene with [Co{N(SiMe3)2}2] afforded the corresponding complex 51 in only 20% isolated yield.85 The structures of the various bis(amido) complexes were determined by X-ray diffraction, and all feature a three-coordinate CoII center in a slightly distorted trigonal-planar coordination environment. A slight decrease of the Co–CNHC bond length from ca. 2.15 to 2.06 A˚ (Table 3) was observed in the series SIPr, SIMes, IPr, IMes, ItBu, consistent with a decreasing steric demand of the NHC ligands. The Co–CNHC bond length of 2.130(1) A˚ in the CycAAC complex 51 is similar to the Co-CNHC separation in the NHC complexes 50au/s – 50g, consistent with only a weak contribution of p-bonding interaction with the Cy cAAC ligand. The values of the magnetic moments of 50u/s – 50g in solution are in the range 4.5–5.3 mB, suggesting high-spin CoII centers (S ¼ 3/2) with a substantial contribution from the orbital moment to the effective magnetic moment. The 1H NMR spectra of 50au/s – 50d indicate that the idealized C2 symmetry observed in the solid state is retained in solution. Some structural distortions are observed in the complexes bearing non-symmetrical ligands (50e and 50g), possibly the result of weak secondary interactions. In the course of investigations on the thermal behavior of 50au, 50bu and 50c (Scheme 16),84 it was found that heating a solution of 50bu in toluene led to the formation of 52 after rearrangement of the NHC ligand from the ‘normal’ C-2 to the ‘abnormal’ C-4 coordination mode. Complex 52 was isolated in moderate yield (30%) and represents the first ‘abnormal’ NHC-Co complex. The crystal structure determination of 52 established a trigonal-planar environment for the metal, with a Co–CNHC bond length slightly shorter than that in the precursor complex 50bu (2.059(2) vs. 2.119(3) A˚ ). The steric bulk exerted by the IPr ligand in its ‘abnormal’ coordination mode is significantly reduced. It is noteworthy that the less sterically hindered IMes analogue, 50au, does not undergo a thermally-induced ‘normal-to-abnormal’ rearrangement, further supporting a sterically induced reactivity. The ItBu complex 50c has only moderate thermal stability and when it was heated at 80  C, activation of a tert-butyl substituent and formation of a 1-tert-butylimidazole CoII complex occurred.84

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N-Heterocyclic Carbene Complexes of Cobalt

Scheme 16 Thermal behavior and reactivity of the bis(amido) complexes 50.

Aminolysis of the N(SiMe3)2 ligand in the mono(silylamido) complexes 48as and 48b or ligand exchange with bulky amides provided access to other NHC–Co complexes (Scheme 17).82,83 Aminolysis of the N(SiMe3)2 group of 48as also occurred upon reaction with N,N’-bis(cyclohexyl)acetamidine and 2,6-diisopropylaniline and yielded 53 and 54, respectively.82 The distorted tetrahedral environment around the cobalt center in 53 contains one NHC, one chloride and one k2-amidinato ligand. A very weak interaction occurs with the amidinato C atom (Co–Camidinato ¼ 2.433(3) A˚ ). The structure of the dinuclear complex 54 contains metal centers in a distorted tetrahedral coordination geometry. Each Co is coordinated by bridging chlorides, one SIMes ligand and one terminal, bent anilido ligand. The long Co


Co intermetallic distance (ca. 3.35 A˚ ) precludes a direct metal-metal interaction. Aminolysis of the coordinated amide also occurred when 48b was reacted with 2,6-di-tert-butyl-4-methylphenol (BHT–H) and afforded the three-coordinate trigonal-planar phenoxide complex 55 in 71% yield.83 Reaction of 48b with 2,6-diisopropylaniline (NH2(DiPP)) or with its deprotonated derivative Li[NH(DiPP)] did not lead to the corresponding mono-substituted derivative 56, but rather to the bis(aryl-amido) complex 57. A ligand disproportionation of 56 into 57 and [{Co(m-Cl)Cl(IPr)}2] (49au) may account for this result. The crystal structure analysis of 57 revealed a pseudo trigonal-planar coordination environment for the metal with close Co-H contacts involving the CHMe2 atoms of two aryl-amido groups. Reaction of 48b with the bulkier lithium 2,6-dimesitylanilide (LiNHDmp) resulted in an amide exchange reaction with formation of 58 which, in contrast to 56, is stable towards disproportionation. It could also be obtained by reaction of [{Co(m-Cl)Cl(IPr)}2] (49au) with 2 equiv. of LiNHDmp. The solid-state structure of 58 revealed a pyramidalization of the cobalt center due to an additional interaction with a flanking aryl group

N-Heterocyclic Carbene Complexes of Cobalt

657

Scheme 17 Reactivity of three-coordinate mono(amido) complexes 48.

of the NDmp ligand (Co–Cipso separation of 2.577(3) A˚ ). The magnetic moment of 58 in solution (meff ¼ 3.7(1) mB)83 is substantially lower than that in the three-coordinate amido complexes 50, 51 and 57 (Table 3), likely resulting from the pyramidalization of the metal center. Aminolysis of one or both N(SiMe3)2 ligands of the bis(silylamido) complexes of type 50 was also a successful strategy to access the aryl-amido complexes 59 and 60au/s – 60b,85 and the phosphinidene complex 61 (Scheme 16).86 Interestingly, a sequential substitution of the N(SiMe3)2 groups by NH(DiPP) anilido ligands is possible. Reaction of 50bu with 1 equiv. NH2(DiPP) led to the mixed silylamide/aryl-amide complex 59, while using an excess of NH2(DiPP) resulted in the formation of the symmetrical bis(anilido) complex 60au. In the case of the phosphine functionalized PCNHCP complex 50d, only the symmetrical complex 60b was isolated, whether 1 equiv. or excess NH2(DiPP) was used. Similarly, only the synthesis of the bis(anilido) complex 60as proceeded cleanly. The various complexes were characterized by X-ray diffraction and they all feature a trigonal-planar coordination geometry with relatively long Co–CNHC separations. Substitution of the N(SiMe3)2 ligands by NH(DiPP) groups leads to a decrease in the Co–CNHC bond distances, probably for steric reasons. As observed with the mono(anilido) complex 58, close contacts between the Co center and CHMe2 protons of the NH(DiPP) groups were observed in 59 and 60au/s – 60b. Magnetic and EPR

658

N-Heterocyclic Carbene Complexes of Cobalt

studies of the different complexes are consistent with high-spin CoII centers (S ¼ 3/2) featuring a strong anisotropy and large values for the zero-field splitting parameter D. It is noteworthy that the transamination reactivity from N(SiMe3)2 to NH(DiPP) does not follow simple acid-base considerations but may involve a combination of electronic and steric effects.85 The reaction of 50f (isolated or prepared in situ) with PH2Mes was also based on the reactivity of the N(SiMe3)2 ligands and afforded the centrosymmetric, phosphinidene-bridged dinuclear complex 61 (Scheme 16).86 The 31P NMR spectrum of 61 contains a singlet at d 449.1 ppm, consistent with the formulation of the bridging ligands as phosphinidenes. Surprisingly, 61 is diamagnetic, as confirmed by variable-temperature SQUID magnetization measurements, probably owing to a strong antiferromagnetic coupling between the two metal centers through the bridging ligands. Interestingly, Ghadwal et al. used recently the (C-2)-arylated imidazolium salt IPrPh
HI as a precursor to ‘abnormal’ NHC–CoII complexes (Scheme 18).88 The reaction of IPrPh
HI with [Co{N(SiMe3)2}2] yielded the bis ‘abnormal’ NHC complex 62 in good isolated yield (79%), which will be described in the corresponding Section 7.11.2.3.1.2.5 (Complexes of type [CoX2(NHC)2]). The bis(amido) ‘abnormal’ NHC complex 63 was obtained from an indirect route involving NHC transfer from an ‘abnormal’ NHC-borane adduct, the latter synthesized by reaction of IPrPh
HI with NaBHEt3. In the solid state, the metal center in 63 is in a distorted trigonal-planar environment, similar to that observed in the ‘normal’ NHC analogues 50. The Co–CNHC bond distance of 2.075(2) A˚ is comparable to that in the ‘abnormal’ IPr analogue 52.

Scheme 18 Synthesis of the ‘abnormal’ NHC–CoII complexes 62 and 63.

Recently, the related tetrahedral CoII bis(triazolylidene) complexes 64a–c have been reported (Scheme 19) and their magnetic properties were investigated.89 The values for the magnetic moments at room temperature are significantly larger than the spin-only value for a high-spin CoII center, which suggests a large second-order orbital contribution to the moment.

Scheme 19 Synthesis of triazolylidene CoII complexes.

The formation of cobalt complexes with remote-substituted amino-carbene ligands (Scheme 20) was investigated by reaction of the neutral ligands with [Co{N(SiMe3)2}2].90 It should be noted that the amino-carbene (ACR) form is in a tautomeric equilibrium with the corresponding zwitterionic imidazolium-amido (IAR) form and the position of the equilibrium depends on the nature of

N-Heterocyclic Carbene Complexes of Cobalt

659

Scheme 20 Synthesis of CoII complexes with an amino-NHC ligand.

the substituent (R ¼ tBu, Cy, Mes). When R ¼ tBu, the reaction of the amino-carbene ligand with 1 equiv. of [Co{N(SiMe3)2}2] or CoCl2 led to metallation of the CNHC site, resulting in the formation of 65 and 66, respectively. The identity of both complexes was established by X-ray crystallography, but low-quality data prevented meaningful discussion of the metrical data in 65. Thermolysis of 65 resulted in a unique rearrangement of the complex, leading to ring-opening of the N-heterocycle and formation of 67 with a metallated ketenimine motif. Depending on the steric and electronic properties of the remote amine substituent, diverse coordination modes can be observed. Indeed, when R ¼ Cy, metallation at the N site occurred, resulting in the CoII imidazolium-amido complex 68. In the case of a Mes substituent, reaction of the neutral amino-carbene ligand with 2 equiv. of [Co{N(SiMe3)2}2] at high temperature led to the formation of the dinuclear mixed-valent complex 69, in which both Co centers adopt distorted planar T-shaped geometries (N-CoII-N and C-CoI-N angles of 148.92(6) and 164.92(6) , respectively). Analysis of the bond distances around the metal centers led a tentative assignment of the metal oxidation states, with a CoII ion coordinated to the remote-amido group and an aNHC-metallated CoI center (Co-CaNHC bond distance of 1.951(1) A˚ ).90

•

Alkyl complexes

The three-coordinate bis(alkyl) complex 70 was obtained by alkylation of the dinuclear complex [{Co(m-Cl)Cl(IPr)}2] (49au) with 4 equiv. Mg(CH2SiMe3)Cl (Scheme 21).91

Scheme 21 Synthesis of the bis(alkyl) complex 70 and reactivity towards silanes.

660

N-Heterocyclic Carbene Complexes of Cobalt

The 1H NMR spectrum of 70 showed paramagnetically shifted resonances and is consistent with an average C2v symmetry for the complex in solution.91 The solution magnetic moment of 5.1 mB indicates a high-spin CoII center with significant orbital contribution, which is further supported by EPR measurements. The solid-state structure of 70 revealed a trigonal-planar coordination environment for the CoII center, which is coordinated by two alkyl and one IPr ligands. The lack of reaction upon exposure of this high-spin CoII alkyl complex 70 to CO over short periods of time (15–30 min) suggested a relatively low reactivity. The cyclic voltammetry of 70 revealed several irreversible reduction events, suggesting that the in situ-generated reduced complexes are prone to further chemical events.91 Further studies of 70 indicated a large magnetic anisotropy for the trigonal planar cobalt center (D ¼ +73.7 cm–1).171 Dynamic magnetic measurements showed slow relaxation of the magnetization below 4 K with an energy barrier (Ueff) of 22.5 K. Reactions of the dialkyl complex 70 with silanes led to different silyl-bridged dinuclear cobalt complexes depending on the steric bulk of the silane reagents used (Scheme 21).166 In the case of hexylsilane (SiH3Hex), the diamagnetic complex 71 was isolated in high yield and features four bridging m-SiH2Hex groups. The distance between the two CoII centers is short enough (2.4465(9) A˚ ) to indicate the existence of a Co–Co single bond, which explains the apparent diamagnetism of the complex. Reaction of 70 with SiH3Ph resulted in the formation of 72, isolated as a mixture of two isomers. Multinuclear NMR and IR spectroscopic data supported the presence of three bridging m-SiH2Ph ligands and rapid exchange of agostic Si-H and hydride donors. Reaction with the bulkier SiH2Ph2 silane yielded 73 which contains two bridging hydrides and two bridging silyl ligands displaying agostic Si-H interactions with the CoII centers. In addition, the short Co-Co separation of 2.4527(8) A˚ supports the presence of a single Co–Co bond.166 Bis(alkyl) CoII NHC complexes can also be conveniently synthesized from the [CoR2(TMEDA)] (R ¼ CH2CMe3, CH2SiMe3, CH2CMe2Ph) precursors, the latter being obtained in high yield and purity by treatment of [Co(acac)2(TMEDA)] with MgR2 (Scheme 22).73 The TMEDA ligand is readily displaced by NHC donors and this procedure offers an alternative to the use of thermally sensitive [CoR2(py)2] precursors. The three-coordinate CoII alkyl complexes 74a – 74e are high spin CoII (S ¼ 3/2) species with effective magnetic moments in the range 4.47–5.04 mB. During the synthesis of 74a and 74e, small amounts ( 84%) were obtained upon pretreatment of the cobalt dichloride salt with KN(SiMe3)2 at low temperature, followed by addition of the imidazolium salt. Attempts of transmetallation from the corresponding silver or copper complexes proved unsuccessful. Complex 110 is paramagnetic with a solution magnetic moment of 4.24 mB, consistent with three unpaired electrons and a high-spin CoII center. Two stretching vibrations were detected at 1685 and 1652 cm–1 in the IR spectrum of 110, indicating that only one imine group is bound to the metal center. In contrast, the imidazolium proligand displayed one single IR absorption band at 1698 cm–1 for both C]N groups. The X-ray diffraction analysis of 110 confirmed a bidentate coordination mode of the ligand. The metal center is in a distorted tetrahedral environment, coordinated by one imine, one NHC and two chloride ligands. As expected, the C]N bond distance for the coordinated imine (1.273(4) A˚ ) is slightly longer than that of the dangling imine (1.253(4) A˚ ), as a result of p-backdonation. Combined steric and electronic effects are probably responsible for the lack of tridentate coordination mode of the ligand.110

Scheme 35 Synthesis of the imine-donor functionalized complex 110.

The hexacoordinated complex 111 was obtained in 56% yield by reaction of the corresponding naphthyridine-based triazolium proligand with Ag2O in acetonitrile, followed by transmetallation with [CoCl2(PPh3)2] (Scheme 36).111 This paramagnetic complex

Scheme 36 Synthesis of 111 by transmetallation from the Ag NHC complex.

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N-Heterocyclic Carbene Complexes of Cobalt

was characterized by EPR spectroscopy, revealing a low-spin CoII center. A structural analysis by X-ray diffraction established a distorted octahedral coordination environment with two chelating naphthyridine-functionalized NHC ligands and two additional acetonitrile ligands in trans position with respect to the NHC donors.111 CoII complexes bearing picolyl-functionalized NHC ligands were prepared by treatment of the corresponding free carbene ligand with different amounts of [Co{N(SiMe3)2}2] (Scheme 37).112 Using a metal/ligand ratio of 1:2, metallation at room temperature gave 112 in relatively high yield. Formation of this distorted tetrahedral CoII complex results from the deprotonation of each picolyl CH2 spacer with concomitant dearomatization of the pyridine rings. Performing the metallation reaction in a 1:1 metal/ligand ratio led to the three-coordinate complexes 113a and 113b after a few minutes of reaction at room temperature. Structural analysis by X-ray diffraction revealed trigonal planar complexes with coordination of the CNHC donor to the cobalt center while the pyridine group remained dangling. Slow conversion of 113b to 114 was observed at room temperature and complete conversion could be achieved upon heating to 90  C. The coordination environment around the cobalt center is trigonal pyramidal and contains one silylamide ligand and one chelating, dearomatized picolyl-NHC ligand. Dearomatization of the ligand was evidenced by analysis of the bond distances within the picolyl framework.112 Similar ligand dearomatization was reported earlier in the reaction of a phosphino-picoline-NHC ligand precursor with [Co{N(SiMe3)2}2] (see below, Scheme 43).119,120

Scheme 37 Synthesis of pyridyl-NHC CoII complexes and ligand dearomatization.

The CoII complexes 115a,b bearing bidentate amine-functionalized NHC ligands were obtained by transmetallation of the corresponding in situ generated AgI complexes with CoCl2 (Scheme 38).113 Both complexes feature a distorted tetrahedral coordination geometry around a high-spin CoII center (meff ¼ 4.3 and 3.7 mB for 115a and 115b, respectively). They efficiently catalyze the hydrogenation of ethyl 3,3-dimethylacrylate, a model trisubstituted alkene substrate, with a 1 mol% catalyst loading at an H2 working pressure of 10 bar and in the presence of 3 mol% of NaBHEt3. Catalytic systems obtained in situ from the imidazolium salt precursors, CoCl2 and NaBHEt3 were found to proceed with activities similar to that of the well-defined complexes 115a,b.113

Scheme 38 Synthesis of amine-donor functionalized CoII NHC complexes by transmetallation.

N-Heterocyclic Carbene Complexes of Cobalt

671

Fig. 3 CoII NHC complexes discussed in other sections.

Complexes 13 (Scheme 3, Section 7.11.2.1.1.2.1), 27 (Scheme 7, Section 7.11.2.1.1.2.3) and 116 (Scheme 84, Section 7.11.2.6.1.1) belong to this category (Fig. 3) and are discussed in the respective sections.

7.11.2.3.5.2 Tridentate ligands 7.11.2.3.5.2.1 Symmetrical CNHCNCNHC pincers The air-sensitive CoII complex 117 bearing a CNHCNCNHC pincer ligand was obtained by the aminolysis reaction of [Co {N(SiMe3)2}2] with the pincer imidazolium salt proligand (Scheme 39).114 Reaction of 117 with thallium triflate afforded the air-stable 118 featuring a distorted octahedral geometry with the triflate anions coordinated trans to each other. Treatment of 117 with NBr(SiMe3)2 resulted in the oxidation of the metal center, and formation of the diamagnetic tris-bromo CoIII complex 119.114

Scheme 39 Synthesis and reactivity of the pentacoordinated CoII complex 117.

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N-Heterocyclic Carbene Complexes of Cobalt

Reduction of the metal center occurred upon treatment of 117 with Na/Hg, which gave the diamagnetic CoI complex 120a, or by reaction of 117 with organolithium or organomagnesium reagents, which afforded 121.114 The reduction of 117 to 120a was also achieved using a stoichiometric amount of NaBHEt3.115 The high air-sensitivity of 120a is illustrated by the instantaneous formation of the diamagnetic [CoBr(2-O2)(CNHCNCNHC)] complex upon exposure to air.114 Direct reaction of 117 with excess MeLi (4 equiv.) led to the square-planar Co-alkyl complex 121 which can also be synthesized by alkylation of 120a with slight excess of MeLi. Interestingly, the 1H NMR spectra of the reduced Co complexes 120a and 121 display an unusual downfield shift for the para pyridine hydrogen (d 9.55 and 10.65 ppm, respectively), which may be ascribed to the redox non-innocence of the ligand.114,115 The chemistry of the reduced CoX-CNHCNCNHC (X ¼ halide, methyl, hydride) complexes and their electronic structures will be further discussed in the Section 7.11.2.4.2.2.1 where an alternative bonding description as [CoII(CNHCNCNHC)–I]-type complexes will be presented. The cationic CoII alkyl complex 122 was obtained in low yield by one-electron oxidation of 121 with [FeCp2]BArF4 (Scheme 39) and exhibited a solution magnetic moment of 1.8(5) mB, consistent with an S ¼ ½ ground state. The solid-state structure of 122 confirmed a square-planar coordination environment around the Co center. Analysis of this complex by EPR spectroscopy revealed a remarkable anisotropy of the g tensor and large hyperfine coupling constants, consistent with a cobalt-centered spin and a square-planar CoII complex.115 The CoII complexes 123 and 124 bearing a flexible aliphatic bis(NHC) pincer ligand were obtained by addition of tBuOK to a mixture of the corresponding imidazolium salt and CoCl2 (Scheme 40).116 In an alternative synthesis, 124 was prepared by reaction of the ligand precursor with [Co{N(SiMe3)2}2(THF)] (Scheme 40).173 The cobalt center in this high-spin CoII complex (meff ¼ 4.2 mB) lies in a distorted tetrahedral environment, coordinated by the chelating tridentate ligand and one chloride. Complex 124 is an efficient (pre)catalyst for the hydrogenation of carbonyl compounds (aldehydes, ketones and esters) under 30 bar of H2, using 2 mol% of the complex and in the presence of a base.116,173 Interestingly, catalytic systems formed in situ from the ligand precursor, CoCl2 and tBuOK as a base display efficiencies similar to that of isolated 124.116,173 In the presence of excess base, deprotonation of the central amine group on the pincer ligand may lead to a cobalt amido complex that further activates H2 through a cooperative metal-ligand bond activation process. The involvement of the ligand N–H moiety in the catalytic steps was demonstrated by the very low activity observed for the corresponding system based on the N-methyl-substituted pincer ligand. Further DFT studies revealed the key role of this N–H unit in the catalyst regeneration step, i.e. in the formation of the active catalytic species through heterolytic H2 splitting.174 It should be noted that 124 also catalyzes the transfer hydrogenation of ketones to alcohols using iPrOH as hydrogen source.175

Scheme 40 Synthesis of CoII complexes with an aliphatic bis(NHC) pincer ligand.

Using the same synthetic approach as depicted in Scheme 40, the 2,6-lutidine derivative of 124 bearing DiPP substituents was prepared by aminolysis of [Co{N(SiMe3)2}2] with the corresponding imidazolium proligand.176 Alternatively, the transmetallation procedure from Ag to Co has also been employed to access lutidine-based bis(NHC) CoII pincer complexes.177 The isolated CoII pincer complex was found to be an efficient (pre)catalyst for the selective semi-hydrogenation of terminal alkynes into the corresponding alkenes under mild conditions (room temp., 5 bar H2).176 Interestingly, an active catalytic system was also generated in a one-pot procedure by mixing the ligand precursor and CoCl2 in the presence of a base (tBuOK). The system was further used in the migratory hydrogenation of terminal alkynes through a tandem alkyne isomerization/semi-hydrogenation reaction. This one-pot two-step procedure involves a sequential base-catalyzed isomerization of terminal alkynes to internal 2-alkynes and a cobalt-catalyzed semi-hydrogenation of the 2-alkyne intermediates into (Z)-2-alkenes.176

7.11.2.3.5.2.2 Symmetrical CNHCCCNHC pincers The CoII and CoIII complexes 125a,b and 126a,b, respectively, bearing an anionic CNHCCCNHC pincer ligand were obtained using a similar route as described above for the CNHCNCNHC complex 117 (Scheme 41).117 The ‘aminolysis procedure’ was selected due to unsuccessful attempts to cleanly generate the lithiated anionic pincer ligand by addition of nBuLi to the CNHCCCNHC free carbene. The one-pot reaction between the imidazolium salt and [Co{N(SiMe3)2}2(py)2] with one equivalent of base, in the absence or presence of trityl chloride as oxidant, afforded in good yield (75–93%) the CoII and CoIII complexes 125a and 126a,b, respectively. Surprisingly, the same procedure did not afford the mesityl-substituted CoII complex 125b, which was obtained in low isolated yield (28%) by reduction of the CoIII analogue 126b with half an equivalent of the magnesium anthracene complex [Mg(C14H10)


N-Heterocyclic Carbene Complexes of Cobalt

673

Scheme 41 Synthesis and reactivity of the pincer complexes 125a,b and 126a,b.

3THF]. The 1H NMR spectra of the diamagnetic CoIII complexes 126a,b indicated C2-symmetric species in solution. X-Ray diffraction data established an octahedral coordination environment for the CoIII center, the latter containing one anionic tridentate CNHCCCNHC chelate, two chlorides trans to each other and one additional pyridine ligand.117 Both paramagnetic low-spin CoII complexes 125a and 125b were characterized by EPR spectroscopy and X-ray diffraction. The cobalt center lies in a square-pyramidal coordination environment, with the CNHCCCNHC ligand and one pyridine in the basal plane, and one chloride in the apical position. Cyclic voltammetry measurements revealed a reversible cathodic event at E1/2 ¼ –1.46 and –1.56 V vs. Fc+/Fc for 125a and 125b, respectively, suggesting the formation of the corresponding reduced CoI complexes. Chemical reduction of 126a with magnesium anthracene afforded the square-planar diamagnetic CoI-N2 complex 127 in good yield. In the presence of triphenylphosphine, reduction of 125a,b with magnesium anthracene or Na/Hg led to the square-pyramidal low-spin CoI complexes 128a,b. In the IR spectra of 127 and 128a,b, the coordinated N2 molecule gave rise to a strong absorption band at 2063, 2117 and 2112 cm–1, respectively, indicating a very weak activation of the dinitrogen ligand upon coordination. Addition of one or two equivalents of trityl chloride (CClPh3) as oxidant to 127 or 128a,b regenerated the corresponding CoIII and CoII complexes, respectively, showing that the interconversion between the three oxidation states can be easily achieved.117

674

N-Heterocyclic Carbene Complexes of Cobalt

7.11.2.3.5.2.3 Symmetrical OCNHCO pincers The number of metal complexes with oxygen-functionalized NHC ligands has rapidly increased during the past few years.57 The redox non-innocent OCNHCO pincer ligand featuring two di-tert-butyl-phenolate moieties and a central NHC core stabilizes a series of Co complexes in high oxidation states (Scheme 42).118 The pincer complexes 129u/s and 130 were obtained by deprotonation of the imidazolium salt precursors with 3 equiv. NaOMe and subsequent treatment with a stoichiometric amount of CoCl2. In all three complexes, the cobalt center has an approximate square-planar coordination geometry, defined by the anionic OCNHCO chelate and an additional solvent molecule (MeCN or THF) bound trans to the NHC donor. The labile THF ligand is readily displaced by acetonitrile upon dissolution in MeCN. The relatively short Co–CNHC bond distances (Table 3) can be traced back to geometric constraints imposed by the chelating ligand and the strong s-donor ability of the NHC donor. Slight differences are observed between the metrical data of the unsaturated and saturated chelates in 129u/s and 130. A shorter Co–CNHC bond distance and an elongation of the Co–N bond involving the MeCN ligand were observed in the saturated 129s, reflecting enhanced p-backdonation from the metal to the saturated carbene. For all three complexes, the bonding metrics within the phenolate groups are consistent with two dianionic phenoxide donors and a formally CoII center. All three complexes are paramagnetic, with solution magnetic moments in the range 1.82–1.90 mB. Such values are slightly higher than the spin-only value for an S ¼ ½ center but consistent with square-planar d7 CoII centers exhibiting a substantial orbital contribution to the magnetic moment. Computational data further supported the formulation of 129u/s and 130 as containing CoII centers bound to closed-shell [OCNHCO]2– dianions.

Scheme 42 Cobalt complexes with the redox non-innocent OCNHCO pincer ligand.

To investigate whether oxidized deriatives of 129s can be accessed, cyclic voltammetry measurements were performed and revealed three quasi-reversible one-electron oxidations at potentials below 1.2 V vs. Fc+/Fc, indicating formation of a formally CoV species. However, the true oxidation state, as observed by spectroscopic and physical methods, is much lower (vide infra). Interestingly, a strong variation (up to 400 mV) in the oxidation potentials was observed depending on the nature of the NHC backbone. The redox-potential can thus be conveniently tuned for further reactivity investigations towards small molecules.118 The mono-cationic complex 131 was obtained by oxidation of 129s with one equiv. of AgOTf followed by anion metathesis with NaBPh4. The crystal structure determination of 131 revealed a Co center in a pseudo-square-pyramidal environment and an elongation of the Co–CNHC bond distance from 1.789(2) A˚ in 129s to 1.849(3) A˚ in 131. Analysis of the metrical data within the phenolate rings reveals quinoid-type bond alternations, pointing to some degree of ligand oxidation and the presence of phenoxyl radicals. The solution magnetic moment meff ¼ 2.88 mB is consistent with the spin-only value for an S ¼ 1 center, suggesting either the formulation of 131 as a closed-shell [OCNHCO]2– dianionic ligand bound to an S ¼ 1 CoIII center, or as a low-spin CoII ion ferromagnetically coupled to a monoanionic radical [OCNHCO]•–. In the latter case, a single unpaired electron would be delocalized over the whole ligand system. Computational and solid-state magnetism data were consistent with both formulations, rendering the ligand in 131 truly non-innocent.118 The dicationic complex 132 was successfully prepared by oxidation of 129s with 2.1 equiv. of [N(p-C6H4Br)3]PF6. Its crystal structure shows a pseudo-octahedral coordination geometry around to Co center with three THF molecules completing the coordination sphere. The ligand metrical data within the phenoxide moieties clearly revealed a ligand-based oxidation and a quinoid-type pattern. Complex 132 is therefore best described as a charge-neutral, closed-shell, doubly oxidized ligand coordinated

N-Heterocyclic Carbene Complexes of Cobalt

675

to a formal CoII metal center. This view of the electronic structure of the complex was further supported by computational data. The solution magnetic moment meff ¼ 2.51 mB is substantially higher than that for a low-spin d7 S ¼ ½ ion, suggesting strong spin-orbit coupling. The synthesis of the tricationic complex observed by cyclic voltammetry was however unsuccessful, resulting only in intractable mixtures under various conditions.118 Reaction of 129s (L ¼ MeCN) with AgCF3 in MeCN afforded the trifluoromethyl CoIII complexes 133 and 134, which could be isolated depending on the crystallization conditions (Scheme 42).147 The two complexes differ by the number of coordinated MeCN molecules and their interconversion occurs in solution depending on the donor nature of the solvent. Complex 133 results from the loss of one coordinated MeCN ligand located trans to the CF3 moiety in 134, which may be due to the strong trans influence of the CF3 group. In both complexes, the CF3 group is orthogonal to the mean OCNHCO-Co plane, with a very short Co–CF3 bond length (1.8692(16) A˚ ). Spectroscopic data support a bonding description involving low-spin CoIII centers coordinated by a closed-shell [OCNHCO]2– dianionic ligand. The two complexes have different colors in the solid state (red for 133, green for 134) and exhibit a different stability towards light. Although 134 is light stable, exposure of 133 to ambient light regenerates 129s with formation of a •CF3 radical. Both complexes are however thermally stable and photoexcitation is necessary to observe the homolytic cleavage of the CoIII–CF3 bond. The easy activation of the thermodynamically strong Co–CF3 bond was used to perform radical C–H trifluoromethylation reactions. Using a 16 W 2700 K (soft white) compact fluorescent lamp, quantitative mono trifluoromethylation of (hetero)arenes was observed. The differences in reactivity between 133 and 134 towards homolytic cleavage of the Co–CF3 bond were thought to arise from their different geometries and the redox-active nature of the OCNHCO ligand. The loss of the MeCN ligand situated trans to the CF3 group lowers the energy of the empty dz2 orbital. Upon photoexcitation, one electron is transferred from the ligand to the dz2 orbital, which weakens the Co–CF3 bond. Photoexcitation thus occurs at lower energy in 133 than in 134, which explains the higher reactivity and lower stability of 133 towards light.147 7.11.2.3.5.2.4 Non-symmetrical PNCNHC pincers Interesting reactivity was observed with a non-symmetrical phosphino-picoline-NHC (PNCNHC) ligand, originating from the acidity of the a-CH2P protons (Scheme 43).119 Upon side-arm deprotonation, the neutral ligand is transformed into the anionic dearomatized P NaCNHC pincer ligand which features one vinylic phosphine (P ) and one anionic nitrogen (Na) donors. The corresponding P NaCNHC-CoBr pincer complex, 135, was accessed either via transmetallation of the dearomatized potassium salt with [CoBr2(THF)2], or by double aminolysis of [Co{N(SiMe3)2}2] using the imidazolium bromide salt precursor. The CoII complex 135 is paramagnetic in solution with an effective magnetic moment meff ¼ 2.1(2) mB, consistent with the presence of a low-spin d7 CoII center. The structural analysis of 135 revealed a CoII center in a nearly square-planar coordination geometry, chelated by the anionic pincer ligand and further coordinated by a bromide donor. Perusal of the bond distances within the heterocycle clearly evidences the dearomatization of the picoline moiety, with alternating double and single bonds. Monitoring the reaction between the imidazolium salt precursor and [Co{N(SiMe3)2}2] by EPR and NMR spectroscopy revealed the formation of an intermediate species, assigned to the 5-coordinate CoII complex [CoBr{N(SiMe3)2}(PNCNHC)].120

Scheme 43 Synthesis and reactivity of the CoII complex 135 bearing a dearomatized, tridentate phosphino-picoline-NHC (PNCNHC) ligand.

Reaction of 135 with one additional equivalent of [CoBr2(THF)2] led to an unusual metallation at the nucleophilic a-CHP side-arm and afforded the dinuclear CoII complex 136 bearing a re-aromatized PNCNHC ligand.119 The central CoII metal is in a distorted square-pyramidal environment, chelated by the neutral PNCNHC ligand and further coordinated by one terminal and one

676

N-Heterocyclic Carbene Complexes of Cobalt

bridging bromide. The coordination environment of the lateral CoII center is tetrahedral, with one sp3-hybridized a-C benzylic donor, one bridging and one terminal bromides, and one THF ligand. The value of the magnetic moment of 136 in solution (meff ¼ 4.8(2) mB) is lower than the theoretical value expected for two non-interacting high-spin CoII centers (5.48 mB), suggesting some degree of antiferromagnetic coupling between the two metals. Alkylation of 135 with LiCH2SiMe3 proceeded neatly and led to the square-planar low-spin CoII complex 137 which was characterized by X-ray diffraction and EPR spectroscopy.120 The reduction chemistry of 135 will be described below in Scheme 64, Section 7.11.2.4.2.2.3. 7.11.2.3.5.3 Ligands with higher denticity 7.11.2.3.5.3.1 Tripodal ligands—Phenolate/bis(NHC)amine The CoII complex 138 bearing a hybrid tripodal bis(NHC)-monophenolate ligand was obtained by reaction of the corresponding potassium salt with CoCl2 (Scheme 44).121,122 The solid-state structure of 138 revealed a distorted trigonal-pyramidal coordination geometry around the cobalt center, solely coordinated by the tripodal ligand. The Co–N bond separation of 2.141(2) A˚ suggests the presence of a coordination bond with the anchoring N atom, while no coordination with the chloride counteranion is observed. Salt metathesis with sodium azide yielded the CoII complex 139. Azide coordination was confirmed by IR spectroscopy with the presence of intense absorption bands centered at 2081, 2044 and 1999 cm–1. In the molecular structure of 139, the CoII center displays a trigonal-pyramidal coordination geometry, chelated in its base by the two NHC and the phenolate O donors, while the azide is located in axial position. The Co. . .N separation of 2.659(2) A˚ involving the anchoring N atom in 139 is too long to suggest coordination to the metal. Analysis of the 1H NMR spectra of 138 and 139 in different solvents suggested that the Cl– and N–3 anions are coordinated to the metal center in THF solution but are not coordinated in MeCN and chloroform solutions. The results of variable-temperature SQUID magnetization measurements were consistent with high-spin CoII complexes (S ¼ 3/2). The magnetic moments for 138 and 139 (4.3–4.4 mB) are significantly larger than the spin-only value for a triplet ground-state (3.87 mB), suggesting a large contribution from spin-orbit coupling.

Scheme 44 Synthesis and reactivity of 139 bearing a hybrid tripodal ligand.

Photolytic cleavage of the azide was observed upon irradiation of the complex at l ¼ 310 nm, in the presence of 2,4,6-tristert-butylphenol as a sacrificial hydrogen donor.122 The transient nitrido intermediate complex 140 (see below) led to the CoII complex 141 after metathesis with NaBPh4. The latter complex exhibits a modified ligand structure, resulting from an N-migratory insertion. The CoII center displays a four-coordinate trigonal-pyramidal geometry showing a Co-N separation with the anchoring N atom of 2.106(2) A˚ , in line with a bonding interaction. The transformation of the azido complex 139 into 141 is evident by comparison of the corresponding IR spectra, where the intense azide absorption bands have been replaced by bands at 3340 and 1608 cm–1 corresponding to the imine N–H and C]N stretching vibrations, respectively. Analysis of 141 by EPR spectroscopy and SQUID magnetization measurements supported a high-spin CoII center with an S ¼ 3/2 ground state. The isolation of 141 upon irradiation of 139 follows the formation of a transient nitrido complex, 140, the insertion of the nitrido ligand into one Co–CNHC bond, and finally an H-atom abstraction step. Low-temperature (10 K) photolysis experiments on 139 were monitored by EPR

N-Heterocyclic Carbene Complexes of Cobalt

677

spectroscopy and supported the formation of the highly reactive nitrido complex 140. The EPR spectrum was consistent with the formation of a S ¼ ½ low-spin d5 CoIV nitrido complex, which was further supported by DFT computational analysis. This nitrido intermediate complex readily reacts at temperatures higher than 50 K, which prevented its isolation in the solid state. The mechanism of the transformation was investigated by DFT calculations. The release of N2 from 139 leading to 140 involves a high activation barrier of 46.8 kcal mol–1, consistent with a photochemically-induced N2 elimination step. The following N-migratory insertion step into the Co–CNHC bond to form a Co-imido intermediate has a very low activation barrier of 2.2 kcal mol–1. Finally, the more stable carbene-imine-phenolate complex 141 is obtained after abstraction of an H-atom from 2,4,6-tris-tert-butylphenol.122 7.11.2.3.5.3.2 Pentadentate ligands The CoII complex 142 bearing a pentadentate pyridine-substituted ligand was accessed by transmetallation from the corresponding silver complex with CoII triflate under mild conditions (Scheme 45).123 Attempts to generate the free NHC carbene by deprotonation of the imidazolium salt precursor did not proceed cleanly. In the solid state, the pentadentate ligand of 142 features a square-pyramidal coordination mode with the NHC donor in the apical position. Analysis by EPR spectroscopy established a low-spin CoII center, showing the tendency of the ligand to stabilize low-spin electronic configurations.

Scheme 45 Synthesis of 142 by transmetallation from the Ag complex.

Additional CoII complexes will be described below in Section 7.11.2.4 dealing with mononuclear CoI complexes. In particular, this will be the case for 156 – 158, 184, 198, 199, 201, 203a – 203d and 238a,b shown in Fig. 4.

Fig. 4 Structures of the CoII complexes relevant to Section 7.11.2.4.

678

Table 3

N-Heterocyclic Carbene Complexes of Cobalt Selected structural and magnetic data for CoII complexes. Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)a

Spin state

References

13

1.893(2)

1.880(2) Co-N 1.967(2) Co-Cbenzyl

3.1(1)

LS CoII

69

14au

1.953(6)

1.691(6) Co¼N

4.8(1)

HS CoII

69

14as

1.971(5)

1.682(4) Co]N

4.6(1)

HS CoII

70

14b

1.949(4) [1.959(4)]b

1.675(3), [1.677(3)]b Co¼N

5.1(1)

HS CoII

70

20

1.962(4)

2.2363(13) Co-P 2.069(4)−2.263(3) Co-Carene

2.3(1)

LS CoII

78

21

1.912(5) 1.907(5)

2.277(2) Co-P 2.046(5) Co-Calkyl

2.6(1)

LS CoII

78

22

2.025(2)

1.842(2) Co-Oterminal 1.836(2) Co-Oside-on 1.977(2) Co-Nside-on

3.4(1)

LS CoII See text

79

Complex number

Formula

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

679

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

23

2.046(4)

1.724(3) Co-N 2.020(3) Co-Caryl

2.6(1)

HS CoII See text

79

26

1.937(4) 1.909(4)

1.47(4) Co-H 2.249(1) Co-Si

nd

71

27

1.906(2) 1.902(2)

2.292(1) Co-Si 2.051(2) Co-Cbenzyl

nd

71

37a

1.940(2)

1.870(2) Co-N

3.0(2)

LS CoII

75

37b

1.936(17)

1.8672(15) Co-N

2.9(1)

LS CoII

75

41

1.947(7) 1.964(7) 1.981(7) 1.969(6)

–

2.4

LS CoII

80

45

1.899(3)

1.910(3) Co-N 1.40(4) Co-H 1.67(3) Co-HSi 2.390(1) Co-Si

3.0(1)

LS CoII

69

46

1.922(2)

1.896(2) Co-N 1.62 Co-HSi 1.961(2) Co-Cbenzyl 2.407(1) Co-Si

3.0(1)

LS CoII

69

(Continued )

680

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

47

2.078(2)

1.918(1) Co-N 1.959(2) Co-Calkyne

4.5(1)

HS CoII

69

48au

2.054(4)

1.900(4) Co-N 2.222(2) Co-Cl

4.9

HS CoII

81,82

48as

No crystal structure

4.8

HS CoII

81

48b

2.055(2)

1.905(2) Co-N 2.2324(6) Co-Cl

5.0(1)

HS CoII

83

50au

2.105(6)

1.952(6) Co-N 1.950(6) Co-N

5.1

HS CoII

84

50as

2.127(2) [2.141(2)]b

1.950(2), [1.950(2)]b Co-N 1.955(2), [1.951(2)]b Co-N

4.5; 5.58c

HS CoII

85

50bu

2.119(3)

1.966(4) Co-N 1.958(3) Co-N

4.7

HS CoII

84

50bs

2.153(6)

1.968(5) Co-N, 1.971(5) Co-N

4.8; 5.34c

HS CoII

85

50c

2.064(5)

1.971(4) Co-N, 1.964(5) Co-N

4.7

HS CoII

84

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

681

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

50d

2.128(2), [2.122(2)]b

1.964(2), [1.960(2)]b Co-N 1.959(2), [1.961(2)]b Co-N

4.7; 5.46c

HS CoII

85

50e

2.117(1)

1.961(1) Co-N 1.977(1) Co-N

4.8; 5.66c

HS CoII

85

50f

2.083(4)

1.951(2) Co-N

4.8(2)

HS CoII

86

50g

2.075(2)

1.953(1) Co-N 1.927(1) Co-N

5.3(1)

HS CoII

87

51

2.130(1)

1.948(1) Co-N 1.972(1) Co-N

5.34c

HS CoII

85

52

2.059(2)

1.960(2) Co-N 1.930(2) Co-N

4.7

HS CoII

84

53

2.054(4)

2.033(3) Co-N 2.039(3) Co-N 2.2670(11) Co-Cl

nd

55

2.033(2)

1.8756(14) Co-O 2.2061(6) Co-Cl

5.1(1)

82

HS CoII

83

(Continued )

682

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

57

2.061(2)

1.8934(13) Co-N

4.5(1)

HS CoII

83

58

2.070(4)

1.901(3) Co-N 2.2391(12) Co-Cl 2.577(3) Co-Cipso

3.7(1)

HS CoII

83

59

2.078(2)

1.937(2) Co-Nsilyl 1.907(2) Co-Naryl

nd

85

60au

2.058(3)

1.896(2) Co-N

nd

85

60as

2.068(2)

1.898(2) Co-N

nd

85

60b

2.027(5)

1.888(4) Co-N 1.902(4) Co-N

4.0

HS CoII

85

62

2.021(2) 2.037(2)

2.6067(5) Co-I 2.6259(6) Co-I

4.05c

HS CoII

88

63

2.075(2)

1.9484(15) Co-N 1.9342(14) Co-N

nd

HS CoII

88

64a

2.061(2) 2.062(2)

2.2559(7) Co-Cl 2.2620(6) Co-Cl

4.37

HS CoII

89

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

683

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

64b

2.045(4) 2.054(4)

2.4085(7) Co-Br 2.4156(7) Co-Br

4.61

HS CoII

89

64c

2.086(5)

2.6084(6) Co-I

4.74

HS CoII

89

65

Low-quality X-ray data

–

4.5

HS CoII

90

70

2.097(3)

2.099(2) Co-Calkyl

5.1(2)

HS CoII

91

74a

2.057(2)

2.044(2) Co-C 2.031(2) Co-C

4.74c

HS CoII

73

74b

2.057(3)

2.048(3) Co-C 2.061(3) Co-C

4.90c

HS CoII

73

74c

2.059(2)

2.045(2) Co-C 2.045(3) Co-C

5.04c

HS CoII

73

74d

2.057(3) [2.058(3)]b

2.056(3) [2.054(3)]b Co-C 2.061(3) [2.061(3)]b Co-C

4.47c

HS CoII

73

74e

2.0582(13)

2.043(2) Co-C 2.044(2) Co-C

nd

73

(Continued )

684

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

75

2.0833(15)

2.0417(16) Co-C 1.9909(11) Co-O 1.9849(12) Co-O

nd

73

76

2.0872(11)

2.0470(12) Co-C 1.9961(8) Co-O 2.0040(9) Co-O

nd

73

77a

2.051(3)

2.044(2) Co-N 2.2414(8) Co-Cl 2.2414(8) Co-Cl

4.35c

HS CoII

92

77b

2.057(3)

2.065(3) Co-N 2.3888(7) Co-Br 2.3866(7) Co-Br

4.43c

HS CoII

92

77c

2.043(3)

2.066(3) Co-N 2.5869(7) Co-I 2.6012(6) Co-I

4.72c

HS CoII

92

77d

2.058(8)

2.050(6) Co-N 2.595(1) Co-I 2.591(1) Co-I

nd

HS CoII

93

79

1.919(2)

2.1910(13) Co-S

1.73

LS CoII

94

80

1.922(2)

2.1741(6) Co-S

nd

Spin state

References

94

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

81

2.011(2)

2.003(3) Co-Ccage

nd

82

2.048(2)

2.0001(8) Co-O 2.0582(8) Co-O

4.06c

HS CoII

96

83

2.050(2)

2.394(1) Co-Br 2.419(1) Co-Br 2.414(1) Co-Br

4.3

HS CoII

97

84a

2.069(2) 2.089(2)

2.2738(7) Co-Cl 2.2713(7) Co-Cl

3.9(1)

HS CoII

91

84b

2.053(2) 2.051(2)

2.2606(6) Co-Cl 2.2686(6) Co-Cl

4.2(1)

HS CoII

98

84c

No crystal structure

4.2c

HS CoII

99

84d

2.082(3)

2.414(1) Co-Br

4.2

HS CoII

97

84e

2.076(3) 2.076(3)

1.899(2) Co-O 1.888(2) Co-O

4.29

HS CoII

100

Spin state

685

References

95

(Continued )

686

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

85

2.084(6) 2.086(6)

2.2863(14) Co-S 2.2875(16) Co-S

4.2c

HS CoII

99

86

1.928(2)

1.997(2) Co-C

2.4(2)

LS CoII

101

87

1.915(2)

2.037(2) Co-C

2.4(1)

LS CoII

91

89

1.971(2) 2.002(2)

–

3.47 (see text); 1.95c

LS CoII

102

90

2.058(5) 2.055(5) 2.065(5) 2.061(5)

–

3.62 (see text); 4.16c

HS CoII

102

91a

2.016(2) 2.044(2) 2.045(2)

2.2581(7) Co-Cl

4.2(3)

HS CoII

103

92a

2.062(2) 2.069(2)

2.2838(8) Co-Cl 2.2916(9) Co-Cl

3.9(3)

HS CoII

103

92b

2.037(3) 2.055(3)

2.2664(9) Co-Cl 2.2653(8) Co-Cl

nd

103

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

687

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

93

2.061(2) 2.067(2) 2.035(2)

2.042(2) Co-CMe

4.1(3)

HS CoII

103

94

2.064(7) 2.099(6) 2.128(7)

1.886(7) Co-N

4.0(3)

HS CoII

104

95

2.014(2) 2.038(2) 2.045(2)

1.876(2) Co-O

4.13

HS CoII

105

96

2.056(4) 2.057(4)

–

4.9

HS CoII

106

99

1.907(5) 1.958(5) 1.913(5) 1.957(5)

2.401(7) Co-O

1.8

LS CoII

107

100

1.887(2)

2.025(2) Co-Cbenzyl

2.6(2)

LS CoII

108

103

1.895(4) 1.901(4)

2.057(5) Co-Cbenzyl 1.946(4) Co-CC¼N

nd

108

104

1.940(6) 1.905(7)

2.014(6) Co-Cbenzyl 1.973(5) Co-N

nd

108

(Continued )

688

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

105a

1.929(5) 1.891(5)

2.048(5) Co-Cbenzyl 2.300(2) Co-Si

2.6(1)

LS CoII

101

105b

1.931(2) 1.912(2)

2.067(2) Co-Cbenzyl 2.309(1) Co-Si

2.6(1)

LS CoII

101

105c

1.900(2) 1.907(2)

2.070(2) Co-Cbenzyl 2.327(1) Co-Si

2.7(1)

LS CoII

101

105d

No crystal structure

2.7(1)

LS CoII

109

107

1.928(3) 1.948(3)

2.274(1) Co-Si 2.169(4) Co-B 1.64(3) Co-H 1.68(3) Co-H

2.4(1)

LS CoII

101

108

1.896(3)

2.206(1) Co-Si 2.148(3) Co-N 2.001(2) Co-N

nd

110

2.030(3)

2.077(3) Co-N 2.224(1) Co-Cl 2.229(1) Co-Cl

4.24

HS CoII

110

111

1.914(2) 1.915(2)

2.402(2) Co-Ntriaz 2.430(2) Co-Ntriaz 1.937(2) Co-NMeCN 1.944(7) Co-NMeCN

nd

LS CoII

111

109

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

112

1.961(4) 2.00(1)

1.971(3) Co-Npicolyl 1.970(3) Co-Npicolyl

4.0

HS CoII

112

113a

2.077(2)

1.930(2) Co-Nsilylamide 1.956(2) Co-Nsilylamide

4.6

HS CoII

112

113b

2.078(3)

1.929(3) Co-Nsilylamide 1.961(2) Co-Nsilylamide

4.5

HS CoII

112

114

1.982(3)

1.902(2) Co-Nsilylamide 1.962(2) Co-Npicolyl

4.4

HS CoII

112

115a

2.005(4)

2.071(3) Co-N 2.2428(10) Co-Cl 2.2657(10) Co-Cl

4.3

HS CoII

113

115b

2.013(2)

2.084(2) Co-N 2.2449(7) Co-Cl 2.2474(7) Co-Cl

3.7

HS CoII

113

116

2.034(3)

1.885(2) Co-NH 1.883(2) Co-NCC

3.9(1)

HS CoII

66

117

No crystal structure

2.00

LS CoII

114

118

1.942(6) 1.941(6)

2.396(4) Co-O 2.358(4) Co-O

1.85

LS CoII

114

122

1.948(2) 1.943(2)

1.954(2) Co-CMe

1.8(5)

LS CoII

115

(Continued )

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

123

2.011(3)

2.091(3) Co-N 2.2752(12) Co-Cl 2.2500(14) Co-Cl

nd

124

2.020(3) 2.045(3)

2.094(2) Co-N 2.2544(8) Co-Cl

4.2

HS CoII

116

125a

1.963(4) 1.948(4)

1.871(3) Co-Caryl 2.4465(9) Co-Cl 2.025(3) Co-N

nd

LS CoII

117

125b

1.948(3) 1.930(3)

1.872(3) Co-Caryl 2.456(1) Co-Cl 2.011(3) Co-N

nd

LS CoII

117

129u

1.830(8)

1.809(5) Co-O 1.816(5) Co-O

1.82

LS CoII

118

129s

1.811(2)

1.8110(15) Co-O 1.8132(15) Co-O

1.88

LS CoII

118

130

1.829(8)

1.809(4) Co-O

1.90

LS CoII

118

135

1.919(3)

2.218(1) Co-P 1.897(2) Co-N 2.343(1) Co-Br

2.1(2)

LS CoII

119

137

1.947(2)

2.235(1) Co-P 1.938(1) Co-N 1.999(2) Co-Calk

2.2(1)

LS CoII

120

Spin state

References

116

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

691

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

138

2.021(2) 2.028(2)

2.141(2) Co-Nanchor

4.28

HS CoII

121

139

2.081(2) 2.060(2)

2.659(2) Co-Nanchor 2.039(2) Co-Nazide

4.42

HS CoII

121

141

2.008(3)

2.106(2) Co-Nanchor 1.949(2) Co-Nimine 1.893(2) Co-O

4.27

HS CoII

122

142

1.845(6)

2.214(5) Co-Npy 2.056(5) Co-Npy 2.073(5) Co-Npy 2.238(5) Co-Npy 1.958(5) Co-NMeCN

1.7(1)

LS CoII

123

156

No crystal structure

4.6(2)

HS CoII

124

157

1.939(2)

2.025(2) Co-Calk 2.032(2) Co-Cmesityl 2.2129(7) Co-P

2.4(2)

LS CoII

124

158

1.888(5)

1.934(9) Co-Cadamantyl 2.046(6) - 2.107(6) Co-CCp

2.0(1)

LS CoII

124

184

1.917(4) 1.978(4)

2.515(1) Co-Br 1.915(3) Co-N 2.052(4) Co-Calkene 2.087(4) Co-Calkene

nd

LS CoII

125

(Continued )

692

N-Heterocyclic Carbene Complexes of Cobalt

Table 3

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

meff (in mB)

Spin state

References

198

2.078(5) 2.080(5) 2.074(5)

2.256(2) Co-Cl

4.23

HS CoII

126

199

2.028(3) 2.033(3) 2.047(3)

2.035(3) Co-NMeCN

4.83

HS CoII

126

201

2.052(2), 2.049(2), 2.047(2)

1.938(2) Co-Nazide

nd

127

203b

2.033(3), 2.033(3), 2.199(2),

2.199(2) Co-Ntripod 1.990(2) Co-Nimine

nd

127

203c

2.043(2), 2.019(2),

2.169(2) Co-Ntripod 1.997(2) Co-Nimine

nd

127

238a

1.998(5) 1.997(5)

1.940(4) Co-N 1.971(3) Co-N

nd

–

128

238b

1.987(5) 1.990(5)

1.909(5) Co-N 1.944(4) Co-N

nd

–

128

a

Magnetic moment in solution, unless otherwise stated. Two or more crystallographically independent molecules in the asymmetric unit. c In the solid state. b

N-Heterocyclic Carbene Complexes of Cobalt

7.11.2.4 7.11.2.4.1

693

Mononuclear CoI complexes Monodentate carbene ligands

7.11.2.4.1.1 Homoleptic complexes 7.11.2.4.1.1.1 Complexes of type [Co(NHC)2]+(A–) The homoleptic two-coordinate CoI complexes 143au/s bearing (S)IMes ligands were obtained in good yield from the corresponding tri-coordinate [CoCl{(S)IMes}2] (101u/s) complexes by chloride abstraction (Scheme 46).108,130 Synthesis of the IAd analogue, 143b, was achieved by reaction of 144 with equimolar amounts of NaBArF4 and IAd free carbene.130 The 1H NMR spectra of the paramagnetic 143au/s–b indicated an idealized C2 symmetry for the [Co(NHC)2]+ cation in solution. Interestingly, the effective magnetic moments of 143au/s at room temperature (meff ¼ 5.40 and 5.10 mB, respectively) are much higher than that of the IAd complex 143b (meff ¼ 3.94 mB, Table 4), indicating a substantial contribution of unquenched orbital angular momentum in the former complexes. The solid-state structures of 143au/s–b were established by X-ray crystallography and revealed structural similarities in the three complexes, all featuring a linear CNHC-Co-CNHC arrangement. While no secondary intra- or inter-molecular interactions were observed in the (S)IMes complexes, secondary interactions between the Co center and the adamantyl group were found in 143b. Another interesting difference between the three structures lies in the value of the dihedral angle between the two mutually trans NHC rings. With the bulky IAd ligand, an orthogonal arrangement of the NHC planes is observed, while, with the less sterically hindered IMes and SIMes ligands, the dihedral angles (39.6 and 35.0 , respectively) are smaller. Ab initio calculations showed that changes in the dihedral angle between the two NHC rings dramatically affect the spin-orbit coupling splitting, owing to modifications in the Co-NHC p-interactions.

Scheme 46 Synthesis of the homoleptic CoI NHC complexes 143au/s–143c.

SQUID magnetization measurements on the IMes complex 143au revealed a notably large magnetic anisotropy. Furthermore, 143au exhibits a slow magnetic relaxation behavior under an applied dc field and was considered to be the first d8 single-ion magnet.130 In contrast, the magnetic anisotropy in 143as and 143b is lower and these complexes do not present single-ion-magnet behavior under applied dc fields. Such a disparity in the magnetic properties may originate from slightly different Co-NHC interactions, depending on the saturated/unsaturated character of the NHC ligand, the dihedral angle between the two NHC planes and secondary metal-ligand intramolecular interactions. The high-spin (meff ¼ 3.2 mB) homoleptic two-coordinate CoI complex 143c bearing two Et2cAAC ligands was obtained upon halide abstraction from 2b using NaBArF4 (Scheme 46).65 Its crystal structure revealed a slightly bent CcAAC–Co–CcAAC bonding angle, in contrast to the linear CNHC–Co–CNHC arrangement observed in 143au (168.35(9) vs. 178.58 , respectively). This bending contrasts with the perfectly linear arrangement observed in the Me2cAAC analogue 3.64 Cyclic voltammetry measurements on 143c revealed a reversible reduction process at E1/2 ¼ –1.79 V vs. Fc+/Fc, corresponding to the formation of the formal Co0 complex 1b (see Scheme 1 in Section 7.11.2.1.1.1).

694

N-Heterocyclic Carbene Complexes of Cobalt

Fig. 5 Structures of the CoI complexes 30 and 145.

Cationic bis(NHC) CoI complexes could also be obtained by substitution of the weakly bound ortho-difluorobenzene ligand in the cationic sandwich complex [Co(o-C6H4F2)2]+ featuring the weakly-coordinating [Al(OR)4]– (R ¼ C(CF3)3) anion.132 Using this procedure, [Co(IPr)2]{Al(OR)4} (143d) was isolated and structurally characterized. 7.11.2.4.1.1.2 Complexes of type [Co(NHC)3]+(A–) The homoleptic three-coordinate 30 was already described in Section 7.11.2.1.1.2.4 (Scheme 8).75 The structure of the trigonal-planar CoI complex 145 featuring an extremely bulky amido counter-anion has been published but no associated experimental data have been reported (Fig. 5).134 7.11.2.4.1.1.3 Complexes of type [Co(NHC)4]+(A–) The homoleptic CoI complexes 42a–c were obtained in good yield by reaction of the corresponding free NHCs with [CoCl(PPh3)3], followed by anion exchange with NaBPh4 (Scheme 13).80 The structures of 42a–c were determined by X-ray diffraction and established that the CoI center is in a square-planar coordination environment with four monodentate NHC ligands. A slight elongation of the Co–CNHC separations (Table 4) is observed when increasing the steric bulk on the NHC alkyl wingtips (on aver. 1.913(4), 1.931(3) and 1.970(2) A˚ for the Me-, Et- and iPr-substituted complexes, respectively). Cyclic voltammetry studies on 42a revealed two quasi-reversible oxidation processes at –1.26 V and 1.37 V, corresponding to the oxidation of the metal center in its divalent and trivalent states, respectively. In addition, an irreversible oxidation process was observed at ca. 0.90 V, which was associated with the oxidation of the borate anion.80 As mentioned above in Section 7.11.2.3.1.1 (Scheme 13), the corresponding CoII complex 41 could be obtained by oxidation of 42a with organic halides such as 3,5-dimethylphenyl iodide, benzyl bromide and 2-methyl-1,2-dichloropropane, indicating an easy interconversion between the +I and +II oxidation states in the [Co(Me2IEt)4]+/2+ core.80 The activity of the tetra(NHC) cationic CoI complexes 42 was evaluated in the reaction of alkenes with silanes. Notably, the sterically less hindered [Co(Me2IMe)4](BPh4) (42b) catalyzes the hydrogenation of alkenes using SiH2Ph2 as the terminal hydrogen source.178 7.11.2.4.1.2 Heteroleptic complexes 7.11.2.4.1.2.1 Complexes of type [CoX(NHC)] The first neutral two-coordinate CoI complex 146 bearing a Me2IiPr donor and a very bulky amide ligand was published in 2015 by the group of Jones; it was obtained in moderate yield from the corresponding [Co(amide)(Z6-C6H6)] precursor by substitution of the coordinated benzene (Scheme 47).134 This complex exhibits an almost linear coordination geometry around the CoI center, with an N–Co–CNHC angle of 175.2(1) . Depending on the crystallization conditions, another isomer (147) was isolated. Its crystal structure revealed a bent arrangement (N–Co–C angle of 140.2(1) ), with an additional intramolecular interaction with the ipso carbon of one phenyl group from the amido ligand. The Co-CNHC separation in the two-coordinate 146 (1.879(2) A˚ ) is slightly

Scheme 47 Synthesis of the two-coordinate isomeric CoI complexes 146 and 147.

N-Heterocyclic Carbene Complexes of Cobalt

695

shorter than that in the arene-coordinated 147 (1.979(3) A˚ ). Both complexes exhibit similar paramagnetically shifted resonances in their 1H NMR spectra, suggesting an easy interconversion between the two isomers in solution. Their solution magnetic moment at room temperature (meff ¼ 2.6(1) mB) is consistent with a high-spin S ¼ 1 ground state.134 Very recently, the synthesis of the two-coordinate, formally CoI, complex 148 was achieved using different synthetic strategies (Scheme 48).87,135 Starting from the mono(amido) complex 48b, reduction with KC8 or NaBHEt3 afforded paramagnetic 148 in moderate yield after recrystallization (40–50%). In an alternative, higher yielding procedure, 148 was obtained from the bis(silylamido) complex 50bu by reaction with PH2Mes (Mes ¼ 2,4,6-(tBu)3C6H2), along with an organic phosphinidene by-product (Scheme 48). No further reaction between 148 and excess PH2Mes was noted, in contrast to the aminolysis reactivity between PH2Mes and [Co{N(SiMe3)2}(Me2IMe)] in Scheme 16. Interestingly, the attempted reduction of the IMes substituted [CoCl{N(SiMe3)2}(IMes)] (48au) (Scheme 15, Section 7.11.2.3.1.2.1) with KC8 failed to give the expected [Co{N(SiMe3)2}(IMes)] complex, but the three-coordinate CoI complex [CoCl(IMes)2] (101u) (Scheme 33, Section 7.11.2.3.5.1.1) was isolated instead. The solid-state structure of 148 was established by X-ray diffraction and revealed a nearly linear coordination geometry at the metal center (N–Co–CNHC angle of 178.83(7) ). The Co–CNHC bond distance (1.942(2) A˚ ) is significantly shorter than that in the bis(silylamido) precursor 50bu (2.119(3) A˚ ). The magnetic properties of 148 were investigated by SQUID magnetometry, which revealed high magnetic moments.135 Furthermore, combined spectroscopic (XPS, XANES) and computational (DFT) analyses indicated that the electronic structure of 148 is best described by a high-spin CoII center coupled to one electron, the latter delocalized on the IPr ligand. Consequently, the IPr ligand is electronically non-innocent and exhibits a radical anionic character.

Scheme 48 Synthesis and reactivity of the two-coordinate, formally CoI complex 148.

Interestingly, the formal CoI complex 148 could also be accessed by treatment of 50bu with SiH(OEt)3, generating the silylamide Si(OEt)3[N(SiMe3)2] and dihydrogen as by-products (Scheme 48).87 This reaction corresponds to a rare example of CoII to CoI reduction by silane reagents. A possible mechanism for the formation of 148 involves the generation of a CoII hydride intermediate [CoH{N(SiMe3)2}(IPr)], followed by homolytic cleavage of the Co–H bond. Reaction of 148 with an additional equivalent of SiH(OEt)3 in benzene afforded the diamagnetic CoI hydride complex 149 in high yield, for which the hydride resonance was observed in the 1H NMR spectrum at d –21.6 ppm (Scheme 48). The driving force for both steps may be related to the formation of the by-product Si(OEt)3[N(SiMe3)2] which features a strong Si–N bond. In further reactivity studies, the reaction of 148 with NH2Mes afforded the dinuclear complex 150 after aminolysis of the Co-N(SiMe3)2 bond.135 The structure of this centrosymmetric complex was established by X-ray diffraction and revealed the presence of two bridging (NHMes)- anilido ligands between the two CoI centers. The Co-Co separation of 2.5765(4) A˚ is consistent with some degree of metal-metal interaction. 7.11.2.4.1.2.2 Complexes of type [CoX(NHC)L] The three-coordinate CoI complex 144 was prepared by reaction of IAd with [CoCl(PPh3)3] and isolated in good yield (70%) after recrystallization (Scheme 49).133 Its solid-state structure revealed secondary interactions between the Co center and two H atoms

696

N-Heterocyclic Carbene Complexes of Cobalt

Scheme 49 Synthesis and reactivity of the CoI alkyl complex 151.

from the adamantyl NHC wingtips. The corresponding CoI alkyl complex 151 was obtained by alkylation of 144 with LiCH2SiMe3. The coordination geometry around the metal in 151 is trigonal-planar, with a large CNHC–Co–Calkyl angle of 137.32(7) . The solution magnetic moment of this paramagnetic complex, meff ¼ 3.7(1) mB, is considerably larger than the spin-only value for a high-spin S ¼ 1 CoI center (mso ¼ 2.83 mB). The CoI alkyl complex 151 was used to access other (pseudo) tri-coordinate CoI complexes (Scheme 49).133 Reaction of 151 with SiH2Ph2 led to the first three-coordinate CoI silyl complex, 152, in moderate yield. The crystal structure of 152 and elemental analysis data confirmed the identity of the complex but further characterization was hampered by its fast decomposition in C6D6 solution. Treatment of 151 with PhC^CPh led to the CoI-alkyne-alkyl complex 153 featuring both an Z2-coordinated alkyne donor and a s-alkyl group. The Calkyne-Calkyne bond distance for the coordinated diphenylacetylene is relatively long (1.271(3) A˚ , to be compared to ca. 1.20 A˚ in free PhC^CPh), with a bent CPh-Calkyne-Calkyne arrangement (148 ), resulting from strong metalto-ligand back-donation. As observed in 144, short-contact interactions between the Co center and one or two H atoms of the adamantyl wingtip substituents were identified in the crystal structures of 151 – 153. Complex 151 is highly efficient and selective in the catalytic hydrosilylation of alkynes with SiH2Ph2 to give alkenylsilanes. Similar performances were observed for the silyl complex 152, suggesting formation of the latter complex within the catalytic cycle.133 Interestingly, the chlorido complex 144 efficiently catalyzes the anti-Markovnikov (i.e. 2,1-addition) hydrosilylation of monosubstituted alkenes with SiH2Ph2.178 The three-coordinate CoI complex 154 bearing the non-symmetrical Me2IAdMes ligand was obtained in high yield by metallation of the free NHC ligand with [CoCl(PPh3)3] (Scheme 50),124 by applying a procedure similar to that used for the synthesis of 144. Analysis of the molecular structure of this paramagnetic high-spin CoI complex (meff ¼ 3.5(1) mB) revealed a trigonal planar geometry around the metal center, with short contacts between two adamantyl C–H bonds and the metal. The corresponding distances are much shorter in 154 than in 144, which may result from steric repulsion between the adamantyl group and the NHC methyl backbone substituents.

N-Heterocyclic Carbene Complexes of Cobalt

697

Scheme 50 Synthesis and reactivity of the CoI complex 154.

Reaction of 154 with organometallic reagents afforded cyclometallated CoI and CoII NHC complexes in which C(sp3)–H bond activation occurred at one NHC wingtip (Scheme 50).124 For example, reaction of 154 with one equiv. LiCH3 led to the clean formation of the diamagnetic CoI complex 155 in 90% isolated yield. This complex results from a C(sp3)–H bond activation at the adamantyl NHC substituent, leading to the formation of a five-membered metallacycle with Co–CNHC and Co–Cadamantyl bond distances of 1.869(4) and 1.995(5) A˚ , respectively. The PPh3 ligand is coordinated to the cobalt center through an Z6-arene interaction. In contrast, treatment of 154 with LiCH2SiMe3 led to two different CoII complexes, depending on the reaction conditions. Using a 1:1 molar ratio, 156 was isolated and characterized spectroscopically, featuring 1H NMR resonances similar to those observed in the IPr analogue 70 (Scheme 21, Section 7.11.2.3.1.2.1). Reaction of 154 with two equiv. LiCH2SiMe3 resulted in another CoII complex, 157, isolated in moderate yield (36%). In this low-spin CoII complex (meff ¼ 2.6(2) mB), C(sp3)–H bond activation occurred at the mesityl NHC substituent, leading to the formation of a six-membered metallacycle with Co-CNHC and Co-Cbenzyl bond distances of 1.939(2) and 2.032(2) A˚ , respectively. Formation of the CoI alkyl intermediate [Co(CH2SiMe3) (Me2IAdMes)(PPh3)], an analogue of the alkyl complex 151, may be invoked in the synthesis of 156 and 157. This intermediate would further undergo a disproportionation reaction with formation of the putative Co0 complex [Co(Me2IAdMes)(PPh3)2], which may explain the moderate isolated yields (4 equiv.) SiH2Ph2 led to reduction of the cobalt center and formation of the diamagnetic CoI complex 195 in low yield (Scheme 64).120 The crystal structure of 195 indicated re-aromatization of the heterocycle and silylation of the pincer backbone at the a-CHP position. A possible mechanism involves, first, heterolytic cleavage of the Si-H bond through a metal-ligand cooperative process, generating a CoII-hydride intermediate. Homolytic cleavage of the CoII–H bond and concomitant reduction of the metal center would lead to the formation of 195.120 Such a homolytic Co–H bond cleavage has also been postulated in the formation of the CoI amido complex [Co{N(SiMe3)2}(IPr)] (148) from the [CoH{N(SiMe3)2}(IPr)] intermediate (see Scheme 48 in Section 7.11.2.4.1.2.1).87

710

N-Heterocyclic Carbene Complexes of Cobalt

7.11.2.4.2.3 Ligands with higher denticity: Tripodal ligands—Tris(NHC)amine The tris-carbene CoI complexes 196a,b were synthesized by reaction of the corresponding free carbene ligands with the CoI precursor [CoCl(PPh3)3] (Scheme 65).126 The 1H NMR spectra of the complexes are consistent with C3-symmetric species in solution. The tripodal ligand in 196a,b creates a protecting cavity which allows the coordination of an additional donor. Exposure of 196a to excess CO gas (1 atm) yielded the CoI–CO complex 197. The terminal bonding mode of the CO ligand was confirmed by the IR absorption band centered at 1927 cm–1. The magnetic moments of 196a (3.65 mB) and 197 (3.49 mB) at room temperature were determined by SQUID magnetization measurements and are consistent with high-spin CoI complexes (d8, S ¼ 1) with a large orbital angular momentum contribution (spin-only value of 2.83 mB for a triplet ground state).126 Although stable in the solid state under inert atmosphere, oxidation of 196a into the high-spin CoII complex 198 readily occurred in solution, especially in the presence of benzyl chloride or chlorinated solvents such as dichloromethane or chloroform. Treatment of 198 with NaBPh4 in acetonitrile afforded the dicationic high-spin CoII complex 199. The latter could also be obtained by reaction of 196a with NaBPh4 in MeCN but, in this case, several days were required for the reaction to reach completion. The molecular structures of 198 and 199 display a distorted trigonal-pyramidal geometry around the CoII center, with the chloride or the acetonitrile ligands in the axial position, respectively. The different complexes were further characterized by SQUID magnetization measurements and UV/ vis absorption spectroscopy.126

Scheme 65 Reactivity of 196a,b bearing a tripodal ligand.

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

Selected data for NHC CoI complexes.

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)a

2a

1.920(2), 1.932(2) [1.936(3), 1.920(3),]

2.2413(9) Co-Cl [2.2661(8) Co-Cl]

–

nd

2b

1.942(2) 1.936(2)

2.2884(6) Co-Cl

–

2.9

HS CoI

65

2c

1.926(2) 1.938(2)

2.3899(3) Co-Br

–

4.4(1)

HS CoI

64

3

1.944(3)

–

–

4.8(1)

HS CoI

64

18a

1.892(3)

2.2254(9) Co-P 2.044(3)−2.184 (3) Co-Carene

195.3

0

LS CoI

78

18b

No crystal structure

Co-P

187.5

0

LS CoI

78

19

1.970(2)

2.3701(7) Co-P 1.780(3) Co-CO 1.786(2) Co-CO 1.805(2) Co-CO

187.4

0

LS CoI

78

Spin state

711

References

63,64,129

(Continued )

712

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

28

2.030(5) 2.003(5) 2.009(5)

2.3890(13) Co-Cl

–

3.4(1)

HS CoI

75

30

1.962(2) 1.990(2) 1.983(3)

–

–

3.6(1)

HS CoI

75

42a

1.923(3) 1.928(3) 1.935(3) 1.936(3)

–

198.6

0

LS CoI

80

42b

1.910(4) 1.912(4) 1.914(4) 1.914(4)

–

201.3

0

LS CoI

80

42c

1.957(2) 1.970(2) 1.974(2) 1.978(2)

–

199.3

0

LS CoI

80

78

1.880(2)

2.1015(7) Co-P

191.3

0

LS CoI

94

101u

1.955(5) 1.953(5)

2.244(2) Co-Cl

–

4.4(1)

HS CoI

108

101s

1.914(2) 1.964(2)

2.303(1) Co-Cl

–

nd

HS CoI

130

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

713

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

102

1.930(4) 1.938(4)

2.184(4) Co-Cbenzyl

nd

0

LS CoI

108

120a

1.917(5) 1.909(5)

2.3169(8) Co-Br

nd

0

LS CoIIb

114,115

120b

1.904(3) 1.906(3)

2.2005(8) Co-Cl

178.2

0

LS CoI

115

121

1.914(4) 1.898(4)

1.951(4) Co-CMe

nd

0

LS CoIIb

114

127

1.911(3) 1.899(3)

1.872(2) Co-Caryl 1.802(2) Co-N

nd

0

LS CoI

117

128a

No crystal structure

210.8

0

LS CoI

117

128b

1.915(1) 1.900(1)

1.875(1) Co-Caryl 1.827(1) Co-N 2.2483(4) Co-P

208.9

0

LS CoI

117

128c

1.903(2) 1.908(2)

1.873(2) Co-Caryl 1.871(2) Co-N 2.213(1) Co-P

209.6

0

LS CoI

131

(Continued )

714

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

143au

1.937(2)

–

–

4.2(1); 5.40c

HS CoI

108,130

143as

1.936(2)

–

–

5.10c

HS CoI

130

143b

1.943(3)

–

–

3.94c

HS CoI

130

143c

1.957(2) 1.957(2)

–

–

3.2

HS CoI

65

143d

1.9858(11) 1.9859(11)

–

nd

nd

144

1.9708(12)

2.2294(3) Co-P 2.2448(3) Co-Cl

–

4.1(1)

145

1.991(3) 1.973(3) 1.996(3)

–

experimental procedure 146

1.962(3)

147

2.009(4)

134 1.879(2) Co-N

1.979(3) Co-N 2.103(3) Co-Cipso

132

HS CoI

133

No

–

2.6(1)

HS CoI

134

–

2.6(1)

HS CoI

134

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

715

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

148

1.942(2); 1.943(2)

1.873(2); 1.875(1) Co-N

–

4.1 5.1(1)

HS CoIIb

87,135

149

1.850(2)

1.44(3) Co-H

nd

0

LS CoI

87

151

2.000(2)

2.041(2) Co-Calkyl 2.236(1) Co-P

–

3.7(1)

HS CoI

133

152

1.980(6)

2.342(2) Co-Si 2.226(2) Co-P

–

nd

153

2.018(2)

1.952(2) Co-Calkyne 1.938(2) Co-Calkyne 2.034(2) Co-Calkyl

–

3.5(2)

HS CoI

133

154

1.961(4)

2.226(2) Co-Cl 2.215(2) Co-P

–

3.5(1)

HS CoI

124

155

1.869(4)

1.995(5) Co-Cadamantyl

185.2

0

LS CoI

124

159

1.943(2) [1.946(2)]d

–

–

2.6(1)

HS CoI

136

160a

1.895(3); 1.898(2)

1.689(3); 1.698(2) Co-CCO

183.3 (C6D6) 182.5 (acetoned6)

0

LS CoI

94,137

133

(Continued )

716

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

160b

1.907(2)

1.720(3) Co-CCO

190.1 (C6D6) 199.0 (acetoned6)

0

LS CoI

137

160c

1.875(8)

1.643(7) Co-CCO

nd

0

LS CoI

138

160d

No crystal structure

nd

0

LS CoI

138

160e

1.888(3)

1.699(4) Co-CCO

nd

0

LS CoI

139

160f

1.902(5)

1.695(4) Co-CCO

nd

0

LS CoI

136

162a

1.891(2)

1.995(2) Co-Cethylene 1.998(2) Co-Cethylene

189.3

0

LS CoI

137

162b

1.912(2)

1.991(2) Co-Cethylene 1.998(2) Co-Cethylene

193.8

0

LS CoI

137

162c

No crystal structure

191.5

0

LS CoI

138

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

193.7

0

LS CoI

138

191.1

0

LS CoI

140

183.0

0

LS CoI

140

162d

No crystal structure

163

1.906(2)

169a

No crystal structure

169b

1.897(5)

2.000(5) Co-Calkyne 1.943(6) Co-Calkyne

196.6

0

LS CoI

140

170

1.929(8)

1.97(2) Co-Cethylene 1.91(2) Co-Cethylene

241.4

0

LS CoI

141

171a

1.961(2)

2.071(2) Co-CH3 1.770(2) Co-CCO 1.774(3) Co-CCO 1.790(3) Co-CCO

187.2

0

LS CoI

142

171c

1.981(2)

1.964(2) Co-CF3 1.777(2) Co-CCO 1.799(2) Co-CCO 1.802(2) Co-CO

nd

0

LS CoI

143

171d

1.998(2)

2.079(2) Co-B 1.773(2) Co-CCO 1.762(2) Co-CCO 1.786(2) Co-CO

190.5

0

LS CoI

144

2.003(2) Co-Calkene 2.034(2) Co-Calkene

717

(Continued )

718

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

172a

2.004(2)

2.016(2) Co-COMe 1.770(3) Co-CCO 1.788(3) Co-CCO 1.815(2) Co-CO

185.0

0

LS CoI

142

172b

No crystal structure

186.5

0

LS CoI

142

173

No crystal structure

188.8

0

LS CoI

142

174

1.873(3)

2.015(2) Co-Cbenzyl

195.1

0

LS CoI

82

176

1.927(2) (not cyclometallated) 1.905(2) (cyclometallated)

1.991(2) Co-Cbenzyl 1.760(2) Co-N

203.5, 202.9

0

LS CoI

108

178

No crystal structure Unstable in solution

–

187.6

0

LS CoI

115

179

No crystal structure Unstable in solution

–

194.4

0

LS CoI

115

180

1.902(2) 1.898 (2)

1.750(2) Co-N2

194.1

0

LS CoI

115

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

719

d 13C NHC

meff (in mB)

Spin state

References

181a

No crystal structure

179.9

0

LS CoI

115

181b

No crystal structure

179.7

0

LS CoI

115

182

1.913(3) 1.903(3)

1.801(3) Co-N2

188.3

0

LS CoI

115

185

1.912(4) 1.913(3)

1.945(3) Co-N 2.041(3) Co-Calkene 2.097(3) Co-Calkene 2.037(3) Co-Calkene 2.079(3) Co-Calkene

191.1

0

LS CoI

125

189

1.901(3) 1.901(3)

1.855(4) Co-Caryl 1.962(2) Co-Npy

nd

0

LS CoI

145

191a

1.893(2) 1.892(2)

1.867(2) Co-Caryl 1.893(2) Co-N 2.240(1) Co-P

nd

0

LS CoI

145

191b

1.892(6) 1.908(5)

1.870(5) Co-Caryl 1.892(5) Co-N 2.221(2) Co-P

207.2

0

LS CoI

145

(Continued )

720

N-Heterocyclic Carbene Complexes of Cobalt

Table 4

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

193

1.886(3)

1.904(2) Co-Npy 2.225(1) Co-P 1.789(3) Co-N2

191.8

0

LS CoI

119

194

1.914(4) (P NaC) 1.760(5) (PNC)

2.202(1) Co-P 2.207(1) Co-P 1.904(3) Co-Na 1.918(4) Co-Npy 1.960(4) Co-Cbackbone 1.742(4) Co-N2

191.5 179.7

0

LS CoI

119

195

1.892(2

1.859(2) Co-Npy 2.1841(6) Co-P 2.3394(4) Co-Br

nd

0

LS CoI

120

196a

No crystal structure

–

3.65

HS CoI

126

197

2.035(2) 2.055(2) 2.027(2)

1.846(2) Co-CCO

–

3.49

HS CoI

126

229

1.910(2) 1.904(2)

2.2283(7) Co-Si 1.790(2) Co-N

198.3

0

LS CoI

146

a

Magnetic moment in solution, unless otherwise stated. See text, redox-active ligand. c In the solid state. d Two or more independent molecules in the asymmetric unit. b

N-Heterocyclic Carbene Complexes of Cobalt

721

Clean formation of the diamagnetic CoIII–O2 addition product 200 occurred upon exposure of 196a to O2 at room temperature. Side-on coordination of the peroxo ligand was indicated by an O–O stretching frequency at 890 cm–1 in the IR spectrum of 200, and further confirmed by X-ray diffraction analysis. The O–O distance of 1.429(3) A˚ lies in the typical range for peroxide complexes (1.4–1.5 A˚ ). DFT calculations support the formulation of the complex as a low-spin CoIII metal center (d6, S ¼ 0) with a dative peroxo ligand. The main contribution in the HOMO arises from the dioxygen p orbitals, suggesting some nucleophilic character for the coordinated dioxygen ligand, and a possible reactivity towards electron-deficient organic substrates. The reaction of 200 with benzoyl chloride in acetonitrile led to an oxygen-transfer reaction with quantitative formation of phenyl benzoate and the CoII complex 199. The reaction of 200 was also investigated with electron-deficient alkenes. However, only the reaction with benzylidenemalonitrile occurred cleanly, leading to 199 and benzyl aldehyde (Scheme 65). The lack of electrophilicity of 200 was confirmed by the absence of reactivity towards styrene, cyclohexene or triphenylphosphine. Accordingly, 200 was classified as a Class II nucleophile.181 In the attempted synthesis of a CoIII imido complex, 196a was treated with trimethylsilyl azide (Scheme 65).127 However, no oxidation of the metal center was observed, but rather formation of the one-electron oxidized azido species 201 occurred, which could be successfully crystallized after anion metathesis by treatment with tetrabutylammonium tetraphenylborate. Reaction of 196a,b with aryl azides at low temperature (–35  C) led to the corresponding mononuclear CoIII imido complexes 202a–d in almost quantitative yields. The 1H NMR spectra of these diamagnetic CoIII complexes suggest a C3-symmetry in solution. Analysis of the crystal structure of 202c reveals a pseudo-tetrahedral coordination geometry around the Co metal center. The short Co]Nimido distance of 1.675(2) A˚ , in the expected range for CoIII imido complexes, reflects a strong Co]N multiple bond character. DFT calculations indicate that the LUMOs of the complex consist of the empty dxy and dyz orbitals of the metal which are destabilized by a strong p-bonding interaction with the imido p-lone pairs. The Co]N bonding in 202c is best described as a formal double bond.127 Although stable in the solid state and in solution at –35  C, 202a–d react at room temperature and insertion of the imido group into one of the cobalt-carbene bonds yielded the CoII imino complexes 203a–d. This reaction possibly involves a CoI imino intermediate that further disproportionates into 203a–d, metallic Co0 and other unidentified organic products. The solid-state structures of 203b and 203c were established by X-ray diffraction and confirmed the coordination of two carbene units, the anchoring nitrogen atom and the newly formed imine N donor. The observed imido insertion reactivity shows that the cobalt-imido complexes 202a–d are highly electrophilic. However, no intermolecular imido transfer was observed upon treatment of 202a–d with nucleophiles such as styrene or tetramethylimidazol-2-ylidene (Me2IMe), suggesting that the intramolecular process is faster.127

7.11.2.5 7.11.2.5.1

Mononuclear CoIII complexes Monodentate carbene ligands

7.11.2.5.1.1 Heteroleptic complexes of type [CoX3(NHC)L3] The CoIII complexes 161,136 164,137 165,138 and 166 (Scheme 66)148 were obtained by oxidative addition, starting from [Co(Z5-C5R5)(NHC)(L)] (R ¼ H, Me; L ¼ no ligand, CO, C2H4), as described in Section 7.11.2.4.1.2.4. Among them, 164 and 166 have been structurally characterized.

Scheme 66 Oxidative addition reactivity of Cp NHC CoI complexes.

722

N-Heterocyclic Carbene Complexes of Cobalt

Reaction of 165 with MgMe2 led to the CoIII dimethyl complex 204a in low yield (< 10%), which was characterized by NMR spectroscopy and mass spectrometry.138 The corresponding IPr complex, 204b, was obtained by displacement of the phosphine donor in [CoMe2Cp(PPh3)] with free IPr (Scheme 67).139 However, only partial substitution was observed, probably the result of the high steric demand of the IPr ligand. The pseudo-tetrahedral piano-stool complex 204b could be isolated from the equilibrium mixture by chromatography. Determination of the equilibrium constants revealed a large negative contribution from the entropic factor (DH ¼ –24.4 +/– 1.7 kJ mol–1 and DS ¼ –65.8 +/– 5.5 J mol–1 K–1), which can be explained by the high steric crowding in the NHC complex. Addition of the smaller and more basic PMe3 phosphine donor to 204b resulted in a complete substitution of the IPr ligand and formation of [CoMe2Cp(PMe3)].139

Scheme 67 Equilibrium mixture in the synthesis of 204b.

7.11.2.5.1.2 Heteroleptic complexes of type [CoX2(NHC)L3]+(A–) The intense purple NHC CoIII complex 205 was obtained by decarboxylation of N,N’-dimethylimidazolium-2-carboxylate in the presence of meso-tetraphenylporphyrin CoIII chloride (Scheme 68).150 Coordination of the NHC ligand in 205 was established by NMR spectroscopy. The signal for the Me wingtip substituents of the NHC donor is high field shifted (d –0.63 ppm) because of the porphyrin ring current. Even in the presence of excess imidazolium carboxylate, only the mono-NHC-substituted complex 205 was isolated. Halide abstraction with AgBF4 afforded 206 which added MeOH or EtOH to give the structurally characterized alcohol complexes 207a,b, respectively. The CoIII center lies in an octahedral coordination environment with the porphyrin ligand

Scheme 68 Synthesis of porphyrin NHC CoIII complexes.

N-Heterocyclic Carbene Complexes of Cobalt

723

occupying the equatorial positions while the NHC and alcohol ligands are located in the axial positions. Distortion of the porphyrin macrocycle is observed, probably due to the short Co–CNHC bond (1.96 A˚ ), preventing the coordination of a second NHC ligand. Addition of 1,2-dimethylimidazole or 2,4-dimethylimidazole to 205 led to the CoIII NHC-imidazole complexes 208a,b which were isolated after purification by preparative thin-layer chromatography and characterized spectroscopically.150

7.11.2.5.2

Tris-carbenes complexes

The homoleptic CoIII tris(carbene)borate complexes 209a–c bearing tripodal NHC ligands were obtained by in situ deprotonation of the corresponding tris(imidazolium)borate salts, followed by addition of CoCl2 or [Co(acac)2] (Scheme 69).151,152 Air was passed through the reaction mixture in order to ensure oxidation of the metal to its trivalent state. In the IR spectra of 209a,b, the B–H stretching vibrations were detected at 2400–2500 cm–1. The solid-state structures of 209b and 209c were unambiguously established by X-ray diffraction studies, revealing a CoIII metal center in a coordination environment of approximate S6 symmetry, coordinated by six NHC donors. The heteroleptic CoIII complex 211 was obtained by metallation of the in situ generated tris(NHC) borate free carbene ligand with one equiv. [Co(acac)2] in the presence of 1-methylimidazole, followed by aerobic oxidation (Scheme 69).152 This complex efficiently catalyzes the room temperature oxidation of cyclohexane with meta-chloroperbenzoic acid (mCPBA).

Scheme 69 Synthesis of CoIII complexes with tripodal NHC-borate ligands.

Treatment of the tris(NHC)borate CoII amido complex 94 (Scheme 30 in Section 7.11.2.3.3) with the stable 2,4,6-tri(tertbutyl)phenoxy radical resulted in the formation of the diamagnetic CoIII imido complex 212 along with 2,4,6-tri(tert-butyl) phenol (Scheme 70).104 The X-ray structure of 212 establishes a short Co–N bond distance (1.660(3) A˚ ) and a linear Co–N–C

Scheme 70 The tris(NHC)borate scaffold stabilizes CoIII-imido and -oxo complexes.

724

N-Heterocyclic Carbene Complexes of Cobalt

bond angle (179.7(3) ), consistent with the occurrence of an imido complex. The formation of 212 involves both a proton-transfer (PT) and an electron-transfer (ET) step, likely occurring through a concerted proton-electron transfer (CPET) pathway. Attempts to generate intermediates arising solely from a PT or ET step were unsuccessful, pointing towards a concerted mechanism, which was further supported by DFT calculations. The gas-phase N–H BDE (enthalpy) of 94 was estimated at 75 kcal mol–1 (to be compared with the O–H BDE of 2,4,6-tri(tert-butyl)phenol of 81.2 kcal mol–1), consistent with a spontaneous reaction. Interestingly, TEMPO (TEMPO–H, O–H BDE ¼ 69.7 kcal mol–1) does not react with 94, possibly due to a high kinetic barrier.104 Chemical oxidation of the tris(NHC)borate CoII hydroxo complex 95 (see Scheme 30 in Section 7.11.2.3.3) with [FeCp2] BF4 led to the corresponding diamagnetic CoIII complex 213 (Scheme 70).105 Clean conversion back to 95 was possible upon reduction of 213 with cobaltocene, indicating the reversibility of the redox process, as previously suggested by cyclic voltammetry studies. Although 213 features a limited thermal stability, structural data could be obtained at low temperature and revealed a CoIII center in a highly distorted tetrahedral coordination environment. The slightly shorter Co–O bond distance in 213 (1.776(7) A˚ ) compared to that in 95 (1.876(2) A˚ ) is consistent with the oxidation of the metal center. Treatment of 213 with the strong base LiN(SiMe3)2 led to 214 featuring a highly unusual terminal CoIII-oxo group. The molecular structure of the complex was determined by X-ray diffraction and revealed a tetranuclear arrangement with coordination of one equivalent of LiBF4 per monomer. Decoordination and precipitation of the lithium salt resulted from addition of 2,2,1-cryptand, leading to the mononuclear and salt-free CoIII-oxo complex 215 (Scheme 70).105 The latter features a Co–O bond distance of 1.682(6) A˚ , ca. 0.1 A˚ shorter than that in the hydroxo complex 213, which supports an increased formal bond order in 215. Although the CoIII-oxo complexes 214 and 215 are thermally unstable, their relatively slow rates of decomposition (half-lives of ca. 6.5–8 h at room temp.) did not hamper the study of their reactivity towards O- and H-atom transfer reagents. Reaction with 9,10-dihydroanthracene as an H-atom source led to a net H-atom transfer reactivity with formation of anthracene and 95. Reaction of 214 or 215 with PMe3 resulted in the formation of O]PMe3, i.e. a net O-atom transfer from Co to P, although the nature of the cobalt complex formed is unclear.105 A bond dissociation free energy (BDFE) of 84.6 kcal mol–1 for the O-H bond in 95 was determined through a precise measurement of the pKa of the complex (ca. 25.6 in MeCN).182 This BDFE value is consistent with the instantaneous reaction of 215 with 2,4,6-tri(tert-butyl)phenol, leading to the corresponding phenoxyl radical and 95.105 The relatively high pKa value for 95 implies an unusual high basicity for the CoIII-oxo complex 215 in comparison with other transition metal oxo complexes, which may have a direct influence on the mechanism of H-atom transfer reactions.182 Indeed, C–H activation reactions induced by transition metal oxo complexes are usually governed by the BD(F)Es of the C–H bond being broken and the O–H bond being formed, substrates with stronger C–H bonds reacting with slower rates. In contrast, the C–H activation rates of various H-atom donors with 215 were found to correlate with the pKa of the substrates, rather than the BDEs of the C–H bonds cleaved. This observation suggests a basicity-controlled mechanism of H-atom transfer, which could be consistent with either a stepwise proton and electron transfer, or a basic asynchronous coupled proton-electron transfer (CPET). Distinction between these two mechanisms was possible on the basis of combined experimental and DFT computational studies. The resulting data showed that C–H activation by the CoIII-oxo complex 215 does not occur via a stepwise process but rather involves a pKa-driven concerted asynchronous mechanism.182 DFT calculations further revealed similarities in the bonding situation between 215 and the isoelectronic imido complex 212.104,105

7.11.2.5.3

Functionalized NHCs

As shown below, cobalt(III) complexes featuring diverse functionalized NHC ligands have been obtained by different methods such as transmetallation reactions from AgI, NiII and ZrIV complexes, treatment of imidazolium salts with CoCl2 in the presence of base (with further oxidation), oxidation of CoII complexes and oxidative addition from CoI complexes.

7.11.2.5.3.1 Bidentate ligands 7.11.2.5.3.1.1 Nitrogen-donor functionalized The CoIII complexes 216 and 217a bearing nitrogen-donor functionalized NHC ligands were obtained using the NHC transfer strategy from the corresponding AgI or NiII complexes, respectively (Scheme 71).153,154 For the synthesis of 216, the silver complex was generated in situ by reaction of the imidazolium salt precursor with Ag2O and further treated with [CoCl2(PPh3)2] under air atmosphere.153 Although a CoII precursor was used for the transmetallation, a trivalent CoIII complex was isolated, which can be explained by a spontaneous oxidation of the metal center in air. Analysis of the complex by 1H NMR spectroscopy revealed a mixture of two isomers of 216 in solution. However, only one isomer was isolated in the solid state and it features a CoIII center in an octahedral environment with the two NHC donors arranged in a cis fashion.153

N-Heterocyclic Carbene Complexes of Cobalt

725

Scheme 71 Synthesis of CoIII complexes with bidentate NHC-based ligands.

The synthesis of 217a involves an unusual transmetallation from nickel to cobalt (Scheme 71).154 The NiII intermediate complex was prepared in moderate yield by reaction of the corresponding imidazolium salt with Ni metal in air. Further treatment with CoCl2 afforded the CoIII complex 217a, which was isolated in 31% yield. No crystal structure was reported and the metal-to-ligand ratio in the complex was determined on the basis of elemental analysis data. As for the synthesis of 216, a CoIII complex, rather than the expected CoII analogue, was isolated owing to oxidation by air. Such a transmetallation from Ni to Co offers an alternative to the use of NHC-Ag complexes as carbene transfer reagents but is sometimes hampered by relatively low yields in comparison with the latter route. Another strategy was used for the synthesis of the corresponding complex bearing a butenyl NHC wingtip substituent, 217b (Scheme 71).155 Treatment of the imidazolium salt precursor with [Co(OAc)2
4H2O] and K2CO3 afforded the diamagnetic CoIII complex 217b
KPF6 in moderate yield (60%) after recrystallization. The Ag to Co transmetallation strategy was also used to access the mesoionic pyridylcarbene complex 219 (Scheme 72).156 After the Ag-NHC complex was prepared in situ by reaction of the triazolium salt proligand with Ag2O, addition of the chlorido-bridged dinuclear CoIII precursor [CoCp (m-Cl)Cl]2 afforded 219 in 34% yield. The X-ray structure of this complex revealed a three-legged

Scheme 72 Synthesis of 219 bearing a bidentate pyridyl-NHC ligand.

726

N-Heterocyclic Carbene Complexes of Cobalt

piano-stool-type coordination geometry with one 5-coordinated Cp ring, one chloride and one bidentate pyridylcarbene ligand. Two reduction processes were observed by cyclic voltammetry. The first reduction wave was detected at –1.1 V vs. Fc+/Fc, which probably corresponds to the reduction of the metal center with dissociation of one chloride ligand. The second reduction at –1.59 V is possibly associated with ligand reduction. The activity of 219 in electrocatalytic H2 production was investigated and the CoIII complex proved to be an efficient electrocatalyst at very low overpotential and high turnover. A remarkable robustness of the complex was observed with no detected decomposition even in the presence of 50 equiv. acetic acid over 1 day.156

7.11.2.5.3.1.2 Sulfur-donor functionalized The CoIII complex 220 bearing three thiolate-functionalized 1,2,4-triazolylidene ligands was prepared in 68% yield by reaction of the corresponding 1,2,4-triazolium salt with anhydrous CoCl2 and excess K2CO3 (Scheme 73).157 As observed with the nitrogen-donor functionalized NHC complexes 216 and 217a,b (Scheme 71), oxidation of the metal center occurred, probably by reaction with either traces of O2 or by generation of H2. The solid-state structure of 220 revealed a distorted octahedral coordination environment with three bidentate ligands. Two NHC donors are in mutual trans position while the third NHC ligand is coordinated trans to a sulfur donor. This non-symmetrical arrangement leads to two different carbene resonances at d 177.2 and 179.8 ppm in the 13C{1H} NMR spectrum. In addition, the Co–CNHC separations for the trans CNHC–Co–CNHC donors (1.946(3) and 1.969(3) A˚ ) are slightly longer than that of the NHC located trans to the sulfur group (1.918(3) A˚ ), which reflects the stronger trans influence exerted by the NHC ligand.157

Scheme 73 Synthesis of 220 bearing a bidentate sulfur-NHC ligand.

7.11.2.5.3.2 Tridentate ligands The syntheses of CoIII complexes grouped below in Fig. 8 for convenience have already been described in previous sections (119 in Scheme 39, Section 7.11.2.3.5.2.1,114 126a,b in Scheme 41, Section 7.11.2.3.5.2.2,117 131 in Scheme 42, Section 7.11.2.3.5.2.3,118 and 187 and 188 in Scheme 62, Section 7.11.2.4.2.2.2).131

Fig. 8 CoIII complexes with tridentate ligands.

7.11.2.5.3.2.1 Symmetrical CNHCCCNHC pincers Reaction of the formally CoI pincer complex 127 (Scheme 41, Section 7.11.2.3.5.2.2)117 with silanes resulted in the oxidative addition of one Si–H bond across the metal center (Scheme 74).158 While a rapid decomposition of the CoIII products was observed

N-Heterocyclic Carbene Complexes of Cobalt

727

Scheme 74 Formation of silyl complexes by reaction of 127 with SiH2Ph2.

after treatment of 127 with SiH3Ph or SiHMe2Ph, reaction with SiH2Ph2 gave 221 which was stable enough to be characterized in situ by 1H and 29Si NMR spectroscopy. The value of the 2JSiH coupling constant (ca. 13 Hz) is consistent with an oxidative addition of the Si-H bond, rather than a simple Z2-coordination of the silane. In the course of attempts to isolate 221, a few crystals of the bis(silyl) CoIII complex 222 were formed and their identity was unambiguously determined by X-ray diffraction studies. However, further isolation and characterization of this complex proved difficult. The formation of 222 probably results from a s-bond metathesis of 221 with an Z2-coordinated silane moiety.158 Various CoIII-CNHCCCNHC complexes were obtained by transmetallation of the corresponding anionic pincer ligand from Zr to Co (Scheme 75).159 The Zr complex was generated in situ by reaction of the imidazolium salt precursor with 2.5 equiv. of [Zr(NMe2)4]. Subsequent addition of [Co(acac)3] afforded a complex mixture from which 223 was isolated and structurally characterized. The presence of other products, assigned to 224 and 225, was established by comparison of the 1H NMR spectra of the mixture with that of authentic samples (vide infra). In the solid-state structure of 223, the CoIII center is in a distorted octahedral environment containing the anionic tridentate pincer, two chlorides and one neutral amine ligand. Although the precursor imidazolium salt contained iodides as counter-anions, the presence of chloride ligands may be due to an exchange

Scheme 75 Synthesis of CoIII pincer complexes by transmetallation from Zr.

728

N-Heterocyclic Carbene Complexes of Cobalt

with dichloromethane. In order to avoid such halide scrambling, the imidazolium chloride proligand was treated with 2.5 equiv. of [Zr(NMe2)4], followed by addition of CoCl2 (Scheme 75). This reaction resulted in the formation of another CoIII complex, identified as 224 by mass spectrometry and NMR spectroscopy. The oxidation of the metal center may be due to adventitious reaction with traces of oxygen. Reaction of the isolated ZrI2(NMe2)-CNHCCCNHC complex with [Co(acac)3] led to the formation of air-stable 225, which features a distorted octahedral coordination geometry around the CoIII center,159 along with small amounts of 224.160 The Co–Caryl bond distance of 1.874(5) A˚ in 225 is noticeably shorted than that in 223 (2.107(4) A˚ ) (Table 5). It should be noted that, upon extended periods of crystallization, 223 dimerizes by decoordination of the dimethylamino ligand, leading to the halide-bridged dimer 226.160

7.11.2.5.3.2.2 Symmetrical CNHCSiCNHC pincers Oxidation of 105a (Scheme 33, Section 7.11.2.3.5.1.1)101 with [FeCp2]BPh4 afforded the diamagnetic CoIII complex 227 in good yield (Scheme 76).146 A hydride resonance was detected in its 1H NMR spectrum at d –33.4 ppm. Although crystals of the etherate complex 227 suitable for X-ray diffraction studies could not be obtained, exchange of the coordinated Et2O molecule with MeCN afforded 228 whose solid-state structure revealed the formation of a CoIII hydrido-silyl complex bearing a new NHC-based pincer ligand with a central anionic silyl donor. This new ligand scaffold originates from the coupling of a silyl-NHC chelate and a benzyl-NHC chelate. This coupling reaction is induced by the one-electron oxidation of the metal center, but the detailed mechanism remains elusive. The cobalt center in 228 lies in a square-pyramidal coordination geometry, with the tridentate CNHCSiCNHC pincer ligand chelating the metal center in a meridional fashion.146

Scheme 76 Synthesis and reactivity of the CoIII complex 227 bearing a CNHCSiCNHC pincer ligand.

N-Heterocyclic Carbene Complexes of Cobalt

729

Reduction of 227 with LiBHEt3 under N2 atmosphere afforded the CoI complex 229 in 22% yield. Its solid-state structure revealed a distorted square-planar geometry around the metal center which is coordinated by the anionic tridentate CNHCSiCNHC chelate and a terminal N2 ligand. Surprisingly, the Si–Co–N angle of 131 differs from the linear arrangement usually observed in CoI-N2 pincer complexes. This unusual seesaw geometry may be a consequence of the strong trans influence of the silyl group. In the IR spectrum of 229, the stretching vibration of the coordinated N2 ligand was detected at 2004 cm–1 (Table 9), indicating the high s-donation ability of the pincer ligand.146 The reactivity of the electron-rich CoI complex 229 was investigated towards the oxidative addition of C–H, N–H and O–H bonds (Scheme 76).146 Upon loss of N2, a reversible cyclometallation reaction was observed, leading to the CoIII complex 230 as a result of formal oxidative addition of one benzylic C–H bond across the CoI metal center. Reaction of 229 with 2-hydroxypyridine and 1-naphthylamine resulted in the formation of the CoIII complexes 231 and 232, respectively, through irreversible intermolecular N–H and O–H bond activation processes. 7.11.2.5.3.2.3 Symmetrical NCNHCN pincers Using the same strategy as for the synthesis of 217a (Scheme 71, Section 7.11.2.5.3.1.1), the CoIII complex 233 bearing two tridentate NCNHCN pincer-type ligands was obtained by transmetallation from the corresponding NiII complex with CoCl2 (Scheme 77).154 Oxidation of the metal center may be due to reaction with traces of oxygen.

Scheme 77 Synthesis of 233 by transmetallation from Ni.

7.11.2.5.3.2.4 Non-symmetrical NCNHCN pincers Complex 218 (Scheme 71, Section 7.11.2.5.3.1.1) was obtained by transmetallation from an in situ generated AgI complex.153 Here also, isolation of the CoIII complex was due to a spontaneous oxidation of the metal center in air. Analysis of the 1H NMR spectrum of 218 revealed a rapid dissociation/association of the ligand in solution.

7.11.2.5.3.2.5 Non-symmetrical NNCNHC pincer Using a non-symmetrical NHC ligand featuring pyridine and phenanthroline donors, and following the same strategy as above, transmetallation of the in situ generated NHC silver complex with [CoCl2(PPh3)2] afforded the air stable CoIII complex 234 (Scheme 78).161 Analysis of its molecular structure revealed a distorted octahedral geometry with two ligands bound to the Co

Scheme 78 Synthesis of a CoIII complex with a tridentate NHC-phenanthroline ligand.

730

N-Heterocyclic Carbene Complexes of Cobalt

center in a pincer NNCNHC fashion while the pyridine group is dangling. The Co–N distances corresponding to the N donors located trans to the NHCs are slightly elongated (0.14–0.17 A˚ , Table 5), which can be ascribed to the larger trans influence of the NHC moieties. No significant catalytic performance of the complex was observed in tandem click/Sonogashira reaction.161 The CoIII complexes 235 and 236 bearing tridentate pyridyl-amide-NHC ligands were obtained in moderate yield (25–30%) by reaction of the corresponding imidazolium salts with CoII acetate, irrespective of the metal/ligand stoichiometry, followed by oxidation in air (Scheme 79).162 Structural analysis by X-ray diffraction revealed CoIII centers in octahedral coordination geometries with meridional binding of the tridentate ligands, while the additional pyridyl and benzyl groups in 235 and 236, respectively, remain dangling. X-Ray quality crystals of 236 could only be obtained after anion exchange with NH4PF6 but complete refinement of the structure was hampered by severe anion disorder. Analysis of the complexes by cyclic voltammetry revealed two reversible redox processes at –1.20 V and –1.85 V vs. AgCl/Ag for 235, with a similar behavior observed for 236. These redox events were tentatively assigned to the CoIII/CoII and CoII/CoI redox couples. The electrocatalytic activity of the complexes in proton reduction was examined in DMF, using acetic acid as the proton source.162

Scheme 79 Synthesis of CoIII complexes with a tridentate NHC-pyridyl-amide ligand.

7.11.2.5.3.2.6 Non-symmetrical CNHCNC ligands The CoIII pincer hydrides 237a,b were obtained in low isolated yields (7–14%) by deprotonation of the non-symmetrical CNHCNC ligand precursor with nBuLi, followed by reaction with [CoMe(PMe3)4] (Scheme 80).128 The paramagnetic CoII complexes 238a,b

Scheme 80 Synthesis of non-symmetrical CoIII CNHCNC hydride complexes.

N-Heterocyclic Carbene Complexes of Cobalt

731

featuring two trans-coordinated bidendate CNHC-N ligands were identified as side-products and structurally characterized (Scheme 80). Formation of these two different types of cobalt complexes may arise from an intermediate CoI complex that further evolves either through a Csp2–H bond activation reaction, affording the CoIII pincer complexes 237a,b, or through disproportionation and ligand exchange reactions leading to the formation of the bis-chelate CoII complexes 238a,b and [Co(PMe3)4]. The latter was identified by IR monitoring of the reaction. In the IR spectra of 237a and 237b, the Co–H stretching vibrations were detected at 1904 and 1920 cm–1, respectively. The 1H NMR hydride signals appeared as triplets (2JPH ¼ 69 Hz), upfield shifted in the range d –21 to –22 ppm. The molecular structure of 237b revealed a Co center in a distorted octahedral geometry, surrounded by the chelating CNHCNC ligand, two PMe3 trans to each other and one hydrido ligand. The CoIII pincer complexes 237a,b are efficient (pre)catalysts in the hydrosilylation of alkenes using SiH2Ph2 and addition of pyridine N-oxide was beneficial for the activity, facilitating the dissociation of PMe3 through formation of the corresponding phosphine oxide. Substrate-dependent regioselectivity was observed, with high Markovnikov selectivity for the hydrosilylation of aryl alkenes and anti-Markovnikov selectivity when using alkyl alkenes as substrates.128 The CoIII complex 242 containing two tridentate CNHCCNHCN ligands will be discussed below in Scheme 83. 7.11.2.5.3.2.7 Non-symmetrical ONCNHC ligand The CoIII complex 239 (Scheme 81) was obtained through an unusual ring-opening reaction of a N-N-bonded bis-triazolium salt.163 Reaction of the azolium salt with CoCl2 and K2CO3 in acetonitrile resulted in the ring opening of one of the two triazolyl groups, leading to the formation of a tridentate CNHC,N,O ligand. The ring-opening reaction may be due to the presence of trace amounts of water in the solvent. Reactions with either larger amounts of water (ca. 0.1% water) in MeCN or in the presence of molecular sieves resulted in lower isolated yields of the product. Oxidation of the metal center to its trivalent state was found to occur even if the reaction was carried out under inert atmosphere. Such an oxidation may be due to a side-reaction with the azolium precursor itself. In cyclic voltammetry measurements, two irreversible oxidation processes for 239 were detected at +0.15 and +0.31 V vs. Fc+/Fc and correspond to the oxidation of the I– counteranion. In addition, a reversible oxidation was recorded at +0.92 V, probably corresponding to the oxidation of the metal center to CoIV. However, the synthesis of the corresponding CoIV complex by chemical oxidation of 239 was not successful.163

Scheme 81 Ring-opening of a bis-triazolium salt affording 239.

The air-stable CoIII complexes 240a–c were obtained by metallation of the corresponding free carbenes, generated in situ, with CoBr2 (Scheme 82).164 As observed in other instances, although a CoII precursor was used, trivalent metal complexes were

Scheme 82 Synthesis of CoIII complexes bearing tridentate ONCNHC ligands.

732

N-Heterocyclic Carbene Complexes of Cobalt

exclusively isolated. The activity of 240a–c was evaluated in catalytic transfer hydrogenation of ketones using 2-propanol as both solvent and hydrogen donor. A possible mechanism was suggested on the basis of spectroscopic and electrochemical studies.164 7.11.2.5.3.3 Ligands with higher denticity: Tetradentate ligands The CoIII complex 241 was obtained by transmetallation from the corresponding AgI complex with [CoCl2(PPh3)2] and spontaneous oxidation of the metal center (Scheme 83).153 Although this complex is not stable in solution and converts into the CoIII complex 242, which contains two tridentate CNHCCNHCN ligands, a few crystals of 241 suitable for X-ray diffraction analysis could be obtained. The CoIII center is in an octahedral environment, with the two NHC donors arranged in a cis fashion, and two chloride anions trans to each other.153 Reaction of the AgI complex with excess Co metal in air was also reported, leading to a structurally characterized square-planar CoII complex.183 However, the spectroscopic data may suggest the formation of a diamagnetic CoIII complex instead.

Scheme 83 Synthesis of 241 and 242 bearing a tetradentate ligand.

Table 5

Selected data for NHC CoIII complexes. Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)a

Spin state

References

109a

1.939(4)

2.275(1) Co-Si 1.973(3) Co-N 1.912(3) Co-N 1.930(3) Co-N 2.290(3) Co-O

173.7

0

LS CoIII

109

109d

1.951(2)

2.267(1) Co-Si 1.926(2) Co-N 1.934(2) Co-N 1.969(2) Co-N 2.221(2) Co-O

172.7

0

LS CoIII

109

119

1.962(7) 1.962(7)

2.4262(12) Co-Br 2.4025(12) Co-Br 2.3894(11) Co-Br (trans N)

nd

0

LS CoIII

114

126a

1.981(2) 2.000(2)

1.880(2) Co-Caryl 2.098(2) Co-N 2.2787(6) Co-Cl 2.3175(6) Co-Cl

195.3

0

LS CoIII

117

Complex number

Formula

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

733

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

126b

1.961(4) 1.958(4)

1.871(4) Co-Caryl 2.090(3) Co-N 2.275(1) Co-Cl 2.265(1) Co-Cl

196.7

0

LS CoIII

117

131

1.849(3)

1.801(2) Co-Oaryl 1.803(2) Co-Oaryl

–

2.88

HS CoIII or LS CoII b

118

133

1.8321(14)

1.8267(10) Co-Oaryl 1.8346(11) Co-Oaryl 1.8692(16) Co-CF3

–

0

LS CoIII

147

134

1.8428(13)

1.8702(10) Co-Oaryl 1.8758(10) Co-Oaryl 1.9189(14) Co-CF3

–

0

LS CoIII

147

164

1.939(5)

1.938(3) Co-O 1.934(3) Co-O

164.0

0

LS CoIII

137

166

1.935(3)

2.2286(8) Co-P 2.2383(8) Co-P

183.6

0

LS CoIII

148

187

1.922(3) 1.919(3)

1.857(3) Co-Caryl 2.223(1) Co-P 1.40(5) Co-H 2.306(1) Co-Cl

nd

0

LS CoIII

131

188

No crystal structure

nd

0

LS CoIII

131

192

1.918(11)

208.3

0

LS CoIII

149

1.865(3) Co-Caryl 1.53(4) Co-H 1.85(4) Co-H 2.000(3) Co-N

(Continued )

734

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

200

2.003(4) 1.934(4) 1.950(4)

2.137(3) Co-N 1.855(2) Co-O 1.906(2) Co-O

164.8

0

LS CoIII

126

202c

1.940(3), 1.938(3), 1.962 (3),

1.675(2) Co¼N

nd

0

LS CoIII

127

204b

1.913(2)

1.979(3) Co-CMe 1.951(3) Co-CMe

nd

0

LS CoIII

139

207a

1.930(3)

2.059(2) Co-O 1.933(3) Co-N 1.918(2) Co-N 1.934(3) Co-N 1.923(3) Co-N

nd

0

LS CoIII

150

207b

1.933(4)

2.067(3) Co-O 1.923(3) Co-N 1.914(3) Co-N 1.935(3) Co-N 1.911(3) Co-N

nd

0

LS CoIII

150

209a

No crystal structure

179.9

0

LS CoIII

151

209b

1.943(4)–1.959 (5)

184.3

0

LS CoIII

151

209c

1.932(2), 1.946(2), 1.952(2)

187.7

0

LS CoIII

152

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

735

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

211

1.908(2) 1.922(2) 1.943(2)

1.947(2) Co-O 1.964(2) Co-O 1.996(2) Co-N

191.5, 191.6

0

LS CoIII

152

212

1.949(4) 1.952(4) 1.988(4)

1.660(3) Co-N

nd

0

LS CoIII

104

213

1.888c

1.776(7) Co-Oc

nd

0

LS CoIII

105

214

1.913c

1.657 Co-Oc

nd

0

LS CoIII

105

215

1.918(7) 1.934(6) 1.935(6)

1.682(6) Co-O

nd

0

LS CoIII

105

216

1.907(3) 1.915(3)

1.942(3) Co-N 1.950(3) Co-N 2.277(1) Co-Cl 2.297(1) Co-Cl

180.3

0

LS CoIII

153

217a

No crystal structure

nd

0

LS CoIII

154

(Continued )

736

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

217b

1.902(2)

1.989(2) Co-N

185.1

0

LS CoIII

155

218

1.815(4) 1.913(4)

1.972(3) Co-N 1.976(3) Co-N 2.005(3) Co-N (trans NHC) 2.253(1) Co-Cl

nd

0

LS CoIII

153

219

1.932(2)

2.022(2) Co-N 2.2645(5) Co-Cl

nd

0

LS CoIII

156

220

1.946(3) 1.969(3) 1.918(3)

2.299(1) Co-S 2.297(1) Co-S 2.287(1) Co-S

177.2 179.8

0

LS CoIII

157

222

1.940(2) 1.948(2)

2.3507(7) Co-Si 2.3467(7) Co-Si 1.866(2) Co-Caryl 1.840(2) Co-N

nd

0

LS CoIII

158

223

2.174(4) 2.203(4)

2.411(1) Co-Cl (cis C) 2.360(1) Co-Cl (trans C) 2.135(5) Co-N

nd

nd

nd

159

1.913(2) Co-Caryl 1.913(2) Co-Caryl

183.9

0

LS CoIII

159

224 1.960(3) 1.954(2) 1.958(3) 1.965(3)

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

737

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

225

1.971(4) 1.965(4)

2.5685(6) Co-I 2.002(3) Co-O (trans C) 1.941(3) Co-O (trans I)

188.6 188.0

0

LS CoIII

159,160

227

No crystal structure

181.6 181.5

0

LS CoIII

146

228

1.925(4) 1.916(4)

2.1962(12) Co-Si 1.40(4) Co-H 1.909(4) Co-N

183.0

0

LS CoIII

146

230

1.886(2) 1.903(2)

2.2366(7) Co-Si 1.39(3) Co-H 2.032(2) Co-Cbenzyl

nd

0

LS CoIII

146

231

1.906(2) 1.924(2)

2.2497(8) Co-Si 1.34(3) Co-H 2.061(2) Co-N 2.106(2) Co-O

193.3

0

LS CoIII

146

232

1.916(3)1.921(3)

2.185(1) Co-Si 1.37(3) Co-H 1.924(3) Co-N

194.0

0

LS CoIII

146

233

1.941(4)

1.914(3) Co-N 1.923(3) Co-N

182.2

0

LS CoIII

154

(Continued )

738

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

234

1.927(6) 1.949(6)

1.871(4) Co-N 1.874(4) Co-N 2.016(5) Co-N (trans NHC) 2.043(5) Co-N (trans NHC)

nd

0

LS CoIII

161

235

1.917(2)

1.987(2) Co-Npyridyl 1.924(2) Co-Namide

nd

0

LS CoIII

162

236

1.907

2.009 Co-Npyridyl 1.902 Co-Namide

nd

0

LS CoIII

162

237a

No crystal structure

158.3

0

LS CoIII

128

237b

1.906(1)

1.34(4) Co-H 2.165(1) Co-P 2.209(1) Co-P 2.001(1) Co-N 1.942(1) Co-C

158.4

0

LS CoIII

128

239

1.899(4) 1.883(4)

1.878(3) Co-N 1.894(3) Co-N 1.966(3) Co-O 1.993(3) Co-O

150.4

0

LS CoIII

163

240a

1.966(3)

1.931(3) Co-N 1.904(2) Co-O

nd

0

LS CoIII

164

N-Heterocyclic Carbene Complexes of Cobalt

Table 5

(Continued)

Complex number

Formula

739

Co-CNHC bond distance (in ˚A)

Co-X bond distance (in ˚A)

d 13C NHC

meff (in mB)

Spin state

References

240c

1.963(3)

1.935(3) Co-N 1.902(2) Co-O

nd

0

LS CoIII

164

241

1.823(5) 1.828(4)

2.059(4) Co-N 2.077(4) Co-N 2.226(2) Co-Cl 2.231(1) Co-Cl

nd (not stable in solution)

nd

nd

153

a

Solution magnetic moment. See text, redox-active ligand. c Aver. value, two or more crystallographically independent molecules in the asymmetric unit. b

7.11.2.6 7.11.2.6.1

Mononuclear CoIV and CoV complexes Monodentate carbene ligands

7.11.2.6.1.1 Heteroleptic complexes of type [CoX4(NHC)] Recently, Deng’s group reported the synthesis of the CoIV imido complex 12 by reaction of the Co0 alkene complex 8a with 2 equiv. of the bulky DiPPN3 azide (Schemes 3 and 84).66 This paramagnetic complex was isolated in 55% yield after recrystallization and exhibited in solution a magnetic moment of 2.2(1) mB at room temperature, consistent with an S ¼ ½ ground state. In the EPR spectrum, a characteristic hyperfine structure due to coupling with 59Co was observed, but the hyperfine coupling with nitrogen was unresolved. The X-ray structure of 12 revealed a trigonal-planar coordination geometry around the CoIV metal center with an overall C2v symmetry. The Co–Nimido–Caryl moiety is slightly bent (173.0(3) ) and the Co–Nimido bond distance (1.665(3) A˚ ) is in the upper range of distances reported for Co imido complexes (1.61–1.68 A˚ ). The Nimido–Caryl bond distance (1.357(4) A˚ ) lies between the values found for N–C double and single bonds (1.28 and 1.45 A˚ , respectively), suggesting substantial electron delocalization onto the aryl moiety. As a result, the Co–N bond is weakened, which may account for the relatively long Co–N separation observed

Scheme 84 Synthesis and reactivity of the NHC CoIV complex 12.

740

N-Heterocyclic Carbene Complexes of Cobalt

experimentally. DFT calculations support the experimentally observed S ¼ ½ ground state for 12 and indicate a net Mayer bond order of 1.39 for the Co–N bonds. Further characterization by cyclic voltammetry revealed a reversible oxidation process occurring at –0.16 V vs. SCE, suggesting the possible formation of a CoV imido species. Complex 12 is stable in solution and in the solid state at room temperature but undergoes intramolecular C–H bond amination reaction when heated in benzene at 50  C. As a result, the CoII diamido complex 116, which features longer Co–N separations (1.883(2) and 1.885(2) A˚ ), was isolated. The formation of 116 is thought to proceed via radical processes involving H-atom abstraction of a benzylic C–H by one imido group.66 7.11.2.6.1.2 Heteroleptic complexes of type [CoX4(NHC)]+(A–) As suggested by cyclic voltammetry studies, a CoV imido complex might be accessed by the one-electron oxidation of 12. Chemical oxidation of 12 with [FeCp2](BArF4) resulted in the formation of 243 which was isolated in 88% yield (Scheme 84).66 The 1H and 13 C NMR spectra of this diamagnetic CoV complex suggest a C2v symmetric [Co(NDiPP)2(IMes)]+ cation in solution. The S ¼ 0 ground spin state was further supported by DFT calculations. Analysis of the crystal structure of 243 revealed a trigonal-planar geometry around the Co center, similarly to what was observed in 12. The average Co–Nimido and Nimido–Caryl bond distances in 243 (1.642(3) and 1.351(5) A˚ , respectively) are slightly shorter than those in 12 (Table 6). Accordingly, a higher C–N bond order of 1.47 in 243 (vs. 1.39 for 12) was established from DFT calculations, resulting from the removal of the electron in one antibonding singly occupied orbital in 12. The Co–CNHC bond distance in 243 is slightly longer (1.941(4) A˚ , Table 6), possibly reflecting the larger trans influence exerted by the imido ligands in the oxidized complex. Contrary to the reactivity observed for 12, no thermal decomposition of 243 occurred upon heating a THF solution of the complex to 60  C.66

7.11.2.6.2

Functionalized carbene ligands

Complexes 132 (Scheme 42, Section 7.11.2.3.5.2.3) and 140 (Scheme 44, Section 7.11.2.3.5.3.1) were already mentioned in the respective sections (Fig. 9).

Fig. 9 Structures of complexes 132 and 140.

Table 6 Complex number

Selected data for the formal CoIV and CoV NHC complexes. meff (in mB)a

Spin state

References

–

2.2(1)

LS CoIV

66

1.818(2) Co-O 1.825(2) Co-O

–

2.51

LS CoIIb

118

1.640(3) Co-N 1.642(3) Co-N

176.8

0

LS CoV

66

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

d

12

1.879(5)

1.665(3) Co-N

132

1.840(2)

243

1.941(4)

a

Formula

Solution magnetic moment. See text, redox-active ligand.

b

13

C NHC

N-Heterocyclic Carbene Complexes of Cobalt

7.11.2.7 7.11.2.7.1

741

Polynuclear homometallic complexes Binuclear complexes

7.11.2.7.1.1 Monodentate carbene ligands 7.11.2.7.1.1.1 Halide complexes and reactivity The paramagnetic dinuclear complexes [{Co(m-Cl)Cl(NHC)}2] (49a–d) were prepared by addition of one equiv. NHC to CoX2 (X ¼ Cl, Br, I) or [CoBr2(THF)2] in THF (Scheme 85).82,91,92,97 While the complexes bearing unsaturated NHCs were isolated in ca. 30–55% yield after crystallization from THF, lower yields (8–10%) were reported for the saturated 49as and 49ds, due to the formation of large amounts of insoluble materials. Unexpectedly, 49du was also obtained in low yield in an attempt to oxidize 84a with Ag+ or NO+ (Scheme 28, Section 7.11.2.3.1.2.5), but the latter reaction did not proceed cleanly.91 Due to the extreme air-sensitivity of dinuclear 49a–d, the partially reprotonated 83 was typically identified as a decomposition by-product. Complexes 49au–c were characterized by SQUID magnetization measurements and XPS, EPR and UV-vis spectroscopy, which established a tetrahedral coordination environment with a high-spin CoII (S ¼ 3/2) ground state.92 X-Ray diffraction studies confirmed the tetrahedral coordination geometry around the CoII center, involving one NHC, one terminal and two bridging halides. The large separation between the two cobalt atoms (2.95–3.42 A˚ , Table 7) precludes any direct metal-metal interaction. Only slight variations in the Co–CNHC bond distances were observed by changing the nature of the NHC and/or the halide ligand.

Scheme 85 Synthesis and reactivity of the dinuclear CoII complexes 49a–d.

Splitting of the halide bridge occurred upon addition of pyridine or [Mg(CH2SiMe3)Cl], leading to the four-coordinate 77a–d (Scheme 23, Section 7.11.2.3.1.2.2) and the trigonal-planar dialkyl complex 70 (Scheme 21, Section 7.11.2.3.1.2.1), as depicted in Scheme 85.91–93 As indicated, both complexes were described above in the corresponding sections. Addition of 4 equiv. [Mg(benzyl)2(THF)2] to 49b did not lead to the dibenzyl analogue of 70 but 174 was isolated instead, as a result of the reduction of the metal center and coupling between the two benzyl ligands (see Section 7.11.2.4.1.2.4).82

742

N-Heterocyclic Carbene Complexes of Cobalt

The dinuclear complexes 177a,b were obtained by treatment of free Me2cAAC with CoBr2 and CoCl2, respectively (Scheme 86).129,184 In the crystal structure of 177a, the metal adopts a pseudo-tetrahedral coordination geometry, involving one Me2 cAAC donor, one terminal and two bridging bromides. No structure was reported for 177b due to decomposition in solution.

Scheme 86 Synthesis of the dinuclear cAAC complexes 177a,b and reduction with KC8.

The behavior of 177b toward reduction was investigated in detail and, interestingly, highly reduced complexes exhibiting a Co–Co bond were characterized (Scheme 86).63,129 Reaction of 177b, generated in situ, with 1 equiv. of free Me2cAAC and one equiv. of KC8 led to the paramagnetic mononuclear CoI complex 2a.63,129 Characterization by cyclic voltammetry revealed a quasireversible one-electron reduction at E1/2 ¼ –0.57 V, indicating the possible formation of a further reduced, formally Co0 complex (see Scheme 1, Section 7.11.2.1.1.1). Reduction of 177b with 4 equiv. KC8 led to the diamagnetic 244, isolated in 65% yield. The crystal structure of the complex revealed the presence of two cobalt atoms, in the formal oxidation state zero, bridged by two cAAC ligands in a head-to-tail manner. Each cobalt center features a half-sandwich coordination environment, with one Me2cAAC donor and an additional Z6-interaction with the DiPP aryl ring from the other cAAC ligand. The experimental Co–Co internuclear distance of 2.6550(6) A˚ is slightly longer than the value obtained by DFT calculations, probably resulting from the strong p-accepting properties of the cAAC ligand. The Co–CcAAC bond distances (1.854(2) and 1.853(2) A˚ ) are shorter than the reported values for Co–CNHC separations (1.87 to 2.09 A˚ ). Elongation of the CcAAC–N bond is observed in 244, as a possible result of strong backdonation from the cobalt to the carbene carbon atom. Cyclic voltammetry measurements on 244 in CH2Cl2 displayed a one-electron reversible reduction at E1/2 ¼ –0.80 V and a one-electron reversible oxidation at E1/2 ¼ +0.45 V, suggesting that both the radical anionic and radical cationic derivatives of 244 might be synthetically accessible.129

N-Heterocyclic Carbene Complexes of Cobalt

743

The radical cation 245 was synthesized as a triflate salt in 90% yield by reduction of 177b with 3 equiv. KC8 in the presence of LiOTf (Scheme 86). The overall structure of 245 is similar to that of 244 but with a shorter Co–Co bond length of 2.4610(6) A˚ . In comparison with 244, a slight decrease in the CcAAC–N (1.328(3) and 1.338(3) A˚ ) and increase in the Co–CcAAC (1.891(2) and 1.920(2) A˚ , Table 7) bond distances can be noticed. This observation is consistent with a reduced back-bonding interaction from the cobalt centers to the carbenes, owing to the lower electron density at the metal in the oxidized complex. The radical cation 245 was further characterized by EPR spectroscopy. The spectrum recorded at 115 K in frozen solution was consistent with the coupling of one electron spin with two equivalent 59Co (I ¼ 7/2) nuclei and suggested metal-metal spin delocalization and a mixed-valence (Co0.5)2 species. Computational analyses of the electronic structures of 244 and 245 revealed a donor-acceptor character for the Co–CcAAC interactions with significant p back-donation. Calculations further indicated that the shortening of the internuclear Co–Co separation in 245 may only be caused by intermolecular interactions. The bonding situation in 244 is best described as involving two Co atoms with an 18-electron configuration (resulting from the overall donation of 8 electrons from the phenyl group and the cAAC ligand to each Co atom of the Co2 fragment) and forming a Co–Co single bond rather than a weak Co–Co closed-shell interaction. The Co–Co separation in 245 is slightly longer than that for a typical single bond (2.66 vs. 2.52 A˚ ), which is possibly due to the strong p-accepting properties of the cAAC ligand.129 7.11.2.7.1.1.2 Carbonyl complexes Some of the first examples of NHC-Co complexes obtained by metallation of a free NHC ligand (and not via electron-rich olefins) include the dinuclear cobalt carbonyl complexes depicted in Scheme 87.165,167,185 The diamagnetic complex 246 was obtained by addition of [Co2(CO)8] to a solution of in situ generated free IMes in THF and was characterized spectroscopically.185 However, this complex is highly sensitive and decomposition was observed upon crystallization attempts. Likewise, the saturated SIMes analogue was found too unstable to be isolated. Substitution of one CO ligand by a PPh3 donor gave the more stable complexes 247u/s which were structurally characterized. The NHC and phosphine ligands are trans to each other and the Co–Co separations (2.682(1) A˚ in 247u and 2.686(1) A˚ in 247s) are similar to those reported for the analogous bis(phosphine) complexes [Co2(CO)6(phosphine)2].185

Scheme 87 Synthesis of NHC carbonyl Co0 complexes.

The reaction between two equiv. of free IMes and [Co2(CO)8] in THF under a CO atmosphere did not result in the expected dinuclear [{Co(IMes)(CO)3}2] (44b) but the salt 175 was isolated instead.167 Formation of the [Co(CO)4]– anion was readily evidenced by IR spectroscopy, with a very strong absorption band at 1886 cm–1 (Table 8). The crystal structure of 175 revealed a discrete anion-cation pair. The cation consists of a CoI center in a distorted trigonal-bipyramidal environment with the three CO ligands in the equatorial plane and two IMes ligands in the apical positions. The tetrahedral [Co(CO)4]- anion displays slightly shorter Co-carbonyl separations than those in the cation, resulting from the formal negative charge on the Co center.167 In order to form the dinuclear 44b, an indirect synthesis involving the preparation of the phosphine complex [Co(PCyPh2) (CO)3}]2 was considered (Scheme 87).165 Substitution of the two phosphines by IMes ligands proceeded neatly and resulted in the diamagnetic 44b which was spectroscopically and structurally characterized. Complex 44b is highly air sensitive and only stable when kept under a CO atmosphere. In comparison with 175, the Co–CNHC bond distances in 44b are significantly shorter (in aver. 1.986(4) A˚ in 175 vs. 1.926(10) A˚ in 44b), reflecting the large trans influence exerted by the NHC ligands.165 The non-symmetrical dinuclear complex 248 was obtained in 54% yield by substitution of one CO ligand by IPr in the corresponding aldehyde-functionalized Z2-alkyne-(hexacarbonyl)dicobalt(0) complex (Scheme 88).186 The change in the

744

N-Heterocyclic Carbene Complexes of Cobalt

Scheme 88 Synthesis of the non-symmetrical dinuclear 248.

electronic properties of the complex upon ligand substitution was evidenced by IR spectroscopy. A red shift of the CO absorptions was observed in 248 compared to those in the precursor bis-cobalt alkyne complex. The aldehyde stretching frequency is also a good probe with an absorption band detected at 1638 cm–1 for 248 and 1653 cm–1 for the PPh3 analogue, consistent with the higher electron-donation ability of the IPr ligand. The structure of the complex was unambiguously determined by X-ray diffraction studies.186 7.11.2.7.1.1.3 Silyl complexes The silyl complex 15 was obtained by reaction of 9e with excess SiH2Ph2 (Scheme 3, Section 7.11.2.1.1.2.1, and Scheme 89) and its crystal structure revealed an overall C2 molecular symmetry.71 The outcome of the reaction was found to depend on the nature of the NHC ligand and on the stoichiometry of the SiH2Ph2 reagent. Reaction of 9b and 9f with two equiv. of SiH2Ph2 afforded the dinuclear silyl complexes 16a,b, respectively.71

Scheme 89 Synthesis of dinuclear cobalt silyl complexes.

Each cobalt center in 15 is coordinated by one NHC, three bridging SiH2Ph2 and one hydride ligand, suggesting a dinuclear CoII silyl hydride complex. Analysis of the NMR spectra of 15 indicated fluxional behavior in solution and a diamagnetic complex. Thermal decomposition of 15 gave the paramagnetic complex 249, resulting from the loss of one SiH2Ph2 and one H2 molecule. This complex is analogous to complexes 16 discussed above (Scheme 3, Section 7.11.2.1.1.2.1). In the molecular structure of 249, two hydrosilyl ligands bridge the two metal centers through a Co–Si s-bond with one Co center and a 3c-2e Co-(Z2-Si-H) interaction with the second Co. Reaction of 249 with tBuNC led to the diamagnetic and formally Co0-CoII complex 250 (Scheme 89).71 Some dinuclear cobalt complexes described in previous sections and depicted in Fig. 10 include the CoII anilido complexes 54 (Scheme 17, Section 7.11.2.3.1.2.1)82 and 150 (Scheme 48, Section 7.11.2.4.1.2.1),135 the CoII phosphinidene-bridged complex 61 (Scheme 16, Section 7.11.2.3.1.2.1),86 and the CoIII Cp complexes 167 and 168 (Scheme 53, Section 7.11.2.4.1.2.4).148

N-Heterocyclic Carbene Complexes of Cobalt

745

Fig. 10 Structures of dinuclear complexes previously described.

7.11.2.7.1.2 Bridging bis-carbene and pincer ligands The dinuclear CoII complex 251 was obtained in low isolated yield (11%) by reaction of the imidazolium salt precursor with 2 equiv. of [Co{N(SiMe3)2}2] (Scheme 90).187 Attempts to generate the free carbene ligand by deprotonation of the imidazolium salt proligand with sodium tert-butoxide, sodium hydride or sodium bis(trimethylsilyl)amide were unsuccessful and only resulted in intractable mixtures. In the crystal structure of 251, the two CoII centers are situated on opposite regions of the space with respect to the central arene and coordinated by one NHC, one chloride and one bis(trimethylsilyl)amide ligand. The Co–Carene separation of 2.95 A˚ is too long to correspond to any direct metal-arene interaction.187

Scheme 90 Synthesis of 251 bearing a bridging bis-carbene ligand.

The dinuclear pincer complexes 136 and 194 depicted in Fig. 11 were already described in previous sections (Scheme 43, Section 7.11.2.3.5.2.4 and Scheme 64, Section 7.11.2.4.2.2.3, respectively).119

Fig. 11 Structures of complexes 136 and 194 previously described.

746

N-Heterocyclic Carbene Complexes of Cobalt

7.11.2.7.2

Tetranuclear complexes

The tetranuclear CoII cluster 252 (Scheme 91) was prepared in good yield by phosphine displacement from the corresponding cubane [CoS(PiPr3)]4 (either isolated or generated in situ).99 The latter was obtained by a self-assembly reaction between CoCl2, PiPr3 and S(SiMe3)2. The crystal structure of 252 is also of the cubane-type with the CoII centers in distorted tetrahedral coordination environments. SQUID magnetization measurements established a Stotal ¼ 3 ground state, involving three parallel and one antiparallel S ¼ 3/2 spins. The magnetic moment is close to the spin-only value for S ¼ 3 in the range 10–100 K (mSO ¼ 2 (3
4)1/2 ¼ 6.93 mB) and then decreases with the temperature, reaching 5.2 mB at 300 K. Characterization by cyclic voltammetry revealed two quasi-reversible oxidation processes but no reduction was observed up to –2.0 V. In comparison with the precursor [CoS(PiPr3)]4, the oxidation potentials of 252 are lower, owing to the better s-donor NHC ligands. Oxidation of 252 to give the monocationic 253 was achieved by treatment with tropylium hexafluorophosphate [C7H7]PF6, and the structure of the cubane cluster was only very slightly affected compared to that of 252. No distinction between the four metal sites could be detected by perusal of the metrical data (Table 7), supporting electron delocalization and formation of a formal mixed-valence (3CoII + CoIII) [Co4S4]1+ core.99

Scheme 91 Synthesis of the tetranuclear cubane clusters 252 and 253.

7.11.2.8

Polynuclear heterometallic complexes

The uranium/cobalt complex 254 was obtained via transmetallation of a lithium uranyl imidazolyl complex with CoCl2 (Scheme 92).188 The former was prepared by reaction of [UO2(DiPPnacnac)Cl]2 with 1-methylimidazole (MeIm) followed by the addition of lithiated MeIm. The bimetallic complex 254 is paramagnetic with a solution magnetic moment of 4.42 mB, suggesting the presence of a high-spin CoII center. It should be noticed that an isomerization of the imidazolyl ligand occurred upon addition of CoCl2. While the C-2 carbon binds to the uranium center before transmetallation, cobalt-carbene bonds are formed in 254 and the imidazolyl nitrogen is coordinated to the uranyl ion. The reason for such an isomerization was rationalized in terms of ‘hard/ soft’ donors, with preferred bond formation between the N donor and the harder uranyl fragment.188

Scheme 92 Synthesis of the uranium/cobalt bimetallic complex 254.

The 3d-4f heterobimetallic NHC complex 255 was obtained by substitution of the pyridine ligands in the corresponding pre-organized Co2La core (Scheme 93).189 The crystal structure of 255 confirmed retention of the trinuclear arrangement upon ligand substitution, with the La3+ cation located between two CoII ions, the metal centers bound together via six bridging pivalate ligands. Distorted tetrahedral environments are observed around the cobalt metal centers with Co–CNHC bond distances of 2.071(3) A˚ .

N-Heterocyclic Carbene Complexes of Cobalt

747

Scheme 93 Synthesis of the lanthanum/cobalt bimetallic complex 255.

Table 7

Selected data for the polynuclear NHC Co complexes.

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

Co-Co separation (in ˚A)

d 13C NHC

meff (in mB)a

References

15

1.932(3) 1.929(3)

2.325(1), 2.362 (1), 2.255(1) Co1-Si; 2.340(1), 2.246 (1), 2.366(1) Co2-Si

2.578(1)

189.1

0

71

16a

1.927(2)

2.2280(6) Co-Si 2.3165(6) Co-Si

2.4971(5)

–

nd

71

44a

1.958(3)

1.777(4), 1.780 (4), 1.789(4) Co-CCO

2.764(1)

186.3

0

69

44b

1.902(10) 1.949(11)

1.756(9), 1.759 (10), 1.783(10), 1.738(9), 1.760 (9), 1.764(11) Co-CCO

2.697(2)

183.2

0

165

49au

2.0298(13)

2.2238(4) Co-Cl (terminal) 2.3245(4), 2.3144(4) Co-Cl (bridging)

3.127(1)

–

5.2(1); 4.27b

91,92

49as

2.041(6)

2.219(2) Co-Cl (terminal); 2.306(2), 2.319 (2) Co-Cl (bridging)

3.139(1)

–

nd

82

49b

2.032(4); 2.027(2)

2.362(1), 2.3559 (4) Co-Br (terminal); 2.448(1), 2.455 (1), 2.4506(4), 2.4463(4) Co-Br (bridging)

3.155(1) 3.213(1)

–

5.2; 4.06b

92,97

(Continued )

748

N-Heterocyclic Carbene Complexes of Cobalt

Table 7

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

Co-Co separation (in ˚A)

d 13C NHC

meff (in mB)

References

49c

2.038(7)

2.574(1) Co-I (terminal); 2.631(1), 2.637 (1) Co-I (bridging)

3.416(2)

–

4.03b

92

49du

2.031(2)

2.2209(6) Co-Cl (terminal); 2.3230(7), 2.3444(7) Co-Cl (bridging)

3.157(1)

–

nd

91

49ds

2.035(4)

2.2553(13) Co-Cl (terminal); 2.080(4), 2.094 (4) Co-Cl (bridging)

2.951(1)

–

nd

82

54

2.059(3)

1.910(2) Co-N 2.3435(8) Co-Cl 2.3921(8) Co-Cl

ca. 3.35

nd

nd

82

61

1.895(3) 1.901(3)

2.1628(9) Co-P

2.524(1)

nd

0

86

66

2.045(5)

ca. 3.22

nd

nd

90

69

1.9512(14)

2.5628(3)

nd

nd

90

71

1.912(3)

2.224(2) Co-Cl (terminal); 2.327(2), 2.353 (2) Co-Cl (bridging) 1.8662(12) Co-Nsilylamide (coordinated to remote N) 1.8861(12) Co-Nremote 1.8711(14) Co-Nsilylamide (coordinated to aNHC) 2.2534(11) Co-Si 2.2808(13) Co-Si 2.3923(11) Co-Si 2.4045(13) Co-Si

2.4465(9)

nd

0

166

N-Heterocyclic Carbene Complexes of Cobalt

Table 7

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

Co-Co separation (in ˚A)

749

d 13C NHC

meff (in mB)

References

197.3 and 198.5c

0

166

72

No crystal structure

73

1.939(2)

2.2267(8) Co-Si 2.3274(8) Co-Si

2.4527(8)

nd

nd

166

136

1.942(8)

2.348(2) Co1-Br (trans to N) 2.760(2) Co1-Br (bridging) 2.116(8) Co2-C

3.726(2)

–

4.8(2)

119

150

1.8897(14)

2.0421(12) Co-N 2.0387(12) Co-N

2.5765(4)

–

3.8

135

167

1.923(3) 1.936(3)

2.2711(13) Co-P 2.2480(13) Co-P 2.2480(13) Co-P 2.2659(17) Co-P

nd

186.7

0

148

168

1.932(4)

2.2407(12) Co-P 2.2298(13) Co-P

nd

186.7

0

148

175

1.984(4) 1.988(4)

1.790(6) (CoCCO)aver in the cation 1.765(6) (CoCCO)aver in the anion

nd

163.1

0

167

(Continued )

750

N-Heterocyclic Carbene Complexes of Cobalt

Table 7

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

Co-Co separation (in ˚A)

d 13C NHC

meff (in mB)

References

177a

2.073(4), 2.097(4)

2.378(1), 2.391 (1) Co-Br (terminal) 2.500(1), 2.501 (1), 2.502(2), 2.485(1) Co-Br (bridging)

3.581(1)

–

4.7(1) in THF-d8 4.9 in CD2Cl2

184

194

1.914(4)

dearomatized pincer 1.760(5) rearomatized pincer

1.742(4) Co-N2 1.960(4) Co-Cbackbone

nd

191.5 179.7

0

2.001(3), 1.991(3)

1.879(3) Co-Caryl, 2.5955(8) Co-I (terminal), 2.6074(8), 2.7454(9) Co-I (bridging)

ca. 3.95

nd

nd

160

244

1.854(2) 1.853(2)

In the range 2.075(2) 2.187(2)

2.6550(6)

nd

0

129

245

1.891(2) 1.920(2)

In the range 2.052(2) 2.251(2)

2.4610(6)

–

nd

129

119 226

N-Heterocyclic Carbene Complexes of Cobalt

Table 7

(Continued)

Complex number

Formula

751

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

Co-Co separation (in ˚A)

d 13C NHC

meff (in mB)

References

247u

1.944(3)

2.183(1) Co-P

2.682(1)

181.3

0

185

247s

1.941(5)

2.188(2) Co-P

2.686(1)

214.1

0

185

248

1.969(2)

1.953(2) CoNHCCMe 1.922(2) CoNHCCald 1.798(2) CoNHCCCO 1.803(2) CoNHCCCO

2.495(1)

186.7

0

186

249

1.931(2)

2.246(1) Co-Si 2.336(1) Co-Si

2.599(1)

–

2.7(2)

71

250

1.934(4)

2.303(2) Co-Si 2.283(2) Co-Si 1.761(4) Co-Cisonitrile

2.657(1)

195.6

0

71

251

2.039(2)

1.915(2) Co-N 2.2481(7) Co-Cl

–

nd

187

252

1.988(2) 2.000(2) 1.973(2) 1.979(2)

2.25(1) aver. Co-S

2.69(2) aver. Co-Co

–

5.2 at r.t., see textb

99

253

1.977(3) 1.984(3) 1.984(3) 1.964(3)

2.22(1) aver. Co-S

2.66(2) aver. Co-Co

–

nd

99

752

N-Heterocyclic Carbene Complexes of Cobalt

Table 7

(Continued)

Complex number

Formula

Co-CNHC bond distance (in ˚A)

Co-coligand bond distance (in ˚A)

Co-Co separation (in ˚A)

d 13C NHC

meff (in mB)

References

254

2.047(6)

2.034(6) Co-N 2.330(2) Co-Cl

–

–

4.42

188

255

2.071(3)

1.972(2) Co-O 1.985(2) Co-O 1.968(2) Co-O

7.981(1)

–

nd

189

a

Magnetic moment in solution, unless otherwise stated. In the solid state. c The complex exists as two different isomers, see text. b

Table 8

Characteristic IR absorption bands of carbonyl and related complexes.

Complex Number

Formula

CO complexes 19

31a 31b 31c 31d 31e 31f 31g 31h 32a 32b 32d 32e 32f 32g 32h 32i 32j 34 35a 35b 35c 35d 35e 35f 35g 44a 44b

[Co(NO)(IiPr)2(CO)] [Co(NO)(InPr)2(CO)] [Co(NO)(ICy)2(CO)] [Co(NO)(IMe)2(CO)] [Co(NO)(Me2IiPr)2(CO)] [Co(NO)(Me2IMe)2(CO)] [Co(NO)(IMeiPr)2(CO)] [Co(NO)(IMetBu)2(CO)] [Co(NO)(IiPr)(CO)2] [Co(NO)(InPr)(CO)2] [Co(NO)(IMe)(CO)2] [Co(NO)(Me2IiPr)(CO)2] [Co(NO)(Me2IMe)(CO)2] [Co(NO)(IMeiPr)(CO)] [Co(NO)(IMetBu)(CO)2] [Co(NO)(IPr)(CO)2] [Co(NO)(IMes)(CO)2] [Co(NO)(Me2cAAC)(CO)2] [Co(NO)(IiPr)(CO)(PMe3)] [Co(NO)(IiPr)(CO)(PEt3)] [Co(NO)(IiPr)(CO)(PHiPr2)] [Co(NO)(Me2IiPr)(CO)(PMe3)] [Co(NO)(Me2IMe)(CO)(PMe3)] [Co(NO)(IMeiPr)(CO)(PMe3)] [Co(NO)(IMetBu)(CO)(PMe3)] [{Co(IPr)(CO)3}2] [{Co(IMes)(CO)3}2]

n(CO, NO) (in cm–1)

References

2028, 1962, 1943 (KBr)

78

1613 (vs, NO), 1865 (vs, CO) (neat, ATR) 1621 (vs, NO), 1868 (vs, CO) (neat, ATR) 1633 (vs, NO), 1878 (vs, CO) (neat, ATR) 1613 (vs, NO), 1873 (vs, CO) (neat, ATR) 1623 (vs, NO), 1870 (vs, CO) (neat, ATR) 1620 (vs, NO), 1865 (vs, CO) (neat, ATR) 1610 (vs, NO), 1873 (vs, CO) (neat, ATR) 1619 (vs, NO), 1868 (vs, CO) (neat, ATR) 1707 (vs, NO), 1937 (vs, CO (B1)), 2011 (vs, CO (A1)) (neat, ATR) 1707 (vs, NO), 1936 (vs, CO (B1)), 2011 (vs, CO (A1)) (neat, ATR) 1701 (vs, NO), 1929 (vs, CO (B1)), 2011 (vs, CO (A1)) (neat, ATR) 1703 (vs, NO), 1923 (vs, CO (B1)), 2011 (vs, CO (A1)) (neat, ATR) 1732 (vs, NO), 1932 (vs, CO (B1)), 2009 (vs, CO (A1)) (neat, ATR) 1710 (vs, NO), 1938 (vs, CO (B1)), 2011 (vs, CO (A1)) (neat, ATR) 1705 (vs, NO), 1933 (vs, CO (B1)), 2007 (vs, CO (A1)) (neat, ATR) 1722 (vs, NO), 1945 (vs, CO (B1)), 2010 (vs, CO (A1)) (neat, ATR) 1715 (vs, NO), 1929 (vs, CO (B1)), 2001 (vs, CO (A1)) (neat, ATR) 1710 (vs, NO), 1933 (vs, CO (B1)), 2004 (vs, CO (A1)) (neat, ATR) 1643 (vs, NO), 1886 (vs, CO) (neat, ATR) 1643 (vs, NO), 1886 (vs, CO) (neat, ATR) 1654 (vs, NO), 1896 (vs, CO) (neat, ATR) Not isolated 1643 (vs, NO), 1888 (vs, CO) (neat, ATR) 1653 (vs, NO), 1892 (vs, CO) (neat, ATR) 1656 (vs, NO), 1886 (vs, CO) (neat, ATR) 1960 (m), 1937 (s) (KBr) 1973 (w), 1958 (w), 1941 (vs) (THF)

76 76 76 76 76 76 76 76 76 76 76 76 76 76 76 76 76 76 77 77 77 77 77 77 77 69 165

N-Heterocyclic Carbene Complexes of Cobalt

Table 8

753

(Continued)

Complex Number

Formula

102

n(CO, NO) (in cm–1)

References

1953(s), 1895(vs) (KBr)

108

160a 160b 160c 160d 160e 160f 171a 171c 171d 172a 172b 173 175 197

[CoCp(IiPr)(CO)] [CoCp (IiPr)(CO)] [CoCp(IMes)(CO)] [CoCp (IMes)(CO)] [CoCp(IPr)(CO)] [CoCp (IPr)(CO)] [CoMe(IMes)(CO)3] [Co(CF3)(SIPr)(CO)3] [Co(boryl)(IPr)(CO)3] [Co(COMe)(IMes)(CO)3] [Co(COEt)(IMes)(CO)3] [Co(H)(IMes)(CO)3] [Co(IMes)2(CO)3]+[Co(CO)4]-

1892 (KBr); 1911 (hexane) 1877 (KBr) 1918 (pentane); 1896 (dichloromethane) 1896 (pentane); 1875 (dichloromethane) 1921 (pentane) 1884 (ether) 1926 (s, CO), 1940 (s, CO), 2018 (w, CO) (nujol, KBr) 1975 (s, CO), 1990 (sh, CO), 2061 (w, CO) (nujol, KBr) 1925 (s, CO), 1952 (s, CO), 2024 (w, CO) (nujol, KBr) 1652 (m, COMe), 1930 (s, CO), 1964 (s, CO), 2038 (w, CO) (nujol, KBr) 1600 (m, COEt) 1648 (m, COEt), 1928 (s, CO), 1966 (s, CO), 2050 (w, CO) (nujol, KBr) 1936 (s, CO), 1944 (s, CO), 2023 (w, CO) (nujol, KBr) 1981 (vw), 1886 (vs) (THF) 1927 (KBr)

94,137 137 138 138 139 136 142 143 144 142 142 142 167 126

246 247u 247s 248

[Co2(IMes)(CO)7] [Co2(IMes)(CO)6(PPh3)] [Co2(SIMes)(CO)6(PPh3)]

2074 (m), 2022 (sh), 1988 (s), 1937 (m) (CHCl3) 1969 (sh), 1949 (s), 1922 (sh) (CHCl3) 1969 (sh), 1949 (s), 1922 (sh) (CHCl3) 2060, 2015, 1959 (neat)

185 185 185 186

[CoCp (CO3)(IiPr)]

1612 (KBr)

137

CO3 complexes 164

Table 9

Characteristic IR absorption bands of N2 complexes and related.

Complex Number

Formula

N2 (free) Dinitrogen complexes Co–I complexes 36a

K[Co(ICy)2(N2)2]

36b

Rb[Co(ICy)2(N2)2]

36c

Cs[Co(ICy)2(N2)2]

38

K(18-c-6)[Co(ICy)2(N2)2]

Co0 complexes 29 CoI complexes 127

[Co(ICy)3(N2)]

n(N2) (in cm–1)

References

2331

1807, 1881 (KBr) 1817, 1890 (THF) 1804, 1888 (KBr) 1816, 1891 (THF) 1811, 1882 (KBr) 1817, 1894 (THF) 1812, 1892 (KBr) 1828, 1906 (THF)

75

1917 (KBr); 1921 (THF)

75

2063 (neat, ATR)

117

75 75 75

(Continued )

754

Table 9

N-Heterocyclic Carbene Complexes of Cobalt

(Continued)

Complex Number

Formula

n(N2) (in cm–1)

References

128a

2117 (neat, ATR)

117

128b

2112 (neat, ATR)

117

128c

2114 (neat, ATR)

131

176

2006 (KBr)

108

193

2057 (KBr)

119

229

2004 (KBr)

146

N3 complexes 139

2081, 2044, 1999 (KBr)

121

201

2045 (KBr)

127

N-Heterocyclic Carbene Complexes of Cobalt

7.11.3

755

General conclusion

In the recent years, the spectacular expansion of the chemistry of cobalt NHC organometallics has generated remarkable and unprecedented structural and reactivity diversity and uncovered interesting physical and magnetic properties of single molecules with potential catalytic and materials applications. The early scepticism of the scope of application of the NHC paradigm in 3d organometallics has proven unfounded, thanks to the thoughtful and sophisticated tuning of the NHC donors to the desired ends. Based on the currently established foundations, the area is expected to grow further in the foreseeable future. This coincides with the renaissance of the interest in the usage of base metals (including Co) to tackle sustainability issues in catalysis and other areas associated with intensive use of metals. The manipulation of electronic and steric tuning handles in the NHC ligand paradigm has introduced new ligand design concepts, such as manipulating the singlet-triplet gap in s-/p- metal-donor interactions, flexible sterics, NHC functionalization etc. It is remarkable and unprecedented that the same tuneable donor system has been successful in stabilizing all oxidation states from Co–I to CoV. The impact of monodentate NHCs in the isolation of reactive Co organometallics in low coordination geometries (high or low oxidation states) is a recent highlight with anticipated knock-on effects in catalysis and magnetic materials. The facile access of coordinated reactive functionalities e.g. alkyls, hydrides, boryls, silyls, imido, carbenes, dinitrogen complexes etc. stabilized by NHC ligands will be invaluable for the development of new synthetic transformations with Co. The functionalization and integration of NHCs in bidentate, tridentate, pincer and macrocyclic spectator architectures in order to increase stability and alleviate unfavorable Co-C BDEs is responsible for recent milestones in Co catalysis e.g. in hydrogenation, hydrosilylation, cross coupling, hydroarylation etc. It is reasonable to expect that these principles and pathways to new developments are fully open to explore further. Electrocatalysis and the relevant catalytic tuning by geometrical rigidity control, s-donicity and redox non-innocence are elements in NHC ligand design with future applications in Co organometallics. Attempts to increase stability of Co NHC organometallics and transfer their versatility in adverse reaction media (e.g. water, air, acid etc.) may also reside on the creative NHC ligand design ideas that are emerging (macrocycles, concave-shaped architectures etc.). As mentioned previously, the molecular design of NHC donors has reached a state of maturity, encompassing heterocyclic rings with a range of sizes and number/nature of heteroatoms, aromaticity etc. but additional progress is anticipated on this front. Although the imidazol-based NHC family appears to be the most intensely studied on Co, better s-donοr and modulated p-acceptor properties may be within reach by choosing alternative substituents, heteroatoms and functionalities of the NHC. The latter can influence the energies of the s- and “pp“-orbitals of the divalent NHC carbon by mesomeric and/or inductive mechanisms. This trend, currently more clearly noticeable in heavier transition metals, will certainly enter the mainstream of 3d NHC (and Co) organometallic chemistry. Additional progress is expected in the organization and tuning of the ’flexible sterics’ of the NHCs, again by virtue of the creative use of aromatic or aliphatic substituents. The management of non-covalent interactions (NCI) to organize the space at the coordination sphere should have implications on reactivity, regio- and enantio-selectivity and the stabilization of electronically or sterically unsaturated Co complexes. Finally, the impact of classical donor functionalities, being part of the NHC ligand designs and operating in synergy with the NHC donor(s), cannot be underestimated. Numerous new ligand development opportunities are opening up, being accessible with relatively facile synthetic methodologies. The controlled assembly of multinuclear homo- or hetero-metallic Co structures assisted by the suitably tuned NHC constitutes another direction of exploration.

Acknowledgments We are grateful to the CNRS, Université de Strasbourg, USIAS, Région Alsace and Communauté Urbaine de Strasbourg for the award of fellowships and a Gutenberg Excellence Chair (2010–11) to AAD. We thank the CNRS and the MESR (Paris) for funding of our own research in the area.

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Soc. 2014, 136, 15072–15078. Smith, J. M.; Long, J. R. Inorg. Chem. 2010, 49, 11223–11230. Gao, Y.; Chen, Q.; Leng, X.; Deng, L. Dalton Trans. 2019, 48, 9676–9683. Tian, Y.; Maulbetsch, T.; Jordan, R.; Törnroos, K. W.; Kunz, D. Organometallics 2020, 39, 1221–1229. Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 13464–13473. Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322–16323. Xie, S.; Li, X.; Sun, H.; Fuhr, O.; Fenske, D. Organometallics 2020, 39, 2455–2463. Mondal, K. C.; Samuel, P. P.; Roesky, H. W.; Carl, E.; Herbst-Irmer, R.; Stalke, D.; Schwederski, B.; Kaim, W.; Ungur, L.; Chibotaru, L. F.; Hermann, M.; Frenking, G. J. Am. Chem. Soc. 2014, 136, 1770–1773. Meng, Y.-S.; Mo, Z.; Wang, B.-W.; Zhang, Y.-Q.; Deng, L.; Gao, S. Chem. Sci. 2015, 6, 7156–7162. Tokmic, K.; Markus, C. R.; Zhu, L.; Fout, A. R. J. Am. Chem. Soc. 2016, 138, 11907–11913. Meier, S. C.; Holz, A.; Kulenkampff, J.; Schmidt, A.; Kratzert, D.; Himmel, D.; Schmitz, D.; Scheidt, E.-W.; Scherer, W.; Bülow, C.; Timm, M.; Lindblad, R.; Akin, S. T.; ZamudioBayer, V.; von Issendorff, B.; Duncan, M. A.; Lau, J. T.; Krossing, I. Angew. Chem. Int. Ed. 2018, 57, 9310–9314. Mo, Z.; Xiao, J.; Gao, Y.; Deng, L. J. Am. Chem. Soc. 2014, 136, 17414–17417. Hicks, J.; Jones, C. Organometallics 2015, 34, 2118–2121. Danopoulos, A. A.; Braunstein, P.; Yu Monakhov, K.; van Leusen, J.; Kogerler, P.; Clemancey, M.; Latour, J.-M.; Benayad, A.; Tromp, M.; Rezabal, E.; Frison, G. Dalton Trans. 2017, 46, 1163–1171. Andjaba, J. M.; Tye, J. W.; Yu, P.; Pappas, I.; Bradley, C. A. Chem. Commun. 2016, 52, 2469–2472. Dürr, S.; Zarzycki, B.; Ertler, D.; Ivanovic-Burmazovic, I.; Radius, U. Organometallics 2012, 31, 1730–1742. Fooladi, E.; Dalhus, B.; Tilset, M. Dalton Trans. 2004, 3909–3917. Simms, R. W.; Drewitt, M. J.; Baird, M. C. Organometallics 2002, 21, 2958–2963. Lubitz, K.; Radius, U. Organometallics 2019, 38, 2558–2572. Foerstner, J.; Kakoschke, A.; Goddard, R.; Rust, J.; Wartchow, R.; Butenschön, H. J. Organomet. Chem. 2001, 617–618, 412–422. Llewellyn, S. A.; Green, M. L. H.; Cowley, A. R. Dalton Trans. 2006, 4164–4168. Harrison, D. J.; Daniels, A. L.; Korobkov, I.; Baker, R. T. Organometallics 2015, 34, 4598–4604. Frank, R.; Howell, J.; Campos, J.; Tirfoin, R.; Phillips, N.; Zahn, S.; Mingos, D. M. P.; Aldridge, S. Angew. Chem. Int. Ed. 2015, 54, 9586–9590. Tokmic, K.; Jackson, B. J.; Salazar, A.; Woods, T. J.; Fout, A. R. J. Am. Chem. Soc. 2017, 139, 13554–13561. Sun, J.; Luo, L.; Luo, Y.; Deng, L. Angew. Chem. Int. Ed. 2017, 56, 2720–2724. Harris, C. F.; Kuehner, C. S.; Bacsa, J.; Soper, J. D. Angew. Chem. Int. Ed. 2018, 57, 1311–1315. Dürr, S.; Ertler, D.; Radius, U. Inorg. Chem. 2012, 51, 3904–3909. Nugent, J. W.; García-Melchor, M.; Fout, A. R. Organometallics 2020, 39, 2917–2927. Albrecht, M.; Maji, P.; Häusl, C.; Monney, A.; Müller-Bunz, H. Inorg. Chim. Acta 2012, 380, 90–95. Fränkel, R.; Kernbach, U.; Bakola-Christianopoulou, M.; Plaia, U.; Suter, M.; Ponikwar, W.; Nöth, H.; Moinet, C.; Fehlhammer, W. P. J. Organomet. Chem. 2001, 617–618, 530–545. Nishiura, T.; Takabatake, A.; Okutsu, M.; Nakazawa, J.; Hikichi, S. Dalton Trans. 2019, 48, 2564–2568. Xi, Z.; Liu, B.; Lu, C.; Chen, W. Dalton Trans. 2009, 7008–7014.

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N-Heterocyclic Carbene Complexes of Cobalt Liu, B.; Liu, X.; Chen, C.; Chen, C.; Chen, W. Organometallics 2012, 31, 282–288. Saha, S.; Kaur, M.; Singh, K.; Bera, J. K. J. Organomet. Chem. 2016, 812, 87–94. van der Meer, M.; Glais, E.; Siewert, I.; Sarkar, B. Angew. Chem. Int. Ed. 2015, 54, 13792–13795. Holm, S. C.; Rominger, F.; Straub, B. F. J. Organomet. Chem. 2012, 719, 54–63. Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. ACS Catal. 2016, 6, 3589–3593. Reilly, S. W.; Webster, C. E.; Hollis, T. K.; Valle, H. U. Dalton Trans. 2016, 45, 2823–2828. Denny, J. A.; Lamb, R. W.; Reilly, S. W.; Donnadieu, B.; Webster, C. E.; Hollis, T. K. Polyhedron 2018, 151, 568–574. Gu, S.; Xu, D.; Chen, W. Dalton Trans. 2011, 40, 1576–1583. Luo, S.; Siegler, M. A.; Bouwman, E. Eur. J. Inorg. Chem. 2019, 2019, 617–627. Wang, G.-F.; Song, X.-J.; Chen, F.; Li, Y.-Z.; Chen, X.-T.; Xue, Z.-L. Dalton Trans. 2012, 41, 10919–10922. Abubakar, S.; Bala, M. D. ACS Omega 2020, 5, 2670–2679. van Rensburg, H.; Tooze, R. P.; Foster, D. F.; Slawin, A. M. Z. Inorg. Chem. 2004, 43, 2468–2470. Ishizaka, Y.; Nakajima, Y. Organometallics 2019, 38, 888–893. van Rensburg, H.; Tooze, R. P.; Foster, D. F.; Otto, S. Inorg. Chem. 2007, 46, 1963–1965. Liu, Y.; Cheng, J.; Deng, L. Acc. Chem. Res. 2020, 53, 244–254. Bunting, P. C.; Atanasov, M.; Damgaard-Møller, E.; Perfetti, M.; Crassee, I.; Orlita, M.; Overgaard, J.; van Slageren, J.; Neese, F.; Long, J. R. Science 2018, 362, eaat7319. Bryan, A. M.; Long, G. J.; Grandjean, F.; Power, P. P. Inorg. Chem. 2013, 52, 12152–12160. Saber, M. R.; Przyojski, J. A.; Tonzetich, Z. J.; Dunbar, K. R. Dalton Trans. 2020, 49, 11577–11582. Verma, P. K.; Sethulekshmi, A. S.; Geetharani, K. Org. Lett. 2018, 20, 7840–7845. Shao, Z.; Zhong, R.; Ferraccioli, R.; Li, Y.; Liu, Q. Chin. J. Chem. 2019, 37, 1125–1130. Wang, J.; Wu, K.; Qi, X. Cat. Sci. Technol. 2019, 9, 5315–5321. Ibrahim, J. J.; Reddy, C. B.; Fang, X.; Yang, Y. Eur. J. Org. Chem. 2020, 2020, 4429–4432. Liu, X.; Liu, B.; Liu, Q. Angew. Chem. Int. Ed. 2020, 59, 6750–6755. Ibrahim, H.; Bala, M. D. J. Organomet. Chem. 2015, 794, 301–310. Gao, Y.; Wang, L.; Deng, L. ACS Catal. 2018, 8, 9637–9646. Verma, P. K.; Mandal, S.; Geetharani, K. ACS Catal. 2018, 8, 4049–4054. Verma, P. K.; Prasad, K. S.; Varghese, D.; Geetharani, K. Org. Lett. 2020, 22, 1431–1436. Sisemore, M. F.; Selke, M.; Burstyn, J. N.; Valentine, J. S. Inorg. Chem. 1997, 36, 979–984. Goetz, M. K.; Anderson, J. S. J. Am. Chem. Soc. 2019, 141, 4051–4062. Liu, B.; Xia, Q.; Chen, W. Angew. Chem. Int. Ed. 2009, 48, 5513–5516. Pelties, S.; Wolf, R. Z. Anorg. Allg. Chem. 2013, 639, 2581–2585. Gibson, S. E.; Johnstone, C.; Loch, J. A.; Steed, J. W.; Stevenazzi, A. Organometallics 2003, 22, 5374–5377. Poulton, A. M.; Christie, S. D. R.; Fryatt, R.; Dale, S. H.; Elsegood, M. R. J.; Andrews, D. M. Synlett 2004, 2004, 2103–2106. Tsui, E. Y.; Agapie, T. Polyhedron 2014, 84, 103–110. Schettini, M. F.; Wu, G.; Hayton, T. W. Chem. Commun. 2012, 48, 1484–1486. Nikolaevskii, S. A.; Petrov, P. A.; Sukhikh, T. S.; Yambulatov, D. S.; Kiskin, M. A.; Sokolov, M. N.; Eremenko, I. L. Inorg. Chim. Acta 2020, 508, 119643.

7.12

Organocobalt Complexes in C–H Bond Activation

Naohiko Yoshikai, Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai, Japan © 2022 Elsevier Ltd. All rights reserved.

7.12.1 Introduction 7.12.2 C–H activation promoted by low-valent cobalt complexes 7.12.2.1 Chelation-assisted C–H functionalization 7.12.2.1.1 Reaction with alkynes and alkenes 7.12.2.1.2 Reaction with electrophiles 7.12.2.1.3 Reaction with organometallic reagents 7.12.2.2 Non-chelation-assisted C–H functionalization 7.12.2.2.1 Reaction with alkynes and alkenes 7.12.2.2.2 Reaction with electrophiles 7.12.2.2.3 C–H borylation 7.12.3 C–H activation promoted by high-valent cobalt complexes 7.12.3.1 C–H activation promoted by Cp Co(III)-type complexes 7.12.3.1.1 Addition to polar C]X bonds and Michael acceptors 7.12.3.1.2 Reaction with alkynes, alkenes, and allenes 7.12.3.1.3 Reaction with nitrene or carbene precursors 7.12.3.1.4 Reaction with E–X-type electrophiles 7.12.3.1.5 Miscellaneous transformations 7.12.3.1.6 Enantioselective C–H functionalization 7.12.3.2 C–H activation assisted by bidentate directing group 7.12.3.2.1 Reaction with alkynes, alkenes, and allenes 7.12.3.2.2 Dehydrogenative C–H functionalization 7.12.3.2.3 C–H carbonylation and related transformations 7.12.3.2.4 Miscellaneous transformations 7.12.3.3 Miscellaneous reactions 7.12.4 Conclusion Acknowledgments References

7.12.1

759 759 759 761 766 768 768 768 771 771 774 774 774 776 785 789 792 794 795 795 803 806 807 808 810 810 810

Introduction

The transition metal-catalyzed C–H activation has had a transformative impact on organic synthesis over the last decades. Cobalt, the first-row transition metal of Group 9, has played a unique role in the development of this important subject. In 1955, Murahashi reported a Co2(CO)8-catalyzed ortho-carbonylation reaction of a Schiff base, which is the first example of transition metal-mediated C–H activation (Scheme 1).1 However, this seminal report was followed by a long dormant period, during which a few notable examples, such as ortho-alkenylation of azobenzene derivative with diphenylacetylene (Scheme 2)2 and intermolecular hydroacylation of vinylsilane (Scheme 3),3 were reported.4 Besides these examples of catalytic C–H activation, stoichiometric chelation-assisted C–H activation (cyclometallation) reactions5 using well-defined CoI complex [CoMe(PMe3)4] (Scheme 4)6 or cyclopentadienyl (Cp)–CoIII complex7 (Scheme 5) were reported. A simple CoIII salt was also found to promote stoichiometric C(sp3)–H activation of a multidentate substrate (Scheme 6).8 This chapter describes an overview of cobalt-catalyzed or -mediated C–H activation reactions, most of which have been reported during the last decade. The reactions discussed are limited to those that (most likely) involve organocobalt species generated via C–H bond cleavage at the inner sphere of cobalt. The chapter is organized firstly by the loosely defined oxidation state (i.e. low-valent or high-valent) of the cobalt complex responsible for C–H activation and secondary by the type of the reaction partner and/or the transformation. More specifically, “low-valent” is used to describe cobalt species generated from CoII or CoIII salts or complexes and reductants or well-defined complexes with Co0 or CoI electronic configuration. Meanwhile, “high-valent” refers to well-defined CoIII complexes such as Cp CoIII derivatives or CoII or CoIII salts, which are often used together with oxidants. This cobalt-catalyzed C–H activation has been reviewed many times, often with specific focus on the type of the catalysts or the transformations.9

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00140-2

759

760

Organocobalt Complexes in C–H Bond Activation

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

Scheme 6

Organocobalt Complexes in C–H Bond Activation

7.12.2

C–H activation promoted by low-valent cobalt complexes

7.12.2.1

Chelation-assisted C–H functionalization

7.12.2.1.1

761

Reaction with alkynes and alkenes

Yoshikai reported that a low-valent cobalt catalyst generated in situ from a cobalt(II) salt, a monodentate phosphine, and a Grignard reagent promotes the addition of 2-arylpyridine to an internal alkyne via chelation-assisted C–H activation (Scheme 7).10 The reaction affords the ortho-alkenylated product with syn-selectivity, with transfer of the ortho-hydrogen atom to the vinylic position of the product. The regioselectivity of the addition is governed by the steric size of the alkyne substituents, with the C–C bond formation preferentially taking place at the less hindered acetylenic carbon. With modification of the ligand and the Grignard reagent, aryl ketimines (Scheme 8),11 aryl aldimines,12 and N-pyrimidylindoles (Scheme 9)13 can also be used as the substrates for the directed C–H activation/hydroarylation to alkynes under room-temperature conditions. a,b-Unsaturated imines also undergo imine-directed olefinic C–H functionalization with alkynes, which is followed by 6p electrocyclization to give dihydropyridine derivatives (Scheme 10).14

Scheme 7

Scheme 8

Scheme 9

Scheme 10

762

Organocobalt Complexes in C–H Bond Activation

Scheme 11 illustrates a proposed catalytic cycle for the imine-directed alkyne hydroarylation. Reduction of the cobalt precatalyst by Grignard reagent would generate an active low-valent cobalt species [Co0L]. The alkyne and the imine would reversibly coordinate to the cobalt species, followed by rate-determining oxidative addition of the ortho C–H bond. The alkyne would then undergo migratory insertion into the Co–H bond, and subsequent C–C reductive elimination would afford the hydroarylation product along with regeneration of [Co0L]. The preference for the C–C bond formation at the less hindered acetylenic carbon may be ascribed to minimization of the steric repulsion during the alkyne insertion step.

Scheme 11

Petit reported that a well-defined Co0 complex [Co(PMe3)4] catalyzes some of the above alkyne hydroarylation reactions.15 For example, the reaction of aryl ketimine under microwave irradiation (170  C) gave the anti-hydroarylation product, which was shown to form through isomerization of initially formed syn-hydroarylation product (Scheme 12). Notably, a DFT study on this system suggested a concerted, so-called ligand-to-ligand hydrogen transfer (LLHT) mechanism16 for the C–H bond cleavage and the alkyne insertion. Furthermore, Suslick and Tilley demonstrated that a well-defined Co–I complex [Co(N2)(PPh3)3][Li(THF)3] also serves as a viable catalyst for the imine-directed alkyne hydroarylation under mild conditions (Scheme 13).17 The Grignard-free catalyst tolerated functional groups such as ester and cyano groups. Their extensive mechanistic studies on this and related reaction systems shed light on the complexity of the reaction kinetics involving reversible coordination of alkyne and imine molecules and led to a proposal of a phosphine-assisted concerted metalation–deprotonation (CMD) as the mechanism for the C–H activation.

Scheme 12

Scheme 13

Organocobalt Complexes in C–H Bond Activation

763

Yoshikai and coworkers reported several cobalt-based catalytic systems for the chelation-assisted hydroarylation of styrene derivatives. The addition of 2-arylpyridine to styrene using PCy3 as the supporting ligand afforded the branched adduct as the major regioisomer (Scheme 14, top).18 By contrast, the use of bulky N-heterocyclic carbene (NHC) such as IMes led to linear-selective addition (Scheme 14, bottom). Deuterium-labeling experiments on these catalytic systems suggested that the reaction involves reversible C–H activation and olefin insertion and that the regioselectivity is determined in the C–C bond-forming reductive elimination, which was also supported by computational studies.19 The branched selectivity appeared to reflect the intrinsic preference of cobalt for the formation of Z3-benzyl-coordinated intermediate, while the IMes ligand was proposed to favor the formation of a less congested alkylcobalt species leading to the linear product.

Scheme 14

Using a simple triarylphosphine such as P(p-Tol)3, aryl aldimines underwent branched-selective addition to styrenes under mild conditions (Scheme 15).20 Aryl ketimines could also be used as substrates for the branched-selective addition with a simple triarylphosphine ligand and the use of a secondary Grignard reagent as the reductant (Scheme 16, top).21 On the other hand, structural modification of the triarylphosphine allowed a switch to linear selectivity for aryl ketimines (Scheme 16, bottom).22 Using a chiral phosphoramidite as the supporting ligand, the branched-selective addition of 3-iminoindole derivative to styrene was achieved with moderate to good enantioselectivity (Scheme 17).23

Scheme 15

Scheme 16

764

Organocobalt Complexes in C–H Bond Activation

Scheme 17

Nakamura reported ortho-alkylation of secondary benzamides with alkyl-substituted olefins using a catalyst generated from Co(acac)2, CyMgCl, and DMPU (N,N0 -dimethylpropylene urea) (Scheme 18).24 Notably, the reaction using an internal olefin resulted in the primary alkylation product. Yoshikai developed a cobalt–phenanthroline catalyst for the ortho-alkylation of N-aryl ketimines with vinylsilanes and alkyl olefins (Scheme 19).25 On extending such imine-directed hydroarylation, Yoshikai used 3-iminoindoles to demonstrate intramolecular cyclization under cobalt–NHC catalysis (Scheme 20)26 and chain-walking hydroarylation to allyl-, homoallyl-, or bishomoallylbenzene to give 1,1-diarylalkanes (Scheme 21).27 Meanwhile, Petit showed the competence of Co(PMe3)4 as a catalyst for inter- and intramolecular hydroarylation of 3-iminoindole (Scheme 22).15c N–H imine also proved to serve as a powerful directing group for the olefin hydroarylation under cobalt–phosphine catalysis (Scheme 23).28 In some of the above-mentioned hydroarylations, metallic magnesium could be used as the reductant instead of the Grignard reagent.29

Scheme 18

Scheme 19

Scheme 20

Organocobalt Complexes in C–H Bond Activation

765

Scheme 21

Scheme 22

Scheme 23

Besides (hetero)aromatic and olefinic C–H bonds, aldehydic C–H bonds have also been subjected to chelation-assisted C–H functionalization with alkenes or alkynes under cobalt catalysis. The addition of N-3-picolin-2-yl aldimine to 1-alkenes was promoted by a low-valent catalyst generated from CoBr2, an electron-rich diphosphine dippf, and Zn (Scheme 24).30 Meanwhile, another cobalt catalyst bearing a different electron-rich diphosphine (dcype) effected the annulation of salicylaldehydes and alkynes to form either chromone (Scheme 25) or chromanone derivatives, depending on the alkyne used.31

Scheme 24

Scheme 25

766

Organocobalt Complexes in C–H Bond Activation

7.12.2.1.2

Reaction with electrophiles

Nakamura reported cobalt-catalyzed ortho-alkylation of secondary benzamides with alkyl chlorides (Scheme 26),32 where the excess Grignard reagent (CyMgCl) served as a reductant for the cobalt precatalyst (Co(acac)2) and a base to remove the amide N–H and the ortho C–H protons. The reaction became sluggish with a secondary alkyl chloride, while the use of tBuCl resulted in an isobutylated product. Ackermann developed cobalt–NHC catalysts for chelation-assisted C–H functionalization of 2-arylpyridines and N-pyridylindoles with a broad range of organic electrophiles including aryl sulfamates, carbamates, and chlorides, benzyl phosphates, alkyl chlorides, and alkenyl acetates, where CyMgCl was also used as the reductant/base (Scheme 27).33 Similar catalytic systems were shown to promote the ortho C–H arylation and alkylation of secondary benzamides, aryl tetrazoles, and 2-aryloxazoline derivatives.34 Yoshikai developed ortho C–H functionalization of aryl imines and related substrates with alkyl (pseudo)halides, aryl chlorides, benzyl phosphates, and alkenyl phosphates under cobalt–NHC or cobalt–phosphine catalytic systems (Scheme 28).35 Zeng reported cobalt-catalyzed ortho-allylation of 2-arylpyridines and aryl imines with allylic carbonates under ligand-free conditions (Scheme 29).36

Scheme 26

Scheme 27

Scheme 28

Organocobalt Complexes in C–H Bond Activation

767

Scheme 29

Scheme 30 shows a catalytic cycle proposed for the cobalt–NHC-catalyzed ortho-alkylation of aryl ketimines. An alkyl cobalt species derived from the cobalt precatalyst and the Grignard reagent would undergo cyclometalation of the aryl imine. The resulting intermediate would undergo single electron transfer (SET) to the alkyl halide, which would be followed by C–C coupling to afford the alkylation product. Meanwhile, transmetalation with the Grignard reagent would regenerate the alkyl cobalt species. The radical nature of the cobalt–NHC catalytic system was exploited to develop a tandem radical cyclization/ortho-alkylation reaction between aryl imine and tethered bromo-alkene (Scheme 31).37

Scheme 30

Scheme 31

Besides organic halides and pseudohalides, N-aryl aldimines and aziridines also serve as electrophiles for cobalt-catalyzed chelation-assisted aromatic C–H functionalization (Scheme 32).38 Again, the combination of a cobalt–NHC catalyst and a Grignard reagent proved to effect both the transformations.

768

Organocobalt Complexes in C–H Bond Activation

Scheme 32

7.12.2.1.3

Reaction with organometallic reagents

Wang and Shi reported an ortho-arylation reaction of 2-arylpyridine derivatives with aryl Grignard reagents in the presence of catalytic Co(acac)3, TMEDA and 2,3-dichlorobutane (DCB), where DCB acted as the oxidant (Scheme 33).39 The same catalytic system was shown to promote methylation of ferrocenes containing nitrogen directing groups using MeMgBr as the methylating agent.40 Nakamura developed ortho-alkylation of 2-arylpyridines and secondary benzamides with alkyl Grignard reagents in the presence of catalytic Co(acac)2 and excess DMPU under aerobic conditions (Scheme 34).41 Xu reported oxidative ortho-methylation and ethylation of bidentate N-(8-quinolinyl)benzamide with the corresponding trialkylaluminum using DCB as the oxidant.42

Scheme 33

Scheme 34

7.12.2.2 7.12.2.2.1

Non-chelation-assisted C–H functionalization Reaction with alkynes and alkenes

Yoshikai demonstrated that a low-valent cobalt–diphosphine catalyst generated by reduction with a Grignard reagent promotes C2-alkenylation of (benz)oxazole and (benzo)thiazole derivatives with internal alkynes (Scheme 35).43 The reaction of C2-deuterated benzoxazole resulted in clean transfer of the deuterium atom to the vinylic position of the product, indicating that the relatively acidic C2 position was not deprotonated by the Grignard reagent but instead underwent C–H activation on the cobalt catalyst. More recently, Yoshikai reported that a catalytic system comprised of a cobalt–diphosphine complex and an aluminum Lewis acid such as AlMe3 promotes the C(sp2)–H alkenylation of Lewis basic substrates such as formamides, pyridones, pyridines, imidazopyridines, and oxazole/thiazole derivatives with internal alkynes (Scheme 36).44 Pyridones reacted at both the C4 and C6

Organocobalt Complexes in C–H Bond Activation

769

Scheme 35

Scheme 36

positions, while pyridines displayed selectivity toward the C4 position. Imidazo[1,2-a]pyridine exclusively reacted at the C5 position. Mechanistic studies on the reaction of formamide suggested that the reaction involves concerted C–H cleavage and alkyne insertion via a LLHT mechanism. Low-valent cobalt–diphosphine catalysts generated using metallic reductant such as zinc, manganese, or indium have been shown to promote hydroacylation reactions via aldehyde C–H activation. Yoshikai disclosed enantioselective cyclization of ortho-acyl or -alkenyl benzaldehyde using cobalt–chiral diphosphine catalysts to form phthalide or indanone derivative, respectively (Scheme 37).45 While the latter indanone formation was rather limited in terms of the alkene substituent, modification of the reductant and the solvent proved effective for the intramolecular hydroacylation of trisubstituted alkene (Scheme 38).46 Regardless of the E/Z ratio of the C]C bond, various ortho-alkenylbenzaldehydes could be transformed into trans-2,3-disubstituted indanones with high diastereo- and enantioselectivities. An achiral cobalt–diphosphine catalyst engaged benzaldehydes bearing ortho-cyclopropylvinyl or cyclopropylidenemethyl group in intramolecular hydroacylation involving cyclopropane cleavage, affording eight- or seven-membered cyclic ketones, respectively (Scheme 39).47 While all the above hydroacylation required aromatic aldehydes, Dong demonstrated that the chiral cobalt–diphosphine catalyst could also promote enantioselective intramolecular hydroacylation of a,a-bisallyl-substituted aldehydes, affording cyclobutanone derivatives with high enantio-, diastereo- and regioselectivities (Scheme 40).48

Scheme 37

770

Organocobalt Complexes in C–H Bond Activation

Scheme 38

Scheme 39

Scheme 40

Yoshikai found that the reaction of an arylzinc reagent with an internal alkyne in the presence of a cobalt–diphosphine (Xantphos) catalyst afforded an ortho-alkenylaryl zinc species, as demonstrated by its interception with iodine or other electrophiles (Scheme 41).49 This reaction is considered to proceed through insertion of the alkyne into an aryl cobalt species, vinyl-to-aryl 1,4-cobalt migration, and cobalt-to-zinc transmetalation. The ortho-alkenylarylzinc species or its iodinated product has proved to serve as a precursor for the synthesis of various benzo[b]heterole derivatives.50 Yoshikai also demonstrated the feasibility of 1,4-cobalt migration in the addition of arylzinc reagents to norbornene51 and in intramolecular settings (Scheme 42).52

Scheme 41

Organocobalt Complexes in C–H Bond Activation

771

Scheme 42

7.12.2.2.2

Reaction with electrophiles

Mita and Sato reported that a cobalt–diphosphine catalyst promotes, in the presence of AlMe3, carboxylation of an allylic C(sp3)–H bond with CO2 (Scheme 43).53 The reaction system allowed the reaction to take place at the terminal position of allyl arenes and 1,4-dienes, affording cinnamic acid and related derivatives. The same catalytic system also promoted direct addition of allyl arenes and a-olefins to ketones54 and cyclization of ketoalkenes into 2-vinylcycloalkanols (Scheme 44).55

Scheme 43

Scheme 44

7.12.2.2.3

C–H borylation

Chirik reported C–H borylation of heteroarenes and arenes using a PNP pincer-ligated cobalt alkyl complex as catalyst (Scheme 45).56 Electron-rich heteroarenes such as furan, thiophene, and indole derivatives could be borylated using HBpin

Scheme 45

772

Organocobalt Complexes in C–H Bond Activation

under mild conditions. By contrast, borylation of pyridines and simple arenes need to be performed using B2pin2 under more forcing conditions. An active catalyst could also be generated by in situ-reduction of a PNP-ligated cobalt(II) dichloride complex with NaBEt3H, and an electron-rich PNP ligand was found to give rise to the highest catalytic activity.57 A terpyridine cobalt(II) diacetate also serves as an air-stable precatalyst for the C–H borylation.58 Chirik’s mechanistic study on the Co-PNP-catalyzed C–H borylation revealed distinct catalytic cycles for six-membered59 and five-membered60 (hetero)arenes, as illustrated in Scheme 46. The reaction of the former substrate class was found to involve borylation of the pyridine C4-position of the pincer ligand. Oxidative addition of B2pin2 to a cobalt(I) hydride species, reductive elimination of HBpin, oxidative addition of the arene C–H bond to cobalt(I) boryl species, and C–B bond-forming reductive elimination are the major elementary steps. The arene oxidative addition was identified as the turnover-limiting step (TLS), which became slower by the influence of the C4-boryl group. With this insight, Chirik developed an improved pincer ligand bearing a pyrrolidinyl group at the C4 position. The reaction of the latter substrate class using HBpin instead of B2pin2 was also proposed to involve a CoI/CoIII catalytic cycle, while reductive elimination of H2 from cobalt(III) dihydride boryl was determined as the TLS. (A)

(B)

Scheme 46

Organocobalt Complexes in C–H Bond Activation

773

The cobalt–PNP catalyst was found to display unique site selectivity in the C–H borylation of fluorinated arenes (Scheme 47).61 The cobalt catalyst targeted the acidic and less hindered ortho-position of the fluorine atom of meta-substituted fluoroarenes with high site-selectivity. By contrast, a conventional iridium catalyst, with sterics as the primary selectivity controlling factor, showed poor site selectivity toward the same substrates.

Scheme 47

Chirik also reported that a cobalt a-diimine complex catalyzes benzylic C–H borylation of toluene derivatives, identifying different reaction conditions for the selective mono-, di-, and tri-borylation (Scheme 48).62 Furthermore, branched alkyl arenes were found to undergo C–H borylation at the homobenzylic position. Chirik also demonstrated the capability of the cobalt-diimine catalyst to promote hydrogen isotope exchange of alkyl arene derivatives (Scheme 49).63 The isotope exchange occurred most efficiently at the benzylic C(sp3)–H bonds, while exchange at other C(sp3)–H bonds and aromatic C–H bonds were also observed to some extent. The stereochemical configuration of a chiral nonracemic alkyl arene was retained.

Scheme 48

Scheme 49

774

Organocobalt Complexes in C–H Bond Activation

7.12.3

C–H activation promoted by high-valent cobalt complexes

7.12.3.1

C–H activation promoted by Cp Co(III)-type complexes

7.12.3.1.1

Addition to polar C]X bonds and Michael acceptors

Matsunaga and Kanai reported that a cationic CoIII complex [Cp Co(C6H6)](PF6)2 catalyzes chelation-assisted addition of 2-arylpyridines to electrophilic acceptors such as N-sulfonyl aldimine and a,b-unsaturated ketone (Scheme 50).64 Both acyclic and cyclic enones could be used as the latter electrophile. In addition, N-acyl pyrrole could also be employed as a Michael acceptor. A neutral CoIII complex [Cp CoCl2]2 could also be made catalytically active by activation with a silver salt such as AgPF6. Under the Cp CoIII catalysis, N-pyrimidylindoles also underwent the chelation-assisted addition of the C2 position to imines.65

Scheme 50

A proposed catalytic cycle for the addition of 2-phenylpyridine to aldimine is shown in Scheme 51. Dissociation of the benzene ligand from the cationic Cp CoIII complex would be followed by chelation-assisted C–H activation of 2-phenylpyridine. The resulting cobaltacycle intermediate would undergo insertion of the aldimine, and subsequent protodemetalation and C–H

Scheme 51

Organocobalt Complexes in C–H Bond Activation

775

activation would afford the product along with regeneration of the cobaltacycle intermediate. While the cationic Cp CoIII complex alone was used as the catalyst in this original report, as described below, many of the following reports on Cp CoIII-catalyzed C–H functionalization employed cationic Cp CoIII catalysts, either derived from well-defined cationic complexes or the combination of neutral Cp CoIII complexes and silver salts, along with carboxylate salts or carboxylic acids as additives. For such reaction systems, cationic species such as [Cp CoIII(OCOR)]+ have often been proposed as catalytical active species, which would undergo C–H activation through a concerted metalation-deprotonation mechanism with the carboxylate as the internal base. Hummel and Ellman reported that a cationic Cp CoIII catalyst promotes chelation-assisted addition of azobenzenes to aldehydes, which is followed by dehydrative cyclization to afford indazole derivatives (Scheme 52).66 A cationic Cp CoIII complex bearing non-coordinating B(C6F5)4 anions and AcOH constituted the efficient catalytic system. The reaction of unsymmetrical azobenzene would produce a mixture of products arising from C–H activation of the different aryl groups, while the site of the reaction could be controlled when one of the aryl group was made sterically less accessible. The same catalytic system was shown to promote the addition of a,b-unsaturated oxime ethers to aldehydes, which afforded furans through deaminative cyclization of the initially formed hydroxy/oxime intermediate (Scheme 53). Similar to the above indazole synthesis, Cp CoIII-catalyzed addition of 2-arylpyridines and aldehydes and subsequent dehydrative cyclization were reported to give indolizine derivatives.67

Scheme 52

Scheme 53

Besides aldimines, aldehydes, and a,b-unsaturated ketones, a variety of C ¼ X-type compounds and Michael acceptors have been demonstrated to serve as electrophiles in the Cp CoIII-catalyzed, chelation-assisted C–H functionalization. These include C ¼ X substrates such as isocyanates (Scheme 54),68 ketenimines,69 and glyoxylate,70 and Michael acceptors such as maleimides/maleate esters (Scheme 55)71 and acrolein.72 The scope of aromatic and olefinic substrates used in this type of transformation is also broad. For example, the reaction using maleimide as the Michael acceptor has been reported for N-pyrimidylindoles, enamides, azobenzenes, oxime ethers, and aryl ketones as the reaction partners. In addition, 8-methyl- and ethylquinolines were also shown to participate in Cp CoIII-catalyzed, chelation-assisted C(sp3)–H activation and addition to maleimide.71h,71i

Scheme 54

Scheme 55

776

Organocobalt Complexes in C–H Bond Activation

Ellman reported a Cp CoIII-catalyzed three-component coupling of arenes, a,b-unsaturated ketones, and aldehydes through chelation-assisted C–H activation, Michael addition, and aldol reaction (Scheme 56).73 For example, the reaction of 1-arylpyrazole with a near equimolar amount of enone and an excess amount of aldehyde proceeded smoothly at room temperature to afford the three-component product in good yield with high diastereoselectivity, which was also accompanied by a varying amount of the Michael adduct. The reaction using an analogous Cp RhIII catalyst predominantly afforded the Michael adduct and gave only a trace amount of the three-component product, which would imply the higher nucleophilicity of the cobalt enolate intermediate formed upon the initial Michael addition. In addition to 1-arylpyrazoles, pyridylisoquinolinone, aryl ketimine, and vinylpyrazole were also shown to participate in the three-component coupling. The reaction using chiral N-sulfinyl imine in place of aldehyde also took place to afford the Mannich-type product with high diastereoselectivity (Scheme 57).

Scheme 56

Scheme 57

7.12.3.1.2

Reaction with alkynes, alkenes, and allenes

Matsunaga and Kanai reported a cationic Cp CoIII complex-catalyzed coupling reactions between N-carbamoylindoles and internal alkynes, which afford either C2-alkenyl indoles or pyrroloindolone derivatives (Scheme 58).74 With an N,N-dimethylcarbamoyl group, high concentration, and lower temperature, the reaction gave C2-alkenyl indoles in good yield with high regio- and stereoselectivity. By contrast, the indole substrate bearing a morpholine group, at lower concentration and higher temperature, underwent deaminative annulation to produce a pyrroloindolone derivative. The latter transformation was particularly notable as it was not observed using a Cp RhIII catalyst. N-Carbamoylpyrroles also participated in the C2-alkenylation with internal alkynes, where Cp CoIII showed higher efficiency and selectivity than Cp RhIII.75

Scheme 58

Upon further investigation, Matsunaga found that the above annulation reaction involves migration of the carbamoyl directing group to form a tetrasubstituted alkenyl amide as an intermediate, and thus developed reaction conditions that could selectively afford the alkenyl amide (Scheme 59).76 The reaction was applicable to a series of aryl-substituted internal alkynes. Overall, the Cp CoIII-catalyzed coupling reactions between N-carbamoylindole and alkyne commonly involve directed C–H activation and alkyne insertion to give an alkenyl cobalt species, which would undergo protodemetalation or intramolecular carbonyl addition. The unique success of the latter process with the Cp CoIII catalyst may be ascribed to the higher nucleophilicity of the organocobalt

Organocobalt Complexes in C–H Bond Activation

777

Scheme 59

species. Ackermann reported a related case of directing group migration for the Cp CoIII-catalyzed annulation of N-(2-pyridyl) pyridones and propargylic carbonates, which afforded indolizinone derivatives (Scheme 60).77 The migration of the pyridyl group is triggered by the nucleophilc attack of an alkenylcobalt intermediate on the C2 position.

Scheme 60

Sundararaju, Matsunaga/Kanai, and Ackermann independently reported Cp CoIII-catalyzed annulation reactions between aryl oxime derivatives and alkynes to give isoquinolines.78 Sundararaju used unprotected oxime as the directing group (Scheme 61), while Matsunaga/Kanai and Ackermann employed oxime acetate. These reactions were applicable to a variety of internal alkynes including diarylalkynes, dialkylalkynes, and aryl(alkyl)alkynes as well as terminal alkynes. Aryl(alkyl)alkynes preferentially underwent C–C bond formation at the acetylenic carbon proximal to the alkyl group often with imperfect regioselectivity, whereas terminal alkynes formed the C–C bond exclusively at the unsubstituted carbon. Matsunaga and Kanai found that oxime derivatives bearing a meta-substituent such as chloro, bromo, iodo, ester, methyl, or CF3 group underwent C–H activation at the less hindered position with high selectivity (15:1 to > 20:1) (Scheme 62). This observation highlighted the uniqueness of the Cp CoIII catalyst, as Cp RhIII catalyst displayed poor regioselectivity for the same substrates. The high regioselectivity of Cp CoIII was rationalized by the shorter ionic radius of cobalt. Similar to the oxime groups, various directing groups including N-hydroxybenzimidamide,79 N-Bochydrazones,80 N-sulfinylimines,81 unprotected hydrazones,82 3-aryloxadizolones,83 3-aryloxadiazoles,84 1,2-(diarylidene)hydrazines,85 and N-Cbz hydrazones86 have been employed for Cp CoIII-catalyzed redox-neutral isoquinoilne synthesis through directed C–H activation/alkyne annulation. Meanwhile, isoquinoline synthesis via C–H activation/alkyne annulation of N–H imines and benzimidates under oxidative conditions was also reported.87

Scheme 61

Scheme 62

778

Organocobalt Complexes in C–H Bond Activation

Li, Glorius, and Zhang independently reported Cp CoIII-catalyzed dehydrative annulation reactions between anilides and alkynes for the synthesis of quinoline derivatives.88 Common to their catalytic systems was the use of Lewis acid such as AgNTf2, BF3
OEt2, B(C6F5)3, or Zn(OTf )2 as a crucial additive. The catalytic systems using AgNTf2 or BF3•OEt2 are shown in Scheme 63. The reaction was proposed to involve amide-directed aromatic C–H activation, insertion of the alkyne into the aryl–CoIII bond, intramolecular nucleophilic addition of the alkenyl cobalt species to the amide carbonyl group, and dehydrative aromatization. Among these steps, the nucleophilic addition would be facilitated by the Lewis acid. Yi reported a quinoline synthesis via Cp CoIII-catalyzed condensation of simple anilines with acetophenone and paraformaldehyde,89 while the mechanism is apparently different from that of the above annulation. The same group also described Cp CoIII-catalyzed coupling of aniline, arylacetylene, and DMSO to afford 4-arylquinolines90 or 3-arylquinolines.91

Scheme 63

In the study of the annulation of anilides and alkynes, Glorius revealed a switch of the reaction outcome to indole formation by modification of the reaction conditions and the N-modifying substituent (Scheme 64).88b Thus, change of the acyl group to a carbamoyl group, along with the use of Ag2O as oxidant, resulted in an exclusive formation of N-carbamoyl indole. The same transformation was independently reported by Shi, where Ag2CO3 was used as the oxidant.92 Meanwhile, the oxidizing directing group strategy93 has been exploited for the development of a series of Cp CoIII-catalyzed, redox-neutral indole-forming annulation reactions. Thus, aniline derivatives containing preinstalled oxidizing moieties, such as Boc-protected arylhydrazine (Scheme 65),94 N-nitrosoaniline (Scheme 66),95 arylhydrazine,96 and N-arylnitrones,97 underwent annulation with alkynes to afford the corresponding NH indoles or N-substituted indoles.

Scheme 64

Scheme 65

Scheme 66

Organocobalt Complexes in C–H Bond Activation

779

Cheng reported Cp CoIII-catalyzed oxidative annulation reactions of 2-arylpyridines, azobenzenes, and related substrates with internal alkynes for the synthesis of the corresponding polycyclic quaternary ammonium salts (Scheme 67).98 These reactions employed a stoichiometric amount of AgBF4 as the oxidant as well as the source of BF4 counteranion for the ammonium salt product. Related reactions were also reported by Wang99 and Li.100 Furthermore, Choudhury described oxidative annulation between N-(2-pyridyl)imidazole and internal alkynes, and characterized a pyridine-chelated Cp CoIII–NHC complex as the reaction intermediate (Scheme 68).101

Scheme 67

Scheme 68

Benzoic acid derivatives have also been used as substrates for Cp CoIII-catalyzed C–H activation/alkyne annulation. Zhang and Li independently reported redox-neutral, dealkoxylative annulation of aromatic esters with alkynes for the synthesis of indenone derivatives (Scheme 69).102 Both the reaction systems predominantly used diarylalkynes as the reaction partners. Jeganmohan, Pawar, and Sundararaju described isoquinolone synthesis from N-methoxybenzamides and alkynes or 1,3-diynes, where the N-OMe group served as the internal oxidant (Scheme 70).103 The reaction of 1,3-diynes took place with exclusive regioselectivity, and the product could be used for the second annulation with another molecule of N-methoxybenzamide for the synthesis of

Scheme 69

780

Organocobalt Complexes in C–H Bond Activation

Scheme 70

bis-isoquinolones. Glorius reported an intramolecular variant of this isoquinolone synthesis (Scheme 71).104 Thus, Nalkoxybenzamides bearing tethered alkyne moieties underwent regioselective intramolecular annulation via N–O bond cleavage to afford isoquinolone and pyridone derivatives. The number of the methylene groups between the oxygen atom and the alkynyl group could be between two and five. The pendant hydroxyl group could be utilized to transform the products into a variety of aromathecin, protoberberine, and tylophora alkaloids. N-Chloroamide105 and N-pyridinium ylide106 have also been used as directing groups for analogous annulation reactions for the synthesis of isoquinolones. Sundararaju reported oxidative annulation of benzoic and acrylic acids with internal alkynes for the synthesis of isocoumarin and pyrone derivatives (Scheme 72).107

Scheme 71

Scheme 72

Besides the above-described reactions, a variety of Cp CoIII-catalyzed annulation reactions between aromatic or olefinic substrates and alkynes via C–H activation have been reported. Thus, enamides,108 cyclic sulfonyl ketimines,109 2-arylimidazoles,110 2-arylquinazolinones,111 and a-ketosulfoxonium ylides112 have been used as the substrates for the annulation with alkynes for the synthesis of pyrroles, spirocyclic indenyl benzosultams, imidazo[2,1-a]isoquinolines, fused quinazolinones, and 1-naphthols, respectively. 2-Alkenylphenols and ynamides were demonstrated to undergo [5 +2] annulation or dearomative spiroannulation to give 2-aminobenzoxepines113 or spiro[4,5]decatetraenone derivatives,114 respectively. Vinylene carbonate could be used as an acetylene surrogate for Co CoIII-catalyzed annulation with benzamides and acrylamides for the synthesis of isoquinolones and pyridones (Scheme 73).115

Scheme 73

Organocobalt Complexes in C–H Bond Activation

781

Pérez-Temprano studied the mechanism of the oxidative annulation of 2-arylpyridine and alkyne (Scheme 67) in detail to reveal a few key catalytic intermediates and the kinetic profile of the reaction (Scheme 74).116 A cationic cyclometalated complex of 2-phenylpyridine bearing an acetonitrile ligand was synthesized through oxidative addition of 2-(2-iodophenyl)pyridine to a Cp CoI complex, and it was demonstrated to be catalytically competent. Furthermore, a seven-membered metallacycle formed through insertion of diphenylacetylene into the aryl–CoIII bond was unambiguously characterized. Overall, a catalytic cycle involving cyclometalation with Cp CoIII, alkyne insertion into the aryl–CoIII bond, C–N reductive elimination to form the product, and reoxidation of Cp CoI to Cp CoIII was supported. The oxidative addition of aryl iodide to Cp CoI has also enabled the synthesis of weakly coordinated Cp CoIII metalacycles that are difficult to synthesize via C–H activation.117 This approach proved useful to probe Cp CoIII-catalyzed C–H functionalization reactions of weakly coordinating substrates. Pérez-Temprano also revealed beneficial effects of MeCN and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) in the C–H metalation by Cp CoIII.118 Thus, MeCN was found to stabilize the cobaltacycle intermediate and hence to suppress the reverse reaction of the C–H metalation, whereas HFIP proved to significantly accelerate the C–H metalation.

Scheme 74

In addition to the C2-alkenylation of N-carbamoylindoles and -pyrroles with alkynes (Scheme 58), several other examples of Cp CoIII-catalyzed, chelation-assisted C–H alkenylation with alkynes have been reported. Yu reported linear-selective alkenylation of 2-arylpyridine, N-pyrimidylindole, and 6-arylpuridine derivatives with arylacetylenes to afford trans-stilbene-type products (Scheme 75).119 Sundararaju reported C(sp3)–H alkenylation of 8-methylquinoline derivatives with internal alkynes (Scheme 76).120 Maji reported ortho-alkenylation of benzamides bearing a bulky N-tert-butyl group with internal alkynes.121 An intramolecular variant of this reaction, using benzamide substrates bearing a propargyl ether group at the meta position, afforded benzofuran derivatives.122 Li observed that the regioselectivity of C2-alkenylation of N-pyrimidylindole with a terminal alkyne largely depends on the alkyne substituent. Thus, in contrast to the linear-selective alkenylation (Scheme 75), exclusively

Scheme 75

782

Organocobalt Complexes in C–H Bond Activation

Scheme 76

branched-selective alkenylation took place with propargyl alcohol or amine derivatives as well as with alkynyl silanes bearing a bulky silyl group (Scheme 77).123 DFT calculations suggested that the regioselectivity for the latter substrates was determined in the protodemetalation of alkenylcobalt intermediate. Sundararaju reported pyrazolyl group-directed arene C–H alkenylation using various alkynes, and synthesized and characterized a seven-membered chelated alkenylcobalt complex (Scheme 78).124 Prabhu reported decarboxylative and desilylative ortho-alkenylation reactions of benzamide and arylpyrazole derivatives with alkynylcarboxylic acids and alkynylsilanes, respectively.125

Scheme 77

Scheme 78

Alkenylcobalt(III) species formed through chelation-assisted C–H activation and subsequent alkyne insertion could be terminated by processes other than the above-discussed heterocycle formation and protodemetalation. Sundararaju reported Cp CoIII-catalyzed coupling of quinoline N-oxides and internal alkynes via C–H alkenylation and oxygen atom transfer, which afforded a-(8-quinolinyl)ketone derivatives (Scheme 79).126 Li reported a C–H alkenylation/intramolecular Diels–Alder cascade between N-pyrimidylindole and a substrate containing terminal alkyne and 2,5-cyclohexadienone moieties, which afforded a fused polycyclic indole derivative (Scheme 80).127 Ellman demonstrated that the alkenylcobalt(III) species can be intermolecularly intercepted by electrophilic halogenating agents. Thus, a three-component coupling between an arene bearing a directing group, an alkyne or allene, and N-halosuccinimide was achieved, affording a functionalized alkenyl halide in a stereochemically controlled fashion (Scheme 81).128

Scheme 79

Organocobalt Complexes in C–H Bond Activation

783

Scheme 80

Scheme 81

Several examples of Cp CoIII-catalyzed directed C–H functionalization using less polar or nonpolar alkenes have been reported. Matsunaga and Kanai reported ortho C–H alkenylation of secondary benzamides with acrylate using AgOAc as an oxidant (Scheme 82).129 Acetanilide was also a viable substrate for the alkenylation. Furthermore, such C–H alkenylation has been extended to Weinreb amide130 and ketone131 as directing groups. Glorius reported intermolecular carboamination of acrylate with phenoxyacetamide to afford a-amino acid derivatives (Scheme 83).132 The reaction was proposed to involve chelation-assisted C–H activation, alkene insertion, and C–N reductive elimination to form a six-membered intermediate, which would be followed by oxidative addition of the O–N bond to CoI and subsequent protodemetalation to furnish the product. In contrast to this mode of action of the Cp CoIII catalyst, under Cp RhIII catalysis, the same substrates afforded a Heck-type product due to facile b-hydride elimination. Propiolates and strained bicyclic alkenes have also been demonstrated to undergo analogous carboamination under Cp CoIII catalysis.133 Similar to the isoquinolone synthesis via [4 + 2] annulation using alkynes (Scheme 70), [4 +2]-type annulation reactions between N-methoxybenzamides and oxa- or azabicyclic alkenes (Scheme 84)134 and between N-chlorobenzamides and maleimides135 have been reported. A spirocyclization between imidate and maleimide that proceeds via C–H alkenylation and intramolecular Michael addition was also reported.136

Scheme 82

784

Organocobalt Complexes in C–H Bond Activation

Scheme 83

Scheme 84

Ackermann reported Cp CoIII-catalyzed C2-alkylation of N-pyridylindoles with unactivated 1-alkenes, which took place in a regiodivergent fashion depending on the reaction conditions employed (Scheme 85).137 A catalytic system comprised of Cp Co (CO)I2 and AgSbF6 afforded, at the reaction temperature of 120  C, the linear alkylation product with exclusive regioselectivity. On the other hand, the reaction could be promoted at a lower temperature (50  C) by adding a bulky carboxylic acid (1-AdCO2H), importantly with a reversal of the regioselectivity, affording the branched adduct as the major product. According to experimental and computational mechanistic studies, the observed regiodivergence was ascribed to distinct mechanisms of proto-demetalation of alkyl cobalt intermediates that would be formed via C–H metalation and alkene insertion. A Cp CoIII catalyst was also reported to promote intramolecular, amide-directed hydroarylation of unactivated alkenes to produce dihydrobenzofuran derivatives (Scheme 86).138

Scheme 85

Scheme 86

Allenes have also been used as coupling partners for Cp CoIII-catalyzed, chelation-assisted C–H functionalization. Cheng reported [5+ 1]-type annulation reaction between ortho-alkenylphenols and allenes to afford 2H-chromene derivatives via olefinic C–H activation (Scheme 87).139 Ackermann described directed C–H alkenylation of N-pyrimidylindoles, 1-arylpyrazoles, and

Organocobalt Complexes in C–H Bond Activation

785

Scheme 87

2-arylpyridines with 1,1-disubstituted allenes (Scheme 88).140 Kinetic studies on this reaction revealed first-order, inverse first-order, and zero-order dependence on Cp CoIII catalyst, 1-arylpyrazole, and allene, respectively, which suggested that coordination and dissociation of an extra molecule of 1-arylpyrazole occurred prior to the rate-limiting C–H activation. Cheng reported [3 +3]-type annulation reaction between anilides and allenes to afford dihydroquinoline derivatives141 and [4 + 1]-type annulation reaction between benzamides and allenes to afford isoindolinone derivatives.142

Scheme 88

Analogous to the three-component C–H activation/Michael/aldol cascade (Scheme 56), Ellman reported three-component coupling reaction of benzamide derivatives, 1,3-dienes, and aldehydes to afford multisubstituted homoallyl alcohols with high regio- and stereoselectivities (Scheme 89).143 Parent butadiene and 1,3-dienes bearing one or two substituents at the terminal position could be employed, and both aromatic and aliphatic aldehydes smoothly participated in the reaction. The reaction was proposed to proceed through amide-directed C–H metalation, insertion of the diene into the Co–aryl bond, isomerization of the Co–allyl species through b-hydride elimination and re-insertion, and allylation of the aldehyde through a six-centered transition state. The scope of this three-component coupling was extended to internally substituted dienes. Thus, the reaction of 1-arylpyrazole, 2-substituted or 1,2-disubstituted diene, and aldehyde afforded homoallyl alcohols containing quaternary carbons with high diastereoselectivity (Scheme 90).144 Benzamides and 2-arylpyridines could also be used as arene substrates. Besides aldehydes, activated ketones such as isatin derivatives, oxetanones, and azetidinones could also be used as carbonyl electrophiles.

Scheme 89

Scheme 90

7.12.3.1.3

Reaction with nitrene or carbene precursors

Matsunaga and Kanai described Cp CoIII-catalyzed directed C–H sulfonamidation of the C2 position of N-pyrimidylindole with tosyl azide (Scheme 91).145 This work was the first example where an active cationic catalyst was generated from air-stable Cp Co(CO)I2 and a silver co-catalyst. They also reported C–H phosphoramidation of the same indole substrate using phosphoryl azide.146 Chang reported ortho C–H amidation of 2-arylpyridines and related substrates with tert-butyl acetoxycarbamate as the amidating agent (Scheme 92).147 Chang and Jiao independently reported the use of dioxazolone as the amidating agent for

786

Organocobalt Complexes in C–H Bond Activation

Scheme 91

Scheme 92

Cp CoIII-catalyzed, chelation-assisted arene C–H amidation (Scheme 93).148 Besides anilides, various aromatic substrates including benzamides, 2-arylpyridines, and 6-arylpurines could be amidated with this amidating agent. Compared with analogous Cp RhIII and Cp IrIII catalysts, the Cp CoIII catalyst was found to exhibit superior catalytic activity in the amidation of anilides. A generally proposed mechanism for this amidation involves C–H metalation, coordination of dioxazolone to the cobaltacycle, amido insertion with concomitant decarboxylation, and protodemetalation of the amido-cobalt species.

Scheme 93

A wide variety of aromatic and olefinic substrates have proved amenable to the Cp CoIII-catalyzed directed C–H amidation with dioxazolone. Thus, 2-aryloxazolines,149 N-pyrimidylindoles,149–150 enaminones,151 2-pyridylferrocenes,152 acrylamides,153 2-aryltriazoles,154 4-aryltriazoles,155 ferrocenyl thioketones,156 azobenzenes,157 ferrocenyl amides,158 ketosulfoxonium ylides,159 and N-pyrimidylindolines160 were efficiently amidated using more or less similar catalytic systems (Scheme 94). Weakly coordinating

Scheme 94

Organocobalt Complexes in C–H Bond Activation

787

(hetero)aromatic ketones could also be amidated at the ortho position, especially when the Lewis basicity of the carbonyl oxygen was increased with an electron-donating substituent (e.g. para-methoxy group).161 Li and Sundararaju independently reported the use of an aniline derivative as a cocatalyst or additive along with Cp CoIII catalyst for the ortho C–H amidation of less coordinating aromatic aldehydes through transient formation of an aldimine directing group (Scheme 95).162 Sundarararaju and Dixon/Seayad described Cp CoIII-catalyzed directed amidation of C(sp3)–H bond using 8-methylquinoline163 and tertiary alkyl thioamides (Scheme 96),164 respectively. The latter substrates could also be aminated using anthranil as the aminating agent.165

Scheme 95

Scheme 96

Li, Glorius, and Ackermann independently developed Cp CoIII-catalyzed C–H amidation/dehydrative cyclization using benzimidates and dioxazolones, which affords 4-alkoxyquinazolines (Scheme 97).166 Li demonstrated that N-sulfinylimines could also be used as substrates for analogous quinazoline synthesis. The superior catalytic activity of Cp CoIII compared with Cp RhIII and Cp IrIII was observed by Glorius. Ackermann’s study demonstrated the compatibility of this reaction with a variety of nitrogen-containing heterocycles, such as pyrazole, pyridine, and pyrimidine, which were attached as the substituents to the benzimidate substrates. Intermolecular competition experiments revealed relative directing group ability of various nitrogenbased functional groups and heterocycles, where the imidate group outcompeted other directing groups. Analogous C–H amidation/cyclization reactions were developed for the synthesis of thiadiazine 1-oxides,167 1,2,3-benzotriazin-4(3H)-ones,168 pyridimidones,153 3-methoxyquinazolinones,169 and 4H-3,1-benzoxazin-4-ones170 from NH-sulfoximines, benzamides, acrylamides, N-methoxybenzamides, and Weinreb amides, respectively. In addition, Li developed ortho-selective C–H amination/oxidative cyclization reaction between benzimidates and anthranils, which was cooperatively catalyzed by Cp CoIII and Cu(OAc)2 catalysts to give 1H-indazole derivatives (Scheme 98).171

Scheme 97

Scheme 98

788

Organocobalt Complexes in C–H Bond Activation

Glorius reported Cp CoIII-catalyzed reactions between 2-arylpyridine derivatives and diazomalonate and related diazo compounds, affording nitrogen-containing p-conjugated compounds through directed C–H functionalization and intramolecular cyclization (Scheme 99).172 The products displayed bright fluorescence in solution and in the solid state, with a broad range of emission wavelengths depending on the substituents. Glorius extended this C–H functionalization/cyclization approach to N-H, N-alkyl, and N-aryl imines and thus developed a synthetic method for isoquinolin-3-one derivatives (Scheme 100).173 Instead of a commonly utilized silver salt, B(C6F5)3 was used in combination with neutral Cp CoIII precatalyst to achieve high reaction efficiency. DFT studies on this reaction shed light on the precatalyst activation process, where oxophilic B(C6F5)3 would capture an acetate anion rather than halide to assist in the formation of cationic Cp CoIII species.174 Analogous to the above reactions, Li and Ackermann developed Cp CoIII-catalyzed C–H functionalization/cyclization of arylamidines and diazo ketoesters to afford aminoisoquinoline derivatives (Scheme 101).175 Besides these heterocycle syntheses, Cp CoIII-catalyzed directed C–H activation/ insertion reactions of diazomalonates were reported for 1-arylpyrazoles,176 N-pyrimidylindoles,176 and 8-methylquinolines177 as the substrates. In addition, Zeng reported oxindole synthesis from N-nitrosoaniline and diazo ketoester via C–H functionalization and Wolff rearrangement (Scheme 102).178 In place of diazo compounds, sulfoxonium ylides would also serve as carbene precursors for Cp CoIII-catalyzed directed arene C–H alkylation (Scheme 103).179

Scheme 99

Scheme 100

Scheme 101

Scheme 102

Scheme 103

Organocobalt Complexes in C–H Bond Activation

7.12.3.1.4

789

Reaction with E–X-type electrophiles

Cp CoIII catalysts have been demonstrated to promote chelation-assisted, formal SN-type C–H functionalization using a variety of electrophiles containing leaving groups (E–X). Glorius and Ackermann independently reported ortho C–H cyanation reaction of 2-arylpyridines, N-pyrimidylindoles and related (hetero)arenes with N-cyano-N-phenyl-p-toluenesulfonamide as a cyanating agent under cationic Cp CoIII catalytic systems (Scheme 104).180 In a related study, Chang used N-cyanosuccinimide for Cp CoIII-catalyzed C–H cyanation of 2-arylpyridine, 6-arylpurine and related aromatic compounds.181 The C–CN bond formation in these reactions was proposed to occur through insertion of the C  N bond into a cyclometalated arylcobalt(III) intermediate and subsequent b-elimination. Glorius also disclosed directed C–H halogenation of aromatic and olefinic substrates bearing pyridyl or amide directing group using electrophilic halogen sources such as N-iodosuccinimide and N-bromophthalimide (Scheme 105).180a The scope of the C–H iodination and bromination was further extended to 6-arylpurines.182

Scheme 104

Scheme 105

Glorius reported Cp CoIII-catalyzed directed C–H allylation of N-pyrimidylindoles using allyl carbonate as the allylating agent (Scheme 106).180a The reaction would involve insertion of the C]C bond into cyclometalated arylcobalt(III) species and subsequent b-oxygen elimination. Glorius extended the scope of the C–H allylation to substituted allyl carbonates as well as to benzamide and acrylamide derivatives.183 Meanwhile, Ackermann developed pyrimidyl-directed allylation of indole, pyrrole, and arene derivatives using allyl acetate as the allylating agent.184 This C–H allylation proved feasible for enamides,71b 1-arylpyrazoles,185 and aryl ketones186 as the substrates. Furthermore, Ackermann succeeded in performing the C–H allylation on N-pyridylindole derivatives containing complex peptide chains at the C3 position, demonstrating late-stage functionalization and macrocyclic peptide synthesis (Scheme 107).187 Matsunaga and Kanai developed Cp CoIII-catalyzed dehydrative C–H allylation using allylic alcohols (Scheme 108).188 Interestingly, the reaction using Cp RhIII catalyst instead of Cp CoIII gave much lower yields. The authors attributed the superiority of the Cp CoIII catalyst to its harder nature, which would facilitate b-hydroxy elimination rather than competitive b-hydride elimination.189 The scope of the dehydrative allylation was further extended to 6-arylpurines and benzamides.190 Sundararaju also demonstrated the uniqueness of Cp CoIII compared with Cp RhIII in dehydrative allylation of quinoline N-oxide (Scheme 109).191

Scheme 106

Scheme 107

790

Organocobalt Complexes in C–H Bond Activation

Scheme 108

Scheme 109

Electrophiles other than allyl alcohol derivatives have been employed in Cp CoIII-catalyzed C–H functionalization reactions that involve migratory insertion/b-oxygen elimination processes. Thus, C–H allylation of N-pyrimidylindoles and related substrates with vinyloxiranes,192 dehydrative C–H naphthylation of N-pyrimidylindoles and 2-arylpyridines with 7-oxabenzonornornadiene,192–193 and dehydrative C–H allenylation of 1-arylpyrazoles with tertiary propargylic alcohols (Scheme 110)194 were reported. In the latter reaction, Cp CoIII proved to be more efficient catalyst than Cp RhIII, and also promoted bis-allenylation of sterically unhindered 1-arylpyrazoles using excess propargylic alcohols. Vinylethylene carbonate was used as an electrophile for domino decarboxylative/ dehydrative C–H/N–H allylation of benzimidates, affording dihydroisoquinoline derivatives (Scheme 111).195 The reaction using allenylmethyl carbonate allowed for the installation of a dienyl group onto the C8 position of quinoline-N-oxides (Scheme 112).196

Scheme 110

Scheme 111

Scheme 112

Organocobalt Complexes in C–H Bond Activation

791

While mechanistically distinct from these reactions that involve migratory insertion followed by direct b-oxygen elimination, vinyl acetate was used as an electrophile for the Cp CoIII-catalyzed ortho C–H vinylation of aryl ketones.197 The reaction likely proceeds via 1,2-migratory insertion of vinyl acetate into a cobaltacycle intermediate, b-hydride elimination, reinsertion and b-acetoxy elimination. An alkene insertion/b-nitrogen elimination process was also demonstrated in Cp CoIII-catalyzed reactions between 2-arylpyridine and vinylaziridines198 and between 8-methylquinoline to 7-azanorbornadienes (Scheme 113).199

Scheme 113

Fluorinated alkenes have been shown to participate as electrophiles in Cp CoIII-catalyzed directed C–H functionalization via C–F bond cleavage. Li reported C2-alkenylation of N-pyrimidylindole with gem-difluorostyrenes, which afforded Z-configured trisubstituted alkenyl fluorides with exclusive stereoselectivity in most cases (Scheme 114).200 Other aromatic substrates such as 2-arylpyridines, 1-arylpyrazoles, and benzimidates also participated in the reaction, where the latter substrates underwent concomitant conversion of the imidate functionality into a cyano group. Ackermann also disclosed analogous C–H alkenylation of N-pyridylindoles using gem-difluorostyrenes. In the same study, C–H allylation with perfluoroalkyl olefin was also achieved under Cp CoIII catalysis, which afforded Z-alkenyl fluorides as the major stereoisomer albeit with imperfect stereoselectivity (Scheme 115).201 These reactions likely proceed via C–H metalation, alkene insertion, and stereoselective b-fluorine elimination, which was supported by DFT calculations in Ackermann’s study. Yoshino and Matsunaga also reported analogous Cp CoIII-catalyzed reactions 6-arylpurines with gem-difluorostyrenes and perfluoroalkyl olefins.202 Li reported allylation of 8-methylquinoline with perfluoroalkyl olefins via C(sp3)–H activation.203

Scheme 114

Scheme 115

Ackermann reported Cp CoIII-catalyzed directed C–H allylation reaction of N-pyridylindoles with vinylcyclopropane bearing a diester moiety via C–C bond cleavage of the cyclopropane ring (Scheme 116).204 2-Arylpyridines and 1-arylpyrazoles could also be used as the substrates. The reaction afforded thermodynamically less stable, Z-configured 1,2-disubstituted olefins with good stereoselectivity. In contrast, the same reaction using Cp RhIII catalyst afforded the E-product as the major product, albeit with only moderate stereoselectivity. According to the experimental and computational mechanistic studies, the diastereoselectivity of the reaction would be determined in the b-carbon elimination step, in which the cobalt and rhodium catalysts would exhibit opposite diastereoselectivity. Yoshino and Matsunaga used the same activated vinylcyclopropane for imidate-directed C–H allylation.205

792

Organocobalt Complexes in C–H Bond Activation

Scheme 116

Several other electrophiles have been used for Cp CoIII-catalyzed directed C–H functionalization reactions. Shi reported C2-alkynylation of N-pyrimidylindoles with triisopropylsilyl (TIPS)-protected ethynylbenziodoxolone (Scheme 117).206 Meanwhile, Ackermann used TIPS-substituted bromoalkyne to achieve analogous C–H alkynylation reaction.207 Yoshino and Matsunaga developed C–H trifluoromethylthiolation reaction of 2-arylpyridines and 6-arylpurines using N-trifluoromethylthiodibenzenesulfonimide (CF3SN(SO2Ph)2) (Scheme 118).208 The trifluoromethylthiolation of 2-arylpyridines was also achieved using AgSCF3 as the trifluoromethylthiolating agent under oxidative conditions.209

Scheme 117

Scheme 118

7.12.3.1.5

Miscellaneous transformations

Glorius reported Cp CoIII-catalyzed dehydrogenative C–S coupling between N-pyrimidylindoles and benzenethiols, which employed Cu(OAc)2 and benzoquinone as stoichiometric additives and In(OTf )3 as a catalytic additive (Scheme 119).210 Cu(OAc)2 was suggested to convert the thiol into copper(I) thiolate species for thiolate transfer to cobalt, while it would also serve as oxidant, along with benzoquinone, for the catalyst turnover. Cp RhIII catalyst proved ineffective for this reaction, which was ascribed to catalyst deactivation by the thiol. Anbarasan reported Cp CoIII-catalyzed cyclization of ortho-vinylanilines and -phenols under aerobic conditions to give indoles and benzofurans (Scheme 120).211 Wang reported Cp CoIII-catalyzed dehydrogenative and carbonylative conversion of 2-alkenylphenols into coumarins under CO atmosphere and oxidative conditions (Scheme 121).212 Dehydrogenative bond formation was also achieved using N-pyrimidylindoles and free indoles under Cp CoIII catalysis, which afforded unsymmetrical 2,2’-biindoles (Scheme 122).213

Scheme 119

Organocobalt Complexes in C–H Bond Activation

793

Scheme 120

Scheme 121

Scheme 122

Johansson and Ackermann reported Cp CoIII-catalyzed or -mediated directed arene C–H methylation using trimethylboroxine under oxidative conditions (Scheme 123).214 The reaction was applicable to a wide variety of directing groups and also displayed compatibility to various functional groups present in the substrate or externally added. These attributes enabled late-stage methylation of biologically active natural products and pharmaceutically relevant molecules that contain coordinating functional groups.

Scheme 123

794

Organocobalt Complexes in C–H Bond Activation

7.12.3.1.6

Enantioselective C–H functionalization

Enantioselective C–H functionalization reactions using Cp CoIII and analogous catalysts have been developed by two distinct strategies. One is the use of a chiral carboxylic acid in combination with an achiral Cp CoIII catalyst, and the other is the use of a chiral cyclopentadienyl–CoIII catalyst.215 In the former strategy, the chiral carboxylic acid serves as a carboxylate base to discriminate enantiotopic C–H bonds or selectively protonates one of enantiomeric organocobalt species. On the other hand, the latter strategy, in theory, allows enantioinduction in any bond-forming or -cleaving steps that occur at the inner sphere of the cobalt center. Yoshino and Matsunaga rendered the C(sp3)–H amidation of tertiary thioamides with dioxazolones (see Scheme 96) enantioselective using the combination of an achiral CoIII catalyst and a chiral carboxylic acid derived from tert-leucine, with enantioselectivity greater than 90:10 for most cases (Scheme 124).216 The cyclopentadienyl ligand on cobalt was modified from the parent Cp by replacing one of the methyl group with a tert-butyl group, which slightly improved the enantioselectivity. Mechanistic experiments confirmed that the enantioselectivity was determined in irreversible C(sp3)–H activation of the enantiotopic methyl groups. Yoshino and Matsunaga developed a planar chiral, 2-arylferrocenyl carboxylic acid for the same type of enantioselective C(sp3)–H amidation of thioamides bearing aryl groups at the a-position (Scheme 125).217 Compared to their earlier catalytic system, the reaction proceeded at a lower temperature, with enantioselectivity slightly lower than 90:10. Shi reported another example of Cp CoIII/chiral carboxylic acid-catalyzed enantioselective C–H amidation using a ferrocene bearing a thioamide directing group as the substrate (Scheme 126).218 A catalytic system comprised of cationic Cp CoIII complex and a monoprotected amino acid promoted the amidation reaction under mild conditions, thus affording planar-chiral amidated ferrocenes with moderate enantioselectivity (up to 77.5:22.5 er).

Scheme 124

Scheme 125

Scheme 126

Organocobalt Complexes in C–H Bond Activation

795

Ackermann succeeded in making the C2-alkylation of N-pyridylindole with allylbenzene derivatives (see Scheme 85) enantioselective using an achiral Cp CoIII catalyst and a chiral C2-symmetric carboxylic acid derived from a chiral diamine (Scheme 127).219 Experimental and computational mechanistic studies suggested that the enantioselectivity is determined in the protodemetalation of the alkylcobalt intermediate formed upon alkene insertion, rather than the alkene insertion step itself. Yoshino and Matsunaga reported enantioselective 1,4-addition of N-pyrimidylindoles to maleimides using a Cp CoIII/axially chiral carboxylic acid system, albeit with moderate enantioselectivity (up to 81:19 er),220 which also featured the establishment of the stereogenic center in the protodemetalation step (Scheme 128).

Scheme 127

Scheme 128

Distinct from the above examples, Cramer developed chiral cyclopentadienyl cobalt complexes for the enantioselective [4 +2]-type annulation between N-chlorobenzamides and alkenes to form dihydroisoquinolones (Scheme 129).221 A chiral binaphthyl-based trisubstituted cyclopentadienyl cobalt complex displayed excellent enantioselectivity (99.5:0.5 er) for the annulation of styrene, outperforming analogous chiral rhodium catalyst. Besides styrenes, the reaction was applicable to a wide variety of alkenes including acrylates, alkyl alkenes, cyclic dienes, and bicyclic alkenes, where enantioselectivity higher than 95:5 er was achieved in many cases.

Scheme 129

7.12.3.2 7.12.3.2.1

C–H activation assisted by bidentate directing group Reaction with alkynes, alkenes, and allenes

Daugulis reported cobalt-catalyzed [4 +2]-type dehydrogenative annulation reaction between N-(8-quinolinyl)benzamide and alkyne as the first example of using a bidentate directing group in cobalt-catalyzed C–H functionalization (Scheme 130).222 A catalytic system comprised of Co(OAc)2
4H2O, NaOPiv, and Mn(OAc)2 afforded isoquinolone derivatives in trifluoroethanol under air. The reaction tolerated various functional groups and both internal and terminal alkynes, where the C–C bond formation

796

Organocobalt Complexes in C–H Bond Activation

Scheme 130

preferentially took place at the sterically less hindered acetylenic carbon. The proposed mechanism of the annulation involves bidentate amide-assisted C–H metalation with a CoIII species generated by oxidation of Co(OAc)2 with Mn(OAc)2 and/or O2, alkyne insertion into the aryl–Co bond, C–N bond-forming reductive elimination, and catalyst reoxidation. The same transformation was later achieved by using PivOH as a stoichiometric additive instead of Mn(OAc)2 and NaOPiv under air223 or with the aid of visible light photoredox catalyst (Na2[EosinY]) under O2.224 A picolinamide of benzylamine also acted as a bidentate substrate to undergo annulation with 2-butyne, affording a dihydroisoquinoline derivative albeit with much lower efficiency (Scheme 131). 1,3-Diynes,225 alkynylsilanes,226 and 4-hydroxy-2-alkynoates227 could also be used as reaction partners for the [4 +2] annulation of 8-quinolinyl benzamide.

Scheme 131

Besides the 8-quinolinyl group, Ackermann and Zhai described the use of 1-oxypyridin-2-yl228 and 2-pyridyl(methyl)amino229 groups, respectively, as the substituents for benzamides to promote cobalt-catalyzed, bidentate chelation-assisted [4 +2] annulation with alkynes (Schemes 132 and 133). Unlike the Q group, these directing groups could be readily removed by simple reagents (KOtBu for the former and SmI2 for the latter), thus allowing for facile preparation of unprotected isoquinolones. In addition, pyridin-2-ylmethoxyamide was introduced as a traceless directing group for the [4 + 2] annulation with alkynes, which directly afforded unprotected isoquinolones (Scheme 134).230

Scheme 132

Scheme 133

Scheme 134

Organocobalt Complexes in C–H Bond Activation

797

The groups of Ackermann and Lei developed electrochemical conditions for this class of [4 +2] isoquinolone synthesis to eliminate the need for chemical or molecular oxidant.231 Ackermann reported the annulation assisted by the 1-oxypyridin-2-yl (PyO) directing group, which featured the reaction setup using undivided cell and H2O/MeOH solvent system (Scheme 135). Meanwhile, Lei reported the annulation of benzamide and acrylamide derivatives bearing the 8-quinolinyl (Q) group with parent acetylene (1 atm), affording the corresponding isoquinolone and pyridone products (Scheme 136). Ackermann extended the electrochemical conditions to Zhai’s hydrazide directing group (see Scheme 133) and also demonstrated electrochemical removal of this directing group.

Scheme 135

Scheme 136

Besides the benzamide derivatives, [4+ 2] annulation reactions with alkynes have been achieved for other types of bidentate aromatic compounds, such as those bearing N-(8-quinolinyl)sulfonamide,232 N-(8-quinolinyl)phosphinamide,233 N-(2-pyridyl) hydrazone (Scheme 137),234 and picolinamide (Scheme 138).235 The latter two could be used as traceless directing groups to allow for the synthesis of isoquinolines. A related [5 +2] annulation between 2-(picolinamido)biphenyl and alkyne was reported to give dibenzo[b,d]azepine derivatives.236

Scheme 137

Scheme 138

Bidentate benzamides and terminal alkynes would undergo alternative reaction, that is, [4 +1] annulation, to produce 3-alkylideneisoindolinone derivatives. Niu and Song employed the 1-oxypyridin-2-yl directing group to achieve [4 +1] annulation using a reaction system comprised of catalytic cobalt(II) oxalate and superstoichiometric AgOAc (Scheme 139).237 The silver additive was proposed to act as a terminal oxidant as well as to convert the terminal alkyne into silver acetylide, which would undergo transmetalation with a cyclometalated cobalt intermediate. A variety of arylacetylenes were tolerated, whereas alkylacetylenes did not participate in the reaction. Zhang reported that the 8-quinolinyl group could also be used as the directing group for the analogous [4 + 1] annulation.238 In the same study, Zhang also achieved [4 + 1] annulation of tertiary alkyl carboxamides with

798

Organocobalt Complexes in C–H Bond Activation

Scheme 139

terminal alkynes via C(sp3)–H activation (Scheme 140). Silylated alkynes was found to undergo [4 +1] annulation with 8-quinolinyl-substituted benzamides via desilylation.239 Later, Niu and Song reported that PyO-appended benzamides underwent either [4 + 2] or [4+ 1] annulation with arylpropiolic acid via decarboxylation to give isoquinolones or 3-alkylideneisoindolinones, respectively, depending on the reaction conditions (Scheme 141).240 One of the major difference between the reaction systems for these [4 +2] and [4 +1] annulation reactions was the amount of Ag2O additive, which was used in a catalytic quantity for the former and in a super stoichiometric quantity for the latter. The dual role of silver additive was again proposed for the [4 +1] annulation. Gao and You reported that bidentate acrylamides bearing 8-quinollinyl directing group and triisopropylsilylacetylene underwent olefinic C–H alkynylation under oxidative cobalt catalysis rather (Scheme 142).241 A competitive [4+ 1] annulation was observed for acrylamides without b-substituent, while those substituted at the b-position underwent exclusive alkynylation.

Scheme 140

Scheme 141

Scheme 142

Commonly proposed catalytic cycles for the cobalt-catalyzed [4+ 2] and [4+ 1] annulation reactions between bidentate benzamides and alkynes are shown in Scheme 143. Both the reactions were assumed to involve an active CoIII-species and bidentate chelation-assisted C–H metalation to afford a cyclometalated arylcobalt(III) intermediate. Alkyne insertion into this intermediate and subsequent C–N reductive elimination would afford the [4 +2] annulation product, followed by reoxidation of CoI to CoIII. On the other hand, the [4+ 1] annulation would proceed through the reaction between the cyclometalated arylcobalt(III) intermediate and terminal alkyne (or silver acetylide), which would generate a cobalt(III) acetylide species. Aryl–alkynyl reductive elimination of this species would give ortho-alkynylated benzamide as the primary product, which would then undergo intramolecular addition of the amide nitrogen to the alkynyl group to furnish the [4 +1] annulation product.

Organocobalt Complexes in C–H Bond Activation

799

Scheme 143

Ribas explored the cobalt-mediated reaction of a tridentate microcyclic arene substrate with terminal alkynes as a model system to gain insight into the mechanisms of the catalytic [4 +2] and [4+ 1] annulations (Scheme 144).242 They managed to obtain a well-defined cyclometalated arylcobalt(III) complex and observed its divergent reactivity depending on the nature of the alkyne. Thus, parent phenylacetylene afforded the [4 +2] product with high selectivity, while arylacetylene bearing a strongly electronwithdrawing nitro group mainly gave rise to the [4 +1] product. Contrary to the above-mentioned mechanisms (Scheme 143), both the [4 + 2] and [4 +1] annulation reactions using a terminal alkyne were suggested to proceed via aryl(alkynyl)cobalt(III) species, its reductive elimination, and cobalt-assisted intramolecular cyclization. Ribas also explored C–H functionalization of the same macrocyclic substrate with ethyl diazoacetate (Scheme 145).243 By reacting the cyclometalated cobalt(III) carboxylate and the diazoester, they were able to fully characterize the reaction intermediate formed upon carbene insertion into Co(III)–carboxylate, which was regarded as aryl-Co(III)-alkyl enolate. The aryl–alkyl bond formation from this intermediate was promoted by H2O or Lewis acid with concomitant elimination of carboxylate.

Scheme 144

Scheme 145

800

Organocobalt Complexes in C–H Bond Activation

The cobalt-catalyzed reaction between N-(8-quinolinyl)benzamide and alkene gave rise to different products depending on the nature of the alkene and the reaction conditions. Daugulis reported oxidative [4 +2] annulation reaction using air as oxidant and Mn(OAc)3•2H2O as a cocatalyst, affording dihydroisoquinolone derivatives (Scheme 146).244 The reaction tolerated ethylene and various terminal alkenes including styrene, alkyl olefin, allyl alcohol, and vinyl ether. Internal alkenes such as cyclopentene, cyclooctene, and cinnamyl alcohol also participated in the reaction. Lei achieved the same annulation using ethylene under electrochemical conditions,231b while Rueping and Sundararaju demonstrated the use of a photoredox catalyst (Na2Eosin Y) and O2 to effect the annulation using various 1-alkenes.245 Nickolls described the [4 +2] annulation using benzo[b]thiophene 1,1-dioxide as a specific reaction partner.246 Zhai reported analogous [4 + 2] annulation between hydrazide-based bidentate thiophene and maleimide.247

Scheme 146

Ackermann reported oxidative coupling reaction between N-(8-quinolinyl)benzamide and acrylate to afford isoindolinone derivative (Scheme 147).248 The reaction was proposed to give ortho-alkenylated benzamide as the initial product, which would undergo Michael-type cyclization. Besides acrylate esters, methyl vinyl ketone and acrylonitrile participated in the reaction to afford the corresponding isoindolinone derivatives. Analogous transformations were also achieved using maleimides as starting materials to give spirocyclic products (Scheme 148).249

Scheme 147

Scheme 148

Maiti, Jeganmohan, and Chatani independently described oxidative ortho C–H allylation of N-(8-quinolinyl)benzamides with alkyl-substituted olefins (Scheme 149).250 Benzamide substrates bearing ortho substituents predominantly underwent allylation rather than alkenylation, whereas alkenylation was a preferred pathway for the substrates without ortho substituent. The selectivity toward allylation was ascribed to lower steric strain required in the corresponding b-hydride elimination step. Maiti obtained a

Scheme 149

Organocobalt Complexes in C–H Bond Activation

801

well-defined cyclometalated cobalt(III) complex derived from the bidentate ortho-toluamide substrate, which was coordinated by another molecule of the amide as N,N-bidentate ligand, demonstrated its competence as a catalyst. Maiti expanded the scope of this reaction to internal alkenes by employing a 1,5-chelating, oxazoline-based directing group (Scheme 150).251 Maiti also extended the oxidative C–H allylation method to bidentate 2-biphenylamine derivatives for the selective functionalization of the 2’-position.252

Scheme 150

Cheng reported a cobalt-catalyzed deaminative [3+ 2]-type annulation between N-(8-quinolinyl)benzamide and 7-oxabenzonorbornadiene for the synthesis of polycyclic indanone derivatives (Scheme 151).253 Other strained bicyclic alkenes including norbornene, norbornadiene, and 7-azabenzonorbornadiene derivatives also participated in the reaction. The reaction was proposed to proceed through C–H metalation, insertion of the alkene, and intramolecular attack of the resulting alkyl cobalt species to the amide carbonyl group, followed by release of 8-aminoquinoline. The thus-generated 8-aminoquinoline was found to cause strong catalyst inhibition, and for this reason the reaction required a relatively high catalyst loading. Zhai reported an analogous [3 +2]-type annulation of benzamide bearing a bidentate hydrazide directing group with oxabicyclic alkene, where the initial annulation product underwent in situ dehydrative aromatization to give a benzo[b]fluorenone derivative (Scheme 152).254 Oxabicyclic alkenes were also used for the oxidative [4 +2]-type annulation reaction assisted by the bidentate phosphinamide group.255

Scheme 151

Scheme 152

Li and Kwong reported cobalt-catalyzed ortho C–H functionalization of N-(8-quinolinyl)benzamide with benzylidenecyclopropane, which involved cleavage of the cyclopropane C–C bond and the aromatic C–H bond of the benzylidenecyclopropane (Scheme 153).256 As the result, a dihydronaphthalene moiety was installed at the ortho position. The reaction was proposed to proceed through bidentate chelation-assisted C–H metalation, insertion of the alkylidene moiety, b-carbon elimination, intramolecular C–H activation of the proximal aryl ring, and C–C bond-forming reductive elimination. On the other hand, Volla reported a different catalytic system that promoted [4+ 2] annulation between N-(8-quinolinyl)benzamide and benzylidenecyclopropane without cyclopropane cleavage, affording spirocyclic products (Scheme 154).257 The reaction was performed under much lower temperature than in Kwong’s reaction.

802

Organocobalt Complexes in C–H Bond Activation

Scheme 153

Scheme 154

Volla, Rao, and Cheng independently reported [4 + 2] annulation reactions between N-(8-quinolinyl)benzamide and allenes, the regioselectivity of which depended on the substituent of the allene substrate (Scheme 155).258 Thus, aryl-substituted allenes and 1,1-disubstituted allenes reacted at the internal C]C bond to afford 4-methylene-dihydroisoquinolone derivatives. On the other hand, allenyl phosphonate and alkyl-substituted allenes reacted at the less substituted C]C bond to furnish isoquinolone products. Similar [4+ 2] annulation reactions were also reported for N-(8-quinolinyl)sulfonamide259 and N-(8-quinolinyl)phosphinamide260 as directing groups. Bidentate hydrazide was also used for oxygenative [4+ 2] annulation261 and electrooxidative [4 +2] annulation of allenes (Scheme 156).262 The latter reaction tolerated monosubstituted allenes bearing phosphine oxide, phosphonate, ester, and aryl groups, and proved also applicable to a cyclic allene.

Scheme 155

Scheme 156

Organocobalt Complexes in C–H Bond Activation

803

Besides the intermolecular C–H functionalization reactions using unsaturated hydrocarbons, Shi reported a bidentate amide-assisted intramolecular C(sp3)–H functionalization reaction of aliphatic carboxamides with alkene (Scheme 157).263 Tertiary alkyl carboxamides bearing a pendant alkene moiety underwent intramolecular cyclization in the presence of cobalt precatalyst and silver oxidant, thus affording bicyclo[4.1.0]heptane derivatives, tolerating various alkyl and aryl groups as the substituent R. Substrates bearing an alkene pendant shorter by one carbon also underwent the same reaction to give bicyclo[3.1.0] hexanes, whereas those bearing a longer pendant failed to give the expected product. The reaction was proposed to proceed through bidentate chelation-assisted C(sp3)–H activation of the methyl group, migratory insertion of the alkene moiety to form the cyclopentyl or cyclohexyl ring, second C(sp3)–H activation of the methylene group, and reductive elimination to form the cyclopropyl ring.

Scheme 157

7.12.3.2.2

Dehydrogenative C–H functionalization

Niu and Song reported cobalt-catalyzed, bidentate chelation-assisted C–H alkoxylation of N-(1-oxy-2-pyridyl)benzamides in alcoholic solvents using Ag2O as the oxidant (Scheme 158).264 The reaction tolerated a variety of substituted benzamides as well as a series of primary alcohols. Acrylamides bearing the same directing group also participated in the alkoxylation of the olefinic C–H bond. The proposed mechanism of this alkoxylation involves SET between an alkoxy cobalt(III) species and the substrate, transfer of the alkoxy group, and rearomatization by C–H cleavage. Cp Co(CO)I2 also served as a precatalyst for the reaction.265 Ackermann devised electrochemical conditions for the same alkoxylation (Scheme 159).266 Mechanistic studies on this electrochemical system indicated that the reaction involves oxidation-induced reductive elimination of an aryl(alkoxy)cobalt(IV) intermediate.267 Chelation-assisted C–H alkoxylation of picolinoyl-protected 1-naphthylamines was also reported using a cobalt catalyst and a silver oxidant.268 Zhang and Chatani independently reported ortho C–H acyloxylation of N-(8-quinolinyl)benzamide with carboxylic acid using silver salts as oxidant.269 Ackermann was again successful in developing electrochemical setup for the same transformation, thus eliminating the need for chemical oxidant.270

Scheme 158

Scheme 159

Similar to the above C–H alkoxylation, Niu and Song achieved chelation-assisted C–H amination of N-(1-oxy-2-pyridyl) benzamides with secondary amines using AgNO3 as the oxidant (Scheme 160).271 The reaction tolerated morpholine and related six-membered cyclic secondary amines. However, most of other cyclic and acyclic amines reacted sluggishly or failed to give the desired amination product. Zhang reported the use of N-(8-quinolinyl)benzamides for analogous C–H amination.272 The groups of Ackermann273 and Lei274 developed electrooxidative conditions for these amination reactions using 1-oxy-2-pyridyl and 8-quinolinyl directing groups, respectively, to eliminate the need for chemical oxidant (Scheme 161). Ackermann employed g-valerolactone (GVL) as renewable solvent. Niu and Song reported ortho C–H amination of N-(8-quinolinyl)benzamide with aniline, where aniline underwent twofold C–N bond formation to afford triarylamine product.275

804

Organocobalt Complexes in C–H Bond Activation

Scheme 160

Scheme 161

Ge reported cobalt-catalyzed, bidentate chelation-assisted C(sp3)–H amidation reactions of aliphatic amides bearing 8-quinolinyl group.276 Various tertiary and secondary carboxamides participated in intramolecular C(sp3)–H amidation of the sterically least hindered b-position or benzylic b-position, thus affording the corresponding b-lactam derivatives (Scheme 162). When the substrate contained both methyl and benzyl groups at the a-position, amidation at the methyl group was preferred. A substrate bearing phenyl and methyl groups at the a-position afforded indolinone derivative as the major product as a result of aromatic C–H activation, accompanied by a minor amount of C(sp3)–H amidation product. The proposed mechanism involves bidentate chelation-assisted C(sp3)–H metalation with CoIII, oxidation of the resulting cobaltacycle, and C–N reductive elimination of the putative CoIV species. The authors also developed intermolecular C(sp3)–H amidation of tertiary carboxamides bearing a-methyl group using a perfluoroalkyl carboxamide as the amide source (Scheme 163).

Scheme 162

Scheme 163

Organocobalt Complexes in C–H Bond Activation

805

Daugulis reported cobalt-catalyzed, bidentate chelation-assisted dehydrogenative C–C homocoupling of benzamides (Scheme 164).277 Using a semi-stoichiometric amount of Co(acac)2 in combination with Mn(OAc)2/O2, N-(8-quinolinyl)benzamide afforded the corresponding biaryl product. The cross-coupling between electronically distinct benzamide substrates was also achieved with moderate selectivity. You reported cobalt-catalyzed cross-coupling between N-(8-quinolinyl)arylamide and azole-type heteroarenes for the synthesis of heterobiaryls (Scheme 165).278 Using slight excess of azole (1.5 equiv.), the cross-coupling product was obtained with good chemoselectivity, along with small amount of homocoupling product of the amide substrate. A variety of heteroaryl amides including (benzo)thiophene, furan, indole, and pyrrole as well as benzamide derivatives participated in the reaction with benzoxazole. Meanwhile, various azole-type heterocycles such as (benz)oxazoles, (benzo)thiazoles, imidazoles, purines, and caffeine could be used to functionalize 2-thiophenecarboxamide. The chemoselective heterobiaryl formation was explained by the sequence of chelation-assisted C–H activation followed by deprotonative activation of the relatively acidic C–H bond of azole. The former step was proposed to involve an electron transfer from the aryl group to CoIII.

Scheme 164

Scheme 165

Niu and Song reported cobalt-catalyzed dehydrogenative cross-coupling between N-(8-quinolinyl)benzamide and 2-arylpyridine (Scheme 166).279 The reaction afforded pyridine-containing ortho-teraryls bearing a variety of substituents. The CoIII catalyst was supposed to play two distinct roles. The first is to oxidize N-(8-quinolinyl)benzamide by one electron to generate a cation radical species, and the second is the cyclometallation of 2-arylpyridine via concerted metalation-deprotonation mechanism. The thus-generated two species would merge to give rise to a diarylcobalt(IV) intermediate, followed by C–C reductive elimination. Later, Zhang reported analogous biaryl coupling employing other directing groups such as oxime and using catalytic Co and Mn salts.280 Lu and Li reported cobalt-catalyzed dehydrogenative ortho-alkylation of 2-pyridylisopropyl-(PIP)-protected benzamides with cycloalkanes, used as solvent, in the presence of peroxide oxidant (Scheme 167).281 Benzamide substrates bearing an ortho substituent afforded the monoalkylated product, while those without ortho substituents gave a mixture of mono- and dialkylation products. The reaction was applicable to other aliphatic substrates such as methylarenes, alkyl ethers, and alkyl sulfides, and was assumed to involve a coupling between cyclometalated CoIII species and alkyl radical. A similar reaction was also reported by Liu.282

Scheme 166

806

Organocobalt Complexes in C–H Bond Activation

Scheme 167

7.12.3.2.3

C–H carbonylation and related transformations

Grigorjeva and Daugulis reported cobalt-catalyzed C(sp2)–H carbonylation/cyclization reactions of N-(8-quinolinyl)benzamide under oxidative conditions and CO atmosphere, affording phthalimide derivatives (Scheme 168).283 Acrylamide derivatives also participated in the carbonylation. The 8-quinolinyl group could be removed by ammonia in MeOH at room temperature. Since this report, cobalt-catalyzed, bidentate chelation-assisted C(sp2)–H carbonylation has been extended in terms of the substrate scope and the carbonyl source. Daugulis extended the scope of the reaction using N-(8-quinolinyl)sulfonamides.284 Zhang introduced azodicarboxylate as alternative carbonyl source to eliminate toxic CO gas, albeit at a higher reaction temperature (Scheme 169),285 whereas Lei reported electrochemical conditions for the carbonylation of (8-quinolinyl)benzamide with CO (Scheme 170).286 Lei also achieved electrochemical aminocarbonylation of the bidentate arylamide in the presence of secondary or primary amine and CO. Azodicarboxylate was also used in cobalt-catalyzed C(sp2)–H carbonylation/cyclization of picolinoyl-protected benzylamines,287 2-biphenylamines,288 and phenylglycinols,289 where the picolinoyl group acted as a traceless directing group. Zhai employed a bidentate benzhydrazide directing group for the carbonylation of benzamides, where the directing group could be readily removed by hydrogenolysis under Raney Ni.290 Wu reported the use of benzene-1,3,5-triyl triformate (TFBen) as a CO surrogate for peri-carbonylation of picolinoyl-protected 1-naphthylamines (Scheme 171)291 and ortho-carbonylation of 2-picolyl benzamides.292

Scheme 168

Scheme 169

Scheme 170

Organocobalt Complexes in C–H Bond Activation

807

Scheme 171

Sundararaju, Gaunt, and Lei independently developed cobalt-catalyzed, bidentate chelation-assisted C(sp3)–H carbonylation of aliphatic amides to afford succinimide derivatives (Scheme 172).293 The reaction was particularly efficient with tertiary alkyl carboxamides, while secondary and primary alkyl carboxamides also participated in the C–H carbonylation. Sundararaju’s system employed CO pressure of 2 atm, while Gaunt and Lei performed the reaction with 1 atm CO. Sundararaju reported on the use of CO generated by in situ deoxygenation of CO2 with disilane and CsF under a two-chamber setup for the bidentate amide-assisted C(sp2)–H and C(sp3)–H carbonylation.294

Scheme 172

Analogous C(sp2)–H activation/cyclization of bidentate benzamides with isocyanides was reported using the 8-quinolinyl directing group by Hao, Ji, and Sundararaju295 and the bidentate hydrazide group by Zhai.296 Sundararaju further reported that, in a polar protic solvent (TFE), the 8-quinolinyl group on the amide substrate underwent swapping with the tert-butyl group of isocyanide (Scheme 173).297 A reaction mechanism involving alcohol-assisted intramolecular trasamidation was proposed. Ackermann described cobalt-catalyzed insertion of isocyanides and carbon monoxide using the bidentate benzhydrazide directing group under electrooxidative conditions (Scheme 174).298

Scheme 173

Scheme 174

7.12.3.2.4

Miscellaneous transformations

Besides the above-discussed reactions, miscellaneous cobalt-catalyzed, bidentate amide-assisted C(sp2)–H functionalization reactions have been reported. Balaraman reported ortho C–H alkynylation of N-(8-quinolinyl)benzamide with TIPS-protected bromoacetylene (Scheme 175).299 Benzamide substrates without ortho-substituents reacted with 2 equiv. of the bromoacetylene to

808

Organocobalt Complexes in C–H Bond Activation

Scheme 175

afford bis-alkynylated products in good yield, unless the second alkynylation was retarded by a relatively bulky meta-substituent. Analogous ortho-alkynylation was achieved using benzylamines bearing N-picolinoyl chelating group.300 Li and Lu reported ortho C–H methylation of PIP-protected benzamides with cumoyl peroxide.301 Chatani reported quinolinylamide-assisted C–H iodination with I2 (Scheme 176).302 To prevent undesirable iodination of the relatively electron-rich 5-position of the quinolinyl group, preinstallation of a Cl substituent to this position was effective. Chatani further improved the efficiency and scope of this C–H iodination using a 2-aminophenyloxazoline-based bidentate amide directing group.303 Gui reported ortho C–H methylthiolation of N-(8-quinolinyl)benzamide using DMSO as a thiolating agent.304 Ackermann reported oxidative ortho C–H arylation of N-(8quinolinyl)benzamide with aryl(trimethoxy)silanes (Scheme 177).305 The additives in this reaction, CuF2 and CsF, were proposed to act as an oxidant and to facilitate the transmetalation between Si and Co, respectively. Prior to this report, Tan described analogous arylation using arylboronic acids, albeit using a stoichiometric amount of cobalt salt.306 Deb reported acetoxylation of N-(8-quinolinyl)benzamide using Mn(OAc)3 as oxidant and acetoxy source.307 Besides these amide-assisted reactions, Das reported nitration of N-(2-pyridyl)aniline with AgNO2.308

Scheme 176

Scheme 177

7.12.3.3

Miscellaneous reactions

A few examples of high-valent cobalt-catalyzed C–H functionalization reactions that do not rely on either Cp CoIII catalyst or bidentate directing groups have been reported. Niu and Song reported C2-arylation of N-pyrimidylindoles with arylboronic acids using CoII catalyst and MnII cooxidant under air (Scheme 178).309 The proposed mechanism of the reaction involved cyclometallation of N-pyrimidylindole with CoIII, merger of the resulting arylcobalt(III) species with an aryl radical generated via oxidation of the arylboronic acid, and reductive elimination of the diarylcobalt(IV) species, where the Mn salt, in combination with O2, would facilitate the oxidation of the boronic acid as well as the catalyst reoxidation. A similar catalytic system also promoted C7-arylation of N-pyrimidylindolines.310 Daugulis reported oxidative [4 +2] annulation between simple benzoic acids and alkynes or styrenes

Scheme 178

Organocobalt Complexes in C–H Bond Activation

809

using Co(hfacac)2 as a precatalyst, (Me3Si)2NH as a base, and Ce(SO4)2/O2 as the oxidizing system (Scheme 179).311 Both terminal and internal alkynes participated in the annulation, while their reaction mechanisms might not be the same. Thus, involvement of a cobalt acetylide was suggested for the reaction of terminal alkynes. Cobalt-catalyzed C2-nitration of Boc-protected indoles with t-BuNO2 was also reported, which represented another example of C–H functionalization of monodentate substrates.312

Scheme 179

Cheng reported low valent cobalt–diphosphine-catalyzed hydroarylative cyclization of 1,6-enynes with aromatic ketones that likely involves carbonyl-assisted ortho C–H activation on high-valent cobalt (Scheme 180).313 Aromatic esters also proved to be excellent substrates for this tandem cyclization/ortho C–H alkylation. The reaction was proposed to involve oxidative cyclization of the enyne on CoI, carbonyl-assisted cyclometalation with the resulting high-valent cobaltacycle intermediate, and C–C reductive elimination. Upon extending this hydroarylative cyclization to aromatic aldehydes, Cheng observed ligand-dependent divergent pathways (Scheme 181).314 The originally used dppp ligand effected hydroacylative cyclization to give a,b-unsaturated ketones, whereas the hydroarylative cyclization was promoted using cis-1,2-bis(diphenylphosphino)ethylene (dppen). Lautens succeeded in rendering the hydroarylative cyclization of 1,6-enynes enantioselective using N-pyridylindoles as chelating arene substrates and a chiral diphosphine as a supporting ligand (Scheme 182).315 High enantioselectivities, typically higher than 90:10 er, were achieved across a variety of substituted 2-pyridylindoles and 1,6-enyne substrates. N-pyridylpyrroles reacted with equal efficiency and enantioselectivity, while 2-phenylpyridine displayed somewhat lower enantioselectivity. Similar to Cheng’s system, acetophenone was also a viable substrate, but the enantioselectivity was largely diminished.

Scheme 180

Scheme 181

810

Organocobalt Complexes in C–H Bond Activation

Scheme 182

C–H functionalization of unfunctionalized (hetero)arenes using simple cobalt catalysts has been limited to date. For example, CoCl2•6H2O was reported to promote C3-arylation of imidazo[1,2-a]pyridines with aryl iodides.316 However, several other transition metal catalysts, including commercially available simple salts, are known to promote the same transformation. As such, the cobalt-mediated system does not seem of have particular uniqueness with respect to the mechanism or advantage in terms of the synthetic scope.

7.12.4

Conclusion

This Chapter has summarized the development of cobalt-catalyzed C–H bond activation/functionalization reactions, most of which has been made since 2010. Low-valent cobalt catalysts generated with the aid of strong reductant or those having well-defined Co(0) or Co(–I) oxidation state promote chelation-assisted arene C–H functionalizations such as hydroarylation, C–H/C–X coupling, and C–H/C–M coupling. Cobalt–diphosphine catalysts generated using milder reductant promote inter- or intramolecular hydroacylation, which may be made enantioselective using chiral diphosphine ligand. Well-defined cobalt complexes supported by pincer ligands prove to serve as catalysts for C–H borylation, displaying unique regioselectivity compared with iridium catalysts. Cp CoIII-type catalysts promote an extremely wide variety of chelation-assisted C–H functionalizations including C–H addition to polar C¼ X bond, addition or annulation reactions with alkynes, alkenes, and other unsaturated hydrocarbons, C–H insertion with nitrene or carbene surrogates, and SN-type reaction with various E–X-type electrophiles. Besides their utility as inexpensive alternative to Cp RhIII and Cp IrIII congeners, Cp CoIII catalysts often display unique reactivities and selectivities owing to the higher nucleophilicity, harder nature, and shorter ionic radius of CoIII. The combination of an achiral CpCoIII-type catalyst and a chiral carboxylic acid or a CoIII catalyst supported by a chiral Cp-type ligand have been developed for enantioselective C–H functionalizations. Bidentate directing groups enable simple cobalt salts, often in combination with chemical oxidants, to promote C(sp2)–H and C(sp3)–H functionalizations such as addition/annulation with unsaturated hydrocarbons, dehydrogenative C–C and C–heteroatom cross-coupling, and carbonylation. In many of such reactions, the chemical oxidant may be eliminated by adopting electrochemical setup. Further studies on the fundamental organometallic chemistry of cobalt, along with its productive merger with photo- and electrochemistry, may lead to the development of even wider range of catalytic C–H functionalizations in the future.

Acknowledgments Our work on cobalt-catalyzed C–H functionalization has been supported by the Ministry of Education, Singapore (MOE2016-T2-2-043) and Japan Society for the Promotion of Science (KAKENHI Grant Number 20K23375).

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7.13

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Hugo Valdés, Rebeca Osorio-Yañez, Ernesto Rufino-Felipe, and David Morales-Morales, Instituto de Química, Universidad Nacional Autónoma de México, Coyoacán, Ciudad de México, Mexico © 2022 Elsevier Ltd. All rights reserved.

7.13.1 Introduction 7.13.2 Cobalt pincer complexes 7.13.2.1 Cross-coupling reactions 7.13.2.2 Hydroboration 7.13.2.3 Silylation/hydrosilylation and hydrophosphination 7.13.2.4 Hydrogenation and dehydrogenation 7.13.3 Rhodium pincer complexes 7.13.4 Iridium pincer complexes 7.13.4.1 Alkane dehydrogenation 7.13.4.2 Olefin isomerization 7.13.4.3 Tandem reactions involving alkane dehydrogenation 7.13.4.4 Dehydrogenation of substrates with heteroatoms 7.13.4.5 Dehydrogenative coupling 7.13.4.6 Dehydrogenation of carboxylic acids 7.13.4.7 Dehydrogenation of alcohols 7.13.4.8 Hydrogenation of CO2 7.13.4.9 Hydrogenation of alkenes 7.13.4.10 Hydrogenation of benzoquinones and nitroarenes 7.13.4.11 Hydroboration and carbonylation 7.13.4.12 Miscellaneous reactions 7.13.5 Conclusions Acknowledgments References

7.13.1

816 817 817 819 822 825 834 853 854 856 856 857 857 858 859 859 860 861 861 862 863 863 863

Introduction

The present chapter reviews the advances that the pincer chemistry of group 9 transition metals has had in the last decade or so. The chemistry of pincer compounds in general has had a tremendous advancement since their discovery, being evident that its progress was of interest for different areas of chemistry this being particularly true in the case of catalysis. Along the discovery of new applications of this privileged ligand platform, advances also in the better understanding of the role of the ligands have become clear and the design of new functional pincer ligands has become fundamental for the recent advances of pincers applications. As expected, the chemistry of these most interesting complexes has been reviewed abundantly in the last decade, including their uses and applications, and most recently research in this field has been abundant on the shift on the use of the traditional precious metals for more abundant non-precious transition metals, such is the case of cobalt. Although most of the chemistry developed using this metal started using their derivatives as polymerization catalyst, their design has quickly evolved for other applications in other relevant processes such as cross coupling reactions and CdH activation, most of the times trying to found chemical similarities with its iridium and rhodium counterparts. Initially difficult, but hurdles have been quickly overcome; cobalt pincer chemistry has had an impressive takeoff in the last decade and has become a hot topic of research. However, this has not hampered the advance of the more “traditional” iridium and rhodium pincer chemistry, exhibiting also some amazing advances, leaving clear the old saying that “an old dog can learn new tricks.” In this sense, several great reviews have been made along this decade and previously too, reviews which are often referred along this chapter, that have paved the way to produce this chapter. In fact, in 2020 in Advances in Organometallic Chemistry, Sola and Martin produced a very nice, extensive review on the synthesis and structural features, as well as some of their reactivity towards small molecules of group 9 pincer complexes. In this context, we have focused this chapter on recent advances on the applications of these complexes in catalysis. We believe that both perspectives will nicely complement each other to have a comprehensive information of the current state of the art of this most important type of complexes. We sincerely hope the readers agree with us and that the information described on this chapter serve as inspiration for yet future developments using group 9 transitions metals and the privileged ligand platform called pincer ligands. Hence, this chapter has been, naturally divided in three sections for cobalt, rhodium and iridium, respectively, and each section subdivided on the different catalytic processes where these species have found important applications.

816

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00145-1

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

7.13.2

817

Cobalt pincer complexes

Pincer chemistry and catalysis have advanced hand in hand; thus, it is not rare that pincer complexes have found use in a plethora of catalysis transformations including cross-coupling reactions, hydrogenations, dehydrogenations, polymerizations, among other processes for the production of high added value products such as pharmaceutical principles, advanced materials, and agrochemicals.1–20 The well-known high stability and outstanding catalytic properties of pincer complexes have been mainly attributed to the three coordinated, usually mer fashion of the ligand to the metal, giving place to two fused usually five- or six-member metallacycles. This privileged fashion is, in a major extent, responsible for the high stability of the pincer complexes, necessary to maintain the catalytic active entity along all the catalytic process. Notably, in some cases, and every time more often, the pincer ligands are non-innocent, playing also an active role in the catalytic reaction mechanism.21–25 Another feature that makes this privileged ligand platform so successful, is the facility in which electronics and sterics can be tuned, allowing access to a wide variety of ligands and their corresponding complexes often in high yields from commercially available starting materials.26–31 Thus, pincer ligand may be customized for virtually any transition metal for a given catalytic application. On the other hand, transition metals of group nine have been connected to the development and catalytic applications of pincer complexes from the introduction of this ligand class. First used in Ir and Rh complexes described for hydrogenations, dehydrogenations and other similar reactions from the mid 1990’s,32,33 pincer complexes have experienced a tremendous development since then. In contrast, the use of Co pincer complexes as catalysts is a more recent advance. Such Co complexes underwent an initial slow development about a decade later than its heavier Rh and Ir counterparts; however, nowadays we can find extraordinary catalytic applications using these compounds.34 The different timeframes for development of Co and Rh/Ir complexes is probably a consequence of the particular chemical properties of each group 9 transition metal. In this sense, Rh and Ir exhibit chemical similarities, having common +I and +III oxidation states. This M(I)/M(III) couple enables elementary steps crucial in several catalytic reaction pathways such as oxidative addition and reductive elimination reactions. In fact, these pathways are critical in the synthesis of several pincer complexes. Conversely, the common oxidation states of cobalt are +II and +III, although careful choice of the proper ligand may provide easy access to stabilized +I and +III oxidation states, thus improving the catalytic applications of these Co pincer species. Hence, we have seen an enormous development of the catalytic applications of cobalt pincer complexes in the last two decades,17,34–38 mainly due to their unique chemical and redox properties.39 This includes both one-electron oxidation of Co(II) to Co(III) while the strongest-field pincer ligands promote the capability to cycling between Co(I) and Co(III) oxidation states via two-electron transformations. These properties are key steps in some catalytic mechanisms, and consequently, have inspired the establishment of new catalytic methodologies. Additionally, cobalt is more abundant and cheaper than their noble family members, thus their use as catalyst has become very attractive.

7.13.2.1

Cross-coupling reactions

Bhat and co-workers reported the first example of Suzuki-Miyaura couplings catalyzed by a series of Co(II) pincer complexes under mild conditions (80  C, in air).40 They established structure-activity relationships by varying the side arm of the pincer ligands, finding that the presence of nitrogen atoms enhanced the catalytic activity due to their higher donating properties (Table 1). This assumption matched the proposed reaction mechanism, which starts with the reduction of Co(II) to Co(0), followed by an Table 1

Suzuki-Miyaura couplings catalyzed by Co(II) pincer complexes.

Substrate NC-C6H4-Br NC-C6H4-Br OHC-C6H4-Cl

Yield [%] 81 87 64

83 90 68

89 92 71

818

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

oxidative addition of the aryl halide. This last step being favored by electron rich metal centers produced by strong donating ligands such as PNNNP pincers. The reaction mechanism follows with an ion-exchange, a transmetallation of the organoborane, and finally, a reductive elimination process releases the product and regenerates the catalytic active species (Scheme 1a).

(B)

(A)

HN

N

NH

R 2P

Co

PR2

(II)

N R 2P

Ar Ph

HN

N

NH

Co

PR2

(0)

Cl 4a, R = iPr

-OTf

ArX

HN

N

NH

HN

N

NH

R2P

Co

PR2

R 2P

Co

PR2

Ar

N R 2P

-OR

(II)

(OH)2BCO3) PhB(OH)2

(II)

Ar

Ph

X CsCO3

HN

N

NH

R 2P

Co

PR2

(II)

Ar

PR2

S=1

AcO OAc 3a, R = Ph

R 2P

(I)

Co

CsX

(I)

Co

PR2

(RO)2BOR

OR

N R2 P S=1

(RO)2BAr

(I)

Co

N PR2

R2P

(I)

Co

Ar

OTf

Ar'-Ar

PR2

S=0

Ar'-OTf

CO3

Scheme 1 Reaction mechanism for Suzuki-Miyaura coupling using (A) Co(II)- and (B) Co(I)-pincer catalyst.

Interestingly, Co(I) PNP pincer complexes also catalyze the cross-coupling between aryl triflate and heteroaryl boron nucleophiles.41 However, the reaction mechanism operates in a different manner in comparison with the Co(II) pincer complexes (Scheme 1B). In this case, the reaction starts with the exchange of the halide with an alkoxide ligand, followed by a transmetallation of the neutral organoborane to yield a Co(I)-aryl compound. This step produces a modification in the field strength, going from high (S ¼ 1) to low (S ¼ 0) spin, and consequently, the geometry around the metal also changes, going from tetrahedral to square planar. Thus, the interaction between the electrophile (aryl triflate) and the metal center is favored, hence promoting the CdC bond formation, and releasing the product and regenerating the active catalytic species. Noteworthy is the fact that the metal does not change its oxidation state when the Co(I) pincer complex is used, in contrast to the Co(II) compounds. On the other hand, catalytic reactions involving CdH bond activation are highly desirable because they increase the atom economy of the whole reaction, decreasing the amount of waste and energy.19 In this sense, Co(I) pincer complexes were used to catalyze the borylation of heterocyclic compounds and arenes (Scheme 2).42–46 Complexes 5 and 6 showed moderate to high activity under mild conditions, reaching conversions up to 98%. However, the catalysts may easily decompose by the substitution of the 4-position of the pyridine moiety in the pincer ligand. Thus, the second-generation of these sorts of catalysts included a substituent at the 4 position, such as methyl or pyrrolidine, proving to be very active towards fluorine aromatic compounds.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

819

Scheme 2 Borylation of aromatic compounds catalyzed by Co pincer complexes.

7.13.2.2

Hydroboration

The hydroboration process is another synthetic strategy that allows the incorporation of a boron moiety in a compound. Since the pioneering work of Chirik and co-workers in 2013,47 the use of new Co pincer complexes has attracted much attention.48–55 Scheme 3 shows some relevant examples of Co pincer complexes used in this transformation. Usually, the reaction is carried out using a catalyst loading that goes from 0.05 (16a) to 2.5 (20a) mol%, an organic solvent such as THF, MTBE, Et2O or benzene, and temperatures close to room temperature (22  to 25  C). Interestingly, the reaction may proceed in the absence of solvent using complexes 5c, 5d and 16a as catalysts. In some cases the presence of an additive (NaBHEt3, LiBHEt3, NaOtBu) increases the performance of the catalyst, for example the hydroboration of styrene with pinacolborane was performed in >99% yield in 3 min at 25  C, using catalyst 16a (0.05 mol%) and NaBHEt3 (2 mol%). Under these conditions, the anti-Markovnikov product was only observed.

820

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 3 Hydroboration catalyzed by Co pincer complexes.

In addition, the first enantioselective hydroboration of 1,1-disubstituted alkenes was reported by Huang and co-workers.55 The reaction of a–methylstyrene with HBPin required the previous catalyst activation with NaBHEt3 when complexes 14a-c were used. The anti-markovnikov product was obtained in up to 91% yield with a 99% ee. However, using complex 15a produced better results achieving 95% yield and 99% ee with lower catalyst loadings, shorter reaction times and avoiding the requirement of previous catalyst activation. Interestingly, the hydroboration of cis- or trans-4-octene with HBPin using complexes 5c-d and 10a afforded 1-octylboronic ester as result of an isomerization-hydroboration sequence. The most active complex was 10a, achieving 98% yield in 1.5 h. This strategy was then extended to hindered alkenes, such as trisubstituted alkenes with linear or branched alkyl fragments, obtaining isolated yields up to >75%. Chirik et al. developed a methodology for the hydroboration of terminal alkynes, and studied the structure-activity relationship of the pincer ligand over the obtained product.56 The hydroboration of 1-octyne with HBPin at 23  C in the presence of complex 5c, containing the 2,6-diisopropyl fragments, affords the (E)-isomer in a >98% yield. While under similar conditions and using the complex with cyclohexyl substituents (5b), the (Z)-isomer is observed in a higher proportion (92:8) (Scheme 4).

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

821

Scheme 4 Hydroboration of terminal alkynes by Co pincer complexes.

An elegant strategy to obtain borylated-alkanes from internal alkynes was developed by Lu.57 Complex 21a catalyzed the hydroboration/hydrogenation of internal alkynes with HBPin and hydrogen in a one pot methodology. The reaction was highly regio- and enantioselective. Scheme 5 shows some representative examples that exemplify the high tolerance of functional groups, such as hydroxy, ether, ester and amines.

Scheme 5 Hydroboration/hydrogenation of internal alkynes by Co pincer complexes.

The hydroboration of nitriles to bis(borylated)amines was carried out using the electron-rich Co pincer complex 20a, on benzene at 70  C for 16 h.57 Under these reaction conditions aliphatic, aromatic, heteroaromatic nitriles were borylated in yields ranging from 52 to 85% yield (Scheme 6). This catalytic protocol represents a good alternative for the reduction of nitriles, which typically require stoichiometric amounts of toxic and dangerous reagents, such as LiAlH4 or NaBH4.

822

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 6 Hydroboration of nitriles by Co pincer complex 20a.

7.13.2.3

Silylation/hydrosilylation and hydrophosphination

The silylation/hydrosilylation of alkenes,52,58–60 alkynes52,61–67 and carbonyls68–72 has been successfully achieved by Co pincer catalysts. Chirik and co-workers described a methodology to obtain allylsilanes or alkylsilanes from alkenes (Scheme 7).58,59 The synthesis of allylsilanes was performed with complex 5d which incorporates relative bulky imine groups, while the preparation of alkylsilanes was carried out using a complex with smaller imine fragments 21a. Smaller substituents promote the bimolecular reaction of the silane with a cobalt alkyl rather than the unimolecular b-hydrogen elimination, this step in the reaction mechanism is fundamental for the formation of the product. Fout and co-workers used a bis(NHC) pincer ligand for the chemoselective hydrosilylation of alkenes. The catalyst (20a) was so selective that tolerates functional groups such as hydroxyl, amino, ester, epoxide, ketone, formyl and nitrile, obtaining the anti-Markovnikov product (Scheme 7).

Scheme 7 Silylation and hydrosilylation catalyzed by Co pincer complexes.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

823

The hydrosilylation of alkynes is another effective strategy to synthesize allylsilanes. The addition of the Si and H atoms to the alkyl group may generate three isomers (Scheme 8), namely b-(E)-, b-(Z)- and a-allylsilanes. The selectivity of the reaction is highly influenced by steric factors, for instance using bulky silyl groups and complex 23a the b-(Z)-isomer is favored,63 while using a less steric demanding complex 22a the b-(E)-isomer is formed.52 The a-isomer was observed using catalysts 24a-27a mainly due to steric effects that favored the 1,2-insertion of the alkyne into the CodSi bond.61,64,67,73 The reaction mechanism starts with the reaction of the Co catalyst with the silicon source forming a CodSi species, followed by alkyne coordination to Co and insertion into the CodSi bond. This step is crucial for the selectivity of the product being the steric repulsions between the “arms” of the pincer ligand, the silyl groups and the alkyne substituents determinant on the isomer formation. Then, the silyl-alkenyl-cobalt complex reacts with the silicon source, regenerating the CodSi species and releasing the product.

Scheme 8 Hydrosilylation of alkynes catalyzed by Co pincer complexes.

Chen and Xia reported the hydrosilylation of 1,3-diyne compounds with a Co pincer complex to selectively form 1,3-enynes (Scheme 9).66 The reactions were performed using 2 mol% of catalyst and 6 mol% of NaBEt3H at room temperature for 5 min.

Scheme 9 Hydrosilylation of 1,3-diynes to form 1,3-enynes.

824

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Under these conditions, several functional groups were tolerated, such as heterocycles, amines, halogen, cyano, etc. This example represents one of the most effective catalyst for the hydrosilylation of 1,3-diynes reported this far. The hydrosilylation of carbonyls is a good strategy to obtain secondary alcohols since the OdSi bond may be easily hydrolyzed under basic conditions. In 2012, Gade reported a new series Co NNN-pincer complexes derived from the chiral 1,3-bis(2-pyridylimino)isoindolates 29a-b for the hydrosilylation of prochiral alkyl aryl ketones (Scheme 10).68 The series of catalysts were very active and enantioselective, reaching yields up to 100% and 91% ee. Since then, some other examples of pincer complexes have been described for this purpose, including CNC,69 PONOP, PNP,70 and NNN.70,72

Scheme 10 Hydrosilylation of ketones using Co pincer complexes.

Geer and Kays described the first example of hydrophosphination of activated alkenes and imines catalyzed by Co pincer compounds (Scheme 11).74 The reaction produces selectively the b-anti-Markovnikov product, and NMR studies demonstrated that complex 33a is in fact the catalytic active species, and that the metal center does not changes its oxidation state along the whole reaction mechanism. Thus, the reaction starts with the coordination of the R group to Co, following by the regioselective nucleophilic attack of the phosphine to the olefin. Then, the product is released and the catalyst regenerated after a proton transfer.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

825

Scheme 11 Hydrophosphination of activated alkenes and imines catalyzed by Co pincer complex 33a.

7.13.2.4

Hydrogenation and dehydrogenation

Since the report by Caulton and co-workers describing one of the first Co pincer complexes for molecular hydrogen activation,75 Co pincer compounds have gained a privileged place as valuable catalysts for the hydrogenation/dehydrogenation reaction processes20 allowing the synthesis of high value products and valuable materials, being a milestone for the development of hydrogen storage systems such as liquid organic hydrogen carriers (LOHC). Interestingly, pincer ligands have demonstrated not be only spectator ligands, but playing a fundamental role, actively participating in the reaction mechanisms of hydrogenation/dehydrogenation processes by “activating” the substrates.23 Scheme 12 illustrates the three common operational fashions in which pincer ligands usually get involved in the activation of the substrates. Pyridine-based pincer ligands can be dearomatized by reacting with a base (i.e., KOtBu), making this species capable of activating bonds such as HdH, HdOR, HdNR2, etc. regenerating the aromatized pincer ligand in a metal-ligand cooperation process (Scheme 12 i). The reaction mechanism with pyridine-based pincer complexes also operated via a labile arm-ligand (hemilability), which may be exchanged by the substrate (Scheme 12 ii). Finally, non-aromatic pincer ligands mainly follow an outer-sphere mechanism, where the central NdM bond can be easily interconverted to a N]M bond (Scheme 12 iii). These operational modes are commonly observed for Co, Ir, Ru, among other transition metals.

826

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 12 Common operational modes for the substrate activation by non-inocent pincer ligands. (i) Metal-ligand cooperation via dearomatization/aromatization process. (ii) Hydrogenation via labile ligand. (iii) Metal-ligand cooperation via outer-sphere mechanism.

The hydrogenation of alkenes, alkynes, ketones, aldehydes, imines, nitriles and esters has been achieved by Co pincer complexes in the presence of molecular hydrogen. Table 2 shows the reaction conditions and the catalysts employed in each methodology. In general, Co(I) pincer complexes do not need the presence of an additive, in contrast Co(II)-based complexes required the addition of an additive such as TIBA (triisobutylaluminum), H[BArF4](Et2O)2, NaOtBu or NaBHEt3, which produces the active catalytic species by reducing the oxidation state of the Co, going from (II) to (I). In 2012, Budzelaar and co-workers evaluated the catalytic activity of complex 22b in the hydrogenation of 1-octene using an excess of TIBA, attaining a conversion in the order of 10,000 mol/mol catalyst/bar/H.76 Some years later, Chirik and co-workers used the enantiopure C1-symmetric complex 5e for the stereoselective hydrogenation of geminal-disubstituted alkenes and cyclic alkenes.77,78 The catalyst performed very well, using a hydrogen pressure of 4 atm affording yields up to 98% and >90% ee. Lu and co-workers used complex 14b for the hydrogenation of 1,1-diarylethanes and a-alkylstyrenes. The catalyst was very active, such that works under a mere H2 balloon pressure, maintaining high yields and excellent enantiomeric excess.79 The hydrogenation of a-vinylsilanes to chiral tertiary silanes was achieved using a PNN Co(II) catalysts and NaBHEt3.80 The products were obtained in high yields and enantioselectivities of up to 99% ee. And more challenging substrates such as 1,1-diborylalkenes and indenyl boronates were reduced employing complex 41a.81 The reaction was carried out using a hydrogen pressure of 4 atm at 24  C for 17 h. This methodology provides a facile route for the preparation of 1,1-diboron species, which are excellent synthons in enantioselective Suzuki-Miyaura couplings. Hanson and co-workers described an elegant method for the hydrogenation of C]C, C]O and C]N bonds using the aliphatic pincer complex 34a.82 The hydrogenation of terminal and internal alkenes occurs in 24 h at 25  C. Interestingly, the catalyst selectively hydrogenated the terminal position of the (R)-(+)-limonene, keeping intact the internal alkene. On the other hand, the hydrogenation of ketones and imines required longer reaction times and a slightly increase in temperature, hence Kempe and co-workers described a chemoselective procedure for the hydrogenation of ketones and aldehydes83 using catalyst 37a, that nicely tolerates the presence of alkenes, producing the alcohol as the only product. In 2016, Fout and co-workers reported the hydrogenation of disubstituted alkynes.84 The CCC Co pincer complex 38a produced selectively the E product when one of the substituents was an aryl group. However, when the substituents were changed by alkyl chains the selectivity decreased, leading to an E:Z mixture > 81:19. The first example of a hydrogenation of esters to alcohols with a Co catalyst was described by Milstein and co-workers.93 The reaction required harsh conditions, such as high hydrogen pressure (50 bar) and higher temperatures (180  C), as well as the addition of both NaHBEt3 and KOtBu. The possible mechanism involves the formation of an ester enolate intermediate. Later on,

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Table 2

827

Hydrogenation of alkenes, alkynes, ketones, aldehydes, imines, nitriles and esters catalyzed by Co pincer complexes.76–98

Catalyst (catalyst loading)

Representative substrate

Hydrogen pressure/ Temperature/Time

Additive/solvent

Product (Yield, ee)

1-octene (neat)

5–50 bar

TIBA

n-octane (10,000 mol/mol catalyst/bar/h)

4 atm/22  C/24 h or 4 atm/25  C/16 h

Benzene or toluene or Et2O

(ca. 0.5 mmol)

or

or

(5 mol%) (41–98%, >90% ee) 1 atm/25  C/24–42 h

H[BArF4](Et2O)2 (2 mol%) /THF (80–100%)

1–4 atm/25–60  C/24–65 h H[BArF4](Et2O)2 (2 mol%)/THF

(2 mol%) (86–100%) 4 atm/60  C/42–72 h

H[BArF4](Et2O)2 (2 mol%)/THF

1 atm/25  C/24 h

THF

(65–88%)

(98–100%)

(mol%) 1 atm/5.5 to R.T.  C

Benzene

(TOF ¼ 1000/h)

(2 mol%) (Continued )

828

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Table 2

(Continued)

Catalyst (catalyst loading)

Representative substrate

Hydrogen pressure/ Temperature/Time

Additive/solvent

20 bar/20  C/24 h

NaOtBu (2.0 eq with respect to the catalyst)/2-methyl-2-butanol

Product (Yield, ee)

(64 to >99%)

(0.25–3.0 mol%) 4 atm/R.T./2–22 h

benzene

4 atm/30  C/17 h

THF

H2 balloon/R. T./3–12 h

NaBHEt3 (15 mol%) / toluene

(>99%)

(2–5 mol%) (64–96%, E:Z > 99)

or

or

(5 mol%) (82–99%, 58–99% ee) 2–4 bar/R. T./5 h

NaBHEt3 (10 mol %)/n-pentane

(86–98%, 46–99% ee)

(5 mol%) 10 bar/R.T.–60  C/ 5 min–48 h

Benzene (59 to >99%)

(2 mol%) 4 atm/24  C/17 h

(5 mol%)

Benzene

(75–98%, 93–98% ee)

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Table 2

(Continued)

Catalyst (catalyst loading)

Representative substrate

Hydrogen pressure/ Temperature/Time

Additive/solvent

4 atm/22  C/1–120 h

benzene

Product (Yield, ee)

(15 to >95%)

(5 mol%) 50 bar/130  C/38–70 h

NaHBEt3 (8 mol%), KOtBu (25 mol %)/THF

30 bar/135  C/36–60 h

NaHBEt3 (2 mol%), NaOEt (4.4 mol %)/benzene

(50–85%) (4 mol%) for esters (2 mol%) for nitriles

or

or

(30–99%)

55 bar/120  C/20 h

THF

10 atm/120  C/2 d

THF

50 bar/120  C/6–24 h

NaOMe (20 mol%)/dioxane

4 atm/115  C/8 h

KOtBu (6 mol%), NaHBEt3 (4 mol%)/ toluene

(15–97%)

(0.1–2 mol%) for esters (5 mol%) for N-heterocycles

(19–99%)

(26–99%)

(5 mol%)

or

or

(2 mol%)

(39–99%)

829

830

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Jones and co-workers reported the hydrogenation of esters and g-valerolactone catalyzed by complex 18a under additive-free conditions,95 however the experimental conditions of high temperature and pressure were similar to those described by Milstein. The reaction mechanism proposed under this additive-free conditions (Scheme 13) consists in the formation of the hydride-Co species, which then reacts with the ester to form a metal-bound hemiacetal. Then, a dihydrogen molecule is coordinated to the metal center producing an acidic hydrogen atom that may protonated the hemiacetal, and the subsequent formation of the “first” alcohol and the Co-aldehyde species. Insertion of the aldehyde into the CodH bond produces an alkoxide, and the production of a vacant coordination site, which can be occupied by a dihydrogen molecule, triggering the protonation and formation of the “second” alcohol. A more recent example was described by Beller et al., in which they used complex 44a as catalyst in the presence of NaOMe for the hydrogenation of a series of aromatic, aliphatic and cyclic esters.97

Scheme 13 Reaction mechanism for the hydrogenation of esters using Co pincer complexes under additive-free conditions.

On the other hand, the first example of hydrogenation of nitriles to primary amines using Co pincer catalyst was described by Milstein and co-workers.94 They reported the use of catalyst 43a in the presence of NaBHEt3 and NaOEt under 30 bar (29.6 atm) of hydrogen, in this way a series of aryl, benzylic and aliphatic nitriles were selectively reduced to the corresponding primary amines in moderate to excellent yields. A further improvement was described by Fout and co-workers.98 They employed a C(carbene) CC(carbene) Co pincer complex 45a for the hydrogenation of nitriles under milder conditions by reducing the hydrogen pressure, from 29.6 atm to 4 atm. Also, demonstrated that the reaction of NaHBEt3 with the catalyst precursor produces the Lewis acid BEt3 which plays an important role in the reaction mechanism. Additionally, Jones and co-workers described the reversible hydrogenation and dehydrogenation of N-heterocycles using the Co pincer catalyst 18a.96 The hydrogenation of quinolones derivatives was performed using a hydrogen pressure of 10 atm, under heating at 120  C during 2 d, under these reaction conditions the N-heterocyclic moiety was hydrogenated exclusively, maintaining

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

831

the whole carbonated aromatic cyclic, with yields of about >98%. In the case of isoquinoline and 2-methylindole the pressure, catalyst loading, and temperature were increased, obtaining 53 and 19% yield, respectively. Interestingly, the hydrogenated products may be dehydrogenated, regenerating the starting materials and releasing molecular hydrogen. This strategy represents a good alternative for the design of hydrogen storage systems. Furthermore, the hydrogenation mediated by Co pincer complexes was also employed for the transformation of CO2 into high value products. It is well known that CO2 is one of the greenhouse gases considered as the mayor contributor of climate change (global warming) and thus its transformation may bring a beneficial impact to the environment and society. Some such reports of transformations with CO2 have been described in the literature.99–103 For instance, Bernskoetter and co-workers prepared the novel aliphatic Co(II) PNP-pincer complex 46a, which was capable to transform CO2 and H2 into formate in the presence of LiOTf and DBU (Scheme 14).100,102 Interestingly, when the N-methyl group of the pincer backbone ligand was exchanged by a hydrogen atom, the catalytic activity decreased dramatically (Scheme 14), ranging from 29,000 TON for 46a to 450 TON for 47a.

Scheme 14 Transformation of CO2 to formate catalyzed by Co pincer complexes.

Furthermore, the reaction of CO2, H2 and amines in the presence of the Co(II) pincer catalyst 48a is a useful strategy to synthesize formamides.103 Here, the addition of a reducing agent (NaHBEt3) and a base (KOtBu) was necessary for the reaction to proceed to the formation of an unsaturated intermediate, which is actually the active catalytic species (Scheme 15). Then, the latter specie can activate H2, forming a Co(I) hydride complex, followed by the insertion of CO2 into the CodH bond. Finally, the excess of amine results in the formation of the formate salt, liberating water and releasing the product, thus regenerating the catalyst.

P R2

H N Co

Cl

PR2

NaHBEt3 P R2

Cl 48a, R = R = iPr2 H N H

-H2O

P R2

NH3+HCOO-

R R

Cl

KOtBu

O

NH2

P R2

H N Co

N Co

PR2

PR2

P R2

OCHO

CO2 Scheme 15 Reaction mechanism for the formation of formamides catalyzed by Co pincers.

H2 H N Co H

PR2

NaHBEt3

R

H N Co

PR2

832

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Another strategy to hydrogenate multiple bonds consists in using a transfer hydrogen agent instead of dihydrogen. In this sense, alkenes, alkynes, ketones, aldehydes and imines have been hydrogenated using this procedure (Scheme 16). Here, the aliphatic Co(II) pincer complex 18a promotes the hydrogenation of alkenes, ketones, aldehydes and imines using isopropanol as transfer hydrogenation agent.104,105 Unfortunately, the catalyst was no selective in the presence of olefin, aldehydes or ketone functional groups in the substrate, leading to hydrogenation of both functional groups. However, the NNN Co(II)-pincer complex 49a successfully hydrogenated alkynes using H3N:BH3106 selectively producing the Z-isomer, achieving Z/E ratios of 100/0.

Transfer hydrogenation agent

2 mol % [18a]

R'

R O

OH

R

OH

2 mol % [18a]

+

r.t., 24 h

R'

R

R'

R

C, 24 h

N

2 mol % [18a]

R'

R

N H

C, 24 h R'

H3N:BH3 +

R'

R'

2-4 mol % [49a] 50-80 C, 10-24 h

R

R'

R

R

+

R'

R E-isomer

Z-isomer H N Co

R2P

BArF4 PR2

CH2SiMe3 18a, R = Cy

N N Co N Br Br O O 49a

Scheme 16 Hydrogenation of multiple bonds with Co catalysts.

Based on the results presented above, Liu and Zhou expanded the substrate scope to nitriles using the same strategy (Scheme 17).107 Interestingly, when the hydrogenation of nitriles was carried out using complex 50a as catalysts, in hexane at

Transfer hydrogenation agent

a] [50 % ol e 1 m exan h nH3N:BH3 + R

N N

H N Co

R

NH2

N 0.5 mol % [51a] R HFIP 0.5 mo l% HFI [5 1 a P, R ] 'R''N R H

PR2

Cl Cl 50a, R = tBu Scheme 17 Hydrogenation of nitriles catalyzed by Co pincer complexes.

N

H N Co

N H

N R''

PR2

Cl Cl 51a, R = tBu

R

R'

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

833

50  C, primary amines were obtained. However, when the reaction was performed using complex 51a as catalysts and hexafluoroisopropanol as solvent, symmetric secondary amines were produced, noteworthy the fact that presence of secondary amines lead to the formation of asymmetric tertiary amines. The applicability of this strategy allowed the synthesis of more than 70 amines with several distinct functional groups such as heterocycles, aryls, alkyls, esters, among others. Another relevant application of Co pincer complexes is their use as catalysts in the acceptorless dehydrogenation of HCdYH bonds (Y ¼ heteroatom), leading to the formation of the desired dehydrogenated product and molecular dihydrogen.86,87 Hanson and co-workers described the synthesis of imines from alcohols and amines (Scheme 18).108 Thus, complex 34a catalyzed the dehydrogenation of secondary alcohols at 120  C during 24 h, generating the corresponding ketone and dihydrogen as products. They tested a series of asymmetric alcohols including aryl and aliphatic groups. Taking advantage of the reaction conditions, they were able to perform the reaction in the presence of amines, obtaining the corresponding imine in good to excellent yields (56–98%). As expected, the lower yields were produced with secondary alcohols, while using primary alcohols lead to higher yields. When the reaction was carried out in the presence of molecular sieves the formation of amines was preferred over the imines by a dehydrogenation/condensation/hydrogenation-transfer reaction, the so-called dehydrogenative coupling reaction.109 In other examples, however, the addition of KOtBu instead of molecular sieves was preferred.110,111

Scheme 18 Acceptorless dehydrogenation of alcohols catalyzed by Co pincer catalyst.

In addition, the dehydrogenative coupling reaction has allowed the synthesis of heterocyclic compounds (Scheme 19).112,113 For instance, the reaction of diols with amines affords the corresponding 1,2,5-substituted pyrroles. Whereas, when the reaction is carried out with 1,2-diaminobenzene and an alcohol the formation of 2-substituted benzimidazoles takes place. Interestingly, water and dihydrogen are the only by-products, thus representing a nice, efficient and green alternative for the synthesis of heterocycles.

Scheme 19 Synthesis of heterocycles by dehydrogenative coupling catalyzed by Co pincer complexes.

834

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Kempe and co-workers described an elegant three step procedure for the alkylation of amides and esters with alcohols (Scheme 20).114 First, the alcohol is dehydrogenated, a “condensation” type reaction then occurs, followed by the hydrogenation of the alkene via borrowing hydrogenation. This sequence of reactions has also allowed the synthesis of a–alkylation of ketones with primary alcohols115 and the attaining of secondary amines via the N-alkylation of amines with other amines.116

Scheme 20 Dehydrogenation/condensation/hydrogenation via “hydrogen-borrowing.”

7.13.3

Rhodium pincer complexes

Rhodium complexes have exhibited a plethora of applications in different chemical fields, this being particularly true in the case of catalysis due to its unique chemical properties. For instance, rhodium easily changes its oxidation state, going from (I) to (III), allowing some fundamental reaction steps such as transmetalation, oxidative addition, insertion and reductive elimination. Besides, rhodium is reactive toward polar and non-polar bonds, i.e. H2, SidH, BdH and CdH. Hence, the combination of rhodium with pincer ligands gives a wide entrance for the generation of highly active and selective catalysts. Ozerov and co-workers have prepared a POCOP-Rh(III) (54a) pincer-complex and use it for Kumada–Tamao–Corriu couplings between Grignard reagents and arylhalides (Scheme 21).117 The reactions proceeded very fast using non-sterically hindered aryl iodides and 1 mol% catalyst loading. As expected, decreasing the catalyst loading to 0.1 mol%, increased the reaction time. Furthermore, most of the reactions were carried out under mild conditions at 22  C in short reaction times ranging from 2 to 5 min. However, the catalyst exhibited low activity towards bromofluorobenzene and aryl-chlorides.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

835

Scheme 21 Kumada-Tamao-Corriu couplings catalyzed by the POCOP-Rh(III) (54a) pincer complex.

The proposed reaction mechanism for the Kumada-Tamao-Corriu couplings catalyzed by the Rh(III) pincer complex is shown in Scheme 22. The catalytic active species is formed by the reduction of Rh(III) to Rh(I) and the creation of vacant coordination sites by dehydrochlorination of pincer complex 54a with the Grignard reagent. Then, a typical CdC cross-coupling mechanism is observed; i.e. oxidative addition of the aryl halide occurs, then a transmetalation reaction followed by a reductive elimination that yields the product and regenerates the catalyst to start the catalytic cycle once again.

Scheme 22 Reaction mechanism for the Kumada-Tamao-Corriu couplings catalyzed by the POCOP-Rh(III) (54a) pincer complex.

Ozerov and co-workers also used a series of Rh(I) complexes (55a–58a) including POCOP, PCP and PNP pincer ligands for the dimerization of terminal alkynes, i.e., 4-ethynyltoluene, 1-hexyne, or trimethysilylacetylene (Scheme 23).118 The reactions were carried out using 1.0 mol% of the corresponding catalyst in C6D6 at 80  C. All Rh(I) complexes were active, achieving moderate to high conversions. In general, the products were a mixture of E- and gem-enyne isomers, with small amounts of oligomers in some cases. The Z-enyne isomers were only observed in two reactions in low yields (1 and 8%). In general, none of the catalysts showed selectivity for E or gem-enyne products. The symmetric POCOP-based complex is the faster catalyst for these processes with TON’s up to 20,000, performing better than previous reports.119

836

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Substrate

[Rh]

t (h)

gem:E:Z:oligomers

% Conv.

4-MeC6H4CLCH

55a

3

29:69:0:2

>97

4-MeC6H4CLCH

56a

3

24:70:0:6

>97

4-MeC6H4CLCH

57a

36

22:56:0:22

94

4-MeC6H4CLCH

58a

70

2:73:0:25

74

t

BuCLCH

55a

3

73:19:0:8

97

t

BuCLCH

56a

3

62:31:0:7

95

t

BuCLCH

57a

36

65:18:0:17

63

t

BuCLCH

58a

70

29:71:0:0

25

Scheme 23 Dimerization of terminal alkynes catalyzed by Rh pincer complexes.

In a similar approach, Bezuidenhout and co-workers evaluated the catalytic activity of a Rh(I)–oxygen adduct complex derived from a CNC-pincer ligand in the alkyne homo-dimerization and hydrothiolation reactions in the absence of base or any additive (Scheme 24).120 In general, complex 59a exhibited a high functional group tolerance and high selectivity, achieving exclusively the gem-enyne isomer with moderate to excellent conversions (48–99%). On the other hand, the alkyne hydrothiolation reactions were carried out using 1 mol% of catalysts 59a in C6D6 at 80  C. Under these reaction conditions an excellent selectivity towards the a-vinyl sulfide isomer was observed without the formation of dimerization products (91–100% of a-product). Unfortunately, low activities and selectivities were observed when both substrates contained an aromatic substituent (40% a-product). Furthermore, non-symmetric bis-a,a´-vinyl sulfides were selectively obtained via a one-pot stepwise process via alkyne hydrothiolation reactions catalyzed by 59a of a dithiol with different alkynes attaining yields from 59 to 99%.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

837

Scheme 24 Alkyne dimerization and alkyne hydrothiolation reactions catalyzed by the pincer Rh(I) complex 59a.

In 2013, Song and coworkers prepared a series of chiral C2-symmetrical Rh(III) complexes including bis(imidazolinyl)phenyl pincer ligands.121 The complexes were active catalysts for the alkynylation of ethyl and methyl trifluoropyruvate with aromatic and heteroaromatic terminal alkynes (Scheme 25). The optimized catalytic conditions consisted in using 3 mol% of catalyst in Et2O at 25  C. The best catalysts were complexes 60d and 60e, and in particular 60e showed high yields and excellent enantioselectivity (84–99% yield and 94–98% ee). Using phenylacetylene and substituted phenylacetylenes as substrates high ee values (>96%) were observed, tolerating both electron-withdrawing and electron-donating groups at the 2-, 3- or 4-position of the alkynyl group. Similarly, the alkynylation reaction using complex 60e as catalysts was performed using various aliphatic terminal-alkynes, achieving good enantioselectivities (ee 97–99%) albeit with low yields. In order to improve the catalytic performance, the reactions were carried out at higher temperature (70  C) in a mixture of toluene/Et2O. The yields reached under these conditions were of 68–85% with ee values around 52–95%.

838

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 25 Alkynylation catalyzed by Rh(III) pincer complexes.

In a similar approach, Nishiyama described the functionalization of terminal alkynes with fluoroalkyl-substituted ketones catalyzed by a series of CCN Rh(III) pincer complexes (61a–61c).122 The reactions were carried out using 5 mol% of catalyst, ketone (1.0 eq), alkyne (2.0 eq) at 60  C for 24 h. Under these conditions, complex 61c was the most active catalyst, achieving from moderate to good yields (35–85%) with good enantioselectivities for the (S)-products (78–93% ee) (Scheme 26).

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

839

Scheme 26 Functionalization of terminal alkynes with fluoroalkyl-substituted ketones catalyzed by a series of CCN Rh(III) pincer complexes.

The reaction mechanism of the functionalization of terminal alkynes with fluoroalkyl-substituted ketones is depicted in Scheme 27. The mono(alkynyl) intermediate 61-I is obtained from the reaction of 61 with an alkyne. Then, a second CdH activation of an alkyne affords the bis(alkynyl) intermediate 61-II. Later, the species 61-III is formed by the dissociation of the AcOH ligand, followed by the coordination of a ketone to give the intermediate 61-IV. Then, the insertion of a ketone into the Rh-alkynyl bond produces the intermediate 61-V. Finally, protonation of 61-V with AcOH yielded the S-enantiomer product.

840

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 27 Reaction mechanism for the alkynylation with ketones catalyzed by complex 61.

In 2013, Song and co-workers performed a slight structure modification to complex 60, exchanging the OAc by chlorine ligands (62a–62f).123 The resulting complexes were evaluated as catalysts in the asymmetric allylation of aromatic, heteroaromatic and aliphatic aldehydes with allyltributyltin (Scheme 28). After optimization of the catalytic reactions, they determined that using 5 mol % of catalyst in the presence of molecular sieves (4 A˚ ) in DCM at room temperature for 6 h, the best catalyst was complex 62b, affording yields between 78 and 99% of the products. Benzaldehydes with both electron-donating and electron-withdrawing groups were well tolerated, affording good to excellent enantioselectivities (84− 97%).

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

841

Scheme 28 Allylation and carbonyl-ene reaction catalyzed by Rh(III) pincers.

Complexes 62a–62f were also tested in the carbonyl-ene reaction of methyl or ethyl trifluoropyruvates with alkenes to prepare the corresponding a-hydroxyl-a- trifluoromethyl esters (Scheme 29).123 In this case, complex 62f was the most active catalyst of the series, achieving from moderate to high yields (60–89%) with good to excellent enantioselectivities (65–95% ee). The reaction required the presence of Ag(I) species to create vacant coordination sites via halide abstraction. Thus, the reactions were carried out using 3 mol% of catalyst and AgOTf (2 eq) in DCM at room temperature for 16 h. The reaction proceeds in high enantioselectivities when electron-withdrawing groups are present in the 2-arylpropane substrates (91 −95% ee), while electron-donating groups such as alkyl or phenyl groups provided lower stereoselectivities (74 −89% ee). The proposed mechanism for the asymmetric carbonyl-ene reaction is displayed in Scheme 29.124 Following activation of the Rh-complex with AgOTf (62-I), one molecule of ethyl trifluoropyruvate coordinates to the rhodium(III) center by “Re-face attack” of the alkene to the keto carbonyl group in 62-II with 1,5-hydrogen migration to afford in the transition state 62-III (transition state). Finally, decomplexation of the desired product affords again the active catalytic species 62-I completing the catalytic cycle.

842

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 29 Reaction mechanism for the asymmetric carbonyl-ene reaction catalyzed by the Rh(III) pincer complex 62.

In 2015, Arai and co-workers used a series of chiral bis(imidazolidine)-derived NCN–rhodium pincer complexes for the catalytic asymmetric-Mannich reactions of malononitrile with various aromatic-aldehyde-derived N-Boc imines to give the desired chiral Mannich adducts.125 These reactions were carried out using N-Boc imine (1.0 eq), malonitrile (1.5 eq) and 3 mol% of catalyst in toluene at room temperature (Scheme 30). On this occasion, complex 63c was the best catalyst, achieving conversions between 54 and 99% with ee values ranging from 73 to 94%. In general, the reactions with the imines derived from electron-rich benzaldehydes were converted more smoothly than imines with electron-withdrawing groups.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

843

Scheme 30 Mannich type reaction catalyzed by Rh pincer complexes.

The proposed reaction mechanism for the Mannich type reactions catalyzed by 63c is shown in Scheme 31. Here, first one molecule of malononitrile coordinates to the Rh(III)-center to give the 63c-I adduct. Then, the acetoxy ligand abstracts a proton from the malononitrile ligand to generate the enolate 63c-II. Afterwards, the nucleophilic addition of malononitrile to the N-Boc

Scheme 31 Catalytic reaction mechanism of the Mannich type reaction catalyzed by the Rh(III) pincer complex 63c.

844

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

imine takes place affording intermediate 63c-III. Finally, protonation of the rhodium carbamate with acetic acid produces the Mannich adduct with the concomitant regeneration of the catalyst. In 2011, Nakamura and co-workers reported the use of m-carborane-based chiral NBN-Rh(III) pincer complexes for the asymmetric conjugate reduction of a,b-unsaturated esters and reductive aldol reactions (Scheme 32).126 Among these catalysts, the complex 64a was the most active for both catalytic reactions. Excellent conversions were obtained for the reduction reactions (89–96%,) with high enantioselectivities (94–98% ee). In the case of the reductive reaction, it proceeded using 1 mol% of 64a and (EtO)2MeSiH (1.5 eq) in toluene at 60  C, affording a 98% yield with a 9:1 ratio of anti:syn products.

Scheme 32 Asymmetric conjugate reduction of a,b-unsaturated esters and reductive aldol reaction catalyzed by Rh complexes.

Later on, de Bruin and coworkers prepared two series of ionic and neutral rhodium carbonyl pincer complexes of the type [(CO)Rh(PNN)] and explored their catalytic activity in ketene and ketene imine production using ethyl diazoacetate or sodium 2-benzylidene-1-tosylhydrazin-1-ide as the carbene precursors (Scheme 33).127 Complexes 66b and 66e were the most active

Scheme 33 Synthesis of ketene and ketene imines catalyzed by Rh pincer complexes.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

845

catalysts of the series. In the amide synthesis with EDA, complex 66e afforded the best result with almost full conversion and very high selectivity to the amide derivative (ratio > 50), whereas for the synthesis of the amide using the sodium N-tosyl hydrazone salt, complex 66b was the best, achieving a 68% yield. Besides, these complexes were also tested in the Staudinger ketene-imine cycloaddition of the N-tosyl hydrazone salt with N-benzylidenemethanamine to prepare b-lactams. The reactions were carried out using 2.5 mol% of catalyst and 20 bar of CO in ClCH2CH2Cl at 55  C for 20 h. In these reactions, complex 66b produced the best yields of the desired b-lactam product (67.7%), while catalysts 66e afforded a moderate 33.0% yield. The mechanism for the ketene formation is depicted in Scheme 34. Firstly, the metallocarbene intermediate 66-II is formed by coordination of the ethyl diazoacetate to 66-I via N2 loss. Subsequently, intermediate 66-II undergoes insertion of CO into the M] CHCO2Et bond, affording the metal-ketene species 66-III. Then, two possible pathways may occur in the coordination sphere of the metal center for the generation of intermediate 66-IV and the product; the nucleophilic coupling or a [2 + 2] imine-ketene cyclization. Finally, the exogenous CO coordinates to 66-IV, completing the catalytic cycle.

Scheme 34 Reaction mechanism for the catalytic ketene formation with coupling reaction mediated by complexes 66a-66d.

In 2014, Messerle and co-workers described a series of NCN-Rh and NCN-Ir pincer complexes containing a central NHC donor linked with two pyrazole groups and their use as catalyst for the cyclization of 4-pentynoic acid to form g-methyleneg-butyrolactone via hydrocarboxylation (Scheme 35).128 The reactions were performed using 5 mol% of catalyst in THF-d8 at 80  C for 24 h. The most active catalyst of this series was 67a, achieving complete conversion of the substrate after 18 h. Complex 67a was also evaluated in the hydrosilylation of 1-phenylpropyne or phenylacetylene with triethylsilane. However, in both reactions, a low conversion was observed (31–37%). Only with phenylacetylene as substrate, high selectivity for the b-trans product was observed (b-trans:b-cis:a ¼ 24:5:2).

846

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 35 Cyclization and hydrosilylation reactions catalyzed by pincer Rh complexes.

In another report, Hollis and coworkers found that CCC-NHC Rh(III) pincer complexes were effective catalysts for the b-boration of acyclic and cyclic a,b-unsaturated carbonyls with B2pin2 (Scheme 36).129 These reactions were performed using 4 mol% of catalyst, 1 eq of alkenes and 2.5 eq of B2pin in methanol or ethanol at 22  C. Under these conditions, the most active catalyst was complex 69c, affording the products in moderate to high yields; however, the yields were higher with the acyclic alkenes substrates (81–99%).

Scheme 36 Hydroboration of alkenes catalyzed by Rh pincer complexes.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

847

Another interesting borylation reaction was recently described by Tan and coworkers. They evaluated a series of Nishiyama’s catalysts [(S,S)-Rh(Phebox)(OAc)2
H2O] for asymmetric borylative cyclization of cyclohexadienone-containing 1,6-dienes.130 They determined the optimal reaction conditions to be the use of 5 mol% of the catalyst in presence of NaOtBu (10 mol%) and MeOH (2.5 eq) in THF at room temperature for 24 h (Scheme 37). Under these conditions, the best results were obtained with complexes 70b and 70c, affording optically pure cis-bicyclic skeletons bearing various contiguous stereocenters in good yields (25−93%) and excellent diastereoselectivities (>20:1 dr) and enantioselectivities (90−99% ee). This method was also applicable to cyclohexadienone-tethered mono-, 1,1-di-, and (E)-1,2-disubstituted alkenes, and tolerated a wide range of functional groups on both cyclohexadienone and alkene components. Especially, with cyclohexadienone-tethered terminal alkene substrates, the products displayed exclusively anti-Markovnikov regioselectivity.

Scheme 37 Borylation reaction catalyzed by Rh pincer complexes.

Additionally, a reaction mechanism was proposed using SAESI-MS experiments and DFT studies (Scheme 38). Thus, the Rh(III)-complex (70c-I) is obtained by s-metathesis from 70c with B2pin2, following by the decoordination of water and reductive elimination of X-Bpin, providing the catalytic active species 70c-II. From this reaction step, there are two possible catalytic cycles. In path A, the complex 70c-II undergoes an oxidative addition with B2pin2 to give the species 70c-III (confirmed by SAESI −MS). Then, the cycloenone is coordinated and inserted into complex 70c-III, and the generated complex 70c-V undergoes successive intramolecular conjugate addition, reductive elimination, and protonation, affording the borylative cyclization product and regenerating complex 70c-II. While in path B, the cycloenone coordinates to complex 70c-II to afford complex 70c-VIII (confirmed by SAESI −MS), which undergoes an oxidative cyclometalation to form complex 70c-VIII. Next, complex 70c-VIII produces the final borylative cyclization product and regenerates 70c-II through s-metathesis, reductive elimination and protonation reactions.

848

70c

Path A

Path B O

O

N conjugate addition

sBu

Bpin

N

Rh

sBu

N Bpin

O

Bpi n

Rh

H

MeOH

MeOBpin

70c-V

sBu

secondary interaction

Rh

Bpin

N

Me

Bpin

sBu

O

Bpin

Me 70c-VI

O

O O

O

O Me

N sBu

Rh

Bpin

N Bpin

sBu

70c-III

O

detected by SAESI-Ms O

Me

Proposed reaction mechanisms for the asymmetric borylative reaction catalyzed by 70c.

B2pin2

V-metathesis

Bpin O MeOH

H O

oxidative addition

O

O

O

protonation

O

Me

sBu

O

Me

sBu

70c-II

N

H

Bpin

H O

N

Rh

Bpin

N sBu

O

MeOBpin

Me

Rh H O Me 70c-VIII

coordination

70c-IV

Scheme 38

Rh

B2pin2

H

reductive elimination

O

O

N Bpin s Bu H

O

N sBu

Bpin

Bpin O

protonation

O N

X

sBu

dissociation and R.E. X-Bpin H2O H

O

insertion

O

OH2

O

O

N

70c-VI

O

Me

Rh

X=OAc or OtBu 70c-I

Bpin sBu

Bpin

sBu

Bpin

sBu

N

O

O Me

O

O

N

O

O

oxidative cyclometalation

Me O

O N

Rh

N sBu

sBu

O Me 70c-VII

O

detected by SAESI-Ms

N O

sBu

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

B2pin2 tBuONa

V-metathesis

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

849

Recently, Esteruelas et al. prepared the pincer complex POP-Rh(I) (71a) and explored its catalytic activity in the dehalogenation of chlorocyclohexane using 1 mol% of the catalyst and NaHCO3 (1.2 eq) in 2-propanol at 82  C for 24 h, achieving 85% yield of the cycloalkane (Scheme 39).131 Interestingly, acetone was not found in the reaction mixture, indicating that the solvent did not participate in the dehalogenation.

Scheme 39 Dehalogenation of cycloalkanes catalyzed by a Rh pincer complex.

Esteruelas also evaluated the catalytic activity of the pincer POP-Rh(III) complex (72a) in the monoalcoholysis of diphenylsilane with a wide range of alcohols.132 The reactions were performed in toluene at 32  C, using 0.17 mol% of catalyst, silane and alcohol concentrations of 0.3 M (Scheme 40). Under these conditions, alkoxysilanes HSi(OR)Ph2 were selectively formed in high yields (71–92%), with high turnover frequencies at 50% conversion (TOF50%), in a range from 4,000 to 76,500 h−1.

Scheme 40 Monoalcoholysis of H2SiPh2 catalyzed by the Rh complex 72a

The authors proposed a mechanism for the alcoholysis which is represented in Scheme 41. In this proposal, the rhodium(I) intermediate 72a-I is obtained from the reductive elimination of HSi(OH)Ph2 from 72a, which undergoes oxidative addition of diphenylsilane to give 72a-II. Then, complex 72a-IV is produced via intermediate 72a-III by elimination of molecular hydrogen. After this, the nucleophilic addition of ROH to 72a-IV takes place to form intermediate 72a-V. Finally, the alkoxysilane product is formed by reductive elimination to form 72a-V, generating again the Rh(I)-complex 72a-I.

850

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 41 Catalytic cycle for the monoalcoholysis of diphenylsilane catalyzed by 72a.

In a similar approach, Ito and Nishiyama prepared a series of CCN-Rh pincer complexes containing NHC-oxazoline hybrid ligands and explored their catalytic activity in the conjugate reduction of ethyl b-methylcinnamate with HSi(OEt)2Me.133 The reactions were carried out using 1 mol% of catalyst in presence of KOtBu (2 mol%) in toluene at 60  C for 1 h (Scheme 42). The most active catalyst was complex 73b, achieving high conversions (91%), albeit with low enantioselectivity (49% ee). On the other hand, complexes 74a-c and 75a-b were tested in the hydrogenation of aromatic ketones. In general, all complexes were very active (95–99%), with modest enantioselectivity (19–60% ee). The hydrogenation of 9-acetylanthracene using 74a was unexpectedly found to give a mixture of hydrogenated products.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

851

Scheme 42 Hydrogenation of alkenes and ketones catalyzed by Rh pincer complexes.

Early in 2020, Young and coworkers demonstrated that the deoxygenation of amine and pyridine N-oxides may be performed by PCcarbeneP-Rh pincer complexes.134 The best yields were obtained using 5 mol% of complex 76a in iPrOH at 80  C for 24 h (Scheme 43). The catalytic scope was explored towards a wide range of N-oxides with electron-donating and electron-withdrawing groups, showing moderate to excellent yields (36− 99%). Complex 76a was also tested for alcohol oxidation reactions in the presence of trimethylamine oxide, achieving modest to good conversions to provide 14–78% yields of the corresponding ketone.

852

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 43 Deoxygenation and hydrogenation catalyzed by Rh pincer complexes.

In 2015, Kunz and coworkers explored the catalytic activity of various rhodium–NHC–pincer complexes (77a to 79a) in the Meinwald rearrangement of epoxides into methylketones (Scheme 44).135,136 The complex 79a demonstrated to be the best catalyst compared with 77a and 78a complexes. The reactions were carried out using 1 mol% of catalyst and 10 mol% LiBr at room temperature. The catalytic system shows an excellent functional group tolerance, and a wide array of epoxides was tested. The corresponding methyl ketones were obtained in moderate to good yields (7–99%) with excellent chemo- and regio-selectivities, favoring methylketones over aldehydes.

Scheme 44 Meinwald rearrangement of epoxides catalyzed by Rh pincer complexes.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

853

The proposal for the reaction mechanism with catalyst 77a is displayed in Scheme 45. Here, the intermediate 77a-I is first obtained by oxidative addition (path b), formation that was confirmed through NMR spectroscopy. From 77a-I, there are two possible pathways: concerted 1,2-H shift under reductive elimination of the catalyst (path c) or b-hydride elimination (path d) to form the Rh-hydride intermediate Int-1, which subsequently undergoes reductive elimination (path e) to release the desired product and produces the catalyst. However, Int-1 was not observed during the catalytic reaction. Possibly, the N-homoallyl moiety of catalyst could be inserted into the Rh-H intermediate Int-1 to give the rhodium alkyl intermediate Int-2, which could undergo a D/H exchange in the case where the deuterated phenyl oxirane was used.

Scheme 45 Catalytic cycle for Meinwald rearrangement of epoxides catalyzed by the Rh(I) pincer complex 77a.

7.13.4

Iridium pincer complexes

Almost twenty five years ago, the first example using an Ir pincer-type catalyst for the dehydrogenation of alkanes was reported by Kaska and Jensen.32,33 This report encouraged the scientific community to study iridium pincer-type complexes in the catalytic dehydrogenation of alkanes, since its conversion is of major relevance for organic synthesis. Starting from there, two major areas of chemistry merged and have permitted the synthesis of valuable substrates in tandem processes such as alkane-alkene coupling reactions, alkyl group metathesis, borylation of alkanes, alkyl-aryl coupling reactions and dehydrogenation of hetero-compounds. In this sense, there are excellent reviews that have covered at different times the advances in the realm of Ir pincer chemistry.7,15,137,138 Thus, in the next paragraphs we described the latest achievements in catalytic transformations promoted by Ir pincer complexes.

854

7.13.4.1

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Alkane dehydrogenation

The selective synthesis of alkenes from alkanes is considered a keystone in organic chemistry, but this transformation possesses several drawbacks, starting with the fact that it is a thermodynamically disfavored process. However, in the work published by Jensen and Kaska in 1996, they succeeded it using an Ir pincer complex.32 Since this pioneering work, several examples have been reported up to date.7,139–167 The dehydrogenation of cyclooctane (COA) in the presence of tert-butylethylene (TBE) as hydrogen acceptor is the “benchmark reaction” for testing catalysts in the alkane dehydrogenation reaction (Scheme 46). The conditions reported by Jensen consist in heating the reaction mixture at 150–200  C for 24 h or even a week in the presence of 0.3 mol% of a PCP-Ir pincer complex as catalyst (80a).32,33 This reaction conditions have served for comparison to the upcoming reports. Noteworthy is that they found excess of TBE and TBA in the reaction mixture interferes with the activity of the catalyst. Therefore, it is important to make small additions of TBE to the reaction mixture.

Scheme 46 Benchmark dehydrogenation reaction of COA.

In 2017, Kumar and Goldman reported the synthesis of piperylene from pentane by a PCP-Ir complex (81a) using TBE as hydrogen acceptor and reported >1000 TON with a 95% yield.163 They demonstrated that the less-bulky pincer ligands are, the transformation becomes much more effective. The yield reported in this reaction was ca. 100 times higher than that obtained with the tert-butyl derivative reported by Jensen.32,33 Wendt and co-workers explored the reactivity of two phosphinite pincer complexes cis-(POCOP)IrHCl and trans-(POCOP)IrHCl (82a).164 The thermal stability of these cyclohexane-based pincer complexes is lower than that of their aromatic-based analogous.168 Interestingly, their aromatic counterparts can be obtained through successive intramolecular dehydrogenation reactions by heating the cyclohexane-based complexes at high temperatures.169 Nevertheless, the cyclohexane-based complexes are moderately active (TON 1684) in the alkane dehydrogenation of COA with TBE (180  C for 32 h). Similarly, Ozerov and coworkers reported a phosphine donor pincer ligand with a boron as a central donor atom.165 The PBP-Ir pincer complex (83a) also showed moderate activity for alkane dehydrogenation (TON 221, 200  C, 26 h). On the other hand, Gelman and coworkers described a more sophisticate pincer complex based on dibenzobarrelene, which contains an extra hemilabile ligand (84a).170 This series of complexes exhibited a TON of 150 for the dehydrogenation of COA at 120  C and using catalysts loadings of 0.5 mol%. Asymmetric tert-butyl substituted pincer ligands with a 1,3-diaminobenzene and 3-aminophenol scaffolds are an alternative to alkane dehydrogenation presumably due to a combined electronic and steric effect of the NH/O hybrid linker and different alkyl groups at phosphorous. Liu reported asymmetric ligands containing a NH linker in the transfer dehydrogenation of COA, obtaining high yields.171 Additionally, an overall improvement of the reaction conditions could be attributed to the use of a Fischer-Porter bottle reactor (Scheme 47).

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

(A)

Jensen and Kaska, 1996

(D)

(B)

(C)

(E)

(G)

855

(F)

(H)

Scheme 47 Recently reported iridium pincer complexes for alkane dehydrogenation.

Jones and coworkers attempted the dehydrogenation of n-octane using para-benzoquinones as hydrogen acceptors, resulting in poor yields.172 Interestingly, the dehydrogenation of 2,3-dihydrobenzofuran (DHBF) using 2,6-tBu2BQ as acceptor was completed after heating the mixture in toluene at 120  C for 3 h. The same PCP-Ir catalyst was used for the dehydrogenation of isopropanol in moderate yields at 150  C for 39 h with a TON >5000. Yamashita et al. synthesized a series of pincer iridium complexes with a pyrrole-based core (85a) and reported low activity in the dehydrogenation of alkanes (TON 4).173 The temperature of the reaction in this case was raised up to 220  C. In order to improve the catalytic process of alkane dehydrogenation, Celik and Goldman studied a continuous-flow system.174 The Ir pincer complexes that previously showed excellent activity in the dehydrogenation of COA were supported on silica. These materials tolerate reaction temperatures up to 340  C. Taking into account previous reports on the dehydrogenation of alkane in gas phase,154 they explored the dehydrogenation of shorter-chain olefins. In 2019 Tada, Muratsugu and Yamashita reported a pincer Ir(V) complex bearing a P-Al-P ligand (86a).175 They used an aluminum atom as anionic donor in their ligand since it has strong s-donating ability compared to their previous boron examples,158,176 and a very low electronegativity. Thus, in the dehydrogenation of COA with 1-hexene as hydrogen acceptor, the catalytic activity of complex 86a was modest with TON’s of 31 observed after heating the reaction mixture at 180  C. This is the first example of a pincer complex bearing an alumanyl moiety used for the transfer alkane dehydrogenation. Liu and Huang presented a series of asymmetric NCP-type pincer iridium complexes, one of them (87a) showed a high robustness and activity in the dehydrogenation of COA, n-octane and heterocycles.177 Complex 87a was also active in the olefin isomerization reaction with good regio- and stereoselectivity,166 and transfer hydrogenation of alkenes and alkynes using ethanol as H donor.178,179 Complex 87a reached a TON of 3440 in the dehydrogenation of COA at 200  C for 18 h. This result is similar to that described by Jensen using the POCOP-Ir pincer complex (TON 2000). The dehydrogenation of n-octane resulted in a mixture of octenes due to a olefin isomerization process, and the dehydrogenation of heterocycles reached above 60% of conversion in almost all cases. A second report by the same research group revealed the effects of the linker in pincer-type iridium complexes in the dehydrogenation of linear and cyclic alkanes.180 The thermal stability was also highly influenced for the linkers and they demonstrated that the incorporation of N-linkers provides an overall positive effect even in the acceptorless dehydrogenation of 1,2,3,4-tetrahydronaphtalene.

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Recently, the formation of enamines was explored by Goldman and coworkers.181 The catalytic mechanism was studied by DFT calculations, consisting of an oxidative addition of the amine a-CdH bond followed by a b-elimination. In agreement with several reports, the di-isopropylphosphino substituted pincer iridium complex resulted to be more active for the dehydrogenation of tertiary amines than the di-t-butylphosphino substituted species.32 The catalyst loading for this transformation was only 2% (in comparison to 10% for tBu analogue) but longer reaction times (32–48 h) and higher temperatures were required. The increment in the temperature using the tBu derivative lowered the yields, therefore this fact can be excluded as the reason for the increment in the yield using the iPr analogue.

7.13.4.2

Olefin isomerization

Huang reported an asymmetric Ir pincer complex (87a) which is highly active as catalyst in the isomerization of alkenes and permits regio- and stereoselectivity.166 The monoisomerization of 1-alkenes to trans-2-alkenes was achieved with TON’s up to 19,000 at a 100 g scale using 0.005% mol of catalysts. The more bulky pyridine-phosphine Ir complexes (see previous report182) possesses a dual agostic interaction with the metal and allows even challenging isomerizations in linear a-olefins. An epoxide isomerization was recently reported by Kunz.183 In a comparison between Rh,135 Co and Ir pincer complexes bearing an CNC-type pincer complexes (Scheme 48) the Rh derivative showed higher activity.

Scheme 48 Epoxide isomerization catalyzed by C(NHC)NC(NHC) iridium pincer complex 88a.

7.13.4.3

Tandem reactions involving alkane dehydrogenation

Tandem reactions are highly desirable processes since they may allow the reduction of byproducts or wastes, reaction time, and energy. In this sense, the catalytic versatility of Ir pincer complexes has allowed the development of tandem processes.184–199 Hence, Hintermann pioneered in 2018 the hydrogen-autotransfer alkylation (HAT) process, using a base and an iridium pincer complex as catalyst.200 This approach permits the alkylation of pyrroles in a systematic fashion according to Hans Fischer experiments. The latter consist in the reaction of alkyl-alcohols with pyrroles at high temperature and pressure (>200  C in a sealed tube). The PNP-Ir pincer complexes with a bis(phosphinylamino)triazine scaffold, similar to those described by Kempe,201 resulted in the more active catalyst for this reaction. These conditions ([Ir(COD)Cl2] with Kempe’s ligand, 24 h, 110  C) allowed the synthesis of a vast variety of alkylated pyrroles including pyrrole carboxylic esters (Scheme 49).

Scheme 49 HAT alkylation of 2,5-dimethylpyrrole with Kempe’s ligand.

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857

In 2020, Yagishita reported the oxidative coupling of benzylic amines into imines in a photocatalytic system. The reaction proceeds at room temperature with yields from 47 to 99% when the LED light was on.202 Two possible mechanisms could occur during this transformation, either a photoredox pathway or a singlet oxygen production pathway (Scheme 50).

Scheme 50 Oxidative coupling of benzylic amines into imines.

7.13.4.4

Dehydrogenation of substrates with heteroatoms

The dehydrogenation of compounds with heteroatoms has been achieved using Ir pincer complexes.99,201,203–221 In 2017, Yamashita reported one of the very few metal-catalyzed dehydrogenation of an amine-borane with a catalyst containing a boron ligand (90a).176 The catalytic dehydrogenation of dimethylamine-borane (DMAB) yields the cyclic dimer, dihydrogen gas and the dihydride complex of the catalyst (Scheme 51). The percentage of conversion using 0.5 mol% of catalyst loading was up to 97% by heating it at 60 C for 3 h in a concentrated solution in THF.

Scheme 51 Catalytic dehydrogenation of amine boranes catalyzed by Yamasita’s or Belkova’s iridium pincer complexes.

Shubina, Giambastiani and Belkova reported a comparative study in reactivity between iridium pincer hydride complexes bearing PCN- (91a) and PCP-type (92a) ligands.222,223 The asymmetrical Ir pincer complex showed higher catalytic activity in the amineborane dehydrogenation reaction than the symmetrical derivative. At 25  C, above 60% of conversion was observed for the dehydrogenation of DMAB, tBuNH2
NH3 and NH3BH3. However, the activity of complex (91a) was lower than that of the POCOP-Ir pincer complex.203

7.13.4.5

Dehydrogenative coupling

The first example of enantioselective CdH insertion of a-aryl-a-diazoacetates by an iridium pincer catalyst was reported by Song and Gong for the functionalization of indoles by an NCN pincer complex (93a) bearing a bis(imidazolinyl)phenyl scaffold (Scheme 52).224 While optimized conditions for the transformation yielded the desired products in moderate enantioselectivities, ranging from 37% to 86% ee, thus the catalyst offers inferior stereocontrol than that observed for its Fe, Rh, Pd and Cu analogues.

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Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 52 Functionalization of indoles with a-aryl-a-diazoacetates via CdH activation catalyzed by Ir(III) pincer ligands.

The dehydrogenative coupling of vinyl arenes to form CvinyldCvinyl bonds was studied by Goldman and Jones.225 The approach consisted in a tandem reaction performed by iridium pincer complexes through a CdH addition and a CdC bond coupling which can be inter- or intra-molecular (Scheme 53). The yields of the intramolecular coupling were up to 80%. Additionally, this report led two years later to the first example of an iridium pincer catalyst for the Geurbet reaction of ethanol.226

Scheme 53 Dehydrogenative coupling of vinyl arenes and Geurbet conversion.

7.13.4.6

Dehydrogenation of carboxylic acids

Oro and Iglesias described the dehydrogenation of formic acid with an Ir pincer complex based on N-heterocyclic olefin (NHO) (96a), under solvent free conditions.227 This green approach highlights the advantages of using formic acid instead of other more toxic hydrogen carriers such as boranes or methanol. This report also shows that the chelating bis-phosphine ligands without an NHO moiety are less active than the NHO-based derivatives. Additionally, this is the only complex reported so far that is active under solvent-less conditions and in water (Scheme 54).

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

859

Scheme 54 Dehydrogenation of formic acid by Ir complexes.

7.13.4.7

Dehydrogenation of alcohols

Wendt and coworkers tested the acceptorless dehydrogenation of benzyl alcohol using a carbene (POCOP)¼IrCl complex (98a) and found good percentages of conversion by heating the reaction mixture at 190  C for 16 h (Scheme 55).164 However, a mixture of benzylbenzoate, benzaldehyde and other products were observed. Other alcohols examined produced low conversions (around 15%).

Scheme 55 Dehydrogenation of alcohols by Ir pincer complexes.

The dehydrogenation of methanol was presented by Beller, in which a low concentration of the base was enough for the conversion in high yields (Scheme 55). 228 This is an important improvement since previous reports required harsher conditions and the presence of several additives. The catalyst for this transformation is an iridium-PNP pincer complex (99a) and it was possible the identification of CO organometallic species that were responsible for the deactivation of the catalyst.

7.13.4.8

Hydrogenation of CO2

The catalytic hydrogenation of some substrates using iridium complexes is very well known.229–241 In particular, the hydrogenation of CO2 allows the synthesis of formic acid, which is an important raw material used in several industries, as well as it shows a great potential as hydrogen-storage material. However, the hydrogenation of CO2 remains as a challenging transformation and a robust and highly active catalyst have not yet been identified. In this sense, Nishibayashi and Nozaki addressed this challenge using a PC(carbene)P-Ir pincer complex (101a) (Scheme 56) and demonstrated the strong donating ability of the carbene in the pincer skeleton.242 Moreover the turnover numbers were higher than those observed for the PONOP-Ir derivative (100a) in 46 h (2,30,000 and 54,000, respectively). Similarly, Kang studied the reversible electrochemical conversion of CO2 and formate with a PONOP-Ir pincer complex (100a). This is the first electrocatalyst that can work for both reactions with high stability.243

860

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 56 Hydrogenation of CO2.

7.13.4.9

Hydrogenation of alkenes

As mentioned above, Huang and co-workers studied the hydrogenation of alkenes using EtOH and a NCP pincer iridium catalyst (87a) (Scheme 57).178 This was the first example that uses EtOH as hydrogen source in the transfer hydrogenation of unactivated alkenes. The hydrogenation of 1,3-butadiene and propylene was achieved with supported Ir(III) pincer complexes.244 Two catalysts were active in this transformation, however, the sulfated zirconia-supported (102a) achieved higher selectivity under the optimal conditions studied (80  C, 52 h, 2.5 mg cat.) than the analogous silica-supported. The sulfated zirconia-supported catalyst showed

Scheme 57 Hydrogenation of alkenes catalyzed by Ir pincer complexes.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

861

no formation of nanoparticles after the reaction (evaluated by HAADF and STEM) at 120  C while the silica-supported catalyst exhibited some leaching, which resulted in less selectivity. Therefore, the hypothesis regarding the catalytic species in the cycle is confirmed, since the formation of nanoparticles using the catalyst supported on silica are the real responsible of the hydrogenation of the alkenes.

7.13.4.10 Hydrogenation of benzoquinones and nitroarenes Jones and coworkers successfully achieved the catalytic hydrogenation of para-benzoquinones with a PCP-Ir iridium pincer catalyst (103a) using 1 atm of H2 (Scheme 58).172 After optimization of the reaction conditions (60–120  C, toluene, 0.5–1 mol% cat), they were able to hydrogenate several para-substituted benzoquinones in high yields (TON ca. 200). However, using substrates with the adequate geometry to form chelating ligands such as 2,3-dihydroxybenzoquinone and tetra-tert-butyldiphenoquinone were poorly hydrogenated (ca. 0.5%), probably due to the formation of stable complexes with the catalyst, turning it catalytically inactive.

Scheme 58 Hydrogenation of benzoquinones and nitroarenes.

Gelman presented the transfer hydrogenation of nitroarenes to anilines using excess of formic acid as hydrogen source, avoiding the use molecular hydrogen as reductant.245 The selectivity to aniline was also high when using the PCP-Ir pincer catalysts (104) and the reaction mixture was heated at 80  C (Scheme 58). After finding the optimal conditions for this transformation, the hydrogenation of electron-rich (such as p-nitroanilines) and electron poor (such as o- and p-nitrochlorobenzene) anilines was achieved in high selectivity and yields.

7.13.4.11 Hydroboration and carbonylation The transformation of CO2 to valuable feedstock is considered as one of the most important challenges in modern chemistry. In this sense, Rendón and Suárez reported the hydroboration of CO2 to form methoxyboranes under mild conditions.246 Methoxyboranes have been used as formate source for synthetic purposes, representing an alternative for the exploitation of CO2. The reduction of CO2 was carried out using a 1–2 bar pressure of CO2, catecholborane (HBcat) or pinacolborane (HBpin), and the tautomer C(carbene)NP-Ir pincer complex (105a) at 30  C (Scheme 59). Interestingly, the presence of water produced an increase in the catalytic activity, for example using 1 mol% of water a TON of 150 was obtained, while increasing the amount to 7 mol% the TON was higher, reaching 415.

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Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

Scheme 59 Hydroboration and carbonylation catalyzed by Ir pincer complexes.

Miller and coworkers described the salt-promoted catalytic methanol carbonylation in the presence of an Ir pincer complex bearing a crown ether fragment (106a) (Scheme 59).247 It is very well known that crown ether moieties can easily hold Lewis acids, which can be beneficial to some catalytic processes. So, they determined that the presence of some metal-based salts produce a positive effect in the catalytic performance of the Ir complex, the higher rates being observed using LiCl and HfCl4. Whereas, the presence of La3+ and Ce3+ salts inhibited the catalytic reaction. However, the intramolecular cation-crown interaction was not crucial for the catalytic performance, since the Ir analogous complex with a non-crown ether pincer ligand exhibited a similar activity when a salt and a crown-ether was added to the reaction.

7.13.4.12 Miscellaneous reactions Ir pincer complexes have been employed for other interesting catalytic reactions.132,248–255 For instance, Huang and coworkers used a series of symmetric and non-symmetric iridium pincer complexes for the dehydrogenation of alkyl azides to prepare nitriles in the presence or absence of a hydrogen acceptor (Scheme 60).256 This transformation results very interesting because allows the cyanide-free preparation of nitriles without carbon chain elongation. Interestingly, when using the asymmetric (PSCOP)Ir pincer complex (107b) as catalyst no hydrogen acceptor is needed.

Scheme 60 Dehydrogenation of primary azides.

Organometallic Pincer Complexes of Cobalt, Rhodium, and Iridium

863

The silane deuteration via CdD bond activation was described by Tobita and coworkers (Scheme 61).257 They deuterated a series of hydrosilanes with C6D6 at room temperature using a SiNN-Ir pincer complex (108a), which one arm is expected to be hemilabile. All the tris-alkyl substrates were deuterated in yields above 80 % and in relatively short times (1–72 h), however, dihydrosilane and siloxy substrates were poorly deuterated.

Scheme 61 Silane deuteration by Ir pincer complexes.

7.13.5

Conclusions

Although the pincer chemistry of cobalt can still be considered sort of new, it has had a tremendous development in recent years. The attractiveness of this area is not only rooted in cost of the metal itself, but in the potential that their pincer complexes may offer for organic transformations and bond activation. This produced in part by the not common one electron processes involved while using this metal complexes, but also the potential opportunities that can be produced by properly combine cobalt with the myriad of pincer ligands available, or even better, having this metal serving as motif for the further development of new functional, non-innocent pincer ligands. Thus, the pincer chemistry of cobalt shown in this paper represents only a glimpse of what may come in the next few years application wise. However, let’s not forget about rhodium and iridium, that have paved the way for cobalt to succeed on a very well-developed area found for these metals on pincer chemistry, were although many classic applications have been well developed, some others, based on the very well understanding of their mechanistics have been emerging in recent years, producing novel transformations and activation of some bonds either under milder conditions or with greater selectivity. In fact, it is logic to think that the pincer chemistry of cobalt may follow the same path as it was the case of their iridium and rhodium counterparts, finding in the future a lot of other possible applications based on proper understanding of their mechanistics, thus leading to a better understanding of the fundamental steps to further unleash, in a rational manner, the full potential of cobalt pincer derivatives. Thus, the future of the chemistry of pincer compounds of group 9 looks bright and promising, nicely mixing some traditional well-known chemistry with rhodium and iridium pincer derivatives and some new, surely rich and abundant pincer cobalt chemistry, opening a wide door of opportunities for future research.

Acknowledgments The financial support by PAPIIT-DGAPA-UNAM (PAPIIT IN210520) and CONACYT A1-S-33933 and FORDECYT-PRONACES FON.INST 22/2020 (FOINS 307152) is gratefully acknowledged. R. N. O.-Y. (CONACYT, Project ID: 98646, Clave: BP-PA20200603135404104-98646) thank CONACYT for a postdoctoral scholarship of the program Estancias Posdoctorales por México and E.R.F. would like to thank Programa de Becas Postdoctorales-DGAPA-UNAM for a postdoctoral scholarship (Oficio: CJIC/ CTIC/4851/2021). H. V. thanks CONACYT (CVU: 410706) and Generalitat de Catalunya (Beatriu de Pinós H2O2 MSCA-Cofund 2019-BP-0080).

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