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Tin Chemistry
Tin Chemistry: Fundamentals, Frontiers, and Applications Edited by Marcel Gielen, Alwyn Davies, Keith Pannell and Edward Tiekink © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51771-0
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Tin Chemistry Fundamentals, Frontiers, and Applications
Edited by
ALWYN G. DAVIES University College London, UK
MARCEL GIELEN Free University of Brussels VUB, Belgium
KEITH H. PANNELL University of Texas at El Paso, USA
EDWARD R. T. TIEKINK University of Texas at San Antonio, USA
A John Wiley and Sons, Ltd, Publication
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This edition first published 2008 C 2008 John Wiley & Sons, Ltd Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The Publisher and the Author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of fitness for a particular purpose. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the Publisher nor the Author shall be liable for any damages arising herefrom. Library of Congress Cataloging-in-Publication Data Tin chemistry : fundamentals, frontiers, and applications / edited by Marcel Gielen . . . [et al.]. p. cm. Includes bibliographical references and index. ISBN 978-0-470-51771-0 (cloth : alk. paper) 1. Organotin compounds. I. Gielen, M. (Marcel), 1938– QD412.S7T56 2008 546 .686–dc22 2008010997 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 9780470517710 Typeset in 10/12pt Times by Aptara, New Delhi, India Printed in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire
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Dedicated to the memory of Des Cunningham, 1942–2006
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Preface List of Contributors 1
2
Introduction and Overview 1.1 Introduction Alwyn G. Davies 1.1.1 History, Occurrence, Production, and Applications 1.1.2 The Element 1.1.3 Structure and Bonding 1.1.4 Organotin Compounds, Rn Snm 1.1.5 Organotin Compounds With Electronegative Ligands, Rn SnX4-n 1.1.6 Preparation of Organotin(IV) Compounds 1.1.7 Preparation of Organotin Compounds in Lower Valence States 1.1.8 Literature References Fundamentals in Tin Chemistry 2.1 NMR Spectroscopy of Tin Compounds Bernd Wrackmeyer 2.1.1 Introduction 2.1.2 Experimental 2.1.3 Nuclear Spin Relaxation 2.1.4 Chemical Shifts δ 119 Sn 2.1.5 Indirect Nuclear Spin–Spin Coupling Constants n J (119 Sn,X) References 2.2 Tin(II) Clusters Tristram Chivers and Dana J. Eisler 2.2.1 Introduction
xvii xix 1 1 1 3 4 5 9 10 13 13 14 17 17 17 17 20 21 39 45 53 53
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2.3
2.4
2.5
2.6
2.7
2.2.2 Imidotin Cubane Clusters 2.2.3 The Seco-Cubane Sn3 (μ3 -NtBu)(μ-NtBu)(μ-NHtBu)2 2.2.4 Double-Cubane Clusters 2.2.5 Imidotin Chalcogenides 2.2.6 Phosphido– and Arsenido–Tin Clusters 2.2.7 Summary and Future Prospects References Mono-Organotin Oxo-Clusters Franc¸ois Ribot 2.3.1 Introduction 2.3.2 Molecular Structures 2.3.3 Syntheses and Formation Mechanisms 2.3.4 Conclusions Acknowledgments References Organotin Carboxylate and Sulfonate Clusters Vadapalli Chandrasekhar, Puja Singh, and Kandasamy Gopal 2.4.1 Introduction 2.4.2 Organotin Carboxylates 2.4.3 Organotin Sulfonates 2.4.4 Conclusion Acknowledgments References Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds Herbert H¨opfl 2.5.1 Introduction 2.5.2 Metallosupramolecular Chemistry with Tin 2.5.3 Formation of Tin Macrocycles and Extended Networks 2.5.4 Conclusions and Perspectives Acknowledgments References Deltahedral Zintl Ions of Tin: Synthesis, Structure, and Reactivity Slavi C. Sevov 2.6.1 Introduction 2.6.2 Background 2.6.3 Geometry, Charge, Electron Count, and Electronic Structure 2.6.4 Reactions With Nine-Atom Deltahedral Zintl Anions of Tin 2.6.5 Solution Studies by NMR 2.6.6 Concluding Remarks Acknowledgments References Stable Stannylium Cations in Condensed Phases Joseph B. Lambert 2.7.1 Introduction 2.7.2 Pentacoordination (Trigonal Pyramids) 2.7.3 Tetracoordination: Binding with Solvent or Anion
53 60 61 62 65 66 66 69 69 70 78 89 90 90 93 93 93 105 113 114 114 117 117 117 120 133 133 133 138 138 138 140 143 148 150 150 150 152 152 154 155
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2.8
2.9
2.10
2.11
2.12
2.7.4 Tricoordination: NMR Evidence for Free Stannylium Ions 2.7.5 Tricoordination: Crystallographic Evidence and Computational Confirmation of Free Stannylium Ions 2.7.6 Summary References Preparation and Coordination Chemistry of Mono- and Bidentate Benzannulated N-Heterocyclic Stannylenes Including Some Germanium and Lead Analogs Alexander V. Zabula and F. Ekkehardt Hahn 2.8.1 Introduction 2.8.2 Stable N-Heterocyclic Stannylenes, Germylenes, and Plumbylenes 2.8.3 Complexes of Bidentate N-Heterocyclic Germylenes and Stannylenes References Stannenes, Distannenes, and Stannynes Yoshiyuki Mizuhata and Norihiro Tokitoh 2.9.1 Introduction 2.9.2 Distannenes (Sn Sn) 2.9.3 Stannenes (Sn C) 2.9.4 Silastannenes (Si Sn) and Germastannenes (Ge Sn) 2.9.5 Stannynes (Sn C) 2.9.6 Distannynes (Sn Sn) and Their Reduced Species References Tetraorganodistannoxanes: Simple Chemistry From a Personal Perspective Klaus Jurkschat 2.10.1 Introduction 2.10.2 Unsymmetrically Substituted Tetraorganodistannoxanes 2.10.3 Looking for the Third Dimension 2.10.4 Variation of R 2.10.5 Variation of Spacer Z 2.10.6 Variation of the Electronegative Substituents X and Y 2.10.7 Miscellaneous Acknowledgments References Unusual Bonds and Coordination Geometries M´onica Moya-Cabrera, Vojtech Jancik and Raymundo Cea-Olivares 2.11.1 Introduction 2.11.2 Unusual Bonds 2.11.3 Unusual Coordination Geometries References Tin(II) Heterobimetallic and Oligometallic Derivatives Muhammad Mazhar and Imtiaz-ud-Din 2.12.1 Introduction 2.12.2 General Synthetic Procedures 2.12.3 Characterization Techniques 2.12.4 Stoichiometric and Structural Aspects of Tin(II) Heterobimetallic and Oligometallic Compounds 2.12.5 Conclusion References
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156 157 158 159 160 160 160 166 174 177 177 177 183 193 195 196 198 201 201 202 209 212 212 221 221 229 229 231 231 231 241 247 251 251 254 256 258 267 267
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2.13 Computational Methods for Organotin Compounds Sarah R. Whittleton, Russell J. Boyd, and T. Bruce Grindley 2.13.1 Introduction 2.13.2 Relativistic Effects in Heavy Elements 2.13.3 Effective Core Potentials 2.13.4 Other Computational Methods Available for Tin 2.13.5 Current State of Computational Organotin Chemistry 2.13.6 Structure Prediction 2.13.7 Reaction Pathways and Mechanisms 2.13.8 Thermochemistry 2.13.9 Bond Strengths and Bond Dissociation Enthalpies 2.13.10 Spectroscopic and Related Properties 2.13.11 Conclusion Acknowledgments References 3
Materials Chemistry and Structural Chemistry of Tin Compounds 3.1 Tin Compounds For CVD (Chemical Vapor Deposition) Geraldo M. de Lima 3.1.1 Introduction 3.1.2 General Aspects of CVD 3.1.3 Organometallic Chemical Vapor Deposition (MOCVD) 3.1.4 Tin Compounds For CVD 3.1.5 Tin(IV) Oxide and Related Materials 3.1.6 Tin Sulfides 3.1.7 Tin(II) Selenide and Telluride 3.1.8 Tin (IV) Phosphide 3.1.9 Tin Alloys 3.1.10 Summary References 3.2 Class II Tin-Based Hybrid Materials Prepared From Alkynyltin Precursors Thierry Toupance 3.2.1 Introduction 3.2.2 Functionalization of Oxide Surfaces 3.2.3 Self-Assembled Tin-Based Hybrid Materials 3.2.4 Nanoporous Nanostructured Tin Dioxide Materials 3.2.5 Conclusion Acknowledgments References 3.3 Organotin Compounds as PVC Stabilizers Esen Arkis 3.3.1 Introduction 3.3.2 Types of Organotin Stabilizers 3.3.3 Tin Carboxylates 3.3.4 Tin Mercaptides 3.3.5 The Mechanism of Stabilization
269 269 269 270 271 272 272 274 276 276 277 278 278 279 285 285 285 285 287 287 287 290 291 292 292 292 293 296 296 297 301 306 309 310 310 312 312 313 314 315 315
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3.4
3.5
3.6
3.7
3.8
3.3.6 Operational Considerations 3.3.7 Evaluating Stability 3.3.8 Conclusion References Organotin Compounds as Anion-Selective Carriers in Chemical Sensors Nikos Chaniotakis 3.4.1 Introduction to Chemical Sensors 3.4.2 Potentiometric Ion Selective Electrodes (ISEs) 3.4.3 The Ionophores 3.4.4 Organotin-Mediated Anion Partitioning into Liquid Polymeric Membranes 3.4.5 Anion Selective Organotin-based ISEs 3.4.6 Conclusions References Tin Compounds as Flame Retardants and Smoke Suppressants Paul A. Cusack 3.5.1 Introduction 3.5.2 Tin Treatments For Fibers 3.5.3 Zinc Stannates 3.5.4 Recent Developments 3.5.5 Fire-Retardant Mechanism 3.5.6 Summary References Quadratic Non-Linear Optical Properties of Tin-Based Coordination Compounds Pascal G. Lacroix and Norberto Farf´an 3.6.1 Introduction 3.6.2 Basic Concepts of Quadratic Non-Linear Optics 3.6.3 Tin-Based Materials in Quadratic Non-Linear Optics 3.6.4 Concluding Remarks Acknowledgments References Monoorganotin Precursors For Hybrid Materials Bernard Jousseaume 3.7.1 Introduction 3.7.2 Functional Trialkynylorganotins 3.7.3 Bridged Ditins 3.7.4 Conclusion Acknowledgments References Organotin Polymers and Related Materials Hemant K. Sharma and Keith H. Pannell 3.8.1 Introduction 3.8.2 Synthesis of Linear Oligostannanes 3.8.3 Synthesis of Polystannanes 3.8.4 Properties of Polystannanes 3.8.5 Polymers with Tin in the Backbone
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317 320 322 323 324 324 324 326 326 329 336 337 339 339 339 340 343 346 348 348 351 351 351 352 358 359 359 361 361 361 368 374 374 374 376 376 376 378 381 386
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3.8.6 Polymers with Tin as a Pendant Group Acknowledgments References 3.9 Intermolecular Tin . . . π-Aryl Interactions: fact or artifact? A New Bonding Motif For Supramolecular Self-Assembly in Organotin Compounds Ionel Haiduc, Edward R. T. Tiekink, and Julio Zukerman-Schpector 3.9.1 Introduction 3.9.2 Discussion 3.9.3 Conclusions and Outlook Acknowledgments References
388 389 389
Medicinal/Biocidal Applications of Tin Compounds and Environmental Aspects 4.1 The Cardiovascular Activity of Organotin Compounds Mala Nath 4.1.1 Introduction 4.1.2 Cardiovascular Activity of Organotin Compounds 4.1.3 Conclusion References 4.2 Organotins: Insecticidal/Larvicidal Activities and Quantitative Structure–Activity Relationships George Eng and Xueqing Song 4.2.1 Overview: Organotins 4.2.2 Larvicidal/Insecticidal Activities 4.2.3 Quantitative Structure–Activity Relationships 4.2.4 Food For Thought References 4.3 Anti-Fungal Activity of Organotin Compounds Heloisa Beraldo and Geraldo M. de Lima 4.3.1 Introduction 4.3.2 Biological Applications of Organotin Compounds 4.3.3 Fungi and Fungal Infections 4.3.4 Mechanisms of Biological Action of Organotin Compounds 4.3.5 Structure–Activity Relationships 4.3.6 Anti-Fungal Screening 4.3.7 Conclusions References 4.4 Chemical and Biotechnological Developments in Organotin Cancer Chemotherapy Claudio Pettinari and Fabio Marchetti 4.4.1 Introduction 4.4.2 Developments in the Design of Organotin Anti-Cancer Compounds 4.4.3 Conclusion References 4.5 Impact of Organotin Compounds on the Function of Human Natural Killer Cells Margaret M. Whalen 4.5.1 Introduction
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392 392 393 409 409 409
413 414 425 427 430 430 430 435 439 439 443 443 443 443 444 445 446 452 452 454 454 455 465 465 469 469
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4.5.2 Effects of n-butyltin Chlorides on Human NK Cell Function: Tri-n-Butyltin Chloride (TBTC) 4.5.3 Di-n-Butyltin Chloride (DBTC) 4.5.4 Effects of Trimethyltin Chloride (TMTC) on Human NK Cell Function 4.5.5 Effects of Dimethylphenyltin Chloride (DMPTC) on Human NK Cell Function 4.5.6 Effects of Methyldiphenyltin Chloride (MDPTC) on Human NK Cell Function 4.5.7 Effects of Triphenyltin Chloride (TPTC) on Human NK Cell Function 4.5.8 Summary References 4.6 Biological Aspects of Organotins: Perspectives in Structural and Molecular Biology Hiram I. Beltr´an, Rosa Santillan and Norberto Farf´an 4.6.1 Introduction 4.6.2 Interaction of Organotin Compounds with Biological Systems and Mimic Xenobiotics 4.6.3 Interaction of Organotin Compounds in Real and Model Membranes 4.6.4 Roles of Organotin Compounds in Cell Function 4.6.5 Aspects of Organotins in Structural and Molecular Biology 4.6.6 Perspectives Acknowledgments References 5
Tin in Organic Synthesis 5.1 Applications of Organotin Derivatives for Carbohydrate Synthesis T. Bruce Grindley 5.1.1 Introduction 5.1.2 Preparation 5.1.3 Structures 5.1.4 Reactions References 5.2 Reactions of SE Substitution for Organostannanes in Organic Synthesis David R. Williams, and Partha P. Nag 5.2.1 Introduction 5.2.2 Mechanistic Considerations and a Predictive Model for Reactions with Aldehydes 5.2.3 Allylation Reactions of Substrate Control 5.2.4 Allylation Reactions Exhibiting α-Chelation Control 5.2.5 Allylation Reactions Exhibiting β-Chelation Control 5.2.6 Reactions of γ -(Alkoxy)allylstannanes 5.2.7 Reactions of Oxocarbenium Cations 5.2.8 Reactions of N-Acyliminium Cations 5.2.9 Reactions with α, β-Unsaturated Carbonyl Compounds 5.2.10 Reactions of Allylic Stannanes with Imines 5.2.11 Transmetalation Reactions of Allylic Stannanes 5.2.12 Reactions of Chiral Allylic Stannanes 5.2.13 Reactions of Allenylstannanes 5.2.14 Transmetalation Reactions of Allenylstannanes
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470 472 474 475 475 476 477 479 482 482 482 484 488 491 492 492 492 497 497 497 498 500 504 512 515 515 516 519 521 524 526 527 531 532 533 535 541 543 547
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5.3
5.4
5.5
5.6
5.2.15 Reactions of Propargylic Stannanes 5.2.16 Enantioselective Reactions with Chiral Lewis Acids 5.2.17 Conclusion and Future Outlook References Cross-Coupling of Organotin Compounds for Carbon Carbon Bond Formation Pablo Espinet and Miroslav Genov 5.3.1 Introduction 5.3.2 Mechanistic Aspects and Consequences 5.3.3 Catalysts and Ligands 5.3.4 Ligandless Coupling 5.3.5 Copper Effect 5.3.6 Microwave-Assisted Reactions 5.3.7 Natural Product Synthesis 5.3.8 Conclusion References Stille Cross-Coupling for the Synthesis of Natural Products Sergio Pascual and Antonio M. Echavarren 5.4.1 Introduction 5.4.2 Alkenyl–Alkenyl Stille Coupling 5.4.3 Alkenyl–Alkynyl Stille Coupling 5.4.4 Alkenyl–Aryl Stille Coupling 5.4.5 Aryl–Aryl Stille Coupling 5.4.6 sp3 –sp2 Coupling Reactions 5.4.7 Couplings for the Synthesis of Ketones 5.4.8 Summary and Outlook References New Trends in the Synthesis of Solid-Supported Organotin Reagents and Interest of their Use in Organic Synthesis in a Concept of Green Chemistry Jean-Mathieu Chr´etien, Jeremy D. Kilburn, Franc¸oise Zammattio, Erwan Le Grognec, and Jean-Paul Quintard 5.5.1 Introduction 5.5.2 Removal of Tin Residues by Partition between Two Phases 5.5.3 Solid-Supported Organotin Reagents 5.5.4 Use of Supported Organotins in Organic Synthesis 5.5.5 Perspectives and Conclusions References Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing Pre- and In Situ-Formed Organostannanes Ron Grigg and Visuvanathar Sridharan 5.6.1 Introduction 5.6.2 Mono-Cyclization–Anion Capture Processes 5.6.3 Bis-Cyclization–Anion Capture Processes 5.6.4 Cyclization–Anion Capture Processes Involving Relay Switches 5.6.5 Summary References
550 552 555 556 561 561 561 564 570 571 572 574 575 575 579 579 579 587 587 593 594 599 600 602 607 607 607 608 608 613 617 618 622 622 623 630 634 637 638
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5.7 Carbostannylation of Carbon Carbon Unsaturated Bonds Eiji Shirakawa 5.7.1 Introduction 5.7.3 Carbostannylation of Alkynes 5.7.4 Carbostannylation of Alkenes 5.7.5 Carbostannylation of Dienes 5.7.6 Conclusion References 5.8 Green Organotin Chemistry: an Oxymoron? David Young 5.8.1 Introduction 5.8.2 How Hazardous Are Organostannanes? 5.8.3 Removing Organotin By-Products 5.8.4 Modified Organotin Reagent for Easy Separation 5.8.5 Solid Phase Tin Reagents 5.8.6 Less Toxic Organotin Reagents 5.8.7 No-Tin Reagents 5.8.8 Conclusion References 6
Tin in Catalysis 6.1 Green Organotin Catalysts Junzo Otera, Monique Biesemans, Vanja Pinoie, Kevin Poelmans, and Rudolph Willem 6.1.1 Introduction 6.1.2 Fluorous Distannoxane Catalysts 6.1.3 Grafted Organotin Catalysts 6.1.4 Conclusion References 6.2 Organotin Catalysts for Isocyanate Reactions Werner J. Blank and Edward T. Hessell 6.2.1 Introduction 6.2.2 Mechanism of Urethane Catalysis by Tin Compounds 6.2.3 Structure of the Tin Catalyst 6.2.4 Mechanisms 6.2.5 Synergism of Tin Compounds with Amine Catalysts: DBDTL and 1,4-Diazabicyclo[2.2.2]octane 6.2.6 Mechanism of Catalysis with Blocked Isocyanates 6.2.7 Organotin Catalyst Composition 6.2.8 Catalysis with Organotin Compounds 6.2.9 Applications 6.2.10 Blocked Isocyanates 6.2.11 Catalyst Interactions 6.2.12 Polymer Synthesis 6.2.13 Organotin Replacements References
xv
640 640 641 647 650 651 651 653 653 654 654 656 659 661 663 664 664 667 667
667 668 672 678 678 681 681 681 681 682 685 686 686 687 691 694 696 696 698 698
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6.3 Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides by Lithium Perchlorate Wojciech J. Kinart and Cezary M. Kinart 6.3.1 Catalysis of Metalloene Reactions of Allylstannanes by Lithium Perchlorate 6.3.2 Catalysis of reactions of triorganotin phenoxides with diethyl azodicarboxylate, bis(trichloroethyl) azodicarboxylate and diethyl acetylenedicarboxylate References Index
701 701
709 719 721
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Preface
The chemistry of tin has now grown to the stage where a dedicated monograph can do little more than trace the structure of the subject and provide a guide to the literature. We felt that authoritative, in-depth, reviews were now needed of those branches, both pure and applied, where developments have been most pronounced, and the present volume is the result. Each chapter, by specialists in the field, deals with one important aspect of tin chemistry, and gives a detailed account of its present standing. Both inorganic and organic aspects are covered, though progress has been most extensive in organotin chemistry which is living up to its reputation of being studied by more techniques, and finding more applications, than the organic derivatives of any other metal. Advances in techniques include sophisticated NMR methods, both in solution and the solid-state, and computational methods, and these, coupled with X-ray diffraction and other established methods, have been applied to the study of a wide variety of structures. Topics covered in the book include Sn(II) clusters, tin Zintl ions, Sn(II) heterobimetallic compounds, R3 Sn+ cations, stannylenes (R2 Sn:), stannenes (R2 Sn SnR2 and R2 Sn CR2 ), stannynes (RSn SnR), organotin oxide, carboxylate and sulfonate clusters, dendrimers and macrocycles, organotin polymers, Sn-π interactions, unusual bondings and structures, and compounds with non-linear optical properties. Non-metallurgical uses of tin reflect the biological activity of organotin compounds, and the nontoxicity of inorganic tin. Inorganic tin compounds are used in flame-retardants and smoke suppressants, and SnO2 for coating glass (though usually deposited from organotin compounds). The principal use for organotin compounds is still as a stabiliser for PVC, and a small but important application is as ionophores in sensors, and as precursors for hybrid organic-inorganic nanometric materials. The use as marine antifoulants is being phased out because of its effect on other marine life, but organotin compounds are showing promise as larvicides, insecticides, and fungicides, and, particularly, in cancer therapy. Organotin compounds find wide applications in organic synthesis, and their dominance in some homolytic mechanisms has been referred to as the tyranny of tin. In particular, tin hydrides still hold the field in ring-closing cyclisations. The established use of organotin compounds as reactants has been developed in carbon-carbon crosslinking catalysed by transition metals, in carbohydrate synthesis, in the conjugative electrophilic substitution of allylstannanes, and in the of the reaction of allyltin compounds and organotin phenoxides catalysed by lithium perchlorate. They are used as catalysts in a number of reactions,
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particularly esterification and transesterification. Relatively new applications are in carbostannylation of multiple bonds, and in the anionic capture of the intermediates from palladium-catalysed ring-closing reactions. At the same time, there can be concerns about the disposal of organotin residues, and of traces of toxic organotin residues remaining in the products, and fluorous, polymer-bound, and solid-supported organotin compounds are being increasingly used to avoid the problem. We hope that the chapters of this book will enable readers to keep abreast of these rapidly developing fields. A. G. Davies, M. Gielen, K. H. Pannell, E. R. T. Tiekink
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List of Contributors
Esen Arkis
Izmir Department Institute of Technology, Chemical Engineering Department, Turkey
Hiram I. Beltr´an Departamento de Ciencias Naturales, Divisi´on de Ciencias Naturales e Ingenier´ıa, Universidad Aut´onoma Metropolitana – Cuajimalpa, Mexico Heloisa Beraldo Inorganic Medicinal Chemistry Laboratory, Departamento de Qu´ımica – Universidade Federal de Minas Gerais, Brazil Monique Biesemans
High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium
Werner J. Blank 89 Spectacle Lane, Wilton, CT, USA Russell J. Boyd Department of Chemistry, Dalhousie University, Halifax, Canada Raymundo Cea-Olivares
Instituto de Qu´ımica, Universidad Nacional Aut´onoma de Mexico, Mexico
Vadapalli Chandrasekhar
Department of Chemistry, Indian Institute of Technology Kanpur, India
Nikos Chaniotakis Iraklion, Greece
Laboratory of Analytical Chemistry, Department of Chemistry, University of Crete,
Tristram Chivers Department of Chemistry, University of Calgary, Canada Jean-Mathieu Chr´etien Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Paul A. Cusack Tin Technology, St. Albans, UK
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List of Contributors
Alwyn G Davies
Chemistry Department, University College London, UK
Geraldo M. de Lima Tin Chemistry Laboratory, Departamento de Qu´ımica – Universidade Federal de Minas Gerais, Minas Gerais, Brazil Antonio M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain
Dana J. Eisler Department of Chemistry, University of Calgary, Canada George Eng DC, USA
Department of Chemistry and Physics, University of the District of Columbia, Washington,
Pablo Espinet Spain
IU CINQUIMA/Qu´ımica Inorg´anica, Facultad de Ciencias, Universidad de Valladolid,
Norberto Farf´an Departamento de Qu´ımica, Facultad de Qu´ımica, Universidad Nacional Aut´onoma de Mexico, Mexico Miroslav Genov IU CINQUIMA/Qu´ımica Inorg´anica, Facultad de Ciencias, Universidad de Valladolid, Spain Kandasamy Gopal
Department of Chemistry, Indian Institute of Technology Kanpur, India
Ron Grigg Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry, Leeds University, UK T. Bruce Grindley Department of Chemistry, Dalhousie University, Halifax, Canada Imtiaz-ud-Din
Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
F. Ekkehardt Hahn Institut f¨ur Anorganische und Analytische Chemie, Westf¨alische WilhelmsUniversit¨at M¨unster, Germany Ionel Haiduc
Department of Chemistry, Babes-Bolyai University, Cluj-Napoca, Romania
Edward T. Hessell
King Industries Inc. Norwalk, CT, USA
Herbert H¨opfl Centro de Investigaciones Qu´ımicas, Universidad Aut´onoma del Estado de Morelos, Mexico Vojtech Jancik Instituto de Qu´ımica, Universidad Nacional Aut´onoma de Mexico, Mexico Bernard Jousseaume Klaus Jurkschat
Groupe Mat´eriaux, Universit´e Bordeaux 1, Talence, France
Lehrstuhl f¨ur Anorganische Chemie der Universit¨at, Dortmund, Germany
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List of Contributors
Jeremy D. Kilburn Cezary M. Kinart
xxi
School of Chemistry, University of Southampton, UK Department of Chemistry, University of Lodz, Poland
Wojciech J. Kinart Department of Organic Chemistry, University of Lodz, Poland Pascal G. Lacroix Laboratoire de Chimie de Coordination du CNRS, Toulouse, France Joseph B. Lambert Department of Chemistry, Northwestern University, IL, USA Erwan Le Grognec Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Fabio Marchetti Dipartimento di Scienze Chimiche, University of Camerino, Italy M. Mazhar Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan Yoshiyuki Mizuhata Institute for Chemical Research, Kyoto University, Japan M´onica Moya-Cabrera Mexico
Instituto de Qu´ımica, Universidad Nacional Aut´onoma de Mexico,
Partha P. Nag Department of Chemistry, Indiana University, Bloomington, IN, USA Mala Nath Department of Chemistry, Indian Institute of Technology Roorkee, India Junzo Otera Department of Applied Chemistry, Okayama University of Science, Japan Keith H. Pannell Department of Chemistry, University of Texas at El Paso, TX, USA Sergio Pascual
Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain
Claudio Pettinari Dipartimento di Scienze Chimiche, University of Camerino, Italy Vanja Pinoie High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium Kevin Poelmans High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium Jean-Paul Quintard Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Fran¸cois RiBOT
Chimie de la Mati`ere Condens´ee de Paris, Universit´e Pierre et Marie Curie, Paris
Rosa Santillan Departamento de Qu´ımica, Centro de Investigaci´on y de Estudios Avanzados del Instituto Polit´ecnico Nacional, Mexico.
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List of Contributors
Slavi C. Sevov Department of Chemistry and Biochemistry, University of Notre Dame, IN, USA Hemant K. Sharma Eiji Shirakawa Puja Singh
Department of Chemistry, University of Texas at El Paso, TX, USA
Department of Chemistry, Graduate School of Science, Kyoto University, Japan
Department of Chemistry, Indian Institute of Technology Kanpur, India
Xueqing Song Department of Chemistry and Physics, University of the District of Columbia, Washington, DC, USA V. Sridrahan Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry, Leeds University, UK Edward R.T. Tiekink Norihiro Tokitoh
Department of Chemistry, The University of Texas at San Antonio, TX, USA
Institute for Chemical Research, Kyoto University, Japan
Thierry Toupance Institut des Sciences Mol´eculaires, Groupe Mat´eriaux, University of Bordeaux 1, Institut des Sciences Mol´eculaires, France Margaret M. Whalen Department of Chemistry, Tennessee State University, Nashville, TN, USA Sarah R. Whittleton Department of Chemistry, Dalhousie University, Halifax, Canada Rudolph Willem High Resolution NMR Centre (HNMR), Vrije Universiteit Brussels, Belgium David R. Williams Bernd Wrackmeyer
Department of Chemistry, Indiana University, Bloomington, IN, USA Anorganische Chemie II, Universit¨at Bayreuth, Germany
David Young Eskitis Institute of Cell and Molecular Therapies, Griffith University, Queensland, Australia Alexander V. Zabula Institut f¨ur Anorganische und Analytische Chemie, Westf¨alische WilhelmsUniversit¨at M¨unster, Germany Fran¸coise Zammattio Laboratoire de Synth`ese Organique (LSO), UFR des Sciences et des Techniques, Universit´e de Nantes, France Julio Zukerman-Schpector
Department of Chemistry, Universidade Federal de S´ao Carlos, Brazil
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1 Introduction and Overview 1.1
Introduction
Alwyn G. Davies Chemistry Department, University College London, UK
1.1.1
History, Occurrence, Production, and Applications
Tin has been known as a metal since time immemorial, and the discovery, in about 3500 bc, that it formed a strong, hard alloy with copper, started the Bronze Age, which lasted until about 1200 bc. The abundance of tin in the Earth’s surface is about 2 ppm, significantly less than that of zinc (94 ppm), copper (63 ppm), or lead (12 ppm). The most important ore is cassiterite, SnO2 , which occurs as placer (alluvial) deposits. The breakdown of the current production of tin by area is shown in Figure 1.1.1. About 75% of the world’s production comes from China and South East Asia, and about 18% from South America, but the annual figures are sensitive to political, social, and economic factors.1 The cassiterite ore is obtained by dredging, open-cast mining, or gravel-washing, in which the ore is washed out of the deposit with high-pressure jets of water. The cassiterite has a density 2.5 times that of sand, and the ore is concentrated by gravity. It is roasted to remove arsenic and sulfur, and to convert metal sulfides into oxides, then it is reduced by smelting with coal or fuel oil in a reverberatory, rotary, or electric furnace (Equation 1.1.1). SnO2 + 2CO −−−→ Sn + 2CO2
(1.1.1)
An increasing amount of tin is also being recovered, by melting, from food and drink cans and industrial scrap. In 2005 and 2006, the total annual production of refined tin was about 350 000 tonnes. At the time of writing (April, 2008), demand exceeds supply, and the price of high-grade tin has just hit a record high of US$22 150/tonne. The applications of tin are shown in the pie chart in Figure 1.1.2. About half the production of tin is used in solders, and this is increasing with the increasing production of telecommunication and electronic equipment, and the need to eliminate lead, because of its toxicity. Tin Chemistry: Fundamentals, Frontiers, and Applications Edited by Marcel Gielen, Alwyn Davies, Keith Pannell and Edward Tiekink © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51771-0
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Tin Chemistry: Fundamentals, Frontiers and Applications China
37.2%
Indonesia 23.2% Malaysia
Figure 1.1.1
6.5%
Thailand
7.9%
Bolivia
4.0%
Brazil
2.5%
Peru
11.7%
Belgium
2.3%
Russia
1.1%
Other
3.7%
World production of tin
Conventional tin/lead solders have the approximate composition Sn63/Pb37 by weight, corresponding to the eutectic mixture, which is close to Sn3 Pb, with a melting point of 183 ◦ C. Lead-free solders are often composed of tin with 3–4% silver and 0.5–1% copper, and have a melting point of 215–220 ◦ C. Some 20% of the production of tin goes into tinplate, which is produced by hot-dipping or electroplating; its use in canning has reduced because of the increasing competition from aluminium cans, and protective polymer layers for steel cans. As an alloy with lead, tin has been used also in pewters, for making organ pipes, and, alloyed with copper, for making bronze. Babbitt metal, used in bearings, commonly contains about 90% of tin, together with a small amount of harder metals such as copper or antimony. About 14% of the production of tin goes into tin chemicals; a further breakdown is not possible because of the commercial sensitivity of the information. Tin tetrachloride and butyltin trichloride are used for coating glass with SnO2 (see Chapter 3.1), and float glass is produced on a molten pool of tin. The first organotin compound, diethyltin dichloride, was prepared by Frankland in 1849 by heating ethyl chloride with metallic tin, and this is often taken to mark the beginning of organometallic chemistry. The first application of organotin compounds came in about 1943, when they were used first for the stabilization of PVC against heat during processing, and a variety of industrial and biological applications were subsequently developed, although, in recent years these have been somewhat curtailed by concerns about toxicity. Solders 49.7% Tinplate 18.3% Chemicals 14.1% Brass and bronze 5.6%
Figure 1.1.2
Glass
1.8%
Other
10.4%
Applications of tin
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Introduction Table 1.1.1
3
Properties of tin
Property
Value
Property
Value
Atomic number Atomic mass Melting point Boiling point Density (white tin) Density (grey tin)
50 118.710 232 ◦ C 2625 ◦ C 5.769 g cm–3 7.280 g cm–3
Electronegativity Atomic radius Covalent radius van der Waals radius
1.96 (Pauling) 1.45 pm 1.41 pm 2.17 pm
In the last half century, there has been much more research activity in the organometallic chemistry of tin than in its inorganic chemistry, and this is reflected in the contents of this book. 1.1.2
The Element
Selected properties of the element are shown in Table 1.1.1. It is in Group 14 of the Periodic Table, with the electronic configuration [Kr] 4d10 5s2 5p2 ; its principal valence state is Sn(IV), though Sn(II) inorganic compounds are common, and many stannous organic compounds, with specially designed structures, have been prepared in recent years. Tin has 10 stable isotopes (Table 1.1.2), which is the largest number for any element, and results in very characteristic mass spectra. The 117 Sn and 119 Sn isotopes, each with spin 1/2, are used in NMR spectroscopy. The γ-active 119m Sn isotope, which is prepared by the neutron-irradiation of enriched 118 Sn, is used in M¨ossbauer spectroscopy. Metallic tin exists in two allotropes. White tin, or β-tin, is a silvery-white, electrically conducting, metal, with a distorted cubic structure. Below about 10 ◦ C, it slowly coverts into grey tin, or α-tin, with a 26% increase in volume, which creates excrescences on the surface, called tin pest or plague. α-Tin is a semiconductor with a diamond structure, with Hf = 2.51 kJ mol–1 compared with metallic tin.2
Table 1.1.2 Isotope
Tin isotopes Mass
Abundance (%)
Spin
112
111.90494
0.95
0
114
113.90296
0.65
0
115
114.90353
0.34
1/2
116
115.90211
14.24
0
117
116.90306
7.57
1/2
118
117.90179
24.01
0
119
118.90339
8.58
1/2
120
119.90213
32.97
0
122
121.90341
4.17
0
124
123.90524
5.98
0
Mass Spectrum
112
114
116
118
120
122
124
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Tin Chemistry: Fundamentals, Frontiers and Applications
White tin is inert to air at room temperature, but at 200 ◦ C it is oxidized to SnO2 . Samples for microanalysis for C, H, and N, by combustion in oxygen, are usually sealed in tin capsules. The heat of combustion of the tin (Hc = −142 kJ mol–1 ) raises the temperature from about 1000 ◦ C to 1800 ◦ C, and the SnO2 which is formed acts as an oxidation catalyst. Tin shows no reaction with water and dilute acids, but concentrated hydrochloric acid reacts to give SnCl2 and hydrogen, and concentrated sulfuric acid gives SnSO4 and SO2 . In ether, HCl gas reacts to give solvated H2 SnCl2 , which, together with HSnCl3 from HCl and SnCl2 , finds some use in organic synthesis. Hot aqueous alkali, MOH, reacts to give M2 [Sn(OH)6 ]. If electrons are added into the crystal structure, they lead to the breaking of the Sn–Sn bonds, each atom carrying an unshared electron pair, and ultimately to the formation of isolated anionic clusters with triangular faces (deltahedra), known as Zintl ions (e.g. 1 and 2; see Chapter 1.6).3 These compounds are diamagnetic and poor conductors. They can be prepared either by reduction of tin with an alkali metal or electrochemically, and are soluble in polar, basic solvents. For example, [Na+ ]4 [Sn9 ]4– can be prepared from the reaction of tin with sodium in ethylenediamine, or in the presence of a crown ether to associate with the sodium cations. The most common structure is a nine-atom cluster 2.4
Surprisingly little work appears to have been carried out on the reaction of these cluster anions with organic electrophiles, but electrically neutral organotin clusters can be prepared, usually by the reduction of organotin(II) compounds carrying bulky aryl groups. Examples are Sn5 Ar6 , with a propellane structure (3), and Sn8 Ar4 (4) and Sn8 Ar8 , with cubic structures.5
1.1.3
Structure and Bonding
Both the Sn(II) and Sn(IV) states are stable. The Sn(II) state uses mainly the 5p orbitals for bonding, leaving the unshared singlet pair in the largely 5s state, with a little p character, and compounds SnX2 (5, the stannylenes) have an XSnX angle of about 90–100◦ . These compounds are most stable when there are strongly electron-attracting ligands, which make loss of the remaining electron pair more difficult (e.g. :SnF2 , :SnCl2 ), or when the ligands X are very bulky, and sterically protect the tin against further ligation (e.g. :Sn[N(SiMe3 )2 ]2 ). Otherwise, oxidation readily occurs to the Sn(IV) state, where the tin is sp3 hybridized, and the SnX4 (8, stannane) molecule has tetrahedral symmetry. However, both the stannylenes and the stannanes have vacant 5d orbitals, which can accept one or more further ligands, The stannylenes readily form the pyramidal sp3 complexes :SnX3 (6), and the trigonal bipyramidal sp3 d complexes :SnX4 (7), and the stannanes form the trigonal bipyridamidal sp3 d complexes SnX5 (9) or octahedral sp3 d2 complexes SnX6 (10). All of these may carry charges corresponding to the charge of the new ligands X. These basic structures are often distorted, and higher coordination states are sometimes formed.
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Introduction
Sn X
Sn
X
X
X
X
X
X
Sn
X
X
X
X
X
p2
sp3
X sp3d
5
6
7
X X
Sn X
X
sp3
Sn X
X
Sn
X
X
X sp3d
sp3d2
9
10
8
5
X X
Structures of Sn(IV) compounds
Structures of Sn(II) compounds
The X groups themselves may act as these further ligands, resulting in intramolecular coordination, or, particularly in condensed phases, intermolecular association, to give oligomers or polymers. A variety of techniques are available for determining these structures [119m Sn M¨ossbauer spectroscopy, 117 Sn or 119 Sn NMR spectroscopy in the solid or liquid state (see Chapter 2.1),6 IR spectroscopy, X-ray diffraction etc.] and structural studies have been a major aspect of inorganic and organic tin chemistry. For example, in the vapor phase, Sn(II)F2 is a monomer with an FSnF angle of 94◦ (11),7 whereas, in the solid state, it exists as cyclic tetramers, held together by weaker Sn–F interactions (12).8 Within the ring, the average FSnF angle is 83.7◦ , and outside the ring it is 82.8◦ . In the gas-phase, Me3 SnCl is a ˚ in solution, the monomer is in equilibrium with oligomers, tetrahedral monomer with r SnCl 2.306(3) A; but in the crystal it is associated into a zig-zag polymer (13), with approximately trigonal bipyramidal ˚ 9 tin, and r SnCl 2.43 and 3.27 A. F Sn F F Sn F
F
F
Sn
Sn F
F Sn
Me F
Me
Cl Sn Me Me
Cl
Sn
Cl
Me Me
F
11
12
13
Some values of bond lengths and bond dissociation energies are given in Table 1.1.3, but it must be emphasized that these are only indicative values, which are dependent on the physical state. 1.1.4
Organotin Compounds, Rn Snm
The various known organotin species and their structures, where the tin is bonded to only carbon or tin, and without functional groups, are shown in Table 1.1.4. •+ Stannane radical cations SnMe•+ 4 (and SnH4 ) have been generated in frozen Freon matrices by irradiation with γ -rays. The ESR spectra, with the backing of MNDO and PM3 calculations, have been interpreted as implying distortion of the tetrahedral structure of the stannane into a C3v configuration, with an almost planar trigonal base, and one long one-electron SnMe bond. Alkylstannane radical anions can be prepared by γ -irradiation of the stannane in a matrix of Me4 Si, and arylstannane radical anions by reduction with an alkali metal. The radical anion, Me4 Sn•– , appears to have a trigonal bipyramidal structure, with the unpaired electron located in an equatorial, largely sp2 , orbital.
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Tin Chemistry: Fundamentals, Frontiers and Applications
Table 1.1.3
SnX bond lengths, dissociation energies, and stretching frequencies
Bond
r/pm
Me3 Sn H Me3 Sn Me Me3 Sn Et Me3 Sn CH CH2 Me3 Sn Ph Me3 Sn SnMe3 R2 Sn SnAr2 RSn SnR Me3 SnCl Me3 SnBr Me3 SnI Me3 SnOH a
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For (CH CH2 )4 Sn.
b
For Ph4 Sn.
171 218 220 212a 214b 277.6 ca. 280 267–307 235.1 249 272 196 c
BDE/ kJ mol–1 322 ±17 295 ±17 281 340 358 291 90c — 425 ±7 381 ±17 320 ±17 488 ±17
ν/cm–1 1846 526, 506 — — 241 192 — — 331 234 189 3620 (OH)
For R = (Me3 Si)2 CH
Hypervalent pentaorganostannate anions, such as Me5 Sn– , are formed when an organolithium compound is added to a tetraorganostannane in the presence of a ligand to solvate the lithium cation. The 119 Sn NMR signal occurs about 300 ppm upfield from that of the parent stannane. The six isomeric Phn Me5−n Sn− anions can be observed in equilibrium, and the NMR spectra imply that the anions have a trigonal bipyramidal structure, with the phenyl groups in the apical positions.10 These anions are formed particularly readily from the stannacyclopentanes and stannacyclopentadienes, perhaps because ring strain is relieved by rehybridization of the tin. The UV spectra of the oligostannanes, (R2 Sn)n , show a strong absorption maximum, with a red shift with increasing chain length. Conjugation between the σSnSn bonds produces a series of molecular orbitals analogous to the π-orbitals of a conjugated polyene. The bonding orbitals have no nodes at the midpoints of bonds, and an increasing number of nodes at the tin atoms; the antibonding orbitals have nodes at the midpoints of bonds, and again an increasing number of nodes at tin. The HOMO has a node at each tin atom, as shown in Table 1.1.4. A metal-like electronic band is formed, with the implication of useful electronic and optical properties (see Chapter 3.8).11 The three-coordinate anion Ph3 Sn– is pyramidal, with an average CSnC angle of 96.9◦ , and the electron pair in a predominantly sp3 orbital.12 The hindered SnSi-bonded anion, (But2 MeSi)3 Sn– , is less pyramidal, with average SiSnSi angles of 111.6◦ , presumably because of the increased steric hindrance.13 Prolonged attempts to characterize free organotin cations culminated in 2003, in the isolation of the tris(2,4,6-triisopropylphenyl)stannyl cation by the reaction shown in Equation (1.2) (see Chapter 2.7).14 (1.1.2) The cation is planar about the tin, with the aryl rings twisted out of the plane, propeller-fashion, by an average of 61.1◦ ; In the NMR spectrum, the value of δSn is 714. The SiSn-bonded cation (But2 MeSi)3 Sn+ is similarly planar, and shows δSn at 2653 ppm.15
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Table 1.1.4 Rn Snm
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Structures of compounds with only SnSn or SnC bonds Structure
Evidence
Name
R4 Sn
X-Ray
Stannane
R4 Sn•+
ESR
Stannane radical cation
R4 Sn•−
ESR
Stannane radical anion
R3 Sn-SnR3
X-Ray
Distannane
(R2 Sn)n
X-Ray
Oligostannane
R5 Sn−
NMR
Hypervalent stannate anion
R3 Sn−
X-Ray
Stannate anion
R3 Sn+
X-Ray
Stannylium cation (see Chapter 2.7)
R3 Sn•
ESR
Stannyl radical
R2 Sn:
X-Ray
Stannylene (see Chapter 2.8)
Cp2 Sn:
X-Ray
Stannocene
R2 Sn SnR2
X-Ray
Distannene (see Chapter 2.9)
RSn SnR
X-Ray
Distannyne (see Chapter 2.9)
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Tin Chemistry: Fundamentals, Frontiers and Applications
In a radical centered on an element X, the magnitude of the ESR hyperfine coupling to X, a(X), if X has nuclear spin, gives a measure of the s-electron density at the nucleus. In a planar, sp2 hybridized radical, X will lie in the nodal plane of the p orbital which contains the unpaired electron, and show a low coupling constant. On the other hand, a pyramidal, sp3 hybridized radical, with the unpaired electron in an sp3 orbital with a substantial amount of s character, will show a large value of a(X). Simple stannyl radicals, R3 Sn•, have large a(119 Sn) values, e.g. Me3 Sn•,161.1 mT; Ph3 Sn•,186.6 mT, implying that the radicals are pyramidal and not planar, as alkyl radicals are. This is supported by the fact that a(Sn) shows a negative temperature coefficient, and that optically active organotin compounds retain their asymmetry when they react through intermediate stannyl radicals. Only one stannyl radical has been isolated and had its structure determined by X-ray diffraction. The reaction of But2 MeSiNa with SnCl2 gives the radical (But2 MeSi)3 Sn•, which shows a(117/119 Sn) 32.9 mT, very much less than the value for the simpler organostannyl radicals. In the crystal, it has a trigonal planar structure (like the corresponding cation), which is doubtless imposed by the the steric requirements of the bulky silyl groups.15 Simple alkyl and aryl stannylenes, R2 Sn:, exist only as short-lived intermediates, but kinetic stability can be achieved with bulky substituent groups. The first to be isolated was [(Me3 Si)2 CH]2 Sn: (‘Lappert’s stannylene’).16 In the gas phase, it is a singlet monomer with the angle SiSnSi 97(2)◦ , but, in solution, the monomer is in equilibrium with the (distannene) dimer. Several other persistent stannylenes have been prepared, usually from an aryllithium compound and SnCl2 . Typical aryl groups are 2,4,6-triisopropylphenyl, 2,4,6-tri-t-butylphenyl, and 2,4,6-tristrifluoromethylphenyl. They can be regarded as having notionally sp2 hybridized tin, with a vacant 5p orbital (Table 1.1.4), and can behave as both Lewis acids and Lewis bases (see Chapter 2.9). If suitable intramolecular substitutents, particularly amino groups, are present, the stannylenes may form pyramidal (sp3 ) 3-coordinate complexes, or trigonal bipyramidal (sp3 d) complexes. Stannocene, Cp2 Sn:, similarly can be described as having sp2 hybridized tin, with the non-bonding electron pair as a phantom ligand. The angle between the planes of the cyclopentadienyl rings is 46◦ , but if the rings contain alkyl substituents, this angle decreases.17 Bis(pentamethylcyclopentadienyl)tin(II) shows an angle of 36◦ , and in the bis(pentaphenylcyclopentadienyl) compound it is zero, with parallel rings. The distannene dimer of Lappert’s stannylene, [(Me3 Si)2 CH]2 Sn Sn[CH(SiMe3 )2 ]2 , has an SnSn ˚ with a CSnC angle of 109.2◦ , and an angle of 41◦ between the SnSn bond and bond length of 2.768(1) A, the CSnC plane. The other known distannenes have a similar trans structure, with SnSn bond lengths ˚ (see Chapter 2.9). The bonding is interpreted in terms of mutual overlap of the sp2 orbital up to 3.09 A carrying the electron pair, with the vacant 5p orbital, as illustrated in Table 1.1.4. A few stannenes, R2 Sn CR 2 , have been prepared, but not enough yet to allow confident interpretation of the nature of the bonding. The distannynes, ArSn SnAr, Ar = 2,6-(2,6-Pri2 C6 H3 )2 C6 H3 , and Ar = 2,6-(2,4,6-Pri3 C6 H2 )2 C6 H3 ˚ and a CSnSn angle of 125.2◦ , and have trans-bent structures, with an SnSn bond length of 2.67 A ◦ ˚ distances 2.78–2.82 A and angles 93.6–98.0 , respectively. This bonding may be interpreted in terms of one σ -bond, and two of the type described above for the distannenes, as shown in Table 1.1.4.18,19 These bond lengths and angles, however, and indeed the mode of bonding, are very sensitive to the structure of the aryl groups. Even the introduction of an Me3 Si group into the 4-position of the phenyl ring of the former of these two compounds appears to change the structure into one closer to two singly bonded Ar2 Sn: molecules.20 Reduction of the distannyne with an alkali metal gives the radical anion and then the dianion, ˚ and a CSnSn angle of 107.5◦ . ArSn SnAr= 2K+ , with an SnSn distance of 2.78 A
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Introduction
1.1.5
9
Organotin Compounds With Electronegative Ligands, Rn SnX4-n
Some examples of organotin compounds with electronegative ligands, which are intra- or intermolecularly hypercoordinated, are given in formulae 14–19. The complex of pyridine with trimethyltin chloride, with trigonal bipyramidal coordination, as shown in 14, was the first such compound for which the structure was determined by X-ray diffraction.21 The ligand may be present as a functional group in an aromatic ring or alkyl chain on the tin (e.g. 15), and stannylenes, which behave as both Lewis acids and Lewis bases, may exhibit a similar intra-molecular association (e.g. 16).
N Me Me 2.43 Å
Sn Me Cl
14
NMe2 Sn Br
15
Ph Me
(Me3Sn)2C
N SnSn(SiMe3)3
16
The carboxylates R3 SnOCOR , in the crystal, are usually inter-molecularly bonded through the carbonyl groups into linear polymers, (e.g. 17)22 and are 4-coordinate monomers only when the R groups are bulky (e.g. phenyl, cyclohexyl, or neophyl).23,24 When difunctional organotin compounds R2 SnX2 , are hydrolyzed, the first isolable compound is often the difunctional distannoxane, XR2 SnOSnR2 X (see Chapter 2.10).25 These compounds have attracted a lot of attention because of their ability to catalyze reactions such as esterification and transesterification. When R is a simple alkyl or aryl group, these distannoxanes are associated into dimmers (e.g. 18),26 but many more complex variants on this theme have been synthesized, largely in a search for more effective catalysts. Mono-organotin oxo compounds assemble into larger oligomeric clusters (see Chapter 3.2). In the crystal, trimethyltin methoxide is a linear polymer, and dimethyltin dimethoxide is a dimer. The cyclic dialkyl-1,3,2-dioxastannolanes, which are formed from 1,2-diols, are important in regiospecific synthesis, particularly in carbohydrate chemistry. Their degree of association is determined by steric factors: in the solid state, 1,1-di-n-butyl-1,3,2-dioxastannolane is a 6-coordinate ribbon polymer, but the di-tert-butyl analog is a 5-coordinate dimer (19). Methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-αd-glucopyranoside is a 5-cordinate dimer, but methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-α-dmannopyranoside is a pentamer containing three, medial, 6-coordinate and two, terminal, 5-coordinate tin atoms.
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1.1.6
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Tin Chemistry: Fundamentals, Frontiers and Applications
Preparation of Organotin(IV) Compounds
Scheme 1.1.1 illustrates the principal routes into the major classes of organotin(IV) compounds through the nucleophilic alkylation of tin tetrachloride with an organometallic reagent. In the laboratory, this is usually a Grignard reagent or an organolithium compound, RM. The R group may be alkyl, allyl, aryl, alkenyl, or alkynyl. Unless R is very bulky, it is difficult to stop the reaction at the stage of an organotin halide, and the product is usually R4 Sn. In industry, the reaction is usually carried out with the cheaper alkylaluminium compounds, which need no ethereal solvent, and, as these are prepared by the Ziegler growth reaction between Et3 Al and ethylene, it is the organotin compounds with alkyl groups having even numbers of carbon atoms, such as butyl and octyl, that are readily available. Most commonly, organic syntheses are carried out with n-butyltin compounds, such as tri-n-butyltin hydride or di-n-butyltin dichloride, because they are cheaper and less toxic than the corresponding methyltin compounds. Under the appropriate conditions, the alkylation with R3 Al can be stopped at the stage of an intermediate alkyltin halide. Cyclopentadienyltin(IV) compounds can be prepared by treating tin tetrachloride, or an organotin chloride, with CpNa. These compounds are fluxionally σ -bonded.17 They are photosensitive, and the stannanes CpSnR3 , on irradiation with UV light, undergo homolysis to give Cp• and R3 Sn• radicals. Many additives have been investigated for promoting the direct reaction of organic halides with tin metal, but the reaction has limited use. Dimethyltin dichloride can be prepared from the reaction of methyl chloride with molten tin, but, in the laboratory, the reaction is used principally for preparing diallyl- or dibenzyl-tin dichlorides. The reaction between tin(II) halides and organic halides to give organotin trihalides, similarly finds little practical application. The tetraorganotin compounds are converted into the organotin chlorides by the Kocheshkov reaction, which involves the disproportionation reaction with SnCl4 , the basis of which is the non-additivity of bond dissociation energies caused by the interaction of RSn with RSn, RSn with SnCl, and SnCl with SnCl bonds. The chlorides can then be converted into other functional compounds Rn SnX4-n , by anion exchange. The dialkyltin dithiolates, (e.g.(n-C8 H17 )2 Sn(SCH2 CO2 CH2 CHEtBu)2 , ‘isooctylthioglycollate’), and maleates (e.g. [n-Bu2 SnOCOCH CHCO2 ]n ), are used for stabilising PVC against elimination of HCl when it is processed at high temperature. Di-n-butyltin dilaurate is used in catalyzing the formation of polyurethanes from diols and diisocyanates, though, for some purposes, stannous octoate is preferred. Hydrolysis of the halides and other functionally substituted compounds gives the hydroxides, which are often unstable and spontaneously dehydrate to give the oxides. The first hydrolysis products of the difunctional compounds R2 SnX2 are usually the difunctional distannoxanes, XR2 SnOSnR2 X, which have attracted much attention (see above). Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides.27 These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. R3 Sn• + R X −→ R3 SnX + R •
(1.1.3)
R • + R3 SnH −→ R H + R3 Sn•
(1.1.4)
R3 Sn• + C C −→ R3 SnC−C• R3 SnC−C• + R3 SnH −→ R3 SnC−CH + R3 Sn•
(1.1.5) (1.1.6)
RSnCl 3 L2
R2 SnCl 2L 2
R 2SnH 2
L
L
HO -
X-
HO -
X-
X-
HO -
R2 SnCl 2
R3 SnCl
RSnCl 3
LiAlH 4
L
LiAlH 4
RX
RSn(OH)Cl 2
RSnX3
HO -
SnCl 2
Sn
(R2SnO) n
RI
[RSn(OH)O] n
(R2SnO)n
R2 SnX 2
R 2 SnX 2
R 3 SnOSnR 3
(X = OCOR', OR', NR' 2 , SR' etc)
XR2 SnOSnR 2 X
R2 SnXCl
R3 SnOH
R3 SnX
Introduction
Scheme 1.1.1 Synthesis of organotin(IV) compounds based on alkylation of SnCl4 with an organometallic reagent, and the Kocheshkov redistribution reaction
or Pdo
base
R3 SnH
SnCl 4 200 o C
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R 3Sn C C H
R3 Sn C C
Hydrostannation
H CC
R 3 SnH
LiAlH4
R3 SnCl
C C
Pd o
BuLi, LDA, KOBut, etc M
X
R 3Sn hν
R 3SnSnR 3
R 3SnM
R3 SnX
R'X
R 3SnR' Scheme 1.1.2 compounds
Synthesis of organotin compounds based on the reactions of tin hydrides or stannylmetallic
Carbostannation of alkynes, and 1,2- and 1,3-dienes can be brought about by acyl, allyl, or alkynylstannanes in the presence of a nickel(0) catalyst.28 Reaction of the hydrides with a base or with a Pd0 compound leads to the elimination of hydrogen, and the formation of Sn Sn-bonded oligomers or polymers. The principal alternative route into the organotin(IV) manifold is based on the reaction of tin hydrides or stannylmetallic compounds, as shown in Scheme 1.1.2. The most common (radical) mechanism of hydrostannation of alkenes and alkynes with organotin hydrides is outlined, for alkenes, in Equations (1.1.5) and (1.1.6). Hydrostannation can also be catalyzed by palladium compounds. Other tin hydrides may react by a polar rather than a radical mechanism. A series of PVC stabilisers (the Estertins) have been developed by the AKZO company, and which are prepared by the reaction of HCl with metallic tin or SnCl2 , to give the solvated chlorotin hydrides H2 SnCl2 and HSnCl3 , respectively, which add, in a heterolytic Michael fashion to acrylic or propargylic esters [e.g. Equations (1.7) and (1.8)];29 the chlorides are then converted by anion exchange into the thioglycollates. H2 C CHCO2 Me Et2 O Sn + HCl −−−→ [H2 SnCl2 ] −−−−−−−−−−−−−−→ Cl2 Sn(CH2 CH2 CO2 Me)2 HC CCO2 Me Et2 O SnCl2 + HCl −−−→ [HSnCl3 ] −−−−−−−−−−−−−−→ Cl3 SnCH CHCO2 Me
(1.1.7) (1.1.8)
Tin hydrides react with strong metallic bases such as BuLi, NaH, LDA, or RMgX, by deprotonation to give the corresponding stannylmetallic compounds, R3 SnM, which will react with organic electrophiles to create new Sn C bonds [e.g. Equations (1.1.9) and (1.1.10)]. Me3 SnLi + CCl4 −→ (Me3 Sn)4 C Bu3 SnMgCl +
−→ Bu3 SnCH2 CH2 OH
(1.1.9) (1.1.10)
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Introduction
13
SnCl2 RCp2SnX
CpM
RX
R*Sn SnR*
RM M
CpSnX
HX
.. RSnCl
Cp2Sn: X2
RLi
Cp2SnX2
(R2Sn)n R2Sn ..
Cp = cyclopentadienyl
Scheme 1.1.3
SnR*2
R*2 Sn CR2
Me N R*2Sn
R*2 Sn R2 C..
MeN3
1.1.7
.. R*SnH
SnR*2 SnR*2
R* = bulky group
Preparation of lower-valence organotin compounds
Preparation of Organotin Compounds in Lower Valence States
Routes to lower-valence organotin compounds are shown in Scheme 1.1.3. Alkylation or arylation of stannous chloride gives stannylenes, R2 Sn:, which normally self react to give linear or cyclic oligomers, but if the R groups are very bulky (t-butylphenyl, or 2,6-bis(2,4,6-triisopropylphenyl)phenyl), kinetically stable stannylenes can be obtained (see Chapter 2.8). In the solid state, the stannylenes usually dimerize to give the distannenes, R*2 Sn SnR*2 , where R* denotes a bulky group. Reaction of a stannylene with a carbene gives a stannene, R*2 Sn CR2 . Reduction of chlorostannenes with potassium or sodium has recently given the first distannynes, R*Sn SnR* (see Chapter 2.9). Stannocene, Cp2 Sn: (see above), was prepared in 1956 as an air-sensitive white solid, m.p. 105 ◦ C, by the reaction of cyclopentadienyllithium with SnCl2 .30 1.1.8
Literature
There are no monographs on the inorganic chemistry of tin, but the topic is covered in Comprehensive Inorganic Chemistry (1973, E.W. Abel)31 , in both editions of Comprehensive Coordination Chemistry (1987, P.G. Harrison;32 2004, J. Parr33 ), and in Encyclopedia of Inorganic Chemistry (J.L. Wardell),34 and there is a chapter on the inorganic chemistry in both editions of Chemistry of Tin (P.G. Harrison; P.J. Smith).35,36 Organotin chemistry is much better served. It was first surveyed in Krause and von Grosse’s Die Chemie der Metal-organischen Verbindungen,37 published in 1937, and reprinted in 1965. The first generally available article in English was Ingham, Rosenberg, and Gilman’s Chemical Review,38 in 1960 – a time when the field was beginning to expand rapidly. Copies were widely circulated, and introduced the subject to a wide audience. Neumann’s Die Organische Chemie des Zinns, appeared in 1967,39 and was translated and updated as The Organic Chemistry of Tin in 1970,40 and Poller’s The Chemistry of Organotin Compounds,41
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also appeared in 1970. This was supplemented by Kocheshkov, Zemlyanskii, Sheverdina, and Panov’s Metodi Elemento-organiicheskoi Khimii, Germanii, Olovo, Svinesh (1968) for readers in Russia,42 where research into organotin chemistry was very active. By 1971, the subject could not be covered comprehensively by one author, and Organotin Chemistry, edited by Sawyer, appeared in three volumes with articles by 14 authors.43 The three editions of Comprehensive Organometallic Chemistry included chapters on tin (1982, 1995, and 2007 respectively);44–46 2000 copies of the first of these chapters were distributed, and again helped to popularize the subject. Two volumes in the series of reviews edited by Patai (1978) and by Rappoport (2002), and the two editions of Chemistry of Tin, edited by Harrison35 and by Smith,36 deal largely with organotin compounds. There is a short review in Encyclopedia of Inorganic Chemistry.47 Much of the activity in the organotin field has been directed towards the use of organotin compounds in organic synthesis. This topic was covered in Pereyre, Quintard, and Rahm’s Tin in Organic Synthesis 1987,48 and supplemented by Jousseaume and Pereyre49 in Smith’s Chemistry of Tin. Articles in Houben Weyl,50 and its successor, Science of Synthesis,51 also deal mainly with the use of organotin compounds in synthesis. Organotin compounds are covered in 25 volumes of Gmelin, which is also available on line, in a different format, and there have been two editions (1997 and 2004) of Davies’ Organotin Chemistry.52,53 This last book gives a more extensive bibliography. References 1. I am grateful to the ITRI Ltd (www.itri.co.uk) for providing current data on the production and applications of tin. 2. M. J. P. Musgrave, Proc. Roy. Soc. 227A, 503 (1963). 3. T. F. F¨assler, in Metal Clusters in Chemistry, P. Braunstein, L. A. Ora, P. R. Raithby (Eds), Wiley-VCH, Weinheim, 1999, Vol. 3, 1612–1642. 4. T. F. F¨assler, Coord. Chem. Rev. 215, 347 (2001). 5. N. Wiberg and P. Power, in Molecular Clusters of the Main Group Elements, M. Driess and H. N¨oth (Eds), Wiley-VCH, Weinheim, 200, 188–208. 6. J. C. Martins, M. Biesemans, and R. Willem, Progr. NMR Spectrosc. 36, 271 (2000). 7. R. H. Hauge, J. W. Hastie, and J. L. Margrave, J. Mol. Spectrosc. 45, 420 (1973). 8. R. C. McDonald, H. H. K. Hau, and K. Eriks, Inorg. Chem. 15, 762 (1976). 9. M. B. Hossain, J. L. Lefferts, K. C. Molloy, D. Van der Helm, and J. J. Zuckerman, Inorg. Chim. Acta 36, L409 (1979). 10. H. J. Reich, and N. H. Phillips, Pure Appl. Chem. 59, 1021 (1987). 11. L. R. Sita, K. W. Terry, and K. Shibata, J. Am. Chem. Soc. 117, 8049 (1995). 12. T. Birchall, and V. Manivannan, J. Chem. Soc., Dalton Trans. 2671 (1985). 13. T. Fukawa, M. Nakamoto, V. Y. Lee, and A. Sekiguchi, Organometallics 23, 2376 (2004). 14. J. B. Lambert, L. Lin, and S. Keinan, J. Am. Chem. Soc. 125, 6022 (2003). 15. A. Sekiguchi, T. Fukawa, V. Y. Lee, and M. Nakamoto, J. Am. Chem. Soc. 125, 9250 (2003). 16. P. J. Davidson, D. H. Harris, and M. F. Lappert, J. Chem. Soc., Dalton Trans. 2268 (1976). 17. P. Jutzi, and N. Burford, Chem. Rev. 99, 969 (1999). 18. N. Wiberg, S. K. Vasisht, G. Fischer, and P. Mayer, Z. Anorg. Allgem. Chem. 630, 1823 (2004). 19. P. P. Power, Appl. Organomet. Chem. 19, 488 (2005). 20. R. C. Fischer, L. Pu, J. C. Fettinger, M. A. Brynda, and P. P. Power, J. Am. Chem. Soc. 128, 11366 (2006). 21. I. R. M. Beattie, G. P., and Hulme, R. Chemistry & Industry, 1429 (1962). 22. M. A. Saeed, A. Badshah, M. K. Rauf, D. C. Craig, and S. Ali, Acta Cryst. E62, m469 (2006). 23. E. R. T. Tiekink, Appl. Organomet. Chem. 5, 1 (1991).
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Introduction 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53.
15
E. R. T. Tiekink, Trends in Organomet. Chem. 1, 71 (1994). A. G. Davies, J. Chem. Research, 309 (2004). P. G. Harrison, M. J. Begley, and K. C. Molloy, J. Organomet. Chem. 186, 213 (1980). A. G. Davies, J. Chem. Res. 141 (2006). Y. Nakao, E. Shirakawa, T. Tsuchimoto, and T. Hiyama, J. Organomet. Chem. 689, 3701 (2004). R. E. Hutton, and V. Oakes, in Organotin Compounds: New Chemistry and Applications, ed. J. J. Zuckerman (Ed.), American Chemical Society: Washington, 1976, 123–133. E. O. Fischer, and H. Grubert, Z. Naturforsch. 11B, 423 (1956). E. W. Abel, in Comprehensive Inorganic Chemistry, J. C. Bailar, H. J. Emeleus, R. S. Nyholm, and A. F. Trotman-Dickenson, (Eds), Pergamon, Oxford, 1973, Vol. 2, Chapter 17, 43–104. P. G. Harrison and T. Kikabbai, in Comprehensive Coordination Chemistry, G. Wilkinson, R. D. Gillard, and J. A. McCleverty (Eds), Pergamon, Oxford, 1987, Vol. 2. J. Parr, in Comprehensive Coordination Chemistry II, J. A. McCleverty and T. J. Meyer (Eds), Elsevier, Oxford, 2004, Vol. 3, 545–608. J. L. Wardell, in Encyclopedia of Inorganic Chemistry, R. B. King (Ed.), Wiley, Chichester, 2005, Vol. 9, 5590–5605. P. G. Harrison (Ed.), Chemistry of Tin. Blackie, Glasgow, 1989. P. J. Smith (Ed.), Chemistry of Tin, 2nd edn., Blackie, London, 1998. E. Krause, and A. von Grosse, Die Chemie der Metal-organischen Verbindungen. Borntraeger, Berlin, 1937. R. K. Ingham, S. D. Rosenberg, and H. Gilman, Chem. Rev. 60, 459 (1960). W. P. Neumann, Die Organische Chemie des Zinns. Ferdinand Enke Verlag, Stuttgart, 1967. W. P. Neumann, The Organic Chemistry of Tin. John Wiley & Sons, Ltd. Chichester, 1970. R. C. Poller, The Chemistry of Organotin Compounds. Logos Press, London, 1970. K. A. Kocheshkov, N. N. Zemlyansky, N. I. Sherevdina, and E. M. Panov, Metodi Elemento-organicheskoi Khimii. Germanii, Olovo, Svine. Nauka, Moscow, 1968, 162–530. A. K. Sawyer, Organotin Compounds. Marcel Dekker, New York, 1971. A. G. Davies, and P. J. Smith, in Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone, and E. W. Abel (Eds), Pergamon Press, Oxford, 1982, Vol. 2, 519–627. A. G. Davies, in Comprehensive Organometallic Chemistry II, E.W. Abel, F. G. A. Stone, and G. Wilkinson, Pergamon Press, Oxford, 1995, Vol. 2, 217–303. A. G. Davies, in Comprehensive Organometallic Chemistry III, R. H. Crabtree and D. M. Mingos (Eds), Elsevier, Oxford, 2007, Vol. 3, 809–883. J. L. Wardell and G. M. Spencer, in Encyclopedia of Inorganic Chemistry, R. B. King (Ed.), Chichester, 2005, Vol. 9, 5590–5605. M. Pereyre, J. P. Quintard, and A. Rahm, Tin in Organic Synthesis. Butterworth, London, 1987. B. Jousseaume, and M. Pereyre, in Tin, 2nd edn., P. J. Smith (Ed), Chapman and Hall, London, 1998, 290–387. G. B¨ahr and S. Pawlenko, in Houben Weyl, Methoden der Organische Chemie, Thieme, Stuttgart, 1978, Vol. 13/6. E. J. Thomas, in Science of Synthesis. Vol. 5. Compounds of Group 14 (Ge, Sn, Pb), M. G. Moloney (Ed.), Thieme, Stuttgart, 2003, 195–607. A. G. Davies, Organotin Chemistry. VCH, Weinheim, 1997. A. G. Davies, Organotin Chemistry, 2nd edn. Wiley-VCH, Weinheim, 2004.
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2 Fundamentals in Tin Chemistry 2.1
NMR Spectroscopy of Tin Compounds
Bernd Wrackmeyer Anorganische Chemie II, Universit¨at Bayreuth, D-95440 Bayreuth, Germany
2.1.1
Introduction
The element tin possesses numerous naturally occurring isotopes, of which three (115 Sn, 117 Sn, 119 Sn) are magnetically active spin-1/2 nuclei (Table 2.1.1). Owing to their fairly high natural abundance and favorable nuclear magnetic properties, 117 Sn and 119 Sn nuclei are highly attractive for NMR experiments, both in solution and in the solid state. Usually, the 119 Sn isotope is the target, although 117 Sn NMR spectra can be measured likewise if desired. 119 Sn NMR has become an extremely valuable, sometimes indispensable, routine tool in almost every field of tin chemistry. This development is aided by various sophisticated NMR techniques which have greatly enhanced the information that can be obtained from NMR measurements of spin-1/2 nuclei in general.1 The progress in 119 Sn NMR has been reviewed repeatedly.2–8 Moreover, the presence of magnetically active tin isotopes is also mirrored in the NMR spectra of other nuclei, such as 1 H, 13 C, 19 F, or 31 P to name some prominent examples. The volume of useful 119 Sn NMR data produced so far is prohibitive for a complete assessment. However, this overview attempts to give a general outline of the field, and shows applications of NMR measurements in solution and in the solid state, aiming at representative and stimulating examples that can be studied by standard experiments. In this context, NMR parameters, such as chemical shifts δ 119 Sn (in ppm; this attribute will not be further used here) and coupling constants n J (119 Sn,X) (in Hz) are of great importance. This review excludes 119 Sn NMR parameters of paramagnetic compounds, alloys, and various other inorganic solids. 2.1.2
Experimental
Referencing
With modern NMR spectrometers the definition of a fixed frequency (Table 2.1.1) as the reference for chemical shifts δ is always recommended9 when an internal reference cannot be generally applied. The Tin Chemistry: Fundamentals, Frontiers, and Applications Edited by Marcel Gielen, Alwyn Davies, Keith Pannell and Edward Tiekink © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51771-0
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Table 2.1.1 NMR properties of some important spin-1/2 nuclei including the magnetically active tin isotopes 115 Sn, 117 Sn and 119 Sna Nucleus 1
H C 15 N 19 F 29 Si 31 P 77 Se 103 Rh 115 Sn 117 Sn 119 Sn 125 Te 183 W 187 Os 195 Pt 199 Hg 207 Pb 13
N. A. (%) 99.985 1.108 0.37 100 4.70 100 7.58 100 0.35 7.61 8.58 6.99 14.28 1.64 33.8 16.84 22.6
RCb
γ [107 rad s−1 T−1 ]
[MHz]
26.7522 6.7283 −2.7126 25.1815 −5.3190 10.8394 5.1214 −0.8468 −8.8014 −9.589 −10.0318 −8.5087 1.1283 0.6193 5.8383 4.8458 5.6264
100.000000 25.145020 10.136767 94.094003 19.867187 40.480747 19.071523 3.160000 32.718780 35.632295 37.290665 31.549802 4.166398 2.282343 21.400000 17.910841 20.920597
5.67 103 1.00 2.19 10−2 4.73 103 2.10 3.77 102 3.02 0.180 0.707 19.9 25.7 12.8 6.08 10−2 1.15 10−3 19.9 5.68 11.9
Reference standard SiMe4 , 1% in CDCl3 SiMe4 1% in CDCl3 MeNO2 (neat) CFCl3 (neat) SiMe4 1% in CDCl3 H3 PO4 . 85 % aq Me2 Se (neat) No compound SnMe4 (neat) SnMe4 (neat) SnMe4 (neat) Me2 Te (neat) Na2 WO4 in D2 O, 1 M OsO4 (CDCl3 ) No compoundc HgMe2 (neat)d PbMe4 (neat)
a
Most data taken from Reference 9. R means receptivity relative to that of 13 C at natural abundance. δ Pt([PtCl6 ]2− ) = +4533. d (199 Hg) = 17.870535 MHz for Hg(ClO4 )2 (0.1 M in 0.1 M HClO4 ). b C
c 195
frequency (1 H) for TMS = 100.000 000 MHz can be compared with the 1 H NMR frequency of TMS for individual NMR spectrometers. Thus, it is not necessary to handle toxic SnMe4 as a chemical, and the choice of the fixed frequency (119 Sn) for neat SnMe4 (Table 2.1.1) guarantees precise reproduction of δ 119 Sn data. Other referencing techniques should be discouraged. For solid-state 119 Sn NMR spectra, indirect referencing is required,10,11 and several solid tin compounds can be used. A sharp 119 Sn NMR signal is observed for solid Sn(C6 H11 )4 (δ 119 Sn −97.3) under conditions of MAS, cross-polarization, and 1 H decoupling. This is a commercial chemical, easy to handle in air and the FID after even one pulse can be used to optimize experimental conditions. Techniques for Measurement of 119 Sn Resonances
As a result of the relatively high sensitivity to the NMR experiment (Table 2.1.1), 119 Sn resonance signals can be observed in most cases directly, using the PFT technique. If 1 H decoupling is required, it may prove necessary to use inverse gated 1 H decoupling in order to suppress the NOE (maximum NOE ηmax = −1.34) which may partially or completely cancel 119 Sn NMR signals. Frequently, there is a significant dependence of the positions of the 119 Sn NMR signals on temperature gradients in the sample. Therefore, the 1 H decoupling power should be carefully adjusted, and a constant temperature is required to ensure that the line widths of the 119 Sn NMR signals are not controlled by experimental conditions. Polarization transfer pulse sequences (INEPT 12,13 or DEPT 14 ) can be applied successfully if the approximate magnitude of coupling constants n J (119 Sn,1 H) is known (see Figure 2.1.1). Indirect detection of 119 Sn NMR signals, in general by 1 H NMR, using various types of twodimensional (2D) NMR techniques1,6 (HMQC, HMBC, HSQC) enables one to study fairly dilute solutions
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Figure 2.1.1 186.5 MHz 119 Sn NMR spectra (recorded by the refocused INEPT pulse sequence with CPD 1 H decoupling) of Me2 Sn(C C-SiMe3 ) in C6 D6 (23 ◦ C; ca. 5%); result of eight transients, showing the 13 C and 29 Si satellites.86
for the observation of 119 Sn resonances within a short timeframe (increase in sensitivity by a factor [γ (1 H)/γ (119 Sn)]3/2 = 4.4, when compared with INEPT, or [γ (1 H)/γ (119 Sn]5/2 = 11.8, when compared with normal detection). These experiments may be further improved by the application of pulsed field gradients.1,6,15 Indirect detection of 119 Sn NMR signals is also helpful for severely broadened 119 Sn NMR signals [e.g. by partially relaxed scalar coupling of 119 Sn with a quadrupolar nucleus (Table 2.1.2) as shown in Figure 2.1.2], since the relevant 1 H NMR signals may remain sharp. Table 2.1.2 Nucleus 2
H Li 7 Li 9 Be 11 B 14 N 17 O 35 Cl 37 Cl 73 Ge 79 Br 81 Br 127 I 6
a
NMR properties of some selected quadrupolar nucleia N.A. (%)
0.015 7.42 92.58 100 80.42 99.63 0.037 75.53 24.47 7.76 50.54 49.46 100
Spin I
Q [10−28 m2 ]
γ [107 rad s−1 T−1 ]
[MHz]
1 1 3/2 3/2 3/2 1 5/2 3/2 3/2 9/2 3/2 3/2 5/2
2.87 10−3 −6.4 10−4 −3.7 10−3 5.3 10−2 4.1 10−2 1.67 10−2 6.11 10−2 −8.2 10−2 −6.5 10−2 −0.17 0.33 0.27 −0.79
4.1066 3.9371 10.3976 −3.7606 8.5847 1.9338 −3.6280 2.6242 2.1844 −0.9360 6,7256 7.2498 5.3896
15.350609 14.716106 38.863790 14.051820 32.083971 7.226324 13.556430 9.797931 8.155764 3.488315 25.053752 27.007028 20.008632
Most data taken from Reference 9.
Reference standard Si(CD3 )4 (D12 -TMS) LiCl, D2 O ≈ 9.7 M LiCl, D2 O ≈ 9.7 M BeSO4 , D2 O ≈ 0.45 M F3 B-OEt2 (CDCl3 ) MeNO2 (neat) D2 O (liquid) KCl, D2 O ≈ 2.2 M KCl, D2 O ≈ 2.2 M GeMe4 (neat) NaBr, D2 O, ≈ 10 M NaBr, D2 O, ≈ 10 M KI, D2 O, ≈ 6 M
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Tin Chemistry: Fundamentals, Frontiers and Applications 2 Me2Sn
4 5
FeA C(9)
3
1 Fe
C(8) Fe
2 C(2A) C(1A)
C(2) C(4)
C(3)
C(1) Sn
C(14) C(13)
δ119
Sn
0
−100 −200 −300 −400 −500
Figure 2.1.2 149.2 MHz solid-state 119 Sn{1 H} VACP MAS NMR spectrum (MAS = 9000 Hz) of Me2 Sn(C C-Fc)2 showing two different tin sites [δiso −144.0 (minor), −150.5 (major) are marked by filled circles; δ 119 Sn (CD2 Cl2 solution) −152.8]. In contrast to the bulk crystalline material, the crystal studied by X-ray diffraction had only one type of molecule in the unit cell, as shown. (Adapted from Reference 228, with permission from Wiley-VCH.)
In the solid state, high resolution 119 Sn NMR spectra of samples containing hydrogen10,11 can be readily obtained, in most cases, by cross polarization/magic angle spinning (CP/MAS) techniques with optimized contact times, or by using variable contact times16 (Figure 2.1.3). Owing to the high NMR sensitivity, even single pulse techniques will provide meaningful 119 Sn NMR spectra of diamagnetic solids within a reasonable time. 2.1.3
Nuclear Spin Relaxation
In solution, the four most relevant relaxation mechanisms (DD = dipole–dipole, SC = scalar, SR = spin-rotation, and CSA = chemical shift anisotropy) can contribute to 119 Sn nuclear spin relaxation at moderate field strength (B0 = 2.11–5.97 T). The CSA mechanism may take over for T1 (119 Sn) at higher field strengths (B0 2 dependence!) which are more common nowadays,17 and since this affects T2 (119 Sn) likewise, significant broadening of 119 Sn NMR signals is observed, in particular in the case of highly anisotropic surroundings of the 119 Sn nuclei. Thus, CSA relaxation of 119 Sn nuclei is already dominant at moderate field strength for two-coordinate tin nuclei, e.g. in bis(amino)stannylenes.18 However, for larger molecules of organotin compounds or at low temperatures, dipole–dipole relaxation may become competitive. Therefore, the negative NOE has to taken into account (see above). (T1 )−1 = (T1 DD )−1 + (T1 SC )−1 + (T1 SR )−1 + (T1 CSA )−1
(2.1.1)
The resolution of indirect nuclear 119 Sn-X spin–spin coupling either in the 119 Sn or in the X NMR spectrum depends on the magnitude of the coupling constants and relaxation rates of the 119 Sn or the X nuclei. In the latter case, quadrupolar nuclei X are concerned [e.g. X = 11 B (see Figure 2.1.2, 2.1.3), 14 N (see Figure 2.1.4), 35/37 Cl]. In the former case, broadening of 117/119 Sn satellites in spin- 12 X NMR
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Figure 2.1.3 Contour plot of the 2D 500.1 MHz 1 H detected 1 H/119 Sn shift correlation (HSQC) of a sample of a reaction mixture containing three different tin–boron compounds. The mixture could not be analyzed by direct 119 Sn NMR methods. (Adapted from Reference 152.) The F1 projection shows the 119 Sn NMR spectrum with its broad lines as a result of 1 J(119 Sn,11 B) ≈ 1000 Hz. The overlapping 119 Sn resonances are well resolved in the contour plot, and the unstable species Me3 SnB(OMe)2 (marked by lines towards F1 and F2 ) can be readily identified. The insert shows the influence of relatively slow 11 B quadrupolar relaxation in [Me3 SnBH3 ]− giving rise to fairly sharp 119 Sn NMR signals. (Reproduced from Reference 204, with permission form Wiley-VCH.)
spectra, recorded at high field strength B0 , indicates anisotropic surroundings of the tin nuclei.18,19 Such properties are, of course, readily verified by solid-state 119 Sn NMR spectra confirming the information on the chemical shift anisotropy.19 2.1.4
Chemical Shifts δ 119 Sn
General 119
Sn chemical shifts of diamagnetic tin compounds cover a range of approximately 6500 ppm, from ca. +4000 to −2500 with δ 119 Sn (SnMe4 ) = 0. Such a large range of chemical shifts indicates great sensitivity of this parameter even to small structural changes (Figure 2.1.6). Major changes in 119 Sn nuclear magnetic shielding are due to the paramagnetic shielding constant σ p (119 Sn) which is determined
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Figure 2.1.4 111.9 MHz solid-state 119 Sn{1 H} CP/MAS NMR spectrum (recycle delay 5 s; contact time 1 ms) of a 1-stanna-4-bora-cyclohexadiene derivative,86 showing a broad line accompanied by spinning side bands (2230 Hz) with splitting in the top corresponding to 3 J ( 119 Sn,11 B)
by magnetic field-induced currents, mixing ground and excited electronic states. The electronic structures around the respective 119 Sn nuclei depend in a complex way, both on the coordination number (2 to 10) and formal oxidation state (+2 or +4 in most cases). Although there is some progress in the calculation of 119 Sn nuclear magnetic shielding,20,21 the data are not reliable with respect to small changes.
Figure 2.1.5 149.2 MHz solid-state 119 Sn{1 H} VACP MAS NMR spectrum of a spirocyclic ferrocenophane derivative. In agreement with the X-ray structural analysis, there is a single tin site (δ 119 Sn (C6 D6 solution) 107.5). The splitting of the center band is caused by non-averaged dipolar interactions with the quadrupolar nuclei 14 N and 11 B, as well as by 119 Sn-14 N and 119 Sn-11 B spin–spin coupling. (Adapted from Reference 229, copyright 2003, Elsevier.)
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Figure 2.1.6 74.6 MHz 119 Sn{1 H-inverse gated} NMR spectrum of an equilibrated reaction mixture of (Me2 SnSe)3 and (Ph2 SnS)3 (23 ◦ C in CDCl3 ). After complete exchange of all Me2 Sn Se and Ph2 Sn S fragments, the spectrum should show 40 119 Sn NMR signals, of which 39 are clearly resolved.86
The relation between experimental isotropic δ 119 Sn data for tin compounds in solution and in the solid-state forms a link to crystallographic data with direct structural information, which is becoming increasingly available. Frequently, the 119 Sn NMR signals are shifted to lower frequency in the solid state as compared to solution. Intra- and in particular inter-molecular interactions, fairly weak in solution, may become stronger in the solid state, and the tin atoms adopt a higher coordination number, which is sometimes more relevant from the crystallographic point of view.22–26 In any case, solid state 119 Sn NMR spectra provide additional information on the tensor components of nuclear shielding. These data can be used to discuss the bonding situation, as has been shown in the example of the dimeric stannylene 1,27 other tin(II) compounds,28 and the stannynes 3.29 In these cases and also in the tristannaallene 230 (Scheme 2.1.1), the bonding situation differs markedly from the straightforward picture derived from the structures of alkenes, allenes, or alkynes. Coordination Number of Tin and Electronic Structure
In tin compounds, irrespective of the formal oxidation sate +2 or +4 of tin, the increase in the coordination number is accompanied by a marked increase in 119 Sn nuclear shielding. Nowadays, a large number of examples have proven to be consistent with this observation. This also includes rather weak intra- or
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Tin Chemistry: Fundamentals, Frontiers and Applications CH(SiMe3)2 Sn
(Me3Si)2HC (Me3Si)2HC δ
119
Sn
But3Si
But3Si
SiBut3
2 30 δ Sn 503 (1), 2233 (2); 1 119 J( Sn,119Sn) = 4302 Hz
SiBut3
Sn (2)
SiBut3
119
Sn 2328 (monomer in solution), 692 (293 K) to 613 (77 K) (in solid-state) 1 119 J( Sn,119Sn) = 1402 Hz
(3) Sn
(3) Sn
But3Si
1 27
But3Si
(2) Sn
(1) Sn
CH(SiMe3)2
Ar = C6H3- 2,6(C6H3-2,6-Pri2) Ar
SiBut3
Sn
Sn (1) 3 30
Ar
δ119Sn 412 (1), -694 (3); 1 119 J( Sn(1),119Sn(2)) = 2208 Hz 1 119 J( Sn(1),119Sn(3)) = 2223 Hz
Scheme 2.1.1
Sn 4 29
δ119Sn 381.4 (in toluene olution) 86 335.1 (in solid-state) 1 119 J( Sn,119Sn) not observed
Examples with Sn Sn ‘double’ and ‘triple’ bonds
inter-molecular interactions, when the δ 119 Sn values change as a function of the solvent, of temperature,31,32 or of the steric requirements of remote substituents.31,32 In the case of tin(II) compounds, the lowest 119 Sn nuclear shielding is observed for monomeric dialkylstannylenes (Scheme 2.1.2).27,33a,b However, it has been suggested that agostic B H..Sn interactions significantly increase 119 Sn nuclear shielding.33c For diarylstannylenes, one observes a wide range of δ 119 Sn data,34 and this may be caused by intra- or inter-molecular association. The latter is also evident from crystal structures.34a Me3Si
SiMe3
(Me3Si)2HC
Ar Sn
Sn
(Me3Si)2HC 5 27 δ119Sn 2325 or 2328
δ119Sn 2208 34a Sn Ar = C6H2-2,4,6-(SiMe3)3
Ar Me3Si
SiMe3
6
33a
7
δ119Sn 723 34c Ar = C6H2-2,4,6-(CF3)3
δ119Sn 2323
Scheme 2.1.2
Diorganostannylenes
Bis(amino)stannylenes are also known to be monomers when bulky amino groups are linked to the tin atom.35 The δ 119 Sn data are typical (Scheme 2.1.3), and there is no significant difference between cyclic and non-cyclic derivatives, although in the former, one might be tempted to discuss SnN(pp)π-bonding. The 119 Sn nuclear shielding is markedly increased when compared with that for the dialkylstannylenes. Therefore, it is suggested that the relative energies of electrons in the Sn C and Sn N σ bonds are responsible for the above observation.36 In the bis(amino)stannylenes, the energy difference between ground states and relevant excited states is larger, giving rise to a smaller contribution to the paramagnetic shielding term and hence to increased 119 Sn nuclear magnetic shielding in the tin(II) amides (Scheme 2.1.3), when compared with dialkylstannylenes (Scheme 2.1.2). This assumption is further supported by δ 119 Sn = 277 for (Me3 Si)2 N-Sn-OAr (Ar = C6 H2 -2,6-But2 -4-Me).37 In transition metal complexes of
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25
bis(amino)stannylenes38 (Scheme 2.1.3), the lone pair of electrons at tin is used to form the M Sn bond. Since the formally empty pz -orbital at the tin atom is still present, there is no dramatic shift of the 119 Sn resonance in comparison with the free Sn(II) amide. This is also indicated by δ 119 Sn of bis(alkoxy)stannylene complexes in the absence of further association,40 although there are no data to compare with the free ligand. However, in dimers of stannylenes or in adducts of stannylenes, a marked increase in 119 Sn nuclear shielding is observed, since the Sn-pz orbital is now used in donor–acceptor interactions.40,41
(Me3Si)2N (Me3Si)2N
But
B
Sn
N
Me3Si
8
Me2Si Sn
δ119Sn 776 36
B
N
Me2Si
10 t
Me3Si(Bu )N
SiMe3
Sn
δ119Sn 791.5 39
Me3Si(But)N
N
(Me3Si)2N (
Sn N
11
12 38
But δ119Sn 759 36
δ119Sn 885 (M = Pd) 815 (M = Pt)
9 δ119Sn 850 36
Scheme 2.1.3
Sn)3M (Me3Si)2N
Diaminostannylenes
An increasing number of tin compounds are becoming available, in which tin is involved in multiple bonding with other elements, such as carbon, silicon, germanium,42,43 or phosphorus,44a sulfur, or selenium,44b and this type of bonding causes magnetic deshielding of 119 Sn nuclei (Scheme 2.1.4). However, tin usually avoids multiple bonding, and this is reflected by δ 119 Sn data as in 15, pointing towards a greater contribution of the zwitterionic structure. But
But Me3Si
N
B
Si
Sn[CH(SiMe3)2]2 Me3Si
N
B But
N(SiMe3)2 Ar Sn
But 14
13
δ119Sn 835 43a
Pri2N
Ar = C6H3-2,6-(NMe2)2 δ119Sn 621 42b Pri2N
+ i
Sn[N(SiMe3)2]2 15
Pr 2N δ
119
Scheme 2.1.4
Sn – 44.7
Sn[N(SiMe3)2]2 i
Pr 2N 42a
Multiple bonding between tin and carbon or silicon
In stannocenes, the tin atoms formally possess the coordination number 10, and the 119 Sn magnetic shielding is high (Scheme 2.1.5), in contrast to the stannylenes. Numerous stannocenes are instructive examples, for which the molecular structures have been determined in the solid state, and the 119 Sn NMR spectra have been measured both in the solid state and in solution.
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16 Sn(η5-C5H5)2
δ119Sn solution solid-state –2199 45 –2162.8, 2224.4 45
21 Sn[η5-C5(CH2Ph)5]2
δ119Sn solution solid-state –2188 50 –2288 50
17 Sn(η5-C5H3-2,5-But2)
–2100 46
22 Sn(η5-C5Ph5)2
–
18 Sn(η5-C5Me5)2
–2129 47 –2136.6, –2140.2 47 23 Sn(η5-NC4H2-2,5-But2)2
5
i
19 Sn(η -C5Pr 5)2
–
5
t
20 Sn(η -C5Me4-SiMe2Bu )2 –2204
49
–2362
48
–2236
49
Scheme 2.1.5
24 [Sn(η
–1889
5
-C5H3-2,5-But2)]+[BF4]–
–2215 50 51
–2337.7
46
-
Stannocenes and related derivatives
The δ 119 Sn values for stannocenes are found within a narrow range, apparently independent of the structure (ring-centroid-Sn-ring-centroid angles vary from ca. 140◦ to 180◦ ), and there is no great difference between the data for solutions and the solid state.45,47,49,50 The latter fact points towards similar structures in both phases, proving that the η5 -coordination of the cyclopentadienyl rings is retained in solution. In 23, two N -pyrrolyl anions replace the cyclopentadienyl ligands, and the δ 119 Sn value of 23 is in agreement with η5 -coordination, as suggested by the solid-state structure.51 The major reason for the highly shielded 119 Sn nuclei in all these compounds (Scheme 2.1.5) appears to be the position of the tin atom (in the formal oxidation state +2) above the five-membered ring (in an apical position) with an energetically low-lying orbital for the lone pair of electrons at tin. Thus, in the cationic 24 only one cyclopentadienyl ring is coordinated to tin,46 and neither the positive charge nor the lower coordination number reduces the 119 Sn nuclear shielding. Tin(IV) compounds are known with tin coordination numbers of 3, 4, 5, 6, and in some cases >6. The search for ‘free’ triorganotin cations (stannylium ions) continues,52 and a few examples (e.g. 2553 and 2654 ) have been reported where three-coordinate tin atoms are likely to be present both in the solid state and in solution (Scheme 2.1.6). The extreme 119 Sn nuclear deshielding in the tris(silyl)stannylium ion 2654 is caused by B0 -induced currents involving σ − π* magnetic dipole-allowed transitions, for which the energy difference is much smaller when compared with the triarylstannylium ion 25. Examples of another new class of organotin compounds are also shown in Scheme 2.1.6. The positively charged tin centers in these zwitterionic species 27 and 28 are stabilized by coordination with C C bonds.55 Pri
Pri Pri
3
+ Sn
Pri 25
δ119Sn 714 53
(But2MeSi)3Sn + 26 δ119Sn 2653 54
Me
–
+ Me2Sn Pri δ
119
BEt2 27
Me
–
+ Sn+
Et2B
Et
Sn 215.4
Et
55a
Me Me 28
–
BEt2 Et
δ119Sn 165.6 55b
Scheme 2.1.6 bonds
‘Free’ stannylium cations and stannylium ions stabilized by side-on coordination with C
C
The change in coordination number from 4 to 5 is frequently observed for many tin compounds. This may be enforced by the appropriate chemistry (Scheme 2.1.7). The negative 119 Sn values are typical, although their magnitude varies with the system under investigation. Very often intra-molecular association, adduct formation or inter-molecular aggregation (e.g. in
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NMR of Tin Compounds
Me Me
Me Sn Me Me
– Ph Ph
Ph Sn Ph
29
Ph 30
δ119Sn -277 56 Δ
119
Sn -277
–
Bu3 Sn
– Me3Sn
H2 N
Me 32
27
– BEt2 Et
δ119Sn -303 56
31 δ119Sn -44 57
δ119Sn -175.6 58
Δ119Sn -174.2
Δ119Sn -37.6
Δ119Sn -291.6
Scheme 2.1.7 Anionic complexes of tetraorganotin compounds. The values (coordination shifts) refer to SnMe4 , SnPh4 , 1,1-dibutyl-1-stanna-indene, and the corresponding dihydro-1,2,5-azoniastannaboratole, respectively
a vast number of organotin carboxylates and related derivatives), sometimes as temperature-dependent equilibria, are observed using 119 Sn NMR spectroscopy, or interactions with donor solvents (e.g. DMSO) have to be taken into account. There are numerous examples of neutral or ionic tin compounds for which the structural information from X-ray analysis is confirmed by 119 Sn NMR data for the compounds in solution. Depending on the Lewis-acidity of the respective tin compound and the availability of suitable donors, the coordination number of tin may readily increase to 6 in solution or even to 7 or 8 in the solid state. Tin halides have been studied on many occasions since they are important starting materials for the synthesis of tin compounds. The δ 119 Sn NMR data for the solid state and frequently also in solution indicate inter- or intra-molecular coordination if suitable donor sites are available.59–65 However, predictions about Sn..Cl..Sn bridges should be made with great caution: see, for example, the Sn..Cl..Sn bridge in an anionic chloro complex 66 and the absence of such a bridge in 1,1 -bis(trichlorostannyl)ferrocene (δ 119 Sn23.2). In the case of the latter, the crystal structure has been determined,67 showing neither appreciable intra- or inter-molecular Sn..Cl..Sn interactions. Adducts of organotin or tin halides with Lewis bases may be fluxional, e.g. with respect to cis/trans isomerization in octahedral complexes, and in the case of dimethylsulfide adducts of SnCl4 , this process is readily analyzed by magnetization transfer experiments using 119 Sn NMR spectroscopy.68 Although weak association in the solid state is readily seen from the results of crystal structures, it may not always be obvious from solid-state 119 Sn NMR spectra, since the isotropic δ 119 Sn values are similar to those in solution. However, considering the respective principal structure and careful analysis of the tensors of the chemical shifts usually reveal such weak interactions. The dialkyltin tropolonates are instructive examples in this respect (Figure 2.1.7).69 Tin compounds with tin–chalcogen bonds are highly abundant and possess an amazing variety of structural features, in particular for Sn O bonds if the oxygen is part of a carboxylate unit or of another acid.70–80 Diorganotin oxides are polymers or trimers, depending on the steric requirements of the organyl group. In the case of sulfur and selenium, structures of trimers and dimers have been determined, and numerous NMR spectroscopic studies in solution and in the solid-state have been carried out.81,82 Substituent Effects and Effects of Cyclic Structures
The influence of various substituents on δ 119 Sn for series of tetra-coordinate tin(IV) compounds SnX4-n Yn (n = 0–4) can be graphically depicted as U-shaped curves. These substituent effects are not linear, but approximately pair-wise additive. Some representative 119 Sn NMR data are given in Table 2.1.3 (tin hydrides), Table 2.1.4 (tetraorgano. tin), Table 2.1.5 (tin halides), Table 2.1.6 (tin chalcogenides), and Table 2.1.7 (compounds with Sn N, Sn P, Sn As, Sn Sb, and Sn Bi bonds).
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Tin Chemistry: Fundamentals, Frontiers and Applications C36 08 C34
C35 05 C21
Sn2
C27
07 C28
06
δiso −202 δ119Sn (solution) −195.5 δiso −269.5 δ119Sn (solution) −233
C5
C1A C2A
01
C3A Sn1A
δ119Sn
04A
Sn1 04 03
01A
200
−200
0
−400
−600
C5A
−800
02
C1
Figure 2.1.7 Comparison of 111.9 MHz solid-state 119 Sn{1 H} CP/MAS NMR spectra and solid-state molecular structures of dimethyltin- (upper trace; MAS 5500 Hz) and dibutyltin tropolonates (lower trace; MAS 8000 Hz). In the latter, tin has coordination number 7, and the 119 Sn chemical shift anisotropy is much larger (δ11 = +195.8, δ22 = −153.4, δ33 = −850.6; η = 0.60), when compared with that for the Me2 Sn derivative (δ11 = −23.4, δ22 = −79.6, δ33 = −498; η = 0.19), where the tin atom is six-coordinate. (Adapted from Reference 69, with permission from John Wiley & Sons, Ltd.) Table 2.1.3
119
Sn NMR parameters of some tin hydridesa
Compound
δ 119 Sn
1 J (119 Sn,1 H)
Me3 SnH Bu3 SnH Ph3 SnH (CF3 )3 SnH Fe(η5 -C5 H4 -SnMe2 H)2 [b]
−104.5 −91.4 −164.5 −309.8 −102.4
1744 1604.4 1935.8 2798 1824
Bu2 Sn(Cl)H Me2 SnH2 Ph2 SnH2 (CF3 )2 SnH2 Fe(η5 -C5 H4 -SnMeH2 )2 [b]
−18.3 −225.0 −234.0 −303.1 −210.5
2178 1797 1928 2536 1876
MeSnH3 PhSnH3 H2 C CH-SnH3 H2 C C CH-SnH3 HC C-SnH3 CF3 SnH3 Fe(η5 -C5 H4 -SnH3 )2
−346.0 −320.0 −361 −338.4 −320.6 −351.6 −330.7
1852 1920
a
See References 5 and 8 for more data.
2010.3 2242.4 2207 1947
J (119 Sn,13 C) 352 912 371.8 (Me) 514.4 (C-1) 451 485 777 387 (Me) 546.6 (C-1)
488.0 517.2 653 583.2
Ref. 5 5 5 142 143 144 5 5 142 143 5 5 145 146 146 142 143
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SnNMR parameters of some tetraorganotin compounds a δ 119 Sn
Compound SnMe4 SnEt4 SnPr4 SnPri4 SnBu4 Sn(CH2 CH CH2 )4 Sn(CH2 Ph)4 SnPh4 Sn(CH CH2 )4 Sn(C CMe)4 Sn(C CSiMe3 )4 Sn(C C-Fc)4 Sn(CF3 )4 (E)-Me3 Sn-CH C(Me)Hb
0 1.4 −16.8 −43.9 −11.5 −47.9 −35.8 128.8 −157.4 −345.4 −384.9 −342.2 −350.4 −32.5
(Z)-Me3 Sn-CH C(Me)H
−51.2
(E)-Me3 Sn-CH CH-SnMe3
−52.3
(Z)-Me3 SnCH CH-SnMe3
−60.6
(Me3 Sn)2 C C CH2
−9.6
Me3 Sn-Ph
−28.6
Me3 Sn-2-thienyl
−27.5
Me3 Sn-2-pyridyl
−52.9
Me3 Sn-2-thiazolyl
−32.6
Fe(η5 -C5 H4 SnMe3 )2
−4.2
Me3 Sn-C CMe (C6 D6 )
−73.8
Me3 Sn-C C-Cl (CDCl3 )
−56.1
Me3 Sn-C C-SnMe3 (CDCl3 )
−80.9
Me3 Sn-C C-C C-SnMe3 (CDCl3 )
−59.0
Me2 Sn(CH CH2 )2
−79.4
Me2Sn
CMe2
Me2 SnPh2 Me2 Sn(C CH)2 MeSn(CH CH2 )3 MeSnPh3 c MeSn(C CMe)3 a
−161.7 −60 −154.5 −124.0 5 −98 −250.4
See ref.5,8 for further data. See Reference 231 for numerous alkenyltin compounds. c See Reference 160 for similar compounds. b
1
J (119 Sn,13 C)
336.6 320.0 316.0 308.5 314.0 264.9 258.8 531.1 519.3 1167.7 1036.0 1167.3 1001.0 352.0 (Me) 478.4 (C ) 346.9 (Me) 464.6 (C ) 337.0(Me) 409.0 (C ) 340.0(Me) 496.0(C ) 496.1 (C ) 348.2 (Me) 261.7 (C ) 347.5 (Me) 474.4 (Ph) 373.3 (Me) 395.5 (C ) 346.6 (Me) 605.5 (C ) 373.8 (Me) 461.5 (C ) 357.8(Me) 492.5 (C-1) 404.1 (Me) 502.9 (C ) 406.8 (Me) 385.7 (C ) 400.7 (Me) 388.5 (C ) 405.0 (Me) 372.6 (C ) 369.4 (Me) 466.8 (C )
2
J (119 Sn,13 C) – 23.0 30.0 15.0 20.0 48.3 37.8 35.5 <4 242.5 171.9 241.8 542 (19 F) 77.8
30.5
3
J (119 Sn,13 C)
– – 51.0 – 52.0 51.3 23.6 53.1, 11.7 (4 J ) 20.1 18.6 (29 Si)
5,104a 5, 147 5,147 5,104a 4,147 5,147 148 149 5,147 150 150 228 142 136
49.3
136
1013.0 (119 Sn)
151
491.0(119 Sn)
151
60.6
104e 5, 147
32 37.1
45.2 (C-4) 17.3 (C-5) 93.2
68.6
153 154 155
51.8
40.3
156
107.4
11.0
157
76.6 45.2 83.0
379.2 (Me)
86 47.6 (117 Sn)
157
18.5; 8.0 (4 J ) 158 19.0 (117 Sn,5 J ) 5, 104 88.0
440.0 (C )
88 5 104,157
501.7 (Me) 606.7 (C ) 617.1(Me) 873.9 (C )
Ref.
184.2
16.2
5 159
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Table 2.1.5
119
Sn NMR parameters of some tin(IV) halidesa
Compound
δ 119 Sn
1 J (119 Sn,13 C)
2 J (119 Sn,13 C) 3 J (119 Sn,13 C)
+139 −[2298 (19 F)] 124.5 − [2374 (19 F)] −40.8 − [2463 (19 F)] −261.6 − [2920 (19 F)] 16.4 −274.0 (solid) − [3000 (19 F)] −803 − [1625 (19 F)] [SnF6 ]2− 2− [PhSnF5 ] −692 − [1180, 2430 (19 F)] [Ph2 SnF3 ]− −402 − [2310, 2250 (19 F)] Ph3 SnF-OP(NMe2 )3 −272.0 − [2040 (19 F)] Me3 Sn-Cl (CDCl3 , 33 mol%) 165.7 379.0 [h] (CDCl3 , 15 mol%) 171.2 (acetone, 30.5 mol%) 119.8 Et3 Sn-Cl (CCl4 ) 155 347.3 28.1 Et34 Sn-Cl (THF) 103 300 32 Bu3 Sn-Cl (CDCl3 ) 152.0 337.0 23.2 But3 Sn-Cl 50 (PhCH2 )3 Sn-Cl 51.3 293.0 42.7 Ph3 Sn-Cl (CDCl3 ) −44.7 (HC C)3 Sn-Cl (CDCl3 ) −263.2 1312.7 268.6 (CF3 )3 Sn-Cl −255.8 1179 636 (19 F) (Bu2 SnCl)2 O −139.8 and −92 n.r. 73.2 (117 Sn) N(CH2 CH2 CH2 )3 Sn-Cl 19.5 476.6 28.9 N[CH2 CH2 N(Me)]3 Sn-Cl −180.2 9.1 −192 (solid) −577.9 [HB(3-t Bu-5-Mepz)3 ]Sn-Clb,c Me2 SnCl2 (CH2 Cl2 ) 137.0 Et2 SnCl2 (CCl4 /CH2 Cl2 ) 121.0 439.5 Et2 SnCl2 (THF) 9 586 50 Bu2 SnCl2 (CDCl3 ) 126.3 419.9 36.6 But2 SnCl2 (CCl4 ) 56 (H2 C CH)2 SnCl2 −30.2 (CDCl3 ) 750.7 −394.1 (DMSO) 1469.7 Ph2 SnCl2 (CH2 Cl2 ) −32.0 [Ph2 SnCl3 ]− −250 [Ph2 SnCl4 ]2− −425 (HC C)2 SnCl2 (CDCl3 ) −194.5 1466.1 300.6 (CF3 )2 SnCl2 −179.1 1498 773 (19 F) (Et2 N)2 SnCl2 −100.1 [(Me3 Si)2 N]2 SnCl2 −143.8 BuSnCl3 (CDCl3 ) 6.1 645.0 40.0 BuSnCl3 (DMSO) −457.1 n.r. 71.6 PhSnCl3 (CH2 Cl2 ) −65.0 1118.4 79.1 (PhCMe2 CH2 )3 Sn-F (CDCl3 ) (PhMe2 Si)3 CSnMe2 -F (Me3 Si)3 CSnPh2 -F N[CH2 CH2 N(Me)]3 Sn-F
CF3 SnCl3
−141.5
2055
966 (19 F)
26.8
Ref. 5 162 162 26c 163 163 163 164 5,8,165
64.5 31.8
38.5 23.1
85.4
120.0 210.3 125.5, 25.8 (4 J )
5,86 236 5,86 5 166 5 86 142 167 64b 26c 168 5 5 236 169 5 170 5 163 163 86 142 171 171 172 172 5,86 142
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NMR of Tin Compounds Table 2.1.5
31
Continued
Compound SnCl4 [SnCl6 ]2− Me3 Sn-Br (C6 H6 or CHCl3 ) Et3 Sn-Br Bu3 Sn-Br (CDCl3 ) Ph3 Sn-Br (CDCl3 ) Me2 SnBr2 (C6 H6 or CHCl3 ) Et2 SnBr2 (CCl4 ) Bu2 SnBr2 (neat) MeSnBr3 (C6 H6 or CHCl3 ) BuSnBr3 (CDCl3 ) BuSnBr3 (DMSO) SnBr4 (CS2 ) [SnBr6 ]2− Me3 Sn-I (C6 H6 or CHCl3 ) Bu3 Sn-I (CDCl3 ) Me2 SnI2 (C6 H6 or CHCl3 ) Bu2 SnI2 (neat) MeSnI3 (CCl4 ) SnI4 (CS2 )
δ 119 Sn
1 J (119 Sn,13 C)
2 J (119 Sn,13 C)
−150 ±2 −732 (Cl) 128 148 138.5 −59.8 70.0 96 89.7 −165 −114.7 −604.1 −638 −2070 38.6 86.9 −159 −56.1 −699.5 −1701
–
3
J (119 Sn,13 C)
Ref.
–
–
326.7
23.2
65.0
391.8
34.2
85.4
590.6 n.r.
56.8 69.7
117.8 218.9
5 173 5 5 86 5 5 5 169 5 172
313.0
23.2
64.5
347.9
33
83.6
5 5 5 86 169 5 5
a
See Reference 161 for compilations with 1 J (119 Sn,19 F). See References 5 and 8 for more data. c pz = pyrazolyl. b
The bond angles at the tin atom play an important role. This is most obvious for different ring sizes,83,85,89,90 and the nature of the elements adjacent to the tin atom will, of course, also determine structural properties. 119 Sn nuclear deshielding is typical of tin atoms in five-membered rings (Scheme 2.1.8) when compared with non-cyclic structures and particularly with six-membered or larger rings. In order to allow the assessment of the influence of four- or three-membered rings, the δ 119 Sn data set is still insufficient. It appears that 119 Sn nuclear shielding is fairly high for tin atoms in three-membered rings.84 119 Sn NMR spectroscopy, together with NMR data from other nuclei, serves most conveniently to monitor molecular rearrangements and helps with unambiguous structural assignment. An example is shown in Scheme 2.1.992 (note again the differences in the δ 119 Sn data for five- and six-membered rings). Similarly, the complex products arising from the hydrolysis of butyltin alkoxides were studied by 119 Sn NMR in the liquid and solid states,93 and monitoring of the hydrolysis of BuSnCl3 was also instructive.94 Examples of 119 Sn data for tin compounds with Sn–Group-14 element, Sn B, Sn Tl, and Sn Li bonds are given in Table 2.1.8. Although there are no typical ranges of chemical shifts (with a few exceptions, such as for the Zintl anions), the characterization of these compounds is straightforward, in general, considering the spin–spin coupling (see below) between 117/119 Sn and the magnetically active isotopes of most of these elements (including tin itself).
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Table 2.1.6
119
Sn NMR parameters of compounds with tin–chalcogen bondsa
Compound
δ 119 Sn
Me3 Sn-OH (CH2 Cl2 ) Me3 Sn-OMe (C6 H6 ) Me3 Sn-OPri (C6 H6 ) Me3 Sn-OBut (C6 H6 ) Me3 SnOPh Me3 Sn-O-SiMe3 (Me3 Sn)2 O (C6 D6 ) Me3 Sn-O-ReO3 Bu3 Sn-OMe (C6 D6 )
118 129 109 91 151.0 121.0 110.6 91.8 98.5
Bu3 Sn-OPri Bu3 Sn-OBut Bu3 Sn-OC(O)Me (Bu3 Sn)2 O (C6 H6 ) (But3 Sn)2 O (C6 H6 ) Ph3 Sn-OH (CH2 Cl2 ) (Ph3 Sn)2 O (CDCl3 )
92.1 71.7 103.8 82 −39.3 −86 −85.5
Me2 Sn(OMe)2 (C6 H6 ) Me2 Sn(OBut )2 Me2 Sn(acac)2 (CHCl3 ) Bu2 Sn(OMe)2 (CCl4 ) Bu2 Sn(OBut )2 (But2 SnO)3 (CDCl3 ) MeSn(OEt)3 Pri Sn(OPri )3 BuSn(OEt)3 (C6 H6 ) BuSn(OBut )3 (C6 H6 ) Sn(OPri )4 (CDCl3 ) Sn(OBu)4 (CDCl3 ) Sn(OBui )4 Me3 Sn-SMe (C6 H6 or CHCl3 ) Me3 Sn-SBut Me3 Sn-SC(S)NMe2 (Me3 Sn)2 S (C6 H6 ) Me3 Sn-S-Fcb Bu3 Sn-SBu Bu3 Sn-SBut Ph3 Sn-SMe (CH2 Cl2 ) (Ph3 Sn)2 S (CDCl3 ) Me2 Sn(SMe)2 (C6 H6 or CHCl3 ) Me2 Sn(SCH2 CH2 S) Me2 Sn(SCH2 CH2 CH2 S) Me2 Sn(SCH CHS) (Me2 SnS)3 (C6 H6 ) (Bu2 SnS)3 (CCl4 ) But2 Sn(SCH2 )2
−126.3 −1.8 −365 −159 −34 −84.3 −434 −217.9 (−364.2 solid) −432 −199 −596 −583, −616, −629.6 −373 90 55.5 25.0 86.5 91.3 74.1 50.4 −47 −53.7 144 193.0 (66.9 solid) 149 231 128 126.9 171 (171 solid)
1
J (119 Sn, 13 C)
2
J (119 Sn,13 C)
407.7 480.7 365.1 363.8 363.8 360.1
627.4
20.7, 57.8 (3 J ), 8.7 (4 J ) 19.5 18.4 18.5
45.2, 59.5 (3 J ), 12.2 (4 J )
866.3
41.9
n.r.
n.r.
348.9 330.7 330.8
15.7 21.2 20.8
567.4
44.2, 59.2 (3 J ), 13.1 (4 J )
372.0 445.0 (solid) 368.7 407
<5(13 C) 191 (117 Sn)
Ref. 5 5 5 5 174 175 5 176 86 71b 71b 71b 5 5 5 5,86 5 5 5 5 5 5 5 24 5 5 177 177 177 5 5 178 5 179 71b 71b 5 5,86 5 180 5 86 5 5 181
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NMR of Tin Compounds Table 2.1.6
Continued
Compound (t Bu2 SnS)2 Ph2 Sn(SMe)2 (CH2 Cl2 ) (Ph2 SnS)3 (CDCl3 ) MeSn(SMe)3 (C6 H6 or CHCl3 ) PhSn(SMe)3 (CH2 Cl2 ) (4-Me-C6 H4 Sn)4 S6 d (C6 F5 Sn)4 S6 (SCH2 CH2 S)2 Sn Sn(SPh)4 Me3 Sn-SeMe Me3 Sn-SePh Me3 Sn-Se-Fcb (Me3 Sn)2 Se fc(SnMe2 )2 Sec (Ph3 Sn)2 Se (CDCl3 ) Me2 Sn(SeMe)2 (Me2 SnSe)3 (t Bu2 SnSe)2
δ 119 Sn 124.0 38.5 16.8 167 107 86.4 46.4 277.0 (241.0 solid) 49.5 45.6 55 53.6 44.5 28.1 −78.5 57.1 42 51.2
(Ph2 SnSe)3 MeSn(SeMe)3 Sn(SeMe)4 Sn(SePh)4 Sn(Se-Fc)4 b [Sn4 Se10 ]4−d (Me3 Sn)2 Te (CH2 Cl2 ) (Me2 SnTe)3 (C6 D6 ) (But2 SnTe)e2
−44 14.8 −80.5 −132.7 −218.3 −350.1 −66.8 −195 −125.2
Sn(TePh)4 [SnTe3 ]2− [SnTe4 ]4− [Sn2 Te6 ]4−d
−570.5 −1170.1 −1823.6 −1675.0
[Sn2 Te7 ]4−
−1344.9
a
33
1
J (119 Sn, 13 C) 363
2
J (119 Sn,13 C)
Ref.
114 (117 Sn)
183 5 5 5 5 182 182 180 184 5 5 179
260.7 (117 Sn) 290.0
333.2 977.6 (77 Se) 1060 (77 Se) 364.1 (Me) 491.1 (fc) 1133(77 Se) 544.0 1213 (77 Se) 1228 (77 Se) 923 (77 Se) 1380 (77 Se) 1520 (77 Se) 1584 (77 Se) 1700 (77 Se) 2274, 1536 (77 Se) 1385 (125 Te) 3098 (125 Te) 289 2117 (125 Te) 3379 (125 Te) 4535 (125 Te) 2851 (125 Te) 3998, 1633 (125 Te) 4023, 2902, 2319 (125 Te)
19.3 60.5 (fc) 204.8(117 Sn) 44.8, 58.0 (3 J ), 11.8 (4 J ) 333 17 (117 Sn)
11.9 342 (117 Sn) 29 (117 Sn)
124a 185 5, 86 5 5 183 5 5 5 184 179 187 124a 5 183 184 187 187 188 188
See References 5 and 8 for more data; see Reference 44b for three-coordinate tin compounds with Sn S (δ 119 Sn 531) and Sn Se bonds (δ 119 Sn 440). b Fc = C5 H4 FeCp (1-ferrocenyl). c fc = 1,1’-ferrocenediyl. d For 119 Sn NMR data of chalcogenide derivatives of imidotin cage complexes, see Reference 231. e See Reference 234 for other four-membered rings (R2 SnTe)2 .
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Tin Chemistry: Fundamentals, Frontiers and Applications
Table 2.1.7
119
Sn NMR parameters of compounds with Sn N, Sn P, Sn As, Sn Sb and Sn Bi bondsa 1
Compound
δ
119
Sn
1
119
J(
13
Sn, C)
Me3 Sn-NMe2 Me3 Sn-NEt2 Me3 Sn-N(H)Ph Me3 Sn-N(Me)Ph Me3 Sn-NC4 H4 (Me3 Sn)2 NMe (Me3 Sn)2 NPh (Me3 Sn)2 NSiMe3
75.5 60 46.4 73.0 72.9 81 63.0 66
378.1
(Me3 Sn)3 N
86.3
366.2
But3 Sn-NH2 (But3 Sn)2 NH (Me3 Sn)2 NC(O)Me solid-state (C6 H11 )3 Sn-NCS Me2 Sn(NMe2 )2 Me2 Sn[N(Me)Ph]2 (MeC C)2 Sn(NEt2 )2 MeSn(NMe2 )3 MeSn[N(Me)Ph]3 PhC C-Sn[N(Me)CH2 CH2 ]3 N Sn(NMe2 )4 Sn[N(Me)Ph]4 [Sn(NCS)6 ]2− Me3 Sn-PEt2 (C6 D6 ) Me3 Sn-PBut2 (C6 D6 ) Me3 Sn-PPh2 (C6 H6 ) (Me3 Sn)2 PBut (C6 D6 ) (Me3 Sn)2 PPh (CH2 Cl2 ) (Me3 Sn)3 P (C6 D6 ) (Me3 Sn)3 P-Cr(CO)5 (Me3 Sn)3 P-W(CO)5 fc(SnMe2 )2 PPhb Me2 Sn(PPh2 )2 (2,4,6-i Pr3 -C6 H2 )2 Sn PArc [(Me3 Si)2 CH]2 Sn PArc Sn[P[Si(F)(C6 H2 −2,4,6-Pri3 )2 ]2 Cl4 Sn(PBu3 )2 (Me3 Sn)3 As (Me3 Sn)3 As-Co(CO)2 NO (Me3 Sn)3 Sb (Me3 Sn)3 Bi a
−27.9 −16.1 40.7, 126.6 0.3, −19.4 −3.2 58.8 30.5 −176.9 −15.1 −66.0 −173.9 −172 (solid) −121.8 −175.6 −842 −11.1 −37.8 −3.0 −0.1 14.2 37.2 64.2 63.4 1.3 −11.5 499.5 658.3 1551 −573 6.2 31.7 −90 −110.0
1
J (119 Sn,15 N) J (119 Sn,31 P)
other coupling constants
−26.3 +2.2 −37.2
389.6
−41.7 −83.7 113.3 138.5
n.r.
136 (14 N)
1009.6
+24.0 146.8
785.5
+87 n.r.
288.8 303
+175 150 (14 N) 665.3 792 586 816 724 829 409.5 375.5 735.4 808 2208 2295 1682 2395
68.5 (117 Sn,2 J ) 8.3 (29 Si,2 J ), (117 Sn, 2 J ) 6.5(13 C,3 J ) 186.8(117 Sn,2 J ) 16.5(1 H,2 J ) 35.0 (1 H,2 J )
2
135.9 J (119 SnC
13
C)
144 (117 Sn) 365.4 (117 Sn)
Ref. 5 5 114a 114a 5, 106a 5 114a, 119 107 107, 119 190 190 191 192 5 118 193 5 118 26c 5 118 194 5 5 5 5 5 122 122 122 195 5 44 44 196 5 198 198 5 5
See References 4 and 8 for more data; see Reference 233 for three-coordinate tin–nitrogen compounds: β-diketiminatotin(II) halides, amides, chalcogenides. fc = 1,1’-ferrocenediyl. c Ar = 2,4,6-But3 -C6 H2 . b
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NMR of Tin Compounds δ119Sn
δ119Sn SnMe2
Et
SnMe2 34 SnMe2 35
SnMe2 8.4 86 36
53.5 83
33
Et
SnMe2 S
SnMe2 S 40
Et
–42.5 83
SnMe2 133.5 37 SiMe3
Et2B
Me2Sn
SnMe2 38
87
190.0 83 231.0 86
C Se
105.8 91
Et N Me2Si SnMe2 Me2Si N 44 Et
C Se
–177.5 88
δ119Sn
SiMe3 N SnMe2 N 43 SiMe3
39
S
SiMe3
72.4 85
δ119Sn
S
SnMe2 213.2 90 41
35
73.9 91
S SnMe2 S
Scheme 2.1.8
149.0 83
42
Examples of five- and six-membered stannacycles
Numerous compounds with Sn M bonds, where M is a transition metal, have become available by various routes (e.g. insertion of SnCl2 into M Cl bonds, oxidative addition, or simply by metathesis accompanied by alkali metal halide formation). Examples are given in Table 2.1.9. For the tin coordination number 4, one observes an increase in 119 Sn nuclear shielding by a bond between tin and more heavy metals; a similar trend is well known for 13 C and 31 P nuclear shielding in C M and P M bonds, respectively. Tin Compounds With a Formal Oxidation State < +2
The tin nuclei in Zintl anions95 of the type [Sn5 ]2− , [Sn9 ]4− and others such as [Sn8 Tl]5− and [Sn9−n Pbn ]4− , and similar species containing Pt,96 are highly shielded (δ 119 Sn values range from around −1100 to around −1900). The fluxional cluster anion [Sn9 ]4− functions as a ligand in transition metal complexes (e.g. [1-M(CO)3 (η4 -Sn9 )]4− with M = Cr, Mo, W 97 ). These complexes are anionic clusters with a closo structure (bicapped quadratic antiprisms), and are rigid on the NMR timescale, in contrast to the fluxional Zintl anions. Therefore, they provide a wealth of NMR data (δ 119 Sn and coupling constants) for describing the bonding situation. On the other hand, the least shielded 119 Sn nuclei known so far belong to transition metal complexes containing trigonal-planar coordinated tin atoms in the formal oxidation state 0 (48)98 or −2 (4999
Me
Et
Me2Sn
BEt2 N
45
Me
Et
Me2Sn
BEt2 S
S
Me2Sn 47
15.2
77.3
δ11B
–9.6
40.6
2.5 –92
–147
Et
B
Et S
δ119Sn –106.5 δ14N
Scheme 2.1.9 NMR92
Et
N
N
46
Me
–344
An examples of twofold intra-molecular rearrangement monitored by
119
Sn,
11
B and
14
N
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Tin Chemistry: Fundamentals, Frontiers and Applications
Table 2.1.8 119 Sn NMR parameters of compounds with tin–silicon, –germanium, –tin, –lead, –boron, –lithium and –potassium bondsa Compound Me3 Sn-SiMe3 (Me3 Sn)4 Si (Ph3 Sn)2 SiPh2 (Me3 Sn)4 Ge (Ph3 Sn)2 GePh2 Me3 Sn-SnMe3 Me3 Sn-SnPh3 CH2 (SnMe2 SnMe2 )2 CH2 Me2 (Ph)Sn-Sn(Ph)Me2 fc(SnMe2 )2 b
δ 119 Sn −126.7 −34 −167 −25.2 −147 −113 −91.5, −153 −78.5 −120.2 −45.3
(Me3 Sn)2 SnMe2 fc(SnMe2 )3 b
−99.5, −261.7 −102.5 (C Sn) −249.3 ( Sn )
(Me3 Sn)3 SnMe (Me3 Sn)4 Sn Ph3 Sn-SnPh3 (Me2 Sn)6 c (But2 Sn)4 (100 ◦ C) [MeN(CH2 CH2 CH2 )2 SnMe]2 [Sn4 ]2− (en) [Sn9 ]4− (en) [Sn8 Tl]5− (en)
−89.5, −489.7 −80, −739 −143.6 −241.4 99.0 −101.9 −1895 −1230 −1167
Me3 Sn-PbMe3 Me3 Sn-PbBut3 Ph3 Sn-PbMe3 [Me3 Sn-BH3 ]Li (THF) Me3 Sn-B(NMe2 )2 (Me3 Sn)2 BNMe2 2,3-μ-Ph3 Sn-B5 H8 1-Ph3 Sn-B5 H8 2-Ph3 Sn-B5 H8 SnC2 Me2 B9 H9 d [Sn(C2 Me2 B9 H9 )2 ]2−d Me3 Sn-Li (THF) Bu3 SnLi / Et2 O (But CH2 )3 SnK a
−57.0 21.0 −119.5 −28.5 −150 −149 −98.3 −89.2 −87.5 −379 449 −183 −155 −221; −211 (solid)
1
J (119 Sn,13 C)
413 258.1 419
227.0 (Me) 338.9 (C-1) 244.7 (Me) 185.0 (Me) 362.9 (C-1)
162
n.r.
1
J (119 Sn,M)
Ref.
656 (29 Si) 220 (29 Si) 515 (29 Si) 724 (2 J , 117 Sn) 37.5 (73 Ge) 725 (2 J , 117 Sn) 4460 (117 Sn) 4240 (119 Sn) 4245 (117 Sn) 4153 (117 Sn) 5274 (117 Sn)
114a 131 134
2873 (119 Sn) 2875 (119 Sn) 537.2 (2 J , 117 Sn) 1733 (119 Sn) 881 (119 Sn) 4470 (117 Sn) 1176 (117 Sn) 1195 (117 Sn) 4060 (117 Sn) 1281 (117 Sn) 266 (117 Sn) 429 (117 Sn) 800 (203,205 Tl) 3570 (207 Pb) 1637 (207 Pb) 2800 (207 Pb) 554 (11 B) 953 (11 B) 657 (11 B) 1117 (11 B) 1061 (11 B)
402.5 (7 Li) 289 (39 K)
115 134 114a 114a 198 86 199 5 200 200 5 5,131 5 201 202 203 5 5 5 114 117 114b 204 204 204 205 205 205 206 206 110 208 209
See References 5 and 8 for more data; n.r. means not reported. fc = 1,1 -ferrocenediyl. c These data have been reported for the cyclic pentamer; however, the intensity pattern of 117 Sn satellites (2:2:1) correspond to the hexamer. d For other borane clusters containing tin, see e.g. Reference 208. b
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NMR of Tin Compounds Table 2.1.9
119
37
Sn NMR parameters of compounds with a transition metal–tin bonda
Compound
δ
119
Sn
[Me3 Sn-Ti(CO)6 ]− [Ph3 Sn-Ti(CO)6 ]− [(Me3 Sn)2 Zr(CO)5 ]2− [(Me3 Sn)4 Zr(CO)4 ]2− [(Ph3 Sn)4 Zr(CO)4 ]2− [(Ph3 Sn)4 Hf(CO)4 ]2− Cp2 Zr[Sn(CHSiMe3 )2 ]2 Me3 Sn-Ta(Cp)2 H2 Me3 Sn-Cr(Cp)(CO)3 Me3 Sn-Mo(Cp)(CO)3 Me3 Sn-W(Cp)(CO)3 b Cl2 Sn(PBut3 )-W(CO)5 (2-Ph2 PCH2 -C6 H4 )2 SnW(CO)5 Me3 Sn-Mn(CO)5 (Ph3 Sn)2 Mn(Cp’)(CO)2 Ph3 Sn-Mn(Cp’)(H)(CO)2 Me2 Sn[Mn(CO)5 ]2 MeSn[Mn(CO)5 ]3 Me3 Sn-Re(CO)5 Me2 Sn[Re(CO)5 ]2 (Me3 Sn)2 Fe(CO)4 (Me3 Sn)2 Fe(CO)3 CS (Me2 ClSn)2 Fe(CO)4 [Me2 SnFe(CO)4 ]2
35.0 23.5 16.4 49.5 337.0 301.0 +1677.6 53 161 121 43.1 2.9 −7.5
{Me2 Sn[Fe(CO)4 ]2 }2 Sn
123.8 (Me2 Sn) 287.7 (Sn) 1532.0 144.1 689.6 889.9 −34.6
Sn[Fe(CO)4 ]4 Me3 Sn-FeCp(CO)2 [Sn(Cl)Fe(Cp)(CO)]2 Sn[FeCp(CO)2 ]2 [Ru(SnCl3 )6 ]4− (3M HCl)
63 61.1 58.6 150 284 −89 −223 80.4 78.5 258.5 87.8
[Os(SnCl3 )5 Cl]4− (MeNO2 )c
−531.8 (eq) −581.8 (ax)
Me3 Sn-Co(CO)4 Me2 Sn[Co(CO)4 ]2 MeSn[Co(CO)4 ]3 (OEP)Rh-SnBu3 d (OEP)Rh-SnPh3 d (Bu3 Sn-μ-H)2 Rh(H)(PPh3 )2 [Rh(SnCl3 )5 ]4−
151 293 483 118.3 −121.6 −11.7 8.5
[J (119 Sn,M)] { J (119 Sn,117 Sn)} 2
– – – – {1100} – {630.0}
[150] [1470] [894.0]
J (119 Sn,13 C)
J (119 Sn,31 P)
Ref.
94.0 (2 J ) 97.0 (2 J ) 49.0 (2 J ) 92.0 (2 J ) 100.0 (2 J ) 93.0 (2 J )
– – – – – – –
210 210 211 211 212 212 213 5 5 5 5 5 214
179.0
(1 J
)
136.0(2 J , 1 H)
{349.8} {332.3} {302.0} [64.0] {1798} {1580} [36.0] [42.7] 2981 (1 J(Sn,Sn)) {2474 (cis) 13460 (trans)} [1290] {2270 (cis, eq, eq), 1614 (eq, ax), 19465 (trans, eq,eq)}
[314.2] [412.9] [102.9] [806] {3803}
274.4 (1 J ) 282.3 (1 J ) – 86.0, 48.0 28.0 (2 J ) – 42.0 (2 J ) – 95 (2 J ) 2474 (2 J cis) 13460 (trans)
n.r. n.r.
1560 (1 J ) 443.0
–
– – – – – – – –
5 215 215 5 5 5 5 216 216 217 217 217 217 217 217 218 218 219 219 5
5 5 5 221 221 223 5 (Continued )
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Table 2.1.9
119
Sn NMR parameters of compounds with a transition metal–tin bonda δ
Compound [Rh(H)(SnCl3 )5 ]3− [Ir(H)(SnCl3 )5 ]3− cis-(Me3 Sn)2 Pt(PPh3 )2 fcSnMe2 Pt(PPh3 )2 SnMe2 e
119
[J (119 Sn,M)] { J (119 Sn,117 Sn)} J (119 Sn,13 C) 2
Sn
77.8(ax) −13.7 (eq) −185.6 (ax) −318.8 (eq) −16.5 −27.5
[532,ax] [596,eq] [e] [8532.0] [9056.0]
cis-Me3 Sn(Me3 SnC=C)Pt- −57.5 (SnPt) [8140] (PPh3 )2 −88.9 (SnC ) 278.8 (3 J (Pt,Sn) −144.0 [15077] [etpPt-SnCl3 ]− f cis-[PtCl2 (SnCl3 )2? ]2− [Pt(SnCl3 )5 ]2−
−375 −126.4
[27627] {2936} [15791] {6057}
(Continued)
J (119 Sn,31 P)
Ref.
–
224 224 224
– −
652.3[f] 141,224 +206.0 (cis) 141 −1574.5 (trans) 111.0 (cis) 141 1606 (trans) 190.0 (cis) 225 2044.0 (trans) 226 – 227
– – – – – –
a
See References 5 and 8 for more extensive compilations. A relationship between the W Sn bond length and the magnitude of 1 J (183 W,119 Sn) has been proposed for tungstenocene–stannyl complexes.235 c For some other complexes with Os Sn bonds, see Reference 220. d OEP = 2,3,7,8,12,13,17,18-Octaethylporphyrinato ligand. e 1,3-Distanna-2-platina-[3]ferrocenophane. f etp = tris(2-diphenylphosphanoethyl)phosphane. b
(Scheme 2.1.10). This deshielding is related to the presence of an energetically low-lying π * LUMO available for B0 -induced rotation of charge, causing an increase in the paramagnetic contribution to magnetic shielding.
(OC)2(Cp)Mn
Mn(Cp)((CO)2
Cr(CO)5
Sn
Sn Mn(Cp)(CO)2
48
δ119Sn 3301
Scheme 2.1.10 Two examples of extreme metal complexes
(OC)5Cr
2-
Cr(CO)5 49
δ119Sn 3924 119
Sn nuclear magnetic deshielding in low valent tin transition
Isotope-Induced 119 Sn Chemical Shifts
There are no primary isotope shifts for the magnetically active tin nuclei.100 As expected for a heavy nucleus, such as 119 Sn, secondary isotope effects may be quite large and can be readily detected. In tributyltin tritide, the 1 1/3 H(119 Sn) value was determined as −2.7 ppm (to be compared with 1 1/2 H(119 Sn) = − 1.8 ppm),101 whereas in stannane SnH4−n Dn , the 1 1/2 H(119 Sn) values are ca.−0.403 ppm/D.102 Weak intra-molecular association can be deduced from long-range isotope effects n 1/2 H(119 Sn), e.g. in an RO(H). . . Sn fragment.103
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Numerous data have been collected for 1 12/13 C(119 Sn).104 These isotope-induced shifts possess the expected negative sign (shifts of the 119 Sn NMR signals to lower frequency for the heavier isotope) as well as a positive sign, depending on the other substituents at the tin atom. In addition to mass-dependent vibrational effects, responsible for part of the isotope-induced chemical shifts, there is also a strong contribution arising from electronic effects.105 Typically, the latter effects become increasingly important and noticeable for heavy nuclei. The application of HEED extended pulse sequences106 has opened the way to the measurement of isotope shifts 1 14/15 N(X) in natural abundance of 15 N (0.37 %). Expectedly, the isotope-induced effect of the different tin isotopes 117 Sn and 119 Sn on chemical shifts of other nuclei is very small, considering the small difference in their masses. However, precise measurements of 13 C104d,e and 15 N NMR spectra107 revealed these tiny effects on 13 C and 15 N nuclear shielding, respectively [1 117/119 Sn(13 C), 1 117/119 Sn(15 N), 1 115/119 Sn(15 N)].
2.1.5
Indirect Nuclear Spin–Spin Coupling Constants n J(119 Sn,X)
General
Indirect nuclear spin–spin coupling constants n J (119 Sn,X) can be measured either from 119 Sn NMR spectra, as splitting or as satellites (dependent on the natural abundance of X), or from X NMR spectra as 117/119 Sn satellites accompanying the central X NMR signal. The presence of two magnetically active spin-1/2 isotopes of the same element, 119 Sn and 117 Sn in fair natural abundance, allows for the measurement of 119 Sn–117 Sn spin–spin coupling, if the two tin atoms are in identical surroundings, or, additionally, of 119 Sn–119 Sn spin–spin coupling for different surroundings (Figure 2.1.8). The J values possess a sign, positive or negative, which frequently is not given, although the signs are known in the majority of cases. In case of ambiguity, the respective sign can be determined by appropriate 1D or 2D NMR experiments7 (see Figure 2.1.9 for an example). One should keep in mind that γ (119 Sn) < 0 (see Tables 2.1.1 and 2.1.2 for the signs of other γ values). Therefore, it might be more convenient to use the notation of reduced coupling constant n K (119 Sn,X), n
K (119 Sn,X) = n J (119 Sn,X) 4π 2 (γ (119 Sn) γ (X) h)−1
(2.1.2)
since J and K values possess opposite signs if γ (X) > 0. Examples for various coupling constants involving 119 Sn can be found in the Tables 2.1.3–2.1.10. One-Bond Coupling Constants 1 J(119 Sn,X)
The sign of 1 J (119 Sn,1 H) is negative (1 K (119 Sn,1 H) > 0), typical of one-bond spin–spin coupling involving 1 H. The changes in the magnitude of 1 J (119 Sn,1 H) (Table 2.1.3) appear to depend on the sum of the electronegativities of the other substituents at tin, as expected, if the concept of rehybridization108 is valid. This simple relationship might become more complicated in the case of five-coordinate tin hydrides,109 where the Sn H bond in solution might prefer, on average, either axial or equatorial positions. In the known organotin(IV) compounds, the coupling constants 1 J (119 Sn,13 C) are negative (1 K (119 Sn,13 C) > 0), although the concept of rehybridization is no longer strictly valid (see Scheme 2.1.11 for some selected 1 J (119 Sn,13 C) data). The sign of 1 J (119 Sn,13 C) changes to positive (1 K (119 Sn,13 C) < 0) in triorganotin lithium derivatives.110 In organotin(II) compounds, the influence of the lone pair of
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Figure 2.1.8 186.5 MHz 119 Sn{1 H} NMR spectrum (refocused INEPT pulse sequence with CPD 1 H decoupling) of an organometallic-substituted alkene (23 ◦ C in C6 D6 ) bearing two stannyl groups (see Figure 2.1.10 for the different line widths). The 117 Sn satellites (arrows) corresponding to 2 J ( 119 Sn,117 Sn) = 811 Hz are readily identified as part of an AX spin system, whereas the 119 Sn satellites corresponding to 2 J ( 119 Sn,119 Sn) form an AB-spin system, of which only the inner lines (asterisks) are visible.86
electrons111 at the tin atom will cause a negative sign of 1 K (119 Sn,13 C).36 The bond angle CSnC (θ) appears to be related to the magnitude of 1 J(119 Sn,13 C) for dimethyltin(IV) [|1 J (119 Sn,13 C) | = 10.7 (± 0.5) θ − 778 (± 64)] 112 or dibutyltin(IV) compounds [1 J (119 Sn,13 C) | = 9.99 (± 0.73) θ − 746 (± 100)].113 The sign of 1 J (119 Sn,29 Si) is positive in silylstannanes (1 K (119 Sn,29 Si) > 0),114 (see Scheme 2.1.12), and also a positive sign of 1 K(119 Sn,73 Ge) in (Me3 Sn)4 Ge has been determined.115 In the case of the tris(trimethylsilyl)stannyl anion, the magnitude of 1 J (119 Sn,29 Si) is dependent on the cation and the solvent.133 It is likely that sign inversion has to be considered, which was experimentally verified for 1 119 J ( Sn119 Sn) in the corresponding tris(trimethylstannyl)stannyl anion.135 In the case of polystannanes, sign inversion of 1 J (119 Sn,119 Sn) from positive (1 K (119 Sn,119 Sn) > 0) to negative (1 K (119 Sn,119 Sn) < 0) has been proposed as a result of the increase in the Sn Sn bond length, when it exceeds 285 pm.116 In the case of plumbylstannanes, positive and negative 1 K (207 Pb,119 Sn) values have been determined experimentally (e.g. 1 J (207 Pb,119 Sn) = −3570 Hz for Me3 Sn–PbMe114b and +1637 Hz for 3 Me3 Sn-PbBut3 117). Owing to the polar Sn N bond and the presence of the lone pair of electrons at the nitrogen atom, coupling constants 1 J (119 Sn,15 N) may be of either sign (Scheme 2.1.13), and in the case of triorganostannylamines, changes in sign may even have to be considered for different R groups linked
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Figure 2.1.9 Contour plots of the 50.4 MHz 15 N/ 1 H (left) and 186.5 MHz 2D119 Sn/ 1 H shift correlations of 15 N enriched (ca. 10%) PhN(SnMe3 )2 . The experiments serve for comparison of signs of reduced coupling constants K , and positive and negative tilts of the cross peaks indicate alike or different signs of K , respectively. In the formulae A to D, the arrows indicate the path of 1 H polarization transfer, and the dashed lines from the respective passive to the active nuclei show which reduced coupling constants are compared with respect to their signs. Thus, in the 15 N/ 1 H correlation, 119 Sn or 117 Sn is the passive nucleus, and in the 119 Sn/ 1 H correlation 15 N (labeled ‘C’) and 117 Sn (labeled ‘D’) are the passive nuclei. Since it is known that 2 K (Sn, 1 H Me ) < 0, all other signs (see ‘results’) follow unambiguously. (Adapted from Reference 119.)
J(119Sn,13C) [Hz] (Me) 294.0 436.6 Me3SnBut 353.5 450.9 Me3SnCH=CH2 Me3SnPh 347.5 474.4 CH Me3SnC 410.3 414.8 Me3SnH 349.5 Me3SnCl 379.0 396.3 (Me3Sn)2O Me3SnNEt2 381.0 1
1
J(119Sn,13C) [Hz]
336.6 SnMe4 320.0 SnEt4 308.5 SnPri4 258.8 Sn(CH2Ph)4 Sn(CH=CH2)4 519.3 531.1 SnPh4 Sn(C CH)4 1176.2 1001.0 Sn(CF3)4
Scheme 2.1.11
J(119Sn,13C) [Hz] (Me) 369.4 466.8 Me2Sn(CH=CH2)2 1
Me2Sn(C
CH)2
(Me3Sn)4C Me2SnH2 Me2Sn(NEt2)2 MeSn(C CH)3 MeSn(NEt2)3
503.0 316.3 485.1 469.7 630.9 592.4
576.2 108.3
833.0
Some representative examples of one-bond 119 Sn–13 C spin–spin couplings
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Me3SnSiMe3 (Me3Sn)2SiMe2 (Me3Sn)4Si
J(119Sn,29Si) [Hz]
656 114a 504 131 (Ph3Sn)2SiPh2 t 220 131 Bu 2Sn(SiPh3)2 132
Me2Sn(SiMe3)2
525
Sn(SiMe3)4
351 133
Ph2Sn(SiMe3)2
–
[Sn(SiMe3)3] [Cs(2.2.2)crypt]+
Scheme 2.1.12
515 134 357 132 520 132
Me2 Me2 Si t But2Sn SiSiMe Bu 2Sn SiMe2 2 t t Bu 2Sn SiMe2 Bu 2Sn Si Si Me2 Me2 214 189b 217 189b
355 133
Selected examples of one-bond 119 Sn–29 Si spin–spin couplings
to the nitrogen atom.114a,118,119 The magnitude of 1 J (119 Sn,15 N) in monomeric bis(amino)stannylenes is large,26,120 and a negative sign (1 K (119 Sn,15 N) < 0!) can be safely assumed, since there are lone pairs of electrons at both nitrogen and tin atoms. In any case, 119 Sn–15 N coupling may serve, together with δ 119 Sn data, as an indication of coordinative Sn–N bonding.109,121 1
Me3SnN(H)Ph Me3SnN(Me)Ph Me3SnN(SiMe3)2 1-Me3Sn-pyrrole 1-Me3Sn-indole 8-Me3Sn-carbazole Me3SnN(PMe2)Ph Me3SnN[P(S)Me2]Ph
J(119Sn,15N) [Hz]
–26.3 +2.2 –52.5 –37.2 –45.3 –49.3 +9.5 –21.0
(Me3Sn)2NBut (Me3Sn)2NPh (Me3Sn)2NSiMe3 (Me3Sn)2NGeMe3 (Me3Sn)3N (Me3Sn)2NBC8H14 (Me3Sn)2NNMe2
–38.0 –41.4 –59.0 –68.4 –83.7 –31.9 –45.5
Me2Sn[N(Me)Ph]2 +24.0 MeSn[N(Me)Ph]3 +87.0 +175.0 Sn[N(Me)Ph]4 Sn[N(But)SiMe3]2 –398.9 –366.0 Sn[N(SiMe3)2]2 But N SiMe2 Sn SiMe2 N
–382
But
Scheme 2.1.13 Sign and magnitude of various coupling constants 1 J ( 119 Sn15 N) for tin(IV)- and tin(II)nitrogen compounds
Except for the phosphane adducts of tin(IV) halides, the experimental evidence points towards a positive sign of 1 J (119 Sn,31 P) (1 K(119 Sn,31 P) < 0!) in all tin–phosphorus(III) compounds,122 a consequence of the dominant influence of the lone pair of electrons at the phosphorus atom. Even for transition metal complexes of stannylphosphanes,122 or in borane complexes of stannylphosphanes,123 the coupling sign remains positive. A negative sign of 1 K (119 Sn,77 Se)124a and 1 K (125 Te,119 Sn)124a is likely for all tin–selenium and tin– tellurium compounds,124 and this can also be assumed for 1 K (119 Sn,19 F). In contrast, a positive sign can be attributed to the 1 K (119 Sn,M) (M = transition metal nuclei) values known so far. In the case of Sn X bonds and quadrupolar nuclei X, the 119 Sn NMR signal may be broadened without resolved splitting, as a result of scalar relaxation of the second kind.125 In this case, the coupling constants can be calculated, if the relaxation time of the X nucleus is known.126 In favorable cases, the pattern of the solid-state 119 Sn NMR spectra can be analyzed with respect to the isotropic indirect nuclear spin–spin coupling (e.g. in Ph3 SnCl, 1 J (119 Sn,35 Cl) = +280 and +275 Hz).127
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Two-Bond (Geminal) Coupling Constants 2 J(119 Sn,X)
There is a wealth of data for 2 J (119 Sn,1 H) in organotin compounds, mainly for SnMe groups, and in this case a smoothly curved relationship between 2 J (119 Sn,1 H) and 1 J (119 Sn,13 CMe ) has been noted.110 The sign of 2 J (119 Sn,1 H) is positive, in general (2 K (119 Sn,1 H) < 0!), for intervening aliphatic carbon atoms, in contrast with the negative sign of 2 J (119 Sn,1 H) if the intervening carbon atom is olefinic (e.g. for all vinyl tin compounds). The general trend of data and sign of 2 J (119 Sn,13 C) data is analogous to 2 J (119 Sn,1 H). However, since the magnitude of 2 J (119 Sn,13 C) is less predictable than that of 2 J (119 Sn,1 H), it is advisable to determine the sign of 2 J (119 Sn,13 C) prior to discussion.128 This advice applies similarly to other coupling constants 2 119 J ( Sn,X), and the relevant NMR techniques have been reviewed.7 In five-membered stannacycles, the coupling pathways across two and three bonds have to be considered, and since the respective contributions are of opposite signs, in most cases, the interpretation of the magnitude of these coupling constants is not always straightforward. If the intervening nuclei are carbon atoms, the contribution from 2 J is usually small when compared with that of 3 J . For transition metal complexes with two or more SnCl3 ligands, the magnitude of the coupling constants 2 119 J ( Sn,117 Sn) across the metal can range from small values to several hundred and several thousand Hz, depending on the stereochemistry, other substituents and the intervening metal atom. As expected, a trans-coupling pathway usually leads to very large coupling constants. Three-Bond (Vicinal) Coupling Constants 3 J(119 Sn,X)
The magnitude of vicinal coupling constants depends on the dihedral angle in the respective fragment (Karplus-type dependence). This relationship holds also for 3 J (119 Sn,X) (e.g. X = 1 H, 11 B, 13 C, 15 N) in many tin compounds (see Scheme 2.1.14) for examples with X = 13 C), and has been widely exploited for structural assignments.4,5,7,85,129,136 3
Me3Sn
J(119Sn,13C) [Hz]
CH3
Me3Sn
CH2Me 64.3 130 BEt2
Me3Sn
BEt2 83.0 130 CH3
49.3 136 Me3Sn CH3 Me3Sn
CH3
Me3Sn
79.0 136 Me3Sn 106.0 (trans) 137 77.8 (cis) 137 Me3Sn
CH3
141.5 (trans) 138 BEt2 118.2 (cis) 138 2
Me2Sn
CMe2
88.0 88
Me2Sn 5
Me2Sn
H2 C
4.2 85
B
Dihedral angle Sn-C-C-CH2 close to 90°
Scheme 2.1.14
Me2Sn
63.0 86 CH2CH3 3 2 3 86 4 66.0 (3) ( J + J ) 61.0 (4) ( 2J + 3J ) 86 H2 C
68.4 85
B
Dihedral angle Sn-C-C-CH2 close to 180°
Stereochemical dependence of three-bond (vicinal) 119 Sn–13 C spin–spin couplings
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Figure 2.1.10 76.4 MHz 119 Sn[1 H} NMR spectrum of a mixture of isomers of alkenylstannanes, in which a boryl group is either in cis- or in trans-position relative to the stannyl group.130 The line widths of the 119 Sn NMR signals are a function of the residual broadening owing to partial relaxation of three-bond 119 Sn 11 B spin–spin coupling.86
So far, most signs determined for 3 K (119 Sn,X) are positive, with very few exceptions. If X is a quadrupolar nucleus such as 11 B, differential broadening of the 119 Sn NMR signals (see Figures 2.1.3 and 2.1.10 for partially relaxed scalar 119 Sn–11 B coupling) indicates the relative magnitudes of |3 J(119 Sn,11 B)|.130 Long-Range Coupling Constants n J(119 Sn,X) with n > 3
As is typical for heavy spin-1/2 nuclei with an appreciably large magnetic moment, 119 Sn–X spin–spin coupling across four or more bonds is often observed (e.g. X = 1 H, 13 C, 19 F). However, these data are diagnostic only in the case of series of analogous compounds. One problem in this context is that the sign of such coupling constants may change readily, and a second problem is the fact that these data receive only scant attention. In general, n J (119 Sn,X) data are measured if: (i) multiple equivalent coupling pathways through polycyclic structures, (ii) π -bonding systems, or (iii) a metal, or both a metal and π -bonding systems are involved. Thus, in most benzyltin compounds 5 J (119 Sn,13 C) can be measured. The same is true for 2-picolyltin derivatives.106b The π-system of allenes is also well known to mediate spin–spin couplings across four or five bonds, and various types of long-range coupling constants n J (119 Sn,1 H), n J (119 Sn,13 C) and n 119 J ( Sn,119 Sn) (n = 4,5; see Reference 139 for other n J (Sn,Sn) data) have been determined, including the respective signs.138 Two examples for Sn–Sn coupling across six and four bonds with intervening metals and π-systems are given in Scheme 2.1.15. In principle, coupling constants n J (119 Sn,X) (n > 3) have to be considered as valuable parameters for structural assignments. Moreover, in particular for X = 1 H, they can serve for improving NMR
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Me3Sn
PEt3 Pt PEt3
6J(119Sn,117Sn)
Scheme 2.1.15
SnMe3 25.0 Hz 140
Me3Sn
45
SnMe3 Pt PPh3 PPh3
4J(119Sn,117Sn)
17.5 Hz 141
Long range Sn–Sn spin–spin coupling constants in platinum(II) complexes
experiments in inverse 1 H/119 Sn HMBC experiments.1 Furthermore, various selective 1D and 2D polarization transfer experiments can be based on small coupling constants n J (119 Sn,1 H). References 1. 2. 3. 4. 5. 6.
7.
8. 9. 10. 11.
12.
13. 14. 15. 16. 17.
18. 19. 20.
J. C. Martins, M. Biesemans, and R. Willem, Progr. NMR Spectrosc. 36, 271 (2000). J. D. Kennedy and W. McFarlane, Rev. Silicon, Germanium, Tin, Lead Compounds 1, 235 (1974). P. J. Smith and A. P. Tupciauskas, Annu. Rep. NMR Spectrosc. 8, 291 (1976). J. D. Kennedy and W. McFarlane, in Multinuclear NMR, J. Mason (Ed.), Plenum Press, New York, 1987, pp. 305–333. B. Wrackmeyer, Annu. Rep. NMR Spectrosc. 16, 73 (1985) F. Kayser, M. Biesemans, M. Gielen, and R. Willem, in Physical Organometallic Chemistry – Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem, and B. Wrackmeyer (Eds.),Vol. 1, John Wiley & Sons, Ltd, Chichester, 1996, pp. 45–86. B. Wrackmeyer, in Physical Organometallic Chemistry – Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem, and B. Wrackmeyer (Eds.),Vol. 1, John Wiley & Sons, Ltd, Chichester, 1996, pp. 87–122. B. Wrackmeyer, Annu. Rep. NMR Spectrosc. 38, 203 (1999). R. K. Harris, E. D. Becker, S. M. Cabral de Menezes, R. Goodfellow, and P. Granger, Encyclopedia of Nuclear Magnetic Resonance 9, 5–19, John Wiley & Sons, Ltd, Chichester, 2002. A. Sebald, in Solid-State NMR II – NMR-Basic Principles and Progress, P. Diehl, E. Fluck, H. G¨unther, R. Kosfeld, and J. Seelig (Eds.), Vol. 31, Springer, Berlin, 1994, pp. 91–131. A. Sebald, in Physical Organometallic Chemistry – Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem, and B. Wrackmeyer (Eds.),Vol. 1, John Wiley & Sons, Ltd, Chichester, 1996, pp. 123–157. (a) G.A. Morris and R. Freeman, J. Am. Chem. Soc. 101, 760 (1979); (b) G.A. Morris, J. Am. Chem. Soc. 102, 428 (1980); (c) G.A. Morris, J. Magn. Reson. 41, 185 (1980); (d) D.P. Burum, R.R. Ernst, J. Magn. Reson. 39, 163 (1980). V. Gouron, B. Jousseaume, M. Ratier, J.-C. Lartigue, and M. Petraud, Magn. Reson. Chem. 28, 755 (1990). (a) D. T. Pegg, D. M. Doddrell, W. M. Brooks, and M. R. Bendall, J. Magn. Reson. 44, 32 (1981); (b) D. T. Pegg, D. M. Doddrell, and M. R. Bendall, J. Chem. Phys. 77, 2745 (1982). T. Parella, Magn. Reson. Chem. 36, 467 (1998). (a) G. Metz, X. Wu, and S. O. Smith, J. Magn. Reson. Series A 110, 219 (1994); (b) O. B. Peersen, X. Wu, and S. O. Smith, J. Magn. Reson. Series A 106, 127 (1994). (a) S. J. Blunden, A. Frangou, and D. G. Gillies, Org. Magn. Reson. 20, 170 (1982); (b) A. Lauksonen, and R. E. Wasylishen, J. Am. Chem. Soc. 117, 392 (1995); (c) T. B. Grindley, R. D. Curtis, R. Thangarasa, and R. E. Wasylishen, Can. J. Chem. 68, 2103 (1990); (d) T. Birchall, V. Manivannan, Can. J. Chem. 63, 2211 (1985). B. Wrackmeyer, C. Stader, K. Horchler, H. Zhou, and D. Schlosser, Inorg. Chim. Acta, 176, 205 (1990). B. Wrackmeyer, G. Kehr, and R. Boese, Chem. Ber., 125, 643 (1992). (a) A.C. de Dios, Magn. Reson. Chem. 34, 773 (1996); (b) N. Bertazzi, G. Casella, F. Ferrante, L. Pellerito, A. Rotondo, and E. Rotondo, Dalton Trans. 2007, 1440.
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S. Adams and M. Dr¨ager, J. Organomet. Chem. 323, 11 (1987). J. D. Kennedy and W. McFarlane, J. Chem. Soc. Dalton Trans. 1976, 1219. T. N. Mitchell and C. Kummetat, J. Organomet. Chem. 157, 275 (1978). T. N. Mitchell and A. Amamria, J. Organomet. Chem. 252, 47 (1983). B. Wrackmeyer, U. D¨orfler, G. Kehr, H.E. Maisel, and W. Milius, J. Organomet. Chem. 524, 169 (1996). T.N. Mitchell and C. Nettelbeck, Magn. Reson. Chem. 25, 879 (1987). B. Wrackmeyer, K. Horchler von Locquenghien, E. Kupce, and A. Sebald, Magn. Reson. Chem. 31, 45 (1993). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, Chem. Eur. J. 4, 1027 (1998). R. Eujen, N. Jahn, and U. Thurmann, J. Organomet. Chem. 434, 159 (1992). H. Schumann, C. Janiak, E. Hahn, C. Kolax, J. Loebel, M.D. Rausch, J.J. Zuckerman, and M.J. Heeg, Chem. Ber. 119, 2656 (1986). T. Kawakami, T. Sugimoto, A. Baba, H. Matsuda, and N. Sonoda, J. Org. Chem. 60, 2677 (1995). T. Janati, J.-C. Guillemin, and M. Soufiaoui, J. Organomet. Chem. 486, 57 (1995). L. Lasalle, T. Janati, and J.-C. Guillemin, Chem. Commun. 1995, 699. B.E. Mann and B.F. Taylor, 13 C NMR Data of Organometallic Compounds, Academic Press, London 1981. J. Holecek, A. Lycka, K. Handlir, and M. Nadvornik, Collect. Czech Chem. Commun. 53, 571 (1988). I. Wharf, Inorg. Chim. Acta, 1989, 159, 41 (1089). B. Wrackmeyer, G. Kehr, D. Wettinger, and W. Milius, Main Group Met. Chem. 16, 433 (1993). T.N. Mitchell, A. Amamria, H. Killing, and D. Rutschow, J. Organomet. Chem. 304, 257 (1986). B. Wrackmeyer, E. Kupce, and J. K¨ummerlen, Magn. Reson. Chem. 30, 403 (1992). B. Wrackmeyer, Z. Naturforsch. Teil B, 34, 235 (1979). B. Wrackmeyer, H.E. Maisel, and H. Zhou, Main Group Met. Chem. 16, 475 (1993). I. I. Padilla-Martinez, M. de Jesus Rosalez-Hoz, R. Contreras, S. Kerschl, and B. Wrackmeyer, Chem. Ber. 127, 343 (1994). F.H. K¨ohler, W.A. Geike, and N. Hertkorn, J. Organomet. Chem. 334, 359 (1987). B. Wrackmeyer, J. Magn. Reson. 42, 287 (1981). F. H¨olzl and B. Wrackmeyer, J. Organometal. Chem. 179, 397 (1979). B. Wrackmeyer, G. Kehr, and D. Wettinger, Inorg. Chim. Acta, 220, 161 (1994). R. N. Kapoor, P. Apodaca, M. Montes, F. D. Gomez, and K. H. Pannell, Appl. Organomet. Chem. 19, 518 (2005). J. W. Emsley, L. Phillips, and V. Wray, Prog. NMR Spectrosc. 10, 716 (1977). S. S. Al-Juaid, S. M. Dhaher, C. Eaborn, P. B. Hitchcock, and J. D. Smith, J. Organomet. Chem. 325, 117 (1987). D. Dakternieks and H. Zhu, Organometallics, 11, 3820 (1992). M. Jang and A. F. Janzen, J. Fluorine Chem. 66, 129 (1994). S. .E. Johnson and C. B. Knobler, Organometallics,11, 3684 (1992). A. Lycka, J. Jirman, A. Kolonicny, and J. Holecek, J. Organomet. Chem. 333, 305 (1987). C. Picard, P. Tisnes, and L. Casaux, J. Organomet. Chem. 315, 277 (1986). D. L. Reger, S. S. Mason, J. Takats, X. W. Zhang, A. L. Rheingold, and B. S. Haggerty, Inorg. Chem. 32, 4345 (1993). J. Holecek, M. Nadvornik, K. Handlir, and A. Lycka, J. Organomet. Chem. 315, 299 (1986). J. Holecek, K. Handlir, M. Nadvornik, S.M. Teleb, and A. Lycka, J. Organomet. Chem. 339, 61 (1988). B. Wrackmeyer, G. Kehr, and H. Zhou, S. Ali, Inorg. Chim. Acta, 197, 129 (1992). V. Pejchal, J. Holecek, M. Nadvornik, and A. Lycka, Collect. Czech. Chem. Commun. 60, 1492 (1995). K. B. Dillon and A. Marshall, J. Chem. Soc. Dalton Trans. 1987, 315. M.-C. Ye and J.G. Verkade, Energy & Fuels, 8, 172 (1994). B. Wrackmeyer, U. Klaus, and W. Milius, Chem. Ber. 128, 679 (1995). W. A. Herrmann, J. G. Kuchler, J. K. Felixberger, E. Herdtweck, and W. Wagner, Angew. Chem. 100, 420 (1988); Angew. Chem. Int. Ed. 27, 394 (1988).
141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176.
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C.D. Chandler, G.D. Fallon, A.J. Koplick, and B.O. West, Aust. J. Chem. 40, 1427 (1987). O.-S. Jung and Y.S. Sohn, Bull. Korean. Chem. Soc. 9, 365 (1988). M. Herberhold, M. H¨ubner, and B. Wrackmeyer, Z. Naturforsch. Teil B, 48, 940 (1993). A. G. Davies, S. D. Slater, D. C. Povey, and G. W. Smith, J. Organomet. Chem. 352, 283 (1988). P. A. Bates, M. B. Hursthouse, A. G. Davies, and S. D. Slater, J. Organomet. Chem. 363, 45 (1989). H. Berwe and A. Haas, Chem. Ber. 120, 1175 (1987). H. Puff, G. Bertram, B. Ebeling, M. Franken, R. Gattermayer, R. Hundt, W. Schuh, and R. Zimmer, J. Organomet. Chem. 379, 235 (1989). P. A. W. Dean and R. V. S. Srivastava, Inorg. Chim. Acta, 105, 1 (1985). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, J. Organomet. Chem. 533, 109 (1997). J. Campbell, D. P. DiCiommo, H. P. A. Mercier, A. M. Pirani, G. J. Schrobilgen, and M. Willuhn, Inorg. Chem. 34, 6265 (1995). R. C. Burns, L. A. Devereux, P. Granger, and G. J. Schrobilgen, Inorg. Chem. 24, 2615 (1985). J. Campbell, L. A. Devereux, M. Gerken, H. P. A. Mercier, A. M. Pirani, and G. J. Schrobilgen, Inorg. Chem. 35, 2945 (1995). (a) U. Hermann, G. Reeske,M. Sch¨urmann, and F. Uhlig, Z. Anorg. Allg. Chem. 627, 453 (2001); (b) U. Hermann, M. Sch¨urmann, and F. Uhlig, J. Organomet. Chem. 585, 211 (1999). A. D¨orr, D. Gudat, D. H¨anssgen, H. Hens, and E. Stahlhut, Bull. Soc. Chim. Fr. 131, 674 (1994). S. Geetha, M. Ye, and J.G. Verkade, Inorg. Chem. 34, 6158 (1995). K. C. Molloy, K. Quill, S. J. Blunden, and R. Hill, J. Chem. Soc. Dalton Trans. 1986, 875. B. Wrackmeyer, G. Kehr, H. Zhou, and S. Ali, Main Group Met. Chem. 15, 89 (1992). S. J. Blunden, P. A. Cussack, and D. G. Gillies, J. Magn. Reson. 60, 114 (1984). M. Herberhold, U. Steffl, and B. Wrackmeyer, Z. Naturforsch. Teil B,54, 57 (1999). M. Drieß, R. Janoschek, H. Pritzkow, and U. Winkler, Angew. Chem. 107, 1746 (1995); Angew. Chem. Int. Ed. 34, 1614 (1995). H. Schumann and K.-H. K¨ohricht, J. Organomet. Chem. 373, 307 (1989). T. N. Mitchell, R. Faust, B. Fabisch, and R. Wickenkamp, Magn. Reson. Chem. 28, 82 (1990). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, Angew. Chem. 108, 1927 (1996); Angew. Chem. Int. Ed. 35, 1803 (1996). M. Herberhold, U. Steffl, W. Milius, and B. Wrackmeyer, Z. Anorg. Allg. Chem. 624, 386 (1998). B. Watta, W.P. Neumann, and J. Sauer, Organometallics, 4, 1954 (1985). H. Puff, C. Bach, W. Schuh, and R. Zimmer, J. Organomet. Chem. 312, 313 (1986). K. Jurkschat, A. Tzschach, C. M¨ugge, J. Piret Meunier, M. van Meerssche, G. van Binst, C. Wynants, M. Gielen, and R. Willem, Organometallics, 7, 593 (1988). W. Biffar, H. N¨oth, H. Pommerening, R. Schwerth¨offer, W. Storch, and B. Wrackmeyer, Chem. Ber. 114, 49 (1981). D. K. Srivastava, N. P. Rath, and L. Barton, Organometallics, 11, 2263 (1992). P. Jutzi, D. Wegener, H.-G. Stammler, A. Karaulov, and M.B. Hursthouse, Inorg. Chim. Acta 198, 369 (1992). T. G¨adt, F. M. Schappacher, R. P¨ottgen, and L. Wesemann, Inorg. Chem. 46, 2864 (2007). H. J. Reich, J. P. Borst, and R. R. Dykstra, Organometallics, 13, 1 (1994). P. B. Hitchcock, M. F. Lappert, G. A. Lawless, and B. Royo, Chem. Commun. 1993, 554. J. E. Ellis and P. Yuen, Inorg. Chem. 32, 4998 (1993). J. E. Ellis, P. Yuen, and M. Jang, J. Organomet. Chem. 507, 283 (1996). J. E. Ellis, K.-M. Chi, A.-J. DiMaio, S. R. Frerichs, J. R. Stenzel, A. L. Rheingold, and B. S. Haggerty, Angew. Chem. 103, 196 (1991); Angew. Chem. Int. Ed. 30, 194 (1991). W. E. Piers, R. M. Whittal, G. Ferguson, J. F. Gallagher, R. D. J. Froese, H. J. Stronks, and P. H. Krygsman, Organometallics, 11, 4015 (1992). K. Jurkschat, H.-B. Abicht, A. Tzschach, and B. Mahieu, J. Organomet. Chem.309, C47 (1986). U. Schubert, E. Kunz, B. Harkers, J. Willnecker, and J. Meyer, J. Am. Chem. Soc. 111, 2572 (1989). W. Petz, B. Wrackmeyer, and W. Storch, Chem. Ber. 122, 2261 (1989).
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217. 218. 219. 220.
B. Wrackmeyer, B. Distler, and M. Herberhold, Z. Naturforsch. Teil B, 47, 1749 (1992). Z. Duan, D. Lei, and M. J. Hamdensmith, Polyhedron, 10, 2105 (1991). M. Moriyama, T. Aoki, S. Sjimota, and Y. Saito, J. Chem. Soc. Chem. Commun. 1982, 500. M. M. M¨ohlen, C. E. F. Rickard, W. R. Roper, G. R. Whittell, and L. J. Wright, J. Organomet. Chem. 691, 4065 (2006). T. Mizutani, T. Uesaka, and H. Ogoshi, Organometallics, 14, 341 (1995). L. Carlton and R. Weber, Inorg. Chem. 32, 4169 (1993). T. Yamakawa, S. Shinoda, Y. Saito, H. Moriyama, and P. S. Pregosin, Magn. Reson. Chem. 23, 202 (1985). Y. Obora, Y. Tsuji, K. Nishiyama, M. Ebihara, and T. Kawamura, J. Am. Chem. Soc. 118, 10922 (1996). P. Br¨uggeller, Z. Naturforsch. Teil B, 41, 1561 (1986). J. H. Nelson, W. L. Wilson, L. W. Cary, N. W. Alcock, H. J. Clase, G. S. Jas, L. Ramsey-Tassin, and J. W. Kenney, III, Inorg. Chem. 35, 883 (1996). K. R. Koch, Magn. Reson. Chem. 30, 158 (1992). B. Wrackmeyer, B. H. Kenner-Hofmann, W. Milius, P. Thoma, O. L. Tok, and M. Herberhold, Eur. J. Inorg. Chem. 2006, 101. B. Wrackmeyer, H. E. Maisel, W. Milius, and M. Herberhold, J. Organomet. Chem. 680, 271 (2003). T. Endo, F. Sasaki, H. Hara, J. Suzuki, S. Tamura, Y. Nagata, T. Iyoshi, A. Saigusa, and T. Nakano, Appl. Organomet. Chem. 21, 183 (2007). D. J. Eisler and T. Chivers, Chem. Eur. J. 12, 233 (2006). P. B. Hitchcock, J. Hu, M. F. Lappert, and J. R. Severn, Dalton Trans. 2004, 4193. T. Tajima, N. Takeda, T. Sasamori, and N. Tokitoh, Eur. J. Inorg. Chem. 2005, 4291. T. A. Mobley, R. Gandour, E. P. Gillis, K. Nti-Addae, R. Palchaudhuri, P. Rajbhandari, N. Tomson, A. Vargas, and Q. Zheng, Organometallics, 24, 3897 (2005). P. A. Chugunov, N. A. Troitskii, K. S. Nosov, M. P. Egorov, and O. M. Nefedov, Russ. Chem. Bull. Int. Ed. 53, 2327 (2004).
221. 222. 223. 224. 225. 226. 227. 228. 229. 230. 231. 232. 233. 234. 235.
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Tin(II) Clusters
2.2
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Tin(II) Clusters
Tristram Chivers and Dana J. Eisler Department of Chemistry, University of Calgary, Alberta, Canada
2.2.1
Introduction
The chemistry of tin(II) clusters has undergone remarkable development since Veith’s initial report on the synthesis of the imidotin cubane [Sn(μ3 -NR)]4 in 1979.1 A number of synthetic strategies for these compounds have been developed, and a wide array of examples have been well characterized. There are three main structural motifs observed in imidotin clusters, although the cubane cluster [Sn(μ3 -NR)]4 , I (Scheme 2.2.1), is by a wide margin the most common. The seco-cubane structure, II, in which a single corner of the cube is vacant, has only been well-characterized in a single case,2 but a number of related structures have been observed.3−6 More recently, the double-cubane clusters, III, have been reported by Wright,7−9 but have so far only been obtained when heterocyclic substituents are present on the imido-nitrogen centers. The first studies on imidotin clusters focused mainly on synthesis and structures, although a few reactivity studies were performed. The potential of the cubane clusters to act as multidentate Lewis-base ligands was recognized early on, and investigations into the coordination chemistry of these compounds have been undertaken.10,11 More recent studies have focused on the reactions of imidotin clusters with chalcogens, with the aim of generating single-source precursors that can be used for the deposition of tin chalcogenides.12,13
Scheme 2.2.1
The purpose of this chapter is to provide the reader with an overview of the significant findings in tin(II) cluster chemistry up to mid-2007. Although the majority of investigations have involved imidotin clusters, there have been important developments in the last 10 years in our knowledge of related phosphidotin clusters (see Section 2.2.6). 2.2.2
Imidotin Cubane Clusters
Synthesis of [Sn(μ3 -NR)]4
(1) Transamination reactions. Most of the known imidotin cubanes of the type [Sn(μ3 -NR)]4 have been prepared via transamination reactions between a tin amide reagent and a primary amine. The first example was reported by Veith in 1979 and involved the reaction of the cyclic diazastannylene 1 (Scheme 2.2.2) with tert-butylamine at 50 ◦ C.1 However, the initial reaction of these reagents produces either the tricyclic compound 2 or the seco-cubane 3, depending on the reaction stoichiometry (from molar ratios of 1:tBuNH2 of 2:1 or 3:4, respectively). Compounds 2 and 3 are converted to the cubane
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Scheme 2.2.2
cluster [Sn(μ3 -NtBu)]4 , 4, at high temperatures (200 ◦ C).1,14 Alternatively, the reaction of 3 with one equivalent of 1 at 210 ◦ C for 2 days produces 4. In all cases, the cubane 4 was obtained in yields ranging from 83–92%. By contrast, the reactions of 1 with less bulky amines, such as iso-propylamine or N,N dimethylhydrazine, generate the cubanes 5 and 6 in 90% yield under milder conditions (5: 70 ◦ C, 15 min.; 6: spontaneous formation under ambient conditions).15 More recently, Veith has extended this synthetic protocol to generate the series of silyl-substituted cubanes 7–11 via the reaction of 1 with the appropriate primary amines, under conditions similar to those used in the formation of 4.5 With the exception of 7, analogs of the seco-cube 3 were isolated as intermediates in each case; pyrolysis at 250 ◦ C/0.01 torr was required to produce the cubanes [Sn(μ3 -NR)]4 .5 The cubane 7 was isolated by sublimation at 200 ◦ C/0.01 torr, so the possibility of a seco-cube intermediate cannot be ruled out. Since Veith’s early work, a wide variety of imidotin cubane clusters [Sn(μ3 -NR)]4 have been reported and several different synthetic methodologies have been employed, with varying degrees of success. The synthesis of the cubanes 12 and 13 (Scheme 2.2.3), with bulky aromatic substituents on the nitrogen centers, was achieved by the reaction of Sn[N(SiMe3 )2 ]2 with the appropriate primary amine, under relatively mild conditions (13: boiling hexane, 1 h, 35% yield; 12: 50 ◦ C in the melt 5 min, 80% yield).16 It has been suggested that the versatility of the reaction of primary amines with Sn[N(SiMe3 )2 ]2 is limited, since it requires the use of relatively acidic amines,17 however, this claim has not been substantiated by any reported experimental studies. Wright and coworkers have used the powerful base Sn(NMe2 )2 to prepare the cubanes 14 and 15 in 60% and 38% yields, respectively.17 One of the advantages of using the highly reactive reagent Sn(NMe2 )2 is that it allows for the formation and isolation of thermally unstable cubanes, e.g. 15, since the synthesis can be conducted at 20 ◦ C or below.17 Wright has also prepared [Sn(μ3 -NtBu)]4 , 4, from Sn(NMe2 )2 and tBuNH2 in boiling toluene.18 Transamination of Sn(NMe2 )2 has also been employed to synthesize a series of [Sn(μ3 -NR)]4 cubanes that contain
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Scheme 2.2.3
methoxy-substituted phenyl rings, by reaction with the appropriate primary amine.3,19 However, many of these compounds were isolated in low yields (8–15%); notable exceptions are 18, 22 and 23 (82, 68, and 37% yields, respectively). Compound 24 was also obtained in 23% yield by this synthetic route.3 (2) Reactions of Sn(II) Reagents with Metal Amides and Metal Imides. A few [Sn(μ3 -NR)]4 cubanes have been synthesized by methods other than transamination reactions with primary amines. The compounds 25 and 26 (Scheme 2.3.3) were obtained in 21 and 51% yields, respectively, from the reaction of SnCl2 with [(Me3 Sn)(Me3 M)NLi·(Et2 O)]2 (M = Ge, Sn) with the elimination of Me3 SnCl at room temperature.20 The analogous trimethylsilyl-substituted cubane 7 has also been prepared by this route, using the reagent [(Me3 Sn)(Me3 Si)NLi·(Et2 O)]2 .21 The imidotin cubane 27 containing the bulky adamantyl-substituted diiso-propylphenyl substituents was prepared in 76% yield from the room temperature reaction of SnCl2 with the corresponding lithiated amine.22 The known cubane 14 was also generated in 42% yield from the reaction of SnCp2 with lithium cyclohexylamide.23 Finally, the phenylsubstituted cubane 28 was obtained in 42% yield from the reaction of SnCl2 with the magnesium imide [(THF)MgNPh]6 in boiling THF for 18 h; compound 28 was also isolated as a cocrystallized complex with MgCl2 (THF)4 , for which a crystal structure verified the presence of a cubane core.24 Synthesis of Related Imidotin Cubane Clusters
The basketane-like compound 29 (Scheme 2.2.4) was obtained as a minor by-product in the reaction of the cyclic diazastannylene, 1, with Me2 Si(NHMe)2 .25 While 29 has been well characterized, including by an X-ray structural determination, no direct synthetic route to this compound has been reported.25 The
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Scheme 2.2.4
cubane 30a, in which one imido substituent in the cluster is replaced by an oxo group was first prepared (in 44% yield) in 1980 by the stoichiometric hydrolysis of the seco-cubane 3 in boiling tert-butylamine.26 More recently, an improved synthesis of 30a (in 89% yield) from the hydrolysis of the cubane 4, prepared in situ at –78 ◦ C, in a mixture of THF and acetonitrile, was reported.27 The sulfur analog, 30b, obtained from the reaction of 3 with H2 S, has been mentioned in a review.28 However, no details of the synthetic procedure or characterization of this compound were given. Reaction Pathways for the Formation of [Sn(μ3 -NR)]4
The reaction pathway for the formation of [Sn(μ3 -NR)]4 from the cyclic diazastannylene 1 has been discussed previously by Veith.29 More recent studies on the synthesis of imidotin cubanes from Sn(NMe2 )2 have resulted in the isolation and structural characterization of important intermediates.3,4 For example, the reaction of this reagent with the bulky amines 2,6-diiso-propylaniline and 2,4,6-trimethylaniline produces the mixed amido/imido compounds 31 and 32 (Scheme 2.2.5) rather than the expected [Sn(μ3 NR)]4 cubanes. These clusters exhibit high thermal stability, as demonstrated by the observation that the known diiso-propylphenyl-substituted cubane 12 was only obtained from this reaction after refluxing in toluene for 2.5 days.4 The reluctance of the compounds 31 and 32 to proceed to the cubane structures was attributed to the high steric bulk of the aromatic substituents. A configurational isomer of these compounds, 33, was isolated during the synthesis of the cubane 21.3 The tricyclic compound 33 can be considered to consist of an [Sn(μ-NR)]2 dimer coordinated to an Sn(NMe2 )2 monomer. The possibility
Scheme 2.2.5
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Tin(II) Clusters
Scheme 2.2.6
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Possible reaction pathways for the formation of [Sn(μ3 -NR)]4
that [Sn(μ3 -NR)]4 cubanes are formed stepwise in solution, through the aggregation of [Sn(μ-NR)]2 dimers has been considered previously by Veith.6 In this context Veith carried out the synthesis of the cubane 4 in the presence of [Sn(OtBu)2 ]2 and isolated the analogous tricyclic complex 34, in which the [Sn(μ-NtBu)]2 dimer is coordinated to a Sn(OtBu)2 monomer.6 The identification of the compounds 31–34 provides significant insight into the pathways involved in the formation of tin cubanes; a possible route (pathway i in Scheme 2.2.6) has been previously proposed by Wright.3,4 A second possibility, (pathway ii in Scheme 2.2.6) is presented here for the first time, although aspects of this proposal have been discussed previously by Veith.29 The primary difference between the two pathways is in the stoichiometry of the first step. Pathway i requires a 1:1 molar ratio of reactants [tBuNH2 :Sn(NMe2 )2 ], whereas that ratio is 2:1 in pathway ii. Pathway i does not account for the formation of 33, while pathway ii does not produce complexes of the type 31 and 32. An explanation of the formation of the two isomeric structures represented by 31, 32, and 33 must involve an intramolecular rearrangement of the amido and imido substituents. For example, cleavage of two Sn–N bonds in B and formation of a new Sn–N bond to give A would be entropically driven by the loss of steric strain involved in the conversion of two four-membered rings into two six-membered rings (Scheme 2.2.6).
Reaction Chemistry
(1) Reactions of [Sn(μ3 -NR)]4 . An intriguing aspect of the chemistry of imidotin cubanes lies in their potential to act as multidentate Lewis-base donors to form coordination complexes. While this
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Scheme 2.2.7
possibility has been investigated by Veith, only a few complexes have been reported. The first example was the formation of [Sn(μ3 -NtBu)]4 ·2AlCl3 , 35 (Scheme 2.2.7).10 Treatment of the cubane 4 with AlCl3 in molar ratios ranging from 1:1 to 1:4 gave only the bis-adduct 35, which was identified by X-ray crystallography. The AlCl3 ligands in 35 exchange rapidly between the four tin centers in solution, as indicated by the observation of a single resonance for the tert-butyl substituents in the 1 H NMR spectrum.10 Transition-metal complexes of [Sn(μ3 -NR)]4 cubanes have also been reported. The reaction of Fe2 (CO)9 with [Sn(μ3 -NSiEt3 )]4 , 8, produces the complex [Sn(μ3 -NSiEt3 )]4 ·2Fe(CO)4 , 36, in 82% yield, even when an excess of Fe2 (CO)9 is used.5 The presence of only two broad signals for the ethyl substituents in the 1 H NMR spectrum suggests that the iron centers scramble in solution, cf. 35. A series of related transition-metal complexes of 4 have been mentioned in reviews, however no details have been reported for the complexes 37a–d.11,29 The reaction of the cubane 4 with an excess of SO2 results in the rapid formation of the dimeric compound 38, in which two imidotin cubanes are bridged by two sulfito ligands. The retention of the cubane structure is unexpected, since SO2 is known to insert readily into Sn–NR2 bonds.30 The dimer 38 is the only known complex in which two [Sn(μ3 -NR)]4 cubane clusters are strongly linked. The reaction of 4 with P(SiMe3 )3 was attempted with a view to preparing the phosphorus-containing cluster 39 (Scheme 2.2.7), but instead resulted in the formation of the phosphido-centered double cubane
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40, in an unspecified low yield.31 When the same reaction was carried out in the presence of 30a, the yield of 40 was increased to 14%. The solid-state structure of 40 revealed a distorted six-coordinate geometry at the phosphorus center. The phosphorus atom is strongly bound to five of the tin atoms, with Sn–P ˚ while the contact to the unique tin center is considerably distances in the range 2.580(4)–2.725(3) A, ˚ 31 longer, at 3.298(3) A. In contrast to the previous examples, a number of reactions in which the cubane core is not retained have been reported. Early work by Veith showed that the reaction of the cubane 4 with HCl results in the complete decomposition of the cluster.32 The addition of HCl to 4 readily produces the dimeric compound [(NHtBu)Sn(μ-Cl)]2 , as well as the adduct tBuNH2 ·SnCl2 and the salt [tBuNH3 ][SnCl3 ], all of which were well characterized. More recently, Wright has investigated the interaction of 4, prepared in situ, with lithiated amines and phosphines.18 The reaction of three equivalents of LiNHC10 H7 with 4 in boiling toluene produced the cubane 41, in which only a single NtBu substituent of the parent cluster has been retained. In addition, one of the tin centers has been replaced by two lithium ions, only one of which is accommodated within the anionic cubane core, while the second is present as a THF-solvated counterion. The outcome of the reaction of 4 with six equivalents of LiPHC6 H11 is even more dramatic, with all of the tert-butylimido groups being replaced by phosphinidine substituents, to give the cluster 42, which can be considered to consist of an [Sn4 (μ4 -PCy)6 ]4− tetra-anion with four lithium cations.18 Finally, Wright and coworkers have reported that the reaction of the cubane 21, prepared in situ, with KOtBu and MeN(Li)CH2 CH2 N(Li)Me produces the heteroleptic stannate complexes 43 and 44 (Scheme 2.2.8), respectively.33 (2) Reactions of Sn4 (μ3 -NR)3 (μ3 -O). An interesting substitution reaction of the oxo-cubane 30a has been reported by Wright.27 The treatment of 30a with LiNHC10 H7 produces the heteroleptic imidotin cubane 45 (Scheme 2.2.9) in which the oxo group has been replaced by an imido group. The substitution occurs exclusively at the oxygen center owing to the higher polarity of the Sn–O bonds (relative to Sn–N) and is thermodynamically driven by the formation of LiOH.27 The isolation of the cluster 45 demonstrates the potential of this synthetic route to generate mixed-substituent imidotin cubanes of the type Sn4 (μ3 -NR)3 (μ3 -NR ), which are not accessible by direct synthetic procedures. However, to date, 45 remains the only example of this type of cluster.
Scheme 2.2.8
The oxo cluster 30a has also been used to prepare a handful of metal complexes. The aluminium complex 46 was obtained in 81% yield from the reaction of 30a with 1.5 equivalents of AlMe3 ; no coordination to the tin centers was observed.26 Thermolysis of 46 resulted in the generation of the cubane 4.26 The complexes 47 and 48 were prepared in THF by the reaction of 30a with LiCl and FeCl2 , respectively, in a 3:1 molar ratio.34 Both complexes exhibit the coordination of three oxo cubanes to the metal center; strong metal–oxygen bonding is indicated by the short metal–oxygen bond distances [47,
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Scheme 2.2.9
˚ 48, Fe–O 2.026(7) A]. ˚ The oxo cubane 30a behaves as a stronger O-donor ligand than Li–O 1.94(1) A; THF, presumably as a result of the localization of negative charge on the oxygen center.34 2.2.3
The Seco-Cubane Sn3 (μ3 -NtBu)(μ-NtBu)(μ-NHtBu)2
Synthesis of seco-Sn3 (μ3 -NtBu)(μ-NtBu)(μ-NHtBu)2
The seco-cubane 3 was first prepared in 98% yield from the reaction of three equivalents of 1 with four equivalents of tert-butylamine.1 The identity of this compound was later unambiguously established by an X-ray structural determination.2 Alternatively, 3 can be obtained from the reaction of Sn[N(SiMe3 )2 ]2 with tert-butylamine.35 While this latter reaction produces 3 in lower yield (71%), it has the advantage of using the more conveniently accessible reagent Sn[N(SiMe3 )2 ]2 . Veith has reported the observation of a series of silyl-substituted analogs of 3, which were subsequently converted into the cubane clusters 8–11, although no characterization of these intermediates was given in that work (see Transamination Reactions above).5 Undoubtedly, there is considerable scope for the synthesis of seco-cubanes related to 3, provided that appropriately bulky substituents are present on the nitrogen centers. Reactions of seco-Sn3 (μ3 -NtBu)(μ-NtBu) (μ-NHtBu)2
The reaction of the seco-cubane 3 with hydrogen halides has been investigated by Veith and coworkers. The 1:1 reaction of dilute solutions of either HCl or HBr with 3 readily produces the complexes 49a and 49b (Scheme 2.2.10) in nearly quantitative yield.36 These complexes are comprised of the cationic cluster [Sn3 (μ3 -NtBu)(μ-NHtBu)3 ]+ , which is coordinated to a halide anion via three hydrogen bridges. Their identity was unambiguously established in both cases by X-ray structural determinations. Treatment of 3 with an excess of HX resulted in the complete decomposition of the cluster, giving rise to products similar to those observed in the analogous reaction with the cubane 4.32,36 The iodo compound 49c was not obtained directly from HI and 3, but by a simple anion-exchange reaction between the chloro compound 49a and sodium iodide; the bromo derivative 49b could also be
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Scheme 2.2.10
prepared similarly by reaction with sodium bromide.36 It was noted by Veith that these latter reactions are bi-phasic, suggesting the possibility that the compounds 49a–c could have potential as phase-transfer catalysts.36 The vacant corner inherent in the structure of the seco-cubane 3 provides the possibility of generating mixed-metal clusters, such as 50a,b, by further reactions with suitable metallating reagents. The reaction of 3 with the germanium and lead analogs of 1, were found to require forcing conditions to generate the closed cubane compounds, e.g. 50a, 190 ◦ C for 15 h; 50b, 210 ◦ C.37 While 50a is thermodynamically stable, scrambling occurs in the formation of the lead derivative 50b to give the heterometallic cubanes 51 and 52 (Scheme 2.2.10) and the symmetrical cubanes [M(μ3 -NtBu)]4 (M = Sn, Pb), on the basis of mass spectrometric studies.37 The presence of the two amido protons in 3 also provides the opportunity to generate heterobimetallic clusters by reaction with organoalkali reagents. The magnesiated cubane 53 is produced in 69% yield from the reaction of dibutylmagnesium with 3 in hot THF.38 The reaction of 3 with two equivalents of butyllithium at –78 ◦ C in THF generates 54 in 39% yield.13 The structure of the solvent-separated ion pair 54 is analogous to that of 41.
2.2.4
Double-Cubane Clusters
A fascinating recent development in imidotin-cluster chemistry involves the isolation of a series of double cubanes, which contain an Sn7 (μ3 -NR)8 core. Wright and coworkers have demonstrated that when pyridinyl or pyrimidinyl groups are present on the imido nitrogen centers, the unusual doublecubane clusters 55–60 are obtained (Scheme 2.2.11), rather than the [Sn(μ3 -NR)]4 cubanes.8 These clusters are comprised of two interlocked [Sn(μ3 -NR)]4 cubanes, which share one tin vertex. The central tin center is formally in the +4 oxidation state, so that these double cubanes may be viewed as involving the coordination of two [Sn3 (μ3 -NR)4 ]2− anions, such as those present in the clusters 53 and 54, to a central Sn+4 cation. The presence of both Sn(II) and Sn(IV) centers was verified by 119 Sn NMR spectroscopy.7 The deposition of tin metal was observed during the syntheses of 55–60, suggesting that
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Scheme 2.2.11
the oxidation of one Sn(II) center to Sn(IV) is accompanied by the reduction of another Sn(II) center to tin metal.7,8 The compounds 55–60 have all been prepared, albeit in low yields (8–26%), from the reaction of Sn(NMe2 )2 with the appropriate primary amine, under conditions which produce cubanes for nonheterocyclic containing amines.7,8 Thus, the formation of the double-cubane structural motif presumably arises from the nature of the heterocyclic substituents on the imido nitrogen centers. Inspection of the solid-state structures of these compounds revealed the presence of short intramolecular heterocyclic ˚ which may play a role in the formation of the douN· · ·Sn contacts, ranging from 2.812(4) to 3.188(6) A, 7 ble cubanes. The likely formation of double-cubane clusters, which do not contain heterocyclic imido substituents, by metathetical reactions of two equivalents of 53 or 54 with main-group or transition-metal tetrahalides has not yet been investigated. The reaction of a primary amine and Sn(NMe2 )2 in the presence of water generated a related double cubane, 61 (Scheme 2.2.12), in which one of the NR groups in each cubane was replaced by an oxygen atom.9 The interesting bis-amido-μ-imidoditin(II) acceptor ligand, [(NHR)Sn]2 (μ-NR), provides a bridge between the two oxygen centers.
Scheme 2.2.12
2.2.5
Imidotin Chalcogenides
The first imidotin-chalcogenide cluster containing a terminal tin–chalcogen bond to be reported was the tellurium complex 62 (Scheme 2.2.13), which was unexpectedly obtained from the reaction of
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Scheme 2.2.13
[Li2 Te(NtBu)3 ]2 with SnCl2 .39 In solution the terminal tellurium atom was found to exchange rapidly between the tin(II) and tin(IV) sites, as determined by variable temperature NMR spectroscopy, cf. 35 and 36. The discovery of compound 62 prompted investigations into reactions of imidotin clusters [Sn(μ3 -NR)]4 with chalcogens, with a view to preparing single-source precursors for the deposition of tin-chalcogenide materials, via the thermodynamically favourable elimination of tBuN=NtBu.12 Initial studies focused on the reaction of the cubane 4 with sulfur, selenium and tellurium. In the case of tellurium, only a single tin center is oxidized to give the mono-telluride 63b, even when 4 was refluxed in toluene for two days with an excess of tellurium.12 Oxidation of 4 was more facile with selenium, resulting in the generation of 64, the first example of a compound containing more than one terminal tin– chalcogen bond. The mono-sulfide 63a was prepared by the reaction of 4 with one equivalent of sulfur. Further reaction with sulfur produced only insoluble products, presumably sulfido-bridged oligomers.12 The selenide and telluride clusters 64 and 63b are fluxional in solution as a result of rapid exchange of the terminal chalcogen centers between the tin sites, cf. 62. Reactions of the seco-cubane 3 with chalcogens were also investigated. While mono-chalcogenides are readily produced in the case of sulfur, selenium and tellurium, further reaction with either sulfur or selenium produced only insoluble products.35 The solid-state structures of the mono-selenide and mono-telluride 65a and 65b (Scheme 2.2.14) revealed that one of the Sn(NtBu)2 (NHtBu) sites was oxidized in preference to the unique Sn(NtBu)(NHtBu)2 tin center. Solution-state NMR studies indicated that chalcogen exchange occurs exclusively between the two Sn(NtBu)2 (NHtBu) sites.35 In the case of selenium, in addition to 65a, the selenido-bridged dimer 66 was present in solution as indicated by 1 JSn-Se coupling constants (65a, 3220 Hz; 66, 779 Hz).
Scheme 2.2.14
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Reaction of the heterobimetallic cluster 53 with selenium or tellurium in hot THF produced the monochalcogenides 67a and 67b (Scheme 2.2.15), which each contain a terminal tin–chalcogen bond.38 In solution, chalcogen exchange occurs between the three tin sites. A dramatic increase in the susceptibility of the tin centers towards oxidation by chalcogens was observed for the anionic cluster of 54. The reaction of 54 with one or two equivalents of selenium or tellurium occurs spontaneously at room temperature, to give the corresponding mono- and di-chalcogenide clusters 68, 69, 70a and 70b. While a monomeric cluster was formed in the solid state for the mono-telluride 68, a dimeric structure was observed for the mono-selenide 69, presumably owing to the localization of anionic charge on the more electronegative selenium centers.38 The marked enhancement in the reactivity of the anionic cluster in 54 (compared to that of the seco-cubane 3) was further demonstrated by the isolation and structural characterization of the tri-chalcogenide clusters 71a and 71b in high yields (Scheme 2.2.15).13 Pure samples of the mono- and di-chalcogenides may be obtained by the reaction of the tri-chalcogenide clusters 71a or 71b with 54 in the appropriate stoichiometric ratio. This chalcogen-transfer process can be used to chalcogenate otherwise unreactive imidotin clusters. For example, although the neutral cubane 12 does not react with selenium in boiling toluene, oxidation of the tin centers to give 72 occurs readily at room temperature when the reaction is carried out in the presence of a catalytic amount of 54.38 Complex 72 is a dimer in which two [Sn(μ-NR)]2 four-membered rings are bridged by two selenido ligands.
Scheme 2.2.15
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Tin(II) Clusters
2.2.6
65
Phosphido– and Arsenido–Tin Clusters
During the past 10 years there have been significant developments in the chemistry of tin(II) clusters that incorporate phosphorus and, to a lesser extent, arsenic, primarily from the groups of Westerhausen, Wright, and Driess.40,41 The reaction of Sn[N(SiMe3 )2 ]2 with H2 ESiiPr3 (E = P, As) in a 1:1 molar ratio produces the hexagonal prisms [Sn(μ3 -ESiiPr3 )]6 (73a, E = P; 73b, E = As; Scheme 2.2.16) as blackred crystals in very high yields,42,43 whereas the corresponding reaction with H2 PSitBu3 generates the cubane [Sn(μ3 -PSitBu3 )]4 , 74.44 The phosphorus or arsenic analogs of [Sn(μ3 -NtBu)]4 are not known. When the reaction of Sn[N(SiMe3 )2 ]2 with H2 PSi(iPr3 )2 (2,4,6-iPr3 C6 H2 ) is carried out in the presence of SnCl2 ,the Sn3 P2 Cl2 cluster 75, in which SnCl2 is trapped by a dimeric [Sn(μ-PR)]2 molecule, is isolated in 44% yield.42
Scheme 2.2.16
The transmetallation of [Sn(μ3 -PSitBu3 )]4 74 with barium metal produces the cubanes 76–78 (Scheme 2.2.17) in which one, two, or three tin atoms are replaced by barium, cf. the mixed Sn–Pb clusters 50–52.45,46 The cubanes 76–78 are also accessible from the reaction of a mixture of Sn[N(SiMe3 )2 ]2 and Ba[N(SiMe3 )2 ]2 with H2 PSitBu3 . The substitution of tin by barium in these clusters results in a very large high-field shift of the RP ligand bonded to three tin atoms (δ 31 P = –529).41 Another cluster type that involves Sn(II) and an alkaline earth metal is represented by 79 (M = Ca, Ba) in which the dianion [(Me3 Si)2 PSn(μ-PSiMe3 )2 SnP(SiMe3 )2 ]2− coordinates to the divalent metal as a tetradentate ligand.47,48
Scheme 2.2.17
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Scheme 2.2.18
The reactions of the Sn(II) reagent Sn(NMe2 )2 with alkali metal primary phosphides RPH(M) have been studied extensively by Wright and coworkers.40,49−54 The structures of the phosphido tin(II) clusters obtained via this approach depend markedly on the organic substituent (R) and the alkali metal (M). For less bulky R groups and/or for the heavier alkali metals, the formation of P–P and Sn–Sn bonded complexes is commonly observed. Some examples of this behavior include the anions [{Sn(μ-PMes)}2 (μMesPPMes)]2− 80,51 [{Sn(μ-PCy)}3 ]2− 81,50 and [Sn3 (μ3 -PtBu)(μ-PtBu)3 ]2− 82 (Scheme 2.2.18).53 The ˚ in the trimeric cluster 81 are in the typical range of a Zintl anion.50 This tin–tin bond lengths (3.15–3.18 A) unusual structure is described by an electron-deficient (two-electron–three-center) bonding arrangement involving p-orbitals on the three tin atoms. The dianion in 82 is the phosphorus analog of the dianions present in the heterometallic imidotin cubanes 53 and 54. Tetraanions of the type [Sn4 (μ4 -PR)6 ]4− (R = tBu, Cy) are present in the structure of 42 (see Reactions of [Sn(μ3 -NR)]4 above).
2.2.7
Summary and Future Prospects
Our knowledge of the fundamental chemistry of tin(II) clusters has progressed significantly since Veith’s initial report on the formation of the imidotin cubane [Sn(μ3 -NtBu)]4 almost 30 years ago. In addition to the refinement of synthetic strategies to the well-known [Sn(μ3 -NR)]4 cubanes, a wide variety of other structural motifs have been established. Compared to the imidotin systems, recent investigations of phosphidotin(II) clusters have revealed significant differences in the types of cluster structures that are accessible and synthetic routes to a variety of novel, anionic clusters have been established. The reaction chemistry of these potentially versatile reagents is essentially undeveloped. The incorporation of other main group elements or transition metals into tin(II) clusters via metathetical reactions of metal halides with anions such as [Sn3 (μ3 -NtBu)(μ-NtBu)3 ]2− (E = N, P) is an obvious target. The transfer of the fundamental knowledge on tin(II) clusters described in this chapter to practical applications in materials science is an important, but worthwhile, challenge for the future. References 1. M. Veith, M.L. Sommer, and D. J¨ager, Chem. Ber., 112, 2581 (1979). 2. M. Veith, Z. Naturforsch., 35b, 20 (1980). 3. A. Bashall, N. Feeder, E.A. Harron, M. McPartlin, M.E.G. Mosquera, D. S´aez, and D.S. Wright, J. Chem. Soc., Dalton Trans., 4104 (2000). 4. R.E. Allan, M.A. Beswick, G.R. Coggan, P.R. Raithby, A.E.H. Wheatley, and D.S. Wright, Inorg. Chem., 36, 5202 (1997).
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5. M. Veith, M. Ops¨older, M. Zimmer, and V. Huch, Eur. J. Inorg. Chem., 1143 (2000). 6. M. Veith, and W. Frank, Angew. Chem. Int. Ed. Engl., 23, 158 (1984). 7. D.R. Armstrong, F. Benevelli, A.D. Bond, N. Feeder, E.A. Harron, A.D. Hopkins, M. McPartlin, D. Moncrieff, D. S´aez, E.A. Quadrelli, A.D. Woods, and D.S. Wright, Inorg. Chem., 41, 1492 (2002). 8. F. Benevelli, E.L. Doyle, E.A. Harron, N. Feeder, E.A. Quadrelli, D. S´aez, and D.S. Wright, Angew. Chem. Int. Ed., 39, 1501 (2000). 9. N. Feeder, E.A. Harron, M.E.G. Mosquera, A.D. Woods, and D.S. Wright, Chem. Commun., 1010 (2001). 10. M. Veith and W. Frank, Angew. Chem. Int. Ed. Engl., 24, 223 (1985). 11. M. Veith, Phosphorus, Sulfur, and Silicon, 41, 195 (1989). 12. T. Chivers, T.J. Clark, M. Krahn, M. Parvez, and G. Schatte, Eur. J. Inorg. Chem., 1857 (2003). 13. T. Chivers and D.J. Eisler, Angew. Chem. Int. Ed., 43, 6686 (2004). 14. M. Veith and O. Recktenwald, Z. Naturforsch., 38b, 1054 (1983). 15. M. Veith and G. Schlemmer, Chem. Ber., 115, 2141 (1982). 16. H. Chen, R.A. Bartlett, H.V. Rasika Dias, M.M. Olmstead, and P.P. Power, Inorg. Chem., 30, 3390 (1991). 17. R.E. Allan, M.A. Beswick, A.J. Edwards, M.A. Paver, M.A. Rennie, P.R. Raithby, and D.S. Wright, J. Chem. Soc., Dalton Trans., 1991 (1995). 18. R.E. Allan, M.A. Beswick, N.L. Cromhout, M.A. Paver, P.R. Raithby, A. Steiner, M. Trevithick, and D.S. Wright, Chem. Commun., 1501 (1996). 19. A. Bashall, A. Ciulli, E.A. Harron, G.T. Lawson, M. McPartlin, M.E.G. Mosquera, and D.S. Wright, J. Chem. Soc., Dalton Trans., 1046 (2002). 20. J.F. Eichler, O. Just, and W.S. Rees, Jr., Inorg. Chem., 45, 6706 (2006). 21. J.F. Eichler, O. Just, and W.S. Rees, Jr., Phosphorus, Sulfur, and Silicon, 179, 715 (2004). 22. Y. Tang, L.N. Zakharov, A.L. Rheingold, and R.A. Kemp, Inorg. Chim. Acta, 359, 775 (2006). 23. R.E. Allan, M.A. Beswick, M.K. Davies, P.R. Raithby, A. Steiner, and D.S. Wright, J. Organomet. Chem., 550, 71 (1998). 24. W.J. Grigsby, T. Hascall, J.J. Ellison, M.M. Olmstead, and P.P. Power, Inorg. Chem., 35, 3254 (1996). 25. M. Veith, M. Grosser, and O. Recktenwald, J. Organomet. Chem., 216, 27 (1981). 26. M. Veith and H. Lange, Angew. Chem. Int. Ed. Engl., 19, 401 (1980). 27. B. Gal´an, M.E.G. Mosquera, J.S. Palmer, P.R. Raithby, and D.S. Wright, J. Chem. Soc., Dalton Trans., 1043 (1999). 28. M. Veith, Chem. Rev., 90, 3 (1990). 29. M. Veith, Angew. Chem. Int. Ed. Engl., 26, 1 (1987). 30. T. Chivers, T.J. Clark, M. Parvez, and G. Schatte, Dalton Trans., 2107 (2003) 31. M.E.G. Mosquera, A.D. Hopkins, P.R. Raithby, A. Steiner, A. Rothenberger, A.D. Woods, and D.S. Wright, Chem. Commun., 327 (2001). 32. M. Veith, M. Jarczyk, and V. Huch, Chem. Ber., 121, 347 (1988). 33. A.D. Bond, E.A. Harron, G.T. Lawson, M.E.G. Mosquera, M. McPartlin, and D.S. Wright, J. Chem. Soc., Dalton Trans., 3525 (2002). 34. C. Brown, M.E.G. Mosquera, J.S. Palmer, P.R. Raithby, A. Steiner, and D.S. Wright, J. Chem. Soc., Dalton Trans., 487 (2000). 35. T. Chivers, D.J. Eisler, and J.S. Ritch, Z. Anorg. Allg. Chem., 630, 1941 (2004). 36. M. Veith, J. Fischer, T.R. Prout, M. N¨otzel, P. Hobein, and V. Huch, Inorg. Chem., 30, 4130 (1991). 37. M. Veith and M. Grosser, Z. Naturforsch., 37b, 1375 (1982). 38. D.J. Eisler and T. Chivers, Chem. Eur. J., 12, 233 (2006). 39. T. Chivers and G. Schatte, Chem. Commun., 2264 (2001). 40. F. Garc´ıa, M.L. Stead, and D.S. Wright, J. Organomet. Chem., 691, 1673 (2006). 41. M. Driess, R. E. Mulvey, and M. Westerhausen, in Molecular Clusters of the Main Group Elements, M. Driess and H. N¨oth (Eds), Wiley-VCH, Weinheim, 2004, Chapter 3.6. 42. M. Driess, S. Martin, K. Merz, V. Pintchouk, H. Pritzkow, H. Gr¨utzmacher, and M. Kaupp, Angew. Chem. Int. Ed. Engl., 36, 1894 (1997).
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43. M. Westerhausen, N. Makropoulos, H. Piotrowski, M. Warchhold, and H. N¨oth, J. Organomet. Chem., 614–615, 70 (2000). 44. M. Westerhausen, M. Krofta, N. Wiberg, J. Knozek, H. N¨oth, and A. Pfitzner, Z. Naturforsch., 53B, 1489 (1998). 45. M. Westerhausen, M. Krofta, S. Schneiderbauer, and H. Piotrowski, Z. Anorg. Allg. Chem., 1391 (2005). 46. M. Westerhausen, Dalton Trans., 4755 (2006). 47. M. Westerhausen, H.D. Hausen, and W. Schwarz, Z. Anorg. Allg. Chem., 621, 877 (1995). 48. M. Westerhausen and W. Schwarz, Z. Anorg. Allg. Chem., 622, 903 (1996). 49. A.D. Bond, A. Rothenberger, A.D. Woods, and D.S. Wright, Chem. Commun., 525 (2001). 50. P. Alvarez-Bercedo, A.D. Bond, R. Haigh, A.D. Hopkins, G.T. Lawson, M. McPartlin, D. Moncrieff, M.E.G. Mosquera, J.M. Rawson, A.D. Woods, and D.S. Wright, Chem. Commun., 1288 (2003). 51. F. Garcia, A.D. Hopkins, S.M. Humphrey, M. McPartlin, C.M. Pask, A.D. Woods, and D.S. Wright, Organometallics, 23, 4821 (2004). 52. F. Garc´ıa, A.D. Hopkins, R.A. Kowenicki, M. McPartlin, C.M. Pask, M.L. Stead, A.D. Woods, and D.S. Wright, Organometallics, 24, 1813 (2005). 53. F. Garc´ıa, J.P. Hehn, R.A. Kowenicki, M. McPartlin, C.M. Pask, A. Rothenberger, M.L. Stead, and D.S. Wright, Organometallics, 25, 3275 (2006). 54. P. Alvarez, F. Garc´ıa, J.P. Hehn, F. Kraus, G.T. Lawson, N. Korber, M.E.G. Mosquera, M. McPartlin, D. Moncrieff, C.M. Pask, A.D. Woods, and D.S. Wright, Chem. Eur. J, 13, 1078 (2007).
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Mono-Organotin Oxo-Clusters
2.3
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Mono-Organotin Oxo-Clusters
Franc¸ois Ribot Chimie de la Mati`ere Condens´ee de Paris, Universit´e Pierre et Marie Curie-Paris6, Paris, France
2.3.1
Introduction
Metal oxo-clusters are finite polymetallic aggregates, the architecture of which is based on a metal– oxygen framework. A great diversity exists in these species. They range from purely inorganic polyoxometalates1−5 to organometallic oxides6 in which a Mx O y core has its surface protected by organic groups. Mono-organotin derivatives exhibit a rich oxo-cluster chemistry and numerous compounds with different structural types have been described. For the sake of classification, mono-organotin oxo-clusters can be divided in two main families. The first one includes all the oxo-clusters which also bear multidentate ligands (e.g. carboxylates, phosphinates, phosphonates, etc.). In these species, the architecture of the metal–oxygen framework is strongly influenced by the binding characteristics of the ligands, as well as the ligand-to-tin ratio. This first family is still a very active field of research, that has been the subject of several reviews,7-14 and thus will not be discussed here. The second family, the subject of the present chapter, contains the mono-organotin oxo-clusters which only possess oxo- and/or hydroxo- bridges and various types of terminal groups (hydroxy, halogeno, etc.), in addition to the organic residue directly bonded to each tin atom through an Sn C link. They correspond to the general formula (RSn)n Ox (OH) y Clz . A third type of compounds which can be considered to be mono-organotin oxo-clusters are polyoxometalates that include in their metal–oxygen framework one or more SnR units, isolated or connected through Sn O Sn linkages.15-23 In these derivatives, tin is not the most abundant element and the properties are mainly related to the other metal (e.g. tungsten) or to the organic group bonded to tin. These compounds, though interesting, will not be discussed here. Mono-organotin oxo-clusters have shown good potential in the field of catalysis24 and hybrid organic– inorganic materials.25-31 They are also fundamentally interesting because of their relation to organostannonic acid, RSnO(OH), a compound active in transesterification reactions,32-37 that generally is an ill-defined polymer.7,38 Indeed, these oxo-clusters share with organostannonic acid a general preparation scheme based on the hydrolysis-condensation of organotin trihalides, or similar molecular precursors,39-44 and they represent stable intermediates, or dead ends, in the synthesis of polymeric RSnO(OH). A good description of their structures and a better understanding of their formation are likely to provide significant insight into the ill-defined and still unknown structure of organostannonic acid. With its two main NMR-active isotopes, i.e. 119 Sn (I = 1/2, 8.58%, 45 = 37.290665 MHz) and 117 Sn (I = 1/2, 7.61%, 45 = 35.632295 MHz), tin is a friendly and versatile spectroscopic probe, complementary to single crystal X-ray diffraction, to study tin-based compounds, both in solution and in the solid state.46-49 In solution, the similar natural abundance of both isotopes gives generally rich satellite patterns in which the coupling between two equivalent nuclei can be observed through the n J (119 Sn–117 Sn). The chemical shift anisotropy data, that can only be obtained from solid-state 119 Sn NMR, are also valuable information to characterize the local geometry.48,49 In the context of organotin oxo-clusters, solution 119 Sn or 117 Sn NMR is one of the most valuable techniques to prove whether a given metal–oxygen framework, evidenced in a crystalline material, is preserved upon dissolution. Solid-state 119 Sn NMR, which is becoming more and more accessible, allows one to establish that a given organotin oxo-cluster is present in a solid, even an amorphous solid,29,31 and that it is not mixed with any ill-defined oxo-polymer.50 Finally, the
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comparison of solid-state and solution 119 Sn NMR data is a way to identify weak coordinative contacts and structural evolutions between the crystallized and the dissolved species.50 The aim of this chapter is to review and discuss the molecular structures, syntheses, and possible formation mechanisms of the organotin oxo-clusters that follow the formula (RSn)n Ox (OH) y Clz . 2.3.2
Molecular Structures
This first section presents the molecular structures of all the known derivatives. Many of them adopt a cage structure with the organic groups pointing outward. The drawings (Figures 2.3.1–4, 2.3.7, and 2.3.8) are all based on crystalline structures. They use the fractional coordinates that have been either published or obtained from CCDC51 or FIZ Karlsruhe.52 To ease the readability, all the organic groups are omitted and only the Sn C bonds are shown. The compounds discussed also include species without confirmatory X-ray crystal structures but sufficient spectroscopic evidence exists to unambiguously relate them to a known molecular structure. There are several ways to describe a structure and its metal–oxygen framework. One of them relies on sub-units,53 and a very common one in many organotin oxo-clusters is the so-called O-capped trimer, in which three tin atoms are connected all together with a μ3 -O and two-by-two with three μ2 -O(H/R ) (Scheme 2.3.1). For five- or six-coordinate metals this sub-unit corresponds to three distorted trigonal bipyramids or octahedra connected through a common vertex and three edges. This tri-metallic sub-unit, which is, for example, found in {[(RSn)(OH)(O2 PR2 )]3 O}(O2 PR2 ),9 {[(RSn)3 (OR )3 O]2 (HPO3 )4 }, 10,11 and {(PhSn)3 P2 W15 O59 }9- ,22 occurs in the structural chemistry of many other metal oxo-clusters, such as polyoxometalates (POMs)1–5 and titanium oxo-clusters.54
Scheme 2.3.1
Sn
O framework of the O-capped trimer sub-unit
{RSn(μ2 -OH)Cl2 (H2 O)}2
The dimer {RSn(OH)Cl2 (OH2 )}2 is the smallest mono-organotin oxo-cluster. Its molecular structure (Figure 2.3.1a) is based on two octahedra joined via two μ2 -OH bridges defining a common edge. This double bridge results in a distannoxane ring (Sn2 O2 ). The four other vertices of each octahedron are occupied by two chloride atoms (cis-position), the first carbon of the R group (equatorial position, trans to the shorter Sn OH), and one water molecule (axial position). Such a dimer has been isolated for methyl-,55 ethyl-,56 isopropyl-,57 n-butyl-,58 and isobutyltin.57 The n-butyl- and methyltin derivatives have been studied by solution 119 Sn NMR. For the n-butyltin compounds, the observation of a major resonance at −408.6 ppm (in acetone-d6 ) is consistent with six-coordinate tin atoms and with the preservation of the dimeric structure in solution.58 A similar conclusion does not seem to hold for the methyltin derivative, for which 119 Sn NMR indicates a drop in the coordination of tin (−175.9 ppm in CDCl3 ).55 Such a dimer has also been observed containing bridged organotins. Indeed, the hydrolysis of 1,3bis(trichlorostannyl)propane, in water, yields a solid where the dimers are associated by the alkyl spacer in infinite chains.59
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Figure 2.3.1 Molecular structures of {RSn(OH)Cl2 (OH2 )}2 (a),55−59 {(Me3 Si)3 CSnO(OH)}3 (b),60 {[(Me3 Si)3 CSn]4 O6 } (c),62 and {2,4,6-i Pr3 C6 H2 SnO(OH)}6 (d).63 Tin is presented as small black circles, O (or H2 O) as medium white circles, OH as medium striped circles, and Cl as large hatched circles. The same color code applies subsequently, as appropriate.
{(Me3 Si)3 CSn(μ2 -O)(OH)}3 and {(Me3 Si)3 CSn(μ2 -O)Cl}3
For tris(trimethylsilyl)methyltin, a structure based on three four-coordinate tin atoms connected through μ2 -O bridges in a Sn3 O3 cycle has been observed.60 This structure is reminiscent of the one observed for several diorganotin oxides7 and stannasiloxanes.61 The oxy-hydroxy derivative (Figure 2.3.1b) adopted an ideal chair conformation with the organic groups in equatorial positions and a cis-arrangement of the three axial hydroxy groups. The 119 Sn NMR spectra in C6 D6 ({(Me3 Si)3 CSn(μ2 -O)(OH)}3 : −156 ppm with 2 J (117 Sn,119 Sn) = 719 Hz; {(Me3 Si)3 CSn(μ2 -O)Cl}3 :−141.0 ppm with 2 J (117 Sn,119 Sn) = 829 Hz and −133.0 ppm with 2 J (117 Sn,119 Sn) = 822 Hz) are consistent with a tetrahedral coordination of the tin atoms and indicate that the structures are preserved in solution. The number of signals (one for the hydroxy derivative and two for the chloro derivative) indicates that these two compounds exhibit different conformations.60 {[(Me3 Si)3 CSn]4 (μ2 -O)6 }
Tris(trimethylsilyl)methyltin also affords a fully condensed species, the structure of which is based on four Sn3 O3 cyclo-tristannoxanes rings ‘fused’ together in a regular hetero-adamantane Sn4 O6 core with tetrahedral tin atoms and μ2 -oxo bridges only (Figure 2.3.1c).62 In such a compound, the oxo-core is completely covered by the bulky organic residues. The same compact architecture has also been reported for tris(trimethylsilyl)methyltin and n-butyltin compounds, but with sulfur or selenium instead of oxygen.62
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On the basis of 119 Sn NMR, this oxo-cluster is said to be stable in solution. However, the reported chemical shift (94.3 ppm in C6 D6 ) is quite different from those observed for other CSnO3 environments with the same organotin.60 {(2,4,6-i Pr3 C6 H2 Sn)6 (OH)4 (μ3 -O)4 (μ2 -O)2 (μ2 -OH)2 }
With the bulky 2,4,6-tris(isopropyl)phenyl group, a hexamer has been isolated.63 Its structure (Figure 2.3.1d), which contains only five-coordinate tin atoms (distorted trigonal bipyramid), is based on a Sn6 O8 twisted ribbon made of six four-membered [Sn2 O2 ] rings. The oxygen atoms are distributed in between μ3 -O, μ2 -O, and μ2 -OH bridges and terminal hydroxy groups. An intra-molecular set of hydrogen bonds ‘closes’ both ends of this bracelet-like framework. Alternatively, the metal–oxygen framework can be described by the connection of two O-capped trimers modified by the opening of one of the three μ2 -O bridges. This structure is kinetically labile in solution, as seen by the appearance of ten 119 Sn NMR signals (from −309.5 to −538.7 ppm) when the compound is dissolved in deuterated chloroform. However, this decomposition is reversible and the hexamer is quantitatively recovered upon crystallization.63 The solid-state 119 Sn NMR (CP-MAS) signal of this oxo-cluster contains only two isotropic chemical shifts (−213 and −355 ppm) while three different tin atoms exist in the structure.63 However, according to distances and angles they can be grouped in only two environments, one tin atom exhibiting a more distorted trigonal bipyramid than the two others. This stronger distortion likely explains the 140 ppm difference in the 119 Sn chemical shifts of two five-coordinate environments. {(i PrSn)9 (μ3 -O)8 (μ2 -OH)6 Cl5 }
The pyramidal cage structure of {(i PrSn)9 O8 (OH)6 Cl5 } is based on five six-coordinate (distorted octahedron) and four five-coordinate (distorted trigonal bipyramid) tin atoms linked by μ3 -oxo and μ2 -OH bridges (Figure 2.3.2).64 A terminal chloride atom is also connected to each six-coordinate tin atom. No study has ever checked if this product of hydrolysis was preserved in solution or could only be isolated in the solid state.
Figure 2.3.2 Molecular structure of {( i PrSn)9 O8 (OH)6 Cl5 }. (Reproduced from reference 64, copyright 1989, Elsevier.)
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Figure 2.3.3 Ball and stick (left) and polyhedral (right) molecular structure of {(RSn)12 O14 (OH)6 }2+ .29,50,65−69 The polyhedra around the five-coordinate tin atoms are striped.
{(RSn)12 (μ3 -O)14 (μ2 -OH)6 }2+ and Related Compounds
The macrocation {(RSn)12 O14 (OH)6 }2+ is the most documented mono-organotin oxo-cluster. Its molecular structure (Figure 2.3.3) is based on six five-coordinate (distorted square pyramid), located at the equator of this almost spherical cage, and six six-coordinate (distorted octahedron) tin atoms, linked by μ3 -O and μ2 -OH bridges.29,50,65−69 To better describe this metal–oxygen framework, it can be envisaged as being comprised of three sub-units. Two of them are O-capped trimers that contain all six-coordinate tin atoms and the hydroxy bridges. These sub-units define the positive poles of this oxo-cluster. The other sub-unit is a macro-ring containing all five-coordinate tin atoms connected through their basal oxygen atoms. The anions required to balance the positive charge always interact through hydrogen bonds with one or several μ2 -OH links. A similar molecular structure is found for the titanium(IV) oxo-cluster, Ti12 O16 (Oi Pr)16 ,70,71 and a vanadium(V/IV) oxo-cluster, {(VO)12 O12 F2 (OH)6 }6− .72 In the titanium derivative, Ti-Oi Pr moieties replace the RSn groups and four out of six μ2 -OH groups are also substituted by alkoxy groups; the last two original μ2 -OH groups are transformed into μ2 -O, to yield a neutral compound. The anionic vanadium derivative is purely inorganic, with terminal oxo groups on its surface, and incorporates a μ3 -F to replace the internal μ3 -O that connects the six-coordinate metal atoms at each pole. This similarity between a vanadium and an organotin oxo-cluster has prompted Reuter et al. to look for mixed organotin-vanadium compounds and they found several where one, two, three, or six V O groups replace the square-pyramidally coordinated organotin groups.73 Two of these mixed compounds {(i PrSn)11 (OVIV )O14 (OH)6 }Cl,73 and {(n BuSn)9 (OVV )3 O14 (OH)6 Cl2 (DMSO)2 },74 have been structurally well-characterized (Figures 2.3.4a and b). The metal–oxygen framework of the first structure does not differ from that of {(RSn)12 O14 (OH)6 }2+ , but bears a single positive charge. For the second structure, because the three [OVO4 ] units are condensed together and V O single bonds are shorter than Sn O bonds, the original metal–oxygen framework is disrupted in two places, with two μ3 -O groups transformed into μ2 -O, and allows two equatorial tin atoms to increase their coordination to six, each with a Sn Cl bond and an Sn O coordinative bond with a DMSO molecule.74 One additional difference between these two mixed organotin–vanadium oxo-clusters is found in the oxidation state of vanadium; +4 in the derivative with a single vanadium and +5 in the derivative with three vanadium atoms.
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Figure 2.3.4 Ball and stick (left) and polyhedral (right) molecular structures of {( i PrSn)11 (OV)O14 (OH)6 }+ ,73 and {( n BuSn)9 (OV)3 O14 (OH)6 Cl2 DMSO2 }.74 Additional color code: vanadium as small hatched circles. The polyhedra around the five-coordinate tin atoms are striped.
The structure of {(RSn)12 O14 (OH)6 }2+ is fully preserved in solution, where it acquired a perfect 3-fold symmetry (D3d ), as proved many times by its distinctive 119 Sn NMR signature composed of two resonances, one for each coordination, and up to three two-bonds scalar couplings in between tin atoms of different or identical coordination (Figure 2.3.5, Table 2.3.1).29,43,50,66,67,69,75,76 As presented in the Introduction, solid-state 119 Sn NMR is a complementary tool to X-ray diffraction for the study of organotin oxo-clusters. Mono-organotin derivatives generally exhibit large chemical shift anisotropies and their spectra contain many spinning side bands.29,31,50,67,75,77 The 119 Sn NMR (MAS) spectra of {(n BuSn)12 O14 (OH)6 }(OH)2 (HOi Pr)4 and {(n BuSn)12 O14 (OH)6 }(pTs)2 (Dioxane) are shown as examples (Figure 2.3.6). From such spectra, an anisotropy and an asymmetry can be measured for each chemical shift.78 For five-coordinate tin atoms (isotropic chemical shift ca. −280 ppm), the anisotropy equals 380 ± 20 ppm, while for six-coordinate tin atoms (isotropic chemical shift ca. −460 ppm), the anisotropy is smaller at 300 ± 20 ppm The asymmetry for both coordinations can range from 0.15 to
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Figure 2.3.5 Solution 119 Sn NMR spectrum of {(n BuSn)12 O14 (OH)6 }(pTs)2 in CDCl2 at 93.3 MHz. : 2 J ( 119 Sn-119/117 Sn) between a five- and a six-coordinate tin atom linked through a single oxo-bridge, : 2 J ( 119 Sn-119/117 Sn) between a five- and a six-coordinate tin atom linked through a double oxo-bridge, ∗ : 2 J ( 119 Sn-117 Sn) between two six-coordinate tin atoms linked through a double oxo-hydroxo bridge, and : 2 J ( 119 Sn-117 Sn) between two five-coordinate tin atoms linked through a double oxo-bridge.50
0.45, depending on the compound. For {(n BuSn)12 O14 (OH)6 }(pTs)2 (Dioxane), 119 Sn NMR has clearly provided evidence for the weak interaction that takes place in the solid state between some of the five-coordinate tin atoms and the oxygen atoms of the dioxane molecules. Indeed a low frequency shift of about 20 ppm is observed for the tin atoms experiencing this extra contact, characterized by an Sn O ˚ 50 Solid-state 119 Sn NMR was also used to unambiguously show the preservation distance of 3.3 A. Table 2.3.1
119
Sn NMR data for various {(RSn)12 O14 (OH)6 }X2 derivatives
{(RSn)12 O14 (OH)6 }X2
Solvent
δ(119 Sn)/ppm and [2 J (119 Sn–119 Sn)]/Hz
Ref.
R = n Bu, X = Cl R = n Bu, X = OH R = n Bu, X = OH R = n Bu, X = pTsa R = n Bu, X = O2 PPh2 R = n Bu, X = AMPSb R = n Bu, X = AcO R = Me3 SiCH2 , X = Cl R = pBST,c X = OH R = AcO(CH2 )5 , X = OH R = PentCro,d X = OH R = C6 H5 , X = OH/Cl
CD2 Cl2 C6 D6 CDCl3 CD2 Cl2 CD2 Cl2 CDCl3 CD2 Cl2 CDCl3 CDCl3 CDCl3 CDCl3 CD2 Cl2
−283.1 [425, 156], −468.1 [425, 188] −280.1 [383, 177, 40], −447.4 [383, 205, 40] −282.2 [380, 177], −449.0 [380, 205] −282.8 [423, 162, 42], −461.8 [423, 176, 42] −283.3 [412, 158], −457.7 [412, 174] −283.0 [423, 153, 42], −462.0 [423, 172, 42] −281 [410], −458 [410] −269.6 [434, 183], −460.5 [434, 191] −280.0 [unresolv.], −470.4 [unresolv.] −280.7 [373, 178], −443.7 [373, 207] −282.2 [unresolv.], −449.0 [unresolv.] −354 [420, 220], −520 [420, 246]
66 67 43 50 75 29 31 69 43 43 43 76
a c
pTs : 4-CH3 C6 H4 SO3 ; b : AMPS : H2 C CHCONHC(CH3 )2 CH2 SO3 , : pBST : 4-(H2 C CH)C6 H4 (CH2 )4 , d : PentCro : MeCH CHCO2 (CH2 )5
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Figure 2.3.6 Solid-state 119 Sn NMR MAS spectra of {( n BuSn)12 O14 (OH)6 }(OH)2 (HOi Pr)4 (a)67 and {( n BuSn)12 O14 (OH)6 }(pTs)2 (Dioxane) (b)50 at 111.9 MHz (ν M AS = 13 kHz). Isotropic chemical shifts are indicated with arrows.
of the {(n BuSn)12 O14 (OH)6 }2+ oxo-core in insoluble amorphous materials prepared from functional oxo-clusters.29,31 [Na ⊂{(i PrSn)12 (μ4 -O)4 (μ2 -OH)24 }]5+
The structure of [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ is based on the association, around a central sodium ion, of four identical O-capped trimers ‘(RSn)3 (μ3 -O)(μ2 -OH)3 (OH)6 ,’ through μ2 -OH bridges only (Figure 2.3.7).79 The trimeric sub-units are oriented so that only two of them share an edge; all the other trimer–trimer contacts are through vertices. Such an arrangement of the trimers is also found for the polyoxometalate [PW10 V2 O40 ]5− and corresponds to the γ isomer of the Keggin’s structure.5,80
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Figure 2.3.7 Ball and stick (left) and polyhedral (right) molecular structure {( i PrSn)12 O4 (OH)24 }]5+ .79 Additional color code: sodium as small hatched circles.
of
[Na
77
⊂
There is no evidence for the stability of such species in solution and it might be only encountered in the solid state where it cocrystallizes in between infinite chains of [Ag7 I11 ]4− . {(2,4,6-i Pr3 C6 H2 Sn)8 (μ4 -O)2 (μ3 -O)8 (μ2 -O)4 (μ2 -OH)8 (SnOH)4 }
{(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } does not strictly belong to the same family because it includes four purely inorganic tin atoms which come from a dearylation process during its synthesis.63 However, its structure is worth discussing here. It can be described with a central adamantane-type inorganic core, {(HOSn)4 O6 }, capped by two identical and perpendicular organostannoxane arcs, {(2,4,6i Pr3 C6 H2 Sn)O(OH)}4 (Figure 2.3.8). The four inorganic tin atoms are six-coordinate and each bear a terminal OH. The organostannoxane arcs are based on two central six-coordinate tin atoms and two external five-coordinate ones (distorted trigonal bypiramid). An alternate description of this metal–oxygen framework can be one based on four identical O-capped trimers, each based on a six-coordinate inorganic tin, a six-coordinate aryltin moiety, and a five-coordinate one. This structure includes five different types
Figure 2.3.8 Ball and stick (left) and polyhedral (right) molecular structure of {(2,4,6Pr3 C6 H2 Sn)8 (HOSn)4 O14 (OH)8 }.63 Additional color code: inorganic tin as small hatched circles. The polyhedra around the five-coordinate tin atoms are striped.
i
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of oxygen atoms: two μ4 -O, eight μ3 -O, four μ2 -O, eight μ2 -OH, and four terminal OH. It incorporates two features not observed in any of the previously described organotin oxo-clusters: oxygen atoms between four tin atoms and face-sharing RSnO5 octahedra. The latter feature is also observed in a POM derivative [{(PhSn)2 O}2 H(α-AsW9 O33 )2 ]9− .15 Its signature 119 Sn NMR spectrum, three resonances (−574.1, −438.2, and −338.0 ppm, for SnO6 , CSnO5 , and CSnO4 environments, respectively) in a 1:1:1 ratio, is fully consistent with the stability of its metal–oxygen framework in solution.63 Solid-state 119 Sn NMR (CP-MAS), that shows three isotropic chemical shifts (−576, −448, −343 ppm) very close to those observed in solution, indicates that almost no changes take place when this organotin oxo-cluster is dissolved. 2.3.3
Syntheses and Formation Mechanisms
Syntheses of Organotin Oxo-Clusters
As with organostannonic acid, mono-organotin oxo-clusters are generally prepared by hydrolysiscondensation of molecular precursors such as organotin trihalides (RSnCl3 , RSnBr3 ), organotin trialkoxides (RSn(OR )3 ),39 and organotin trialkynides (RSn(C≡CR )3 ).40−44 Organotin trichlorides are more accessible (several are commercially available or can be prepared via Kotcheskov’s redistribution),81 but they generally yield compounds that still contain chloride, either directly bonded to tin (not hydrolyzed) or as charge compensating anion with cationic species. In some cases, already hydrolyzed/condensed precursors, of unknown structures (i.e. i PrSn(OH)2 Cl · 3/4H2 O and n BuSnO(OH)), are also used. The synthesis of {(n BuSn)12 O14 (OH)6 }2+ from n BuSnO(OH) and sulfonic acid is inspired by the significant amount of work on oxo-clusters with carboxylate, phosphinate, or phosphonate ligands.7−9 Yet the poor complexing ability of sulfonates results in the formation of an oxo-cluster that does not include any sulfonate in its architecture. The experimental conditions reported are quite diverse and they are summarized in Tables 2.3.2 and 2.3.3. There are no clear trends that allow the confident prediction of the composition of the final compounds, especially when considering that some organotin oxo-clusters are serendipitous products. The influence of the organic group appears very strong. For example, tris(trimethylsilyl)methyl, a very bulky group, yields compounds based on Sn3 O3 tristannoxane rings and only four-coordinate tin atoms.60 2,4,6Tris(isopropyl)phenyl, another bulky substituent, can yield a hexamer and a dodecamer that contain only five-coordinate and five- and six-coordinate tin atoms, respectively; in the latter, one third of the tin atoms have undergone dearylation.63 The bulkiness of the organic substituents is not the only parameter; intra- or inter-molecular interactions involving the tin atom can modify the reaction outcome. For example, while the hydrolysis-condensation of AcO(CH2 )5 Sn(C C-n Bu)3 yields the {(RSn)12 O14 (OH)6 }2+ macrocation, with AcO(CH2 )3 Sn(C C-n Bu)3 a strong intra-molecular interaction,82,83 involving the alkoxy oxygen, probably prevents the formation of the oxo-cluster and instead a soluble oxo-polymer is obtained.43 Solvent is also of importance to control the outcome of the hydrolysis-condensation reaction. With exactly the same precursor (i PrSn(OH)2 Cl · 3/4H2 O), the same amount, and the same hydrolysis procedure (atmospheric moisture), {(i PrSn)9 O8 (OH)6 Cl5 } is obtained in DMSO,64 while {(i PrSn)12 O14 (OH)6 }Cl2 is obtained in DMF or DMPU.65 The parameter with the least influence seems to be the nature of the hydrolyzable group. Indeed, the macrocation {(RSn)12 O14 (OH)6 }2+ has been obtained, with the same organic R group, from organotin trichlorides, trialkoxides, and trialkynides. Yet, organotin trichlorides need harsher conditions to be fully hydrolyzed. A comment needs to be made on the yields of reactions. Indeed, hydrolysis-condensation reactions can yield well-defined organotin oxo-clusters, but they can also yield ill-defined organotin oxo-polymers.
EtSnCl3 n BuSnCl3 i PrSnCl3 i BuSnCl3 MeSnCl3 Cl3 Sn(CH2 )3 SnCl3 (Me3 Si)3 CSnCl3 {(Me3 Si)3 CSnOCl}3 (Me3 Si)3 CSnBr3 2,4,6-i Pr3 C6 H2 Sn(C CMe)3
{EtSnOHCl2 (H2 O)}2 {n BuSnOHCl2 (H2 O)}2 {i PrSnOHCl2 (H2 O)}2 {i BuSnOHCl2 (H2 O)}2 {MeSnOHCl2 (H2 O)}2 {Sn(CH2 )3 Sn(OH)2 Cl4 (H2 O)2 } {(Me3 Si)3 CSnOCl}3 {(Me3 Si)3 CSnO(OH)}3 {[(Me3 Si)3 CSn]4 O6 } {2,4,6-i Pr3 C6 H2 SnO(OH)}6
Synthesis conditions
a
: Secondary product in the synthesis of {(2,4,6-i Pr3 C6 H2 Sn}8 O14 (OH)8 (SnOH)4 }; b : Unknown structure, prepared by the hydrolysis of i PrSnCl3 in toluene
48 94 75 75 80 –a 56 – – 20
– 10
56 58 57 57 55 59 60 60 62 63 63 64 79 63
Yield Ref.
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Acidic water Pure in air for a week CHCl3 , air moisture, slow evaporation CHCl3 , air moisture, slow evaporation CH2 Cl2 , H2 O/Sn = 2, 1,3-xylyl-18-crown-5 (0.25 eq.), 24h Water, H2 O/Sn = 550, slow evaporation Pentane, H2 O/Sn = 6.3, PhNH2 (3.6 eq), reflux 21 days Hexane, LiOH (6 eq.), reflux 28 days THF/NH3 (liq), Na2 O (1.5eq.), −78 ◦ C, 6 hours THF, H2 O/Sn = 12, reflux 9 days t AmylOH, H2 O/Sn=24, 60 ◦ C, 9 days i i b PrSn(OH)2 Cl·3/4H2 O DMSO, air moisture, few weeks {( PrSn)9 O8 (OH)6 Cl5 } i PrSn(OH)2 Cl·3/4H2 Ob DMSO, NaI and AgI, air moisture, few weeks [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ {(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } 2,4,6-i Pr3 C6 H2 Sn(C CMe)3 THF, H2 O/Sn = 12, reflux 9 days
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Synthesis conditions
Crystalsa
c
a
i
i
i
65 65 68 68 66 69 69 67 67 76 43 43 43 43 50 29
– – – – 84 85 93 67 – – – – – – 60 15
: in the absence of X-ray structure, the formation of {(RSn)12 O14 (OH)6 }2+ is confirmed by 119 Sn NMR; b : unknown structure, prepared by the hydrolysis of i PrSnCl3 in toluene; : crystallized in acetone/acetonitrile; d : crystallized in chloroform; e : crystallized in dioxane containing 0.5 % H2 O; f : crystallized in acetonitrile
Pr/Cl Pr/Cl i Pr/{(i PrSn)4 (MoO4 )4 O(OH)3 }
i
Yield Ref.
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PrSn(OH)2 Cl·3/4H2 Ob DMF, air moisture, few weeks 1 r3H2 O and 1 r2DMF PrSn(OH)2 Cl·3/4H2 Ob DMPU, air moisture, few weeks 1 r4DMPU·4H2 O i b PrSn(OH)2 Cl·3/4H2 O DMSO, (NH4 )2 MoO4 (1.25 eq.), air moisture, 1 r6DMSO few days i i Pr/{(i PrSn)4 (MoO4 )4 O(OH)3 } PrSn(OBu)3 DMF, H3 [P(Mo3 O10 )4 ] (0.12 eq.), air moisture, 1 r10DMF few days n n Bu/Cl BuSnCl3 Water, slow addition of 1M KOH till pH = 4 1 r2H2 Oc Me3 SiCH2 SnCl3 Water, slow addition of 1M KOH till pH = 4 1d Me3 SiCH2 /Cl Me3 SiCH2 /Cl Me3 SiCH2 SnCl3 Toluene/NH3 aq. (15%), Me3 SiCl (6.7 eq.) No n n i Bu/OH BuSn(Oi Pr)3 PrOH, H2 O/Sn = 10, 15 hours 1 r4i PrOH n n t t Bu/OH BuSn(O Amyl)3 AmylOH, H2 O/Sn = 10 No Ph/OH + Cl PhSn(Ot Amyl)x Cl3−x t AmylOH, H2 O/Sn = 10 No n n Bu/OH BuSn(C Cn Bu)3 CHCl3 /THF/H2 O No 4-(H2 C CH)C6 H4 (CH2 )4 /OH RSn(C Cn Bu)3 CHCl3 /THF/H2 O No AcO(CH2 )5 /OH RSn(C Cn Bu)3 CHCl3 /THF/H2 O No RSn(C Cn Bu)3 CHCl3 /THF/H2 O No MeCH CHCO2 (CH2 )5 /OH n n Bu/4-CH3 C6 H4 SO3 BuSnO(OH) toluene, R’SO3 H/Sn= 3.5, reflux 48 hours 1 rDioxanee n Bu/H2 C CHCONHC(CH3 )2 CH2 SO3 n BuSnO(OH) toluene, R’SO3 H/Sn= 3.5, reflux 24 hours 1f
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These latter species can be highly soluble in organic solvents, but, because of reorientation time, they can be 119 Sn NMR silent,43,50 especially if they are mixed with 119 Sn NMR ‘loud’ compounds, the signals of which are very narrow. Moreover, these oxo-polymers generally have a mean composition close to the one of the oxo-clusters (for n BuSnO(OH) Sn : 56.8%, C : 23.0%, and H : 4.8%; for {(n BuSn)12 O14 (OH)6 }(OH)2 Sn : 57.7%, C : 23.3%, and H : 4.7%). Accordingly, a yield simply measured from the weight of recovered solid, especially if there is an amorphous part, with the hypothesis, based on 119 Sn solution NMR, that there is no other compound can be definitely overestimated. To avoid such problems, quantitative 119 Sn solution NMR with an internal or external concentration reference should be performed to make sure that all the dissolved solid participates in the observed resonances. For instance, in the synthesis of {(n BuSn)12 O14 (OH)6 }(pTs)2 from n BuSnO(OH), the 119 Sn NMR spectrum of the crude materials looks exactly the same as the crystallized compound. However, quantitative 119 Sn NMR has shown that this crude material contains only ca. 50% {(n BuSn)12 O14 (OH)6 }(pTs)2 .50 To address these problems of yield determination, 119 Sn solid-state MAS NMR and size-exclusion chromatography can also be practical tools.50,84 Inter-conversions between different species have been reported for oxo-clusters with complexing ligands.8,9 But nothing similar has been observed for {(RSn)n Ox (OH) y Clz } compounds, except for {2,4,6-i Pr3 C6 H2 SnO(OH)}6 , that transforms into unknown species in solution, but can be quantitatively recovered upon crystallization.63 Another related case concerns the formation of {(Me3 Si)3 CSnO(OH)}3 upon treating {(Me3 Si)3 CSnOCl}3 with LiOH.60 However, this transformation proceeds without any modification of the metal–oxygen framework, which remains based on a Sn3 O3 tristannoxane ring. Changing the composition while keeping the metal–oxygen framework has also been observed several times with the substitution of the anions in {(RSn)12 O14 (OH)6 }X2 .28,30,31,75,85 This exchange reaction appears controlled by the acidity of the anions. Indeed hydroxyls can be replaced easily by many different anions [Equation (2.3.1)],31,75 and this approach has been used to prepare oxo-clusters with functional anions.28 By contrast, when the anion to exchange is derived from a strong acid (HCl or RSO3 H), the reaction with two equivalents of a weaker acid does not yield the exchange product, but the partial destruction of the metal–oxygen framework and the formation of different oxo-clusters that include complexing ligands [Equations (2.3.2) and (2.3.3)].75,85 Yet this destruction can be avoided if salts of the weak acids are used [Equation (2.3.4)].85,86 Finally, whatever the anion, if an excess of carboxylic or phosphinic acid is used, the metal–oxygen framework is transformed [Equations (2.3.5) and (2.3.6)].31,66 For Y = RSO3 , Cl, RCO2 , R2 PO2 {(n BuSn)12 O14 (OH)6 }(OH)2 + 2HY → {(n BuSn)12 O14 (OH)6 }Y2 + 2H2 O
(2.3.1)
For X = RSO3 , Cl {(n BuSn)12 O14 (OH)6 }X2 + 2RCO2 H → 5/6{(n BuSn)12 O14 (OH)6 }X2 + 1/3HX + 4/3H2 O + 1/3{n BuSnO(O2 CR)}6
(2.3.2)
For X = RSO3 , Cl {(n BuSn)12 O14 (OH)6 }X2 + 2R2 PO2 H → 7/8{(n BuSn)12 O14 (OH)6 }X2 + 1/4HX + 1/2H2 O + 1/2{[n BuSn(OH)(O2 PR2 )]3 O}(O2 PR2 )
(2.3.3)
For Y = OH, R2 PO2 {(n BuSn)12 O14 (OH)6 }(O3 SR)2 + 2Me4 NY → {(n BuSn)12 O14 (OH)6 }Y2 + 2Me4 N(O3 SR)
(2.3.4)
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For X = RSO3 , Cl, OH {(n BuSn)12 O14 (OH)6 }X2 + 12RCO2 H → 2{n BuSnO(O2 CR)}6 + 2HX + 8H2 O
(2.3.5)
{(n BuSn)12 O14 (OH)6 }X2 + 16R2 PO2 H → 4{[n BuSn(OH)(O2 PR2 )]3 O}(O2 PR2 ) + 2HX + 4H2 O (2.3.6) Because, in {(RSn)12 O14 (OH)6 }X2 , the anions do not interact directly with any tin atom but with the μ2 -OH that constitute the charged poles, the exchange of the anions or their ionic dissociation cause only small changes on the 119 Sn chemical shifts (Table 2.3.1). 1 H–1 H NOESY/ROESY (homonuclear Overhauser correlation spectroscopy) and 31 P–1 H HOESY (heteronuclear NOESY) have been used to study the ionic dissociation of {(RSn)12 O14 (OH)6 }X2 (X : O3 SR and O2 PR2 ), which depends on the solvent nature and the anion.50,75 More recently, pulsed field gradient (DOSY) 1 H NMR has also shown its remarkable versatility to probe association/dissociation and exchange of the charge compensating anions of {(RSn)12 O14 (OH)6 }X2 .86 There is another type of reaction with {(RSn)12 O14 (OH)6 }X2 that preserves its structure. Even though no species were isolated, the possible replacement of two μ2 -OH, out of six, by methanol has been clearly shown by electrospray mass spectrometry (ES-MS).66 Reflections on Possible Formation Mechanisms
The diversity observed in the molecular structure of organotin oxo-clusters is difficult to explain, but is likely related to a subtle balance between the nature of the organic group and the hydrolysis conditions (solvent, hydrolysis ratio, etc.). There is even a case where a unique reaction mixture yields two different oxo-clusters with fairly different metal–oxygen frameworks, i.e. {2,4,6-i Pr3 C6 H2 SnO(OH)}6 and {(2,4,6i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 }.63 In order to clarify this diversity, a reflection on possible formation mechanisms involving a limited number of ‘plausible intermediates’ is interesting. A leading idea in this reflection is the strong tendency for mono-organotin derivatives to increase the coordination of tin upon hydrolysis-condensation. Already in mono-organotin trialkoxides this tendency exists and tin coordination is increased by the formation of alkoxy bridges.87 Except for oxo-clusters involving the very bulky (Me3 Si)3 C group,60 the coordination of tin in all the oxo-clusters is five or six. Such coordination numbers are also classically observed for organotin oxo-clusters that contain complexing ligands.8,9 This increase of coordination, related to the Lewis acidity of mono-organotin, is achieved by the formation of bridges and adducts. The smallest organotin oxo-cluster {RSn(OH)Cl2 (OH2 )}2 combines both features, and compounds isolated for R : Me and —(CH2 )3 — show that tin can form water adducts, up to a coordination of six, prior to the hydrolysis of the first chloride.55,59 A second leading idea is the occurrence of μ3 -O bridges in many organotin oxo-clusters. These considerations lead to propose the trimers {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)2+x X3−x L y } (X : Cl, OR , C CR , and L : H2 O, ROH, THF, DMSO, etc.) as plausible intermediates. In this formula, X represents a hydrolyzable group not yet removed, and L a molecule that can bind to tin atoms through a dative bond and participate to their coordination expansion (y ≤ 3 for CNmax = 6). These molecules, being generally very labile, will not be considered in the proposed mechanisms nor drawn in the schemes and figures, to avoid their overcrowding. The formation of such trimers (Scheme 2.3.2) can be rationalized by the condensation of a dimer {RSn(μ2 -OH)(OH)X}2 and a monomer {RSn(OH)2 X}, two species likely related by a concentration-controlled association/dissociation equilibrium. The condensation of such trimers, or their derivatives, makes it possible to generate several known oxo-clusters. For instance, the ‘tilted’ face-to-face condensation of two trimers {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)5 } yields exactly the metal–oxygen framework of {2,4,6-
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Scheme 2.3.2 Proposed mechanism for the formation of {(RSn)3 ( μ3 -O) (μ2 -OH)2 (OH)2+x X3−x } (X : Cl, OR’, C CR’) i
Pr3 C6 H2 SnO(OH)}6 [Figure 2.3.9, Equation (2.3.7)]. The condensation of four trimers {(RSn)2 (HOSn)(μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 }, derived from the previous one through an intramolecular condensation and the hydrolysis of the organic group of one of the three tin atoms (dearylation process), yields exactly the metal–oxygen framework of {(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } [Figure 2.3.10, Equations (2.3.8–10)]. The aggregation around a central sodium atom of four trimers, {(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 }, followed by their connection
Figure 2.3.9
Proposed mechanism for the formation of {2,4,6-i Pr3 C6 H2 SnO(OH)}6 .
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Figure 2.3.10
Proposed mechanism for the formation of {(2,4,6-i Pr3 C6 H2 Sn)8 (HOSn)4 O14 (OH)8 }.
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through the conversion of each terminal hydroxy group into a bridging one (μ2 -OH) and the protonation of the four surface μ2 -O bridges, results in the formation of [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ [Figure 2.3.11, Equation (2.3.11)]. The protonation step that turns μ2 -O into μ2 -OH is likely favoured by the delocalization of the positive charge on three adjacent μ2 -OH. However, the reason why this latter compound corresponds only to the γ -isomer of the Keggin’s structure does not appear in the proposed mechanism. In the proposed mechanisms, the trimeric intermediates for {(2,4,6-i Pr3 C6 H2 Sn)8 O14 (OH)8 (SnOH)4 } and [Na ⊂ {(i PrSn)12 O4 (OH)24 }]5+ are similar, but their organic groups are oriented differently, opposite to the face or on the face defined by the three μ2 -O/OH bridges. This difference is in line with the bulkiness of the organic groups. 2{(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)5 } → {(RSn)6 (μ3 -O)4 (μ2 -O)2 (μ2 -OH)2 (OH)4 } + 4H2 O
(2.3.7)
{(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)5 } → {(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + H2 O
(2.3.8)
{(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + H2 O → {(RSn)2 (HOSn)(μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + RH (2.3.9) 4{(RSn)2 (HOSn)(μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } → {(RSn)8 (μ4 -O)2 (μ3 -O)8 (μ2 -O)4 (μ2 -OH)8 (SnOH)4 } + 6H2 O
(2.3.10)
4{(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + Na+ + 4H+ → {Na ⊂ (RSn)12 (μ4 -O)4 (μ2 -OH)24 }5+ (2.3.11) The sole consideration of {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)2+x Cl3−x } trimers does not allow us to build the metal–oxygen framework of {(i PrSn)9 O8 (OH)6 Cl5 }. However, its construction can be rationalized with the help of three dimeric units {RSn(μ2 -OH)(OH)Cl}2 and one trimer {(RSn)3 (μ3 -O)(μ2 OH)2 (OH)2 Cl3 } [Figure 2.3.12, Equation (2.3.12)]. The dimeric intermediate considered here has already been proposed to rationalize the formation of several phosphinate-based organotin derivatives and, moreover, it has the formula of the starting material used to prepare {(i PrSn)9 O8 (OH)6 Cl5 }.13,64 The condensation of the same dimer and trimer, in which the chloride can be substituted for an alkoxy or an alkynide group, or their fully hydrolyzed versions, allows the building up of the metal oxygen framework of the macrocation {(RSn)12 O14 (OH)6 }2+ [Figure 2.3.13, Equation (2.3.13)]; again the last step here is the protonation of the μ2 -O. 3{RSn(μ2 -OH)(OH)Cl}2 + {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)3 Cl2 } → {(RSn)9 (μ3 -O)8 (μ2 -OH)6 Cl5 } +4H2 O + 3HCl
(2.3.12)
3{RSn(μ2 -OH)(OH)X}2 + 2{(RSn)3 (μ3 -O)(μ2 -O)(μ2 -OH)2 (OH)3 } + 2H+ → {(RSn)12 (μ3 -O)14 (μ2 -OH)6 }2+ + 6H2 O + 6HX
(2.3.13)
The formation of {(Me3 Si)3 CSnOCl}3 , {(Me3 Si)3 CSnO(OH)}3 , and {[(Me3 Si)3 CSn]4 O6 } with their four-coordinate tin atoms, appears completely different. The following mechanisms can be
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Figure 2.3.11
Proposed mechanism for the formation of [Na ⊂ {( i PrSn)12 O4 (OH)24 }]5+ .
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Figure 2.3.12
Proposed mechanism for the formation of {( i PrSn)9 O8 (OH)6 Cl5 }.
87
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Figure 2.3.13
Proposed mechanism for the formation of {(RSn)12 O14 (OH)6 }2+ .
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Scheme 2.3.3
89
Proposed mechanism for the formation of {(Me3 Si)3 CSnOCl}3 and {(Me3 Si)3 CSnO(OH)}3
proposed (Scheme 2.3.3). Instead of condensation, a dimer {RSn(μ2 -OH)(OH)Cl}2 and a monomer {RSn(OH)2 Cl} can fuse together to yield a cyclic trimer {RSn(μ2 -OH)(OH)Cl}3 .13 Then, because of the very bulky tris(trimethylsilyl)methyl groups, which prevents any intermolecular condensation and induces a steric stress on five-coordinate tin atoms, the trimer undergoes intra-molecular dehydration to yield {RSn(μ2 -O)Cl}3 . Then, {RSn(μ2 -O)(OH)}3 is simply formed by hydrolyzing the chlorides, as observed experimentally.60 The formation of the adamantane-like core of {[(Me3 Si)3 CSn]4 O6 } is likely the result of the condensation of {RSn(μ2 -O)OH}3 with RSn(OH)x X3−x . Careful studies, especially by 119 Sn solution NMR and MS, of the solutions from which the various oxo-clusters are obtained, will be necessary to confirm these suggested mechanisms. Yet, without explaining everything, they manage to rationalize, with only a limited number of plausible intermediates, the formation of all the metal–oxygen frameworks discussed in this chapter. The main types of intermediate considered are the dimers, {(RSn)2 (μ2 -OH)2 (OH)2+x X2−x } (X : Cl, OR , C CR ) and the trimers {(RSn)3 (μ3 -O)(μ2 -OH)2 (OH)2+x X3−x } (X : Cl, OR , C CR ). 2.3.4
Conclusions
A variety of organotin oxo-clusters, with the general formula (RSn)n Ox (OH) y Clz and different molecular structures, have been reported during the last 30 years. The {(RSn)12 O14 (OH)6 }2+ cluster is the most studied, and the only one that has been evaluated for its catalytic properties24 or ability to generate advanced hybrid organic–inorganic materials.27−31 Similar work is needed on the other derivatives to determine the potentialities of this class of compounds. The diversity of structures, even if they might be derived from a limited number of intermediates, reflects the complexity that is present in ill-defined polymeric organostannonic acids. Oxo-clusters can probably help to understand these materials, but more as spectroscopic references than as ready-to-use solutions. Indeed, two very different oxo-clusters, both with bulky organic groups, have been reported with the formula RSnO(OH).60,63 The first one, {(Me3 Si)3 CSnO(OH)}3 , was even presented as the answer to an 80-year-old question about the basic structural elements of mono-organotin acids.60 However, these two oxo-clusters exhibit only four- or five-coordinate tin atoms, while solid-state 119 Sn NMR has shown that n BuSnO(OH) contains mostly (>80%) six-coordinate tin atoms that moreover present spectroscopic characteristics very similar to the ones found for the six-coordinate tin atoms of {(n BuSn)12 O14 (OH)6 }2+ .77 Hopefully, the story shall continue and this family of oxo-clusters will grow, as recently demonstrated by the discovery of two new members.63 With this perspective, a closer look at the solutions from which the oxo-clusters are recovered might be very fruitful, as the species that have been isolated so far may not be the only ones to form, but simply the only ones to crystallize/precipitate. Mono-organotin compounds with complex or functional organic groups, as well as new synthesis conditions (e.g. solvothermal or supercritical),88 could also yield metal–oxygen frameworks of yet unknown architectures. In the context of new nanostructured materials, mono-organotin derivatives with a cleavable organic group (e.g. aryl, benzyl)63,89 are especially exciting because they could yield oxo-clusters that might be subsequently
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assembled by condensation, after the cleavage of the surface Sn C bonds, to form unknown tin oxide structures. Acknowledgments The author would like to acknowledge the students, post-doctoral fellows, and colleagues who have contributed over the years to work on organotin oxo-clusters. Their names can be found in the references. CNRS, Universit´e P. et M. Curie-Paris6, and EU are also thanked for financial support. References 1. M.T. Pope, in Comprehensive Coordination Chemistry II, J.A. McCleverty, and T.J. Meyer (Eds), Elsevier, Oxford, 2004, pp. 635. 2. C.L. Hill, in Comprehensive Coordination Chemistry II, J.A. McCleverty and T.J. Meyer (Eds), Elsevier, Oxford, 2004, pp. 679. 3. Polyoxometalate Chemistry for Nano-Composite Design, T. Yamase and M.T. Pope (Eds), Kluwer Academic/Plenum Publishers, New York, 2002. 4. Polyoxometalate Chemistry: from Topology via Self-assembly to Applications, M.T. Pope and A. Müller (Eds), Kluwer Academic Publishers, Dordrecht, 2001. 5. M.T. Pope and A. Müller Angew. Chem. Int. Ed., 30, 34 (1991). 6. H.W. Roesky, I. Haiduc and N.S. Hosmane, Chem. Rev., 103, 2579 (2003). 7. V. Chandrasekhar, S. Nagendran and V. Baskar, Coord. Chem. Rev., 235, 1 (2002). 8. V. Chandrasekhar and K. Gopal, Appl. Organomet. Chem., 19, 429 (2005). 9. R.R. Holmes, Acc. Chem. Res., 22, 190 (1989). 10. V. Chandrasekhar, V. Baskar and J.J. Vittal, J. Am. Chem. Soc., 125, 2392 (2003). 11. V. Chandrasekhar, V. Baskar, K. Gopal and J.J. Vittal, Organometallics, 24, 4926 (2005). 12. K.C.K. Swamy, S. Nagabrahmanandachari and K. Raghuraman, J. Organomet. Chem., 587, 132 (1999). 13. K.C.K. Swamy, R.O. Day and R.R. Holmes, Inorg. Chem., 31, 4184 (1992). 14. S.Y. Song, J.F. Ma, J. Yang L.L. Gao and Z.M. Su, Organometallics, 26, 2125 (2007). 15. G. Sazani, M.H. Dickman and M.T. Pope, Inorg. Chem., 39, 939 (2000). 16. G. Sazani and M.T. Pope, Dalton Trans., 1989 (2004). 17. N. Belai and M.T. Pope, Polyhedron, 25, 2015 (2006). 18. S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacote, S. Thorimbert and M. Malacria, Angew. Chem. Int. Ed., 42, 3404 (2003). 19. S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacote, S. Thorimbert and M. Malacria, J. Am. Chem. Soc., 127, 6788 (2005). 20. F.B. Xin and M.T. Pope, Inorg. Chem., 35, 5693 (1996). 21. F.B. Xin, M.T. Pope, G.J. Long and U. Russo, Inorg. Chem., 35, 1207 (1996). 22. F.B. Xin and M.T. Pope, Organometallics, 13, 4881 (1994). 23. X.H. Wang and J.F. Liu, J. Coord. Chem., 51, 73 (2000). 24. S. Durand, K. Sakamoto, T. Fukuyama, A. Orita, J. Otera, A. Duthie, D. Dakternieks, M. Schulte, and K. Jurkschat, Organometallics, 19, 3220 (2000). 25. C. Sanchez, G.J. de Soler-Illia, F. Ribot, T. Lalot, C.R. Mayer, and V. Cabuil, Chem. Mater., 13, 3061 (2001). 26. F. Ribot and C. Sanchez, Comments Inorg. Chem., 20, 327 (1999). 27. L. Angiolini, D. Caretti, R. De Vito, F.T. Niesel, E. Salatelli, C. Carlini, F. Ribot and C. Sanchez, J. Inorg. Organomet. Polym., 7, 151 (1997). 28. F. Ribot, F. Banse, C. Sanchez, M. Lahcini and B. Jousseaume, J. Sol-Gel Sci. Technol., 8, 529 (1997). 29. F. Ribot, D. Veautier, S.J. Guillaudeu and T. Lalot, J. Mater. Chem., 15, 3973 (2005). 30. F. Ribot, A. Lafuma, C. Eychenne-Baron and C. Sanchez, Adv. Mater., 14, 1496 (2002). 31. F. Ribot, F. Banse, F. Diter and C. Sanchez, New J. Chem., 19, 1145 (1995).
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32. Z.P. Du, W.K. Kang, T. Cheng, J. Yao, and G.Y. Wang, J. Mol. Catal., A Chem., 246, 200 (2006). 33. O.A. Mascaretti, R.L.E. Furlan, C.J. Salomon and E.G. Mata, Phosphorus Sulfur Silicon Relat. Elem., 150–151, 89 (1999). 34. R.L.E. Furlan, E.G. Mata and O.A. Mascaretti, Tetrahedron Lett., 39, 2257 (1998). 35. M. Noda, Prep. Biochem. Biotechnol., 29, 333 (1999). 36. P.C. Rooney, US Patent [5,166,310] 1992. 37. S.D. Barnicki, C.E. Sumner Jr., H.C. Williams, US Patent [5,512,691] 1996. 38. J.G.A. Luitjen, Recl. Trav. Chim. Pays Bas, 85, 873 (1966). 39. D.P. Gaur, G. Srivastava, and R.C. Mehrotra, J. Organomet. Chem. 63, 221 (1973). 40. B. Jousseaume, M. Lahcini, M.-C. Rascle, F. Ribot and C. Sanchez, Organometallics, 14, 685 (1995). 41. C. Franc, B. Jousseaume, M. Linker and T. Toupance, Chem. Mater., 12, 3100 (2000). 42. H. Elhamzaoui, B. Jousseaume, H. Riague, T. Toupance, P. Dieudonne, C. Zakri, M. Maugey and H. Allouchi, J. Am. Chem. Soc., 126, 8130 (2004). 43. P. Jaumier, B. Jousscaume, M. Lahcini, F. Ribot and C. Sanchez, Chem. Commun., 369 (1998). 44. B. Jousseaume, H. Riague, T. Toupance, M. Lahcini, P. Mountford, and B.R. Tyrrell, Organometallics, 21, 4590 (2002). 45. Resonance frequency of neat tetramethyltin for a 1 H resonance frequency of 100.000 000 MHz. 46. J.C. Martins, M. Biesemans and R. Willem, Prog. Nucl. Magn. Reson. Spectrosc., 36, 271 (2000). 47. (a) B. Wrackmeyer, in Annual Reports on NMR Spectroscopy (vol. 38), G. A. Webb (Ed), Academic Press, San Diego, 1999, pp. 203–264; (b) B. Wrackmeyer, in Tin Chemistry–Fundamentals, Frontiers and Applications, A. Davies, M. Gielen, K. Pannell and E.R.T. Tiekink (Eds), Wiley, 2007, chapter 2. 48. A. Sebald, in Advanced Applications of NMR to Organometallic Chemistry, M. Gielen, R. Willem and B. Wrackmeyer (Eds), Wiley, Chichester, 1996, pp. 123–157. 49. T.N. Mitchell, in Chemistry of Tin (2nd edn), P.J. Smith (Ed), Blackie Academic & Professional, London, 1998, pp. 480–495. 50. C. Eychenne-Baron, F. Ribot, N. Steunou, C. Sanchez, F. Fayon, M. Biesemans, J.C. Martins and R. Willem, Organometallics, 19, 1940 (2000). 51. Cambridge Crystallographic Data Centre, http://www.ccdc.cam.ac.uk. 52. FIZ Karlsruhe, http://www.fiz-karlsruhe.de/home.html. 53. A sub-unit used to describe a structure does not have to exist as an isolated chemical species. It is simply a set of atoms with a specific connection scheme. 54. L. Rozes, N. Steunou, G. Fornasieri and C. Sanchez, Monatsh. Chem., 137, 501 (2006). 55. S.E. Johnson and C.B. Knobler, Organometallics, 13, 4928 (1994). 56. C. Lecomte, J. Protas and M. Devaud, Acta Crystallogr. Section B, 32, 923 (1976). 57. H. Puff and H. Reuter, J. Organomet. Chem., 364, 57 (1989). 58. R.R. Holmes, S. Shafieezad, V. Chandrasekhar, J.M. Holmes, and R.O. Day, J. Am. Chem. Soc., 110, 1174 (1988). 59. B. Zobel, A. Duthie, D. Dakternieks and E.R.T. Tiekink, Organometallics, 20, 2820 (2001). 60. J. Janssen, J. Magull and H.W. Roesky, Angew. Chem. Int. Ed., 41, 1365 (2002). 61. J. Beckmann and K. Jurkschat, Coord. Chem. Rev., 215, 267 (2001). 62. K. Wraage, T. Pape, R. Herbst-Irmer, M. Noltemeyer, H.-G. Schmidt, and H.W. Roesky, Eur. J. Inorg. Chem., 869 (1999). 63. G. Prabusankar, B. Jousseaume, T. Toupance and H. Allouchi, Angew. Chem. Int. Ed., 45, 1255 (2006). 64. H. Puff and H. Reuter, J. Organomet. Chem., 368, 173 (1989). 65. H. Puff and H. Reuter, J. Organomet. Chem., 373, 173 (1989). 66. D. Dakternieks, H. Zhu, E.R.T. Tiekink and R. Colton, J. Organomet. Chem., 476, 33 (1994). 67. F. Banse, F. Ribot, P. Toledano, J. Maquet and C. Sanchez, Inorg. Chem., 34, 6371 (1995). 68. G. Kastner and H. Reuter, Main Group Met. Chem., 23, 383 (2000). 69. J. Beckmann, K. Jurkschat, U. Kaltenbrunner, S. Rabe, M. Schuermann, D. Dakternieks, A. Duthie, and D. Mueller, Organometallics, 19, 4887 (2000).
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Tin Chemistry: Fundamentals, Frontiers and Applications V.W. Day, T.A. Eberspacher, W.G. Klemperer and C.W. Park, J. Am. Chem. Soc., 115, 8469 (1993). N. Steunou, F. Robert, K. Boubekeur, F. Ribot, and C. Sanchez, Inorg. Chim. Acta, 279, 144 (1998). A. Muller, R. Rohlfing, E. Krickemeyer, and H. Bogge, Angew. Chem. Int. Ed., 32, 909 (1993). G. Kastner and H. Reuter, J. Organomet. Chem., 598, 381 (2000). M. Izaaryene, G. Kastner and H. Reuter, Z. Kristallogr., 220, 622 (2005). F. Ribot, C. Sanchez, R. Willem, J.C. Martins and M. Biesemans, Inorg. Chem., 37, 911 (1998). F. Ribot and E. Dien, unpulished results. F. Ribot, C. Eychenne–Baron, F. Fayon, D. Massiot and B. Bresson, Main Group Met. Chem., 25, 115 (2002). Anisotropy (ζ ) and asymmetry (η) are defined as follows: ζ = σ33 –σiso and η = (σ22 –σ11 )/(σ33 –σiso ), with σiso = (σ11 + σ22 + σ33 )/3. σ11 , σ22 , and σ33 are the three components of the shielding tensor expressed in its principal axis system with the convention, |σ33 –σiso | > |σ11 –σiso | > |σ22 –σiso |. R.K. Harris, S.E. Lawrence, S.W. Oh, V.G.K. Das, J. Molecular Struct., 347, 309 (1995). H. Reuter, Angew. Chem. Int. Ed., 30, 1482 (1991). P.J. Domaille and R.L. Harlow, J. Am. Chem. Soc., 108, 2108 (1986). K. Molloy, in Chemistry of Tin (2nd edn), P.J. Smith (Ed), Blackie Academic & Professional, London, 1998, pp. 138–175. M. Biesemans, R. Willem, S. Damoun, P. Geerlings, E.R.T. Tiekink, P. Jaumier, M. Lahcini and B. Jousseaume, Organometallics, 17, 90 (1998). M. Biesemans, R. Willem, S. Damoun, P. Geerlings, M. Lahcini, P. Jaumier, and B. Jousseaume, Organometallics, 15, 2237 (1996). F. Ribot, C. Eychenne-Baron, and C. Sanchez, Phosphorus Sulfur Silicon Relat. Elem., 150–151, 41 (1999). C. Eychenne-Baron, F. Ribot and C. Sanchez, J. Organomet. Chem., 567, 137 (1998). F. Ribot, V. Escax, J.C. Martins, M. Biesemans, L. Ghys, I. Verbruggen, and R. Willem, Chem. Eur. J., 10, 1747 (2004). J.D. Kennedy, J.Chem.Soc. Perkin Trans., 2, 242 (1977). 87. G.L. Zheng, J.F. Ma, Z.M. Su, L.K. Yan, J. Yang, Y.Y. Li, and J.F. Liu, Angew. Chem. Int. Ed., 43, 2409 (2004). V. Chandrasekhar, K. Gopal, P. Sasikumar and R. Thirumoorthi, Coord. Chem. Rev., 249, 1745 (2005).
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Organotin Carboxylate and Sulfonate Clusters
2.4
93
Organotin Carboxylate and Sulfonate Clusters
Vadapalli Chandrasekhar, Puja Singh, and Kandasamy Gopal Department of Chemistry, Indian Institute of Technology, Kanpur, India
2.4.1
Introduction
Organotin carboxylates and sulfonates form an important class among the family of organotin compounds.1−3 These compounds possess rich structural diversity. A range of structures from simple mononuclear compounds to complex polynuclear cages and clusters are formed.4 The structural diversity of these compounds emanates from several features. These include: (1) The ability of tin to have varied coordination geometries and coordination numbers. (2) The versatility of the carboxylate ligand to engage in different modes of binding from monodentate to bidentate; in the case of the latter from chelating to bridging. (3) The varied coordination response of the sulfonate ligand, which varies from non-coordinating to polydentate coordination. The theme of this chapter revolves around clusters of organotin carboxylates and sulfonates. For the sake of completion other structural types are mentioned briefly. 2.4.2
Organotin Carboxylates
Among organotin carboxylates, clusters and cages are formed mainly in di- and monoorganotin compounds.2 Among triorganotin carboxylates, the predominant structures are chain and discrete structures, although some macrocycles are also known.2−3 Triorganotin Carboxylates
Triorganotin carboxylates, R3 SnO2 CR , formed generally in the reactions of R3 SnOH or (R3 Sn)2 O with a carboxylic acid R CO2 H, usually possess two main types of structures:2 (a) chain structures; (b) discrete structures. Polymeric or chain structures are the most common structural types known for triorganotin carboxylates. These are formed in three situations (Figure 2.4.1): (1) When the carboxylate ligand binds in a bridging anisobidentate manner and interconnects adjacent tin centers (Figure 2.4.1(a)); (2) When a dicarboxylate ligand bridges two tin centers (Figure 2.4.1(b)); (3) When a carboxylate ligand containing an additional coordination site in the form of a heteroatom bridges two adjacent tin centers, binding one tin atom by the heteroatom and the other by the carboxylate oxygen atom (Figure 2.4.1(c)). In all of these cases the geometry around tin is trigonal bipyramidal. The axial sites are occupied by the electronegative substituents. The O–Sn–O angles in compounds belonging to the type shown in Figure 2.4.1(a) are usually in the range of 170 to 175◦ . The two Sn–O distances are generally not equivalent; the Sn–O (carboxyl) distance is slightly longer. For example, in Me3 SnO2 CC10 H7 , the shorter ˚ 5 Owing to this bond distance variation, Sn–O distance is 2.14(1) while the longer distance is 2.57(1)A. the tin atom is displaced from the equatorial plane towards the covalently bound oxygen. Discrete structures are found generally for Ar3 SnO2 CR (Ar = Aryl). Many examples of triphenyltin carboxylates, with some exceptions, are known to adopt these structures. In these compounds the tin is
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R CO2H
Sn
O
1:2
R O
(a) R R HO2C R3SnOSnR3
CO2H
O
R
R
O
R n
R
Sn
O
R
O
1:1
Sn
O
R
O
Sn
O (b)
R
O n
R
R R Sn
O CO2H
X 1:2
O R
R
R
X O
Sn
(c) R
Figure 2.4.1
X n
Formation of chain structures in triorganotin carboxylates
bound covalently to three carbon atoms and one oxygen atom, see Figure 2.4.2. A fifth weak intramolecular Sn–O interaction is often found with the participation of the carbonyl oxygen atom. The Sn–O covalent ˚ 2 The bond in these compounds is typical of a single bond distance and varies from 2.03 to 2.12 A. 2 ˚ Sn–O coordinate bond, on the other hand varies, from 2.45 to 3.11A. In general, the preference of the carboxylate ligand to adopt a bridging coordination mode seems to drive the formation of the coordination polymer. In situations where this cannot be accomplished, discrete structures are realized.14 Other types of structures found for triorganotin carboxylates include dimeric structures, as seen for [(Ph3 SnO2 C-C6 H4 -2-Cl)2 .H2 O]15 and Ph3 Sn(O2 C-C6 H4 -2-S)2 SnPh3 .16 In the former, a water molecule blocks the fifth coordination site at tin and prevents polymer propagation. In the latter, one of the tin atoms is bound by the carboxylate ligand, while the other is bound by a thiolate ligand. Macrocyclic structures are known for some triorganotin carboxylates. Thus, n-Bu3 SnO2 C-C6 H3 -2,6-F2 is a tetranuclear macrocycle,17 while Ph3 Sn(NPG) (NPG = N-phthalyl glycinate) is a hexanuclear macrocycle.13 Representative examples of the various structural types known for triorganotin carboxylates are summarized in Figure 2.4.2. Diorganotin carboxylates
The simplest of the diorganotin carboxylates are the dicarboxylates of the type R2 Sn(O2 CR )2 .2 These compounds are formed in a 1:2 reaction between an organotin substrate such as [R2 SnO]n and a carboxylic acid R CO2 H. In general, these have monomeric structures. The tin is hexacoordinate,
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Organotin Carboxylate and Sulfonate Clusters Ph Ph Ph
O
Sn
O2N
N
O
Sn
O
O N
(b)
Ph Ph
O
N
O
Sn
O
O
Me
O
Sn
O
O
Me
O2N
Ph
HO
n
NO2
n
NO2 O
O
Ph
O
(f)
S
O Ph Ph H2O
n-Bu
O
Ph
O
Sn
O Sn
O2N
O Sn
n-Bu
O
n-Bu n-Bu Sn
O
N Ph
n-Bu n-Bu
O n-Bu
N
O
n-Bu Sn n-Bu
n-Bu O
Sn
Ph
O
Ph NO2
n-Bu n-Bu
O
Ph Ph
O
O
Ph
Ph
Me
N
O
S
N
(d)
Ph
O
Me Sn
(e)
(c)
N
N
Sn
Sn
O
Me Me
O Ph
Me
Me
H2O
(a)
Sn
NO2 Me
S
95
O
Ph
O
O
Ph
Sn
Sn Ph Ph
NO2
N O
O
Ph Ph
O O
O
(g)
O Sn
Sn
O
N O
Ph Ph
Ph Ph O O
Ph
Ph
O
O
O
Ph N
N
Sn Sn
O
O
O
Ph Ph
N
O
O (h)
Figure 2.4.2 Representative examples of various structural forms of triorganotin carboxylates: (a)–(c) represent discrete structures; 6−8 (d) represents a chain structure where the carboxylate ligand bridges two tin centers; 9 (e) represents a chain structure where a heteroatom and a carboxylate ligand bridge two tin centers; 10 (f) represents a dimeric structure; 11 and (g)–(h) represent macrocyclic structures. 12−13
skew-trapezoidal bipyramidal, with a 2C, 4O coordination environment in which the tin-bound organic groups lie over the weaker Sn←O bonds. Each carboxylate ligand binds to the tin in a chelating anisobidentate mode. Other diorganotin carboxylates are of the type [(R2 SnO2 CR )2 O]2 .2 These are formed in 1:1 reactions of [R2 SnO]n and the carboxylic acid. The solid-state structures of these compounds reveal that they are dimeric and possess ladder structures (Figure 2.4.3). This topic has been dealt with in detail in many comprehensive review articles.1−4,18−24 The reader may refer to these articles for further details. Ladder structures have been classified depending on subtle variation of the coordination of the carboxylate ligands. These are depicted in Figure 2.4.3. The common structural motif of all the ladder structures is a central four-membered Sn2 O2 ring. Some recent representative examples of [(R2 SnO2 CR )2 O]2 are summarized in Table 2.4.1.25−63 Hydroxyl-bridged dimeric structures are readily formed in the reactions of [t-Bu2 SnO]3 with carboxylic acids.64−65 Representative examples are shown in Figure 2.4.4. Among these compounds, [t-Bu2 Sn(μ-OH)(O2 CC5 H4 -Fe-C5 H5 )]2 (Figure 2.4.4(b)) is an electroactive compound54 while [t-Bu2 Sn(μ-OH)(O2 C-Fl)]2 (Fl = fluorenyl) (Figure 2.4.4(c)) is a photoactive compound.56 Some trinuclear clusters are also known among diorganotin carboxylates.66−73 These are summarized in Table 2.4.2. It can be seen that the carboxylic acids involved in the formation of these compounds are mono/dicarboxylic acids containing other heteroatoms (Table 2.4.2, entries 1–5, 8 and 9) or dicarboxylic acids without other heteroatoms (Table 2.4.2, entries 6 and 7). Among the cyclic compounds,
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O R
R
O
R
O
R
O
O O
O Sn
O
Sn
R
O
O
R
R R
R
O
O O
Sn
R
R
R
O Sn
Sn
O
R
O
O
O
R Sn
R
R
O Sn
O
Sn
Sn
Sn O
R
O O
O
O
O
O
R
Sn R R
O
R
R
R
O
O
R
Sn
R
L4
O
O
O
R
O
R
O
O
O
R L3
O
R
L2
R
Sn
Sn
O Sn
R
R
L1
O
O Sn
O
R
O
O O
R
O R
O
Sn
O
R
Sn
Sn
O
Sn
R
O
O
O
R
O
R
O
R
R
R
Sn
Sn
O
R
O
R
Sn
Sn
O
R
O
R
R
O
L5
L6
R
n
O
O Sn R
R
N
O
O O
R
Sn
Sn
R
R
O O
R Sn O
R
L7
N
N
N
Sn
HO O Sn
Sn N
O
Sn
Sn
R R R
O O
Sn
R
O O
R L8 R
OH
R
O
O
O R
R
Sn
O
R
R
O
R
O
Sn
O
R
O
N
Sn R
R
O
O
O
R
Sn O
O
R
R
O
R
O L9
N N
Figure 2.4.3 Various types of ladder structures ( L1 –L9 ) found in [(R2 SnO2 CR’)2 O]2 (see Table 2.4.1 for specific examples)
[{(n-Bu2 )2 Sn(2,5-PDC)}3 ] (2,5-PDC = 2,5-pyridinedicarboxylate, D = Lewis base, such as DMSO, H2 O)68 and [{(n-Bu2 )2 Sn(1,3-BDC)(D)}3 ] (1,3-BDC = 1,3-benzenedicarboxylate)70 (Table 2.4.2, entries 4 and 6) deserve special mention. Both of these are macrocyclic compounds formed as a result of intermolecular bridging by the dicarboxylate ligands. Among these, [{(n-Bu2 )2 Sn(2,5-PDC)}3 ] forms a three-dimensional hybrid supramolecular network which traps hydrophobic molecules in its microporous channels.68 Recently, there have been reports on other macrocyclic compounds involving diorganotin carboxylates.74−78 Some examples are shown in Figure 2.4.5. A 48-membered macrocycle containing 18
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Organotin Carboxylate and Sulfonate Clusters Table 2.4.1 L9 ]a,b Entry
R
Representative examples of diorganotin carboxylates, [(R2 SnO2 CR’)2 O]2 [Ladder types L1 to R (carboxylate ligand)
Ref.
Entry
R (carboxylate ligand)
R
Ladder L1 1.
97
Me
25
4.
S
n-C8 H17
N S
N
O
Ref.
28
S
S
N
2.
n-Bu
26
N
5.
S
n-Bu
Me2N
N 3.
Me
27
OH
6.
29
S
n-Bu
30
Fe
N
O Ladder L2
7.
n-Bu
31
OH
11.
n-Bu
N
N
O 8.
n-Bu
35
O
S
32
12.
n-Bu
33
13.
n-Bu, n-Pr
N
O PPh2
36
S
9.
n-Bu
O
10.
n-Bu
O2N
30
Fe
Ph
34
14.
n-Bu
37
Cl
Cl
O (Continued )
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Table 2.4.1 Entry
20:38
(Continued) R
R (carboxylate ligand)
n-Bu, n-Pr
Ref.
Entry
R
38
20.
n-Bu
Ref.
SH
42
N
Fe
16.
R (carboxylate ligand)
S
H3C
n-Bu
Ph
39
21.
n-Bu
40
22.
n-Bu
O2N
NO2
43
MeO 17.
n-Bu
Me
29
S Et2N
HN
S
Me 18.
n-Bu
41
N N
23.
n-Bu
44
N N
19.
n-Bu
41
Me N N
Me
Me N N Me
24.
n-Bu
45
S
N
Ladder L3 25.
n-Bu
46
O Ph
S Ladder L4 26.
n-Bu
26
28.
O
CH3
O
Me, n-Bu
N
48
Cl OMe
27.
n-Bu
47
O
N S
29.
PhCH2
49
O S
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Organotin Carboxylate and Sulfonate Clusters Table 2.4.1 Entry 30.
99
(Continued)
R
R (carboxylate ligand)
n-Bu
OH CH3 H3C
Ref.
Entry
R
50
35.
n-Bu
R (carboxylate ligand)
Ref. 53
OH
N
N 31.
n-Bu
OH CH3
50
36.
n-Bu
N
32.
Me
54
Fe
51
37.
n-Bu
55 O
33.
n-Bu
Cl
52
38.
n-Bu
56
26
39.
n-Bu
56
Cl 34.
n-Bu
N S N
Ladder L5 40.
Me
57
H3C
Ladder L6 41.
n-Bu
58
Fe Ladder L7 42.
Me
59
Me
HN
44.
Ph
Cl
Cl
61
Cl
Me 43.
Me
Me
Me
60
Me (Continued )
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Table 2.4.1 Entry
July 4, 2008
(Continued) R (carboxylate ligand)
R
Ref.
Entry
R (carboxylate ligand)
R
Ref.
Ladder L8 45.
n-C8 H17
62
N N
Ladder L9 46.
a b
PhCH2
63
These examples are derived from literature published after 2003. For older examples see references 1–4 and 18–24. See Figure 2.4.3 for structural depiction of ladder structures L1 –L9 .
tin atoms (Figure 2.4.5(a)) has been assembled in a reaction between 2-mercaptonicotinic acid and di-nbutyltin dichloride.74 The macrocycle contains an outer rim of four hexa-coordinate tin centers which are linked to a pair of central hydrolyzed hexa-tin ladders. The concerted coordination action of the multiple coordinating sites (N,S,O) assist in the assembly of the macrocycle. Other macrocycles involving hydrolyzed organostannoxane motifs include the bridging of two penta-tin organostannoxane ladders by a pair of ferrocene dicarboxylate ligands (Figure 2.4.5(c))63 or bridging a pair of tetra-tin organostannoxane ladders by 2-mercapto-4-methyl-5-thiazole acetate ligand (Figure 2.4.5(e)).77 Figure 2.4.5(b) shows an interesting macrocycle containing a central distannoxane linked to the periphery of a tetra-tin containing macrocycle.75 In this compound, the multiple coordination action of m-mercaptobenzoate influences the formation of the multinuclear cage. Macrocycles of the type shown in Figure 2.4.5(d) containing six tin centers have been realized in some instances.75−76 The use of o-hydroxy benzoates as ligands has also been productive in the formation of hexa-tin-containing macrocyclic rings (Figure 2.4.5(f)).78
t-Bu O
Sn
Me C O t-Bu t-Bu O C O
H O
H O
Sn t-Bu
O
Fe
t-Bu
O
H O
Sn
C O t-Bu
t-Bu
O H (b)
Ph
O O F3C
t-Bu
Figure 2.4.4
t-Bu
O C O
Sn
C Sn
O H (c)
O C Me
Sn O H (a)
t-Bu
t-Bu
CF3 O Sn C CF3 O Ph
Fe
t-Bu
H O
Ph Sn
O H (d)
O F3C C O F3C
Ph
Hydroxyl-bridged dinuclear derivatives.63,53,55,64
CF3
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Organotin Carboxylate and Sulfonate Clusters Table 2.4.2
Entry 1.
101
Representative examples of trinuclear diorganostannoxane macrocycles
R and X
R (carboxylate ligand)
Me and Cl
Ref. Entry 66
N
R (carboxylate ligand)
R and X
6. n-Bu and no X
70 O
O
N O
Ref.
Sn
Sn
O O
O
Ph
Ph
Sn
Sn 2.
n-Bu and Cl
N
66
7. n-Bu and no X
O
N
Ph P
O
Sn
71
Ph
P
O
O O O
Sn
Sn
3.
n-Bu and Cl
67
O N
8. Me and no X
72 O
O
O
Sn
Sn
N O
O
Sn 4.
n-Bu and H2 O Sn
O
Sn
O
O N
O
68
O
9. n-Bu and no X
N O
Ph and no X
69
N
O
Sn
O
S
Sn
73
Me O
O
Sn
Sn 5.
Sn
Me Sn
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Tin Chemistry: Fundamentals, Frontiers and Applications R O
Sn
R
R R
S
R R
R R
R
R R
Sn
R
O
Sn
Sn
R
O
O
Sn
S
O
Sn
S
R
R
N
Sn
HO
R
Sn
R
Sn
O
O
O
O
R
R
R
R
Sn
OH
R
R
O
O
R
O
Sn
Sn
Sn
R
O
R
R Sn
R
R
O Sn R
R
R
R
R
Sn
Sn O
R
O
O R
Sn
O Sn
O
Sn
O
Sn
O O
R = Me
R Sn
R
R HO
O
O Sn
O
R R (b)
R
O O
R
O
S O
O R
R R
O O
OH
R
O
R = n-Bu
O R
R
R O
R = CH2Ph ;
R Sn
R
O
R = n-Bu
R (a)
Sn O
O
O
S R
Sn
R
O
N
N
O R
S
R
O
R
Sn
R Sn
O O
S R
Sn
O S
Sn
N
S R
O
Sn
O
O R R
Sn
O
R
Sn
R
O
Sn
R R
R O
O
R
Sn O
O
R
O
O
Sn
O
Sn
R
N
Sn
O R R
O
R
O
R
R
O
S
O
Sn S
R
Sn
S
O
N
N
N
R R
=
Fe
(c-1)
O O
(c-2)
S ;
O S
=
O O (d-2)
(d-1) i-Pr
N S
O
R R O
R R
R Sn R MeO
N
Sn MeO
Sn O
O
Sn O
R
R
Sn
Sn
R
O R R O
S
S
O
S
R = Me
i-Pr
i-Pr
Sn
O
R
R
i-Pr
O
R Sn
O
R = n-Bu
Sn R
N
O
O
Sn
i-Pr
R
O
O
O
O
(e)
Figure 2.4.5
R
O
R
i-Pr
S
N
R
O
R
R
O O
O
Sn
Sn
O
OMe
O
R
Sn R R
Sn
O
O O
OMe
O
i-Pr
R
i-Pr
R R
O
S
S S
i-Pr
O
i-Pr
R i-Pr
(f)
i-Pr
Various multi-nuclear diorganotin macrocycles
R
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Organotin Carboxylate and Sulfonate Clusters R
R O
O
O
O
Sn O HO2C
R
CO2H
103
Sn O
O
(a)
R
O n
or
2:1
R O
O
Sn
O
R
[R2SnO]n
O
R Sn
O
O
(b) R
R
CO2H Sn
O
X
O Sn
X 1:2
n
O
X
O
Sn
R (c)
Figure 2.4.6
Formation of one-dimensional polymeric diorganotin carboxylates
In addition to macrocycles, one- and two-dimensional polymers are also known among diorganotin carboxylates (Figures 2.4.6 and 2.4.7). Several examples of two-dimensional polymers are known among diorganotin carboxylates (Figure 2.4.7(a–e)).3,79 The reaction of pyrazole-3,5-dicarboxylic acid with dibenzyltin dichloride affords a two-dimensional polymer (Figure 2.4.7(d)).79 The repeat unit of the twodimensional polymer contains two symmetrically related tri-tin halves. Hydrolysis of the two-dimensional polymer affords a polymeric tape containing, alternately, the hexa-tin macrocycle and a tetra-tin ladder (Figure 2.4.7(e)). Monoorganotin Carboxylates
The reaction of monoorganotin oxide-hydroxide [RSn(O)OH]n , with a carboxylic acid R CO2 H in a 1:1 stoichiometry affords hexanuclear compounds, [RSn(O)O2 CR ]6 .2,4 This reaction is very general and a number of such hexanuclear compounds, also known as drums, have been prepared (Figure 2.4.8, Table 2.4.3).56,61,80−96 In some cases drums have been formed as a result of alkyl or aryl cleavage preceding hydrolysis of R2 Sn(O2 CR )89,93 or during 1:2 condensation reaction of (R3 Sn)2 O with car2 boxylic acids.86−87 All of the drums have a central Sn6 O6 cage, which is formed by the fusion of two Sn3 O3 rings. Alternate tins are bridged by bidentate carboxylate ligands. The sides of the drum consist of six four-membered Sn2 O2 rings. The Sn–O bond distances found in these compounds, as ˚ 80 Many of the exemplified for [n-BuSn(O)O2 CFc]6 (Fc = ferrocenyl), range from 2.06 to 2.14 A. drum compounds also exhibit a rich supramolecular chemistry as a result of several non-covalent interactions.92 The generality of the drum synthesis has been utilized to prepare stannoxane-cored dendrimer-like molecules containing an interesting electroactive, photoactive, or coordinating periphery (Table 2.4.3,
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Tin Chemistry: Fundamentals, Frontiers and Applications Sn O
O
Sn
O O2N
O
Sn
O O O O2N
Sn
Sn
O
O
NO2 O
O O
Sn
O O
Sn
O
O
O
Sn
O
Bn Sn
O N
O N
N N
N Sn Bn
O
O
O
R Sn
O
Bn
N
R O
Sn
RR R
O
R n
(d)
Bn Sn
Bn n
(c) O
Bn = -CH2Ph O
Bn Bn Sn
Sn
O O
O N
N N
Bn
Bn Sn
Bn
N
O O
HO
N O
Sn Bn
Bn
O N N O
Bn (e)
Figure 2.4.7
Sn
Sn
Bn O
Sn
N
N
O
OH
Bn N
Bn O
O
O
O Bn Bn Bn O
Bn
Bn
Sn
O
HO
Sn Bn
Bn
Sn
OH
O
Sn Bn
Sn
O
O
N N H
N
Sn
Sn
O
O
Bn Sn
Sn
Bn
H N N
R R
O
HO
O
Cl
Sn
O
O
O
Bn O
Sn
Sn
O Bn
Bn Bn
N
O
R
R Bn
O
OH Sn
n
Cl
O
O
Sn
O
(b)
O
Bn Bn
N N
O
Sn
O
O
Sn
Sn
Bn
Bn
O
Sn
(a)
O
Sn
Sn O
O Sn
O
O
O
O
n
O
Sn
Sn
NO2 O
O
Sn
O
O
O
Sn
O
Sn
Sn
Sn
O
O Bn n
One- and two-dimensional polymers formed among diorganotin carboxylates3,79
O
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Organotin Carboxylate and Sulfonate Clusters
R
R C
COOH
O
O O
O
O
R
C
O O
R = n-Bu
R
R
C O
O Sn
R Sn O
O O
Sn
O O
O O
R
C R
C R
Figure 2.4.8
Sn
Sn
-6 H2O
R
O
R
Sn
6 [RSn(O)OH]n + 6
C
O
R
105
Drum
Formation of hexanuclear organostannoxane drums
entries 1–2, 9, 12–14 and 27–30).56,80−81,85 More recently, multi-porphyrin- and multi-buckminister fullerene-containing organostannoxanes have been prepared by utilizing the drum synthesis (Figure 2.4.9; Table 2.4.3).95−96 The metallated derivative of the former has been used as a catalyst for plasmid cleavage.95 Although drum compounds are the must ubiquitous forms found in monoorganotin carboxylates, hexanuclear open-chain ladder compounds (Figure 2.4.10(a) and (b))2 and trinuclear compounds (Figure 2.4.10(c))97 are also found in some instances. The hexanuclear ladders can be converted to drums by a controlled hydrolysis.2 In an interesting recent development the reaction of [n-BuSn(O)(OH)]n with 9-hydroxy-9fluorenecarboxylic acid (LH2 ) afforded a new hexanuclear cage [n-BuSn(μ-OH)(L)]6 .3H2 O.2CHCl3 ] (Figure 2.4.11).98 This cage consists of three [Sn2 (μ-OH)2 ] units which are bridged together by the tridentate carboxylate ligand, L. The six tin atoms of this cage occupy the vertices of a trigonal prism. Another interesting aspect of these cages is that they form guest-assisted three-dimensional columnar supramolecular structures when the guests are phenols. 2.4.3
Organotin Sulfonates
The sulfonate ligand is weakly coordinating in comparison to the strongly coordinating carboxylate ligand. Therefore, is not surprising that the chemistry of the organotin sulfonates is much less developed than that of the corresponding organotin carboxylates. In many instances the reaction of an organotin oxide with a sulfonic acid results in the formation of the hydrated organotin cations (Figure 2.4.12).99 Such hydrated organotin cations are quite interesting in their own right in terms of their structural and catalytic chemistry. However, these are not covered in this section nor are the monomeric organotin sulfonates with the focus being on organotin sulfonates, where at least two tin atoms are present.100−114
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Table 2.4.3 Entry
20:38
R
Representative examples of hexanuclear monoorganostannoxane drumsa R’ (carboxylate ligand)
n-Bu
Ref.
Entry
80
11.
n-Bu
83
81
12.
n-Bu
56
82
13.
n-Bu
81
14.
n-Bu
R
R’ (carboxylate ligand)
Ref.
Fe 2.
n-Bu
Fe
3.
n-Bu
4.
n-Bu
Me Me
Me
5.
Me
6.
n-Bu
7.
n-Bu
8.
n-Bu
9.
n-Bu
10.
n-Bu
85
N N Me
Me
N N
Me
n-Bu
N N
85
N N
Me
Me
82
15.
PhCH2
83
16.
PhCH2
87
82
17.
Ph
88
82
18.
Ph
56
19.
Me3 SiCH2
N
86
Me
H2N
NH2
84
20.
Ph
Cl Cl
Cl
H3C H N
61
89
90
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Organotin Carboxylate and Sulfonate Clusters Table 2.4.3 Entry 21.
107
(Continued)
R
R’ (carboxylate ligand)
PhCH2
N
Ref.
Entry
86
26.
PhCH2
91
27.
n-Bu
TTP-H2b
95
R
R’ (carboxylate ligand)
NH
Ref. 63
O 22.
PhCH2
O 23.
Me
H3C
92
28.
n-Bu
TTP-Mb
95
24.
i-Pr
H3C
93
29.
n-Bu
Ful-1b
96
25.
PhCH2
94
30.
n-Bu
Ful-2b
96
a b
NH
See Figure 2.4.8 for structural depiction of drum. See Figure 2.4.9 for structural depiction of the carboxylate ligand.
R'' N
TTP-H2
R''
TTP-M N
HN
NH
R''
R''
O
O
N
N R''
R'' =
(a)
O
N M = Cu (b-1) Pd (b-2)
R''
Ful-2 R'' O
O
O
O
O
O R'' = C16H33
(c)
Me
Ful-1 R''
O
N M
R''
R''
O
O
O
O
O
R'' O
R'' O
O O
O O
O
(d)
Figure 2.4.9 Schematic representation of the carboxylate ligands in the drum compounds containing the porphyrin or buckminister fullerene periphery
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Figure 2.4.10
Monoorganotin carboxylates with open structures R
R OH
O
OH
O
R
6 [RSn(O)OH]n
Sn
OH
OH
HO CO2H
Sn
O
O
O
Sn
OH
O O O O
6
O R
O O +
O
Sn
Sn
O OO O Sn
O
OH R = n-Bu R
Figure 2.4.11
R
Formation of hexanucular organotin trigonal prism n-Bu H2O
[n-Bu2SnO]n +
H2O SO3H
Figure 2.4.12
2+ OH2
Sn OH2
·2 SO3−
Formation of hydrated organotin cation
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Organotin Carboxylate and Sulfonate Clusters
109
Tri- and Di-Organotin Sulfonates
Polymeric or chain structures are formed among some triorganotin sulfonates (Figure 2.4.13).84,100−102 In these compounds, sulfonate ligands act as bridging bidentate ligands, similar to a carboxylate ligand, and generate a coordination polymer. A number of diorganotin sulfonates containing the four-membered distannoxane [Sn2 (μ-OH)2 ] motif are known.24 These are monomeric (Figure 2.4.14(a–e)), oligomeric (Figure 2.4.14(f)) or polymeric (Figure 2.4.14(g–i)). The sulfonate ligand in these compounds is either coordinating (Figure 2.4.14(a–c) and 2.4.14(g–i)) or non-coordinating (Figure 2.4.14(d–f)). Compounds such as {[n-Bu2 Sn(μ-OH)(O3 SC6 H3 -2,5-Me2 )]2 }n are two-dimensional polymers.103 This compound contains a [Sn2 (μ-OH)2 ] unit as its repeat unit. Such four-membered distannoxanes are linked to each other by an anisobidentate coordination action of the sulfonate ligand to afford a 20-membered macrocyclic ring. The macrocyclic rings, in turn, are linked to each other to afford a coordination polymer (Figure 2.4.15). Diorganotin sulfonates, containing motifs other than [Sn2 (μ-OH)2 ] are summarized in Figure 2.4.16. These include oligomeric and polymeric compounds.101,104−109 In all of these cases the sulfonate ligand is involved in coordination to the tin atoms. In an interesting example, the reaction of 1,5-napthalenedisulfonic acid (LH2 ) with (n-Bu3 Sn)2 O leads to a Sn–C bond cleavage to afford [n-Bu2 Sn(H2 O)3 (L)Sn(H2 O)3 n-Bu2 ]2+ [L]2− (Figure 2.4.17).102 In this compound one of the disulfonate ligands is involved in bridging two hydrated tin centers. The other disulfonate is present as the counter anion. The crystal structure of this compound shows that it possesses a three-dimensional pillared structure, formed as a result of intricate intramolecular O–H—O bonds.
Ph O
Sn
S
O
O
Ph
CF3
Ph
Sn Ph
O Ph
Sn
Ph
n
O H
CH3 = H3C
O
; (a-1)
Sn
O
Ph
Ph (b)
Sn
Ph
Ph O
Sn
O
O
Sn
S Ph
OH2
Ph
Ph
O
S O
O
Sn
n
(c)
Figure 2.4.13
O
O
(a-2)
O
S
Polymeric triorganotin sulfonates106−109
n
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n-Bu H2O
O
O H (a)
O S n-Bu O2
n-Bu
P
Sn
Sn O H (e)
Ph Et
NMe2
n-Bu
Et .2 CF3SO3-
O
4+ OH2
O H
H2O
SiMe3 SiMe3
H O Sn
Sn
OH2
O H
Me3Si
(f)
n-Bu O
H O
n
.4n CF3SO3−
O S
O
Sn
R
Sn O H
O n-Bu
S
CH2
Ph
Sn R
t-Bu .2 CF3SO3-
Sn
Sn
2+ OH2
Sn O H (d)
H O
H2O
Sn O
Sn
Me3Si
OH2
N O
t-Bu
H O
t-Bu
Ph
H O
H2O
t-Bu
CF3
2+
Et
Et
O
n-Bu
O .2 n-Bu4N+
NMe2
Me2N
Ph
O
H2O O
Me
O
O H (b)
n-Bu
Sn
n-Bu
S O2
O2 S
O 2− S
Sn
O S
O
O H (c)
O
n-Bu O
H O
O
Me
n-Bu
H O
Sn F3C
OH2
O
P
Me2N
O
CF3
n-Bu Sn
n-Bu
NMe2 n-Bu O
Me2N
O N
Sn
Sn F3C
O2 S
n-Bu
H O
Sn n-Bu
(g)
O
O n R = 2,4,6-(Me)3C6H2-; 2,5-(Me)2C6H3-
Sn
n-Bu
O
H O
Sn O
Me
n-Bu
S O Sn
O
n-Bu O O
O S
Me
Sn O H (h)
n-Bu
O n
Sn n-Bu
H O
H2O Sn
O H
n-Bu
S
O
O
O
n-Bu
n-Bu H O
Sn F 3C
Figure 2.4.14
O S
CF3
Sn
O
F 3C
n-Bu O
O O H S n-Bu O2 (i)
OH2 O2 S
n-Bu O
CF3
Sn n-Bu
n O
Diorganotin sulfonates containing four-membered [Sn2 ( μ-OH)2 ] units24
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Organotin Carboxylate and Sulfonate Clusters O
n-Bu n-Bu
n-Bu
O
H O
S Sn
Sn n-Bu
O
O H
O
OH
n-Bu
Sn
O
n-Bu
n-Bu O n
Two-dimensional coordination polymer of {[n-Bu2 Sn(μ-OH)(O3 SC6 H3 -2,5-Me2 )]2 }n Ph R
O O O S O
O O
O Sn
O
R
O
F3C
S
Sn
S
O
Sn
Sn O H (b)
Ph
O
Sn
O
CF3
O
O
Sn
O
S
F3C
Ph Ph
O O
R H2 C F
(a-1)
(c)
(a-2)
Sn
n
Ph
n-Bu Me
n-Bu
O
O
Sn
Sn
O Sn
n-Bu
Sn O
O
S
O n-Bu
O Sn
Ph
S O
F3C
Ph O O Sn
N n-Bu
Ph
S O
O Sn Ph
(f)
Me
Figure 2.4.16
Ph P O Sn Ph
Ph O n
(g)
Examples of di- and triorganotin sulfonates
S
CF3 O
n-Bu Sn
O H (e)
CF3
O
O
Sn
n-Bu
O
O
Sn
O
Ph
O
O
n-Bu O
(d)
Sn O H
n-Bu O H O
Sn
O
Ph
n-Bu
Sn O
Ph
O
n-Bu
O
N O
S
Sn
O
O
F3C
Me
n-Bu
O
n-Bu
O
O
Ph S O
n-Bu
S
Sn
O
F3C
Ph
H O
Ph
CH3
Ph
O
Sn
R O
Ph P
Ph Ph
Ph
Sn
P
Ph
Ph
Ph Ph
O
Sn
O
Ph
O
O S
O
Ph
O
S
Ph
R
O
R
n-Bu
Sn
O
Ph
Ph
H O
O
O
S
O
O R
O
Sn
O
O
O O
O
O
O
S
R
Sn
F 3C
Ph
Ph
O
R
R
n-Bu
O H
n-Bu
O
R
Sn
Sn
S
O
Sn
O
H O
O
O
OH
n-Bu
O
n-Bu
n-Bu
Sn HO
Sn
n-Bu
S
O
Figure 2.4.15
O
O
S
n-Bu
n-Bu
HO
O
O
Sn
O
n-Bu
O
O
111
n-Bu n-Bu
n
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n-Bu
H2O
S
O
n-Bu3Sn)2O + SO3H
Sn
H2O
n-Bu
O OH2
SO3–
OH2 ⋅
Sn
O
2+
n-Bu
H2O
O
O
S
OH2
n-Bu
O
SO3–
SO3H
Figure 2.4.17
Formation of dinuclear organotin cation
A similar Sn–C bond cleavage reaction as above has been observed in the reaction of (Ph3 Sn)2 O with triflic acid. The resulting product [(Ph2 Sn)2 (μ3 -O)(μ-OH)(O3 SCF3 )]2 is a tetramer (Figure 2.4.16(d)).106 Monoorganotin Sulfonates
The reaction of [n-BuSn(O)OH]n with aryl sulfonic acids leads to the formation of the dodecanuclear organooxotin cage {[(n-BuSn)12 (μ3 -O)14 (μ-OH)6 ]2+ .2RSO− 3 } (R = 4-MeC6 H4 -; 2,5-Me2 C6 H3 -; CH2 =CHC(O)-NH-C(Me)2 CH2 -) (Figure 2.4.18).110−112 Such dodecanuclear organooxotin cages with other counter anions are also known. The dodecanuclear organooxotin cage has a ‘football’ shaped geometry and comprises of two tri-tin poles and a hexa-tin equator. The three tin atoms present in the poles are linked to each other by a μ3 -O. Four μ-OH ligands bridge alternate tin atoms in the tri-tin subunit. This overall coordination leads to a coordination number of six (1C, 5O) for the tin atoms located at the poles. In contrast, the six tin atoms situated in the equator are five-coordinate (1C, 4O). Further, the equatorial belt of the football comprises of Sn2 O2 distannoxane rings. The utility of the football cages in catalyzing esterification reactions111 as well as in forming a nano building block for tin-based inorganic–organic hybrid materials has been investigated.113
R R
3 [RSn(O)OH]n + R' SO3H
Toluene Reflux
Sn
O
R
HO
Sn
R = n-Bu R
Sn Sn R
O O
O
O O O
Sn R
Sn
O
Sn
O
R
O Sn R
O O
HO
R' =
O Sn
HO
R
Sn
R
R O
2+ OH OH
Sn Sn
·2
R' SO3−
OH
R foot-ball cage
NH O
Figure 2.4.18
Formation of dodecanuclear football shaped macrocations
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Organotin Carboxylate and Sulfonate Clusters
113
R Sn
SO3H
H2O
H2O
SO3H
+
R OH
R
Sn
HO
HO
O H
O HO
R
Sn
OH2 R
HO
HO
O
O
Sn HO Sn
(a)
R OH
Sn
R ·4
R
O H O
SO3−
O
OH
O
OH O2S O
Sn
Sn
S
HO
Sn
Sn
R
O
O
Sn
OH
O
O
HO
O S
R
O OH
4+
OH2 O 2 S O Sn
HO
Sn
H O
Sn
HO
HO OH2
H O
SO3−
O S
R=
·
(b)
O O-capped cluster
R
Figure 2.4.19
Formation of cationic monoorganotin sulfonates
Recently the hydrolysis of aryltrialkyltin 2,4,6-i-Pr3 C6 H2 Sn(CCMe)3 in the presence of aryl sulfonic acids has been investigated (Figure 2.4.19).114 A decanuclear tetracationic cage {[(2,4,6i-Pr3 C6 H2 Sn)7 Sn3 (μ-OH)16 (OH)3 (μ3 -O)3 (μ4 -O)(4-MeC6 H4 SO3 )2 ]4+ .[4-MeC6 H4 SO− and 3 ]4 .3H2 O} a trinuclear O-capped cluster {[(2,4,6-i-Pr3 C6 H2 Sn)3 (μ3 -O)(μ-OH)3 (2,5-Me2 C6 H3 SO3 )3 ]+ .[2,5Me2 C6 H3 SO− 3 ].2C6 H5 Me} were isolated in these reactions (Figure 2.4.19). The latter structural type is quite commonly observed, both independently and as part of other larger organooxotin cages.2 2.4.4
Conclusion
Organotin carboxylates and sulfonates have a rich structural chemistry. In many instances complex cluster/cage structures are realized. Often the formation of such cages is the result of a partial hydrolysis, resulting in the in situ generation of hydroxide or oxide ligands, which hold multiple tin centers together through their bridging coordination action. The role of the carboxylate and sulfonate ligands is to provide structural support, in many instances by a bridging coordination mode. Depending on the type of bridging action, molecular cages/clusters or polymers (one- or two-dimensional) are formed. The presence of additional coordination sites in the carboxylate ligand provides an opportunity for enhancing the versatility of the ligand and can result in the formation of new structural types. The use of nanometric organooxotin cages in the formation of new materials is another aspect of contemporary research. Finally the predictable assembly of an organooxotin cage with a definite nuclearity and dimension is allowing this methodology to be adapted for the preparation of stannoxane-cored starburst type molecules. In such compounds the stannoxane core serves as an inert support around which a desired functional periphery is built. Instances of electroactive, photoactive, and coordination peripheries are already known. 54,56,85,95
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Acknowledgments V. C. is thankful to the Department of Science and Technology, New Delhi for funding and for a J. C. Bose Fellowship. P. S. and K. G. thank the Indian Institute of Technology Kanpur and the Council of Scientific and Industrial Research, New Delhi for financial support. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
Davies A. G., Organotin Chemistry, WILEY-VCH, Verlag, GmbH and Co. KGaA, Weinheim, 2004. Chandrasekhar V., Nagendran S., and Baskar V., Coord. Chem. Rev. 2002; 235: 1. Chandrasekhar V., Nagendran S., Gopal K., Sasikumar P., and Moorthi R., Coord. Chem. Rev. 2005; 249: 1745. Chandrasekhar V., Gopal K., and Thilagar P., Acc. Chem. Res. 2007; 40: 420. Begley M. J., Sowerby D. B., Kapoor P., and Kapoor R., Polyhedron 1995; 14: 1937. Yin H.-D. and Wang C-H., Appl. Organometal. Chem. 2004; 18: 411. Teoh S. G., Tan T. S., Yeap G.-Y., and Fun H.-K., Z. Kristallogr., New Cryst. Struct. 1999; 214:161. Yin H. D., Wang Q.-B.Nightmare, and Xue S.-C., J. Organomet. Chem. 2005; 690: 435. Renamy S. V., Bassene S., Diop C. A. K., Sidibe M., Diop L., Mahon M. F., and Molloy K. C., Appl. Organometal. Chem. 2004; 18: 455. Yin H., Wang C., Wang Y., and Ma C.-L., Indian J. Chem., Sect.A 2003; 42: 48. Ng S. W. and Hook J.M., Acta Crystallogr., 1999; C55: 312. Sadiq-ur-Rehman, Ali S., Badshah A., Malik A., Ahmed E., Jin G.-X., and Tiekink E. R. T., Appl. Organometal. Chem. 2004; 18: 401. Ng S. W., Das V. G. K., Pelizzi G., and Vitali F., Heteroatom. Chem. 1990; 1: 433. Willem R., Bouhdid A., Meddour A., Camacho C. C., Mercier F., Gielen M., and Biesemans M., Organometallics 1997; 16: 4377. Holmes R. R., Day R.O., Chandrasekhar V., Volano J. F., and Holmes J. M., Inorg. Chem. 1986; 25: 2490. Ng S. W., Chin K. L., Wei C., Das V. G. K., and Mak T. C. W., J. Organomet. Chem. 1989; 365: 207. Gielen M., Khloufi A. E., Biesemans M., Kayser F., Willem R., Mahieu B., Maes D., Lisgarten J. N., Wynes L., Moreira A., Chattopadhya T. K., and Palmar R. A., Organometallics 1994; 13: 2849. Holmes R, R., Acc. Chem. Res. 1989; 22: 190. Tiekink E. R. T., Appl. Organometal. Chem. 1991; 5: 1. Jain V. K., Coord. Chem. Rev. 1994; 135/136: 809. Gielen M., Biesemans M., Willem R., and Tiekink E. R. T., Eur. J. Inorg. Chem. 2004; 445. Chandrasekhar V. and Gopal K., Appl. Organometal. Chem. 2005; 19: 429. Beckmann J., Appl. Organometal. Chem. 2005; 19: 494. Chandrasekhar V., Singh P., and Gopal K., Appl. Organometal. Chem. 2007; 21: 483. Benetollo F., Lobbia G. G., Mancini M., Pellei M., Santini C., J. Organomet. Chem. 2005; 690: 1994. Ma C.-L., Han Y., and Zhang R., J. Organomet. Chem. 2004; 689: 1675. Tian G.-R., Zhang R.-F., Ma C.-L., and Ng S. W., Acta Crystallogr., 2005; E61: m2528. Yin H.-D., Gao Z.-J., Li G., Xu H.-L., and Hong M., Chin. J. Inorg. Chem. 2006; 22:157. Yin H.-D., Xue S.-C., and Liu G.-F., Acta Chim. Sinica 2004; 62: 603. Peng B., Sun L.-J., Chang W.-X., and Xie Q.-L., Chin. J. Struct. Chem. 2003; 22: 647. Chen M.-S., Kuang D.-Z., Deng Y.-F., Zhang C.-H., and Feng Y.-L., Chin. J. Inorg. Chem. 2006; 22: 367. Yin H., Xue S.-C., and Wang Q.-B., Indian J. Chem., Sect.B. 2005; 44: 1040. Tian L., Yu Q., Zheng X., Shang Z., Liu X., and Qian B., Appl. Organometal. Chem. 2005; 19: 672. Kapoor R., Gupta A., Kapoor P., and Venugopalan P., Appl. Organometal. Chem. 2003; 17: 600. Zhou Y., Jiang T., Ren S., Yu J., and Xia Z., J. Organomet. Chem. 2005; 690: 2186. Phadnis P. P., Dey S., Jain V. K., Nethaji M., and Butcher R. J., Polyhedron 2006; 25: 87. Li F.-H., Yin H.-D., Huang W.-B., and Wang Y.-W., Acta Crystallogr., Sect.E: Struct. Rep. Online 2006; 62: m919.
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Organotin Carboxylate and Sulfonate Clusters 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81.
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82. Chandrasekhar V., Gopal K., Nagendran S., Steiner A., and Zacchini S., Cryst. Growth & Des. 2006; 6: 267. 83. Chandrasekhar V., Schmid C. G., Burton S. D., Holmes J. M., Day R. O., and Holmes R. R., Inorg. Chem. 1987; 26:1050. 84. Chandrasekhar V., Baskar V., Boomishankar R., Gopal K., Zacchini S., Bickley J. F., and Steiner A., Organometallics 2003; 22: 3710. 85. Chandrasekhar V., Thilagar P., and Sasikumar P., J. Organomet. Chem. 2006; 691: 1681. 86. Yin H., Wang C., and Wang Y., Indian J. Chem., Sect.B. 2004; 43: 612. 87. Yin H., Wang C., and Wang Y. ,Indian J. Chem., Sect.B. 2004; 43: 1493. 88. Chandrasekhar V., Day R. O., and Holmes R. R., Inorg. Chem. 1985; 24: 1970. 89. Beckmann J., Dakternieks D., Duthie A., Thompson L., and Tiekink E. R. T., Acta Crystallogr., 2004; E60: m767. 90. Yin H.-D., Wang C.-H., Ma C.-L., and Fang H.-X., Chin. J. Org. Chem. 2003; 23: 291. 91. Yin H.-D., Wang C.-H., Ma C.-L., and Fang H.-X., Chin. J. Chem. 2003; 21: 452. 92. Day R. O., Chandrasekhar V., Swamy K. C. K., Holmes J. M., Burton S. D., and Holmes R. R., Inorg. Chem. 1988; 27: 2887. 93. Kuan F.-S., Dakternieks D., and Tiekink E. R. T., Acta Crystallogr., 2002; E58: m301. 94. Yin H.-D., Wang C.-H., and Ma C.-L., Chin. J. Org. Chem. 2003; 23: 470. 95. Chandrasekhar V., Nagendran S., Azhakar R., Kumar R. M., Srinivasan A., Ray K., Chandrashekar T. K., Madhavaiah C., Verma S., Priyakumar U. D., and Sastry G. N., J. Am. Chem. Soc. 2005; 127: 2410. 96. Hahn U., Gegout A., Duhayon C., Coppel Y., Saquet A., and Nierengarten J.-F. Chem. Commun 2007; 516. 97. Swamy K. C. K, Nagabrahmanandachari S., and Raghuraman K., J. Organomet. Chem. 1999; 587: 132. 98. Chandrasekhar V., Thilagar P., Bickley J. F., and Steiner A., J. Am. Chem. Soc. 2005; 127: 11556. 99. Chandrasekhar V., Boomishankar R., Singh S., Steiner A., and Zacchini S., Organometallics 2002; 21: 4575. 100. Diop C. A. K., Bassene S., Sidibe M., Sarr A. D., Diop L., Molloy K. C., Mahon M. F., and Toscano R. A,. Main Group Met. Chemistry 2002; 25: 683. 101. Beckmann J., Dakternieks D., Duthie A., and Mitchell C., Organometallics 2004; 23: 6150. 102. Chandrasekhar V., Boomishankar R., Singh S., Steiner A., and Bickley J. F., Organometallics 2003; 22: 3342. 103. Chandrasekhar V., Singh P., and Gopal K., Organometallics 2007; 26: 2833. 104. Allen FH, Lerbscher JA, Trotter J. J. Chem. Soc. (A) 1971; 2507. 105. Shankar R., Kumar M., Narula S. P., Chadha R. K., J. Organomet. Chem. 2003; 671: 35. 106. Beckmann J., Dakternieks D., Duthie A., and Mitchell C., Appl. Organometal. Chem. 2004; 18: 51. 107. Orita A., Xiang J., Sakamoto K., and Otera J., J. Organomet. Chem. 2001; 624: 287. 108. Shankar R., Kumar M., Chadha R. K., and Hundal G., Inorg. Chem. 2003; 42: 8585. 109. Beckmann J., Dakternieks D., Duthie1 A., Mitchell C., Ribot F., de la Caillerie J.B.d’E., and Revel B., Appl. Organometal. Chem. 2004; 18: 353. 110. Eychenne-Baron C., Ribot F., Steunou N., Sanchez C., Fayon F., Biesemans M., Martins J. C., and Willem R., Organometallics 2000; 19: 1940. 111. Chandrasekhar V., Boomishankar R., Gopal K., Sasikumar P., Singh S., Steiner A., and Zacchini S., E. J. Inorg. Chem. 2006; 4129. 112. Ribot F., Veautier D., Guilladeau S. J., and Lalot T., J. Mater. Chem. 2005; 15: 3975. 113. Sanchez C., Soler-Illia G. J. D. A. A. , Ribot F. , Grosso D., C. R. Chimie 2003; 6: 1131 114. Prabusankar G., Jousseaume B., Toupance T., and Allouchi H., Dalton Trans. 2007; 3121.
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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds
2.5
117
Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds
Herbert H¨opfl Centro de Investigaciones Qu´ımicas, Universidad Aut´onoma del Estado de Morelos, Cuernavaca, M´exico
2.5.1
Introduction
In 2006, the Cambridge Structural Database revealed 7062 crystal structures containing tin(II) or tin(IV), of which only a very limited number has discrete macrocyclic structures or extended polymeric networks.1 Since the number of publications on such systems has increased exponentially during the last ten years, a brief overview of the progress reached so far in this field is worthwhile. The purpose of the present survey is to examine and visualize the potential that organotin(IV) complexes can have in metal-directed self-assembly of macrocycles and extended networks. Their application in the formation of cage- and cluster-type structures is only mentioned superficially, since this chemistry is described in other sections within this chapter.
2.5.2
Metallosupramolecular Chemistry with Tin
Metal-Directed Self-Assembly
The combination of metal ions and appropriate ligands to form macrocyclic, cage-like, and extended network structures has become a powerful tool for the construction of systems having cavities, pores, or channels, and is currently one of the most important topics in chemical and material sciences.2 Applications are visualized for a wide range of fields, such as ion and molecular recognition, sorption, filtration, storage, transport, catalysis, etc.2b,f,i,k,m The ultimate goal in the design of such functional materials is the generation of methods that allow control over size, shape, and function of the corresponding cavities. For this purpose metal–organic architectures prepared from tailored building blocks are ideal, since a wide range of structural modifications can be carried out. Concerning the metal ions, parameters such as atom size, coordination number, coordination geometry, Lewis acidity, etc., can be varied, either for the same element or by selection of another metal atom, giving a whole range of different molecular and supramolecular structures.2b,f Regarding the organic building blocks, the number, Lewis basicity, spatial separation, and geometric orientation of the metal-coordinating donor atoms can be modified. Furthermore, substituents attached to the organic connectors or additional ligands coordinated to the metal ion (e.g. blockers) can be employed in order to: (i) modulate the hydrophilic/hydrophobic character of the cavity walls; (ii) cover them with certain functional groups; and (iii) provide the cavities with a chiral shape.2b,c,f,m Through judicious choice of metal ions, connectors (spacers) and blockers, a large variety of discrete structures and network topologies can be prepared, i.e.: (i) zero-dimensional triangles, squares, or parallelograms; (ii) one-dimensional chains, helices, or ladders; (iii) two-dimensional sheets, grids, honeycomb-like assemblies, or so-called brick-wall structures; and (iv) three-dimensional cubic or diamandoid networks (Figure 2.5.1). The nuclearity, shape, and size of the rings or cages, which are present in the resulting supramolecular architectures, depend strongly on the donor–metal–donor and donor– connector–donor bond angles, e.g. systems involving 90◦ angles give squares and cubes, while systems with 120◦ angles give hexagons and honeycomb-like arrangements.2a,b,c,e,k
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Figure 2.5.1 Schematic representation of some of the discrete and extended supramolecular architectures that can be prepared by metal-directed self-assembly: (a) 1D polymeric chains; (b) discrete macrocyclic structures; (c) two-dimensional grid- and honeycomb-like networks; (d) three-dimensional diamandoid open frameworks
So far, mainly transition metals have been explored for metal-directed self-assembly and little is known about supramolecular architectures with main group elements,2e,2o,3 which is somewhat surprising considering the impact elements such as aluminium and silicon have in industry. Additionally, several main-group elements that form kinetically and thermally stable organometallic compounds, in particular boron, silicon, sulfur, phosphorus, and tin, are very interesting building blocks for metal-directed selfassembly, because the organic substituents attached to the metal ions can be varied almost indefinitely in size, shape, and functionality. So far, little attention has been paid to the utilization of tin(II) and tin(IV) compounds in this direction, and, therefore, only a limited number of macrocycles or coordination polymers is known. Moreover, many of these compounds have been prepared with another objective or obtained accidentally. Primary and Secondary Tin Building Blocks
As already mentioned, the supramolecular structure of coordination assemblies is ruled by geometric factors, such as the metal coordination environment and, in particular, the number and geometry of coordination sites available for the interaction with organic building blocks.
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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds a)
R
R
R X
Sn X
b)
X
X
X
R
c)
X
X X
X
Sn X
R
Sn
X
X
Sn
X R
X X
X
X
R
Sn R
Sn X
119
X
R
R X
X
X
X
X Sn R
X X
R X
Sn R
X R
Figure 2.5.2 Most common coordination environments for tin macrocycles, cages and coordination polymers: (a) monoorganotin; (b) diorganotin; (c) triorganotin derivatives. R = organic group, X = ligand with functional groups containing C, N, O, S, etc.
For tin(IV), the most frequent coordination geometries are based on tetrahedral, trigonal-bipyramidal, octahedral, and pentagonal-bipyramidal polyhedra.4 In organometallic tin(IV) complexes there is a tendency for the tin atoms to enhance their coordination number either via intra-molecular coordination or inter-molecular association. The final coordination number depends strongly on the number of organic substituents attached to the tin atom and the nature of the donor atoms of the ligands.5 Considering that the formation of metallocyclic polygons and polyhedra requires at least two free coordination sites, the number of possible bonding environments is limited. Figure 2.5.2 summarizes the coordination geometries found most frequently in organotin(IV) macrocycles and coordination polymers. For the construction of discrete macrocycles, cages, and extended assemblies, it is also possible to employ di- or oligo-nuclear metal building blocks, in which two or more metal centers are connected to form so-called secondary building units (SBUs). Examples are stannoxane-derived organotin dimers, clusters and cages (Figure 2.5.3).6 All examples shown in this figure contain at least one Sn2 O2 ring, and are frequently observed in the solid-state structures of organotin carboxylates, as illustrated by the examples shown in the same figure. Diorganotin dicarboxylates frequently associate through the combination of two skewed-trapezoidal bipyramids forming a single Sn2 O2 core (Figure 2.5.3a).7 Systems having a Sn4 O2 core are common for partially hydrolyzed diorganotin carboxylates, in which the central bis(tetraorgano)stannoxane fragment, [XR2 Sn-O-SnR2 Y]2 , generally contains additional bridges between the exo and endo tin atoms. If either X or Y is a single atom, a ladder-type structure having a chair conformation is obtained. The bridging ligands generally are carboxylates, however, systems containing alkoxides, halides, or phosphonates are also known.7 The carboxylate can function either as monodentate or bidentate bridging ligand, and all possible combinations have been described.7 With monoorganotin derivatives, a series of oligotin clusters has been described, of which the drum structure containing a Sn6 O6 core has received most attention.6b,c So far, there are few reports on macrocyclic and supramolecular assemblies using organotin(IV) (see below).
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Tin Chemistry: Fundamentals, Frontiers and Applications a)
b)
O Sn
R Sn
Sn O
Sn Y
O
Sn2O2
Sn
R
Sn
R X
Sn R
O
R Sn
O
R Y
R
X, Y = Hal-, -O2CR'', etc.
R O
O
c)
O
R R O
X
Sn
Sn4O2
Sn
O
Sn
O
O
R
O
Sn
Sn
O Sn O
R O
Sn
O
O Sn
O
Sn Sn
O Sn6O6
Figure 2.5.3 Frequent tin-containing secondary building units (SBUs) that can be used for the generation of macrocyclic or extended supramolecular assemblies: (a) Sn2 O2 dimers; (b) Sn4 O2 X2 Y2 -distannoxanes; (c) Sn6 O6 drums Methodologies for Tin-Directed Self Assembly
Considering the current state-of-the-art for metal-directed self-assembly, and the coordination facilities of tin(IV) species, three general methodologies can be proposed for the construction of supramolecular assemblies containing tin: (1) Assembly of mono-, di- or oligonuclear tin compounds via inorganic connectors; (2) Assembly of mono-, di- or oligonuclear tin compounds via organic connectors; (3) Combination of the afore-mentioned options. Since the synthetic methods employed for the generation of the supramolecular tin(IV) assemblies described herein are very similar to those known for monomeric tin(IV) complexes, in the following, mainly structural aspects in relation to the ring and network composition, as well as the cavities, if existing, are described. 2.5.3
Formation of Tin Macrocycles and Extended Networks
Application of Inorganic Connectors
Functional groups containing electronegative atoms such as fluorine, chlorine, oxygen, sulfur, and nitrogen are frequently capable of bridging tin moieties, and oligo- or polymeric structures are obtained. For R4−n Sn(IV)m+ moieties, the most common bridging functions are X = F− , Cl− , O2− , S2− , OH− , OR− , 3− 2− − − 8 CN− , RSO− 3 , PO4 , RPO3 , R2 PO2 and RCO2 .
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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds O− Xn−
O
E
E R
O− −
X = Hal , Chalc , OH−, OR−, SR−, CN−, N3−, SCN−, etc. n−
E = C, N
2−
+
O−
O
O
R
R
E
O−
−
M
O
−
O
121
O−
O−
O
M = V , CrVI, MoVI, WVI, TcVII,ReVII, etc. V
E = P, As E = S, Se, Te R = alkyl, aryl, OR, O−, NR2, etc.
Figure 2.5.4 Functional groups with bridging capability that might be candidates for the formation of tin macrocycles or extended networks
Of the anionic functional groups outlined in Figure 2.5.4, only some have been applied as bridging ligands for tin atoms; therefore, there might be additional promising candidates for the formation of tin macrocycles or extended networks. In functions with π-electron density, delocalization can occur, and in this case the binding energies between the donor atoms of the ligand and the tin atom will be intermediate between those of the corresponding covalent and coordinative bonds (symmetrical or isobidentate bridging). Some of the bridging ligands can have different configurations, as shown in Figure 2.5.5 for bidentate ligands. Depending on the energy differences between the different configurations, the ligands can be more or less flexible in the variation of the M–ligand–M bond angle, an aspect that is important for the formation of cyclic structures, since it makes it possible to reduce steric strain. When sulfur replaces oxygen, chelating begins to dominate over bridging, as it occurs in complexes with ligands derived from dithiophosphate derivatives9 and dithiocarbamates.10 (1) Oxygen-Containing Bidentate or Oligodentate Connectors. Although monomeric and 1D polymeric R4−n Sn(IV)m+ complexes are known for many of the ligands shown in Figure 2.5.4, so far macrocyclic and extended network structures have been reported only for a limited number, and interestingly, most of them are triorganotin derivatives. This can be explained by the requirement of a subtle balance of electronic and steric effects of the organic groups attached to the tin and ligand moieties,5 and may be, in part, also due to the relatively small number of research groups dedicated to a systematic exploration of supramolecular tin chemistry. Concerning the oxygen-containing bidentate ligands [Rn EO2+x ] y− and MOm− 4 , discrete dimeric, tetrameric and hexameric ring assemblies have been reported for organotin carboxylate,11 phosphinate,12 phosphonate monoester,13 phosphate diester,14 sulfonate,12d,15 and tungstate16 derivatives. In these macrocyclic structures, the composition of the central inorganic ring system is [SnO2 E]n or [SnO2 M]n , respectively, with n = 2 for the dimer, n = 4 for the tetramer, and n = 6 for the hexamer, thus R
R
X
X
X
M
M
M
syn-syn
Figure 2.5.5
R
X
syn-anti
M
M
X
X
M
anti-anti
Possible configurations for bidentate bridging ligands
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b)
R'
R' R
R O
O O R
S
R''
O
O
Sn O
S
R
R
R'
O
C
R'
O R R
C
R
O
R
O
Sn
O
R'
Sn
R
O
O
O
Sn
R''
R
O
O
R
C
R
Sn R
R'
O
Sn
R
O C
R
R' R = Et; R' = Me, Ph; R'' = Me
c)
R''
R = nBu; R' = 4−NO2C6H4, 2,6−C6H3F2
R'
R
R
Sn
P
Sn
P O
O
O
R
R
R
R''
R
O R
R R'
R'' R'
Sn R
O
O P
P O
R
O
R
R Sn
Sn O
R
P
O
O P
Sn O
R
R
R R
R' R''
R'' R'
R
R'
R''
R = Ph; R', R'' = Me, Ph, OMe, OPh
Figure 2.5.6 Examples of: (a) dimeric; (b) tetrameric; (c) hexameric and organotin assemblies bridged by inorganic oxygen-containing connectors
giving rings of 8, 16, and 24 members (for examples, see Figure 2.5.6). Generally, the bridging ligands adopt an approximate syn-anti conformation. In triorganotin derivatives, the tin atoms exhibit trigonalbipyramidal coordination geometries, in which the organic substituents are located in the equatorial plane. NMR spectroscopic, cryoscopic, and related studies have shown that macrocyclic triorganotin tetramers and hexamers undergo transformations fast on the NMR timescale in solution.11b,12c Two-dimensional and three-dimensional structures containing analogous and related macrocyclic ring systems are known for most of the ligand types that form discrete macrocyles.17 Additionally, there are a few examples for two-dimensional and three-dimensional coordination polymers containing sulfate, phosphate, vanadate, chromate, molybdate, and tungstate,18 indicating that these are also possible candidates for the formation of discrete macrocycles.
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In the skeletons of these two-dimensional and three-dimensional networks, the [SnO2 E(M)]n rings are linked either in the vertex-to-vertex or edge-to-edge mode. Besides the ring dimensions already observed for discrete macrocycles (n = 2, 4, 6), in several cases, larger rings are involved (up to n = 10),18e thus confirming that large cyclo-oligomeric systems may exist in solution, as has been proposed by some authors.12c With monoorganotin derivatives, some of the oxygen-containing tri- and tetra-dentate ligands shown in Figure 2.5.4 have been employed for the formation of interesting cages, ladders, and clusters.6 (2) Other Inorganic Connectors. Cyclic, cluster, and polymeric organotin assemblies are also known for other inorganic ligands, mainly F− , Cl− , O2− , S2− , OH− , OR− , CN− and ON(R)NO− , but there are only a few examples for discrete macrocyclic structures.19 Figure 2.5.7 shows some examples that illustrate that there is a strong preference for 12-membered rings, albeit the number of tin and bridging atoms varies. This is also true for the macrocyclic ring composed only of metal-tin bonds, [Ph2 SnOs(CO)4 ]6 .19c Extended networks of organotin(IV) fragments are known only for a small number of these inorganic connectors, the most important examples being [Me2 SnF2 ]n ,20 and the pseudohalide derivatives [Me2 Sn(C2 N3 )]n and [Me3 Sn(N C N)]n .21 The combination of metal cyanide anions with di- and triorganotin moieties gives heterobimetallic coordination polymers of the general formula [(Rm SnIV )x {M(CN)n } y ] that have been explored very systematically. So far, mainly complexes with square-planar (Ni),22 tetrahedral (Cu),23 octahedral (Fe, Co, Ru, Os),24 and square antiprismatic (Mo, W)24i,25 [M(CN)n ]m− coordination environments have been studied in combination with different triorganotin cations (R3 Sn+ with R = Me, Et, Pr, nBu, Ph, etc.). The triorganotin fragments function as linear connectors between the [M(CN)n ]m− nodes, whereby the anions coordinate to the axial positions in the bridging trigonal-bipyramidal tin environment. It has been possible to prepare either neutral or negatively charged open-framework structures with cubic, diamandoid, or related distorted cages, whose cavities or channels have been filled with diverse neutral and cationic species, such as solvent molecules,24b,d,g tetraalkylammonium ions,22a,b,23a,d,24f,h metallocenes,24c and others.24j The composition of the rings present in these architectures is [M(CN)2 Sn]n with 18, 24, 30, 36, etc. members (Figure 2.5.8). Within these coordination polymers the topology of the macrocycles depends on the coordination geometry of the [M(CN)n ]m− node, the organic substituents attached to the tin atoms, or the included guest molecules.24e,g In combination with bipodal nitrogen bases, principally pyridine derivatives, a new generation of mixed-ligand heterobimetallic systems has been prepared, in which the nitrogen bases form either part of the coordination polymer framework (M←N coordination) or participate in the network through hydrogen bonding interactions with partially hydrated Me3 Sn+ fragments.23b,c,e,24k,i This methodology permits modulation of the distance between the metal nodes, and provides, in combination with the varying steric volume of the R3 Sn+ fragments, a powerful tool for the generation of tailored zeolite-like materials (organometallic zeolites).23a,24h Application of Organic Connectors
R4−n Sn(IV)m+ complexes can be connected by an almost infinite number of organic fragments or molecules through the formation of either covalent Sn X or coordinative Sn←D bonds. The most important elements for the formation of these bonds are carbon, nitrogen, oxygen, and sulfur. Although polymeric structures predominate, a considerable number of tin macrocycles and cryptands have been prepared from organic connectors during the last 10 years. The most explored compound classes are systems connected through (CH2 ) Y (CH2 ) , O Y O , S Y S , O2 C Y CO2 (Y = aliphatic or aromatic bridge), and pyridine- or imidazole-containing organic bridges. Of the coordination polymers reported within this group of ligands, only a small number has an extended two- or
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a) S
R Y
OMe
Sn
R
R Sn S
MeO
Sn
Y
OH
R R = CH(SiMe 3)2; Y = −CH2CH2−
Ph
b) N
R
N
O
R Sn
O
Ph
O
R
N Sn
R
R
R
R O
O R
Sn
R
N R
O
O
Sn
O
N
R
t-Bu t-Bu
R
t-Bu Sn t-Bu
O
O Te
O
O
R
O O
Te
N
N Ph
Sn R
OMe
R = CH 2Ph
R
O
OH
R
c)
Y
HO
R
R S
Sn
O
O Sn
Sn
R Sn
OH
HO
S
MeO OMe S
Sn
R Sn
Sn
OMe S
OH
Sn
R
Sn
O R
R
O
R = 4−C6 H4−OMe, 4−C6H4−NMe2
N Ph
R = Me d) Cl R' R'
Sn
Cl
R R
Ga
Cl
Cl R'
Ga
R' R' R'
Ga
R
R' Cl
Ga
R Sn
R'
Cl
R R = Me, (R')2 = 1,8−C10H6
Figure 2.5.7 Figure 2.5.6
Ph
e)
R
Ph
Ph
R
Sn
Ph
Ph Sn
R
Os R Ph Sn R Ph R R R R Os R R R Ph R R Sn R R Os Os Ph Sn R R R Ph Ph Os
R R
R
Os Sn
Ph R
R R
R = CO
Discrete tin rings formed with inorganic bridging connectors different from those shown in
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R M
M C R R
N
R
M
Sn
N
N R
N
C
M C N
R
R
N C
Sn
C
R
R
R C
N
R
R
R
N C
Sn
N
R Sn R
Sn
N
C
C
N
C
125
Sn
R
N R
C
C
M M
R
C
N
Sn R
N
C
M
R
M R
C
R
R
C
N
R
N
Sn N
Sn R
C
N
R
C
M
M
C
C
N
N R
R Sn
R
R
Sn R
R N
N
C
C M
M C
R
N
R
Sn R M = Fe, Co, Ni, Cu, Mo, W, Ru, Os
N Sn
N
R
C
N C
C
R R
M
R = Me, Et, Pr, nBu, Ph, etc.
Figure 2.5.8 Trinuclear, tetranuclear, and hexanuclear macrocyclic rings present in two-dimensional and three-dimensional networks of triorganotin cyanometalates
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b)
S
S
(CH2)n H3C N N P N
N
P
Cl
O
CH3 N
O
N
Sn Cl
Cl
+
N
H
CH3 H O 2− O Sn Cl Cl
Cl
N
+
CH3
n = 2, 3
Figure 2.5.9
Mononuclear tin macrocycles
three-dimensional structure,26,27 most of them being bis(triorganotin) dicarboxylates. Open-framework structures are almost unknown so far,28 and therefore, in the following, emphasis is placed only on systems having macrocyclic structures. There have been reports on four types of macrocyclic organotin(IV) assemblies formed from neutral or anionic bidentate organic connectors: (i) mononuclear R4−n Sn(IV)m+ macrocycles; (ii) di- and oligonuclear R4−n Sn(IV)m+ macrocycles; (iii) dinuclear cryptands; and (iv) macrocycles and cryptands formed from oligonuclear secondary building blocks. (1) Mononuclear R4−n Sn(IV)m+ Macrocycles. There are only a few structural reports on tin macrocycles, in which ring closure has been achieved through the combination of an R4−n Sn(IV)m+ fragment with a single bidentate ligand (Figure 2.5.9).29 The ring size varies from 11 to 20 members and it is interesting to note that the cavities of the derivatives shown in Figure 2.5.9a are chiral, making them of potential interest for asymmetric synthesis.29a,b Mononuclear cyclooligomers have been also proposed for diorganotin poly(ethyleneglycol) derivatives.30 (2) Di- and Oligonuclear R4−n Sn(IV)m+ Macrocycles. As shown in Figure 2.5.10, neutral di- or oligonuclear R4−n Sn(IV)m+ macrocycles are obtained when connectors such as (CH2 )n , O Y O , S Y S , and related bridging groups (Y = aliphatic or aromatic bridge) are combined with two or more diorganotin (R2 Sn2+ ) fragments (Figure 2.5.10).31−34 Interestingly, organometallic macrocycles with aromatic connectors are very rare. In Figure 2.5.10a cyclophane and porphyrin-type derivatives are shown.31b,e Dicarboxylates or carboxylates containing an additional anionic function (e.g. RS− , RO− , etc.) capable of coordination to tin atoms (functionalized monocarboxylates) are ideal candidates for the construction of discrete di- and oligonuclear organotin macrocycles, as is shown by a series of organotin carboxylate macrocycles having dimeric,28,35 trimeric,36 tetrameric,37 and hexameric38 compositions (Figure 2.5.11). Dicarboxylate ligands that do not contain heteroatoms capable of coordination to the tin atoms via chelate ring formation, are coordinating in an anisobidentate manner to diorganotin moieties. The resulting diorganotin carboxylates have either a skewed-trapezoidal bipyramidal coordination environment, or a pentagonal-bipyramidal geometry, if an additional coordination site in the equatorial plane is occupied.28,36c,e Together with the spatial orientation of the coordinating functions, these geometric parameters determine the nuclearity and size of the macrocyclic ring. Both planar and corrugated macrocycles have been reported (Figure 2.5.11a). Macrocycles obtained from functionalized dicarboxylates capable of chelate ring formation are generally smaller than the analogous macrocycles obtained from the unsubstituted dicarboxylate (compare
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b)
H
CH3 Et
Cl
O
Et
Sn
O
Et
Cl
P
O
CH3
O O
P
Et
O
O O
CH3 O
O
Cl CH3
O O
Sn Cl
Et
P
Et
NH
O NMe2
P
+ N
Et
NMe2
Et
127
MeO
Bu
MeO
Bu
Cl
O
− Sn
Cl
O
Bu
OMe
Sn − Bu
OMe
O NH N +
H
Figure 2.5.10 Examples for di- and oligononuclear R4−n Sn(IV) m+ macrocycles with connectors such as: (a) (CH2 )n ; (b) O Y O ; (c) S Y S , and related bridging groups (Y = aliphatic or aromatic bridge)
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c)
Sn S
Ph S P S
tBu
S
S
tBu Sn
Sn tBu
N
S
S
S
S
tBu
P
S N
Ph
S
S Sn R R = Me, tBu
CH3
H3C
S
S
CH3
S
Sn
Sn N
N
S
S
CH3 S
S
N
N
N
N
S
S CH3 CH3
CH3
Sn
Sn
N
N S
S
N
N
CH3
S
S S
S Sn
CH3 CH3
Figure 2.5.10
(Continued)
analogous or related entries in Figures 2.5.11a and 2.5.11b). For example, when comparing the trinuclear macrocycles [nBu2 Sn(isophthalate)]3 , [nBu2 Sn(2,5-pyridinedicarboxylate)]3 , and [Me2 Sn(2,6˚ for the first complex to 8.5 and 5.0 A ˚ pyridinedicarboxylate)]3 , the Sn· · ·Sn distances decrease from 9.0 A for the latter two derivatives. However, it appears that chelate ring formation contributes in a positive manner to the formation of macrocyclic ring structures, since a relatively large number of functionalized carboxylates has given macrocyclic instead of polymeric structures (Figure 2.5.11b).
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The tetranuclear macrocycle shown in Figure 2.5.11c has been obtained by a very interesting and promising synthetic strategy, which consists of the application of a functionalized organotin complex in the self-assembly process.37 In assemblies where the coordination number can be enhanced from six to seven, frequently intermolecular association occurs in the solid state through the formation of O· · ·Sn interactions (Sn2 O2 ring formation) that can generate interesting two- or three-dimensional coordination polymers, including open frameworks.26,28 There are also reports on extended networks formed through extensive hydrogen-bonding interactions between the macrocyclic ring systems, in particular when water molecules are coordinated to the tin atoms.28,36e The trimeric di-n-butyltin macrocycles derived from
a)
Ph O
O R R
O
O
Sn
Ph
Sn
R Ph
O
O
O
O
Ph
O
O
O
O
O
P Ph
O
Sn
O
O
O
O R
R R O
R O
Sn
Ph
O Sn
R O
P
Sn
P
Ph
O
R R R
Ph
P
O Ph
R
Ph
P P
Ph
Ph
R = nBu
R = nBu O O
O O
L
R
N
L Sn
R Sn
O
R
O
N CH3
O
Sn
Sn
O O
R
O
N
O O Sn L
O O
CH3 O
N
R
O CH3 N
CH3
O
O
O
CH3 O
N
Sn
R O
R = nBu, Ph L = H 2O, DMSO
O
O
CH3
O
Figure 2.5.11 Di-, tri-, tetra- and hexanuclear macrocycles can be prepared from carboxylate-derived connectors using: (a) dicarboxylates; (b) functionalized monocarboxylates; and (c) organotin compounds functionalized with a carboxylate group
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N
R'
R'
N R
Cl
O
Sn O
R
Cl
Sn
R'
N
R
O
O
Cl
R R
R'
O
O
N
O R R'
R
Sn O
Sn
N
O
R
R
N
R Sn
N O
Sn
N O
R O
N
O
O
O R
R'
R = Me, nBu R = Et, nBu R' = H, Me
PhH2C PhH2C
O Sn
O
N HN
O
Ph
Ph
O
O
Ph
O O
N Sn
S
Sn
O O
NH N
O
N
Sn
S
CH2Ph
S
O
CH2Ph
O
Sn
O Ph R'
N Ph
R' S R'
R R'
O S
O
Sn
Sn R
R
R O R'
O
R' O
R = Me, nBu R' = H, iPr
R Sn S
R
Sn
R
R
O R'
O
O
R
R'
R
Sn
O Sn R'
S
O
O
R
O
R
S
R'
Figure 2.5.11
R'
(Continued)
S
R'
Ph O
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c) R
O
Sn
R R
Sn
R
O O
O
O
O
O
R R
Sn R
Sn
O
R R = nBu
Figure 2.5.11
(Continued)
2,5-pyridinedicarboxylic acid, [nBu2 Sn(2,5-pyridinedicarboxylate)]3 , assemble to a three-dimensional hydrogen-bonded structure that contains spherical assemblies built from eight macrocycles. These cap˚ 3 that sules are stabilized by a total of 36 hydrogen bonds and have an accessible inner cavity of 1850 A, can be occupied by organic molecules such as diphenyl- and triphenylmethane, triptycene, and sodium tetraphenylborate.28,36e Imidazole- and pyridine- derived ligands are important organic connectors and have been applied for the assembly of a large number of metallomacrocycles.2 However, with tin, only very few examples have been reported so far.39 Dithiocarbamate ligands have some structural similarities with carboxylates and their coordination chemistry has been explored extensively.10 Interestingly, their application in metallosupramolecular chemistry is almost unexplored so far, 41 and there are only three reports on structurally characterized macrocycles derived from bis(dithiocarbamates) (Figure 2.5.12).41
H N
N
H
S S S S R Sn R Sn R R S S S S
H
N
N
H
R = Me, nBu, t Bu, Cy, Ph
Figure 2.5.12
N
N S R R
S
S Sn
Sn S
S
S
S
R R S
N
N R= Me, n Bu
Organotin macrocycles derived from bis(dithiocarbamates) as organic connectors
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c)
(CH2) n-2
R
Sn
Sn
(CH2) n
O O
R'
R'
N
N
N R' N R'
P
M
P
O 2- O O Sn O O
R 2R Sn R
Ph Ph
L'
R = Ph, Cl n = 5-10
O
L'
R
(CH2) n-2
b)
Ph Ph
L'
P Ph O Ph
R' N O
M L'
P O Ph O Ph
L'
O
N O
O 2- O O Sn O
2- R Sn R R
P
M
P Ph
Ph
Ph
L'
Ph
M = Pd, Ag L ' = Br, DMF
R'
R = Cl, Br R ' = Me, Cl; (R')2 = C4H6
Figure 2.5.13
Tin cryptands obtained from
(CH2 ) Y (CH2 ) ,
O Y O
and related connectors
(3) Dinuclear Cryptands. The combination of monoorganotin moieties and (CH2 ) Y (CH2 ) or O Y O connectors gives neutral42 or charged cryptands43,44 (Figure 2.5.13), of which the latter have been applied for the inclusion of metal ions. Halogen-bridged cryptands related to the systems shown in Figure 2.5.13a have also been reported.31f Fourfold-bridged dinuclear tin cryptands are unknown so far. It is possible to modulate gradually the ring size of the cavity when changing the length of the connectors. For the cryptands shown in Figure 2.5.13a the Sn· · ·Sn distances have been varied ˚ 42 from 5.5 to 11.1 A. (4) Macrocycles and Cryptands Formed from Oligonuclear Secondary Building Blocks. Some research groups have developed synthetic methodologies that allow them to link ladder-type secondary building blocks to form either oligonuclear macrocyclic or cryptand structures. Thus, it has been possible to combine the pentanuclear stannoxane ladder shown in Figure 2.5.14a with inorganic bridging functions such as nitrate, carbonate, sulfate, hydrogen phosphate, and trifluoromethanesulfonate.45 The Sn· · ·Sn ˚ In combination with organic conseparations in the macrocyclic structures range from 5.67 to 6.33 A. nectors, such as carboxylic acid derivatives, the macrocycle ring size could be expanded up to 18.3 ˚ 45c,46 There is also an example for a bis(heptanuclear) ladder system, which, in addition to the central A. macrocycle, contains two smaller lateral macrocycles.47 A ladder-derived twin macrocycle has been also reported.48
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a) R X R
Sn
O
O
Sn
Sn O
R
O
R R
O
R R Sn
Sn
X
R
R = CH 2Ph, nBu; X = O, OH L = NO, CO, P(OH)(O), SO2, S(O)CF3, C(O)−(CH2)4−C(O), C(O)−2,4−C6 H4NO2−C(O)−, C(O)−ferrocene−C(O), etc.
O L
Sn L
Sn R Sn R
O
R
O
Cl R
R Cl
O
Sn
O
L
R
O
R
Sn R
Sn
Sn Cl
R Sn
R
R X
R
Cl
L
X O
R
Cl R Sn
R
R
O
Sn
O
Sn
R R
L
R
133
Cl Sn
L Cl
R Sn Cl
R = −CH2tBu, −CH2iPr, −CH2SiMe3 L = −(CH 2)n− with n = 1−10; 1,3−CH2−C6H4−CH2−
Figure 2.5.14 Representative examples for oligonuclear: (a) macrocycles and (b) cryptands that have been prepared from ladder-type secondary organotin building blocks linked through organic and/or inorganic connectors
Furthermore, a series of cryptands have been prepared consisting of ladders of different nuclearity (n = 4, 6, 8) that have been linked pair-wise through a varying number of (CH2 )n (n = 3 12) and related connectors.49 A triple ladder is also known.50 2.5.4
Conclusions and Perspectives
This survey has shown that during the last few years, the number of reports on tin macrocycles and coordination polymers having cavities, pores, or channels has considerably increased. The exploration of organotin-derived assemblies is particularly promising, since the organic substitutents attached to the tin atoms can be employed in different directions such as the modification of functionality, size, and shape of the cavity. Applications in host–guest chemistry, molecular recognition, filtration, sorption, storage, catalysis, transport, etc., are practically unexplored to date.
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40. (a) O.D. Fox, M.G.B. Drew, and P.D. Beer, Angew. Chem. Int. Ed., 39, 136 (2000); (b) O.D. Fox, M.G.B. Drew, E.J.S. Wilkinson, and P.D. Beer, Chem. Commun., 391 (2000); (c) P.D. Beer, N. Berry, M.G.B. Drew, O.D. Fox, M.E. Padilla-Tosta, and S. Patell, Chem. Commun., 199 (2001); (d) S.W. Lai, M.G.B. Drew, and P.D. Beer, J. Organomet. Chem., 89, 637 (2001); (e) L.H. Uppadine, J.M. Weeks, and P.D Beer, Dalton Trans., 3367 (2001); (f) M.E. Padilla-Tosta, O.D. Fox, M.G.B. Drew, and P.D. Beer, Angew. Chem. Int. Ed., 40, 4235 (2001); (g) P.D. Beer, N.G. Berry, A.R. Cowley, E.J. Hayes, E.C. Oates, and W.W.H. Wong, Chem. Commun., 2408 (2003); (h) P.D. Beer, A.G. Cheetham, M.G.B. Drew, O.D. Fox, E.J. Hayes, and T.D. Rolls, Dalton Trans., 603 (2003); (i) P.R.A. Webber, M.G.B. Drew, R. Hibbert, and P.D. Beer, Dalton Trans., 1127 (2004). 41. (a) O.-S. Jung, Y. S. Sohn, and J. A. Ibers, Inorg. Chem., 25, 2273 (1986); (b) W.H. Lee, O.-S. Jung, Y.S. Sohn, P. Kim, Bull. Korean Chem. Soc., 7, 421 (1986); (c) R. Reyes-Mart´ınez, J.A. Guerrero-Alvarez, H. H¨opfl, and H. Tlahuext, Arkivoc, 19 (2008). 42. (a) M. Newcomb, M.T. Blanda, Y. Azuma, and T.J. Delord, Chem. Commun., 1159 (1984); (b) M. Newcomb, J.H. Horner, M.T. Blanda, and P.J. Squattrito, J. Am. Chem. Soc., 111, 6294 (1989); (c) H. Horner, P.J. Squatritto, N. McGuire, J.P. Riebenspies, and M. Newcomb, Organometallics, 10, 1741 (1991). 43. (a) X. Sun, D.W. Johnson, D.L. Caulder, R.E. Powers, K.N. Raymond, and E.H. Wong, Angew. Chem. Int. Ed., 38, 1303 (1999); (b) X. Sun, D.W. Johnson, D.L. Caulder, K.N. Raymond, and E.H. Wong, J. Am. Chem. Soc., 123, 2752 (2001); (c) X. Sun, D.W. Johnson, K.N. Raymond, and E.H. Wong, Inorg. Chem., 40, 4504 (2001). 44. (a) S.V. Lindeman, Y.T. Struchkov, and Y.Z.Voloshin, J. Coord. Chem., 28, 319 (1993); (b) S.V. Lindeman, Y.T. Struchkov, and Y.Z. Voloshin, J. Coord. Chem., 34, 203 (1995); (c) Y.Z. Voloshin, V.K. Belsky, and V.V. Trachevskii, Polyhedron, 11, 1939 (1992). 45. (a) A. Orita, K. Sakamoto, H. Ikeda, J. Xiang, and J. Otera, Chem. Lett., 40 (2001); (b) C.L. Ma, J. Zhang, Q. Jiang, and R. Zhang, Inorg. Chim. Acta, 357, 2791 (2004); (c) G.-L. Zheng, J.-F. Ma, J. Yang, Y.-Y. Li, and X.-R. Hao, Chem. Eur. J., 10, 3761 (2004); (d) D. Ballivet-Tkatchenko, S. Chambrey, R. Keiski, R. Ligabue, L. Plasseraud, P. Richard, and H. Turunen, Catal. Today, 115, 80 (2006). 46. (a) R.-H. Wang, M.-C. Hong, J.-H. Luo, R. Cao, and J.-B. Weng, Eur. J. Inorg. Chem., 2082 (2002); (b) C.L. Ma, and J. Sun, Dalton Trans.,1785 (2004). 47. C.L. Ma, Q. Jiang, R. Zhang, and D.Wang, Dalton Trans., 2975 (2003). 48. R. Zhang, J. Sun, and C.L. Ma, Inorg. Chim. Acta, 357, 4322 (2004). 49. (a) M. Mehring, M. Schurmann, I. Paulus, D. Horn, K. Jurkschat, A. Orita, J. Otera, D. Dakternieks, and A. Duthie, J. Organomet. Chem., 574, 176 (1999); (b) D. Dakternieks, F.S. Kuan, A. Duthie, and E.R.T. Tiekink, Main Group Met. Chem., 23, 731 (2000); (c) B. Costisella, D. Dakternieks, K. Jurkschat, M. Mehring, I. Paulus, and M. Schurmann, Khim. Get. Soedin., SSSR (Russ.) (Chem. Heterocycl. Compd.), 1535 (2001); (d) M. Mehring, I. Paulus, B. Zobel, M. Schurmann, K. Jurkschat, A. Duthie, and D. Dakternieks, Eur. J. Inorg. Chem., 153 (2001); (e) D. Dakternieks, A. Duthie, B. Zobel, K. Jurkschat, M. Schurmann, and E.R.T. Tiekink, Organometallics, 21, 647 (2002); (f) J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, and E.R.T. Tiekink, Organometallics, 22, 4399 (2003); (g) J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, and E.R.T.Tiekink, J. Organomet. Chem., 688, 56 (2003); (h) J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, K. Jurkschat, M. Schurmann, and E.R.T. Tiekink New J. Chem., 28, 1268 (2004). 50. M. Mehring, M. Schurmann, H. Reuter, D. Dakternieks, and K. Jurkschat, Angew. Chem. Int. Ed., 36, 1112 (1997).
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Tin Chemistry: Fundamentals, Frontiers and Applications
2.6
Deltahedral Zintl Ions of Tin: Synthesis, Structure, and Reactivity
Slavi C. Sevov Department of Chemistry and Biochemistry, University of Notre Dame,USA
2.6.1
Introduction
One little-known area of chemistry is the chemistry of heavy main-group p elements, mostly metals and semimetals, in negative oxidation states. While the chemistry of Sn2+ and Sn4+ , for example, is very well studied and we all learn about it very early in our education, not many people are aware of the fact that tin atoms can accept electrons and exhibit completely different reactivity when negatively charged. For example, catenation and clustering are uncommon for cationic tin, but is the norm for anions, both in inter-metallics and molecular compounds. Thus, cyclopentadienyl-like Sn6− 5 is found in the inter-metallic compounds Na8 AeSn6 (Ae = Ba, Eu), Li9−x AeSn6+x (Ae = Ca, Eu), Li5 Ca7 Sn11 , and Li6 Eu5 Sn9 ,1 isolated tetrahedra of Sn4− 4 and alkali-metal cations constitute the structures of A4 Sn4 (A = alkali metal),2 while Na4 CaSn6 and Li2 Ln5 Sn7 (Ln = Ce, Pr, Sm, Eu) contain infinite anionic chains and heptane-like Sn16− oligomers, respectively.3,4 Notice that, as might be expected, the negative 7 oxidation states are achieved when the more electronegative p element, tin in this case, is combined with much more electropositive s or f element such as the alkali, alkaline-earth, or rare-earth metals. Notice also that, for the purpose of structure rationalization and systematic description, the formal oxidation state assignments assume complete electron transfer from the more electropositive atoms to the p element. Such polar inter-metallic compounds, called Zintl phases when electronically balanced, are known for all post-transition metals and semimetals.5 Negatively charged clusters of some of the p-metals and semimetals are also stable in solutions and can be crystallized from them as ionic molecular compounds. Known, at present, are a number of such ligand-free anionic clusters, called Zintl ions.6 Some examples are the clusters with general formulas 3− 4− 2− Pn3− 7 , E9 , E9 , and E5 where Pn = P, As, Sb and E = Si, Ge, Sn, Pb. This chapter will limit its attention to the synthesis, structure, and functionalization of molecular anionic clusters of tin. Although both nine- and five-atom clusters are known (Figure 2.6.1), the focus will be predominantly on the nine-atom 4− deltahedral clusters Sn3− 9 and Sn9 which are the most readily prepared and most extensively studied species. 2.6.2
Background
The history of the Zintl ions starts in 1891 when Joannis experimented with liquid ammonia solutions of sodium and their reactivity towards heavy p-block elements.7 He noticed that Pb and Sb dissolve in such solutions, and the original blue color (due to the solvated free electrons) changed to green for lead and to brown for antimony. Furthermore, he was able to estimate the ratio of Na : Pb as approximately 1 : 2 and this is very close to 4 : 9 (1 : 2.25) of the nowadays well-known clusters Pb4− 9 . In 1917 this ratio was measured electrolytically more precisely by Smyth and reported as 1 : 2.26.8 In the meantime, Kraus showed that tin also dissolves in such solutions with a similar color change, this time to red.9 However, there was no clear understanding of why and under what form these p elements dissolve. Speculated in some cases was the existence of metal-salt aggregates such as Na4 Sn·Sn8 , Na4 Pb·Pb8 , and Na3 Sb·Sb6 . Ten or so years later, in the 1930s, Edward Zintl conducted a series of more systematic studies of these systems.10 He carried out potentiometric titrations of liquid ammonia solutions of alkali metals with various p metal salts, typically halides. Thus, the titration of a sodium solution with lead(II) iodide revealed that the green anionic species in solution are Pb4− 9 . Zintl and coworkers also discovered that
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b)
3− Figure 2.6.1 The two known anionic clusters of tin: (a) the nine-atom Sn4− 9 and Sn9 , and (b) the five-atom 2− Sn5 . Both clusters are deltahedral, although the nine-atom species has an open square face (the bottom). It is derived from the complete ten-atom deltahedron, a bicapped square antiprism, by removal of one of the capping vertices. The five-atom cluster is a trigonal bipyramid
the same polyatomic anions could be extracted in solutions from binary alloys of the corresponding p element with an alkali metal mixed in appropriate proportions. Based on these extraction studies they proposed the existence of a number of anions in solution although, at the time, it was assumed that the alloys did not contain these anions, an assumption that was proven wrong more than 65 years later.11 3− Some of the proposed species, such as E4− 9 for E = Sn, Pb and Pn7 for Pn = As, Sb, were structurally 3− characterized later by other groups, while the existence of other proposed species, for example Pb4− 7 , Bi7 , 3− 3− 6 Pn5 , Pn3 for Pn = As, Sb, Bi, was never confirmed. Nonetheless, the recently synthesized mixedatom clusters [Bi3 M2 (CO)6 ]3− (M = Cr, Mo), [Bi3 Ni4 (CO)6 ]3− , [Sb3 Ni4 (CO)6 ]3− , and [Bi3 Ni6 (CO)9 ]3− 3− 12 suggest indirectly that the proposed Bi3− 3 and Sb3 may exist in solutions. On the other hand, species 2− 2− like the square-like Bi4 and the double-bonded oxygen-like Bi2 were not originally proposed, but have been structurally characterized.13 The ions mentioned above and, generally, all polyatomic anions of post-transition metals and semimetals are now known as Zintl ions. Among them, the deltahedral Zintl ions form a special subclass of polyatomic clusters with geometries made of triangular faces. At the time of their discovery, however, the shapes of these anions were unknown. It took more than 30 years after Zintl’s work for the first, although partial, structural report to appear. In 1970 Kummer and Diehl reported that, by dissolving an alloy of NaSn2.4−2.5 in ethylenediamine (= en) they could crystallize a compound with an overall formula Na4 Sn9 r7en.14 However, it was Corbett who, in 1975, reported the first single-crystal structure with a deltahedral cluster, that of Sn4− 9 in the compound [Na-(2,2,2-crypt)]4 Sn9 , crystallized by using 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo-[8.8.8]-hexacosane) as a sequestering agent of the alkali-metal cations, a method that is now the most common in crystallization of Zintl ions.15 A year later, Kummer and Diehl reported the structure of Na4 Sn9 r7en with the same Sn4− 9 deltahedral Zintl ion.16 Most of the effort during the following 25–30 years was focused primarily on improving
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the synthesis and crystallization of Zintl ions, on rationalization of subtle differences in their geometries and electronic structures, and NMR studies of their solutions. In addition to 2,2,2-crypt as a sequestering agent, 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) and even smaller crown ethers were found to help crystallization in some cases as well. The first reaction with deltahedral Zintl ions, which happened to be Sn4− 9 , was reported in 1988 by Eichhorn, Haushalter, and Pennington.17 It was a simple ligand exchange reaction in which the labile 6 ligand of a transition-metal complex was replaced by the Zintl ion. Thus, Sn4− 9 readily replaced the η 4− mesitylene in (mes)Cr(CO)3 and formed the corresponding transition-metal complex [Sn9 M(CO)3 ] . 2.6.3
Geometry, Charge, Electron Count, and Electronic Structure
The most stable clusters of tin, as well as of the rest of the elements in this group except carbon, are, by far, the nine-atom clusters. As mentioned above, Sn4− 9 was the first deltahedral cluster to be structurally characterized. While Ge9 was added very soon after,18 nine-atom clusters of lead and silicon were found respectively 20 and 30 years later.19,20 Thus, the following discussion of geometry, charges, cluster-bonding, and electronic structure is valid not only for tin clusters, but also for those of silicon, germanium, and lead. The bonding in the deltahedral clusters cannot be rationalized with simple 2-center–2-electron bonds because it is achieved through delocalized electrons. These clusters are analogous to the well-known cage-like boranes both structurally and electronically, and they similarly follow the Wade–Mingos rules for electron counting.21 Each BH in the boranes corresponds to a naked atom of group 14 where the B H bonding pair of electrons is replaced by a lone pair of electrons. For example, E2− 5 has 4− 2n + 2 = 12 cluster bonding electrons (each vertex provides two electrons) as closo-B5 H2− 5 , while E9 (E = group 14 element) is a nido-cluster with 2n + 4 = 22 cluster-bonding electrons and corresponds to a nido-B9 H4− 9 . While experimenting with different synthetic and crystallization techniques for the E4− 9 clusters, it was discovered that the same clusters, but with a charge of 3 , i.e. E3− 9 , can be crystallized as well. However, it was not clear what specific conditions lead to one or the other type. Sn3− 9 was characterized initially in [K-(2,2,2-crypt)]3 Sn9 r1.5en and then in [K-(2,2,2-crypt)]6 (Sn9 )2 r1.5en r0.5tol (tol = toluene).22 The re15 16 r ported compounds with Sn4− 9 , on the other hand, are many more: [Na-(2,2,2-crypt)]4 Sn9 , Na4 Sn9 7en, 23 24 25 r r K[K-(2,2,2-crypt)]3 Sn9 , K[K-(18-crown-6)]3 Sn9 en, Rb2 [Rb-(18-crown-6)]2 Sn9 1.5en, Cs7 [K(2,2,2-crypt)](Sn9 )2 r3en,26 [K-(12-crown-4)2 ]2 [K-(12-crown-4)]2 Sn9 r4en,27 and Li4 Sn9 r(NH3 )17 .28 Very similar lists of compounds exist for germanium and lead while for silicon, the cluster with a charge of 4 has not been reported.6 In addition to the clusters with 3 and 4 charges, a silicon cluster 29 with a charge of 2 , i.e. Si2− 9 , was also structurally characterized. Detailed studies carried out for the germanium system revealed that the nine-atom clusters with different charges are in equilibria between themselves and solvated electrons.30 The same equilibria most 3− 2− likely exist for tin, i.e. Sn4− 9 and Sn9 (and possibly Sn9 ) may coexist in solution in an equilibrium such 4− 3− as Sn9 Sn9 + e− (solv). It was realized later that the size and the shape of the available counter cations defines which of these species crystallizes from the solutions. Thus, excess of 2,2,2-crypt in solutions with K+ leads to the presence of only the very large cations [K-(2,2,2-crypt)]+ , and apparently only three such large cations can pack in a crystal lattice with a nine-atom cluster. This means that only Sn3− 9 clusters can be crystallized in these cases and, indeed, this is observed in [K-(2,2,2-crypt)]3 Sn9 r1.5en and [K-(2, 2,2-crypt)]6 (Sn9 )2 r1.5en r0.5tol.22 Deficiency of 2,2,2-crypt, on the other hand, results in the availability of small naked alkali-metal cations. This allows for packing of four cations, some naked and some sequestered, with a cluster anion and this selectively extracts Sn4− 9 in K[K-(2,2,2-crypt)]3 Sn9 and
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Cs7 [K-(2,2,2-crypt)](Sn9 )2 r3en.23,26 Apparently, the flat crown ethers have a similar size and shape effect.24,25,27 In addition, complete absence of a sequestering agent, as in Na4 Sn9 r7en and Li4 Sn9 r(NH3 )17 , logically leads to the same clusters in the solid state.16,28 Finally, the crystallization of Sn4− 9 with [Na-(2,2,2-crypt)] maybe due to the smaller size of the sodium cation and its effect on the overall size of the cryptated cation.15 Five-atom clusters with a charge of 2 are known for all heavier elements of this group,20,31 but their synthesis is quite erratic, i.e. there are no clear guidelines for exactly how to crystallize these species. The first Sn2− 5 clusters, reported by Corbett et al., were crystallized from ethylenediamine solutions of precursors, with nominal compositions anywhere between NaSn and NaSn1.7 .31a The same clusters were later crystallized from a solution of a precursor with a nominal composition Na1.7 Sn.31b Although the 4− nine-atom tin clusters Sn3− 9 and Sn9 are typically extracted from similar solutions of precursors with compositions close to NaSn2.25 , it is not clear how important the different precursor stoichiometries are. 3− Keep in mind that the average charge per tin atom in Sn2− 5 , 0.4 , is between those of Sn9 , 0.33 , and 4− Sn9 , 0.44 , and these numbers do not correlate in any way with the average charge per tin atom in the precursors (assuming complete electron transfer from Na to Sn). The specific geometry and electronic structure of the nine-atom clusters are very well suited for handling different charges with very small structural distortions and for easy inter-conversion between species with different charges. The overall shape of the clusters can be viewed as that of a tricapped trigonal prism (Figure 2.6.2) in which one, two, or three of the trigonal prismatic edges parallel to the three-fold axis (vertical in Figure 2.6.2) are elongated to some extent.6a It has been shown for germanium that lengthening and shortening of these edges involves very little energy, yet greatly affects the electronic structure of the 6a cluster, particularly the energy of the HOMO of the E4− 9 . This orbital (Figure 2.6.3) is bonding within the triangular bases of the trigonal prism, but is antibonding between them, most strongly along the same trigonal prismatic edges that change distance. Thus, one or more long edges reduce the antibonding character of the orbital and it is occupied and the HOMO for E4− 9 , a nido-cluster with 22 cluster-bonding electrons. Shortening of the edge(s) pushes the orbital higher in energy because of increasing antibonding character. It becomes occupied by only one electron in E3− 9 , a radical cluster with 23 cluster-bonding electrons. Further shortening of the edges pushes the orbital even higher in energy, to become empty
Figure 2.6.2 The overall shape of a nine-atom cluster viewed as a tricapped trigonal prism (vertical threefold axis) with one, two, or three elongated trigonal prismatic edges parallel to the three-fold axis. Shown is a cluster with one elongated edge (an open bond), which results in a pseudo-square face. This cluster is identical to the cluster in Figure 2.6.1a, but the pseudo-square face in the latter is at the bottom
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Sng4−
Sng3−
Sng2−
Figure 2.6.3 Schematic MO diagrams (left) of nine-atom tin clusters with different charges and the frontier orbital (encircled in the MOs and shown to the right) that changes energy upon cluster distortion. This orbital 3− 2− 2− 2− is the HOMO in Sn4− 9 , the SOMO in Sn9 , and the LUMO in a hypothetical Sn9 (known are Si9 and Ge9 , although the latter has never been structurally well characterized). Notice that the orbital is π -bonding within the two triangular bases of the trigonal prism (top and bottom triangles), but is σ -antibonding between them along the vertical trigonal prismatic edges. Shortening of the latter increases the antibonding character and pushes the orbital higher in energy as shown
and the LUMO for E2− 9 , a closo-cluster with 20 cluster-bonding electrons. This charge flexibility has been extensively exploited in the case of germanium clusters and it is believed to be responsible for their diverse chemistry of adding a variety of substituents, both nucleophiles and electrophiles.6a Also of interest for later discussion are the three filled orbitals below the HOMO of E4− 9 . These orbitals are shown in Figure 2.6.4 for a cluster with one elongated edge that generates a pseudo-square open face (the front face in Figure 2.6.4). This face can be capped by an additional vertex, in which case one of
Figure 2.6.4 The three filled molecular orbitals below the HOMO in E94− with the cluster shown with its pseudo-square open face in front. If an additional atom is to cap this open face, the MO shown in (a) is perfectly positioned for σ -interactions while those in (b) and (c) can participate in π -interactions
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the three orbitals has the appropriate symmetry for σ -overlap (Figure 2.6.4a), while the other two can form π -bonds (vertical and horizontal in Figures 2.6.4b and c, respectively), with suitable orbitals on the capping vertex. 2.6.4
Reactions With Nine-Atom Deltahedral Zintl Anions of Tin
17 As mentioned already, Sn4− 9 was the very first deltahedral cluster to undergo a reaction of any kind. 4− 4− The reaction, Sn9 + (mes)Cr(CO)3 → [Sn9 Cr(CO)3 ] + (mes), can be described as a simple ligand exchange reaction in which the cluster replaces a six-electron donating group at the Cr atom and forms [Sn9 Cr(CO)3 ]4− (Figure 2.6.5). The Cr(CO)3 fragment coordinates at the open pseudo-square face of the 2 Sn4− 9 cluster. Its three frontier and empty d orbitals z , xz, and yz are perfectly positioned to interact with the three filled cluster orbitals shown in Figure 2.6.4 resulting in a σ -interaction between z 2 and the orbital in Figure 2.6.4a, and π-interactions between xz and yz on Cr(CO)3 and the MOs shown in Figures 2.6.4b and c. Therefore, the cluster behaves as a six-electron donating ligand such as mesitylene, benzene, etc. Focusing on the organometallic part of the anion, these six additional electrons are needed to completing the 18-electron environment at the Cr atom. From cluster perspective, on the other hand, this reaction can be viewed as adding the missing vertex in the nido-Sn4− 9 to form the corresponding closo-species [Sn9 Cr(CO)3 ]4− . The latter has the classical shape for a closo-cluster with ten vertices, namely a bicapped square antiprism, where chromium is one of the two capping atoms. The fragment Cr(CO)3 is isolobal with CH3+ and, therefore, has three empty frontier orbitals and donates zero electrons to the cluster bonding. This means that the capped cluster retains the original 22 cluster-bonding electrons and, with its ten vertices, corresponds to a closo-species according to Wade–Mingos rules.21 Notice that, overall, the original cluster does not undergo a redox process, it just adds a vertex without loss or gain of electrons. It was shown later that similar reactions can be carried out with the heavier congeners in the Cr group, 4− Mo and W. It is now known that both Sn4− 9 and Pb9 can be derivatized with M(CO)3 where M = Cr, Mo, or W to form bicapped square antiprisms where the transition metal caps one of the squares.17,32−35 (It is somewhat surprising that no analogous [Ge9 M(CO)3 ]4− clusters have been characterized to date,
Cr
Figure 2.6.5 [Sn9 Cr(CO)3 ]4− , the first derivative of a deltahedral Zintl anion. The Cr(CO)3 fragment caps the open pseudo-square face of the original Sn4− 9 cluster and does not donate electrons for cluster bonding. The hetero-atomic cluster is a closo-species according to both shape, a bicapped square antiprism, and electron count. The three empty d orbitals z 2 , xz, and yz of Cr(CO)3 overlap with the filled orbitals shown in Figure 2.6.4 and, therefore, the cluster behaves as a six-electron donating ligand
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presumably due to subtle differences in the sizes and electronic properties of the clusters.) However, more recent reports suggest that both [Sn9 M(CO)3 ]4− and [Pb9 M(CO)3 ]4− may be fluxional in solution (below) with the possibility of the M(CO)3 occupying other positions in the cluster.33−35 Rotation of a triangular face of the cluster was suggested as a possible mechanism for such fluxionality, although such a process would be very complex and involve a series of steps, each associated with a bond cleavage or bond creation. It was later structurally proved that another isomer indeed exists for both tin and lead. The transition metal in the new isomers, η5 -[Sn9 M(CO)3 ]4− and η5 -[Pb9 M(CO)3 ]4− , occupies a position in the square antiprism and is five-coordinate (Figure 2.6.6).33,35 Apparently the two tin isomers, η4 -[Sn9 W(CO)3 ]4− and η5 -[Sn9 W(CO)3 ]4− , cocrystallize from the reaction with W(CO)3 (mes). On the other hand, only η4 -[Pb9 Mo(CO)3 ]4− is made by the reaction with the corresponding Mo(CO)3 (mes), while η5 -[Pb9 Mo(CO)3 ]4− is produced exclusively from the reaction with Mo(CO)3 (MeCN).35 Another report of NMR studies in solutions, however, questions whether the two isomers inter-convert from one to another in solution (dynamic model) or are formed irreversibly during the synthesis (static model).34 Let us look more closely at these isomers and try to rationalize some possible mechanisms. As already discussed, the open pseudo-square face of these clusters is the most electron-rich place in the free cluster, and it is logical to assume that strong electrophiles, such as M(CO)3 will coordinate there (Figure 2.6.7). At this point the new intermediate can undergo two different distortions. The lower energy path is to optimize its geometry by perfecting the two capped faces to squares via further elongation of the former long trigonal prismatic edge and shortening of the other two edges, as shown in Figure 2.6.7b. The resulting cluster is the much more common isomer η4 -[E9 M(CO)3 ]4− (Figure 2.6.7c). The second choice for the capped cluster is to undergo a more energy-demanding rearrangement that involves at least one diamond–square–diamond (DSD) transformation (Figure 2.6.7d). This involves breaking the bond between atoms 5–9, as numbered in Figure 2.6.7d, and creating a new bond between the transition metal M and atom 6. A potential second DSD may be occurring at the face made of atoms 1–2–5–6. However,
W
Figure 2.6.6 The isomer η5 -[Sn9 W(CO)3 ]4− , a bicapped square antiprism (the pseudo four-fold axis is vertical), where the transition metal is a part of the square prism and is five-coordinate with the cluster
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M caps square face
b)
c) ?
a) M caps square face (different rearrangement)
d)
e)
Figure 2.6.7 Proposed mechanisms for: (a)→(b)→(c) the formation of the more common isomer η4 [Sn9 M(CO)3 ]4− and (a)→(d)→(e) the more exotic isomer η5 -[Sn9 M(CO)3 ]4−
as already discussed, the nine-atom clusters may have one, two, or three elongated prismatic edges, and therefore atoms 1–6 may already be far apart. The described rearrangement followed by geometry optimization would produce the more exotic isomer η5 -[E9 M(CO)3 ]4− (Figure 2.6.7e). This proposed mechanism is consistent with both the dynamic and static models, i.e. it allows for inter-conversion and equilibrium between the two isomers via the same two intermediates as shown with a question mark in Figure 2.6.7. In addition to the tricarbonyl fragments, tin clusters can also be capped by ZnPh to form [Sn9 ZnPh]3− (Figure 2.6.8).36 The reaction is carried out with ZnPh2 , and the fragment caps the cluster at the same open pseudo-square face. However, unlike the zero-electron donating tricarbonyl fragments, ZnPh is isolobal with CH2+ and donates one electron to the cluster. This results in a lower charge for the resulting species,
Zn
Figure 2.6.8 The ZnPh-capped cluster [E9 ZnPh]3− for E = Si, Ge, Sn, and Pb made by reaction of the clusters with ZnPh2 . The ZnPh fragment is a one-electron donor and reduces the charge of the cluster by one, compared to clusters capped with M(CO)3 . The ZnPh fragment is also an electrophile and caps the same open pseudo-square face as M(CO)3
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i.e. 3– instead of 4–, although the number of cluster-bonding electrons remains 22. There is, however, an example of a capped germanium cluster with 21 bonding electrons, [Ge9 Ni(CO)]3− , where the capping Ni(CO) fragment is a zero-electron donor.37 It suggests that not only E4− 9 clusters can be capped, but also those with a lower charge, E3− . 9 Besides capping with organometallic fragments, the nine-atom deltahedral clusters can undergo a second type of modification, namely insertion of a transition-metal atom inside the cluster. For example, reactions of Pb4− 9 with Pt(PPh3 )4 and Ni(COD)2 (COD = 1,5-cyclooctadiene) produce Pt-centered icosahedra [Pt@Pb12 ]2− and Ni-centered bicapped square antiprisms [Ni@Pb10 ]2− , respectively.38 Clearly, the nine-atom clusters have somehow reassembled into 12- and 10-atom species. This behavior, however, seems to be specific only to lead clusters. Thus, similar reactions of Gen− 9 clusters with Ni(COD)2 39 produce [Ni@Ge9 ]3− , which is a centered version of the empty Ge3− species. It should be pointed 9 out that the inserted atoms of this group, i.e. Ni, Pd, and Pt, have a closed-shell d 10 configuration and do not contribute electrons for cluster-bonding. However, they provide orbitals that overlap with the cluster-bonding orbitals and thus contribute to the overall bonding within the cluster. Although similar centered single-cluster species of tin have not been structurally characterized yet, there is a very good proof that such insertion of a transition-metal atom occurs for them as well. Thus, reaction of tin clusters with Ni(COD)2 produces a dimer of Ni-centered Sn9 clusters fused via a common vertex.40 The anion [Ni2 Sn17 ]4− (Figure 2.6.9) is made of two tricapped trigonal prisms of tin, each of the kind with two elongated prismatic edges (shown lighter in Figure 2.6.9). The two clusters share the tin atom that caps the trigonal prismatic faces with the elongated edges and are positioned at 90◦ with respect to each other. As shown in Figure 2.6.9, the left prism has its pseudo three-fold axis vertical, while the one to the right has its axis along the viewing direction. Finally, the nine-atom tin clusters can undergo a third type of modification which is a combination of the two types already discussed, i.e. insertion of a transition-metal atom combined with capping by a fragment of a transition-metal complex. The insertion of the transition metal is most likely the first step of this process. This has been demonstrated for germanium clusters, where the reaction was carried out stepwise by first inserting an Ni atom and then using the product for capping with a NiCO fragment.37 The inserted transition metal appears to affect the overall electronic structure of the cluster in a subtle way that seems to change the cluster site for preferred coordination of capping fragments, from an open
Figure 2.6.9 The dimer [Ni2 Sn17 ] 4− made of two Ni-centered clusters fused via a common tin vertex. Each half is a tricapped trigonal prism with two elongated prismatic edges (shown lighter), and the common vertex is the atom capping the prismatic face with two elongated edges in each half. The two clusters are rotated around the Ni Sn Ni axis by 90 ◦ with respect to each other, so that the pseudo three-fold axis of the cluster to the left is vertical, while it is along the viewing direction for the cluster to the right
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Figure 2.6.10 The Pt-centered and Pt(PPh3 )-capped [Pt@(Sn9 Pt-PPh3 )]2− . The Sn9 cluster, a tricapped trigonal prism (vertical pseudo three-fold axis), with three elongated edges (shown lighter), is capped at the upper trigonal prismatic base. The capped base opens up as the capping Pt-atom is pressed towards the center of the cluster. The final geometry is very close to spherical, i.e. all atoms, including the capping Pt atom, are at similar distances to the central atom
pseudo-square face to one of the triangular bases of the tricapped trigonal prism. This is observed for all centered and capped germanium clusters [Ni@(Ge9 Ni-PPh3 )]2− ,41 [Ni@(Ge9 Ni-CO)]2− ,37 [Ni@(Ge9 NiC≡CPh)]3− ,37 [Ni@(Ge9 Ni-en)]3− ,37 and the dimer [(Ni@Ge9 )Ni(Ni@Ge9 )]4− ,39 as well as for one of the two known capped and centered tin clusters, [Pt@(Sn9 Pt-PPh3 )]2− (Figure 2.6.10).42 The capping atom in these clusters is apparently pulled towards the center of the cluster, perhaps by interactions with the central atom, to a distance from the center that is similar to those of the nine tin atoms (Figure 2.6.11a, b). This distortion, in turn, causes the capped triangular base to open so much that the corresponding three atoms are no longer in contact with each other (Figure 2.6.11c). The resulting cluster is nearly spherical, with all atoms, including the capping atom, at similar distances from the center. The range of such ˚ 42 distances in [Pt@(Sn9 Pt-PPh3 )]2− is very narrow, 2.70–2.80 A. Electronically, the centered and capped clusters are very similar to the empty clusters and can handle different numbers of cluster-bonding electrons, perhaps by small changes in their geometry. They seem to be most stable with 20 cluster-bonding electrons, a number that corresponds to a capped closo-species, according to the Wade–Mingos rules, which prescribe 2n electrons for such cases. The number of vertices n in [M@(E9 M-L)]2− is 10 and the 20 electrons are provided by the nine atoms of group 14 and the charge of 2 . It should be pointed out that despite its higher charge [Ni@(Ge9 Ni-C≡CPh)]3− has the
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Figure 2.6.11 The two types of capping observed for centered tin clusters: the path (a)→(b)→(c) represents capping of a triangular base of the tricapped trigonal prismatic cluster; the path (a)→(d)→(e) represents capping of the open pseudo-square face of the cluster as observed also for empty clusters (Figure 2.6.7). The capped triangular base in (b) opens up upon insertion of the capping atom towards the center of the cluster, in order to interact with the central atom, and this results in the cluster shown in (c)
same electron count, simply because the one extra negative charge is for the anionic ligand [C≡CPh]− .37 There are, however, two examples of centered and capped clusters that carry 21 electrons, [Ni@(Ge9 Nien)]3− and [Ni@(Sn9 Ni-CO)]3− .37,42 Again, they should be viewed as derivatives of the corresponding empty E3− 9 clusters. While the germanium product conforms to the common geometry described above and shown in Figure 2.6.10, the tin species represents the first and only centered cluster that is capped at the open pseudo-square face, as are the empty clusters (Figure 2.6.12). With just this one example it is difficult to even speculate about the reasons for the different geometry in this case. The formation of the species again most likely starts with insertion of the centering atom followed by capping of the face and pulling in the capping atom by interactions with the central atom (Figure 2.6.11a, d, e). The final geometry is not as spherical as for the species with a capped triangular face because the tin atom opposite ˚ from the center, than the rest of the atoms with distances in the capping Ni atom is further out, at 2.83 A ˚ 42 the range 2.62–2.68 A. 2.6.5
Solution Studies by NMR
The first extensive NMR studies of deltahedral Zintl ions were carried out by Rudolph and coworkers.43 Using 117 Sn, 119 Sn, and 207 Pb NMR they were the first to show that the homo- and hetero-atomic nine-atom deltahedral clusters Sn9 , Pb9 , Sn9−x Gex , and Sn9−x Pbx are fluxional in solution. The homoatomic clusters exhibit single resonances while multiple signals were observed for the hetero-atomic species due to their different stoichiometries, with one signal per stoichiometry. In addition to this, all the clusters show single spin–spin couplings of the types 117 Sn 119 Sn, 117 Sn 207 Pb, and 119 Sn 207 Pb. These observations indicate that, on the NMR timescale, all atoms of the cluster are equivalent by dynamic and
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Ni Ni
Figure 2.6.12 The Ni-centered and Ni(CO)-capped cluster [Ni@(Sn9 Ni-CO)]3− , where the open pseudosquare face of the cluster is capped, as in capped empty clusters (Figures 2.6.5 and 2.6.8). This is the only centered and capped cluster with this geometry, all others have the triangular base of the tricapped trigonal prism capped, as in Figure 2.6.10
fast intra-molecular exchange at room temperature. The absence of a signal for Sn3− 9 can be understood in light of its unpaired electron and paramagnetic character, which will broaden and greatly shift the eventual resonance. Rudolph and coworkers were also the first to explore reactions of nine atom clusters with transitionmetal complexes by NMR, specifically reactions of Sn9 and Pb9 clusters with Pt(PPh3 )4 .44 The 119 Sn and 207 Pb NMR experiments again showed single resonances for the corresponding derivatives, in addition to signals from unreacted E9 clusters. According to the authors, the observed spin–spin couplings between the cluster and 195 Pt suggested a single platinum atom per cluster. Almost 20 years later the product of this reaction with tin clusters was structurally characterized as [Pt@(Sn9 Pt-PPh3 )]2− (discussed above, Figure 2.6.10).42 It was confirmed that in solution the tin atoms are in dynamic exchange while the Pt Pt PPh3 fragment stays rigid, almost like a molten tin drop on a stick. According to the 195 Pt NMR, spin–spin coupling between Sn and Pt occurs only with the central platinum atom and this is consistent with the earlier observations for only one Pt atom per cluster, based on 119 Sn NMR alone.42 Finally, two recent NMR studies of the W(CO)3 capped tin cluster [Sn9 W(CO)3 ]4− (discussed above, Figures 2.6.5 and 2.6.6) suggest different behavior for these species33,34 While Eichhorn et al. imply dynamic behavior and therefore equilibrium between clusters with differently positioned W(CO)3 , i.e. between η4 and η5 coordination,33 the report by Schrobilgen et al. claims that such an interpretation is misleading, due to poor resolution of the NMR spectra.34 The latter group presents both 117 Sn and 119 Sn NMR spectra that show three different environments for the tin atoms and match the theoretically calculated ones for a static cluster. Similar spectra were observed for the analogous tin clusters capped with Cr(CO)3 and Mo(CO)3 .34 The same authors, on the other hand, acknowledge possible dynamic behavior for the Mo(CO)3 -capped lead cluster [Pb9 Mo(CO)3 ]4− , and migration of the Mo(CO)3 from the η4 to the η5 position.34 This was later confirmed by the structural characterization of η5 -[Pb9 Mo(CO)3 ]4− made by reaction of lead clusters with Mo(CO)3 (MeCN).35 Obviously more studies are needed in order to resolve the dispute. One strong piece of evidence in support of the exchange theory is the structurally characterized η5 -[Sn9 W(CO)3 ]4− coexisting with η4 -[Sn9 W(CO)3 ]4− in the reaction product.33
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Concluding Remarks
The nine-atom deltahedral Zintl ions of group 14 are fascinating species in many respects: unusual bonding, aesthetically pleasing and yet flexible geometry, and very rich chemistry. The observed reactivity of the tin clusters with transition-metal compounds is similar to that of germanium clusters and this suggests that many more similarities may be expected between the two systems. The chemistry of the germanium clusters has been much more extensively studied at this stage.6a It has been shown that they participate in additional reactions such as oligo- and polymerization, functionalization with main-group organometallic fragments (SnR3 , GeR3 , SbR2 , BiR2 ) and with various organic groups, addition of alkenes by reactions with alkynes, etc. All this indicates that similar reactions with tin clusters may result in the corresponding functionalized tin analogs or, perhaps, even unexpected and potentially more interesting species. Acknowledgments I would like to thank my former and current coworkers Angel Ugrinov, Jose Goicoechea, Michael Hull, and Donald Chapman for their many discoveries in the chemistry of the Zintl ions. The financial support by the National Science Foundation (CHE-0446131 and CHE-0742365) is greatly appreciated. References 1. (a) Todorov, I. and Sevov, S. C., Inorg. Chem. 2004, 43, 6490; (b) Todorov, I. and Sevov, S. C., Inorg. Chem. 2005, 44, 5361. 2. Sch¨afer, H. and Eisnemann, B., Rev. Mat. Sci. 1985, 15, 1. 3. Todorov, I. and Sevov, S. C., Inorg. Chem. 2006, 45, 4478. 4. Todorov, I. and Sevov, S. C., Inorg. Chem. 2007, 46, 4044. 5. (a) Sevov, S. C. in Intermetallic Compounds Principles and Practice: Progress, Eds. J. H. Westbrook and R. L. Fleischer, John Wiley & Sons, Ltd., Chichester, England, 2002, pp. 113–132; (b) Chemistry, Structure, and Bonding of Zintl Phases and Ions, Ed. S. M. Kauzlarich, VCH Publishers, Inc., New York, NY, 1996; (c) Pottgen, R., Z. Naturforsch. 2006, 61, 677. 6. Recent reviews: (a) Sevov, S. C. and Goicoechea, J. M., Organometallics 2006, 25, 567;. (b) F¨assler, T. F., Coord. Chem. Rev. 2001, 215, 377; (c) Corbett, J. D., Angew. Chem. Int. Ed. 2000, 39, 670; (d) Corbett, J. D., Chem. Rev. 1985, 85, 383; (e) Corbett, J. D., Struct. Bonding 1997, 87, 157. 7. (a) Joannis, A., Hebd. Seances Acad. Sci. 1891, 113, 795; (b) Joannis, A., Hebd. Seances Acad. Sci. 1892, 113, 587; (c) Joannis, A., Ann. Chim. Phys 1906, 7, 75. 8. Smyth F. H., J. Am. Chem. Soc. 1917, 39, 1299. 9. (a) Kraus, C. A., J. Am. Chem. Soc. 1907, 29, 1571; (b) Kraus, C. A., J. Am. Chem. Soc. 1922, 44, 1216; (c) Kraus, C. A., Trans. Am. Electrochem. Soc. 1924, 45, 175; (d) Kraus, C. A., J. Am. Chem. Soc. 1925, 47, 43. 10. (a) Zintl, E., Goubeau, J.and Dullenkopf, W. Z. Phys. Chem., Abt. A 1931, 154, 1; (b) Zintl, E.and Harder, A. Z. Z. Phys. Chem., Abt. A 1931, 154, 47; (c) Zintl, E. and Dullenkopf, W., Z. Phys. Chem., Abt. B 1932, 16, 183; (d) Zintl, E. and Kaiser, H., Z. Anorg. Allg. Chem. 1933, 211, 113; (e) Zintl, E., Harder, A., and Neumayr, S., Z. Phys. Chem., Abt. A 1931, 154, 92. 11. Queneau, V. and Sevov, S. C., Angew. Chem. Int. Ed. Engl. 1997, 36, 1754. 12. (a) Goicoechea, J. M., Hull, M. W., and Sevov, S. C., J. Am. Chem. Soc., in press; (b) Xu, L., Ugrinov, A., and Sevov, S. C., J. Am. Chem. Soc. 2001, 123, 4091. 13. (a) Cisar, A. and Corbett, J. D. Inorg. Chem. 1977, 16, 2482; (b) Xu, L., Bobev, S., El-Bahraoui, J., and Sevov, S. C., J. Am. Chem. Soc. 2000, 122, 1838. 14. Kummer, D. and Diehl, L. Angew. Chem., Int. Ed. Engl. 1970, 9, 895. 15. (a) Corbett, J. D. and Edwards, P. A., J. Chem. Soc., Chem. Commun. 1975, 984; (b) Corbett, J. D. and Edwards, P. A., J. Am. Chem. Soc. 1977, 99, 3313. 16. Diehl, L., Khodadadeh, K., Kummer, D., and Str¨ahle, J., Chem. Ber. 1976, 109, 3404.
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17. Eichhorn B. W., Haushalter, R. C., and Pennington, W. T., J. Am. Chem. Soc. 1988, 110, 8704. 18. Belin, C. H. E., Corbett, J. D., and Cisar, A., J. Am. Chem. Soc. 1977, 99, 7163. 19. (a) F¨assler, T. F. and Hunziker, M., Inorg. Chem. 1994, 33, 5380; (b) Campbell, J., Dixon, D. A., Mercier, H. P. A., and Schrobilgen, G. J., Inorg. Chem. 1995, 34, 5798. 20. Goicoechea, J. M. and Sevov, S. C., J. Am. Chem. Soc. 2004, 126, 6860. 21. (a) Wade, K. J., Adv. Inorg. Chem. Radiochem. 1976, 18, 1; (b) Wade, K., J. Chem. Soc. D 1971, 792; (c) Mingos, D. M. P., Nat. Phys. Sci. 1972, 99, 236; (d) Mingos, D. M. P., Acc. Chem. Res., 1984, 17, 311. 22. (a) Critchlow, S. C. and Corbett, J. D., J. Am. Chem. Soc. 1983, 105, 5715; (b) F¨assler, T. F. and Hunziker, M., Z. Anorg. Allg. Chem. 1996, 622, 837. 23. Burns, R. C. and Corbett, J. D., Inorg. Chem. 1985, 24, 1489. 24. F¨assler, T. F. and Hoffmann, R., Angew. Chem. Int. Ed. 1999, 38, 543. 25. Hauptmann, R. and F¨assler, T. F., Z. Anorg. Allg. Chem. 2002, 628, 1500. 26. Hauptmann, R., Hoffmann, R., and F¨assler, T. F., Z. Anorg. Allg. Chem. 2001, 627, 2220. 27. Ugrinov, A. and Sevov, S. C., Appl. Organomet. Chem. 2003, 17, 373. 28. Korber, N. and Fleischmann, A., J. Chem. Soc., Dalton Trans. 2001, 383. 29. Goicoechea, J. M. and Sevov, S. C., Inorg. Chem. 2005, 44, 2654. 30. Ugrinov, A. and Sevov, S. C., Chem. Eur. J. 2004, 10, 3727. 31. (a) Edwards, P. A. and Corbett, J. D., Inorg. Chem. 1977, 16, 903; (b) Birchall, T., Burns, R. C., Devereux, L. A., and Schrobilgen, G. J., Inorg. Chem. 1985, 24, 890; (c) Campbell, J. and Schrobilgen, G. J. Inorg. Chem. 1997, 36, 4078; (d) Somer, M., Carrillo-Cabrera, W., Peters, E., Peters, K., Kaupp, M., and von Schnering, H. G., Z. Anorg. Allg. Chem. 1999, 625, 37. 32. Eichhorn, B. W. and Haushalter, R. C., J. Chem. Soc., Chem. Commun. 1990, 937. 33. Kesanli, B., Fettinger, J., and Eichhorn, B. W., Chem. Eur. J. 2001, 7, 5277. 34. Campbell, J., Mercier, H. P. A., Franke, H., Santry, D. P., Dixon, D. A., and Schrobilgen, G. J. Inorg. Chem. 2002, 41, 86. 35. Yong, L., Hoffmann, S. D., and F¨assler, T. F., Eur. J. Inorg. Chem. 2005, 3663. 36. Goicoechea, J. M. and Sevov, S. C., Organometallics, 2006, 25, 4530. 37. Goicoechea, J. M. and Sevov, S. C., J. Am. Chem. Soc. 2006, 128, 4155. 38. (a) Esenturk, E. N., Fettinger, J., Lam, Y.-F., and Eichhorn, B. W. Angew. Chem. Int. Ed. 2004, 43, 2132; (b) Esenturk, E. N., Fettinger, J., and Eichhorn, B. W., Chem. Commun. 2005, 247. 39. Goicoechea, J. M. and Sevov, S. C., Angew. Chem. Int. Ed. 2005, 44, 2. 40. Esenturk, E. N., Fettinger, J., and Eichhorn, B. W., J. Am. Chem. Soc. 2006, 128, 12. 41. (a) Gardner, D. R., Fettinger, J., and Eichhorn, B. W., Angew. Chem., Int. Ed. Engl. 1996, 35, 2852; (b) Esenturk, E. N., Fettinger J., and Eichhorn B. W., Polyhedron, 2006, 25, 521. 42. Kesanli, B., Fettinger, J., Gardner, D. R., and Eichhorn, B. W., J. Am. Chem. Soc. 2002, 124, 4779. 43. (a) Rudolph, R. W., Wilson, W. L., Parker, F., Taylor, R. C., and Young, D. C. J. Am. Chem. Soc. 1978, 100, 4629; (b) Rudolph, R. W., Taylor, R. C., and Young, D. C. J. Am. Chem. Soc. 1981, 103, 2480; (c) Wilson, W. L., Rudolph, R. W., Lohr, L. L., Taylor, R. C., and Pyykko, P., Inorg. Chem. 1986, 25, 1535. 44. Teixidor, F., Luetkens, M. L., Jr., and Rudolph, R. W., J. Am. Chem. Soc. 1983, 105, 149.
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2.7
Stable Stannylium Cations in Condensed Phases
Joseph B. Lambert Department of Chemistry, Northwestern University, Evanston, Illinois , USA
2.7.1
Introduction
Tricoordinate tin, until recently, has been unknown in condensed phases, due to its high electrophilicity. Thermodynamic stability in the gas phase testifies to the kinetic roots of its instability, particularly in the liquid phase. The ideal tricoordinate geometry is planar and trigonal, as viewed in 1 from above or below, and in 2 from the side on. R
R Sn
R
Sn
R R
R
1
2
It is expected that each R–Sn–R angle is very close to 120◦ , so that they must sum to 360◦ . The empty 4p orbital should have identical, large lobes above and below the plane defined by the tin atom and its three attached atoms. Such materials have been assigned the name stannylium ions by analogy with the 1993 IUPAC recommendations for carbylium (e.g., methylium), silylium, and germylium ions.1 These terms imply cationic trivalency and are to be distinguished from methanium, silanium, and stannanium, which imply cationic pentavalency.1 Convenient general terms, without implications of coordination number or geometry, are carbocation, silyl cation, germyl cation, and stannyl cation. As will become evident, current nomenclature does not provide accurate coordination or geometric information for some cationic organotin forms. The long and tortuous path to the preparation of stable tricoordinate silicon cations (silylium ions) in condensed phases2,3 presaged the creation of analogous germanium and tin species. In hindsight, four technical breakthroughs were required for the preparation of stable silylium ions, and consequently for stable stannylium ions: (1) solvents of low nucleophilicity, capable of dissolving ionic species; (2) anions of very low nucleophilicity; (3) substituents that stabilize positive charge by resonance (mesomerism), while also sterically hindering the approach of nucleophiles; and (4) novel methodologies for generation of the cation. Although numerous solvents have long been known that are capable of creating and dissolving organic ions, for the most part they are appreciably nucleophilic. Thus classic solvolysis experiments studied carbocations in water, alcohols, and carboxylic acids, which all proved entirely inappropriate for silylium and stannylium ions because of their nucleophilic (i.e., silaphilic or stannaphilic) properties. Various so-called ‘magic acid’ formulations proved equally ineffective, because of the presence of oxygen and halogen (particularly fluoro) functionalities with strong nucleophilicity. Dipolar, aprotic (more aptly, nonhydroxylic) solvents, such as acetonitrile and hexamethylphosphoramide (HMPA) invariably led to some kind of coordination between the solvent and the silicon or tin center. Early experiments with solvents of low nucleophilicity emphasized halogenated hydrocarbons, dichloromethane in particular, which have proved successful in some cases. Probably the most important breakthrough was the observation, reported in 1992, that aromatic solvents such as benzene dissolve stannyl cations.4 Although this observation was made by Lambert and Kuhlmann in the context of stannyl cations, it also was critical in the preparation of the first stable silylium ions in condensed phases. All successful preparations of stable, free stannylium
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ions have used aromatic solvents. These solvents, however, were not successful without careful choice of substituents on tin, as well as of the anion. Much research has been carried out in many contexts to create anions with low nucleophilicity.5 In the specific context of low valent silyl cations, the anion families that have proved most successful have been perfluoroaromatics introduced by Lambert and Zhang6 and carboranes introduced by Reed and coworkers,7 reported in adjacent publications in 1993. Prior to use of these anions, unsuccessful investigations were reported with anions such as perchlorate, tetrafluoroborate, tetraphenylborate, and hexachloroantimonate. All successful preparations of stannylium cations have used perfluoroaromatic anions, particularly tetrakis(pentafluorophenyl)borate (TPFPB). Coordination with solvent or anion can generate several different geometries. When a stannylium ion coordinates with a neutral solvent molecule S, either a tetracoordinate species (3, referred to as a solvated stannyl cation) or a pentacoordinate species (4) can form. Although these materials qualify as stannyl S
S
R R
Sn
R
Sn R
R R
S
3
4
cations, they are not stannylium ions, which require tricoordination. The neutral solvent S brings in additional coordination, but does not change the overall charge. An anion A− can produce similar species (5 and 6), but with altered charge. The tetracoordinate species 5 has its formal charge neutralized by the A Sn R
R
Sn
R R
5
A
A R R
R
Sn
S
A
6
7
R R
presence of the anion. The pentacoordinate species 6 contains one solvent molecule and one anion, so charge is also neutralized. Pentacoordination also can involve a pair of anions, leading to 7, which is anionic. None of these three species, therefore, is a stannyl cation, much less a stannylium cation. It is noted that the geometry around tin in 4, 6, and 7 is trigonal with respect to the organic ligands R, with some analogy to the stannylium ion 2, but with variable charge and higher coordination. The nature of the bonding between tin and either solvent S or anion A may be more complex than implied by structures 3–7, in which only formal charge is depicted. In the first place, full bonding may not occur between the tin atom and either the solvent S or the anion A (8–12). As a result, the Sn S or the Sn A bond order decreases, as the bond length increases. Secondly, more positive charge is present on tin in these five structures (and more negative charge on the anion), compared with the fully bonded analogs 3–7. Thirdly, in forms 8 and 9, the geometry around tin flattens with respect to that in 3 or 5. Thus, whereas the sum of the R Sn R angles may be ca. 328◦ (three tetrahedral angles) for 3 and 5, it occurs variably within the range >328◦ to <360◦ for 8 and 9. Such molecules have intermediate bonding and should be considered to have some stannylium cation character. Systems 10–12 have lengthened bonds to solvent or anion within the pentacoordinated geometry, and higher positive charge density on tin, than in 4, 6, and 7. These species (10–12) are not stannylium cations, because tin has higher coordination than
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S
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9
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R
A
R R
R
Sn
A
R R
R
Sn
S
S
A
10
11
12
R R
three. The geometry around tin, however, remains trigonal. There is no accepted nomenclature for these variants. 2.7.2
Pentacoordination (Trigonal Pyramids)
The trigonal bipyramids illustrated by 4, 6, and 7 (or 10–12) with solvent S or anion A− in the apical positions have been found often, particularly when at least one apical atom is highly electron withdrawing (O, N, F, Cl).8 Such pentacoordinate structures are not of central interest in the current context, but some examples are worth discussing. Two nuclear magnetic resonance criteria have been used extensively.9 Decreased coordination from the ideal tetragonal geometry of tetramethyltin generally moves the 119 Sn chemical shift to a larger positive value (higher frequency, lower field, deshielded), as expected for a pure, trigonal stannylium ion 1 or its partial version 8. Increased coordination, as in the various trigonal bipyramids, results in larger negative values (lower frequency even to negative values, higher field, shielded). A second important NMR criterion is the one bond 1 J (119 Sn–13 C) coupling constant, which responds to the geometry around tin. Flattening of the geometry to the extreme of sp2 hybridization, as expected either in a silylium ion 1 or in the trigonal bipyramids (4, 6, 7, 10–12), results in an increase in the coupling constant, with respect to tetrahedral models (3, 5, 8, 9).10 Thus for trigonal bipyramidal systems, we expect 119 Sn chemical shifts that move into the negative range and one-bond coupling constants with increased magnitude. One of the earliest claims for a free stannylium ion was the very simple system SnH+ 3 , formed by reaction of tin tetrahydride with fluorosulfonic acid at −78 ◦ C.11 The resulting ion exhibited a chemical shift of δ−186. The negative chemical shift, however, indicates higher, not lower, coordination. Cremer et al. modeled the system with coordination to one or two water molecules and concluded that pentacoordination was likely.12 Bagno et al. carried out more definitive calculations, not only at higher levels, relativistically, and with spin–orbit coupling, but also by modeling coordination with fluorosulfonic acid rather than water.13 They calculated the 119 Sn chemical shift for SnH3 (FSO3 H)+ 2 to be δ−142, in good agreement with the observed value. This early, proposed stannylium ion11 therefore proved to be pentacoordinate, presumably with a pair of apical fluorosulfonic acid molecules coordinated through oxygen, i.e., 4 or 10. In another early study, Edlund et al. were able to demonstrate the existence of an equilibrium between tetracoordinated (3 or 8) and pentacoordinated (4 or 10) stannyl cations.14 They prepared their materials
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by the method used by Lambert et al. in attempts to generate silylium cations, by reaction of tin hydrides with the trityl cation (R3 Sn—H + Ph3 C+ → Ph3 C—H + ‘R3 Sn+ ’). For tributyltin hydride, the chemical shift of the resulting species, with perchlorate as the anion, varied with solvent: δ 156 (CH2 Cl2 ), 54 (CH3 CN), and −43 (HMPA). With tetrafluoroborate as the anion, the respective figures were δ 220, 44, and −44. Although definitive structures could not be described, the trend clearly suggests that the species with the less coordinating solvent, dichloromethane, is tetracoordinate (3 or, more likely, 8). A second solvent molecule is definitely introduced with HMPA and is likely for acetonitrile (4 or 10). The chemical shifts for the materials formed from triphenyltin hydride were in the range δ−211 to −276 for both anions, with the lower negative end of this range for acetonitrile and the higher negative end for HMPA. These materials must be pentacoordinate. The authors also measured the 1 J (119 Sn–13 C) coupling constants for the product for tributyltin hydride with perchlorate as the anion. The values were 339 (CH2 Cl2 ), 424 (CH3 CN), and 491 (HMPA) Hz, indicating geometrical alterations towards the expected sp2 hybridization (3 → 4 or 8 →10) as the coupling constant increases. Another purported tin cation was produced by the reaction of CB11 Me12 r with Bu6 Sn2 .15 Although the authors referred to the product as Bu3 Sn+ CB11 Me− 12 , the X-ray structure revealed an infinite column of alternating anions and cations. A methyl group of the anion was present at the apical positions, above ˚ compared with a value of and below the plane of the tributyltin portion. The Sn–C distance is 2.81 A, ˚ 2.14 A for the normal Sn–C covalent bond length. These values suggest that the best representation of this species in the solid is 12. The 119 Sn chemical shift was observed to be δ 454, whereas the value expected for trialkylstannylium ions is closer to δ 1700, as discussed in the next two sections. This slightly deshielded value indicates residual bonding to the anion, again as in 12. This material thus resembles the pentacoordinate structure 12 in both the solid and solution. 2.7.3
Tetracoordination: Binding with Solvent or Anion
11 119 In the same study as claims for SnH+ Sn chemical 3 , Birchall and Manivannan also reported that the + shift for a purported SnMe3 ion was δ 322. This material was prepared by the reaction of trimethyltin hydride with fluorosulfonic acid. At the time (1985), this value could be considered unusually deshielded, so that this species clearly had at least some stannylium ion character. No further structural data, however, were available. In 1992, Lambert and Kuhlmann reported several different methods in attempts to generate stannylium ions.4 The most successful, and novel, was the reaction of tributyltin hydride with tris(pentafluorophenyl)borane, which abstracts the hydride to create an anion H(C6 F5 )3 B− with very low nucleophilicity. In a second approach to reducing the overall nucleophilicity of the environment, they used benzene for the solvent, the first example of the use of aromatic solvents in the silylium/germylium/stannylium context. The 119 Sn chemical shift for this tributyl system in benzene proved to be δ 360, the most deshielded value at the time. The perchlorate of the same substrate (prepared by hydride abstraction from tributyltin hydride with trityl perchlorate) exhibited a shift of δ 245 in CD2 Cl2 , 231 in benzene, and 150 in sulfolane. Trimethyltin hydride, when reacted with trityl perchlorate, gave a species with chemical shifts of δ 249 in CD2 Cl2 and δ 234 in benzene. The authors ambiguously called these species tricoordinated, as well as perturbed by solvent, which requires a fourth coordination. Shortly thereafter (1994), Kira, Sakurai, and coworkers reported a very similar reaction of tributhyltin hydride with trityl tetrakis[3,5-di(trifluoromethyl)phenyl]borate in CD2 Cl2 .16 The resulting species, which they called a trivalent tin cation, had a chemical shift of δ 356 at −20 ◦ C. The material decomposed above this temperature and reacted with ethyl ether at −70 ◦ C to give a more solvated species [‘Bu3 Sn(Et2 O)+ ’] with a chemical shift of δ 165 at −20 ◦ C.
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The difficulty in interpreting these results, at the time, arose from the lack of reliable points of comparison for the expected 119 Sn chemical shifts of stannylium ions. There was no doubt that values higher than 300 were deshielded to an unprecedented extent, but were these shifts sufficient to demonstrate tricoordination or trivalency? Computation in the early 1990s could not provide a reliable answer. Arshadi et al.10 bypassed the calculational problem in 1996 by publishing a remarkable empirical correlation between structurally analogous silicon and tin compounds (29 Si chemical shift vs 119 Sn chemical shift). Since reliable calculations were available for the 29 Si chemical shifts of trialkylsilylium ions, this plot could provide at least an indication of the expected 119 Sn chemical shifts of trialkylstannylium ions, which proved to be δ ca. 1700. The species observed by Birchall, Lambert, and Sakurai thus were very far from the expected chemical shift and hence from the ideal tricoordinate geometry of the stannylium ion. Since the values are deshielded to some extent, pentacoordination could be ruled out. The best description of the structures observed by all these authors, therefore, is the bond-stretched, solvent-coordinated stannyl cation 8. Lambert and Kuhlmann observed high conductivity, so that the neutral anion-coordinated variant 9 could be eliminated. Such structures (8) also apply to those reported in 1992 by Edlund et al.14 as the tetrahedral part of the equilibrium with pentacoordinate species. The most deshielded material within this category (8) was reported in 2002 by Piers and coworkers.17 They used a novel, highly fluorinated borate anion, the solvent benzene, and hydride abstraction with trityl to produce the species Bu3 Sn(benzene)+ with a 119 Sn chemical shift of δ 434. By this time, authors were referring to such species as solvated stannyl cations. The first X-ray structure of a species 8 was reported by Lambert et al. in 1995.18 Reaction of triethylstannane with trityl tetrakis(perfluorophenyl)borate (TPFPB) in benzene produced a material best represented as Et3 Sn(benzene)+ , which reacted with water to produce (Et3 Sn)2 OH+ TPFPB− . The Sn–O ˚ Whereas the Sn–C length is normal [as in (Ph3 Sn)2 O with 2.14 and Sn–C bond lengths were both 2.12 A. ˚ ˚ in (Ph3 Sn)2 O]. As calculated from A], the Sn–O bond clearly is lengthened [in comparison with 1.96 A ˚ the Pauling equation, the lengthening of 0.16 A corresponds to an Sn–O covalent bond order of only 0.54 (or ionic bond order of 0.46). The C–Sn–C angles sum to 352.8◦ , indicating a very flat tetrahedron, as in 8 rather than 3. This family of stannyl cations (8) has a flattened tetrahedral structure and somewhat deshielded 119 Sn chemical shifts (values up to δ 434). The bond between tin and the fourth coordination site is lengthened, and tin clearly has considerable positive charge density. Although one may speak of stannylium ion character, these materials, nonetheless, are not free, tricoordinate stannylium ions. 2.7.4
Tricoordination: NMR Evidence for Free Stannylium Ions
Lambert and Zhao reported preparation of a free, tricoordinate silylium ion in 1997, based on NMR chemical shift data.19 They used their technique of nucleophilic isolation provided by aromatic solvents and by the TPFPB anion. In addition, the substituents on silicon were bulked up by the use of 2,4,6trimethylphenyl (mesityl) to prevent access of even the weak solvent and anion nucleophiles to the silicon atom. Hydride abstraction had proved impossible with trimesitylsilane, because of steric hindrance to access of electrophiles. Consequently, they developed a novel method to generate the cation by using allyl as the leaving group instead of hydride. This procedure is illustrated in Equation (2.7.1) with tin instead of silicon. + Mes3 SnCH2 CH CH2 + E+ → Mes3 SnCH+ 2 CHCH2 E → Mes3 Sn + CH2 CHCH2 E
(2.7.1)
The allyl group extends outside the steric sphere imposed by the mesityl groups, so that an electrophile such as trityl has easy access to the double bond. The tetracoordinate tin intermediate shown in the equation is sterically relieved as it expels the neutral leaving group to produce the stannylium (or silylium
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or germylium) ion. The observed 29 Si chemical shift δ 225 of the silylium ion formed in this way agreed closely with that of calculations (δ 230).20 This very successful method was applied to tin, according to Equation (2.7.1), in 1999 by Lambert et al.21 Allyltributylstannane was treated with trityl TPFPB in benzene to produce a species with a 119 Sn chemical shift of only δ 263, the same value as when hydride was abstracted from tributyltin hydride. Such a species is clearly solvated, Bu3 Sn(benzene)+ (an example of 8), and possesses possibly 20% stannylium character. The butyl groups are insufficient to prohibit partial coordination with solvent. Reaction of the much bulkier allyltrimesitylstannane with electrophiles, however, produced a species with a 119 Sn chemical shift of δ 806. Such a value is much further deshielded than all previous examples. Moreover, the normal shielding effect of the aryl rings should offset some of the cationic deshielding and cause much lower values (more shielded) than for the trialkyl cases. Unfortunately, no 119 Sn chemical shift calculation was known at the time, so the authors only had recourse to the empirical correlation between silicon and tin chemical shifts.10 If this correlation is strictly linear, the expected 119 Sn chemical shift for trimesitylstannylium should be about δ 1100, as calculated from the observed 29 Si value for trimesitylsilylium of δ 225. Since this end of the plot could well be non-linear, the estimate of 1100 could be either too high or too low. The linear assumption indicated that the authors had achieved an ion with 70–80% stannylium character, but not a free stannylium ion. To test whether the chemical shift was being shielded by interaction with solvent, the authors altered the nucleophilicity of the solvent.21 They found the value of δ 806 to be invariant to changes to better donors such as toluene and p-xylene or to worse donors such as 1,2-dichlorobenzene. These results seemed to indicate little solvent interaction. They also succeeded in synthesizing allylphenyldi(2,4,6triisopropylphenyl)stannane,21 which on treatment with electrophiles produced a cation with a 119 Sn chemical shift of δ 697, more shielded than the trimesityl case. The presence of the single phenyl group, however, may permit better access of solvent, even with the two bulkier groups. In 2001, Lambert and Lin examined the duryl substituent in this context.22 It was thought that the second (meta) pair of methyl groups would provide steric buttressing to prevent the ortho methyl groups from bending away from the tin atom, thereby enhancing steric protection around tin in comparison with the effect of mesityl. The tridurylsilylium cation proved to have a nearly identical 29 Si chemical shift (δ 227) to that of trimesitylsilylium (δ 225), indicating similar silylium ion character. Treatment of allyltridurylstannane with electrophiles produced a cation with a 119 Sn chemical shift of only δ 720, more shielded than that of trimesitylstannylium (δ 806). This puzzling result seemed to indicate lower stannylium character. The NMR results alone were inconclusive. Clearly, highly deshielded chemical shifts were being observed, indicative of high stannylium ion character. In the absence of a good point of reference from reliable calculations, however, it was impossible to conclude whether these were entirely free (1) or partially solvated (8) stannylium ions. 2.7.5
Tricoordination: Crystallographic Evidence and Computational Confirmation of Free Stannylium Ions
Lambert and coworkers reported the first crystallographic evidence for a free stannylium ion in 2003.23 Their breakthrough was the synthesis of the highly crowded starting material allyltris(2,4,6triisopropylphenyl)stannane. The X-ray structure revealed a tricoordinate species of the type 1 without interaction between tin and either the solvent benzene or the anion TPFPB. No atom from the anion ˚ and no solvent was present. The sum of the three C Sn C angles was 359.9(2)◦ , was closer than 4 A, experimentally planar. The angle of twist of the aryl rings with respect to the plane was 61.1◦ , somewhat
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higher than the twist angle in trimesitylsilylium of 49.2◦ . The observed 119 Sn chemical shift in solution was δ 714, yet another value apparently short of the expected δ ca. 1100 from the empirical correlation.10 High-level calculations, however, were now possible. In the gas phase, the calculated value proved to be δ 763. The authors commented that, ‘Calculations at this level are systematically higher frequencies than are observation values.’ They concluded that the observed chemical shift was entirely in accord with a free, tricoordinate stannylium ion, unencumbered by any interaction with solvent or anion. The upper (deshielded) limit for triarylstannylium ions thus proved to be δ ca. 700–800 rather than δ ca. 1100. The 29 Si/119 Sn correlation10 must be non-linear at the high frequency end. Such a conclusion suggested that the previously observed trimesitylstannylium (δ 806), tridurylstannylium (δ 720), and phenyldi(2,4,6-triisopropylphenyl)stannylium (δ 697) ions were also entirely free. The later and even higher-level calculations of Bagno et al. confirmed this conclusion.13 They calculated a value of δ 840 for trimesitylstannylium at the scalar level and of δ 910 at the spin–orbit level. Both values are in good agreement with the observation (δ 806). In general, their calculations gave slightly more deshielded values than did those observed for all tin species. They presented a plot of calculations at the spin–orbit level vs. observation for the 199 Sn chemical shifts, which was remarkably linear for a wide range of tin structures. The value for the trimesitylstannylium ion was the deshielded extreme on this plot and fell very close to the linear fit. These calculations on molecules in the gas phase provide strong support that the observed trimesitylstannylium ion, for which no crystal structure is available, is indeed free and tricoordinate, as likely are tridurylstannylium22 and phenyldi(2,4,6-triisopropylphenyl)stannylium.21 In a subsequent study, Lambert, Herber, and their coworkers reported, by M¨ossbauer spectroscopy, that the motion of the tin atom within tris(2,4,6-triisopropylphenyl)stannylium tetrakis (pentafluorophenyl)borate is isotropic.24 Very soon after the above report of the first stable stannylium ion, Sekiguchi et al. reported the preparation of a stable stannylium ion by an entirely novel method of generation.25 They first prepared the stable free radical, tris(di-tert-butylmethylsilyl)stannyl [(t-Bu2 MeSi)3 Sn]. Treatment of this material with trityl TPFPB in benzene (thus using the same methods of nucleophilic isolation as previously developed) led to the stable tris(di-tert-butylmethylsilyl)stannylium ion. The sum of the three C–Sn–C angles was 360.0◦ , indicative of planarity and trigonality. Closest approach to atoms of the anion was greater than ˚ The 119 Sn chemical shift proved to be a very remarkable δ 2653 in dichloromethane. Deshielding 5 A. was enhanced by the silyl substituents (and by the absence of aryl substituents). The cation was modeled with trisilylstannylium ion [(H3 Si)3 Sn+ ], for which a chemical shift of δ 2841 was calculated, in good agreement, considering the usual overestimation associated with these calculations. 2.7.6
Summary
Several stable, tricoordinate aryl-substituted stannylium ions have been prepared by Lambert and coworkers,21−23 and a stable tricoordinate silyl-substituted stannylium ion has been prepared by Sekiguchi and coworkers.25 These materials were prepared respectively by the allyl-leaving group and radical oxidation methods. Both groups employed nucleophilic isolation by use of aromatic solvents and the TPFPB anion, and both groups used substituents that were both electronically stabilizing and sterically bulky to prohibit approach of solvent or anion. The cations were characterized by X-ray crystallography, NMR chemical shifts, and supporting calculations. Eleven years had elapsed since the breakthrough of aromatic solvents,4 which had been followed quickly by the development of perfluorinated anions6 and much later of the allyl leaving group method.19 Many years were required before all the pieces were in place, but there is no doubt now that tricoordinate stannylium ions may be prepared that are stable at room temperature in both liquid and solid phases.
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References 1. Paragraphs RC-82.2.2.1 and RC-82.1.1.1, A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), Blackwell Scientific Publications, Oxford, UK, 1993. 2. J. B. Lambert, L. Kania, and S. Zhang, Chem. Rev., 95, 1191 (1995). 3. J. B. Lambert, Y. Zhao, and S. M. Zhang, J. Phys. Org. Chem., 14, 370 (2001). 4. J. B. Lambert and B. Kuhlmann, J. Chem. Soc., Chem. Commun., 931 (1992). 5. S. H. Strauss, Chem. Rev., 93, 927 (1993). 6. J. B. Lambert and S. Zhang, J. Chem. Soc., Chem. Commun., 383 (1993). 7. Z. Xie, D. J. Liston, T. Jelinek, V. Mitro, R. Bau, and C. A. Reed, J. Chem. Soc., Chem. Commun., 384 (1993). 8. Davies, A. G. Organotin Chemistry, 2nd ed., Wiley-VCH, Weinheim, Germany, 2004. 9. B. Wrackmeyer, Annu. Rep. NMR Spectrosc., 38, 203 (1999). 10. M. Arshadi, D. Johnels, and U. Edlund, Chem. Commun., 1279 (1996). 11. T. Birchall and V. Manivannan, J. Chem. Soc., Dalton Trans., 2671 (1985). 12. D. Cremer, L. Olsson, R. Reichel, and E. Kraka, Isr. J. Chem., 33, 369 (1993). 13. A. Bagno, G. Casella, and G. Saiella, J. Chem. Theory Comp., 21, 37 (2006). 14. U. Edlund, M. Arshadi, and J. Johnela, J. Organomet. Chem., 456, 57 (1993). 15. I. Zharov, B. T. King, Z. Havlac, A. Pardi, and J. Michl, J. Am. Chem. Soc., 122, 10253 (2000). 16. M. Kira, T. Oyamada, and H. Sakurai, J. Organomet. Chem., 471, C4 (1994). 17. L. D. Henderson, W. E. Piers, G. J. Irvine, and R. McDonald, Organometallics, 21, 340 (2002). 18. J. B. Lambert, S. M. Ciro, and C. L. Stern, J. Organomet. Chem., 499, 49 (1995). 19. J. B. Lambert and Y. Zhao, Angew. Chem., Int. Ed. Engl., 36, 400 (1997). 20. T. M¨uller, Y. Zhao, and J. B. Lambert, Organometallics, 17, 278 (1998). 21. J. B. Lambert, Y. Zhao, H. Wu, W. C. Tse, and B. Kuhlmann, J. Am. Chem. Soc., 121, 5001 (1999). 22. J. B. Lambert and L. Lin, J. Org. Chem., 66, 8537 (2001). 23. J. B. Lambert, L. Lin, S. Keinan, and T. M¨uller, J. Am. Chem. Soc., 125, 6022 (2003). 24. J. B. Lambert, L. Lin, C. Nowik, and R. H. Herber, Inorg. Chem., 43, 405 (2004). 25. A. Sekiguchi, T. Fukawa, V. Y. Lee, and M. Nakamoto, J. Am. Chem. Soc., 125, 9250 (2003).
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2.8
Preparation and Coordination Chemistry of Mono- and Bidentate Benzannulated N-Heterocyclic Stannylenes Including Some Germanium and Lead Analogs
Alexander V. Zabula and F. Ekkehardt Hahn Institut f¨ur Anorganische und Analytische Chemie, Westf¨alische Wilhelms-Universit¨at, M¨unster, Germany
2.8.1
Introduction
Stannylenes and their lighter silicon and germanium, as well as the heavier lead analogs are singlet species of divalent silicon, germanium, tin, or lead, which possess an unshared electron pair in the ground state and the vacant p-orbital of the divalent group 14 elements. Particularly in the case of tin and germanium, they are key intermediates in numerous reactions and are most likely more significant for the chemistry of these elements than carbenes are in organic chemistry. Germylenes and stannylenes were first considered as heavy analogs of carbenes in 1962 by Volpin.1 Later, Nefedov developed and expanded this description.2 Although simple germylenes and stannylenes are reactive species that can be characterized by low-temperature spectroscopic techniques, or by the products of their reactions,3 the use of sterically demanding groups or/and substitution of germanium and tin atoms with heteroatoms (N, O, S) significantly stabilizes the reactive divalent state and a great number of germylenes and stannylenes, stable at room temperature, have been isolated over the last 30 years.4 Due to their electronic structure, these analogs of carbenes are capable of forming complexes with Lewis bases and Lewis acids (Scheme 2.8.1). Like carbenes, they are also prospective ligands in transition-metal complexes.
Scheme 2.8.1 Electronic structure of dialkyl- and diamino-substituted carbene analogs and their adducts with Lewis bases and Lewis acids
2.8.2
Stable N-Heterocyclic Stannylenes, Germylenes, and Plumbylenes
The longest known stable tin(II) organyl-bis(cyclopentadienyl)tin, or stannocene, was synthesized by Fischer and Grubert in 19565 by the reaction between CpLi and SnCl2 . The molecular structure determination showed the angle between the normals to the Cp rings to measure around 145◦5 which was taken as an indication of the presence of an unshared electron pair at the tin atom. However, a strong dependence of the (R5 )Cp–Sn–Cp(R5 ) angle on the R substituents (Me, Bz, Ph) at the cyclopentadienyl rings5c,6 with
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angles up to 180◦6b was later found. The first stable dialkyl7 and diaminostannylenes A7,8 (Scheme 2.8.2), with sterically demanding groups that provide stabilization of the tin atom, were synthesized by Lappert. N-heterocyclic stannylenes (NHSn) of type B were first reported by Veith.9 These cyclic stannylenes are monomeric or dimeric in solution and in the solid state, depending on the N -substituents R. Some years thereafter the cyclic diaminostannylenes C H (Scheme 2.8.2), which contain the tin atom as part of an aromatic system have been described. They are normally obtained from the corresponding amines by a transamination reaction using Sn[N(SiMe3 )2 ]2 , or by the reaction of the lithiated amines with SnCl2 (Scheme 2.8.2).
Scheme 2.8.2
Preparation and types of N-substituted diaminostannylenes
The first benzimidazolin-2-stannylenes of type C (R = SiMe3 , SiMe2 tBu, CH2 tBu) were synthesized and characterized in 1995.10,11 The presence of bulky N ,N -substituents led to an efficient shielding of the tin(II) center. A different type of benzannulated stannylene D with a pyrido-annulated five-membered ring was prepared by Heinecke et al.12 The diaminostannylene E, which is an analog of Arduengo’s carbene,
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Scheme 2.8.3
Dimerization of NHSns with sterically demanding N,N -substituents
was isolated recently.13 This shows remarkable chemical properties, such as the transfer of the tin atom from one stannylene to a diazadiene with formation of a new stannylene of type E. The benzannulated N ,P-substituted stannylene F was described recently14 and stannylenes derived from diaminonaphthalene G15 and cationic stannylenes H16 containing a 10 π -electron ring system are also known. The empty p-orbital in N-heterocyclic diaminostannylenes normally acts as a strong Lewis acid. This property influences the aggregation behavior of NHSns. Stannylenes of type C, with sterically demanding N ,N -substituents (R = CH2 tBu) form a dimeric aggregate in the solid state with contacts between the tin atom and the aromatic six-membered ring of an adjacent molecule. The N ,N -(SiMe3 )2 -substituted benzannulated NHSn dimerizes in the presence of tetramethylethylenediamine (TMEDA) via a donor– acceptor interaction of the nitrogen atoms of TMEDA with the empty p-orbitals at two tin(II) atoms10 (Scheme 2.8.3). A different aggregation behavior has been found for NHSns with sterically non-demanding or donorfunctionalized N ,N -substituents. The NHSn 1 with N ,N -(CH3 )2 -substituents dimerizes in the solid state via two strong inter-molecular Sn N interactions11 (Figure 2.8.1). Only weak intra-molecular Sn←O and no inter-molecular interactions were observed for the monomeric N ,N -ether-functionalized stannylene 211 . The stannylene with n-propylamine substituents 3 exhibits both intra-molecular and inter-molecular Sn←N interactions, thereby forming a bimolecular aggregate in the solid state.11 119 Sn NMR spectroscopy provides useful information about the Lewis acidic properties of benzannulated stannylenes. The 119 Sn chemical shifts for various benzannulated N-heterocyclic stannylenes fall in a broad range from −5–269 ppm11 depending on the N ,N -substituents and the solvent. It is obvious that the 119 Sn chemical shift reflects the amount of electron density at the tin atom and a high-field shift would indicate a gain in electron density at the tin center. The difference in the 119 Sn chemical shift recorded in non-polar and donor solvents is indicative of the donor strengths of the N ,N -functional groups. A significant difference in the 119 Sn chemical shift has been observed for stannylene 1 in benzene (222 ppm) and THF (107 ppm) (Scheme 2.8.4). Apparently, the dimer observed for 1 in the solid state (Figure 2.8.1) breaks up in solution, and the tin center is stabilized in THF solution by coordination of THF molecules, causing the observed high-field shift. This high-field shift is not observed in the 119 Sn NMR spectrum of 1 recorded in non-coordinating benzene. For the stannylenes with donor-functionalized N ,N substituents, the difference in the 119 Sn chemical shift in THF and benzene can serve as an indication of the strengths of intra-molecular donor-acceptor interaction. The difference in the 119 Sn chemical shift is larger in 2 compared to 3. This indicates a weak interaction of the oxygen donors with the tin center in benzene, which is replaced by a Sn· · ·OTHF interaction in THF solution (Scheme 2.8.4). Essentially
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Figure 2.8.1 Molecular structures of benzannulated N-heterocyclic stannylenes with sterically nondemanding or Lewis base functionalized N,N -substituents
no dependence of the chemical shift upon the type of solvent was observed for stannylene 3 which was taken as evidence for the persistence of the Sn· · ·NMe2 donor interaction, even in THF solution.11 Another indication for the Lewis acidic properties of NHSns is provided by the reaction of dibenzotetraazafulvalenes with stannylenes.17 Electron-rich enetetramines can be cleaved into N-heterocyclic carbenes.18 Pioneering work by Lappert has demonstrated that this cleavage, in the presence of an electrophilic metal center, leads to NHC complexes.18a,19 Hahn and coworkers have used the electrophilic tin center in the NHSn 4 to cleave the dibenzotetraazafulvalene 5 with the formation of the zwitterionic stannylene-carbene adduct 6 (Scheme 2.8.5).17 Similar carbene adducts of diaminogermylenes and diaminostannylenes have been described by Schumann et al. and Lappert et al.20 and even transient diorganogermylenes have been stabilized by formation of an adduct with an N-heterocyclic carbene.21 Cyclic and non-cyclic polydentate dialkyl- and diaminostannylenes have been described in the literature, 22,23 some of which can act as chelating ligands in transition-metal complexes.23a Veith et al.
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Scheme 2.8.4 Dependence of the 119 Sn resonance for 1–3 from the solvent (values recorded in C6 D6 and [D8 ]THF in parentheses)
have shown that polycyclic bis- or tris-stannylenes undergo a transformation upon heating to cubane-type tetra-stannylenes with an Sn4 N4 core.24 Such tin(II)-containing cubanes are potential synthons for iminostannylenes R N Sn.25 Benzannulated N-heterocyclic bis-stannylenes with a CH2 CMe2 CH2 bridge and sterically demanding or donor-functionalized N -substituents, such as CH2 tBu or CH2 CH2 CH2 NMe2 have been described recently.26 The bis-stannylene with the donor-functionalized N -substituent 7 shows both an intra-molecular interaction of the amino group with the vacant p-orbital at each of the tin atoms, and an inter-molecular interaction between this orbital and the π-system of the benzene ring of an adjacent molecule. These interactions lead to a polymeric arrangement of the bis-stannylenes in the crystal lattice (Figure 2.8.2). Germanium compounds analogous to the N-heterocyclic stannylenes A H (Scheme 2.8.2) are known. The first stable diorgano- and diaminogermylenes, Ge[C(SiMe3 )3 ]2 and Ge[N(SiMe3 )2 ]2 , apart
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Scheme 2.8.5 Cleavage of the dibenzotetraazafulvalene 5 by NHSn 4 with formation of the zwitterionic stannylene-carbene adduct 6
from some germanocenes,5c,27 have been described by Lappert.7 The reactive germanium atom in the diaminogermylenes is not only stabilized by the steric bulk of the substituents at the GeII atom, but also by electron-donation from the trigonal-planar nitrogen atoms to the vacant p-orbital of the germanium atom (push-pull effect). The subsequent report9 of the first heterocyclic germylene (NHGe) I (Scheme 2.8.6) predates Arduengo’s description of the first N-heterocyclic carbene28 by almost 10 years. Later, the first stable NHGes derived from saturated imidazoles J were reported.29 The molecular structure of the first benzannulated NHGe K, with a germanium atom as a part of an aromatic system, was described in 1989,30
Figure 2.8.2 Molecular structure of the donor-functionalized bis-stannylene 7 (top) and arrangement of the molecules in the crystal lattice (bottom)
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Scheme 2.8.6
Selected N-heterocyclic germylenes
again some 10 years before its carbene analog was prepared.31 The NHGe analog of Arduengo’s carbene L was first described in 1992.32 Based on spectroscopic data and quantum-chemical calculations, it was estimated that a cyclic delocalization of the six π -electrons occurs in the planar heterocycle of L.33 It appears that aromaticity is much more pronounced in NHGes of type L than in the related N-heterocyclic silylenes. Pyrido-,12 naphtho-,34 and tropo-annulated35 N-heterocyclic germylenes have been described recently. Even cationic36 and ring-functionalized NHGes exhibiting betaine reactivity37 are known. Some bis- and polygermylenes, none of which can act as a chelating ligand, have been described.22,38 Recently novel N-heterocyclic bis-germylenes of type M, which are potentially bidentate ligands, have been prepared. In these derivatives two benzannulated germylene rings are linked together by a bridge between two ring-nitrogen atoms.39 The arrangement of the bis-germylenes in the solid state strongly depends on the type of bridge and the steric demand of the substituents at the nitrogen atoms of the heterocycle. The bis-germylene with an n-butylene bridge exists as monomer 8 (Figure 2.8.3), when the second nitrogen atom of the heterocycle is substituted with the sterically demanding neopentyl group.39 Replacement of the neopentyl group with the less bulky ethyl group leads to the bimolecular aggregate 9 exhibiting weak intermolecular Ge· · ·N interactions (Figure 2.8.3). The bis-germylene with an 1,2-(CH2 )2 (C6 H4 ) linker 10 exists in a polymeric arrangement in the crystal lattice with interactions between the empty p-orbital of the germanium atom and the aromatic system of the benzene ring of a parallel-oriented adjacent molecule (Figure 2.8.3). The bis-germylene 11 with the lutidine bridging unit exhibits a potential pincer topology in the solid state and can serve as a Ge N Ge pincer ligand.40 This bisgermylene shows an interesting intramolecular Ge· · ·Ge interaction in the solid state (Figure 2.8.3). The first N-heterocyclic benzannulated plumbylenes 12 (N ,N -substituents CH2 tBu) and 13 (N ,N substituents iBu) have recently been described. Depending on the steric bulk of the N ,N -substituents, they form two different bimolecular aggregates in the solid state (Figure 2.8.4).41 2.8.3
Complexes of Bidentate N-Heterocyclic Germylenes and Stannylenes
Complexes of germylenes and stannylenes with transition metals have attracted considerable attention, and this chemistry has been reviewed.42 Homoleptic complexes of three-coordinated platinum or palladium with Ge[N(SiMe3 )2 ]2 or Sn[N(SiMe3 )2 ]2 have been prepared by Lappert.43 They react with carbon monoxide to give the tri-nuclear clusters [M(Ge, Sn)[N(SiMe3 )2 ]2 )(CO)3 ] (M = Pt, Pd).44 Heteroleptic complexes of three-coordinated Pd with the Ge[N(SiMe3 )2 ]2 ligand and different phosphines are able to reversibly bind H2 and CO2 .45 Even an example of intramolecular C H activation has been described for the iridium(I) complex with the Ge[N(SiMe3 )2 ]2 ligand.46 The N-heterocyclic germylenes and stannylenes of type Me2 Si(NtBu)2 E (E = Ge, Sn) form complexes [M(Me2 Si(NtBu)2 E)4 (μ-X)2 ] (M = Ni, X = Br; M = Pd, Pt, X = Cl) starting from the metal halides.
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Figure 2.8.3
Figure 2.8.4
Arrangement of bis-germylenes of type M in the solid state
Molecular structures of the N-heterocyclic plumbylenes 12 and 13
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In addition, the germylene Ge[N(SiMe3 )2 ]2 is capable of replacing all the triphenylphosphine ligands in [RhCl(PPh3 )3 ] to give a rhodium(I) tetra-germanium compound (two GeII and two GeIV ).47 The dehydrohalogenating properties of dialkylgermylenes were used for the transformation of a carbene ligand into a carbyne at a ruthenium metal center.48 A naphthannulated germylene reacts with [Ni(cod)2 ] to give the homoleptic complex [Ni(NHGe)4 ]34a while the sterically more demanding NHGe J (R = tBu) in the same reaction gave the three-coordinated complex [Ni(J)3 ].32 Some adducts of stannylenes with Lewis acids, such as boron compounds49 and AlCl350 are known. More common are adducts of stannylenes with Lewis bases, such as 1,4-dioxane51 or phosphines. Stannylenes form adducts with carbenes (Scheme 2.8.5) and with carbene analogs, such as gallium(I) derivatives52 or even other stannylenes.53 In the latter adducts between two tin(II) atoms, one stannylene acts as a Lewis base and the other one as a Lewis acid. Stannylenes as ligands in transition-metal complexes act as Lewis bases. In many cases the empty porbital at the coordinated stannylene center accepts electron density from donor ligands.42 The homoleptic platinum(0) complex of diaminostannylene Sn[N(SiMe3 )2 ]2 A was obtained by substitution of the cod ligand in [Pt(cod)2 ].43,54 The reaction of Sn[N(SiMe3 )2 ]2 with [PtCl(μ-Cl)(PEt3 )]2 gave insertion of the stannylene in the Pt Cl bond, with formation of a tin(IV) complex.54 The stannylene analog of nickel tetracarbonyl described by Veith et al. exhibits a tetrahedral nickel(0) atom in the solid state, and NMR investigations show that the coordination number four is preserved in solution.55 Heteroleptic complexes of the unsaturated NHGe of type L (R = CH2 tBu) with the tetracarbonyl or tricarbonyl molybdenum complex fragments have been described.56 Upon complex formation, a shortening of Ge N bond distances within the coordinated germylene was observed. A similar shortening of the Ge N bond lengths occurs in the {Mo(CO)4 } complex 14 with a benzannulated bis-germylene (Figure 2.8.5)39 relative to the free bis-germylenes of type M. In addition, the N Ge N angle in the coordinated NHGe is larger than in the free ligand. These observations are in accordance with theoretical predictions.57 In the molybdenum complex 14, both germylene donor groups are coordinated in cis-positions, an arrangement identical to that observed for the analogous bis-carbene complex.18a Upon cooling, the protons of methylene groups in 14 become diastereotopic due to the restricted movement of the germylene ligands. At 240 K four well-resolved doublets are observed in the 1 H NMR spectrum of 14 (Figure 2.8.5).
Figure 2.8.5
Molecular structure of 14 and its variable temperature 1 H NMR spectra
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Scheme 2.8.7
169
Preparation of the homoleptic complexes 15–18 with bis-stannylene ligands
Bis-stannylenes with different bridging groups react with M0 precursors ([Ni(cod)2 ] or [Pt(nbe)3 ]) to give the homoleptic tetrakis-stannylene complexes 15–18 (Scheme 2.8.7).58 The molecular structures of 15 and 16 are depicted in Figure 2.8.6 together with the 119 Sn NMR spectra of the complexes recorded in [D8 ]THF and [D8 ]toluene. The 119 Sn chemical shift of 15 clearly depends on the solvent used for recording the NMR spectrum, with the 119 Sn resonance shifted upfield in [D8 ]THF. Only one resonance was detected for 15, both in [D8 ]THF and [D8 ]toluene. These observations are consistent with the presence of four identical tin(II) atoms in 15 with the upfield shift in [D8 ]THF caused by coordination of the solvent into the empty p-orbital at the tin atom (see also Scheme 2.8.4). The structure analysis (Figure 2.8.6, left) confirms the presence of four identical tin centers in 15, all of which are easily accessible for interactions with solvent molecules. 119 Sn NMR spectra of complex 16 exhibit two resonances in [D8 ]THF and [D8 ]toluene, one of which is significantly shifted upfield in [D8 ]THF. The molecular structure of 16 (Figure 2.8.6, right) explains this observation. Complex 16 possesses two different types of tin(II) atoms. One type (Sn1, Sn3) is sterically protected by the phenylene bridge between two stannylene units, the other type (Sn2, Sn4) is much less shielded. In non-coordinating [D8 ]toluene, two almost identical 119 Sn resonances around 450 ppm are observed. In [D8 ]THF the sterically unprotected tin atoms (Sn2 and Sn4) are coordinated by solvent molecules causing the upfield shift to 390 ppm while the other two tin atoms (Sn1 and Sn3) are inaccessible for solvent coordination. The X-ray diffraction analysis shows the nickel(0) atoms in 15 and 16 coordinated in a slightly distorted tetrahedral fashion by four stannylene donors. The Ni Sn distances fall in the normal ˚ for both complexes. range of 2.3654(15)–2.3827(14) A The platinum(0) complexes 17 and 18 also contain a tetrahedrally coordinated transition metal center. The molecular structures of 17 and 18 (Figure 2.8.7) are similar to those found for the analogous complexes 15 and 16. The sterically unprotected stannylene tin atoms in 17 are coordinated by THF molecules in the solid state (these THF molecules are not shown in Figure 2.8.7).
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Figure 2.8.6 Molecular structures of the homoleptic complexes 15 (left) and 16 (right) and their 119 Sn NMR spectra in [D8 ]THF and [D8 ]toluene
Figure 2.8.7 Molecular structures of the homoleptic platinum (0) complexes of bis-stannylenes 17 (left, coordinated THF molecules are not shown for clarity) and 18 (right)
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When [Pt(PPh3 )4 ] was reacted in the presence of O PPh3 with a bis-stannylene, a heteroleptic tetrahedral platinum(0) complex 19 with a P2 Sn2 donor set was obtained (Scheme 2.8.8). One of the two stannylene donors in 19 is coordinated by the oxygen atom of a molecule of O PPh3 (Scheme 2.8.8), again underlining the Lewis acidity of the p-orbital at the tin atom. In spite of coordination of O PPh3 to one tin atom only, both Pt Sn bond lengths are almost equidistant and only one resonance was observed ˚ compared to related in the 119 Sn NMR spectrum. The Sn1 O1 bond distance is rather short (2.270(4) A) 51 ˚ ether-adducts of stannylenes (2.527(5) A ). The nature of the M EII bond (E = C, Si, Ge, Sn) in transition-metal complexes is still the subject of discussion.57 It is believed that stannylenes are weaker π -acceptors than carbenes. In general, the σ contribution to the M E bond is believed to be larger than the π contribution, although the latter should not be neglected.57 We have compared the bond parameters in the bis-stannylene complex 20 (Figure 2.8.8, left) with those measured for the analogous bis-carbene complex 2118a and the bis-germylene complex 1439 (Figure 2.8.5). The bond parameters for the molybdenum complexes with N ,N -alkylsubstituted bis-carbenes, bis-germylenes and bis-stannylenes (Table 2.8.1) were also compared to those of the N donor-functionalized bis-stannylene 7 (Figure 2.8.2) and its molybdenum complex 2226 (Figure 2.8.8, right). The X-ray data for the bis-carbene complex 21, the bis-germylene complex 14, and the bis-stannylene analog 20 show the expected elongation for the Mo E distances (E = C: 2.324(3); E = Ge: 2.5204(6), ˚ The Mo−E distance increases by about 0.2 A ˚ from E = C 2.5189(6); E = Sn: 2.6850(3), 2.6736(4) A). ˚ → E = Ge and by another 0.15 A from E = Ge → E = Sn (Table 2.8.1). A different trend was observed ˚ for E = C → E = Ge and for the intra-heterocycle E N bond lengths. These elongated by about 0.45 A ˚ by 0.2 A for E = Ge → E = Sn. The intra-heterocycle E N bond lengths increased much more than the Mo E bond lengths.26 This could indicate that the heavier analogs of benzannulated carbenes do not act exclusively as σ -donors, but, owing to the Lewis acidity of the empty p-orbital of the stannylenes, also
Scheme 2.8.8 Preparation and molecular structure of complex 19. Phenyl groups at the phosphorus atoms have been omitted for clarity
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Table 2.8.1
Bond parameters and vibrational data for complexes 14, 20–22, and bis-stannylene 7
N
N N
E
O C
E
N
Mo C O
C O
C O
2118a (E = Ccarbene )
1439 (E = Ge)
2026 (E = Sn)
2226
r (Mo–E), A˚
2.324(3)
r (E–N), A˚ r (Sn–NMe2 ), A˚
1.385(4) 1.367(4) –
2.5204(6) 2.5189(6) 1.823(3)– 1.827(3) –
2.6850(3) 2.6736(4) 2.0182(2)– 2.0398(2) –
2.7046(6) – 2.033(2), 2.0586(2) 2.390(2)
(E–Mo–E), ◦ (N–E–N), ◦
95.77(14) 103.3(2) 1889a 2002a 1857a 1799a
89.734(12) 81.48(7) 81.25(7) 1947b 2038b 1929b –
86.87(2) 80.49(7)
ν(CO), A1 , cm−1 ν(CO), A1 , cm−1 ν(CO), B1 , cm−1 ν(CO), B2 , cm−1
90.691(2) 87.93(14), 87.73(14) 1950b 2029b 1925b 1908a
a b
1928 2008 – –
726
2.063(3)– 2.090(3) 2.561(4) 2.530(4) – 78.56(13) 78.57(12) – – – –
Infrared data Raman data (λexc = 632.8 nm)
function as π-acceptor ligands. The π -acceptor capability appears to increase in turn from carbenes, to germylenes, and to stannylenes. This assumption is corroborated by an inspection of the ν(CO) stretching modes in 21, 14, and 20. The wave numbers for the trans-CO-stretching modes increase, starting from the bis-carbene complex to the bis-germylene complex to reach a maximum for the bis-stannylene complex. This indicates reduced back-bonding to the CO ligands, which can be caused by enhanced back-bonding to the germylene and stannylene ligand or, less likely, is caused by their reduced σ -donor capacity. An evaluation of the π-acceptor capability of NHGe relative to phosphites has recently been published.59 An interesting observation was made with complex 22, containing the N -donor-functionalized bisstannylene ligand. Due to interaction of tin atom with the N -propylamine-donor (N3, Figure 2.8.8) stabilizing back-bonding from the molybdenum atom is no longer required, resulting in an expansion of ˚ relative to 20 (2.6850(3), 2.6736(4) A). ˚ Stabilization of the p-orbital the Mo Sn bond in 22 (2.7046(6) A) og the tin atom by the Me2 N donor group also leads to a reduction in the intra-heterocycle N1 Sn N2 delocalization and concurrent elongation of the intra-ring Sn N bond lengths in 22, relative to 20 (Figure 2.8.8, Table 2.8.1). The lutidine-bridged bis-stannylene 23 can act as a trapping agent for Sn O and Pb O generated in situ.60 Hydrolysis of E[N(SiMe3 )2 ]2 (E = Sn, Pb) leads to tin(II) and lead(II) monoxides which were trapped by the bis-stannylene 23 (Scheme 2.8.9). In both cases the bis-stannylene acts as a pentadentate
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Figure 2.8.8 Molecular structures of the molybdenum complexes with the N-alkyl-substituted bisstannylene 20 (right) and the N-donor-substituted bis-stannylene 22 (right)
pincer-ligand towards Pb O and Sn O. The inter-atomic distances Sn1(Sn2) O1 and Sn3(Pb) N in 24 and 25 (Scheme 2.8.9) fall in the range between donor-acceptor and covalent bond lengths; however, the choice between covalent and coordination bonds, in the case of complexes 24 and 25, seems somewhat arbitrary. The 119 Sn M¨ossbauer spectrum of complex 24 shows two absorptions for two different divalent tin atoms.
Scheme 2.8.9
Preparation and molecular structures of the complexes 24 and 25
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23. (a) M. Henn, M. Sch¨urmann, B. Mahieu, P. Zanello, A. Cinquantini, and K. Jurkschat, J. Organomet. Chem., 691, 1560 (2006); (b) H. Braunschweig, C. Drost, P. B. Hitchcock, M. F. Lappert, and L. J.-M. Pierssens, Angew. Chem., 109, 285 (1997); Angew. Chem., Int. Ed. Engl., 36, 261 (1997). 24. (a) M. Veith, M.-L. Sommer, and D. J¨ager, Chem. Ber., 112, 2581 (1979); (b) M. Veith and H. Lange, Angew. Chem., 92, 408 (1980); Angew.
