Comprehensive Organometallic Chemistry IV. Volume 2: Groups 1 to 2 [2] 9780128202067

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COMPREHENSIVE ORGANOMETALLIC CHEMISTRY IV

COMPREHENSIVE ORGANOMETALLIC CHEMISTRY IV EDITORS-IN-CHIEF

GERARD PARKIN Department of Chemistry, Columbia University, New York, NY, United States

KARSTEN MEYER Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität, Erlangen, Germany

DERMOT O’HARE Department of Chemistry, University of Oxford, Oxford, United Kingdom

VOLUME 2

GROUPS 1 TO 2 VOLUME EDITOR

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

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

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

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

CONTENTS OF VOLUME 2 Editor Biographies

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Contributors to Volume 2

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Preface 2.01

xv

Introduction to Groups 1 to 2

1

Simon Aldridge

2.02

Organometallic Complexes of the Alkali Metals

5

Eva Hevia, Marina Uzelac, and Andryj M Borys

2.03

Organometallic Complexes of the Alkaline Earth Metals

71

Sharanappa Nembenna, Nabin Sarkar, Rajata Kumar Sahoo, and Sayantan Mukhopadhyay

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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 2 Simon Aldridge Department of Chemistry, University of Oxford, Oxford, United Kingdom Andryj M Borys Departement für Chemie, Biochemie und Pharmazie, Universität Bern, Bern, Switzerland Eva Hevia Departement für Chemie, Biochemie und Pharmazie, Universität Bern, Bern, Switzerland Sayantan Mukhopadhyay School of Chemical Sciences, National Institute of Science Education and Research (NISER), HBNI Bhubaneswar, India

Rajata Kumar Sahoo School of Chemical Sciences, National Institute of Science Education and Research (NISER), HBNI Bhubaneswar, India Nabin Sarkar School of Chemical Sciences, National Institute of Science Education and Research (NISER), HBNI Bhubaneswar, India Marina Uzelac Departement für Chemie, Biochemie und Pharmazie, Universität Bern, Bern, Switzerland

Sharanappa Nembenna School of Chemical Sciences, National Institute of Science Education and Research (NISER), HBNI Bhubaneswar, India

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

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Introduction to Groups 1 to 2

Simon Aldridge, Department of Chemistry, University of Oxford, Oxford, United Kingdom © 2022 Elsevier Ltd. All rights reserved.

Perhaps in a more noticeable way than for any other part of the Periodic Table, the chemistry of organometallic and related compounds of main group elements has undergone a dramatic transformation since the publication of the last edition of Comprehensive Organometallic Chemistry. In the period since 2006, significant advances in the fields of s- and p-block metal chemistry, in particular relating to low-valent compounds, and to systems offering two-site cooperative reactivity have led to seminal contributions in the fields of small molecule activation, and to promising (hitherto unimagined) applications in homogeneous catalysis. As such, patterns of useful chemical reactivity once thought to be the preserve of more ‘exotic’ transition elements have been transposed into the main group.1 Main group metals have traditionally been regarded as ‘poor relations,’ both with respect to the lighter elements of the p-block (witness the diversity, functionality and versatility of carbon chemistry) and the metals of the d-block, for which powerful applications in catalytic processes for the construction of new chemical bonds have long been recognized. In particular, the prevalence of main group compounds having wide HOMO-LUMO gaps - and the consequent lack of frontier orbitals of the correct symmetry occurring in an appropriate energy range to interact with small molecule substrates—had long been identified as a critical shortcoming for numerous chemical applications. Moreover, the associated restriction in redox and coordinative flexibility had seemed to rule out the opportunity for carrying out the sorts of fundamental chemical steps (oxidative addition, reductive elimination, insertion/metathesis processes) central to a wide range of catalytic processes familiar from the chemistry of d-block elements. More recently however, advances in ligand design leading to the development of strongly electron releasing, highly sterically encumbered ancillary donors, has allowed for the synthesis of a broad range of coordinatively unsaturated main group species. Moreover, the isolation of heavier p-block compounds (in particular) featuring geometric and electronic structures which diverge from their lighter (n ¼ 2) congeners has been shown to offer an exciting paradigm for the binding and activation of industrially relevant small molecule substrates (e.g., H2, alkenes/alkynes, CdH bonds, CO, N2, and NH3). Within this sphere, a key development was reported towards the very end of the period covered by Comprehensive Organometallic Chemistry III. Power and co-workers—making use of the very sterically demanding terphenyl family of ligands which they pioneered—had developed a viable chemical strategy for the isolation of heavier group 14 analogues of alkynes. Among these geometrically distinct (trans bent) systems, a digermyne was shown in 2005 to be capable of the room temperature activation of dihydrogen (Scheme 1).2 This compound represented the first system from the s or p-blocks of the Periodic Table to show such capabilities. While this particular molecule yields a mixture of mono-, di- and trihydride products (depending on the hydrogenation conditions) the frontier orbital basis for the cleavage of the HdH bond was readily understood by elegant comparison with the models of sigma donation/pi back-bonding developed for the transition metal activation of H2 (Fig. 1). As such, the starting gun was fired on the development of main group metal systems capable of binding/activating related (isolobal) small molecule substrates, and this area has subsequently become a major theme in main group chemistry. The initial unsymmetrical addition of H2 at one of the germanium metal centers to give RGeGe(H)2R implied by these orbital considerations could subsequently be demonstrated for a related digermyne system,3 and reversibility in the activation of H2 even shown for a related distannyne.4 An alternative frontier orbital platform for the activation of small molecules based on a single element center involves (singlet) carbene-like systems, which possess a lone pair and orthogonal formally vacant p-type orbital (Fig. 1C). The activation of H2 (and furthermore NH3) by carbene compounds themselves5 was mirrored by the development of silylene, germylene and stannylene systems possessing a similar orbital manifold (and small HOMO-LUMO separations) and which cleave dihydrogen at a single site to give the corresponding E(IV) dihydrides.6–8 The ability of other small molecules such as alkenes and CO to bind to metal centers via superficially similar orbital interactions to H2 suggested that main group compounds featuring other archetypal organometallic ligands such as ethene or carbon monoxide might also be viable. In due course examples of both di- and mononuclear systems featuring coordination of ethene were demonstrated crystallographically (Fig. 2).9,10 The binding of CO by main group metal/metalloid systems in ‘transition metallike’ fashion (notwithstanding ‘simple’ sigma-only binding to highly electrophilic boranes),11 has been established for non-metal systems such as carbenes and phosphinidenes,12,13 and by transient systems such as borylenes and silylenes,14–16 which possess a suitably narrow HOMO-LUMO separation. Orbital analogies also suggest that main group compounds capable of the activation of even more challenging substrates, such as CdH bonds and dinitrogen ought to be accessible. The activation of the CdH bond in benzene was one of the first to be demonstrated in intermolecular fashion by a d-block system,17 and the corresponding oxidative addition reaction (and the analogous CdC cleavage process) was recently demonstrated for a main group metal.18,19 Dinitrogen capture and transformation has also been achieved, with Braunschweig exploiting the small HOMO-LUMO separation in a transient borylene to capture (and indeed couple) N2.20,21 Moreover, such chemistry is also not the exclusive preserve of p-block systems. The use of reductive methodologies has also led to the synthesis of tractable MdM bonded M(I) compounds within group 2, focusing on magnesium (Fig. 3).22 Related calcium chemistry undertaken in the presence of N2, gives rise to a bridging [N2]2− complex, the structure of which has obvious parallels with early d-block and f-element chemistry.23

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00175-X

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Introduction to Groups 1 to 2

Scheme 1 Room temperature activation of dihydrogen by a molecular main group species (Dipp ¼ 2,6-iPr2C6H3).2

(A)

(B)

(C)

Fig. 1 Key frontier orbital interactions in the activation of dihydrogen (A) by a generic transition metal complex; (B) by a trans bent digermyne; (C) by a carbene (or heavier group 14 analogue).

Fig. 2 Reversible uptake of ethene by a distannyne (Trip ¼ 2,4,6-iPr3C6H2).9

Fig. 3 Dimeric magnesium(I) (left) and calcium dinitrogen complexes (right) synthesized via reductive methodologies (Dpp ¼ C6H3(CHEt2)2-2,6).

Introduction to Groups 1 to 2

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Fig. 4 Heterolytic cleavage of dihydrogen by a single component phosphine/borane Frustrated Lewis Pair (FLP).

Ammonia is often cited as an example of a substrate which is difficult to activate using d-block systems, in part due to the unfavorable coordination equilibrium, which typically leads to the formation of classical Werner complexes rather than NdH oxidative addition. While NdH bond cleavage by transition metal systems therefore remains very rare,24 a number of low-valent main group systems have been shown to be capable of analogous activation chemistry.5,25 Quantum chemical and experimental studies suggest that a coordination/proton migration pathway offers an alternative mechanistic route which yields the E(H) NH2 function.7,8,26 No account of the chemistry of the main group elements in the past 15 years would be complete without considering the development of Frustrated Lewis Pair (FLP) chemistry. The combination of a unquenched Lewis acid and Lewis base offers an alternative strategy for the design of a system bearing a low-lying LUMO and energetically elevated HOMO. While the realization that steric factors could lead to the uncoupling of a classical Lewis adduct dates from the work of HC Brown,27 the application of such systems to the heterolytic cleavage of H2, and subsequently to the capture and activation of a range of other small molecules (and ultimately to metal-free catalysis) was pioneered from 2006 onwards by Stephan and Erker (Fig. 4).28 Catalytic processes employing FLPs have typically made use of the heterolytic H2 cleavage process brought about by a phosphine/borane FLP enable the delivery of H+/H− to polar substrates, and so effect hydrogenation. Wider application of this strategy has led to the development of a raft of new metal-free catalytic reactions, albeit typically (as yet) with lower performance than traditional d-block systems.29 More generally, catalysis has been viewed as the driver for the development of a range of new main group organometallic systems, endeavoring to take main group catalysis from the realm of simple Lewis acids, to single site constant oxidation state processes based on insertion/metathesis, and even to systems relying on redox shuttling at the main group metal. In this regard, a recently reported system operating via a Bi(III)/Bi(V) shuttle represents a significant breakthrough in the field.30,31 This volume of Comprehensive Organometallic Chemistry IV incorporates the contents of two volumes from the previous iteration—namely those focusing on (i) groups 1, 2, 11 and 12; and (ii) groups 13–15. In addition, given recent developments in the field it also includes chapters that are completely new to the fourth edition—such as the chapter focusing on Frustrated Lewis Pairs written by Campos and co-workers.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Power, P. P. Nature 2010, 463, 171–177. Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 12232–12233. Li, J.; Schenk, C.; Goedecke, C.; Frenking, G.; Jones, C. J. Am. Chem. Soc. 2011, 133, 18622–18625. Wang, S.; Sherbow, T. J.; Berben, L. A.; Power, P. P. J. Am. Chem. Soc. 2018, 140, 590–593. Frey, G.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439–441. Protchenko, A. V.; Birjkumar, K. H.; Dange, D.; Schwarz, A. D.; Vidovic, D.; Jones, C.; Kaltsoyannis, N.; Mountford, P.; Aldridge, S. J. Am. Chem. Soc. 2012, 134, 6500–6503. Peng, Y.; Guo, J.-D.; Ellis, B. D.; Zhu, Z.; Fettinger, J. C.; Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2009, 131, 16272–16282. Protchenko, A. V.; Bates, J. I.; Saleh, L. M. A.; Blake, M. P.; Schwarz, A. D.; Kolychev, E. L.; Thompson, A. L.; Jones, C.; Mountford, P.; Aldridge, S. J. Am. Chem. Soc. 2016, 138, 4555–4564. Peng, Y.; Ellis, B. D.; Wang, X.; Fettinger, J. C.; Power, P. P. Science 2009, 325, 1668–1670. Rodriguez, R.; Gau, D.; Kato, T.; Saffon-Merceron, N.; De Cózar, A.; Cossío, F. P.; Baceiredo, A. Angew. Chem. Int. Ed. 2011, 50, 10414–10416. Burg, A. B.; Schlesinger, H. I. J. Am. Chem. Soc. 1937, 59, 780–787. Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Angew. Chem. Int. Ed. 2006, 45, 3488–3491. Hansmann, M. M.; Bertrand, G. J. Am. Chem. Soc. 2016, 138, 15885–15888. Braunschweig, H.; Dewhurst, R. D.; Hupp, F.; Nutz, M.; Radacki, K.; Tate, C. W.; Vargas, A.; Ye, Q. Nature 2015, 522, 327–330. Ganesamoorthy, C.; Schoening, J.; Wölper, C.; Song, L.; Schreiner, P. R.; Schulz, S. Nat. Chem. 2020, 12, 608–614. Reiter, D.; Holzner, R.; Porzelt, A.; Frisch, P.; Inoue, S. Nat. Chem. 2020, 12, 1131–1135. Green, M. L. H.; Knowles, P. J. J. Chem. Soc. Chem. Commun. 1970, 1677. Hicks, J.; Vasko, P.; Goicoechea, J. M.; Aldridge, S. Nature 2018, 557, 92–95. Hicks, J.; Vasko, P.; Goicoechea, J. M.; Aldridge, S. J. Am. Chem. Soc. 2019, 141, 11000–11003. Légaré, M.-A.; Bélanger-Chabot, G.; Dewhurst, R. D.; Welz, E.; Krummenacher, I.; Engels, B.; Braunschweig, H. Science 2018, 359, 896–900. Légaré, M.-A.; Rang, M.; Bélanger-Chabot, G.; Schweizer, J. I.; Krummenacher, I.; Bertermann, R.; Arrowsmith, M.; Holthausen, M.; Braunschweig, H. Science 2019, 363, 1329–1332. Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754–1757. Rösch, B.; Genter, T. X.; Langer, J.; Färber, C.; Eyselein, J.; Zhao, L.; Ding, C.; Frenking, G.; Harder, S. Science 2021, 371, 1125–1128. Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080–1082.

4

25. 26. 27. 28. 29. 30. 31.

Introduction to Groups 1 to 2

Peng, Y.; Ellis, B. D.; Wang, X.; Power, P. P. J. Am. Chem. Soc. 2008, 130, 12268–12269. Brown, Z.; Guo, J.-D.; Nagase, S.; Power, P. P. Organometallics 2012, 31, 3768–3772. Brown, H. C.; Schlesinger, H. I.; Cardon, S. Z. J. Am. Chem. Soc. 1942, 64, 325–329. Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124–1126. Stephan, D. W.; Erker, G. Angew. Chem. Int. Ed. 2015, 54, 6400–6441. Dunn, N. L.; Ha, M.; Radosevich, A. T. J. Am. Chem. Soc. 2012, 134, 11330–11333. Planas, O.; Wang, F.; Leutzsch, M.; Cornella, J. Science 2020, 367, 313–317.

2.02

Organometallic Complexes of the Alkali Metals

Eva Hevia , Marina Uzelacb, and Andryj M Borysa, aDepartement für Chemie, Biochemie und Pharmazie, Universität Bern, Bern, Switzerland; bEastCHEM School of Chemistry, University of Edinburgh, Edinburgh, United Kingdom a

© 2022 Elsevier Ltd. All rights reserved.

2.02.1 2.02.2 2.02.2.1 2.02.2.1.1 2.02.2.1.2 2.02.2.1.3 2.02.2.1.4 2.02.2.1.5 2.02.2.2 2.02.2.3 2.02.2.4 2.02.2.5 2.02.2.6 2.02.2.7 2.02.3 2.02.3.1 2.02.3.2 2.02.4 2.02.5 References

Introduction Structural diversity of alkali metal organometallics Alkyl derivatives Methyl, propyl and butyl Secondary and tertiary alkyl derivatives Benzyl, diphenyl, and triphenylmethane derivatives Silyl substituted alkyl derivatives Phosphorus and sulfur substituted alkyl derivatives Aryl derivatives Cyclopentadienide, indenide and fluorenide derivatives Alkali metal interactions with p-systems Alkynyl derivatives Ylide, yldiide, and methandiide derivatives Metallocene derivatives Assessing aggregation of alkali metal organometallics using NMR spectroscopy DOSY NMR studies Solution constitution of lithium amides Mixed alkyl/alkoxide aggregates of alkali metals Summary

6 7 7 8 13 14 18 27 33 38 40 45 46 57 59 59 60 62 64 64

Abbreviations 12-c-4 15-c-5 18-c-6 2c2e 2-MeTHF 3c2e Ad Am bipy CCP COT Cp Cp Cy DABCO DFT Dipp dman DMDAC DMDAP DME DOPT DOSY DPE dpp-bian ECC Et2O Flu HMDS(H)

12-Crown-4 15-Crown-5 18-Crown-6 Two-center-two-electron 2-Methyltetrahydrofuran Three-center-two-electron Adamantyl Amyl ¼ pentyl 2,20 -Bipyridine Cycloparaphenylene Cyclooctatetraene Cyclopentadienide/cyclopentadienyl Pentamethylcyclopentadienide Cyclohexyl 1,4-Diazabicyclo[2.2.2]octane Density Functional Theory 2,6-Diisopropylphenyl 2-Dimethylamino-methyl-naphthalene 1,3-Dimethyl-1,3-diazacyclohexane 1,3-Dimethyl-1,3-diazacyclopentane 1,2-Dimethoxyethane 5,6,11,12-Di-o-phenylene-tetracene Diffusion Ordered Spectroscopy Diphenoxyethane 1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphthene External Calibration Curve Diethyl ether Fluorenide/fluorenyl Hexamethyldisilazane

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00160-8

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6

Organometallic Complexes of the Alkali Metals

HOESY hppH i Bu ICC IMe Ind i Pr LDA LUMO Me Me3TACD Me6TREN Mes n Bu NHC NMR Np OMe OPTBCOT PAH Pent PGSE Ph PMDETA PMP Py rac SMP t Bu TECDA TEEDA TEMCDA THF THP Tipp TBME TMCDA TMEDA TMP TMPDA TMTAC Tol TtBuTAC

2.02.1

Heteronuclear Overhauser Enhancement Spectroscopy 1,3,4,6,7,8-Hexahydro-2Hpyrimido[1,2-a]pyrimidine Iso-butyl Internal Calibration Curve 1,3,4,5-Tetramethylimidazole-2-ylidene Indenide/indenyl Iso-propyl Lithium diisopropylamide Lowest unoccupied molecular orbital Methyl 1,4,7-Trimethyl-1,4,7,10-tetraazacyclododecane Tris[2-(dimethylamino)ethyl]amine Mesityl ¼ 2,4,6-trimethylphenyl n-Butyl N-Heterocyclic carbene Nuclear Magnetic Resonance Neopentyl Methoxy Octaphenyltetrabenzocyclooctatetraene Polyaromatic hydrocarbon Pentyl Pulsed Gradient Spin-Echo Phenyl N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine 2,2,4,6,6-Pentamethylpiperidide Pyridine Racemic (S)-Methoxypyrrolidine Tert-butyl N,N,N0 ,N0 -Tetraethylcyclohexane-1,2-diamine N,N,N0 ,N0 -Tetraethylethylenediamine N0 ,N0 ,N00 -Triethyl-N00 -methylcyclohexane-1,2-diamine Tetrahydrofuran Tetrahydropyran 2,4,6-Triisopropylphenyl Tert-butyl methyl ether N,N,N‘,N‘-Tetramethylcyclohexane-1,2-diamine N,N,N0 ,N0 -Tetramethylethylenediamine 2,2,6,6-Tetramethylpiperidide N,N,N0 ,N0 -Tetramethyl-1,3-propanediamine 1,3,5-Trimethyl-1,3,5-triazacyclohexane Toluene Tri-tert-butyl-1,3,5-triazacyclohexane

Introduction

Epitomized by commercial organolithium reagents, Group 1 organometallic complexes have long played a pivotal role in synthesis.1 Utilized in chemistry laboratories worldwide, they are especially in high demand for functionalizing unsaturated organic molecules and so are indispensable in the chemical industry (e.g., for the manufacture of agrochemicals, electronic materials, pharmaceuticals and other medicines, and fine chemicals in general).2,3 Part of their popularity lies in their high reactivity, as a consequence of the large polarity of their metal-carbon bonds, making the use of these reagents ubiquitous in a multitude of carbon-carbon bond forming processes as well as in metallation chemistry (via Li-halogen exchange or deprotonative metallation).4,5 In contrast, with the exception of Lochmann-Schlosser superbase combinations,6 applications in organic chemistry of the heavier alkali-metal organometallics have been significantly less developed. This can be attributed to the lower stabilities and solubilities of these more polar compounds, which places restrictions on the conditions necessary for their employment. However, new unconventional alkali-metal mediated applications have emerged that hint at the vast potential that these systems might offer with future development.7 For instance, arylsodium compounds, obtained by reaction of aryl chlorides with sodium dispersion,

Organometallic Complexes of the Alkali Metals

7

have been found to be valuable precursors in palladium catalyzed cross-coupling reactions,8 while direct sodiation of aromatic molecules can be accomplished in continuous flow chemistry using a hexane-soluble alkylsodium reagent.9 Although the number of studies that systematically compare the reactivity of a whole series of Group 1 (Li-Cs) organometallic compounds is scarce, a clear alkali-metal effect has been noticed in transfer hydrogenation reactions of alkenes.10 Interestingly, and counter-intuitively, sodium and potassium appear to outperform lithium in several of these new studies.11,12 At the core of these innovations, is the close structure/reactivity interplay in Group 1 organometallics. Studies shedding light on the role of solvent effects and their influence on the aggregation and constitution of these compounds,13 as well as advancing the understanding on their preferred modes of interaction with the relevant organic substrates14 have fast forwarded the development of new synthetic applications under conditions traditionally considered forbidden for Group 1 organometallics. This includes, for example, the use of organolithium compounds under air, at room temperature using bioinspired solvents such as Deep Eutectic Solvents (DES), glycerol or water, where the kinetic activation of the RLi reagents allows fast and chemoselective reactivities to be realized towards a range of unsaturated substrates such as ketones, imines and nitriles, among others.15–18 While for decades nearly all of the applications of these types of organometallic compounds were confined to stoichiometric reactions, alkali-metal complexes have already shown excellent potential in catalysis,19 emerging as sustainable alternatives to precious transition metal catalysis in different hydroelementation reactions. Focussing on structural and bonding aspects, this chapter summarizes advances made in alkali-metal organometallic chemistry, an area renowned for its large diversity of structures. Adopting a systematic approach, the discussion is grouped according to the different types of carbon-based ligands present in complexes whose structures have been authenticated by X-ray crystallographic techniques. Underpinning these diverse structural motifs, it is the strong tendency of these compounds towards aggregation which is directly related to the nature of the carbon-based ligand, and the participation of Lewis donors, as well as the identity of the specific alkali-metal. Experimental conditions of temperature and concentration, as well as the donor ability of the solvent should also be taken into account when assessing the composition of Group 1 organometallics in solution. Increasing use of the two-dimensional technique Diffusion-Ordered NMR Spectroscopy (DOSY)20 has allowed for the investigation and estimation of solvation and aggregation states of alkali-metal organometallic compounds.21,22 A selection of representative studies using DOSY NMR in combination with other NMR experiments is covered in this chapter to illustrate how these techniques can inform on the solution constitution of Group 1 organometallic and amide complexes, including Lochmann-Schlosser superbase combinations. These studies outline the development of DOSY NMR as a valuable probe in organolithium chemistry, particularly during the past decade that enable the confident estimation of the molecular weight (MW) of several Group 1 metal complexes in solution and thus help guide reaction mechanisms.

2.02.2

Structural diversity of alkali metal organometallics

Organolithiums are ubiquitous reagents in preparative chemistry and since the beginning of investigations into their structures in the 1960s, many of their physical and chemical properties became comprehensible.23 A prominent structural feature of these reagents is their tendency for oligomerisation both in solution and the solid state. The root of this affinity is in the electron deficiency of a single LiR molecule where the number of valence electrons is not sufficient for two-electron two-center bonding with all available Li valence orbitals. To compensate, aggregates with multicenter bonds (e.g., two-electron four-center) are formed. The rich structural diversity in the types of aggregates include monomers, dimers (head-to-tail arrangement of monomers), trimers (rings), tetramers (heterocubanes and rings), hexamers (pseudo-octahedra), decamers, dodecamers (stacked rings) and finally, chain polymers. The most decisive role in determining the type of aggregate formed are the nature of the ligand, the solvent, and the presence and nature of additional neutral donors. However, concentration and temperature also have a significant role, especially in terms of complex solution-state equilibria which include both intramolecular bond fluctuation and intermolecular exchange. One of the key features of organolithiums, that perhaps helped enhance their popularity, and set them apart from their heavier alkali-metal analogs or alkaline-earth reagents such as Grignard reagents, is their solubility in hydrocarbon solvents. While it is certainly correct that covalent character contribution to the MdC bond is higher for Li than any other alkali metal, structural studies have shown that agostic type intermolecular forces influence the solubility of a given organolithium in nonsolvating media. For that reason, although MeLi and tBuLi adopt very similar solid state structures, the stronger intermolecular forces in MeLi make it insoluble in nonsolvating media, as opposed to the soluble tBuLi. The heavier alkali-metal organometallics are still lagging behind organolithiums. The difficulties in handling these reagents primarily arise from their higher, often erratic, reactivity, and poor solubility, attributed to the larger ionic radii which allow for greater hapticity promoting 2D and 3D aggregates. This section will introduce the new findings in the structural diversity of alkali-metal organometallics, with these general characteristics in mind.

2.02.2.1

Alkyl derivatives

The alkyl derivatives are presented in order of their complexity starting with the simplest all-carbon alkyls to heteroatom substituted alkane frameworks. The majority of presented structures are organolithiums, however we will see that the more elaborate ligand systems (e.g., Si-substituted alkyls) can help increase the solubility and lead to the isolation of heavier alkali-metal complexes. The employed donor systems are various, covering a range of denticities, donor atoms and flexibility, but an emerging interest is the

8

Organometallic Complexes of the Alkali Metals

incorporation of chiral ligand scaffolds in order to access enantiomerically rich organolithium reagents. Finally, a significant amount of work, supported by the isolation and structural elucidation of organometallic intermediates, has shown that the regularly employed donor ligands are not always inert to the strongly basic alkyl-lithiums.

2.02.2.1.1

Methyl, propyl and butyl

Several new adducts of methyl-lithium have been recently characterized, including [(MeLi)
Et2O]4 (1),24 an adduct with a racemic mixture of (S,S)-TMCDA and (R,R)-TMCDA [MeLi
trans-TMCDA]2 (2),24 and [(MeLi)4(TMTAC)3]1 (4).25 While 2 is isostructural with its chiral counterpart 17 (vide infra), 1 is structurally analogous to the previously obtained THF adduct26 giving rise to a Li4 tetrahedron in which each Li3-face is m3-capped by the methyl group, and each Li-atom is additionally apically coordinated by the diethyl ether solvent. In 4 a chain adduct of (MeLi)4 tetramers is formed and as such can be viewed as intermediate aggregation between the 3D-polymeric network of pure crystalline MeLi27 and the solvated heterocubane tetramers such as 1. The three non-connected Li-atoms are saturated by binding to only one of the three N-atoms of the terminal TMTAC ligands (Fig. 1).25 Solid state structures of donor-unsupported [{iBuLi}6] (5)28 and with commonly used diamine ligands [iPrLi
TMEDA]2 (6),29 i [( PrLi)3
(TEEDA)2] (7)29 and [iPrLi
(R,R)-TECDA] (8)29 have been reported. Determined by the means of X-ray powder diffraction, 5 contains distorted Li6 octahedra where six of the eight Li3 faces are capped by iso-butyl groups arranged in a propeller-like orientation. Dictated by the steric demands of the ligand, 6–8 show transitions from dimeric 6 through to non-symmetrical 7 with an uneven number of lithium units, to monomeric 8 (Fig. 2).29 6 comprises a central Li2C2 four-membered ring with only slight deformation from planarity and is comparable to other known dimeric lithium alkyl structures.30 In 7, the central Li2C2 four-membered ring is now linked with another isopropyl unit and the structure of 7 can formally be considered as an isopropyl-lithium dimer into which an additional molecule of iPrLi has been inserted. In monomeric 8, the Li-center exhibits three contacts—two to the N-atoms of the ligand and one to the carbanionic center of the iso-propyl group. Close inspection of

Fig. 1 Tetrameric structure of 4.

Fig. 2 Structures of diamine-ligated iPrLi complexes 6–8.

Organometallic Complexes of the Alkali Metals

9

7 and 8 revealed that the arrangement of the Et groups on N-atoms of the TEEDA and TECDA ligands influences the degree of aggregation. In 7 one Et group of each N-atom is arranged in the direction of the iso-propyl-lithium molecules and one towards the ligand, whereas in 8 none of Et groups are directed towards the cyclohexane backbone, as such an orientation would lead to strong repulsion with the axial H-atoms of the ring. New examples of structurally authenticated butyl-lithium derivatives (Fig. 3) include [{(nBuLi)
(1,2-dipiperidinoethane)}2] (9),31 [{(nBuLi)2
PMDETA}2] (10),32 [tBuLi
TEEDA] (11)33 [(tBuLi)2
(N,N0 -dimethylpiperazine)]1 (12),34 as well as complexes with cyclic triaminals [(tBuLi)3
TMTAC] (13),35 [(TtBuTAC)(tBuLi)] (14)36 and [(TtBuTAC)(nBuLi)2]2 (15).36 While both 9 and 10 exhibit dimeric structures with a central Li2C2 core, the differing denticity of the donors employed affect the ratio of ligand to nBuLi units, causing structural differences. In 9, employing a bidentate ligand gives rise to a 1:1 ratio and a structure with a ligand-capped Li2(nBu)2 core, typical for dimeric alkyl-lithium compounds,30 as for instance found in previously reported TMEDA26 and (−)-sparteine37 supported complexes. In 10,32 employing a tridentate ligand induces a 1:2 ratio affording a structure in which two capping monomeric nBuLi
PMDETA units have short Li⋯C contacts to the Li-centers of the central Li2(nBu)2 unit. 12 is the first coordination polymer of tBuLi with zig-zag arranged chains built by tBuLi dimers which are coupled through two ligand molecules. Both 11 and 14 are monomeric adducts, but incorporating diamine TEEDA in 11,33 Li is tricoordinate similar to previously reported tBuLi
(−)-sparteine complex,38 while 1436 incorporates cyclic triaminal 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane and the Li center is tetracoordinate. Decreasing the steric bulk of donor ligand, utilizing 1,3,5-tri-methyl-1,3,5-triazacyclohexane (TMTAC) affords a coordination complex 1335 where the three molecules of tBuLi are coordinated by the tridentate ligand giving a highly symmetrical molecule (Fig. 4). The three Li atoms form an almost equilateral triangle and each has three contacts: two to the carbanion centers and one

Fig. 3 Selected examples of donor-supported nBuLi and tBuLi complexes.

Fig. 4 Molecular structure of coordination complex 13.

10

Organometallic Complexes of the Alkali Metals

to the N-atom of the ligand. Unlike in oligomeric organolithiums such as 1 where Li3 faces are m3-capped by the carbanion, in 13 tBu anions coordinate to only two Li-atoms thus forming a six-membered Li-C ring. Employing sterically less demanding alkyl-lithium, affords dimeric 1536 which consists of a tetrameric ladder-type nBuLi fragment with two TtBuTAC ligands bonded at the two outer Li atoms. Slowly decaying 15 transforms into [TtBuTAC-Li (m-nBu)]2[Li3(nBu)2N(tBu)(n-pent)] (16)36 with central [Li3(nBu)2N(tBu)(n-pent)] unit in a ladder-type arrangement. The complexation of chiral donor ligands (Fig. 5) with alkyl-lithium bases has become an important tool for the generation of enantiomerically enriched alkyl-lithiums. One of the most utilized chiral donors is (R,R)-TMCDA which gives access to [MeLi
(R,R)-TMCDA]2 (17),39 [iPrLi
(R,R)-TMCDA]2 (18),39 [nBuLi
(R,R)-TMCDA]2 (19),40 [(nBuLi)2
(R,R)-TMCDA]2 (20),40 [sBuLi
(R,R)-TMCDA] (21),39 and [tBuLi
(R,R)-TMCDA] (22).34 While adducts 17–19 are again dimeric with a central four-membered Li2C2 ring (vide supra), 20 adopts a ladder-type arrangement of three four-membered Li2C2 rings which show deformation from planarity and are not arranged rectangularly to each other as seen in 15. The two external Li-centers are capped by the two (R,R)-TMCDA molecules and have further two contacts to the carbanion centers, whilst the internal Li-centers possess only three contacts to the carbanion centers. An isomer of 20 has been isolated when racemic amine was used with the only difference being the orientation of butyl units.40 Monomeric 21 and 22 display the expected shortened Li-C and Li-N distances compared to those found in comparable dimeric compounds,30 and the space filling model shows that the Li-center is barely shielded by the ligand and positively polarized. Having a free coordination site for electrophiles makes these complexes highly reactive. Closely

Fig. 5 Representative structural arrangements for alkyl-lithium complexes supported by chiral donor ligands.

Organometallic Complexes of the Alkali Metals

11

related tBuLi adducts of Et-substituted and mixed Me/Et substituted derivatives of TMCDA, namely [tBuLi
(R,R)-TECDA] (23)41 and [tBuLi
(R,R)-TEMCDA] (24)41 have also been structurally characterized as monomeric in the solid state.41 Homodimeric methyl-lithium complexes of (−)-sparteine 2542 and (+)-sparteine surrogate (1R,2S,9S)-11-methyl-7,11-diazatricyclo[7.3.1.02,7]tridecane 2642 and bidentate allylic amine (S)-methoxypyrrolidine (SMP) 2743 display a structural motif with the central Li2C2 ring and the Li-center further chelated by the ligand. On the contrary, related N,N0 -bismethylated 2,7-diazabicyclo [4.4.1]undecane in 2844 does not bind lithium in a bidentate fashion, but bridges between two different lithium atoms of neighboring MeLi cubic aggregates. Utilizing N,O-functionalized silicon-stereogenic silane as a donor ligand afforded monomeric, chiral tBuLi complex 29,45 where the asymmetry of SC,SSi-configured tridentate ligand was able to force a single and precise configuration (RLi) at the metal center. Bonded through two O- and one N-atom, the Li-center is found in a distorted tetrahedral coordination environment. Mixed anionic, enantiopure (+)-(1S,2S,4R)-anisyl fencholate MeLi aggregate 3046 adopts a solid state structure with a 3:1 stoichiometry of the lithium fencholate/MeLi ratio giving rise to a pseudo-C3 propeller-like arrangement. The central structural motif is a distorted cubic Li4O3C1 core in which three Li-centers bind to the methylide unit. Methoxy-free aryl fencholate aggregate of nBuLi 3147 maintains the 3:1 aggregate stoichiometry and the Li4O3C1 core, but the endomethyl groups of the bicyclic [2,2,2]-heptane moieties compensate for the missing methoxy coordination through close contacts to the butylide-binding Li-atoms. The ligand systems used to coordinate to alkyl-lithium bases are not always inert and can undergo a-lithiation and b-deprotonation of tertiary methyl and ethyl amines (Fig. 6). Examples of a-lithiated TMEDA 32,33 PMDETA 33,32 (R,R)-TMCDA 3434 and racemic trans-TMCDA 3548 have all been structurally authenticated. In the solid state 32 and 35 exist as a S4 symmetric tetramer giving rise to a distorted Li-C eight-member ring.33 In racemic 35 both (R,R)- and (S,S)-TMCDA alternate, however moving to enantiopure (R,R)-TMCDA in 34 resulted in a structural change and 34 is isolated as a trimer. 33 adopts a centrosymmetric dimeric arrangement with a central N2C2Li2 ring in the chair conformation.32 Unlike in 32 and 34–35 where each Li-center coordinates to two N-atoms and two carbanions, in 33 each Li-center interacts with three N-atoms and only one carbanion. Rather than undergoing a second metallation, 34 forms a co-complex with tBuLi (36),48 which adopts a dimeric structure with three Li2C2 four-membered rings connected by one common edge.

Fig. 6 Molecular structures of selected examples of a-lithiated diamines: (A) TMEDA (32); (B) PMDETA (33); (C) (R,R)-TMCDA (34) and (D) (R,R)-TMCDA co-complexed with tBuLi (36). Hydrogen atoms except on metalated C omitted for clarity.

12

Organometallic Complexes of the Alkali Metals

Apart from the disfavored decomposition of donor ligand, this can also serve as a synthetic route to amino-functionalized organometallics. Examples includes complexes 37 and 38 (Fig. 7), obtained from the reaction of lithiated TMEDA or PMDETA with chloro(dimethylvinyl)silane followed by carbolithiation. This sequential route allowed for selective synthesis of a-silylorganolithium reagents with longer alkyl chains next to the metallated carbon center—compounds which are generally very difficult to access.49 Cyclic polyamines 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC),50 1,3-dimethyl-1,3-diazacyclohexane (DMDAC)50 and 1,3-dimethyl-1,3-diazacyclopentane (DMDAP)51 can be deprotonated by tBuLi exclusively at the position between the two N-atoms affording complexes 39,50 4050 and 41,51 respectively. 39 and 41 (Fig. 8) consist of two equivalents of the deprotonated species and one equivalent of the free polyamine ligand affording a chain aggregate of dimers of the lithiated product linked by a non-lithiated aminal molecule, while 40 is tetrameric and co-crystallizes with tBuLi tetramers. Similar outcomes are observed in systems with two cyclic units joined via methylene groups such as bis(3-methyl-1,3diazacyclohex-1-yl)methane and bis(3-methyl-1,3-diazacyclopent-1-yl)methane. Here, doubly lithiated dimers H2C[NCH(Li)NMe(CH2)3]2 (42)52 and H2C[NCH(Li)NMe(CH2)2]2 (43)52 are obtained. A partially lithiated product 44,52 which is an aggregate that contains two units of the singly deprotonated substrate molecule and a non-reacted equivalent of tBuLi, was also isolated and can be seen as an intermediate on the way to 42. Examples of molecules undergoing a reaction with an alkyl-lithium followed by co-complexation have been extended to [{(iPr3Si)2SiLi2}2
2tBuLi] (45),53 [Li3(nBu)(Me3TACD)2] (46)54 or [Li4(nBu)2(Me3TACD)2] (47)54 and [{dpp-bian(nBu)Li}n BuLi]2 (48).55 45 represents the first known co-aggregate of a silyl-lithium and an alkyl-lithium—consisting of four molecules; two gem-dilithiosilanes, (iPr3Si)2SiLi2, and two tBuLi molecules all obtained from the lithiation of a single molecule (Fig. 9). The central structural motif of 45 is a unique Li6 core composed of two distorted tetrahedra sharing a common Li-Li edge. Each (iPr3Si)Si group bridges simultaneously to two triangular faces formed by four Li-atoms, while each tBu group bridges only two lithium atoms.53

Fig. 7 Structures of a-silylorganolithium reagents 37 and 38.

Fig. 8 Structures of complexes 41, 42 and 44.

Fig. 9 Molecular structure of complex 45.

Organometallic Complexes of the Alkali Metals

13

Addition of excess nBuLi to cyclic polyamine 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane or to diimine 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) affords complexes 46, 47,54and 4855 respectively. 46 and 47 are obtained by co-complexation of lithiated cyclic polyamine [Li2(Me3TACD)2] with nBuLi in different stoichiometries, and both show a ladder-type motif, while 48 (Fig. 10) is formed by alkylation of the diimine affording an amido-imino ligand that then co-complexes with nBuLi exhibiting a dimeric structure.55

2.02.2.1.2

Secondary and tertiary alkyl derivatives

In the solid state, unsolvated cyclopentyl-lithium (49)56 exists as a discrete hexameric aggregate (Fig. 11A) similar to n-butyllithium57 or cyclohexyl-lithium,58 where the six Li-atoms are arranged in a trigonal anti-prismatic conformation with each of the six faces m3-capped by a single cyclopentyl anion. Addition of donor solvent THF affords the tetrameric aggregate 5056 where each Li-atom of the Li4-tetrahedra is coordinated by a molecule of THF, whilst the addition of chiral lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine affords the mixed aggregate 5159 which contains two chiral lithium amide and two alkyllithium subunits with a Li2N2 core, and adopts a four-rung ladder structure (Fig. 11B). Extensive NMR solution studies, including 1 H DOSY experiments, demonstrated that the solid state constitutions of both 50 and 51 persist in toluene solution, while 49 exists as an equilibrium between the hexameric and tetrameric aggregates, with the hexamer being the major species. Examples of structurally authenticated substituted cyclopropyl-lithiums include [Li(THF)2(m-c-CPhC2H4)2Li(THF)] (52),60 [Li(THF)2{m-c-C(SPh)C2H4}2Li(THF)] (53),61 [{m-c-C(SiMe3)C2H4}Li]4 (54)61 and [Li(THF)2(m2-c-CPhC2H4)(m3-NMe2)Li]2 (55).60 52 and 53 feature a non-centrosymmetric structure with the central Li2C2 ring built by two 1-phenylcyclopropyl and 1-(phenylthio)

Fig. 10 Dimeric structure of 48.

Fig. 11 Molecular structures of cyclopentyl-lithium complexes: (A) 49 and; (B) 51.

14

Organometallic Complexes of the Alkali Metals

Fig. 12 Schematic representation of complexes 52–54 with Li⋯Ca-Cb agostic interactions labeled in dashed lines.

cyclopropyl groups, respectively, with one three- and one four-coordinate lithium cation (Fig. 12). The three-coordinate Li-center further shows a short contact with one of the b-C-atoms of the cyclopropyl ligand resulting in Li⋯Ca-Cb agostic distortion that provides additional electron density to the Li-atom.60 The same stabilizing Li⋯Ca-Cb agostic distortion is present in 53 despite the interaction between Li-center and the donating sulfur from the phenylthio-group, emphasizing the strength of these interactions. Solution-state studies suggest that the solid-state structures are preserved in benzene and the distortion is reflected in reduced JCa-Cb coupling constants.61 Mixed aggregate 5560 adopts a ladder-type dimeric structure with a central planar Li2N2 motif of two dilithium moieties. Within each subunit, the two Li-centers are bridged by 1-phenylcyclopropyl and a dimethylamido ligand. Although the bridging phenylcyclopropyl group is again not symmetrically bound to the two Li-centers, the two hard dimethyl amino groups which interact strongly with Li significantly decrease the electrostatic Cb⋯Li interaction. Unsupported 1-(trimethylsilyl)cyclopropyl-lithium 54 is built from two-coordinate Li-centers assembled into a square arrangement by a fourfold screw-axis.61 The solid state structure appears to be preserved in non-coordinating, non-polar solvents, indicated by DOSY NMR analysis. Due to both the short Li⋯Cb contacts and CadCb bond lengthening, it is proposed that the short Li⋯Hb distances arise from Li⋯Ca-Cb interactions rather than from direct Li⋯C-H interactions, thus completing the coordination sphere around each lithium by C–C agostic interactions (Fig. 13). The isolation of 54 with formally two-coordinate Li-cations in the absence of Lewis donating ligands, shows that when CdC bonds are sufficiently destabilized as they are in cyclopropyl derivatives, electron density becomes available for significant secondary interactions which are strong enough to allow the stabilization of formally two-coordinate Li cations.61 Molecular structures of strained bicyclic Li(1-norbornane) 5662 and tricyclic phenyltetrahedryl-lithium 5763 have also been reported (Fig. 14). In tetrameric 56, a-carbons of each 1-norbornyl unit are equidistant to each Li3 face of the Li4 tetrahedron with further stabilizing interactions between the Li-atoms and the b-carbons present. In 57, a pseudo-dimeric structure is observed where the carbon-bound Li-center is chelated by one molecule of TMEDA and further coordinated to one N-atom of the second molecule of TMEDA that acts as a bridge between two symmetry-related units.

2.02.2.1.3

Benzyl, diphenyl, and triphenylmethane derivatives

Several different benzyl-lithium derivatives are known, obtained by deprotonation of toluene and supported by different donors, including monodentate, bidentate and bridging ligands. More recent additions to the family of structurally characterized benzyllithium derivatives include [(MeO(CH2)2OMe)Li(CH2C6H5)]2 (58)64 and [{Me2N(CH2)2OMe}
(LiCH2C6H5)]4 (59)65 (Fig. 15). In both cases, bidentate donor molecules have been employed which cap lithium atoms as chelating rather than bridging ligands. However, in 58 a dimeric complex with a central Li2C2 ring is formed, whereas 59 exists a tetrameric complex with a planar Li4C4 ring. Employing the related, tridentate donor base PMDETA, a monomeric derivative [(PMDETA)LiCH2C6H5] (60) was obtained,65 whilst complexation with 2-(1-pyrrolidyl)ethoxide afforded [(C4H8N-CH2CH2OLi)6
(C6H5CH2Li)2] (61)66 which displays a C6(OR)6-type structure doubly capped with the benzyl-lithium moieties. Moving to the tetradentate Me6TREN ligand, a series of monomeric complexes [(Me6TREN)MCH2C6H5] (M ¼ Li (62), Na (63) and K (64))67 (Fig. 16) and [(Me6TREN)MCH2C6H3Me2] (M ¼ Li (65), Na (66), K (67))68 could be accessed. Upon comparison,

Fig. 13 Molecular structure of 54 with TMS group shown wire-frame for clarity. Dashed lines represent Li⋯Ca-Cb agostic interactions.

Organometallic Complexes of the Alkali Metals

15

Fig. 14 Molecular structures of (A) 56 with Li–Cb interactions represented in dashed lines and; (B) pseudo-dimeric 57.

Fig. 15 Molecular structures of selected LiCH2Ph complexes: (A) 59 and; (B) 61. Hydrogen atoms except benzylic omitted for clarity.

Fig. 16 Molecular structures of [(Me6TREN)MCH2C6H5] complexes: (A) M ¼ Li (62); (B) M ¼ Na (63); (C) M ¼ K (64). All hydrogen atoms except benzylic hydrogens have been omitted for clarity.

the change in interaction of alkali-metals with the benzyl anion is evident—from Li–C s-interaction with a pyramidalized CH2 unit in 62 and 65, to an exclusively p-interaction with the K-center and a planar CH2 moiety in 64 and 67. The interaction with sodium in 63 and 66 is intermediate between these two extremes. Both s- and p-interactions of the benzyl anion with Cs-centers have been observed in the first crystal structure of benzyl-cesium, serendipitously incorporated within the decomposition product 488 (see Section 2.02.2.6).69 The benzyl anions are sandwiched between four Cs cations—two that bind to the benzylic carbon and two that engage in p-aryl interactions, similar to that previously observed in the crystal structures of [PhCH2M(PMDETA)] (M ¼ K or Rb).70

16

Organometallic Complexes of the Alkali Metals

Other examples of structurally authenticated group 1 benzylic species include a-substituted complexes 68–77 (Fig. 17). All of these novel benzyl-lithium derivatives are monomeric with the tetracoordinate Li center completing its coordination sphere by bonding to multidentate Lewis donors. In examples featuring the tridentate PMDETA ligand (68–69),71 an Z1-coordination mode of the benzylic anion to the Li-cation is observed, whilst employing bidentate TMEDA and TMPDA ligands (70–76)71,72 gives rise to a Z2-coordination mode. An interesting example is lithiated phenyl 1-phenylethylsulfide which in the presence of TMEDA affords two different complexes, 76 and 77. Whereas 76 displays the expected Z2-mode of coordination of the benzylic anion, 77 adopts a more unusual73,74 solvent separated ion pair structural motif without Li-C interactions. Incorporation of substituents was intended to allow access to enantiopure a-substituted benzyl-lithium complexes, with variations including methyl,71 phenylsulfide71 and trialkylsilyl71,72 substituents, but only racemic crystals were obtained. Similar attempts were made with lithiated benzylsilane 78,75 in which additional chiral (R,R)-TMCDA ligand was employed to cap the Z3-coordinated Li-cation; however the asymmetric unit was found to consist of both diastereomers differing in the configuration of the benzylic carbanionic center.75 Further work has shown that enantiomerically enriched lithium complexes (including benzyl-lithiums) are indeed accessible when functionalized silyl substituents are employed (see Section 2.02.2.1.4). Incorporation of neutral, donating moieties such as amino or phosphinimino groups in the ortho position of the Ph group furnished chelated species [{(2-Me2NCH2)C6H4CH(SiMe3)}Li
TMEDA] (79),76 [Li{CH2C6H4(PPh2]NSiMe3)}
TMEDA] (80),77 [Li{CH(SiMe3)C6H4(PPh2]NSiMe3)}
OEt2] (81)77 and [Li{CH(SiMe3)C6H4(PPh2]NSiMe3)}]2(m-TMEDA) (82)77 (Fig. 18). Direct carbolithiation of piperidinoallylamine (Fig. 19) afforded solvent-free, dimeric 83 displaying a unique structure in which the two Li cations within the same molecule are found in different chemical environments.78 While one is Z1-coordinated by the benzylic C-atoms of the two monomers and by the two N-centers of the piperidine, the other one is unusually Z6-coordinated by the Ph groups of two monomers. In the presence of THF, the coordination by the p-system is lost and monomeric 84 is obtained in which the benzylic group is less shielded and the negative charge is more concentrated on the carbanionic center.

Fig. 17 Structures of a-substituted benzyl-lithium species 68–78.

Fig. 18 Examples of a-substituted benzyl-lithium species (79–82) with incorporated neutral donors.

Organometallic Complexes of the Alkali Metals

17

Fig. 19 Schematic representation of carbolithiated piperidinoallylamine species 83–84.

Direct benzylic metallation of (2-phenylethyl)-dimethylamine with LICKOR afforded benzyl-potassium polymer 85 which features two different K-cation environments (Fig. 20).79 One is doubly Z6-coordinated by the Ph rings of two separate deprotonated phenethylamine moieties and two THF molecules, while the other one forms contacts with one THF molecule and is doubly coordinated by the N-centers and Z3-coordinating benzylic units of two adjacent phenethylamines. Related 2-N-methyl1,2,3,4-tetrahydroisoquinoline afforded benzyl-lithium compound 86 and benzyl-potassium species 87 when treated with BuLi/ TMEDA or LICKOR in THF, respectively (Fig. 20). In 86, the TMEDA-capped Li-center forms a four-membered LiCCN ring through contact with the N-atom in the b-position of the substrate. Similar contacts are observed in 87, but the additional Z6-coordination of the K-center of each unit to the aromatic moiety of the tetrahydroisoquinoline scaffold of the respective next unit gives rise to a coordination polymer. LICKOR metallation of diphenylmethane affords [{Ph2CHK
(THF)0.5}1] (88),80 which is the only example of a crystallographically characterized potassium diphenylmethanide (Fig. 21). In the 3D-structure of the complex, the distorted tetrahedral environment of each K-atom is formed by two benzhydryl methine carbanions, one phenyl ring and one THF molecule which acts as a bridge between the two neighboring K-atoms. Adding to the family of benzhydryl ligands, a series of alkali-metal complexes [{2,2-(4-MeC6H4NMe2)2CH}Li(TMEDA)] (89), [{2,2-(4-MeC6H4NMe2)2CH}Na(THF)3] (90) and [{2,2-(4-MeC6H4NMe2)2CH}K(THF)2] (91) and [{2,2-(4-MeC6H4NMe2)2C (SiMe3)} K]1 (92) were reported (Fig. 22).81 Whilst the smaller Li and Na congeners afford monomeric complexes featuring a Z4-CCCN coordination of the tridentate benzhydryl scaffold, the larger K analog affords dimeric complex 91 and 1D-polymeric 92. In 91, the two K-centers are sandwiched between the two tridentate benzhydryl ligands featuring m-Z5-pentadienyl:k3-CNN and m-k3-CNN:Z6-arene bonding modes. Attaching a Me3Si group to the methanide carbon of benzhydryl scaffold affects the mutual arrangement of the phenyl rings affording a m-Z3:Z6 bridging coordination mode of the ligand to the K-centers in 92.

Fig. 20 Schematic representation of complexes 85–87.

Fig. 21 A portion of a polymeric structure of 88. All hydrogen atoms except methine hydrogens omitted for clarity.

18

Organometallic Complexes of the Alkali Metals

Fig. 22 Molecular structures of complexes (A) 89; (B) 90 and; (C) 91. All hydrogen atoms except the one on the methanide carbon omitted for clarity.

Sodium triphenylmethanide [(Ph3C)Na(THF)3] (93) was unexpectedly obtained from sodium naphthalenide induced cleavage of one of the CdN bonds of bis(amino)cyclodiphosphazane.82 In addition to coordination by the three THF molecules and the central carbon atom of the trityl moiety, the Na-center further interacts with all three ipso-carbons in a nearly symmetrical fashion. Functionalization of the triphenylmethane scaffold gave access to mixed aryloxy/trityl complexes [Li2{k2-O,C-OC6H2-2-C (C6H5)2-4,6-tBu2}(OEt2)]2 (94), [Li2{k2-O,C-OC6H2-2-C(3,5-Me2C6H3)2-4,6-tBu2}]2 (95) and [Na2{k2-O,C-OC6H2-2-C (C6H5)2-4,6-tBu2}(THF)2.5]2 (96)83 (Fig. 23). Whereas in 94 each Li-atom interacts with the ligand in a Z4-OCCC fashion, in 95 three Li-centers are clustered together by two ligands through diverse Z3-metal-ligand p-interactions, while the fourth Li-center is sandwiched in a symmetric Z6:Z6-arene bonding mode. The lack of symmetry in bonding in 95 compared to 94 is attributed to the increased steric bulk of the ligand scaffold. In 96, the two ligands are bridged by three Na-centers forming a Na3O2 cluster, while the final fourth Na-atom coordinates to only one central carbon atom of the trityl group. The coordination spheres of three Na-centers are completed by coordination of THF molecules whereas one Na-atom engages in Z2:Z2-arene bonding.

2.02.2.1.4

Silyl substituted alkyl derivatives

Incorporation of silyl substituents within the alkyl ligand framework offers several key features that have made ligands such as [CH2SiMe3]−, [CH(SiMe3)2]−, [C(SiMe3)3]− and their derivatives widespread in organometallic chemistry across the Periodic Table.84 Trimethylsilyl-substituted ligands are lacking hydrogen atoms in the b-position, thereby evading the possibility for decomposition of organometallic species via b-hydrogen elimination pathways. Furthermore, the size of the ligand drastically increases with every trimethylsilyl substitution, which helps to kinetically stabilize the organometallic species and precludes aggregation. The presence of a large number of methyl groups also increases the lipophilicity of carbanion allowing for better solubility in hydrocarbon solvents and thus accessibility of donor-free organometallics. This section will start by discussing the structural diversity of silyl-substituted alkyl complexes of alkali-metals and finish with functionalized silanes which have allowed access to enantiomerically enriched lithium complexes. Deaggregation of parent hexameric (trimethylsilyl)methyl-lithium85,86 has been achieved with an array of Lewis donating ligands affording novel tetrameric, dimeric and monomeric adducts. In the presence of monodentate ethers, asymmetric tetramers [(Et2O)2
(LiCH2SiMe3)4] (97)87 and [(tBuOMe)2
(LiCH2SiMe3)4] (98)87 were formed (Fig. 24). Within the distorted tetrahedral Li4 core, two Li-centers are donor solvated and two are donor free, and these donor-free Li-centers benefit from the secondary electron donation from the C-atoms at the g-position relative to the metalated C-atom. Addition of bidentate ligands DME or TMEDA afforded dimeric adducts [(DME)
(LiCH2SiMe3)]2 (99)87 and [(TMEDA)
LiCH2SiMe3]2 (100)88 with a central Li2C2 four-membered ring in which each Li-center is coordinated by the chelating ligands and the carbanions. The central Li2C2 ring in 99 is nearly planar, while 100 shows an envelope conformation. In both structures, the silyl groups bend away to the opposite sides of the ring leaving one side of the dimer more open. Dimeric aggregates with a central Li2C2 core were also obtained using a monodentate, strained quinuclidine donor ligand with a strongly directed lone pair, and with chiral chelating tert-butyl[(2S)-2-(methoxymethyl)-1-pyrrolidinyl]silane. Quinuclidine afforded symmetrical and unsymmetrical dimers 101 and 102 in which the number of donor ligands varies from two to three, respectively.89 In 103 (Fig. 25), the chiral dimethoxysilane coordinates to the Li-center solely by the methoxymethyl side arm and the silicon-bound, pro-S methoxy group, while the tBu group shows a syn-type configuration with respect to complexed Me3SiCH2Li.90 Chiral diamines (−)-sparteine and (R,R)-TMCDA yielded dimeric [{(−)-sparteine}
LiCH2SiMe3]2 (104)88 and monomeric [(R, R)-TMCDA
LiCH2SiMe3] (105)89 adducts. The nearly planar Li2C2 parallelogram of 104 exhibits two significantly different LidC bond distances (2.147(3) and 2.661(3) A˚ ) which can lead to interpretation of 104 as weak electrostatic dimerisation of two [{(−)sparteine}
LiCH2SiMe3] units. In monomeric 105, the Li-cation is coordinated by only the two N-atoms and one carbanion resulting in an incomplete coordination sphere, with a coordination number of three.89 Inspection of the SidCMe bonds showed no significant differences indicating no presence of the stabilizing a-effect of the neighboring Si-atom, but that the negative charge

Organometallic Complexes of the Alkali Metals

Fig. 23 Molecular structures of mixed aryloxy/trityl complexes: (A) 94; (B) 95 and; (C) 96.

Fig. 24 Molecular structure of 97.

19

20

Organometallic Complexes of the Alkali Metals

Fig. 25 Molecular structure of 103.

in 105 is stabilized by the polarization of the Si-center stressed by a short Si-Ccarbanion distance. Tridentate PMDETA ligand affords monomeric adduct [(PMDETA)
LiCH2SiMe3] (106),88 however here the Li-center is coordinated by three N-atoms of the donor base and the carbanion affording a distorted tetrahedral coordination sphere. The crystal structure of unsolvated (trimethylsilyl)methyl-sodium 10791 (Fig. 26) has been revealed to consist of distorted Na4 tetrahedral core units as previously observed in classical organometallic reagents such as MeLi or MeNa.23 Each face of the tetrahedron is m3-capped by an alkyl group, two of which also interact with the Na-centers of two neighboring Na4 units connecting them into a chain arrangement, but also inducing the asymmetry in the structure with one three- and one four-coordinate Na-cation. Addition of TMEDA afforded polymeric [{(TMEDA)NaCH2SiMe3}1] (108)92 in which the Na-atoms are coordinated by two N-atoms of TMEDA and CH2 groups of two different trimethylsilyl groups with alternating short and long NadC bonds (Fig. 27). The crystal structure of unsolvated (trimethylsilyl)methyl-potassium is still unknown, but amine supported complexes [{(PMDETA)KCH2SiMe3}1] (109)92 and [(TMEDA)3(KCH2SiMe3)4] (110)92 have been isolated and structural characterized. 109 exhibits an infinite helical chain structure similar to sodium complex 108, with the difference that the larger K-cation is coordinated by the three N-atoms of PMDETA and two carbon atoms of two different trimethylsilyl groups with alternating potassium-carbon distances. 110 was identified as the donor-deficient heterocubane pseudo-tetramer where only three of four K-atoms are chelated by TMEDA, while the fourth, TMEDA-uncoordinated K-atom displays intermolecular interactions with a peripheral methyl group of a neighboring tetramer forming infinite chains.92 Alkali-metal complexes containing the bis(trimethylsilyl)methyl ligand supported by donor ligands such as [{LiCH (SiMe3)2
THF}2] (111),93 [{LiCH(SiMe3)2
TMEDA}] (112),93 [{NaCH(SiMe3)2
THF}1] (113)93 and [{NaCH(SiMe3)2}2
{TMEDA}3] (114)93 have also been structurally characterized. 111 is dimeric with a planar Li2C2 core with coordination of one additional THF ligand to each Li-center showing an overall approximate trigonal planar environment. Monomeric 112 displays also a tri-coordinate

Fig. 26 Portion of the polymeric chain structure of 107 with two connected, neighboring tetrameric units.

Organometallic Complexes of the Alkali Metals

21

Fig. 27 Structures of TMEDA supported complexes: (A) portion of the polymeric chain of 108 and; (B) pseudo-tetramer of 110.

Fig. 28 Molecular structure of 114.

Li-center afforded by bidentate coordination of the TMEDA ligand in addition to the carbanionic ligand. Despite both 111 and 112 containing a tri-coordinate Li-center, the LidC bond distance in 112 is a two-center two-electron type and thus shorter than three-center two-electron bonds present in 111. 113 displays a zig-zag polymeric chain very similar to the arrangement observed in 108 (vide supra), while 114 (Fig. 28) can be described as a dimer of TMEDA-coordinated monomers of alkylsodium units connected by a bridging TMEDA group. A closely related 1,4-dilithio-1,1,4,4-tetrakis(trimethylsilyl)butane (115)94 that incorporates a dianionic ligand containing bis(trimethylsilyl)-substituted carbanions on each end has also been structurally characterized (Fig. 29).94 Employing a methoxy-substituted analog of the bis(trimethylsilyl)methyl ligand previously afforded Li[CH(SiMe3){SiMe (OMe)2}]95 which when reacted with TlCl undergoes fragmentation yielding [Li4(m-[CH(SiMe3){SiMe(OMe)2}]−)3(m3-OMe)] 116 as a side product.96 In the solid state, 116 exhibits a fused tris(hexacyclic) core with the O-atom from the m3-OMe ligand as the locus of the three O3Li2Si rings, while the fourth Li ion is bound to the Si atoms of the three rings via a C(H)SiMe3 moiety (Fig. 30).96 A potassium complex of tris(dimethylsilyl)methyl ligand supported by TMEDA [{KC(SiHMe2)3(TMEDA)}2] (117) has been isolated and crystallographically characterized (Fig. 31).97 In the dimeric solid state structure, each K-center is coordinated by a bidentate TMEDA ligand and carbanion, and further three hydrosilyl interactions which can be described as SiH analogs of agostic

Fig. 29 Molecular structure of 115.

22

Organometallic Complexes of the Alkali Metals

Fig. 30 Schematic representation of 116.

Fig. 31 Schematic representation of 117. End-on K⋯H-Si coordination represented by dashed lines, and side-on K⋯H-Si interactions represented by arrows.

and anagostic structures.98 Two types of three-center, two-electron K⋯H-Si interactions include “side-on” coordination of Si-H moiety from the C(SiHMe2)3 ligand, and two “end-on” K⋯H-Si interactions from two H-Si groups in the second C(SiHMe2)3 group connecting the two units into a dimer.97 Although the solution state studies on samples of 117 suggest potassium-silylhydride interactions, the data is not in agreement with the solid state structure, but instead indicates a fluxional solution-phase structure. Other examples of structurally authenticated derivatives of sterically demanding “trisyl” ligands include [Na{C(SiMe3)Ph2} (THF)3] (118),99 guanidine-functionalized [LiC(SiMe3)2(SiMe2hpp)] (119),100 and [KC(SiMe3)2(SiMe2hpp)] (120).100 The monomeric system 11899 (Fig. 32) exhibits a solid state structure very similar to closely related [Na{CPh3}(THF)3] (93) (vide supra) where the Na-center, in addition to coordination to three THF molecules and carbanion, engages in secondary interactions with Cipso of the Ph substituent. Incorporation of a strongly basic guanidine donor into the ligand framework enables strong intramolecular bonding of imino N-atom to the metal center affording six-membered metallacycles 119100 and 120100 (Fig. 33). In 119, the two-coordinate Li-atom engages in secondary interactions, comparable in magnitude to those found in methyl-lithium, with the Me group of the adjacent molecule linking the monomeric units into a chain arrangement. NMR spectroscopic data suggests that the molecular structure is

Fig. 32 Molecular structure of 118.

Organometallic Complexes of the Alkali Metals

23

Fig. 33 (A) Molecular structure of 119; (B) section of the polymeric chain arrangement of 120.

Fig. 34 Schematic representation of complexes 121–123.

preserved in benzene solution based on inequivalent amido and imino nitrogen atoms. The molecular structure of 120 is very similar to 119, but here both N-atoms interact with the metal and the coordination at the carbanionic center is planar, reflecting the greater ionic character of the K-carbanion interaction.100 The neighboring units are held together through potassium cation and carbanion interactions forming a C⋯K⋯C backbone with the hpp groups of the adjacent units located on the opposite sides. Examples of methylpyridine-substituted bis(trimethylsilyl) and “trisyl” ligands have also been reported and include [{2(6-Me-pyr)(Me3Si)}CHLi
OEt2]2 (121),101 [{6-Me(2-Pyr)}(Me3Si)2CNa(PMDETA)] (122),102 and [{6-Me(2-Pyr)}(Me3Si)2CK]1 (123)102 (Fig. 34). In dimeric 121, the two Li centers exhibit two different bonding modes—Z1 alkyl and Z3-azaallyl to the anionic moiety. In both heavier alkali-metal complexes 122 and 123, the metal centers are again engaged in Z3 1-aza-allyl interactions with the anionic moiety. In monomeric 122, the Na-center completes its coordination sphere by interacting with three N-atoms of the PMDETA donor ligand. In 123, the K center engages in additional Z3 allylic interactions, giving rise to a linear polymeric chain with numerous agostic inter- and intra-molecular K⋯MeSi interactions. Oxygen functionalization of silyl-substituted alkanes allowed for isolation and characterization of complexes [{(Me3Si)2(Me2MeOSi)C}K]1 (124),103 [[{(Me3Si)2C(SiMe2)}2O]K2(OEt2)]1(125),103 and [{(Me3Si)(Me2MeOSi)C(SiMe2CH2)}2Li][Li(DME)3] (126).103 Complexes 124 and 125 (Fig. 35) adopt polymeric structures with alternating K-cations and essentially planar carbanions. In 124, each K-center is coordinated by two adjacent monocarbanionic centers and by the O-atom of one of the adjacent ligands, generating a four-membered chelate ring. The coordination sphere of potassium is completed by a total of five short Si-Me⋯K contacts. 125 contains a dicarbanionic ligand and two K-cations with differing coordination spheres. The first potassium ion simultaneously coordinates to the siloxane oxygen and the two carbanionic centers generating two four-membered chelate rings. The coordination sphere of potassium is completed by the oxygen atom of a molecule of Et2O and by short contacts to two Me groups from each of the adjacent carbanion centers. The second K-atom in the chain is coordinated by the two adjacent carbanion centers without any interactions with the oxygen center, completing its coordination sphere through short contacts to three Me groups from each half of the ligand. It might be tempting to describe 125 as potassium dialkylpotassiate, but based on the K-C distances and the planarity of the carbanion centers, the more accurate description is as a polymer with K-cations both folded within the dianions and linking them together. Complex 126 (Fig. 36) crystalises as a discrete solvent separated ion pair with one cation and one half of each of the two crystallographically independent anions, which are an enantiomeric pair of RR and SS diastereomers. Each anion consists of a Li ion coordinated by both carbanionic centers of the ligand giving rise to a seven-membered chelate ring. Further coordination by the two O-atoms of the OMe groups at the periphery of the same ligand generates additional two four-membered chelate rings.103

24

Organometallic Complexes of the Alkali Metals

Fig. 35 Schematic representation of portions of the polymeric chain structures 115 and 116.

Fig. 36 Schematic representation of solvent separated ion pair structure of 126.

Further examples of structurally authenticated Si-containing alkyl-lithium complexes include lithium carbatrane 127104 and dilithium complex of vicinal C-N dianion 128105 (Fig. 37). 127 contains a tetradentate tripodal ligand with a robust SiMe2 linker coordinated to Li-center in an approximately trigonal mono-pyramidal fashion. The transannular LidC bond distance is larger than the sum of the covalent radii, indicative of an antrane compound, while the planarity of the [CSi3] moiety suggests a significant degree of zwitterionic character with negative charge located on the carbon center.106,107 Synthetically important precursors, 127 can be employed to access carbatrane complexes of transition metals. Monomeric 128105 comprises vicinal dianion with both Li-atoms three coordinate: one is coordinated to both central C and N atoms forming a unique lithiaazacyclopropane and one THF molecule, and the other is coordinated to only N-atom and two molecules of THF. As previously mentioned (see Section 2.02.2.1.3), incorporation of trialkylsilyl or mixed (alkyl/aryl)silyl substituents at the a-position has been attempted in order to access enantiomerically enriched benzyl-lithium complexes. Enantiopure alkyl-lithium compounds are highly desirable reagents; however, since the inversion barrier for the stereogenic carbanionic center is very low, their stereochemical integrity is often lost at room temperature.108 Despite the initial lack of success, further work has shown that the combination of polarization via a silicon atom and “side-arm complexation” leads to isolation of diastereomerically enriched

Fig. 37 Molecular structures of (A) 127 and; (B) 128.

Organometallic Complexes of the Alkali Metals

25

Fig. 38 Portion of a polymeric chain of [(R,S)-129] in the solid state. All hydrogen atoms except benzylic hydrogens omitted for clarity.

lithiated (benzylsilyl)methylamine [(R,S)-129].109 The Li-center is coordinated by the two donor atoms of the (methoxymethyl) pyrrolidine (SMP) ligand and the benzylic carbanion, and completed by an Z3-Ph substituent of the adjoining silane molecule. Coordination of the SMP ligand transfers the stereochemical information onto the metal fragment, while the Li⋯Ph p-interactions build up an infinite chain structure of [(R,S)-129] molecules in the solid state (Fig. 38). From this result it is evident that the presence of additional donating Lewis base is not necessary for the stereoselectivity, but its presence will prevent formation of polymeric chain structures as evidenced by isolation of dimeric TMEDA adduct [(R,S)-130]110 (Fig. 39) and monomeric DABCO [(R,S)-131]110 or quinuclidine [(R,S)-132]111 adducts (Fig. 40). In each case, as observed in [(R,S)-129], the Li-center is coordinated by the benzylic C-atom, and by the donor atoms of the SMP side arm that keeps it in a fixed position. Additional coordination by the N-atom of employed N-donor affords overall tetrahedral coordination. High resolution X-ray diffraction analysis and topological analysis of enantiomerically enriched benzylsilane [(R,S)132]111 has estimated that the influence of the Si center in stabilization of stereogenic information on carbanion is higher than the mesomeric stabilization. The negative charge generated at the carbanion hardly couples into the phenyl ring and is counterbalanced by the pronounced positive charge of the neighboring Si-atom, revealing that the a-effect of the silicon atom is a combination

Fig. 39 Dimeric structure of [(R,S)-130]. All hydrogen atoms except benzylic hydrogens omitted for clarity.

Fig. 40 Molecular structures of diastereomerically enriched benzyl-lithium (131–132) and alkyl-lithium (133–135) complexes.

26

Organometallic Complexes of the Alkali Metals

of polarization of the electron density and an electrostatic bond reinforcement.111 The stabilizing a-Si effect is evident in the shortening of SidCcarbanion bond (1.813 A˚ ) compared to the other SidC bonds present and in comparison to the standard SidC bond length (1.87 A˚ ).112 Solution state 13C NMR studies displayed only one quartet for C-Li coupling indicating a fixed Li-C contact at room temperature. Further supporting the finding that the polarization via silicon atoms contributes more than the mesomeric stabilization to the retention of stereogenic information, highly diastereomerically enriched silyl-substituted alkyl-lithium compounds [(R,S)133]113 and [(R,S)-134],113 and [(R,S)-135]114 have also been successfully isolated (Fig. 40). In all three complexes, the Li-center is coordinated by the carbanion, and the N- and O-atom of the SMP ligand, while the fourth coordination site is either occupied by THF or short contacts with the H-atoms of the methyl group of the neighboring molecule formally forming a coordination polymer.113 With a smaller Me substituent (vs. SiMe3 found in 133 and 134) on the ligand scaffold, 135 fulfills the coordination sphere of Li by adopting a dimeric structure with a central four-membered Li2C2 ring. All isolated complexes 129–135 show R configuration at the lithiated carbon center and were shown to undergo reaction with electrophiles (e.g., trimethyltin chloride) with inversion of the configuration at the metalated carbon center. By contrast, replacing the chiral SMP side arm with an achiral monodentate piperidinomethyl side arm afforded 13675 (Fig. 41) in which both diastereomers are present within the asymmetric unit. Here, like in above examples, Li is tetrahedrally coordinated, however the incorporation of achiral side arm despite further addition of chiral, chelating donor ligand (R,R)-TMCDA, has not been successful in delivering diastereomerically enriched benzylsilane.75 Further supporting this observation, incorporation of a chiral side-arm in benzylsilane upon metallation afforded 13775 (Fig. 41), consisting of two epimers that differ in the configuration at the carbanionic center. The benzyl anions so generated show sp2 hybridization and delocalisation of the anionic charge is evident in further interactions between the benzyl moieties and Li-centers giving rise to a non-centrosymmetric aggregate of the two epimers. One of the two bridging benzyl units Z1-coordinates both Li-centers on the same face causing a bent aromatic system, while the other benzyl unit binds the two Li-centers from different faces and the Ca-center shows a trigonal-pyramidal coordination mode. Addition of a donating ether (Et2O or TBME) that acts as a fourth ligand to the Li-cation, breaks the aggregate with almost clean chirality transfer affording monomeric (RN,R,R,R)-13875 and (RN,R,R,R)-13975 (Fig. 41). Finally, it has been shown that methyl-substituted silanes such as dimethyl-bis(2,4,6-trimethyl-2,4,6-triaza-cyclohex-1-yl)silane or aminomethyl-functionalized oligosilanes can undergo direct a-lithiation exemplified by isolation of complexes 140,115 and rac-141 and rac-142,116 respectively. The presence of two (2,4,6-triazacyclohex-1-yl) substituents increases the steric hindrance of the silane and 140 adopts a dimeric structure with a central Li2C2 motif (Fig. 42) in which the two central Li atoms are pushed apart from one, as another evidenced by the deformation of the Li2C2 ring into a rhombus.115

Fig. 41 Structures of complexes 136–139.

Fig. 42 Dimeric structure of 140.

Organometallic Complexes of the Alkali Metals

27

A similar dimeric structure with a central Li2C2 four-membered ring is again observed in sterically encumbered trisilane rac-142, whilst the smaller disilane in rac-141 allows for formation of tetrameric structure where each Li3 face of the central Li-tetrahedron is m3-capped by the carbanionic centers (Fig. 43).116 When the methodology is applied to enantiomerically pure analogs, chiral at Si-complexes are accessible, such as those found in 143116 and 144.116 Both 143 and 144 are mixed aggregates exhibiting tetrameric structures (Fig. 44) in which the central Li4 tetrahedron is built of two molecules of a-lithiated disilane or silagermane, and two sterically less bulky tBuLi units. Comparison of the four related structures reveals that although the structure with the central Li4 tetrahedron is preferred (141), upon increase of steric demand mixed-aggregates will form (143–144), and when the steric congestion at the Si-center is too great, a dimeric structure will result (142). Stereogenic Si-centers with absolute configuration RSi have been incorporated in complexes 14590 (which contains a chiral SMP side-arm) and 146,116 containing (R,R)-TMCDA-derived side-arm (Fig. 45). In both cases a dimeric compound with a central Li2C2 ring is formed, as found in many other dimeric alkyl-lithium compounds. Enantiomerically pure (R,R,RSi)-146 is accessible only at temperatures as low as −120  C, whereas on warming both possible diastereomers [(R,R,RSi)-146 + (R,R,SSi)-146] were obtained.116 Crystallographically, this is evident in the asymmetric unit—in the enantiomerically pure complex only one half of the dimer is found which through C2 symmetry assembles into the dimer, while the complex containing both diastereomers contains one whole molecule of the dimer.116

2.02.2.1.5

Phosphorus and sulfur substituted alkyl derivatives

Examples of alkyl complexes of alkali-metals with other heteroatoms in the b-position include mostly phosphanyl- and phosphino-borane moieties. Although the examples are not numerous, these complexes could serve as precursors to allow further functionalization of the parent phosphine, yielding new, more complex phosphine ligands for use in catalysis.

Fig. 43 Solid state structures of (A) tetrameric 141 and; (B) dimeric 142.

Fig. 44 Tetrameric structures of mixed aggregates (A) 143 and (B) 144.

28

Organometallic Complexes of the Alkali Metals

Fig. 45 Dimeric solid state structures of complexes (A) (SC,SC,RSi)-145 and; (B) (R,R,RSi)-146.

Fig. 46 Portion of a polymeric structure of 147. Hydrogen atoms omitted and tBu group represented as wire-frame for clarity.

Examples of structurally characterized phosphanyl-substituted organolithiums include [Li{CH2P(tBu)2}] (147)117 and [(PMDETA)Li{CH2PPh2}] (148).118 The solid-state structure of solvent-free 147 was determined by high-resolution X-ray powder diffraction, revealing an infinite double-chain of dimers motif built out of alternating edge-sharing six- and four-membered rings (Fig. 46).117 By contrast, donor-supported 148118 crystallizes as a discrete monomer. Another interesting example is [(Et2O)3Li2{C(H)Py}2PPh]2 (149),119 formed upon double lithiation of a picolyl-substituted phosphine, which contains a phosphorus center as a soft donor and two nitrogen centers as hard donors. The N(py)-Li coordination generates a head-to-tail dimer with a central eight-membered (LiNC2)2 ring. Both Li-centers are tetra-coordinated, but found in different environments—one P,N-coordinated by the ligand and solvated by two molecules of Et2O, while the other is supported by two carbanions of the ligand, one N-atom of the second ligand and one molecule of Et2O (Fig. 47).119 Deprotonation of tertiary phosphine-boranes led to isolation of a-metallated species [{Me2P(BH3)CH2}Na(THF)]1 (150)120 and [{PhP(BH3)(CHPh)(CH2Ph)}K(OEt2)]2 (151),121 in which the metal centers show contacts to both the carbanions and the BH3 moiety. In 150, the ligand simultaneously chelates one metal center and bridges to an adjacent metal, giving rise to a ribbon type polymer of alternating Na2C2 and Na2(BH3)2 squares. In centrosymmetric dimer 151, each potassium is coordinated by the carbanion, an Z1-BH3 contact from the same ligand and an Z2-BH3 contact from the second ligand. The coordination sphere is completed by Z6 coordination of the Ph ring of the non-deprotonated benzyl group of the ligand, and one short K⋯ Me contact with the terminal Et2O ligand (Fig. 48). Attempts to isolate Li-analogs or to effect polymetallations with excess alkyl-lithium base showcased the propensity of the formed species to undergo C-O and Si-O cleavage with ethereal solvents and siloxanes. Isolated clusters included [PhP(BH3) (CHPh)2]Li3(OSiMe2CH2CH2CH2CH3)(OEt2)3 (152),121 [PhP(BH3)(CHPh)2]Li3(OEt)(OEt2)3 (153),121 [PhP(BH3)(CHPh)2] Li3(OMe)(OtBuMe)3 (154)121 and [PhP(BH3)(CHPh)2]Li3(OiPr)(OEt2)3 (155)121 and [{Me2P(BH3)CHSiMe2OLi}4Li4(Et2O)2.75(THF)1.25] (156).120 Clusters 152–155 are structurally very similar, with the core of a distorted cube formed by the three Li centers, two carbanions, a P-BH3 moiety and an O-atom of siloxide or alkoxide. 156120 contains a new siloxy-functionalized alkyl ligand formed by insertion of a dimethylsiloxane fragment from the grease into a LidC bond. The presence of such ligands leads to the formation of a central Li6O4 adamantoid core, on which two Li2C2 squares are fused (on opposite sides) via a Li-vertex bridged by a carbanion to another terminal Li-ion (Fig. 49). Lithiation of prochiral phosphine-boranes afforded a-monophosphinoalkyl-lithium borane complex 157122 and doubly lithiated dimethylphosphine borane 158 (Fig. 50).123 The presence of the sterically encumbered bidentate DPE ligand in 157122

Organometallic Complexes of the Alkali Metals

29

Fig. 48 Molecular structures of (A) 150 and; (B) 151. All hydrogen atoms except BH3 groups omitted for clarity.

Fig. 47 Molecular structure of 149.

Fig. 49 Molecular structures of (A) 153 and; (B) 156. Hydrogen atoms except BH3 unit omitted for clarity.

led to isolation of a monomeric trans-1-phenyl-2-phospholanyl-lithium borane complex featuring a tricoordinate lithium center. However, extensive solution state NMR spectroscopic studies (including 1H-7Li HOESY) indicate that the isolated trans isomer exists in equilibrium with its cis isomer and with a solvent separated ion pair species. 158123 crystallizes as two molecules of a dilithiated complex with three molecules of R,R-TMCDA. The central structural motif consists of two Li2C2 four-membered rings fused through a common Li-vertex found in an almost square-planar environment through contacts with all four carbanionic centers. The two end Li-centers coordinate to two carbanions and are each further capped by a molecule of TMCDA. The final Li atom is located above the ring unit via Z1-BH coordination of the two ligand units and coordinated by another molecule of TMCDA. The unusually facile and controlled deprotonation of both Me-substituents of dimethylphenylphosphine borane is credited to Li ⋯HB interactions.

30

Organometallic Complexes of the Alkali Metals

Fig. 50 Structures of 157 and 158.

Fig. 51 Structures of 159–161.

Replacing the Ph group of dimethylphenylphosphine borane with a trimethylsilylmethyl group yields upon deprotonation a carbanion which is isoelectronic with the well-established [(Me3Si)3C]− ligand. Alkali-metal complexes incorporating the [(Me3Si)2{Me2P(BH3)}C]− carbanion include Li (159),124 Na (160125–161126), K (162–166), Rb (167–168) and Cs (169)125 species, in which a variety of coordination modes are found, but where M⋯HB interactions are always present (Fig. 51). In the solid state, 159124 exists as a discrete dimer in which one Li is bound by the carbanion centers of two discrete phosphine-borane units and has a short Z1-BH contact to one carbanion ligand, while the BH3 group of the second carbanion ligand Z3-bridges to the second Li center capped by three molecules of THF.124 In the sodium congeners 160125 and 161,126 the ligand is C- and Z2-BH3-coordinating, bridging between neighboring Na-centers in 160, giving rise to a 1D zigzag polymer, but chelating in monomeric 161. Solvent-free K (162)125 and Rb (167)125 complexes are isostructural and isomorphous crystallizing as a complex 2D sheets with two distinct metal environments and two distinct ligand environments (Fig. 52A). One ligand simultaneously chelates M1 and bridges between M1 and M2 in a m2-Z2:Z3-BH3 group, whereas the second ligand acts as both an m2-Z2:Z2-BH3 bridge between M1 and M10 and as a bridge between M1/10 and a symmetry equivalent M2 via its carbanion center and the BH3 group. The coordination spheres of the metal centers are completed by short agostic type M⋯Me-Si contacts.125 Addition of THF donor to complex 162126 still affords a polymer, but in this case a 1D polymeric arrangement is found in 163 which incorporates half a molecule of THF (Fig. 52B).126 Bidentate TMEDA and tridentate PMDETA afford dimers 164126 and 165,126 respectively. The higher denticity of PMDETA does not lead to further deaggregation but only increases the coordination number of potassium. In both instances, coordination of carbanion, Z2-BH3 group of one ligand and an Z1-BH3 group from the second ligand gives rise to a central K2(BH3)2 rhombus. The coordination sphere of each K-atom is completed by N-atoms of the donor ligands and by short K⋯Me contacts.126 Addition of 12-crown-4 sequesters K+ and a separated ion pair [K(12-c-4)2][(Me3Si)2{Me2P(BH)3}C] (166)126 crystallizes, in which the two molecules of crown-ether coordinate the K-cation without any short contacts between the K-ion and phosphine-borane-stabilized carbanion. PMDETA-supported complexes K (165), Rb (168) and Cs (169) are essentially isostructural with the only difference being an increased number of M⋯Me agostic interactions as the ionic radius of the metal center increases (Fig. 53). Charge-stabilizing silyl groups in conjunction with a phosphine-borane moiety are capable of stabilizing two adjacent carbanion centers, as exemplified by the isolation and characterization of (THF)2Li{(Me3SiCH)2P(BH3)Ph}Li(THF)3 (170).127 In this contact ion-pair ate complex, the formed 1,3-dicarbanion acts as a bidentate C,C0 -donor ligand to one Li-center generating a dialkyl-lithiate “anion,” while simultaneously Z2-BH3 bridging to the lithium center. Both lithium centers are overall located in a distorted tetrahedral environment completed by terminal THF molecules (Fig. 54).

Organometallic Complexes of the Alkali Metals

31

Fig. 52 Molecular structures of (A) 162 and; (B) 163. Hydrogen atoms except BH3 units omitted for clarity.

Fig. 53 Molecular structures of a) 166 and; b) 169. All hydrogen atoms except BH3 units omitted for clarity.

Fig. 54 Structures of complexes 170 and 171.

Bis(phosphine-borane) ligands are found in lithium [[{Ph2P(BH3)}(Me3Si)C(CH2)]Li(THF)3]2
2THF (171)128 and potassium [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2K2(THF)4]1 (172)128 complexes. In 171, each Li-atom is only Z2-BH3 coordinated by the ligand, and shows no short contacts with the carbanion centers, most probably due to the high steric encumbrance of the substituents. Potassium complex 172 is a chain polymer of centrosymmetric units linked via BH3⋯K contacts. Each potassium is coordinated by one of the carbanion centers, in addition to Z2-BH3 contacts, two molecules of THF and additional short contacts with the methylene carbons of the ligand backbone and THF.128 Other examples of structurally elucidated alkali-metal complexes with bis(phosphine-borane) ligands include lithium 173–176 and potassium 177–179 complexes (Figs. 55–56).129 Similarly, as previously seen in 159,124 the Li-cations in complexes 173–176129 are coordinated by the carbanion centers and an Z1-BH3 group of the phosphine-borane stabilized carbanions. However, in 173–175129 planar carbanions are found due to extensive delocalisation of charge in the aromatic ring and the P-C

32

Organometallic Complexes of the Alkali Metals

Fig. 55 Representation of structures of complexes (173–178) incorporating the bis(phosphine-borane) ligand scaffold with rigid backbone.

Fig. 56 Molecular structures of complexes incorporating flexible bis(phosphine-borane) dianions: (A) 176 and; (B) 179. All hydrogen atoms except BH3 units omitted for clarity.

s -orbitals of o-phenylene-bridged bis(phosphine-borane) ligand. In 177,129 the monocarbanion formed coordinates to the K-ion, which also engages in Z2-BH3 contacts with both borane groups and short contacts with the ipso-carbon of the Ph ring, while in 178,129 a dicarbanion is formed and the two K centers show Z1-BH3 contacts, interactions with the carbanions, and Z3 and Z5 contacts with the aromatic backbone. In both cases, the K-centers complete their coordination spheres with three N-atoms of PMDETA and short contacts with the Me-substituents of PMDETA. The flexible backbone of the ligand in 179129 allows for the formation of a polycyclic structure in which each BH3 group bridges between the two potassium ions in a m2-Z1:Z1 fashion generating a (KHBH)2 cycle. In addition to deprotonation of phosphine-boranes, examples of Ca–metalated phosphine sulfide130 phosphine oxide,131 phosphinimine132 and N-phenylphosphazene species133 have been structurally authenticated. Lithiated dimethyl-substituted phosphine sulfide 180130 adopts a monomeric structure where (−)-sparteine-capped lithium shows contacts to both the carbanion and the S-atom. In dimeric [(LiCMe2)tBu2PNSiMe3] (181)132 two four-membered P-N-Li-C rings are formed,132 while [{LiCH2P (Ph)2]NPh}4] (182)133 crystalises as a tetramer which, unlike previously reported examples of phosphazenyl lithium complexes, contains lithium exclusively coordinated to carbon atoms (Fig. 57). This unique tetrameric aggregate incorporates two distinct Li-coordination modes—one in which the Li is tetra-coordinated by two N and two C-donors, and the other where the four-coordinate Li is bound exclusively to four sp3 hybridized carbanionic centers. Extensive NMR studies have shown that the structure persists in toluene solution.133 Metallation of phosphine oxide in the presence of an appropriately sized crown ether allowed isolation of alkali-metallated organophosphine oxides [{Ph2P(O)CH2}K
(18-crown-6)] (183)131 and [{Ph2P(O)CH2}Na
(15-crown-5)] (184).131 Both display contact ion-pair structures, however in 183, the potassium center is coordinated solely through the O-center of the organophosphine oxide, while in 184, the phosphine oxide acts as a chelating bidentate ligand coordinating to sodium via both O- and C-atoms.

Organometallic Complexes of the Alkali Metals

33

Fig. 57 Molecular structures of (A) 180; (B) 181 and; (C) 182.

Finally, it is worth mentioning that sulfones can also be easily deprotonated by alkyl-lithiums resulting in a-sulfonyl functionalized alkyl carbanions. In most cases, however, solid state structures show no direct LidC bonds and coordination of the SO2 groups is observed instead. For instance a-lithiated sulfonyls [{(THF)Li{CH(CH2Ph)SO2Ph}}1] (185)134 and [{(THF)2Li {CMe(Ph)SO2Ph}}2] (186)135 adopt structures with central Li2S2O4 eight-membered rings. Rare examples of a-lithiated sulfones with direct LidC bonds include [{(THF)Li{CH(Me)SO2Ph}}1] (187)134 with a polymeric ladder-like structure built up by eight-membered Li2C2S2O2 rings and [{LiCH(CH2CH2OMen)SO2Ph}4] (188)135 (MenOH ¼ (−) menthol) showing a heterocubane Li4S4 core (Fig. 58). In [Li{CH(SPh)CH2CH2OtBu}(TMEDA)] (189),136 the Li-cation is C,O-coordinated by the O-functionalized alkyl phenyl sulfide, as is the potassium center in the deprotonated benzyl phenyl sulfoxide K[18-crown-6] (THF){PhS(]O)CHPh} (190).137

2.02.2.2

Aryl derivatives

Alkali-metal aryl compounds are common precursors to access more complex organometallic species. Since 2005, a plethora of new substituted aryl derivatives have been prepared, many of which have been characterized by X-ray diffraction, highlighting the broad structural diversity available in this class of compound. The simplest alkali-metal aryl compound, PhLi, has been investigated in detail in solution138 and in the solid-state, where it can adopt a range of solvates and aggregates including polymeric,139 tetrameric,140 dimeric141 and monomeric,142 depending on the choice of donor. Solid-state structures of unsolvated alkali-metal aryl derivatives are rare due to their poor solubility in hydrocarbon solvents, and therefore synchrotron powder X-ray diffraction measurements are often required for structural elucidation. Donor-free ortho-(191) and para-tolyl lithium (192) are polymeric in the solid-state (Fig. 59)143; the structures are comprised of {ArLi}2 dimers which extend out in an infinite coordination polymer via Z6-arene-lithium contacts. The same structural motif is surprisingly still observed for MesLi (Mes ¼ 2,4,6-trimethylphenyl) (193) despite increased steric demands.144

Fig. 58 Molecular structure of 188.

34

Organometallic Complexes of the Alkali Metals

Fig. 59 Polymeric structure of [p-Tol-Li]1 (192).

Increasing the steric demands even further to terphenyl-lithium derivatives, [2,6-(3,5-Me2-C6H3)2C6H3Li]2 (194) and [2,6(2,4,5-Me3-C6H2)2C6H3Li]2 (195), enables the isolation of hydrocarbon soluble, donor-free species that form discrete dimers in the solid-state (Fig. 60).145,146 Here, the lithium centers are supported by interactions with the ipso- and ortho-carbons of the peripheral aryl substituents. The addition of donors to terphenyl lithium compounds leads to rare examples of monomeric species.147 It is well established that the addition of external donors to alkali-metal organometallic reagents typically breaks down larger donor-free aggregates to well defined solvates with reduced aggregation. Common donors include Et2O, THF, TMEDA and PMDETA, but chiral donors including (−)-sparteine and cis-TMCDA have more recently been introduced to alkali-metal organometallic chemistry. Solvated dimers of PhLi (Fig. 61) are obtained with (−)-sparteine148 (196) and cis-TMCDA149 (197), both of which bear similar structural features to the TMEDA solvated PhLi dimer.141 The tridendate donor 1,2,3-trimethyl1,3,5-triazacyclohexane (198) also gives a dimeric structure with PhLi,35 which contrasts with the monomeric species obtained using the more flexible tridentate donor, PMDETA.142 Dimeric structures are also obtained using tetrahydropyran (THP) (199), an alternative ethereal solvent with reduced ring-strained compared to THF.64 Alkali-metal aryl derivatives bearing a single ortho-donor substituent can form internally solvated aggregates. For aryl derivatives bearing simple ortho-donors such as OMe (200)150 or NMe2 (201),151 the formation of mono-solvated tetramers akin to [PhLi(Et2O)]4140 is commonly observed. Many recent examples of di-solvated dimers have been reported for aryl-lithium species containing ortho-nitrogen152–155 (202–207) or oxygen donors156 (208) (Fig. 62); in each of these cases, the coordination sphere of the lithium center is satisfied by an external donor (Et2O, THF or TMEDA). Although both non-centrosymmetric and centrosymmetric dimers are observed in the solid-state for related compounds, insight into whether this behavior is retained in solution is limited.152 Detailed solution-state NMR studies on 208 reveal that it exists as a mixture of three diastereomeric chelated homo- and/or heterochiral bridged dimers in slow interconversion at 180 K in Et2O solution.156 Aryl-lithiums bearing amide donors (209) adopt tetrameric structures which contain two unique lithium environments157; the two central lithium atoms are arranged in a trigonal planar conformation bonded to three ipso-carbons, whilst the peripheral lithium atoms are in a distorted tetrahedral environment bonded to one ipso-carbon, two carbonyl oxygen donors and a single Et2O molecule (Fig. 63). Structurally diversity can be common for compounds bearing both oxygen and nitrogen donors. For example, aryl-lithiums bearing an ortho-imine substituent and fused 3,4-methylenedioxy group (210–217) can adopt polymeric, tetrameric or dimeric structures depending on the N- and C-substituents, and choice of solvent.155 Moving beyond single ortho-donors, the rise of organometallic pincer complexes based on 2,6-disubtituted aryl compounds has led to an increased number of reports of structurally characterized aryl-lithium precursors. These include aryl-lithiums bearing ether (218–219),158,159 sulfone (220),160 amine (221 −222),161,162 imine (223)163 and oxazoline (224)164 donors in the 2,6-positions (Fig. 64). In all cases, disolvated dimeric structures are obtained in the solid-state, but in the case of 218,158 the inflexibility of the

Organometallic Complexes of the Alkali Metals

Fig. 60 Dimeric structure of donor-free [2,6-(3,5-Me2-C6H3)2C6H3Li]2 (194).

Fig. 61 Examples of dimeric PhLi aggregates obtained using less traditional donors and solvents.

Fig. 62 Dimeric ArLi structures bearing a single ortho-nitrogen or oxygen donor.

Fig. 63 Tetrameric structure of ortho-lithiated 2-isopropyl-(N,N-diisopropyl)-benzamide (209).

35

36

Organometallic Complexes of the Alkali Metals

Fig. 64 Selected examples of 2,6-disubstituted aryl-lithium compounds bearing intramolecular oxygen (218–220) or nitrogen donors (221–224).

phenyl-ether substituent requires an external Et2O donor to satisfy the coordination vacancy of lithium. Related compounds bearing OMe (225) or OtBu (226) donors at the 2,6-positions adopt tetrameric165 or trimeric166 aggregates in the absence of external donor solvents. Heteroatom-alkali-metal interactions are not just limited to classical oxygen and nitrogen centered Lewis donors and examples with Li⋯F (227)167 and Na⋯F (228)168 contacts have been reported for alkali-metal aryl species bearing ortho-CF3 substituents (Fig. 65). These contacts are relatively weak however, and are easily disrupted when stronger, polydentate donors such as PMDETA are added (229).168 Compounds 228–229 also represent rare examples of structurally characterized aryl-sodium species; closely related dimeric TMEDA solvates bearing ortho-dialkylamino substituents (230 −231) have been documented.169,170 A tetrameric aryl-sodium bearing 2,6-OiPr substituents (232) has also been reported.171 A subset of internally solvated aryl-lithium compounds includes ortho-substituents that are also prone to deprotonative metallation, to give a unique family of di- or tri-metallated species that can serve as polyanionic pincer precursors. The ortho-lithiated silylamide 233 (Fig. 66) forms a THF solvated dimer in the solid-state, but in the absence of a coordinating solvent, a tetrameric, octanuclear species (234) is obtained in which one of the NMe2 groups on silicon acts as a neutral donor.172 Depending on the choice of aryl-substituents at nitrogen and the donor solvent, 2,6-disubstituted benzylamide derivatives can form tightly bound dimeric hexanuclear compounds (235) or solvent-separated tetranuclear carbanions (236) with Li(DME)3 counter-cations.173 Several related examples bearing tethered lithiated alkenes,174,175 thiols176 and thioethers177 have also been reported and structurally characterized. p-Extended aryl-lithiums can serve as unique model systems to study 2,6-disubstituted benzene derivatives. Anthracenyl-lithium forms a Et2O solvated dimer (237) which undergoes sequential donor addition and exchange.178 Structural and computational investigations on the conformation and LidCaryl bond length in the (mixed) solvates reveal that reactivity does not always scale linearly when strong donors are added, suggesting that mixed donors may allow for fine tuning of the reactivity of organolithiums to enhance selectivity.178 Ortho-substituted naphthyl-lithium derivatives generally adopt similar structures to their benzene counterparts, with or without external donors. Notably however, the two regioisomers of ortho-lithiated 2-dimethylamino-methyl-naphthalene (dman), 1-lithio-dman (238) and 3-lithio-dman (239), show very different chemistry towards donors in solution and in the solid-state.179 Donor-free 1-lithio-dman (238), which in insoluble in non-donor solvents, converts into the thermodynamically favored and hydrocarbon soluble 3-lithio-regioisomer (239) at 90  C; this is proposed to proceed through the involvement of heteroleptic intermediates. Extensive and combined crystallographic and solution-state studies have also been applied to simple heteroaryl-lithium derivatives. The aggregation of 2-thienyl-lithium with various donors has been investigated by single crystal X-ray diffraction, as well as 1D and 2D NMR spectroscopy (including DOSY NMR) to confirm that the solid-state aggregation is retained in solution.180

Organometallic Complexes of the Alkali Metals

37

Fig. 65 Dimeric structure of [(2-CF3-1-Na-C6H4)(TMEDA)]2 (228).

Fig. 66 Examples of functionalized aryl-lithium compounds bearing ortho-amide substituents.

This is an important consideration in organolithium chemistry, since it is the solution-state aggregation which dictates reactivity and hence selectivity.181 In a similar fashion to PhLi, 2-thienyl-lithium forms mono-solvated tetramers (with Et2O) (240), disolvated dimers (with THF, DME and TMEDA) (241–243) or trisolvated monomers (with PMDETA) (244) (Fig. 67). By contrast, deprotonative lithiation of thiophene in the presence of bis(2-methoxyethyl)ether (aka diglyme) affords a solvent-separated lithium lithiate species containing an anionic (C4H3S)3Li2(diglyme) fragment and a Li(diglyme)2 cation (245).182 Deprotonative lithiation of furan results in a 2-furyl-lithium tetramer solvated by free furan (246),183 reflecting the hard-hard match between oxygen donors and lithium centers, that is not observed with the softer sulfur donor in thiophene and its derivatives.

Fig. 67 Solid-state structures of 2-thienyl-lithium; (A) Et2O-solvated tetramer (240); (B) DME-solvated dimer (242); (C) PMDETA-solvated monomer (244).

38

Organometallic Complexes of the Alkali Metals

2.02.2.3

Cyclopentadienide, indenide and fluorenide derivatives

The first organometallic complex containing the cyclopentadienide (Cp ¼ C5H−5) ligand was reported in 1900184 and structural studies on this class of compound have continued to be fruitful for almost all metals across the Periodic Table, and for a range of cyclopentadienide derivatives. This section will primarily focus on alkali-metal complexes reported in the last 15 years, but several comprehensive reviews discussing alkali-metal cyclopentadienide complexes are available.185,186 The most common structural motif for alkali-metal Cp derivatives is an unsolvated or solvated linear polymer consisting of alternating Cp rings and alkali-metal cations. This motif has now been reported for all alkali-metals; Li (247)187; Na (248)187; K (249)187; Rb (250)188 and Cs (251)189 (Fig. 68). High-pressure synchrotron studies of [CpLi]1 (247) and [CpK]1 (249), reveal a high compressibility and significant reduction in unit cell volume with increasing pressure; for [CpK]1 (249), this unit cell reduction is manifested in an increased K+⋯Cp− bend angle from 45 (at 0 GPa) to 51 (at 3.9 GPa).190 Unsolvated linear polymers are also obtained when switching to pentamethyl-cyclopentadienide (Cp ¼ C5(CH3)5); Li (252)191; Na (253)192; Rb (254)193 and Cs (255).193 The linear polymer is typically not disrupted when simple monodendate donors such as THF194–196 (256–258) or pyridine (259)197 are added; the larger ionic radius of the alkali metal down group 1 is reflected in the higher coordination of K (257)195 and Rb (258)196 compared to Na (256).194 Monomeric alkali-metal cyclopentadienide derivatives can be obtained by the addition of suitable macrocyclic donors. For the parent Cp, these have now been reported and structurally authenticated for all alkali-metals using appropriately sized crown ethers; CpLi(12-c-4) (260),188 CpNa(15-c-5) (261)198,199; CpK(18-c-6) (262)200; CpRb(18-c-6) (263)201; CpCs(18-c-6) (264).201 Many related examples with modified Cp ligands, crown ethers, or solvates have been recently documented.202–210 The cyclopentadiene ligand is still available to act as a suitable donor, which can result in multi-decker sandwich cations of the formula [18-c-6⋯M⋯Cp⋯M⋯18-c-6]+ (265),211 and are frequently observed counter-cations in transition-metal and main-group chemistry.212–219 By contrast, anionic alkali-metal sandwich complexes of the formula [Cp-M-Cp]− have also been recently reported for Li (266)220 and K (267)221; the latter species is obtained from 1,3-(SiMe3)2-C5H3 in the presence of [2,2,2-cryptand]. Many examples of functionalized cyclopentadienide ligands have now been documented. These range from simple alkyl and aryl substituents that primarily influence the electronic and steric properties, tethered donors that can coordinate to the metal center, or functional groups that can engage in onward reactivity. A complete series (LidCs) of bulky pentaaryl-cyclopentadienide complexes bearing alkyl substituents (iPr, nBu or tBu) in the para-position have been documented (268–282, Fig. 69).222,223 Several

Fig. 68 Unsolvated and solvated linear polymers of CpAM and Cp AM (Cp ¼ C5H5; Cp ¼ C5(CH3)5; AM ¼ Li, Na, K, Rb, Cs).

Fig. 69 Pentaaryl-cyclopentadienide alkali-metal complexes (268–287).

Organometallic Complexes of the Alkali Metals

39

of these have been characterized by single-crystal X-ray diffraction, and whilst the unsolvated (275)222 or solvated (283–285)223 linear polymer remain the prototypical solid-state motif, donor-solvents can lead to charge-separated ion pairs, [(CpArtBu 5 )2Li] tBu 223 [Li(THF)4] (286), oligomers [(CpArtBu Func5 )2K3(CH3CN)10][(CpAr5 )2K] (287), or pseudo-charge-separated polymers (288). tionalized cyclopentadienide alkali-metal complexes bearing partially unsaturated cyclic (289)224 and saturated bicyclic (290)225 alkyl substituents have also been reported. Donor-functionalized cyclopentadienide complexes bearing CH(CH3)N(CH3)2 (290)226 and CN (291–292)227 substituents also adopt polymeric structures; the Li complex (290) is a 1D-polymer,226 whilst the K and Cs complexes (291–292) are lattices featuring intermolecular CN⋯AM interactions.227 Cyclopentadienide lithium complexes decorated with flexible and chelating N,O, O-donors are monomeric in the solid-state (293, Fig. 70A), and NMR studies show that this aggregation and internal coordination is retained in solution. Monomeric lithium complexes have also been reported for the ester-functionalized cyclopentadienide complex (294),228 albeit in the presence of TMEDA, whilst 1,2-diester-funtionalised Na-cyclopentadienide complex (295)229 is polymeric due to intermolecular C]O⋯Na interactions. The related 1,2-bis-amidine functionalized Na-cyclopentadienide (296) complex (N,N0 -fulvenealdiminates) forms a head-to-tail dimer.230 Lithium cyclopentadienide complexes functionalized with bulky phosphines are monomeric in the presence of DME, a chelating ethereal donor solvent (297–298, Fig. 70B). Several examples of cyclopentadienide phosphonium (299) and phosphonium diylide complexes (300–303)231,232 have been reported and structurally characterized with different substituents on phosphorus and the Cp ring. In the solid-state, head-to-tail dimers are obtained due to intermolecular coordination of the ylidic-carbon to the lithium center, whilst in THF solution, charge-separated ion pairs are observed.231 Cyclopentadienide complexes that contain organic substituents amendable to onward functionalization are attractive synthetic targets. Alkali-metal cyclopentadienide complexes bearing alkenes (304)233 and alkynes (305–307)234,235 (Fig. 71) have been reported and structurally characterized; solvated monomers or polymers are observed with no evidence of alkali-metal

(A)

(B)

Fig. 70 (A) Crystal structure of 293; (B) Lithium cyclopentadienide complexes functionalized with phosphines (297–298) and phosphonium ylides (300–303).

Fig. 71 Alkali-metal cyclopentadienide complexes bearing carbon-based (304–307) and anionic heteroatom functional groups (308–312).

40

Organometallic Complexes of the Alkali Metals

p-interactions in the solid-state. Whilst the onward reactivity of the alkyne functional group in 305–306234 was not explored, compounds 304233 and 307235 undergo a photolytic [2 + 2] cycloaddition to give the corresponding metallocenophanes. Treatment of 1,4-dilithiobutadienes with CO provides direct access to oxycyclopentadienyl dianions (308–309), which are solvated dimers in the solid-state.236 The alkylamido-cyclopentadienide alkali-metal complexes (310 −311)237 adopt THF-solvated oligomeric chain structures; compound 310 (Ar ¼ 4-OMe-C6H4) consists of repeating head-to-tail monomeric units, whilst 311 (Ar ¼ 2-OMe-C6H4) features an additional internal OMe-donor leading to a head-to-head arrangement. Direct two-electron reduction of anilydenecyclopentadiene with lithium metal affords the amide-functionalized dianionic complex (312),238 which adopts a charge-separated head-to-tail dimeric structure in the solid-state. Alkali-metal cyclopentadienide complexes bearing tethered aryl or benzyl substituents often display additional solid-state interactions between the arene and alkali-metal which can lead to unique structural motifs and diverse supramolecular assemblies. Donor-free benzyl cyclopentadienyl-lithium (313) self-assembles in the solid-state to give linear polymers; the benzyl substituents are orientated anti along the chain, with neighboring chains attracted by edge-to-face aromatic C-H⋯p interactions.239 For substituted Cp complexes featuring the larger and softer potassium cation (314–317),240–242 weak Z1 interactions between the pendant-arene and K+ are observed in the solid-state (K⋯CArene ¼ 3.23–3.692 A˚ ). Meta-terphenyl substituted cyclopentadienide complexes adopt cyclic oligomeric structures with large internal cavities. The potassium analog (318, Fig. 72A) is tetrameric whilst the cesium analog (319, Fig. 72B) is a cyclic hexamer, with both supramolecular assemblies fabricated from Z3 coordination to the cyclopentadienide rings and additional Z6 interactions with the flanking mesityl substituents. Indenide (Ind) and fluorenide (Flu) derivatives can be considered as benzo- and dibenzo-fused cyclopentadienide ligands respectively, and show similar aggregation in solution and in the solid-state. Novel monomeric alkali-metal complexes of the parent and substituted Ind and Flu ligands have recently been reported for Li (320− 322),243,244 Na (323)245 and K (324–325),246 with a range of different donors including N-heterocyclic carbenes (NHC), (−)-sparteine, 2,20 -bipyridine (bipy), and 18-crown-6 (Fig. 73A). Notably, the hapticity of the fluorenide coordination to the potassium center in 324–325246 is offset from the central five-membered ring and therefore deviates from the typical Z5 coordination (324, R ¼ H ¼ Z3; 325, R ¼ SiMe3 ¼ Z2). Linear polymeric structures have been documented for potassium indenide (326)247 and fluorenide (327)248 complexes, even in the presence of the tridendate donor, PMDETA. Several examples of lithium indenide and fluorenide complexes (328–330)249–251 functionalized with intramolecular tethered nitrogen-donors have been documented (Fig. 73B). In compounds 328249 and 329,250 the lithium coordination sphere is satisfied by an external molecule of Et2O or THF, whilst in 330,251 the bulky diisopropylphenyl (Dipp) group prevents additional solvent coordination leading instead to weak CH3⋯Li interactions in the solid-state. The lithium indenide complex 331,252 which is decorated with bulky substituents (SiMe3, CH2SiMe3 and Ph), adopts a intermolecular dimeric structure in the solid-state to give a bimetallic sandwich with syn-Z5:Z6-facial Li-coordination to the coplanar five-membered and benzo-fused rings (Fig. 73C).

2.02.2.4

Alkali metal interactions with p-systems

The interaction of alkali-metals with p-systems has gathered significant interest across a range of fields, with the simpler alkali-metal allyl complexes and related derivatives being primarily exploited as precursors in organometallic chemistry, whilst polycyclic hydrocarbons that are reduced directly with alkali-metals show unique physical and electronic properties, and therefore have promising applications within materials chemistry. Alkali-metal allyl complexes represent the simplest p-delocalised organometallic species, and as such have been widely studied in solution and the solid-state,253 as well as by theoretical methods.254,255 Similarly to cyclopentadienide, indenide and fluorenide alkali-metal complexes (see Section 2.02.2.3), polymeric structures are commonly obtained for allyl complexes, whilst the addition of suitable donors, or changing the nature of the substituents often

Fig. 72 Crystal structures of ArMesCp-AM cyclic oligomers. (A) Potassium tetramer (318); (B) Cesium hexamer (319).

Organometallic Complexes of the Alkali Metals

41

(A)

(B)

Fig. 73 Selected examples of non-polymeric alkali-metal indenide and fluorenide complexes bearing (A) external donors (320–325); (B) intramolecular donors (328–330); (C) intermolecular donors (331).

gives rise to well-defined aggregates. The parent allyl-lithium [LiC3H5] is monomeric in the presence of the bidentate donor (R, R)-TMCDA (332),149 whilst alkyl- and silyl-substituted lithium allyl derivatives often adopt dimeric structures, with or without donors (333–335).255–257 Many other functionalized (alkenyl (336–337),258 silyl (338–339),259,260 amine (340–341)261 and phosphine (342)262) alkali-metal allyl complexes are monomeric in the presence of TMEDA, PMDETA, toluene or intramolecular N,O,O-chelating donors (343),263 whilst monodendate ethereal donors afford polymeric structures (344–345).257,261 In some cases, the coordination of the lithium cation to the allyl-fragment changes from Z3 to Z2 or Z1 hapticity,258,261 with the latter being best described as an alkyl-lithium. This should also be distinguished from alkenyl-lithium complexes, in which the lithium is directly bonded to a non-conjugated sp2 carbon without an a-CHR2 group; several recent examples of solvated dimers (346–349), cyclic oligomers (350) and polymers (351) have been documented.264–266 Polymeric structures have been recently reported for SiMe3-substituted allyl complexes267 of the heavier alkali-metals (Na, K and Cs). Diverse supramolecular assemblies are observed in the solid-state with donor-free [Z3-K-1,3-(SiMe3)2-C3H3]1 (Fig. 74, 352)

Fig. 74 Diverse polymeric structures of donor-free (352) and solvated potassium (353–354) and cesium (355) 1,3-(SiMe3)2-C3H3 complexes.

42

Organometallic Complexes of the Alkali Metals

adopting a helical polymeric motif,255 whilst the corresponding DME (353)268 and THF (354)269 solvates adopt zig-zag structures with K-K0 -K and K0 -K-K0 angles of 141.9 /153.3 and 103.2 /170.2 respectively. By contrast, the THF-solvated Cs-analog (355)269 is a perfectly linear 1D-polymer whilst the THF-solvated Na-analog (356)270 is a cyclic tetramer. A cyclic tetramer has also been reported for a closely related lithium-allyl complex (357) functionalized with a silylamine (-SiMe2NC5H10) which acts as an intermolecular donor, whilst subsequent deprotonative lithiation affords a dodecanuclear cyclic hexamer (358) with an internal C6Li6 core.271 Pentadienide complexes can be considered as intermediary structures between allyl and cyclopentadienide complexes. [2,4-tBu2-C5H5K(THF)]1 (359)272 is polymeric with Z5 coordination of the potassium cation to the delocalised anion, whilst enantiomerically pure pentadienyl complexes derived from (1R)-(−)-myrtenal (360–362)273–275 are monomeric in the presence of 18-crown-6, although the hapticity of the K-coordination can vary between Z5, Z4 and Z3 depending on the C1 or C2 substituents (H, Me or SiMe2NiPr2). 1,4-Dilithiobutadienes are common precursors to access five-membered hetero- and metallocycles. In the solid-state, the two lithium cations lie above and below the dianionic C4 plane with uniform bond distances to both C1 and C4. A trimeric hexanuclear aggregate has been reported for donor-free 1,2,3,4-tetraethyl-1,4-dilithiobutadiene (363, Fig. 75A)276 whilst derivatives with bulkier silyl- or aryl-substituents adopt dimeric tetrahedral structures with (364–365)276,277 or without (366, Fig. 75B)278 external donor solvents. Monomeric 1,4-dilithiobutadiene complexes are obtained through the addition of TMEDA (367)279 or when functionalized with flexible intramolecular N,O,O-chelates (368).280 Cyclic p-bonded carbanions beyond five-membered cyclopentadienide ligands have found widespread use in organometallic chemistry, particularly in f-block chemistry. Reduction of 1,2,3,4-tetrakis(trimethylsilyl)cyclobuta-1,3-diene with potassium affords the corresponding [K2{Z4-C4(SiMe3)4}] complex (339)260 which is essentially monomeric in the solid-state with weak contacts between the potassium centers and CH3 groups of neighboring molecules in the extended polymeric crystal lattice. Cyclooctatetraene (COT) also forms a delocalised dianion that can be accessed by direct alkali-metal reduction or deprotonative metallation of readily accessible precursors. Silyl-substituents are commonly added to improve solubility, and structurally characterized examples have been recently reported for Li, Na and K analogs.281,282 The lithium complex of 1,4-bis(trimethylsilyl)-cyclooctatetranide (369, Fig. 76A)281 is a tetrametallic structure consisting of two lithium cations sandwiched between two non-planar COT rings with two lithium cations Z3-coordinated to the outer COT faces and solvated by THF. By contrast, the sodium analog (370, Fig. 76B)281 is a 1D-polymer with Z8 Na-coordination to planar COT rings and bridging coplanar THF donors. Similar polymeric structures are obtained for 1-, 1,3- and 1,4-trimethylsilyl substituted potassium COT complexes in the presence of DME (371–372) or toluene (373).282 The bulkier [K2(DME)2{Z8-C8H6-1,4-(SiPh3)2}] (374, Fig. 76C)282 is monomeric in the solid-state, but one of the potassium centers is coordinated to only three of the four available oxygen donors of the chelating ethereal donor solvent, and additional weak intra- and intermolecular CAr-H⋯K interactions (2.9676–3.7907 A˚ ) are also observed. A complete alkali-metal series (Li-Cs) of dibenzo[a,e]cyclooctatetraene dianions have been reported283; the Li complex (375) is a THF solvated monomer whilst the Na (376), K (377), Rb (378), and Cs (379) analogs all adopt THF solvated polymeric structures. The addition of 18-crown-6 to 377 affords a monomeric species (380), whereas a solvent separated ion-triple is obtained with [2.2.2]-cryptand (381).283 The p-expanded system, octaphenyltetrabenzocyclooctatetraene (OPTBCOT) can be reduced with lithium metal to a stable tetraanion (382),284 which contains two solvent separated Li cations and two internally coordination Li cations; one is sandwiched between two peripheral phenyl rings in an asymmetric Z6-fashion, whilst the other resides deeply within the central cavity. The potassium complex of the cyclononatetraenyl anion (383) adopts a Et2O-solvated polymer consisting of a disordered mixture of both the cis,cis,cis,cis- and cis,cis,cis,trans-isomers.285 Radical anions of aromatic hydrocarbons are widely used as powerful reducing agents. Solid-state studies into monocyclic derivatives are limited however due to their instability and poor solubility. 1,4-bis(trimethylsilyl)benzene is reduced to the

Fig. 75 Molecular structures of oligomeric 1,4-dilithiobutadienes illustrating the (A) octahedral Li6 core in the donor-free trimer (363) and; (B) tetrahedral Li4 core in the donor-free dimer (366).

Organometallic Complexes of the Alkali Metals

43

Fig. 76 Solid-state structures of alkali-metal 1,4-silyl-substituted cyclooctatetranide (COT) complexes; (A) lithium dimer (369); (B) sodium polymer (370); (C) potassium monomer (374).

corresponding radical anion with potassium metal in the presence of 18-crown-6 (384)286; the solid-state structure of the monomer (Fig. 77A) reveals an approximate Z5-coordination of the K+ cation to the planar anion which displays significant bond length alterations when compared to the neutral arene.287 When 1,2-diphenylbenzene is reduced with rubidium metal in THF, the Birch-type reduction product 385 (Fig. 77B) is obtained.288 Despite the use of THF as the reaction solvent, or addition of strong chelating donors, the solvent-free solid-state structure instead reveals a number of stabilizing intermolecular p-arene contacts to the soft Rb cations, leading to a three-dimensional coordination polymer. The potassium salt of the naphthalenide radical anion is a THF-solvated 3D-polymer (386)289 [K2(C10H8)2(THF)]1 or 18-crown-6 solvated 1D-polymer (387),290 whilst the lithium analog [Li+(TMEDA)2][C10H8•-] is a solvent-separated ion-pair in the presence of TMEDA (388).291 Naphthalene can be further reduced to the dianion for which the dilithium salt has been reported as the TMEDA solvated monomer (389, Fig. 77C) with each Li cation Z4-coordinated above and below the puckered C10H8 core.291 Many reduced polycyclic hydrocarbons contain a five-membered ring system and therefore can be viewed as arene-fused derivatives of cyclopentadienide (see Section 2.02.2.3) with extensive p-conjugation. Pentalane (C8H6) is a polycyclic hydrocarbon comprised of two fused cyclopentadiene rings. The pentalene dianion was first isolated in 1962,292 35 years before the neutral hydrocarbon,293 but only structurally characterized in 1985 as the DME solvate (390), [Li2(DME)2C8H6].294 The hexamethylpentalene dianion has more recently been reported as the TMEDA solvate (391, Fig. 78),295 and this dianionic pentalene framework also lies at the core of several benzo-fused polycyclic systems. The dibenzo[a,e]pentalene dianion (392–393) can be accessed, albeit in low yields, by direct lithium metal reduction of phenyl silyl acetylenes.278,296 Improved yields are obtained when using K to afford the corresponding dipotassium dibenzopentalenide, which has been structurally characterized as the DME solvate (394).297 By contrast, the [a,f]-isomer of dibenzopentalene requires a multi-step organic synthetic route, with the final stage being the two-electron oxidation of the dilithium salt (395).298

Fig. 77 Selected examples of reduced aromatic hydrocarbons: (A) monomeric potassium benzene radical anion (384); (B) polymeric rubidium anion (385); (C) monomeric dilithium naphthalane dianion (389).

44

Organometallic Complexes of the Alkali Metals

Fig. 78 Dianionic polycyclic hydrocarbons derived from pentalene (391–395).

Macrocyclic donors such as 18-crown-6 are frequently employed to aid in the isolation and structural elucidation of reduced polycyclic hydrocarbons, particularly for the heavier alkali-metals (K, Rb or Cs). However, because the negative charge is effectively delocalised across the conjugated p-system, solvent separated ion structures are commonly obtained. Indeno[1,2-b]fluorene is sequentially reduced to the dianion which can be structurally characterized as the 18-crown-6 solvated Rb complex (396, Fig. 79A).299 The crystal structure contains two independent molecules in the asymmetric unit in which the two Rb cations are either Z2- or Z6-coordinated above or below the central six-membered ring. The isomeric indeno[1,2-a]fluorene dianion300 has been characterized as the THF-solvated Cs complex (397), which contains two unique Cs environments; one Cs cation coordinates to the terminal six-membered ring and is solvated by two molecules of THF, whilst the second cation coordinates to both five- and six-membered rings—further contacts to neighboring dianions lead to a 2D polymeric ribbon in the crystal lattice. The tetracene derivative, 5,6,11,12-di-o-phenylene-tetracene (DOPT), is likewise reduced to its dianion using Rb or Cs metal in the presence of a suitable polydentate donor; the Rb complex (398) is a monomeric 18-crown-6 solvate, whilst the Cs complex (399) is polymeric with bridging tetraglyme [CH3O(CH2CH2O)4CH3] donors.301 Heteroatom containing polycyclic hydrocarbons and their reduced analogs have also been widely studied. The pyrene-fused azaacene dianion (400, Fig. 79B), isolated as the 18-crown-6 potassium salt, is an open-shell diradical with a small singlet-triplet energy gap.302 A series of reduced diindenosiloles and their p-extended derivatives, which contain one or three five-membered silicon heterocycles at its core, have been prepared and their physically properties studied.303 Alkali-metal reduction of the pentacyclic diindenosilole with lithium metal affords the dimeric dianion (401) in which two lithium cations are sandwiched between two antiparallel dianions through Z5-coordination to the cyclopentadienide rings, with the other Li cations coordinated to the open faces and solvated by THF. The sodium (402) and potassium (403) analogs

(A)

(B)

Fig. 79 (A) Selected examples of dianionic polycyclic hydrocarbons with heavy alkali-metals (396–398). (B) Selected examples of dianionic poly(hetero)cyclic hydrocarbons (400–404).

Organometallic Complexes of the Alkali Metals

45

are polymeric in the solid-state but adopt similar structural features to the lithium dimer. By contrast, the heptacyclic system which contains three fused-siloles at its core, is monomeric when reduced to the corresponding dianion with excess Li metal in THF (404).303 Polyaromatic hydrocarbons (PAHs) can serve as molecular models for sp2-hybridized carbon allotropes, such as graphite, fullerenes and carbon nanotubes, which are promising candidates as anode materials for energy storage devices and other organic electronics.304 Corannulene, C20H10, is a bowl-shaped subunit of C60 fullerene which, with its doubly degenerate lowest unoccupied molecular orbital (LUMO), can accept up to four electrons to form a stable tetraanion.305 The lithium salt of [C20H10]4− forms a sandwich structure with five Li cations intercalated between two eclipsed tetraanionic corannulene decks (405, Fig. 80).306 The remaining three lithium cations are sequestered away from the anionic fragment by 12-crown-4 and/or THF, whilst in the absence of 12-crown-4, two Li cations bind to the external surface of the supercharged sandwich (406–407), with only one solvent-separated [Li(solv)n]+.306,307 A collection of corannulene tetraanions featuring mixed alkali-metal sandwiches has been reported, each displaying unique structural features and supramolecular motifs. The LiK5 (408)308 and LiRb5 systems (409)309 feature five solvated K or Rb cations occupying the peripheral interior sites and encapsulated by the six-membered rings, akin to 405, with an additional Li cation at its core and sandwiched between the central five-membered rings. Significant curvature of the corannulene surface is observed when moving to the heavier alkali-metals, and the Li3K3 (410),308 Li3Rb3 (411),309 Li4Rb2 (412)309 and Li3Cs3 (413)310 complexes all adopt a clamshell structure in which the sandwich is hinged opened to accommodate up to three of the larger cations. Several examples of corannulene-based monoanions, dianions and trianions have also been structurally characterized,307,311–317 as have the accessible anions of the related bowl-shaped sumanene (C21H12).318–320 Alkali-metal fullerides MxC60 (M ¼ Li-Cs; x ¼ 1–3) possess many unique physical properties, but detailed structural analyzes of these materials has been hindered due to difficulty in growing single crystals suitable for X-ray diffraction studies. The potassium fulleride radical anion [C60K(THF)5] (414)321,322 is monomeric with the solvated K cation coordinated to a hexagonal face, whilst the cesium analog [C60Cs(THF)4]1 (415)323 is polymeric, leading to a Jahn-Teller distortion (i.e., a significant deviation from the ideal Ih symmetry). Alkali-metal cations can also be encapsulated inside fullerenes, and whilst solid-state studies reveal that the lithium cation in [Li@C60][SbCl6] (416)324 is located in an off-center position within the carbon-cage, solution state 13C NMR spectroscopy shows only a single peak indicative of a symmetrical yet dynamic coordination environment. Reduced cycloparaphenylenes (CCP), which are molecular nanohoops with a structural relationship to carbon nanotubes, have also attracted experimental and theoretical interest.325 Dianions of [6]CCP (417),326 [8]CCP (418),327 10[CCP] (419)327 and [12] CCP (420)327 have been structurally characterized as potassium solvates; in compounds 417–419, the K cations are coordinated to opposite sides of the exo (exterior) surface, whilst 420 is a solvent separated ion-triple. [8]CCP can be further reduced to a tetraanion (421)328 which engages in both exo and endo (interior) p-coordination to the potassium cations, two of which are encapsulated within the open cavity of the highly distorted carbon nanobelt.

2.02.2.5

Alkynyl derivatives

The tendency of alkali-metal alkynyls towards aggregation is well-documented, with donor ligands playing a key role in determining the degree of aggregation.30 Addition of 1,3,4,5-tetramethylimidazole-2-ylidene (IMe) to tert-butylethynyl-lithium gives a tetrameric aggregate [tBuC^CLi
IMe]4 (422)329 which displays a common heterocubane structural motif (Fig. 81).30 The terminal carbanionic sp-Ca atom m3 bridges each of the four Li3 triangles, while each Li-atom of the Li4-tetrahedra is further coordinated by the carbon atom of the carbene ligand. Another typical type of aggregation is the C6Li6-type core, which has been observed in the solid state structures of lithiated oligoacetylenes [{CH2Si(Me)(C2Li)}3(THF)3] (423) and [{CH2Si(Ph)(C2Li)}3(THF)3] (424).330

Fig. 80 (A) Molecular structure of the corannulene tetraanionic sandwich complex (405). The three solvent-separated lithium cations have been omitted for clarity. (B) Top-down space-filling view of 405, illustrating the position of each Li cation within the interior of the sandwich.

46

Organometallic Complexes of the Alkali Metals

Fig. 81 Solid state structures of lithium alkynyls (A) tetrameric 422 and; (B) dimeric 423.

Intercalation of three uniformly oriented lithium acetylide functions of the all-cis-forms of the 1,3,5-triethynyl1,3,5-trisilacyclohexane framework of one monomer between the carbanions of a second molecule gives rise to a C6Li6-type dimeric structure. The increased polarisability and larger ionic radii of heavier alkali-metals results in extensive aggregation that requires the use of large, multidentate donor ligands to facilitate isolation and characterization of these species. Employing a K-selective polydentate 18-crown-6, that can saturate the coordination sphere of the metal center, afforded a monomeric potassium tolylacetylide [CH3PhC^CK(18-c-6)] (424).331

2.02.2.6

Ylide, yldiide, and methandiide derivatives

Since their discovery over a century ago, ylides,332 and in particular phosphonium ylides, have proved to be an extremely important class of nucleophile.333–335 In addition to their widespread synthetic utility,336,337 the electronic structure of these carbon bases has also been a matter of great interest and often dispute.338,339 Although ylide chemistry is now well established, studies of their metal derivatives are gaining momentum, and this section of the chapter will introduce pertinent findings of this emerging area from the perspective of alkali-metal derivatives. Fig. 82 summarizes the nomenclature and structural representation of ylide-based derivatives and related carbon bases discussed herein. A large number of multifunctional ylide-based ligands exist including diylides, triylides and bisylides.340 More recent developments have seen reports on a-metalated ylides, or yldiides, which due to their unique structures should act as ligands with s- and p-donor abilities. Currently there are not many examples of structurally characterized yldiides due to the associated synthetic challenges, but these valuable species can be viewed as a link between the two unique families of carbon bases, namely bisylides and methandiides. Methandiides with their dianionic carbon center have attracted considerable research interest in the last 20 years

Fig. 82 Nomenclature and structural representations of ylide derivatives and related species.

Organometallic Complexes of the Alkali Metals

47

due to their utility as precursors for new carbene complexes of metals across the periodic table.341–344 These (and their monoanionic precursors, i.e., methanides) are typically accessible by deprotonation of corresponding substituted methanes by very strong bases. Considering the highly ionic character of AMdC bond, the stability of these geminal carbanionic species is challenging but is attained by the strong anion-stabilizing ability of the P(V) moiety and a donor side arm. Although most examples include nitrogen donor-side arms, examples with donating groups of weaker coordinating ability (e.g., a sulfonyl moiety) have also been realized. The presence or absence of additional donor functions within the methanide and methandiide ligands leads to diverse and interesting structural properties of formed alkali-metal species. New examples of lithium phosphonium diylides include [(TMEDA)Li(CH2-PPh2-CH-PPh2)] (425),345 [(TMEDA)Li {CH2-PPh2-Flu}](426),346 [(PMDETA)Li{CH2-PPh2-Flu}] (427),346 and [(Et2O)Li{CH2-PMe2-Flu}]2 (428)347 (Fig. 83), as well as previously mentioned Cp-functionalized examples 300–303231 (see Section 2.02.2.3). In monomeric 425,345 TMEDA-capped Li is further coordinated by a phosphorus atom and a carbon atom of the monoanionic ylidic ligand, exhibiting an overall distorted tetrahedral coordination environment. Observed bond lengths within the formed five-membered ring are indicative of overall electron delocalization and partial multiple bond character within the CPCP fragment.345 Lithium complexes 426 and 427 contain the same [{CH2PPh2Flu}−] diylidic ligand, but due to the differences in the denticity of the Lewis base donor, distinct coordination to the Li cation is evident. In 426,346 where bidentate TMEDA is capping Li, diylidic ligand shows chelating coordination to Li via its methylene carbon and a Z1 electrostatic interaction with the fluorenylidene moiety. Switching to tridentate PMDETA in 427346 affords again tetrahedral coordination environment for the Li-cation, but the diylide is acting as a monodentate ligand coordinated through methylene carbon. In the presence of smaller Me substituents on the phosphorus atom and monodentate donor ligand, a dinuclear complex 428 is formed.347 In 428 the pendant methylene group acts as a chelating and bridging donor forming a four membered Li2C2 ring exhibiting a two-electron-three-center bonding. The coordination sphere of Li-center is completed by a Z3-coordination of fluorenyl moiety and a molecule of diethyl ether. Examples of structurally authenticated triylides are fewer and include [(Et2O)Li2{(CH2)2-PMe-Flu}]2 (429)347 and [Li2{PhP (CH2)3}
2THF]2 (430)348 (Fig. 84). Phosphonium triylide 429 adopts a dimeric structure exhibiting a fused Li4C4P2heterodicubane structural motif where the cubes share a common Li2C2 face. This cluster type has been previously observed in [(THF)Li2{H2CS(NtBu)2}]2 (431).349 Like in diylide counterpart 428, in 429 fluorenyl moiety again engages in Z3-bonding mode and ylidic methylene functionality in three-center two-electron type bonding. Triylide 430348 also displays a dimeric arrangement, where the fully lithiated, inverted umbrella-shaped phosphoylide dianions [PhP(CH2)3]2− are assembled together by four THF-solvated Li-cations.

Fig. 83 Examples of structurally characterized phosphonium diylides 425–428.

Fig. 84 Structures of triylides 429 and 430.

48

Organometallic Complexes of the Alkali Metals

Despite these examples, lithiation of phosphorus ylides can prove challenging. Typically employed are strong lithium bases such as tBuLi or sBuLi, whereas lithium amide LiHMDS was shown to be ineffective, and an adduct with phosphonium ylide 432350 was isolated (Fig. 85A). Another interesting example is the isolation of 433,351 where rather than the expected a-lithiation of the ylide Ph3PC(H)CH3, deprotonation in the ortho-position of the phenyl substituent took place (Fig. 85B). Metallation of sulfonyl-, sulfinyl- or cyanide-functionalized phosphonium ylides afforded new examples of yldiides which could be isolated and structurally characterized in the form of alkali-metal salts. Within symmetrical dimer {(Ph3P-C-SO2Tol)3Na3(THF)}2 (434),352 six yldiide units are assembled in two face-connected (NaO)4 cubes in which Na-centers form contacts with O-atoms of the sulfonyl moiety and with the ylidic C-atoms (Fig. 86A). In [(18-c-6)
K
(Ph3P-C-SO2Tol)] (435),352 the K-atom is analogously coordinated by the yldiide, but the crown-ether fills its coordination sphere affording overall a monomeric complex (Fig. 86B). Lithium yldiide 436353 (Fig. 87) also adopts a dimeric structure with a Li2O2 core, however the two Li-centers display different coordination spheres. One is tetra-coordinated by the two yldiide ligands (through O- and C-atoms), whereas the other Li-center shares the coordination to the two O-atoms of the yldiide and completes its coordination with two terminal THF molecules.353 In unsymmetrical aggregates [{(Ph3P–C–CN)-Li}3(LiHMDS)5] (437)354 (Fig. 88) and [{(Ph3P–C–CN)-Na}8(NaHMDS)2] (438)354; and in dimeric [{(Ph3P–C–CN)-Li}3(15-c-5)]2 (439),354 the alkali-metal cations feature contacts with both ylidic Cand cyanide N-atoms. The molecular structure of 439 consists of six yldiides and two crown ethers, with a central (Li–C–C–N)2 eight membered ring connected to two Li2N2 four-membered rings, while [{(Ph3P–C–CN)–Li}4(18-c-6)]1(440)354 (Fig. 89A) exhibits a polymeric structure consisting of two different dimeric subunits of Li2N2 four-membered rings. Interestingly, in one of these units, the yldiides again feature mixed C/N coordination of the metal, while in the other unit Li is coordinated through the nitrogen end of the CN and by the crown ether. By contrast, [{(Ph3P–C–CN)–Na}(15-c-5)] (441)354 and [{(Ph3P–C–CN)–K}(18-c-6)] (442)354 (Fig. 89B) exhibit monomeric structures, where the harder Na-atom coordinates solely to the N-atom of the nitrile, while the softer K-atom prefers the interaction with the ylidic and nitrile carbon atoms in an unusual Z2-binding to the C-C linkage.

Fig. 85 Molecular structures of (A) LiHMDS adduct of phosphonium ylide 432 and; (B) ortho-deprotonated Ph3PC(H)CH3 ylide 433.

Fig. 86 Molecular structures of (A) 434 and; (B) 435. Non-coordinating THF molecules and H-atoms omitted for clarity. Ph substituents of PPh3 fragment and p-Tol on S in 434 represented as wire-frame for clarity.

Organometallic Complexes of the Alkali Metals

Fig. 87 Molecular structure of 436. Tipp substituent on S represented wire-frame and all hydrogen atoms omitted for clarity.

Fig. 88 Molecular structure of 437. All hydrogen atoms omitted for clarity and Ph-substituents of PPh3 fragment represented wire-frame for clarity.

Fig. 89 Molecular structures of (A) 440 (portion of polymeric arrangement) and; (B) 442. All hydrogen atoms omitted and Ph substituents of PPh3 fragment represented as wire-frame for clarity.

49

50

Organometallic Complexes of the Alkali Metals

A range of bis(iminophosphoranyl)methanide alkali-metal complexes has been prepared, and many of these include N-bound trimethylsilyl, adamantyl, phenyl or mesityl substituents. Examples include [Li{HC(PPh2NSiMe3)2}]2 (443),355 [Li{HC (PPh2NAd)2}]2 (444)356 and [Li{HC(8-C9H6N)P(iPr2)]NtBu}]2 (445),357 which in the solid state display a dimeric arrangement with the methanide carbons bridged by two Li-centers forming a four-membered ring. In 443 and 444 (Fig. 90A), four nearly planar LiNPC four-membered rings are fused to the core metallacycle, two of which project above and two below the central Li2C2 plane. In 445 (Fig. 90B) two of these four-membered LiNPC rings are replaced by the two quinolyl groups. Sodium and potassium congeners [Na{HC(PPh2NAd)2}(THF)2] (446)356 and [K{HC(PPh2NAd)2}(THF)2] (447)356 are also accessible by direct deprotonation of the parent ligand with benzyl species, while the heavier group 1 methanides [Rb{HC(PPh2NSiMe3)2}(THF)2] (448)356 (Fig. 90C), [Cs{HC(PPh2NSiMe3)2}(DME)2] (449)356 [Rb{HC(PPh2NAd)2}(THF)2] (450)356 and [Cs{HC(PPh2NAd)2}(DME)2] (451)356 are obtained by trans-metallation of lithium derivatives 443 or 444 with MOR (M ¼ Rb, Cs; OR ¼ 2-ethylhexoxide). Complexes 446–451 display similar structures with the bis(iminophosphorano)methanide ligands coordinating to the metal center through the nitrogen lone pairs forming six-membered chelate rings without any methanide C⋯M contacts. Metal cations satisfy their coordination spheres through coordination of two molecules of solvent and by engaging in electrostatic interaction with one of the P-phenyl rings. Structurally characterized alkali-metal complexes of N-aryl substituted bis(iminophosphoranyl)methanides include the mesityl (Li—Cs, 452–456) and 2,6-diisopropyl (Li—Cs, 457–463) series. All mesityl-substituted complexes (Fig. 91) adopt a six-membered metallocyclic structure in which the methanide ligand behaves as a bidentate chelating ligand without metal⋯methanide contacts. In monomeric Li (452),358 Na (453)359 and Rb (455)359 complexes the coordination sphere of the alkali-metal is completed by solvent molecules ranging from one molecule of ether in 452 to two molecules of bidentate DME in 455 as the ionic radii of the metal increases. Larger Na and Rb also engage in secondary Na⋯Cipso and Rb-Z2-aryl interactions. Solvent free K (454)360 and Cs (456)359 congeners adopt dimeric and hexameric structures in the solid state, respectively, through Z6-M-aryl interactions. Such extensive intermolecular interactions involving each Cs center make the formation of hexameric wheel structure of 456 favored even in the presence of donor solvents such as THF. A solvent-free lithium bis(iminophosphoranyl)methanide complex that adopts a monomeric structure was achieved by increasing the steric demands of the N-substituents to 2,6-diisopropylphenyl (Dipp). In complex [Li{CH(PPh2NDipp)2}] (457),361 the Li-center again shows no interaction with the methanide center and is coordinated by the two imino N-atoms, to affords an almost planar CP2N2Li ring, and a weak interaction with a Dipp-Cipso center (Fig. 92A). The potassium congener, [K{HC(PPh2NDipp)2}]1 (458),362 on the other hand, is polymeric and the K-center is now C,N-chelated by the ligand with further Z6 and Z2 interactions with surrounding arene groups of the non-coordinating imino-arm (Fig. 92B). The larger ionic radius of K and its greater propensity to engage in electrostatic interactions in comparison to Li is the most likely reason behind the formation of polymeric 458 compared to monomeric 457.

Fig. 90 Molecular structures of (A) 444; (B) 445 and; (C) 448.

Fig. 91 Structures of complexes 452–456.

Organometallic Complexes of the Alkali Metals

51

Fig. 92 Molecular structures of (A) 457 and; (B) portion of the polymeric chain of 458.

Solvated complexes 459 (Na),359 462 (Rb)359 and 463 (Cs)359 display very similar monomeric structures (Fig. 93) as seen in previously mentioned examples, with the differences in the size of alkali-metal being reflected in an increased number of interactions. With K, two different THF-solvates have been structurally characterized: monomeric [K{HC(PPh2NDipp)2}(THF)2] (460),359 and dimeric [{K[HC(PPh2NDipp)2](THF)}2] (461)359 (Fig. 93). In monomeric 460, the methanide ligand chelates the K-center via one imino-nitrogen and the methanide carbon, while the other imino-nitrogen is non-coordinating. The coordination sphere of K is completed by two molecules of THF, an Z6-interaction with the Dipp substituent of non-coordinating imino arm, and by one Cipso⋯K contact with the other Dipp substituent. Aged hexane solutions of 460 deposited crystals of the loose dimer 461, as a result of losing a molecule of THF. In 461, the K-center is coordinated by the two imino-nitrogen centers, lacking any methanide-K interactions, and with a molecule of THF. The coordination sphere of K is completed by a weak interaction with Dipp-Cipso and Ph-Cortho interactions from the coordinated ligand, and Dipp-Cmeta interactions from another ligand giving rise to a dimeric structure. Removing one of the iminophosphoranyl arms, can still afford a stable and isolable lithium methanide as evidenced by the isolation and structural characterization of [(THF)2Li{CH(PPh2)(PPh2]NSiMe3)}] (464).363 In monomeric 464 (Fig. 94), the phosphine (iminophosphoranyl)methanide coordinates to the metal center via the N–P–C backbone in a heteroallylic fashion affording a four-membered metallacycle, with two molecules of THF completing the coordination sphere of Li.

Fig. 93 Structures of complexes 459–463.

Fig. 94 Molecular structure of 464.

52

Organometallic Complexes of the Alkali Metals

Treating bis(iminophosphoryl)methane ligands with two equivalents of alkali-metal bases leads to the formation of dilithio methandiide complexes [Li2{C(PPh2NAd)2}]2 (465),364 [Li2{C(PPh2N(CHMeiPr))2}]2365 (466), [Li2{C(PPh2NMes)2}]2366 (467), [Li2{C(PPh2NSiMe3)2}]2 (468)367,368 and [Li2{C(PPh2N(p-Tol))2}
THF]2 (469),364or disodio complex [Na2{C (PPh2NSiMe3)2}]2 (470).369 Mixed metal derivatives [LiNa{C(PPh2NSiMe3)2}]2 (471),369 [LiNa3{C(PPh2NSiMe3)2}2] (472),370 [LiK{C(PPh2NSiMe3)2}]2 (473),370 [NaK{C(PPh2NSiMe3)2}]2 (474),370 [Na3K{C(PPh2NSiMe3)2}2] (475)370 are also accessible by transmetallation or sequential metallation. The central structural motif consists of four coplanar metal centers capped by ligands forming distorted M4C2 octahedral cores (Fig. 95). In homometallic complexes 465–470, the four M-atoms form an approximately square M4 unit in which all four atoms form reasonably strong interactions with the carbanions. In heterometallic complexes 471–475, the metal atoms are arranged in a rhombohedral manner which can, based on interactions with the ligand, be described as a contact ion pair in which the lighter metals interact strongly with the carbanions, while the heavier metals are pushed to the periphery of the structure. The mixed-metal mixed-anion complex [NaK{CH(PPh2NSiMe3)2}2{N(SiMe3)2}
Tol] (476)370 was serendipitously isolated, and forms due to the inability of sodium amide NaHMDS to deprotonate the parent bis(iminophosphoryl)methane ligand (Fig. 96).

Fig. 95 Representation of complexes 465–475.

Fig. 96 Molecular structure of mixed-metal mixed-anion complex 476.

Organometallic Complexes of the Alkali Metals

53

The increased steric bulk of the N-bound 2,6-diisopropylphenyl substituent in [{Li2C(PPh2NDipp)2}2] (477)371 still allows for the formation of the dimeric structure as found in complexes 466–468, albeit with somewhat decreased symmetry. However, if lithiation is performed in the presence of the chelating TMEDA ligand, unusual, monomeric [Li2{C(PPh2NDipp)2}
TMEDA] (478)361 is obtained. In 478 (Fig. 97A), the two Li-centers are found in different environments. While one Li cation maintains its N,N0 -chelated position, showing relatively short contact to the methandiide center, the other TMEDA-capped Li-center forms an ionic bond with the same methandiide center, resulting in an overall trans-planar geometry.361 Addition of donating THF to 477 afforded monomeric complexes [Li2{C(PPh2NDipp)2}
(THF)2] (479)371 (Fig. 97B) and [Li2{C(PPh2NDipp)2}
(THF)3] (480),371 while the addition of PMDETA afforded [Li2{C(PPh2NDipp)2}
PMDETA] (481),371 in which steric constraints cause unsymmetrical bonding of tridentate PMDETA. Comparison of the closely related structures 478–481 shows that the methandiide ligand can adopt one of two isomeric forms—W in which two four-membered chelate rings are formed around a central, spirocyclic C-atom (found in 479 and 481), or a U-form in which a six-membered chelate ring and a further LidC bond are formed (found in 478 and 480). The use of excess lithium base in the synthesis of 477 can lead to the isolation of small quantities of methanide side product [HC(Ph2PNDipp)(Ph2P)Li(THF)2] (482)371 (Fig. 98A), in which one of the P(V) centers is reduced to P(III), yielding a solid state structure very similar to complex 464. More controlled and reproducible further lithiation of slurries of 477 is also possible. Adding an further equivalent of alkyl-lithium to 477 at elevated temperatures afforded [Li3C{(PPh2NDipp)(PPh(C6H4) NDipp)}]2 (483)371 and [Li3C{(PPh2NDipp)(PPh(C6H4)NDipp)}
(THF)4] (484),371 in which ortho-lithiation of the P-bound Ph substituent takes place. The core of 483 (Fig. 98B) features two W-form trianionic ligands and six Li-cations, in which each ligand now contributes the N- and two C-atoms to bonding. Addition of the donor THF to 483 induces monomerisation affording 484 (Fig. 98C), in which the methandiide moiety is again in the U-form, N,N0 -chelating the Li(THF) fragment. Metallation in the ortho-phenyl position with highly electropositive metal ions has been previously observed for cesium.69 Seeking to isolate methandiide complexes of the heaviest alkali-metals (K, Rb and Cs), [(K2{C(PPh2NPh)2})2(THF)4] (485),69[(Rb2{C(PPh2NPh)2})2(C6H6)4] (486)69 and a mixed metal [LiCs{C(PPh2NSiMe3)2}
THF]2 (487)69 were obtained when

Fig. 97 Molecular structures of (A) 478 and; (B) 479.

Fig. 98 Molecular structures of (A) 482; (B) 483 and; (C) 484.

54

Organometallic Complexes of the Alkali Metals

the parent ligand was treated with benzyl-metal reagents. All three complexes adopt a similar dimeric aggregate, previously seen in complexes 465–474. However, likely due to the larger size of these cations, in the above examples (e.g., complexes 465–470), only the “inner” two M+ ions are connected to the C-atom and the dimers are held together solely by N-M interactions. Attempts to isolate an all-cesium analog yielded a cyclic decomposition product 48869 proposed to form by ortho-metallation of the dicaesiated methandiide followed by nucleophilic attack on the other phosphonium unit to effect ring-closure. Co-crystallizing with benzylcaesium in 488, the benzyl anions bridge to neighboring symmetry-equivalent units of cyclic bis(amidophosphorano)methandiide via a dicaesiated C-atom (Fig. 99). Replacing one of the imino arms with a softer sulfur or selenium donor affords a ligand system with two donor sites of different hardness. Metallation of such ligands afforded methanide complexes [Li{CH(PPh2]NSiMe3)(PPh2]S)}] (489),372 [K{CH(PPh2]NSiMe3)(PPh2]S)}]2 (490)373 (Fig. 100A) [K{CH(PPh2]NSiMe3)(PPh2]Se)}]2 (491)374 and methandiide [Li2{C(PPh2]NSiMe3)(PPh2]S)}]2
THF (492)372 (Fig. 100B). Monomeric 489 contains the six-membered metallacycle formed by the ligand and lithium center in a twisted-boat conformation, while dimeric 492 contains a central Li2C2 plane formed by Li-centers bridged by two geminal dianionic ligands. These two central Li-atoms are further coordinated by one N- and one S-atom of each ligand. The remaining two Li-atoms are on the periphery coordinated by two S- and two N-atoms respectively. [K{CH(PPh2]NSiMe3)(PPh2]Se)}]2 (491) exists as a centrosymmetric dimer with a central K2Se2 ring built through weak secondary K⋯Se interactions between monomeric units. Each K-center further engages in intramolecular (Z3 and Z6) interactions with neighboring phenyl groups.374 A very similar structure is adopted by the sulfur analog [K{CH(PPh2]NSiMe3) (PPh2]S)}]2 (490),373 in which only Z6 interactions between K and neighboring Ph group are observed.

Fig. 99 Molecular structure of [488
{C6H5CH2Cs}2
4(THF)].

Fig. 100 Molecular structures of (A) 490 and; (B) 492.

Organometallic Complexes of the Alkali Metals

55

Deprotonation of a-imino- or a-thiophosphinoyl-substituted sulfone afforded methanides [{(NDippPPh2)CHSO2Ph}K] (493)375 [{(NDippPPh2)CSO2Ph}Li2] (494),375 [{[PPh2S][SO2Ph]CH}Li(THF)2]2 (495)376 and [{[PPh2S][SO2Ph]C}2Li4(THF)3]2 (496),376 in which a combination of hard/soft coordinating sites can again be found. In 493, the asymmetric unit contains a dimeric structural motif where the two monomeric subunits are connected via two K-atoms, both of which show slightly different coordination environments, whilst none of the K-atoms show contact to the methanide center. In 494, the central structural motif is formed by two almost planar (SO2Li)2 eight-membered rings which are connected via the methandiide C-atoms and additional four Li-atoms. Both 495 and 496 exhibit dimeric structures. In 495 (Fig. 101), the Li-center is not coordinated by the methanide C-atom or thiophosphinoyl moiety, but by coordination of the two sulfone and two THF O-atoms giving rise to a central (OSOLi)2 eight-membered ring in a chair-like conformation. The solid state structure of 496 consists of four methandiide ligands and six coordinating THF molecules arranged so that Li-atoms form 4-, 6-, or 8-membered rings with the donor atoms which are connected with each other by one common side. Unlike in 495, in 496 the Li-atoms show contacts to the methandiide C-atom and the thiophosphinoyl moiety. Isoelectronic replacement of one the sulfonyl oxygen atoms with an NMe group affords a chiral sulfoximine within this ligand framework. Double lithiation of this ligand allows for the preparation and isolation of enantiomerically pure dilithiomethane [{[PPh2S][PhSO(NMe)]C}Li2THF]2 (497)377 (Fig. 102), in which the metallated C-atom exhibits two contacts to the Li-atoms.

Fig. 101 Molecular structure of 495.

Fig. 102 Molecular structure of dimeric S-497 which co-crystalizes with two molecules of [Me3SiOLi
THF] which have been omitted for clarity.

56

Organometallic Complexes of the Alkali Metals

Examples of thiophosphinoylmethanide complexes include lithium [(TMEDA)
Li{CH(PPh2]S)}2] (498)378 and [(THF)(Et2O)
Li{CH(PPh2]S)}2] (499),379 [{(PPh2S)(PPh2O)CH}Li
py]2 (500)380 and [{(PPh2S)(SiMe3)CH}Li
OEt2]2 (501)381 and potassium [{(PPh2]S)2CH}K]1 (502)378,382 [{1,3-C6H4(PhP]S)2CH}K
3THF] (503)382 derivatives. Selenophosphinoylmethanide complex [K{CH(PPh2]Se)}2]1 (504)378 has also been prepared and structurally authenticated. Symmetrically substituted 498378 and 499379 adopt monomeric structures with the Li-center tetrahedrally coordinated by the two S-atoms of the thiophosphinoyl groups, and completed by the solvent molecules. 500 and 501 contain the unsymmetrically substituted methanide ligand, and adopt dimeric structures with central Li2E2 rings (E ¼ O in 500 and E ¼ S in 501). In the mixed P]S/P]O thiophosphinoyl methanide 500,380 the pyridine solvated Li center is S,O-chelated by the methanide ligand and further coordinated to the O-donor of one the of the neighbouring units affording a central Li2O2 plane. In 501381 (Fig. 103A), where only one donating heteroatom is present, ether solvated Li-center is S,C-chelated by the methanide ligand, with the S-atom of the symmetry generated second unit giving rise to a central Li2S2 ring.381 As seen in imino derivatives, within the monomeric units of 502378,382 and its Se-congener 504378 (Fig. 103B), potassium coordinates to the donor atoms of the thiophosphinoyl substituents forming a twisted six-membered metallacycle without any interaction with the methanide center. The six-membered P2E2CK rings (E ¼ S, Se) connect into a polymeric ladder structure through coordination of K-centers to the chalcogen atoms of adjacent molecules. In 502, both sulfur atoms are coordinated to the K-center of a neighboring unit, whilst in 504 only one Se-center interacts with K, which is reflected in the number of Z6-Ph interactions—one in 502 and two in 504. In monomeric 503,382 the tris-THF solvated K-center is coordinated by both S-atoms and methanide center of the ligand constructing a KSPCPS metallacycle in a pseudo-boat conformation. Reported examples of thiophosphinoylmethandiide complexes include symmetrical-substituted [{(PPh2S)2C}Li2]2
(Et2O)n (505)383 where n ¼ 2 or 3, and unsymmetrically-substituted [Cy2P(S)CP(S)Ph2]Li2 (506),384 as well as mixed P]S/P]O {[PPh2S][PPh2O]C}Li2 (507).380 All three complexes adopt a dimeric structure in which the belt of four Li-atoms is capped top and bottom by mutually orthogonal methandiide ligands. This structural motif differs significantly from the bis(iminophosphorane)methanediide dimers such as 465–468 (vide supra) with a pseudo-octahedral Li4C2 core cluster. Combining phosphonate or thiophosphinoyl moiety with a boranophosphorano side-arm afforded examples of lithium methanides [{(PPh2O)(PPh2BH3)CH}Li
OEt2]2 (508),385 [{(PPh2S)(PPh2BH3)CH}Li
THF]2 (509),385 and lithium methandiide [{(PPh2S)(PPh2BH3)C}Li2
2DME] (510).385 In dimeric 508 and 509 (Fig. 104), the lithium atoms are not directly coordinated to

Fig. 103 Molecular structures of (A) 501 and; (B) portion of a polymeric chain of 504.

Fig. 104 Molecular structure of 509. All hydrogen atoms except BH3 units omitted for clarity.

Organometallic Complexes of the Alkali Metals

57

Fig. 105 Molecular structure of 511. All hydrogen atoms except BH3 units omitted for clarity.

Fig. 106 Molecular structures of (A) 515 and; (B) 517.

the methanide center but are supported by two O- or S-atoms of two units, one BH3 moiety and a molecule of solvent. Methandiide complex 510 crystalises as a monomer with the two Li-cations coordinated to the central dianion and their coordination spheres completed by the chelating solvent molecule. A lithium methanide with two adjacent phosphine-borane groups, [{(PPh2BH3)2CH}Li
(Et2O)0.75] (511)386 (Fig. 105), displays a dimeric structure in the solid state. The two Li-centers are found in different coordination environments, but both engage in Z2-BH3 contacts with both boranophosphorano side-arms of the same ligand. Finally, a series of diphosphinomethanide complexes has been reported. Solvent-free potassium complex [K{CH(PPh2)2} (OEt)0.5]387 has been prepared, but eluded structural characterization in the absence of donor ligands. Treatment with excess tetrahydropyran yielded polymeric [K{CH(PPh2)2}(THP)]1 (512), whilst neutral bidentate ligands afforded discrete dinuclear complexes [K{CH(PPh2)2}(DME)]2 (513) and [K{CH(PPh2)2}(TMEDA)]2 (514), and oligodentate ligands such as diglyme and 18-crown-6 afford [K{CH(PPh2)2}(diglyme)2] (515) (Fig. 106A) and [K{CH(PPh2)2}(18-c-6)][K{CH(PPh2)2}(18-c-6)(THF)] (516).388 Addition of oligodentate ligands reduces the interactions between the K-center and the diphosphanylmethanide anion from both P-atoms, methanide C-atom and two p-Ph interactions found in 512–514 to only one P-atom in 515 and 516. Switching from potassium to the harder Lewis acid Li with higher affinity for coordination to the hard O-atom of THP ligands leads to increased donor ligand content and the isolation of monomeric [Li{CH(PPh2)2}(THP)3] (517).388 In 517 (Fig. 106B), and structurally comparable [Li{CH(Ph2P)2}(EtO2)2] (518),389 no interaction with the methanide C-atom is evident, but electrostatic interactions with ligand through the P-atoms are observed.

2.02.2.7

Metallocene derivatives

The metallation of metallocenes provides access to a rich class of organometallic chemistry. The 1,10 -dilithiation of ferrocene using nBuLi and TMEDA was first reported in 1967,390 and the product crystallized as the trimeric solvate [Fe(Z5-C5H4)2]3Li6(TMEDA)2 (519) in 1985391; this compound has been adopted as a readily accessible and widely used precursor to synthesize functionalized ferrocenediyl-derivatives. Recrystallisation of 519 from tetrahydrofuran affords the THF solvated dimer

58

Organometallic Complexes of the Alkali Metals

[Fe(Z5-C5H4)2]2Li4(THF)6 (520),392 which possesses structural features similar to the dimeric PMDETA solvate [Fe(Z5-C5H4)2]2Li4(PMDETA)2 (521),393 illustrating the significant influence that donors can have on the solid-state structure. The selective monolithiation of ferrocene can be achieved by using the mixed tBuLi/KOtBu superbase, or by lithium-halogen exchange at low temperatures. Mixed-metal bases have also been successfully employed for the regioselective mono-, di- and tetrametallation of metallocenes. Donor-free monolithioferrocene is poorly soluble in non-coordinating solvents, and is proposed to adopt a polymeric structure in the solid-state, akin to [CpLi]1 or [PhLi]1, whilst the THF solvated complex [Fe(Z5-C5H4) (Z5-C5H5)]2Li2(THF)4 (522, Fig. 107A) is dimeric, similar to many aryl-lithiums (see Section 2.02.2.2). Related head-to-tail dimeric motifs have been reported for ferrocenophane (523–524, Fig. 107B)394 and ferrocene (525–527)395 derivatives containing dimethylamino-substituents, whereas a head-to-head dimeric structure is observed for the Et2O mono-solvate (528, Fig. 107C).395 The donor-free monolithioferrocene {Fe(Z5-C5H5)(Z51,3-Ar22-LiC5H3)}2 (where Ar ¼ 3,5-tBu2-C6H3; 529), which contains bulky flanking aryl-rings, is also dimeric in the solid-state.396 Transition-metal sandwich complexes beyond ferrocene can be 1,10 -dilithiated in a similar fashion, furnishing versatile starting materials for the synthesis of ansa-bridged complexes and metallocenophanes after quenching with a suitable electrophile. The PMDETA solvated dimers of 1,10 -dilithiated osmocene (530)397 and ruthenocene (531)397 adopt similar solid-state structures to the ferrocene derivative (521, Fig. 108A).392 This same dimeric motif is also observed for many heteroleptic sandwich complexes in which one of the cyclopentadienide rings is replaced with benzene, Z6-C6H6 (532–533, Fig. 108B),398,399 cycloheptatrienyl,

Fig. 107 Examples of dimeric monolithioferrocene derivatives containing inter- and intramolecular donors: (A) 522; (B) 523; (C) 528.

(A)

(B)

(C)

(D)

Fig. 108 Examples of 1,10 -dlithiated metallocenes that adopt PMDETA solvated dimers; (A) homoleptic species derived from [M(Z5-C5H5)2] (521, 530–531); (B) heteroleptic species derived from [M(Z5-C5H5)(Z6-C6H6)] (532–533); (C) heteroleptic species derived from [Ti(Z5-C5H5)(Z7-C7H7)] (534); (D) heteroleptic species derived from [Ti(Z5-C5H5)(Z8-C8H8)] (535).

Organometallic Complexes of the Alkali Metals

59

Z7-C7H7 (534, Fig. 108C),400 or cyclooctatetraene, Z8-C8H8 (535, Fig. 108D).401 Several THF solvated dimeric 1,10 -dilithiated metallocenes,397,402 including bis(benzene)molybdenum, [Mo(Z6-C6H5)2]2Li4(THF)6 (536),403 have also been recently reported. Interestingly, although troticene [Ti(Z7-C7H7)(Z5-C5H5)] is exclusively monolithiated on the seven-membered ring with nBuLi in Et2O within 3 h,404,405 extended reaction times (36 h) or the addition of PMDETA affords the thermodynamically favored species, [Ti(Z7-C7H7)(Z5-C5H4Li-PMDETA)] (537), which is monomeric in the solid-state.400

2.02.3

Assessing aggregation of alkali metal organometallics using NMR spectroscopy

The solvation and aggregation of alkali-metal organometallic species has a profound influence on their reactivity, which ultimately dictates selectivity and yields.181 This important factor has driven the development of new experimental techniques targeted to elucidate and understand the solution-state composition, which may not always reflect what is observed in the solid-state (see Section 2.02.2). Traditionally, the aggregation of organolithiums in solution has been investigated using colligative property measurements such as cryoscopy, or has exploited low temperature 13C and 6/7Li NMR spectroscopy to determine solution-state composition based on Li-C J coupling. Kinetic studies have also been successfully applied to correlate the aggregation of alkali-metal organometallics to reaction rates, whilst more recent DFT calculations have provided new insights into the mechanism and barriers associated with solvation processes.

2.02.3.1

DOSY NMR studies

Diffusion-Ordered NMR Spectroscopy (DOSY) is a 2D NMR experiment that, with the aid of the Pulsed Gradient Spin-Echo (PGSE) method, enables the separation and measurement of diffusion coefficients of individual species in solution. This can be directly correlated to the hydrodynamic radii (i.e., the size and shape) of molecules in solution using the Stokes-Einstein equation. By introducing suitable internal references, complications arising from changes in temperature, viscosity and concentration can be eliminated, and molecular weights can be predicted based on the measured diffusion coefficients.406 Although this internal calibration curve (ICC) method has been successfully applied to a range of organometallic systems, the necessity for three internal references comes with several drawbacks and limitations. More recently, the external calibration curve (ECC)22 method has been introduced which exploits a single internal reference and normalized diffusion coefficients at a defined concentration, which in turns provides a more practical method for estimating molecular weights. 1 H DOSY NMR studies have recently been undertaken to reinvestigate the solvation and aggregation of nBuLi, which complements prior spectroscopic407,408 and solid-state studies26,57 of this ubiquitous organolithium reagent. In hydrocarbon solvents, a temperature-dependant equilibrium between an octamer {nBuLi}8 and hexamer {nBuLi}6 was established, whilst in ethereal solvents (Et2O or THF), the tetra-solvated tetramer {nBuLi(THF)}4 is the predominant species in solution, which further deaggregates to the tetra-solvated dimer {nBuLi(THF)2}2 at high THF concentrations.409 By contrast, sBuLi exists as a mixture of hexamers and tetramers in hydrocarbon solvents, however the presence of small amounts of sBuOLi due to adventitious O2 leads to the formation of mixed sBuOLi/sBuLi hexameric and octameric aggregates.410 The hexamer-tetramer equilibrium has also been established by 1H DOSY NMR spectroscopy for cyclopentyl-lithium, which crystallizes as a hexamer from hydrocarbon solvents, whilst the crystalline THF-solvated tetramer {C5H11Li(THF)}4 retains its solid-state aggregation in solution.56 The bulkier, silyl-substituted alkyl-lithium LiCH(SiMe3)2 is proposed to be dimeric in hydrocarbon solvents, whilst the Na congener exists as a mixture of tetramer and hexamer depending on the concentration.93 1H DOSY NMR studies also demonstrate that the crystalline DME87 and Me2N(CH2)2OMe65 solvated dimers of LiCH2SiMe3 retain their dimeric aggregation in solution. Interestingly, whilst the Me2N(CH2)2OMe solvate of benzyl-lithium adopts a cyclic tetrameric arrangement in the solid-state, 1H DOSY NMR suggests that it deaggregates to a dimer in toluene,65 which is uncommon for non-coordinating solvents. Heteronuclear DOSY NMR studies using 6/7Li or 13C isotopically enriched species can be used to overcome the drawbacks of overlapping resonances commonly observed in the narrow frequency range of 1H NMR. These methods have now been successfully applied to elucidate the solution-state aggregation of simple alkyl-lithiums which have previously precluded analyzes using traditional techniques. At low temperatures (−103  C) in THF, 6Li NMR DOSY shows that iPrLi exists as a 1:2 mixture of the tri-solvated monomer {iPrLi(THF)3} and the di-solvated dimer {iPrLi(THF)}2, with an interconversion DG{ value (barrier) of 38 kJ mol−1.411 13C DOSY NMR studies confirm that MeLi exists as tetra-solvated tetramers {MeLi(Et2O)}4 in diethyl ether, whilst di-solvated dimers {MeLi(amine)}2 are adopted in the presence of a range of chelating amines including TMEDA, trans-TMCDA, N, N0 -dimethylbispidine, PMDETA, and (−)-sparteine. 7Li DOSY NMR studies have been used to investigate exchange processes in heteroleptic organolithium aggregates. Specifically, the crystalline mixed alkyl/aryl lithium {tBuLi}4.4{Me2NC6H4Li}4 exists as a non-statistical mixture containing the two homoleptic and three possible heteroleptic tetramers, with a thermodynamic preference for the homoleptic species.151 The aggregation of donor-base stabilized 2-thienyl-lithium {C4H3SLi(donor)x}y has been probed in both the solid-state (see Section 2.02.2.2) and in non-coordinating solvents.180 1H DOSY NMR studies confirm that the solid-state structure of the monomeric PMDETA adduct, and dimeric TMEDA and DME adducts are retained in toluene, whilst partial dissociation of THF from the crystalline tetra-solvated dimer {C4H3SLi(THF)2}2 has been attributed to an equilibrium with the tetra-solvated tetramer {C4H3SLi(THF)}4. 1H and 7Li DOSY NMR studies on the separated ion-pair 2-thienyl-lithium lithiate solvated by diglyme, [Li(diglyme)2]+[(diglyme)Li2(C4H3S)3]−, also show that this constitution exists in solution.182 Combined crystallographic and

60

Organometallic Complexes of the Alkali Metals

DOSY NMR studies have further been utilized on the 4-lithiated substituted pyridine (4-Li-2,6-Mes2-py), which is a tetra-solvated dimer in the solid-state but deaggregates in THF to the tri-solvated monomer.412 The ECC DOSY NMR spectroscopy method has enabled the solution-state chemistry of the complete series of alkali-metal cyclopentadienides to be studied in THF solution.413 Whilst polymeric structures are commonly adopted for alkali-metal cyclopentadienide complexes in the solid-state (see Section 2.02.2.3), discrete solvated monomers {CpM(THF)n} were found in bulk THF for the lithium, sodium, potassium and rubidium analogs. Two molecules of THF coordinate to the cation in the lithium analog, which increases to three for sodium and potassium as the cation size increases. Surprisingly however, only two molecules of THF coordinate to the monomeric {CpRb(THF)2} system, illustrating the fading donor ability of THF with increasing softness on descending group 1. By contrast, the heaviest cesium analog was estimated to have a molecular weight >1500 g mol−1, and was proposed to adopt a cyclic oligomeric structure in solution, {CpCs(THF)2}n (n ¼ 5 or 6).

2.02.3.2

Solution constitution of lithium amides

Due to their widespread use in organic synthesis, there has been considerable interest and research efforts aimed to elucidate the solution-state constitution of the trio of utility lithium amides414; lithium diisopropylamide (LDA), lithium hexamethyldisilazide (LiHMDS) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP). Many crystallographic, computational, and solution-state studies on these species have now been documented, however this section will focus on more recent developments using NMR spectroscopy, which has greatly deepened our understanding of these synthetically valuable reagents. Early NMR spectroscopy studies of LiHMDS identified a tetramer-dimer equilibrium in hydrocarbon solvents, and a dimer-monomer equilibrium in ethereal media. Interestingly, donor-free LiHMDS exists as cyclic trimer in the solid-state, highlighting the contrasting aggregation behavior in the two phases. Arene solvents still possess considerable donor ability, which leads to the dimer {LiHMDS}2 being the sole species observed in toluene solution. More recent 6Li and 15N NMR studies have revealed the intricacy of the THF-dependent aggregation of LiHMDS, which firstly identifies mono- and di-solvated dimers with 0.3–1 equivalents of THF, and further deaggregation to a solvated monomer {(THF)nLiHMDS} (n ¼ 3, 4) at high THF concentrations (Fig. 109). The monomer-dimer equilibrium is also very sensitive to the steric profile of the ethereal donor; with THF the monomer-dimer ratio is 1:3 whilst for 2,2-dimethyltetrahydrofuran the ratio is 35:1. Further substitution to 2,2,5,5-tetramethyltetrahydrofuran however refavours the dimeric aggregate, a process that has been attributed to competing donor/amide steric interactions. In the presence of the chelating amine TMEDA, a solvated monomeric structure is adopted in both the solid-state and in solution. Complementing these prior studies, the aggregation and binding constants of a range of oxygen-based donors towards LiHMDS has recently been investigated by 1H DOSY NMR spectroscopy.415 6 Li-15N double labeling NMR experiments on LDA in non-donor solvents imply that it exists as a mixture of cyclic oligomers, including the dimer and trimer among other proposed higher oligomers. This again contrasts with the solid-state structure of LDA which was determined to adopt a polymeric helical chain motif. DOSY NMR studies have been carried out to further elucidate the aggregation of LDA in non-donor solvents. At room temperature in toluene, a 2:1 mixture of the cyclic trimer {LiNiPr2}3 and cyclic tetramer {LiNiPr2}4 was observed, whilst at 100  C the trimer is the favored species, with no evidence of the previously proposed donor-free dimer. On cooling to −50  C, a pentameric species can be identified by 1H NMR spectroscopy with excellent molecular weight correlation. An additional minor species can be observed on further cooling, which was attributed to a hexamer, but the poor signal-to-noise ratio precluded a measurement of its diffusion coefficient and subsequent molecular weight determination.

Fig. 109 THF-dependant aggregation and solvation of LiHMDS.

Organometallic Complexes of the Alkali Metals

61

In THF solution, LDA exists predominantly as the di-solvated dimer {LDA(THF)}2, whilst mono- and di-solvated dimers and trimers can be identified with sub-stoichiometric quantities of THF in non-donor solvents. The addition of bidentate donor TMEDA does not lead to the expected solvated monomer, and instead the cyclic dimer with only a single nitrogen coordinated to each lithium center is observed. Furthermore, the coordination of TMEDA is only favored at low temperatures and readily dissociates or is displaced by THF, reflecting the clear preference for monodendate donors. The aggregation of LiTMP in non-donor solvents was initially investigated using 6Li and 15N NMR spectroscopy, revealing that it exists as a mixture of cyclic trimers and tetramers, with several different chair conformations possible for each oligomer. DFT studies predicted that these oligomeric isomers have similar relative energies, hinting at the possibility of polymorphs, which was shown experimentally through the characterization of the cyclic trimer in the solid-state, 30 years after the cyclic tetramer. DOSY NMR studies support the notion that these two oligomers coexist in solution, regardless of which crystalline polymorph is dissolved; however a temperature dependent equilibrium was established in which the trimer predominates at low temperatures. Furthermore, it was found that the trimer is significantly more stable in non-arene solvents, as evidenced by a significantly slower conversion and equilibration to the tetramer in cyclohexane when compared to benzene. In THF, 7Li NMR spectroscopy shows a concentration-dependent monomer-dimer equilibrium for LiTMP. Similar monomer-dimer equilibria were observed for the related amide LiPMP (lithium 2,2,4,6,6-pentamethylpiperidide) in the presence of TMEDA, in which the dimer adopts an “open” configuration (which has been determined for LiTMP in the solid-state), whilst the monomeric species {(TMEDA)LiPMP} is favored with a 10-fold excess of the bidentate amine donor. The aggregation of LiTMP with precise stoichiometric THF additive has also been assessed. Combined 7Li and DOSY NMR spectroscopy studies reveal that the crystalline species {(THF)LiTMP}2 dissociates in cyclohexane into a solvated dimer and unsolvated higher oligomer, with trace amounts of a solvated monomer present in solution. The solvated dimer was proposed to be either the closed or open dimer, whilst the unsolvated species could either be the cyclotrimer or cyclotetramer, illustrating the further complexity of organolithium aggregation in mixed donor/non-donor solvents (Fig. 110). DOSY NMR studies utilizing both the ICC406 and ECC22 methods have been utilized to examine the solution-state composition of N-substituted lithium anilides.416 Heteronuclear (6Li, 13C and 15N) NMR spectroscopy417 and solid-state studies418 have previously identified a multi-stage progression from di-solvated dimer to tri-solvated monomer (Fig. 111), which is influenced by the sterics of the amide and donor solvent. Nevertheless, DOSY NMR studies in THF or 2-MeTHF indicate that tri-solvated monomers (LiAD3) were adopted in all cases for a range of N-substituted lithium anilides, despite the Li2A2D3 or Li2A2D4 structures observed in the solid-state. These findings, based on experiments that were performed under synthetically relevant conditions, helped to rationalize the observation that it was the formation of small kinetically activated aggregates that was responsible for the observed reactivity, and not pre-coordination of the substrate to the lithium amide.416

(A)

(B)

(C)

(D)

Fig. 110 Different aggregation states of LiTMP: (A) cyclotetramer; (B) cyclotrimer; (C) di-solvated cyclodimer; (D) “Open” di-solvated dimer.

Fig. 111 Solution-state constitution of N-substituted lithium anilides. A ¼ amide (-NArR); D ¼ donor.

62

2.02.4

Organometallic Complexes of the Alkali Metals

Mixed alkyl/alkoxide aggregates of alkali metals

The binary mixture of n-butyl-lithium and potassium t-butoxide, widely known as the Lochmann-Schlosser superbase or LICKOR, displays remarkable utility in synthetic chemistry as a broad-spectrum base with exceptional metalating properties. Its practical utility stems from carefully balanced combination of Brønsted basicity (higher than that of butyl-lithium) and kinetic stability (higher than that of butyl-potassium). It is intriguing that LICKOR has been widely utilized for challenging (poly)deprotonations since 1967,419,420 yet its constitution still remains elusive. Recently, however, some light has been shed on these mixtures with the isolation of the first bimetallic-aggregate that contains all constituents of the LICKOR base and a simple metallated hydrocarbon substrate. The post-metallation product of the reaction between LICKOR and benzene was isolated in the form of discrete dilithiumtetrapotassium cluster complex of formula [(PhK)4(PhLi)(tBuOLi)(THF)6(C6H6)2] (538)421 (Fig. 112). The central four-membered ring comprises one phenyl-lithium and one lithium tert-butoxide fragment, which are surrounded by a twisted eight-membered ring of four phenyl-potassium units. Within the core of the aggregate, the two Li-atoms are coordinated by “hard” anions—one tert-butoxide and three phenyl anions with “in-plane” interactions of significant s character. By contrast, larger and softer K-atoms, are arranged on the periphery of this core between the four outer phenyl groups, with out-of-plane p-Ph-K interactions. Further employing 538 for lateral metallation of toluene can illustrate its own behavior as a superbase. One of the main obstacles in the characterization of LICKOR mixtures is the extremely low solubility of the corresponding alkyl-potassium which is ultimately formed. Its precipitation shifts the equilibrium to the product side, precluding the characterization of relevant compounds present in only low concentrations. Replacing the butyl group of the classical Lochmann-Schlosser superbase, with a higher neopentyl (CHt2Bu, Np) homolog, ensured good solubility in hexanes, and thermal stability at room temperature thus enabling characterization of [LixKyNpz(OtBu)x+ y− z] mixtures.422,423 The structural similarity of OtBu and CHt2Bu (Np) leads to statistical replacement of OtBu with Np groups, decreasing the symmetry of resulting molecule and enhancing its solubility. Due to this structural analogy between OtBu and Np groups, the basic structure of the mixture [Li4K4Np3(OtBu)5] (539) exhibits both statistical substitutional disorder and conformational/positional disorder, therefore the constitution and interatomic distances are only approximate, but the molecular connectivity is conclusive (Fig. 113). 539 comprises of a [K4]4+ square plane

Fig. 112 Molecular structures of (A) 538 and; (B) core motif of 538 with terminally coordinated THF and C6H6 molecules omitted.

Fig. 113 Molecular structure of 539 with disordered components omitted for clarity.

Organometallic Complexes of the Alkali Metals

63

Fig. 114 Stepwise exchange of tert-butoxy groups by neopentyl groups from all-alkoxide Li4K4(OtBu)8 to 539, observable by 1H NMR concentration studies.

capped from both sides with [NpLi(OtBu)2LiNp]2− dianions positioned in a staggered fashion to each other. The arrangement of eight metal centers outlines the corners of a truncated tetrahedron, giving rise to four LiK2 triangles capped mainly by Np groups and four Li2K2 trapeziums capped exclusively by OtBu groups. Solution-state studies of varying NpLi/tBuOK stoichiometries mixtures indicate a complex equilibria of compounds ranging from all-alkoxide [Li4K4(OtBu)8]424 to 539 (Fig. 114). Adding an equivalent of LiNp to[Li4K4(OtBu)8] alters the 1H NMR spectrum, and leads to a decrease in the intensity of the two distinguishable OtBu signals of the all-alkoxide species, alongside concomitant increase of the singlet resonances of Np group, suggesting the presence of Li4K4Np(OtBu)7. Addition of a second equivalent of LiNp leads to the formation of Li4K4Np2(OtBu)6, observable in the 1H NMR spectrum as two isomers. The first isomer in which the second LiNp group is incorporated in the same Li(OtBu)2Li centered unit gives rise to a singlet, whereas the second isomer in which the LiNp group is placed next to the Li(OtBu)2Li unit on the other side of K4 plane is observed in the 1H NMR spectrum as two symmetric doublets, due to the diastereotopic splitting of the CH2 signal. Upon further increasing the LiNp content to yield 539, the NMR spectra become more complicated, and in addition to the broad signal corresponding to 539, signals for Li4K4Np2(OtBu)6 are also found, indicating that 539 partially dissociates in solution. Further increasing the Np content results in reduced stability of the formed cluster, eventually leading to separation of the Li and K species and yielding charge-separated structures [Li4(OtBu)3]+[K3Np3(OtBu)]− (540a) or [Li4(OtBu)3]+[K3Np4]− (540b), formed by elimination of one KNp or KOtBu unit from 539. The combined units within 540a or 540b form a Li3K3 octahedron with opposing Li3 and K3 triangular faces. The Li2K faces are capped by OtBu groups which also connect to the fourth lithium atom capping the Li3 triangle. The K3 face is capped by the m3-bonded Np group that shows conformational disorder, while the groups capping the LiK2 faces feature substitutional disorder between OtBu and Np. In solution, the 1H NMR signals of 539, 540a and 540b are indistinguishable and cannot be assigned due to their participation in fast-exchange equilibria. It is worth noting, that the nBu compound with a similar formulation to 539 loses its structural integrity at an even earlier stage. Varying the content of both metals and alkyl/alkoxy ratio, two other alkane-soluble alkyl/alkoxy mixed aggregates could be structurally characterized—heterometallic [{Li4KNp2(OtBu)3}2] (541) and homometallic [{K4Np(OtAm)3}2] (542).425 From a LiNp:LiOtBu:KOtBu ¼ 1.5:1.0:1.0 mixture in hexane, lithium-rich complex 541 was isolated (Fig. 115). Within each monomeric unit, the five metal centers are arranged at the corners of a square pyramid affording the square Li4 base and Li2K triangle faces. The Li4 face is m4-capped by a OtBu-group, while the each Li2K triangle is m3-capped by either two OtBu or two Np-groups. The two neighboring Li4K units are assembled into a dimeric arrangement through the interaction of neighboring CH2-fragement and the potassium atom.

Fig. 115 Molecular structure of dimeric 541 with disordered components omitted for clarity.

64

Organometallic Complexes of the Alkali Metals

Fig. 116 Molecular structure of dimeric 542 with disordered components omitted for clarity.

Targeting an all-potassium alkyl/alkoxy cluster, KOtBu was replaced by more soluble KOtAm,426,427 mixing LiNp:KOtAm in 1.0:4.5 ratio in hexane afforded 542 (Fig. 116). Aggregation of one KNp and three KOtAm units affords a K4O3C heterocubane framework where four potassium atoms occupy four diagonal corners of the cube. The seven corners occupied by K and O-atoms are fairly symmetrical, but the linear arrangement of Np group opens up another coordination site through additional K–CH2 interactions which leads to edge-linked dimers.

2.02.5

Summary

Since their discovery over a century ago, organolithium reagents have attracted widespread interest and have established themselves as reagents of choice on the frontiers of synthetic chemistry. Such privileged position has been brought on by their unrivaled combination of both reactivity and structural diversity. Building on this, and facilitated by the progress of available techniques and instrumentation, a greater understanding of bonding and secondary interactions has been obtained. This has allowed for more accurate and detailed structure-reactivity correlations to be established. Within this context, the development of techniques such as DOSY NMR, that are now routinely applied to organolithium complexes, has helped to bridge the gap between the solid-state structures and solution state identity of these complexes. Furthermore, the organometallic chemistry of the heavier Group 1 metals has continued to grow at steady pace, leading to a greater understanding on the bonding and stability of these systems. This is remarkable as the isolation and characterization of these compounds have been traditionally very challenging due to their greater tendency to form highly aggregated structures and undergo decomposition in many common organic solvents. Knowledge accrued by many of these structural studies has undoubtedly inspired recent advances on the applications of Group 1 organometallic complexes, not only as key precursors for the synthesis of organometallic complexes of other metals but also as valuable reagents in organic synthesis and catalysis. We hope the findings included in this chapter may encourage more investigations into the organometallic chemistry of these metals which over the past 15 years has continued to grow and to expand without showing any signs yet of lacking in novelty or impact.

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2.03

Organometallic Complexes of the Alkaline Earth Metals

Sharanappa Nembenna, Nabin Sarkar, Rajata Kumar Sahoo, and Sayantan Mukhopadhyay, School of Chemical Sciences, National Institute of Science Education and Research (NISER), HBNI Bhubaneswar, India © 2022 Elsevier Ltd. All rights reserved.

2.03.1 2.03.2 2.03.2.1 2.03.2.2 2.03.2.2.1 2.03.2.2.2 2.03.2.2.3 2.03.2.2.4 2.03.2.3 2.03.2.3.1 2.03.2.3.2 2.03.2.4 2.03.3 2.03.3.1 2.03.3.2 2.03.3.2.1 2.03.3.2.2 2.03.3.2.3 2.03.3.2.4 2.03.3.2.5 2.03.3.3 2.03.3.3.1 2.03.3.3.2 2.03.3.3.3 2.03.3.3.4 2.03.3.3.5 2.03.3.3.6 2.03.3.3.7 2.03.3.3.8 2.03.3.4 2.03.3.5 2.03.3.5.1 2.03.3.5.2 2.03.3.6 2.03.3.7 2.03.4 2.03.4.1 2.03.4.1.1 2.03.4.1.2 2.03.4.1.3 2.03.4.1.4 2.03.4.2 2.03.4.3 2.03.4.3.1 2.03.4.3.2 2.03.4.3.3 2.03.4.4 2.03.4.5 2.03.4.6 2.03.4.7

Introduction Beryllium Aryl and allyl compounds Carbene stabilized organoberyllium compounds N-Heterocyclic carbene (NHC) stabilized beryllium compounds Cyclic (alkyl) (amino) carbene (CAAC) stabilized organoberyllium compounds Carbodicarbene stabilized organoberyllium compounds Synthesis of carbodiphosphorane-stabilized organoberyllium compounds with a metal-carbon double bond Organoberyllium compounds with group 15 donor ligands N-donor stabilized organoberyllium compounds P-donor stabilized organoberyllium compounds Miscellaneous compounds Magnesium Organocyclopentadienyl derivatives of magnesium Carbene stabilized organomagnesium compounds ‘Normal’ NHC coordinated organomagnesium compounds ‘Abnormal’ NHC-supported organomagnesium compounds CAAC-stabilized organomagnesium compounds Alkoxy-functionalized NHC-stabilized organomagnesium compounds Amido-functionalized NHC-stabilized organomagnesium compounds Organomagnesium compounds with group 15 bonded ligands Synthesis of N-donor ligand supported four, five, and six-membered ring organomagnesium compounds Synthesis of high-membered rings and mixed-donor (N, O, and N, P, etc.) supported homoleptic and heteroleptic organomagnesium compounds Synthesis of mixed metal organomagnesium compounds Chiral organomagnesium compounds related to N-donor ligands Other N-donor organomagnesium compounds Preparation of organomagnesium compounds by using low oxidation state Mg(I) complexes Preparation of organomagnesium compounds by using BDIdMg(II) hydride Magnesiation of simple organic compounds Organomagnesium compounds with oxygen bonded ligands Cationic organomagnesium complexes Group-14 stabilized organomagnesium cationic compounds Group-15 stabilized organomagnesium cationic compounds Organomagnesium p-arene complexes Application of organomagnesium complexes as catalysts Calcium C-donor stabilized organocalcium compounds Alkyl stabilized organocalcium compounds Aryl stabilized organocalcium compounds NHC stabilized organocalcium alkyls NHC stabilized organocalcium amides/halides Cyclopentadienyl stabilized organocalcium compounds N-donor stabilized organocalcium compounds b-Diketiminate stabilized organocalcium compounds Tp stabilized organocalcium compounds Pincer stabilized organocalcium compounds Cationic organocalcium compounds Mixed metal organocalcium compounds p-Arene stabilized organocalcium compounds Applications of organocalcium complexes as catalysts

Comprehensive Organometallic Chemistry IV

https://doi.org/10.1016/B978-0-12-820206-7.00173-6

74 75 75 77 77 84 88 90 91 91 96 96 98 98 101 101 114 115 118 119 120 120 130 134 138 140 144 145 149 152 153 153 156 161 165 165 166 166 171 177 178 185 186 186 192 194 195 196 201 203

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2.03.5 Strontium and barium 2.03.5.1 Alkyl and alkynyl compounds 2.03.5.2 Carbene stabilized strontium and barium organometallic complexes 2.03.5.2.1 NHC stabilized compounds 2.03.5.2.2 CAAC supported compounds 2.03.5.2.3 Bis-iminophosphorano barium carbene complexes 2.03.5.3 Cyclopentadienyl derivatives of strontium and barium 2.03.5.4 Group 15 ligand supported organometallic compounds 2.03.5.4.1 N-donor ligand supported four, five, and six-membered ring organostrontium compounds 2.03.5.4.2 Tp ligand supported organobarium complexes 2.03.5.4.3 Miscellaneous N-donor ligand stabilized organostrontium compounds 2.03.5.4.4 Chiral N-donor ligand stabilized organobarium compounds 2.03.5.5 Cationic organostrontium and barium complexes 2.03.5.6 Heterobimetallic organostrontium and barium compounds 2.03.5.7 Organometallic (M]Sr and Ba) p Arene complexes 2.03.6 Concluding remarks Acknowledgments References

Abbreviations Ad Ae aIPrPh ASCP BDI Bdpmp BIAN bImMe Bn Bpy Bz CAAC CarMesPyCarMes CDC CDP COSY COT Cp Cp CpBIGt-Bu Cy Cy CAAC DAB DABCO Dep Dep BDI DFB DiPeP DiPeP BDI Dipp Dipp BDI DMAT DME Dmp DOSY DPE

Adamantyl Alkaline earth 1,3-Bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene 2,2,5,5-Tetramethyl-2,5-disila-1-azacyclopent-1-yl ({(Me2)SiCH2}2) b-Diketiminate (CH[C(CH3)N-R]2) Bis-2,6-(diphenylmethyl)phenyl (2,6-(Ph2CH)2dC6H3) Bis(imino)acenaphthene N-methylbenzimidazole Benzyl (CH2Ph) Bipyridine Benzoyl, C6H5CO− Cyclic (alkyl) (amino) carbene 2,6-bis(3-mesitylimidazol-2-ylidene)pyridine Carbodicarbene ([:C(CR2)]) Carbodiphosphorane (C:(PR3)2) Correlated spectroscopy Cyclooctatetraenyl Cyclopentadienyl (Z5-C5H5) Pentamethylcyclopentadienyl (Z5-C5Me5) C5(4-tBudPh)5 Cyclohexyl 2-(2,6-Diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4,5] decan-1-ylidene 1,4 Diazabuta-1,3-diene 1,4-Diazabicyclo [2.2.2]octane 2,6-Et2-C6H3 [HC{C(Me)(NDep)}2]− 1,2-difluorobenzene 2,6-iPe2-C6H3 [HC{C(Me)(NDiPep)}2]− 2,6-iPr2-C6H3 [HC{C(Me)(NDipp)}2]− 2-dimethyl-amino-a-trimethylsilylbenzyl ((Me2NPh)CH2SiMe3) Dimethyloxyethane (CH3OCH2CH2OCH3) 2,6-Dimesityl-phenyl (2,6-Mes2C6H3) Diffusion ordered spectroscopy Diphenyl ethylene

204 205 208 208 212 212 213 219 219 223 226 227 227 230 233 237 237 237

Organometallic Complexes of the Alkaline Earth Metals

Dppp DXE EPR Et Et CAAC HMBC HMDS IiPr2Me2 IMe2 IMe4 IMes ImMes ImtBu i Pe IPr i Pr ItBu L’1 L1 L10H L11H L12H L13 L14H L15 L16 L17 L18H L19 L2 L20 L21 L22 L23H L24 L25 L3 L4 L5 L6 L7 L8 L9 Me Me4TACD Me CAAC Mes Mes BDI Napth Nbd n Bu NHC NMR NN-H2 NOESY

1,3-Bis(diphenylphosphino)propane, Ph2PdC3H6dPPh2 Dixylylethylene Electron paramagnetic resonance Ethyl (CH2CH3) 1-(2,6-diisopropylphenyl)-3,3-diethyl-5,5-dimethylpyrrolidine-2-ylidene Heteronuclear multiple bond correlation Hexamethyldisilazide; [N(SiMe3)2]2 1,3-Diisopropyl-4,5-dimethyl-imidazol-2-ylidene, C{N(iPr)CMe}2 C{N(CH3)CH}2 C{N(CH3)CCH3}2 C{N(2,4,6-Me3C6H3)CH}2 1-Mesityl-imidazole 1-Tert-butyl-imidazole Isopentyl, [(CH3CH2)2CH] 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene, C{N(2,6-iPr2C6H3)CH}2 Isopropyl (CH(CH3)2) 1,3-Bis(tert-butyl)imidazole-2-ylidene Me2NC2H4NC(CH2)CHC(Me)O Me2NC2H4NC(Me)CHC(Me)O 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol ¼ (CH3)(C5H3N)C2H2(CH3)2OH 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol ¼ (CH3)(C5H3N)C2H2(C6H5)2OH DippNHPPh2 ¼ (2,6-iPr2-C6H3)NHPPh2 [CH3C(2,6-iPr2-C6H3N)CHC(CH3)(NCH2CH2PPh2)] (R,R)-tert-butyl-2-(((-2-(dimethylamino)cyclohexyl)(methyl)amino)methyl)-6-(triphenylsilyl)phenol ¼ R, R-tBu(NMe2)C6H10(CH2)NCH3(SiPh3)C6H2OH [CH3C[NCH(CH3)Ph]CHC(NDipp)CH3] [CH3C[NCH(CH3)Napth]CHC(NAr)CH3] [CH{C[NCH(CH3)Naph]CH3}2] HC(3-Ad-5-Mepz)3[3-Ad-5-Mepz ¼ 3-(1-adamantyl)-5-methylpyrazolyl] ¼ HC [(CH)3(CH2)6C(MeC3HN2)]3 HC(tBu2pz)2SiMe2NR ¼ HC(tBu2C3HN2)2SiMe2NR Me2NC3H6NC(Me)CHC(Me)O 1,4-diisopropyl;-1,4,7-triazacyclononane ¼ (iPr)2(NCH2CH2)3 Me3TACD, [(Me3TACD) ¼ 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane)] ¼ Me3(NCH2CH2)4 4-tert-butyl-2,6-bis(morpholinomethyl)phenoxy ¼ tBu-2,6-[N(CH2CH2)2O]2(CH2)2C6H2O 2,6-di-tert-butyl-methylphenol ¼ 2,6-tBu2MeC6H2OH 4,6-(MesN]PPh2)2dibenzofuran ¼ (2,4,6-Me3dC6H2NPPh2)2(C6H3)2O [MeC(NDipp)CHCRNCH2CH2N(Me)dCH2CH2NMe2]− [OCMe2CH2{CNCH2CH2NAr}] [N{CH2CH2(CNCHCHNMes)}2] 1,5,9-trimesityldipyrromethene ¼ 1,5,9-tris(2,4,6-Me3dC6H2) C9H5N2 (DippBDI)2(m-CH2CH2) ¼ {CH3C(2,6-(iPr)2C6H3N)CHC(CH3)(NCH2)}2 CH(8dC9H6N)(iPr2P]NtBu) CH3C(2,6-(iPr)2C6H3N)CHC(CH3)(NCH2CH2dD) (D]NMe2, N((CH2CH2)2CH2)) 4-tert-butyl-6-(triphenylsilyl)-2-[R((3-(dimethylamino)-propyl)amino)methyl]phenoxyl ¼ {tBu(SiPh3)[R(NMe2)(CH2)3NCH2]C6H2O}, R ¼ (CH2)3NMe2, R ¼ CH2Ph Methyl 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidine-2-ylidene 2,4,6-Me3dC6H2 [HC{C(Me)(NMes)}2]− Napthalenide Norbornadiene n-butyl (CH3(CH2)2CH3) N-heterocyclic carbene Nuclear magnetic resonance MeC(NAr)CHC(Me)(NH)-(N)C(Me)CHC(Me)(NHAr) Nuclear overhauser effect spectroscopy

73

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Organometallic Complexes of the Alkaline Earth Metals

OCP PCy3 PhAmisopropy Phen p-tol AmDipp p-tolAmBdpmp Pz SD SIPr t Bu tBu DAB TEMPO THF THP TismPriBenz TMC TMEDA TMP TMTA Tol ToM ToT TpAd,iPr TPB Tph TPHN TpMe,Me TptBu,Me Tren Tripp TROP Xyl

2.03.1

2-Phosphaethynolate Tricyclohexyl phosphine Phenyl-(diisopropyl)-amidinate Phenylethylene Para totyl dipp amidinate Para totyl bdpmp amidinate Pyrazol-1-yl-methyl Stilbene dianion SIPr ¼ 1,3-bis(2,6- diisopropyl)phenyl-4,5-dihydroimidazol-2-ylidene [C{N(2,6-iPr2C6H3)CH2}2] Tertiary butyl (CH(CH3)3) 1,4-Bis(2,6-diisopropylphenyl)-1,4-Diazabuta-1,3-diene 2,2,6,6-Tetramethylpiperidine-1-oxyl Tetrahydrofuran ((CH2)3CH2O) Tetrahydropyran (C5H10O) Tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]-methyl Tetramethyl carbene, 1,3,4,5-tetramethyl-imidazol-2-ylidene (C{N(CH3)CCH3}2) N,N,N0 ,N0 tetramethylethylenediamine (CH₃)₂NCH₂CH₂N(CH₃)₂ 2,2,6,6-Tetramethylpiperidine 1,3,5-Trimethyl-1,3,5-triazinane (CH₃NCH₂)₃ Toluene Tris(4,4-dimethyl-2-oxazolinyl)- phenyl borate Tris(4-t-butyl-2-oxazolinyl)- phenyl borate Hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate Triphenylbenzene ((Ph)3C6H3) 2-(2,4,6-triisopropylphenyl)phenyl 4,40 -Bis- (carboxyphenyl)-2-nitro-1,10 -biphenyl Tris(3,5-Me2-pyrazolyl)borate Tris(3-tBu-5-Me-pyrazolyl)borate Tris(2-aminoethyl)amine 2,4,6-Triisopropylphenyl (2,4,6-iPr3C6H2) [5H]Dibenzo[a,d]cyclohepten-5-yl 2,6-Me2dC6H3

Introduction

Alkaline earth (Ae) elements are silvery shiny metals with ns2 electronic configuration. Due to their low electron affinity and ionization energies, these elements typically exist as divalent metal ions M2+ upon oxidation. Selected atomic properties of nonradioactive alkaline earth elements BedBa are summarized in Table 1. Table 1

Selected atomic and physical data for nonradioactive alkaline earth elements Be-Bad

Element

Be

Mg

Ca

Sr

Ba

Atomic Number Atomic mass No. of naturally occurring isotopes Electronic configuration Melting point oC Boiling point oC Ionization energy (kJ mol−1)

4 9.012 1 [He]2 s2 1278 2500 899.4 (1st); 1757 (2nd) 1.85 1.576 0.96(3) 0.45 2.32 −1.97

12 24.305 3 [Ne]3 s2 649 1105 737.7 (1st); 1451 (2nd) 1.74 1.293 1.41(7) 0.72 2.45 −2.36

20 40.078 6 [Ar]4 s2 839 1482 589.8 (1st); 1145.4 (2nd) 1.54 1.034 1.76(10) 1.00 2.77 −2.84

38 87.620 4 [Kr]5 s2 768 1380 549.2 (1st); 1064 (2nd) 2.63 0.963 1.95(10) 1.18 2.90 −2.89

56 137.327 7 [Xe]6 s2 710 1537 502.7 (1st); 965 (2nd) 3.65 0.881 2.15(11) 1.35 3.05 −2.92

Density/g cm−3 Electronegativity Covalent radius (A˚ ) Ionic radius (A˚ ) CN 6 Vander Waals radii (A˚ ) in RM type molecules Redox potential (M2+/M)V

Ref.

1–3 1–3 4 5 6,7 8 9

Organometallic Complexes of the Alkaline Earth Metals

75

Organometallic compounds bearing an MdC bond (M]BedBa), particularly organomagnesium compounds, are well-known nucleophilic reagents. In 1900, Grignard reported the synthesis of organomagnesium compounds by reacting several organic halides with Mg turnings in ether. In 1912, he was awarded a Nobel Prize. However, the preparation of heavier alkaline earth organometallics is quite challenging. Despite that, organometallic compounds of alkaline earths chemistry have been flourishing in recent years. Since 2006,10 a number of excellent books, book chapters, and review articles have been documented in this field. A listing of the several areas covered by these books, book chapters, and review articles is incorporated in Table 2. This chapter deals with an overview of reports on organometallic compounds of non-radioactive group 2 elements (BedBa) since 2006, with a focus on MdC (M]BedBa) systems or systems featuring pi arene interactions. In addition, this chapter covers the synthesis, structures, bonding, and reactivity of group 2 organometallic compounds, including the mechanisms of reactions. We hope this will give readers and researchers in the area a fast and accessible glimpse into the flourishing field of alkaline earth organometallic chemistry.

2.03.2

Beryllium

Despite the high toxicity of beryllium81, there has been tremendous growth in organometallic beryllium compounds in recent years—including landmark discoveries in the isolation of low valent organoberyllium compounds and their reactivity studies.82,83 Caution: Beryllium (Be), Be-based coordination, and organometallic compounds or reagents are incredibly toxic. These compounds should only be handled using more appropriate safety precautions.

2.03.2.1

Aryl and allyl compounds

Bulky terphenyl ligands are used to synthesize monomeric aryl-substituted beryllium halide compounds. Synthesis of a tri-coordinated [{C6H3Ph2-2,6}BeBr(OEt2)] compound 1 was carried out recently, analogously to previously reported:84 [ArBeX(OEt2)] (X]Cl or Br, Ar]{C6H3(Mes)2-2,6}) was synthesized by the reaction of 1 equiv. Ar’Li; (Ar’]C6H3Ph2-2,6), with the BeX2(OEt2)2.85 The 9Be NMR and X-ray crystallographic analysis of the compound have revealed the three-coordinate beryllium center. (Fig. 1). Table 2

Selected published books, book chapters, reviews based on organometallic compounds of the alkaline earth elements.

Areas of study

Ref.

Coordination chemistry of alkaline earth metal compounds Alkaline earth metal reagents Alkaline earth metals in homogeneous catalysis Soluble molecular alkaline earth metal hydrides Low oxidation state with MdM bonded alkaline earth metal compounds NHC stabilized alkaline earth metal compounds Alkaline earth metal in asymmetric catalysis

6,11–29

Fig. 1 The solid-state structure of the aryl organoberyllium compound [{C6H3Ph2-2,6}BeBr(OEt2)] 1.

30–42 43–59 60–63 64–69 70–76 77–80

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Organometallic Complexes of the Alkaline Earth Metals

Synthesis of the diphenyl beryllium compound 2, [BePh2] has been known for many years, but the tri-nuclear complex structure has come to light recently. Though dialkylberyllium compounds are readily formed by treating ethereal beryllium dihalide with alkyl lithium compounds, this is not the case for the synthesis of diphenyl beryllium compounds. The attached ether molecules within the obtained products cannot be removed. Compound 2 was synthesized86 by reacting HgPh2 with elemental beryllium at 140  C in benzene, resulting in the crystallization of [BePh2] molecule in a linear tri-nuclear structure with m2-linked phenyl group between the beryllium centers (Fig. 2). However, the soluble monomeric adduct (IiPr2Me2)BePh2 [IiPr2Me2 ¼ C{N(iPr)CMe}2] has been synthesized by treating 2 with nucleophilic IiPr2Me2 group and structurally characterized by X-ray crystallography.87 In 2, the two-terminal beryllium atoms are in a trigonal planar geometry, while the central beryllium atom is surrounded tetrahedrally by four Ph groups. The addition of Et2O to BePh2 followed by removal of volatiles in vacuum affords Be2Ph4
Et2O. Reaction of PhLi with BePh2 results in the formation of Li[BePh3]. The anionic species [BePh3]− is hard to isolate in the solid-state; the reaction of the compound with 12-crown-4 forms [Li(12-crown-4)][BePh3] in which the [BePh3]− shows no interaction with the Li/crown counterion. After Fischer and co-workers’ synthesis of [BeCp2] in 1959,88 the chemistry of the beryllocene compounds remained unexplored for decades. Attachment of the related indenyl group to beryllium is accomplished by a mechanochemical approach. A thorough grinding of the 2:1 mixture of [K(C9H7)] and BeBr2 followed by filtration and evaporation of the hexane extract of the ground mixture results in [Be(C9H7)2] (3a; Fig. 3).89 The 1H NMR spectrum showed the presence of the indenyl group, whereas the single peak in the 9Be NMR spectrum at 19.1 ppm is consistent with the signal of the [BeCp2] molecule (18.5 ppm). Later, X-ray crystallography revealed the structure of the di(indenyl)beryllium compound with mixed Z5/Z1 hapticity. Considering the structural differences between the [BeCp2] and [BeCp 2] approaches to synthesize a more crowded indenylberyllium compound have been made. A similar mechanochemical route was followed, i.e., grinding of a mixture of K[1,3(SiMe3)2 C9H7] and BeBr2, followed by filtration and evaporation affords the mono(indenyl) species [Be{1,3-(SiMe3)2C9H7}Br] 3b (Fig. 4).89 1H and 9Be NMR signals are consistent with the indenyl-coordinated beryllium compound. X-ray crystallographic analysis has revealed that the structure is monomeric, featuring an Z5 coordination mode.

Fig. 2 The trimeric structure of the compound [BePh2] 2.

Fig. 3 The structure of the di-indenyl beryllium complex [Be(C9H7)2] 3a.

Organometallic Complexes of the Alkaline Earth Metals

77

Fig. 4 The solid-state structure of the compound [Be{1,3-(SiMe3)2(C9H7)}Br] 3b.

Fig. 5 The solid-state structure of the compound [{1,3(SiMe3)2C3H3}2Be(Et2O)] 4.

The benzene ring present in the indenyl group results in slippage of hapticity from Z5 to Z3 in the indenyl-beryllium compound, a contrast to [BeCp2]. DFT calculations (at the B3PW91-D3BJ/def2TZVP functional/basis set) was performed to examine the binding of the indenyl group attached to the beryllium atom. The previous experience of difficulties establishing the binding in the analogous [BeCp2] was because of the marginal difference in energies of the Z5/Z5 and Z5/Z1 conformations. However, in compound 3a, a similar kind of DFT calculations suggest that the Z5/Z5 conformation of the compound is available, though it is less than 5 kcal/mol higher in energy than the Z5/Z1 conformation. Incorporating bulky allyl ligands into organoberyllium chemistry is highly useful due to their significant utility in suppressing oligomerization and the power to enhance the solubility of a large number of metal complexes. The diethyl ether adduct of [{1,3(SiMe3)2C3H3}2Be] 4 has been synthesized via salt metathesis reaction of BeCl2 and K[1,3-(SiMe3)2C3H3] in diethyl ether (Fig. 5).90 The highly soluble, air and moisture-sensitive colorless complex [Be{1,3-(SiMe3)2C3H3}2(Et2O)] was structurally characterized by X-ray crystallography. Subsequently, a DFT study was performed on the binding mode. The result of which indicates the occurrence of both the Z1 and the Z3 binding of the allylic counterpart, in equilibrium in the solution. This observation points towards the viability of beryllium compounds with p-allyl groups without additional coordinated bases, i.e., Et2O.

2.03.2.2 2.03.2.2.1

Carbene stabilized organoberyllium compounds N-Heterocyclic carbene (NHC) stabilized beryllium compounds

An approach to reduce IPr [IPr]C{N(2,6-iPr2C6H3)CH}2] stabilized beryllium dibromide in toluene by potassium napthalenide [K2(C10H8)2(THF)] resulted in the formation of IPr stabilized [IPr
Be(C10H8)] 5 (Fig. 6).91 A two-step reaction can be invoked to explain product formation. A double reduction of the beryllium dihalide was initially made, followed by reducing the naphthalene with the preformed (IPr)Be(0). The 1H NMR spectrum exhibited four signals consistent with the naphthalene proton associated with the beryllium coordinated saturated carbons occurring at a high field (d ¼ 2.73 ppm). The 13C{1H} NMR spectrum shows five signals of naphthalene, in which one of the signals is at a high field. The 9Be NMR spectrum shows a resonance at −4.2 ppm, i.e., similar to the signal (1.7 ppm) for [(MeCAAC)Be(C14H10)], reported by Braunschweig and co-workers (vide infra). X-ray crystallographic analysis revealed that the structure features a five-coordinate beryllium center ligated by one IPr and one naphthalene ligand via a Z4 binding mode. The bending of the naphthalendiyl group at C1 and C2 suggests the loss of planarity and aromaticity of the naphthalene group. Five coordinated NHC-stabilized IPr
Be(BH4)2 6 was prepared by the reaction of IPr
BeCl2 with LiBH4 (Scheme 1).92 The solid-state structure revealed the colorless prism-shaped crystals to feature a five coordinated distorted square pyramidal geometry at Be with each [BH4]− unit attached in a bidentate fashion (Fig. 7). In contrast to the pyrophoric nature of (CH3)3N
Be(BH4)2, compound 6 exhibits air stability for several days.

78

Organometallic Complexes of the Alkaline Earth Metals

Fig. 6 The solid-state structure of beryllium napthalenediyl compound [IPr:Be(C10H8)] 5.

Scheme 1 Synthesis of NHC stabilized beryllium borohydride complex.

Fig. 7 The solid-sate structure of the compound [IPr:Be(BH4)2] 6.

Bis(diazaboryl)beryllium compound 7 has been synthesized by reacting BeCl2 with Yamashita’s93 lithium diazaborolide. The reaction of one equivalent of the IMe2 [IMe2 ¼ C{N(CH3)CH}2] with the bis(diazaboryl)beryllium complex 7 generated the trigonal mixed base NHC-stabilized bis(diazaboryl) beryllium compound 8 (Scheme 2).94 A broad signal at 28 ppm in the 9 Be NMR spectrum and the downfield shift of the 11B NMR spectrum to 38 ppm indicates the ionic character of the BedB bond (Fig. 8). This reactivity of the bis(diazaboryl) beryllium compound reflects the electrophilic nature of the beryllium center in the compound.

Organometallic Complexes of the Alkaline Earth Metals

79

Scheme 2 Synthesis of beryllium bis(diazaborolyl) compound.

Fig. 8 The solid-state structures of [Be{B(DippNCH2)2}2] 7 and [IMe:Be{B(DippNCH2)2}2] 8.

Recent advances in ligand stabilization strategies have led to a tremendous breakthrough in alkaline earth metal chemistry development. In particular, highly stabilizing neutral ligands, i.e., N-heterocyclic carbenes (NHCs) and cyclic (alkyl)( amino) carbene (CAACs), have played a crucial role in synthesizing low-valent beryllium compounds. This low-valent organometallic framework plays a vital role in exploring beryllium redox chemistry. A doubly reduced NHC-stabilized beryllium compound, SIPr
Be(bpy) [SIPr]C{N(2,6-iPr2C6H3)CH2}2] 9 was synthesized by adding 2 equiv. of KC8 to a mixture of SIPr
BeCl2 and bipyridine (Scheme 3).95 Upon stirring the mixture over 16 h, a dark orange-colored solid product was formed. The 1H NMR spectrum shows one septet signal at 3.11 ppm consistent with the methine group in SIPr. X-ray crystal analysis of the compound shows a distorted trigonal planar geometry at Be. The long bond distance of 1.429(4) A˚ for both the N(1)-C(1) and N(2)-C(2) bonds and short C(1)-C(2) bond distance of 1.384(6) A˚ in compound 9 (compared to the free bipyridine molecule) is consistent with a doubly reduced bipyridine ligand attached to beryllium center (Fig. 9).

Scheme 3 Double reduction of a-diimine by NHC-beryllium dihalide/KC8.

80

Organometallic Complexes of the Alkaline Earth Metals

Fig. 9 The solid-state structure of the compound [(SIPr)
Be(II)(bpy)] 9.

Bulky DAB substituted beryllacycles [Be(DippDAB)(OEt2)] (DippDAB ¼ [(DippNCH)2]2, Dipp ¼ 2,6-iPr2C6H3) have been synthesized by adding a solution of [Li2(DippDAB)] to a toluene solution of [BeI2(OEt2)2]. The ether coordinated beryllacycle [Be(DippDAB)(OEt2)] reacts with other neutral ligands leading to substitution of ether by the incoming ligand e.g., IiPr2Me2 and tetramethyl carbene(TMC) or IMe4 [IMe4](C{N(Me)CMe}2)] afforded compounds [Be(DippDAB)(IiPr2Me2)] (10a) and [Be(DippDAB)(IMe4)] 10b respectively (Scheme 4).96 The compounds were characterized by 1H, 13C{1H}, and 9Be NMR, which suggests that the molecule does not show dimeric character in solution. Subsequently, X-ray crystallographic analysis proved the structures of the compounds to feature monomeric tri-coordinated beryllium centers, similar to the doubly reduced bipyridine system 9.

Scheme 4 Synthesis of beryllacycles ligated by bulky DAB ligand.

The reaction of 2 equiv. of lithium diisopropylphenoxide (LiODipp) with a mononuclear NHC-stabilized BeCl2 adduct in THF forms the compound IiPr2Me2
Be(ODipp)2; [IiPr2Me2]C{N(iPr)CMe}2] 11 (Scheme 5). 1H NMR spectroscopy of the obtained white solid showed two septet signals at 4.87 and 3.55 ppm, representing a new environment for the IPr and ODipp ligands, respectively. The structure of the compound has been revealed by X-ray crystallography, where the IPr group is coordinated to Be(ODipp)2 in a distorted trigonal planar form, with the largest OdBedC angle of 124.99(17)o. By contrast, the reaction between the NHC adduct of BeCl2 and the less sterically demanding alkoxide salt, NaOEt gives [(IiPr2Me2)
Be(OEt)Cl]2 12 (Scheme 5).97 X-ray crystallographic studies revealed an ethoxide bridged dimeric structure with one Cl and one IiPr2Me2 coordinated to each beryllium center (Fig. 10).

Organometallic Complexes of the Alkaline Earth Metals

81

Scheme 5 Synthetic protocol for NHC stabilized beryllium alkoxides.

Fig. 10 The molecular structures of compounds [(IiPr2Me2)
Be(ODipp)2] 11 and [(IiPr2Me2)
Be(OEt)Cl]2 12.

Beryllium dihalides are very well-known precursors to a wide range of organometallic complexes. For example, they undergo salt metathesis reactions with various alkali metal alkyls and aryls to form beryllium dialkyl and diaryls. The synthesis of the CAAC stabilized low-valent Be(0) compound reported by Braunschweig and co-workers is carried out by reducing CAAC by a BeX2 (X]Cl, Br) compound.98 A series of NHCs such as IiPr2Me2, IPr, and IMes coordinated beryllium dihalides 13a-13c and 1491 are reported to be synthesized by reaction of 1 or 2 equiv. of NHC with a solution of [BeX2(OEt2)2]. The compounds have been structurally characterized by X-ray crystallography. The reaction of IPr
BeBr2 with (DippBDI)Al [DippBDI]HC{C(Me)(NDipp)}2; Dipp ¼ 2,6-diisopropylphenyl] led to the IPr-coordinated BedHdBe dimer [{IPr:Be(m2-H)Br}2] 15.91 X-ray crystallography revealed a distorted tetrahedral beryllium center. The reaction of diamidocarbene (DAC) [DAC ¼ C{CON(Mes)}2{C(Me)2}] with an equimolar amount of BeCl2 forms a tri-coordinated compound (DAC)
BeCl2 16 (Scheme 6 and Figs. 11–14).99 The 9Be NMR spectrum of the compound reveals the three coordinate nature of the beryllium center.

82

(B)

(D)

Scheme 6 Synthesis of NHC stabilized beryllium dihalides and mixed halide/hydride compounds.

Organometallic Complexes of the Alkaline Earth Metals

(C) (A)

Organometallic Complexes of the Alkaline Earth Metals

Fig. 11 The solid-state structures of the compounds [(IPr)
BeI2] 13a and [(IMes)
BeBr2] 13b.

Fig. 12 The solid- structure of the compound [(IiPr2Me2)2
BeI2] 14.

Fig. 13 The solid-state structure of the compound [{IPr
Be(m2-H)Br}2] 15.

Fig. 14 The solid-state structure of the compound [(DAC)
BeCl2] 16.

83

84

Organometallic Complexes of the Alkaline Earth Metals

Apart from being potent two-electron neutral donor ligands and well-known for stabilizing low oxidation state main group metals, N-heterocyclic carbenes (NHC) are also susceptible to activation of CdH, CdN, and CdC bonds in peripheral or extracyclic positions. Insertion of beryllium into the CdN bond of an NHC has been reported via the formation of dimeric BedHdBe bridged compounds [{NHC
(Me)Be(m-H2)}2] [NHC]IPr (18a); IMes (18b)]. The addition of 2 equiv. of MeLi to a mixture of IPr and BeCl2 led to the formation of IPr
Be(Me)2 17a, which upon reaction with two equiv. of PhSiH3 at room temperature formed [{IPr
(Me)Be(m-H2)}2], 18a. This dimeric adduct is sparingly soluble in O-donor solvents. An NMR-scale reaction of the THF solvated dimer compound with two equiv. of PhSiH3 at 80  C for 6 h led to a red solution, yielding large red crystals of the compound 19a (Scheme 7). The 1H NMR spectrum of 19a showed an unexpected asymmetry in the IPr ring and isopropyl methane resonances. The presence of one sharp singlet at 2.01 ppm, which is further correlated with the 13C NMR spectrum as a new methylene resonance, is consistent with the insertion of the IPr coordinated beryllium atom into the CdN bond of the other NHC. This hypothesis was proven by X-ray crystallography, showing that the beryllium atom has inserted into the CdN bond of the NHC, yielding a tri-coordinate beryllium center bound to a chelating dianionic alkyl amido group and one NHC. The exact mechanism of the reaction is unclear; however, DFT calculations are performed by Dutton and co-workers on the reaction mechanism, which suggests that the pathway involves a hydride shift to the carbene carbon followed by beryllium insertion into the elongated CdN bond.

Scheme 7 Reactivity of NHC stabilized beryllium dialkyl compound.

The NHC
BeMe2 are low temperature (−18  C) stable species; analogous monomeric dihydrides are not observed. Approaches to synthesize monomeric hydrides have been unsuccessful. However, which result in mixed alkyl-hydride beryllium species. Arrowsmith and co-workers reported that this ring expansion process proceeds via a bis-NHC beryllium dihydride intermediate, as previously postulated by Dutton and co-workers. Steric congestion at the beryllium center of the intermediate is the driving force for hydride migration. They prepared IMes
Be(Me)2, 17b similarly to previously reported IPr
Be(Me)2, 17a and reacted with two equiv. of PhSiH3 to form the analogous compound [{IMes
(Me)Be(m-H2)}2], 18b. When the dimeric compound 18b is reacted with two equiv. of IMes in the absence of any hydride donor, which leads to the formation of ring-expanded beryllium alkyl amido compound 19b, occurring via a Me shift (Scheme 7).100 The presence of a 3H doublet signal at 1.36 ppm and the absence of the expected methine singlet at ca. 2.32 ppm in the 1H NMR spectrum differentiates compound 19b from the ring-expanded beryllium alkyl amido compound 19a.

2.03.2.2.2

Cyclic (alkyl) (amino) carbene (CAAC) stabilized organoberyllium compounds

2.03.2.2.2.1 CAAC-stabilized low-oxidation state organoberyllium compounds Cyclic(alkyl)(amino)carbenes are well known for their good p electron acceptor behavior.101 Moreover, these neutral ligands have been employed for the isolation of low valent metal complexes.102–104 The synthesis of the first zero-valent CAAC-stabilized Be(0) compound was carried out by reacting (CAAC)
BeX2 with a free CAAC ligand along with KC8. On the other hand, (CAAC)
BeX2 adducts are synthesized by either adding the free CAAC ligand to a BeX2 solution or by the in situ deprotonation of the iminium salt followed by treatment with BeX2. Reaction of [(MeCAAC)
BeCl2] {MeCAAC ¼ 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidine-2-ylidene} with 1 equiv. of MeCAAC leads to the formation of the [(MeCAAC)2
Be], (20, Scheme 8). The 9Be NMR of 20 shows a broad signal at 32 ppm, consistent with computationally predicted bis-CAACdBe model ([Be(RCAAC)2]) type of compounds.

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Scheme 8 CAAC supported low valent organoberyllium species.

The reaction of [(CyCAAC)
BeCl2] {CyCAAC ¼ 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4,5] decan-1-ylidene} with 1 equiv. of MeCAAC formed a mixed carbene complex [(CyCAAC)(MeCAAC)Be], (21, Scheme 8), which has been characterized crystallographically and presence of non-equivalent resonances of the 2,6-diisopropylphenyl group at 1:1 ratio in the 1H NMR spectrum.98 The absence of the ligand redistribution reflects the strong covalent character of the BedC bonds in this compound. Computational studies of the CdBedC bonding in 20 depict the BedC bond as a donor-acceptor interaction between the ground state singlet of MeCAAC with the Be(0) in a 1s22s02p2 configuration (Fig. 15). This chemistry supports the description of CAAC ligands as excellent p-acceptors.

Fig. 15 The molecular structures of the compounds [Be(MeCAAC)2] 20 and [Be(CyCAAC)(MeCAAC)] 21.

2.03.2.2.2.2 CAAC stabilized neutral radicals and radical cations Synthesis of first-ever paramagnetic beryllium radical cation [(EtCAAC)2Be•]+ 22 stabilized by cyclic(alkyl)(amino)carbene has been carried out by reacting the low-valent, neutral species [(EtCAAC)2Be] [EtCAAC ¼ 1-(2,6-diisopropylphenyl)-3,3-diethyl-5,5-dimethylpyrrolidine-2-ylidene] with 4 equiv. of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, Scheme 9).105 The resulting radical cation is partnered with a three-coordinate beryllium anion [Be(II)TEMPO]− [TEMPO ¼ 2,2,6,6-tetramethylpiperidin-1-oxyl] with a trigonalplanar geometry (Fig. 16). The radical cation is completely insoluble in cyclic and aromatic hydrocarbon solvents and is prone to decomposition with ethereal solvents. Treatment with two equiv. of Na[BArF4] (ArF ¼ 3,5-bis(trifluoromethyl)phenyl) in diethyl ether results in the formation of a deep blue-green solution. Subsequently, this product has been characterized crystallographically as a similar radical cation with different counter-anion, i.e., [BArF4]−. The EPR spectra of compound 22 reveal its paramagnetic, radical character. Subsequently, a DFT study indicated a significant degree of spin density on the beryllium center.

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 9 CAAC stabilized paramagnetic beryllium radical cation.

Fig. 16 The molecular structure of the compound [(EtCAAC)2Be•]+[Be(II)TEMPO3]− 22.

The CAAC-stabilized, neutral Be radical 24 was prepared 1 year later by reducing (CAAC)
BeCl2 precursors. Addition of 1 equiv. of L-Selectride to a mixture of MeCAAC and (MeCAAC)
BeCl2 formed the (MeCAACH)(MeCAAC)BeCl, 23 (Scheme 10).98. The 9Be NMR chemical shift of the complex 23 occurs at 19 ppm. Reduction of compound 23 by lithium sand for 10 min led to radical compound 24 (Scheme 10).106 The EPR signal for 10 was characterized by a hyperfine coupling constant of 11.6 MHz to 9Be. Computational calculations have shown that the unpaired electron is delocalized over the BedCAAC p bond.

Scheme 10 Synthesis of neutral beryllium radical.

2.03.2.2.2.3 Reactivity of CAAC stabilized beryllium dihalide compounds CAAC stabilized beryllium dihalide complexes are well known for their role as potential precursors for various low-valent organoberyllium compounds. One of their most crucial reactions is adduct formation via salt metathesis reactions. The reaction of 1 equiv. of lithium amidinate salt, [Li(NiPr)2CPh] with [(MeCAAC)
BeCl2] readily forms MeCAAC stabilized beryllium amidinate complex 25 by a salt elimination reaction (Scheme 11). Moreover, a similar kind of salt elimination reactivity is observed upon treatment of [(MeCAAC)
BeCl2] with magnesium anthracene derivative [Mg(C14H10)(THF)3], leading to the formation of adduct [(MeCAAC)
Be(C14H10)] 26 (Scheme 11). The reaction of 1 equiv. of 1,3,2-diazaboryllithium with [(MeCAAC)
BeCl2], affords the adduct [(MeCAAC)
BeCl{B(DippNCH2)2}2], 27, also via a salt elimination reaction (Scheme 11 and Fig. 17).99 A 1H NMR study of compound 27 shows a 1:1 ratio of MeCAAC and diazaboryl groups. The 11B NMR spectrum shows broad

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resonance at 34 ppm, consistent with the presence of 1,3,2-diazaboryl moiety. The reaction of dilithium tetraphenylbutadiene with [(MeCAAC)
BeCl2] affords MeCAAC-coordinated antiaromaticberyllate compound 28. CAAC stabilized beryllium dihalides are potent stabilizers of doubly reduced diimines like DAB or bipyridine. The reaction of [(EtCAAC)
BeCl2] with bipyridine in the presence of 2 equiv. of KC8 at room temperature leads to the formation of the two-electron reduced bipyridine ligated (EtCAAC)Be compound 29 (Scheme 11)99,107 (Fig. 18), in a similar fashion as in the case of previously discussed reduction of bipyridine by KC8/NHCBeCl2 to obtain compound 9 (Scheme 3, vide supra).

Scheme 11 Reactivity of CAAC-stabilized organoberyllium dihalide.

Fig. 17 The solid state structures of the compounds [(MeCAAC)Be{CPh(NiPr)2}] 25, [(MeCAAC)Be(C14H10)] 26, and [(MeCAAC)BeCl{B(DippNCH2)2}2] 27.

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Fig. 18 The solid state structures of the compounds [(MeCAAC)
Be(CPh)4] 28 and [(EtCAAC)
Be(bpy)] 29.

2.03.2.2.3

Carbodicarbene stabilized organoberyllium compounds

Carbodicarbenes (CDCs) are carbones in which two neutral NHC ligands stabilize the central carbon atom. The first carbodicarbene-stabilized Be compound was synthesized by reaction of the free CDC ligand, C[C{MeN(iPr)NC6H4}]2 with (Et2O)2
BeCl2 at room temperature leading to the compound (CDC)
BeCl2. The 13C NMR spectrum of (CDC)
BeCl2 features a signal at 160.3 ppm, consistent with the beryllium-ligated carbone carbon. Dropwise addition of K[N(SiMe3)2] to a suspension of [(CDC)
BeCl2] in toluene readily affords [(CDC)
Be{N(SiMe3)}2Cl], 30. Broad septet in 1H NMR spectrum at 4.80 ppm is consistent with the methine proton at 4.71 ppm in the starting material. The appearance of an N(SiMe3) group at 0.33 ppm is consistent with compound 30.108 The 13C NMR spectrum shows resonance at 160.8 ppm is consistent with the carbone-carbon. Treatment of (CDC)
BeCl2 with LiBH4 in toluene solution leads to a BedHdB bridged hydride complex [(CDC)
Be(BH4)Cl] 31109 (Scheme 12). X-ray crystallographic analysis of compound 31 revealed a distorted tetrahedral beryllium center with three-center two-electron BedHdB bonds.

Scheme 12 Synthesis of organoberyllium species supported by carbodicarbenes.

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The reaction of compound 30 with one equivalent of K[N(SiMe3)2] resulted in five-membered beryllacyclic compound 32 (Scheme 13).110 The 1H NMR spectrum of 32 showed two new septet environments at 5.57 and 3.36 ppm that are characteristic of the methine protons of an unsymmetrical CDC-containing product. A possible mechanism for the formation of 32 would be beryllium mediated sp3 CdH bond activation, followed by cyclization/chloride metathesis. Another way of inducing this reaction is via radical-based cyclization, by the treatment of 30 with the reducing agent KC8. X-ray crystallography confirmed the formation of the same five-membered beryllacycle compound 32, in which the beryllium center is in a distorted trigonal planar geometry (Fig. 19).

Scheme 13 Reactivity of mixed amido/halide beryllium(II) complex stabilized by CDC.

Fig. 19 The solid-state structure of the compound 33.

The low reactivity of the five-membered beryllacycle compound 33 with CAAC and NHC donors is due to the overcrowded nature of the beryllium center. Treatment of beryllium dichloride dietherate (Et2O)2
BeCl2 with compound 32 in toluene/hexane mixture solution results in a salt metathesis product 33 (Scheme 13).110 The 1H NMR spectrum of compound 33 shows two septets at 5.58 and 3.31 ppm are attributed to methine protons. The other two doublets, 2.84 and 2.79 ppm, attributed to the diastereotopic methylene protons on the carbon atom of compound 33 were shifted downfield region in compared to beryllium complex of 32 (2.75 and 2.60 ppm). The reaction of (EtCAAC) and IiPri2Me2 in toluene solution with compound 33 leads to different reactivity. X-ray crystal analysis of the reaction products shows the CAAC inserted into the BedC bond of the metallacycle to form a six-membered beryllacycle (compound 34). In contrast, IPr directly coordinates to the beryllium center in compound 35 (Scheme 14 and Fig. 20).110

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 14 Unusual reactivity of beryllacycle 33.

Fig. 20 The molecular structures of the compounds 34 and 35.

2.03.2.2.4

Synthesis of carbodiphosphorane-stabilized organoberyllium compounds with a metal-carbon double bond

Carbodiphosphoranes are the phosphorus analogs of CDCs, in which the central neutral carbon is directly attached to two phosphine ligands. Free CDP [C:(PR3)2; R]Me, Ph] ligand when treated with 2 equiv. of nBuLi results in doubly ortho-lithiated compound 36 (Scheme 15). 36 reacts with one equiv. of BeCl2 to formed doubly orthoberyllated CDP compound [(Ph3P)2CBeLi2] (37, Fig. 21).111 X-ray crystal analysis revealed that the BedC bond length in compound 37 1.704(2) A˚ is shorter than most BedC single bonds [1.720(3) −1.748(6) A˚ ], indicating BedCDP multiple bond character.

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Scheme 15 Synthesis of di-ortho-beryllated carbodiphosphorane complex.

Fig. 21 The solid-state structure of the compound [(Ph3P)2C:Be] 37.

2.03.2.3 2.03.2.3.1

Organoberyllium compounds with group 15 donor ligands N-donor stabilized organoberyllium compounds

N donor ligand stabilized organoberyllium compounds can be achieved smoothly by alkane elimination reactions at low temperatures. For example, the reaction of BeEt2 with 1 equiv. of the bulky amidine [tBuC(NAr)2H] at −78  C affords amidinate coordinated organoberyllium compound [tBuC(NAr)2BeEt] 38 (Scheme 16 and Fig. 22).112

Scheme 16 Synthesis of bulky amidinate organoberyllium complex.

Fig. 22 The molecular structure of the compound [tBuC(NAr)2BeEt] 38.

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Organometallic Complexes of the Alkaline Earth Metals

Reactions of H2C[PPh2NR]2 (R]SiMe3, Ph, 2,6-iPr2C6H3) with BeEt2 a 1:2 stoichiometric ratio also afford alkane elimination products, i.e., combination of H2C[PPh2NR]2 (R]SiMe3, Ph) with 2 equiv. of BeEt2 gave the binuclear bis(diphenylphosphinimino)methanediide beryllium complexes [C(PPh2NR)2](BeEt)2 (R]SiMe3 39a, Ph 39b), respectively (Scheme 17 and Fig. 23).113

Scheme 17 Synthesis of dinuclear organoberyllium compound.

Fig. 23 The solid-state structure of the compound [C(PPh2NPh)2](BeEt)2 39b.

Incorporation of neutral ligands into 1,3,2-diazaberyllacycles is discussed above with respect to NHCs; different reactivity of the same beryllacycle is observed using nucleophilic alkali metal alkyls instead of neutral ligands. The reactions of an ethereal adduct of the diazaberyllacycle with 1 equiv. of MeLi or nBuLi led to the formation of anionic beryllate salts 40a and 40b in medium-high yield (Scheme 18). The 1H NMR spectra of the beryllate salt compounds reveal that the alkyl group (R]Me 40a, nBu 40b) is coordinated to Be, along with the presence of the one or two molecules of Et2O coordinated at Li within the [Li(OEt2)n]+ counterion (n ¼ 1, 2).96

Scheme 18 Synthesis of heterocyclic beryllate salts.

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93

Alkane elimination reactions of a wide range of BeR2 (R]Me, nBu and iBu) compounds with b-diketiminate protio ligands reveal similar reactivity to amidinates, resulting in b-diketiminate coordinated organoberyllium compounds (Scheme 19; Table 3).114 The 9Be NMR spectra of compounds 41a-h show broad resonances in the downfield region, between 14.6 and 17.1 ppm, consistent with three-coordinate Be species. All of these compounds except one are crystalline and mildly air and moisture-sensitive. X-ray crystal analysis is also consistent with the presence of three-coordinate beryllium centers with consistent BedC and BedN bond lengths (Figs. 24–26).

Scheme 19 Preparation of three coordinate organoberyllium compounds chelated by b-diketiminate ligand.

Table 3

Bond distances and angles of NHC organoberyllium compounds.

Compound

R

Ar

Ar0

NdBedN [o]

˚] BedC [A

41a 41b 41c 41d 41e 41f 41 g 41 h

Me Me n Bu i Bu Me n Bu Me Me

Mes Dep Xyl Mes Dipp Dipp Ph Xyl

Mes Dep Xyl Mes (S)-(−)-CHMePh (S)-(−)-CHMePh Ph Xyl

108.44(9) 108.55(18) 107.89(14) 108.20(6) 109.20(6) – – –

1.721(18) 1.722(4) 1.731(3) 1.735(3) 1.707(13) – – –

Data from Ref. Paparo, A.; de Bruin-Dickason, C. N.; Jones, C. Aust. J. Chem. 2020, 73, 1144–1148.

Fig. 24 The solid-state structure of compound [HC{(Me)C(2,4,6-CH3C6H3)N}2 Be(Me)] 41a.

Fig. 25 The solid state structure of the compound [HC{(Me)C(2,6-(CH3)2C6H3)N}2 Be(nBu)] 41c.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 26 The solid-state of the compound [{HC(CMe)2(NDipp)(NCH(CH3)Ph)}Be(Me)] 41e.

An alternative route to synthesizing the b-diketiminate stabilized organo-beryllium compounds is via the reaction of the protio ligand precursor with in situ generated beryllium dialkyls. Addition of 2 equiv. of MeLi or nBuLi to a mixture of DippBDIH [DippBDI ¼ HC{MeC(DippN)}2H; Dipp ¼ 2,6-diisopropylphenyl] and BeCl2 leads to the formation of BeMe2 or Be(nBu2), followed by the formation of a six-membered metallacycle [{DippBDI}Be(R)] (RdMe 42a, nBu) with alkane release (Scheme 20).115

Scheme 20 Synthesis of heteroleptic b-diketiminato beryllium alkyl compounds.

The equimolar reaction of previously discussed H2C[PPh2NDipp]2 with BeEt2 at −78  C readily forms the thermally stable mononuclear, six-membered bis(diphenylphosphinimino)methanediide beryllium compound [CH(PPh2N-2,6-iPr2C6H3)2]BeEt 43 via elimination of ethane, via a similar mechanism as in the previous case, Scheme 17 (Scheme 21 and Fig. 27).113

Scheme 21 Synthesis of six-membered mononuclear beryllium ethyl compound.

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Fig. 27 The solid-state structure of the compound [{CH(PPh2N-2,6-iPr2C6H3)2}BeEt] 43.

Attempts to synthesize a beryllium diazaboryl complex, have been made via the reaction between (DepBDI)BeBr [DepBDI ¼ HC {MeC(DepN)}2H; Dep ¼ 2,6-diethylphenyl] with [(THF)2Li{B(DAB)}]. This approach, however, results in the formation of 1,3,2-diazaborole substituted compound [(DepBDI)Be{B(DippNC)2}H] 44 (Scheme 22). Although the exact mechanism of the reaction is not clear, a possible pathway involves deprotonation of the sp2 carbon of the DAB backbone. The compound 44 has been characterized by multinuclear NMR and single-crystal X-ray structural analyses (Fig. 28). In compound 44, the boron heterocyclic unit is anionic, and therefore an isostructural analog of an abnormal NHC.116

Scheme 22 Synthesis of diazaboryl complex of beryllium.

Fig. 28 The solid-state structure of the compound [HC{(2,6-EtC6H3)NC(Me)}2 Be{B(DippNC)2}H] 44.

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Organometallic Complexes of the Alkaline Earth Metals

2.03.2.3.2

P-donor stabilized organoberyllium compounds

Alkylation of the four-coordinate beryllium compounds [(dppp)
BeCl2]; (dppp ¼ bis(diphenylphosphino)propane), [Ph2PdC3H6dPPh2] and (Me3P)2
BeCl2 has been carried out, leading to the formation of [(dppp)Be(nBu)2] (45) and [(Me3P)Be(nBu2)] (46). Upon reaction of 2 equiv. of nBuLi into a solution of [(dppp)BeCl2], instant precipitation of fine, colorless solid occurs (Scheme 23).

Scheme 23 Synthesis of dibutyl beryllium phosphine complex.

A similar reactivity is shown by (Me3P)BeCl2 upon adding two equiv. of nBuLi, leading to the substitution of Cl atom by the bulkier nBu group, and loss of one phosphine ligand to give three-coordinated compound (Me3P)Be(nBu)2 46 (Scheme 24).117 The volatile phosphine ligand can be removed entirely in a vacuum to produce dimeric Be(nBu)2 (47), completely free of oxygen atom-containing solvents, and therefore of potential use for semiconductor applications.

Scheme 24 Preparation of three-coordinate dimeric organoberyllium species.

2.03.2.4

Miscellaneous compounds

The formation of neutral and anionic 1,3,2-diazaberyllacyles coordinated by alkyl and NHC ligands have been discussed earlier in this chapter. In order to probe the reactivity of differently substituted dilithiodiazabutadienediyls, reactions with beryllium dihalides were carried out; the reaction of [Li2(tBuDAB)] with BeBr2(Et2O)2 in a 1:1 M ratio leads to the formation of the unusual trimeric 1,3,2-diazaberyllacyle 48 in trace yield. X-ray crystallographic analysis revealed the structure of the compound, in which one monomeric beryllacycle is cleaved upon two beryllacycles forming trimeric compound 48 (Fig. 29).96 This result highlights the challenge for synthesizing monomeric 1,3,2-diazaberyllacyles containing (tBuDAB)Li2.

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97

Fig. 29 The solid-state structure of the compound [Be{(tBu)NCH}2]3 48.

Though the most widely used method to synthesize compounds containing the BeMe2 unit is via the reaction of ethereal beryllium dihalide with MeLi, the reaction often generates a mixture of side products. Attempted synthesis of the complex [(TMEDA)BeMe2], [TMEDA ¼ N, N, N0 , N0 tetramethylethylenediamine] via reaction with TMEDA with BeMe2 solutions results in a mixture of products, with one of them shown to be the TMEDA adduct of the salt Li2[BeMe4], 50. Upon changing the reactant stoichiometry ratio (to 1:4:2 for [BeI2(OEt2)2], LiMe and TMEDA, respectively), the polymeric lithium tetramethylberyllate salt 51 is formed (Scheme 25).114 Both these colorless, air and moisture-sensitive solids have been structurally characterized by X-ray crystallography (Fig. 30).

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 25 Synthesis of heterobimetallic organoberyllium complexes.

Fig. 30 The solid-state structures of the compounds[(TMEDA)2Li2(BeMe4)] 50 and [(TMEDA)Li3(BeMe4)] 51.

2.03.3

Magnesium

Magnesium is the sixth most abundant element on Earth. It is an essential element in biology; thus, it is considered biocompatible. For example, it is crucial to cells or enzymes of living organisms for synthesizing DNA and RNA, and green plants -chlorophylls, which are responsible for photosynthesis.118,119 For the synthesis and application of organometallic compounds, Grignard37 was awarded the Nobel Prize in 1912. Since then, there has been massive progress in organomagnesium chemistry. The following section is divided based on the identity of the organometallic ligand bound at magnesium.

2.03.3.1

Organocyclopentadienyl derivatives of magnesium

In 2016 Cano et al. reported neutral cyclopentadienylmagnesium complexes [Mg{Z5-C5H3-1,3-(CH2CH2NiPr2)(SiMe2dNPh2)} X(THF)] (X]Benzyl 52a; N(SiMe3)2 52b), which are synthesized by reaction between one equivalent of [C5H4(CH2CH2NiPr2) (SiMe2NPh2)] with MgX2(THF)2 (X]Bz or N(SiMe3)2) in toluene at 85–125  C (Scheme 26).120 Furthermore, compound 52a reacts with [HNMe3][BPh4] in mixture of C6D6/C6D5N to afford the highly unstable cationic organomagnesium complex [Mg {Z5-C5H3-1,3- (CH2CH2NiPr2)(SiMe2NPh2)}][BPh4] 53.

Scheme 26 Synthesis of the cationic organomagnesium complex.

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99

In 2020 Schulz et al.121 showed how different ligand systems help to isolate a new type of cyclopentadienyl magnesium compounds—as shown in Scheme 27; heteroleptic L1 L’1 Mg2Cp 54 (L1]Me2NC2H4NC(Me)-CHC(Me)O, L’1]Me2NC2H4NC (CH2)CHC(Me)O) and L22Mg2Cp2 55 (L2]Me2NC3H6NC(Me)CHC(Me)O) complexes are synthesized by deprotonation of corresponding b-ketoiminate ligands (L1H and L2H) in toluene. The molecular structure of 54 indicates that Mg1 and Mg2 adopt tetrahedral and octahedral geometries, respectively, due to the different binding modes of L1 (Fig. 31). However, for 55, ligand L2 acts as only a bidentate ligand, resulting in a Mg2O2 core with one side-arm of the ligand not connected with any magnesium center.

Scheme 27 Heteroleptic b-ketoiminate organocyclopentadienyl derivatives of magnesium.

Fig. 31 The solid-state structures of [L1L0 1Mg2Cp ] 54 and [L22Mg2Cp2] 55.

In 2015, Honrado et al. synthesized the racemic mixed cyclopentadienyl/scorpionate magnesium alkyls 56a-56f by deprotonation of the lithiated salt of NNCp-scorpionate ligand with Grignard reagents, RMgCl at 1:1 M ratio in 88–95% isolated yield (Scheme 28).122 The existence of two types of pyrazole rings was demonstrated by 1H and 13C{1H} NMR spectroscopy. The solid-state structure of 56b and 56c are shown in Fig. 32, which reveals two pyrazole rings located in cis and trans positions with respect to the tert-butyl group.

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 28 Racemic NNCp-scorpionate magnesium alkyl complexes.

Fig. 32 The solid-state structures of [Mg(Et)(k2 -Z5 -bpztcp)] 56b and [Mg(CH2SiMe3)(k2 -Z5 -bpztcp)] 56c.

In additional experiments, the authors first synthesized optically active fulvene and further converted it into lithiated bis(3,5-dimethylpyrazol-1-yl)methane. When treated with Grignard reagents RMgCl, these chiral salts afforded single enantiopure magnesium alkyls 57a-57c (Scheme 29).122

Scheme 29 Single enantiopure organomagnesium compounds.

Organometallic Complexes of the Alkaline Earth Metals

2.03.3.2 2.03.3.2.1

101

Carbene stabilized organomagnesium compounds ‘Normal’ NHC coordinated organomagnesium compounds

N-heterocyclic carbenes are excellent supporting ligands for the isolation of new, unusual organomagnesium compounds. In 2010 Robertson, Mulvey and coworkers123 found that reactions between simple magnesium amides like Mg(HMDS)2 [HMDS] N(SiMe3)2] and Mg(TMP)2 [TMP ¼ 2,2,6,6-tetramethylpiperidine] with IPr in a 1:1 M ratio afforded the mononuclear carbene adducts [IPrMgnBu(HMDS)] 58a and [IPrMgnBu(TMP)] 58b in the solid-state (Scheme 30). Compound 58b, however, which equilibrates in toluene-d8 solution, via decoordination of the carbene ligand (Scheme 31), due to the combined steric effects of the n Bu and TMP groups which weaken coordination of the IPr ligand to the Mg center. The solid-state structures of 58a and 58b (Fig. 33) reveal central Mg atoms in distorted trigonal planar geometries with short Mg-Ccarbene bond lengths (58a, 2.254(2) A˚ ; 58b, 2.268(2) A˚ ), similar to previously reported carbene stabilized alkyl adducts of magnesium.

Scheme 30 Carbene stabilized mononuclear alkyl-amide adducts.

Scheme 31 Dynamic equilibrium of 58b in toluene-d8.

Fig. 33 The solid-state structures of [IPrMgnBu(HMDS)] 58a and [IPrMgnBu(TMP)] 58b.

Furthermore, these authors found that MgnBu2, when treated with IPr, results unexpectedly in the formation of the tetranuclear alkyl adduct [(IPr)2Mgn4Bu8] 60 (Scheme 32). Both X-ray and NMR studies indicate a linear MgnBu2 unit bridged by an n-butyl group (Fig. 34). Alternatively, compound 60 can be prepared by lithiation of heteroleptic IPrMgnBuCl 59.

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 32 Preparation of NHC stabilized tetranuclear alkyl-magnesium complex.

Fig. 34 The solid-state structure of [(IPr)2Mgn4Bu8] 60.

In Scheme 33,124 it is shown that neutral [IPrMg(Me)(Br)
THF] 61 when methylated with equimolar MeLi at −35  C results in the magnesium alkyl adduct [IPrMgMe2
THF] 62a in good yield. Furthermore, it was found that removal of THF results in the bridged dinuclear compound [{(Mg(IPr)(Me))}2(m-Me)2] 62b. In complex 62b, both Mg centers adopt tetrahedral geometries, in which the bridging Mgd(m-Me) bonds (2.256(2)–2.262(3) A˚ ) are found to be significantly longer than the terminal MgdMe bonds (2.141(2) A˚ ) (Fig. 35). Compound 62b is an analog of the previously reported bridged Mg-ethyl compound [{(Mg(IPr) (Et))}2(m-Et)2].125

Scheme 33 NHC stabilized bridged dinuclear magnesium methyl complex.

Organometallic Complexes of the Alkaline Earth Metals

103

Fig. 35 The solid-state structure of [{(Mg(IPr)(Me))}2(m-Me)2] 62b.

Alternatively, when IiPr2Me2 and MgnBu2 are mixed in a 2:1 M ratio, the stable mononuclear complex [(IiPr2Me2)2Mg(nBu)2] 63 is afforded, featuring a terminal butyl group (Scheme 34).126 The X-ray structure indicates that the Mg center adopts a distorted tetrahedral geometry linked to two carbenes and two butyl ligands (Fig. 36).

Scheme 34 Isolation of terminal magnesium dibutyl adduct of carbene.

Fig. 36 The solid-state structure of [(IiPr2Me2)2Mg(nBu)2] 63.

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Organometallic Complexes of the Alkaline Earth Metals

In 2019 Fremont, ´ Dagorne, and coworkers125 found that in a THF-free environment, equimolar reactions between simple NHCs and the Grignard reagent MeMgBr afford unsolvated dimeric complexes featuring bridging methyl substituents [{IMesMg(Br)}2(m-Me)2] 64 or [{(IPr)Mg(Br)}2(m-Me)2] 65 in excellent yields (Scheme 35). In the respective 1H NMR spectra, a resonance found at d −1.96 and −2.07 ppm corresponds to the bridging MgdCH3 group. The dimeric structures of 64 and 65 were confirmed by 1H DOSY NMR spectroscopy and single-crystal X-ray diffraction (Fig. 37).

Scheme 35 NHC supported mixed alkyl-halide magnesium complexes.

Fig. 37 The solid-state structures of [{(IMes)Mg(Br)}2(m-Me)2] 64 and [{(IPr)Mg(Br)}2(m-Me)2] 65.

NHC ligands are also a powerful tool for isolating molecular Grignard complexes. As shown in Schemes 36 and 126 the dual coordination of two IiPr2Me2 ligands can be used to synthesize the monomeric adduct of methyl magnesium bromide [(IiPr2Me2)2MgBrMe] 66 (Fig. 38).

Scheme 36 Synthesis of the stable mononuclear alkyl-halide adduct.

Organometallic Complexes of the Alkaline Earth Metals

105

Fig. 38 The solid-state structure of [(IiPr2Me2)2MgBrMe] 66.

As it is well known that THF act as a suitable donor solvent that can coordinate easily to metal complexes. Therefore when one equivalent of MgCl2 is mixed with an equimolar amount of IMes or IPr in THF, it results in the THF-coordinated dihalide adducts of, [IMes
MgCl2] 67a or [IPr
MgCl2] 67b, respectively, in good yields (Scheme 37).126,127 In toluene, a dimeric complex [{IPrMgCl} (m-Cl)]2 68a was isolated, featuring bridging by chloride ligands. However, the reaction is kinetically prolonged, taking 5 days to reach a 96% yield. Structural characterization was carried out by X-ray diffraction (Fig. 39); due to the absence of THF, the MgdCl bond length [2.213(4) A˚ ] in 68a is longer than that in 67b (2.202(6) A˚ ). In addition, Ghadwal et al. 128 found that 68b can be synthesized from the direct reaction of IPr with anhydrous MgI2 at room temperature.

Scheme 37 Synthetic procedures of carbene-supported magnesium dihalides.

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Fig. 39 The solid-state structures of [IPrMgCl2
THF] 67b and [{IPrMgCl}(m-Cl)]2 68a.

The double coordination of two IiPr2Me2 precursors has also been used to synthesize mononuclear halides. In 2019 Gilliard et al. synthesized the magnesium dihalide adducts [(IiPr2Me2)2MgX2] 69a-69c by treatment of one equivalent of MgX2 (]Cl, Br, I) in THF with two equivalents of IiPr2Me2 with no sign of dimerization even under a THF environment (Scheme 38).126 For compound 69a, the MgdCcarbene bonds are found to be equal in length (2.2345(12) A˚ ) and similar to those found in both 67a (2.202(6) A˚ ) and 67b (2.213(4) A˚ ), as shown in Fig. 40. However, in 69a, the MgdCl bond length [Mg1dCl1: 2.3278(5) A˚ , Mg1dCl2: 2.3215(5) A˚ ] is found to be more extended than in 67a and 67b, indicating that the central Mg is more electron-rich in this bis-carbene environment. A similar observation is found for compounds 69b and 69c (Table 4).

Scheme 38 IiPr2Me2 coordinated a series of mononuclear magnesium dihalides.

Fig. 40 The solid-state structures of [(IiPr2Me2)2MgCl2] 69a, [(IiPr2Me2)2MgBr2] 69b and [(IiPr2Me2)2MgI2] 69c.

Organometallic Complexes of the Alkaline Earth Metals

Table 4

107

WBI, Bond distances, yields of compounds 69a-69c.

Compound i

(I Pr2Me2)2MgCl2 69a (IiPr2Me2)2MgBr2 69b (IiPr2Me2)2MgI2 69c

MgdX

MgdCcarbene

Yield(%)

0.450 0.491 0.592

0.281 0.280 0.278

75 76 79

Data from Ref. Wong, Y. O.; Freeman, L. A.; Agakidou, A. D.; Dickie, D. A.; Webster, C. E.; Gilliard, R. J. Organometallics 2019, 38, 688–696.

To isolate unsolvated metal complexes, the choice of solvent is critical. Wilson, Gillard, and coworkers in 2019 reported the unsolvated carbene-stabilized magnesium bromide complex [{SIPrMgBr}(m-Br)]2 70 by mixing a toluene suspension of SIPr ¼ 1,3-bis(2,6- diisopropyl)phenyl-4,5-dihydroimidazol-2-ylidene [C{N(2,6-iPr2C6H3)CH2}2] with equimolar MgBr2 at room temperature (Scheme 39).95 Due to the presence of two bulky o-dipp groups in 70, the terminal Mg1dBr2 bond is found perpendicular [Br3dMg1dC1dN2 ¼ 95.8(3) ] to the plane containing the SIPr imidazole ring (Fig. 41).

Scheme 39 Synthesis of SIPr adduct of magnesium dibromide.

Fig. 41 The solid-state structure of [{SIPrMgBr}(m-Br)]2 70.

It is well known that imidazolium salts can also be used to synthesize carbene adducts of magnesium halides. Two such examples are shown in Scheme 40, where the chloride bridged adducts [{(IMes)MgX}2(m-Cl)] (X]Br, 71; I, 72) are synthesized by the direct reaction of the salt [IMesH]Cl with EtMgBr (or MeMgI) at room temperature.124,128 In Fig. 42, the molecular structure of 71 and 72 reveals that the four-coordinate Mg center has a distorted tetrahedral geometry, with a longer bridging MgdCl bond (av. 2.388(2) A˚ ) than the terminal MgdCl bond (ca. 2.28(2) A˚ ).

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Scheme 40 Synthesis of NHC-supported mixed halide magnesium compounds.

Fig. 42 The solid-state structure of [{(IMes)MgBr}2(m-Cl)] 71.

In 2009 Roesky, Stalke and coworkers reported the synthesis of the NHC-supported magnesium alkynyl complex [(tBuC^C) 2 Mg
IMe4]2
2toluene 73 in 54% yield (Scheme 41).129 In the IR spectrum, the C^C bond stretching frequency is found at 2060 cm−1, whereas in 13C{1H} spectrum reveals the typical C^CC and MgC^C resonance signals at d117.5 and 130.1 ppm.

Organometallic Complexes of the Alkaline Earth Metals

109

Scheme 41 Synthesis of dinuclear magnesium alkynyl complex.

In the molecular structure of 73, the Mg center shows an interesting coordination geometry around the metal center where the Mg center bonded with NHC and alkynyl groups (Fig. 43). Two alkynyl groups coordinated via end-on coordination with one of the Mg centers and one of which by side-on with another Mg center.

Fig. 43 The solid-state structure of [{(tBuC^C)2Mg
IMe4}2
2C7H8] 73.

In 2009, Hill and coworkers130 reported a NHC-stabilized magnesium hydride cluster for the first time. The synthetic protocol is shown in Scheme 42, where magnesium amide IPr
Mg(N(SiMe3)2)2 74 was reacted with four equiv. of phenylsilane to afford the hydride-containing cluster [Mg4H6(IPr)2{N(SiMe3)2}2] 75. In the molecular structure of 75, four magnesium centers conjoined by six hydrogen atoms result in MgdH bond lengths in the range 1.858(13)–1.902(11)A˚ (Fig. 44). The Mg


Mg distance in 75 (3.1764 (12)–3.3905(12) A˚ ) is significantly longer than that reported for dimeric Mg(I) dimeric compounds (2.8508(12),131 2.8457(8) A˚ ), and even for MgdH bridged dimeric (2.890(2) A˚ ) complexes,132 consistent with the absence of any significant MgdMg interaction in 75.

Scheme 42 Synthesis of soluble magnesium hydride cluster.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 44 The solid-state structure of [Mg4H6(IPr)2{N(SiMe3)2}2] 75.

In 2011 Robertson, Mulvey and coworkers133 synthesized a new type of carbene adduct of magnesium alkyl compounds, [IPr
Mg(CH2SiMe3)2] 76a and [IPr
Mg{CH(SiMe3)2}2] 76b, by the direct reaction between IPr and the corresponding magnesium silyl alkyl (Scheme 43). Complexes 76a and 76b are isostructural monomers (Fig. 45), and the MgdCcarbene bond lengths (76a, 2.267(3) A˚ and 76b, 2.288(5) A˚ ) are compatible with the reported three-coordinate silyl alkyl -magnesium adduct of carbenes (2.254(2)–2.285(2) A˚ ).123,125,130

Scheme 43 Carbene supported magnesium dialkyls.

Fig. 45 The solid-state structures of [IPr
Mg(CH2SiMe3)2] 76a and [IPr
Mg{CH(SiMe3)2}2] 76b.

Organometallic Complexes of the Alkaline Earth Metals

111

In 2014 Nembenna and coworkers134 reported the structurally characterized NHC-bis-amido adduct [ItBu
Mg(N(SiMe3)2)2] 77 (where ItBu]1,3-bis(tert-butyl)imidazole-2-ylidene),. 77 was prepared by treatment of an equimolar quantity of ItBu with Mg {(N(SiMe3)2)}2 in dry toluene with good yield (Scheme 44). The molecular structure obtained from X-ray diffraction is shown in Fig. 46.

Scheme 44 Synthesis of magnesium bis-amido adduct of carbene.

Fig. 46 The solid-state structure of [ItBuMg{N(SiMe3)2}2] 77.

Further, in 2017 the same research group reported the synthesis of NHC-stabilized aryl magnesium amides, i.e., [IMes
Mg(Ar) (N(SiMe3)2)] {Ar]Xyl, 78a; Mes, 78b} by the reaction between [IMes
Mg(Ar)Br]2 and LiN(SiMe3)2 under inert atmosphere (Scheme 45).135 Both amido compounds (78a and 78b) are fully characterized by multinuclear NMR and X-ray techniques (Fig. 47). For both 78a and 78b, the magnesium center sits in a distorted trigonal planar geometry with MgdCaryl bond lengths (Mg1dC2; 2.1538(18) A˚ , 78a, and 2.1552(16) A˚ , 78b) which are found to be shorter than that in previously reported (C6H5) MgBr2. Et2O (2.20 A˚ ).136 In addition, the same research group, reported the structurally characterized NHC-bis-amide, IEt2Me2
Mg(N(SiMe3)2)2.137

Scheme 45 Synthesis of NHC stabilized aryl magnesium amides.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 47 The solid state structures of [IMesMg(Xyl)(N(SiMe3)2)] 78a and [IMesMg(Mes)(N(SiMe3)2)] 78b.

In 2019 Gilliard et al. reported other examples of NHC adducts of magnesium amides, i.e., [(IiPr2Me2)Mg(HMDS)2] 79 and [(IiPr2Me2)Mg(ASCP)2] 80 (ASCP ¼ 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopent-1-yl) (Scheme 46).138 The solid-state structures reveal the monomeric structures of compounds 79 and 80. (Fig. 48).

Scheme 46 Synthesis of NHC stabilized magnesium di-silylamide.

Fig. 48 The solid-state structures of [(IiPr2Me2)Mg(HMDS)2] 79 and [(IiPr2Me2)Mg(ASCP)2] 80.

Organometallic Complexes of the Alkaline Earth Metals

113

Again in 2019, the same research group reported NHC-supported magnesium hydrides, [(IiPr2Me2)Mg(m-H)(HMDS)]2 81 and [(I Pr2Me2)Mg(m-H)(ASCP)]2 82. The reaction of the NHC-stabilized magnesium bisamides, 79 and 80 with 2.1 equivalent of PhSiH3 in hexane for 16 h results in carbene coordinated bridged hydrides (81, 82), respectively, in 66–73% isolated yield (Scheme 47).138 X-ray crystallographic analysis of the crystals confirmed their identity as mono(hydrido) magnesium species (81 and 82), present as centrosymmetric dimers having a [Mg2(m-H)2]2+ core (Fig. 49). The MgdCcarbene bond lengths in these hydrides (81; 2.2239(17) A˚ and 82; 2.2250(16) A˚ ) do not differ very much from the amide precursor 79 [MgdCcarbene: 2.2120(19) A˚ ]. i

Scheme 47 Synthesis of NHC supported mixed amido-hydride magnesium compounds.

Fig. 49 The solid-state structures of [(IiPr2Me2)Mg(m-H)(HMDS)]2 81 and [(IiPr2Me2)Mg(m-H)(ASCP)]2 82.

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Organometallic Complexes of the Alkaline Earth Metals

2.03.3.2.2

‘Abnormal’ NHC-supported organomagnesium compounds

In 2015 Mulvey et al.139 found that magnesiation of IPr by inverse-crown [KMg(TMP)2(nBu)]6 in a THF/methylcyclohexane mixture affords the bis-carbene magnesiate [KMg(IPr−)2(nBu)(THF)]1 83, as shown in Scheme 48. In additional experiments, the authors found that when the IPr ligand is treated with one equivalent of [(TMEDA)NaMg(TMP)2(nBu)] in n-hexane/THF produces the tris(mesionic)carbene adduct [(THF)3Na(m-IPr−)Mg(THF)(IPr−)2] 85 (Scheme 23, Method A). In method B, three equivalents of IPr reacted with one equivalent of [(TMEDA)NaMg(TMP)2(nBu)] in n-hexane, gives intermediate 84 [(THF)NaMg(THF)(IPr−)2(nBu)] which once dissolved in dissolved in n-hexane/THF at −30  C forms 85. The molecular structures of compounds 83 and 85 are shown in Fig. 50.

Scheme 48 Alkali mediated magnesiation of normal carbenes.

Fig. 50 The solid-state structures of [KMg(IPr−)2(nBu)(THF)]1 83 and [(THF)3Na(m-IPr−)Mg(THF)(IPr−)2] 85.

In 2016 Ghadwal et al.128 reported the abnormal NHC-supported magnesium diiodide compound [(aIPrPh)MgI2(OEt2)] 86 [aIPrPh ¼ 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene, (CPh{N(2,6-iPr2C6H3)CH}2)] by treatment of C2-arylated imidazolium salt with MeMgI at 80  C (Scheme 49). Furthermore, the reaction between compound 86 and 2 equiv. of K{N(SiMe3)2} in toluene at room temperature yields [(aIPrPh)Mg{N(SiMe3)2}2] 87. Both 86 and 87 are monomeric and 86 is the first structurally characterized abnormal NHC-supported magnesium iodide complex (Fig. 51).

Organometallic Complexes of the Alkaline Earth Metals

115

Scheme 49 Synthesis of aIPr supported magnesium dihalide and bis-amides.

Fig. 51 The solid-state structure of [(aIPrPh)MgI2(OEt2)] 86.

2.03.3.2.3

CAAC-stabilized organomagnesium compounds

In 2015 Turner and coworkers140 reported the first CAAC {cyclic(alkyl)(amino)carbene} supported magnesium amides [(RCAAC) Mg{N(SiMe3)2}2] (R]Me 88a, Cy 88b). The synthetic protocol shown in Scheme 50 involves adding one equivalent of K {N(SiMe3)2}2 to a mixture of cyclic iminium salts and 0.5 equiv. of Mg{N(SiMe3)2}2 to afford 88a and 88b. Both compounds are mononuclear with distorted trigonal planar geometries at the metal and MgdC carbene bond length of 2.2931(12) A˚ (88a) and 2.2989(12) A˚ (88b) (Fig. 52).

Scheme 50 Isolation of CAAC stabilized magnesium bis-amides.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 52 The solid-state structures of [((MeCAAC)Mg{N(SiMe3)2}2)] 88a and [((CyCAAC) Mg{N(SiMe3)2}2)] 88b.

In 2019 Wilson, Gillard, and coworkers95 reported the dimeric complex (Et2CAAC)MgBr2, 89, from the reaction of the Et2CAAC ligand with a toluene suspension of anhydrous MgBr2 at room temperature, and is regarded as the first CAAC supported magnesium halide complex (Scheme 51). X-ray data demonstrates the dimeric structure of 89 (Fig. 53).

Scheme 51

Et2

CAAC supported dimeric magnesium bromide complex.

Fig. 53 The solid-state structure of [(Et2CAAC)MgBr2] 89.

Organometallic Complexes of the Alkaline Earth Metals

117

Additional reactivity of compound 89 has been investigated by adding bifunctional N-donors (DippDAB and bpy) in the presence of KC8 in toluene, which gives rise to complexes (Et2CAAC)MgBr(DippDAB) 90 and (Et2CAAC)MgBr(bpy) 91, which are bright orange and dark red respectively and have been characterized by X-ray crystallography (Scheme 52)95 (Fig. 54).

Scheme 52 Reduction of organic substrates by KC8 in CAAC magnesium dihalides.

Fig. 54 The solid-state structures of [(Et2CAAC)MgBr(DippDAB)] 90 and [(Et2CAAC)MgBr(bpy)] 91.

The reaction of free Me2CAAC with KC8 in THF at 25  C gives a pale yellow solid of [(CMe2CH2CMe2)C]NDipp(THF)] 92. 13C {1H} NMR confirms the absence of a carbene carbon resonance and formation of imine C]N (d ¼ 183.8 ppm). Treatment of 92 with 2 equivalents of KC8 and (Me2CAAC)
MgCl2, affords the orange-red crystalline solid [(Me2CAAC)Mg(CMe2CH2CMe2C] NDipp)(THF)], 93, in 47% yield. The molecular structure of 93 is shown in Fig. 55. In method B, the direct reaction of two Me2 CAAC ligands with MgCl2 followed by KC8 reduction at −78  C also affords 93 (Scheme 53).141

118

Organometallic Complexes of the Alkaline Earth Metals

Fig. 55 The solid-state structure of [(Me2CAAC)Mg(CMe2CH2CMe2C]NDipp)(THF)] 93.

Scheme 53 Different methods for ligand rearrangement of CAAC.

2.03.3.2.4

Alkoxy-functionalized NHC-stabilized organomagnesium compounds

Treatment of Mg{N(SiMe3)2}2(THF)2 with one equivalent of the alkoxy functionalized NHC, L3H {L3][OCMe2CH2{CNCH2CH2NAr}] and Ar]Dipp} in non-polar solvents affords magnesium amide [L3Mg{N(SiMe3)2}2]2 94 in good yield (Scheme 54). The absence of any identifiable midazolidine CH resonance confirms the formation of this dimeric compound, which features a bridging alkoxy group, as established by X-ray diffraction (Fig. 56).142

Scheme 54 Synthesis of functionalized carbene stabilized magnesium amide complex.

Organometallic Complexes of the Alkaline Earth Metals

119

Fig. 56 The solid-state structure of [L3Mg{N(SiMe3)2}2]2 94.

2.03.3.2.5

Amido-functionalized NHC-stabilized organomagnesium compounds

In 2008, Arnold et al.143 reported a series of amido functionalized organomagnesium compounds. As shown in Scheme 55, the reaction of L4H4Cl3 {L4][N{CH2CH2(CNCHCHNMes)}2]} with thee equivalents of methyl magnesium chloride in THF, gives a colorless powder Mg3(L4H)Cl6 95 in 71% yield. 1H NMR confirms the presence of ammonium protons and two imidazolium protons. Heating 95 in THF for 2 h at 80  C yields a dark purple-colored compound Mg2(L4)Cl3 96 with the elimination of HCl and MgCl2.. Further reactivity studies of 95 have been developed, where this compound reacts with two (or three) equivalents of LiN(SiMe3)2 in THF at room temperature over 12 h to give rise to complexes [Mg2(L4)Cl2N(SiMe3)2] 97 or [Mg2(L4)Cl {N(SiMe3)2}2] 98 respectively. Compound 97 can transform into 98 upon reaction with K{N(SiMe3)2}. In 97 and 98, geometry around magnesium centers is distorted tetrahedral (Fig. 57).

Scheme 55 Synthesis of a series of functionalized NHC-supported organomagnesium compounds.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 57 The solid-state structures of [Mg2(L4)Cl2N(SiMe3)2] 97 and [Mg2(L4)Cl{N(SiMe3)2}2] 98.

2.03.3.3 2.03.3.3.1

Organomagnesium compounds with group 15 bonded ligands Synthesis of N-donor ligand supported four, five, and six-membered ring organomagnesium compounds

In 2013, Coles et al. reported the amidinate supported magnesium acetylide complex [Mg(MesC{NCy}2)(C^CPh)(THF)] 100, which is prepared by hydroelementaion of magnesium amide 99 with phenylacetylene.144 Alternatively, the authors found that 100 can be synthesized from mixed amidinate-guanidinate type complex 101 in the presence of one equivalent of phenylacetylene (Scheme 56). For 100, the 13C{1H} NMR features a characteristic acetylide carbon (dMgC^CPh) peak at d 117.7 ppm in C6D6.

Scheme 56 Amidinate stabilized solvated four-membered magnesium acetylide complex.

Organometallic Complexes of the Alkaline Earth Metals

121

In 2020, Bakewell et al. reported the heteroleptic bulky amidinate magnesium amide compound 102 (Scheme 57). This compound can easily be accessed by the reaction between p-tol AmDipp ligand {where p-tol AmDipp]4dMeC6H4dNHArdCH]NAr, Ar]2, 6diPr2dC6H3} and Mg[N(SiMe3)2]2 in toluene at room temperature (Scheme 57).145 Later, it was reported that dissolving compound 102 in phenylsilane at 80  C afforded magnesium hydride complex 103, which ultimately converts into bridged magnesium acetylide complex 104 in the presence of one equivalent of phenylacetylene. Under similar conditions, 102 affords 104, as evidenced by X-ray crystallography (Fig. 58).

Scheme 57 Synthesis of organo-magnesium amidinate complex.

Fig. 58 The solid-state structure of [(4-MeC6H4)C{NAr}2MgC^CPh]2 104.

In an additional experiment, the same research group found that the equimolar reaction between extremely bulky amidinate ligand p-tolAmBdpmp {where p-tolAmBdpmp]4-MeC6H4dNHArdCH]NAr, Ar]2,6d(CHPh2)2dC6H3} and MgMe2 in THF immediately formed the solvated magnesium methyl complex 105 in 72% yield (Scheme 58). Interestingly the same reaction is not complete in toluene due to the poor solubility of magnesium precursor. At harsh conditions (110–115  C and longer reactions time, 3 days), bulky amidinate supported magnesium amide 106 and hydride 107 were isolated using p-tolAmBdpmp (deprotonation methods). The terminal magnesium alkylidine complex 108 was prepared by treating compound 106 with excess PhC^CH at room temperature. Under similar conditions, 107 is not converted into 108, but heating at 80  C for 2 h in benzene-d6 does afford this product. Interestingly, contrary to 103, no H2 gas liberation is found, which strongly indicates the formation of Mg-alkynyl intermediates through alkyne insertion into the MgdH bond (Scheme 58).145 This short-lived intermediate finally converts into 108 on reacting with another equivalent of phenylacetylene with the liberation of one styrene molecule.

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 58 Bulky amidinate supported four-membered magnesium heterocycles.

In 2011 Mashima, Anwander, and coworkers reported the heteroleptic organodimagnesium complex [{PhCH(Me) NCH2CH2N]CHPh}Mg(Me)]2 109 synthesized by 1,2-addition of MgMe2 across imino moiety (N]C) of a bis-N, N’-diimine ligand (Scheme 59).146 In the dimeric molecular structure of 109 (Fig. 59), bond lengths of 2.084(1) and 2.102(1) A˚ are associated with the amido NdMg moiety, while terminal, the MgdC bond length is between 2.146(2) and 2.175(2) A˚ .

Scheme 59 Addition of MgMe2 across C]N bond.

Fig. 59 The solid-state structure of [{PhCH(Me)NCH2CH2N]CHPh}Mg(Me)]2 109.

In 2014 Hill et al. reported dearomatization of neutral a-diimines DippL {where L]bis(imino)-acenaphthene, C36H40N2} with Mg{CH(SiMe3)2}2(THF)2 to give the heteroleptic magnesium alkyl complex [DippL Mg{CH(SiMe3)2}(THF)] 110 at 60  C (Scheme 60).147 Complex 110 is stable towards Schlenk type equilibrium along with thermal decomposition.

Organometallic Complexes of the Alkaline Earth Metals

123

Scheme 60 Synthesis of the five-membered organomagnesium complex.

In 2015 Mashima et al. synthesized the five-membered homoleptic organomagnesium complex [iPr2NCH2CH2N (CHPhCH2Ph)Mg(CH2Ph)]2 111 stabilized by a monoanionic N,N-diisopropylamineimine ligand (Scheme 61).148 The structure of 111 features an anti-arrangement of the magnesium-coordinated benzyl groups, which results in a four-membered {Mg2(m-N)2}core.

Scheme 61 Synthesis of a five-membered dinuclear organomagnesium compound.

Surprisingly, the reaction of another N,N-bidentate ligand with the similar reagent [Mg(CH2Ph)2(THF)2] leads to the formation of two different isomers of magnesium benzyl complex [Me2NCH2CH2N(CHPhCH2Ph)Mg(CH2Ph)]2 112 (Scheme 62)148 with 90% yield. The authors found that prolonged heating at 80  C in a toluene/pyridine mixture results in selective formation of the thermodynamically stable anti-isomer 112b over syn-isomer 112a with an 87% yield. The molecular structure of anti-112b is shown in Fig. 60, which reveals that MgdN1 bond length in anti-112b is longer (2.232(3) A˚ ) than anti-111 (MgdN1; 2.179(4) A˚ ) due to the bulk of the isopropyl groups of the N,N-diisopropylamineimine ligand.

Scheme 62 Isolation of syn and anti-isomers of magnesium alkyl compounds.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 60 The solid-state structure of [iPr2NCH2CH2N(CHPhCH2Ph)Mg(CH2Ph)]2, anti-111, and [Me2NCH2CH2N(CHPhCH2Ph)Mg(CH2Ph)]2, anti-112b.

In 2011 Strohmann et al. introduced a new class of magnesium alkyl silylamides [{(CH3)2Si(CH2NC5H10)(NR)}Mg (nBu)]2 (R]tBu, 113a; iPr, 113b) by deprotonation of silazane ligand with magnesium dibutyl at −30  C (Scheme 63).149 In the molecular structure of 113a, two magnesium centers are attached by two bulky butyl groups resulting in a C- bridged structure. In contrast, in 113b, magnesium centers are cojoined by two silylamides leading to an N-type bridge (Figs. 61 and 62), resulting in an extended MgdN bond length of 113b in comparison to113a.

Scheme 63 Butyl-magnesium silylamides with C-type and N-type bridge

Fig. 61 Bridging mode 113a and 113b.

Fig. 62 The solid-state structures of [{(CH3)2Si(CH2NC5H10)(NtBu)}Mg(nBu)]2 113a and [{(CH3)2Si(CH2NC5H10)(NiPr)}Mg(nBu)]2 113b.

Organometallic Complexes of the Alkaline Earth Metals

125

In 2009 Hill et al. reported six-membered magnesium methyl complex [(DippBDI)Mg(Me)(THF)] 114a150 [BDI ¼ b-diketiminate ligand, (CH[C(CH3)NdR]2)] by salt metathesis of DippBDI ligand with the Grignard regent MeMgBr in THF (Scheme 64). In 2011 and 2013, similar solvated mononuclear alkyl compounds [(DippBDI)Mg(nBu)(Solvent)] (Solvent ¼ 2-MePy 114b151, 2-MeTHF 114c152) were reported by Hill and Fraenkel et al. respectively. Both 114b and 114c are synthesized by deprotonating the b-diketiminate protio ligand with dibutyl magnesium. The molecular structures of 114b and 114c are presented in Fig. 63.

Scheme 64 Synthesis of b-diketiminate supported organomagnesium complexes.

Fig. 63 The solid-state structures of [(DippBDI)Mg(nBu)(2-MePy)] 114b and [(DippBDI)Mg(nBu)(2-MeTHF)] 114c.

In 2012 Chisholm et al. 153 synthesized a series of three-coordinate magnesium alkyl complexes (115a-115 g) via reactions between bidentate N-donor ligands ((DippBDI and L5H, where L5 ¼ 1,5,9-tri mesityl dipyrromethene)) with magnesium alkyl reagents (Schemes 65 and 66). The molecular structure of one representative example (116a) is shown in Fig. 64.

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 65 Synthesis of THF coordinated magnesium alkyl compounds.

Scheme 66 Three-coordinate organomagnesium compounds bear dipyrromethene ligand.

Fig. 64 The solid-state structure of [L5MgnBu(THF)] 116a.

Organometallic Complexes of the Alkaline Earth Metals

127

In 2014 Stasch, Jones, Hill, and coworkers reported the unsolvated three-coordinate magnesium methyl complex BDIMgMe [Bdpmp ¼ bis-2,6-(diphenylmethyl)phenyl (2,6-(Ph2CH)2-C6H3)] 117 (Scheme 67).154 Signals at d −1.27 ppm 1 ( H NMR) and d −18.1 ppm (13C{1H} NMR) correspond to the MgdMe moiety. In addition, these authors successfully isolated THF-free magnesium n-butyl complex BdpmpBDIMgBu 119 by removing coordinated THF from pre-synthesized BdpmpBDIMgBu (THF) 118 complex at 60  C. The compound 119 can also be synthesized by deprotonating BdpmpBDIH with nBu2Mg in a noncoordinating solvent (hexane/toluene mix) in a 1:1 M ratio.

Bdpmp

Scheme 67 Synthesis of solvent-free three-coordinate magnesium alkyl compounds.

These authors extended the reactivity of 119 towards alkynes. As shown in Scheme 67, monomeric acetylide [L7MgC^CnBu] 120 complex was isolated by the insertion of 1-hexyne into the MgdBu bond of 119. Compound 120 has been characterized by 13C {1H} NMR, in which signals at d 103.5 and d 111.9 ppm correspond to the MgC^CnBu and MgC^CnBu moieties. The molecular structure of 117–120 is shown in Fig. 65.

Fig. 65 The solid-state structures of [(BdpmpBDI)MgMe] 117, [(BdpmpBDI)MgBu(THF)] 118, [(BdpmpBDI)MgBu] 119 and [(BdpmpBDI)MgC^CnBu] 120.

In 2013 Sirsch, Harder, and coworkers employed the b-diketiminato ligand, NN-H2 ¼ [MeC(NAr)CHC(Me)(NH)-(N)C(Me)CHC(Me)(NHAr)] to isolate the terminal magnesium butyl complex [NN-(MgnBu)2]2 121 by a simple deprotonation technique (Scheme 68; for the sake of convenience to the readers Chemdraw structures are drawn in monomeric form).155 The molecular structure of 121 reveals that four magnesium centers are bridged by two butyl groups, leading to two separate Mg2(m-Bu)2 cores (Fig. 66). In addition, the author also isolated the mixed hydride/alkyl complex [NN-(MgH)(MgnBu)]2 122 as a yellow powder with 41% yield by heating 121 with PhSiH3 in a 1:1 M ratio at 60  C. Compound 122 is proved by 1H NMR spectrum where d −0.44 and 3.59 ppm correspond to MgdnBu and MgdH protons.

128

Organometallic Complexes of the Alkaline Earth Metals

Scheme 68 Stable tetranuclear magnesium hydride cluster.

Fig. 66 The solid-state structure of [{MeC(NAr)CHC(Me)(N) (N)C(Me)CHC(Me)(NAr)}Mgn2Bu2] 121.

In 2021 Kretschmer et al. used the related bis b-diketiminato ligand L6H2, [where L6 ¼ (DippBDI)2(m-CH2CH2)] to isolate the heteroleptic dimagnesium butyl complex [{(DippBDI)Mg(THF)(nBu)}2(m-CH2CH2)] 123 albeit in relatively low yield (35%) (Scheme 69).156 In the molecular structure of 123, coordinated THF is coordinated to magnesium axially to the six-membered ring plane, mimicking the related mononuclear analog (Fig. 67).

Scheme 69 Heteroleptic six-membered dinuclear magnesium butyl complex.

Organometallic Complexes of the Alkaline Earth Metals

129

Fig. 67 The solid-state structure of [{(DippBDI)Mg(THF)(nBu)}2(m-CH2CH2)] 123.

In 2011 Stasch et al. reported the b-diketiminate supported magnesium alkynyl complex [{(DippBDI)MgC^CMe}2] 124, which was isolated by salt metathesis between [(DippBDI)MgI(OEt2)] and LiC^CMe in a 1:1 M ratio (Scheme 70).157. The 13C{1H} NMR signals at d 112.8 and 115.8 ppm correspond to the MgC^CCH3 and MgC^CCH3 moieties, respectively. The solid-state structure of 124 is shown in Fig. 68, where magnesium coordination with the second acetylide p system leads to dimerization.

Scheme 70 Magnesium acetylide complex stabilized by b-diketiminate ligand.

Fig. 68 The solid-state structure of [{(DippBDI)MgC^CMe}2] 124.

Similarly, in 2013, Hill, Mahon, and coworkers reported a series of four-coordinate magnesium acetylides [{(DippBDI) MgC^CR}2] (R](nBu)125a, tBu(125b), (Cy)125c, (Ph)125d, (p-tol)125e, (CH2OCH3)125f, (CH2OPh)125 g and (CH2NMe2) 125 h), synthesized by s-bond metathesis between terminal alkynes and (DippBDI)MgN(SiMe3)2(THF) in a non-polar solvent (Scheme 71).158 All metal acetylides were characterized by NMR and single-crystal X-ray diffraction techniques (Fig. 69).

130

Organometallic Complexes of the Alkaline Earth Metals

Scheme 71 Synthesis of series of terminal magnesium acetylides.

Fig. 69 The solid-state structure of [(DippBDI)MgC^CtBu]2 125b.

2.03.3.3.2 Synthesis of high-membered rings and mixed-donor (N, O, and N, P, etc.) supported homoleptic and heteroleptic organomagnesium compounds In 2007 Wang et al.159 synthesized terminal magnesium ethyl compound [Mg(Et){CH(8dC9H6N)(Pri2P]NtBu)}] 126 by simple deprotonation. As shown in Scheme 72, deprotonation of the protio-ligand, L7 [L7]CH(8dC9H6N)(Pri2P]NtBu)] by MgEt2 with the liberation of ethane gas affords 126 in 67% yield. In the molecular structure of 126, it is found that the MgdCethyl bond is more extended than the MgdCligand bond due to ring strain (Fig. 70). In additional experiments, 126 was reacted with benzyl alcohol at 0  C to afford the magnesium alkoxide 127. Due to ring strain, the metal-bound CH group is protonated rather than the coordinated magnesium ethyl fragment.

Scheme 72 Organo-magnesium compounds bear iminophosphorano ligands.

Organometallic Complexes of the Alkaline Earth Metals

131

Fig. 70 The solid-state structure of [Mg(Et){CH(8dC9H6N)(Pri2P]NtBu)}] 126.

In 2010 Chen et al. 160 employed the neutral tridentate N-donor ligand L8H {where L8]CH3C(2,6-(iPr)2C6H3N)CHC(CH3) (NCH2CH2dD) (D]NMe2, N((CH2CH2)2CH2))} to synthesize solvent-free mononuclear butyl compounds L8MgnBu (R]Me 128a, −(CH2)5 128b). Both compounds were isolated in 87–92% yield as white solids (Scheme 73). The molecular structure of compound 128a is shown in Fig. 71.

Scheme 73 Isolation of unsolvated mononuclear magnesium butyl complex.

Fig. 71 The solid-state structure of [L8MgnBu] 128a.

In 2010 Hultzsch et al. employed the tridentate phenoxyamine ligand, L9H [L9]4-tert-butyl-6-(triphenylsilyl)-2-[R ((3-(dimethylamino)-propyl)amino)methyl]phenoxyl], (R](CH2)3NMe2, R]CH2Ph) to isolate monomeric magnesium isopropyl compounds L9MgiPr (R](CH2)3NMe2 129a, CH2Ph 129b), which are analogous to previously reported multidentate ligands (dN and dO) (Scheme 74).161. In the molecular structure of 128a, the central magnesium atom is connected to a phenoxy ligand and isopropyl fragments in a distorted tetrahedral coordination environment (Fig. 72).

Scheme 74 Synthesis of N, N, O- chelated magnesium alkyl complex.

132

Organometallic Complexes of the Alkaline Earth Metals

Fig. 72 The solid-state structure of [L9MgiPr; R](CH2)3NMe2] 129a.

In 2012 Cui and coworkers reported the dimeric magnesium butyl complexes L10MgnBu, 130a and L11MgnBu, 130b by deprotonation of pyridine substituted alkoxy ligand L10H {L10H ¼ 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol}and L11H {L11H ¼ 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol} with one equivalent of MgnBu2 in toluene (Scheme 75).162 In the dimeric structure of 130a, two monoanionic ligands bridge magnesium butyl fragments across the Mg2O2 core (Fig. 73).

Scheme 75 N, O- chelated dinuclear magnesium monobutyl compounds.

Fig. 73 The solid-state structure of [L10MgnBu] 130a.

Reaction between one equivalent of the neutral bidentate phenolate ligand RPz(CH2)tBuPhOH {where Pz ¼ pyrazol-1yl-methyl} with one equivalent of MgnBu2 in toluene leads to dinuclear n-butyl compound [(RPz(CH2)tBuPhOMgBu)2] {R]H 131a, Me 131b} with elimination of butane gas (Scheme 76).163 The dimeric structure of 131a is supported by two m-oxygen atoms derived from different ligands, with distorted tetrahedral geometries at the magnesium centers (Fig. 74).

Organometallic Complexes of the Alkaline Earth Metals

133

Scheme 76 Synthesis of bidentate phenolate ligated mononuclear alkyls.

Fig. 74 The solid-state structure of (MePz(CH2)tBuPhOMgBu)2 131a.

In 2016 Stasch and coworkers found that the phosphinoamide ligand (L12H ¼ DippNHPPh2) can be used to synthesize the heteroleptic binuclear butyl complex [L12Mg2(nBu)] 132 in good yield (Scheme 77).164 Compound 132 has been characterized by multinuclear NMR. In addition, the authors found that when an equimolar solution of ligand L12H, MgnBu2, and PhSiH3 is stirred at 60  C, it affords a magnesium hydride/alkyl complex [(L12)2Mg2(nBu)H2]2, 133. The molecular structure of 132 is shown in Fig. 75.

Scheme 77 Heteroleptic magnesium hydride/alkyl complex.

134

Organometallic Complexes of the Alkaline Earth Metals

Fig. 75 The solid-state structure of [L12Mg2(nBu)] 132.

In 2020, Zhao, Maron, Xu, and coworkers reported a series of heteroleptic magnesium alkyls L13MgR; {where L13 ¼ [CH3C(2, 6- Pr2-C6H3N)CHC(CH3)(NCH2CH2PPh2)]} (R]nBu 134a, Cy 134b, tBu 134c, CH2Ph 134d) supported by monoanionic ligand (NNP) either by deprotonation or salt metathesis methods. (Scheme 78).165 The monomeric molecular structures of 134a and 134d are presented in Fig. 76. i

Scheme 78 Organomagnesium complexes stabilized by a tridentate b-diketiminato ligand.

Fig. 76 The solid-state structures of [L13MgCy] 134b and [L13MgnBu] 134c.

2.03.3.3.3

Synthesis of mixed metal organomagnesium compounds

Alkali metals are beneficial for magnesiation to synthesize stable magnesium reagents. One such process reported by Mulvey et al.166a in 2008, as shown in Scheme 79, reveals that the dropwise addition of the heterogeneous mixture of TMPH and sodium butyl solution in Mg(CH2SiMe3)2 and TMEDA affords colorless the magnesiate reagent [(TMEDA)Na(CH2SiMe3)(TMP)Mg(TMP)] 135. In the crystal structure of 135, one TMP bridges between the sodium and magnesium centers which is additionally connected to the second TMP ligand (Fig. 77).

Organometallic Complexes of the Alkaline Earth Metals

135

Scheme 79 Synthesis of heteroleptic alky amido reagent.

Fig. 77 The solid-state structure of [(TMEDA).Na(CH2SiMe3)(TMP)Mg(TMP)] 135.

A previous report166b–d showed that alkali-metal-Mg(TMP) reagents are suitable magnesiation reagents for both heteroaromatic and aromatic molecules. However, very few reports were found with potassium until 2009, when Mulvey et al. showed simple magnesiation in the ortho position of anisole by potassium magnesiate reagent 136 on the NMR scale (Scheme 80). After 22 h, the final magnesiate product [(PMDETA)K(m-TMP)(o-C6H4OMe)Mg(TMP)] 138 is formed via the short-term intermediate 137.167 The molecular structures of 136–138 are shown in Fig. 78.

Scheme 80 Magnesiation of anisole.

Fig. 78 The solid-state structures of [(PMDETA)K(m-TMP) (m-CH2SiMe3)Mg-(TMP)] 136, [(PMDETA)K(m-TMP)(o-C6H4OMe)Mg(CH2SiMe3)] 137 and [(PMDETA)K(m-TMP)(o-C6H4OMe)Mg(TMP)] 138.

In 2014 O’Hara et al. structurally characterized the potassium-based pre-inverse crown [KMg(TMP)n2Bu]6 139 (Scheme 81).168 Compound 139 is characterized by 1H NMR, in which the MgdCH2 resonance protons found at the upfield region d −0.83 ppm along with DOSY experiments. In additional experiments, compound 139 was shown to deprotonate the C2 position of naphthalene in non-polar solvents to afford a new inverse crown [KMg(TMP)2(2-C10H7)]6 2 in 41% yield. The molecular structures of 139 and 140 are shown in Fig. 79, with 24-membered (KNMgN)6 rings common to both molecules.

136

Organometallic Complexes of the Alkaline Earth Metals

Scheme 81 Synthesis of alkali metal magnesiates.

Fig. 79 The solid-state structures of [KMg(TMP)n2Bu]6 139 and [KMg(TMP)2(2-C10H7)]6 140.

In 2018 Blair and coworkers169 employed the sodium-based magnesiate reagent [(TMEDA)- Na(TMP)2Mg(CH2SiMe3)] 141 to deprotonate the C2 position of N-alkylated indoles at 25  C (Scheme 82). It was found that for bulkier substituents such as i Pr, a long reaction time (16 h) is required to afford monoindol-2-yl magnesiate compound [(TMEDA)dNa(TMP) (a-C11H12N)Mg (TMP)] 142 in 81% yield. For methyl and ethyl substituents, only a 15 min reaction time was needed to complete the reaction to yield tetraindol-2-yl type complexes [Na(TMEDA)2MgR4] {R]a-C9H8N5 143a, a-C10H11N6 143b}. To increase the yields, the authors used an in situ method in which Na(TMP) and magnesium amide reagents were mixed in a 2:1 M ratio with indoles to afford 143a and 143b in 70–80% isolated yields. The molecular structure of 142, 143a-143b is shown in Fig. 80.

Scheme 82 Magnesiation of indoles.

Organometallic Complexes of the Alkaline Earth Metals

137

Fig. 80 The solid-state structure of [(TMEDA)dNa(TMP)(a-C11H12N)Mg(TMP)] 143a and [Na(TMEDA)2Mg(a-C9H8N5)4] 143b.

Again in 2011, Mashima, Anwander, and coworkers reported the mixed metal methyl compound [{PhCH(Me)NCH2CH2N] CHHPh}Mg(AlMe4)(AlMe3)] 144 by using the same bis-N,N’-diimine ligand (Scheme 83).146 In C6D6, 144 exhibits a signal at d 7.21 ppm for the N]CH Ph group, while at that at d ¼ − 0.32 ppm corresponds to the AldCH3 protons. The molecular structure of 144 is shown in Fig. 81.

Scheme 83 Synthesis of the heterobimetallic organomagnesium complex.

Fig. 81 The solid-state structure of [{PhCH(Me)NCH2CH2N]CHHPh}Mg(AlMe4)(AlM3)] 144.

In 2011 Anwander et al. reported another heteroleptic mixed metal complex, [(TpMe,Me)Mg(AlMe4)] 145, formed in 75% isolated yield by salt metathesis of KTpMe,Me precursor with one equiv. of Mg(AlMe4) (Scheme 84).170 In the 1H NMR spectrum, the authors found a signature peak for AldCH3 protons at d ¼ 0.18 ppm, while a broad peak at d ¼ 4.58 ppm corresponds to the BH proton.

138

Organometallic Complexes of the Alkaline Earth Metals

Scheme 84 Example of heterobimetallic organomagnesium alkyl complex.

2.03.3.3.4

Chiral organomagnesium compounds related to N-donor ligands

In 2009 Hultzsch et al.171 reported the chiral binuclear magnesium butyl complexes [{(R,S,S)-DABN(MeProline)2}Mgn2Bu2] 146a and [{(S,S,S)-DABN(MeProline)2}Mgn2Bu2] 146b, which are synthesized by the reaction between chiral proline {(R,S,S)-1 and (S,S,S)-1} with two equivalent of MgnBu2 in 61–76% isolated yield (Scheme 85). The methylene (a-CH2 and b-CH2) groups of the Mg-butyl fragment behave diasterotopically; two multiplets for each isomer from d ¼ −0.12 to −1.60 ppm in the upfield region of the 1H are observed.

Scheme 85 Synthesis of chiral organomagnesium butyls.

In 2011 Sadow et al. synthesized the chiral terminal magnesium methyl complex ToTMgMe 147, supported by the tridentate oxazolinylborate (H[ToT]) ligand (Scheme 86).172 The authors established the presence of C3 symmetry via a single resonance for the dC(CH3)3 groups at d ¼ 0.72 ppm (1H NMR) in C6D6, while the MgdCH3 signal was found in the upfield region at −0.65 ppm. Additionally, the IR spectrum shows one distinct peak at 1585 cm−1 for the C]N group of 147 compared with two C]N peaks, at 1635 cm−1 and 1601 cm−1 for the parent precursor. The structure of 147 was confirmed by the single-crystal X-ray diffraction (Fig. 82).

Scheme 86 Chiral terminal magnesium alkyl complex.

Organometallic Complexes of the Alkaline Earth Metals

139

Fig. 82 The solid-state structure of [ToTMgMe] 147.

In 2012 Hultzsch et al. found that the reaction between the chiral phenoxyamine protio-ligand, L14H (R, R) ligand (L14H ¼ (R,R)-tert-butyl-2-(((−2-(dimethylamino)cyclohexyl)(methyl)amino)methyl)-6-(triphenylsilyl)phenol) (Scheme 87) and equimolar Mg(CH2Ph)2(THF)2 reagent results in the diastereomeric magnesium benzyl complexes (R, R)-148a-MgR and 148b-MgS in a 9:1 ratio at room temperature which gradually decreases to a 5:1 ratio at 80  C (Scheme 87).173 The solid-state structure of racemic 148 is shown in Fig. 83, in which the central magnesium atom adopts a tetrahedral geometry surrounded by achiral ligand analog to reported zinc complexes.174

Scheme 87 Diasteromic benzyl magnesium compounds.

Fig. 83 The solid-state structure of racemic 148.

In 2019 the Ren research group reported a library of magnesium butyl complexes (149, 150a-150c, 151) stabilized by chiral BDI ligands (L15HdL17H); {where L15][CH3C[NCH(CH3)Ph]CHC(NDipp)CH3], L16][CH3C[NCH(CH3)Napth]CHC(NAr)CH3] Ar]Dipp (150a), Dep (150b), Xyl (150C), L17][CH{C[NCH(CH3)Naph]CH3}2]}. The synthetic route involves deprotonating the respective chiral protio-ligands (Scheme 88).175 All compounds were isolated in 50–70% yield, and the solid-state structure of each is shown in Fig. 84.

140

Organometallic Complexes of the Alkaline Earth Metals

Scheme 88 Synthesis of a series of chiral organomagnesium complexes bearing BDI ligand.

Fig. 84 The solid-state structures of [L15MgnBu(THF)] 149, [L16MgnBu(THF)] 150b, and [L16MgnBu(THF)] 150c.

2.03.3.3.5

Other N-donor organomagnesium compounds

In 2015 the Sadow research group utilized an oxazoline ligand to synthesize the mononuclear magnesium methyl complex, ToMMgMe (ToM ¼ tris(4,4-dimethyl-2-oxazolinyl)-phenyl borate) 152 within ca. 88% yield (Scheme 89).176 In the 1H NMR spectrum, a broad signal at d ¼ −0.6 ppm corresponds to terminal MgdCH3 protons, while the respective carbon signal is found at d ¼ −17.2 ppm in C6D6. Additionally, an 11B NMR signal at d ¼ − 0.6 ppm and 15N{1H} NMR resonance for the CNCMe2CH2O moiety at d ¼ −155.4 is consistent with the proposed structure of 152.

Scheme 89 Synthesis of oxazoline supported terminal magnesium methyl complex.

Organometallic Complexes of the Alkaline Earth Metals

141

Reaction between the potassium salt of the tris(3-tert-butyl-5-pyrazolyl)hydroborato ligand (KTpR,Me)170 with a toluene solution of dimethylmagnesium in a 1:1 ratio led to the first heteroleptic terminal magnesium methyl compound (TpR,Me)Mg(Me) t {R]Me 153a, Bu 153b} (Scheme 90). Compound 153a was isolated by Anwander et al. in 2012 in 80% yield, while the related system 153b was reported by Parkin et al.177 in 2016. The characteristic singlet signal (1H NMR) for the MgdCH3 protons of 153a was found in the upfield region (d ¼ − 0.13 ppm), while for 153b, it is slightly downfield shifted (d ¼ 0.08 ppm).

Scheme 90 Synthetic methods for pyrazolyl-hydroborato stabilized magnesium methyls.

In 2015 Stasch and Jones et al. found an unexpected result when tris(pyrazolyl)methane protio-ligand L18H {where L18 ¼ C(3Ad-5-Mepz)3} [3-Ad-5-Mepz ¼ 3-(1-adamantyl)-5-methylpyrazolyl] was reacted with one equivalent of methyl magnesium iodide: an ionic complex [L18MgMe]I 154 was formed instead of L18MgI (Scheme 91).178 This unexpected apparently outcome occurs due to the steric bulk of the ligand. Moreover, one equivalent of MeLi successfully converts ionic complex 154 into neutral mononuclear alkyl complex L18MgMe 155. Surprisingly, when protio-ligand L18H is allowed to react with toluene suspension of MgnBu2, it affords terminal magnesium methyl complex L18MgBu 156. The heteroleptic complex 156 is accessible due to the high steric demands of ligand, in contrast to previously reported homoleptic compound [{C(3,5-Me2pz)3}2Mg], formed under similar reaction conditions. In the 1H NMR spectrum, the magnesium alkyl protons resonate between d ¼ −0.20 to 0.61 ppm in C6D6 for 154–156, while the corresponding carbon signals shift towards upfield region (for 154: d ¼ − 3.05 ppm and 155: d ¼ −4.02 ppm). The molecular structures of 155 and 156 are shown in Fig. 85.

Scheme 91 Synthesis of tris(pyrazolyl)methane ligand supported organomagnesium compounds.

Fig. 85 The solid-state structures of [L18MgMe] 155 and [L18MgnBu] 156.

142

Organometallic Complexes of the Alkaline Earth Metals

In 2019, Turner, Mountford, and coworkers employed scorpionate protio-ligands L19H{where L19]HC(Bu2pz)2SiMe2NR} to isolate heteroleptic magnesium alkyl complexes L19MgMe (157a-157b) and L19MgnBu (158a-158f) with no sign of dimerization (Scheme 92).179 However, for 157a-157b, very low yields are found (7–24%) compared to 158a-158f (47–74%). Multinuclear NMR along with 2-D COSY experiments are consistent with the reported structures. The molecular structures of 157a and 157b determined crystallographically are illustrated in Fig. 86, showing that the magnesium center is tetrahedrally connected in each case, with a scorpionate ligand bound in k3 fashion and a terminal alkyl group.

Scheme 92 Preparation of a series of magnesium alkyls by using scorpionate ligand.

Fig. 86 The solid-state structures of [L19MgMe] 157a and [L19MgnBu] 158d.

In 2017 Okuda et al.180 found that the reactions of protio-ligand L20H (L20 ¼ 1,4-diisopropyl;-1,4,7-triazacyclononane) with either one equivalent of Mg(C3H5)2 or MgiBu2 gives complexes [L20Mg(Z1-allyl)]2159 or [L20MgiBu]2160 by eliminating propane or isobutane respectively (Scheme 93). The methine resonance of the allyl group of 159 is detected at d ¼ 6.34 ppm as a quintet, whereas the allyl protons come to appear as two sets of doublets around d ¼ 3.81 and 3.65 ppm in the 1H spectrum. The authors performed additional experiments of 160 with AliBu3 in benzene at 25  C, giving colorless crystals of L20(AliBu3)MgiBu 161 in excellent yield. The molecular structure of 160 is shown in Fig. 87.

Molecular organomagnesium compounds supported by triazacyclononane ligand.

Organometallic Complexes of the Alkaline Earth Metals

Scheme 93

143

144

Organometallic Complexes of the Alkaline Earth Metals

Fig. 87 The solid-state structure of [L20MgiBu]2 160.

The MgdMe bond distances of selected terminal organomagnesium methyl complexes are shown in Table 5, along with corresponding NMR data. Table 5

Selected data for terminal magnesium methyl complexes.

Compound

13

−0.42 −1.27 −0.65 −0.13 −0.20 −0.20

−11.5 −18.1 −13.6 −17.2 −4.0 −3.6

H ppm

[{PhCH(Me)NCH2CH2N]CHPh}Mg(Me)]2 109 Ar LMgMe 117 ToTMgMe 147 (TpMe,Me)Mg(Me) 153 [{k3-N-C(3-Ad-5-Mepz)3}MgMe] 155 Mg{HC(tBu2pz)2SiMe2N(IPr)}Me 157a

2.03.3.3.6

1

C{1H} ppm

˚] MgdMe [A

Ref.

2.146(2) A˚ 2.1142(18) A˚ 2.102(1) A˚ 2.212(2) A˚ 2.127(3) A˚ 2.145(2) A˚

146 154 172 170 178 179

Preparation of organomagnesium compounds by using low oxidation state Mg(I) complexes

In 2007 Stasch, Jones and coworkers131 reported the prototype low oxidation state Mg(I) dimer 162 stabilized by bulky N-donor b-diketiminate ligands. Subsequently, the same research group reported a series of Mg(I) dimers (Scheme 94).181 These compounds have been used as two-electron reducing agents182 to prepare a range of organic compounds. As shown in Scheme 67, compound 162 affects the double reduction of simple organic molecules like cyclooctatetraene, carbodiimide, and anthracene to afford organomagnesium compounds 163–165183 at low temperatures.182 Interestingly, only the 9- and 10-positions of anthracene are reduced. In addition, compound 162 causes cleavage of esters to afford dimeric organomagnesium compounds (166a-166e)184 under harsh conditions.

Scheme 94 Synthesis of organomagnesium compounds by using low oxidation Mg(I) complex

Organometallic Complexes of the Alkaline Earth Metals

145

When two equiv. of tert-butyl nitrile reacts with 162, reductive CdC bond cleavage affords [(DippBDI)Mg(tBu)(NCtBu)] (167). By contrast, in the case of aryl isonitriles under similar conditions, compound 162 effects reductive CdC bond coupling to give the yellow crystalline product [{(ArBDI)Mg}2{m-(XylN]Cd)2}] {Ar]Dipp 168a, Mes 168b}.185 The compound 162 can also be used for the activation of weakly acidic CdH protons. In the presence of catalytic bis(tricyclohexylphosphine)palladium(0) ([Pd(PCy3)2]), CdH bond exchange of benzene containing reducing agent 162 affords organomagnesium hydride complex 169 in 76% isolated yield, as reported by Crimmin et al. in 2018.186 Similarly, 162 can be used for the activation of arenes 170187 via CdH bond activation. Moreover N-heterocyclic carbenes also undergo CdH bond activation by low valent Mg(I) dimer 162 to afford [(ArBDI)Mg(IMes-H)] {Ar]Xyl 171a, Mes 171b}188 at 80  C. Insertion of the Mg unit derived from 162 into the CdF bond of fluoroarenes results in organomagnesium fluoro compound 172189 in a manner analogous to classical Grignard reagent synthesis. Another interesting insertion reaction occurs with alkenes. It was found that addition of one equivalent of 1,2-diphenylethylene across Mg(I) dimer 162 yields 1,2-dimagnesioethane compound [{(ArBDI)Mg}2(m-CH2CPh2)] {Ar]Mes 173a, Dep 173b}.190 Compound 173 is very reactive towards dihydrogen and carbon monoxide. Additionally, the ethylene mixture and two-fold TMC react with 162 affords 1,4-dimagnesiobutane 174191 in moderate yield. Selected molecular structures are shown in Fig. 88. Moreover, in 2015, Jones et al. reported the reaction of [depBDIMg]2 or [mesBDIMg]2 with ketenimine MesN]C]CPh2 afforded MgdMg cleavage products [depBDIMg]2 [m-k2dNdCd(Mes)NCCPh2] and [mesBDIMg]2 [m-k2dNdCd(Mes)NCCPh2], respectively.192

Fig. 88 The solid state-structures of [{(MesBDI)Mg}2(anthracene)] 165, [{(DippBDI)Mg}2{m-(XylN]Cd)2}] 168a, [(MesBDIMg)2(H)(C6H5)] 169, [(XylBDI)Mg(IMes-H)] 171, [(DippBDIMg)(C6F5)(THF)] 172, [{(MesBDI)Mg}2(m-CH2CPh2)] 173a.

2.03.3.3.7

Preparation of organomagnesium compounds by using BDIdMg(II) hydride

Similarly, Mg(II) hydride[(BDI)Mg(m-H)2Mg(BDI)] 170181 has also introduced itself as an excellent reducing agent. Scheme 95 shows that 175 undergoes functional organic transformations such as activation and insertion type reactions to afford reduced organic compounds (176–180).188,193 The molecular structures of selected examples are shown in Fig. 89.

146

Organometallic Complexes of the Alkaline Earth Metals

Scheme 95 Formation of organomagnesium compounds using magnesium (II) hydride for activation and insertion type reactions.

Fig. 89 The solid-state structures of [(DippBDI)Mg{CH2CH(CH2)4}] 177, [(DepBDI)Mg(m-H)(m-Imid)Mg(DepBDI)] 179a, [(DepBDI)Mg(IMes-H)] 180.

As previously shown, b-diketiminate supported magnesium compounds can activate fluoroarenes via CdH (and CdF) bond activation. For complete regioselective functionalization, magnesium amide complexes can be used, as they are kinetically more active than analogous alkyl compounds. The Hevia research group demonstrated one such example in 2017. In this context, it is found that when BDIMg(TMP) complex 181 reacts with benzofuran, it leads to magnesiation of the a CdH bond and immediate

Organometallic Complexes of the Alkaline Earth Metals

147

formation of organomagnesium compound 182 in 78% isolated yield (Scheme 96).194 X-ray diffraction confirmed the molecular structure of 182 (Fig. 90). Adding one equivalent of 2-(2,4- fluorophenyl)pyridine in THF solution to 182 induces chemoselective ortho CdF bond activation to afford the cross-coupled product 183. The side-product is the fluoride bridged magnesium dimer.

Scheme 96 Regioselective magnesiation.

Fig. 90 The solid-state structure of [(MesBDIMg)(THF)(Benzofuran)] 183.

In 2016 Okuda et al. reported the protonolysis of magnesium hydride complex [(L21.AliBu3) MgH]; (L21 ¼ Me3TACD) [(Me3TACD) ¼ 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane] with terminal alkynes (Me3SiC^CH, PhC^CH) to afford magnesium alkynyl compounds L21Mg(C^CSiMe3) 184a and L21Mg (C^CPh) 184b in more than 80% yield (Scheme 97).195 In addition, insertion of 1,1-diphenylethylene across the MgdH bond leads to different regio-isomers L21Mg(CPh2CH3) 185a and L21Mg(CH2CHPh2) 185b.

Scheme 97 Insertion and protonolysis reaction.

148

Organometallic Complexes of the Alkaline Earth Metals

In 2013 Hazari et al. reported the reaction between tripodal ligand (Me6tren) supported dialkyl magnesium 186 with excess phenylacetylene to afford magnesium-diacetylide complex (Me6tren)Mg(CCPh)2 187 (Scheme 98).196 In the 13C{1H} NMR spectrum, the characteristic acetylide carbon peak is found at d ¼ 110.1 ppm with no signal for the MgdMe group, consistent with the formation of 187.

Scheme 98 Synthesis of magnesium-diacetylide complex 187.

In 2017 Parkin et al. reported the [TismPriBenz]-stabilized magnesium methyl complex [TismPriBenz]MgMe 188, which was then used to synthesize various organomagnesium derivatives, e.g., by protonation of the magnesium-methyl bond by amine (both 1o and 2o) and H2S to afford [TismPriBenz]MgN(H)Ph 189, [TismPriBenz]MgNR {R]Ph2 190a, C4H8 190b} and [TismPriBenz]MgSH 191 (Scheme 99).197 Compound 188 also undergoes insertion reactions with CO2 and CS2 to furnish acetate [TismPriBenz]Mg (k2-O2CMe) 192a and dithioacetate [TismPriBenz]Mg(k2-S2CMe), 192b. The crystal structure of 192a indicates that the acetate moiety is coordinated to the magnesium center in k2–manner (Fig. 91). Reaction with terminal alkynes led to acetylide derivatives [TismPriBenz]MgC^CR {R]Ph 193a, Bu 193b}. Metathesis of 188 with metal-halide reagents such as Me3SnX results in strings of magnesium-halide complexes [TismPriBenz]MgX {X]F 194a, Cl 194b, Br 194c, I 194d}. Moreover, an interesting metathesis reaction involves the synthesis of terminal magnesium hydride [TismPriBenz]MgH 195 by using phenylsilane. In the 1H NMR spectrum, the MgdH signal is found at d ¼ 6.78 ppm.

Scheme 99 Activation of small molecules by magnesium carbatrane 188.

Organometallic Complexes of the Alkaline Earth Metals

149

Fig. 91 The solid-state structure of [TismPriBenz]Mg(k2 -O2CMe) 193a.

The same authors extended studies of the chemical reactivity of 188 towards the insertion of terminal alkenes such as styrene, which leads to the formation of [TismPriBenz]MgCH(Me)Ph 196 (Scheme 100).197,198 Interestingly, compound 194c represents the first example of alkene insertion into a terminal MgdH bond.

Scheme 100 Organomagnesium compound by the insertion of styrene into MgdH bond.

2.03.3.3.8

Magnesiation of simple organic compounds

In 2014 Hevia et al. found that laboratory-synthesized sodium magnesiate 197 can deprotonate the acidic proton of N-methyl benzimidazole (in C2 position) to afford [{Na(THF)5}2+{(Ph2Si(NAr )2)Mg(bImMe )}2−] 198 {bImMe ¼ N-methylbenzimidazole} a bridged bimetallic complex. Butane gas evolved via MdH exchange interaction (Scheme 101).199 The 13C NMR spectrum of the product shows the characteristic deprotonated C2 carbon peak at d ¼ 194.0 ppm (in THF-d8) compared to the (protonated) ligand (142.7 ppm). The molecular structure of 198 is presented in Fig. 92, which shows that the central magnesium atom adopts a distorted tetrahedral geometry, ligated by carbon and nitrogen atoms of different bridging N-methyl benzimidazole ligands to give a six-membered ring, i.e., MgCNMgCN. Due to the presence of this ring, the magnesium-carbon bond length (av. 2.233(3) A˚ ) is found to be longer than in the parent precursor (M-Cbutyl 2.124(3) A˚ ).

Scheme 101 Synthesis of magnesiate cation.

150

Organometallic Complexes of the Alkaline Earth Metals

Fig. 92 The solid-state structure of [{Na(THF)5}2+{(Ph2Si(NAr)2)Mg(bImMe )}2−] 198.

The same research group used BDI-supported magnesium amido complex 181 for C2- magnesiation of challenging substrates such as benzothiazole, N-heterocyclic molecule, and substituted 2-pyridines to affords new magnesiate products 199–204 in good yields (Schemes 102–104).200,201 All substrates undergo regioselective magnesiation at the a-position. The molecular structures of 199 and 201–204 are presented in Figs. 93 and 94.

Scheme 102 Magnesiation of benzothiazole.

Scheme 103 Regioselective magnesiation of N-heterocyclics.

Organometallic Complexes of the Alkaline Earth Metals

151

Scheme 104 Magnesiation of substituted 2-pyridines.

Fig. 93 The solid-state structure of [{Ar NC(Me)CHC(Me)- NAr }2Mg2{btz }2] 199.

Fig. 94 The solid-state structures of [{(DippBDI)Mg(C14H10N3)}2] 201, [{(DippBDI)Mg(C3H4N3)}2] 202, [{(DippBDI)Mg(C5H6N)}2] 203, [(DippBDI)MgTHF(C11H6F2N)] 204.

152

2.03.3.4

Organometallic Complexes of the Alkaline Earth Metals

Organomagnesium compounds with oxygen bonded ligands

In 2009 and 2011 Carpentier, Sarazin, and coworkers synthesized oxygen-supported organomagnesium compounds L22MgR {R]nBu 205,202 Me 206203} by deprotonation of phenoxy protio-ligand L22H (where L22 ¼ 4-tert-butyl-2,6- bis(morpholinomethyl)phenoxy) with suitable magnesium alkyl reagents (Scheme 105). In the 1H NMR spectrum, the absence of phenolic protons and the upfield shifted Mg-alkyl signal (−0.15 ppm and −1.65 ppm) are consistent with the formation of compounds 205 and 206.

Scheme 105 Organomagnesium compounds bear phenoxy ligand.

In 2016 Nifant’ev et al. isolated alkyl magnesium phenolate complex [L23MgnBu]2 207; {L23H ¼ 2,6-di-tert-4butyl-methylphenol} via simple deprotonation of L23H with MgBu2 in heptane with 97% yield (Scheme 106).204 The authors further allowed 207 to dissolve in THF to afford mononuclear solvated magnesium butyl complex L23MgnBu(THF) 208. Both 207 and 208 were fully characterized by NMR and single-crystal X-ray techniques (Fig. 95).

Scheme 106 Example of alkyl magnesium phenolate.

Fig. 95 The solid-state structures of [L23MgnBu]2 207 and [L23MgnBu(THF)] 208.

Organometallic Complexes of the Alkaline Earth Metals

153

Metal-organic frameworks (MOFs) have been employed to support reagents for various organic transformations205 like carbonyl and imine hydroboration. In 2016 Lin et al. synthesized a Zr-based single-site MOF containing mononuclear magnesium methyl species TPHN-MOF-MgMe 206 with libration of methane gas (Scheme 107).206 The TPHN (¼ 4,40 -bis(carboxyphenyl)-2-nitro-1,10 -biphenyl) fragment acts as a subordinate building unit in the assembly of heteroleptic compound 0209.

Scheme 107 MOF stabilized magnesium methyl.

2.03.3.5 2.03.3.5.1

Cationic organomagnesium complexes Group-14 stabilized organomagnesium cationic compounds

In 2019 Fremont, Dagorne, and coworkers synthesized NHC supported cationic magnesium species [{IMesMg(Me)(THF)2}]+ 210 and [{IPrMg(Me)(THF)2}]+ 211 from the corresponding neutral precursors 64 and 65 by the use of Na[BPh4] (Scheme 108).124 In the 1H NMR spectrum (C6D6) spectrum, the signal for MgdCH3 moiety shifts towards the upfield region (d ¼ −1.62 ppm, 210 and −1.70 ppm, 211 respectively) due to the greater Lewis acidity of the cationic magnesium center compared to the parent precursor. Compound 210 crystallizes as a mononuclear species, i.e., [{IPrMg(Me)(THF)2}]+ with the Mg center in a distorted tetrahedral geometry (Fig. 96). However, due to the lower steric bulk of IMes over IPr, 211 crystallizes as a dinuclear dication [{(Mg(IMes) (THF))}2(m-Me)2]2+ (2100 ). Cation 2100 accommodates two magnesium centers in distorted tetrahedral geometries bridged by CH3 substituents. Interestingly two different Mgd(m-Me) bond distances are measured crystallographically{Mg1dC1 ¼ 2.205(4) A˚ and Mg1dC2 ¼ 2.338(4) A˚ }.

Scheme 108 NHC supported organomagnesium cation.

154

Organometallic Complexes of the Alkaline Earth Metals

Fig. 96 The solid-state structure of [{(Mg(IMes)(THF))}2(m-Me)2]2+ 2100 and [{IPrMg(Me)(THF)2}]+ 211.

In addition, the same research group isolated a polynuclear dicationic species [[{[Mg(IMes)(Me)][Mg(IMes)]}(m-Me)2]2(m-Br)2]2+ 212 by treatment of neutral halide adduct 64 with Li[B(C6F5)4] in bromobenzene (Scheme 109).124 As confirmed crystallographically, 212 consists of two MesNHCdMg units bridged by two methyl substituents and linked by two bromides (Fig. 97). The whole system possesses C2v-symmetry, which is further confirmed by 1H NMR, with one set of signals for the MgdCH3 moiety at d ¼ −2.63 ppm in C6D5Br.

Scheme 109 Isolation of polynuclear organomagnesium dication.

Fig. 97 The solid-state structure of [[{[Mg(IMes)(Me)][Mg(IMes)]}(m-Me)2]2(m-Br)2]2+ 212.

Organometallic Complexes of the Alkaline Earth Metals

155

It was previously found133 that a double carbene binding strategy helps isolate NHC-supported magnesium dihalide (I Pr2Me2)2MgBr2 66. The same strategy was used by Webster, Gillard, and coworkers in 2020 to synthesize bis-carbene-supported organomagnesium dication [{(IiPr2Me2)2Mg}2(m-Me)2][BArF4]2 213 in 63% yield (Scheme 110).207 In C6D5Br 213 exhibits a characteristic MgdCH3 1H NMR signal at d ¼ − 1.02 ppm, which can be compared to that of the parent entity 66 (d ¼ − 0.82 ppm). The authors presented complete structural characterization of compound 213 (Fig. 98), in which two magnesium centers are coordinated by IPrNHC ligands and unsymmetrically bridged by methyl groups. Due to distortion, both MgdCcarbene (2.209(10)−2.233(12) A˚ ) and Mg − (m-Me) (av. 2.242(11) A˚ ) are found to shorten compared to parent carbene adduct 65. i

Scheme 110 Synthesis of bis-carbene supported organomagnesium dication.

Fig. 98 The solid-state structure of [{(IiPr2Me2)2Mg}2(m-Me)2][(BArF4)2] 213.

In 2021 Gilliard et al. synthesized tris-carbene stabilized magnesium methyl cation [(IiPr2Me2)3Mg]Br 214 as a colorless solid in 89% yield (Scheme 111),208 by treatment of 66 with an additional carbene molecule. Compound 66 was also reacted with [Na(dioxane)2][OCP] in C6D5Br, resulting in the immediate formation of new products. One is methyl magnesium ethoxide complex [(IiPr2Me2)MgMe(m-OEt)]2 215 (25% yield); the second product is identified as magnesium phosphaethynolate charge-separated ion pair [(IiPr2Me2)3MgMe][OCP] 216, formed in 75% yield. The molecular structures of 214–216 are shown in Fig. 99.

Scheme 111 Tris-carbene ligated magnesium cations.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 99 The solid-state structures of [(IiPr2Me2)3Mg]Br 214, [(IiPr2Me2)MgMe(m-OEt)]2 215 and [(IiPr2Me2)3MgMe][OCP] 216.

2.03.3.5.2

Group-15 stabilized organomagnesium cationic compounds

In 2013 Sadow et al. isolated the organomagnesium silyl cations [(TMEDA)Mg{Si(SiMe3)3}][MeB(C6F5)3] 218a and [(dpe)Mg {Si(SiMe3)3}][MeB(C6F5)3] 218b by reaction of the corresponding precursor (217a or 217b) with the strong Lewis acid B(C6F5)3 at room temperature (Scheme 112).209 In the 1H NMR spectrum of 218a, the BdCH3 signal is shifted downfield (d ¼ 1.67 ppm) compared to its precursor (d ¼ −1.03 ppm). The boron-methyl interaction is supported by 1Hd11B HMBC experiments, giving a unique cross peak signal at d −14.3 ppm. For the Si(SiMe3)3 fragments, a large chemical shift is found compared to TMEDA. These data support the notion of methyl group interaction with the electrophile B(C6F5)3. A similar observation is found for the analogous magnesium cation 218b. The solid-state structures of 218a and 218b show a zwitterionic type bridging MgdMedB(C6F5)3 unit (Fig. 100). Due to this bridging, both MgdC (av. 0.23 A˚ ) and MgdN (av. 0.05 A˚ ) bond lengths in cationic species are shorter than in the parent precursor. However, the MgdSi bond separation is identical in each case.

Scheme 112 Synthesis of organomagnesium silyl cations.

Fig. 100 The solid-state structures of [(TMEDA)Mg{Si(SiMe3)3}MeB(C6F5)3] 218a and [(dpe)Mg{Si(SiMe3)3}MeB(C6F5)3] 218b.

Often strong Lewis acids are stabilized by the coordination of bases like THF, pyridine, etc., leading to modulated reactivity at the metal center. Thus, Lewis base free cationic magnesium species have been targeted. As (BDI)Mg+ cations behave as stronger Lewis acids than commercially available electrophiles like B(C6F5)3. This can help in isolating organomagnesium cationic alkyne adduct [(BDI)Mg+ EtC^CEt][B(C6F5)−4] 219 as reported by Harder and coworkers in 2018 (Scheme 113).210

Organometallic Complexes of the Alkaline Earth Metals

157

Scheme 113 Isolation of cationic organomagnesium alkyne.

In the solid structure of 219, it is found that the magnesium center is connected to [B(C6F5)4]− in Z1 fashion while being attached to the alkyne through a weak p interaction (Fig. 101). This results in only minor perturbance of the C^C bond; the CdC^C fragments are bent, indicating the flow of the p electron cloud of the alkyne towards the cationic magnesium center.

Fig. 101 The solid-state structure of [(BDI)Mg+ EtC^CEt][B(C6F5)−4 ] 219.

For further understanding of Mg-alkyne interaction, additional experiments were carried out by the same author in 2019. In situ synthesized magnesium cation 220 was reacted with terminal alkynes such as PhC^CH and Me3SiC^CH, which ultimately led to magnesium-alkynyl compounds [(BDIdH)MgR]2[B(C6F5)−4]2 {R ¼ dC^CPh 221, dC^CSiMe3 222} in good yields (Scheme 114).211 Interestingly, the BDI behaves as a neutral imine type ligand in both alkynyl cations, as confirmed by IR spectroscopy for C]N bond stretching frequencies (221; 1617 cm−1 and 222; 1620 cm−1). In 1H NMR of complex 222, two doublets at d ¼ 3.79 and 3.92 ppm correspond to the backbone CH2 protons while 13C{1H} NMR spectrum features a signal at d ¼ 45.8 ppm, which is slightly upfield compared to the backbone CH2(d ¼ 45 ppm) of the BDI ligand. A similar observation is found for 218b.

Scheme 114 Homoleptic magnesium alkyne cations.

In 2010 Hayes et al. found that bis(phosphinimine) ligands (L24 ¼ 4,6-(MesN]PPh2)2dibenzofuran) can also stabilize organomagnesium monocations, viz. [L24MgnBu]+[BR4]− (R]C6F5 224a, Ph 224b) formed as white solids in good yield (Scheme 115).212

158

Organometallic Complexes of the Alkaline Earth Metals

The reaction procedure involves simple deprotonation between MgnBu2 with activated cationic precursors 223a and 223b. In the 1H NMR spectrum a resonance at −0.13 ppm (both 223a and 223b) corresponds to the MgdCH2 protons, while the 31P{1H} spectra feature downfield shifted resonances (d ¼ −17.6 to 23.0 for 224a, d ¼ −17.6 to 23.2 for 224b). The molecular structure of 224b reveals a magnesium center possessing a trigonal planar geometry with k2 coordination of the ligand (Fig. 102).

Scheme 115 Organomagnesium monocation bearing bis(phosphinimine) ligand.

Fig. 102 The solid-state structure of [L24MgnBu]+[BPh4]− 224b.

In 2013 Hazari et al. synthesized tripodal ligand (Me6tren) supported monocationic magnesium species [(Me6tren)MgMe] BArF{BArF ¼ tetrakis(3,5-bis(trifluoromethyl)phenyl borate} 225 and [(Me6tren)MgMe]Br 226 by reaction of the neutral parent precursor 184b with a suitable electrophile at room temperature (Scheme 116).196 Compound 225 is more thermally stable (up to 1 h) than 226, decomposing instantly (stable up to 5 min at room temperature). The molecular structure of precursor 184b is shown in Fig. 103.

Scheme 116 Tripodal ligand chelated magnesium methyl cations.

Organometallic Complexes of the Alkaline Earth Metals

159

Fig. 103 The solid-state structure of [(Me6tren)MgMe2] 184b.

In 2018 Venugopal et al. proposed an alternate synthetic route for the generation of tripodal ligand-supported alkyl magnesium cations. When [PhNMe2H][B(C6F5)4] is introduced into an ethereal solution of Me6TREN and MgnBu2 and stirred at room temperature, it affords butylmagnesium cation [(Me6TREN)MgnBu][B(C6F5)3] 227 in excellent yield (Scheme 117).213 Similar to other Me6tren stabilized alkyl magnesium cations, the MgdMe signal was found at d –0.59 ppm in the 1H NMR spectrum. The solid-state structure of 227 shows a distorted trigonal pyramidal geometry at the magnesium center, with an MgdC bond length similar to previously described analogous magnesium cations (Fig. 104).

Scheme 117 Alternative route for tripodal ligand stabilized organomagnesium cation.

Fig. 104 The solid-state structure of [(Me6TREN)MgnBu] [B(C6F5)3] 227.

In 2021 Harder et al. analyzed the synthesis and thermal decomposition of cationic magnesium complexes of dicyclopentadiene and norbornadiene. The cationic norbornadiene complex [(DippBDI)Mg+(nbd)][B(C6F5)−4] 228 was synthesized in 85% yield from the corresponding precursor (DippBDI)MgnBu (Scheme 118).214 The crystal structure of 228 shows Z4 coordination of the norbornadiene moiety at magnesium (Fig. 105). It is also noteworthy to mention that one of the alkenes donors in norbornadiene is significantly closer to Mg due to additional Mg⋯ FdC contacts with the [B(C6F5)−4] counterion.

160 Organometallic Complexes of the Alkaline Earth Metals

Scheme 118 Thermal decomposition and synthesis of organomagnesium cations bearing dicyclopentadiene and norbornadiene.

Organometallic Complexes of the Alkaline Earth Metals

161

Fig. 105 The solid-state structure of [(DippBDI)Mg+(nbd)][B(C6F5)−4 ] 228, [(DippBDI)Mg(Cp)Mg(BDI)+][B(C6F5)−4 ] 229, [(ippBDI)Mg+(dcpd)][B(C6F5)−4 ] 230.

The high temperature (60  C) 1H NMR spectrum analysis of 229 revealed that the norbornadiene signals disappeared entirely and formed a well-defined BDI complex with a sharp singlet at d 5.62 ppm. A study of the crystal structure of the decomposition product showed the presence of a cationic dinuclear Mg complex in which a Cp anion bridges two (BDI)Mg+ cations in 229 (Fig. 105). In another similar reaction designed to obtain the magnesium dicyclopentadiene complex, the product [(BDI)Mg(dcpd)] [B(C6F5)−4] (dcpd ¼ dicyclopentadiene) 230 was synthesized in 45% yield. In this case, the crystal structure shows the dcpd ligand bound at magnesium in Z2-fashion (Fig. 105).

2.03.3.6

Organomagnesium p-arene complexes

In 2018 Maron, Hill and coworkers found that reaction between [(RBDI)Mg(n-Bu)] and [Ph3C][Al{OC(CF3)3}4] in either benzened6 or toluene-d8 results in the immediate formation of two immiscible liquid phases containing [(MeBDI)Mg(R)]+ {R ¼ C6D5CD3 231a and C6D6 231b} and [(tBuBDI)Mg(C6D5CD3)]+ 231c (Scheme 119).215 Although analysis of the mixture by 19 F{1H} NMR spectroscopy provides a single sharp peak at d −74.9 ppm, the 1H and 13C{1H} NMR spectra are mainly uninformative. Slow diffusion of hexane into solutions at room temperature resulted in colorless crystals of 231a and 231b, suitable for single-crystal X-ray diffraction (Fig. 106).

Scheme 119 Isolation of magnesium clathrates 231a-231c.

Fig. 106 The solid-state structure of [(MeBDI)Mg(C6D5CD3)]+ 231a and [(MeBDI) Mg(C6D6)]+ 231b.

In 2018 Harder et al. studied the metal p-arene interaction in the monocationic magnesium complexes [(DippBDI)Mg(arene)] [B(C6F5)4] {arene ¼ benzene 233a, toluene 233b, m-xylene 233c and mesitylene 233d}. All compounds 233a-233d were prepared from precursor 232 using trityl cation [Ph3C][B(C6F5)4] in 63–94% isolated yield (Scheme 120).216 Molecular structures of 233a233d are shown in Fig. 107, which reveal that the p-arene is bound to the magnesium center in Z3 fashion while maintaining weak Mg


F interactions with the counter-anion [B(C6F5)4]−. However, in complex 232, the mesitylene ring is attached to the magnesium center in Z6 fashion. Nevertheless, due to the high steric bulk of durene, no p-arene complexation occurs in this case.

162

Organometallic Complexes of the Alkaline Earth Metals

Scheme 120 Analysis of metal p-arene interaction of magnesium monocations bearing BDI ligand.

Fig. 107 The solid-state structure of [(DippBDI)Mg+.toluene][B(C6F5)−4 ] 233b, [(DippBDI)Mg+.m-xylene][B(C6F5)−4 ] 233c and [(DippBDI)Mg+. mesitylene][B(C6F5)−4 ] 233d.

The same research group in 2019 utilized the more bulky cation [(tbuBDI)Mg][B(C6F5)4] 234 to coordinate various p-arene molecules by analogy to the previously described complex [(MeBDI)Mg][B(C6F5)4] 232 (Scheme 121).217 It was found that with smaller arene systems, compound 234 can easily coordinate the arene in Z2 fashion to afford solvated cationic complexes [(tbuBDI)Mg(arene)][B(C6F5)4] {arene ¼ benzene 235a, toluene 235b, m-xylene 235c} in 58–90% yield (Table 6). Meanwhile, the more bulky compound 234 fails to bind mesitylene rather than coordinate chlorobenzene from the reaction mixture, as evident from the chlorobenzene adduct structure obtained from X-ray diffraction (Fig. 108).

Scheme 121 Investigation of metal p-arene interaction of bulky magnesium monocations.

Organometallic Complexes of the Alkaline Earth Metals

Table 6

163

p-Bonding of cationic organomagnesium compounds.

Complex +

[(BDI)Mg. benzene] 233a [(BDI)Mg. toluene]+ 233b [(BDI)Mg. m-xylene]+ 233c [(BDI)Mg. meistylene]+ 233d [(tBuBDI)Mg. benzene]+ 235a [(tBuBDI)Mg. toluene]+ 235b [(tBuBDI)Mg. m-xylene]+ 235c

Yield (%)

Mg-Arene binding

86 74 82 63 77 58 90

Z3 Z3 Z3 Z6 Z2 Z2 Z2

Data from Ref. Pahl, J.; Friedrich, A.; Elsen, H.; Harder, S. Organometallics 2018, 37, 2901–2909.; Friedrich, A.; Pahl, J.; Elsen, H.; Harder, S. Dalton Trans. 2019, 48, 5560–5568.

Fig. 108 The solid-state structure of [(tbuBDI)Mg(benzene)][B(C6F5)4] 235a, [(tbuBDI)Mg(m-xylene)][B(C6F5)4] 235c and [(tbuBDI)Mg(chlorobenzene)][B(C6F5)4] 235d.

In 2019 Harder et al. tested the reactivity of complex 232 towards terminal and internal alkynes. When compound 232 was treated with hex-3-yne, it afforded cationic magnesium alkynyl complex [(BDI)Mg(EtC^CEt)][B(C6F5)4] 236. However, with internal alkynes, it leads to the formation of the [4 + 2] cycloaddition product [MgC(Ph)]C(R)C{C(Me)]NDipp}2]2[B(C6F5)4]2, where R]Ph 236, Me 238 (in 53–73% yield; Scheme 122).211

Scheme 122 Synthesis of organomagnesium alkynyl cations.

In 2020 the same research group introduced three new ligands substituted with TROP groups (where TROP ¼ [5H]dibenzo[a,d] cyclohepten-5-yl) to isolate organomagnesium compounds. In the first category, simple deprotonation of the (TROP)(DIPP)NH ligand with basic Mg(nBu)2 at 70  C was shown to afford the yellow crystalline solid [(TROP)(DIPP)N]2Mg 239 in 51% yield (Scheme 123).214,218. In a second experiment, the symmetric magnesium iodide (TROPBDI)MgI 240 was prepared by salt metathesis of (TROPBDI)Li with one equivalent of MgI2 in benzene. The NOESY spectra of 240 in THF-d8 indicate an endo conformation of the TROP-substituents. To cope with the poor solubility of 240 in non-coordinating aromatic solvents, the third type of asymmetric b-diketiminate ligand (TROP–DIPPBDI) was introduced in the complex (TROP–DIPPBDI)MgI. Analysis of the crystal structures of 240 and 241 reveals that the TROP ligand is always bound to the Mg center in an endo fashion (Fig. 109). This type of attachment forms a concave pocket inside which the alkene double bond stays in the close vicinity of the metal.

164 Organometallic Complexes of the Alkaline Earth Metals

Scheme 123 Isolation of organomagnesium complexes ligated by TROP type ligand.

Organometallic Complexes of the Alkaline Earth Metals

165

Fig. 109 The solid-state structure of [(TROPBDI)MgI] 240 and [(TROP–DIPPBDI)MgI] 241.

2.03.3.7

Application of organomagnesium complexes as catalysts

In recent years, molecular organomagnesium compounds have been utilized as catalysts for various organic transformations.54,57,219 In 2014, Nembenna et al. reported that organomagnesium catalyzed the synthesis of guanidines 77.134 The first CAAC-supported magnesium bis-amide 88140 was used for lactide polymerization to corresponding dihydroxy polylactides under mild conditions. In 2015 Mashima et al. first reported that the organomagnesium compound [IPr 2 NCH2CH2N(CHPhCH2Ph)Mg(CH2Ph)]2 (anti-111)148 catalyzed isomerization of terminal alkynes into allenes at 80  C with moderate yield. Furthermore, anti111 was also employed for the synthesis of difficult internal alkynes from corresponding terminal alkynes. Hill and coworkers reported BDI stabilized mononuclear magnesium butyl complex 119154 as a precatalyst for various hydroelementation reactions. Like compound 119, BDI stabilized magnesium acetylide compound 125158 was also employed to transform substituted alkynes into allenes and selectively into 1,3-enynes. Among chiral compounds, tris(oxazolyl)borate-supported magnesium methyl compound 147172 has been used to cyclize amino alkenes to the corresponding racemic organic pyrrolidines with a yield of 36%. Sadow et al. reported heteroleptic organomagnesium methyl complex 152176 as a catalyst for hydroboration reactions. Mountford and coworkers used heteroscorpionate organomagnesium complex 158179 for the ring-opening polymerization of cyclic esters such as rac-lactide and e-caprolactone with high conversion rates in protic aprotic solvents. Hevia and coworkers employed magnesium amido compound BDIMg(TMP) 181194 for regioselectively magnesiation at the a-position of some challenging substrates such as N-heterocyclic and 2-pyridine with the quantitative conversion. Other classes of the heteroleptic compounds, for instance phenoxy-stabilized magnesium butyl 205202 has been used for the ring-opening polymerization of cyclic esters. Similarly, organomagnesium monocations [L24MgnBu]+[BR4]− R]C6F5 224a, Ph 224b212 act as initiators for the polymerization of simple organic monomers such as e-caprolactone (Fig. 110).

Fig. 110 Molecular organomagnesium compounds for organic transformations.

2.03.4

Calcium

Humphry Davy discovered element 20, calcium, in the year 1808. Like magnesium, calcium is also a cheap, abundant, and biocompatible element.220 The synthesis and applications of calcium-based organometallic compounds are growing.

166

Organometallic Complexes of the Alkaline Earth Metals

2.03.4.1 2.03.4.1.1

C-donor stabilized organocalcium compounds Alkyl stabilized organocalcium compounds

Tris(dimethylsilyl)methyl calcium(II) 242 has been prepared from CaI2 and two equiv. KC(SiHMe2)3 in THF (Scheme 124).221 X-ray crystallography shows that the Ca


Si distance is shorter than in Ca(C(SiMe3)2)2 due to a b-agostic SiH interaction, but the CadC bond distance of 242 is longer (2.616(7) A˚ in comparison to 2.459(9) A˚ ; Fig. 111).222

Scheme 124 Synthesis of the homoleptic organocalcium complex.

Fig. 111 The solid-state structure of [Ca(C(SiHMe2)3)2(THF)2] 242.

[Ca(C(SiHMe2)3)2THF2] reacts with B(C6F5)3 in toluene at ambient temperature to give [CaC(SiHMe2)3(HB(C6F5)3)THF2] 243 by elimination of (Me2SiC(SiHMe2)2)2 (disilacyclobutane) (Scheme 125).221

Scheme 125 Synthesis of alkyl calcium complex and disilacyclobutane.

The air and moisture-sensitive compound bis(trimethylsilyl)Ca(THF)2 244 was synthesized by the salt metathesis reaction of [K {CH(SiMe3)2(THF)}] with CaI2 (Scheme 126).223

Scheme 126 Synthesis of the homoleptic organocalcium complex.

Organometallic Complexes of the Alkaline Earth Metals

167

[2,20 -(4-MeC6H3NMe2)2CH]2Ca(THF)2] 245b has been synthesized by a salt metathesis reaction of {[2,20 -(4-MeC6H3NMe2)2CH]K(THF)}2 and CaI2(THF)2 in toluene, and recrystallized from n-hexane. Compound 245b soluble in benzene by heating at 70  C and evaporating the solvent under high vacuum at 80  C to afford the THF free compound [2,20 -(4-MeC6H3NMe2)2CH]2Ca 245a (Scheme 127).224 Single-crystal X-ray diffraction analysis revealed that the ligand offers tridentate coordination, with one central carbon and two nitrogen atoms of NMe2 groups coordinated to the metal center to give a pincer coordination complex. However, when treated with THF, the coordination site changes from pincer type to bidentate k2-CN (Fig. 112).

Scheme 127 Example of organocalcium of pincer type diarylmethanido complex.

Fig. 112 The solid-state structures of [2,20 -(4-MeC6H3NMe2)2CH]2Ca 245a and [2,20 -(4-MeC6H3NMe2)2CH]2Ca(THF)2] (carbon atoms of THF molecules are omitted for clarity) 245b.

The salt metathesis reaction of CaI2(THF)2 with 2 equiv. [(p-tBudC6H4)2CH]Na(TMEDA) in toluene resulted in the formation of [(p-tBudC6H4)2CH]2Ca(TMEDA) 246 (Scheme 128).225 The 1H NMR in C6D6 shows that methanide protons give a singlet at 4.06 ppm and the crystal data shows bond distances of Ca(1)-C(1A) and Ca(1)-C(1B) of 2.647(2) and 2.545(2) A˚ , respectively (Fig. 113).

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 128 Homoleptic organocalcium complex bearing benzhydryl ligand.

Fig. 113 The solid-state structure of [[(p-tBu-C6H4)2CH]2Ca(TMEDA)] 246.

Establishing a heavier calcium analog of a Grignard reagent is full of challenges due to the high activation energy needed for calcium metal and the high reactivity of the organocalcium compounds, leading to degradation or CdC coupling type reaction. The synthesis of alkyl and aryl calcium halides by a straightforward reduction of alkyl and aryl halides has been brought into the light. The reaction of trimethylsilyl methyl iodide and bromide with activated calcium in THF forms trimethylsilyl methyl calcium iodide and bromide, respectively. Subsequent treatment of compounds 247a and 247b with tetrahydropyran (THP) leads to more robust adducts 248a and 248b (Fig. 114). The reaction of cyclopropyl iodide with activated calcium at 10  C in THP has been shown to smoothly afford cyclopropyl calcium iodide complexe with a yield of 70%. A highly shielded resonance for the a-hydrogen at −1.65 ppm in 1H NMR spectrum and two resonances for the diasterotopic ring hydrogen atom between 0.20 and 0.75 are consistent with the formation of the proposed organo-calcium moiety. Cyclopropylcalcium iodide 249 can be converted into the THP coordinated di(cyclopropyl)calcium complex 250 and calcium diiodide by a Schlenk-type equilibrium (Scheme 129).226

Fig. 114 The solid-state structure of [Me3SiCH2Ca(THP)4Br] 248b.

Organometallic Complexes of the Alkaline Earth Metals

169

Scheme 129 Synthesis of heavier Grignard reagents.

Approaching the reaction mentioned above with sterically demanding tri-isopropylsilylmethyl bromide and activated calcium leads to dimeric m2-Br bridged compound 251 (Scheme 130).226

Scheme 130 Synthesis of compound 251 by reducing (bromomethyl)triisopropylsilane with activated calcium.

Attempts to synthesize a mixed alkyl-aryl calcium compound by reacting phenyl calcium iodide and KCH2SiMe3 in THF in the presence of a LiCl additive obtains compound [Me3SiCH2Ca(THF)3(m-Cl)]2 252 by elimination of (THF)4CaPh2 (Scheme 131).226

Scheme 131 Synthesis of organocalcium halide reagent.

Dimethyl calcium compounds are readily obtained by treating calcium bis-amide in Et2O with commercially available MeLi. The CaMe2 compound is entirely insoluble in hydrocarbon solvents, i.e., n-hexane, benzene, and sometimes decomposes violently upon adding pyridine and acetonitrile solvents. A saturated solution of CaMe2 in THF, when cooled down to −35  C, yields crystalline material as a heptametallic polymeric structure [(THF)10Ca7Me14] 253, with three different Ca environments featuring differently bridged methyl groups. Subsequently, the reaction of CaMe2 with an equimolar amount of CaI2 in THF was shown to result in the formation of (THF)4CaI2 and shows slow decomposition in contrast to CaMe2 in THF. If this mixture is kept for 3 weeks, the formation of another distinct dimeric methyl calcium iodide complex, [(THF)3Ca(Me)(I)]2, 254 has been observed. Further experiments using the relatively weakly coordinating solvent THP results in a similar type of (THP)4CaI2 crystals, which upon further investigation after some days, leads to isolation of the trimetallic complex (THP)5Ca3(Me)5(I), 255 (Scheme 132).227 X-ray crystallographic analysis of the compound has revealed that the structure has a triangular arrangement of three calcium atoms, bicapped with two methyl groups in axial positions, and three methyl groups equatorially attached to the calcium centers making a six-membered ring-type complex (Fig. 115).

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 132 Reactivity of polymeric dimethylcalcium.

Fig. 115 The solid-state structure of [(THF)3Ca(Me)(I)]2 254.

By varying the substituents on 1,3-butadienediide reagents, different reactivity and product structures can be demonstrated upon reaction with CaI2. The room-temperature reaction of the compound [LiC(SiMe3)C(Ph)]2, 256 with CaI2 in THF smoothly affords iodide-bridged 1-calcio-4-lithio-1,3-butadiene compound 257 (Scheme 133).228 X-ray crystallographic analysis of the compound has shown a monomeric structure with calcium and lithium atoms coordinated to the two opposite faces of the butadiene carbon backbone.

Scheme 133 Example of butadienyl heavy Grignard reagent.

Organometallic Complexes of the Alkaline Earth Metals

171

Further reaction of 2,3-dimethyl substituted analog of 1,3-butadienediide with CaI2 in THF affords a mixture of products, i.e., iodide bridged 1,4-dicalcio-butadiene derivative 258, and 2 equiv. of the LiI-ligated dimer of the dicalcio analog 259 (Scheme 134). X-ray crystallographic analysis of compound 259 shows similar bond parameters (CadC bond distance 2.545(6)–2.584(6) A˚ ) as in the iodide bridged 1-calcio-4-lithio-1,3-butadiene compound 257.

Scheme 134 Synthesis of butadiene derivative stabilized organocalcium halides.

Variable temperature NMR study of the compound mixture indicates the conversion of the dimeric compound into the monomeric dicalcio compound 258. Further investigation of the conversion from compound 258 to compound 260 has been carried out at 70  C: stirring for 1 h leads to the gradual formation of red crystalline compound 260 (Scheme 135).228 X-ray crystallographic analysis of 260 shows an unexpected centrosymmetric inverse crown ether Ca4[O] compound. The four calcium centers are situated at the vertices of a square, while the O atom occupies the center of the square; the CadO bond lengths show in the range of 2.332(2)–2.342(9) A˚ .

Scheme 135 Synthesis of inverse crown ether organocalcium complex.

2.03.4.1.2

Aryl stabilized organocalcium compounds

Heavier Grignard analogs of aryl calcium halides 261 are synthesized by treating aryl halides with activated calcium in THF. However, increasing the temperature up to −30  C allows for ether cleavage. PhCaI(THF)4 reacts with KR (R]N(SiMe3)2 or PPh2) in THF to yield [PhCa{N(SiMe3)2}(THF)3] 262a and [PhCa(PPh2)(THF)4] 262b, respectively (Scheme 136).229

Scheme 136 Heteroleptic aryl calcium complexes.

The metathesis reaction of PhCaI(THF)4 with N, N’-bis(trimethylsilyl)benzamidinate potassium salt readily affords dinuclear calcium compound {Ca(THF)3}2(m-I){m,m-[4,4-Ph2-2,6-(C6H4)2N3C3]} 263 via slow release of the benzonitrile (Scheme 137).229 X-ray crystallographic analysis of compound 263 has revealed that the ortho-metallated phenyl groups bridge between seven-coordinate calcium centers (Fig. 116). Due to steric overcrowding at calcium, these centers are defined by longer bond lengths CadC (2.687(6) and 2.668(6) A˚ ) and CadI (3.243(2) and 3.258(2) A˚ ) than in aryl calcium iodides with six-coordinate metal centers.

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 137 Dinuclear aryl organocalcium compound.

Fig. 116 The solid-state structure of [{Ca(THF)3}2(m-I){m,m-(4,4dPh2d2,6-(C6H4)2N3C3)}] 263.

The reaction mechanism is thought to involve the formation of diphenylimide by the treatment of phenylcalcium iodide with benzonitrile. Subsequently, insertion of two equivalents benzonitrile followed by ring closure of the 1,2-dihydrotriazine derivative and a 1,3-CaI shift forms 1,4-dihydrotriazine compound. Furthermore, upon adding phenylcalcium iodide, the phenyl group ortho CH moieties are activated, leading to two-fold deprotonation and the formation of compound 263 (Scheme 138).229

Scheme 138 Solvated organocalcium halide complex bearing a dihydrotriazine derivative.

Organometallic Complexes of the Alkaline Earth Metals

173

The dimetallated aryl derivative Me2Si{C6H4Ca(THF)4I}2 264 has been synthesized by reducing bis(4-iodophenyl)dimethylsilane with calcium in THF. The calcium atoms are shown to be in an octahedral environment with iodide and phenyl groups trans to each other (Scheme 139).230

Scheme 139 Dinuclear aryl organocalcium.

The reaction of excess activated calcium with para-substituted iodobenzene in THP gave heavier Grignard reagents (C6H4-4-R)CaI(THP)4 265a-265e (R]CH3, Cl, Br, I, OCH3), which are crystalline substances (Scheme 140).231 Due to electrostatic reasons, the iodide ligands and the anionic aryl ligands are positioned trans to each other. These monomeric compounds have molecular C2 symmetry due to enormously different proximal and distal CadCdC angles, and the calcium center is in a distorted-octahedral environment. The CadO bond distances are longer (ranging between 2.402(3) and 2.416(3) A˚ ) in all of these complexes. The solid-state structure of the iodide complex exhibits a shorter CadI bond length, which is due to the partial occupation of this position by bromide.

Scheme 140 Synthesis of a series of heavier Grignard reagents.

The compound PhCaI(THF)4, when dissolved in DME, gives a pale yellow-orange solution of the cationic compound [PhCa(DME)3]I 266, which is highly reactive (Scheme 141).232 Due to strong electrostatic interactions, the mean CadC bond distance is 2.517 A˚ , despite the seven-coordinate nature of the metal center (Fig. 117).

Scheme 141 Synthesis of cationic aryl calcium complex.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 117 The solid-state structure of [PhCa(DME)3]I 266.

The reaction of activated calcium with 4-iodobiphenyl in tetrahydropyran gives a colorless crystalline solid, (4-PhdC6H4)CaI (THP)4 267 (Scheme 142).233 This compound possesses C2 symmetry, and the metal center is in a distorted octahedral environment. Similarly, the reaction of activated calcium with 9-bromophenanthrene gives a green reaction mixture containing (Phen)CaBr(THP)4 268. The X-ray data reveal that 268 is a mononuclear octahedral complex with a bond length of 2.559(3) A˚ between calcium and carbon. Further recrystallization using THF solvent at ambient temperature yields the dinuclear species [(Phen)Ca(m-Br)(THF)3]2 269 (Fig. 118).

Scheme 142 Synthesis of solvated aryl calcium halide 267 and equilibrium between solvated mononuclear and dinuclear aryl calcium bromide.

Organometallic Complexes of the Alkaline Earth Metals

175

Fig. 118 The solid-state structures of [(4-PhdC6H4)CaI(THP)4] 267, [(Phen)CaBr(THP)4] 268 and [(Phen)Ca(m-Br)(THF)3]2 269.

Diarylcalcium complex (b-Naph)2Ca(THF)4 270 is synthesized from a 1:1 mixture of [CaI(b-Naph)(THF)4] and potassium tert-butoxide in THF, eliminating KI (Scheme 143).234 The crystallographic data for 270 shows that the two aryl groups are in trans positions due to electrostatic reasons and that the metal center adopts an octahedral geometry with a bond distance between CdCa of ca. 2.635(3) A˚ (Fig. 119).

Scheme 143 Six-coordinate diaryl calcium complex.

Fig. 119 The solid-state structure of [Ca(b-Naph)2(THF)4] 270.

The treatment of activated calcium with iodo-2,6-di(4-tolyl)-benzene and further addition of 0.5 equivalent of iodine at −60  C yields {2,6-(4-tol)2C6H3}CaI(THF)3 271 which is pale-violet in color. This complex has a distorted trigonal bipyramidal geometry in which the bulky aryl group adopts an equatorial position and iodide and THF axial positions. The bond distances between calcium-carbon (2.517(4) A˚ ) and calcium-iodine (3.0754(7) A˚ ) are very short because of the low coordination number around the metal center.

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Organometallic Complexes of the Alkaline Earth Metals

The reaction of triphenylbenzene (TPB)Br with one equiv. of activated calcium in THF forms {2,4,6-Ph3C6H2}CaBr(THF)3 273. Similarly, the reaction of (TPB)Br with 2.5 equivalents of activated calcium or reaction of TPBBr in the presence of calcium powder with TPB gives an inverse sandwich complex, [(THF)3Ca(m-C6H3-1,3,5-Ph3)Ca(THF)3] Further reaction of this compound with TMTA (1,3,5-trimethyl-1,3,5-triazinane) in toluene, gives (TMTA)2Ca(CH2C6H5)2 274 with the elimination of hydrogen gas (Scheme 144).235 The calcium coordination number in this complex is eight, which induces steric strain, and causes a longer calcium-carbon bond length, around 2.623(3) A˚ , than the six-coordinated calcium complex having an average CadC bond length of 2.601(3) A˚ (Fig. 120).

Scheme 144 Synthesis of aryl and inverse sandwich-type organocalcium species.

Fig. 120 The solid-state structures of [{2,6-(4-tol)2C6H3}CaI(THF)3] 271 and [(TMTA)2Ca(CH2C6H5)2] 274.

Organometallic Complexes of the Alkaline Earth Metals

177

The reaction of 2 equiv. of styryldilithium with two equiv. CaI2 in THF at room temperature over 10 min affords styryltricalcium complex 275 after crystallization from ether/THF. The X-ray crystal data show that this complex is trimetallic, with all three calcium atoms bridged by iodide styryl ligands. Mechanistic studies imply that 276 is generated from an intermediate styryldicalcium complex with the elimination of CaI2 via the Schlenk equilibrium. A similar method has also been used to synthesize biphenylcalcium 277 complex via the metathesis reaction of biphenyl2,20 -dilithium with one equivalent of CaI2 in THF (Scheme 145).236 This complex is dimeric, in which two calcium centers are bridged by two biphenyl rings to resemble a calciafluorene complex (Fig. 121).

Scheme 145 Structurally characterized styryl and biaryl organocalcium species.

Fig. 121 The solid-state structure of biphenylcalcium 277.

2.03.4.1.3

NHC stabilized organocalcium alkyls

IiPr2Me2 carbene reacts with Ca[CH(SiMe3)2]2(THF)2 in toluene at ambient temperature via ligand-substitution reaction to produce (IiPr2Me2)2Ca[CH(SiMe3)2]2 (Scheme 146).237 This compound represents the first reported neutral NHC-supported calcium dialkyl complex. 278 was recrystallized from a mixture of toluene/hexane (1:2) at −30  C, giving colorless crystals; the X-ray

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Organometallic Complexes of the Alkaline Earth Metals

analysis data confirmed that compound 278 is a pseudo-tetrahedral complex, coordinated by two NHC and alkyl ligands (Fig. 122). The CadC1 and CadC2 bond lengths associated with the NHC ligands are consistent and appear in a range (2.65(3)–2.66(3) A˚ ). Similarly, the alkyl CadC3 and CadC4 bond lengths at (2.48(1)–2.53(3) A˚ ) are longer than the THF-coordinated dialkyl calcium complex (2.49(3) A˚ ) due to the steric hindrance of the NHC ligands.

Scheme 146 Dialkyl calcium adduct of carbene.

Fig. 122 The solid-state structure of [(IiPr2Me2)2Ca{CH(SiMe3)2}2] 278.

2.03.4.1.4

NHC stabilized organocalcium amides/halides

(IMes)Ca{N(SiMe3)2}Cl 279 can be synthesized by the reaction between 1 equiv. of 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and 1 equiv. of [Ca{N(SiMe3)2}2] in toluene. Similarly, the IMesCa{N(SiMe3)2}2 280 can be synthesized by the reaction between 1 equiv. of 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and 2 equiv. of calcium bis(bis(trimethylsilyl)amide) in toluene at ambient temperature (Scheme 147).238 Crystal data reveals that both compounds 279 and 280 are monomeric. In each case, the metal center shows an ideal trigonal geometry (Fig. 123).

Scheme 147 NHC stabilized calcium amides.

Organometallic Complexes of the Alkaline Earth Metals

179

Fig. 123 The solid-state structure of [(IMes)Ca{N(SiMe3)2}2] 280.

The reaction between IPr and 1 equiv. of Ca{N(SiMe3)2}2 in C6D6 gives (IPr)Ca{N(SiMe3)2}2 281 (Scheme 148).238 281 can be crystallized from benzene or toluene at 5  C, and features a three-coordinate trigonal planar metal center (Fig. 124).

Scheme 148 Synthesis of NHC stabilized calcium bis-amide complex.

Fig. 124 The solid-state structure of [IPrCa{N(SiMe3)2}2] 281.

{H2B(ImtBu)2}Ca{N(SiMe3)2}(THF) 282 (ImtBu ¼ 1-tert-Butyl-imidazole) was synthesized by combining three equiv. of KN(SiMe3)2, one equiv. of [H2B(ImtBu)2]I and one equiv. of CaI2 in THF at −78  C. The 13C{1H} NMR signal of the carbene carbon of this complex appears at 195.0 ppm. Similarly, the [H2B(ImMes)2]I ligand reacts with 1.5 equiv. of Ca{N(SiMe3)2}2 (THF)2 to gives {H2B(ImMes)2}Ca{N(SiMe3)2}(THF) 283 (Scheme 149).239

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 149 Bis-carbene supported calcium mono-amides.

The reaction of the same [H2B(ImtBu)2]I ligand with two equiv. of KN(SiMe3)2 and one equiv. of CaI2, leads to the formation of the heteroleptic calcium iodide, i.e., [{H2B(ImtBu)2}CaI(THF)] 284. The compound 284 undergoes redistribution to give the homoleptic species of {H2B(ImtBu)2}2Ca(THF) 285 (Scheme 150)239 (Fig. 125).

Scheme 150 Bis-NHC stabilized heteroleptic and homoleptic organocalcium complexes.

Fig. 125 The solid-state structure of [{H2B(ImtBu)2}CaI(THF)] 284.

Organometallic Complexes of the Alkaline Earth Metals

181

Treatment of tris(imidazolin-2-ylidene-1-yl)borate bromide with calcium iodide and four equiv. of KN(SiMe3)2 results in the formation of the heteroleptic organocalcium amide {HB(ImtBu)3}Ca{N(SiMe3)2} 287. The 13C{1H} NMR signal of the carbene carbon of this complex appears at 196.2 ppm. Similarly, the reaction of tris(imidazolin-2ylidene-1-yl)borate bromide with three equiv. of KN(SiMe3)2 and CaI2 leads to the formation of a 9:1 mixture of {HB(ImtBu)3}CaI(N-ImtBu) 288a and {HB(ImtBu)3} CaI(THF) 288b, respectively. The crystal data for 288b reveal that the borate ligand adopts a k3 binding mode to the metal center and that the calcium center has a distorted square-pyramidal geometry. [{HB(ImtBu)3}CaBr]2 289 has been synthesized by treatment of tris(imidazolin-2-ylidene-1-yl)borate bromide with 1.5 equivalent of Ca{N(SiMe3)2}2(THF)2 in THF at ambient temperature (Scheme 151).240 X-ray diffraction studies of this compound reveal that the ligand coordinates to the calcium center via a k3 binding mode via three NHC s-donors (Fig. 126).

Scheme 151 Heteroleptic organocalcium amide and halides are bearing tris(pyrazolyl)borate ligand.

Fig. 126 The solid-state structures of [{HB(ImtBu)3}CaI(THF)] 288b and [{HB(ImtBu)3}CaBr]2 289. (all hydrogen and tert-butyl carbon methyl atoms are removed for clarity).

(Me3SiCH2)CaBr(THF)4 has been synthesized by reacting (bromomethyl)trimethylsilane with activated calcium in THF. The reaction solution was added to (2,6-bis(3-isopropyl)imidazol-2-ylidene)benzene in THF at −78  C leading to the formation of [(THF)2Ca(Br)C6H3-2,6-(NHCiPr)2] 290. Similarly, the bis(THP) adduct of 2,6-bis(3-isopropylimidazol-2-ylidene)phenyl calcium iodide, [(THP)2Ca(I)C6H3-2,6-(NHCiPr)2] 291 was synthesized by addition of (Me3SiCH2)CaI(THP)4 to a solution of

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Organometallic Complexes of the Alkaline Earth Metals

(2,6-bis (3-isopropyl)imidazol-2-ylidene)benzene in THF at −78  C (Scheme 152).241 The X-ray crystal data revealed that both complexes possess a distorted-octahedral geometry with six coordination sites consisting of two ether molecules (THF or THP), one halide ion, and one k3 aryl-dicarbene ligands attached to the central metal atom (Fig. 127).

Scheme 152 Direct ortho metalation of (trimethylsilyl methyl)calcium halides.

Fig. 127 The solid-state structures of [(THF)2Ca(Br)C6H3-2,6-(NHCiPr)2] 290 and [(THP)2Ca(I)C6H3-2,6-(NHCiPr)2] 291.

(2,6-Bis(3-isopropyl)imidazol-2-ylidene)benzene reacts with Ca(CH2SiMe3)2 in THF to give bis{2,6-bis(3-isopropylimidazol-2-ylidene)phenyl}calcium, [Ca{C6H3-2,6-(NHCiPr)2}2] 292 (Scheme 153).241 The crystal data show that the CadCipso bond distance (2.493(3) A˚ ) for compound 292 is elongated in comparison to the corresponding bond distances of compound 290 (2.470 (4) A˚ ) and 291 (2.446(7) A˚ ) due to intramolecular strain (Fig. 128). Comparison of 13C{1H} NMR data for 1,3-bis(3-isopropylimidazol-2-ylidene)benzene and its calciated derivatives can be found in Table 7.

Scheme 153 Synthesis of CCC-pincer type calcium complex.

Organometallic Complexes of the Alkaline Earth Metals

183

Fig. 128 The solid-state structure of [Ca{C6H3-2,6-(NHCiPr)2}2] 292.

Table 7 derivatives.

13

C{1H} NMR chemical shifts (ppm) of 1,3-bis(3-isopropylimidazol-2-ylidene)benzene and its calciated

Compound iPr

C6H4-1,3-(NHC )2 [(THF)2Ca(Br)C6H3-2,6-(NHCiPr)2] 290 [(THP)2Ca(I)C6H3-2,6-(NHCiPr)2] 291 [Ca{C6H3-2,6-(NHCiPr)2}2] 292

d(Ccarbene)

d(Cic)

d(Coc)

d(Cmc)

d(Cpc)

213.2 197.3 197.1 199.9

128.1 166.7 165.8 170.2

142.5 150.4 150.2 151.3

110.8 108.9 108.9 108.7

115.5 124.9 125.0 124.3

Data from Ref. Koch, A.; Krieck, S.; Görls, H.; Westerhausen, M. Organometallics 2017, 36, 2811–2817.

The 2,6-bis(3-mesitylimidazol-2-ylidiene)pyridine ligand reacts with 1 equiv. of [CaI(THF)5][AlPh4] in THF produce the [(CarMesPyCarMes)(THF)2CaI][AlPh4] 293. The calcium ion was found to be hexacoordinated with distorted octahedral geometry. Similarly, addition of CaI2 in THF to an equimolar amount of 2,6-bis(3-mesitylimidazol-2-ylidiene)pyridine yields (CarMesPyCarMes)(THF)CaI2 294, ((CarMesPyCarMes) ¼ 2,6-bis(3-mesitylimidazol-2-ylidene)pyridine; Scheme 154).242 X-ray crystal data revealed that the calcium ion in 294 is also hexacoordinate, being bound to two iodide ions, one tridentate pyridine bis-carbene ligand, and one THF molecule. The environment around Ca is distorted octahedral with both iodide atoms in a trans position with an I1dCa1dI2 bond angle of 174.60(2) (Fig. 129).

Scheme 154 Hexacoordinated organocalcium halides.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 129 The solid-state structure of [(CarMesPyCarMes)(THF)CaI2] 294.

The 1:1 stoichiometric reaction between the 2,6-bis(3-mesitylimidazol-2-ylidiene)pyridine ligand and (THF)4Ca(NPh2)2 in THF afforded the organocalcium product, (CarMesPyCarMes)2(THF)Ca(NPh2)2 295 (Scheme 155).242 X-ray diffraction analysis reveals that the calcium ion has a distorted octahedral geometry (Fig. 130).

Scheme 155 Hexacoordinated solvated organocalcium.

Fig. 130 The solid-state structure of (CarMesPyCarMes)2(THF)Ca(NPh2)2 295.

(DippN](Ph)2P)2CH2 reacts with (para-tBubenzyl)2Ca
(THF)4 via double deprotonation to give the light yellow colored compound {(DippN](Ph)2P)2C}Ca(THF)2 296 (Scheme 156).243 The compound has been crystallized from a benzene/hexane mixture, and the X-ray data show that the Ca center has a square pyramidal geometry (Fig. 131).

Scheme 156 Synthesis of monomeric calcium carbene complex.

Organometallic Complexes of the Alkaline Earth Metals

185

Fig. 131 The solid-state structure of [{(DippN](Ph)2P)2C}Ca(THF)2] 296.

2.03.4.2

Cyclopentadienyl stabilized organocalcium compounds

BIGtdBu

(Cp )2Ca (where CpBIGtdBu ¼ C5(4-tBudPh)5) 297 can be synthesized by the reaction of 1 equiv. of calcium metal in mercury with 2 equiv. of CpBIGtdBu. In THF. The solution forms a yellow-colored solution of (CpBIGtdBu)2Ca upon stirring for 12 h at room temperature in quantitative yield (Scheme 157).244 Crystallographic data shows that two Z5-coordinated CpBIGt-Bu ligands are linearly coordinated with the central calcium metal atom.

Scheme 157 Synthesis of sandwich-type organocalcium.

In 2017, Roesky et al. synthesized halide-bridge dimeric compound [CaCpMe4(C^CSiMe3)I(THF)2]2 298 by the salt metathesis reaction of [KCpMe4(C^CSiMe3)(THF)2]n with an equivalent amount of CaI2 in THF (Scheme 158).245 The X-ray data revealed that this compound has a halide-bridged dimeric structure with different CadI bond distances: Ca1dI1 3.0920(7) A˚ , Ca1dI1’ 3.2039(7) A˚ .

Scheme 158 Binuclear calcium iodide species supported with ethynyl substituted cyclopentadienyl ligand.

The borohydride compound Ca(BH4)2(THF)2 was reacted with 1 equiv. of KCp in THF at room temperature for 18 h resulting in the formation of [Cp Ca(BH4)(THF)n]2, 299 (where Cp ¼ (Z5-C5Me5)) (Scheme 159).246 The 1H NMR of this compound shows the resonance of (Z5-C5Me5) at 2.00 ppm, and BH4 shows a quartet peak at −0.44 ppm.

Scheme 159 Synthesis of calcium borohydride.

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Organometallic Complexes of the Alkaline Earth Metals

2.03.4.3

N-donor stabilized organocalcium compounds

In 2011, Hill et al. synthesized heavier organoalkaline earth metal complexes using bulky substituted bis(imino)acenaphthene (BIAN) derivatives by reacting Ca{CH(SiMe3)2}2(THF)2 with a suspension of the bulky ligand BIAN in C6D6 or toluene (Scheme 160).247

Scheme 160 Dearomatization of BIAN ligand.

The bulky calcium amidinate complex tBuAmDIPPCaH2 (tBuAmDipp ¼ tBuC[N(2,6-iPr-C6H3)]2) reacts with diphenylacetylene in benzene at 80  C to yield brown-red [tBuAmDippCa]2(SD) {SD ¼ stilbene dianion, [Ph(H)CdC(H)Ph]2−} 301 (Scheme 161).248 The bond distance between the bridging carbon and the ipso carbon of the phenyl substituent is around 1.437(2) A˚ , indicating some multiple bond character and the transfer of electron density to the phenyl groups. Therefore, the crystal structure of this calcium stilbene complex shows that a highly delocalized stilbene dianion is present, bridging two Ca2+ ions.

Scheme 161 Example of calcium stilbene compound.

2.03.4.3.1

b-Diketiminate stabilized organocalcium compounds

The reaction of calcium bis-benzyl complex [(DMAT)2Ca
(THF)2, (DMAT ¼ 2-dimethylamino-a-trimethylsilylbenzyl)] with a homoleptic calcium bis-DippBDICa complex leads to the formation of a heteroleptic benzyl complex, 302, via intramolecular CdH activation. Subsequent heating of 302 to 50  C generates dimeric complex 303. Dialkyl calcium complex Ca{CH (SiMe3)2}2(THF)2 when treated with free DippBDI protio-ligand in hexane, forms the heteroleptic complex [DippBDI(Ca)CH (SiMe3)2(THF)] 304 after 12 days at 60  C along with by-product H2CH(SiMe3)2 (Scheme 162).249

Scheme 162 Organocalcium compounds are bearing b-diketiminato ligand.

Organometallic Complexes of the Alkaline Earth Metals

187

The reaction of [(DippBDI)CaH]2 with less bulky alkenes gives calcium n-alkyl derivatives. Treatment of [(DippBDI)CaH]2 with an excess of ethene, but-1-ene or 3 equiv. of hex-1-ene in C6D6 at room temperature yields complexes [(DippBDI)CaR]2 (R ¼ ethyl 306a, n-butyl 306b, n-hexyl 306c) (Scheme 163).250 The 1H NMR spectrum confirms complete consumption of the hydride ligand and the appearance of an upfield resonance of a-methylene (comes around −0.7 to −0.8 ppm) as a quartet (306a) or triplet signal (306b, 306c), respectively. The solid-state structures obtained crystallographically confirm that all the compounds are centrosymmetric dimers. These compounds show unsymmetrical Ca-a-methylene bond lengths, which display as 2.4847(19) and 2.5733(19) A˚ (Fig. 132).

Scheme 163 BDI supported calcium alkyl complex.

Fig. 132 The solid-state structure of [HC{(Me)CN(2,6-iPr2C6H3)}2Ca(CH2CH3)]2 306a.

Further investigation of the reactivity of terminal alkenes with [(DippBDI)CaH]2 has been carried out by reacting the liquid hydrocarbon with [(DippBDI)CaH]2, resulting in the formation of mixed alkyl hydride compounds, followed by the formation of the desired dimeric alkyl compounds [(DippBDI)Ca(CH2)2R]2 308a-308d (Scheme 164).251 The appearance of new BDI-methine proton resonance for 307a-307d (4.79–4.72 ppm) in the 1H NMR spectrum with an intensity ratio of 2:1 for the hydride signal suggests the formation of the mixed alkyl hydride intermediate. However, the formation of the dimeric calcium alkyl compounds is identified by the newly formed BDI methine proton resonances (4.70–4.52 ppm for 308a-308d) with a 1:2 intensity ratio to the a-methylene group. X-ray crystallographic analysis of the compounds 308a-308d shows centrosymmetric structures with symmetrically bridged alkyl groups between (BDI)Ca moieties that resemble the previously discussed mixed alkyl hydride dimeric alkyl compounds (Fig. 133). The bond distances and bond angles for compounds 308a-308d can be found in Table 8.

Scheme 164 A series of BDI supported calcium alkyls.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 133 The solid-state structures of [(BDI)Ca{CH2(CH2)6CH3}]2 308a, [(BDI)Ca{(CH2)2C(CH3)3}]2 308b, [(BDI)Ca{(CH2)3Ph}]2 308c, and [(BDI)Ca{(CH2)4Ph}]2 308d. Table 8

Crystallographic parameters of compounds 308a-308d.

Bond parameters

308a

308b

308c

308d

Ca1dN1 [A˚ ] Ca1dN2 [A˚ ] Ca1dC30 [A˚ ] Ca1dC30’ [A˚ ] N1dCa1dN2 [ ] Ca1dC30dCa1’ [ ] C31dC30dCa1 [ ] C31dC30dCa1’ [ ]

2.3427(13) 2.3209(13) 2.4947(18) 2.5716(18) 82.88(5) 81.74(6) 161.79(12) 80.78(10)

2.3458(11) 2.3409(11) 2.5948(14) 2.5073(14) 83.11(4) 81.69(4) 78.58(8) 156.77(10)

2.3286(9) 2.3390(9) 2.4923(12) 2.5768(12) 82.86(3) 82.43(4) 164.12(9) 81.96(6)

2.3185(9) 2.3311(10) 2.4743(13) 2.5634(13) 83.79(3) 82.08(4) 162.05(9) 80.03(7)

Data from Ref. Wilson, A. S. S.; Hill, M. S.; Mahon, M. F. Organometallics 2019, 38, 351–360.

The reaction of the [(DippBDI)CaH]2 with diphenyl acetylene affords purple-colored insoluble stilbene dianion compound 309 (Scheme 165). X-ray crystallographic analysis of compound 309 shows a tetrahedral calcium environment around the stilbene dianion moiety. Two different calcium centers are present in which Ca(1) is attached to two nitrogen atoms of b-diketiminate and the phenyl ring of the stilbene group in a Z5 manner, whereas the other Ca(2) center is attached to one bridging H atom along with a b-diketiminate nitrogen atom, and close contacts with the C1, C1’ and C2 centers (Fig. 134).

Organometallic Complexes of the Alkaline Earth Metals

189

Scheme 165 Binuclear calcium hydrido-alkyl species.

Fig. 134 The solid-state structures of [{(BDI)Ca}4(H)2(PhHCCHPh)] 309 (iPr groups are removed for clarity) and [{(BDI)Ca}2(H)(PhCHCH2Ph)] 310.

Further reaction of [(DippBDI)CaH]2 with trans stilbene provides an orange solution of a single compound 310 (Scheme 165).251 H NMR spectrum of compound 310 shows a diagnostic triplet resonance at 2.44 ppm for the benzyl proton, which integrates 1:2 with the BDI methine proton at 4.77 ppm, consistent with insertion of the stilbene group into the calcium hydride moiety. The reaction of 3 equiv. of norbornene with 1 equiv. of [(DippBDI)CaH]2 in C6D6 leads the dicalcium norbornyl hydride compound 311. X-ray crystallographic analysis of 311 has revealed that the calcium centers are ligated with BDI ligand, hydride, and norbornyl groups. Steric crowding arranges the system in a distorted tetrahedral geometry in which the calcium atom is 1.9 A˚ out of the plane of the BDI backbone of 311 (Fig. 135). 1

Fig. 135 The solid-state structure of [{(BDI)Ca}2(H)(CH(C6H10))] 311.

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Organometallic Complexes of the Alkaline Earth Metals

However, the reaction of 3 equivalents of norbornene with [DippBDICaH]2 in methylcyclohexane solvent at 80  C for 16 h afforded crystalline compound 313 (Scheme 166).251 Although the exact mechanism of the formation of this compound is unclear. The X-ray crystal analysis suggests a possible mechanism via intramolecular ligand deprotonation followed by CdH activation of a methyl proton of the Dipp group of the BDI ligand.

Scheme 166 Dicalcium norbornyl hydride complex.

Reactivity of BDI-supported dimeric calcium hydride compound [DippBDICaH]2 with terminal and internal alkenes, diphenyl alkynes, and norbornene are summarized in the following diagram (Scheme 167). 250,251

Scheme 167 Reactivity of BDI calcium hydride complex.

The reaction of [(DippBDI)Ca{N(SiMe3)2}2THF] with PhSiH3 in hexane at 60  C affords the structurally characterized dimeric compound [(DippBDI)CaH(THF)]2 314.252 The reaction of myrcene with the compound 314 results in the stereoselective functionalization of the calcium hydride at the monosubstituted double bond, and to the formation of compound 315.253 NMR study of 315 shows rapid fluxional exchange; however, at low temperatures (−50  C), the endo isomer predominates. Compound 314 reacts with 1,1-diphenylethene at 60  C, resulting in the formation of the compound [(DippBDI)Ca(CPh2Me) (THF)] 316. Similarly, the reaction of cyclohexadiene with 314 at 40  C readily affords a cyclohexenylcalcium compound 318, whereas the reaction of the same calcium hydride with cyclohexyl isocyanide affords dimeric aldimide compound 317 (Scheme 168).63,254 X-ray crystallographic analysis of the aldimide compound 317 shows the [RN](H)C] units bridge between two Ca centers with CadC bond lengths of 2.556(2) and 2.625(2) A˚ .

Organometallic Complexes of the Alkaline Earth Metals

191

Scheme 168 Activation of small molecules with [DippBDICaH.THF]2.

The one-pot reaction of KCH(SiMe3)2, bis(iminoaniline) ligand, [{N^N}H ¼Ortho-C6H4{NH(C6Hi3Pr2)}(CH]NC6Hi3Pr2)] and CaI2 in a 2:1:1 ratio results in the formation of the yellow heteroleptic iminoanilide alkyl complex [{N^N}Ca{CH(SiMe3)2} (THF)] 319 in moderate yield (Scheme 169).255 It is noteworthy to mention that in contrast to [BDI]− or bis(imino)acenapthene ligands which respectively undergo deprotonation or dearomatization in the presence of the basic Ca ⋯{CH(SiMe3)}− moiety, here the {N^N}− ligand framework is chemically inert towards CH(SiMe3)2 which can be attributed to the steric shielding around to the metal center by the significantly hindered {N^N}− ligand.

Scheme 169 Heteroleptic organocalcium amide bearing iminoanilide ligand.

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Organometallic Complexes of the Alkaline Earth Metals

The potassium salt of ligand L25 [L25 ¼ [MeC(NDipp)CHCRNCH2CH2N(Me)-CH2CH2NMe2]−], (R]Me or tBu) reacts with CaI2 in THF at room temperature, resulting in the formation of calcium iodides, 320a and 320b, in 90% and 89% yields, respectively. Subsequently, calcium alkyl complexes 321a and 321b can be synthesized by reacting 320a and 320b, respectively, with K[CH2SiMe3] in benzene at room temperature (Scheme 170).256. Single-crystal X-ray diffraction studies confirmed the solid-state structures of compounds 321a and 321b. In these, it can be seen that the calcium adopts a distorted square pyramidal geometry with the four nitrogen atoms of the ligand forming the basal plane and a carbon atom of the attached alkyl group in the apical position (Fig. 136).

Scheme 170 Organocalcium species stabilized by b-diketiminato-based tetradentate ligands.

Fig. 136 The solid-state structure of [{MeC(NDipp)CHC(Me)NCH2CH2N(Me)-CH2CH2NMe2}CaCH2SiMe3] (iPr groups are removed for clarity) 321a.

2.03.4.3.2

Tp stabilized organocalcium compounds

In 2018, Anwander et al. reported the first compounds containing terminal-bonded calcium methyl moieties (TptBu,Me)Ca(Me) (S) (322a: S]THF), synthesized by the reaction of a [CaMe2]n suspension in THF added with HTptBu,Me (where, TptBu,Me ¼ tris(3-t Bu-5-Me-pyrazolyl)borate). Similarly, (TptBu,Me)Ca(Me)(S) (322b: S]THP) was synthesized by the reaction of a [CaMe2]n suspension in THP with HTptBu,Me (Scheme 171).227 The 1H NMR resonance for the CadCH3 moieties of these compounds are singlets at −0.53 ppm for (TptBu, Me)Ca(Me)(THF) and −0.46 ppm for (TptBu, Me)Ca(Me)(THP), respectively. The crystal data reveal that both compounds feature a calcium coordination number of five (Fig. 137).

Scheme 171 Solvated terminal calcium methyl complexes.

Organometallic Complexes of the Alkaline Earth Metals

193

Fig. 137 The solid-state structures of [(TptBu, Me)Ca(Me)(THF)] 322a and [(TptBu, Me)Ca(Me)(THP)] 322b.

(TpAd,iPr)Ca(H)(THP) {where, TpAd,iPr ¼ hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate} reacts with an equimolar amount of trans-stilbene at ambient temperature to afford [(TpAd,iPr)Ca(PhCHCH2Ph)] 324. The crystal data of this compound reveals that the C1dC2 bond distance becomes 1.510(3) A˚ , implying that the stilbene C]C bond is reduced to a CdC single bond (Fig. 138). Similarly, (TpAd,iPr)Ca(H)(THP) reacts with an equivalent amount of DPE (DPE ¼ 1,1-diphenyl-ethene) in benzene at ambient temperature to give [(TpAd,iPr)Ca{CPh2(Me)}] 325. Similarly, (TpAd,iPr)Ca(H)(THP) reacts with an equivalent amount of 4-phenyl-1-butene in hexane at ambient temperature to give [(TpAd,iPr)Ca{(CH2)4Ph}(THP)] 326 (Scheme 172).257 The crystal data shows that the Ca center 5-coordinate with a distorted trigonal-bipyramidal geometry (Fig. 138).

Fig. 138 The solid-state structures of [(TpAd,iPr)Ca(PhCHCH2Ph)] 324 and [(TpAd,iPr)Ca{(CH2)4Ph}(THP)] 326.

194

Organometallic Complexes of the Alkaline Earth Metals

Scheme 172 Mononuclear calcium terminal hydride and unsubstituted n-alkyl complexes.

2.03.4.3.3

Pincer stabilized organocalcium compounds

The bis(imino)pyridine ligand [2,6-{(Dipp)NC(CH3)}2C5H3N] reacts with Ca{CH(SiMe3)2}2(THF)2 in C6D6 at ambient temperature to give [Ca{2,6-{2,6-(iPr2C6H3)NCCH2}2(C5H3N)(THF)2}] 329 (Scheme 173).258 Compound 329 has a five-coordinated calcium center ligated by a tridentate diamide ligand and two THF molecules (Fig. 139).

Scheme 173 Preparation of pincer supported organocalcium compounds.

Fig. 139 The solid-state structure of [Ca{2,6-{2,6-(iPr2C6H3)}NdC]CH2}2(C5H3N)(THF)2] 329 (iPr groups are removed for clarity).

Organometallic Complexes of the Alkaline Earth Metals

2.03.4.4

195

Cationic organocalcium compounds

Ca(CH2Ph)2(THF)4 in THF reacts with Me4TACD at ambient temperature resulting in the formation of Ca(Me4TACD)(CH2Ph)2 330 (Scheme 174).259 330 has been characterized by IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (Fig. 140).

Scheme 174 Dibenzyl organocalcium compound stabilized by NNNN-type ligand.

Fig. 140 The solid-state structure of [Ca(Me4TACD)(CH2Ph)2] 330.

In 2017, Okuda et al. synthesized cationic organocalcium alkyl complexes featuring weakly coordinating anions. The complex Ca(Me4TACD)(CH2Ph)2 (330) reacts with [NEt3H][BAr4] (Ar]C6H4d4dtBu; C6H3d3,5dMe2) in THF at ambient temperature to produce the cationic complex [Ca(Me4TACD)(CH2Ph)(THF)][BAr4] (331a: Ar]C6H4d4dtBu; 331b: Ar]C6H3d3,5dMe2) (Scheme 175).259

Scheme 175 Organocalcium alkyl cations. tBu AmDippH (tBuAmDipp]tBuC(NDipp)2) reacts with Jutzi’s Acid [H(OEt2)2]+[B(C6F5)4]− in chlorobenzene resulting in the formation of [tBuAmDippH2][B(C6F5)4]. The subsequent reaction of [tBuAmDippH2][B(C6F5)4] with Ca(p-tBu-benzyl)2 in a mixture of chlorobenzene and benzene forms [tBuAmDippCa(C6H6)][B(C6F5)4] 332 (Scheme 176).260 X-ray crystallography is consistent with a three-coordinate cationic calcium amidinate complex in which the ligand is bound to the calcium center by N, aryl chelation (Fig. 141).

196

Organometallic Complexes of the Alkaline Earth Metals

Scheme 176 Synthesis of naked calcium cation bearing amidinate ligand.

Fig. 141 The solid-state structure of [tBuAmDippCa(C6H6)]+[B(C6F5)4]− 332.

2.03.4.5

Mixed metal organocalcium compounds

Addition of [Ca(AlMe4)2]n to a solution of KTptBu,Me in toluene at room temperature gives [(TptBu,Me)Ca(AlMe4)] 333. Similarly, AlEt3 was added to (TptBu, Me)Ca(N(SiMe3)2) solution in pentane, and the resulting solution was stirred at ambient temperature, affording [(TptBu, Me)Ca(AlEt4)] 334 (Scheme 177).261 The 1H NMR spectrum of [(TptBu, Me)Ca(AlMe4)] features a resonance for the AldMe groups at −0.17 ppm, and the Al(CH2) groups of [(TptBu, Me)Ca(AlEt4)] give rise to a signal at 0.26 ppm. Crystallographic data reveal that both compounds have distorted trigonal bipyramidal geometries around the calcium atoms (Fig. 142).

Scheme 177 Isolation of organocalcium alkylaluminate.

Organometallic Complexes of the Alkaline Earth Metals

197

Fig. 142 The solid-state structures of [(TptBu,Me)Ca(AlMe4)] 333 and [(TptBu,Me)Ca(AlEt4)] 334.

333 was subsequently reacted (separately) with THF and DMAP (DMAP ¼ 4-(dimethylamino)-pyridine) in toluene leading to the formation of [(TptBu,Me)Ca(AlMe4)(THF)2] 335 and [(TptBu,Me)Ca(AlMe4)(DMAP)] 336, respectively (Scheme 178).261

Scheme 178 Reactivity of calcium tetraalkylaluminate.

[((BDIDipp)Ca(m-Ph)2BPh2)] 337 has been synthesized by the reaction of (DippBDI)Ca{N(SiMe3)2} with [HNEt3][BPh4] in toluene at ambient temperature. The solid-state structure of compound 337 shows that tetraphenylborate anions connect with the calcium center via Ca


m-PhB interactions. Subsequently, a solution of 337 in toluene was treated with [Al{SiNDipp}K]2; [{SiNDipp}]{CH2SiMe2N(Dipp)}2] in toluene at ambient temperature to form {SiNDipp}AldCa(DippBDI) 338. The solid-state structures crystallized in hexane at −30  C and revealed the presence of direct calcium to aluminum bond. Furthermore, 1,3,5,7-cyclooctatetraene reacting with 338 in C6D6 at ambient temperature leading to the formation of the {SiNDipp} Al-COTdCa(DippBDI) 339 (Scheme 179).262 The X-ray crystal structure revealed that compound 339 could be regarded as a heterobimetallic inverse sandwich species (Fig. 143).262

198

Organometallic Complexes of the Alkaline Earth Metals

Scheme 179 Synthesis of calcium alumanyl species.

Fig. 143 The solid-state structure of [(BDIDipp)Ca(m-Ph)2BPh2] 337, [{SiNDipp}AldCa(DippBDI)] 338 and [{SiNDipp}AldCOTdCa(DippBDI)] 339.

A solution of [Ca(AlMe4)2]n in THF reacts with [Li{NH(Dipp)}] or [K{NH(SiPh3)}] to afford (THF)4Ca(m2-NR)(m2-Me) AlMe2 (340a: R]Dipp; 340b: R]SiPh3; Scheme 180).263 X-ray crystallography revealed that both compounds show a distorted octahedral geometry at Ca, featuring bridging methyl and imido groups and four THF molecules. The CadmdCH3 bond distances for 340a/b (2.651(2) and 2.668(2) A˚ ) are longer in comparison to other six-coordinate complexes (e.g., (phen)Ca(AlMe4)2, 2.584(7) A˚ ).

Scheme 180 Heteroleptic mixed metal alkyls.

Organometallic Complexes of the Alkaline Earth Metals

199

340b reacts with an equivalent amount of PhC^CH, leading to methane gas evolution and formation of [(THF)Ca(NSiPh3) {AlMe2(CCPh)}]2 341. X-ray crystallography revealed that the compound was dimeric. (THF)4Ca{(m2dMe)AlMe2(N(SiPh3) (SiH2Ph))}2 342 was synthesized by the reaction of 340b with PhSiH3 in toluene (Scheme 181).263

Scheme 181 Synthesis of heterobimetallic organocalcium compounds.

In 2010, Hill et al. synthesized [k3-{ArNC(Me)C(AlEt3)HC(Me)NAr}Ca{Et4Al}] 343 by treating a solution of {ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)2}(OEt2) with 2 equiv. of triethylaluminium at ambient temperature (Scheme 182).264 X-ray crystallographic analysis reveals that three-centered-two-electron interactions of the ethyl groups bridge the calcium and aluminum atoms. The AlEt4 moieties are of two types (m3 and m2), and the resulting isomers are assigned as 343a and 343b, respectively. The CadC bond length in 343a (2.878(10)-2.910(15) A˚ ) is longer than 343b (2.606(12) and 2.611(16) A˚ ) because of the increased coordination number at calcium in 343a (Fig. 144).

Scheme 182 Heterobimetallic calcium aluminate complexes.

Fig. 144 The solid-state structure of [k3-{ArNC(Me)C(AlEt3)HC(Me)NAr}Ca{Et4Al}] 343a.

[(Z5-C5Me5)Ga] reacts with calcocene [(Z5-C5Me5)2Ca] resulting in the formation of [(Z5-C5Me5)2CaGa(Z5-C5Me5)] 344 (Scheme 183).265 1H NMR data show two singlets: at 1.90 ppm for the [(Z5-C5Me5)Ga] moiety and 1.96 ppm for the [(Z5-C5Me5)2Ca] unit. Crystallographic data reveal that all C5Me5 groups in this compound coordinate in Z5 fashion (Fig. 145).

200

Organometallic Complexes of the Alkaline Earth Metals

Scheme 183 Calcium-gallium donor-acceptor sandwich.

Fig. 145 The solid-state structure of [(Z5-C5Me5)2CaGa(Z5-C5Me5)] 344.

A solution of GaMe3(Et2O) in toluene reacts with (TptBu, Me)Ca(AlMe4) in toluene at room temperature resulting in the formation of [(TptBu,Me)Ca(GaMe4)] 345. This compound can also be synthesized by the reaction between (TptBu,Me)Ca {N(SiMe3)2} and GaMe3 in pentane by amide elimination (Scheme 184).261 [(TptBu,Me)Ca(GaMe4)] crystalizes in toluene at −35  C, and crystal data reveal that compound 345 is isostructural with complex 333 (Fig. 146).

Scheme 184 Synthesis of calcium tetramethylgallate complex.

Organometallic Complexes of the Alkaline Earth Metals

201

Fig. 146 The solid-state structure of [(TptBu,Me)Ca(GaMe4)] 345.

Reaction of dimeric calcium hydride [(BDI)CaH]2 with 2 equivalents of Ph3SnH in a C6D6 solution, gives complex [(BDI)Ca(m2-H)(m2-SnPh3)Ca(BDI)] 346. In this complex, two{(BDI)Ca} are connected via hydride and triphenylstannyl ligands, each m2-bridging, and the geometry around the tin center is trigonal pyramidal. Furthermore, compound 346 can be treated with additional triphenyltin hydride, giving dimeric calcium triphenylstannyl complex 347 (Scheme 185).266 In complex 347, the calcium centers are connected to the BDI ligand, a single Sn atom, and engage in a Z6 interaction with the phenyl ring of the SnPh3 unit of the other component of the dimer.

Scheme 185 Organocalcium stannyl complex ligated by BDI.

2.03.4.6

p-Arene stabilized organocalcium compounds

Calcium (I) complexes are unusual and not well known in the literature. However, such complexes can be synthesized by two routes. One is by direct addition of excess activated calcium complex with triphenylbenzene (TPB)Br in THF at −60  C; another involves the addition of 1,3,5-triphenylbenzene with activated calcium in the presence of the catalytic amount of 2,4,6-Ph3C6H2Br, gives [(THF)3Ca{m-Ph3C6H3}Ca(THF)3] (vide supra) 348, is black. In the first route, the insertion of calcium metal into the CdBr bond, followed by cleavage of ether and activated calcium reduction, gives rise to complex 348 (Scheme 186).267 This complex is highly air and moisture-sensitive and also pyrophoric. X-ray crystallographic analysis confirms that two calcium centers lie on a C3-axis above and below a central arene ring, in the manner of an inverse sandwich complex. The arene ring lies between two metal centers, also ligated by three THF molecules. Due to the anionic character of tpb, the CdC bond distance between the ring system is tpb2− (tpb ¼ 1,3,5-triphenylbenzene) 1.435(7) A˚ , which is smaller than that of neutral tpb 1.487 A˚ . (Fig. 147).

Scheme 186 Inverse sandwich calcium complex.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 147 The solid-state structure of [(THF)3Ca{m-Ph3C6H3}Ca(THF)3] 348.

The reaction of CaI2 with two equiv. K(C3H5) in THF at 25  C gives a non-pyrophoric compound Ca(C3H5)2 349, which is polymeric and off-white. Furthermore, compound 349 treats with one equiv. of triglyme in THF at room temperature, to yield bis(allyl)complex [Ca(Z3-C3H5)2(triglyme-k4)] 350 (Scheme 187).268 The allyl ligands are attached to the calcium center in a Z3-fashion.

Scheme 187 Example of organocalcium allyl complex. DiPeP

BDICaI [DiPePBDI ¼ HC{C(Me)(NDiPep)}2]−, {DiPeP ¼ 2,6-iPe2-C6H3} 351 complex reacts with KC8 forming putative calcium(I) complex (DippBDI)CadCa(DippBDI) in situ. The (DiPePBDI)CaCa(DiPePBDI) reacts with benzene, toluene or p-xylene, gives rise to black products (DiPePBDI)Ca(arene)Ca(DiPePBDI) 352a-352c (arene ¼ benzene, toluene, p-xylene), featuring p-arenes interactions to both calcium centers (Scheme 188).269 The formal calcium oxidation state changes from +1 to +2, when (DiPePBDI)CaCa(DiPePBDI) reacts with benzene, toluene, or p-xylene, produces (DiPePBDI)Ca(arene)Ca(DiPePBDI), as the additional 2- charge is associated with the arene system. Compound 352a is highly reducing, and it decomposes at ambient temperature to the formation of compound 353 and H2 gas.

Scheme 188 Activation of aryl hydrocarbons by in situ generated low oxidation state calcium complex.

Organometallic Complexes of the Alkaline Earth Metals

2.03.4.7

203

Applications of organocalcium complexes as catalysts

In 2019 Trifonov et al.225 reported catalysis involving [{(p-tBuC6H4)2CH}2Ca(TMEDA)] 246, for the regio-, and chemoselective hydrophosphination of double bond. In the case of the hydrophosphination of styrene, heating at 70  C for 72 h gives quantitative yield (100%) of hydrophosphinated product (Scheme 189).

Scheme 189 Calcium-catalyzed hydrophosphination of styrene derivatives.

Harder and coworkers270 reported the use of (DMAT)2Ca
(THF)2 as an active catalyst for imine reduction with phenylsilane at 25–60  C with 2.5 mol% catalyst loading (Scheme 190).

Scheme 190 Calcium- catalyzed imine hydrosilylation.

The NHC stabilized dialkyl calcium complex [{(IiPr2Me2)2Ca{CH(SiMe3)2]2}] 278 has been used as a highly efficient chemoselective catalyst for the cross-dehydrocoupling reaction of amines and silanes under 5 mol% catalyst load at 25  C in C6D6 (Scheme 191).237

Scheme 191 Dehydrocoupling of amine with silane.

The scorpionate-based organocalcium complex [(TpAd,iPr)Ca(p-CH2C6H4dMe)(THP)] has been used as an effective catalyst for the hydrogenation of terminal and internal alkenes under mild conditions (5 mol% catalyst loading in C6D6 at 40  C) (Scheme 192).257

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Organometallic Complexes of the Alkaline Earth Metals

Scheme 192 Alkene hydrogenation.

Moreover, Cheng et al. recently reported the regioselective CdH silylation of aromatic ethers with hydrosilanes using [(TpAd,iPr)Ca(p-CH2C6H4dMe)(THP)] as a precatalyst (Scheme 193).271

Scheme 193 Ortho CdH silylation.

In 2021, Chen et al. showed that N-donor stabilized organocalcium alkyl complexes 321a-321b can act as efficient catalysts for redistributing ArSiH3 to Ar3SiH and for the cross-dehydrocoupling between electron-withdrawing Ar(R)SiH2 and Ar0 (R)SiH2 under 1 mol% catalyst loading in C6D6 (Scheme 194).256

Scheme 194 Redistribution of hydrosilanes.

2.03.5

Strontium and barium

Organometallic compounds of strontium and barium are relatively understudied compared with those of their lighter group 2 congeners, beryllium, magnesium, and calcium. The main reasons for this are their large ionic radii, high reactivity, and increased polarity of the MdC bond on descending group 2.272,273. Moreover, isolation of heteroleptic heavier organoalkaline earth organometallic compounds is challenging due to the increased propensity for ligand redistribution via the Schlenk equilibrium (Eq. 1). 2LAeX Ð AeX2 +LAeL Ae ¼ Be < Mg < Ca < Sr < Ba

(1)

Organometallic Complexes of the Alkaline Earth Metals

205

Nevertheless, there has been increasing attention on the study of organometallic strontium and barium complexes, particularly with respect to their synthesis and structural properties. For example, NHC- and group 15 ligand-stabilized organostrontium and barium compounds are known238,247,255.

2.03.5.1

Alkyl and alkynyl compounds

The synthesis of bis(trimethylsilyl)methyl stabilized organostrontium, and barium compounds was achieved by the salt metathesis reactions of MI2 (M]Sr, Ba) with K{CH(SiMe3)2}(THF) in THF (Scheme 195).223 Both compounds Sr{CH(SiMe3)2}2(THF)3 354 and Ba{CH(SiMe3)2}2(THF)3 355 are highly air and moisture sensitive. The 1H NMR spectra of 354 and 355 show characteristic methine proton resonances for the CH(SiMe3)2 group at −1.59 and −1.49 ppm, respectively. The solid-state structures reveal that compounds 354 and 355 obtain five-membered and tris-THF adducts. Compound 355 is isostructural to strontium analog 354 (Fig. 148).

Scheme 195 Bis(trimethylsilyl) methyl stabilized organostrontium and barium compounds.

Fig. 148 The solid-state structures of the compounds [Sr{CH(SiMe3)2}2(THF)3] 354 and [Ba{CH(SiMe3)2}2(THF)3] 355.

In 2015, Izod et al. reported heavier alkaline-earth benzyl compounds via the reaction between MI2 (M]Sr or Ba) and benzyl potassium, PhCH2K in a 1:2 stoichiometric ratio in THF solvent. The orange-colored dibenzylstrontium [(PhCH2)4Sr2(THF)3] 356 and dibenzylbarium [(PhCH2)6Ba3(THF)4] 357 products were obtained in good yields (Scheme 196).274 The solid-state structures displayed showed that compounds 356 and 357 crystallize as extended 2D networks (Fig. 149).

Scheme 196 Salt metathesis of benzyl potassium with strontium and barium dihalides.

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Organometallic Complexes of the Alkaline Earth Metals

Fig. 149 The molecular structures of the compounds [(PhCH2)4Sr2(THF)3] 356 and [(PhCH2)6Ba3(THF)4] 357.

The same research group further extended the structural diversity of heavier alkaline earth organometallics. Thus, reactions between phosphine-borane PhP(CH2SiMe3)2
BH3 with one equiv. of either dibenzylstrontium, (PhCH2)2Sr(THF) or dibenzyl barium, (PhCH2)2Ba provided the related dimeric complexes [(PhP(BH3){CH(SiMe3)}2)Sr(THF)1.75(OEt2)0.25]2 358 and [(PhP(BH3){CH(SiMe3)}2)Ba(OEt2)1.75(THF)0.25]2 359, respectively (Scheme 197).275 X-ray crystallographic analysis of these compounds (Fig. 150) shows the distinct structures of each molecule reflecting the chirality of the carbanion center and the bridging mode of the ligands. In compound 358, the two Sr centers are bridged by each of the carbanion centers of the dicarbanion ligand, forming a four-membered ring structure. Each Sr center is coordinated by two carbanion centers, the borane’s H atoms, and a disordered mixture of THF and diethyl ether. The Ba analog shows some differences from the Sr compound with the borane hydrogen bridging in an m2-Z1:Z2 manner between the two Ba centers.

Scheme 197 Strontium and barium alkyls.

Fig. 150 The solid-state structures of the compounds [[PhP(BH3){CH(SiMe3)}2]Sr(THF)1.75(OEt2)0.25]2 358 and [[PhP(BH3){CH(SiMe3)}2]Ba(OEt2)1.75(THF)0.25]2 359.

Organometallic Complexes of the Alkaline Earth Metals

207

In 2013, Hill et al. reported organostrontium amido boranes reacting a strontium dialkyl with secondary amine boranes. The reactions of strontium bis(trimethylsilyl) methyl, [Sr{CH(SiMe3)2}2(THF)2] with dialkylamino boranes (either dimethylamine borane, Me2NH
BH3 or pyrrolidine borane, (CH2)4NH
BH3) afforded the thermally stable strontium alkyl/dialkylamidoborane products [Sr{CH(SiMe3)2}{NR2BH3}(THF)n]2 {R]Me 360a; n ¼ 1, (CH2)2 360b; n ¼ 0}, respectively (Scheme 198).276 Moreover, the authors observed the formation of dialkylamidoborane compounds produced by b-H elimination.

Scheme 198 Isolation of organostrontium amido boranes.

In 2012, Ruhlandt-Senge et al. reported a series of organobarium di- and triphenylmethanides by two synthetic routes. The barium diphenylmethanide, Ba(18-crown-6)(CHPh2)2 361a, was synthesized by treating highly basic Ba(CH2Ph)2 with two equiv. of CH2Ph2 in 18-crown-6. By contrast, Ba(DME)2(CHPh2)2 361b was prepared by the treatment of [Ba[N(SiMe3)2]2(THF)2 with two equiv. of LiCHPh2(TMEDA) in dimethoxyethane (DME) solvent (Scheme 199).277 The formation of 361a occurs via toluene elimination, while transmetallation affords compound 361b. Both compounds 361a and 361b display distorted pseudo-octahedral geometry (Fig. 151).

Scheme 199 Synthetic methods for organobarium diphenylmethanides.

Fig. 151 The solid-state structure of [Ba(DME)2(CHPh2)2] 361b.

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Organometallic Complexes of the Alkaline Earth Metals

The same research group reported the formation of organostrontium and barium acetylides by a transamination method. Thus, the 18-crown-6 stabilized strontium acetylides, Sr(18-crown-6)(CCR)2; (R]tBu 362a or 4-tBuC6H4 362b), and barium acetylides Ba(18-crown-6)(CCR)2 (R]tBu 363a or 4-tBuC6H4 363b), can be accessed by the reactions between the corresponding metal bis-amide and alkynes (Scheme 200).278 Alternatively, compounds 362a-362b, and 363a-363b can be produced in excellent yields and high purity by reacting dibenzyl metal reagents with alkynes. Both approaches are established on the pKa difference between the acidic organic alkyne (pKa 29) and the released NHSiMe3 (pKa 30) or toluene (pKa 41). The molecular structures of representative examples of compounds 362b and 363a are depicted in Fig. 152.

Scheme 200 Synthesis of a series of calcium acetylides.

Fig. 152 The solid-state structures of [Sr(18-crown-6)(CCd4dtBuC6H4)2] 362b and [Ba(18-crown-6)(CCdtBu)2] 363a.

2.03.5.2 2.03.5.2.1

Carbene stabilized strontium and barium organometallic complexes NHC stabilized compounds

In 2008, Barrett, Hill, and co-workers reported the NHC-stabilized metal bis-amide compounds of strontium and barium (IMes)M {N(SiMe3)2}2 (M]Sr 364, Ba 365). Compounds 364 and 365 have been prepared by adding heavier group 2 bis(trimethylsilyl) amides to the imidazolium salt (Scheme 201) or by the direct addition of free carbene to the solvent-free metal bis-amides. In the prior method, the reaction is presumed to occur through deprotonation of the imidazolium salt to obtain a mixed halide-amide NHC complex, followed by either carbene/unreacted metal bis-amide adduct formation or a salt metathesis reaction. The latter synthetic method for preparing NHC-metal bis-amides has been demonstrated for the homoleptic calcium amide. These compounds are moisture sensitive and very soluble in hydrocarbon solvents. The solid-state structures showed that compounds 364 and 365 are three-coordinate monomeric complexes, in which the metal is surrounded by one sigma donating L type NHC ligand and two X-type amide ligands (Fig. 153).

Scheme 201 Synthesis of IMes stabilized strontium and barium bis-amides.

Organometallic Complexes of the Alkaline Earth Metals

209

Fig. 153 The solid-state structures of the compounds [(IMes)M{N(SiMe3)2}2 M ¼ Sr 364, Ba 365]]

In 2009, Hill et al. also reported the homo and heteroleptic tris(imidazoline-2-ylidene-1-yl)borate complexes of the heavier group 2 elements strontium and barium. One-pot synthesis of heteroleptic tris(imidazoline-2-ylidene-1-yl)borate-supported amides (366; Sr and 367; Ba) has been achieved by adding THF at −78  C to a mixture of 1 equiv. of the ligand precursor, four equiv. KN(SiMe3)2 and either SrI2 or BaI2. By contrast, a homoleptic tris(imidazoline-2-ylidene-1-yl)borate Sr(II) compound 368 was obtained by adding THF to a mixture of 1:1.5 ratio of ligand and strontium bis-amide (Scheme 202).240 The authors reasoned that the formation of the homoleptic compound was due to the larger ionic radius of the Sr center and less sterically crowded halide co-ligand. The solid-state structures revealed that the heteroleptic complexes 366 and 367 are adducts of 1-tert-butylimidazole; the latter neutral ligand originates from the fragmentation of the starting ligand (Fig. 154).

Scheme 202 Homoleptic and heteroleptic strontium and barium complexes.

210

Organometallic Complexes of the Alkaline Earth Metals

Fig. 154 The molecular structure of the compound [{HB(ImtBu)3}2Sr] 368.

In 2017, Westerhausen et al. reported the neutral, tridentate ligand 2,6-bis(3-mesitylimidazol-2-ylidiene)pyridine (CarMesPyCarMes) and its use in stabilizing heavier group 2 metal dihalide complexes. A 1:1 stoichiometric ratio in the reaction between (CarMesPyCarMes) and MI2 (M]Sr or Ba) in THF yielded (CarMesPyCarMes)(THF)2MI2 (M]Sr 369 and Ba 370) in good yields (Scheme 203).242 X-ray crystallographic analysis of compounds 369 and 370 revealed that both compounds show distorted pentagonal-bipyramidal geometry and consist of two iodide counterions, two THF molecules, and one (CarMesPyCarMes) ligand attached to the metal center (Fig. 155).

Scheme 203 Synthesis of Sr and Ba diiodide chelated by tridentate ligand.

Fig. 155 The solid-state structures of the compounds [(CarMesPyCarMes)(THF)2MI2]; M]Sr 369, Ba 370.

Hill et al. synthesized heteroleptic-organostrontium amide compound, [{H2B(ImtBu)2}Sr{N(SiMe3)2}(THF)2] 371 by the addition of [H2B(ImtBu)2]I or [H2B(ImtBu)2]Br to one equiv. of SrI2 and three equiv. KN(SiMe3)2 in THF at −78  C (Scheme 204).239 As revealed by the X-ray structure, the metal center of compound 371 resides in distorted pseudo-octahedral coordination geometry with a facial O2N donor set furnished by two molecules of THF and amido ligand. The other face is resided by bis(imidazoline-2ylidene-1-yl)borate ligand, which acquires a boat conformation and Z3-coordination mode. The SrdH bond distance is 2.87(3) A˚ , resembling several established borohydride substituted strontium derivatives (Fig. 156).

Organometallic Complexes of the Alkaline Earth Metals

211

Scheme 204 Synthesis of heteroleptic organostrontium amide 371.

Fig. 156 The solid-state structure of the compound [{H2B(ImtBu)2}Sr{N(SiMe3)2}(THF)2] 371.

Very recently, Roesky et al. reported NHC-supported strontium dihalide by employing a redox transmetalation method. Bis NHC-supported strontium (II) complex [(IMe2)2SrI2(THF)2] 372 was prepared by the redox-transmetalation reaction of an (NHC) AgI complex with zero-valent strontium metal in the presence of THF (Scheme 205). 279 X-ray crystal analysis of compound 372 revealed a distorted octahedral geometry (Fig. 157).

Scheme 205 NHC adduct of strontium diiodide by using redox transmetalation procedure.

Fig. 157 The solid-state structure of the compound [(IMe)2SrI2(THF)2] 372.

212

Organometallic Complexes of the Alkaline Earth Metals

2.03.5.2.2

CAAC supported compounds

Turner et al. reported the first structurally characterized CAAC supported strontium, and barium bis-amide complexes, [(RCAAC)M {N(SiMe3)2}2 (M]Sr; R]Me 373, Ba; R]Me 374a, Cy 374b)] (Scheme 206).140 This synthesis was achieved by reacting in situ generated CAAC with M{N(SiMe3)2}2. The 13C{1H} spectra show carbene signals at 283.8 and 303.3 ppm for the Sr and Ba compounds, respectively. The molecular structures determined crystallographically have revealed the isostructural and monomeric nature of compounds 373 and 374a (Fig. 158).

Scheme 206 CAAC stabilized Sr and Ba bis-amides.

Fig. 158 The solid-state structures of the compounds [(RCAAC)M{N(SiMe3)2}2(THF) (M]Ba)]; R]Me 374a, Cy 374b.

2.03.5.2.3

Bis-iminophosphorano barium carbene complexes

Harder and co-workers synthesized the monomeric barium carbene complex, [{(2,6-iPr2C6H3-NPPh2)2C}Ba(THF)3] 375 by the reaction of bis-iminophosphoranomethane (DippNPPh2)2CH2 with dibenzylbarium in THF. The reaction proceeds smoothly via double deprotonation of the bis-iminophosphoranomethane. Only one doublet at 1.04 ppm for the methyl proton of the Dipp groups in the 1H NMR spectrum points to fast rotation around the CdN bond at room temperature. Moreover, to study the reactivity of the comparatively less sterically demanding bis-iminophosphoranomethane, the same research group treated strongly basic dibenzylbarium with (Me3SiNPPh2)2CH2, which readily affords the dimeric compound [(Me3SiNPPh2)2CBa.THF]2 376 via double deprotonation (Scheme 207).280 X-ray crystallographic analysis of compound 376 has revealed a centrosymmetric dinuclear structure (Fig. 159).

Organometallic Complexes of the Alkaline Earth Metals

213

Scheme 207 Organobarium complex ligated by bis-iminophosphoranomethane ligand.

Fig. 159 The molecular structure of the compound [{(2,6-iPr2C6H3-NPPh2)2C}Ba(THF)3] 376.

2.03.5.3

Cyclopentadienyl derivatives of strontium and barium

A mixed pentamethylcyclopentadienyl (Cp )/guanidinate complex has been generated via a facile transamination route; treatment of SrCp 2 with one equiv. of iPrN]C(NR2)N(H)iPr at 20  C affords dimeric [Cp Sr(m2,Z2: Z2 -iPrN]C(NR2)NiPr)2SrCp ]; (R]Me 377a or iPr 377b) with 68% and 80% yields, respectively (Scheme 208).281 Here, two five-coordinate Sr atoms are bridged by two m2,Z2:Z2-guinidinate ligands. Reaction of BaCp 2(THF)1.7 with iPrN]C(N(iPr)2)N(H)iPr at 20  C afford the barium analog Cp Ba(m2,Z2:Z2-iPrN]C(NR2)NiPr)2BaCp (R]iPr 378). X-ray crystallographic analysis reveals strong similarities between the barium and strontium complexes (Fig. 160).

214

Organometallic Complexes of the Alkaline Earth Metals

Scheme 208 Synthesis of mixed ligand-supported organostrontium and barium compounds.

Fig. 160 The molecular structures of the compounds [(Cp )M(m2,Z2:Z2-iPrN]C(NR2)NiPr)2M(Cp )] (M]Sr; R]iPr 377b); (M]Ba; R]iPr 378).

The synthesis of bulky cyclopentadienyl sandwich compounds of the heavier alkaline earth metals (Sr, Ba) has been carried out by reacting cyclopentadiene ligands with highly basic dibenzyl strontium and barium, e.g., by heating a mixture of two equiv. (4dnBudC6H4)5C5H and one equiv. of (2dMe2NdadMe3Sidbenzyl)2Sr.(THF)2 in benzene at 60  C for 3 days, to afford the super-bulky sandwich strontium complex [{(4dnBudC6H4)5C5}2Sr] 379. Moreover, the reaction of dibenzylbarium with the (4dnBudC6H4)5C5H smoothly affords the barium analog [{(4dnBudC6H4)5C5}2Ba] 380 by deprotonation in a much shorter timeframe (2 h; Scheme 209).282 The solid-state structures of compounds 379 and 380 reveal highly symmetric structures featuring parallel Cp rings sandwiched around metal atoms along with a CdH⋯ C(p) network present between two cyclopentadienyl ligand (Fig. 161).

Scheme 209 Well-defined bulky cyclopentadienyl sandwich Sr and Ba complexes.

Organometallic Complexes of the Alkaline Earth Metals

215

Fig. 161 The solid-state structures of the compounds [{(4dnBudC6H4)5Cp}2M (M]Sr 379, Ba 380)].

The reaction of two equiv. of NHSi (N-heterocyclic silylene) with 1 equiv. of (Z5-C5Me5)2M(OEt2) (M]Sr, Ba) results in the organostrontium and -barium compounds [(Z5-C5Me5)2M{Si(NtBuCH)2} (M]Sr 381; Ba 382)] (Scheme 210).283 Furthermore the same (Z5-C5Me5)2M(OEt2) precursors react with 1,4-diazabuta-1,3-diene (DAB) to form DAB-chelated complexes [(Z5C5Me5)2M(k2-{NtBu]CHCH]NtBu})] (M]Sr 383, Ba 384) (Scheme 210). A donor-acceptor interaction is formulated between the divalent Si and metal centers in the former class of compound. X-ray crystallographic analysis shows these compounds are monomeric in the solid-state (Fig. 162).

Scheme 210 Organostrontium and barium adducts of N-heterocyclic silylene and 1,4-diazabuta-1,3-diene.

216

Organometallic Complexes of the Alkaline Earth Metals

Fig. 162 The molecular structures of the compounds [(Z5-C5Me5)2Sr{Si(NtBuCH)2}] 381 and [(Z5C5Me5)2Sr(k2-{NtBu]CHCH]NtBu})] 383.

In 2019, Cheng and co-workers reported a study of the hydrogenolysis of strontium and barium alkyl compounds. They carried out the hydrogenolysis of the half-sandwich pentaarylcyclopentadienyl ligand stabilized systems [(CpAr)M{CH(SiMe3)2}(THF) (CpAr]Ar5C5, Ar]3,5diPr2dC6H3)] for both strontium and barium, resulting in the formation of bulky organometallic hydride complexes. Treatment of [(CpAr)Sr[CH(SiMe3)2](THF)] 385 in the presence of H2 at 10 atm affords hydride bridged compound [(CpAr)Sr(m-H)(THF)]2 385 (Scheme 211).284 The 1H NMR spectrum of compound 386 confirms a hydride signal around 6.64 ppm. Moreover, the barium analog of compound 385 undergoes hydrogenolysis in the presence of 2 equiv. DABCO in hexane and form [(CpAr)Ba(m-H)(DABCO)]2 387. The authors further examined the reactivity of compound 386 with two equiv. of DABCO in hexane and obtained the DABCO-coordinated strontium analog [(CpAr)Sr(m-H)(DABCO)]2 388. X-ray crystallographic analysis of compounds 387 and 388 has revealed the hydride bridged dimeric structure of the compounds (Fig. 163).

Scheme 211 Synthesis of half-sandwich heavy alkaline-earth metal hydrides.

Fig. 163 The solid-state structures of the compounds [(CpAr)Ba[CH(SiMe3)2](THF)] 385, [(CpAr)Sr(m-H)(DABCO)]2 387 and [(CpAr)Ba(m-H)(DABCO)]2 388.

Organometallic Complexes of the Alkaline Earth Metals

217

The metallocenes [(CpBIGtdBu)2M] (M]Sr 389, Ba 390; CpBIGtdBu]C5(4dtdBudC6H4)5) can be synthesized by taking two equiv. of CpBIGtdBu. in THF with one equiv. of strontium or barium metal in mercury. In the case of strontium, this forms a yellow-colored solution of (CpBIGtdBu)2Sr 389 in quantitative yield upon stirring for 12 h at room temperature (Scheme 212).244 Crystallographic data shows that these compounds 389 and 390 are monoclinic with a propeller-type conformation of the aryl groups due to CH ⋯p interactions (Fig. 164). In each case, there are two Z5-coordinated CpBIGtdBu ligands at the metal, which are linearly coordinated.

Scheme 212 Direct synthesis of Sr and Ba metallocenes.

Fig. 164 The solid-state structures of the compounds [(CpBIGtdBu)2M (M]Sr 389; Ba 390)].

Roesky and co-workers reported the first example of a cyclopentadienyl strontium borohydride complex, [Cp Sr(BH4)(THF)2]2, formed by the 1:1 stoichiometric reaction of Sr(BH4)2(THF)2 and KCp in THF at room temperature (Scheme 213).285 Multinuclear NMR spectral analysis (1H and 11B NMR) indicates the presence of the [BH4]− ligand in the product. The molecular structure of this compound (391) is dimeric, with interactions between Sr and the borohydride ligands reflecting a m-{k2(H):k3(H)} mode of ligation (Fig. 165).

Scheme 213 Synthetic route for cyclopentadienyl Sr borohydride 391.

218

Organometallic Complexes of the Alkaline Earth Metals

Fig. 165 The molecular structure of the compound [Cp Sr(BH4)(THF)2]2 391.

Decamethylbarocene, (Z5-C5Me5)2Ba, undergoes CdH activation to form [(C5Me4CH2C5Me5)(C5Me5)Ba] 393 with intramolecular metal olefin bonding, upon reaction over several days with the half sandwich In(I) compound (Z5-C5Me5)In at high temperature (140–150  C) (Scheme 214).286 The reaction mechanism is suggested to proceed via two possible ways. Initial insertion of (C5Me5)In into the CdH bond of one of the methyl groups of the Cp ring of (Z5-C5Me5)2Ba is proposed to occur, followed by the release of “InH”. Another possible mechanism follows a radical pathway, starting from the homolytic cleavage of (C5Me5)In, and formation of the Cp radical, which initiates CdH bond cleavage in (Z5-C5Me5)2Ba, followed by combination with the Cp radical. The molecular structure of the resulting compound (C5Me4CH2C5Me5)(C5Me5)Ba 393 is shown in Fig. 166.

Scheme 214 CdH bond activation of decamethylbarocene.

Fig. 166 The molecular structure of the compound Ba-olefin complex [(C5Me4CH2C5Me5)(C5Me5)Ba] 393.

Organometallic Complexes of the Alkaline Earth Metals

2.03.5.4 2.03.5.4.1

219

Group 15 ligand supported organometallic compounds N-donor ligand supported four, five, and six-membered ring organostrontium compounds

Heteroleptic metal pentafluorophenyl triazine compounds, [(Dmp)(Tph)N3M(C6F5)(THF)n];(Dmp ¼ 2,6-Mes2C6H3; Mes ¼ 2,4, 6-Me3C6H2; Tph ¼ 2-TripC6H4; Trip ¼ 2,4,6-iPr3C6H2; n ¼ 1, 2; M]Sr 394, Ba 395) were synthesized by the Niemeyer group in 2005 by treating aryl tri-azene compounds with bis(pentafluorophenyl)mercury followed by reaction with the corresponding metal.287 Compounds 394 and 395 undergo s-bond metathesis in the presence of PhSiH3 to afford the unsolvated homoleptic triazines [M{N3Dmp(Tph)}2] (M]Sr 396; Ba 397; Scheme 215).288 The structures of these compounds were confirmed by X-ray crystallography (Fig. 167): the metal centers interact with the flanking Mes ring of each terphenyl group, which supplements trazenide coordination to the metal centers.

Scheme 215 Synthesis of homoleptic triazine-based organostrontium and barium complexes.

Fig. 167 The solid-state structures of compounds [M{N3Dmp(Tph)}2 (M]Sr 396, Ba 397)]. All Hydrogen and mesityl methyl groups of Sr 396 are removed for clarity.

Amidinate ligands have been used for the isolation of main group metal hydrides. Due to a narrower bite angle than BDI ligands, it becomes unsuitable for stabilizing heavier alkaline earth metal complexes. To address this challenge and to utilize amidinate ligands for the isolation of discrete SrdH compounds, the Harder group has carried out the reaction of the bulky amidine p-TolAmH, (p-tolyl)C(NAr)2H (Ar ¼ 2,6-Ph2CH-4-iPr-phenyl) with Sr[N(HSiMe2)2]2 in C6D6 at 60  C to yield the heteroleptic amidinate stabilized strontium amide compound [(p-TolAm)Sr{N(SiHMe2)2}] 398 via deprotonation of the amidine. Further treatment of the 398, with PhSiH3 in a 1:1 M ratio in C6D6 solvent at room temperature, led to the intermediate formation of the strontium hydride complex (p-TolAm)SrH 399, which readily undergoes dehydrogenation in the presence of tetrahydrofuran via deprotonation of the Ph2CH substituents of the Dipp group to form compound 400 (Scheme 216 and Fig. 168).289

220

Organometallic Complexes of the Alkaline Earth Metals

Scheme 216 Synthesis of bulky amidinato organostrontium compound.

Fig. 168 The solid-state structure of complex 400.

Kays et al. reported the first-ever example of a homoleptic monomeric barium-complex ligated by an NCN-type ligand. The reaction of guanidine [MesN{C(NCy2)}N(H)Mes] with two equiv. of barium in the presence of 0.5 equiv. HgPh2 and THF at room temperature results in the elimination of benzene and Hg and the formation of barium-complex [Ba{MesN{C(NCy2)}NMes}2] 401 (Scheme 217).290 1H NMR data reveal an upfield shifted resonance for the ortho-methyl substituents of the Mes group at 2.16 ppm consistent with an aryl ring interaction with metal. There is an Z6-arene interaction of the barium center with a pendant aryl ring can also be crystallographically (Fig. 169). The arrangement of the ligands in complex 401 is nearly orthogonal: the dihedral angle between the NCN planes is ca. 88.5 degrees.

Scheme 217 Synthesis of guanidinate supported organobarium complex.

Organometallic Complexes of the Alkaline Earth Metals

221

Fig. 169 The solid-state structure of the compound [Ba{MesN{C(NCy2)}NMes}2] 401.

Hill and co-workers studied the Schlenk-type redistribution of heavier organoalkaline earth metal complexes using bulky substituted bis(imino)acenaphthene (BIAN) derivatives, 402. Treating Sr{CH(SiMe3)2}2(THF)2 with a suspension of bulky Dipp BIAN ligand in C6D6 or toluene resulted in the instant solubilization of the starting materials and a color change from bright orange to dark green (Scheme 218).247 Analysis of the 1H NMR spectrum of this solution indicated complete consumption of the starting material Sr{CH(SiMe3)2}2(THF)2 and the ligand and clean formation of 402, involving alkyl group transfer to the BIAN ligand backbone (Scheme 218).

Scheme 218 Synthesis of BIAN supported organostrontium alkyl complex.

Iminoanilide-supported alkyl strontium and barium complexes [mAnDiPP M(X)(THF)2 (X]CH(SiMe3)2)]; (M]Sr 403, Ba 404) have been synthesized by the one-pot reaction of the Dipp-functionalized iminoanilide and KCH(SiMe3)2 with MI2 (M]Sr, Ba) in a 1:2:1 ratio (Scheme 219).255 Compounds 403 and 404 have been characterized by NMR spectroscopy, and X-ray crystallographic analysis indicates that both compounds are five-coordinate at the metal possessing near-perfect square pyramidal geometries with the CH(SiMe3)2 ligand at the apical position (Fig. 170).

Scheme 219 Iminoanilide ligand supported organostrontium and barium amide compounds.

222

Organometallic Complexes of the Alkaline Earth Metals

Fig. 170 The solid-state structures of the compounds [{mAnDiPP}M(X)(THF)2 (X]CH(SiMe3)2) (M]Sr 403; Ba 404)].

BDI supported strontium alkynyl complex [(DippBDI)Sr(THF)(m-C^CPh)]2 405 has been prepared by the addition of [( BDI)SrN(SiMe3)2(THF)] to two equiv. of phenylacetylene at −5  C in THF (Scheme 220).129 X-ray crystallography reveals a dimeric structure in which the strontium atoms are additionally coordinated by two nitrogen atoms of the BDI ligand and a THF molecule. Compound 405 contains an unprecedented bis-all-p- and bis-all-s-coordinated strontium atoms in the same dinuclear species (Fig. 171). Dipp

Scheme 220 Synthesis of strontium(II) acetylide complex.

Fig. 171 The molecular structure of the compound [(DippBDI)Sr(THF)(m-C^CPh)]2 405.

Organometallic Complexes of the Alkaline Earth Metals

223

Bulky DiPePBDI-stabilized dimeric strontium hydride complex 406 has been synthesized by the reaction of [(DiPePBDI)H with Sr {N(SiMe3)2}2] in n-hexane followed by treatment with PhSiH3 at 80  C. The potent nucleophilicity of this compound is evident from the high rate of deuterium exchange (HdD exchange) in benzene[D6] at room temperature via a nucleophilic substitution reaction. The hydride complex 406 reacts with ethylene in benzene[D6] at 1 bar pressure at room temperature, resulting in the formation of the highly reactive strontium alkyl complex [(DiPePBDI)SrEt]2 408, with competing deuterium exchange product [(DiPePBDI)SrD]2 407 (Scheme 221).291 The dimeric alkyl strontium complex reacts further with ethylene and forms low molecular weight oligomers, along with long-chain polyethylenes characterized by GC–MS analysis. The solid-state structure of the compound [(DiPePBDI)SrEt]2 408 is shown in Fig. 172.

Scheme 221 Reactivity of bridged hydride of strontium complex chelated by b-diketiminate ligand.

Fig. 172 The solid-state structure of the compound [(DiPePBDI)SrEt]2 408.

2.03.5.4.2

Tp ligand supported organobarium complexes

Super-bulky scorpionate ligand TpAd,iPr (TpAd,iPr ¼ hydrotris(3-adamantyl-5-isopropylpyrazolyl)borate) stabilized dimeric barium hydride [(TpAd,iPr)Ba(m-H)]2 409 has been reported to react with one equiv. of carbon monoxide to afford the cis-ethendiolatedianion 410 (Scheme 222).292 Further investigation of the reactivity of compound 409 with one equiv. of 1,4-diphenyl1,3-butadiyne affords [{(TpAd,iPr)Ba}2(PhCHdC^CdCHPh)] 411. In both reactions, the disappearance of the hydride signal at 10.39 ppm in the 1H NMR spectrum confirms the complete consumption of starting material and the formation of products 410 and 411. Both the complexes are dimeric (Fig. 173).

224 Organometallic Complexes of the Alkaline Earth Metals

Scheme 222 CO and alkyne insertion across BadH bond of 409.

Organometallic Complexes of the Alkaline Earth Metals

225

Fig. 173 The solid-state structures of compounds 410 and 411. All hydrogens and iPr/adamantyl group carbon atoms are removed for clarity.

Compound 409 reacts with 2 equiv. of diphenyl ethylene (DPE) in benzene resulting in the formation of dark red mono DPE adduct (TpAd,iPr)Ba(Z6-Ph)C(Ph)Me 412. Moreover, compound 412 reacts with an excess amount of diphenyl ethylene to form dark red colored (TpAd,iPr)Ba(Z6-Ph)C(Ph)dCH2dC(Ph)2Me 413 via a reversible equilibrium (Scheme 223).293 In both cases, the Ba center is bound to the phenyl rings of the diphenyl ethylene unit through a Ph. . ..Ba p interaction, as shown in Fig. 174.

Scheme 223 Synthesis of DPE adduct organobarium.

226

Organometallic Complexes of the Alkaline Earth Metals

Fig. 174 The solid-state structures of the compounds [(TpAd,iPr)Ba(Z6-Ph)C(Ph)Me] 412 and [(TpAd,iPr)Ba(Z6-Ph)C(Ph)dCH2dC(Ph)2Me] 413.

2.03.5.4.3

Miscellaneous N-donor ligand stabilized organostrontium compounds

Formation of the heteroleptic Sr ⋯olefin complex [{RO1}SrN(SiMe2H)2]2 414 has been accomplished by treating Sr[N(SiMe2H)2]2(THF) with 1 equiv. of a,a-(CF3)2 alcohol [a,a-(CF3)2C(OH)CH2N(iPr)CH2CH2CHCH2] (which has terminal olefinic tether) at room temperature (Scheme 224).294 Complex 414 crystallizes as a centrosymmetric O-bridged dimeric unit stabilized by a secondary Sr ⋯ FdC and b-agostic Sr ⋯ HdSi interactions (Fig. 175). A single resonance at 4.87 ppm in the 1H NMR spectrum is for the SidH unit is consistent with retention of the Sr ⋯ HdSi agostic interaction in the solution phase. 1H DOSY NMR spectroscopy has revealed that the complex 414 is dimeric.

Scheme 224 Synthesis of heteroleptic organostrontium olefin complex.

Fig. 175 The solid-state structure of compound [{RO1}SrN(SiMe2H)2]2 414.

Organometallic Complexes of the Alkaline Earth Metals

2.03.5.4.4

227

Chiral N-donor ligand stabilized organobarium compounds

Harder and co-workers have synthesized [(DMAT)2Ba(THF)2] 415; by the salt metathesis reaction between two equiv. of (DMAT)K, with BaI2 in THF in a yield of 83% (Scheme 225).295 X-ray crystallographic analysis of the yellow crystalline solid reveals that compound 415 consists of diastereomeric heterochiral benzylic carbon atoms with R and S configuration with an average bariumcarbon bond distance of around 2.951(2) A˚ . NMR data and structural features indicate strong delocalization of negative charge in the aryl ring of the DMAT ligand. The crystallographic analysis of complex 415 also reveals that two THF molecules bind to the barium center (Fig. 176).

Scheme 225 Synthesis of the chiral organobarium complex.

Fig. 176 The solid-state structure of the compound [(DMAT)Ba(THF)2] 415.

2.03.5.5

Cationic organostrontium and barium complexes

Lewis acidic [(DippBDI)M]+ (M]Sr, Ba) cations can be obtained by double deprotonation of [(BDI)H2][B(C6F5)4] (which is itself formed by protonating the corresponding b-diketimine with [H(OEt2)2][B(C6F5)4]) using homoleptic base M{N(SiiPr3)2}2 (M]Sr, Ba). In less coordinating polar solvents, this generates complexes of the type [(BDI)M][B(C6F5)4]. Due to clathrate formation, the compounds could not be crystallized, but the addition of pyrene yielded crystals of [(BDI)M(pyrene)][B(C6F5)4] (M]Sr 416, Ba 417). Lewis base-free monodentate amide-stabilized cations [(amide)Ba(toluene)2]+ can be accessed by an amide abstraction pathway. For less bulky amides of barium, for example, Ba{N(SiMe3)2}2, reaction with [Ph3C][B(C6F5)4] readily affords [{(Me3Si)2N}Ba(toluene)2][B(C6F5)4] 418 in a moderately high yield. For bulky amides of both strontium and barium, e.g., M{N(SiiPr3)2}2, reactions with the anilinium salt [PhNMe2H][B(C6F5)4] affords [{(iPr3Si)2N}Ba(toluene)][B(C6F5)4] 420 and Sr analog [{(iPr3Si)2N}Sr(PhNMe2)][B(C6F5)4] 419 (Scheme 226).296 NMR studies of the obtained compounds show free solvent signals, which indicates that the toluene and the PhNMe2 components are coordinated to the metal center very weakly.

228

Organometallic Complexes of the Alkaline Earth Metals

Furthermore, X-ray crystallographic analysis shows that the toluene and PhNMe2 moieties are attached in the form of p-arene complexes (Figs. 177–179). These compounds contain Lewis basic amide ligands; they can act as potential precursors for synthesizing Lewis base free cations of the heavier alkaline earth metals.

Scheme 226 Preparation of a series of cationic organostrontium and barium complexes.

Fig. 177 The solid-state structures of [(BDI)M(pyrene)][B(C6F5)4] (M]Sr 416; Ba 417).

Organometallic Complexes of the Alkaline Earth Metals

229

Fig. 178 The solid-state structure of [{(Me3Si)2N}Ba(tol)2][B(C6F5)4] 418.

Fig. 179 The molecular structure of [{(iPr3Si)2N}Sr(PhNMe2)][B(C6F5)4] 419.

Okuda and group reported that the reaction of free Me4TACD with [SrH2]n in the presence of THF readily affords (Me4TACD) Sr(CH2Ph)2 421 (Scheme 227).297 Further reactivity of complex 421 with one equiv. of the Bronsted acid [NEt3H][B(C6H3-3,5-Me2)4] in THF affords the macrocyclic cationic benzyl complex [(Me4TACD)Sr(CH2Ph)(THF)] [B(C6H3-3,5-Me2)4] 422. X-ray crystallographic analysis of compound 422 reveals Z1-coordination of the benzyl ligand in the solid-state (Fig. 180). However, a similar reaction in the presence of THP instead of THF affords [(Me4TACD)Sr(CH2Ph)] [B(C6H3-3,5-Me2)4] 423 in lower yield and purity. The crystallographic study, in this case, reveals Z6-coordination of the benzyl ligand in compound 423.

230

Organometallic Complexes of the Alkaline Earth Metals

Scheme 227 Organostrontium cations chelated by NNNN-type macrocycle.

Fig. 180 The crystal structure of the cationic Sr complex [(Me4TACD)Sr(CH2Ph)(THF)] [B(C6H3-3,5-Me2)4] 423.

2.03.5.6

Heterobimetallic organostrontium and barium compounds

The reaction of SrI2 with KN(SiMe3)2 and K2C8H8 in a 2:2:1 ratio in THF allows for the synthesis of a cyclooctatetraenyl (COT) strontium amido complex, and subsequently, the colorless inverse sandwich amido-distrontium compound [{(Me3Si)2N} Sr(THF)2]2(m-C8H8) 424 (Scheme 228).298 The 1H and 13C NMR spectra of 424 feature a C8H8 resonance at 6.30 and 90.3 ppm, respectively, in THF-d8 solvent. Moreover, manipulation of the stoichiometric ratio to 2:4:1 (with the same starting materials) gives [K[{(Me3Si)2N}2Sr]2(m-C8H8)] 425. X-ray crystallographic analysis shows that 425 is dimeric, having an inversion center. The distance of Sr-COT of complex 425 is 2.19 A˚ which is 0.05 A˚ shorter than 424 (Fig. 181).

Scheme 228 COT stabilized mixed metal amido organostrontium compounds.

Organometallic Complexes of the Alkaline Earth Metals

231

Fig. 181 The solid-state structures of cyclooctatetraenyl (COT) strontium amido complexes [[{(Me3Si)2N}M(THF)2]2(m-C8H8)] 424 and [[K{(Me3Si)2N}2Sr]2(m-C8H8)] 425.

The reaction of PhP(CH2SiMe3)2
BH3 with two equiv. of SrI2 and two equiv. of 4-tBuC6H4CH2K, affords [(PhP{CH (SiMe3)}2
BH3)2Sr3K2(OEt2)(THF)2]2
Et2O 426 (Scheme 229).275 X-ray crystallographic analysis reveals the presence of two distinct types of strontium center. Each of the two peripheral Sr(1) is surrounded by four carbanion centers from two dicarbanion ligands and adopts distorted trigonal bipyramidal geometries. By contrast, the central Sr(2) is only connected to the hydrogen atoms of four BH3 and two THF molecules, giving an overall pseudo-octahedral geometry (Fig. 182).

Scheme 229 Synthesis of mixed metal organostrontium hydride cluster.

232

Organometallic Complexes of the Alkaline Earth Metals

Fig. 182 The solid-state structure of [[PhP(BH3){CH(SiMe3)}2]2Sr3K2(Oet2)(THF)2]2
Et2O 426.

Anwander and co-workers synthesized the first isolable barium tetra alkyl gallate complexes [Ba(GaR4)2(toluene)]2 (R]Me 427a, Et 427b) by mixing [Ba(AlEt4)2]n with six equiv. of GaR.3Et2O in toluene (Scheme 230).299 The solid structure of [Ba(GaEt4)2(toluene)]2 exhibits Z2 and m2-Z2:Z1 coordination modes for the terminal and bridging tetraethylgallate ligands, respectively (Fig. 183). However, in the case of the corresponding methyl compound 427a, a white precipitate is formed and dried to form [Ba(GaMe4)2]n, which is insoluble in toluene.

Scheme 230 Synthesis of organobarium gallate compounds.

Fig. 183 The solid-state structure of heterobimetallic peralkylated barium-complex [Ba(GaEt4)2(toluene)]2 427b.

Organometallic Complexes of the Alkaline Earth Metals

233

An unusual molecular barium oxide has been prepared by the reaction of [Ba{N(SiHMe2)2}2]n with Lewis acidic trimethyl gallium in toluene, affords [Ba(m4-O)(GaMe3)(toluene)]2 428 (Scheme 231)300. The solid-state structure of 428 reveals a dibarium complex, in which the metal centers are each connected to a molecule of toluene (with Z6-binding mode) while [OGaMe3] ligands bridge between the metal centers (Fig. 184).

Scheme 231 Synthesis of the mixed metal organobarium cluster.

Fig. 184 The solid-state structure of heterobimetallic barium-complex [Ba(m4-O)(GaMe3)(toluene)]2 428.

2.03.5.7

Organometallic (M]Sr and Ba) p Arene complexes

Sarazan and the group have reported the synthesis of heteroleptic bulky boryloxides and phenoxide species of barium. Treatment of [Ba{m2-OSi(SiMe3)3}{N(SiMe3)2}]2 429 with two equiv. of (Tripp)2BOH affords the oxygen-bridged dimeric boryl oxide [Ba{m2-OB(Tripp)2}{OSi(SiMe3)3}]2 430 in high yield. Moreover, a similar type of reaction of [Ba{N(SiMe3)2}2(THF)2] with two equiv. of (Tripp)2BOH forms the related system [Ba{m2-OB(Tripp)2}{OB(Tripp)2}]2 432 in 74% yield (Scheme 232).301 Compounds 430 and 432 are centrosymmetric dimers in which the barium centers are bridged through O-boryloxide ligands (Fig. 185). Further investigation of the reactivity of compound 429 with two equiv. of 2,6-Ph2C6H3OH leads to the formation of heteroleptic phenoxide species [Ba{m2-O(2,6dPh2dC6H3)}{OSi(SiMe3)3}]2 431.

234

Organometallic Complexes of the Alkaline Earth Metals

Scheme 232 Synthesis of barium boryloxides and phenoxide complexes.

Fig. 185 The solid-state structure of boryloxide complex [Ba{m2-OB(Tripp)2}{OB(Tripp)2}]2 432.

Organometallic Complexes of the Alkaline Earth Metals

235

The first example of strontium ansa-arene complex [Sr(DXE)(o-DFB)2][Al(ORF)4]2 {where DXE ¼ dixylylethylene and DFB ¼ 1,2-difluorobenzene}433 has been synthesized via a salt metathesis reaction between excess SrI2 and Ag[Al(ORF)4] in the presence of 1,2-difluorobenzene and dixylylethylene (Scheme 233).302 The molecular structure of 433 reveals that both the aromatic rings adopt an Z6-coordination and SrdC bond distances range between 2.945(3)-3.117(3)A˚ , while the difluorobenzene ligand is coordinated in k2 fashion (Fig. 186).

Scheme 233 Preparation of strontium ansa-arene compound 433.

Fig. 186 The solid-state structure of ansa-arene complex [Sr(DXE)-(o-DFB)2]2+([Al(ORF)4]−)2 433.

Xi and co-workers synthesized the first organobarium species in which the barium center is attached to the organic framework by an Z8 binding mode. They reacted 1,4-dilithio-1,3-butadiene with Ba{N(SiMe3)2}2 in hexane and obtained barium dibenzopentalenide 434 in a moderately high yield (Scheme 234).303 Deep red crystals of 434 were analyzed crystallographically, indicating that the p-framework is bent along the bridgehead CdC’ backbone at about an angle of 13.5o (Fig. 187).

Scheme 234 Synthetic route for barium dibenzopentalenide 434.

236

Organometallic Complexes of the Alkaline Earth Metals

Fig. 187 The solid-state structure of the compound barium dibenzopentalenide 434.

Reaction of [Ba{N(SiMe3)2}2(THF)2] with {(Me3Si)2CH}2BOH affords compound [Ba{m2-N(SiMe3)2}(OB{CH(SiMe3)2}2)]2, which further reaction with two equiv. of {(Me3Si)2CH}2BOH in toluene to form the toluene adduct [Ba(OB{CH (SiMe3)2}2)2
C7H8] 435 in moderately high yield. This product can also be accessed by the reaction of [Ba{N(SiMe3)2}2(THF)2] with four equiv. of {(Me3Si)2CH}2BOH in toluene (Scheme 235).304. Single-crystal analysis of compound 435 reveals that the tri-coordinate barium center is ligated by the toluene moiety in Z6 manner (Fig. 188).

Scheme 235 Synthesis of barium boryloxide complexes.

Fig. 188 The solid-state structure of the compound [Ba(OB{CH(SiMe3)2}2)2
C7H8] 435.

Organometallic Complexes of the Alkaline Earth Metals

2.03.6

237

Concluding remarks

Since the publication of COMC III in the year 2007, an overview of the recent developments on organometallic compounds of nonradioactive alkaline earth metals has been documented. In recent years, the coordination chemistry of the alkaline earth elements has been rapidly growing. As far as the organometallic compounds of alkaline earth elements are concerned, there has been tremendous growth in the 21st century. However, many open questions305 still need to be addressed by synthetic chemists in the coming years. Isolation of chiral low oxidation state organomagnesium compounds, heavier alkaline earth organometallic compound containing M-M bonds, the synthesis of Ae]Ae306–309 or Ae]E310 (E ¼ any other elements such as N, P, etc.) double-bonded compounds, etc. We hope that a comprehensive overview of organometallic compounds of alkaline earth metals presented here will convince researchers around the globe that these cheaper, non-toxic, biocompatible systems (except beryllium) are valuable in homogenous, heterogeneous, asymmetric catalysis, materials, and polymer sciences.

Acknowledgments The authors are grateful to the National Institute of Science Education and Research (NISER), HBNI, DAE, Govt. of India for their generous support of their work. The authors thank A Ganesh Patro for the part of the preparation of the artwork.

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256. 257. 258. 259. 260. 261. 262. 263. 264. 265. 266. 267. 268. 269. 270. 271. 272. 273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295. 296. 297. 298. 299. 300. 301. 302. 303. 304. 305. 306. 307. 308. 309. 310.

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