Tin Chemistry: Fundamentals, Frontiers and Applications

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OTE/SPH JWBK199-FM

OTE/SPH JWBK199-Gielen

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Printer: Yet to come

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 François 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 Höpfl 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 Mónica 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 Farfán 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

413 413

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. Beltrán, Rosa Santillan and Norberto Farfán 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 Chrétien, Jeremy D. Kilburn, Françoise 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

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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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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. Beltrán Departamento de Ciencias Naturales, División de Ciencias Naturales e Ingenier´ıa, Universidad Autónoma 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 Autónoma 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 Chrétien Laboratoire de Synthèse Organique (LSO), UFR des Sciences et des Techniques, Université de Nantes, France Paul A. Cusack Tin Technology, St. Albans, UK

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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 Inorgánica, Facultad de Ciencias, Universidad de Valladolid,

Norberto Farfán Departamento de Qu´ımica, Facultad de Qu´ımica, Universidad Nacional Autónoma de Mexico, Mexico Miroslav Genov IU CINQUIMA/Qu´ımica Inorgánica, Facultad de Ciencias, Universidad de Valladolid, Spain Kandasamy Gopal


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 für Anorganische und Analytische Chemie, Westfälische WilhelmsUniversität Münster, Germany Ionel Haiduc

Department of Chemistry, Babes-Bolyai University, Cluj-Napoca, Romania

Edward T. Hessell

King Industries Inc. Norwalk, CT, USA

Herbert Höpfl Centro de Investigaciones Qu´ımicas, Universidad Autónoma del Estado de Morelos, Mexico Vojtech Jancik Instituto de Qu´ımica, Universidad Nacional Autónoma de Mexico, Mexico Bernard Jousseaume Klaus Jurkschat

Groupe Matériaux, Université Bordeaux 1, Talence, France

Lehrstuhl für Anorganische Chemie der Universität, Dortmund, Germany

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Jeremy D. Kilburn Cezary M. Kinart

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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 Synthèse Organique (LSO), UFR des Sciences et des Techniques, Université 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 Mónica Moya-Cabrera Mexico

Instituto de Qu´ımica, Universidad Nacional Autónoma 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 Synthèse Organique (LSO), UFR des Sciences et des Techniques, Université de Nantes, France Fran¸cois RiBOT

Chimie de la Matière Condensée de Paris, Université Pierre et Marie Curie, Paris

Rosa Santillan Departamento de Qu´ımica, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Mexico.

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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


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 Moléculaires, Groupe Matériaux, University of Bordeaux 1, Institut des Sciences Moléculaires, 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, Universität Bayreuth, Germany

David Young Eskitis Institute of Cell and Molecular Therapies, Griffith University, Queensland, Australia Alexander V. Zabula Institut für Anorganische und Analytische Chemie, Westfälische WilhelmsUniversität Münster, Germany Fran¸coise Zammattio Laboratoire de Synthèse Organique (LSO), UFR des Sciences et des Techniques, Université de Nantes, France Julio Zukerman-Schpector

Department of Chemistry, Universidade Federal de Sáo 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 Mössbauer 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 Mössbauer 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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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


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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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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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. Fässler, 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. Fässler, Coord. Chem. Rev. 215, 347 (2001). 5. N. Wiberg and P. Power, in Molecular Clusters of the Main Group Elements, M. Driess and H. Nöth (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. Bähr 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, Universität 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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NMR of Tin Compounds

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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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21

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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23

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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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 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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41

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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Tin Chemistry: Fundamentals, Frontiers and Applications 1

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.

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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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NMR of Tin Compounds 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199. 200. 201. 202. 203. 204. 205. 206. 207. 208. 209. 210. 211. 212. 213. 214. 215. 216.

51

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. Hübner, 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. Schürmann, and F. Uhlig, Z. Anorg. Allg. Chem. 627, 453 (2001); (b) U. Hermann, M. Schürmann, and F. Uhlig, J. Organomet. Chem. 585, 211 (1999). A. Dörr, D. Gudat, D. Hänssgen, 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. Köhricht, 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. Mügge, J. Piret Meunier, M. van Meerssche, G. van Binst, C. Wynants, M. Gielen, and R. Willem, Organometallics, 7, 593 (1988). W. Biffar, H. Nöth, H. Pommerening, R. Schwerthöffer, 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. Gädt, F. M. Schappacher, R. Pöttgen, 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. Möhlen, 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. Brüggeller, 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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2.2

53

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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Scheme 2.2.6

57

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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Tin(II) Clusters

63

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. Jäger, 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. Sáez, 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. Opsölder, 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. Sáez, 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. Sáez, 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. Galán, 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. Nötzel, 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. Nöth (Eds), Wiley-VCH, Weinheim, 2004, Chapter 3.6. 42. M. Driess, S. Martin, K. Merz, V. Pintchouk, H. Pritzkow, H. Grützmacher, 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. Nöth, J. Organomet. Chem., 614–615, 70 (2000). 44. M. Westerhausen, M. Krofta, N. Wiberg, J. Knozek, H. Nöth, 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

69


François Ribot Chimie de la Matière Condensée de Paris, Université 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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71

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.

20:38

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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Table 2.3.2


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.

20:38

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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Table 2.3.3


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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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83

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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85

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, Université 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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79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89.

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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


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


n-Bu, n-Pr

Ref.

Entry

R

38

20.

n-Bu

Ref.

SH

42

N

Fe

16.


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


n-Bu

OH CH3 H3C

Ref.

Entry

R

50

35.

n-Bu


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

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


Me and Cl

Ref. Entry 66

N


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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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

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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


PhCH2

N

Ref.

Entry

86

26.

PhCH2

91

27.

n-Bu

TTP-H2b

95

R


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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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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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.

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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.

115

Peng B., Sun L.-J., Chang W.-X., and Xie Q.-L., Chin. J. Org. Chem. 2003; 23: 344. Parvez M., Sadiq-ur-Rehman, Shahid K., Ali S., and Mazhar M., Acta Crystallogr., 2004; E60: m1465. Dokorou V., Demertzis M. A., Jasinski J. P., and Kovala-Demertzi D., J. Organomet. Chem. 2004; 689: 317. Wen Z.-K., Song H.-B., Du M., Zhai Y.-P., and Tang L.-F., Appl. Organometal. Chem. 2005; 19: 1055. Zhang R., Sun J., and Ma C.-L., Inorg. Chim. Acta 2004; 357: 4322. Li F.-H., Yin H.-D., Gao Z.-J., and Wang D.-Q., Acta Crystallogr., 2006; E62: m788. Yin H. D., Wang C., and Ma C.-L. Indian J. Chem., Sect.B. 2004; 43: 2623. Yin H.-D., Xue S.-C., and Wang Q.-B. Chin. J. Inorg. Chem. 2004; 20: 421. Ng S. W., Wei C., and Das V. G. K., J. Organomet. Chem. 1991; 412: 39. Zhang R., Sun J., and Ma C.-L., J. Organomet. Chem. 2005; 690: 4366. Galani A., Kovala-Demertzi D., Kourkoumelis N., Koutsodimou A., Dokorou V., Ciunik Z., Russo U., and Demertzis M. A., Polyhedron 2004; 23: 2021. Yin H., Ma C.-L., Wang Y., and Zhang R., Indian J. Chem., Sect.B. 2003; 42: 889. Baul T. S. B., Masharing C., Basu S., Rivarola E., Holcapek M., Jirasko R., Lycka A., de Vos D., and Linden A., J. Organomet. Chem. 2006; 691: 952. Ahmad A., Khan A., Ali S., and Parvez M., Acta Crystallogr., 2006; E62: m1167. Amini M. M., Abadi S. H., Mirzaee M., Yang S.-Y., and Ng S. W., Acta Crystallogr., 2003; E59: m876. Yin H.-D., Wang Q.-B., and Xue S.-C., J. Organomet. Chem. 2005; 690: 435. Chandrasekhar V., Gopal K., Nagendran S., Singh P., Steiner A., Zacchini S., and Bickley J. F., Chem.-Eur. J. 2005; 11: 5437. Wang Y., Yin H.-D., Xue S.-C., and Wang Q.-B., Chin. J. Inorg. Chem. 2006; 22: 656. Chandrasekhar V., Thilagar P., Steiner A., and Bickley J. F., Chem.-Eur. J. 2006; 12: 8847. Beckmann J., Dakternieks D., Kuan F. S., and Tiekink E. R. T., J. Organomet. Chem. 2002; 659: 73. Tao J., Xiao W., and Yang Q., J. Organomet. Chem. 1997; 531: 223. Dokorou V., Ciunik Z., Russo U., and Kovala-Demertzi D., J. Organomet. Chem. 2001; 630: 205. Vatsa C., Jain V. K., Kesavadas T., and Tiekink E. R. T., J. Organomet. Chem. 1991; 408: 157. Alcock N. W. and Roe S. M., J. Chem. Soc., Dalton Trans. 1989; 1589. Yin H.D., Li G., Gao Z.-J., and Xu H.-L., J. Organomet. Chem. 2006; 691: 1235. Zheng G.-L., Ma J.-F., Yang J., Li Y.-Y., and Hao X-R. Chem.-Eur. J. 2004; 10: 3761. Mokal V. B., Jain V. K., and Tiekink E. R. T., J. Organomet. Chem. 1992; 431: 283. Chandrasekhar V., Nagendran S., Gopal K., Steiner A., and Zacchini S., Chem. Commun. 2003; 862. Ma C.-L., Han Y., Zhang R., and Wang D., Dalton Trans. 2004; 1832. Beltran H. I., Zamudio-Rivera L. S., Mancilla T., Santillan R., and Farfan N., Chem.-Eur.J. 2003; 9: 2291. Garcia-Zarracino R. and Höpfl H., Angew. Chem., Int. Ed. 2004; 43:1507. Ma C.-L., Jiang Q., and Zhang R., J. Organomet. Chem. 2003; 678: 148. Garcia-Zarracino R., Ramos-Quinones J., and Höpfl H., Inorg. Chem. 2003; 42: 3835. Bowen R. J., Caddy J., Fernandes M. A., Layh M., Mamo M. A., Meijboom R., J. Organomet. Chem. 2006; 691: 717. Ma C.-L., Li J., Zhang R., and Wang D., Inorg. Chim. Acta 2005; 358: 4575. Baul T. S. B., Masharing C., Willem R., Biesemans M., Holcapek M., Jirasko R., and Linden A., J. Organomet. Chem. 2005; 690: 3080. Ma C.-L., Jiang Q., Zhang R., and Wang D., Dalton Trans. 2003; 2975. Ma C.-L., Zhang Q., Zhang R., and Wang D., Chem.-Eur. J. 2006; 12: 420. Lockhart T. P., Organometallics 1988; 7: 1438. Ma C.-L. and Sun J., Dalton Trans. 2004; 1785. Prabusankar G. and Murugavel R., Organometallics 2004; 23: 5644. Chandrasekhar V., Thirumoorthi R., and Azhakar R., Organometallics 2007; 26: 26. Chandrasekhar V., Nagendran S., Bansal S., Kozee M. A., and Powell D.R., Angew. Chem., Int. Ed. 2000; 39: 1833. Chandrasekhar V., Nagendran S., Bansal S., Cordes A. W., and Vij A. Organometallics 2002; 21: 3297.

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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


Herbert Höpfl Centro de Investigaciones Qu´ımicas, Universidad Autónoma del Estado de Morelos, Cuernavaca, México

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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Tin Chemistry: Fundamentals, Frontiers and Applications a) R'

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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123

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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Tin Chemistry: Fundamentals, Frontiers and Applications R

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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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds R

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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Tin Chemistry: Fundamentals, Frontiers and Applications R

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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129

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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Tin Chemistry: Fundamentals, Frontiers and Applications b)

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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131

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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Macrocyclic and Supramolecular Chemistry of Organotin(IV) Compounds b)

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.

Acknowledgments The author thanks CONACyT for financial support. References 1. The Cambridge Structural Database: A Quarter of a Million Crystal Structures and Rising, Cambridge Crystallographic Data Centre, version 5.28, November 2006, Cambridge, UK; F.H. Allen, Acta Cryst., B58, 380 (2002). 2. For reviews see: (a) P.J. Stang, Chem. Eur. J., 4,19 (1998); (b) G.F. Swiegers, and T.J. Malefetse, Chem. Rev., 100, 3483 (2000); (c) M. Fujita, Structure and Bonding, 96, 177 (2000); (d) R.W. Saalfrank, E. Uller, B. Demleitner, and I. Bernt, Structure and Bonding, 96, 149 (2000); (e) I. Haiduc, and F.T. Edelmann, Supramolecular Organometallic Chemistry, Wiley-VCH, Weinheim (2000); (f) B.J. Holliday, and C.A. Mirkin, Angew. Chem.

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3.

4. 5. 6.

7. 8. 9. 10. 11.

12.

13. 14.

15. 16. 17.

18.

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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. Höpfl, 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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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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Deltahedral Zintl Ions of Tin: Synthesis, Structure, and Reactivity a)

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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. Schäfer, 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) Fässler, 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 Strähle, 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) Fässler, 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) Fässler, 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. Fässler, T. F. and Hoffmann, R., Angew. Chem. Int. Ed. 1999, 38, 543. 25. Hauptmann, R. and Fässler, T. F., Z. Anorg. Allg. Chem. 2002, 628, 1500. 26. Hauptmann, R., Hoffmann, R., and Fässler, 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 Fässler, 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 E EC CSn Sn E E < (E, E are heavier group 14 elements). The first stable compound with an Sn Sn double bond, 1, was reported by Lappert et al. in 1973.2 Since then, distannenes 6–17 have been synthesized and characterized,14–25 and some of their parameters are summarized in Table 2.9.1. The molecular structures of 1 and 14 are shown in Figures 2.9.2a and b, respectively. However, almost all of the distannenes (R2 Sn SnR2 ) known to date undergo ready dissociation in solution to form two molecules of the corresponding stannylenes (R2 Sn:). Tetrasilylsubstituted distannene 14, reported by Sekiguchi et al.,22 and tristannacyclopropene 17, reported by Wiberg et al.,25 form stable double bonds both in the solid state and in solution. The syntheses of distannenes are generally achieved by the dimerization of stannylenes (see Section 2.8). As a unique example, distannene 6 is obtained by the thermal or photochemical reaction of tristannacyclopropane 18.14 It was reported that distannene 6 exists in equilibrium between 6 and 18, or stannylene 19 depending on the temperature (Scheme 2.9.3). In addition, the cyclic distannene 17 is obtained as a result of isomerization of tristannaallene 2125 (see below).

Tip

RA RB RC Mes RA RB Si(SiMe3 )3 SiMe(t-Bu)2

6

7 8 9 10 11 12 13 14 15

a

E

E

θ The angles θ and τ are defined by b distannene (plane); stannylene (italic)/except for 15 and 16 c Two peaks were observed. d anion radical species of 14 e double-bond lengths; f two independent molecules.

E

SiMe(t-Bu)2

τ

(E = Sn).

3.087(2) 2.582(4) [2.601(3)]e, f 2.8978(3) 2.684(1), 2.675(1) [2.682(1), 2.675(1)] f

2.910(1) 3.639(1) 2.7705(8) 2.702 2.7914(4) 2.833(1) 2.8247(6) 2.6683(10) 2.961(1)

–

2.768(1)

19.50(4), 60.05(4) 24/46

Sn Sn ), 2233 (>Sn Sn ) /toluene-d8

−30, 275 412/C6 D6

740, 725c /Et2 O or toluene/−108 ◦ C 2315/102 ◦ C 427.3/methylcyclohexane-d14 / −68 ◦ C 1420/toluene-d8 /40 ◦ C 1401/toluene-d8 /100 ◦ C not available 1205.7/C6 D6 not available 1506/C6 D6 168/C6 D6 not observed 630.7/C6 D6 −637.19, 1264.44/(THF/C6 D6 )

δ Sn (ppm)b /solvent/temperature

10:3

20d 21

RA RB CH2 (4-t-Bu-C6 H4 ) Si(SiMe3 )3 Si(SiMe3 )3 Si(SiMe3 )3 Si(SiMe3 )3 SiMe(t-Bu)2

Tip

Dis

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R2

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Tin Chemistry: Fundamentals, Frontiers and Applications R1

(t-Bu)3Si

R2

Sn Sn R2

Si(t-Bu)3

R

Sn

R1

(t-Bu)3Si

Sn Sn

6-14 a

R'

Si(t-Bu)3

17

R Mes: R = R' = Me Tip: R = R' = i-Pr RB: R = R' = CF3 RC: R = 2,6-(i-Pr) 2C6H4, R' = H

N N N Me3Si Sn Cl2Sn

SiMe3

N (t-Bu)H2C

Sn N

Sn

N

N CH2(t-Bu)

t-Bu

N RA = 15

16

Scheme 2.9.2

Stable distannenes. a R1 and R2 : see Table 2.9.1 Tip Tip Sn

Tip 6

Tip Sn Sn Tip Tip Tip 18

Scheme 2.9.3

+

Sn Tip

19

Equibrium between distannene 6 and tristannacyclopropane 18 or stannylene 19

(a)

(b)

Figure 2.9.2

Molecular structures of (a) 1, (b) 14, and (c) 20

(c)

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Stannenes, Distannenes, and Stannynes

181

These distannenes, except 14 and 17, possess trans-bent structures with substantial out-of-plane an˚ approximately equal to or greater than those of the gles and Sn–Sn distances [2.768(1)–3.639(1) A] corresponding Sn–Sn single bond. However, the Sn Sn double-bond lengths in 14 and 17 are very short ˚ (two independent molecules), respectively]. Moreover, the sp2 Sn [2.6683(10) and 2.582(4)/2.601(3) A atoms have planar geometry [the sums of the bond angles around them are 360.0◦ for 14 and 350.8/355.8◦ and 359.6/360.0◦ for 17 (two independent molecules), respectively] along with very small bent angles. Due to the distannene–stannylene equilibrium in solution, the 119 Sn NMR signals of these distannenes, except 14 and 17, are only observed at low temperature or not at all (only the signals assignable to stannylenes were reported). The signals assignable to the central tin atom of distannenes are observed at 740/725 (1), 427 (6), 630.7 (14), and 412 (17) ppm. Except for 14, the reactivity of acyclic distannenes reflecting the Sn Sn double-bond character has not been investigated because of their equilibrium in solution, and these distannenes react as the stannylene in many cases (see Section 2.8). Although some [2+2]-cycloaddition reactions were reported (Scheme 2.9.4),2e,f,14b these cycloadducts could be formed either by [2+2]-cycloaddition or via the insertion of the second stannylene unit into the initially formed three-membered ring compound.

1

t-Bu

Dis Dis Dis Sn Sn Dis

P

P

Dis Dis Dis Sn Sn Dis

t-Bu 1

6

Tip Tip Tip Sn Sn Tip

Ph

Ph R R R Sn Sn R

[2+2]

1 or 6

E R' R R'

E

Sn R R

R Sn R

Scheme 2.9.4

R

Sn [1+2] R'

E

Reactions of distannenes, 1 and 6

Therefore, it is important to examine the reactivity of distannene 14, which does not undergo dissociation into stannylenes in solution. Distannene 14 reacts with carbon tetrachloride and phenylacetylene to afford the corresponding adducts, respectively, which retain the Sn Sn bond (Scheme 2.9.5).22

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(t-Bu)2MeSi Cl (t-Bu)2MeSi Sn Sn SiMe(t-Bu) 2 Cl SiMe(t-Bu) 2

CCl4

(t-Bu)2MeSi SiMe(t-Bu) 2 (t-Bu)2MeSi Sn Sn SiMe(t-Bu) 2

Ph

14

Ph

Scheme 2.9.5

Addition reactions of distannene 14

The reduction of 14 with a potassium mirror in the presence of [2.2.2]cryptand resulted in the formation of the corresponding anion radical 20 (Scheme 2.9.6, Figure 2.9.2c).22 Its structure displays a highly ˚ which is 0.23 A ˚ longer than that in 14, indicating twisted Sn Sn bond with a bond length of 2.8978(3) A, the decrease in the Sn Sn bond order (Table 2.9.1). The negative charge and the unpaired electron of 20 are separated between the two tin atoms, both in the solid state or in solution. In the solid state, the geometries of the two tin atoms of 20 are different from each other. One of them has a highly pyramidal configuration, reflecting the tin anion character, and the other has a planar geometry reflecting, the tin radical character. In solution, the EPR spectrum of 20 shows a single resonance (g = 2.0517) with two distinct pairs of satellite signals with hyperfine coupling constant values of 34.0 mT (α-119,117 Sn) and 18.7 mT (β-119,117 Sn), respectively, implying localization of the unpaired electron on one of the two Sn atoms.

14

SiMe(t-Bu) 2

K/[2.2.2]cryptand

[K([2.2.2]cryptand)]+

THF

Scheme 2.9.6

(t-Bu)2MeSi Sn Sn SiMe(t-Bu) 2 (t-Bu)2MeSi

20

Reduction of distannene 14

Tristannaallene

The thermolabile tristannaallene 21 was prepared by the reaction of Sn[O(t-Bu)]2 or Sn[N(SiMe3 )2 ]2 with (t-Bu)3 SiNa in pentane/benzene at −25 ◦ C by Wiberg et al. 25 Compound 21 isomerized to give tristannacyclopropene 17 at room temperature (τ 1/2 = 9.8 h) (Scheme 2.9.7). The structure of the cyclic distannene 17 has been already discussed earlier. The X-ray crystallographic analysis of 21 (Figure 2.9.3) shows that the framework is bent (156◦ ) and the terminal tin atoms have pyramidal geometries. The Sn Sn ˚ are approximately equal to the shortest Sn Sn bond length of the bonds in 21 (average value 2.68 A)

Si[N(SiMe3)2]2 or Si[O(t-Bu)] 2

NaSi(t-Bu) 3

(t-Bu) 3Si

Sn Sn Sn

(t-Bu)3Si

Si(t-Bu) 3

Si(t-Bu) 3

half-life period 9.8 h at 25 °C

21 Sn

Sn Sn

Sn

Scheme 2.9.7

Sn

Sn

Synthesis of tristannaallene 21

17

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Figure 2.9.3

183

Molecular structure of 21 (one of the two independent molecules)

˚ In the 119 Sn NMR spectrum of 21, two signals at low field (δ acyclic distannene 14 [2.6683(10) A]. 503, 2233, intensity ratio = 2:1) are observed in toluene-d8 solution. In particular, the latter chemical shift, which was assigned to the central tin atom, is characteristic of those reported for stannylenes, indicating that the bonding situation in 21 can be described by the resonance contributions shown in Scheme 2.9.7. 2.9.3

Stannenes (Sn C)

Stannenes,7,10,26,27 doubly-bonded tin–carbon compounds, are viewed as ‘bridge’ compounds to elucidate the similarities and differences between olefins (>C CSn Sn – 20 °C

R

R

t-BuLi

R

Sn Sn

Et2O

Sn Sn

F

H

R

F

–LiF 26 or 27 Li

Tip

Sn C C Sn

Tip Tip 32

27 No Change 80 °C

Scheme 2.9.9

Synthesis of stannenes, 26 and 27

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(Me3Si)2HC

Me

Sn C (Me3Si)2HC

Me

33

R

R

R

Sn C

R

HO

H

R

R

Sn C

R MeO

H

R

R

R

R R

Cl

R

x2 26, 34

H

LiAlH4 26

MeOH

R

R

26

C Sn R R

R R R Sn C R O

Ph Ph

R

26, 27, 33, 34

R

R

H

R

34

MeI 26

26

R

PhC CH

H 26

R R R Sn C R

(t-Bu)2MeSiN3

PhNH2

R

R Sn C

R

26

R R Sn C

HCl

R R R Sn C R R

Ph2C=O

Sn C

R Ph(H)N

R

26

H

R

R

Sn C H

Sn C

R

SiMe3 34

H2O 26, 27, 33

SiMe3

Sn C

R

R

Sn C I

Me

(t-Bu)2MeSi

N

N N

R R R Sn C R

R 27, 34

Ph

Scheme 2.9.10

Reactivities of stannenes, 26, 27, 33, and 34

28–30,34–36 have Sn C bonds markedly longer than the typical Sn C single-bond lengths and are considered as formal Sn C double-bond compounds. The 119 Sn NMR signals of stannenes 22, 24, 25, and 27, bearing a short Sn C bond, are observed at low field, and a similar tendency is observed in the case of 29 Si NMR chemical shifts of silenes.20,27 By contrast, those of stannenes 28 and 29, bearing a long Sn C bond, are observed at relatively high field. The exception of 30 is considered to reflect the character of the stannylene–carbene complex rather than the stannene. The reactivity of heteroatom-substituted stannenes has not been fully reported so far. On the other hand, those of tetraarylstannenes 26 and 27 and transient stannenes 3338 and 3439 have been widely investigated (Scheme 2.9.10). These compounds readily undergo 1,2-addition with various protic reagents and iodomethane. Their reactions with multiple-bond compounds such as ketones, butadienes, and azides result in the formation of the corresponding [2+n]-cycloadducts (n = 2, 3, 4).

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187

Cumulative double-bond compounds (Sn C N, Sn C C)

1-Stannaketenimine 31 was successfully synthesized by the reaction of bis[2,4,6-tris(trifluoromethyl) phenyl]stannylene with mesityl isocyanide, by Grützmacher et al. (Scheme 2.9.11).40 The X-ray crystallographic analysis of 31 (Figure 2.9.6) shows that the Sn C N frame is very bent (154◦ ) and the tin atom ˚ is much has a pyramidal coordination geometry. The Sn C bond length in the SnCN unit [2.397(3) A] ˚ In solulonger than the Sn C single bond lengths in sterically encumbered tin compounds (2.22–2.30 A). tion, 31 dissociates into the corresponding stannylene and isocyanide, as evidenced by a trapping experiment using 2,3-dimethyl-1,3-butadiene and the temperature-dependent chemical shifts in the 119 Sn NMR spectra, which change between ca. −150 and 400 ppm (−80 to + 70 ◦ C). Measurement of the temperaturedependence of the 119 Sn NMR chemical shift allowed the determination of the dissociation enthalpy of 29.6 kJ mol−1 and the dissociation entropy of 90 JK−1 mol−1 . These results strongly suggest that 31 could be described as a stannylene–isocyanide complex rather than a cumulative double-bond compound.

R

R Sn

+

C

Sn C

N Mes

R

R

Mes

Mes Sn

N

31 1-stannaketenimine

C

N

R R 31' stannylene-isocyanide complex

F3 C R=

CF3 R F3 C

R

Scheme 2.9.11

Sn

+

Synthesis and reactivity of 1-stannaketenimine 31

Figure 2.9.6

Molecular structure of 31

C N Mes

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Generation of 1-stannaallene 35 was suggested as an intermediate in the synthesis of the first stable distannirane 36, by Escudié et al. (Scheme 2.9.12).41 Because of the probable lability of the Sn C double bond, 35 could behave as a stannylene-vinylidene carbene complex, as observed in the related 1-stannaketenimine 31, which behaves as a stannylene-isocyanide complex. Therefore, the generation of the final product of this reaction, 36, should be most likely interpreted in terms of the [2+1]-cycloaddition of 35 with the stannylene 37, which should be generated in an equilibrium amount.

t-BuLi

Tip C Tip Sn C F Cl

Tip

Tip Sn Sn

Tip

C

C

Tip 36

Tip

Tip Sn C

Sn

C

+

C C

Tip

Tip 35

Scheme 2.9.12

37

Generation of 1-stannaallene 35

Stannaaromatic Compounds

Stannaaromatic compounds, which contain a tin atom instead of a skeletal carbon atom in aromatic hydrocarbons, have a formal Sn C double bond in their resonance forms. To date, tin analogs of cyclopentadienyl anions and dianions, phenanthrene, and naphthalene have been reported. Some stannole anions 38–46 have been reported by Saito et al.42–44 The syntheses of dianions were accomplished by transmetallation or by the reduction of bi(1,1-stannole)s or stannoles. Monoanions were synthesized by the reaction of the dianion with t-butyl chloride or by the reduction of bi(1,1-stannole)s under milder conditions than in the synthesis of the corresponding dianion, for example, as in the synthesis and reactivity of tetraphenyl-substituted stannole anions, shown in Scheme 2.9.13. The reversible redox behavior of dianion 38 is noteworthy.42h,i The reduction of dianion 38 by an equimolar amount of oxygen or [Cp2 Fe]+ [BF4 ]− results in the formation of the 1,2-dianion 41, and the treatment of 41 with lithium metal regenerates dianion 38. Crystallographic analysis of 38 (Figure 2.9.7a) reveals not only the planarity of the stannole ring, but also the unique η5 –η5 interaction mode (Figure 2.9.8).42g The lengths of the C C bonds in the

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Stannenes, Distannenes, and Stannynes Ph

Ph

Ph

Ph

Sn Li

Ph

Ph

Ph

Li

R

n-Bu

R

Sn Ph

Ph R

Ph

M

45: M = Li 46: M = K

Ph Li 38 Ph

MeI Li

O2 R = t-Bu Ph

Ph

Ph

R

O2 or [Cp2Fe]+[BF4]-

41

Ph

Sn Me

Ph

Sn Ph

t-BuCl

39

Ph

Li

40 R = Ph

Ph

Scheme 2.9.14

Ph

Stannole dianions and monoanions

Li

Sn

R Ph Ph

Li Ph 41

Sn M

Li

43: R = t-Bu 44: R = Ph

Scheme 2.9.13 Ph

Ph

Sn

Li 42

Ph

Ph

n-Bu

Ph

Sn Li

Ph

Li

39: R = t-Bu 40: R = Ph

38

Ph

Sn Sn

Ph

Sn

Ph Ph Li

Ph

189

Me

Synthesis and reactivities of tetraphenylstannole anions, 38–41

stannole ring are almost equal to each other, and the negative charges are considerably delocalized on the stannaaromatic ring (Figure 2.9.9). Although these results indicate that 38 forms a 6π -electron system, such compounds should be classified as special stannaaromatic compounds with negative charges. In addition, the lengths of the two Sn C ˚ ˚ which are close to the typical Sn C single bond length (2.14 A). bonds are 2.133(4) and 2.179(4) A, Compound 41 is the first X-ray-characterized tin analog of the cyclopentadienyl anion. Contrary to the stannole dianion 39, bond alternation for the C C bonds is observed in the five-membered ring of 41, indicating that the 1,2-dianion 41 has considerable diene character (Figure 2.9.8).42h The two lengths of ˚ are longer than, not only those of dianion 41, but also that of the typical Sn C Sn C bonds [2.184(4) A] ˚ The geometry around the tin atoms is clearly pyramidalized, as judged by the single-bond length (2.14 A). ◦ angle of 110 between the C4 Sn plane and the Sn Sn bond. These results indicate the lower aromaticity of 41 compared to dianion 39.

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(a)

(b)

(c)

Figure 2.9.7 Molecular structures of (a) [38·2(Et2 O)(benzene)] (benzene molecule is omitted for clarity), (b) 48, and (c) 49

The 119 Sn NMR signals for stannole anions appeared in the range from +163 to −105 ppm (Table 2.9.3), which is higher field than those reported for stannenes bearing a short Sn C double bond. Neutral stannaaromatic compounds, bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group, have been reported by Tokitoh et al. The generation of 9-stannaphenanthrene 47 in the reaction of the corresponding chlorostannane with lithium 2,2,6,6-tetramethylpyperidide was suggested using a trapping experiment.45 However, 9-stannaphenanthrene 47 is thermally unstable and readily undergoes dimerization at room temperature (Scheme 2.9.15). The instability of 47 is in sharp contrast to the high stability of the Tbt-substituted 9-sila-46 and germa-phenanthrenes,47 which are stable at 100 ◦ C in C6 D6 .

R

M

R

R

M

R

R

E

M

E M

R M R

R E

R

R

M

R

R

η5-η5 interaction

R R

E

R

R

η1-η5 interaction

R

delocalized structure

dianionic species

Figure 2.9.8

R

localized anion

anionic species

Possible interaction modes of metallole anions (E = Group 14 elements, M = alkali metals) 1.446(6) Ph 1.442(5)

2.133(4) Sn

Ph

Figure 2.9.9

Ph 1.473(4)

2.179(4)

Ph 1.422(6)

1.378(4)

Ph

38

Ph 1.361(4)

Ph 2.184(4) Sn

Sn

2.184(4) Ph

˚ of 38 and 41 Selected bond lengths (A)

41

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191

The δ Sn values of stannaaromatic compounds

Table 2.9.3 No.

δ Sn (ppm)

38 39 40 41 42 43 44 46 48 49

163.3 30.4 −30.3 −80.4 −9.2 −30.3 −104.8 −21.8 264 106

solvent Et2 O Et2 O THF THF Et2 O Et2 O Et2 O Et2 O C6 D6 C6 D6

Li

Cl

N

Tbt

Tbt

Tbt Sn

Sn

Sn Sn

rt

THF, –78 °C

Tbt 47

Scheme 2.9.15

Generation of 9-stannaphenanthrene 47

By contrast, 2-stannanaphthalene 48 is thermally stable.48 The synthesis of 48 was achieved by the reaction of the corresponding bromostannane with lithium diisopropylamide (Scheme 2.9.16). The X-ray crystallographic analysis of 48 (Figure 2.9.7b) shows the planar geometry of the 2-stannanaphthalene ring, and the bond alternation, which are essentially the same features as those of a parent naphthalene. The 119 Sn NMR signal of 48 is observed at 264 ppm, which is characteristic of a low-coordinated tin atom (Table 2.9.3). In the 1 H and 13 C NMR spectra, all the signals assigned to the 2-stannanaphthalene ring are observed in the typical aromatic region. Tbt 2.029(6) Tbt Sn Br

LiN(i-Pr)2

1.394(8)

t-Bu

Sn

2.081(6)

t-Bu 1.372(9) 1.443(9)

1.436(9) 48

Scheme 2.9.16

˚ of 2-stannanaphthalene 48 Synthesis and selected bond lengths (A)

The main reason for the difference of the stability between 47 and 48 may be the introduction of an additional substituent (t-Bu) on 48. However, theoretical studies for stannaaromatic compounds implied

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the general conclusions that: (i) 9-stannaphenanthrene has lower aromaticity than 2-stannanaphthalene, and (ii) 2-stannanaphthalene has sufficient aromaticity comparable to the parent naphthalene.45,46 Although the reactivity of 47 and 48 indicate their character as Sn C double-bond compounds (Scheme 2.9.17), the ligand exchange reaction of 48 with [Cr(CH3 CN)3 (CO)3 ] results in the formation of the (1,2,3,4,4a,8a-η)-2-stannanaphthalene complex, 49, regioselectively, as for conventional arene systems (Scheme 2.9.18 ).46 The X-ray crystallographic analysis (Figure 2.9.7c) revealed that the two endocyclic Sn C bonds of the complex of 49 are somewhat elongated when compared to those of free 2-stannanaphthalene 48, but 49 still keeps the planarity of the 2-stannanaphthalene moiety. In the 1 H, 13 C, and 119 Sn NMR spectra of 49, signals corresponding to the atoms in the SnC5 ring are shifted upfield relative to those for free 2-stannanaphthalene. The IR spectrum of the 2stannanaphthalene complex recorded in the solid-state shows the presence of three intense bands (1941, 1862, 1851 cm−1 ) assigned to the carbonyl stretchings, which are observed in a similar region to those of the η6 -naphthalene(tricarbonyl)chromium complex (1941, 1864 cm−1 ). This result suggests that 2stannanaphthalene has a coordination ability as an arene ligand similar to that of naphthalene. Mes* C

N O

Mes*CNO

Sn

Tbt

D OMe Sn

MeOD Tbt

47, at –78 °C

Sn Tbt t-Bu t-Bu

Mes* =

Mes

t-Bu

C MesCNO

N O Sn Tbt t-Bu

OH Sn Tbt t-Bu

H2O

48

Sn Tbt t-Bu

Scheme 2.9.17

Reactivities of 9-stannaphenanthrene 47 and 2-stannanaphthalene 48

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Stannenes, Distannenes, and Stannynes Tbt 2.035(5) 48

Sn Tbt

[Cr(CH3CN)3(CO)3] OC OC

Cr

t-Bu

1.425(6)

CO

2.093(4) t-Bu 1.379(6) 1.443(6)

1.427(6)

49

Scheme 2.9.18

Sn

193

Cr(CO)3

˚ of 2-stannanaphthalene complex 49 Synthesis and selected bond lengths (A)

Judging from the structural features, NMR, UV/vis, and Raman spectra, and the chemical reactivities, 2-stannanaphthalene 48 is considered to have significant aromatic character. 2.9.4

Silastannene (Si Sn) and Germastannenes (Ge Sn)

Since the late 1990s, new classes of heavy alkene analogs containing tin, i.e., silastannenes and germastannenes, have been investigated.27 The generation of the first germastannene 50 was reported by Escudié et al.49 However, 50 was stable only below −20 ◦ C, and it underwent ready dissociation into Tip2 Sn: (Tip = 2,4,6-triisopropylphenyl) and Mes2 Ge: at room temperature, to afford germadistannirane 51 as the final isolable product (Scheme 2.9.19). Germastannenes, 52–55, stable at ambient temperature, were reported by Weidenbruch et al. and Sekiguchi et al. (Scheme 2.9.20).50–52 The most striking feature of 53 is its ready isomerization to the symmetrically substituted germastannene 54 by heating at 50 ◦ C in C6 D6 solution. Some parameters of the germastannenes 52–55 are summarized in Table 2.9.4. The experimental results show that the dissociation behavior of germastannenes is totally controlled by their substituents; germastannenes 50 and 52, having only aryl groups, undergo dissociation in solution at ambient temperature, whereas all three germastannenes 53–55 bearing silyl substituents form stable double bonds, both in the solid state and in solution, as suggested by their NMR data and reactivities. The 119 Sn NMR spectra of all the germastannenes 52-55 show a signal at 268–525 ppm, which is the region characteristic of doubly bonded tin derivatives.

Mes F Mes Ge Sn Tip H Tip

t-BuLi

Mes

– LiF

Mes

Tip

Tip

Mes

Ge Sn rt

Tip

Ge Mes

+

Sn Tip

50 stable below –20 °C

Scheme 2.9.19

Mes

Mes

Ge Tip Sn Sn Tip Tip

51

Tip

Generation of germastannene 50

The first stable silastannene 56 was reported by Sekiguchi et al. (Scheme 2.9.21, Figure 2.9.10).53 The Sn NMR spectrum of 56 shows a downfield-shifted resonance of the doubly bonded Sn atom at +516.7 ppm, which is characteristic of an sp2 Sn atom. The sp2 Si atom in 56 resonates at +27.4 ppm, which is an unusually high-field shifted chemical shift for a doubly bonded Si atom. Such a phenomenon should be definitely ascribed to the inverted polarity Si+ Sn− due to the electronic environments around the double

119

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Tip

GeCl2 •diox SnCl2 TipMgBr

Tip

Tip

Ge Sn Tip

Tip

Tip

Sn Tip Sn Sn Tip

solution (gradually)

Tip 52

Tip

+

Tip

Tip

Tip

Ge Ge Tip

Tip Tip Tip Ge Sn Tip

O2

O O

(t-Bu)2MeSi

Li

Cl +

Ge (t-Bu)2MeSi

Li

(t-Bu)2MeSi

Tip

Cl

Tip

(t-Bu)2MeSi

Ge Sn

Sn Tip

(t-Bu)2MeSi

Tip Ge Sn

50 °C

Tip

Tip

SiMe(t-Bu) 2 54

53 S

S

(t-Bu)2MeSi Ge (t-Bu)2MeSi

(t-Bu)2MeSi

SiMe(t-Bu)2 E

(t-Bu)2MeSi

E' Si

(t-Bu)2MeSi

CCl4

Cl Si Si SiMe(t-Bu)2 (t-Bu)2MeSi Cl 55

SiMe(t-Bu) 2

E = Si, E' = Ge or E = Ge, E' = Si

Scheme 2.9.20

SiMe(t-Bu) 2 Ge Sn

SnCl2 • diox

Tip Sn

S

Tip

Cl SiMe(t-Bu)2 (t-Bu)2MeSi Ge Sn Cl Cl Si Si SiMe(t-Bu)2 (t-Bu)2MeSi Cl

Synthesis and reactivities of germastannenes, 52–55

bond, that is, the electron-donating silyl substituents on the sp2 Si atom and the electron-withdrawing aryl groups on the sp2 Sn atom. ˚ which is within the typical The length of the Si Sn double bond 56 was determined as 2.4188(14) A, ˚ ˚ values of Si Si (2.14–2.29 A) and Sn Sn (2.59–3.09 A) bond lengths. The shortening of the Si Sn ˚ is by ca. 7%. double bond of 56 compared with the Si Sn single bond (average value 2.60 A) The Si Sn double bond of 56 is rather strong, but this species does not dissociate in solution into a silylene and a stannylene, judging from the spectrum and the reactivity. Table 2.9.4

Selected parameters of germastannenes 52–55 and silastannene 56

No.

˚ d(Sn Si or Ge) (A)

θ (Sn, Si or Ge, deg)a

τ (deg)a

δ Sn (ppm)b

52 53 54 55 56

2.5065(5) – – – 2.4188(14)

43.3, 30.2 – – 55 9.6, 26.2

– – 28 34.6

268 525.1 373.4 440 516.7 (δ Si : 27.4)

E, E' E a b

The angles θ and τ are defined by Measured in C6 D6 at room temperature

E'

θ

τ (E, E’ = Sn and Ge or Si).

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Stannenes, Distannenes, and Stannynes (t-Bu)2MeSi

Li

(t-Bu)2MeSi

Cl +

Si Li

(t-Bu)2MeSi

Tip

Si

Sn Cl

Tip

(t-Bu)2MeSi 56

Scheme 2.9.21

PhEH (E = O, S)

(t-Bu)2MeSi (t-Bu)2MeSi

Tip

Tip

Si Sn Tip H

EPh

Synthesis and reactivities of silastannene 56

Figure 2.9.10

2.9.5

Tip Sn

195


Stannynes (Sn C)

Many experimental and theoretical studies have been made of the chemistry of triple-bonded compounds of heavier Group 14 elements.54 Recently, a series of homonuclear acetylene analogs of heavier Group 14 elements have been isolated as stable compounds (see Section 2.9.6).55–60 However, no stable heteronuclear analogs, e.g. stannyne, have been reported so far. The generation of stannyne 58 by the photolysis of diazomethyl-substituted stannylene 57 was reported by Kira et al. (Scheme 2.9.22).61 The formation of 59 is explained by the intermediacy of stannyne 58, followed by the intramolecular insertion of the carbene moiety of 58 into the proximate methyl C H bond of the isopropyl group. This result afforded evidence not only for the generation of 58 but also for its high carbene-like reactivity. i-Pr

i-Pr

Ar* i-Pr

Sn C Ar*

N2 Sn C

hν

Si(i-Pr)3 58'

i-Pr

i-Pr Si(i-Pr) 3 Sn C

Si(i-Pr)3 57

i-Pr Ar* = i-Pr

Me Ar* Sn C

Si(i-Pr)3

i-Pr

i-Pr

58 i-Pr

Scheme 2.9.22

Generation of stannyne 58

59

i-Pr

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196


2.9.6

Distannynes (Sn Sn) and Their Reduced Species

As described in Section 2.9.5, a series of homonuclear acetylene analogs of heavier group 14 elements (Si Si, Ge Ge, Sn Sn, Pb Pb) have been isolated as stable compounds.55–60 Among them, distannynes 60–62,59,62,63 tin analogs of acetylene, and their anion radicals 63–6662,64 and dianions 67–6962,65 were synthesized by Power et al. and characterized by UV-vis, 1 H and 13 C NMR, Na sodium anthracenide or KC8

Ar = Ar*, Ar', 4-SiMe3-Ar' M = Na, K

K

R Ar Sn

Ar

K

Sn Sn

Cl

K

Ar

Ar M+

Sn Sn

Ar

M+2

2–

Sn Sn

Ar

R'

Ar R

Ar

Ar

Ar Sn Sn

M+

Sn Sn

Table 2.9.5 No.

2–

Sn Sn Ar

Ar

Ar

Scheme 2.9.23

M+2

Synthesis of distannynes and their anion radicals and dianions

Selected parameters of distannynes 60–62, their anion radicals 63–66, and dianions 67–69 M+

Ar

Neutral species 60 Ar’ 61 Ar* 62 4-SiMe3 -Ar’

˚ C(ipso) Sn Sn (deg) λmax (nm),{ε (Lmol–1 cm–1 )}/solventa d(Sn Sn) (A) 2.6675(4) – 3.0660(10)

125.24(7) – 99.25(14)

Monoanions 63 Ar* 64 Ar’ 65 Ar* 66 Ar*

Na(THF)3 K(THF)6 K(THF)6 K(D)(THF)b2

2.8107(13) 2.8081(9) 2.8123(9) 2.782(1)

97.9(3), 98.0(4) 97.91(16) 95.20(13) 95.0(4)

Dianions 67 Ar* 68 Ar’

Na2 K2

2.789(1) 2.7754(3)

104.8(2) 106.02(5)

K2

2.7763(9)

107.5(1)

69 a b

Ar*: R = Tip; R' = H Ar': R = Dip; R' = H 4-SiMe3-Ar': R = Dip; R' = SiMe3 Tip: R = R' = i-Pr Dip: R = i-Pr; R' = H

Ar*

measured at room temperature O O

O

D= O

O O

410 (2000), 597 (1700)/hexane 409 (1800), 593 (1400)/hexane 416 (4700), 608 (1200)/hexane 709 (140)/hexane 461(1900)/THF

408 (3900), 556 (2200), 875 (152)/benzene 412 (4200), 552 (3200)/toluene

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(a)

Figure 2.9.11 for clarity)

(b)

197

(c)

Molecular structures of (a) 60, (b) 65, and (c) [69.2(benzene)] (benzene molecules are omitted

and EPR spectroscopy, together with X-ray crystallography. Their synthetic routes and some parameters were summarized in Scheme 2.9.23 and Table 2.9.5, respectively. The molecular structures of 60, 65, and 69 are shown in Figures 2.9.11a, b, and c, respectively. The syntheses of distannynes 60–62 were achieved by the reactions of bulky terphenyl-substituted chlorostannylenes and potassium. The structure of 60 is not linear, but bent, and the Sn Sn bond length ˚ is similar to the shortest Sn Sn distance in the acyclic distannene [2.6683(10) A ˚ in 11]. [2.6675(4) A] On the other hand, the introduction of a trimethylsilyl substituent into the para-position of the central ˚ which is ca. aryl ring causes large structural differences. The Sn Sn bond length in 62 is 3.0660(10) A, ˚ longer than that in 60, indicating the single-bond character of the Sn Sn bond in 62. In addition, 0.4 A the dihedral angles of Sn Sn C(ipso) C(ortho) in 62 are 91.0◦ and −101.1◦ , which are quite different from those in 60 (177.0◦ and 3.1◦ ) Unfortunately, the electronic state of the Sn atom in 60–62 cannot be elucidated in detail, since the solution 119 Sn NMR signals have not been detected. The solid-state 119 Sn NMR and Mössbauer spectra of 60 and 6166 suggest that the structure of 61 in the crystalline state is more trans-bent and 61 has a longer Sn Sn bond than those measured for 60 by X-ray crystallography. Ph N

N

Ph Ph

N Ar'

i-Pr

Ph N

Sn

Sn Ar'

i-Pr Ar' =

60

i-Pr Me3Si

N

N

SiMe3

N

Scheme 2.9.24

Ar'

Sn

N

Sn

Ar'

Reactivities of distannyne 60

i-Pr

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The reduction of the chlorostannylenes or distannynes with alkali metals or metal arenides affords the corresponding anion radicals or dianions. They preserve the trans-bent geometry seen in the neutral precursor 60. While all the anion radical species 63–66 formed separate ion pairs with a solvated alkali metal cation, all the dianion salts 67–69 are obtained as contact ion triples with two alkali metal cations complexed between aryl rings. Single and double reduction of the neutral species results in the narrowing of the C(ipso) Sn Sn angles and changes in the Sn Sn bond lengths. Some reactivity of 60 has been reported.67 The reactions of 60 with azobenzene or trimethylsilylazide results in Sn Sn bond cleavage, affording bisstannylenes (Scheme 2.9.24). Although the germanium analog of 60 reacts with trimethylsilylacetylene or diphenylpentadiyne to give the corresponding [2+2]cycloadducts, distannyne 60 does not react with these alkynes. These results suggest a lower reactivity for 60 than the germanium analog and an increase in the lone pair character at each tin atom. References 1. For a review, see: L. E. Gusel’nikov and N. S. Nametkin, Chem. Rev., 6, 529 (1979). 2. (a) P. J. Davidson and M. F. Lappert, J. Chem. Soc., Chem. Commun., 317 (1973); (b) D. E. Goldberg, D. H. Harris, M. F. Lappert, and K. M. Thomas, J. Chem. Soc., Chem. Commun., 261 (1976); (c) D. E. Goldberg, P. B. Hitchcock, M. F. Lappert, K. M. Thomas, A. J. Thorne, T. Fjeldberg, A. Haaland, and B. E. R. Schilling, J. Chem. Soc., Dalton Trans., 2387 (1986); (d) K. W. Zilm, G. A. Lawless, R. M. Merrill, J. M. Millar, and G. G. Webb, J. Am. Chem. Soc., 109, 7236 (1987); (e) A. H. Cowley, S. W. Hall, C. M. Nunn, and J. M. Power, Angew. Chem., Int. Ed. Engl., 27, 838 (1988); (f) L. W. Sita, I. Kinoshita, and P. Lee, Organometallics, 9, 1644 (1990). 3. T. C. Klebach, R. Lourens, and F. Bickelhaupt, J. Am. Chem. Soc., 100, 4886 (1978). 4. A. G. Brook, F. Abdesaken, B. Gutekunst, G. Gutekunst, and R. K. Kallury, J. Chem. Soc., Chem. Commun., 191 (1981). 5. M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, and T. Higuchi, J. Am. Chem. Soc., 103, 4587 (1981). 6. R. West, M. J. Fink, and J. Michl, Science, 214, 1343 (1981). 7. (a) P. P. Power, Chem. Rev., 99, 3463 (1999); (b) P. P. Power, J. Chem. Soc., Dalton Trans., 2939 (1998). 8. For reviews of disilenes, see: (a) G. Raabe and J. Michl, Chem. Rev., 85, 419 (1985); (b) R. Okazaki and R. West, Adv. Organomet. Chem., 39, 231 (1996); (c) M. Kira and T. Iwamoto, J. Organomet. Chem., 611, 236 (2000). 9. For reviews of digermenes, see: (a) J. Escudié, C. Couret, H. Ranaivonjatovo, and J. Satgé, Coord. Chem. Rev., 130, 427 (1994); (b) N. Takeda and N. Tokitoh, Digermenes and digermanes, in Science of Synthesis, HoubenWeyl Methods of Molecular Transformations, Vol. 5. Organometallics, Compounds of Group 14 (Ge, Sn, Pb), M. G. Moloney (Vol. Ed.), D. Belllus, E. N. Jacobsen, S. V. Ley, R. Noyori, M. Regitz, E. Schaumann, I. Shinkai, E. J. Thomas, and B. M. Trost (Eds), George Thieme Velag, Stuttgart, New York, 2003. 10. For reviews of heavier group 14 multiple bondings, see: (a) T. Tsumuraya, S. A. Batcheller, and S. MasamuStrained-ring and double-bond systems consisting of the group-14 elements Si, Ge, and Sn, Angew. Chem., Int. Ed. Engl., 30, 902 (1991); (b) M. Driess and H. Grützmacher, Angew. Chem., Int. Ed. Engl., 35, 829 (1996); (c) M. Weidenbruch, Eur. J. Inorg. Chem., 373 (1999); (d) N. Tokitoh and R. Okazaki, Multiply bonded germanium, tin, and lead compounds, in The Chemistry of Organic Germanium, Tin, and Lead Compounds, Z. Rappoport (Ed.), John Wiley & Sons, Inc., New York, 2002; (e) K. W. Klinkhammer, Recent advantages in structural chemistry of organic germanium, tin and lead compounds, in The Chemistry of Organic Germanium, Tin and Lead Compounds, Z. Rappoport (Ed), John Wiley & Sons, Ltd, Chichester, 2002; (f) M. Weidenbruch, Organometallics, 22, 4348 (2003); (g) Sasamori, T.; Tokitoh, N., Group 14 multiple bonding, in Encyclopedia of Inorganic Chemistry, 2nd Edn, R. Bruce King (Ed.), John Wiley & Sons, Chichester, 2005. 11. (a) H. Jacobsen and T. Ziegler, J. Am. Chem. Soc., 116, 3667 (1994); (b) K. D. Dobbs and W. J. Hehre, Organometallics, 5, 2057 (1986); (c) H. Grützmacher and T. F. Fässler, Chem. Eur. J., 6, 2317 (2000). 12. Y. Apeloig, R. Pauncz, M. Karni, R. West, W. Steiner, and D. Chapman, Organometallics, 22, 3250 (2003). 13. M. Karni and Y. Apeloig, J. Am. Chem. Soc., 112, 8589 (1990).

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199

14. (a) S. Masamune and L. R. Sita, J. Am. Chem. Soc., 107, 6390 (1985); (b) M. Weidenbruch, A. Schäfer, H. Kilian, S. Pohl, W. Saak, and H. Marsmann, Chem. Ber., 125, 563 (1992). 15. M. Weidenbruch, H. Kilian, K. Peters, H. G. Vonschnering, and H. Marsmann, Chem. Ber., 128, 983 (1995). 16. U. Lay, H. Pritzkow, and H. Grützmacher, J. Chem. Soc., Chem. Commun., 260 (1992). 17. C. Stanciu, A. F. Richard, and P. P. Power, J. Am. Chem. Soc., 126, 4106 (2004). 18. J. Klett, K. W. Klinkhammer, and M. Niemeyer, Chem. Eur. J., 5, 2531 (1999). 19. M. Sturmann, W. Saak, K. W. Klinkhammer, and M. Weidenbruch, Z. Anorg. Allg. Chem., 625, 1955 (1999). 20. K. W. Klinkhammer, T. F. Fässler, and H. Grützmacher, Angew. Chem., Int. Ed., 37, 124 (1998). 21. W. Klinkhammer and W. Schwarz, Angew. Chem., Int. Ed. Engl., 34, 1334 (1995). 22. (a) T. Fukawa, V. Y. Lee, M. Nakamoto, and A. Sekiguchi, J. Am. Chem. Soc., 126, 11758 (2004); (b) V. Y. Lee, T. Fukawa, M. Nakamoto, A. Sekiguchi, B. L. Tumanskii, M. Karni, and Y. Apeloig, J. Am. Chem. Soc., 128, 11643 (2006). 23. W. -P. Leung, W. -H. Kwok, F. Xue, and T. C. W. Mak, J. Am. Chem. Soc., 119, 1145 (1997). 24. C. Drost, P. B. Hitchcock, and M. F. Lappert, Angew. Chem., Int. Ed. Engl., 38, 1113 (1999). 25. N. Wiberg, H. W. Lerner, S. K. Vasisht, S. Wagner, K. Karaghiosoff, H. Noth, and W. Ponikwar, Eur. J. Inorg. Chem., 1211 (1999). 26. For a review of metallenes, see: J. Escudié, C. Couret, and H. Ranaivonjatovo, Coord. Chem. Rev., 180, 565 (1998). 27. For a review of heteronuclear heavy alkenes, see: V. Y. Lee and A. Sekiguchi, Organometallics, 23, 2822 (2004). 28. A. Berndt, H. Meyer, G. Baum, W. Massa, and S. Berger, Pure. Appl. Chem., 59, 1011 (1987). 29. H. Meyer, G. Baum, W. Massa, S. Berger, and A. Berndt, Angew. Chem., 99, 559 (1987). 30. M. Weidenbruch, H. Kilian, M. Stuermann, S. Pohl, W. Saak, H. Marsmann, D. Steiner, and A. Berndt, J. Organomet. Chem., 530, 255 (1997). 31. M. Stürmann, W. Saak, M. Weidenbruch, A. Berndt, and D. Sclesclkewitz, Heteroatom Chem., 10, 554 (1999). 32. (a) G. Anselme, H. Ranaivonjatovo, J. Escudié, C. Couret, and J. Satgé, Organometallics, 11, 2748 (1992); (b) G. Anselme, J. -P. Declercq, A. Dubourg, H. Ranaivonjatovo, J. Escudié, and C. Couret, J. Organomet. Chem., 458, 49 (1993). 33. Y. Mizuhata, N. Takeda, T. Sasamori, and N. Tokitoh, Chem. Commun., 5876 (2005). 34. H. Schumann, M. Glanz, F. Girgsdies, F. E. Hahn, M. Tamm, and A. Grzegorzewski, Angew. Chem., Int. Ed. Engl., 38, 2232 (1997). 35. N. Kuhn, T. Kratz, D. Bläser, and R. Boese, Chem. Ber., 128, 245 (1995). 36. A. Schäfer, M. Weidenbruch, W. Saak, and S. Pohl, J. Chem. Soc., Chem. Commun., 1157 (1995). 37. K. M. Mackay, in The Chemistry of Organic Germanium, Tin and Lead Compounds, S. Patai (Ed.), John Wiley & Sons, Ltd, Chichester, 1995. 38. G. Anselme, C. Couret, J. Escudié, S. Richelme, and J. Satgé, J. Organomet. Chem., 418, 321 (1991). 39. (a) N. Wiberg and S. K. Vasisht, Angew. Chem., Int. Ed. Engl., 30, 1993 (1991); (b) N. Wiberg, S. Wagner, and S.-K. Vasisht, Chem. Eur. J., 4, 2571 (1998). 40. H. Grützmacher, S. Freitag, R. Herbert-Irmer, and G. S. Sheldrick, Angew. Chem. Int. Ed. Engl., 31, 437 (1992). 41. L. Baiget, H. Ranaivonjatovo, J. Escudié, and H. Gornitzka, J. Am. Chem. Soc. 126, 11792 (2004). 42. (a) M. Saito, R. Haga, and M. Yoshioka, Heteroatom Chem., 12, 349 (2001); (b) M. Saito, R. Haga, and M. Yoshioka, Main Group Met. Chem., 25, 81 (2002); (c) M. Saito, R. Haga, and M. Yoshioka, Chem. Commun., 1002 (2002); (d) M. Saito, R. Haga, and M. Yoshioka, Chem. Lett., 32, 912 (2003); (e) M. Saito, R. Haga, and M. Yoshioka, Phosphorus, Sulfur and Silicon and the Relat. Elem., 179, 703 (2004); (f) M. Saito, R. Haga, and M. Yoshioka, Eur. J. Inorg. Chem., 3750 (2005); (g) M. Saito, R. Haga, M. Yoshioka, K. Ishimura, and S. Nagase, Angew. Chem. Int. Ed., 44, 6553 (2005); (h) R. Haga, M. Saito, and M. Yoshioka, J. Am. Chem. Soc., 128, 4934 (2006); (i) R. Haga, M. Saito, and M. Yoshioka, Eur. J. Inorg. Chem., 1297 (2007). 43. M. Saito, M. Shimosawa, M. Yoshioka, K. Ishimura, and S. Nagase, Organometallics, 25, 2967 (2006). 44. M. Saito, M. Shimosawa, M. Yoshioka, K. Ishimura, and S. Nagase, Chem. Lett., 35, 940 (2006). 45. Y. Mizuhata, N. Takeda, T. Sasamori, and N. Tokitoh, Chem. Lett., 34, 1088 (2005).

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46. N. Tokitoh, A. Shinohara, T. Matsumoto, T. Sasamori, N. Takeda, and Y. Furukawa, Organometallics, 26, 4048 (2007). 47. T. Sasamori, K. Inamura, W. Hoshino, N. Nakata, Y. Mizuhata, Y. Watanabe, Y. Furukawa, and N. Tokitoh, Organometallics, 25, 3533 (2006). 48. Y. Mizuhata, T. Sasamori, N. Takeda, and N. Tokitoh, J. Am. Chem. Soc., 128, 1050 (2006). 49. M.-A. Chaubon, J. Escudié, H. Ranaivonjatovo, and J. Satgé, Chem. Commun., 2621 (1996). 50. A. Schäfer, W. Saak, and M. Weidenbruch, Organometallics, 22, 215 (2003). 51. A. Sekiguchi, R. Izumi, V. Y. Lee, and M Ichinohe, Organometallics, 22, 1483 (2003). 52. (a) V. Y. Lee, K. Takanashi, M. Ichinohe, and A. Sekiguchi, J. Am. Chem. Soc., 125, 6012 (2003); (b) V. Y. Lee, K. Takanashi, M. Nakamoto, and A. Sekiguchi, Russ. Chem. Bull., Int. Ed., 53, 1102 (2004). 53. A. Sekiguchi, R. Izumi, V. Y. Lee, and M. Ichinohe, J. Am. Chem. Soc., 124, 14822 (2002). 54. For reviews of triply bonded compounds of heavier main group elements, see: (a) P. P. Power, Chem. Commun., 2091 (2003); (b) M. Weidenbruch, J. Organomet. Chem., 646, 39 (2002); (c) P. Jutzi, , Angew. Chem. Int. Ed., 39, 3797 (2000). 55. N. Wiberg, W. Niedermayer, G. Fischer, H. Nöth, and M. Suter, Eur. J. Inorg. Chem., 1066 (2002). 56. A. Sekiguchi, R. Kinjyo, and M. Ichinohe, Science, 305, 1755 (2004). 57. M. Stender, A. D. Phillips, R. J. Wright, and P. P. Power, Angew. Chem. Int. Ed., 41, 1785 (2002). 58. Y. Sugiyama, T. Sasamori, Y. Hosoi, Y. Furukawa, N. Takagi, S. Nagase, and N. Tokitoh, J. Am. Chem. Soc., 128, 1023 (2006). 59. A. D. Phillips, R. J. Wright, M. M. Olmstead, and P. P. Power, J. Am. Chem. Soc., 124, 5930-5931 (2002). 60. L. H. Pu, B. Twamley, and P. P. Power, J. Am. Chem. Soc., 122, 3524 (2000). 61. W. Setaka, K. Hirai, H. Tomioka, K. Sakamoto, and M. Kira, J. Am. Chem. Soc., 126, 2696 (2004). 62. L. Pu, A. D. Phillips, A. F. Richards, M. Stender, R. S. Simons, M. M. Olmstead, and P. P. Power, J. Am. Chem. Soc., 125, 11626 (2003). 63. C. F. Roland, L. Pu, C. F. James, A. B. Marcin, and P. P. Philip, J. Am. Chem. Soc., 128, 11366 (2006). 64. M. M. Olmstead, R. S. Simons, and P. P. Power, J. Am. Chem. Soc., 119, 11705 (1997). 65. L. Pu, M. O. Senge, M. M. Olmstead, and P. P. Power, J. Am. Chem. Soc., 120, 12682 (1998). 66. H. S. Geoffrey, R. G. Jason, P. A. Matthew, I. Nowik, H. H. Rolfe, and P. P. Philip, Inorg. Chem., 45, 9132 (2006). 67. C. Cui, M. O. Marilyn, C. F. James, H. S. Geoffrey, and P. P. Power, J. Am. Chem. Soc., 127, 17530 (2005).

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Tetraorganodistannoxanes: Simple Chemistry From a Personal Perspective

2.10

201


Klaus Jurkschat Lehrstuhl für Anorganische Chemie der Technischen Universität, Dortmund, Germany Dedicated to Professor Dainis Dakternieks on the occasion of his retirement.

2.10.1

Introduction

Among organometallics, organotin compounds probably show the most diverse range of applications ranging from all sorts of biological activity to catalysts for value-creating technical processes.1−3 An important class of organotin compounds are tetraorganodistannoxanes.4 These compounds can formally be interpreted as intermediates along the hydrolysis pathway of diorganotin compounds R2 SnX2 (X = electronegative substituent) and indeed, controlled hydrolysis is one way to obtain them (path a, Scheme 2.10.1). Alternative procedures for the synthesis of these compounds include the reaction of diorganotin oxides with acids (paths b,c), the oxidation of functional distannanes with oxygen (path d) and the treatment of diorganotin oxides with an equimolar quantity of diorganotin compounds R2 SnX2 (X = electronegative substituent) (path e). The latter path probably is the most elegant one, as no by-products are formed that need to be removed. Tetraorganodistannoxanes, in all their variations, are usually crystalline, sharp-melting solids that are, depending on the substituent pattern at the tin atoms, sparingly to well soluble in common organic solvents. The chemistry of tetraorganodistannoxanes is as old as organotin chemistry itself, with the first reports made by Löwig5 and Cahours.6 However, it took more than hundred years until the true nature of this class of compounds was unravelled. The first proposal concerning the connectivity of the atoms in tetraorganodistannoxanes was made by Pfeiffer and Brack, that is Et2 XSnOSnXEt2 (X= Cl, Br), without knowing about their dimeric nature.7 A breakthrough in understanding tetraorganodistannoxanes was made in the early sixties of the last century by the pioneering work of Okawara et al. 8,9 and Davies et al.,10 including molecular weight determinations and the first solid-state structure, as determined by single crystal X-ray diffraction analysis. Their findings initiated further research activities and substantial contributions to the elucidation of structures in solution and in the solid-state were made by the groups of Harrison,11 Puff,12 Holmes,13 Gross,14 Jain,15 Michel,16 and Hasha,17,18 whereas applications in organic syntheses were especially developed by Otera and coworkers.19,20

Scheme 2.10.1

Synthesis of tetraorganodistannoxanes

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The molecular structures of tetraorganodistannoxanes in the solid state are characterized by: (i) the ladder-type motif formally realized by dimerization via O→Sn donor-acceptor interactions and (ii) the almost planar Sn4 X4 O2 layer (Scheme 2.10.1). In context with the rather simple synthetic procedures for making tetraorganodistannoxanes (Scheme 2.10.1), their catalytic activity in a variety of organic reactions, and their structural peculiarities, this class of organotin compounds has become among the most extensively investigated. It is not the scope of this account to thoroughly review all of these activities, because this has been already done by others,4,19−21 but to summarize the contributions we have made to the field and, where possible, to put these in context with related work. 2.10.2

Unsymmetrically Substituted Tetraorganodistannoxanes

Until 1991, there had been no structurally characterized unsymmetrically substituted tetraorganodistannoxanes, although the compound Et2 BrSnOSnBrn-Pr2 had been mentioned in an early report.22 We decided to start our investigations with t-butyl-substituted tin compounds because I had experience with these concerning good crystallization properties and their simple 1 H NMR spectra. The reaction of t-Bu2 SnCl2 with 1/n (R2 SnO)n or of R2 SnCl2 with 1/3 (t-Bu2 SnO)3 in toluene provided almost quantitatively [t-Bu2 ClSnOSnClR2 ]2 (1, R = Me; 2, R = n-Bu), the schematic molecular structure of which is shown in Scheme 2.10.2.23 The most striking features of the structures of these compounds are that: (i) the more bulky t-Bu2 Sn-moieties are in the exocyclic positions and (ii) the chlorine atoms are more strongly bound to the exocyclic than to the endocyclic tin atoms. Formally, this structure can be interpreted in terms of a four-membered diorganotin oxide, cyclo-(R2 SnO)2 , being stabilized by two Lewis-acidic t-Bu2 SnCl2 -molecules through O→Sn and Cl→Sn interactions. Although a formalism, this way of looking at the structures of compounds 1 and 2 is meaningful as it supports the interpretation of the behavior of these compounds in solution. Thus, both tetraorganodistannoxanes 1 and 2 proved to

Scheme 2.10.2 Schematic representation of the tetraorganodistannoxanes 1, 2, 6, 7, 10, and 11 including ˚ selected bond distances (A)

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Scheme 2.10.3 spectroscopy

Equilibrium between the tetraorganodistannoxanes 1 and 3, as studied by

119 Sn

203

NMR

be kinetically labile in solution, that is, the 119 Sn NMR spectrum of a sample of analytically pure singlecrystalline 1 that had been dissolved in CDCl3 displayed 16 resonances of different integral ratios. Not all of these signals could be assigned, but based on 1 H, 13 C, 119 Sn NMR spectroscopic and electrospray mass spectrometric studies, the equilibrium shown in Scheme 2.10.3 was established. Addition of Me2 SnCl2 or t-Bu2 SnCl2 shifts the equilibrium to the right or to the left, respectively, to the extent that either compound 1 or the tetramethyldistannoxane [Me2 ClSnOSnClMe2 ]2 (3) are exclusively present in the equilibrium mixture. In the course of these studies we also learned that reaction of t-Bu2 SnCl2 with 1/3 cyclo-(t-Bu2 SnO)3 gives a mixture consisting of t-Bu2 ClSnOSnClt-Bu2 ) (4) and the ‘three quarter’ ladder t-Bu2 Cl2 SnO(t-Bu2 Sn)2 O (5) which, however, could not be separated. On the other hand, the fluorine-substituted tetraorganodistannoxane [t-Bu2 (F)SnOSn(F)t-Bu2 ]2 (6) (Figure 2.10.1) was obtained from the reaction of cyclo-(t-Bu2 SnO)3 with (t-BuF2 Si)2 , along with cyclo-[t-BuFSiOSntBu2 ]2 O.24 In contrast to compounds 1 and 2, in solution the fluoro-substituted tetraorganodistannoxane 6, as well as its non-symmetrically substituted analog [t-Bu(F)SnOSn(F)n-Bu2 ]2 (7),25 are kinetically inert on the 1 H, 13 C, and 119 Sn NMR timescales, indicating superior bridging capacity of fluoride over chloride

Figure 2.10.1

Molecular structure of [(t-Bu2 FSn)2 O]2 (6), a fluoro-substituted tetraorganodistannoxane

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Scheme 2.10.4 Schematic representation of the five possible isomers for the tetraorganodistannoxanes (PhR(Cl)SnOSn(Cl)RPh]2 (8, R = Me3 SiCH2 ; 9, R = Me2 (CH2 CH)SiCH2 )

anions. Support in understanding this kinetic inertness comes again from the inspection of the molecular structures. It shows for both 6 and 7 a shorter Sn–F distance to the endocyclic (c in Scheme 2.10.2) than to the exocyclic (a in Scheme 2.10.2) tin atoms. Nevertheless, these distances should not be over-interpreted, as Sn–F bonds are known to have a high ionic character.26 The hydrolysis of the non-symmetrically substituted diorganotin dichloride (Me3 SiCH2 )PhSnCl2 gives rise to formation of the tetraorganodistannoxane [(RPh(Cl)SnOSn(Cl)PhR]2 (8, R Me3 SiCH2 ), which in solution is kinetically labile on the laboratory timescale, but inert on the 119 Sn NMR timescale. Thus, in solution, all five possible isomers of 8 are detected (Scheme 2.10.4). Two of these isomers (8a, 8b) were characterized by single crystal diffraction analysis.27 It is not clear yet whether these two isomers exclusively crystallized or whether the crystal fraction the crystals were taken from contained the other three isomers as well. The former variant is more likely, as an independent study on the hydrolysis of [Me2 (CH2 CH)SiCH2 ]PhSnCl2 gave the corresponding tetraorganodistannoxane [RPh(Cl)SnOSn(Cl)PhR]2 (9, R Me2 (CH2 CH)SiCH2 ) for which, in solution, five isomers were also observed (Scheme 2.10.4) and from which the related isomers 9a and 9b, shown in Figure 2.10.2, crystallized.28 Bearing in mind the similarity of fluoride and hydroxide ions, we were also interested in rational syntheses of perhydroxy-substituted tetraorganodistannoxanes. Thus, the careful treatment of the diorganotin dichloride (Me3 SiCH2 )2 SnCl2 with NaOH and the reaction of cyclo-(t-Bu2 SnO)3 with (Me3 SiCH2 )2 Sn(OSiMe3 )2 , in the presence of water, gave compounds [(Me3 SiCH2 )2 (OH) SnOSn(OH)(CH2 SiMe3 )2 ]2 (10) and [t-Bu2 (OH)SnOSn(OH)(CH2 SiMe3 )2 ]2 (11) (Figure 2.10.3), respectively.29,30 Like the chloro-substituted tetraorganodistannoxanes 1 and 2, in solution both compounds 10 and 11 are kinetically labile. Thus, 10 is in equilibrium with the corresponding diorganotin oxides [(Me3 SiCH2 )2 SnO]n (12, n = 3; 13, n = 4). Compound 12 was completely characterized (Figure 2.10.4),29 whereas the identity of 13 was tentatively concluded from an additional 119 Sn NMR signal at δ 47.5 with 2 119 J ( Sn-O-117 Sn) of 437 Hz and a satellite-to-signal-to-satellite integral ratio of 8:84:8. Tetraorganodistannoxane 11 is in equilibrium with cyclo-(t-Bu2 SnO)3 (14) and the mixed diorganotin oxides cyclo-R2 Sn(OSnt-Bu2 )2 O, (15, R = Me3 SiCH2 ), and cyclo-t-Bu2 Sn(OSnR2 )2 O (16,

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205

Figure 2.10.2 Molecular structures of the trans- and cis-isomers of [(RPhClSn)2 O]2 (R = Me2 (CH2 CH)SiCH2 , 9a (left), 9b (right)

R = Me3 SiCH2 ) which, however, could not be isolated. We tentatively concluded two pathways to account for formation of these diorganotin oxides; one involving the ‘three-quarter’ ladder tBu2 Sn(OH)2 ·O(R2 Sn)2 O (17, R = Me3 SiCH2 ) and t-Bu2 Sn(OH)2 (18), and the other the monomeric tetraorganodistannoxane t-Bu2 (OH)SnOSn(OH)R2 (19, R = Me3 SiCH2 ) (Scheme 2.10.5).30 Worth mentioning in this context is that t-Bu2 Sn(OH)2 does exist and has been characterized31 and that there are also examples of monomeric tetraorganodistannoxanes, i.e., [(Me3 Si)2 CH](OH)SnOSn(OH)[CH(SiMe3 )2 ] (20)32 and [2,6-(t-BuOCH2 )2 C6 H3 Sn(OH)]2 O (21).33

Figure 2.10.3 Molecular structure of the perhydroxy-substituted tetraorganodistannoxane [t-Bu2 (OH) SnOSn(OH)(CH2 SiMe3 )]2 (11)

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Figure 2.10.4

Molecular structure of the diorganotin oxide [(Me3 SiCH2 )2 SnO]3 (12)

In the course of these investigations it was also shown that cohydrolysis of diorganotin dichlorides, or related species, with trimethylchlorosilane provides a variety of organostannasiloxanes. Depending on the identity of R, either mononuclear organotin compounds R2 Sn(OSiMe3 )2 (22, R = i-Pr; 23, R = Me3 SiCH2 ; 24, R = t-Bu; 25, R = Cp(CO)3 W; 26, R = Cp(CO)2 Fe) or tetraorganodistannoxanes [R2 (Me3 SiO)SnOSn(OSiMe3 )R2 ]n (27, R = Me, n = 2; 28, R = Et, n = 2; 29, R = t-Bu, n = 1; 30, R = Me3 SiCH2 , n = 1) are obtained.34 Notably, the tetraorganodistannoxanes [t-Bu2 (OH)SnOSnClR2 ]2 (31, R = cyclohexyl [35]; 32, R = Me3 SiCH2 [30]) containing both strongly and weakly coordinating anions, such as hydroxide and chloride, respectively, are, at least on the 119 Sn NMR timescale, kinetically inert and this is in line with the observations made by Hasha on related compounds.17 An interesting aspect when looking at the ladder-type structures is the idea that four-membered diorganotin oxides can be stabilized by making the tin atoms five- or six-coordinate. Indeed, this idea was shown to be correct by isolation of di-2-aminopyrimidinium tetraphenyldichloro-bis(μoxo)distannate, (C4 H6 N3 )2 [Ph2 ClSnO]2 (33) (Scheme 2.10.6)36 and of [R(PhS)SnO]2 (R = 4-t-Bu-2,6[P(O)(O-i-Pr)2 ]2 C6 H2 ). Finally, in context with the identification in situ of the ‘three quarter’ ladder t-Bu2 SnCl2 ·O(t-Bu2 SnO)2 (5) and the utilization of cyclo-(t-Bu2 SnO)3 , we investigated in more detail compounds of the type t-Bu2 SnX2 ·O(t-Bu2 SnO)2 E (35) (E = Ph2 Si, Ph2 P(O)+ ; X = OH, F).31,37,38 The latter were obtained by two different reactions (Scheme 2.10.7), but they might formally be interpreted as being derived from the reaction of compound 5 with an element or organoelement oxide, E=O. This view might initiate further activities in this direction. As one example, the molecular structure of t-Bu2 SnF2 ·O(t-Bu2 SnO)2 SiPh2 is shown in Figure 2.10.5.

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Scheme 2.10.5 and 16

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Two pathways proposed to account for the formation of mixed diorganotin oxides 15

Scheme 2.10.6 Schematic presentation of the anion in (C4 H6 N3 )2 [Ph2 ClSnO]2 (33) including selected bond ˚ and bond angles (◦ ) distances (A)

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Scheme 2.10.7

Different approaches to obtain compounds of type t-Bu2 SnX2 ·O(t-Bu2 SnO)2 E (35)

Figure 2.10.5

Molecular structure of t-Bu2 SnF2 ·O(t-Bu2 SnO)2 SiPh2

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2.10.3

209

Looking for the Third Dimension

Structural modifications of tetraorganodistannoxanes (R2 XSnOSnYR2 )2 can be realized by variation of the organic substituents R and/or of the electronegative groups X and Y. One tempting idea that had been around for many years was to create organostannoxanes with a higher tin nuclearity, that is, for instance, to link tetraorganodistannoxanes via spacers (Scheme 2.10.8).

Scheme 2.10.8

Strategies to link tetraorganodistannoxanes to give supramolecular networks

Scheme 2.10.9

Schematic representation of the anion in [NHEt3 ][(SnMe2 Cl)5 O3 ]

At that time there were only two reports in this direction, i.e., [NHEt3 ][(SnMe2 Cl)5 O3 ],39 a pentanuclear hydrolysis product of dimethyltin dichloride (Scheme 2.10.9) and the crystal structure of [(Me2 (AcO)SnOSn(OAc)Me2 ]2 revealing the latter to be a polymer realized by intermolecular carboxylate bridges.40 In a first attempt and with the results of the non-symmetrically substituted tetraorganodistannoxanes 1 and 2 in mind, we looked at the reaction of the trimethylene-bridged ditin compound RCl2 Sn(CH2 )3 Sn Cl2 R (36, R = Me3 SiCH2 ) with cyclo-(t-Bu2 SnO)3 (Scheme 2.10.10). Surprisingly, we did not obtain the expected product, t-Bu2 (Cl)SnOSn(R)(Cl)(CH2 )3 Sn(R) (Cl)OSn(Cl)t-Bu2 , but t-Bu2 SnCl2 in quantitative yield and a sharp melting compound of empirical formula {[R(Cl)Sn(CH2 )3 Sn(Cl)R]O}n (37, R = Me3 SiCH2 ) (Scheme 2.10.10). From molecular weight determination we found compound 37 to be tetrameric (n = 4) in solution, but it was only the single crystal X-ray diffraction analysis that provided the key to understanding the true nature of this compound (Figure 2.10.6) and revealed that the idea of linking ladder-type structural motifs via covalent bonds had been realized.41

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Scheme 2.10.10

Synthesis of the spacer-bridged tetraorganodistannoxane 37

In the solid state, compound 37 is a prism-type entity in which two almost planar Sn4 Cl4 O2 layers are linked together by four trimethylene spacers. This solid state structure is retained in solution, as is evidenced by: (i) two equally intense 119 Sn and four pairs of equally intense 13 C NMR signals, (ii) electrospray mass spectrometry, and (iii) molecular weight determination. Subsequent investigations showed that the most efficient approach for this class of compounds is again the reaction of the α, ω-bis(dichloroorganostannyl) alkane with its corresponding oxide (Scheme 2.10.11).42 The investigations also revealed cyclo-(t-Bu2 SnO)3 to be a superior O2− source over (Me2 SnO)n (Scheme 2.10.11).

Figure 2.10.6

Molecular structure of {[R(Cl)Sn(CH2 )3 Sn(Cl)R]O}4 (37, R = Me3 SiCH2 )

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Scheme 2.10.11

Scheme 2.10.12

211

Different synthetic pathways to obtain spacer-bridged tetraorganodistannoxanes (DL)

Possible structures for spacer-bridged tetraorganodistannoxanes {[R(X)Sn-Z-Sn(X)R]O}n

Once the first double-ladder-type compound had been established it was apparent that systematic study of this class of compounds should be conducted and a number of questions in context with the structures (A), (B), and (C) shown in Scheme 2.10.12 be addressed. Thus, we were and still are interested in studying the influence the identity of the spacers Z, the organic substituents R, and the electronegative substituents X and Y have on which structure is actually observed in solution and in the solid state, and whether these structures can be inter-converted. Furthermore, we looked at the possibility of extending the concept from double to multiple ladder-type structures and made the first attempts to assemble tetraorganodistannoxanes with double ladder-type structures to give supramolecular networks. Associated with the tasks mentioned above, a great number of spacer-bridged ditin compounds of type RX2 Sn-Z-SnX2 R (38) (X = halogen, OCOR ; Z = (CH2 )n with n = 1–12, CH2 SiMe2 CH2 , 1,3(CH2 CH2 )2 C6 H4 ; R = Ph, Me3 SiCH2 , Cp(CO)2 Fe) had to be prepared and most of them were also characterized by single crystal X-ray diffraction analysis. In general, these compounds feature both intraand intermolecular Sn-X· · ·Sn (X = halogen) bridges, the extent of which depends on the identity of X and R, and on the lengths of the spacer Z, and which give rise to supramolecular networks. One representative example of many is shown in Figure 2.10.7.43 The structure of the spacer-bridged tetraorganodistannoxanes actually observed in solution and in the solid state is the sum of the influences caused by the substituents R, X, and Y, and by the length and composition of the spacer Z. In the following, we look for some trends that are induced by each of these variables (see Scheme 2.10.12 and Table 2.10.1).

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Figure 2.10.7

2.10.4


Molecular structure of [Cp(CO)2 Fe(Cl2 )SnCH2 ]2 CH2

Variation of R

For a given spacer and given electronegative substituents X and Y, variation of the substituent R controls the type of structure that is actually observed in the solid state and in solution. Thus, the bulky substituent (Me3 Si)2 CH (55, entry 17), as well as the electron-rich Cp(CO)2 Fe-moiety (56, entry 18), favor a monomeric C-type structure in solution that is likely to be retained in the solid state as well. Replacement of the Cp(CO)2 Fe-substituent in 56 (entry 18) by the less electron-releasing (CO)5 Mn-moiety in 57 (entry 19) causes a B-type structure (cis–trans equilibrium) in solution. The closely related substituents Me3 SiCH2 , Me2 (CH2 CH)SiCH2 , and Me3 CCH2 have similar effects on the structures to the extent that for compounds 46–48, A-type structures are observed in the solid state and in solution (entries 8–10). For compounds 58, 61, 62 (entries 20, 23, 24), A-type structures are found in the solid state, but equilibria between A- and B-type arrangements occur in solution. Finally, the compounds 71 and 72 (entries 33 and 34) show exclusively cis-B-type structures, whereas the derivatives 73 and 74 (entries 35, 36) adopt transB-type structures in the solid state, but B C equilibria in solution. Interestingly, all phenyl-substituted compounds (39–43, entries 1–5) exclusively adopt A-type double ladder structures, regardless of spacer length and the identity of X and Y. 2.10.5

Variation of Spacer Z

The mono- and oligomethylene-bridged compounds [Z = (CH2 )n , n = 1, 3–8, 10, 12; entries 1–11, 14, 20– 25, 27, 29, 30, 38) exhibit exclusively double ladder structures (A-type) in the solid state with separations ˚ (68, entry 30). between the two Sn4 X2 Y2 O2 layers ranging from approximately 3.5 (40, entry 2) to 15 A The molecular structures of compounds 40 and 68 are shown in Figures 2.10.8 and 2.10.9, respectively. In solution, most of these compounds are kinetically labile and involved in A B equilibria, with the latter being shifted towards the B-type structures as the spacer length increases (entries 7, 11, 20–24, 26–30, 38). Related to this trend is the observation that attempts at preparing tetraorganodistannoxanes containing different R groups by mixing equimolar amounts of [{Me3 SiCH2 (Cl)Sn(CH2 )3 Sn(Cl)CH2 SiMe3 }O]4 (46) (entry 8) and [{Me3 CCH2 (Cl)Sn(CH2 )3 Sn(Cl)CH2 CMe3 }O]4 (48) (entry 10) failed, whereas reaction of the tetramethylene-bridged analogs under the same conditions quantitatively provided the corresponding tetraorganodistannoxane 61, containing both the Me3 SiCH2 - and Me3 CCH2 -substituents (entry 23) (Scheme 2.10.13, Figure 2.10.10).

R

Ph Ph Ph Ph Ph Me3 SiCH2 Me3 SiCH2 Me3 SiCH2 Me2 (CH2 CH)SiCH2 Me3 CCH2 Me3 SiCH2 Me3 SiCH2 Me3 SiCH2 Me3 SiCH2 Me3 SiCH2 (Me3 Si)2 CH (Me3 Si)2 CH Cp(CO)2 Fe

1/39 2/40 3/41 4/42 5/43 6/44 7/45 8/46 9/47 10/48 11/49 12/50 13/51 14/52 15/53 16/54 17/55 18/56

OH OH 2OH/2Cl OH OH 2OH/2Cl OH Cl Cl Cl OAc Cl PhC(O)O 2,5-Me2 C6 H3 C(O)O p-NH2 C6 H4 C(O)O F Cl Cl

X I Br Cl Br I Cl Me3 SiO Cl Cl Cl OAc Cl PhC(O)O 2,5-Me2 C6 H3 C(O)O p-NH2 C6 H4 C(O)O F Cl Cl

Y A A A A A A A A A A A not known B (cis) A B (cis) B (trans) not known not known

A B A A A A B B B C B C B B C C

A A A A A

51 52 46 46 46 46 46 41 46 42 42 45 58 58 46 44 44 45

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CH2 CH2 (CH2 )3 (CH2 )3 (CH2 )3 (CH2 )3 (CH2 )3 (CH2 )3 (CH2 )3 (CH2 )3 (CH2 )3 CH2 SiMe2 CH2 (CH2 )3 (CH3 )3 (CH2 )3 (CH2 )3 (CH3 )3 CH2 SiMe2 CH2

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Solution (NMR spectroscopy, molecular weight determination)

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Table 2.10.1 Spacer-bridged tetraorganotindistannoxanes {[R(X)Sn-Z-Sn(Y)R]O}n . Structures in solution and in the solid state. The letters A, B, C refer to Scheme 2.10.12


Cl Cl F OAc Cl Cl Cl Cl Cl Cl Cl Cl Cl 1Cl/1OH OH OH Cl Cl OH OH Cl Cl Cl

Cl Cl Cl OAc Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl H2 O/OTf Cl Cl Cl

not known A A A A A A not known A not known A A B(cis) B (cis) B (cis) B (cis) B (trans) B (trans) B (trans) A A A A

B(cis, trans) A B A B A B A B A B A A B A B A B A B A B B (one isomer) B B B B C B C B (cis, trans) A B not known A complex

45 42 46 49 47 47 53 53 53 53 53 53 48 49 50 50 49 50 50 57 54 54 56

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(CH2 )3 (CH2 )4 (CH2 )4 (CH2 )4 (CH2 )4 (CH2 )4 (CH2 )5 (CH2 )6 (CH2 )7 (CH2 )8 (CH2 )10 (CH2 )12 CH2 (Me2 )SiC2 Si(Me2 )CH2 CH2 (Me2 )SiC2 Si(Me2 )CH2 CH2 (Me2 )SiC2 Si(Me2 )CH2 CH2 (Me2 )SiC2 Si(Me2 )CH2 CH2 (Me2 )SiOSi(Me2 )CH2 CH2 (Me2 )SiOSi(Me2 )CH2 CH2 (Me2 )SiOSi(Me2 )CH2 (CH2 )3 p-(CH2 Me2 Si)C6 H4 m-(CH2 CH2 )2 C6 H4 (CH2 )3 O(CH2 )3

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19/57 20/58 21/59 22/60 23/61 24/62 25/63 26/64 27/65 28/66 29/67 30/68 31/69 32/70 33/71 34/72 35/73 36/74 37/75 38/76 39/77 40/78 41/80

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Figure 2.10.8

Figure 2.10.9

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Molecular structure of {[Ph(Br)SnCH2 Sn(OH)Ph]O}4 (40)

Molecular structure of {[R(Cl)Sn(CH2 )12 Sn(Cl)R]O}4 (68, R = Me3 SiCH2 )

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Scheme 2.10.13 Syntheses of the tetraorganodistannoxane double ladder 61 containing different substituents R and R’

The kinetic inertness of the trimethylene-bridged tetraorganodistannoxane 46 (entry 8) on one hand and the kinetic lability of the tetramethylene-bridged species 58 (entry 23) on the other hand also becomes evident with the observation that the former does not react with the 1,3-bis(organodichlorostannyl)propane RCl2 Sn(CH2 )3 SnCl2 R (R = Me3 CCH2 ), whereas the latter shows exchange upon reaction with 1,4bis(organodichlorostannyl)butane, RCl2 Sn(CH2 )4 SnCl2 R (R = Me3 CCH2 ) (Scheme 2.10.14).47 A rather interesting situation was observed for the tetraorganodistannoxanes 69–72 containing acetylene-based spacers (entries 31–34). All these species exclusively crystallize as cis-B-type structures. The molecular structure of 72 is shown in Figure 2.10.11. In solution, they each show one isomer only, which is most likely the cis one. All these compounds are chiral, but they crystallize as racemic mixtures and no efforts have yet been made to separate the enantiomers. Formal replacement of the acetylene bridge in the compounds mentioned above by an oxygen atom provides the disiloxanyl-bridged tetraorganodistannoxanes shown in entries 35–37. Like their acetylene-bridged analogs, they adopt B-type structures, but exclusively crystallize as trans-isomers. The

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Figure 2.10.10

217

Molecular structure of the unsymmetrically substituted tetraorganodistannoxane 61

molecular structure of trans-([{R(Cl)SnCH2 SiMe2 }2 O]O)2 (74, R = Me3 CCH2 , entry 36) is shown in Figure 2.10.12. The two tetraorganodistannoxanes 77 and 78, containing phenyl-derived spacers, are listed in entries 39 and 40, respectively. Both compounds adopt A-type double ladder structures in the solid state (Figure 2.10.13) and for 78 this structure is retained in solution. Given the position of tetraorganodistannoxanes as intermediates on the hydrolysis pathway from diorganotin dichloride to diorganotin oxide, as discussed above, it should be possible to prepare spacerbridged molecular diorganotin oxides as well. We could not directly demonstrate this relation for the two tetraorganodistannoxanes shown in entries 39 and 40, but achieved the goal by increasing the steric bulk of the organic substituent R. Thus, [ p-{R(O)SnCH2 SiMe2 }2 C6 H4 ]3 [R = (Me3 Si)2 CH2 ] is the first example of a spacer-bridged molecular organotin oxide (Figure 2.10.14).55 ˚ Most interestingly, The distance between the two Sn3 O3 layers amounts to approximately 12.2 A. as with some of the tetraorganodistannoxanes (see discussion below) the molecule shows a twist (37◦ ) between the upper and lower Sn3 O3 rings making it chiral.

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Scheme 2.10.14 former

Reaction of compound 58 with [Me3 CCH2 (Cl2 )SnCH2 ]2 proving the kinetic lability of the

From an inspection of the A-type double ladder structures it becomes evident that there are cavities between the Sn4 O2 X2 Y2 layers, the size of which depend on the spacer lengths, and the question arises whether these cavities can be employed to host guest molecules. However, so far, such a property has not been verified for any of the compounds investigated. With the idea of increasing the potential for cation inclusion, the tetraorganodistannoxane 80 (entry 41) was prepared. In the solid state it adopts the A-type structure in which the two Sn4 O2 Cl4 -layers are linked by four di-npropylether chains with the ether-oxygen atoms being thought of as potential donor sites that could complex metal cations. However, even for this compound, no enhanced affinity forwards Li+ , Na+ , Cu2+ , or Mg2+ is observed. In solution, compound 80 is kinetically labile and falls apart to give a number of species which are in equilibrium. The identity of these species could not be established unambiguously.

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Figure 2.10.11 Molecular structure of the acetylene-bridged tetraorganodistannoxane {[R(Cl)SnCH2 SiMe2 C2 SiMe2 CH2 Sn(OH)R]O}2 (72, R = Me3 CCH2 )

Figure 2.10.12 CCH2 )

Molecular structure of trans-{[R(Cl)SnCH2 SiMe2 OSiMe2 CH2 Sn(Cl)R]O}2 (74, R = Me3

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Figure 2.10.13

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Molecular structure of the m-diethylphenyl-bridged tetraorganodistannoxane 78

Figure 2.10.14 Molecular structure of the spacer-bridged diorganotin oxide [ p-{R(O)SnCH2 SiMe2 }2 C6 H4 ]3 (R = (Me3 Si)2 CH)

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2.10.6

221

Variation of the Electronegative Substituents X and Y

As with the identity of the organic substituents R and the spacers Z, the electronegative substituents X and Y also have a profound influence on the structure of spacer-bridged tetraorganodistannoxanes. Like the parent compounds, enhanced kinetic stability in solution is observed for those cases in which the substituents are different (X = Y). This is especially pronounced for the phenyl-substituted derivatives 39–43 (entries 1–5) and also valid for the acetylene-bridged compounds (entries 32–34). The compounds 39, 40, 67, and 77 (entries 1, 2, 29, and 39) show a twist between the upper and the lower Sn4 O2 layers, whereas the layers of the other compounds are superimposable. A further interesting aspect of the A-type structures is related to the positions of the bridging and non-bridging substituents X and Y. Two cases, (I) and (II), can be distinguished. In case (I), X and Y of the upper and lower layers are superimposable (entries 3, 6–11, 20, 22–25, 27, 38, and 41), whereas in case (II) they are opposites (entries 1, 2, 4, 5, 21, and 29) (Scheme 2.10.15).

Scheme 2.10.15 Relative positions of the electronegative substituents X and Y in the upper and lower Sn4 O2 X2 Y2 -layers

Probably the most striking features concerning the influence of X and Y on the structures are demonstrated for compounds 51– 53 (entries 13–5). The benzoate, as well as the p-aminobenzoate derivatives 51 and 53, respectively, each show a cis-B-type structure in the solid state (Figure 2.10.15), whereas the only slightly modified 2,4-dimethylbenzoate derivative 52 adopts the A-type structure (Figure 2.10.16). Apparently, both structures differ little in energy and this makes it understandable why they and other compounds listed in Table 2.10.1 form A B-equilibria in solution. Finally, replacement of the chlorine atoms in 55 (entry 17) by fluorine atoms in 54 (entry 16) causes the latter to adopt the B-type structure, even with the bulky (Me3 Si)2 CH-substituent, which again indicates the excellent bridging capacity of the fluoride ion. 2.10.7

Miscellaneous

The cohydrolysis of Me3 Si(Cl2 )Sn(CH2 )3 Sn(Cl2 )CH2 SiMe3 and trimethylchlorosilane, Me3 SiCl, under basic conditions affords the trimethylsiloxy-substituted tetraorganodistannoxane [{R(Me3 SiO) Sn(CH3 )3 Sn(OH)R}O]4 (45) (entry 7), which shows an A-type double ladder structure. Most surprisingly, reaction under the same conditions of the tetramethylene-bridged ditin compound, Me3 Si(Cl2 )SnCH2 )4 Sn(Cl2 )CH2 SiMe3 provides the novel deca-nuclear organotinoxo cluster 81, in which

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Figure 2.10.15 Molecular structure of the benzoate-substituted {[R(PhC(O)O)Sn(CH2 )3 Sn(OC(O)Ph)R]O}2 (51, R = Me3 SiCH2 )

tetraorganodistannoxane

cis-

Figure 2.10.16 Molecular structure of the m-dimethylbenzoate-substituted tetraorganodistannoxane {[R(2,4-Me2 C6 H3 COO)Sn(CH2 )3 Sn(OOCC6 H3 -2,4Me2 )R]O}4 (52, R = Me3 SiCH2 )

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two Sn5 O5 layers are connected by three tetramethylene spacers (Scheme 2.10.16) and in which the two trimethylsiloxy-substituents at each layer are cis (Figure 2.10.17).46

Scheme 2.10.16

Figure 2.10.17

Synthesis of the decanuclear organotinoxo cluster 81

Molecular structure of the decanuclear organotinoxo cluster 81

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Figure 2.10.18

Molecular structure of the decanuclear organotinoxo cluster 82

The structural motif of the Sn5 O5 layer strongly resembles that of Alcock’s Sn5 O3 Cl2 -layer in [NHEt3 ] [(Me2 SnCl)5 O3 ]39 and it appears that it is more common than originally anticipated. Thus, J.-F. Ma et al. have prepared [(R2 SnO)3 (R2 SnOH)2 Z]2 [82, Z = CO3 ; 83, Z = 1,1’-ferrocenedicarboxylic acid; 84, Z = OC(O)(CH2 )4 C(O)O)], in which two Sn5 O5 moieties are linked by two Z spacers.59 The molecular structure of the carbonate-bridged compound 82 is shown in Figure 2.10.18. In a rather spectacular piece of work, C. Ma et al. modified this basic structural unit even further by surrounding it with a belt containing eight tin atoms alternately linked by eight 2-mercapto-nicotinic acid anions to give the eighteen-tin-nuclear organotinoxo cluster 85, the molecular structure of which is shown in Figure 2.10.19.60 Most recently, the same group presented a methoxy-substituted Me10 Sn5 O5 ladder 86, which is stabilized by a 1,2,4-triazole-based organic backbone.61 In addition, a two-dimensional supramolecular assembly of the ladder units is realized by intermolecular C-H· · ·S and C-H· · ·O hydrogen bridges. The alert reader might have noticed that in Table 2.10.1 compounds that contain a dimethylene spacer are missing. The reaction of (RCl2 SnCH2 )2 (R = Me3 SiCH2 ) with either sodium hydroxide or cyclo-(tBu2 SnO)3 gives no well-defined tetraorganodistannoxane. However, the (Me3 Si)2 CH-substituted analog provides, under the same reaction conditions, a crystalline material that turns out to be the cyclic organotinoxo cluster 87, consisting of six tin atoms connected by a combination of dimethylene, oxygen, and hydroxo bridges. The cluster also contains two water molecules that are involved in hydrogen bridges to the cluster oxygen atoms (Figure 2.10.20).

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Figure 2.10.19

Molecular structure of the eighteen tin-nuclear organotinoxo cluster 85

Figure 2.10.20

Molecular structure of the hexanuclear organotinoxo cluster 87

225

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Scheme 2.10.17

Synthesis of the hexanuclear organotinoxo cluster 90

In solution, compound 87 is involved in an equilibrium with its corresponding tetranuclear diorganotin oxide [R(O)Sn(CH2 )2 Sn(O)R]2 (88, R = (Me3 Si)2 CH) and water.44 Another hexanuclear organotinoxo cluster based on diorganotin moieties is Molloy’s 1,5-diazido1,1,3,3,5,5-hexa-tristannoxane dimer [R2 (N3 )2 Sn(R2 SnO)2 N3 ]2 (89, R = n-Bu).62 Looking at the structure of this compound in a similar manner as we did for [t-Bu2 ClSnOSnClR2 ]2 (R = Me, n-Bu) might suggest it to be composed of a tetranuclear (R2 SnO)4 moiety that is stabilized by two R2 Sn(N3 )2 molecules in a Lewis acid–Lewis base relationship. Indeed, in solution this cluster is kinetically labile and shows at least 20 119 SnNMR resonances, but no systematic studies were made to assign these signals. The hexanuclear organotinoxo cluster 90 that is based on a spacer-bridged ditin compound is obtained by reaction of the10-membered ring cyclo-CH2 [CH2 Sn(Cl2 )CH2 SiMe2 ]2 O with cyclo-(t-Bu2 SnO)3 (Scheme 2.10.17). Its molecular structure is shown in Figure 2.10.21.50 It features a cis-configuration and, at first sight, can be seen as a central (RR SnO)4 moiety that is stabilized by two t-Bu2 SnCl2 molecules. Comparison

Figure 2.10.21

Molecular structure of the hexanuclear organotinoxo cluster 90

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227

Figure 2.10.22 Molecular structure of the tetraorganodistannoxane 94, the only one that shows a triple ladder structure

of the Sn–Cl distances shows, however, that the bridging chlorine atom is more strongly bound to the ˚ tin atom. In solution, compound 90 is kinetically inert on the 119 Sn MNR endocyclic (Sn-Cl 2.598 A) timescale and shows three equally intense resonances, as expected. The relatively simple procedures that allow the synthesis of a great number of tetraorganodistannoxanes with double ladder structures and the esthetic beauty of the latter provoked efforts to prepare related triple and quadruple ladders. In order to do so, the three and four tin atom-containing derivatives Cl2 Sn[(CH2 )3 SnCl2 R]2 (91, R = Me3 SiCH2 ), Me2 Si[CH2 Si(Cl2 )(CH2 )3 SnCl2 R]2 (92, R = Me3 SiCH2 ), and CH2 [CH2 Sn(Cl2 )(CH2 )3 SnCl2 R]2 (93, R = Me3 SiCH2 ) were prepared. The reaction of compound 91 with cyclo-(t-Bu2 SnO)3 gave t-Bu2 SnCl2 and the tetraorganodistannoxane 94, the X-ray diffraction analysis of which revealed indeed the first and as yet only triple ladder structure (Figure 2.10.22).63 The reaction, under the same conditions, of compound 92 with cyclo-(t-Bu2 SnO)3 gave a tetraorganodistannoxane 95 that exhibits a ‘folded’ double ladder structure (Figure 2.10.23)63 rather than a quadruple ladder. Apparently, the dimethylsilyl group in the spacer supports cyclization to give the intermediate 92a (Scheme 2.10.18) that in turn dimerizes to yield compound 95, which is the head-to-tail dimer of intermediate 92a. There is also spectroscopic indication for the existence of the head-to-head dimer 95b, but this isomer has not been isolated.

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Figure 2.10.23

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Molecular structure of the tetraorganodistannoxane 95, a ‘folded’ double ladder

Scheme 2.10.18 Suggested pathway that accounts for the formation of the head-to-tail and head-to-head dimers 95a and 95b, respectively

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The reaction of compound 93 with its corresponding oxide {CH2 [CH2 Sn(O)(CH2 )3 Sn(O)R]2 }n (96) provides the tetraorganodistannoxane 97 of which the elemental analysis and the 119 Sn NMR spectrum (four equally intense resonances) are in favor of a quadruple ladder. However, so far, this has not been substantiated by single crystal X-ray diffraction analysis.50 Future efforts might be devoted to employing tetraorganodistannoxanes with double and triple ladder structures as organometallic secondary building units in supramolecular chemistry. Acknowledgments I cordially acknowledge my coworkers and colleagues from Deakin University Geelong (Australia) and Dortmund University of Technology (Germany), whose names appear in the references, for their enthusiastic work. I am grateful to the Australian Research Council (Grant to D. Dakternieks), the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for financial support. Dr. G. Reeske is acknowledged for technical 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.

W. P. Neumann, The Organic Chemistry of Tin, John Wiley & Sons, Ltd, Chichester, 1970. P. J. Smith (Ed.), Chemistry of Tin, 2nd edn, Blackie, London, 1998. A. G. Davies, Organotin Chemistry, Wiley-VCH, Weinheim, 2004 (including comprehensive bibliography). A. G. Davies, J. Chem. Research, 309 (2004). C. Löwig, Liebigs Ann. Chem. 84, 308 (1852). A. Cahours, Liebigs Ann. Chem., 354 (1860). P. Pfeiffer and O. Brack, Z. Anorg. Allg. Chem., 87, 229 (1914). R. Okawara, Proc. Chem. Soc. 383 (1961). R. Okawara, D.G. White, K. Fujitami, and H. Sato, J. Am. Chem. Soc., 83, 1342 (1961). D. L. Alleston, A.G. Davies, M. Hancock, and R.F.M. White, J. Chem. Soc., 5469 (1963). P.G. Harrison, M.J. Begley, and K.C. Molloy, J. Organomet. Chem., 186, 213 (1980). H. Puff, I. Bung, E. Friedrichs, and A. Jansen, J. Organomet. Chem., 254, 23 (1983). J.F. Vollano, R.O. Day and R.R. Holmes, Organometallics, 3, 745 (1984). D.C. Gross, Inorg. Chem., 28, 2355 (1989). V.K. Jain, V.B. Mokal and P. Sandor, Magn. Res. Chem., 30, 1158 (1992). O. Primel, M.-F. Lauro, R. Pétiaud and A. Michel, J. Organomet. Chem., 558, 19 (1998). D.L. Tierney, P.J. Moehs and D.L. Hasha, J. Organomet. Chem., 620, 211 (2001). D.L. Hasha, J. Organomet. Chem., 620, 296 (2001). J. Otera, Chem. Rev., 93, 1449 (1993). J. Otera, Acc. Chem. Res., 37, 288 (2004). V. Chandrasekhar, K. Gopal and P. Thilagar, Acc. Chem. Res., 40, 420 (2007). T. Harada, Rep. Sci. Res. Inst. Tokyo, 24, 177 (1948). D. Dakternieks, K. Jurkschat, S. van Dreumel and E.R.T. Tiekink, Inorg. Chem., 36, 2023 (1997). J. Beckmann, M. Biesemans, K. Hassler, K. Jurkschat, J.C. Martins, M. Schürmann and R. Willem, Inorg. Chem. 37, 4891 (1998). S. Durand, K. Sakamoto, T. Fukuyama, A. Orita, J. Otera, A. Duthie, D. Dakternieks, M. Schulte and K. Jurkschat, Organometallics, 19, 3220 (2000). J. Beckmann, D. Horn, K. Jurkschat, F. Rosche, M. Schürmann, U. Zachwieja, D. Dakternieks, A. Duthie, and A.E.K. Lim, Eur. J. Inorg. Chem., 164 (2003) J. Beckmann, D. Dakternieks, A. Duthie and E.R.T. Tiekink, Dalton Trans., 755 (2003). H. Zhou, M. Schürmann, and K. Jurkschat, unpublished results.

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29. J. Beckmann, M. Henn, K. Jurkschat, M. Schürmann, D. Dakternieks and A. Duthie, Organometallics, 21, 192 (2002). 30. J. Beckmann, K. Jurkschat, S. Rabe, M. Schürmann and D. Dakternieks, Z. Anorg. Allg. Chem., 627, 458 (2001). 31. J. Beckmann, K. Jurkschat, B. Mahieu and M. Schürmann, Main Group Met. Chem., 21, 113 (1998). 32. M.A. Edelmann, P.B. Hitchcock, and M.F. Lappert, J. Chem. Soc., Chem. Commun., 1116 (1990). 33. B. Kasná, R. Jambor, M Schürmann, and K. Jurkschat, J. Organomet. Chem., 692, 3555 (2007). 34. J. Beckmann, K. Jurkschat, U. Kaltenbrunner, S. Rabe, M. Schürmann, D. Dakternieks, A. Duthie and D. Müller, Organometallics, 19, 4887 (2000). 35. U. Baumeister, D. Dakternieks, K. Jurkschat and M. Schürmann, Main Group Met. Chem., 25, 521 (2002). 36. N. Kourkoumelis, A. Hatzidimitriou, and D. Kovala-Demertzi, J. Organomet. Chem., 514, 163 (1996). 37. J. Beckmann, K. Jurkschat, M. Schürmann, D. Suter and R. Willem, Organometallics, 21, 3819 (2002). 38. J. Beckmann, D. Dakternieks, A. Duthie, K. Jurkschat, M. Mehring, C. Mitchell and M. Schürmann, Eur. J. Inorg. Chem., 4356 (2003). 39. N.W. Alcock, M. Pennington and G.R. Willey, J. Chem. Soc., Dalton Trans., 2683 (1985). 40. T. P. Lockhart, W. F. Manders and E. M. Holt, J. Am. Chem. Soc., 108, 6611 (1986). 41. D. Dakternieks, K. Jurkschat, D. Schollmeyer and H. Wu, Organometallics, 13, 4121 (1994). 42. M. Mehring, M. Schürmann, I. Paulus, D. Horn, K. Jurkschat, A. Orita, J. Otera, D. Dakternieks and A. Duthie, J. Organomet. Chem., 574, 176 (1999). 43. B. Zobel, M. Schürmann, and K. Jurkschat, unpublished results. 44. B. Zobel, M. Schürmann, K. Jurkschat, D. Dakternieks and A. Duthie, Organometallics, 17, 4196 (1998). 45. B. Zobel, PhD thesis, Dortmund University 1997. 46. I. Paulus, PhD thesis, Dortmund University 2005. 47. M. Mehring, I. Paulus, B. Zobel, M. Schürmann, K. Jurkschat, A. Duthie and D. Dakternieks, Eur. J. Inorg. Chem., 153 (2001). 48. M. Schulte, M. Schürmann, D. Dakternieks and K. Jurkschat, Chem. Commun., 1291 (1999). 49. M. Schulte, M. Mehring, I. Paulus, M. Schürmann, K. Jurkschat, D. Dakternieks, A. Duthie, A. Orita and J. Otera, Phosphorus, Sulfur and Silicon, 150, 201 (1999). 50. M. Schulte, PhD thesis, Dortmund University 2000. 51. M. Mehring, G. Gabriele, S. Hadjikakou, M. Schürmann, D. Dakternieks, and K. Jurkschat, Chem. Commun., 834 (2002). 52. M. Mehring, M. Schürmann, and K. Jurkschat, unpublished results. 53. J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan, K. Jurkschat, M. Schürmann and E.R.T. Tiekink, New J. Chem., 29, 1268 (2004). 54. D. Dakternieks, A. Duthie, B. Zobel, K. Jurkschat, M. Schürmann and E.R.T. Tiekink, Organometallics, 21, 647 (2002). 55. D. Dakternieks, B. Zobel, K. Jurkschat, M. Schürmann and E.R.T. Tiekink, Organometallics, 22, 1343 (2003). 56. J. Beckmann, D. Dakternieks, A. Duthie, F.S. Kuan and E.R.T. Tiekink, J. Organomet. Chem., 688, 56 (2003). 57. J. Beckmann, D. Dakternieks, A. Duthie and F.S. Kuan, and E.R.T. Tiekink, Organometallics, 22, 4399 (2003). 58. B. Costisella, D. Dakternieks, K. Jurkschat, M. Mehring, I. Paulus and M. Schürmann, Chemistry of Heterocyclic Compounds, 37, 1405 (2001). 59. G.-L. Zheng, J.-F. Ma, J. Jang, Y.-Y. Li and X.-R. Hao, Chem. Eur. J., 10, 3761 (2004). 60. C. Ma, Q. Jiang, R. Zhang and D. Wang, Dalton Trans., 2975 (2003). 61. C. Ma, Y. Han and R. Zhang, J. Inorg. Organomet. Polym. Mater., 17, 541 (2007). 62. M. Hill, M.F. Mahon and K.C. Molloy, Main Group Chemistry, 1, 309 (1996). 63. M. Mehring, M. Schürmann, H. Reuter, D. Dakternieks and K. Jurkschat, Angew. Chem. Int. Ed., 36, 1112 (1997).

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Unusual Bonds and Coordination Geometries

2.11

231


Mónica Moya-Cabrera, Vojtech Jancik and Raymundo Cea-Olivares Instituto de Qu´ımica, Universidad Nacional Autónoma de México

2.11.1

Introduction

This chapter deals with structurally characterized tin(II) and tin(IV) compounds containing either unusual bonds to tin atoms or unique geometric arrangements around the tin center. While we emphasize molecular compounds, selected examples of supramolecular assemblies featuring unusual structural traits are also discussed. Strictly inorganic compounds will not be considered, as they are amply described elsewhere.1 2.11.2

Unusual Bonds

Bonding to Elements from Groups 1 and 2

Organotin hydrides are extremely important reducing agents in organic synthesis, with the vast majority of tin hydrides involving Sn(IV). Currently, there are only two examples of tin(II) hydrides that have been structurally authenticated (Figure 2.11.1). The use of the bulky terphenyl substituent enabled the synthesis of the first Sn(II) hydride [Ar*Sn(μ-H)]2 (1) (Ar* = C6 H3 -2,6-Trip2 ; Trip = 2,4,6-i Pr3 -C6 H2 ) along with its deuterium analog [2,6-[Ar*Sn(μ-D)]2 (2).2 Compound 1 behaves as a monomer in solution, which can be assessed by the dramatic color change from orange in the solid state to dark blue when dissolved in hydrocarbon solvents. This color shift is related to an absorption at 608 nm, which can be attributed to an n → p electronic transition. The structure of 1 features pyramidalized tin atoms consistent with the presence of a stereochemically active lone pair of electrons at the metal. Treatment of [SnCl{HC(CMeNAr)2 }] with AlH3 ·NMe3 yielded the first monomeric and terminal tin (II) hydride [SnH{HC(CMeNAr)2 }] (3).3 DFT calculations show that the electron density of the lone pair of electrons contributes to the Sn H bond and further analyses exhibit a 70% participation of the

H1

Sn1A H1A

H1

N1

Sn1

Sn1

1Å 1Å

1

Figure 2.11.1

3

Molecular structures of compounds 1 and 3

N2

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Na1 Si4

N1 Sn1

Si10

Li1

Si1 N2

Si11 Si9

Sn1

N3

Si5

Si2

Si12

1Å

Si3

Si8

Si6 Si7 1Å

4

Figure 2.11.2

16

Molecular structures of compounds of compounds 4 and 16

hydrogen atom in this bond. 1 H-NMR data confirm a highly deshielded hydrogen atom, as a consequence of the lack of efficient delocalization of the electron density from the ligand to the tin atom. Structural data for alkali metal derivatives of tin (containing alkali−metal–tin contacts) are scarce. The examples in Figure 2.11.2 illustrate two compounds bearing such a type of bonding. Although stannyllithium compounds are important as sources of nucleophilic stannyl anions, only a few examples of compounds bearing Li Sn bonds have been fully characterized (Table 2.11.1). Indeed, stannyllithium compounds have common structural features, such as coordination to Lewis bases to form adducts of type Li(Lewis base)−SnR3 : Li(pmdeta)SnPh3 (pmdeta = MeN(CH2 CH2 NMe2 )2 ) (4),4 Li(thf)3 Snt Bu3 (5),5 Li(thf)2 Sn(SiMet Bu2 )3 (6),6 Li(toluene)Sn(SiMet Bu2 )3 (7),6 and Li(thf)3 Sn(SiMe3 )3 (8).7 The average ˚ is only slightly longer than expected for pure covalent Li Sn bond length in these compounds (2.88 A) ˚ 8 bonding (sum of covalent radii of Li and Sn, ca. 2.74 A). The tin atom in 4 exhibits a significant pyramidal geometry, with C Sn C angles of 96.1◦ , suggesting that a high degree of p−character is employed in the Sn C bonding. As a result, the lone pair on the Sn center has mainly an s character. In contrast, the C Sn C angles in 5 (av. 103◦ ) are most likely due to the presence of sterically more demanding t Bu groups. Compounds 6 and 7 exhibit long Sn Si bonds, which keep the bulky substituents away from the anion ˚ The geometry around the center; thereby leaving sufficient room for close Li Sn contacts (av. 2.80 A). tin center in 7 is not highly pyramidalized; with the sum of the Si Sn Si bond angles corresponding to 338.6◦ ; this contrasts with the highly pyramidal structure in 8 (296.3◦ ). On the other hand, the tendency of 6 and 7 to planarity can be rationalized in terms of the high steric demands of the bulky Si substituents. Moreover, 7 exhibits a tin–lithium–arene π interaction; such a structural trait is also observed in the dimeric compound (LiSnPh2 Ar*)2 (Ar* = C6 H3 -2,6-Trip2 ; Trip = 2,4,6-i Pr3 -C6 H2 ) (9).9 The Li Sn bonding in the latter structure is closely related to the trimetallic compound LiAr*Sn(Me)SnAr* (10).10

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Table 2.11.1 Compounds comprising M Sn bonds (M = Li, Na) Compound

˚ M Sn (A)

Metal coord. #

Ref.

Li(pmdeta)SnPh3 (pmdeta = MeN(CH2 CH2 NMe2 )2 ) (4) Li(thf)3 Snt Bu3 (5) Li(thf)2 Sn(SiMet Bu2 )3 (6) Li(toluene) Sn(SiMet Bu2 )3 (7) Li(thf)3 Sn(SiMe3 )3 (8) (LiSnPh2 Ar*)2 (Ar* = C6 H3 –2,6-Trip2 ; Trip = 2,4,6-i Pr3 -C6 H2 )(9) LiAr*Sn(Me)SnAr* (Ar* = C6 H3 -2,6-Trip2 ; Trip = 2,4,6-i Pr3 -C6 H2 ) (10) [LiSn(Sit Bu2 Me)3 ]2 (11) [Li(thf)Sn(SiMe3 )3 ]2 (12)

2.86, 2.88 2.88 2.83 2.77 2.87 2.81 2.69

4 4 3 2 4(η6 -Ar) 2(η2 -Ar) 3(η1 -Ar, η2 -Ar)

4 5 6 6 7 9 10

2.99, 3.14 2.76, 2.80, 2.86, 2.93 2.89, 2.97 3.06 2.76, 277 3.07 3.10 3.08 3.70 3.27 3.24 3.30, 3.31

2 2, 3

6 11

4 4 2(η5 -Ar) 2(η6 -Ar) 3 6 5 5 4 4(η6 -Ar, η6 -Ar)

12 13 14 15 16 17 18 19 20, 21 21

Li(thf)3 Sn{HC[Me2 SiN( p-tolyl)]3 }(13) Li(thf)3 Sn{HC[Me2 SiN[(S)-1-(1-naphthyl)ethyl)]3 }(14) Li2 [SnC4 Ph4 ] (15) Na(toluene)Sn[Si(SiMe3 )3 ]3 (16) [Na(thf)2 ]2 Sn8 R6 (R = Sit Bu3 ) (17) Na(15-crown-5)Sn(SiMe3 )3 (18) [Na2 (pmdeta)2 (thf)][Sn3 (Pt Bu)4 ] (19) [Na(pmdeta)4 ][(PhP PPh)Sn(μ-PPh)]2 (20) [Na(thf)3 ](Ar*SnSnAr*) (21) Na2 Ar*SnSnAr* (22)

˚ in 10, is extremely short in comparison to those observed in 9 (2.81 A) ˚ and The Li Sn distance (2.69 A) the previous examples. It is possible that the simultaneous binding of the Li+ ion by the ortho−Trip groups of the terphenyls plays a role in the shortening of the distance. Apparently, the chelating η6 Trip rings create a more tightly bound lithium atom, consequently leading to a slightly weaker Li Sn interaction. Other examples of structures bearing Li Sn bonds include the dimeric structures [LiSn(Sit Bu2 Me)3 ]2 (11)6 and [Li(thf)Sn(SiMe3 )3 ]2 (12),11 as well as the tripodal triamido stannates Li(thf)3 Sn{HC[Me2 SiN( p-tolyl)]3 } (13)12 and Li(thf)3 Sn{HC[Me2 SiN[(S)-1-(1-naphthyl)ethyl)]3 } (14).13 Another interesting example is the aromatic dianion Li2 [SnC4 Ph4 ] (15), obtained by the reduction of hexaphenylstannole with an excess of lithium.14 The structure of 15 shows two lithium atoms that lie above and below the stannole ring, with each Li atom coordinated in a η5 fashion. The stannole dianion has a planar structure with almost equal C C distances within the ring, and hence the negative charge is considerably delocalized in the ring. A strong low−frequency resonance was observed in 7 Li-NMR spectroscopy, due to the diatropic ring current from the 6π -electron system. Compounds containing M Sn bonds with the heavier alkali metals (M = Na, K, Rb, Cs) are less common than those with lithium, with only a few examples of Na Sn bonds structurally characterized (Table 2.11.1). The sodium tris(hypersilyl)stannide, Na(toluene)Sn[Si(SiMe3 )]3 (16),15 exhibits a sodium cation coordinated to the negatively charged tin atom and a slightly distortedg η6 -coordinated toluene ˚ is shorter than the sum of the covalent molecule (Figure 2.11.2). The observed Na Sn distance (3.07 A) ˚ 8 and can be attributed to a highly polarized bond. This value is comparable to those radii (ca. 3.28 A) ˚ 16 and Na(15observed in the sodium octastannanediide [Na(thf)2 ]2 Sn8 R6 (R = Sit Bu3 ) (17) (3.10 A) 17 ˚ In particular, structures bearing Sn(II) atoms exhibit weaker Na Sn crown-5)Sn(SiMe3 )3 (18) (3.08 A).

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˚ 18 [Na(pmdeta)4 ][(PhP PPh)Sn(μ-PPh)]2 interactions: [Na2 (pmdeta)2 (thf)][Sn3 (Pt Bu)4 ] (19) (3.70 A), ˚ 19 [Na(thf)3 ](Ar*SnSnAr*) (21) (3.24 A) ˚ 20,21 Na2 Ar*SnSnAr* (22) (3.30 and 3.31 A). ˚ 21 (20) (3.27 A), Compounds 21 and 22 were both obtained from [Ar*Sn(μ-Cl)]2 and sodium under different reaction conditions. So far, only a handful of structurally characterized examples of compounds featuring K Sn contacts have been reported. The molecular structures of two polymorphs of [K(dme)2 Sn(μ-OSiPh3 )3 ] (dme = MeOCH2 CH2 OMe ) (23a, b) were obtained under identical conditions from the reaction of the bis-triphenylsilyloxytin(II) dimer, [Sn(OSiPh3 )(μ-OSiPh3 )]2 ,with [K(THF)n (OSiPh3 )], followed by crystallization from dme.22 The two differ only by the presence of two dme ligands (monoclinic) against ˚ for one bidentate dme (triclinic). The K Sn distances differ only slightly in both compounds, 3.46 A ˚ for 23b (triclinic). On the other hand, the reaction of [Sn(OSiPh3 (μ23a (monoclinic) and 3.48 A OSiPh3 )]2 with two equivalents of [K(18-crown-6)Sn(OSiPh3 )] in toluene yields [K(18-crown-6)(η2 toluene)2 ] [K(18-crown-6){Sn(OSiPh3 )3 }2 ] (24).22 Treatment of [Sn2 (CHt2 Bu)6 ] with K in THF afforded [K{Sn(CHt2 Bu)3 }(η6 -toluene)3 ] (25) after crystallization from toluene.23 In a similar way, K2 Ar SnSnAr (Ar = C6 H3 -2,6-Dip2 ; Dip = C6 H3 -2,6-i Pr2 ) (26) was obtained by the reduction of Ar SnSnAr with ˚ are slightly shorter than those an excess of potassium.21 The K Sn distances observed in 24 (3.49 A) ˚ and 26 (3.58 and 3.59 A). ˚ present in 25 (3.55 A) Several interesting examples of polyatomic anions [Sn9 ]4− (Zintl ion) bearing M Sn (M = Na, K, Rb, Cs) have been synthesized.24−29 Among these compounds are [K(18-crown-6)]4 Sn9 (27) and [K(18crown-6)]3 KSn9 (en) (en = H2 NCH2 CH2 NH2 ) (28), which were obtained from the reaction of K and Sn in liquid 18-crown-6.24 X-ray crystallography shows two types of crystals with the composition K4 Sn9 , differing only in the ratio of K to 18-crown-6. In compound 28 all the K atoms have contact with the nine-atom tin clusters, whereas in 27 only two of the four K atoms are directly bound to the [Sn9 ]4− . The ˚ and vary K Sn distances of the [K(18-crown-6)] unit in 27 and 28 are in the range of 3.54 to 4.15 A around the value observed for the Zintl phase KSn.30 Using a similar synthetic strategy, the [Rb(18-crown-6)]2 Rb2 [Sn9 ](en)1.5 (en = H2 NCH2 CH2 NH2 ) ˚ in the case of Rb(1), and (29) was prepared.28 The Rb Sn bond lengths range from 3.71 to 4.23 A ˚ for Rb(2), giving an average of 4.02 and 3.87 A, ˚ respectively. Therefore each from 3.73 to 3.99 A [Sn9 ]4− cluster anion has contact with four Rb atoms that are not coordinated by the 18-crown-6. The Rb atoms that are coordinated to the 18-crown-6 are also bound to the [Sn9 ]4− anions, with mean Rb Sn ˚ for Rb(3) and 3.77 A ˚ for Rb(4). Consequently, each [Sn9 ]4− anion has six Rb atoms distances of 3.87 A in the first coordination sphere. There is just one example of the Zintl anion [Sn9 ]4− with a Cs Sn interaction, {K([2.2.2]crypt)]Cs7 [Sn9 ]2 ·(en)3 (30).29 The crystal structure of 30 reveals an arrangement build from layers with composition (Cs7 [Sn9 ]2 )− , which are separated by [K-([2.2.2]crypt]+ units. The Cs Sn ˚ which are comparable with those observed in the cesium contacts are in the range of 3.84 to 4.18 A, ˚ 31 benzyldihypersilylstannanide 31, 4.04 and 4.28 A. With the exception of the bis(trimethylstannyl)calcium 32,32 compounds bearing metal alkaline earth– tin contacts are virtually unknown (Figure 2.11.3). Compound 32 is formed when hexamethyldistannane reacts with calcium in THF, and can be isolated as crystalline Ca(thf)4 (SnMe3 )2 which exhibits a Ca Sn ˚ The Ca Sn distance lies between the broad range observed in binary Ca Sn phases contact of 3.27 A. ˚ (3.18–3.34 A). Bonding to f-Block Elements

The reaction between YbI2 and K[Sn(CHt2 Bu)3 ] affords the diamagnetic ytterbium bis−stannyl derivative Yb(SnNep3 )2 (thf)2 (Nep = 2,2-dimethylpropyl) (33).33 On the other hand, the reaction of naphthaleneyt-

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Sn1A O1A

Ca1

O2A

01

02

Sn1

1Å

Figure 2.11.3

Molecular structure of compound 32

terbium with SnPh4 in THF yields the distannylytterbium complex Yb(SnPh3 )2 (thf)4 (34)34 along with the heteroleptic complex Ph3 SnYb(thf)2 (μ-Ph)3 Sn(thf)3 (35),35 which can be isolated by crystallization from THF/ether solution. Polynuclear complexes Ln[Sn(SnMe3 )3 ]2 (thf)4 [Ln = Sm (36), Yb (37)] have been synthesized by reacting Me3 SnCl with samarium or ytterbium in THF.36 A similar organotin complex with ytterbium having the same composition, but a different structure to 37 was isolated from the reaction of MeSnCl3 with ytterbium, Yb[Sn(SnMe3 )3 ]2 (thf)4 (37a). The Ln Sn distances in 36 ˚ are longer than those in 37 (Ln = Yb), 3.29, 3.30 A ˚ and in 37a 3.30 A, ˚ by ca. (Ln = Sm), 3.40 and 3.39 A ˚ 0.1 A. The difference between Sm Sn and Yb Sn distances are close to the difference between the ionic ˚ respectively.37 The Yb Sn distances found radii of six-coordinate Sm(II) and Yb(II), 1.11 and 1.02 A, in 37 and 37a are longer than that in Yb(SnNep3 )2 (thf)2 (33), but shorter than those in Yb(SnPh3 )2 (thf)4 ˚ and Ph3 SnYb(thf)2 (μ-Ph)3 Sn(thf)3 (35) (3.38 A). ˚ (34) (3.31 A) The only known example of a molecule incorporating an actinide–tin bond corresponds to Cp3 USnPh3 (Cp = η5 -C5 H5 ) (38)38 (Figure 2.11.4). Compound 38 was prepared from Cp3 UNEt2 and ˚ compares well with the calculated HSnPh3 in toluene at room temperature. The U Sn distance of 3.17 A 39 ˚ U Sn distance in the model compound (H2 N)3 U SnH3 (3.20 A). In addition, the U Sn molecular orbital was determined to comprise mainly valence Sn pz (42%), U dz2 (16%) and U s(12%) atomic orbitals. Bonding to d-Block Metals

Transition metal–tin bonds are the most common type of bonds after those with p-block elements, namely C, N, O, and halogens. The less common transition metal–tin bonds are associated with early transition metals, namely Ti, Zr, Hf, V, Ta, and Nb. A fundamental difficulty in this bond formation is the existence of energetically low-lying electron transfer reaction pathways from the tin to the early transition metal. An interesting approach to the synthesis of group 4 metal–tin bonds comprises the use of tripodal amido

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Sn1

U1

1Å

Figure 2.11.4


ligands. Triamidostannates were found to bind to highly oxidizing metal centers to give thermally stable heterobimetallic compounds (43–46, 55). The electronegative nitrogen substituents at the divalent tin center appear to stabilize the metal with respect to oxidation by the heterometal center to which they are bonded. Table 2.11.2 summarises the most important features of compounds bearing early transition metal–tin bonds. Compounds 39, 60, and 74, outlined in Figure 2.11.5, represent rare examples of compounds comprising early transition metal–tin bonds. Bonding to Post-Transition Elements (Groups 11 and 12)

The most important compounds bearing group 11 metal–tin bonds are listed in Table 2.11.3. The molecular structure of [(Me3 Si)3 Si]CuSn[Si(SiMe3 )3 ](C6 H3 -2,6-Mes2 ) Mes = 2,4,6-Me3 C6 H3 ] (75) can be interpreted as a donor–acceptor complex.67 The complex exhibits consequently a trigonal planar tin atom and a linearly coordinated copper atom with a Si Cu Sn angle of 179.4◦ . The Cu Sn distance in 75 ˚ is shorter than the sum of covalent radii (ca. 2.96 A), ˚ 68 but slightly longer than that in 76 (2.45 A). ˚ (2.50 A) The cluster [Sn(mit)6 Cu4 ] (mit = 1-methylimidazol-2-thiolate) (77) exhibits a [SnCu4 ] core stabilized ˚ bonding between the endo lone pair of the Sn center and the tetrahedral by Cu(I) Sn(II) (2.73, 2.90 A) Cu4 fragment.70 The heterobimetallic compounds 79–83 comprise Ag(I)–Sn(II) covalent bonds, with similar Ag Sn bonding distances (Table 2.11.3). In particular, compounds 79 and 80 give dimeric arrangements, while in for 81–83, the use of silver fluorinated tris(pyrazolyl)borates and tin(II) N–alkyl-2– (alkylamino)troponiminates allow the isolation of monomeric compounds. The only known example of a compound comprising a Au(II)–Sn(II) bond corresponds to 87. This compound consists of a Au(II) eight-membered diaurocycle adopting a chair conformation, linked to two tripodal tris(amido)tin fragments. This generates a nearly linear Sn Au Au Sn unit linked by covalent ˚ and thus significantly longer than the Au Sn metal–metal bonds. The Au Sn bond length is 2.68 A ˚ (Figure 2.11.6). distance in the Au(I) derivative [{MeSi[Me2 SiN( p-tolyl)]3 }SnAu(PPh3 )] (84) (2.57 A) This observation was rationalized as the result of the steric repulsion between the periphery of the tripodal amido ligand and the bis(ylide) ligands coordinated to the Au2 unit. The Au Sn distance in 84 is slightly

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Table 2.11.2 Compounds bearing early transition metal tin bonds Compound

˚ M Sn (A)

Metal coord. #

Cp2 Ti(Cl)SnPh3 (39) [K(15-corona-5)2 ]2 [Ti(η4 -C10 H8 )2 (SnMe3 )2 ] (40) [NEt4 ][Ti(CO)5 (SnPh3 )2 ] (41) [K[2.2.2.crypt][Ti(CO)6 SnCy3 ] (Cy = cyclohexyl) (42) Cp2 ClZrSn{HC[Me2 SiN( p -tolyl)]3 } (43) Cp2 MeZrSn{HC[Me2 SiN( p -tolyl)]3 } (44) Cp2 (COMe)ZrSn{HC[Me2 SiN( p -tolyl)]3 } (45) (Me-C5 H4 )2 Zr[Sn{CH(SiMe3 )2 }2 ]2 (46) Cp2 Zr[N(Me2 -C6 H3 ){C(CH2 Ph)(CHPh)}]SnMe3 (47) [NEt4 ][Zr(CO)4 (SnPh3 )4 ] (48) [K(15-crown-5)2 ]2 [Zr(CO)4 (SnMe3 )2 ] (49) [K(15-crown-5)2 ][Zr(CO)4 (Ph2 P(CH2 )PPh2 )SnMe3 ] (50) Cp2 Hf(Cl)SnPh3 (51) Cp(Cp*)Hf(Me)SnPh3 (52) Cp(Cp*)Hf(NMe2 )SnPh3 (53) Cp(Cp*)Hf(OMe)SnPh3 (54) Cp2 ClHfSn{HC[Me2 SiN( p-tolyl)]3 } (55) Cp(Cp*)Hf(Cl)(SnH(Mes)2 (Mes = mesityl) (56) Hf(toluene)2 SnMe3 (57) [(C5 H4 )2 C(Me)2 ]Hf(NMe2 )SnPh3 (58) [NPr4 ][(Hf(CO)2 (SnPh3 )] (59) CpV(Nt Bu)(NHt Bu)Sn(SiMe3 )3 (60) [NEt4 ][V(CO)5 (SnPh3 )2 ] (61) V(CO)6 SnMe3 (62) Cp2 Nb(CO)SnPh3 (63) (Me-C5 H4 )2 Nb(CO)SnCl3 (64) [(Me-C5 H4 )2 Nb(CO)]2 SnCl2 (65) [Cp2 Nb(H)]2 Sn(Et)Cl (66) [K(18-crown-6)][Cp2 Nb(SnMe3 )2 ] (67) [{Cp2 Nb(SnMe3 )}2 (μ-O)] (68) [K[2.2.2.crypt][{(Me-C5 H5 )2 Nb}2 Sn6 ] (69)

2.84 2.87, 2.87 2.84, 2.81 2.92 3.02 3.04 3.01, 3.02 2.87 2.97 3.09 3.01 3.06 2.97 2.97 2.97 2.96 3.00 3.01 2.95 2.94 3.06 2.77 2.76, 2.79 2.94 2.85 2.76 2.84 2.85, 2.86 2.82, 2.83 2.86 2.85, 2.84, 2.85 2.81, 2.80 2.83 2.82 3.02

4 4 6 7 4 4 4 4 4 6 6 7 4 4 4 4 4 4 4 4 8 4 7 7 4 4 4 4 4 4 4 4 5 5 5 6

40 41 42 43 44 45 45 46, 47 48 49 50 50 51 51 51 51 44 52 53 51 49 54 55 56 57 57 58 58 59 60 61 61 62 63 64 65

2.75

5

66

Cp2 Nb(H)(SnMe2 Cl)2 (70) [(Me3 Si-C5 H4 )2 Nb(H)2 SnPh3 (71) [NEt]4 [CpNb(CO)3 SnPh3 ] (72) [(μ2 :η3 ,η3 -cyclo-SnP2 ){Nb[N(Np)(3,5-Me2 -C6 H3 )]3 }2 ] (Np = neopentyl) (73) Cp2 TaH2 (SnMeCl2 ) (74)

Ref.

˚ and considerably shorter than those in the tri−coordinate Au(I) shorter than that found for 89 (2.63 A), ˚ as well as those in the polynuclear compounds 86 (2.81 A) ˚ and 89 (2.97, 2.82 A), ˚ complex 85 (2.88 A), for which weak Au Sn bonds are proposed. Other synthetic strategies employed in the formation of Au(I)–Sn(II) bonds involve the use of the stannaborane [SnB11 H11 ]2− with phosphine gold electrophiles.81−83 In contrast to the examples above, compounds bearing Group 12 metal–tin bonds are much less known. Compounds M[Sn{MeSi[Me2 SiN( p-tolyl)]3 }]2 [M = Zn (90), Cd (91), Hg (92)] are obtained by reaction of the lithium triamidostannate Li(OEt2 )Sn{MeSi[Me2 SiN( p-tolyl)]3 } with 0.5 molar equivalents of

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C11

Si1 N2 Sn1

H1

V1

Sn1

Sn1

Ta1

Ti1 C11A

N1 Si3

H1A

Si2 C11

1Å

1Å

1Å

39

60

Figure 2.11.5

74

Molecular structures of compounds 39, 60, and 74

MCl2 (M = Zn, Cd, Hg) in toluene.84 Compounds 91 and 92 are the first examples of Cd Sn and ˚ Hg Sn bonded complexes that have been structurally characterized. The Cd Sn bond length (2.68 A) ˚ in 91 is longer than the Hg Sn bond in 92 (2.66 A). The structural organization in 90 comprises two geometrically unequivalent [Sn{MeSi[Me2 SiN( p-tolyl)]3 }] metal–ligand units. The tin atom in one of these metallacages is directly bonded to the zinc atom to form an unsupported Zn Sn bond with a bond ˚ Comparable values are observed in the other two compounds bearing such a type of bond length of 2.58 A. ˚ 85 and {K[2.2.2.crypt]}2 ZnPh[Sn9 ] (94) (av. 2.71 A). ˚ 86 [(PhCO)2 CH]2 ZnSn[(CH2 )3 NMe2 ]2 (93) (2.63 A) Table 2.11.3 Compounds comprising group 11 metal–tin bonds

Compound [(Me3 Si)3 Si]CuSn[Si(SiMe3 )3 ](C6 H3 -2,6-Mes2 ) (75) [MeB(3-(CF3 )pz)3 ]CuSn(Cl)[(Bn)2 ATI] {pz = pyrazolyl; [(Bn)2 ATI] = N-benzyl-2-(benzylamino)-troponiminate} (76) [Sn(mit)6 Cu4 ] (77) [SnEt(2-py)3 ]CuBr (78) [(NCS)Ag(thf)Sn{CH(SiMe3 )2 }2 ]2 (79) [{MeSi[Me2 SiN( p-tolyl)]3 }SnAg]2 (80) [HB(3,5-CF3 )2 pz)3 ]AgSn(Cl)[(n-Pr)2 ATI] {pz = pyrazolyl; [(n-Pr)2 ATI] = N − n-propyl-2-(n-propylamino)-troponiminate} (81) [HB(3,5-CF3 )2 pz)3 ]AgSn(I)[(n-Pr)2 ATI] (82) [HB(3,5-CF3 )2 pz)3 ]AgSn(N3 )[(n-Pr)2 ATI] (83) [{MeSi[Me2 SiN( p-tolyl)]3 }SnAu(PPh3 ) (84) (PMe2 Ph)AuSnCl3 (85) [Mn2 Au2 SnCl2 (CO)6 (μ-Ph2 PCH2 PPh2 ){P( p -tolyl)3 }2 ] (86) (CH2 PPh2 CH2 )2 [{MeSi[Me2 SiN( p -tolyl)]3 }Sn)2 Au2 ] (87) [Au4 (PPh3 )4 (μ2 -Cl3 Sn)2 ] (88) [Au8 (PPh3 )7 (SnCl3 )][SnCl6 ] (89)

˚ M Sn (A)

Metal coord. #

Ref.

2.50 2.45

2 4

67 69

2.73, 2.90 3.19 2.60 2.66 2.59

5 5 3 3 4

70 71 72 73 74

2.59 2.59 2.57 2.88 2.81, 2.82 2.68 2.97, 2.82 2.63

4 4 2 3 3 4 5 4, 5, 8

74 75 76] 77 78 76 79 80

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Si2

Si1

N1 Si3 Si4 N2 N3

Sn1 Au1

P1

1Å

Figure 2.11.6

Molecular structure of compound 84

Bonding to p-Block Elements (Groups 13–15)

As already mentioned, compounds bearing p-block element–tin bonds are the most common type of compounds observed with tin. This is particularly true for the lighter elements (B, C, N, O) and halogens. However, compounds bearing bonds between Sn and elements such as Al, Ga, Pb, Sb, and Bi are extremely scarce, while bonding to rare gases in unknown. So far, [Cl3 Al(SnNtBu)]4 (95) represents the only structurally characterized compound containing ˚ in 95 corresponds to that an Al Sn bond (Figure 2.11.7).87 The Al(III)–Sn(II) bond distance (2.78 A) expected from the sum of the atomic radii. On the other hand, reaction of the anionic gallium(I) N -heterocyclic carbene analog [K(tmeda)] [Ga{[N(Ar)C(H)]2 }] [tmeda = (CH3 )2 NCH2 CH2 N(CH3 )2 ; Ar = 2,6-i Pr2 -C6 H3 ] with the alkene analog {(Me3 Si)2 CH}2 Sn Sn{CH(SiMe3 )2 }2 leads to [K(tmeda)][Sn{CH(SiMe3 )2 }2 Ga{[N(Ar)C(H)]2 }] ˚ lies outside of the sum of the covalent radii of (96).88 The Ga Sn bond distance in 96 (2.72 A) ˚ and therefore, may be considered as weak. Other closely related comthe two elements (2.65 A) ˚ (97), pounds [K(tmeda)][Sn{CH(SiMe3 )2 }[Ga{[N(Ar)C(H)]2 }]2 [Ar = 2,6-i Pr2 -C6 H3 ] (2.66, 2.64 A) ˚ (98), [K(tmeda)][Sn(Ar )2 Ga{[N(Ar)C(H)]2 }] [Ar = 2,6-i Pr2 -C6 H3 , Ar = 2,4,6-i Pr3 -C6 H2 ] (2.67 A) ˚ (99) exhibit shorter Ga Sn and [(Giso)SnGa{[N(Ar)C(H)]2 }] [Giso = [i Pr2 NC{N(Ar)}2 ]− ] (2.69 A) bond lengths (Figure 2.11.7).88 Compounds bearing tin atoms bonded to metalloids of Group 14 are much more common than those with metals of Group 13, particularly in the cases of Sn Si and Sn Sn bonds, and to a lesser extent Sn Ge bonds; while for Sn Pb, there are only two compounds structurally characterized bearing such a bond. Compound 100 (Figure 2.11.8) represents the first structurally authenticated example of a silastannene ˚ which was reported (>Si Sn Sn(O2 CR)n > Sn(β-diketonate)n , whereas the solubility of these materials decreases as: Sn(OR)n > Sn((β-diketonate)n > Sn(O2 CR)n . 2.12.2

General Synthetic Procedures

The general synthetic procedures for heterometallic derivatives and the chemical concepts which are involved are as follows: Lewis Acid–Base Interactions

The Sn(II) heterobimetallic and oligometallic complexes, in general, follow the Lewis acid–base neutralization reaction mechanism. The electronegativity difference between two or more metal atoms in a

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Tin(II) Heterobimetallic and Oligometallic Derivatives

255

complex compound implies that one metal center will act as a strong acceptor of the electron density and the other will act as the donor to generate an heterometallic complex. These compounds can be obtained by simple mixing of the homometallic alkoxide, as first recognized by Meerwein and Bersin,22 and based on the neutralization of acidic and basic alkoxides.23,24 Ba(OR)2 + 2Sn(OR)2 −→ Ba[Sn(OR)3 ]2

(2.12.4)

2Sn(OBut )2 + Ba(OBut )2 −→ Ba[Sn(OBut )3 ]2

(2.12.5)

Sn(Ot Bu)2 + Tl(OBut ) −→ Tl[Sn(OBut )3 ]

(2.12.6)

The synthesis of the mixed valence compound is another example of a Bronsted acid–base reaction; the product is soluble in both polar and non-polar solvents.25 [Sn(OPri )4(HOPri )] 2 Sn[N(SiMe3)2]2 + HOPri

Ether

SnIISnIV(OPri )6 + 2HN(SiMe3)2

(2.12.7)

Metathesis Reactions

Another plausible mechanism through which these species could be prepared is a salt elimination or metathesis reaction, in which component parts of different molecules are exchanged to yield new structures. The prerequisite for the formation of Sn(II) heterometallic complexes is that the two or more combining metals must have nearly the same electropositive character as that of tin and also have the capability to enhance their coordination sphere upon complexation under suitable reaction conditions. The easiest approach to prepare Sn(II) heterometallic molecules is the substitution of an anionic ligand by a suitable halo-alkoxometallate ligand through the salt elimination mechanism.26−28 SnCl2 + 2MM (OR)n −→ Sn[M (OR)n ]2 + 2MCl

(2.12.8)

M = Na, K; M = Zr, Nb, Co, Mn; n = 4.6 SnCl2 + 2KSb(OCH3 )4 −→ [Sn[Sb(OCH3 )4 ]2 ] + 2KCl ↓

(2.12.9)

[Zr2 (Oi Pr)9 ]SnCl + KAl(OPri )4 −→ [Zr2 (OPri )9 ]Sn[Al(OPri )4 ] + KCl ↓

(2.12.10)

[Zr2 (OPri )9 ]3 SnCl + KNb(OPri )6 −→ [Zr2 (Oi Pr)9 ]3 Sn[Nb(OPri )6 ] + KCl ↓

(2.12.11)

Na2 Sn2 (OBut )6 + MgCl2 −→ MgSn2 (OBut )6 + 2NaCl Polymetallic Sn(II) alkoxides can be synthesized following the sequential route.

(2.12.12) 29

SnCl2 + K[Zr2 (OPri )9 ] −→ ClSn[Zr2 (OPri )9 ] + KCl ClSn[Zr2 (OPri )9 ] + K[Al(OPri )4 ] −→ [Al(OPri )4 ]Sn[Zr2 (OPri )9 ] + KCl

(2.12.13) (2.12.14)

The synthesis of the Sn(II) bis-alkoxide complex [Sn(OSiMe3 )2 ]2 can be brought about through an oxo-transfer reaction that involves a metathetical exchange between carbon dioxide and the divalent tin bis-amides.30

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Tin Chemistry: Fundamentals, Frontiers and Applications SiMe3

SiMe3 Me3Sn 2 Me3Sn

N Sn + 3CO2

Pentane, 250C

Me3SiN C

O Me3SiO

N

Sn

Sn O

SiMe3

NSiMe3

OSiMe3 + 2Me3SiN C

O

SiMe3

(2.12.15)

Elimination Reactions

Another selective approach to the synthesis of heterobimetallic compounds is the reaction of bivalent tin with a commercially available hydroxide or acetate, with the elimination of a small molecule, such as alcohol, or a volatile byproduct, such as CO.31 Sn(OR )n + M(OZ)n −→ (R O)n -1 Sn–O–M(OZ)n-1 + R OZ Z = H, CH3 CO; M = Cd, Pb; n = 2; n = 2, 4

(2.12.16)

Co2 Sn2 (OBut )8 + Fe2 (CO)9 −→ Co2 Sn2 (OBut )8 [Fe(CO)4 ]2 + CO

(2.12.17)

Mixed metal oxo-clusters can be prepared by ester elimination between metal alkoxides [M(OR)n ] and metal carboxylates [M (O2 CR)n ] as follows:32 M(OR1 )n + M (O2 CR2 )n −→ MM (μ3 -O)x (OR1 )n-x (O2 CR2 )n-x + xR1 O2 CR2 (M = Sn, M = Pb, n = 2, 4; R1 = But ; R2 = CH3 )

(2.12.18)

Thus, the methodology to synthesize heterometallic compounds mainly depends on both the nature of the elements and the ligand(s) employed. 2.12.3

Characterization Techniques

X-Ray Crystallography

The most authentic information gathered about the structure of a novel heterometallic compound is based on single-crystal X-ray crystallography, in spite of some limitations. The first and foremost task is to collect the best looking single crystal from a batch that may contain crystals of more than one morphology. Twinning, the intergrowth of unit cells in two distinct orientations, could be a genuine crystallographic difficulty in this case which has to be addressed by the crystallographer. Heterometallic Sn(II) derivatives, particularly alkoxides, are moisture sensitive and extremely soluble in common organic solvents. Consequently low temperature crystallographic data collection is a prerequisite. As these materials are potential molecular precursors of multicomponent oxides and while carrying out CVD studies, the solid-state structural information remains valid in the vapor phase, which could be considered to support the CVD mechanism. Few selected illustrative examples of molecular structures of Sn(II) heterometallic derivatives and their ORTEP drawings are described in subsequent pages. Nuclear Magnetic Resonance

Multinuclear NMR is not only a potential and unique technique for the structural and stereochemical characterization of homometallic Sn(II) complexes, but also for oligometallic derivatives having more than one metallic element, provided that the other metal(s) is(are) NMR active. There are several NMR

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active nuclei including 29 Si, 119 Sn, 121 Sb, 205 Tl, 115 In, 207 Pb, 17 O, 19 F, 31 P, 11 B, etc. in addition to 1 H and 13 C probes for the determination of the coordination environment around the metal atom(s) in the heterometallic species, both in solution and solid state.29,33,34 29 Si and 17 O NMR spectral studies also facilitate the identification of various types of bridged oxygen atoms (μ2 , μ3 etc.) in alkoxide compounds. However, in the case of Sn(II) alkoxides, the NMR studies are rather complex in view of the exchange phenomenon that may arise due to intra-molecular bridge–terminal or terminal–terminal site exchange. Mössbauer Spectroscopy

The solid-state structure of heterometallic compounds containing nuclei such as 119 Sn, 57 Fe, 119 Sb, 129 I, etc. can be deduced by this technique. The Mössbauer parameters, isomer shift, quadruple splitting, and magnetic hyperfine splitting explicitly describe the chemical state of the tin (element). 119m Sn isomer shift data33 show that a value greater than 2.1 mm s−1 indicates an Sn(II) derivative and a value smaller than 2.1 mm s−1 an Sn(IV) compound (both relative to SnO2 ), as in (acac)2 SnCr(CO)5 ; IS = 1.81 and QS = 2.28 mm s−1 . 57 Fe Mössbauer spectroscopy can also find applications for heterometallic derivatives containing iron because of the low sensitivity and paramagnetic nature of the 57 Fe nucleus. Mass Spectrometry

Mass spectrometry can give valuable information on the structural details of Sn(II) heterometallic derivatives, subject to stability with respect to the disproportionation reaction in the vapor phase. The mass spectral results demonstrate that fast atom bombardment mass spectrometry (FABMS) could find immense applications in the characterization of heterometallic derivatives, particularly of non-volatile, high molecular weight compounds, such as those of tin. IR Spectroscopy

IR data, in principle, bring valuable information for systems with functionalized ligands, but falls behind in the structural analysis of tin(II) heterometallic derivatives because of the problem of masking of important bands. However, IR spectroscopy can be used as an auxiliary technique for the identification of the functional groups present in the heterometallic compounds rather than for detailed structural analysis. The coordination mode of carboxylate to metal can be predicted by determining the parameter ν = [ν(COO)asy – ν(COO)sy ] by IR spectroscopy; if ν is less than 200 cm−1 , the carboxylate behaves as a bridging ligand.34,35 This technique has also been employed to identify the presence of various alkoxy groups, as every metal–ligand bond gives a characteristic absorption, such as νOMe ∼ 1180; νOEt ∼ 1025 and 1070; νOi Pr ∼ 840, 1125 and 1160 cm−1 , respectively, in various mixed metal alkoxides. The terminal and bridging alkoxy groups in various heterometallic compounds may be identified on the basis of IR absorption bands present in the region around 1020–1180 and 940–1070 cm−1 respectively. Heterometallic β-diketonates exhibit two IR absorption bands around 1567–1575 cm−1 due to C O and at 1509–1512 due to C O stretching modes. A band observed in the range 407–418 cm−1 may be assigned to Sn–O stretching vibrational band for these compounds.33 Elemental Analysis

Finally, elemental analysis results may pose difficulties in arriving at the correct composition of tin(II) heterometallic compounds due to changes in metal–ligand ratio. However, with the help of X-ray structural analysis, the CHN results may prove fruitful to prove the stoichiometry of the heterometallic species.

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Stoichiometric and Structural Aspects of Tin(II) Heterobimetallic and Oligometallic Compounds

The stoichiometry of heterobimetallic and oligometallic compounds is only known after isolation and characterization, mostly by single crystal X-ray diffraction, as these reactions are mainly governed by thermodynamic parameters. The single source bimetallic complex Sn(dmae)2 Cd(acac)2 (1) was prepared by mixing equimolar quantities of Sn(dmae)2 and Cd(acac)2 as follow:36 ∼60 ◦ C

2 Sn(dmae)2 + 2[Cd(acac)2 ] −−−−−→ [Sn(dmae)2 Cd(acac)2 ]2 Toluene

dmae = OCH2 CH2 NMe2

acac = CH3 COCH2 COCH3

(2.12.19)

The X-ray analysis of (1) showed the structure to be centrosymmetric with an inversion center at the heart of the central Sn2 O2 ring. The crystal has a monoclinic unit cell with space group P21 /n. The overall coordination sphere around tin is trigonal bipyramidal, whereas two chelating acac groups complete the coordination geometry around cadmium as cis-octahedral (Figure 2.12.2). Tin(II) tert-butoxide furnishes an excellent acid-base system that reacts with another metal alkoxides to yield mixed metal alkoxides. The skeletal structure of [Sn(OBut )3 ]2 Sr (2) shows that the polyhedron is built from two SnO3 Sr trigonal bipyramids connected via a common apex with retention of the three-fold axis. The apical position, which at the same time is a center of inversion, is occupied by the strontium atom with a distorted octahedral environment (Figure 2.12.3).37 The Sn atoms are trigonal bipyramidally coordinated (O–Sn–O: 82.3(1)◦ ) ˚ respectively. and the average Sn–O and Sr–O distances are 2.078(3) and 2.523(3) A The structure of indium tri-tert-butoxystannate can be described as a cage molecule having SnO3 M trigonal bipyramidal skeleton and is monomeric in contrast to the alkali metal derivatives (Figure 2.12.4).17 The molecular structure of the silver and tin mixed-metal complex [Ag(SCN)(Sn(CH(SiMe3 )2 )2 (OC4 H8 )2 ] (4) possesses a triclinic unit cell with space group PI (Figure 2.12.5).38 The structure is comprised of three-coordinated silver atoms in a planar ring, and the tin has a slightly distorted tetrahedral geometry. The novel feature of this structure is the existence of a covalent bond between silver and tin. Sn(II) phosphinidine chemistry has been reviewed recently by Wright39 and provides exciting prospects, not only for the low-temperature synthesis of materials, but also for discovering new structural

Figure 2.12.2 Molecular structure of Sn(dmae)2 Cd(acac)2 (1)36 . (Reproduced from reference 36, with permission from Wiley, UK.)

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Figure 2.12.3 Crystal structure of Sn(O t Bu)3 Sr(O t Bu)3 Sn (2)37 . (Reproduced from reference 37, with permission the American Chemical Society.)

Figure 2.12.4 Skeleton of Sn(OBut )3 In (3)24 . (Reproduced from reference 24, with permission from the American Chemical Society.)

Figure 2.12.5 Crystal structure of [Ag(SCN)(Sn(CH(SiMe3 )2 )2 (OC4 H8 )2 ] (4)38 . (Reproduced from reference 38, with permission from the American Chemical Society.)

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Figure 2.12.6 ORTEP diagram of [Li(thf)4 ]+ [( t BuN)(C10 H7 N)3 Sn3 Li-thf].thf.toluene (5)41 . (Reproduced from reference 41, with permission from the Royal Society of Chemistry.)

features and mechanisms that are involved in phosphorous–phosphorous and metal–metal bond formation. In contrast to their ubiquitous amide counterparts, the phosphide-based ligands (–PR2 ) have been explored to a lesser degree because the lone pair of electrons in the phosphide ligand remains stereochemically active owing to the larger barrier to inversion associated with phosphorus40 [PH3 155 kJ mol−1 ; NH3 24 kJ mol−1 ]. This favors the formation of bridging arrangements when bound to metals. Heterometallic complexes of tin(II) imido and phosphinidine anions have been prepared by the reactions of the imidotin(II) cubane, [SnNt Bu]4 , with three equivalents of lithium1-naphthylamide and lithium cyclohexylphosphine. The products, containing divalent tin, of composition [Li(thf)4 ]+ [(t BuN)(C10 H7 N)3 Sn3 Li-thf].thf.toluene where thf = OC4 H8 (5) and {[Sn2 (PC6 H11 )3 ]2 Li4 .4thf}.2thf (6) have been isolated.41 A crystallographic study of (5) indicates it to be an ion-separated species (Figure 2.12.6). The [(t (BuN)(C10 H7 N)3 Sn3 Si.thf] anion has a cubane structure, regarded as a Li-substituted Sn4 N4 unit, in which the three C10 H7 N2− imido ligands have similar environments, each bridging the Li center and two Sn centers within the cubane. The coordination of the Li+ cation by a thf ligand ensures that it assumes a typical pseudo-tetrahedral geometry. The structure of [[Sn2 (PC6 H11 )3 ]2 Li4 .4thf].2thf (6) is that of a mixed metal cage complex that contains a fourteen-membered [Sn4 P6 Li4 ] core (Figure 2.12.7) with two lattice-bound molecules of thf per complex molecule. Similar core geometries to [Li(thf)4 ]+ [(t BuN)(C10 H7 N)3 Sn3 Li-thf].thf.toluene have been observed in [[Sn2 (PC6 H11 )3 ]2 Li4 .4thf].2thf. The molecules of (6) are constructed from the association of the tin(II) phosphinidine tetraanion [{Sn2 (PC6 H11 )3 }]4− with four thf-solvated Li+ cations. The tetra-anionic unit can be visualized as four tris(phosphido)stannate (SnP3 ) moieties inter-linked into a metallocyclic structure. The four Li+ cations are bound to the phosphorus centers of the Sn2 (μ-PC6 H11 )2 dimer units and to the phosphorus centers of the dimer-bridging PC6 H11 groups in the Sn4 plane of the [{Sn2 (PC6 H11 )3 }2 ]4− anion. The molecular structure of [Cl2 Sn(μ-NC)Mn(CO)dppm)2 ] (7) comprises a slightly distorted octahedral manganese core coordinated to a dichlorotin(II) moiety, with tin having a pyramidal geometry (Figure 2.12.8).42 Another conspicuous aspect of the structure of 7 is the existence of two independent molecules in a unit cell.

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Figure 2.12.7 Structure of [[Sn2 (PC6 H11 )3 ]2 Li4 .4thf].2thf (6). (Reproduced from reference 41, with permission from The Royal Society of Chemistry.)

Figure 2.12.8 Molecular structure of [Cl2 Sn(μ-NC)Mn(CO)dppm)2 ] (7)42 . (Reproduced from reference 42, with permission from the Royal Society of Chemistry.)

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Figure 2.12.9 Crystal structure of [{SnI(Zr2 (OPri )9 )}2 ] (8). (Reproduced from reference 43, by permission of The Royal Society of Chemistry.)

The molecular structure of [{SnI[Zr2 (OPri )9 ]}2 ] (8) is monoclinic with space group P21/n , having two molecules per unit cell.43 Due to the presence of stereochemically active lone pair of electrons at Sn(II), only one terminal OPri group of the Zr2 (OPri )9 unit interacts with Sn(II) in a tridentate manner. The geometry around each tin atom can be described as distorted octahedral, in which one of the axial sites is occupied by a pair of non-bonding electrons and the central Sn2 I2 O6 unit can be visualized as a fusion of two SnI2 O3 octahedra. All the zirconium atoms are six coordinated, each with slightly distorted octahedral geometry due to the constraints of the M2 (OR)9 framework. Tin(II) oligomeric linear and cyclic complexes of ligands comprising nitrogen and oxygen as main components are also rich in structural chemistry and were reviewed by Veith and Mehotra.17,18 The polymeric Sn(II) neo-pentoxide [Sn(ONeoPent)2 ]∞ (9), prepared by reacting [Sn(NMe2 )2 ]2 with HONeoPent (ONeoPent = OCH2 CMe3 ) through the use of an amide–alcohol exchange reaction, is an important synthetic intermediate in tin(II) chemistry.44 [Sn(NMe2 )2 ]2 + 4NeoPentOH −→ 2/∞ [Sn(ONeoPent)2 ]∞ + 4HNMe2

(2.12.20)

The polymeric structure of [Sn(ONeoPent)2 ]∞ is shown in Figure 2.12.10. Within linear oligometallic alkoxide chains, each tin center is coordinated by four ONeoPent ligands through μ2 –O, forming a chain. The propensity of tin(II) to form cyclooligomeric derivatives is similar to those exhibited by early transition metals, alkaline earth metals, and bismuth.46 The elements with a large metallic radius and a small valence, such as divalent Ba, and Sr and trivalent Ln, In, Al, and Fe, favor the stability of oxoderivatives rather than alkoxide oligomers. The oxo ligands facilitate the formation of polynuclear species, for example, yttrium and lanthanum triisopropoxides are actually pentanuclear oxoisopropoxides with a central bridging oxo ligand. The controlled hydrolysis of titanium compounds produces various types of Ti–O clusters quite analogous to tin(II) oxoalkoxides. The crystal structure of Ti4 (O)4 (OCH2 CH2 NMe2 )8 (10) demonstrates the central Ti4 O4 core which is governed by the nature of the ligand employed, for instance, the bidentate dmae ligand (Figure 2.12.11). Ti(OPri )4 + 4HOCH2 CH2 NMe2 −→ Ti(OCH2 CH2 NMe2 )4 + 4Pri OH

(2.12.21)

4 Ti(OCH2 CH2 NMe2 )4 + 4H2 O −→ Ti4 (O)4 (OCH2 CH2 NMe2 )8 + 8HOCH2 CH2 NMe2

(2.12.22)

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Figure 2.12.10 Polymeric structure of [Sn(ONeoPent)2 ]∞ (9)44 . (Reproduced from reference 44, with permission from the American Chemical Society.)

Figure 2.12.11 Molecular structure of Ti4 (O)4 (OCH2 CH2 NMe2 )8 (10). (Reproduced from reference 46, by permission of The Royal Society of Chemistry.)

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Figure 2.12.12 Crystal structure of Sn5 ( μ3 -O)2 ( μ-ONeoPent)6 (11)44 . (Reproduced from reference 44, with permission from the American Chemical Society.)

The versatile coordination mode of oxoligands, which can bind up to six metal centers, is the main reason for the formation of cyclooligometallic derivatives. Although hydrolytic susceptibility decreases with increase in the number of oxoligands, even then the complex remains moisture-sensitive, perhaps due to the incomplete coordination sphere of the metal. These oligomeric complexes may take several weeks to crystallize at low temperature. Upon controlled hydrolysis, the polymeric [Sn(ONeoPent)2 ]∞ molecule yields products (11) and (12).44 5/2[Sn(ONeoPent)2 ]2 + 2H2 O−−−−−→Sn5 (μ3 -O2 (μ-ONeoPent)6 + 4HONeoPent

(2.12.23)

(11) 3[Sn(ONeoPent)2 ]2 + 4H2 O−−−−−→Sn6 (μ3 -O)4 (μ-ONeoPent)4 + 8HONeoPent

(2.12.24)

(12) The structure of (11) comprises an asymmetrical molecule that has five tin atoms arranged in a squarebased pyramidal geometry, linked by four basal (μ-ONeoPent) ligands, two facial μ3 –O and two facial μ-ONeoPent ligands. Because of the presence of the free electron pairs and the weak ligand–metal interactions, the various tin metal centers possess either a distorted trigonal bipyramidal or tetrahedral geometry. The compound (12) has six tin atoms arranged in octahedral geometry with an S4 axis of symmetry, (Figure 2.12.13). Considering the lone pair in the geometrical arrangement, both the axial and equatorial atoms are in five-coordinate square-based pyramidal geometries, and in contrast to other reported structures,45 no μ3 -ONeoPent bonds are formed due to the steric bulk of ONeoPent.

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Figure 2.12.13 Molecular structure of Sn6 ( μ3 -O)4 ( μ-ONeoPent)4 (12)44 . (Reproduced from reference 44, with permission from the American Chemical Society.)

Tin(II) dimethoxide, an important synthetic intermediate in bivalent tin chemistry, is readily prepared by the reaction of anhydrous Sn(II) dichloride and dried methanol in presence of triethylamine.47 Et3 N/Ar

SnCl2 + 2MeOH −−−−−→ Sn(OMe)2 −Et3 NHCl

(2.12.25)

Sn(dmae)2 , synthesized from Sn(OMe)2 by a trans-alcoholysis, can be employed as a reagent for the preparation of both homoleptic and heteroleptic species, in which the dmae ligand facilitates the

Figure 2.12.14

ORTEP diagram of Sn6 (O)4 (OMe)4 (13)48

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Figure 2.12.15 Crystal structure of Sn6 (O)4 (dmae)4 (14)36 . (Reproduced from reference 36, with permission from Wiley, UK.)

coordination of the second metal. Sn(OMe)2 undergoes controlled hydrolysis to give a cage compound Sn6 (O)4 (OMe)4 (13) with an adamantane-type structure (Figure 2.12.14).47 4H2O

6Sn(OMe)2 −−−−−→Sn6 (O)4 (OMe)4 −8MeOH

(2.12.26)

When compound (13) is refluxed in the presence of dmaeH, a cage complex, Sn6 (O)4 (dmae)4 (14), is formed36 by elimination of methanol. −4dmaeH

Sn6 (O)4 (OMe)4 −−−−−→ Sn6 (O)4 (dmae)4 −4MeOH

(2.12.27)

The structure of (14) is similar to that of Sn6 (O)4 (OC2 H5 )4 (15), which is isolated from the reaction of Sb(OEt)3 and Sn(dmae)2 , possibly through a group exchange reaction, followed by hydrolysis. The structure of (15) adopts a cage arrangement, which comprises two six-membered Sn3 O3 rings fused by Sn–O links between the two faces. The tin is trigonal bipyramidal with one equatorial site occupied by a lone pair of electrons; Figure 2.12.16. Compound (14) assumes the same general structure as (15), but in a far less regular fashion (Figure 2.12.15).36 Here the capping μ3 -dmae group is clearly monodentate and the Sn–OCH2 CH2 NMe2 distances show much variability in comparison to the Sn–O bonds in the Sn3 O3 rings in both structures.

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Figure 2.12.16 Crystal structure of Sn6 (O)4 (OC2 H5 )4 (15)36 . (Reproduced from reference 36, with permission from Wiley, UK.)

These oligometallic species prove to be excellent precursors due to their high solubility, low decomposition temperature, ease of modification, hydrolysis cross-linking behavior, and commercial availability.35,48 2.12.5

Conclusion

Tin oxide, SnO2 , or mixed metal oxides, such as indium tin oxide and cadmium tin oxide, have attracted great attention due to their application as gas sensors, in solar cells, and as electronic devices. The efficiency of the sensor depends on several parameters, like homogeneity of sensor and cosensor particles, surface area, and particle size of the material. Chemists seek suitable precursor materials that could deliver the desired end product, a metal oxide/mixed metal oxide of specific stoichiometry and composition, preferably in a single step. Therefore, efforts are being made to synthesize suitable precursors using soft chemistry (hydrolytic or non-hydrolytic sol-gel processing and metal organic deposition in solution or chemical vapor deposition, all being summarized as soft chemistry) approaches capable of delivering the target compound in a single step. The distinguishing features of bimetallic and oligometallic compounds of tin(II) are that each metal center is coordinatively saturated by use of chelating ligands, such as alkoxides/functionalized alkoxides, β-diketonates, and carboxylates to force the polymetallic tin complexes into a molecular regime, reducing the possibility of interaction between monomeric units. Thus, the metal oxide core of the end product is covered by organic surroundings making heterobimetallic and oligometallic complexes of tin(II) soluble in organic solvents, and ideal for CVD and other technological applications. References 1. 2. 3. 4. 5. 6. 7.

P. Braunstein, C. Charles, and R.D. Adams, C.R. Chimie, 8, 1873 (2005); and references cited therein. M. Okazaki, S. Ohshitanai, M. Iwata, H. Tobita, and H. Ogino, Coord. Chem. Rev., 226, 167 (2002). M. Mohamedi, S.J. Lee, D. Takahash, M. Nishizawa, T. Itoh, and I. Uchida, Electrochim Acta, 46, 1161 (2001). M. Veith, Eur. J. Inorg. Chem., 9, 1883 (2000). C. Tatsuyama and S. Ichimura, Jpn. J. Appl. Phy., Part 1, 9, 1012 (1970). S.K. Das and G.C. Morris, J. Appl. Phys., 73, 782 (1993). S.J. Laverty, H. Feng, and P.J. Maguire, Electrochem. Soc., 144, 2165 (1997).

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8. C.G. Granqvist, Materials Science for Solar Energy Conversion Systems, C.G. Granqvist (Ed.), Pergamon, Oxford, 1991, p. 106. 9. D.S. Ginley and C. Bright, MRS Bull., 25, 15 (2000). 10. W. Song, S.K. So, and L. Cao, Appl. Phys. A, 72, 361 (2001). 11. W. Gopel and K.D. Schierbaum, Sens. Actuat. B., 26, 1 (1995). 12. P.T. Moseley, Meas. Sci. Technol., 8, 223 (1997). 13. M.P.R. Panicker, M. Knaster, and F.A. Kroger, J. Electrochem. Soc., 125, 556 (1978). 14. C.G. Granqvist and A. Hultaker, Thin Solid Films, 411, 1 (2002). 15. Y.K. Lin and C.J. Wu, Surf. Coat. Technol., 88, 239 (1996). 16. L.G. Hubert-Pfalzgraf, Inorg. Chem. Commun., 6, 102 (2003) and references cited therein. 17. M.Veith, Chem. Rev., 90, 3 (1990). 18. R.C. Mehrotra, A. Sing, and S. Sogani, Chem. Rev., 94, 1643 (1994). 19. H.O. Davies, A.C. Jones, T.J. Leedham, P. O’Brien, A.J.P. White, and D.J. Williams, J. Mater. Chem., 8, 2315 (1998). 20. A.C. Jones, J. Mater. Chem., 12, 2576 (2002). 21. L.G. Hubert-Pfalzgraf, J. Mater. Chem., 14, 3113 (2004). 22. H. Meerwein and T. Bersin, Ann., 475, 113 (1929). 23. M. Veith and S.M. Reimer, Chem. Ber., 123, 1941 (1990). 24. G. Grag, R.K. Dubey, A. Singh, and R.C. Mehrotra, Polyhedron, 10, 1733 (1991). 25. D.J. Teff, C.D. Minear, D.V. Baxter, and K.G. Caulton, Inorg. Chem., 37, 2547 (1998). 26. T. Athar, R. Bohra, and R.C. Mehrotra, Main Group Met. Chem., 10, 399 (1987). 27. S. Mathur, A. Singh, and R.C. Mehrotra, Polyhedron, 11, 341 (1992). 28. M.J. Hampden-Smith, T.A. Wark, J.C. Jones, and C. Brinker, J . Ceram. Trans., 25, 187 (1992). 29. S. Mathur, A. Singh, and R.C. Mehrotra, Polyhedron, 12, 1073 (1993). 30. L.R. Sita, J.R. Bahcock, and R. Xi, J. Am. Chem. Soc., 118, 10912 (1996). 31. M. Veith, D. Kafer, J. Koch, P. May, L. Stahl, and V. Huch, Chem. Ber., 125, 1033 (1992). 32. J. Caruso and N.E. Hampden-Smith J., J. Chem. Soc., Chem. Commun., 1041 (1995). 33. R.C. Mehrotra, R. Bohra, and D.P. Gaur, Metal β-Diketonates and Allied Derivatives, Academic Press, Inc., London (1978). 34. I. Wakeshime and I. Kijima, Chem. Lett., 325 (1972). 35. (a) G.B. Deacon and P. Huber, Inorg. Chim. Acta, 104, 41 (1985). (b) G.B. Deacon and R.J. Phillips, Coord. Chem. Rev., 33, 227 (1980). 36. N. Hollingsworth, G.A. Horley, M. Mazhar, M.F. Mahon, K.C. Molloy, P.W. Haycock, P. Myers, and G.W. Critchlow, Appl. Organometal. Chem., 20, 687 (2006). 37. M. Veith, D. Kafer, and V. Huch, Angew. Chem., 25, 375 (1986). 38. P.B. Hitchcock, M.F. Lappert, and J.M. Pierssens, Organometallics, 17, 2686 (1998). 39. F. Garcia, M.L. Stead, and D.S. Wright, J. Organomet. Chem., 691, 1673 (2006). 40. F.A. Cotton, G. Wilkinson, C.A. Murillo, and M. Bochmann, Advanced Inorganic Chemistry, 6th edn., John Wiley & Sons, Inc., New York (2001) p. 338. 41. R.E. Allan, M.A. Beswick, N.L. Cromhout, MA. Paver, P.R. Raithley, A. Steiner, M. Trevithick, and D.S. Wright, Chem. Commun., 1501 (1996). 42. K.M. Anderson, N.G. Connelly, N.J. Goodwin, G.R. Lewis, M.T. Moreno, A.G. Orpen, and A.J. Wood, J. Chem. Soc., Dalton Trans., 1421 (2001). 43. M. Veith, S. Mathur, and V. Huch, J. Chem. Soc., Dalton Trans., 2485 (1996). 44. T.J. Boyle, T.M. Alam, M.A. Rodriguez, and C.A. Zechmann, Inorg. Chem., 41, 2574 (2002). 45. M.M. Olmstead and. P.P. Power, Inorg. Chem., 23, 413 (1984). 46. B.F.G. Johnson, M.C. Klunduk, T.J. O’Connell, C. McIntosh, and J. Ridland, J. Chem. Soc., Dalton Trans., 1553 (2001). 47. P. Harrison, B.J. Haylett, and. T.J. King, J.C.S. Comm., 112 (1978). 48. C.D. Chandler, C. Roger, and. J.J. Hampden-Smith, Chem. Rev., 93, 1205 (1993).

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Computational Methods for Organotin Compounds

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269


Sarah R. Whittleton, Russell J. Boyd, and T. Bruce Grindley Department of Chemistry, Dalhousie University, Halifax, Canada B3H 4J3

2.13.1

Introduction

The primary focus of computational chemistry is to employ theoretical methods to solve problems related to chemical systems. An important part of computational chemistry involves choosing the most appropriate method,1 based on the type of chemical system and the chemical properties of interest. Because there are an endless number of methods available, choosing computational methods for organotin systems epitomizes this dilemma. Organotin compounds are of interest because of their important applications in industry, biology, and agriculture as well as their use in organic synthesis and materials science. In this chapter, computational organotin chemistry is introduced by outlining the factors that need to be considered in this branch of theoretical chemistry, followed by methods currently available to perform theoretical studies of organotin species. Finally, a review of the most recent organotin computational chemistry literature is presented, separated into the types of specific chemical topics that have attracted attention. These topics include structure prediction, reactions pathways, thermochemistry, bond energies, and spectroscopy. It should be noted that for the purpose of this review, the definition of organotin chemistry is not strictly defined as the chemistry involving tin–carbon bonds, but is altered to include tin binding with other elements of the first, second, and third rows of the periodic table. Organotin species that include transition and heavy main group metals may also be included, if binding with organic groups is also present. However, this chapter does not include computational studies involving the vast field of tin oxides in materials science, instead focusing on more conventional organotin species. 2.13.2

Relativistic Effects in Heavy Elements

Tin is one of the heavier elements and, therefore, a number of obstacles arise when employing theoretical and computational chemistry techniques to study organotin systems. Heavy elements contain large numbers of electrons, which increase the demands on computational resources required for calculations.2 In addition, accounting for electron correlation using some correlation methods becomes computationally unreasonable because of the large numbers of electrons. This is problematic because inclusion of electron correlation becomes more important for heavy main group elements.3 Finally, to ensure complete accuracy for an electronic structure calculation, the effects of relativity need be considered when studying heavy-element systems.4−7 A number of effects arise from the theory of relativity, in particular, time dilation, mass increase, and length contraction. Mass increase is an important relativistic effect that can influence the molecular properties of heavyelement systems.8 Inner electrons of heavy atoms are subjected to the large positive nuclear charge, and therefore acquire large speeds to maintain the balance in the electrostatic field. These speeds (v) are comparable to the speed of light (c), which creates an increased mass (m) as shown in Equation (2.13.1). This mass increase causes the inner s-orbitals to contract, resulting in a 1s orbital contraction of 8% for tin,9 moving core electrons closer to the nucleus.8 This contraction is a direct relativistic effect and to maintain the core-valence orthogonality, the outer s-orbitals also contract. Contraction is greatest for s- and p-orbitals, affecting the ns orbitals more than the np orbitals. Contraction causes lowering of both orbital and total energies, and affects ionization energies, excitation energies, electron affinities, and

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electronegativity.9 Furthermore, this contraction increases nuclear shielding, causing orbitals of higher angular momentum to increase in size.10 mo m= (2.13.1) 2 1 − vc2 The radial expansion of valence orbitals introduces substantial alterations in the chemical bonding and valence spectroscopic properties of heavy-element systems.4−7,11 Therefore to perform computational calculations on heavy elements such as tin, it is mandatory to include relativistic effects to determine electronic structure and to ensure quantitative agreement with experimental data.12 Accounting for relativistic effects in computational organotin studies becomes complicated, because Hartree–Fock (HF), density functional theory (DFT), and post-HF methods such as n-th order Møller– Plesset perturbation (MPn), coupled cluster (CC), and quadratic configuration interaction (QCI) methods are non-relativistic. Relativistic effects can be incorporated in quantum chemical methods with Dirac–Hartree–Fock theory,13 which is based on the four-component Dirac equation.14 Unfortunately the four-component Hamiltonian in the all-electron relativistic Dirac–Fock method makes calculations time consuming,8,15,16 with calculations becoming 100 times more expensive.17 The four-component Dirac equation can be approximated by a two-component form, as seen in the Douglas–Kroll (DK) Hamiltonian18 or by the zero-order regular approximation (ZORA).16,19−24 To address the electron correlation problem, there are many elaborations of relativistic DFT,25,26 which involve modification of the Kohn–Sham equations to yield the relativistic Dirac–Kohn–Sham equations.27−30 Nonetheless, because of the large number of electrons in organotin systems, all-electron calculations incur high computational costs.31 Consequently, in order to reduce the computational time, most organotin studies employ effective core potentials, which incorporate relativistic effects into formally non-relativistic theoretical methods such as HF, DFT, MP2, etc. 2.13.3

Effective Core Potentials

The principle of the effective core potential (ECP) method is to separate the electronic system into core and valence electrons. The core electrons are replaced with an effective core potential, and only valence electrons are treated explicitly in the quantum chemical calculation.32 The ECP method assumes that the core electrons are chemically inert, that the atomic orbitals of core electrons do not change from the free atom for which they were derived to an atom in a molecule, and that valence electrons dictate the chemistry of the system such as bonding, structure, and reactivity.2,33,34 The complexities of ECPs are beyond the scope of our abridged discussion. The full details on the mathematical description of ECPs and their applications in theoretical chemistry are documented in several excellent references.2,8,12,15,34,35 Effective core potentials address the aforementioned problems that arise when using theoretical methods to study heavy-element systems. First, ECPs decrease the number of electrons involved in the calculation, reducing the computational effort, while also facilitating the use of larger basis sets for an improved description of the valence electrons. In addition, ECPs indirectly address electron correlation because ECPs may be used within DFT, or because fewer valence electrons may allow implementation of post-HF, electron correlation methods.8 Finally, ECPs account for relativistic effects by first replacing the electrons that are most affected by relativity, with ECPs derived from atomic calculations that explicitly include relativistic effects via Dirac–Fock calculations.2 Because ECPs incorporate relativistic effects, they may also be termed relativistic effective core potentials (RECPs). There are a variety of basis sets and corresponding effective core potentials that have been parameterized for tin; those used most commonly in the literature are briefly discussed in this chapter. These effective

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core potentials differ in many ways, including the number of core electrons described in the ECP, the manner in which the ECP was constructed and parameterized, and the basis set chosen to describe the valence electrons. Effective core potentials most prevalent in the current organotin literature include LANL2, SBD, SBKJC, and CRENBL and their respective basis sets. Most organotin systems use large-core ECPs, where 46 core electrons are described by the ECP and the remaining four valence electrons are treated explicitly in the calculations. The LANL2 effective core potential developed by Hay and Wadt36 at the Los Alamos National Laboratory (LANL) is a large-core ECP. The basis set used to describe the four valence electrons can be minimal (LANL2MB) or double-zeta (LANL2DZ). The LANL2DZ basis set is of 3-21G quality and is most popular for use with the LANL2 ECPs. Additional diffuse p-functions and/or d-type polarization functions, developed by Sunderlin and coworkers,37 may be included in this basis set (LANL2DZdp). Another popular ECP is the larger-core Stuttgart–Dresden–Bonn ECP constructed by Dolg and coworkers.38 This ECP can be used with the Stuttgart relativistic large core basis set (Stuttgart RLC or MWB) or can be used with the correlation consistent basis sets developed by Martin and Sundermann.39 These SDB-cc-pVnZ (n = D, T, Q) basis sets can be double-, triple-, or quadruple-zeta and may be augmented (aug) with diffuse functions, and are much larger than the LANL2 basis sets. It should be noted that a new series of these large basis sets has been developed40 to be used with 28 electron small-core ECPs.41,42 These basis sets, denoted cc-pVnZ-PP, describe the 32 ‘valence’ electrons, but have not been widely used in the literature. Use of the SBKJC and CRENBL ECPs appear to be less prevalent in the current literature. The SBKJC ECP, also called the consistent effective potential (CEP), developed by Stevens and coworkers43 is also large core. The CRENBL ECP developed by La John44−46 and coworkers is small core, where the 10 4d electrons are also classified as valence electrons, leaving 36 electrons to be treated as core electrons in the ECP. Each ECP is unique in the way it is developed, and generally the method used to construct effective core potentials is either the shape-consistent method or the energy-adjusted extraction method.47 The former method defines the ECPs by solving an eigenvalue problem from the all-electron reference calculation, while the latter involves constructing ECPs so that they reproduce observables. The LANL2, SBKJC, and CRENBL ECPs are all deemed shape-consistent, while the SDB ECP is energy-adjusted.35 2.13.4

Other Computational Methods Available for Tin

Although most computational studies of organotin systems employ ECPs, other methods can be used to describe tin. These methods include semi-empirical methods, all-electron relativistic methods, and hybrid energy methods, such as Morokuma’s ONIOM method48 and hybrid quantum mechanical and molecular mechanics methods (QM/MM). Semi-empirical methods can also be used to describe tin in computational organotin studies and are typically reserved for large systems. For mid-sized systems, semi-empirical optimized geometries are often used as starting geometries for optimizations performed with a higher level of theory. Those most frequently used in the current literature, that are parameterized for tin, are parameterized model number 3 (PM3),49 Austin model 1 (AM1),50 and modified neglect of differential overlap (MNDO)51 semi-empirical methods. Because semi-empirical methods are constructed from HF theory, they are not relativistic, although like HF and DFT, these methods have been modified to include relativistic effects. However, application of relativistic semi-empirical methods is uncommon, and most of those reported in the literature are non-relativistic.

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However, semi-empirical methods are also employed in ONIOM calculations of organotin systems. ONIOM and QM/MM are hybrid techniques that partition very large chemical systems into two or more parts, each employing a different level of theory.52,53 In the ONIOM method48,54 the real system is subdivided, where the model system contains the most important atoms, those thought to be actively involved in the reaction. The model system is treated with a high level of theory, such as MP2 or DFT. The remaining atoms in the real system are treated with a lower level of theory, such as semi-empirical methods or molecular mechanics force fields. The use of ONIOM for organotin systems allows for highlevel, relativistic treatment of tin by employing ECPs in the model system. Effective core potentials can also be used in QM/MM,55 which is a more specific hybrid method where quantum mechanics (QM) is paired with molecular mechanics (MM) and is used less frequently in current computational organotin chemistry. The zero-order regular approximation (ZORA),16,21 a two-component form of the fully-relativistic Dirac equation, is currently used for organotin computational calculations using basis sets specifically designed for ZORA. It should be noted that while all-electron calculations, whether non-relativistic or relativistic, can be used for organotin systems, the 6-31G Pople basis set is not available for tin and therefore, most all-electron calculations involving tin employ the smaller 3-21G basis set. Valence bond (VB) theory may be used as an alternative to molecular orbital (MO) theory for computational organotin studies.56−58 Most MO calculations of organotin systems use Gaussian,59 GAMESS,60,61 or Amsterdam density functional (ADF)62 program suites. A variety of VB methods exist, and although VB wavefunctions are more difficult to calculate, some VB methods can also be implemented in these programs.58 2.13.5

Current State of Computational Organotin Chemistry

There are several reviews on theoretical studies of Sn;9,31,63 however, these chapters include theoretical aspects of compounds containing group 14 atoms, Si–Pb. Because these previous reviews provide an extensive background to the topic of computational tin chemistry, the present chapter reviews computational organotin studies since the publication of these reviews. Unlike previous reviews that separate discussion by the types of organotin system being studied, this chapter is organized according to the topics of interest, such as structure prediction, reactions pathways, thermochemistry, bond energies, and spectroscopy. Although there has been an exponential growth of computational organotin studies in the last 30 years, a substantial portion of the current publications combine both computational and experimental techniques. This chapter includes examples of this type along with those based on only computational methods. While the majority of the current literature is included, in depth discussion is reserved for the most intriguing studies to give an overview of the current state of computational organotin chemistry. 2.13.6

Structure Prediction

Structure determination is an important part of computational chemistry because the geometries of chemical species dictate their chemical properties. Most computational chemistry studies begin with geometry optimizations to obtain energy minima. The optimized geometry is used to obtain other features of the chemical systems including thermochemistry, spectroscopy, and reaction mechanisms. However, structure prediction is still the primary focus of many current computational organotin studies, which are typically paired with experimental techniques such as X-ray crystallography or gas-phase electron diffraction. Numerous studies optimize systems with two or more different levels of theory, including HF, MP2, DFT, CC, or different ECPs, or density functionals,64−77 and then compare the different methods

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Computational Methods for Organotin Compounds Bu O

iPr iPr

Bu Sn

OO

O

Si O iPr

iPr Si iPr

O

Bu

Bu O

iPr

273

Si OO

O

Si O iPr

Si

O

iPr

Figure 2.13.1 The di-n-butylstannylene acetal (left) and the silicon-containing analog (right) used in the SPMC conformational search and subsequent AM1 optimizations.97

for their abilities to reproduce experimental measurements. Computational efficiency may be considered along with computational accuracy, because larger tin basis sets, although more accurate, require significantly greater computational resources.70,71 While many studies determine which method most accurately reproduces experimental results, few studies explicitly decide which method is optimal for further calculations. Fully relativistic treatment of tin is rare,78 unless the organotin systems being studied are small, due to heavy demands on computational resources. Instead, most studies that focus on structure prediction of organotin systems use effective core potentials to account for relativistic effects, and these ECPs are implemented within density functional theory.76,77,79−96 One unique study examined the conformations of stannylene acetals (Figure 2.13.1) using both DFT and Monte Carlo methods, hoping to shed light on the observed decrease in regioselectivity of alkylation reactions in this set of compounds.97 A systematic pseudo Monte Carlo (SPMC) search was performed on one stannylene acetal to generate a number of conformers. Because tin was not parameterized for the SPMC search, the atom was replaced by silicon, and these silicon-analog structures were optimized using AM1. From the 131 conformers, two of the lowest energy structures were selected and the silicon atom was replaced with tin. These tin structures were used as initial geometries for the DFT-B3LYP98−100 calculations using the SDB effective core potential with the default basis set. The differences in relative energy between the two tin species were calculated to be small and the authors suggest that species inter-conversion is possible, which may have caused the low regioselectivity of the reaction. While AM1 was used to perform a rapid conformational search, it can also be employed in larger organotin systems, in particular using ONIOM methods. The stabilities of donor–acceptor complexes formed between tin(IV) species and triazolopyrimidine (tp) derivatives were evaluated using the ONIOM method.101 The organotin system of interest was Bu2 Sn(tpO2 ), a model that mimics a truncated mono-dimensional polymer. In the two-layer ONIOM calculation, the two tpO2− 2 ligands coordinated to the central di-n-butyltin moiety in two different octahedral modes (Figure 2.13.2) were described by the B3LYP DFT functional, using the DZVP basis set. For the lower layer, semi-empirical AM1 was employed. ONIOM calculations are justified in this case because these systems have ∼120 atoms, including three tin atoms. However, while a high-level method such as DFT was employed for the higher layer, neither large basis sets nor ECPs were used, and thus the relativistic effects associated with tin were not accounted for. Work by Manogaran and Ramachandran illustrated how larger organotin systems can be studied at a high level of theory, by using effective core potentials to incorporate relativistic effects and also to account for electron correlation.102−104 These authors investigated the electronic structure and vibrational spectra of X20 H20 , X10 H16 , X14 H20 , X18 H24 , X22 H28 , and X26 H30 ; X=C, Si, Ge, Sn. Tin calculations used the DFT (B3LYP) level of theory, where the LANL2DZ ECP and basis set were used to describe the tin

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Tin Chemistry: Fundamentals, Frontiers and Applications O O H H O

N N H tpO2

O

N H

H

N O

N

Sn

H

N N

O

N

tpO22-Sn

H

N N N

H

N

Sn

tpO22-Sn

Figure 2.13.2 The tpO2 molecule and its potential bonding modes to tin. Note that cis and trans geometries are possible for both bonding modes with octahedral geometry at n-Bu2 Sn.101

electrons. ECPs are ideal here, because systems including up to 26 Sn atoms may be computationally strenuous even when using a small, all-electron basis set. Another unique study examined the inhibitory effects of a tin compound on the catalytic peroxidation of linoleic acid to hydroperoxylinoleic acid by the enzyme lipoxygenase (LOX).105 This distinct docking study investigated the complex–protein interactions with an organotin inhibitor. Prior to docking, X-ray structures of the complexes were optimized using PM3, and the simulation revealed that interactions occur between the rigid protein and the flexible ligands. Here, computational results elaborated on experimental kinetics, and it was shown that the tin inhibitor binds to the enzyme at a site away from the substrate binding site, which causes a decrease in the catalytic activity of the enzyme. Ideally, computational methods would include both electron correlation and relativistic effects. Instead, some studies address ether electron correlation106−112 or relativistic effects, but not both.113−115 2.13.7

Reaction Pathways and Mechanisms

Computational studies investigate reaction mechanisms and pathways by constructing potential energy profiles. This involves exploring reaction thermodynamics and kinetics, by examining reactants and products as well as the transition states geometries and activation energy barriers. Like those seen in structure prediction, most current studies implement effective core potentials and density functional theory to perform calculations.116−140 However, ECPs can be paired with MP2 to account for electron correlation; thus far, this approach has only been used for smaller chemical systems.141 Furthermore, solvation methods such as the polarizable continuum model (PCM)142−147 can be employed to examine organotin reactions, often to mimic experimental conditions.119,135,136,146 Recent work by Hu and coworkers theoretically investigated the geometries and energetics associated with the rearrangement of XSn Y to Sn Y–X (Y = N, P, As, Sb, or Bi) and the effects of a variety of substituents X (X=H, Li, BeH, BH2 , CH3 , NH2 , OH, and F). The first study explored the relative stabilities of different types of tin–nitrogen bonds, and illustrated how computational methods can examine organotin properties that may not be able to be determined using experimental techniques.147 The study used MP2 and QCISD, with the LANL2DZ+dp basis set and ECP. Parallel studies where Y= P, As, Sb, or Bi also examined Ar Sn Y species, that may self-dimerize when Ar is a bulky substituent such as C6 H3 -2,6−[C6 H2 -2,4,6−C(SiH3 )3 ]2 (Figure 2.13.3).148−150 For these larger systems, ONIOM (B3LYP/ LANL2DZ: PM3) was used to optimize structures, to reduce computational time and to reduce the memory requirements for the calculation. The use of B3LYP with ECPs facilitated the incorporation of both electron correlation and relativistic effects.

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C(SiH3)3 (H3Si)3C C(SiH3)3

Ar" C(SiH3)3 (H3Si)3C C(SiH3) 3

Figure 2.13.3 The bulky substituent, Ar”, used in computational studies of the self-dimerizing species Ar”Sn Y, where Y = P, As, Sb, and Bi136,145,146

Another interesting series of reactions involves oxidations and reduction facilitated by tin catalysis.135,151−154 The first study examined the reaction pathway for the Baeyer–Villiger oxidation, in which a ketone is oxidized to a carboxylic acid by a peroxide.135 Using acetone with hydrogen peroxide acting as an oxidant, the study compared the non-catalyzed reaction to the tin−catalyzed reaction, by investigating the reaction pathways and thermodynamics. Four different tin-catalyzed mechanisms were examined, each involving tin acting as a Lewis acid. The investigation was performed using B3LYP, by modeling tin with the LANL2DZdp basis set and ECP on tin. Solvation effects were also accounted for in selected cases by IEF-PCM. The thermodynamics of these reaction pathways were examined, revealing that the tin catalyst lowers the Gibbs activation energy barrier by 15.4 kcal mol−1 . Furthermore, the tin center acting as a Lewis acid may enhance the hydroxyl group’s ability as a leaving group in the Baeyer–Villiger rearrangement. Another study of the Baeyer–Villiger oxidation investigated the large pore synthetic Sn-beta zeolite as the organotin catalyst.152 This study employed molecular mechanics as well as quantum-chemical calculations. A molecular mechanics force field was first parameterized to account for Si to Sn substitution in the zeolite. Once optimized, the active site cluster was excised and this cluster was used as a model catalyst in DFT calculations with B3PW91and the LANL2DZ basis set for tin. The reaction mechanism was studied at this higher level of theory, with and without the tin catalyst, and a variety of possible mechanisms were examined, similar to the other study of the Bayer–Villiger oxidation by Root et al.135 An additional investigation of the Sn-beta zeolite system evaluated the two sites of Lewis acidity.143 Acetonitrile was employed as an adsorption probe for acidity, as it coordinates to each Lewis acid with different binding strengths. However, unlike previous studies,135,152 the ONIOM approach was used, where the model system contains moieties thought to be actively involved in the reaction, including acetonitrile, Sn, and the immediately surrounding Si, O, and H atoms. For the model system, B3PW91/LANL2DZ98,155 was used to optimize Sn, with N, C, O, Si, and H optimized with B3PW91/6-31G(d,p). The remaining atoms in the real system were treated with the semi-empirical MNDO method. Using ONIOM allows for relativistic treatment of tin while not employing large basis sets for atoms far from the catalytic site. Another series of calculations was performed using small model, zeolite–acetonitrile complexes with fewer atoms, again using the aforementioned DFT method. The ONIOM and all DFT calculations confirmed experimental findings that acetonitrile coordinates to one Lewis-acid site more strongly than the other.

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In addition, the stretching vibrations from the DFT frequency analysis agreed with the experimental values. ONIOM was also used to explore the Meerwein–Ponndorf–Verley (MPV) reduction reaction catalyzed by Sn-beta zeolites.153 DFT was used to examine the model system, but here the lower level treatment employed for the rest of the system was the HF/3-21G level of theory. Again, all DFT calculations were performed for small model complexes, in this case, Sn(OSiH3 )3 OH. One distinct study examined the tin-Peterson olefination of aldehydes using B3LYP and three different basis sets within the system.146 This study is unique because the authors used 6-31G(d) and small core SDB ECPs for the critical part of the reacting systems, that is, for tin, while all remaining atoms which are involved through steric effects were described using 3-21G(d). In addition, some calculations were performed to include bulk solvation using the PCM method. Although the chemical system was divided into more than two parts and different basis sets were used for each part, the method used was not a hybrid method, but was unique and different from ONIOM. This study demonstrates that relatively large systems can be calculated with computational accuracy by including relativistic effects and electron correlation and still maintain a high-level computational model. 2.13.8

Thermochemistry

As with the structure determination studies, thermodynamics of tin systems have not been the focus of organotin studies, but have instead been used to shed light on organotin reaction mechanisms. The results of many of the current calculations are compared to experimentally determined thermochemical data. As in many previous studies, relativistic effects are often not included,156,157 and in one study using MP2/3-21G(d) for all atoms, enthalpies associated with the thermal decomposition of SnCl4 were overestimated by 5–15% compared to experimental results.158 The authors attribute this large discrepancy to the small basis set employed. This topic is an excellent example of an area where effective core potentials could be employed to allow for larger basis sets for both tin and non-tin atoms, with the expectation of providing results that more accurately reflect experimental measurements. However, most authors reporting calculations of thermochemical parameters recently have employed ECPs and thus accounted for relativistic effects.159−161 Many thermochemical studies of organotin systems have employed high-level QM methods, as most chemical systems are of small relative size.162 One theoretical study investigates the structure, vibrational properties, thermodynamics, and charge distribution properties for the binary NaCl-SnCl2 system using the CRENBL ECP and corresponding basis set for tin in these CCSD(T)//MP2 and CCSD(T)//DFT calculations.161 2.13.9

Bond Strengths and Bond Dissociation Enthalpies

Bond strengths and bond dissociation enthalpies are valuable to chemists because they shed light on the chemistry and reactivity, thus it is important to determine the strengths of bonds involved in organotin compounds. Generally, small molecules appear to be the focus of recent bond energy studies, allowing for high-level calculations, often with small-core ECPs or even with fully relativistic treatment of tin. One group examined the correlation between bond length and bond strength in a variety of fluorinesubstituted group 14 ethane homologs computationally. In an initial study,163 MP2 optimized Sn2 Hx F y geometries were reoptimized using CCSD, and it was shown that bond length and strength are not correlated in these species. More recently, bond lengths, dissociation energies, and force constants were shown to not be generally correlated for the group 14 ethane homologs.164 This small system allowed utilization of small-core SDB ECPs and large augmented basis sets.

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These smaller systems also allow for full relativistic treatment of tin. Recent examples include the analysis of bonding in the Fischer- and Schrock-type tin homologs performed using the BP86 functional.165−167 The energy decomposition analysis was performed with the BP86 functional, using ZORA to consider scalar relativistic effects. Similar methods were used later to examine the nature of bonding in tin analogs of carbenium ions.168 A recent study by Sunderlin and coworkers examined the effects of substituents on A–Cl− bond − 169 strengths in the hypervalent systems, ACl− MP2 and 5 , ACl4 F , and A(CH3 )3 Cl2 ; A = Si, Ge, and Sn. B3LYP were used to perform the calculations, employing both SDB-aug-cc-pVTZ and LANL2DZdp basis sets with ECP. For some compounds, additional calculations were performed for geometry optimizations using the PW91 functional with a scalar-relativistic ZORA Hamiltonian to include relativistic effects. Calculated BDEs were smaller than experimental results with B3LYP, while the incorporation of electron correlation via MP2 calculations resulted in better agreement with experimental measurements. All-electron calculations were performed with B3LYP/DZP to investigate the donor–acceptor bond strengths in a series of group 14 complexes, MX4 ·nL (M = Si, Ge, Sn; X = F, Cl, Br; L = NH3 , Py, 2,2’-bipy, 1,10-phen).170 While basis set superposition errors and reorganization energies were included, this study did not include the effects of relativity that are known to be particularly important for Sn. We are currently investigating similar species using different basis sets and ECPs on tin to account for relativistic effects and explore which ECP is best suited to reproduce donor–acceptor bond strengths.171 Shaik and coworkers have carried out extensive studies of tin bonding by means of a valence bond (VB) approach which results in ‘charge-shift’ bonds.56,57,172,173 These bonds are neither ionic nor covalent, but are a resonance mixture of the two forms. This approach has been used to examine hydrogen transfer between X groups, where X = CH3 , SiH3 , GeH3 , SnH3 , and PbH3 .174,175 with the CCSD(T) and MP2 methods and the LANL2DZ ECP. Their studies explored the reactivity of the tin species by examining the bond energies, excitation energies, and polar effects, calculated in terms of VB theory. 2.13.10

Spectroscopic and Related Properties

NMR, NQR, Raman, and infrared spectra of organotin systems have been calculated recently. These studies have been linked to experimental studies, either to validate the computational method and ensure accurate prediction of spectroscopic properties, or to use as a tool to provide supporting evidence for experimental studies.176 Poleshchuk and coworkers have paired experimental and theoretical chemistry techniques to examine nuclear quadrupole resonance (NQR) parameters of SnCl4 L2 complexes. Their earlier studies used the semi-empirical PM3 method to optimize geometries, and to calculate effective charges that are correlated with NQR parameters.177 Later studies used B3LYP with ECPs or all-electron basis sets on tin to calculate NQR parameters such as the quadrupolar splitting, the quadrupolar coupling constant (QCC), and NMR chemical shifts for SnCl4 L2 complexes.178 Motivation for higher-level calculations was provided by the quality of reproduction of the NQR parameters calculated with PM3. However, the standard basis sets and ECPs used in the latter study still showed substantial deviation in QCCs from those measured experimentally.178 Further investigation using different tin species also showed that nuclear quadrupolar coupling constant values calculated with ECPs yield values that are lower than those obtained from experiment.179 In addition, all-electron calculations with the 3-21G(d) basis set suggest that a larger basis set is required to reproduce experimental energies.179 119 Sn NMR chemical shifts were examined with the B3PW91 functional for the first large-scale systematic study of a large series of Sn compounds, CH3 SnRR R ; R, R , R g = halogen, alkyl, halogenated alkyl, alkoxy, or thio alkyl groups.180 This non-relativistic study used the IGLO II basis set for tin to calculate chemical shifts that were comparable to experiment. However, the authors suggest that this is

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likely due to the cancellation of relativistic and solvent effects. In addition, for species containing Sn–Br or Sn–I bonds, chemical shifts are overestimated using non-relativistic methods. Quantum mechanical methods have been used to calculate 119 Sn NMR properties such as chemical shifts and coupling constants, for stannane, tetramethylstannane, methyltin halides, tin halides, and some stannyl cations.181 Relativistic effects were included by using the ZORA method. Each method allows the possibility of including only scalar effects or spin orbit coupling as well. 119 Sn chemical shifts and spin–spin couplings were calculated and compared to experimental values. A favorable correlation was shown for the chemical shifts, except for organotin species where heavy atoms are bound to tin, in which case a good correlation was obtained only at the spin-orbital level. Therefore, it is clear that relativistic effects must be considered for these heavy-element tin systems. The electric field gradients of 34 organotin compounds were calculated using the DFT method.182 The electric field gradient was used to construct the quadrupolar splitting (E) parameter, which was compared to the E determined experimentally by means of Mössbauer–Zeeman spectroscopy. Geometries were optimized using B3LYP and with double-zeta valence plus polarization basis set for all atoms, including tin. ECPs were not used to incorporate relativistic effects because the authors initially believed that the core electrons only were important for calculating the electric field gradients of these systems. While correlation between calculations and experiment was deemed good, the authors suggested that relativistic effects should be considered to obtain a better description of the electronic structure. However, they emphasized that the goal of the study was not to quantitatively determine E, but to establish a method that can be used as a supporting tool for the structural assignment of tin compounds. They suggested this method may be particularly useful when X-ray crystallography cannot be performed. Many of the recent studies that examine Raman and infrared spectroscopy have been mentioned in previous sections of this chapter.65,66,183,184 However, a vibrational spectroscopic study by Comerlato and coworkers used HF and B3LYP with the SBKJC EPC for tin to examine IR and Raman spectra of the anionic [NEt4 ]2 [Sn(dmit)3 ] complex.183 Comparison of the calculated scaled frequencies to experimental values revealed that the B3LYP method is more accurate than the HF method. The latter method is well known to overestimate frequencies by about 10%. Other related chemical properties of organotin systems are also of recent interest, including dipole polarisability and second hyperpolarisability,185 atomic charge, electrostatic potential, ionization potentials, electron affinities,186 and electronegativities.187 2.13.11

Conclusion

The ability of computational chemistry to provide insight into the properties and reactions of organotin compounds has been affected by advances in all aspects of computational chemistry. Treatment of electron correlation, inclusion of relativistic effects, inclusion of solvent effects, new methods for treating large systems, and faster computer processors have collectively led to a dramatic increase in the predictive capability of computational organotin chemistry. In this chapter, we have focused on the methods currently employed to examine organotin properties, including structure prediction, reaction pathways, thermochemistry, bond energies, and spectroscopy. We are confident that the advances in the next decade will outstrip those of the past two decades. Acknowledgments RJB and TBG gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support.

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Computational Methods for Organotin Compounds 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187.

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M. Boronat, A. Corma, and M. Renz, J. Phys. Chem. B, 110, 21168 (2006). M. Boronat, P. Concepcion, A. Corma, M. Renz, and S. Valencia, J. Catal., 234, 111 (2005). J. P. Perdew and Y. Wang, Physical Review B, 45, 13244 (1992). W. Setaka, K. Hirai, H. Tomioka, K. Sakamoto, and M. Kira, J. Am. Chem. Soc., 126, 2696 (2004). E. I. Davydova, A. Y. Timoshkin, T. N. Sevastianova, A. V. Suvorov, and G. Frenking, Phosphorus Sulfur Silicon, 179, 953 (2004). K. Takahashi, A. Kunz, D. Woiki, and P. Roth, J. Phys. Chem. A, 104, 5246 (2000). Y. Wang, J. Ma, and S. Inagaki, Tetrahedron Lett., 46, 5567 (2005). K. Hilpert, S. Roszak, J. Saloni, M. Miller, P. Lipkowski, and J. Leszczynski, J. Phys. Chem. A, 109, 1286 (2005). J. Saloni, S. Roszak, M. Miller, and J. Leszczynski, J. Phys. Chem. A, 110, 12535 (2006). I. M. B. Nielsen, C. L. Janssen, and M. D. Allendorf, J. Phys. Chem. A, 107, 5122 (2003). M. Kaupp, B. Metz, and H. Stoll, Angew. Chem. Int. Ed., 39, 4607 (2000). M. Kaupp and S. Riedel, Inorg. Chim. Acta, 357, 1865 (2004). A. D. Becke, Physical Review A, 38, 3098 (1988). J. P. Perdew, Physical Review B, 33, 8822 (1986). M. Lein, A. Szabo, A. Kovacs, and G. Frenking, Farad. Dis., 124, 365 (2003). I. Fernández and G. Frenking, J. Phys. Chem. A, 111, 8028 (2007). C. T. Hao, J. D. Kaspar, C. E. Check, K. C. Lobring, T. M. Gilbert, and L. S. Sunderlin, J. Phys. Chem. A, 109, 2026 (2005). E. I. Davydova, A. Y. Timoshkin, T. N. Sevastianova, A. V. Suvorov, and G. Frenking, THEOCHEM, 767, 103 (2006). S. R. Whittleton, R. J. Boyd, and T. B. Grindley, J. Phys. Chem. A, to be published. A. Shurki, P. C. Hiberty, and S. Shaik, J. Am. Chem. Soc., 121, 9768 (1999). P. C. Hiberty, R. Ramozzi, L. C. Song, W. Wu, and S. Shaik, Farad. Dis., 135, 261 (2007). S. Shaik, W. Wu, K. M. Dong, L. C. Song, and P. C. Hiberty, J. Phys. Chem. A, 105, 8226 (2001). L. C. Song, W. Wu, K. M. Dong, P. C. Hiberty, and S. Shaik, J. Phys. Chem. A, 106, 11361 (2002). A. Sekiguchi, T. Fukawa, V. Y. Lee, and M. Nakamoto, J. Am. Chem. Soc., 125, 9250 (2003). O. K. Poleshchuk, J. N. Latosinska, and J. Koput, J. Mol. Struct., 516, 71 (2000). O. K. Poleshchuk, E. L. Kalinina, Y. A. Shanina, and G. Frenking, Russ. J. Coord. Chem., 29, 53 (2003). O. K. Poleshchuk, E. L. Shevchenko, V. Branchadell, M. Lein, and G. Frenking, Int. J. Quant. Chem., 101, 869 (2005). R. Vivas-Reyes, F. De Proft, M. Biesemans, R. Willem, and P. Geerlings, J. Phys. Chem. A, 106, 2753 (2002). A. Bagno, G. Casella, and G. Saielli, J. Chem. Theory Comput., 2, 37 (2006). G. Barone, A. Silvestri, G. Ruisi, and G. La Manna, Chem. Eur. J., 11, 6185 (2005). G. B. Ferreira, E. Hollauer, N. M. Comerlato, and J. L. Wardell, Spectrochim. Acta A, 62, 681 (2005). J. O. Jensen, Spectrochim. Acta A, 60, 2561 (2004). A. Alparone, A. Millefiori, and S. Millefiori, Chem. Phys., 298, 75 (2004). A. H. Holm, T. Brinck, and K. Daasbjerg, J. Am. Chem. Soc., 127, 2677 (2005). K. T. Giju, F. De Proft, and P. Geerlings, J. Phys. Chem. A, 109, 2925 (2005).

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3 Materials Chemistry and Structural Chemistry of Tin Compounds 3.1

Tin Compounds For CVD (Chemical Vapor Deposition)

Geraldo M. de Lima Tin Chemistry Laboratory, Departamento de Qu´ımica – Universidade Federal de Minas Gerais, Brasil

3.1.1

Introduction

Nanoscale materials are those with dimensions less than 100 nm. Most of the nanomaterials used, such as oxides, sulfides, nitrides, and others are well known, in many cases since the beginning of civilization. In recent decades, it has been observed that specific properties of these materials, useful in biomedical,1 electromagnetic,2 mechanical,3 and catalytic areas,4 can be enhanced by reducing particle size to nanoscale dimensions. Many synthetic strategies have been developed in order to obtain nanometric materials with specific properties. Thin films of powders, in particular, have been the subject of current investigations. Studies of new synthetic approaches for nanometric films are intimately connected with the development of the chemical vapor deposition technique, which has widespread acceptance and is used for the production of important supplies for semiconductor electronic applications.5

3.1.2

General Aspects of CVD

The chemical vapor deposition (CVD) process involves the deposition of a stable solid film, produced by chemical reactions of gaseous reactants in an activated (heat, light, plasma) environment. The chemical source materials, containing elements of which the thin film is to be made, can be gaseous, liquid, or solid.6 Figure 3.1.1 displays a general CVD set-up. In this case an inorganic material, MX2 , can be employed as the source for the deposition of a thin metal (M) film on a substrate. The precursor MX2 is firstly heated for transfer in the gas phase and carried to the deposition chamber by an inert or reacting gas (H2 ). 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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MX2 in the gas phase (2) ..... .....

X2(g)

Exhaust

(1) MX2 in solid or liquid phase

(2) (4)

(3)

Figure 3.1.1

(2)

(1) Carrier gas (2) Heater (3) Substrate (4) Thin metal(M) film

General set-up of Chemical Vapor Deposition process

The film is formed in contact with the hot substrate. If the reacting gas is O2 , for example, the film will be the corresponding metal oxide. The unique advantages of CVD justify its continuous expansion and development into the most important method for producing films for solid-state devices.7 The first examples of CVD thin film experiments involved the deposition of W onto carbon lamp filaments by reduction of WCl6 with H2 , as reported in a patent at the end of the nineteenth century.8 Afterwards Ti, Ni, Zr, Ta and other pure metallic films were obtained by CVD processes (Equations 3.1.1 and 3.1.2).7 Til4(s)

1200 ◦ C −→ Ti(s) + 212(g)

(3.1.1)

◦

Ni(CO)4(g)

150 C −→ Ni(s) + 4CO(g)

(3.1.2)

The technique made significant progress at the end of the 1930s, when the deposition of refractory compounds (metal borides, carbides, nitrides, oxides, and silicides), pigments (silicon dioxide and titanium dioxide), and other materials (sulfides, selenides, tellurides, and alloys) became industrially important.9 The increasing need for germanium and indium antimonides, and highly pure germanium and silicon in the 1950s resulted in the CVD technique becoming the main synthetic tool for the preparation of semiconductors. Tremendous expansion in the field was attained when Ge was replaced by Si as a semiconductor material. Today, CVD technology spreads over various fields (aerospace, military, basic science, and engineering) and its development also advanced preparative approaches for bulk materials, as well as composites, coatings, and films.7 The chemical reactions in CVD of films and coatings can involve the following types: thermal decomposition (pyrolysis), reduction, oxidation, hydrolysis, disproportionation, or synthesis. In spite of the advantages of CVD techniques, such as versatility, adaptability, compatibility, quality, simplicity, reproducibility, productivity, and cheapness, some drawbacks are observed, mainly those concerning chemical hazards due to toxicity and instability of precursors. Other disadvantages relate to difficulties concerning the deposition of multicomponent materials using more than one precursor with different vapor rates. However, in a small number of experiments, this difficulty has been overcome by using single-source precursors. In addition, some sophisticated variants of CVD can increase the fabrication cost of materials.6

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Tin Compounds For CVD

3.1.3

287

Organometallic Chemical Vapor Deposition (MOCVD)

The various forms of CVD processes are classified according to the type of activation energy: thermally activated (TACVD), plasma-enhanced (PECVD), photo-assisted (PACVD), laser-induced and electronbeam assisted CVD. MOCVD or low-pressure organometallic CVD is a special type of TACVD where organometallic compounds, employed as precursors, are thermally decomposed at normal pressure, producing thin films (10–20 nm or less). The gas flow can vary according to the need for a specific atmosphere (air, oxygen, argon, or reducing environment, H2 ). In some cases, organometallic compounds offer the advantage of lower reaction and deposition temperatures, less toxicity, and thermodynamic instability compared to inorganic materials. They are employed mainly in the deposition processes of group 2–16 and 13–15 semiconductors, superconductors, insulators, conductive, and resistive layers in amorphous, polycrystalline, or single crystalline forms, mainly used in electronics. Since most organometallic compounds are volatile, precise control of heating rates, gas flows, and deposition pressures are required. 3.1.4


Thin films of optically semi-transparent and electrically conducting materials are required in many optoelectronic devices. The deposition of transparent conducting films is usually achieved by means of APCVD (atmospheric pressure CVD) by hydrolysis of the metal chlorides or pyrolysis of organometallic compounds. The latter process allows the deposition of materials (composites or metal films) at lower temperatures compared to other methods. Superior properties of these films, namely high conductivity and optical transmission, lack of contamination, non-stoichiometry, smooth and flawless surface morphology, fine-grained polycrystalline structure, increased environmental stability, and easy etchability are required in sophisticated applications. The preparation, properties, and applications of transparent conducting films were reviewed in the 1980s.10 However, a considerable amount of new research has, subsequently, been reported. Most of the work concerning tin-based technological materials relates to chalcogenidecontaining powders. 3.1.5

Tin(IV) Oxide and Related Materials

Tin (IV) oxide, SnO2 , (rutile-type structure), a well-established n-type semiconductor with a wide band gap (E gap = 3.6 eV at 300 K) also has potential applications as a catalyst support,11 as transparent conducting electrodes,12 and as a gas sensor.13 This material possesses many advantages, such as: (i) high thermodynamic stability in air (at least up to 500 ◦ C), (ii) low cost, and (iii) the possibility of the introduction of catalysts or dopants to enhance the sensitivity or selectivity.14 Several methods, such as CVD,15 electrodeposition,16 electron beam evaporation,17 pyrolysis,18 sputtering of Sn targets,19 hydrothermal,20 reactions in liquid ammonia,21 pulsed laser deposition,22 mechano-chemical,23 and sol-gel precipitation24 have been investigated for the preparation of tin(IV) chalcogenides.25 Organotin(IV) oxides have been employed in the preparation of nanoscale particles of SnO2 .26 The nature of the organic group attached to the Sn center plays an important role in the decomposition process.27 Tin(IV) compounds are attractive as single-source materials for the formation of SnO2 in CVD experiments. Thus, at the end of the 1960s, thin films were deposited using SnCl2 28 or SnCl4 29 in the presence O2 . Afterwards, tin(IV) oxide was produced by pyrolysis of organotin(IV) derivatives, such as SnMe4 ,30 SnEt4 ,31 Sn(n-Bu)2 (OAc)2 32 and SnMe2 Cl2 ,33 Sn(n-Bu)Cl3 or Sn(n-Bu)2 (OAc)2 ,34 SnEt2 (NEt2 )2 , and Sn(NMe2 )4 .35 Thin films of SnO2 have been deposited epitaxially on α-Al2 O3 using the SnI4 -O2 precursor combination.36 Synthetic approaches have been recently reviewed.37 Also an interesting paper has

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O NMe2

O Cd

Me

O O Me

O Sn

O

O Sn NMe2

NMe2

O

Me Me O

O

Cd NMe2 O

O Me

Me

Figure 3.1.2

Structure of the complex [Sn(dmae)2 Cd(acac)2 ]

been published discussing results of experimental and theoretical works, carried out in order to elucidate reaction mechanisms for the pyrolysis, oxidation, and hydrolysis of organotin compounds during CVD experiments for SnO2 production.38 The chemical features of tin(IV) alkoxides, such as pre-existing metal–oxygen bonds in molecular units, high volatility and low decomposition temperatures make them attractive precursors for deposition of SnO2 .39 The heterometallic complex [Sn(dmae)2 Cd(acac)2 ], Figure 3.1.2, (acac = 2,4-pentanedionato; dmae = N,N’-dimethylamino-ethanoate) has been decomposed in aerosol-assisted chemical vapor deposition conditions, producing amorphous tin(IV) oxide films with no detectable cadmium.40 More advanced materials have also been prepared. Tetraethyltin(IV) was used as a single-source precursor for deposition of SnO2 films on Ni support, employing a special CVD technique (FBCVD – fluidized bed-CVD).41 The SnO2 films deposited on the Ni surface exhibited a dense nodular surface morphology similar to that previously observed on flat substrates. They exhibited satisfactory thickness uniformity from one particle to another, although traces of NiO were found at the SnO2 /Ni interface. Finally, the SnO2 CVD coated-Ni particles were tested as anodes in an electrochemical cell, Figure 3.1.3. An interesting experiment was performed to deposit SnO2 on multiwalled carbon nanotubes (MWCNTs) by CVD at 550 ◦ C, using SnH4 as the tin source.42 This technique may provide a good way to produce tunable SnO2 –MWCNT composites (Figure 3.1.4). Tin(II) t-butoxide, [(Sn(O-But )2 )2 ], as well as the corresponding heterometal alkoxides [M{Sn(OBut )3 }2 ] (M = Ca, Sr, Ba), have been employed as precursors for chemical vapor deposition processes, generating either SnO2 or MSnO3 (M = Ca, Sr, Ba).43 Thin films of Pt-doped-SnO2 have been deposited in the temperature range 320–440 ◦ C by MOCVD, using SnEt4 and Pt(hfa)2 {hfa = hexafluoroacetylacetonato} in an O2 atmosphere. This material was much more sensitive than SnO2 for ethanol detection in dry air.44 In order to enhance conductivity and other properties, SnO2 has been doped with cations containing Sb, In, Cd, Bi, Mo, B, P, Te, or W and/or with F or Cl anions.45

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Figure 3.1.3

289

Scanning electron microscopic image of the cross-section of Ni/SnO2 particle

Sn-doped In2 O3 (ITO) is the most widely used transparent conducting oxide (TCO) material. This can be satisfactorily prepared by pyrolitic oxidation of a mixture of In(dpm)3 and Sn(n-Bu)2 (OAc)2 at 550 ◦ C in a oxygen–nitrogen or nitrogen saturated with water vapor atmosphere,46 [Equation (3.1.3) {dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate}]. In(dpm)3 + SnBu2 (OAc)2

CVD −→ ITO + decomposition products

dpm = 2, 2, 6, 6-tetramethyl1-3-5-heptanedionate OAc = acetate

Figure 3.1.4

Scanning electron microscopic image of SnO2 —MWCNT particle

(3.1.3)

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But

O

O

Bu t Bu

N

O

O

O

O

But

But

O

O

O

O

Bu t

But

[Mg(dpm) 2(TMED)] Figure 3.1.5

[In(dpm) 3]

Sn

O

In

Mg N

But

t

Bu t

O

O O

[Sn(acac)2]

Structure of the complexes [Mg(dpm)2 (TMEDA)], [In(dpm)2 ] and [Sn(acac)2 ]

In spite of the vast technological applications of ITO, several limitations can be encountered, such as relatively low transmission in the blue-green spectral region and suboptimal conductivity. Therefore, a search for ITO-alternative materials by combining various transition and/or main group metal oxides has been stimulated. The chemical vapor deposition of acetylacetonato metal complexes seems to be the most successful synthetic approach. The films of Mg- and Sn-doped In2 O3 , MgIn14.3 Sn6.93 Oz , have been grown by chemical vapor deposition of a mixture of [In(dpm)3 ], [Sn(acac)2 ], and [Mg(dpm)2 (tmeda)] (Figure 3.1.5 [acac = 2,4-pentanedionato; tmeda = N, N, N’, N’-tetramethylethylenediamine]). High conductivity was found for the film, making it a promising material for near-IR optoelectronic applications.47 ZITO films with composition ZnIn2.0 Sn1.5 Oz have been prepared by CVD experiments employing [In(dpm)3 ], [Sn(acac)2 ], and [Zn(hfa)2 (diamine)], (hfa = hexafluoroacetylacetonato). Such films were used in the fabrication of polymer light-emitting diodes, which exhibited light outputs and current efficiencies almost 70% greater than those of commercial Sn-doped indium oxide (ITO) films.48 Cadmium stannate, used as electrodes in photogalvanic cells, is another example of a transparent conductor oxide (TCO) having desirable properties, such as good durability and chemical resistance. It can be produced by the spray pyrolysis CVD method with organic solutions of CdCl2 and SnCl4 49 or [Cd(hfa)2 (TMEDA)] and [Sn(acac)2 ].50 It also shows the unexpected effect of improving transparency with increasing film conductivity higher than tin-doped indium oxide. Zirconium-tin-titanate (ZTT) is a potential alternative dielectric material.51 ZTT can be deposited using a solvent-free precursor mixture of Zr(OBut )4 , Sn(OBut )4 , and Ti(OBut )4 .52 A ZTT film of composition Zr0.97 Sn0.12 Ti0.05 O3.33 was obtained by MOCVD of a mixture of Ti{OR}3 {N(SnMe3 )2}] {R = Pri and But } or [Ti{OPri }3 {N(SnMe3 )(SiMe3 )}] and Zr(OBut )4 .53 3.1.6

Tin Sulfides

The synthesis and characterization of narrow-bandgap semiconductors, especially SnS2 and SnS, have received much attention in the last few years, due to their optical and electronic properties.54 Tin sulfides comprise an interesting class of semiconductor materials. A variety of phases are known, such as SnS (herzenbergite), SnS2 (berndtite – 70 polytypes known), Sn1+x Sn (non-stoichiometric),

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Sn2 S3 (ottemannite, three polytypes), Sn4 S5 , and a number of alkaline and alkaline earth tin-based polysulfides.55 Tin(II) sulfide displays a distorted rock-salt layered structure similar to GeS (orthorhombic space group Pbnm) in which six sulfur atoms surround each tin center with three short Sn–S bonds within the layer and three long bonds connecting two neighboring SnS layers.55 SnS (n-type or p-type) has an optical bandgap of 1.3 eV, located between those of Si and GaAs (1.08 to 1.51 eV).56 Hence, its films have potential as photovoltaic materials,57 holographic recording systems,58 and solar control devices.59 SnS2 , a wider bandgap (2.07–2.18 eV)60 n-type semiconductor, possesses a layered structure, similar to those of PbI2 and CdI2 , in which each metal atom lies at the center of an octahedron, and is bonded to six sulfur atoms.55 , 61 This structural arrangement allows intercalation of alkali metals and metallocenes62 with resulting increases in conductivity. Sn2 S3 , a direct forbidden semiconductor63 with a bandgap of 0.95 eV,55 is a mixed-valence Sn(II)/Sn(IV) compound with the same local order as other tin sulfides, but with a ribbon-like structure.64 A number of synthetic approaches have been proposed in order to prepare nanosized grains of tin(II) and tin(IV) chalcogenides. Thermal decomposition of {(PhCH2 )2 SnX}3 , (X = Se and Te) in an inert atmosphere has been used to prepare SnSe, SnTe, and Sn(Se1−x Sx ).25 , 60 , 65 Tin(II) sulfide semiconductor nanometric particles have been prepared by the thermal decomposition at 350 ◦ C in air of R4 Sn4 S6 (R = Me, n-Bu and Ph).66 Further heating to 500 ◦ C in an N2 atmosphere led to the pure orthorhombic Sn2 S3. 67 The thermal decomposition of the Ph2 Sn{S2 CN(CH2 )4 }2 or Ph3 Sn{S2 CN(CH2 )4 } compounds yielded nanometric SnS and Sn2 S3 68 Pure phase SnS has been obtained in pyrolysis experiments, at 350 ◦ C in N2 , of [SnPhx (S2 CNEt2 )4−x ] (x = 2 or 3) or [SnRx (S2 CNC4 H8 )4−x ] (x = 1, R = Ph and x = 2, R = n-Bu).69 In contrast to Sn-based oxide films, widely prepared by CVD techniques, this methodology has been less utilized for tin-sulfide-containing materials.70 Most of the approaches still use a toxic H2 S atmosphere. Tin(IV) sulfide has been prepared by CVD experiments of a mixture of SnCl4 and H2 S.15 , 71 The same precursors have been used to prepare a heteroelectrical junction based on SnS and SnS2 , by plasma-enhanced chemical vapor deposition (PECVD) on a glass with a transparent conductor oxide (TCO) as substrate. A glass/TCO/n-type SnS2 /p-type SnS/Al diode structure has been observed for the final product of this experiment.72 Tin chloride and thiourea have been employed for the deposition of Sn2 S3 thin films by spray pyrolysis.73 The tin sulfide Sn4 S6 -bonded cluster has been anchored on acid Y-zeolite by using MOCVD, employing SnMe4 /H2 S as precursors.74 Unsymmetric dithiocarbamato-organotin(IV) complexes [SnMe3 {S2 CN(n-Bu)(Me)}] and [SnPh{S2 CN(n-Bu)(Me)}3 ] generated SnS and Sn2 S3 films on glass substrates by employing CVD at 350–550 ◦ C, in an H2 S atmosphere.75 The heteroleptic tin(IV) thiolate and dithiocarbamato-complex, [Sn(SCy)2 (S2 CNEt2 )] (Cy = cyclohexyl), has been tested as a single-source precursor in CVD experiments yielding SnS2 rather than SnS films. 3.1.7

Tin(II) Selenide and Telluride

Tin(II) selenide and telluride, and ternary alloys, such as Pb1−x Snx X (X = Se and Te) are promising materials for the fabrication of mid-IR photodetectors, light-emitting diodes, diode lasers, and memories in switching devices.76 Linear and cyclic organotin(IV) chalcogenides (R3 Sn)2 E (R = Ph, -CH2 Ph and E = Se, and Te) and (R2 SnE)3 (R = Ph, -CH2 Ph and E = Se) and pyridineselenolato-complexes such as [Sn(μ−SePy)2 ]2 , (py = pyridine) have been tested as single-source precursors for the preparation of powders of SnSe and SnTe by pyrolysis.77

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MOCVD experiments have been conducted using two air- and light-stable organotin(IV) derivatives, [Sn{(SiMe3 )2 CH}2 {E}] (E = Se and Te). The experiments were conducted at 300–600 ◦ C and 1 Torr in a flow of a 1:1 He/H2 mixture. Thin films of SnSe and SnTe were satisfactorily obtained.78 3.1.8

Tin (IV) Phosphide

The preparation of tin phosphides has received attention due to their interesting mechanical, optical, and electrical properties79 and catalyst applications.80 Atmospheric pressure chemical vapor deposition (APCVD) of tin phosphide thin films was achieved on glass substrates from the reaction of SnCl4 or SnBr4 with Rx PH3−x (R = Cy or Ph) at 500–600 ◦ C. The films showed good uniformity and surface coverage.81 3.1.9

Tin Alloys

Rhodium–tin bimetallic particles have been deposited in a NaY zeolite. They were obtained by chemical vapor deposition with subsequent H2 reduction of SnR4 (R = C2 H5 or C6 H5 ) onto reduced Rh/NaY; samples prepared by ion exchange (IE) or by chemical vapor deposition (CVD). The resultant product was used in the selective hydrogenation of α, β-unsaturated aldehydes.82 New types of Ge(1−y) Sn y semiconductors has been obtained by CVD. PhSnH3 or a mixture of SnD4 and hydrogen (20% in volume) have been employed as tin sources (SnH4 is insufficiently stable and pure SnD4 decomposes readily in Sn and D2 at the experiment temperature, 22 ◦ C). The Ge(1−y) Sn y samples were grown by ultra-high vacuum chemical vapor deposition (UHV-CVD) reactions of the Sncontaining materials with commercial Ge2 H6 . The growth was conducted on Si(100) and Si(111) between 250 and 350 ◦ C and 2 × 10−3 Torr. Highly uniform Ge(1−y) Sn y layers with smooth and continuous surface morphologies were obtained. The final product was used as a substrate for subsequent growth of Ge(1−y) Six Sn y , which exhibited high thermal stability, superior crystallinity and unique optical and strain properties, such as adjustable bandgaps, and controllable strain states (compressive, relaxed, and tensile). The Ge–Si–Sn system also represents a new class of ‘designer’ templates for the monolithic integration of 13-15 and 2-16 semiconductors with Si electronics.83 Particles of Ni3 Sn, Ni3 Sn2 , and Ni3 Sn4 , have been formed on silica by chemical vapor deposition (CVD) of tetramethyltin on Ni/SiO2 . The Ni–Sn/SiO2 exhibited high catalytic activity in the dehydrogenation reaction of cyclohexane, however lower than that of Ni/SiO2 catalyst. The selectivity of Ni3 Sn/SiO2 and Ni3 Sn2 /SiO2 for the formation of benzene was almost 100%, even at higher conversions.84 3.1.10

Summary

CVD is a complex process, since it is still based on empirical results and experience rather than on an understanding of the scientific fundaments of the mechanisms involved in the technique. Nevertheless, it owns a wider range of thin film and coating applications than any other deposition or coating technique. The use of online monitoring and diagnostic tools, together with an improved understanding of the thermodynamics, kinetics, fluid dynamics, rate control, and mass transport of the CVD process would help to minimize the influence of non-controlled parameters during the CVD process. The high costs and the environmental concerns connected to CVD process are other drawbacks to be overcome. The use of single-source precursors has contributed to minimize some of the CVD process parameters that need control. The literature shows that tin compounds (inorganic or organometallic complexes) are among the best materials for low temperature CVD experiments. Tin-containing materials, oxides, sulfides, phosphides, etc., and their metal-mixed composites, can be employed in a multitude of applications, ranging from low-emissivity windows, solar cells, transparent conducting oxides, diodes, and anti-static layers for catalysts.37 , 38 , 55

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For each application a different set of properties are required. Most of them can be modulated by using a ‘particular’ synthetic approach. Recent work shows that in some cases tin oxides, sulfides, etc. exhibit less interesting properties than the corresponding heterometallic materials. Therefore, the search for heterometallic single-source precursors is an open field of investigation. Tin-containing alkoxides, siloxides, thiolates, phosphinates, etc., mixed with group 13, 15, or transition metals, might represent the classes of compounds to be investigated and tested in CVD experiments. The development of advanced tin-based materials will widen the scope of engineering and technological applications of the metal. References 1. (a) J. Fan, J. G. Lu, R. S. Xu, R. Jiang, and Y. Gao, J. Coll. Inter. Sci. 266, 215 (2003); (b) A. G. Barrientos, J. M. De La Fuente, T. C. Rojas, A. Fernandez, and S. Penades, Chem. Eur. J., 9, 1909 (2003). 2. (a) C. Wang, Z. X. Deng, G. H. Zhang, S. S. Fan, and Y. D. Li, P. Technol., 125, 39 (2002); (b) X. B. Wang, Y. Q. Liu, W. F. Qiu, and D. B. Zhu, J. Mater. Chem., 12, 1636 (2002); (c) G. Ferey, Angew. Chem. Inter. Ed., 42, 2576 (2003); (d) H. Yang, L. C. Shen, L. J. Zhao, L. Z. Song, J. Z. Zhao, Z. C Wang, L. Wang, and D. Zhang, Mater. Lett., 57, 2455 (2003); (e) T. Oku and M. Kuno, Diam. Rel. Mater., 12, 840 (2003). 3. C. T. Sun and H. T. Zhang, J. Appl. Phys., 93, 1212 (2003). 4. (a) H. Yang, R. Lu, L. C. Shen, L. Z. Song, J. Z. Zhao, Z. C. Wang, and L. Wang, Mater. Lett., 57, 2572 (2003); (b) J. A. Wang, M. A. Valenzuela, S. Castillo, J. Salmones, and M. Moran-Pineda, J. Sol-Gel Sci. Tech., 26, 879 (2003). 5. (a) L. Eckertova, Physics of Thin Films, Plenum Press, New York, 2nd edn., 1986; (b) C. E. Morosanu, Thin Films by Chemical Vapour Deposition, in Thin Films Science and Technology, Vol. 7, Elsevier, Amsterdam, 1990 6. (a) Stephen M. Gates, Chem. Rev., 96, 1519 (1996); (b) V. Hopfe, D. W. Sheel, C. I. M. A. Spee , R. Tell , P. Martin , A. Beil , M. Pemble , R. Weiss, U. Vogt, and W. Graehlert, Thin Solid Films, 442, 60 (2003); (c) C. Vahlas, B. Caussat, P. Serp, and G. N. Angelopoulos, Mat. Sci. Eng. R, 53, 1 (2006). 7. K.L. Choy, Prog. Mat. Sci., 48, 57 (2003). 8. J. S. De Lodyguine, Illuminant for Incandescent Lamps, US patent 575002 (1893). 9. K. L. Choy, Handbook of Nanostructured Materials and Nanotechnology, in Synthesis and Processing, Vol. 1, Academic Press, San Diego (CA), 2000. 10. J. L. Vossen, Phys. Today, 33, 26 (1980). 11. (a) S. R. Wang, J. Huang, Y. Q. Zhao, S. P. Wang, X. Y. Wang, T. Y. Zhang, S. H. Wu, S. M. Zhang, and W. P. Huang, J. Mol. Catal. A-Chem., 259, 245 (2006); (b) G. M. Maksimov, M. A. Fedotov, S. V. Bogdanov, G. S. Litvak, A. V. Golovin, and V. A. Likholobo, J. Mol. Catal. A-Chem., 158, 435 (2000); (c) D. E. Williams and K. F. E. Pratt, J. Chem. Soc. Faraday 94, 3493 (1998); (d) J. Y. Wei, Y. X. Zhu, and Y. C. Xie, Chin. J. Catal., 24, 137 (2003). 12. (a) J. D. Shindler and R. M. Suter, Rev. Sci. Instrum., 63, 5343 (1992); (b) T. E. Moustafid, H. Cachet, B. Tribollet, and D. Festy, Electrochim. Acta, 47, 1209 (2002); (c) I. Kurisawa, M. Shiomi, S. Ohsumi, M. Iwata, and M. Tsubota J. Power Sources, 95, 125 (2001). 13. (a) A. R. Phani, S. Manorama, and V. J. Rao, Mater. Chem. and Phys., 58, 101 (1999); (b) G. J. Li, X. H Zhang, and S. Kawi, Sensors Actuat. B-Chem., 60, 64 (1999); (c) J. B. Sanchez, F. Berger, M. Fromm, M. H. Nadal, and V. Eyraud, Thin Solid Films, 436, 132 (2003). 14. W. Gopel and K.D. Schierbaum, Sensors Actuat. B-Chem., 1, 26 (1995). 15. (a) J. C. Alonso, M. Garcia, A. Ortiz, and J. Toriz, Semicond. Sci. Technol., 11, 243 (1996); (b) L. S. Price, I. P. Parkin, A. M. E. Hardy, R. J. H. Clark, T. G. Hibbert, and K. C. Malloy, Chem. Mater., 11, 1792 (1999); (c) S. H. Park, V. C. Son, W. S. Willis, S. L. Suib, and K. E. Creasy, Chem. Mater., 10, 2389 (1998). 16. (a) Z. Zainal, M. Z. Hussein, A. Kassim, and A. Ghazali, J. Mater. Sci. Lett., 16, 1446 (1997); (b) B. Subramanian, T. Mahalingan, C. Sanjeeviraja, M. Jayachandran, and M. J. Chockalingan, Bull. Electrochem., 14, 398 (1998). 17. D. Barreca, S. Garon, P. Zanella, and E. Tondello, J. Phys. IV, 9, 667 (1999). 18. E. Shauti, A. Banerjee, V. Dutta, and K. L. Chopra, J. Appl. Phys., 53, 1615 (1982).

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3.2

Class II Tin-Based Hybrid Materials Prepared From Alkynyltin Precursors

Thierry Toupance University de Bordeaux, Institut des Sciences Moléculaires, Groupe Matériaux, Talence Cedex, France

3.2.1

Introduction

Over recent decades, the need of ever higher performance materials has stimulated many efforts to synthesize functional nanostructured materials. In this context, organic–inorganic hybrid materials, which combine at the nanometer scale active inorganic and organic or bioactive components, have received worldwide attention owing to their potential or demonstrated applications in fields such as electronics, optics, catalysis, and medicine.1 Different synthetic methods have therefore been adopted to develop tailor-made hybrid materials, since their chemical and physical properties are mostly ruled by their degree of organization and local structure. For example, it is worth mentioning the sol-gel route, using organofunctional or bridged metal alkoxides as precursors,2 the assembling of well-defined nanobuilding blocks,3 template-directed self-assembly procedures, with or without nanobuilding blocks,4 integrative approaches associating the previous methods with micro-molding routes,5 and, also, the grafting of an organic functionality onto oxide surfaces.6 These materials have been categorized into two main different classes: (i) class I hybrids, which correspond to all systems where no covalent or iono-covalent bonds exist between the organic and inorganic networks; (ii) class II hybrids, where strong covalent or iono-covalent bonds connect at least a fraction of the organic and inorganic components together.1 Class II hybrid materials therefore take up a very important place owing to their expected high chemical and mechanical stabilities. Thus, the hydrolysiscondensation of bis(trialkoxysilyl)alkylene, arylene, and benzylene derivatives in the presence of organic templates yield bridged silsesquioxane hybrid materials exhibiting controlled texture and morphology, both at the mesoscopic and molecular levels.7 In some cases, the organic spacer could be removed by air oxidation at high temperature, perhaps preceded by a chemical treatment, leading to mesoporous silicas.8 The hydrolysis of similar organosilanes has also furnished long-range ordered structures when the organic bridge between the two silicon atoms contains urea functionalities able to induce the self-assembly of the organic linker via hydrogen bonding.9 Furthermore, surface functionalization and modification of oxide particles and films have been achieved with chloro-,10 hydrido-,11 or allylsilanes12 to confer new properties to oxides such as water6 and/or fat13 repulsion, catalyst immobilization,12 and ion detection.14 However, the narrow range of precursors available has hampered the development of a similar approach for transition or main group metal-based hybrid materials, and most studies have focused on silica-based materials. Although, tetragonal tin dioxide (with a rutile-like structure) is one of the most fascinating smart and functional materials for technological and industrial applications because of its unique chemical and mechanical stabilities combined with two specific characteristics, semiconductivity and optical transparency, very few examples of tin-based hybrids have been reported up to now. The main one concerned the assembly of tin-oxo hydroxo clusters using telechelic organic biscarboxylic acid derivatives.15 Consequently, since tin is one of the rare metals capable of forming stable metal–carbon bonds under conventional sol-gel hydrolytic conditions, original synthetic routes towards hydrolyzable organotins have been established for sol-gel chemistry and chemical modification of oxide surfaces in order to widen the scope of the precursors available to design functional hybrid materials.16 Alkynylorganotins have been chosen as target precursors for both fundamental and practical reasons: (i) hydrolysis-condensation processes should remove the alkynyl functionality as an inert gas or liquid; (ii) the formation of gels should be favoured by their hydrolysis rate, which lies between those of the

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corresponding chlorides and alkoxides; and (iii) their reaction with hydroxylated species should be readily monitored by IR spectroscopy. Functional trialkynylorganotins 117 , 18 , 19 and organically bridged α,ω-bis(trialkynyltin) compounds 220,21 have therefore been synthesized successfully in order to prepare tin-based hybrid materials. In this section, the chemical modification of silica and tin dioxide surfaces using 1 (Scheme 3.2.1) is presented first, along with some physical properties of the resulting materials. (H9C4-C

C)3Sn

R

1

C8F17

R=

C22H45

R=

1a 1b

R=

R=

(

(

)2

1c

)3

1d

Scheme 3.2.1 Chemical formulae of trialkynylorganotin derivatives 1 used for chemical modification of oxide surfaces

The synthesis and the structure of self-assembled tin-based hybrid materials prepared from 2 (Scheme 3.2.2) are then reported, as well as the textural and structural properties of the tin dioxide materials obtained after annealing these hybrids. 2a

R=

2b

R=

(H3C-C C)3Sn

R

Sn(C

C-CH3)3

R=

2f

R=

2g

2c

R=

R=

2

(

2d

R=

(

)2

2e

(

)2

O ( )4

R= R=

)2

( ) 4O

2h 2i

Scheme 3.2.2 Chemical formulae of organically bridged α,ω-bis(trialkynyltin) compounds 2 used for the preparation of self-assembled tin-based hybrid materials

3.2.2

Functionalization of Oxide Surfaces

The irreversible chemisorption of a functional trialkynylorganotin onto metal oxides was first achieved by reacting 1a in solution with a non-porous silica powder.18 According to quantitative FTIR measurements and elemental analyses, a careful control of the concentration of 1a in the grafting solution and of the reaction time made it possible to tune the 1H ,1H ,2H ,2H -heptadecafluorodecyl chain content in the surface-modified silica in the range 0.05 and 0.28 mmol g−1 , i.e. 0.3 and 1.7 chain nm−2 (Table 3.2.1). Although the reaction rate is increased by a rise in temperature, the denser and more reproducible coatings were prepared by reacting 0.28 mmol g−1 of 1a at room temperature for 17 hours, the final chain loadings remaining constant when a higher starting content of 1a was used.

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Table 3.2.1

Loadings of 1a-b and chain densities on non-porous Biosepra 100 silica (100 m2 g−1 )

Precursor

Mole number of precursor introduced (mmol g−1 )

Reaction time (h)

Loading (mmol g−1 )

Chain density (chain nm−2 )

1a 1a 1a 1a 1b 1b 1b (F17 H4 C10) SiCl3 (F17 H4 C10) SiCl3 (F17 H4 C10) SiCl3

0.20 0.40 0.68 1.23 0.16 0.34 0.68 0.20 0.68 1.28

17 17 17 17 72 72 72 17 17 17

0.18 0.22 0.24–0.27 0.28 0.16 0.34 0.34 0.19 0.44 0.44

1.05 1.30 1.40–1.60 1.70 1.00 2.10 2.10 1.15 2.65 2.65

The presence of the fluorous chain at the silica surface was unambiguously demonstrated by TGA-MS, XPS, and 13 C MAS NMR measurements, and the release of three hexyne molecules per chemisorbed chain was shown by quantitative FTIR measurements. 117 Sn CP-MAS spectroscopy on the chemisorbed tin species showed a single isotropic resonance at –390 ppm, typical of five- or six-coordinate monoalkyltin sites bound to oxygenated ligands. Moreover, 29 Si MAS and CP-MAS NMR spectra of non-porous silica before and after reaction with 1a revealed the presence of more condensed Q3 and Q4 species,22 i.e., the decrease of the amount of surface hydroxyl moieties, in the surface modified silica, which was in agreement with grafting of the precursor molecule 1a (Figure 3.2.1). These data are therefore consistent with an irreversible chemisorption of 1a occurring via the release of the three alkynyl functionalities to

Q3 Q3 Q4 Q4 Q2 Q2 a b ppm

0

-50

-100

-150

Figure 3.2.1 29 Si CP-MAS NMR spectra of non-porous Biosepra 100 silica: (a) before grafting; (b) after reaction with 1a. Q4 : siloxane group Si(O0.5 )4 ; Q3 : single hydroxyl silanol group Si(O0.5 )3 OH; Q2 : double hydroxyl silanol group Si(O0.5 )2 (OH)2

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yield a 1H ,1H ,2H ,2H -heptadecafluorodecyl thin layer, probably a monolayer, covalently grafted to the oxide surface via Sibulk O Sn C linkages, involving five- or six-coordinate tin centers as suggested in Scheme 3.2.3.18 According to the X-ray crystal structures of organotin clusters,23 dative bonds between the tin atoms and either unreacted silanol and/or stannol groups, or neighboring Sn-O-Sn bridges account for the coordination of the tin atoms that is observed. C8F17

F17C8

F17C8

Sn

O

O O O

O

Sn

Sn O

H Si

O

O

O

H

O

O

O

O Sn O

O

O

Si

Si

C8F17

Si

O

Si

H

O

SiO2

Scheme 3.2.3 100 silica

Schematic representation of the species formed after reaction of 1a with non-porous Biosepra

As far as the influence of the chain nature is concerned, the same conclusions can be drawn for 1b bearing the electron-releasing docosyl chain. Compound 1b leads to surface modified silicas with docosyl group contents as high as 0.34 mmol g−1 , to be compared to 0.24–0.27 mmol g−1 determined for 1a under similar experimental conditions. Therefore, the alkyl moiety gives higher loadings, an observation in agreement with the steric hindrance of the chains. Furthermore, the ratio between the alkyl and fluorinated chain contents, ca. 1.25–1.5, is in close agreement with the one between the cross-sectional areas of a ˚ 2 , respectively, according to perfluorinated chain and an alkyl chain which is estimated to be 30 and 20 A 19 the X-ray crystal structures of related organotins. Nonetheless, compound 1b reacts much slower than 1a, more than three days being necessary to reach the maximum chain loading. This suggests that the nucleophilic attack of hydroxylated species, such as adsorbed water or surface hydroxyl groups, at the tin center is favoured with 1a since an electron-withdrawing fluorinated chain makes the tin site more electrophilic than the electron-releasing alkyl group does. As a consequence, the electronic demand and the bulkiness of the chain linked to the tin atom governs the reactivity (i.e. chain loading and reaction time) of trialkynylorganotins endowed with flexible linear organic groups towards silica supports. However, the trialkynylorganotin 1a provides less dense layers than those prepared from the corresponding silicon trichloride, since the maximum loading of the organically modified silica synthesized from 1a only represents 66% of that reached under the same experimental conditions with the perfluorinated trichlorosilane analog (Table 3.2.1). Furthermore, the latter reacts much faster than 1a, as the maximum chain loading is attained after 2 h instead of 17 h (Figure 3.2.2). This behavior is related to the higher hydrolysis rate of the trichloroorganosilanes, which will favour surface reactions and selforganization of the chains at the oxide surface and hinder polycondensation reactions in solution, as previously proposed.24 The chemical modification of oxide surfaces with trialkynylorganotins 1 is not restricted to silica substrates. Indeed, nanoporous and nanocrystalline F-doped and undoped tin dioxide powders, prepared by a sol-gel route,25 , 26 can be chemically modified with organotins 1a and 1c–d to provide fluorous or dyesensitized SnO2 nanohybrids.26 , 27 , 28 As for silica, the amount of deposited organic group progressively rises to a plateau value as the concentration of 1 in the grafting solution is increased (Table 3.2.2).27

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0.40 0.35 0.30

a

0.25 0.20 0.15 0.10 0.05 0.00 0

5

10

15

20

25

30

35

40

45

50

Reaction time (h) Figure 3.2.2 Loadings, as a function of the reaction time, obtained after reaction at room temperature with non-porous Biosepra 100 silica of: (a) 1a; (b) (F17 H4 C10 )SiCl3 . Mole number of starting precursor: 0.68 mmol g−1

Elemental analysis, solid-state, and solution FTIR measurements demonstrate the removal of three molecules of hexyne per organometallic group deposited, along with the formation of Snbulk –O–Sn–C bonds. The presence of a maximum chain density from a certain precursor concentration dismisses the possibility of any continuous polycondensation of 1 at the oxide surface, which is consistent with the irreversible chemisorption of 1 as a monolayer. Moreover, the modified SnO2 materials show remarkable chemical stability both in organic and aqueous media in the pH range 3–10. A promising application of this new functionalization method of metal oxides relates to the photosensitization of semiconducting oxides to convert light into electricity. Thus, using the cavity microelectrode technique, a significant photocurrent is produced under blue light illumination by 1c- and 1d-modified, F-doped and undoped SnO2 nanoporous powders, in the presence of an aqueous electrolyte,26 , 28 the Table 3.2.2

Loadings of 1a, 1d and chain densities on porous tin dioxide powders (50 m2 g−1 )

Precursor

Mole number of precursor introduced (mmol g−1 )

Loading (mmol g−1 )

Chain density (chain nm−2 )

1a 1a 1a 1d 1d 1d 1d 1d

0.058 0.173 0.580 0.046 0.080 0.104 0.164 0.405

0.056 0.168 0.172 0.041 0.076 0.092 0.131 0.125

0.67 2.02 2.07 0.49 0.92 1.10 1.57 1.50

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1.4

18 16

1.2

14

10

0.8

8

0.6

6

0.4

4 0.2

2 0 340

Optical density (a.u.)

1.0

12

IPCE (%)

OTE/SPH

360

380

400

420

440

460

480

0.0 500

Wavelength (nm) Figure 3.2.3 Incident photon-to-current efficiency (left, circles) and UV-visible absorption spectrum (right, solid line) of 1 μm thick SnO2 film modified with 1c

intensity of the current increasing almost linearly with the light power. In addition, with an aqueous NaBr solution, the photopotential generated by these systems reaches 700 mV, which is the highest ever reported for SnO2 -based systems.29 This approach was then generalized for the surface modification of oxide films. For instance, chemical modification of nanoporous SnO2 thin film coated onto a transparent semi-conducting electrode with 1c, furnishes highly colored layers. Under white light illumination, the resulting photoelectrochemical cell involving an (I− /I− 3 )-based organic electrolyte, shows a short-circuit current density (Jsc ) of 240 μA cm−2 , an open-circuit photovoltage (Voc ) of -0.36 V, and a fill factor of 0.35, corresponding to an overall conversion efficiency of about 0.037%. Furthermore, a maximum quantum yield of 16% is measured for this system as evidenced by the incident photon-to-current efficiency (IPCE) plot (Figure 3.2.3). The rather good agreement between the photocurrent action spectrum and the UV-visible absorption of the 1c-modified SnO2 electrode confirms that the hybrid nanoporous thin films actually achieve photovoltaic conversion. Compound 1d behaves similarly, but the overall conversion efficiency is significantly lower, i.e. 0.022%. As a result, although no π-conjugation between the perylene dye and the tin dioxide nanoparticles exists, electron transfer actually occurs, probably through a bridge-assisted mechanism as previously proposed for dye-excited states weakly coupled to semi-conducting particles.30 On the basis of this mechanism, the lower cell efficiency found for 1d-modified electrodes could be due to the longer hexylene linker in 1d, even though the overall conversion efficiency of a dye-sensitized solar cell depends on many different factors (oxide crystallinity, electrolyte nature, etc.). 3.2.3

Self-Assembled Tin-Based Hybrid Materials

Hydrolysis-condensation of organically bridged α,ω-bis(trialkynyltin) compounds 2 was the second route investigated to prepare class II tin-based hybrid materials. At first, hydrolysis conditions of 2 were determined to get transparent gels. As gelling of neutral solutions is very slow, acidic or basic conditions were employed to accelerate the process. Whatever the compound 2 and the water amounts used, base-catalyzed

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Figure 3.2.4 Transparent gel obtained by acid-catalyzed hydrolysis of 2 after ageing for several weeks at room temperature

hydrolysis systematically furnishes a heterogeneous mixture of gel and precipitate. By contrast, acidcatalyzed hydrolyses with a large excess of water yields gels within several days which remain transparent for several weeks (Figure 3.2.4). Gelling times of 2 are several orders of magnitude longer than those described for stabilized dialkoxytins,25 which could be related to the lower reactivity of the tin-C(sp-hybridized) bond in the alkynylorganotins toward hydroxylated species such as water or alcohols. In addition, only transparent gels are solely obtained when hydrolysis is conducted under acidic conditions. Thus, acid catalysis probably favours an increase in the hydrolysis rate of the tin–alkynyl bond and a concomitant decrease in the condensation rate of the hydroxyl groups leading to the formation of low molecular weight species and, as a consequence, to transparent media. Regardless of the nature of the bridge, elemental analysis, FTIR, and TGA-MS studies reveal, without ambiguity, that the xerogels obtained after drying are composed of condensed oxo-hydroxo organotin species, all alkynyl groups having been removed. The Sn–Clinker bonds are also maintained in the xerogels and the organic units are structural components of the framework. These findings establish for the first time that transparent gels can be readily prepared from perorganometallic compounds, alkynyl leaving groups replacing advantageously the traditional chloride or alkoxide groups usually involved in the sol-gel chemistry. Therefore, this approach promotes the use of precursors that are easy to purify and handle, and involves leaving groups which are eliminated as an inert gas, i.e. propyne, instead of as an acid or an alcohol, and which prevent undesirable side reactions from taking place and the presence of unwanted organics in the xerogels.31 The organic linker organization at the nanometre level in these xerogels was then studied. For distannylated derivatives including flexible or short rigid linkers, no features typical of any spacer self-assembly at the nanometer level could be detected in the corresponding powder XRD pattern. By contrast, when precursors containing a semi-rigid or long rigid spacer such as 2b, 2c, 2e, and 2h are used, a notable diffraction feature is detected at low angle, i.e. between 4 and 6.5◦ , corresponding to distances between diffractive planes ranging between 1.4 and 2.1 nm (Table 3.2.3). Furthermore, this value closely matches that calculated from the tin–tin distance determined in the X-ray crystal structures of the precursors. A structural model where layers of tin dioxide alternate with layers of hydrophobic organic chains has therefore been proposed to rationalize these results, a tilt angle of the organic spacer being postulated,

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Table 3.2.3 Structural data for hybrid xerogels X120 prepared in homogeneous conditions after drying at 120◦ C, y where y stands for the precursor nature Xerogel

2θ (deg)

dexp (nm)

dcal (nm)

αa (deg)

Domain sizeb (nm)

X120 2a

–

–

–

–

–

X120 2b X120 2c X120 2d X120 2e X120 2f X120 2g X120 2h X120 2i

5.7

1.55

1.54

0

3.3

4.8

1.84

1.97

21

4.8

–

–

–

–

–

6.3

1.40

1.48

19

4.2

–

–

–

–

–

–

–

–

–

–

4.3

2.05

2.05

0

4.2

23

3.5

5.2

c

1.70

d

2.9 & 1.85

a

Tilt angle; b Determined from X-ray line broadening ( ± 0.5 nm); c Calculated postulating a fully extended linker; d Calculated assuming a doubly folded linker

in some cases, to fit the experimental and the calculated tin oxide plane distances. Weak hydrophobic interactions between the spacers are thus sufficient to favour the organization of the long aromatic or mixed aromatic–aliphatic spacers in the xerogels.32 Whilst the ordered domains represent only two or three layers, the hydrolysis under homogeneous conditions of 2b, 2c, 2e, and 2h yields spontaneously self-assembled tin-based hybrid materials.33,34 Such an organization is thus consistent with the formation of kinetically controlled xerogels as previously proposed for organically bridged hybrid silica.35 Interestingly, when the semi-rigid linker contains oxygen atoms, as in 2i, a unique intra-molecular coordination phenomenon is demonstrated in the corresponding self-assembled hybrid material (Figure 3.2.5). Indeed, the experimental distance found between the polystannoxanne walls is far shorter, i.e. 1.7 nm, than that expected from a fully extended organic

Sn

O O

Sn O

O Sn O Sn O O Sn O O Sn O O Sn O Sn O Sn O Sn O

O

Sn

O

α dcal O Sn O

Figure 3.2.5

O

O

O Sn O Sn O O

O Sn O Sn O O

dexp

Sn O Sn O

Schematic representation of the structural model proposed for xerogel X120 2i

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Figure 3.2.6 Molecular structure of (4,4 -bis(trichlorostannyl)-n-butyloxymethyl)biphenyl as determined by X-ray crystallography

chain, i.e. 2.9 nm and is close to the distance, i.e. 1.85 nm, calculated from the measured tin–tin separation determined in the X-ray crystal structure of the 4,4 -bis(trichlorostannyl)butyloxymethyl)biphenyl (Figure 3.2.6). This suggests the presence of a fully condensed network of tin oxide without coordinated water. Assuming a tilt angle α of about 23◦ , this indicates the formation of layers of stacked organic spacers alternating with tin oxide planes, where the spacer is doubly folded by intra-molecular coordination. The formation of such a structure is interpreted as follows. During the hydrolysis of uncoordinated 2i, the electrophilicity of the tin atoms is sufficiently enhanced by the substitution of alkynyl functions by oxygenated groups to cause a stabilizing coordination expansion at tin by the oxygen atom of the linker, which is in an appropriate position. The formation of two [1,2]oxastanninane-like six-membered rings is therefore induced by this intra-molecular coordination, as observed in the solid state for (4,4 bis(trichlorostannyl)-n-butyloxymethyl)biphenyl. The formation of these two rings reduces the distance between the tin atoms, which leads to the short distance between the tin oxide walls, as determined by powder XRD. This finding is confirmed by the result obtained in the case of a non-coordinating spacer of similar length, which shows that the alkylene chains of the hybrid prepared from 2h are fully extended. Hydrolyses have also been conducted in micro-emulsions, using non-ionic surfactants. In this case, selfassembled tin-based hydrid materials are prepared, regardless of the nature of the precursor molecule (Table 3.2.4). Micro-emulsion conditions are therefore required to organize the hybrid containing the flexible alkylene spacers. Table 3.2.4 Structural data for hybrid xerogels X120 prepared in microemulsion after drying at 120 ◦ C, y where y stands for the precursor nature Xerogel

2θ (deg)

dexp (nm)

dcal (nm)

αa (deg)

X120 2b

5.6

1.57

1.54

0

3.6

X120 2e X120 2f X120 2g X120 2i

6.2

1.42

1.48

16

4.0

8.1

1.09

1.10

0

2.3

7.6

1.16

1.26

23

3.7

4.7

1.87

1.85

0

3.9

a

Tilt angle; b Determined from X-ray line broadening ( ± 0.5 nm)

Domain sizeb (nm)

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Table 3.2.5 BET specific surface area of xerogels prepared under different hydrolysis conditions

Xerogel

SBET homogeneous medium (m2 ·g −1 )

SBET microemulsion (m2 ·g −1 )

X120 2a

7

X120 2b X120 2e X120 2f X120 2g X120 2i

2

8

3

10

–

2

165

118

275

2

12

The textural properties of the hybrid materials are also strongly dependent upon the hydrolysis conditions and the nature of the linker. After hydrolysis in a homogeneous medium, nitrogen adsorption and TEM measurements indicate the formation of non-porous solids with BET specific surface areas lower than 7 m2 g−1 , except for 2g. Variation of the precursor concentration does not strongly affect these values. In contrast, hydrolysis under microemulsion conditions leads to a significant increase in the specific surface areas, mainly in the case of the flexible alkylene linker (Table 3.2.5). In the latter case, the N2 adsorption-desorption isotherms were typical of mesoporous materials with mean pore size diameters in the range 5–10 nm. Even though many other parameters are known to govern the textural properties, a possible reason for the improvement observed under micro-emulsion hydrolysis conditions might arise from the adsorption of the surfactant onto the particles. Indeed, the poly(ethylene oxide) chains of the surfactant used could interact with the polar surface of the hybrid nanoparticles. This interaction could prevent inter-particle condensation to furnish a more divided hybrid, which would result in a higher specific area. However, this phenomenon seems to be less pronounced for rigid and semi-rigid linkers, likely due to the stronger interactions existing between organic spacers, which would give denser hybrid xerogels. To gain better insight into the particle growth mechanism, hydrolyses of 2a, 2d and 2f under microemulsion conditions were investigated by dynamic light scattering.36 Regardless of the nature of the precursor, only one population of scattering objects of diameter 18 nm, attributed to the micelles formed under the experimental conditions used, can be detected in solution at low reaction time. After a short period, another particle population appears, which grows slowly during the first six hours, and then rapidly, as shown in Figure 3.2.7. For the butylene precursor 2f, particles as large as 3 μm are formed within 10 hours, leading to precipitation, whereas the 18 nm micelle population still remains unchanged from the beginning. Consequently, the coexistence of micelles and large oxide particles strongly suggests that the condensation process occurs in the aqueous phase, as previously reported for the hydrolysis of tetraethoxysilane,37 which is a precursor molecule completely different from the perorganotins used here. Compounds 2a and 2d, which contain arylene and 4,4 -dimethylenebiphenyl, respectively, behave similarly, but the main difference lies in the size of the particle formed after 24 hours, which is much smaller than that measured for 2f. Furthermore, the hydrolysis rate of the latter is much lower, as evidenced

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Tin Chemistry: Fundamentals, Frontiers and Applications 1600 1400 Hydrodynamic radius (nm)

OTE/SPH

1200 1000 800 600 400 200 0 0

2

4

6

8

10

12

Time (hour)

Figure 3.2.7 Evolution of the particle hydrodynamic radius as a function of the reaction time during the hydrolysis of 2f under microemulsion conditions: micelles (squares) and large oxide particles (circles)

by FTIR measurements. According to the two main reactions involved in the sol-gel process, i.e. hydrolysis of the precursor followed by condensation of the species formed, the experimental differences observed as a function of the precursor nature can be rationalized as follows. The electron-releasing effect of the alkyl substituents slows down the nucleophilic attack of water molecules on the metal center, but enhance the nucleophilicity of the resulting hydroxyl function, which leads to a higher rate of condensation. By contrast, the electron-withdrawing effect of the phenyl and benzyl substituents make the oxygen atoms bound to the tin atom less nucleophilic, which results in a lower rate of condensation and a slower particle growth of the corresponding starting precursors. As a consequence, the particle growth is mostly governed by the condensation step. 3.2.4

Nanoporous Nanostructured Tin Dioxide Materials

To obtain tin dioxide materials, xerogels prepared from 2 in homogeneous media have to be annealed to remove the organic linker and to crystallize the oxide particles. According to TGA-MS measurements, temperatures higher than 400 ◦ C are required to eliminate the organics. The hybrid xerogels display relative thermal stability in the order p-phenylene < p-xylene = butylene, the same trend having been found for longer spacers.31 This stability order is likely to result from antagonistic contributions making the removal of the p-phenylene spacer easier under the pyrolysis conditions used. Indeed, no obvious relationships can be drawn with the cleavage ability of tin–carbon bonds towards ionic or radical reagents, which is known to decrease in the order Sn-CH2 Ph > Sn-Ph > Sn-CH2 CH3 . Similarly, this order does not follow that expected from the bond dissociation enthalpies, which increase in the order Sn-CH2 Ph < Sn-CH2 CH3 < Sn-Ph.38 Calcination of the xerogels at 400 and 500 ◦ C under an oxygen atmosphere induces the removal of the organic spacer, as confirmed by FTIR spectroscopy. Whereas the dried xerogels X120 are clearly amorphous (Figure 3.2.8b), sintering and particle growth provokes x an increase in crystal size, as shown by the XRD features of the annealed xerogels, which become progressively sharper with increasing temperature of the thermal treatment (Figure 3.2.8c and d). The XRD patterns recorded after calcination at high temperature account for the formation of tin dioxide

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307

d c

10

Figure 3.2.8

20

30

40

50 2θ (°)

60

70

b a 80

Powder XRD patterns of (a) cassiterite SnO2 (JCPDS 41-1445), (b) X2b120 , (c) X2b400 , and (d) X2b500

nanocrystalline particles, the average crystallite sizes of which are evaluated from the Scherer formula (Table 3.2.6). Annealing also provokes drastic changes in the textural properties of the resulting materials, and the texture and the morphology of samples calcined at a given temperature is strongly dependent upon the starting xerogel. For each xerogel studied, the detection of structural water by TGA/MS demonstrates that Table 3.2.6 Textural properties and crystallinity of annealed samples Xzy where y stands for the precursor nature and z for the calcination temperature

Sample

SaB E T (m2 g−1 )

Total Pore Volumea (cm3 g−1 )

Mean Pore Diametersa (nm)

Mean Crystallite Sizec (nm)

X400 2a

110

0.14

5.0

4

3.9

X500 2a

70

0.12

6.5

7.5

2.6

X400 2b

62

0.09

6, 9.5

5

< 3.9

X500 2b

25

0.05

6.5, 13, 20

31

< 3.9

X400 2d

150

0.13

< 3.5

7.5

34.9

X500 2d

45

0.15

12

12

2.5

X400 2f

65

0.07

4.5

5

22.7

X500 2f

40

0.08

7.5

9

6.3

X400 2g

45

0.08

8.5

12

8.2

X500 2g

26

0.05

–b

31

8.9

a b

C/Sn (at%)

Surface areas were determined by BET, mean pore diameters by BJH theory (adsorption branch), and pore volumes by single-point analysis; BJH theory cannot be used to evaluate to mean pore size in this case; c Determined from X-ray line broadening ( ± 0.5 nm)

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condensation of neighboring surface hydroxyl groups occurs, creating new tin–oxygen–tin bridges via oxolation reactions.31 These reactions are activated by various processes, such as the network shrinkage induced by the annealing and the decrease in steric hindrance around the tin centers due to the pyrolysis of the organic spacers. At different stages of the thermal treatment, both processes may force the close matching of reactive hydroxyl groups yielding a continuous water emission. This is well known to cause a severe drop in the specific surface area by closure of the porosity and/or particle growth. However, the formation of pores arising from the decomposition of the organic spacer clearly competes with this process. As a matter of fact, annealing not only permits the elimination of the organics from the xerogels with the concomitant formation of tin dioxide particles, but also improves significantly the textural properties of most of the final materials. Except for the xerogel prepared from 2g, the specific surface areas indeed increase by more than one order of magnitude after annealing at 400 ◦ C, BET surface areas as high as 110–150 m2 g−1 being reached in the case of the materials synthesized from X120 2a and X120 . Although the size of the pores is larger than the length of the organic spacer removed during the 2d calcination step, the pore size distribution remains narrow, with mean pore sizes of 3.5–5 nm. However, other xerogels lead to significantly lower surface areas with larger pore size distribution. Organically bridged ditin hexaalkynides 2 are therefore useful sol-gel precursors of mesoporous (or nanoporous) tin dioxide materials. Indeed, for each sample studied, the N2 adsorption-desorption isotherm is a type IV isotherm with a type H2 hysteresis loop, which is typical of mesoporous solids, according to the IUPAC classification (Figure 3.2.9).39 The corresponding porosity clearly falls into the mesoporous range, i.e. pore diameters between 2 and 50 nm (Table 3.2.6). According to TEM images (Figure 3.2.10) and XRD patterns (Figure 3.2.8), the annealed materials are therefore composed of a porous network of aggregated cassiterite tin dioxide nanoparticles, the mesoporosity that is detected stemming from the inter-particle space, as previously proposed for other SnO2 materials prepared by sol-gel techniques.25 Among all the precursors studied, precursor 2a appears to be the most useful precursor of nanocrystalline mesoporous SnO2 for application purposes. In comparison with 2d and 2f, compound 2a provides pure cassiterite tin dioxide materials with very low amounts of carbon from calcination at 400 ◦ C, owing to the rather facile elimination of the p-phenylene spacer. After calcination at 400 and 500 ◦ C, the BET surface area (110 and 70 m2 g−1 ), the mean pore size diameter (5 and 6.5 nm) and the mean crystallite size (4.5 and 7 nm) are close to those reported for tin dioxide materials prepared by the ‘template’ method after 110

0.035

100 -1

90

0.030

3

80

-1

dV/dRp (cm .g .nm )

60 50 40 30 20

0.020 0.015 0.010 0.005

10 0 0.0

0.025

-1

70

3

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OTE/SPH

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Relative Pressure (P/P0 )

0.8

0.9

1.0

0.000 0

5

10

15

20

25

30

Pore Diameter (nm)

Figure 3.2.9 Nitrogen gas adsorption-desorption isotherm (left) and BJH (adsorption branch) pore-size distribution (right) of X400 2a

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Figure 3.2.10

309

Transmission electron image of X500 2a

500 treatment in the same temperature range.40 Moreover, the specific surface areas of X400 2a and X2a are large 2 −1 since they are equivalent to 300 and 190 m g , respectively, for siliceous materials after normalization to account for the difference in density between silica and tin dioxide. It is also worth mentioning that unusual pore size distributions are found for materials prepared from 2b. Indeed, the hysteresis loops of 500 the N2 adsorption-desorption isotherm of X400 2b and X2b show several features leading to polymodal pore size distributions which might be due to different pore families (Table 3.2.6). Indeed, X500 2b contains both nanoparticles, i.e. 10–30 nm, and much larger particles with a size greater than 200 nm according to the TEM images. The polymodal pore size distributions could therefore be related to the inter-particle space existing between particles of different sizes.34 In summary, the nature of the hybrid therefore allows, to some extent, the control of both texture and morphology of the tin dioxide materials obtained after calcination. Nevertheless, treatment at 500 ◦ C leads to similar BET surface area and mean crystallite size, which shows that annealing at very high temperature erases completely the ‘fingerprint’ of the organic linker.

3.2.5

Conclusion

In the field of functional organic-inorganic hybrid materials, recent trends in the chemistry based on alkynylorganotins have been reviewed. First of all, a new functionalization method based on trialkynylorganotins has allowed the addition of specific organic functionalities to silica and tin dioxide surfaces to yield hydrophobic, lipophobic, or perylene-dye modified oxide materials, which have found promising applications in the field of photovoltaic conversion. This new approach constitutes the first example of grafted organotins on oxide supports synthesized by a solution route. Furthermore, self-assembled tinbased class II hybrid materials have been prepared for the first time by hydrolysis of organically bridged α,ω-bis(trialkynyltin) derivatives, the organization at the nanometre level being tuned by the precursor nature and the hydrolysis conditions. Annealing of these hybrids provides nanoporous, nanostructured

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tin dioxide, the textural and morphological properties of which are controlled by the precursor nature and the temperature of the thermal treatment. This work therefore widens the range of functional hybrid materials available and may be extended to the preparation of other tin-based hybrid materials bearing organic groups showing catalytic or luminescence properties. Acknowledgments The author is indebted to Dr. B. Jousseaume for fruitful discussions and Dr. G. Vila¸ca, H. Riague, Dr. H. El Hamzaoui, Dr. M. de Borniol, Dr. C-H. Han, O. Babot and M-C. Rascle for their crucial contribution to this work, and wishes to thank, Prof. M. Biesemans, Prof. R. Willem, Dr. H. Allouchi, Dr. C. Zakri, Dr. G. Campet, Dr. C. Labrugère, Dr. V. Vivier, Dr. H. Cachet and Dr. J. Brötz for their fruitful collaboration. References 1. P. Gomez-Romero and C. Sanchez, Functional Hybrid Materials, Wiley-VCH: Weinheim, 2003. 2. D.A. Loy and K.J. Shea, Chem. Rev., 95, 1431 (1995). 3. C. Sanchez, G.J.A.A. Soler-Illia, F. Ribot, C. Mayer, V. Cabuil, and T. Lalot, Chem. Mater., 13, 3061 (2001); G. Fornasieri, L. Rozes, S. Le Calvé, B. Alonso, D. Massiot, M-N. Rager, M. Evain, K. Boubekeur, and C. Sanchez, J. Am. Chem. Soc., 127, 4869 (2005); U. Diaz, A. Cantin and A. Corma, Chem. Mater., 19, 3686 (2007). 4. G.J.A.A. Soler-Illia, C. Sanchez, B. Lebeau, and J. Patarin, Chem. Rev., 102, 4093 (2002). 5. C. Sanchez, H. Arribart, and M.M. Giraud-Guille, Nat. Mater., 2, 277 (2005); R. Backov, Soft Matter, 2, 452 (2006). 6. J. Sagiv, J. Am. Chem. Soc., 102, 92 (1980). 7. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, and O. Terasaki, J. Am. Chem. Soc., 121, 9611 (1999); B.J. Melde, B. Hollande, C.F. Blanford, and A. Stein, Chem. Mater., 11, 3302 (1999); T. Asefa, M.J. MacLachlan, N. Coombs, and G.A. Ozin, Nature, 402, 867 (1999). 8. R. Corriu, Polyhedron, 17, 925 (1998). 9. J.J.E. Moreau, L. Vellutini, M. Wong Chi Man, and C. Bied, J. Am. Chem. Soc., 123, 1509 (2001); J.J.E. Moreau, L. Vellutini, J-L. Bantignies, M. Wong Chi Man, C. Bied, P. Dieudonné, and J-L. Sauvajol, J. Am. Chem. Soc., 123, 7957 (2001); J.J.E. Moreau, B.P. Pichon, M. Wong Chi Man, C. Bied, H. Pritzkow, J-L. Bantignies, P. Dieudonné, and J-L. Sauvajol, Angew. Chem. Int. Ed. 43, 203 (2004). 10. N. Tillman, A. Ulman, J.S. Schildkraut, and T.L. Penner, J. Am. Chem. Soc., 110, 6136 (1988). 11. A.Y. Fadeev and T. Mc Carthy, J. Am. Chem. Soc., 121, 12184 (1999). 12. T. Shimada, K. Aoki, Y. Shimoda, T. Nakamura, N. Tokunaga, S. Inagaki, and S. Hayashi, J. Am. Chem. Soc., 125, 4688 (2003). 13. N. Yoshino, Y. Yamamoto, K. Hamano, and T. Kawase, Bull. Chem. Soc. Jpn., 66, 1754 (1993). 14. H. Perrot, N. Jaffrezic-Renault, and P. Clechet, J. Electrochem. Soc., 137, 598 (1990). 15. F. Ribot, A. Lafuma, C. Eychenne-Baron, and C. Sanchez, Adv. Mater., 14, 1496 (2002); F. Ribot, D. Veautier, S. Guillaudeu, and T. Lalot, J. Sol-Gel Sci. & Technol., 32, 37 (2004); F. Ribot, D. Veautier, S.J. Guillaudeu, and T. Lalot, J. Mater. Chem., 15, 3973 (2005). 16. B. Jousseaume, see Chapter 3.7. 17. G. Vila¸ca, K. Barathieu, B. Jousseaume, T. Toupance, and H. Allouchi, Organometallics, 22, 4584 (2003). 18. S. Boutet, B. Jousseaume, T. Toupance, M. Biesemans, R. Willem, C. Labrugère, and L. Delattre, Chem. Mater., 17, 1803 (2005). 19. M-L. Dumartin, H. Elhamzaoui, B. Jousseaume, M-C. Rascle, T. Toupance, and H. Allouchi, Organometallics, 26, 5576 (2007). 20. B. Jousseaume, H. Riague, T. Toupance, M. Lahcini, P. Mountford, and B.R. Tyrell, Organometallics, 21, 4590 (2002)

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21. H. Elhamzaoui, B. Jousseaume, T. Toupance, and H. Allouchi, Organometallics, 26, 3908 (2007). 22. S. Léonardelli, L. Facchini, C. Frétigny, P. Tougne, and A.P. Legrand, J. Am. Chem. Soc., 114, 6412 (1992). 23. F. Banse, F. Ribot, P. Tolédano, J. Maquet, and C. Sanchez, Inorg Chem., 34, 6371 (1995); V. Chandrasekhar, S. Nagendran, and V. Baskar, Coord. Chem. Rev., 235, 1 (2002); V. Chandrasekhar, P. Singh, and K. Gopal, Appl. Organometal. Chem., 21, 483 (2007). 24. T. Vallant, J. Kattner, H. Brunner, U. Mayer, and H. Hoffmann, Langmuir, 15, 5339 (1999); R. Resh, M. Grassenbauer, G. Friedbacher, T. Vallant, H. Brunner, U. Mayer, and H. Hoffmann, Appl. Surf. Sci., 140, 168 (1999). 25. T. Toupance, O. Babot, B. Jousseaume, and G. Vila¸ca, Chem. Mater., 15, 4691 (2003). 26. C-H. Han, B. Jousseaume, M-C. Rascle, T. Toupance, H. Cachet, and V. Vivier, J. Fluor. Chem., 125, 1247 (2004). 27. G. Vila¸ca, B. Jousseaume, C. Mahieux, C. Belin, H. Cachet, M-C. Bernard, V. Vivier, and T. Toupance, Adv. Mater., 18, 1073 (2006). 28. H. Cachet, V. Vivier, and T. Toupance, J. Electroanal. Chem., 572, 249 (2004). 29. S. Ferrere, A. Zaban, and B.A. Gregg, J. Phys. Chem. B, 101, 4490 (1997); A. Kay and M. Grätzel, Chem. Mater., 14, 2930 (2002). 30. J.B. Asbury, E. Hao, Y. Wang, and T. Lian, J. Phys. Chem. B, 104, 11957 (2000); N.A. Anderson, X. Ai, D. Chen, D.L. Mohler, and T. Lian, J. Phys. Chem. B, 107, 14231 (2003). 31. T. Toupance, H. El Hamzaoui, B. Jouseaume, H. Riague, I. Saadeddin, G. Campet, and J. Brötz, Chem. Mater, 18, 6364 (2006). 32. H. Elhamzaoui, B. Jousseaume, H. Riague, T. Toupance, P. Dieudonné, C. Zakri, M. Maugey, and H. Allouchi, J. Am. Chem. Soc., 126, 8130 (2004). 33. H. Elhamzaoui, B. Jousseaume, T. Toupance, C. Zakri, M. Biesemans, R. Willem, and H. Allouchi, Chem. Commun., 1304 (2006). 34. T. Toupance, M. de Borniol, H. Elhamzaoui, and B. Jousseaume, Appl. Organomet. Chem., 21, 514 (2007). 35. G. Cerveau, R.J.P. Corriu, and E. Framery, Chem. Mater., 13, 3373 (2001). 36. H. Elhamzaoui, T. Toupance, M. Maugey, C. Zakri, and B. Jousseaume, Langmuir, 23, 785 (2007). 37. C. Lesaint, B. Lebeau, C. Marichal, J. Patarin, and R. Zana, Langmuir, 21, 8923 (2005). 38. A.G. Davies, Organotin Chemistry, Wiley-VCH: Weinheim, 2004, p. 27. 39. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, and T. Siemieniewska, Pure Appl. Chem., 57, 603 (1985). 40. D.N. Srivastava, S. Chappel, O. Palchik, A. Zaban, and A. Gedanken, Langmuir, 18, 4160 (2002).

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3.3

Organotin Compounds as PVC Stabilizers

Esen Arkis Izmir Institute of Technology, Chemical Engineering Department, Izmir, Turkey

3.3.1

Introduction

When poly(vinyl chloride) is fabricated, it passes between rollers (calendars) at about 200 ◦ C, which causes elimination of some HCl at allylic defects in the polymer, as shown below (Figure 3.3.1). Furthermore, the released HCl induces further elimination, giving a polyolefin structure with a yellow coloration, which turns red, and then black, after which the polymer becomes brittle. This degeneration can be inhibited by organotin stabilizers, often organotin mercaptides that appear to have two principal functions. Firstly, they react with the HCl to give organotin chlorides, which do not catalyze the elimination process. Secondly, they substitute the chloride at the reactive sites, introducing other groups, such as mercaptide groups, which are not easily eliminated. Organotin maleates may also remove diene units by the Diels–Alder reaction. The first generation tin mercaptide stabilizers were dialkyltin long-chain mercaptans (1), traditional mercaptoacetate products (2), and mercaptoacetic ester products, having an average tin content of 18% in the stabilizer. When a stabilizer with a high content of a monoalkyltin compound derived from a mercaptoethanol ester (3), was demonstrated to provide outstanding early color and color retention in white pipe, the industry quickly accepted this departure from traditional mercaptoacetate products. Second generation products were mixed mono-/di-alkyl tin long-chain mercaptans. The stabilizer is synthesized directly from an appropriate combination of mono-n-butyltin trichloride and di-n-butyltin dichloride, which is reacted with a suitable quantity of i-octyl mercaptoacetate and sodium sulfide. Although the tin content in the stabilizer is increased to about 22%, these products are often used in diluted form to aid in their handling, while reducing losses. Recently, a third generation of stabilizers has been developed, which are monoalkyltin short-chain, and/or functionalized mercaptans or sulfides. Neither dialkyltin nor monoalkyltin sulfides by themselves are particularly good stabilizers, in spite of their very high tin content. However, when the alkyltin sulfides are used in conjunction or admixture with the alkyltin mercaptides, a synergistic effect on performance is obtained. Thus, the combination of dialkyltin bis(i-octylmercaptoacetate) with monoalkyltin sulfide (4), or monoalkyltin tris(i-octylmercaptoacetate) with dialkyltin sulfide (5), affords high tin-content products with powerful stabilizing properties. Another approach to such a stabilizer composition would be a direct synthesis from an appropriate combination of mono-n-butyltin trichloride and di-n-butyltin dichloride, which is reacted with a suitable quantity of i-octyl mercaptoacetate and sodium sulfide. This technology has been extended to include methyltins as well as n-butyltins and mercaptoethanol esters as well as i-octyl mercaptoacetate.1

C H

H C

H H CH CH CH CH Cl Cl

-HCl C H

H C

C H

H C

H CH CH Cl

-HCl

etc.

-HCl C H

Figure 3.3.1

H C

C H

H C

C H

H C

Dehydrochlorination of PVC

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R2Sn(SR')2 (1)

R2Sn(SCH2CO2R')2 (2)

Figure 3.3.2

R2Sn(SCH2CH2OCOR')2 (3)

S RSn S

R Sn S

S

Sn S R

SnR

R2Sn

(4)

313

SnR S Sn 2 S R S 2 (5)

Formulae for sulfur containing organotin heat stabilizers

The stabilizers stated above have the following formulae (Figure 3.3.2). This section deals with the stabilization of PVC by organotin stabilizers having different alkyl groups (methyl, ethyl, n-butyl, n-octyl, etc.) and different ligands (carboxylates, mercaptides, etc.), the mechanism of reaction and the evaluation of stability by yellowness index and conductivity. 3.3.2

Types of Organotin Stabilizers

The Alkyl Group

Most of the organotin stabilizers in general use have structures of the type Rn SnX4−n where R is normally an alkyl group, usually methyl, n-butyl, or n-octyl, and X is one of a large group of saturated or unsaturated carboxylates or mercaptide derivatives. The carboxylates are frequently esters or half esters of maleic acid and the thiol residues often derived from alkyl thioglycolates, HSCH2 COOR. There are three major types of tin stabilizers (organotin stabilizers), distinguished by their respective alkyl groups: n-octyl, n-butyl, and methyl: 1. n-Octyltin stabilizers have either one or two octyl groups bonded to the tin atom. Some n-octyltin stabilizer formulations are approved for food contact applications by most regulatory agencies worldwide. 2. n-Butyltin stabilizers have one or two butyl groups bonded to the tin atom. n-Butyltins are used in rigid applications, such as siding and window profiles, and are approved for use in drinking-water pipe in the United States and Canada. 3. Methyltin stabilizers have one or two methyl groups bonded to the tin atom.2 Dialkyltin compounds of the above groups, particularly the di-n-butyl derivatives, are the most effective stabilizers. Their solubility in the PVC resin and in almost all plasticizers imparts a clarity to the finished product which is unmatched by any other group of stabilizers currently in use. Each particular di-nbutyl derivative has special properties for each method of processing. Therefore, the finished product usually contains a mixture of several stabilizers, designed for a specific product, its processing scheme, and its environmental exposure. Varying the structure of the R groups in dialkyltin compounds R2 SnX2 has relatively little effect on the stabilizer efficiency, and the nature of X is much more important. For di-n-butyltin compounds, for example, the diacetate is a poor stabilizer, the dilauryl mercaptide is much better, and the di(isooctyl mercaptoacetate) is extremely good. In general, mercaptoacetates or mercaptopropionates confer good thermal properties but no photoprotection. Maleate esters on the other hand do confer a degree of photostability, in addition to the fact that they are sulfur-free, which has advantages in some applications. The Ligands

The most effective stabilizer systems in practice are subdivided into: 1. Sulfur-containing stabilizers (organotin mercaptides and sulfides). 2. Sulfur-free stabilizers usually containing Sn-O bonds.

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Sulfur-containing stabilizers can be used in mixtures with lubricants for PVC to be used in foodstuff packaging and for pipes for drinking water. In the manufacture of high-transparency foils, this stabilizer class can be used in practice only when UV absorbers are included. Stabilizers containing Sn–S bonds may be of many types, for example organotin mercaptides, organotin mercaptoalcohols, organotin thioglycollates, and organotin polymers containing sulfur. In contrast to sulfur-containing tin compounds, tin carboxylates have to be used with antioxidants. The advantage of the use of sulfur-free stabilizers is the good photostability and the lack of odor. These stabilizers, too, need additional lubricants, since they tend to stick to the metal parts of calendars and extruders.3 3.3.3

Tin Carboxylates

The earliest tin stabilizers, di-n-butyltin dilaurate and di-n-butyltin maleate, are typical of sulfur-free stabilizers. These products, and subsequent developments in tin carboxylates, provided clarity to PVC and a much lower order of toxicity than lead- or cadmium-containing stabilizers. One stabilizer in this group, di-n-octyltin maleate, is sanctioned by the United States FDA for rigid PVC food-contact applications. Tin carboxylates are used in rigid PVC–acetate copolymer applications, but do not offer the degree of stability required to process rigid PVC homopolymers. They do, however, exhibit excellent light stability properties and are sometimes used in combination with the sulfur-containing organotins to enhance the outdoor weathering capabilities of rigid PVC. Organotin derivatives of maleic acid may have an additional stabilizer function with the Diels–Alder reaction (Figure 3.3.3) by scavenging function.4 Their performance is good in all types of vinyl chloride polymerization, i.e. suspension, emulsion, and bulk. Optimum results are obtained when they are combined with small amounts of phenolic antioxidants particularly in plasticized PVC, impact-modified PVC, and PVC copolymers. Because stabilizers containing maleic acid occasionally lead to eye and mucous membrane irritations, there have been many attempts to replace them with other systems, and for many years, organotin stabilizers, free of maleic acid, have been on the market. These consist of a combination of organotin carboxylates, e.g. laurates, and a small amount of an organotin mercaptide. Just as with sulfur-free organotin stabilizers, when used in suitable formulations, this combination gives rigid PVC high transparency and excellent weathering stability. In the melt, PVC stabilized with alkyltin maleates tends to stick to hot contact areas of the processing equipment, but this problem can be prevented by suitable lubricants. The effect of dialkyltin maleates and laurates on the thermal dehydrochlorination of PVC has been compared in 1,2,4-trichlorobenzene solution by IR analysis, and showed that tin laurates are superior to tin maleates in replacing the labile chloride atoms in PVC. Attempts to trace intermediate monochlorotin derivatives in the case of maleates by polarography and Mössbauer spectroscopy were not conclusive. Di-n-butyltin laurate prevents the formation of longer polyene sections, retarding the colouration of the PVC. Di-n-butyltin laurate cannot prevent the formation of short polyene sections. Thermal

+

Diene

+

Dienophile

Figure 3.3.3

Diels–Alder reaction

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dehydrochlorination still occurs, but its rate is decreased and longer polyenes are formed. The higher the di-n-butyltin laurate content, the greater the concentration of shorter polyene sequences. 3.3.4

Tin Mercaptides

The thio-organotins were introduced in the early 1950s and provided a considerable improvement in clarity and heat stability. Although they exhibited poor light stability and some odour, and were more costly than lead or barium–cadmium stabilizers, tin mercaptides soon gained acceptance in the United States for the difficult stabilization of rigid PVC, and have been the most widely used stabilizers in rigid PVC pipe and profile extrusion, injection, and blow molding up to the present time. Tin mercaptides offer a unique set of properties for rigid PVC processing: classical vinyl stabilization and antioxidant functions combined with fusion promotion and melt viscosity reduction. Melt rheology studies have shown that in addition to excellent color stability, organotin–sulfur bonded compounds furnish a lower melt viscosity in rigid PVC than structurally equivalent organotin–oxygen bonded compounds (tin carboxylates) of similar viscosity, molecular weight, and compatibility. They also impart lower melt viscosities than Group II A and B metal carboxylates (barium–cadmium, calcium–zinc stabilizers). A possible explanation lies in the ability of sulfur to internally satisfy the secondary bonding capabilities of tin to a greater extent than oxygen, thus preventing secondary cross-linking or ‘melt stiffening’ of rigid PVC or copolymers which would otherwise occur through the coordination of the tin atom with groups on the polymer chain.5 3.3.5

The Mechanism of Stabilization

Organotin mercaptide stabilizers have an anti-oxidative action. This contributes to the stabilization in as much as the dehydrochlorination is much faster in the presence of air (oxygen) than in the presence of inert gas, and the loss of HCl is noticeably retarded by phenolic antioxidants. Organotin mercaptide stabilizers decompose hydrogen peroxides, providing a secondary antioxidant effect (Figure 3.3.4). O iOct Me

O

S

Me

50°C

+

Sn S

OOH

O iOct O

O O

iOct

S

Me2SnO + i Oct

S

O

+ OH

O

Figure 3.3.4 Reaction of an organotin stabilizer with t-butyl hydroperoxide. (Reproduced from reference 6, copyright 2003, John Wiley & Sons, Ltd.)

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Organotin mercaptide stabilizers also break autoxidation chains, and, compounds of this type are even patented as antioxidants for other plastics. Organotin mercaptides are able to bind or to neutralize HCl, which – as already mentioned – has an auto-catalytic effect on dehydrochlorination, especially in presence of oxygen, thus providing a further preventive function (Scheme 3.3.1). nOct2 Sn(S-CH2 -CO-O-iOct)2 + 2HCI Scheme 3.3.1

−→

nOct2 SnC12 + 2 HS-CH2 -CO-O-iOct

Binding of the HCl by an organotin mercaptide stabilizer6

The induction period – the axial section on the time coordinate of the dehydrochlorination curve – is a common criterion of all heat stabilizers. Normally, within this period, the processing takes place. The length of the induction period may be considered simply as a measure of the heat stability of PVC. However, this does not – and this must be emphasized – allow any definitive conclusions to be drawn concerning the initial color, which is of the utmost importance from the practical standpoint. From the shape of the dehydrochlorination curve, especially from its gradient, very important conclusions can be drawn, particularly concerning the interactions of stabilizer conversion products with PVC, and also with each other. In Figure 3.3.5, the dehydrochlorination curves for a di-n-octyltin mercaptide stabilizer are shown. It can be seen that the induction period increases almost linearly with the stabilizer concentration. As a first approximation, the length of the induction period may also be considered as a measure of the binding capacity of the stabiizer for hydrogen chloride. Furthermore, the gradient of the curves decreases with increasing concentration of the stabilizer, and this can be seen as an indication that the conversion products have a favorable effect on the heat stability of the PVC. The exchange of the labile chlorine atoms (which act as initial sites for dehydrochlorination, for less easily removable thiolate groups) is an extremely important stabilizing function, which has also preventive character. This exchange reaction has first been proposed and established by Frye et al. (Scheme 3.3.2).

Loss of hydrogen chloride, %

OTE/SPH

0% 0.5%

3

1% 2 2% 1

0

1

2

3

4

5

6

7

Time, h

Figure 3.3.5 Thermal dehydrochlorination curves of PVC at 175 ◦ C in the presence of different amounts of di-n-octyltin-bis-i-octylthioglycolate. (Reproduced from reference 6, copyright 2003, John Wiley & Sons, Ltd.)

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nBu2Sn(S-CH2-COO-iOct)2 + 2 HCl nBu2SnCl2 + 2 HS-CH2-COO-iOct

- 1/2 nBu2 Sn(S-CH2-COO- iOct)2 +

PVC

Cl 1/2 nBu2SnCl2 + PVC

S-CH2-COO-iOct

Scheme 3.3.2

Exchange of labile chlorine atoms in PVC by organotin mercaptide stabilizers6

This results from the fact that the tin mercaptide possess both nucleophilic and electrophilic properties, which permit substitution by the cyclic mechanism shown in Scheme 3.3.3, rather than the elimination which is normally observed with such structures. Just like the ability to bind HCl, this exchange reaction is a general characteristic of all efficient PVC heat stabilizers and stabilizer systems. An essential condition of this exchange reaction, is of course, that the transferred groups – in this case a mercaptocarbonic acid ester group – have a lower tendency to be eliminated than the chlorine atom. The two possible reactions of organotin mercaptide stabilizers, both ending in the formation of dialkyltin dichloride, are shown in Schemes 3.3.1 and 3.3.2. The formation of dialkyltin dichloride may thus be considered, as a first approximation, to be a measure of the total stabilizer conversion. Accordingly the diagram in Figure 3.3.6 shows the correlation between the stabilizer conversion and the time of heat treatment at 180 ◦ C. Finally Figure 3.3.7 shows the relative stabilizer conversion versus stabilizer concentration curves for different heating times. It can be clearly seen that the stabilizer conversion decreases with concentration in a characteristic way; we can assume that with long reaction times, concentrations of all stabilizers will be 100% converted. When all the stabilizer is consumed, the PVC will continue to degrade.7 3.3.6

Operational Considerations

It is apparent from static heat stability tests on a rigid PVC system, that the degree of alkylation is directly related to the type of degradation observed. Using n-butyltin mercaptoacetates, the performance of din-butyltin bis(i-octylmercaptoacetate), (n-(C4 H9 )2 Sn(SCH2 CO2 C8 H17 )2 , may be taken as standard. The two extreme compounds, tetra-n-butyltin and tin tetra(i-octylmercaptoacetate), provide no stability, the former behaving as though no stabilizer was present and the latter giving severe early blackening, which is characteristic of strong Lewis acids. These results may be rationalized in terms of the coordination mechanism which is involved in stabilization by organotin compounds. The reaction of organometallic compounds in a polar medium, such as organotin compounds in molten PVC, involves coordination of labile chloride atoms in a PVC molecule by the tetravalent organotin compound, and the subsequent displacement and allylic rearrangement seems to be the primary mode of action (Scheme 3.3.3).

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Tin Chemistry: Fundamentals, Frontiers and Applications H C

H C C H

H2 C CH

CH

C H

Cl

S

Cl

R

+

Sn

H C

H C

CH

CH

CH

CH2 CH

S R +

Cl Sn

Scheme 3.3.3 Proposed mechanism of exchange of chloride atoms in PVC by mercaptide groups of organotin stabilizers:6 coordination of allylic chlorine by organotin mercaptide with subsequent rearrangement to a non-allylic structure

Mono-n-butyltin tris(i-octylmercaptoacetate) combines improved early color with shortened longterm stability. This is due to its intermediate Lewis acid character between that of diisooctyl bis(ioctylmercaptoacetate) and tin tetra(i-octylmercaptoacetate). The decreased effectiveness of tri-n-butyltin i-octylmercaptoacetate may be ascribed to the increased reluctance to coordination by the tin atom, due to presence of the third alkyl (n-butyl) group. Additionally, the use of trialkyltin derivatives in PVC stabilization has been avoided because of their toxicity. Whether the alkyl group attached to tin is methyl, n-butyl, n-octyl, or carboxymethylethyl would appear to have minor significance in that all four types may be satisfactorily employed in rigid PVC structures. In the United States it is the methyltin or n-butyltin types which predominate. Because of their high cost, octyltin derivatives are utilized only where FDA approval is required, thus severely limiting their market. The ‘estertins,’ which have been recently introduced, will be successful to the extent that they can compete with the established methyltin and n-butyltin stabilizers on a cost-performance basis. An estertin mercaptoethanol ester, so-called reverse estertin, has following ligand structure: S-CH2 -CH2 -OCO-alkyl.8 However, differences in performance do exist, which are dependent on the alkyl substituent present and which must be taken into consideration by stabilizer manufacturers when developing new products, and by end users when formulating and processing PVC compounds with these products. In the case of dialkyltin bis(i-octylmercaptoacetate), as the alkyl chain length decreases from octyl to methyl, the

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0.5% Stabilizer 30

Stabilizer conversion, %

JWBK199-3.1

OTE/SPH

20 1% Stabilizer

10

0

2% Stabilizer

5

10 Time, min

15

20

Figure 3.3.6 Stabilizer conversion in PVC versus heating time curves in a continuous milling test at 180 ◦ C (stabilizer: di-n-octyltin bis(i-octylthioglycolate). (Reproduced from reference 6, copyright 2003, John Wiley & Sons, Ltd.)

30

Stabilizer conversion, %

OTE/SPH

20

20 min 10 10 min 5 min 0

0.5

1.0

1.5

2.0

Stabilizer concentration, %

Figure 3.3.7 Stabilizer conversion versus stabilizer concentration curves for different heating times in a continuous milling test at 180 ◦ C (stabilizer: di-n-octyltin bis(i-octylthioglycolate). (Reproduced from reference 6, copyright 2003, John Wiley & Sons, Ltd.)

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stabilizers become increasingly compatible with the resin, and, in fact, become better stabilizers for the resin since a greater part of the stabilizer molecule is the i-octyl mercaptoacetate ligand. This is manifested by lower melt viscosities, and often lubrication adjustments are required when changing stabilizers. The estertins also demonstrate increased compatibility with PVC due to the presence of a polar ester group in the alkyl chain. Methyltin stabilizers appear to be more sensitive to the amount of monomethyl species present in the product. Put another way, less monomethyl species are required for color improvement than is the case with n-butyltin or n-octyltin compounds. This may be explained by the greater Lewis acid strength of monomethyltin trichloride and by the enhanced ability of methyltin compounds to coordinate labile chloride atoms in the PVC molecule, due to less steric hindrance offered by the smaller methyl groups compared to the bulkier n-butyl and n-octyl groups. This steric effect is most pronounced when comparing methytin derivatives with n-butyltin analogs as catalysts in the production of polyurethanes. Moreover the thermal stability of dimethyltin bis(i-octylmercaptoacetate) itself is superior to the di-nbutyltin, di-n-octyltin, and the estertin analogs. Breakdown of the stabilizer occurs less readily in a press stability test, indicating the suitability of this methyltin stabilizer for injection molding applications. For many years, the principal mercaptan used in organotin stabilizers was i-octyl mercaptoacetate. Speciality applications, where a more highly lubricating stabilizer was desirable, called for the use of dodecyl mercaptan. The linear aliphatic chain with no compatibilizing ester groups provided a greater degree of lubrication. However, the yellow color obtained with di-n-butyltin bis(dodecylmercaptide) precluded its use for most applications. The ability of the tin atom in di-n-butyltin bis(dodecylmercaptide) to coordinate labile chloride atoms in PVC is evidently inferior to that in di-n-butyltin bis(dodecylmercaptoacetate), with consequent poorer performance. Di-n-butyltin bis(i-octylmercaptoacetate) is an internally coordinated compound, the structure of which facilitates coordination of labile chloride atoms, while the lack of any restrained configuration in di-n-butyltin bis(dodecylmercaptide) results in steric hindrance to coordination by the tin atom. Sulfur coordinating with tin is exchanged with chloride, and the sulfur is bonded to the PVC C atom where chloride was bonded previously. If one compares di-n-butyltin bis(dodecylmercaptide) with the dimethyltin analog, one observes significantly improved color stability with the dimethyltin compound, presumably due, again, to substantially decreased steric hinderance about the tin atom. When di-n-butyltin compounds derived from esters of mercaptoethanol were evaluated, color stability results were obtained similar to that of isooctylmercaptoacetate.1 3.3.7

Evaluating Stability

Yellowness Index and Transparency

Films having an organotin stabilizer have a lower yellowness index than those of control films heated at different periods at 160 and 180 ◦ C (Figure 3.3.8). The higher the organotin stabilizer concentration and heating temperature are, the higher is the yellowness index (more tin, more color). The color of the film with 2.5% organotin stabilizer has a very small yellowness index up to 30 minutes at both 160 and 180 ◦ C, indicating that a safe induction period for dehydrochlorination is present. While control and 2.5% organotin stabilizer containing films are transparent, films with 5% organotin stabilizer are opaque white, indicating limited solubility of organotin stabilizer in PVC.6 HCl Evolution from PVC and PVC with Di-n-Octyltin Bis(i -Octylthioglycollate) Stabilizer Films

Dehydrochlorination of the films at 140 and 160 ◦ C can be determined in a Metrohm 763 PVC thermomat. In this instrument HCl evolved by heating PVC is carried by N2 gas through a fixed amount of water. The conductivity change of water vapor versus time is measured. The conductivity is proportional to the amount of HCl evolved. Figure 3.3.9 shows HCl evolution from films of PVC, and of PVC with

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80 PVC 160°C PVC 180°C 2.5% Sn500K160°C 5% Sn500K160°C 2.5% Sn500K180°C 5% Sn500K180°C

70

60

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Yellowness index of films versus heating time. (Reproduced from reference 6, copyright 2003, HCI evolution at 140 and 160°C 80 3 70 Conductivity (μs/cm)

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Time (hour)

Figure 3.3.9 HCl Evolution from films heated at 140 ◦ C and 160 ◦ C: (1) PVC without di-n-octyltin bis(isooctylthioglycollate) at 160 ◦ C, (2) PVC without di-n-octyltin bis(isooctylthioglycollate) at 140 ◦ C, (3) PVC with di-n-octyltin bis(isooctylthioglycollate) at 160 ◦ C, (4) PVC with di-n-octyltin bis(isooctylthioglycollate) at 140 ◦ C. (Reproduced from reference 9, with permission from Elsevier.)

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di-n-octyltin bis(i-octylthioglycollate) stabilizer, at 140 and 160 ◦ C respectively. This shows that HCl evolution from a PVC film without the di-n-octyltin bis(i-octylthioglycollate) stabilizer starts after a short induction period. HCl evolution from the PVC with di-n-octyltin bis(i-octylthioglycollate) stabilizer showed longer induction periods, and at 140 ◦ C, no HCl was evolved from the film over the time of the experiment. HCl evolution increased with temperature, being more rapid at 160 ◦ C than at 140 ◦ C. The onset times for HCl evolution were 14.3 and 2.5 h at 140 and 160 ◦ C, respectively, for PVC film without di-n-octyltin bis(i-octylthioglycollate) stabilizer. On the other hand, the film with di-n-octyltin bis(isooctylthioglycollate) stabilizer did not evolve HCl in 30.3 h at 140 ◦ C. HCl started to evolve from the films with di-n-octyltin bis(isooctylthioglycollate) stabilizer in 14.3 h at 160 ◦ C.9 3.3.8

Conclusion

The choice of a stabilizer depends on: 1. 2. 3. 4.

The requirements for processing. The properties for the end-use. Cost constraints within which (1) and (2) must be constrained. Other formulation components which may interact with the stabilizer.

The basic type of stabilizer system selected is often dictated by end-use or regulatory constraints (NSF – National Science Foundations, PPI – Plastics Pipe Institute, United States FDA, and German BGA). Subsequent choice of a specific stabilizer should be made with the major objective of achieving optimum cost-performance – how much processing stability is available per dollar of stabilizer cost? Or conversely, what will be the lowest cost choice to furnish the required processing stability for a particular process – including all safety factors, such as regrind extrusion, power failures, and end-use stability needs? A level of stability much beyond the ‘necessary’ level can translate to significant unnecessary costs. There are laboratory tests for PVC: the yellowness index of samples treated with shear and heat is measured in a Brabender plastograph or a roller mill, and with heat only, in a static test oven as a function of time, to test thermal stability. These laboratory tests (Brabender, mill stability, and heat stability) can furnish an indication of comparative cost-performance, but the final decision really should be based on production extrusion runs and subsequent evaluation of either residual stability, or regrind extrusion, which is also conveniently done with the heat stability testing oven. Within the organotin mercaptide group of stabilizers, the most recent ‘reverse ester’ methyltin stabilizers have demonstrated a unique ability to impart extremely good initial color stability, and to hold that color even during 100% regrind extrusion to a much greater degree and at lower costs than were previously possible. In other words, the useful processing stability of rigid PVC – to the point of product rejection – is extended. Conversely, as the stabilizer level is reduced, the decrease in stability time (seen as color development) is much less with these new methyltin stabilizers than with n-butyltin or mixed-metal tin stabilizers. This means that a given level of stability (necessary to run an extrusion plant, including a regrind safety factor) can be achieved with less stabilizer in the compound. Typical use levels for twin screw pipe extrusion are in the 0.3–0.4 phr (stabilizer parts per hundred parts of PVC resin) range, and 0.7–1.2 phr for single screw pipe extrusion. The stabilizing efficiency furnished by such methyltins, especially at lower use levels, is significant, when comparing actual performance data and a schematic comparison of methyltin and lead-stabilized pipe compounds at various equivalent cost levels. In this case, a level of stability much above the ‘necessary’ level would add unnecessary cost to the formulation and return no real benefits.5

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References 1. L.R. Brecker, Pure and Applied Chemistry 53, 577 (1981). 2. http:// www.tinstabilizers.com. (2007). 3. C.A. Heiberger, Encyclopedia of Chemical Technology, Kirk-Othmer (Eds), 23, John Wiley and Sons, New York (1983). 4. http.//en.wikipedia.org/wiki (2007). 5. http://www.plastics.com/articles/10/3/RIGID-PVC-EXTRUSION-HANDBOOK-RAW-MATERIALSSELECTION (2007). 6. E. Arkis, and D. Balkose, Adv. Polymer Technol., 21, 65 (2003). 7. H.O. Wirth and H. Andreas, Pure Appl. Chem., 49, 627 (1977). 8. www.kunststoff.web.de. (2007). 9. E. Arkis, and D. Balkose, Polymer Degrad. Stabil., 88, 46 (2005).

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3.4

Organotin Compounds as Anion-Selective Carriers in Chemical Sensors

Nikos Chaniotakis Laboratory of Analytical Chemistry, Department of Chemistry, University of Crete

3.4.1

Introduction to Chemical Sensors

The science of chemical sensors is a multidisciplinary and multidimensional field that utilizes specific characteristics of chemistry and engineering for the specific purpose of designing direct sensing systems for specific analytes. It is usually the case that the chemical sensor can monitor the activity of chemical species that are in contact with its sensing element. The fascinating characteristic of a chemical sensor is the fact that it selectively ‘sees’ a chemical compound, and subsequently passes this information quantitatively to the analyst. This is achieved by the employment of three specific, but highly interconnected, physicochemical processes. These are the chemical recognition, the transduction, and the display, as shown in Figure 3.4.1. Of these, the transduction and the display of the measurement are mainly physical and electronic processes. The transduction is a chemical-to-physical metamorphosis phenomenon during which the physicochemical characteristics of the analyte are transformed into a physical property, such as potential, current, resistance, light, etc. While this process is very important, and can influence dramatically the observed characteristics of the sensor, it will not be addressed here.

3.4.2

Potentiometric Ion Selective Electrodes (ISEs)

According to IUPAC 1 , an ISE is: ‘An electrochemical sensor, based on thin films or selective membranes as recognition elements. . .The potential difference response has, as its principal component, the Gibbs energy change associated with permselective mass transfer (by ion-exchange, solvent extraction or some other mechanism) across a phase boundary.’ The schematic diagram of an ISE shown in Figure 3.4.2 provides some experimental details for the design of the sensors, while it provides the principle behind the idea of measuring the membrane potential for quantitative work. As can be seen in Figure 3.4.2, the measured potential is developed at the test solution–membrane interface, based on the ability of the sensing membrane to extract charged species from the solution into the sensing element or membrane, using differences in chemical energy.

Signal Transduction

Display

Analyte Signal Conditioning Potentiostat

Ionophore

Figure 3.4.1

Schematic diagram of a sensor system

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To Potentiostat Internal Reference Solution (M+ X−)

Reference Element (Ag, AgCl)

Sensing Membrane

SnR4

SnR4 SnR4

SnR4

SnR4 SnR4

SnR4 CI K+

Figure 3.4.2

SnR4

SnR4

CI

SnR4

SnR4

CI+

K+

SnR4 CI K+

SnR4

Schematic representation of an ISE

The theory that governs the operation of ISEs was described early on by the Nernst equation:2 E membrane = −

RT 0.05912 [X − ]inner = ln − log X − sample nF [X ]sample n

(3.4.1)

and subsequently by the Nikolski–Eisenman Equation,3 which also takes into account the selectivity of the sensor towards the primary X over that of interfering ions Y: E membrane = −

RT n pot (Y m )n/m sample ln X + k x,y nF

The use of such direct analytical systems as ISEs has significant advantages over other indirect or separation analytical techniques. The most important advantages are: (a) the speed of analysis, (b) the simple operation, (c) the relatively fast response, and (d) the low cost of operation and analysis. The existence of such characteristics has provided the drive for the development of a range of ISEs, and now many of these systems are commercially available. On the other hand, current analytical requirements such as detection at trace levels, monitoring of toxic substances in harsh environments, and online or in vivo monitoring of medical or illicit substances, demand the continuous improvement of existing ISEs. There is also a continuing need for sensors selective for even more analytes. At the same time, there is the need for chemical analysis of a large number of samples in a minimum amount of time, to be performed by non-specialized personnel, with minimum or no reagent use, using very small sample volumes. These are some of the additional challenges that face scientists involved in the design of ISEs.

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3.4.3

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The Ionophores

Fundamentally, ISE operation is largely based on the selective recognition that takes place at the sensing element–sample interface. Before any discussion takes place on this issue, it must be understood that this process is based purely on the second law of thermodynamics, and specifically on the fact that differences in chemical activity between the sensing element and the sample solution tend to even out, assuming that the system is isolated. Based on this requirement, any changes in the activity of the analyte in the test solution must also be distributed evenly in the sensing element membrane. For this to happen there must be selective mass transfer of the analyte from the solution into the membrane, a process that is controlled by the ionophore used. An ionophore, from the Greek words ion and fero (carry), is a compound (organic or organometallic) which can selectively and reversibly coordinate to a specific ion and can thus, based on differences in Gibbs energy, transport the ion from the aqueous solution into a membrane. Up-to-date knowledge in the area of ionophores suggests that there are some very important physicochemical characteristics that a compound must have in order to be a candidate for use as an ionophore in ISEs. The most important of these are: (1) The ionophore should have a binding constant with the analyte that is neither too low, nor too high.4 (2) The ionophore should coordinate selectively, but reversibly, with the analyte. (3) The ionophore should be stable both in the membrane phase, and when in contact with aqueous environments. (4) The ionophore must have high lipophilicity so that the leaching rate, and thus the signal drift, is kept to a minimum.5 Figure 3.4.3 shows some very well known and extensively used cation and anion ionophores, which fulfill most of the above requirements. It is the aim of this chapter is to present the efforts made worldwide for the development of chemical sensors based on the unique chemical recognition capabilities of organotin structures. In particular, we will examine in a time-based flowchart the progress of the design and application of Sn(IV)-based ionophores and their application in the development of anion selective chemical potentiometric sensors. 3.4.4

Organotin-Mediated Anion Partitioning into Liquid Polymeric Membranes

One of the most important categories of ion selective chemical sensors is based on what are called ‘liquid membranes.’ This term was first used in 19616 to describe a matrix that is not water soluble; it contains either anionic or cationic sites (liquid ion exchangers), which can selectivity facilitate the exchange of inorganic ions. In order to study the active carrier-mediated ion transport through these liquid membranes, a cell such as the one shown in Figure 3.4.4 has been employed. The role of these relatively large and sparingly soluble ion exchangers introduced into the liquid membrane was to facilitate the partitioning of ions of the opposite charge into the liquid membrane, and to actively transfer them into the receiving solution. This idea was based on much earlier work by Nernst, in which a similar system was composed of two aqueous solutions on either side of an organic phase.7 In this pioneering work, Nernst actually measured the imposed potential difference between the different phases, based on the selective partitioning of different ions. This was the first synthetic membrane in which electric properties and selective ion permeabilities were studied. It was much later, and after the aforementioned work of Rosaro6 and Kunin,8 in which it was proven that various synthetic membranes doped with different species could function as ion exchangers, and that these systems were used for quantitative work. Indeed, the properties of these membranes were of

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Figure 3.4.3 (a) Valinomycin, a potassium ionophore; (b) ETH 1001, a calcium ionophore; (c) NiPhenantroline, a nitrate ionophore; and (d) Mn(III)TPP, a chloride ionophore

great interest due to the fact that they could be used as models in order to elucidate the mechanisms of ions permeating through biological membranes. In a case where ion partitioning is large, the species partitioned can be readily analyzable, and thus chemical sensors could be constructed. The idea of using organotin compounds as ionophores was based on the fact that since, like carbon, tin forms covalent bonds via sp3 hybridization, and with the presence of empty d orbitals, it can coordinate with up to three extra electron-donating substituents, such as Lewis-basic anions. It was Selwyn, in 1970,9 , 10 that took advantage of this property and showed clearly the direct role of the trimethyltin, tri-n-propyltin, tri-n-butyltin, and triphenyltin chlorides on the active chloride transport mediated in mitochondrial membranes, as shown in Figure 3.4.5. It was also shown in this study that the mediation is based on chloride-hydroxide ‘antiporter’ transport. This fact was verified many years later, as Simon showed, based on NMR and other studies, that indeed these compounds act as neutral carriers in liquid polymeric membranes.11 The next issue to be resolved was the selectivity of the organotin compounds. Data indicating that these ionophores act with specific selectivity was reported for the first time in 1979.12 It was now evident that anion transport is due to the permeation of electroneutral ion pairs. At the same time it was proven that the exchange rate of chloride is related to the lipophilicity of the organic ligands, with the tri-n-butyltin

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Figure 3.4.4 membrane

Schematic diagram of the cell used to study the mediated transport of ions through a liquid

X Receiving Aqueous Phase

Organic Phase Liquid Membrane

X Source Aqueous Phase

X−

R3SnOH

X−

OH−

R3SnX

OH−

Figure 3.4.5 The neutral carrier scheme is illustrated. It is important to note that the ionophore can promote active transport based on the ion exchange mechanism since the electro neutrality principle must be maintained within the bulk of the membrane

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ionophore being the most effective. Within this period, the selectivity of the transport was also elucidated, and it was shown that it is in the order of: F– Cl– < Br– < I– = SCN– OH– . The powerful capability of organotin to coordinate with oxoions was suggested to be very useful in the remediation of contaminated fields. As suggested by Zolotov,13 dialkyltin salts are suitable as ‘extractants’ for various oxygen-containing anions, such as arsenate, phosphate, and other doubly charged anions, based on the formation of inner-sphere complexes. These results set the groundwork for the development of methods for the separation of arsenic, phosphorus, and silicon, as well as spectrophotometric applications for the quantitative determination of phosphorus and arsenic in various solutions. These studies established the foundation for the application of organotin ionophores as chemical sensors. 3.4.5

Anion Selective Organotin-based ISEs

It took scientists about 10 years after the first use of organotin compounds for selective coordination and transport, to apply these ionophores for the development of chemical sensors. The efforts were directed towards controlling the parameters that influence the anion coordination properties of the organotin carriers, which could then lead to the development of a variety of mono- and bidentate organotin ionophores. It was in 1980 that the first report on an anion-selective sensor based on an organotin ionophore was reported. Zolotov et al.14 used a dialkyltin dinitrate, developed initially for arsenate extraction, in order to develop a chemical sensor. In this work a liquid membrane comprising As(V)-dilauryltin(IV), chloroform, and 10% decanol was used to obtain calibration curves of arsenate anion at pH 5.0 (citrate buffer solution, μ = 0.1, Na2 SO4 ); and pH 9.2 (borate buffer solution, μ = 0.1 Na2 SO4 ). The results indicated that the calibration curve at the lower pH had a much larger, close to theoretical, slope, while the response at higher pH had a much smaller slope. This was a clear indication that the interaction of the organotin carriers with hydroxide is very strong. The hydroxide interference still remains an issue today, and care must be taken during measurements to keep the pH constant and at the lowest possible value. Meanwhile, the science of chemical sensors was developing fast. The technology of polymer-supported liquid membranes was already a mature science, as it was more than 15 years since it was first reported.15 The use of plasticized PVC allowed for the construction of membrane-based sensors with great ease, and gave sensor technology a new boost. It was the same basic technology that was subsequently used for the development of liquid-polymeric-based ISEs. Based on this plasticized PVC membrane technology, Simon’s group published, in 1984, a milestone manuscript, in which it is clearly shown that there is indeed a very high selectivity obtained with sensors based on membranes doped with tri-n-octyltin chloride,16 as shown in Figure 3.4.6. The selectivity was also shown to be due to the direct interaction of the tin center with the anion in solution, as was proven by NMR studies, shown in Figure 3.4.7. In addition, it was shown in this paper that the tri-n-octyltin chloride acts as a neutral carrier in the membrane, based on electrodialysis experiments. Following this report, the area of the selective anion recognition and sensing has bloomed, with many scientists using and optimizing the existing organotin ionophores or designing new organotin carriers for anion detection and monitoring. The efforts of Simon’s group to elucidate the mechanism and the selectivites of the organotin compounds continued.17 As shown in Table 3.4.1, a series of triorganotin compounds of the type R3 SnY and R1 R2 SnY was evaluated with respect to their anion selectivities exhibited in solvent polymeric membranes. The study of such a large number of carriers allowed for the elucidation of the effect of both the amount of carrier in the membrane, as well as the effect of the ligand nature on the observed selectivities. It was thus concluded that the selectivity was influenced by the size of the constituent R groups, suggesting steric hindrance in the penta-coordination of the corresponding ionophores by analyte anions.

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EMF [mV] 20 % TOTCI 40 % DMSNE 40 % PVC

logK 5

4

3 SCN− 2 Cl04−

NO 3− 50 mV

1

0 −1

−3

−2

−1

0 log aX

SCN−

SCN−

N03−

N03−

I−

−

CIO− 4 Br− HCO3− NO3−

I

HCO3− CIO4− −

−

Br

Br

CI−

CI−

HCO3− −

HCO3− −

OAc 5O42−

6% MTDDACI

−

SCN

SCN−

I−

I−

HPO42− −2

−4

−

Cl04

Cl04−

−

CI−

−5

Pot

CLX

Br Cl− NO3−

OAc HPO42− SO42− 3% MTDDACI

CI−

OAc− HPO42− SO42−

OAc− HPO42− SO42−

3% TOTCI

20% TOTCI

65% DBP

49% DMSNE

49% DMSNE

40% DMSNE

29% PVC

48% PVC

48% PVC

40% PVC

Figure 3.4.6 (left) Electrode functions of a cell assembly with a membrane based on tri-n-octyltin chloride for four different anions. The spaces between the curves are not shown to scale, but their order corresponds to Pot the selectivity sequence; (right) Selectivity factors, log KCl/x for membranes based on a classical ion exchanger (columns 1 and 2) and on a tin organotin compound (columns 3 and 4), as determined by the separate solution method. (Adapted from reference 11, the American Chemical Society.)

Even though the use of organotin based carriers in ISEs was well established by the end of 1980s, the observed selectivities were not adequate for these sensors to be used for real sample analysis. This changed with the work published in 1989 by Chaniotakis et al.,18 in which a porphyrin was used as the organic ligand for the Sn(IV) center. The response properties of a 5,10,15,20-tetraphenyl (porphyrinato)tin(IV)dichloride, (Sn[TPP]CI) doped, plasticized PVC membrane electrode indicated a very high selectivity toward salicylate, an anionic breakdown product of aspirin (acetyl salicylate). It was shown for the first time that an ISE could exhibit an anion selectivity pattern, with high specificity for salicylate over lipophilic inorganic anions (perchlorate, periodate, thiocyanate, iodide, etc.) and biological organic anions (citrate, lactate, and acetate). The observed selectivity for salicylate was confirmed as being due to the ionophore, based on radiotracer uptake experiments using labelled [14 C] salicylate, as shown in Figure 3.4.8. The exceptional selectivity of this sensor allowed for the direct monitoring of salicylate in blood and serum with very good precision and accuracy.

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Organotin Compounds as Anion-Selective Carriers δ13C

δ119Sn

[ppm]

[ppm]

331

(119Sn-13C) [Hz]

(119Sn-13C)

26

480 120 13C-1

24

440

80 22

40

400

Sn CI

20

0

360 119Sn

18

−40

320

0

05

10

15

20

25

30

MOLES CI–/MOLES LIGAND 119 Figure 3.4.7 Chemical shifts δ Sn and δC13 for the tin center and the carbon in the α position, respectively, 1 and one-bond coupling constants J ( 119 Sn-13 C) between these two nuclei for solutions with different ratios of Kryptofix 222 potassium chloride salt to tri-n-octyltin chloride. (Adapted from reference 11, the American Chemical Society.)

Table 3.4.1 Organotin anion selective carriers substituted with different non-labile (R1 , R2 ) and labile (Y) ligands. (Adapted from reference 17 Wiley-VCH) (R1 )2 R2 YSn(IV)

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

R1

R2

Y

CH3 -(CH2 )3 CH3 -(CH2 )3 CH3 -(CH2 )7 CH3 -(CH2 )3 -CH(Et)CH2 CH3 -CH13 CH3 -CH2 CH3 -CH2 CH3 -CH2 CH3 -(CH2 )3 Cyclohexyl Ph PhCH2

CH3 -(CH2 )3 CH3 -(CH2 )3 CH3 -(CH2 )7 CH3 -(CH2 )3 -CH(Et)CH2 CH3 -(CH2 )13 CH3 -(CH2 )5 CH3 -(CH2 )7 CH3 -(CH2 )13 CH3 -(CH2 )17 Cyclohexyl Ph PhCH2

Cl AcO Cl Cl AcO AcO AcO AcO AcO Br Cl Cl

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CPM Thousands

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2

1

4

6 Time, Min.

8

10

0 0

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40

60 Time, Min.

80

100

Figure 3.4.8 [14 C] Salicylate uptake of PVC membranes as a function of time: (•) blank-PVC; (+) [TPP]H2 PVC; () Sn[TPP]Cl2 -PVC. Insert: radioactivity taken up by Sn[TPP]Cl2 -PVC membrane plotted as a function of the square root of time. (Adapted from reference 18.)

The fact that the organotin carriers were proven to be anion selective in their neutral form provided the required information for the development of an optical sensor. Based on the coextraction principle,19 an optical sensor membrane for the determination of chloride was described. For this, tri-n-octyltin chloride was used as the ionophore, and together with the appropriate pH sensitive chromoionophore, allowed for the development of an optical sensor system for the monitoring of chloride levels in blood and serum. It was now evident that the structure of the ligands surrounding the coordinating tin(IV) center was very critical to the observed selectivity of the sensor. Based on this, Glasier et al. 20 targeted increasing selectivity towards phosphate ions by using bis( p-methylbenzyl)tin dichloride, dibenzyltin dichloride, and bis( p-chlorobenzyl) tin dichloride. It was shown that the p-chloro derivative provided the best detection limit and the highest degree of selectivity for phosphate, sufficient for real sample analysis. In order to improve the selectivity towards phosphate, one must overcome two fundamental obstacles: the very high hydrophilicity of orthophosphate, and the relatively weak basicity of the phosphate oxygen atoms. Even though the monodentate ionophores designed by Arnold had a significantly improved selectivity,20 it was not sufficiently high for real sample analysis. For this, the use of multidentate organotin ionophores were proposed by Chaniotakis et al. in 199321 and 1994.22 In this work, a series of multistannyl derivatives were studied in order to determine the parameters that will allow for the best possible phosphate selectivity. It was shown that a drastically different potentiometric behavior was observed depending on the distance between the tin(IV) coordinating centers, as well as the type of ligands attached to them, as shown in Table 3.4.2 and Figure 3.4.9. A careful design of the distanyl carriers can thus provide them with better selectivity for phosphate over even the most

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Table 3.4.2 Multi-dentate organotin compounds with different organic substituents attached to the Sn centers, studied in order to investigate the electron-withdrawing effect and the steric effect on the observed selectivity XY2 Sn-(CH2 )n -SnY2 X X 1 2 3 4 5 6 7 8 9

Figure 3.4.9

Ph Ph Me3 SiCH2 Me3 SiCH2 Cl Cl Bu Cl Cl

Y

n

Br Br Cl Cl Me Me Cl Ph Ph

1 2 3 4 1 3 1 1 2

pot Experimental selectivity coefficients kH P O− ,X − of multidentate organotin compounds with 2

4

different organic substituents attached to the Sn centers showing: (a) the electron-withdrawing effect and (b) the steric effect of the organic substituents. (Adapted from reference 22, with permission from Wiley-VCH.)

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lipophilic anions, such as perchlorate and thiocyanate. The parameters that control the selectivity of these ionophores are: (1.) There must be one electron-withdrawing organic substituent on the Sn centers. (2.) The number of CH2 groups between Sn centers must be either one or three. (3.) Two electronegative substituents (Cl, Br) on each Sn atom show better selectivity for phosphate. It has been known since the initial studies involving organotin ionophores, that the main drawback of these carriers is their instability in aqueous environments. Chemical sensors are employed in aqueous solution analysis, and for this reason the ionophore-containing polymeric membrane is saturated with water. Exposing organotin ionophores to water vapor makes them susceptible to hydrolysis. Both the tin–halide and the tin–carbon bonds are prone to nucleophilic attack by hydroxide. This process leads to the breakdown of the ionophore structure, a fact that is evident in the analytical characteristics of the sensor. In addition, the limited lipophilicity of these ionophores aids in the relatively fast leaching of the native ionophore from the membrane phase into the aqueous test solution. Large baseline drift, in addition to a progressive deterioration of the sensor selectivity, is a direct result of these processes. To solve this problem there are two main routes of approach. The first involves the covalent bonding or grafting of the ionophore to the polymer substrate. It is expected that increasing the molecular weight and lipophilicity of the ionophore, will result in a decrease in the rate of leaching out of the membrane. The other, more elegant, approach is to use either intra-molecularly stabilized or inter-molecularly polymerizable organotin ionophores. These latter approaches can also provide a way to fine tune the tin acidity and thus render the ionophore more resistant to hydrolysis. Grafting to a polymer backbone is a method that has been successfully employed previously for the stabilization of a variety of carriers.23 , 24 It was thus expected that this grafting procedure would also work in the case of triorganostannane ionophores on a polymer backbone. Initial studies25 , 26 indicated that this approach does not actually work, due, according to the authors, to the fact that the lipophilic barrier induced by the polystyrene backbone seriously hinders the required special interaction between the ionophore tin centers and the analytes. The phase boundary between the aqueous solution and the organic membrane phase is too large for efficient ion partitioning. In addition, the observed initial response diminished within 24 h of sensor operation. Another approach was undertaken,27 in which polyethylene glycols (PEGs) with tri-n-butyltin carboxylate end groups were studied as ionophores. It was shown that this approach might slightly improve upon the stability problem. The study of macromolecular compounds with a range of molecular weight (e.g. PEG-58-Sn, PEG-102-Sn, and PEG-0-Sn) suggested that the increase of the chain length leads to an improvement in the overall electrode performance. Despite these promising results, the sensor based on this type of ionophore also lost its activity after 24–36 hours of operation. An alternative way of controlling the Lewis acidity of the tin center in conjunction with the intramolecular stabilization and polymerization was then undertaken28 , 29 . For this, a series of intra-molecularly coordinated tri-n-butyltin and triphenyltin benzoates, triphenyltin acetates and cinnamates, with or without the perfluorophenyl group in the carboxylate moiety, were investigated in order to correlate the potentiometric chloride response with the Lewis acidity of the organotin carrier (Figure 3.4.10). Based on these studies, the potentiometric response was found to be inversely related to the Lewis acidity of the organotin carrier. It was finally concluded that small association constants (highly reversible systems) lead to better potentiometric behavior. On the other hand, large association constants are not desirable, since they may lead to electrode surface overload and possible ionophore analyte precipitation.

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335

Figure 3.4.10 Intra-molecularly coordinated tri-n-butyl- and triphenyl-tin benzoates, phenylacetates, and cinnamates investigated in order to correlate the potentiometric chloride response with the Lewis acidity of the organotin carrier

A similar approach is used for the design of novel fluoride ionophores 30 . The important characteristic of these ionophores is not only the very good selectivity, but also the exceptional stability over a period of 30 days, as shown in Figure 3.4.11. Very recently new directions in the design of organotin-based ionophores have been attempted. One such effort entails the use of Schiff base complexes31 , 32 or salophens33 of Sn(IV). The response mechanism

120 Detection limit

5,0x10−7

100 1,0x10−6 80 1,5x10−6 60

2,0x10−6

Slope (mV/decade)

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Detection limit (M)

OTE/SPH

40

2,5x10−6

Slope

3,0x10−6

20 0

5

10 15 20 25 Days of continuous operation

30

Figure 3.4.11 Stability of ISE based on carriers I, II and III in terms of detection limit (-◦-) and Slope (--) of the sensor over time under continuous monitoring at pH 5.5. (Adapted from reference 30 , with permission from the American Chemical Society)

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Figure 3.4.12

Fluoride- (right) and arsenate- (left) selective ionophores. (Adapted from references 36 and 37.)

of the ionophores was proven to be that of a charged carrier and the resulting electrodes have been successfully applied to biological samples. By now it is established that bidentate tin ionophores are the ideal generic substructure upon which ionophores for a specific anion can de designed.34 , 35 Based on this idea, selective ionophores, such as those shown in Figure 3.4.12, have been proposed for fluoride36 and arsenate. 37

3.4.6

Conclusions

Organotin compounds have only recently being employed as ionophores in chemical sensors. Despite their relatively short career in the ISE area, their contribution has already been very significant. Organotin ionophores are at this time a promising class of compounds for selective anion recognition. The plasticity of the chemical environment of the tin(IV) center allows for the relatively easy manipulation of two fundamental parameters that control the ligand binding characteristics. Those parameters are the tin acidity, and the spatial environment of the coordination sphere. Up to now it has been shown that these parameters can indeed control the selectivity of the organotin ionophores to a great extent, providing sensors that are selective to even the most hydrophilic anions, such as phosphate, or to the smallest anions, such as fluoride. The unique characteristic of tunability together with the existing possibility for stabilization against hydrolysis via intra-molecular polymerization has set the groundwork for a bright future of organotin ionophores in chemical sensor science. It is expected that as synthetic tin chemistry progresses, new ionophores will be designed and synthesized with even higher selectivity, better stability to hydrolysis, and higher lipophilicity, all of which will lead to a wider range of ISE applications.

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337

References 1. R. Buck and E. Lindner, Recommendations for Nomenclature of Ion-Selective Electrodes, Pure App. Chem., 66, 2527 (1994). 2. W. Nernst, Reasoning of theoretical chemistry: Nine papers (1889–1921) (Begründung der Theoretischen Chemie: Neun Abhandlungen, 1889–1921), Verlag Harri Deutsch, Frankfurt am Main. 3. B. P. Nicolsky, M. M. Schulz, A. A. Belijustin, and A. A. Lev, in GIass Electrodes for Hydrogen and Other Cations, G. Eisenman, (Ed.), M. Dekker, New York, 1967. 4. J. K. Tsagatakis, N. A. Chaniotakis, K. Jurkschat, S. Damoun, P. Geerlings, A. Bouhdid, M. Gielen, I.Verbruggen, M. Biesemans, J. C. Martins, and R. Willem, Helv. Chim. Acta, 82 531 (1999). 5. O. Dinten, U. E. Spichiger, N. Chaniotakis, P. Gehrig, B. Rusterholz, W. E. Morf, and W. Simon, Anal. Chem. 63, 596 (1991). 6. L. Henri, P. D. Rosano, and J. H. Schulman J. Phys. Chem., 65 1704 (1961). 7. W. Nernst and E. H. Riesenfeld, Ann. Phys., 8, 600 (1902). 8. R. Kunin and A. G. Winger, Angew. Chem. Intern. Ed. En, 1, 149 (1962). 9. M. J. Selwyn, A. P. Dawson, M. Stockdale, and N. Gains. Eur. J. Biochem. 14, 120 (1970). 10. M. J. Selwyn, in Organotin Compounds: New Chemistry and Applications, J. J. Zuckerman (Ed.), ACS, Washington DC (1976). 11. U. Wuthier, H. V. Pham, R. Zünd, R. J. J. Funck, A. Bezegh, D. Ammann, E. Pretsch and W. Simon, Anal. Chem., 56, 535 (1984). 12. J. O. Wieth and M. T. Tosteson J. GzN. Physiol. 73 765 (1979). 13. V. M. Shkinev, B. Y. Spivakov, G. A. Vorobeva and Y. A. Zolotov, Anal. Chim. Acta, 167, 145 (1985). 14. V. A. Zarinskii, L. K. Shpigun, V. M. Shkinev, B. Ya, Spinakov, and Yu A. Zolotov, Zhurnal Analiticheskoi Khimii 35, 2143 (1980). 15. R. Bloch, A. Shatkay, and H. A. Saroff, Biophys. J. 7, 865 (1967). 16. U. Wuthier, H. V. Pham, R. Zünd, R. J. J. Funck, A. Bezegh, D. Ammann, E. Pretsch and W. Simon, Anal. Chem., 56, 535 (1984). 17. U. Wuthier, H.-V. Pham, B. Rusterholz and W. Simon, Helv. Chim. Acta 69, 1435 (1986). 18. N. A. Chaniotakis, S. B. Park and M. E. Meyerhoff, Anal. Chem. 61, 566 (1989). 19. S. S. S. Tan, P. C. Hauser, K. Wang, K. Fluri, K. Seiler, B. Rusterholz, G. Suter, M. Krüttli, U. E. Spichiger and W. Simon, Anal. Chim. Acta, 255, 35 (1991). 20. S. A. Glazier and M. A. Arnold, Anal. Chem., 63, 754 (1991). 21. N. A. Chaniotakis, K. Jurkschat and A. Rühlemann, Anal. Chim. Acta, 282, 345 (1993). 22. I. Tsagatakis; N. A. Chaniotakis and K. Jurkschat, Helv. Chim. Acta, 77, 2191 (1994). 23. G. J. Moody, E. E. Saad and J. D. R Thomas, Sel. Electrode Rev., 70, 71 (1988). 24. S. Daunert, S. Wallace, A. Florido and L. G. Bachas, Anal. Chem. 63, 1676 (1991). 25. H. Dalil, M. Biesemans, R. Willem, L. Angiolini, E. Salatelli, D. Caretti, N. A. Chaniotakis and K. Perdikaki, Helv. Chim. Acta, 85, 852 (2002). 26. L. Angiolini, E. Salatelli, D. Caretti, M. Biesemans, H. Dalil, R. Willem, N. A. Chaniotakis, E. Gouliaditi and K. Perdikaki, Macromol. Chem. Phys., 203, 219 (2002). 27. D. Tonelli, I. Carpani, L. Mazzocchetti, L. Angiolini, D. Caretti, E. Salatelli and F. Tarterini, Electroanalysis: 18, 1055 (2006). 28. J. K. Tsagatakis, N. A. Chaniotakis, K. Jurkschat, S. Damoun, P. Geerlings, A. Bouhdid, M. Gielen, I.Verbruggen, M. Biesemans, J. C. Martins and R. Willem, Helv. Chim. Acta, 82, 531 (1999). 29. K. Perdikaki, J. K. Tsagatakis and N. A. Chaniotakis, Mikrochim. Acta, 136, 217 (2001). 30. S. Chandra, A. Ruzicka, P. Svec and H. Lang, Anal. Chim. Acta, 577, 91 (2006). 31. L. Xu, R. Yuan, Y.-Q. Chai, and X.-L. Wang, Ana.l Bioanal. Chem., 381, 781 (2005). 32. L. Xu, R. Yuan, Y.-Z. Fu and Y.-C. Chai, Anal. Sci., 21, 287 (2005). 33. S. Shahrokhian, M. K. Amini, R. Kia and S. Tangestaninejad, Anal. Chem., 72, 956 (2000). 34. D. Lieu, W. C. Chen, R. H. Yang, G. L. Shen and R. Q. Yu, Anal. Chim. Acta, 338, 209 (1997).

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35. I. Tsagatakis, N. Chaniotakis, R. Altmann, K. Jurkschat, R. Willem, J. C. Martins, Y. Qin and E. Bakker. Helv. Chim. Acta, 84, 1952 (2001). 36. K. Perdikaki, I. Tsagatakis, N. A. Chaniotakis, R. Altmann, K. Jurkschat and G. Reeske, Anal. Chim. Act., 467, 197 (2002). 37. N. A. Chaniotakis, K. Jurkschat, G. Reeske and A. Volosirakis, Anal. Chim. Acta, 553, 185 (2005).

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Tin Compounds as Flame Retardants and Smoke Suppressants

3.5

339

Tin Compounds as Flame Retardants and Smoke Suppressants

Paul A. Cusack ITRI Innovation Limited, St Albans, UK

3.5.1

Introduction

Fire is one of the main problems affecting modern life, its destructive potential manifesting itself principally in terms of human suffering and loss of property. Every year, over 4000 deaths and 40 000 severe injuries are attributed to fire in Europe, with direct costs of fire damage exceeding € 25 billion.1 Flame retardants are chemicals which, when added to polymeric materials, inhibit the combustion process, thereby delaying ignition, preventing fire spread and, in some cases, reducing the amount of smoke and toxic gases produced during the burning process. The European Commission has stated that, over the last 10 years, a 20% reduction in fire deaths has resulted from the use of flame retardants.2 Worldwide consumption of flame retardant chemicals has recently been estimated at 1.6 million tonnes per annum,3 the main types being based on relatively few elements – bromine, chlorine, phosphorus, nitrogen, aluminium, magnesium, and antimony. Although tin compounds have been known to exhibit flame-retardant properties since the mid-19th century4 and despite the fact that a wide range of inorganic and organotin compounds have subsequently been reported as active flame retardants,5 only a few tin-based systems have reached commercialization. However, recent concerns about the potential toxicity of certain flame retardants and their damaging effects in the environment,6 have led to an intensified search for safer additives. In this connection, the generally accepted low toxicity of inorganic tin compounds7 has been a major factor in their emergence as viable flame retardants and smoke suppressants for many polymeric materials. This chapter reviews the key application areas for tin-based flame retardants and smoke suppressants, discusses recent developments including ultrafine/colloidal and ‘coated filler’ products, and summarizes current knowledge relating to the mechanistic action of tin fire-retardant additives.

3.5.2

Tin Treatments For Fibers

Natural Fibers

The first reported use of a tin-based flame retardant dates back to 1859, when a process involving the in situ precipitation of hydrous tin(IV) oxide was developed to impart flame-resistant properties to cotton and other cellulosic materials.4 This concept was modified and extended at the turn of the century,8 and Perkin’s ‘Non-Flam’ process, involving impregnation of cotton flannelette with sodium hydroxystannate solution followed by immersion in aqueous ammonium sulfate, produced an insoluble deposit of hydrous SnO2 in the fabric, which was claimed to be permanently resistant to laundering.9 Further work on inorganic tin deposits continued during the 1930s and 1940s, the best system being ‘stannic tungstate’,10,11 the species deposited during the aqueous reaction of tin(IV) chloride and sodium tungstate. The development of organophosphorus-based flame retardants, which were directly bonded on to the cellulosic polymer backbone, resulted in the tin treatments being largely replaced during the latter half of the 20th century. However, a multistage ‘stannate–phosphate’ process for conferring flame retardancy to cotton fabrics has recently been commercialized in India.12,13 During the 1970s, aqueous solutions containing tin(IV) chloride and ammonium bifluoride found commercial application as flame-resistant treatments for woollen sheepskins and rugs,14,15 where they had the advantage over competitive titanium and zirconium fluoride systems of not imparting any yellow

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coloration, nor attacking the leather backing. 19 F NMR studies showed that the major tin species present 16 in the treating formulation is SnF2− 6 , these anions being attracted to the amino groups in the wool, the amino groups themselves being protonated in the highly acidic treating medium. The tin treatment was found to be unaffected by dry cleaning and was used in New Zealand for treating over 4000 sheepskins per annum. More recently, work undertaken at The International Tin Research Institute (ITRI) and supported by UK Government funding under the Civil Aircraft Research & Technology Demonstration (CARAD) program, has shown that a number of experimental tin-based solutions or colloids match or even outperform a commercial potassium fluorozirconate (PFZ) treatment, when applied to a typical aircraft cabin wool–nylon blend fabric.17 Particular benefits were observed with regard to reduced emissions of smoke and carbon monoxide – the major cause of death in virtually all fire situations and further evaluation of the inorganic tin systems appears to be justified. Synthetic Fibers

Studies of tin compounds as flame retardants for synthetic fibers have been rather limited and have generally concluded that the tin is only effective when used as a synergist with a halogen source. Hence, whereas ITRI found that tin halides, including SnCl2 , SnBr2 , and SnCl4 , were all effective flame retardants when incorporated at levels of 10–15% into nylon-6 fibers,18 other studies using the same fiber material have focused on the use of SnO2 or metal hydroxystannates in conjunction with halogen additives, such as 2,4,6-tribromoaniline.19 Similarly, certain diorganotin compounds, including dioctyltin thioglycollate (in polypropylene fibers)20 and dibutyltin oxide (in polyacrylonitrile fibers),21 are effective synergists when used with commercial brominated flame retardants. A proprietary system (‘Sandoflam 5070’) comprising a brominated organic compound, a phosphate and an organotin derivative, was claimed to be a very effective flame retardant for polypropylene fibers and reached commercialization during the early 1980s.22 3.5.3

Zinc Stannates

Manufacture, Characterization and Consumption Data

By far the most important tin-based fire retardants are the zinc stannates – zinc hydroxystannate (ZHS) and its anhydrous analog, zinc stannate (ZS). Originally developed at ITRI during the mid-1980s, these additives are now being marketed worldwide as non-toxic flame retardants and smoke suppressants for use in a wide range of polymeric materials.23 ZHS is manufactured industrially by the aqueous reaction of sodium (or potassium) hydroxystannate with zinc chloride: Na2 Sn(OH)6 + ZnCl2 → ZnSn(OH)6 + 2NaCl

(3.5.1)

The white precipitate product is washed free of sodium chloride and dried in air at ca. 105 ◦ C. ZS is manufactured by controlled thermal dehydration of ZHS, usually at a temperature in the range 300– 400 ◦ C: ZnSn(OH)6 → ZnSnO3 + 3H2 O

(3.5.2)

Although there is generally little difference in the effectiveness of ZHS and ZS in terms of their fireretardant properties, ZS is the preferred additive for polymers which are processed at temperatures above ca. 200◦ C. Some important properties of ZHS and ZS are given in Table 3.5.1.

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Tin Compounds as Flame Retardants and Smoke Suppressants Table 3.5.1

341

Properties of zinc hydroxystannate and zinc stannate

Property

ZHS

ZS

Chemical formula Molecular weight CAS Number ELINCS Number TSCA Listed Appearance Analysis (typical):

ZnSn(OH)6 286.12 12027-96-2 404-410-4 Yes White powder 41% Sn 23% Zn TET (63 μM) > TMT (280 μM). For 45 Ca uptake, it followed the same order, i.e., TBT (0.35 μM) > TET (10 μM) > TMT (440 μM). In agreement with the in vitro results, both cardiac SR Ca2+ ATPase and 45 Ca uptake were significantly inhibited in vivo in rats treated with these organotin compounds

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The Cardiovascular Activity of Organotin Compounds CARDIAC 100 % INHIBITION

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45Ca−UPTAKE

SARCOPLASMIC RETICULUM

TBT

419

TET

TMT

80 60 40

IC50 − 10 µM

IC50 − 0.35 µM

IC50 − 440 µM

20 0 0.0

0.5

1.0

0

5 10 15 20 Concentration (µM)

0

200

400

600

800

Figure 4.1.2 In vitro effects of triorganotin compounds on rat cardiac SR 45 Ca uptake. (Reproduced from Reference 41, with kind permission of Springer Science and Business Media.)

at all doses, when compared to control rats (Figure 4.1.4), indicating that these tin compounds inhibit cardiac SR Ca2+ -transport.41 Further, TET and TMT appeared to exert dose-dependent effects, while TBT did not show a dose–response relationship (Figure 4.1.4). cAMP-dependent 32 P-binding to trichloroacetic acid-precipitable cardiac SR proteins in the absence and presence of different concentrations of TBT, TET, and TMT are presented in Figure 4.1.5. c-AMP significantly elevated (70–80%) the 32 P-binding to SR proteins in vitro in the absence of any organotin. In the presence of organotins, cAMP-stimulated 32 P-binding to proteins was significantly reduced, but the decrease was concentration-dependent only at lower concentrations (Figure 4.1.5). The order of potency is TBT > TET > TMT. In agreement with in vitro studies, cAMP-dependent 32 P-binding to proteins was significantly reduced in vivo in rats treated with these tin compounds at all doses (Figure 4.1.6). SDS-polyacrylamide gel electrophoresis of the cardiac SR revealed at least 30 Coomassic blue-stainable bands, ranging from 9 to 120 kDa.

CARDIAC

Ca2+−ATPase

SARCOPLASMIC RETICULUM

TBT

80 % INHIBITION

OTE/SPH

TET

60

TMT

IC50 − 2 µM

IC50 − 250 µM

40 IC50 − 63 µM

20 0

2

4

6

8

10 0

20

40

60

80

0

200

400

600

800

Concentration (µM)

Figure 4.1.3 In vitro effects of triorganotin compounds on rat cardiac SR Ca2+ -ATPase. (Reproduced from Reference 41, with kind permission of Springer Science and Business Media.)

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45 Ca−Uptake Ca2+−ATPase

TET

TBT

100 % of Control



120

Control

TMT

Control

Control

80 60 40 20 0.75

1.5

2.5

0.5

1.0

1.5

0.75

1.5

2.5

Dosage of Tin compounds (mg/kg/d. for 6 days)

Figure 4.1.4 In vivo effects of triorganotin compounds on rat cardiac SR 45 Ca uptake and Ca2+ -ATPase; ∗ Significantly different from control. (Reproduced from Reference 41, with kind permission of Springer Science and Business Media.) 32

P– BOUND TO PROTEINS – cAMP DEPENDENT

180 % of Control

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TET

TMT

140

100

60

0

2

4

6

8

10 0

25 50 75 Concentration (µM)

100 0

200

400

600

800

Figure 4.1.5 In vitro effects of triorganotin compounds on rat cardiac SR protein phosphorylation. (Reproduced from Reference 41, with kind permission of Springer Science and Business Media.) 32P–

pmol/mg/protein/min

OTE/SPH

TBT

BOUND TO PROTEINS — cAMP DEPENDENT TET

TMT

30

20

10

0

Ctrl

0.75

1.5

2.5

Ctrl

0.5

1.0

1.5

Ctrl

0.75

1.5

2.5

Dosage of Tin compounds (mg/kg/d. for 6 days)

Figure 4.1.6 In vivo effects of triorganotin compounds on rat cardiac SR protein phosphorylation; ∗ Significantly different from control. (Reproduced from Reference 41, with kind permission of Springer Science and Business Media.)

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The Cardiovascular Activity of Organotin Compounds

421

Autoradiographs from samples incubated in the presence of cAMP indicated 32 P incorporation in seven bands. Of the seven bands, the band corresponding to about 24 kDa molecular weight protein decreased in intensity with the treatment of organotin compounds in vitro as well as in vivo. These results suggest that triorganotin compounds may be affecting Ca2+ pumping mechanisms through the alteration of phosphorylation of specific proteins corresponding to phospholamban in rat cardiac SR.41 Furthermore, the effects of TBT, TET, and TMT on rat cardiac ATPases and catecholamine binding have also been investigated by Desaiah et al.,42 since these phenomena are involved in cardiac function. All three organotin compounds inhibited cardiac Na+ K+ -ATPase, [3 H]ouabain binding, K+ -activated p-nitrophenyl phosphatase (K+ -PNPPase), and oligomycin-sensitive (o.s.) and oligomycin-insensitive (o.i.) Mg2+ -ATPase in a concentration-dependent manner. K+ -PNPPase was less sensitive to these triorganotins when compared to Na+ K+ -ATPase, suggesting that triorganotin compounds affect the Na+ pump activity by acting on the Na+ -dependent phosphorylation process. Mg2+ -ATPase (o.s.) was more sensitive to these triorganotin compounds when compared to Mg2+ -ATPase (o.i), confirming their potent effect on the enzymes of oxidative phosphorylation.42 The order of potency is TBT > TET > TMT. Further, TET and TMT, but not TBT, inhibited [3 H]norepinephrine and [3 H]dopamine binding to cardiac membranes in a concentration-dependent manner, the effect being more with TET. These results suggest that triorganotin compounds inhibit sodium pump activity as well as ATP synthesis. Since Na+ K+ -ATPase is involved in the active transport of catecholamines, triorganotin compounds not only inhibited the catecholamines transport, but also, to some extent, affected catecholamine binding, thus interfering with cardiac function.42 Kang et al. also reported43,44 that TET and triphenyltin chloride (TPT) dose-dependently induced Ca2+ release from the isolated sarcoplasmic reticulum membrane vesicles and inhibited the Ca 2+ -ATPase activity, while TBT had comparatively less potency and efficacy. TPT induced Ca2+ release in ruthenium red-sensitive and -insensitive ways, with EC50 values of 75 and 270 μM, repectively.44 TPT exerted dual effects on the apparent [3 H]ryanodine binding; TPT (0.5–10 μM) dose-dependently potentiated the [3 H]ryanodine binding, however, the [3 H]ryanodine binding decreased as the concentration of TPT increased. The dissociation of bound [3 H]ryanodine was facilitated by TPT. Recently, it has been reported45 that exposure to a low-leaching rate TBTO-based anti-fouling paint induces significant tachycardia (elevated heart rate) in the sub-tropical mussel, Perna viridis (L.) and the response is likely to be associated with organotin detoxication mediated by the action of the heart. A high ratio of TBT:DBT is present in the tissues suggesting that although partial detoxication is evident, P. viridis is inefficient at metabolizing organotins.45 As heart serves several homeostatic functions, TBT accumulation and detoxication will be energetically costly and P. viridis inhabiting areas that have high shipping densities are likely to experience chronic and sub-lethal stress.45 Cardiovascular Activity of Organotin(IV) Derivatives of Amino Acids

It is an established fact that when a drug is administrated there is a change in blood pressure with the passage of time. Thus, a drug, which lowers the blood pressure for longer duration, is considered to be a more effective anti-hypertensive than the one for which lowering occurs in shorter duration. Nath et al. investigated cardiovascular activity of a large number of organotin derivatives of amino acids, dipeptides, triglycine, thymidine, ascorbic acid, and umbelliferone, when administrated intravenously in either adult cats (body weight 3–4 kg) or mongrel dogs (body weight 10–20 kg) of either gender.46−55 Di-n-butyltin and diphenyltin derivatives of L-proline (HPro),46 triphenyltin derivatives of D-penicillamine (H2 Pen),47 trans-hydroxy-L-proline (HHyp), and glutamine (HGlu),46 tri-n-butyltin(Pro), and trimethyltin derivatives of HPro, HHyp, and HGlu46 exhibited mild and delayed anti-hypertensive activity of varying degree

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Table 4.1.3

Cardiovascular activity data of organotin(IV) derivatives of amino acids Cardiovascular activitya LD50 (mg kg−1 )

Dose (mg kg−1 )b i.v.

n-Bu2 Sn(Pen) Ph2 Sn(Pen) Ph3 Sn(HPen) Ph3 Sn(HPen).H2 O

>500 >500 >500 >1000

2 2 1 2.5

n-Bu2 SnCl(HPen).H2 O n-Bu3 Sn(Pro) Me3 Sn(Pro) n-Bu2 Sn(Pro)2 Ph2 Sn(Pro)2 Me3 Sn(Hyp) Ph3 Sn(Hyp) Ph2 Sn(Hyp)2 Me3 Sn(Glu) Ph3 Sn(Glu) Captopril

>1000

2.5

n.d. n.d. n.d. n.d. n.d. >1000 >2000 n.d. >1000 n.d.

5 5 5 5 5 5 2.5 5 5 2.5

Complex/ Reference drug

Change in blood pressure (mm Hg) Immediate

Delayed

NC NC NC –20 (126.2 ± 4.32)c,d –51 (81.6 ± 9.07)c,e NC NC NC NC NC NC –30 NC NC –10

–40 –50 –10 +9 (134.6 ± 6.37)c NC –10 –7 –10 –5 –5 –6 –90 –5 –6.5 –60

Duration (min) 20 45 5 9 (8.8 ± 1.92)c 20 (19.6 ± 1.67)c 4 3 5 2 2 6 60 1 9 1440

a

In comparison to control; b i.v., Intravenously; NC, no change; c Mean ± standard error; d Control 145.6 ± 6.5; e Control 132.6 ± 7.88; p < 0.05; Animals: either adult mongrel dogs (body weight 10–20 kg) or cats (body weight 3–4 kg) of either gender

and duration without affecting the carotid occlusion (CO) and noradrenaline (NA) pressure responses (Table 4.1.3). Also, none of these studied complexes had shown bradycardia (decrease in heart beat rate) as well as tachycardia, and hence no change in the resting heart beat rate. This suggests that these complexes may act as direct vasodilators on the smooth muscles of blood vessels. Ph2 Sn(Pen), Ph2 Sn(Hyp)2 , n-Bu2 SnCl(HPen).H2 O, and n-Bu2 Sn(Pen) showed potent anti-hypertensive activities.46,47 In the case of Ph2 Sn(Hyp)2 ,46 the initial fall in blood pressure (30 mmHg) was further followed by a potent and gradual decrease in blood pressure (90 mmHg), that lasted for 60 min, and a bradycardia of 2–3 beats per min was observed. In addition, it was associated with inhibition of the CO without affecting the NA response, which might be suggestive of a central site of action for this complex. Considering its potentiality, it was further studied in detail at three gradual doses (1.25, 2.5, and 5.0 mg kg−1 i.v.). At a dose of 1.25 mg kg−1 i.v., it showed a transient fall of 10 mmHg followed by a potent fall of 45 mmHg in blood pressure, and no change in heart rate was observed. This complex partially inhibited the CO response without affecting the NA response. In higher dose (5.0 mg kg−1 i.v.), this complex initially elicited a marked hypotensive activity (60 mmHg) followed by a very potent fall in blood pressure (120 mmHg) that lasted for about 90 min, and a bradycardia of 5–6 beats per min was also observed. In addition, there was a complete blockage of pressure responses evoked either by bilateral carotid occlusion or noradrenaline injection.46 However, the behavior of Ph3 Sn(HPen).H2 O and n-Bu2 SnCl(HPen).H2 O was different,47 as Ph3 Sn(HPen).H2 O showed an immediate fall in blood pressure (–20 mmHg), followed by moderate and gradual increase (+9 mmHg) in blood pressure that lasted for about 9 min and was associated with inhibition of both the CO and NA responses, which might be suggestive of a peripheral site of action, whereas n-Bu2 SnCl(HPen).H2 O showed an immediate potent fall in blood pressure of ∼51 mmHg in ∼20 min without affecting the CO and NA responses.47

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Cardiovascular Activity of Organotin(IV) Derivatives of Dipeptides, Triglycine, and Thymidine

Di- and tri-organotin(IV) derivatives of a large number of dipeptides,47−51 viz., glycyltryptophane (H2 Gly-Trp), valylvaline (H2 Val-Val), alanylvaline (H2 Ala-Val), glycyltyrosine (H2 Gly-Tyr), glycylphenylalanine (H2 Gly-Phe), leucyltyrosine (H2 Leu-Tyr), leucylleucine (H2 Leu-Leu), leucylalanine (H2 Leu-Ala), β-alanyl-l-histidine or carnosine (H2 Ala-His), glycylglycine (H2 Gly-Gly), glycylleucine (H2 Gly-Leu), glycylisoleucine (H2 Gly-Ile), glycylvaline (H2 Gly-Val), and triglycine (H3 Gly-Gly-Gly), except Ph2 Sn(Leu-Ala),48 Me3 Sn-(HVal-Val)/(HAla-Val)/(HGly-Leu),49 Ph3 Sn-(HAla-Val)/(HGlyTyr)/(HGly-Gly)/(HGly-Val),50 n-Bu2 SnCl(H2 Gly-Gly-Gly).H2O,47 and Ph3 Sn(H2 Gly-Gly-Gly).H2O,47 exhibited delayed anti-hypertensive activity of varying degree and duration (Tables 4.1.4 and 4.1.5) without affecting the CO and NA pressure responses, which suggests that these complexes may act as direct vasodilators on the smooth muscles of blood vessels. Moreover, none of these complexes induced bradycardia as well as tachycardia. Furthermore, among Ph2 Sn(IV) derivatives, Ph2 Sn(Ala-His) is found to be the most effective and among n-Bu2 Sn(IV) derivatives, n-Bu2 Sn(Leu-Tyr) is found to be the most effective.48 However, an immediate drop of 160 mmHg compared to control has been observed in Ph2 Sn(Leu-Ala) at a dose of 1.0 mg kg−1 i.v. causing the sudden death of the Table 4.1.4

Cardiovascular activity data of diorganotin(IV) derivatives of dipeptides and triglycine Cardiovascular activity Change in blood pressure (mm Hg)b,c

Complex/ Reference drug Captopril n-Bu2 Sn(Gly-Trp) Ph2 Sn(Gly-Trp) n-Bu2 Sn(Val-Val) Ph2 Sn(Val-Val) n-Bu2 Sn(Ala-Val) Ph2 Sn(Ala-Val) n-Bu2 Sn(Gly-Tyr) Ph2 Sn(Gly-Phe) n-Bu2 Sn(Leu-Tyr) n-Bu2 Sn(Leu-Leu) Ph2 Sn(Leu-Ala) n-Bu2 Sn(Ala-His) Ph2 Sn(Ala-His) n-Bu2 Sn(HGlyGly-Gly) n-Bu2 SnCl(H2 Gly-GlyGly).H2 O Ph2 Sn(HGly-GlyGly).MeOH a

LD50 Dose (mg kg−1 ) (mg/kg)a i.v.

Control (Mean ± SE)

Immediate (Mean ± SE)

Delayed (Mean ± SE)

Duration (min) (Mean ± SE)

– >500 >500 >400 >300 >500 >400 >500 >200 >800 >400 >200 >500 >500 >500

2.5 2.0 2.0 2.0 1.0 2.0 2.0 4.0 1.0 2.0 2.0 1.0 2.0 2.0 2.0

160.0 ± 9.45 150.0 ± 6.95 100.0 ± 10.45d 1440 ± 30.0 134.8 ± 4.42 NC 124.9 ± 4.87e 5.0 ± 1.38 136.4 ± 6.24 NC 116.8 ± 6.54 f 10.0 ± 1.94 135.2 ± 4.21 NC 105.4 ± 4.94e 10.0 ± 1.85 131.4 ± 5.96 NC 111.6 ± 5.25e 15.0 ± 1.75 131.6 ± 5.08 NC 91.8 ± 5.45e 20.0 ± 1.45 137.4 ± 6.44 NC 117.6 ± 6.14e 10.0 ± 1.65 137.1 ± 4.52 NC 107.5 ± 4.38e 5.0 ± 1.15 136.8 ± 5.12 NC 121.6 ± 5.63e 10.0 ± 1.67 145.4 ± 4.86 NC 95.6 ± 4.75e 35.0 ± 1.44 132.6 ± 4.31 NC 112.7 ± 4.75e 10.0 ± 1.24 g g 148.9 ± 5.72 5.0 ± 1.20 138.4 ± 7.42 NC 117.9 ± 4.85 f 10.4 ± 1.08 134.8 ± 8.02 NC 104.4 ± 7.35 f 10.1 ± 1.39 133.8 ± 7.94 NC 118.6 ± 8.21 f 5.0 ± 1.82

>1000

2.5

135.0 ± 7.90

60.0 ± 8.09

120.0 ± 8.39 f

28.6 ± 2.19

>500

2.0

131.6 ± 6.31

NC

91.8 ± 6.53e

20.2 ± 0.94

i.v., Intraveneously; b No change in heart rate (bpm), except n-Bu2 SnCl(H2 Gly-Gly-Gly).H2 O; c No effect on carotid occlusion and noradrenaline pressor responses except n-Bu2 SnCl(H2 Gly-Gly-Gly).H2 O; SE denotes the standard error; d p < 0.001; e p < 0.05; f p < 0.01; g Immediate fall in blood pressure (–160 mmHg) has been observed; Animals: either adult mongrel dogs (body weight 10–20 kg) or cats (body weight 3–4 kg) of either gender

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Table 4.1.5

Cardiovascular activity data of triorganotin(IV) derivatives of dipeptides and triglycine Cardiovascular activity Change in blood pressure (mm Hg)b,c

Reference drug/ Complex

LD50 Dose (mg kg−1 ) (mg/kg)a i.v.

Captopril n-Bu3 Sn(HGly-Trp) Me3 Sn(HVal-Val) Ph3 Sn(HVal-Val) Me3 Sn(HAla-Val) Ph3 Sn(HAla-Val)

– >1000 >1000 >500 >1000 >2000

Me3 Sn(HGly-Tyr) n-Bu3 Sn(HGly-Tyr) Ph3 Sn(HGly-Tyr) n-Bu3 Sn(HGly-Gly) Ph3 Sn(HGly-Gly) Me3 Sn(HLeu-Tyr) Me3 Sn(HLeu-Leu) Ph3 Sn(HLeu-Leu) Me3 Sn(HLeu-Ala) n-Bu3 Sn(HLeu-Ala) Ph3 Sn(HLeu-Ala) Me3 Sn(HGly-Leu) n-Bu3 Sn(HGly-Leu) Ph3 Sn(HGly-Leu) Me3 Sn(HGly-Ile) n-Bu3 Sn(HGly-Ile) Ph3 Sn(HGly-Ile) Ph3 Sn(HGly-Val) Ph3 Sn(HAla-His) Ph3 Sn(H2 Gly-GlyGly).H2 O

>1000 >1000 >1000 >1000 >600 >1000 >1000 >500 >1000 >1000 >400 >1000 >1000 >500 >1000 >1000 >500 >600 >400 >400

2.5 2.5 2.5 1.0 2.5 1.25 2.5 5.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 1.0 2.5 2.5 1.0 2.5 2.5 1.0 2.5 2.5 1.0 1.0 1.0 1.0

Control (Mean ± SE) 160.0 ± 9.45 129.6 ± 7.95 137.6 ± 7.66 140.4 ± 6.84 132.2 ± .8.13 135.0 ± 7.90 143.2 ± 8.16 130.0 ± 7.90 131.2 ± 10.25 137.0 ± 8.42 139.6 ± 8.64 130.0 ± 7.90 147.8 ± 6.49 133.2 ± 6.45 131.6 ± 6.69 139.4 ± 5.43 131.6 ± 6.98 133.8 ± 9.80 136.4 ± 6.24 137.6 ± 8.29 132.2 ± 6.01 135.8 ± 7.89 132.0 ± 7.04 141.4 ± 9.20 142.0 ± 8.43 138.2 ± 11.54 139.6 ± 9.46 140.2 ± 6.48



150.0 ± 6.95 100.0 ± 10.45d NC 123.6 ± 7.89e 129.0 ± 6.55 f 118.2 ± 6.18d NC 100.2 ± 6.21 f 122.0 ± 8.68e 113.0 ± 6.32d f 125.0 ± 7.38 105.0 ± 7.38d 74.0 ± 6.81d 114.0 ± 6.74 f f 20.0 ± 7.38d 79.6 ± 8.38 NC 71.0 ± 11.18e NC 107.8 ± 7.88e 110.4 ± 8.17e 125.8 ± 6.26 f NC 119.4 ± 8.41e 64.8 ± 6.41e 128.5 ± 4.66e NC 84.0 ± 4.52 f NC 120.4 ± 7.02 f NC 109.3 ± 5.03e NC 122.6 ± 4.83 f NC 121.4 ± 9.60e NC 111.3 ± 5.93e 118.4 ± 7.40d 108.4 ± 6.54e NC 127.2 ± 6.37e NC 115.6 ± 6.84e NC 92.8 ± 6.41e NC 131.4 ± 10.23e NC 97.1 ± 7.92e 104.6 ± 11.61e 123.0±10.36e NC 104.2 ± 7.48e NC 120.1 ± 6.89e

Duration (min) (Mean ± SE) 1440 ± 30.0 5.6 ± 0.89 29.2 ± 2.28 60.0 ± 1.03 9.2 ± 2.28 59.6 ± 1.67 119.4 ± 2.19 149.6 ± 1.67 39.6 ± 2.19 9.6 ± 2.60 40.2 ± 3.49 3.6 ± 1.14 59.8 ± 2.86 20.0 ± 1.41 9.2 ± 2.28 30.0 ± 2.09 5.4 ± 1.14 5.2 ± 1.09 40.0 ± 1.39 24.0 ± 3.08 4.6 ± 1.14 20.0 ± 1.20 41.4 ± 2.60 9.2 ± 2.28 40.0 ± 2.17 49.6 ± 1.67 40.2 ± 1.26 20.4 ± 3.64

a

i.v., Intraveneously; b No change in heart rate (bpm), and no effect on noradrenaline (NA) pressor response; c No effect on carotid occlusion (CO) pressor response, except Ph3 Sn(HGly-Tyr), Ph3 Sn(HGly-Gly) and Ph3 Sn(HGly-Val); SE denotes the standard error; d p < 0.001; e p < 0.05; f p < 0.01

animal due to cardiac arrest because of its higher toxicity (LD50 > 200 mg kg−1 ). The behavior of Ph3 Sn-(HGly-Tyr)/(HGly-Gly)/(HGly-Val) is different, as it showed immediate moderate to potent fall in blood pressure (∼30–83 mmHg) as compared to the control value, followed by moderate and gradual increase in blood pressure (∼15–64 mmHg) that lasted for about 40–60 min and was associated with inhibition of the CO response without affecting the NA response, which might be suggestive of a central site of action.50 Moreover, Ph3 Sn(HGly-Tyr) and Ph3 Sn(HGly-Gly) induced bradycardia (4–5 beats per min), while Ph3 Sn(HGly-Val) first induced bradycardia followed by tachycardia. Further, Me3 Sn-(HVal-Val)/(HAla-Val)/(HGly-Leu)49 and Ph3 Sn(HAla-Val)51 showed an immediate fall in blood pressure (∼9–30 mmHg) at a dose of 2.5 mg kg−1 i.v., as compared to the control value, that lasted for

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9–120 min without affecting the CO and NA responses and heart rate, except for Ph3 Sn(HAla-Val). However, Ph3 Sn(HAla-Val) decreased the heart rate by 5 beats per min. Since this complex showed promising anti-hypertensive activity, it was further assayed at three gradual doses (1.25, 2.5, and 5.0 mg kg−1 i.v.). These data indicated that it lowered blood pressure by 110 mmHg in 150 min at a 5.0 mg kg−1 dose (Table 4.1.5).51 These observations indicate that the anti-hypertensive activity is influenced by the structural features of the side chain at the methylene carbon atom adjacent to either the O C O and/or the amino group in the dipeptide anion coordinated to the di- and triorganotin(IV) moiety, as well as on the tin-bound organic group. In general, the di-n-butyltin(IV) dipeptides exhibited greater anti-hypertensive activity than the diphenyltin analogs, whereas the Ph3 Sn(IV) derivatives showed potent activity of longer duration than the Me3 Sn(IV) and n-Bu3 Sn(IV) analogs. In the case of n-Bu2 SnCl(H2 Gly-Gly-Gly).H2 O, an immediate fall in blood pressure (∼75 mmHg) was followed by a moderate and gradual increase in blood pressure (∼60 mmHg) that lasted for ∼29–30 min and was associated with inhibition of both CO and NA responses, which might be suggestive of a peripheral site of action.47 Further, it first decreased the resting heart rate (3–4 beats per min) followed by an increase (3–4 beats per min). Ph3 Sn(H2 Gly-Gly-Gly).H2 O showed a fall in blood pressure of ∼20 mmHg in 20 min and inhibited the CO, but potentiated the NA pressure response.47 Triphenyltin thymidinate exhibited insignificant anti-hypertensive activity (a drop of 5 mmHg of blood pressure as compared to control) that lasted for two minutes without causing any effect on the heart rate, CO, and NA responses.52 Some of triphenyltin(IV) or trimethyltin(IV) derivatives of dipeptides, viz. Ph3 Sn(HVal-Val), Ph3 Sn(HAla-Val), Ph3 Sn(HGly-Ile), Ph3 Sn(HGly-Val), Ph3 Sn(HGly-Gly), Me3 Sn(HGly-Tyr), Me3 Sn(HGly-Ile), and Me3 Sn(HLeu-Tyr), (Scheme 4.1.2) have potent anti-hypertensive activity, comparable to that of Captopril, but the duration of efficacy is much shorter than that for Captopril. Since these compounds have low toxicity (LD50 >1000–2000 mg kg−1 ), they may be considered as good anti-hypertensive drugs. Cardiovascular Activity of Organotin(IV) Derivatives of Ascorbic Acid and Umbelliferrone

Organotin(IV) ascorbates exhibited mild anti-hypertensive activity (a fall of 6–10 mmHg in blood pressure) which lasted for 3–5 min only, at a dose of 5.0 mg kg−1 i.v., without affecting CO and NA response, thus it can be said that these compounds do not change blood pressure appreciably.53 Di- and tri-organotin(IV) derivatives of umbelliferrone exhibited mild anti-hypertensive activity of varying degree and duration (Table 4.1.6) without affecting the CO and NA responses.54,55 Such a profile of pharmacological effect is indicative of the direct vasodialator action of these compounds. The 1,10-phenanthroline adducts of these organotin(IV) derivatives of umbelliferrone exhibited potent anti-hypertensive activity of varying degree and duration. Thus, n-Bu2 Sn(Umb)2 .phen showed potent initial hypotensive activity (75 mmHg) of gradual onset (+44 mmHg), which was followed by a bradycardia of 5 beats per min then by tachycardia of 3–4 beats per min (Table 4.1.6). The cardiovascular activity of this compound lasted for about 45 min and was associated with marked inhibition of CO and NA responses.54 Such a cardiovascular profile is suggestive of a peripheral site of action. Ph3 Sn(Umb).phen showed hypotensive activity (23 mmHg), which lasted for about 16 min, and inhibited the NA response without affecting CO response, whereas Me3 Sn(Umb).phen lowered blood pressure by 20 mmHg with inhibition of both CO and NA responses. Diorganotin(IV) derivatives of umbelliferrone and their 1,10-phenanthroline adducts are found to lower blood pressure more effectively than triorganotin analogs. 4.1.3

Conclusion

Trialkyl- and triphenyltin compounds interfered with heme metabolism as well as the cardiovascular system, caused a fall in blood pressure that resulted from a depression of the vascular smooth muscle, altered

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Scheme 4.1.2

Structures of organotin(IV) derivatives exhibiting potent cardiovascular activity

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Table 4.1.6 Cardiovascular activity data of organotin(IV) derivatives of umbelliferone and their 1,10-phenanthroline adducts Cardiovascular activitya Change in blood pressure (mm Hg) Reference drug/ Complex Captopril Me3 Sn(Umb)b Ph3 Sn(Umb)b n-Bu2 Sn(Umb)b2 Ph2 Sn(Umb)b2 Me3 Sn(Umb).Phenc Ph3 Sn(Umb).Phend n-Bu2 Sn(Umb)2 .Phenc

LD50 Change (mg kg−1 ) in HR – >1000 >1000 >1000 >1000 >1000 >1600 >1000

– – – – – ↓↑ ↓↑ ↓↑


Duration (min) (Mean ± SE)

Control (Mean ± SE)


160.0 ± 9.45 132.0 ± 6.7 131.8 ± 10.0 135.0 ± 14.6 138.4 ± 6.3 137.0 ± 6.6 143.8 ± 9.6 135.0 ± 7.9

150.0 ± 6.95 100.0 ± 10.45 1440 ± 30.0e − 122.0 ± 6.4g 4.9 ± 1.1 e − 112.4 ± 8.2 12.4 ± 2.5 − 115.6 ± 13.2 f 13.8 ± 2.2 f − 118.4 ± 5.4 10.0 ± 2.4 − 117.6 ± 4.6 f 13.2 ± 3.3 120.0 ± 7.2e 124.0 ± 9.3 f 16.2 ± 1.3 60.6 ± 6.8 f 104.6 ± 9.2e 45.0 ± 2.2

Dose = 2.5 mg kg−1 i.v.; b No effect on carotid occlusion and noradrenaline pressor responses; c Inhibited CO and NA responses; d No effect on carotid occlusion but inhibited NA response; e p < 0.001; f p < 0.05; g p < 0.01; SE denotes the standard error; HR denotes heart rate (bpm); ↓decrease in heart beat rate, ↑ increase in heart beat rate

a

the blood composition, and resulted in a decrease in organ/heart ratio in rats/mice. They also inhibited Mg2+ -ATPase (o.s.) and Ca2+ -ATPase activities in beef heart mitochondria, basal- and isoproterenolstimulated cardiac sarcoplasmic reticulum (SR) 45 Ca2+ uptake and Ca2+ -ATPase in vivo as well as in vitro in rat heart ventricular membrane vesicles, and sodium pump activity, as well as ATP synthesis. These studies indicated that triorganotin compounds may be affecting Ca2+ pumping mechanisms through the alteration of phosphorylation of specific proteins corresponding to phospholamban in rat cardiac SR and thus interfering with cardiac function, since SR Ca2+ and Na+ transport are involved in cardiac function. Several di- and triorganotin derivatives of amino acids, dipeptides, triglycine, and umbelliferrone showed potent anti-hypertensive activity comparable to that of Captopril with or without affecting the carotid occlusion (CO) and noradrenaline (NA) pressure responses and heart rate, but the duration of efficacy is much shorter than that for Captopril. Since these compounds have low toxicity (LD50 >1000–2000 mg kg−1 ), they may be considered as good anti-hypertensive drugs. References 1. G. J. M. van der Kerk, Organotin Chemistry: Past, Present, and Future, in Organotin Compounds, New Chemistry and Applications, J. J. Zuckerman (Ed.), Advances in Chemistry Series 157, American Chemical Society, Washington, DC, 1976. 2. K. Fent, Crit. Rev. Toxicol., 26, 1 (1996). 3. Tin Compounds, in Kirk-Othmer, Encyclopedia of Chemical Technology, J. I. Kroschwitz (Ed.), 4th Edn, Vol. 24, John Wiley & Sons, Inc, New York, 1997. 4. R. C. Poller, The Chemistry of Organotin Compounds, Logos Press Limited, London, 1970. 5. D. Liu, W.-C. Chen, G.-L. Shen, and R.-Q., Yu, Analyst, 121, 1495 (1996). 6. I. Tsagkatakis, N. Chaniotakis, R. Altmann, K. Jurkschat, R. Willem, J. C. Martins, Y. Qin, and E. Bakker, Helv. Chim. Acta, 84, 1952 (2001). 7. J. Duncan, Pharmacol. Ther., 10, 407 (1980).

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8. S. J. Blunden, P. A. Cusak, and R. Hill (Eds), The Industrial Uses of Tin Chemicals, Royal Society of Chemistry, London, UK, 1985. 9. W. T. Piver, Environ. Health Perspect., 4, 61 (1973). 10. M. Hoch, Appl. Geochem., 16, 719 (2001). 11. P. J. Craig, G. Eng, and R. O. Jenkins, Occurrence and Pathways of Organometallic Compounds in the Environment – General Considerations, in Organometallic Compounds in the Environment, 2nd Edn, P. J. Craig (Ed.), John Wiley & Sons, Ltd., Chichester, 2003. 12. J. T. Byrd and M. O.Andrae, Science, 218, 565 (1982). 13. A. Fait, A. Ferioli, and F. Barbieri, Toxicology, 91, 77 (1994). 14. P. J. Smith, Toxicological Data on Organotin Compounds, International Tin Research Institute Publications, Uxbridge, 538 (1978). 15. S. Evans and G. J. Nicholson, Sci. Total Environ., 258, 73 (2000). 16. K. Fent, Toxicol. Lett., 140–141, 353 (2003). 17. W. N. Aldridge, B. W. Sreet, and D. N. Skilleter, Biochem.J ., 168, 353 (1977). 18. K. S. Prasada Rao, C. S. Chetty, C. H. Trottman, J. E. Uzodinma, and D. Desaiah, Cell Biochem. Funct.,3, 267 (1985). 19. A. H. Penninks, F. Kuper, B. J. Spit, and W. Seinen, Immunopharmacology, 10, 1 (1985). 20. N. J. Soneij, A. H. Penninks, and W. Seinen, Int. J. Immunopharmacol., 10, 891 (1988). 21. N. J. Soneij, A. A. van Iersel, A. H. Penninks, and W. Seinen, Toxicol. Appl. Pharmacol., 81, 274 (1985). 22. D. W. Rosenburg, G. S. Drummond, and A. Kappas, Mol. Pharmacol., 21, 150 (1982). 23. H. B. Stoner, J. M. Branes, and J. I. Duff, Br. J. Pharmacol. Chemotherapy, 10, 16 (1955). 24. Y. Hiroshi and I. Michiko, Bull. Natl. Inst. Ind. Health, 7, 7 (1962). 25. R. Truhaut, Y. Chauvel, J. P. Anger, L. N. Phu, J. van den Driessche, and L. R. Guesnier, Euro. J. Toxicol. Envir. Hyg., 9, 31 (1976). 26. G. Tauberger, Med. Exp., 9, 393 (1963). 27. H. G. Verschuuren and G. J. van Esch, Unpublished Report of the National Institute of Public Health, Utrecht, 1964. 28. H. G. Verschuuren, G. J. van Esch, and A. M. Arnoldussen, Unpublished Report of the National Institute of Public Health 161/162, Utrecht, 1962. Refs. 27 and 28 are cited in WHO (1965), Evaluation of the Toxicity of Pesticide Residues in Food, Food and Agriculture Organization of the United Nations, FAO Meeting Report No. PL/1965/10, WHO/FOOD Add./26.65. 29. B. D. Mehrotra, K. S. Prasada Rao, and D. Desaiah, Toxicol. Lett., 26, 25 (1985). 30. P. D. Boyer, B. Chance, L. Ernester, P. Mitchell, E. Racker, and E. C. Slater, Ann. Rev. Biochem., 46, 955 (1977). 31. R. P. Holmes, M. Mahfouz, B. D. Travis, N. L. Yoss, and M. J. Keenan, Ann. N. Y. Acad. Sci., 414, 44 (1983). 32. I. Ishaaya, J. L. Engel, and J. E. Casida, Pest. Biochem. Physio., 6, 270 (1976). 33. E. I. Krajnc, P. W. Wester, J. G. Loeber, F. X. R. van Leeuwen, J. G. Vos, H. A. M. G. Vaessen, and C. A. van der Heuden, Toxicol. Appl. Pharmacol., 75, 363 (1984). 34. M. Tada, T. Yamamoto, and Y. Tonomura, Physiol. Rev., 58, 1 (1978). 35. C. F. Louis, M. Maffitt, and B. Jarvis, J. Biol. Chem., 257, 15182 (1982). 36. M. Tada and M. Inui, J. Mol. Cell Cardiol., 15, 565 (1983). 37. L. R. Jones, S. W. Maddock, and D. R. Hathaway, Biochim. Biophys. Acta, 641, 242 (1981). 38. E. G. Kranias, F. Mandel, T. Wang, and A. Schwartz, Biochemistry, 19, 5434 (1980). 39. I. K. Sahib and D. Desaiah, Cell Biochem. Funct., 5, 149 (1987). 40. I. Kabeer, A. Sahib, and D. Desaiah, J. Biochem. Toxicol., 1, 55 (1986). 41. P. R. S. Kodavanti, J. A. Cameron, P. R. Yallapragada, P. J. S. Vig, and D. Desaiah, Arch. Toxicol., 65, 311 (1991). 42. J. A. Cameron, P. R. S. Kodavanti, S. N. Pantyala, and D. Desaiah, J. Appl. Toxicol., 11, 403 (1991). 43. J. J. Kang, S. H. Liu, I. L. Chen, Y. W. Cheng, S. Lin, and Y. Shoei, Pharmacol. Toxicol., 82, 23 (1998). 44. J. J. Kang, I. L. Chen, and Y. W. Cheng, J. Biochem.(Tokyo), 122, 173 (1997). 45. S. Nicholson, Aust. J. Ecotoxicol., 9, 137 (2003). 46. M. Nath, R. Jairath, G. Eng, X. Song, and A. Kumar, Spectrochim. Acta Part A, 62, 1179 (2005).

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47. M. Nath, S. Pokharia, G. Eng, X. Song, and A. Kumar, J. Organomet. Chem., 669, 109 (2003). 48. M. Nath, S. Pokharia, G. Eng, X. Song, and A. Kumar, Synth. React. Inorg. Met.-Org. Chem., 34, 1689 (2004). 49. M. Nath, S. Pokharia, G. Eng, X. Song, A. Kumar, M. Gielen, R. Willem, and M. Biesemans, Appl. Organomet. Chem., 18, 460 (2004). 50. M. Nath, S. Pokharia, G. Eng, X. Song, and A. Kumar, Eur. J. Med. Chem., 40, 289 (2005). 51. M. Nath, S. Pokharia, G. Eng, X. Song, and A. Kumar, Spectrochim. Acta, 63, 66 (2006). 52. M. Nath, R. Jairath, G. Eng, X. Song, and A. Kumar, Inorg. Chem. Commun., 7, 1161 (2004). 53. M. Nath, R. Jairath, G. Eng, X. Song, and A. Kumar, Spectrochim. Acta, 61, 77 (2006). 54. M. Nath, R. Jairath, G. Eng, X. Song, and A. Kumar, Spectrochim. Acta, 61, 3155 (2006). 55. M. Nath, R. Jairath, G. Eng, X. Song, and A. Kumar, J. Organomet. Chem., 690, 134 (2005).

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4.2

Organotins: Insecticidal/Larvicidal Activities and Quantitative Structure–Activity Relationships

George Eng and Xueqing Song Department of Chemistry and Physics, University of the District of Columbia, Washington, DC, USA

4.2.1

Overview: Organotins

Organotin (IV) compounds characterized by the presence of one or more carbon–tin bonds have various biocidal activities, and show a toxicity which is dependent on both the number and nature of the organic groups attached to the tin atom. In general, for the series of organotins, Rn SnX4-n , where n = 1–4, those containing three Sn C bonds have the highest biological activities, and the toxicity tends to decrease with the successive decrease in the number of organic groups attached to the tin atom. Thus, diorganotins are more toxic than their monoorganotin analogs, which are considered to be non-toxic.1,2 On the other hand, toxicity of tetraorganotins arises from the decomposition of these compounds into their triorganotin derivatives.1,2 Triorganotin compounds are toxic to various organisms and their biocidal properties are well documented in the literature.1−5 Toxicity of triorganotin compounds has also been reported to be speciesspecific and is a function of the organic group attached to the tin atom.1−5 For example, trimethyltins have a high toxicity towards insects and mammals, triethyltins are most effective against mammals, tri-npropyltins are effective against Gram-negative bacteria, while tri-n-butyltin and triphenyltin compounds are effective against fungi. Aquatic species such as fish and molluscs are sensitive to tri-n-butyltins and triphenyltins. While triorganotins are toxic against various mosquitoes and their larvae, there does not appear to be a definitive single R group that is most active, but three organic groups that have been shown to have high activities are the n-butyl, phenyl, and cyclohexyl groups.6−8 Increasing the chain length has been reported to decrease the activity of trialkyltins, with tri-n-octyltins being effectively non-toxic to all living organisms.9 Early reports indicated that the anionic group X, within a series of R3 SnX compounds, does not play a major role in the toxicity of the compounds,1−5 and this is the generally accepted view. Reports, however, have indicated that if the X group itself is biologically active, then the activity of the compound may be enhanced.2,5 Increased activities of R3 SnX compounds have also been reported when the X group increases the aqueous solubility of the compound.10 On the other hand, complexes where the X group forms a five-coordinated chelated triorganotin monomer, or a polymer, tend to have decreased activities.11 More interestingly, a study on the insecticidal effects of triorganotins on the diamondback moth, Plutella xylostells (Linnaeus), indicated that changes in the anionic X group had both effects. Changes in the X group for a series of tricyclohexyltins had little effect on their activities while significant changes were observed for a series of triphenyltins in the same study.12 Thus, the effect of the anionic group X, within a particular series of R3 SnX compounds, has been cited as being both insignificant and important. Replacing the alkyl groups with aryl or cyclohexyl substituents converts the triorganotin compounds into effective agricultural fungicides due to their anti-feedant and/or less phytotoxic behaviors.2,5,12 4.2.2

Larvicidal/Insecticidal Activities

The literature contains extensive studies, as well as reviews, on triorganotin pesticidal12–18 activities, with the majority being focused on their agrochemical applications. Thus, the focus of this section will be on organotin larvicidal/insecticidal activities pertaining to insects of non-agricultural importance.

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One of the first reported claims of organotin compounds as having entomological properties was in a series of patents citing the mothproofing capabilities for this class of compounds.19−22 The compounds included both alkyl and aryl organotin derivatives. As early as 1964, a comprehensive review of the insecticidal applications of organotins was compiled by Ascher and Nissim, which included both agricultural and non-agricultural insect species.23 Various non-agricultural insects discussed included houseflies and mosquitoes. The anti-feedant and chemosterility properties of organotins to various species of insects were also discussed.23 In light of the many biocidal properties of triorganotins, various investigators have attempted to tailor this class of compounds to be more species-specific, as well as to increase their biocidal properties. Two non-agricultural species that are commonly used to evaluate the insecticidal efficacy of triorganotins are flies and mosquitoes. Many methodologies have been used to evaluate the efficacy of triorganotins on these two insects. A common unit of activity, in parenthesis, has been included with the original data for easier comparison. Flies

While flies are pollinators, they are generally regarded as a nuisance. In addition, adults of many species are capable of carrying various pathogens that cause a host of human diseases. Thus, excessive fly populations are not only an annoyance, but may pose a public health issue. Flies are also often used as a test medium in organotin chemistry. There are many genera of flies and the one most commonly used in organotin studies is the Musca. Musca: The housefly, Musca domestica Linnaeus, is a species that is commonly associated with humans or human activities. Studies by Blum and Bower24 showed that triethyltin hydroxide and several of its esters, applied topically, caused rapid paralysis and death of houseflies with dosages ranging from 0.25 to 1.28 μg fly−1 (1.0 to 3.5 × 10−9 mol fly−1 ). The effective dosage was dependent on whether the flies were DDT-susceptible or resistant. A later study by these authors using 42 organotin compounds indicated that maximum toxicity against houseflies was achieved with triorganotins, R3 SnX, with the anionic X group playing a minor role in the toxicity.25 The LD50 (the dose that kills 50% of the test population) values reported for the compounds, in mol fly−1 , ranged from a low of 4.5 × 10−10 to a high of 24.0 × 10−10 . Similar findings that triorganotins were the most effective against houseflies were also reported by Saxena and Crowe.26 Kochkin et al.27 determined that the most effective compound against various insects, including houseflies, was trimethyltin acetate with a minimal concentration of 0.01 g m−2 (5 × 10−5 mol m−2 ). Georghiou et al.28 showed that trimethyltin chloride was effective against an insecticide-susceptible strain of housefly, as well as for three insecticide-resistant strains. The LD50 value for the susceptible strain was 6.0 × 10−9 mol fly−1 , while the insecticide-resistant strains ranged from 5.1–8.0 × 10−9 mol fly−1 . On the other hand, houseflies have also been reported to be resistant to triorganotin insecticides, such as tri-n-butyltin chloride.29 Insecticidal properties against houseflies were also found for a series of triorganotin derivatives of cyclic olefins and hydrocarbyl-substituted cyclic olefins30 using a micro-drop technique. For example, 0.02 g ml−1 or 2 × 104 parts per million (ppm) of the triphenyltin derivatives, when applied to the thorax or abdomen of the fly, had kill rates between 96–99%. A 0.2% (2 × 103 ppm) test solution of tetraorganotins containing a vinyl group had an effective mortality rate between 90 and 100% for 29 of 33 compounds after a 24 h period.31 The effectiveness of a series of trialkylstannyl-1,2,4-triazoles was also evaluated against houseflies at 5 × 102 ppm and ranged from 0 to 100% depending on the organic group attached to the tin atom.32 A series of tricyclohexyltin thiophosphates33 was effective, with LD50 values ranging from 50 to 100 μg per 25 flies (2–3 × 10−9 mol fly−1 ) of the toxicant with one exception. Several trimethylstannylmethyl ethers were found to be active against houseflies, and the insecticidal mode of action was reported to

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be similar to that of pyrethroid insecticides, rather than of organotin insecticides.34 Diorganotins, such as a series of dialkyltin diphenoxides, have also been found to be active against houseflies.35 Not all ‘designer’ triorganotin compounds, however, were found to be effective. For example, the pesticidal activity of the tricyclohexyltin derivative of 2-mercapto-benzothiazole was significantly lower than that of organic pesticides, such as chlorpyrifos and permethrin.36 Mosquitoes

Another insect that is commonly used in the evaluation of the biocidal activities of triorganotins is the mosquito. Mosquitoes are one of the most important blood-sucking arthropods in the world. They are not only annoying, but are responsible for the transmission of various diseases. For example, certain species of Aedes mosquitoes are responsible for the transmission of yellow fever, dengue, and other pathogenic viruses, while mosquitoes in the genus Anopheles are vectors of malaria. A third group is the Culex mosquitoes, which are important in the transmission of West Nile virus. While, from 1928 to 1930, organotins were reported as being effective mothproofing agents,19−22 it was not until several decades later that the efficacy of organotin compounds against mosquitoes was investigated. A series of di- and triorganotins was screened against an unspecified species of mosquitoes by Kochkin et al.,27 and showed that the most effective compound was trimethyltin acetate, with 100% mortality at a concentration of 0.01 g m−2 (5 × 10−5 mol m−2 ). Another study, which also did not specify the mosquito species, concluded that two of the 10 dichloromaleimide triorganotins tested at the 10 ppm levels against adult mosquitoes had a 100% mortality rate, while the others ranged between 0 and 70%,37 the toxicity being dependent on the alkyl groups attached to the tin atom. Mosquito larvae are often used in evaluating the toxicity of triorganotin compounds. The growth development of larvae is referred to as larval stages, of which there are four. For example, the first larval stage is known as the first instar, with each successive moulting leading to the next instar stage. Upon maturity, the fourth instar larvae molts into the pupal stage from which the adult mosquito emerges directly. Aedes: With the reporting that triorganotins were effective in combating mosquito larvae, investigators have attempted to develop other organotin compounds for this mission. Mosquito larvae are often used to evaluate the effectiveness of triorganotins. For example, a series of triorganotin borate compounds was found to be effective against Aedes aegypti (Ae. aegypti) mosquito larvae, even though boric acid itself was not effective. Tris(triphenyltin) borate had a 100% mortality at a concentration of 0.2 ppm while boric acid had a zero kill rate at a concentration higher than 0.2 ppm.38 A series of hexaalkylditins showed a range of mortality against the Ae. aegypti larvae, depending on the concentrations of the toxicants; the most effective was hexamethylditin with a 100% mortality after a 48 h exposure to a concentration of 0.1 ppm,39 but, at this test concentration, no mortality was reported for the other homologs. An emulsion, as well as a wettable powder of hexamethylditin, also showed 100% mortality, but the concentration needed to achieve these results was 10 times higher. Tricyclopentyltin derivatives screened against the Ae. aegypti larvae showed that tricyclopentyltin bromide, at a concentration of 1 ppm, was toxic to 95% of the third or early fourth instar larvae.40 Another class of triorganotins that are effective mosquito larvicides are the trimethyltin sulfinates, (CH3 )3 SnOS(O)R, where R is a substituted phenyl ring.41 A series of 11 triorganotin compounds was evaluated against the Ae. aegypti larvae in the fourth instar stage; the most effective compounds were those in which the R group was either 4-bromophenyl or 4-cyclohexylphenyl, which demonstrated 100% effectiveness at the 1 ppm level. With a 10-fold increase in concentration, all the compounds showed 100% effectiveness, with the exception of the phenyl substituent (82%).

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Attempts have been made to prolong the biocidal activities of organotins by incorporating them into a rubber matrix and allowing them to be released slowly. For example, rubber compositions containing organotin additives such as bis(tri-n-butyltin) oxide were reported to have a 100% kill rate of mosquito larvae at concentrations of less than 1 ppm.42 Using tri-n-butyltin fluoride (TBTF) as the active agent in another slow-release study, the reverse phenomenon was observed for the LC50 (the concentration that kills 50% of the test organisms) values for the third instar stage larvae of Ae. aegypti.43 The LC50 dosage was reached within two days at concentrations of 0.02 and 0.1 ppm TBTF, but at a concentration of 0.2 ppm TBTF, the LC50 dosage was not reached until seven days. This reverse phenomenon was attributed to the ability of the organism to detect chronic intoxication at the higher concentration and set up a defence mechanism. At the lower concentration, chronic intoxication was not detected and no defence mechanism was initiated until it was too late.43 The experiment was repeated using second instar stage larvae and an LC50 value was obtainable after 24 h. None of the larvae survived for more than five days, and none developed into the third instar stage, suggesting that the compounds interfered with the protein synthesis of the target vectors.43 Another series of tetraorganotins in which the tin atom is bonded to a vinylic carbon atom also showed good activity against the early fourth instar stage larvae of the Ae. aegypti mosquito. With the exception of one compound, all showed final mortality rates between 90 and 100%, with solutions containing up to 3 ppm of the compound.31 A larvicidal study involving 25 triorganotins indicated that the most effective compounds were dimethyl-n-octyltin and diethyl-n-octyltin acetates6 followed by tris-( ptolyltin) chloride and their pyridine and triphenylphosphine oxide adducts. The LC50 values were 0.095 ppm or less for the acetates and ranged from 0.11 to 0.19 ppm for the p-tolyltins. Fifteen triorganotin compounds were evaluated against the fourth instar stage of the Ae. aegypti larvae.44 The LC50 values were between 0.84 and 3.35 ppm,44 and were dependent on the organic group, with the following order of activity: n-butyl > phenyl > cyclohexyl > methyl. Several series of di- and triorganotin carboxylates were also screened.45,46 The triorganotin carboxylates had activities of an order of magnitude higher than those of the diorganotin derivative.45 It was also determined that the carboxylates were not as effective as organophosphorus insecticides. Their advantages lie in their biodegradability to a non-toxic tin species and the lack of known resistance of mosquitoes to organotins.45,46 Recently, a series of tris(para-substituted-phenyl)tins was screened against the Ae. aegypti larvae.47 Compounds with a fluoro substituent on the phenyl ring were the most active while those with a CH3 Ssubstituent were the least effective. The fluoro substituents had an average LC50 value of 0.71 ppm, while the values for the least effective CH3 S- compounds were an order of magnitude higher. A statistical test showed that there was no significant difference between the efficacies of triphenyland tricyclohexyl-tin dithiocarbamates at the 95% confidence level.48 The authors further concluded that the toxicity was dependent on both the compound and species of mosquito larvae involved. A similar conclusion was drawn for the toxicity evaluations between several series of triorganotin carboxylates and the second instar larval stage of the Ae. aegypti mosquito.49−51 Two series of novel triorganotins esters of modified pyrethroid acids were found to be effective against the Ae. aegypti larvae,52,53 but no definitive order of toxicity was observed, based on the organic group attached to the tin atom. Tin(II) complexes have also been screened against this species of mosquito larvae;54 the organotin(II) complexes were not as effective as organotin(IV) complexes. Anopheles: Another species of mosquitoes that are transmitters of human diseases are the Anopholine mosquitoes. They are vectors for human malaria, but the literature contains relatively few studies on this species of mosquitoes and organotin compounds. Fourteen triorganotins were screened against the second to the fourth instar larval stages of the Anopheles stephensi (An. stephensi) mosquitoes, with tricyclohexyltin compounds being the most effective, with

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a LC50 value of less than 0.01 ppm8 The larvicidal activities of the triorganotins were also compared to other larvicides: tricyclohexyltin chloride and tri-n-butyltin chloride were comparable to some synthetic insecticides. The efficacy of the compounds declined from the second to the fourth instar larval stages, and this decrease was attributed to the formation of a thicker chitin covering. Attempts to design more effective larvicides/insecticides to combat various species of mosquitoes have resulted in new classes of triorganotin compounds being synthesized and screened. For example, a series of tris(para-substituted-phenyl)tins, (X-C6 H4 )3 SnY, where X = Cl-, F-, CH3 -, SCH3 - and Y = Cl− , OH− and OAc− ), was screened against An. stephensi larvae.47 Compounds with the highest toxicity were those that contained a single atom substituent on the phenyl ring, and the efficacy of the compounds was related to the size of the para-substituent attached to the phenyl ring rather than on the anionic Y group. Insecticidal activities on the larvae of this species of mosquito have also been determined for a series of triphenyl- and tricyclohexyltin dithiocarbamates.48 The study showed that the dialkyldithiocarbamate derivatives were more toxic than the monoalkyldithiocarbamate compounds. Some tri-n-butyltin complexes,49,50 and a series of triphenyl- and tricyclohexyltin para-substituted benzoates51 were effective larvicides against the An. stephensi larvae. Two series of triorganotin esters of modified pyrethroid acids were effective against the second instar stage of this species of mosquito larvae,52,53 but no common order of activity was observable, based on the organic group attached to the tin atom, as was observed with the Aedes aegypti mosquito larvae.52,53 Other studies involving An. stephensi larvae were residual and delay evaluations. The residual activity of a compound is a measure of its effectiveness over a given time period, while delay studies give an indication of the long-term effects of the compounds. Both residual and delay studies were conducted on the An. stephensi larvae employing a variety of triorganotins.55 The residual activity studies indicated that the compounds were effective between one to 10 weeks with triphenyltin chloride having the longest effect, and the residual effectiveness of several of the compounds was comparable to or better than that of some commonly used larvicides. Delayed effect results indicated that triphenyltin chloride was the most effective, delaying pupation until the tenth week. Adulticidal studies using An. stephensi mosquitoes have also been investigated. A series of organotin compounds which included tri-n-butyltins, phenyltins, and tricyclohexyltin hydroxide was tested against the adult females of this species; the triorganotins were the most effective, the tri-n-butyltins were more so than the triphenyltins, followed by tricyclohexyltin hydroxide.26 In a later study, 19 commercial triorganotins (R3 SnX) were evaluated against the An. stephensi adult mosquitoes.55 The activities were a function of the organic group attached to the tin atom in the order trimethyl > tri-n-butyl > triphenyl > tricyclohexyl. The anion group played a significant role in the toxicity of the compounds, with the fluoro derivatives being the most active.55 Culex: Another species of mosquitoes is the Culex pipiens, which is the northern house mosquito, and is the most common species found in urban areas. It is responsible for the transmission of West Nile Virus. In an early study, Castel et al.7 screened a series of organotins against the larvae of the Culex pipiens berbericus mosquito; the most toxic compounds were those that contained three alkyl/aryl groups, and triphenyltin chloride was the most effective with an LC50 value of 0.25 ppm. However, these were less effective than insecticides such as DDT, lindane, and malathion by a factor of 10. Subsequently, a study by Gras and Rioux evaluated a series of 24 organotins against this same species of mosquito larvae.56 Again, the most effective compounds were the triorganotins. Most of the R3 SnX compounds had LC50 values less than 0.50 ppm, and variations in the anion group on the tin atom did not play a role in the toxicity. Attempts to develop a controlled release of triorganotins as a long-term larvicide resulted in impregnating rubber matrices with various triorganotins. The incorporation of tri-n-butyltins into several rubber

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materials was found to be of little value for the larvicidal activities against the Culex pipiens quinquefasciatus (Cx. P. quinquefasciatus) larvae.57 Boike and Rathburn58 reported that when neoprene rubber was incorporated with bis(tri-n-butyltin) oxide, it was toxic against mosquito larvae when immersed in clear tap water, but its effectiveness was reduced when the tap water contained organic debris. Tri-n-butyltin fluoride was found to be an effective larvicide agent against the first and second instar larval stages of Cx. P. quinquefasciatus larvae, when incorporated into a thermoplastic matrix.59 Tri-n-butyltin fluoride and bis(tri-n-butyltin) oxide were effective as a controlled releasing agent against the same species,60 and the LT100 (lethal time for 100% mortality) data observed for these compounds were similar to those of pesticides such as temephos and chlorpyrifos. A series of triorganotin silatrane derivatives, at a concentration of 3.5 ppm, was evaluated against Culex pipiens larvae.61 The death rates varied from less than 10% to a high of over 90%. Emergence inhibition rates, in some cases, were greater than 90%, suggesting that compounds with low kill rates would not be good candidates as larvicides. Recently, a series of triorganotin butyrates and cyclopropanecarboxylates were reported to be effective against the second instar stage of the Cx. P. quinquefasciatus mosquito.52,53 The toxicities, in ppm, ranged from 0.39 to 3.21 for the butyrates,52 while the cyclopropanecarboxylate compounds had toxicities that ranged from 0.31 to 1.09 ppm.53 Summary:

r Organotins are effective larvicides/insecticides against various species of non-agricultural insects, such as flies and/or mosquitoes. r Compounds with three organic groups attached to the tin atom have the highest toxicity. r Organic groups that have high activities against mosquitoes include: triphenyl, tricyclohexyl, and trin-butyl. r Triorganotin toxicity is more dependent on the R group than the X group. r Effectiveness decreases with later larval stages. r Toxicity towards mosquitoes depends on the species of mosquito. r Houseflies have been reported to be resistant to some triorganotins. r No mosquito species have been reported to be resistant to any organotin compound. 4.2.3

Quantitative Structure–Activity Relationships

The quantitative structure–activity relationship (QSAR) is a common technique used to assist in the development and/or design of a compound to meet the requirements for a specific application. QSAR is a regression equation that relates a measurable biological property of a compound to a molecular descriptor(s) of the chemical. The success of a QSAR will depend on the quality of the data set and on the suitability of the descriptor(s) selected. Molecular descriptors can be classified into various types of parameters including: physicochemical, topological, geometrical, and electronic.62 QSAR methodologies, as well as the various types of descriptors, have been reviewed,62−67 and there are numerous publications in the literature, as well as thousands of web sites pertaining to this topic.64 The pharmaceutical industry was one of the first to employ this technique to reduce the cost of developing and testing new drugs, which can cost thousands to hundreds of thousands of dollars depending on the tests and time involved.63,68 Another common use of QSARs is in the area of environmental risk assessments. During the past few decades, the use of organotins has increased dramatically, most likely due to their diverse biocidal properties. In fact, organotins have a higher commercial usage than any other organometallic system.5 In turn, this has led to an increased concern about the fate of the compounds and their degradation

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products as environmental pollutants. Thus, the development of reliable QSARs for organotins is of great value. It would not only allow prediction of the toxicity of new and/or untested compounds, but would also assist in the prioritization of which compounds would be best suited for initial trials. Early correlation studies have employed descriptors such as Hammett sigma constants and Taft parameters.63 Octanol/water partition coefficient parameters (P), which are more closely associated with biological activities, were later developed by Leo et al.69 and are one of the more common used descriptors in biological QSAR studies.63 With the rapid advances in current computer technology, the calculations of various descriptors for new and/or untested compounds have become commonplace. Currently, there are commercial computer programs dedicated to this specific application, while others contain a section devoted to QSAR methodology. For organotin compounds, the vast majority of QSAR studies are related to their biological activities. Schönfelder and Thust used Free–Wilson as well as Hansch analyses to develop a series of QSARs for the acaricidal and algicidal activities for a series of organotins, R3 SnR , where R = alkyl/aryl group and R = ligand.70 Free–Wilson analysis is a regression technique, which uses the presence or absence of substituents/groups on a molecule as the only descriptor in the QSAR. The Free–Wilson analyses yielded correlation coefficients r greater than 0.89, while the Hansch equations using hydrophobic, electronic, and steric parameters gave coefficients between 0.87 and 0.99, depending on the compounds. It was further determined that the fourth group (R ) had little effect on the activities of the compounds. Using the Hansch log P parameters, Wong et al.71 were able to generate a good QSAR between the partition coefficients (log P) of a series of triorganotins and their toxicities against the freshwater alga, Ankistrodesmus falcatus (A. falcatus). Studies involving a series of di- and triorganotins with zoeal mud crabs, Rhithropanopeus harrisii (R. harrisii),72−74 showed good linear correlations between the natural log of the LC50 values and the Leo fragment constant,72 the Hansch π parameter,73,74 and the total surface area73,74 of the compounds. For example, the results of the regression analyses using the Hansch π parameters yielded a correlation coefficient, r 2 , of 0.94 for the triorganotins and 0.97 for the diorganotins, while the total surface area descriptor resulted in a correlation coefficient value of 0.94 for the triorganotins.74 Correlation coefficient values of 0.90 and 0.94 were obtained for the diorganotins, depending on the coordination of the tin atom. It was further concluded that the partitioning behavior of the organotins plays a major role in the toxicity of the compounds. Vighi and Calamari75,76 used descriptors such as pK a and log P values, as well as the first-degree molecular connectivity indices, to develop QSARs between these descriptors and the toxicity, for a series of organotins against Daphnia magna. The equations generated gave significant correlations and high predictive capabilities. While the first-degree molecular connectivity indices can be correlated to the toxicity of the compounds, Singh and Sharma77 reasoned that the third connectivity indices should also play a major role in the toxicity of a compound. This was confirmed in their study between a series of trialkyltin acetates and fungi. The authors were able to obtain good correlations between four fungi and a series of trialkyltin acetates using the third connectivity indices of the molecules. The toxicity for a series of di- and triorganotins was also found to correlate well with the hydrophobic characteristics (log P or Hansch π) of the compounds against two mammalian cell lines (BALB/c mouse fibroblast 3T3 and mouse neuroblastoma N2 a cells).78 The sequence of the cytotoxicity for the organotins was similar to those observed in earlier studies.71,74,75 Another study by Babich and Borenfreund79 using bluegill sunfish BF-2 cell lines showed that there was a direct linear correlation between the cytotoxicity of a series of diorganotins and the lipophilicity of the compounds, with a correlation coefficient of 0.958. A QSAR approach was used to determine the anti-tumor activities of a host of organotins.80 Several classes of diorganotins were screened against P-388 lymphocytic leukaemia in mice. The study showed

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that the activities of the compounds could be correlated to the lipophilicity or log P value of the organic group attached to the tin atom. Descriptors and/or QSARs that previously required mainframe computer time to calculate, or were not possible at all, can now be routinely done on a desktop computer. A common descriptor that is easily calculable is the total surface area (TSA), since a broad database of bond distances, angles, and van der Waals radii is readily available in the literature. Using literature values, Brinckman et al.73,74 were able to calculate the TSA values of various individual organotin molecules. Calculations for the individual molecules involved various degrees of coordination, charge, or likely conformations of the compound. In addition to the calculation of the individual organotin molecules, the mean fragment TSA values for several organic groups and labile inorganic ligands were also assembled by Brinckman.81 In addition to the study on R. harrisii,73,74 Brinckman et al. found a good correlation between a series of triorganotins and A. falcatus81 using TSA values as the descriptor. A similar finding was observed for the uptake of trialkyltin chloride by Escherichia coli.82 In addition to the TSA correlation, the results indicated that the uptake also correlated well with the Hansch π gnd Taft–Hammett parameters. Eng et al. were able to find high linear correlations between several distinct types of organisms and a series of diorganotins, using TSA as the descriptor.83 The correlation coefficients ranged from 0.85 to 0.98, indicating that TSA is an excellent predictor of toxicity in organotin compounds, provided that the toxicity process is primarily related to the hydrophobicity of the compound. Another study by these authors indicated that TSA was again a satisfactory predictor of toxicity for a series of Group IVA organometals against the bacteria, Escherichia coli, and the alga, Selenastrum capricornutum,84 provided that the toxicity is a function of the hydrophobicity of the organometallic compound and that no solubility problems arise. Correlation coefficients for the various organometals ranged from 0.87 to 0.99. Boopathy et al.85 observed opposite results from those observed by Eng and Brinckman in their study involving methanogenic bacteria and a series of organotin chlorides and sulfates. Their study indicated that the toxicity of the compounds increased as the TSA of the molecule decreased. A similar finding, reported by Lascourrèges et al.,86 showed a negative correlation in their studies between a series of organotins and three pure strains of sulfate-reducing bacteria isolated from marine sediments. While TSA may be a good indicator of hydrophobicity,83 Boopathy et al.85 suggested that it might not be a uniform descriptor in predicting organotin toxicity to all organisms. Another topological descriptor closely related to the total surface area parameter is the molecular volume, since both descriptors use the radius of the molecule in their calculations. Luedke et al.87 show that both descriptors can be used with an equal level of confidence for a series of di- and triorganotins screened against several different types of organisms. Despite the different types of organism involved, the authors were able to generate regression equations with correlation coefficient values ranging from 0.745 to 0.996 for the diorganotins and 0.634 to 0.989 for the triorganotins. Huang et al.88 investigated the inhibition effects of 12 organotin compounds on two green algae, Scenedesmus obliquus and Platymonas sp. QSAR studies were performed using physicochemical and topological descriptors such as log P, TSA, and connectivity indices, and the authors concluded that the toxicity of the compounds is determined primarily by the lipophilicity of the compounds. Sun et al.89 also determined the toxicity of several organotin compounds against the rotifer Brachionus plicatilis. The authors concluded that the toxicity of the organotins was primarily dependent on the lipophilicity of the compounds, but that the electronic property also played a role. In addition to traditional parameters, other parameters have been employed in QSAR studies. For example, Nagase et al.90 were unable to generate acceptable linear correlations between common physicochemical and topological descriptors such as molecular weight, log P, molecular connectivity indices,

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Wiener number information content, etc. and the toxicity of 29 organotins against the red killifish, Oryzias latipes. The correlation coefficients ranged from –0.230 to 0.510, leading the authors to conclude that the toxicity of the compounds against the red killifish was not a function of the hydrophobicity of the organotin compounds. To solve this dilemma, a new descriptor, index value (IV), which is related to the number of phenyl or alkyl groups attached to the tin atom, was created. Using this parameter, excellent regression equations were obtainable to predict the toxicity of the compounds. The correlation coefficients using the new descriptor ranged from 0.857 to 0.907. In a later study, the haemolytic activities of 27 organotins were studied with rabbit erythrocytes.91 In addition to the IV parameter, a mean information and molecular connectivity index descriptor was used to develop an acceptable regression QSAR (0.854) for predicting the haemolytic effects of the organotins. The authors further suggested that the haemolytic activity due to the organotins might be related to the lethal factor in the earlier red killifish study.90 Todeschini et al.92 proposed three-dimensional molecular indices, WHIM (weighted holistic invariant molecular), which uses the whole molecular structure to predict the toxicity of 15 organotins against Daphnia magna. Quantum chemical descriptors derived from the PM3 Hamiltonian have also been used in QSAR studies.93 The toxicities of Daphnia magna and two green algae were correlated to various quantum chemical descriptors for a series of organotins. An increase in toxicity was observed for the organotins, when correlated with the energy of the lowest unoccupied molecular orbitals of the compound against Daphnia magna. A similar increase was observed using the highest occupied molecular orbitals against the green algae. The authors concluded that these findings indicated that the donor–acceptor interactions between the organotin and target organism played an important role in the toxicity mechanism.93 Computer programs dedicated to QSAR analyses or with a QSAR component have also been used to assist in the development of suitable QSARs for various applications. Using one such package, Samuel et al.94 were able to generate comparative molecular field analysis (CoMFA) models to predict the cytotoxicity of a series of dibenzyltin(IV) derivatives against two human cancer cell lines, MCF-7, a mammary carcinoma, and WiDr, a colon carcinoma. Using a different program, Eng et al. were able to generate various QSARs in their mosquito work.45,47,48,51,53 The first was for a series of triphenyl- and tri-n-butyltin complexes against the Ae. aegypti larvae.45 A reasonable QSAR was obtainable between the toxicity and the molecular connectivity indices of the compounds with a correlation coefficient of 0.82. However, the authors cautioned that the QSAR model should be used with care, due to the limited number of compounds used in generating the QSAR. Another acceptable QSAR was obtained between the Ae. aegypti larvae and a series of tris(parasubstituted-phenyl)tins. The toxicity of the compounds correlated well with the kappa shape index (k1) and the kappa alpha shape index (ka2), which is a modified version of the kappa shape index. Both of these indices are attributes related to the molecular shape encoded in the molecule.47 It was also possible to develop a QSAR between two descriptors of the molecules for a series of triorganotin dithiocarbamates and the toxicity of An. stephensi larvae.48 The two descriptors used were log P and the chi path cluster 4 index (χ pc4) of the molecule. The χ pc4 descriptor is related to the skeletal branching in the molecule. In that study it was concluded that the regression equation generated had a correlation coefficient of 0.815 with a cross validation of 65.08, indicating that the constructed model could be used, with care, to predict the LC50 values. A reasonable QSAR was also developed between the formula weight and valences third-order path chi index of a series of triphenyl- and tricyclohexyltin benzoates and the LC50 values of the An. stephensi larvae.51 The chi index is a parameter which encodes branching pattern information of the substitution on the ring. Cross validation of the training set also indicated that the constructed model could be used to predict the LC50 value for this series of compounds.

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In later studies, various QSARs were developed between a series of triorganotin carboxylates and three species of mosquito larvae, Ae. aegypti, An. stephensi, and Cx. P. quinquefasciatus.53 The best QSAR model was obtained for the Cx. P. quinquefasciatus larvae, with the toxicity of the compounds being related to the surface area of the molecule. For Ae. aegypti larvae, a QSAR was generated between the toxicity of the compounds and the principal moment of inertia along the X-axis of the molecule. The last mosquito larvae studied, An. stephensi, correlated well with the dipole moment of the molecule along the X-axis of the molecule. The fact that QSARs were obtainable using different descriptors of the molecules suggests that the interactions between the molecules and larvae are different for each species.53 The authors were also unable to generate a single QSAR for all three species of larvae, leading to the conclusion that the kill mechanism is different for each species of larvae. This hypothesis has been advanced for a series of triorganotin dithiocarbamates.48 While QSAR studies normally result in linear regression equations, this is not always the case. For example, using molecular descriptors, Schüu¨ rmann and Röderer95 were unable to correlate the toxicity of trialkyltins against the fungus, Botrytis allii, using a linear regression analysis. However, the authors were able to obtain a parabolic QSAR model with high correlation coefficients. Eng et al.,96 in a study involving a series of triaryltin chlorides and the fungus (Ceratocystis ulmi), the causative agent of Dutch elm disease, was also unable to generate linear correlation equations using either topological or partitioning parameters. However, a concave-down curve was obtainable between the inhibitory concentration of the compounds and Hammett sigma values of the substituents on the phenyl ring. Concave-down plots are obtained when there is a change in the rate-determining step in multistep processes. Summary:

r r r r

QSAR may assist in the designing of new and untested compounds. QSAR may assist in the estimation of various parameters/properties of a chemical. QSAR may assist in the determination of the environmental impact of a chemical. QSAR may reduce the cost in the development of a compound.

4.2.4

Food For Thought

There is a plethora of publications on the biological activities of organotins, while the number of publications pertaining to their mode of action is limited. Knowing the mode of action could assist in designing more effective compounds. In addition, organotins would be more environmental friendly if their aqueous solubility could be increased, thus reducing the amount of the compound needed to achieve the desired results. Ultimately these areas of research need to be investigated more thoroughly. References 1. D. P. Miller and P. J. Craig, in Chemistry of Tin, 2nd Edn, P. J. Smith (Ed.), Blackie Academic & Professional, London, 1998, and references therein. 2. S. J. Blunden and A. Chapman, in Organometallic Compounds in the Environment: Principles and Reactions, P. J. Craig (Ed.), John Wiley & Sons, Inc., New York, 1986, and references therein. 3. A. G. Davies and P. J. Smith, in Comprehensive Organometallic Chemistry, Vol. 2, G. Wilkinson, F. G. A. Stone, and E. W. Abel (Eds), Pergamon Press Ltd., Oxford, 1982, and references therein. 4. R. C. Poller, The Chemistry of Organotin Compounds, Academic Press, New York, 1970, and references therein. 5. S. J. Blunden, P. A. Cusack, and R. Hill, The Industrial Uses of Tin Chemicals, The Royal Society of Chemistry, London, 1985, and references therein. 6. V. G. Kumar Das, L. Y. Kuan, K. I. Sudderuddin, C. K. Chang, V. Thomas, C. K. Yap, M. K. Lo, G. C. Ong, W. K. Ng, and Y. Hoi-Sen, Toxicology, 32, 57 (1984).

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440 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. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48.

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Tin Chemistry: Fundamentals, Frontiers and Applications P. Castel, G. Gras, J.-A. Rioux, and A. Vidal, Trav. Soc. Pharm. Montpellier, 23, 45 (1963). G. Eng, C. Whitmyer, B. Sina, and N. Ogwuru, Main Group Met. Chem., 22, 311 (1999). P. J. Smith, International Tin Research Institute, Publication No. 538, 1 (1978). A. J. Kuthubutheen, R. Wickneswari, and V. G. Kumar Das, Appl. Organometal. Chem., 3, 231 (1989). S. J. Blunden, P. S. Smith, and B. Sugavanam, Pestic. Sci., 15, 253 (1984). N. W. Ahmad, S.-A. Mohd, S. Balabaskaran, and V. G. Kumar Das, Appl. Organometal. Chem., 7, 583 (1993). A. K. Saxena, Appl. Organometal. Chem., 1, 39 (1987). S. Nicklin and M. W. Robson, Appl. Organometal. Chem., 2, 487 (1988). A. J. Crowe, Appl. Organometal. Chem., 1, 143 (1987). A. J. Crowe, Appl. Organometal. Chem., 1, 331 (1987). B. Sugavanam, Tin and Its Uses, No. 126, 4 (1980). M. H. Gitlitz, in Organotin Compounds: New Chemistry and Applications, J. J. Zuckerman (Ed.), Advances in Chemistry Series 157, American Chemical Society, Washington DC, 1976, and references therein. I. G. Farbenindustrie AG, Great Britain Patent, 303 092 (1928). I. G. Farbenindustrie AG, Dutch Patent, 20 570 (1929). E. Hartmann, P. Kümmel, and M. Hardtman (for I. G. Farbenindustrie AG), German Patent, 485 646, (1929). E. Hartmann, M. Hardtmann, and P. Kümmel (for I. G. Farbenindustrie AG), US Patent, 1 744 633 (1930). K. R. S. Ascher and S. Nissim, World Rev. Pest Control, 3, 188 (1964), and references therein. M. S. Blum and F. A. Bower, J. Econ. Ent. 50, 84 (1957). M. S. Blum and J. J. Pratt, Jr. J. Econ. Ent. 53, 445 (1960). P. N. Saxena and A. J. Crowe, Appl. Organometal. Chem., 2, 185 (1988). D. A. Kochkin, V. I. Vashkov, and V. P. Dremova, J. Gen. Chem., Moscow, 34, 325 (1964). G. P. Georghiou, R. L. Metcalf, and E. P. Von Zboray, Bull. World Health Organ., 33, 479 (1965). R. F. Hoyer and F. W. Plapp, Jr., J. Econ. Ent., 61, 1269 (1968). J. P. Pellegrini, Jr. and I. J. Spilners, (for Gulf Research and Development Co.), US Patent, 3 519 666 (1970). R. H. Davis, M. E. Schroeder, and T. N. Mitchell (for Shell Internationale Research Maatschappij BV), Great Britain Patent, 2 112 644 (1983). X. Xie, C. Chen, Q. Xie, and X. Xu, Yingyong Huaxue, 9, 52 (1992). D. R. Baker (for Stauffer Chemical Co.) US Patent, 3 919 418 (1975). K. Tsushima, T. Yano, K. Umeda, N. Matsuo, M. Hirano, and N. Ohno, Pestic. Sci., 25, 17 (1989). Stauffer Chemical Co., Great Britain Patent, 1 048 918 (1964). K. C. Molloy, T. G. Purcell, D. Cunningham, P. McCardle, and T. Higgins, Appl. Organometal. Chem., 1, 119 (1987). Q. Xie and Y. Zhu, Yingyong Huaxue, 11, 96 (1994). G. Weissenberger (for Monsanto Co.), US Patent 3 312 725 (1967). H. Q. Smith and E. E. Ivy, (for Pennsalt Chemical Co.), US Patent 3 400 202 (1968). M & T Chemicals, Inc., Great Britain Patent, 1 581 269 (1980). R. J. Strunk and W. L. Hubbard (for Uniroyal, Inc.), US Patent 4 209 452 (1980). N. F. Cardarelli (for B. F. Goodrich Co.), US Patent 3 417 181 (1968). L. R. Sherman and J. C. Jackson, in Controlled Release of Pesticides and Pharmaceuticals, D. H. Lewis (Ed), Plenum Press, New York, 1981. T. T. Nguyen, N. Ogwuru, and G. Eng, Appl. Organometal. Chem., 14, 345 (2000). T. S. Basu Baul, S. Dhar, E. Rivarola, F. E. Smith, R. Butcher, X. Song, M. McCain, and G. Eng, Appl. Organometal. Chem., 17, 261 (2003). T. S. Basu Baul, K. S. Singh, X. Song, A. Zapata, G. Eng, A. Lycka, and A. Linden, J. Organometal. Chem., 689, 4702 (2004). X. Song, Q. Duong, E. Mitrojorgji, A. Zapata, N. Nguyen, D. Strickman, J. Glass, and G. Eng, Appl. Organometal. Chem., 18, 363 (2004). G. Eng, X. Song, Q. Duong, D. Strickman, J. Glass, and L. May, Appl. Organometal. Chem., 17, 218 (2003).

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49. T. S. Basu Baul, K. S. Singh, M. Holˇcapek, R. Jirásko, A. Linden, X. Song, A. Zapata, and G. Eng, Appl. Organometal. Chem., 19, 935 (2005). 50. T. S. Basu Baul, K. S. Singh, A. Lyˇcka, A. Linden, X. Song, A. Zapata, and G. Eng, Appl. Organometal. Chem., 20, 788 (2006). 51. Q. Duong, X. Song, E. Mitrojorgji, S. Gordon, and G. Eng, J. Organometal. Chem., 691, 1775 (2006). 52. G. Eng, X. Song, A. Zapata, A. C. de Dios, L. Casabianca, and R. D. Pike, J. Organometal. Chem., 692, 1398 (2007). 53. X. Song, A. Zapata, J. Hoerner, A. C. de Dios, L. Casabianca, and G. Eng, Appl. Organometal. Chem., 21, 545 (2007). 54. A. Jain, S. Saxena, A. K. Rai, P. N. Saxena, and J. V. Rao, Metal-Based Drugs, 6, 183 (1999). 55. N. Ogwuru, Q. Duong, X. Song, and G. Eng, Main Group Met. Chem., 24, 775 (2001). 56. G. Gras and J.-A. Rioux, Arch. Inst. Pasteur Tunis, 42, 9 (1965). 57. H. A. Schultz and A. B. Webb, Mosq. News, 29, 38 (1969). 58. A. H. Boike, Jr. and C. B. Rathburn, Jr., Mosq. News, 33, 501 (1973). 59. N. F. Cardarelli (for Environmental Chemicals, Inc.), US Patent 4 237 114 (1980). 60. N. F. Cardarelli, Mosq. News, 38, 328 (1978). 61. K. S. Osaka, T. N. Sakai, H. M. Minoo, K. K. Osaka, I. N. Kawanishi, and T. K. Takarazuka (for Nitto Kasei Co., Ltd.), US Patent 4 654 368 (1987). 62. R. Dagani, Chem. Engin. News, March 9, 26 (1981). 63. N. Nirmalakhandan and R. E. Speece, Environ. Sci. Technol., 22, 606 (1988) and references therein. 64. C. Hansch, D. Hoekman, A. Leo, D. Weininger and C. D. Selassie, Chem. Rev., 102, 783 (2002), and references therein. 65. D. J. W. Blum and R. E. Speece, Environ. Sci. Technol., 24, 284 (1990), and references therein. 66. C. D. Selassie, in Burger’s Medicinal Chemistry and Drug Discovery, 6th Edn, Vol. 1: Drug Discovery, D. J. Abraham (Ed.), John Wiley & Sons, Inc., New York, 2003, and references therein. 67. C. Hansch, D. Hoekman, and H. Gao, Chem. Rev., 96, 1045 (1996). 68. C. Hansch, S. H. Unger and A. B. Forsythe, J. Med. Chem., 16, 1217 (1973). 69. A. Leo, C. Hansch, and D. Elkins, Chem. Rev., 71, 525 (1971). 70. D. Schönfelder and U. Thust, in Chemical Structure-Biological Activity Relationships: Quantitative Approaches, F. Darvas (Ed.), Pergamon Press, New York, 1980. 71. P. T. S. Wong, Y. K. Chau, O. Kramar, and G. A. Bengert, Can. J. Fish Aquat. Sci., 39, 483 (1982). 72. R. B Laughlin, Jr. and O. Lindén, Ambio 14, 88 (1985). 73. R. B. Laughlin, Jr., W. French, R. B. Johannesen, H. E. Guard, and F. E. Brinckman, Chemosphere, 13, 575 (1984). 74. R. B. Laughlin, Jr., R. B. Johannesen, W. French, H. Guard, and F. E. Brinckman, Environ. Toxicol. Chem., 4, 343 (1985). 75. M. Vighi and D. Calamari, Chemosphere, 14, 1925 (1985). 76. M. Vighi and D. Calamari, Chemosphere, 16, 1043 (1987). 77. P. P. Singh and K. K. Sharma, Indian J. Chem., 32B, 551 (1993). 78. E. Borenfreund and H. Babich, Cell Biol. Toxicol., 3, 63 (1987). 79. H. Babich and E. Borenfreund, Fund. Appl. Toxicol., 10, 295 (1988). 80. R. Barbieri, Inorg. Chim. Acta, 191, 253 (1992). 81. P. J. Craig and F. E. Brinckman in Organometallic Compounds in the Environment, Principles and Reactions, P. J. Craig (Ed.), John Wiley & Sons, Inc., New York, 1986, and references therein. 82. F. E. Brinckman, G. J. Olson, W. R. Blair, and E. J. Parks, in Aquatic Toxicology and Hazard Assessment: 10th Volume, ASTM STP 971, W. J. Adams, G. A. Chapman, and W. G. Landis (Eds), American Society for Testing and Materials, Philadelphia, 1988. 83. G. Eng, E. J. Tierney, J. M. Bellama, and F. E. Brinckman, Appl. Organometal. Chem., 2, 171 (1988). 84. G. Eng, E. J. Tierney, G. J. Olson, F. E. Brinckman, and J. M. Bellema, Appl. Organometal. Chem., 5, 33 (1991).

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442 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96.

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Tin Chemistry: Fundamentals, Frontiers and Applications R. Boopathy and L. Daniels, Appl. Environ. Microbiol, 57, 1189 (1991). J. F. Lascourrèges, P. Caumette, and O. F. X. Donard, Appl. Organometal. Chem., 14, 98 (2000). E. Luedke, E. Lucero, and G. Eng, Main Group Met. Chem., 14, 59 (1991). G. Huang, H. Sun, and S. Dai, Bull. Environ. Contam. Toxicol., 58, 299 (1997). H.-W. Sun, G.-L. Huang, S.-G. Dai, and T.-Y. Chen, Toxicol. Environ. Chem., 60, 75 (1997). H. Nagase, T. Hamasaki, T. Sato, H. Kito, Y. Yoshioka, and Y. Ose, Appl. Organometal. Chem., 5, 91 (1991). T. Hamasaki, H. Masumoto, T. Sato, H. Nagase, H. Kito, and Y. Yoshioka, Appl. Organometal. Chem., 9, 95 (1995). R. Todeschini, M. Vighi, R. Provenzani, A. Finizio, and P. Gramatica, Chemosphere, 32, 1527 (1996). J.-W. Chen, S.-J. Wang, X. Quan, S. Chen, D.-M. Xue, and Y. Z. Zhao, J. Dalian Univ. Tech., 40, 305 (2000). P. M. Samuel, D. de Vos, D. Raveendra, J. A. R. P. Sarma, and S. Roy, Bioorg. Med. Chem. Lett., 12, 61 (2002). G. Schüu¨ rmann and G. Röderer, in Heavy Metals in the Hydrological Cycle, M. Astruc and J. N. Lester (Eds), Selper Ltd, London, 1988. G. Eng, Y. Z. Zhang, D. Whalen, R. Ramsammy, L. E. Khoo, and M. DeRosa, Appl. Organometal. Chem., 8, 445 (1994).

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Anti-Fungal Activity of Organotin Compounds

4.3

443


Heloisa Beraldoa and Geraldo M. de Limab a

Inorganic Medicinal Chemistry Laboratory, Departamento de Qu´ımica, Universidade Federal de Minas Gerais, Brasil b Tin Chemistry Laboratory, Departamento de Qu´ımica, Universidade Federal de Minas Gerais, Brasil

4.3.1

Introduction

Organotin compounds (SnX4-n Rn , 1 ≤ n ≤ 4), first reported in 1849,1 result from the addition of organic fragments to tin. These compounds present a wide range of industrial and biological applications. Tin may have more of its organometallic derivatives in use than any other element.2 Today organotin compounds are produced on the industrial scale, at about 50 kilotons annually.3 4.3.2

Biological Applications of Organotin Compounds

One of the most important bioinorganic chemistry research areas as regards organotin compounds is the investigation of their cytotoxic/antitumour activities. Organotin complexes with a variety of ligands such as benzoates, phenylacetates, and cinnamates, proved to be active in vitro and in vivo against several tumor cell lines.4−8 Moreover, tin(IV) complexes with thiosemicarbazones proved to be cytotoxic against human tumor cell lines, inducing cellular apoptosis.9 These are but a few examples of organotin compounds with anti-tumor or cytotoxic activities. The literature contains many other examples, which are beyond the scope of the present chapter. Organotin compounds have a wide range of other biological activities such as anti-oxidant,10 antiinflammatory and vasodepressant11 amoebicidal, and anti-leishmanial, which have been described in a series of review articles.12 Furthermore, organotin compounds are amongst the organometallic compounds most widely used as biocides.13 Triorganotin(IV) compounds (TOT) are exploited mainly for their biocidal properties, serving as preservatives, bactericides, fungicides, molluscicides, and insecticides.2,14,15 Tri-n-butyltin oxide (TBTO) was the first organotin compound to be used as a biocidal agent,16 in anti-fouling paints for ships.17,18 However, in recent years, high concentrations of TBTO have been found in different aquatic environments.2,19−21 The environmental presence of TBT results largely from its use in antifouling paints, and its toxic effects on non-target organisms has resulted in contamination of various ecosystems in recent decades.2,14,15 It is well known that organotin compounds exhibit anti-microbial activities against different colonies of bacteria and fungi.22,23,24,25 The anti-fungal properties of organotin compounds will be discussed in this chapter. 4.3.3

Fungi and Fungal Infections

Fungi are a large group of organisms that are prevalent in terrestrial habitats. The fungi kingdom includes some of the most important organisms, in terms of both their ecological and economic roles. Ecologically, this kingdom is important (along with certain bacteria) as decomposers and recyclers of nutrients in ecosystems by breaking down dead organic material. In addition, most vascular plants grow with symbiotic fungi that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs, such as antibiotics (e.g. penicillin was isolated from the fungus Penicillium), foods, such as mushrooms, and the bubbles in bread, champagne, and beer.26 Fungi also cause a number of plant and animal diseases. Because fungi are more chemically and genetically similar to animals than other organisms, fungal diseases are very difficult to treat. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and may cause severe damage

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to crops. Plants have three types of association with fungi: endophytes of the leaves, stems and roots, mycorrhizal fungi of the roots, and plant pathogens. Only a few of the enormous array of plant pathogens are known to cause disease. With few exceptions, veterinary and human mycologies deal with the same fungal pathogens. Fungal diseases are called mycoses and those affecting humans can be divided into four groups: superficial mycoses are caused by fungi that grow only on the surface of the skin or hair; cutaneous mycoses include such infections that occur only in the superficial layers of skin, nails, or hair; subcutaneous mycoses penetrate the skin to involve the subcutaneous, connective, and bone tissue; systemic or deep mycoses are able to infect internal organs and become widely disseminated throughout the body. This latter type is often fatal.27 Systemic infections caused by fungi are cryptococcus meningitis or endocardites, pulmonary and cerebral aspergilloses, blastomycoses, histoplamosis, coccidiomycosis, and paracoccidiomycosis. Superficial infections are classified into dermatomycosis (infections of the skin, hair, and nails) and candidiasis (infections with yeast that affects mucous membranes of the mouth, vagina, or skin). Among the drugs employed in the treatment of fungal infections are the anti-fungal antibiotics, such as anphotericin and nystatin, and synthetic anti-fungal agents such as flucytosine, tolnaftate, and the azoles. Anphotericin binds cell membranes and interferes with permeability and transport functions. It binds more strongly to the membranes of fungi because they have more affinity for the fungal membrane ergosterol than for cholesterol, the main sterol in the plasma membrane of animal cells. Nystatin has a similar mechanism of action. Flucytosine is a synthetic anti-fungal agent that is converted to 5-fluorouracil, an antimetabolite that inhibits DNA syntheses. Its spectrum of action is narrower than that of amphotericin B. Tolnaftate is a synthetic anti-fungal agent that is active only against growing cells and does not affect Candida species. Azoles are a group of broad-spectrum synthetic anti-fungal agents consisting of a five-membered azole ring (imidazole or triazole) attached by a carbon–nitrogen bond to other aromatic rings. The mechanism of action of the azoles involves the blocking of ergosterol synthesis.27,28 In the last 20–30 years there has been a steady increase in fungal infections, not only by known pathogenic fungi, but also by fungi previously thought to be innocuous, with the concomitant appearance of opportunistic infections. Among the causes of the problem is the widespread use of broad-spectrum antibiotics, which decreases the populations of non-pathogenic bacteria that compete with fungi. The increasing use of immunosuppressant drugs and the HIV epidemic are also responsible for the increase in opportunistic diseases.27 These factors have resulted in an increase in the number of patients at risk of fungal diseases. The appearance of azole-resistant organisms and the rise in the number of patients at risk of mycotic infections has created new challenges. The search for new anti-fungal drug candidates is therefore very important, and metal-based drugs might represent an alternative therapeutic route. Tin could, in principle, be a metal of choice, considering that organotin compounds present anti-microbial properties. 4.3.4

Mechanisms of Biological Action of Organotin Compounds

The number and nature of the organic groups bonded to the metal center influence the toxicity towards microorganisms, which, in general, decreases in the order R3 SnX > R2 SnX2 > RSnX3 . R4 Sn presents little toxicity. However, the order of toxicity depends on the microorganism, and varies from strain to strain.2 Some authors have proposed that toxicity in the R3 Sn series correlates with total molecule surface (TSA) and hence n-propyl-, n-butyl-, n-pentyl-, phenyl-, and cyclohexyl-substituted tin should be more

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toxic than ethyl- and methyltin. Moreover, if the toxic effects are exerted intra-cellularly, following transport through the cell membrane, a correlation should exist between toxicity and lipophilicity.29 Mechanisms of Organotin Uptake

Organotins may act as cationic species and as organic compounds in solution. Therefore microbial uptake of organotins may occur as a result of metal–biomass or lipophilic interactions. Cell surfaces are predominantly anionic due to the presence of ionic groups such as carboxylate, hydroxyl, and phosphate in the cell wall polymers, and these groups can act as ligands, binding metals at the cell surface. Hence, uptake comprises initially a metabolic-independent biosorption process through adsorption, complexation, precipitation, or crystallization within the cell wall, followed by the metabolic-dependent bioaccumulation by means of the transport of the metal across the membrane to the interior of the cell. Organotin compounds may show lipophilic interactions with cellular membranes. Uptake by membrane diffusion may occur, in addition to or instead of uptake of the free metal ion. Hydrophobic non-polar species may dissolve into the membrane and enter the cytosol. On entering the cytosol the lipophilic complex may become distributed among the cellular components and membranes intact, or the metal may be released from the complex and bind to other sites.2 Mechanisms of Toxicity

The mechanism of biological action of organotin derivatives is still not clear. In spite of that, some tentative proposals have emerged. Efforts to understand how these compounds interact with microorganisms have been made in order to assess the risks of organotin pollution. These studies have provided information on the mechanism of action of organotin compounds. Due to their lipophilicity, organotins are membrane-active and the cytoplasmic membrane is an obvious target of action. Disruption of membrane integrity may occur because of organotin binding or insertion into the membrane. Organotins can also act intra-cellularly and intact organelles may be disrupted.2 Studies of tin interactions with the yeast Candida maltosa revealed that the cell wall is the dominant site of Sn(IV) interactions with yeast, while lipophilic interactions play an important role in uptake and toxicity of tri-n-butyltin compounds, TBT. TBT uptake resulted in cell death and extensive K+ leakage, while Sn(IV) uptake had no effect. Trimethyltin compounds, (TMT) did not interact with cells. Of the three kinds of compounds, TBT alone altered membrane fluidity.2 If the organometallic species enters the cytosol, the metal can dissociate from the ligand and bind to internal sites. Sn(IV) is able to form complexes with biological molecules, thiols, peptides,12 proteins,30 amino acids, nucleic acids, carbohydrates,12 and steroids.31 The redox potential for the transformation Sn2+ → Sn4+ , −0.154 V, (compared to standard hydrogen electrode) lies within the physiological range found for several enzymes and thiols. Therefore it is possible that enzymatic processes or interaction with thiols as reducing agents are involved in the biological activity of organotin compounds.32 4.3.5

Structure–Activity Relationships

Many studies on the bioactivities of organotin compounds present work on structure–activity relationships. A recent review article summarizes some of the results of these investigations.33 The toxicity of organotin compounds has been found to be a function of the number of organic groups attached to the tin atom, as well as of the nature of the organic group. It is well known that triorganotin compounds have the highest biocidal activities. The nature of the organic group determines the species to which the triorganotin is most toxic. Thus trimethyltin compounds are highly toxic to insects and mammals, while triphenyl derivatives have high toxicities towards fish, fungi, and molluscs.

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In general, compounds with alkyl groups are more toxic than compounds with aryl groups. In R3 SnX derivatives, X itself can be biologically active or can assist the transport of the molecule to the active site. One way to reduce the number of organotin derivatives discharged into the environment, and to reduce the cost of developing more effective tin-based biocides, is to design more effective compounds. Quantitative structure–activity relationship (QSAR) studies appear to be an interesting strategy to address this problem. As a first example, Eng et al. reported the toxicities of a series of aryltins against the fungus, Ceratocystis ulmi, the causative agent of Dutch elm disease. Neither topological nor partitioning effects were important in determining the activity of the triaryltin chlorides. An equation was obtainable as a function of the Hammett sigma (σ ) values of the substituents on the phenyl ring. It was proposed, in view of the Hammett correlation, obtained by plotting log 1/C (C = IC50 ) against the QSAR quantitative structure–activity relationship constant σ s that the triphenyltin fragment is responsible for the biological activity. The biotoxicity was observed to be a function of how readily the triphenyltin cation forms, which in turn is a function of the leaving ability of the carboxylate ion.34 Also, Singh and Sharma reported a linear relationship between the toxicity of trialkyltin acetates and several fungi using the first and third-order connectivity indices of the organotin. The third-order connectivity indices, as well as the first order, for organotins played a significant role in determining the toxicity of the trialkyltin acetates against the fungi.35 4.3.6

Anti-Fungal Screening

Organotin Complexes with Esters, Carboxylates, Amino Acids, and Peptides

Organotin(IV) derivatives of amino acids have been of interest as possible biocides, and as intermediates in peptide synthesis. Hence, di-n-butyltin complexes of Schiff bases derived from amino acids, Figure 4.3.1. (n-Bu)2 SnL (L = dianion of tridentate Schiff bases derived from amino acids), when tested against Candida albicans, Crytococcus neoformans, Sporotrichum shenckii, Trichophyton mentafrophytes, and Aspergillus fumigatus fungi exhibited moderate activity compared to the starting material {(n-Bu)2 SnO}3 . The complex with the 2-hydroxyl-1-naphthaldehyde derivative exhibited the highest activity.36

O

O Bu

Sn N

Sn

Bu N

O

Bu Bu O

R

R O

O

R = -CHR´- (R´= Pr,i CH 2CH2SMe, CH2Pr i); or -(CH2)n- (n = 1, 2 or 3)

Figure 4.3.1

Structures of organotin complexes derived of amino acids

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R´´ CH

O

NH NH

Sn

R R

R´

CH

O C

O R´´ = R´ = CH2Ph; R´´ = CH3 and R´ CH2Ph; R´´ = H and R´= CH2Pri; R´´ = PhCH2 and R´= CH2Pri R = Ph or Bun

Figure 4.3.2

Structure of diorganotin(IV) complexes of dipeptides

A comprehensive review article describes investigations on the antimicrobial properties of organotin complexes with amino acids and peptides:37 diorganotin(IV) complexes of the general formula R2 SnL (R = n-Bu, Ph; L = dianion of dipeptides alanylphenylalanine, phenylalanylleucine, phenylalanylphenylalanine, glycylleucine, glycylisoleucine), Figure 4.3.2, were tested against colonies of Aspergillus niger, Penicillium chrysogenum, Aureobasidium pullulans, and Verticillium dahliae. The results indicated that the complexes possess high fungicidal activities. Di-n-butyltin complexes were found to be more active than (n-Bu)2 SnO, whereas diphenyltin derivatives were less active than Ph2 SnCl2 . The data revealed that the di-n-butyltin cation imparted greater activity than the phenyl analog. The relationship between the activity and the nature of the substituents (electron-withdrawing or electrondonating) present in the dipeptide chain was also investigated.38 R3 SnL, R2 Sn(L)2 complexes with amino acids (R = Me, Ph, n-Bu ; R = n-Bu, Ph; HL = various amino acids) were active against a wide spectrum of fungi. The activity order was: triphenyl > diphenyl > di-n-butyl > trimethyltin. Because of their high anti-fungal activities, Ph3 Sn derivatives of a few amino acids, and (n-Bu)2 Sn(l-tyr) and Ph2 Sn(dl-Asp), have been screened in vivo against a multi-infection fungal model in mice. The compounds were active at a dose of 50 mg kg−1 .37 Triorganotin(IV) derivatives of dipeptides with the general formula R3 Sn(HL) [R = Ph, HL = monoanion of glycylisoleucine (Gly-Ile), valylvaline (Val-Val), alanylvaline (Ala-Val), leucylalanine (Leu-Ala), leucylleucine (Leu-Leu); R = n-Bu, HL = monoanion of glycylisoleucine (Gly-Ile), leucylalanine (LeuLeu); and R = Me, HL = monoanion of leucylalanine (Leu-Ala)] have been synthesized and tested for fungicidal activity. Only Sn(n-Bu)3 (Gly-Ile) and SnPh3 (Ala-Val) exhibit satisfactory fungicidal activity against Candida albicans, Microsporum gypseum, and Euglena gracilis.11,39 The anti-fungal activity of complexes of Sn(n-Bu)2 Cl2 with heterocyclic β-diketones and N -phthaloyl amino acids, Figure 4.3.3, were tested against the growth of Fusarium oxysporium, Alternaria alternata, and Alternaria solani. It was evident that increasing anti-fungal activity is correlated with the presence of tin. This has been attributed to the lipophilic character of the complexes, responsible for their permeation through the lipid layer of the fungal membrane.40 The organotin esters Me2 SnL2 , Me3 SnL, (n-Bu)2 SnL2 , (n-Bu)3 SnL, Ph3 SnL, (PhCH2 )2 SnL2 , [(Me2 SnL)2 O]2 , Et2 SnL2 , and n-Oct2 SnL2 , (L = (E)-3-(3-fluorophenyl)-2-(4-chlorophenyl)-2-propenoate) have shown low fungicidal activities against Trichophyton longifusus, Candida albicans, Candida glaberata, Microsporum canis, Aspergillus flavus, and Fusarium solani. The complexes were less active than the clinically used drugs Amphotericin B and Miconazole.41

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Tin Chemistry: Fundamentals, Frontiers and Applications R H 3C

n O Bu

O

R´

Sn N N

O

n

CH

Bu O

N

O

O

R = 4-F-C6H4; 4-Cl-C6H4; 4-Br-C6H4; CF3 R' = H; CH3; CH(CH3)2

Figure 4.3.3

Structure of heterocyclic β-diketonato-di-n-butyl complexes

Lipophilic properties have also been suggested in order to explain the anti-fungal activity of diphenyltin(IV) and di-n-butyltin(IV) carboxylates against Aspergillus niger, Aspergilluus flavus, and Pencillium citrinum. The compounds exhibited greater fungitoxicity than the diorganotin(IV) dichlorides and the carboxylic acids from which they were synthesized. The anti-fungal activity of the complexes was attributed to the organotin part of the molecule, whereas the carboxylic group influences the delivery process of the R2 Sn moiety. An additive effect of the organometallic fragment and the free carboxylic acids has been suggested to result in the enhancement of the activity of the complexes in comparison with that of the individual components.42 Organotin(IV) complexes with phthalimido-4-methyl pentanoate ligand were tested against a variety of isolates of human pathogens (Aspergillus flame, Trichophyton schoenlem, Pseudallescheria boydii, Candida albicans, Aspergillus niger), animal pathogens (Microsporum canis, Trichophyton mantagrophytes, Trichophyton rubrum, Trichophyton) and plant pathogens (Fusarium oxysponumvarlycopersici, Fusanum solanivarlycopersici, Macrophormina phaseolina, Rhizoctonia solani). Both di- and triorganotin complexes showed anti-fungal activity. The best results were observed for the triorgananotin complexes.43 Dialkyltin complexes with N -arylidene-α-amino-acid-triphenyltin adducts of N -alkylsalicylideneimines and triphenyltin thiolate complexes, (C6 H5 )3 SnSR, have been tested towards colonies of Ceratocystis ulmi, responsible for Dutch Elm Disease (DED). The first series proved to be less active, in accordance with the general observation that diphenyltin complexes are not effective inhibitors of fungi. The second class was an effective inhibitor of C. ulmi. This study, as well as earlier results, suggests that the species responsible for the inhibition of C. ulmi is the Ph3 Sn+ cation or its hydrated species. For the last series the low IC50 values were attributed to the addition of a biologically active group to the triphenyltin moiety. An alternative explanation would be that the non-dissociating S-group might assist in carrying the biocidal tin center through the cell wall of the fungus.44 In addition, the fungicidal activities of triorganotin esters of N -arylidene-omega-amino acids of general formula R3 SnOCO(CH2 )n N=CHAr (R = Ph, n-Bu; Ar = 2-HOC6 H4, 2-HOC10 H6 , n = 1, 2, 3, and 5) were tested. The tri-n-butyl- and triphenyltin complexes were effective inhibitors of Ceratocystis ulmi, with activities superior to those of commercially available tri-n-butyl- and triphenyltin fungicides. While tri-n-butyltin derivatives are known to induce phytotoxic effects on plants, this series of compounds

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Figure 4.3.4 Transmission electron microscopy (TEM) images of C. albicans cells exposed to [Sn(nBu)2 (O2 CC6 H4 )2 NH2 ]

proved to have no effect for concentrations up to 20 mg l−1 , which are far above their minimum inhibitory concentrations. The studied compounds are more effective than the commercially known tri-n-butyl- or triphenyltin based anti-microbials. The results suggest that the studied compounds can be considered as candidates for the control of Dutch Elm disease.45 Also triphenyltin carboxylates and several triphenyltin chloride adducts with 2,3-disubstituted thiazolidin-4-ones were screened in vitro against C. ulmi. Both classes showed inhibitory activity against the growth of the fungus. The organotin carboxylate Ph3 SnOCOR, {R = 2-C4 H3 O, 2-C4 H3 S, 4-CH3 OC6 H4 , Ph, 4-CH3 C6 H4 , 4-NH2 C6 H4 , 4-NO2 C6 H4 } showed to be effective in reducing colonies of Ceratocystis ulmi responsible for Dutch Elm disease. Lower IC50 values were observed, in comparison to Ph3 SnOH or Ph3 SnCl, commercial fungicides.34 The in vitro anti-fungal activities of [Sn(CH3 )2 (2-OC6 H4 CHNC6 H4 CO2 )], [Sn(CH3 )2 O2 C (C6 H4 NH2 )2 ]2 Sn2 O2 (CH3 )4 , and [Sn(n-Bu)2 (O2 CC6 H4 )2 NH2 ] have been tested against Candida albicans. The effect of the complexes on the cellular activity of this yeast were investigated. No changes in DNA integrity or in the mitochondria function were observed. However, all complexes reduced the ergosterol biosynthesis. Special techniques used for morphological investigations such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested that the organotin complexes act on the cell membrane, in view of the observed cytoplasm leakage and strong deterioration of the cellular membrane, Figure 4.3.4. The data indicate a mechanism of action similar to that of azole drugs clinically used in Candida infections.46 Organotin Complexes with Hydrazones and Thiohydrazones

The organotin(IV) complexes, ( p-ClC6 H4 )3 Sn(L)Cl, and ( p-ClC6 H4 )2 Sn(L)Cl2 , obtained by reacting the organotin halide with salicylaldehyde aniline-N -thiohydrazone and cinamaldehyde aniline-N -thiohydrazone, respectively, have been tested for anti-fungal activity against the pathogenic Rhizoctonia bataticola strain. The results showed that these organotin(IV) complexes inhibit the growth of Rhizoctonia bataticola colonies.47 The biological activity of a series of organotin complexes with pyrrole-2-carboxaldehyde, 2hydroxybenzoylhydrazone (H3 mfps), and pyrrole-2-carboxaldehyde-2-picolinohydrazone was investigated against Aspergilus niger. Sn(H3 mfps)(C6 H5 )2 Cl2 .2H2 O, Figure 4.3.5, strongly inhibited the growth of the fungal colony.48

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O Ph

Cl

Sn N

OH

N Ph

H

Cl

C H

N H

Figure 4.3.5

Structure of Sn(H3 mfps)(C6 H5 )2 Cl2 .2H2 O

The reaction of diphenyltin dichloride with Schiff bases derived from the condensation reaction of S-benzyldithiocarbazate (NH2 NHCS2 CH2 C6 H5 ) and heterocyclic aldehydes yielded five- and six- coordinate organotin(IV) complexes. The organotin complexes were tested for their anti-fungal activity against Aspergillus niger, Rhizoctonia phaseoli, and Penicillium crysogenes. Complexes were more active than the starting materials.49 The dithiocarbazate organotin complexes were more active than the related semicarbazone and thiosemicarbazone complexes. Organotin Complexes with Triazoles

Triorganotin(IV) complexes of the triazolo-pyrimidine derivatives 4,5-dihydro-5-oxo-[1,2,4]triazolo[1,5a]pyrimidine (5HtpO), 4,7-dihydro-5-methyl-7-oxo-[1,2,4]triazolo-[1,5a]pyrimidine (HmtpO), and 4,5,6,7-tetrahydro-5,7-dioxo-[1,2,4]triazolo-[1,5a]pyrimidine (H2 tpO2 ), Figure 4.3.6 and the diorganotin derivative (n-Bu)2 Sn(tpO2 ), were tested against both strains of Candida albicans and Candida tropicalis. Good anti-yeast activity was shown by Sn(n-Bu)3 (HtpO2 ) and its precursor Sn(n-Bu)3 (OCH3 ). An antigerm-tube formation test was also performed. Colonies of C. albicans were grown in the presence of Sn(n-Bu)3 (HtpO2 ), Sn(nBu)3 (OCH3 ), and SnPh3 (HtpO2 ) at concentrations equal to MIC, 2 × MIC and 4 × MIC. The tri-n-butyl complex displayed significant activity at concentrations of 3 and 1.5 μg ml−1 , corresponding to 4 × MIC and 2 × MIC, respectively, but a low activity at 0.78 μg ml−1 , equal to MIC. The best results were found for its precursor (n-Bu)3 Sn(OCH3 ), which also showed an interesting activity at a MIC concentration of 0.78 μg ml−1 , while Ph3 Sn(HtpO2 ) was not effective against germ-tube formation at the studied concentrations. Germ-tube formation is an important factor in the formation of O

O N O

N

N

N

N H3C

N

N

N

N O

N

H

H

H

5HtpO

HmtpO

H2tpO2

N

N

Figure 4.3.6 Structure of 4,5-dihydro-5-oxo-[1,2,4]triazolo-[1,5a]pyrimidine (5HtpO), 4,7-dihydro-5methyl-7-oxo-[1,2,4]triazolo-[1,5a]pyrimidine (HmtpO), and 4,5,6,7-tetrahydro-5,7-dioxo-[1,2,4]triazolo[1,5a]pyrimidine (H2 tpO2 )

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biofilms of C. albicans, a form of yeast resistant to a wide range of current anti-fungal agents. Anti-biofilm properties of the tri-n-butyltin complex were only detected at concentrations of 16 × MIC or more.24 A series of organotin compounds synthesised from the reaction of tribenzyltin chloride with quinines, barbiturates, and triazoles were tested. All compounds showed anti-fungal activity against Aspergillus wentii, Aspergillus caespitosum, Aspergillus candidus, and Aspergillus awamori. The best results were found for the organotin derivatives of thiobarbiturate and benzoquinone.50 Organotin Complexes with Sulfides, Thiosemicarbazones and Dithiocarbamates

Triphenyltin lupinylsulfide displayed good biological activity against C. albicans and A. niger in comparison to triphenyltin chloride.51 The anti-fungal activities of organotin complexes of 2-benzoylpyridine-N (4)-phenylthiosemicarbazone (H2 Bz4 Ph)], [(n-Bu)Sn(L)Cl2 ] and [(n-Bu)2 Sn(L)Cl] were tested against Candida albicans. The free thiosemicarbazone proved to be more active than its tin(IV) complexes.52 Chloro-diorganotin(IV) complexes of 4-methyl-1-piperidinedithiocarbamate have been synthesized and assayed for their anti-fungal activity against six different plant and human pathogens: Trichophyton longifusus, Candida albicans, Aspergillus flavus, Microsporum canis, Fusarium solani, and Candida glaberata. The complexes were more active than the free ligand.53 Pyrrolidinedithiocarbamate derivatives of organotin halides [Sn{S2 CN(CH2 )4 }2 Cl2 ], [Ph2 Sn {S2 CN(CH2 )4 }2 ], [Ph3 Sn{S2 CN(CH2 )4 }], and [(n-Bu)2 Sn{S2 CN(CH2 )4 }2 ] have been tested against Candida albicans. The microorganism presented resistance to the dithiocarbamate ligand and all tin(IV) complexes assayed were active. The highest activity was found for the first and the last compounds.54 Chlorodiorganotin(IV) complexes of 4-methyl-1-piperidine carbodithioic acid have been synthesized. The complexes exhibited higher anti-fungal activity than the free ligand.22 Organotin Complexes with Schiff Bases

Organotin complexes of Schiff bases derived from sulpha drugs were found to be highly active against Penicillium crysogenum, Aspergillus niger, and Fusarium oxysporum.55 Anti-fungal activity tests were carried out with Candida albicans, Cryptococcus neoformans, Sporotrichum schenckii, Trichophyton mentagrophytes, and Aspergillus fumigatus in the presence of the organotin complexes, R3 SnL (R = Me, Ph) and R2 SnL2 (R = Ph, n-Bu) [L = the anion of Schiff bases derived from condensation reaction of 2amino-5-(o-anisyl)-l,3,4-thiadiazole with salicylaldehyde, 2-hydroxynaphthaldehyde or 2-hydroxyacetophenone}, Figure 4.3.7. The antimicrobial activity data, in

N

N R1 N

S

R2

OCH3 OH R1 = H or CH3 R2 =

OH

or

Figure 4.3.7 Structure of the condensation product of 2-amino-5-(o-anisyl)-l,3,4-thiadiazole with salicylaldehyde or 2-hydroxynaphthaldehyde

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terms of MIC, showed that all the Schiff bases exhibit similar activity and most of the tested complexes were more active than the ligands, displaying the following order of activity: Ph2 SnL2 > Me3 SnL > Ph3 SnL > (n-Bu)2 SnL2 , independent of the nature of the ligand.56 4.3.7

Conclusions

Organotin compounds are useful as anti-fungal agents against a variety of fungal strains. Although environmental problems may arise by the indiscriminate use of these species, one way to reduce the number of organotin derivatives discharged into the environment is to design more effective compounds by means of quantitative structure–activity relationship (QSAR) studies. The design of effective tin-based anti-fungal agents involves the appropriate choice of n and R in the SnX4-n Rn scaffold. In addition, a strategy to obtain additional or synergistic effects is to combine the anti-fungal activities of both the organotin moiety and the organic groups used in the preparation of organotin complexes. Strategies to avoid or diminish toxicity to the host involve the design and preparation of organotin compounds with selective affinity for the fungal membrane. 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.

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25. A. Bacchi, M. Carcelli, P. Pelagatti, G. Pelizzi, M.C. Rodriguez-Arguelles, D. Rogolino, C. Solinas, and F. Zani J. Inorg. Biochem, 99, 397 (2005). 26. G.J. Tentora, B.R. Funke, and C.L. Case, Microbiology: an Introduction, 8th Edn, Pearson Education, Inc., Benjamin Cummings Copyright, Upper Saddle River, NJ, 2004. 27. H.P. Hang, J.M. Ritter, and M.M. Dale, Pharmacology, 3rd Edn, Churchill Livingstone, Oxford, 1995. 28. B.G. Katzung, Basic & Clinical Pharmacology, 8th Edn, McGraw Hill, Inc., Columbus, OH, 2001. 29. J. J. Cooney and S. Wuertz, J. Industrial Microbiol and Biotechnol 4, 375 (2005). 30. B.A. Buck-Koehntop, F. Porcelli, J.L. Lewin, C.J. Cramer, and G. Veglia, J. Organomet. Chem., 691, 1748 (2006), and references therein. 31. K. C. Molloy, Bioorganotin compounds, in The Chemsitry of the Metal-Carbon Bond, Vol. 5, F. R. Hartley (Ed.), John Wiley & Sons, Ltd, Chichester, 1989. 32. J. M. Tsangaris and D. R. Williams, Appl. Organomet.Chem., 6, 3 (1992). 33. X. Song, A. Zapata and G. Eng, J. Organomet. Chem., 691(8), 1756 (2006). 34. G. Eng, D. Whalen, P. Musingarimi, J. Tierney, and M. DeRosa, Appl. Organomet. Chem., 12, 25 (1998). 35. P.P. Singh and K.K. Sharma, Indian J. Chem.. 32B 551 (1993). 36. M. Nath, R. Yadav, M. Gielen, H. Dalil, D. de Vos, and G. Eng, Appl. Organomet. Chem., 1, 727 (1997). 37. M. Nath, S. Pokharia, and R. Yadav, Coord. Chem. Rev. 215, 99 (2001). 38. M. Nath, R. Yadav, G. Eng, T. Nguyen, and A. Kumar J. Organomet. Chem., 577, 1 (1999). 39. M. Nath, S. Pokharia, G. Eng, X. Q. Song, A. Kumar, M. Gielen, R. Willem, and M. Biesemans, Appl. Organomet. Chem., 18, 460 (2004). 40. A. Joshi, S. Verma, R. B. Gaur, and R. R. Sharma, Bioinorg. Chem. Applications, 3, 201 (2005). 41. K. Sadiq-ur-Rehman, S. Shahid, M. H. Ali, Bhatti, and M. Parvez, J. Organomet. Chem. 690, 1396 (2005). 42. J. J. Bonire, G. A. Ayoko, P. F. Olurinola, J. O. Ehinmidu, N. S. N. Jalil, and A. A. Omachi, Metal Based Drugs, 5, 233 (1998). 43. M. Ashfaq, J. Organomet. Chem. 691, 1803 (2006). 44. G. Eng, D. Whalen, Y.Z. Zhang, A. Kirksey, M. Otieno, L.E. Khoo, and B.D. James, Appl. Organomet. Chem. 10, 501 (1996). 45. N. K. Goh, C. K. Chu, L. E. Khoo, D. Whalen, G. Eng, F. E. Smith, and R. C. Hynes, Applied Organomet. Chem., 12, 457 (1998). 46. F. T. Vieira, D. C. Menezes, G. M. de Lima, M. E. Cortés, G. A. B. Silva, A. Vilas-Boas, and J. R. S. Maia, Appl. Organoment, Chem. (2008), in press. 47. A.K. Mishra, N. Manav, and N.K. Kaushik, Spectrochim. Acta Part A 61, 3097 (2005). 48. G. Bergamaschi, A. Bonardi, E. Leporati, P. Mazza, P. Pehzgatti, C. Pelizzi, G. Pelizzi, M. C. Rodriguez Argiielles, and F. Zuni, J. Inorg. Biochem. 68, 295 (1997). 49. H. L. Singh and A. K. Varshney, Bioinorg. Chem. Applications, 4, 1 (2006). 50. M. Kidwai, B. Dave, P. Misra, R. K. Saxena, and M. Singh, Inorg. Chem. Comm., 3, 465 (2000). 51. F. Novelli, M. Recine, F. Sparatore, and C. Juliano, Farmaco 54, 237 (1999). 52. H. Beraldo, G. M. de Lima, A. P. Rebolledo, L. N. Gambi, N. L. Speziali, J. D. Ardisson, D. F. Maia, C. B. Pinheiro, and M. E. Cortés, Appl. Organomet. Chem., 17, 945 (2003). 53. S. Shahzadi, S. Ali, M. H. Bhatti, M. Fettouhi, and M. Athar, J. Organomet. Chem. 691, 1797 (2006). 54. G. M. de Lima, A. O. Porto, J. D. Ardisson, F. T. Vieira, M. E. Cortés, T. E. Albrecht-Schmitt, and D. C. Menezes, Eur. J. Med. Chem., 40, 1277 (2005). 55. M. K. Gupta, H. L. Singh, S. Varshney, and A.K. Varshney, Bioinorg. Chem. Applications, 1, 309 (2003). 56. M. Nath and S. Goyal, Metal Based Drugs, 2, 297 (1995).

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4.4

Chemical and Biotechnological Developments in Organotin Cancer Chemotherapy

Claudio Pettinari and Fabio Marchetti Dipartimento di Scienze Chimiche, University of Camerino, Italy

4.4.1

Introduction

Organotin(IV) derivatives have potential in several important fields, from catalysts to biological agents. The toxicity of organotins was reported as early as 1886,1 but it was not until the 1950s that their toxicities were explored systematically.2 In particular, many organotin complexes have been shown to possess interesting anti-tumor activity,3 and the National Cancer Institute (NCI) has tested about 2000 tin-based compounds, the largest number ever tested among metal complexes.4 The first in vitro active complexes were designed to emulate the cisplatin framework,5 with the composition SnR2 X2 and SnR2 X2 L2 (X = halide or pseudohalide, R = organic group, and L = a nitrogen ligand such as py, or L2 = a bidentate nitrogen ligand such as en, bipy, phen, etc.), but they showed several disadvantages, among them low solubility in water, a disappointingly low activity, and high toxicity in vivo. During recent decades, several organotins with different structures and anti-neoplastic activity toward the mammarian tumor MCF-7 and a colon carcinoma WiDr, have been developed by Gielen et al.6 The anti-tumor activity of most organotin compounds is displayed via different mechanisms at the molecular level. Many of them seem to bind DNA through the nitrogens of DNA bases, while the phosphate groups of the DNA sugar backbone usually act as anchoring sites for tin. Moreover it has been pointed out that the R2 Sn2+ and R3 Sn+ moieties (R = alkyl or aryl group) are probably the ultimate reactive species of the di- and trisubstituted organotin species, where tin is bonded to halide, pseudohalide, oxygen, nitrogen, or sulfur, as Sn C bonds are the more hydrolytically stable. It has been also suggested that organotin compounds exert their effects through binding to thiol groups of proteins.7 However, recent studies8 have shown that low doses of organotins can exhibit anti-tumoral activity and have suggested a mode of action via a gene-mediated pathway in the cancer cells, opening a new research sub-area on organotin compounds. A number of metal compounds able to activate apoptosis directly involved in the apoptotic pathway, such as p53 tumor suppressor, TRAIL receptor, caspases, and the Bcl-2 family of proteins have been recently developed. Since there are two primary modes of apoptosis, i.e. extrinsic and intrinsic, metal-induced apoptosis is thought to be initiated intracellularly, the mitochondria being most pertinent in mediating apoptosis via metal-induced reactive oxygen species.9 The di-n-butyltin and tri-n-butyltin chloride compounds also induce apoptosis in vitro in rat thymocytes, through inhibition of DNA synthesis while increasing RNA synthesis.10 The apoptotic pathway induced by the di-n-butyltin and tri-n-butyltin chlorides starts with an increase of Ca2+ ions, then the release of the cytochrome c from mitocondria occurs, followed by activation of caspases and finally by DNA fragmentation.11 The compound diethyltindichloro(1,10-phenanthroline) has been shown to inhibit, in vitro, cancer cell growth and also to change the surface of the cancer cell membrane.12 In general, the toxicity of organotin compounds seems to increase with the chain length of the organic alkyl groups, which are often more active than aryl ones, and to follow the order R3 Sn > R2 Sn > RSn. More recent results indicate that when designing new anti-tumor tin compounds it is necessary to individuate a balance between solubility and lipophilicity features in order to achieve efficacy. Also, the reduction of side effects, such as neurotoxicity and immune suppression, should be considered an important goal. It is, in fact, known that triethyltin compounds are able to attack the myelin of the central nervous system and trimethyltin compounds cause neuronal hyperexcitation.13

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Developments in Organotin Cancer Chemotherapy

Figure 4.4.1

4.4.2

455

Structures of diorganotin carboxylates and pyridinecarboxylates

Developments in the Design of Organotin Anti-Cancer Compounds

The first systematic studies of structure–activity relationships in anti-tumor organotins were carried out by Gielen’s group, on several di- and triorganotin(IV) carboxylates, including 2,3-pyridinecarboxylates (Figure 4.4.1), and their cytotoxicity.6 Organotin compounds containing steroidal moieties have been synthesized and their in vitro activity tested toward a series of human tumor cell lines. These results were compared with a parent steroid and two model compounds, and show that organotin steroids I–III depicted in Figure 4.4.2 exhibit promising in vitro activity. In particular, III is highly effective towards cancer cells and may be employed as a model for further investigation on structure–activity relationships in anti-tumor organotin compounds.14 A large number of organotin camphorates, steroidcarboxylates, and carboxylates containing the polyoxaalkyl moiety, and thereby soluble in water (Figures 4.4.3 and 4.4.4), have been reported by Gielen and coworkers, and their cytotoxicity studied in aqueous media against several human cancer cells.15 Two triphenyltin carbohydrates have been reported by Caruso et al.16 Their anti-tumor activity has been rationalized by the authors as follows: (a) the triphenyltin carbohydrates are less active than Ph3 SnCl with respect to their capacity to interfere with DNA, RNA, and protein synthesis of isolated rapidly proliferating thymocytes, protein synthesis being found to be most sensitive; (b) the in vitro tests toward the mouse

Figure 4.4.2

Structure of organotin steroids reported in Ref. 14

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Figure 4.4.3

Structures of di- and tri-organotin steroidcarboxylates

tumor cell lines MOPC315, P815, SL2, and L1210 showed that the two triphenyltin carbohydrates are less effective than Ph3 SnCl. Moreover it was found that Sn C bonded triphenyltin carbohydrates are less active than Ph3 SnCl in vitro; the Ph3 Sn–carbohydrate (I) (Figure 4.4.5) is more active than Ph3 SnCH2 –carbohydrate II, and this may be related to the long Sn C(carbohydrate) bond distance in the former compound, which shows a striking biological activity in contrast to the normal inactivity of tetraorganotins. The low aqueous solubility of tin compounds seems to pose an important difficulty, which may be partially addressed through the use of hydrophilic leaving groups. Formulation may require a colloidal suspension or DMSO to enhance solubility.13 Hydrolysis seems to be necessary for the activity of organotin compounds. The lipophilicity of the more stable C-bound groups on tin is important in controlling their toxicity, where the n-butyl groups are apparently the most lipophilic.

Figure 4.4.4

Structures of organotin polyoxaalkylcarboxylates

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Figure 4.4.5

457

General structures of triphenyltin carbohydrates

A recent advance in this field has been reported by Yin,17 who has synthesized six new di-n-butyltin bis-heteroaromatic carboxylates (Figure 4.4.6) and tested them in vitro against the human mammary tumor cell line MCF-7. They are soluble in water and display quite high activity, much higher than that of cisplatin. Di- and triorganotin compounds containing the anion of 3-maleimidopropionic acid have been reported to show promising cytotoxicity against MCF-7 breast cancer, EVSA-T breast cancer, WiDr colon cancer, IGROV ovarian cancer, and M226 non-small cell lung cancer.18 The same researchers have recently expanded their study by reporting the synthesis, characterization, and cytotoxicity effects of a series of organotin(IV) derivatives of N -maleoylglycine19 and new organotin-2-maleimidopropanoates, which show in vitro activity against seven tumor cell lines. The use of the 2-maleimidopropanoic ligand increases the hydrophilicities of these complexes, which possess significant activities.20

Figure 4.4.6

Structure of di-n-butyltin bis-heteroaromatic carboxylates

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Figure 4.4.7

Triethyltin lupinylsulfide hydrochloride

A triorganotin quinolizidine compound, triethyltin lupinylsulfide hydrochloride (Figure 4.4.7), has been reported to show quite good solubility in ethanol/water and to be a potent anti-proliferative against three different human cancer cell lines: teratocarcinoma of the ovary (PA-1), colon carcinoma (HCT-8), and glioblastoma (A-172).21 The cytocidal effects due to this compound seem consistent with necrosis or delayed cell death rather than apoptosis. An innovative class of diorgano- and dichlorotin-containing anti-tumor agents based on hydroxamic acids as ligands (Figure 4.4.8) has been reported by Pombeiro et al.22 These compounds, soluble in alcohols and hydroalcoholic solutions, exhibit cytotoxicities against a series of human tumor cell lines, which, in some cases, are identical to, or even higher than, that of cisplatin. It has been shown that for the dialkyltin complexes, the activity increases with the length of the carbon chain of the alkyl ligand and is higher in the case of the chloro-substituted benzohydroxamato ligand. The [Bun2 Sn(L1 )2 ] complex displays a high in vivo activity against H22 liver and BGC-823 gastric tumors, and has relatively low toxicity. Also, electron-withdrawing substituents (X = NO2 or F) increase the anti-tumor activity. A series of R2 SnCl2 adducts with Schiff bases (Figure 4.4.9) has been recently screened against the three tumor cell lines, L929, K562, and HeLa, and the results being very similar or even better than those of oxaliplatin.23 In searching for new structural variables for tin anti-tumor compounds, Gielen has proposed a number of perfluoroalkanecarboxylates, three of them being very active against seven tumor cell lines.24 His recent review on organotin compounds is an effort to systematize all kinetic and stereochemical evidence on most of their compounds with respect to results derived from cytotoxicity assays.25 A series of water soluble (3-methoxypropyl)stannanes has been prepared by Lebl and shown to possess promising cytotoxicity and trypanocidal activities.26 Pruchnik et al. also reported that hexakis(2-cyanoethyl)ditin(III) derivatives possess modest cytotoxic activity against A549 and HSMC cells.27 Some triorganotin derivatives of 2-phenyl-1,2,3-triazole-4-carboxylates have been reported to possess good anti-tumor activity against three human tumor cell lines (Hela, CoLo205, and MCF-7).28 The same authors have also reported a novel binuclear organotin complex containing N -[(3,5-dibromo-2-hydroxyphenyl)methylene]valinate (Figure 4.4.10) displaying good in vitro cytotoxicity against the same three tumor cell lines.29

Figure 4.4.8

The hydroxamic acid ligands used for the synthesis of the organotins in Ref. 22

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Figure 4.4.9

Figure 4.4.10 valinate

459

Schiff bases ligands employed in Ref. 23

Structure of the triphenyltin complex of N-[(3,5-dibromo-2-hydroxyphenyl)methylene]

In the course of the last decades, Sordo and his group have reported several tin(IV) and organotin(IV) adducts showing significant anti-tumor activity in vitro against human carcinoma cell line KB, based on N,N-chelating ligands.30 Interestingly, some of these systems contain unsymmetric dinitrogen donors, such as mepirizole31 and 3,5-dimethyl-1-(2 -pyridyl)pyrazole32 (Figure 4.4.11), the former being a wellknown anti-inflammatory agent.33 All these active organotins possess a specific structural feature, namely

Figure 4.4.11

Structure of mepirizole, of 3,5-dimethyl-1-(2’-pyridyl)pyrazole and of its R2 SnCl2 adducts

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Figure 4.4.12

Structure of bis[(di-n-butyl-3,6-dioxaheptaneato)tin] and tri-n-butyltin 3,6,9-trioxodecanoate

an average Sn N bond length higher than 239 pm, whereas the inactive compounds have Sn N bond lengths less than 239 pm. This seems to imply that predissociation of the ligand may be an important step in the mode of action of active systems; on the other hand the coordinated ligand may favour transport of tin compounds through cell membranes to the site of action in the cells, and then organotin moieties are released by hydrolysis. Other recent research has explored the interaction between diorganotin(IV) and triorganotin(IV) halides and glycyltirosine, glycyltryptophane, leucytirosine, leucylleucine, valylvaline, and alanylvaline. These ligands act in dianionic form as tridentate donors coordinating through COO− , NH2 , and N-peptide groups toward R2 Sn(IV), whereas they act as bidentate monoanionic donors coordinating through COO− and NH2 groups toward R3 Sn(IV). All these systems have been tested against several human cancer cell lines, providing significantly higher activities than that of etoposide and cisplatin.34 Messori has recently studied the interactions, in vitro, between DNA and two organotin(IV) systems, bis[(di-n-butyl-3,6-dioxaheptanoato)tin] and tri-n-butyltin 3,6,9-trioxodecanoate (Figure 4.4.12).35 These complexes have been reported by Gielen to possess high cytotoxicity.36 The interaction of these compounds with DNA was investigated by using circular dichroism spectroscopy, DNA melting experiments, and gel mobility shift assays. On the basis of the experimental results, it has been suggested that their interaction with DNA is not sequence- or base-specific and therefore most likely occurs at the level of external phosphate groups. Similar results have been also obtained by solution NMR studies of the interaction between diorganotin(IV) derivatives of azoles with some nucleotides by Pettinari et al.37 Ionic diorganotin(IV) N,C,N-chelates, containing a five coordinated tin center (Figure 4.4.13), have been reported to show cytotoxicity against seven human tumoral cell lines (MCF-7, EVSA-T, WiDr, IGROV, M19MEL, A498, H226).38

Figure 4.4.13

Structures of [2,6-bis(dimethylaminomethyl)phenyl]diorganostannyl salts

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Figure 4.4.14 Dibutyltin di(carboranecarboxylate), dicarboranetin (2,6-pyridine dicarboxylate) and dicarboranetin di(5-oxopyrrolidine-2-carboxylate)

A particular class of organotin carborane-containing derivatives has been reported by Gielen et al.39 to possess high anti-neoplastic activity. They can be constituted by carboranecarboxylates coordinated to tin through the COO, or with a carborane moiety directly bonded to tin, together with an additional tridentate (2,6-pyridine dicarboxylate) or two monodentate 2-l-pyrrolidone-5-carboxylates (Figure 4.4.14). These boron-containing systems are of potential interest in anti-cancer therapy by neutron capture. A new class of anti-tumor compounds, based on organotin(IV)-porphyrin derivatives, containing transition and Sn metal ions has been recently reported.40 The porphyrin acts as intercalating agent, which, by an attachment to the ring of specific linkers, could selectively bring the organotin moiety to the site where the lesion must be operated. The interaction of the organotin (IV) porphinate complexes towards DNA has been investigated as well as their cytotoxicity against P388 and A-549 tumor cells, with the percentage inhibitory effect of this family of compounds also being described. It seems that the cytotoxicity of organotin(IV)-porphinate could be related to the solubility in water of the compounds and the central ion in the porphyrin ring. The organotin[meso-tetra(4-sulfonatophenyl)]porphinate, organotin[meso-tetra(4carboxyphenyl)]porphinate and organotin(IV)chloro protoporphyrin IX derivatives (Figure 4.4.15) were also previously tested for their cytotoxicity toward immortalized mouse embryonic fibroblasts (NIH-3T3) and toward early developing embryos of Anilocra physodes.41 Diethyltin-N -(2-pyridylmetylene)-4-toluidine dichloride was tested against P338 leukemia in mice and showed anti-neoplastic effects. This compound has induced significant delay in cell cycles in mouse bone marrow cells.

Figure 4.4.15

Structure of dibutyltin porphinate derivatives

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The cellular glutathione (GSH) is a depleting agent: when the GSH level is low the extent of delay in cell cycle is reduced.42 The anti-cancer activity of Norfloxacin (TM) organotin polymers against normal Babl/3T3 cells has been reported,43 the order of the activity being the following: n-butyltin n-propyltin ethyltin > methyltin = n-octyltin = lauryltin Due to the non-toxic nature of the organotin polymers, they are prime candidates in the battle against cancer. They are currently undergoing further tests against various cancer cell lines. The in vivo anti-proliferative and anti-tumor activity of di-n-butyl and tri-n-butyl tin species towards Ehrlich ascites tumor IMC carcinoma, P388, and Sarcoma 180 has been reported. The cellular mechanism of the anti-proliferative activities reveals that the di-n-butyl- and tri-n-butyl-tin species selectively accumulate near to the nucleus, golgi apparatus, and endoplasmic reticulum in the cell and then destroy the structure of Golgi apparatus and endoplasmic reticulum, inhibiting the ceramide metabolism function, inositol triphosphate (IP3)-induced intracellular Ca2+ mobilization, and finally stopping the membrane-mediated signal transduction leading to DNA synthesis.44 Cytotoxic effects of organotin complexes of 2-(2,4-dichloroanilinocarbonyl)benzoic acid have been correlated to the geometry on tin.45 Tetrameric distannoxanes, which are the product of condensation between di-n-butyltin oxide and HOOCR (R = 2,3,4-(OCH3 )3 C6 H2 -, 4-(NH2 )-2-(OH)C6 H2 -, O(CH2 CH2 )2 NC(O)SCH2 -),46 have been shown to display very high activities, particularly the latter44a (Figure 4.4.16), which is highly active against MCF-7, EVSA-T, WiDr, IGROV, M19 MEL, A498, and H226 cell lines.

Figure 4.4.16

The tetrameric distannoxane 2-[(2,4-dichloroanilinocarbonyl)benzoate

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Figure 4.4.17

463

Structure of diorganotin(IV) salicyloxamates

Two diorganotin(IV) salicyloxamate derivatives (Figure 4.4.17), containing a tetracoordinate tin, have been reported to possess high potency compared with cisplatin, 5-fluorouracil, and etoposide against several human tumor cell lines, but less active than methotrexate and doxorubicin.47 Di-n-butyltin(IV) complexes of bis(carboxymethyl)benzylamines (Figure 4.4.18) have been reported to possess cytotoxicity toward several human tumor cell lines higher than that of cisplatin, 5-fluorouracil, and etoposide.48 Flufenamic acid (fluH) and organotin adducts have been evaluated for anti-proliferative activity. Among the compounds tested, [n-Bu2 (flu)SnOSn(flu)-n-Bu2 ] and [n-Bu2 Sn(flu)2 ] exhibited high cytotoxic activity against the cancer cell line A549.49 The cytotoxic activity of di- and triorganotin(IV) systems containing the O,S-donor O-cholesterylO-phenyl phosphorothioate ligand (Figure 4.4.19) have been investigated against KB, OVCAR-5, and SQC-1 UISO.50 This research originated from the knowledge that phosphoramidates themselves show considerable anti-cancer activity.51 Organotin(IV) systems, containing S-bonded ligands to tin, have also been explored as anti-tumor agents. Gielen et al. reported ortho-aminophenyl- and 2-pyridyl-thiolate triphenyl tin compounds and their anti-tumor activity,52 and later also those of some n-butyltin(IV) cysteaminates and N,Ndimethylcysteaminates.53 Diorganotin(IV) derivatives of 2-mercapto-6-nitrobenzothiazole have recently been reported and shown to exist in distorted six-octahedral and cis-trigonal bipyramidal geometry (Figure 4.4.20).54 Their in vitro activity tests show good inhibition rates of Ehrlich ascites carcinoma, the dibutyl tin compound being the most active. It is noteworthy that sulfur-bonded organotin anti-cancer complexes seem to be very rare; an interesting study was previously reported by Keppler on the anti-tumor activity of diorganotin(IV) dithiophosphorus derivatives against P388 leukemia.55 A three-dimensional QSAR study on a number of dibenzyltin(IV) dichloride and dibenzyltin(IV)diisothiocyanate derivatives with N,S-donor ligands (Figure 4.4.21) has been reported by Roy et al., showing significant cytotoxic activities against human cancer cell lines, compared to analogous

Figure 4.4.18

Di-n-butyltin(IV) complexes of bis(carboxymethyl)benzylamines

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Figure 4.4.19 iolate ligand

Structures of organotin(IV) derivatives containing the O-cholesteryl-O-phenyl phosphoroth-

Figure 4.4.20

Figure 4.4.21

Structures of diorganotin(IV) derivatives of 2-mercapto-6-nitrobenzothiazole

Structures of the organotin systems investigated in the 3-D QSAR study reported in Ref. 56

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Figure 4.4.22

465

Structures of cis-dihalotin(IV) di-acylpyrazolonates

dialkyltin(IV) derivatives through comparative molecular field analysis (CoMFA). Models operate on a set of compounds superimposed to reflect their common bonding orientation and describe the relative change in magnitude of the electrostatic and steric fields as a function of compound, sampled as function of spatial position around the compound set, and accounting for the variance in measured biological activity; these studies are used to produce the three-dimensional models which indicate the regions that effect the biological activity with a change in the chemical substitution.56 As the organic groups on tin have been recognized to play a key role on the toxicity side effects, a different approach on the design of tin anti-tumor systems has been proposed by Pettinari,57 involving the use of strong chelating acyl-pyrazolones as spectator ligands and halotin(IV) acceptors, similarly to previous Ti(IV), Zr(IV), Hf(IV), and Sn(IV) β-diketonates tested by Keppler.58 The systems reported are cis-dihalotin(IV) with an octahedral geometry in which two O,O- or N,N-chelating ligands are bonded to tin (Figure 4.4.22), some of them displaying moderate but definite anti-proliferative effects on some human melanoma cell lines, particularly toward SK-MEL-5, which is intrinsically resistant to all conventional treatment modalities. 4.4.3

Conclusion

This overview has demonstrated that tin and organotin derivatives could be applied for the development of new anti-tumor drugs. However a number of significant points need to be addressed: (i) most of the compounds reported to date are not soluble in water at physiological pH, so much effort should be directed to overcoming this obstacle; (ii) significant toxic side effects prevent their effective use as drugs, so the search for species which are more selective, and efficient at low dosage, represents a challenge; (iii) the action mechanisms of organotin on biological targets are often different or not well known, so new techniques need to be applied to understand where the organotin compounds exert their biological activity. References 1. W. N. Aldridge and J. E. Cremer, Biochem. J., 61, 406 (1955) and references therein; R. D. Kimbrough, Environmental Health Perspectives, 14, 51 (1976) and references therein. 2. G. J. M. Van der Kerk, and J. G. A. Luijten, J. Appl. Chem., 4, 314 (1954); G. J. M. Van der Kerk and J. G. A. Luijten, J. Appl. Chem., 6, 56 (1956). 3. M. Gielen and E. R. Tiekink, 50 Sn Tin Compounds and Their Therapeutic Potential in Metallotherapeutic Drugs and Metal-Base Diagnostic Agents: The Use of Metals in Medicine, M. Gielen and E. R. T. Tiekink (Eds), John Wiley & Sons, Ltd., Chichester, 2005, Ch. 22, pp. 421–439.

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4. (a) A. Penninks, M. Bol-Schoenmakers, and W. Seinen, Cellular Activity of Organotin Compounds in Relation to their Antitumor Series in Tin-Based Antitumor Drugs, M. Gielen (Ed.), NATO ASI Series, Springer, Berlin, 1990, p. 169; (b) C. Pettinari, F. Marchetti, A. Cingolani, A. Lonrenzotti, E. Mundorff, M. Rossi, and F. Caruso, Inorg. Chim. Acta, 262, 33 (1997). 5. (a) A. J. Crowe, Antitumor ctivity of Tin Compounds in Metal Compounds in Cancer Therapy, S. P. Fricker (Ed.), Chapman & Hall, London, 1994, pp. 147–179; (b) A. K. Saxena and F. Huber, Coord. Chem. Rev., 95, 109 (1989). 6. M. Gielen, Coord. Chem. Rev., 151, 41 (1996). 7. A. H. Penninks and W. Seinen, Vet. Q., 6, 209 (1984). 8. (a) M. Gielen, J. Braz. Chem. Soc., 14, 870 (2003), and the references cited therein; (b) N. Hoeti, J. Ma, S. Tabassum, Y. Wang, and M. Wu, J. Biochem., 134, 521 (2003); (c) N. M. Xanthopoulou, S. K. Hadjikakou, N. Hadjiliadis, M. Schurmann, K. Jurkschat, A. Michaelides, S. Skoulika, T. Bakas, J. Binolis, S. Karkabounas, and K. Charalabopoulos, J. Inorg. Biochem., 96, 425 (2003). 9. F. Chen, V. Vallyathan, V. Castranova, and X. Shi, Mol. Cell. BioChem., 222, 183 (2001). 10. A. Gennari, R. Bleumink, B. Vivani, C. L. Galli, M. Marinovich, R. Pieters, and E. Corsini, Toxicol. Appl. Pharmacol., 181, 27 (2002). 11. A. Gennari, R. Bleumink, B. Vivani, C. L. Galli, M. Marinovich, R. Pieters, and E. Corsini, Toxicol. Appl. Pharmacol., 169, 185 (2000). 12. J. Xiao, J. Cui, Y. Su, J. He, and J. Yao, J. Ch. Pharm. Sci., 2, 45 (1993). 13. D. De Vos, R. Willem, M. Gielen, K. E. Van Wingerden, and K. K. Nooter, Met. Based Drugs, 5, 179 (1998). 14. M. Gielen, P. Lelieveld, D. de Vos, H. Pan, R. Willem, M. Biesemans, and H. H. Fiebig, Inorg. Chim. Acta, 196, 115 (1992). 15. (a) M. Gielen, Appl. Organometal. Chem. 16, 481 (2002) and references reported therein; (b) M. Gielen, M. Biesemans, D. de Vos, and R. Willem, J. Inorg. Biochem., 79, 139 (2000); (c) F. P. Pruchnik, M. Banbula, Z. Ciunik, M. Latocha, B. Skop, and T. Wilczok, Inorg. Chim. Acta, 356, 62 (2003); (d) M. Gielen, R. Willem, H. Dalil, D. de Vos, C. M. Kuiper, and G. J. Peters, Met. Based Drugs, 5, 83 (1998); (e) M. Gielen, H. Dalil, B. Mahieu, D. de Vos, M. Biesemans, and R. Willem, Met. Based Drugs, 5, 275 (1998); (f) M. Gielen, A. El Khloufi, M. Biesemans, A. Bouhdid, D. de Vos, B. Mahieu, and R. Willem, Met. Based Drugs, 1, 305 (1994); (g) M. Gielen, H. Ma, A. Bouhdid, H. Dalil, M. Biesemans, and R. Willem, Met. Based Drugs, 4, 193 (1997); (h) M. Kemmer, M. Gielen, M. Biesemans, D. de Vos, and R. Willem, Met. Based Drugs, 5, 189 (1998). (i) J. Koshy, V. G. Kumar Das, S. Balabaskaran, S. W. Ng, and N. Wahab, Met. Based Drugs, 7, 245 (2000); (j) M. Gielen, H. Dalil, M. Biesemans, B. Mahieu, D. De Vos, and R. Willem, Appl. Organometal. Chem. 13, 515 (1999); (k) M. Gielen, H. Dalil, B. Mahieu, M. Biesemans, and R. Willem, Appl. Organometal. Chem. 12, 855 (1998). (m) M. Kemmer, L. Ghys, M. Gielen, M. Biesemans, E. R. T. Tiekink, and R. Willem, J. Organomet. Chem. 582, 195 (1999); (n) M. Kemmer, M. Biesemans, M. Gielen, E. R. T. Tiekink, and R. Willem, J. Organomet. Chem. 634, 55 (2001). 16. F. Caruso, M. Bol-Schoenmakers, and A. H. Penninks, J. Med. Chem., 36, 1168 (1993). 17. H.-D., Yin, Z.-J., Gao, and C.-H. Wang, Ch. J. Chem., 23, 928 (2005). 18. M. I. Khan, M. K. Baloch, and M. Ashfad, J. Organomet. Chem., 689, 3370 (2004). 19. M. I. Khan, M. Kaleem Baloch, and M. Ashfaq, Appl. Organometal. Chem., 19, 132 (2005). 20. M. I. Khan, M. K. Baloch, M. Ashfad, and Obaidullah, Appl. Organometal. Chem., 20, 463 (2006). 21. (a) F. Barbieri, F. Sparatore, M. Cagnoli, C. Bruzzo, F. Novelli and A. Alama, Chem.-Biol. Interact., 134, 27 (2001); (b) F. Barbieri, M. Viale, F. Sparatore, A. Favre, M. Cagnoli, C. Bruzzo, F. Novelli and A. Alama, Anticancer. Res., 20, 977 (2000). 22. (a) Q. Li, M. F. C. Guedes da Silva and A. J. L. Pombeiro, Chem. Eur. J., 10, 1456 (2004); (b) Q. Li, M. F. C. Guedes da Silva, Z. Jinghua and A. J. L. Pombeiro, J. Organomet. Chem., 689, 4584 (2004). 23. T. A. K. Al-Allaf, L. J. Rashan, A. Stelzner and D. R. Powell, Appl. Organometal. Chem., 17, 891 (2003). 24. M. Kemmer, H. Dalil, M. Biesemans, J. C. Martins, B. Mahieu, E. Horn, D. de Vos, E. R. T. Tiekink, R. Willem, and M. Gielen, J. Organomet. Chem., 608, 63 (2000). 25. M. Gielen, M. Biesemans, and R. Willem, Appl. Organometal. Chem., 19, 440 (2005).

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26. T. Lebl, A. Smicka, J. Brus, and C. Bruhn, Eur. J. Inorg. Chem., 143 (2003). 27. F. P. Pruchnik, M. Barbula, Z. Ciunik, H. Chojnacki, B. Skop, M. Latocha, and T. Wilczok, J. Inorg. Biochem., 90, 149 (2002). 28. L. Tian, Y. Sun, H. Li, X. Zheng, Y. Cheng, X. Liu, and B. Qian, J. Inorg. Biochem., 99, 1646 (2005). 29. L. Tian, Y. Sun, B. Qian, G. yang, Y. Yu, Z. Shang, and X. Zheng, Appl. Organometal. Chem., 19, 1127 (2005). 30. (a) A. Sanchez Gonzalez, J. S. Casas, J. Sordo, U. Russo, M. I. Lareo, and B. J. Regueiro, J. Inorg. Biochem., 39, 227 (1990); (b) J. S. Casas, A. Castineiras, E. Garcia Martinez, P. Rodriguez Rodriguez, U. Russo, A. Sanchez, A. Sanchez Gonzalez, and J. Sordo, Appl. Organometal. Chem., 13, 69 (1999); (c) P. Alvarez-Boo, J. S. Casas, M. D. Couce, E. Freijanes, A. Furlani, V. Scarcia, J. Sordo, U. Russo, and M. Varela, Appl. Organometal. Chem., 11, 963 (1997). 31. P. Alvarez Boo, J. S. Casas, E. E. Castellano, M. D. Couce, E. Freijanes, A. Furlani, U. Russo, V. Scarcia, J. Sordo, and M. Varela, Appl. Organometal. Chem., 15, 75 (2001). 32. P. Alvarez-Boo, J. S. Casas, A. Castineiras, M. D. Couce, E. Freijanes, A. Furlani, U. Russo, V. Scarcia, J. Sordo, and M. Varala, Inorg. Chim. Acta, 353, 8 (2003). 33. Y. Oshima, T. Akimoto, W. Tsukada, T. Yamasaki, K. Yamaguchi, and H. Kojima, Chem. Pharm. Bull., 17, 1492 (1969). 34. M. Nath, S. Pokharia, X. Q. Song, G. Eng, M. Gielen, M. Kemmer, M. Biesemans, R. Willem, and D. De Vos, Appl. Organometal. Chem., 17, 305 (2003). 35. A. Casini. L. Messori, P. Orioli, M. Gielen, M. Kemmer, and R. Willem, J. Inorg. Biochem., 85, 297 (2001). 36. M. Kemmer, M. Gielen, M. Biesemans, D. de Vos, and R. Willem, Met. Based Drugs, 5, 189 (1998). 37. C. Pettinari, F. Marchetti, and Q. Li, Main Group Met. Chem., 24, 53 (2001). 38. L. Dostal, A. Ruzicka, R. Jambor, V. Buchta, P. Kubanova, and J. Holecek, Met. Based Drugs, 9, 91 (2002). 39. (a) V. I. Bregadze1, S. A. Glazun, P. V. Petrovskii, Z. A. Starikova, V. Ya. Rochev, H. Dalil, M. Biesemans, R. Willem, M. Gielen., and D. de Vos, Appl. Organometal. Chem., 17, 453 (2003); (b) M. Gielen, F. Kayser, O. B. Zhidkova, V. Ts. Kampel, V. I. Bregadze, D. de Vos, M. Biesemans, B. Mahieu, R. Willem, Metal-Based Drugs, 2, 37 (1995); (c) M. Gielen, A. Bouhdid, R. Willem, V. I. Bregadze, L. V. Ermanson, E. R. T. Tiekink, J. Organometal Chem., 501, 277 (1995); (d) E. R. T. Tiekink, M. Gielen, A. Bouhdid, R. Willem, V. I. Bregadze, L. V. Ermanson, S. A. Glazun, Met. Based Drugs, 4, 75 (1997). 40. G. Han and P. Yang, J. Inorg. Biochem., 91, 230 (2002). 41. (a) G. Mirisola, A. Pellerito, T. Fiore, G. C. Stocco, L. Pellerito, A. Cestelli, and I. Di Liegro, Appl. Organomet. Chem., 11, 499 (1997); (b) A. Pellerito, T. Fiore, F. Maggio, A. M. Giuliani, L. Pellerito, and C. Mansueto, Appl. Organometal. Chem., 11, 707 (1997). 42. C. Syngai, B. Basu, S. Tushar, and A. Chatterjee, J. Environ. Pathol. Toxicol. Oncol., 20, 333 (2001). 43. W. D. Siegmann-Louda, E. C. Carraher Jr., M. Graham, R. Doucette, and L. Lanz, in: Proceedings of the 224th ACS National Meeting, Boston, MA, United States, August 18–22, 2002, PMSE-170 (Abstracts). 44. C. Pellerito, L. Nagy, L. Pellerito, and A. Szorcisk, J. Organomet. Chem., 691, 1733 (2006) and references therein. 45. S. Shahzadi, K. Shahid, S. Ali, M. Mazhar, and K. M. Khan, J. Iran. Chem. Soc., 2, 277 (2005). 46. (a) S. W. Ng, J. M. Hook, and M. Gielen, Appl. Organometal. Chem. 14, 1 (2000); (b) S. W. Ng, W. Chen, and V. G. Kumar Das, J. Organomet. Chem., 412, 39 (1991). 47. M. Gielen, H. Dalil, D. de Vos, M. Biesemans,, and R. Willem, Met. Based Drugs, 5, 265 (1998). 48. T. Mancilla, L. Carrillo, L. S. Zamudio Rivera, C. Camacho Camacho, D. de Vos, H. Rahier, M. Gielen, M. Kemmer, M. Biesemans, and R. Willem, Appl. Organometal. Chem., 15, 593 (2001). 49. D. Kovala-Demertzi, V. N. Dokorou, J. P. Jasinski, A. Opolski, J. Wiecek, M. Zervou, and M. A. Demertzis, J. Organomet. Chem., 690, 1800 (2005). 50. M. Lòpez-Cardoso, P. Garcia y Garcia, R. Cea-Olivares, and M.- L. Villareal, Met. Based Drugs., 8, 333 (2002). 51. K. Pankiewicz, R. Kinas, W. J. Stec, A. B. Foster, M. Jarman, J. M. S. Van Maanen, J. Am. Chem. Soc., 101, 7712 (1979). 52. M. Gielen, A. Bouhdid, E. R. T. Tiekink, D. de Vos, R. Willem, Met. Based Drugs., 3, 75 (1996).

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M. Gielen, K. Handlir, M. Hollein, and D. de Vos, Met. Based Drugs., 7, 233 (2000). C. Ma, Q. Jiang,, and R. Zhang, Appl. Organometal. Chem., 17, 623 (2003). B. K. Keppler, C. Silvestru, and I. Haiduc, Met. Based Drugs., 1, 73 (1993). P. M. Samuel, D. de Vos, D. Raveendra, J. A. R. P. Sarmac, and S. Roy, Bioorg. Med. Chem. Lett., 12, 61 (2002). C. Pettinari, F. Caruso, N. Zaffaroni, R. Villa, F. Marchetti, R. Pettinari, C. Phillips, J. Tanski, and M. Rossi, J. Inorg. Biochem., 100, 58 (2006). 58. (a) B. K. Keppler, C. Friesen, H. Vongerichten, and E. Vogel, in Metal Complexes in Cancer Chemotherapy, B. K. Keppler (Ed.), VCH Publishers, Weinheim, Germany, 1993, pp. 297–323; (b) T. Schilling, B. K. Keppler, M. E. Heim, G. Niebch, H. Dietzfelbinger, J. Rastetter, and A. R. Hanauske, Invest. New Drugs, 13, 327 (1996); (c) D. Searle, P. J. Smith, N. A. Bell, L. A. March, I. W. Nowell, and J. D. Donaldson, Inorg. Chim. Acta, 162, 143 (1989). 53. 54. 55. 56. 57.

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Impact of Organotin Compounds on Human Natural Killer Cells

4.5

469

Impact of Organotin Compounds on the Function of Human Natural Killer Cells

Margaret M. Whalen Department of Chemistry, Tennessee State University, Nashville, USA

4.5.1

Introduction

Humans have significant exposure to organotins (OTs) in food, drink, and various household products. OTs have been widely used in consumer and industrial products.1−4 Butyltins (BTs) and phenyltins (PTs) are two groups of OTs that have been shown to significantly contaminate the environment.1 Trin-butyltin compounds (TBTs) were painted onto ships and barges as an anti-fouling agent.1 It has been found in baking parchments made from siliconized paper and was found to transfer into cookies placed on TBT-containing baking parchments.4 It is found in fish that we eat, 5−9 and the average human intake of TBTs was estimated to be 2.29 μg per day in a market basket survey done in Japan in 1997.10 Di-nbutyltin compounds (DBTs) are used as stabilizers in PVC plastic products 2,11 and measurable levels have been found in plastic food containers.12 Beer, wine, and fruit juices stored in PVC containers during manufacturing contain BT residues.13,14 Drinking water also contains BTs due to leaching from PVC pipes.15 DBTs have also been used as a deworming agent for poultry, and turkey liver samples have been shown to contain DBT.16 Levels of TBTs as high as 198 μg kg−1 and DBTs as high as 216 μg kg−1 have been found in seafood in certain areas.17 Triphenyltin compounds (TPTs) are used as fungicides on major food and food-stock crops.18,19 Recent yearly usage of TPTs in the US (1999 and 2000 agricultural data combined) was about 136 360 kg year−1 .20,21 TPTs have also been used in anti-fouling paints.22,23 Due to widespread use, TPT contamination has been reported in water, sediment, and fish from both freshwater and marine environments in the United States, Europe, and Japan.23−28 Trimethyltin compounds (TMTs) have been found in some PVC products at levels of 8.5–24.9 μg g−1 .29 There have been documented cases of accidental acute exposures to TMTs.30,31 Additionally, measurable levels of TMTs have been found in urine samples from humans who had not experienced an acute exposure.32 Both TBTs and DBTs have toxic effects in animal cells, including immunotoxic effects.33−40 Rats fed TPTs showed diminished thymus-dependent immune responses.41 Additionally, TPTs produced tumors in rats and mice.41,42 TMTs have appeared to be primarily neurotoxic.43−45 Humans are accumulating BTs. They have been found in human blood 46,47 and liver samples.4,48 Levels found in human blood samples ranged from undetectable to 261 nM (85 ng ml−1 ) for TBTs and from undetectable to 309 nM (94 ng ml−1 ) for DBTs.46,47 NK cells are a sub-set of lymphocytes that destroy (lyse) tumor cells, virally infected cells, and antibodycoated cells. They are defined by the absence of the T cell receptor/CD3 complex and the presence of CD56 and/or CD16 on their surface.49 NK cells lyse the above mentioned target cells without prior sensitization, putting them at the forefront in our immune defense against tumor and virally infected cells.49−52 Binding of tumor cells stimulates a series of enzyme activities leading to activation of mitogen-activated protein kinases (MAPKs).51−75 Activation of the MAPK p44/42 (ERK1/2) appears to be involved in the release of cytotoxic proteins (granzyme B and perforin) onto targets.67−70 Other MAPKs, p38 and Jun N-terminal kinase (JNK), also have a role in the regulation of NK cells.68,71 We have shown that TBTs, DBTs, TPTs, and TMTs, as well as other OTs, can significantly reduce the lytic function of human natural killer cells.46,76−81 Thus, OTs may promote cancer development and viral infection in humans, due to their capacity to interfere with the ability of NK cells to lyse their targets. This chapter is a summary of studies examining the effects of several OTs on the function of human NK cells. These include the effects of the various OTs on lytic function, ability to bind to target cells, intra-cellular ATP levels, and the functions of enzymes (MAPKs) that are critical to the lytic function of NK cells.

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Table 4.5.1 Effects of tri-n-butyltin chloride on the lytic function, binding function, ATP levels, CD16 levels, CD56 levels, granzyme B levels, and perforin levels of human NK cellsa % Decrease

[TBTC] nM

Length of exposure

Lysis

Binding

ATP

CD16

CD56

Granzyme B

Perforin

500 400 300 200

1h 1h 1h 1h 24 h 48 h 6 days 24 h 48 h 6 days 24 h 48 h 6 days 24 h 48 h 6 days 1 h/24 hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db

81 54 36 NS 95 87 99 56 65 99 33 45 81 22 26 59 76 74 90 64 78 85 24 34 24 NS NS 18 NS NS 16

– – NS NS 39 – – – – – – – – – – – NS 32 – – – – – – – – – – – – –

20 – NS NS 25 70 97 NS 24 70 NS 25 50 NS 10 27 14 33 100 NS 47 88 NS 26 50 NS 19 30 NS NS 15

– – NS – 37 – – – – – – – – – – – 18 13 – – – – – – – – – – – – –

– – NS – NS – – – – – – – – – – – NS 26 – – – – – – – – – – – – –

– – NS – 51 – – – – – – – – – – – 44 58 – – – – – – – – – – – – –

– – NS – 47 – – – – – – – – – – – 52 59 – – – – – – – – – – – – –

100 50 25 300 200 100 50 25

a

Data summarized from references 46,76,77,82,88–92 NK cells were exposed to TBTC for 1 h after which the TBTC was removed and the cells were washed twice in TBTC-free media and then suspended for 24 h, 48 h, or 6 days in TBTC-free media; NS, not significant; –, not determined b

4.5.2

Effects of n-butyltin Chlorides on Human NK Cell Function: Tri-n-Butyltin Chloride (TBTC)

Effects on Lytic Function

Exposure of NK cells to 300 and 500 nM TBTC for 1 h produces significant decreases in their lytic function (Table 4.5.1).76,82 A concentration of 300 nM TBTC decreased lytic function by 36%, while 500 nM caused about an 80% decrease in this function. NK cells exposed to TBTC for 24 h showed significant decreases in lytic function at 25–200 nM TBTC (Table 4.5.1),82 with 200 nM TBTC causing a greater than 90% loss of lytic function and 100 nM causing about a 60% decrease. Decreases in lytic

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471

function after 48 h exposures to 25–200 nM TBTC were similar to those seen after 24 h exposures. However, after a 6 d exposure to TBTC there was a loss of lytic function of about 60% at concentrations as low as 25 nM.82 We have also found that exposure of NK cells to TBTC persistently and progressively inhibited lytic function after the compound was removed. Exposure to 300 nM TBTC for 1 h caused a 36 % decrease in lytic function.76,77,82 However, cells exposed to 300 nM TBTC for 1 h, followed by removal of the TBTC-containing media and incubation in TBTC-free media for 24 h, 48 h, or 6 d prior to assaying for lytic activity, showed greater losses of lytic function than those tested immediately following the 1 h exposure (76, 74, and 90% respectively, Table 4.5.1). A similar progressive loss of lytic function was seen following a 1 h exposure to 25–200 nM TBTC. These results have implications regarding the dangers of an acute exposure to TBTC. Effects on Target-Binding Function

The ability of an NK cell to lyse its target is dependent on the capacity of the NK cell to physically bind to the target. Thus, it is important to determine if a loss of binding function can account for the losses in lytic function that are seen with TBTC exposures. When the ability of NK cells to bind to tumor target cells was examined after a 1 h exposure to 300 nM TBTC there was no loss of binding function, this was true in the 24 h period following a 1 h exposure to 300 nM TBTC as well (Table 4.5.1).46,76,77 A 24 h exposure to 200 nM TBTC caused a 39% decrease in binding function, but caused a 95% decrease in lytic function (Table 4.5.1). Thus, it appears that while TBTC can interfere with binding function to some extent, this interference is not sufficient to account for the loss of lytic function. Effects on ATP Levels

TBTC has been shown to decrease ATP levels in rodent cells 83,84 and to inhibit ATP synthesis in rat liver mitochondria.85 It has been shown to bind to the Fo portion of F-ATP synthase in bacteria86 and to inhibit the ATP synthase of bovine heart sub-mitochondrial particles.87 Thus, TBTC may be decreasing ATP levels in NK cells and the lowered ATP levels may be, at least in part, responsible for the loss of lytic activity. ATP levels were measured in TBTC-exposed NK cells and the percentage decrease in ATP levels, as compared to control cells, after various TBTC-exposures is given in Table 4.5.1.82,88 The results from these studies indicate that TBTC exposures can result in decreases in ATP levels. However, TBTC-induced decreases in lytic function do not show a significant association with decreased ATP levels, except after 48 h and 6 days. These results indicate that the loss of lytic function induced by exposures to TBTC cannot be accounted for solely by TBTC-induced decreases in ATP levels. Effects on Protein Expression

Our recent studies have shown that a consequence of TBTC exposure is alteration of the expression of specific proteins of functional importance to the NK cell.89−92 These include CD16, CD56, granzyme B, and perforin (Table 4.5.1). Granzyme B and perforin are proteins secreted by the NK cell onto the target cell that lead to the death of the target cell,49 while CD16 and CD56 are cell surface proteins that define NK cells and may be involved in NK binding to certain targets.49 Exposure to higher TBTC concentrations for 1 h caused no immediate changes in the levels of expression of any of these proteins. But, there were notable decreases in expression of CD16, granzyme B, and perforin in the 24 and 48 h periods following a 1 h exposure (Table 4.5.1).89−91 These decreases were not the result of a generalized decrease in the expression of proteins in the NK cells, as several other proteins that were examined showed no change in expression.89 A 24 h continuous exposure to a lower concentration (200 nM TBTC) also resulted in

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Tin Chemistry: Fundamentals, Frontiers and Applications Table 4.5.2 Effects of tri-n-butyltin chloride on the activation of the mitogen-activated protein kinases (MAPKs), p44/42, p38, and JNK in human NK cellsa Fold activation compared to control

[TBTC] nM

Length of exposure

p44/42

p38

JNK

300 200 100 50 25 300 200 100 50 25

10 min 10 min 10 min 10 min 10 min 6h 6h 6h 6h 6h

8.8 5.3 2.7 1.6 NS 5.3 4.6 NS NS NS

2.9 1.8 1.8 2.0 NS 2.1 1.6 NS NS NS

1.8 1.8 2.0 2.4 1.5 NS NS NS NS NS

a

Data summarized from references 93 and 94

significant decreases in expression of all of these proteins (Table 4.5.1).89−91 These results indicate that a consequence of exposure to both higher and lower levels of TBTC is a disruption of protein expression for specific proteins. Effects on Mitogen-Activated Protein Kinase (MAPK) Activity

MAPKs have been shown to regulate the release of cytotoxic granules (regulated by p44/42 activation), as well as protein synthesis (p44/42, p38, and JNK).67,68,72 Thus, alterations of their activation state by TBTC would disrupt the cytotoxic function of NK cells. Exposure to 300 nM TBTC activates p44/42 by 8.8 ± 4 fold within 10 min and decreases cytotoxic function by approximately 40% within 1 h of exposure (Tables 4.5.1 and 4.5.2).93,94 This level of p44/42 (ERK1/2) activation may leave an inadequate pool of p44/42 available to be activated when the NK cell subsequently comes into contact with a tumor target cell. Thus, a decrease in NK cytotoxic function would be seen, essentially, immediately after such an exposure. However, exposure to 200 nM TBTC causes a smaller activation of p44/42 (6 ± 3 fold) and the decrease in cytotoxic function takes longer to occur [no decrease after 1 h, ∼90% decrease after 24 h or 24 h following a 1 h exposure (Table 4.5.2)].82 p38 and JNK activation predominate at concentrations of TBTC below 100 nM.93,94 The activation of the various MAPKs seen after 10 min persists out to 1 h, but begins to decrease by 6 h (Table 4.5.2). 4.5.3

Di-n-Butyltin Chloride (DBTC)

Effects on Lytic Function

Table 4.5.3 summarizes the effects of various DBTC exposures on the lytic function of human NK cells. Exposure of NK cells to 10 μM DBTC for 1 h produced a significant decrease in their lytic function.46,76,78,95 A 6 d exposure to as little as 500 nM DBTC caused significant decreases in lytic function. As with TBTC, exposure to DBTC caused a persistent loss of lytic function.76,78,95 In general, DBTC appears to be about 5 to 10 times less effective at decreasing lytic function.

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Table 4.5.3 Effects of di-n-butyltin chloride on the lytic function, binding function, ATP levels, CD16 levels, CD56 levels, granzyme B levels, and perforin levels of human NK cellsa % Decrease

[DBTC] μM

Length of exposure

Lysis

Binding

ATP

CD16

CD56

Granzyme B

Perforin

10 5 2.5 1.5 1

1h 1h 1h 24 h 24 h 48 h 6 days 24 h 48 h 6 days 24 h 48 h 6 days 24 h 48 h 6 days 1h/24hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db 1 h/24 hb 1 h/48 hb 1 h/6 db

92 NS NS 98 93 94 95 46 62 95 NS NS NS NS NS NS 91 98 100 56 67 99 NS NS 40 NS NS NS

– NS – 33 – – – – – – – – – – – – 48 80 – – – – – – – – – –

NS –NS NS – 48 97 100 NS 60 99 NS 24 53 NS NS NS 51 85 99 NS 22 85 NS 18 31 NS NS NS

– – – 56 – – – – – – – – – – – – 60 77 – – – – – – – – – –

– – – NS – – – – – – – – – – – – 29 55 – – – – – – – – – –

– – – 62 – – – – – – – – – – – – 35 49 – – – – – – – – – –

– – – 49 – – – – – – – – – – – – 70 67 – – – – – – – – – –

0.5 0.2 0.1 5 2.5 1 0.5

a

Data summarized from references 46, 76, 78, 95, 97, 98 NK cells were exposed to DBTC for 1 h after which the DBTC was removed and the cells were washed twice in DBTC-free media and then suspended for 24 h, 48 h, or 6 days in DBTC-free media; NS, not significant; –, not determined b

Effects on Target-Binding Function

As mentioned above, if a compound is able to diminish the ability of NK cells to lyse target cells, then it is important to determine if that compound is having any effect on the capacity of the NK cells to bind to target cells. The data in Table 4.5.1 show that TBTC caused decreases in the ability of NK cells to bind to target cells after a 24 h continuous exposure and in the 48 h period following a 1 h exposure. Table 4.5.3 shows that DBTC has similar effects on binding (although it takes higher concentrations of DBTC) to those seen with TBTC.46,76,78 As with TBTC, the loss of lytic function cannot be entirely accounted for by DBTC-induced decreases in binding function. Effects on ATP Levels

DBTC, like TBTC, appears to interfere with the function of ATP synthase.96 Table 4.5.3 indicates the decreases in ATP levels that occurred when NK cells were exposed to DBTC.88,95 These decreases did not

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associate with decreased lytic function until the 48 h time point. Like TBTC, the decreases in ATP levels seen with DBTC exposures tended to take a longer time to manifest than did decreases in lytic function.95 Thus, DBTC-induced decreases in ATP could not entirely account for the losses of lytic function. Effects on Protein Expression

Exposure of NK cells to DBTC resulted in alteration of the expression of specific proteins of functional importance to the NK cell.97,98 The percentage decrease in the expression of the lytic proteins, granzyme B and perforin, and the cell surface proteins, CD16 and CD56, in response to DBTC exposures are given in Table 4.5.3. NK cells exposed to DBTC at a concentration of 5 μM for 1 h showed no immediate changes in the levels of expression of any of these proteins. But, there were notable decreases in expression of CD16, granzyme B, and perforin in the 24 and 48 h periods following a 1 h exposure to 5 μM DBTC (Table 4.5.3). A 24 h continuous exposure to a lower concentration (1.5 μM DBTC) also resulted in significant decreases in expression of each of these proteins (Table 4.5.3). These results indicate that, like TBTC, exposure to both higher and lower levels of DBTC caused a disruption of protein expression for specific proteins. Effects on Mitogen-Activated Protein Kinase (MAPK) Activity

In preliminary experiments, we have shown that DBTC exposure, like TBTC, is able to increase the activation of MAPK. The activations of the MAPK, p44/42, seen with DBTC appear to be less intense than those seen with TBTC, while the activations of p38 and JNK seem to be similar to those seen with TBTC. 4.5.4

Effects of Trimethyltin Chloride (TMTC) on Human NK Cell Function

The following series of triorganotins differ significantly in their hydrophobicity: trimethyltin< dimethylphenyltin<methyldiphenyltin B > T, while the decrease for LAURDAN was T > M > B. The DPH anisotropy values decreased in the presence of 30 μM of (CH3 CH2 CH2 CH2 )3 SnCl, for the T fraction by 95%: for the M fraction by 85% and for the B fraction by 93%. Concerning the LAURDAN values, in the presence of 30 μM (CH3 CH2 CH2 CH2 )3 SnCl, the T fraction decreased by 22%, the M fraction by 25%, and the B fraction was not affected by the presence of the organotin. Simple organotin structures such as (C6 H5 )4 Sn, (C6 H5 )3 SnCl, and (C6 H5 )2 SnCl2 , were studied for their interactions in model DPPC membranes and their effect on red blood cells, using hemolytic tests.16 In order to test whether phenyltin derivatives have to penetrate the lipid bilayer to inflict damage on the plasma membrane, the following experiments were carried out using DPPC vesicles. The location assays were monitored by steady-state 1 H NMR and steady-state fluorescence spectroscopy (employing fluorescent probes). The (C6 H5 )4 Sn did not partition into the membrane, therefore the damage that it caused to the stability of the vesicle was negligible as was the result of the hemolytic test. Although (C6 H5 )3 SnCl and (C6 H5 )2 SnCl2 absorbed almost equally into the lipid bilayer, the former was specific towards the head-group regions, while the latter was specific for the hydrophobic-tail regions. Another

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Biological Aspects of Organotins

TPTC

TPTC

Polar section Sn

DPTD

487

TPT

DPTD Apolar section

DPTD

TPTC

Sn Cl

DPTD DPTD Sn

Cl

TPTC

TPTC

Figure 4.6.3

Cl

Polar section

Selectivity of organotin compounds towards model bilayer

important evidence in support of these findings was found in hemolytic potency. This seemed to be directly related to the location of the specific phenyltin derivative in the lipidic membrane. The (C6 H5 )3 SnCl, which remained at the surface (head group DPPC region) of the bilayer, possessed the highest degree of hemolysis, and the (C6 H5 )2 SnCl2 , which penetrated much deeper into the inter-layer region caused a smaller degree of hemolysis (Figure 4.6.3). These results clearly indicate a correlation between hemolysis and positioning of the tin compounds in the bilayer. The analysis of the chemical structure of phenyltin compounds is thus directly linked to their ability to absorb and penetrate the biological membranes. In general, phenyltins contain hydrophobic phenyl rings bonded to a positively charged tin atom. Since it has been demonstrated that they act differently because they are located in different parts of the membrane, a study to determine the effect of (C6 H5 )2 SnCl2 and (C6 H5 )3 SnCl on the permeability of S2 O2− 4 ions across a phosphatidylcholine liposome membrane, and the location of the two phenyltin compounds was reported by Gabrielska and coworkers.14 The ability of phenyltin compounds to facilitate S2 O2− 4 ion transfer was evaluated by a fluorescent quenching assay, which showed that both derivatives affect ion transfer, but that the dependence of dithionite transport on phenyltin concentration is different; (C6 H5 )3 SnCl is more efficient than (C6 H5 )2 SnCl2 above 20 μl. Since the results of the studies of organotin compounds on hemolysis were not simple, and a feasible mechanism of action had not been reported, in 2002 Burda and coworkers42 proposed a molecular mechanism for some triorganotin and triorganolead compounds for comparison purposes. The receptors were erythrocytes obtained from fresh hog blood. The trends in hemolytic activity were as follows: (C6 H5 )3 PbCl > (CH3 CH2 CH2 )3 SnCl = (C6 H5 )3 SnCl > (CH3 CH2 )3 PbCl > (CH3 )3 SnCl.42 The trend directly indicates that the hemolytic activity increases with the hydrophobicity of the organic ligands, as well as supporting the premise that compounds containing lead are more toxic than their analogous tin compounds. The (C6 H5 )3 SnCl compound was very effective for the lysis of hog erythrocytes. This species was determined by119 Sn Mössbauer spectroscopy to interact with the protein components of these erythrocyte membranes in a highly selective way, and interaction of (C6 H5 )3 SnCl with hemoglobin was detected by these spectroscopic studies. The doublet signal fitting of the Mössbauer spectra carried out

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for the lyophilized fractions of supernatants containing hemoglobin, sediments containing erythrocyte membranes, and intact red blood cells, led to the conclusion that Nhet from histidine and/or Sthiol from cysteine were the only possible donor atoms for (C6 H5 )3 SnCl species. Despite the fact that (C6 H5 )3 SnCl caused a high degree of hemolysis, it was not detected in the supernatant. This observation is an important proof that hog hemoglobin does not have strong binding sites for organotin compounds. So far, only rat and cat hemoglobins have been found to strongly bind organotin compounds through the Sthiol and Nhet donor atoms.78−80 These observations strongly suggest that, due to its location in the hydrocarbon core of the bilayer, the hydrophobic (C6 H5 )3 SnCl may interact with the integral proteins, as well as with cytoskeleton proteins, such as ankyrin and β-spectrin42 in which the amino acid sequences have cysteine (C) and histidine (H) residues. Another important variable in the systematic analysis of organotin compounds is the surface charge depolarization of real and model membranes observed for model neutral membranes.81 This led to a comparative study of the influence of the membrane surface charge on its interaction, not only with organotin compounds, but also with organolead derivatives, using dodecyltrimethylammonium bromide and dodecylsulphonate to modify the bilayer lipid membranes (BLMs). The main conclusion was that the effectiveness of depolarization in alkyltin compounds increased with increasing molecular weight.81 For the membranes studied, the trialkyltin(IV) derivatives were the most active. Also, a strong correlation was found between the depolarization activity of the compounds tested and the lipophilicity of the hydrolysis products of the organotin, while the effect of the surface charge was secondary. In the case of phenyltin and phenyllead derivatives the activity/depolarization presented the following trend (C6 H5 )3 PbCl > (C6 H5 )3 SnCl > (C6 H5 )2 SnCl2 82 denoting also that the activity of these compounds is dependent on the lipophilicity of their water reaction products. 4.6.4

Roles of Organotin Compounds in Cell Function

Organotins are xenobiotic to human cells and affect them to different levels, depending on the cell type and the structure of the tin-containing molecule. Nonetheless tin has been categorized as an ultratrace essential element at concentrations of 2 × 10−6 % (w/w) in the human body. Given that many organotin compounds have been widely employed as anti-foulings agents and as biocides, and due to their importance as prototype anti-neoplastic agents, the effects produced by organotin compounds in normal and abnormal cells have been the subject of research for the last four decades. Although organotins have become prototypes for anti-tumor pharmaceuticals, (CH3 CH2 CH2 CH2 )3 SnCl exposure has negative effects on the immune system in aquatic organisms tested both in vivo85 and in vitro.86,87 In order to track the effects of organotin compounds on various cells and cell functioning, their interaction with human natural killer (NK) cells, which are important for the regulation of malignancies, was investigated.88 NKs are a sub-set of lymphocytes capable of killing tumor cells, virally infected cells and antibody-coated cells.88 The mechanism of action of NK cells involves the release of granules containing the cytotoxic proteins perforin and granzyme B. Perforin release is stimulated by the contact of NKs with target cells and is thus polymerized into the membrane of the target cell. As denoted by its name, it has the ability to form pores through the membrane,89 which are utilized by granzyme to enter the cytosol of the target cell.90 In this way, the NKs are believed to be part of one of the first mechanisms against cancer and other malignancies. Since measurable levels of (CH3 CH2 CH2 CH2 )3 SnCl have been detected in human blood samples,55,91 studies illustrating the ability of this organotin to decrease the capability of NKs to destroy target cells in vitro92 are of great importance. The studies showed that 1 h exposure of NK cells to (CH3 CH2 CH2 CH2 )3 SnCl caused persistent inhibition of the NK cells’ ability to destroy tumor cells for 24 and 48 h periods following exposure. Moreover, loss of function could

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be significantly prevented and/or reversed if the NK-stimulatory interleukins (IL)-2 or -12 were present during the 24 and 48 h periods, as shown by Whalen and coworkers.92 It was further demonstrated by Thomas25,93 that (CH3 CH2 CH2 CH2 )3 SnCl exposure may disrupt the transcription of the genes of the cytotoxic granzyme B and perforin. Therefore, (CH3 CH2 CH2 CH2 )3 SnCl was able to significantly decrease the mRNA, the protein levels, and the phosphorylation of cAMP-response-element-binding protein(CREB) under these conditions. As stated by this group25,88,93 IL-2 appeared to prevent/reverse (CH3 CH2 CH2 CH2 )3 SnCl-induced decline in perforin protein levels, in the 24 h period following a 1 h exposure to 300 nM (CH3 CH2 CH2 CH2 )3 SnCl; however, the granzyme B protein, its mRNA, and CREB phosphorylation, were not reversed by the presence of either IL-2 or IL-12. Certainly, other relevant effects are those concerning triggering of apoptosis by organotin compounds. Trisubstituted organotin compounds increase intra-cellular Ca2+ levels and subsequent DNA fragmentation, causing cell death, for example during exposure to (CH3 CH2 CH2 CH2 )3 SnCl in thymocytes.94,95 The induction of apoptosis in tunicate hemocytes with 10 μM (CH3 CH2 CH2 CH2 )3 SnCl96 has also been demonstrated. The apoptosis of these cells induced by (CH3 CH2 CH2 CH2 )3 SnCl was monitored by the condensation of chromatin, fragmentation of DNA, translocation of phosphatidyl serine, and the loss of membrane permeability. The presence of (CH3 CH2 CH2 CH2 )3 SnCl and (C6 H5 )3 SnCl in PC-12 (pheochromocytoma of the adrenal medulla) cells caused a rapid increase in calcium levels and apoptosis, while (CH3 CH2 )3 SnCl caused only slight increments of intra-cellular calcium, and (CH3 )3 SnCl had no effect.97 In particular, in HL-60 (promyelocytic) cells, exposure to 5 μM (C6 H5 )3 SnCl26 increased the intra-cellular Ca2+ within seconds, followed by actine depolymerization, activation of transcription factor (NF-κB), apoptotic bodies, and DNA fragmentation. A final step in the cascade of (C6 H5 )3 SnCl effects in PC-12 was the release of the tumor necrosis factor-α (TNF-α). In the case of Jurkat T cells, the presence of 2 μM (CH3 CH2 CH2 CH2 )3 SnCl also increased the intra-cellular calcium levels; this was followed by mitochondrial hyperpolarization, with subsequent loss of membrane potential, associated with the rapid release of mitochondrial cytochrome C and delayed activation of type II caspases.34 Consequently, these cells avoided the normal induction of apoptosis, thus suggesting that the mitochondrial permeability is not directly coupled to the presence of (CH3 CH2 CH2 CH2 )3 SnCl. In this study34 it was shown that (CH3 CH2 CH2 CH2 )3 SnCl also hindered, through the rapid elevation of calcium contents, the production of glycolytic ATP. As stated previously, organotin compounds interfere with the production of ATP, and since ATP is the energy provider of living cells and most of it is synthesized by F1 F0 ATP synthase (ATPase), a specific attempt was made to ascertain a plausible mechanism of action of organotin compounds with ATPase.9 The (CH3 CH2 CH2 CH2 )3 SnCl inhibits ATP hydrolysis by Na+ translocation of ATPase from Ilyobacter tartaricus, or the H+ translocation counterpart for Escherichia coli, within an apparent inhibition constant of 200 nM. In this study, the inhibition site was targeted by using a tritium-labelled (CH3 CH2 CH2 CH2 )3 SnCl derivative, the diaziridinebenzyloxymethyldi-n-butyltin chloride (DDBTC, Figure 4.6.4).

N TBTC, R=nBu Sn

Figure 4.6.4

R Cl

DDBTC, R=

N CF3

O 3 H

Molecular design employed for targeting of the active site of ATPase

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X Sn X

Figure 4.6.5

L

X Cl, NCS Cl, NCS Cl, NCS Cl, NCS Cl, NCS Cl, NCS

L --Pyridine Imidazole Thiophene Bipyridine Phenanthroline

Di-benzyl-tin-X2 -L derivatives employed for in vitro tests in Na+ /K+ -ATPase system

The in vitro effects on Na+ /K+ -ATPase from rat brain synaptic membranes of two series of di-benzyltin-X2 -L derivatives (Figure 4.6.5), where the X substituents were chloride and isothiocyanate, L was pyridine, imidazole, thiophene, bipyridine, and phenanthroline, or was not present in the case of the free organotin, have been investigated.10 The order of potency of the activity of ATPase was Cl > SCN, showing IC50 values that ranged from 64 to 252 μM for the former, and from 132 to 399 μM for the latter. In the chloride series, the toxicity followed the order thiophene > phenanthroline > bipyridine > pyridine > imidazole. Almost the reverse tendency was found for the thiocyanate derivatives, namely pyridine > imidazole > thiophene > phenanthroline > bipyridine. In both series, the free organotin compounds were less toxic, implying biological reaction mechanisms through substitution kinetics and a high dependence on solubility and coordination number. Another interesting study was carried out in an effort to validate a cellular test system based on cultured networks as an effective test platform for applications in neurotoxicology, using two different mice CNS tissues. These tissues exhibited characteristic changes in electrophysiological activity upon treatment with (CH3 )3 SnCl as the neurotoxicant.51 It was found that in the presence of 1–2 μM (CH3 )3 SnCl, the spinal cord networks began to respond and shut off activity at 4–7μM, and auditory cortex cultures started a response at 2–3 μM and shut off at 4–7 μM. These results suggested that neuronal networks cultured on microelectrode arrays might allow quantitative determination of acute functional toxicity. In particular, these networks could provide data on functional damage, reversible morphological damage, and irreversible damage, allowing the correlation of functional and structural damage. Hepatocytes are cells involved in many biochemical processes, such as the syntheses of proteins, phospholipids, and bioconjugate molecules, protein storage, carbohydrate transformation, and detoxification, modification and excretion of exogenous and endogenous substances, etc. Additionally, organotin compounds are known for their hepatotoxicity,98 immunotoxicity,99 and as mitochondrial toxins.98,100 In order to understand the metabolic mechanisms of organotin compounds, their interaction with hepatic cells needs to be investigated. Therefore, very recent efforts to correlate hepatotoxicity with mitochondrial damage were carried out by Jurkiewicz and coworkers,30 by studying the effects of (CH3 CH2 CH2 CH2 )3 SnCl in rat hepatocytes. This group included a detailed biochemical pathway, which main steps of this pathway can be summarised as follows: (i) the exposure of hepatocytes to 2.5 μM (CH3 CH2 CH2 CH2 )3 SnCl released cytochrome c from the mitochondria to the cytosol after 15 min; (ii) the activities of the main effector caspase-3, caspase-8, and caspase-9 initiator increased after 30 min of (CH3 CH2 CH2 CH2 )3 SnCl exposure (the latter resembles mitochondria pathway activation); (iii) the activation of the death receptor pathway by the mobilization of the adaptor protein FADD from the cytosol to the membrane occurred after 15 min; (iv) a decrease of pro-apoptotic proteins in the cytosol and an increase in their cleaved forms occurred; (v) the appearance of apoptotic substrates (poly(ADP-ribose) polymerase, DNA fragmentation factor-45) dismantled the cell due to their cleavage by caspase-3; and

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(vi) the disappearance of the DNA fragmentation factor-45 was also noted. This effort is a good example of the chain of events caused by organotins in the functioning of cells. The (CH3 CH2 CH2 CH2 )3 SnCl and (C6 H5 )3 SnCl compounds have been proposed as potential competitive inhibitors of aromatase (which is related to the androgen-to-estrogen conversion by cytochrome P450) and in this particular case their active concentrations are quite high for real applications, since they are toxic to mammalian cells.23 Investigations on female Nucella lapillus suggested an association between the change in testosterone titer and exposure to (CH3 CH2 CH2 CH2 )3 SnCl,101 these events are propitiated through imposex endocrine disruption, and they have been observed at very low concentrations (0.5 ng l−1 of (CH3 CH2 CH2 CH2 )3 SnCl.102 Furthermore, organotin compounds, such as (CH3 CH2 CH2 CH2 )3 SnCl and (C6 H5 )3 SnCl, are low dose active species in human ovarian granulose cells, suppressing the aromatase activity at nanomolar quantities (about 60 nM) and also restraining the gene expression.28 On the other hand, in human choriocarcinoma cells, organotins enhance estrogen biosynthesis and the activation of aromatase at the same low concentrations, decreasing DNA and protein synthesis.23,29,72 These discrepancies found in the action of organotin compounds against aromatase are highly dependent on the type of tissue in which the enzyme is expressed and clearly show that specific toxicities are correlated to the type of cells exposed. 4.6.5

Aspects of Organotins in Structural and Molecular Biology

The molecular design of traditional pharmaceuticals, involving evolutionary algorithms coupled with heuristic and robust fitting procedures, could provide novel lead compounds that may be very different from the traditional chemicals employed to counterattack a disease, even in cases where the molecular structure of a possible substrate or biological receptor, or both, are not known. This disadvantage has been due to the limited variability of organotin compounds employed for structural and biomolecular studies. Another important point to take into account is the diversified behavior of metals in biological systems, which hinders the construction of reliable approaches to model or predict the interactions of compounds containing them, or even their in vitro/in silico testing with biological receptors.103 Due to the non-specificity of common organotin compounds towards biologic systems, further systematic studies would require the coupling of known therapeutic agents or strategically substituted molecules with organotin fragments in order to track variables and understand the tendencies in observed biological activities;103−105 an initial effort to ascertain this has been reported recently.106 Structure–activity relationship studies of tin compounds have been a new area of research,106,107 originally recognized two decades ago by Saxena.108 An important finding for tin-based drug design was that these chemicals have long been known to localize in tumor tissue.108−110 Since then, tin compounds have proven promising in the fight against cancer, leading to important advances to modulate toxicity,111 to describe their interactions with model112−114 and real cell membranes,42 and for the systematic modulation of lyophilic properties115−118 and other pharmaceutical goals,119−121 such as their interactions with cell functioning and specific parts of the cell.8,9,33,122,125 The main variations led to inclusion of heteroelements, and of different Caliphatic or Caromatic substituents at the tin center124−127 moving away from the traditional organotin compounds possessing halogens or other simple substituents. These structure– activity correlations were identifiers of inverse toxicity responses between superior and inferior organisms and decisive for biological activity investigations.67−70 In particular, the toxicity in mammals has been extensively proven for di- and tri-n-butyltin (IV) compounds and just recently it has been emphasized that diaryltin(IV) compounds could be even less toxic than most tin moieties since minimum damage at the lipid membrane occurred in erythrocytes.113 The di- and tri-phenyltin(IV) compounds have been less studied.128,129

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Calixarene diorganotin(IV) derivatives have been used as protective compounds against blood and tissue oxidative stress in lead exposure. The results suggest that tin compounds significantly protect against lead-induced biochemical alterations, indicative of a heme synthetic pathway and therefore exhibiting only a moderate effect on tissue oxidative stress.130 These beneficial effects were attributed mainly to the preference of the target sites/tissues to recognize tin(IV) compounds instead of those containing lead. Alignment procedures, usually applied to macromolecular entities such as genes or proteins, have been recently applied to a family of diphenyltin derivatives with N,S-donor ligands, denoting important cytotoxic activities against two human tumor cell lines, MCF-7, a mammary carcinoma, and WiDr, a colon carcinoma.107 From this criterion of multi-alignment, structure–activity relationships for the family of organotin derivatives have been found, showing predictive performance and a suitable pathway for further relationships among molecules containing organotin moieties and experimental activities. Very recently a family of diphenyltin(IV) compounds was tested in vitro against human tumor cell lines to assess their cytotoxic activity,106 and the same family of compounds was also tested for antioxidant efficiency in rat brain homogenate, showing that there is a structure–activity relationship in the latter case. The correlation suggests that selected molecular variables are prototype tracers for the calculation of inhibitory concentrations. Moreover there seems to be an inverse structure–response behavior among activities, since the most hydrophobic organotin molecule is the least active compound for cytotoxic assays, while it is the best in anti-oxidant tests. 4.6.6

Perspectives

Important advances have led to the design of organotin-containing molecules, e.g. for one of the most common therapeutic applications, anti-neoplastic agents.104,116,124−126 Other applications are related to their biocide activities, coupled with corrosion inhibition industrial applications,128,131 or the systematic design of chromophoric organotin compounds106,132−135 for further photodynamic therapy applications. These potential applications justify a continuous development of new molecules to eliminate the misconception that all organotin-containing molecules should be banned due to their toxicity. Some other metal-containing drugs such as cisplatin are widely used, despite their side effects. As mentioned, this should motivate researchers to perform molecular design-based research of organotin compounds to attain biological activities at ultra-trace levels, trespassing from the selectivity to the specificity level, in order to avoid toxicity and undesirable side effects. This may be attainable, as has been shown for the different biological actions of (CH3 )3 SnCl and (CH3 CH2 )3 SnCl as neurotoxins, since the toxicity of the former is localized in the central nervous system (hippocampus and neocortex),136,137 while the latter is localized in the spinal cord and thus induces damage only in the peripheral nervous system.136,137 Acknowledgments The authors acknowledges financial support from CONACYT. N. Farfán thanks UNAM (PAPIIT IN203207) and H. I. Beltrán SEP-PROMEP (46168-M, 7242/040061) and UAM. References 1. J. Hladyszowski, J. Gabrielska, P. Ordon, S. Przestalski, and M. Langner, J. Membr. Biol. 189 (2002) 213. 2. J. Gabrielska, M. Soczynska-Kordala, R. Zylka, and S. Przestalski, Eur. Biophys. J. Biophys. Lett. 34 (2005) 697. 3. A. Ambrosini, E. Bertoli, and G. Zolese, Appl. Organometal. Chem. 10 (1996) 53. 4. J. J. Chicano, A. Ortiz, J. A. Teruel, and F. J. Aranda, Biochim. Biophys. Acta-Biomembr. 1558 (2002) 70.

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Biological Aspects of Organotins 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98.

99.

100.

101. 102. 103. 104. 105. 106. 107. 108. 109. 110.

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K. R. Siebenlist and F. Taketa, Biochem. J. 233 (1986) 471. D. Zielinska, H. Radecka, and J. Radecki, Chemosphere 40 (2000) 327. H. Radecka, D. Zielinska, and J. Radecki, Sci. Total Environ. 234 (1999) 147. (a) Z. Qi, Z. Gao, and G. Zhang, Zhonghua Yufang Yixue Zazhi 22 (1988) 352; (b) F. H. Nielsen, Trace Elem. Man Anim. 7: Monogr., Proc., Round Tables Discuss. Int. Symp., 7th (1991) 35/32–35/34. J. M. Tsangaris and D. R. Williams, Appl. Organomet. Chem. 6 (1992) 3. R. S. Anderson, M. A. Unger, and E. M. Burreson, Mar. Environ. Res. 42 (1996) 177. E. L. Cooper, V. Arizza, M. Cammarata, L. Pellerito, and N. Parrinello, Comp. Biochem. Physiol. C-Pharmacol. Toxicol. Endocrinol. 112 (1995) 285. W. S. Fisher, A. Wishkovsky, and F. L. E. Chu, Arch. Environ. Contam. Toxicol. 19 (1990) 354. L. D. Thomas, H. Shah, A. D. Bankhurst, and M. M. Whalen, Arch. Toxicol. 79 (2005) 711. E. R. Podack, Curr. Top. Microbiol. Immunol. 178 (1992) 175. R. V. Talanian, X. H. Yang, J. Turbov, P. Seth, T. Ghayur, C. A. Casiano, K. Orth, and C. J. Froelich, J. Exp. Med. 186 (1997) 1323. K. Kannan, K. Senthilkumar, and J. P. Giesy, Environ. Sci. Technol. 33 (1999) 1776. M. M. Whalen, T. B. Williams, S. A. Green, and B. G. Loganathan, Environ. Res. 88 (2002) 199. L. D. Thomas, H. Shah, S. A. Green, A. D. Bankhurst, and M. M. Whalen, Chem. Res. Toxicol. 17 (2004) 1782. T. Y. Aw, P. Nicotera, L. Manzo, and S. Orrenius, Arch. Biochem. Biophys. 283 (1990) 46. M. Raffray, D. McCarthy, R. T. Snowden, and G. M. Cohen, Toxicol. Appl. Pharmacol. 119 (1993) 122. F. Cima and L. Ballarin, Appl. Organometal. Chem. 13 (1999) 697. B. Viviani, A. D. Rossi, S. C. Chow, and P. Nicotera, Neurotoxicology 16 (1995) 19. (a) S. Ueno, T. Kashimoto, N. Susa, Y. Shiota, M. Okuda, K. Mutoh, F. Hoshi, K. Watanabe, S. Tsuda, S. Kawazoe, T. Suzuki, and M. Sugiyama, Toxicol. Sci. 75 (2003) 201; (b) S. Ueno, T. Kashimoto, N. Susa, M. Ishii, T. Chiba, K. Mutoh, F. Hoshi, T. Suzuki, and M. Sugiyama, Archiv. Toxicol. 77 (2003) 173; (c) T. Kawanishi, T. Kiuchi, H. Asoh, R. Shibayama, H. Kawai, H. Ohata, K. Momose, and T. Hayakawa, Biochem. Pharmacol. 62 (2001) 863; (d) S. Ueno, T. Suzuki, N. Susa, Y. Furukawa, and M. Sugiyama, Archiv., Toxicol. 71 (1997) 513. (a) J. B. Nielsen and J. Strand, Environ. Res. 88 (2002) 129; (b) M. M. Whalen, B. G. Loganathan, and K. Kannan, Environ. Res. 81 (1999) 108; (c) F. Cima, L. Ballarin, G. Bressa, A. Sabbadin, and P. Burighel, Marine Chem. 58 (1997) 267; (d) C. D. Rice, M. M. Banes, and T. C. Ardelt, Archiv. Environ. Contamin. Toxicol. 28 (1995) 464; (e) M. Raffray and G. M. Cohen, Archiv. Toxicol. 67 (1993) 231; (f) G. Bressa, L. Cima, R. Hinton, P. Grasso, S. Price, and S. Ahmed, J. Toxicol. Clin. Exp. 11 (1991) 289; (g) G. Bressa, R. H. Hinton, S. C. Price, M. Isbir, R. S. Ahmed, and P. Grasso, J. Appl. Toxicol. 11 (1991) 397. (a) M. L. Billingsley, J. Yun, B. E. Reese, C. E. Davidson, B. A. Buck-Koehntop, and G. Veglia, J. Cell. Biochem. 98 (2006) 243; (b) B. A. Buck-Koehntop, F. Porcelli, J. L. Lewin, C. J. Cramer, and G. Veglia, J. Organomet. Chem. 691 (2006) 1748; (c) J. D. Robertson and S. Orrenius, Toxicol. 181 (2002) 491; (d) A. Nishikimi, Y. Kira, E. Kasahara, E. F. Sato, T. Kanno, K. Utsumi, and M. Inoue, Biochem. J. 356 (2001) 621; (e) A. Gennari, B. Viviani, C. L. Galli, M. Marinovich, R. Pieters, and E. Corsini, Toxicol. Appl. Pharmacol. 169 (2000) 185. N. Spooner, P. E. Gibbs, G. W. Bryan, and L. J. Goad, Mar. Environ. Res. 32 (1991) 37. M. J. J. Ronis and A. Z. Mason, Mar. Environ. Res. 42 (1996) 161. M. J. Abrams and B. A. Murrer, Science 261 (1993) 725. M. Gielen, Appl. Organometal. Chem. 16 (2002) 481. L. J. Tian, Q. S. Yu, X. L. Zheng, Z. C. Shang, X. L. Liu, and B. C. Qian, Appl. Organometal. Chem. 19 (2005) 672. H. I. Beltran, C. Damian-Zea, S. Hernández-Ortega, A. Nieto-Camacho, and M. T. Ram´ırez-Apan, J. Inorg. Biochem. 101 (2007) 1070. P. M. Samuel, D. de Vos, D. Raveendra, J. Sarma, and S. Roy, Bioorg. Med. Chem. Lett. 12 (2002) 61. A. K. Saxena and F. Huber, Coord. Chem. Rev. 95 (1989) 109. A. Atsushi, K. Hisada, and I. Ando, Radioisotopes 22 (1973) 7. M. Yamaguchi, K. Sugii, and S. Okada, J. Toxicol. Sci. Jpn. 5 (1981) 238.

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111. J. F. Lascourreges, P. Caumette, and O. F. X. Donard, Appl. Organometal. Chem. 14 (2000) 98. 112. A. Olzynska, M. Przybylo, J. Gabrielska, Z. Trela, S. Przestalski, and M. Langner, Appl. Organometal. Chem. 19 (2005) 1073. 113. A. Miszta, J. Gabrielska, S. Przestalski, and M. Langner, Appl. Organometal. Chem. 19 (2005) 736. 114. M. Langner, J. Gabrielska, and S. A. Przestalski, Appl. Organometal. Chem. 14 (2000) 25. 115. M. Gielen, R. Willem, M. Biesemans, M. Kemmer, and D. de Vos, 2001, Tin polyoxaalkanecarboxylates and compositions containing them., US20010039270A1, US. 116. M. Gielen, J. Braz. Chem. Soc. 14 (2003) 870. 117. G. Y. Han and P. Yang, J. Inorg. Biochem. 91 (2002) 230. 118. M. Kemmer, L. Ghys, M. Gielen, M. Biesemans, E. R. T. Tiekink, and R. Willem, J. Organomet. Chem. 582 (1999) 195. 119. C. P. Berg, A. Rothbart, K. Lauber, G. M. Stein, I. H. Engels, C. Belka, R. U. Janicke, K. Schulze-Osthoff, and S. Wesselborg, Oncogene 22 (2003) 775. 120. W. Fiers, R. Beyaert, W. Declercq, and P. Vandenabeele, Oncogene 18 (1999) 7719. 121. C. R. Jan, B. P. Jiann, Y. C. Lu, H. T. Chang, W. R. Su, W. C. Chen, C. C. Yu, and J. K. Huang, Life Sci. 70 (2002) 1337. 122. N. Hoti, J. Ma, S. Tabassum, Y. Wang, and M. Wu, J. Biochem. 134 (2003) 521. 123. M. Bragadin, D. Marton, S. Manente, G. Scutari, and A. Toninello, Inorg. Chim. Acta 348 (2003) 225. 124. M. Gielen, Coord. Chem. Rev. 151 (1996) 41. 125. M. Gielen (Ed.), Tin-Based Antitumour Drugs, Springer-Verlag, Berlin, 1990. 126. M. Gielen, E. R. T. Tiekink (Eds), Metallotherapeutic Drugs and Metal-Based Diagnostic Agents: The Use of Metals in Medicine, John Wiley Sons, Chichester, 2005. 127. M. Gielen (Ed.), Metal-Based Anti-Tumour Drugs, Freund Pub. House, London, 1988. 128. L. S. Zamudio-Rivera, R. George-Tellez, G. López-Mendoza, A. Morales-Pacheco, E. Flores, H. Höpfl, V. Barba, F. J. Fernandez, N. Cabirol, and H. I. Beltran, Inorg. Chem. 44 (2005) 5370. 129. Q. S. Li, M. da Silva, and A. L. L. Pombeiro, Chem. Eur. J. 10 (2004) 1456. 130. A. Ali, S. J. S. Flora, G. Saxena, E. Kolehmainen, B. Mahieu, and C. P. Rao, J. Inorg. Biochem. 100 (2006) 206. 131. V. Barba, E. Vega, H. Höpfl, L. S. Zamudio-Rivera, M. A. Dominguez-Aguilar, R. George-Tellez, F. GodinezSalomon, J. M. Hallen-López, and H. I. Beltran, Eur. J. Inorg. Chem. (2007) 927. 132. J. M. Rivera, D. Guzmán, M. Rodr´ıguez, J. F. Lamère, K. Nakatani, R. Santillan, P. G. Lacroix, and N. Farfán, J. Organomet. Chem. 691 (2006) 1722. 133. J. M. Rivera, H. Reyes, A. Cortes, R. Santillan, P. G. Lacroix, C. Lepetit, K. Nakatani, and N. Farfán, Chem. Mat. 18 (2006) 1174. 134. N. Farfán, T. Mancilla, R. Santillan, A. Gutiérrez, L. S. Zamudio-Rivera, and H. I. Beltran, J. Organomet. Chem. 689 (2004) 3481. 135. H. Reyes, C. Garc´ıa, N. Farfán, R. Santillan, P. G. Lacroix, C. Lepetit, and K. Nakatani, J. Organomet. Chem. 689 (2004) 2303. 136. B. Viviani, C. L. Galli, and M. Marinovich, Neurosci. Res. Comm. 23 (1998) 139. 137. M. Aschner and J. L. Aschner, Neurosci. Biobeh. Rev. 16 (1992) 427.

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5 Tin in Organic Synthesis 5.1

Applications of Organotin Derivatives for Carbohydrate Synthesis

T. Bruce Grindley Department of Chemistry, Dalhousie University, Halifax Canada B3H 4J3

5.1.1

Introduction

Organotin derivatives have been used extensively as intermediates for the regioselective substitution of the hydroxyl groups of carbohydrates and other diols and polyols. These organotin intermediates react with a wide variety of electrophiles and reactions proceed much faster or under milder conditions than with the parent diols. The reactions of diols and polyols are widely employed because they yield monosubstituted products reliably, often with spectacular regioselectivity. Although the causes of the regioselectivity are not fully understood, generalizations have been established, which allow reliable predictions to be made as to what types of structures will give regioselective reactions and which locations are most favorable. Newer trends include the use of catalytic tin reagents and the acceleration of both the formation of the organotin intermediates and the subsequent reactions with electrophiles by microwave irradiation. This chapter will summarize the types of regioselectivity observed and will outline what is known about the underlying causes of the regioselectivity. Two types of tin derivatives have been used mainly to achieve these reactions; dialkylstannylene acetals are formed by reaction of dialkyltin oxides with diols; tributyltin ethers are formed by reaction of bis(tributyltin) oxide with alcohols. Some aspects of the regioselectivity achieved with these two types of intermediates are the same, while others are different, as will be outlined in the sections to follow. Where the reaction outcomes are similar, the dialkyltin reagents are strongly preferred because of their lower toxicity.1 As these reagents are heavily employed, the topic has been reviewed several times,2−7 but significant new observations continue to be made. This chapter is not intended to be a comprehensive summary of all applications of these intermediates, but will highlight examples of each type of selectivity. IUPAC nomenclature for dialkylstannylene acetals varies with ring size. These derivatives are named by the Hantzsh–Widmann system using Rule R-2.3.3.1 from the 1993 IUPAC Recommendations for the


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Nomenclature of Organic Compounds (see Table 5.1.4): 2,2-dialkyl-1,3,2-dioxastannolanes if the ring is five-membered; 2,2-dialkyl-1,3,2-dioxastanninanes if the ring is six-membered; or 2,2-dialkyl-1,3,2dioxestannepanes if the ring is seven-membered.8 Trialkyltin ethers are also termed trialkyltin alkoxides or trialkylstannyl ethers. The first reactions using both dibutylstannylene acetals and tributyltin ethers as intermediates were performed in 1974 by Wagner et al.,9 who formed benzoates and p-toluenesulfonates from dibutylstannylene acetals of nucleosides with good, but often opposite, regioselectivity and also formed phosphates from bis(tributyltin) ethers (Scheme 5.1.1). Shortly afterwards, David reacted dibutylstannylene acetals of diols with bromine to give mono-keto products highly selectively.10 O N O N

H N

HO O

N

O Bu SnO 2

N

O

MeOH

O

O

78% (recrystallized) OBz OH O

Sn Bu

Bu

N MeOH

HO

N

5.1.2

H

O

O 62% (recrystallized) OH

Scheme 5.1.1

O

N O

BzCl, Et3N

TsCl O

OH

H

HO

MeOH OH

HO

O

H

OTs

The first use of dibutylstannylene acetals9

Preparation

Dialkylstannylene Acetals

Dialkylstannylene acetals are usually prepared by heating diols with dialkyltin oxide in methanol or in benzene or toluene with azeotropic removal of water (see Scheme 5.1.2). The latter conditions probably result in complete conversion in two hours at reflux although they are normally left 4 to 24 hours. Addition of a Soxhlet apparatus containing molecular sieves towards the end of the reaction ensures complete removal of water. Reaction in methanol, where dialkyldimethoxytin is an intermediate for the preparation, is more rapid, often being complete in one hour; however, one group observed that yields were lower and that starting material remained after reaction workup if the dibutylstannylene acetal was formed by this method.11 The technique used for preparation may also have been a factor in the higher yields reported by Kovác and Edgar,12 who formed the stannylene acetal in toluene, than reported for identical reactions by earlier workers, who formed it in methanol.13 Most reactions in which dialkylstannylene acetals are formed in methanol give excellent yields; probably it is sufficient to be very careful to remove the methanol and the by-product water completely at the end of the dialkylstannylene acetal formation to ensure complete conversion.

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Organotin Derivatives for Carbohydrate Synthesis OH n OH

O +

Bu2 SnO

n O

n = 0-2

Scheme 5.1.2

499

Bu +

Sn

H 2O

Bu

Formation of dibutylstannylene acetals

The use of pre-formed dibutyltin dimethoxide is advantageous, if considerably more expensive;14,15 reaction of diols are complete in 5 to 15 minutes in benzene at reflux. This technique avoids the problems with working in methanol mentioned above and gives better yields. Stannylene acetals have also been prepared in situ using sub-stoichiometric dibutyltin dichloride.16 The preparation of stannylene acetals is also accelerated by microwave irradiation17 and selective reactions can be performed in low yields on some diols and polyols by microwave irradiation in the presence of a catalytic amount of dibutyltin oxide.18,19 Microwave irradiation in the presence of stoichiometric dibutyltin oxide gives excellent yields of stannylene acetals in 5 min at 150 ◦ C.20 It has also been found that p-toluenesulfonation reactions can be conducted with catalytic dibutyltin oxide if a stoichiometric amount of base, normally triethylamine, is present.21 These conditions can also be used for acylation of 1-phenyl-1,2-ethanediol with dibutyltin dialkoxides as catalysts22 and is also effective in the acylation of glycerophosphoryl choline, but only when 2-propanol is the solvent (Scheme 5.1.3).23 OCO(CH 2 )14CH3

HO HO

Bu2 SnO (5 mol %)

O O

P

CH3 (CH 2)14COCl (1.2 eq)

HO

O

O—

O 2-propanol (reflux)

O

Et 3N (1.2 eq) rt, 15 min

P

O—

O 90%

N+

N+

Scheme 5.1.3

Use of catalytic dibutyltin oxide23

Dibutyltin oxide is the dialkyltin oxide used most commonly, although both di-t-butyltin oxide and fluorous tin oxide [(C6 F13 (CH2 )2 )2 SnO] are available commercially from speciality companies. Use of the latter compound allows the tin-containing products to be removed by fluorous extraction.24 It can also be prepared on a large scale24,25 using the convenient general method developed for the preparation of dialkyltin oxides, including the synthetically useful hexamethylenetin oxide (see Scheme 5.1.4).26 Polymer-supported equivalents of dialkyltin oxides are also now becoming available.27,28,29 ClCH2 CO 2H

1. Mg 2

X R

Ph2SnR 2

(ClCH 2CO 2 )2SnR 2

NaOH

[OSnR 2 ]n

2. Ph2SnCl 2 X = Br or I

Scheme 5.1.4 A general synthetic method for the preparation of dialkyltin oxides. Examples of XR used include ICH2 CH2 C6 F13 ,24 BrCH2 CH(CH3 )2 ,26 BrCH(CH3 )2 ,26 Br(CH2 )6 Br26

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Tributylstannyl Ethers

Tributylstannyl ethers are normally prepared by reaction of alcohols with hexabutyldistannoxane, more commonly known as bis(tributyltin) oxide, in benzene or toluene at reflux, with removal of water. Holzapfel et al. noted that the reaction in benzene requires only 0.5 molar equivalents of bis(tributyltin) oxide to go to completion, but takes 16 h at reflux.30 They can also be prepared under mild conditions by reaction of the alcohol with tributylstannane in the presence of catalytic triflic acid, or better with allyltributyltin (Scheme 5.1.5).31,32 Polymer supported versions of tributyltin ethers have been prepared.33 SnBu3

HO O

BnO

SePh

AcO CF3SO 3H (0.3 eq) /CH2Cl2, rt, 2 h

OAc

Scheme 5.1.5

O

BnO

SePh

AcO

5.1.3

Bu3SnO

1.3 eq

OAc

Preparation of tributylstannyl ethers under mild conditions31

Structures

Dialkylstannylene Acetals

The structures adopted by dialkylstannylene acetals are dependent on the degree of substitution on the acetal, the physical state, and the temperature. 2,2-Dibutyl-1,3,2-dioxastannolane,34 its (4R,5R)-4,5dimethyl derivative,35 and 2,2-dibutyl-1,3,2-dioxastanninane36 exist as infinite polymers in the solid state with octahedral hexacoordinate tin atoms. Compounds having larger substituents are less aggregated in the solid; 2,2-di-t-butyl-1,3,2-dioxastannolane,37 methyl 4,6-O-benzylidene-2,3-O-dibutylstannyleneα-d-glucopyranoside (1),38,39 benzyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-β-d-glucopyranoside (2),35 and methyl 4,6-O-benzylidene-2,3-O-di-tert-butylstannylene-α-d-mannopyranoside (3)35 are dimers, while methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-α-d-mannopyranoside (4)40 is a pentamer (Scheme 5.1.6).

Ph

Ph

O

O

O

O

O

OBn

O

O

O

O Bu

Sn

1

OMe Bu

R

R

O

Bu

Bu

O

O O

Sn

2

Sn Ph

O O

3 R = t-Bu 4 R = Bu

OMe

Scheme 5.1.6 Structures of benzylidene-2,3-O-dibutylstannylene-α-D-glucopyranoside, benzyl 4,6O-benzylidene-2,3-O-dibutylstannylene-β-D-glucopyranoside, methyl 4,6-O-benzylidene-2,3-O-di-tertbutylstannylene-α-D-mannopyranoside, and methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-α-Dmannopyranoside

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Organotin Derivatives for Carbohydrate Synthesis

501

An important factor for reaction regioselectivity is that the butyl groups lie approximately perpendicular to the plane defined by the O2 Sn2 ring(s). In the structure of methyl 4,6-O-benzylidene-2,3O-dibutylstannylene-α-d-glucopyranoside (1), the butyl groups on tin were highly mobile, even in the crystal at –70 ◦ C.39 Formation of the dimer requires that one oxygen atom becomes tricoordinate, while the other remains dicoordinate (see Scheme 5.1.7). The tricoordinate oxygen atoms are attached to two tin atoms, each bearing highly mobile butyl groups that will hinder approach of electrophiles and reduce reactivity. Branching of the alkyl groups on tin also reduces reactivity in general.41 Another important aspect of these structures is related to the geometries of the dimers or terminal units of oligomers of stannylene acetals.37,39,40 The tin atoms adopt a distorted trigonal bipyramidal geometry with the alkyl groups in equatorial orientations. The tricoordinate oxygens are equatorial to one tin atom, but apical to the other. The dicoordinate oxygen atoms are apical (see Scheme 5.1.7) and this may contribute to their reactivity.2

O Bu

n

Sn

O

O

Sn

n = 0-2

Bu Bu Bu

O n

Scheme 5.1.7 An idealized structure of a dibutylstannylene acetal dimer illustrating the fact that the two oxygen atoms become differentiated on dimer formation

These compounds are less aggregated in solution. For instance, 2,2-dibutyl-1,3,2-dioxastannolane, a polymer in the solid state,34 has been shown, by variable temperature 119 Sn NMR spectroscopy, to be a mixture of dimers, trimers, and tetramers in solution, with dimers predominating at room temperature and above.42,43 This technique has also indicated that most carbohydrate-derived stannylene acetals are present predominantly as dimers in solution.44,45,46 Supporting evidence has been obtained from mass spectral studies44 and by comparison of solid-state NMR spectra with those of solutions.45 Some dibutylstannylene acetals derived from cis-diols contain an observable proportion of higher oligomers; benzyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-α-d-mannopyranoside is present as a mixture of a dimer and a trimer in chloroform-d at –60 ◦ C but, in the less polar solvent toluene-d8 , is mainly present as a tetramer at that temperature.47 1,3,2-Dioxastannolanes or 1,3,2-dioxastanninanes show a much increased tendency to exist as species containing pentacoordinate or hexacoordinate tin atoms than do acyclic dialkyltin dialkoxides.48,49 This tendency is caused38 by the small size of the bond angles imposed on the tin atom by formation of a ring ˚ are much longer than the other bonds in the ring in which the two Sn O bond lengths (∼2.0–2.1 A) 34,36−40 ˚ (C C and C O, 1.4–1.55 A). In the solid, O Sn O bond angles were observed to be 78–80◦ 37−40 34,35,40 for both fiveand six-coordinate tin atoms in 1,3,2-dioxastannolane rings, and 93.2◦ for the six-coordinate tin atom in the one 1,3,2-dioxastanninane ring studied by X-ray crystallography.36 These bond angles are closer to the approximately 90◦ bond angles needed for trigonal bipyramidal geometry or octahedral geometry of five- and six-coordinate tin, but are much smaller than those in the tetrahedral geometry favored by tetracoordinate tin. The O Sn O bond angles are forced to have these small values

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because of the geometry of rings that have two adjacent sides (O Sn bonds) much longer than the others. In dimers, oligomers, or polymers, the Sn O bonds inside the monomer units are markedly shorter, 1.98 ˚ than those between monomer units, which are 2.23 to 2.27 A, ˚ if the tin atom is pentacoordinate, to 2.13 A, 34,40 ˚ or 2.43 to 2.60 A, if the tin atom is hexacoordinate. The geometries of the stannylene acetals oligomers explain why dibutylstannylene acetals formed from trans-diols do not form higher aggregates than dimers.6 The atoms in the 1,3,2-dioxastannolane rings are approximately in the same plane as the central O2 Sn2 rings. If the two oxygen atoms are equatorial on pyranose rings, the substituents on carbons 4 and 5 of the 1,3,2-dioxastannolane rings, that is the remainders of the pyranose rings, are also roughly in the same plane. Formation of oligomers higher than dimers is severely hampered by steric interactions between alternate 1,3,2-dioxastannolane rings in the oligomers (see Scheme 5.1.8). If one of the oxygen atoms is axial, as with methyl 4,6-O-benzylidene2,3-O-dibutylstannylene-α-d-mannopyranoside (4), the substitutent on the carbon bearing that oxygen lies perpendicular to the central plane, and higher oligomers are sterically accessible.

O

O Sn Bu

Bu Bu

Bu

dimer

Scheme 5.1.8

Sn

Sn O

O

O

O

Bu

Bu Bu

Sn

Sn O

O

Bu

O

O

Bu Bu

trimer

Steric interactions in a dibutylstannylene acetal trimer

Only when a considerable number of large substituents are present do stannylene acetals exist to any extent as the monomer; 2,2-dibutyl-4,4,5,5-tetraethyl-1,3,2-dioxastannolane is present almost entirely as the dimer at room temperature in chloroform at 20 ◦ C but the monomer–dimer equilibrium gradually shifts to favor the monomer above about 80 ◦ C. 2,2-Di-t-butyl-4,4,5,5-tetramethyl-1,3,2-dioxastannolane is a monomer at room temperature.50 When the two oxygen atoms involved in the stannylene acetal are not related by symmetry, three dimers, two with C2 symmetry, can be formed, as shown in Scheme 5.1.9 for those obtained from (2R)-1,2-propanediol. Dimers are named by means of the numbers of the tricoordinate oxygen atoms.45 Steric effects appear to be the most important factor in determining the relative populations of the three dimers. In particular, stannylene acetals derived from trans-diols with one adjacent axial substituent exist in solution to the level of detection of 119 Sn NMR spectroscopy as the symmetric dimer in which the tricoordinate oxygen atom is not adjacent to the axial substituent.45 Similarly, dialkylstannylene acetals from carbohydrate-derived terminal 1,2-diols exist predominantly as symmetric dimers with the primary oxygen atoms tricoordinate.46 Simple 2,2-dibutyl-1,3,2-dioxastannolanes form solid complexes of monomer units with nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1:1 stoichiometry and pentacoordinate tin atoms.51 Such complexes are less stable for more substituted stannylene acetals, e.g., those derived from carbohydrates.51 Addition of nucleophiles to solutions of stannylene acetals in non-polar solvents has been found to markedly increase the rates of reaction with electrophiles52 and transient 1:1 complexes of this type are

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Organotin Derivatives for Carbohydrate Synthesis Me

1

1

2

O

1

O

1

O

Sn

O Me

O

2

2

2

1

Me 1,1-Dimer

Scheme 5.1.9

Sn

Me

O

Sn

O

Sn

O

2

O

Me

O

Sn

O

Sn

2

O

1

503

Me 1,2-Dimer

2,2-Dimer

Dimer nomenclature for the stannylene acetals derived from (2R)-1,2-propanediol

likely intermediates. Tetrabutylammonium iodide was the nucleophile used first52 but a wide variety of nucleophiles have been used subsequently; tetraalkylammonium halides, N -methylimidazole30 and cesium fluoride53,54 are used most. Nucleophilic solvents, such as N ,N -dimethylformamide and ethers, probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in non-polar solvents.30,55 Tributylstannyl Ethers

In the solid state, trimethyltin methoxide56 and trimethyltin hydroxide57,58 are linear polymers. The tin atoms are pentacoordinate with distorted trigonal bipyramidal geometries having apical oxygen atoms. In non-polar solvents, simple and more complex trialkyltin alkoxides exist predominantly as monomers with tetrahedral tetracoordinate tin atoms. This was determined by molecular weight measurements and from 119 Sn NMR chemical shifts, which are diagnostic for coordination status.48,59,60 Formation of tributylstannyl ethers from a polyol using less than a stoichiometric amount of bis(tributyltin) oxide yields a mixture that contains all possible tributylstannyl ethers.59,60 These tributyltin ethers do not inter-convert rapidly on the NMR timescale in benzene up to 100 ◦ C, but do inter-convert rapidly under reaction conditions.60 The presence of added nucleophiles, such as tetrabutylammonium halides61 or N -methylimidazole (NMI),30,62 markedly accelerates reactions with electrophiles. In the presence of NMI, the 119 Sn nuclei are more shielded, consistent with the changes in chemical shift observed when tin nuclei assume higher coordination.60 The shift changes are relatively small on addition of one equivalent of NMI at room temperature ( 98:2 using these procedures. The rationale for face selectivity is not fully understood, since the mechanism appears to involve a homochiral dimer [BINOL]2 Ti2 X2 .120 Three examples applied to functionalized aldehydes are illustrated in the formation of 382, 384, and 387 (Scheme 5.2.84).121

Scheme 5.2.84

Examples of enantioselective allylations utilizing nonracemic BINOL catalysts

Some improvements have been described for the enantioselective addition of allyltri-n-butylstannane to simple aldehydes using a catalyst prepared from BINOL and Zr(O-tBu)4 in toluene.122 Reactions proceed

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at −78 to −60 ◦ C, however, the catalyst structure is unknown and is expected to involve dimeric BINOL complexes. The chiral zirconium catalyst derived from (R)-BINOL has been used for enantioselective allylations of imines prepared from ortho-hydroxyaniline.123 This nitrogen substituent is proposed to coordinate the chiral catalyst, and reactions produce good yields of amines (74–91%) using several substituted allylic stannanes with moderate asymmetric induction. Loh and coworkers have also presented results for enantioselective allylations of aromatic aldehydes with allyltri-n-butylstannane in the presence of (R)-BINOL-InCl3 complexes.124 The enantioselective BINOL/Ti(IV)-mediated propargylation of simple aldehydes with allenyltri-nbutylstannane was first reported by Keck.125 Reactions suffered from high catalyst loading and long reaction times. Yu has found that reactions proceed with 10 mol% catalyst in the presence of stoichimetric additives such as i-PrSBEt2 .126 Recently, Yu has also reported the use of the BINOL/Ti(IV) protocol for enantioselective condensations of 1-tri-n-butylstannyl-2-butyne with simple aldehydes, to yield allene 388. The reaction requires a disubstituted alkyne since terminal propargylic stannanes have produced only homopropargylic alcohols. Additionally, allenyl-tri-n-butylstannane gives alcohol 388 under similar conditions in comparable yield and enantioselectivity, suggesting a facile equilibration mechanism for the starting stannanes (Scheme 5.2.85).126

Scheme 5.2.85

Asymmetric catalysis in the preparation of nonracemic alcohol 388

Recently, Akiyama has reported the catalytic enantioselective propargylation of α-imino esters 389 with allenylstannane, in presence of 1 mol% [Cu(MeCN)4 ]ClO4 /(R)-tol-BINAP to generate amine 390 in good yield and high enantioselectivity,127 and related efforts by Jorgensen and coworkers123 have examined the reaction of 389 with (E)-crotylstannane (Scheme 5.2.86). H • NTs

H

H SnBu3

NHTs

H

[Cu(MeCN)4]ClO4/(R)-tol-BINAP EtO2C

H 389

Scheme 5.2.86

Et2O, –30 °C, 5 h, 96%, 86% ee

EtO2C 390

Enantioselective propargylation of 389 using BINAP-derived catalyst

Using the same catalyst system, Akiyama has demonstrated the catalytic asymmetric allenylation of α-amino ester 389 with propargylstannane 391 to synthesize allenic amine 392 in good yield and high enantiopurity (Scheme 5.2.87).127 Finally, Denmark128 has reported the combination of SiCl4 and a catalytic amount of chiral bisphosphoramide (R,R)-393 for highly enantioselective allylations of aromatic and unsaturated aldehydes, yielding examples such as 394. The use of SiCl4 alone does not promote the reaction. A hexacoordinate complex of silicon is suggested as a working hypothesis in which the bisphosphoramide ligand

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S E Substitution for Organostannanes in Organic Synthesis

Scheme 5.2.87

555

Catalytic asymmetric allylation of imine 389 via BINAP-derived catalyst

provides a chiral lattice for the approach of the allylstannane. The chemistry has been extended to include allenyltri-n-butylstannane for the enantioselective formation of homopropargylic alcohols 395 (Scheme 5.2.88).

Scheme 5.2.88

5.2.17

Enantioselective allylations of aromatic aldehydes

Conclusion and Future Outlook

In the past 30 years, we have witnessed the development of SE allylation processes from the status of interesting intellectual studies of mechanism and reactivity, to become a powerful idea serving as the central theme in the construction of complex molecules. Organostannanes are important reagents for useful methodology in SE reactions for the construction of homoallylic and homopropargylic alcohols. In fact, high levels of stereochemical control and functional group compatibility have been demonstrated in the reactions of a variety of allylic, propargylic, and allenic stannanes. The thematic requirement of these processes recognizes the role of a heteroatom-stabilized carbocation as the electrophilic partner in an effective condensation. This simple concept has been documented by the remarkable variety of applications, owing to the ingenuity and creativity of investigators in the field. Our understanding of reactivity and our ability to predict outcomes have led to stellar achievements in the synthesis of complex natural products, as well as the sophistication of the approach to retrosynthetic analysis. A number of challenging issues raise substantial interest and opportunities for future research. Emphasis will be placed on the need for higher levels of diastereoselection and the ability to exercise stereocontrol as applications are pursued in a demanding context. Efficient processes will be explored for the preparation of reagents, particularly as the organostannane must meet the requirements imposed for a synthesis pathway. Organostannane reagents which embody dual functionalization provide an important linkage to effect reaction cascades for building molecular complexity. Future investigations will gain greater appreciation for the metal coordination structure and issues of stereochemistry with respect to the metal

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center, which affect reactivity and stereoselection. Finally, the discovery of chiral catalysts will fuel the development of enantioselective reactions and new methods for asymmetric induction. It is our hope that this overview of the ideas and the achievements of leading investigators in the field will stimulate and encourage new contributions to these important endeavors.

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73. (a) D.R. Williams, K.G. Meyer, K. Shamim, and S. Patnaik, Can. J. Chem., 82, 120 (2004); (b) D.R. Williams, D.A. Brooks, K.G. Meyer, and M.P. Clark, Tetrahedron Lett., 39, 7251 (1998). 74. D.R. Williams, D.A. Brooks, and M.A. Berliner, J. Am. Chem. Soc., 121, 4924 (1999). 75. (a) D.R. Williams, S. Patnaik, and S.V. Plummer, Org. Lett., 5, 5035 (2003); (b) D.R. Williams, S.V. Plummer, and S. Patnaik, Angew. Chem., Int. Ed., 42, 3934 (2003). 76. (a) D.R.Williams, A.A. Kiryanov, U. Emde, M.P. Clark, M.A. Berliner, and J.T. Reeves, Proc. Natl. Acad. Sci. USA, 101, 12058 (2004); (b) D.R. Williams, A.A. Kiryanov, U. Emde, M.P. Clark, M.A. Berliner, and J.T. Reeves, Angew. Chem., Int. Ed., 42, 1258 (2003). 77. (a) T.-P. Loh and G.-L. Chua, Chem. Commun., 2739–2749 (2006); (b) V. Nair, S. Ros, C.N. Jayan and B.S. Pillai, Tetrahedron, 60, 1959 (2004); (c) S. Araki and T. Hirashita, in Main Group Metals in Organic Synthesis, H. Yamamoto and K. Oshima (Eds),Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim, Germany, 2004, Vol. 1, 323–386; (d) B.C. Ranu, Eur. J. Org. Chem., 2347 (2000); (e) J. Podlech and T.C. Maier, Synthesis, 633 (2003); (f) F. Fringuelli, O. Piermatti, F. Pizzo, and L. Vaccaro, Curr. Org. Chem., 7, 1661 (2003); (g) A.N. Pae and Y.S. Cho, Curr. Org. Chem., 6, 715–737 (2002); (h) C. Li and T.-H. Chan, Tetrahedron, 55, 11149 (1999); (i) J.A. Marshall, Chemtracts-Organic Chemistry, 10, 481 (1997); (j) P. Cintas, Synlett, 1087 (1995). 78. J.A. Marshall and K.W. Hinkle, J. Org. Chem., 60, 1920 (1995). 79. (a) J.A. Marshall and K.W. Hinkle, J. Org. Chem., 62, 5989 (1997); (b) J.A. Marshall and M. Chen, J. Org. Chem., 62, 5996 (1997); (c) J.A. Marshall and A.W. Garofalo, J. Org. Chem., 61, 8732 (1996); (d) J.A. Marshall and K.W. Hinkle, J. Org. Chem., 61, 4247 (1996); (e) J.A. Marshall and K.W. Hinkle, J. Org. Chem., 61, 105 (1996). 80. J.A. Marshall, G.S. Welmaker, and B.W. Gung, J. Am. Chem. Soc., 113, 647 (1991). 81. J.A. Marshall, G.S. Welmaker, and B.W. Gung, J. Am. Chem. Soc., 113, 647 (1991). 82. B.W. Gung, D.T. Smith, and M.A. Wolf, Tetrahedron Lett., 32, 13 (1991). 83. J.A. Marshall and W.-Y. Gung, Tetrahedron Lett., 29, 1657 (1988). 84. J.A. Marshall, J.A. Jablonowski, and G.P. Luke, J. Org. Chem., 59, 7825 (1994). 85. M. Lequan and G.J. Guillerm, J. Organomet. Chem., 54, 153 (1973). 86. T. Mukaiyama and T. Harada, Chem. Lett., 10, 621 (1981). 87. K. Ruitenberg, H. Westmijze, J. Meijer, C.J. Elsevier, and P. Vermeer, J. Organomet. Chem., 241, 417 (1983). 88. (a) J.A. Marshall and C.A. Sehon, J. Org. Chem., 62, 4313–4320 (1997); see also (b) J.A. Marshall and X.-J. Wang, J. Org. Chem., 56, 6264 (1991). 89. E.L. Eliel and S.H. Wilen, Stereochemistry of Organic Compounds, Wiley-Interscience: New York, 1994; 1120–1121. 90. J.A. Marshall and X.-J. Wang, J. Org. Chem., 55, 6246 (1990). 91. (a) J.A. Marshall, R.H. Yu, and J.F. Perkins, J. Org. Chem., 60, 5550 (1995); (b) J.A. Marshall, J.F. Perkins, and M.A. Wolf, J. Org. Chem., 60, 5556 (1995). 92. J.A. Marshall and M.R. Palovich, J. Org. Chem., 62, 6001 (1997). 93. J.A. Marshall and X.-J. Wang, J. Org. Chem., 56, 3211 (1991). 94. J.A. Marshall and X.-J. Wang, J. Org. Chem., 57, 1242 (1992). 95. (a) T. Sato, S. Okura, J. Otera, and H. Nozaki, Tetrahedron Lett., 28, 6299 (1987); (b) T. Takeda, H. Oshima, M. Inoue, A. Togo, and T. Fujiwara, Chem. Lett., 16, 1345 (1987). 96. D.R. Williams, L. Mi, R.J. Mullins, and R.E. Stites, Tetrahedron Lett., 43, 4841 (2002). 97. S.G. Nelson, W.S. Cheung, A.J. Kassick, and M.A. Hilfiker, J. Am. Chem. Soc., 124, 13654 (2002). 98. E.A. Colby, K.C. O’Brien, and T.F. Jamison, J. Am. Chem. Soc., 127, 4297 (2005). 99. R. Yamaguchi, M. Moriyasu, I. Takase, M. Kawanisi, and S. Kozima, Chem. Lett., 16, 1519 (1987). 100. (a) J. Haruta, K. Nishi, K. Kikuchi, S. Matsuda, Y. Tamura, and Y. Kita, Chem. Pharm. Bull., 37, 2338 (1989); see also (b)W.F.J. Karstens, F.P.J.T. Rutjes, and H. Hiemstra, Tetrahedron Lett., 38, 6275–6278(1997). 101. (a) J. Haruta, K. Nishi, S. Matsuda, Y. Tamura, and Y. Kita, J. Chem. Soc., Chem. Commun., 1065 (1989); (b) J. Haruta, K. Nishi, S. Matsuda, S. Akai, Y. Tamura, and Y. Kita, J. Org. Chem., 55, 4853 (1990).

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102. E.J. Corey and K. Liu, Tetrahedron Lett., 38, 7491 (1997). 103. S.T. Staben, J.J. Kennedy-Smith, D.C. Huang, K. Britton, R.L. LaLonde, and F.D. Toste, Angew. Chem., Int. Ed., 45, 5991 (2006). 104. E. Keinan and M. Peretz, J. Org. Chem., 48, 5302 (1983). 105. (a) A. Pommier, V. Stepanenko, K. Jarowicki, and P.J. Kocienski, J. Org. Chem., 68, 4008 (2003); (b) B.M. Trost and G.A. Doherty, J. Am. Chem. Soc., 122, 3801 (2000). 106. J.A. Marshall and B.A. Johns, J. Org. Chem., 63, 7885 (1998). 107. J.A. Marshall and G.M. Schaaf, J. Org. Chem., 68, 7428 (2003). 108. J.A. Marshall and M.P. Bourbeau, J. Org. Chem., 67, 2751 (2002). 109. J.A. Marshall and K.C. Ellis, Org. Lett., 5, 1729 (2003). 110. J.A. Marshall and M.M. Yanik, J. Org. Chem., 66, 1373 (2001). 111. I. Kadota, D. Hatakeyama, K. Seki, and Y. Yamamoto, Tetrahedron Lett., 37, 3059 (1996). 112. (a) T.B. Durham, N. Blanchard, B.M. Savall, N.A. Powell, and W.R. Roush, 126, 9307 (2004); (b) K.A. Scheidt, T.D. Bannister, A. Tasaka, M.D. Wendt, B.M. Savall, G.J. Fegley, and W.R. Roush, J. Am. Chem. Soc., 124, 6981 (2002); (c) B.M. Savall, N.A. Powell, and W.R. Roush, Org. Lett., 3, 3057 (2001). 113. (a) L.S. Hegedus, P. Ranslow, M. Achmatowicz, C. de los Rios, C. Hyland, E.M. Garcia-Frutos, and S. Salman, Pure Appl. Chem., 78, 333 (2006); (b) C. de los Rios, and L.S. Hegedus, J. Org. Chem., 70, 6541 (2005); (c) P.B.D. Ranslow, L.S. Hegedus, and C. de los Rios, J. Org. Chem., 69, 105 (2004). 114. (a) J. Kjellgren, H. Sunden, and K.J. Szabo, J. Am. Chem. Soc., 127, 1787 (2005); (b) D.Young, in Science of Synthesis: Houben–Weyl Methods of Molecular Transformations, Organometallics, Compounds of Group 14 (Ge, Sn, Pb), M.G. Moloney (Ed.), Thieme Verlag, Stuttgart, (2003), Vol. 5, 549–557; (c) S. Okamoto, S.-i. Matsuda, D.K. An, and F. Sato, Tetrahedron Lett., 42, 6323 (2001). 115. (a) K. Furuta, M. Mouri and H. Yamamoto, Synlett., 561 (1991); (b) K. Ishihara, M. Mouri, Q. Gao, T. Maruyama, K. Furuta, and H. Yamamoto, J. Am. Chem. Soc., 115, 1490 (1993). 116. J.A. Marshall and M.R. Palovich, J. Org. Chem., 63, 4381 (1998). 117. S. Aoki, K. Mikami, M. Terada, and T. Nakai, Tetrahedron, 49, 1783 (1993). 118. G.E. Keck, K.H. Tarbet, and L.S. Geraci, J. Am. Chem. Soc., 115, 8467 (1993). 119. (a) A.L. Costa, M.G. Piazza, E. Tagliavini, C. Trombini, and A. Umani-Ronchi, J. Am. Chem. Soc., 115, 7001 (1993); see also (b) A. Yanagisawa, A. Ishaba, H. Nakashima, and H. Yamamoto, Synlett., 88 (1997); (c) A. Yanagisawa, H. Nakashima, A. Ishiba, and H. Yamamoto, J. Am. Chem. Soc., 118, 4723 (1996); (d) P. Bedeschi, S. Casolari, A.L. Costa, E. Tagliavini, and A. Umani-Ronchi, Tetrahedron Lett., 36, 7897 (1995). 120. J.W. Faller, D.W.I. Sams, and X. Liu, J. Am. Chem. Soc., 118, 1217 (1996). 121. (a) P.A. Evans and V.S. Murthy, Tetrahedron Lett., 39, 9627 (1998); (b) D. Meng, P. Bertinato, A. Balog, D.-S. Su, T. Kamenecka, E.J. Sorensen, and S.J. Danishefsky, J. Am. Chem. Soc., 119, 10073 (1997); (c) J.-Y. Park, I. Kadota, and Y. Yamamoto, J. Org. Chem., 64, 4901 (1999). 122. (a) S. Casolari, P.G. Cozzi, P.A. Orioli, E. Tagliavini, and A. Umani-Ronchi, Chem. Commun. 2123 (1997); (b) M. Kurosu and M. Lorca, Tetrahedron Lett., 43, 1765 (2002). 123. X. Fang, M. Johannsen, S.Yao, N. Gatherhood, R.G. Hazell, and K.A. Jorgensen, J. Org. Chem., 64, 4844 (1999). 124. Y.-C. Teo, J.-D. Goh, and T.-P. Loh, Org. Lett., 7, 2743 (2005). 125. (a) G.E. Keck, D. Krishnamurthy and X. Chen, Tetrahedron Lett., 35, 8323 (1994); (b) S. Konishi, H. Hanawa, and K. Maruoka, Tetrahedron: Asymmetry, 14, 1603 (2003); (c) J.M. Brunel, Chem. Rev., 105, 857 (2005). 126. (a) C.-M. Yu, S.-K. Yoon, H.-S. Choi, and K. Baek, Chem. Commun., 763 (1997); (b) C.-M. Yu, S.-K. Yoon, K. Baek, and J.-Y. Lee, Angew. Chem., Int. Ed., 37, 2392 (1998). 127. H. Kagoshima, T. Uzawa, and T. Akiyama, Chem. Lett., 31, 298 (2002). 128. S.E. Denmark and T. Wynn, J. Am. Chem. Soc., 123, 6199 (2001).

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Cross-Coupling of Organotin Compounds for C C Bond Formation

5.3

561

Cross-Coupling of Organotin Compounds for Carbon Carbon Bond Formation

Pablo Espinet and Miroslav Genov IU CINQUIMA/Qu´ımica Inorgánica, Facultad de Ciencias, Universidad de Valladolid, Spain

5.3.1

Introduction

With close links to the large family of palladium- and nickel-catalyzed cross-coupling reactions, based on transmetalations of a variety of hard or soft organometallic nucleophiles1 (e.g. Suzuki2 , Hiyama3 , Sonogashira,4 Kumada,5 and Negishi,6 and related reactions7 , 8 ), the Stille reaction [Equation (5.3.1)] is a versatile C–C bond formation reaction between organotin compounds and organic electrophiles (mainly halides or pseudo-halides). This reaction has established itself as one of the most general and most selective palladium-catalyzed cross-coupling reactions.9 , 10 , 11 , 12 Typically, R2 in Equation (5.3.1) is an unsaturated group like vinyl, aryl, heteroaryl, alkynyl, or allyl. R is a group, which is difficult to transfer such as Bu (n-butyl), or Me. Organic halides, triflates, and sulfonates R1 X are used as electrophiles. R1 X + R2 SnR3

Pd0

−→

R1 − R2 + XSnR3

(5.3.1)

The first examples of the coupling of organotin compounds were reported during the period 19761977, by the research groups of Eaborn13 and Kosugi.14,15 The extensive synthetic and mechanistic work carried out by Stille and coworkers since 197816 soon turned this reaction into a standard methodology for organic synthesis.17 The Stille reaction ranks today amongst the more general organic transformations,18 especially for the synthesis of complex molecules,19 where the Stille coupling is often superior, displaying high selectivity and broad scope. Its tolerance towards many functional groups makes Stille coupling particularly effective for transformations of highly functionalized molecules and it has been applied to the construction of a variety of ring systems bearing sensitive functional groups.20 Notable examples are the synthesis of dynemicin A by Danishefsky and coworkers21 and rapamycin by Nicolaou and coworkers,22 in which double couplings afford the formation of two C C bonds in a single step. 5.3.2

Mechanistic Aspects and Consequences

In his review of 1986,9 Stille proposed a mechanism based primarily on data obtained from the coupling of benzoyl chloride with tri-n-butyl(phenyl)tin. This proposal already clearly stated four main steps of the catalytic cycle: oxidative addition, transmetalation, isomerization, and reductive elimination. The active catalytic species in the Stille proposal was assumed to be a [PdL2 ] (L = PPh3 ) complex, which reacts with the organic electrophile R1 -X to give complex 1 (Scheme 5.3.1). The transmetalation following was believed to lead to complex 2. A trans to cis isomerization was then required for a reductive elimination, resulting in the organic product R1 − R2 . Stille noted that the existing data for the transmetalation supported an electrophilic cleavage of the Sn C bond (SE 2), with the PdII complex 1 acting as the electrophile. From his studies on the coupling of benzoyl chloride with (S)PhCHDSnBu3 catalyzed by [Pd(CH2 Ph)Cl(PPh3 )2 ], which took place ‘certainly with more than 65% net inversion,’9 , 23 Stille proposed the open transition state shown in the center of Scheme 5.3.1 for the SE 2 transmetalation step.

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Tin Chemistry: Fundamentals, Frontiers and Applications [PdL2]

R1 R2

R1X oxidative addition

reductive elimination

δ− Cl H D δ+ δ− R1 Pd C SnR3 L Ph

R2 R1 Pd

L

L

L R1 Pd

X

L 1

3

R2SnR3 isomerization

transmetalation

L R1 Pd

XSnR3

R2

L 2

Scheme 5.3.1

The original proposal for the mechanism of the Stille reaction

This proposal did not fit later observations, amongst them the observation of some intermediates and the occurrence of either retention24 or inversion23 , 25 of the configuration of alkyltin compounds with a stereogenic Cα centre, and has been modified and completed (Scheme 5.3.2), based on more recent results. A thorough mechanistic account has been published recently,26 which has been complemented with theoretical studies.27 , 28 R1 X

R1 R2

R

Transition states

R1

[PdLn]

X Pd L

cyclic

L C Bu3Sn

R1

1

R2 Pd

R2 Pd L

Bu3Sn

R

L

X

1

R1 L Pd L X

for the cyclic

R1L Pd C L Y

SnBu3

for the open

L Pd L R2SnBu3

X

Y = s, L open through cationic species

open

L Pd L + R R2

Scheme 5.3.2

R2SnBu3

R1

R1 2

Pd L

L

R1 L Pd L (S)

X

Catalytic cycles for transmetalation with cyclic and open transition states

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563

The oxidative addition of organic electrophiles (halides, sulfonates, and related compounds)29 to Pd0 is the first step in the cross-coupling reactions. The stereochemical course of the oxidative addition of allyl halides with a halogen directly attached to a stereogenic carbon (like 4, Scheme 5.3.3) depends on the nature of the solvent and the metal-bound ligands, so the configuration of the product can be pre-determined. Thus, the coupling of 4 with RSnBu3 occurs with overall retention of configuration when the reactions are performed in benzene with a Pd0 complex made in situ from [Pd(η3 C3 H5 )Cl] and maleic anhydride (Scheme 5.3.3). The configuration of the product is a consequence of the oxidative addition step (Scheme 5.3.2), which occurs with complete or predominant retention of configuration in less-coordinating solvents, such as benzene, CH2 Cl2 , THF, or acetone.30 , 31 However, complete or nearly complete inversion was observed in more polar, coordinating solvents, such as MeCN or DMSO.

CO2Me

CO2Me +

[Pd0]

CO2Me +

R SnBu3

Cl

R

R

4 R = Ph: benzene MeCN R = vinyl: benzene MeCN

96

4

0

100

92

8

0

100

Scheme 5.3.3 Stereochemical course of the oxidative addition of allyl halides with a halogen directly attached to a stereogenic carbon

The other step with stereochemical consequences is the transmetalation step. The enantioselective synthesis of tin derivatives with chiral carbons attached at tin is not easy, and the transfer of alkyl groups is more difficult than that of aryl or sp2 ; there are only three reports in the literature where groups with a stereogenic sp3 carbon attached to tin have been successfully transferred in a Stille reaction. Stille and Labadie found the reaction to proceed with predominant inversion of the configuration [Equation (5.3.2)].23

H

D C

Ph

O SnBu3 +

Cl Ph

[Pd(CH2Ph)Cl(PPh3)2] 4 mol% HMPA, 85 oC

D H C Ph

Ph

(5.3.2) O

Recently Chong and Kells demonstrated that R-sulfonamidobenzyltin compounds can be easily prepared in high enantiomeric purity and can undergo Stille type couplings with benzoyl chloride to give the expected ketones in high yields, with inversion of stereochemistry [Equation (5.3.3)].25 This application is not only of mechanistic, but of synthetic interest, since it allows access to a wide

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variety of enantiomerically pure amine derivatives, such as R-amino acids, R-amino ketones, and arylmethylamines.

HN

Bus

Bus

NH

O +

SnBu3

Cl Ph

Ph

Pd2dba3, CuCN 5 mol% Toluene, Ligand, 70 oC

(5.3.3) O

X

X X = H, p-Me, p-MeO, p-Cl, p-CF3

up to 98% yield

L = Ph3P, (o-tol)3P, dppe, tris(2,4,6-trimethoxyphenyl)phosphine Bus = tert-butanesulfonyl protecting group dba = dibenzylideneacetone

On the other hand, Falk and coworkers reported complete retention of configuration in the related coupling process [Equation (5.3.4)]24

BzO

H C

O SnBu3

+

Cl

Me(CH2)6

Ph

[PdCl2(PPh3)2], CuCN 2 mol% Toluene, 75 oC

BzO

H

Ph

C Me(CH2)6

(5.3.4) O

The key to the different stereochemical outcome of the reactions depicted with Equations (5.3.1–5.3.4) is the transmetalation step of the coupling reaction, and it was mostly the need to accommodate this dual stereochemical behavior that led to the proposal by Espinet and Casado of a catalytic cycle with two different pathways (Scheme 5.3.2), and two transition states: one cyclic (accounting for retention) and one open (accounting for inversion).32 , 33 The development of alkyl tin derivatives chiral at Cα , and the stereochemical control of the oxidative addition and transmetalation steps might significantly enhance the synthetic power of the Stille coupling reaction.34 5.3.3

Catalysts and Ligands

Both Pd0 and PdII complexes can be used as catalysts in the cross-coupling reaction. PdII complexes are air stable, but they need to be reduced to Pd0 before entering the catalytic cycle. This is achieved, as shown in Scheme 5.3.4, by reaction of the PdII pre-catalyst with the organic nucleophile (the tin derivative, in the case of the Stille reaction) and produces undesired by-products, which could be a problem in the case of high pre-catalyst loadings. Pd0 , although air and/or light sensitive in many cases, should be the choice catalyst in such cases. Alternatively, the use of PdII complexes [PdR XL2 ] having the same R group as the organic electrophile used in the reaction (R X), if accessible, also solves the problem of contamination by the by-product. Some of the most commonly used catalysts or pre-catalysts are [Pd(PPh3 )4 ],35 [Pd2 (dba)3 ] (dba = dibenzylideneacetone),36 [PdCl2 (NCMe)2 ],37 [PdCl2 (PPh3 )2 ],38 [Pd(CH2 Ph)Cl(PPh3 )2 ],39 [PdCl2 (dppf)]

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Cross-Coupling of Organotin Compounds for C C Bond Formation precatalyst PdX2L2

565

catalyst organic nucleophile

PdR2L2

R R PdL2 +

+ RM PdR'RL2

PdR'XL2

R' R byproduct

Scheme 5.3.4

Formation of by-products associated to the use of Pd II precatalysts

(dppf = 1,1-bis(diphenylphosphino)ferrocene),40 and [(μ-Cl)2 Pd2 (C3 H5 )2 ].41 Historically the use and choice of these fairly common complexes has been mostly dictated by trial and error, ease of availability, or imitation of previous reports. Sometimes mixtures of complexes or ligands have been used. The present mechanistic knowledge allows for a better understanding of the advantages and shortcomings of the different ligands. Thus, good donating ligands facilitate the oxidative addition, but render more difficult the transmetalation and the reductive elimination steps. On the other hand, weak ligands facilitate the transmetalation step at the expense of making the oxidative addition more difficult.26, 32 Catalysts having weak ligands are often applied together with strong ligands (typically PPh3 or diphosphines), the weak ligand complexes then acting just as convenient precursors of the corresponding complexes with strong ligands which are conveniently generated in situ. Especially useful is bis(dibenzylideneacetone)dipalladium(0) [Pd2 dba3 ]·(sol) (dba = dibenzylideneacetone, sol = dba or solvent molecule)36 , which is commercially available and stable, and can be used in conjunction with a large number of ligands. Since olefins are good ligands for Pd0 , but bad for PdII , dba plays an important role in stabilizing the catalyst in the form of Pd(dba)L2 , in the steps where Pd is reduced to Pd0 , whereas it is easily displaced by strong L ligands in the steps where the intermediates are in the form of PdII . Besides the dominant triphenylphosphine (PPh3 ) or diphosphines as ligands, the less electron-donating tri(2-furyl)phosphine and triphenylarsine (AsPh3 ) have been used successfully. In more recent times, the use of sterically bulky phosphines has revolutionized the scope of many cross-coupling reactions, particularly for the coupling of less reactive substrates such as aryl chlorides42 , 43 and alkyl electrophiles.44 Thus, these phosphines combine the properties of facilitating the oxidative addition step with less oxidising organic electrophiles, and protecting the alkyl palladium intermediates from the undesired β-H elimination along the reaction pathway. Particularly useful for the activation of aryl chlorides are palladium complexes of the bulky phosphine P(t-Bu)3 ,45 which is readily available. The Stille coupling at room temperature of alkenyl stannanes with alkyl bromides possessing βhydrogen atoms is possible in the presence of the bulky monodentate phosphine, Pt Bu2 Me. In this case, the addition of a fluoride salt is necessary to promote the reactivity of the tin compound [Equation (5.3.5)]. Using the diaminophosphine ligand, PCy(pyrrolidinyl)2 , the reaction can be extended to the coupling of aryltin compounds.46 [{(π-allyl)PdCl}2], PtBu2Me

Ralkyl Br + Bu3Sn R

Me4NF, THF, r.t

(5.3.5)

Ralkyl R

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Bedford and co-workers have demonstrated that palladium complexes of the simple, inexpensive tricyclohexylphosphine display very high activity in the Stille coupling of non-activated and deactivated aryl chlorides [Equation (5.3.6)].47 The use of K3 PO4 is necessary to promote the coupling.

2 Cl + R SnBu3

1

R

Pd(OAc)2,PCy3

2

K3PO4,1,4-dioxane,100 °C,18h R1

1

R = substituted aryl 2 R = phenyl,vinyl

R

(5.3.6)

32%–100%conversion

Recently the group of Verkade introduced a family of proazaphosphatrane ligands to produce Pd2 (dba)3 /P(RNCH2 CH2 )3 N catalyst systems, highly effective for the Stille cross-coupling of a broad range of functionalized aryl chlorides with aryl, vinyl, and allyl tin reagents [Equation (5.3.7)].48 The methodology is compatible with functional groups such as ester, nitro, trifluoromethyl, keto, cyano, and aldehyde. The system is also active for the synthesis of sterically hindered biaryls (di-, tri-, and tetra-ortho-substituted), allows for room temperature coupling of aryl bromides, and also permits aryl triflates and vinyl chlorides to participate in the Stille reaction. CsF or Me4 NF ware used as additives.

X

+ R SnBu3

FG

Pd2(dba)3, L CsF or Me4NF, 1,4-dioxane, 60–110 °C

X = Cl, Br, TfO R = aryl, vinyl and allyl Ph i-Bu i-Bu i-Bu N N N Ph i-Bu P N i-Bu P N P N Ph i-Bu Ph N N N L= N N N

R FG up to 98% yield

(5.3.7)

Ph P NN N

Ph

N

N -heterocyclic carbenes have demonstrated their utility as ligands in a variety of cross-coupling reactions, including the Stille reaction.49 Nolan and coworkers found that a Pd(OAc)2 /imidazolium chloride system mediates the catalytic cross-coupling of aryl halides with organotin compounds. The imidazolium salt iPr·HCl (iPr = 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene), in combination with TBAF (tetra-n-butylammonium fluoride, nBu4 NF), was found to be most effective for the crosscoupling of aryl bromides and electron-deficient aryl chlorides with aryl and vinyl tin compounds. In fact, the role of the TBAF additive (base) in these transformations is three-fold: (i) the strong base F– initially deprotonates the imidazolium chloride to form the free carbene ligand in situ, which coordinates to Pd; (ii) it also facilitates the transmetalation step by forming more reactive five-coordinated tin species, as the organotin compounds react with the fluoride anion to afford these five-coordinated [FSnR3 R ]− species, which are more effective in the transmetalation step than the organotin compounds

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SnR3 R themselves;33 and (iii) finally it helps the removal of tin by-products from the reaction mixture [Equation (5.3.8)].50

+ R

X

SnR'3

Y X = Cl, Br

Pd(OAc)2, L jHCl TBAF, dioxane/THF

R Y up to 98% yield

R = Ph, vinyl R' = Me, n-Bu

R N

L jHCl =

+

N R

(5.3.8)

F

Me

NBu4

Sn Me

Me

Cl five-coordinated organostannane

Triphenylarsine can be advantageous in metal complex-catalyzed processes, compared to PPh3 . As discussed above, the lower coordinating strength of AsPh3 facilitates transmetalation, although using AsPh3 instead of PPh3 can also frustrate other steps (e.g. oxidative addition) and, consequently the whole process.26 The first advantageous application of AsPh3 as a ligand in catalysis was by Farina and Krishnan in the palladium-catalyzed Stille cross-coupling of aryl electrophiles and tin nucleophiles.14 They also demonstrated that a Pd-based catalyst with AsPh3 is superior to one with PPh3 , in the coupling between aryl triflates and tetramethyltin [Equation (5.3.9)] or olefinic tin compounds [Equation (5.3.10)].52 Interestingly, PPh3 , the classical ligand in many cross-coupling reactions, including the Stille protocol, performs poorly in terms of rates of reaction. This fact, along with the air sensitivity of Pd(PPh3 )4 , makes the use of the air-stable Pd0 complex Pd2 (dba)3 together with AsPh3 an especially attractive catalyst. Typically, vinyl and aryl halides or triflates can be coupled in NMP (N -Methyl-2-pyrrolidone) at room temperature. Allylic halides also react more cleanly in THF at room temperature, when AsPh3 is used as the ligand.

O OTf

+

Me4Sn

Ph3As, NMP, 60 oC

(5.3.9)

O

Pd2dba3 95%

O OTf

+

SnBu3

Ph3As, NMP

O

o

(5.3.10)

Pd2dba3, 40 C 95%

The reaction of 8-bromoguanines with aryl- and hetero-aryl tin compounds in the presence of a palladium catalyst leads to the formation of the corresponding 8-aryl(heteroaryl)guanines. It was found

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that the addition of AsPh3 strongly reduces the reaction time and increases the yields [Equation (5.3.11)].53 O

O

N

HN

Br N

HN

O N

HN

Aryl

Aryl-SnBu3, Pd(PPh3)4

N

O O

N

HN

Ph3As, xylene, reflux

O

i-Pr

i-Pr

N O

O 86 %

i-Pr

O

(5.3.11) O i-Pr

Aryl =

O(i-Pr)

,

S

Me

The formation of a 16-membered macrolide intermediate in the synthesis of bafilomycin A1 proved to be very difficult to achieve by an intra-molecular Stille reaction. AsPh3 turned out to be the only ligand making this reaction possible. The best conditions found involve the use of Pd2 (dba)3 /AsPh3 /i-Pr2 NEt in DMF, at 40 ◦ C, to afford the desired macrocycle in 28% yield [Equation (5.3.12)].54 OMe

OMe I OH

O

n-Bu3Sn

O

HO

O

Pd2dba3 (10 mol%), Ph3As (80 mol%)

O

i-Pr2NEt, DMF, 40 oC, 30 h

ODMT

ODMT OMe

OMe

28 %

(5.3.12) The undesired presence of contaminant tin by-products in the coupling products has promoted the search for procedures to remove them. Along with the use of fluoride, already commented upon, the use of polymer-supported tin reagents is an interesting possibility. Very recently, Zammattio and Quintard reported the synthesis of two new vinyltin reagents grafted onto an insoluble macroporous polymer. These reagents were used in the palladium-catalyzed Stille cross-coupling reaction with aryl halides [Equation (5.3.13)]. In all reactions, the conversion of the starting aryl halide is high and the amount of organotin by-product is very low. At the end of the catalytic run, the amount of Sn is up to 16 ppm in the crude reaction mixture with the insoluble polymer removed, and it is less than 1 ppm in the product purified by chromatography on silica gel.55 SnBu2

R1

X +

R1 = CH(OEt)2, H immobilized on Amberlite XE 305

R2

R1

Pd(PPh3)4 toluene, 110 oC, 40h R2

X = Br, I

conversion 80–100%

R2 = -OMe, -NO2, -CHO

(5.3.13)

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569

O O CO

CO

CO

CO

CO

CO

OEt

OEt

OEt

OEt

OEt

O

OEt

O O

O Pd

O

O

O

OEt

OEt OC

O

CO

O EtO

OEt OEt CO CO

OEt CO

O O

O O O

CO OEt

Figure 5.3.1

CO

CO

CO

CO

OEt

OEt

OEt

OEt

The polymer-supported palladium complex Pd-pol

Pd is also a potential contaminant and Nobile and coworkers recently employed a polymersupported palladium complex (Pd-pol, Figure 5.3.1), obtained by copolymerization of Pd(AAEMA)2 with ethyl methacrylate and ethylenene glycol dimethacrylate (AAEMA– is the deprotonated form of 2(acetoacetoxy)ethyl methacrylate), as a catalyst in Stille cross-coupling reactions [Equation (5.3.14)].56 The reaction can be performed in air without any activating ligand and with non-dried solvents. The catalyst can be recycled several times.

R1

I

R1 = NO2, Me, H

+ R2 SnR 3

Pd-pol DMF, 70 oC

R2 = Me, Ph, PhC≡C

R

R'

(5.3.14)

Yield 80–99%

Chiu and coworkers have prepared the soluble non-cross-linked polystyrene (NCPS)-bound reagent NCPS-AsPh3 from 4-styryldiphenylarsine [Equation (5.3.15)] and applied it as a palladium ligand in Stille cross-coupling reactions. The catalytic system performed equally well compared to free AsPh3 as ligands in the Stille cross-coupling reaction of organic electrophiles and organotin compounds, with the

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advantage that it could be conveniently and efficiently separated from the reaction by precipitation, and recycled for further use [Equation 5.3.16)].57

( )1 ( )8 Ph

AIBN, toluene,

+

NCPS-AsPh3

85 oC

AsPh2

(5.3.15)

AsPh2

AIBN = Azobisisobutyronitrile

Pd(PhCN)2Cl2 R1

X

+

R1 = CHO, OMe, H, Cl

NCPS-AsPh3

R2 SnR3

dioxane, reflux, 18 h

R1

R2 = Ph, vinyl

R2

(5.3.16)

Yield 33–99%

X = Br, I, OTf

Nitrogen-based ligands have been used in the Stille cross-coupling reaction, but their scope and utility has not yet been well established.58 However, there are interesting precedents. Recently, Li and coworkers developed an efficient Pd(OAc)2 /1,4-diazabicyclo[2.2.2]octane (DABCO)-catalyzed Stille procedure.59 In the presence of Pd(OAc)2 and DABCO, various aryl halides, including aryl iodides, aryl bromides, and activated aryl chlorides, were coupled efficiently with organotin compounds to afford the corresponding biaryls, alkene, and alkynes in good to excellent yields. High turnover numbers for the Stille crosscoupling reaction (TONs up to 980 000 for the coupling reaction of 1-bromo-4-nitrobenzene and furan-2yltributyltin) were observed [Equation (5.3.17)]. Bu4 NF or KF were used as reaction promoters. Generally Bu4 NF was more efficient than KF, although KF performed better in the case of arylhalides having an oxygen-containing group, except for NO2 .

X

+

R2 SnBu3

R1

Pd(OAc)2/DABCO Base, Dioxane, 100 oC

R1

R2

(5.3.17)

X = Cl, Br, I

TONs: up to 980 000

R1 = COMe, OMe, H, NO2, Me

5.3.4

Ligandless Coupling

The coupling reaction of aryl and vinyl iodides, bromides, and triflates with organotin compounds can also be very effectively conducted using palladium on carbon as a source of Pd0 . The yield and rate of

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571

reactions are significantly improved by the addition of copper iodide as cocatalyst. Further improvement was found by addition of AsPh3 , while addition of PPh3 was deleterious,60 [Equation (5.3.18)]. R1 X +

R2SnBu3

Pd/C, CuI, Ph3As o

R1 R2

NMP, 80 C

(5.3.18)

Fouquet and coworkers have shown that monoalkyltins activated by a fluoride source are as reactive as their vinyl or aryl analogs in Stille coupling, thus providing an easy access to the palladium-catalyzed formation of Csp3 Csp2 bonds. This interesting approach holds several advantages, such as: (i) a quantitative preparation of stable and easy to handle alkyltin reagents, (ii) a simple coupling procedure without any phosphine ligand added under neutral conditions, and (iii) a facile purification step of the organic products from the non-toxic inorganic tin by-products [Equation (5.3.19)].61

Sn[N(TMS)2]2

1)

R1 X

2) Bu4NF

F R1 Sn F

N(TMS)2

R2 X

N(TMS)2

Pd2dba3, 1%

R1 R2

(5.3.19)

R1 = alkyl R2 = aryl, vinyl, alkynyl TMS = trimethylsilyl

The coupling reaction is believed to proceed via an activated hypervalent organostannate intermediate prepared in situ by adding a fluoride source. Comparison between PPh3 , P(i-Pr)3 , and AsPh3 , in conjunction with Pd2 dba3 , showed that the highest yields in the desired products are obtained under ‘ligandless’ conditions. 5.3.5

Copper Effect

A remarkable phenomenon in the Stille reaction is the effect of the addition of CuI or other CuI salts, which accelerate couplings catalyzed by [PdL4 ] with variable success.24,26,51 The nature of the ‘copper effect’ can be two-fold. Under mild conditions, the copper salt acts as scavenger of ligand in solution, in competition with Pd. This was first qualitatively studied by the groups of Farina and Liebeskind,62 and then quantitatively by Espinet and Casado.64 Thus, ligands such as PPh3 , that coordinate strongly to PdII , and have a strongly retarding effect on the transmetalation step, are effectively scavenged by CuI , which forms even stronger complexes. On the other hand, for weaker ligands such as AsPh3 , which do not show a strong retarding effect on transmetalation, when present in excess, the copper effect is small and, therefore, is not worth using as an additive, as addition of CuI salts produces minimal rate accelerations. A second possible action of copper(I) salts is that they can transmetalate with the organotin reagent, producing organocopper reagents in solution. This seems to be favored in the case of more stringent reaction conditions, e.g. higher boiling solvents. Recently, Baldwin and coworkers suggested a tandem use of CuI and CsF as a very efficient method for acceleration of the cross-coupling of a variety of iodides and triflates.65 Assuming that a transmetalation equilibrium exists between Bu3 SnR and CuI, to give Bu3 SnI and CuR, the transmetalation of R to Pd could occur from either Bu3 SnR or CuR. Removal of Bu3 SnI in the form of insoluble Bu3 SnF should favor the formation of the organocopper species CuR, more reactive towards transmetalation to Pd than the organotin species Bu3 SnR, this resulting eventually in enhancement of the coupling

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reaction. The remarkable increase in yield reported in the last entry of Equation 5.3.20 seems to support this idea. NO2 SnBu3

I +

Pd catalyst DMF, 40 oC, 2 h

O2N

Pd(PPh3)4

2%

Pd(PPh3)4, CsF

8%

Pd(PPh3)4, CuI

46 %

Pd(PPh3)4, CsF and CuI

98 %

(5.3.20)

Baldwin et al. demonstrated that the combination of CuI and CsF can significantly enhance the Stille reaction with a large number of examples. The PdCl2 /PtBu3 catalytic system with CuI and CsF in DMF is most effective for the coupling of aryl bromides, while [Pd(PPh3 )4 ] in combination with CuI and CsF is optimal when coupling iodides and triflates.65 Finally, a new effect of the addition of copper salts was noted recently. The regioselectivity of the Stille coupling of 3,5-dibromo-2-pyrone can be modified in the presence of CuI in DMF as the solvent, leading to the products shown in Scheme 5.3.5.66

O

O Br

O

[Pd(PPh3)4]

O Ph

O

+

O Br

O

+

Ph

O

PhSnBu3 Br

Ph

Ph

toluene, 100 oC (0.5 h)

81

trace

16

toluene, 100 oC, CuI (1 equiv.) (2 h)

71

6

4

DMF, 50 oC (4 days)

41

2

1

DMF, 50 oC, CuI (1 equiv.) (2 h)

trace

75

trace

Scheme 5.3.5

5.3.6

Br

Effect of the CuI on the regioselectivity of the Stille coupling of 3,5-dibromo-2-pyrone

Microwave-Assisted Reactions

Microwave irradiation has recently become a possible method to improve reaction yields and dramatically shorten reaction times.67 Numerous types of reaction with highly enhanced rates have been found, and very high yields and clean reactions have been obtained by applying only small amounts of energy.68

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573

Stille reactions have also shown to be suitable for these flash heating conditions, and rapid and successful Stille couplings were reported by Hallberg and Larhed in 1996 [Equation (5.3.21)].69 OTf

SnBu3

Ph

Pd2dba3, AsPh3

+

LiCl, NMP O

(5.3.21) O

classic: 70 hours

82 %

microwave: 2.8 min, 50 W

68 %

The same authors also reported a solid-phase version of this reaction [Equation (5.3.22)].70 OTf

Ph SnBu3

H N

1) Pd2dba3, AsPh3, NMP

+

RAM

H 2N

2) TFA O

O

NMP = (N-Methyl-2-pyrrolidone) microwave: 3.8 min, 40 W

85 %

(5.3.22) In order to cope with the toxicity and difficult separation of the tin by-products, the use of highly organofluorinated tin reactants was introduced by Curran and Hallberg. The main advantage is that inorganic, regular organic, and organofluorinated components can be efficiently separated via three-phase extractions with water, regular organic, and fluorocarbon solvents, respectively. A difficulty associated with this approach is that normally the ‘fluorous Stille’ couplings require long reaction times of 1 day at 80 ◦ C. This drawback has been successfully overcome using the microwave heating technique, which allows the shortening of the reaction time to less than 2 min using F-13 tagged organotins (F-13 = CH2 CH2 C6 F13 ) [Equation (5.3.23)].71 Cl +

(C6F13CH2CH2)3Sn

Cl

(5.3.23)

PdCl2(PPh3)3 Br

2.0 min, 50 W 90 %

Again, the conventional heating approach delivered only poor yields with other more fluorous-tagged organotins (F-21), whereas the microwave-accelerated reaction delivered 75% yield after 6 min [Equation (5.3.24)].72

Br (C10F21CH2CH2)3Sn

+

Pd(OAc)2, P(m-PhSO3Na)3 6.0 min, 50 W, LiCl, DMF

O

O

75 %

(5.3.24)

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5.3.7

Natural Product Synthesis

The Stille reaction is not only a powerful inter-molecular carbon–carbon bond formation process, but has found widespread use in the generation of cyclic structures.73 The intra-molecular Stille reaction was first reported by Piers and coworkers in 1985, applied for generating a variety of ring sizes and especially for microcycles.74 Today, the Stille reaction is a very reliable and frequently used method for the construction of carbocyclic and heterocyclic ring structures, due its remarkable functional group tolerance, mentioned earlier.75 An elegant example of that type of reaction is the stitching-cyclization reaction by Nicolaou and coworkers for the construction of rapamycin from the bis(vinyl iodide) precursor and trans-1,2-distannyl ethylene (Scheme 5.3.6).22 The last step of this total synthesis involved a double Stille coupling and proceeded from the precursor with no need for protection of the hydroxyl groups, with 20 mol% [PdCl2 (MeCN)2 ] and i-Pr2 NEt in a dilute DMF/THF solution at room temperature, probably via an iodotin intermediate.

Me

H

Me

I Me

O

O

Me O

OH

OMe O

N

O

O

OMe Me

Me

Me

H

O

O

OH

OMe O

N

SnBu3

[PdCl2(NCMe)2] (20 mol%) OMe

i-Pr2NEt DMF/THF, 25 oC

OH

intramolecular Stille coupling

H

SnBu3

I

OH O

Me

Bu3Sn

H

I

H

Me

O

O

Me O

Me

O Me

H

OH

OMe Me

OMe OH

Me

precursor intramolecular Stille coupling

(27%)

Me

H

Me

O

O OH

OMe O

N H O

Me

O

O Me

O

O

OMe Me

Me

H

OMe OH

Me Rapamycin

Scheme 5.3.6

Approach to the total synthesis of Rapamycin: Nicolaou’s ‘stitching cyclization’

In addition to intra-molecular cyclizations, Stille coupling is also a powerful methodology for the selective introduction of fragments in complex molecules. The use of this protocol has been demonstrated with the total synthesis of gambierol (Scheme 5.3.7) by the Sasaki76 and Yamamoto77 groups. In both approaches, a Stille reaction was used to append the delicate triene-containing chain into a fully constructed non-protected polycyclic ether precursor. The Sasaki group used the corresponding vinyl bromide precursor, while the Yamamoto group utilized the iodide. In both cases, the presence of CuI salts was necessary to promote the transmetalation step and to increase the reaction rates. The coupling product was formed with retention of the alkene geometry.

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Cross-Coupling of Organotin Compounds for C C Bond Formation OH Me

O

Me

H

O

H H O

H

O

H

H

O

H O Me

OH

H

O H Me

H

H Me

SnBu3

Yamamoto et al. Rainier et al.

[Pd(PPh3)4 (80 mol%)

P(2-furyl)3 (160 mol%) CuI, DMSO, 40 oC

CuCl, LiCl DMSO/THF, 60 oC

72–75%

43%

H

O

H H O

H

O

H

H

O

H

Scheme 5.3.7

5.3.8

H H

O H Me

O Me

OH

X

X = Br

[Pd2dba3]jCHCl3 (40 mol%)

Me

OH

Sasaki et al.

X=I

O

H

O

+

OH Me

575

H

O H Me

OH

Appendage of the triene-containing side chain to complete the total synthesis of gambierol

Conclusion

In summary, despite of the growing development of other Pd-catalyzed reactions using more polar organometallics, the Stille reaction continues to be an interesting alternative with no obvious replacement when substrates with active groups are involved. Moreover, the new improvements introduced by the use of bulky ligands, the better understanding of the reaction mechanism, and the effect of additives and ligands, as well as the efforts in developing procedures to avoid the problem of tin residues and to develop enantioselective protocols hold the promise of a long life for this catalytic process. References 1. E.-I. Negishi, J. Organomet. Chem. 2002, 653, 34. 2. N. Miyaura, T. Ishiyama, H. Sasaki, M. Ishikawa, M. Satoh, and A. Suzuki, J. Am. Chem. Soc. 1989, 111, 314. 3. (a) Y. Hatanaka and T. Hiyama, Pure Appl. Chem. 1994, 66, 1471; (b) Y. Hanataka and T. Hiyama, Synlett 1991, 845; (c) T. Hiyama in Metal-Catalyzed Cross-Coupling Reactions, F. Diederich and P. J. Stang (Eds), Wiley-VCH, Weinheim, 1998, Chapter 10; (d) T. Hiyama and E. Shirakawa, Topp. Curr. Chem. 2002, 219, 61.

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4. (a) K. Sonogashira, J. Organomet. Chem. 2002, 653, 46; (b) R. R. Tykwinski, Angew. Chem. Int. Ed.2003, 42, 1566. 5. (a) K. Tamao, K. Sumitani, and M. Kumada, J. Am. Chem. Soc. 1972, 94, 4374; (b) M. Yamamura, I. Moritani, and S.-I. Murahashi, J. Organomet. Chem. 1975, 91, C39; (c) J. F. Fauvarque and A. Jutand, Bull. Soc. Chim. Fr. 1976, 765; (d) A. Sekiya and N. Ishikawa, J. Organomet. Chem. 1976, 118, 349. 6. (a) E. Negishi, Acc. Chem. Res. 1982, 15, 340; (b) E. Negishi, M. Qian, F. Zeng, L. Anastasia, and D. Babinski, Org. Lett. 2003, 5, 1597, and references therein; (c) E. Negishi and L. Anastasia, Chem. Rev. 2003, 103, 1979. 7. Leading references on palladium-catalyzed couplings of organoindium compounds: (a) I. Pérez, J. P. Sestelo, and L. A. Sarandeses, J. Am. Chem. Soc. 2001, 123, 4155; (b) K. Takami, H. Yorimitsu, H. Shinokubo, S. Matsubara, and K. Oshima, Org. Lett. 2001, 3, 1997; (c) U. Lehmann, S. Awasthi, and T. Minehan, Org. Lett. 2003, 5, 2405; (d) P. Ho Lee, S. W. Lee, and D. Seomoon, Org. Lett. 2003, 5, 4963. 8. Iron-catalyzed cross-couplings of alkyl-Grignards: (a) A. Fürstner and A. Leitner, Angew. Chem. 2002,114, 632; Angew. Chem. Int. Ed. 2002, 41, 609; (b) A. Fürstner, A. Leitner, M. Méndez, and H. Krause, J. Am. Chem. Soc. 2002, 124, 13856. 9. J. K. Stille, Angew. Chem. 1986, 98, 504; Angew. Chem. Int. Ed. Engl 1986, 25, 508. 10. V. Farina in Comprehensive Organometallic Chemistry II, Vol. 12, E.W. Abel, F. G. A. Stone, and G. Wilkinson (Eds), Pergamon, Oxford, 1995, Chapter 3.4. 11. (a) T. N. Mitchell, Synthesis 1992, 803; (b) T. N. Mitchell in Metal-Catalyzed Cross-Coupling Reactions , F. Diederich and P. J. Stang (Eds), Wiley-VCH, New York, 1998, Chapter 4. 12. V. Farina, V. Krishnamurthy, and V. J. Scott, The Stille Reaction, Wiley, New York, 1998. 13. D. Azarian, S. S. Dua, C. Eaborn, and D. R. M. Walton, J. Organomet. Chem. 1976, 117, C55. 14. (a) M. Kosugi, K. Sasazawa, Y. Shimizu, and T. Migita, Chem. Lett. 1977, 301; (b) M. Kosugi, Y. Shimizu, and T. Migita, Chem. Lett. 1977, 1423. 15. M. Kosugi and K. Fugami, J. Organomet. Chem. 2002, 653, 50. 16. D. Milstein and J. K. Stille, J. Am. Chem. Soc. 1978, 100, 3636. 17. V. Farina, V. Krishnamurthy, and W. K. Scott, Organic Reactions, Vol. 50, John Wiley & Sons, Inc., New York, 1997. 18. J. Tsuji, Palladium Reagents and Catalysts, John Wiley & Sons, Ltd, Chichester, 1995. 19. (a) K. C. Nicolaou and E. J. Sorensen, Classics in Total Synthesis, VCH, Weinheim, 1996; (b) K. C. Nicolaou and S. A. Snyder, Classics in Total Synthesis II, Wiley-VCH, Weinheim, 2003. 20. M. A. J. Duncton, and G. Pattenden, J. Chem. Soc. Perkin Trans. 1 1999, 1235. 21. M. D. Shair, T. Y. Yoon, K. K. Mosny, T. C. Chou, and S. J. Danishefsky, J. Am. Chem. Soc. 1996, 118, 9509. 22. (a) K. C. Nicolaou, T. K. Chakraborty, A. D. Piscopio, N. Minowa, and P. Bertinato, J. Am. Chem. Soc. 1993, 115, 4419; (b) K. C. Nicolaou, A. D. Piscopio, P. Bertinato, T. K. Chakraborty, N. Minowa, and K. Koide, Chem. Eur. J. 1995, 1, 318. 23. J. W. Labadie and J. K. Stille, J. Am. Chem. Soc. 1983, 105, 6129. 24. (a) J. Ye, R. K. Bhatt, and J. R. Falck, J. Am. Chem. Soc. 1994, 116, 1; (b) J. Ye, R. K. Bhatt, and J. R. Falck, Tetrahedron Lett. 1993, 34, 8007. 25. (a) J. W. Labadie and J. K. Stille, J. Am. Chem. Soc. 1983, 105, 6129; (b) K. Kells and J. M. Chong, J. Am. Chem. Soc. 2004, 126, 15666. 26. P. Espinet and A. Echavarren, Angew. Chem. Int. Ed. 2004, 43, 4704, and references cited herein. 27. R. Alvarez, O. N. Faza, C. S. Lopez, and A. R. de Lera, Org. Lett. 2006, 35. 28. A. Nova, G. Ujaque, F. Maseras, A. Lledós, and P. Espinet, J. Am. Chem Soc 2006, 128, 14571. 29. (a) A minireview on the use of anhydrides, esters, and carbonic acid derivatives as substrates for the oxidative addition to Pd0 : A. Zapf, Angew. Chem. 2003, 115, 5552; Angew. Chem. Int. Ed. 2003, 42, 5394; (b) Stille coupling of sulfonyl chlorides: S. R. Dubbaka and P. Vogel, J. Am. Chem. Soc. 2003, 125, 15292. 30. (a) H. Kurosawa, S. Ogoshi, Y. Kawasaki, S. Murai, M. Miyoshi, and I. Ikeda, J. Am. Chem. Soc. 1990, 112, 2813; (b) H. Kurosawa, H. Kajimura, S. Ogoshi, H. Yoneda, K. Miki, N. Kasai, S. Murai, and I. Ikeda, J. Am. Chem. Soc. 1992, 114, 8417. ˚ 31. A. Vitagliano, B. Akermark, and S. Hansson, Organometallics, 1991, 10, 2592.

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577

32. A. L. Casado, P. Espinet, and A. M. Gallego, J. Am. Chem. Soc. 2000, 122, 11771. 33. A. L. Casado and P. Espinet, J. Am. Chem. Soc. 1998, 120, 8978. 34. For a review about the mechanisms of the cleavage of carbon-tin bonds, see M. Gielen, J. Braz. Chem. Soc. 2003, 14, 870. 35. D. Coulson, Inorg. Synth. 1972, 13, 121. 36. This is a dinuclear complex of formula [Pd2 (dba)3 ·dba]: Y. Takahashi, T. Ito, S. Sakai, and Y. Ishii, J. Chem. Soc., Chem. Commun 1970, 1065. After recrystallization, [Pd2 (dba)3 ·(solv)] (solv = CH2 Cl2 , CHCl3 , benzene, toluene) are obtained: T. Ukai, H. Kawazura, Y. Ishii, J. J. Bonnet, and J. A. Ibers, J. Organomet. Chem. 1974, 65, 253; M. C. Mazza and C. G. Pierpont, J. Chem. Soc., Chem. Commun. 1973, 207; M. C. Mazza and C. G. Pierpont, Inorg. Chem. 1974, 13, 1891. See also: A. M. Echavarren and J. K. Stille, J. Organomet. Chem. 1988, 356, C35. Tris(dibenzylideneacetone)palladium(0), [Pd(dba)3 ], has been prepared by heating [Pd(dba)2 ] with excess dba in benzene: M. C. Mazza and C. G. Pierpont, Inorg. Chem. 1973, 12, 2955. 37. M. Kharash, R. Seyler, and F. Mayo, J. Am. Chem. Soc. 1938, 60, 882. 38. A. Schoeberg, I. Bartoletti, and R. Heck, J. Org. Chem. 1974, 39, 3318. 39. a) K. Lau, P. Wong, and J. K. Stille, J. Am. Chem. Soc., 1976, 98, 5832; (b) P. Fitton, J. McKeon, and B. Ream, J. Chem. Soc., Chem. Commun. 1969, 370. 40. T. Hayashi, M. Konishi, Y. Kobori, M. Kumada, T. Higuchi, and K. Hirotsu, J. Am. Chem. Soc, 1984, 106, 158. 41. W. Dent, R. Long, and A. Wilkinson. J. Chem. Soc. 1964, 1585. 42. Review of cross-coupling reactions of aryl chlorides: A. F. Littke and G. C. Fu, Angew. Chem. 2002, 114, 4350–4386; Angew. Chem. Int. Ed. 2002, 41, 4177. 43. The Stille coupling of activated aryl fluorides has been recently found to be possible simply by using [Pd(PPh3 )4 ] as the catalyst: Y. M. Kim and S. Yu, J. Am. Chem. Soc. 2003, 125, 1696. 44. (a) D. J. Cárdenas, Angew. Chem. 1999, 111, 3201–3203; Angew. Chem. Int. Ed. 1999, 38, 3018; (b) D. J. Cárdenas, Angew. Chem. 2003, 115, 398; Angew. Chem. Int. Ed. 2003, 42, 384. 45. (a) A. Littke and G. Fu, Angew Chem., Int. Ed. 1999, 38, 2411; (b)A. Littke, L. Schwarz, and G. Fu, J. Am. Chem. Soc. 2002, 124, 6343. 46. H. Tang, K. Menzel, and G. C. Fu, Angew. Chem. Int. Ed. 2003, 42, 5079. 47. R. Bedford, C. Cazin, and S. Hazelwood (née Welch), Chem. Commun. 2002, 2508. 48. (a) W. Su, S. Urgaonkar, and J. Verkade, Org. Lett. 2004, 6, 1421; (b) W. Su, S. Urgaonkar, P. McLaughlin, and J. Verkade, J. Am. Chem. Soc. 2004, 126, 16433. 49. A. Hillier, G. Grasa, M. Viciu, H. Lee, Ch. Yang, and S. Nolan, J. Organomet. Chem. 2002, 653, 69, and references cited herein. 50. G. Grasa and S. Nolan, Org. Lett. 2001, 3, 119. 51. V. Farina and B. Krishnan, J. Am. Chem. Soc. 1991, 113, 9585. 52. V. Farina and G. Roth, Tetrahedron Lett. 1991, 32, 4243. 53. P. Arsenyan, M. Ikaunieks, and S. Belyakov, Tetrahedron Lett. 2007, 48, 961. 54. E. Quéron and R. Lett, Tetrahedron Lett. 2004, 45, 4539. 55. J.-M. Chrétien, A. Mallinger, F. Zammattio, E. Le Grognec, M. Paris, G. Montavonc, and J.-P. Quintarda, Tetrahedron Lett. 2007, 48, 1781. 56. M. M. Dell’Anna , A. Lofù, P. Mastrorilli, V. Mucciante, and C. F. Nobile, J. Organomet. Chem 2006, 691, 131. 57. K. Lau and P. Chiu, Tetrahedron Lett. 2007, 48, 1813. 58. (a) H. Azinian, C. Eaborn, and A. Pidcock, J. Organomet. Chem. 1981, 215, 49; (b) R. Sustman, J. Lau, and M. Zipp, Tetrahedron Lett. 1986, 27, 5207; (c) R. van Asselt and C. Elsevier, Tetrahedron 1994, 50, 323. 59. J.-H. Li, Y. Liang, D.-P. Wang, W.-J. Liu,Y.-X. Xie, and D.-L. Yin, J. Org. Chem. 2005, 70, 2832. 60. G. Roth and V. Farina, Tetrahedron Lett. 1995, 36, 2191. 61. A. Herve, A. Rodriguez, and E. Fouquet, . Org. Chem. 2005, 70, 1953. 62. (a) V. Farina and G. P. Roth, Adv. Metalorg. Chem. 1996, 5, 1; (b) V. Farina, Pure Appl. Chem. 1996, 68, 73. 63. V. Farina, S. Kapadia, B. Krishnan, C. Wang, and L. S. Liebeskind, J. Org. Chem. 1994, 59, 5905. 64. A. L. Casado and P. Espinet, Organometallics, 2003, 22, 1305–1309. 65. S. Mee, V. Lee, and J. Baldwin, Angew. Chem. Int. Ed. 2004, 43, 1132.

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W-S. Kim, H-J. Kim, and C-G. Cho, J. Am. Chem. Soc. 2003, 125, 14288. O. Kappe, Angew. Chem. Int. Ed. 2004, 43, 6250–6284, and references cited herein. M. Larhed, C. Moberg, and A. Hallberg, Acc. Chem. Res. 2002, 35, 717, and references cited herein. M. Larhed and A. Hallberg, J. Org. Chem 1996, 61, 9582. M. Larhed, G. Lindenberg, and A. Hallberg, Tetrahedron Lett. 1996, 37, 8219. M. Larhed, M. Hoshino, S. Hadida, D. Curran, and A. Hallberg, J. Org. Chem. 1997, 62, 5583. K. Olofsson, S. Kim, M. Larhed, D. Curran, and A. Hallberg, J. Org. Chem 1999, 64, 4539. K. Nicolaou, P. Bulger, and D. Sarlah, Angew. Chem. Int. Ed. 2005, 44, 4442, and references cited herein. E. Piers, R. Friesen, and B. Keay, J. Chem. Soc. Chem. Commun. 1985, 809. For a recent review for the application of the Stille reaction in the synthesis of complex natural products see: M. Vin´ıcius and N. de Souza, Curr. Org. Synth. 2006, 3, 313. 76. H. Fuwa, N. Kainuma, K. Tachibana, and M. Sasaki, J. Am. Chem.Soc. 2002, 124, 14983, and references therein. 77. (a) I. Kadota, H. Takamura, K. Sato, A. Ohno, K. Matsuda, and Y. Yamamoto, J. Am. Chem. Soc. 2003, 125, 46; (b) I. Kadota, H. Takamura, K. Sato, A. Ohno, K. Matsuda, M. Satake, and Y. Yamamoto, J. Am. Chem. Soc. 2003, 125, 11893.

66. 67. 68. 69. 70. 71. 72. 73. 74. 75.

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Stille Cross-Coupling for the Synthesis of Natural Products

5.4

579


Sergio Pascual and Antonio M. Echavarren Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain

5.4.1

Introduction

The Stille coupling of organic electrophiles with organostannanes1 , 2 , 3 plays a prominent role in current organic synthesis, in spite of the concern raised by the toxicity of tin compounds. The high reliability of this coupling procedure compares with other robust methods such as the Wittig and Julia olefinations, as well as ring-closing and cross-metathesis procedures for the joining of large, highly functionalized fragments in target-oriented organic synthesis. One of the particular advantages of this reaction is the fact that, unlike the Suzuki–Miyura coupling,4 the reaction proceeds under essentially neutral conditions, compatible with most functional groups. Several reviews cover in depth applications in synthesis,5 , 6 , 7 , 8 , 9 , 10 , 11 organometallic chemistry,12 and the mechanistic aspects of this reaction.13 Significant improvements made by Farina using tri-2furylphosphine and triphenylarsine as ligands,14 and recent advances in cross-coupling methodology make it possible to effect couplings of organostannanes under mild conditions.15 , 16 , 17 , 18 Attention has also been given to the effect of additives, particularly copper19 , 20 , 21 and halide anions, in this reaction.22 A noteworthy development was the discovery that copper(I) salts can indeed catalyze certain couplings of organostannanes in the absence of palladium.23 In this chapter we focus on applications of the Stille coupling reaction for the synthesis of complex natural products published in recent years (mainly the last 10 years). The chapter is organized by the type of bond being formed in the cross-coupling reaction. 5.4.2

Alkenyl–Alkenyl Stille Coupling

The stereospecific Stille coupling of alkenyl bromides or iodides with alkenylstannanes is one of the most widely applied methods for the synthesis of dienes. Thus, a slight modification of the original method24 was applied in the coupling of alkenyl iodide 1 with (Z )-alkenylstannane 2, stereospecifically building a polyene as the last step in the synthesis of (−)-stipiamide (Scheme 5.4.1).25 Me

Me

Me

Me I Me

OH

Me PdCl2(MeCN)2, NMP

+ (80%) O

SnBu3

OH

O Me

N

HO

H Me

Me N

Me

Me Me

1

OH (-)-Stipiamide (3)

H 2

Scheme 5.4.1

Synthesis of (−)-stipiamide (Bu: n-butyl, NMP: 1-methyl-2-pyrrolidinone)

A similar coupling of 4 and 5 was applied for the synthesis of the macrolide anti-tumor agent rhizoxin D (7) by White and coworkers (Scheme 5.4.2).26 Pattenden’s group also accomplished the total

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Tin Chemistry: Fundamentals, Frontiers and Applications Me

Me O

O

Me N

O

O

O

O

Me

O TIPSO

SnMe3

Me

TIPSO

5

Me

PdCl2(MeCN)2, DMF (84%)

O

O

Me I

O

N

Me

Me O

Me

Me

OMe

O

OMe 4

6

HF·py, THF 0°C to r.t. (59%) Me

HO

Me

Me N

Me

O

Me

O Me

O

OMe Rhizoxin D (7)

Scheme 5.4.2 py: pyridine)

Synthesis of the macrolide rhizoxin D (TIPS: triisopropylsilyl, DMF: N,N-dimethylformamide,

synthesis of rhizoxin D (7) by an intra-molecular alkenyl–alkenyl coupling.27 This type of coupling has been used frequently in the final steps of several total syntheses and synthetic approaches. Selected examples include the synthesis of reveromycin B,28 phthoxazolin A,29 (S, S)-sapinofuranone B,30 (+)-rottnestol, (+)-raspailol A and (+)-raspailol B,31 nafuredin-γ ,32 and pseudotrienic acid B,33 as well as in the synthesis of fragments of disorazole C1 34 and viridenomycin,35 and the monomeric counterpart of marinomycin A.36 White applied the alkenyl–alkenyl coupling of 8 with dienyl stannane 9 under standard conditions, for the build up of the triene fragment of polycavernoside A (10), a lethal toxin of a red alga (Scheme 5.4.3).37 It is noteworthy that this Stille coupling was efficiently performed as the last step of the synthesis, using the full glycosidated aglycon 8 as the electrophilic partner. The synthesis of the potent immunosuppressive agent (−)-pateamine A (13) was reported by Romo’s group, using a Stille coupling between 11 and aminostannane 12 (Scheme 5.4.4).38 It is interesting that the alkenyl–alkenyl coupling succeeded in the presence of an allyl carboxylate, which could have formed a π -allyl intermediate. Pattenden’s group completed a total synthesis of (−)-pateamine A (13) using both of the intra-molecular Stille alkenyl–alkenyl coupling reactions to elaborate the E, Z -diene macrolide core 14, and then an inter-molecular coupling using stannane 15 to form the all-E-polyene side chain portion of the natural product.39

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581

Me Me I

Me O

HO

O O

O O

Me

O

O OH

Me

Me

O MeO

Me Me

PdCl2(MeCN)2 DMF

8

(75%)

OMe

OMe

O

HO

O

O O

O O

OMe

Me

O

+

MeO Me

Bu3Sn

O OH

O

O

OMe Polycavernoside A (10)

OMe

OMe

9

Scheme 5.4.3

Stille coupling in the last step of the synthesis of polycavernoside A Me 1.

S Me

Br

N

O

O

TCBocHN Me O

O

Me

Me

Bu3Sn

NMe2

Me S

12 Pd2(dba)3jCHCl3 AsPh3, THF, 25°C (27%, 57% based on recovered 11)

NMe2 Me

N

O

O

Me

H2N Me

2. Cd/Pb, 1M NH4OAc THF, 25°C, (80%)

O

11

O

Me

(-)-Pateamine A (13) Me

1. Bu3Sn

NMe2 15

PdCl2(MeCN)2 DMF, r.t., 6 h (36%) 2. 10% Cd/Pb, 1M NH4OAc THF, r.t., 5 h (73%) S Me

I

N

O

O

Me

TCBocHN Me O

O

Me 14

Scheme 5.4.4 Two approaches for the preparation of (−)-pateamine A (dba: dibenzylideneacetone, TCBoc: β,β,β-trichloro-tert-butoxycarbonyl)

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The synthesis of alisamycin (18) and members of the manumycin family of antibiotics, reported by Taylor, relied on the Stille coupling between 17 and the stannane 16 (Scheme 5.4.5).40 Similar methodology was also employed to complete the total synthesis of the antibiotic nisamycin,40 asuka-mABA, 2880-II, and limocrocin.41 Wipf’s group applied a similar strategy for the synthesis of nisamycin.42 The synthesis of (+)-inthomycin B, based on the Stille coupling of a stannyl-diene with an oxazole vinyl iodide, has also been reported recently by Taylor and coworkers.43 O

HO Br

O

O

O

H N O

17

HO

N H

O

O

O HO

[PdCl2(PPh3)2/DIBAL-H] (5 mol%) THF-DMF, r.t. (64%)

SnBu3

H N

Alisamycin (18)

16 O OH HN

O

Scheme 5.4.5

Synthesis of alisamycin (DIBAL-H: diisobutylaluminum hydride)

de Lera’s group has reported the synthesis of retinoids by coupling of alkenyl iodides or triflates with organostannanes.44 Similar approaches have been reported by other researchers.45 , 46 The coupling of cyclic alkenyl triflates with cyclic alkenylstannanes has been used by de Meijere and coworkers as the key step for concise syntheses of steroids.47 A remarkably concise preparation of β-carotene (21) and (3R,3 R)-zeaxanthin (22) was reported by de Lera using a two-fold Stille cross-coupling reaction, with the C12-pentaenylbis-stannane 19 as the central linchpin (Scheme 5.4.6).48 Me

Me

Me

Me

I SnBu3

Bu3Sn 19

+ R

Me

Me 20a (R= H) 20b (R= OH)

PdCl2(PhCN)2 iPr2NEt, BHT THF/DMF (1:1), 25°C

Me

R

Me

Me

Me

Me

Me

Me

Me

R

Me

Me

β,β-carotene (21: R= H) (73%) (3R, 3´R)-zeaxanthin (22: R= OH) (46%)

Scheme 5.4.6 Linchpin strategy for the synthesis of β,β-carotene and zeaxanthin (BHT: 2,6-di-tert-butyl-4methylphenol)

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583

The linchpin strategy was also applied by Coleman for the synthesis of the anti-tumor agent lucilactaene49 and the gymnoconjugatins A and B,50 by sequential Stille and Suzuki–Miyaura couplings. The same group also made use of the Stille coupling for the synthesis of strobilurin B.51 A similar strategy, using a 1,3,5-hexatriene metalated with boron and tin at the C-1 and C-6 terminii, was applied for the synthesis of 2 -O-methylmyxalamide D and (6E)-2 -O-methylmyxalamide D.52 As expected, the alkenylstannane of the bismetalated linchpin is more reactive and undergoes selective coupling with a Pd(0) catalyst (Pd2 (dba)3 , AsPh3 ). The synthesis of 6’-epi-peridinin (31), the allene epimer of the carotenoid butenolide peridinin, is representative of the state-of-the-art in this field (Scheme 5.4.7).53 , 54 , 55 The new strategy developed by the groups of de Lera and Brückner uses three Stille couplings, including a halogen-selective Stille crosscoupling of the ylidenebutenolide fragment 28, and highly functionalized alkenylstannanes 24 and 27.

Me H Me

Bu3Sn

Me C OH Me

AcO

S O2 24

Br

N

Me

Me C

25 NaHMDS THF, -78°C then 26, -78°C, 2h (70%)

SnBu3

OHC 26 Me Me

O O

Me C

Me

Me

Br

H

Me

Br

Pd2(dba)3·CHCl3, AsPh3 Bu4N+Ph2PO2-, BHT THF, 25°C, 5.5 h (82%)

OH

AcO

+ 5É-27

H

SnBu3

5´Z-27 (Z:E=75:25) O

O

Me

Pd2(dba)3·CHCl3, AsPh3 Bu4N+Ph2PO2-, BHT THF, 55°C, 31 h (72%)

SnBu3 O HO

Me C

29

Me

5´

I 28

Me

OH Me

AcO

H

OH Me

AcO

PdCl2(PhCN)2 iPr2NEt, DMF/THF 40°C, 1.5 h (64%)

23

SO2BT

Me

S

Me 30 O Me

O

Me

Me

HO Me

Me 6´-epi-peridinin (31)

Scheme 5.4.7

Me

C

O HO

H

Me Me

OAc

Three Stille cross-couplings applied on the successful synthesis of 6’-epi-peridinin

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The recent syntheses of gambierol (34), accomplished by the groups of Sasaki56 and Yamamoto,57 are also illustrative (Scheme 5.4.8). Both syntheses use the Stille coupling reaction in the last step for building the Z , Z -1,3-diene functionality with the same stannane 33. These selective couplings used different catalysts, ligands, or additives (LiCl), but both were carried out in the presence of Cu(I), an additive employed to facilitate the transmetalation.58 One of the diene side chains of the related natural product (−)-brevenal was assembled by using copper(I) thiophen-2-carboxylate (CuTC)-promoted Stille coupling.59 HO

Me

O

Me

H

O

H H O H

HO H

O

H

H

O

H

O H Me

H

O Me

O

33

O

Me

Pd(PPh3)4, CuCl LiCl, DMSO/THF (1:1) 60°C, 2 d, (43%) (X= Br) or Pd2dba3, P(furyl)3, CuI DMSO, 40°C, (72%) (X= I)

SnBu3

Me

Me

H

O

H H O H

HO H

O

H

H

O

H

H

O H Me

O Me

(-)-Gambierol (34)

Scheme 5.4.8

X OH

H

32a (X= Br) 32b (X= I)

HO

H

H

O H Me

OH

Formation of the trienic side chain in gambierol by using the Stille reaction

Cu(I) was also used as a cocatalyst for the key Stille cross-coupling reaction in the syntheses of (+)hamabiwalactone,60 cochleamycin A,61 strobilurins G, M, and N,62 cyercene A and the placidenes,63 elysiapyrones A and B,64 and (−)-SNF4435 C and (+)-SNF4435 D.65 In these last cases, after the Stille coupling, the resulting tetraenes underwent a cascade of 8π - and 6π -electrocyclizations, mimicking the biosynthesis of these compounds.66 A synthesis of panepophenanthrin has been accomplished by Baldwin via a biomimetic Diels–Alder dimerization as a key step. The key monomeric precursor was assembled by a Stille cross-coupling of two readily available building blocks.67 The enantioselective syntheses of ircinol A (38) and the related manzamine alkaloids ircinal A and manzamine A were accomplished by Martin (Scheme 5.4.9).68 The concise synthesis of ircinol A (38) highlights the strategy for assembling the tricyclic ABC ring core via a domino Stille coupling/Diels– Alder reaction.

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Stille Cross-Coupling for the Synthesis of Natural Products Br

H COOMe

COOMe N N Boc

O

SnBu3 Pd(PPh3)4 toluene, Δ

N Boc

O

OTBDPS

OTBDPS

35

OTBDPS OTBDPS

OTBDPS

OTBDPS

N Boc

O

(68%)

COOMe H

N

N

585

36

37

13 steps

H

CH2OH OH

N H N

Ircinol A (38)

Scheme 5.4.9 ircinol A

Domino Stille coupling/Diels-Alder reaction in one of the initial steps of the synthesis of

The cross-coupling/Diels–Alder cycloaddition has been used in other syntheses. Thus, in work towards the synthesis of the marine alkaloids zoanthamine and norzoanthamine, reported by Tanner, the Diels–Alder precursors were constructed by a Pd(0)- and Cu(I)-catalyzed coupling.69 A synthesis of the decahydrofluorene nucleus of GKK1032s was achieved using a Stille-coupling/intra-molecular Diels– Alder reaction.70 The synthesis of the 36-membered macrolide dermostatin A (42), carried out by Rychnovsky, is remarkable for the complexity of this natural product and its acid- and light-sensitivity (Scheme 5.4.10).71 Several approaches to the synthesis of amphidinolides also make use of alkenyl–alkenyl Stille coupling reactions.72 , 73 , 74 , 75 , 76 In the synthesis of callipeltoside A (45) by Trost, the coupling of (E)-3-(tributylstannyl)prop-2-en-1-ol with dibromoalkene 43 proceeded with concomitant dehydrobromination to afford conjugated dienyne 44 (Scheme 5.4.11).77 , 78 Other important natural products, such as crocacin D (46),79 , 80 bafilomycin A1 (47),81 , 82 polycephalin C (48),83 dictyostatin (49),84 (+)-tubelactomicin A (50),85 fostriecin (51a) and 8-epi-fostriecin (51b),86 , 87 archazolid B (52),88 and apoptolidin (53)89 have been synthesized by using the alkenyl–alkenyl coupling are shown in Figure 5.4.1. The alkenyl-alkenyl coupling has been frequently used in the intra-molecular fashion for the synthesis of large macrocycles, as an alternative to the most common lactonization procedures.8 Selected important

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Tin Chemistry: Fundamentals, Frontiers and Applications O

OEt P OEt

O

OTBS I O

O Me

Bu3Sn

+

OH

Me O

O

O

O

O

O

40

O Pd2(dba)3·CHCl3 iPr2NEt, AsPh3, THF (77%)

39

OTBS

HO OR

O Me

Me O

O

O

O

O

O

O 41

(R=

O

O P(OEt)2

)

3 steps

O

OH O

OH Me

Me OH

OH

OH

OH

OH

OH

OH

Dermostatin A (42)

Scheme 5.4.10

Alkenyl–alkenyl Stille coupling in the preparation of dermostatin A

natural products synthesized by this method are: elaiolide (54),90 14,15-anhydropristinamycin IIB (55),91 macrolactin A (56),92 sanglifehrin A (57),93 , 94 concanamycin F (58),95 , 96 and sarain A (59)97 (Figure 5.4.2). Mycotrienol I (60) (Figure 5.4.2) was synthesized by Pannek using two consecutive Stille couplings (inter- and intra-molecular) with (E)-1,2-bistributylethene as the linchpin.98 A new method for the preparation of indoles from α-haloenones and α-(trialkylstannyl)enecarbamates has been developed by Funk.99 The method is based on a Stille coupling followed by an electrocyclic ring closure and oxidation. A 6π-electrocyclic ring closure of a substituted 2,3-divinylpyrroline, available by a Stille coupling reaction, was also used by Funk for the synthesis of cis-trikentrins A and B.100 Most alkenyl–alkenyl couplings proceed uneventfully. However, Suffert has shown in several examples that a proximal triple bond can interfere, leading to a carbocyclization instead of the expected direct Stille

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Stille Cross-Coupling for the Synthesis of Natural Products HO

SnBu3

Pd2(dba)3·CHCl3, (4-MeOPh)3P DIPEA, DMF, 80°C

Br

HO

Cl

Br

587

Cl

44

(66%) 43

6 steps O Me O

NH

MeO O

O

Me

Me Me H MeO

O

OH O

O

Me Callipeltoside A (45)

Scheme 5.4.11

Cl

Synthesis of callipeltoside A

coupling.101 In the example shown in Scheme 5.4.12, using bromoalkenes 61a,b and stannane 62, an 8π -electrocyclization was used to construct eight-membered rings in a one-pot 4-exo-dig cyclocarbopalladation/Stille coupling/electrocyclization sequence to form tetracycles 63 and 64. 5.4.3

Alkenyl–Alkynyl Stille Coupling

Fewer examples of the alkeny–alkynyl Stille cross-couplings have been reported. Coupling of tri-nbutyl(3-methylbut-3-en-1-ynyl)stannane with iodoenone 65 afforded 66, which was transformed into the antimicrobial compound (−)-asperpentyn (Scheme 5.4.13).102 Similarly, the first synthesis of the trienyne (±)-taxifolial A (71) was achieved from iodoalkene 68 and enenylstannane 69.103 The first stereoselective synthesis of (−)-ichthyothereol (75), which bears a conjugated enetriyne, was completed from iodoalkene 72 and triynylstannane 73.104 The synthesis of furocaulerpin also used the alkenyl– alkynyl cross-coupling under standard conditions.105 5.4.4

Alkenyl–Aryl Stille Coupling

There are numerous examples of this type of coupling for the formation of alkenyl–aryl and alkenyl– heteroaryl bonds. Only a few examples within the context of complex natural product synthesis are highlighted here. The synthesis of (+)-phorboxazole A (79) was reported by the group of Smith III by means of the coupling of fragments 76 and 77 (Scheme 5.4.14).106

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Tin Chemistry: Fundamentals, Frontiers and Applications OMe Me

Me

O Me

Me

OH

HO NH

OMe OMe

Me

O

OH

Me

O

OMe

N H

O

OH

O

Me

O

Me

Me

OMe

Me

Bafilomycin A1 (47)

Crocacin D (46) O Me

HO

COOH

N HO OH

OH

O

Me

Me

H

HO

O O

Me

O

O H

H O Me

OH OH

H

OH

Me OH

N

HO

Me

Dictyostatin (49)

O

(+)-Tubelactomicin A (50)

Polycephalin C (48) Me

Me

MeO NaO HO O

O

P

O O

Me

OH

OH

OH

Me

Me R1

R2

O

Me R1=

OH

Me

R2=

Me, OH ; Fostriecin (51a) R1= OH, R2= Me ; 8-epi-Fostriecin (51b)

O N S

OH HO MeO

Me

MeHN

Me

O O Me Me

O O

Archazolid B (52)

Me O

OH Me O MeO HO

OH H O

Me

OMe Me

O

O

OH Apoptolidin (53)

Me Me O

O

OH OMe

HO Me

Figure 5.4.1 Selected natural products synthesized by intermolecular alkenyl-alkenyl Stille coupling highlighting the bond formed in the key cross-coupling

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589

O Me OH

HO

O Me

Me

O OH

O

O

Me OH

O

Me

N H

Me

Me

Me

Me

Me

Me

O

O

O

N

N

OH

OH

Elaiolide (54)

O

Me O

H

O

14,15-Anhydropristinamycin IIB (55) Me

OH

Me

Me

HO

O

Me Me

O

HO

OH OH

O

O

Me

Me NH O

O

HO

OH

O

NH O

HN

O

O

N NH

Me Macrolactin A (56)

Sanglifehrin A (57)

OH

Me O

Me Me Me OH

OMe OH

O Me

CHO

Me N

O

HO Me

O

Me

Me

OH

N

OH

Me

NH

OH HO

Me

O O

HO

OMe

HO

(+)-Concanamycin F (58)

OMe Mycotrienol I (60)

Sarain A (59)

Figure 5.4.2 Selected natural products synthesized by intra-molecular alkenyl–alkenyl Stille coupling highlighting the bond formed in the key cross-coupling

Bu3Sn HO HO

X

SiR3 Br

HO 62 (X= C(COOMe)2 C(CH2OH)2, O, NTs)

SiR3

HO

SiR3

O

+ 61a (R= Me) 61b (R= Et)

Scheme 5.4.12

Pd(PPh3)4 benzene, 85°C (11-63%)

H

X 63

H

X 64

Interference by a proximal triple bond with the Stille coupling leading to a carbocyclization

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O

O I

Bu3Sn

O

Me

OH

O OTBS

Me

2 steps O

PdCl2(PPh3)2, AsPh3 CuI, THF, r.t. (98%)

OTBS

65

OH

66

(-)-Asperpentyn (67)

OTBDMS OTBDMS SnMe3

Me

69

Me I OAc OAc 68

PdCl2(MeCN)2 (2 mol%) DMF, r.t. (99%)

OAc OAc 70

2 steps

Me

AcO CHO OAc

Taxifolial (71)

TBDMSO H H

TBDMSO H H

SnBu3

Me 73

Me I

H O H

PdCl2(PPh3)2 (5 mol%) THF, r.t. (95%)

H O H 74

72 TBAF, THF, r.t. (100%) HO H H Me

H O H (-)-Ichthyothereol (75)

Scheme 5.4.13 Alkenyl-alkynyl Stille couplings in the synthesis of different natural products (TBDMS: tertbutyldimethylsilyl)

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591

MeO OTBS TMS

O N

Me MeO

+

OTIPS O N

O

I

SnMe3

O

Me

O

H3CO

O

O CH3 Me

O 76

77 Pd2(dba)3·CHCl3 AsPh3, DIPEA NBu4Ph2PO2, DMF, r.t. (68%) MeO

TMS OTBS O Me

MeO

N

O

OTIPS O

O

Me

O

H3CO

O

N O O

CH3 Me 78

3 steps MeO

Br OH O Me

MeO

N

O

OH O

O

Me

O

HO

O

N O O

Me

Me

(+)-Phorboxazole A (79)

Scheme 5.4.14 Synthesis of phorboxazole A (DIPEA: diisopropylethylamine, TBS: tert-butyldimethylsilyl, TIPS: triisopropylsilyl)

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The first total syntheses of higher-order members of the polypyrrolidinoindoline alkaloid family, quadrigemine C (83) and psycholeine (84), developed by Overman, uses a double Stille cross-coupling of diiodide 80 with stannane 81 (Scheme 5.4.15).107 The efficiency of this coupling is remarkable, as 81 bears an aryl triflate that could react intra-molecularly with the stannane. This coupling was also performed in the total synthesis of the hodgkinsines carried out by the same group.108 NMeTs

I

OTf

H H Me N N

Bn N

NBn O

NMeTs

81

OTf O

Pd2(dba)3·CHCl3 P(2-furyl)3, CuI NMP, r.t. (71%) I

H H Me N N

O

SnBu3

N N H H Me

NBn

N N H H Me

OTf

NMeTs

80 82

3 steps

H

Me N H

HN H N

NH MeN

Me N

H

H NMe

N AcOH (0.1 N) N H

NMe

N N H H Me

N N H H Me

HN H

Psycoleine (84)

Scheme 5.4.15

100°C

N Me Quadrigemine C (83)

Synthesis of polypyrrolidinoindoline alkaloids (NMP: 1-methyl-2-pyrrolidinone)

A difficult Stille reaction of bromoester 85 with a stannylpyridine gave the tri-Me ester of ent-thallusin 86 in 54–92% yield, using stoichiometric Pd(PPh3 )4 .109 The use of CuI and water as additives was crucial for the success of this coupling for the synthesis of ent-thallusin 87 (Scheme 5.4.16). The enantioselective total synthesis of the diazobenzofluorene antibiotic (−)-kinamycin C (91) via the coupling of bromoenone 88 with naphthylstannane 89, has been reported by Porco and coworkers

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Br COOMe Me

Me

O

H

COOH

N

N COOMe

COOMe N

85

COOH Me

O

H

Alkenyl-aryl coupling in the synthesis of ent-thallusin

SnBu3

Br O

+

Pd2(dba)3, AsPh3, CuCl, iPr2NEt

OMOM O

MOMO MOMO

CH3 OH

MOMO

OMOM

O

CH3CN, 70°C, 4 h (70%)

CH3 TBSO

88

O

ent-Thallusin (87)

86

OMOM

O

Me H

H

microwave, 90°C, 30 min

Scheme 5.4.16

Me H

(1.5 equiv.)

COOMe SnBu3

COOH

COOMe

then, CuI (1.5 equiv.)

H

TBSO

COOMe Pd(PPh3)4 (1 equiv.) DMF, 0.1% H2O microwave, 90°C, 15 min

Me

593

89

OH

90

11 steps

O

OAc

AcO

CH3 OH OAc

OH

O

N N

Kinamycin C (91)

Scheme 5.4.17

Stille coupling in the synthesis of kinamycin C (MOM: methoxymethyl)

(Scheme 5.4.17).110 It is interesting, in the context of synthesis of more simple members of this family of natural products, that one of the first examples of the beneficial effect of Cu(I) in the Stille reaction was discovered.111 5.4.5

Aryl–Aryl Stille Coupling

The aryl–aryl, aryl–heteroaryl, and heteroaryl–heteroaryl couplings have also been extensively applied in synthesis. Only a few examples of the most recent results within the context of complex natural product synthesis will be summarized here.

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Tin Chemistry: Fundamentals, Frontiers and Applications COOtBu

TBSO Me3Sn

NCbz N H Cbz

COOtBu

TBSO I

H

93

NCbz N H Cbz

COOtBu

TBSO

CbzN

Pd2(dba)3, AsPh3 DMF, 45°C (83%)

92

Cbz N

NCbz

OTBS

tBuOOC

N H Cbz

94 12 steps

HO O

H N H

HN HO

O N H

N

O

O O O

HN

H N

H

N

O

O N

OH NH

O

HO

H

N H

N

O

H N OH

O NH

H N H

O Himastatin (95)

O

O

OH

Scheme 5.4.18 Aryl–aryl Stille coupling in the synthesis of himastatin (Cbz: benzyloxycarbonyl, TBS: tertbutyldimethylsilyl)

Several examples of synthesis of acetogenic isoquinoline alkaloids have been reported in Bringmann’s group using the aryl–aryl Stille coupling.112 In another example, Danishefsky reported the coupling of 92 with stannane 93 to give biaryl 94, an intermediate in the total synthesis of himastatin (95) (Scheme 5.4.18).113 Panek reported the syntheses of the anti-fungal agents cystothiazoles A (100a) and B (101) from nonsymmetrical bisthiazoles 98a,b, which were prepared by the regioselective coupling of bistriflate 96 with stannanes 97a,b (Scheme 5.4.19).114 This synthesis also made use of a second Stille coupling of 98a,b with alkenylstannane 99. The construction of the dragmacidin E core ring system 107 proceeds via two Stille cross-couplings with N -tosyl-3-(tri-n-butylstannyl)-indole and 105 (Scheme 5.4.20).115 This synthesis also features an application of a new indole annulation reaction.99 In the synthesis of the thiazolyl peptide GE2270 A (111), developed by Bach and coworkers, a remarkable intra-molecular Stille reaction on 109 leads to macrocycle 110 in a highly efficient manner (Scheme 5.4.21).116 5.4.6

sp3 –sp2 Coupling Reactions

Compared with the previous coupling types, much less has been reported with these relatively more difficult couplings. Exceptionally, couplings with allyl electrophiles, usually allyl acetates or halides, have been often used in the synthesis of complex natural products.117 A noteworthy example is the synthesis of the azaspiracids 1–3 (115a–c), neurotoxins isolated from mussels, whose structure was determined by total synthesis by Nicolaou and coworkers.118 This synthesis features a notable Stille

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Stille Cross-Coupling for the Synthesis of Natural Products Bu3Sn

595

N R S

TfO

97a (R= H) 97b (R= OTBS)

N

TfO

N

S

OTf Pd(PPh3)4, LiCl Dioxane, 100°C

S 96

N

S

R 98a (R= H) (72%) 98b (R= OTBS) (68%)

OMe OMe Pd(PPh3)4, LiCl Dioxane, 100°C

SnBu3 MeO

O

Me

OMe OMe

OMe OMe N

MeO

O

99

Me

S

N

TBAF, THF

S

r.t. (98%)

N

MeO

OH

S

S

N R

Cystothiazole B (101)

Scheme 5.4.19

O

Me

Cystothiazole A 100a: R= H (85%) 100b (R= OTBS) (72%)

Synthesis of cystothiazoles A and B (TBS: tert-butyldimethylsilyl) SnBu3

N Br

OTIPS

OTIPS

OTIPS I OMe

N

NTs

N 4 steps

Br

Pd(PPh3)4 CuCl, LiCl (85%)

102

NTs

N

N Ts

N

OMe

N

I

OMe

O 103

104 SnMe3

O Br H N

105

NH

N H

OTIPS

OTIPS

NH N

HO

Pd(PPh3)4 CuCl, LiCl DMSO (71%)

NBoc

NH

N

O

N H Dragmacidin E (108)

Scheme 5.4.20

NTs

N BnO

N

OMe O

N Ac 107

NTs

N

6 steps

O NBoc 106

Stille couplings in the construction of dragmacidin E core

OMe

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Tin Chemistry: Fundamentals, Frontiers and Applications COOtBu S

COOtBu SnMe3

N

N

S Br

N

N S

S

O

OTBDMS

Pd(PPh3)4 (20 mol%) PhMe, 85°C

N

S

S

S

N

OH

(75%)

NH

O

HN

O

O H N

N S O

NH

O

NH

MeHN

N

N

N

N

S

N S

O

H N

N

OMe MeHN

N

O

S

S

Me 109

NH

O

NH

OMe

Me 110

O

3 steps

O O

H2N O N

N S

N

N N

N

S

S

S

N

OH HN

O

O H N

N S MeHN

NH

O

NH

N S

O OMe

Me

O GE2270 A (111)

Scheme 5.4.21 Intramolecular macrocyclization by using the Stille reaction in the synthesis of the thiazolyl peptide GE2270 A (TBDMS: tert-butyldimethylsilyl)

reaction of allylic acetates 112a–c with a vinylstannane 113 to give 114a–c, the core structures of the azaspiracids (Scheme 5.4.22). The coupling of benzyl halides with stannanes has been occasionally used in synthesis.119 One illustrative example is the total syntheses of piericidin A1 (119) and B1 (120).120 In these syntheses a heterobenzylic Stille cross-coupling reaction of 116 with alkenylstannane 117 proceeded in the presence of a Pd(0) complex bearing bulky PtBu3 as the ligand to give key intermediate 118 (Scheme 5.4.23).

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Stille Cross-Coupling for the Synthesis of Natural Products R1 O O OAc

H

H

Teoc

H OH R2

O

N

O HO

Me

H

O

Me

Me

H Me

112a: R1= H, R2= Me 112b: R1= R2= Me 112c: R1= R2= H

O

+ O

113

AcO Pd2(dba)3, AsPh3 LiCl, iPr2NEt R1 H

O O

H OH R2

O

O HO

Me

H

OAc

O

H H OTES

Teoc

Me O

N O H Me

Me

Me

114a: R1= H, R2= Me 114b: R1= R2= Me 114c: R1= R2= H

R1 O HO

O H

O

H

H OH R2

O

O HO

Me

H

O H

H NH

Me

O O

Me O Me

H

Me

Azaspiracid-1: R1= H, R2= Me (115a) Azaspiracid-2: R1= R2= Me (115b) Azaspiracid-3: R1= R2= H (115c)

Scheme 5.4.22

OTES

Synthesis of azaspiracids 1–3

Me

SnMe3

597

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Tin Chemistry: Fundamentals, Frontiers and Applications OH

MeO MeO

OTBS Me Bu3Sn

+

Br

N

Me Me

Me

Me

Me

117

116

Pd2(dba)3, PtBu3 LiCl, dioxane, 70 °C, 18 h (74%) OH MeO MeO

OH Me

OTBS

3 steps Me

N Me

Me

Me

Me

OMe

N

Me Me

Me

Me

Piericidin B1 (120)

Bu4NF, THF 50 °C, 12h (93%)

OH

MeO

MeO

Me

Me

118

MeO

MeO

Me

OH

N

Me Me

Me

Me

Me

Piericidin A1 (119)

Scheme 5.4.23

Stille coupling strategy for the preparation of piericidins A1 and B1

A noteworthy sp3 –sp2 coupling was reported in the synthesis of the antibiotic carbapenem 124, which was developed by chemists at Merck (Scheme 5.4.24).121 In this reaction, carbapenem triflate 121 reacted with stannatrane 122 to give 123. The stannatranes were designed by Vedejs to facilitate the selective coupling of alkyl groups.122 In a similar vein, in a bold application of Stille chemistry, Danishefsky completed the synthesis of eleutherobin by the remarkable coupling of triflate 125 with (trimethylstannylmethyl)arabinose donor 126 (Scheme 5.4.25).123 With other stannanes RSnR3 (R, R = alkyl), the selective transfer of the desired alkyl group is not possible, which limits the use of this type of coupling to symmetrical R4 Sn reagents. As an example, in the total synthesis of the potent anti-tumor agent ecteinascidin (ET-743), a methyl group was introduced on an aromatic ring by coupling of Me4 Sn with an aryl triflate.124 However, allylations with allylSnR3 (R = Me, Bu) have been frequently used in synthesis as a means to introduce a three-carbon chain, which can be used in olefin metathesis, or be easily transformed into a –CH2 CHO fragment by oxidative cleavage. Examples can be found in the syntheses of taspine,125 santiagonamine,126 (R)-(+)-lasiodiplodin and zeranol.127 Reaction with substituted allylstannanes occurs at the least substituted terminus of the allyl.128 Thus, in the synthesis of (±)-A80915G (132), a member of the napyradiomycin family of antibiotics,129 two consecutive palladium allylations were performed (Scheme 5.4.26). In the first reaction, a geranyl chain was introduced selectively, by coupling with the more reactive iodide of substrate 129 to give 130, using PdCl2 (dppf) as a pre-catalyst (dppf: 1,1 bis(diphenylphosphino)ferrocene). In the second cross-coupling, a prenyl was added on to a sterically hindered position to give 131.

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TESO H H Me

+ N

Me

N +

OTf

N O

599

+ CONH2

N

COOPNB

2 TfO

SO2

N Sn 121

122 Pd2(dba)3, P(2-furyl)3 iPr2NEt, NMP, 60°C, 3 h (98%) + N N

TESO H H

+ CONH2

Me

Me

N

2 TfO

SO2

N O

COOPNB 123

+ N N HO H H

Me

+ CONH2

Me N

SO2

Cl

N O

COO

124

Scheme 5.4.24 Stille reaction in the synthesis of the antibiotic carbapenem 124 by using stannatrane 122 as coupling partner (TES: triethylsilyl)

The coupling of 11 C-labeled CH3 I with organostannanes130 with stoichiometric amounts of palladium has been used extensively to prepare 11 C-labeled metabolites for positron emission tomography (PET). 5.4.7

Couplings for the Synthesis of Ketones

Ketones can be obtained by coupling of acid chlorides with organostannanes or by carbonylative couplings. The first reaction is one of the older and more general Stille coupling reactions131 , 132 and continues to be used under essentially the original conditions.133 Carbonylative couplings are less commonly used, despite the conciseness of the method.134 Recently, it has been found that addition of Cu(I) salts significantly improves the efficiency of this coupling.135 An interesting example has been reported by Couladouros’ group (Scheme 5.4.27).136 The stereoselective

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Me

OTBS Me

H

O H

OMe OSO2CF3 125

Pd(PPh3)4, LiCl 2-amino-5-chloropyridine THF, Δ

Me H

OTBS Me O

+ H

O O

O

O Bu3Sn

OAc

O

O

O OMe

(40-50%)

AcO

O

127

126 3 steps N

Me H

NMe

O Me O O OH

O H

OMe O

AcO

OH

Eleutherobin (128)

Scheme 5.4.25

Synthesis of eleutherobin by alkyl-alkenyl coupling

synthesis of the pair of natural macrolides, trans- and cis-resorcylide (135a,b), was performed using ring-closing metathesis on dienes 134a,b, prepared by the carbonylative Stille coupling of benzyl chlorides 133a,b. 5.4.8

Summary and Outlook

The Stille coupling reaction plays a prominent role in the synthesis of complex natural products. As a robust method, this coupling is often used to join large fragments in key carbon–carbon bond forming reactions, usually at late stages of the synthesis. In addition to the standard procedures developed by Stille,1 which are still applied in many couplings, the use of triphenylarsine as a ligand,14 and the addition of Cu(I) salts as cocatalysts19 , 21 have led to significant improvements in reaction rates and overall efficiency. The use of bulky phosphines, such as tri-tert-butylphosphine,15 , 16 as ligands for palladium is becoming more widespread. The use of sterically hindered ligands and the discovery of totally new types of catalysts137 are expected to have an important impact on future applications of the Stille reaction in organic synthesis.

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Stille Cross-Coupling for the Synthesis of Natural Products Bu3Sn

Me

OMe Cl

Me

OMe

Me

Br

(3 equiv.)

I

PdCl2(dppf)2 (0.1 equiv.) DMF, 125 °C, 24 h (73%)

OMe

601

Br

Cl

Me OMe

Me

129

Me

130 Bu3Sn

Me Me

PdCl2(dppf)2 (0.2 equiv.) DMF, 125 °C (67%)

(3 equiv.) OH

O

OMe

Me 5 steps

Me

O

Me

Cl

Me

Me

HO O

Me

Me

Me

OMe

Me

A80915G (132)

Scheme 5.4.26

Me

131

Consecutive palladium-catalyzed-allylations for the synthesis of (±)-A80915G

OH O

O

O

O

4 steps

Bu3Sn

BnO

Cl

OBn

O O OBn

trans-Resorcylide (135a)

134a

O Bu3Sn

HO

O

BnO Pd(PPh3)4, P(2-furyl)3 CO, HMPA, 80 °C, 2 h (60%)

133a

O

O

HO

Cl

OH 133b

Scheme 5.4.27

H

O

O

H RCM

O Pd(PPh3)4, P(2-furyl)3 CO, HMPA, 80 °C, 1.5 h (74%)

O

O

O O

O OH

OH 134b

cis-Resorcylide (135b)

Carbonylative couplings in the preparation of cis- and trans-resorcylide

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Tin Chemistry: Fundamentals, Frontiers and Applications and B. B. Snider, Org. Lett., 5, 4385 (2003); (d) M. S. Shanmugham and J. D. White, Chem. Commun., 44 (2004). (a) K. C. Nicolaou, D. Y.-K. Chen, Y. Li, W. Qian, T. Ling, S. Vyskocil, T. V. Koftis, M. Govindasamy, and N. Uesaka, Angew. Chem. Int. Ed., 42, 3649 (2003); (b) K. C. Nicolaou, T. V. Koftis, S. Vyskocil, G. Petrovic, W. Tang, M. M.O. Frederick, D. Y.-K. Chen, Y. Li, T. Ling, Y. Yamada, and M. A. Yoichi, J. Am. Chem. Soc., 128, 2859 (2006); (c) K. C. Nicolaou, M. O. Frederick, E. Z. Loizidou, G. Petrovic, K. P. Cole, T. V. Koftis, V. Theocharis, Y. Yamada, and M. A. Yoichi, Chem. Asian J., 1, 245 (2006). Selected examples: (a) H. Sugiyama, F. Yokokawa, and T. Shioiri, Tetrahedron, 59, 6579 (2003); (b) G. A. Holloway, H. M. Hügel, and M. A. Rizzacasa, J. Org. Chem., 68, 2200 (2003); (c) S. Yamaguchi, N. Tsuchida, M. Miyazawa, and Y. Hirai, J. Org. Chem., 70, 7505 (2005). (a) M. J. Schnermann and D. L. Boger, J. Am. Chem. Soc., 127, 15704 (2005); (b) M. J. Schnermann, F. A. Romero, I. Hwang, E. Nakamaru-Ogiso, T. Yagi, and D. L. Boger, J. Am. Chem. Soc., 128, 11799 (2006). M. S. Jensen, C. Yang, Y. Hsiao, N. Rivera, K. M. Wells, J. Y. L. Chung, N. Yasuda, D. L. Hughes, and P. J. Reider, Org. Lett., 2, 1081 (2000). E. Vedejs, A. R. Haigt, and W. O. Moss, J. Am. Chem. Soc., 114, 6556 (1992). X.-T. Chen, B. Zhou, S. K. Bhattacharya, C. E. Gutteridge, T. R. R. Pettus, and S. J. Danishefsky, Angew. Chem. Int. Ed., 37, 789 (1998). E. J. Corey, D. Y. Gin, and R. S. Kania, J. Am. Chem. Soc., 118, 9202 (1996). T. R. Kelly and R. L. Xie, J. Org. Chem., 63, 8045 (1998). M. D. Markey, Y. Fu, and T. R. Kelly, Org. Lett., 9, 3255 (2007) (a) A. Fürstner, G. Seidel, and N. Kindler, Tetrahedron, 55, 8215 (1999); Approach to salicylhalamides: (b) J. T. Feutrill, G. A. Holloway, F. Hilli, H. M. Hugel, and M. A. Rizzacasa, Tetrahedron Lett., 41, 8569 (2000). (a) T. Bach and L. Kruger, Eur. J. Org. Chem., 2045 (1999) (b) K. Krohn, P. Frese, and C. Freund, Tetrahedron, 56, 1193 (2000); (c) Y.-S. Jung, B.-Y. Joe, C.-M. Seong, and N.-S. Park, Bull. Korean Chem. Soc., 21, 463 (2000); (d) S. Takaoka, K. Nakade, and Y. Fukuyama, Tetrahedron Lett., 43, 6919 (2002). S. Takemura, A. Hirayama, J. Tokunaga, F. Kawamura, K. Inagaki, K. Hashimoto, and M. Nakata, Tetrahedron Lett., 40, 7501 (1999). Leading references: (a) M. Suzuki, H. Doi, K. Kato, M. Bjorkman, B. Langstrom, Y. Watanabe, and R. Noyori, Tetrahedron, 56, 8263 (2000); (b) F. Karimi, J, Barletta, and B. Langstrom, Eur. J. Org. Chem., 2374 (2005); (c) M. Yu, W. Tueckmantel, X. Wang, A. Zhu, A. P. Kozikowski, and A.-L. Brownell, Nucl. Med. Biol., 32, 631 (2005); (d) I. Bennacef, C. Perrio, M.-C. Lasne, and L. Barre, J. Org. Chem., 72, 2161 (2007); (e) T. Bourdier, G. Poisnel, M. Dhilly, J. Delamare, J. Henry, D. Debruyne, and L. Barre, Bioconjugate Chem., 18, 538 (2007). M. Kosugi, Y. Shimizu, and T. Migita, Chem. Lett., 1423 (1977); (b) M. Kosugi, Y. Shimizu, and T. Migita, J. Organomet. Chem., 129, C36 (1977). (a) D. Milstein and J. K. Stille, J. Am. Chem. Soc., 100, 3636 (1978); (b) D. Milstein, and J. K. Stille, J. Org. Chem., 44, 1613 (1979). See, inter alia: (a) B. K. Nabbs and A. D. Abell, Bioorg. Med. Chem. Lett., 9, 505 (1999); (b) T. V. Hansen and L. Skattebol, Tetrahedron Lett., 45, 2809 (2004); (c) T. Ichige, S. Kamimura, K. Mayumi, Y. Sakamoto, S. Terashita, E. Ohteki, N. Kanoh, and M. Nakata, Tetrahedron Lett., 46, 1263 (2005); (d) H. Takihiro, Y. Uruma, Y. Usuki, A. Miyake, and H. Iio, Tetrahedron: Asymm., 17, 2339 (2006); (e) J. Deska and U. Kazmaier, Angew. Chem. Int. Ed., 46, 4570 (2007). An interesting application for the synthesis of C(1→4)-linked disaccharides: P. Steunenberg, V. Jeanneret, Y.-H. Zhu, and P. Vogel, Tetrahedron: Asymm., 16, 337 (2005). R. D. Mazzola, S. Giese, C. L. Benson, and F. G. West, J. Org. Chem., 69, 220 (2004). E. A. Couladouros, A. P. Mihou, and E. A. Bouzas, Org. Lett., 6, 977 (2004). A. M. Echavarren, Angew. Chem. Int. Ed., 44, 3962 (2005).

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New Trends in the Synthesis of Solid-Supported Organotin Reagents

5.5

607

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 Chrétien,a,b Jeremy D. Kilburn,b Françoise Zammattio,a Erwan Le Grognec,a and Jean-Paul Quintard a a b

University de Nantes, CNRS “Laboratoire de Synthèse Organique-UMR 6513”, Nantes, France. School of Chemistry, University of Southampton, UK

5.5.1

Introduction

Organotin reagents are well recognised as efficient tools in modern organic synthesis due to their versatility as reagents allowing chemo-, regio- and stereoselective reactions.1 Because of their high tolerance for numerous functionalities, tin hydrides have proved to be invaluable reagents in such simple chemical reactions as the reduction of functional groups1,2 and in many other reactions which proceed via free radical mechanisms. Such has been the reliance on using organotin reagents in free radical chemistry that when efficient homolytic reactions are described without the use of tin reagents, the work is frequently published with phrases such as ‘tin-free radical reactions’ in the title!3 Similar efficiency is also observed when organotin reagents are used as coupling partners in palladiumcatalyzed cross-coupling reactions for formation of new carbon–carbon bonds. Such reactions give good control of the stereochemistry in resulting vinylic systems and a high tolerance for a large range of functional groups.4 For instance, cross-coupling of ‘umpolung’ reagents (according to the Seebach concept) with acyl halides has provided highly efficient syntheses of dicarbonyl compounds.5 Stille crosscoupling reactions have also been used in many cases as the key step in the total synthesis of complex molecules.6 Numerous other useful applications of organotin reagents have been described, for example: r The use of allyltins and γ-substituted allyltins in the stereocontrolled synthesis of homoallylic alcohols.7 r The use of organotin alkoxides and dialkoxides as powerful directing groups for O-substitutions in sugar chemistry.1,8 r The use of organotin azides as key reagents in the synthesis of tetrazoles of pharmaceutical interest.9 However, in spite of such impressive synthetic potential, organotin reagents often appear to be used as a last resort. They undoubtedly have a bad reputation because of their toxicity and the difficulties of removing tin residues from reaction products. The first point (toxicity) is often over-emphasized. In practice, while triethyl- and trimethyl-tin derivatives are highly toxic (LD50 < 15 mg kg−1 ), the more commonly used tri-n-butyltin derivatives with a LD50 generally in the range 100–300 mg kg−1 need not be considered as highly toxic compounds10 and can be handled by a competent organic chemist without any major risk. Nonetheless, the contamination of reaction products, and the environment generally, by tri-n-butyltin residues must be avoided because of their biological toxicity (exemplified, for instance, by their antifouling properties as algaecides and molluscicides).11,12 One approach to limit organotin pollution has been to use such reagents as a catalyst, or at least in sub-stoichiometric amounts. For example, the regeneration of an active organotin reagent is often possible in situ using hydrogenosilanes or by reduction with sodium borohydride,13 but even in these conditions it is impossible to completely avoid tin contamination, even with careful purification.

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5.5.2

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Removal of Tin Residues by Partition between Two Phases

Due to the difficulties encountered with the purification of products from reactions involving organotin reagents, numerous methods have been proposed in order to improve the purification step. Conversion of tri-n-butyltin halides to insoluble tri-n-butyltin fluoride at the end of the reaction has been proposed14 and in some cases, the presence of the fluoride anion in the reaction itself may simultaneously increase the reactivity of the organotin reagents.15 A recent improvement using liquid chromatography on a KF/silica gel mixture has also been shown to allow very clean and simple purification.16 Partition methods between non-miscible phases such as hexanes and acetonitrile have also been used to simplify the purification when polar products are obtained.17 In this case, tri-n-butyltin derivatives remain in the hexane phase, while products of the reaction are recovered in acetonitrile. Clearly, the greater the difference in the polarity of the organotin by-product (e.g. tetraalkyltin), and the synthesized product (e.g. aminoalcohols or aminoacids), the better the partitioning of the tin residues, which can reduce tin contamination levels to 2 carbon units), another route has been developed consisting of initial grafting of a terminal chloroalkyl spacer (on Amberlite XE 305). The addition of an appropriate stannyl anion followed by a halogenation reaction affords the polymer C (Scheme 5.5.4).40,49 H 1. n-BuLi/TMEDA 2. Br(CH2)nCl Amberlite XE-305

n = 3, 4, 6

Scheme 5.5.4

Cl n

MH

1. Bu2SnPhLi 2. I2

SnBu2I n

SnBu2H n

Polymer C

Incorporation of an organotin function through a 3C–6C spacer

Using this route, longer spacers can be used and the tin hydride can be obtained directly by reaction of the chloroalkyl resin with Bu2 SnHLi.50 However, using Bu2 PhSnLi provides better reproducibility, allowing a loading of organotin hydride of ∼1.2 mmol g−1 when the polymer C is reduced with NaBH4 ,40 a value which can be improved to 1.4 mmol.g−1 using LiAlH4 in appropriate experimental conditions.51 Starting from polymer C, simple supported allyltins,52 vinyltins,51 aryltins, or heteroaryltins53 can be obtained via substitution by Grignard reagents. When more functionalized reagents are required, reaction of polymer C with organozinc reagents can be very helpful for the synthesis of allyl or aryl reagents containing a cyano or an ester group52 and hydrostannylation of alkynes provides a route to functionalized vinyltins.51 Spacers of four carbon units are often long enough to allow NMR characterization, and reactivities of the supported reagents have a good analogy with those of tri-n-butyltin analogs. Longer spacers are sometimes required, for instance when self-association of the grafted functional organotin may occur. In this case, the longer spacer has a direct impact, both on the reactivity and on the characterization of the compounds by solid-state MAS NMR due to the higher mobility, as seen, for example, with tin oxides prepared from polymer D (Scheme 5.5.5).54−56 It should also be mentioned that polymers A–D have to be washed correctly in order to remove the soluble organotin species resulting from the synthesis. This aspect has been studied in detail for polymer C and when it was used for the reduction of organic halides, soluble organotin residues

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New Trends in the Synthesis of Solid-Supported Organotin Reagents H

1. n-BuLi/TMEDA Cl 2. Br(CH2)nCl

Amberlite XE-305

n

H2O

1. Ph2BuSnLi

SnBuCl2

2. HCl (g)

n

MeOH

1/2

611

SnBuCl-O n 2

n = 4, 6, 11 Polymer D

Scheme 5.5.5

Incorporation of a tin oxide function

were found to come from the synthesis of Bu2 SnPhLi.57 Accordingly, using these polymers in synthesis generally gives some organotin contamination in the products which decreases on repeated use of the resin and typically stabilizes after three cycles. (ii) Copolymerization of Styrene and DVB with an Appropriate Organotin Reagent. The first report of a copolymerization was between styrene/DVB and γ-(chlorodibutylstannyl) propyl methacrylate. The resulting polymer was used to prepare γ-butyrolactones via a free radical cyclization of appropriate halides, after the in situ generation of the hydride with NaBH4 .58 In this case, the functionalized polymer (P CO2 –(CH2 )3 SnBu2 Cl) had a loading of 0.5 mmol g−1 of tin chloride. Further examples were developed more than 10 years later using styrene, DVB, and styrene grafted with a β-stannylated ethyl chain (Scheme 5.5.6). SnX 3 +

DVB

+

Styrene

SnX3

when SnX3 = SnBu2 Cl : Polymer E

Scheme 5.5.6

Copolymerization with incorporation of a stannylated unit on a 2C spacer

Through this route, several reagents have been prepared with a tin incorporation that depends on the nature of the SnX3 moiety. A loading of 0.9 mmol g−1 was obtained for SnBr3 compared with 0.7 mmol g−1 for Sn(OMe)3 , 1 mmol g−1 for SnCl(OMe)2 and 1.5 mmol g−1 for SnBr(OMe)2 .59 When SnBu2 Cl was used (copolymer E), more detailed studies were published and depending on the experimental conditions and on the DVB rate, tin loadings of 0.7 to 1.4 mmol g−1 were obtained (values comparable with those obtained through the attachment of a side chain on a polymer such as Amberlite XE 305, see above). Cross-linking levels were in the range of 20– 45% when conventional aqueous suspension polymerizations were performed in the presence of 2-ethyl-hexanol or decane as porogens.60,61 The stannylated substituent was obtained by hydrostannylation of divinylbenzene with Bu2 SnHCl. Radical copolymerization of styrene and 10% DVB together with various triorganotin-4vinylbenzoates (using trimethyl-, tri-n-butyl-, and triphenyl-tin) leads to the corresponding polymer-supported tin carboxylates.62 In order to modify the properties of the supported reagents, (E)-1,4-bis-(4-vinylphenoxy)but2-ene has been used in place of DVB as a cross-linking reagent,60 giving a polymer having higher swelling abilities in organic solvents, but the tin loading appears to be lower (0.45–1 mmol g−1 ). Improved stability of the stannylated polymers is observed when allylic ether I63 or styryl ether II64 are used as a monomer (Scheme 5.5.7). The allylic ether monomer I has also been copolymerized with N -aryl maleimides, but in this case the tin loading is quite low (0.5 mmol g−1 ) and the polymer appears to be less stable, especially at higher temperature.63

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Tin Chemistry: Fundamentals, Frontiers and Applications O O

I (R = n-Bu or Ph)

Scheme 5.5.7 5.5.3.2

SnPh2Cl

SnR2Cl II

Stannylated precursors with O-containing spacers used in copolymerization

Gel-Type Resins

In this series, Merrifield resin was generally the starting material and modifications were carried out to obtain the desired supported organotin reagents. The most commonly used strategy consists of the substitution of the chloride by an O-alkenyl group followed by hydrostannylation (Scheme 5.5.8).40,65−69 OM n

Cl

R2SnHCl O

n = 1,2

Merrifield resin

M = Li, Na

Scheme 5.5.8

n

AIBN

O

n

SnR2Cl

R = Bu, Me

Stannylation of Merrifield type resins with 5- or 6- atom spacers

According to this route, organotin chlorides are easily obtained and steric effects around the tin atom can be modified (use of R = Me instead of R = Bu) when the reactivity appears to be sensitive to steric requirements.66−68 An alternative method used to modify the Merrifield resin involves transformation into a vinyl function which is subsequently hydrostannylated (Scheme 5.5.9).70

1. K2CO3, DMSO

Bu2SnCl2 Bu2SnH2

Cl 2. H2C

PPh3, THF

Merrifield resin

Scheme 5.5.9

SnBu2Cl

AIBN, hν toluene

Stannylation of the Merrifield type resin with a 2C spacer

These supported tin chlorides can be modified by substitution to give tin hydroxide (or bis stannyloxide),65 allyl-, aryl-, or vinyl-tins (by reaction with appropriate organometallic reagents69,70 ) or more often to give tin hydrides. The tin hydrides can be used in reduction or hydrostannylation reactions66 and can also be dehydrogenated to give supported ditin species using Pd(PPh3 )4 catalysis.68 Apart from these potentially reusable supported organotin reagents, it is worth noting that gel-type resins can also be used in a reverse mode where the organotin residues are released into the liquid phase and washed away before release of the newly formed organic compound. In such cases, gel-type carboxypolystyrene resins have been used, for example in the reaction of crotylstannanes with aldehydes (Scheme 5.5.10).71,72 This method can also be extended to allenyltin analogs.73

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New Trends in the Synthesis of Solid-Supported Organotin Reagents

613

OH O COOH

Bu3Sn

O

RCHO BF3·OEt2

Me O

EDCI, DMAP CH2Cl2

Me ∗ O

CH2Cl2, –78 °C Bu3Sn

Scheme 5.5.10

∗ R OH

Me

Temporary supported organotins on Gel-type resins

Similarly, there are several examples using resin bound aryl- or vinyl-stannanes in Stille couplings, which leave the coupled product on the solid support and the tin by-products are released into solution and washed away, as described in a recent review.74 5.5.3.3

Soluble Polymers

When a non-cross-linked polystyrene (obtained by copolymerization of a styrene monomer together with an organotin functionalized styrene monomer without divinylbenzene) is used as a matrix, it can be solubilized allowing the desired reaction to be carried out in a homogeneous phase, and the polymeric by-product is recovered from the reaction as a precipitate upon cooling and addition of methanol.75−77 In this series, reagents of the type ‘PS’ CH2 O(CH2 )3 SnBu2 R, (R = H or allyl) have been prepared and used in free radical reductions and allylations. In both cases, the recovery of the products is much easier than with soluble tri-n-butyltin analogs and low tin contamination was observed (95% recovery of reagent at each stage.

Scheme 5.8.9

Catalytic reduction with a fluorous tin hydride

Some degradation of the fluorous tin hydride does appear to occur with multiple recycling. The first four reductions of bromoadamantane were complete in 4 hours while the last two required 5 hours. Nevertheless, the ability to use 1–10% tin catalyst with almost complete recovery substantially improves the ‘green credentials’ of tin hydride reductions. Most importantly, tin contamination of the product of fluorous tin hydride-mediated reactions cannot be detected to an estimated detection limit of 1%. Fluorous reagents, unlike more polar catalysts and reagents, are highly soluble in supercritical CO2 (scCO2 ) and the same reduction of bromoadamantane has also been performed in this more environmentally friendly solvent.34 Workup, however, involved partitioning between perfluorohexanes and benzene to separate reagent and product and the authors admit that there is little advantage to the use of an environmentally friendly solvent if isolation of the product involves extraction with volatile organic solvents

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Green Organotin Chemistry

Figure 5.8.2

659

Fluorous silica

such as benzene. It may be possible, however, to exploit the large solubility difference between fluorous and organic compounds in scCO2 to develop both a reaction and a separation protocol in this solvent. The reactions involving fluorous tin reagents discussed above also require fluorous reaction solvents and/or extraction solvents. These solvents are expensive and hazardous to human health and to the environment.34 Preliminary investigations35 towards an easily contained alternative have involved a fluorous monolayer on silica (Figure 5.8.2). A fluorous surfactant covalently tethered to silica provides a thin film of perfluorinated solvent for reactions and/or extractions. This material was used for the small-scale hydrocyclization of 6-bromo1-hexene with NaBH4 and a catalytic amount of a fluorous tin bromide in 1-butanol. The yield of methylcyclopentane was modest, however, and this technology is a long way from being viable on an industrial scale. The same idea has been more successfully employed with fluorous silica-supported tin Lewis acid catalysts for Baeyer–Villiger oxidations.36 5.8.5

Solid Phase Tin Reagents

Polymer supported tin hydrides37 ensure complete removal of tin by-products, but their synthesis is not trivial. They have been used for reductions of halides and for ring closures, and the resulting polymerbound tin halide can be easily filtered from the reaction mixture, regenerated, and reused. Even more successful has been the use of solid-supported tin reagents for Stille coupling.38 The development of these reagents is based on a catalytic variation of the stoichiometric Stille coupling39 involving in situ reduction of the triakyltin halide with polymethylhydrosiloxane and concomitant hydrostannylation of an alkyne to generate the vinylstannane, which is coupled to the organic halide with regeneration of the trialkyltin chloride (Scheme 5.8.10). The more toxic trimethyltin chloride performs significantly better than tri-n-butyltin chloride, but the catalytic cycle requires as little as 5 mol%.

Scheme 5.8.10

Catalytic Stille coupling

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A modified protocol has been developed40 involving less hazardous trimethyltin fluoride (Scheme 5.8.11).

Scheme 5.8.11

Catalytic Stille coupling with Me3 Sn F generated in situ

The trimethyltin fluoride is generated in situ or can be added initially. The advantage of this reagent is that it is less volatile than trimethyltin chloride, not easily absorbed through the skin and can be filtered off at the end of the reaction, rather than distributed in both the organic and aqueous waste streams. The resin-bound trialkyltin halide developed for this reaction38 required higher catalytic loadings (30–100%) to obtain reasonable yields of coupled product, (Scheme 5.8.12), presumably reflecting the availability of the catalytic site in the biphasic mixture. Tin contamination of the column purified product, however, was 500 ppm obtained for the free trimethyltin chloride-catalyzed reaction.

Scheme 5.8.12

Catalytic Stille with resin-bound reagent

Distannanes can be homolytically cleaved under mild conditions to yield stannyl radicals41 and have been widely used for atom-transfer cyclisations.20 Comparable yields have been obtained with a resin bound hexaorganoditin (Scheme 5.8.13), which can be easily removed by filtration with very low tin contamination (99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99b >99b >99 >99

100 100 100 100 98 100 99 100 100 99 99 100

Reaction conditions: RCOOR’ (1.0 mmol); R”OH (1.0 mmol); 1 or 2 (0.05 mmol); FC-72 (5 ml); 150 ◦ C, 16 h; washing with toluene (1 ml × 2). b At 160 ◦ C

a

(Table 6.1.2). Although the 2 mol% catalyst loading is enough for the completion of the reaction as described above, it is actually not necessary to decrease the amount of the catalyst to such a level because of its facile recovery and recycling (see below). Thus, 5 mol% catalyst was used; this is easier for exact weighing of the catalyst than 2 mol % loading. The reaction proceeded perfectly even using the reactants in a 1:1 ratio. No starting materials were detected by GLC analysis after 16 hours of reaction. Aliphatic, α, β-unsaturated, and aromatic derivatives could be used as ester components. A variety of alcohols such as primary, secondary, allylic, and propargylic alcohols could be used. Geraniol suffered neither isomerization nor the cyclization that easily occurs under acidic conditions. Other acid-sensitive functions like propargylic, tetrahydropyranyl (THP), and tert-butyldimethylsilyl (TBS) groups survived. Apparently, the neutral reaction conditions favor the tolerance of these functional groups. The same technology could be applied to esterification as well. As shown in Scheme 6.1.2, esterification of an equimolar mixture of RCOOH and R’OH was conducted in the presence of 1 (5 mol%) in FC-72.25g Perfect conversion was realized. With less sterically demanding reactants, no sign of the reactant

Scheme 6.1.2

Esterification of an equimolar mixture of RCOOH and R’OH in the presence of 1 in FC-72

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Green Organotin Catalysts

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alcohol was detected in the reaction mixture, and only a single peak assignable to the ester was observed. Consistently, the isolated yields were also quantitative. The further practical usefulness of this protocol is apparent from the tolerance of various functional groups. Evaporation of the FC-72 layer led to complete recovery of the catalyst, which could be reused for a subsequent reaction. However, the separated catalyst solution could be more conveniently used directly for the next reaction. Washing the FC-72 solution twice with toluene was sufficient to remove the product remaining on the surface of FC-72 layer. The separated FC-72 solution could be forwarded to other reactions. Thus, many of the reactions shown in Table 6.1.2 were conducted with the same catalyst solution. On the other hand, washing would not be necessary if the same reaction is repeated. For instance, the reaction between Ph(CH2 )2 COOEt and PhCH2 OH was repeated 10 times with a single catalyst solution. The yield of the first run was found to be 95% without washing. Furthermore, a >99% yield was constantly obtained, on the basis of GLC analysis, during the second and tenth runs, indicating that nearly the same amount of the product remained on the FC-72 surface in each run. After the tenth run, 97 % of the catalyst was recovered. Binary Solvent System

If the use of an organic solvent is preferable, an FC-72/organic solvent binary system can be used.21,25h The reaction was conducted in a 1:1 mixture of FC-72 and toluene. However, a mixture of equimolar reactants failed to give complete conversion in this protocol. The use of a slight excess of alcohol (1.2–1.3 equiv.) was required for satisfactory yields (>99%). The catalyst was recovered without loss (>99%) from the FC-72 layer. Notably, however, a control experiment without the catalyst afforded only a 23% yield. Complete conversion was achieved when the substrate alcohol was used as cosolvent. When the reaction was performed in a mixture of FC-72 (4 ml) and alcohol (2 ml), the desired esters (2 mmol) were obtained in 100% yields. The operation was quite simple, as evaporation of the organic layer to remove the lowboiling point alcohol left pure esters. Notably, the reaction between Ph(CH2 )2 COOEt and CH3 OH was repeated 20 times. The GLC yield was constantly over 99% each time, and 91% of the catalyst was recovered after the twentieth run, indicative of virtually no loss and no deactivation of the catalyst during repeated operations. Single Organic Solvent System

The fluorous biphase technology has also been invoked to recover catalysts in conventional transesterification.21,25h The reaction was conducted with fluoroalkyldistannoxane catalysts (5 mol%) in refluxing toluene. However, the use of equimolar amounts of Ph(CH2 )2 COOMe and PhCH2 OH failed to achieve perfect conversion (91% yield), but when the alcohol component was employed in excess (1.2 equiv.) quantitative yields were obtained. Such a smooth reaction was rather unexpected since the catalysts are insoluble in toluene at room temperature, yet the reaction mixture looked to be homogeneous at reflux temperature. This finds strong support from the fact that catalysts which were powdery before use turned crystalline in toluene after the reaction mixture cooled down. The catalysts were as active as in the previous protocols and, thus, there was no difference in activity between FC-72 and toluene solvents. Quantitative yields of the desired esters were obtained in various combinations of reactant esters and alcohols. The same chemoselectivity as found formerly held in the present case as well, and of more synthetic significance was the successful use of ethyl acetoacetate. This substrate was not employable in the reaction using FC-72 because the reaction at 150 ◦ C induced thermal decomposition of the ester. By contrast, no decomposition occurred in refluxing toluene. After 16 h, the reaction mixture was washed with FC-72. Evaporation of the FC-72 solution recycled the catalyst, with recovery yields of 100% for

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1 and 99% for 3. This difference was increased upon recycled use of the catalysts. Apparently, catalyst with higher fluorine content was more efficiently recovered. 6.1.3

Grafted Organotin Catalysts

The synthesis of organotin compounds anchored to a solid support can be tackled by different approaches. The most widely used is the one in which a commercial cross-linked polystyrene is employed as a starting material. Compounds of the type [P-H](1−t) [P-(CH2 )n -SnBuX2 ]t ,24,32−35 [P-H](1−t) {[P-(CH2 )n -SnBuY]2 O}t/2 (n = 4, 6, or 11)30,31 and [P-H](1−t) [P-(CH2 )n -SnZ3 ]t , (n = 4 or 11)34,36 in which [P–H] represents the monomer unit of the cross-linked polystyrene matrix, X = Ph, Cl, OH, OOCCH3 , Y = Cl, OH, OOCCH3 , Z = Ph, Cl and t is the molar fraction of organotin functionalized styrene monomers, were synthesized using this approach and will be discussed extensively in what follows. An alternative approach involves the polymerization of a suitable tin-containing monomer,24 and was recently employed by Angiolini and coworkers,37 with the preparation of grafted organotin carboxylates by copolymerization of triorganotin derivatives of p-vinylbenzoic acid ( p-VBA) with styrene and 1,4divinylbenzene. Synthesis and Characterization

(1) [P–H](1−t) [P-(CH2 )n -SnBuX2 ]14,29−32 and [P–H](1−t) {[P-(CH2 )n -SnBuY]2 O}[30,31] (n = 4, 6 or 11). t t/2 The synthesis of insoluble polymer-supported organotin reagents, mainly hydrides, was reported several years ago. A comprehensive review on the state-of-the-art up to 1998 is given by Delmond and Dumartin.24 It was demonstrated that such compounds exhibit a reactivity comparing favourably with reagents in homogeneous solutions, and that they were significantly less polluting. The main drawback at the time the review was written was, being insoluble, the compounds were difficult to characterize completely, thus hampering a good understanding of their reactivity at the solid–liquid interface. In 1996, solid-state tin NMR was first introduced for qualitative and quantitative analysis of the different tin species present in the polymers.38 Since that time a major breakthrough in the characterization of grafted organotins was achieved both by developing a statistical method, enabling the determination of the functionalization degree t from elemental analysis data,29 and by the application of high resolution magic angle spinning (hr-MAS) NMR, a technique allowing the characterization of rotationally mobile molecular moieties grafted onto a swollen solid support, directly in situ, at the heterogeneous solid–liquid interface.32,40 The general reaction sequence for the preparation of the title compounds is given in Scheme 6.1.3.30,31

Scheme 6.1.3 The general reaction sequence for the preparation of [P-H]( 1−t) [P-(CH2 )n -SnBuX2 ] and [P-H]( 1−t) {[P-(CH2 )n -SnBuY]2 O}t/2 ( n = 4, 6 or 11)

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Figure 6.1.2 Diffusion filtered 1 H hr-MAS spectra, from top to bottom, of [P-H]( 1−t) [P-(CH2 )11 Cl]t , [P-H]( 1−t) [P-(CH2 )11 Sn-n-BuPh2 ]t and [P-H]( 1−t) [P-(CH2 )11 Sn-n-BuCl2 ]t . (Reproduced from Chemistry – A European Journal 2005, 11, 4552–4561. Copyright (2005) with permission from Wiley-VCH.)

The first step determines the functionality degree and can be tuned according to the needs of the target (trans)esterification reaction. The succession of the reaction conversions, as well as the assessment of the completeness of the reactions can be conveniently monitored by 1 H hr-MAS NMR,14 as illustrated in Figure 6.1.2, which shows the disappearance of the -CH2 Cl resonance, the introduction of the -SnPh2 group, and the substitution of the Ph groups by Cl atoms. Whereas one-dimensional 119 Sn hr-MAS spectroscopy was successful as a means of detecting and identifying possible grafted impurities or incomplete conversions, at the level of the tin functionality (Figure 6.1.3b), this technique could not be applied to the grafted distannoxane (Figure 6.1.3d), as the additional interface cross-linking arising from the Sn O Sn distannoxane bridging induced a high conformational rigidity, hampering local rotational mobility of the spacer-organotin moiety, the fundamental necessary condition for hr-MAS NMR to be feasible. Figure 6.1.3 shows the solid-state MAS 117 Sn and hr-MAS 119 Sn spectra of both the grafted chlorodistannoxane derivative and its corresponding dialkyldichlorotin precursor. The solid-state spectrum of the grafted dialkyltin dichloride (Figure 6.1.3a) displays a single, albeit broad, 117 Sn resonance at its isotropic chemical shift, identical within experimental error to its chemical shift in the 119 Sn hr-MAS spectrum (Figure 6.1.3b), indicating that the tin atom has the same

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Figure 6.1.3 117 Sn MAS spectrum (a, c) and 119 Sn hr-MAS NMR spectrum (b, d) of [P-H]( 1−t) [P(CH2 )11 SnBuCl2 ]t and [P-H]( 1−t) {[P-(CH2 )11 SnBuCl]2 O}t/2 respectively. (Reproduced with permission from Appl. Organomet. Chen. 2007, 21, 504–513. Copyright (2007) John Wiley & Sons Limited)

four-coordination sphere in the dry solid state and at the solid–liquid interface. This is in contrast to the 117 Sn solid-state MAS spectrum of the grafted distannoxane (Figure 6.1.3c), which displays a very broad anisotropy pattern, with two isotropic chemical shifts at −94 and −151 ppm, very similar to those of the molecular analog in solution (at −92 and −139 ppm), providing evidence for the existence of the characteristic pairs of endo and exo tin atoms in the Sn2 O2 ring core of the dimeric ladder structure (Figure 6.1.4), which therefore appears to be common to both the non-grafted molecular and the C11-grafted distannoxanes. The existence of such an Sn2 O2 ring core has not been formally identified, so far, in the grafted C4- and C6- analogs.39 (2) Synthesis of [P-H](1−t) [P-(CH2 )n -SnZ3 ]t , (n = 4 or 11).31,33 The grafted tin trichloride with a C4spacer was synthesized by reaction of the corresponding grafted tricyclohexyltin moiety with SnCl4 in dry

Figure 6.1.4

Dimeric ladder structure of tetraorganodistannoxanes

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toluene,31 according to the Kocheshkov reaction.40 An alternative synthetic route towards a functionally pure, as assessed by hr-MAS NMR, organotin trichloride grafted onto cross-linked polystyrene with the C11-spacer, [P-H](1−t) [P-(CH2 )11 -SnCl3 ]t , was elaborated by extending the method used earlier for the SnBuPh2 analog (Scheme 6.1.4)41 The general reaction scheme for the synthesis of the polystyrenegrafted alkyltin trichloride compound, given in Scheme 6.1.4, avoids a priori potential tin contamination of the final catalysts that might result from the work-up procedures of the heterogeneous reaction mixture having contained SnCl4 .

Scheme 6.1.4 compound

The general reaction scheme for the synthesis of the polystyrene-grafted alkyltin trichloride

The success of reaction conversions, as well as the assessment of their completeness could again be monitored successfully by 1 H and 119 Sn hr-MAS NMR. Catalytic Activity

(1) Model reaction. The catalytic activity of all mentioned compounds has been investigated in the transesterification of ethyl acetate with n-octanol [Equation (6.1.1)],29−33 and in some cases with other alcohols as well. CH3 COOEt + OctOH −→ CH3 COOct + EtOH

(6.1.1)

In general, not unexpectedly, better results were obtained with the longer spacer and with the SnCl3 functionality, having the highest Lewis acidity. While in early studies,29−32 the thermodynamically reversible equilibrium reaction was driven to completion by using a seven-fold excess of starting ethyl acetate, while distilling off the ethanol generated, more recent studies on [P-H](1−t) [P-(CH2 )11 -SnCl3 ]t were performed with a stoichiometric 1:1 mixture of ester and alcohol in order to keep optimal time variations of concentration under control, and ensure in this way reproducible determination of conversion degrees.33 The resulting half-life times towards thermodynamic equilibrium, obtained for 10 successive catalytic runs of recycled catalyst, averaged at 0.47 ± 0.04 h, indicating both a high catalytic activity and a good recyclability of the catalyst under the transesterification conditions used, as well as consistent activity from run to run. Investigation of the catalyst after the consecutive runs, revealed a lowering in signal-to-noise ratio and an increase in glass transition temperature, Tg , of the material. The overall Tg increasing trend upon increasing number of runs (Figure 6.1.5, short-dashed line) fairly parallels the overall trend of decreasing signal-to-noise ratio of the 119 Sn hr-MAS resonance (Figure 6.1.5, long-dashed line). These trends suggest that cross-linking continues upon successive transesterification runs, increasing the conformational rigidity of the C11-spacer at the interface and, consequently, resulting in the disappearance of its 119 Sn hr-MAS signal and an overall decrease in the signal-to-noise ratio for the 119 Sn resonance of the unaltered SnCl3 catalyst fraction.

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Figure 6.1.5 Glass transition temperature Tg () and signal-to-noise ratio S/N () of the 119 Sn hr-MAS NMR spectra of the C11-SnCl3 catalyst, as a function of the number of successive transesterification runs; subsequent thorough Soxhlet extraction of the sample after the tenth run (; , on the right vertical axis). Short-dashed and long-dashed lines show the increasing Tg and decreasing S/N trends upon increasing number of catalytic runs. (Reprinted with permission from Organometallics 2007, 26, 6718–6725. Copyright (2007) American Chemical Society)

The chemical modification of the organotin functionality did not influence the catalytic activity. Indeed, conversion half-life times remained almost unchanged in up to nine subsequent runs (ca. half an hour on average), demonstrating the excellent recyclability of the grafted organotin trichloride catalyst. When comparing the performance of the supported tin trichloride to that of the soluble molecular butyltin trichloride, the grafted system showed slightly higher catalytic activity under identical conditions. Also, after filtering off the catalyst, further refluxing of the reaction mixture did not lead to any additional reagent conversion, indicating unambiguously the grafted catalyst to be the catalytically active species and excluding any activity from organotins having leached, if any, into the reaction mixture. (i) Tin leaching in the model transesterification [P-H ](1−t) [P-(CH2 )11 -SnCl3 ]t . Under conditions in which ethyl acetate is in four-fold excess, the reaction attains equilibrium in ca. two hours, with a conversion degree averaging at ca.75%. For eight out of ten catalytic runs, the residual tin content averages at ca. 5 ppm, reflecting a satisfactorily low leaching degree. However, another set of experiments shows, however, that the degree of tin leaching is extremely sensitive to the contact time between the catalyst and the refluxing reaction mixture.36 (ii) Mechanistic aspects of the grafted catalysis in the model transesterification.36 The high sensitivity of the 119 Sn chemical shift to interactions between the tin atom and a nucleophilic substrate enables one to monitor the catalytic mechanism, in particular the coordination of the organotin catalyst in situ at the solid–liquid interface (Figure 6.1.6), using 119 Sn hr-MAS NMR, under sample conditions partly or completely mimicking the transesterification. The low-frequency 119 Sn shift observed in all cases indicates a coordination expansion of the tin atom of the grafted C11-SnCl3 catalyst from a four-coordinate state in its pure, unaltered form, to five- and/or six-coordination in the presence of ester and/or alcohol, from which it can be deduced that both the ester and alcohol coordinate the tin atom, which brings them in close mutual vicinity, favoring the nucleophilic attack of the oxygen of the alcohol onto the carbonyl carbon atom of the ester needed for the transesterification.

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f e d c b a 200

100

0

–100

–200

ppm

Figure 6.1.6 119 Sn hr-MAS spectra in CDCl3 of pure, unused grafted C11-SnCl3 catalyst without any added component (a); unused catalyst in the presence of solely ethyl acetate (b); same, in the presence of solely n-octanol (c); same, in the presence of a 1:1 mixture of ethyl acetate and n-octanol (d); same, in the presence of a 1:1:1:1 mixture of ethyl acetate, ethanol, n-octyl acetate and n-octanol mimicking the equilibrium mixture (e); used catalyst, in contact with its own equilibrium mixture after five catalytic runs (f). In order to improve signal-to-noise ratio, the quantity of tin was higher in all samples (a) to (f) than in the actual reaction mixture. (Reprinted with permission from Organometallics 2007, 26, 6718–6725. Copyright (2007) American Chemical Society)

(2) Ring-opening polymerization. The ring-opening polymerization (ROP) of ε-caprolactone to polyε-caprolactone [Equation (6.1.2)] was investigated using [P-H](1−t) [P-(CH2 )n -SnBuCl2 ]t and [PH](1−t) {[P-(CH2 )n -SnBuCl]2 O}t/2 (n = 6 or 11),14,42 as well as [P-H](1−t) [P-(CH2 )11 -SnCl3 ]t as catalysts.42

(6.1.2)

It was reported that the long C11 spacer is important to keep the polydispersity index under control.[14] As for the case of the model transesterification, the grafted tin trichloride proved to be the most efficient catalyst. Quantitative conversion was complete after less than two hours under reaction conditions in which conversion was incomplete for [P-H](1−t) [P-(CH2 )11 -SnBuCl2 ]t and [P-H](1−t) {[P-(CH2 )11 SnBuCl]2 O}t/2 . Even after 15 minutes of reaction, very high conversions are achieved.42 In spite of chemical changes at the organotin catalyst after the first catalytic run, it could be recycled in nine subsequent consecutive ROP runs without any major loss of catalytic activity (Table 6.1.3). The decreasing trend for the conversions after the sixth run was not observed when the reaction time was fixed at a longer duration of two hours. This was explained by a gradual increase of polymer remnants inside the pores of the beads after each run, hampering an optimal diffusive access of fresh monomer and initiator towards the tin atom, confirmed by the fact that full conversion was again reached after the catalyst was submitted to a thorough Soxhlet extraction. The yields of isolated polymer after three precipitationdissolution cycles followed the same trend as the conversion degrees, but were systematically lower, due to mass losses into the filtrate during the polymer purification process.

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Table 6.1.3 Conversion and yield of synthesized PCL in subsequent runs of ROP of ε-CL initiated by n-PrOH and catalyzed by the grafted C11-SnCl3 catalyst in toluene at 100 ◦ C for a reaction time of 15 minutes ([ε-CL ]0 = 6.28 mol/l, [ε-CL ]0 / [n-PrOH]0 = 10, [n-PrOH]0 / [Sn] = 20) Run

1

2

3

4

5

6

7

8

9

10

Conversion [%] Yield [%]

98 79

97 75

95 74

96 74

95 75

95 75

89 69

82 62

81 61

74 57

(i) Tin leaching in the ring-opening polymerization. For each of the ten consecutive runs mentioned in the table, an average tin content of ca. 15 ppm was detected in the reaction mixture, corresponding to an average loss of ca 0.3 % with respect to the amount of tin used for the run.42 Though somewhat higher than for the model transesterifications where they amounted to ca. 5 ppm, these undesired tin amounts in the reaction products correspond to 0.2 % of the amount under standard homogeneous catalysis condition, the benefits from grafting the catalyst being therefore undisputable. Furthermore, this limited tin leaching appears to be not harmful to the catalytic activity, since the grafted tin trichloride catalyst did not show any decrease in catalytic activity after two hours of reaction, being then 100%, and did not provide any change in control over the polymerization rate and the polymer polydispersity data, which did not vary significantly with the number of catalytic runs. As for the transesterification reaction, it was again observed that the degree of tin leaching was sensitive to the contact time between the catalyst and the reaction mixture and therefore reaction conditions needed to be carefully chosen. 6.1.4

Conclusion

It goes without saying that green chemical processes will play a pivotal role in chemical industry in the 21st century. The technology for highly efficient recovery of catalysts is one of the key issues in this context. Organotin catalysts or promoters are employed for many synthetic reactions and even for industrial processes, but some of them are toxic, especially when they are volatile and/or trialkyltin derivatives. It should be noted, however, that not all of organotin compounds are harmful, and, in fact, dialkyltin derivatives have been used for a long time as stabilizers for poly(vinyl chloride) and as catalysts for curing silicones without any problems. Nevertheless, it is highly desirable to establish an efficient technology to recover organotin catalysts from their reaction mixtures. In this chapter, two strategies to immobilize the organotin catalysts have been described: immobilization by liquid-phase on one hand and by solid-phase on the other. The former has the advantage of performing reactions under homogeneous conditions, while the latter can suppress the catalyst leaching to the minimum level. Needless to say, there are other methods to recover catalysts, and the need for the catalyst immobilization will be constantly increasing. References 1. 2. 3. 4.

J. Otera (Ed.), Esterification, Wiley-VCH: Weinheim, 2003. M. Tüter, H. A. Aksoy, E. E. Gilbaz, E. Kursun, Eur. J. Lipid Sci. Technol. 2004, 106, 513. J. Aburto, I. Alric, E. Borredon, Starch/Stärke 2005, 57, 145. G. Deshayes, F. A. G. Mercier, P. Degée, I. Verbruggen, M. Biesemans, R. Willem, P. Dubois, Chem. Eur. J. 2003, 9, 4346.

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5. D. Payne, G. Ross, H. Zhang, A. Morton, C. Valentine, Patent number US 2005260409; Application number US 2004-986167 20041112; Chem. Abstr. 2005, 143, 479293. 6. M. S. Huda, L. T. Drzal, M. Misra, A. K. Mohanty, J. Appl. Polym. Sci. 2006, 102, 4856. 7. L. E. Trapasso, P. L. Meisel, L. B. Meisel, W. K. Chwang, Patent number US 5606103; Application number US 1995-580181 19951228; Chem. Abstr. 1995, 126, 238802. 8. S. Shyamroy, B. Garnaik, S. Sivaram, J. Polym. Sci. 2005, 43, 2164. 9. C. Flosbach, K.-F. Doessel, W. Lenhard, O. Reis, T. Fey, Patent number US 2003026895; Application number US 2001-921815 20010803; Chem. Abstr. 2001, 138, 123954. 10. L. Katz, G. W. Ashley, Chem. Rev. 2005, 105, 499. 11. S. Li, M. Vert, Biodegradation of aliphatic polyesters (Ed. : G. Scott), Kluwer, Dordrecht, 2002, pp. 71 and 132. 12. A.-C. Albertsson, I. K. Varma, Biomacromolecules 2003, 4, 1466. 13. A. Sodergard, M. Stolt, Prog. Polym. Sci. 2002, 27, 1123. 14. G. Deshayes, K. Poelmans, I. Verbruggen, C. Camacho-Camacho, P. Degée, V. Pinoie, J. C. Martins, M. Piotto, M. Biesemans, R. Willem, P. Dubois, Chem. Eur. J. 2005, 11, 4552. 15. P. J. Smith (Ed.), Chemistry of Tin, Chapman & Hall, St Edmundsbury Press: Suffolk, 1998. 16. A. G. Davies, Organotin Chemistry, Wiley-VCH: Weinheim, 2004. 17. O. A. Mascaretti, R. L. E. Furlán, Aldrichimica Acta 1997, 30, 55. 18. C. J. Evans, S. Karpel Organotin Compounds in Modern Technology, Elsevier, Amsterdam, 1985. 19. I. Omae, Applications of Organometallic Compounds, Wiley, Chichester, 1998. 20. A. Orita, A. Mitsutome, J. Otera, J. Org. Chem. 1998, 63, 2420. 21. J. Xiang, A. Orita, J. Otera, Adv. Synth. Catal. 2002, 344, 84. 22. J. Otera, Chem. Rev. 1993, 93, 1449. 23. W. P. Neumann, M. Peterseim, React. Polym. 1993, 20, 189. 24. B. Delmond, G. Dumartin, in Solid-State Organometallic Chemistry: Methods and Applications (Eds.: M. Gielen, R. Willem, B. Wrackmeyer), Wiley, Chichester, 1999, pp. 445–471, and references cited therein. 25. a) D. L. An, Z. Peng, A. Orita, A. Kurita, S. Man-e, K. Ohkubo, X. Li, S. Fukuzumi, J. Otera, Chem. Eur. J. 2006, 12, 1642–1647; b) A. Orita, S. Man-e, J. Otera, J. Am. Chem. Soc. 2006, 128, 4182; c) X. Li, A. Kurita, S. Man-e, A. Orita, J. Otera, Organometallics 2005, 24, 2567; d) J. Xiang, A. Orita, J. Otera, J. Organomet. Chem. 2002, 648, 246; e) J. Otera, Acc. Chem. Res. 2004, 37, 288; f) Y. Imakura, S. Nishiguchi, A. Orita, J. Otera, Appl. Organometal. Chem. 2003, 17, 795; g) J. Xiang, A. Orita, J. Otera, Angew. Chem. Int. Ed. 2002, 41, 4117; h) J. Xiang, S. Toyoshima, A. Orita, J. Otera, Angew. Chem. Int. Ed. 2001, 40, 3670. 26. A. G. Hernán, V. Guillot, A. Kuvshinov, J. D. Kilburn, Tetrahedron Lett. 2003, 44, 8601. 27. G. Dumartin, M. Pourcel, B. Delmond, O. Donard, M. Pereyre, Tetrahedron Lett. 1998, 39, 4663. 28. L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, M. Biesemans, Appl. Organomet. Chem. 2005, 19, 841. 29. J. A. Gladysz, D. P. Curran, I. T. Horváth (Ed.) Handbook of Fluorous Chemistry, Wiley-VCH: Weinheim, 2004. 30. J. Otera, In Advances in Detailed Reaction Mechanisms; J. M. Coxon (Ed.), JAI Press Inc.: Greenwich, Conn, 1994; Vol. 3, pp. 167–197. 31. J. Otera, N. Dan-oh, H. Nozaki, J. Org. Chem. 1991, 56, 5307. 32. F. A. G. Mercier, M. Biesemans, R. Altmann, R. Pintelon, J. Schoukens, B. Delmond, G. Dumartin, R. Willem, Organometallics 2001, 20, 958. 33. M. Biesemans, F. A. G. Mercier, M. Van Poeck, J. C. Martins, G. Dumartin, R. Willem, Eur. J. Inorg. Chem. 2004, 2908. 34. C. Camacho-Camacho, M. Biesemans, M. Van Poeck, F. A. G. Mercier, R. Willem, K. Darriet-Jambert, B. Jousseaume, T. Toupance, U. Schneider, U. Gerigk, Chem. Eur. J. 2005, 11, 2455. 35. J. C. Martins, F. A. G. Mercier, A. Vandervelden, M. Biesemans, J.-M. Wieruszeski, E. Humpfer, R. Willem, G. Lippens, Chem. Eur. J. 2002, 8, 3431. 36. V. Pinoie, K. Poelmans, H. E. Miltner, I. Verbruggen, M. Biesemans, G. Van Assche, B. Van Mele, J. C. Martins, R. Willem, Organometallics 2007, 26, 6718.

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37. L. Angiolini, D. Caretti, L. Mazzocchetti, E. Salatelli, R. Willem, M. Biesemans, J. Organomet. Chem. 2006, 691, 3043. 38. G. Dumartin, J. Kharboutli, B. Delmond, M. Pereyre, M. Biesemans, M. Gielen, R. Willem, Organometallics 1996, 15, 19. 39. K. Poelmans, V. Pinoie, I. Verbruggen, M. Biesemans, G. Van Assche, G. Deshayes, P. Degée, P. Dubois, R. Willem, Appl. Organomet. Chem. 2007, 21, 504. 40. K. A. Kocheshkov, Ber. Deutsch. Chem. Gesell. 1926, 62, 996. 41. G. Ruel, N. K. The, G. Dumartin, B. Delmond, M. Pereyre, J. Organomet. Chem. 1993, 444, C18. 42. K. Poelmans, V. Pinoie, I. Verbruggen, M. Biesemans, G. Deshayes, P. Degée, P. Dubois, R. Willem, Organometallics 2008, 27, 1841.

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6.2

681


Werner J. Blanka and Edward T. Hessellb a b

Independent Consultant, Wilton, CT, USA King Industries Inc., Norwalk, CT, USA

6.2.1

Introduction

Polyurethanes formed by the reaction of polyisocyanates with hydroxyl compounds are a major group of polymers1 used in coatings, adhesives/sealants/binders, elastomers, encapsulants, elastomeric fibers, films, gels, composites, microcellar elastomers, rubbers/millable gums, and foams. Depending on their structure, these polyurethane polymers can give alternatively soft and flexible, tough, hard, and durable exterior films, foams, or coatings. Organotin catalysts are the most commonly used organometallic compounds for catalyzing the reaction of the isocyanate group with a hydroxyl group. Changes in the structure of the catalyst permit the delay in onset of reaction, change in potlife, and also improve hydrolytic stability of the catalyst. Organotin compounds are also excellent catalysts for the reaction of blocked isocyanates with hydroxyl compounds. The versatility of organotin compounds in catalyzing a large variety of isocyanate reactions makes these catalysts very easy to use, but can also create problems because less desirable side reactions are also promoted. For environmental and health reasons there is an interest in replacing organotin catalysts in certain applications with more benign catalysts. 6.2.2

Mechanism of Urethane Catalysis by Tin Compounds

The mechanism by which tin compounds catalyze urethane formation from an isocyanate and a hydroxyl compound has been exhaustively studied over 30 years, and a number of excellent reviews have been written on this subject.2 It has been demonstrated that the mechanism of the reaction may differ, depending on the type of tin catalyst used, as well as the absolute and relative concentrations of both the catalyst and reagents. Furthermore, the catalyst may coordinate additional ligands or undergo ligand exchange with solvent or reagents to form new active catalytic compounds3 or exist as a dimer or oligomer in solution.4 Finally, the urethane reaction product can also function as a catalyst, which complicates studies of its kinetics. For this last reason it has been proposed that an ‘uncatalyzed’ isocyanate/hydroxyl reaction does not exist.5 Despite these difficulties, a wealth of investigations have been conducted, many involving model compounds, that have revealed details of the mechanisms of this important reaction. 6.2.3

Structure of the Tin Catalyst

The most active and commonly used catalysts contain tin(IV). Such compounds usually adopt a tetrahedral geometry and the analogy to carbon is obvious. But it must be kept in mind that tin is a metal that has empty 5d orbitals that can expand its coordination number through hypervalent interactions with polar compounds having strong electronegative atoms with non-bonding electron pairs, most notably oxygen or nitrogen. For example, complexes containing alkoxy groups can exist as either monomers or dimers in solution, depending on the steric bulk of the ligands (Figure 6.2.1). Another exception to the tetrahedral geometry for tin complexes are the dialkyltin dicarboxylates, such as di-n-butyltin dilaurate (DBTDL), where the carboxylate can be bidentate and a skew-trapezoidal bipyramidal geometry results (Figure 6.2.2). Tin atoms are capable of coordinating additional molecules in solution (Figure 6.2.3a), as well as undergoing associative exchange of certain labile ligands with other compounds in solution (Figure 6.2.3b).

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Figure 6.2.1

Tin(IV) dimer complex with expanded coordination number about tin

Figure 6.2.2

Structure of a dialkyltin dicarboxylate

So, the notion of an unsolvated tetrahedral tin compound in solutions containing both alcohols and isocyanates is an over-simplification and such ligand association and exchange processes form the basis for any proposed mechanism of catalysis. 6.2.4

Mechanisms

There are at least three distinct mechanisms proposed thus far for tin catalysts in the isocyanate/hydroxyl reaction.6 Thiele and Becker have categorized tin compounds as either ‘insertion’ or ‘Lewis Acid’ catalysts (Table 6.2.1).4 The authors report that one can easily determine the mechanism by which any catalyst works by observing their behavior in the presence of isocyanate alone. Catalysts that undergo an exothermic reaction with the isocyanate to generate isocyanurates are insertion catalysts. Insertion Mechanism

One of the first mechanisms proposed for the tin catalyzed formation of urethane is shown in Figure 6.2.4. This mechanism involves three basic steps: (1) Associative exchange of alcohol into the tin compound (solvolysis). (2) Coordination and insertion of the isocyanate into the Sn-O bond of the tin alkoxide. (3) Associative exchange of the intermediate N-stannylcarbamate with more alcohol regenerating the intermediate solvated tin complex.

Figure 6.2.3

(a) Coordination of additional ligands; (b) associative ligand exchange

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Table 6.2.1 Classification of organotin compounds according to their mechanism of catalysis Insertion Catalysts

Lewis Acid Catalysts

Oxides Alcoholates Thiolates Amine compounds Phosphine compounds β-Dicarbonyl compounds

Carboxylates Halides Salts of Inorganic Acids Sulfides

The major evidence in favor of this mechanism was provided by Davies, who demonstrated that tri-n-butyltin methoxide undergoes clean insertion of phenyl isocyanate to generate the tri-n-butyltin carbamates (Figure 6.2.5).7 It was also demonstrated that reaction of 6 with ethanol leads cleanly to methyl N -phenylcarbamate and tributyltin ethoxide (Figure 6.2.6). There is no report of any cross-over products. Others have argued that mixed carboxylate/alkoxides such as 7 are the active species, but it was admitted that the carboxylate had a higher affinity for tin than alkoxide and thus formation of the critical first catalytic intermediate in the process is not favorable.8 Indeed, 13 C NMR was used to measure an equilibrium constant of 8.3 × 10–4 for exchange of 2-propanol into di-n-butyltin dilaurate9 (DBTDL) (K1 of Figure 6.2.7). But at the extremely high hydroxyl/catalyst ratios observed in real systems, this can result in a significant conversion of the catalyst to the first intermediate proposed in the insertion mechanism.

Figure 6.2.4

Insertion mechanism for tin catalyzed urethane formation

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Figure 6.2.5

Insertion of phenyl isocyanate into the Sn-OCH3 bond of methoxytri-n-butyltin

Figure 6.2.6

Formation of methyl-N-phenylcarbamate and tributyltin ethoxide

Figure 6.2.7

Exchange of alcohol for carboxylate in dialkyltin dicarboxylates

Lewis Acid Mechanism

The Lewis acid mechanism of catalysis is shown in Figure 6.2.8. In this mechanism, the tin compound functions as a classic Lewis acid, coordinating to the isocyanate either through the carbonyl oxygen or nitrogen in the initial step to polarize the carbonyl. This increases the electrophilicity of the isocyanate carbon toward nucleophilic attack by alcohol. It has been shown by cryoscopy that there is a strong association of isocyanate with di-n-butyltin dicarboxylates.10

Figure 6.2.8

Lewis acid mechanism for tin catalyzed urethane formation

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685

Although it is known that complexes such as tin tetrafluoride will not undergo exchange with alcohols to form alkoxides, they can still function as catalysts for urethane formation.11 Finally, the fact that there is an observable Hammett effect for reactions run with various phenyl isocyanates substituted in the para-position suggests that coordination of the isocyanate is involved in the rate-determining step. It has been suggested that this mechanism dominates in cases where ligand exchange is not possible, or relatively slow. Houghton and Mulvaney have shown that the related distannoxane carboxylates do not undergo exchange with alcohols in solution, yet are effective catalysts.12 Evidence of a ternary complex between DBTDL, isocyanate and alcohol has been observed by NMR.13 Ionic Mechanism

A less conventional mechanism was first proposed by Van der Weij for the reaction of phenyl isocyanate with methanol using di-n-butyltin diacetate as catalyst,14 which is illustrated in Figure 6.2.9.

Figure 6.2.9

Ionic mechanism for tin catalyzed urethane formation

This mechanism was based on two pieces of evidence. First, a kinetic study showed that the order of the reaction with respect to both the catalyst and alcohol was 0.5. -d[PhNCO]/dt = +d[urethane]/dt = k[tin catalyst]0.5 [CH3 OH]0.5 [PhNCO]

(6.2.1)

Secondly, the reaction was inhibited by both strong and weak acids. Strong acids, such as HBF4 , completely stopped the reaction. Weaker acids, such as acetic acid, had a much less pronounced and concentration-dependent effect. It has been suggested that the concept of the ionic mechanism must be viewed with some degree of caution, since the reaction proceeded faster in non-polar solvents, such as cyclohexane, compared with a dipolar aprotic solvent, such as dimethylformamide, whereas one would expect that the polarity of the solvent would significantly stabilize the ionic catalyst intermediates.15 However, Urban et al. have demonstrated that an ionic mechanism is likely operative in the reaction of hexamethylene diisocyanate with an acrylic polyol, using DBTDL as catalyst.16 6.2.5

Synergism of Tin Compounds with Amine Catalysts: DBDTL and 1,4-Diazabicyclo[2.2.2]octane

It is well known that tin catalysts work synergistically with amines in the isocyanate/hydroxyl reaction. It has been argued that the two compounds work independently, with the tin acting as a classic Lewis

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Figure 6.2.10

Amine catalysis of tin alcoholysis

acid to polarize the isocyanate, as shown in Figure 6.2.85 and the base acting to deprotonate the alcohol thus making the resulting alkoxide more nucleophilic.17 ROH + R 3 N : RO− + R 3 N+ -H

(6.2.2)

However, Bechara18 has proposed an alternate mechanism involving an amine–tin complex. He concluded that the coordination of the amine to the tin aids in the alcoholysis of the tin carboxylate as shown in Figure 6.2.10. This mechanism was based on the kinetics of the reaction of phenyl isocyanate and n-butanol conducted at various absolute and relative concentrations of DBTDL and 1,4-diazabicyclo[2.2.2]octane (DABCO). 6.2.6

Mechanism of Catalysis with Blocked Isocyanates

Wicks19 has reviewed the various reaction mechanisms with blocked isocyanates. There are two general mechanisms (addition–elimination and elimination–addition) by which the blocked isocyanate reacts with a hydroxyl compound (Figure 6.2.11, A and B). It is possible that a particular type of blocked isocyanate can function by either mechanism, depending on such factors as the type of blocking group, type of hydroxyl compound, temperature, and the polarity of the solvent. Tin catalysts such as DBTDL are often included in such formulations, but higher concentrations are required than in reactions with isocyanates and the role of the catalyst is not always well defined. It is generally accepted that the tin compounds do not assist the elimination step for blocked isocyanates that function by the elimination–addition mechanism, but it is conceivable that tin catalysts can act as classic Lewis acids to promote the ‘transesterification,’ in the case where the addition–elimination mechanism is operative. Tin catalysts have been shown to act synergistically with other metal catalysts active in promoting blocked isocyanate reactions such as metal acetylacetonate. Tin catalysts have also been found to act synergistically with amines in circumstances where the elimination–addition mechanism was confirmed.20 6.2.7

Organotin Catalyst Composition

A large variety of organotin compounds are commercially available and registered in the toxic inventories of many countries.21 Most of the catalysts used in the reaction of isocyanates are based on dimethyltin,

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Figure 6.2.11

687

Two general mechanisms for reaction of a blocked isocyanate with a hydroxyl compound

di-n-butyltin and di-n-octyltin, and are derivatives, such as the alkyl mercaptopropionate, alkyl mercaptoacetate, dodecyl mercaptide, octanoate, alkyl thioglycolate, 2-ethylhexanoate, mercaptoacetate, alkyl maleate, dodecanoate, benzoate, salicylate, alkoxide, acetyl, phenoxide, sulfide, borate, sulfate, chloride, iodide, acrylate, phthalate, 2,4-pentanedionate, 2-mercaptosuccinate, fluoride, salts, or oxides. In a complex formulation there is high likelihood that the counterion to the tin compound is exchanged with the other ions in the formulation. Exceptions are the ions that bind stronger or chelate tin; these salts are more hydrolytically stable and can also show different rates of catalysis. 6.2.8

Catalysis with Organotin Compounds

Isocyanates are very versatile chemicals, which can undergo many reactions. Table 6.2.2 shows the commercially used reactions of isocyanates. Table 6.2.2

Reactions of isocyanates

Reactant

Catalysts

Product

Alcohol22,23 Water23 Mercaptan24 Carboxyl25 Urea26 Urethane Activated CH compounds27 Amine Epoxy28 Isocyanate29,30 Isocyanate31 Isocyanate32

Organotin, t-amine Organotin, t-amine Base, t-amine t-amine Acids pKa 0.1–0.8 Acids pKa 0.1–0.8 Strong base No catalyst Strong base, salts Quat. ammonium salt, K-salt Phosphine Phosphine oxides

Urethane Urea, carbon dioxide Thiourethane Amide, urea Biuret Allophanate Amide Urea Oxazolidone Isocyanurate Dimer, Diazetidine-2,4-dione Carbodiimide

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Tin Chemistry: Fundamentals, Frontiers and Applications Table 6.2.3 Relative reaction rates of phenylisocyanate with methanol a Catalyst

Relative rate

None Triethylamine n-Butyltin triacetate Di-n-butyltin diacetate Tri-n-butyltin acetate Tetra-n-butyltin

1 11 1400 30 000 500 80

a

Reaction conditions: equal molar amounts of phenyl isocyanate and methanol in n-butyl ether; 1 mol% catalyst per isocyanate at 30 ◦ C.

The reaction of aromatic isocyanates with hydroxyl groups is very responsive to tertiary amine catalysis and these catalysts are predominant amongst the catalysts used. Because many tertiary amines also catalyze the water side reaction, they are often used in conjunction with organometallic catalysts, which are more selective in catalyzing the isocyanate–hydroxyl reaction. Aliphatic isocyanates, on the other hand, are not very responsive to amine catalysis and require organometallic catalysts. Combinations of tertiary amines with organotin compounds are synergistic and give enhanced reaction rates. Table 6.2.3 compares the relative reaction rate33 of phenyl isocyanate with methanol. Table 6.2.4 illustrates the change in reaction rate33 of di-n-butyltin compounds with different ligands. Stronger acids or more strongly bonded tin compounds substantially reduce the reaction rate of phenyl isocyanate with methanol. Table 6.2.5 shows the gel time34 in a polyester/isocyanate composition with a range of di-n-butyltin catalysts, and combination of DBTDL with an amine catalyst and stannous, zirconium, and zinc octanoate.35

Table 6.2.4

Relative reaction rates of phenyl isocyanate with methanol a

Di-n-butyltin catalyst None Di-n-butyltin sulfide Di-n-butyltin difluoride Di-n-butyltin dihydride Di-n-butyltin dichloride Di-n-butyltin bis(octyl maleate) Di-n-butyltin dibenzenesulfonamide Di-n-butylbis(2,4-pentanedionato-O,O )-tin 3,3’-[(Di-n-butylstannylene)bis(thio)] bis(dodecanethiol) Di-n-butyltin diacetate Di-n-butyltin dibutoxide Di-n-butyltin oxide Di-n-butyltin maleate Di-n-butyltin dilaurate a

Relative rate 1 16 67 69 200 15 000 18 000 20 000 25 000 30 000 40 000 40 000 40 000 40 000

Reaction conditions equal molar amounts of phenyl isocyanate and methanol in n-butylether; 1 mol% catalyst per isocyanate at 30 ◦ C

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Table 6.2.5 Gel time (minutes) of polyestera Catalyst (1% parts by weight) None Di-n-butyltin S,S-dibutyldithio-carbonate Di-n-butyltin dilauryl mercaptide (DBTDLM) Di-n-butyltin dilaurate (DBTDL) Di-n-butyltin bis-o-phenylphenate Di-n-butyltin diacetate Di-n-butyltin maleate Dimethyltin dichloride (DMTDC) Stannous octanoate 1,4-Diazabicyclo[2.2.2]octane (DABCO) DBTDL and DABCO (1:1) Zirconium octanoate Zinc octanoate Bismuth stearate Triphenylantimony dichloride

Me, %

HDI

IPDI

H12-MDI

MDI

TDI

0 0.14 0.19 0.19 0.21 0.34 0.34 0.54 0.29 0 0.09 0.14 0.18 0.21 0.29

220 180 1 2 3 1 1 1 8 25 1 120 30 30 120

>240 >240 6 15 10 5 6 6 90 30 7 >240 120 60 >240

>240 >240 3 5 8 2 3 2 15 40 3 >240 120 90 >240

100 30 9/1) was used unless otherwise noted; b The double bond geometry was Z; c (Z)-But-2-enyltri-n-butyltin (E /Z = 3/7) was used

bulk of But is likely to have more influence than in a looser acyclic transition state. The results observed by Nishigaichi14 parallel that observed in the thermal reaction15 of but-2-enyltri-n-butytin, where the cyclic six-membered transition state (see Scheme 6.3.7) is believed to occur. It was assumed that this effect may be explained by taking into consideration the contribution of the more congested cyclic transition state, stabilized by the metal perchlorates, not acting as a conventional Lewis acid. The marked decrease in reactivity of but-2-enyltri-n-butyltin also compares with the thermal (high pressure) reaction.10 H

R

SnBu3

O

R

OH

(Z)-crotyltin

(10a)

H

R

O

SnBu3

R

OH

(E)-crotyltin Scheme 6.3.7

(10b)

The six-membered transition state for allylation of aldehydes with allyltin compounds

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Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides

709

Kinart16 compared the reactivity of allyldi-n-butyltin chloride and allyltri-n-butyltin towards n-hexanal in 4 M LiClO4 in diethyl ether. The reaction mixtures were prepared by dissolving 1 mmol of n-hexanal and an equimolar amount of allyltin compound in 2 cm3 of diethyl ether or in 2 cm3 of 4 M LPDE. The reactions were carried out over 24 h at room temperature. The obtained results show that the presence of lithium perchlorate in the solution improves considerably the yield of alcohol formed during the allylstannation. In pure diethyl ether at 25 ◦ C the addition proceeds for allyldi-n-butyltin chloride with only 50% yield, whereas in 4 M LPDE the yield of reaction increases to 100%, and for allyltri-n-butyltin it achieves a value of 20%. 6.3.2

Catalysis of reactions of triorganotin phenoxides with diethyl azodicarboxylate, bis(trichloroethyl) azodicarboxylate and diethyl acetylenedicarboxylate

The use of O-metallation of alcohols or enols to enhance their reactivity towards electrophiles, such as aldehydes or alkyl or acyl halides has been reported by Davies.17 The reaction of triorganotin alkoxides with other polar multiply-bonded acceptors was also reported (Scheme 6.3.8). R3SnOR + A=B

R3Sn-A-B-OR

where: A = B is RNC = O, RNC = S, O = CO, S = CS, RN = C = NR, EtO2C - C C - C - CO2Et

Scheme 6.3.8

The reaction of triorganotin alkoxides with polar multiply bonded acceptors

The polarity of the Mδ+ Oδ− Ar bond would be expected to promote the reaction with enophiles such as DEAD, bis(trichloroethyl) azodicarboxylate, and diethyl acetylenedicarboxylate, whatever the detailed structure of the metal phenoxide (which may be associated in solution), or the mechanism of the substitution. Organotin phenoxides (n-Bu3 SnOAr) are very useful reagents because it is easy to introduce or remove the organotin group, and because of the pronounced polarity of the Sn–O bond. Kobayashi and Yamaguchi18 found that the reaction of phenol with trimethylsilylacetylene at the ortho-position is catalyzed by SnCl4 -BuLi. It was suggested that the reaction occurs through the tin phenoxide and this can be written as shown in Scheme 6.3.9. Cl3Sn O

OH

SiMe3 OSnCl3

SiMe3 SiMe3

+

SnCl4-BuLi

H

(12) Scheme 6.3.9

(13)

The reaction of phenol with trimethylsilylacetylene catalyzed by SnCl4 -BuLi

The tri-n-butyltin phenoxides can be easily prepared by azeotropic dehydration of a mixture of phenol and bis(tri-n-butyltin) oxide (TBTO) in toluene.19 In experiments reported by Kinart20,21 the tin phenoxides and DEAD were added to a 4 M solution of LiClO4 in diethyl ether at 23 ◦ C. For measuring the half-lives of the reactions, the concentrations of the reagents were 0.0254 M and the absorbance of DEAD at 410 nm was monitored. On a preparative scale, the reaction was followed by TLC or NMR and the products were isolated by chromatography, which also served to remove the n-Bu3 Sn group. Table 6.3.7

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Table 6.3.7 Amination pf phenols with DEAD catalyzed by LiClO4 (half-lives of the reaction for 0.0254 mol dm−3 solutions)

OH

0

OSnBu3

1520

CH3

CH3

SnBu3

4

200

N

N

C

O

O

C

OEt

EtO OH

0

OSnBu3

2200 CH3 SnBu3

4

240

CH3

N

N

C

O

C

OEt

EtO

O OSnBu3

0

7500

O

OEt

OH

C

OEt

N

C N

4

O

SnBu3

270

CH3

CH3 OH

OSnBu3

0

1520

4

200

OCH3

OCH3 SnBu3 N

N

C

O

O

C

OEt

EtO

OH

0

OH

– H

4

124

N O

N

C

C

OEt

EtO

OEt

O OH

0

C

–

OEt

N H

4

621

O

C N

O

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Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides Table 6.3.7

711

(Continued)

OH

OSnBu3

0

1050 CH3 SnBu3

4

N

135

O

CH3

N

C

OEt

EtO

OEt

O

OSnBu3

0

–

O

C

OH

OEt N

C

N

CH3

4

>1200

0

720

CH3

O

SnBu3 CH3 CH3

OH OSnBu3

CH3 CH3

CH3 SnBu3

CH3

4

130

N O

N

C

O

C

EtO OEt

OH

0

OSnBu3

920

H3C

H 3C

CH3

4

CH3 SnBu3

200

N

N O

C

C

O

EtO OEt OEt

O C

OSnBu3

OEt

N

0

5760

Bu3Sn

C N

O OH

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shows the half-lives of the reactions and the structure of the products, which were obtained in essentially quantitative yields. Exceptionally, the yield of the reaction of tri-n-butyl-(3,4-dimethylphenoxy)tin was 30%, and this seems to be associated with the fact that its substitution was exclusively possible in the ortho-position. Additionally, Kinart observed that tri-n-butyl-(2,6-dimethoxyphenoxy)tin was completely inactive, and no product with DEAD was formed. Also, no reaction occurred between the parent phenols and DEAD under these conditions; the presence of LiClO4 reduced the half-lives of the reactions of tin phenoxides by a factor varying from 7 up to 27. Kinart also proved that these ring amination reactions could similarly be carried out catalytically, and indeed the reactions occur equally well when alcohol and DEAD are used together with 10 mol% of TBTO, thus avoiding the need to prepare the tin phenoxide. Indeed, the results showed that the reactions of 1- and 2-naphthols with DEAD are strongly catalyzed by TBTO. (1-Hydroxy-4-naphthyl)-hydrazine-N,N -dicarboxylic acid diethyl ester and (2-hydroxy-1-naphthyl)-hydrazine-N,N -dicarboxylic acid diethyl ester were respectively formed with excellent yields, and the half-lives of the reactions were similar to that of tri-n-butyltin naphthoxides.20 The cycle of reactions that is involved is shown in Scheme 6.3.10 and examples of the reaction in Table 6.3.7. OH

OH

EtO2C NNHCO2Et OSnBu3

OH

OSnBu3

(Bu3Sn)2O

EtO2C NNHCO2Et EtO2CN=NCO2Et

Scheme 6.3.10

Amination of tri-n-butylphenoxytin with DEAD

The mechanism of the reaction when the amination occurs in the para-position to the stannyloxy group may well be a simple electrophilic substitution via a Wheland intermediate as shown in Scheme 6.3.11. The reaction of 1-naphthylamine, however, has been written in the form of an ene reaction,22 and two reasonable mechanisms, apart from the above electrophilic substitution, can be proposed for the ortho-amination of 2-(tri-n-butylstannyloxy)naphthalene. First, it could follow a metalloene mechanism, as shown in Scheme 6.3.12.

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Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides O+SnBu3

OSnBu 3

OSnBu 3

713

OSnBu3

+ DEAD

_ EtO2CN

H EtO2CNNHCO 2Et NCO _ 2Et

Scheme 6.3.11 intermediate

The mechanism of amination of tri-n-butylphenoxytin with DEAD via a Wheland

CO2Et

CO2Et EtO2C

EtO2C

N N

EtO2C

SnBu3

N N

H

H OSnBu3

N N

SnBu3

CO2Et

O

O

SnBu3 OH

DEAD

Scheme 6.3.12 The mechanism of amination of 2-(tri-n-butylstannyloxy)naphthalene with DEAD. The metalloene mechanism

Second, the tin could act as a Lewis acid in stabilizing the Wheland intermediate by coordination to an anionic nitrogen, as shown in Scheme 6.3.13. CO2Et EtO2C

N N

_

CO2Et EtO2C

SnBu3

N N

H

H OSnBu3

O

OSnBu3

DEAD

Scheme 6.3.13 The mechanism of amination of 2-(tri-n-butylstannyloxy)naphthalene. Tin acts as a Lewis acid in stabilizing the Wheland intermediate by coordination to anionic nitrogen

In an attempt to obtain further evidence of the mechanism, the behavior of 2-methoxynaphthalene and 2-trimethylsiloxynaphthalene under standard conditions was studied. Neither compound would be expected to take part in an ene reaction, nor involve a Lewis acid-stabilized transition state, and if a reaction did occur, it would suggest a conventional electrophilic aromatic substitution. However, neither compound showed any reaction with DEAD in the presence of LiClO4 at room temperature for some

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weeks, and the mechanism of the reaction must still be regarded as an open question. Hydrazocarboxylate esters can readily be hydrolyzed and then reduced to amines. Hence, the reaction discussed above provides a means for introducing the amino group and its derivatives into a phenolic ring under very mild conditions. Leblanc23 has shown that reactions of electron-rich arenes with bis(2,2,2-trichloroethyl) azodicarboxylate in diethyl ether and acetone are strongly catalyzed by lithium perchlorate (3 M solutions of LiClO4 in Et2 O and acetone). Although it is more expensive and less stable than DEAD, it is also more reactive with electron-rich arenes. Its application may sometimes be an alternative to DEAD. Reactions reported by Leblanc with this azo enophile were carried out at elevated temperatures.23 However, heating solutions of LiClO4 in Et2 O or acetone may be hazardous. Kinart proved that the use of organotin phenoxides instead of pure phenols, and a more concentrated solution of LiClO4 makes it possible to carry out reactions with bis(trichloroethyl) azodicarboxylate with quantitative yield at room temperature (see Table 6.3.8).24 Kinart compared the yields of the reaction of four phenols and obtained from them tri-n-butytyltin phenoxides in 5 M solution of LiClO4 in Et2 O at room temperature. The aryl hydrazides prepared during this study were easily converted to their corresponding anilines by reduction with zinc dust in acetic acid. According to Kinart, LiClO4 plays two roles in the present reaction, i.e. the activation of the azo compound and stabilization of the intermediate complex. The 1.42 × 10−3 M solution of bis(2,2,2-trichloroethyl) azodicarboxlate in diethyl ether, exhibits an absorption maximum at 243 nm. An increase in the concentration of LiClO4 in Et2 O results in the decrease in the molar absorptivity at 243 nm. For example, its ratio in the solution of LiClO4 in Et2 O changes from approximately 1.1 to 1.6 with increasing concentration of LiClO4 in the range 1–1.9 mol dm−3 . This seems to reflect the lowering of the LUMO of the azo enophile by association with aggregates of LiClO4 . In 1908, pure ortho-vinylphenol was synthesised for the first time by decarboxylation of orthohydroxycinnnamic acid.25 Since then, a number of other methods have been developed.26,27 Kinart28 studied the possibility of using organotin phenoxides for the synthesis of both vinylphenols and phenyl vinyl ethers. The tri-n-butyltin phenoxides and diethyl acetylenedicarboxylate were added to a 5 M solution of LiClO4 in diethyl ether at 25 ◦ C. They were stored at room temperature for 2 days. The progress of the reaction was monitored by TLC (using 7:3 v/v petroleum–ethyl acetate mixture as eluent). The yields of the reactions and products of the additions of different tri-n-butyltin phenoxides with diethyl acetylenedicarboxylate carried in 5 M solutions of LiClO4 in diethyl ether at 25 ◦ C are collected in Table 6.3.9. According to Kinart28 the reaction between studied organotin phenoxides and diethyl acetylenedicarboxylate proceeds according to two possible mechanisms, which may compete (Scheme 6.3.14). As the result, a mixture of a pair of ortho-vinylphenols and the analogous pair of phenyl vinyl ethers can be obtained, as shown below. The mechanism of the reaction of vinylation in the ortho-position to the stannyloxy group of different organotin phenoxides must still be regarded as an open question as to whether it is an ene reaction or a simple aromatic substitution. The reaction of tin phenoxides with diethyl acetylenedicarboxylate gives a mixture of products. Additionally, Kinart28 has found that the yield of vinylphenols obtained as products of the reaction of tri-nbutyltin phenoxides increases in the following order: tri-n-butyl-(2-methoxyphenoxy)tin < tri-n-butyl-(otolyloxy)tin ≈ tri-n-butylphenoxytin < tri-n-butyl-( p-tolyloxy)tin. Although kinetic studies have not been carried out, the comparison of the yields seems to indicate that tri-n-butyl-( p-tolyloxy)tin is the most reactive of the four phenoxides mentioned above. The yields and the ratio of ethers and vinylphenols that were obtained from each reaction are given in Table 6.3.9. Again, as was observed for amination, tri-n-butyl(2,6-dimethoxyphenoxy)tin exhibited different behavior in comparison to other phenoxides. Its reaction

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715

Table 6.3.8 Amination of phenols and tri-n-butyltin phenoxides with bis(trichloroethyl) azo-dicarboxylate carried out in 5 mol dm−3 solution of LiClO4 in Et2 O

OH H3CO

OCH3

––

0

OH H3CO

OCH 3

OSnBu 3 H3CO

OCH3

SnBu 3 N

N

O

60

O

Cl3C

CCl 3

OH CH3

OH CH 3

H N

60

N

O

O

Cl3C

CCl3

OH CH3

OSnBu 3 CH3

SnBu3 N

N

O

100

O

Cl3C

CCl3 OH

OH CH3 H N

CH3

70

N

O

O

Cl3C

CCl3

(Continued )

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Table 6.3.8

(Continued)

OH

OSnBu 3 CH3 SnBu3 N

CH3

100

N

O

O

Cl3C

CCl3 OH CH3

OH CH3

CH3 H N

CH3

40

N

O

O

Cl3C

CCl3

OH CH3

OSnBu3 CH3

CH3 SnBu3 N

CH3

100

N

O

O

Cl3C

CCl3

O

OH H OH

CCl3 N N CCl3

50

O

CH 3

CH3 O

OSnBu 3

Bu3Sn OH

CCl3 N N CCl3 O

CH3

CH3

100

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Catalysis of Reactions of Allyltin Compounds and Organotin Phenoxides Table 6.3.9

717

Catalytic vinylation of different organotin phenoxides

EtOOC

EtOOC

H

COOEt

O

O OSnBu 3 H3CO

H OCH3

H3CO

COOEt OCH3

H3CO

60%

OCH3

(12)

(13) (12) : (13) = 1 : 1

EtOOC

EtOOC H

COOEt O

O COOEt

H

OSnBu3

CH3

CH3

(15)

(14) OH

60%

COOEt

OH

COOEt H

COOEt H

COOEt

(16)

(17) (14) : (15) : (16) : (17) = 1 : 1 : 1 : 1 EtOOC

EtOOC

H

COOEt

O

O

COOEt

H

CH3

CH3 OSnBu 3

(18)

(19)

OH

100% OH

COOEt

COOEt H

COOH CH3

COOEt

H

CH3

CH3

(20)

(21) (18) : (19) : (20) : (21) = 1 : 1 : 1 : 1

(Continued )

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Table 6.3.9

(Continued)

EtOOC

EtOOC COOEt

H O

O H

H 3C

COOEt

H3C

OSnBu3 H3C

(22)

(23)

OH

80%

COOEt

H3C

OH

COOEt

COOEt H3C

H

H

COOEt

(24)

(25)

(22) : (23) : (24) : (25) = 1.5 : 1.5 : 1 : 1 EtOOC

EtOOC COOEt

H O

O H

MeO

OSnBu3

(26)

MeO OH

COOEt

MeO

(27)

COOEt

MeO

COOEt

80% OH

MeO

H COOEt

H

(28)

COOEt

(29)

(26) : (27) : (28) : (29) = 2 : 2 : 1 : 1

OTE/SPH JWBK199-6.1


July 7, 2008

15:40



719

OSnBu3CO2Et

OSnBu3 CO2Et

_

H

CO2Et

+

H

CO2Et H

OSnBu3

+

CO2Et

OH

CO2Et EtO2C

CO2Et

H

CO2Et

CO2Et

H

SnBu3 O

O CO2Et H+

Scheme 6.3.14

CO2Et

The mechanism of vinylation of tri-n-butylphenoxytin with diethyl acetylenedicarboxylate.

with diethyl acetylenedicarboxylate gave only an equimolar mixture of 2-(2,6-dimethoxyphenoxy)maleic acid diethyl ester and 2-(2,6-dimethoxyphenoxy)fumaric acid diethyl ester in 80% yield (see Table 6.3.9).28 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

H.-S. Dang and A. G. Davies, J. Chem. Soc. Perkin Trans., 2, 721 (1991). H.-S. Dang and A. G. Davies, Tetrahedron Lett., 32, 1745 (1991). A. G. Davies and W. J. Kinart, J. Chem. Soc. Perkin Trans., 2, 2281 (1993). W. J. Kinart, C. M. Kinart, and I. Tylak, J. Organomet. Chem., 608, 49 (2000). W. J. Kinart, J. Chem. Research (S), 486, (1994). ´ c, I. Tylak, and C. M. Kinart, Phys. Chem. Liq., 38, 193 (2000). W. J. Kinart, E. Snie´ W. J. Kinart, I. Tylak, and C. M. Kinart, J. Chem. Research (S), 46 (1999). R. H. Young, K. Wehrly, and R. L. Martin, J. Am. Chem. Soc., 93, 5774 (1971). Y. Yamamoto, T. Komatsu, and K. Muruyama, J. Organomet. Chem., 285, 31 (1985). Y. Yamamoto, K. Muruyama, and K. Matsumoto, J. Chem. Soc., Chem. Commun., 489 (1983). K. J. Henry, P. A. Grieco, and C. T. Jagoe, Tetrahedron Lett., 33, 1817 (1992). J. Ipaktschi, A. Heydari, and H.-O. Kalinowski, Chem. Ber., 127, 905 (1994). Y. Nishigaichi, A. Takuwa, K. Iihama, and N. Yoshida, Chem. Lett., 693 (1991). Y. Nishigaichi, N. Nakano, and A. Takuwa, J. Chem. Soc., Perkin Trans.1, 1203 (1993). Y. Yamamoto, Acc. Chem. Res., 20, 243 (1987), and references cited therein. W. J. Kinart, C. M. Kinart, and M. Gruszczynska, Main Group Met. Chem., 25, 527 (2002).

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720 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

July 7, 2008

15:40


Tin Chemistry: Fundamentals, Frontiers and Applications A. G. Davies, J. Chem. Soc. Perkin Trans. 1, 1997 (2000). K. Kobayashi and M. Yamaguchi, Org. Lett., 3, 241 (2001). A. G. Davies, Organotin Chemistry, Wiley-VCH, Weinheim, 1997. W. J. Kinart and C. M. Kinart, J. Organomet. Chem., 665, 233 (2003). W. J. Kinart, C. M. Kinart, Q. T. Tran, and R. Oszcz¸eda, Main Group Met. Chem., 27, 241 (2004). J. A. Berson, Chemical Creativity, Wiley-VCH, New York, 1999 I. Zaltsgendler, Y. Leblanc, and M. A. Bernstein, Tetrahedron Lett., 34, 2441 (1993). W. J. Kinart, C. M. Kinart, Q. T. Tran, R. Oszcz¸eda, and R. Nazarski, Appl. Organomet. Chem., 18, 398 (2004). K. Fries and G. Fickewirth, Chem. Ber., 41, 367 (1908). E. T. Everhart and J. C. Craig, J. Chem Soc. Perkin Trans. 1, 1701 (1991). Y. Rollin, G. Meyer, M. Troupel, J.-F. Fauvarque, and J. Perichon, J. Chem. Soc. Chem. Commun., 793 (1983). W. J. Kinart, C. M. Kinart, Q. T. Tran, and R. Oszcz¸eda, 2Appl. Organomet. Chem., 19, 147 (2005); W. J. Kinart and C. M. Kinart, J. Organomet. Chem., 691, 1441 (2006).

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July 26, 2008

16:50


Index

References to figures are given in italic type. References to tables are given in bold type. absorption spectra, organotin compounds 6 acetates, cyclization 623 achazolid B 585 acids, reactions with tin 4 actinide 235 acylation, stannylene acetals 507 N -acyliminium cations 531–532 acylmethylstannylation alkenes 648 alkynes 644–645 acylstannylation 644–645 1,2,dienes 650 1,3,dienes 650 Aedes mosquitoes 432–434 aldehydes and allylic stannanes 516–519 allylstannane reactions 705–709 alisamycin 582 alkenes, carbostannylation 647–649 alkenylstannylation intramolecular alkenes 649 alkynes 645 alkyl groups, in PVC stabilizers 313, 318

alkylation dialkylstannylene acetals 504–505 stannylene acetals 509–511 alkynes carbostannylation 641–647 dimerization 645 alkynylstannylation 643–645, 645 1,2,dienes 650 alkynyltins, functionalization of metal oxide surfaces 297–301 allene, as relay switch 637 allenylstannanes 543–547 allotropes, tin 3 alloys applications 2 chemical vapor deposition (CVD) 292 allyl halides, cyclization 623–626 allylation allylic stannanes 517–518, 519–521, 538–539 exhibiting α-chelation control 521–524 exhibiting β-chelation control 524, 525 allylstannanes reactions with aldehydes 705–709 reactions with DEAD 703


OTE/SPH JWBK199-ind


722

July 26, 2008

16:50


Index

allylstannanes (Contd. ) reactions with α, β-unsaturated carbonyl compounds 532–533 reactions with 4-phenyl-1,2,4,-triazoline-3,5-dione (PTAD) 701 transmetalation 535–540 allylstannylation 641–643 alumina trihydrate (ATH) 345 Amberlite XE 305 610 amino acid derivatives 422 cardiovascular activity 421–425 fungicidal properties 447 anion partitioning 326–329 Anophles mosquitoes 433–435 anphoreticin 444 anti-tumor activity 454 antibiotics 582 apoptolidin 585 apoptosis, organotin-induced 489 applications isocyanate catalysts 691–694 medical 413 mono-organotin oxo-clusters 69 tin 2 tin oxide 267 tin(II) sulfide 291 aromatase 491 aromatic compounds 188–193 with group I metals 233 arsenic, clusters with tin 65–66 aryl iodides 631–633 cyclization 626–629 arynes, alkynyl and alkenylstannylation 645 ascorbic acid derivatives, cardiovascular activity 425 atom economy 653 ATP (adenosine triphosphate) 489–490 effect of n-butyltin chlorides 471 effect of di-n-butyltin 473–474 Baeyer-Villiger oxidation, computational studies 275 bafilomycin A 585 benzene rings 392 benzyl halides, cyclization 623–626 benzyne, as relay switch 636, 637 -tin 3 blood calcium transport 417–418 effects of organotin compounds 486–487 bonding 4–5, 6 bis-stannylenes and complexes 7

computational studies 276–277 tin to cadmium 238 tin to d-block elements 235–236 tin to f-block elements 234–235 tin to group I and II elements 231–234 within Zintl ions 140 Brevetoxin B 530 bromine, in flame retardants 343 but-2-enyltri-n-butyltin, reactions with aldehydes 708 n-butyltin chlorides, effect on NK cell function 470–472, 470 by-products, removal 654–656 cadmium, bonds with tin 238 caesium 234 calcium transport, in blood 417–418 carbamyl chlorides 623 carbohydrate synthesis, overview 497–498 carbon dioxide, as supercritical solvent 658–659 carbon monoxide, as cyclization relay switch 634–636 carbonyl compounds 532–533 carbostannylation 640–651 alkenes 647–649 alkynes 641–647 dienes 650–651 carboxylates diorganotin 97–100 diorganotin polymers 104 monorganotin, with open structures 108 as PVC stabilizers 314–315 cardiovascular system 414–421 cassiterite 1, 251 catalysis alkynylstannylation 644 allylstannane reactions, by lithium perchlorates 701–704 allylstannylation 642–643 by metal perchlorates 707–708 computational studies 275 cross-coupling 564–570 fluourous, distannoxanes 668–672 hydrostannation 12 insertion reactions 682–685, 683 isocyanate reactions 681–698 Lewis acid catalysts 683 with lithium perchlorate 701–719 replacements for organotins 698 triorganotin phenoxides with diethyl carboxylates 709–719 with zirconocene 380

OTE/SPH JWBK199-ind


July 26, 2008

16:50


Index cell function cell membranes 484–488 effect on NK cell function 488–491 di-n-butyltin chloride 472–475 dimethylphenyltin chloride 475–476 chalcogenides, imidotin 62–64 characterization techniques, tin(II) derivatives 256–257 chemical properties, polystannanes 381–382 chemical shift, isotope-induced 38–39 chemical shift anisotropy (CSA) 20–21 chemical vapor deposition (CVD) overview 285–286 compound properties 287 organometallic 287 tin alloys 292 tin(IV) oxide 287–290 chemotherapy 454 chirality allylic stannanes 541–542 and NLO properties 352 clusters 53–66 imidotin cubane 53–59 coated polymer fillers 344–346, 347 coatings 691–692 computational chemistry overview 269 ECP methods 269–271 NLO (non-linear optical) properties 357 other methods 271–272 reaction mechanisms 274–276 spectroscopic parameters 277 computational studies bond strengths and bond disassociation enthalpies 276–277 thermochemistry 276 connectors inorganic 120–123 organic 123–133 consistent effective potential (CEP) 271 coordination chemistry tin(II) derivatives 252–254 unusual coordination geometries 241–246 hepta-coordinated tin atoms 244 octa-coordinated tin atoms 244 penta-coordinated tin atoms 244 tetra-coordinated tin atoms 243–244 tri-coordinated tin atoms 243 coordination number, and NMR behaviour 23–27 coupling constants long-range 44–45

723

one-bond 39–42 three-bond 43–44 two-bond 43 coupling reactions see cross-coupling; Wurtz coupling crocacin D 585 cross-coupling reactions alkenyl-alkenyl 579–587 catalysts and ligands 564–570 copper effect 571–573 mechanisms 561–564 natural product synthesis 574–575 Stille coupling 561 alkenyl-alkenyl 587 alkenyl-aryl 587–593 aryl-aryl 593–594 by-products 654–656 copper effect 571–572 and green chemistry 653 for natural product synthesis 579–587 sp3-sp2coupling 594–599 without ligands 570–571 cryptands 132 crystallography, stannyllium ions 157–158 cubanes double-cubane clusters 61–62 seco-cubanes 60–61, 63 Culex mosquitoes 434–435 CVD see chemical vapor deposition cyclic structures, NMR spectroscopy properties 27–35 cyclization 622–623 mono-cyclization 623–629 bis-cyclization 630–633 tris-cyclization 633 acetates 623 allyl halides 623–626 aryl iodides 626–629 benzyl halides 623–626 involving relay switches 634–637 cyclooligomeric derivatives 262–267 cyclopentadienyltin(IV) compounds 10 d-block metals, bonding 235–236 DEAD (diethyl azodicarboxylate) 702, 703 dehydrochlorination, PVC 320–322 dehydropolymerization, polystannane synthesis 378–381 density functional theory (DFT) 278 di-n-butyltins, environmental impact 469 di-n-butyltin chloride, effect on NK cell function 472–475

OTE/SPH JWBK199-ind


724

July 26, 2008

16:50


Index

dialkylation, stannylene acetals 510 dialkylstannylene acetals, structure 500–503 diaminostannylenes 25, 161–162 diazabicycloundecene (DBU) 655 dibutyltin dilaureate (DBTDL) 693 dibutyltin oxide, as catalyst 499 dictostatin 585 Diels-Aler dimerization 584–585 dienes, carbostannylation and acylstannylation 650–651 diethyl actylenedicarboxylate 709–719 diethyl azodicarboxylate (DEAD) 702, 703 -diketonates 253 dimethylphenyltin chloride (DMPTC), effects on NK cell function 475–476 diorganostannylenes 24 diorganotins, as building block for self–assembly 119 dipalmitoyl phosphatidylcholine (DPPC) 484–488, 485 dipeptides organotin derivatives 423–425 fungicidal properties 447 Dirac equation 270 distannane 7 distannene 7 distannenes 177–183 physical parameters 180 synthesis 178–181 distannynes 7, 8, 196–198 physical parameters 196 DNA, organotin compounds 482–483 drums 106–107 monoorganotin carboxylates 103–105 effective core potential (ECP) 270–271 effective mass yield 653 electrocoating 695 electron structure, Zintl ions 141–143 electronic configurations, computational modelling 270–272 elimination reactions, tin(II) derivatives 256 environmental impact, di-n-butyltins 469 equilibrium reactions, stannylene acetals 508–509 erythrocytes 486–487 esterification 667–672 estertins 12 fibers, flame-retardant treatments 339–340 flame retardants 339–348 coated fillers 344–346

inorganic tin colloids 343–344 ultrafine powders 344 flies 431–432 flucytosine 444 fostriecin 585 fungi 443–444 fungicides 443–452 complexes with dithiocarbamates 451 complexes with hydrazones and thiohydrazones 450–451 complexes with Schiff bases 451–452 complexes with sulfides 451 complexes with thiosemicarbamates 451 complexes with triazoles 450–451 mechanisms of action 444–445 gels, preparation 301–306 germastannenes 193 germylenes diorgano- and diamino- 164–166 heterocyclic 167 glassmaking 2 gold 236 grafted catalysts, catalytic activity 675–678 green chemistry overview 653–656 catalysts 668–672 reagents low toxicity 661–663 without tin 663–664 grey tin 3 Group 1 elements 233 Group 11 elements 236 bonds with tin 238–239, 238 halides 27 boiling points 346 hepatotoxicity 490–491 heterometallic compounds, tin(II) 251–267 histone, acetylation 483–484 homoleptic compounds, N-heterocyclic stannocenes 169–171 hybrid materials classification 296 nanoporous 306–309 precursors bridged ditins 368–374 functional trialkynylorganotins 361–368, 363–364 hydrazones, complexed with organotins 449–450

OTE/SPH JWBK199-ind


July 26, 2008

16:50


Index hydrides 231–234 and alkyl halide reduction 614–615 NMR spectroscopy parameters 28 as reagents 607 hypervalent stannate ion 7 imidotin cubane clusters 53–59 reaction chemistry 57–59 imines, and allylic stannanes 533–534 index value (IV) 438 indium-tin oxide (ITO) 251 insecticides flies 431–432 mosquitoes 432–435 quantitative structure-activity relationship (QSAR) 435–439 insertion catalysis 682–685 ion selective electrodes (ISE) 324–325, 325 anion selective 329–336 ionophores 326 selectivity 334 selectivity control 334–335 ionic catalysis 685 ionic supramolecular complexes 397–398 ionophores, fluoride 335–336 IR (infrared) spectrometry 257 isocyanates blocked 694–696 catalytic mechanisms 687 catalytic mechanisms 686–691 reactions 687 isomerism tetraorganodistannoxanes 204 Zintl ion compounds 144 isotopes 3, 17 ITO (indium-tin oxide) 289–290 IUPAC nomenclature, dialkylstannylene acetals 497–498 ketones, synthesis using cross-coupling reactions 599–600 Kocheschkov reaction 10, 11 ladder structures, diorganotin carboxylates 95, 96, 100 Lappert’s stannylene 8 leaching 676, 678 Lewis acid-base interactions, Tin(II) derivatives 254–255 Lewis acids as allylation catalysts 552–555

725

benzannulated stannylenes 162–163 catalysts 683 ligands dithiocarbamate, in macrocycles 131–132 imidazole- derived, in macrocycles 131 inorganic, in macrocycles 120–123, 124 pyridine- derived, in macrocycles 131 stannylenes as 168 tin(II) derivatives 252–254 liquid membranes 326–329 lithium 233 lytic function effect of n-butyltin chlorides 470–471 effects of di-n-butyltin chloride 472–474 macrocycles 125 di- and oligonuclear 125, 126–133, 127, 128 with diorganotin carboxylates 96–97, 101–103 formation 120–133 inorganic connectors 124 ligands dithiocarbamate 131–132 inorganic 120–123, 124 mononuclear 126 organic connectors 123–133 macroporous polymers 609–613 magnesium hydroxide (MH) 345 mass increase 269–270 mass spectrometry 257 matrices polystyrene, insoluble 609–613 soluble polymers 613 membranes, cell 484–488 mercaptides for PVC, mechanism of stabilization 315–317 as PVC stabilizer 315 metallocenes 386 metathesis reactions, tin(II) derivatives 255 methyl acylate 647–649 methyldiphenyltin chloride (MDPTC), effects on NK cell function 475–476 microwave irradiation, and cross-coupling reactions 572–573 mitochondria 490–491 mitogen-activated protein kinases (MAPK) 472 MOCVD 287 molecular volume, and toxicity 437 monocarbonylation 634–636 monoorganotin, as building block for self-assembly 119 mosquitoes 432–435

OTE/SPH JWBK199-ind


726

July 26, 2008

16:50


Index

Mössbauer Spectroscopy 257 Musca domestica 431–432 mycoses 444 nano-tin 661–662 nanoporous materials 306–309 National Cancer Institute (NCI) 454 natural product synthesis 579–601 Nernst equation 325 NK cells 469 effects of organotins 488–491 NLO (non-linear optical) properties molecular materials 353–354 overview 351–352 NMR spectroscopy benzannulated stannylenes 162–163 compounds with oxidation state

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